High-strength steel sheet and method for producing same

Abstract

Provided is a high-strength steel sheet having a tensile strength of 1180 MPa or more, excellent bendability and delayed fracture resistance, and a wide optimal gap range for bending and forming a sheared end surface. The high-strength steel sheet comprises, in mass%, 0.030 to 0.500% of C, 0.50 to 2.50% of Si, 1.50 to 5.00% of Mn, 0.100% or less of P, 0.0200% or less of S, 1.000% or less of Al, 0.0100% or less of N, 0.0100% or less of O, and 0.005 to 0.100% of Nb, with the remainder comprising Fe and unavoidable impurities, and has a martensite content of 70% or more, a retained austenite content of 3 to 20%, and a total of a ferrite content and a bainitic ferrite content of 10% or less, the instability index k of the retained austenite is less than 6.1, and the instability index d of the retained austenite at the initial stage of processing is less than 5.7.

Description

Technical Field

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

[0002] To achieve both improved fuel efficiency (reduction in CO2 emissions) through lightweighting and enhanced crashworthiness, the strength of thin steel sheets for automobiles is being increased, and new regulations are being introduced.

[0003] In recent years, in order to increase vehicle body strength, high-strength steel sheets having a tensile strength (TS) of 1180 MPa or more (see, for example, Patent Documents 1 to 3) have been increasingly used in major structural parts of automobiles.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2015-193897

[0007] Patent Document 2: International Publication No. 2019 / 187090

[0008] Patent Document 3: International Publication No. 2020 / 174805 Summary of the Invention

[0009] Conventionally, research has been focused on hot press forming of high-strength steel sheets, but recently, research has begun on cold press forming from the perspectives of cost and productivity.

[0010] However, when a component is obtained by cold forming a high-strength steel sheet having a TS of 1180 MPa or more, there is a possibility that delayed fracture may occur due to increased residual stress in the component or deterioration in the delayed fracture resistance of the steel sheet itself.

[0011] Delayed fracture refers to a phenomenon in which, when a formed part is placed in a hydrogen-permeated environment, hydrogen permeates into the steel sheets that make up the part, reducing the interatomic bonding strength or causing local deformation, resulting in the formation of microcracks, which then propagate and eventually fracture.

[0012] Furthermore, from the viewpoint of formability, high-strength steel sheets used in automobiles are less likely to crack at the bending ridges when bending is required (ie, they have excellent bendability).

[0013] Furthermore, many end faces of automotive frame parts are formed by shearing, and these sheared end faces are also required to be free from cracks caused by bending (bending).

[0014] The morphology of the shear end face depends on the shear gap, and the crack generation on the shear end face also depends on the shear gap.

[0015] Therefore, high-strength steel sheets used in automobiles require a wide range of optimal gaps for bending the sheared end faces.

[0016] Therefore, an object of the present invention is to provide a high-strength steel sheet having a tensile strength (TS) of 1180 MPa or more, excellent bendability and delayed fracture resistance, and a wide range of optimal gaps for bending of sheared end faces, and a method for producing the same.

[0017] The present inventors have conducted intensive studies and, as a result, have found that the above-mentioned object can be achieved by adopting the following configuration, thereby completing the present invention.

[0018] That is, the present invention provides the following [1] to [5].

[0019] [1] A high-strength steel plate comprising a steel plate having a composition and microstructure comprising, in mass%, 0.030% to 0.500% C, 0.50% to 2.50% Si, 1.50% to 5.00% Mn, 0.100% or less P, 0.0200% or less S, 1.000% or less Al, 0.0100% or less N, 0.0100% or less O, and 0.005% to 0.100% Nb, with the remainder being Fe and unavoidable impurities, wherein the microstructure comprises 70% or more martensite, 3% to 20% retained austenite, and 10% or less ferrite and bainitic ferrite in total, wherein the retained austenite instability index k is less than 6.1, and the retained austenite instability index d at an early stage of processing is less than 5.7.

[0020] [2] The high-strength steel sheet according to [1], wherein the chemical composition further contains, in 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.

[0021] [3] The high-strength steel sheet according to [1] or [2], wherein the surface of the steel sheet further includes a plating layer.

[0022] [4] A method for manufacturing a high-strength steel plate, which is a method for manufacturing the high-strength steel plate described in [1] or [2] above, comprising: maintaining a steel slab having the composition described in [1] or [2] above at a slab heating temperature of 1220°C or higher, and then performing hot rolling to obtain a hot-rolled steel plate; cooling the hot-rolled steel plate at an average cooling rate v1 of 30°C / s or higher from 800°C to 600°C; then performing pickling and cold rolling to obtain a cold-rolled steel plate; subjecting the cold-rolled steel plate to a heat treatment A at a temperature T1 of 800°C or higher for 10 seconds or longer, and then cold rolling the cold-rolled steel plate to a cooling stop temperature Ta of 100°C or higher and (Ms point - 80°C) or lower; wherein in the heat treatment A, the average cooling rate v2 from 750°C to 600°C is 20°C / s or higher, and the average cooling rate v from the Ms point to the cooling stop temperature Ta is 20°C / s or higher. 3 is 150°C / s 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 to 100 MPa, after the heat treatment A, the cold-rolled steel sheet is subjected to a heat treatment B at a temperature T2 that is higher than the cooling stop temperature Ta and lower than 450°C, maintained for 5 seconds to 1000 seconds, and then cooled, after the heat treatment B, the cold-rolled steel sheet is subjected to a heat treatment C that is heated to a temperature T3 that is higher than 150°C and lower than 400°C, and then cooled without maintaining at the temperature T3, in the heat treatment C, an average cooling rate v4 from 150°C to 50°C is 1.0°C / h to 50.0°C / h, the cold-rolled steel sheet is worked after the heat treatment A and before the heat treatment C, and an equivalent plastic strain of 0.10% to 5.00% is applied to the cold-rolled steel sheet.

[0023] However, the Ms point is obtained from the following formula (a), and the unit is °C.

[0024] Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]···(a)

[0025] In the above formula (a), [%M] is the content of the element M in the above composition, and is 0 when the element M is not contained.

[0026] [5] The method for producing a high-strength steel sheet according to [4] above, wherein the cold-rolled steel sheet is subjected to a plating treatment.

[0027] According to the present invention, a high-strength steel sheet having a tensile strength (TS) of 1180 MPa or more, excellent bendability and delayed fracture resistance, and a wide range of optimal gaps for bending of sheared end faces can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1Schematic diagram showing a test piece used in a tensile test.

