High strength steel plate and method for manufacturing same

Abstract

The present invention provides a high-strength steel sheet that has a tensile strength of not less than 1,180 MPa, that has excellent bendability and excellent resistance to delayed fractures, and that has a wide range of clearance optimal for bend-forming at a sheared end surface. The high-strength steel sheet comprises a steel sheet, wherein the steel sheet contains, in mass%, 0.030-0.500% C, 0.50-2.50% Si, 1.50-5.00% Mn, not more than 0.100% P, not more than 0.0200% S, not more than 1.000% Al, not more than 0.0100% N, not more than 0.0100% O, and 0.005-0.100% Nb, with the remainder consisting of Fe and unavoidable impurities, the martensite amount is not less than 70%, the retained austenite amount is 3-20%, the total of the ferrite amount and the bainitic ferrite amount is not more than 10%, retained austenite has an instability index k of less than 6.1, and the retained austenite has an instability index d of less than 5.7 in the initial stage of processing.

Description

[Technical field]

[0001] The present invention relates to a high-strength steel plate and a method for producing the same. [Background technology]

[0002] In order to achieve both improved fuel efficiency (reduced CO2 emissions) through lighter vehicle bodies and improved crash resistance, progress is being made in increasing the strength of thin steel sheets for automobiles, and new legal regulations are being introduced one after another. In recent years, there has been an increasing number of cases in which high-strength steel sheets having a tensile strength (TS) of 1180 MPa or more (see, for example, Patent Documents 1 to 3) are used in major structural parts of automobiles in order to increase the strength of the vehicle body. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2015-193897 A [Patent Document 2] International Publication No. 2019 / 187090 [Patent Document 3] International Publication No. 2020 / 174805 Summary of the Invention [Problem to be solved by the invention]

[0004] Conventionally, hot press forming of high strength steel sheets has been actively studied, but recently, from the viewpoints of cost and productivity, cold press forming has been considered again. However, when a high-strength steel plate with a TS of 1,180 MPa or more is cold-pressed to obtain a part, there is a risk of delayed fracture occurring due to increased residual stress in the part or deterioration of the delayed fracture resistance of the steel plate itself. Delayed fracture occurs when a formed part is placed in a hydrogen penetration environment, where hydrogen penetrates into the steel plate that makes up the part, reducing the interatomic bonding strength and causing localized deformation, resulting in microcracks that then propagate and lead to fracture.

[0005] Moreover, from the viewpoint of formability, high-strength steel sheets used in automobiles are required to be less susceptible to cracks occurring at bending ridges during bending (i.e., to have excellent bendability).

[0006] Furthermore, automotive frame parts have many end faces that are formed by shear processing, and it is required that these shear end faces do not develop cracks due to bending (bending). The shape of the sheared edge depends on the shear clearance, and the occurrence of cracks at the sheared edge also depends on the shear clearance. For this reason, high-strength steel sheets used in automobiles are required to have a wide optimum clearance range for bending of the shear end faces.

[0007] Therefore, an object of the present invention is to provide a high-strength steel plate having a tensile strength (TS) of 1180 MPa or more, excellent bendability and resistance to delayed fracture, and a wide optimum clearance range for bending of the shear end face, and a manufacturing method thereof. [Means for solving the problem]

[0008] As a result of extensive investigations, the present inventors have found that the above object can be achieved by employing the following configuration, and have completed the present invention.

[0009] That is, the present invention provides the following [1] to [5]. [1] A high-strength steel plate comprising: a composition containing, by mass%, C: 0.030% or more and 0.500% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, O: 0.0100% or less, and Nb: 0.005% or more and 0.100% or less, with the balance being Fe and unavoidable impurities; and a microstructure in which the amount of martensite is 70% or more, the amount of retained austenite is 3% or more and 20% or less, and the sum of the amount of ferrite and the amount of bainitic ferrite is 10% or less, and the non-stability index k of retained austenite is less than 6.1, and the non-stability index d of retained austenite at an initial stage of processing is less than 5.7. [2] The high-strength steel plate according to the above item [1], wherein the chemical composition further contains, by mass%, at least one element selected from the group consisting of Ti: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% 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 a surface of the steel sheet. [4] A method for producing a high-strength steel plate according to the above [1] or [2], comprising: holding a steel slab having the composition according to the above [1] or [2] at a slab heating temperature of 1220°C or more; then performing hot rolling to obtain a hot-rolled steel plate; cooling the hot-rolled steel plate under conditions in which an average cooling rate v1 from 800°C to 600°C is 30°C / s or more; and then performing pickling and cold rolling to obtain a cold-rolled steel plate; holding the cold-rolled steel plate at a temperature T1 of 800°C or more for 10 seconds or more; and then performing a heat treatment A in which the cold-rolled steel plate is cooled to a cooling stop temperature Ta of 100°C or more (Ms point - 80°C) or less; a tension F applied to the cold-rolled steel sheet from the Ms point to the cooling stop temperature Ta is 5 MPa or more and 100 MPa or less; after the heat treatment A, a heat treatment B is performed in which the cold-rolled steel sheet is held at a temperature T2 of 5 seconds or more and 450°C or less for 5 seconds or more and 1000 seconds or less and then cooled; after the heat treatment B, the cold-rolled steel sheet is heated to a temperature T3 of 150°C or more and 400°C or less and then cooled without being held at the temperature T3, and in the heat treatment C, an average cooling rate v4 from 150°C to 50°C is 1.0°C / h or more and 50.0°C / h or less; and after the heat treatment A and before the heat treatment C, the cold-rolled steel sheet is processed to impart an equivalent plastic strain of 0.10% to 5.00% to the cold-rolled steel sheet. Here, the Ms point is expressed in °C and is calculated by the following formula (a). Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]...(a) In the above formula (a), [%M] is the content of element M in the above composition, and is 0 when element M is not contained. [5] A method for producing a high-strength steel sheet according to the above [4], further comprising subjecting the cold-rolled steel sheet to a plating treatment. Effect of the Invention

[0010] According to the present invention, there is obtained a high-strength steel plate having a tensile strength (TS) of 1180 MPa or more, excellent bendability and resistance to delayed fracture, and a wide optimum clearance range for bending of the sheared end face. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a schematic diagram showing a test piece used in a tensile test. [Diagram 2] 1 is a graph showing a schematic diagram of the relationship between the tensile stress and the tensile strain applied to a test piece during a tensile test. [Diagram 3] 1 is an example of a graph showing the relationship between tensile strain ε and the amount of retained austenite. [Figure 4] 1 is another example of a graph showing the relationship between tensile strain ε and the amount of retained austenite. [Diagram 5] 11 is yet another example of a graph showing the relationship between tensile strain ε and the amount of retained austenite. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] [High strength steel plate] The high-strength steel plate of this embodiment (hereinafter also referred to as the "present high-strength steel plate") includes a steel plate, and may further include a plating layer on the surface of this steel plate, as described below. The steel plate of the present high strength steel plate has a component composition and a microstructure (steel structure) described below, and satisfies the instability index k and the instability index d described below. High strength means a tensile strength (TS) of 1180 MPa or more.

