WO2025154509A1 - Steel sheet, component including same, and method for manufacturing steel sheet - Google Patents

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 for manufacturing steel plate

The present invention relates to a steel plate, a part including the steel plate, and a method for manufacturing the steel plate.

In recent years, in order to address environmental issues, there has been a demand for weight reduction in automotive parts in order to reduce _CO2 gas emissions and improve fuel efficiency. At the same time, social demands for improved collision safety are also increasing. In order 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, so there is a need for steel materials that simultaneously improve both strength and workability.

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 morphology in which the volume fraction of the total structure is 10-40% ferrite and 60-90% martensite, and in which the ferrite fractions (shown in "area %) at the four corners and 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, Vα5, respectively, and when the average value of these five points is Vαm, all of the ferrite fractions Vα1, Vα2, Vα3, Vα4, Vα5 are within the range of Vαm ±5 (area %). Patent Document 1 also 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 small variation in the ferrite fraction within the coil can be obtained, which in turn makes it possible to realize a high-strength cold-rolled steel sheet coil with small strength variation 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 automotive steel sheets, and specifically discloses in the examples that in addition to improving tensile strength, total elongation and hole expandability are also improved.

JP 2010-159453 A

As mentioned above, it is known that the workability of steel decreases with increasing strength, and properties such as hole expandability as described in Patent Document 1 decrease. If the hole expandability decreases, it may not be possible to process 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 according to the application, such as uniform elongation in addition to the hole expandability mentioned above. For example, in automobile suspension parts and other parts having complex shapes such as lower arms and trailing arms, the workability of the steel itself may decrease with increasing strength, or work hardening may occur when forming into a complex shape, causing strain to be applied to the vicinity of the maximum load after part formation. Therefore, even when the strength is increased to the same level or higher than before, there is still a high demand for materials that maintain a high and stable load during a collision and therefore do not or are less likely to cause fracture accompanied by a decrease in load.

In addition, 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 the crash safety of automobiles, it is necessary to improve not only the tensile strength but also the yield strength, and therefore there is a demand to increase the yield ratio, which is the ratio of yield strength to tensile strength.

The present invention was made in consideration of these circumstances, and its purpose is to provide a steel plate with a new structure 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 that includes the same, and a method for manufacturing the steel plate.

In order to achieve the above object, the inventors conducted a study focusing on the metal structure of steel plate, particularly hot-rolled steel plate. As a result, the inventors discovered that by forming the metal structure of a hot-rolled steel plate having a predetermined chemical composition mainly from ferrite and martensite and utilizing precipitation strengthening by adding Ti, it is possible to improve the strength, uniform elongation and yield ratio, and reduce the difference in hardness between ferrite and martensite to improve hole expandability, and further, by uniformly distributing ferrite in the plate thickness direction, more specifically, by equally dividing a 150 μm×150 μm region at 1/4 of the plate thickness position on a cross section perpendicular to the plate surface of the steel plate into nine parts, calculating the number density of ferrite in each divided region, and taking the average value of these as Nαm, it is possible to significantly suppress the load reduction after uniform elongation by uniformly distributing ferrite in the metal structure so that the difference in the number density of ferrite in each divided region adjacent to each other in the plate thickness direction is Nαm×0.60 or less, and thus completed the present invention.

The present invention, which has achieved the above object, is as follows.
(1) 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 to 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 to 0.0100%, and the balance: Fe and impurities;
The metal structure is, in area percent,
Ferrite: 10-40%,
Martensite: 60-90%,
Bainite: 0 to 10%, and at least one of pearlite and retained austenite: 0 to 5% in total;
A steel plate characterized in that, when a 150 μm x 150 μm region at 1/4 of the plate thickness position on 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 of these 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 all Nαm x 0.60 or less.
(2) The chemical composition is, in mass%,
B: 0.0001 to 0.0030%,
Cr: 0.001-0.70%,
Mo: 0.001-0.12%,
Cu: 0.001-0.40%,
Ni: 0.001 to 0.30%,
V: 0.001-0.300%,
Sn: 0.001 to 0.040%,
As: 0.001 to 0.100%,
Zr: 0.001 to 0.050%,
Ca: 0.0001 to 0.0010%,
Mg: 0.0001 to 0.0010%,
Bi: 0.001 to 0.010%,
Co: 0.001 to 0.010%,
W: 0.001-0.100%,
Zn: 0.001 to 0.010%, and REM: 0.0001 to 0.0100%
The steel sheet according to (1) above, characterized in that it contains at least one of the following:
(3) The steel sheet according to (1) or (2) above, characterized in that Nαm is 0.050 pieces/μm2 or ^more .
(4) The steel plate 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 sheet thickness of 1.0 to 8.0 mm.
(6) A part, comprising the steel sheet according to any one of (1) to (5) above.
(7) A heating step including heating a slab having the chemical composition described in (1) or (2) above and holding it at a temperature of 1180 to 1350° C. for 6000 seconds or more;
A hot rolling process including finish rolling the slab using a tandem rolling mill having four or more rolling stands, the hot rolling process satisfying the following conditions (a) to (c):
(a) the rolling temperature in each of the rolling passes in the two stages immediately preceding the latter two stages is 960 to 1080°C, and the rolling reduction in each of the rolling passes 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 rolling passes immediately preceding the latter two rolling passes; and (c) a rolling reduction rate in each rolling pass of the latter two rolling passes is 10 to 40%. A method for producing a steel plate, comprising a cooling step including: water-cooling the finish-rolled steel plate, 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 said temperature range for 2.0 to 8.0 seconds, and water-cooling the steel plate to 50°C or less within 13 seconds after air-cooling.

The present invention provides a steel plate that has high strength, high uniform elongation, hole expandability, and yield ratio, and is capable of suppressing load reduction during a collision, as well as a part that includes the steel plate and a method for manufacturing the steel plate.

1A and 1B are schematic diagrams for explaining 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.