[0029] Figure 2 This is a graph schematically showing the relationship between the tensile stress and the tensile strain applied to the test piece during the tensile test.

[0030] Figure 3 : is an example of a graph showing the relationship between the tensile strain ε and the amount of retained austenite.

[0031] Figure 4 : is another example of a graph showing the relationship between the tensile strain ε and the amount of retained austenite.

[0032] Figure 5 This is another example of a graph showing the relationship between the tensile strain ε and the amount of retained austenite. DETAILED DESCRIPTION

[0033] [High-strength steel plate]

[0034] The high-strength steel sheet according to the present embodiment (hereinafter also referred to as “the present high-strength steel sheet”) includes a steel sheet, and as described later, the surface of the steel sheet may further include a plating layer.

[0035] The steel sheet included in the present high-strength steel sheet has a chemical composition and a microstructure (steel structure) described below, and satisfies an instability index k and an instability index d described below.

[0036] High strength means a tensile strength (TS) of 1180 MPa or more.

[0037] The present high-strength steel sheet has a tensile strength (TS) of 1180 MPa or more, is excellent in bendability and delayed fracture resistance, and has a wide range of optimum gaps for bending the sheared end surface.

[0038] Hereinafter, the optimal gap range for bending the sheared end surface will also be simply referred to as the "optimal gap range."

[0039] Briefly, the present inventors conducted intensive studies and found the following.

[0040] (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 1180 MPa or more can be achieved.

[0041] (2) By setting the retained austenite amount to 3% or more, excellent bendability can be achieved.

[0042] (3) Excellent delayed fracture resistance can be achieved by setting the instability index d of retained austenite at the initial stage of working to less than 5.7 and the amount of retained austenite to 20% or less.

[0043] (4) By making the instability index k of retained austenite less than 6.1 and the instability index d of retained austenite at the initial stage of processing less than 5.7, a wider optimum gap range can be achieved.

[0044] By applying this high-strength steel sheet to, for example, automotive structural parts, it is possible to improve fuel efficiency by reducing the weight of the vehicle body, and therefore has great industrial value.

[0045] Steel Plate

[0046] First, the steel sheet included in this high-strength steel sheet will be described.

[0047] The thickness of the steel plate is not particularly limited, and is, for example, 0.5 mm to 3.0 mm.

[0048] Ingredients

[0049] The chemical composition of the steel sheet included in the present high-strength steel sheet (hereinafter also referred to as "present chemical composition") will be described.

[0050] “%” in the composition means “mass %” unless otherwise specified.

[0051] (C: 0.030% to 0.500%)

[0052] 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.

[0053] If the amount of C is too low, 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 higher. Therefore, the C content is 0.030% or higher, preferably 0.050% or higher, and more preferably 0.100% or higher.

[0054] On the other hand, if the C content is too high, the martensite becomes brittle and the delayed fracture resistance decreases. Therefore, the C content is 0.500% or less, preferably 0.400% or less, and more preferably 0.350% or less.

[0055] (Si: 0.50% to 2.50%)

[0056] Si is one of the important basic components of steel and is an element that affects TS and the amount of retained austenite.

[0057] If Si is too low, the strength of martensite decreases, making it difficult to achieve a TS of 1180 MPa or higher. Therefore, the Si content is 0.50% or higher, preferably 0.55% or higher, and more preferably 0.60% or higher.

[0058] On the other hand, if Si is too much, retained austenite increases excessively, and delayed fracture resistance decreases. Therefore, the Si content is 2.50% or less, preferably 2.00% or less, and more preferably 1.80% or less.

[0059] (Mn: 1.50% to 5.00%)

[0060] 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.

[0061] If Mn is too low, 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 higher. Therefore, the Mn content is 1.50% or higher, preferably 2.00% or higher, and more preferably 2.20% or higher.

[0062] On the other hand, if the Mn content is too high, the martensite becomes brittle and the delayed fracture resistance decreases. Therefore, the Mn content is 5.00% or less, preferably 4.50% or less, and more preferably 4.00% or less.

[0063] (P: 0.100% or less)

[0064] P segregates at prior austenite grain boundaries, embrittles the grain boundaries, and embrittles the steel sheet, thereby reducing delayed fracture resistance. Therefore, the P content is 0.100% or less, preferably 0.070% or less, and more preferably 0.030% or less.

[0065] There is no particular lower limit. Furthermore, P is a solid solution strengthening element that 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.

[0066] (S: 0.0200% or less)

[0067] S exists as sulfides and embrittles the steel sheet, thereby reducing delayed fracture resistance. Therefore, the S content is 0.0200% or less, preferably 0.0050% or less, and more preferably 0.0025% or less.

[0068] The lower limit is not particularly limited, but due to production technology constraints, the S content is preferably 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0069] (Al: 1.000% or less)

[0070] Al exists as oxides, embrittles the steel sheet, and thus reduces delayed fracture resistance. Therefore, the Al content is 1.000% or less, preferably 0.500% or less, more preferably 0.150% or less, and even more preferably 0.070% or less.

[0071] There is no particular lower limit. Furthermore, Al inhibits carbide formation during the heat treatment described below and promotes 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.

[0072] (N: 0.0100% or less)

[0073] Nitrogen, present as nitrides, embrittles the steel sheet and reduces delayed fracture resistance. Therefore, the N content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.

[0074] The lower limit is not particularly limited, but due to production technology constraints, the N content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0075] (O: 0.0100% or less)

[0076] O exists as oxides, which embrittles the steel sheet and reduces delayed fracture resistance. Therefore, the O content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.

[0077] The lower limit is not particularly limited, but due to production technology constraints, the O content is preferably 0.0001% or more, more preferably 0.0007% or more, and even more preferably 0.0015% or more.

[0078] (Nb: 0.005% to 0.100%)

[0079] As a result of repeated intensive studies, the present inventors have discovered that Nb affects the instability index k of retained austenite.

[0080] If Nb is too low, the structure will coarsen after the heat treatment described later, increasing the instability index k of the retained austenite. This will narrow the optimal gap range. Therefore, the Nb content is 0.005% or more, preferably 0.008% or more, and more preferably 0.010% or more.

[0081] On the other hand, if Nb is too much, martensite becomes embrittled and delayed fracture resistance decreases. Therefore, the Nb content is 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[0082] (Other elements)

[0083] The present component composition may further contain at least one element (other element) selected from the elements described below in terms of mass %.