[0013] This high-strength steel plate has a tensile strength (TS) of 1180 MPa or more, excellent bendability and resistance to delayed fracture, and a wide optimum clearance range for bending of the shear end face. Hereinafter, the optimum clearance range for bending of the sheared end surface will also be simply referred to as the "optimum clearance range."

[0014] In summary, the present inventors have conducted extensive research and have found the following. (1) By setting the amount of martensite to 70% or more and the total amount of ferrite and bainitic ferrite to 10% or less, a TS of 1,180 MPa or more can be realized. (2) By setting the amount of retained austenite to 3% or more, excellent bendability can be achieved. (3) By keeping the instability index d of the retained austenite at the initial stage of processing to less than 5.7 and the amount of retained austenite to 20% or less, excellent delayed fracture resistance can be achieved. (4) A wide optimum clearance range can be realized by setting the instability index k of the retained austenite to less than 6.1 and the instability index d of the retained austenite at the initial stage of processing to less than 5.7.

[0015] By applying this high strength steel sheet to, for example, structural parts of an automobile, it is possible to reduce the weight of the vehicle body and thereby improve fuel efficiency, and therefore the industrial value of this steel sheet is extremely great.

[0016] <Steel plate> First, the steel plate provided 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 also referred to as "the present chemical composition") will be described. Unless otherwise specified, "%" in the component composition means "% by mass."

[0018] (C: 0.030% or more and 0.500% or less) C is one of the important basic components of steel, and is an element that affects the amount of martensite and the total amount of ferrite and bainitic ferrite. If the C content is too low, the amount of martensite decreases, and the total amount of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the C content is 0.030% or more, preferably 0.050% or more, and more preferably 0.100% or more. On the other hand, if the C content is too high, the martensite becomes embrittled and the delayed fracture resistance deteriorates, so the C content is 0.500% or less, preferably 0.400% or less, and more preferably 0.350% or less.

[0019] (Si: 0.50% or more and 2.50% or less) Silicon (Si) is one of the important basic components of steel, and is an element that affects TS and the amount of retained austenite. If the Si content is too low, the strength of martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the Si content is 0.50% or more, preferably 0.55% or more, and more preferably 0.60% or more. On the other hand, if the Si content is too high, the amount of retained austenite increases excessively, and the delayed fracture resistance property decreases, so the Si content is set to 2.50% or less, preferably 2.00% or less, and more preferably 1.80% or less.

[0020] (Mn: 1.50% or more and 5.00% or less) Mn is one of the important basic components of steel, and is an element that affects the amount of martensite and the total amount of ferrite and bainitic ferrite. If the Mn content is too low, the amount of martensite decreases, and the total amount of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the Mn content is 1.50% or more, preferably 2.00% or more, and more preferably 2.20% or more. On the other hand, if the Mn content is too high, martensite becomes embrittled and the delayed fracture resistance deteriorates, so the Mn content is 5.00% or less, preferably 4.50% or less, and more preferably 4.00% or less.

[0021] (P:0.100% or less) P segregates at prior austenite grain boundaries, embrittling the grain boundaries and embrittling the steel sheet, thereby reducing the delayed fracture resistance, so the P content is 0.100% or less, preferably 0.070% or less, and more preferably 0.030% or less. The lower limit is not particularly limited. However, P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the P content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0022] (S:0.0200% or less) S exists as sulfide and embrittles the steel sheet, thereby reducing the delayed fracture resistance, so the S content is 0.0200% or less, preferably 0.0050% or less, and more preferably 0.0025% or less. Although there is no particular lower limit, due to constraints on production technology, the S content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0023] (Al: 1.000% or less) Since Al exists as an oxide and embrittles the steel sheet, the delayed fracture resistance is deteriorated, and therefore the Al content is 1.000% or less, preferably 0.500% or less, more preferably 0.150% or less, and further preferably 0.070% or less. The lower limit is not particularly limited. However, Al can suppress the formation of carbides during the heat treatment described below and promote the formation of retained austenite. Therefore, the Al content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0024] (N:0.0100% or less) N exists as a nitride and embrittles the steel sheet, thereby reducing the delayed fracture resistance, so the N content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less. Although there is no particular lower limit, due to constraints on production technology, the N content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0025] (O:0.0100% or less) O exists as an oxide and embrittles the steel sheet, thereby reducing the delayed fracture resistance, so the O content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less. Although there is no particular lower limit, due to constraints on production technology, the O content is preferably 0.0001% or more, more preferably 0.0007% or more, and even more preferably 0.0015% or more.

[0026] (Nb: 0.005% or more and 0.100% or less) As a result of extensive investigations, the present inventors have found that Nb affects the instability index k of retained austenite. If the Nb content is too low, the structure after the heat treatment described below becomes coarse, and the instability index k of the retained austenite increases. This narrows the optimum clearance range. For this reason, the Nb content is 0.005% or more, preferably 0.008% or more, and more preferably 0.010% or more. On the other hand, if the Nb content is too high, the martensite becomes embrittled and the delayed fracture resistance deteriorates, so the Nb content is 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[0027] (other elements) This component composition may further contain, by mass %, at least one element (other element) selected from the group consisting of the elements described below.

[0028] (Ti and V) Too much Ti and V generates a large amount of coarse precipitates and inclusions, embrittling the steel sheet and reducing the delayed fracture resistance, so that the Ti and V contents are each 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less. The lower limit is not particularly limited. However, Ti and V increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment described later. Therefore, the contents of Ti and V are each preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.015% or more.

[0029] (Ta and W) If the Ta and W contents are too high, a large amount of coarse precipitates and inclusions are formed, the steel sheet becomes embrittled, and the delayed fracture resistance property is deteriorated. Therefore, the Ta and W contents are each 0.10% or less, preferably 0.09% or less, and more preferably 0.08% or less. The lower limit is not particularly limited. However, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment described later. Therefore, the contents of Ta and W are preferably 0.01% or more, more preferably 0.03% or more, and further preferably 0.05% or more, respectively.

