Steel plate, parts containing the same, and method for manufacturing steel plate

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

[Technical Field]

[0001] This invention relates to steel plates, parts containing the same, and methods for manufacturing steel plates. [Background technology]

[0002] In recent years, in response to environmental issues, there has been a growing demand for lighter automotive parts to reduce CO2 emissions and improve fuel efficiency. At the same time, there is an increasing social demand for improved collision safety. To achieve both weight reduction and improved collision safety, increasing the strength of steel materials is an effective means. However, increasing the strength of steel materials usually reduces their workability; therefore, there is a need for steel materials that can improve both strength and workability simultaneously.

[0003] In relation to improving strength and workability, for example, Patent Document 1 describes a cold-rolled steel sheet coil made of a composite structure steel sheet mainly composed of ferrite and martensite, having a microstructure in which ferrite accounts for 10-40% and martensite for 60-90% of the total structure, and when the ferrite fractions (indicated as "area %)" at the four corners and the centroid of an 800mm x 800mm steel sheet cut from any position in the coil are Vα1, Vα2, Vα3, Vα4, Vα5, and the average value of these five points is Vαm, all of the above ferrite fractions Vα1, Vα2, Vα3, Vα4, and Vα5 are within the range of Vαm ± 5 (area %). Furthermore, Patent Document 1 teaches that by strictly defining the conditions for heat treatment of the base steel sheet (cold-rolled steel sheet), a high-strength cold-rolled steel sheet coil with small variation in ferrite fraction within the coil can be obtained, thereby realizing a high-strength cold-rolled steel sheet coil with small strength variation within the coil and a tensile strength of 980 MPa or higher. It also teaches that such a high-strength cold-rolled steel sheet coil can be reliably formed as a steel sheet for automobiles, and specifically discloses in the examples that, in addition to improved tensile strength, overall elongation and hole-expandability are also improved. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2010-159453 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] As described above, it is known that as strength increases, the workability of steel materials decreases, and properties such as hole-expandability, as described in Patent Document 1, decline. When hole-expandability decreases, it may not be possible to process the material into the desired shape, for example, in automobile suspension parts. For this reason, in the development of high-strength steel sheets, it is important to increase strength while ensuring a certain level of properties appropriate to the application, such as uniform elongation in addition to the hole-expandability mentioned above. For example, in automobile suspension parts such as lower arms and trailing arms, which have complex shapes, the workability of the steel material itself decreases with increased strength, or work hardening occurs when forming the complex shape, resulting in strain being applied to the part near the maximum load after forming. Therefore, even when increasing strength to the same or greater level than conventional materials, there is still a high need for materials that can maintain a high and stable load during collision and thus do not cause or are less likely to cause fracture accompanied by a decrease in load.

[0006] Furthermore, for components requiring impact resistance, plastic deformation occurs when subjected to an impact exceeding the yield strength. Therefore, from the perspective of ensuring the crash safety of automobiles, it is necessary to improve not only the tensile strength but also the yield strength. Consequently, it is necessary to increase the yield ratio, which is the ratio of yield strength to tensile strength.

[0007] This invention has been made in view of the above circumstances, and its objective is to provide a steel plate, a component containing the same, and a method for manufacturing the steel plate, which, through a novel configuration, has high strength, high uniform elongation, hole-expanding properties, and a yield ratio, and can suppress load reduction during collision. [Means for solving the problem]

[0008] To achieve the above objective, the inventors focused on the microstructure of steel sheets, particularly hot-rolled steel sheets, and conducted research. As a result, the inventors found that by constructing the microstructure of a hot-rolled steel sheet having a predetermined chemical composition mainly from ferrite and martensite, and utilizing precipitation strengthening by the addition of Ti, it is possible to improve strength, uniform elongation, and yield ratio, as well as improve hole-expanding properties by reducing the hardness difference between ferrite and martensite. Furthermore, they discovered that by uniformly arranging ferrite in the microstructure, specifically by dividing a 150 μm × 150 μm region at the 1 / 4 thickness position of the cross section perpendicular to the surface of the steel sheet into nine equal parts, calculating the number density of ferrite in each divided region, and taking the average value of these as Nαm, the difference in the number density of ferrite in each adjacent divided region in the thickness direction is all Nαm × 0.60 or less, thereby significantly suppressing the load reduction after uniform elongation, and thus completed the present invention.

[0009] The present invention, which has achieved the above objectives, is as follows. (1) The chemical composition is expressed in mass%, C: 0.060~0.300%, Si: 0.30~1.50%, Mn: 1.00~2.70%, P: 0.100% or less, S: 0.0300% or less, sol.Al: 0.001~0.500%, O: 0.0100% or less, N: 0.0070% or less, Ti: 0.070~0.170%, Nb: 0.001~1.000%, B: 0~0.0030%, Cr: 0~0.70%, Mo: 0~0.12%, Cu: 0~0.40%, Ni: 0~0.30%, V: 0~0.300%, Sn: 0~0.040%, 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, in area%, 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. A steel plate characterized by this. (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%, W: 0.001 to 0.100%, Zn: 0.001~0.010%, and REM: 0.0001~0.0100% The steel plate according to (1) above, characterized in that it includes at least one of the following. (3) 0.050 Nαm particles / μm 2 The steel plate described in (1) or (2) above, characterized in that it is as described above. (4) A steel sheet as described in any one of the above items (1) to (3), characterized by having a tensile strength of 1180 MPa or more. (5) A steel plate according to any one of the above items (1) to (4), characterized by having a plate thickness of 1.0 to 8.0 mm. (6) A component characterized by including a steel plate as described in any one of the above items (1) to (5). (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 process that includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and satisfies the following conditions (a) to (c): (a) The rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is 960 to 1080°C, and the reduction ratio in each rolling pass is 30 to 50%. (b) Cool the rolled material to a cooling stop temperature of 800-910°C within 0.20 seconds after the rolling pass of the two stages immediately preceding the two stages of the subsequent stage, and (c) The reduction ratio in each of the two subsequent rolling passes is 10-40%. A cooling process comprising: water cooling the finish-rolled steel sheet; cooling it to a temperature range of 600-750°C within 4.0 seconds from the start of water cooling; then air cooling in the said temperature range for 2.0-8.0 seconds; and water cooling the steel sheet to 50°C or below within 13 seconds after air cooling. A method for manufacturing steel plates, including the method described above. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a steel plate that has high strength, high uniform elongation, hole-expanding properties and yield ratio, and can suppress load reduction during collision, a part containing the same, and a method for manufacturing the steel plate. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram illustrating the characteristics of the number density of ferrite, where (a) shows an example that does not satisfy the characteristics of the number density of ferrite according to the present invention, and (b) shows an example that satisfies those characteristics. [Modes for carrying out the invention]

[0012] <Steel plate> The steel sheet according to the embodiment of the present invention, in particular the hot-rolled steel sheet, has a chemical composition in mass%. C: 0.060~0.300%, Si: 0.30~1.50%, Mn: 1.00~2.70%, P: 0.100% or less, S: 0.0300% or less, sol.Al: 0.001~0.500%, O: 0.0100% or less, N: 0.0070% or less, Ti: 0.070~0.170%, Nb: 0.001~1.000%, B: 0~0.0030%, Cr: 0~0.70%, Mo: 0~0.12%, 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: 0~0.010%, W: 0~0.100%, Zn: 0~0.010%, REM: 0~0.0100%, and The remainder consists of Fe and impurities. The metallic structure, in area percentage, Ferrite: 10-40%, Martensite: 60-90% Baynite: 0-10%, and At least one of perlite and retained austenite: totaling 0-5%, The method is characterized by the fact that when a 150 μm × 150 μm region at the 1 / 4 thickness position of a cross-section perpendicular to the plate surface is evenly divided into 9 sections, the number density of ferrite in each divided section is calculated, and the average value of these is taken as Nαm, the difference in the number density of ferrite in each adjacent divided section in the thickness direction is all Nαm × 0.60 or less.

