JP7787482B2 - Steel plates and outer panel components - Google Patents

Description

The present invention relates to steel sheets, and more specifically to steel sheets and exterior panel members with excellent appearance, primarily for use in, for example, automobile exterior panel members.

In order to reduce carbon dioxide emissions from automobiles, efforts are underway to reduce the weight of automobile bodies while ensuring safety by using high-strength steel sheets. While the increase in the strength of automotive steel sheets has been remarkable for automobile frame components, steel sheets with a tensile strength of 300 MPa or less are still primarily used for exterior panel components such as doors and hoods, and no progress has been made in increasing strength. These exterior panel components require high formability and appearance. Generally, increasing the strength of steel sheets reduces their formability and appearance after forming. Therefore, it is difficult to achieve both strength and formability and appearance, particularly appearance after forming, in high-strength steel sheets. Several methods have been proposed to address these issues.

For example, Patent Document 1 describes a steel sheet for hot-dip galvanizing containing, by mass%, C: 0.02-0.3%, Si: 0.1-2.0%, Mn: less than 1.0%, Cr: more than 1.0% but not more than 3.0%, P: 0.02% or less, S: 0.02% or less, Al: 0.014% or less, and N: 0.001-0.008%, satisfying the following conditions: 2.5% or less than 1.5% Mn% + Cr%, 4.1% - 2.3% Mn% - 1.2% Cr% or less Si%, with the balance being Fe and unavoidable impurities. Patent Document 1 also teaches that by optimizing the amounts of Mn, Cr, and Si added, it is possible to achieve both the workability of a steel sheet for hot-dip galvanizing with a tensile strength of 390 MPa or more and an appearance after processing that is suitable for use as an automotive exterior panel. Furthermore, Patent Document 1 teaches that by setting the area ratio of the ferrite, which is the main phase, to 70% or more and the area ratio of the hard second phase containing martensite to 30% or less, it is possible to keep all of the strength, yield strength, yield ratio, and strength-ductility balance within a good range.

Patent Document 2 states that the mass percentages are C: 0.0005-0.01%, Si: 0.2% or less, Mn: 0.1-1.5%, P: 0.03% or less, S: 0.005-0.03%, Ti: 0.02-0.1%, Al: 0.01-0.05%, N: 0.005% or less, Sb: 0.03% or less, and Cu: over 0.005% and 0.03% or less, and that Ti* = (Ti%) - 3.4 x (N%) - 1.5 x (S%) - 4 x (C%). [0005] Patent Document 2 discloses a cold-rolled steel sheet having a composition containing Ti* in a range satisfying 0<Ti*<0.02 and further satisfying (Sb%)≧(Cu%)/5, with the balance consisting of Fe and unavoidable impurities, wherein the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the surface layer portion up to 10 μm from each surface on both sides of the steel sheet is 9% or less of the total Ti content (mass%) in the steel sheet. Patent Document 2 also teaches that by setting the content (mass%) of Ti element contained in precipitates less than 20 nm in size in the surface layer portion up to 10 μm from each surface on both sides of the steel sheet to 9% or less of the total Ti content (mass%) in the steel sheet, it is possible to avoid the occurrence of uneven appearance due to such fine Ti-based precipitates and obtain a cold-rolled steel sheet with excellent surface properties, and further teaches that the cold-rolled steel sheet can be suitably used for parts that require excellent surface quality after forming, such as automotive exterior panels.

JP 2009-249737 A International Publication No. 2011/142473

For example, in the case of dual-phase steels having a metal structure containing soft ferrite and hard martensite, as described in Patent Document 1, non-uniform deformation is likely to occur during processing such as press forming, in which the soft ferrite and its surrounding areas deform preferentially. Therefore, when using dual-phase steels composed of soft and hard structures, minute irregularities may appear on the steel sheet surface after forming, resulting in appearance defects known as ghost lines. Furthermore, thin, wide-width steel sheets are relatively often required for automotive exterior panel applications, as described in Patent Documents 1 and 2. However, such thin, wide-width steel sheets are prone to heat buckling, a type of bending, during the manufacturing process, for example, when they are threaded through a continuous annealing processing line (CAPL). Continuing operations while heat buckling occurs can lead to sheet fracture, which necessitates the production line being stopped and recovery work being carried out, resulting in significant damage.

Therefore, the present invention aims to provide a steel sheet with a novel configuration that can suppress the occurrence of heat buckling during the manufacturing process while achieving both strength and good appearance after forming.

To achieve the above objective, the inventors conducted research focusing on both the chemical composition and metallographic structure of the steel sheet. As a result, they discovered that optimizing the chemical composition of the steel sheet and appropriately controlling the form and amount of Nb carbonitrides can improve the high-temperature strength of the steel sheet, thereby suppressing the occurrence of heat buckling during the manufacturing process. Furthermore, by uniformly dispersing martensite contained in a predetermined proportion in the metallographic structure in both the micro- and macro-regions of the metallographic structure, the desired high strength can be achieved based on this hard structure, and the generation of minute irregularities on the steel sheet surface can be significantly suppressed even when strain is imparted by press forming or the like, thereby completing the present invention.

The present invention, which has achieved the above object, is as follows.
(1) in mass %,
C: 0.030-0.100%,
Mn: 0.70-3.00%,
Si: 0.005-1.500%,
P: 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0010-0.0150%,
O: 0.0100% or less,
Nb: 0.005-0.200%,
Cr: 0-1.00%,
Mo: 0 to 0.80%,
B: 0 to 0.0100%,
Ti: 0-0.200%,
V: 0 to 0.500%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0-1.00%,
Ta: 0-0.10%,
Co: 0-3.00%,
Sn: 0-1.00%,
Sb: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0-0.0500%,
As: 0 to 0.10%, and the balance: Fe and impurities, and has a chemical composition in which index A represented by the following formula 1 is 0.50% or more,
In area %,
Ferrite: 75 to 95%,
Martensite: 5 to 25%, and the remaining structure: 0 to 10% in total,
the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
The average grain spacing of the martensite is 2.5 μm or less,
A steel sheet characterized by having a metal structure in which the standard deviation of the area ratio of martensite in a direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained.
(2) The chemical composition is, in mass%,
Cr: 0.001-1.00%,
Mo: 0.001-0.80%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.200%,
V: 0.001-0.500%,
Ni: 0.001 to 1.00%,
Cu: 0.001 to 1.00%,
W: 0.001-1.00%,
Ta: 0.001 to 0.10%,
Co: 0.001-3.00%,
Sn: 0.001 to 1.00%,
Sb: 0.001-0.200%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%,
REM: 0.0001-0.0100%,
Bi: 0.0001 to 0.0500%, and As: 0.001 to 0.10%
The steel sheet according to (1) above, characterized in that it contains at least one of the following:
(3) The steel sheet according to (1) or (2), characterized in that the ferrite has an average grain size of 3.0 to 25.0 μm, the martensite has an average grain size of 1.0 to 5.0 μm, and the martensite has an average aspect ratio of 2.5 or more.
(4) An outer panel member comprising the steel plate according to any one of (1) to (3) above.

The present invention makes it possible to provide a steel sheet that can suppress the occurrence of heat buckling during the manufacturing process and that can achieve both strength and good appearance after forming.

<Steel plate>
The steel plate according to the embodiment of the present invention comprises, in mass%,
C: 0.030-0.100%,
Mn: 0.70-3.00%,
Si: 0.005-1.500%,
P: 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0010-0.0150%,
O: 0.0100% or less,
Nb: 0.005-0.200%,
Cr: 0-1.00%,
Mo: 0 to 0.80%,
B: 0 to 0.0100%,
Ti: 0-0.200%,
V: 0 to 0.500%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0-1.00%,
Ta: 0-0.10%,
Co: 0-3.00%,
Sn: 0-1.00%,
Sb: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0-0.0500%,
As: 0 to 0.10%, and the balance: Fe and impurities, and has a chemical composition in which index A represented by the following formula 1 is 0.50% or more,
In area %,
Ferrite: 75 to 95%,
Martensite: 5 to 25%, and the remaining structure: 0 to 10% in total,
the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
The average grain spacing of the martensite is 2.5 μm or less,
The steel sheet is characterized by having a metal structure in which the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained.

In recent years, there has been an increasing need to reduce the weight of automotive exterior panel components (roofs, hoods, fenders, doors, etc.). Therefore, similar to the case of structural components, there is a demand for higher strength and thinner thicknesses for these exterior panel components. However, dual-phase steel (DP steel), which has a relatively low yield strength, is often used for these exterior panel components to avoid surface defects known as surface distortions that occur during press forming and other processes. However, DP steel, which contains a mixture of soft ferrite and hard martensite, is prone to non-uniform deformation during processes such as press forming, in which the soft structure and its surrounding area deform preferentially. This can lead to minute irregularities on the steel sheet surface after forming, resulting in appearance defects known as ghost lines. More specifically, during processes such as press forming, the soft ferrite structure deforms significantly, resulting in depressions on the steel sheet surface. On the other hand, the hard martensite structure deforms less. Therefore, the hard structure does not deform as much on the steel sheet surface as the soft structure, but rather protrudes. As a result, variations in the amount of deformation occur, particularly in the width direction of the steel sheet (the direction perpendicular to the rolling direction and the thickness direction), resulting in the formation of band-shaped (striped) ghost lines. Meanwhile, as steel sheets become stronger, elements such as Mn are sometimes added in relatively large amounts to improve the hardenability of the steel sheet. Mn is an element that is prone to streak-like segregation in steel sheets. More specifically, Mn-enriched regions such as center segregation and microsegregation are formed during casting, and these enriched regions are elongated in the rolling direction by hot rolling or cold rolling, resulting in Mn segregation in streak-like segregation. Therefore, due to this Mn segregation, regions with high and low hardenability exist in the steel sheet. As a result, a relatively large number of striped hard structures are formed in the metallographic structure of the steel sheet after quenching. In this case, the occurrence of ghost lines becomes particularly noticeable. On the other hand, if Mn segregation in steel sheets could be sufficiently suppressed, the formation of such striped hard structures would be reduced, making it possible to more uniformly disperse the hard structures in the metallographic structure. In this case, even when strain is imparted by press forming or the like, it is believed that the generation of minute irregularities on the steel sheet surface can be sufficiently reduced, and the occurrence of ghost lines can be suppressed. However, in particular when the amount of Mn added to the steel sheet increases in accordance with the demand for higher strength, it is actually very difficult to reliably and sufficiently suppress Mn segregation. For this reason, it is generally quite difficult to achieve both strength and good appearance after forming.

