Steel sheet and component comprising same

Abstract

The present invention provides: a steel sheet that has high strength, excellent ductility, excellent hole expandability, and excellent crack resistance; and a component that comprises the same. Provided is a steel sheet characterized by having a prescribed chemical composition and a metallographic structure which, in terms of area%, consists of 70-97% of bainite, 3-30% fresh martensite and tempered martensite combined, less than 5% of ferrite, and not more than 5% of the remaining structure, such that, in a surface layer region, a Taylor factor (Ml) in L-axis bending is not more than 3.50, a Taylor factor (Mc) in C-axis bending is not more than 3.50, and Ml / Mc is not less than 0.98 and such that, in an internal region other than the surface layer region, the average particle size of prior austenite grains is not more than 50 μm, and the average flatness of the prior austenite grains is not more than 6.5. Also provided is a component comprising the steel sheet.

Description

Steel sheet and component including the same

[0001] The present invention relates to a steel sheet and a component including the same.

[0002] In recent years, in the automobile industry, weight reduction of vehicle bodies has been demanded from the perspective of improving fuel efficiency. To achieve both weight reduction of vehicle bodies and collision safety, increasing the strength of steel sheets used is one effective method, and against this background, development of high-strength steel sheets has been promoted.

[0003] On the other hand, it is known that the formability of steel sheets decreases as strength increases. Therefore, even for high-strength steel sheets, excellent formability (for example, ductility, hole expandability, bendability, etc.) is required.

[0004] In relation to this, Patent Document 1 discloses that a steel sheet having a predetermined chemical composition has a steel structure whose main phase is martensite and bainite with a total area fraction of 80 to 100%, the total area fraction of martensite in bainite is 2 to 20%, among the martensite in bainite, the area fraction of martensite in which the orientation difference between the crystal orientation of the martensite and the crystal orientation of at least one piece of adjacent bainite is 15° or more is more than 50% relative to the total martensite, when a region surrounded by a boundary where the orientation difference between adjacent crystals is 15° or more is defined as a crystal grain, 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 is 2.0 or less. Patent Document 1 teaches that according to the above configuration, a high-strength hot-rolled steel sheet which is suitable as a material for automobile components and is excellent in ductility and bending-unbending properties can be provided.

[0005] Patent Document 2 discloses a high-strength hot-rolled steel sheet having a predetermined chemical composition, with a bainite phase comprising 85% or more area as the main phase, a martensite phase or martensite-austenite mixed phase comprising 15% or less area as the second phase, and the remainder being a ferrite phase, the average grain size of the second phase being 3.0 μm or less, the average aspect ratio of prior austenite grains being 1.3 or more and 5.0 or less, the area ratio of recrystallized prior austenite grains to unrecrystallized prior austenite grains being 15% or less, and precipitates with a diameter of less than 20 nm precipitated in the hot-rolled steel sheet comprising 0.10% or less by mass%. Patent Document 2 teaches that with the above configuration, a high-strength hot-rolled steel sheet can be obtained with a tensile strength TS of 980 MPa or more and excellent punchability and hole-expanding properties.

[0006] Patent Document 3 describes a metal having a predetermined chemical composition, in which, along the thickness direction, the metal structure at a depth of 1 / 4 of the plate thickness from the surface is 0-40% by area ratio of ferrite and bainite, 50-100% of martensite, and 0-10% of pearlite and retained austenite. In the metal structure of the soft surface region, which is the area from the surface to 20 μm along the thickness direction, the metal structure is 50% or more by area ratio of ferrite, and martensite, bainite, pearlite and A steel sheet is disclosed wherein the total of one or more types of retained austenite is 0 to 50%, the ratio of the Vickers hardness Hs of the soft surface portion to the Vickers hardness Hc at the 1 / 4 depth position, Hs / Hc, is 0.65 or less, the ratio of the Taylor factor M value ML of the L section to the Taylor factor M value MC of the C section, ML / MC, in the ferrite of the soft surface portion is 0.95 or more, and the aspect ratio of the island-like hard phase of the soft surface portion is 3.0 or less and the major axis is 5.0 μm or less. Patent Document 3 teaches that the above configuration makes it possible to provide a steel sheet with excellent bendability and low bending anisotropy.

[0007] Patent Document 4 discloses a high-strength steel sheet having a predetermined chemical composition, wherein the microstructure includes, in the surface region from the steel sheet surface to the 1 / 10 thickness position, upper bainite accounting for 80% or more by area ratio and fresh martensite and / or retained austenite accounting for 2% or more by total area ratio, upper bainite accounting for 70% or more by area ratio and fresh martensite and / or retained austenite accounting for 3% or more by total area ratio in the internal region from the 1 / 10 thickness position to the 3 / 10 thickness position, the average grain size in the surface region from the steel sheet surface to the 1 / 10 thickness position is 6 μm or less, and the difference (HV2-HV1) between the hardness of the surface region from the steel sheet surface to the 1 / 10 thickness position and the hardness of the internal region from the 1 / 10 thickness position to the 3 / 10 thickness position is 5% to 15% of [0.3 × tensile strength (MPa)]. Patent Document 4 teaches that, according to the above configuration, a high-strength steel sheet can be obtained that possesses a tensile strength of 980 MPa or more, press formability, and bendability.

[0008] Patent Document 5 describes a steel having a predetermined chemical composition, in a range of 1 / 8 to 3 / 8 thickness centered at a position 1 / 4 thickness from the surface, containing, by volume fraction, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%, total of pearlite and cementite: 0% to 10%, with the remainder being hard ferrite, and in the aforementioned range of 1 / 8 to 3 / 8 thickness, the proportion of retained austenite with an aspect ratio of 2.0 or more to the total retained austenite is 50% or more, and the region having a hardness of 80% or less of the hardness in the aforementioned range of 1 / 8 to 3 / 8 thickness is soft. A steel sheet is disclosed characterized in that, when defined as a layer, a soft layer with a thickness of 1 to 100 μm exists in the thickness direction from the surface, the volume fraction of crystal grains with an aspect ratio of less than 3.0 among the ferrite contained in the soft layer is 50% or more, the volume fraction of retained austenite in the soft layer is less than 50% of the volume fraction of retained austenite in the range of 1 / 8 thickness to 3 / 8 thickness, and when the emission intensity of the wavelength indicating Si is analyzed by high-frequency glow discharge analysis from the surface in the thickness direction, a peak of emission intensity of the wavelength indicating Si appears in the range of more than 0.2 μm and 5.0 μm or less from the surface. Patent Document 5 teaches that with the above configuration, a high-strength steel sheet can be provided that has excellent ductility and hole-expanding properties, excellent chemical conversion treatment properties and plating adhesion, and good bendability after processing.

[0009] International Publication No. 2022 / 244707, International Publication No. 2017 / 017933, International Publication No. 2024 / 128245, International Publication No. 2022 / 209839, International Publication No. 2019 / 187090

[0010] As mentioned above, it is generally known that the formability of steel sheets decreases as their strength increases. When ductility and hole-expandability decrease, it may become impossible to process them into the desired shape, for example, in automotive suspension parts. In addition, automotive parts require resistance to cracking under various impacts in the operating environment.

[0011] Therefore, the present invention aims to provide a steel plate and a component containing the same that is high in strength, has excellent ductility and hole-expanding properties, and has excellent crack resistance.

[0012] To achieve the above objective, the inventors focused on and investigated the microstructure of steel sheets, particularly hot-rolled steel sheets. Specifically, the inventors first found that by configuring the microstructure of a steel sheet having a predetermined chemical composition to be mainly composed of bainite, and containing fresh martensite, tempered martensite, and ferrite in specific proportions, it is possible to achieve high strength in the steel sheet, for example, a tensile strength of 940 MPa or higher, while also achieving the desired ductility. Next, the inventors found that by controlling the Taylor factor (Ml) in L-axis bending and the Taylor factor (Mc) in C-axis bending in the surface region, more specifically from the "surface" to "a position 200 μm from the surface in the thickness direction," bending can be improved, thereby improving crack resistance while maintaining high strength. In addition, the inventors have discovered that by controlling the average grain size and average flatness of prior austenite grains in the internal region, more specifically in regions other than the surface region, it is possible to achieve the desired hole expansion properties while maintaining high strength and significantly improve crack resistance, thus completing the present invention.

