Steel sheet, member, and component, and method for

Abstract

Provided is a high-strength steel sheet having excellent member strength, stretch flangeability, bendability, and delayed fracture resistance under atmospheric corrosion and in a coated state. The steel sheet has a prescribed component composition, microstructure and the like, the average value [tau q] of the plastic deformation initiation stress [tau q] measured by nanoindentation at a position 1 / 4 of the sheet thickness of the steel sheet is 2.50-4.10 GPa, the standard deviation [sigma q] of the tau q is 0.30 GPa or less, and the average value [tau q] of the plastic deformation initiation stress [tau q] is 0.30 GPa or less. When [tau s] is the average value of the plastic deformation start stress [tau] s measured by the nanoindentation method at a position 10 [mu] m from the surface of the steel sheet, the ratio of measurement points less than 0.85 * [tau s] is 25.0% or less.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet, particularly a high-strength steel sheet excellent in component strength, stretch flange formability, bendability, and delayed fracture resistance under atmospheric corrosion and in a coated state, and a method for producing the same. The steel sheet of the present application can be favorably used as a structural member for a vehicle member or the like. BACKGROUND

[0002] In order to achieve both reduction in CO2 emission and improvement in crashworthiness due to weight reduction of a vehicle, high-strength thin steel sheets for vehicles are being promoted, and new legal regulations are being introduced.

[0003] Therefore, in the main structural members forming the frame of a vehicle cabin, the application of high-strength steel sheets having a tensile strength (TS) of 1180 MPa or more is increasing.

[0004] High component strength is sometimes required for high-strength steel sheets used in reinforced members and frame structural members of a vehicle. In this case, a high yield ratio (YR = yield strength YS / tensile strength TS x 100) is sometimes required, but the members such as a crash box have a blanked end face and a bent processed portion. Therefore, the steel sheets used for these members are also required to have good stretch flange formability and bendability.

[0005] Here, in high-strength steel sheets having a tensile strength of 1180 MPa or more, there is a problem that a delayed fracture phenomenon in which a component suddenly breaks due to hydrogen permeating from an atmospheric corrosion environment in which a vehicle runs occurs.

[0006] That is, in a steel sheet for a vehicle, stress is applied at the time of press working and at the time of assembly of a member, and then there is a risk that hydrogen permeates into the steel sheet from the environment, and therefore it is required to improve the delayed fracture resistance of a high-strength steel sheet.

[0007] In addition, in a vehicle member, the risk of delayed fracture occurrence is high in a bent processed portion and a sheared end face portion, and therefore it is required to improve the delayed fracture resistance of a bent processed portion having a sheared end face portion.

[0008] In particular, a vehicle member is exposed to an atmospheric corrosion environment in a coated state. The coated state is different from the uncoated state, and a defective portion of coating inevitably occurs. In an environment in which dry and wet cycles are repeated, corrosion locally occurs in the surface layer of the steel sheet from the defective portion of coating described above, hydrogen permeates from the corrosion portion, and this can lead to the occurrence of delayed fracture.

[0009] Therefore, it is required to improve the resistance to delayed failure of bent sections with sheared ends in a painted state under conditions of repeated wetting and drying (hereinafter also referred to as "resistance to delayed failure under atmospheric corrosion and in the painted state"). Therefore, in order to increase the application rate of high-strength steel sheets in automotive parts, it is desirable to comprehensively satisfy the above-mentioned characteristics.

[0010] In response to the above requirements, for example, Patent Document 1 discloses an ultra-high strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement and a tensile strength of more than 1300 MPa, and a method for manufacturing the same.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: International Publication No. 2018 / 011978 Summary of the Invention

[0014] The high-strength steel sheet described in Patent Document 1 exhibits excellent delayed-destruction characteristics on the punched end face in a hydrochloric acid aqueous solution at pH 1. However, it does not consider the characteristics of atmospheric corrosion environments, i.e., the delayed-destruction characteristics of the coated state under conditions of alternating wet and dry conditions throughout the day and night.

[0015] Therefore, in order to widely apply high-strength steel sheets to automotive parts, in addition to excellent component strength, elongation flange properties, and bending properties, it is also required to improve the resistance to delayed failure under atmospheric corrosion and in the painted state.