[0030] ((B)) If the content of B is too much, cracks are generated inside the steel sheet during casting or hot rolling, the steel sheet becomes embrittled, and the delayed fracture resistance property is deteriorated. Therefore, the content of B is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less. The lower limit is not particularly limited. However, B segregates at the austenite grain boundaries during the heat treatment described below and improves hardenability. Therefore, the B content is preferably 0.0003% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0031] ((Cr, Mo and Ni)) If the Cr, Mo and Ni contents are too high, the amount of coarse precipitates and inclusions increases, the steel sheet becomes brittle, and the delayed fracture resistance deteriorates. Therefore, the Cr, Mo and Ni contents are each 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less. However, since Cr, Mo and Ni are elements that improve hardenability, the contents of Cr, Mo and Ni are each preferably 0.01% or more, more preferably 0.04% or more, and further preferably 0.08% or more.

[0032] ((Co)) Too much Co increases coarse precipitates and inclusions, embrittling the steel sheet and reducing delayed fracture resistance, so the Co content is 0.010% or less, preferably 0.008% or less, and more preferably 0.006% or less. The lower limit is not particularly limited. However, since Co is an element that improves hardenability, the Co content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0033] ((Cu)) If the Cu content is too high, the amount of coarse precipitates and inclusions increases, the steel sheet becomes brittle, and the delayed fracture resistance decreases, so the Cu content is 1.00% or less, preferably 0.80% or less, and more preferably 0.60% or less. The lower limit is not particularly limited. However, since Cu is an element that improves hardenability, the Cu content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0034] ((Sn)) If the Sn content is too high, cracks are generated inside the steel sheet during casting or hot rolling, the steel sheet becomes embrittled, and the delayed fracture resistance is deteriorated. Therefore, the Sn content is 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less. The lower limit is not particularly limited. However, since Sn is an element that improves hardenability, the Sn content is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.

[0035] ((Sb)) If the Sb content is too high, the amount of coarse precipitates and inclusions increases, the steel sheet becomes brittle, and the delayed fracture resistance decreases, so the Sb content is 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less. The lower limit is not particularly limited. However, since Sb is an element that controls the softened surface thickness and enables strength adjustment, the Sb content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0036] ((Ca, Mg and REM)) If the amount of Ca, Mg, and REM (rare earth metals) is too large, the amount of coarse precipitates and inclusions increases, the steel sheet becomes brittle, and the delayed fracture resistance property decreases. Therefore, the amount of Ca, Mg, and REM is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less, respectively. The lower limit is not particularly limited. However, Ca, Mg and REM are elements that make the shape of nitrides and sulfides spheroidal and improve the ultimate deformability of the steel sheet. Therefore, the contents of Ca, Mg and REM are each preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0037] ((Zr and Te)) If the Zr and Te contents are too high, the amount of coarse precipitates and inclusions increases, the steel sheet becomes brittle, and the delayed fracture resistance deteriorates. Therefore, the Zr and Te contents are each 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less. The lower limit is not particularly limited. However, Zr and Te are elements that make the shape of nitrides and sulfides spheroidal and improve the ultimate deformability of the steel sheet. Therefore, the content of Zr and Te is preferably 0.001% or more, more preferably 0.008% or more, and further preferably 0.015% or more, respectively.

[0038] ((Hf)) If the Hf content is too high, the amount of coarse precipitates and inclusions increases, the steel sheet becomes brittle, and the delayed fracture resistance decreases, so the Hf content is 0.10% or less, preferably 0.09% or less, and more preferably 0.08% or less. The lower limit is not particularly limited. However, Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet. Therefore, the Hf content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0039] ((Bi)) If the Bi content is too high, the amount of coarse precipitates and inclusions increases, the steel sheet becomes brittle, and the delayed fracture resistance decreases, so the Bi content is 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less. The lower limit is not particularly limited. However, since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more, more preferably 0.020% or more, further preferably 0.050% or more, and particularly preferably 0.090% or more.

[0040] Regarding the above-mentioned other elements, when the content of each is less than the above-mentioned preferable lower limit, the effect of the present invention is not impaired, and therefore they are regarded as unavoidable impurities.

[0041] (Remainder) The balance in this composition is Fe and unavoidable impurities.

[0042] Microstructure Next, the microstructure of the present high strength steel plate (hereinafter also referred to as "the present microstructure") will be described.

[0043] (Total amount of ferrite and bainitic ferrite: 10% or less) If the amount of ferrite and bainitic ferrite is too large, it becomes difficult to achieve a TS of 1180 MPa or more, and therefore the total amount of ferrite and bainitic ferrite is 10% or less, preferably 9% or less, and more preferably 8% or less. The lower limit is not particularly limited.

[0044] The amount of ferrite and the amount of bainitic ferrite are determined as follows. The steel plate is polished to expose the L-section at 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) as the observation surface. The observation surface is corroded with 3 volume % nital, and then observed in 10 fields of view at 2000x magnification using a scanning electron microscope (SEM), and SEM images of each field of view are obtained. In the SEM images, ferrite and bainitic ferrite are confirmed as structures with flat concave interiors. The area ratios (unit: %) of ferrite and bainitic ferrite in each SEM image are calculated, and the average of the area ratios calculated for the 10 fields of view is taken as the total amount of ferrite and bainitic ferrite.

[0045] (Residual austenite content: 3% to 20%) Because excellent bendability can be achieved, the amount of retained austenite is 3% or more, preferably 5% or more, more preferably 7% or more, and even more preferably 8% or more. On the other hand, the amount of retained austenite is 20% or less, preferably 15% or less, and more preferably 13% or less, because excellent delayed fracture resistance can be achieved.

[0046] The amount of retained austenite is determined as follows. The steel plate is polished to expose an L-section at a position 0.1 mm deeper than the 1 / 4 position of the plate thickness. This L-section is further polished by 0.1 mm in the depth direction by chemical polishing to obtain an observation surface. For the observation surface, an X-ray diffraction (XRD) device is used to determine the integrated intensity ratio of the diffraction peaks using CoKα radiation. More specifically, the integrated intensity ratios of the diffraction peaks of each of the {200}, {220}, and {311} planes of fcc iron to each of the {200}, {211}, and {220} planes of bcc iron are determined. The average value of the nine integrated intensity ratios is taken as the volume fraction (unit: %) of the retained austenite amount, and this is taken as the amount of retained austenite.