[0013] The present invention, which has achieved the above objectives, is as follows. (1) In mass%, C: 0.045-0.130%, Si: 0.30-1.50%, Mn: 1.20-2.60%, Al: 0.005-0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0-0.180%, Nb: 0-0.070%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-1.000%, B: 0-0.0100%, Ca: 0-0.0500%, Mg: 0-0.050% It has a chemical composition consisting of REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.100%, Sn: 0-0.100%, and the remainder: Fe and impurities, and in area percent, it consists of bainite: 70-97%, fresh martensite and tempered martensite: 3-30% in total, ferrite: less than 5%, and the remainder structure: 5% or less. A steel sheet characterized by having a microstructure in which, in the surface region, the Taylor factor (Ml) in L-axis bending is 3.50 or less, the Taylor factor (Mc) in C-axis bending is 3.50 or less, and Ml / Mc is 0.98 or more, and in the internal region other than the surface region, the average particle size of prior austenite grains is 50 μm or less, and the average flatness of the prior austenite grains is 6.5 or less.(2) The chemical composition is, in mass%, Ti: 0.001 to 0.180%, Nb: 0.001 to 0.070%, V: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001 to 2.000%, Mo: 0.001 to 3.000%, Ni: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0500%, Mg: 0.0001 to 0.050%, REM: 0.0001 to 0.1000%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, (1) The steel sheet according to (1) above, characterized in that it contains at least one of the following: Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.100%, and Sn: 0.001 to 0.100%. (3) The steel sheet according to (1) or (2) above, characterized in that in the surface region, the average particle size of the prior austenite grains is 30 μm or less, and the average flatness of the prior austenite grains is 4.5 or less. (4) The steel sheet according to any one of (1) to (3) above, characterized in that it has a tensile strength of 940 MPa or more. (5) A steel plate as described in any one of (1) to (4) above, characterized by having a plate thickness of 1.0 to 8.0 mm. (6) A component as described in any one of (1) to (5) above, characterized by including a steel plate.

[0014] According to the present invention, it is possible to provide a steel plate and a component containing the same that has high strength, excellent ductility and hole-expanding properties, and excellent crack resistance.

[0015] Figure 1 is a diagram illustrating the crack resistance test (Figure 1(A) before crack resistance test, Figure 1(B) after crack resistance test).

[0016] <Steel Plate> The steel plate according to the embodiment of the present invention has the following composition in mass%, C: 0.045 to 0.130%, Si: 0.30 to 1.50%, Mn: 1.20 to 2.60%, Al: 0.005 to 0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0 to 0.180%, Nb: 0 to 0.070%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0 to 2.000%, Mo: 0 to 3.000%, Ni: 0 to 1.000%, B: 0 to 0.0100%, Ca: 0 to 0.0500%. It has a chemical composition consisting of Mg: 0-0.050%, REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.100%, Sn: 0-0.100%, and the remainder being Fe and impurities. In area percent, it consists of bainite: 70-97%, fresh martensite and tempered martensite: 3-30% in total, ferrite: less than 5%, and the remainder being 5% or less. The metal structure is characterized by having a surface region in which the Taylor factor (Ml) in L-axis bending is 3.50 or less, the Taylor factor (Mc) in C-axis bending is 3.50 or less, and Ml / Mc is 0.98 or more, and in the internal region other than the surface region, the average particle size of the prior austenite grains is 50 μm or less, and the average flatness of the prior austenite grains is 6.5 or less.

[0017] As mentioned earlier, it is generally known that properties such as ductility and hole-expandability decrease with increasing strength. Furthermore, in automotive parts and the like, in addition to high strength, resistance to cracking during deformation under various impacts in the operating environment is required. Therefore, the inventors investigated not only the appropriate chemical composition of the steel sheet, but also the metallic structure of the steel sheet in particular. To explain in more detail, the inventors first found that by configuring the metallic structure of a steel sheet with an optimized chemical composition to be mainly bainite, and containing fresh martensite, tempered martensite, and ferrite in specific proportions, more specifically, by configuring it so that in area percent, bainite: 70-97%, fresh martensite and tempered martensite: 3-30% in total, ferrite: less than 5%, and the remaining structure: 5% or less, it is possible to achieve the desired high strength, more specifically a tensile strength of 940 MPa or more, while also achieving the desired ductility.

[0018] Next, the inventors focused on the microstructure of the surface region in order to improve crack resistance while maintaining high strength. The inventors investigated controlling the Taylor factor (Ml) in L-axis bending to 3.50 or less and the Taylor factor (Mc) in C-axis bending to 3.50 or less in the surface region, more specifically, the region from the "surface" to "a position 200 μm from the surface in the plate thickness direction". Here, in the direction of a small Taylor factor, slip deformation is easy, and uniform slip deformation can be completed during bending without the assistance of deformation by shear bands. On the other hand, in the direction of a large Taylor factor, the stress required for uniform slip deformation during bending increases significantly, so non-uniform local deformation, so-called shear band formation, is easier than uniform slip deformation, and irregularities may occur. In this study, the inventors found that by controlling Ml and Mc to 3.50 or less, bendability is improved, and as a result, crack resistance is improved. In addition, the inventors have found that, as will be explained in detail later in relation to the manufacturing method, by controlling the shape ratio difference (X4-X1), which is the difference between the shape ratio X1 of the first stage and the shape ratio X4 of the fourth stage, to less than 1.20 to 3.00 in the hot rolling process, an appropriate strain gradient can be introduced into the surface region of the steel sheet, thereby controlling the Ml / Mc ratio in the surface region to 0.98 or higher. Here, the strain gradient corresponds to the dislocation density gradient, and these dislocations become nucleation sites for recrystallization. Therefore, by controlling the shape ratio difference (X1-X4) to less than 1.20 to 3.00, an appropriate dislocation density gradient is introduced, and by making these dislocations nucleation sites for recrystallization, the nucleation of recrystallization in the surface region becomes uniform, thereby controlling the Ml / Mc ratio to 0.98 or higher. Thus, by controlling Ml and Mc to be 3.50 or less in the surface region, and by controlling Ml / Mc to be 0.98 or more, that is, by controlling Ml and Mc to be roughly the same, the in-plane anisotropy in the surface region is reduced, further improving the flexibility, and thereby further improving crack resistance while maintaining high strength.

[0019] Furthermore, the inventors focused on the microstructure of the internal region to improve hole-expanding properties and crack resistance while maintaining high strength. The inventors found that by controlling the internal region, more specifically the region other than the surface region, so that the average particle size of the prior austenite grains is 50 μm or less and the average flatness of the prior austenite grains is 6.5 or less, the desired hole-expanding properties can be achieved and crack resistance can be significantly improved. Although not intended to be bound by any particular theory, it is thought that by controlling the average particle size of the prior austenite grains to 50 μm or less, cracks caused by deformation such as bending are reduced compared to the case where the average particle size of the prior austenite grains is large, and therefore crack propagation is suppressed, thereby achieving the desired hole-expanding properties and improving crack resistance while maintaining high strength. Furthermore, by controlling the average flattening of the prior austenite grains to 6.5 or less, the prior austenite grains were not excessively flattened. As a result, strain concentration in a specific direction was suppressed, which is thought to have enabled the achievement of the desired hole expansion properties and improved crack resistance while maintaining high strength.

[0020] As described above, the steel sheet according to the embodiment of the present invention achieves high strength of 940 MPa or more, while also achieving excellent ductility and hole-expanding properties, as well as excellent crack resistance. Therefore, the steel sheet according to the embodiment of the present invention has high strength, excellent ductility, hole-expanding properties, and crack resistance, making it particularly useful in applications in the automotive sector where these properties are required.

[0021] The steel sheets according to embodiments of the present invention will be described in more detail below. In the following description, "%", which is the unit for the content of each element, means "mass%" unless otherwise specified. In this specification, "~", which indicates a numerical range, is used to mean that the numbers written before and after it are included as the lower limit and upper limit, respectively, unless otherwise specified.

[0022] [C: 0.045-0.130%] Carbon (C) is an element that increases strength inexpensively and is an important element for controlling the strength of steel. To obtain this effect sufficiently, the C content should be 0.045% or more. The C content may be 0.050% or more, 0.055% or more, or 0.060% or more. On the other hand, excessive C content may lead to a decrease in ductility. For this reason, the C content should be 0.130% or less. The C content may be 0.120% or less, 0.110% or less, 0.100% or less, 0.090% or less, 0.080% or less, or 0.070% or less.

[0023] [Si: 0.30-1.50%] Si is an effective element for increasing strength as a solid solution strengthening element. To obtain this effect sufficiently, the Si content should be 0.30% or more. The Si content may be 0.40% or more, 0.50% or more, 0.60% or more, or 0.70% or more. On the other hand, excessive Si content may lead to a decrease in ductility along with an increase in steel strength. For this reason, the Si content should be 1.50% or less. The Si content may be 1.40% or less, 1.30% or less, 1.20% or less, 1.10% or less, 1.00% or less, 0.90% or less, or 0.80% or less.

[0024] [Mn: 1.20-2.60%] Mn is an element that enhances the hardenability of steel and is effective in increasing its strength. To obtain these effects fully, the Mn content should be 1.20% or more. The Mn content may be 1.40% or more, 1.60% or more, 1.80%, or 2.00% or less. On the other hand, excessive Mn content may lead to a decrease in ductility along with an increase in steel strength. For this reason, the Mn content should be 2.60% or less. The Mn content may be 2.50% or less, 2.40% or less, 2.30% or less, 2.20% or less, or 2.10% or less.

