JP7836014B2 - Steel plate and its manufacturing method - Google Patents

Description

This invention relates to steel plates and methods for manufacturing the same.

In recent years, there has been a growing demand for lighter automotive parts to address environmental issues, such as reducing _CO2 emissions and improving fuel efficiency. At the same time, there is an increasing social demand for improved collision safety. To achieve both weight reduction and improved collision safety, increasing the strength of steel materials is an effective means. However, increasing the strength of steel materials usually reduces their workability; therefore, there is a need for steel materials that can improve both strength and workability simultaneously.

In relation to improving strength and workability, for example, Patent Document 1 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, having a bainite phase with an area ratio of more than 95% throughout the entire thickness direction, and having an average grain size of the bainite phase in the region from the surface to the 1/4 position of the thickness direction, which is 5 μm or less in the thickness cross-section parallel to the rolling direction and 4 μm or less in the thickness cross-section perpendicular to the rolling direction, and further having a structure in which there are 7 or fewer crystal grains that are elongated in the rolling direction with an aspect ratio of 5 or more in the thickness direction, with a tensile strength TS of 780 MPa or more. Patent Document 1 also teaches that, according to the above configuration, a high-strength hot-rolled steel sheet with a tensile strength TS of 780 MPa or more and significantly improved punching workability can be easily and inexpensively manufactured.

Patent Document 2 describes a cold-rolled and annealed steel sheet having a predetermined chemical composition and a microstructure consisting of martensite and/or lower bainite in surface proportions, wherein the martensite includes fresh martensite and/or self-tempered martensite, and the total surface proportions are in the range of 60 to 95% for martensite and lower bainite, in the range of 4 to 35% for low carbide-containing bainite, in the range of 0 to 5% for ferrite, and less than 5% for island-type retained austenite. Patent Document 2 also teaches that with the above configuration, it is possible to achieve a yield strength in the range of 800 to 970 MPa before skin pass operation, a tensile strength in the range of 1180 to 1320 MPa, an elongation at the break point of at least 5%, and a hole expansion ratio of 30% or more (Ac%).

Patent Document 3 describes a hot-rolled steel sheet having a predetermined chemical composition, wherein the total area ratio of the martensite phase and the lower bainite structure at the 1/4 thickness position is 85% or more, the average grain size of crystal grains surrounded by boundaries with a crystal orientation difference of 15° or more is 20 μm or less, the area ratio of crystal grains with an aspect ratio of 0.30 or less is 50% or less, and the average value of the X-ray random intensity ratio of the {100}<011> to {211}<011> orientation group at the center position of the thickness is 6.0 or less, and the maximum value is 8.0 or less. Furthermore, Patent Document 3 teaches that, according to the above configuration, a high-strength hot-rolled steel sheet with high strength, as well as excellent hole expandability and low-temperature toughness, can be stably manufactured.

Patent Document 4 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, in which the steel structure is mainly composed of martensite and bainite, with a total area ratio of 80-100%, a total area ratio of martensite in bainite of 2-20%, and among the martensite in bainite, the area ratio of martensite where the crystal orientation difference between the martensite and the crystal orientation of at least one bainite adjacent to the martensite is less than 15° is 50% or more of the total martensite. Furthermore, Patent Document 4 teaches that according to the above configuration, a high-strength hot-rolled steel sheet with excellent ductility, end crack resistance, and hole expansion properties, suitable as a material for automobile parts, can be provided.

Patent Document 5 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, in which the steel structure is mainly composed of martensite and bainite, with a total area ratio of 80-100%, the total area ratio of martensite in bainite being 2-20%, and among the martensite in bainite, the area ratio of martensite where the crystal orientation difference between the martensite and the crystal orientation of at least one bainite adjacent to it is 15° or more exceeds 50% of the total martensite, and when the region surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more is considered a crystal grain, the average aspect ratio of the crystal grains in the region from the surface of the steel sheet to a depth of 5 μm is 2.0 or less. Furthermore, Patent Document 5 teaches that according to the above configuration, a high-strength hot-rolled steel sheet with excellent ductility and bending/recovery properties, suitable as a material for automobile parts, can be provided.

Japanese Patent Publication No. 2012-062562 Special Publication No. 2017-507241 Japanese Patent Publication No. 2017-057472 International Publication No. 2022/244706 International Publication No. 2022/244707

As described above, it is known that as strength increases, the workability of steel materials decreases, and properties such as hole-expandability, as described in Patent Documents 2 to 4, decline. When hole-expandability decreases, it may become impossible to process the material into the desired shape, for example, in automobile suspension parts. Therefore, in the development of high-strength steel sheets such as high-strength hot-rolled steel sheets, it is important to achieve high strength while ensuring a certain level of properties appropriate to the application, such as uniform elongation in addition to the hole-expandability mentioned above. For example, in automobile suspension parts, such as lower arms and trailing arms, which have complex shapes, the decrease in workability due to increased strength may cause necking in the molded parts, resulting in a decrease in their function.

Furthermore, for components requiring impact resistance, plastic deformation occurs when subjected to an impact exceeding the yield strength. Therefore, from the perspective of ensuring the crash safety of automobiles, it is necessary to improve not only the tensile strength but also the yield strength. For this reason, it is necessary to increase the yield ratio, which is the ratio of yield strength to tensile strength.

This invention has been made in view of the above circumstances, and its objective is to provide a steel sheet and a method for manufacturing the same that, through a novel structure, has high strength, high uniform elongation, hole-expanding properties and yield ratio, and can suppress the occurrence of necking during forming.

To achieve the above objective, the inventors focused on the microstructure of steel sheets, particularly hot-rolled steel sheets, and conducted research. As a result, the inventors discovered that by constructing the microstructure of a hot-rolled steel sheet having a predetermined chemical composition with a structure controlled within a predetermined range, while being mainly composed of martensite, it is possible to achieve high strength and improved uniform elongation. Furthermore, by including a predetermined amount of a specific granular bainite in the microstructure, it is possible to improve the yield ratio and hole-expanding properties while significantly suppressing the occurrence of necking during forming. Moreover, by utilizing precipitation strengthening through the addition of Ti, the yield ratio can be further increased, and the hardness difference between each phase in the microstructure can be reduced. The combination of this reduction in hardness difference and the improvement in hole-expanding properties due to the specific granular bainite can further significantly improve the hole-expanding properties, thus completing the present invention.

The present invention, which has achieved the above objectives, is as follows.
(1) The chemical composition is, in mass%,
C: 0.060-0.200%,
Si: 0.30-2.00%,
Mn: 1.20-2.70%,
P: 0.100% or less,
S: 0.0300% or less,
Sol. Al: 0.001–0.500%,
Nb: 0.001-1.000%,
O: 0.0100% or less,
N: 0.0070% or less,
Ti: 0.070-0.200%,
B: 0 to 0.0030%,
Cr: 0-0.90%,
Mo: 0 to 0.12%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
V: 0-0.300%,
Sn: 0 to 0.040%,
As: 0 to 0.100%,
Zr: 0 to 0.050%,
Ca: 0-0.0010%,
Mg: 0 to 0.0010%,
Bi: 0 to 0.010%,
Co: 0 to 0.010%,
W: 0-0.100%,
Zn: 0 to 0.010%,
REM: 0-0.0100%, and the remainder: Fe and impurities.
The metallic structure, in area percentage,
Martensite: 60.0–85.0%
The material contains 10.0-30.0% granular bainite and 20.0% or less ferrite, where the maximum orientation difference at 0.1 μm intervals within grains surrounded by grain boundaries with an orientation difference of 15° or more is 3.5° or less, and the intra-grain orientation difference is 10° or more.
A steel plate characterized in that the average spacing between granular bainite grains is 50.0 μm or less.
(2) The chemical composition is, in mass%,
B: 0.0001 to 0.0030%,
Cr: 0.001-0.90%,
Mo: 0.001-0.12%,
Cu: 0.001-0.40%,
Ni: 0.001 to 0.30%,
V: 0.001-0.300%,
Sn: 0.001 to 0.040%,
As: 0.001 to 0.100%,
Zr: 0.001 to 0.050%,
Ca: 0.0001 to 0.0010%,
Mg: 0.0001 to 0.0010%,
Bi: 0.001 to 0.010%,
Co: 0.001 to 0.010%,
W: 0.001-0.100%,
Zn: 0.001–0.010%, and REM: 0.0001–0.0100%
The steel plate according to (1) above, characterized in that it includes at least one of the following.
(3) The steel sheet according to (1) or (2) above, characterized in that the metallic structure further contains, by area percent, at least one of bainite, pearlite, and retained austenite: in total of 20.0% or less.
(4) The steel sheet according to any one of (1) to (3) above, characterized in that the average particle size of the granular bainite grains is 5.0 to 30.0 μm.
(5) A component characterized by including the steel plate described in any one of the above items (1) to (4).
(6) A heating step comprising heating a slab having the chemical composition described in (1) or (2) above and holding it at a temperature of 1180 to 1320°C for 6000 seconds or more.
A hot rolling process that includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and that satisfies the following conditions (a) to (c): (a) the rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is 960 to 1080°C, and the reduction ratio in each rolling pass is 30 to 40%.
A method for manufacturing a steel sheet, comprising a cooling step including: (b) cooling the rolled material to 910°C or below at an average cooling rate of 400°C/second or more within 0.20 seconds after the two rolling passes immediately preceding the two subsequent stages, and (c) having a reduction ratio of 20 to 30% in each rolling pass of the two subsequent stages.