<Steel plate>
The steel sheet according to the embodiment of the present invention, particularly the hot-rolled steel sheet, has a chemical composition in mass%:
C: 0.060-0.300%,
Si: 0.30-1.50%,
Mn: 1.00-2.70%,
P: 0.100% or less,
S: 0.0300% or less,
sol. Al: 0.001 to 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 to 0.0100%, and the balance: Fe and impurities;
The metal structure is, in area percent,
Ferrite: 10-40%,
Martensite: 60-90%,
Bainite: 0 to 10%, and at least one of pearlite and retained austenite: 0 to 5% in total;
A 150 μm x 150 μm region at 1/4 of the plate thickness on a cross section perpendicular to the plate surface is equally divided into nine portions, the number density of ferrite is calculated in each divided region, and the average value of these is taken as Nαm. In this case, the difference in the number density of ferrite in adjacent divided regions in the plate thickness direction is all Nαm x 0.60 or less.

As mentioned above, it is known that the properties such as hole expandability decrease with increasing strength of steel. For example, in order to manufacture parts with complex shapes such as lower arms and trailing arms among the suspension parts of automobiles, a steel plate with excellent hole expandability while maintaining high strength, for example, a tensile strength of 1180 MPa or more that enables weight reduction, is required. From the viewpoint of increasing strength, it is preferable that the metal structure of the steel plate is composed mainly of martensite. However, although martensitic steel has excellent strength, excessive inclusion of martensitic steel reduces properties such as uniform elongation, so that there is a problem that it is generally poor in workability. In addition, in parts with complex shapes such as lower arms and trailing arms, the workability of the steel itself decreases with increasing strength, and work hardening occurs when forming into a complex shape, so that strain may be applied to near the maximum load after part forming. Therefore, there remains a strong need for high-strength steel plates that have improved properties such as hole expandability and uniform elongation, can maintain a high and stable load during a collision even when forming parts with complex shapes, and therefore can suppress the occurrence of fractures accompanied by a drop in load, and also have a high yield ratio from the perspective of automobile collision safety, etc.

The inventors therefore conducted research focusing on the metal structure of the steel plate, in addition to determining the appropriate chemical composition of the steel plate. First, the inventors discovered that by configuring the metal structure of a steel plate having a specified chemical composition to a structure mainly composed of soft ferrite and hard martensite, more specifically, a structure containing, by area percentage, 10-40% ferrite and 60-90% martensite, it is possible to improve strength and uniform elongation through the action mechanism of so-called DP (Dual Phase) steel.

On the other hand, since DP steel generally has a low yield ratio, the inventors have found that the yield ratio can be increased and the hole expandability can be significantly improved by utilizing precipitation strengthening through the addition of Ti. Although it is not intended to be bound by any particular theory, it is believed that the improvement in hole expandability due to such precipitation strengthening is due to the reduction in the hardness difference between ferrite and martensite in the metal structure. To explain in more detail, in the steel sheet according to the embodiment of the present invention, as described above, the metal structure is composed mainly of ferrite and martensite, and the soft structure ferrite can be contained up to 40% by area. In this case, the hardness difference between ferrite and martensite in the metal structure increases, and the hole expandability decreases. However, in the steel sheet according to the embodiment of the present invention, by controlling the Ti content in the steel to 0.070 mass% or more, the soft structure of ferrite is precipitation strengthened by Ti precipitates, thereby reducing the hardness difference between ferrite and martensite in the metal structure, and therefore it is believed that the hole expandability can be significantly improved.

Next, the inventors conducted a study on the assumption 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 uniform elongation, which corresponds to the elongation at the maximum load point in a uniaxial tensile test. This is because by suppressing the work softening after uniform elongation, it is possible to effectively absorb the collision energy from the maximum load to fracture during a collision by the plastic deformation of the steel sheet. First, in an experiment conducted by the inventors, it was found that when the absolute value of the work softening rate after uniform elongation ( ^d2σ / ^dε2 ) (σ: true stress, ε: true strain) is controlled to 250,000 MPa or less, the occurrence of fracture can be completely or significantly suppressed even when the formed part is subjected to a drop weight test. Therefore, in order to stably realize 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 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 by uniformly distributing ferrite in the thickness direction of the steel plate, more specifically, by uniformly dividing a 150 μm×150 μm region at 1/4 of the thickness position of a cross section perpendicular to the plate surface of the steel plate into nine parts, calculating the number density of ferrite in each divided region, and arranging ferrite uniformly in the metal structure so that the absolute value of the difference in the number density of ferrite in each divided region adjacent to each other in the thickness direction is Nαm×0.60 or less when the average value of the ferrite density in each divided region adjacent to each other in the thickness direction is Nαm×0.60 or less. In relation to this, the inventors have found that even when the steel plate is formed into a part, particularly a part having a complex shape such as a lower arm or a trailing arm, the load reduction during a collision can be suppressed and the occurrence of fracture can be significantly suppressed.

Fig. 1 is a schematic diagram for explaining the characteristics of the number density of ferrite, and Fig. 1(a) shows an example that does not satisfy the characteristics of 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 of a cross section perpendicular to the plate surface is evenly 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. 1(b) shows an example that satisfies the characteristics. Referring to Fig. 1(a), first, a 150 μm × 150 μm region at the 1/4 position of the plate thickness of a cross section perpendicular to the plate surface of the steel plate is evenly 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 / 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 are calculated (i.e., the difference between Nα1 and Nα4 (0.045 pieces/ ^μm2 ), the difference between Nα4 and Nα7 (0.033 pieces/ ^μm2 ), the difference between Nα2 and Nα5 (0.017 pieces/ ^μm2 ), the difference between Nα5 and Nα8 (0.012 pieces/ ^μm2 ), the difference between Nα3 and Nα6 (0.008 pieces/ ^μm2 ), and the difference between Nα6 and Nα9 (0.014 pieces/ ^μm2 ) (Figure 1 (a) (ii)). If all of the differences in number density among these six are 0.60 times or less (Nαm × 0.60 or less) the average value Nαm (0.058 particles/ ^μm2 ), that is, if the values obtained by dividing the differences in number density by Nαm are all 0.60 or less, the feature regarding number density of ferrite according to the present invention is satisfied. However, in Fig. 1(a), the value obtained by dividing the difference between Nα1 and Nα4 (0.045 particles/ ^μm2 ) by Nαm (0.058 particles/ ^μm2 ) is 0.78, which does not satisfy the feature.

