Steel sheet, component including same, and method for manufacturing steel sheet

Abstract

Provided is a steel sheet characterized by having a prescribed chemical composition and in that: the metal structure thereof contains, in terms of area%, 10-40% of ferrite, 60-90% of martensite, 0-10% of bainite, and a total of 0-5% of at least one among pearlite and retained austenite; and when a 150 μm × 150 μm region at a position at 1 / 4 of the sheet thickness in a cross section perpendicular to the sheet surface is equally divided into nine regions, the number density of ferrite in each of the divided regions is calculated, and the average value thereof is indicated by Nαm, the difference in the number density of ferrite between divided regions adjacent to each other in the sheet thickness direction is Nαm × 0.60 or less in all cases. Also provided are a component including the steel sheet, and a method for manufacturing the steel sheet.

Description

Steel plate, part including same, and method of manufacturing steel plate

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

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

[0003] In relation to improvements in strength and workability, for example, Patent Document 1 describes a high-strength cold-rolled steel sheet coil made of a dual-phase steel sheet mainly composed of ferrite and martensite, which has a structural form in which, in terms of space factor relative to the entire structure, ferrite is 10 to 40% and martensite is 60 to 90%. When the ferrite fractions (shown as "area %) at the four corners and the center of gravity of an 800 mm x 800 mm steel sheet cut out from any position on the coil are Vα1, Vα2, Vα3, Vα4, and Vα5, respectively, and the average value of these five points is Vαm, all of the ferrite fractions Vα1, Vα2, Vα3, Vα4, and Vα5 are within the range of Vαm ± 5 (area %). Furthermore, Patent Document 1 teaches that by strictly specifying the conditions for heat treatment of the base steel sheet (cold-rolled steel sheet), a high-strength cold-rolled steel sheet coil with a small variation in the ferrite fraction within the coil can be obtained, thereby realizing a high-strength cold-rolled steel sheet coil with a small variation in strength within the coil and a tensile strength of 980 MPa or more, and that such a high-strength cold-rolled steel sheet coil can be stably formed into a steel sheet for automobiles, and specifically discloses in the examples that in addition to improving tensile strength, total elongation and hole expandability are also improved.

[0004] JP 2010-159453 A

[0005] As described above, it is known that the workability of steel decreases with increasing strength, resulting in a decrease in properties such as hole expandability, as described in Patent Document 1. Decreased hole expandability may make it impossible to process the steel into the desired shape, for example, in automobile suspension parts. Therefore, in the development of high-strength steel sheets, it is important to achieve high strength while ensuring certain properties appropriate for the application, such as uniform elongation in addition to the hole expandability. For example, in automobile suspension parts with complex shapes, such as lower arms and trailing arms, the workability of the steel itself may decrease as the strength is increased, or work hardening may occur during forming into the complex shape, resulting in strain to near the maximum load after part formation. Therefore, there remains a high demand for materials that can maintain a high and stable load during a collision, even when strength is increased to the same level or higher than conventional materials, and therefore do not or are less likely to cause fractures accompanied by a decrease in load.

[0006] Furthermore, for components that require impact resistance, plastic deformation occurs when they receive an impact that exceeds their yield strength. Therefore, from the perspective of ensuring automobile collision safety, it is necessary to improve not only tensile strength but also yield strength, and therefore it is necessary to increase the yield ratio, which is the ratio of yield strength to tensile strength.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel sheet having a novel configuration that has high strength, high uniform elongation, hole expandability, and yield ratio, and that can suppress load reduction during a collision, as well as a part including the same, and a method for manufacturing the steel sheet.

[0008] In order to achieve the above object, the present inventors have conducted research focusing on the metallographic structure of steel sheets, particularly hot-rolled steel sheets. As a result, the present inventors have found that by forming the metallographic structure of a hot-rolled steel sheet having a predetermined chemical composition mainly from ferrite and martensite and utilizing precipitation strengthening by adding Ti, it is possible to improve strength, uniform elongation, and yield ratio, and reduce the hardness difference between ferrite and martensite to improve hole expandability. Furthermore, by uniformly distributing ferrite in the thickness direction, more specifically, by equally dividing a 150 μm × 150 μm region at a quarter-thickness position of a cross section perpendicular to the plate surface of the steel sheet into nine, calculating the number density of ferrite in each divided region, and averaging the calculated number density of ferrite in each divided region in the thickness direction, the difference in the number density of ferrite in each divided region is Nαm × 0.60 or less. This finding has led to the completion of the present invention.

[0009] The present invention has achieved the above object as follows: (1) A steel sheet having a chemical composition, in mass %, of C: 0.060 to 0.300%, Si: 0.30 to 1.50%, Mn: 1.00 to 2.70%, P: 0.100% or less, S: 0.0300% or less, sol. Al: 0.001 to 0.500%, O: 0.0100% or less, N: 0.0070% or less, Ti: 0.070 to 0.170%, Nb: 0.001 to 1.000%, B: 0 to 0.0030%, Cr: 0 to 0.70%, Mo: 0 to 0.12%, Cu: 0-0.40%, Ni: 0-0.30%, V: 0-0.300%, Sn: 0-0.040%, As: 0-0.100%, Zr: 0-0.050%, Ca: 0-0.0010%, Mg: 0-0.0010%, Bi: 0-0.010%, Co: 0 to 0.010%, 1. A steel sheet comprising: W: 0-0.100%, Zn: 0-0.010%, REM: 0-0.0100%, and the balance: Fe and impurities; and a metallographic structure consisting, in area %, of ferrite: 10-40%, martensite: 60-90%, bainite: 0-10%, and at least one of pearlite and retained austenite: 0-5% in total; wherein a 150 μm × 150 μm region at a 1 / 4 position in the sheet thickness of a cross section perpendicular to the sheet surface is equally divided into nine regions, the number density of ferrite is calculated for each divided region, and the average value is taken as Nαm, and all differences in the number density of ferrite between adjacent divided regions in the sheet thickness direction are Nαm × 0.60 or less.(2) The chemical composition is, in mass%, B: 0.0001 to 0.0030%, Cr: 0.001 to 0.70%, Mo: 0.001 to 0.12%, Cu: 0.001 to 0.40%, Ni: 0.001 to 0.30%, V: 0.001 to 0.300%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, Zr: 0.001 to 0.050%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, Bi: 0.001 to 0.010%, Co: 0.001 to 0.010%, The steel sheet according to the above (1), characterized in that it contains at least one of W: 0.001 to 0.100%, Zn: 0.001 to 0.010%, and REM: 0.0001 to 0.0100%. (3) Nαm is 0.050 pieces / μm. 2 (4) The steel sheet according to any one of (1) to (3) above, characterized in that it has a tensile strength of 1180 MPa or more. (5) The steel sheet according to any one of (1) to (4) above, characterized in that it has a plate thickness of 1.0 to 8.0 mm. (6) A part, characterized in that it includes the steel sheet according to any one of (1) to (5) above. (7) A heating step comprising heating a slab having the chemical composition described in (1) or (2) above and holding it at a temperature of 1180 to 1350°C for 6000 seconds or more; A hot rolling step comprising finish-rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and satisfying the following conditions (a) to (c): (a) the rolling temperature in each of the two rolling passes immediately preceding the last two rolling passes is 960 to 1080°C, and the reduction in each of the rolling passes is 30 to 50%, (b) the rolled material is cooled to a cooling stop temperature of 800 to 910°C within 0.20 seconds after the rolling pass in the two rolling passes immediately preceding the last two rolling passes, and (c) the reduction in each of the last two rolling passes is 10 to 40%. a cooling step in which the finish-rolled steel sheet is water-cooled to a temperature range of 600 to 750°C within 4.0 seconds from the start of water cooling, and then air-cooled in the temperature range for 2.0 to 8.0 seconds, and water-cooled the steel sheet to 50°C or less within 13 seconds after air-cooling.

[0010] According to the present invention, it is possible to provide a steel plate that has high strength, high uniform elongation, hole expandability, and yield ratio, and that can suppress load reduction during a collision, a part including the steel plate, and a method for manufacturing the steel plate.

[0011] 1A and 1B are schematic diagrams illustrating the characteristics related to the number density of ferrite, in which (a) shows an example that does not satisfy the characteristics related to the number density of ferrite according to the present invention, and (b) shows an example that satisfies the characteristics.

