High-strength steel sheet and method for manufacturing the same

Abstract

The object of the present invention is to provide a high-strength steel plate with a TS of 1320 MPa or higher, a YR of 85% or higher, and an excellent appropriate gap range for delayed fracture, and a method for manufacturing the same. A high-strength steel plate contains specific components, has a specific microstructure, and the microstructure satisfies the formulas specified in (1) and (2) below. KAM(S) / KAM(C)<1.00……(1) Here, KAM(S) represents the KAM (core average orientation difference) value of the surface portion of the steel plate, and KAM(C) represents the KAM value of the center portion of the steel plate. Hv(Q)-Hv(S)≥8……(2) Here, Hv(Q) represents the hardness of the 1 / 4 portion of the plate thickness, and Hv(S) represents the hardness of the surface portion of the steel plate.

Description

Technical Field

[0001] This invention relates to a high-strength steel sheet with excellent tensile strength and resistance to delayed fracture, and a method for manufacturing the same. The high-strength steel sheet of this invention is suitable for use as structural components such as automotive parts. Background Technology

[0002] With the aim of reducing CO2 emissions through vehicle lightweighting and improving crashworthiness through body lightweighting, there is a growing trend towards using high-strength thin steel sheets in automobiles, and new regulations have been introduced accordingly. As a result, the use of high-strength steel sheets with a tensile strength (TS) of 1320 MPa or higher in the main structural components of automobiles is increasing, with the goal of increasing body strength.

[0003] For high-strength steel sheets used in automobiles, from the perspective of component performance, a good yield ratio (YR = yield strength YS / tensile strength TS) is required. For example, for skeletal components such as automobile bumpers, excellent impact absorption during a collision is required; therefore, steel sheets with a good YR value related to impact absorption are preferred.

[0004] Furthermore, automotive frame components contain numerous end faces formed through shearing processes. The morphology of these sheared end faces depends on the shearing gap. The morphology of the sheared end faces affects resistance to delayed fracture. Here, delayed fracture refers to the phenomenon where, when a formed component is placed in a hydrogen-infiltrating environment, hydrogen penetrates the steel sheet constituting the component, reducing interatomic bonding forces and causing localized deformation, thereby generating microcracks that eventually lead to fracture. For high-strength steel sheets used in automobiles, a wide appropriate gap range for delayed fracture is required.

[0005] For these requirements, for example, Patent Document 1 provides a high-strength steel sheet with a tensile strength of 980 MPa or more and excellent bending workability, as well as a method for manufacturing the same. However, the technology described in Patent Document 1 does not consider the tensile strength (YR) and an appropriate gap range for delayed fracture. Furthermore, none of the steel sheets described in Patent Document 1 achieve a YR ≥ 85%.

[0006] For example, Patent Document 2 provides a high-strength steel plate with a tensile strength of 1320 MPa or more and excellent resistance to delayed fracture at the shear end face, as well as a method for manufacturing the same. However, the technology described in Patent Document 2 does not consider an appropriate gap range for delayed fracture.

[0007] For example, Patent Document 3 provides a high-strength steel sheet with a tensile strength of 1100 MPa or more, and excellent YR, surface properties, and weldability, as well as a method for manufacturing the same. However, the technology described in Patent Document 3 does not consider an appropriate gap range for delayed fracture.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent No. 6354909

[0011] Patent Document 2: Japanese Patent No. 6112261

[0012] Patent Document 3: Japanese Patent No. 6525114 Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] The present invention was developed in view of the above circumstances, and aims to provide a high-strength steel plate with a TS of 1320 MPa or more, a YR of 85% or more, and an excellent appropriate gap range for delayed fracture, and a method for manufacturing the same.

[0015] Methods for solving problems

[0016] In order to achieve the above-mentioned goals, the inventors have conducted in-depth research and have discovered the following insights.

[0017] (1) By making the tempered martensite content above 85%, a TS of above 1320MPa can be achieved.

[0018] (2) By making the retained austenite less than 5%, KAM(S) / KAM(C) less than 1.00, and Hv(Q)-Hv(S) greater than 8, it is possible to achieve more than 85% YR.

[0019] (3) By making KAM(S) / KAM(C) less than 1.00 and Hv(Q)-Hv(S) greater than 8, an excellent appropriate gap range for delayed fracture can be achieved.

[0020] This invention is based on the above-mentioned insights. That is, the main structure of this invention is as follows.

[0021] [1] A high-strength steel plate having a composition, by mass%, comprising C: 0.15% or more and 0.45% or less, Si: 0.10% or more and 2.00% or less, Mn: 0.5% or more and 3.5% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 1.000% or less, N: 0.0100% or less, H: 0.0020% or less, with the balance being Fe and unavoidable impurities; and having a structure of tempered martensite of 85% or more in area fraction, retained austenite of less than 5% in volume fraction, and the total of ferrite and bainitic ferrite of 10% or less in area fraction, satisfying the formulas specified in (1) and (2) below.

[0022] KAM(S) / KAM(C)<1.00……(1)

[0023] Here, KAM(S) represents the KAM (Kernel Average Misorientation) value of the surface layer of the steel plate, and KAM(C) represents the KAM value of the center layer of the steel plate.

[0024] Hv(Q)-Hv(S)≥8……(2)

[0025] Here, Hv(Q) represents the hardness of the 1 / 4 section of the plate thickness, and Hv(S) represents the hardness of the surface layer of the steel plate.

[0026] [2] The high-strength steel plate according to [1] further contains, as a component, one or more elements selected from Ti: less than 0.100%, B: less than 0.0100%, Nb: less than 0.100%, Cu: less than 1.00%, Cr: less than 1.00%, V: less than 0.100%, Mo: less than 0.500%, Ni: less than 0.50%, Sb: less than 0.200%, Sn: less than 0.200%, As: less than 0.100%, Ta: less than 0.100%, Ca: less than 0.0200%, Mg: less than 0.0200%, Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%, and REM: less than 0.0200%.

[0027] [3] The high-strength steel plate according to [1] or [2], wherein a coating is provided on the surface of the steel plate.

