JP7820682B2 - Steel plates and outer panel components - Google Patents

Description

The present invention relates to a steel plate.

In the automotive industry, there is a demand for lighter vehicle bodies in order to improve fuel efficiency. In order to achieve both lighter vehicle bodies and collision safety, one effective method is to increase the strength of the steel plates used, and against this background, development of high-strength steel plates is progressing.

In this regard, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of a steel sheet substrate, wherein the substrate contains, by mass%, C: 0.02-0.20%, Si: 0.7% or less, Mn: 1.5-3.5%, P: 0.10% or less, S: 0.01% or less, Al: 0.1-1.0%, N: 0.010% or less, and Cr: 0.03-0.5%, and the surface oxidation index A during annealing, defined by the formula A = 400Al/(4Cr + 3Si + 6Mn) where the contents of Al, Cr, Si, and Mn are the same as those in the formula, is 2.3 or more, with the balance being Fe and unavoidable impurities, and the structure of the substrate is composed of ferrite and a second phase, with the second phase being mainly martensite. Furthermore, Patent Document 1 describes that the high-strength hot-dip galvanized steel sheet has excellent surface quality and a tensile strength of 590 MPa or more, making it suitable for use mainly as structural parts of automobiles such as members and rockers.

Japanese Patent Application Laid-Open No. 2005-220430

In recent years, in connection with the demand for further improvements in fuel efficiency, there has been a growing need to reduce the weight of not only structural components such as the members described in Patent Document 1, but also exterior panel components such as roofs, hoods, fenders, and doors. Unlike the structural components described above, these exterior panel components are visible to the public, and therefore, in addition to properties such as strength, design and surface quality are important. Therefore, excellent appearance after forming is required. Meanwhile, in connection with this demand for weight reduction, the steel sheets used for these exterior panel components are also required to be even stronger and thinner. Additionally, as the shapes of these exterior panel components become more complex, unevenness tends to occur more easily on the steel sheet surface after forming, and if such unevenness occurs, the appearance quality will be reduced.

Specifically, for example, DP steel (Dual Phase steel) composed of a soft ferrite phase (first phase) and a hard second phase primarily composed of martensite, as described in Patent Document 1, has low yield strength (hereinafter sometimes referred to as "YS") and is excellent in formability during press forming and other forming processes. However, it is prone to non-uniform deformation, in which the soft phase composed of ferrite and its surrounding areas deform preferentially during forming. For this reason, when such DP steel is used for exterior panel components, minute irregularities appear on the steel sheet surface after forming, which can result in appearance defects known as ghost lines and stretcher strain.

Furthermore, even if attempts were made to optimize various conditions during steel plate manufacturing in order to improve these poor appearances, depending on the conditions during steel plate manufacturing, the low YS characteristic of DP steel may not be achieved, and there is a risk that workability during forming may be reduced.

Therefore, the present invention aims to provide a high-strength steel sheet with a novel configuration that has excellent appearance after forming and workability during forming.

The present invention includes the following aspects:

(Aspect 1)
A steel plate,
The chemical composition of the steel plate is, in mass%,
C: 0.030-0.100%,
Mn: 1.00-2.50%,
Si: 0.005-1.500%,
Al: 0.005-0.700%,
P: 0.100% or less,
S: 0.0200% or less,
N: 0.0150% or less,
O: 0.0100% or less,
Cr: 0-0.80%,
Mo: 0 to 0.50%,
B: 0 to 0.0100%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.50%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0-1.00%,
Sn: 0-1.00%,
Sb: 0 to 0.200%,
As: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0 to 0.0100%, and the balance: Fe and impurities;
The area ratio of the hard phase in a region having a depth from the surface of the steel plate to a 1/8 depth position of the plate thickness is 0.20Vm to 0.80Vm, where Vm is the area ratio of the hard phase in a region having a depth from a 3/8 depth position to a 5/8 depth position of the plate thickness,
The metal structure in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness contains, in area %, 75 to 97% ferrite and 3 to 25% hard phase, and the standard deviation of the area ratio of the hard phase in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness in a direction perpendicular to the rolling direction and the plate thickness direction is 0.85% or less.

(Aspect 2)
The chemical composition is, in mass %,
Cr: 0.01-0.80%,
Mo: 0.01-0.50%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.100%,
V: 0.01-0.50%,
Ni: 0.01-1.00%,
Cu: 0.01 to 1.00%,
W: 0.01-1.00%,
Sn: 0.001 to 1.00%,
Sb: 0.001-0.200%,
As: 0.001-0.200%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
The steel sheet according to the above aspect 1, characterized in that it contains one or more selected from the group consisting of:

(Aspect 3)
The steel sheet according to the above aspect 1 or 2, wherein an index A represented by the following formula (1) is 0.45% or more:
A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb]...(1)
Here, [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass %, and when the corresponding element is not contained, it is 0%.

(Aspect 4)
The steel sheet according to any one of Aspects 1 to 3, characterized in that the following formula (2) is satisfied:
(TS-180000/TS)/Vm≧35...(2)
Here, TS represents the tensile strength in MPa, and Vm represents the area ratio of the hard phase in the region having a depth ranging from 3/8 to 5/8 of the plate thickness.

(Aspect 5)
The average grain size of the ferrite in a region having a depth from a 3/8 depth position to a 5/8 depth position of the plate thickness is 5.0 to 30.0 μm,
A steel sheet according to any one of Aspects 1 to 4, characterized in that the hard phase has an average grain size of 1.0 to 5.0 μm in a region having a depth from a 3/8 depth position to a 5/8 depth position of the sheet thickness.

(Aspect 6)
Aspect 6. The steel sheet according to any one of Aspects 1 to 5, wherein the hard phase in a region having a depth from a 3/8 depth position to a 5/8 depth position of the sheet thickness is composed of at least one of martensite, bainite, tempered martensite, and pearlite.

(Aspect 7)
An outer panel member comprising the steel plate according to any one of Aspects 1 to 6.

The present invention makes it possible to provide high-strength steel sheets that have excellent appearance after forming and excellent workability during forming.

Preferred embodiments of the steel sheet of the present invention are described in detail below. Note that, in this specification, various numerical ranges include the upper and lower limits unless otherwise specified.

DP steel, which has a relatively low yield strength (YS), is often used for exterior panel components such as roofs and doors to avoid surface defects known as surface distortions that occur during press forming and other processes. However, as mentioned above, DP steel is prone to non-uniform deformation, in which soft phases and their surrounding areas deform preferentially during processes such as press forming. Therefore, when such DP steel is used for exterior panel components, minute irregularities can appear on the steel sheet surface after forming, resulting in appearance defects known as ghost lines. To explain the occurrence of these ghost lines in more detail, first, during processes such as press forming, the soft phases composed of ferrite deform in a concave manner, while the hard phases composed mainly of martensite deform in a convex manner, rather than in a concave manner. This results in the formation of minute irregularities on the steel sheet surface after forming. These minute irregularities are formed by convex portions extending generally along the rolling direction and concave portions extending generally along the rolling direction, aligned in the sheet width direction, perpendicular to the rolling direction. When the surface of the steel sheet after forming is polished, the convex portions of the minute irregularities on the surface of the steel sheet are removed, resulting in the appearance of ghost lines, which are streaky patterns extending in the rolling direction of the steel sheet. The rolling direction can be easily identified based on the extension direction of the crystal grains of the steel sheet. In this specification, the direction orthogonal to the rolling direction and the thickness direction means a direction perpendicular to each of the rolling direction and the thickness direction.

To improve the appearance of ghost lines, the inventors first conducted detailed studies focusing on the morphology of hard phases in the metallographic structure of steel sheets. As a result, the inventors discovered that in steel sheets with a mixture of soft and hard phases, such as DP steel, the presence of hard phases connected in stripes (striped hard phases) in the metallographic structure significantly increases the severity of ghost lines. Furthermore, the inventors discovered that by suppressing the formation of such striped hard phases and dispersing the hard phases more uniformly in the metallographic structure, it is possible to suppress the formation of minute irregularities on the steel sheet surface after forming while fully maintaining the high strength provided by the hard phases, thereby ultimately suppressing the occurrence of ghost lines.

To explain in more detail, the inventors discovered that reducing Mn segregation during solidification in the slab casting process, in which molten steel is solidified to cast slabs, is an effective way to suppress the formation of banded hard phases. Therefore, the inventors conducted further detailed studies on methods for reducing Mn segregation from two perspectives: center segregation and microsegregation.

The inventors first considered that suppressing the flow of molten steel during slab casting would be an effective way to reduce center segregation of Mn, and conducted various studies. More specifically, when molten steel solidifies, it solidifies from the surface, with the center solidifying last. As the solid phase is expelled from the liquid phase, Mn in the liquid phase becomes concentrated during this stage. If the molten steel flows during solidification, this concentrated Mn tends to eventually collect in the center during the solidification process, resulting in significant center segregation of Mn. The inventors then discovered that by appropriately controlling the solidification conditions and suppressing the flow of molten steel when manufacturing steel plate, center segregation of Mn can be significantly suppressed.

Next, the inventors conducted various studies, believing that promoting Mn diffusion during solidification would be effective in reducing Mn microsegregation. To promote Mn diffusion, it is effective to create a structure in which Mn diffuses easily. Therefore, the inventors focused on the δ phase, in which Mn has a fast diffusion rate, and experimentally investigated the influence of each element in the steel on Mn microsegregation in order to achieve δ solidification. As a result, the inventors found that when the C and Mn contents are high, δ solidification does not occur during solidification, but the Mn diffusion rate decreases, increasing microsegregation. On the other hand, when the Si, Al, Cr, and Mo contents are high, Mn diffusion during solidification is promoted, reducing microsegregation.