1(b), all of the differences in number density of ferrite in each divided region adjacent to each other in the sheet thickness direction calculated in the same manner are 0.60 times or less (Nαm×0.60 or less) the average value Nαm (0.056 particles/μm ² ), and therefore satisfy the feature regarding the number density of ferrite according to the present invention. In the embodiment of the present invention, by thus homogenizing the number density of ferrite in the sheet thickness direction of the steel sheet, that is, by uniformly disposing the ferrite in the sheet thickness direction of the steel sheet, 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 sheet is formed into a part having a complex shape such as a lower arm or a trailing arm, it becomes possible to significantly suppress the occurrence of fracture during a collision.

Without intending to be bound by any particular theory, it is believed that by distributing ferrite uniformly in the metal structure, it is possible to suppress local deformation even during a collision, thereby suppressing the load drop during the collision and maintaining a high and stable load, and as a result, it is possible to significantly suppress the occurrence of fracture. To explain in more detail, when a steel plate is composed of a structure mainly composed of soft ferrite and hard martensite, the strength is generally ensured by the hard martensite, and deformation is borne by the soft ferrite. For this reason, deformation during a collision tends to concentrate in the ferrite, which is a soft structure. In such a case, if there is an area in which ferrite is relatively abundant locally in the metal structure, it is believed that strain during deformation will concentrate in such an area. As a result, it is not possible to suppress local deformation, and it is believed that the load during the collision will decrease due to such deformation. Here, considering the deformation mode during a collision, it is important to suppress the load drop during bending deformation in particular during a collision, and in bending deformation, fracture occurs in the thickness direction of the steel plate starting from the surface layer. For this reason, 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 plate thickness direction. In fact, it has been found that by uniformly arranging ferrite in the plate thickness direction of the steel plate as shown in FIG. 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 even when the steel plate is formed into a part having a complex shape such as a lower arm or a trailing arm, it is possible to significantly suppress the occurrence of fracture during a collision. The fact that the work softening rate after uniform elongation can be reduced by uniformly arranging ferrite in the plate thickness direction of a steel plate 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 known in the past, and was revealed for the first time by the present inventors. Therefore, according to the embodiment of the present invention, for example, despite the high strength of the tensile strength of 1180 MPa or more, it is possible to have high uniform elongation, hole expandability and yield ratio, and to significantly suppress the occurrence of fracture accompanied by load reduction during a collision, and therefore the steel plate according to the embodiment of the present invention is particularly useful for use in the automotive field.

Below, 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 written before and after it are included as the lower and upper limits, unless otherwise specified.

[C:0.060-0.300%]
C is an element effective in increasing the strength of the steel plate. In addition, C forms carbides and/or carbonitrides with Nb in the steel, and also contributes to refining the structure due to the pinning effect of the precipitates formed. In order 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, if C is contained excessively, the uniform elongation may decrease. 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.

[Si: 0.30-1.50%]
Si is an element that suppresses the formation of iron carbides and contributes to improving strength and formability. In order to fully obtain such effects, 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, if Si is contained excessively, the ferrite fraction becomes high, which may cause a large difference in hardness between ferrite and martensite, resulting in a decrease in 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.

[Mn: 1.00-2.70%]
Mn is an element that is effective in increasing strength as a hardenability and solid solution strengthening element. In order 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, if Mn is contained excessively, the ferrite fraction may decrease due to excessive improvement in hardenability, and uniform elongation may decrease. 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.

[P: 0.100% or less]
If P is contained excessively, workability may decrease 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 lead to an increase in costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[S: 0.0300% or less]
If S is contained excessively, a large amount of sulfides such as MnS may be generated, 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 lead to an increase in costs. Therefore, the S content may be 0.0001% or more, 0.0010% or more, or 0.0030% or more.

[sol. Al: 0.001 to 0.500%]
Sol. Al is an element that acts as a deoxidizer for molten steel. In order to obtain such an 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, if sol. Al is contained excessively, the ferrite fraction becomes high, which may cause a large difference in hardness between ferrite and martensite, resulting in a decrease in 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 means acid-soluble Al, and refers to solid-solution Al present in the steel in a solid solution state.

[O: 0.0100% or less]
O is an element that is mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed, which may reduce 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 time for refining, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.

[N: 0.0070% or less]
If N is contained excessively, coarse nitrides are formed, and slab cracks may occur 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 lead to an increase in costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.

[Ti: 0.070 to 0.170%]
Ti is an element that precipitates in steel as Ti carbides such as TiC, strengthens soft structures such as ferrite by precipitation strengthening, and contributes to improving strength and yield ratio. Furthermore, Ti can reduce the hardness difference between ferrite and martensite in the metal structure due to precipitation strengthening, so it is also effective in improving hole expandability. In order to fully obtain these effects, the Ti content is 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, if Ti is contained excessively, coarse carbides and the like are generated in the steel, which may cause slab cracks during hot rolling or reduce the workability of the steel sheet. Therefore, the Ti content is 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.

[Nb: 0.001 to 1.000%]
Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to the refinement of prior austenite grains and thus to the high strength of steel sheet by the pinning effect. In order to fully obtain such effects, the Nb content is set to 0.001% or more. The Nb content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, if Nb is excessively contained, coarse carbides and the like may be generated in the steel, which may reduce 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.

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

[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 in order to obtain such an 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, even if B is contained excessively, the effect is saturated and there is a risk of increasing the manufacturing cost. 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.

[Cr: 0-0.70%]
Cr is an element that enhances the hardenability of steel and contributes to improving strength and/or corrosion resistance. The Cr content may be 0%, but in order 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, even if Cr is contained excessively, the effect is saturated and there is a risk of increasing the manufacturing cost. 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.

[Mo: 0 to 0.12%]
Mo is an element that enhances the hardenability of steel and contributes to improving strength. The Mo content may be 0%, but in order to obtain such an 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, if Mo is contained excessively, the deformation resistance during hot working may increase, and the equipment load may become large. 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.

[Cu: 0-0.40%]
Cu is an element that contributes to improving strength by precipitation strengthening or solid solution strengthening. The Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.001% or more. 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 excessively, the effect is saturated and there is a risk of increasing the manufacturing cost. 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.