[0028] [4] A method for manufacturing a high-strength steel plate, which is the method for manufacturing a high-strength steel plate described in [1] or [2] above, wherein,

[0029] Cold-rolled steel sheets, produced by hot rolling, pickling, and cold rolling of steel billets, are annealed at a temperature T1 of 850°C or higher and 1000°C or lower, and at a holding time t1 of 10 seconds or higher and 1000 seconds or lower.

[0030] Then, cool to below 100°C.

[0031] Processing begins when the temperature reaches 100°C and the elapsed time t2 is less than 1000 seconds.

[0032] The starting temperature T2 for the above processing is below 80℃.

[0033] Processing is carried out under conditions where the equivalent plastic strain is greater than 0.10% and less than 5.00%.

[0034] Then, tempering is performed under the conditions of a temperature T3 of 100°C or higher and 400°C or lower, and a holding time t3 at T3 of 1.0 second or higher and 1000.0 seconds or lower.

[0035] Cooling is performed under the condition that the cooling rate θ1 from T3 to 80°C is less than 100°C / second.

[0036] [5] According to the manufacturing method of high-strength steel plate described in [4], the strain imparting based on the processing is performed twice or more in the processing steps before tempering, and the processing is performed under the condition that the total of the equivalent plastic strain of each processing is 0.10% or more.

[0037] [6] The method for manufacturing high-strength steel plates according to [4] or [5], wherein plating is performed during or after annealing.

[0038] Invention Effects

[0039] According to the present invention, high-strength steel sheets with a strength (TS) of 1320 MPa or higher, a yield (YR) of 85% or higher, and excellent appropriate gap range for delayed fracture can be obtained. Furthermore, by applying the high-strength steel sheets of the present invention, for example, to automotive structural components, it is possible to improve fuel efficiency through vehicle body lightweighting. Therefore, it has significant industrial applicability. Detailed Implementation

[0040] The embodiments of the present invention will be described below.

[0041] First, the appropriate range of the composition of high-strength steel plates and the reasons for limiting it will be explained. It should be noted that, in the following explanation, unless otherwise specified, the "%" indicating the content of the constituent elements of steel refers to "mass %".

[0042] C: Above 0.15% and below 0.45%

[0043] Carbon (C) is one of the essential components of steel, and especially in this invention, it is a crucial element affecting the ultimate tensile strength (TS). When the C content is less than 0.15%, it is difficult to achieve a TS of 1320 MPa or higher. Therefore, the C content is set to 0.15% or more. Preferably, the C content is 0.16% or more. More preferably, it is 0.17% or more. Even more preferably, it is 0.18% or more. Most preferably, it is 0.19% or more. On the other hand, when the C content exceeds 0.45%, it reduces the steel's ultimate deformation capacity and decreases the appropriate gap range for delayed fracture. Therefore, the C content is set to 0.45% or less. Preferably, it is 0.40% or less. More preferably, it is 0.35% or less. Even more preferably, it is 0.30% or less. Most preferably, it is 0.26% or less.

[0044] Si: 0.10% or more and 2.00% or less

[0045] Si is one of the important basic components of steel, and especially in this invention, Si is a crucial element affecting TS (steel strength) and retained austenite. When the Si content is less than 0.10%, it is difficult to achieve a TS of 1320 MPa or higher. Therefore, the Si content is set to 0.10% or more. The Si content is preferably 0.15% or more. The Si content is more preferably 0.20% or more. The Si content is further preferably 0.30% or more. The Si content is most preferably 0.40% or more. On the other hand, when the Si content exceeds 2.00%, the retained austenite increases excessively, making it difficult to achieve a YR (retained austenite ratio) of 85% or more. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.80% or less. The Si content is more preferably 1.60% or less. The Si content is further preferably 1.50% or less. The Si content is most preferably 1.20% or less.

[0046] Mn: 0.5% or more and 3.5% or less

[0047] Mn is one of the important basic components of steel, and especially in this invention, Mn is an important element affecting the ferrite and bainite fractions. When the Mn content is less than 0.5%, the ferrite and bainite fractions increase, making it difficult to achieve a steel strength (TS) of 1320 MPa or higher, and also difficult to achieve a yr content (YR) of 85% or higher. Therefore, the Mn content is set to 0.5% or more. The Mn content is preferably 0.7% or more. The Mn content is more preferably 1.0% or more. The Mn content is further preferably 1.1% or more. The Mn content is most preferably 1.5% or more. On the other hand, when the Mn content exceeds 3.5%, macroscopic segregation of Mn occurs, reducing the steel's ultimate deformation capacity and thus reducing the appropriate gap range for delayed fracture. Therefore, the Mn content is set to 3.5% or less. The Mn content is preferably 3.3% or less. The Mn content is more preferably 3.1% or less. The Mn content is further preferably 3.0% or less. The Mn content is most preferably 2.8% or less.

[0048] P: below 0.100%

[0049] When the phosphorus (P) content exceeds 0.100%, P segregates at grain boundaries, causing embrittlement of the steel plate and thus reducing the appropriate gap range for delayed fracture. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less. More preferably, the P content is 0.060% or less. It should be noted that the lower limit of the P content is not particularly limited, but due to limitations in production technology, it is preferably 0.001% or more.

[0050] S: below 0.0200%

[0051] When the sulfur content exceeds 0.0200%, it exists in the form of sulfides, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate gap range for delayed fracture. Therefore, the sulfur content is set to 0.0200% or less. The sulfur content is preferably 0.0100% or less. More preferably, the sulfur content is 0.0050% or less. It should be noted that while there is no particular limitation on the lower limit of the sulfur content, it is preferably 0.0001% or more due to limitations in production technology.

[0052] Al: Above 0.010% and below 1.000%

[0053] By including Al, the strength of the steel sheet increases, making it easier to achieve a strength tolerance (TS) of 1320 MPa or higher. To achieve this effect, the Al content needs to be 0.010% or higher. Therefore, the Al content is set to 0.010% or higher. The Al content is preferably 0.012% or higher. The Al content is more preferably 0.015% or higher. The Al content is even more preferably 0.020% or higher. On the other hand, when the Al content exceeds 1.000%, the ferrite and bainite fractions increase, making it difficult to achieve a TS of 1320 MPa or higher, and also difficult to achieve a yr content (YR) of 85% or higher. Therefore, the Al content is set to 1.000% or lower. The Al content is preferably 0.500% or lower. The Al content is more preferably 0.100% or lower.

