High-strength steel plates, high-strength galvanized steel plates, their manufacturing methods and components

Abstract

This invention provides a high-strength steel plate with excellent tensile flange properties, bending and LME resistance, and capable of manufacturing parts with high dimensional accuracy. The high-strength steel plate of the present invention has the following composition: it contains C, Si, Mn, P, S, Al, N, Nb, Ti and B in specified amounts, such that [%Si] / [%Mn] is 0.10 to 0.60, the amount of free Ti is 0.001% by mass or more, and the remainder is composed of Fe and unavoidable impurities; it has the following steel structure: the total amount of Nb dissolved in the surface layer and the amount of precipitation below 100 nm is 0.002% by mass or more, at the 1 / 4 position of the plate thickness, the area fraction of martensite is 78% or more, the area fraction of ferrite is 10% or less, the volume fraction of retained austenite is less than 10.0%, the ratio of the grain size in the rolling direction to the grain size in the thickness direction of the original austenite grains is 2.0 or less, and the hardness variation frequency per 1500 μm in the width direction at the 200 μm position of the plate thickness is 10 times or less; and the high-strength steel plate has a tensile strength of 1180 MPa or more.

Description

Technical Field

[0001] This disclosure relates to high-strength steel plates, high-strength galvanized steel plates, methods for manufacturing the same, and components thereof. Background Technology

[0002] To balance reducing CO2 emissions through vehicle lightweighting and improving crashworthiness through body lightweighting, the use of high-strength steel sheets in automobiles is underway, and new laws and regulations are constantly being introduced. Therefore, to increase body strength, the use of high-strength steel sheets with a tensile strength (TS) of 1180 MPa or higher in the main structural components forming the car body frame is increasing.

[0003] High-strength steel sheets used in automotive reinforcing components and frame structures require excellent formability. Furthermore, the formed components must exhibit excellent dimensional accuracy. For example, components such as crash boxes have punched ends and bent sections; therefore, from a formability perspective, steel sheets with high tensile flange strength and bendability are preferred. Additionally, from a component performance perspective, increasing the yield ratio of the steel sheet (YR = yield strength YS / tensile strength TS) enhances impact energy absorption during collisions. Furthermore, from a dimensional accuracy perspective, controlling the yield ratio (YR) of the steel sheet within a certain range suppresses springback after forming, thus controlling the component's dimensional accuracy. To increase the application rate of high-strength steel sheets in automotive components, these characteristics must be comprehensively met.

[0004] Furthermore, it has recently been confirmed that during spot welding of high-strength galvanized steel sheets, the zinc coating diffuses into the grain boundaries of the steel surface, causing liquid metal embrittlement (LME) and resulting in grain boundary cracks (LME cracks). Even in high-strength steel sheets without a galvanized coating, LME cracks will occur as long as the welded object is galvanized steel; therefore, this must be considered a problem in all high-strength steel sheets. Consequently, when using high-strength steel sheets for skeletal components, high-strength steel sheets with excellent LME resistance are required.

[0005] To address these requirements, for example, Patent Document 1 provides a high-strength steel sheet with a tensile strength of 980 MPa or higher that exhibits excellent ductility, tensile flange properties, bending properties, and LME resistance, and is capable of manufacturing parts with high dimensional accuracy.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 2020 / 184154 Summary of the Invention

[0009] The high-strength steel sheet described in Patent Document 1 comprehensively satisfies the requirements for strength, ductility, tensile flange properties, bending properties, and LME resistance, and can be used to manufacture parts with high dimensional accuracy. However, the TS of the high-strength steel sheet described in Patent Document 1 is 980 MPa, and there is room for further improvement in strength.

[0010] This disclosure was developed in view of the above circumstances, and its object is to provide a high-strength steel sheet with a strength of 1180 MPa or more that can produce parts with excellent tensile flange properties, bending and LME resistance and can be manufactured with high dimensional accuracy, and to provide an advantageous manufacturing method for the high-strength steel sheet.

[0011] It should be noted that in this disclosure, the ability to manufacture parts with high dimensional accuracy (high dimensional accuracy during molding) refers to a yield ratio (YR) of 65% to 90%. It should be noted that YR is calculated by the following formula (2).

[0012] YR=YS / TS×100····(2)

[0013] In addition, excellent tensile flange properties refer to a hole expansion ratio (λ) of 25% or higher, which is an indicator of tensile flange properties.

[0014] Bending performance is evaluated by the pass rate of the bending test. Five samples are subjected to bending tests at the maximum R value where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) is less than 5. Then, the presence of cracks at the edge of the bending apex is evaluated. Only when none of the five samples break, i.e., only when the pass rate is 100%, is the bending performance judged as excellent.

[0015] Furthermore, regarding LME resistance, the fracture portion of the test piece after the high-temperature tensile test described in the examples was cut off using the plate thickness section (L section) parallel to the tensile direction of the test piece as the observation surface. The plate thickness section was observed, and the plate thickness t at a position 400 μm from the tensile fracture end was determined. If the reduction in plate thickness obtained by substituting this plate thickness t into the following formula (3) is 0.20 or more, it is judged to have excellent LME resistance.

[0016] Plate thickness reduction rate = (t0 - t) / t0···(3)

[0017] Here, t0 is the initial plate thickness of the notched tensile test piece before the high-temperature tensile test, and t is the plate thickness at a position 400 μm from the tensile fracture front end towards the holding part after the high-temperature tensile test. For example, in Figure 1 In the L-section of the fractured part shown in the figure, t is determined as shown in the figure.

[0018] It should be noted that when the thickness reduction rate is large, that is, when the plate breaks after significant shrinkage during high-temperature tensile testing, it is judged to have excellent LME resistance.

[0019] In order to achieve the above-mentioned problem, the inventors conducted repeated and in-depth research and obtained the following insights.

[0020] (1) By making a structure with martensite (quenched martensite and tempered martensite) as the main body, the tensile flange property can be achieved to more than 25%.

[0021] (2) By making the average ratio of the grain size in the rolling direction of the original austenite grains to the grain size in the thickness direction of the plate less than 2.0, the YR, which is an indicator of the dimensional accuracy of the component, can be achieved to 65% to 90%.

[0022] (3) The ratio of [%Si] to [%Mn] satisfies the relationship of 0.10 to 0.60, and the hardness variation frequency in the width direction of the plate at the 200μm thickness position is less than 20 times per 1500μm, thereby achieving good bending performance.

[0023] (4) The ratio of [%Si] / [%Mn] satisfies the relationship of 0.10 to 0.60, and the total amount of Nb solid solution in the thick surface layer and the amount of precipitation below 100nm is greater than 0.0020%, thereby achieving good LME resistance.

[0024] This disclosure is based on the above-mentioned insights. That is, the main points of this disclosure are as follows.

[0025] [1] A high-strength steel plate having the following composition and the following steel structure, and having a tensile strength of 1180 MPa or more; the composition, in mass %, contains C: 0.090% to 0.390%, Si: 0.01% to 2.50%, Mn: 2.00% to 4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 0.100%, N: less than 0.0100%, Nb: 0.002% to 0.100%, Ti: 0.005% to 0.100%, and B: 0.0002% to 0.0100%, and [%Si] / [%Mn] satisfies a relationship of 0.10 to 0.60, the amount of free Ti obtained by the following formula (1) satisfies a relationship of more than 0.001% by mass, and the remainder consists of Fe and unavoidable impurities;

[0026] In the steel microstructure, the total amount of Nb dissolved in the surface layer and the amount of Nb precipitated below 100 nm is 0.002% by mass or more.

[0027] At the 1 / 4 position of the plate thickness

[0028] The martensite area ratio is over 78%.

[0029] The area fraction of ferrite is less than 10%.

[0030] The volume fraction of retained austenite is less than 10.0%.

[0031] The average ratio of the grain size in the rolling direction to the grain size in the thickness direction of the original austenite grains is less than 2.0.

[0032] The hardness variation frequency in the width direction of the plate at the 200μm thickness position is less than 20 times per 1500μm.

[0033] remember

[0034] Free Ti content (%) = [%Ti] - (47.9 / 14.0) × [%N] - (47.9 / 32.1) × [%S] ... (1)

[0035] It should be noted that [%X] in formula (1) represents the content (mass%) of element X in the steel, which is 0 when element X is not present.

[0036] [2] According to the high-strength steel plate described in [1] above, the surface softening thickness of the steel structure is 10 μm to 100 μm.

[0037] [3] The high-strength steel plate according to [1] or [2] above, wherein the above composition further contains, by mass %, at least one element selected from O: less than 0.0100%, V: less than 0.200%, Ta: less than 0.10%, W: less than 0.10%, Cr: less than 1.00%, Mo: less than 1.00%, Ni: less than 1.00%, Co: less than 0.010%, Cu: less than 1.00%, Sn: less than 0.200%, Sb: less than 0.200%, Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%, Zr: less than 0.100%, Te: less than 0.100%, Hf: less than 0.10%, and Bi: less than 0.200%.

[0038] [4] A high-strength coated steel sheet, wherein at least one side of the high-strength steel sheet described in any one of [1] to [3] above has a coating.

[0039] [5] A method for manufacturing a high-strength steel plate, wherein a steel slab having the composition described in [1] or [3] is held at a slab heating temperature of 1150°C or higher for at least 100 minutes.

[0040] Next, the steel slab is subjected to hot rolling at a roughing end temperature of 1050°C or higher and a finishing start temperature of 1000°C or higher to produce a hot-rolled plate.

[0041] Next, the hot-rolled plates are pickled.

[0042] Next, the hot-rolled sheet is cold-rolled to produce a cold-rolled sheet by cold rolling with a cumulative reduction rate of 20% to 75%.