[0016] The present invention was developed in view of the above circumstances, and its object is to obtain a high-strength steel sheet with excellent component strength, elongation flange properties, bending properties, and resistance to delayed failure under atmospheric corrosion and in a painted state, and to provide an advantageous method for manufacturing the high-strength steel sheet. Furthermore, its object is to provide components made of the high-strength steel sheet and automotive frame structural components or automotive reinforcing components made using the components.

[0017] In this invention, "high-strength steel plate" refers to a steel plate with a tensile strength (TS) of 1180 MPa or higher as determined by the tensile test described later.

[0018] "Excellent component strength" means that the yield ratio (YR) obtained through the tensile test described later exceeds 75%.

[0019] "Excellent extension flange properties" means that the expansion ratio (λ) obtained by the expansion test described later is more than 30%.

[0020] "Excellent bending performance" means that the ultimate bending radius (R / t) determined by the bending test described later is less than 5.0.

[0021] "Excellent resistance to delayed failure under atmospheric corrosion and in the coated state" means that after the test piece with shear end face described later was subjected to stress and chemical conversion electrodeposition coating, no cracks were observed after 30 days of repeated wet and dry corrosion cycle test.

[0022] The inventors conducted in-depth research and found that the above-mentioned objectives could be achieved by adopting the structure described below, thus completing the present invention.

[0023] That is, the main idea of ​​this invention is as follows.

[0024] 1. A steel plate having, by mass percent, the following composition: C: 0.090% to 0.390%, Si: 0.01% to 2.00%, Mn: 2.00% to 4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 1.000%, N: less than 0.0100%, and O: less than 0.0100%, with the remainder consisting of Fe and unavoidable impurities.

[0025] The microstructure at 1 / 4 of the steel plate thickness is as follows:

[0026] The area ratio of tempered martensite is above 75%, the area ratio of fresh martensite is below 15%, the combined area ratio of ferrite and bainite is below 15%, and the area ratio of retained austenite is below 15%.

[0027] Furthermore, the average value of the plastic deformation initiation stress τq at the 1 / 4 position of the plate thickness, measured by nanoindentation, is 2.50 GPa to 4.10 GPa, and the standard deviation σq of the plastic deformation initiation stress τq is less than 0.30 GPa.

[0028] The microstructure at a distance of 10 μm from the surface of the steel plate is as follows:

[0029] The area ratio of tempered martensite is below 40%, the area ratio of pearlite is below 15%, and the combined area ratio of ferrite and bainite is above 60%.

[0030] When the average value of the plastic deformation initiation stress τs at the above 10μm position measured by nanoindentation method is set as [τs], the proportion of measurement points with a value less than 0.85 × [τs] is less than 25.0%.

[0031] 2. The steel plate according to claim 1 above, wherein the above composition further contains, by mass %, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[0032] 3. The steel plate according to 1 or 2 above, wherein the surface of the steel plate has a coating.

[0033] 4. A component comprising a steel plate as described in any one of 1 to 3 above.

[0034] 5. A frame structure component or reinforcing component of an automobile, comprising the components described in 4 above.

[0035] 6. A method for manufacturing a steel plate, comprising performing a hot rolling process on a steel billet having the composition described in 1 or 2 above: the number of passes in a temperature region of 1000°C or higher being 4 or more, the reduction rate of each pass being 15% or more, and the average strain rate being 9 × 10⁻⁶. －4 / s~1×10 －2 After rough rolling under conditions within the range of / s, finish rolling is performed, followed by coiling to obtain a hot-rolled sheet; the hot-rolled sheet is then pickled and cold-rolled to obtain a cold-rolled sheet. After the pickling and cold-rolling process, the cold-rolled sheet is annealed and cooled to below 150°C, and then reheated.

[0036] The above annealing is carried out under the following conditions: the heating temperature is above 800°C, and within the heating temperature range T1 above 800°C, the dew point is above -25°C, and the following equation 1 is satisfied.

[0037] 2.0≤K≤60.0...Equation 1

[0038] It should be explained that

[0039]

[0040] In addition, among them,

[0041] [%C] represents the carbon content in the steel plate.

[0042] The time when the temperature of the cold-rolled sheet first reaches 800°C during the annealing heating process is defined as t = 0 (s), and t = tE (s) is the time when the annealing is completed and the temperature of the cold-rolled sheet reaches 800°C again. t (°C) represents the average temperature of the cold-rolled sheet during the time interval t: t-1 to t (s).