According to the present invention, it is possible to provide a steel sheet, particularly a hot-rolled steel sheet, and a method for manufacturing the same, which has high strength, high uniform elongation, hole-expanding properties, and a yield ratio, and which can suppress the occurrence of necking during forming.

<Steel plate>
The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition in mass%.
C: 0.060-0.200%,
Si: 0.30-2.00%,
Mn: 1.20-2.70%,
P: 0.100% or less,
S: 0.0300% or less,
Sol. Al: 0.001–0.500%,
Nb: 0.001-1.000%,
O: 0.0100% or less,
N: 0.0070% or less,
Ti: 0.070-0.200%,
B: 0 to 0.0030%,
Cr: 0-0.90%,
Mo: 0 to 0.12%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
V: 0-0.300%,
Sn: 0 to 0.040%,
As: 0 to 0.100%,
Zr: 0 to 0.050%,
Ca: 0-0.0010%,
Mg: 0 to 0.0010%,
Bi: 0 to 0.010%,
Co: 0 to 0.010%,
W: 0-0.100%,
Zn: 0 to 0.010%,
REM: 0-0.0100%, and the remainder: Fe and impurities.
The metallic structure, in area percentage,
Martensite: 60.0–85.0%
It contains 10.0-30.0% granular bainite and 20.0% or less ferrite, where the maximum orientation difference at 0.1 μm intervals within a grain surrounded by grain boundaries with an orientation difference of 15° or more is 3.5° or less, and the intra-grain orientation difference is 10° or more.
It is characterized by an average spacing of granular bainite grains of 50.0 μm or less.

As mentioned earlier, it is known that properties such as hole-expandability decrease as the strength of steel increases. For example, in order to manufacture parts with complex shapes such as lower arms and trailing arms in the suspension of automobiles, a steel sheet is required that has high strength, such as a tensile strength of 1180 MPa or more that enables weight reduction, while also having excellent hole-expandability. From the viewpoint of increasing strength, it is preferable that the metal structure of the steel sheet be composed mainly of martensite. However, although martensitic steel has excellent strength, excessive inclusion of martensite reduces properties such as uniform elongation, so it generally has the problem of poor workability. In addition, in parts with complex shapes such as lower arms and trailing arms, the decrease in workability due to increased strength may cause necking in the formed parts, which may reduce their function. Therefore, there is a need for a high-strength steel sheet that can improve properties such as hole-expandability and uniform elongation, suppress the occurrence of necking even when forming parts with complex shapes, and have a high yield ratio from the viewpoint of automobile crash safety, etc.

Therefore, the inventors investigated not only the appropriate chemical composition of steel sheets, particularly hot-rolled steel sheets, but also the metallic structure of the hot-rolled steel sheets. First, the inventors found that by constructing the metallic structure of a hot-rolled steel sheet having a predetermined chemical composition with a structure mainly composed of hard martensite, more specifically, a structure containing 60.0 to 85.0% martensite by area percentage, it is possible to achieve high strength, such as a tensile strength of 1180 MPa or more, while significantly improving the uniform elongation of the resulting hot-rolled steel sheet.

Next, the inventors found that by including a predetermined amount of specific granular bainite in the metal structure, more specifically, by including 10.0 to 30.0% by area of granular bainite in grains surrounded by grain boundaries with an orientation difference of 15° or more, where the maximum orientation difference at 0.1 μm intervals is 3.5° or less, the intra-grain orientation difference is 10° or more, and the average distance between adjacent grains is 50.0 μm or less, the yield ratio and hole-expanding properties can be improved while significantly suppressing the occurrence of necking during molding. Although not intended to be bound by any particular theory, it is thought that the characteristic orientation changes of granular bainite contribute in particular to suppressing the occurrence of necking. More specifically, the characteristic that "within grains surrounded by grain boundaries with an orientation difference of 15° or more, the maximum orientation difference at 0.1 μm intervals is 3.5° or less, and the intra-grain orientation difference is 10° or more" means that although the orientation change within the crystal grain of granular bainite is relatively gradual and continuous, the overall orientation difference within the crystal grain is relatively large. For example, bainite has many different interfaces within its crystal grains, which results in discontinuous and abrupt orientation changes. On the other hand, ferrite has relatively small orientation changes within its crystal grains, and therefore is continuous, but the overall orientation difference within the crystal grain is also relatively small. Therefore, granular bainite can be considered to have characteristics between bainite and ferrite in terms of orientation change. While bainite structures are prone to necking due to discontinuous orientation changes, granular bainite, despite exhibiting relatively large orientation differences throughout the grain, shows continuous orientation changes as described above, unlike bainite and martensite, which similarly has many interfaces within its grains. Therefore, it is believed that this characteristic orientation change in granular bainite significantly suppresses the occurrence of necking during forming. In addition, the inventors have found that including granular bainite at an area percentage of 10.0% or more can also improve hole-expanding properties. Although not intended to be bound by any particular theory, it is believed that the presence of a predetermined amount of granular bainite, which has properties intermediate between martensite or bainite and ferrite, in the steel suppresses the generation of voids from different phase interfaces during hole-expanding, thereby improving hole-expanding properties.

However, as mentioned above, granular bainite also possesses characteristics similar to ferrite. Therefore, if the amount of granular bainite becomes too high in a martensite-dominant microstructure, it is thought that the microstructure will resemble that of so-called DP steel (composite structure steel), which is composed of martensite and ferrite, thus leading to a decrease in the yield ratio. Even when the amount of granular bainite is appropriate, if the amount of ferrite becomes excessively high, or if the amount of martensite decreases, resulting in a relatively high total amount of granular bainite and ferrite, the microstructure will similarly resemble that of DP steel, leading to a decrease in the yield ratio. Therefore, from the perspective of sufficiently suppressing necking while maintaining a high yield ratio, it is necessary to include an appropriate amount of granular bainite in the microstructure, while maintaining a martensite area ratio of 60.0% or higher to control the total amount of granular bainite and ferrite within an appropriate range. In addition, the inventors conducted further investigations and found that, although the reason is not entirely clear, the hole-expanding properties of the steel sheet can be improved by arranging granular bainite grains at appropriate intervals, more specifically by controlling the average spacing of granular bainite grains to 50.0 μm or less. Furthermore, they found that controlling the average spacing of granular bainite grains in this way is also important in suppressing the occurrence of necking during forming. Based on these findings, according to the steel sheet embodiment of the present invention, by including 10.0 to 30.0% in area percent of granular bainite in the metal structure, in which the maximum orientation difference at 0.1 μm intervals is 3.5° or less within grains surrounded by grain boundaries with an orientation difference of 15° or more, and the intra-grain orientation difference is 10° or more, and by controlling the average spacing of granular bainite grains to 50.0 μm or less, it is possible to significantly suppress the occurrence of necking during forming while improving the yield ratio and hole-expanding properties.

In addition, the inventors have found that the yield ratio can be further increased by utilizing precipitation strengthening through the addition of Ti, and that this hole-expanding property can be more significantly improved by combining it with the improvement in hole-expanding property caused by the specific granular bainite mentioned above. Although we do not intend to be bound by any particular theory, it is thought that this improvement in hole-expanding property due to precipitation strengthening is due to a reduction in the hardness difference between each phase in the metal structure. To explain in more detail, in the steel sheet according to the embodiment of the present invention, although the metal structure is composed mainly of martensite as described above, it also includes other structures that are softer than martensite, for example, it may contain up to 20.0% of ferrite, which is a soft structure, by area percent. In this case, the hardness difference between each phase in the metal structure becomes high, and the hole-expanding property decreases. However, in the steel sheet according to the embodiment of the present invention, by controlling the Ti content in the steel to 0.070 mass% or more, the Ti precipitates strengthen the soft structure such as ferrite, thereby reducing the hardness difference between each phase in the metallic structure. It is believed that the combination of this reduction in hardness difference and the improvement in hole-expanding properties caused by the specific granular bainite described above can make the hole-expanding properties even more significantly improved.