[0043] Next, the above-mentioned cold-rolled sheet is subjected to the following annealing process to obtain a high-strength steel sheet. The annealing process is carried out under the condition that the average heating rate in the temperature range of 250°C to 700°C is 10°C / s or more and the residence time from 750°C to the heating temperature is 40s or more, and the sheet is heated to a heating temperature of 820°C or more, and the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C / s or more, and the sheet is cooled to 150°C or less.

[0044] [6] In the method for manufacturing high-strength steel plate according to [5] above, the oxygen concentration of the atmosphere at the heating temperature is 2 ppm to 30 ppm by volume, and the dew point of the atmosphere is -35°C or higher.

[0045] [7] A method for manufacturing a high-strength coated steel sheet, wherein after the annealing process described in [5] or [6] above, a coating process is performed on at least one side of the high-strength steel sheet to obtain a high-strength coated steel sheet.

[0046] [8] A component is made of at least a portion of a high-strength steel plate as described in any one of [1] to [3] above.

[0047] [9] A component is made of at least a portion of the high-strength coated steel sheet described above [4].

[0048]

[10] The component described in [8] or [9] above is used as a frame structure component of an automobile or as a reinforcing component of an automobile.

[0049] According to this disclosure, a high-strength steel plate with a strength of 1180 MPa or higher and a component with excellent tensile flange properties, bending and LME resistance can be provided, and parts can be manufactured with high dimensional accuracy. Attached Figure Description

[0050] Figure 1 This is a schematic diagram used to illustrate the measurement of the reduction in plate thickness.

[0051] Figure 2 This is a graph used to illustrate the measurement of the frequency of hardness variation. Detailed Implementation

[0052] The embodiments of this disclosure will now be described. It should be noted that this disclosure is not limited to the following embodiments.

[0053] First, the appropriate range of the steel plate's composition and the rationale for defining it will be explained. It should be noted that, unless otherwise specified, the "%" indicating the content of the constituent elements in the steel plate refers to "mass %". Similarly, unless otherwise specified, "ppm" refers to "mass ppm". Furthermore, in this specification, the numerical range indicated by "~" refers to the range of values ​​before and after "~" as the lower and upper limits.

[0054] [C: 0.090%~0.390%]

[0055] Carbon (C) is one of the essential components of steel, and in particular, in this disclosure, it is a crucial element affecting the area fraction of martensite and ferrite, as well as the volume fraction of retained austenite. If the C content is less than 0.090%, the area fraction of martensite decreases, leading to an increase in the area fraction of ferrite, making it difficult to achieve a steel strength (TS) of 1180 MPa or higher. On the other hand, if the C content exceeds 0.390%, the carbon concentration in the retained austenite increases excessively, resulting in a significant increase in the hardness of the martensite formed from the retained austenite during punching. This promotes cracking during hole expansion, reducing the hardness (λ). Furthermore, bending performance also decreases. Therefore, the C content is between 0.090% and 0.390%. The C content is preferably 0.100% or more. The C content is preferably 0.360% or less. The C content is more preferably 0.110% or more. The C content is more preferably 0.350% or less.

[0056] [Si: 0.01%~2.50%]

[0057] Si is one of the essential components of steel, and particularly in this disclosure, it is an element that affects the volume fraction of retained austenite because it inhibits carbide formation during annealing and promotes the formation of retained austenite. Furthermore, Si exhibits significant resistance to tempering softening below 400°C, thus affecting the frequency of hardness variation per 1500 μm in the width direction at a thickness of 200 μm. If the Si content is less than 0.01%, the hardness distribution of martensite in the width direction becomes uneven, resulting in a hardness variation frequency exceeding 20 times per 1500 μm at a thickness of 200 μm, leading to reduced flexibility. On the other hand, if the Si content exceeds 2.50%, the carbon concentration in the retained austenite increases excessively, significantly increasing the hardness of the martensite transformed from retained austenite during punching. This increases void formation during punching and reaming, reducing λ. Furthermore, flexibility also decreases. Consequently, LME resistance also decreases. Therefore, the Si content is between 0.01% and 2.50%. The Si content is preferably 0.10% or more. The Si content is preferably 2.00% or less. The Si content is more preferably 0.15% or more. The Si content is more preferably 1.50% or less.

[0058] [Mn: 2.00%~4.00%]

[0059] Mn is one of the essential components of steel, and in particular, in this disclosure, it is a crucial element affecting the martensite area ratio. If the Mn content is less than 2.00%, the martensite area ratio decreases, leading to an increase in the ferrite area ratio, making it difficult to achieve a steel strength (TS) of 1180 MPa or higher. On the other hand, if the Mn content exceeds 4.00%, the hardness distribution of martensite in the width direction of the plate becomes uneven, resulting in a hardness variation frequency exceeding 20 times per 1500 μm in the width direction at a plate thickness of 200 μm, thus reducing flexibility. Therefore, the Mn content is between 2.00% and 4.00%. The Mn content is preferably 2.20% or more. The Mn content is preferably 3.80% or less. The Mn content is more preferably 2.50% or more. The Mn content is more preferably 3.60% or less.

[0060] [P: below 0.100%]

[0061] If phosphorus (P) is excessive, it segregates at the original austenite grain boundaries, causing grain boundary embrittlement and reducing the ultimate deformation capacity of the steel sheet, thus decreasing λ. Furthermore, flexibility also decreases. Therefore, the P content needs to be 0.100% or less. It should be noted that there is no particular lower limit for the P content, but since P is a solid solution strengthening element and can improve the strength of the steel sheet, it is preferably 0.001% or more. Therefore, the P content is 0.100% or less. Preferably 0.001% or more. Preferably 0.070% or less.

[0062] [S: below 0.0200%]

[0063] Sulfur (S) exists in the form of sulfides, reducing the steel's ultimate deformation capacity and thus decreasing its bending strength (λ). Furthermore, its flexibility is also reduced. Therefore, the S content needs to be 0.0200% or less. It should be noted that while there is no specific lower limit for the S content, due to limitations in production technology, the S content is preferably 0.0001% or more. Therefore, the S content is 0.0200% or less. The S content is preferably 0.0001% or more. The S content is preferably 0.0050% or less.

[0064] [Al: below 0.100%]

[0065] If Al is in excess, the A3 phase transformation point increases, and the microstructure contains a large amount of ferrite, making it difficult to achieve the desired YR. Therefore, the Al content needs to be 0.100% or less. It should be noted that there is no specific lower limit for the Al content, but to suppress carbide formation during continuous annealing and promote the formation of retained austenite, the Al content is preferably 0.001% or more. Therefore, the Al content is 0.100% or less. The Al content is preferably 0.001% or more. The Al content is preferably 0.050% or less.

[0066] [N: below 0.0100%]

[0067] Nitrogen (N) exists in the form of nitrides, reducing the ultimate deformation capacity of the steel sheet, thus decreasing λ. Furthermore, its bendability also decreases. Therefore, the N content needs to be 0.0100% or less. It should be noted that there is no particularly specified lower limit for the N content, but due to limitations in production technology, the N content is preferably 0.0005% or more. Therefore, the N content is preferably 0.0100% or less. The N content is preferably 0.0005% or more. The N content is preferably 0.0050% or less.

[0068] [Nb: 0.002%~0.100%]

[0069] Nitrogen (Nb) improves the strength of steel sheets by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Furthermore, by adding Nb, the combined amount of Nb dissolved in the surface layer and the amount precipitated below 100 nm in the thicker layer can be increased, achieving good resistance to LME (Low Metallic Effluent). To obtain this effect, the Nb content needs to be 0.002% or more. On the other hand, if the Nb content exceeds 0.100%, the carbides, nitrides, or carbonitrides become coarse, and their quantity increases, thus reducing flexibility. Therefore, the Nb content is 0.002% to 0.100%. The Nb content is preferably 0.003% or more. The Nb content is preferably 0.060% or less. The Nb content is more preferably 0.004% or more. The Nb content is more preferably 0.030% or less.

[0070] [Ti: 0.005%~0.100%]

[0071] Ti improves the strength of steel sheets by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Furthermore, by adding Ti, the total amount of Nb dissolved in the surface layer and the amount of Nb precipitated below 100 nm in the thickest layer can be increased, achieving good LME resistance. To obtain this effect, the Ti content needs to be 0.005% or more. On the other hand, if the Ti content exceeds 0.100%, the carbides, nitrides, or carbonitrides become coarse, and their quantity increases, thus reducing flexibility. Therefore, the Ti content is 0.005% to 0.100%. The Ti content is preferably 0.008% or more. The Ti content is preferably 0.060% or less. The Ti content is more preferably 0.010% or more. The Ti content is more preferably 0.030% or less.

[0072] [B: 0.0002%~0.0100%]

[0073] Boron (B) is an element that can improve hardenability without lowering the martensitic transformation initiation temperature and can suppress ferrite formation during the cooling process of annealing. To achieve this effect, the B content needs to be 0.0002% or more. On the other hand, if the B content exceeds 0.0100%, cracks will form inside the steel sheet during hot rolling, reducing the ultimate deformation capacity of the steel sheet, and thus decreasing λ. Furthermore, the bendability also decreases. Therefore, when adding B, its content is 0.0002% to 0.0100%. The B content is preferably 0.0003% or more. The B content is preferably 0.0050% or less. The B content is more preferably 0.0004% or more. The B content is more preferably 0.0040% or less.

[0074] [[%Si] / [%Mn]: 0.10~0.60]

[0075] This is a crucial inventive element of this disclosure. By controlling [%Si] / [%Mn] within a desired range, good bending performance and LME resistance can be achieved. If [%Si] / [%Mn] is less than 0.10, the hardness distribution of martensite in the width direction of the plate becomes uneven, resulting in a hardness variation frequency exceeding 20 times per 1500 μm in the width direction at a plate thickness of 200 μm, thus reducing bending performance. On the other hand, if [%Si] / [%Mn] exceeds 0.60, LME resistance decreases. Therefore, [%Si] / [%Mn] is between 0.10 and 0.60. [%Si] / [%Mn] is preferably 0.12 or more. [%Si] / [%Mn] is preferably 0.50 or less. [%Si] / [%Mn] is more preferably 0.13 or more. [%Si] / [%Mn] is more preferably 0.40 or less.