[0084] ((Ti and V))

[0085] If Ti and V are excessive, a large amount of coarse precipitates and inclusions will be generated, causing embrittlement of the steel sheet and reducing delayed fracture resistance. Therefore, the contents of Ti and V are each 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less.

[0086] There is no particular lower limit. Furthermore, Ti and V increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment, as described later. Therefore, the Ti and V contents are each preferably 0.001% or greater, more preferably 0.005% or greater, and even more preferably 0.015% or greater.

[0087] ((Ta and W))

[0088] If Ta and W are excessive, a large amount of coarse precipitates and inclusions will be generated, causing embrittlement of the steel sheet and reducing delayed fracture resistance. Therefore, the contents of Ta and W are each 0.10% or less, preferably 0.09% or less, and more preferably 0.08% or less.

[0089] There is no particular lower limit. Furthermore, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or heat treatment, as described later. Therefore, the Ta and W contents are each preferably 0.01% or greater, more preferably 0.03% or greater, and even more preferably 0.05% or greater.

[0090] ((B))

[0091] If B is too high, cracks will form inside the steel sheet during casting or hot rolling, making the steel sheet brittle and reducing delayed fracture resistance. Therefore, the B content is 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0060% or less.

[0092] The lower limit is not particularly limited. In addition, B segregates at the austenite grain boundaries during the heat treatment described below, thereby improving 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.

[0093] ((Cr, Mo and Ni))

[0094] If Cr, Mo, and Ni are too high, coarse precipitates and inclusions increase, the steel sheet becomes brittle, and delayed fracture resistance decreases. Therefore, the contents of Cr, Mo, and Ni are each 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0095] The lower limit is not particularly limited. In addition, 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 even more preferably 0.08% or more.

[0096] ((Co))

[0097] If the Co content is too high, coarse precipitates and inclusions increase, the steel sheet becomes brittle, and delayed fracture resistance decreases. Therefore, the Co content is 0.010% or less, preferably 0.008% or less, and more preferably 0.006% or less.

[0098] The lower limit is not particularly limited. In addition, 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.

[0099] ((Cu))

[0100] If Cu is too much, coarse precipitates and inclusions increase, the steel sheet becomes brittle, and delayed fracture resistance decreases. Therefore, the Cu content is 1.00% or less, preferably 0.80% or less, and more preferably 0.60% or less.

[0101] The lower limit is not particularly limited. In addition, 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.

[0102] ((Sn))

[0103] If Sn is too high, cracks will form inside the steel sheet during casting or hot rolling, making the steel sheet brittle and reducing delayed fracture resistance. Therefore, the Sn content is 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less.

[0104] The lower limit is not particularly limited. In addition, 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.

[0105] ((Sb))

[0106] If the Sb content is too high, coarse precipitates and inclusions increase, the steel sheet becomes brittle, and delayed fracture resistance decreases. Therefore, the Sb content is 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less.

[0107] The lower limit is not particularly limited. In addition, since Sb is an element that controls the softened thickness of the surface layer and can adjust the strength, the Sb content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0108] ((Ca, Mg and REM))

[0109] If Ca, Mg, and REM (rare earth metals) are excessive, coarse precipitates and inclusions increase, the steel sheet becomes brittle, and delayed fracture resistance decreases. Therefore, the contents of Ca, Mg, and REM are each 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.

[0110] There is no particular lower limit. Furthermore, Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides, thereby improving 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.

[0111] ((Zr and Te))

[0112] If Zr and Te are too high, coarse precipitates and inclusions increase, the steel sheet becomes brittle, and delayed fracture resistance decreases. Therefore, the contents of Zr and Te are each 0.100% or less, preferably 0.080% or less, and more preferably 0.060% or less.

[0113] There is no particular lower limit. Furthermore, Zr and Te are elements that spheroidize the shape of nitrides and sulfides, thereby improving the ultimate deformability of the steel sheet. Therefore, the Zr and Te contents are each preferably 0.001% or greater, more preferably 0.008% or greater, and even more preferably 0.015% or greater.

[0114] ((Hf))

[0115] If Hf is too high, coarse precipitates and inclusions increase, the steel sheet becomes brittle, and delayed fracture resistance decreases. Therefore, the Hf content is 0.10% or less, preferably 0.09% or less, and more preferably 0.08% or less.

[0116] There is no particular lower limit. Furthermore, Hf is an element that spheroidizes the shape of nitrides and sulfides, thereby improving 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.

[0117] ((Bi))

[0118] If Bi is too much, coarse precipitates and inclusions increase, the steel sheet becomes brittle, and delayed fracture resistance decreases. Therefore, the Bi content is 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less.

[0119] The lower limit is not particularly limited. In addition, 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.

[0120] When the content of each of the above-mentioned other elements is less than the above-mentioned preferred lower limit, the effects of the present invention are not inhibited and therefore they are regarded as unavoidable impurities.

[0121] (Remainder)

[0122] The remainder of this component composition is composed of Fe and inevitable impurities.

[0123] Micro-organization

[0124] Next, the microstructure of the steel sheet included in the present high-strength steel sheet (hereinafter also referred to as "present microstructure") will be described.

[0125] (Total amount of ferrite and bainitic ferrite: 10% or less)

[0126] If ferrite and bainitic ferrite are too much, it is difficult to achieve a TS of 1180 MPa or more. Therefore, the total amount of ferrite and bainitic ferrite is 10% or less, preferably 9% or less, and more preferably 8% or less.

[0127] The lower limit is not particularly limited.

[0128] The amount of ferrite and the amount of bainitic ferrite were determined as follows.

[0129] The steel plate was polished to expose the L-section at the 1 / 4 thickness position (the position equivalent to 1 / 4 of the thickness in the depth direction from the steel plate surface) as the observation surface. After the observation surface was etched with 3% by volume of nitric alcohol, 10 fields of view were observed at a magnification of 2000 times using a scanning electron microscope (SEM), and SEM images of each field of view were obtained. In the SEM images, ferrite and bainitic ferrite were identified as internally flat concave structures. The area ratios of ferrite and bainitic ferrite in each SEM image were calculated (unit: %), and the average value of the calculated area ratios for the 10 fields of view was taken as the total amount of ferrite and bainitic ferrite.

[0130] (Retained austenite content: 3% to 20%)

[0131] 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.

[0132] 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.

[0133] The amount of retained austenite is determined as follows.