[0013] As mentioned earlier, it is known that properties such as hole-expandability decrease as the strength of steel increases. For example, in order to manufacture parts with complex shapes such as lower arms and trailing arms in the suspension of automobiles, a steel sheet is required that has high strength, such as a tensile strength of 1180 MPa or more that enables weight reduction, while also having excellent hole-expandability. From the viewpoint of increasing strength, it is preferable that the metal structure of the steel sheet be composed mainly of martensite. However, although martensitic steel has excellent strength, if it is included in excess, properties such as uniform elongation decrease, so it generally has the problem of poor workability. In addition, in parts with complex shapes such as lower arms and trailing arms, the workability of the steel material itself decreases as the strength increases, or work hardening occurs when forming the complex shape, so that the strain is applied up to near the maximum load after the part is formed. Therefore, there is still a high demand for high-strength steel sheets that have improved properties such as hole-expandability and uniform elongation, can maintain a high and stable load during collisions even when forming parts with complex shapes, thereby suppressing the occurrence of fractures accompanied by load reduction, and have a high yield ratio from the perspective of automobile collision safety, etc.

[0014] Therefore, in addition to making the chemical composition of the steel sheet appropriate, the inventors focused particularly on the metallographic structure of the steel sheet. First, the inventors found that by constructing the metallographic structure of a steel sheet having a predetermined chemical composition with a structure mainly composed of soft ferrite and hard martensite, more specifically, a structure containing 10-40% ferrite and 60-90% martensite by area percentage, the strength and uniform elongation can be improved through the mechanism of action of so-called DP (Dual Phase) steel.

[0015] On the other hand, since DP steel generally has a low yield ratio, the inventors have found that by utilizing precipitation strengthening through the addition of Ti, the yield ratio can be increased and hole-expanding properties can be significantly improved. Although we do not intend to be bound by any particular theory, it is thought that this improvement in hole-expanding properties due to precipitation strengthening is due to a reduction in the hardness difference between ferrite and martensite in the microstructure. To explain in more detail, in the steel sheet according to the embodiment of the present invention, as described above, the microstructure is composed mainly of ferrite and martensite, and may contain up to 40% by area percentage of ferrite, which is a soft microstructure. In this case, the hardness difference between ferrite and martensite in the microstructure becomes high, and hole-expanding properties decrease. However, in the steel sheet according to the embodiment of the present invention, by controlling the Ti content in the steel to 0.070 mass% or more, the soft microstructure of ferrite is precipitation-strengthened by Ti precipitates, thereby reducing the hardness difference between ferrite and martensite in the microstructure, and therefore it is thought that hole-expanding properties can be significantly improved.

[0016] Next, the inventors considered that in order to maintain a high and stable load even during a collision and suppress the occurrence of fracture, it is necessary to suppress the work softening after the uniform elongation, which corresponds to the elongation at the point of maximum load in a uniaxial tensile test, and conducted an investigation. This is because, by suppressing the work softening after the uniform elongation, it becomes possible to effectively absorb the collision energy from the maximum load to fracture during a collision through the plastic deformation of the steel plate. First, in experiments conducted by the inventors, the work softening rate after the uniform elongation (d 2 σ / dε 2 It was found that when the absolute value of (σ: true stress, ε: true strain) was controlled to 250,000 MPa or less, fracture could be completely or significantly suppressed when the molded parts were subjected to a drop weight test. Therefore, in order to stably achieve such a processing softening rate, the inventors focused on ferrite, a soft structure in which deformation tends to concentrate during impact, and conducted further investigations. As a result, the inventors found that by uniformly arranging ferrite in the thickness direction of the steel plate, more specifically by dividing a 150 μm × 150 μm region at the 1 / 4 thickness position of the cross section perpendicular to the surface of the steel plate into nine equal parts, calculating the number density of ferrite in each divided region, and taking the average value of these as Nαm, uniformly arranging ferrite in the metal structure such that the absolute value of the difference in the number density of ferrite in adjacent divided regions in the thickness direction, or more specifically, the absolute value of the difference in the number density of ferrite in adjacent divided regions in the thickness direction, is all Nαm × 0.60 or less, 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. In connection with this, the inventors found that even when the steel plate is formed into parts, particularly parts with complex shapes such as lower arms and trailing arms, the load reduction during collision can be suppressed and the occurrence of fracture can be significantly suppressed.

[0017] Fig. 1 is a schematic diagram for explaining the characteristics regarding the number density of ferrite. Fig. 1(a) shows an example that does not satisfy the characteristics regarding the number density of ferrite according to the present invention, that is, "when a 150 μm × 150 μm region at the 1 / 4 position of the plate thickness in a cross-section perpendicular to the plate surface is equally divided into 9 parts, the number density of ferrite is calculated in each divided region, and when the average value thereof 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". Fig. 1(b) shows an example that satisfies the said characteristics. Referring to Fig. 1(a), first, a 150 μm × 150 μm region at the 1 / 4 position of the plate thickness in a cross-section perpendicular to the plate surface of the steel plate is equally divided into 9 parts, and the number density of ferrite is calculated in each divided region, that is, each of Nα1 to Nα9, and their average value Nαm = 0.058 pieces / μm 2 is determined (Fig. 1(a)(i)). Next, the differences in the number density of ferrite in each divided region adjacent in the plate thickness direction, that is, the difference between Nα1 and Nα4 (0.045 pieces / 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 ) are calculated (Fig. 1(a)(ii)). If all of these 6 differences in the number density are Nαm (0.058 pieces / μm 2 ) or less (Nαm × 0.60 or less), that is, if the value obtained by dividing each difference in the number density by Nαm is all 0.60 or less, then the characteristics regarding the number density of ferrite according to the present invention are satisfied. However, in Fig. 1(a), the value obtained by dividing the difference between Nα1 and Nα4 (0.045 pieces / μm 2 ) by Nαm (0.058 pieces / μm 2 ) is 0.78, and it can be seen that the said characteristics are not satisfied.

[0018] On the other hand, referring to Figure 1(b), all of the differences in the number density of ferrite in each adjacent divided region in the thickness direction, calculated in the same way, are equal to the average value Nαm (0.056 particles / μm). 2 The number density is 0.60 times or less (Nαm × 0.60 or less), and therefore it can be seen that the characteristics regarding the number density of ferrite according to the present invention are satisfied. In the embodiment of the present invention, by making the number density of ferrite uniform in the thickness direction of the steel plate, that is, by uniformly arranging ferrite in the thickness direction of the steel plate, the absolute value of the work softening rate after uniform elongation in the uniaxial tensile test can be reliably reduced to 250,000 MPa or less, and therefore even when the steel plate is formed into parts having complex shapes such as lower arms and trailing arms, it is possible to significantly suppress the occurrence of fracture during collision.