[0043] Furthermore, in the aforementioned cooling process,

[0044] The average cooling rate v2 in the temperature range T2 of 600℃~750℃ is 1.0℃ / s~15.0℃ / s.

[0045] The average cooling rate in the temperature range above 500°C and below 600°C exceeds the aforementioned v2.

[0046] The residence time in temperature range T3 (above 400℃ but below 500℃) is set to 10s–150s.

[0047] The average cooling rate v4 in the temperature range T4 from Ms -100℃ to Ms℃ is made to be greater than 3.0℃ / s.

[0048] Furthermore, the aforementioned reheating is carried out under the condition that the highest temperature reached during the reheating, i.e., temperature X, and the holding time Y above temperature X-10°C satisfy the following equation 2.

[0049] 8000≤(273+X)×(20+Log(Y / 3600))≤12000...Equation 2

[0050] The unit of temperature X is °C, and the unit of holding time Y is seconds.

[0051] 7. The method for manufacturing the steel plate according to 6 above, wherein a plating process is further performed during the cooling process.

[0052] 8. A method for manufacturing a component, comprising the following steps: performing at least one of forming or joining processing on the steel plate described in any one of 1 to 3 above to produce the component.

[0053] According to the present invention, a high-strength steel sheet with excellent component strength, elongation flange properties, flexural properties, and resistance to delayed failure under atmospheric corrosion and in the coated state can be provided.

[0054] In addition, it is possible to provide components made of the aforementioned steel plates.

[0055] Furthermore, according to the present invention, a method for manufacturing the aforementioned steel plate and components can be provided.

[0056] Furthermore, according to the present invention, it is possible to provide a car frame structure component or a car reinforcement component made using the above-described components. Detailed Implementation

[0057] The high-strength steel plate and its manufacturing method of the present invention will be described in detail below, including their composition, microstructure and manufacturing method.

[0058] First, the appropriate range of the composition and the rationale for its limitation shall be explained. It should be noted that in the following explanation, the "%" indicating the content of the constituent elements of steel, unless otherwise specified, means "mass %".

[0059] [C: 0.090%~0.390%]

[0060] C is one of the important basic components of steel. In this high-strength steel plate, it affects the area ratio of tempered martensite at the 1 / 4 position of the plate thickness and the resistance to delayed failure.

[0061] If the carbon content is too low, the area ratio of tempered martensite at the 1 / 4 position of the plate thickness will decrease, making it difficult to achieve a temper strength (TS) of 1180 MPa or higher. Therefore, the carbon content is 0.090% or higher. The carbon content is preferably 0.115% or higher, and more preferably 0.140% or higher.

[0062] On the other hand, if the carbon content is too high, the strength of the tempered martensite at the 1 / 4 position of the plate thickness increases significantly, promoting cracking progress during delayed failure tests and reducing the resistance to delayed failure under atmospheric corrosion and in the coated state. Therefore, the carbon content is 0.390% or less. The carbon content is preferably 0.375% or less, and more preferably 0.360% or less.

[0063] [Si: 0.01% to 2.00%]

[0064] Si increases the strength of steel sheets by suppressing the precipitation of cementite in tempered martensite and fresh martensite, as well as through solid solution strengthening. To achieve this effect, the Si content is 0.01% or more. Preferably, the Si content is 0.05% or more, and more preferably 0.10% or more.

[0065] On the other hand, if the Si content is too high, it significantly inhibits the precipitation of carbides during bainitic and martensitic phase transformations, leading to an excessive increase in retained austenite at the 1 / 4 position of the plate thickness. This results in a significant increase in the hardness of the work-induced martensite formed from the retained austenite during shearing. Consequently, the resistance to delayed failure under atmospheric corrosion and in the coated state is reduced. Furthermore, the elongation flangeability is decreased. Therefore, the Si content is 2.00% or less. Preferably, the Si content is 1.75% or less, more preferably 1.50% or less.

[0066] [Mn: 2.00%~4.00%]

[0067] Mn is one of the important basic components of steel, and in particular, it significantly affects the area ratio of tempered martensite, resistance to delayed failure, and extension flange properties in this invention.

[0068] If the Mn content is too low, the area ratio of tempered martensite will decrease, making it difficult to achieve a temper strength (TS) of 1180 MPa or higher. Therefore, the Mn content is 2.00% or higher. The Mn content is preferably 2.20% or higher, and more preferably 2.40% or higher.

