Steel sheet and method for manufacturing same

Abstract

Provided are a steel sheet and a method for manufacturing same. The steel sheet has a predetermined chemical composition and has a metallic microstructure containing, in area%, 60.0-85.0% of martensite, 10.0-30.0% of granular bainite, in which the maximum misorientation is 3.5° or less at 0.1 μm intervals and the in-grain misorientation is at least 10° within a grain surrounded by grain boundaries having a misorientation of at least 15°, and 20.0% or less of ferrite, wherein the average spacing between grains of the granular bainite is 50.0 μm or less.

Description

Steel plate and its manufacturing method

[0001] The present invention relates to a steel sheet and a method for manufacturing the same.

[0002] In recent years, in response to environmental issues, there has been a demand for lighter automotive parts in order to reduce CO2 gas emissions and improve fuel efficiency. At the same time, societal demands for improved collision safety are also increasing. Increasing the strength of steel materials is an effective way to achieve both weight reduction and improved collision safety. However, increasing the strength of steel materials usually results in a decrease in workability, so steel materials that simultaneously improve both strength and workability are needed.

[0003] In relation to improvements in strength and workability, for example, Patent Document 1 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, a bainite phase of more than 95% in area throughout the thickness direction, an average grain size of the bainite phase in a region from the surface to a quarter position in the thickness direction in the thickness direction of 5 μm or less in a thickness cross section parallel to the rolling direction and 4 μm or less in a thickness cross section perpendicular to the rolling direction, and a structure in which there are seven or fewer crystal grains elongated in the rolling direction and having an aspect ratio of 5 or more in a region whose width in the thickness direction is 1 / 10 of the thickness centered at the thickness center, and a tensile strength TS of 780 MPa or more. Patent Document 1 also teaches that the above configuration enables easy and inexpensive production of a high-strength hot-rolled steel sheet having a tensile strength TS of 780 MPa or more and excellent punching workability, with significantly improved punching workability.

[0004] Patent Document 2 describes a cold-rolled annealed steel sheet having a predetermined chemical composition and a microstructure consisting of martensite and / or lower bainite in a surface abundance ratio, the martensite including fresh martensite and / or self-tempered martensite, with the total surface abundance ratio being in the range of 60 to 95% for martensite and lower bainite, 4 to 35% for low-carbide-containing bainite, 0 to 5% for ferrite, and less than 5% for island-form 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 the skin-pass operation, a tensile strength in the range of 1180 to 1320 MPa, an elongation at break of at least 5%, and a hole expansion ratio Ac % of 30% or more.

[0005] Patent Document 3 describes a hot-rolled steel sheet having a predetermined chemical composition, in which the total area ratio of martensite and lower bainite at a quarter-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 having 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 of the sheet thickness is 6.0 or less and the maximum value is 8.0 or less. Patent Document 3 also teaches that the above configuration enables the stable production of high-strength hot-rolled steel sheet that has high strength and excellent hole expandability and low-temperature toughness.

[0006] Patent Document 4 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, a steel structure in which martensite and bainite account for 80 to 100% of the total area ratio of the main phase, the total area ratio of martensite in the bainite being 2 to 20%, and the area ratio of martensite in the bainite, in which the difference in orientation between the crystal orientation of the martensite and the crystal orientation of at least one bainite adjacent to the martensite is less than 15°, is 50% or more relative to the total martensite. Patent Document 4 also teaches that the above configuration can provide a high-strength hot-rolled steel sheet with excellent ductility, end cracking resistance, and hole expandability, which is suitable as a material for automotive parts.

[0007] Patent Document 5 describes a high-strength hot-rolled steel sheet having a predetermined chemical composition, a steel structure in which martensite and bainite account for 80 to 100% of the total area ratio of the main phase, the total area ratio of martensite in the bainite being 2 to 20%, the area ratio of martensite in the bainite having a crystal orientation misorientation of 15° or more between the martensite and at least one bainite adjacent to the martensite being more than 50%, and the average aspect ratio of the crystal grains present in a region from the surface of the steel sheet to a depth of 5 μm being 2.0 or less when the region surrounded by the boundary between adjacent crystals having a misorientation of 15° or more is defined as a crystal grain. Patent Document 5 also teaches that the above configuration can provide a high-strength hot-rolled steel sheet with excellent ductility and bending / rebending property suitable as a material for automotive parts.

[0008] Japanese Patent Application Publication No. 2012-062562 Japanese Patent Application Publication No. 2017-507241 Japanese Patent Application Publication No. 2017-057472 International Publication No. 2022 / 244706 International Publication No. 2022 / 244707

[0009] As described above, it is known that the workability of steel declines with increasing strength, resulting in a decline in properties such as hole expandability, as described in Patent Documents 2 to 4. A decline in hole expandability may make it impossible to process the steel into the desired shape, for example, in automobile undercarriage parts. For this reason, 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 for the application, such as uniform elongation in addition to the hole expandability. For example, in automobile undercarriage parts with complex shapes, such as lower arms and trailing arms, the decline in workability associated with increased strength can cause necking in the formed parts, resulting in a decline in their functionality.

[0010] Furthermore, for components that require impact resistance, plastic deformation occurs when they receive an impact that exceeds their yield strength. Therefore, from the perspective of ensuring automobile collision safety, it is necessary to improve not only tensile strength but also yield strength, and therefore it is necessary to increase the yield ratio, which is the ratio of yield strength to tensile strength.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel sheet having a novel configuration that has high strength, high uniform elongation, hole expandability, and yield ratio, and that can suppress the occurrence of necking during forming, and a method for manufacturing the same.

[0012] In order to achieve the above object, the present inventors have conducted research focusing on the metallographic structure of steel sheets, particularly hot-rolled steel sheets. As a result, the present inventors have discovered that by configuring the metallographic structure of a hot-rolled steel sheet having a predetermined chemical composition with a structure mainly composed of martensite but controlled within a predetermined range, it is possible to achieve high strength and improved uniform elongation, and that by including a predetermined amount of specific granular bainite in the metallographic structure, it is possible to improve the yield ratio and hole expandability while significantly suppressing the occurrence of necking during forming, and further, by utilizing precipitation strengthening by adding Ti, it is possible to further increase the yield ratio and reduce the hardness difference between each phase in the metallographic structure, and by combining such a reduction in hardness difference with the improvement in hole expandability due to the specific granular bainite, it is possible to more significantly improve the hole expandability, and have completed the present invention.

