WO2025225091A1 - High-strength steel sheet and manufacturing method therefor - Google Patents

Abstract

The purpose 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 is excellent in delayed fracture resistance at a place where stress generated by deformation by press working of a steel sheet is distributed in a complex manner; and to provide a method for manufacturing the same.　This high-strength steel sheet is characterized in having a predetermined composition and in having a structure in which: the total area ratio of tempered martensite and bainite is 55.0% to 100.0%; the area ratio of fresh martensite is 0% to 25.0%; the area ratio of the remaining structure including at least one of ferrite, pearlite, and residual austenite is 0% to 20.0%; the total amount of Nb present as a solid solution of the Nb contained in steel is 0.005% or more; and regarding the amount of Nb (Nb₁₀₀) present as a precipitate of 100 nm or more and the total Nb amount (Nb) contained in steel, the ratio (Nb₁₀₀/Nb) is 0.25 or less.

Description

High strength steel plate and method for manufacturing the same

The present invention relates to steel sheets and manufacturing methods thereof. In particular, it relates to high-strength steel sheets and manufacturing methods that are suitable for use in industrial fields such as automobiles and electrical machinery, which are formed by cold pressing, and that have excellent press workability and delayed fracture resistance in areas where stress is distributed in a complex manner, such as protrusions created by press working.

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. Therefore, there is a demand for the use of high-strength steel sheets with a TS of 1320 MPa to auto body frame components.

However, as the strength of steel sheet increases, ductility tends to decrease, making it more difficult to press into more complex shapes. Good elongation and good stretch-flangeability are required as indicators of this forming. Stretch-flangeability is considered good when the hole expandability, which is the average value of the hole expansion ratio determined by hole expansion tests, is high.

Furthermore, when high-strength steel plate with a TS of 1,320 MPa or higher is cold-pressed to form parts, there is a risk of delayed fracture due to increased residual stress within the part and a deterioration in the delayed-fracture resistance of the steel plate 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 plate, weakening interatomic bonding strength and causing localized deformation, resulting in the formation of microcracks, which then propagate and destroy the part. Furthermore, press forming is thought to involve bending in two or more directions, rather than just one direction, 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 to determine whether cracks occur, are thought to be unable to adequately evaluate the delayed fracture properties of locations with complex stress distributions resulting from such processing.

For example, Patent Document 1 discloses a technology that achieves high-strength steel sheets with 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 sheets and appropriately controlling the state of Ti and Nb in the cold-rolled steel sheets after annealing.

Furthermore, Patent Document 2 discloses a high-strength thin 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 thin steel sheet with a tensile strength of 980 MPa or more and excellent hydrogen embrittlement resistance and stretch flangeability.

Furthermore, Patent Document 3 discloses a technology that achieves high-strength steel with excellent delayed fracture resistance by specifying the prior austenite grain size and the amounts of Fe and Cr contained in carbides.

JP 2016-37651 A International Publication No. 2017/203994 Japanese Patent Application Laid-Open No. 2017-210645

However, the technology described in Patent Document 1 is limited to a strength of approximately 1180 MPa at most, and makes no mention of 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 document 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.

Furthermore, the technology described in Patent Document 2 uses SSRT to measure hydrogen embrittlement resistance. However, in order to investigate the delayed fracture resistance of automotive parts manufactured by press working, it is necessary to investigate the effects of bending by press working. Therefore, it is difficult to determine whether this technology has sufficient delayed fracture resistance for automotive parts that require bending.

Furthermore, the technology described in Patent Document 3 evaluates delayed fracture resistance by measuring the fracture life through a cathodic charge four-point bending test using flat test specimens. However, actual press-formed products are subjected to bending and deformation in multiple directions, not just one direction. For this reason, it is difficult to say that the delayed fracture resistance of actual press-formed products can be evaluated using the usual delayed fracture evaluation method using four-point bending.

The present invention therefore aims to provide cold-rolled or plated steel sheets that have high strength and good formability, as well as excellent resistance to delayed fracture in areas where stress generated by deformation during press working of the steel sheet is distributed in a complex manner, and methods for manufacturing them.