Although ghost lines can be improved to a certain extent by the above-described methods for reducing Mn segregation, in order to obtain a more sufficient effect of improving ghost lines, the present inventors have investigated further methods for improving ghost lines in addition to the above-described methods for reducing Mn segregation. As a result, the present inventors have found that ghost lines can be improved by reducing the hard phase fraction of a steel sheet, even if centerline segregation of Mn remains to a certain extent.
The present inventors have also found that the occurrence of ghost lines is also greatly influenced by the solidification structure, and that even if the center segregation of Mn is small, if coarse equiaxed crystals are formed in the solidification structure, negative segregation of Mn occurs, increasing the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the plate thickness direction, making ghost lines more likely to occur. Furthermore, the present inventors have found that a method different from conventional measures against center segregation, namely a method of reducing the equiaxed crystal fraction and controlling the solidification structure to a columnar crystal structure, can suppress negative segregation of Mn and improve ghost lines.

Based on these findings, the inventors discovered that ghost lines can be significantly improved while adequately maintaining high strength by controlling the solidification structure during casting to a columnar crystal structure, reducing the central segregation of Mn during solidification, which is the cause of the formation of banded hard phases, and reducing the hard phase fraction and its variation.

Although the above-mentioned methods have sufficiently improved ghost lines, it has been found that a visual defect known as stretcher strain can still occur. Furthermore, even if attempts are made to control the various conditions during steel plate manufacturing within specific ranges as described above in order to improve these visual defects, it has been found that, depending on the conditions, the low YS characteristic of DP steel cannot be achieved, and workability during forming may be reduced.

Therefore, the inventors conducted extensive research into a method for suppressing poor appearance after forming, including not only ghost lines but also stretcher strain, while also achieving excellent workability during forming. As a result, the inventors discovered that a high-strength steel sheet with excellent appearance after forming and excellent workability can be obtained by controlling, within specific ranges, the area fraction of the hard phase in the region extending from the surface of the steel sheet to a depth position of 1/8 of the sheet thickness (i.e., the hard phase fraction in the surface layer of the steel sheet), and the area and variation of the hard phase in the region extending from a depth position of 3/8 to a depth position of 5/8 of the sheet thickness (i.e., the hard phase fraction in the center of the steel sheet).

The present invention was completed based on the above findings and includes the following embodiments.

The following describes in detail a preferred embodiment of the steel plate of the present invention.

<Steel plate>
The steel plate according to one embodiment of the present invention has a chemical composition, in mass%,
C: 0.030-0.100%,
Mn: 1.00-2.50%,
Si: 0.005-1.500%,
Al: 0.005-0.700%,
P: 0.100% or less,
S: 0.0200% or less,
N: 0.0150% or less,
O: 0.0100% or less,
Cr: 0-0.80%,
Mo: 0 to 0.50%,
B: 0 to 0.0100%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.50%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0-1.00%,
Sn: 0-1.00%,
Sb: 0 to 0.200%,
As: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0 to 0.0100%, and the balance: Fe and impurities.
In the steel plate of this embodiment, the area ratio of the hard phase in a region having a depth from the surface of the steel plate to a depth position of 1/8 of the plate thickness is 0.20Vm to 0.80Vm, where Vm is the area ratio of the hard phase in a region having a depth from a depth position of 3/8 to a depth position of 5/8 of the plate thickness.
Furthermore, in the steel plate of this embodiment, the metal structure in a region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness contains, in area %, 75 to 97% ferrite and 3 to 25% hard phase, and the standard deviation of the area ratio of the hard phase in the direction perpendicular to the rolling direction and the plate thickness direction in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness is 0.85% or less.

As described above, the steel sheet of this embodiment has a specific chemical composition, and the metallographic structure in the region from 3/8 to 5/8 of the thickness (i.e., the center of the sheet thickness) has a lower hard phase fraction than conventional DP steel, and the standard deviation of the hard phase fraction (i.e., the variation in hard phase fraction) in the direction perpendicular to the rolling direction and the thickness direction at the center of the sheet thickness is small. This specific metallographic structure can be obtained by adopting a specific chemical composition and casting conditions, as described below.

As described above, the steel sheet of this embodiment has such a unique metal structure, namely, a metal structure with small central segregation of Mn during solidification and a small hard phase fraction and its variation, making it possible to suppress the generation of minute irregularities on the steel sheet surface after forming while adequately maintaining high strength. As a result, the steel sheet of this embodiment can significantly suppress the occurrence of ghost lines and stretcher strain while adequately maintaining high strength.

Furthermore, in the steel sheet of this embodiment, the area ratio of the hard phase in the region extending from the surface of the steel sheet to a depth position of 1/8 of the sheet thickness, i.e., the hard phase fraction in the surface layer, is 0.20 Vm to 0.80 Vm relative to the area ratio Vm of the hard phase in the center of the sheet thickness. When the hard phase fraction in the surface layer is 0.20 Vm or more, low yield strength (YS) and high tensile strength (TS) can be ensured, resulting in a high-strength steel sheet with excellent workability during forming. This also makes it easier to suppress stretcher strain that occurs during forming of the steel sheet. Furthermore, when the hard phase fraction in the surface layer of the steel sheet is 0.80 Vm or less, good bendability can be ensured, resulting in a steel sheet with excellent workability during forming.

In order to control the hard phase fraction in the surface layer of the steel sheet within the range of 0.20 Vm to 0.80 Vm, it is necessary to adopt specific conditions, as described below, in the hot rolling process, pickling process, and annealing process during steel sheet manufacturing.

Here, a hard phase fraction of "0.20 Vm" in the surface layer of a steel plate means that it is 0.20 times (0.20 x Vm) the hard phase fraction Vm in the center of the plate thickness. The same applies to 0.80 Vm, etc. Note that in this specification, the hard phase area fraction (%) and hard phase fraction (%) are synonymous.

As described above, the steel sheet of this embodiment is a high-strength steel sheet that combines suppression of poor appearance after forming, including not only ghost lines but also stretcher strain, with excellent workability during forming, i.e., a high-strength steel sheet that is excellent in both appearance after forming and workability during forming.

The steel sheet of this embodiment will be described in more detail below. In the following description, the unit "%" for the content of each element and the index A means "mass%" unless otherwise specified. Furthermore, in this specification, the term "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.
In addition, in this specification, the term "a depth position x/y in the sheet thickness (where x and y are natural numbers satisfying x<y)" refers to a position moved from the surface in the sheet thickness direction of the steel sheet by a distance (depth) of x/y of the sheet thickness toward the center of the steel sheet in the sheet thickness direction. For example, when the sheet thickness of a steel sheet is 2 mm, the "depth position ⅛ of the sheet thickness" refers to a position that is 0.25 mm deep from the surface of the steel sheet in the sheet thickness direction. Note that, when a steel sheet has a coating layer on its surface, the "surface of the steel sheet" in the definition of "a depth position x/8 of the sheet thickness from the surface of the steel sheet" above refers to the interface between the steel sheet and the coating layer, and the "sheet thickness" refers to the thickness of the steel sheet (base material) excluding the coating layer.

(chemical composition)
As described above, the steel sheet of this embodiment has
C: 0.030-0.100%,
Mn: 1.00-2.50%,
Si: 0.005-1.500%,
Al: 0.005-0.700%,
P: 0.100% or less,
S: 0.0200% or less,
N: 0.0150% or less,
O: 0.0100% or less,
Cr: 0-0.80%,
Mo: 0 to 0.50%,
B: 0 to 0.0100%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.50%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0-1.00%,
Sn: 0-1.00%,
Sb: 0 to 0.200%,
As: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0 to 0.0100%, and the balance: Fe and impurities.

Below, we will explain each element in this chemical composition in detail.

[C:0.030-0.100%]
C is an element that increases the strength of steel sheet. To fully obtain this effect, the C content is set to 0.030% or more. The C content may be 0.032% or more, 0.034% or more, or 0.035% or more. On the other hand, if C is contained excessively, the diffusion of Mn during solidification may be inhibited, and the microsegregation of Mn may not be sufficiently suppressed. Therefore, the C content is set to 0.100% or less. The C content may be 0.095% or less, 0.090% or less, or 0.080% or less.

[Mn: 1.00-2.50%]
Mn is an element that improves the hardenability of steel and contributes to improving its strength. To fully obtain this effect, the Mn content is set to 1.00% or more. The Mn content may be 1.02% or more, 1.04% or more, or 1.05% or more. On the other hand, if Mn is contained excessively, the diffusion of Mn during solidification may be hindered, and microsegregation of Mn may not be sufficiently suppressed. Therefore, the Mn content is set to 2.50% or less. The Mn content may be 2.40% or less, 2.30% or less, or 2.20% or less.

[Si: 0.005 to 1.500%]
Si is a deoxidizing element for steel and a solid-solution strengthening element effective for increasing the strength of steel sheet without impairing its ductility. Si is also an element effective for promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully obtain these effects, the Si content is set to 0.005% or more. The Si content may be 0.008% or more, 0.010% or more, or 0.012% or more. On the other hand, excessive Si content may reduce the peelability of scale and cause surface defects. Therefore, the Si content is set to 1.500% or less. The Si content may be 1.200% or less, 1.000% or less, or 0.800% or less.