[0013] The present invention has achieved the above object as follows: (1) A steel sheet having a chemical composition, in mass %, of C: 0.060 to 0.200%, Si: 0.30 to 2.00%, Mn: 1.20 to 2.70%, P: 0.100% or less, S: 0.0300% or less, sol. Al: 0.001 to 0.500%, Nb: 0.001 to 1.000%, O: 0.0100% or less, N: 0.0070% or less, Ti: 0.070 to 0.200%, B: 0 to 0.0030%, Cr: 0 to 0.90%, Mo: 0 to 0.12%, Cu: 0-0.40%, Ni: 0-0.30%, V: 0-0.300%, Sn: 0-0.040%, As: 0-0.100%, Zr: 0-0.050%, Ca: 0-0.0010%, Mg: 0-0.0010%, Bi: 0-0.010%, Co: 0 to 0.010%, 1. A steel sheet comprising W: 0-0.100%, Zn: 0-0.010%, REM: 0-0.0100%, and the balance: Fe and impurities, wherein the metallographic structure comprises, in area %, 60.0-85.0% martensite, 10.0-30.0% granular bainite, the maximum misorientation at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more being 3.5° or less and the intragranular misorientation being 10° or more, and 20.0% or less ferrite, and wherein the average spacing of the 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 to 0.90%, Mo: 0.001 to 0.12%, Cu: 0.001 to 0.40%, Ni: 0.001 to 0.30%, V: 0.001 to 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%, The steel sheet according to (1) above, characterized in that it contains at least one of W: 0.001 to 0.100%, Zn: 0.001 to 0.010%, and REM: 0.0001 to 0.0100%.(3) The steel sheet according to (1) or (2) above, characterized in that the metal structure further contains, by area percentage, at least one of bainite, pearlite, and retained austenite: a total of 20.0% or less. (4) The steel sheet according to any one of (1) to (3) above, characterized in that the average grain size of the granular bainite grains is 5.0 to 30.0 μm. (5) A part, characterized in that it includes the steel sheet according to any one of (1) to (4) above. (6) A heating step including 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 step including finish-rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and satisfying the following conditions (a) to (c): (a) the rolling temperature in each of the two rolling passes immediately preceding the last two passes is 960 to 1080°C, and the reduction in each of the rolling passes is 30 to 40%, (b) the rolled material is cooled to 910°C or less at an average cooling rate of 400°C / second or more within 0.20 seconds after the rolling pass in the two passes immediately preceding the last two passes, and (c) the reduction in each of the last two passes is 20 to 30%. a cooling step of water-cooling the finish-rolled steel sheet to a temperature range of 500 to 650°C within 4.0 seconds from the start of water-cooling, then air-cooling for 2.0 to 6.0 seconds in the temperature range, and water-cooling the steel sheet to 50°C or less within 13 seconds after air-cooling.

[0014] According to the present invention, it is possible to provide a steel sheet, particularly a hot-rolled steel sheet, which has high strength, high uniform elongation, hole expandability, and yield ratio, and can suppress the occurrence of necking during forming, and a manufacturing method thereof.

[0015] <Steel Sheet> The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition, in mass %, of C: 0.060 to 0.200%, Si: 0.30 to 2.00%, Mn: 1.20 to 2.70%, P: 0.100% or less, S: 0.0300% or less, sol. Al: 0.001 to 0.500%, Nb: 0.001 to 1.000%, O: 0.0100% or less, N: 0.0070% or less, Ti: 0.070 to 0.200%, B: 0 to 0.0030%, Cr: 0 to 0.90%, Mo: 0 to 0.12%, Cu: 0-0.40%, Ni: 0-0.30%, V: 0-0.300%, Sn: 0-0.040%, As: 0-0.100%, Zr: 0-0.050%, Ca: 0-0.0010%, Mg: 0-0.0010%, Bi: 0-0.010%, Co: 0 to 0.010%, The alloy is characterized by comprising W: 0-0.100%, Zn: 0-0.010%, REM: 0-0.0100%, and the balance: Fe and impurities, and the metal structure comprises, in area %, 60.0-85.0% martensite, 10.0-30.0% granular bainite, which has a maximum misorientation of 3.5° or less at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more and an intragranular misorientation of 10° or more, and 20.0% or less ferrite, and the average spacing of the granular bainite grains is 50.0 μm or less.

[0016] As mentioned above, it is known that properties such as hole expandability decrease with increasing strength of steel. For example, to manufacture automobile suspension parts with complex shapes, such as lower arms and trailing arms, a steel sheet is required that has high strength, for example, a tensile strength of 1180 MPa or more, which enables weight reduction, while also having excellent hole expandability. From the perspective 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 martensitic content reduces properties such as uniform elongation, which generally results in poor workability. Furthermore, in parts with complex shapes, such as lower arms and trailing arms, the decrease in workability associated with increasing strength can cause necking in the formed parts, resulting in a decrease in their functionality. Therefore, there is a demand for high-strength steel sheets that can improve properties such as hole expandability and uniform elongation, suppress necking even when forming parts with complex shapes, and further have a high yield ratio from the perspective of automobile collision safety, etc.

[0017] Therefore, the present inventors conducted research focusing on the metallographic structure of a hot-rolled steel sheet, in addition to making the chemical composition of the steel sheet, particularly a hot-rolled steel sheet, appropriate. First, the present inventors discovered that by configuring the metallographic structure of a hot-rolled steel sheet having a predetermined chemical composition to 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, for example, a tensile strength of 1180 MPa or more, while significantly improving the uniform elongation of the resulting hot-rolled steel sheet.

[0018] Next, the inventors discovered 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 which the maximum misorientation at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more is 3.5° or less, the intragranular misorientation is 10° or more, and the average spacing between adjacent grains is 50.0 μm or less, the yield ratio and hole expandability can be improved while significantly suppressing the occurrence of necking during forming. While not intending to be bound by any particular theory, it is believed that the characteristic orientation change of granular bainite particularly contributes to suppressing the occurrence of necking. More specifically, the characteristic that "within grains surrounded by grain boundaries with a misorientation of 15° or more, the maximum misorientation at 0.1 μm intervals is 3.5° or less, and the intragranular misorientation is 10° or more" means that although the orientation change within the grains of granular bainite is relatively gradual and continuous, the misorientation within the entire grain is relatively large. For example, bainite has many different interfaces within the grains, which results in discontinuous and abrupt orientation changes. On the other hand, ferrite has a characteristic that the orientation change within the grains is relatively small, and therefore continuous, but the misorientation within the entire grain is also relatively small. Therefore, granular bainite can be considered to have characteristics between bainite and ferrite in terms of orientation change. While necking is likely to occur in bainite structures due to discontinuous orientation changes, granular bainite, despite showing a relatively large misorientation within the entire grain, exhibits a continuous orientation change as described above, unlike bainite and martensite, which also have many interfaces within the grain. Therefore, it is believed that the occurrence of necking during forming can be significantly suppressed due to such characteristic orientation changes of granular bainite. In addition, the inventors have discovered that hole expandability can also be improved by including 10.0% or more granular bainite by area percent.Although it is not intended to be bound by any particular theory, it is believed that the presence of a certain amount of granular bainite, which has properties intermediate between martensite or bainite and ferrite, in steel suppresses the generation of voids from the interface between different phases during hole expansion processing, thereby improving hole expandability.

[0019] However, as described above, granular bainite has characteristics similar to those of ferrite. Therefore, if the amount of granular bainite is too large in a metal structure mainly composed of martensite, it is thought that the metal structure will be similar to so-called DP steel (dual phase steel) composed of martensite and ferrite, thereby resulting in a decrease in the yield ratio. Even if the amount of granular bainite is appropriate, if the amount of ferrite is excessively large or the amount of martensite is reduced so that the total amount of granular bainite and ferrite is relatively large, the metal structure will similarly be similar to DP steel, resulting in a decrease in the yield ratio. Therefore, from the perspective of maintaining a high yield ratio while sufficiently suppressing the occurrence of necking, it is necessary to have an appropriate amount of granular bainite present in the metal structure, while maintaining the area fraction of martensite at 60.0% or more to control the total amount of granular bainite and ferrite within an appropriate range. In addition, the inventors conducted further studies and found that, although the reason is not entirely clear, arranging granular bainite grains at an appropriate interval, more specifically, controlling the average interval of the granular bainite grains to 50.0 μm or less, can improve the hole expandability of the steel sheet, and further that controlling the average interval of the granular bainite grains in this manner is also important for suppressing the occurrence of necking during forming. Based on these findings, according to a steel sheet according to an embodiment of the present invention, by including 10.0 to 30.0% by area of ​​granular bainite in the metal structure, the maximum misorientation at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more is 3.5° or less, and the intragranular misorientation is 10° or more, and by controlling the average interval 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 expandability.