[Ni: 0-0.30%]
Ni is an element that contributes to improving strength by precipitation strengthening or solid solution strengthening. The Ni content may be 0%, but in order to obtain such an effect, the Ni content is preferably 0.001% or more. 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 excessively, the effect is saturated and there is a risk of increasing the manufacturing cost. 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.

[V: 0-0.300%]
V is an element that contributes to improving strength by precipitation strengthening, etc. The V content may be 0%, but in order 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 is saturated and there is a risk of increasing the manufacturing cost. 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.

[Sn: 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0 to 0.0010%, Mg: 0 to 0.0010%, Bi: 0-0.010%, Co: 0-0.010%, W: 0-0.100%, Zn: 0-0.010%, and REM: 0-0.0100%]
Sn, As, Zr, Ca, Mg, Bi, Co, W, Zn, and REM may be 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: 0-0.040% 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%. Regarding the lower limit of these elements, for example, the Sn, As, Zr, Bi, Co, W and Zn contents may be 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.

In the steel plate according to the embodiment of the present invention, the remainder other than the above elements consists of Fe and impurities. Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel plate is industrially manufactured.

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

[Metal structure]
[Ferrite: 10-40%]
The metal structure of the steel plate according to the embodiment of the present invention contains ferrite: 10 to 40% in area%. The desired uniform elongation can be achieved by containing 10% or more of ferrite, which is a soft structure, in area%. From the viewpoint of further improving the uniform elongation, the higher the area ratio of ferrite, the more 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 ratio 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, and the hole expandability may decrease. Therefore, the area ratio of ferrite is set to 40% or less. From the viewpoint of further increasing the strength, yield ratio, and/or hole expandability, the lower the area ratio of ferrite, the more preferable, for example, 38% or less, 35% or less, 32% or less, 30% or less, 28% or less, or 26% or less.

[Martensite: 60-90%]
The metal structure of the steel plate according to the embodiment of the present invention contains, in terms of area%, 60 to 90% martensite. By configuring the metal structure of the steel plate with a structure containing hard martensite within such a range, high strength, for example, high strength with a tensile strength of 1180 MPa or more, can be achieved. From the viewpoint of further increasing strength, the higher the area ratio of martensite, the more preferable it is, and 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, uniform elongation may decrease. Therefore, the area ratio of martensite is 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.

[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 ratio of bainite is too high, the uniform elongation may decrease. Therefore, the area ratio 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 ratio of bainite may be 0%, and may be, for example, 0.5% or more, 1% or more, or 2% or more.

[Remaining organization]
The remaining structure other than ferrite, martensite, and bainite may be 0% in terms of area percent, but if a remaining structure exists, the remaining structure may be at least one of pearlite and retained austenite. If the area ratio of at least one of pearlite and retained austenite exceeds 5% in total, it may lead to a decrease in uniform elongation, or it may become impossible to control ferrite and/or martensite within a desired range. Therefore, the total area ratio of at least one of pearlite and retained austenite is 5% or less, and may be, 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 ratio 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.

[Identification of metal structure and calculation of area ratio]
Next, the identification of metals and the calculation of the area ratio will be described. The structure observation is performed on the plate thickness cross section in the direction perpendicular to the plate surface. Although the plate thickness cross section is preferably parallel to the rolling direction, in cases where the rolling direction of the steel plate cannot be specified, the plate thickness cross section does not necessarily need to be parallel to the rolling direction. Specifically, first, a test piece is taken from the steel plate, and the cross section of the test piece is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror surface using a dilution solution such as alcohol or a liquid in which diamond powder with a grain size of 1 to 6 μm is dispersed in pure water. Next, this cross section is polished for 8 minutes at room temperature using colloidal silica with a grain 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 plate thickness direction and 150 μm in the direction perpendicular to the plate thickness direction, centered at 1/4 of the plate thickness position from the steel surface, is measured by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information.

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). In this case, it is preferable that the degree of vacuum in the EBSD device is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62, and 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

[Identification of retained austenite and calculation of area ratio]
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 the region having the fcc crystal structure, and the area ratio of this region is calculated, thereby obtaining the area ratio of the retained austenite.

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

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

[Identification of pearlite and calculation of area ratio]
The pearlite area ratio is calculated by observing a secondary electron image by FE-SEM after corrosion using a Nital reagent. In order to observe a secondary electron image by FE-SEM in the same region as the EBSD measurement region, a Vickers indentation is stamped near the observation position. Then, the surface contamination is polished away, leaving the structure of the observation surface, and Nital etching is performed. Next, the same field of view as the EBSD observation surface is observed by FE-SEM at, for example, 800 times magnification. The pearlite area ratio is calculated by performing image analysis on a structure photograph obtained by using the FE-SEM in the same region as the EBSD measurement region, that is, 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 1/4 position of the plate thickness from the steel plate surface. Here, a structure in which plate-shaped ferrite and Fe-based carbides are layered is regarded as pearlite.

[Identification of martensite and calculation of area ratio]
The area ratio of martensite is calculated by subtracting the area ratios of the retained austenite, ferrite, bainite, and pearlite from 100%. Metal structures other than martensite are sequentially identified, and the last remaining metal structure is regarded as martensite.

[Difference in number density of ferrite in adjacent divided regions in the plate thickness direction: all 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 the 1/4 position of the plate thickness in a cross section perpendicular to the plate surface is divided into nine equal parts, the number density of ferrite is calculated in each divided region, and the average value of the ferrite is taken as Nαm, the difference in the number density of ferrite in each divided region adjacent to each other in the plate thickness direction, more specifically, the absolute value of the difference in the number density of ferrite in each divided region adjacent to each other in the plate thickness direction is all controlled to be Nαm×0.60 or less. By uniformly disposing the ferrite in the plate thickness direction of the steel plate in this manner, 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, thereby making it possible to suppress the load reduction during a collision. As a result, even when the steel plate according to the embodiment of the present invention is formed into an automobile part, particularly an automobile suspension part, such as a lower arm or a trailing arm having a complex shape, it is possible to significantly suppress the occurrence of fracture during a collision.