[0025] [Al: 0.005-0.400%] Al is an element that acts as a deoxidizing agent for steel and has the effect of making steel sound. To obtain this effect sufficiently, the Al content should be 0.005% or more. The Al content may be 0.010% or more or 0.020% or more. On the other hand, if Al is included in excess, coarse Al oxide may be generated, which may reduce the ductility of the steel sheet. For this reason, the Al content should be 0.400% or less. The Al content may be 0.200% or less, 0.100% or less, 0.080% or less, 0.060% or less or 0.050% or less.

[0026] [P: 0.080% or less] P is an element that segregates at grain boundaries and promotes steel embrittlement. A lower P content is preferable, so ideally it should be 0%. However, excessive reduction of the P content can lead to a significant increase in cost. For this reason, the P content may be 0.0001% or more, or 0.001% or more, or 0.002% or more. On the other hand, excessive P content can lead to steel embrittlement due to grain boundary segregation, as described above. Therefore, the P content should be 0.080% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0027] [S: 0.0100% or less] S is an element that generates nonmetallic inclusions such as MnS in steel, leading to a decrease in the ductility of the steel. A lower S content is preferable, so ideally it should be 0%. However, excessive reduction of the S content can lead to a significant increase in cost. For this reason, the S content may be 0.0001% or more, or 0.0005% or more, or 0.0010% or more. On the other hand, excessive S content can lead to cracking during cold forming, starting from nonmetallic inclusions. Therefore, the S content should be 0.0100% or less. The S content may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0028] [N: 0.0150% or less] N is an element that forms coarse nitrides in steel sheets, reducing the workability of the steel sheet. A lower N content is preferable, so ideally it should be 0%. However, excessive reduction of the N content can lead to a significant increase in manufacturing costs. For this reason, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content can form coarse nitrides as described above, reducing the workability of the steel sheet. Therefore, the N content should be 0.0150% or less. The N content may also be 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0029] [O: 0.0100% or less] O is an element that, when mixed in during the manufacturing process, forms coarse inclusions and reduces the workability of the steel sheet. A lower O content is preferable, so ideally it should be 0%. However, excessive reduction of the O content can lead to a significant increase in manufacturing costs. For this reason, the O content may be 0.0001% or more, or 0.0005% or more, or 0.0010% or more. On the other hand, if the O content is excessive, as mentioned above, it may form coarse inclusions and reduce the workability of the steel sheet. Therefore, the O content should be 0.0100% or less. The O content may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0030] 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, if necessary, contain at least one of the following optional elements in place of a portion of the remaining Fe.

[0031] [Ti: 0-0.180%] Ti has the effect of improving the strength of steel sheets by forming carbonitrides in the steel and strengthening through precipitation. The Ti content may be 0%, but in order to obtain such an effect, the Ti content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Ti content is excessive, the effect will saturate, and including more Ti in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Ti content should be 0.180% or less. The Ti content may be 0.160% or less, 0.140% or less, 0.120% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0032] [Nb: 0-0.070%] Nb has the effect of improving the strength of steel sheets by forming carbonitrides in the steel and strengthening through precipitation. The Nb content may be 0%, but in order to obtain such an effect, the Nb content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Nb content is excessive, the effect will saturate, and including more Nb in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Nb content should be 0.070% or less. The Nb content may be 0.060% or less, 0.050% or less, 0.040% or less, 0.030% or less, or 0.020% or less.

[0033] [V: 0-1.000%] V has the effect of improving the strength of steel sheets by forming carbonitrides in the steel and strengthening through precipitation. The V content may be 0%, but in order to obtain such an effect, the V content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the V content is excessive, the effect will saturate, and including more V in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the V content should be 1.000% or less. The V content may be 0.500% or less, 0.250% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0034] [Cu: 0-1.000%] Cu is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Cu content is excessive, the effect will saturate, and including more Cu in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Cu content should be 1.000% or less. The Cu content may be 0.500% or less, 0.250% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0035] [Cr: 0-2.000%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength. The Cr content may be 0%, but to obtain such effects, it is preferable that the Cr content be 0.001% or more. The Cr content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Cr content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, the Cr content should be 2.000% or less. The Cr content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.250% or less, 0.100% or less, or 0.050% or less.

[0036] [Mo: 0-3.000%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. The Mo content may be 0%, but to obtain such an effect, the Mo content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if the Mo content is excessive, the effect will saturate, and including more Mo in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Mo content should be 3.000% or less. The Mo content may be 1.500% or less, 1.000% or less, 0.750% or less, 0.500% or less, 0.250% or less, or 0.100% or less, or 0.050% or less.

[0037] [Ni: 0-1.000%] Ni is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if Ni is included in excess, the effect will saturate, and including more Ni in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Ni content should be 1.000% or less. The Ni content may be 0.500% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.040% or less.

[0038] [B: 0-0.0100%] B improves low-temperature toughness by segregating at grain boundaries and increasing grain boundary strength. The B content may be 0%, but to obtain this effect, it is preferable that the B content be 0.0001% or more. The B content may be 0.0002% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, the B content should be 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.

[0039] [Ca: 0-0.0500%] Ca is an element that can control the morphology of nonmetallic inclusions. The Ca content may be 0%, but to obtain such an effect, it is preferable that the Ca content be 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, if Ca is included in excess, the effect will saturate, and including more Ca in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Ca content should be 0.0500% or less. The Ca content may be 0.0200% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.

[0040] [Mg: 0-0.050%] Mg is an element that can control the morphology of nonmetallic inclusions. The Mg content may be 0%, but to obtain such an effect, the Mg content is preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. On the other hand, if the Mg content is excessive, the effect will saturate, and including more Mg in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the Mg content should be 0.050% or less. The Mg content may also be 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0041] [REM: 0-0.1000%] REM is an element that can control the morphology of nonmetallic inclusions. The REM content may be 0%, but to obtain such an effect, the REM content is preferably 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, if REM is included in excess, the effect will saturate, and including more REM in the steel sheet than necessary will lead to an increase in manufacturing costs. Therefore, the REM content should be 0.1000% or less. The REM content may also be 0.0500% or less, 0.0200% or less, 0.0100% or less, 0.0075% or less, or 0.0050% or less. In this specification, REM refers to the collective term for 17 elements, including scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content is the total content of these elements.

[0042] [Bi: 0-0.100%] Bi is an element effective in improving corrosion resistance. The Bi content may be 0%, but to obtain such an effect, it is preferable that the Bi content be 0.001% or more. The Bi content may also be 0.005% or more, or 0.010% or more. On the other hand, if Bi is included in excess, the effect will saturate, and including more Bi in the steel plate than necessary will lead to an increase in manufacturing costs. Therefore, the Bi content should be 0.100% or less. The Bi content may also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0043] [Ta: 0 to 0.100%] Ta is an element effective for controlling the morphology of carbides and increasing strength. The Ta content may be 0%, but to obtain these effects, the Ta content is preferably 0.001% or more. The Ta content may be 0.005% or more, or 0.010% or more. On the other hand, even if Ta is contained excessively, the effect saturates, and containing unnecessary Ta in the steel sheet leads to an increase in manufacturing cost. Therefore, the Ta content is set to 0.100% or less. The Ta content may be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0044] [Zr: 0 to 0.500%] Zr is an element capable of controlling the morphology of non-metallic inclusions. The Zr content may be 0%, but to obtain such effects, the Zr content is preferably 0.001% or more, and may be 0.005% or more or 0.010% or more. On the other hand, even if Zr is contained excessively, the effect saturates, and containing unnecessary Zr in the steel sheet leads to an increase in manufacturing cost. Therefore, the Zr content is set to 0.500% or less. The Zr content may be 0.200% or less, 0.100% or less, 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0045] [Co: 0 to 3.000%] Co is an element effective for increasing the strength of steel sheets. The Co content may be 0%, but to sufficiently obtain such effects, the Co content is preferably 0.001% or more. The Co content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, even if Co is contained excessively, the effect saturates, and containing unnecessary Co in steel leads to an increase in manufacturing cost. Therefore, the Co content is set to 3.000% or less. The Co content may be 1.500% or less, 1.000% or less, 0.750% or less, 0.500% or less, 0.250% or less, 0.100% or less, or 0.050% or less.