Generally, automotive steel sheets are often processed into the desired part shape by press forming. Typically, press forming is carried out in multiple steps, resulting in a relatively large number of areas where, for example, strain accumulates within the steel sheet after primary deformation, and then undergoes further deformation. However, since steel sheets work harden and become stronger when strain is introduced, the processability in subsequent processes generally decreases, and necking can occur in the formed area. In this study, the inventors found that improving the bendability after pre-strain is effective in suppressing such necking in the formed area during forming. More specifically, they found that necking can be reproduced by applying a 10% pre-strain to a steel sheet test piece in a certain direction using uniaxial tension, and then performing a 90° bending test in a direction intersecting that direction. In particular, the steel sheet exhibits poor ductility in the C direction (direction perpendicular to the rolling direction). In connection with this, a tensile test was performed in the C direction, followed by a bending test in the L direction (rolling direction). The results showed that if necking did not occur in the bending test specimen, it was possible to improve necking in the actual forming of the parts. According to the steel sheet according to the embodiment of the present invention, by including the above-mentioned specific granular bainite in the metal structure at an area percentage of 10.0 to 30.0%, the occurrence of necking can be reliably suppressed even in such bending tests after pre-straining. The fact that such bending tests after pre-straining can reproduce the occurrence of necking in the actual forming of parts, and furthermore, that including granular bainite exhibiting the above-mentioned characteristic orientation change at an area percentage of 10.0% or more can significantly suppress the occurrence of necking in bending tests after pre-straining, was previously unknown and has now been revealed for the first time by the present inventors. Therefore, according to the embodiments of the present invention, for example, despite having high strength with a tensile strength of 1180 MPa or more, it has high uniform elongation, hole-expanding properties and yield ratio, and can reliably suppress the occurrence of necking even in the forming of actual parts. Thus, the steel sheet according to the embodiments of the present invention is particularly useful for use in the automotive field.

The steel sheet according to the embodiment of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. Also, in this specification, "~" which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower limit and upper limit, respectively, unless otherwise specified.

[C:0.060-0.200%]
Carbon (C) is an effective element for increasing the strength of steel plates. Furthermore, C forms carbides and/or carbonitrides with Nb in the steel, contributing to microstructural refinement through the pinning effect of the formed precipitates. To fully obtain these effects, the C content should be 0.060% or higher. The C content may also be 0.070% or higher, 0.080% or higher, 0.100% or higher, or 0.120% or higher. On the other hand, excessive C content may reduce hole-expanding properties. Therefore, the C content should be 0.200% or lower. The C content may also be 0.180% or lower, 0.160% or lower, 0.150% or lower, or 0.140% or lower.

[Si:0.30-2.00%]
Si is an element that suppresses the formation of iron carbides and contributes to improved strength and formability. To fully obtain these effects, the Si content should be 0.30% or more. The Si content may be 0.40% or more, 0.50% or more, 0.60% or more, 0.70% or more, 0.85% or more, 1.00% or more, or 1.20% or more. On the other hand, if the Si content is excessive, the ferrite fraction will increase, which may reduce hole-expanding properties. Also, a high ferrite fraction increases the total amount of granular bainite and ferrite, resulting in a metal structure similar to DP steel, which may reduce the yield ratio. Therefore, the Si content should be 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, 1.50% or less, or 1.40% or less.

[Mn: 1.20-2.70%]
Mn is an effective element for increasing strength as a hardenability and solid solution strengthening element. To fully obtain these effects, the Mn content should be 1.20% or more. The Mn content may be 1.30% or more, 1.50% or more, 1.60% or more, 1.80% or more, or 2.00% or more. On the other hand, if the Mn content is excessive, the granular bainite fraction decreases, the hole-expanding properties decrease, and the occurrence of necking during molding may not be sufficiently suppressed. Therefore, the Mn content should be 2.70% or less. The Mn content may be 2.60% or less, 2.50% or less, 2.40% or less, 2.30% or less, or 2.20% or less.

[P: 0.100% or less]
If phosphorus (P) is present in excessive amounts, processability may decrease due to grain boundary segregation, etc. Therefore, the P content should be 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[S: 0.0300% or less]
Excessive sulfur content can lead to the formation of many sulfides such as MnS, which can reduce processability. Therefore, the sulfur content should be 0.0300% or less. The sulfur content may also be 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the sulfur content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the sulfur content may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[sol. Al: 0.001–0.500%]
Sol. Al is an element that acts as a deoxidizing agent for molten steel. Sol. Al is also an effective element for increasing the granular bainite fraction. To obtain these effects, the sol. Al content should be 0.001% or higher. The sol. Al content may be 0.010% or higher, 0.020% or higher, 0.030% or higher, 0.050% or higher, or 0.100% or higher. On the other hand, excessive sol. Al content can increase the ferrite fraction, potentially reducing hole-expanding properties. Furthermore, a higher ferrite fraction increases the total amount of granular bainite and ferrite, resulting in a metal structure similar to DP steel, which can decrease the yield ratio. Therefore, the sol. Al content should be 0.500% or lower. The sol. Al content may be 0.400% or lower, 0.300% or lower, or 0.200% or lower. Al refers to acid-soluble Al, specifically solid-solution Al present in steel.

[Nb: 0.001 to 1.000%]
Nb is an element that contributes to the refinement of prior austenite grains and, consequently, to the increased strength of steel sheets by forming carbides, nitrides, and/or carbonitrides in steel through a pinning effect. Nb is also an effective element for increasing the fraction of granular bainite and controlling its morphology. To fully obtain these effects, the Nb content should be 0.001% or higher. The Nb content may be 0.005% or higher, 0.010% or higher, 0.050% or higher, 0.100% or higher, 0.200% or higher, or 0.300% or higher. On the other hand, excessive Nb content can lead to the formation of coarse carbides in the steel, reducing the workability of the steel sheet. Therefore, the Nb content should be 1.000% or lower. The Nb content may be 0.800% or lower, 0.600% or lower, 0.500% or lower, or 0.400% or lower.

[O: 0.0100% or less]
O is an element that is introduced during the manufacturing process. Excessive oxygen content can lead to the formation of coarse inclusions, which can reduce the workability of the steel sheet. Therefore, the oxygen content should be 0.0100% or less. The oxygen content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the oxygen content is not particularly limited and may be 0%, but reducing it to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the oxygen content may be 0.0001% or more, or 0.0005% or more.

[N: 0.0070% or less]
If nitrogen is present in excessive amounts, it can form coarse nitrides, which may cause slab cracking during hot rolling. Therefore, the nitrogen content should be 0.0070% or less. The nitrogen content may also be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the nitrogen content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the nitrogen content may be 0.0001% or more, or 0.0005% or more.

[Ti: 0.070-0.200%]
Ti precipitates in steel as Ti carbides such as TiC, and through precipitation strengthening, it strengthens soft structures such as ferrite, contributing to improvements in strength and yield ratio. Furthermore, because Ti reduces the hardness difference between phases in the metal structure due to precipitation strengthening, it is also effective in improving hole expansion properties. To fully obtain these effects, the Ti content should be 0.070% or higher. The Ti content may be 0.080% or higher, 0.090% or higher, 0.100% or higher, or 0.120% or higher. On the other hand, if Ti is included in excess, coarse carbides may be formed in the steel, which may cause slab cracking during hot rolling or reduce the workability of the steel sheet. Therefore, the Ti content should be 0.200% or lower. The Ti content may be 0.180% or lower, 0.170% or lower, 0.160% or lower, or 0.150% or lower.

The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may optionally contain at least one of the following elements in place of a portion of the remaining Fe.

[B: 0 to 0.0030%]
B is an element that enhances the hardenability of steel and contributes to improving its strength. The B content may be 0%, but to obtain such effects, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the B content be 0.0030% or less. The B content may also be 0.0025% or less, 0.0020% or less, 0.0015% or less, or 0.0010% or less.

[Cr: 0-0.90%]
Cr is an element that enhances the hardenability of steel and contributes to improving its strength and/or corrosion resistance. The Cr content may be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Cr content is excessive, the effect will saturate, and there is a risk of increased manufacturing costs. Therefore, the Cr content is preferably 0.90% or less, and may be 0.70% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[Mo: 0 to 0.12%]
Mo is an element that enhances the hardenability of steel and contributes to improving its strength. The Mo content may be 0%, but to obtain such effects, it is preferable that the Mo content be 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, if the Mo content is excessive, the deformation resistance during hot working may increase, and the load on the equipment may increase. Therefore, it is preferable that the Mo content be 0.12% or less. The Mo content may be 0.10% or less, 0.08% or less, 0.06% or less, or 0.05% or less.

[Cu: 0-0.40%]
Cu is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Cu content may be 0%, but to obtain such an effect, it is preferable that the Cu content be 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if these elements are included in excess, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the Cu content be 0.40% or less. The Cu content may be 0.30% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[Ni: 0-0.30%]
Ni is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but to obtain such an effect, it is preferable that the Ni content be 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if these elements are included in excess, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the Ni content be 0.30% or less. The Ni content may be 0.20% or less, 0.15% or less, 0.10% or less, or 0.08% or less.

[V: 0-0.300%]
V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but to obtain such an effect, it is preferable that the V content be 0.001% or more. The V content may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if the V content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the V content be 0.300% or less. The V content may be 0.200% or less, 0.100% or less, or 0.080% or less.