From the viewpoint of further suppressing load reduction during a collision, it is preferable that the difference in number density of ferrite in each divided region adjacent in the plate thickness direction is as small as possible. For example, the difference in number density of ferrite in each divided region adjacent 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. There is no particular lower limit, but for example, the difference in number density of ferrite in each divided region adjacent 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.

[Nαm: 0.050 pieces/μm ² or more]
According to a preferred embodiment of the present invention, Nαm is 0.050 pieces/μm ² or more. In the steel plate according to the embodiment of the present invention, as described above, it is important to uniformly arrange ferrite in the plate thickness direction of the steel plate, and from the viewpoint of increasing the uniformity of the arrangement of ferrite, 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 to each other in the plate thickness direction is reduced, and the load reduction at the time of collision is suppressed. From the viewpoint of making such an effect more remarkable, the higher Nαm is preferable, and may be, for example, 0.055 pieces/μm ² or more, 0.060 pieces/μm ² or more, 0.065 pieces/μm ² or more, 0.070 pieces/μm ² or more, 0.075 pieces/μm ² or more, or 0.080 pieces/μm ² or more. The upper limit is not particularly limited, but may be, for example, 0.200 particles/μm ² or less, 0.150 particles/μm ² or less, or 0.120 particles/μm ² or less.

[Measurement of Nαm and number density of ferrite in each divided region]
The measurement of Nαm and the number density of ferrite in each divided region is performed by EBSD in the same manner as in the case of the identification of ferrite and the calculation of the area ratio, as follows. Specifically, first, a sample is taken from the steel plate so that the plate thickness cross section perpendicular to the plate surface is 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 plate cannot be specified, the plate thickness cross section does not necessarily have to be parallel to the rolling direction. Next, EBSD analysis is performed at a measurement interval of 0.1 μm on a rectangular region of 150 μm in the plate thickness direction and 150 μm in the direction perpendicular to the plate thickness direction, centered on a 1/4 position of the plate thickness from the steel plate surface, to obtain crystal orientation information of this rectangular region. The EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL). Next, from the crystal orientation information of this rectangular region, the orientation difference (GAM value) within the grain is calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. Next, the region with a GAM value of 0.5° or less is identified as ferrite, and the number of ferrite particles in each of the divided regions into which the 150 μm×150 μm region is equally divided into 9 is counted to calculate the number density Nα1 to Nα9, and the average value thereof is determined as Nαm (FIGS. 1(a)(i) and (b)(i)). The number density of ferrite is calculated by counting all the number of ferrite particles detected and identified under the above measurement conditions. When counting, if ferrite exists on the boundary between multiple divided regions or in multiple divided regions across the boundary, the ferrite is counted in each of the multiple divided regions. For example, if one ferrite exists in the divided regions A and B on the boundary between the two divided regions A and B or across the boundary, the one ferrite is counted in both divided regions A and B. Similarly, if one ferrite exists in the divided regions A, B, and C on the boundary between the three divided regions A, B, and C or across the boundary, the one ferrite is counted in each of the divided regions A, B, and C. Finally, the difference in number density of ferrite in each divided region adjacent in the plate 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)).

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

As described above, the steel plate according to the embodiment of the present invention has high uniform elongation, hole expandability and yield ratio despite its high strength, and can significantly suppress the occurrence of fracture accompanied by load reduction during collision, and in particular, it is possible to suppress the occurrence of fracture accompanied by load reduction during collision even when forming a part having a complex shape. Therefore, the steel plate according to the embodiment of the present invention can reliably achieve a high level of the contradictory properties of high strength and excellent workability, and can achieve excellent impact resistance. Therefore, the steel plate 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 that includes the steel plate according to the embodiment of the present invention. Examples of automotive suspension parts include lower arms and trailing arms. These automotive parts, particularly automotive suspension parts, only need to include the steel plate according to the embodiment of the present invention in at least a part of these parts, and therefore at least a part of these parts satisfy the above-mentioned chemical composition and metal structure characteristics. In areas of steel sheets that do not come into direct contact with the die during press forming or other forming processes and that are relatively lightly processed, the characteristics of the metal structure do not change significantly before and after forming.

[Mechanical properties]
[Tensile strength (TS) and uniform elongation (uEl)]
According to the steel plate 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 plate according to the embodiment of the present invention, despite having such a very high tensile strength, the specific combination of the chemical composition and metal structure described above can significantly suppress the load drop during collision while improving the uniform elongation and hole expandability. The upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel plate may be 1780 MPa or less, 1470 MPa or less, 1400 MPa or less, or 1300 MPa or less. In addition, according to the steel plate according to the embodiment of the present invention, a high uniform elongation can be achieved, specifically a uniform elongation of 5.0% or more can be achieved. 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.

[Hole expansion ratio (λ)]
According to the steel plate having the above chemical composition and metal structure, high hole expandability, specifically, a hole expansion ratio of 40% or more can be achieved. 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 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 having a width of 100 mm and a length of 100 mm is taken from the steel plate, and a punch 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, the initial hole is expanded with a conical punch with an apex angle of 60° until a crack penetrating the plate thickness occurs, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio λ (%) of each test piece is calculated by the following formula. This hole expanding test is carried out three times, and the average value thereof is determined as the hole expanding ratio λ.
λ=100×{(d1-d0)/d0}

[Yield ratio (YR)]
According to the steel plate having the above chemical composition and metal structure, in addition to high tensile strength, the yield ratio can be increased, and more specifically, a yield ratio of 75% or more can be achieved. 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 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 piece from a direction (C direction) in which the longitudinal direction of the test piece is preferably parallel to the rolling perpendicular direction of the steel plate and performing a tensile test in accordance with JIS Z 2241:2022. When the rolling direction of the steel plate cannot be specified, the JIS No. 5 test piece may be taken from any direction in the plate plane of the steel plate.
Yield ratio YR = 0.2% yield strength / tensile strength TS x 100

<Method of manufacturing steel sheet>
Next, a preferred method for manufacturing the steel sheet according to the embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing the steel sheet according to the 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, the following will specifically show the manufacture of a hot-rolled steel sheet, but the steel sheet according to the embodiment of the present invention includes any steel sheet having the above-described chemical composition and metal structure, 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 the embodiment of the present invention is a hot-rolled steel sheet.