[0020] In addition, the inventors have discovered that the yield ratio can be further increased by utilizing precipitation strengthening through the addition of Ti, and that the hole expandability can be more significantly improved by combining this with the improvement in hole expandability due to the specific granular bainite. While not intending to be bound by any particular theory, it is believed that the improvement in hole expandability due to precipitation strengthening is due to a reduction in the hardness difference between each phase in the metallographic structure. More specifically, in the steel sheet according to the embodiment of the present invention, as described above, the metallographic structure is primarily composed of martensite, but also contains other structures softer than martensite, such as ferrite, which may contain up to 20.0% by area of ​​a soft structure. In this case, the hardness difference between each phase in the metallographic structure increases, resulting in a decrease in hole expandability. However, in the steel plate 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 soft structures such as ferrite, thereby reducing the hardness difference between the phases in the metal structure, and it is believed that the combination of this reduction in hardness difference and the improvement in hole expandability due to the specific granular bainite can more significantly improve the hole expandability.

[0021] Generally, automotive steel sheets are often processed into the desired part shape by press forming. Because press forming is typically performed in multiple steps, there are relatively many locations where, for example, primary deformation accumulates strain within the steel sheet, resulting in additional deformation. However, because steel sheets undergo work hardening and high strength upon strain introduction, their workability in subsequent processes generally deteriorates, potentially resulting in necking in the formed portion. The present inventors have found that improving bendability after pre-straining is effective in suppressing necking in the formed portion during forming. More specifically, they found that necking can be reproduced by applying a 10% pre-strain to a steel sheet test specimen in a uniaxial tensile test in a certain direction and then performing a 90° bending test in a direction perpendicular to that direction. In particular, the ductility of steel sheets is poor in the C direction (the direction perpendicular to the rolling direction). Therefore, they conducted a tensile test in the C direction followed by a bending test in the L direction (the rolling direction). They found that if necking did not occur in the bend test specimen, necking could also be improved in actual part forming. According to the steel sheet of the present invention, by including the specific granular bainite in the metal structure at 10.0 to 30.0% by area, the occurrence of necking can be reliably suppressed even in such bending tests after pre-straining. The fact that necking can be reproduced in actual part forming by such bending tests after pre-straining, and further the fact that the occurrence of necking in bending tests after pre-straining can be significantly suppressed by including 10.0% or more by area of ​​granular bainite exhibiting the characteristic orientation change described above, was not previously known and has now been revealed for the first time by the present inventors. Therefore, according to the embodiment of the present invention, despite having a high strength of, for example, tensile strength of 1180 MPa or more, the steel sheet has high uniform elongation, hole expandability, and yield ratio, and can reliably suppress the occurrence of necking in actual part forming. Therefore, the steel sheet of the present invention is particularly useful for use in the automotive field.

[0022] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0023] [C: 0.060 to 0.200%] C is an element effective in increasing the strength of steel sheet. Furthermore, C forms carbides and / or carbonitrides with Nb in steel, contributing to refinement of the structure due to the pinning effect of the formed precipitates. To fully obtain these effects, the C content is set to 0.060% or more. The C content may be 0.070% or more, 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, excessive C content may reduce hole expandability. Therefore, the C content is set to 0.200% or less. The C content may be 0.180% or less, 0.160% or less, 0.150% or less, or 0.140% or less.

[0024] [Si: 0.30 to 2.00%] Si is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. To fully obtain this effect, the Si content is set to 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, excessive Si content may increase the ferrite fraction and reduce hole expandability. Furthermore, an increase in the ferrite fraction increases the total amount of granular bainite and ferrite, resulting in a metal structure similar to that of DP steel, which may reduce the yield ratio. Therefore, the Si content is set to 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.

[0025] [Mn: 1.20 to 2.70%] Mn is an element that is effective in increasing hardenability and strength as a solid solution strengthening element. To fully obtain these effects, the Mn content is set to 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, excessive Mn content reduces the granular bainite fraction, reduces hole expandability, and may not be able to sufficiently suppress the occurrence of necking during forming. Therefore, the Mn content is set to 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.

[0026] [P: 0.100% or less] Excessive P content may result in reduced workability due to grain boundary segregation, etc. Therefore, the P content is set to 0.100% or less. The P content may 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 increase costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0027] [S: 0.0300% or less] Excessive S content may result in the formation of large amounts of sulfides such as MnS, which may reduce workability. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the S content may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[0028] [Sol. Al: 0.001 to 0.500%] Sol. Al is an element that acts as a deoxidizer for molten steel. Sol. Al is also an element that is effective in increasing the granular bainite fraction. To achieve these effects, the sol. Al content is set to 0.001% or more. The sol. Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive sol. Al content may increase the ferrite fraction and reduce hole expandability. Furthermore, increasing the ferrite fraction increases the total amount of granular bainite and ferrite, resulting in a metal structure similar to that of DP steel, which may reduce the yield ratio. Therefore, the sol. Al content is set to 0.500% or less. The Al content may be 0.400% or less, 0.300% or less, or 0.200% or less. Sol. Al means acid-soluble Al, and indicates solute Al that is present in the steel in a solid solution state.

[0029] [Nb: 0.001 to 1.000%] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, thereby contributing to the refinement of prior austenite grains through a pinning effect, thereby increasing the strength of the steel sheet. Nb is also an element that is effective in increasing the fraction of granular bainite and controlling its morphology. To fully obtain these effects, the Nb content is set to 0.001% or more. The Nb content may be 0.005% or more, 0.010% or more, 0.050% or more, 0.100% or more, 0.200% or more, or 0.300% or more. On the other hand, excessive Nb content may cause the formation of coarse carbides and the like in the steel, resulting in a decrease in the workability of the steel sheet. Therefore, the Nb content is set to 1.000% or less. The Nb content may be 0.800% or less, 0.600% or less, 0.500% or less, or 0.400% or less.

[0030] [O: 0.0100% or less] O is an element that is mixed in during the manufacturing process. Excessive O content may form coarse inclusions, reducing the workability of the steel sheet. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long refining time, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more or 0.0005% or more.

[0031] [N: 0.0070% or less] Excessive N content may form coarse nitrides, which may cause slab cracking during hot rolling. Therefore, the N content is set to 0.0070% or less. The N content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[0032] [Ti: 0.070 to 0.200%] Ti precipitates in steel as Ti carbides such as TiC, strengthening soft structures such as ferrite through precipitation strengthening and contributing to improved strength and yield ratio. Furthermore, Ti can reduce the hardness difference between phases in the metal structure due to precipitation strengthening, and is therefore also effective in improving hole expandability. To fully achieve these effects, the Ti content is set to 0.070% or more. The Ti content may be 0.080% or more, 0.090% or more, 0.100% or more, or 0.120% or more. On the other hand, excessive Ti content can produce coarse carbides in the steel, which can cause slab cracking during hot rolling or reduce the workability of the steel sheet. Therefore, the Ti content is set to 0.200% or less. The Ti content may be 0.180% or less, 0.170% or less, 0.160% or less, or 0.150% or less.

[0033] 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 contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0034] [B: 0 to 0.0030%] B is an element that improves the hardenability of steel and contributes to improving strength. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0030% or less. The B content may be 0.0025% or less, 0.0020% or less, 0.0015% or less, or 0.0010% or less.

[0035] [Cr: 0 to 0.90%] Cr is an element that improves the hardenability of steel and contributes to improving 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 Cr is contained excessively, the effect saturates and there is a risk of increasing 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.

[0036] [Mo: 0 to 0.12%] Mo is an element that improves the hardenability of steel and contributes to improving strength. The Mo content may be 0%, but to achieve this effect, the Mo content is preferably 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, excessive Mo content may increase deformation resistance during hot working and increase equipment load. Therefore, the Mo content is preferably 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.

