High strength steel plate and method for manufacturing the same

Abstract

The object of the present invention is to provide a cold-rolled steel sheet or a plated steel sheet which has high strength and good formability, and which has excellent resistance to delayed fracture at locations where stress generated by deformation during press working of the steel sheet is distributed in a complex manner, and a method for manufacturing the same. It has a specified chemical composition, the total area ratio of tempered martensite and bainite is 55.0% or more and 100.0% or less, the area ratio of fresh martensite is 0% or more and 25.0% or less, the area ratio of the residual structure containing at least one of ferrite, pearlite and retained austenite is 0% or more and 20.0% or less, and the total amount of Nb present in a solid solution state among the Nb contained in the steel is 0.005% or more, and the amount of Nb present as precipitates of 100 nm or more (Nb 100 ) and the total amount of Nb contained in the steel (Nb), the ratio (Nb 100 A high-strength steel plate having a structure in which the Si content (Si / Nb) is 0.25 or less.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a manufacturing method thereof, and more particularly to a high-strength steel sheet that is suitable for use in industrial fields such as automobiles and electrical machinery, which are formed by cold pressing, and has excellent press workability and delayed fracture resistance at locations where stress is distributed in a complex manner, such as protrusions formed by press working, and a manufacturing method thereof. [Background technology]

[0002] In recent years, there has been a growing need to reduce the weight of automobile bodies in order to improve fuel efficiency and protect the global environment. This has led to a demand for the application of high-strength steel sheets with a TS of 1320 MPa to body frame components.

[0003] However, as the strength of steel sheets increases, their ductility tends to decrease, making press forming into more complex shapes more difficult. Good elongation and good stretch-flangeability are sometimes required as indicators of this forming. Good stretch-flangeability is considered good when the hole expandability, which is the average value of the hole expansion ratio determined by a hole expansion test, is high.

[0004] Furthermore, when high-strength steel sheets with a TS of 1320 MPa or higher are cold-pressed to form parts, delayed fracture may occur due to increased residual stress within the part or a deterioration in the delayed fracture resistance of the steel sheet itself. Here, delayed fracture refers to a phenomenon in which, when a stressed part is placed in a hydrogen intrusion environment, hydrogen penetrates into the steel sheet, weakening interatomic bonding strength and causing localized deformation, resulting in the formation of microcracks, which then propagate and destroy the part. Furthermore, press forming involves bending in not only one direction but also two or more directions, resulting in the presence of locations with complex stress distributions. Conventional delayed fracture evaluation methods, such as low-load tests or hydrogen charging of samples subjected to unidirectional bending loads and evaluating whether cracks occur, are thought to be inadequate for assessing the delayed fracture resistance of such processed locations with complex stress distributions.

[0005] For example, Patent Document 1 discloses a technology for realizing a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent delayed fracture resistance in laser welds by strictly adjusting the chemical composition of the steel sheet and appropriately controlling the state of Ti and Nb in the cold-rolled steel sheet after annealing.

[0006] Patent Document 2 discloses a high-strength steel sheet having a tempered martensite single-phase structure or a dual-phase structure consisting of ferrite and tempered martensite. In Patent Document 2, the hardness and area fraction of the tempered martensite, the distribution of precipitates containing one or more of Nb, Ti, and Zr precipitated in the tempered martensite, and the grain size of ferrite surrounded by high-angle grain boundaries are appropriately controlled. This technology realizes a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent in both hydrogen embrittlement resistance and stretch flangeability.

[0007] Furthermore, Patent Document 3 discloses a technique for realizing high-strength steel with excellent delayed fracture resistance by specifying the prior austenite grain size and the Fe and Cr amounts contained in carbides. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-37651 [Patent Document 2] International Publication No. 2017 / 203994 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-210645 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the technology described in Patent Document 1 is limited to a strength of approximately 1180 MPa at most, and does not mention elongation. Furthermore, since the steel sheet inevitably contains TiN, it is expected that stretch flangeability will be poor. Furthermore, with regard to delayed fracture properties, the technology only mentions the delayed fracture resistance of laser welds, and it is difficult to say that the delayed fracture resistance of automotive parts manufactured by press working is sufficient.

[0010] Furthermore, the technology described in Patent Document 2 uses SSRT to measure hydrogen embrittlement resistance. However, to investigate the delayed fracture resistance of automobile parts manufactured by press working, it is necessary to investigate the influence of bending by press working. Therefore, it is difficult to determine whether the delayed fracture resistance is sufficient for automobile parts that are subjected to bending using this technology.

[0011] Furthermore, in the technology described in Patent Document 3, delayed fracture resistance is evaluated by measuring the fracture life using a cathodic charge four-point bending test on a flat test piece. However, actual pressed products are subjected to bending and deformation in multiple directions, not just one direction. Therefore, it is difficult to say that the delayed fracture resistance of actual pressed products can be evaluated using the usual four-point bending test.

[0012] Therefore, an object of the present invention is to provide a cold-rolled steel sheet or a plated steel sheet which has high strength and good formability, and which has excellent resistance to delayed fracture at a location where stress generated by deformation during press working of the steel sheet is distributed in a complex manner, and a method for manufacturing the same.

[0013] In the present invention, "high strength" refers to a TS of 1320 MPa or more, and "good formability" refers to an elongation of 10.0% or more and a hole expansion ratio of 30.0% or more. In addition, in the present invention, delayed fracture refers to the delayed fracture resistance properties at locations where stresses caused by press working are distributed in a complex manner. [Means for solving the problem]

[0014] The inventors carefully controlled the slab heating conditions, the temperature and time of finish rolling in hot rolling, the cooling rate from hot rolling to coiling, the coiling temperature, and the annealing conditions to control the state of Nb contained in the steel and evaluated the delayed fracture resistance of the steel sheet itself. As a result, it was found that Nb improves delayed fracture resistance whether it is present in a solid solution state or a precipitated state. While the reason why solute Nb improves delayed fracture resistance is unclear, it is thought that the grain boundary segregation of solute Nb improves grain boundary strength, making it less likely for microcracks to occur due to hydrogen penetration, thereby improving delayed fracture resistance. Furthermore, precipitated Nb is expected to form incoherent precipitates relative to the matrix, and these incoherent precipitates have a high hydrogen trapping ability, which is thought to be the reason for the improved delayed fracture resistance.

[0015] On the other hand, when press working is performed, stress and strain are introduced during deformation. If a large amount of coarse Nb precipitates are present, voids are likely to form at the interface between the precipitates and the steel matrix. These voids act as crack propagation paths for delayed fracture, adversely affecting delayed fracture resistance.

[0016] Therefore, in order to improve the delayed fracture resistance in areas where stress and strain caused by press working are distributed in a complex manner, it was considered important to include Nb in a solid solution or precipitated state and to create a structure in which coarse Nb precipitates do not exist more than necessary.

[0017] The present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] The composition is, in mass%, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.80% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, and Nb: 0.010% or more and 0.500% or less, with the balance being Fe and unavoidable impurities. The microstructure at the 1 / 4 position of the plate thickness is a steel in which the total area ratio of tempered martensite and bainite is 55.0% or more and 100.0% or less, the area ratio of fresh martensite is 0% or more and 25.0% or less, the area ratio of the residual structure containing at least one of ferrite, pearlite, and retained austenite is 0% or more and 20.0% or less, and the total amount of Nb present in a solid solution state among the Nb contained in the steel is 0.005% or more, and the amount of Nb present as precipitates of 100 nm or more (Nb 100 ) and the total amount of Nb contained in the steel (Nb), the ratio (Nb 100 / Nb) is 0.25 or less. [2] The high-strength steel plate according to [1], further containing, as a chemical composition, one or more selected from, by mass%, Ti: 0.200% or less, V: 0.500% 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, Co: 1.00% or less, Ni: 1.00% 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] A high-strength steel sheet according to [1] or [2], having a plating layer on the surface of the steel sheet. [4] The high-strength steel sheet according to [3], wherein the plating layer is an alloyed plating layer. [5] A method for producing a high-strength steel plate according to [1] or [2], wherein a steel material having the above-mentioned composition is heated to a temperature T sol After the slab heating process, which involves heating at ℃ or higher for 1.0 hour or more, T solA method for producing a high-strength steel sheet, comprising: a hot rolling step in which finish rolling is performed, starting at -100°C or higher and finishing at 800°C or higher; the number of rolling stands used in the finish rolling is four or more; the time required from the start of finish rolling to the end of finish rolling is 20 seconds or less; and the time required from the start of rolling in the third rolling stand to the end of finish rolling is 10 seconds or less; followed by cooling to 650°C at a cooling rate of 50°C / s or more; a coiling step in which a coiling temperature is 650°C or less; heating; a soaking temperature of 780°C to 950°C; a soaking time of 10 seconds to 600 seconds; and an annealing step in which cooling to 650°C at a cooling rate of 20°C / s or more. (Formula 1)T sol =(7900 / (3.42+(-log([Nb%][C%])))-273 Here, [Nb%] and [C%] are the amounts of Nb and C contained in the steel, respectively. [6] A method for producing a high-strength steel sheet according to [5], wherein after holding in the annealing step, a plating treatment is carried out. [7] The method for producing a high-strength steel sheet according to [6], wherein the plating treatment is an alloying plating treatment. [Effects of the Invention]

[0018] According to the present invention, it is possible to obtain a high-strength steel sheet which has high strength and good formability, and which is excellent in delayed fracture resistance at locations where stress is distributed in a complex manner, such as bulged or drawn portions formed during press working. Therefore, the present invention is of great utility in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of automobile body frame parts. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention will be specifically described below. Note that "%" representing the content of component elements means "% by mass" unless otherwise specified.

[0020] The reason why the composition of the steel in the present invention is limited to the above range will be explained below.

[0021] C: 0.030% or more and 0.500% or less Carbon (C) is an element necessary for increasing the area ratio of tempered martensite, bainite, and fresh martensite, thereby increasing strength. To fully obtain this effect, the C content must be at least 0.030%. On the other hand, if the C content exceeds 0.500%, the hardness of martensite becomes too high, and the delayed fracture resistance of the press-formed portion deteriorates. Therefore, the C content is set to 0.030% or more and 0.500% or less. The preferred lower limit is 0.050% or more, more preferably 0.070% or more. The preferred upper limit is 0.400% or less, more preferably 0.300% or less.

[0022] Si: 0.01% or more and 2.50% or less Silicon (Si) is an element that suppresses the excessive formation and growth of carbides in steel, increases the fraction of retained austenite, and improves ductility. If the Si content is less than 0.01%, this effect is reduced and good formability is not achieved, so the lower limit is set to 0.01% or more. However, if the Si content exceeds 2.50%, the amount of Si segregation increases, resulting in a decrease in delayed fracture resistance. Therefore, the Si content is set to 0.01% or more and 2.50% or less. The preferred lower limit is 0.05% or more, more preferably 0.10% or more. The preferred lower limit is 2.00% or less, more preferably 1.80% or less.

[0023] Mn: 0.80% or more and 5.00% or less Mn is an element that affects the area ratio of tempered martensite, bainite, and fresh martensite by improving hardenability. If the Mn content is less than 0.80%, soft phases such as ferrite are excessively formed, making it impossible to obtain the desired area ratio of tempered martensite, bainite, or tempered martensite, resulting in insufficient steel sheet strength. On the other hand, if the Mn content exceeds 5.00%, MnS increases, making it easier for cracks to form from the MnS, resulting in poor stretch flangeability. Therefore, the Mn content is set to 0.80% or more and 5.00% or less. The preferred lower limit is 1.00% or more, more preferably 1.20% or more. The preferred upper limit is 4.50% or less, more preferably 4.00% or less.

[0024] P:0.100% or less P may segregate at grain boundaries and cause embrittlement, which can adversely affect stretch flangeability, so its amount must be 0.100% or less. Therefore, the P content is set to 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more, and more preferably 0.003% or more.

[0025] S: 0.0200% or less S segregates at grain boundaries, embrittling steel during hot working, and may also adversely affect stretch flangeability through the formation of sulfides, so its content must be 0.0200% or less. Therefore, the S content is set to 0.0200% or less, preferably 0.0180% or less, and more preferably 0.0150% or less. There is no particular lower limit for the S content, but due to production technology constraints, it is preferably 0.0001% or more, and more preferably 0.0003% or more.

[0026] Al: 0.100% or less Al acts as a deoxidizer and is an effective element for reducing inclusions in steel, so it is preferable to add it in the deoxidation process. However, Al raises the austenitization transformation point and causes ferrite to be included in the microstructure, so if the Al content exceeds 0.100%, it becomes difficult to achieve the desired TS. Therefore, the Al content is set to 0.100% or less, preferably 0.080% or less. There is no particular lower limit for the Al content, but it is preferably 0.001% or more, and more preferably 0.005% or more.

[0027] N: 0.0100% or less N has a negative effect on ductility by forming coarse nitrides, and if the N content exceeds 0.0100%, a large amount of coarse nitrides is formed, resulting in significant deterioration of stretch flangeability. The smaller the content, the better, so the N content is set to 0.0100% or less. Preferably, it is 0.0090% or less. More preferably, it is 0.0080% or less. There is no particular lower limit for the N content, but due to constraints on production technology, it is preferably 0.0005% or more. More preferably, it is 0.0010% or more. Even more preferably, it is 0.0020% or more.

[0028] O: 0.0100% or less O exists as an oxide and reduces the stretch flangeability of steel sheets. Therefore, the O content must be 0.0100% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is set to 0.0100% or less. The preferred upper limit is 0.0050% or less.

[0029] Nb: 0.010% or more and 0.500% or less Nb is an element that improves delayed fracture resistance. To fully achieve this effect, the Nb content must be 0.010% or more. On the other hand, if the Nb content exceeds 0.500%, coarse precipitates such as Nb carbides and nitrides are formed, making new cracks more likely to occur originating from the coarse precipitates, and it is quite possible that the delayed fracture resistance and stretch flangeability of the press-formed portion will be reduced. Therefore, the Nb content is set to 0.010% or more and 0.500% or less. The preferred lower limit is 0.012% or more, more preferably 0.015% or more. The preferred upper limit is 0.400% or less, more preferably 0.350% or less.

[0030] A high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance including Fe and unavoidable impurities. Preferably, a high-strength steel sheet according to one embodiment of the present invention has a composition containing the above-mentioned components, with the balance consisting of Fe and unavoidable impurities. That is, a steel sheet according to one embodiment of the present invention preferably contains only the above-mentioned basic components and the balance, with the balance being Fe (iron) and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, and As. A total of 0.100% or less of these impurities is permitted.

[0031] In addition to the above components, the alloy may contain one or more elements selected from, by mass%, Ti: 0.200% or less, V: 0.500% 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, Co: 1.00% or less, Ni: 1.00% 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.

[0032] Ti: 0.200% or less Ti contributes to precipitation strengthening and further refines the prior austenite grain size, which in turn refines tempered martensite and bainite, thereby effectively improving steel strength. Therefore, when Ti is contained, the content is set to 0.001% or more. However, if Ti is added in an amount exceeding 0.200%, Ti may remain in an undissolved state when the steel material is heated before hot rolling, increasing the number of coarse precipitates and reducing stretch flangeability. Therefore, the Ti content is set to 0.200% or less, preferably 0.180% or less. To achieve the above-mentioned effects, the lower limit is preferably 0.001% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.

[0033] V:0.500% or less V contributes to precipitation strengthening and further refines the prior austenite grain size, which in turn refines tempered martensite and bainite, thereby effectively improving steel strength. Therefore, when V is contained, the V content is set to 0.001% or more. However, if V is added in an amount exceeding 0.500%, V may remain in an undissolved state when the steel material is heated before hot rolling, increasing the number of coarse precipitates and reducing stretch flangeability. Therefore, the V content is set to 0.500% or less, preferably 0.400% or less, and more preferably 0.350% or less. To achieve the above-mentioned effects, the lower limit is preferably 0.001% or more, and more preferably 0.005% or more.

[0034] Ta: 0.10% or less Like Ti, Ta contributes to high strength by forming alloy carbides and alloy carbonitrides. Furthermore, Ta partially dissolves in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb,Ta)(C,N). Therefore, Ta can be added as needed to significantly suppress precipitate coarsening and stabilize the contribution of precipitation strengthening to strength. Therefore, when Ta is included, its content should be 0.01% or more. However, excessive Ta addition saturates the precipitate stabilization effect and increases inclusions, causing surface and internal defects and significantly reducing ductility. Therefore, the Ta content should be 0.10% or less, preferably 0.08% or less, and more preferably 0.07% or less. To achieve the aforementioned effects, the lower limit is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0035] W: 0.10% or less W can be added as needed to improve the hardenability of steel and further improve steel strength by refining tempered martensite and bainite. Therefore, when W is contained, the W content is set to 0.01% or more. However, if the W content exceeds 0.10%, the amount of coarse precipitates such as WN and WS remaining in an undissolved state during slab heating in hot rolling may increase, resulting in poor stretch flangeability. Therefore, the W content is set to 0.10% or less, preferably 0.08% or less, and more preferably 0.07% or less. To achieve the above-mentioned effects, the lower limit is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.05% or more.

[0036] B: 0.0100% or less B is an element that can improve hardenability by segregating at austenite grain boundaries, forming a structure mainly composed of tempered martensite and bainite, and improving the strength of the steel sheet, so it can be added as needed. Therefore, when B is contained, the content is set to 0.0001% or more. However, if the content exceeds 0.0100%, coarse precipitates are formed and stretch flangeability deteriorates. Therefore, the B content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0070% or less. In order to achieve the above-mentioned effects, the preferred lower limit is 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0005% or more.

[0037] Cr:1.00% or less Cr has the effect of improving the balance between strength and ductility, so it can be added as needed. Therefore, when Cr is contained, the Cr content is set to 0.01% or more. However, if added in excess of 1.00%, the area fraction of fresh martensite becomes excessive, and stretch flangeability and ductility decrease. Therefore, the Cr content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less. In order to achieve the above-mentioned effects, the preferred lower limit is 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0038] Mo: 1.00% or less Mo has the effect of improving the balance between strength and ductility, so it can be added as needed. Therefore, when Mo is contained, the content is set to 0.01% or more. However, if added in excess of 1.00%, the area fraction of fresh martensite becomes excessive, and stretch flangeability and ductility decrease. Therefore, the Mo content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less. In order to achieve the above-mentioned effects, the preferred lower limit is 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0039] Co: 1.00% or less Co is an element effective in improving hardenability and strengthening steel, so it can be added as needed. Therefore, when Co is contained, the content is set to 0.01% or more. However, if the addition exceeds 1.00%, the area fraction of fresh martensite becomes excessively large, resulting in reduced stretch flangeability and ductility. Therefore, the Co content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.60% or less. In order to achieve the above-mentioned effects, the preferred lower limit is 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0040] Ni: 1.00% or less Ni increases the strength of steel through solid solution strengthening, so it can be added as needed. However, if added in excess of 1.00%, the area fraction of fresh martensite becomes excessive, resulting in reduced stretch flangeability and ductility. Therefore, when Ni is contained, the content is set to 0.01% or more. Therefore, the Ni content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.60% or less. In order to achieve the above-mentioned effects, the preferred lower limit is 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0041] Cu: 1.00% or less Cu is an element effective in strengthening steel and can be added as needed. Therefore, when Cu is contained, the content is set to 0.01% or more. However, if Cu is added in an amount exceeding 1.00%, the area fraction of fresh martensite becomes excessive, resulting in reduced stretch flangeability and ductility. Therefore, the Cu content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.60% or less. In order to achieve the above-mentioned effects, the preferred lower limit is 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0042] Sn: 0.200% or less, Sb: 0.200% or less Sn and Sb can be added as needed to suppress decarburization in a region of several tens of micrometers in the surface layer of the steel sheet, which occurs due to nitriding or oxidation of the steel sheet surface, prevent a decrease in the area ratio of tempered martensite on the steel sheet surface, and ensure strength. Therefore, when Sn and Sb are contained, their contents are set to 0.001% or more. However, excessive addition of either of these elements in an amount exceeding 0.200% may embrittle the steel sheet and reduce ductility. Therefore, the Sn and Sb contents are set to 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less. The preferred lower limit for achieving the above-mentioned effects is 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

[0043] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less If the content of Ca, Mg, and REM is 0.0100% or less, the amount of coarse precipitates and inclusions will not increase and the presence of Nb will not be affected, so delayed fracture resistance will not deteriorate. Therefore, the contents of Ca, Mg, and REM are preferably 0.0100% or less. Although there are no particular lower limits for the contents of Ca, Mg, and REM, because these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the contents of Ca, Mg, and REM are more preferably 0.0005% or more. Therefore, when Ca, Mg, and REM are contained, their contents should be 0.0100% or less, more preferably 0.0005% or more, and even more preferably 0.0050% or less. REM is a collective term for Sc, Y, and 15 other elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.

[0044] Zr: 0.100% or less, Te: 0.100% or less If Zr and Te are each 0.100% or less, the amount of coarse precipitates and inclusions will not increase, and they will not affect the state of Nb, so delayed fracture resistance will not deteriorate. Therefore, the Zr and Te contents are preferably 0.100% or less. Although there are no particular lower limits for the Zr and Te contents, since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, it is more preferable that the Zr and Te contents be 0.001% or more. Therefore, when Zr and Te are contained, their contents should be 0.100% or less, more preferably 0.001% or more, and even more preferably 0.080% or less.

[0045] Hf: 0.10% or less If Hf is 0.10% or less, the amount of coarse precipitates and inclusions will not increase, and the presence of Nb will not be affected, so delayed fracture resistance will not deteriorate. Therefore, the Hf content is preferably 0.10% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is more preferably 0.01% or more. Therefore, if Hf is contained, its content should be 0.10% or less, and more preferably 0.08% or less.

[0046] Bi:0.200% or less If Bi is 0.200% or less, coarse precipitates and inclusions do not increase, and the presence of Nb is not affected, so delayed fracture resistance does not deteriorate. Therefore, the Bi content is preferably 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, if Bi is contained, its content should be 0.200% or less, and more preferably 0.100% or less.

[0047] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferred lower limit, the effect of the present invention is not impaired, and therefore these elements are included as unavoidable impurities.

[0048] Next, the microstructure will be described. The method for evaluating the microstructure will be as described in the examples.

[0049] Total area ratio of tempered martensite and bainite: 55.0% to 100.0% Tempered martensite and bainite contribute to the strength of the steel sheet. Furthermore, having a steel sheet mainly comprise tempered martensite and bainite is effective in maintaining high strength. To fully achieve this effect, the sum of the area fractions of bainite and tempered martensite must be at least 55.0% or more, preferably 57.0% or more. Furthermore, the upper limit of the total area fraction of tempered martensite and bainite is 100.0%.

[0050] Area ratio of fresh martensite: 0% to 25.0% Fresh martensite is a very hard phase, improving steel strength. While fresh martensite is not necessarily required if steel sheet strength is ensured, the inclusion of fresh martensite in the steel sheet structure further improves steel strength, making it possible to achieve even higher strength. Therefore, the area fraction of fresh martensite is set to 0% or more. On the other hand, because fresh martensite is very hard, voids are likely to occur around the fresh martensite during punching, which reduces the stretch flangeability of the steel and makes it difficult to achieve good formability. Therefore, the area fraction of fresh martensite must be set to 25.0% or less. Therefore, the area fraction of fresh martensite must be set to 0% or more and 25.0% or less. The preferred lower limit is 2.0% or more. The preferred upper limit is 23.0% or less.

[0051] In the present invention, even if a residual structure such as ferrite, pearlite, or retained austenite other than tempered martensite, bainite, and fresh martensite is present, the effects of the present invention are not impaired. Therefore, the area ratio of the residual structure containing at least one of ferrite, pearlite, and retained austenite is set to 0% or more and 20.0% or less. It is more preferably 18.0% or less. It is even more preferably 15.0% or less.

[0052] The amount of Nb contained in steel that exists in solid solution is 0.005% or more If the amount of solute Nb is less than 0.005%, the grain boundary strength is not sufficiently improved by the grain boundary segregation of Nb, making it difficult to obtain good delayed fracture resistance. Therefore, the amount of Nb present in the steel as a solid solution is set to 0.005% or more. The upper limit is preferably 0.100% or less, more preferably 0.085% or less.

[0053] The amount of Nb present as precipitates of 100 nm or more (Nb 100 ) and the ratio of the total amount of Nb contained in the steel (Nb 100 / Nb) is 0.25 or less The amount of Nb contained in the coarse precipitates in the steel sheet (Nb 100 ) and the ratio of the total Nb content (Nb 100 If the Nb content (Nb) in the coarse precipitates in the steel sheet exceeds 0.25, the proportion of the coarse precipitates derived from hard Nb increases. Therefore, the presence of precipitates during press working makes it easier for voids to form. These voids become crack propagation paths for delayed fracture, deteriorating the delayed fracture resistance and also adversely affecting stretch flangeability. Therefore, the Nb content (Nb 100 ) and the ratio of the total Nb content (Nb 100 / Nb) is set to 0.25 or less, preferably 0.22 or less, and more preferably 0.20 or less.

[0054] plating layer The high-strength steel sheet of the present invention may have a plating layer on its surface. Furthermore, the plating layer may be an alloyed plating layer. An example of the plating layer is a zinc plating layer. This zinc plating layer preferably contains 0.08% to 0.30% Al. Furthermore, the effects of the present invention remain unchanged even if this zinc plating layer contains elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn in addition to Zn, Al, Mg, and Si. Furthermore, this zinc plating layer may be an alloyed zinc plating layer that has been subjected to an alloying treatment.

[0055] Next, the manufacturing conditions will be described.

[0056] [Steel slab (steel material) manufacturing process] First, a steel material having the above-described composition is melted to produce a slab (also referred to as a steel material or steel slab). The method for melting the steel material is not particularly limited, and any known melting method, such as a converter or electric furnace, can be used. The slab is preferably produced by a continuous casting method to prevent macrosegregation, but it can also be produced by an ingot casting method or a thin slab casting method. After the slab is produced, it is cooled to room temperature and then reheated, as is the conventional method. Alternatively, an energy-saving process can be applied, such as charging the hot slab into a heating furnace without cooling, or direct rolling, in which the slab is rolled immediately after a short heat retention period. Note that all temperatures are surface temperatures unless otherwise specified.

[0057] [Hot rolling process] Heating conditions for steel material (slab): Temperature T expressed by (Equation 1) sol Heat at ℃ or higher for 1.0 hours or more The heating temperature of the steel material is T expressed by the following formula (Formula 1): sol Precipitates present during the heating stage of the steel material will remain as coarse precipitates in the final steel sheet, which will have a negative effect on stretch flangeability. For this reason, it is necessary to re-dissolve as many coarse precipitates as possible during casting. solIf the heating time is less than 1.0 hour, Nb will exist as precipitates from the heating stage of the steel material. As a result, the precipitates will grow in the subsequent process, and the amount of Nb existing as coarse precipitates will increase. As a result, the amount of Nb existing as precipitates of 100 nm or more (Nb 100 ) and the ratio of the total amount of Nb contained in the steel (Nb 100 / Nb) exceeds 0.25, resulting in poor delayed fracture resistance and stretch flangeability. Although there is no particular upper limit for the heating temperature of the steel material, if it exceeds 1500°C, the amount of oxidation increases and scale loss may increase, so the heating temperature of the steel material is preferably 1500°C or less. (Formula 1)T sol =(7900 / (3.42+(-log([Nb%][C%])))-273 Here, [Nb%] and [C%] are the amounts of Nb and C contained in the steel, respectively (each unit is mass%).

[0058] Finishing rolling start temperature for hot rolling: T sol -100°C or higher After heating, the steel material is hot-rolled to produce hot-rolled steel sheets. sol If the temperature is less than -100°C, the Nb dissolved during heating of the steel material will precipitate excessively, the amount of dissolved Nb will be less than 0.005%, and the delayed fracture resistance will be deteriorated. sol -100℃ or higher. Preferably, T sol -70°C or higher. More preferably, T sol It is above -50°C.

[0059] On the other hand, the upper limit of the finish rolling start temperature is not particularly limited, but if the upper limit of the finish rolling delivery temperature exceeds 1300°C, scale loss during slab preheating may increase. In such cases, this may cause the sheet to crack during hot rolling. For this reason, the finish rolling start temperature for hot rolling is preferably 1300°C or lower, more preferably 1250°C or lower, and even more preferably 1200°C or lower.

[0060] Hot rolling finish temperature: 800°C or higher The heated steel material is hot-rolled to form a hot-rolled steel sheet. If the finishing temperature is less than 800°C, the Nb dissolved during heating of the steel material will precipitate excessively. As a result, the amount of dissolved Nb will be less than 0.005%, which will not only result in poor delayed fracture resistance but also increase the rolling load and the rolling load, which will hinder cold rolling. Therefore, the finish rolling delivery temperature in hot rolling is set to 800°C or higher, more preferably 820°C or higher, and even more preferably 850°C or higher.

[0061] Although there is no particular upper limit for the finish rolling end temperature, if the finish rolling end temperature exceeds 1100°C, the amount of oxide (scale) generated increases rapidly, the interface between the base steel and the oxide becomes rough, and the surface quality after pickling and cold rolling may deteriorate. Furthermore, the crystal grain size may become excessively coarse, which may cause the surface of the pressed product to become rough during processing. Therefore, the temperature is preferably 1100°C or lower, more preferably 1050°C or lower, and even more preferably 1000°C or lower.

[0062] The number of rolling stands used in finishing rolling is 4 or more. When the number of rolling stands used in finish rolling is less than four, the reduction ratio applied in one rolling pass increases, and a large amount of strain is introduced into the surface layer of the steel sheet. As a result, Nb acts as a nucleation site for precipitates in the subsequent process, the amount of solute Nb becomes less than 0.005%, and the delayed fracture resistance becomes poor. Therefore, the number of rolling stands used in finish rolling is set to four or more. There is no particular upper limit on the number of rolling stands used in finish rolling, but it is preferably 10 or less, more preferably 8 or less.

[0063] Time required from start to finish of finish rolling: 20 seconds or less If the time required from the start to the end of finish rolling exceeds 20 seconds, excessive precipitation of Nb-containing precipitates will result in a solute Nb content of less than 0.005%, deteriorating delayed fracture resistance. Therefore, the time required from the start to the end of finish rolling is set to 20 seconds or less, more preferably 17 seconds or less, and even more preferably 15 seconds or less. There is no particular lower limit to the required time, but due to production technology constraints, the required time is preferably 1 second or more, more preferably 2 seconds or more, and even more preferably 3 seconds or more.

[0064] The time required from the start of rolling to the end of finishing rolling in the third rolling stand is 10 seconds or less If the time required from the start of rolling to the end of finish rolling in the third rolling stand exceeds 10 seconds, excessive precipitation of Nb-containing precipitates will result in a solute Nb content of less than 0.005%, deteriorating the delayed fracture resistance. Therefore, the time required from the start of rolling to the end of finish rolling in the third rolling stand is set to 10 seconds or less, more preferably 8 seconds or less, and even more preferably 7 seconds or less. The third rolling stand means the third rolling stand from the entry side of finish rolling.

[0065] [Winding process] Cooling rate of 50°C / s or more to 650°C If the cooling rate from the end of finish rolling in the hot rolling process to 650°C is slower than 50°C / s, solute Nb begins to precipitate. As a result, the amount of solute Nb contained in the steel becomes less than 0.005%, making it impossible to obtain good delayed fracture resistance. In order to minimize the precipitation of Nb during manufacturing, the cooling rate to 650°C is set to 50°C / s or more. It is preferably 52°C / s or more, and more preferably 55°C / s or more. There is no particular upper limit to the cooling rate, but it is preferably 250°C / s or less, and more preferably 200°C / s or less.

[0066] Winding temperature: 650℃ or less When the coiling temperature after hot rolling exceeds 650°C, the growth of Nb proceeds excessively, and the amount of Nb precipitates of 100 nm or more increases. As a result, the amount of Nb present as precipitates of 100 nm or more (Nb 100 ) and the ratio of the total amount of Nb contained in the steel (Nb 100 Therefore, the coiling temperature after hot rolling is set to 650°C or lower, preferably 630°C or lower, and more preferably 600°C or lower.

[0067] Although there is no particular lower limit for the coiling temperature, if the temperature is lower than 300°C, the strength of the hot-rolled sheet increases, the rolling load in cold rolling increases, and defects in the sheet shape occur, resulting in reduced productivity. Therefore, the lower limit of the coiling temperature is preferably set to 300°C or higher, more preferably 320°C or higher, and even more preferably 340°C or higher.

[0068] The obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to treatment such as pickling, if necessary. The pickling method for the hot-rolled coil may be a conventional method. Furthermore, the hot-rolled coil may be subjected to skin-pass rolling in order to correct the shape and improve the pickling properties.

[0069] After hot rolling and / or intermediate heat treatment and / or pickling, the steel sheet may be heat-treated directly, or may be cold-rolled and then heat-treated. When cold-rolling is performed, the cold reduction is preferably 25% or more or 30% or more. On the other hand, excessive reduction increases the rolling load, leading to an increase in the load on the cold-rolling mill, so the upper limit is preferably 75% or 70%. Here, intermediate heat treatment refers to optional heating for the purpose of softening the steel sheet when it is too hard when cold-rolling is performed, and such heat treatment can also be performed according to conventional methods.

[0070] [Heating process] Soaking temperature: 780℃ or higher and 950℃ or lower When the steel is held at a temperature below 780°C, the area ratio of tempered martensite and bainite in the final structure is less than 60.0% because the steel is held in the two-phase region, and sufficient steel sheet strength cannot be ensured. On the other hand, when the steel is held at a temperature above 950°C, the growth of precipitated Nb proceeds excessively, and the amount of Nb precipitates of 100 nm or more increases. As a result, the amount of Nb present as precipitates of 100 nm or more (Nb 100 ) and the ratio of the total amount of Nb contained in the steel (Nb 100 / Nb) exceeds 0.25, resulting in poor delayed fracture resistance and stretch flangeability. Therefore, the soaking temperature is set to 780°C or higher and 950°C or lower. The lower limit is preferably 800°C or higher, more preferably 820°C or higher. The upper limit is preferably 940°C or lower, and even more preferably 920°C or lower.

[0071] Soaking time: 10s or more and 600s or less If the soaking time is less than 10 seconds, the austenitization of the steel sheet does not progress sufficiently, the area ratio of tempered martensite and bainite becomes less than 55.0%, and TS: 1320 MPa or more cannot be achieved. On the other hand, if the soaking time exceeds 600 seconds, the growth of Nb progresses excessively, and the amount of Nb precipitates increases. As a result, the amount of Nb present as precipitates of 100 nm or more (Nb 100 ) and the ratio of the total amount of Nb contained in the steel (Nb 100 / Nb) exceeds 0.25, resulting in poor delayed fracture resistance and stretch flangeability. Therefore, the soaking time is set to 10 seconds or more and 600 seconds or less. The lower limit is preferably 15 seconds or more, more preferably 20 seconds or more. The upper limit is preferably 580 seconds or less, and even more preferably 550 seconds or less.

[0072] Cooling to 650°C at a rate of 20°C / s or more If the cooling rate to 650°C is less than 20°C / s, the solute Nb contained in the steel will precipitate, the amount of solute Nb will be less than 0.005%, and the delayed fracture resistance will be poor. Therefore, the cooling rate to 650°C is set to 20°C / s or more, preferably 22°C / s or more, and more preferably 24°C / s or more. There is no particular upper limit to the cooling rate to 650°C, but due to constraints on production facilities, it is preferably 100°C / s or less, and more preferably 80°C / s or less.

[0073] The cooling conditions after this cooling step are not particularly specified, but the martensite may be self-tempered during this cooling step. Alternatively, after the cooling step is completed, the martensite may be tempered by holding the material at, for example, 200 to 450°C for 10 seconds or more. Alternatively, the martensite may be tempered by cooling to, for example, 100 to 350°C and then reheating to 200 to 450°C. In either case, it is sufficient that the martensite is tempered.

[0074] Plating The obtained high-strength steel sheet may be subjected to a plating treatment, if necessary. Here, the type of plating metal used for the plating treatment is not particularly limited, such as Zn plating or Al plating. Examples of Zn plating treatment include hot-dip galvanizing treatment and electrogalvanizing treatment. When hot-dip galvanizing treatment is performed, the steel sheet that has been subjected to the annealing treatment is immersed in a galvanizing bath at a temperature of 440°C or higher and 500°C or lower to perform the hot-dip galvanizing treatment. Thereafter, the coating weight is adjusted by gas wiping or the like.

[0075] There are no special restrictions on the plating conditions, but the plating weight (amount of plating per side) is set at 20 g / m from the viewpoint of corrosion resistance and plating weight control. 2 It is preferable that the thickness is 120 g / m or more from the viewpoint of adhesion. 2 The coating weight is preferably 25 g / m or less. 2 More preferably, it is 30 g / m or more. 2 It is even more preferable that the plating coverage is 100 g / m or more. 2It is more preferable that the density is 70 g / m or less. 2 It is even more preferred that:

[0076] It is preferable that a galvanizing bath containing 0.08% to 0.30% Al is used for hot dip galvanizing. Furthermore, the effects of the present invention remain unchanged even if the galvanizing bath contains elements other than Al, Mg, and Si, such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn.

[0077] When hot-dip galvanizing is performed, the hot-dip galvanizing treatment is followed by the hot-dip galvanizing treatment in a temperature range of 450°C to 600°C. When the hot-dip galvanizing treatment is performed at a temperature exceeding 600°C, untransformed austenite transforms to pearlite. Untransformed austenite becomes fresh martensite upon final cooling, but if the amount of pearlite transformation increases, the area ratio of the remaining structure may exceed 20.0%, which may impair the effects of the present invention. Therefore, when hot-dip galvanizing treatment is performed, it is preferable to perform the hot-dip galvanizing treatment in a temperature range of 450°C to 600°C. The Fe concentration in the coating layer of the alloyed hot-dip galvanized steel sheet is preferably 8 to 17%. [Example]

[0078] Steel having the chemical composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter and formed into a slab by continuous casting. The obtained slab was subjected to the hot rolling process, coiling process, and heating process under the conditions shown in Table 2. If reheating was performed, the slab was cooled to a cooling stop temperature of 100°C to 350°C, then reheated to a holding temperature of 200°C to 450°C, and held thereafter, before being cooled to room temperature. If reheating was not performed, high strength steel was obtained by holding the holding temperature at 200°C to 450°C for 10 seconds or more, and then cooling to room temperature. Heat-drying Steel plate ( HR) was obtained. Furthermore, hot-dip galvanizing treatment was performed to obtain hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA). For the hot-dip galvanizing bath, a zinc bath containing 0.19 mass% Al was used for the hot-dip galvanized steel sheets (GI), and a zinc bath containing 0.14 mass% Al was used for the galvannealed steel sheets (GA), and the bath temperature was 465°C. The coating weight was 45 g / m per side. 2 The GA was adjusted so that the Fe concentration in the plating layer was within the range of 9 mass % or more and 12 mass % or less.

[0079] [Table 1]

[0080] [Table 2]

[0081] [Table 3]

[0082] The area fractions of fresh martensite, tempered martensite, and bainite were determined by polishing a thickness cross section (L cross section) parallel to the rolling direction of the steel plate and then etching it with 3 vol.% nital. Next, 10 fields of view were observed at 2000x magnification using a scanning electron microscope (SEM) at a 1 / 4 thickness position (a position corresponding to 1 / 4 of the thickness in the depth direction from the steel plate surface). The obtained microstructural images were used to calculate the area fractions of each structure (the sum of tempered martensite and bainite, and fresh martensite). In addition, in the above microstructural images, fresh martensite was considered to be the light gray region of the structure, and tempered martensite and bainite were considered to be the dark gray region where carbides precipitated.

[0083] The tensile test was carried out in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece, which was sampled so that the tensile direction was perpendicular to the rolling direction of the steel sheet, and TS (tensile strength) and EL (total elongation) were measured. In the examples, those that did not satisfy the conditions of TS (tensile strength) of 1320 MPa or more and EL (total elongation) of 10.0% or more were treated as comparative examples.

[0084] The hole expansion ratio was measured in accordance with JIS Z 2256 (2010). Each steel plate was cut into a 100 mm × 100 mm piece, and a 10 mm diameter hole was punched with a clearance of 12% ± 1%. A 60° conical punch was then pressed into the hole using a die with an inner diameter of 75 mm and a blank holding force of 9 tons to measure the hole diameter at the crack initiation limit. The critical hole expansion ratio λ (%) was calculated using the following equation (3), and the hole expandability was evaluated from this critical hole expansion ratio value. In the examples, those that did not satisfy the condition of a hole expansion ratio of 30.0% or more were treated as comparative examples. (Equation 3) Limit hole expansion ratio λ (%) = {(D f -D0) / D0}×100 However, in the above formula, D f is the hole diameter (mm) when the crack occurs, and D0 is the initial hole diameter (mm).

[0085] The delayed fracture resistance of areas with complex stress distribution, such as overhangs, was evaluated using a four-point bending immersion test. Steel plates were V-bent and then flattened using a block clamp. From the flattened steel plates, samples measuring 85 mm wide x 20 mm long were sheared so that the ridge of the V-bend was parallel to the width direction and centered in the length direction. Both end faces in the width direction were then ground. Both end faces in the length direction were then milled to obtain test specimens measuring 75 mm wide x 16 mm long. Three test specimens were prepared for each steel, and these were tightened using a four-point bending jig with a dial gauge method to achieve the target load stresses equivalent to YS, TS, and TS + 200 MPa, respectively. The immersion test was conducted in a 0.1 wt.% ammonium thiocyanate aqueous solution supplemented with 50 vol.% McIlvaine buffer solution at pH 6. The test temperature was constant at 25°C, and the test time was 96 hours. After 96 hours, if no cracks were found visually on any of the stressed steel plates, it was judged as ◎, and if cracks were found at a tightening stress equivalent to TS + 200 MPa, it was judged as 〇. Furthermore, if cracks were found at a tightening stress equivalent to TS, it was judged as △, and if cracks were found at a tightening stress equivalent to YS, it was judged as ×. Here, steel plates with excellent delayed fracture resistance are those judged as ◎, ○, or △.

[0086] The amounts of dissolved Nb and Nb present as precipitates of 100 nm or more in the steel sheets were measured by the following procedure.

[0087] Several test pieces of cold-rolled or galvanized steel sheets were prepared, each approximately 20 × 50 mm in size. The galvanized steel sheets were then removed from the surface using a router (precision grinder). The surfaces of the specimens were then electrolytically polished to a depth of approximately 50 μm to obtain a fresh surface. The resulting specimens were electrolyzed using 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol. The resulting electrolyte was collected and the Nb concentration was quantified in mass% using ICP atomic emission spectrometry. This was taken as the amount of dissolved Nb. The electrolytic residues on the metal samples were then immersed in separately prepared methanol, and the residues remaining on the metal samples were collected in a container using ultrasonic vibration. The electrolytic solution and the methanol containing the residues remaining on the metal samples were then filtered using an alumina filter with a 100 nm pore size to capture residues with a particle size of 100 nm or greater. The collected residues were then acid-decomposed, and the Nb concentration was quantified in mass% using ICP atomic emission spectrometry. This is the amount of Nb present as precipitates of 100 nm or more (Nb 100 ) was decided.

[0088] The high-strength steel sheets of the invention examples all have high strength, good formability, and excellent delayed fracture resistance, while the comparative examples are inferior in at least one of strength, formability, and delayed fracture resistance.

Claims

1. The component composition is in mass%: C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.80% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, O: 0.0100% or less, Nb: 0.010% or more and 0.500% or less, The balance is composed of Fe and unavoidable impurities, The microstructure at the 1 / 4 position of the plate thickness is The total area ratio of tempered martensite and bainite is 55.0% or more and 100.0% or less, The area ratio of fresh martensite is 0% or more and 25.0% or less, The area ratio of the residual structure containing at least one of ferrite, pearlite, and retained austenite is 0% or more and 20.0% or less, The total amount of Nb present in a solid solution state among the Nb contained in the steel is 0.005% or more, The amount of Nb present as precipitates of 100 nm or more (Nb １００ ) [mass%] and the total Nb content (Nb) [mass%] contained in the steel, the ratio (Nb １００ A high-strength steel plate having a structure in which the Si content (Si / Nb) is 0.25 or less.

2. Furthermore, the component composition is as follows: Ti: 0.200% or less, V: 0.500% 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, Co: 1.00% or less, Ni: 1.00% 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, The high-strength steel plate according to claim 1, further comprising one or more selected from the following:

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

4. The high-strength steel sheet according to claim 3 , wherein the plating layer is an alloyed plating layer.

5. The method for producing a high-strength steel plate according to claim 1 or 2, The steel material having the above-mentioned composition is heated to a temperature T ｓｏｌ After performing a slab heating step of heating at ℃ or higher for 1.0 hour or more, T ｓｏｌ Finish rolling begins at -100°C or higher, Hot rolling is performed to finish the rolling at 800°C or higher. The number of rolling stands used in finishing rolling is four or more, The time required from the start of finish rolling to the end of finish rolling is 20 seconds or less, Furthermore, after performing a hot rolling process in which the time required from the start of rolling to the end of finish rolling in the third rolling stand is 10 seconds or less, Cooling is performed at a cooling rate of 50°C / s or more to 650°C, Thereafter, a winding step is performed at a winding temperature of 650°C or less, followed by heating, The soaking temperature is set to 780°C or more and 950°C or less, and the soaking time is set to 10 seconds or more and 600 seconds or less, A method for producing a high-strength steel sheet, comprising an annealing step in which cooling to 650°C is performed at a cooling rate of 20°C / s or more. (Equation 1) T ｓｏｌ =7900 / (3.42+(-log([Nb%][C%])))-273 Here, [Nb%] and [C%] are the amounts of Nb and C contained in the steel, respectively.

6. The method for producing a high strength steel sheet according to claim 5, wherein a plating treatment is carried out after the holding in the annealing step.

7. The method for producing a high-strength steel sheet according to claim 6, wherein the plating treatment is an alloying plating treatment.

Images