As mentioned above, thin, wide-width steel sheets are often required for automotive exterior panel applications. However, such thin, wide-width steel sheets are prone to heat buckling during the manufacturing process, for example, when passing through a continuous annealing and processing line (CAPL). More specifically, CAPLs are typically equipped with hearth rolls with a crown that creates a convex shape in the center. Therefore, when a steel sheet passes through a CAPL, compressive stress is applied to the steel sheet's widthwise center due to the convex crown of the hearth roll. Meanwhile, because continuous annealing is performed at relatively high temperatures, the yield stress of the steel sheet decreases as the sheet temperature increases. Therefore, particularly when thin, wide-width steel sheets are continuously annealed in a CAPL, the decrease in yield stress at relatively high temperatures can cause the steel sheet to be unable to adequately resist the compressive stress. In such cases, a phenomenon known as heat buckling occurs, in which the sheet breaks and wrinkles. Continuing operations while heat buckling is occurring can lead to plate breakage, and if plate breakage occurs, the production line must be stopped and recovery work carried out, resulting in significant damage.

Therefore, the present inventors first investigated ways to improve the high-temperature strength of steel sheets from the viewpoints of both the chemical composition and metallographic structure of the steel sheets in order to suppress or reduce the occurrence of heat buckling. As a result, the present inventors found that, from the viewpoint of the chemical composition of the steel sheets, the high-temperature strength of the steel sheets can be improved by controlling the index A, expressed by the following formula 1, to 0.50% or more, thereby suppressing or reducing the occurrence of heat buckling.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained.

While not intending to be bound by any particular theory, it is believed that controlling index A to 0.50% or more not only improves the high-temperature strength of the steel sheet but also lowers the Ac3 point of the steel sheet. As will be described in detail later in connection with the steel sheet manufacturing method, in embodiments of the present invention, in the first heat treatment step corresponding to the CAPL after cold rolling, the steel sheet must be heated to a temperature higher than the Ac3 point at which the steel sheet becomes austenite single phase, more specifically, to Ac3 + 10°C or higher. Therefore, lowering the Ac3 point allows the heating temperature in the first heat treatment step to be lowered, which in turn makes it possible to suppress the decrease in yield stress of the steel sheet that accompanies an increase in sheet temperature during heating. Therefore, controlling index A to 0.50% or more significantly improves the resistance of the steel sheet to the compressive stress that causes heat buckling by actively improving the high-temperature strength of the steel sheet itself and suppressing the decrease in yield stress due to the lower heating temperature in the first heat treatment step.

Furthermore, the inventors have discovered that, from the perspective of the metallographic structure of the steel sheet, controlling the morphology and amount of Nb carbonitrides so that a relatively large amount of Nb carbonitrides of an appropriate size is present in the steel sheet; more specifically, controlling the morphology and amount of Nb carbonitrides so that the Nb content of all Nb carbonitrides is 0.004% or more and the Nb content of Nb carbonitrides with a particle size of 20 nm or more is 60% or more of the total Nb carbonitrides can improve the high-temperature strength of the steel sheet. Controlling the morphology and amount of Nb carbonitrides within this range allows for the presence in the steel of a sufficient amount of Nb carbonitrides with a particle size of 20 nm or more, which is effective in improving high-temperature strength. Therefore, in combination with controlling the chemical composition using Index A described above, the high-temperature strength of the steel sheet can be significantly improved, and as a result, the occurrence of heat buckling during the steel sheet manufacturing process can be significantly suppressed or reduced.

Next, in order to achieve both strength and good post-forming appearance, the inventors investigated ways to optimize the ratio of ferrite, a soft structure, to martensite, a hard structure, in the metallographic structure to achieve the desired high strength while also improving the post-forming appearance. Specifically, the inventors focused on the distribution of martensite, a hard structure, in the metallographic structure, and more specifically, investigated ways to control the martensite distribution from a perspective other than reducing Mn segregation. As a result, as will be described in detail later with regard to the manufacturing method of steel sheet, the inventors discovered that by configuring the metallographic structure of a steel sheet before final annealing to a structure primarily composed of bainite and/or martensite, and then final annealing the steel sheet having such a metallographic structure under specified conditions, it is possible to uniformly distribute martensite in both micro and macro regions in the final metallographic structure, regardless of the presence or degree of Mn segregation. More specifically, the inventors discovered that by subjecting a steel sheet having a metallographic structure consisting of bainite and/or martensite to final annealing under predetermined conditions, the average particle spacing of martensite can be controlled to 2.5 μm or less in the microregion, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction can be controlled to 1.5% or less in the macroregion. By controlling the average particle spacing of martensite to 2.5 μm or less, hard structures can be densely and uniformly dispersed in the microregion. By controlling the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less, the variation in hard structures in the macroregion can be significantly reduced. By satisfying both of these requirements, a metallographic structure in which martensite, a hard structure, is finely and uniformly dispersed throughout the steel sheet can be formed. As a result, the steel sheet according to the embodiment of the present invention can achieve a more uniform deformation amount of the steel sheet, particularly in the width direction, even during forming such as press forming, and can achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed. For example, even if the uniformity of martensite is ensured in the microscopic region, if the uniformity of martensite is not ensured in the macroscopic region, it is not possible to form a metallographic structure in which martensite is finely and uniformly dispersed throughout the steel sheet. Similarly, even if the uniformity of martensite is ensured in the macroscopic region, if the uniformity of martensite is not ensured in the microscopic region, martensite may exist unevenly locally, making it impossible to form a metallographic structure in which martensite is finely and uniformly dispersed throughout the steel sheet. Therefore, in order to achieve an excellent post-forming appearance in which appearance defects such as ghost lines are significantly suppressed in the steel sheet according to the embodiment of the present invention, it is necessary to satisfy both requirements: controlling the average particle spacing of martensite to 2.5 μm or less, and controlling the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction to 1.5% or less.

While not intending to be bound by any particular theory, it is believed that in order to achieve a fine and uniform dispersion of martensite throughout the entire steel sheet in the metallographic structure of the final steel sheet, it is extremely important to form a large number of highly dispersed austenite nucleation sites during heating in the final annealing. In this regard, martensite structures contain substructures such as packets, blocks, and laths within prior austenite grains, and therefore have many different internal interfaces compared to structures such as ferrite. Bainite, like martensite, also has many different internal interfaces. Therefore, by forming the metallographic structure of the steel sheet before final annealing with bainite and/or martensite, it is possible to generate a large number of dispersed carbides that can serve as austenite nucleation sites at these interfaces during heating of such a metallographic structure in the final annealing. Therefore, after forming many carbides at the interfaces, further heating to the two-phase region of ferrite and austenite is believed to enable the fine and uniform formation of austenite throughout the steel sheet. Finally, by rapidly cooling a steel sheet having such a metallographic structure, martensite is generated from the austenite, and in the final metallographic structure, the average particle spacing of martensite is controlled to 2.5 μm or less, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. In other words, it is believed that a metallographic structure in which martensite is uniformly dispersed in both micro- and macro-regions can be obtained. By performing such heat treatment, it is believed that martensite can be finely and uniformly dispersed throughout the steel sheet to such an extent that the effects of Mn segregation are counteracted. Conventionally, it has been considered common to consider control of the distribution of hard structures from the perspective of reducing Mn segregation itself. Therefore, it is extremely unexpected and surprising that martensite can be uniformly dispersed in both micro- and macro-regions in the final metallographic structure, regardless of the presence or degree of Mn segregation.

In addition to the above-mentioned findings related to suppressing the occurrence of heat buckling and ghost lines, the steel sheet according to an embodiment of the present invention ensures good formability by controlling the area ratio of ferrite, a soft structure, to 75-95%, while controlling the area ratio of martensite, a hard structure, to 5-25% and further controlling the chemical composition of the steel sheet within a specified range to ensure high strength of 540 MPa or more. As a result, it is possible to suppress the occurrence of heat buckling during the manufacturing process and achieve a high level of both strength and appearance after forming.

The steel sheet according to an embodiment of the present invention will be described in more detail below. In the following description, the unit of content of each element, "%," means "mass %" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values before and after it are included as the lower and upper limits, unless otherwise specified.

[C:0.030-0.100%]
C is an element that ensures a predetermined amount of martensite and improves the strength of the steel sheet. C is also an austenite-stabilizing element and is effective in lowering the Ac3 point. To fully obtain these effects, the C content is set to 0.030% or more. The C content may be 0.040% or more or 0.050% or more. On the other hand, excessive C content may result in excessively high strength, resulting in reduced elongation, or in the inability to control the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction within a desired range. For this reason, the C content is set to 0.100% or less. The C content may be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[Mn: 0.70-3.00%]
Mn is an element that improves hardenability and contributes to improving steel sheet strength. Furthermore, Mn is also an austenite-stabilizing element and is effective in lowering the Ac3 point. To fully obtain these effects, the Mn content is set to 0.70% or more. The Mn content may be 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. In a preferred method for producing a steel sheet, which will be described later, the metallographic structure of the steel sheet before final annealing must be composed of a structure mainly composed of bainite and/or martensite in order to uniformly disperse martensite in both the micro and macro regions in the final metallographic structure. Therefore, improving hardenability by adding Mn is also important for improving the appearance after forming. On the other hand, excessive Mn content may not fully counteract the effects of Mn segregation, making it impossible to control the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction within the desired range. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[Si: 0.005 to 1.500%]
Si is an element that improves the strength of steel sheets through solid solution strengthening. To fully obtain this effect, the Si content is set to 0.005% or more. The Si content may be 0.010% or more, 0.100% or more, 0.200% or more, 0.300% or more, or 0.400% or more. On the other hand, excessive Si content may make it difficult to remove scale formed during hot rolling, resulting in deterioration of appearance. Therefore, the Si content is set to 1.500% or less. Furthermore, since Si is a ferrite stabilizing element, reducing the Si content can lower the Ac3 point. Therefore, the Si content may be 1.200% or less, 1.000% or less, 0.900% or less, 0.800% or less, 0.700% or less, or 0.600% or less.