[Sn: 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0 to 0.0010%, Mg: 0 to 0.0010%, Bi: 0-0.010%, Co: 0-0.010%, W: 0-0.100%, Zn: 0-0.010%, and REM: 0-0.0100%]
Sn, As, Zr, Ca, Mg, Bi, Co, W, Zn, and REM may be included in the steel sheet as optional elements, or may exist in the steel sheet as trump elements. The content of these elements may be as follows: Sn: 0 to 0.040% or 0.020%, As: 0 to 0.100% or 0.050%, Zr: 0 to 0.050% or 0.030%, Ca: 0 to 0.0010% or 0.0008%, Mg: 0 to 0.0010% or 0.0008%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100% or 0.050%, Zn: 0 to 0.010%, and REM: 0 to 0.0100% or 0.0050%. Regarding the lower limits for these elements, for example, the content of Sn, As, Zr, Bi, Co, W, and Zn may be 0.001% or more, 0.005% or more, or 0.008% or more, respectively. Similarly, the content of Ca, Mg, and REM may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

In the steel sheet according to the embodiment of the present invention, the remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities are, for example, components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when manufacturing steel sheets industrially .

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

[Metal structure]
[Martensite: 60.0–85.0%]
The metal structure of the steel sheet according to the embodiment of the present invention contains 60.0 to 85.0% martensite by area percentage. By constructing the metal structure of the steel sheet with a structure containing hard martensite within this range, it is possible to achieve high strength, for example, a tensile strength of 1180 MPa or more, while significantly improving the uniform elongation of the resulting steel sheet. From the viewpoint of further increasing strength, a higher area percentage of martensite is preferable, for example, it may be 62.0% or more, 65.0% or more, 68.0% or more, or 70.0% or more. From the viewpoint of further improving uniform elongation, a lower area percentage of martensite is preferable, for example, it may be 82.0% or less, 80.0% or less, 78.0% or less, or 75.0% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[Granular bainite in which the maximum orientation difference at 0.1 μm intervals within a grain surrounded by a grain boundary with an orientation difference of 15° or more is 3.5° or less, and the intra-grain orientation difference is 10° or more: 10.0–30.0%]
The metallic structure of the steel sheet according to the embodiment of the present invention contains 10.0 to 30.0% granular bainite, in area percent, in which the maximum orientation difference at 0.1 μm intervals within grains surrounded by grain boundaries with an orientation difference of 15° or more is 3.5° or less, and the intragranular orientation difference is 10° or more. Here, the structure referred to as granular bainite in the prior art does not necessarily have the characteristics of "a maximum orientation difference at 0.1 μm intervals within grains surrounded by grain boundaries with an orientation difference of 15° or more, and an intragranular orientation difference of 10° or more." The structure referred to as granular bainite in the prior art is often not sufficiently defined, and therefore, simply calling it granular bainite does not mean that it refers to the same thing as the granular bainite according to the embodiment of the present invention. In embodiments of the present invention, it is extremely important that the metal structure of the steel sheet contains 10.0% to 30.0% by area of a specific granular bainite having the above-mentioned characteristics, in other words, granular bainite characterized by "relatively smooth and continuous orientation changes within the crystal grains, yet a relatively large orientation difference across the entire crystal grain." This technical matter and the effects obtained thereby were discovered for the first time by the inventors. As mentioned above, by including 10.0% or more by area of granular bainite characterized by relatively smooth and continuous orientation changes within the crystal grains, yet a relatively large orientation difference across the entire crystal grain, it is possible to significantly suppress the occurrence of necking during forming due to these characteristic orientation changes. In addition, as mentioned above, it is believed that including 10.0% or more by area of the granular bainite suppresses the generation of voids from the interphase interface during hole expansion processing, and consequently, it is possible to improve hole expansion performance. From the viewpoint of further suppressing necking and/or further improving hole expansion properties, a higher area ratio of granular bainite is preferable, for example, it may be 12.0% or more, 15.0% or more, or 18.0% or more. On the other hand, as mentioned above, granular bainite has characteristics similar to ferrite, so if the area ratio of granular bainite becomes too high in a martensite-dominant microstructure, it will result in a microstructure similar to so-called DP steel, leading to a decrease in the yield ratio. Therefore, from the viewpoint of maintaining a higher yield ratio, a lower area ratio of granular bainite is preferable, for example, it may be 28.0% or less, 25.0% or less, or 22.0% or less.

In this study, the inventors found that when a 10% pre-strain was applied to a steel sheet in the C direction (perpendicular to the rolling direction) using uniaxial tension, followed by a 90° bending test in the L direction (rolling direction), if no necking occurred in the bending test specimen, it was possible to improve necking in the actual forming of the parts. According to the steel sheet according to the embodiment of the present invention, by including the above-mentioned specific granular bainite in the metal structure at an area percentage of 10.0 to 30.0%, the occurrence of necking can be reliably suppressed even in such bending tests after pre-straining. Therefore, according to the embodiment of the present invention, the occurrence of necking can be reliably suppressed even in the later stages of deformation in forming operations that are divided into multiple processes, such as actual press forming of automotive steel sheets. Thus, the steel sheet according to the embodiment of the present invention is particularly useful for use in the automotive field.

[Ferrite: 20.0% or less]
The metal structure of the steel sheet according to the embodiment of the present invention contains 20.0% or less of ferrite by area percentage. If the ferrite, which is a soft structure, can be limited to 20.0% or less by area percentage, the hardness difference between each phase in the metal structure can be sufficiently reduced by strengthening the soft structure containing the ferrite with Ti precipitates. Therefore, by combining this reduction in hardness difference with the improvement in hole-expanding properties resulting from the control of the average spacing of granular bainite, which will be explained later, it is possible to significantly improve the hole-expanding properties. If the area percentage of ferrite exceeds 20.0%, it may not be possible to sufficiently improve the hole-expanding properties even if precipitation strengthening by Ti precipitates and control of the average spacing of granular bainite are combined. In addition, if the area percentage of ferrite exceeds 20.0%, the total amount of granular bainite and ferrite increases, resulting in a metal structure similar to DP steel, which may lead to a decrease in the yield ratio. From the viewpoint of increasing pore-expanding properties and/or yield ratio, a lower ferrite area ratio is preferable, for example, 18.0% or less, 15.0% or less, 12.0% or less, 10.0% or less, 8.0% or less, 5.0% or less, or 3.0% or less. The lower limit of the ferrite area ratio is not particularly limited and may be 0%, for example, 0.5% or more or 1.0% or more.

[Remaining organization]
The remaining tissue other than martensite, the specific granular bainite and ferrite described above may be 0% by area percentage. However, if the remaining tissue is present, it may contain at least one of bainite, pearlite, and retained austenite in total, up to 20.0% by area percentage. If the total area percentage of at least one of bainite, pearlite, and retained austenite exceeds 20.0%, it may lead to a decrease in uniform elongation or make it impossible to control other tissues such as martensite and granular bainite within the desired range. Therefore, a smaller area percentage of the remaining tissue is preferable. For example, the total area percentage of at least one of bainite, pearlite, and retained austenite may be 15.0% or less, 10.0% or less, 8.0% or less, 5.0% or less, or 3.0% or less. On the other hand, the lower limit is not particularly limited, and the total area ratio of at least one of bainite, pearlite, and retained austenite may be 0%, or for example, 0.1% or more, 0.5% or more, or 1.0% or more.

[Average spacing between granular bainite grains: 50.0 μm or less]
In the metal structure of the steel sheet according to the embodiment of the present invention, the average spacing of granular bainite grains is controlled to 50.0 μm or less. Here, granular bainite grains refer to granular bainite grains (crystal grains) that are surrounded by grain boundaries with an orientation difference of 15° or more, have a maximum orientation difference of 3.5° or less at 0.1 μm intervals, and have an intra-grain orientation difference of 10° or more. By controlling the average spacing between granular bainite grains exhibiting the above-mentioned characteristic orientation change to 50.0 μm or less, in combination with precipitation strengthening by ferrite and Ti precipitates of 20 area % or less as described above, it is possible to significantly improve the hole-expanding properties of the steel sheet. In addition, the average spacing of granular bainite grains is also a factor that determines the arrangement of the granular bainite structure, and therefore, if there is a bias in the spacing of granular bainite grains, even if granular bainite exhibiting the above-mentioned characteristic orientation change is contained in an area % of 10.0% or more, it may not be possible to reliably suppress the occurrence of necking during forming. From the viewpoint of further improving hole expansion and more reliably suppressing necking, the smaller the average spacing between granular bainite grains, the better. For example, it may be 35.0 μm or less, 30.0 μm or less, 28.0 μm or less, 25.0 μm or less, or 23.0 μm or less. The lower limit is not particularly limited, but for example, the average spacing between granular bainite grains may be 5.0 μm or more, 7.0 μm or more, 10.0 μm or more, or 15.0 μm or more.