[0076] [Free Ti content = [%Ti] - (47.9 / 14.0) × [%N] - (47.9 / 32.1) × [%S]: ≥0.001%]...(1)

[0077] By ensuring that the amount of free Ti, as calculated by equation (1) above, is a certain level or higher, the combined amount of Nb dissolved in the thick surface layer and the amount of Nb precipitated below 100 nm increases, thus achieving good LME resistance. To achieve this effect, the amount of free Ti is 0.001% or higher. It should be noted that there is no specific upper limit for the amount of free Ti, but since carbides, nitrides, or carbonitrides become coarser and their quantity increases, resulting in decreased flexibility, the amount of free Ti is preferably 0.040%. Therefore, the amount of free Ti is 0.001% or higher. The amount of free Ti is preferably 0.002% or higher. The amount of free Ti is preferably 0.040% or lower.

[0078] One embodiment of the high-strength steel plate of the present invention has a composition containing the aforementioned elements, with the remainder consisting of Fe and unavoidable impurities. Furthermore, preferably, one embodiment of the high-strength steel plate of the present invention has a composition containing the aforementioned elements, with the remainder consisting of Fe and unavoidable impurities. Here, unavoidable impurities include Zn, Pb, and As. These impurities are permissible as long as their total content is less than 0.100%.

[0079] In addition to the aforementioned essential components, the high-strength steel plate disclosed herein may contain, by mass percent, at least one element selected from the following: O: 0.0100% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[0080] [O: below 0.0100%]

[0081] O, existing in the form of oxides, reduces the ultimate deformation capacity of the steel sheet. Therefore, if the O content exceeds 0.0100%, λ decreases. Furthermore, bendability also decreases. Therefore, the O content is 0.0100% or less. It should be noted that there is no specific lower limit for the O content, but due to limitations in production technology, the O content is preferably 0.0001% or more. Therefore, when added, the O content is 0.0100% or less. The O content is preferably 0.0001% or more. The O content is preferably 0.0050% or less.

[0082] [V: below 0.200%]

[0083] A large amount of v (V) generates coarse precipitates and inclusions, reducing the ultimate deformation capacity of the steel sheet. Therefore, if the v content exceeds 0.200%, λ (the steel's resistance to deformation) decreases. Furthermore, bendability also decreases. Therefore, the v content is 0.200% or less. It should be noted that there is no specific lower limit for the v content, but by ensuring the v content is 0.001% or more, fine carbides, nitrides, or carbonitrides are formed during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet. Additionally, by adding v, the recrystallization temperature during the annealing heating process increases, lowering the average ratio of the original austenite grain size in the rolling direction to the grain size in the thickness direction, thus allowing the γR (grain size ratio) to be controlled within the desired range. Therefore, the v content is preferably 0.001% or more. Therefore, when added, the v content is 0.200% or less. The v content is preferably 0.001% or more. The v content is preferably 0.100% or less.

[0084] [Ta: below 0.10%]

[0085] [W: below 0.10%]

[0086] If the contents of Ta and W each exceed 0.10%, a large amount of coarse precipitates and inclusions will be generated, reducing the ultimate deformation capacity of the steel plate, and thus reducing λ. Additionally, the bendability also decreases. Therefore, the contents of Ta and W are each 0.10% or less. It should be noted that there is no specific lower limit for the contents of Ta and W, but by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing to improve the strength of the steel plate, the contents of Ta and W are preferably 0.01% or more each. Therefore, when added, the contents of Ta and W are preferably 0.10% or less each. The contents of Ta and W are preferably 0.01% or more each. The contents of Ta and W are preferably 0.08% or less each.

[0087] [Cr: less than 1.00%]

[0088] [Mo: 1.00% or less]

[0089] [Ni: below 1.00%]

[0090] If the contents of Cr, Mo, and Ni each exceed 1.00%, coarse precipitates and inclusions increase, reducing the ultimate deformation capacity of the steel plate and thus decreasing λ. Furthermore, bendability also decreases. Therefore, the contents of Cr, Mo, and Ni are each 1.00% or less. It should be noted that there is no specific lower limit for the contents of Cr, Mo, and Ni, but since they are elements that improve hardenability, the contents of Cr, Mo, and Ni are preferably 0.01% or more each. Therefore, when added, the contents of Cr, Mo, and Ni are each 1.00% or less. The contents of Cr, Mo, and Ni are preferably 0.01% or more. The contents of Cr, Mo, and Ni are preferably 0.80% or less.

[0091] [Co: less than 0.010%]

[0092] If the Co content exceeds 0.010%, coarse precipitates and inclusions increase, reducing the steel plate's ultimate deformation capacity and thus decreasing λ. Additionally, bendability also decreases. Therefore, the Co content is 0.010% or less. It should be noted that there is no specific lower limit for the Co content, but since Co is an element that improves hardenability, the Co content is preferably 0.001% or more. Therefore, when added, the Co content is 0.010% or less. The Co content is preferably 0.001% or more. The Co content is preferably 0.008% or less.

[0093] [Cu: less than 1.00%]

[0094] If the Cu content exceeds 1.00%, coarse precipitates and inclusions increase, reducing the steel plate's ultimate deformation capacity and thus decreasing λ. Furthermore, bendability also decreases. Therefore, the Cu content is 1.00% or less. It should be noted that there is no specific lower limit for the Cu content, but since Cu is an element that improves hardenability, the Cu content is preferably 0.01% or more. Therefore, when added, the Cu content is 1.00% or less. The Cu content is preferably 0.01% or more. The Cu content is preferably 0.80% or less.

[0095] [Sn: below 0.200%]

[0096] If the Sn content exceeds 0.200%, cracks will form inside the steel sheet during casting or hot rolling, reducing the steel sheet's ultimate deformation capacity and thus decreasing λ. Additionally, bendability also decreases. Therefore, the Sn content is 0.200% or less. It should be noted that there is no specific lower limit for the Sn content, but since Sn is an element that improves hardenability, the Sn content is preferably 0.001% or more. Therefore, when added, the Sn content is 0.200% or less. The Sn content is preferably 0.001% or more. The Sn content is preferably 0.100% or less.

[0097] [Sb: below 0.200%]

[0098] If the Sb content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the steel plate's ultimate deformation capacity and thus decreasing λ. Furthermore, bendability also decreases. Therefore, the Sb content is 0.200% or less. It should be noted that there is no specific lower limit for the Sb content, but since Sb is an element that controls the surface softening thickness and can adjust strength, the Sb content is preferably 0.001% or more. Therefore, when added, the Sb content is 0.200% or less. The Sb content is preferably 0.001% or more. The Sb content is preferably 0.100% or less.

[0099] [Ca: below 0.0100%]

[0100] [Mg: less than 0.0100%]

[0101] [REM: below 0.0100%]

[0102] If the contents of Ca, Mg, and REM each exceed 0.0100%, coarse precipitates and inclusions increase, reducing the ultimate deformation capacity of the steel plate, and thus decreasing λ. Furthermore, bendability also decreases. Therefore, the contents of Ca, Mg, and REM are each 0.0100% or less. It should be noted that there is no specific lower limit for the contents of Ca, Mg, and REM, but since these elements are responsible for spheroidizing the shape of nitrides and sulfides and improving the ultimate deformation capacity of the steel plate, the contents of Ca, Mg, and REM are preferably 0.0005% or more. Therefore, when added, the contents of Ca, Mg, and REM are each 0.0100% or less. The contents of Ca, Mg, and REM are preferably 0.0005% or more. The contents of Ca, Mg, and REM are preferably 0.0050% or less.

[0103] [Zr: below 0.100%]

[0104] [Te: below 0.100%]

[0105] If Zr and Te each exceed 0.100%, coarse precipitates and inclusions increase, reducing the steel plate's ultimate deformation capacity and thus decreasing λ. Furthermore, bendability also decreases. Therefore, the content of Zr and Te needs to be 0.100% or less each. It should be noted that there is no specific lower limit for the content of Zr and Te, but since they are elements that spheroidize nitrides and sulfides and improve the ultimate deformation capacity of the steel plate, the content of Zr and Te is preferably 0.001% or more each. Therefore, when added, the content of Zr and Te is 0.100% or less. The content of Zr and Te is preferably 0.001% or more each. The content of Zr and Te is preferably 0.080% or less each.

[0106] [Hf: below 0.10%]

[0107] If the Hf content exceeds 0.10%, coarse precipitates and inclusions increase, reducing the steel plate's ultimate deformation capacity and thus decreasing λ. Additionally, bendability also decreases. Therefore, the Hf content is 0.10% or less. It should be noted that there is no specific lower limit for the Hf content, but since it is an element that spheroidizes nitrides and sulfides and improves the ultimate deformation capacity of the steel plate, the Hf content is preferably 0.01% or more. Therefore, when added, the Hf content is 0.10% or less. The Hf content is preferably 0.01% or more. The Hf content is preferably 0.08% or less.

[0108] [Bi: below 0.200%]

[0109] If the Bi content exceeds 0.200%, coarse precipitates and inclusions increase, reducing the steel plate's ultimate deformation capacity and thus decreasing λ. Furthermore, bendability also decreases. Therefore, the Bi content is 0.200% or less. It should be noted that there is no specific lower limit for the Bi content, but since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more. Therefore, when added, the Bi content is 0.200% or less. The Bi content is preferably 0.001% or more. The Bi content is preferably 0.100% or less.

[0110] It should be noted that the presence of O, V, Ta, W, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi as unavoidable impurities does not impair the effectiveness of the present invention when their contents are below the preferred lower limit.

[0111] Next, the steel structure of the steel plate will be explained.