[0054] N: below 0.0100%

[0055] When the nitrogen (N) content exceeds 0.0100%, the cast steel billet becomes brittle and prone to cracking, significantly reducing productivity. Therefore, the N content is set to be 0.0100% or less. The N content is preferably 0.0080% or less. More preferably, it is 0.0070% or less. Even more preferably, it is 0.0060% or less. Most preferably, it is 0.0050% or less. It should be noted that while there is no particular limitation on the lower limit of the N content, due to limitations in production technology, it is preferably 0.0010% or more.

[0056] H: below 0.0020%

[0057] When the hydrogen (H) content exceeds 0.0020% or falls below this level, the steel's ultimate deformation capacity decreases, reducing the appropriate gap range for delayed fracture. Therefore, the H content is set to be 0.0020% or less. Preferably, the H content is 0.0015% or less. More preferably, it is 0.0010% or less. It should be noted that there is no particular limitation on the lower limit of the H content, but the lower the H content, the higher the appropriate gap range for delayed fracture; therefore, it can be 0%.

[0058] The high-strength steel plate of the present invention preferably contains, in addition to the above-mentioned composition, one or more elements selected by mass percentage from Ti: less than 0.100%, B: less than 0.0100%, Nb: less than 0.100%, Cu: less than 1.00%, Cr: less than 1.00%, V: less than 0.100%, Mo: less than 0.500%, Ni: less than 0.50%, Sb: less than 0.200%, Sn: less than 0.200%, As: less than 0.100%, Ta: less than 0.100%, Ca: less than 0.0200%, Mg: less than 0.0200%, Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%, and REM: less than 0.0200%.

[0059] Ti: below 0.100%

[0060] When the Ti content exceeds 0.100%, the cast steel billet becomes brittle and prone to cracking, significantly reducing productivity. Therefore, when adding Ti, its content is set to 0.100% or less. The Ti content is preferably 0.075% or less. More preferably, it is 0.050% or less. The Ti content is even more preferably less than 0.050%. On the other hand, by containing Ti, the strength of the steel plate increases, making it easier to achieve a strength strength (TS) of 1320 MPa or higher. To obtain this effect, the Ti content is preferably 0.001% or more. More preferably, it is 0.005% or more. The Ti content is even more preferably 0.010% or more.

[0061] B: Below 0.0100%

[0062] When the boron content exceeds 0.0100%, the cast steel billet becomes brittle and prone to cracking, significantly reducing productivity. Therefore, when adding boron, its content is set to 0.0100% or less. The boron content is preferably 0.0080% or less, and more preferably 0.0050% or less. On the other hand, by containing boron, the strength of the steel plate increases, making it easier to achieve a strength tolerance (TS) of 1320 MPa or higher. To obtain this effect, the boron content is preferably 0.0001% or more, and more preferably 0.0002% or more.

[0063] Nb: below 0.100%

[0064] When the Nb content exceeds 0.100%, the rough cast steel billet becomes brittle and prone to cracking, significantly reducing productivity. Therefore, when Nb is added, its content is set to 0.100% or less. The Nb content is preferably 0.090% or less. More preferably, it is 0.050% or less. Even more preferably, it is 0.030% or less. On the other hand, by containing Nb, the strength of the steel plate increases, making it easier to achieve a strength tolerance (TS) of 1320 MPa or higher. To obtain this effect, it is preferable to set the Nb content to 0.001% or more. More preferably, it is 0.002% or more.

[0065] Cu: below 1.00%

[0066] When the Cu content exceeds 1.00%, the cast steel billet becomes brittle and prone to cracking, significantly reducing productivity. Therefore, when Cu is added, the Cu content is set to 1.00% or less. The Cu content is preferably 0.50% or less. On the other hand, by containing Cu, the intrusion of hydrogen into the steel sheet is suppressed, improving the appropriate gap range for delayed fracture. To achieve this effect, the Cu content is preferably 0.01% or more. The Cu content is preferably 0.03% or more. The Cu content is more preferably 0.10% or more.

[0067] Cr: less than 1.00%

[0068] When the Cr content exceeds 1.00%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate clearance range for expansion deformation. Therefore, when Cr is added, its content is set to 1.00% or less. The Cr content is preferably 0.70% or less. The Cr content is more preferably 0.50% or less. On the other hand, Cr not only acts as a solid solution strengthening element, but also stabilizes austenite during the cooling process of continuous annealing, suppressing ferrite formation and thus increasing the strength of the steel sheet. To achieve this effect, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.02% or more.

[0069] V: Below 0.100%

[0070] When the content of each element exceeds 0.100%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate clearance range for hole expansion deformation. Therefore, when adding V, its content is set to 0.100% or less, preferably 0.060% or less. On the other hand, V increases the strength of the steel sheet. To achieve this effect, the V content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.

[0071] Mo: 0.500% or less

[0072] When the Mo content exceeds 0.500%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate clearance range for expansion deformation. Therefore, when adding Mo, its content is set to 0.500% or less. The Mo content is preferably 0.450% or less. The Mo content is more preferably 0.400% or less. On the other hand, Mo not only acts as a solid solution strengthening element, but also stabilizes austenite during the cooling process of continuous annealing, suppressing the formation of ferrite and thus increasing the strength of the steel sheet. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.020% or more.

[0073] Ni: below 0.50%

[0074] When the Ni content exceeds 0.50%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate clearance range for hole expansion deformation. Therefore, when Ni is added, its content is set to 0.50% or less. The Ni content is preferably 0.45% or less, and more preferably 0.30% or less. On the other hand, Ni stabilizes austenite during the cooling process of continuous annealing, suppressing ferrite formation and thus increasing the strength of the steel sheet. To achieve this effect, the Ni content is preferably 0.01% or more, and more preferably 0.02% or more.

[0075] Sb: below 0.200%

[0076] When the Sb content exceeds 0.200%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate clearance range for hole expansion deformation. Therefore, when adding Sb, its content is set to 0.200% or less. The Sb content is preferably 0.100% or less. The Sb content is more preferably 0.050% or less. On the other hand, Sb inhibits the formation of surface softening, increasing the strength of the steel sheet. To achieve this effect, the Sb content is preferably 0.001% or more. The Sb content is more preferably 0.005% or more.