[The average particle size of granular bainite grains is 5.0–30.0 μm]
In the metal structure of the steel sheet according to the embodiment of the present invention, the average particle size of the granular bainite grains is preferably 5.0 to 30.0 μm. By controlling the average particle size of the granular bainite grains within the range of 5.0 to 30.0 μm, a fine and uniform granular bainite structure can be obtained, thereby further improving the bendability after pre-straining. For example, the average particle size of the granular bainite grains may be 6.0 μm or more, 8.0 μm or more, or 10.0 μm or more. Similarly, the average particle size of the granular bainite grains may be 25.0 μm or less, 22.0 μm or less, 20.0 μm or less, or 18.0 μm or less.

[Identification of martensite, bainite, pearlite, and retained austenite, and calculation of their area proportions]
The identification and calculation of the area percentages of martensite, bainite, pearlite, and retained austenite are performed by optical microscopy observation and X-ray diffraction after etching with Nital reagent or Repera solution. Microscopic microscopy is performed on the thickness cross section perpendicular to the plate surface. Preferably, the thickness cross section is parallel to the rolling direction. Specifically, first, a sample is taken from the steel plate and the observation surface of the sample is etched with Nital. Next, by performing image analysis on a microscopic image obtained at a depth of 1/4 of the plate thickness in a 300 μm × 300 μm field of view using an optical microscope, the total area percentages of martensite and bainite, and the area percentage of pearlite are calculated. Next, using a sample whose observation surface has been Repera-etched, the total area percentages of martensite and retained austenite are calculated by performing image analysis on a microscopic image obtained at a depth of 1/4 of the plate thickness in a 300 μm × 300 μm field of view using an optical microscope. Next, using a sample that has been surface-machined to a depth of 1/4 of the plate thickness from the direction normal to the rolling surface, the volume fraction of retained austenite is calculated by X-ray diffraction measurement. Since the volume fraction of retained austenite is equivalent to the area fraction, this is taken as the area fraction of retained austenite. The area fraction of martensite is calculated by subtracting the obtained area fraction of retained austenite from the total area fraction of martensite and retained austenite calculated earlier. Finally, the area fraction of bainite is calculated by similarly subtracting the obtained area fraction of martensite from the total area fraction of martensite and bainite calculated earlier.

[Identification of ferrite and calculation of area ratio]
The identification of ferrite and the calculation of its area fraction are performed by electron backscatter diffraction (EBSD) as follows. Specifically, first, a sample is taken from a steel plate so that the thickness cross section perpendicular to the plate surface becomes the observation surface. It is preferable that the thickness cross section is parallel to the rolling direction. Next, EBSD analysis is performed at measurement intervals of 0.2 μm on a rectangular region centered at 1/4 of the plate thickness from the steel plate surface, with a length of 200 μm in the thickness direction and 400 μm in the direction perpendicular to the thickness direction, to obtain crystal orientation information for this rectangular region. The EBSD analysis is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points/second. Next, using the crystal orientation information of this rectangular region, the grain average misorientation (GAM value) is calculated using the software "OIM Analysis®" included with the EBSD analyzer. Finally, regions with a GAM value of 0.5° or less are identified as ferrite, and their area fraction is calculated. Here, the "GAM value" is the average of the grain averages between adjacent pixels in a region enclosed by grain boundaries with a grain average of 15° or more.

[Identification of granular bainite and calculation of area ratio]
The identification and area fraction of granular bainite are performed by EBSD as follows. Specifically, first, a sample is taken from a steel plate so that the thickness cross section perpendicular to the plate surface becomes the observation surface. Preferably, the thickness cross section is parallel to the rolling direction. Next, EBSD analysis is performed at measurement intervals of 0.1 μm on a rectangular region centered at 1/4 of the plate thickness from the steel plate surface, with a length of 200 μm in the thickness direction and 400 μm in the direction perpendicular to the thickness direction, to obtain crystal orientation information for this rectangular region. The EBSD analysis is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points/second. Next, using the crystal orientation information of this rectangular region, the software "OIM Analysis®" included with the EBSD analyzer is used to define regions enclosed by grain boundaries with an orientation difference of 15° or more as crystal grains. The intra-grain orientation difference of these crystal grains is calculated, and crystal grains with a maximum orientation difference of 3.5° or less at 0.1 μm intervals, and an intra-grain orientation difference, more specifically, a maximum orientation difference within the grain of 10° or more, are identified as granular bainite, and their area fraction is calculated. The average of the area fractions obtained for three arbitrary intra-grain lines is determined as the area fraction of the granular bainite. The "maximum intra-grain orientation difference" for granular bainite is determined by "Grain Reference Orientation Deviation (GROD)". The maximum orientation difference within a grain is determined as the misorientation between other pixels within the same crystal grain, using the orientation of the pixel with the minimum KAM value (Karnel Average Misorientation) as the reference. In the embodiment of the present invention, the reference crystal orientation is the orientation with the minimum KAM value within the same crystal grain. The GROD and KAM values can be calculated using the software "OIM Analysis® Version 7.0.1" attached to the EBSD analyzer.

[Method for determining the average spacing and average particle size of granular bainite grains]
The average spacing of granular bainite grains is determined by measuring the distance between the centroid of a granular bainite grain identified by EBSD and the centroid of the nearest granular bainite grain. The average value of 100 or more distance measurements is then used as the average spacing of the granular bainite grains. Additionally, the average equivalent diameter of all granular bainite grains for which 100 or more distance measurements were taken is used as the average grain size of the granular bainite grains.

[Plate thickness]
The steel plate according to the embodiment of the present invention is not particularly limited, but generally has a thickness of 1.0 to 8.0 mm. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

The steel sheet according to the embodiment of the present invention can suppress necking even when forming parts with complex shapes, and therefore can reliably achieve a high level of both high strength and excellent workability, which are conflicting properties. Accordingly, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields where the coexistence of these properties is required, and is particularly useful for use in parts in the automotive field. For this reason, in a preferred embodiment, an automotive part, particularly an automotive undercarriage part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automotive undercarriage parts include lower arms and trailing arms. These automotive parts, particularly automotive undercarriage parts, only need to include the steel sheet according to the embodiment of the present invention in at least a part of these parts, and therefore at least a part of these parts satisfies the characteristics of the chemical composition and metal structure described above. In parts of the steel sheet that undergo relatively little processing during forming such as press forming, the characteristics of the steel sheet do not change particularly before and after forming. Parts of the steel sheet that undergo relatively little processing are determined by characteristics such as having a smooth shape without deformation such as bending, and having a small rate of increase or decrease in plate thickness. In parts with complex shapes, such as lower arms and trailing arms, multiple forming operations can cause necking or constriction, reducing the rigidity of specific areas. Therefore, these parts cannot be manufactured from a single sheet of steel, requiring specific parts to be separate components, which can increase part costs. However, according to the steel sheet according to the embodiment of the present invention, even in parts with complex shapes, such as lower arms and trailing arms, multiple forming operations can be performed from a single sheet of steel without causing necking, which is also economically advantageous.

[Mechanical properties]
[Tensile strength (TS) and uniform elongation (u-El)]
According to the steel sheet having the above chemical composition and metal structure, particularly the hot-rolled steel sheet, a high tensile strength, specifically a tensile strength of 1180 MPa or more, can be achieved. The tensile strength is preferably 1200 MPa or more, 1220 MPa or more, or 1240 MPa or more. According to the steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, the occurrence of necking during forming can be significantly suppressed while improving uniform elongation and hole expansion properties, through the specific combination of chemical composition and metal structure described above. There is no particular upper limit to the tensile strength, but for example, the tensile strength of the steel sheet may be 1780 MPa or less, 1470 MPa or less, or 1400 MPa or less. Furthermore, according to the steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, a high uniform elongation can be achieved, specifically a uniform elongation of 5.0% or more. The uniform elongation is preferably 5.2% or more, 5.5% or more, 5.8% or more, or 6.0% or more. There is no particular upper limit to the uniform elongation, but for example, the uniform elongation of a steel plate may be 15.0% or less, 10.0% or less, or 8.0% or less. Tensile strength and uniform elongation are measured by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2011. For example, if it is difficult to take a JIS No. 5 test specimen due to dimensional constraints, other test specimens described in JIS Z 2241:2011 may be used. However, if the plate thickness is less than 0.5 mm, 0.5 mm should be set as the lower limit for proper evaluation. For example, if it is difficult to take a JIS No. 5 test specimen due to dimensional constraints and it is also difficult to use other test specimens described in JIS Z 2241:2011, a micro-Vickers test in accordance with JIS Z 2244-1:2020 can be performed, and the hardness (HV) can be converted to a tensile strength value and used. A sample for the micro-Vickers test can be prepared as follows. First, a sample is cut from any position at least 50 mm away from the edge of the steel plate (if a sample cannot be taken from this position, a position avoiding the edge) so that the thickness cross section perpendicular to the plate surface can be observed. The thickness cross section is preferably parallel to the rolling direction. The size of the sample should be such that approximately 10 mm can be observed in the direction perpendicular to the thickness direction, although this depends on the measuring device. After polishing the cross section of the sample using silicon carbide paper from #600 to #1500, it is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the observation surface is finished by electropolishing. For the micro-Vickers test, 30 measurements are taken at a position 1/4 of the plate thickness with a load of 500 gf, and the average value is used. The conversion can be done using the following formula.
Tensile strength [MPa] = 3.12 × Vickers hardness [HV] + 16

