High-strength steel plate and method for manufacturing the same, and member and method for manufacturing the same

Abstract

Provided is a high strength steel sheet which has a yield strength of 800 MPa or more and exhibits excellent workability, collision yield strength, and crack-stopping properties. The diffusible hydrogen amount in the steel is 0.50 ppm by mass or less. The constituent composition of the steel contains, in terms of mass%, 0.150-0.500% of C, 0.01-3.00% of Si, 1.50-4.00% of Mn, 0.100% or less of P, 0.0200% or less of S, 0.100% or less of Al, 0.0100% or less of N and 0.0100% or less of O, with the remainder comprising Fe and unavoidable impurities. The total areal ratio of tempered martensite and bainite is 55-95%. The existence ratio (A / B) of a structure A having a nanohardness of 7 GPa or more and a structure B having a nanohardness of 6 GPa or less is 0.8-2.5. The solid solution carbon concentration in retained austenite is 0.50-0.90 mass%.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel plate having a yield strength (YS) of 800 MPa or more, a method for manufacturing the same, a member, and a method for manufacturing the same. [Background technology]

[0002] In recent years, for example in the automobile industry, there has been a desire to improve the fuel efficiency of automobiles in order to reduce carbon dioxide (CO2) emissions from the perspective of preserving the global environment. Reducing the weight of a vehicle body is an effective way to improve fuel efficiency, but it is necessary to do so while maintaining the body's strength. Reducing the body's weight can be achieved by increasing the strength of the steel sheets used in automobile parts and simplifying the body structure to reduce the number of parts.

[0003] For example, Patent Documents 1 to 3 disclose high-strength steel sheets having a yield strength of 800 MPa or more. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-21231 [Patent Document 2] Patent No. 2018-21233 [Patent Document 3] Patent No. 2017-214647 Summary of the Invention [Problem to be solved by the invention]

[0005] High strength steel sheets with a yield strength of 800 MPa or more must have good formability in order to be formed into automotive parts. Furthermore, steel sheets used as automobile parts are required to have excellent strength during a collision (hereinafter referred to as "collision strength") and to be able to suppress the growth of cracks generated by external forces during a collision (excellent crack retention properties).

[0006] Therefore, an object of the present invention is to provide a high-strength steel sheet having a yield strength of 800 MPa or more and excellent formability, impact resistance and crack arrestability. [Means for solving the problem]

[0007] The present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

[0008] That is, the present invention provides the following [1] to

[10] . [1] A steel plate is provided, wherein the amount of diffusible hydrogen in the steel of the steel plate is 0.50 mass ppm or less, and the steel plate has a component composition and a microstructure, and the component composition is, in mass%, C: 0.150 to 0.500%, Si: 0.01 to 3.00%, Mn: 1.50 to 4.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0. A high-strength steel plate containing 0.0100% or less of Fe, with the remainder consisting of Fe and unavoidable impurities, wherein the microstructure has a total area ratio of tempered martensite and bainite of 55 to 95%, an abundance ratio A / B of structure A having a nano-hardness of 7 GPa or more and structure B having a nano-hardness of 6 GPa or less is 0.8 to 2.5, and a solute carbon concentration in the retained austenite is 0.50 to 0.90 mass%. [2] The above-mentioned composition further includes, in mass%, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less, Mo: 1.000% or less, Cr: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Zr: 0.1000% or less, Te: 0.100% or less, Cu The high-strength steel plate according to the above [1], containing at least one element selected from the group consisting of: C: 1.000% or less, Ni: 1.000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Co: 0.010% or less, Ta: 0.10% or less, Hf: 0.10% or less, and Bi: 0.200% or less. [3] The high-strength steel sheet according to the above [1] or [2], further comprising a plating layer on the surface of the steel sheet. [4] The high-strength steel sheet according to [3] above, wherein the plating layer is a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, or an electrogalvanized layer. [5] A method for producing a high-strength steel sheet according to the above item [1], comprising: hot rolling a steel slab having the chemical composition according to the above item [1] to obtain a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; heating the cold-rolled steel sheet at a heating temperature T1 of 750 to 950°C for 10 to 500 s, cooling to a cooling stop temperature T2 of 120°C or higher and lower than 280°C, reheating to a reheating temperature T3, recooling without holding at the reheating temperature T3, and holding at a temperature T4 lower than the reheating temperature T3 for 1 s or more; a heat input influence index J from the cooling stop temperature T2 to the reheating temperature T3, which is represented by the following formula (1), is 1500 to 4000; and the cold-rolled steel sheet held at the temperature T4 is temper-rolled using rolls having a surface roughness of 1.5 to 5.0 μm. J = (T3 - T2) (log(9t) + 20) ... (1) In the above formula (1), t is the heating time from the cooling stop temperature T2 to the reheating temperature T3° C., and is expressed in seconds. [6] A method for producing a high-strength steel sheet according to the above item [2], comprising: hot rolling a steel slab having the chemical composition according to the above item [2] to obtain a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; heating the cold-rolled steel sheet at a heating temperature T1 of 750 to 950°C for 10 to 500 s, cooling to a cooling stop temperature T2 of 120°C or higher and lower than 280°C, reheating to a reheating temperature T3, recooling without holding at the reheating temperature T3, and holding at a temperature T4 lower than the reheating temperature T3 for 1 s or more; a heat input influence index J from the cooling stop temperature T2 to the reheating temperature T3, which is represented by the following formula (1), is 1500 to 4000; and the cold-rolled steel sheet held at the temperature T4 is temper-rolled using rolls having a surface roughness of 1.5 to 5.0 μm. J = (T3 - T2) (log(9t) + 20) ... (1) In the above formula (1), t is the heating time from the cooling stop temperature T2 to the reheating temperature T3° C., and is expressed in seconds. [7] The method for producing a high-strength steel sheet according to [5] or [6] above, wherein the cold-rolled steel sheet is subjected to a plating treatment. [8] The method for producing a high-strength steel sheet according to [7] above, wherein the plating treatment is a hot-dip galvanizing treatment, a galvannealed hot-dip galvanizing treatment, or an electrogalvanizing treatment. [9] A member made using the high-strength steel plate according to any one of [1] to [4] above.

[10] A method for producing a component, comprising subjecting the high-strength steel plate according to any one of the above [1] to [4] to at least one of forming and joining to obtain a component. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a high-strength steel sheet having a yield strength of 800 MPa or more and excellent formability, crash resistance and crack arrestability. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a chart showing an example of heat treatment. [Figure 2A] FIG. 3 is a cross-sectional view showing the hat member. [Figure 2B] FIG. 2 is a schematic diagram showing a hat member subjected to a three-point bending test. DETAILED DESCRIPTION OF THE INVENTION

[0011] [High strength steel plate] The high-strength steel sheet of this embodiment (hereinafter also referred to as "the present high-strength steel sheet") includes a steel sheet, and may further include a plating layer on the surface of this steel sheet, as described below. The steel plate of the present high strength steel plate has a chemical composition and a microstructure as described below, and satisfies the amount of diffusible hydrogen in steel as described below. High strength means that the yield strength (YS) is 800 MPa or more.

[0012] This high-strength steel plate has a yield strength of 800 MPa or more and is excellent in workability, crashworthiness, and crack arrestability. Because it has sufficient crash strength, it is suitable for use as parts for transportation vehicles such as automobiles.

[0013] The high-strength steel sheet can be formed by any common processing method such as press working without any restrictions, and the high-strength steel sheet can be welded by any common welding method such as spot welding or arc welding without any restrictions.

[0014] <Steel plate> First, the steel plate included in the present high-strength steel plate will be described. The thickness of the steel plate is not particularly limited, and is, for example, 0.5 mm or more and 3.0 mm or less.

[0015] 《Component composition》 The chemical composition of the steel plate of the present high-strength steel plate (hereinafter, for convenience, also referred to as "the present chemical composition") will be described. Unless otherwise specified, "%" in the composition of the components means "% by mass."

[0016] (C: 0.150 to 0.500%) C forms martensite and increases the strength of the steel sheet. If the C content is too low, the total area ratio of tempered martensite and bainite decreases, resulting in a decrease in collision resistance and yield strength. Therefore, the C content is 0.150% or more, preferably 0.180% or more, and more preferably 0.200% or more. On the other hand, if the C content is too high, the amount of structure A with a nanohardness of 7 GPa or more, which is the starting point of cracks, increases, resulting in reduced workability. Therefore, the C content is 0.500% or less, preferably 0.460% or less, and more preferably 0.400% or less.

[0017] (Si: 0.01 to 3.00%) Si suppresses the formation of carbides during heat treatment and affects the hardness of the structure and the solute carbon concentration in the retained austenite. From the viewpoint of ensuring a structure with appropriate nano-hardness and ensuring a certain level or more of solute carbon concentration in the retained austenite, the Si content is 0.01% or more, preferably 0.50% or more, and more preferably 0.80% or more. On the other hand, if the Si content is too high, the concentration of solute carbon in the retained austenite increases excessively, so the Si content is 3.00% or less, preferably 2.60% or less, and more preferably 2.40% or less.

[0018] (Mn: 1.50-4.00%) Mn affects the area ratio of tempered martensite and bainite. From the viewpoint of obtaining good crash resistance and a yield strength of 800 MPa or more, the Mn content is 1.50% or more, preferably 1.90% or more, and more preferably 2.30% or more. On the other hand, if the Mn content is too high, the amount of structure A with a nanohardness of 7 GPa or more, which is the starting point of cracks, increases, and workability deteriorates. Therefore, the Mn content is 4.00% or less, preferably 3.50% or less, and more preferably 3.30% or less.

[0019] (P:0.100% or less) P segregates at prior austenite grain boundaries and embrittles the grain boundaries, which reduces the ultimate deformability of the steel sheet and reduces workability. Therefore, the P content is 0.100% or less, preferably 0.030% or less, and more preferably 0.010% or less. The lower limit of the P content is not particularly limited, but since P is a solid solution strengthening element and increases the strength of the steel sheet, it is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0020] (S:0.0200% or less) S combines with Mn to form coarse MnS, which act as crack initiation sites, reducing workability. Therefore, the S content is set to 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0020% or less. The lower limit of the S content is not particularly limited, but due to constraints on production technology, it is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

[0021] (Al: 0.100% or less) Al increases the A3 transformation point, which increases the amount of ferrite and decreases the area ratio of tempered martensite and bainite. Therefore, the Al content is 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less. The lower limit of the Al content is not particularly limited, but is, for example, 0.010%, and preferably 0.020%, because Al suppresses the formation of carbides during heat treatment and promotes the formation of retained austenite.

[0022] (N:0.0100% or less) N combines with Ti to form TiN, which becomes the starting point for cracks and reduces workability, so the N content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less. The lower limit of the N content is not particularly limited, but due to constraints on production technology, it is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%.

[0023] (O:0.0100% or less) O forms oxides that act as crack initiation sites, reducing workability, so the O content is set to 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020% or less.

[0024] (Other elements) This composition may further contain, in mass %, at least one element selected from the group consisting of the elements described below.

[0025] ((B:0.0100% or less)) B is an element that can improve the hardenability of the steel sheet by segregating at the austenite grain boundaries and increases the yield strength of the steel sheet, so its addition is preferred. However, if the amount of B is too high, Fe 23 (CB)6 is formed, which becomes the starting point of cracks and reduces workability. Therefore, the B content is preferably 0.0100% or less, more preferably 0.0050% or less, further preferably 0.0040% or less, and particularly preferably 0.0030% or less. The lower limit of the B content is not particularly limited, but from the viewpoint of obtaining the effect of adding B, it is, for example, 0.0005%, and preferably 0.0010%.

[0026] ((Ti:0.200% or less)) Ti is preferably added because it forms fine carbides, nitrides or carbonitrides during hot rolling or heat treatment, thereby increasing the yield strength of the steel sheet. However, if the Ti content is too high, the amount of coarse nitrides formed by bonding with N increases and serves as the starting point for cracks, resulting in a decrease in workability. Therefore, the Ti content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The lower limit of the Ti content is not particularly limited, but from the viewpoint of obtaining the effect of adding Ti, it is, for example, 0.005%, and preferably 0.010%.

[0027] ((Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less)) Nb, V and W are preferably added because they form fine carbides, nitrides or carbonitrides during hot rolling or heat treatment, thereby increasing the yield strength of the steel sheet. However, if the amount of these elements added is excessively large, they will not dissolve when the steel slab is heated and will remain as coarse carbides. The coarse carbides will become the starting point for cracks and reduce workability. Therefore, the Nb content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Nb, it is, for example, 0.005%, and preferably 0.010%. The V content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding V, it is, for example, 0.005%, and preferably 0.010%. The W content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding W, it is, for example, 0.010%, and preferably 0.020%.

[0028] ((Mo: 1.000% or less, Cr: 1.000% or less)) Mo and Cr are preferably added because they improve the hardenability of the steel sheet and thereby increase the yield strength of the steel sheet, but if the amount of these elements is too large, hard martensite is formed in excess, increasing the number of crack initiation points and reducing the workability. Therefore, the Mo content is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Mo, it is, for example, 0.010%, and preferably 0.020%. The Cr content is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Cr, it is, for example, 0.010%, and preferably 0.020%.

[0029] ((Sb: 0.200% or less, Sn: 0.200% or less)) Sb and Sn are preferably added because they increase the yield strength of the steel sheet by suppressing decarburization of the steel sheet surface, but if the amount of these elements is excessively large, the steel becomes embrittled and workability deteriorates. Therefore, the Sb content is preferably 0.200% or less, more preferably 0.080% or less, and even more preferably 0.040% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Sb, it is, for example, 0.001%, and preferably 0.002%. The Sn content is preferably 0.200% or less, more preferably 0.080% or less, and even more preferably 0.040% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Sn, it is, for example, 0.001%, and preferably 0.002%.

[0030] ((Zr: 0.1000% or less, Te: 0.100% or less)) Zr and Tellurium are preferably added because they spheroidize the shape of nitrides and sulfides and improve workability, but if the amount of these elements is excessively large, the amount of coarse precipitates remaining in an undissolved state increases when the steel slab is heated during hot rolling, deteriorating workability. Therefore, the Zr content is preferably 0.1000% or less, more preferably 0.0800% or less, and even more preferably 0.0500% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Zr, it is, for example, 0.0050%, and preferably 0.0100%. The Te content is preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Te, it is, for example, 0.005%, and preferably 0.010%.

[0031] ((Cu:1.000% or less)) Cu is preferably added because it improves the hardenability of the steel sheet and thereby increases the yield strength of the steel sheet, although an excessively high Cu content increases the amount of Cu inclusions, deteriorating the workability. Therefore, the Cu content is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Cu, it is, for example, 0.010%, and preferably 0.020%.

[0032] ((Ni:1.000% or less)) Ni is preferably added because it increases the yield strength of the steel sheet by improving the hardenability of the steel sheet, but if the Ni content is too high, the hard martensite increases, deteriorating the workability. Therefore, the Ni content is preferably 1.000% or less, more preferably 0.800% or less, and even more preferably 0.500% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Ni, it is, for example, 0.010%, and preferably 0.020%.

[0033] ((Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less)) Ca, Mg, and rare earth metals (REM) are preferably added because they spheroidize precipitates such as sulfides and oxides, improving workability. However, if the amount of these elements is excessively large, the coarse sulfides become the starting point for cracks, deteriorating workability. Therefore, the Ca content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0040% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Ca, it is, for example, 0.0005%, and preferably 0.0010%. The Mg content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0040% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Mg, it is, for example, 0.0005%, and preferably 0.0010%. The REM content is preferably 0.0100% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding REM, it is, for example, 0.0005%, and preferably 0.0010%.

[0034] ((Co: 0.010% or less, Ta: 0.10% or less, Hf: 0.10% or less, Bi: 0.200% or less)) Co, Ta, Hf, and Bi are preferably added because they make the shape of precipitates spherical and improve workability, but if the amount of these elements is excessively large, the coarse precipitates become the starting point for cracks, deteriorating workability. Therefore, the amount of Co is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.007% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Co, it is, for example, 0.001%, and preferably 0.002%. The Ta content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.07% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Ta, it is, for example, 0.01%, and preferably 0.02%. The Hf content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.07% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding Hf, it is, for example, 0.01%, and preferably 0.02%. The Bi content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.080% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the effect of adding REM, it is, for example, 0.001%, and preferably 0.005%.

[0035] (remainder) The balance in this composition is Fe and unavoidable impurities.

[0036] Microstructure Next, the microstructure of the steel plate of the present high-strength steel plate (hereinafter, for convenience, also referred to as "the present microstructure") will be described. In order to obtain the effects of the present invention, it is not sufficient to merely satisfy the above-mentioned composition of the present components; it is also necessary to satisfy the present microstructure described below. Hereinafter, the area ratio is the area ratio relative to the entire microstructure. The area ratio of each structure is determined by the method described in the Examples below.

[0037] (Total area ratio of tempered martensite and bainite: 55-95%) From the viewpoint of stably ensuring good crash resistance and yield strength, the total area ratio of tempered martensite and bainite is 55% or more, preferably 58% or more, and more preferably 60% or more.

[0038] On the other hand, if the total area ratio of tempered martensite and bainite is too high, the amount of structure A with a nanohardness of 7 GPa or more increases, and the abundance ratio (A / B) of structure A to structure B becomes too large, resulting in deterioration of workability. Therefore, this total area ratio is 95% or less, preferably 92% or less, and more preferably 88% or less.

[0039] (Abundance ratio A / B: 0.8~2.5) Structure A, which has a nano-hardness of 7 GPa or more, is fine and hard, and improves crack arrest by stopping the growth of cracks generated by external forces during a collision. On the other hand, structure B, which has a nanohardness of 6 GPa or less, has high toughness and therefore improves workability. By appropriately controlling the abundance ratio (A / B) of Structure A and Structure B, both crack arrestability and workability are excellent. For the reason that crack arrestability is excellent, the abundance ratio (A / B) is 0.8 or more, preferably 1.0 or more, and more preferably 1.1 or more.

[0040] On the other hand, if the abundance ratio (A / B) of Structure A to Structure B is too large, Structure A, which has a nanohardness of 7 GPa or more, becomes the origin of cracks, deteriorating workability. Therefore, the abundance ratio (A / B) is 2.5 or less, preferably 2.3 or less, and more preferably 2.0 or less.

[0041] The nanohardness is a hardness measured using a nanoindentation method, and specifically, is determined by the method described in the examples below. Hardness other than nanohardness (for example, Vickers hardness) cannot evaluate the plastic deformation resistance of local regions of the tissue at the submicron level.

[0042] (Solute carbon concentration in retained austenite: 0.50 to 0.90 mass%) The microstructure includes retained austenite. If the concentration of solute carbon in the retained austenite becomes high, the hardness of martensite transformed from the retained austenite increases significantly when stress is repeatedly applied, increasing the number of crack initiation sites and deteriorating workability. Therefore, the concentration of solute carbon in the retained austenite is 0.90 mass % or less, preferably 0.85 mass % or less, and more preferably 0.80 mass % or less.

[0043] On the other hand, if the concentration of solute carbon in the retained austenite is too low, the workability decreases. Therefore, the concentration of solute carbon in the retained austenite is 0.50 mass % or more, preferably 0.60 mass % or more, and more preferably 0.70 mass % or more.

[0044] (Remnant tissue) This microstructure may contain a structure (remaining structure) other than tempered martensite, bainite, and retained austenite. Examples of the remaining structure include known structures such as fresh martensite, pearlite, ferrite, iron-based carbonitrides, alloy carbonitrides, and inclusions such as MnS and Al2O3. The area ratio of the remaining structure is preferably 20% or less, more preferably 10% or less, and even more preferably 5% or less. If the area ratio of the remaining structure is within this range, the effects of the present invention will not be impaired.

[0045] <Diffusible hydrogen content in steel: 0.50 mass ppm or less> If the amount of diffusible hydrogen in the steel is too high, delayed fracture occurs and workability deteriorates, so the amount of diffusible hydrogen in the steel is 0.50 ppm by mass or less, preferably 0.30 ppm by mass or less, and more preferably 0.20 ppm by mass or less. The amount of diffusible hydrogen in steel is determined by the method described in the examples below.

[0046] <Plating layer> The high-strength steel sheet may further have a plating layer on the surface of the steel sheet from the viewpoint of improving corrosion resistance and the like. Examples of the plated layer include a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrogalvanized layer. The plated layer is formed by a plating treatment described below.

[0047] [Manufacturing method for high-strength steel plates] Next, a method for manufacturing a high-strength steel plate according to this embodiment (hereinafter also referred to as "the present manufacturing method") will be described. The present manufacturing method is also a method for manufacturing the present high-strength steel plate described above. Unless otherwise specified, the temperatures at which steel slabs, steel plates, etc. are heated or cooled below refer to the surface temperatures of the steel slabs, steel plates, etc. The method for producing molten steel to be used as a steel slab (steel material) is not particularly limited, and known methods using a converter, an electric furnace, or the like can be used. It is preferable to obtain a steel slab from molten steel by a continuous casting method. Steel slabs may also be obtained by other methods such as an ingot casting-blooming rolling method or a thin slab continuous casting method.

[0048] <Hot rolling> In this manufacturing method, first, a steel slab having the above-described present chemical composition is hot-rolled to obtain a hot-rolled steel sheet. When hot rolling, the steel slab may be reheated in a heating furnace before rolling. If the steel slab is kept at a temperature equal to or higher than a predetermined temperature, the steel slab may be directly rolled without being heated. In hot rolling, steel slabs are subjected to rough rolling and finish rolling. Before rough rolling, the steel slab is preferably heated to dissolve carbides in the steel slab. From the viewpoint of dissolving carbides and preventing an increase in rolling load, the temperature when heating the steel slab (steel slab heating temperature) is preferably 1100°C or higher, more preferably 1150°C or higher. On the other hand, from the viewpoint of preventing an increase in scale loss, the steel slab heating temperature is preferably 1300°C or less, and more preferably 1280°C or less. As described above, if the steel slab before rough rolling is maintained at a temperature equal to or higher than a predetermined temperature and the carbides in the steel slab are dissolved, heating the steel slab before rough rolling can be omitted. The conditions for rough rolling and finish rolling are not particularly limited, but for example, the finish rolling end temperature is preferably 700 to 1100°C, more preferably 800 to 1000°C.

[0049] <Cold rolling> Next, the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The rolling ratio of the cold rolling is preferably 30% or more, more preferably 35% or more. The upper limit is not particularly limited, and is, for example, 70% or less, preferably 65% ​​or less.

[0050] <Heat treatment> Next, the cold-rolled steel sheet obtained by cold rolling is subjected to heat treatment. FIG. 1 is a chart showing an example of the heat treatment. In the heat treatment, the cold-rolled steel sheet is generally heated to a heating temperature T1, cooled to a cooling stop temperature T2, reheated to a reheating temperature T3, and re-cooled without being held at the reheating temperature T3. In the re-cooling, the cold-rolled steel sheet is held at a temperature T4 lower than the reheating temperature T3.

[0051] The cold-rolled steel sheet that has been subjected to heat treatment and further to temper rolling, which will be described later, corresponds to the steel sheet that constitutes the present high-strength steel sheet.

[0052] 《Heating temperature T1: 750~950℃, heating time t1: 10~500s》 First, the cold-rolled steel sheet is heated at a heating temperature T1. If the heating temperature T1 is too low or the heating time t1 (the time the cold-rolled steel sheet is held at the heating temperature T1) is too short, the steel sheet will be heated in the two-phase region of ferrite and austenite. In this case, the final microstructure will contain ferrite, and the total area ratio of tempered martensite and bainite will decrease. Therefore, the heating temperature T1 is 750° C. or higher, preferably 800° C. or higher, and more preferably 850° C. or higher. The heating time t1 is 10 seconds or higher, preferably 50 seconds or higher, and more preferably 80 seconds or higher.

[0053] On the other hand, if the heating temperature T1 is too high or the heating time t1 is too long, the hydrogen partial pressure increases, increasing the amount of hydrogen that penetrates into the steel, and therefore the amount of diffusible hydrogen in the steel increases. Furthermore, the total area ratio of tempered martensite and bainite becomes high, the abundance ratio (A / B) becomes too large, and workability deteriorates. Therefore, the heating temperature T1 is equal to or less than 950° C., preferably equal to or less than 930° C., and more preferably equal to or less than 900° C. The heating time t1 is equal to or less than 500 s, preferably equal to or less than 300 s, and more preferably equal to or less than 200 s.

[0054] <Cooling stop temperature T2: 120°C or higher and less than 280°C> Next, the cold-rolled steel sheet heated at the heating temperature T1 is cooled to a cooling stop temperature T2. If the cooling stop temperature T2 is too low, the total area ratio of tempered martensite and bainite becomes high, and the abundance ratio (A / B) becomes too large, resulting in deterioration of workability. Therefore, the cooling stop temperature T2 is 120°C or higher, preferably 140°C or higher, and more preferably 150°C or higher.

[0055] On the other hand, if the cooling stop temperature T2 is too high, the total area ratio of tempered martensite and bainite decreases. Also, the structure B with a nanohardness of 6 GPa or less increases, and the abundance ratio (A / B) becomes too small, resulting in poor crack arrestability. Therefore, the cooling stop temperature T2 is lower than 280°C, preferably 270°C or lower, and more preferably 260°C or lower.

[0056] 《Reheating temperature T3》 Next, the cold-rolled steel sheet cooled to the cooling stop temperature T2 is reheated to a reheating temperature T3 and recooled without being held at the reheating temperature T3. The reheating temperature T3 is not particularly limited as long as it satisfies the heat input influence index J described later. The reheating temperature T3 is, for example, 280°C or higher, preferably 290°C or higher, and more preferably 300°C or higher. On the other hand, the reheating temperature T3 is, for example, 400°C or lower, preferably 380°C or lower, and more preferably 350°C or lower.

[0057] 《Heat input influence index J:1500~4000》 The heat input influence index J from the cooling stop temperature T2 to the reheating temperature T3 for a cold-rolled steel sheet is expressed by the following formula (1). J = (T3 - T2) (log(9t) + 20) ... (1) In the above formula (1), t is the heating time (unit: s) from the cooling stop temperature T2 to the reheating temperature T3°C.

[0058] The nanohardness of each structure that makes up the microstructure changes depending on the state of carbon present in each structure. The heat input effect index J is affected not only by the state of carbon present, but also by the diffusion rate and location of carbon. If the heat input effect index J is too low, the carbon in the structure exists in a solid solution state, so the structure A with a nanohardness of 7 GPa or more increases, the abundance ratio (A / B) becomes too large, and workability deteriorates. In addition, the concentration of solute carbon in the retained austenite becomes too high, which deteriorates workability. Therefore, the heat input influence index J is 1500 or more, preferably 1800 or more, and more preferably 2000 or more.

[0059] On the other hand, if the heat input effect index J is too high, the carbon in the structure exists as carbides, increasing the structure B with a nanohardness of 6 GPa or less, and the abundance ratio (A / B) becomes too small, deteriorating crack arrestability. Also, the solute carbon concentration in the retained austenite becomes too low, deteriorating workability. Therefore, the heat input influence index J is 4000 or less, preferably 3800 or less, and more preferably 3500 or less.

[0060] <Holding time t4 at temperature T4 lower than reheating temperature T3: 1 second or more> In the re-cooling, the cold-rolled steel sheet is held at a temperature T4 which is lower than the reheating temperature T3. If the temperature T4 is equal to or higher than the reheating temperature T3, the abundance ratio (A / B) becomes too small, resulting in deterioration of crack arrestability. The lower limit of the temperature T4 is not particularly limited, but is, for example, 180°C, preferably 200°C, and more preferably 220°C.

[0061] If the cold-rolled steel sheet is not held at temperature T4 (for example, the holding time t4 at temperature T4 is zero), the abundance ratio (A / B) becomes too large, resulting in deterioration of workability. Therefore, the retention time t4 is 1 second or more, preferably 3 seconds or more, and more preferably 5 seconds or more.

[0062] <Temper rolling> Next, the heat-treated (specifically, held at temperature T4 for 1 second or more) cold-rolled steel sheet is subjected to temper rolling using rolls.

[0063] <Roll surface roughness: 1.5 to 5.0 μm> By controlling the surface roughness of the roll, it is possible to adjust the mobile dislocations introduced into localized regions at the submicron level and control the hardness distribution of the structure. If the surface roughness of the roll is too small, the abundance ratio (A / B) becomes too small, resulting in poor crack retention. Therefore, the surface roughness of the roll is 1.5 μm or more, preferably 1.8 μm or more, and more preferably 2.0 μm or more.

[0064] On the other hand, if the surface roughness of the roll is too large, the abundance ratio (A / B) becomes too large, resulting in poor processability. Therefore, the surface roughness of the roll is 5.0 μm or less, preferably 4.5 μm or less, and more preferably 4.0 μm or less.

[0065] The surface roughness of the roll is the arithmetic mean roughness Ra measured in accordance with JIS B 0601.

[0066] <Plating process> In the present manufacturing method, a plating layer may be formed on the surface of the cold-rolled steel sheet that has been subjected to temper rolling by plating the steel sheet. Examples of the plated layer include a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrogalvanized layer. The plating treatment is preferably a hot-dip galvanizing treatment, a galvannealed hot-dip galvanizing treatment or an electrogalvanizing treatment.

[0067] When hot-dip galvanizing is performed, for example, the steel sheet is immersed in a zinc bath at a bath temperature of 440 to 500° C. Thereafter, it is preferable to adjust the coating weight of the plating layer by gas wiping or the like. The zinc bath is preferably one having a component composition in which the Al content is 0.10 to 0.23 mass %, with the remainder being Zn and unavoidable impurities.

[0068] When performing galvannealing treatment, if the alloying temperature is too low, the Zn-Fe alloying rate becomes too slow, which may make alloying extremely difficult. On the other hand, if the alloying temperature is too high, untransformed austenite may transform into pearlite. Therefore, the alloying temperature is preferably 450 to 600°C, more preferably 470 to 550°C, and even more preferably 470 to 530°C.

[0069] An electrogalvanized layer is formed by performing an electrogalvanizing treatment. The electrogalvanized layer is not particularly limited, and a conventionally known electrogalvanized layer can be suitably used. The electrogalvanized layer may be a zinc alloy layer in which an appropriate amount of an element such as Fe, Cr, Ni, Mn, Co, Sn, Pb, or Mo is added to Zn depending on the purpose.

[0070] The coating weight of the coating layer of hot-dip galvanized steel sheet (GI), galvannealed steel sheet (GA) and electrogalvanized steel sheet (EG) is 20 to 80 g / m per side. 2 (Double-sided plating) is preferred.

[0071] The plated steel sheet is cooled to a temperature of, for example, 50° C. or less. The steel sheet cooled to a temperature of 50° C. or less may be rolled at an elongation rate of 0.05 to 1.00%. The elongation rate is preferably 0.08 to 0.70%. This rolling may be performed in an apparatus that is continuous with the apparatus (galvanizing apparatus) for performing the zinc plating treatment, or in an apparatus that is not continuous with the plating apparatus. The target elongation rate may be achieved in a single rolling operation, or the target elongation rate may be achieved in total by performing multiple rolling operations. The rolling described here generally refers to temper rolling, but may be rolling by processing using a leveler or the like as long as it can impart an elongation rate equivalent to that of temper rolling.

[0072] In this manufacturing method, the holding temperatures, such as the heating temperature and reheating temperature, do not have to be constant as long as they are within the above-mentioned temperature range. The cooling rate may be changed during cooling as long as it is within the above-mentioned rate range. As long as the above-mentioned temperature range and other conditions are met, any equipment may be used for the heat treatment.

[0073] [Component] Next, the member of this embodiment (hereinafter also referred to as "the member") will be described. The present member is a member at least partially made of the present high-strength steel plate described above, and is obtained by forming the present high-strength steel plate into a desired shape by processing (pressing, etc.), for example. The present member is preferably a member for an automobile part. Note that the member for an automobile part may contain, as a raw material, a steel plate other than the present high-strength steel plate. As described above, the present high-strength steel plate has a yield strength of 800 MPa or more and is excellent in workability, crashworthiness, and crack arrestability. Therefore, the present member is excellent in workability, crashworthiness, and crack arrestability, and can contribute to reducing the weight of the vehicle body, making it particularly suitable for use in automotive components, particularly as automotive frame structural components or automotive reinforcing components.

[0074] [Material manufacturing method] Next, a method for manufacturing this member will be described. This member can be obtained, for example, by subjecting this high-strength steel plate to at least one of forming and joining. The molding process is not particularly limited, and examples thereof include press working. The joining process is not particularly limited, and examples thereof include general welding such as spot welding and arc welding; and caulking using rivets or the like. [Example]

[0075] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0076] <Steel plate manufacturing> Molten steel having the composition shown in Table 1 below, with the balance being Fe and unavoidable impurities, was produced in a converter, and steel slabs were obtained by continuous casting. The underlined values ​​in Table 1 below indicate values ​​outside the scope of the present invention (the same applies to Tables 2 and 3 below). The obtained steel slab was hot rolled to obtain a hot-rolled steel sheet under the conditions shown in Table 2. Specifically, the steel slab was heated to 1250°C and subjected to rough rolling, and then finish rolling at a finish rolling end temperature of 900°C. The obtained hot-rolled steel sheets were cold-rolled at the rolling ratios shown in Table 2 below to obtain cold-rolled steel sheets (sheet thickness: 1.2 mm). The obtained cold-rolled steel sheets were subjected to heat treatment under the conditions shown in Table 2 below. Furthermore, the heat-treated cold-rolled steel sheets were subjected to temper rolling under the conditions shown in Table 2 below.

[0077] In some cases, both sides of a cold-rolled steel sheet (CR) after temper rolling were subjected to a plating treatment to obtain a hot-dip galvanized steel sheet (GI), a galvannealed steel sheet (GA), or an electrogalvanized steel sheet (EG). As the hot-dip galvanizing bath, a zinc bath containing 0.20 mass% Al, with the balance being Zn and unavoidable impurities, was used when producing GI, and a zinc bath containing 0.14 mass% Al, with the balance being Zn and unavoidable impurities, was used when producing GA. The bath temperature was 470°C for both GI and GA production. When manufacturing GI, the coating weight of the plating layer is 45 to 72 g / m per side. 2 When manufacturing GA, the thickness is 45 g / m per side. 2 It was decided. When producing GA, the alloying temperature was set to 500°C. The composition of the GI plating layer was 0.1-1.0 mass% Fe, 0.2-1.0 mass% Al, with the remainder being Fe and unavoidable impurities. The composition of the GA plating layer was 7-15 mass% Fe, 0.1-1.0 mass% Al, with the remainder being Fe and unavoidable impurities. When manufacturing EG, an electrogalvanizing line is used to achieve a coating weight of 30 g / m per side. 2 Electrogalvanizing treatment was carried out so that

[0078] Hereinafter, the heat-treated cold-rolled steel sheet (CR), hot-dip galvanized steel sheet (GI), galvannealed steel sheet (GA) and electrogalvanized steel sheet (EG) will also be simply referred to as "steel sheet".

[0079] Observation of Microstructure The microstructure of the obtained steel sheet was observed as follows. The results are shown in Table 3 below. In Table 3 below, martensite is represented as "M", bainite as "B", and austenite as "γ".

[0080] <Total area ratio of tempered martensite and bainite> The obtained steel sheet was polished so that the cross section (L cross section) parallel to the rolling direction at a position corresponding to 1 / 4 of the sheet thickness was used as the observation surface. The observation surface was corroded with 1% by volume of nital and then observed at 3000x magnification using a scanning electron microscope (SEM). The observation surface was observed in 10 fields and SEM images were obtained. The obtained SEM images were analyzed to determine the total area ratio (unit: %) of tempered martensite and bainite. More specifically, the dark gray areas in the obtained SEM images were determined to be tempered martensite and bainite, and their area ratios (average area ratios of 10 fields of view) were calculated. Image-Pro, a software product manufactured by Media Cybernetics, was used to analyze the SEM images.

[0081] <<Nano hardness measurement>> The obtained steel sheet was polished so that the cross section (L cross section) parallel to the rolling direction at a position corresponding to 1 / 4 of the sheet thickness was used as the observation surface. The observation surface was mirror-polished using diamond paste, and then finish-polished using colloidal silica. The nanohardness of the observed surface was measured at 225 points using a nanoindentation device equipped with a Berkovich indenter. The measurement conditions were as follows: loading and unloading speeds of 50 μN / s, maximum load of 500 μN, data collection interval of 5 ms, and distance between indentations of 2 μm or more. Of the 225 measurement points, those with a nanohardness of 7 GPa or more were designated as structure A, and those with a nanohardness of 6 GPa or less were designated as structure B. The ratio of the number of measurement points was calculated as the abundance ratio of structure A to structure B (A / B).

[0082] <<Measurement of solute carbon concentration in retained austenite>> The obtained steel plate was ground so that the 1 / 4 position of the plate thickness became the observation surface, and then further polished by 0.1 mm by chemical polishing. For the observed surface, an X-ray diffraction (XRD) device was used with CoKα as an X-ray source to determine the peak angles of the austenite (200), (220), and (311) planes, and the lattice constant a (unit: Å) of the retained austenite was calculated using the following formula (2). The calculated lattice constant a of the retained austenite was substituted into the following formula (3) to determine the carbon (C) content (unit: mass%) in the retained austenite, which was defined as the solute carbon concentration in the retained austenite.

[0083] a=1.79021√2 / sinθ…(2) a=3.572+0.0012[Mn]-0.00157[Si]+0.0056[Al]+0.033[C]…(3) In formula (2), a is the lattice constant of retained austenite (unit: Å), and θ is the value obtained by dividing the diffraction peak angle of the (220) plane by 2 (unit: rad). In formula (3), a is the lattice constant of the retained austenite (unit: Å), and [M] is the content of element M in the retained austenite (unit: mass%). However, the content of element M other than C is the content of each element in the composition of the entire steel sheet (specifically, for example, the main composition described above).

[0084] <Measurement of diffusible hydrogen content in steel> Test pieces measuring 30 mm in length and 5 mm in width were taken from the obtained steel plates. The amount of diffusible hydrogen in the steel was measured for the test pieces using thermal desorption analysis. The heating rate was 200°C / hr. The cumulative amount of hydrogen detected in the temperature range from room temperature (25°C) to less than 210°C was taken as the amount of diffusible hydrogen in the steel (unit: ppm by mass). For steel sheets on which a plating layer had been formed, the plating layer was removed using a router (precision grinder), and then the same measurements were carried out. The results are shown in the following Table 3. The amount of diffusible hydrogen in the steel is preferably 0.50 ppm by mass or less.

[0085] <evaluation> The obtained steel sheets were evaluated by the following methods, and the results are shown in Table 3 below.

[0086] Tensile test No. 5 test pieces according to JIS Z 2241 were taken from the obtained steel sheets, with the longitudinal direction (tensile direction) at an angle of 90° to the rolling direction. Using the taken test pieces, tensile tests in accordance with JIS Z 2241 were carried out five times, and the yield strength (YS) and elongation (El) were calculated from the average values ​​of the five tests. If the YS is 800 MPa or more, it can be evaluated as having high strength. If El is 8.0% or more, the steel can be evaluated as having good ductility and excellent workability.

[0087] <Collision resistance evaluation test> Using the obtained steel plates, members with a hat-shaped cross section (hat members) were fabricated, and a three-point bending test was carried out to determine the maximum load (unit: kN).

[0088] First, the hat member 1 will be described with reference to FIG. 2A. Fig. 2A is a cross-sectional view showing the hat member 1. Fig. 2A shows the dimensions of the hat member 1. The hat member 1 is joined to the flat plate 2 by spot welding (nugget diameter: 4.5√t, spot pitch: 35 mm). The flat plate 2 is a cold-rolled steel plate without a plating layer, and has a tensile strength (TS) of 590 MPa and a plate thickness t that is the same as that of the hat member 1 (1.2 mm).

[0089] Next, the three-point bending test will be described with reference to FIG. 2B. FIG. 2B is a schematic diagram showing the hat member 1 subjected to the three-point bending test. Various dimensions are also shown in FIG. 2B. A flat plate 2 joined to the hat member 1 is supported by a rigid support member 3. In this state, a rigid impactor 4 is moved from above toward the hat member 1 at a speed of 1 m / s. In this manner, the three-point bending test is carried out.

[0090] The three-point bending test was carried out three times for each steel plate, and the average value of the maximum loads obtained in each test was taken as the maximum load for that steel plate. In Table 3 below, the maximum load was marked with "◎" if it was 40 kN or more, "◯" if it was 30 kN or more but less than 40 kN, and "×" if it was less than 30 kN. If the rating is "◎" or "◯", the crashworthiness can be evaluated as excellent.

[0091] <Crack arrestability evaluation test> The obtained steel plate was bent using a 90-degree V-block under the following test conditions to obtain a test specimen. The ridge of the bend apex of the obtained test specimen was observed at 40x magnification using a digital microscope (RH-2000, manufactured by Hirox), and the length of the crack was measured. When there were multiple cracks, the sum of the crack lengths was calculated. In Table 3 below, if the total crack length was 6000 μm or less, it was marked with "◎", if the total crack length was more than 6000 μm but less than 12000 μm, it was marked with "○", and if the total crack length was 12000 μm or more, it was marked with "×". If the rating is "◎" or "◯", it can be evaluated as being excellent in crack arrestability.

[0092] (Test conditions) Test method: Roll support, punch indentation Roll diameter: φ30mm Punch tip R: 0.4 mm Distance between rolls: (plate thickness x 2) + 0.5 mm Stroke speed: 20mm / min Test piece size: 60mm x 60mm Bending direction: Rolling right angle method

[0093] [Table 1] TIFF0007761672000002.tif195146

[0094] [Table 2] TIFF0007761672000004.tif141159

[0095] [Table 3] TIFF0007761672000006.tif157160

[0096] <Summary of evaluation results> As shown in Tables 1 to 3 above, No. 、 Steel plates Nos. 3, 5 to 6, 10, 15 to 17, 22, 30 and 33 to 42 all had a yield strength of 800 MPa or more, and were excellent in workability, impact resistance and crack arrestability. In contrast, the steel plates of Nos. 4, 7 to 9, 11 to 14, 18 to 21, 23 to 29 and 31 to 32 were insufficient in at least one of yield strength, workability, crash strength and crack arrestability.

Claims

1. Equipped with steel plates, The amount of diffusible hydrogen in the steel of the steel plate is 0.50 mass ppm or less, The steel sheet has a chemical composition and a microstructure, The component composition is, in mass%, C: 0.150-0.500%, Si: 0.01-3.00%, Mn: 1.50-4.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less, the balance being Fe and unavoidable impurities; The microstructure is The total area ratio of tempered martensite and bainite is 55 to 95%; The area ratio of retained austenite is more than 0% and 45% or less, the area ratio of the remaining structure, which is a structure other than tempered martensite, bainite, and retained austenite, is 20% or less; the abundance ratio A / B of a structure A having a nanohardness of 7 GPa or more and a structure B having a nanohardness of 6 GPa or less is 0.8 to 2.5; A high-strength steel plate having a yield strength of 800 MPa or more, in which the concentration of solute carbon in retained austenite is 0.50 to 0.90 mass %.

2. The component composition further comprises, in mass%, B: 0.0100% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, W: 0.100% or less, Mo: 1.000% or less, Cr: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Zr: 0.1000% or less, Te: 0.100% or less, Cu: 1.000% or less, Ni: 1.000% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Co: 0.010% or less, Ta: 0.10% or less, Hf: 0.10% or less, and The high-strength steel plate according to claim 1, further comprising at least one element selected from the group consisting of Bi: 0.200% or less.

3. The high-strength steel sheet according to claim 1 or 2, further comprising a plating layer on a surface of the steel sheet.

4. The high-strength steel sheet according to claim 3, wherein the plating layer is a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, or an electrogalvanized layer.

5. A method for producing the high-strength steel plate according to claim 1, A steel slab having the chemical composition according to claim 1 is hot-rolled to obtain a hot-rolled steel sheet, The hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet, The cold-rolled steel sheet is heated at a heating temperature T1 of 750 to 950 ° C. for 10 to 500 s, cooled to a cooling stop temperature T2 of 120 ° C. or higher and lower than 280 ° C., reheated to a reheating temperature T3, recooled without holding at the reheating temperature T3, and held at a temperature T4 lower than the reheating temperature T3 for 1 s or more, provided that the difference between the reheating temperature T3 and the temperature T4 is 2 ° C. or higher, and the temperature T4 is 180 ° C. or higher, A heat input influence index J from the cooling stop temperature T2 to the reheating temperature T3, represented by the following formula (1), is 1500 to 4000, The cold-rolled steel sheet held at the temperature T4 is temper-rolled using a roll having a surface roughness of 1.5 to 5.0 μm. J=(T3-T2)(log(9t)+20)...(1) In the formula (1), t is the heating time from the cooling stop temperature T2 to the reheating temperature T3° C., and is expressed in seconds.

6. A method for producing the high-strength steel plate according to claim 2, A steel slab having the chemical composition according to claim 2 is hot-rolled to obtain a hot-rolled steel sheet, The hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet, The cold-rolled steel sheet is heated at a heating temperature T1 of 750 to 950 ° C. for 10 to 500 s, cooled to a cooling stop temperature T2 of 120 ° C. or higher and lower than 280 ° C., reheated to a reheating temperature T3, recooled without holding at the reheating temperature T3, and held at a temperature T4 lower than the reheating temperature T3 for 1 s or more, provided that the difference between the reheating temperature T3 and the temperature T4 is 2 ° C. or higher, and the temperature T4 is 180 ° C. or higher, A heat input influence index J from the cooling stop temperature T2 to the reheating temperature T3, represented by the following formula (1), is 1500 to 4000, The cold-rolled steel sheet held at the temperature T4 is temper-rolled using a roll having a surface roughness of 1.5 to 5.0 μm. J=(T3-T2)(log(9t)+20)...(1) In the formula (1), t is the heating time from the cooling stop temperature T2 to the reheating temperature T3° C., and is expressed in seconds.

7. The method for producing a high-strength steel sheet according to claim 5 or 6, further comprising the step of subjecting the cold-rolled steel sheet to a plating treatment.

8. The method for producing a high-strength steel sheet according to claim 7, wherein the plating treatment is a hot-dip galvanizing treatment, a galvannealed hot-dip galvanizing treatment, or an electrogalvanizing treatment.

9. A member made using the high-strength steel plate according to claim 1 or 2.

10. A member made using the high-strength steel plate according to claim 3.

11. A member made using the high-strength steel plate according to claim 4.

12. A method for manufacturing a component, comprising subjecting the high-strength steel plate according to claim 1 or 2 to at least one of forming and joining to obtain a component.

13. A method for manufacturing a component, comprising subjecting the high-strength steel plate according to claim 3 to at least one of forming and joining to obtain a component.

14. A method for manufacturing a component, comprising subjecting the high-strength steel plate according to claim 4 to at least one of forming and joining to obtain a component.

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