[0037] [Cu: 0 to 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 effects, the Cu content is preferably 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, even if these elements are contained in excess, the effects may saturate and the manufacturing cost may increase. Therefore, the Cu content is preferably 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.

[0038] [Ni: 0 to 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 effects, the Ni content is preferably 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, even if these elements are contained in excess, the effects may saturate and the manufacturing cost may increase. Therefore, the Ni content is preferably 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.

[0039] [V: 0 to 0.300%] V is an element that contributes to improving strength through precipitation strengthening and the like. The V content may be 0%, but to obtain such an effect, the V content is preferably 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, even if V is contained excessively, the effect saturates and there is a risk of incurring an increase in manufacturing costs. Therefore, the V content is preferably 0.300% or less. The V content may be 0.200% or less, 0.100% or less, or 0.080% or less.

[0040] [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 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100%, Zn: 0 to 0.010%, and REM: 0 to 0.0100%] Sn, As, Zr, Ca, Mg, Bi, Co, W, Zn, and REM may be contained in the steel sheet as optional elements, or may be present in the steel sheet as tramp elements. The contents 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%. The lower limits of these elements may be, for example, 0.001% or more, 0.005% or more, or 0.008% or more, respectively. Similarly, the Ca, Mg and REM contents may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0041] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The 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 industrially manufacturing the steel sheet. Impurities are also allowed to be contained within a range that does not affect the effects of the present invention.

[0042] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0043] [Metal Structure] [Martensite: 60.0 to 85.0%] The metal structure of the steel sheet according to the embodiment of the present invention contains, in area %, 60.0 to 85.0% martensite. By configuring the metal structure of the steel sheet to include hard martensite within this range, high strength, for example, a tensile strength of 1180 MPa or more, can be achieved while significantly improving the uniform elongation of the resulting steel sheet. From the viewpoint of further increasing strength, a higher area fraction of martensite is preferable, and may be, for example, 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 fraction of martensite is preferable, and may be, for example, 82.0% or less, 80.0% or less, 78.0% or less, or 75.0% or less. In the present invention, "martensite" encompasses not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0044] [Granular bainite having a maximum misorientation of 3.5° or less at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more, and an intragranular misorientation of 10° or more: 10.0 to 30.0%] The metallographic structure of a steel plate according to an embodiment of the present invention includes, by area%, 10.0 to 30.0% granular bainite having a maximum misorientation of 3.5° or less at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more, and an intragranular misorientation of 10° or more. Here, the structure referred to as granular bainite in the prior art does not necessarily have the characteristic of "having a maximum misorientation of 3.5° or less at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more, and an intragranular misorientation of 10° or more." The structure referred to as granular bainite in the prior art is often not adequately defined, and therefore, simply referring to granular bainite is not recognized as being identical to the granular bainite of the embodiment of the present invention. In the embodiment 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 ​​the specific granular bainite having the above-mentioned characteristics, in other words, granular bainite having the characteristic that "despite the relatively gradual and continuous orientation change within the crystal grains, the overall misorientation within the crystal grains is relatively large." This technical matter and the effects obtained thereby were discovered for the first time by the inventors. As described above, by containing 10.0% or more by area of ​​granular bainite having the characteristic that, despite the relatively gradual and continuous orientation change within the crystal grains, the overall misorientation within the crystal grains is relatively large, it is possible to significantly suppress the occurrence of necking during forming due to such characteristic orientation change. In addition, as mentioned above, it is believed that the inclusion of 10.0% or more of granular bainite by area% suppresses the generation of voids from the interface between different phases during hole expanding, which in turn makes it possible to improve hole expandability. From the viewpoint of further suppressing the occurrence of necking and / or further improving hole expandability, the higher the area fraction of granular bainite, the more preferable it is, and it may be, for example, 12.0% or more, 15.0% or more, or 18.0% or more.On the other hand, as mentioned above, since granular bainite has characteristics similar to ferrite, if the area fraction of granular bainite becomes too high in a metal structure mainly composed of martensite, the metal structure will become similar to that of so-called DP steel, resulting in a decrease in the yield ratio. Therefore, from the viewpoint of maintaining a higher yield ratio, the lower the area fraction of granular bainite, the more preferable it is, and it may be, for example, 28.0% or less, 25.0% or less, or 22.0% or less.

[0045] In a study conducted by the present inventors, a 10% pre-strain was applied to a steel sheet in the C direction (direction perpendicular to the rolling direction) by uniaxial tension, followed by a 90° bending test in the L direction (rolling direction). It was found that if necking did not occur in the bend test specimen, necking could also be improved in actual part forming. According to a steel sheet according to an embodiment of the present invention, by including 10.0 to 30.0% by area of ​​the specific granular bainite in the metallographic structure, necking can be reliably suppressed even in such a bending test after pre-straining. Therefore, according to an embodiment of the present invention, necking can be reliably suppressed even in the later stages of deformation in forming operations that are performed in multiple steps, such as actual press forming of automotive steel sheets. Therefore, the steel sheet according to an embodiment of the present invention is particularly useful for use in the automotive field.

[0046] [Ferrite: 20.0% or Less] The metal structure of the steel sheet according to the embodiment of the present invention contains, in area %, 20.0% or less of ferrite. If the area % of ferrite, which is a soft structure, can be limited to 20.0% or less, the hardness difference between each phase in the metal structure can be sufficiently reduced by precipitation strengthening the soft structure containing the ferrite with Ti precipitates. Therefore, the hole expandability can be more significantly improved by combining this reduction in hardness difference with the improvement in hole expandability due to the control of the average spacing of granular bainite, which will be described later. If the area fraction of ferrite exceeds 20.0%, the hole expandability may not be sufficiently improved even when precipitation strengthening by Ti precipitates and control of the average spacing of granular bainite are combined. In addition, if the area fraction of ferrite exceeds 20.0%, the total amount of granular bainite and ferrite increases, resulting in a metal structure similar to that of DP steel, which may result in a lower yield ratio. From the viewpoint of further increasing the hole expandability and / or yield ratio, the lower the ferrite area ratio, the more preferable, and may be, 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%, or may be, for example, 0.5% or more or 1.0% or more.

[0047] [Remaining Structure] The remaining structure other than martensite, the specific granular bainite, and ferrite may be 0% by area. However, if a remaining structure is present, the remaining structure may include at least one of bainite, pearlite, and retained austenite in a total area of ​​20.0% or less. If the total area ratio of at least one of bainite, pearlite, and retained austenite exceeds 20.0%, uniform elongation may be reduced, or other structures such as martensite and granular bainite may not be controlled within desired ranges. Therefore, the smaller the area ratio of the remaining structure, the better. For example, the total area ratio 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 may be, for example, 0.1% or more, 0.5% or more, or 1.0% or more.

[0048] [Average spacing of 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 the 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 a misorientation of 15° or more, and have a maximum misorientation of 3.5° or less at 0.1 μm intervals within the grains, and have an intragranular misorientation of 10° or more. By controlling the average spacing between granular bainite grains exhibiting the above-described characteristic orientation change to 50.0 μm or less, the hole expandability of the steel sheet can be significantly improved in combination with the precipitation strengthening due to 20 area % or less of ferrite and Ti precipitates described above. In addition, the average spacing of the granular bainite grains is also a factor that determines the arrangement of the granular bainite structure. Therefore, if the spacing of the granular bainite grains is uneven, even if the granular bainite exhibiting the characteristic orientation change described above is contained in an area percentage 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 the hole expandability and further reliably suppressing the occurrence of necking, the smaller the average spacing of the granular bainite grains, the more preferable it is. 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. Although the lower limit is not particularly limited, for example, the average spacing of the 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.