[Hole expansion ratio (λ)]
According to the steel sheet having the above chemical composition and metal structure, high hole expansion properties, specifically a hole expansion ratio of 40% or more, can be achieved. The hole expansion ratio is preferably 42% or more, more preferably 45% or more, or 50% or more. There is no particular upper limit to the hole expansion ratio, but for example, the hole expansion ratio may be 150% or less, 100% or less, or 70% or less. The hole expansion ratio is determined as follows. First, a test piece measuring 100 mm in width and 100 mm in length is taken from the steel sheet, and a punched hole (initial hole: hole diameter d0 = 10 mm) is made using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole is pushed open with a conical punch with a vertex angle of 60° until a crack that penetrates the thickness of the sheet occurs, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) of each test piece is calculated using the following formula. This hole expansion test is performed three times, and the average value is determined as the hole expansion ratio λ.
λ=100×{(d1-d0)/d0}

[Yield ratio (YR)]
According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the yield ratio can also be increased, and more specifically, a yield ratio of 80% or more can be achieved. The yield ratio is preferably 82% or more, more preferably 85% or more. There is no particular upper limit, but for example, the yield ratio may be 95% or less or 92% or less. The yield ratio is determined by the following formula based on the tensile strength and 0.2% proof stress measured by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel sheet (direction C) and performing a tensile test in accordance with JIS Z 2241:2011.
Yield ratio YR = 0.2% proof stress / Tensile strength TS × 100

<Method of manufacturing steel plates>
Next, preferred manufacturing methods for steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, particularly steel sheets having preferred properties, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below. More specifically, the following describes the manufacturing of hot-rolled steel sheets, but the steel sheets according to embodiments of the present invention include any steel sheets having the chemical composition and metal structure described above, i.e., not only hot-rolled steel sheets, but also cold-rolled steel sheets, plated steel sheets, etc. Therefore, the following description merely describes an example of a preferred manufacturing method when the steel sheet according to embodiments of the present invention is a hot-rolled steel sheet.

A method for manufacturing steel sheets according to an embodiment of the present invention is:
A heating step comprising heating a slab having the chemical composition described above in relation to a steel plate and holding it at a temperature of 1180 to 1320°C for 6000 seconds or more,
A hot rolling process that includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and that satisfies the following conditions (a) to (c): (a) the rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is 960 to 1080°C, and the reduction ratio in each rolling pass is 30 to 40%.
(b) Cooling the rolled material to 910°C or below at an average cooling rate of 400°C/second or more within 0.20 seconds after the two rolling passes immediately preceding the two subsequent stages, and (c) The reduction ratio in each rolling pass of the two subsequent stages is 20-30%. The manufacturing method is characterized by including a cooling step in which the finish-rolled steel sheet is water-cooled, cooled to a temperature range of 500-650°C within 4.0 seconds from the start of water cooling, then air-cooled in the said temperature range for 2.0-6.0 seconds, and water-cooled to 50°C or below within 13 seconds after air-cooling. In the above manufacturing method, the temperatures described for the slab and steel sheet refer to the surface temperature of the slab and the surface temperature of the steel sheet, respectively. The following describes each step in detail.

[Heating process]
First, a slab having the chemical composition described above in relation to the steel plate is heated and held at a temperature range of 1180 to 1320°C for 6000 seconds or more. From the viewpoint of productivity, it is preferable to use a slab obtained by continuous casting, but slabs obtained by casting and ingot division may also be used, and if necessary, slabs that have been hot-worked or cold-worked may also be used. In this manufacturing method, holding at a temperature range of 1180 to 1320°C includes not only cases where the slab temperature is held at a constant temperature within the range of 1180 to 1320°C, but also cases where the slab temperature fluctuates within the range of 1180 to 1320°C. By holding the slab at a temperature range of 1180 to 1320°C for 6000 seconds or more, coarse carbides present in the structure can be completely dissolved, eliminating the initiation points of cracks. If the holding temperature is below 1180°C or the holding time is less than 6000 seconds, the solid solution of coarse carbides will be incomplete. If the solid solution of coarse carbides is incomplete, ferrite and bainite transformations originating from these carbides may occur during the cooling process described later, resulting in a martensite area ratio of less than 60.0%, and consequently, the desired strength may not be achieved. The upper limit of the slab heating temperature should be 1320°C or lower from the viewpoint of heating equipment capacity and productivity. The upper limit of the holding time in the temperature range of 1180 to 1320°C is preferably 10,000 seconds or less.

[Hot rolling process]
[Rough rolling]
In this manufacturing method, for example, a heated slab may be subjected to rough rolling before finish rolling to adjust the plate thickness. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions are ensured.

[Finishing Rolling]
[(a) Rolling temperature in each rolling pass of the two preceding stages of the second stage: 960 to 1080°C, and reduction ratio in each rolling pass of the two preceding stages: 30 to 40%]
The heated slab, or a slab that has been roughly rolled as needed, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of four or more rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, it is necessary to appropriately control the rolling temperature and reduction ratio in each rolling pass of the two preceding stages of the two subsequent stages. Specifically, the rolling temperature in each rolling pass of the two preceding stages of the two subsequent stages is controlled to 960 to 1080°C, and similarly, the reduction ratio in each rolling pass of the two preceding stages of the two subsequent stages is controlled to 30 to 40%. By performing rolling under relatively high pressure and relatively high temperature conditions in each rolling pass of the two preceding stages of the two subsequent stages, recrystallization can be promoted and the austenite grains can be refined. In connection with this, it is possible to reduce the average spacing of granular bainite in the final metal structure to a desired range.

Conversely, if the rolling temperature in each of the two rolling passes immediately preceding the two subsequent stages is less than 960°C, and/or the reduction ratio in each of those rolling passes is less than 30%, recrystallization will not be sufficiently promoted, and it will be impossible to reduce the average spacing of granular bainite in the resulting steel sheet to within the desired range. On the other hand, if the reduction ratio in each of the two rolling passes immediately preceding the two subsequent stages exceeds 40%, flattened austenite grains will be formed due to the introduction of excessive strain, and similarly, it will be impossible to reduce the average spacing of granular bainite in the resulting metal structure to within the desired range. Furthermore, if the rolling temperature in each of the two rolling passes immediately preceding the two subsequent stages exceeds 1080°C, the austenite grains will coarseen, and it will be impossible to obtain the desired microstructure fraction even with subsequent rolling and cooling control, or in addition, it will be impossible to control the average spacing and/or average grain size of granular bainite to within the desired range.

[(b) Cool to 910°C or below at an average cooling rate of 400°C/second or more within 0.20 seconds after the rolling pass of the two stages immediately preceding the last two stages.]
In this manufacturing method, the rolled material is cooled to 910°C or below at an average cooling rate of 400°C/second or more within 0.20 seconds after the two rolling passes immediately preceding the two subsequent stages. By cooling the rolled material to 910°C or below relatively quickly after the two rolling passes immediately preceding the two subsequent stages, grain growth after recrystallization can be suppressed, thereby making it possible to reduce the average spacing of granular bainite in the final resulting metal structure to within a desired range. If the time to cool to 910°C or below after the two rolling passes immediately preceding the two subsequent stages exceeds 0.20 seconds, grain growth after recrystallization cannot be sufficiently suppressed, and even if appropriate cooling is performed in the subsequent cooling process, it becomes impossible to control the average spacing and/or average grain size of granular bainite within a desired range.

Furthermore, the average cooling rate between the two subsequent rolling passes and the two preceding rolling passes is extremely important for producing granular bainite with the desired morphology within a predetermined range. More specifically, if the average cooling rate during this period is less than 400°C/second, the maximum orientation difference at 0.1 μm intervals within grains surrounded by grain boundaries with an orientation difference of 15° or more may exceed 3.5°. Therefore, it becomes impossible to produce 10.0% or more of granular bainite with a maximum orientation difference of 3.5° or less and an intra-grain orientation difference of 10° or more. The average cooling rate between the two subsequent rolling passes and the two preceding rolling passes is preferably 500°C/second or higher. Furthermore, if the cooling stop temperature is higher than 910°C, it may become impossible to produce 10.0% or more of granular bainite where the maximum orientation difference at 0.1 μm intervals is 3.5° or less and the intra-granular orientation difference is 10° or more within grains surrounded by grain boundaries with an orientation difference of 15° or more.