[0112] [Martensite area ratio: 78% or higher]

[0113] By using martensite as the main phase, a strength (TS) of 1180 MPa or higher can be achieved. To obtain this effect, the martensite area fraction is 78% or higher. It should be noted that while the effects of this disclosure can be achieved even with a martensite area fraction of 100%, 99% or lower is preferred to improve ductility. Therefore, the martensite area fraction is 78% or higher. The martensite area fraction is preferably 79% or higher. The martensite area fraction is preferably 99% or lower. The martensite area fraction is more preferably 80% or higher. The martensite area fraction is more preferably 98% or lower. The martensite area fraction is further preferably 82% or higher. The martensite area fraction is further preferably 97% or lower. It should be noted that martensite can also be defined as quenched martensite, tempered martensite, and bainite. It should be noted that, as described later, the observation position for the martensite area fraction is at 1 / 4 of the plate thickness.

[0114] [Ferrite area fraction: below 10%]

[0115] By making the ferrite area fraction 10% or less, TS and YR can be controlled within the desired range. Furthermore, λ and bendability can also be improved. It should be noted that the effects of this disclosure can be obtained even if the ferrite area fraction is 0%, but to improve ductility, it is preferable to make the ferrite area fraction 1% or more. Therefore, the ferrite area fraction is 10% or less. The ferrite area fraction is preferably 1% or more. The ferrite area fraction is preferably 8% or less. The ferrite area fraction is more preferably 2% or more. It should be noted that ferrite can also be defined as bainitic ferrite. It should be noted that, as described later, the observation position of the ferrite area fraction is at 1 / 4 of the plate thickness.

[0116] Here, the method for determining the area ratio of martensite (quenched martensite, tempered martensite, and bainite) and ferrite (bainitic ferrite) is as follows.

[0117] After cutting the specimen with the plate thickness section (L section) parallel to the rolling direction of the steel plate as the observation surface, the observation surface was mirror-polished with diamond plaster, then finely polished with colloidal silica, and further etched with 3 vol.% nitric acid alcohol to expose the microstructure. Under an accelerating voltage of 1 kV, a SEM (Scanning Electron Microscope) equipped with an InLens detector was used, with the steel plate thickness at 1 / 4 position as the observation position, observing three fields of view at 5000x magnification within a 17 μm × 23 μm field of view. The obtained microstructure images were analyzed using Adobe Photoshop from Adobe Systems. The area ratio of each constituent microstructure (ferrite (bainitic ferrite), martensite (tempered martensite, bainite, and quenched martensite)) in the three fields of view was calculated by dividing the area by the measured area. These values ​​were averaged and used as the area ratio of each microstructure. Furthermore, in the aforementioned microstructure images, ferrite (bainitic ferrite) is a flat structure with concave areas and no carbides; tempered martensite and bainite are concave structures containing fine carbides; and quenched martensite is a convex structure with fine internal irregularities. These structures are mutually identifiable. It should be noted that the combined area ratio of tempered martensite, tempered martensite, and bainite is used as the area ratio of martensite, therefore they may not be mutually identifiable.

[0118] [Volume fraction of retained austenite: less than 10.0%]

[0119] When the volume fraction of retained austenite is 10.0% or more, it is difficult to achieve the desired YR (Yellow Reduction). Furthermore, due to the increased amount of martensite transformed from retained austenite during punching, the formation of voids during punching and reaming increases, and λ (thickness radius) decreases. Therefore, the volume fraction of retained austenite is less than 10.0%. The volume fraction of retained austenite is preferably 0.5% or more. The volume fraction of retained austenite is preferably 8.0% or less. The volume fraction of retained austenite is more preferably 1.0% or more. The volume fraction of retained austenite is more preferably 6.0% or less.

[0120] Here, the method for determining the volume fraction of retained austenite is as follows.

[0121] After grinding with the observation surface at a position 1 / 4 of the plate thickness from the surface of the steel plate (equivalent to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate), the surface was further ground by 0.1 mm through chemical grinding. For this surface, the integrated reflection intensity of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron was measured using an X-ray diffraction device with a Co Kα ray source. The volume fraction of austenite was calculated based on the intensity ratio of the integrated reflection intensity of fcc iron (austenite) from each plane to that of bcc iron from each plane, and this ratio was used as the volume fraction of retained austenite.

[0122] [The average ratio of the grain size in the rolling direction to the grain size in the thickness direction of the original austenite grains: below 2.0]

[0123] This is a crucial inventive element of this disclosure. By reducing the average value of the ratio of the original austenite grain size in the rolling direction to the grain size in the thickness direction, even if the original austenite grains have a nearly equiaxed shape, the YR (grain diameter) can be controlled within a desired range. To achieve this effect, the average value of the ratio of the original austenite grain size in the rolling direction to the grain size in the thickness direction needs to be 2.0 or less. It should be noted that there is no particular limitation on the lower limit of the ratio of the original austenite grain size in the rolling direction to the grain size in the thickness direction, but it is preferably 0.5 or more to control the YR within the desired range. Therefore, the average value of the ratio of the original austenite grain size in the rolling direction to the grain size in the thickness direction is 2.0 or less. The average value of the ratio of the original austenite grain size in the rolling direction to the grain size in the thickness direction is preferably 0.5 or more. The average value of the ratio of the original austenite grain size in the rolling direction to the grain size in the thickness direction is preferably 1.9 or less.

[0124] Here, the average method for determining the ratio of the austenite grain size along the rolling direction to the grain size along the thickness direction is as follows. After heat-treating the steel plate at 600°C for 10 minutes, a sample is cut with the thickness section parallel to the rolling direction of the obtained steel plate as the observation surface. Next, the observation surface is mirror-polished with diamond paste. Then, the observation surface is further etched with an etching solution containing sulfonic acid, oxalic acid, and ferrous chloride in a saturated aqueous solution of picric acid to expose the austenite grain boundaries. The observation surface in three fields of view is observed at 400x magnification within a 169μm × 225μm field of view using an optical microscope to obtain microstructure images. Using Adobe Photoshop from Adobe Systems, the ratio of the austenite grain size along the rolling direction to the grain size along the thickness direction in the three fields of view is calculated, and the average of these values ​​is taken as the average ratio (aspect ratio) of the austenite grain size along the rolling direction to the grain size along the thickness direction.

[0125] [The total amount of Nb dissolved in the surface layer and precipitated below 100 nm: ≥0.002% by mass]

[0126] This is a crucial element of the invention disclosed herein. The Nb dissolved after annealing and the Nb precipitated at a size of 100 nm or less dissolve during the heating process in the LME test, becoming a solid solution state. That is, by increasing the total amount of Nb dissolved in the surface layer of the plate and the amount of Nb precipitated at a size of 100 nm or less, the amount of Nb dissolved in the surface layer of the plate can be increased during the heating process in the LME test. As a result, the grain boundaries of austenite formed during the heating process in the LME test suppress the intrusion of Zn into the steel plate, thus achieving good LME resistance. To obtain this effect, the total amount of Nb dissolved in the surface layer of the plate and the amount of precipitated at a size of 100 nm or less needs to be 0.002% or more. It should be noted that there is no specific upper limit to the total amount of Nb dissolved in the surface layer of the plate and the amount of precipitated at a size of 100 nm or less; it can be below the Nb content. Therefore, the total amount of Nb dissolved in the surface layer and the amount precipitated below 100 nm is 0.002% or more. Preferably, the total amount of Nb dissolved in the surface layer and the amount precipitated below 100 nm is 0.003% or more. Preferably, the total amount of Nb dissolved in the surface layer and the amount precipitated below 100 nm is less than or equal to the Nb content.

[0127] Here, the method for determining the total amount of Nb dissolved in the surface layer and the amount of Nb precipitated below 100 nm is as follows. The sample is prepared by removing surface contaminants without surface grinding, using the steel plate surface as the analysis surface. For this sample, after electrolytic extraction with 10% acetylacetone as the electrolyte, the electrolyte is collected, and the amount of Nb dissolved and precipitated is analyzed using the quantitative method for solid solution composition in microalloyed steel described in Reference 1. The sample is electrolyzed in a non-aqueous solvent electrolyte for a certain time, and the electrolyte is collected as the sample for solid solution composition analysis. Additionally, the electrolyzed sample is removed from the electrolyte and transferred to a beaker containing methanol, and ultrasonically stirred. After stirring, the methanol is filtered through 100 nm filter paper, and the filtrate passing through the filter is used as the sample for precipitate analysis containing precipitates below 100 nm. For the sample for solid solution composition analysis, the amount of Nb dissolved is determined by ICP-luminescence analysis. In addition, for samples used for precipitate analysis, after the filtrate is dried, nitric acid, perchloric acid, and sulfuric acid are added and heated to dissolve until sulfuric acid fumes appear. After cooling, hydrochloric acid is added, and the solution is diluted with pure water to a certain volume. The amount of Nb precipitated below 100 nm is then determined by ICP-N luminescence analysis. It should be noted that in the case of coated steel sheets, the hot-dip galvanized layer of the sample is removed with alkali before the above analysis is performed.

[0128] [Reference 1] Tetsuya Shiroshiro, Tomoharu Ishida, Masao Inose, Kyoko Fujimoto: Iron and Steel, 99 (2013), 362.

[0129] [Hardness variation frequency per 1500μm in the width direction of the plate at a thickness of 200μm: less than 20 times]

[0130] This is a crucial inventive element of this disclosure. The desired flexibility can be achieved by reducing the frequency of hardness variation per 1500 μm along the width of the plate surface at a position 200 μm thick from the steel plate surface. To achieve this effect, the frequency of hardness variation per 1500 μm along the width of the plate surface at the 200 μm thickness position needs to be 20 times or less. It should be noted that there is no particular limitation on the lower limit of the frequency of hardness variation per 1500 μm along the width of the plate surface at the 200 μm thickness position from the steel plate surface, but a lower frequency is preferred; even 0 times / mm can achieve the effects of this disclosure. Therefore, the frequency of hardness variation per 1500 μm along the width of the plate surface at the 200 μm thickness position is 20 times or less. Preferably, the frequency of hardness variation per 1500 μm along the width of the plate surface at the 200 μm thickness position is 0 times or more. The frequency of hardness variation in the width direction of the plate at the 200μm thickness position is preferably less than 10 times.