[0049] [Average grain size of granular bainite grains: 5.0 to 30.0 μm] In the metal structure of the steel sheet according to the embodiment of the present invention, the average grain size of the granular bainite grains is preferably 5.0 to 30.0 μm. By controlling the average grain 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-strain. For example, the average grain 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 grain 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.

[0050] [Identification of Martensite, Bainite, Pearlite, and Retained Austenite and Calculation of Area Fractions] Identification of martensite, bainite, pearlite, and retained austenite and calculation of their area fractions are performed by optical microscope observation and X-ray diffraction after corrosion using a Nital reagent or a Repeller solution. Microstructural observation using an optical microscope is performed on a thickness cross section perpendicular to the sheet surface. The thickness cross section is preferably parallel to the rolling direction. Specifically, a sample is first collected from the steel sheet, and the observation surface of the sample is etched with Nital. Next, image analysis is performed on a microstructural photograph obtained using an optical microscope with a 300 μm × 300 μm field at a depth of 1 / 4 of the sheet thickness to calculate the total area fraction of martensite and bainite, and the area fraction of pearlite. Next, using a sample whose observation surface has been subjected to Repeller corrosion, image analysis is similarly performed on a microstructural photograph obtained using an optical microscope with a 300 μm × 300 μm field at a depth of 1 / 4 of the sheet thickness to calculate the total area fraction of martensite and bainite. Next, using a sample that has been face-milled from the normal direction of the rolled surface to a depth of 1 / 4 of the plate thickness, 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 previously calculated total area fraction of martensite and retained austenite. Finally, the area fraction of bainite is calculated by subtracting the obtained area fraction of martensite from the previously calculated total area fraction of martensite and bainite.

[0051] [Identification of Ferrite and Calculation of Area Fraction] Identification of ferrite and calculation of area fraction are performed by electron backscattered diffraction (EBSD) as follows. Specifically, first, a sample is taken from the steel sheet so that the observation surface is the thickness cross section perpendicular to the sheet surface. The thickness cross section is preferably parallel to the rolling direction. Next, EBSD analysis is performed at a measurement interval of 0.2 μm on a rectangular region of 200 μm in the thickness direction and 400 μm in the direction perpendicular to the thickness direction, centered at a position ¼ of the sheet thickness from the steel sheet surface, to obtain crystal orientation information of this rectangular region. EBSD analysis is performed using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) at an analysis speed of 50 to 300 points per second. Next, from the crystal orientation information of this rectangular region, the grain average misorientation (GAM value: Grain Average Misorientation) is calculated using the software "OIM Analysis (registered trademark)" attached to 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 value of the misorientation between adjacent pixels in a region surrounded by grain boundaries with a misorientation of 15° or more.

[0052] [Identification of Granular Bainite and Calculation of Area Fraction] Identification of granular bainite and calculation of area fraction are performed by EBSD as follows. Specifically, first, a sample is taken from the steel sheet so that the observation surface is the thickness cross section perpendicular to the sheet surface. The thickness cross section is preferably parallel to the rolling direction. Next, EBSD analysis is performed at measurement intervals of 0.1 μm on a rectangular region centered at a position ¼ of the sheet thickness from the steel sheet surface, measuring 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. EBSD analysis is performed using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) at an analysis speed of 50 to 300 points per second. Next, using the crystal orientation information of this rectangular region and software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, regions surrounded by grain boundaries with a misorientation of 15° or more are defined as crystal grains, the intragranular misorientation of the crystal grains is calculated, and crystal grains with a maximum misorientation of 3.5° or less at 0.1 μm intervals and an intragranular misorientation, more specifically, a maximum intragranular misorientation of 10° or more, are identified as granular bainite, and their area fraction is calculated. The average of the area fractions obtained for any three intragranular lines is determined to be the area fraction of the granular bainite. The "maximum intragranular misorientation" for granular bainite is found using "Grain Reference Orientation Deviation (GROD)." The maximum misorientation value within a grain is determined as the misorientation with other pixels within the grain, using the orientation of the pixel within the same crystal grain at which the KAM value (Kernel Average Misorientation) is minimum as the reference. In an embodiment of the present invention, the reference crystal orientation is the orientation within the same crystal grain at which the KAM value is minimum. The GROD and KAM values ​​can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" provided with the EBSD analyzer.

[0053] [Method of determining the average spacing and average grain size of granular bainite grains] The average spacing of granular bainite grains is determined by measuring the distance between the center of gravity of granular bainite grains identified by EBSD and the center of gravity of the nearest granular bainite grain, and averaging the distances measured at 100 or more points. Furthermore, the average circle-equivalent diameter of all granular bainite grains measured at 100 or more points is determined as the average grain size of the granular bainite grains.

[0054] [Thickness] The steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 8.0 mm, although not particularly limited thereto. 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.

[0055] The steel sheet according to the embodiment of the present invention can suppress the occurrence of necking even when forming parts having complex shapes, thereby reliably achieving a high level of the contradictory properties of high strength and excellent formability. Therefore, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields where both of these properties are required, and is particularly useful for use in parts in the automotive field. Therefore, in a preferred embodiment, an automobile part, particularly an automobile suspension part, including the steel sheet according to the embodiment of the present invention is provided. Examples of automobile suspension parts include lower arms and trailing arms. These automobile parts, particularly automobile suspension parts, may contain the steel sheet according to the embodiment of the present invention in at least a portion thereof, and therefore, at least a portion of these parts satisfy the above-described chemical composition and metallographic characteristics. In a portion of a steel sheet that has been processed relatively lightly in forming, such as press forming, the characteristics of the steel sheet do not change significantly before and after forming. A portion of a steel sheet that has been processed relatively lightly is determined by characteristics such as a smooth shape that has not been subjected to deformation such as bending, and a small rate of change in sheet 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 portions. This can prevent these parts from being manufactured from a single steel plate, resulting in specific portions being separated, which can increase the cost of the parts. However, with the steel plate according to the embodiment of the present invention, multiple forming operations can be performed from a single steel plate without causing necking, even in parts with complex shapes, such as lower arms and trailing arms, which is economically advantageous.

[0056] [Mechanical Properties] [Tensile Strength (TS) and Uniform Elongation (u-El)] Steel sheets, particularly hot-rolled steel sheets, having the above-described chemical composition and metallographic structure can achieve high tensile strength, specifically tensile strength of 1180 MPa or more. The tensile strength is preferably 1200 MPa or more, 1220 MPa or more, or 1240 MPa or more. Despite having such extremely high tensile strength, steel sheets according to embodiments of the present invention can significantly suppress the occurrence of necking during forming while improving uniform elongation and hole expandability due to the specific combination of chemical composition and metallographic structure described above. The upper limit of tensile strength is not particularly limited, but the tensile strength of the steel sheet may be, for example, 1780 MPa or less, 1470 MPa or less, or 1400 MPa or less. Furthermore, steel sheets according to embodiments of the present invention, particularly hot-rolled steel sheets, can achieve high uniform elongation, 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. The upper limit of the uniform elongation is not particularly limited, but for example, the uniform elongation of the steel sheet may be 15.0% or less, 10.0% or less, or 8.0% or less. The tensile strength and uniform elongation are measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction perpendicular to the rolling direction of the steel sheet (C direction) 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 piece due to dimensional constraints, other test pieces described in JIS Z 2241:2011 can be used. However, when the sheet thickness is less than 0.5 mm, the lower limit is set to 0.5 mm in order to perform an appropriate evaluation. For example, when it is difficult to obtain a JIS No. 5 test piece due to dimensional constraints and it is also difficult to use other test pieces 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) converted into tensile strength can be used. A sample to be subjected to the micro-Vickers test can be prepared as follows. First, a sample is cut out from an arbitrary position 50 mm or more away from the end face of the steel plate (if a sample cannot be obtained from this position, a position avoiding the end) so that a cross section of the plate thickness perpendicular to the plate surface can be observed.The thickness cross section is preferably parallel to the rolling direction. The size of the sample, depending on the measuring device, should be large enough to observe approximately 10 mm in a direction perpendicular to the thickness direction. The cross section of the sample is polished using #600 to #1500 silicon carbide paper, and then mirror-finished using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water. The observation surface is then finished by electrolytic polishing. The micro Vickers test is performed at 30 points at 1 / 4 of the thickness of the plate with a load of 500 gf, and the average value is used. Conversion can be performed using the following formula: Tensile strength [MPa] = 3.12 × Vickers hardness [HV] + 16.