[0134] The steel plate was ground to expose an L-section at a position 0.1 mm deeper than 1 / 4 of the plate thickness. The L-section was further ground by chemical grinding in the depth direction by 0.1 mm to obtain an observation surface. For the observation surface, the integrated intensity ratio of the diffraction peaks was determined using CoKα radiation in an X-ray diffraction (XRD) device. More specifically, the integrated intensity ratio of the diffraction peaks of the {200}, {220} and {311} crystal planes of fcc iron and the {200}, {211} and {220} crystal planes of bcc iron was determined. The average value of the 9 integrated intensity ratios was taken as the volume fraction (unit: %) of the retained austenite amount, which was taken as the retained austenite amount.

[0135] (Martensite content: more than 70%)

[0136] Since a TS of 1180 MPa or more can be achieved, the amount of martensite is 70% or more, preferably 75% or more, and more preferably 80% or more.

[0137] In addition, in order to achieve high TS in martensite, the contribution of tempered martensite is important, and the amount of tempered martensite is preferably 80% or more.

[0138] The martensite volume is calculated as follows.

[0139] First, the retained austenite amount, the total amount of ferrite, and the total amount of bainitic ferrite are determined by the above method. Then, the total amount is subtracted from 100%, and the resulting value (unit: %) is defined as the martensite amount.

[0140] Therefore, the amount of martensite referred to here includes both quenched martensite and tempered martensite.

[0141] Note that the retained austenite amount is a volume ratio as described above, which is approximately equal to the area ratio. Therefore, the retained austenite amount is subtracted from 100% along with the total of the ferrite amount and the bainitic ferrite amount calculated as the area ratio.

[0142] 《Instability Index k: less than 6.1》

[0143] The present inventors have conducted intensive studies and have found that the instability index k of retained austenite (also referred to as "instability index k") affects the optimal gap range.

[0144] If the instability index k is too high, the retained austenite becomes less stable and, during shearing, the retained austenite excessively transforms into hard martensite, which reduces the ultimate deformability of the steel sheet and narrows the optimal gap range.

[0145] Therefore, 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.

[0146] The lower limit is not particularly limited, but the instability index k is, for example, 1.0 or greater, preferably 1.5 or greater, and more preferably 2.0 or greater.

[0147] 《Instability Index d: less than 5.7》

[0148] As a result of intensive research, the present inventors have discovered that the instability index d of retained austenite in the early stage of processing (also simply referred to as "instability index d") affects delayed fracture resistance and the optimal gap range.

[0149] If the instability index d is too high, the stability of retained austenite in the initial stage of processing is low. In the initial stage of processing, the retained austenite is excessively transformed into hard martensite, which becomes the starting point of delayed fracture in a hydrogen permeation environment, thereby reducing delayed fracture resistance.

[0150] On the other hand, if the instability index d is too high, the retained austenite will be excessively transformed into hard martensite in the initial stage of processing, thereby reducing the ultimate deformability of the steel sheet and narrowing the optimal gap range.

[0151] Therefore, 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.

[0152] The lower limit is not particularly limited, but the instability index d is, for example, not less than -15.0, preferably not less than -10.0, and more preferably not less than -5.0.

[0153] The instability index k and the instability index d are calculated as follows.

[0154] First, a tensile test (described in detail in [Examples]) described later was carried out, and a test piece of a steel plate (JIS No. 5 test piece) was processed.

[0155] Figure 1 Schematic diagram showing a test piece used in a tensile test. Figure 2 It is a graph schematically showing the relationship between the tensile stress and the tensile strain applied to the test piece 1 during the tensile test.

[0156] like Figure 2 As shown, tensile strain (tensile plastic strain) is applied to the test piece by applying tensile stress. Thus, multiple test pieces with arbitrary tensile strains ε in the range of 0 to 10% (0 to 0.10) are obtained. The tensile strain ε before processing (before applying tensile strain) is 0%.

[0157] Then, the center of each test piece ( Figure 1 The amount of retained austenite when the tensile strain ε was applied was determined by the above method. The obtained results were plotted on a graph (horizontal axis: tensile strain ε, vertical axis: logarithm of the retained austenite amount).

[0158] For the figure (see below Figures 3 to 5 ) is used to obtain a linear approximation (y = -ax + b) using the least squares method. The obtained approximation is substituted into the following equation (1) to determine the slope a of the approximation as the instability index k of retained austenite.

[0159] log(fγ)=-k·ε+log(fγ0)···(1)

[0160] In the above formula (1), fγ is the amount of retained austenite when tensile strain ε is applied, and fγ0 is the amount of retained austenite before working.

[0161] Furthermore, the estimated value of the retained austenite amount before working (when the tensile strain ε is 0%) is obtained from the slice of the above formula (1). Then, the instability index d of the retained austenite before working is obtained based on the following formula (2).

[0162] d=fγ0-fγε0···(2)

[0163] In the above formula (2), fγ0 represents the measured value of the amount of retained austenite before working, and fγε0 represents the estimated value of the retained austenite before working.

[0164] Figure 3 : is an example of a graph showing the relationship between the tensile strain ε and the amount of retained austenite. Figure 4 Here is another example of the same graph. Figure 5 Here is another example of the same graph.

[0165] Will Figure 3 and Figure 4 When comparing, Figure 3 The slope a (instability index k) of the approximate formula is 2.3, which is less than Figure 4 The slope a (instability index k) of the approximate formula is 11.9.

[0166] The small slope a (instability index k) of the approximate formula indicates that the change in the amount of retained austenite during processing is small and the stability of the retained austenite is good.

[0167] Next, observe Figure 5 , the slope a (instability index k) of the approximate formula is 4.0, which is relatively small.

[0168] However, in Figure 5 In the figure, the estimated value of retained austenite before processing, fγε0, is 12.3 (log(fγε0)=1.09). In contrast, the measured value of retained austenite before processing, fγ0, is 18 (log(fγ0)=1.26), and the instability index d (=fγ0-fγε0) is 5.7, which is relatively large.

[0169] When the instability index d is large, the slope of the approximate formula is large only 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.

[0170] Therefore, when both the instability index k and the instability index d satisfy the above ranges, the stability of retained austenite is excellent both during processing and in the initial stage of processing.

[0171] 〈Plating layer〉

[0172] From the viewpoint of improving corrosion resistance, etc., the surface of the steel sheet of the present high-strength steel sheet may further include a plating layer.