[0019] While not intended to be bound by any particular theory, it is believed that by uniformly distributing ferrite within the metal structure, localized deformation can be suppressed even during impact, thereby reducing the load drop during impact and maintaining a high and stable load, and consequently significantly suppressing the occurrence of fracture. To explain in more detail, when a steel sheet is composed mainly of soft ferrite and hard martensite, generally the hard martensite ensures strength, and the soft ferrite is responsible for deformation. Therefore, deformation during impact tends to concentrate in the soft ferrite. In such cases, if there are regions in the metal structure where ferrite is relatively abundant locally, it is thought that the strain during deformation will concentrate in these regions. As a result, localized deformation cannot be suppressed, and it is thought that the load during impact decreases due to such deformation. Considering the deformation modes during impact, it is important to suppress the load drop, especially in bending deformation, and in bending deformation, fracture occurs in the thickness direction starting from the surface layer of the steel sheet. Therefore, in order to suppress the load reduction during impact, it is considered effective to make the arrangement of ferrite uniform in the metal structure, especially in the thickness direction. In fact, as shown in Figure 1, by uniformly arranging ferrite in the 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, and it has been found that even when the steel plate is formed into parts with complex shapes such as lower arms and trailing arms, it is possible to significantly suppress the occurrence of fracture during impact. The fact that uniformly arranging ferrite in the thickness direction of a steel plate composed mainly of ferrite and martensite can reduce the work softening rate after uniform elongation, and furthermore, that this can significantly suppress the occurrence of fracture during impact, was not previously known and has now been revealed for the first time by the present inventors.Therefore, according to embodiments of the present invention, for example, despite having high strength with a tensile strength of 1180 MPa or more, it has high uniform elongation, hole-expanding properties and yield ratio, and can significantly suppress the occurrence of fracture accompanied by load reduction during collisions. Thus, steel sheets according to embodiments of the present invention are particularly useful for use in the automotive sector.

[0020] 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 and upper limits, respectively, unless otherwise specified.

[0021] [C:0.060~0.300%] Carbon (C) is an effective element for increasing the strength of steel sheets. Furthermore, C forms carbides and / or carbonitrides with Nb in the steel, contributing to microstructure refinement through the pinning effect of the formed precipitates. To fully obtain these effects, the C content should be 0.060% or higher. The C content may also be 0.070% or higher, 0.080% or higher, 0.100% or higher, 0.120% or higher, or 0.150% or higher. On the other hand, excessive C content may reduce uniform elongation. Therefore, the C content should be 0.300% or lower. The C content may also be 0.280% or lower, 0.250% or lower, 0.200% or lower, 0.180% or lower, or 0.160% or lower.

[0022] [Si: 0.30~1.50%] Si is an element that suppresses the formation of iron carbides and contributes to improved strength and formability. To fully obtain these effects, the Si content should be 0.30% or more. The Si content may be 0.40% or more, 0.50% or more, 0.60% or more, 0.70% or more, or 0.80% or more. On the other hand, if the Si content is excessive, the ferrite fraction will increase, which may lead to a large difference in hardness between ferrite and martensite, and a decrease in hole expansion properties. Therefore, the Si content should be 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: 1.00~2.70%] Mn is an effective element for increasing strength as a hardenability and solid solution strengthening element. To obtain these effects to the fullest, the Mn content should be 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, if the Mn content is excessive, the ferrite fraction may decrease due to an excessive improvement in hardenability, and the uniform elongation may decrease. Therefore, the Mn content should be 2.70% or less. The Mn content may be 2.60% or less, 2.50% or less, 2.40% or less, 2.30% or less, or 2.20% or less.

[0024] [P:0.100% or less] If phosphorus (P) is present in excessive amounts, processability may decrease due to grain boundary segregation, etc. Therefore, the P content should be 0.100% or less. The P content may also be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0025] [S:0.0300% or less] Excessive sulfur content can lead to the formation of many sulfides such as MnS, which can reduce processability. Therefore, the sulfur content should be 0.0300% or less. The sulfur content may also be 0.0200% or less, 0.0100% or less, or 0.0050% or less. The lower limit of the sulfur content is not particularly limited and may be 0%, but excessive reduction will lead to increased costs. Therefore, the sulfur content may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[0026] [sol.Al:0.001~0.500%] sol.Al is an element that acts as a deoxidizing agent for molten steel. To obtain this effect, the sol.Al content should be 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, if the sol.Al content is excessive, the ferrite fraction will increase, which may lead to a large difference in hardness between ferrite and martensite, and a decrease in hole-expanding properties. Therefore, the sol.Al content should be 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 means acid-soluble Al, and refers to solid-solution Al present in the steel in a solid-solution state.

[0027] [O:0.0100% or less] O is an element that is introduced during the manufacturing process. Excessive O content can lead to the formation of coarse inclusions, which can 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, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing it to less than 0.0001% requires more time for refining, leading to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.

[0028] [N:0.0070% or less] If N is present in excess, it can form coarse nitrides, which may cause slab cracking during hot rolling. Therefore, the N content should be 0.0070% or less. The N content may also 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 lead to increased costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[0029] [Ti: 0.070~0.170%] Ti precipitates in steel as Ti carbides such as TiC, and through precipitation strengthening, it strengthens soft structures such as ferrite, contributing to improvements in strength and yield ratio. Furthermore, because Ti reduces the hardness difference between ferrite and martensite in the metal structure due to precipitation strengthening, it is also effective in improving hole expansion properties. To fully obtain these effects, the Ti content should be 0.070% or higher. The Ti content may be 0.080% or higher, 0.090% or higher, 0.100% or higher, or 0.120% or higher. On the other hand, if the Ti content is excessive, coarse carbides may be formed in the steel, which may cause slab cracking during hot rolling or reduce the workability of the steel sheet. Therefore, the Ti content should be 0.170% or lower. The Ti content may be 0.160% or lower, 0.150% or lower, 0.140% or lower, or 0.130% or lower.

[0030] [Nb:0.001~1.000%] Nb is an element that contributes to the refinement of prior austenite grains and, consequently, to the increased strength of steel sheets by forming carbides, nitrides, and / or carbonitrides in steel through a pinning effect. To fully obtain this effect, the Nb content should be 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, if the Nb content is excessive, coarse carbides and the like may be formed in the steel, which may reduce the workability of the steel sheet. Therefore, the Nb content should be 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 optionally contain at least one of the following elements in place of a portion of the remaining Fe.

[0032] [B: 0~0.0030%] B is an element that enhances the hardenability of steel and contributes to improving its strength. The B content may be 0%, but to obtain such effects, it is preferable that the B content be 0.0001% or more. The B content may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if the B content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the B content be 0.0030% or less. The B content may also be 0.0025% or less, 0.0020% or less, 0.0015% or less, or 0.0010% or less.

[0033] [Cr: 0~0.70%] Cr is an element that enhances the hardenability of steel and contributes to improving its strength and / or corrosion resistance. While the Cr content may be 0%, to obtain these effects, it is preferable that the Cr content be 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Cr content may lead to saturation of the effect and an increase in manufacturing costs. Therefore, it is preferable that the Cr content be 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: 0~0.12%] Mo is an element that enhances the hardenability of steel and contributes to improving its strength. While the Mo content may be 0%, it is preferable that the Mo content be 0.001% or more to obtain such effects. The Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, if the Mo content is excessive, the deformation resistance during hot working may increase, and the equipment load may increase. Therefore, it is preferable that the Mo content be 0.12% or less. The Mo content may be 0.10% or less, 0.08% or less, 0.06% or less, or 0.05% or less.