[P: 0.100% or less]
P is an impurity element that embrittles welds and deteriorates galvanizability. Therefore, the P content is set to 0.100% or less. The P content may be 0.060% or less, 0.040% or less, 0.020% or less, or 0.010% or less. The lower the P content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, reducing the P content to less than 0.0001% in practical steel sheets significantly increases production costs, which is economically disadvantageous. Therefore, the P content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[S: 0.0200% or less]
S is an impurity element that impairs weldability and manufacturability during casting and hot rolling. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0150% or less, 0.0120% or less, 0.0100% or less, 0.0060% or less, or 0.0030% or less. The lower the S content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the S content of a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the S content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[Al: 1.000% or less]
Al functions as a deoxidizer and is effective in increasing the strength of steel. The Al content may be 0%, but to fully obtain these effects, the Al content is preferably 0.001% or more. The Al content may be 0.005% or more, 0.010% or more, 0.025% or more, or 0.050% or more. On the other hand, excessive Al content may form coarse oxides and reduce toughness. Therefore, the Al content is set to 1.000% or less. Furthermore, since Al is a ferrite stabilizing element, reducing the Al content can lower the Ac3 point. Therefore, the Al content may be 0.800% or less, 0.600% or less, or 0.300% or less.

[N: 0.0010-0.0150%]
N is an element that is effective in improving the high-temperature strength of steel sheet by forming carbonitrides with Nb. To fully obtain this effect, the N content is set to 0.0010% or more. The N content may be 0.0015% or more, 0.0020% or more, 0.0025% or more, or 0.0030% or more. On the other hand, excessive N content may cause blowholes during welding. Therefore, the N content is set to 0.0150% or less. The N content may be 0.0120% or less, 0.0100% or less, 0.0080% or less, or 0.0060% or less.

[O: 0.0100% or less]
O is an element that causes blowholes during welding. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower the O content, the more preferable it is, and the lower limit is not particularly limited and may be 0%. On the other hand, if the O content in a practical steel sheet is reduced to less than 0.0001%, the manufacturing cost will increase significantly, which is economically disadvantageous. Therefore, the O content may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[Nb: 0.005-0.200%]
Nb is an element effective in increasing the index A and improving the high-temperature strength of the steel sheet. In particular, by adding Nb, a relatively large amount of Nb carbonitrides having an appropriate size can be formed, thereby further improving the high-temperature strength of the steel sheet. To fully obtain these effects, the Nb content is set to 0.005% or more. The Nb content may be 0.010% or more, 0.015% or more, 0.020% or more, or 0.040% or more. On the other hand, if Nb is contained excessively, the effect may saturate, which may lead to an increase in manufacturing costs, or the formation of coarse carbonitrides in the steel may reduce the toughness of the steel sheet. For this reason, the Nb content is set to 0.200% or less. The Nb content may be 0.150% or less, 0.100% or less, 0.080% or less, or 0.060% or less.

The basic chemical composition of the steel sheet according to an embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary, for the purpose of improving properties. For example, the steel sheet may contain at least one of Cr: 0-1.00%, Mo: 0-0.80%, B: 0-0.0100%, Ti: 0-0.200%, V: 0-0.500%, Ni: 0-1.00%, Cu: 0-1.00%, W: 0-1.00%, Ta: 0-0.10%, Co: 0-3.00%, Sn: 0-1.00%, Sb: 0-0.200%, Ca: 0-0.0100%, Mg: 0-0.0100%, Zr: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0500%, and As: 0-0.10%. These optional elements will be described in detail below.

[Cr: 0-1.00%]
Like Mn, Cr is an element that improves hardenability and contributes to improving the strength of the steel sheet. The Cr content may be 0%, but to obtain the above-mentioned effects, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.10% or more, or 0.20% or more. On the other hand, if Cr is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Cr content is preferably 1.00% or less, and may be 0.80% or less, 0.60% or less, or 0.40% or less.

[Mo: 0-0.80%]
Like Nb, Mo is an element that contributes to improving the high-temperature strength of steel sheets. This effect can be achieved even with trace amounts. The Mo content may be 0%, but to achieve the above effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. Therefore, the Mo content is preferably 0.80% or less. The Mo content may be 0.60% or less, 0.50% or less, 0.40% or less, or 0.20% or less.

[B: 0 to 0.0100%]
B is an element that suppresses the formation of ferrite and pearlite during the cooling process from austenite and promotes the formation of martensite. Furthermore, B is a beneficial element for increasing the strength of steel. These effects can be achieved even with trace amounts. The B content may be 0%, but to achieve the above effects, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more or 0.0010% or more. On the other hand, excessive B content may result in a decrease in toughness and/or weldability. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[Ti: 0 to 0.200%]
Ti is an element effective in controlling the morphology of carbides. Ti can promote an increase in the strength of ferrite. The Ti content may be 0%, but to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.002% or more, 0.010% or more, 0.020% or more, or 0.040% or more. On the other hand, if Ti is contained excessively, the effect saturates and there is a risk of an increase in manufacturing costs. Therefore, the Ti content is preferably 0.200% or less, and may be 0.100% or less, 0.080% or less, or 0.050% or less.

[V: 0-0.500%]
Like Ti, V is an element effective in controlling the morphology of carbides and is also effective in refining the structure to improve the toughness of the steel sheet. The V content may be 0%, but to obtain the above effect, the V content is preferably 0.001% or more. The V content may be 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, excessive V content may cause the formation of a large amount of precipitates, which may reduce toughness. Therefore, the V content is preferably 0.500% or less. The V content may be 0.400% or less, 0.200% or less, or 0.100% or less.

[Ni: 0-1.00%]
Ni is an element effective in improving the strength of steel sheets. The Ni content may be 0%, but to obtain the above effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more or 0.05% or more. On the other hand, excessive Ni content may reduce the weldability of the steel sheets. Therefore, the Ni content is preferably 1.00% or less. The Ni content may be 0.80% or less, 0.40% or less, or 0.20% or less.

[Cu: 0-1.00%]
Cu is an element that contributes to improving the strength of steel sheets. This effect can be achieved even with trace amounts. The Cu content may be 0%, but to achieve the above effect, the Cu content is preferably 0.001% or more. The Cu content may be 0.01% or more or 0.05% or more. On the other hand, excessive Cu content may cause red shortness and reduce productivity in hot rolling. Therefore, the Cu content is preferably 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, 0.30% or less, or 0.20% or less.

[W: 0-1.00%]
W is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. The W content may be 0%, but to obtain these effects, the W content is preferably 0.001% or more. The W content may be 0.01% or more or 0.05% or more. On the other hand, excessive W content may cause a decrease in weldability. For this reason, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.40% or less, or 0.20% or less.

[Ta: 0 to 0.10%]
Ta, like W, is an element effective in controlling the morphology of carbides and improving the strength of steel sheets. The Ta content may be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.01% or more or 0.03% or more. On the other hand, even if Ta is contained in an excessive amount, the effect saturates, and adding more than necessary to the steel sheet increases the manufacturing cost. For this reason, the Ta content is preferably 0.10% or less. The Ta content may be 0.08% or less, 0.06% or less, or 0.04% or less.

[Co: 0-3.00%]
Co, like Ni, is an element effective in improving the strength of steel sheet. The Co content may be 0%, but to obtain the above effect, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Co content may deteriorate hot workability and increase raw material costs. Therefore, the Co content is preferably 3.00% or less. The Co content may be 2.00% or less, 1.00% or less, 0.50% or less, or 0.20% or less.

[Sn: 0-1.00%]
Sn is an element that can be contained in steel sheets when scrap is used as a raw material for the steel sheets. Furthermore, Sn may cause ferrite embrittlement. Therefore, the lower the Sn content, the better, and it is preferably 1.00% or less. The Sn content may be 0.10% or less, 0.040% or less, or 0.02% or less. The Sn content may be 0%, but reducing the Sn content to less than 0.001% results in an excessive increase in refining costs. Therefore, the Sn content may be 0.001% or more, 0.005% or more, or 0.01% or more.

[Sb: 0 to 0.200%]
Like Sn, Sb is an element that can be contained in steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, Sb may strongly segregate at grain boundaries, potentially causing embrittlement of the grain boundaries. Therefore, the lower the Sb content, the better, and it is preferably 0.200% or less. The Sb content may be 0.100% or less, 0.040% or less, or 0.020% or less. The Sb content may be 0%, but reducing the Sb content to less than 0.001% would result in an excessive increase in refining costs. Therefore, the Sb content may be 0.001% or more, 0.005% or more, or 0.010% or more.

[Ca: 0-0.0100%]
[Mg: 0 to 0.0100%]
[Zr: 0 to 0.0100%]
[REM: 0 to 0.0100%]
Ca, Mg, Zr, and REM are elements that contribute to improving the formability of steel sheets. The Ca, Mg, Zr, and REM contents may be 0%, but to achieve these effects, the Ca, Mg, Zr, and REM contents are each preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, excessive inclusion of these elements may reduce the ductility of the steel sheet. Therefore, the Ca, Mg, Zr, and REM contents are each preferably 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content is the total content of these elements.

[Bi: 0 to 0.0500%]
Bi is an element that has the effect of improving formability by refining the solidification structure. The Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0030% or more. On the other hand, even if Bi is contained in an excessive amount, the effect saturates, and adding more Bi than necessary to the steel sheet increases the manufacturing cost. Therefore, the Bi content is preferably 0.0500% or less, and may be 0.0400% or less, 0.0200% or less, 0.0100% or less, or 0.0050% or less.

[As: 0 to 0.10%]
Like Sn and Sb, As is an element that can be contained in steel sheet when scrap is used as a raw material for the steel sheet. Furthermore, As is an element that strongly segregates at grain boundaries, and the lower the As content, the better. The As content is preferably 0.10% or less, and may be 0.04% or less, or 0.02% or less. The As content may be 0%, but reducing the As content to less than 0.001% results in an excessive increase in refining costs. Therefore, the As content may be 0.001% or more, 0.005% or more, or 0.01% or more.

In the steel sheet according to the embodiment of the present invention, the balance excluding the above elements consists of Fe and impurities. Impurities are elements that are mixed in from the steel raw materials and/or during the steelmaking process, and whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to the embodiment of the present invention.