[0173] Examples of the plating layer include a hot-dip galvanizing layer, an alloyed hot-dip galvanizing layer, and an electrogalvanizing layer. The plating layer is formed by a plating treatment described below.

[0174] The coating weight of the plating layer is not particularly limited, but is preferably 20 to 80 g / m2 on one side. 2 .

[0175] [Method for producing high-strength steel sheet]

[0176] Next, a method for producing a high-strength steel sheet according to the present embodiment (hereinafter also referred to as "the present production method") will be described. The present production method is also a method for producing the above-described present high-strength steel sheet.

[0177] The temperatures when heating or cooling the steel slabs, steel plates, etc. shown below refer to the surface temperatures of the steel slabs, steel plates, etc. unless otherwise specified.

[0178] The method for producing molten steel to be used as a billet is not particularly limited, and a known method using a converter, an electric furnace, etc. can be employed. From the viewpoint of preventing macrosegregation, it is preferable to obtain a billet from molten steel by continuous casting.

[0179] Hot Rolling

[0180] In the present production method, first, a steel slab having the above-mentioned present component composition is held at a slab heating temperature described below, and then hot-rolled to obtain a hot-rolled steel sheet.

[0181] 《Slab heating temperature: above 1220℃》

[0182] If the slab heating temperature is too low, the dissolution of inclusions is insufficient, resulting in embrittlement of the steel sheet and a decrease in delayed fracture resistance. Therefore, the slab heating temperature is preferably 1220°C or higher, more preferably 1230°C or higher, and even more preferably higher than 1230°C.

[0183] The upper limit is not particularly limited, but the slab heating temperature is preferably 1300°C or lower, more preferably 1290°C or lower, and even more preferably 1280°C or lower.

[0184] <cool down>

[0185] 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.

[0186] 《Average cooling rate v1: 30℃ / s or more》

[0187] The present inventors have conducted intensive studies and have found that the average cooling rate v1 from 800° C. to 600° C. (also referred to simply as “average cooling rate v1”) affects the instability index k of retained austenite.

[0188] If the average cooling rate v1 is too low, coarse Nb-based carbides will precipitate excessively during cooling, reducing the effect of Nb in reducing the instability index k of retained austenite. As a result, the instability index k increases, narrowing the optimal gap range.

[0189] Therefore, the average cooling rate v1 is 30° C. / s or higher, preferably 35° C. / s or higher, and more preferably 40° C. / s or higher.

[0190] Pickling and cold rolling

[0191] Next, the cooled hot-rolled steel sheet is pickled and cold-rolled to obtain a cold-rolled steel sheet.

[0192] Pickling removes oxides from the surface of hot-rolled steel sheets and is therefore important for ensuring good chemical treatment properties and coating quality in the high-strength steel sheets that serve as the final product. Pickling can be performed once or in multiple passes.

[0193] The hot-rolled steel sheet is pickled to obtain a pickled sheet, which is then dried as needed.

[0194] Cold rolling may be performed on the pickled sheet before drying or after drying.

[0195] The reduction ratio during cold rolling and the thickness of the sheet after rolling are not particularly limited, nor are the number of rolling passes and the reduction ratio in each pass.

[0196] Next, the cold-rolled steel sheet obtained by cold rolling is sequentially subjected to the following heat treatments: A, B, and C. Before the heat treatment C, the following processing is performed.

[0197] The following is a more detailed description.

[0198] Heat Treatment A

[0199] First, heat treatment A is performed on a cold-rolled steel sheet obtained by cold rolling.

[0200] In brief, in the heat treatment A, the cold-rolled steel sheet is held (heated) at a temperature T1 described later, and then cooled to a cooling stop temperature Ta described later.

[0201] Hereinafter, the conditions of the heat treatment A will be described.

[0202] 《Temperature T1: above 800℃》

[0203] First, the cold-rolled steel sheet is held (heated) at temperature T1. If 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.

[0204] Therefore, the temperature T1 is 800°C or higher, preferably 820°C or higher, and more preferably 840°C or higher.

[0205] The upper limit is not particularly limited, but the temperature T1 is, for example, 940° C. or lower, preferably 920° C. or lower, and more preferably 900° C. or lower.

[0206] 《Maintaining time t1: more than 10 seconds》

[0207] If the time (holding time t1) for holding (heating) the cold-rolled steel sheet at the temperature T1 is too short, 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.

[0208] Therefore, the holding time t1 is 10 seconds or longer, preferably 30 seconds or longer, and more preferably 50 seconds or longer.

[0209] The upper limit is not particularly limited, but the holding time t1 is, for example, 300 seconds or less, preferably 250 seconds or less, and more preferably 200 seconds or less.

[0210] Cooling stop temperature Ta: 100°C or higher and (Ms point - 80°C) or lower

[0211] Next, the cold-rolled steel sheet maintained at the temperature T1 is cooled to the cooling stop temperature Ta.

[0212] If the cooling stop temperature Ta is too low, the amount of retained austenite decreases, and the bendability deteriorates. Therefore, the cooling stop temperature Ta is 100°C or higher, preferably 120°C or higher, and more preferably 140°C or higher.

[0213] 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 (Ms point - 80°C) or lower, preferably (Ms point - 90°C) or lower, and more preferably (Ms point - 100°C) or lower.

[0214] 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 fall within the range described below.

[0215] 《Average cooling rate v2: 20℃ / s or more》

[0216] If the average cooling rate v2 from 750°C to 600°C (also referred to simply as "average cooling rate v2") is too low, 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 higher. Therefore, the average cooling rate v2 is 20°C / s or higher, preferably 22°C / s or higher, and more preferably 24°C / s or higher.

[0217] The upper limit is not particularly limited, but 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.

[0218] 《Average cooling rate v3: below 150℃ / s》

[0219] The present inventors have conducted intensive studies and found that the average cooling rate v3 from the Ms point to the cooling stop temperature Ta (also referred to simply as "average cooling rate v3") affects the instability index d of retained austenite in the early stage of working.

[0220] If the average cooling rate v3 is too high, the martensitic transformation rate is rapid, causing retained austenite to coarsen, increasing the instability index d of retained austenite in the early stages of processing. This reduces delayed fracture resistance and narrows the optimal gap range. Therefore, the average cooling rate v3 should be 150°C / s or less, preferably 120°C / s or less, and more preferably 90°C / s or less.

[0221] The lower limit is not particularly limited, but the average cooling rate v3 is, for example, 5° C. / s or higher, preferably 8° C. / s or higher, and more preferably 10° C. / s or higher.