[0077] Sn: below 0.200%

[0078] When the Sn content exceeds 0.200%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate clearance range for hole expansion deformation. Therefore, when Sn is added, its content is set to 0.200% or less. The Sn content is preferably 0.100% or less. The Sn content is more preferably 0.050% or less. On the other hand, Sn inhibits the formation of surface softening, thereby increasing the strength of the steel plate. To achieve this effect, the Sn content is preferably 0.001% or more. The Sn content is more preferably 0.005% or more.

[0079] As: below 0.100%

[0080] When the content of each element (As) exceeds 0.100%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate clearance range for hole expansion deformation. Therefore, when As is added, its content is set to 0.100% or less. The As content is preferably 0.060% or less. The As content is more preferably 0.010% or less. As increases the strength of the steel plate. To achieve this effect, the As content is preferably 0.001% or more. The As content is more preferably 0.005% or more.

[0081] Ta: below 0.100%

[0082] When the Ta content exceeds 0.100%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate clearance range for hole expansion deformation. Therefore, when Ta is added, its content is set to 0.100% or less. The Ta content is preferably 0.050% or less. The Ta content is more preferably 0.010% or less. On the other hand, Ta increases the strength of the steel sheet. To achieve this effect, the Ta content is preferably 0.001% or more. The Ta content is more preferably 0.005% or more.

[0083] Ca: below 0.0200%

[0084] When the Ca content exceeds 0.0200%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate gap range for hole expansion deformation. Therefore, when Ca is added, its content is set to 0.0200% or less. The Ca content is preferably 0.0100% or less. On the other hand, Ca is an element used for deoxidation and is effective in shaping sulfides into spherical forms, improving the ultimate deformation capacity of steel plates, and increasing the appropriate gap range for delayed fracture. To achieve such effects, the Ca content is preferably 0.0001% or more.

[0085] Mg: below 0.0200%

[0086] When the Mg content exceeds 0.0200%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate gap range for hole expansion deformation. Therefore, when Mg is added, its content is set to 0.0200% or less. On the other hand, Mg is an element used for deoxidation and is effective in shaping sulfides into spherical forms, improving the ultimate deformation capacity of steel plates, and increasing the appropriate gap range for delayed fracture. To achieve such effects, the Mg content is preferably 0.0001% or more.

[0087] Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%

[0088] When the contents of Zn, Co, and Zr each exceed 0.020%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate gap range for hole expansion deformation. Therefore, when adding Zn, Co, and Zr, their contents are each set to 0.020% or less. On the other hand, Zn, Co, and Zr are all effective elements for spherizing inclusions, improving the ultimate deformation capacity of steel plates, and increasing the appropriate gap range for delayed fracture. To achieve such effects, the contents of Zn, Co, and Zr are preferably 0.0001% or more.

[0089] REM: below 0.0200%

[0090] When the REM content exceeds 0.0200%, a large number of coarse precipitates and inclusions are generated, reducing the steel's ultimate deformation capacity and thus decreasing the appropriate gap range for hole expansion deformation. Therefore, when adding REM, its content is set to 0.0200% or less. On the other hand, REM is an effective element for spherizing inclusions, improving the ultimate deformation capacity of steel plates, and increasing the appropriate gap range for delayed fracture. To achieve such effects, the REM content is preferably 0.0001% or more.

[0091] The balance other than the above-mentioned components is Fe and unavoidable impurities. It should be noted that for the above-mentioned optional components, the effect of the present invention is not impaired if the content is below the lower limit value. Therefore, when these optional elements are contained in amounts below the lower limit value, these optional elements are included as unavoidable impurities.

[0092] Next, the steel structure of the high-strength steel plate of the present invention will be described.

[0093] Tempered martensite: ≥85% by area fraction

[0094] In this invention, this is an extremely important inventive condition. By using martensite as the main phase, a strength (TS) of 1320 MPa or higher can be achieved. To obtain this effect, it is necessary to have tempered martensite of 85% or more in area fraction. Therefore, the tempered martensite is set to 85% or more in area fraction. Preferably, the tempered martensite is 90% or more in area fraction. More preferably, it is 92% or more in area fraction. Even more preferably, it is 95% or more. On the other hand, there is no particular upper limit, and the tempered martensite can be 100% in area fraction.

[0095] The method for determining tempered martensite is as follows: After grinding the L-section of the steel plate, it was etched with a 3% (v / v) nitric acid ethanol solution. Ten fields of view were observed at 2000x magnification on the 1 / 4 section of the plate (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate). It should be noted that in the above microstructure images, tempered martensite is a microstructure with fine internal irregularities and contains carbides. The tempered martensite can be determined by averaging these values.

[0096] Retained austenite: less than 5% by volume fraction

[0097] In this invention, this is an extremely important inventive condition. When the retained austenite content is 5% or more by volume, it is difficult to achieve a YR content of 85% or more. The reason for the decrease in YR is that the increase in retained austenite causes a decrease in YS due to the processing-induced phase transformation of the retained austenite. Therefore, the retained austenite content is set to be less than 5%. Preferably, it is set to 4% or less. It should be noted that there is no particular limitation on the lower limit of retained austenite content; the lower the retained austenite content, the more preferred, and it can be 0%.

[0098] The method for determining retained austenite is as follows. Retained austenite is determined as follows: After grinding the steel plate to a surface of 0.1 mm from 1 / 4 of its thickness, it is further ground to 0.1 mm using chemical grinding. For the resulting surface, the integral intensity ratios of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron are measured using CoKα rays with an X-ray diffraction apparatus. The nine integral intensity ratios are then averaged to determine the retained austenite.

[0099] The total of ferrite and bainitic ferrite: less than 10% in terms of area fraction.

[0100] In this invention, this is an extremely important inventive condition. When the total content of ferrite and bainitic ferrite exceeds 10%, it is difficult to achieve a yield strength (TS) of 1320 MPa or higher, and it is also difficult to achieve a yield strength (YR) of 85% or higher. The reason for the reduced YR is that, since ferrite and bainitic ferrite are soft structures, early yielding occurs. Therefore, the total content of ferrite and bainitic ferrite is set to 10% or less. Preferably, it is set to 8% or less. More preferably, it is set to 5% or less. It should be noted that there is no particular limitation on the lower limit of the total content of ferrite and bainitic ferrite; the lower the content, the more preferred. The lower limit of the total content of ferrite and bainitic ferrite can be 0%.