[Index A: 0.50% or more]
The chemical composition of the steel sheet according to the embodiment of the present invention is required to have an index A represented by the following formula 1 of 0.50% or more.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] represent the content (mass %) of each element, and are 0% when the element is not present. As described above, for the steel sheet according to the embodiment of the present invention, improving the high-temperature strength of the steel sheet is extremely important in suppressing the occurrence of heat buckling during the manufacturing process. To improve the high-temperature strength of the steel sheet, it is effective to add large amounts of elements such as Nb and Mo, which improve the high-temperature strength of the steel sheet, as well as to lower the heating temperature (i.e., Ac3 + 10°C or higher) required in the primary heat treatment step, which will be described in detail later in connection with the manufacturing method of the steel sheet. Lowering the heating temperature required in the primary heat treatment step can suppress the decrease in the yield stress of the steel sheet that accompanies the increase in sheet temperature during heating, thereby significantly improving the resistance to compressive stress at the width center of the steel sheet, which causes heat buckling. From this perspective, the influence of each element in the steel on the improvement of the high-temperature strength of the steel sheet itself and the decrease in the Ac3 point was experimentally investigated. More specifically, the inventors discovered that by controlling the index A, defined by the contents of these elements together with a coefficient that takes into account the influence of the elements, i.e., the index A expressed by the above formula 1, to 0.50% or more, the resistance of the steel sheet to the compressive stress can be improved based on the active improvement of the high-temperature strength of the steel sheet itself and the suppression of a decrease in yield stress due to a decrease in the heating temperature in the primary heat treatment step, thereby significantly suppressing the occurrence of heat buckling during the manufacturing process. From the viewpoint of suppressing the occurrence of heat buckling, a larger index A is preferable, and may be, for example, 0.55% or more, 0.60% or more, 0.65% or more, 0.70% or more, 0.75% or more, or 0.80% or more. The upper limit of index A is not particularly limited, and may be, for example, 2.00% or less, 1.80% or less, 1.50% or less, 1.30% or less, or 1.10% or less.

The chemical composition of the steel sheet according to the embodiment of the present invention may be measured using a common analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[Ferrite: 75 to 95%]
Ferrite is a soft structure and is easily deformable, contributing to improved elongation. When the area ratio of ferrite is 75% or more, sufficient formability can be obtained. From the viewpoint of improving formability, a higher area ratio of ferrite is preferable, and may be, for example, 78% or more, 80% or more, 82% or more, or 85% or more. On the other hand, if ferrite is contained in an excessive amount, the steel sheet may not achieve the desired strength. Therefore, the area ratio of ferrite is set to 95% or less. The area ratio of ferrite may be 93% or less, 90% or less, or 87% or less.

[Martensite: 5 to 25%]
Martensite is a hard structure with a high dislocation density, and therefore contributes to improving tensile strength. By setting the area fraction of martensite to 5% or more, a tensile strength of, for example, 540 MPa or more can be ensured. From the viewpoint of improving strength, a higher area fraction of martensite is preferable, and may be, for example, 7% or more, 10% or more, or 13% or more. On the other hand, when the area fraction of martensite is 25% or less, formability and appearance can be ensured. The area fraction of martensite may be 22% or less, 20% or less, 18% or less, or 15% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[Remainder of structure: 0 to 10% in total]
The area fraction of the remaining structure other than ferrite and martensite may be 0%, but if a remaining structure is present, the remaining structure is at least one of bainite, pearlite, and retained austenite. From the viewpoint of ensuring the above-mentioned effects based on ferrite and martensite, the area fraction of the remaining structure, i.e., at least one of bainite, pearlite, and retained austenite, may be 10% or less in total, for example, 8% or less, 6% or less, 4% or less, or 2% or less. On the other hand, achieving a 0% area fraction of the remaining structure requires advanced control in the steel plate manufacturing process, which may result in a decrease in yield. Therefore, the area fraction of the remaining structure may be 0.5% or more, or 1% or more.

[Identification of metal structure and calculation of area ratio]
Identification of the metallographic structure and calculation of the area ratio are performed using an FE-SEM (field emission scanning electron microscope), an optical microscope, and X-ray diffraction after corrosion using Nital reagent (3% nitric acid in ethanol). Microstructure observation using an FE-SEM and an optical microscope is performed at a magnification of 500 to 50,000 times on a 100 μm x 100 μm area in the cross section of the steel plate perpendicular to the plate surface. For each metallographic structure, three measurements are taken, and the area ratio is determined by calculating the average of these measurements. For example, if the steel plate being measured is too thin to ensure a 100 μm measurement area in the thickness direction, the length in the thickness direction is reduced to ensure a measurement area of 10,000 ^μm² . For example, a measurement area of 20 μm in the thickness direction and 500 μm in the direction perpendicular to the thickness direction may be observed. However, if the number of crystal grains contained in the thickness direction is too small, measurement accuracy may decrease. Therefore, the measurement length in the thickness direction is set to 10 μm or more, preferably 50 μm or more. The same applies to the "100 μm×100 μm area" in the following description.

In this specification, the term "plate thickness x/y position (where x and y are natural numbers satisfying x < y)" refers to a position moved from the surface (plate surface) in the plate thickness direction of the steel plate toward the center of the steel plate by a distance (depth) of x/y of the plate thickness t. For example, if the plate thickness t of a steel plate is 2 mm, then the "1/8 plate thickness position" refers to a position 0.25 mm deep from the surface of the steel plate in the plate thickness direction. Note that if the steel plate has a coating such as a plating layer on its surface, the "surface of the steel plate" refers to the interface between the steel plate and the coating, and "plate thickness t" refers to the thickness of the steel plate (base material) excluding the coating.

The area fractions of ferrite and martensite are determined using the following procedure. First, the specimen surface is etched with a Nital reagent (3% nitric acid in ethanol). A 100 μm x 100 μm area is then observed using an FE-SEM (e.g., a JEOL JSM-7200F, measured at an accelerating voltage of 15 kV and a magnification of 500-2000x) within the range of 1/8 to 3/8 of the specimen thickness, centered at the 1/4 position. Because martensite and retained austenite are not corroded by nital etching, the area fraction of the uncorroded area corresponds to the total area fraction of martensite and retained austenite. Specifically, the metal structure is binarized based on differences in brightness using image analysis software Image J (Ver. 1.54f). The black areas in the image data represent ferrite, and the uncorroded white areas represent the combined structure of martensite and retained austenite. Therefore, the area fraction of ferrite is calculated from the area fraction of the black area, and the area fraction of martensite is calculated by subtracting the area fraction of retained austenite measured by X-ray diffraction, which will be described later, from the area fraction of the uncorroded area. The area fraction of martensite calculated by this method also includes the area fraction of tempered martensite.

The area fraction of retained austenite is calculated using X-ray diffraction. First, the specimen is mechanically and chemically polished from the plate surface to a depth of 1/4 of the way through the plate thickness. More specifically, the specimen is thinned to the observation position by mechanical polishing, and then thinned to the target position by chemical polishing (with hydrofluoric acid). Next, using an X-ray diffractometer (RINT2500, X-ray output 40 kV-200 mA, for example) manufactured by Rigaku Corporation, the fraction of retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of the (200) and (211) of the bcc phase and the (200), (220), and (311) of the fcc phase, obtained at the 1/4 plate thickness position using MoKα radiation. The commonly used five-peak method is used for this calculation. The calculated fraction of retained austenite is then determined as the area fraction of retained austenite.

As described above, the remaining structures of bainite, pearlite, and retained austenite may be present in a total amount of 0 to 10%. In other words, in the steel sheet according to this embodiment, ferrite and martensite are the primary metallurgical structures, and the remaining structures of bainite, pearlite, and retained austenite are metallurgical structures that may be unavoidably generated during manufacturing. Therefore, there is essentially no positive technical significance in identifying or measuring the area fractions of these remaining structures, such as bainite, pearlite, and retained austenite. From the chemical composition and manufacturing method of the steel sheet described in this specification, it is clear that the remaining structure in the steel sheet according to this embodiment is bainite, pearlite, retained austenite, or a composite thereof. The following methods can be used to identify and measure the remaining structures of bainite and pearlite. The method for measuring the area fraction of retained austenite is as described above.

The identification of bainite and calculation of its area fraction are performed using the following procedure. First, the specimen surface is etched with Nital reagent. Next, a 100 μm x 100 μm region within the 1/8 to 3/8 thickness range, centered at 1/4 of the specimen thickness, is observed using an FE-SEM (e.g., a JEOL JSM-7200F, measured at an accelerating voltage of 15 kV and a magnification of 500 to 2000 times). Based on the location and arrangement of cementite contained within the structure in this observation region, bainite is identified as follows: Bainite is classified as upper bainite and lower bainite. Upper bainite has cementite or retained austenite present at the interface of lath-shaped bainitic ferrite. Lower bainite has cementite present within lath-shaped bainitic ferrite, has a single crystal orientation relationship between bainitic ferrite and cementite, and has the same cementite variant. Based on these characteristics, upper bainite and lower bainite can be identified. In the present invention, these are collectively referred to as bainite, and the area ratio of the identified bainite is calculated based on image analysis.

The identification of pearlite and calculation of its area ratio are carried out using the following procedure. First, the observation surface of the sample is corroded with Nital reagent, and then the area from 1/8 to 3/8 of the plate thickness, centered at 1/4 of the plate thickness, is observed using an SEM (e.g., a JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). Areas in the SEM image where lamellar cementite is observed are identified as pearlite, and the area ratio of this area is calculated based on image analysis.

[The Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides]
According to an embodiment of the present invention, the steel sheet contains Nb carbonitrides in its metal structure, and the Nb content of all Nb carbonitrides is 0.004% or more, and the Nb content of Nb carbonitrides having a particle size of 20 nm or more is controlled to 60% or more of the total Nb carbonitrides. In the present invention, Nb carbonitrides include not only NbCN but also NbC and NbN, and further include NbCN, NbC, and NbN in which a portion of the Nb is substituted with one or more other elements, such as Ti. By controlling the form and amount of Nb carbonitrides within the above range, a sufficient amount of Nb carbonitrides having a particle size of 20 nm or more, which are effective in improving high-temperature strength, can be present in the steel. Therefore, in combination with the control of the chemical composition using the index A described above, the high-temperature strength of the steel sheet can be significantly improved. As a result, it is possible to reliably suppress or reduce the occurrence of heat buckling during the steel sheet manufacturing process. From the viewpoint of improving the high-temperature strength of the steel sheet, the ratio of the Nb content in all Nb carbonitrides to the Nb content in Nb carbonitrides having a particle size of 20 nm or more is preferably as large as possible. For example, the Nb content in all Nb carbonitrides may be 0.006% or more, 0.008% or more, 0.010% or more, or 0.012% or more. The upper limit is not particularly limited, and for example, the Nb content in all Nb carbonitrides may be 0.100% or less, 0.060% or less, 0.040% or less, or 0.030% or less. Similarly, the Nb content in Nb carbonitrides having a particle size of 20 nm or more may be 62% or more, 65% or more, 68% or more, or 70% or more of the Nb content in all Nb carbonitrides. The upper limit is not particularly limited, and for example, the Nb content in Nb carbonitrides having a particle size of 20 nm or more may be 95% or less, 90% or less, or 85% or less of the Nb content in all Nb carbonitrides. If the particle size of the Nb carbonitride is 20 nm or more, the effect of improving high-temperature strength can be obtained, and if the particle size is too large, the effect will not be significantly reduced. Therefore, although there is no particular upper limit on the particle size of the Nb carbonitride, the particle size of the Nb carbonitride may be, for example, 1000 nm or less, i.e., 1 μm or less.