[0046] [Zn: 0 to 0.200%] Zn is an element effective for controlling the morphology of sulfides and improving local ductility and stretch-flange formability. The Zn content may be 0%, but in order to sufficiently obtain such effects, the Zn content is preferably 0.001% or more. The Zn content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Zn content increases inclusions and may cause defects on the surface and inside of the steel sheet. Therefore, the Zn content is set to 0.200% or less. The Zn content may be 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0047] [W: 0 to 0.200%] W is an element that enhances the hardenability of steel and contributes to improving strength. The W content may be 0%, but in order to obtain such effects, the W content is preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, even if W is contained excessively, the effect becomes saturated, and containing more W in steel than necessary leads to an increase in manufacturing cost. Therefore, the W content is set to 0.200% or less. The W content may be 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0048] [Sb: 0 to 0.500%] Sb is an element effective for improving corrosion resistance. The Sb content may be 0%, but in order to obtain such effects, the Sb content is preferably 0.001% or more, and may be 0.005% or more, 0.010% or more, 0.015% or more, or 0.020% or more. On the other hand, even if Sb is contained excessively, the effect becomes saturated, and containing more Sb in the steel sheet than necessary leads to an increase in manufacturing cost. Therefore, the Sb content is set to 0.500% or less. The Sb content may be 0.200% or less, 0.100% or less, 0.050% or less, or 0.030% or less.

[0049] [As: 0-0.100%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain such an effect, it is preferable that the As content be 0.001% or more. The As content may be 0.002% or more or 0.005% or more. On the other hand, if the As content is excessive, the effect will saturate, and including more As than necessary in the steel plate will lead to an increase in manufacturing costs. Therefore, the As content should be 0.100% or less. The As content may be 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0050] [Sn: 0-0.100%] Sn is an element effective in improving corrosion resistance. The Sn content may be 0%, but to obtain such an effect, it is preferable that the Sn content be 0.001% or more. The Sn content may also be 0.002% or more, or 0.005% or more. On the other hand, if the Sn content is excessive, the effect will saturate, and including more Sn in the steel than necessary will lead to an increase in manufacturing costs. Therefore, the Sn content should be 0.100% or less. The Sn content may also be 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less.

[0051] In the steel sheet according to the embodiment of the present invention, the remainder other than the above-mentioned elements consists of Fe and impurities. Impurities are components that are mixed in during the industrial manufacture of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap, and components that are included in a range that does not affect the effects of the present invention.

[0052] The chemical composition of the steel sheet according to the embodiment of the present invention can be measured by general analytical methods. For example, the chemical composition of the steel sheet can be measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0053] [Metal structure] [Bainite: 70-97%] The metal structure of the steel sheet according to the embodiment of the present invention contains bainite: 70-97% by area %. By configuring the metal structure of the steel sheet with a structure mainly composed of bainite in this way, it is possible to increase the strength while increasing the ductility of the steel sheet. From the viewpoint of improving ductility, a higher area ratio of bainite is preferable, and may be 75% or more, 80% or more, or 85% or more. On the other hand, if the area ratio of bainite becomes too high, the total area ratio of fresh martensite and tempered martensite will decrease, and as a result, it may not be possible to achieve the desired strength, for example, a tensile strength of 940 MPa or more. Therefore, the area ratio of bainite is set to 97% or less, and may be, for example, 95% or less, 90% or less, or 88% or less.

[0054] [Fresh martensite and tempered martensite: 3-30% in total] The microstructure of the steel sheet according to the embodiment of the present invention contains, in area percent, fresh martensite and tempered martensite: 3-30% in total. From the viewpoint of improving strength, a higher total area ratio of fresh martensite and tempered martensite is preferable, for example, 5% or more, 10% or more, or 15% or more. On the other hand, from the viewpoint of improving ductility, a lower total area ratio of fresh martensite and tempered martensite is preferable, for example, 25% or less, 22% or less, or 20% or less.

[0055] [Ferrite: Less than 5%] The metal structure of the steel sheet according to the embodiment of the present invention includes less than 5% ferrite by area percentage. If the area percentage of ferrite becomes too high, the total area percentage of bainite, fresh martensite, and tempered martensite will decrease, and as a result, it may not be possible to achieve the desired strength, for example, a tensile strength of 940 MPa or more. Therefore, the area percentage of ferrite is less than 5%, and may be, for example, 3% or less or 2% or less. Alternatively, the area percentage of ferrite may be 0%. On the other hand, from the viewpoint of improving ductility, the area percentage of ferrite may be 0.5% or more or 1% or more.

[0056] [Residual structure: 5% or less] The metallography of the steel sheet according to the embodiment of the present invention includes residual structure: 5% or less in area %. As described above, the metallography of the steel sheet according to the embodiment of the present invention includes bainite, fresh martensite, tempered martensite, and ferrite, and may also include other residual structures, but the area ratio of the residual structure is preferably small, and may be 0%. The area ratio of the residual structure is not particularly limited, but may be, for example, 0 to 5%, 0 to 3%, 0 to 2%, or 0 to 1%. In other words, the total area ratio of bainite, fresh martensite, tempered martensite, and ferrite may be, for example, 95 to 100%, 97 to 100%, 98 to 100%, or 99 to 100%. The area ratio of the residual structure may be 0.5% or more, 1% or more, or 2% or more. If residual structure is present, it may include at least one of pearlite and retained austenite.

[0057] [Identification of Metallographic Structure and Calculation of Area Ratio] [Bainite, Fresh Martensite, Tempered Martensite, Pearlite, and Retained Austenite] The identification of bainite, fresh martensite, tempered martensite, pearlite, and retained austenite, and the calculation of their area ratios, are performed by optical microscopy observation, scanning electron microscopy observation, and X-ray diffraction after etching with Nital reagent or Repera solution. First, a sample is taken from a steel plate so that the thickness cross section perpendicular to the plate surface becomes the observation surface. It is preferable that the thickness cross section is parallel to the rolling direction, but if the rolling direction of the steel plate cannot be determined, the thickness cross section does not necessarily have to be parallel to the rolling direction. Next, the observation surface of the sample is etched with Nital. Then, using an optical microscope, the area ratio of pearlite is calculated by performing image analysis on a rectangular region obtained in the thickness direction, centered at 1 / 4 of the plate thickness from the steel plate surface, with a length of 200 μm and a length of 600 μm perpendicular to the thickness direction. Image analysis is performed on eight or more fields of view, and the area percentage is determined from the average value in each image analysis. Here, a structure in which plate-like ferrite and Fe-based carbides are layered is considered to be pearlite. Subsequently, at the same observation position, only the corroded layer is removed by polishing, the surface is mirror-finished, and then corroded with Repeller solution. After that, the structure is observed using a scanning electron microscope, and the resulting microstructure is analyzed to obtain the total area percentage of fresh martensite and retained austenite, as well as the area percentage of tempered martensite. Here, areas with high brightness and no carbides are judged to be fresh martensite and retained austenite, and areas with high brightness and carbides are judged to be tempered martensite. Since martensite (fresh martensite and tempered martensite) is not sufficiently corroded by Repeller corrosion, it can be distinguished from other corroded structures as described above. However, since retained austenite, like martensite, is not sufficiently corroded, the area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite, which is determined by the X-ray diffraction method described below, from the total area ratio of fresh martensite and retained austenite.

[0058] Using a sample that has been surface-machined from the direction normal to the rolling surface up to 1 / 4 of the plate thickness, the volume fraction of retained austenite is calculated by X-ray diffraction measurement. The volume fraction of retained austenite is defined as the area fraction of retained austenite. Finally, the area fraction of bainite is calculated by subtracting the area fractions of pearlite, fresh martensite, tempered martensite, retained austenite, and ferrite (described later) from 100%.

[0059] [Ferrite] The identification and area fraction of ferrite are performed by electron backscatter diffraction (EBD) as follows. Specifically, first, a sample is taken from the steel plate so that the thickness cross section perpendicular to the plate surface becomes the observation surface. Although it is preferable that the thickness cross section is parallel to the rolling direction, it is not necessary for the thickness cross section to be parallel to the rolling direction if the rolling direction of the steel plate cannot be determined. Next, EBSD analysis is performed at measurement intervals of 0.2 μm on a rectangular region centered at 1 / 4 of the plate thickness from the steel plate surface, with a length of 200 μm in the thickness direction and a length of 600 μm perpendicular to the thickness direction, to obtain crystal orientation information for this rectangular region. EBSD analysis is performed on 8 or more fields of view, and the area fraction of ferrite is determined from the average value in each analysis. EBSD analysis is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points / second. Next, the grain average misorientation (GAM value) is calculated from the crystal orientation information of this rectangular region using the software "OIM Analysis®" included with the EBSD analysis system. Finally, regions with a GAM value of 0.6° or less are identified as ferrite, and their area fraction is calculated. Here, the "GAM value" is the average of the grain averages between adjacent pixels in a region surrounded by grain boundaries with a grain average of 15° or more.