[Al: 0.005-0.700%]
Al functions as a deoxidizer and is a solid-solution strengthening element effective in increasing the strength of steel. Furthermore, Al is also effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. To fully obtain these effects, the Al content is set to 0.005% or more. The Al content may be 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, excessive Al content may deteriorate castability and reduce productivity. Therefore, the Al content is set to 0.700% or less. The Al content may be 0.600% or less, 0.500% or less, 0.400% or less, or 0.300% or less.

[P: 0.100% or less]
P is an element that is mixed in during the manufacturing process. P is also a solid solution strengthening element. The P content may be 0%. However, reducing the P content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the P content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive P content may reduce the toughness of the steel plate. Therefore, the P content is set to 0.100% or less. The P content may be 0.080% or less, 0.075% or less, or 0.070% or less.

[S: 0.0200% or less]
S is an element that is mixed in during the manufacturing process. The S content may be 0%. However, reducing the S content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0008% or more. On the other hand, excessive S content may form Mn sulfides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and/or bendability. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0150% or less, 0.0100% or less, or 0.0080% or less.

[N: 0.0150% or less]
N is an element that is mixed in during the manufacturing process. The N content may be 0%. However, reducing the N content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form nitrides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and/or bendability. Therefore, the N content is set to 0.0150% or less. The N content may be 0.0100% or less, 0.0080% or less, or 0.0050% or less.

[O: 0.0100% or less]
O is an element that is mixed in during the manufacturing process. The O content may be 0%. However, reducing the O content to less than 0.0001% requires time for refining, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0008% or more. On the other hand, excessive O content may form coarse oxides, which may reduce the formability of the steel sheet, such as ductility, hole expandability, stretch flangeability, and/or bendability. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0070% or less, 0.0050% or less, or 0.0030% or less.

The basic chemical composition of the steel sheet of this embodiment is as described above. Furthermore, in this embodiment, the steel sheet may contain one or more of the following optional elements in place of a portion of the remaining Fe, as needed. These optional elements are described in detail below.

[Cr: 0-0.80%]
Cr is an element that improves the hardenability of steel and contributes to improving the strength of steel sheets. Cr is also an element that is effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. The Cr content may be 0%, but to obtain these effects, the Cr content is preferably 0.01% or more. The Cr content may be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, excessive Cr content may cause the formation of coarse Cr carbides that serve as fracture initiation points. Therefore, the Cr content is preferably 0.80% or less. The Cr content may be 0.75% or less, 0.70% or less, or 0.65% or less.

[Mo: 0-0.50%]
Mo is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. Mo is also an element that is effective in promoting the diffusion of Mn during solidification and reducing Mn microsegregation. The Mo content may be 0%, but to obtain these effects, the Mo content is preferably 0.01% or more. The Mo content may be 0.05% or more or 0.07% or more. On the other hand, excessive Mo content may deteriorate hot workability and reduce productivity. Therefore, the Mo content is preferably 0.50% or less. The Mo content may be 0.45% or less, 0.40% or less, or 0.35% or less.

[B: 0 to 0.0100%]
B is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0005% or more, 0.0008% or more, or 0.0010% or more. On the other hand, excessive B content may cause B precipitates to form, reducing the strength of the steel sheet. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[Ti: 0 to 0.100%]
Ti is an element that has the effect of reducing the amounts of S, N, and O, which generate coarse inclusions that act as fracture initiation sites. Ti is also a precipitation strengthening element that refines the structure and improves the strength-formability balance of steel sheets. While the Ti content may be 0%, to obtain these effects, the Ti content is preferably 0.001% or more. The Ti content may be 0.005% or more, 0.007% or more, or 0.010% or more. On the other hand, excessive Ti content may result in the formation of coarse Ti sulfides, Ti nitrides, and/or Ti oxides, which may reduce the formability of the steel sheet. Therefore, the Ti content is preferably 0.100% or less. The Ti content may be 0.080% or less, 0.075% or less, or 0.070% or less.

[Nb: 0 to 0.100%]
Nb is a precipitation strengthening element that contributes to improving the strength of steel sheets due to strengthening by precipitates, grain refinement strengthening by suppressing ferrite grain growth, and/or dislocation strengthening by suppressing recrystallization. The Nb content may be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.003% or more, 0.004% or more, or 0.005% or more. On the other hand, excessive Nb content may increase unrecrystallized ferrite, thereby reducing the formability of the steel sheet. Therefore, the Nb content is preferably 0.100% or less. The Nb content may be 0.080% or less, 0.070% or less, or 0.060% or less.

[V: 0-0.50%]
V is an element that contributes to improving the strength of steel sheets due to strengthening by precipitates, grain refinement strengthening by suppressing ferrite grain growth, and/or dislocation strengthening by suppressing recrystallization. The V content may be 0%, but to obtain these effects, the V content is preferably 0.01% or more. The V content may be 0.02% or more. On the other hand, excessive V content may precipitate a large amount of carbonitrides, which may reduce the formability of the steel sheet. Therefore, the V content is preferably 0.50% or less. The V content may be 0.40% or less, 0.30% or less, or 0.20% or less.

[Ni: 0-1.00%]
Ni is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The Ni content may be 0%, but to obtain such effects, the Ni content is preferably 0.01% or more. The Ni content may be 0.03% or more or 0.05% or more. On the other hand, excessive Ni content may deteriorate the weldability of the steel sheet. Therefore, the Ni content is preferably 1.00% or less. The Ni content may be 0.60% or less, 0.50% or less, or 0.40% or less.

[Cu: 0-1.00%]
Cu is present in steel in the form of fine particles and is an element that contributes to improving the strength of the steel sheet. The Cu content may be 0%, but to obtain such an effect, the Cu content is preferably 0.01% or more. The Cu content may be 0.03% or more or 0.05% or more. On the other hand, excessive Cu content may reduce the weldability of the steel sheet. Therefore, the Cu content is preferably 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[W: 0-1.00%]
W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of the steel sheet. The W content may be 0%, but to obtain such an effect, the W content is preferably 0.01% or more. The W content may be 0.02% or more. On the other hand, excessive W content may deteriorate hot workability and reduce productivity. Therefore, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[Sn: 0-1.00%]
Sn is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sn content may be 0%, but to obtain such an effect, the Sn content is preferably 0.001% or more. The Sn content may be 0.004% or more. On the other hand, excessive Sn content may cause embrittlement of the steel sheet. Therefore, the Sn content is preferably 1.00% or less. The Sn content may be 0.80% or less, 0.60% or less, or 0.40% or less.

[Sb: 0 to 0.200%]
Sb is an element that suppresses coarsening of crystal grains and contributes to improving the strength of the steel sheet. The Sb content may be 0%, but to obtain such an effect, the Sb content is preferably 0.001% or more. The Sb content may be 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, excessive Sb content may cause embrittlement of the steel sheet. Therefore, the Sb content is preferably 0.200% or less. The Sb content may be 0.180% or less, 0.170% or less, or 0.160% or less.

[As: 0 to 0.200%]
Although As does not have as significant an effect as Sb and Sn, it is an element that is effective in improving corrosion resistance in an acidic environment. The As content may be 0%, but to obtain such an effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more, 0.004% or more, or 0.006% or more. On the other hand, excessive As content may deteriorate hot workability. Therefore, the As content is preferably 0.200% or less. The As content may be 0.180% or less, 0.170% or less, or 0.160% or less.

[Ca: 0 to 0.0100%], [Mg: 0 to 0.0100%], [Zr: 0 to 0.0100%] and [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 such effects, the Ca, Mg, Zr, and REM contents are preferably 0.0001% or more. The Ca, Mg, Zr, and REM contents 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 preferably 0.0100% or less. The Ca, Mg, Zr, and REM contents may be 0.0080% or less, 0.0070% or less, or 0.0060% or less, respectively.

In this specification, REM is a collective term for 17 elements: scandium (Sc), atomic number 21; yttrium (Y), atomic number 39; and the lanthanides lanthanum (La), atomic number 57, through lutetium (Lu), atomic number 71. The REM content is the total content of these elements.

Regarding the above optional elements, in this embodiment, the chemical composition of the steel sheet is
Cr: 0.01-0.80%,
Mo: 0.01-0.50%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.100%,
V: 0.01-0.50%,
Ni: 0.01-1.00%,
Cu: 0.01 to 1.00%,
W: 0.01-1.00%,
Sn: 0.001 to 1.00%,
Sb: 0.001-0.200%,
As: 0.001-0.200%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
When the steel sheet contains such optional elements, it is possible to more reliably maintain high strength while significantly suppressing the occurrence of appearance defects after forming, such as ghost lines, and it is also possible to provide a steel sheet with excellent workability during forming.

In the steel sheet of this embodiment, the balance other than the above elements consists of Fe and impurities. Here, impurities refer to components that are mixed in during the industrial production of steel sheet due to various factors in the manufacturing process, including raw materials such as ore and scrap. Examples of impurities include H, Na, Cl, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb, Bi, and Po. The total impurity content may be 0.100% or less.

[Index A: 0.45% or more]
In this embodiment, the chemical composition of the steel sheet preferably has an index A expressed by the following formula (1) of 0.45% or more.
A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb]...(1)
Here, [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass %, and when the corresponding element is not contained, it is 0%.

Index A is determined by the content of the solid solution strengthening elements Si, P, and Al, and the precipitation strengthening elements Ti and Nb. The higher this value, the higher the strength can be obtained with a lower hard phase fraction. By setting Index A to 0.45% or more, high strength can be obtained while controlling the hard phase fraction of the steel plate below a certain level.