The method for producing a steel sheet according to an embodiment of the present invention comprises:
A heating step comprising heating a slab having the chemical composition described above in relation to the steel plate and holding it at a temperature of 1180-1350°C for at least 6000 seconds;
A hot rolling process including finish rolling the slab using a tandem rolling mill having four or more rolling stands, the hot rolling process satisfying the following conditions (a) to (c):
(a) the rolling temperature in each of the rolling passes in the two stages immediately preceding the latter two stages is 960 to 1080°C, and the rolling reduction in each of the rolling passes 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 stages immediately preceding the latter two stages, and (c) the rolling reduction in each rolling pass of the latter two stages is 10 to 40%. The method is characterized by including a cooling step including water-cooling the finish-rolled steel plate, cooling it to a temperature range of 600 to 750 ° C. within 4.0 seconds from the start of water cooling, then air-cooling in the temperature range for 2.0 to 8.0 seconds, and water-cooling the steel plate to 50 ° C. or less within 13 seconds after air-cooling. 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 described in detail below.

[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, but a slab obtained by casting and blooming can also be used, and if necessary, a slab 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 the case where the temperature of the slab is held at a constant temperature within the range of 1180 to 1350 ° C, but also the case where the temperature of the slab is held while fluctuating within the range of 1180 to 1350 ° C. By holding the slab at a temperature range of 1180 to 1350 ° C for 6000 seconds or more, the coarse carbides present in the structure can be completely solid-dissolved, and the starting point of cracks can be eliminated. If the holding temperature is less than 1180 ° C or the holding time is less than 6000 seconds, the solid-dissolution of the coarse carbides is incomplete. If the solid solution of the coarse carbides is incomplete, the area ratio of martensite may become less than 60% due to the occurrence of ferrite or bainite transformation originating from such carbides in the cooling process described below, and as a result, the desired strength may not be obtained. 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 10000 seconds or less.

[Hot rolling process]
[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 of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

[Finish rolling]
[(a) Rolling temperature in each rolling pass of the two stages immediately preceding the last two stages: 960 to 1080° C., and rolling reduction in each rolling pass of the two stages immediately preceding the last two stages: 30 to 50%]
The heated slab or the slab that has been rough-rolled as necessary is then subjected to finish rolling. In this manufacturing method, the 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 the reduction ratio in each rolling pass of the two stages immediately preceding the last two stages, specifically, the rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is controlled to 960 to 1080°C, and similarly, the reduction ratio in each rolling pass of the two 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 "rolling passes of the last two stages". Therefore, in this case, the "rolling passes of the two stages immediately preceding the last two stages" refers to the fourth and fifth rolling passes. By performing rolling under relatively high pressure under relatively high temperature conditions in each of the rolling passes of the two stages immediately preceding the latter two stages, it is possible to promote recrystallization and refine the austenite grains. In relation to this, it is possible to uniformize the number density of ferrite in the plate thickness direction at a desired level in the finally obtained metal structure. To explain in more detail, ferrite mainly nucleates from austenite grain boundaries. Therefore, by promoting recrystallization and refining the austenite grains, it is possible to increase the number of austenite grain boundaries and increase the number of ferrite nucleation sites. As a result, it is possible to generate more ferrite in a uniformly dispersed manner in the metal structure, and it is possible to uniformize the number density of ferrite in the plate thickness direction at a desired level in the finally obtained metal structure.

On the other hand, if the rolling temperature in each of the rolling passes immediately before the last two stages is less than 960°C and/or the reduction ratio in each rolling pass is less than 30%, recrystallization is not sufficiently promoted, unrecrystallized grains remain partially, and the number density of ferrite cannot be sufficiently uniformed in the thickness direction in the finally obtained metal structure. As a result, the work softening rate after uniform elongation decreases, that is, the work softening after uniform elongation becomes noticeable. On the other hand, if the rolling reduction ratio in each of the rolling passes immediately before the last two stages exceeds 50%, flat austenite grains are formed due to the introduction of excessive strain, and the number density of ferrite cannot be sufficiently uniformed in the thickness direction in the finally obtained metal structure. As a result, the work softening after uniform elongation becomes noticeable. In addition, if the rolling temperature in each of the rolling passes immediately before the last two stages exceeds 1080°C, the austenite grains after recrystallization become coarse, the number of austenite grain boundaries decreases, and the ferrite nucleation sites decrease. 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 of the rolling passes in the two stages immediately preceding the last two stages is 1000 to 1060°C.

[(b) Cooling to a cooling stop temperature of 800 to 910 ° C. within 0.20 seconds after the rolling passes of the last two stages immediately before the last two stages]
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 rolling pass of the two stages immediately before the latter two stages. By cooling the rolled material to a cooling stop temperature of 800 to 910 ° C. relatively quickly after the rolling pass of the two stages immediately before the latter two stages, the grain growth of the austenite grains after recrystallization can be suppressed, and the number density of ferrite in the finally obtained metal structure can be made uniform at a desired level in the plate thickness direction. If the time to cool to a cooling stop temperature of 800 to 910 ° C. after the rolling pass of the two stages immediately before the latter two stages exceeds 0.20 seconds or the cooling stop temperature is higher than 910 ° C., the grain growth of the 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 a desired range in the plate thickness direction. On the other hand, if the cooling stop temperature is lower than 800 ° C., excessive generation of ferrite may occur in the finally obtained metal structure. If excessive ferrite is formed, the strength may decrease, or the difference in hardness between ferrite and martensite may not be sufficiently reduced even by precipitation strengthening due to Ti precipitates, resulting in a decrease in hole expandability.

[(c) Reduction rate in each rolling pass in the latter two stages: 10 to 40%]
In this manufacturing method, the reduction rate in each rolling pass of the latter two stages of finish rolling is controlled to 10 to 40%. By introducing strain at such a moderate reduction rate in each rolling pass of the latter two stages, it is possible to increase the driving force of ferrite transformation in the subsequent cooling process. If the reduction rate of each rolling pass of the latter two stages 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 finally obtained metal structure. As a result, the uniform elongation decreases. On the other hand, if the reduction rate of each rolling pass of the latter two stages exceeds 40%, the driving force of ferrite transformation becomes too large, which may lead to excessive generation of ferrite in the finally obtained metal structure. If ferrite is excessively generated, the strength may decrease, or the hardness difference between ferrite and martensite may not be sufficiently reduced even by precipitation strengthening due to Ti precipitates, and the hole expansion property may decrease. Preferably, the reduction rate in each rolling pass of the latter two stages of finish rolling is 15 to 38%.