[0101] The method for determining the total amount of ferrite and bainitic ferrite is as follows: After grinding the L-section of the steel plate, it was etched with a 3% (v / v) nitric acid ethanol solution. Ten fields of view were observed at 2000x magnification at the 1 / 4-thickness section of the plate (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface). It should be noted that in the above microstructure images, the ferrite and bainitic ferrite are concave structures with flat internal structures. The total amount of ferrite and bainitic ferrite can be calculated from the average of these values.

[0102] In addition to all the aforementioned structures, pearlite, fresh martensite, and acicular ferrite were considered. These structures, as long as they do not exceed 5%, will not affect the properties and are therefore permissible.

[0103] KAM(S) / KAM(C) < 1.00

[0104] KAM(S) represents the KAM (core average orientation difference) value of the surface layer of the steel plate, and KAM(C) represents the KAM value of the center layer of the steel plate.

[0105] In this invention, this is an extremely important inventive condition. The surface portion of the steel plate refers to a position 100 μm away from the surface of the steel plate towards the center of the plate thickness. The center of the steel plate refers to the position at half the plate thickness. The inventors' research confirmed that, in order to improve YR and the appropriate gap range for delayed fracture, and to change the dislocation distribution state from the surface portion to the interior, it is effective to have KAM(S) / KAM(C) less than 1.00. Therefore, KAM(S) / KAM(C) is set to be less than 1.00. It should be noted that the lower limit of KAM(S) / KAM(C) is not particularly limited, but due to limitations in production technology, it is preferable to set it to 0.80 or higher.

[0106] The method for determining the KAM value is as follows. First, a test piece for microstructure observation is cut from a cold-rolled steel sheet. Next, the cut test piece is ground using colloidal silica vibration grinding with the rolling direction section (L-section) as the observation surface, making the observation surface mirror-like. Then, electron backscatter diffraction (EBSD) is performed to obtain local crystal orientation data. At this point, the SEM magnification is set to 3000x, the step size to 0.05μm, the measurement area to 20 square μm, and the WD to 15mm. The OIM Analysis7 software is used to analyze the obtained local orientation data. The analysis involves 10 fields of view for each thickness of the target sheet, and the average value is used.

[0107] Before data analysis, a cleaning process was performed sequentially using the Grain Dilation function (grain tolerance angle: 5, minimum grain size: 2, single iteration: on) and the Grain CI Standardization function (grain tolerance angle: 5, minimum grain size: 5) of the analysis software. Then, only measurement points with CI values ​​> 0.1 were used for analysis. A graph of KAM values ​​is shown, and the average KAM value of the bcc phase was calculated. The analysis was performed under the following conditions.

[0108] Nearest neighbor: 1st

[0109] Maximum misorientation: 5

[0110] Perimeter only

[0111] Check the box to set 0-point kernels to maximum misorientation

[0112] Hv(Q)-Hv(S)≥8

[0113] Hv(Q) represents the hardness of the plate at 1 / 4 of its thickness, and Hv(S) represents the hardness of the surface layer of the steel plate.

[0114] In this invention, this is an extremely important inventive condition. The surface layer of the steel plate refers to the position 100 μm away from the surface of the steel plate towards the center of the plate thickness. The inventors' research confirmed that, in order to improve YR (Yellow Ratio), to achieve an appropriate gap range for delayed fracture, and to allow the hardness to change from the surface layer to the interior, it is effective to have Hv(Q)-Hv(S) of 8 or higher. Therefore, Hv(Q)-Hv(S) is set to 8 or higher. There is no particular upper limit to Hv(Q)-Hv(S), but due to limitations in production technology, it is preferable to set it to 30 or lower. It should be noted that the preferred ranges for Hv(Q) and Hv(S) are 400–600 and 400–600, respectively.

[0115] The method for determining hardness is as follows: First, test pieces for microstructure observation are cut from cold-rolled steel sheets. Next, the cut test pieces are ground with the rolling direction section (L-section) as the observation surface until it becomes mirror-like. Then, the hardness is determined using a Vickers hardness tester with a 1 kg load. For the target sheet thickness, 10 points are measured at 20 μm intervals, and the average value of the 8 points excluding the maximum and minimum hardness is used.

[0116] Next, the manufacturing method of the present invention will be described.

[0117] In this invention, the smelting method for the steel raw material (steel billet) is not particularly limited, and known smelting methods such as converters and electric furnaces are suitable. To prevent macroscopic segregation, the steel billet is preferably manufactured by continuous casting.

[0118] In this invention, the billet heating temperature, billet soaking time, and coiling temperature during hot rolling are not particularly limited. Examples of methods for hot rolling billets include methods that involve heating the billet before rolling, methods that involve rolling the billet directly without heating it after continuous casting, and methods that involve subjecting the continuously cast billet to a short-time heat treatment before rolling. The billet heating temperature, billet soaking time, finishing rolling temperature, and coiling temperature during hot rolling are not particularly limited. The billet heating temperature is preferably 1100°C or higher. The billet heating temperature is preferably 1300°C or lower. The billet soaking time is preferably 30 minutes or higher. The billet soaking time is preferably 250 minutes or lower. The finishing rolling temperature is preferably above the Ar3 phase transformation point. Furthermore, the coiling temperature is preferably 350°C or higher. The coiling temperature is preferably 650°C or lower.

[0119] The hot-rolled steel sheets manufactured in this way are pickled. Pickling removes oxides from the surface of the steel sheet, so it is important to ensure good chemical conversion properties and coating quality of the final high-strength steel sheet. Pickling can be done in one step or in multiple steps. Alternatively, cold rolling can be performed while the hot-rolled and pickled sheet is still in the same state, or cold rolling can be performed after heat treatment.

[0120] The reduction rate in cold rolling and the thickness of the rolled plate are not particularly limited, but the reduction rate in cold rolling is preferably 30% or more. The reduction rate in cold rolling is preferably set to 80% or less. It should be noted that the number of rolling passes and the reduction rate for each pass can be adjusted without particular limitation to achieve the desired effect.