[0035] [Cu: 0~0.40%] Cu is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Cu content may be 0%, but to obtain such an effect, it is preferable that the Cu content be 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if these elements are included in excess, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the Cu content be 0.40% or less. The Cu content may be 0.30% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0036] [Ni: 0~0.30%] Ni is an element that contributes to improving strength through precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but to obtain such an effect, it is preferable that the Ni content be 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if these elements are included in excess, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the Ni content be 0.30% or less. The Ni content may be 0.20% or less, 0.15% or less, 0.10% or less, or 0.08% or less.

[0037] [V: 0~0.300%] V is an element that contributes to improving strength through precipitation strengthening, etc. The V content may be 0%, but to obtain such an effect, it is preferable that the V content be 0.001% or more. The V content may also be 0.010% or more, 0.030% or more, or 0.050% or more. On the other hand, if the V content is excessive, the effect will saturate, which may lead to an increase in manufacturing costs. Therefore, it is preferable that the V content be 0.300% or less. The V content may also be 0.200% or less, 0.100% or less, or 0.080% or less.

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

[0039] In the steel sheet according to the embodiment of the present invention, the remainder of the elements other than those mentioned above consists of Fe and impurities. Impurities are components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.

[0040] 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 spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas fusion-thermal conductivity method, and O can be measured using the inert gas fusion-nondispersive infrared absorption method.

[0041] [Metal structure] [Ferrite: 10-40%] The metal structure of the steel sheet according to the embodiment of the present invention contains 10-40% ferrite by area percentage. By including 10% or more of ferrite, which is a soft structure, by area percentage, the desired uniform elongation can be achieved. From the viewpoint of further improving uniform elongation, a higher area percentage of ferrite 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 area percentage of ferrite becomes too high, the strength and / or yield ratio may decrease, 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-expanding properties. Therefore, the area percentage of ferrite should be 40% or less. From the viewpoint of further improving strength, yield ratio and / or hole-expanding properties, a lower area percentage of ferrite 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: 60-90%] The metal structure of the steel sheet according to the embodiment of the present invention contains 60-90% martensite by area percentage. By constructing the metal structure of the steel sheet with a structure containing hard martensite within this range, high strength, such as a tensile strength of 1180 MPa or higher, can be achieved. From the viewpoint of further increasing strength, a higher area ratio of martensite is preferable, for example, it may be 65% or more, 68% or more, 70% or more, 72% or more, or 75% or more. On the other hand, if the area ratio of martensite becomes too high, the uniform elongation may decrease. Therefore, the area ratio of martensite should be 90% or less, 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] [Baynight: 0-10%] The metal structure of the steel sheet according to the embodiment of the present invention may contain bainite. However, if the area ratio of bainite becomes too high, the uniform elongation may decrease. Therefore, the area ratio of bainite should be 10% or less, 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 ratio of bainite may be 0%, for example, 0.5% or more, 1% or more, or 2% or more.

[0044] [Remaining tissue] The remaining microstructure other than ferrite, martensite, and bainite may be 0% in area percentage, but if the remaining microstructure is present, it may be at least one of pearlite and retained austenite. If the combined area percentage of pearlite and at least one of retained austenite exceeds 5%, it may lead to a decrease in uniform elongation, or make it impossible to control ferrite and / or martensite within the desired range. Therefore, the combined area percentage of pearlite and at least one of retained austenite should be 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 combined area percentage of pearlite and at least one of retained austenite may be 0%, for example, 0.1% or more, 0.5% or more, or 1% or more.

[0045] [Identification of metallographic structure and calculation of area ratio] Next, we will explain the identification of metals and the calculation of area ratios. Microstructure observation is performed on the thickness cross section perpendicular to the plate surface. While it is preferable that the thickness cross section be parallel to the rolling direction, it is not necessary for the thickness cross section to be parallel to the rolling direction, for example, when the rolling direction of the steel plate cannot be determined. Specifically, first, a test piece is taken from the steel plate, and the cross section of the test piece is polished using silicon carbide sandpaper from #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, this cross section is polished for 8 minutes at room temperature with colloidal silica with a particle size of 0.25 μm that does not contain alkaline solutions to remove the strain introduced into the surface layer of the test piece. Then, crystal orientation information is obtained by measuring at measurement intervals of 0.1 μm using electron backscatter diffraction in a rectangular region of 150 μm in the thickness direction and 150 μm in the direction perpendicular to the thickness direction, centered at 1 / 4 of the thickness from the steel surface.

[0046] For measurement, it is preferable to use an EBSD apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). In this case, the vacuum level inside the EBSD apparatus should be 9.6 × 10⁻⁶. -5 It is preferable that the Pa is less than or equal to 15kV, the acceleration voltage is 15kV, the irradiation current level is 13, and the electron beam irradiation level is 62. Other observation conditions are preferably as follows. Electronic gun type: Schottky WD (Working Distance): 15mm Objective aperture number: 4 Pixel count: 4096 x 5120 pixels

[0047] [Identification of retained austenite and calculation of area ratio] From the obtained crystal orientation information, the "Phase Map" function included in the "OIM Analysis®" software attached to the EBSD analyzer is used to identify regions with an fcc crystal structure, and the area fraction of these regions is calculated. This yields the area fraction of retained austenite.

[0048] [Identification of ferrite and calculation of area ratio] Next, materials with a bcc crystal structure are identified as "ferrite, martensite, bainite, and pearlite." For these regions, the grain average misorientation (GAM value) is calculated using the "OIM Analysis®" software included with the EBSD analyzer. Finally, regions with a GAM value of 0.5° or less are identified as ferrite, and their area fraction is calculated. Here, the "GAM value" is the average of the grain averages between adjacent pixels in a region enclosed by grain boundaries with a grain average of 15° or more.

[0049] [Identification of bainite and calculation of area ratio] Next, within the remaining region (the region where the "GAM value" is greater than 0.5°), under the condition that the boundary with a crystal orientation difference of 15° is considered a grain boundary, when the maximum value of the "Grain Average IQ" in the ferrite region is Iα, regions where the value is greater than Iα / 2 are extracted as "bainite," and regions where the value is less than or equal to Iα / 2 are extracted as "martensite and pearlite." The area ratio of the extracted bainite is obtained by calculating the area ratio of bainite.

[0050] [Identification of perlite and calculation of area ratio] The area ratio of pearlite is calculated by observing secondary electron images using FE-SEM after etching with Nital reagent. To observe secondary electron images using FE-SEM in the same region as the EBSD measurement area, Vickers indentations are stamped near the observation position. Then, surface contaminants are polished off, leaving the microstructure of the observation surface, and Nital etching is performed. Next, the same field of view as the EBSD observation surface is observed using FE-SEM, for example at a magnification of 800x. The area ratio of pearlite is calculated by performing image analysis on the microstructure obtained using FE-SEM in the same region as the EBSD measurement area, i.e., a rectangular region of 150 μm in the thickness direction and 150 μm perpendicular to the thickness direction, centered at 1 / 4 of the thickness from the steel plate surface. Here, a microstructure in which plate-like ferrite and Fe-based carbides are layered is considered to be pearlite.