[0047] (Martensite content: 70% or more) Because a TS of 1180 MPa or more can be realized, the amount of martensite is 70% or more, preferably 75% or more, and more preferably 80% or more. Among martensite, the contribution of tempered martensite is important in achieving a high TS, and the amount of tempered martensite is preferably 80% or more.

[0048] The amount of martensite is determined as follows. First, the amount of retained austenite, and the total amount of ferrite and bainitic ferrite are calculated by the above-mentioned method, and then the total is subtracted from 100%, and the obtained value (unit: %) is regarded as the amount of martensite. Therefore, the amount of martensite referred to here includes both quenched martensite and tempered martensite. As described above, the amount of retained austenite is a volume fraction, which is approximately equal to the area fraction, so the amount of retained austenite is subtracted from 100% together with the total amount of ferrite and bainitic ferrite, which is an area fraction.

[0049] <Instability index k: less than 6.1> As a result of extensive investigations, the present inventors have found that the instability index k of retained austenite (also simply referred to as the "instability index k") affects the optimum clearance range. If the non-stability index k is too high, the stability of the retained austenite is low and the retained austenite is excessively transformed into hard martensite during shearing, which reduces the ultimate deformability of the steel plate and narrows the optimum clearance range. For this reason, the instability index k is less than 6.1, preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.5 or less. Although there is no particular lower limit, the instability index k is, for example, 1.0 or more, preferably 1.5 or more, and more preferably 2.0 or more.

[0050] <Instability index d: less than 5.7> As a result of extensive investigations, the present inventors have found that the non-stability index d of retained austenite at the initial stage of processing (also simply referred to as "non-stability index d") affects delayed fracture resistance and the optimum clearance range. If the non-stability index d is too high, the stability of the retained austenite in the initial stage of processing is low, and the retained austenite is excessively transformed into hard martensite in the initial stage of processing, which becomes the starting point of delayed fracture in a hydrogen penetration environment, thereby deteriorating the delayed fracture resistance. Furthermore, if the instability index d is too high, the retained austenite is excessively transformed into hard martensite in the initial stage of processing, which reduces the ultimate deformability of the steel sheet and narrows the optimum clearance range. For this reason, the instability index d is less than 5.7, preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.5 or less. Although there is no particular lower limit, the instability index d is, for example, −15.0 or more, preferably −10.0 or more, and more preferably −5.0 or more.

[0051] The instability index k and the instability index d are calculated as follows. First, a tensile test described later (details are given in [Examples]) is carried out to prepare a steel plate test piece (JIS No. 5 test piece). Fig. 1 is a schematic diagram showing a test piece used in a tensile test, and Fig. 2 is a graph showing a schematic relationship between the tensile stress and the tensile strain applied to the test piece 1 during the tensile test. As shown in Fig. 2, a tensile stress is applied to the test piece to impart tensile strain (tensile plastic strain). In this way, a number of test pieces are obtained to which an arbitrary tensile strain ε is imparted in the range of 0 to 10% (0 to 0.10). The tensile strain ε before processing (before the tensile strain is imparted) is 0%. Then, the amount of retained austenite when tensile strain ε is applied to the center of each test piece (shaded area in Fig. 1) is determined by the above-mentioned method, and the obtained results are plotted on a graph (horizontal axis: tensile strain ε, vertical axis: logarithm of the amount of retained austenite). For each plot in the graph (see Figs. 3 to 5 described later), a linear approximation (y = -ax + b) is obtained using the least squares method. The obtained approximation is applied to the following formula (1), and the slope a of the approximation is determined as the instability index k of the retained austenite. log(fγ)=-k·ε+log(fγ0)···(1) In the above formula (1), fγ represents the amount of retained austenite when a tensile strain ε is applied, and fγ0 represents the amount of retained austenite before working.

[0052] Furthermore, an estimated amount of retained austenite before working (when tensile strain ε is 0%) is calculated from the intercept of the above formula (1), and the instability index d of the retained austenite before working is calculated based on the following formula (2). d= A -fγε0···(2) In the above formula (2), A represents the measured value of the amount of retained austenite before working, and fγε0 represents the estimated value of the amount of retained austenite before working.

[0053] Fig. 3 is an example of a graph showing the relationship between tensile strain ε and the amount of retained austenite, Fig. 4 is another example of the same graph, and Fig. 5 is yet another example of the same graph. Comparing FIG. 3 with FIG. 4, the slope a (instability index k) of the approximation equation in FIG. 3 is 2.3, which is smaller than the slope a (instability index k) of the approximation equation in FIG. 4, which is 11.9. A small slope a (instability index k) of the approximation equation indicates that the change in the amount of retained austenite during processing is small and the stability of the retained austenite is good.

[0054] Next, looking at Figure 5, the slope a of the approximation equation (instability index k) is small at 4.0. However, in Fig. 5, the estimated value of the retained austenite before processing, fγε0, is 12.3 (log(fγε0) = 1.09), while the actual measured value of the retained austenite before processing is A is 1 In 8 Yes, the non-stability index d(= A -fγε0) is large at 5.7. When the non-stability index d is large, the slope of the approximation equation is large in the initial stage of processing (the initial stage of the tensile test). In other words, in this case, the stability of the retained austenite in the initial stage of processing is insufficient.

[0055] Therefore, when both the non-stability index k and the non-stability index d satisfy the above-mentioned ranges, the retained austenite has excellent stability both during working and at the initial stage of working.

[0056] <Plating layer> From the viewpoint of improving corrosion resistance and the like, the high-strength steel sheet may further have a plating layer on the surface of the steel sheet. Examples of the plated layer include a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, and an electrolytic galvanized layer. The plated layer is formed by a plating process described later. The coating weight of the plating layer is not particularly limited, and is 20 to 80 g / m per side. 2 is preferred.

[0057] [Manufacturing method for high-strength steel plates] Next, a method for producing a high-strength steel plate according to this embodiment (hereinafter also referred to as the present production method) will be described. The present production method is also a method for producing the present high-strength steel plate described above. 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., unless otherwise specified. The method for producing molten steel to be the steel slab is not particularly limited, and may be a known method using a converter, an electric furnace, etc. From the viewpoint of preventing macrosegregation, it is preferable to obtain a steel slab from molten steel by a continuous casting method.