[0057] [Hole Expansion Ratio (λ)] Steel sheets having the above chemical composition and metallographic structure can achieve high hole expandability, specifically, a hole expansion ratio of 40% or more. The hole expansion ratio may be preferably 42% or more, more preferably 45% or more or 50% or more. The upper limit of the hole expansion ratio is not particularly limited, but may be, for example, 150% or less, 100% or less, or 70% or less. The hole expansion ratio is determined as follows. First, a test piece 100 mm wide x 100 mm long 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 expanded using a conical punch with an apex angle of 60° until a crack penetrating the plate thickness occurs. 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 carried out three times, and the average value is determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0058] [Yield Ratio (YR)] Steel sheets having the above chemical composition and metallographic structure can achieve not only high tensile strength but also a high yield ratio, more specifically, a yield ratio of 80% or more. The yield ratio is preferably 82% or more, more preferably 85% or more. There is no particular upper limit, but the yield ratio may be, for example, 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 piece in a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling direction perpendicular to the steel sheet and conducting a tensile test in accordance with JIS Z 2241:2011. Yield ratio YR = 0.2% proof stress / tensile strength TS × 100

[0059] <Method for Manufacturing Steel Sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, particularly a steel sheet having preferred properties, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, although the following specifically describes the manufacture of a hot-rolled steel sheet, the steel sheet according to an embodiment of the present invention encompasses any steel sheet having the chemical composition and metallographic structure described above, i.e., not only a hot-rolled steel sheet but also a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description merely describes one example of a preferred manufacturing method when the steel sheet according to an embodiment of the present invention is a hot-rolled steel sheet.

[0060] A method for producing a steel plate according to an embodiment of the present invention includes: a heating step, which includes heating a slab having the chemical composition described above in relation to the steel plate, and holding the slab at a temperature of 1180 to 1320°C for 6000 seconds or more; a hot rolling step, which includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and which satisfies the following conditions (a) to (c): (a) the rolling temperature in each of the two rolling passes immediately preceding the last two rolling passes is 960 to 1080°C, and the reduction in each of the rolling passes is 30 to 40%, (b) the rolled material is cooled to 910°C or less at an average cooling rate of 400°C / second or more within 0.20 seconds after the two rolling passes immediately preceding the last two rolling passes, and (c) the reduction in each of the last two rolling passes is 20 to 30%. The method is characterized by including a cooling step in which the finish-rolled steel sheet is water-cooled to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, then air-cooled in the temperature range for 2.0 to 6.0 seconds, and water-cooled the steel sheet to 50°C or less within 13 seconds after air-cooling. In the above manufacturing method, the temperatures described for the slab and steel sheet refer to the surface temperatures of the slab and steel sheet, respectively. Each step will be described in detail below.

[0061] [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. However, slabs obtained by casting and blooming can also be used, and if necessary, slabs obtained by hot working or cold working may be used. In this manufacturing method, holding at a temperature range of 1180 to 1320°C includes not only holding the slab at a constant temperature within the range of 1180 to 1320°C, but also holding the slab at a temperature varying within the range of 1180 to 1320°C. Holding the slab at a temperature range of 1180 to 1320°C for 6000 seconds or more allows the coarse carbides present in the structure to be completely dissolved, eliminating the initiation points of cracks. If the holding temperature is less than 1180°C or the holding time is less than 6000 seconds, the coarse carbides will not be completely dissolved. If the solid solution of coarse carbides is incomplete, ferrite or bainite transformation originating from such carbides occurs during the cooling process described below, resulting in the area ratio of martensite being less than 60.0%, which may result in the failure to obtain the desired strength. The upper limit of the heating temperature of the slab is set to 1320°C or less from the viewpoint of the capacity and productivity of the heating equipment. The upper limit of the holding time in the temperature range of 1180 to 1320°C is preferably 10,000 seconds or less.

[0062] [Hot Rolling Step] [Rough Rolling] In the present manufacturing method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0063] [Finish Rolling] [(a) Rolling temperature in each rolling pass of the two rolling stages immediately preceding the last two stages: 960 to 1080°C, and reduction in each rolling pass of the two rolling stages immediately preceding the last two stages: 30 to 40%] The heated slab, or the slab that has also been rough-rolled as necessary, 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 in each rolling pass of the two rolling stages immediately preceding the last two stages. Specifically, the rolling temperature in each rolling pass of the two rolling stages immediately preceding the last two stages is controlled to 960 to 1080°C, and similarly, the reduction in each rolling pass of the two rolling stages immediately preceding the last two stages is controlled to 30 to 40%. By performing rolling under relatively high pressure and relatively high temperature conditions in the two rolling passes immediately preceding the last two rolling passes, it is possible to promote recrystallization and refine the austenite grains, which in turn makes it possible to reduce the average spacing of granular bainite to within a desired range in the final metal structure.

[0064] In contrast, if the rolling temperature in each of the two rolling passes immediately preceding the last two rolling passes is less than 960°C and / or the reduction ratio in each rolling pass is less than 30%, recrystallization is not sufficiently promoted, and the average spacing of granular bainite in the metallographic structure of the steel sheet obtained at the end cannot be reduced to within the desired range. On the other hand, if the rolling reduction ratio in each of the two rolling passes immediately preceding the last two rolling passes is more than 40%, flat austenite grains are formed due to the introduction of excessive strain, and similarly, the average spacing of granular bainite in the metallographic structure obtained at the end cannot be reduced to within the desired range. Furthermore, if the rolling temperature in each of the two rolling passes immediately preceding the last two rolling passes is more than 1080°C, the austenite grains become coarse, and the desired structure fraction cannot be obtained even by subsequent rolling and cooling control, or in addition, it becomes impossible to control the average spacing and / or average grain size of granular bainite within the desired range.

[0065] [(b) Cooling to 910°C or less at an average cooling rate of 400°C / sec or more within 0.20 seconds after the two rolling passes immediately preceding the last two rolling passes] In this manufacturing method, the rolled material is cooled to 910°C or less at an average cooling rate of 400°C / sec or more within 0.20 seconds after the two rolling passes immediately preceding the last two rolling passes. By cooling the rolled material to 910°C or less relatively quickly in this manner after the two rolling passes immediately preceding the last two rolling passes, grain growth after recrystallization can be suppressed, thereby making it possible to reduce the average spacing of granular bainite to within a desired range in the final metal structure. If the cooling time to 910°C or less after the two rolling passes immediately preceding the last two rolling passes exceeds 0.20 seconds, grain growth after recrystallization cannot be sufficiently suppressed, and even if appropriate cooling is performed in the subsequent cooling step, the average spacing and / or average grain size of granular bainite cannot be controlled within a desired range.