[0222] Tension F: 5MPa~100MPa

[0223] The present inventors have conducted intensive studies and have found that the tension F applied to the cold-rolled steel sheet from the Ms point to the cooling stop temperature Ta (also referred to simply as "tension F") affects the instability index d of retained austenite in the early stage of working.

[0224] If the tension F is too low, the number of nucleation sites for martensite decreases, resulting in coarsening of retained austenite and an increase in the instability index d of retained austenite in the early stages of processing. This reduces delayed fracture resistance and narrows the optimal gap range. Therefore, the tension F is set to 5 MPa or higher, preferably 6 MPa or higher, and more preferably 8 MPa or higher.

[0225] On the other hand, if the tension F is too high, the martensitic transformation proceeds excessively, resulting in a decrease in the amount of retained austenite obtained and a decrease in bendability. Therefore, the tension F is 100 MPa or less, preferably 50 MPa or less, and more preferably 25 MPa or less.

[0226] Ms. Point

[0227] The Ms point (unit: °C) is determined by the following formula (a).

[0228] Ms=519-474×[%C]-30.4×[%Mn]-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]···(a)

[0229] In the above formula (a), [%M] is the content of the element M in the component composition, and is 0 when the element M is not contained.

[0230] Heat Treatment B

[0231] Next, heat treatment B is performed on the cold-rolled steel sheet cooled to the cooling stop temperature Ta.

[0232] In brief, in 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 (eg, room temperature). The room temperature is, for example, 25±5°C.

[0233] Hereinafter, the conditions of the heat treatment B will be described.

[0234] Temperature T2: Cooling stop temperature Ta or higher and 450°C or lower

[0235] First, the cold-rolled steel sheet is held (heated) at temperature T2. This stabilizes the retained austenite. If temperature T2 is too low, the desired amount of retained austenite is not obtained, resulting in reduced bendability. Therefore, temperature T2 is set to be above the cooling stop temperature Ta, preferably above (Ta + 10°C), and more preferably above (Ta + 20°C).

[0236] On the other hand, if the temperature T2 is too high, tempering of martensite proceeds excessively, making it difficult to achieve a TS of 1180 MPa or higher. Therefore, the temperature T2 is 450°C or lower, preferably 420°C or lower, and more preferably 400°C or lower.

[0237] 《Holding time t2: 5 seconds to 1000 seconds》

[0238] If the time 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 retained austenite increases, and the optimal gap range becomes narrower. Therefore, the holding time t2 is 5 seconds or more, preferably 50 seconds or more, and more preferably 80 seconds or more.

[0239] On the other hand, if the holding time t2 is too long, the tempering of martensite will proceed 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.

[0240] 〈Processing (imparting equivalent plastic strain)〉

[0241] After the above-mentioned heat treatment A and before the heat treatment C described later, the cold-rolled steel sheet is worked to impart equivalent plastic strain to the cold-rolled steel sheet.

[0242] The temperature during processing is not particularly limited. For example, processing may be performed while the cold-rolled steel sheet is maintained at temperature T2, or after being maintained at temperature T2 and then cooled to, for example, room temperature.

[0243] 《Equivalent plastic strain: 0.10%~5.00%》

[0244] As a result of intensive research, the present inventors have discovered that the equivalent plastic strain imparted to a cold-rolled steel sheet by working (also simply referred to as "equivalent plastic strain") affects the instability index d of retained austenite in the early stages of working.

[0245] If the equivalent plastic strain is too low, the amount of unstable retained austenite that undergoes phase transformation in the early stages of processing increases, increasing the instability index d, reducing delayed fracture resistance, and narrowing the optimal gap range. Therefore, the equivalent plastic strain is 0.10% or more, preferably 0.15% or more, and more preferably 0.30% or more.

[0246] On the other hand, if the equivalent plastic strain is too large, the work-induced transformation of retained austenite will proceed excessively, the amount of retained austenite obtained will decrease, and the bendability will decrease. Therefore, the equivalent plastic strain is 5.00% or less, preferably 4.00% or less, and more preferably 3.00% or less.

[0247] Number of processing times

[0248] The number of times the cold-rolled steel sheet is processed is not particularly limited.

[0249] That is, the processing may be performed in a plurality of steps as long as the total of the equivalent plastic strains imparted to the cold-rolled steel sheet by each processing step is within the above-mentioned range.

[0250] For example, even if the equivalent plastic strain imparted to the cold-rolled steel sheet by the first working is smaller than the above lower limit, the total equivalent plastic strain may be equal to or greater than (or equal to or less than) the above lower limit by the second and subsequent workings.

[0251] Processing Methods

[0252] Examples of methods for processing cold-rolled steel sheets include temper rolling and using a tension leveler. Examples of levelers include tension levelers, continuous stretch levelers, and roller levelers, with tension levelers being preferred.

[0253] When temper rolling is performed, the equivalent plastic strain is the elongation of the steel sheet (cold-rolled steel sheet), and is obtained from the change in length of the steel sheet before and after processing.

[0254] When working cold-rolled steel sheets using a tension leveler, the equivalent plastic strain is calculated using the method described in Reference 1 below. The calculation uses the following input values: the material's work-hardening behavior is assumed to be linearly hardening elastic-plastic, Bauschinger hardening is neglected, and the reduction in strain due to bending loss is ignored. The Yoshisuke Misaka formula is used as the working curvature formula.

[0255] Plate thickness division number: 31

[0256] Young's modulus: 21000kgf / mm 2

[0257] Poisson's ratio: 0.3

[0258] Yield stress: 111kgf / mm 2

[0259] Plasticity coefficient: 1757kgf / mm 2

[0260] Reference 1: Misaka Yoshisuke, Masukai Ken, Plasticity and Processing, 1976, Vol. 17, pp. 988-994

[0261] Heat Treatment C

[0262] Next, heat treatment C is performed on the cold-rolled steel sheet cooled to room temperature, for example.

[0263] In brief, in the heat treatment C, the cold-rolled steel sheet is heated to a temperature T3 described later, and then cooled to a temperature lower than the temperature T3 (eg, room temperature) without being kept at the temperature T3.

[0264] 《Temperature T3: above 150℃ and below 400℃》

[0265] If the temperature T3 is too low, the stabilization of austenite becomes insufficient, the instability index k of retained austenite increases, and the optimal gap range becomes narrower. Therefore, the temperature T3 is 150°C or higher, preferably 160°C or higher, and more preferably 170°C or higher.