[0069] On the other hand, if the Mn content is too high, the austenite is stabilized, and the retained austenite at the 1 / 4 position of the plate thickness increases excessively. During shearing, the hardness of the process-induced martensite formed from the retained austenite increases significantly. As a result, the resistance to delayed failure under atmospheric corrosion and in the coated state decreases. Furthermore, the elongation flangeability decreases. Therefore, the Mn content is 4.00% or less. The Mn content is preferably 3.70% or less, more preferably 3.50% or less, and even more preferably 3.30% or less.

[0070] [P: below 0.100%]

[0071] Phosphorus (P) segregates at the original austenite grain boundaries, causing grain boundary embrittlement and reducing the ultimate deformation capacity of the steel sheet, thus decreasing its flexibility. Therefore, the P content needs to be 0.100% or less. Preferably, the P content is 0.070% or less. It should be noted that there is no specific lower limit for the P content, but since P is a solid solution strengthening element and increases the strength of the steel sheet, the P content is preferably 0.001% or more.

[0072] [S: below 0.0200%]

[0073] Sulfide (S) exists in the form of sulfides, reducing the ultimate deformation capacity of the steel sheet and thus decreasing its flexibility. Therefore, the S content needs to be 0.0200% or less. Preferably, the S content is 0.0050% or less. It should be noted that there is no particular lower limit for the S content, but due to limitations in production technology, the S content is preferably 0.0001% or more.

[0074] [Al: Below 1.000%]

[0075] Al is used to fully deoxidize and reduce inclusions in the steel.

[0076] If the Al content is too high, a large amount of ferrite will be generated, and delayed fracture cracking will easily progress at the interface between ferrite and tempered martensite or between ferrite and fresh martensite, reducing the resistance to delayed fracture under atmospheric corrosion and in the coated state. Therefore, the Al content is 1.000% or less. The Al content is preferably 0.500% or less, and more preferably 0.100% or less.

[0077] On the other hand, in order to stably carry out deoxygenation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.

[0078] [N: below 0.0100%]

[0079] Nitrogen (N) exists in the form of nitrides, reducing the ultimate deformation capacity of the steel sheet and thus decreasing its flexibility. Therefore, the N content is 0.0100% or less, preferably 0.0050% or less. It should be noted that there is no specific lower limit for the N content, but due to limitations in production technology, the N content is preferably 0.0001% or more.

[0080] [O: below 0.0100%]

[0081] O, existing in the form of oxides, reduces the ultimate deformation capacity of the steel sheet, thus decreasing its flexibility. Therefore, the O content is 0.0100% or less. Preferably, the O content is 0.0050% or less. It should be noted that there is no particular lower limit for the O content, but due to limitations in production technology, the O content is preferably 0.0001% or more.

[0082] The high-strength steel plate of the present invention comprises the above-mentioned components, with the remainder consisting of Fe and unavoidable impurities. It should be noted that unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. A total content of less than 0.100% of these impurities is permissible.

[0083] In addition to the above-mentioned composition, the steel plate of the present invention may further contain, individually or in combination, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

[0084] When Ti, Nb, and V are each 0.200% or less, large coarse precipitates and inclusions are not generated, and the ultimate deformation capacity of the steel sheet is not reduced, thus the bending properties are not decreased. Therefore, when Ti, Nb, and V are present, the contents are preferably 0.200% or less, and more preferably 0.100% or less. On the other hand, there is no particular specification for the lower limit of the contents of Ti, Nb, and V. It should be noted that Ti, Nb, and V increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing. Therefore, the contents of Ti, Nb, and V are preferably 0.001% or more.

[0085] When Ta and W are both 0.10% or less, large coarse precipitates and inclusions are not generated in large quantities, and the ultimate deformation capacity of the steel sheet is not reduced, thus the bending properties are not reduced. Therefore, the contents of Ta and W are preferably 0.10% or less, and more preferably 0.08% or less. On the other hand, there is no particular requirement for the lower limit of the contents of Ta and W. It should be noted that Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing. Therefore, the contents of Ta and W are preferably 0.01% or more.

[0086] When the content of B is 0.0100% or less, cracks do not form inside the steel sheet during casting or hot rolling, and the ultimate deformation capacity of the steel sheet is not reduced, thus the bending properties are not reduced. Therefore, the content of B is preferably 0.0100% or less, more preferably 0.0080% or less. On the other hand, there is no particular specification for the lower limit of the B content. It should be noted that B segregates at the austenite grain boundaries during annealing and is an element that improves hardenability; therefore, the B content is preferably 0.0003% or more.