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. Furthermore, in the present invention, "delayed fracture" refers to the delayed fracture resistance properties in areas where stresses caused by press working are distributed in a complex manner.

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 evaluate the delayed fracture resistance of the steel sheet itself. As a result, they 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 strength is improved by the segregation of solute Nb at grain boundaries, 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 such incoherent precipitates have a high hydrogen trapping ability, which is thought to be the reason for the improved delayed fracture resistance.

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 at this time, voids are likely to form starting from the interface between the precipitates and the steel sheet matrix. These voids act as crack propagation paths for delayed fracture, adversely affecting delayed fracture resistance.

Therefore, in order to improve delayed fracture resistance in areas where stress and strain caused by press working and other processes are distributed in a complex manner, it was thought that it was important to include Nb in a solid solution or precipitate state, and to create a structure in which coarse Nb precipitates are not present more than necessary.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
[1] The composition of the element 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 sheet thickness is a high-strength steel plate having a structure 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 a residual structure including 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, and the ratio ( _Nb100 /Nb) of the amount of Nb present as precipitates of 100 nm or more (Nb100) to the total amount of Nb contained in the steel (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, in 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] The high-strength steel sheet according to [1] or [2], having a plating layer on the steel sheet surface.
[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], comprising: performing a slab heating step in which a steel material having the above-mentioned composition is heated at a temperature _Tsol ° C. or higher represented by Equation 1 for 1.0 hour or more; and then performing a slab heating step in which a steel material having the above-mentioned composition is heated at a temperature Tsol ° _C. or higher represented by Equation 1 for 1.0 hour or more; A 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 ending at 800°C or higher; the number of rolling stands used in the finish rolling is 4 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; thereafter, cooling is performed at a cooling rate of 50°C/s or more to 650°C; thereafter, a coiling step in which a coiling temperature is 650°C or less; thereafter, heating is performed; a soaking temperature is set to 780°C or more and 950°C or less; a soaking time is set to 10 seconds or more and 600 seconds or less; and an annealing step in which cooling is performed at a cooling rate of 20°C/s or more to 650°C.
(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] The method for producing a high-strength steel sheet according to [5], wherein a plating treatment is carried out after holding in the annealing step.
[7] The method for producing a high-strength steel sheet according to [6], wherein the plating treatment is an alloying plating treatment.

The present invention makes it possible to obtain high-strength steel sheets that have high strength and good formability, as well as excellent resistance to delayed fracture in areas where stress is distributed in a complex manner, such as in bulged or drawn sections formed during press working. Therefore, the present invention is of great value in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of automobile body frame parts.

The present invention will now be described in detail. Note that "%" representing the content of component elements means "% by mass" unless otherwise specified.

The reasons for limiting the steel composition to the above range in this invention are explained below.

C: 0.030% or more and 0.500% or less 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% or more. 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.

Si: 0.01% or more and 2.50% or less Si is an element that suppresses the excessive formation and growth of carbides in steel, increases the retained austenite fraction, and improves ductility. If the Si content is less than 0.01%, this effect is reduced and good formability cannot be 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.

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, and 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.

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 0.100% or less. Preferably, it is 0.080% or less. More preferably, it is 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. More preferably, it is 0.003% or more.

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

Al: 0.100% or less Al acts as a deoxidizer and is an effective element for reducing inclusions in steel, and is preferably added in the deoxidation process. However, Al raises the austenitization transformation point and causes ferrite to be included in the microstructure, so a content of more than 0.100% makes it 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.

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.

O: 0.0100% or less O exists as an oxide and reduces the stretch flangeability of the steel sheet. Therefore, the O content needs to 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.

Nb: 0.010% or more and 0.500% or less Nb is an element that improves delayed fracture resistance. To fully obtain this effect, the Nb content needs to 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.

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. In other words, it is preferable that a steel sheet according to one embodiment of the present invention 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 content of 0.100% or less of these impurities is acceptable.