[0266] On the other hand, if the temperature T3 is too high, tempering of martensite proceeds excessively, making it difficult to achieve a TS of 1180 MPa or higher. Therefore, the temperature T3 is 400°C or lower, preferably 350°C or lower, and more preferably 300°C or lower.

[0267] 《Not maintained at temperature T3》

[0268] If the cold-rolled steel sheet is kept at temperature T3, the precipitation of carbides is accelerated, and the carbon that contributes to austenite stabilization is wasted. As a result, austenite stabilization becomes insufficient, the instability index k of retained austenite increases, and the optimal gap range narrows.

[0269] Therefore, as described above, the cold-rolled steel sheet heated to the temperature T3 is not kept at the temperature T3 but is immediately cooled.

[0270] 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 fall within the range described below.

[0271] 《Average cooling rate v4: 1.0℃ / h~50.0℃ / h》

[0272] If the average cooling rate v4 from 150°C to 50°C (also referred to simply as "average cooling rate v4") is too high, carbon enrichment in austenite during cooling becomes insufficient. This leads to insufficient stabilization of austenite, an increase in the instability index k of retained austenite, and a narrowing of the optimal gap range. Therefore, the average cooling rate v4 is set to 50.0°C / h or less, preferably 48.0°C / h or less, and more preferably 45.0°C / h or less.

[0273] On the other hand, due to production technology constraints, the average cooling rate v4 is 1.0° C. / h or higher, preferably 1.2° C. / h or higher, and more preferably 1.4° C. / h or higher.

[0274] The cold-rolled steel sheet subjected to the heat treatment C corresponds to the steel sheet included in the above-mentioned present high-strength steel sheet.

[0275] After heat treatment C, the cold-rolled steel sheet may be processed to impart an equivalent plastic strain of 0.10% to 5.00% to the cold-rolled steel sheet. After processing, the cold-rolled steel sheet may be heated at a temperature of 100°C to 400°C.

[0276] 〈Plating treatment〉

[0277] The cold-rolled steel sheet may be subjected to a plating treatment to form a plating layer.

[0278] The plating treatment is performed, for example, during or after the heat treatment A described above.

[0279] When the plating treatment is performed during the heat treatment A, for example, the cold-rolled steel sheet is subjected to hot-dip galvanizing treatment or alloyed hot-dip galvanizing treatment (alloying treatment performed after hot-dip galvanizing treatment) while being cooled from 750°C to 600°C at an average cooling rate v2 (or after such cooling).

[0280] When the plating treatment is performed after the heat treatment A, for example, the electrogalvanizing treatment is performed after the heat treatment B. Examples of the electrogalvanizing treatment include Zn—Ni alloy electroplating treatment and pure Zn electroplating treatment.

[0281] It should be noted that the plating process is not limited to the above-mentioned hot-dip galvanizing process, alloyed hot-dip galvanizing process and electrogalvanizing process. In addition, the metal species used in the plating process is not limited to Zn, and can also be other metals (for example, Al).

[0282] By performing wiping during the plating process, the adhesion amount of the formed plating layer can be adjusted.

[0283] Other conditions of the plating treatment are not particularly limited, and the treatment may be carried out according to a conventional method.

[0284] From the viewpoint of productivity, the 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).

[0285] Example

[0286] Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples.

[0287] 〈Manufacturing of Steel Plates〉

[0288] Molten steel having the composition shown in Table 1 below, with the remainder consisting of Fe and inevitable impurities, was produced in a converter and continuously cast into steel slabs. The underlined elements in Table 1 below indicate values ​​outside the scope of the present invention (the same applies to Tables 2 and 3 described below).

[0289] The obtained steel slabs were kept at the slab heating temperature shown in Table 2 below and then hot rolled to obtain hot-rolled steel sheets. The obtained hot-rolled steel sheets were cooled from 800°C to 600°C at the average cooling rate v1 shown in Table 2 below.

[0290] The cooled hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet.

[0291] The obtained cold-rolled steel sheets were subjected to heat treatments A to C under the conditions shown in Table 2 below. Before heat treatment C, the cold-rolled steel sheets were processed under the conditions shown in Table 2 below.

[0292] In this manner, cold-rolled steel sheets having the final sheet thickness shown in Table 2 below were obtained.

[0293] In some examples, a cold-rolled steel sheet (CR) is subjected to a plating treatment (hot-dip galvanizing, alloyed hot-dip galvanizing, or electrogalvanizing) to obtain a hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), or electrogalvanized steel sheet (EG).

[0294] Hot-dip galvanizing and alloy hot-dip galvannealing were performed during the cooling of heat treatment A. Electrogalvanizing was performed after heat treatment B (before heat treatment C).

[0295] As the hot-dip galvanizing bath, when producing GI, a zinc bath containing 0.20 mass% Al and the remainder consisting of Zn and inevitable impurities is used, and when producing GA, a zinc bath containing 0.14 mass% Al and the remainder consisting of Zn and inevitable impurities is used.

[0296] The bath temperature was 470° C. in both GI and GA.

[0297] The coating weight is 45 to 72 g / m² per side when manufacturing GI. 2 In the case of GA, the single side is 45g / m 2 .

[0298] When GA is produced, the alloying temperature is 500°C.

[0299] The composition of the GI plating layer is 0.1-1.0 mass% Fe, 0.2-1.0 mass% Al, with the balance being Zn and inevitable impurities. The composition of the GA plating layer is 7-15 mass% Fe, 0.1-1.0 mass% Al, with the balance being Zn and inevitable impurities.

[0300] When manufacturing EG, the coating weight is 30g / m2 per side. 2 The electro-galvanizing treatment is carried out in the form of

[0301] Hereinafter, cold-rolled steel sheets (CR), hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), and electrogalvanized steel sheets (EG) are also referred to simply as “steel sheets”.

[0302] Observation of Microstructure

[0303] The amount of martensite, the amount of retained austenite, and the total amount of ferrite and bainitic ferrite were determined for the obtained steel plate according to the above method. The results are shown in Table 3 below. In Table 3 below, martensite is represented as "M", austenite is represented as "γ", ferrite is represented as "F", and bainitic ferrite is represented as "BF".

[0304] 〈Instability Index k and Instability Index d〉

[0305] The instability index k of retained austenite and the instability index d of retained austenite at the initial stage of working were determined for the obtained steel sheet by the above-mentioned method. The results are shown in Table 3 below.