[0131] Here, the method for determining the frequency of hardness variation per 1500 μm along the width of the plate at a thickness of 200 μm is as follows. Using the 200 μm thickness position (equivalent to 200 μm in depth from the steel plate surface) as the observation surface, the steel plate surface is ground and then mirror-polished using diamond paste. For the mirror-polished observation surface, a Vickers hardness tester is used to measure 100 points along the width of the plate at 15 μm intervals under a load of 50 gf. It should be noted that the measurement position is the center of the plate width. Based on the obtained results, a hardness distribution measured with a Vickers hardness tester along the width of the plate at a thickness of 200 μm is prepared. In this hardness distribution, the maximum hardness value Hv is first calculated. max ) - (Minimum hardness Hv) min The value of )} / 2. With {(maximum hardness Hv) max ) - (Minimum hardness Hv) min The value of )} / 2 is the baseline variation. Any variation in hardness exceeding the baseline variation is counted as one occurrence. The number of hardness variations within the measured hardness area (1500 μm in length) is determined. For example, as... Figure 2 As shown in (a), if the hardness distribution curve varies up and down within a range of 1500 μm in the width direction of the plate to (Hv) max -Hv minIf the number of times greater than 2 (the thick line in the figure) is 2, then the hardness variation frequency is 2 times per 1500μm width direction of the plate. Additionally, for example... Figure 2 As shown in (b), if the hardness distribution curve varies up and down within a range of 1500 μm in the width direction of the plate to (Hv) max -Hv min If the number of times greater than 2 (the thick line in the figure) is 5, then the hardness variation frequency is 5 times per 1500μm width direction of the plate.

[0132] [Surface softening thickness: 10μm~100μm]

[0133] Compared to the 1 / 4 position of the plate thickness, softening the surface layer of the steel plate can further improve its flexibility. To achieve this effect, a surface softening thickness of 10 μm or more is preferable. While there is no particular upper limit to the surface softening thickness, it is preferably 100 μm or less to achieve particularly good flexibility (TS). Therefore, a surface softening thickness of 10 μm or more is preferable, and further preferably 100 μm or less. A surface softening thickness of 12 μm or more is more preferable. A surface softening thickness of 80 μm or less is more preferable. A surface softening thickness of 15 μm or more is even more preferable. A surface softening thickness of 60 μm or less is even more preferable.

[0134] Here, the surface softening thickness is measured as follows.

[0135] Specifically, the surface of the plate thickness section (L-section) parallel to the rolling direction of the steel plate is smoothed by wet grinding. Next, using a Vickers hardness tester under a load of 5 gf, hardness is measured at 5 μm intervals along the plate thickness (depth) from a position 10 μm from the surface of the steel plate to the center of the plate thickness. Then, using the hardness obtained at 1 / 4 of the plate thickness as the reference hardness, a depth position closer to the surface than 1 / 4 of the plate thickness is determined as 0.85 times the reference hardness. The distance (depth) from the surface of the steel plate to this depth position (0.85 times the reference hardness) is then measured, and this measured value is set as the surface softening thickness.

[0136] Furthermore, in the steel microstructure disclosed herein, in addition to the aforementioned martensite (quenched martensite, tempered martensite, bainite), ferrite, and retained austenite, the inclusion of pearlite, cementite, metastable carbides (ε(ε) carbides, η(η) carbides, χ(χ) carbides, etc.), and other substances known as other steel sheet microstructures, as long as they are in the range of 3% or less in terms of area, will not impair the effectiveness of this disclosure.

[0137] High-strength steel plate

[0138] The composition and microstructure of high-strength steel plates are as described above. Furthermore, the thickness of high-strength steel plates is not particularly limited, but is typically 0.3 mm or more and 2.8 mm or less.

[0139] [Glazed Steel Sheet]

[0140] The coated steel sheet disclosed herein is a coated steel sheet having a coating on at least one side of the high-strength steel sheet disclosed herein. The type of coating is not particularly limited, and can be, for example, any of hot-dip galvanizing or electroplating. Alternatively, the coating can be an alloyed coating. The coating is preferably a zinc coating. The zinc coating may contain Al and Mg. Hot-dip galvanized aluminum-magnesium alloy (Zn-Al-Mg coating) is also preferred. In this case, it is preferable that the Al content is 1% to 22% by mass, the Mg content is 0.1% to 10% by mass, and the remainder is Zn. Furthermore, in the case of a Zn-Al-Mg coating, in addition to Zn, Al, and Mg, it may contain one or more elements selected from Si, Ni, Ce, and La, totaling less than 1% by mass. It should be noted that the coating metal is not particularly limited, therefore, in addition to Zn as described above, Al, etc., may also be used.

[0141] Furthermore, the composition of the coating is not particularly limited and can be a general composition. For example, in the case of hot-dip galvanized coatings and alloyed hot-dip galvanized coatings, the composition is generally as follows: containing Fe: less than 20% by mass, Al: 0.001% to 1.0% by mass, further containing a total of 0% to 3.5% by mass of one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the remainder consisting of Zn and unavoidable impurities. In this disclosure, it is preferable to have a coating adhesion amount of 20 to 80 g / m² on each side. 2 The hot-dip galvanized layer is further alloyed to obtain an alloyed hot-dip galvanized layer. Furthermore, in the case of a hot-dip galvanized layer, the Fe content in the coating can be less than 7% by mass; in the case of an alloyed hot-dip galvanized layer, the Fe content in the coating can be 7–20% by mass.

[0142] Next, the manufacturing method of the high-strength steel plate disclosed herein will be described.

[0143] First, a steel slab billet having the above-mentioned composition is melted to produce a steel slab billet. In this disclosure, the melting method for the steel slab billet is not particularly limited, and any known melting method such as a converter or electric furnace can be used. Furthermore, to prevent macroscopic segregation, the steel slab billet (slab) is preferably manufactured by continuous casting, but it can also be manufactured by ingot casting, thin slab casting, etc. In addition to the existing method of temporarily cooling to room temperature and then reheating after manufacturing the steel slab billet, energy-saving processes such as direct rolling or direct rolling, where the slab is loaded into the heating furnace in a warm state without cooling to room temperature or is rolled immediately after a short holding period, can be used without any problems.

[0144] Next, the steel slab is held at a slab heating temperature of 1150℃ or higher for more than 100 minutes.

[0145] [Slab heating temperature: above 1150℃]

[0146] This is a crucial element of the invention disclosed herein. By raising the slab heating temperature to 1150°C or higher, carbides, nitrides, or carbonitrides generated during continuous casting can be dissolved, increasing the total amount of Nb dissolved and the amount of Nb precipitated below 100 nm. Furthermore, by increasing the slab heating temperature, local enrichment of elements in the segregated portions of the slab is reduced, enabling the hardness variation frequency per 1500 μm in the width direction at a thickness of 200 μm to be less than 20 times. To achieve this effect, the slab heating temperature is set to 1150°C or higher. It should be noted that there is no particular upper limit to the slab reheating temperature, but to appropriately prevent an increase in the surface softening thickness after annealing, the TS (slab temperature) is preferably 1300°C or lower, within a more preferred range. Therefore, the slab heating temperature is 1150°C or higher. The slab heating temperature is preferably 1180°C or higher. The slab heating temperature is preferably 1300°C or lower. It should be noted that the slab heating temperature refers to the surface temperature of the steel slab during heating.

[0147] [Slab heating time: 100 minutes or more]

[0148] This is a crucial element of the invention disclosed herein. By extending the slab heating time, carbides, nitrides, or carbonitrides generated during continuous casting can be dissolved, increasing the total amount of Nb dissolved and the amount of Nb precipitated below 100 nm. Furthermore, by extending the slab heating time, the local enrichment of elements in the segregated portions of the slab is reduced, enabling the hardness variation frequency per 1500 μm in the width direction at a thickness of 200 μm to be less than 20 times. To achieve this effect, the slab heating time is 100 minutes or more. It should be noted that there is no particular upper limit to the slab heating time, but to appropriately prevent an increase in the softening thickness of the surface layer after annealing, the TS (heating time) is preferably 600 minutes or less, which is a more preferred range. Therefore, the slab heating time is 100 minutes or more. The slab heating time is preferably 120 minutes or more. The slab heating time is preferably 600 minutes or less.

[0149] It should be noted that under normal conditions, slabs are made into thin slabs by rough rolling. However, from the viewpoint of preventing failures during hot rolling, it is preferable to heat the thin slabs with a strip heater or the like before finishing rolling, while reducing the slab heating temperature.

[0150] [Rough rolling finish temperature: above 1050℃]

[0151] This is a crucial element of the invention disclosed herein. By increasing the rough rolling end temperature, the local enrichment of elements in the segregated portions of the slab is reduced, enabling the hardness variation frequency in the width direction at a 200 μm thickness position to be less than 20 times per 1500 μm. To achieve this effect, the rough rolling end temperature is set to 1050°C or higher. It should be noted that while no specific upper limit is specified for the rough rolling end temperature, it is preferably 1200°C or lower to ensure more uniform austenite grain size after annealing. Therefore, the rough rolling end temperature is 1050°C or higher. The rough rolling end temperature is preferably 1080°C or higher. The rough rolling end temperature is preferably 1200°C or lower. It should be noted that the rough rolling end temperature refers to the surface temperature of the steel plate at the end of rough rolling.