[0058] <Hot rolling> In the present manufacturing method, first, a steel slab having the present chemical composition described above is held at a slab heating temperature explained below, and then hot-rolled to obtain a hot-rolled steel sheet.

[0059] {Slab heating temperature: 1220℃ or higher} If the slab heating temperature is too low, the inclusions are dissolved as impurities, which embrittles the steel sheet and reduces the delayed fracture resistance. Therefore, the slab heating temperature is preferably 1220°C or higher, more preferably 1230°C or higher, and even more preferably over 1230°C. Although there is no particular upper limit, the slab heating temperature is preferably 1300° C. or less, more preferably 1290° C. or less, and further preferably 1280° C. or less.

[0060] <cooling> The hot-rolled steel sheet obtained by hot rolling is cooled. At this time, the average cooling rate v1 from 800° C. to 600° C. satisfies the range described below.

[0061] 《Average cooling rate v1: 30℃ / s or more》 As a result of extensive investigations, the present inventors have found that the average cooling rate v1 from 800° C. to 600° C. (also simply referred to as the "average cooling rate v1") affects the instability index k of retained austenite. If the average cooling rate v1 is too small, excessive coarse Nb-based carbides are precipitated during cooling, and the effect of Nb in reducing the instability index k of the retained austenite decreases, which increases the instability index k and narrows the optimal clearance range. Therefore, the average cooling rate v1 is 30° C. / s or more, preferably 35° C. / s or more, and more preferably 40° C. / s or more.

[0062] Pickling and cold rolling Next, the cooled hot-rolled steel sheet is subjected to pickling and cold rolling to obtain a cold-rolled sheet. Pickling is important for ensuring good chemical conversion treatability and plating quality in the final product, high-strength steel sheet, because it can remove oxides from the surface of the hot-rolled steel sheet. Pickling may be performed once or multiple times. The hot-rolled steel sheet is pickled to obtain a pickled sheet, which is then dried as appropriate. The cold rolling may be performed on the pickled sheet before drying, or may be performed after drying. The reduction ratio in cold rolling and the thickness after rolling are not particularly limited. The number of rolling passes and the reduction ratio of each pass are also not particularly limited.

[0063] Next, the cold-rolled steel sheet obtained by cold rolling is subjected to heat treatment A, heat treatment B, and heat treatment C, which will be described later, in this order. Before heat treatment C, processing, which will be described later, is carried out. This will be explained in more detail below.

[0064] <Heat treatment A> First, a heat treatment A is applied to a cold-rolled steel sheet obtained by cold rolling. Schematically, in the heat treatment A, the cold-rolled steel sheet is held (heated) at a temperature T1, which will be described later, and then cooled to a cooling stop temperature Ta, which will be described later. The conditions of the heat treatment A are explained below.

[0065] 《Temperature T1: 800℃ or more》 First, the cold-rolled steel sheet is held (heated) at a temperature T1. If the temperature T1 is too low, the amount of martensite decreases and the total amount of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the temperature T1 is 800° C. or higher, preferably 820° C. or higher, and more preferably 840° C. or higher. Although there is no particular upper limit, the temperature T1 is, for example, 940° C. or less, preferably 920° C. or less, and more preferably 900° C. or less.

[0066] 《Holding time t1: 10 seconds or more》 If the time (holding time t1) for which the cold-rolled steel sheet is held (heated) at temperature T1 is too short, the amount of martensite decreases and the total amount of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. For this reason, the retention time t1 is 10 seconds or more, preferably 30 seconds or more, and more preferably 50 seconds or more. Although there is no particular upper limit, the retention time t1 is, for example, 300 seconds or less, preferably 250 seconds or less, and more preferably 200 seconds or less.

[0067] 《Cooling stop temperature Ta: 100℃ or more (Ms point -80℃) or less》 Next, the cold-rolled steel sheet held at the temperature T1 is cooled to a cooling stop temperature Ta. If the cooling stop temperature Ta is too low, the amount of retained austenite decreases and the bendability deteriorates, so the cooling stop temperature Ta is 100°C or higher, preferably 120°C or higher, and more preferably 140°C or higher. On the other hand, if the cooling stop temperature Ta is too high, the amount of retained austenite increases excessively, and the delayed fracture resistance decreases. Therefore, the cooling stop temperature Ta is not more than (Ms point -80°C), preferably not more than (Ms point -90°C), and more preferably not more than (Ms point -100°C).

[0068] When the cold-rolled steel sheet is cooled from the temperature T1 to the cooling stop temperature Ta, the average cooling rate in the temperature range described below is controlled to be within the range described below.

[0069] 《Average cooling rate v2: 20℃ / s or more》 If the average cooling rate v2 from 750°C to 600°C (also simply referred to as "average cooling rate v2") is too small, the amount of martensite decreases, the total amount of ferrite and bainitic ferrite increases, and it becomes difficult to achieve a TS of 1180 MPa or more. For this reason, the average cooling rate v2 is 20°C / s or more, preferably 22°C / s or more, and more preferably 24°C / s or more. Although there is no particular upper limit, the average cooling rate v2 is, for example, 65° C. / s or less, preferably 55° C. / s or less, and more preferably 45° C. / s or less.

[0070] 《Average cooling rate v3: 150℃ / s or less》 As a result of extensive investigations, the inventors have found that the average cooling rate v3 from the Ms point to the cooling stop temperature Ta (also simply referred to as the "average cooling rate v3") affects the instability index d of retained austenite at the initial stage of processing. If the average cooling rate v3 is too large, the martensite transformation rate is fast, so that the retained austenite becomes coarse and the instability index d of the retained austenite at the beginning of processing increases. This leads to a decrease in delayed fracture resistance and a narrowing of the optimum clearance range. For this reason, the average cooling rate v3 is 150°C / s or less, preferably 120°C / s or less, and more preferably 90°C / s or less. Although there is no particular lower limit, the average cooling rate v3 is, for example, 5° C. / s or more, preferably 8° C. / s or more, and more preferably 10° C. / s or more.