[0051] [Identification of martensite and calculation of area ratio] The area percentage of martensite is calculated by subtracting the area percentages of retained austenite, ferrite, bainite, and pearlite obtained from 100%. Other metal structures are identified sequentially, and the last remaining structure is considered to be martensite.

[0052] [Difference in ferrite number density in adjacent divided regions in the thickness direction: all less than or equal to Nαm × 0.60] In the steel sheet according to the embodiment of the present invention, a 150 μm × 150 μm region at the 1 / 4 position of the sheet thickness in a cross section perpendicular to the sheet surface is evenly divided into 9 sections. When the number density of ferrite is calculated in each divided section and its average value is taken as Nαm, the absolute value of the difference in the number density of ferrite in adjacent divided sections in the sheet thickness direction, or more specifically, the absolute value of the difference in the number density of ferrite in adjacent divided sections in the sheet thickness direction, is controlled to be Nαm × 0.60 or less. By uniformly arranging ferrite in the sheet thickness direction of the steel sheet in this way, the absolute value of the processing softening rate after uniform elongation in a uniaxial tensile test can be reliably reduced to 250,000 MPa or less, thereby suppressing the load reduction during collision. As a result, even when the steel sheet according to the embodiment of the present invention is formed into automobile parts, particularly automobile suspension parts, such as lower arms and trailing arms, which have complex shapes, the occurrence of fracture during collision can be significantly suppressed.

[0053] From the viewpoint of further suppressing load reduction during collision, it is preferable that the difference in the number density of ferrite in each adjacent segmented region in the thickness direction of the plate is small. For example, the difference in the number density of ferrite in each adjacent segmented region in the thickness direction of the plate 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. The lower limit is not particularly limited, but for example, the difference in the number density of ferrite in each adjacent segmented region in the thickness direction of the plate 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 particles / μm] 2 [End] According to a preferred embodiment of the present invention, Nαm is 0.050 particles / μm 2 This concludes the explanation. In the steel sheet 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 sheet, and from the viewpoint of improving the uniformity of ferrite arrangement, it is preferable to refine the ferrite grains. When ferrite grains are refined, 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 adjacent divided region in the thickness direction is reduced, thereby acting in a direction that suppresses the load reduction during collision. From the viewpoint of making this effect more pronounced, it is preferable for Nαm to be as high as possible, for example, 0.055 grains / μm 2 More than 0.060 pieces / μm 2 More than 0.065 pieces / μm 2 More than 0.070 pieces / μm 2 More than 0.075 pieces / μm 2 Above or above, or 0.080 particles / μm 2 The above is also acceptable. There is no particular upper limit, but for example, Nαm is 0.200 particles / μm 2 Below, 0.150 pieces / μm 2 Less than or 0.120 pieces / μm 2 The following is also acceptable.

[0055] [Measurement of ferrite number density in Nαm and each divided region] The number density of ferrite in Nαm and each divided region is measured by EBSD in the same way as for ferrite identification and area fraction calculation, 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 0.1 μm on a rectangular region centered at 1 / 4 of the plate thickness from the steel plate surface, with a length of 150 μm in the thickness direction and a length of 150 μm perpendicular to the thickness direction, to obtain crystal orientation information for this rectangular region. The EBSD analysis is performed using a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL HIKARI detector) at an analysis speed of 50 to 300 points / second. Next, the orientation difference within the grain (GAM value) is calculated using the "OIM Analysis®" software included with the EBSD analyzer, based on the crystal orientation information of this rectangular region. Then, regions with a GAM value of 0.5° or less are identified as ferrite, and the number of ferrite particles in each of the nine equally divided 150 μm × 150 μm regions is counted to calculate the number densities Nα1 to Nα9, and their average value is determined as Nαm (Figure 1(a)(i) and (b)(i)). The number density of ferrite is calculated by counting all the ferrite particles detected and identified under the above measurement conditions. When counting, if ferrite is present on the boundary of multiple divided regions or straddling the boundary of multiple divided regions, that ferrite is counted in each of those multiple divided regions. For example, if one ferrite particle is present on the boundary of two divided regions A and B or straddling the boundary of divided regions A and B, that one ferrite particle is counted in both divided regions A and B. Similarly, if one ferrite exists on or across the boundary of the three divided regions A, B, and C, that one ferrite shall be counted in each of the divided regions A, B, and C.Finally, the difference in the number density of ferrite in each adjacent divided region in the thickness direction is calculated (Figure 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 (Figure 1(a)(iii) and (b)(iii)).

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

[0057] As described above, the steel sheet according to the embodiment of the present invention has high strength, high uniform elongation, hole-expandability, and yield ratio, and can significantly suppress the occurrence of fracture accompanied by load reduction during collisions. In particular, it can suppress the occurrence of fracture accompanied by load reduction during collisions even when forming parts with complex shapes. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve a high level of balance between the conflicting properties of high strength and excellent workability, and can achieve excellent impact resistance. For this reason, 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 undercarriage part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automotive undercarriage parts include lower arms and trailing arms. These automotive parts, particularly automotive undercarriage parts, only need to include the steel sheet according to the embodiment of the present invention in at least a 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 forming processes such as press forming, the characteristics of the metal structure do not change significantly before and after forming in areas of the steel sheet that do not come into direct contact with the mold and are subjected to relatively minimal processing.

[0058] [Mechanical properties] [Tensile strength (TS) and uniform elongation (uEl)] According to the steel sheet having the above chemical composition and metal structure, a high tensile strength, specifically a tensile strength of 1180 MPa or more, can be achieved. The tensile strength is preferably 1200 MPa or more, 1220 MPa or more, or 1240 MPa or more. According to the steel sheet according to the embodiment of the present invention, despite having such a very high tensile strength, the load reduction during impact can be significantly suppressed while improving uniform elongation and hole expansion properties through the specific combination of 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 1780 MPa or less, 1470 MPa or less, 1400 MPa or less, or 1300 MPa or less. Furthermore, according to the steel sheet according to the embodiment of the present invention, a high uniform elongation can be achieved, specifically a uniform elongation of 5.0% or more. The uniform elongation is preferably 5.2% or more, 5.5% or more, 5.8% or more, or 6.0% or more. There is no particular upper limit to the uniform elongation, but for example, the uniform elongation of the steel sheet may be 15.0% or less, 10.0% or less, 8.0% or less, or 7.0% or less. Tensile strength and uniform elongation are 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.