[0152] [Finishing rolling start temperature: above 1000℃]

[0153] This is a crucial element of the invention disclosed herein. By increasing the finishing rolling start temperature, the local enrichment of elements in the segregated portions of the slab is reduced, enabling the hardness variation frequency in the width direction at a 200 μm thickness position to be less than 20 times per 1500 μm. To achieve this effect, the finishing rolling start temperature is set to 1000°C or higher. It should be noted that there is no particular upper limit to the finishing rolling start temperature, but to ensure more uniform grain size of the original austenite grains after annealing, it is preferably 1180°C or lower. Therefore, the finishing rolling start temperature is 1000°C or higher. Preferably 1020°C or higher. Preferably 1180°C or lower. It should be noted that the finishing rolling start temperature is the temperature of the steel plate surface at the start of finishing rolling.

[0154] Increased rolling load and higher reduction rate of austenite in its non-recrystallized state lead to the development of abnormal microstructures elongated in the rolling direction, which sometimes reduces the workability of the annealed sheet. Therefore, finishing rolling is preferably performed at a finishing rolling temperature above the Ar3 phase transformation point. In addition, to improve the workability after annealing, the winding temperature after hot rolling is preferably above 300°C, and even more preferably below 700°C.

[0155] It should be noted that rough-rolled plates can be joined together during hot rolling and continuously finished rolled. Alternatively, the rough-rolled plates can be wound once. Furthermore, to reduce the rolling load during hot rolling, part or all of the finishing rolling can be lubricated rolling. From the viewpoint of homogenizing the shape and material of the steel plate, lubricated rolling is also effective. It should be noted that the coefficient of friction during lubricated rolling is preferably 0.10 or higher, and more preferably 0.25 or lower.

[0156] The hot-rolled steel sheets manufactured in this way are then pickled. Pickling removes oxides from the surface of the steel sheet, which is important for ensuring good chemical treatment properties and coating quality in the final high-strength steel sheet. Furthermore, pickling can be done in one step or in multiple steps.

[0157] Next, the hot-rolled sheet after pickling or the hot-rolled sheet after pickling and arbitrary heat treatment (hot-rolled annealed sheet) is cold-rolled to produce a cold-rolled sheet. Since strain is introduced uniformly and effectively and a uniform microstructure is obtained, it is preferable to perform cold rolling by multi-pass rolling with two or more passes, such as tandem multi-stand rolling or reverse rolling.

[0158] It should be noted that, in order to introduce processing strain into the surface layer of the steel sheet and increase the combined amount of Nb solid solution and precipitation below 100 nm in the surface layer during annealing, it is preferable to perform bending and back bending processes at least once each before cold rolling. The number of bending and back bending processes before cold rolling is not particularly specified, but it is preferable to perform them at least twice, and more preferably at least three times. It should be noted that bending and back bending processes before cold rolling are generally performed using rolls with a diameter of 300–1500 mm.

[0159] [Cumulative reduction rate of cold rolling: 20%~75%]

[0160] By achieving a cumulative reduction rate of 20% or more in cold rolling, the area fraction of ferrite can be reduced to less than 10%. Furthermore, by increasing the cumulative reduction rate in cold rolling, a large amount of shear bands can be introduced, resulting in a near-equiaxed austenite grain size during heating. As a result, the ferrite refractive index (YR) can be controlled within the desired range. To achieve this effect, the cumulative reduction rate in cold rolling is 20% or more. On the other hand, if the cumulative reduction rate in cold rolling exceeds 75%, the grain size of the austenite formed during annealing becomes fine, and the amount of retained austenite after annealing increases, thus failing to achieve the desired YR. Therefore, the cumulative reduction rate in cold rolling is 20% to 75%. The cumulative reduction rate in cold rolling is preferably 25% or more. The cumulative reduction rate in cold rolling is preferably 70%. The cumulative reduction rate in cold rolling is more preferably 27% or more. The cumulative reduction rate in cold rolling is more preferably 60% or less.

[0161] The cold-rolled sheet obtained as described above is subjected to an annealing process. The annealing conditions are as follows.

[0162] [Average heating rate in the temperature range of 250℃~700℃: 10℃ / s or more]

[0163] By increasing the average heating rate in the temperature range of 250°C to 700°C, the austenite grain size generated during heating can be made uniform, even if the austenite grain size during heating is close to equiaxed. To achieve this effect, the average heating rate in the temperature range of 250°C to 700°C needs to be 10°C / s or more. It should be noted that there is no particular upper limit to the average heating rate in the temperature range of 250°C to 700°C, but from a productivity point of view, it is preferable to be 50°C / s or less, more preferably 40°C / s or less. Therefore, the average heating rate in the temperature range of 250°C to 700°C is 10°C / s or more. The average heating rate in the temperature range of 250°C to 700°C is preferably 12°C / s or more. The average heating rate in the temperature range of 250°C to 700°C is preferably 50°C / s or less. The average heating rate in the temperature range of 250°C to 700°C is more preferably 14°C / s or more. The average heating rate in the temperature range of 250°C to 700°C is more preferably 40°C / s or less. It should be noted that the average heating rate was measured based on the temperature of the steel plate surface.

[0164] [Heating temperature: above 820℃]

[0165] If the heating temperature (annealing temperature) is less than 820°C, the annealing process results in a two-phase region of ferrite and austenite, containing a large amount of ferrite after annealing, making it difficult to achieve the desired TS, YP, λ, and bendability. It should be noted that there is no specific upper limit to the heating temperature, but if the heating temperature increases, the softened thickness of the annealed surface layer increases, and the TS decreases; therefore, 950°C or lower is preferred. Therefore, the heating temperature is 820°C or higher. Preferably 850°C or higher. Preferably 950°C or lower. More preferably 870°C or higher. More preferably 920°C or lower. It should be noted that the heating temperature is measured based on the temperature of the steel sheet surface.

[0166] [Dwell time from 750℃ or above to the heating temperature: 40 seconds or more]

[0167] This is a crucial element of the invention disclosed herein. By increasing the residence time at 750°C or higher up to the heating temperature, carbides, nitrides, or carbonitrides can be dissolved, increasing the combined amount of Nb dissolved in the thick surface layer and the amount precipitated below 100 nm. To achieve this effect, the residence time at 750°C or higher up to the aforementioned heating temperature is 40 seconds or more. It should be noted that there is no particular upper limit to the residence time at 750°C or higher up to the heating temperature, but to ensure the surface layer softening thickness is appropriate and the TS (thickness transition temperature) is more suitable, 400 seconds or less is preferred. Therefore, the residence time at 750°C or higher up to the aforementioned heating temperature is 40 seconds or more. The residence time at 750°C or higher up to the aforementioned heating temperature is preferably 45 seconds or more. The residence time at 750°C or higher up to the aforementioned heating temperature is preferably 400 seconds or less. The residence time at 750°C or higher up to the aforementioned heating temperature is more preferably 50 seconds or more. The residence time at 750°C or higher up to the aforementioned heating temperature is more preferably 300 seconds or less.

[0168] It should be noted that there is no particular limitation on the holding time at the above heating temperature, but it is preferably 10s to 600s.

[0169] [Oxygen concentration in the atmosphere at the heating temperature: 2 ppm to 30 ppm (preferred conditions)]

[0170] By increasing the oxygen concentration at the heating temperature during annealing, decarburization occurs through oxygen in the air, forming a softened layer on the surface of the steel sheet, resulting in better flexibility. To achieve this effect, it is preferable that the oxygen concentration at the heating temperature is 2 ppm or more by volume. On the other hand, as the oxygen concentration at the heating temperature increases, the softened thickness of the annealed surface layer increases, and the TS (steel surface temperature) decreases; therefore, the oxygen concentration at the heating temperature is preferably 30 ppm or less by volume. Thus, the oxygen concentration at the heating temperature is preferably 2 to 30 ppm by volume. More preferably, it is 4 ppm or more by volume. More preferably, it is 28 ppm or less by volume. Further preferably, it is 5 ppm or more by volume. Further preferably, it is 25 ppm or less by volume. It should be noted that the above heating temperature is based on the surface temperature of the steel sheet. That is, the oxygen concentration is adjusted to the above range when the surface temperature of the steel sheet is at the above heating temperature.

[0171] [Dew point of the atmosphere at the heating temperature: above -35°C (preferred condition)]

[0172] By increasing the dew point of the atmosphere at the heating temperature during annealing, decarburization occurs through moisture in the air, forming a softened layer on the surface of the steel sheet, resulting in improved flexibility. To achieve this effect, it is preferable that the dew point at the heating temperature is -35°C or higher. It should be noted that while there is no specific upper limit for the dew point at the heating temperature, to appropriately prevent an increase in the surface softening thickness after annealing and to keep the thickness of the TS (transformation layer) within a more preferable range, the dew point at the heating temperature is preferably 15°C or lower, more preferably 5°C or lower. Therefore, the dew point of the atmosphere at the aforementioned heating temperature is preferably -35°C or higher. The dew point of the atmosphere at the heating temperature is more preferably -30°C or higher. The dew point of the atmosphere at the heating temperature is more preferably 15°C or lower. The dew point of the atmosphere at the heating temperature is further preferably -25°C or higher. The dew point of the atmosphere at the heating temperature is further preferably 5°C or lower. It should be noted that the aforementioned heating temperature is based on the surface temperature of the steel sheet. That is, the dew point is adjusted to the aforementioned range when the surface temperature of the steel sheet is at the aforementioned heating temperature.

[0173] In addition, the average cooling rate below the heating temperature and above 400°C is not particularly limited, but is preferably 5°C / s to 30°C / s.