[0071] 《Tension F: 5MPa or more and 100MPa or less》 As a result of extensive investigations, the inventors have found that the tension F (also simply referred to as "tension F") applied to a cold-rolled steel sheet from the Ms point to the cooling stop temperature Ta affects the instability index d of retained austenite at the initial stage of processing. If the tension F is too low, the number of martensite nucleation sites is reduced, the retained austenite becomes coarse, and the instability index d of the retained austenite at the beginning of processing increases. This reduces the delayed fracture resistance and narrows the optimal clearance range. For this reason, the tension F is 5 MPa or more, preferably 6 MPa or more, and more preferably 8 MPa or more. On the other hand, if the tension F is too high, the martensitic transformation will proceed excessively, resulting in a reduced amount of retained austenite and a deterioration in bendability. Therefore, the tension F is 100 MPa or less, preferably 50 MPa or less, and more preferably 25 MPa or less.

[0072] 《Ms point》 The Ms point (unit: °C) is calculated by the following formula (a). Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]...(a) In the above formula (a), [%M] is the content of element M in the composition, and is 0 when element M is not contained.

[0073] <Heat Treatment B> Next, the cold-rolled steel sheet cooled to the cooling stop temperature Ta is subjected to a heat treatment B. In summary, in the heat treatment B, the cold-rolled steel sheet is held (heated) at a temperature T2 described later, and then cooled to a temperature lower than the temperature T2 (for example, room temperature). The room temperature is, for example, 25±5°C. The conditions of the heat treatment B are explained below.

[0074] 《Temperature T2: Cooling stop temperature Ta or more and 450℃ or less》 First, the cold-rolled steel sheet is held (heated) at a temperature T2. This stabilizes the retained austenite. If the temperature T2 is too low, the desired amount of retained austenite cannot be obtained, and the bendability decreases. For this reason, the temperature T2 is equal to or higher than the cooling stop temperature Ta, preferably equal to or higher than (Ta+10°C), and more preferably equal to or higher than (Ta+20°C). On the other hand, if the temperature T2 is too high, the tempering of martensite proceeds excessively, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the temperature T2 is 450°C or less, preferably 420°C or less, and more preferably 400°C or less.

[0075] 《Holding time t2: 5 seconds or more and 1000 seconds or less》 If the time for which the cold-rolled steel sheet is held at temperature T2 (holding time t2) is too short, the stabilization of austenite becomes insufficient, the instability index k of the retained austenite increases, and the optimal clearance range becomes narrow. For this reason, the holding time t2 is 5 seconds or more, preferably 50 seconds or more, and more preferably 80 seconds or more. On the other hand, if the holding time t2 is too long, the tempering of martensite proceeds excessively, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the holding time t2 is 1000 seconds or less, preferably 800 seconds or less, and more preferably 400 seconds or less.

[0076] <Processing (imparting equivalent plastic strain)> After the above-mentioned heat treatment A and before the later-described heat treatment C, the cold-rolled steel sheet is processed to impart an equivalent plastic strain to the cold-rolled steel sheet. The temperature during processing is not particularly limited. For example, processing may be performed while the cold-rolled steel sheet is held at temperature T2, or may be performed after the cold-rolled steel sheet is cooled to room temperature after being held at temperature T2.

[0077] {Equivalent plastic strain: 0.10% to 5.00%} As a result of extensive investigations, the inventors have found that the equivalent plastic strain (also simply referred to as "equivalent plastic strain") imparted to a cold-rolled steel sheet by processing affects the instability index d of retained austenite in the initial stage of processing. If the equivalent plastic strain is too small, the amount of unstable retained austenite that transforms in the early stage of processing increases, so the non-stability index d increases, the delayed fracture resistance decreases, and the optimum clearance range becomes narrow. Therefore, the equivalent plastic strain is 0.10% or more, preferably 0.15% or more, and more preferably 0.30% or more. On the other hand, if the equivalent plastic strain is too large, the deformation-induced transformation of the retained austenite proceeds excessively, the amount of the obtained retained austenite decreases, and the bendability deteriorates. Therefore, the equivalent plastic strain is 5.00% or less, preferably 4.00% or less, and more preferably 3.00% or less.

[0078] Number of times processed The number of times the cold-rolled steel sheet is processed is not particularly limited. That is, the processing may be carried out in a plurality of steps, as long as the total equivalent plastic strain imparted to the cold-rolled steel sheet by each processing step falls within the above range. For example, even if the equivalent plastic strain imparted to the cold-rolled steel sheet by the first processing is less than the above-mentioned lower limit, the total equivalent plastic strain by the second or subsequent processing may be equal to or greater than the above-mentioned lower limit (or less than the above-mentioned lower limit).

[0079] 《Processing method》 Examples of the method for processing the cold-rolled steel sheet include a method for performing temper rolling on the cold-rolled steel sheet, a method for processing the cold-rolled steel sheet using a tension leveller, etc. Examples of the leveller include a tension leveller, a continuous stretcher leveller, a roller leveller, etc., and a tension leveller is preferred. When temper rolling is performed, the equivalent plastic strain is the elongation rate of the steel sheet (cold-rolled steel sheet) and is determined from the change in length of the steel sheet before and after processing. When processing cold-rolled steel sheets using a tension leveller, the equivalent plastic strain is calculated according to the method described in Reference 1 below. The following data input values ​​are used for the calculation, the work hardening behavior of the material is assumed to be a linear hardening elastoplastic body, Bausinger hardening is ignored, and tension reduction due to bend loss is ignored. The Misaka formula is used as the processing curvature formula. Number of plate thickness divisions: 31 Young's modulus: 21000kgf / mm 2 Poisson's ratio: 0.3 Yield stress: 111kgf / mm 2 Plasticity coefficient: 1757kgf / mm 2 Reference 1: Yoshisuke Misaka, Takeshi Masui, Plasticity and Processing, Vol. 17, 1976, p. 988-994

[0080] <Heat Treatment C> Next, the cold-rolled steel sheet that has been cooled to room temperature is subjected to heat treatment C, for example. In summary, in the heat treatment C, the cold-rolled steel sheet is heated to a temperature T3 described below, and then cooled to a temperature lower than the temperature T3 (for example, room temperature) without being held at the temperature T3.

[0081] 《Temperature T3: 150℃ or more and 400℃ or less》 If the temperature T3 is too low, the stabilization of austenite becomes insufficient, and the instability index k of the retained austenite increases, narrowing the optimum clearance range. Therefore, the temperature T3 is 150°C or higher, preferably 160°C or higher, and more preferably 170°C or higher. On the other hand, if the temperature T3 is too high, the tempering of martensite proceeds excessively, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the temperature T3 is 400°C or less, preferably 350°C or less, and more preferably 300°C or less.