Index A may be 0.48% or more, 0.50% or more, or 0.52% or more. There is no particular upper limit to Index A, but, for example, Index A may be 1.50% or less, 1.20% or less, or 1.00% or less.

The chemical composition of steel plate can be measured using common analytical methods. For example, the chemical composition of steel plate can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). Specifically, a 35 mm square test piece is taken from the steel plate at a depth approximately halfway through the plate thickness, and the chemical composition of the steel plate can be determined by measuring it using a measuring device such as the Shimadzu ICPS-8100 under conditions based on a pre-established calibration curve. C and S, which cannot be measured using ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.

(Metal structure)
[Area ratio of hard phase in a region having a depth from the surface of the steel plate to a depth position of 1/8 of the plate thickness: 0.20 Vm to 0.80 Vm]
In this embodiment, the metal structure of the steel sheet has an area fraction of the hard phase in a region from the surface of the steel sheet to a depth position of ⅛ of the sheet thickness (i.e., the hard phase fraction in the surface layer), which is 0.20 Vm to 0.80 Vm, where Vm is the area fraction of the hard phase in a region from a depth position of ⅜ to a depth position of ⅔ of the sheet thickness (i.e., the hard phase fraction in the sheet thickness center). When the hard phase fraction in the surface layer of the steel sheet is 0.20 Vm or more, a low yield strength (YS) and a high tensile strength (TS) can be ensured, resulting in a high-strength steel sheet with excellent workability during forming. Furthermore, this can also make it easier to suppress stretcher strain that occurs during forming of the steel sheet. Furthermore, when the hard phase fraction in the surface layer of the steel sheet is 0.8 Vm or less, good bendability can be ensured, resulting in a steel sheet with excellent workability during forming. From the viewpoint of further enhancing these effects, the area ratio of the hard phase in the surface layer may be 0.30 Vm or more, 0.35 Vm or more, 0.40 Vm or more, or 0.45 Vm or more. On the other hand, from the viewpoint of further enhancing processability during molding, the area ratio of the hard phase in the surface layer may be 0.79 Vm or less, 0.78 Vm or less, 0.77 Vm or less, or 0.76 Vm or less.

[Metal structure in a region having a depth from 3/8 to 5/8 of the plate thickness: ferrite: 75 to 97% and hard phase: 3 to 25%]
In this embodiment, the metal structure in the region having a depth from the 3/8 depth position to the 5/8 depth position of the steel plate thickness (i.e., the plate thickness center portion) contains, in area %, 75 to 97% ferrite and 3 to 25% hard phase. By making the metal structure in the plate thickness center portion of the steel plate such a specific composite structure, it is possible to maintain the strength of the steel plate within an appropriate range, more specifically, to achieve a tensile strength of 500 MPa or more, while suppressing poor appearance after forming. From the viewpoint of further increasing the strength of the steel plate, the area fraction of the hard phase in the plate thickness center portion may be 4% or more, 5% or more, 6% or more, 7% or more, or 8% or more. Similarly, the area fraction of ferrite in the plate thickness center portion may be 96% or less, 95% or less, or 94% or less. On the other hand, from the viewpoint of further improving the appearance after forming, the area fraction of the hard phase in the plate thickness center portion may be 22% or less, 20% or less, or 18% or less. Similarly, the area fraction of ferrite in the center portion of the sheet thickness may be 78% or more, 80% or more, 82% or more, 85% or more, 90% or more, or 92% or more.

In the steel plate of this embodiment, the hard phase refers to a structure harder than ferrite, and is composed of, for example, at least one of martensite, bainite, tempered martensite, and pearlite. From the perspective of improving the strength of the steel plate, the hard phase in the region having a depth from the 3/8th depth position to the 5/8th depth position of the plate thickness is preferably composed of at least one of martensite, bainite, and tempered martensite, and more preferably composed of martensite. It is preferable that the metal structure of the steel plate contains little retained austenite; specifically, the retained austenite is preferably less than 1% or less than 0.5%, and more preferably 0%, by area.

(Identification of metal structure and calculation of area fraction)
The identification of the metal structure and the calculation of the area fraction are carried out as follows: First, the rolling direction of the obtained steel sheet is identified. The rolling direction of the steel sheet can be determined as follows.
A test piece is taken so that a cross section parallel to the surface of the steel sheet can be observed. The cross section of the test piece taken in the sheet thickness direction is mirror-polished and then observed using an optical microscope. The observation surface is a surface parallel to the surface of the steel sheet at an arbitrary depth position in the sheet thickness direction between a ¼ depth position and a ½ depth position, and the direction parallel to the elongation direction of the crystal grains in the observation surface is determined to be the rolling direction.
The direction perpendicular to the rolling direction and thickness direction of the steel plate is defined as the plate width direction, and the length of the steel plate in the plate width direction is defined as the plate width W. A sample for observing the metal structure (microstructure) is taken from the W/4 position or 3W/4 position of the plate width W (i.e., a position W/4 in the plate width direction from either end of the steel plate in the plate width direction). The size of the sample is, for example, 20 mm in the rolling direction × 20 mm in the plate width direction × thickness of the steel plate.

Next, the metallographic structure is observed using an FE-SEM (field emission scanning electron microscope, for example, JEOL's "JSM-7200F," accelerating voltage: 15 kV, light source: FE, instrument resolution: 1.2 nm). The observation field is a 100 μm x 100 μm area corresponding to the observation target position in the thickness direction of the steel plate. In other words, when observing the metallographic structure of a region extending from the surface of the steel plate to a depth position of 1/8 of the plate thickness, a 50 μm x 100 μm area is observed in the thickness direction of the steel plate from the surface of the steel plate to a depth position of 1/8 of the plate thickness (a 50 μm area in the thickness direction and a 100 μm area in the observation field perpendicular to the thickness direction). On the other hand, when observing the metal structure of a region having a depth from 3/8 to 5/8 of the thickness of the steel plate (i.e., the center of the plate thickness), a 100 μm × 100 μm region between the 3/8 to 5/8 depth positions of the thickness of the steel plate in the plate thickness direction is observed. Note that the resolution for observing any region is 1280 × 960 pixels. To prepare the sample, the plate thickness cross section in the plate width direction is polished as the observation surface and etched by nital corrosion. Next, the "microstructure" is classified from SEM photographs at magnifications of 500 or 1000 times. Ferrite and hard phases can be distinguished by differences in brightness.

For steel plates etched with nital corrosion, the 100 μm x 100 μm area was observed at 500x or 1000x magnification, and image analysis was performed using image analysis software Image J (Ver. 1.54f) to determine the area fraction of the hard phase. Specifically, the secondary electron images of the metallographic structure were binarized based on differences in brightness, and the black areas in the image data were designated as ferrite, while the uncorroded white areas were designated as hard phase, and the area fraction of the hard phase was calculated. For each metallographic structure, three measurement points were measured, and image analysis was performed at these measurement points in the same manner as above to measure the area fraction of the hard phase. These area fractions were then arithmetically averaged to calculate the average value. This average value was designated the area fraction of the hard phase, and the remainder was designated the area fraction of ferrite. In addition, when observing the metal structure of a region having a depth from the surface of the steel plate to a depth position of 1/8 of the plate thickness, and when observing the metal structure of a region having a depth from a depth position of 3/8 to a depth position of 5/8 of the plate thickness of the steel plate, the only difference is the observation field in the plate thickness direction of the steel plate, and the area fraction of the hard phase can be measured in the same way.

If the area fraction of retained austenite needs to be measured, it can be measured by X-ray diffraction on the observation surface. Specifically, using Co-Kα radiation, the integrated intensity of six peaks (α(110), α(200), α(211), γ(111), γ(200), and γ(220)) at a depth of 1/2 the plate thickness is determined, and the volume fraction of retained austenite is calculated using the intensity averaging method. The resulting volume fraction of retained austenite is used as the area fraction of retained austenite.

[Standard deviation of area ratio of hard phase in a direction perpendicular to the rolling direction and the thickness direction in a region having a depth from the 3/8 depth position to the 5/8 depth position of the sheet thickness: 0.85% or less]
In this embodiment, the metal structure of the steel sheet has a standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction in a region having a depth from the 3/8 depth position to the 5/8 depth position of the sheet thickness (i.e., the sheet thickness center portion) of 0.85% or less. Note that the standard deviation of the hard phase fraction means the standard deviation of the area fraction of the hard phase itself. As described above, appearance defects after forming, such as ghost lines, are significantly affected not only by Mn segregation but also by the solidification structure. For example, even if the centerline segregation of Mn is small, if coarse equiaxed crystals are formed in the solidification structure, negative segregation of Mn occurs, which increases the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction, and may worsen the appearance defects after forming. However, in the steel sheet of this embodiment, the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction in the region having a depth from the 3/8 depth position to the 5/8 depth position of the sheet thickness is 0.85% or less, that is, the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction is a certain level or less, so that it is possible to significantly suppress poor appearance after forming.

From the viewpoint of more reliably suppressing poor appearance after forming, the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and the sheet thickness direction in the region having a depth from the 3/8 depth position to the 5/8 depth position of the sheet thickness may be 0.80% or less, 0.75% or less, 0.70% or less, or 0.65% or less. The lower limit of the standard deviation of the hard phase fraction is not particularly limited, and the standard deviation of the hard phase fraction may be, for example, 0.01% or more, 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.