[Cooling process]
[Cool to a temperature range of 600 to 750 ° C. within 4.0 seconds from the start of water cooling, then air cool for 2.0 to 8.0 seconds]
The finish-rolled steel sheet is water-cooled in the next cooling step, 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 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 generation of pearlite and bainite, and therefore it is possible to achieve the desired area fraction of the metal structure in the finally obtained steel sheet. On the other hand, 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 generated, which may result in a decrease in strength and/or hole expansibility.

Furthermore, by air-cooling for 2.0 to 8.0 seconds in the temperature range of 600 to 750°C after water cooling, the transformation to ferrite can be promoted and Ti precipitates can be properly precipitated. Therefore, the air-cooling operation 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 from the viewpoint of the improvement effect of hole expandability 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 a relatively large amount of bainite may be generated. In such a case, the generation of a large amount of bainite reduces the 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 being obtained.

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 generated. In such cases, the strength decreases, and the precipitation strengthening caused by Ti precipitates cannot sufficiently reduce the hardness difference between ferrite and martensite, and the hole expansion ability may decrease. The air cooling temperature is preferably 620 to 730°C, and the air cooling time is preferably 3.0 to 6.0 seconds.

[Water-cool 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., that is, after air cooling is completed, the steel sheet is water-cooled to 50 ° C. or less within 13 seconds. By performing such rapid cooling, martensite can be generated within the desired area ratio range. If the water cooling to 50 ° C. or less exceeds 13 seconds or the cooling stop temperature is higher than 50 ° C., a relatively large amount of bainite may be generated. In such a case, 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. In addition, 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 more or 25 ° C. or more. The water-cooled steel sheet can finally be wound into the form of a hot-rolled coil. The winding conditions are not particularly limited, and the winding can be performed under any appropriate temperature conditions, for example, at room temperature.

　With the steel plate manufactured by the above manufacturing method, the metal structure is composed of a structure containing, in terms of area percentage, 10-40% ferrite and 60-90% martensite, so that high strength, for example, high strength with a tensile strength of 1180 MPa or more, can be achieved 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 precipitation strengthened by Ti precipitates, thereby increasing the yield ratio and reducing the hardness difference between ferrite and martensite in the metal structure, and therefore it is possible to significantly improve the hole expandability. In addition, the number density of ferrite in the thickness direction of the steel plate is made uniform within a predetermined range, so that the load reduction during collision can be suppressed. Therefore, the steel plate manufactured by the above manufacturing method has high uniform elongation, hole expandability, and yield ratio despite its high strength, and can significantly suppress the occurrence of fracture accompanied by a load reduction during collision. Therefore, steel sheets manufactured by the above manufacturing method reliably achieve a high level of both the opposing properties of high strength and excellent workability, while also achieving excellent impact resistance, making them particularly useful in the automotive field where these properties are required.

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

In the following examples, steel sheets according to the embodiments 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 rate after uniform elongation of the obtained steel sheets were investigated.

First, molten steel was cast by continuous casting to form slabs with various chemical compositions shown in Tables 1 and 2. These slabs were heated to a temperature of 1180 to 1350°C and held for a time of 6000 to 10000 seconds, and then hot-rolled. Hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and the finish rolling was performed 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.

The properties of the resulting steel plates were measured and evaluated using the following methods.

[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 rolling direction perpendicular to the rolling direction of the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022.

[Hole expansion ratio (λ)]
The hole expansion ratio (λ) was determined as follows. First, a test piece having a width of 100 mm and a length of 100 mm was taken from the steel plate, and a punching tool having a punch diameter of 10 mm and a die diameter of 10.25 to 11.5 mm (clearance 12.5%) was used to make a punched hole (initial hole: hole diameter d0 = 10 mm). Next, the burr was placed on the die side, and the initial hole was pushed out with a conical punch having an apex angle of 60 ° until a crack penetrating the plate thickness occurred, and the hole diameter d1 mm at the time of the 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}

[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 (C direction) in which the longitudinal direction of the test piece was parallel to the rolling direction perpendicular to the steel plate and conducting a tensile test in accordance with JIS Z 2241:2022.
Yield ratio YR = 0.2% yield strength / tensile strength TS x 100

[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, true stress (MPa) was defined as σ, true strain as ε, and _dσ0 / _dε0 was calculated by differentiating the true stress at uniform elongation with true strain. Next, _dσ1 /dε1 was calculated when the true strain increased by 0.005 from the uniform elongation, and finally, | _dσ0 / _{dε0 - dσ1} / _dε1 |/0.005 was calculated based on these, and the obtained value was determined as the absolute value of the work softening rate (MPa).

Steel plates with 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 were evaluated as steel plates with high strength, high uniform elongation, hole expansion property, and yield ratio, and capable of suppressing load reduction during collision. The results are shown in Table 4. "Maximum difference in number density of ferrite/Nαm" in Table 4 means the maximum value of the six number density differences shown in Figure 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, divided by Nαm. Therefore, if this value is 0.60 or less, the requirement that "the difference in number density of ferrite in each divided region adjacent in the plate thickness direction is all Nαm × 0.60 or less" is met. Also, in the metal structure shown in Table 4, the remaining structure was at least one of pearlite and retained austenite.