[(c) Reduction ratio in each of the two subsequent rolling passes: 20-30%]
In this manufacturing method, the reduction ratio in each of the two subsequent rolling passes of the finish rolling process is controlled to 20-30%. By introducing strain with such an appropriate reduction ratio in each of the two subsequent rolling passes, it is possible to increase the nucleation sites for granular bainite formation in the subsequent cooling process. If the reduction ratio in each of the two subsequent rolling passes is less than 20%, it is not possible to sufficiently form nucleation sites for granular bainite formation, and the desired area ratio of granular bainite cannot be obtained in the final metal structure. On the other hand, if the reduction ratio in each of the two subsequent rolling passes exceeds 30%, flattened austenite grains are formed due to the introduction of excessive strain, and it becomes impossible to reduce the average spacing of granular bainite to the desired range in the final metal structure.

[Cooling process]
[Cool to a temperature range of 500-650°C within 4.0 seconds of starting water cooling, followed by 2.0-6.0 seconds of air cooling.]
The finish-rolled steel sheet is water-cooled in the next cooling step, cooled to a temperature range of 500-650°C within 4.0 seconds of the start of water cooling, and then air-cooled in this temperature range for 2.0-6.0 seconds. First, by cooling to a temperature range of 500-650°C within 4.0 seconds of the start of water cooling, the formation of pearlite can be reliably suppressed, and therefore it is possible to achieve the desired area fraction of the metal structure in the final steel sheet. On the other hand, if the time from the start of water cooling to the temperature range of 500-650°C exceeds 4.0 seconds, a relatively large amount of pearlite is formed, and it becomes impossible to obtain the desired amount of martensite and/or granular bainite in the metal structure of the final steel sheet.

Furthermore, after water cooling, air cooling at a temperature range of 500-650°C for 2.0-6.0 seconds promotes the transformation to granular bainite and allows for the appropriate precipitation of Ti precipitates. Therefore, the air cooling operation at a temperature range of 500-650°C for 2.0-6.0 seconds after water cooling is extremely important not only for suppressing necking by granular bainite, but also for improving hole expansion and other properties due to precipitation strengthening caused by Ti precipitates. For example, if the air cooling temperature is below 500°C, the transformation to granular bainite cannot be sufficiently promoted, and a relatively large amount of bainite may be formed. In such cases, not only is the effect of suppressing necking reduced, but the uniform elongation decreases due to the formation of a large amount of bainite, and furthermore, the formation of martensite decreases in relation to the formation of bainite, which may result in insufficient strength being obtained.

Furthermore, if the air cooling temperature exceeds 650°C, the transformation to granular bainite cannot be sufficiently promoted, while the ferrite transformation is promoted, resulting in the formation of a relatively large amount of ferrite. In addition, sufficient Ti precipitates cannot be precipitated. In such cases, the effect of suppressing necking is reduced, and due to the relatively large amount of ferrite formation and insufficient precipitation strengthening by Ti precipitates, the hole-expanding properties and yield ratio of the resulting steel sheet are reduced. Also, if the air cooling time is less than 2.0 seconds, the transformation to granular bainite cannot be sufficiently promoted, and furthermore, a relatively large amount of martensite may be formed due to subsequent cooling. In such cases, the hole-expanding properties are reduced and/or the effect of suppressing necking is reduced, and the uniform elongation is reduced due to the excessive formation of martensite. On the other hand, if the air cooling time exceeds 6.0 seconds, a relatively large amount of granular bainite may be formed. In such cases, the amount of martensite is reduced, and the total amount of granular bainite and ferrite is relatively high. As mentioned earlier, granular bainite has characteristics similar to ferrite, so in a martensite-dominant microstructure, the combined area ratio of granular bainite and ferrite becomes relatively high, resulting in a microstructure similar to so-called DP steel, which leads to a decrease in the yield ratio. The air cooling temperature is preferably 525 to 625°C, and the air cooling time is preferably 3.0 to 5.0 seconds.

[Water cooling to below 50°C within 13 seconds after air cooling]
After air cooling for 2.0 to 6.0 seconds in the temperature range of 500 to 650°C, the steel sheet is water-cooled to 50°C or below within 13 seconds. This rapid cooling allows for the formation of martensite within the desired area ratio range. If water cooling to 50°C or below takes longer than 13 seconds, or if the cooling stop temperature is higher than 50°C, it may not be possible to achieve a martensite area ratio of 60.0% or more. In such cases, the desired steel sheet strength cannot be achieved. The lower limit of the water cooling time is not particularly limited, but for example, the water cooling time to 50°C or below after air cooling may be 4 seconds or more, or 5 seconds or more. Similarly, the lower limit of the water cooling stop temperature is not particularly limited, but for example, the water cooling stop temperature may be 20°C or higher, or 25°C or higher. Finally, the water-cooled steel sheet can be wound into the form of a hot-rolled coil. The winding conditions are not particularly limited and can be carried out under any suitable temperature conditions.

According to the steel sheet manufactured by the above manufacturing method, by constructing the metal structure with a structure containing 60.0-85.0% martensite by area percentage, it is possible to achieve high strength, such as a tensile strength of 1180 MPa or more, while significantly improving uniform elongation. Furthermore, by including 10.0-30.0% by area percentage of granular bainite in the metal structure, in which the maximum orientation difference at 0.1 μm intervals is 3.5° or less within grains surrounded by grain boundaries with an orientation difference of 15° or more, and the intra-grain orientation difference is 10° or more, and by controlling the average spacing of the granular bainite grains to 50.0 μm or less, it is possible to significantly suppress the occurrence of necking during forming while improving the yield ratio and hole expansion properties. In addition, by controlling the Ti content in the steel to 0.070 mass% or higher, Ti precipitates strengthen soft structures such as ferrite, thereby reducing the hardness difference between phases in the metallic structure. The combination of this reduction in hardness difference and the improvement in hole-expandability caused by the specific granular bainite mentioned above makes it possible to significantly improve the hole-expandability. Therefore, steel sheets manufactured by the above manufacturing method can suppress necking even when forming parts with complex shapes, and thus can reliably achieve a high level of both high strength and excellent workability, which are conflicting properties. This makes them particularly useful in the automotive sector where a balance of these properties is required.

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

In the following examples, steel sheets according to the embodiment of the present invention, particularly hot-rolled steel sheets, were manufactured under various conditions, and the tensile strength (TS), yield ratio (YR), uniform elongation (u-El), hole expansion ratio (λ), and necking occurrence in bending tests after pre-straining of the obtained steel sheets were investigated.

First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Tables 1 and 2. These slabs were heated to a temperature of 1180 to 1320°C and held for 6000 to 10000 seconds, after which hot rolling was performed. Hot rolling was carried out by rough rolling and finish rolling. More specifically, rough rolling was performed under the same conditions for all examples and comparative examples, and finish rolling was performed using a tandem rolling mill consisting of five rolling stands under the conditions shown in Table 3. Next, the finish-rolled steel sheets were water-cooled, air-cooled, and water-cooled under the conditions shown in Table 3 and then wound to obtain steel sheets with a thickness of 2.4 to 3.4 mm.

The properties of the obtained steel plates were measured and evaluated by the following method.

[Tensile strength (TS) and uniform elongation (u-El)]
Tensile strength (TS) and uniform elongation (u-El) were measured by taking a JIS No. 5 test specimen from the orientation where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2011.

[Hole expansion ratio (λ)]
The hole expansion ratio (λ) was determined as follows. First, a test piece measuring 100 mm wide x 100 mm long was taken from the steel plate, and a punched hole (initial hole: hole diameter d0 = 10 mm) was created using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole was expanded using a conical punch with a 60° apex angle until a crack penetrating the plate thickness occurred. The hole diameter d1 mm at the time of crack occurrence was measured, and the hole expansion ratio λ (%) for each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ.
λ=100×{(d1-d0)/d0}

[Yield ratio (YR)]
The yield ratio (YR) was determined by the following formula based on the tensile strength (TS) and 0.2% proof stress measured by taking a JIS No. 5 test specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2011.
Yield ratio YR = 0.2% proof stress / Tensile strength TS × 100

[Presence or absence of necking in bending tests after pre-straining]
First, a tensile test specimen with a parallel section width of 36 mm, a parallel section length of 86 mm, a radius of 36 mm, a gripping section width of 50 mm, and a total length of 372 mm was taken from a steel plate and subjected to a 10% pre-strain under uniaxial tension in the C direction. Next, a 60 mm [C direction] x 30 mm [L direction] specimen was taken from the center of the tensile test specimen and a 90° bending test was performed in the L direction to check for the occurrence of necking. The specimen was evaluated as passing if no necking was observed and failing if necking was observed.

Steel sheets with a tensile strength (TS) of 1180 MPa or higher, a uniform elongation (u-El) of 5.0% or higher, a hole expansion ratio (λ) of 40% or higher, a yield ratio (YR) of 80% or higher, and no necking observed in the bending test after pre-straining were evaluated as steel sheets with high strength, high uniform elongation, hole expansion properties, and yield ratio, while also being able to suppress necking during forming. The results are shown in Table 4. In Table 4, "GB grains" refers to granular bainite grains.