[0174] [Average cooling rate in the temperature range of 250℃~400℃: ≥1.0℃ / s]

[0175] This is a crucial inventive element of this disclosure. By increasing the average cooling rate in the temperature range of 250°C to 400°C, the self-tempering of martensite can be suppressed, resulting in a hardness variation frequency of less than 20 times per 1500 μm in the width direction at a plate thickness of 200 μm. To achieve this effect, the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C / s or more. It should be noted that there is no particular upper limit to the average cooling rate in the temperature range of 250°C to 400°C, but from a productivity point of view, it is preferably 100.0°C / s or less, more preferably 80.0°C / s or less. Therefore, the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C / s or more. The average cooling rate in the temperature range of 250°C to 400°C is preferably 2.0°C / s or more. The average cooling rate in the temperature range of 250°C to 400°C is preferably 100.0°C / s or less. The average cooling rate in the temperature range of 250℃ to 400℃ is preferably 3.0℃ / s or higher. The average cooling rate in the temperature range of 250℃ to 400℃ is preferably 80.0℃ / s or lower. It should be noted that the average cooling rate is measured based on the temperature of the steel plate surface.

[0176] [Cooling stop temperature: below 150℃]

[0177] If the cooling stop temperature exceeds 150°C, it promotes the self-tempering of martensite, making it impossible to achieve the desired hardness variation frequency per 1500 μm along the width of the plate from a position 200 μm thick on the steel plate surface. Consequently, it is difficult to achieve the desired flexibility. It should be noted that while there is no specific lower limit for the cooling stop temperature, from a productivity point of view, it is preferably above room temperature. Therefore, the cooling stop temperature is 150°C or below, preferably 100°C or below. It should be noted that the average stopping speed is measured based on the temperature of the steel plate surface.

[0178] It should be noted that there is no particular requirement for the cooling rate from 250°C or below to the aforementioned cooling stop temperature. However, in order to further improve TS (Transmission Time), the average cooling rate from the time of galvanizing or further galvanizing alloying to the aforementioned cooling stop temperature is preferably 2°C / s or more, and more preferably 5°C / s or more. On the other hand, due to limitations in production technology, the average cooling rate from the time of galvanizing or further galvanizing alloying to the aforementioned cooling stop temperature is preferably 50°C / s or less, and more preferably 40°C / s or less.

[0179] It should be noted that cold-rolled steel sheets can be cooled from the aforementioned cooling stop temperature to room temperature. There is no particular limitation on the cooling rate from the cooling stop temperature to room temperature; any method can be used to cool to room temperature. Cooling methods include gas jet cooling, mist cooling, water cooling, and air cooling.

[0180] As described above, the annealed high-strength steel sheet can be cooled to the aforementioned cooling stop temperature and then rolled with an elongation of 0.05% to 1.00%. By ensuring that the elongation of the rolling after cooling to the aforementioned cooling stop temperature is 0.05% or more, YR can be controlled within a desired range. Furthermore, by ensuring that the elongation of the rolling after cooling to the aforementioned cooling stop temperature is 1.00% or less, the volume fraction of retained austenite can be within a preferred range, and YR and λ can be within a preferred range. Therefore, the elongation of the rolling after cooling to the aforementioned cooling stop temperature is preferably 0.05% to 1.00%. More preferably, it is 0.10% or more. More preferably, it is 0.50% or less.

[0181] Rolling after cooling to the aforementioned cooling stop temperature can be performed on an apparatus continuous with the aforementioned continuous annealing apparatus (online) or on an apparatus discontinuous with the aforementioned continuous annealing apparatus (offline). Furthermore, the target elongation can be achieved in a single rolling pass, or multiple rolling passes can be performed to achieve a total elongation of 0.05% to 1.00%. It should be noted that the rolling described here generally refers to quenching and tempering rolling, but any method using a tension straightener or similar equipment can be employed as long as it imparts the same elongation as quenching and tempering rolling.

[0182] The high-strength steel sheet can be tempered after cooling to the aforementioned cooling stop temperature, or after further rolling following cooling to the aforementioned cooling stop temperature. By setting the tempering temperature to 400°C or below, the volume fraction of retained austenite can be kept within a more preferred range. Therefore, when tempering is performed, the tempering temperature is preferably 400°C or below, and more preferably 350°C or below.

[0183] It should be noted that there is no particular limitation on the cooling rate from the above tempering temperature to room temperature, and any method can be used to cool to room temperature. Cooling methods can include gas jet cooling, mist cooling, water cooling, and air cooling.

[0184] It should be noted that when high-strength steel plates are used as traction objects, they are usually used as traction objects after being cooled to room temperature.

[0185] [Manufacturing method of galvanized steel sheet]

[0186] High-strength steel sheets manufactured as described above can be coated to obtain coated steel sheets. Examples of coating processes include hot-dip galvanizing and alloying after hot-dip galvanizing. Alternatively, annealing and galvanizing can be performed continuously on a single production line. Furthermore, the coating can be formed by electroplating such as Zn-Ni electroplating alloys, or by hot-dip galvanizing an aluminum-magnesium alloy. It should be noted that the above description focuses on galvanizing, but the types of coating metals such as Zn and Al are not particularly limited.

[0187] It should be noted that during hot-dip galvanizing, after immersing the high-strength steel sheet in a galvanizing bath at 440°C to 500°C, the coating adhesion is adjusted by methods such as gas wiping. A galvanizing bath with an Al content of 0.10% to 0.23% by mass is preferably used for hot-dip galvanizing. Furthermore, when performing alloying treatment for galvanizing, the alloying treatment is carried out in a temperature range of 470°C to 600°C after hot-dip galvanizing. If the temperature is below 470°C, the Zn-Fe alloying rate is too slow, resulting in reduced productivity. On the other hand, if alloying treatment is performed at a temperature exceeding 600°C, the untransformed austenite phase may sometimes transform into pearlite, reducing the total sulfide (TS). Therefore, when performing alloying treatment for galvanizing, it is preferable to perform alloying treatment in a temperature range of 470°C to 600°C, and more preferably in a temperature range of 470°C to 560°C. Alternatively, electroplating can also be performed. In addition, the preferred coating adhesion amount is 20-80 g / m² per side. 2 (Double-sided coating) Alloyed hot-dip galvanized steel sheet (GA) is preferably prepared by performing the following alloying treatment to achieve an Fe concentration of 7 to 15% by mass in the coating.

[0188] Skin rolling can be performed on the steel sheet after plating. From the viewpoint of controlling the yield strength (YR) within the desired range, the reduction rate in the skin rolling after plating is preferably 0.05% or more. It should be noted that there is no particular upper limit to the reduction rate in skin rolling, but from a productivity viewpoint, it is preferably 1.50% or less. Skin rolling can be performed online or offline. Furthermore, skin rolling with the target reduction rate can be performed in one go or in multiple stages.

[0189] There are no particular limitations on the conditions of other manufacturing methods, but from a productivity point of view, it is preferable to perform the above-mentioned series of treatments, including annealing, hot-dip galvanizing, and alloying treatment of zinc plating, in a CGL (Continuous Galvanizing Line) that serves as a hot-dip galvanizing production line. After hot-dip galvanizing, wiping can be performed to adjust the weight per unit area of ​​the coating. It should be noted that coating conditions other than those described above can be performed according to conventional methods for hot-dip galvanizing.

[0190] It should be noted that when high-strength coated steel sheets are used as traction objects, they are usually used as traction objects after cooling to room temperature.

[0191] It should be noted that manufacturing conditions other than those mentioned above can be carried out using conventional methods.

[0192] [member]

[0193] Components made at least partially of the aforementioned high-strength steel or high-strength clad steel sheet can be provided. In one example, the aforementioned high-strength steel or high-strength clad steel sheet can be formed into a target shape by stamping to manufacture an automotive part. It should be noted that the automotive part may include steel sheets other than the high-strength steel sheet or high-strength clad steel sheet of this embodiment as sheet blanks. According to this embodiment, high-strength steel sheets with a TS of 1180 MPa or higher and possessing dimensional accuracy, tensile flange strength, bending strength, and LME resistance can be provided. Therefore, it is suitable as an automotive part that contributes to vehicle body lightweighting. This high-strength steel or high-strength clad steel sheet can be well used in automotive parts, especially in all components used as frame structural parts or reinforcing parts.

[0194] Example

[0195] Steel with the composition shown in Table 1, the remainder consisting of Fe and unavoidable impurities, was melted in a converter and produced into slabs by continuous casting. The resulting slabs were reheated, hot-rolled, pickled, and then cold-rolled. The conditions for hot and cold rolling are shown in Tables 2-1 and 2-2. It should be noted that the winding temperature was 500°C, the holding time within the heating temperature range was 100 s, and the average cooling rate below the heating temperature but above 400°C was 20°C / s.

[0196] Next, annealing was performed under the conditions shown in Tables 2-1 and 2-2 to obtain high-strength steel sheet (CR). Then, some of the thin steel sheets were coated to obtain hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), and electro-galvanized steel sheet (EG). For the hot-dip galvanizing bath, a zinc bath containing 0.14–0.19% by mass Al was used in GI, and a zinc bath containing 0.14% by mass Al was used in GA, with a bath temperature of 470°C. The coating adhesion was 45–72 g / m² per side in GI. 2 (Double-sided plating) approximately, and in GA, it is 45g / m² per side. 2 (Double-sided plating) Approximately. Additionally, in GA, the Fe concentration in the plating is 9% to 12% by mass. In EG, where the plating is a Zn-Ni plating, the Ni content in the plating is 9% to 25% by mass.

[0197]

[0198]

[0199]

[0200] The high-strength steel sheet and high-strength coated steel sheet obtained as described above were used as test steels, and their tensile properties, tensile flange properties, bending properties, and LME resistance were evaluated according to the following test methods. The results are shown in Tables 3-1 and 3-2.

[0201] [Tension Test]

[0202] Tensile tests were conducted according to JIS Z 2241. JIS No. 5 test specimens were taken from the obtained steel sheet, perpendicular to the rolling direction, and tested at a crosshead speed of 1.67 × 10⁻⁶. －1 Tensile tests were conducted under conditions of mm / s to determine YS, TS, and E1. It should be noted that in this disclosure, a TS of 1180 MPa or higher is considered acceptable. Furthermore, excellent dimensional accuracy of the component is defined as a yield ratio (YR), an indicator of dimensional accuracy, being between 65% and 90%, which is considered good. It should be noted that YR is calculated using the method described in equation (2) above.