[Measurement of the amount of Nb in all Nb carbonitrides and the ratio of the amount of Nb in Nb carbonitrides with particle sizes of 20 nm or more]
The Nb content in all Nb carbonitrides and the ratio of the Nb content in Nb carbonitrides with a particle size of 20 nm or more are determined as follows. First, a test piece is taken from the 1/2 position in the plate thickness, and the taken test piece is subjected to constant-current electrolysis in an electrolyte (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol). The precipitates attached to the electrolyzed test piece are dispersed in an aqueous sodium hexametaphosphate solution and then filtered and recovered using a porous filter with a pore size of 0.02 μmφ (20 nmφ). Next, the amount of Nb contained in the precipitates on the filter is measured by ICP atomic emission spectroscopy, and the Nb content in the steel precipitated as Nb precipitates with a particle size of 20 nm or more collected on the filter is determined. Furthermore, the amount of Nb in the filtrate that has passed through the filter and has a particle size of less than 20 nm is measured by ICP atomic emission spectroscopy. The total mass of Nb precipitated as Nb carbonitrides is calculated by adding together the amount of Nb in Nb precipitates having a particle size of 20 nm or more and the amount of Nb in Nb precipitates having a particle size of less than 20 nm, and the obtained value is determined as the amount of Nb in all Nb carbonitrides. Furthermore, the amount of Nb precipitated as Nb carbonitrides having a particle size of 20 nm or more is used to calculate the ratio of the amount of Nb precipitated as Nb carbonitrides to the total mass of Nb, and the calculated value is determined as the ratio of the amount of Nb in Nb carbonitrides having a particle size of 20 nm or more to the amount of Nb in all Nb carbonitrides.

[Average particle spacing of martensite: 2.5 μm or less]
In an embodiment of the present invention, the average particle spacing of martensite, which is a hard structure, is controlled to 2.5 μm or less. The average particle spacing of martensite is an index that represents the uniformity of the hard structure distribution in the microregion. The smaller the average particle spacing of martensite, the more densely and uniformly the hard structure is dispersed, and therefore the higher the uniformity. The more uniform the deformation amount of the steel sheet during press forming, particularly in the width direction of the steel sheet, the better the appearance of the steel sheet after press forming. Since the deformation amount of the steel sheet is strongly affected by the distribution state of the hard structure, in order to make the deformation amount of the steel sheet uniform in the width direction of the steel sheet, it is necessary to make the distribution of the hard structure in the metal structure uniform. In addition to controlling the standard deviation of the area fraction of martensite, which will be described later, by controlling the average particle spacing of martensite to 2.5 μm or less, the deformation amount of the steel sheet can be made more uniform in the width direction even during forming, such as press forming, and as a result, a good post-forming appearance can be achieved. The average grain spacing of martensite is preferably 2.4 μm or less, more preferably 2.2 μm or less, and most preferably 2.0 μm or less or 1.8 μm or less. There is no particular lower limit, but the average grain spacing of martensite may be, for example, 0.5 μm or more, 0.8 μm or more, or 1.0 μm or more.

[Measurement of average particle spacing in martensite]
The average grain spacing of martensite is determined as follows. First, a sample having a steel sheet cross section perpendicular to the sheet surface is collected, and the cross section is used as the observation surface. A 100 μm × 100 μm region within the range of 1/8 to 3/8 of the sheet thickness, centered at 1/4 of the sheet thickness, is used as the observation region of this observation surface, and martensite is identified using an FE-SEM (e.g., a JSM-7200F manufactured by JEOL, measured at an acceleration voltage of 15 kV and a magnification of 1000 to 5000 times). Specifically, the metal structure is binarized based on differences in brightness using image analysis software Image J (Ver. 1.54f), and martensite is identified. Specifically, when a nital solution is used, the black portion of the image data represents ferrite, and the uncorroded white portion represents the combined structure of martensite and retained austenite. However, in the steel sheet according to the embodiment of the present invention, the white structure can be considered as martensite. Next, the distance between the centers (centers of gravity) of all adjacent martensite grains among the identified martensite grains is calculated as the particle spacing based on image analysis, and the average of the calculated particle spacings is obtained. This operation is repeated for the other two observation regions, and the average of the three obtained values is determined as the average particle spacing of martensite (strictly speaking, particles containing martensite and/or retained austenite).

[Standard deviation of area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is 1.5% or less]
In an embodiment of the present invention, the standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction is controlled to 1.5% or less. This standard deviation is an index representing the uniformity of the hard structure in the macro region. Appearance, which is an issue during press forming, depends on minute irregularities on the steel sheet surface due to differences in the amount of deformation in the width direction of the steel sheet. Therefore, if there is a large variation in the area fraction of hard structures contained within the steel sheet thickness in the direction perpendicular to the rolling direction and the thickness direction, differences in the amount of deformation in the width direction of the steel sheet will occur, resulting in the generation of minute irregularities on the steel sheet surface. Therefore, it is effective to reduce the standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction, i.e., the width direction of the steel sheet. More specifically, by controlling this standard deviation to 1.5% or less in addition to the aforementioned control of the average particle spacing of martensite, it is possible to further reduce the variation in the amount of deformation in the width direction of the steel sheet even during forming, such as press forming, and as a result, achieve a good post-forming appearance. The standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction is preferably 1.4% or less, more preferably 1.2% or less, and most preferably 1.0% or less. There is no particular lower limit, but the standard deviation may be, for example, 0.1% or more, 0.3% or more, or 0.5% or more.

[Measurement of standard deviation in area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction]
The standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction is determined as follows. First, a metallographic image of a steel sheet cross section in a 50 mm region perpendicular to the rolling direction and the thickness direction is obtained. In the case of an image of 10 mm or smaller, multiple images may be obtained and joined to form a 50 mm region. If the rolling direction is unknown, the cross section is observed at angles of 0°, 45°, 90°, and 135° relative to an arbitrary direction, and the cross section with the highest aspect ratio of precipitates is determined as the cross section parallel to the rolling direction, and the direction perpendicular to this cross section is identified as the direction perpendicular to the rolling direction and the thickness direction. Next, the obtained image is divided into 100 μm (0.1 mm) sections in the direction perpendicular to the rolling direction and the thickness direction, and the area fraction of martensite throughout the thickness is calculated for each divided area. The standard deviation in the area fraction of martensite is calculated based on the martensite area fraction calculated from each of the 500 divided images. The area fraction of martensite in each divided region is calculated according to the procedure described in the section [Identification of Metallic Structure and Calculation of Area Fraction]. Note that, for convenience, the area fraction of retained austenite may be measured in a 50 mm region of the steel sheet cross section in a direction perpendicular to the rolling direction and the sheet thickness direction, instead of the measurement results in each divided region.

[Average grain size of ferrite: 3.0 to 25.0 μm]
According to a preferred embodiment of the present invention, the average grain size of ferrite in the metal structure is 3.0 to 25.0 μm. By controlling the average grain size of ferrite within such a fine range, it is possible to further improve the appearance of the steel sheet, particularly the appearance after forming. The average grain size of ferrite may be 5.0 μm or more, 7.0 μm or more, 8.0 μm or more, 9.0 μm or more, or 10.0 μm or more. Similarly, the average grain size of ferrite may be 22.0 μm or less, 20.0 μm or less, 16.0 μm or less, 14.0 μm or less, or 12.0 μm or less.

The average grain size of ferrite in steel sheet is determined as follows. First, a sample is taken of the steel sheet cross section perpendicular to the sheet surface, and this cross section is used as the observation surface. A 100 μm x 100 μm region of this observation surface is selected within the range of 1/8 to 3/8 of the sheet thickness, centered at the 1/4 position. Martensite is identified using an FE-SEM (e.g., a JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500-2000x). Specifically, the metal structure is binarized based on differences in brightness using image analysis software Image J (Ver. 1.54f), and ferrite is identified. Specifically, when using nital solution, the black areas in the image data represent ferrite, and the uncorroded white areas represent the combined structure of martensite and retained austenite. Next, the circle-equivalent diameters of all identified ferrite are calculated. This operation is repeated for the other two observation regions, and the circle equivalent diameters of all the ferrite particles obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average crystal grain size of the ferrite.

[Average grain size of martensite: 1.0 to 5.0 μm]
According to a preferred embodiment of the present invention, the average grain size of martensite in the metal structure is 1.0 to 5.0 μm. By controlling the average grain size of martensite within such a fine range, it is possible to further improve the appearance of the steel sheet, particularly the appearance after forming. The average grain size of martensite may be 1.2 μm or more, 1.5 μm or more, 1.7 μm or more, or 2.0 μm or more. Similarly, the average grain size of martensite may be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.8 μm or less, 3.6 μm or less, or 3.4 μm or less.

The average grain size of martensite is determined as follows. First, a sample is taken of the steel sheet cross section perpendicular to the sheet surface, and this cross section is used as the observation surface. A 100 μm x 100 μm region of this observation surface is selected within the range of 1/8 to 3/8 of the sheet thickness, centered at the 1/4 position. Martensite is identified using an FE-SEM (e.g., a JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 500 to 2000 times). Specifically, the image analysis software Image J (Ver. 1.54f) is used to binarize the metal structure based on differences in brightness, and martensite is identified. Specifically, when using nital solution, the black areas in the image data represent ferrite, and the uncorroded white areas represent the combined structure of martensite and retained austenite. In steel sheets according to embodiments of the present invention, the white structure is considered to be martensite. Next, the circle-equivalent diameter of all identified martensite is calculated. This operation is repeated in the other two observation regions, and the circle equivalent diameters of all the martensite particles obtained in the three observation regions are arithmetically averaged. The obtained value is determined as the average crystal grain size of martensite (strictly speaking, particles containing martensite and/or retained austenite).

[Average aspect ratio of martensite: 2.5 or more]
According to a preferred embodiment of the present invention, the average aspect ratio of martensite in the metal structure is 2.5 or more. By controlling the average aspect ratio of martensite to 2.5 or more, a state in which greater strain is imparted can be achieved, thereby improving the strength of the steel sheet. The average aspect ratio of martensite may be 2.6 or more, 2.8 or more, or 3.0 or more. There is no particular upper limit, but the average aspect ratio of martensite may be 4.0 or less, 3.8 or less, or 3.6 or less, for example.