[0066] Furthermore, the average cooling rate between the last two rolling passes and the two immediately preceding rolling passes is very important for generating granular bainite having the desired morphology within a predetermined range. More specifically, if the average cooling rate during this period is less than 400°C / s, the maximum misorientation at 0.1 µm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more may exceed 3.5°, making it impossible to generate 10.0% or more of granular bainite with a maximum misorientation of 3.5° or less and an intragranular misorientation of 10° or more. The average cooling rate between the last two rolling passes and the two immediately preceding rolling passes is preferably 500°C / s or more. Similarly, if the cooling stop temperature is higher than 910°C, it may not be possible to generate 10.0% or more of granular bainite, in which the maximum misorientation at 0.1 μm intervals is 3.5° or less and the intragranular misorientation is 10° or more, within grains surrounded by grain boundaries with a misorientation of 15° or more.

[0067] [(c) Reduction in Each Rolling Pass of the Last Two Stages: 20 to 30%] In this manufacturing method, the reduction in each rolling pass of the last two stages of finish rolling is controlled to 20 to 30%. Introducing strain at such an appropriate reduction in each rolling pass of the last two stages makes it possible to increase the number of nucleation sites for generating granular bainite in the subsequent cooling process. If the reduction in each rolling pass of the last two stages is less than 20%, sufficient nucleation sites for generating granular bainite cannot be formed, making it impossible to obtain the desired area ratio of granular bainite in the final metallographic structure. On the other hand, if the reduction in each rolling pass of the last two stages exceeds 30%, flat austenite grains are formed due to the introduction of excessive strain, making it impossible to reduce the average spacing of granular bainite to within the desired range in the final metallographic structure.

[0068] [Cooling Step] [Cooling to a Temperature Range of 500 to 650°C Within 4.0 Seconds from the Start of Water Cooling, followed by Air Cooling for 2.0 to 6.0 Seconds] The finish-rolled steel sheet is then water-cooled in the next cooling step, cooled to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, and then air-cooled at this temperature range for 2.0 to 6.0 seconds. First, by cooling to a temperature range of 500 to 650°C within 4.0 seconds from the start of water cooling, it is possible to reliably suppress the formation of pearlite, and therefore it is possible to achieve the desired area fraction of the metal structure in the finally obtained steel sheet. In contrast, if the time from the start of water cooling to the temperature range of 500 to 650°C exceeds 4.0 seconds, a relatively large amount of pearlite is formed, making it impossible to obtain the desired amount of martensite and / or granular bainite in the metal structure of the finally obtained steel sheet.

[0069] Furthermore, air-cooling for 2.0 to 6.0 seconds in the temperature range of 500 to 650°C after water-cooling promotes the transformation to granular bainite and allows Ti precipitates to be properly precipitated. Therefore, air-cooling for 2.0 to 6.0 seconds in the temperature range of 500 to 650°C after water-cooling is extremely important not only for the suppression of necking due to granular bainite, but also for the improvement of hole expandability and other properties due to precipitation strengthening caused by Ti precipitates. For example, if the air-cooling temperature is less than 500°C, the transformation to granular bainite cannot be sufficiently promoted, and relatively large amounts of bainite may be formed. In such cases, not only is the suppression of necking reduced, but the formation of large amounts of bainite reduces uniform elongation, and further, the formation of bainite reduces the formation of martensite, which may result in insufficient strength.

[0070] Similarly, if the air-cooling temperature exceeds 650°C, the transformation to granular bainite may not be sufficiently promoted, while the ferrite transformation may be promoted, resulting in the formation of a relatively large amount of ferrite. In addition, Ti precipitates may not be sufficiently precipitated. In such cases, the necking suppression effect is reduced, and the hole expandability and yield ratio of the resulting steel sheet are reduced due to the relatively large amount of ferrite formed and the insufficient precipitation strengthening provided by Ti precipitates. Furthermore, if the air-cooling time is less than 2.0 seconds, the transformation to granular bainite may not be sufficiently promoted, and subsequent cooling may result in the formation of a relatively large amount of martensite. In such cases, the hole expandability and / or the necking suppression effect are 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 above, since granular bainite has characteristics similar to those of ferrite, when the total area ratio of granular bainite and ferrite in a metal structure mainly composed of martensite becomes relatively high, the metal structure becomes similar to that of so-called DP steel, resulting in 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.

[0071] [Water Cooling to 50°C or Less 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 less within 13 seconds. This rapid cooling allows martensite to be generated within the desired area ratio range. If the water cooling to 50°C or less takes more than 13 seconds or the cooling stop temperature is higher than 50°C, it may be impossible 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 less after air cooling may be 4 seconds or more or 5 seconds or more. The lower limit of the water cooling stop temperature is also not particularly limited, but for example, the water cooling stop temperature may be 20°C or more or 25°C or more. The water-cooled steel sheet can finally be coiled into a hot-rolled coil. The coiling conditions are not particularly limited, and the coiling can be performed under any appropriate temperature conditions.

[0072] According to the steel sheet manufactured by the above manufacturing method, by configuring the metal structure to a structure containing 60.0 to 85.0% martensite by area%, high strength, for example, tensile strength of 1180 MPa or more, can be achieved while significantly improving uniform elongation. Furthermore, by containing 10.0 to 30.0% by area% granular bainite in the metal structure, which has a maximum misorientation of 3.5° or less at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more and an intragranular misorientation of 10° or more, and by controlling the average spacing of the granular bainite grains to 50.0 μm or less, the yield ratio and hole expandability can be improved while significantly suppressing the occurrence of necking during forming. In addition, by controlling the Ti content in the steel to 0.070 mass% or more, Ti precipitates strengthen soft structures such as ferrite, thereby reducing the hardness difference between phases in the metal structure. The combination of this reduced hardness difference and the improvement in hole expandability due to the specific granular bainite makes it possible to more significantly improve the hole expandability. Therefore, the steel sheet manufactured by the above manufacturing method can suppress the occurrence of necking even when forming parts with complex shapes, and can reliably achieve a high level of the contradictory properties of high strength and excellent workability, making it particularly useful in the automotive field where both of these properties are required.

[0073] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0074] In the following examples, steel sheets according to the embodiments of the present invention, particularly hot-rolled steel sheets, were produced under various conditions, and the tensile strength (TS), yield ratio (YR), uniform elongation (u-El), hole expansion ratio (λ), and occurrence of necking in a bending test after pre-straining of the obtained steel sheets were investigated.

[0075] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions 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, and then hot-rolled. Hot-rolling was carried out by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all Examples and Comparative Examples, and finish rolling was carried out under the conditions shown in Table 3 using a tandem rolling mill consisting of five rolling stands. Next, the finish-rolled steel sheets were water-cooled, air-cooled, and water-cooled under the conditions shown in Table 3, and then coiled to obtain steel sheets having a thickness of 2.4 to 3.4 mm.

[0076]

[0077]

[0078]

[0079] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0080] [Tensile strength (TS) and uniform elongation (u-El)] The tensile strength (TS) and uniform elongation (u-El) were measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241:2011.

[0081] [Hole expansion ratio (λ)] The hole expansion ratio (λ) was determined as follows. First, a test piece 100 mm wide x 100 mm long was taken from the steel plate, and a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%) was used to create a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the burr was placed on the die side, and the initial hole was expanded with a conical punch having an apex angle of 60 ° 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 (%) of 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}

[0082] [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 piece in a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241:2011: Yield ratio YR = 0.2% proof stress / tensile strength TS × 100

[0083] [Presence or absence of necking in bending test after pre-strain] First, a tensile test piece taken from the steel plate, having a parallel portion width of 36 mm, a parallel portion length of 86 mm, a radius of 36 mm, a grip width of 50 mm, and a total length of 372 mm, was subjected to uniaxial tension in the C direction and a pre-strain of 10% was applied. Next, a test piece of 60 mm [C direction] x 30 mm [L direction] was taken from the center of the tensile test piece, and a 90° bending test was performed in the L direction to check for the occurrence of necking. The test piece was evaluated as pass if no necking was observed, and as fail if necking was observed.