[0012] <Steel Sheet> The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition, in mass %, of C: 0.060 to 0.300%, Si: 0.30 to 1.50%, Mn: 1.00 to 2.70%, P: 0.100% or less, S: 0.0300% or less, sol. Al: 0.001 to 0.500%, O: 0.0100% or less, N: 0.0070% or less, Ti: 0.070 to 0.170%, Nb: 0.001 to 1.000%, B: 0 to 0.0030%, Cr: 0 to 0.70%, Mo: 0 to 0.12%, Cu: 0-0.40%, Ni: 0-0.30%, V: 0-0.300%, Sn: 0-0.040%, As: 0-0.100%, Zr: 0-0.050%, Ca: 0-0.0010%, Mg: 0-0.0010%, Bi: 0-0.010%, Co: 0 to 0.010%, The steel sheet is characterized in that the steel sheet comprises W: 0 to 0.100%, Zn: 0 to 0.010%, REM: 0 to 0.0100%, and the balance: Fe and impurities, and the metal structure is, in area %, ferrite: 10 to 40%, martensite: 60 to 90%, bainite: 0 to 10%, and at least one of pearlite and retained austenite: 0 to 5% in total, and when a 150 μm × 150 μm region at a 1 / 4 position of the plate thickness on a cross section perpendicular to the plate surface is equally divided into nine, the number density of ferrite is calculated for each divided region, and the average value is taken as Nαm, the difference in number density of ferrite between adjacent divided regions in the plate thickness direction is all Nαm × 0.60 or less.

[0013] As mentioned above, it is known that properties such as hole expandability decrease with increasing strength of steel. For example, to manufacture automobile suspension parts with complex shapes, such as lower arms and trailing arms, a steel sheet is required that has high strength, for example, a tensile strength of 1180 MPa or more, which enables weight reduction, while also having excellent hole expandability. From the viewpoint of increasing strength, it is preferable that the metal structure of the steel sheet be composed mainly of martensite. However, although martensitic steel has excellent strength, excessive martensitic content reduces properties such as uniform elongation, which generally results in poor workability. Furthermore, in parts with complex shapes, such as lower arms and trailing arms, the workability of the steel itself may decrease as the strength is increased, or work hardening may occur when forming into a complex shape, resulting in strain being applied to near the maximum load after part forming. Therefore, there remains a strong need for high-strength steel sheets that have improved properties such as hole expandability and uniform elongation, and that can maintain a high and stable load during a collision even when forming parts with complex shapes, thereby suppressing the occurrence of fractures accompanied by a decrease in load, and that also have a high yield ratio from the perspective of automobile collision safety, etc.

[0014] Therefore, the inventors conducted research focusing on the metallographic structure of the steel sheet in addition to making the chemical composition of the steel sheet appropriate. First, the inventors discovered that by configuring the metallographic structure of a steel sheet having a predetermined chemical composition to a structure mainly composed of soft ferrite and hard martensite, more specifically, a structure containing, by area percentage, 10 to 40% ferrite and 60 to 90% martensite, it is possible to improve strength and uniform elongation by the action mechanism of so-called DP (Dual Phase) steel.

[0015] On the other hand, because DP steels generally have a low yield ratio, the inventors have discovered that precipitation strengthening through the addition of Ti can increase the yield ratio and significantly improve hole expandability. While not intending to be bound by any particular theory, it is believed that this improvement in hole expandability due to precipitation strengthening is due to a reduction in the hardness difference between ferrite and martensite in the metallographic structure. More specifically, in the steel sheet according to the embodiment of the present invention, as described above, the metallographic structure is primarily composed of ferrite and martensite, and may contain up to 40% by area of ​​ferrite, which is a soft structure. In this case, the hardness difference between ferrite and martensite in the metallographic structure increases, resulting in a decrease in hole expandability. However, in the steel sheet according to the embodiment of the present invention, by controlling the Ti content in the steel to 0.070% by mass or more, Ti precipitates precipitation strengthen the soft ferrite structure, thereby reducing the hardness difference between ferrite and martensite in the metallographic structure, and therefore significantly improving hole expandability.

[0016] Next, the inventors conducted a study based on the idea that in order to maintain a high and stable load during a collision and prevent fractures, it is necessary to suppress work softening after uniform elongation, which corresponds to the elongation at the maximum load point in a uniaxial tensile test. This is because suppressing work softening after uniform elongation makes it possible to effectively absorb the collision energy from the maximum load to fracture during a collision by plastic deformation of the steel sheet. First, in an experiment conducted by the inventors, the work softening rate after uniform elongation (d 2 σ / dε 2It was found that when the absolute value of σ (σ: true stress, ε: true strain) is controlled to 250,000 MPa or less, the occurrence of fracture can be completely or significantly suppressed when the formed part is subjected to a drop weight test. Therefore, in order to stably achieve such a work softening rate, the inventors conducted further studies, focusing on ferrite, a soft structure in which deformation is likely to concentrate during a collision. As a result, the inventors have found that by uniformly distributing ferrite in the thickness direction of a steel sheet, more specifically, by equally dividing a 150 μm × 150 μm region at 1 / 4 of the thickness of a cross section perpendicular to the plate surface of the steel sheet into nine regions, calculating the number density of ferrite in each divided region, and averaging the calculated ferrite density values, the absolute values ​​of the difference in number density of ferrite between adjacent divided regions in the thickness direction, more specifically, the difference in number density of ferrite between adjacent divided regions in the thickness direction, is all Nαm × 0.60 or less, it is possible to reliably reduce the absolute value of the work softening rate after uniform elongation in a uniaxial tensile test to 250,000 MPa or less. In relation to this, the inventors have found that even when the steel sheet is formed into parts, particularly parts with complex shapes such as lower arms and trailing arms, it is possible to suppress load reduction during a collision and significantly suppress the occurrence of fracture.

[0017] 1A is a schematic diagram illustrating the characteristics related to the number density of ferrite, and Fig. 1A shows an example that does not satisfy the characteristics related to the number density of ferrite according to the present invention, i.e., "When a 150 μm × 150 μm region at a 1 / 4 position of the plate thickness in a cross section perpendicular to the plate surface is equally divided into nine, the number density of ferrite is calculated in each divided region, and the average value is Nαm, the difference in the number density of ferrite in each divided region adjacent in the plate thickness direction is all Nαm × 0.60 or less," and Fig. 1B shows an example that satisfies this characteristic. Referring to Fig. 1A, first, a 150 μm × 150 μm region at a 1 / 4 position of the plate thickness in a cross section perpendicular to the plate surface of the steel plate is equally divided into nine, and the number density of ferrite is calculated in each divided region, i.e., Nα1 to Nα9, and the average value Nαm = 0.058 particles / μm 2Next, the difference in the number density of ferrite particles between the divided regions adjacent in the thickness direction, that is, the difference between Nα1 and Nα4 (0.045 particles / μm 2 ), the difference between Nα4 and Nα7 (0.033 pieces / μm 2 ), the difference between Nα2 and Nα5 (0.017 pieces / μm 2 ), the difference between Nα5 and Nα8 (0.012 pieces / μm 2 ), the difference between Nα3 ​​and Nα6 (0.008 pieces / μm 2 ), and the difference between Nα6 and Nα9 (0.014 pieces / μm 2 ) is calculated (FIG. 1(a)(ii)). All of the differences in these six number densities are equal to the average value Nαm (0.058 particles / μm 2 ), that is, when the values ​​obtained by dividing the difference in number density by Nαm are all 0.60 or less, the number density characteristics of the ferrite according to the present invention are satisfied. However, in FIG. 1(a), when the difference between Nα1 and Nα4 (0.045 particles / μm 2 ) to Nαm (0.058 pieces / μm 2 ) is 0.78, which indicates that the characteristic is not satisfied.

[0018] On the other hand, referring to FIG. 1(b), all of the differences in the number density of ferrite in each divided region adjacent in the sheet thickness direction, which were similarly calculated, were within the average value Nam (0.056 particles / μm 2 ) (Nαm × 0.60 or less), and therefore it can be seen that the characteristics related to the number density of ferrite according to the present invention are satisfied. In the embodiment of the present invention, by thus uniforming the number density of ferrite in the thickness direction of the steel sheet, that is, by uniformly disposing ferrite in the thickness direction of the steel sheet, it is possible to reliably reduce the absolute value of the work softening rate after uniform elongation in a uniaxial tensile test to 250,000 MPa or less, and therefore it is possible to significantly suppress the occurrence of fracture during a collision even when the steel sheet is formed into parts having complex shapes such as lower arms and trailing arms.