[0121] The cold-rolled steel sheet obtained as described above is then annealed. The annealing conditions are as follows.

[0122] Annealing temperature T1: 850℃ or higher and 1000℃ or lower

[0123] In this invention, this is an extremely important inventive condition. When the annealing temperature T1 is below 850°C, the combined area fraction of ferrite and bainitic ferrite exceeds 10%, making it difficult to achieve a TS of 1320 MPa or higher, and also difficult to achieve a YR of 85% or higher. Therefore, the annealing temperature T1 is set to 850°C or higher. T1 is preferably 860°C or higher. T1 is more preferably 870°C or higher. On the other hand, when the annealing temperature T1 exceeds 1000°C, the original austenite grain size increases excessively, reducing the appropriate gap range for delayed fracture. Therefore, the annealing temperature T1 is set to 1000°C or lower. Annealing temperature T1 is preferably 970°C or lower. T1 is more preferably 950°C or lower.

[0124] Holding time t1 at annealing temperature T1: ≥10 seconds and ≤1000 seconds

[0125] In this invention, this is an extremely important inventive condition. When the holding time t1 at the annealing temperature T1 is less than 10 seconds, austenitization is insufficient, and the total area fraction of ferrite and bainitic ferrite exceeds 10%, making it difficult to achieve a TS of 1320 MPa or more, and also difficult to achieve a YR of 85% or more. Therefore, the holding time t1 at the annealing temperature T1 is set to 10 seconds or more. The holding time t1 at the annealing temperature T1 is preferably 30 seconds or more. t1 is more preferably 45 seconds or more. t1 is further preferably 60 seconds or more. t1 is most preferably 100 seconds or more. On the other hand, when the holding time at the annealing temperature T1 exceeds 1000 seconds, the original austenite grain size increases excessively, reducing the appropriate gap range for delayed fracture. Therefore, the holding time t1 at the annealing temperature T1 is set to 1000 seconds or less. The holding time t1 at the annealing temperature T1 is preferably 800 seconds or less. t1 is more preferably 500 seconds or less.

[0126] After annealing, cool to below 100°C

[0127] In the cooling process below 100°C, the austenite undergoes a martensitic transformation. To obtain more than 85% martensite, it is necessary to cool to below 100°C after annealing. Therefore, cooling to below 100°C after annealing is required. There is no particular limitation on the lower limit of the cooling end temperature, but due to limitations in production technology, it is preferably above 0°C.

[0128] The elapsed time t2 from the moment the temperature reaches 100°C to the start of processing: less than 1000 seconds.

[0129] In this invention, this is an extremely important inventive condition. When the elapsed time t2 from the moment 100°C is reached to the start of processing exceeds 1000 seconds, the martensitic structure ages, and the strain introduced into the surface and center of the steel plate due to processing changes. Therefore, KAM(S) / KAM(C) is 1.00 or more, and YR and the appropriate gap range for delayed fracture decrease. Therefore, the elapsed time t2 from the moment 100°C is reached to the start of processing is set to 1000 seconds or less. The elapsed time t2 from the moment 100°C is reached to the start of processing is preferably 900 seconds or less. t2 is more preferably 800 seconds or less. It should be noted that the lower limit of the elapsed time t2 from the moment 100°C is reached to the start of processing is not particularly limited, but due to limitations in production technology, it is preferably 5 seconds or more. It should be noted that the inventors' research results found that the elapsed time from the moment 100°C is reached to the end of processing does not affect the strain introduced into the surface and center of the steel plate by processing.

[0130] The processing start temperature T2 is below 80℃

[0131] In this invention, this is an extremely important inventive condition. When the processing start temperature T2 exceeds 80°C, the steel sheet becomes soft, and therefore the strain change introduced by processing to the surface and center of the steel sheet results in a KAM(S) / KAM(C) ratio of 1.00 or higher, reducing YR and the appropriate gap range for delayed fracture. Therefore, the processing start temperature T2 is set to 80°C or lower. Preferably, the processing start temperature T2 is 60°C or lower. More preferably, T2 is 50°C or lower. It should be noted that the lower limit of the processing start temperature T2 is not particularly limited, but due to limitations in production technology, it is preferably 0°C or higher.

[0132] Equivalent plastic strain: ≥0.10% and ≤5.00%

[0133] In this invention, this is an extremely important inventive condition. When the equivalent plastic strain is less than 0.10%, the processing amount is insufficient, KAM(S) / KAM(C) is 1.00 or more, and the range of YR and appropriate gap for delayed fracture decreases. Therefore, the equivalent plastic strain is set to 0.10% or more. The equivalent plastic strain is preferably 0.15% or more. The equivalent plastic strain is more preferably 0.20% or more. When the equivalent plastic strain exceeds 5.00%, the effect of processing is equal in the surface and center portions of the steel plate, KAM(S) / KAM(C) is 1.00 or more, and the range of YR and appropriate gap for delayed fracture decreases. It should be noted that, due to limitations in production technology, the upper limit of the equivalent plastic strain is set to 5.00% or less. Therefore, the equivalent plastic strain is set to 5.00% or less. The equivalent plastic strain is preferably 4.00% or less. The equivalent plastic strain is more preferably 2.00% or less. The equivalent plastic strain is further preferably 1.00% or less.

[0134] In the above-mentioned pre-tempering processing steps, the strain imparting based on the processing is performed in two or more separate processes, preferably when the total equivalent plastic strain of each processing step is 0.10% or more.

[0135] Even if the equivalent plastic strain from the first processing is less than 0.10%, but the total equivalent plastic strain from subsequent processing is 0.10% or more, KAM(S) / KAM(C) is less than 1.00, and YR and the appropriate gap range for delayed fracture are improved. Therefore, strain imparting based on processing can be performed in two or more processing steps before tempering, as long as the total equivalent plastic strain from each processing is 0.10% or more. It should be noted that there is no particular limitation on the time from reaching 100°C to the start of the second or subsequent processing. This is because the first processing reduces the mobility of dislocations in the martensite.