[0082] <<No retention at temperature T3>> When cold-rolled steel sheets are held at temperature T3, the precipitation of carbides is promoted and carbon that contributes to the stabilization of austenite is wasted. This leads to insufficient stabilization of austenite and an increase in the instability index k of the retained austenite, narrowing the optimum clearance range. For this reason, as described above, the cold-rolled steel sheet that has been heated to the temperature T3 is immediately cooled without being held at the temperature T3.

[0083] When the cold-rolled steel sheet is cooled from the temperature T3 to, for example, room temperature, the average cooling rate in the temperature range described below is controlled to be within the range described below.

[0084] 《Average cooling rate v4: 1.0℃ / h or more and 50.0℃ / h or less》 If the average cooling rate v4 from 150°C to 50°C (also simply referred to as "average cooling rate v4") is too large, the concentration of carbon in the austenite during cooling becomes insufficient. This leads to insufficient stabilization of the austenite, and the instability index k of the retained austenite increases, narrowing the optimal clearance range. For this reason, the average cooling rate v4 is 50.0°C / h or less, preferably 48.0°C / h or less, and more preferably 45.0°C / h or less. On the other hand, due to constraints on production technology, the average cooling rate v4 is 1.0° C. / h or more, preferably 1.2° C. / h or more, and more preferably 1.4° C. / h or more.

[0085] The cold-rolled steel sheet after the heat treatment C is applied corresponds to the steel sheet of the present high-strength steel sheet described above. After the heat treatment C, the cold-rolled steel sheet may be processed to impart an equivalent plastic strain of 0.10% or more and 5.00% or less again. After processing, the cold-rolled steel sheet may be heated at a temperature of 100°C or more and 400°C or less.

[0086] <Plating process> The cold-rolled steel sheet may be subjected to a plating treatment, whereby a plating layer is formed. The plating treatment is carried out, for example, during or after the heat treatment A described above. When the plating treatment is carried out during the heat treatment A, for example, a hot-dip galvanizing treatment or an alloying hot-dip galvanizing treatment (a treatment for alloying after the hot-dip galvanizing treatment) is carried out while the cold-rolled steel sheet is being cooled from 750°C to 600°C at an average cooling rate v2 (or after this cooling). When the plating treatment is carried out after the heat treatment A, for example, the electrogalvanizing treatment is carried out after the heat treatment B. Examples of the electrogalvanizing treatment include a Zn-Ni electric alloy plating treatment and a pure Zn electric plating treatment. The plating process is not limited to the above-mentioned hot-dip galvanizing process, alloyed hot-dip galvanizing process, and electrolytic galvanizing process. The metal type used in the plating process is not limited to Zn, and may be other metals (e.g., Al). By performing wiping during plating, the adhesion weight of the plating layer to be formed can be adjusted. Other conditions for the plating treatment are not particularly limited, and the plating treatment may be carried out according to a conventional method. From the viewpoint of productivity, a series of treatments including the above-mentioned heat treatments A to C and the plating treatment is preferably carried out in a continuous hot-dip galvanizing line (CGL). EXAMPLES

[0087] The present invention will be specifically described below with reference to examples, although the present invention is not limited to the examples described below.

[0088] <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 a steel slab was obtained by continuous casting. The underlined parts in Table 1 below indicate that they are outside the scope of the present invention (the same applies to Tables 2 and 3 described below). The obtained steel slab was held at the slab heating temperature shown in Table 2 below, and then hot-rolled to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was cooled. At this time, it was cooled from 800°C to 600°C at an average cooling rate v1 shown in Table 2 below. The cooled hot-rolled steel sheets were pickled and then cold-rolled to obtain cold-rolled steel sheets. The obtained cold-rolled steel sheet was subjected to heat treatments A to C under the conditions shown in Table 2 below. Prior to heat treatment C, the cold-rolled steel sheet was processed under the conditions shown in Table 2 below. In this way, cold-rolled steel sheets having the final thicknesses shown in Table 2 below were obtained.

[0089] In some cases, cold-rolled steel sheet (CR) is subjected to a plating process (hot-dip galvanizing process, galvannealed hot-dip galvanizing process, or electrogalvanizing process) to obtain hot-dip galvanized steel sheet (GI), galvannealed hot-dip galvanized steel sheet (GA), or electrogalvanized steel sheet (EG). The hot-dip galvanizing treatment and the galvannealed hot-dip galvanizing treatment were carried out during the cooling of heat treatment A. The electrolytic galvanizing treatment was carried out after heat treatment B (before heat treatment C).

[0090] 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. The coating weight of the plating layer is 45 to 72 g / m per side when manufacturing GI. 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 Zn 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 Zn and unavoidable impurities. When manufacturing EG, the coating weight of the plating layer is 30g / m per side. 2 Electrolytic zinc plating was performed.

[0091] Hereinafter, cold-rolled steel sheet (CR), hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA) and electro-galvanized steel sheet (EG) will also be referred to simply as "steel sheet".

[0092] Observation of Microstructure For the obtained steel sheets, the amount of martensite, the amount of retained austenite, and the total amount of ferrite and bainitic ferrite were determined according to the above-mentioned method. The results are shown in Table 3 below. In Table 3 below, martensite is represented as "M", austenite as "γ", ferrite as "F", and bainitic ferrite as "BF".

[0093] <Unstable index k and unstable index d> For the obtained steel sheets, the instability index k of the retained austenite and the instability index d of the retained austenite at the initial stage of working were determined according to the method described above. The results are shown in Table 3 below.

[0094] <evaluation> The obtained steel sheets were subjected to the tests described below to evaluate various properties, and the results are shown in Table 3 below.

[0095] Tensile strength (tensile test) From the obtained steel plate, a JIS No. 5 test piece (gauge length: 50 mm, parallel part width: 25 mm) was taken with the direction perpendicular to the rolling direction as the longitudinal direction (tensile direction). Using the taken test piece, -1 A tensile test was carried out in accordance with JIS Z 2241 under the condition of a rate of 100 mm / sec to determine the tensile strength (TS). A TS of 1180 MPa or more was determined to be high strength.

[0096] Bendability (bending test) A test piece (width: 30 mm, length: 100 mm) with the longitudinal direction perpendicular to the rolling direction was taken from the obtained steel plate. A bending test was carried out using the taken test piece according to the V-block method described in JIS Z 2248, and the minimum bending radius R at which no cracks were generated at the bending ridge was measured. The occurrence of cracks was confirmed by observing the bent ridgeline at a magnification of 40 times using a digital microscope (RH-2000, manufactured by Hirox Corporation). When the value (R / t) obtained by dividing the minimum bending radius R by the plate thickness t was 6.0 or less, the bendability was determined to be excellent.