[0059] [Hole expansion ratio (λ)] According to the steel sheet having the above chemical composition and metallic structure, high hole expansion properties, specifically a hole expansion ratio of 40% or more, can be achieved. The hole expansion ratio is preferably 42% or more, more preferably 45% or more, or 50% or more. There is no particular upper limit to the hole expansion ratio, but for example, the hole expansion ratio may be 150% or less, 100% or less, or 70% or less. The hole expansion ratio is determined as follows. First, a test piece measuring 100 mm in width and 100 mm in length is taken from the steel sheet, and a punched hole (initial hole: hole diameter d0 = 10 mm) is made using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole is pushed open with a conical punch with a vertex angle of 60° until a crack that penetrates the thickness of the sheet occurs, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) 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}

[0060] [Yield ratio (YR)] According to the steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the yield ratio can also be increased, more specifically, a yield ratio of 75% or higher can be achieved. The yield ratio is preferably 78% or higher or 80% or higher, more preferably 82% or higher or 84% or higher. There is no particular upper limit, but for example, the yield ratio may be 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 specimen from the 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. Yield ratio YR = 0.2% proof strength / Tensile strength TS × 100

[0061] <Method of manufacturing steel plates> Next, preferred manufacturing methods for steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for manufacturing steel sheets according to embodiments of the present invention, particularly steel sheets having preferred properties, and is not intended to limit the steel sheets to those manufactured by the manufacturing methods described below. More specifically, the following describes the manufacturing of hot-rolled steel sheets, but the steel sheets according to embodiments of the present invention include any steel sheets having the chemical composition and metal structure described above, i.e., not only hot-rolled steel sheets, but also cold-rolled steel sheets, plated steel sheets, etc. Therefore, the following description merely describes an example of a preferred manufacturing method when the steel sheet according to embodiments of the present invention is a hot-rolled steel sheet.

[0062] A method for manufacturing steel sheets according to an embodiment of the present invention is: A heating process including heating a slab having the chemical composition described above in relation to a steel plate and holding it at a temperature of 1180-1350°C for 6000 seconds or more, A hot rolling process that includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and satisfies the following conditions (a) to (c): (a) The rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is 960 to 1080°C, and the reduction ratio in each rolling pass is 30 to 50%. (b) Cool the rolled material to a cooling stop temperature of 800-910°C within 0.20 seconds after the rolling pass of the two stages immediately preceding the two stages of the subsequent stage, and (c) The reduction ratio in each of the two subsequent rolling passes is 10-40%. A cooling process comprising: water cooling the finish-rolled steel sheet; cooling it to a temperature range of 600-750°C within 4.0 seconds from the start of water cooling; then air cooling in the said temperature range for 2.0-8.0 seconds; and water cooling the steel sheet to 50°C or below within 13 seconds after air cooling. It is characterized by including [the following]. In the above manufacturing method, the temperatures described for the slab and steel plate refer to the surface temperature of the slab and the surface temperature of the steel plate, respectively. Each step will be explained 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 1180-1350°C for 6000 seconds or more. From the viewpoint of productivity, it is preferable to use a slab obtained by continuous casting, but a slab obtained by casting and splitting may also be used, and if necessary, slabs that have been hot-worked or cold-worked may also be used. In this manufacturing method, holding at a temperature range of 1180-1350°C includes not only cases where the slab temperature is held at a constant temperature within the range of 1180-1350°C, but also cases where the slab temperature fluctuates within the range of 1180-1350°C. By holding the slab at a temperature range of 1180-1350°C for 6000 seconds or more, coarse carbides present in the structure can be completely dissolved, eliminating the initiation points of cracks. If the holding temperature is below 1180°C or the holding time is less than 6000 seconds, the solid solution of coarse carbides will be incomplete. If the solid solution of coarse carbides is incomplete, ferrite and bainite transformations originating from these carbides may occur during the cooling process described later, resulting in a martensite area ratio of less than 60%, and consequently, the desired strength may not be achieved. The upper limit of the slab heating temperature should be 1350°C or less from the standpoint of heating equipment capacity and productivity. 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 process] [Rough rolling] In this manufacturing method, for example, a heated slab may be subjected to rough rolling before finish rolling to adjust the plate thickness. The conditions for rough rolling are not particularly limited, as long as the desired sheet bar dimensions are ensured.

[0065] [Finishing Rolling] [(a) Rolling temperature in each rolling pass of the two preceding stages of the second stage: 960~1080°C, and reduction ratio in each rolling pass of the two preceding stages: 30~50%] The heated slab, or a slab that has been roughly rolled as needed, is then subjected to finish rolling. In this manufacturing method, finish rolling is performed using a tandem rolling mill consisting of four or more rolling stands. In this manufacturing method, in the finish rolling performed on the heated slab, it is necessary to appropriately control the rolling temperature and reduction ratio in each rolling pass of the two stages immediately preceding the two subsequent stages. Specifically, the rolling temperature in each rolling pass of the two stages immediately preceding the two subsequent stages is controlled to 960-1080°C, and similarly, the reduction ratio in each rolling pass of the two stages immediately preceding the two subsequent stages is controlled to 30-50%. Here, for example, when using a tandem rolling mill consisting of seven rolling stands, the sixth and seventh rolling passes correspond to the "rolling passes of the two subsequent stages." Therefore, in this case, "each rolling pass of the two stages immediately preceding the two subsequent stages" refers to the fourth and fifth rolling passes. By performing rolling under relatively high pressure and relatively high temperature conditions in each of the two rolling passes immediately preceding the last two stages, recrystallization can be promoted, and the austenite grains can be refined. In connection with this, it becomes possible to homogenize the number density of ferrite in the thickness direction of the final metal structure to a desired level. More specifically, ferrite is mainly nucleated from austenite grain boundaries. Therefore, by promoting recrystallization and refining the 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 uniformly dispersed and produced in the metal structure, and it becomes possible to homogenize the number density of ferrite in the thickness direction of the final metal structure to a desired level.

[0066] Conversely, if the rolling temperature in each of the two rolling passes immediately preceding the two subsequent stages is less than 960°C, and / or the reduction ratio in each of those rolling passes is less than 30%, recrystallization is not sufficiently promoted, and some unrecrystallized grains remain, making it impossible to sufficiently homogenize the number density of ferrite in the thickness direction of the final metal structure. As a result, the work softening rate after uniform elongation decreases, i.e., work softening becomes significant after uniform elongation. On the other hand, if the reduction ratio in each of the two rolling passes immediately preceding the two subsequent stages exceeds 50%, flattened austenite grains are formed due to the introduction of excessive strain, and similarly, it becomes impossible to sufficiently homogenize the number density of ferrite in the thickness direction of the final metal structure. As a result, similarly, work softening becomes significant after uniform elongation. Furthermore, if the rolling temperature in each of the two rolling passes immediately preceding the two subsequent stages exceeds 1080°C, the austenite grains after recrystallization become coarser, the number of austenite grain boundaries decreases, and the number of ferrite nucleation sites decreases. As a result, the number density of ferrite in the final metal structure cannot be sufficiently uniform in the thickness direction, and similarly, work softening after uniform elongation becomes significant. Preferably, the rolling temperature in each rolling pass of the two stages immediately preceding the two subsequent stages is 1000 to 1060°C.

[0067] [(b) Cool to a cooling stop temperature of 800-910°C within 0.20 seconds after the rolling pass of the two stages immediately preceding the two stages in the subsequent stage.] In this manufacturing method, the rolled material is cooled to a cooling stop temperature of 800-910°C within 0.20 seconds after the two rolling passes immediately preceding the two subsequent stages. By cooling the rolled material to a cooling stop temperature of 800-910°C relatively quickly after the two rolling passes immediately preceding the two subsequent stages, grain growth of austenite grains after recrystallization can be suppressed, thereby making it possible to uniformly achieve a desired level of ferrite number density in the thickness direction of the final metal structure. If the cooling time to the cooling stop temperature of 800-910°C after the two rolling passes immediately preceding the two subsequent stages exceeds 0.20 seconds, or if the cooling stop temperature is higher than 910°C, it becomes impossible to sufficiently suppress grain growth of austenite grains after recrystallization, and even if appropriate cooling is applied in the subsequent cooling process, it becomes impossible to control the ferrite number density within the desired range in the thickness direction of the sheet. On the other hand, if the cooling stop temperature is lower than 800°C, it may lead to excessive ferrite formation in the final metal structure. If ferrite is formed excessively, the strength may decrease, and even precipitation strengthening due to Ti precipitates may not be sufficient to reduce the hardness difference between ferrite and martensite, which can lead to a decrease in hole expansion properties.