[0306] <evaluate>

[0307] The obtained steel sheets were subjected to the tests described below to evaluate various properties. The results are shown in Table 3 below.

[0308] Tensile Strength (Tensile Test)

[0309] From the obtained steel plate, a JIS No. 5 test piece (mark distance: 50 mm, parallel part width: 25 mm) with the longitudinal direction (tensile direction) perpendicular to the rolling direction was taken. The taken test piece was used to test the steel plate at a crosshead speed of 1.67 × 10 -1A tensile test was conducted according to JIS Z 2241 under the condition of 0.05433 / 0.001 mm / s to determine the tensile strength (TS).

[0310] When TS is 1180 MPa or more, it is judged to be high strength.

[0311] 《Flexibility (bending test)》

[0312] From the obtained steel plate, a test piece (width: 30 mm, length: 100 mm) with its longitudinal side perpendicular to the rolling direction was taken. Using the taken test piece, a bending test was carried out according to the V-block method described in JIS Z 2248 to measure the minimum bending radius R at which no cracks occurred at the bending ridge line.

[0313] The presence or absence of cracks was confirmed by observing the curved ridge portion at a magnification of 40 times using a digital microscope (RH-2000, manufactured by Hirox Corporation).

[0314] When the value obtained by dividing the minimum bending radius R by the plate thickness t (R / t) is 6.0 or less, the plate is judged to have excellent bendability.

[0315] Delayed fracture resistance

[0316] From the obtained steel plate, a test piece (parallel portion width: 6 mm, parallel portion length: 15 mm) with its width parallel to the rolling direction was taken. The test piece was ground across its entire surface to a thickness of 1.0 mm and then tested.

[0317] The test piece was immersed in an aqueous solution containing 3 mass % NaCl and 3 g / L NH4SCN, and the applied current density was set to 0 or 0.05 mA / cm 2 Then, a tensile test (SSRT test) was performed at a tensile speed of 5 μm / min, and the test piece was broken to determine the tensile strength (TS).

[0318] Calculate the current density to be 0.05 mA / cm 2 TS at the time of application is equal to the applied current density of 0 mA / cm 2 The ratio of TS at t is taken as the stress ratio.

[0319] In Table 3 below, a stress ratio of less than 0.70 is indicated as "C," a stress ratio of 0.70 or more and less than 0.80 is indicated as "B," and a stress ratio of 0.80 or more is indicated as "A." A rating of "B" or "A" indicates excellent delayed fracture resistance.

[0320] Optimal clearance range

[0321] The optimum gap range for bending the sheared end surface is determined as follows.

[0322] First, the obtained steel plates were 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 uniformly set to 0°, and the shear gaps were varied to 5%, 10%, 15%, 20%, 25%, 30%, and 35%.

[0323] The sheared test piece was subjected to a bending test using a punch having a minimum bending radius R divided by the plate thickness t (R / t) of 6.0 according to the above method. The sheared end surface of the test piece was bent (bending). The sheared end surface of the test piece was then inspected for cracks (fissures).

[0324] The presence or absence 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).

[0325] In Table 3 below, the shear gap range where the sheared end surface of the test piece did not crack is recorded as "C," the range of 10% or more but less than 15% is recorded as "B," and the range of 15% or more is recorded as "A." "B" or "A" indicates that the optimal gap range for bending the sheared end surface is wide.

[0326]

[0327]

[0328]

[0329]

[0330]

[0331]

[0332]

[0333]

[0334]

[0335] Table 3 (1 / 3)

[0336]

[0337] Table 3 (2 / 3)

[0338]

[0339] Table 3 (3 / 3)

[0340]

[0341] Summary of Evaluation Results

[0342] 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 (inventive examples) have TS of 1180 MPa or more, are excellent in bendability and delayed fracture resistance, and have a wide optimal gap range.

[0343] In contrast, the steel plates No. 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 one of TS, bendability, delayed fracture resistance and optimal gap range.

Claims

1. A high-strength steel plate, comprising: The steel plate has the following composition and microstructure: The composition comprises, in mass%, C: 0.030% to 0.500%, Si: 0.50% to 2.50%, Mn: 1.50% to 5.00%, 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% to 0.100%, with the remainder being Fe and unavoidable impurities. The microstructure is characterized by a martensite content of 70% or more, a retained austenite content of 3% to 20%, and a total of ferrite and bainitic ferrite content of 10% or less; Moreover, the instability index k of retained austenite is less than 6.

1. The instability index d of retained austenite at the initial stage of processing is less than 5.

7.

2. The high-strength steel plate according to claim 1, wherein The composition further contains, in 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 plate according to claim 1 or 2, wherein: The surface of the steel sheet further includes a plating layer.

4. A method for producing a high-strength steel sheet, comprising: producing the high-strength steel sheet according to claim 1 or 2; A steel slab having the composition according to claim 1 or 2 is maintained at a slab heating temperature of 1220° C. or higher and then hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is cooled at an average cooling rate v1 of 30°C / s or higher from 800°C to 600°C, and then pickled and cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet is subjected to the following heat treatment A: the sheet is held at a temperature T1 of 800°C or higher for 10 seconds or longer, and then cooled to a cooling stop temperature Ta of 100°C or higher and (Ms point - 80°C or lower). Furthermore, in the heat treatment A, the average cooling rate v2 from 750°C to 600°C is 20°C / s or more, the average cooling rate v3 from the Ms point to the cooling stop temperature Ta is 150°C / s or less, and the tension F applied to the cold-rolled steel sheet from the Ms point to the cooling stop temperature Ta is 5 MPa to 100 MPa. After the heat treatment A, the cold-rolled steel sheet is subjected to the following heat treatment B: the sheet is kept at a temperature T2 which is higher than the cooling stop temperature Ta and lower than 450° C. for 5 to 1000 seconds, and then cooled. After the heat treatment B, the cold-rolled steel sheet is subjected to the following heat treatment C: the temperature is raised to a temperature T3 of 150° C. or higher and 400° C. or lower, and then the temperature is not maintained at T3 but cooled. Furthermore, in the heat treatment C, the average cooling rate v4 from 150°C to 50°C is 1.0°C / h to 50.0°C / h. 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. in, Ms point is obtained by the following formula (a), the unit is °C, 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 the element M in the above composition, and is 0 when the element M is not contained.

5. The method for producing a high-strength steel plate according to claim 4, wherein: The cold-rolled steel sheet is subjected to a plating treatment.

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