[0097] <Delayed fracture resistance> A test piece (parallel part width: 6 mm, parallel part length: 15 mm) was taken from the obtained steel sheet with the width direction parallel to the rolling direction. The test piece was ground on all sides to a sheet thickness of 1.0 mm, and then the test was carried out. The test piece was immersed in an aqueous solution containing 3 mass% NaCl and 3 g / L NHSCN, and the applied current density was 0 or 0.05 mA / cm 2 After that, a tensile test (SSRT test) was carried out at a tensile speed of 5 μm / min to break the test piece, and the tensile strength (TS) was obtained. The applied current density was 0 mA / cm 2 When the applied current density is 0.05mA / cm for TS, 2 The ratio of TS in this case was calculated as the stress ratio. In the following Table 3, the specimens were given the grade "C" when the stress ratio was less than 0.70, the grade "B" when it was 0.70 or more but less than 0.80, and the grade "A" when it was 0.80 or more. The grades "B" and "A" were judged to have excellent delayed fracture resistance.

[0098] <Optimal clearance range> The optimum clearance range for bending the shear end surface was determined as follows. First, the obtained steel plate was sheared to obtain test pieces (width: 30 mm, length: 100 mm) with the longitudinal direction perpendicular to the rolling direction. The rake angle during shearing was standardized to 0°, and the shear clearance was changed to 5%, 10%, 15%, 20%, 25%, 30%, and 35%. The test pieces obtained by shearing were subjected to bending tests using a punch with a minimum bending radius R divided by plate thickness t (R / t) of 6.0 according to the above-mentioned method, and the sheared end faces of the test pieces were bent (bending process). After that, the sheared end faces of the test pieces were checked for the presence or absence of cracks (fissures). The occurrence of cracks was confirmed by observing the sheared end surface of the test piece at a magnification of 40 times using a digital microscope (RH-2000, manufactured by Hirox Corporation). If the shear clearance range where cracks did not occur on the sheared end face of the test specimen was less than 10%, it was marked as "C", if it was 10% or more but less than 15%, it was marked as "B", and if it was 15% or more, it was marked as "A" in Table 3 below. If it was marked as "B" or "A", it was determined that the optimum clearance range for bending forming of the sheared end face was wide.

[0099] [Table 1] TIFF0007666749000002.tif214108TIFF0007666749000003.tif216113

[0100] [Table 2] TIFF0007666749000005.tif228125TIFF0007666749000006.tif228125TIFF0007666749 000007.tif228125TIFF0007666749000008.tif228125TIFF0007666749000009.tif22893

[0101] [Table 3] TIFF0007666749000011.tif212158TIFF0007666749000012.tif180158

[0102] <Summary of evaluation results> As shown in Table 3 above, the steel plates Nos. 1 to 8, 10 to 12, 14 to 16, 18 to 20, 22, 24, 26, 28 to 32, 34 to 36, 38, 40, 42 to 44, 46 to 48, 50 to 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84 to 85, 87 to 88, 90 to 91 and 93 to 113 (invention examples) had a TS of 1180 MPa or more, were excellent in bendability and delayed fracture resistance, and had a wide optimum clearance range. In contrast, steel plates Nos. 9, 13, 17, 21, 23, 25, 27, 33, 37, 39, 41, 45, 49, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79, 81, 83, 86, 89 and 92 (comparative examples) were insufficient in at least any of TS, bendability, delayed fracture resistance and optimum clearance range.

Claims

1. Equipped with steel plates, The steel plate comprises, in mass%, C: 0.030% or more and 0.500% or less, Si: 0.50% or more and 2.50% or less, Mn: 1.50% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, O: 0.0100% or less and Nb: 0.005% or more and 0.100% or less, and the balance being Fe and unavoidable impurities; A microstructure having an amount of martensite of 70% or more, an amount of retained austenite of 3% or more and 20% or less, and a total amount of ferrite and bainitic ferrite of 10% or less, The instability index k of the retained austenite is less than 6.1; A high-strength steel plate having an instability index d of retained austenite in an initial stage of working of less than 5.

7.

2. The composition further includes, in mass%, Ti: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% 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. A method for producing the high strength steel plate according to claim 1 or 2, A steel slab having the composition according to claim 1 or 2 is held at a slab heating temperature of 1220°C or more, and then hot rolling is performed to obtain a hot-rolled steel sheet, The hot-rolled steel sheet is cooled at an average cooling rate v from 800 ° C to 600 ° C. １ The steel sheet is cooled under a condition where the melting rate is 30° C. / s or more, and then pickled and cold rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is held at a temperature T1 of 800 ° C. or higher for 10 seconds or more, and then cooled to a cooling stop temperature Ta of 100 ° C. or higher (Ms point - 80 ° C.) or lower. Heat treatment A is performed; In the heat treatment A, the average cooling rate v from 750 ° C to 600 ° C ２ is 20 ° C. / s or more, and the average cooling rate v from the Ms point to the cooling stop temperature Ta ３ is 150 ° C. / s or less, and a tension F applied to the cold-rolled steel sheet from the Ms point to the cooling stop temperature Ta is 5 MPa or more and 100 MPa or less, After the heat treatment A, the cold-rolled steel sheet is held at a temperature T2 of 5 seconds to 1000 seconds and then cooled at the cooling stop temperature Ta or higher and 450 ° C. or lower. Heat treatment B is then performed. After the heat treatment B, the cold-rolled steel sheet is heated to a temperature T3 of 150 ° C. or more and 400 ° C. or less, and then a heat treatment C is performed in which the cold-rolled steel sheet is cooled without being held at the temperature T3; In the heat treatment C, the average cooling rate v from 150 ° C to 50 ° C ４ is 1.0 ° C. / h or more and 50.0 ° C. / h or less, A method for producing a high strength steel sheet, comprising: after the heat treatment A and before the heat treatment C, processing the cold rolled steel sheet to impart an equivalent plastic strain of 0.10% or more and 5.00% or less to the cold rolled steel sheet. Here, the Ms point is expressed in °C and is calculated by the following formula (a). Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]...(a) In the formula (a), [% M] is the content of element M in the composition, and is 0 when element M is not contained.

5. The method for producing a high strength steel sheet according to claim 4, further comprising the step of subjecting the cold rolled steel sheet to a plating treatment.

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