Referring to Tables 1-4, it is believed that in Comparative Example 4, recrystallization was not sufficiently promoted because the rolling temperature in each of the two rolling passes immediately preceding the two subsequent stages in the hot rolling process was low. As a result, the average spacing of granular bainite grains in the final resulting metal structure exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 5, it is believed that the austenite grains became coarser because the rolling temperature in each of the two rolling passes immediately preceding the two subsequent stages was high. As a result, the area ratio of granular bainite was less than 10.0%, the average spacing of granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 6, it is believed that recrystallization was not sufficiently promoted because the reduction ratio in the second of the two preceding stages in the two subsequent stages was low. As a result, the average spacing of the granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 7, it is thought that the reduction ratio was high in the first of the two preceding stages of the subsequent stages, resulting in the formation of flattened austenite grains due to the introduction of excessive strain. As a result, the average spacing of the granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 8, it is thought that the cooling time to below 910°C after the two preceding stages of the subsequent stages exceeded 0.20 seconds, which prevented sufficient suppression of grain growth after recrystallization. As a result, the average spacing of the granular bainite grains exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 9, the average cooling rate between the two subsequent rolling passes and the two preceding rolling passes was slow, resulting in a granular bainite area ratio of less than 10.0% exhibiting the predetermined orientation change. Consequently, the martensite area ratio was higher than 85.0%. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 10, the cooling stop temperature during the cooling between the two subsequent rolling passes and the two preceding rolling passes was high, similarly resulting in a granular bainite area ratio of less than 10.0% exhibiting the predetermined orientation change. Consequently, the martensite area ratio was higher than 85.0%. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-straining.

In Comparative Examples 11 and 12, the reduction ratio of the first and second rolling passes in the subsequent two stages was low, which is thought to have prevented the formation of sufficient nucleation sites for granular bainite. As a result, the area ratio of granular bainite was less than 10.0%, and relatedly, the area ratio of martensite was higher than 85.0%, resulting in a decrease in u-El and λ, and necking occurred in the bending test after pre-straining. In Comparative Examples 13 and 14, the reduction ratio of the first and second rolling passes in the subsequent two stages was high, which is thought to have caused the formation of flattened austenite grains due to the introduction of excessive strain. As a result, the average spacing of granular bainite grains exceeded 50.0 μm, resulting in a decrease in λ, and necking occurred in the bending test after pre-straining. In Comparative Example 15, the water cooling time before air cooling in the cooling process was long, resulting in the formation of a relatively large amount of pearlite. As a result, the area ratio of granular bainite was less than 10.0%, λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 16, the low air cooling temperature prevented sufficient promotion of the transformation to granular bainite, and consequently, a relatively large amount of bainite was formed. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 17, the high air cooling temperature prevented sufficient promotion of the transformation to granular bainite, and consequently, a large amount of ferrite was formed. In addition, it is thought that sufficient Ti precipitates could not be precipitated. As a result, λ and YR decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 18, the short air cooling time prevented sufficient promotion of the transformation to granular bainite, and furthermore, a large amount of martensite was formed during subsequent cooling. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-straining. In Comparative Example 19, the long air-cooling time resulted in the formation of a relatively large amount of granular bainite, which in turn reduced the formation of martensite, and the total amount of granular bainite and ferrite was relatively high. As a result, TS and YR decreased. In Comparative Example 20, the long water-cooling time to below 50°C after air-cooling resulted in a martensite area ratio of less than 60.0%, and TS decreased.

Comparative Example 46 showed a decrease in TS due to its low C content. Comparative Example 47 showed a decrease in λ due to its high C content. Comparative Example 48 showed a decrease in u-El due to its low Si content. Comparative Example 49 showed a large amount of ferrite formation due to its high Si content, and consequently, the total amount of granular bainite and ferrite also increased. As a result, λ and YR decreased. Comparative Example 50 showed reduced hardenability due to its low Mn content, resulting in a low martensite area ratio, and consequently, a relatively high total amount of granular bainite and ferrite. As a result, TS and YR decreased. Comparative Example 51 showed a low granular bainite area ratio due to its high Mn content, resulting in a decrease in λ and necking occurring in the bending test after pre-straining. Comparative Example 52 showed a large amount of ferrite formation due to its high sol. Al content, and consequently, the total amount of granular bainite and ferrite also increased. As a result, λ and YR decreased. Comparative Example 53 is thought to have produced coarse carbides and the like due to its high Nb content. As a result, the workability of the steel sheet decreased, u-El and λ decreased, and necking occurred in the bending test after pre-straining. Comparative Example 54 is thought to have not been able to sufficiently exert precipitation strengthening by Ti precipitates due to its low Ti content. As a result, TS and λ decreased. Comparative Example 55 is thought to have produced coarse carbides and the like due to its high Ti content. As a result, the workability of the steel sheet decreased, and λ decreased.

In contrast, in all the examples of the invention, by having a predetermined chemical composition and appropriately controlling each condition in the manufacturing method, it was possible to obtain a steel sheet in which the metal structure, by area percent, contains martensite: 60.0 to 85.0%, granular bainite: 10.0 to 30.0% where the maximum orientation difference at 0.1 μm intervals is 3.5° or less within grains surrounded by grain boundaries with an orientation difference of 15° or more, and the intragranular orientation difference is 10° or more, and ferrite: 20.0% or less, with an average spacing of granular bainite grains of 50.0 μm or less. Furthermore, as a result, despite having a high strength of tensile strength of 1180 MPa or more, it was possible to obtain a steel sheet with high uniform elongation, hole-expanding properties and yield ratio, and the occurrence of necking was reliably suppressed even in bending tests after pre-straining.

Claims (6)

The chemical composition is expressed in mass percent.
C: 0.060-0.200%,
Si: 0.30-2.00%,
Mn: 1.20-2.70%,
P: 0.100% or less,
S: 0.0300% or less,
Sol. Al: 0.001–0.500%,
Nb: 0.001-1.000%,
O: 0.0100% or less,
N: 0.0070% or less,
Ti: 0.070-0.200%,
B: 0 to 0.0030%,
Cr: 0-0.90%,
Mo: 0 to 0.12%,
Cu: 0 to 0.40%,
Ni: 0 to 0.30%,
V: 0-0.300%,
Sn: 0 to 0.040%,
As: 0 to 0.100%,
Zr: 0 to 0.050%,
Ca: 0-0.0010%,
Mg: 0 to 0.0010%,
Bi: 0 to 0.010%,
Co: 0 to 0.010%,
W: 0-0.100%,
Zn: 0 to 0.010%,
REM: 0-0.0100%, and the remainder: Fe and impurities.
The metallic structure, in area percentage,
Martensite: 60.0–85.0%
The material contains 10.0-30.0% granular bainite and 20.0% or less ferrite, where the maximum orientation difference at 0.1 μm intervals within grains surrounded by grain boundaries with an orientation difference of 15° or more is 3.5° or less, and the intra-grain orientation difference is 10° or more.
A steel plate characterized in that the average spacing between granular bainite grains is 50.0 μm or less. The aforementioned chemical composition is, in mass%,
B: 0.0001 to 0.0030%,
Cr: 0.001-0.90%,
Mo: 0.001-0.12%,
Cu: 0.001-0.40%,
Ni: 0.001 to 0.30%,
V: 0.001-0.300%,
Sn: 0.001 to 0.040%,
As: 0.001 to 0.100%,
Zr: 0.001 to 0.050%,
Ca: 0.0001 to 0.0010%,
Mg: 0.0001 to 0.0010%,
Bi: 0.001 to 0.010%,
Co: 0.001 to 0.010%,
W: 0.001-0.100%,
Zn: 0.001–0.010%, and REM: 0.0001–0.0100%
The steel plate according to claim 1, characterized in that it includes at least one of the following. The steel sheet according to claim 1 or 2, characterized in that the metallic structure further contains, by area percentage, at least one of bainite, pearlite, and retained austenite, totaling 20.0% or less. The steel sheet according to claim 1 or 2, characterized in that the average particle size of the granular bainite grains is 5.0 to 30.0 μm. A component characterized by comprising the steel plate described in claim 1 or 2. A heating step comprising heating a slab having the chemical composition described in claim 1 or 2 and holding it at a temperature of 1180 to 1320°C for 6000 seconds or more.
A hot rolling process that includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and that satisfies the following conditions (a) to (c): (a) the rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is 960 to 1080°C, and the reduction ratio in each rolling pass is 30 to 40%.
A method for manufacturing a steel sheet according to claim 1 or 2, comprising a cooling step including: (b) cooling the rolled material to 910°C or below at an average cooling rate of 400°C/second or more within 0.20 seconds after the two rolling passes immediately preceding the two subsequent stages, and (c) having a reduction ratio of 20 to 30% in each rolling pass of the two subsequent stages .