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.

Ti: 0.200% or less Ti contributes to precipitation strengthening and further refines the prior austenite grain size, resulting in refinement of 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 during heating of the steel material 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 preferred lower limit is 0.001% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.

V: 0.500% or less V contributes to precipitation strengthening and further refines the prior austenite grain size, resulting in refinement of tempered martensite and bainite, thereby effectively improving steel strength. Therefore, when V is contained, the 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, it is set to 0.400% or less. More preferably, it is set to 0.350% or less. The preferred lower limit for achieving the above-mentioned effects is 0.001% or more. More preferably, it is set to 0.005% or more.

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 contained, its content is set to 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 is set to 0.10% or less. Preferably, it is set to 0.08% or less. More preferably, it is set to 0.07% or less. To achieve the aforementioned effects, the preferred lower limit is 0.01% or more. More preferably, it is set to 0.02% or more. Even more preferably, it is set to 0.05% or more.

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 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, it is set to 0.08% or less. More preferably, it is set to 0.07% or less. To achieve the above-mentioned effects, the lower limit is preferably 0.01% or more. More preferably, it is set to 0.02% or more. Even more preferably, it is set to 0.05% or more.

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 steel sheet strength, 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 is reduced. 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.

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 content is set to 0.01% or more. However, if added in excess of 1.00%, the area ratio of fresh martensite becomes excessive, and stretch flangeability and ductility decrease. Therefore, the Cr content is set to 1.00% or less. Preferably, it is set to 0.80% or less. More preferably, it is set to 0.50% or less. In order to achieve the above-mentioned effects, the preferred lower limit is set to 0.01% or more. More preferably, it is set to 0.03% or more. Even more preferably, it is set to 0.05% or more.

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, it is set to 0.80% or less. More preferably, it is set to 0.50% or less. In order to achieve the above-mentioned effects, the preferred lower limit is set to 0.01% or more. More preferably, it is set to 0.03% or more. Even more preferably, it is set to 0.05% or more.

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, addition of more than 1.00% results in an excessively large area fraction of fresh martensite, resulting in reduced stretch flangeability and ductility. Therefore, the Co content is set to 1.00% or less. Preferably, it is set to 0.80% or less. More preferably, it is set to 0.60% or less. To achieve the above-mentioned effects, the preferred lower limit is set to 0.01% or more. More preferably, it is set to 0.03% or more. Even more preferably, it is set to 0.05% or more.

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, it is set to 0.80% or less. More preferably, it is set to 0.60% or less. In order to achieve the above-mentioned effects, the preferred lower limit is set to 0.01% or more. More preferably, it is set to 0.03% or more. Even more preferably, it is set to 0.05% or more.

Cu: 1.00% or less Cu is an element effective in strengthening steel, so it can be added as needed. Therefore, when Cu 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, resulting in reduced stretch flangeability and ductility. Therefore, the Cu content is set to 1.00% or less. Preferably, it is set to 0.80% or less. More preferably, it is set to 0.60% or less. In order to achieve the above-mentioned effects, the preferred lower limit is set to 0.01% or more. More preferably, it is set to 0.03% or more. Even more preferably, it is set to 0.05% or more.

Sn: 0.200% or less, Sb: 0.200% or less Sn and Sb 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 can be added as needed to 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. To achieve the above-mentioned effects, the lower limit is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more.

Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less. Ca, Mg, and REM, each present at 0.0100% or less, do not increase coarse precipitates or inclusions, do not affect the state of Nb, and thus do not degrade delayed fracture resistance. Therefore, the Ca, Mg, and REM contents are preferably each 0.0100% or less. While there are no specific lower limits for the Ca, Mg, and REM contents, these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, when Ca, Mg, and REM are contained, their contents should each 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 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.

Zr: 0.100% or less, Te: 0.100% or less If Zr and Te are each 0.100% 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 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 each be 0.100% or less, more preferably 0.001% or more, and even more preferably 0.080% or less.