[0087] When the contents of Cr, Mo, and Ni are all below 1.00%, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, thus the bending properties are not decreased. Therefore, the contents of Cr, Mo, and Ni are preferably below 1.00%, and more preferably below 0.80%. On the other hand, there is no particular specification for the lower limits of the contents of Cr, Mo, and Ni. It should be noted that Cr, Mo, and Ni are elements that improve hardenability; therefore, the contents of Cr, Mo, and Ni are preferably above 0.01%.

[0088] When the Co content is 0.010% or less, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, thus the bending properties are not decreased. Therefore, the Co content is preferably 0.010% or less, more preferably 0.008% or less. On the other hand, there is no particular specification for the lower limit of the Co content. It should be noted that Co is an element that improves hardenability, therefore the Co content is preferably 0.001% or more.

[0089] When the Cu content is 1.00% or less, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, thus the bending properties are not decreased. Therefore, the Cu content is preferably 1.00% or less, more preferably 0.80% or less. On the other hand, there is no particular specification for the lower limit of the Cu content. It should be noted that Cu is an element that improves hardenability, therefore the Cu content is preferably 0.01% or more.

[0090] When the Sn content is 0.200% or less, cracks do not occur inside the steel sheet during casting or hot rolling, and the ultimate deformation capacity of the steel sheet is not reduced, thus the bending properties are not reduced. Therefore, the Sn content is preferably 0.200% or less, more preferably 0.100% or less. On the other hand, there is no particular specification for the lower limit of the Sn content. It should be noted that Sn is an element that improves hardenability, therefore the Sn content is preferably 0.001% or more.

[0091] When the Sb content is 0.200% or less, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, thus the bending performance is not decreased. Therefore, the Sb content is preferably 0.200% or less, and more preferably 0.100% or less. On the other hand, there is no particular specification for the lower limit of the Sb content. It should be noted that, since it is an element that controls the surface softening thickness and adjusts the strength, the Sb content is preferably 0.001% or more.

[0092] When the contents of Ca, Mg, and REM are all below 0.0100%, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, thus the bendability is not decreased. Therefore, the contents of Ca, Mg, and REM are preferably below 0.0100%, and more preferably below 0.0050%. On the other hand, there is no particular specification for the lower limit of the contents of Ca, Mg, and REM. It should be noted that since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformation capacity of the steel plate, the contents of Ca, Mg, and REM are preferably 0.0005% or more.

[0093] When Zr and Te are both 0.100% or less, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, thus the bending properties are not decreased. Therefore, the contents of Zr and Te are preferably 0.100% or less, and more preferably 0.080% or less. On the other hand, there is no particular specification for the lower limit of the contents of Zr and Te. It should be noted that since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformation capacity of the steel plate, the contents of Zr and Te are preferably 0.001% or more.

[0094] When Hf content is 0.10% or less, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, thus the bending properties are not decreased. Therefore, the Hf content is preferably 0.10% or less, more preferably 0.08% or less. On the other hand, there is no particular specification for the lower limit of Hf content. It should be noted that since Hf 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.

[0095] When the Bi content is 0.200% or less, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate is not reduced, thus the bending properties are not decreased. Therefore, the Bi content is preferably 0.200% or less, and more preferably 0.100% or less. On the other hand, there is no particular specification for the lower limit of the Bi content. It should be noted that, since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more.

[0096] It should be noted that for the aforementioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, as long as their contents are below the aforementioned preferred lower limits, the effects of the present invention will not be impaired. Therefore, in this case, they are treated as unavoidable impurities.

[0097] Next, the microstructure at the 1 / 4 position of the thickness of the steel plate of the present invention will be described.

[0098] [Tempered martensite area ratio: 75% or higher]

[0099] In the aforementioned microstructure, by containing a certain amount or more of tempered martensite, a strength (TS) of 1180 MPa or higher can be achieved. Furthermore, the chromium content (YR) can be increased. Therefore, the area fraction of tempered martensite is 75% or higher. This area fraction is preferably 80% or higher, more preferably 85% or higher. On the other hand, there is no particular upper limit to the area fraction of tempered martensite; even if the area fraction of tempered martensite is 100%, the aforementioned effects can be obtained.