[0019] Without intending to be bound by any particular theory, it is believed that uniformly distributing ferrite throughout the metal structure can suppress local deformation during a collision, thereby suppressing load reduction during the collision and maintaining a high and stable load, thereby significantly suppressing the occurrence of fracture. More specifically, when a steel plate is composed of a structure primarily composed of soft ferrite and hard martensite, strength is generally ensured by the hard martensite, and deformation is borne by the soft ferrite. Therefore, deformation during a collision tends to be concentrated in the soft ferrite. In such cases, if there are regions in the metal structure where ferrite is relatively abundant locally, strain during deformation is thought to concentrate in these regions. As a result, local deformation cannot be suppressed, and this deformation is thought to result in a decrease in load during the collision. Considering the deformation mode during a collision, it is particularly important to suppress load reduction during bending deformation. Furthermore, during bending deformation, fractures occur in the thickness direction of the steel plate, originating from the surface layer. Therefore, in order to suppress the load reduction during a collision, it is considered effective to uniformly arrange ferrite in the metal structure, particularly in the sheet thickness direction. In fact, it has been found that by uniformly arranging ferrite in the sheet thickness direction of a steel sheet as shown in Figure 1, the absolute value of the work softening rate after uniform elongation in a uniaxial tensile test can be reliably reduced to 250,000 MPa or less, and that even when the steel sheet is formed into parts having complex shapes such as lower arms and trailing arms, the occurrence of fracture during a collision can be significantly suppressed. The fact that the work softening rate after uniform elongation can be reduced by uniformly arranging ferrite in the sheet thickness direction of a steel sheet composed mainly of a structure of ferrite and martensite, and further the fact that the occurrence of fracture during a collision can be significantly suppressed thereby, was not previously known and has now been revealed for the first time by the present inventors.Therefore, according to the embodiment of the present invention, despite having a high tensile strength of, for example, 1180 MPa or more, the steel sheet has high uniform elongation, hole expandability, and yield ratio, and is able to significantly suppress the occurrence of fractures accompanied by load reduction during a collision, and therefore the steel sheet according to the embodiment of the present invention is particularly useful for use in the automotive field.

[0020] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0021] [C: 0.060 to 0.300%] C is an element effective in increasing the strength of steel sheet. Furthermore, C forms carbides and / or carbonitrides with Nb in steel, contributing to refinement of the structure due to the pinning effect of the formed precipitates. To fully obtain these effects, the C content is set to 0.060% or more. The C content may be 0.070% or more, 0.080% or more, 0.100% or more, 0.120% or more, or 0.150% or more. On the other hand, excessive C content may reduce uniform elongation. Therefore, the C content is set to 0.300% or less. The C content may be 0.280% or less, 0.250% or less, 0.200% or less, 0.180% or less, or 0.160% or less.

[0022] [Si: 0.30 to 1.50%] Si is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. To fully obtain this effect, the Si content is set to 0.30% or more. The Si content may be 0.40% or more, 0.50% or more, 0.60% or more, 0.70% or more, or 0.80% or more. On the other hand, excessive Si content increases the ferrite fraction, which may increase the hardness difference between ferrite and martensite and reduce hole expandability. Therefore, the Si content is set to 1.50% or less. The Si content may be 1.40% or less, 1.20% or less, 1.10% or less, 1.00% or less, or 0.90% or less.

[0023] [Mn: 1.00 to 2.70%] Mn is an element that is effective in increasing hardenability and strength as a solid solution strengthening element. To fully obtain these effects, the Mn content is set to 1.00% or more. The Mn content may be 1.20% or more, 1.50% or more, 1.60% or more, 1.80% or more, or 2.00% or more. On the other hand, excessive Mn content may result in an excessive improvement in hardenability, resulting in a decrease in the ferrite fraction and a decrease in uniform elongation. Therefore, the Mn content is set to 2.70% or less. The Mn content may be 2.60% or less, 2.50% or less, 2.40% or less, 2.30% or less, or 2.20% or less.

[0024] [P: 0.100% or less] Excessive P content may result in reduced workability due to grain boundary segregation, etc. Therefore, the P content is set to 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0025] [S: 0.0300% or less] Excessive S content may result in the formation of large amounts of sulfides such as MnS, which may reduce workability. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the S content may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[0026] [Sol. Al: 0.001 to 0.500%] Sol. Al is an element that acts as a deoxidizer for molten steel. To achieve this effect, the sol. Al content is set to 0.001% or more. The sol. Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, excessive sol. Al content increases the ferrite fraction, which may increase the hardness difference between ferrite and martensite and reduce hole expandability. Therefore, the sol. Al content is set to 0.500% or less. The sol. Al content may be 0.400% or less, 0.300% or less, or 0.200% or less. Sol. Al refers to acid-soluble Al, which refers to solute Al present in the steel in a solid solution state.

[0027] [O: 0.0100% or less] O is an element that is mixed in during the manufacturing process. Excessive O content may form coarse inclusions, reducing the workability of the steel sheet. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long refining time, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more or 0.0005% or more.

[0028] [N: 0.0070% or less] Excessive N content may form coarse nitrides, which may cause slab cracking during hot rolling. Therefore, the N content is set to 0.0070% or less. The N content may be 0.0050% or less, 0.0040% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[0029] [Ti: 0.070 to 0.170%] Ti precipitates in steel as Ti carbides such as TiC, strengthening soft structures such as ferrite through precipitation strengthening and contributing to improved strength and yield ratio. Furthermore, Ti can reduce the hardness difference between ferrite and martensite in the metal structure due to precipitation strengthening, and is therefore also effective in improving hole expandability. To fully achieve these effects, the Ti content is set to 0.070% or more. The Ti content may be 0.080% or more, 0.090% or more, 0.100% or more, or 0.120% or more. On the other hand, excessive Ti content can produce coarse carbides in the steel, which can cause slab cracking during hot rolling or reduce the workability of the steel sheet. Therefore, the Ti content is set to 0.170% or less. The Ti content may be 0.160% or less, 0.150% or less, 0.140% or less, or 0.130% or less.

[0030] [Nb: 0.001 to 1.000%] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, contributing to the refinement of prior austenite grains through a pinning effect, thereby increasing the strength of the steel sheet. To fully achieve this effect, the Nb content is set to 0.001% or more. The Nb content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, excessive Nb content may cause the formation of coarse carbides and the like in the steel, resulting in a decrease in the workability of the steel sheet. Therefore, the Nb content is set to 1.000% or less. The Nb content may be 0.800% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.200% or less.

[0031] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.

[0032] [B: 0 to 0.0030%] B is an element that improves the hardenability of steel and contributes to improving strength. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0030% or less. The B content may be 0.0025% or less, 0.0020% or less, 0.0015% or less, or 0.0010% or less.

[0033] [Cr: 0 to 0.70%] Cr is an element that improves the hardenability of steel and contributes to improving strength and / or corrosion resistance. The Cr content may be 0%, but to obtain these effects, the Cr content is preferably 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if Cr is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Cr content is preferably 0.70% or less, and may be 0.60% or less, 0.50% or less, 0.40% or less, or 0.30% or less.

[0034] [Mo: 0 to 0.12%] Mo is an element that improves the hardenability of steel and contributes to improving strength. The Mo content may be 0%, but to achieve this effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, excessive Mo content may increase deformation resistance during hot working and increase equipment load. Therefore, the Mo content is preferably 0.12% or less. The Mo content may be 0.10% or less, 0.08% or less, 0.06% or less, or 0.05% or less.

[0035] [Cu: 0 to 0.40%] Cu is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Cu content may be 0%, but to obtain such effects, the Cu content is preferably 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if these elements are contained in excess, the effects may saturate and the manufacturing cost may increase. Therefore, the Cu content is preferably 0.40% or less. The Cu content may be 0.30% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0036] [Ni: 0 to 0.30%] Ni is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but to obtain such effects, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if these elements are contained in excess, the effects may saturate and the manufacturing cost may increase. Therefore, the Ni content is preferably 0.30% or less. The Ni content may be 0.20% or less, 0.15% or less, 0.10% or less, or 0.08% or less.

[0037] [V: 0 to 0.300%] V is an element that contributes to improving strength through precipitation strengthening and the like. The V content may be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, even if V is contained excessively, the effect saturates and there is a risk of incurring an increase in manufacturing costs. Therefore, the V content is preferably 0.300% or less. The V content may be 0.200% or less, 0.100% or less, or 0.080% or less.

[0038] [Sn: 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0 to 0.0010%, Mg: 0 to 0.0010%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100%, Zn: 0 to 0.010%, and REM: 0 to 0.0100%] Sn, As, Zr, Ca, Mg, Bi, Co, W, Zn, and REM may be contained in the steel sheet as optional elements, or may be present in the steel sheet as tramp elements. The contents of these elements may be as follows: Sn: 0 to 0.040% or 0.020%, As: 0 to 0.100% or 0.050%, Zr: 0 to 0.050% or 0.030%, Ca: 0 to 0.0010% or 0.0008%, Mg: 0 to 0.0010% or 0.0008%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100% or 0.050%, Zn: 0 to 0.010%, and REM: 0 to 0.0100% or 0.0050%. The lower limits of these elements may be, for example, 0.001% or more, 0.005% or more, or 0.008% or more, respectively. Similarly, the Ca, Mg and REM contents may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.

[0039] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the steel sheet.

[0040] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0041] [Metal Structure] [Ferrite: 10-40%] The metal structure of the steel sheet according to the embodiment of the present invention contains, in area %, 10-40% ferrite. By including 10% or more of ferrite, a soft structure, in area %, the desired uniform elongation can be achieved. From the viewpoint of further improving uniform elongation, a higher ferrite area ratio is preferable, for example, 12% or more, 15% or more, 18% or more, 20% or more, 22% or more, or 25% or more. On the other hand, if the ferrite area ratio is too high, the strength and / or yield ratio may decrease, or the precipitation strengthening due to Ti precipitates may not be able to sufficiently reduce the hardness difference between ferrite and martensite, resulting in a decrease in hole expandability. Therefore, the ferrite area ratio is set to 40% or less. From the viewpoint of further improving strength, yield ratio, and / or hole expandability, a lower ferrite area ratio is preferable, for example, 38% or less, 35% or less, 32% or less, 30% or less, 28% or less, or 26% or less.

[0042] [Martensite: 60 to 90%] The metal structure of the steel sheet according to the embodiment of the present invention contains, in area %, 60 to 90% martensite. By configuring the metal structure of the steel sheet to include hard martensite within this range, high strength, for example, a tensile strength of 1180 MPa or more, can be achieved. From the viewpoint of further increasing strength, a higher area fraction of martensite is preferable, and may be, for example, 65% or more, 68% or more, 70% or more, 72% or more, or 75% or more. On the other hand, if the area fraction of martensite is too high, uniform elongation may decrease. Therefore, the area fraction of martensite is set to 90% or less, and may be, for example, 88% or less, 85% or less, 82% or less, 80% or less, or 78% or less. In the present invention, "martensite" includes not only as-quenched martensite (so-called fresh martensite) but also tempered martensite.

[0043] [Bainite: 0 to 10%] The metal structure of the steel plate according to the embodiment of the present invention may contain bainite. However, if the area fraction of bainite becomes too high, uniform elongation may decrease. Therefore, the area fraction of bainite is set to 10% or less, and may be, for example, 9% or less, 8% or less, 6% or less, 5% or less, or 3% or less. On the other hand, the lower limit is not particularly limited, and the area fraction of bainite may be 0%, or may be, for example, 0.5% or more, 1% or more, or 2% or more.

[0044] [Remaining Structure] The remaining structure other than ferrite, martensite, and bainite may be 0% by area. However, if a remaining structure is present, the remaining structure may be at least one of pearlite and retained austenite. If the area fraction of at least one of pearlite and retained austenite exceeds 5% in total, it may result in a decrease in uniform elongation or may make it impossible to control ferrite and / or martensite within a desired range. Therefore, the total area fraction of at least one of pearlite and retained austenite is set to 5% or less, for example, 4% or less, 3% or less, or 2% or less. On the other hand, the lower limit is not particularly limited, and the total area fraction of at least one of pearlite and retained austenite may be 0%, for example, 0.1% or more, 0.5% or more, or 1% or more.

[0045] [Identification of Metal Structure and Calculation of Area Ratio] Next, the identification of metal and the calculation of area ratio will be described. The structure observation is performed on the thickness cross section perpendicular to the sheet surface. Although the thickness cross section is preferably parallel to the rolling direction, in cases where the rolling direction of the steel sheet cannot be identified, the thickness cross section does not necessarily have to be parallel to the rolling direction. Specifically, first, a test piece is taken from the steel sheet, and the cross section of the test piece is polished using #600 to #1500 silicon carbide paper. Then, a diamond powder with a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water to give a mirror finish. Next, the cross section is polished for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm that does not contain an alkaline solution to remove the strain introduced into the surface layer of the test piece. Next, a rectangular area of ​​150 μm in the thickness direction and 150 μm in the direction perpendicular to the thickness direction, centered at a position 1 / 4 of the thickness from the steel surface, is measured using electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information.

[0046] For the measurement, it is preferable to use an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL). At this time, the degree of vacuum in the EBSD device is 9.6×10 -5 It is preferable that the pressure be 100 Pa or less, the acceleration voltage be 15 kV, the irradiation current level be 13, and the electron beam irradiation level be 62. Other observation conditions are preferably as follows: Electron gun type: Schottky WD (working distance): 15 mm Objective aperture number: 4 Number of pixels: 4096 x 5120 pixels

[0047] [Identification of Retained Austenite and Calculation of Area Fraction] From the obtained crystal orientation information, the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to identify regions with an fcc crystal structure and calculate the area fraction of these regions, thereby obtaining the area fraction of retained austenite.

[0048] [Identification of Ferrite and Calculation of Area Fraction] Next, regions with a bcc crystal structure are determined to be "ferrite, martensite, bainite, and pearlite." For these regions, the grain misorientation (GAM value: Grain Average Misorientation) is calculated using software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Finally, regions with a GAM value of 0.5° or less are identified as ferrite, and their area fraction is calculated. Here, the "GAM value" is the average value of the misorientation between adjacent pixels in a region surrounded by grain boundaries with a misorientation of 15° or more.

[0049] [Identification of Bainite and Calculation of Area Fraction] Next, within the remaining region (region where the "GAM value" exceeds 0.5°), under the condition that boundaries where the crystal orientation difference is 15° are regarded as grain boundaries, when the maximum value of the "Grain Average IQ" of the ferrite region is Iα, regions where Iα / 2 or more are extracted as "bainite" and regions where Iα / 2 or less are extracted as "martensite and pearlite." The area fraction of the extracted bainite is obtained by calculating the area fraction of the bainite.

[0050] [Identification of Pearlite and Calculation of Area Fraction] The pearlite area fraction is calculated by observing a secondary electron image with an FE-SEM after corrosion using a nital reagent. To observe a secondary electron image with an FE-SEM in the same region as the EBSD measurement region, a Vickers indentation is made near the observation position. Subsequently, surface contamination is polished away, leaving the structure of the observation surface, and the specimen is then etched with nital. Next, the same field of view as the EBSD observation surface is observed with an FE-SEM, for example, at a magnification of 800x. The pearlite area fraction is calculated by performing image analysis on a microstructure photograph obtained using the FE-SEM in the same region as the EBSD measurement region, i.e., a rectangular region 150 μm in the thickness direction and 150 μm in the direction perpendicular to the thickness direction, centered at a position ¼ of the thickness from the steel sheet surface. Here, the structure in which plate-like ferrite and Fe-based carbides are layered is considered to be pearlite.

[0051] [Identification of martensite and calculation of area fraction] The area fraction of martensite is calculated by subtracting the area fractions of retained austenite, ferrite, bainite, and pearlite from 100%. Metal structures other than martensite are sequentially identified, and the last remaining metal structure is considered to be martensite.

[0052] [Differences in Number Density of Ferrite in Adjacent Divided Regions in the Plate Thickness Direction: All are Nαm × 0.60 or Less] In the steel plate according to the embodiment of the present invention, when a 150 μm × 150 μm region at 1 / 4 of the plate thickness in a cross section perpendicular to the plate surface is equally divided into nine, the number density of ferrite in each divided region is calculated, and the average value is taken as Nαm, the difference in number density of ferrite in each divided region adjacent in the plate thickness direction, more specifically, the absolute value of the difference in number density of ferrite in each divided region adjacent in the plate thickness direction, is all controlled to be Nαm × 0.60 or less. By uniformly distributing ferrite in this manner in the plate thickness direction of the steel plate, the absolute value of the work softening rate after uniform elongation in a uniaxial tensile test can be reliably reduced to 250,000 MPa or less, thereby suppressing load reduction during a collision. As a result, even when the steel plate according to the embodiment of the present invention is formed into automotive parts, particularly automotive suspension parts, such as lower arms and trailing arms, having complex shapes, it is possible to significantly suppress the occurrence of fracture during a collision.

[0053] From the viewpoint of further suppressing a decrease in load during a collision, it is preferable that the difference in number density of ferrite between adjacent divided regions in the plate thickness direction is as small as possible. For example, the difference in number density of ferrite between adjacent divided regions in the plate thickness direction may all be Nαm×0.55 or less, Nαm×0.50 or less, Nαm×0.50 or less, Nαm×0.45 or less, Nαm×0.40 or less, Nαm×0.35 or less, or Nαm×0.30 or less. Although there is no particular lower limit, for example, the difference in number density of ferrite between adjacent divided regions in the plate thickness direction may all be Nαm×0.03 or more, Nαm×0.05 or more, Nαm×0.08 or more, or Nαm×0.10 or more.

[0054] [Nαm: 0.050 pieces / μm 2 According to a preferred embodiment of the present invention, Nαm is 0.050 particles / μm 2 That's all. In the steel plate according to the embodiment of the present invention, as described above, it is important to uniformly arrange ferrite in the thickness direction of the steel plate, and from the viewpoint of increasing the uniformity of the ferrite arrangement, it is preferable to make the ferrite grains finer. When the ferrite grains are made finer, the number density of ferrite naturally increases. Therefore, by increasing the number density of ferrite, the difference in the number density of ferrite in each divided region adjacent in the thickness direction is reduced, which acts in the direction of suppressing the load reduction during a collision. From the viewpoint of making such an effect more pronounced, the higher Nαm is the more preferable, and it is, for example, 0.055 particles / μm 2 Above, 0.060 pieces / μm 2 Above, 0.065 pieces / μm 2 Above, 0.070 pieces / μm 2 Above, 0.075 pieces / μm 2 or more than 0.080 pieces / μm 2 The upper limit is not particularly limited, but for example, Nαm may be 0.200 particles / μm 2 Below, 0.150 pieces / μm 2 Less than or equal to 0.120 pieces / μm 2 It may be the following:

[0055] [Measurement of Number Density of Ferrite in Nam and Each Divided Region] The measurement of the number density of ferrite in Nam and each divided region is performed by EBSD as follows, as in the case of identifying ferrite and calculating the area ratio. Specifically, first, a sample is taken from the steel sheet so that the plate thickness cross section perpendicular to the plate surface serves as the observation surface. Although the plate thickness cross section is preferably parallel to the rolling direction, in cases where the rolling direction of the steel sheet cannot be identified, the plate thickness cross section does not necessarily have to be parallel to the rolling direction. Next, EBSD analysis is performed at measurement intervals of 0.1 μm on a rectangular region 150 μm in the plate thickness direction and 150 μm in the direction perpendicular to the plate thickness direction, centered at a position ¼ of the plate thickness from the surface of the steel sheet, to obtain crystal orientation information of this rectangular region. EBSD analysis was performed using a device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL) at an analysis speed of 50 to 300 points per second. Next, the crystal orientation information of this rectangular region was used to calculate the intragranular misorientation (GAM value) using the software "OIM Analysis (registered trademark)" provided with the EBSD analyzer. Next, regions with a GAM value of 0.5° or less were identified as ferrite. The 150 μm × 150 μm region was divided into nine equal parts, and the number densities Nα1 to Nα9 were calculated by counting the number of ferrite particles in each divided region. The average value of these numbers was determined as Nαm (Figures 1(a)(i) and (b)(i)). The number density of ferrite was calculated by counting all the ferrite particles detected and identified under the above measurement conditions. When counting, if a ferrite exists on the boundary between multiple divided regions or in multiple divided regions across the boundary, the ferrite will be counted in each of the multiple divided regions. For example, if one ferrite exists on the boundary between two divided regions A and B or in divided regions A and B across the boundary, the one ferrite will be counted in both divided regions A and B. Similarly, if one ferrite exists on the boundary between three divided regions A, B, and C or in divided regions A, B, and C across the boundary, the one ferrite will be counted in each of divided regions A, B, and C.Finally, the difference in number density of ferrite in each divided region adjacent to each other in the thickness direction is calculated (FIGS. 1(a)(ii) and (b)(ii)), and it is determined whether the value obtained by dividing the difference in number density by Nαm satisfies the requirement of 0.60 or less (FIGS. 1(a)(iii) and (b)(iii)).

[0056] [Thickness] The steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 8.0 mm, although not particularly limited thereto. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

[0057] As described above, the steel sheet according to the embodiment of the present invention has high strength, yet high uniform elongation, hole expandability, and yield ratio. It is also capable of significantly suppressing the occurrence of fractures accompanied by load reduction during a collision, particularly when forming parts having complex shapes. Therefore, the steel sheet according to the embodiment of the present invention reliably achieves both the contradictory properties of high strength and excellent formability at a high level, and also achieves excellent impact resistance. Therefore, the steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields where these properties are required, and is particularly useful in the automotive field. In a preferred embodiment, an automotive part, particularly an automotive suspension part, is provided, which includes the steel sheet according to the embodiment of the present invention. Examples of automotive suspension parts include lower arms and trailing arms. It is sufficient for at least a portion of these automotive parts, particularly automotive suspension parts, to contain the steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the above-described chemical composition and metallographic characteristics. In a portion of a steel sheet that is not in direct contact with a mold during forming such as press forming and that is subjected to a relatively small degree of processing, the characteristics of the metal structure do not change particularly before and after forming.

[0058] [Mechanical Properties] [Tensile Strength (TS) and Uniform Elongation (uEl)] Steel sheets having the above-described chemical composition and metallographic structure can achieve high tensile strength, specifically, 1180 MPa or more. The tensile strength is preferably 1200 MPa or more, 1220 MPa or more, or 1240 MPa or more. Despite having such extremely high tensile strength, steel sheets according to embodiments of the present invention can significantly suppress load reduction during collisions while improving uniform elongation and hole expandability through the specific combination of chemical composition and metallographic structure described above. The upper limit of the tensile strength is not particularly limited, but the tensile strength of the steel sheet may be, for example, 1780 MPa or less, 1470 MPa or less, 1400 MPa or less, or 1300 MPa or less. Furthermore, steel sheets according to embodiments of the present invention can achieve high uniform elongation, specifically, a uniform elongation of 5.0% or more. The uniform elongation is preferably 5.2% or more, 5.5% or more, 5.8% or more, or 6.0% or more. The upper limit of the uniform elongation is not particularly limited, but for example, the uniform elongation of the steel sheet may be 15.0% or less, 10.0% or less, 8.0% or less, or 7.0% or less. The tensile strength and uniform elongation are measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is preferably parallel to the rolling direction perpendicular to the rolling direction of the steel sheet (C direction), and performing a tensile test in accordance with JIS Z 2241:2022. When the rolling direction of the steel sheet cannot be specified, the JIS No. 5 test piece may be taken from any direction within the surface of the steel sheet.

[0059] [Hole Expansion Ratio (λ)] Steel sheets having the above chemical composition and metallographic structure can achieve high hole expandability, specifically, a hole expansion ratio of 40% or more. The hole expansion ratio may be preferably 42% or more, more preferably 45% or more or 50% or more. The upper limit of the hole expansion ratio is not particularly limited, but may be, for example, 150% or less, 100% or less, or 70% or less. The hole expansion ratio is determined as follows. First, a test piece 100 mm wide x 100 mm long is taken from the steel sheet, and a punched hole (initial hole: hole diameter d0 = 10 mm) is made using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole is expanded using a conical punch with an apex angle of 60° until a crack penetrating the plate thickness occurs. The hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) of each test piece is calculated using the following formula: This hole expansion test is carried out three times, and the average value is determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0060] [Yield Ratio (YR)] Steel sheets having the above chemical composition and metallographic structure can achieve not only high tensile strength but also a high yield ratio, more specifically, a yield ratio of 75% or more. The yield ratio is preferably 78% or more or 80% or more, more preferably 82% or more or 84% or more. The upper limit is not particularly limited, but the yield ratio may be, for example, 95% or less, 92% or less, 90% or less, or 88% or less. The yield ratio is determined by the following formula based on the tensile strength and 0.2% proof stress measured by taking a JIS No. 5 test piece in a direction (C direction) where the longitudinal direction of the test piece is preferably parallel to the rolling direction of the steel sheet and conducting a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be specified, the JIS No. 5 test piece may be taken from any direction within the steel sheet plane. Yield ratio YR = 0.2% proof stress / tensile strength TS × 100

[0061] <Method for Manufacturing Steel Sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, particularly a steel sheet having preferred properties, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, although the following specifically describes the manufacture of a hot-rolled steel sheet, the steel sheet according to an embodiment of the present invention encompasses any steel sheet having the chemical composition and metallographic structure described above, i.e., not only a hot-rolled steel sheet but also a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description merely describes one example of a preferred manufacturing method when the steel sheet according to an embodiment of the present invention is a hot-rolled steel sheet.

[0062] A method for producing a steel plate according to an embodiment of the present invention includes: a heating step, which includes heating a slab having the chemical composition described above in relation to the steel plate, and holding the slab at a temperature of 1180 to 1350°C for 6000 seconds or more; a hot rolling step, which includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and which satisfies the following conditions (a) to (c): (a) the rolling temperature in each of the two rolling passes immediately preceding the last two rolling passes is 960 to 1080°C, and the reduction in each of the rolling passes is 30 to 50%, (b) the rolled material is cooled to a cooling stop temperature of 800 to 910°C within 0.20 seconds after the two rolling passes immediately preceding the last two rolling passes, and (c) the reduction in each of the last two rolling passes is 10 to 40%. The method is characterized by including a cooling step in which the finish-rolled steel sheet is water-cooled to a temperature range of 600 to 750°C within 4.0 seconds from the start of water cooling, then air-cooled in the temperature range for 2.0 to 8.0 seconds, and water-cooled the steel sheet to 50°C or less within 13 seconds after air-cooling. In the above manufacturing method, the temperatures described for the slab and steel sheet refer to the surface temperatures of the slab and steel sheet, respectively. Each step will be described in detail below.

[0063] [Heating Process] First, a slab having the chemical composition described above in relation to the steel plate is heated and held at a temperature range of 1180 to 1350°C for 6000 seconds or more. From the viewpoint of productivity, it is preferable to use a slab obtained by continuous casting. However, slabs obtained by casting and blooming can also be used, and if necessary, slabs obtained by hot working or cold working may be used. In this manufacturing method, holding at a temperature range of 1180 to 1350°C includes not only cases where the slab temperature is held at a constant temperature within the range of 1180 to 1350°C, but also cases where the slab temperature fluctuates within the range of 1180 to 1350°C. Holding the slab at a temperature range of 1180 to 1350°C for 6000 seconds or more allows the coarse carbides present in the structure to be completely dissolved, eliminating the initiation points of cracks. If the holding temperature is less than 1180°C or the holding time is less than 6000 seconds, the coarse carbides will not be completely dissolved. If the solid solution of coarse carbides is incomplete, ferrite or bainite transformation originating from such carbides occurs during the cooling process described below, resulting in an area ratio of martensite of less than 60%, which may result in the failure to obtain the desired strength. The upper limit of the heating temperature of the slab is set to 1350°C or less from the viewpoint of the capacity and productivity of the heating equipment. The upper limit of the holding time in the temperature range of 1180 to 1350°C is preferably 10,000 seconds or less.

[0064] [Hot Rolling Step] [Rough Rolling] In the present manufacturing method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc. The conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[0065] [Finish Rolling] [(a) Rolling temperature in each rolling pass of the two rolling stages immediately preceding the last two stages: 960 to 1080°C, and reduction in each rolling pass of the two rolling stages immediately preceding the last two stages: 30 to 50%] The heated slab, or the slab that has also been rough-rolled as necessary, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of four or more rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, it is necessary to appropriately control the rolling temperature and reduction in each rolling pass of the two rolling stages immediately preceding the last two stages. Specifically, the rolling temperature in each rolling pass of the two rolling stages immediately preceding the last two stages is controlled to 960 to 1080°C, and similarly, the reduction in each rolling pass of the two rolling stages immediately preceding the last two stages is controlled to 30 to 50%. Here, for example, when a tandem rolling mill consisting of seven rolling stands is used, the sixth and seventh rolling passes correspond to the "last two rolling passes." Therefore, in this case, the "two rolling passes immediately preceding the last two rolling passes" refers to the fourth and fifth rolling passes. By performing rolling under relatively high pressure and relatively high temperature conditions in the two rolling passes immediately preceding the last two rolling passes, recrystallization can be promoted and austenite grains can be refined. In this regard, it is possible to achieve a desired level of uniformity in the number density of ferrite in the thickness direction in the final metallographic structure. More specifically, ferrite nucleates mainly from austenite grain boundaries. Therefore, by promoting recrystallization and refining austenite grains, the number of austenite grain boundaries can be increased, thereby increasing the number of ferrite nucleation sites. As a result, more ferrite can be generated and uniformly dispersed in the metallographic structure, and it is possible to achieve a desired level of uniformity in the number density of ferrite in the thickness direction in the final metallographic structure.

[0066] In contrast, if the rolling temperature in each of the two rolling passes immediately preceding the last two rolling passes is less than 960°C and / or the reduction ratio in each rolling pass is less than 30%, recrystallization is not sufficiently promoted, leaving unrecrystallized grains partially intact, and the number density of ferrite in the final metallographic structure cannot be sufficiently uniform in the thickness direction. As a result, the work-softening rate after uniform elongation decreases, i.e., work-softening after uniform elongation becomes significant. On the other hand, if the rolling reduction ratio in each of the two rolling passes immediately preceding the last two rolling passes exceeds 50%, flat austenite grains are formed due to the introduction of excessive strain, and the number density of ferrite in the final metallographic structure cannot be sufficiently uniform in the thickness direction. As a result, work-softening after uniform elongation becomes significant. Furthermore, if the rolling temperature in each of the two rolling passes immediately preceding the last two rolling passes exceeds 1080°C, the austenite grains after recrystallization become coarse, the number of austenite grain boundaries decreases, and the number of ferrite nucleation sites decreases. As a result, the number density of ferrite cannot be sufficiently uniformed in the thickness direction in the finally obtained metal structure, and similarly, work softening after uniform elongation becomes significant. Preferably, the rolling temperature in each of the two rolling passes immediately preceding the latter two rolling passes is 1000 to 1060°C.

[0067] [(b) Cooling to a Cooling Stop Temperature of 800 to 910°C Within 0.20 Seconds After the Two Rolling Passes Immediately Before the Last Two Rolling Passes] In this manufacturing method, the rolled material is cooled to a cooling stop temperature of 800 to 910°C within 0.20 seconds after the two rolling passes immediately before the last two rolling passes. By cooling the rolled material to a cooling stop temperature of 800 to 910°C in this manner relatively quickly after the two rolling passes immediately before the last two rolling passes, it is possible to suppress the grain growth of austenite grains after recrystallization, and thereby make it possible to uniform the number density of ferrite at a desired level in the plate thickness direction in the finally obtained metal structure. If the cooling time to the cooling stop temperature of 800 to 910 °C after the rolling passes of the two stages immediately preceding the last two stages exceeds 0.20 seconds or the cooling stop temperature is higher than 910 °C, the grain growth of austenite grains after recrystallization cannot be sufficiently suppressed, and even if appropriate cooling is performed in the subsequent cooling process, the number density of ferrite cannot be controlled within the desired range in the plate thickness direction. On the other hand, if the cooling stop temperature is lower than 800 °C, excessive ferrite may be generated in the final metal structure. If excessive ferrite is generated, the strength may decrease, and even precipitation strengthening due to Ti precipitates may not be able to sufficiently reduce the hardness difference between ferrite and martensite, resulting in reduced hole expandability.

[0068] [(c) Reduction in each rolling pass of the last two stages: 10 to 40%] In this manufacturing method, the reduction in each rolling pass of the last two stages of finish rolling is controlled to 10 to 40%. By introducing strain at such a moderate reduction in each rolling pass of the last two stages, it is possible to increase the driving force for ferrite transformation in the subsequent cooling process. If the reduction in each rolling pass of the last two stages is less than 10%, the driving force for ferrite transformation in the subsequent cooling process cannot be sufficiently increased, and the desired ferrite area ratio cannot be achieved in the final metal structure. As a result, uniform elongation decreases. On the other hand, if the reduction in each rolling pass of the last two stages exceeds 40%, the driving force for ferrite transformation becomes too large, which may lead to excessive ferrite formation in the final metal structure. Excessive ferrite formation may result in a decrease in strength, or the hardness difference between ferrite and martensite may not be sufficiently reduced even by precipitation strengthening due to Ti precipitates, resulting in a decrease in hole expandability. Preferably, the reduction ratio in each of the last two rolling passes of the finish rolling is 15 to 38%.

[0069] [Cooling Process] [Cooling to a temperature range of 600 to 750 ° C within 4.0 seconds from the start of water cooling, followed by air cooling for 2.0 to 8.0 seconds] The finish-rolled steel sheet is water-cooled in the next cooling process, cooled to a temperature range of 600 to 750 ° C within 4.0 seconds from the start of water cooling, and then air-cooled at this temperature range for 2.0 to 8.0 seconds. First, by cooling to a temperature range of 600 to 750 ° C within 4.0 seconds from the start of water cooling, it is possible to reliably suppress the formation of pearlite and bainite, and therefore to achieve the desired area fraction of the metal structure in the final steel sheet. In contrast, if the time from the start of water cooling to the temperature range of 600 to 750 ° C exceeds 4.0 seconds, a relatively large amount of ferrite is formed, which may result in a decrease in strength and / or hole expandability.

[0070] Furthermore, air-cooling for 2.0 to 8.0 seconds in the temperature range of 600 to 750°C after water-cooling promotes the transformation to ferrite and allows Ti precipitates to be properly precipitated. Therefore, air-cooling for 2.0 to 8.0 seconds in the temperature range of 600 to 750°C after water-cooling is important not only for the proper generation of ferrite but also for the improvement of hole expandability and other properties due to precipitation strengthening caused by Ti precipitates. For example, if the air-cooling temperature is less than 600°C, the transformation to ferrite cannot be sufficiently promoted, while relatively large amounts of bainite may be generated. In such cases, the generation of large amounts of bainite reduces uniform elongation, and further, the generation of martensite associated with the generation of bainite reduces the generation of sufficient strength, which may result in insufficient strength.

[0071] Similarly, if the air-cooling temperature exceeds 750°C or the air-cooling time is less than 2.0 seconds, the transformation to ferrite cannot be sufficiently promoted, resulting in a decrease in uniform elongation. On the other hand, if the air-cooling time exceeds 8.0 seconds, a relatively large amount of ferrite may be generated. In such cases, the strength decreases, and the precipitation strengthening due to Ti precipitates cannot sufficiently reduce the hardness difference between ferrite and martensite, resulting in a decrease in hole expandability. The air-cooling temperature is preferably 620 to 730°C, and the air-cooling time is preferably 3.0 to 6.0 seconds.

[0072] [Water Cooling to 50°C or Less Within 13 Seconds After Air Cooling] After air cooling for 2.0 to 8.0 seconds in the temperature range of 600 to 750°C, i.e., after the end of air cooling, the steel sheet is water cooled to 50°C or less within 13 seconds. This rapid cooling allows martensite to be formed within the desired area ratio range. If the water cooling to 50°C or less takes more than 13 seconds or the cooling stop temperature is higher than 50°C, a relatively large amount of bainite may be formed. In such cases, the desired uniform elongation cannot be achieved. The lower limit of the water cooling time is not particularly limited, but for example, the water cooling time to 50°C or less after air cooling may be 4 seconds or more or 5 seconds or more. The lower limit of the water cooling stop temperature is also not particularly limited, but for example, the water cooling stop temperature may be 20°C or more or 25°C or more. The water-cooled steel sheet can finally be coiled into the form of a hot-rolled coil. The coiling conditions are not particularly limited, and the coiling can be performed under any appropriate temperature conditions, such as room temperature.

[0073] Steel sheets manufactured by the above manufacturing method have a metal structure containing, by area percentage, 10-40% ferrite and 60-90% martensite, thereby achieving high strength, for example, a tensile strength of 1180 MPa or more, while significantly improving uniform elongation. Furthermore, by controlling the Ti content in the steel to 0.070% by mass or more, Ti precipitates precipitation-strengthen the soft ferrite structure, thereby increasing the yield ratio and reducing the hardness difference between ferrite and martensite in the metal structure, thereby significantly improving hole expandability. In addition, by uniforming the number density of ferrite within a predetermined range in the thickness direction of the steel sheet, load reduction during collision can be suppressed. Therefore, steel sheets manufactured by the above manufacturing method have high uniform elongation, hole expandability, and yield ratio despite their high strength, and can significantly suppress the occurrence of fractures associated with load reduction during collision. Therefore, the steel sheet manufactured by the above manufacturing method reliably achieves both the contradictory properties of high strength and excellent workability at a high level, and also achieves excellent impact resistance, making it particularly useful in the automotive field where these properties are required.

[0074] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0075] In the following examples, steel sheets according to the embodiments of the present invention, particularly hot-rolled steel sheets, were produced under various conditions, and the tensile strength (TS), yield ratio (YR), uniform elongation (uEl), hole expansion ratio (λ), and work softening rate after uniform elongation of the obtained steel sheets were investigated.

[0076] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Tables 1 and 2. These slabs were heated to a temperature of 1180 to 1350°C and held for 6000 to 10000 seconds, and then hot-rolled. Hot-rolling was carried out by performing rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all Examples and Comparative Examples, and finish rolling was carried out under the conditions shown in Table 3 using a tandem rolling mill consisting of five rolling stands. Next, the finish-rolled steel plate was water-cooled, air-cooled, and water-cooled under the conditions shown in Table 3, and then coiled to obtain a steel plate having a thickness of 2.4 to 3.4 mm.

[0077]

[0078]

[0079]

[0080] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0081] [Tensile strength (TS) and uniform elongation (uEl)] The tensile strength (TS) and uniform elongation (uEl) were measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022.

[0082] [Hole expansion ratio (λ)] The hole expansion ratio (λ) was determined as follows. First, a test piece 100 mm wide x 100 mm long was taken from the steel plate, and a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%) was used to create a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the burr was placed on the die side, and the initial hole was expanded with a conical punch having an apex angle of 60 ° until a crack penetrating the plate thickness occurred. The hole diameter d1 mm at the time of crack occurrence was measured, and the hole expansion ratio λ (%) of each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[0083] [Yield Ratio (YR)] The yield ratio (YR) was determined by the following formula based on the tensile strength (TS) and 0.2% proof stress measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022: Yield ratio YR = 0.2% proof stress / tensile strength TS × 100

[0084] [Absolute Value of Work Softening Rate After Uniform Elongation (uEl)] The absolute value of the work softening rate after uniform elongation (uEl) was determined as follows. First, dσ0 / dε0 was calculated by differentiating the true stress at uniform elongation with true strain, where σ is the true stress (MPa) and ε is the true strain. Next, dσ1 / dε1 was calculated when the true strain increased by 0.005 from the uniform elongation. Finally, |dσ0 / dε0 - dσ1 / dε1| / 0.005 was calculated based on these values, and the obtained value was determined as the absolute value (MPa) of the work softening rate.

[0085] A steel sheet having a tensile strength (TS) of 1180 MPa or more, a uniform elongation (uEl) of 5.0% or more, a hole expansion ratio (λ) of 40% or more, a yield ratio (YR) of 75% or more, and an absolute value of the work softening rate after uniform elongation of 250,000 MPa or less was evaluated as having high strength, high uniform elongation, hole expandability, and yield ratio, and being able to suppress load reduction during collision. The results are shown in Table 4. In Table 4, "Maximum difference in number density of ferrite / Nαm" means the value obtained by dividing the maximum of the six number density differences shown in FIG. 1, i.e., the difference between Nα1 and Nα4, the difference between Nα4 and Nα7, the difference between Nα2 and Nα5, the difference between Nα5 and Nα8, the difference between Nα3 ​​and Nα6, and the difference between Nα6 and Nα9, by Nαm. Therefore, when this value is 0.60 or less, the requirement that "the difference in number density of ferrite in each divided region adjacent to each other in the sheet thickness direction is all Nαm × 0.60 or less" is satisfied. In addition, in the metal structure shown in Table 4, the remaining structure was at least one of pearlite and retained austenite.