The average aspect ratio of martensite is determined as follows. First, a sample is taken of the steel sheet cross section perpendicular to the sheet surface, and this cross section is used as the observation surface. A 100 μm x 100 μm region of this observation surface is selected within the range of 1/8 to 3/8 of the sheet thickness, centered at the 1/4 position. Martensite is identified using an FE-SEM (e.g., a JEOL JSM-7200F, measured at an acceleration voltage of 15 kV and a magnification of 1000 to 5000 times). Specifically, the metal structure is binarized based on differences in brightness using image analysis software Image J (Ver. 1.54f), and martensite is identified. Specifically, when using nital solution, the black areas in the image data represent ferrite, and the uncorroded white areas represent the combined structure of martensite and retained austenite. In steel sheets according to embodiments of the present invention, the white structure is considered to be martensite. The aspect ratios of all martensite grains are calculated using the image analysis software Image J (Ver. 1.54f) for the obtained image data. The aspect ratios of particles (crystal grains) on the image can be measured using a function built into the image analysis software Image J (Ver. 1.54f). Next, this operation is performed in the other two observation regions, and the aspect ratios of all martensite grains obtained in the three observation regions are arithmetically averaged, and the obtained value is determined as the average aspect ratio of martensite (strictly speaking, particles containing martensite and/or retained austenite).

[Thickness]
The steel sheet according to the embodiment of the present invention has a thickness of, for example, 0.1 to 2.0 mm, but is not particularly limited thereto. Steel sheets having such a thickness are suitable for use as materials for exterior panel components such as doors and hoods as automotive components. The thickness may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the thickness may be 1.8 mm or less, 1.5 mm or less, 1.2 mm or less, or 1.0 mm or less. For example, by setting the thickness to 0.2 mm or more, it becomes easier to maintain the shape of the molded product flat, and additional effects such as improved dimensional accuracy and shape accuracy can be obtained. On the other hand, by setting the thickness to 1.0 mm or less, the weight reduction effect of the component is significant. The thickness of the steel sheet is measured using a micrometer.

[Plating]
The steel sheet according to the embodiment of the present invention is a cold-rolled steel sheet, but may further include a plating layer on its surface for the purpose of improving corrosion resistance, etc. The plating layer may be either a hot-dip plating layer or an electroplated layer. That is, the steel sheet according to the embodiment of the present invention may be a cold-rolled steel sheet having a hot-dip plating layer or an electroplated layer on its surface. Examples of the hot-dip plating layer include a hot-dip galvanized layer (GI), a galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn—Al alloy plating layer, a hot-dip Zn—Al—Mg alloy plating layer, and a hot-dip Zn—Al—Mg—Si alloy plating layer. Examples of the electroplated layer include an electrogalvanized layer (EG), an electrolytic Zn—Ni alloy plating layer, etc. Preferably, the plating layer is a hot-dip galvanized layer, a hot-dip galvannealed layer, or an electrogalvanized layer. The coating weight of the coating layer is not particularly limited and may be a general coating weight.

[Mechanical properties]
A steel sheet having the above-described chemical composition and metallographic structure can achieve high tensile strength, specifically, a tensile strength of 540 MPa or more. The tensile strength is preferably 570 MPa or more, more preferably 600 MPa or more. The upper limit is not particularly limited, but the tensile strength may be, for example, 980 MPa or less, 900 MPa or less, 850 MPa or less, 830 MPa or less, or 800 MPa or less. Setting the tensile strength to 850 MPa or less has the advantage of easily ensuring formability during press working of the steel sheet. The tensile strength is measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the steel sheet, with the test direction perpendicular to the rolling direction and the sheet thickness direction, and conducting a tensile test in accordance with JIS Z2241:2011.

Steel sheets according to embodiments of the present invention can suppress the occurrence of heat buckling during the manufacturing process and can combine high strength, e.g., a tensile strength of 540 MPa or more, with an excellent appearance after forming, such as press working. Therefore, steel sheets according to embodiments of the present invention are particularly useful for use in parts in technical fields where both of these properties are required. In a preferred embodiment, an exterior panel member, particularly an automotive exterior panel member, is provided that includes a steel sheet according to embodiments of the present invention. Examples of automotive exterior panel members include roofs, hoods, fenders, and doors, which require high design quality. These exterior panel members, particularly automotive exterior panel members, only need to include a steel sheet according to embodiments of the present invention in at least a portion of the exterior panel member, and therefore, at least a portion of these exterior panel members will satisfy the chemical composition and metallographic characteristics described above. In areas of the steel sheet that are relatively lightly processed in forming, such as press forming, the metallographic characteristics do not change significantly before and after forming.

<Steel sheet manufacturing method>
Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, but is not intended to limit the steel sheet to one manufactured by the manufacturing method described below.

A method for manufacturing a steel sheet according to an embodiment of the present invention includes:
A hot rolling process comprising finish rolling a slab having the chemical composition described above in relation to the steel sheet, followed by coiling, wherein the following conditions (a) to (d) are satisfied:
(a) The finish rolling entry temperature is 1000 to 1080°C;
(b) The rolling reduction rate of the rolling pass two passes before the final pass is 30% or more;
(c) a ratio of the reduction rate of the rolling pass two passes before the final pass/the reduction rate of the final pass is 1.5 to 2.5, and (d) a coiling temperature is 520 to 670°C. A cold rolling step of cold rolling the obtained hot-rolled steel sheet at a reduction rate of 70% or more.
The method is characterized by comprising a primary annealing step, which includes heating the obtained cold-rolled steel sheet to a temperature of Ac3+10°C or higher, and a secondary annealing step, which includes heating the cold-rolled steel sheet and holding it at a temperature of (Ac1+20) to 820°C for 10 to 500 seconds. Each step will be described in detail below.

[Hot rolling process]
First, a slab having the chemical composition described above in relation to the steel sheet is subjected to hot rolling. From the viewpoint of productivity, the slab to be used is preferably cast by a continuous casting method, but may also be produced by an ingot casting method or a thin slab casting method. The slab is preferably heated to 1100°C or higher prior to hot rolling. By setting the heating temperature to 1100°C or higher, the rolling reaction force during hot rolling is not excessively large, making it easier to obtain the desired product thickness. The upper limit of the heating temperature is not particularly limited, but from an economic viewpoint, the heating temperature is preferably 1300°C or lower. Furthermore, the heated slab may optionally be subjected to rough rolling before finish rolling for plate thickness adjustment, etc. The conditions for such rough rolling are not particularly limited as long as the desired sheet bar dimensions are ensured.

[(a) Finish rolling entry temperature: 1000 to 1080°C]
The heated slab, or the slab that has been subjected to rough rolling as needed, is then subjected to finish rolling. Finish rolling must be performed under conditions where the entry temperature of the finish rolling is 1000 to 1080°C. Controlling the entry temperature of the finish rolling within this range allows Nb carbonitrides to be properly precipitated in the hot rolling process. Therefore, due to the proper precipitation of Nb carbonitrides, the high-temperature strength of the steel sheet can be sufficiently improved, and the occurrence of heat buckling can be significantly suppressed even by heat treatment at high temperatures in the subsequent primary annealing process, etc. If the entry temperature of the finish rolling is higher than 1080°C, recrystallization is likely to occur, making it difficult to accumulate strain in the later stages of finish rolling, and it becomes impossible to promote the precipitation of Nb carbonitrides having a particle size of 20 nm or more. As a result, it becomes impossible to achieve the desired ratio of the Nb content in Nb carbonitrides having a particle size of 20 nm or more to the Nb content in all Nb carbonitrides. On the other hand, if the finish rolling entry temperature is lower than 1000°C, strain accumulates in the latter stages of finish rolling, but precipitation of Nb carbonitrides itself becomes difficult and the particle size of the Nb carbonitrides becomes small. As a result, it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having particle sizes of 20 nm or more to the amount of Nb in all Nb carbonitrides.

[(b) Reduction rate of the rolling pass two passes before the final pass: 30% or more]
[(c) Ratio of reduction rate of the rolling pass two passes before the final pass/reduction rate of the final pass: 1.5 to 2.5]
In this manufacturing method, finish rolling is performed using multiple rolling stands, for example, a tandem rolling mill consisting of five or more rolling stands. In the finish rolling of this manufacturing method, controlling the reduction ratio in the post-finish rolling stage is important. More specifically, it is important to control the reduction ratio of the rolling pass two passes before the final pass to 30% or more, and to control the ratio of the reduction ratio of the rolling pass two passes before the final pass to the reduction ratio of the final pass to 1.5 to 2.5. By controlling the reduction ratio in the post-finish rolling stage under these conditions, strain is appropriately accumulated in the post-finish rolling stage, allowing Nb nitrides with particle sizes of 20 nm or more to precipitate in the desired proportion. Therefore, the occurrence of heat buckling can be significantly suppressed even during subsequent heat treatments at high temperatures, such as in the primary annealing process. In contrast, for example, if the reduction rate of the rolling pass two passes before the final pass is low and the ratio of the reduction rate of the rolling pass two passes before the final pass to the reduction rate of the final pass is less than 1.5, the accumulation of strain after the reduction in the rolling pass two passes before the final pass will be insufficient, resulting in insufficient precipitation of Nb carbonitrides with particle sizes of 20 nm or more. On the other hand, if the reduction rate of the final pass is high and the ratio of the reduction rate of the rolling pass two passes before the final pass to the reduction rate of the final pass is less than 1.5, the accumulation of strain after the reduction in the final pass will be significant, resulting in an increase in the amount of fine Nb carbonitrides precipitated after finish rolling, i.e., a decrease in the proportion of Nb carbonitrides with particle sizes of 20 nm or more. Therefore, in either case, it will be impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides with particle sizes of 20 nm or more relative to the amount of Nb in all Nb carbonitrides.

On the other hand, for example, if the reduction rate of the rolling pass two passes before the final pass is high or the reduction rate of the final pass is low, and the ratio of the reduction rate of the rolling pass two passes before the final pass to the reduction rate of the final pass exceeds 2.5, the accumulation of strain after the reduction in the rolling pass two passes before the final pass becomes too great, making recrystallization more likely to occur. As a result, it becomes difficult to precipitate Nb carbonitrides with a grain size of 20 nm or more, and it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides with a grain size of 20 nm or more to the amount of Nb in all Nb carbonitrides.

[(d) Winding temperature: 520-670°C]
Next, the finish-rolled material is coiled at a coiling temperature of 520 to 670°C. By appropriately controlling the coiling temperature within this temperature range, scale growth can be suppressed and the metal structure can be made fine and uniform, which is important for obtaining a desired dispersion state of martensite in the final metal structure. If the coiling temperature exceeds 670°C, alloying elements will concentrate in the cementite in the metal structure, and undissolved carbides will remain during heating in the subsequent primary annealing step. As a result, the metal structure cannot be formed from a structure mainly composed of bainite and/or martensite in the primary annealing step, and the desired dispersion state of martensite cannot be obtained even in the subsequent secondary annealing step. More specifically, even after the subsequent secondary annealing step, it is not possible to control the average grain spacing of martensite to 2.5 μm or less, and/or it is not possible to control the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction to 1.5% or less, i.e., it is not possible to obtain a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions. In this case, it is not possible to sufficiently suppress the occurrence of ghost lines, etc., and the appearance after forming is deteriorated.

On the other hand, if the coiling temperature is lower than 520°C, the amount of solute Nb increases, resulting in less precipitation of Nb carbonitrides. As a result, it becomes impossible to achieve the desired amount of Nb in all Nb carbonitrides, and/or it becomes impossible to achieve the desired ratio of the amount of Nb in Nb carbonitrides having particle sizes of 20 nm or more relative to the amount of Nb in all Nb carbonitrides. In such cases, the high-temperature strength of the steel sheet cannot be sufficiently improved, and therefore the occurrence of heat buckling cannot be sufficiently suppressed or reduced during subsequent heat treatments at high temperatures, such as the primary annealing process.

[Cold rolling process]
The obtained hot-rolled steel sheet is subjected to an appropriate pickling treatment to remove scale and then to a cold-rolling process. In the cold-rolling process, the hot-rolled steel sheet is cold-rolled to a rolling reduction of 70% or more. By controlling the rolling reduction within this range, the desired sheet thickness can be ensured, and recrystallization during heating in the subsequent primary annealing process can be completed early, ensuring the desired precipitation state of Nb carbonitrides. If the rolling reduction is less than 70%, recrystallization is delayed during heating in the primary annealing process, and strain remains until the temperature reaches a high temperature. As a result, many fine Nb carbonitrides are precipitated during heating in the primary annealing process, making it impossible to achieve the desired ratio of the Nb content in Nb carbonitrides having a particle size of 20 nm or more to the Nb content in all Nb carbonitrides. On the other hand, it is preferable that the rolling reduction in the cold-rolling process be 90% or less. By setting the reduction to 90% or less, it is possible to prevent the rolling load from becoming excessively large, which makes the rolling difficult. The number of rolling passes and the reduction per pass are not particularly limited, and may be set appropriately so that the reduction of the entire cold rolling is within the above range.

[Primary annealing process]
The obtained cold-rolled steel sheet is heated to a temperature of Ac3+10°C or higher in the next primary annealing step. The Ac3 point (°C) is determined by cutting a small piece from the cold-rolled steel sheet and measuring the thermal expansion of the small piece during heating from room temperature to 1000°C at 10°C/sec. Heating the cold-rolled steel sheet to a temperature of Ac3+10°C or higher promotes austenitization, and then appropriately cooling the sheet, for example, by cooling at an average cooling rate of 30°C/sec or higher in the temperature range up to 200°C, it becomes possible to reliably ensure that the metallographic structure in the cooled steel sheet is composed mainly of bainite and/or martensite, for example, full bainite or full martensite. Here, a structure mainly composed of bainite and/or martensite refers to a structure containing at least one of bainite and martensite in a total area ratio of 90% or more, full bainite refers to a structure composed of 100% bainite in area ratio, and full martensite refers to a structure composed of 100% martensite in area ratio. A bainite and/or martensite structure has many different interfaces therein compared to structures such as ferrite. Therefore, by forming the metal structure of a steel sheet before the secondary annealing process, i.e., the final annealing process, with a structure mainly composed of bainite and/or martensite, it becomes possible to generate a very large number of dispersed carbides that can serve as austenite nucleation sites at these interfaces during heating of such a metal structure in the secondary annealing. As a result, by generating austenite finely and uniformly throughout the steel sheet from these dispersed nucleation sites and then generating martensite from this austenite, the average particle spacing of martensite in the metallographic structure obtained after secondary annealing is controlled to 2.5 μm or less, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction is controlled to 1.5% or less. In other words, it is possible to achieve a metallographic structure in which martensite is uniformly dispersed in both micro- and macro-regions.

If the heating temperature in the primary annealing step is lower than Ac3 + 10°C, austenitization will be insufficient, and the metallographic structure of the steel sheet will not be composed primarily of bainite and/or martensite even after subsequent cooling. This means that the total area fraction of bainite and martensite will not be 90% or greater. As a result, in the final metallographic structure, it will be impossible to control the average grain spacing of martensite to 2.5 μm or less, and/or the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction to 1.5% or less. In this case, the occurrence of ghost lines and other defects will not be sufficiently suppressed, resulting in poor appearance after forming. On the other hand, because heating at higher temperatures reduces productivity, it is preferable that the heating temperature in the primary annealing step be 1050°C or less. The holding time at this heating temperature is preferably 10 to 500 seconds.

[Secondary annealing process (final annealing process)]
The cold-rolled steel sheet after the primary annealing is then heated again in the subsequent secondary annealing step, where it is held at a temperature of (Ac1+20) to 820°C for 10 to 500 seconds. Here, the Ac1 point (°C) is determined, as in the case of the Ac3 point, by cutting a small piece from the cold-rolled steel sheet and measuring the thermal expansion of the small piece during heating from room temperature to 1000°C at a rate of 10°C/second. First, in the stage where the steel sheet after the primary cooling is heated to a temperature of Ac1 to 820°C, carbides can be dispersed and generated at many interfaces contained within bainite and/or martensite in the metal structure. Next, by holding the steel sheet at a temperature of Ac1 to 820°C, which corresponds to the two-phase region of ferrite and austenite, for 10 to 500 seconds, austenite can be generated finely and uniformly from the carbides throughout the steel sheet while maintaining the dispersed state of the carbides at the interfaces. Finally, by appropriately cooling the steel sheet, for example, by cooling at an average cooling rate of 10°C/sec or more in a temperature range up to 500°C, martensite can be appropriately generated from the finely dispersed austenite, and as a result, the average particle spacing of martensite is controlled to 2.5 µm or less, and the standard deviation of the area fraction of martensite in the direction perpendicular to the rolling direction and the plate thickness direction is controlled to 1.5% or less. In other words, it is possible to achieve a metal structure in which martensite is uniformly dispersed in both micro- and macro-regions.

If the heating temperature in the secondary annealing process is below Ac1+20°C or the holding time is less than 10 seconds, the desired metal structure described above cannot be obtained. On the other hand, if the heating temperature exceeds 820°C, the austenite area ratio becomes too high, making it impossible to achieve a ferrite area ratio of 75% or more. Furthermore, the high temperature makes it impossible to maintain the dispersion of carbides at the interfaces, making it impossible to achieve uniform dispersion of martensite in both the micro and macro regions in the final metal structure. Furthermore, if the holding time exceeds 500 seconds, the austenite grains coarsen, and the martensite grains obtained after subsequent cooling also become relatively coarse. In such cases, it becomes impossible to obtain a fine martensite structure in which the average particle spacing of martensite is controlled to 2.5 μm or less.

[Plating process]
For the purpose of improving corrosion resistance, etc., the surface of the obtained cold-rolled steel sheet may be subjected to a plating treatment as necessary. The plating treatment may be a treatment such as hot-dip plating, alloying hot-dip plating, or electroplating. For example, the steel sheet may be subjected to hot-dip galvanizing treatment as the plating treatment, or the hot-dip galvanizing treatment may be followed by an alloying treatment. The specific conditions for the plating treatment and the alloying treatment are not particularly limited and may be any appropriate conditions known to those skilled in the art. For example, the alloying temperature may be 450 to 600°C.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples in any way.

In the following examples, steel sheets according to embodiments of the present invention were manufactured under various conditions, and the occurrence of heat buckling during the manufacturing process, as well as the tensile strength and post-forming appearance characteristics of the resulting steel sheets were investigated.

First, a slab having the chemical composition shown in Table 1 and a thickness of 200 to 300 mm was cast using a continuous casting method. The remainder of the components shown in Table 1 consisted of Fe and impurities. Next, the resulting slab was heated to a temperature of 1100 to 1300°C and then hot-rolled. Hot-rolling was carried out by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same for all examples and comparative examples, and finish rolling was carried out using a tandem rolling mill consisting of seven rolling stands. The reduction ratio of the rolling pass two passes before the final pass in finish rolling (F5 rolling pass) was 30%.

Table 2 shows the cases where the following conditions were met and not met for the hot rolling of each example: Condition I: finish rolling entry temperature 1000-1080°C; Condition II: ratio of the reduction in the rolling pass two passes before the final pass to the reduction in the final pass (F5/F7 reduction ratio) 1.5-2.5; and Condition III: coiling temperature 520-670°C. Specifically, in examples where Condition I was met, the finish rolling entry temperature was 1050°C, while in examples where Condition I was not met, the finish rolling entry temperature was 950°C (Comparative Example 9) or 1120°C (Comparative Example 19). Furthermore, in examples where Condition II was met, the F5/F7 reduction ratio was 2.0, while in examples where Condition II was not met, the F5/F7 reduction ratio was 1.2 (Comparative Example 2) or 3.0 (Comparative Example 20). In the examples that satisfied Condition III, the coiling temperature was 600°C, while in the examples that did not satisfied Condition III, the coiling temperature was 470°C (Comparative Example 10) or 700°C (Comparative Example 21).

The resulting hot-rolled steel sheets were then pickled, followed by cold rolling, primary annealing (holding time of 100 seconds at the specified heating temperature, and an average cooling rate of 40°C/sec to 200°C after annealing), and secondary annealing (holding time of 100 seconds at a heating temperature of 770°C, and an average cooling rate of 15°C/sec to 500°C after annealing) to produce cold-rolled steel sheets with a thickness of 0.4 mm. The heating temperature of 770°C satisfied the requirement of Ac1 + 20°C or higher for all inventive and comparative examples. Table 2 shows the results for cases where cold-rolling condition IV (reduction of 70% or higher) and primary annealing condition V (heating temperature Ac3 + 10°C or higher) were met and not met. Specifically, in examples where condition IV was met, cold rolling was performed at a reduction of 80%, while in examples where condition IV was not met, cold rolling was performed at a reduction of 60% (Comparative Examples 3 and 9). In addition, in examples satisfying condition V, the primary annealing was performed by heating to Ac3+15°C or higher, i.e., 900°C. On the other hand, in examples not satisfying condition V, the primary annealing was performed by heating to a temperature lower than Ac3 (Comparative Examples 4 and 20).

Finally, the surface of the obtained cold-rolled steel sheet was appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrogalvanized layer (EG). For the galvannealed layer (GA), the alloying conditions were 550°C for 20 seconds.

The properties of the obtained steel sheets were measured and evaluated using the following methods. Furthermore, to check for the occurrence of heat buckling during the manufacturing process, the steel sheets were checked for the presence or absence of buckling after the primary annealing. If no buckling occurred, the steel sheets were judged to have no heat buckling and were rated as passed (OK), whereas if buckling occurred, the steel sheets were judged to have had heat buckling and were rated as failed (NG).

[Tensile strength (TS)]
The tensile strength (TS) was measured by taking a No. 5 tensile test piece according to JIS Z2241:2011 from the steel plate, with the longitudinal direction being perpendicular to the rolling direction and the plate thickness direction, and conducting a tensile test in accordance with JIS Z2241:2011.

[Appearance after molding]
The post-forming appearance was evaluated based on the degree of ghost lines that appeared on the surface of the formed door outer. A pressed part was used as a formed part simulating a door outer, which was formed by press-forming a steel plate blanked to a 600 mm square so that the radius of curvature R at the center was 1200 mm. The surface after press-forming was ground with a grindstone, and stripes that appeared on the surface at intervals of several millimeters were judged to be ghost lines, and the degree of occurrence of the stripes was scored from 1 to 5. A random 100 mm x 100 mm area was visually inspected, and a rating of "1" was given if no stripes were observed; a rating of "2" was given if the maximum length of the stripes was 20 mm or less; a rating of "3" was given if the maximum length of the stripes was more than 20 mm but not more than 50 mm; a rating of "4" was given if the maximum length of the stripes was more than 50 mm but not more than 70 mm; and a rating of "5" was given if the maximum length of the stripes was more than 70 mm. A rating of "3" or less was deemed to be excellent in post-forming appearance and was judged to be acceptable. On the other hand, if the evaluation score was "4" or higher, the post-forming appearance was deemed poor and the test piece was deemed to have failed. In this test, the post-forming appearance was evaluated using a pressed part simulating a door outer. However, the evaluation object may also be a formed part that can be estimated to have been strained by 2.5% by press forming, or a test piece taken from a steel plate to which a pre-strain of 2.5% has been applied may also be evaluated. Similar evaluations can be performed using such test methods. In the case of a test piece taken from a steel plate, a JIS No. 5 test piece, the longitudinal direction of which is perpendicular to the rolling direction and the plate thickness direction, can be evaluated by applying a pre-strain of 2.5%.

Steel sheets that did not exhibit heat buckling, had a tensile strength of 540 MPa or greater, and had a post-forming appearance rating of 3 or less were evaluated as being capable of suppressing the occurrence of heat buckling during the manufacturing process and achieving both strength and good post-forming appearance. The results are shown in Table 2. In the metal structure shown in Table 2, the remaining structure was at least one of bainite, pearlite, and retained austenite.

Referring to Tables 1 and 2, in Comparative Example 2, the low F5/F7 reduction ratio in the finish rolling caused significant strain accumulation after reduction in the final pass (F7), which is thought to have increased the amount of fine Nb carbonitrides that precipitated after finish rolling. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more decreased, causing heat buckling in the primary annealing process. In Comparative Example 3, the low reduction ratio in the cold rolling process delayed recrystallization during heating in the subsequent primary annealing process, resulting in residual strain until the material reached a high temperature, which is thought to have caused a large amount of fine Nb carbonitrides to precipitate during heating in the primary annealing process. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more decreased, causing heat buckling in the primary annealing process. In Comparative Example 4, the heating temperature in the first annealing step was low, which resulted in insufficient austenitization, and it is believed that the subsequent cooling did not result in the metallographic structure of the steel sheet being composed mainly of bainite and/or martensite. As a result, in the metallographic structure obtained after the second annealing, the average particle spacing of martensite exceeded 2.5 μm, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction exceeded 1.5%, resulting in a poor appearance after forming. In Comparative Example 9, the temperature at the inlet side of the finish rolling was low, which resulted in insufficient precipitation of Nb carbonitrides. As a result, the proportion of Nb carbonitrides with a particle size of 20 nm or more decreased, and heat buckling occurred in the first annealing step. In Comparative Example 10, the coiling temperature was low, which resulted in a large amount of solute Nb, which is believed to have reduced the precipitation of Nb carbonitrides. As a result, the amount of Nb in all Nb carbonitrides was reduced, and the proportion of Nb carbonitrides with a particle size of 20 nm or more was also reduced, causing heat buckling in the primary annealing step. In Comparative Example 19, the high temperature at the entry side of finish rolling caused recrystallization, resulting in insufficient accumulation of strain in the latter stages of finish rolling, making it impossible to promote the precipitation of Nb carbonitrides with a particle size of 20 nm or more, and resulting in a reduced proportion. As a result, heat buckling occurred in the primary annealing step.

In Comparative Example 20, the high F5/F7 reduction ratio in the finish rolling likely led to excessive strain accumulation after the F5 rolling pass, promoting recrystallization. As a result, the proportion of Nb carbonitrides with particle sizes of 20 nm or more decreased, resulting in heat buckling during the primary annealing process. Furthermore, in Comparative Example 20, as in Comparative Example 4, the low heating temperature during the primary annealing process likely led to insufficient austenitization, preventing the steel sheet from achieving a metal structure primarily composed of bainite and/or martensite, even after subsequent cooling. As a result, the metal structure obtained after secondary annealing had an average martensite particle spacing of more than 2.5 μm and a standard deviation of more than 1.5% in the area fraction of martensite perpendicular to the rolling direction and the thickness direction, resulting in poor appearance after forming. In Comparative Example 21, the high coiling temperature caused alloying elements to concentrate in cementite in the metallographic structure, resulting in undissolved carbides remaining during heating in the subsequent primary annealing step. This presumably prevented the metallographic structure from being composed primarily of bainite and/or martensite. As a result, in the metallographic structure obtained after secondary annealing, the average particle spacing of martensite exceeded 2.5 μm, and the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the sheet thickness direction exceeded 1.5%, resulting in poor appearance after forming. In Comparative Example 27, Nb was not added, so Nb carbonitrides with particle sizes of 20 nm or more did not precipitate, and the value of Index A was also low. As a result, the high-temperature strength of the steel sheet could not be sufficiently improved, and heat buckling occurred in the first annealing step. In Comparative Example 28, the low value of Index A similarly prevented the high-temperature strength of the steel sheet from being sufficiently improved, and heat buckling occurred in the first annealing step. In Comparative Examples 29 and 30, the C or Mn content was high, so the standard deviation of the area ratio of martensite in the direction perpendicular to the rolling direction and the plate thickness direction exceeded 1.5%, resulting in poor appearance after forming. In Comparative Examples 31 and 32, the C or Mn content was low, so sufficient strength was not obtained.

In contrast, all of the steel sheets according to the invention examples have a specified chemical composition and metallurgical structure. By appropriately controlling the proportions of ferrite and martensite in the metallurgical structure, they achieve a TS of 540 MPa or more. In the microscopic region, the average grain spacing of martensite is controlled to 2.5 μm or less, while in the macroscopic region, the standard deviation in the area fraction of martensite in the direction perpendicular to the rolling direction and the thickness direction is controlled to 1.5% or less. This suppresses the generation of minute irregularities on the steel sheet surface and significantly reduces the occurrence of ghost lines, even when strain is imparted by press forming. Cross-sectional observation of the metallurgical structure of the cold-rolled steel sheets according to all of the invention examples before secondary annealing revealed that all were composed of martensite by area fraction of 90% or more. Furthermore, in the steel sheets according to all of the invention examples, the Nb content in all Nb carbonitrides was controlled to 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more was controlled to 60% or more of the Nb content in all Nb carbonitrides, thereby significantly suppressing the occurrence of heat buckling during the manufacturing process of the steel sheets, in combination with controlling the chemical composition using index A. Furthermore, all of the Nb carbonitrides having a particle size of 20 nm or more in the invention examples had particle sizes of 1000 nm or less, i.e., 1 μm or less.

Claims (4)

In mass%,
C: 0.030-0.100%,
Mn: 0.70-3.00%,
Si: 0.005-1.500%,
P: 0.100% or less,
S: 0.0200% or less,
Al: 1.000% or less,
N: 0.0010-0.0150%,
O: 0.0100% or less,
Nb: 0.005-0.200%,
Cr: 0-1.00%,
Mo: 0 to 0.80%,
B: 0 to 0.0100%,
Ti: 0-0.200%,
V: 0 to 0.500%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0-1.00%,
Ta: 0-0.10%,
Co: 0-3.00%,
Sn: 0-1.00%,
Sb: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0-0.0500%,
As: 0 to 0.10%, and the balance: Fe and impurities, and has a chemical composition in which index A represented by the following formula 1 is 0.50% or more,
In area %,
Ferrite: 75 to 95%,
Martensite: 5 to 25%, and the remaining structure: 0 to 10% in total,
the Nb content in all Nb carbonitrides is 0.004% or more, and the Nb content in Nb carbonitrides having a particle size of 20 nm or more is 60% or more of the Nb content in all Nb carbonitrides,
The average grain spacing of the martensite is 2.5 μm or less,
A steel sheet characterized by having a metal structure in which the standard deviation of the area ratio of martensite in a direction perpendicular to the rolling direction and the sheet thickness direction is 1.5% or less.
A=[C]-0.1[Si]+0.3([Mn]-0.5)-0.3[Al]+0.1[Cr]+0.6[Mo]-[Ti]+15[Nb]...Formula 1
Here, [C], [Si], [Mn], [Al], [Cr], [Mo], [Ti], and [Nb] are the contents [mass %] of each element, and are 0% when the element is not contained. The chemical composition is, in mass %,
Cr: 0.001-1.00%,
Mo: 0.001-0.80%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.200%,
V: 0.001-0.500%,
Ni: 0.001 to 1.00%,
Cu: 0.001 to 1.00%,
W: 0.001-1.00%,
Ta: 0.001 to 0.10%,
Co: 0.001-3.00%,
Sn: 0.001 to 1.00%,
Sb: 0.001-0.200%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%,
REM: 0.0001-0.0100%,
Bi: 0.0001 to 0.0500%, and As: 0.001 to 0.10%
The steel sheet according to claim 1, characterized in that it contains at least one of the following: The steel sheet according to claim 1 or 2, characterized in that the average grain size of the ferrite is 3.0 to 25.0 μm, the average grain size of the martensite is 1.0 to 5.0 μm, and the average aspect ratio of the martensite is 2.5 or more. An outer panel member comprising the steel plate according to claim 1 or 2 .