[0068] [(c) Reduction ratio in each rolling pass of the subsequent two stages: 10-40%] In this manufacturing method, the reduction ratio in each of the two subsequent rolling passes of the finish rolling process is controlled to 10-40%. By introducing strain with such an appropriate reduction ratio in each of the two subsequent rolling passes, it is possible to increase the driving force of ferrite transformation in the subsequent cooling process. If the reduction ratio in each of the two subsequent rolling passes is less than 10%, the driving force of 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 ratio in each of the two subsequent rolling passes exceeds 40%, the driving force of ferrite transformation becomes too large, which may lead to excessive ferrite formation in the final metal structure. If ferrite is excessively formed, the strength may decrease, and even precipitation strengthening due to Ti precipitates may not sufficiently reduce the hardness difference between ferrite and martensite, which may lead to decreased hole-expanding properties. Preferably, the reduction ratio in each of the two subsequent rolling passes of the finish rolling process is 15-38%.

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

[0070] Furthermore, after water cooling, air cooling in the 600-750°C temperature range for 2.0-8.0 seconds promotes the transformation to ferrite and allows for the proper precipitation of Ti precipitates. Therefore, the 2.0-8.0 second air cooling operation in the 600-750°C temperature range after water cooling is important not only for the proper formation of ferrite but also for improving properties such as hole expansion due to precipitation strengthening caused by Ti precipitates. For example, if the air cooling temperature is below 600°C, the transformation to ferrite may not be sufficiently promoted, while a relatively large amount of bainite may be formed. In such cases, the formation of a large amount of bainite reduces uniform elongation, and furthermore, the formation of martensite decreases in relation to the formation of bainite, which may result in insufficient strength being obtained.

[0071] Furthermore, 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, and uniform elongation decreases. On the other hand, if the air cooling time exceeds 8.0 seconds, a relatively large amount of ferrite may be formed. In such cases, the strength decreases, and the hardness difference between ferrite and martensite cannot be sufficiently reduced by precipitation strengthening caused by Ti precipitates, which may reduce hole expansion properties. 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 below 50°C 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 air cooling is complete, the steel sheet is water-cooled to 50°C or below within 13 seconds. This rapid cooling allows for the formation of martensite within the desired area ratio. If water cooling to 50°C or below takes longer than 13 seconds, or if 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 below after air cooling may be 4 seconds or more, or 5 seconds or more. Similarly, the lower limit of the water cooling stop temperature is not particularly limited, but for example, the water cooling stop temperature may be 20°C or higher, or 25°C or higher. Finally, the water-cooled steel sheet can be wound into the form of a hot-rolled coil. The winding conditions are not particularly limited and can be carried out under any suitable temperature conditions, for example, at room temperature.

[0073] According to the steel sheet manufactured by the above manufacturing method, by composing the metal structure with a structure containing 10-40% ferrite and 60-90% martensite by area percent, it is possible to achieve high strength, such as 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 mass% or more, the soft structure of ferrite is strengthened by the precipitation of Ti precipitates, thereby increasing the yield ratio and reducing the hardness difference between ferrite and martensite in the metal structure, thus significantly improving hole-expanding properties. In addition, by uniformizing the number density of ferrite within a predetermined range in the thickness direction of the steel sheet, load reduction during impact can be suppressed. Therefore, despite its high strength, the steel sheet manufactured by the above manufacturing method has high uniform elongation, hole-expanding properties, and yield ratio, and can significantly suppress the occurrence of fracture accompanied by load reduction during impact. Therefore, steel sheets manufactured by the above manufacturing method can reliably achieve a high level of balance between the conflicting properties of high strength and excellent workability, while also realizing excellent impact resistance. As such, they are particularly useful in the automotive sector 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 in any way to these examples. [Examples]

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

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

[0077] [Table 1]

[0078] [Table 2]

[0079] [Table 3]

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

[0081] [Tensile strength (TS) and uniform elongation (uEl)] Tensile strength (TS) and uniform elongation (uEl) were measured by taking a JIS No. 5 test specimen from the orientation where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (C direction), and performing 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 measuring 100 mm wide x 100 mm long was taken from the steel plate, and a punched hole (initial hole: hole diameter d0 = 10 mm) was created using a punching tool with a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%). Next, with the burr facing the die side, the initial hole was expanded using a conical punch with a 60° apex angle until a crack penetrating the plate thickness occurred. The hole diameter d1 mm at the time of crack occurrence was measured, and the hole expansion ratio λ (%) for each test piece was calculated using the following formula. This hole expansion test was performed three times, and the average value was determined as the hole expansion ratio λ. λ = 100 × {(d1 - d0) / d0}

[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 specimen from the direction in which the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate (direction C), and performing a tensile test in accordance with JIS Z 2241:2022. Yield ratio YR = 0.2% proof strength / Tensile strength TS × 100

[0084] [Absolute value of work softening rate after uniform elongation (uEl)] The absolute value of the work softening rate beyond uniform elongation (uEl) was determined as follows. First, with true stress (MPa) as σ and true strain as ε, dσ0 / dε0 was calculated by differentiating the true stress with respect to true strain at the time of uniform elongation. Next, dσ1 / dε1 was calculated when the true strain increased by 0.005 from the uniform elongation. Finally, based on these, |dσ0 / dε0-dσ1 / dε1| / 0.005 was calculated, and the obtained value was determined as the absolute value of the work softening rate (MPa).

[0085] Steel plates with a tensile strength (TS) of 1180 MPa or higher, a uniform elongation (uEl) of 5.0% or higher, a hole expansion ratio (λ) of 40% or higher, a yield ratio (YR) of 75% or higher, and an absolute value of the work softening rate after uniform elongation of 250,000 MPa or lower were evaluated as steel plates with high strength, high uniform elongation, hole expansion properties, and yield ratio, and that can suppress load reduction during impact. The results are shown in Table 4. In Table 4, "Maximum difference in ferrite number density / Nαm" refers to the value obtained by dividing the maximum value of the difference in number density of the six types shown in Figure 1, namely 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, if this value is 0.60 or less, the requirement that "the difference in the number density of ferrite in each adjacent divided region in the thickness direction is all Nαm × 0.60 or less" is satisfied. In addition, in the metallographic structure shown in Table 4, the remaining structure consisted of at least one of pearlite and retained austenite.