[0084] Steel sheets with a tensile strength (TS) of 1180 MPa or more, a uniform elongation (u-El) of 5.0% or more, a hole expansion ratio (λ) of 40% or more, a yield ratio (YR) of 80% or more, and no necking observed in a bending test after pre-strain were evaluated as having high strength, high uniform elongation, hole expandability, and yield ratio, and being able to suppress necking during forming. The results are shown in Table 4. "GB grains" in Table 4 refer to granular bainite grains.

[0085]

[0086] With reference to Tables 1 to 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 last two passes in the hot rolling process was low. As a result, the average spacing of granular bainite grains in the final metallographic structure exceeded 50.0 μm, λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 5, it is believed that the rolling temperature in each of the two rolling passes immediately preceding the last two passes was high, resulting in coarsening of the austenite grains. As a result, the area fraction 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-strain. It is believed that in Comparative Example 6, recrystallization was not sufficiently promoted because the reduction rate in the second rolling pass of the two rolling passes immediately preceding the last two passes 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-strain. In Comparative Example 7, it is believed that the high reduction rate in the first rolling pass of the two rolling passes immediately preceding the last two rolling passes resulted in the formation of flat 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-strain. In Comparative Example 8, it is believed that the cooling time to 910°C or less after the two rolling passes immediately preceding the last two rolling passes was longer than 0.20 seconds, which failed to sufficiently suppress 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-strain. In Comparative Example 9, the average cooling rate between the last two rolling passes and the two immediately preceding rolling passes was slow, so the area fraction of granular bainite, which exhibits a predetermined orientation change, was less than 10.0%, and in connection with this, the area fraction of martensite was higher than 85.0%. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-strain.In Comparative Example 10, the cooling stop temperature in the cooling between the last two rolling passes and the two rolling passes immediately preceding them was high, so the area fraction of granular bainite, which also exhibits a predetermined orientation change, was less than 10.0%, and in connection with this, the area fraction of martensite was higher than 85.0%. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-straining.

[0087] In Comparative Examples 11 and 12, the reduction rates in the first and second rolling passes of the latter two stages were low, which is thought to have prevented the sufficient formation of nucleation sites for generating granular bainite. As a result, the area fraction of granular bainite was less than 10.0%, and the area fraction of martensite was higher than 85.0%, resulting in a decrease in u-El and λ and necking in the bending test after pre-strain. In Comparative Examples 13 and 14, the reduction rates in the first and second rolling passes of the latter two stages were high, which is thought to have resulted in the formation of flat austenite grains due to the introduction of excessive strain. As a result, the average spacing of granular bainite grains exceeded 50.0 μm, which reduced λ and caused necking in the bending test after pre-strain. 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 fraction of granular bainite was less than 10.0%, λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 16, the air-cooling temperature was low, so the transformation to granular bainite could not be sufficiently promoted, and as a result, 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-strain. In Comparative Example 17, the air-cooling temperature was high, so the transformation to granular bainite could not be sufficiently promoted, and as a result, a large amount of ferrite was formed. In addition, it is thought that Ti precipitates could not be sufficiently precipitated. As a result, λ and YR decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 18, the air-cooling time was short, so the transformation to granular bainite could not be sufficiently promoted, and further, a large amount of martensite was formed by subsequent cooling. As a result, u-El and λ decreased, and necking occurred in the bending test after pre-strain.In Comparative Example 19, the air-cooling time was long, resulting in the formation of a relatively large amount of granular bainite, which in turn resulted in the formation of less martensite and a relatively large total amount of granular bainite and ferrite. As a result, TS and YR decreased. In Comparative Example 20, the water-cooling time from air-cooling to 50°C or less was long, resulting in the area fraction of martensite being less than 60.0%, and TS decreased.

[0088] In Comparative Example 46, TS decreased due to the low C content. In Comparative Example 47, λ decreased due to the high C content. In Comparative Example 48, u-El decreased due to the low Si content. In Comparative Example 49, a large amount of ferrite was formed due to the high Si content, and the total amount of granular bainite and ferrite was also increased accordingly. As a result, λ and YR decreased. In Comparative Example 50, hardenability decreased due to the low Mn content, resulting in a low area fraction of martensite and a relatively high total amount of granular bainite and ferrite. As a result, TS and YR decreased. In Comparative Example 51, the area fraction of granular bainite was reduced due to the high Mn content, resulting in a low λ and necking in the bending test after pre-strain. In Comparative Example 52, a large amount of ferrite was formed due to the high sol. Al content, resulting in a high total amount of granular bainite and ferrite. As a result, λ and YR decreased. In Comparative Example 53, it is believed that coarse carbides and the like were formed due to the high Nb content. As a result, the workability of the steel sheet deteriorated, u-El and λ decreased, and necking occurred in the bending test after pre-strain. In Comparative Example 54, it is believed that the low Ti content prevented precipitation strengthening by Ti precipitates from fully working. As a result, TS and λ decreased. In Comparative Example 55, it is believed that the high Ti content caused coarse carbides and the like to be formed. As a result, the workability of the steel sheet deteriorated and λ decreased.

[0089] In contrast, all of the steel sheets according to the present invention had a predetermined chemical composition, and by appropriately controlling the manufacturing conditions, the steel sheets had a metal structure containing, by area%, 60.0 to 85.0% martensite, 10.0 to 30.0% granular bainite with a maximum misorientation of 3.5° or less at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more, and 20.0% or less ferrite, and the average spacing of the granular bainite grains was 50.0 μm or less. As a result, despite having a high tensile strength of 1180 MPa or more, the steel sheets had high uniform elongation, hole expandability, and yield ratio, and were able to reliably suppress the occurrence of necking even in bending tests after pre-straining.

Claims

1. The chemical composition, in mass%, is 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 to 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 to 0.0100%, and The balance is Fe and impurities. The metal structure is, in area%, Martensite: 60.0 to 85.0%, Granular bainite in which the maximum misorientation at 0.1 μm intervals within grains surrounded by grain boundaries with a misorientation of 15° or more is 3.5° or less and the intragranular misorientation is 10° or more: 10.0 to 30.0%; and Ferrite: containing 20.0% or less, A steel plate characterized in that the average spacing of 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 to 0.010%, and REM: 0.0001~0.0100% The steel sheet according to claim 1, characterized in that it contains at least one of the following:

3. The steel plate according to claim 1 or 2, wherein the metal structure further contains, by area percentage, at least one of bainite, pearlite, and retained austenite: a total of 20.0% or less.

4. 3. The steel plate according to claim 1, wherein the average grain size of the granular bainite grains is 5.0 to 30.0 μm.

5. A component, characterized in that it comprises a steel sheet according to claim 1 or 2.

6. A heating step comprising heating a slab having the chemical composition according to claim 1 or 2 and holding it at a temperature of 1180 to 1320°C for 6000 seconds or more; a hot rolling step which includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and which satisfies the following conditions (a) to (c); (a) the rolling temperature in each of the rolling passes in the two stages immediately preceding the last two stages is 960 to 1080°C, and the rolling reduction in each of the rolling passes is 30 to 40%; (b) cooling the rolled material to 910°C or less at an average cooling rate of 400°C / sec or more within 0.20 seconds after the rolling passes of the two stages immediately preceding the latter two stages; and (c) The reduction rate in each of the last two rolling passes is 20 to 30%. a cooling step comprising water-cooling the finish-rolled steel sheet to a temperature range of 500 to 650°C within 4.0 seconds from the start of water-cooling, then air-cooling in the temperature range for 2.0 to 6.0 seconds, and water-cooling the steel sheet to 50°C or less within 13 seconds after air-cooling; A method for manufacturing a steel plate, comprising: