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 having a yield strength of at least 800 MPa and excellent collision strength and fracture resistance. The amount of diffusible hydrogen in the steel is 0.50 mass ppm or less. The composition of the steel comprises, in 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 inevitable impurities. The total area ratio of tempered martensite and bainite is 55-95%, the average grain size of retained austenite is 5.0 μm or less, the area ratio of a structure S1 having a carbon concentration of more than 0.1 mass% and 0.3 mass% or less is at least 50.0%, and the area ratio of a structure S2 having a carbon concentration of at least 0.5 mass% is 10.0% or less.

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 usually have improved formability due to the TRIP (Transformation Induced Plasticity) effect of retained austenite. That is, during processing, the retained austenite transforms into martensite due to the TRIP effect, increasing the strength and improving the ductility by increasing the strain dispersion ability.

[0006] However, when such steel sheets are formed into automobile parts, the retained austenite transforms into hard martensite, which can reduce fracture resistance during a collision (hereinafter simply referred to as "fracture resistance").

[0007] Steel sheets used as automobile parts are also required to have excellent strength in the event of a collision (hereinafter referred to as "collision strength").

[0008] Therefore, an object of the present invention is to provide a high-strength steel plate having a yield strength of 800 MPa or more and excellent crash resistance and fracture resistance. [Means for solving the problem]

[0009] The present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

[0010] 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 chemical composition and a microstructure, and the chemical composition includes, 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: 0.0100% or less. and the balance being Fe and unavoidable impurities, wherein the microstructure has a total area ratio of tempered martensite and bainite of 55 to 95%, an average grain size of retained austenite of 5.0 μm or less, an area ratio of a structure S1 having a carbon concentration of more than 0.1 mass % and not more than 0.3 mass % of 50.0% or more, and an area ratio of a structure S2 having a carbon concentration of 0.5 mass % or more of 10.0% or less. [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 [1] or [2] above, 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 the high-strength steel plate 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 plate, cold rolling the hot-rolled steel plate to obtain a cold-rolled steel plate, heating the cold-rolled steel plate 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 but lower than 280°C, reheating to a reheating temperature T3 of 280 to 400°C, and recooling without holding at the reheating temperature T3, such that a heat effect index P from the reheating temperature T3 to (T3-30)°C, as represented by the following formula (1), is 4000 to 6200. P = T3(log(5t)+12)…(1) In the above formula (1), t is the cooling time from the reheating temperature T3 to (T3-30)°C, and is expressed in seconds. [6] A method for producing the high-strength steel plate 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 plate; cold rolling the hot-rolled steel plate to obtain a cold-rolled steel plate; heating the cold-rolled steel plate 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 but lower than 280°C, reheating to a reheating temperature T3 of 280 to 400°C, and recooling without holding at the reheating temperature T3, such that a heat effect index P from the reheating temperature T3 to (T3-30)°C, as represented by the following formula (1), is 4000 to 6200. P = T3(log(5t)+12)…(1) In the above formula (1), t is the cooling time from the reheating temperature T3 to (T3-30)°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]

[0011] According to the present invention, it is possible to provide a high-strength steel plate having a yield strength of 800 MPa or more and excellent crash strength and fracture resistance. [Brief explanation of the drawings]

[0012] [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

[0013] [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.

[0014] The high-strength steel plate has a yield strength of 800 MPa or more and is excellent in crashworthiness and fracture resistance, and therefore has sufficient crash strength, making it suitable for use as parts for transportation vehicles such as automobiles.

[0015] 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.

[0016] <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.

[0017] 《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."

[0018] (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 hard structure with a high carbon concentration (structure S2) increases, which becomes the starting point for cracks during a collision, deteriorating fracture resistance. Therefore, the C content is 0.500% or less, preferably 0.460% or less, and more preferably 0.400% or less.

[0019] (Si: 0.01 to 3.00%) Si suppresses the formation of carbides during heat treatment and increases yield strength. From the viewpoint of obtaining good crash resistance and a yield strength of 800 MPa or more, 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 carbon concentration in the retained austenite increases excessively, and the hard structure with a high carbon concentration (structure S2) increases, which becomes the starting point of cracks during a collision, deteriorating fracture resistance. Therefore, the Si content is 3.00% or less, preferably 2.60% or less, and more preferably 2.40% or less.

[0020] (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 hard tempered martensite and bainite increase excessively, which deteriorates the fracture resistance during a collision. Therefore, the Mn content is 4.00% or less, preferably 3.50% or less, and more preferably 3.30% or less.

[0021] (P:0.100% or less) P segregates at prior austenite grain boundaries, embrittling the grain boundaries and thereby deteriorating fracture resistance during impact, so 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%.

[0022] (S:0.0200% or less) S combines with Mn to form coarse MnS, which can become the starting point for cracks during a collision, deteriorating fracture resistance during a collision. Therefore, the S content is 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%.

[0023] (Al: 0.100% or less) Al acts as a deoxidizer. If the Al content is too high, oxides and nitrides aggregate and coarsen, which can become crack initiation points during a collision, thereby deteriorating fracture resistance during a collision. 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.

[0024] (N:0.0100% or less) N combines with Ti to form TiN. If the N content is too high, the amount of TiN formed increases, which can become the starting point for cracks during a collision, thereby deteriorating fracture resistance during a collision. Therefore, 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%.

[0025] (O:0.0100% or less) O forms oxides, which become crack initiation points during a collision, deteriorating fracture resistance during a collision. Therefore, the O content is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0020% or less.

[0026] (Other elements) This composition may further contain, in mass %, at least one element selected from the group consisting of the elements described below.

[0027] ((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 during a collision, and the fracture resistance during a collision deteriorates. 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%.

[0028] ((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, Ti bonds with N to form coarse nitrides, which deteriorates fracture resistance during a collision. 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%.

[0029] ((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 these elements are added in excessively large amounts, they will not dissolve when the steel slab is heated and will remain as coarse carbides, which can become the starting point for cracks during a collision, thereby deteriorating the fracture resistance during a collision. 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%.

[0030] ((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 high, hard martensite is formed in excess, which reduces the fracture resistance during collision. 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%.

[0031] ((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 its fracture resistance during collision 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%.

[0032] ((Zr: 0.1000% or less, Te: 0.100% or less)) Zr and Te are preferably added because they spheroidize the shape of nitrides and sulfides and improve fracture resistance during collisions, but if the amount of these elements is excessively large, the amount of coarse precipitates remaining in an undissolved state during heating of the hot-rolled steel slab increases, deteriorating fracture resistance during collisions. 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%.

[0033] ((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, but if the Cu content is excessively high, the increase in Cu inclusions will degrade the fracture resistance during collision. 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%.

[0034] ((Ni:1.000% or less)) Ni is preferably added because it improves the hardenability of the steel sheet and thereby increases the yield strength of the steel sheet, but if the Ni content is too high, the hard martensite increases, which reduces the fracture resistance during collision. 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%.

[0035] ((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 fracture resistance during impact. However, if the amount of these elements is excessively large, the coarse sulfides can become the starting point for cracks during impact, deteriorating fracture resistance during impact. 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%.

[0036] ((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 precipitates spherical and improve fracture resistance during impact, but if the amount of these elements is excessively large, the coarse precipitates become the starting points for cracks during impact, deteriorating fracture resistance during impact. 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%.

[0037] (remainder) The balance in this composition is Fe and unavoidable impurities.

[0038] 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.

[0039] (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.

[0040] On the other hand, if the total area ratio of tempered martensite and bainite is too high, the amount of hard structure with a high carbon concentration (structure S2) increases, which becomes the starting point for cracks during a collision, deteriorating fracture resistance. Therefore, this total area ratio is 95% or less, preferably 92% or less, and more preferably 88% or less.

[0041] (Average grain size of retained austenite: 5.0 μm or less) When a steel plate is repeatedly subjected to stress, the retained austenite transforms into hard martensite due to work hardening. If the retained austenite is too large, the martensite transformed from the retained austenite becomes the starting point for cracks during a collision, resulting in a deterioration of fracture resistance. Therefore, the average grain size of the retained austenite is 5.0 μm or less, preferably 4.0 μm or less, more preferably 3.0 μm or less, even more preferably 2.0 μm or less, and particularly preferably 1.0 μm or less.

[0042] (area ratio of structure S1: 50.0% or more) A low-carbon concentration structure has high toughness and high fracture resistance. Furthermore, a low-carbon concentration structure contains at least a portion of retained austenite. Low-carbon concentration retained austenite is prone to martensite transformation. When repeated stress is applied to a steel plate, the low-carbon concentration retained austenite transforms into martensite, which significantly disperses strain during a collision and suppresses the occurrence of cracks during a collision. In other words, fracture resistance is improved. Therefore, the area ratio of the structure S1 having a carbon concentration of more than 0.1 mass % and not more than 0.3 mass % is 50.0% or more, preferably 55.0% or more, and more preferably 60.0% or more. On the other hand, the upper limit of the area ratio of the structure S1 is not particularly limited, and is, for example, 90.0%, and preferably 95.0%.

[0043] (area ratio of structure S2: 10.0% or less) A structure with a high carbon concentration is hard, and therefore becomes the starting point for cracks during a collision, resulting in a deterioration in fracture resistance. Therefore, the area ratio of the structure S2 having a carbon concentration of 0.5 mass % or more is 10.0% or less, preferably 8.0% or less, and more preferably 7.0% or less.

[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 10% or less, more preferably 8% 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 fracture resistance deteriorates. Therefore, 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 cooled from the reheating temperature T3 to at least (T3-30)°C.

[0051] The heat-treated cold-rolled steel sheet corresponds to the steel sheet included in the present high-strength steel sheet described above.

[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, making it difficult to ensure good crashworthiness and yield strength. 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, the average grain size of the retained austenite tends to become coarse, resulting in a decrease in fracture resistance. Also, the increase in hydrogen partial pressure increases the amount of hydrogen that penetrates into the steel, resulting in an increase in the amount of diffusible hydrogen in the steel. Furthermore, if the heating time t1 is too long, the average grain size of the retained austenite tends to become coarse, which reduces fracture resistance. 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 increases, and the structure with a high carbon concentration (structure S2) increases, resulting in a deterioration in fracture resistance. 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, making it impossible to stably ensure good crash resistance and yield strength. 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: 280~400℃》 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. At this time, if the reheating temperature T3 is too low, the structure with a low carbon concentration (structure S1) decreases, and the fracture resistance deteriorates. Therefore, the reheating temperature T3 is 280°C or higher, preferably 290°C or higher, and more preferably 300°C or higher.

[0057] On the other hand, if the reheating temperature T3 is too high, a large amount of iron carbide precipitates in the tempered martensite, reducing the total area ratio of tempered martensite and bainite, thereby reducing the yield strength and crash resistance. Therefore, the reheating temperature T3 is 400°C or less, preferably 380°C or less, and more preferably 350°C or less.

[0058] 《Heat effect index P:4000~6200》 As described above, the cold-rolled steel sheet reheated to the reheating temperature T3 is recooled without being held at the reheating temperature T3. In the re-cooling, the cold-rolled steel sheet is cooled from the reheating temperature T3 to at least (T3-30)° C. At this time, the heat effect index P for the cold-rolled steel sheet from the reheating temperature T3 to (T3-30)° C. is expressed by the following formula (1). P = T3(log(5t)+12)…(1) In the above formula (1), t is the cooling time (unit: s) from the reheating temperature T3 to (T3-30)°C.

[0059] The carbon concentration of each structure that constitutes the microstructure can be controlled by the heat effect index P. That is, in the temperature range from the reheating temperature T3 to (T3-30)°C, the solute carbon in the martensite matrix is ​​diffused into the untransformed austenite. If the heat effect index P is too low, the solute carbon in martensite does not diffuse sufficiently into the untransformed austenite, increasing the amount of the high carbon concentration structure (structure S2) and decreasing the amount of the low carbon concentration structure (structure S1). Therefore, the heat effect index P is 4000 or more, preferably 4200 or more, and more preferably 4400 or more.

[0060] On the other hand, if the heat effect index P is too high, the amount of solute carbon in the untransformed austenite will be too much, and the final microstructure will contain an increased amount of a structure with a high carbon concentration (structure S2). Therefore, the heat effect index P is 6000 or less, preferably 5800 or less, and more preferably 5500 or less.

[0061] <Plating process> In the present manufacturing method, the re-cooled cold-rolled steel sheet may be subjected to a plating treatment to form a plating layer on the surface thereof. 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.

[0062] When the hot-dip galvanizing treatment is carried out, an apparatus configured so that the above-mentioned heat treatment and hot-dip galvanizing treatment can be carried out continuously may be used. 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.

[0063] 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.

[0064] When carrying out the electrogalvanizing treatment, an apparatus configured so that the above-mentioned heat treatment and electrogalvanizing treatment can be carried out successively may be used. 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] [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 collision resistance and fracture resistance. Therefore, the present member has excellent collision resistance and fracture resistance and can contribute to weight reduction of the vehicle body, and is therefore suitable for use in general as automotive components, particularly as automotive frame structural components or automotive reinforcing components.

[0069] [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]

[0070] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the examples described below.

[0071] <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 to 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.

[0072] In some cases, both sides of the heat-treated cold-rolled steel sheet (CR) 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. 2Electrogalvanizing treatment was carried out so that

[0073] 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".

[0074] 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".

[0075] <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.

[0076] <Average grain size of retained austenite> The obtained steel sheet was ground so that the cross section parallel to the rolling direction (L cross section) served as the observation surface, and then buffed using a colloidal silica solution. Then, EBSD (electron beam acceleration voltage: 15 kV, step interval: 0.04 μm) was used to measure 10 areas of 50 μm × 50 μm on the observation surface, and data for obtaining a microstructure image was obtained. The obtained data was processed using OIM Analysis software manufactured by TSL, to obtain a microstructure image. The area of ​​each retained austenite grain was determined using Image-Pro manufactured by Media Cybernetics, and the circle equivalent diameter was calculated. The average of these values ​​was taken as the average grain size of the retained austenite (unit: μm).

[0077] <<Area ratio of texture S1 and texture S2>> The obtained steel sheet was polished with diamond paste so that a cross section parallel to the rolling direction (L cross section) would be the observation surface. The observation surface was mirror-finished by alumina polishing, and then cleaned with a plasma cleaner to remove hydrocarbon contamination (carbon contamination, hereinafter referred to as "contamination") from the observation surface. The cleaned observation surface was measured using an electron beam microanalyzer (FE-EPMA) equipped with a field emission electron gun to obtain data for elemental mapping images. The measurement conditions were an acceleration voltage of 7 kV and a current of 50 nA, in accordance with a non-patent document (T. Yamashita, Y. Tanaka, M. Nagoshi and K. Ishida: Sci. Rep., 6 (2016), DOI: 10.1038 / srep29825.). The steel plate sample was heated and maintained at 100°C, and measurements were performed under conditions that prevented contamination. The measurement data was converted to carbon concentration using a calibration method, and an elemental mapping image of carbon was obtained. Measurements using FE-EPMA were carried out 30 times for each steel sheet, and elemental mapping images were obtained each time. The ratio (area ratio) of the area where the carbon concentration was more than 0.1 mass % and not more than 0.3 mass % to the total area of ​​the obtained elemental mapping image was calculated, and the average value of 30 measurements was taken as the area ratio (unit: %) of structure S1. Similarly, the ratio (area ratio) of the area where the carbon concentration was 0.5 mass % or more to the total area of ​​the obtained elemental mapping image was calculated, and the average value of 30 times was taken as the area ratio (unit: %) of structure S2.

[0078] <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.

[0079] <evaluation> The obtained steel sheets were evaluated by the following methods, and the results are shown in Table 3 below.

[0080] 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, a tensile test in accordance with JIS Z 2241 was carried out five times, and the yield strength (YS) was calculated from the average value of the five tests. If the YS is 800 MPa or more, it can be evaluated as having high strength.

[0081] <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).

[0082] 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).

[0083] 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.

[0084] 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.

[0085] <Break resistance evaluation test> Test pieces (length of parallel portion: 40 mm, width of parallel portion: 20 mm) with the longitudinal direction (tensile direction) at an angle of 90° to the rolling direction were cut out from the obtained steel sheets in accordance with JIS Z 2241. A machined hole with a diameter of 8 mm was formed in the test piece at the center position in the longitudinal direction of the parallel portion and at the center position in the width direction of the parallel portion. Using the test piece with the machined hole formed, a tensile test was carried out five times at a constant tensile speed (2 mm / min) in accordance with JIS Z 2241 to determine the fracture stroke. In Table 3 below, if the breaking stroke (average value of five tensile tests) was 0.85 mm or more, it was marked with "◎", if it was 0.70 mm or more but less than 0.85 mm, it was marked with "○", and if it was less than 0.70 mm, it was marked with "×". If the rating is "◎" or "◯", the specimen can be evaluated as having excellent break resistance.

[0086] [Table 1] TIFF0007761673000002.tif194146

[0087] [Table 2] TIFF0007761673000004.tif172161

[0088] [Table 3] TIFF0007761673000006.tif163161

[0089] <Summary of evaluation results> As shown in Tables 1 to 3 above, the steel plates Nos. 1 to 10, 15 to 17, 22, 25, 30 and 33 to 42 all had a yield strength of 800 MPa or more, and were excellent in collision resistance and fracture resistance. In contrast, the steel plates Nos. 11 to 14, 18 to 21, 23 to 24, 26 to 29 and 31 to 32 were insufficient in at least one of the yield strength, crash strength and fracture resistance.

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 10% or less; The average grain size of the retained austenite is 5.0 μm or less, Structure S having a carbon concentration of more than 0.1 mass% and not more than 0.3 mass% １ The area ratio of the structure S is 50.0% or more and the carbon concentration is 0.5 mass% or more. ２ A high-strength steel plate having a yield strength of 800 MPa or more, wherein the area ratio of

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 of 280 to 400 ° C., and recooled without holding at the reheating temperature T3, A method for producing a high-strength steel plate, wherein a heat effect index P from the reheating temperature T3 to (T3-30) ° C., represented by the following formula (1), is 4000 to 6200. P=T3(log(5t)+12)...(1) In the formula (1), t is the cooling time from the reheating temperature T3 to (T3-30)°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 of 280 to 400 ° C., and recooled without holding at the reheating temperature T3, A method for producing a high-strength steel plate, wherein a heat effect index P from the reheating temperature T3 to (T3-30) ° C., represented by the following formula (1), is 4000 to 6200. P=T3(log(5t)+12)...(1) In the formula (1), t is the cooling time from the reheating temperature T3 to (T3-30)°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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