Hf: 0.10% or less If Hf is 0.10% 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 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. More preferably, it should be 0.08% or less.

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, when Bi is contained, its content is 0.200% or less, and more preferably 0.100% or less.

Furthermore, the effects of the present invention are not impaired when the contents 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 are less than the preferred lower limit values. Therefore, they are considered to be included as unavoidable impurities.

Next, we will explain the microstructure. The method for evaluating the microstructure will be as described in the examples.

Total area ratio of tempered martensite and bainite: 55.0% or more and 100.0% or less 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 obtain this effect, the sum of the area ratios of bainite and tempered martensite must be at least 55.0% or more. It is preferably 57.0% or more. Furthermore, the upper limit of the total area ratio of tempered martensite and bainite is 100.0%.

Fresh martensite area fraction: 0% or more and 25.0% or less. 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 and enables 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.

In addition, in the present invention, the effects of the present invention are not impaired even if residual structures such as ferrite, pearlite, and retained austenite, excluding tempered martensite, bainite, and fresh martensite, are present. 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.

The amount of Nb present in the steel as a 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 preferred upper limit is 0.100% or less, and more preferably 0.085% or less.

The ratio ( _Nb100 /Nb) of the amount of Nb present as precipitates of 100 nm or more ( _Nb100 ) to the total amount of Nb (Nb) contained in the steel is 0.25 or less. If the ratio ( _Nb100 /Nb) of the amount of Nb contained in coarse precipitates in the steel sheet ( _Nb100 ) to the total amount of Nb (Nb) exceeds 0.25, the proportion of coarse precipitates contained in the sheet that are derived from hard Nb increases. Therefore, void formation due to the presence of precipitates during press working is likely to occur. Such voids become crack propagation paths for delayed fracture, deteriorating delayed fracture resistance and also adversely affecting stretch flangeability. Therefore, the ratio (Nb ₁₀₀ /Nb) of the amount of Nb contained in coarse precipitates in the steel sheet (Nb ₁₀₀ ) to the total amount of Nb (Nb) is set to 0.25 or less, preferably 0.22 or less, and more preferably 0.20 or less.

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.

Next, we will explain the manufacturing conditions.

[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, such as direct rolling, can be applied, in which the hot slab is charged into a heating furnace without cooling, or is rolled immediately after a short heat retention period. Note that all temperatures are surface temperatures unless otherwise specified.

[Hot rolling process]
Heating conditions for steel material (slab): Heating for 1.0 hour or more at a temperature _Tsol °C or higher, as expressed by (Equation 1). The heating temperature of the steel material is equal to or higher than _Tsol , as expressed by the following equation (Equation 1). Precipitates present at the heating stage of the steel material exist as coarse precipitates in the final steel sheet, adversely affecting stretch flangeability. For this reason, it is necessary to redissolve as many coarse precipitates as possible during casting. If the heating temperature of the steel material is below _Tsol or 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 grow in subsequent processes, and the amount of Nb present as coarse precipitates increases. As a result, the ratio ( _Nb100 /Nb) of the amount of Nb present as precipitates of 100 nm or larger ( _Nb100 ) to the total amount of Nb contained in the steel (Nb) exceeds 0.25, resulting in poor delayed fracture resistance and stretch flangeability. Although there is no particular upper limit to the heating temperature of the steel material, if the temperature exceeds 1500°C, the amount of oxidation increases and scale loss may increase. Therefore, 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%).

Finish rolling start temperature for hot rolling: _Tsol -100°C or higher The heated steel material is hot rolled to form a hot-rolled steel sheet. If the finish rolling start temperature is less than _Tsol -100°C, Nb dissolved during heating of the steel material will precipitate excessively, resulting in a solute Nb content of less than 0.005%, and the delayed fracture resistance will deteriorate. Therefore, the finish rolling start temperature is set to _Tsol -100°C or higher. Preferably, it is _Tsol -70°C or higher. More preferably, it is _Tsol -50°C or higher.

On the other hand, there is no particular upper limit to the finish rolling start temperature, 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 plate to crack during hot rolling. For this reason, it is preferable that the finish rolling start temperature for hot rolling be 1300°C or lower. More preferably, it is 1250°C or lower. Even more preferably, it is 1200°C or lower.

Finishing temperature of hot rolling: 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, which will hinder cold rolling. Therefore, the finishing rolling delivery temperature of hot rolling is set to 800°C or higher. More preferably, it is 820°C or higher. Even more preferably, it is 850°C or higher.

There is no particular upper limit to the finish rolling temperature, but if the finish rolling temperature exceeds 1100°C, the amount of oxide (scale) produced will increase rapidly, roughening the interface between the base steel and the oxide, and the surface quality after pickling and cold rolling may deteriorate. Furthermore, the crystal grain size may become excessively coarse, causing the surface of the pressed product to become rough during processing. Therefore, the temperature is preferably 1100°C or less. More preferably, it is 1050°C or less. Even more preferably, it is 1000°C or less.

Number of rolling stands used in finish rolling: 4 or more When the number of rolling stands used in finish rolling is less than 4, the reduction rate 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, and the amount of solute Nb becomes less than 0.005%, resulting in poor delayed fracture resistance. Therefore, the number of rolling stands used in finish rolling is set to 4 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.

Time required from the start of finish rolling to the end of finish rolling: 20 seconds or less If the time required from the start of finish rolling to the end of finish rolling exceeds 20 seconds, excessive precipitation of Nb-containing precipitates will result in the amount of solute Nb being less than 0.005%, and delayed fracture resistance will be deteriorated. Therefore, the time required from the start of finish rolling 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.

The time required from the start of rolling to the end of finish 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, the amount of solute Nb becomes less than 0.005% due to excessive precipitation of Nb-containing precipitates, and the delayed fracture resistance deteriorates. 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, it is set to 8 seconds or less, and even more preferably, it is set to 7 seconds or less. Here, the third rolling stand means the third rolling stand from the finish rolling entry side.

[Winding process]
Cooling to 650°C at a cooling rate of 50°C/s or more 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, 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.

Coiling temperature: 650°C or less If the coiling temperature after hot rolling exceeds 650°C, the growth of Nb will proceed excessively, increasing the amount of Nb precipitates of 100 nm or more. As a result, the ratio ( _Nb100 /Nb) of the amount of Nb present as precipitates of 100 nm or more (Nb100) to the total amount of Nb contained in the _steel (Nb) will exceed 0.25, resulting in poor delayed fracture resistance and stretch flangeability. Therefore, the coiling temperature after hot rolling is set to 650°C or less, preferably 630°C or less, and more preferably 600°C or less.

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

The resulting hot-rolled steel sheet (hot-rolled coil) may be subjected to treatments such as pickling, if necessary. The pickling method for the hot-rolled coil may follow conventional methods. Skin-pass rolling may also be performed to correct the shape of the hot-rolled coil and improve its pickling properties.

After hot rolling and/or intermediate heat treatment and/or pickling, heat treatment may be performed directly, or after cold rolling. When cold rolling is performed, the cold reduction is preferably 25% or more or 30% or more. However, excessive reduction increases the rolling load and leads to increased load on the cold rolling mill, so the upper limit is preferably 75% or 70%. Here, intermediate heat treatment is optional heating aimed at softening the steel sheet if it is too hard when cold rolling is performed, and such heat treatment can also be performed according to conventional methods.

[Heating process]
Soaking temperature: 780°C or higher and 950°C or lower. When the steel is held at a temperature lower than 780°C, the steel is held in the two-phase region, resulting in an area ratio of tempered martensite and bainite in the final structure being less than 60.0%, making it impossible to ensure sufficient steel sheet strength. On the other hand, when the steel is held at a temperature higher than 950°C, the growth of precipitated Nb proceeds excessively, increasing the amount of Nb precipitates of 100 nm or larger. As a result, the ratio ( _Nb100 /Nb) of the amount of Nb present as precipitates of 100 nm or larger ( _Nb100 ) to the total amount of Nb contained in the steel (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 preferred lower limit is 800°C or higher, more preferably 820°C or higher. The preferred upper limit is 940°C or lower, even more preferably 920°C or lower.

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

Cooling to 650°C at a cooling 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. It is preferably set to 22°C/s or more, and more preferably set to 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 set to 100°C/s or less. It is more preferably set to 80°C/s or less.

The cooling conditions after this cooling step are not particularly specified, but self-tempering of the martensite may occur during this cooling step. Alternatively, after cooling is completed in this cooling step, 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.

Plating Treatment The obtained high-strength steel sheet may be plated as needed. 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.

There are no particular limitations on the plating conditions, but the coating weight (coating weight per side) is preferably 20 g/ ^m2 or more from the viewpoints of corrosion resistance and coating weight control, and is preferably 120 g/m2 or ^less from the viewpoint of adhesion. The coating weight is more preferably 25 g/m2 or ^more , and even more preferably 30 g/m2 or ^more . The coating weight is more preferably 100 g/m2 ^{or less} , and even more preferably 70 g/m2 or ^less .

For hot dip galvanizing, it is preferable to use a zinc plating bath with an Al content of 0.08% or more and 0.30% or less. Furthermore, the effects of the present invention will not change even if the plating bath contains elements other than Al, Mg, and Si, such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn.

When hot-dip galvanizing is performed, the hot-dip galvanizing alloying treatment is carried out in a temperature range of 450°C to 600°C after the hot-dip galvanizing treatment. If the alloying treatment is carried out at a temperature above 600°C, untransformed austenite will transform into pearlite. The untransformed austenite will become fresh martensite upon final cooling, but if the amount of pearlite transformation is large, the area ratio of the residual structure may exceed 20.0%, which may impair the effects of the present invention. Therefore, when hot-dip galvanizing alloying treatment is carried out, it is preferable to carry out the hot-dip galvanizing alloying treatment in a temperature range of 450°C to 600°C. The Fe concentration in the plating layer of the alloyed hot-dip galvanized steel sheet is preferably 8 to 17%.

Steel having the chemical composition shown in Table 1, with the remainder consisting of Fe and unavoidable impurities, was melted in a converter and formed into slabs by continuous casting. The resulting slabs underwent the hot rolling, coiling, and heating processes under the conditions shown in Table 2. If reheating was performed, they were cooled to a cooling stop temperature of 100°C to 350°C, then reheated to a holding temperature of 200°C to 450°C, held there, and then cooled to room temperature. If reheating was not performed, they were held at a holding temperature of 200°C to 450°C for 10 seconds or more, then cooled to room temperature, yielding high-strength cold-rolled steel sheets (CR). Further, hot-dip galvanizing 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% Al by mass was used for the hot-dip galvanized steel sheet (GI), and a zinc bath containing 0.14% Al by mass was used for the alloyed hot-dip galvanized steel sheet (GA). The bath temperature was 465°C. The coating weight was 45 g/m2 per side (double-sided coating), and for GA, the Fe concentration in the coating layer was adjusted to be within the range of 9% to 12% by mass.

The area ratios of fresh martensite, tempered martensite, and bainite were determined by polishing a cross section (L cross section) of the steel plate parallel to the rolling direction 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 position in the plate thickness direction (a position corresponding to 1/4 of the plate thickness in the depth direction from the steel plate surface). The obtained structural images were used to calculate the area ratio of each structure (fresh martensite, the total of tempered martensite and bainite). Furthermore, in the above structural images, fresh martensite was considered to be the light gray structure region, and tempered martensite and bainite were considered to be the dark gray structure region in which carbides precipitated.

Tensile tests were conducted in accordance with JIS Z 2241 (2011) using JIS No. 5 test pieces, samples taken so that the tensile direction was perpendicular to the rolling direction of the steel plate, and TS (tensile strength) and EL (total elongation) were measured. In the examples, those that did not meet the conditions of TS (tensile strength) of 1320 MPa or more and EL (total elongation) of 10.0% or more were considered comparative examples.

The hole expansion ratio was measured in accordance with JIS Z 2256 (2010). Each steel plate was cut into 100 mm x 100 mm pieces, 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 limiting hole expansion ratio λ (%) was calculated using the following equation (3), and the hole expandability was evaluated from this limiting hole expansion ratio value. In the examples, those that did not meet the condition of a hole expansion ratio of 30.0% or more were designated as comparative examples.
(Equation 3) Limiting hole expansion ratio λ (%)={(D _f −D ₀ )/D ₀ }×100
In the above formula, _Df is the hole diameter (mm) when a crack occurs, and _D0 is the initial hole diameter (mm).

The delayed fracture resistance of areas with complex stress distribution, such as overhangs, was evaluated using a four-point bending immersion test. After V-bending, the steel plates were 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 and centered in the length direction, and both end faces in the width direction were then ground. Next, both end faces in the length direction were 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 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 visually observed in any of the stressed steel plates, they were rated as ◎, and if cracks were observed at a tightening stress equivalent to TS + 200 MPa, they were rated as ○. Furthermore, if cracks were observed at a tightening stress equivalent to TS, they were rated as △, and if cracks were observed at a tightening stress equivalent to YS, they were rated as ×. Here, steel plates with excellent delayed fracture resistance are those rated as ◎, ○, or △.

The amount of dissolved Nb and Nb present as precipitates of 100 nm or larger in the steel sheet was measured using the following procedure.

Multiple test pieces of cold-rolled steel sheet or galvanized steel sheet cut to approximately 20 x 50 mm were prepared, and for the galvanized steel sheet, the plating on the surface of the test piece was removed using a router (precision grinder). The surfaces of the collected test pieces were polished to approximately 50 μm by preliminary electrolytic polishing to obtain a new surface. Electrolysis was performed on the obtained test pieces using 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol. The electrolyte obtained by the above method was collected, and the Nb concentration was quantified in mass% using ICP atomic emission spectrometry. This was taken as the amount of dissolved Nb. Next, residues adhering to the metal sample after electrolysis were immersed in separately prepared methanol, and residues adhering to the remaining part of the metal sample were collected in a container using ultrasonic vibrations. Thereafter, the methanol containing the electrolytic solution and residues adhering to the remaining part of the metal sample was used to collect residues with a particle size of 100 nm or more using an alumina filter with a pore size of 100 nm. The collected residue was acid-decomposed, and the Nb concentration was quantified in mass% using ICP atomic emission spectrometry. This was taken as the amount of Nb present as precipitates of 100 nm or more (Nb ₁₀₀ ).

All of the high-strength steel sheets of the present invention have high strength, good formability, and excellent delayed fracture resistance. On the other hand, the comparative examples are inferior in at least one of the following properties: strength, formability, and delayed fracture resistance.

Claims (7)

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,
A high-strength steel plate having a structure in which the ratio (Nb ₁₀₀ /Nb) of the amount of Nb present as precipitates of 100 nm or more (Nb ₁₀₀ ) to the total amount of Nb contained in the steel (Nb) is 0.25 or less. Further, the composition of the alloy is, in 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,
The high-strength steel plate according to claim 1, further comprising one or more selected from the following: The high-strength steel sheet according to claim 1 or 2, having a plating layer on the surface of the steel sheet. The high-strength steel sheet according to claim 3, wherein the plating layer is an alloyed plating layer. The method for producing a high-strength steel plate according to claim 1 or 2,
After performing a slab heating step in which a steel material having the above-described composition is heated at a temperature _Tsol °C or higher represented by Equation 1 for 1.0 hour or more,
Finish rolling is started at _Tsol -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.
(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. The method for manufacturing high-strength steel sheet according to claim 5, wherein a plating treatment is performed after holding in the annealing process. The method for manufacturing high-strength steel sheet according to claim 6, wherein the plating treatment is an alloying plating treatment.