[0136] Representative processing methods mentioned above include surface rolling and tension leveling. The equivalent plastic strain in surface rolling is the elongation of the steel sheet, which can be determined from the length change of the steel sheet before and after processing. The equivalent plastic strain of the steel sheet during leveling is calculated using the method described in Reference 1. In the calculation, the following data input values ​​are used, assuming the work hardening behavior of the material is a linearly hardened elasto-plastic body, ignoring Walton hardening and the tension reduction caused by bending losses. Furthermore, the Misaka method is used as the processing curvature formula.

[0137] Plate thickness division number: 31

[0138] Young's modulus: 21000 kgf / mm 2

[0139] Poisson's ratio: 0.3

[0140] Yield stress: 111 kgf / mm 2

[0141] Plasticity coefficient: 1757 kgf / mm 2

[0142] [Reference 1] Misaka Keisuke, Masui Ken: Plasticity and Processing (Plasticity and Processing), 17 (1976), 988.

[0143] It should be noted that the above processing can be any general strain application method other than those mentioned above, such as continuous stretching leveling or roller leveling.

[0144] Tempering temperature T3: Above 100℃ and below 400℃

[0145] In this invention, this is an extremely important inventive condition. When the tempering temperature T3 is below 100°C, the carbon diffusion distance is short, therefore, the hardness of the steel plate surface and interior decreases, Hv(Q)-Hv(S) is less than 8, and YR and the appropriate gap range for delayed fracture are reduced. Therefore, the tempering temperature T3 is set to 100°C or higher. The tempering temperature T3 is preferably 150°C or higher. T3 is more preferably 170°C or higher. T3 is even more preferably 200°C or higher. On the other hand, when the tempering temperature T3 exceeds 400°C, the tempering of martensite occurs, making it difficult to achieve a TS of 1320 MPa or higher. Therefore, the tempering temperature T3 is set to 400°C or lower. The tempering temperature T3 is preferably 350°C or lower. T3 is more preferably 300°C or lower. T3 is even more preferably 280°C or lower.

[0146] Holding time t3 at tempering temperature T3: greater than 1.0 second and less than 1000.0 seconds

[0147] In this invention, this is an extremely important inventive condition. When the holding time t3 at the tempering temperature T3 is less than 1.0 second, the carbon diffusion distance is short, therefore, the hardness of the steel plate surface and interior decreases, Hv(Q)-Hv(S) is less than 8, and YR and the appropriate gap range for delayed fracture are reduced. Therefore, the holding time t3 at the tempering temperature T3 is set to 1.0 second or more. The holding time t3 at the tempering temperature T3 is preferably 5.0 seconds or more. t3 is more preferably 50.0 seconds or more. t3 is even more preferably 100.0 seconds or more. On the other hand, when the holding time t3 at the tempering temperature T3 exceeds 1000.0 seconds, the tempering of martensite occurs, making it difficult to achieve a TS of 1320 MPa or more. Therefore, the holding time t3 at the tempering temperature T3 is set to 1000.0 seconds or less. The holding time t3 at the tempering temperature T3 is preferably 800.0 seconds or less. t3 is more preferably 600.0 seconds or less. t3 is further preferably below 500.0 seconds.

[0148] Cooling rate θ1 from tempering temperature T3 to 80°C: below 100°C / second

[0149] In this invention, this is an extremely important inventive condition. When the cooling rate θ1 from the tempering temperature T3 to 80°C exceeds 100°C / second, the carbon diffusion distance is short. Therefore, the hardness of the steel plate surface and interior decreases, Hv(Q)-Hv(S) is less than 8, and YR and the appropriate gap range for delayed fracture are reduced. Therefore, the cooling rate θ1 from the tempering temperature T3 to 80°C is set to 100°C / second or less. The cooling rate θ1 from the tempering temperature T3 to 80°C is preferably 50°C / second or less. It should be noted that the lower limit of the cooling rate θ1 from the tempering temperature T3 to 80°C is not particularly limited, but due to limitations in production technology, it is preferable to set it to 10°C / second or more.

[0150] Cooling below 80°C does not require special specifications; any method can be used to cool to the desired temperature. It should be noted that the desired temperature is preferably approximately room temperature.

[0151] Alternatively, the high-strength steel plate can be further processed to achieve an equivalent plastic strain of 0.10% to 5.00%. The processing to achieve the target equivalent plastic strain can be performed in one go or in multiple stages.

[0152] It should be noted that when high-strength steel plates become the subject of transactions, they are usually cooled to room temperature before they become the subject of transactions.

[0153] High-strength steel sheets can be plated during or after annealing. "During annealing" refers to the process from the end of the annealing temperature T1 (holding time t1) to the end of the tempering temperature T3 (holding time t3), followed by cooling to room temperature. "After annealing" refers to the process after cooling to room temperature.

[0154] As a plating treatment during annealing, examples include hot-dip galvanizing after holding at annealing temperature T1 and cooling to below 100°C, and alloying after hot-dip galvanizing. Alternatively, as a plating treatment after annealing, examples include Zn-Ni electroplating alloying or pure Zn electroplating after holding at tempering temperature T3 for t3 and cooling to room temperature. Electroplating can form a coating, or hot-dip galvanizing-aluminum-magnesium alloying can be performed. It should be noted that the above plating treatments are described primarily in the case of zinc plating; the type of plating metal, such as Zn or Al, is not particularly limited. Other manufacturing methods are not particularly limited; from a productivity point of view, the series of treatments including annealing, hot-dip galvanizing, and zinc alloying are preferably performed using a CGL (Continuous Galvanizing Line) as a hot-dip galvanizing line. After hot-dip galvanizing, wiping can be performed to adjust the weight per unit area of ​​the coating. It should be noted that conditions for plating other than those described above can follow conventional hot-dip galvanizing methods.

[0155] Processing can be performed again during or after annealing and plating, under conditions where the equivalent plastic strain is 0.10% or more and 5.00% or less. Alternatively, processing to achieve the target equivalent plastic strain can be performed in one go or in multiple stages.

[0156] Example

[0157] Steel with the composition shown in Tables 1-1 and 1-2, with the balance being Fe and unavoidable impurities, is smelted in a converter and continuously cast into billets. The resulting billets are then heated, hot-rolled, pickled, and then cold-rolled. The billets undergo annealing, machining, and tempering treatments as shown in Tables 2-1, 2-2, and 2-3 to obtain high-strength cold-rolled steel sheets with a thickness of 0.6–2.2 mm. It should be noted that some steel sheets are manufactured by plating after annealing.