[0060] [In the surface region, Taylor factor (Ml) for L-axis bending: 3.50 or less] [In the surface region, Taylor factor (Mc) for C-axis bending: 3.50 or less] In the surface region of the steel sheet according to the embodiment of the present invention, more specifically, in the region from the "surface" to "a position 200 μm from the surface in the thickness direction", the Taylor factor (Ml) for L-axis bending is 3.50 or less, and the Taylor factor (Mc) for C-axis bending is also 3.50 or less. The Taylor factor is a dimensionless index calculated from the amount of slip deformation required to geometrically satisfy predetermined strain conditions, based on the measured crystal orientation in the crystalline texture and assuming a slip system specific to the crystal structure. In orientations with a small Taylor factor, slip deformation is easy, and uniform slip deformation can be completed during bending without the assistance of deformation by shear bands. On the other hand, in directions with a large Taylor factor, the stress required for uniform sliding deformation during bending increases significantly, making it easier for non-uniform local deformation, so-called shear bands, to form rather than uniform sliding deformation, which can cause unevenness. As explained earlier, our current research has shown that controlling Ml and Mc to 3.50 or less improves bendability and, consequently, crack resistance. From the viewpoint of improving crack resistance, smaller values ​​for Ml and Mc are preferable, and Ml and Mc may be 3.45 or less or 3.40 or less, respectively. On the other hand, the lower limits for Ml and Mc are not particularly limited, but for example, they may be 0.50 or more, 1.00 or more, 1.50 or more, 2.00 or more, 2.50 or more, or 3.00 or more, respectively.

[0061] [In the surface region, Ml / Mc: 0.98 or higher] In the surface region of the steel sheet according to the embodiment of the present invention, more specifically, in the region from the "surface" to "a position 200 μm from the surface in the thickness direction", the ratio of Ml to Mc, Ml / Mc, is 0.98 or higher. By controlling Ml / Mc to be 0.98 or higher, the in-plane anisotropy of the surface region is reduced, the bendability is further improved, and thereby the crack resistance can be further improved. From the viewpoint of improving crack resistance, it is preferable that Ml and Mc are of similar value, and Ml / Mc may be 0.99 or higher or 1.00 or higher, and / or 1.05 or lower, 1.04 or lower, or 1.03 or lower.

[0062] [Measurement of M values ​​(Ml and Mc) in the surface region] The M values ​​(Ml and Mc) in the surface region are measured by EBSD as follows. First, an EBSD analysis is performed at intervals of 5 μm (pitch) in the thickness direction and the rolling direction or the direction perpendicular to the rolling direction in a region (L section) with a depth of 200 μm in the thickness direction, i.e., perpendicular to the steel plate surface, and a length of 1000 μm in the rolling direction, and in a region (C section) with a depth of 200 μm in the thickness direction and a length of 1000 μm perpendicular to the rolling direction, respectively, to obtain crystal orientation information of only the α phase in this rectangular region. If the rolling direction of the steel plate is unknown, cross sections oriented at 0°, 45°, 90°, and 135° with respect to any direction and perpendicular to the surface of the steel plate are observed, and the cross section in which the average major axis of the prior austenite grains was longest is identified as the cross section parallel to the rolling direction. For the crystal orientation data of only the α phase in the field of view, calculations are performed using the "Taylor factor" calculation mode with the "OIM Analysis®" software included with the EBSD analyzer. In this case, only the {110}<111> slip system is considered. Furthermore, the strain tensor represented by Equation 1 is used as the strain tensor for L-axis bending, and the strain tensor represented by Equation 2 is used as the strain tensor for C-axis bending. Here, the strain tensors are RD, TD, and ND from top to bottom. Let Ml be the M value obtained by providing the strain tensor for L-axis bending, and let Mc be the M value obtained by providing the strain tensor for C-axis bending. Using the obtained Ml and Mc, their ratio (Ml / Mc) is calculated to determine Ml / Mc.

[0063] [Average particle size of prior austenite grains in the internal region: 50 μm or less] In the internal region of the steel sheet according to the embodiment of the present invention, more specifically in the region other than the surface region, the average particle size of prior austenite grains is 50 μm or less. By controlling the average particle size of prior austenite grains to be 50 μm or less, cracks that occur due to deformation such as bending are reduced compared to the case where the average particle size of prior austenite grains is large, and therefore crack propagation is suppressed, thereby improving hole-expanding properties and crack resistance. From the viewpoint of improving hole-expanding properties and crack resistance, it is preferable that the average particle size of prior austenite grains in the internal region be as small as possible, and the average particle size may be 45 μm or less, 40 μm or less, or 35 μm or less. The lower limit of the average particle size is not particularly limited, but for example it may be 3 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more.

[0064] [Average flatness of prior austenite grains in the internal region: 6.5 or less] In the internal region of the steel sheet according to the embodiment of the present invention, more specifically in the region other than the surface region, the average flatness of prior austenite grains is 6.5 or less. By controlling the average flatness of prior austenite grains to be 6.5 or less, the prior austenite grains do not become excessively flattened, and the concentration of strain in a specific direction is suppressed, thereby improving hole-expanding properties and crack resistance. From the viewpoint of improving hole-expanding properties and crack resistance, it is preferable that the average flatness of prior austenite grains in the internal region be small, and the average flatness may be 6.4 or less, 6.2 or less, or 6.0 or less. The lower limit of the average flatness is not particularly limited, but for example, it may be 1.0 or more, 1.5 or more, or 2.0 or more.

[0065] [Calculation of average grain size and average flatness of prior austenite grains in the internal region] The average grain size and average flatness of prior austenite grains in the internal region are calculated using a scanning electron microscope (SEM) as follows. Specifically, first, a sample is taken from the steel plate so that the thickness cross section perpendicular to the plate surface becomes the observation surface. The thickness cross section is parallel to the rolling direction. If the rolling direction of the steel plate is unknown, cross sections oriented at 0°, 45°, 90°, and 135° with respect to any direction and perpendicular to the surface of the steel plate are observed, and the cross section in which the average major axis of the prior austenite grains is longest is identified as the cross section parallel to the rolling direction. After mirror polishing the observation surface of the obtained sample, the microstructure of the thickness cross section is revealed using a saturated picric acid aqueous solution and a sodium dodecylbenzenesulfonate etching solution. Next, the particle size of the prior austenite grains is measured from micrographs taken at least three locations in a rectangular region extending 200 μm in the thickness direction and 600 μm perpendicular to the thickness direction, centered at a 500x magnification SEM observation field. The equivalent diameter of one prior austenite grain included in each observation field is calculated. Excluding prior austenite grains whose entirety is not included in the imaging field, such as those at the edges of the imaging field, the above procedure is performed on all prior austenite grains included in each observation field to determine the equivalent diameter of all prior austenite grains in each imaging field. The average particle size of the prior austenite grains is determined by calculating the average of the equivalent diameters of the prior austenite grains obtained in each imaging field. If prior austenite grains with an equivalent diameter of less than 2 μm are included, these are excluded from the above measurement. In addition, the long axis and short axis of at least 20 prior austenite grains with an equivalent diameter of 2 μm or more included in each imaging field are measured. The average long axis and short axis of each prior austenite grain are obtained by calculating the average of the long axis and short axis values ​​obtained from measurements of each prior austenite grain. The average flatness of the prior austenite grain is determined by calculating the ratio of these (average long axis / average short axis).

[0066] [Average particle size of prior austenite grains in the surface region: 30 μm or less] In the surface region of the steel sheet according to a preferred embodiment of the present invention, more specifically, in the region from the "surface" to "a position 200 μm from the surface in the thickness direction", the average particle size of prior austenite grains may be 30 μm or less. By controlling the average particle size of prior austenite grains in the surface region to 30 μm or less, cracks caused by deformation such as bending are reduced, crack propagation is suppressed, and crack resistance can be further improved. From the viewpoint of further improving crack resistance, it is preferable that the average particle size of prior austenite grains in the surface region be as small as possible, and the average particle size may be 28 μm or less or 25 μm or less. The lower limit of the average particle size is not particularly limited, but for example, it may be 1 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more.

[0067] [Average flatness of prior austenite grains in the surface region: 4.5 or less] In the surface region of the steel sheet according to a preferred embodiment of the present invention, more specifically, in the region from the "surface" to "a position 200 μm from the surface in the thickness direction", the average flatness of prior austenite grains may be 4.5 or less. By controlling the average flatness of prior austenite grains in the surface region to be 4.5 or less, the prior austenite grains in the surface region do not become excessively flattened, strain concentration in a specific direction is suppressed, and crack resistance can be further improved. From the viewpoint of further improving crack resistance, it is preferable that the average flatness of prior austenite grains in the surface region be as small as possible, and the average flatness may be 4.4 or less, 4.2 or less, or 4.0 or less. The lower limit of the average flatness is not particularly limited, but for example, it may be 1.0 or more, 1.5 or more, or 2.0 or more.

[0068] [Calculation of average grain size and average flatness of prior austenite grains in the surface region] The average grain size and average flatness of prior austenite grains in the surface region are calculated in the same manner as in "[Measurement of average grain size and average flatness of prior austenite grains in the internal region]", except that the measurement area is defined as a rectangular region from the "surface" of the steel plate to a position 200 μm from the surface in the thickness direction, with a rectangular region of 600 μm perpendicular to the thickness direction.

[0069] [Plate Thickness] The steel plate according to the embodiment of the present invention is not particularly limited, but generally has a plate thickness of 1.0 to 8.0 mm. For example, the plate 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, 5.0 mm or less, or 4.0 mm or less.