Regarding the hard phase fraction, it is preferable that the ratio of the average value of the hard phase area fraction to its standard deviation be 0.10 or less (i.e., standard deviation of hard phase fraction/average value of hard phase area fraction≦0.10). The above ratio is preferably 0.09 or less, 0.08 or less, or 0.07 or less. The lower limit of the above ratio is 0, but the lower limit may be set to 0.01 if necessary.

The standard deviation of the hard phase fraction in the rolling direction and the direction perpendicular to the thickness direction of the metal structure at the center of the sheet thickness can be determined as follows. First, a 100 μm x 100 μm region between 3/8 and 5/8 of the sheet thickness depth position on a cross section parallel to the direction perpendicular to the rolling direction and thickness direction of the steel sheet and perpendicular to the surface of the steel sheet is observed at a magnification of 500x or 1000x using an FE-SEM (field emission scanning electron microscope, for example, JEOL JSM-7200F, acceleration voltage: 15 kV, light source: FE, instrument resolution: 1.2 nm) to obtain a secondary electron image. The resolution is 1280 x 960 pixels. Similar to the hard phase area fraction in the above-mentioned "Identification of Metallic Structure and Calculation of Area Fraction," image analysis is performed on this secondary electron image using image analysis software Image J (Ver. 1.54f), and the hard phase area fraction is measured every 100 μm within an 8 mm range in the direction perpendicular to the rolling direction and thickness direction of the steel plate (plate width direction) from the W/4 position to the 3W/4 position of the plate width W, and the standard deviation is calculated. Note that the observation range in the direction perpendicular to the rolling direction and thickness direction (plate width direction) may be less than 8 mm or may exceed 8 mm. However, the lower limit of the observation range for the standard deviation of the hard phase fraction in the direction perpendicular to the rolling direction and thickness direction (plate width direction) is 4 mm, and the upper limit is 12 mm.

[Relationship between tensile strength of steel sheet and hard phase fraction: (TS-180000/TS)/Vm≧35]
In this embodiment, it is preferable that the steel sheet satisfies the following formula (2).
(TS-180000/TS)/Vm≧35...(2)
Here, TS represents the tensile strength (MPa) of the steel plate, and Vm represents the area ratio (area %) of the hard phase in the region having a depth ranging from 3/8 to 5/8 of the plate thickness.

When a steel sheet satisfies formula (2), even if a certain amount of central segregation of Mn remains, it is easy to suppress the variation in the hard phase fraction in the direction perpendicular to the rolling direction and the thickness direction, which ultimately makes it less likely that poor appearance will occur after the steel sheet is formed.

The tensile strength (TS) of a steel plate can be measured by taking a No. 5 tensile test piece specified in JIS Z2241:2011, with its longitudinal direction perpendicular to the rolling direction and plate thickness direction, from the steel plate and conducting a tensile test in accordance with JIS Z2241:2011. If it is not possible to take a No. 5 tensile test piece from the steel plate to be measured (for example, if the test sample is small), a tensile test piece of any size, with its longitudinal direction perpendicular to the rolling direction and plate thickness direction, can be used instead of the No. 5 tensile test piece.

[Average grain size of ferrite in a region having a depth from 3/8 depth position to 5/8 depth position of the plate thickness: 5.0 to 30.0 μm]
In the steel sheet of this embodiment, the average grain size of ferrite in the metal structure in the region having a depth from the 3/8 depth position to the 5/8 depth position of the steel sheet thickness (i.e., the center portion of the sheet thickness) is preferably 5.0 to 30.0 μm. Controlling the average grain size of ferrite within such a fine range can further improve the appearance of the steel sheet after forming. The average grain size of ferrite may be 6.0 μm or more or 7.0 μm or more. Similarly, the average grain size of ferrite may be 27.0 μm or less, 21.0 μm or less, 15.0 μm or less, or 11.0 μm or less.

The average grain size of ferrite in the center of the sheet thickness is determined as follows.
First, the KAM value of the ferrite is calculated by KAM (Kernel Average Misorientation) analysis and GAIQ (Grain Average Image Quality) analysis in an electron probe microanalyzer (EPMA) measurement, which is a crystal analysis method using an SEM.

The SEM observation device used is a field-emission scanning electron microscope (e.g., JEOL's "JSM-7001F"), and EBSD analysis can be performed using, for example, TSL's "OIM Analysis 7." In EBSD analysis, a 50 μm x 50 μm area between the 3/8th and 5/8th depth positions of a steel plate whose cross-section has been mirror-polished is analyzed at 0.05 μm intervals (pitch). KAM analysis calculates the KAM value of a "certain pixel" by averaging the misorientation (°) between the measurement point and all neighboring pixels. This allows for the creation of a KAM map based on local crystal misorientation. This KAM analysis analyzes the KAM value in ferrite. The presence of ferrite in the EBSD measurement results is determined using the GAIQ analysis described below.

Grain Average Image Quality (GAIQ) analysis is performed on EBSD measurement results obtained under the same measurement conditions as the KAM value to obtain the GAIQ value. Based on this GAIQ value, the region is divided into regions with a GAIQ value of 50,000 or more and regions with a GAIQ value of less than 50,000. Regions (grains) with a GAIQ value of 50,000 or more are determined to be regions containing ferrite. On the other hand, regions (grains) with a GAIQ value of less than 50,000 are determined to be regions containing hard phases. GAIQ analysis is an analysis in which the average value within a single grain of the IQ value, which represents the clarity of the Kikuchi pattern at a "certain pixel" (the measurement point), is used as the GAIQ value of that grain.

In the EBSD measurement results, the circle-equivalent diameters of all crystal grains (ferrite grains) located in the region corresponding to the ferrite determined by the GAIQ analysis are calculated. Here, the crystal grains are defined as the regions surrounded by grain boundaries, which are the boundaries of regions whose crystal orientations differ by 15° or more. The value obtained by arithmetically averaging these diameters is then defined as the average crystal grain size of the ferrite.
The above-described measurement of the average grain size of ferrite is carried out at five locations, and the arithmetic mean value thereof is determined as the average grain size of ferrite.

[Average crystal grain size of the hard phase in the region having a depth from 3/8 depth position to 5/8 depth position of the plate thickness: 1.0 to 5.0 μm]
In the steel sheet of this embodiment, the average crystal grain size of the hard phase in the metal structure in the region having a depth from the 3/8 depth position to the 5/8 depth position of the steel sheet thickness (i.e., the center portion of the sheet thickness) is preferably 1.0 to 5.0 μm. Controlling the average crystal grain size of the hard phase within such a fine range can further improve the appearance of the steel sheet after forming. The average crystal grain size of the hard phase 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 crystal grain size of the hard phase may be 4.7 μm or less, 4.5 μm or less, 4.2 μm or less, 4.0 μm or less, 3.5 μm or less, or 3.0 μm or less.

The average crystal grain size of the hard phase in the center portion of the sheet thickness is determined as follows.
First, a sample having a steel plate cross section perpendicular to the plate surface is collected, and the steel plate cross section is etched with a Nital reagent to obtain an observation surface. A 100 μm x 100 μm region within the 3/8 to 5/8 plate thickness position of this observation surface is used as the observation region, and the hard phase is identified using an FE-SEM (e.g., a JSM-7200F manufactured by JEOL, measured at an acceleration voltage of 15 kV). Specifically, the metal structure is binarized based on differences in brightness using image analysis software Image J (Ver. 1.54f), and the hard phase is identified. Specifically, when a Nital solution is used, the black portions of the image data represent ferrite, and the uncorroded white portions represent the hard phase. Next, the circle-equivalent diameters of all identified hard phases are calculated. This operation is performed for three observation regions, and the circle-equivalent diameters of all obtained hard phases are arithmetically averaged. The obtained value is determined as the average crystal grain size of the hard phase.

(plate thickness)
In this embodiment, the thickness of the steel plate is not particularly limited and can be determined appropriately depending on the type of final product, etc. For example, the steel plate may have a thickness of 0.1 to 2.0 mm. Steel plates having such thicknesses are suitable for use as materials for covering components such as doors and hoods. The thickness of the steel plate may be 0.2 mm or more, 0.3 mm or more, or 0.4 mm or more. Similarly, the thickness of the steel plate 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 making the thickness of the steel plate 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 making the thickness 1.0 mm or less, the weight reduction effect of the component is significant. The thickness of the steel plate is measured using a micrometer.

(Plating layer)
The steel sheet of this embodiment may further have a plating layer on its surface for the purpose of improving corrosion resistance, etc. When the steel sheet has a plating layer, the plating layer may be provided on one or both sides of the steel sheet. The plating layer may be either a hot-dip plating layer or an electroplated layer. Examples of hot-dip plating layers 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 electroplated layers include an electrogalvanized layer (EG) and an electrolytic Zn—Ni alloy plating layer. Among these, the plating layer is preferably a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer. The coating weight of the plating layer is not particularly limited, and a general coating weight may be used.

(Mechanical properties)
According to the steel sheet of this embodiment having the above-mentioned specific chemical composition and metallographic structure, high tensile strength, specifically a tensile strength of 500 MPa or more, can be achieved. The tensile strength of the steel sheet is preferably 540 MPa or more, more preferably 600 MPa or more. The upper limit of the tensile strength is not particularly limited, but may be, for example, 980 MPa or less or 850 MPa or less. By setting the tensile strength to 850 MPa or less, it is easier to ensure workability when press-forming the steel sheet.

Despite having high strength, specifically a tensile strength of 500 MPa or more, the steel sheet of this embodiment has excellent workability in press forming and the like, and can maintain an excellent appearance even after forming. In a preferred embodiment, an exterior panel member, particularly an automobile exterior panel member, is provided that includes a steel sheet according to an embodiment of the present invention. Examples of automobile exterior panel members include roofs, hoods, fenders, and doors, which require high design quality. These exterior panel members, particularly automobile exterior panel members, only need to include a steel sheet according to an embodiment 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 plate 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 plate according to an embodiment of the present invention, but is not intended to limit the steel plate to one manufactured by the manufacturing method described below.

The method for manufacturing steel sheet of this embodiment includes at least a casting process for casting a slab having the above-mentioned specific chemical composition, a hot rolling process for hot rolling the cast slab, a pickling process for removing the oxide film (scale) formed during hot rolling, a cold rolling process for cold rolling the hot-rolled steel sheet after pickling, and an annealing process for holding the cold-rolled steel sheet in a predetermined atmosphere and at a predetermined temperature range.

Furthermore, in addition to the above-mentioned processes, the steel sheet manufacturing method of this embodiment may optionally include other processes such as a cooling process for cooling the cold-rolled steel sheet after annealing, a plating process for forming a plating layer on the surface of the cold-rolled steel sheet after cooling, and a skin-pass rolling process for skin-pass rolling the steel sheet after the plating process.

The preferred conditions for these processes are explained below.

(Casting process)
In the method for producing a steel sheet according to this embodiment, the casting step is a step of casting a slab having the specific chemical composition. The casting step includes soft reduction using a continuous casting machine having a plurality of reduction rolls adjacent to each other in the slab transport direction, the roll pitch between the adjacent reduction rolls being 290 mm or less. In this specification, soft reduction refers to a reduction gradient of 0.6 mm or more per meter in the casting direction.

As described above, the steel sheet of this embodiment must have a lower hard phase fraction than conventional DP steel and a unique metallographic structure with minimal variation in the hard phase fraction in the rolling direction and the direction perpendicular to the thickness direction at the center of the plate. To achieve this unique metallographic structure, it is important to control the solidification structure during casting to form columnar crystals. Specifically, during the casting process, the superheat ΔT (the difference between the molten steel temperature and the solidification temperature of the molten steel) of molten steel having the specific chemical composition described above is set to 25°C or higher, and the segment pressing force is set to 450 tons or higher. This method, which differs from conventional methods for preventing center segregation by controlling the solidification structure to a columnar crystal structure with an equiaxed crystal fraction of 15% or less, can also suppress center segregation. The superheat ΔT is preferably 30°C or higher. Furthermore, the superheat ΔT is preferably 40°C or lower. The molten steel temperature is the temperature of the molten steel in the tundish and can be determined by actual measurement. The solidification temperature can be determined from the chemical composition of the molten steel using a known solidification temperature estimation formula.

Conventional approaches to improving centerline segregation involve minimizing the superheat ΔT (at least below 25°C) and increasing the equiaxed crystal fraction (at least to above 15%), but these conventional approaches do not provide sufficient improvement. In this embodiment, unique casting conditions are adopted that are completely different from conventional approaches, namely, a superheat ΔT of 25°C or higher and a segment pressing force of 450 tons or higher, thereby controlling the solidification structure to a columnar crystal structure and suppressing negative Mn segregation. As a result, Mn microsegregation is reduced, and post-molding appearance such as ghost lines can be significantly improved.

The equiaxed crystal ratio (%) can be calculated by taking an etched print of the thickness cross section of the slab in the width direction (direction perpendicular to the conveying direction and thickness direction), visually determining the boundary between the columnar crystal structure and the equiaxed crystal structure, measuring the thickness (mm) of the equiaxed crystal structure at the center of the slab's thickness and the thickness (mm) of the slab, and dividing the thickness of the equiaxed crystal structure by the thickness of the slab and multiplying by 100.

In addition, during the casting process, soft reduction is performed using a continuous casting machine in which the roll pitch of adjacent reduction rolls is 290 mm or less, which suppresses the flow of molten steel during solidification and reduces the concentration of Mn in the center. This makes it possible to suppress central segregation of Mn. It is preferable that the roll pitch of adjacent reduction rolls is 280 mm or less.

(Hot rolling process)
In the steel sheet manufacturing method of this embodiment, the hot rolling step is a step of hot rolling a cast slab. In this step, it is preferable to heat the slab, whose temperature has dropped to less than 1100°C after casting, to 1100°C or higher prior to hot rolling. By setting the heating temperature to 1100°C or higher, the rolling reaction force does not become excessively large in hot rolling, making it easier to obtain the desired product thickness. There is no particular upper limit to the heating temperature, but from an economical viewpoint, it is preferable that the heating temperature be less than 1300°C. In the hot rolling step, the heated slab is subjected to rough rolling and finish rolling.

It is preferable that the finish rolling end temperature be 950°C or lower. By setting the finish rolling end temperature to 950°C or lower, the average crystal grain size of the hot-rolled steel sheet and the final product can be reduced, ensuring sufficient yield strength and high surface quality after forming.

In order to control the hard phase fraction in the surface layer of the steel sheet within the range of 0.20 Vm to 0.80 Vm, the air-cooling time t (seconds) after finish rolling is set to a time that satisfies the following formula (3).
0.067t≦ln(1300/(FT+273))...(3)
Here, in the formula (3), t represents the air cooling time (seconds) after finish rolling, and FT represents the surface temperature (°C) of the steel sheet after finish rolling.

If the air cooling time after finish rolling satisfies the above formula (3), the internal oxidation layer will be easier to remove in the subsequent pickling process, thereby minimizing the internal oxidation layer.

The hot-rolled steel sheet obtained in this manner is coiled at a predetermined coiling temperature (CT). In order to control the hard phase fraction in the surface layer of the steel sheet within the range of 0.20 Vm to 0.80 Vm, the coiling temperature is set to 580°C or lower. Setting the coiling temperature to 580°C or lower makes it less likely for an internal oxide layer to form. The coiling temperature may also be, for example, 450°C or higher.

(pickling process)
In the steel sheet manufacturing method of this embodiment, the pickling process is a process for removing scale formed during hot rolling. In the pickling process, the hot-rolled steel sheet is continuously transported and immersed in a pickling tank containing an acidic cleaning solution, thereby removing scale formed on the surface of the hot-rolled steel sheet. Examples of the acidic cleaning solution that can be used include hydrochloric acid and sulfuric acid.

In the pickling step, the pickling time and temperature are set according to the chemical components (Si and Mn) of the steel sheet and the coiling temperature (CT). However, in order to control the hard phase fraction in the surface layer of the steel sheet within the range of 0.20 Vm to 0.80 Vm, the pickling time and temperature are set so as to satisfy the following formula (4):
0.4×(T-15)×( _tA -20)≧(5[Si]+0.3[Mn]-0.6)×(CT+273)...(4)
Here, in formula (4), T represents the pickling temperature (°C), _tA represents the pickling time (seconds), [Si] represents the Si content (% by mass), [Mn] represents the Mn content (% by mass), and CT represents the coiling temperature (°C) after hot rolling.

When the pickling time _tA and the pickling temperature T satisfy the above formula (4), the internal oxide layer is more easily removed, and as a result, the internal oxide layer can be reduced as much as possible.

The pickling time _tA can be changed arbitrarily by adjusting the conveying speed of the hot-rolled steel sheet.

(Cold rolling process)
In the method for producing a steel sheet according to this embodiment, the cold rolling step is a step of cold rolling the hot-rolled steel sheet after pickling. In the cold rolling step, the hot-rolled steel sheet is preferably cold-rolled so that the cumulative reduction is, for example, 50 to 90%. By controlling the cumulative reduction within this range, the desired sheet thickness can be ensured, and further, the uniformity of the material properties in the sheet width direction can be sufficiently ensured.

(Annealing process)
In the steel sheet manufacturing method of this embodiment, the annealing step is a step of holding the cold-rolled steel sheet in a predetermined atmosphere at a predetermined temperature range. In the annealing step, it is preferable to perform an annealing treatment in which the cold-rolled steel sheet is heated to a soaking temperature of 750 to 900°C and held there. By setting the soaking temperature to 750°C or higher, ferrite recrystallization and reverse transformation from ferrite to austenite can be sufficiently promoted, making it possible to obtain the desired metal structure in the final product. On the other hand, by setting the soaking temperature to 900°C or lower, crystal grains can be densified and sufficient strength can be obtained.

Furthermore, in this annealing step, in order to control the hard phase fraction in the surface layer of the steel sheet to within a range of 0.20 Vm to 0.80 Vm, conditions such as oxygen potential are controlled by humid annealing, i.e., high dew point annealing, so as to satisfy the following three conditions (i) to (iii):
(i) Heated zone oxygen potential: -0.9≦log(pH ₂ O/pH ₂ )≦-0.3
(ii) isotropic zone oxygen potential: -2.4≦log(pH ₂ O/pH ₂ )≦-0.8
(iii) Temperature of the heating zone outlet and soaking zone: Ac1+20≦T≦Ac3-30
Here, pH ₂ O is the water vapor partial pressure (Pa) of the heating zone atmosphere and the soaking zone atmosphere, pH ₂ is the hydrogen partial pressure (Pa) of the heating zone atmosphere and the soaking zone atmosphere, and T is the temperature (°C) of the outlet side of the heating zone and the soaking zone atmosphere. The Ac1 point (°C) and Ac3 point (°C) are calculated according to the chemical composition of the steel sheet by the following formula:
Ac1=751-27C+18Si-12Mn-23Cu-23Ni+24Cr+23Mo-40V-6Ti+230Nb-169Al
Ac3=911-436C+30Si-25Mn-5Cr+15Mo+136Ti-19Nb+101Al

During the annealing process, by controlling conditions such as oxygen potential so that the above three conditions are met during high dew point annealing, it is possible to more easily control the hard phase fraction in the surface layer and center of the steel sheet within the above specific range.

(cooling process)
In the steel sheet manufacturing method of this embodiment, the cooling step is a step of cooling the cold-rolled steel sheet after annealing. In the cooling step, the cold-rolled steel sheet is preferably cooled so that the average cooling rate from the soaking temperature is 5 to 50°C/s. By setting the average cooling rate to 5°C/s or more, excessive transformation to ferrite is suppressed and the amount of hard phases such as martensite produced is increased, thereby achieving the desired strength. Furthermore, by setting the average cooling rate to 50°C/s or less, the steel sheet can be cooled more uniformly in the sheet width direction (the direction perpendicular to the rolling direction and the sheet thickness direction).

(Plating process)
In the steel sheet manufacturing method of this embodiment, the plating step is a step of forming a plating layer on the surface of the steel sheet by performing a plating treatment on the surface of the cooled cold-rolled steel sheet. Examples of plating treatments include hot-dip plating, alloying hot-dip plating, and electroplating. For example, the plating treatment may involve performing a hot-dip galvanizing treatment on the surface of the steel sheet, 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 any appropriate conditions known to those skilled in the art can be adopted. For example, the alloying temperature may be 450 to 600°C.

In the steel sheet manufacturing method of this embodiment, the plating process is performed for the purpose of improving corrosion resistance, etc., and is not an essential process. Therefore, if the steel sheet does not include a plating layer, there is no need to perform such a plating process.

The present invention is not limited to the above-described embodiments or the following examples, and can be appropriately combined, substituted, modified, etc. within the scope that does not deviate from the purpose and intent of the present invention.

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

In the following examples, steel plates according to one embodiment of the present invention were manufactured under various conditions, and the tensile strength, workability, and post-forming appearance characteristics of the resulting steel plates were examined.

First, a slab having the chemical composition shown in Table 1 was cast using a continuous casting method in which a soft reduction was performed using a continuous casting machine equipped with multiple reduction rolls arranged with a roll pitch of 290 mm or less, with a reduction gradient of 0.6 mm or more per meter in the casting direction. The remainder, other than the components shown in Table 1, consisted of Fe and impurities. The various conditions used in this casting process are shown in Table 2. In Table 2, the casting condition "ΔT" is the condition that superheat ΔT ≧ 25°C, and the casting condition "pressing force" is the condition that segment pressing force ≧ 450 tons. In each example, cases where these conditions were met are marked "OK," and cases where they were not are marked "NG," and are shown in Table 2.

The obtained slab was then subjected to a hot rolling process. The various conditions for the hot rolling process are shown in Table 2.
In Table 2, the "air-cooling time" hot rolling condition for each steel plate is a condition that the air-cooling time after finish rolling satisfies the following formula (3), and the "coiling temperature" hot rolling condition is a condition that the coiling temperature (CT)≦580°C.
0.067t≦ln(1300/(FT+273))...(3)
In the formula (3), t represents the air cooling time (seconds) after finish rolling, and FT represents the surface temperature (°C) of the steel sheet after finish rolling.
In each example, if these conditions are met, they are marked as "OK," and if they are not, they are marked as "NG," and are shown in Table 2.
For steel sheet No. 1, a slab having the chemical composition shown in Table 1 was heated and held at 1230°C for 30 minutes or more, and then subjected to a finish rolling temperature of 900°C, an air cooling time after finish rolling of 1 second, and a coiling temperature of 550°C to produce a hot-rolled steel sheet having a thickness of 2.3 mm.

Furthermore, the hot-rolled steel sheet after the hot rolling process was subjected to a pickling process. Various conditions in the pickling process are shown in Table 2. In Table 2, the "time and temperature" of the pickling conditions are conditions in which the pickling time and pickling temperature satisfy the following formula (4).
0.4×(T-15)×( _tA -20)≧(5[Si]+0.3[Mn]-0.6)×(CT+273)...(4)
Here, in formula (4), T represents the pickling temperature (°C), _tA represents the pickling time (seconds), [Si] represents the Si content (% by mass), [Mn] represents the Mn content (% by mass), and CT represents the coiling temperature (°C) after hot rolling.
In each example, if this condition is met, it is marked as "OK", and if it is not met, it is marked as "NG", and these are shown in Table 2.
For steel sheet No. 1, the hot-rolled steel sheet prepared as described above was pickled with hydrochloric acid at a concentration of 10%, at a temperature of 70°C, and for 100 seconds.

The pickled hot-rolled steel sheets were then subjected to a cold-rolling process (cumulative reduction of 80%), an annealing process (soaking temperature of 800°C), and a cooling process (average cooling rate of 10°C/sec) to produce steel sheets. The surfaces of the obtained steel sheets were appropriately plated to form a hot-dip galvanized layer (GI), a galvannealed layer (GA), or an electrogalvanized layer (EG) consisting of the plating types shown in Table 2.
For steel sheet No. 1, the pickled steel sheet was cold-rolled to a thickness of 0.45 mm. The resulting cold-rolled steel sheet was then subjected to a galvannealing treatment in a continuous hot-dip galvanizing line (CGL). In the CGL, the steel sheet was heated under the following conditions: a heating zone temperature of 780°C, a dew point of -2°C, and a hydrogen concentration of 2.6% by volume; a soaking zone temperature of 780°C, a dew point of -40°C, and a hydrogen concentration of 2.7% by volume. The steel sheet was then subjected to a galvannealing treatment and then to an alloying treatment at 560°C.
In Table 2, steel sheets No. 8 and No. 33 are examples in which the surface of the steel sheet was not plated.

Various conditions in the annealing process are shown in Table 2. In Table 2, the annealing conditions "oxygen potential in the heating zone,""oxygen potential in the soaking zone," and "temperatures on the outlet side of the heating zone and in the soaking zone" are conditions that satisfy the following three conditions (i) to (iii).
(i) Heated zone oxygen potential: -0.9≦log(pH ₂ O/pH ₂ )≦-0.3
(ii) isotropic zone oxygen potential: -2.4≦log(pH ₂ O/pH ₂ )≦-0.8
(iii) Temperature of the heating zone outlet and soaking zone: Ac1+20≦T≦Ac3-30
In each example, if these conditions are met, it is marked as "OK", and if they are not met, it is marked as "NG", and these are shown in Table 2.

Samples were taken from the steel plate manufactured as described above, and their chemical compositions were analyzed. No change was confirmed from the chemical composition of the slab shown in Table 1. Note that underlines next to chemical compositions in Table 1 indicate that the composition is outside the scope of the present invention. Also, underlines next to various values in Table 2 indicate that the composition is outside the scope of the present invention, that the manufacturing conditions do not result in the steel plate of the present invention, or that the steel plate has undesirable properties. Note that the "ratio of hard phase fraction at a depth position between 3/8 and 5/8 of the plate thickness; Vs/Vm" in Table 2 refers to the ratio Vs/Vm of the hard phase fraction Vs in the region extending from the surface of the steel plate to a depth position between 3/8 and 5/8 of the plate thickness, to the hard phase fraction Vm in the region extending from a depth position between 3/8 and 5/8 of the plate thickness. Therefore, this ratio Vs/Vm being 0.20 to 0.80 is synonymous with "the area ratio of the hard phase in the region having a depth from the surface of the steel plate to a depth position of 1/8 of the plate thickness being 0.20Vm to 0.80Vm."

The metal structure and various properties of the surface and center of the plate thickness of the obtained steel plate were measured and evaluated using the methods described above or below.

(Tensile strength and yield ratio)
Tensile test pieces according to JIS Z2241:2011 No. 5, with the longitudinal direction perpendicular to the rolling direction and the thickness direction, were taken from the steel plates, and tensile tests were conducted in accordance with JIS Z2241:2011 to measure the tensile strength (MPa) and yield stress (0.2% proof stress) of the steel plates. Furthermore, the yield ratio YR (%), defined as (yield stress)/(tensile strength), was calculated from these measurement results. From these results, steel plates with a tensile strength of 500 MPa or more were evaluated as high-strength steel plates, and steel plates with a yield ratio of 0.70 or less were evaluated as steel plates with excellent workability.

(Workability)
A close contact bending test was performed using the push bending method of JIS Z2248:2022, a bending test method for metallic materials, to evaluate workability. A rectangular test piece 30 mm wide and 100 mm long was taken from the steel plate so that the axial direction of the bending test was parallel to the rolling direction of the steel plate. After the close contact bending test, the test piece was visually evaluated for the presence or absence of cracks at the ridge line of the bend apex. A test piece without cracks was deemed to have passed, and is shown as "OK" in Table 2. A test piece with cracks observed in the visual evaluation was deemed to have failed, and is shown as "NG" in Table 2.