[0086] [Table 4]

[0087] Referring to Tables 1-4, in Comparative Example 2, the rolling temperature in each of the two rolling passes immediately preceding the last two stages in the hot rolling process was low, which is thought to have prevented sufficient recrystallization, leaving some unrecrystallized grains. As a result, the number density of ferrite in the final metal structure could not be sufficiently uniform in the thickness direction, meaning the maximum difference in ferrite number density / Nαm exceeded 0.60. Related to this, the absolute value of the work softening rate after uEl exceeded 250,000 MPa, meaning that 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 stages was high, which is thought to have caused the austenite grains to coarseen after recrystallization, reducing the number of austenite grain boundaries and thus reducing the ferrite nucleation sites. As a result, the number density of ferrite in the final metal structure could not be sufficiently uniform in the thickness direction, and the absolute value of the work softening rate after uEl exceeded 250,000 MPa. In Comparative Example 4, the reduction ratio in each of the two rolling passes immediately preceding the two subsequent stages was low, which is thought to have prevented sufficient promotion of recrystallization, leaving some unrecrystallized grains. As a result, the number density of ferrite in the final microstructure could not be sufficiently uniform 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 ratio in each of the two rolling passes immediately preceding the two subsequent stages was high, which is thought to have caused the formation of flattened austenite grains due to the introduction of excessive strain. As a result, the number density of ferrite in the final microstructure could not be sufficiently uniform in the thickness direction, and the absolute value of the work softening rate after uEl exceeded 250,000 MPa. In Comparative Example 6, the cooling time to the cooling stop temperature of 800-910°C after the two rolling passes immediately preceding the two subsequent stages was more than 0.20 seconds, which is thought to have prevented sufficient suppression of grain growth of austenite grains after recrystallization. As a result, the number density of ferrite in the final metal structure could not be sufficiently uniform in the thickness direction, 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 the cooling between the two subsequent rolling passes and the two preceding rolling passes was low, resulting in the formation of a large amount of ferrite in the final metal structure. As a result, even with precipitation strengthening due to Ti precipitates, the hardness difference between ferrite and martensite could not be sufficiently reduced, and λ decreased. In Comparative Example 8, the cooling stop temperature during the cooling between the two subsequent rolling passes and the two preceding rolling passes was high, which is thought to have prevented sufficient suppression of austenite grain growth after recrystallization. As a result, the number density of ferrite could not be sufficiently uniform in the thickness direction of the final metal structure, and the absolute value of the processing softening rate after uEl exceeded 250,000 MPa. In Comparative Example 9, the reduction ratio of each of the two subsequent rolling passes was low, which is thought to have prevented sufficient increase in the driving force for ferrite transformation in the subsequent cooling process. 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 reduction ratio in each of the two subsequent rolling passes was high, which is thought to have resulted in an excessively large driving force for ferrite transformation during the subsequent cooling process. As a result, a large amount of ferrite was formed in the final metal structure, and the hardness difference between ferrite and martensite could not be sufficiently reduced even by precipitation strengthening caused by Ti precipitates, 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 the formation of a relatively large amount of ferrite. As a result, λ similarly decreased. In Comparative Example 12, the air cooling temperature was low, resulting in the formation of a relatively large amount of bainite, and uEl decreased. In Comparative Example 13, the air cooling temperature was high, so the ferrite transformation could not be sufficiently promoted, and uEl decreased. In Comparative Example 14, the air cooling time was short, so the ferrite transformation could not be sufficiently promoted, and uEl similarly decreased. In Comparative Example 15, the air cooling time was long, so the ferrite was relatively large, and λ decreased. In Comparative Example 16, the water cooling time to below 50°C after air cooling was prolonged, resulting in the formation of a large amount of bainite and a decrease in uEl.

[0089] Comparative Example 44 is thought to have had insufficient precipitation strengthening due to the low Ti content, resulting in a decrease in TS. Comparative Example 45 is thought to have had coarse carbides and the like due to its high Ti content, resulting in a decrease in the workability of the steel sheet and a decrease in λ. Comparative Example 46 is thought to have had insufficient TS due to its low Nb content. Comparative Example 47 is thought to have had coarse carbides and the like due to its high Nb content, resulting in a decrease in the workability of the steel sheet and a decrease in λ.

[0090] In contrast, in all the steel sheets relating to the invention, by having a predetermined chemical composition and appropriately controlling each condition in the manufacturing method, the metal structure was such that, in area percent, ferrite: 10-40%, martensite: 60-90%, and bainite: 0-10%. When a 150 μm × 150 μm region at the 1 / 4 position of the plate thickness in a cross section perpendicular to the plate surface was evenly divided into 9 sections, and the number density of ferrite in each divided region was calculated, and the average value of these was taken as Nαm, it was possible to obtain a steel sheet in which the difference in the number density of ferrite in each adjacent divided region in the plate thickness direction was all Nαm × 0.60 or less. As a result, despite having a high strength of tensile strength of 1180 MPa or more, it was possible to have high uniform elongation, hole-expanding properties and yield ratio, and the absolute value of the work softening rate after uniform elongation was reliably reduced to 250,000 MPa or less.

Claims

1. The chemical composition is expressed in mass percent. C: 0.060-0.300%, Si: 0.30-1.50%, Mn: 1.00-2.70%, P: 0.100% or less, S: 0.0300% or less, Sol. Al: 0.001–0.500%, O: 0.0100% or less, N: 0.0070% or less, Ti: 0.070 to 0.170%, Nb: 0.001-1.000%, B: 0 to 0.0030%, Cr: 0-0.70%, Mo: 0 to 0.12%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, V: 0-0.300%, Sn: 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0-0.0010%, Mg: 0 to 0.0010%, Bi: 0 to 0.010%, Co: 0 to 0.010%, W: 0-0.100%, Zn: 0 to 0.010%, REM: 0-0.0100%, and The remainder consists of Fe and impurities. The metallic structure, in area percentage, Ferrite: 10-40%, Martensite: 60-90% Baynite: 0-10%, and At least one of perlite and retained austenite: totaling 0-5%, When a 150 μm × 150 μm region at the 1 / 4 thickness position of a cross-section perpendicular to the plate surface is evenly divided into 9 sections, and the number density of ferrite is calculated in each divided region, and the average value of these is taken as Nαm, then the difference in the number density of ferrite in each adjacent divided region in the thickness direction is all less than or equal to Nαm × 0.60, A steel plate characterized by having 0.050 or more Nαm particles per μm².

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

3. The steel plate according to claim 1 or 2, characterized by having a tensile strength of 1180 MPa or more.

4. The steel plate according to claim 1 or 2, characterized by having a plate thickness of 1.0 to 8.0 mm.

5. A component characterized by comprising the steel plate described in claim 1 or 2.

6. A heating step comprising heating a slab having the chemical composition described in claim 1 or 2 and holding it at a temperature of 1180 to 1350°C for 6000 seconds or more. A hot rolling process that includes finish rolling the slab using a tandem rolling mill consisting of four or more rolling stands, and that satisfies the following conditions (a) to (c): (a) The rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is 960 to 1080°C, and the reduction ratio in each rolling pass is 30 to 50%. (b) Cool the rolled material to a cooling stop temperature of 800 to 910°C within 0.20 seconds after the rolling pass of the two stages immediately preceding the two stages of the subsequent stage, and (c) The reduction ratio in each of the two subsequent rolling passes is 10 to 40%. A cooling process comprising: water-cooling a 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 air-cooling it in the aforementioned temperature range for 2.0 to 8.0 seconds; and water-cooling the steel sheet to 50°C or below within 13 seconds after air-cooling. A method for manufacturing a steel sheet according to claim 1 or 2, including the method described above.

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