[0070] As described above, the steel sheet according to the embodiment of the present invention has high strength, yet possesses excellent ductility and hole-expandability, as well as excellent crack resistance. Therefore, it has high strength, ductility, hole-expandability, and crack resistance at a high level. Accordingly, the steel sheet according to the embodiment of the present invention is particularly useful for use in parts in technical fields where these properties are required. In a preferred embodiment, an automobile part, particularly an automobile undercarriage part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automobile undercarriage parts include lower arms and trailing arms. These automobile parts, particularly automobile undercarriage parts, only need to include the steel sheet according to the embodiment of the present invention in at least a portion of these parts, and therefore at least a portion of these parts will satisfy the chemical composition and metallic structure characteristics described above. In parts of the steel sheet that do not come into direct contact with the mold during forming such as press forming, and where the degree of processing is relatively low, the metallic structure characteristics do not change particularly before and after forming.

[0071] [Mechanical Properties] [Tensile Strength (TS)] According to the steel sheet having the above chemical composition and metal structure, high tensile strength, specifically 940 MPa or more, can be achieved. The tensile strength is preferably 980 MPa or more, 1000 MPa or more, or 1050 MPa or more. According to the steel sheet according to the embodiment of the present invention, despite having such very high tensile strength, excellent crack resistance can be achieved in addition to excellent ductility and hole-expanding properties, through a specific combination of the chemical composition and metal structure described above. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel sheet may be 1500 MPa or less, 1400 MPa or less, 1300 MPa or less, or 1180 MPa or less. The tensile strength is measured by taking a JIS No. 5 test piece from a direction (C direction) where the longitudinal direction of the test piece is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel plate cannot be determined, a JIS No. 5 test specimen may be taken from any direction on the surface of the steel plate. If it is difficult to take a JIS No. 5 test specimen from the sample to be measured, a tensile test can be performed on a small test specimen, and the tensile strength can be determined by converting it to the equivalent value for a JIS No. 5 test specimen. For example, an ASTM E8 Sub-size (6 mm wide) can be used as a small test specimen. Needless to say, the test specimen should be taken from a part that is not affected (or is less affected) by work hardening or heat and strain during cutting.

[0072] [Ductility: Total Elongation (EL)] According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, it has excellent ductility, and more specifically, it can achieve a total elongation of 8% or more. The total elongation may preferably be 10% or more. There is no particular upper limit, but for example, the total elongation may be 30% or less or 25% or less. The total elongation is measured by taking a JIS No. 5 test specimen from a direction (C direction) where the longitudinal direction of the test specimen is preferably parallel to the direction perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be determined, a JIS No. 5 test specimen may be taken from any direction within the surface of the steel sheet.

[0073] [Hole Expansion Properties: Hole Expansion Ratio (λ)] According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, it has excellent hole expansion properties, and more specifically, a hole expansion ratio of 30% or more can be achieved. The hole expansion ratio may preferably be 35% or more, and more preferably 40% or more. There is no particular upper limit to the hole expansion ratio, but for example, the hole expansion ratio may be 100% or less or 80% or less. The hole expansion ratio is determined as follows. First, a test piece with a width of 100 mm and a length of 100 mm 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 widened using a conical punch with a 60° apex angle until a crack penetrates the plate thickness. The hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) for each test piece is calculated using the following formula. This hole expansion test is performed three times, and the average value is determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0074] <Method for Manufacturing Steel Sheets> Next, preferred methods for manufacturing steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below. More specifically, the following describes the manufacturing of hot-rolled steel sheets, but the steel sheets according to embodiments of the present invention include any steel sheets having the chemical composition and metal structure described above, i.e., not only hot-rolled steel sheets, but also cold-rolled steel sheets, plated steel sheets, etc. Therefore, the following description merely describes preferred manufacturing methods when the steel sheets according to embodiments of the present invention are hot-rolled steel sheets.

[0075] A method for manufacturing a steel sheet according to an embodiment of the present invention includes heating a slab having the chemical composition described above in relation to a steel sheet, and then finishing rolling the slab using a tandem rolling mill consisting of five or more rolling stands, wherein the hot rolling step satisfies the following conditions (a) to (c): (a) the rolling temperature of the first stage of the tandem rolling mill is 1030°C or higher; (b) in the tandem rolling mill, the shape ratio difference (X4-X1), which is the difference between the shape ratio X1 of the first stage and the shape ratio X4 of the fourth stage, is 1.20 to less than 3.00. (c) The tandem rolling mill is characterized in that, in the fifth stage and beyond, the total reduction ratio in the temperature range of rolling temperatures of 980°C or less is 60% or less, and the cooling process includes accelerating the cooling of the finish-rolled steel sheet to an intermediate air-cooling temperature of 520 to 690°C at an average cooling rate of 60°C / second or more, performing intermediate air-cooling at the intermediate air-cooling temperature for 1.0 to 10.0 seconds, and then cooling to 100°C or less.

[0076] [Hot Rolling Process] [Slab Heating] First, a slab having the chemical composition described above in relation to the steel plate is heated. From the viewpoint of productivity, the slab to be used is preferably cast by the continuous casting method, but it may also be manufactured by the ingot casting method or the thin slab casting method. The slab to be used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. For this reason, it is necessary to heat the slab before subjecting it to hot rolling to solid dissolve the alloying elements in the slab. If the heating temperature is low, the alloying elements will not sufficiently solid dissolve in the slab, leaving coarse alloy carbides, which may cause brittle cracking during hot rolling. For this reason, the heating temperature is preferably 1100°C or higher, and more preferably 1200°C or higher. The upper limit of the heating temperature is not particularly limited, but from the viewpoint of the capacity of the heating equipment and productivity, it is preferably 1300°C or lower. Also, from the viewpoint of solid dissolving the alloying elements in the slab, it is preferable to hold the temperature in the 1100 to 1300°C range for 1000 seconds or more. There is no particular upper limit to the holding time, but from the viewpoint of productivity, it is preferable to keep it at 4000 seconds or less.

[0077] [Rough Rolling] In this manufacturing method, for example, rough rolling may be performed on a heated slab before finish rolling to adjust the plate thickness. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions are secured.

[0078] [(a) Rolling temperature of the first stage of the tandem rolling mill: 1030°C or higher] The heated slab, or the slab that has been roughly rolled in addition as necessary, is subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of four or more rolling stands, more specifically five to eight rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, or the slab that has been roughly rolled in addition as necessary, the rolling temperature of the first stage of the tandem rolling mill is controlled to 1030°C or higher. By setting the rolling temperature of the first stage of the tandem rolling mill to 1030°C or higher, the texture of the surface region is appropriately controlled, and the Ml and Mc of the surface region in the final steel sheet can be controlled to 3.50 or lower. Furthermore, by controlling the first-stage rolling temperature to a higher level, recrystallization is further promoted, and the average particle size of prior austenite grains in the surface region of the final steel sheet can be controlled to 30 μm or less, and the average flatness can be controlled to 4.5 or less. From this viewpoint, it is preferable that the first-stage rolling temperature be 1050°C or higher. On the other hand, there is no particular upper limit to the first-stage rolling temperature, but it may be, for example, 1200°C or lower or 1150°C or lower.

[0079] [(b) In a tandem rolling mill, the shape ratio difference (X4-X1), which is the difference between the shape ratio X1 of the first stage and the shape ratio X4 of the fourth stage: 1.20 to less than 3.00] In this manufacturing method, in a tandem rolling mill, the shape ratio difference (X4-X1), which is the difference between the shape ratio X1 of the first stage and the shape ratio X4 of the fourth stage, is controlled to be between 1.20 and less than 3.00. Controlling this shape ratio difference (X4-X1) is important because it allows control of Ml / Mc in the surface region, thereby improving crack resistance. Here, the shape ratio (Xn) of the nth stage is expressed by the following formula, and means the projected contact arc length of the nth stage (√{R × (hin-hout) / 2}) divided by the average plate thickness of the nth stage ((hin + 2hout) / 3). The difference in shape ratio (X4 - X1) is calculated from the difference between the shape ratio of the first stage (X1) and the shape ratio of the fourth stage (X4). Xn = 3 × √{R × (hin - hout) / 2} / (hin + 2 hout) where, R: Roll diameter of the nth stage rolling mill (mm) hin: Thickness of the entry side plate of the nth stage (mm) hout: Thickness of the exit side plate of the nth stage (mm)

[0080] In this manufacturing method, the shape ratio values ​​for the second and third stages are between the shape ratio values ​​for the first and fourth stages, and it is preferable that the shape ratio values ​​increase sequentially from the first to the fourth stage. Therefore, if the shape ratio difference (X4 - X1) is less than 1.20, the dislocation density gradient introduced into the surface region of the steel sheet becomes gentle, and the dislocation density distribution in the surface region becomes coarse. As a result, the nucleation of recrystallization becomes non-uniform, and the Ml / Mc in the surface region of the final steel sheet may be less than 0.98. Therefore, the shape ratio difference (X4 - X1) should be 1.2 or greater. On the other hand, if the shape ratio difference (X4 - X1) is 3.00 or greater, the dislocation density distribution in the surface region becomes excessively dense, the ferrite transformation from the dislocations proceeds excessively, and the ferrite area ratio in the final steel sheet may be 5% or more. Therefore, the shape ratio difference (X4 - X1) should be less than 3.00.