[0158] In Examples No. 77, 82, 85, 88, and 91, the steel billets fractured during the casting process, thus the tests were interrupted.

[0159] The high-strength cold-rolled steel sheet obtained as described above was used as the test steel, and its tensile properties and resistance to delayed fracture were evaluated according to the following test methods.

[0160] (Organizational Observation)

[0161] Using the method described above, calculate the total of the tempered martensite area fraction, the retained austenite volume fraction, the ferrite area fraction, and the bainitic ferrite area fraction.

[0162] (KAM value)

[0163] Using the method described above, the KAM values ​​of the surface layer and the center layer of the steel plate are calculated.

[0164] (Hardness test)

[0165] Using the method described above, the hardness of the 1 / 4 section of the plate thickness and the hardness of the surface layer of the steel plate are determined.

[0166] (Tension test)

[0167] The tensile test was conducted as follows: JIS No. 5 test pieces (marking distance 50 mm, parallel section width 25 mm) were cut with the length direction perpendicular to the rolling direction as the test piece, and the test was performed according to JIS Z 2241. The crosshead speed was 1.67 × 10⁻⁶. -1 Tensile tests were conducted under conditions of mm / s to determine YS and TS. It should be noted that in this invention, a TS of 1320 MPa or higher is considered acceptable. A yield ratio (YR) of 85% or higher is considered acceptable. It should be noted that YR is calculated using the following formula (3).

[0168] YR=100×YS / TS……(3)

[0169] (Appropriate gap range for delayed fracture)

[0170] The appropriate gap range for delayed fracture was determined using the following method. Test specimens were prepared by cutting pieces 16mm × 75mm in length along the direction perpendicular to the rolling direction. The shearing angle was uniformly 0°, and the shear gap varied at 5%, 10%, 15%, 20%, 25%, 30%, and 35%. Four-point bending was performed according to ASTM (G39-99), with a stress of 1000 MPa applied at the bend apex. The stressed test specimens were then immersed in hydrochloric acid at pH 3 at 25°C for 100 hours. Specimens with a shear gap range of less than 10% without cracking were rated "×"; specimens with a shear gap range of more than 10% but less than 15% without cracking were rated "○"; specimens with a shear gap range of more than 15% without cracking were rated "◎"; and specimens with a shear gap range of more than 10% without cracking were considered to have an excellent appropriate gap range for delayed fracture.

[0171] As shown in Tables 3-1, 3-2, and 3-3, in the examples of the present invention, TS is 1320 MPa or more, YR is 85% or more, and the appropriate gap range for delayed fracture is excellent. On the other hand, in the comparative examples, TS, YR, or any one of the appropriate gap ranges for delayed fracture is poor or worse.

[0172]

[0173]

[0174]

[0175]

[0176]

[0177]

[0178]

[0179]

Claims

1. A high-strength steel plate, comprising: The composition, by mass%, includes: C ≥ 0.15% and ≤ 0.45%, Si ≥ 0.10% and ≤ 2.00%, Mn ≥ 0.5% and ≤ 3.5%, P ≤ 0.100%, S ≤ 0.0200%, Al ≥ 0.010% and ≤ 1.000%, N ≤ 0.0100%, H ≤ 0.0020%, with the balance being Fe and unavoidable impurities; and The microstructure is characterized by tempered martensite comprising 85% or more in area fraction, retained austenite comprising less than 5% in volume fraction, and the total amount of ferrite and bainitic ferrite comprising less than 10% in area fraction, and satisfying the formulas specified in (1) and (2) below. KAM(S) / KAM(C)<1.00……(1) Here, KAM(S) represents the KAM value of the surface layer of the steel plate, and KAM(C) represents the KAM value of the center layer of the steel plate. KAM is the core average orientation difference. The surface layer of the steel plate refers to the position 100 μm away from the surface of the steel plate towards the center of the plate thickness. Hv(Q)-Hv(S)≥8 ……(2) Here, Hv(Q) represents the hardness of the 1 / 4 section of the plate thickness, and Hv(S) represents the hardness of the surface section of the steel plate, which refers to the position 100 μm away from the surface of the steel plate towards the center of the plate thickness.

2. The high-strength steel plate according to claim 1, wherein, As a component, it also contains, by mass%, one or more elements selected from Ti: less than 0.100%, B: less than 0.0100%, Nb: less than 0.100%, Cu: less than 1.00%, Cr: less than 1.00%, V: less than 0.100%, Mo: less than 0.500%, Ni: less than 0.50%, Sb: less than 0.200%, Sn: less than 0.200%, As: less than 0.100%, Ta: less than 0.100%, Ca: less than 0.0200%, Mg: less than 0.0200%, Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%, and REM: less than 0.0200%.

3. The high-strength steel plate according to claim 1 or 2, wherein, The steel plate has a coating.

4. A method for manufacturing a high-strength steel plate, which is the method for manufacturing a high-strength steel plate according to claim 1 or 2, wherein, Cold-rolled steel sheets, produced by hot rolling, pickling, and cold rolling of steel billets, are annealed at a temperature T1 of 850°C or higher and 1000°C or lower, and at a holding time t1 of 10 seconds or higher and 1000 seconds or lower. Then, cool to below 100°C. Processing begins when the temperature reaches 100°C and the elapsed time t2 is less than 1000 seconds. The starting temperature T2 of the process is below 80°C. Processing is carried out under conditions where the equivalent plastic strain is greater than 0.10% and less than 5.00%. Then, tempering is performed under the conditions of a temperature T3 of 100°C or higher and 400°C or lower, and a holding time t3 at T3 of 1.0 second or higher and 1000.0 seconds or lower. Cooling is performed at a cooling rate θ1 of less than 100°C / second from T3 to 80°C.

5. The method for manufacturing high-strength steel plate according to claim 4, wherein, The processing steps prior to tempering are performed in two or more processing-based strain application processes, with the total equivalent plastic strain of each processing step being 0.10% or more.

6. The method for manufacturing high-strength steel plate according to claim 4 or 5, wherein, The plating process is carried out during or after annealing.

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