Referring to Tables 1 to 4, it is considered that in Comparative Example 2, the rolling temperature in each of the two rolling passes immediately before the latter two passes in the hot rolling process was low, so that recrystallization was not sufficiently promoted and unrecrystallized grains remained partially. As a result, the number density of ferrite could not be sufficiently uniformed in the thickness direction in the finally obtained metal structure, that is, the maximum value of the difference in number density of ferrite/Nαm exceeded 0.60, and in connection with this, the absolute value of the work softening rate after uEl exceeded 250,000 MPa, that is, work softening after uEl became significant. In Comparative Example 3, the rolling temperature in each of the two rolling passes immediately before the latter two passes was high, so that the austenite grains after recrystallization became coarse, the number of austenite grain boundaries decreased, and the nucleation sites of ferrite decreased. As a result, the number density of ferrite could not be sufficiently uniformed 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. In Comparative Example 4, the reduction rate in each of the rolling passes immediately before the last two stages was low, so recrystallization was not sufficiently promoted, and unrecrystallized grains were partially left. As a result, the number density of ferrite in the finally obtained metal structure could not be sufficiently uniformed in the plate 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 rolling passes immediately before the last two stages was high, so it is thought that flat austenite grains were formed due to the introduction of excessive strain. As a result, the number density of ferrite in the finally obtained metal structure could not be sufficiently uniformed in the plate 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 rolling passes immediately before the last two stages was more than 0.20 seconds, so it is thought that the grain growth of austenite grains after recrystallization could not be sufficiently suppressed. 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 7, the cooling stop temperature in the cooling between the last two rolling passes and the two immediately preceding rolling passes was low, so a large amount of ferrite was generated in the final metal structure. As a result, the hardness difference between ferrite and martensite could not be sufficiently reduced even by precipitation strengthening due to Ti precipitates, and λ decreased. In Comparative Example 8, the cooling stop temperature in the cooling between the last two rolling passes and the two immediately preceding rolling passes was high, so it is considered that the grain growth of austenite grains after recrystallization could not be sufficiently suppressed. As a result, the number density of ferrite could not be sufficiently uniformed in the plate thickness direction in the final metal structure, and the absolute value of the work softening rate after uEl exceeded 250,000 MPa. In Comparative Example 9, the reduction rate of each rolling pass in the last two stages was low, so it is considered that the driving force of ferrite transformation in the subsequent cooling process could not be 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 of each rolling pass in the latter two stages was high, so it is considered that the driving force of the ferrite transformation in the subsequent cooling process became too large. As a result, a large amount of ferrite was generated in the finally obtained metal structure, and the hardness difference between ferrite and martensite could not be sufficiently reduced even by precipitation strengthening due to Ti precipitates, and λ decreased. In Comparative Example 11, the water cooling time until air cooling in the cooling process was long, so a relatively large amount of ferrite was generated. As a result, λ also decreased. In Comparative Example 12, the air cooling temperature was low, so a relatively large amount of bainite was generated, 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 decreased. In Comparative Example 15, the air cooling time was long, so a relatively large amount of ferrite was generated, and λ decreased. In Comparative Example 16, the time required for water cooling to 50°C or less after air cooling was long, so a lot of bainite was produced and uEl decreased.

In Comparative Example 44, the Ti content was low, which is believed 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 believed 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 believed to have caused TS decreased. In Comparative Example 47, the Nb content was high, which is believed to have caused coarse carbides and the like to form. As a result, the workability of the steel sheet decreased and λ decreased.

In contrast, in all of the steel sheets according to the invention examples, a steel sheet with a predetermined chemical composition and with appropriately controlled manufacturing conditions has a metal structure with, in terms of area percentages, ferrite: 10-40%, martensite: 60-90%, and bainite: 0-10%. When a 150 μm x 150 μm region at 1/4 of the plate thickness position on a cross section perpendicular to the plate surface is equally divided into nine regions, the number density of ferrite is calculated in each divided region, and the average value is taken as Nαm, a steel sheet can be obtained in which the difference in number density of ferrite in adjacent divided regions in the plate thickness direction is all Nαm x 0.60 or less. As a result, despite a high tensile strength of 1180 MPa or more, the steel sheet has 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 (7)

The chemical composition, in mass%, is
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 to 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 to 0.0100%, and the balance: Fe and impurities;
The metal structure is, in area percent,
Ferrite: 10-40%,
Martensite: 60-90%,
Bainite: 0 to 10%, and at least one of pearlite and retained austenite: 0 to 5% in total;
A steel plate characterized in that, when a 150 μm x 150 μm region at 1/4 of the plate thickness position on 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 of these 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 all Nαm x 0.60 or less. The chemical composition, in mass%,
B: 0.0001 to 0.0030%,
Cr: 0.001-0.70%,
Mo: 0.001-0.12%,
Cu: 0.001-0.40%,
Ni: 0.001 to 0.30%,
V: 0.001-0.300%,
Sn: 0.001 to 0.040%,
As: 0.001 to 0.100%,
Zr: 0.001 to 0.050%,
Ca: 0.0001 to 0.0010%,
Mg: 0.0001 to 0.0010%,
Bi: 0.001 to 0.010%,
Co: 0.001 to 0.010%,
W: 0.001-0.100%,
Zn: 0.001 to 0.010%, and REM: 0.0001 to 0.0100%
The steel sheet according to claim 1, characterized in that it contains at least one of the following: The steel sheet according to claim 1 or 2, characterized in that Nαm is 0.050 pieces/μm ² or more. The steel plate according to any one of claims 1 to 3, characterized in that it has a tensile strength of 1180 MPa or more. The steel plate according to any one of claims 1 to 4, characterized in that it has a plate thickness of 1.0 to 8.0 mm. A part comprising the steel plate according to any one of claims 1 to 5. A heating step comprising heating a slab having the chemical composition according to claim 1 or 2 and holding it at a temperature of 1180 to 1350°C for 6000 seconds or more;
A hot rolling process including finish rolling the slab using a tandem rolling mill having four or more rolling stands, the hot rolling process satisfying the following conditions (a) to (c):
(a) the rolling temperature in each of the rolling passes in the two stages immediately preceding the latter two stages is 960 to 1080°C, and the rolling reduction in each of the rolling passes 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 rolling passes immediately preceding the latter two rolling passes; and (c) a rolling reduction rate in each rolling pass of the latter two rolling passes is 10 to 40%. A method for producing a steel plate, comprising a cooling step including: water-cooling the finish-rolled steel plate, 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 said temperature range for 2.0 to 8.0 seconds, and water-cooling the steel plate to 50°C or less within 13 seconds after air-cooling.