[0081] [(c) Total reduction ratio in the temperature range below 980°C in the 5th stage and beyond of the tandem rolling mill: 60% or less] In this manufacturing method, the total reduction ratio in the temperature range below 980°C is controlled to 60% or less in the 5th stage and beyond of the tandem rolling mill. In the 5th stage and beyond of the tandem rolling mill, the finish rolling in the temperature range below 980°C is finish rolling in the non-recrystallized temperature range, and by controlling the total reduction ratio to 60% or less, the average particle size and average flatness of the prior austenite grains in the internal region can be appropriately controlled. If the total reduction ratio in the temperature range below 980°C exceeds 60% in the 5th stage and beyond of the tandem rolling mill, the total reduction ratio above 980°C becomes relatively low, and as a result, refinement by recrystallization does not proceed sufficiently, and the average particle size of the prior austenite grains in the internal region of the final steel sheet may exceed 50 μm. Furthermore, if the total reduction ratio exceeds 60% in the temperature range below 980°C, the austenite grains become excessively flattened, and the average flatness of the prior austenite grains in the interior region of the final steel sheet may exceed 6.5. Therefore, in the fifth stage and beyond of the tandem rolling mill, the total reduction ratio in the temperature range below 980°C should be 60% or less. On the other hand, the lower limit of the total reduction ratio in the temperature range below 980°C is not particularly limited and may be, for example, 15% or more, 20% or more, or 25% or more. Here, "total reduction ratio (%)" is the reduction ratio defined by the following formula, where t0 is the entry plate thickness in the rolling mill that first reaches 980°C or below in the fifth stage and beyond of the tandem rolling mill, and t1 is the exit plate thickness in the final stage of the tandem rolling mill. Total reduction ratio (%) = 100 × {1 - (t1 / t0)} t0: Inlet plate thickness (mm) at the 5th stage and beyond of the tandem rolling mill where the temperature first drops below 980°C t1: Outlet plate thickness (mm) at the final stage of the tandem rolling mill

[0082] [Cooling Process] [Average Cooling Rate to Intermediate Air Cooling Temperature: 60°C / sec or higher] The finish-rolled steel sheet is accelerated cooling to an intermediate air cooling temperature of 520-690°C at an average cooling rate of 60°C / sec or higher. This accelerated cooling makes it possible to achieve the desired metal microstructure fraction. If the average cooling rate is less than 60°C / sec, ferrite transformation will proceed in the temperature range from the cooling start temperature to the intermediate air cooling temperature, which may result in excessive ferrite transformation, and the area ratio of ferrite in the final steel sheet may be 5% or more, and / or the area ratio of bainite may be less than 70%. Therefore, the average cooling rate to the intermediate air cooling temperature should be 60°C / sec or higher. On the other hand, there is no particular upper limit to the average cooling rate to the intermediate air cooling temperature, but it may be, for example, 150°C / sec or less, or 100°C / sec or less.

[0083] [Intermediate air cooling temperature: 520-690°C] [Intermediate air cooling time: 1.0-10.0 seconds] Next, the steel sheet is intermediate air cooled at an intermediate air cooling temperature of 520-690°C for 1.0-10.0 seconds. Intermediate air cooling at an intermediate air cooling temperature of 520-690°C makes it possible to achieve the desired metallographic fraction in the metallographic structure of the final steel sheet. If the intermediate air cooling temperature is below 520°C, bainite will not be sufficiently formed, and the total area ratio of fresh martensite and tempered martensite in the final steel sheet may be 30% or more, and / or the area ratio of bainite may be less than 70%. Therefore, the intermediate air cooling temperature should be 520°C or higher. On the other hand, if the intermediate air cooling temperature exceeds 690°C, ferrite transformation is promoted, bainite will not be sufficiently formed, and the area ratio of ferrite in the final steel sheet may be 5% or more, and / or the area ratio of bainite may be less than 70%. Therefore, the intermediate air cooling temperature should be 690°C or lower.

[0084] If the intermediate cooling time is less than 1.0 second, sufficient bainite may not form, and the total area ratio of fresh martensite and tempered martensite in the final steel sheet may exceed 30%, and / or the area ratio of bainite may fall below 70%. Therefore, the intermediate cooling time should be 1.0 second or longer. On the other hand, if the intermediate cooling time exceeds 10.0 seconds, the area ratio of martensite in the final steel sheet may fall below 3%. Therefore, the intermediate cooling time should be 10.0 seconds or less.

[0085] According to the steel sheet manufactured by the above manufacturing method, by configuring the metal structure of the steel sheet having an optimized chemical composition to include, in area percent, bainite: 70-97%, fresh martensite and tempered martensite: 3-30% in total, ferrite: less than 5%, and the remaining structure: 5% or less, it is possible to achieve high strength, for example, a tensile strength of 940 MPa or more, while also achieving the desired ductility. Furthermore, in the surface region from the "surface" to "a position 200 μm from the surface in the thickness direction of the sheet," by controlling the Taylor factor (Ml) in L-axis bending to 3.50 or less, controlling the Taylor factor (Mc) in C-axis bending to 3.50 or less, and controlling Ml / Mc to 0.98 or more, it is possible to improve crack resistance while maintaining high strength. In addition, in the internal region other than the surface region, by controlling the average particle size of prior austenite grains to 50 μm or less and the average flatness of the prior austenite grains to 6.5 or less, it is possible to achieve the desired hole expansion properties and significantly improve crack resistance while maintaining high strength. Therefore, steel sheets manufactured by the above manufacturing method are high in strength and have excellent ductility, puncture-expandability, and crack resistance, making them particularly useful in applications in the automotive sector where these properties are required.

[0086] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0087] In the following embodiments, steel sheets according to the present invention, particularly hot-rolled steel sheets, were manufactured under various conditions, and the tensile strength (TS), total elongation (EL), hole expansion ratio (λ), and crack resistance of the obtained steel sheets were investigated.

[0088] First, molten steel was cast using a continuous casting method to form slabs with various chemical compositions as shown in Table 1. These slabs were heated to 1100-1300°C and held for 1000-2000 seconds, after which hot rolling was performed. Hot rolling was carried out by finishing rolling using a tandem rolling mill consisting of seven rolling stands. More specifically, the finishing rolling in stages 1-4 (stages 1-4 of the tandem rolling mill) was carried out under the conditions shown in Table 2, and the finishing rolling in stages 5-7 (stage 5 and beyond of the tandem rolling mill) was carried out under the conditions shown in Table 2. Next, the finished-rolled steel plates were accelerated cooling to the intermediate air-cooling temperature shown in Table 2 at the average cooling rate shown in Table 2, and then intermediate air-cooled under the conditions shown in Table 2. Finally, the intermediate-air-cooled steel plates were cooled to below 100°C to obtain steel plates with a thickness of 2.9 mm.

[0089]

[0090]

[0091] The properties of the obtained steel plates were measured and evaluated by the following method.

[0092] [Tensile Strength (TS) and Total Elongation (EL)] Tensile strength (TS) and total elongation (EL) were measured by taking a JIS No. 5 test specimen with a length of 200 mm and a thickness of 2.5 mm from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2022. More specifically, the test was performed at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test specimen, allowing strain to be introduced until fracture occurred.

[0093] [Hole Expansion Ratio (λ)] The hole expansion ratio (λ) was determined as follows. First, a test piece measuring 100 mm in width and 100 mm in length was taken from the steel plate, and a punched hole (initial hole: hole diameter d0 = 10 mm) was created using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole was expanded using a conical punch with a 60° apex angle until a crack penetrating the plate thickness occurred, and the hole diameter d1 mm at the time of crack occurrence was measured. The hole expansion ratio λ (%) for each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0094] [Crack Resistance] Crack resistance was determined as follows. Figure 1 is a schematic diagram illustrating the crack resistance test, with Figure 1(A) showing the material before the crack resistance test and Figure 1(B) showing the material after the crack resistance test. First, a 120 mm x 200 mm test piece 10 was taken from the steel plate with the longitudinal side parallel to the direction perpendicular to the rolling direction (direction C), and a burring section 10a, characteristic of undercarriage parts, was processed onto the test piece 10. The burring section 10a of the test piece 10 was processed so that the internal bending radius was 1.0 mm or 1.5 mm. Next, a ball joint 20 was press-fitted into the burring section 10a. In the crack resistance test (drop weight test), a load P was applied to the load-bearing portion 20a of the ball joint 20 from a direction parallel to the longitudinal direction of the test piece 10, and the test piece 10 was bent near the burring portion 10a (near the location 10b where cracks may occur) as shown in Figure 1(B). In the crack resistance test (drop weight test), the input energy was 1 kJ, the crushing amount was 50 mm, and the test temperature was -40°C. After the test, the presence or absence of cracks in the burring portion 10a (location 10b where cracks may occur) was checked, and the crack resistance was evaluated according to the following criteria. AA: No cracks occurred in the burring portion at both the bending radius 1 mm and 1.5 mm. A: Cracks occurred in the burring portion at the bending radius 1 mm, but no cracks occurred at the bending radius 1.5 mm. B: Cracks occurred in the burring portion at both the bending radius 1 mm and 1.5 mm.

[0095] Steel plates with a tensile strength (TS) of 940 MPa or higher, a total elongation (EL) of 8% or higher, a hole expansion ratio (λ) of 30% or higher, and crack resistance of AA or A were evaluated as having high strength, excellent ductility and hole expansion properties, and excellent crack resistance. The evaluation results are shown in Table 2.

[0096] Referring to Tables 1 and 2, it is thought that in Comparative Example 2, the low carbon content resulted in insufficient martensite hardness. As a result, the TS decreased. On the other hand, in Comparative Example 3, the high carbon content led to excessive martensite formation, resulting in a high total area ratio of fresh martensite and tempered martensite in the final steel sheet. As a result, the EL decreased, and the crack resistance decreased.

[0097] In Comparative Example 4, the low Si content resulted in insufficient solid solution strengthening, leading to a decrease in TS. On the other hand, in Comparative Example 5, the high Si content resulted in excessively high solid solution strengthening, leading to an excessive increase in strength. As a result, EL decreased, and crack resistance declined.

[0098] In Comparative Example 6, the low Mn content resulted in insufficient martensite formation, leading to a lower total area ratio of fresh and tempered martensite in the final steel sheet. As a result, the TS decreased. On the other hand, in Comparative Example 7, the high Mn content resulted in excessive martensite formation, leading to a higher total area ratio of fresh and tempered martensite in the final steel sheet. As a result, the EL decreased, and the crack resistance decreased.

[0099] In Comparative Example 8, the low rolling temperature in the first stage of the hot rolling process prevented proper control of the surface texture, resulting in high Ml and Mc in the surface region of the final steel sheet. Consequently, the crack resistance decreased.

[0100] In Comparative Example 10, during the hot rolling process, the difference in shape ratio (X4-X1), which is the difference between the shape ratio X1 of the first stage and the shape ratio X4 of the fourth stage, was small. As a result, the dislocation density gradient introduced into the surface region of the steel sheet was gentle, the dislocation density distribution in the surface region became coarse, and the nucleation of recrystallization became non-uniform, which is thought to have resulted in a lower Ml / Mc ratio in the surface region of the final steel sheet. Consequently, the crack resistance decreased. On the other hand, in Comparative Example 11, during the hot rolling process, the difference in shape ratio (X4-X1) was large, which is thought to have resulted in an excessively dense dislocation density distribution in the surface region. As a result, the ferrite transformation from these dislocations progressed excessively, and the area ratio of ferrite in the final steel sheet increased. Consequently, the TS decreased.

[0101] In Comparative Example 12, during the hot rolling process, the total reduction ratio in the temperature range below 980°C was high from the 5th stage onward. As a result, the total reduction ratio above 980°C was relatively low, which prevented sufficient recrystallization and is thought to have led to a larger average grain size of prior austenite grains in the interior region of the final steel sheet. Furthermore, the high total reduction ratio in the temperature range below 980°C caused the austenite grains to become excessively flattened, resulting in a higher average flatness of prior austenite grains in the interior region of the final steel sheet. Consequently, λ decreased, and crack resistance declined.

[0102] In Comparative Example 13, the intermediate air cooling temperature was low, resulting in insufficient bainite formation. This likely led to a higher total area ratio of fresh martensite and tempered martensite in the final steel sheet, and a lower area ratio of bainite. As a result, the EL decreased, and crack resistance declined. On the other hand, in Comparative Example 14, the intermediate air cooling temperature was high, which accelerated ferrite transformation. This resulted in insufficient bainite formation, and a higher area ratio of ferrite in the final steel sheet. Furthermore, the insufficient bainite formation likely resulted in a higher total area ratio of fresh martensite and tempered martensite in the final steel sheet. As a result, the TS decreased, the EL decreased, and crack resistance declined.

[0103] In Comparative Example 15, the intermediate air cooling time was too short, resulting in insufficient bainite formation. This likely led to a higher total area ratio of fresh martensite and tempered martensite in the final steel sheet, and a lower area ratio of bainite. As a result, the EL decreased, and the crack resistance declined. On the other hand, in Comparative Example 16, the intermediate air cooling time was too long, which likely resulted in a lower total area ratio of fresh martensite and tempered martensite in the final steel sheet. As a result, the TS decreased.

[0104] In Comparative Example 17, the average cooling rate up to the intermediate air cooling temperature was slow, which likely caused excessive ferrite transformation during cooling. As a result, the area ratio of ferrite in the final steel sheet was high, while the area ratio of bainite was low. Consequently, the TS (Total Score) decreased.

[0105] In contrast, in all the examples of the invention, the steel sheets had a predetermined chemical composition, and by appropriately controlling each condition in the manufacturing method, it was possible to obtain a steel sheet in which the metal structure consisted of, by area percent, bainite: 70-97%, fresh martensite and tempered martensite: 3-30% in total, ferrite: less than 5%, and the remainder structure: 5% or less, with Ml and Mc in the surface region being 3.50 or less, Ml / Mc being 0.98, and in the internal region the average particle size of prior austenite grains being 50 μm or less, and the average flatness of prior austenite grains being 6.5 or less. As a result, the steel sheets had high strength with a tensile strength of 940 MPa or more, excellent ductility and hole-expanding properties, and excellent crack resistance.

[0106] In particular, in Examples 1 and 18-37, the first-stage rolling temperature was controlled to 1050°C or higher during the hot rolling process. This allowed for control to satisfy the following conditions in addition to the above: the average particle size of prior austenite grains in the surface region was 30 μm or less, and the average flatness of prior austenite grains was 4.5 or less. As a result, the crack resistance was AA, indicating superior crack resistance.

[0107] In all of the examples of the invention, the remaining microstructure other than ferrite, fresh martensite, tempered martensite, and bainite consisted of at least one of pearlite and retained austenite.

[0108] 10 Test specimen 10a Burring section 10b Area where cracks may occur 20 Ball joint 20a Load-bearing section

Claims

In mass percent, C: 0.045-0.130%, Si: 0.30-1.50%, Mn: 1.20-2.60%, Al: 0.005-0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0150% or less, O: 0.0100% or less, Ti: 0 to 0.180%, Nb: 0 to 0.070%, V: 0-1.000%, Cu: 0 to 1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0 to 1.000%, B: 0 to 0.0100%, Ca: 0-0.0500%, Mg: 0 to 0.050%, REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0 to 0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0 to 0.500%, As: 0 to 0.100%, Sn: 0-0.100%, and The remainder has a chemical composition consisting of Fe and impurities. In area percentage, Baynite: 70-97%, Fresh martensite and tempered martensite: 3-30% in total. Ferrite: less than 5%, and Remaining tissue: consists of 5% or less. In the surface region, the Taylor factor (Ml) for L-axis bending is 3.50 or less, the Taylor factor (Mc) for C-axis bending is 3.50 or less, and Ml / Mc is 0.98 or more. A steel sheet characterized by having a metallic structure in the internal region other than the surface region, in which the average particle size of the prior austenite grains is 50 μm or less, and the average flatness of the prior austenite grains is 6.5 or less. 　 The aforementioned chemical composition, in mass%, Ti: 0.001 to 0.180%, Nb: 0.001-0.070%, V: 0.001-1.000%, Cu: 0.001 to 1.000%, Cr: 0.001-2.000%, Mo: 0.001-3.000%, Ni: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Ca: 0.0001-0.0500%, Mg: 0.0001-0.050%, REM: 0.0001-0.1000%, Bi: 0.001-0.100%, Ta: 0.001 to 0.100%, Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001-0.200%, W: 0.001-0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.100%, and Sn: 0.001-0.100% The steel plate according to claim 1, characterized in that it includes at least one of the following. 　 The steel sheet according to claim 1 or 2, characterized in that, in the surface region, the average particle size of the prior austenite grains is 30 μm or less, and the average flatness of the prior austenite grains is 4.5 or less. A steel plate according to any one of claims 1 to 3, characterized by having a tensile strength of 940 MPa or more. A steel plate according to any one of claims 1 to 4, characterized in that it has a plate thickness of 1.0 to 8.0 mm. 　 A component characterized by comprising a steel plate as described in any one of claims 1 to 5.

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