[0203] [Drilling Test]

[0204] The hole expansion test was conducted according to JIS Z 2256. After cutting the obtained steel plate into 100mm × 100mm pieces, a hole with a diameter of 10mm was punched out with a gap of 12.5%. Then, using a die with an inner diameter of 75mm, a conical punch with a 60° apex angle was pressed into the hole under a wrinkle-resistant force of 9ton (88.26kN). The hole diameter at which cracking occurs was measured, and the limiting hole expansion rate λ (%) was calculated according to the following formula. The hole expansion performance was evaluated based on the value of this limiting hole expansion rate.

[0205] Limiting porosity: λ(%) = {(D)} f -D0) / D0}×100

[0206] Among them, D f D0 is the initial pore diameter (mm) at which cracks occur. It should be noted that in this disclosure, the pore expansion ratio (λ), an indicator of tensile flange performance, is independent of the strength of the steel plate; a value of λ or higher is considered to indicate good tensile flange performance.

[0207] [Bending Test]

[0208] The bending test was conducted according to JIS Z 2248. A strip-shaped test piece, 30 mm wide and 100 mm long, was taken from the obtained steel plate with the axial direction of the bending test parallel to the rolling direction of the steel plate. Then, a 90°V bending test was performed under a pressing load of 100 kN and a pressing holding time of 5 seconds. It should be noted that in this disclosure, bending performance is evaluated by the pass rate of the bending test. Five samples were subjected to bending tests at the largest R (for example, a bending radius of 6.0 mm when the plate thickness is 1.2 mm) where the value R / t (the ratio of the bending radius (R) to the plate thickness (t)) was 5 or less. Then, the presence of cracks at the edge of the bending apex was evaluated. A case where none of the five samples cracked was judged as having "excellent" bending performance. Furthermore, a case where one or more of the five samples developed microcracks smaller than 200 μm was judged as having "good" bending performance. Furthermore, samples with microcracks larger than 200 μm in one or more of the five samples were judged as having "poor" bending performance. Here, the presence or absence of cracks was evaluated by measuring the ridge of the bending apex using a digital microscope (RH-2000: manufactured by Hirox Co., Ltd.) at 40x magnification.

[0209] [LME Resistance]

[0210] LME resistance was determined through a high-temperature tensile test. For high-strength steel sheets (CR) and various coated steel sheets (GI, GA, EG), a strip-shaped sample with a width of 105 mm and a length of 25 mm was prepared, with the tensile direction perpendicular to the rolling direction of the steel sheet. The end face of the sample was then ground to a width of 99 mm and a length of 20 mm. Next, a tensile test piece was prepared with a shoulder radius of 20 mm, a parallel section width of 5 mm, and a parallel section length of 20 mm. Furthermore, a notch with a radius of 2 mm and a distance of 3 mm between the tips of the notches was made in the center of the parallel section of the tensile test piece, resulting in a notched tensile test piece. It should be noted that, from the viewpoint of ensuring contact with the electrodes, only one side of the steel sheet was ground, and the sheet thickness was adjusted to 1.0 mm. Using the obtained notched tensile test piece, a high-temperature tensile test was performed using a Thermec Master Z thermal processing reproduction apparatus. After heating to 900℃ at 100℃ / s, the sample was immediately cooled with gas at 40℃ / s. Upon reaching 700℃, a tensile test was immediately performed at a crosshead speed of 50mm / s until fracture. After the sample fractured, it was cooled with gas at 100℃ / s to below 200℃.

[0211] For the fracture portion of the test piece supplied for the high-temperature tensile test as described above, the sample for measuring the reduction rate of plate thickness is obtained by cutting it through a section (L section) parallel to the tensile direction of the test piece as the observation surface, and the reduction rate of plate thickness expressed by the above formula (3) is calculated. It should be noted that in this disclosure, when the reduction rate of plate thickness is 0.20 or higher, it is judged to have excellent LME resistance.

[0212] In addition, the area ratio of martensite and ferrite, the volume ratio of retained austenite, the average ratio of the grain size in the rolling direction to the grain size in the thickness direction of the original austenite grains, the frequency of hardness variation per 1500 μm in the width direction of the plate surface at a position 200 μm thick from the surface of the steel plate, and the surface softening thickness were determined according to the above method. Furthermore, the remaining microstructure was also observed using the method described below. After cutting the sample with the plate thickness section (L section) parallel to the rolling direction of the steel plate as the observation surface, the observation surface was mirror-polished with diamond paste, then finely polished with colloidal silica, and further etched with 3 vol.% nitric acid alcohol to expose the microstructure. Under an accelerating voltage of 15 kV, using a SEM (Scanning Electron Microscope), the observation position was 1 / 4 of the plate thickness, and three fields of view were observed at 5000x magnification within a 17 μm × 23 μm field of view. Cementite was identified as the remaining tissue from the obtained tissue images.

[0213]

[0214]

[0215] As shown in Tables 3-1 and 3-2, in the examples of the present invention, TS is 1180 MPa or higher, and the dimensional accuracy, tensile flange properties, bending properties, and LME resistance of the component are all excellent. On the other hand, in the comparative examples, any one or more of the following are poor: strength (TS), dimensional accuracy (YR), tensile flange properties (λ), bending properties, and LME resistance.

[0216] The embodiments of this disclosure have been described above, but this disclosure is not limited to these embodiments. That is, all other embodiments, examples, and techniques applied by those skilled in the art based on these embodiments are included within the scope of this disclosure. For example, in the series of heat treatments in the above manufacturing method, there are no particular limitations on the equipment used to perform the heat treatment on the steel plate, as long as the heat process conditions are met.

[0217] Industrial availability

[0218] By applying the high-strength steel sheet of this disclosure, for example, to structural components such as automotive parts, it is possible to achieve improved fuel efficiency through vehicle body lightweighting.

Claims

1. A high-strength steel plate having the following composition and steel structure, and having a tensile strength of 1180 MPa or higher; The composition comprises, by mass%, C: 0.090%–0.390%, Si: 0.01%–2.50%, Mn: 2.00%–4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 0.100%, N: less than 0.0100%, Nb: 0.002%–0.100%, Ti: 0.005%–0.100%, and B: 0.0002%–0.0100%, and [%Si] / [%Mn] satisfies a relationship of 0.10–0.60, the amount of free Ti obtained by the following formula (1) satisfies a relationship of more than 0.001% by mass, and the remainder consists of Fe and unavoidable impurities; The steel microstructure is defined as having a total Nb solid solution content and Nb precipitates below 100 nm in the surface layer of the steel plate of 0.002% by mass or more. At the 1 / 4 position of the plate thickness The martensite area ratio is over 78%. The area fraction of ferrite is less than 10%. The volume fraction of retained austenite is less than 10.0%. The average ratio of the grain size in the rolling direction to the grain size in the thickness direction of the original austenite grains is less than 2.

0. The hardness variation frequency in the width direction of the plate at the 200μm thickness position is less than 20 times per 1500μm. Free Ti content (%) = [%Ti] - (47.9 / 14.0) × [%N] - (47.9 / 32.1) × [%S] ... (1) It should be noted that [%X] in formula (1) represents the content of element X in the steel, in the form of mass%, which is 0 when element X is not present.

2. The high-strength steel plate according to claim 1, wherein the surface softening thickness of the steel structure is 10μm to 100μm.

3. The high-strength steel sheet according to claim 1 or 2, wherein, The composition further contains, by mass percent, at least one element selected from O: less than 0.0100%, V: less than 0.200%, Ta: less than 0.10%, W: less than 0.10%, Cr: less than 1.00%, Mo: less than 1.00%, Ni: less than 1.00%, Co: less than 0.010%, Cu: less than 1.00%, Sn: less than 0.200%, Sb: less than 0.200%, Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%, Zr: less than 0.100%, Te: less than 0.100%, Hf: less than 0.10%, and Bi: less than 0.200%.

4. A high-strength coated steel sheet, wherein at least one side of the high-strength steel sheet according to any one of claims 1 to 3 has a coating.

5. A method for manufacturing a high-strength steel plate, comprising holding a steel slab having the composition described in claim 1 or 3 at a slab heating temperature of 1150°C or higher for at least 100 minutes. Next, the steel slab is subjected to hot rolling at a roughing end temperature of 1050°C or higher and a finishing start temperature of 1000°C or higher to produce a hot-rolled plate. Next, the hot-rolled plate is pickled. Next, the hot-rolled sheet is cold-rolled to produce a cold-rolled sheet by cold rolling with a cumulative reduction rate of 20% to 75%. Next, the cold-rolled sheet is subjected to the following annealing process to obtain a high-strength steel sheet. The annealing process is carried out under the condition that the average heating rate in the temperature range of 250°C to 700°C is 10°C / s or more and the residence time from 750°C to the heating temperature is 40s or more, the sheet is heated to a heating temperature of 820°C or more, and the average cooling rate in the temperature range of 250°C to 400°C is 1.0°C / s or more, and the sheet is cooled to below 150°C.

6. The method for manufacturing high-strength steel plate according to claim 5, wherein, The oxygen concentration of the atmosphere at the heating temperature is 2 to 30 ppm by volume, and the dew point of the atmosphere is above -35°C.

7. A method for manufacturing a high-strength coated steel sheet, wherein after the annealing process described in claim 5 or 6, a coating process is performed to coat at least one side of the high-strength steel sheet to obtain the high-strength coated steel sheet.

8. A component, at least a portion of which is made of high-strength steel plate as described in any one of claims 1 to 3.

9. A component, at least a portion of which is made of the high-strength plated steel sheet as described in claim 4.

10. The component according to claim 8 or 9, which is for use as a frame structure component of an automobile or as a reinforcing component of an automobile.

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