(Appearance after molding)
The appearance of the steel sheet after forming was evaluated based on the degree of ghost lines that appeared on the surface of the door outer after press forming and the presence or absence of stretcher strain. The surface after press forming was grinded, and stripes that appeared on the surface at intervals of several millimeters and extended in the rolling direction were judged to be ghost lines. The appearance was scored from 1 to 5 based on the following criteria according to the degree of the stripe patterns. A random 100 mm x 100 mm area on the grinded surface of the steel sheet after press forming was visually inspected. A score of "1" was assigned if no stripes extending in the rolling direction were observed; a score of "2" was assigned if the maximum length of the stripes was 20 mm or less; a score of "3" was assigned if the maximum length of the stripes was more than 20 mm but not more than 50 mm; a score of "4" was assigned if the maximum length of the stripes was more than 50 mm but not more than 70 mm; and a score of "5" was assigned if the maximum length of the stripes was more than 70 mm. A score of "3" or less was deemed to be excellent in appearance after forming and was judged to be acceptable. On the other hand, when the evaluation was "4" or more, the molded product was judged to have a poor appearance and was therefore rejected.
Furthermore, when the surface after press molding was visually observed, if surface defects consisting of localized linear or dendritic uneven patterns called stretcher strain were confirmed, the product was rated as "SS" and was judged to have poor appearance after molding and to have failed.

Referring to Table 2, in the comparative example of steel plate No. 3, which was cast under the condition of ΔT = 15°C, the standard deviation of the hard phase fraction in the metal structure at the center of the plate thickness was 0.89%, and ghost lines were generated, resulting in poor appearance after forming.
In the comparative example of steel sheet No. 4, which was subjected to the annealing process under conditions in which the value of the heating zone oxygen potential was −1.5, the ratio Vs/Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure in the sheet thickness center portion was 0.83 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.83Vm), resulting in poor workability.
In the comparative example of steel sheet No. 5, which was subjected to the annealing process under conditions in which the value of the oxygen potential in the soaking zone was −0.6, the ratio Vs/Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure in the sheet thickness center portion was 0.00 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.00 Vm), and the yield ratio was high and stretcher strain occurred, resulting in poor appearance after forming.
In the comparative example of steel sheet No. 6, which was subjected to the annealing process under the condition that the soaking zone temperature was 860°C, the ratio Vs/Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure at the center of the sheet thickness was 0.17 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.17Vm), and the yield ratio was high and stretcher strain occurred, resulting in poor appearance after forming.
In the comparative example of steel plate No. 14, which was cast under the condition of a pressing force of 350 tons, the standard deviation of the hard phase fraction in the metal structure at the center of the plate thickness was 0.88%, and ghost lines were generated, resulting in poor appearance after forming.
In the comparative example of steel sheet No. 15, which was subjected to the hot rolling process under conditions where the finish rolling temperature was 930°C and the air cooling time was 1.7 seconds, i.e., conditions which did not satisfy the above formula (3), the ratio Vs/Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure at the center of the sheet thickness was 0.17 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.17Vm), and the yield ratio was high and stretcher strain occurred, resulting in poor appearance after forming.
In the comparative example of steel sheet No. 16, which was subjected to the hot rolling process under the condition of a coiling temperature of 670°C, the ratio Vs/Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure at the center portion of the sheet thickness was 0.14 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.14Vm), and the yield ratio was high and stretcher strain occurred, resulting in a poor appearance after forming.
In the comparative example of steel sheet No. 17, which was subjected to the pickling process under conditions where the coiling temperature was 570°C, the pickling temperature was 60°C, and the pickling time was 100 seconds, i.e., conditions which did not satisfy the above formula (4), the ratio Vs/Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure at the center portion of the sheet thickness was 0.14 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.14Vm), the yield ratio was high, and stretcher strain occurred, resulting in a poor appearance after forming.
In the comparative example of steel sheet No. 21, in which the hot rolling step was carried out under conditions of a coiling temperature of 690°C, the pickling step was carried out under conditions of a pickling temperature of 65°C and a pickling time of 80 seconds, i.e., conditions that do not satisfy the above formula (4), and the annealing step was carried out under conditions of an oxygen potential value in the soaking zone of -0.5, the ratio Vs/Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure in the sheet thickness center portion was 0.00 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.00Vm), the yield ratio was high, and stretcher strain occurred, resulting in a poor appearance after forming.
In the comparative example of steel sheet No. 22, in which the hot rolling step was carried out under conditions where the finish rolling temperature was 920°C and the air cooling time was 1.5 seconds, i.e., conditions that do not satisfy the above formula (3), and the pickling step was carried out under conditions where the coiling temperature was 570°C, the pickling temperature was 55°C, and the pickling time was 90 seconds, i.e., conditions that do not satisfy the above formula (4), the ratio Vs/Vm of the hard phase fraction Vs of the metallographic structure of the surface layer to the hard phase fraction Vm of the metallographic structure at the center of the sheet thickness was 0.13 (the hard phase fraction Vs of the metallographic structure of the surface layer was 0.13Vm), and the yield ratio was high and stretcher strain occurred, resulting in poor appearance after forming.
In the comparative example of steel plate No. 29 using comparative steel S having a C content of 0.118%, the standard deviation of the hard phase fraction in the metal structure at the center of the plate thickness was 0.87%, resulting in the occurrence of ghost lines and a poor appearance after forming.
In the comparative example of steel plate No. 30 using comparative steel T with a Mn content of 2.71%, the ferrite fraction and hard phase fraction of the metallographic structure at the center of the plate thickness were 73% and 27%, respectively, and the standard deviation of the hard phase fraction of the metallographic structure at the center of the plate thickness was 0.91%, resulting in poor formability and the occurrence of ghost lines, resulting in poor appearance after forming.
In the comparative example of steel sheet No. 31 using comparative steel U having an Si content of 1.620%, stretcher strain occurred, resulting in poor appearance after forming.
In the comparative example of steel plate No. 32 using comparative steel V having a C content of 0.023%, the ferrite fraction and hard phase fraction in the metallographic structure at the center of the plate thickness were 98% and 2%, respectively, resulting in a decrease in tensile strength and inferior strength.
In the comparative example of steel plate No. 33 using comparative steel W with a Mn content of 0.85%, the tensile strength decreased, resulting in poor strength and a high yield ratio.

On the other hand, the steel sheets Nos. 1, 2, 7-13, 18-20, and 23-28, which are examples of the present invention, maintained a high tensile strength of 500 MPa or more while exhibiting excellent workability in press forming and other processes. Even when strain was imparted by press forming, the occurrence of ghost lines and stretcher strain on the steel sheet surface was significantly suppressed.

Claims (6)

A steel plate,
The chemical composition of the steel plate is, in mass%,
C: 0.030-0.100%,
Mn: 1.00-2.50%,
Si: 0.005-1.500%,
Al: 0.005-0.700%,
P: 0.100% or less,
S: 0.0200% or less,
N: 0.0150% or less,
O: 0.0100% or less,
Cr: 0-0.80%,
Mo: 0 to 0.50%,
B: 0 to 0.0100%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.50%,
Ni: 0 to 1.00%,
Cu: 0 to 1.00%,
W: 0-1.00%,
Sn: 0-1.00%,
Sb: 0 to 0.200%,
As: 0 to 0.200%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%,
REM: 0 to 0.0100%, and the balance: Fe and impurities;
The area ratio of the hard phase in a region having a depth from the surface of the steel plate to a 1/8 depth position of the plate thickness is 0.20Vm to 0.80Vm, where Vm is the area ratio of the hard phase in a region having a depth from a 3/8 depth position to a 5/8 depth position of the plate thickness,
The metal structure in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness contains, in area%, ferrite: 75 to 97% and hard phase: 3 to 25%, and the standard deviation of the area ratio of the hard phase in the direction perpendicular to the rolling direction and the plate thickness direction in the region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness is 0.85% or less,
A steel sheet characterized in that an index A represented by the following formula (1) is 0.45% or more .
A=[Si]+10[P]+0.6[Al]+8[Ti]+9[Nb]...(1)
Here, [Si], [P], [Al], [Ti] and [Nb] are the contents of each element in mass %, and when the corresponding element is not contained, it is 0%. The chemical composition is, in mass %,
Cr: 0.01-0.80%,
Mo: 0.01-0.50%,
B: 0.0001 to 0.0100%,
Ti: 0.001 to 0.100%,
Nb: 0.001 to 0.100%,
V: 0.01-0.50%,
Ni: 0.01-1.00%,
Cu: 0.01 to 1.00%,
W: 0.01-1.00%,
Sn: 0.001 to 1.00%,
Sb: 0.001-0.200%,
As: 0.001-0.200%,
Ca: 0.0001-0.0100%,
Mg: 0.0001-0.0100%,
Zr: 0.0001 to 0.0100%, and REM: 0.0001 to 0.0100%
The steel sheet according to claim 1, characterized in that it contains one or more selected from the group consisting of: The steel sheet according to claim 1 or 2, wherein the following formula (2) is satisfied:
(TS-180000/TS)/Vm≧35...(2)
Here, TS represents the tensile strength of the steel plate in MPa, and Vm represents the area ratio of the hard phase in a region having a depth from the 3/8 depth position to the 5/8 depth position of the plate thickness. The average grain size of the ferrite in a region having a depth from a 3/8 depth position to a 5/8 depth position of the plate thickness is 5.0 to 30.0 μm,
The steel plate according to claim 1 or 2, characterized in that the average crystal grain size of the hard phase in a region having a depth from a 3/8 depth position to a 5/8 depth position of the plate thickness is 1.0 to 5.0 μm. The steel plate according to claim 1 or 2, characterized in that the hard phase in the region having a depth ranging from 3/8 to 5/8 of the plate thickness is composed of at least one of martensite, bainite, tempered martensite, and pearlite. An outer panel member comprising the steel plate according to claim 1 or 2.