[0086]

[0087] With reference to Tables 1 to 4, in Comparative Example 2, the rolling temperature in each of the two rolling passes immediately preceding the last two passes in the hot rolling process was low, which is thought to have prevented recrystallization from being sufficiently promoted, resulting in some unrecrystallized grains remaining. As a result, the number density of ferrite in the final metallographic structure could not be sufficiently uniformed in the thickness direction, i.e., the maximum difference in ferrite number density / Nαm exceeded 0.60. In relation to this, the absolute value of the work-softening rate after uEl exceeded 250,000 MPa, i.e., work-softening after uEl was significant. In Comparative Example 3, the rolling temperature in each of the two rolling passes immediately preceding the last two passes was high, which is thought to have caused austenite grains to coarsen after recrystallization, reducing the number of austenite grain boundaries and decreasing the number of ferrite nucleation sites. As a result, the number density of ferrite in the final metallographic structure could not be sufficiently uniformed in the thickness direction, resulting in the absolute value of the work-softening rate after uEl exceeding 250,000 MPa. In Comparative Example 4, the reduction rate in each of the two rolling passes immediately preceding the last two rolling passes was low, which is thought to have prevented recrystallization from being sufficiently promoted, leaving some unrecrystallized grains. As a result, the number density of ferrite in the final metal structure could not be sufficiently uniformed in the thickness direction, and the absolute value of the work-softening rate after uEl exceeded 250,000 MPa. In Comparative Example 5, the reduction rate in each of the two rolling passes immediately preceding the last two rolling passes was high, which is thought to have resulted in the formation of flat austenite grains due to the introduction of excessive strain. As a result, the number density of ferrite in the final metal structure could not be sufficiently uniformed in the thickness direction, and the absolute value of the work-softening rate after uEl exceeded 250,000 MPa. In Comparative Example 6, the time required for cooling to a cooling stop temperature of 800 to 910°C after the two rolling passes immediately preceding the last two rolling passes was longer than 0.20 seconds, which is thought to have prevented the grain growth of austenite grains after recrystallization from being sufficiently suppressed. As a result, the number density of ferrite could not be made sufficiently uniform in the thickness direction in the finally obtained metal structure, and the absolute value of the work softening rate after uEl exceeded 250,000 MPa.

[0088] In Comparative Example 7, the cooling stop temperature during cooling between the last two rolling passes and the two immediately preceding rolling passes was low, resulting in the formation of a large amount of ferrite in the final metal structure. As a result, the precipitation strengthening due to Ti precipitates was unable to sufficiently reduce the hardness difference between ferrite and martensite, and λ decreased. In Comparative Example 8, the cooling stop temperature during cooling between the last two rolling passes and the two immediately preceding rolling passes was high, which is thought to have prevented the grain growth of austenite grains after recrystallization from being sufficiently suppressed. As a result, the number density of ferrite in the final metal structure could not be sufficiently uniform in the plate thickness direction, and the absolute value of the work softening rate after uEl exceeded 250,000 MPa. In Comparative Example 9, the reduction rate of each of the last two rolling passes was low, which is thought to have prevented the driving force of ferrite transformation in the subsequent cooling process from being sufficiently increased. As a result, the desired ferrite area ratio could not be achieved in the final metal structure, and uEl decreased. In Comparative Example 10, the rolling reduction rate in each of the latter two rolling passes was high, which is thought to have caused the driving force for ferrite transformation in the subsequent cooling process to become too large. As a result, a large amount of ferrite was generated in the final metal structure, and precipitation strengthening due to Ti precipitates was unable to sufficiently reduce the hardness difference between ferrite and martensite, resulting in a decrease in λ. In Comparative Example 11, the water cooling time before air cooling in the cooling process was long, resulting in a relatively large amount of ferrite. As a result, λ was also decreased. In Comparative Example 12, the air cooling temperature was low, resulting in a relatively large amount of bainite, resulting in a decrease in uEl. In Comparative Example 13, the air cooling temperature was high, resulting in an insufficient promotion of ferrite transformation, resulting in a decrease in uEl. In Comparative Example 14, the air cooling time was short, resulting in an insufficient promotion of ferrite transformation, resulting in a decrease in uEl. In Comparative Example 15, the air cooling time was long, resulting in a relatively large amount of ferrite, resulting in a decrease in λ. In Comparative Example 16, the time for water cooling to 50°C or less after air cooling was long, so a large amount of bainite was formed and uEl decreased.

[0089] In Comparative Example 44, the Ti content was low, which is thought to have prevented sufficient precipitation strengthening by Ti precipitates. As a result, TS decreased. In Comparative Example 45, the Ti content was high, which is thought to have caused coarse carbides and the like to form. As a result, the workability of the steel sheet decreased, and λ decreased. In Comparative Example 46, the Nb content was low, which is thought to have caused coarse carbides and the like to form in Comparative Example 47, which is thought to have caused coarse carbides and the like to form. As a result, the workability of the steel sheet decreased, and λ decreased.

[0090] In contrast, all of the steel sheets according to the examples of the present invention have a predetermined chemical composition, and by appropriately controlling the conditions in the manufacturing method, the metal structure is, by area%, 10-40% ferrite, 60-90% martensite, and 0-10% bainite. When a 150 μm x 150 μm region at 1 / 4 of the plate thickness in a cross section perpendicular to the plate surface is equally divided into nine, the number density of ferrite in each divided region is calculated, and the average value is taken as Nαm, the difference in number density of ferrite in each divided region adjacent to each other in the plate thickness direction is Nαm x 0.60 or less. As a result, despite having a high tensile strength of 1180 MPa or more, the steel sheets have high uniform elongation, hole expandability, and yield ratio, and the absolute value of the work softening rate after uniform elongation can be reliably reduced to 250,000 MPa or less.

Claims

1. The chemical composition is by mass percentage: C: 0.060 to 0.300%, Si: 0.30 to 1.50%, Mn: 1.00 to 2.70%, P: 0.100% or less, S: 0.0300% or less, sol. Al: 0.001 to 0.500%, O: 0.0100% or less, N: 0.0070% or less, Ti: 0.070 to 0.170%, Nb: 0.001 to 1.000%, B: 0 to 0.0030%, Cr: 0 to 0.70%, Mo: 0 to 0.12%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, V: 0 to 0.300%, Sn: 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0 to 0.0010%, Mg: 0 to 0.0010%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0 to 0.100%, Zn: 0 to 0.010%, REM: 0 to 0.0100%, and the balance: Fe and impurities. The metallographic structure is by area percentage: Ferrite: 10 to 40%, Martensite: 60 to 90%, Bainite: 0 to 10%, and at least one of Pearlite and Retained Austenite: a total of 0 to 5%. When a 150 μm × 150 μm area at the 1 / 4 plate thickness position of a cross-section perpendicular to the plate surface is evenly divided into 9 parts, the ferrite number density is calculated for each divided area, and when the average value thereof is Nαm, the difference in the ferrite number density in each divided area adjacent in the plate thickness direction is all Nαm × 0.60 or less. Steel plate characterized by the above.

2. The steel sheet according to claim 1, wherein the chemical composition contains at least one of the following in mass %: B: 0.0001 to 0.0030%, Cr: 0.001 to 0.70%, Mo: 0.001 to 0.12%, Cu: 0.001 to 0.40%, Ni: 0.001 to 0.30%, V: 0.001 to 0.300%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, Zr: 0.001 to 0.050%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, Bi: 0.001 to 0.010%, Co: 0.001 to 0.010%, W: 0.001 to 0.100%, Zn: 0.001 to 0.010%, and REM: 0.0001 to 0.0100%.

3. The Nam is 0.050 pieces / μm 2 The steel sheet according to claim 1 or 2, characterized in that the above is satisfied.

4. The steel sheet according to any one of claims 1 to 3, having a tensile strength of 1180 MPa or more.

5. The steel sheet according to any one of claims 1 to 4, having a plate thickness of 1.0 to 8.0 mm.

6. A component comprising the steel sheet according to any one of claims 1 to 5.

7. A method for manufacturing a steel sheet, comprising: a heating step of heating a slab having the chemical composition according to claim 1 or 2 and holding it at a temperature of 1180 to 1350°C for 6000 seconds or more; a hot rolling step of finish rolling the slab using a tandem rolling mill composed of 4 or more rolling stands, satisfying the following conditions (a) to (c): (a) the rolling temperature in each rolling pass of the two passes immediately before the last two passes is 960 to 1080°C, and the reduction ratio in each rolling pass is 30 to 50%; (b) cooling the rolled material to a cooling stop temperature of 800 to 910°C within 0.20 seconds after the rolling passes of the two passes immediately before the last two passes; and (c) the reduction ratio in each rolling pass of the last two passes is 10 to 40%; and a cooling step of water-cooling the finish-rolled steel sheet, cooling it to a temperature range of 600 to 750°C within 4.0 seconds from the start of water-cooling, then performing air-cooling for 2.0 to 8.0 seconds in the temperature range, and water-cooling the steel sheet to 50°C or less within 13 seconds after air-cooling.

Images