[0100] Tempered martensite is defined as martensite containing carbides observed in SEM observations, as described later. This carbide comprises cementite (θ), epsilon (ε), eta (η), and chi (χ). Additionally, tempered martensite includes lower bainite formed below the Ms point. It should be noted that the tempered martensite is observed at approximately 1 / 4 of the plate thickness, as described later.

[0101] [Area percentage of fresh martensite: below 15%]

[0102] If there is too much fresh martensite, the yield strength decreases, and the component strength is reduced. Furthermore, the hardness difference between the microstructures increases, reducing the elongation flangeability. Therefore, the total area fraction of fresh martensite is 15% or less. Preferably, the total area fraction of fresh martensite is 13% or less, more preferably 10% or less. It should be noted that the effects of the present invention can be obtained even if the total area fraction of fresh martensite is 0%.

[0103] Fresh martensite is defined as martensite in which no carbides are observed during SEM observation, as described later. It should be noted that the observation location for fresh martensite, as described later, is at 1 / 4 of the plate thickness.

[0104] [Combined area ratio of ferrite and bainite: less than 15%]

[0105] In the aforementioned microstructure, an excess of ferrite and bainite increases the hardness difference between the microstructures, promoting the progression of delayed fracture cracking and reducing resistance to delayed fracture under atmospheric corrosion and in the coated state. Furthermore, the extension flangeability decreases due to the hardness difference between the microstructures. Therefore, the total area fraction of ferrite and bainite is 15% or less. Preferably, the total area fraction of ferrite and bainite is 13% or less, more preferably 10% or less. It should be noted that the effects of the present invention can be obtained even if the total area fraction of ferrite and bainite is 0%.

[0106] Ferrite is a soft BCC iron formed at high temperatures, which includes allotriomorphic ferrite and idiomorphic ferrite.

[0107] Bainite is angular BCC iron containing fine carbides, formed at temperatures above Ms.

[0108] It should be noted that the ferrite and bainite were observed at 1 / 4 of the thickness of the steel plate.

[0109] [Area ratio of retained austenite: less than 15%]

[0110] If there is an excessive amount of retained austenite, a large amount of hard martensite will form from it during shearing. As a result, the hardness difference between the microstructures increases, promoting the progression of delayed fracture cracking and reducing resistance to delayed fracture under atmospheric corrosion and in the coated state. In addition, the elongation flangeability is reduced due to the hardness difference between the microstructures. Furthermore, the yield strength (YS) decreases due to the yielding of retained austenite, resulting in a decrease in component strength.

[0111] Therefore, the area ratio of retained austenite is 15% or less. Preferably, the area ratio of retained austenite is 10% or less.

[0112] On the other hand, there is no particular limitation on the lower limit, and the effects of the present invention can be obtained even if the area ratio of retained austenite is 0%.

[0113] In this invention, the method for determining the area ratio of retained austenite is as follows.

[0114] First, the steel plate to be measured is ground at a position 1 / 4 of its thickness (equivalent to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate), and then further ground by 0.1 mm using chemical grinding to obtain a sample. For the measurement surface of this sample, using an X-ray diffraction apparatus with a Co Kα ray source, the integrated reflection intensity of the (200), (220), and (311) crystal planes of fcc iron (austenite), and the (200), (211), and (220) crystal planes of bcc iron are measured. Then, the intensity ratio of the integrated reflection intensity of each crystal plane of fcc iron to that of bcc iron is calculated, resulting in a total of nine intensity ratios. In this invention, the average of these nine intensity ratios is taken as the volume fraction of retained austenite. Furthermore, in this invention, the volume fraction of retained austenite is considered as the area fraction of retained austenite.

[0115] [Remaining Organization]

[0116] The microstructure at the 1 / 4 position of the steel sheet thickness of the present invention may have a microstructure other than the tempered martensite, fresh martensite, ferrite, bainite, and retained austenite described above (the remaining microstructure). However, for the purpose of not impairing the effects of the present invention, the area ratio of the remaining microstructure is preferably 5% or less. Examples of the remaining microstructure include pearlite, alloyed carbonitrides precipitated in ferrite, and other microstructures known as microstructures of steel sheets.

[0117] [The average value of the stress τq at the onset of plastic deformation is 2.50 GPa to 4.10 GPa.]

[0118] The stress at the 1 / 4 position of the steel plate thickness where plastic deformation begins is an important component in this invention.

[0119] Here, the stress at the onset of plastic deformation, as described later, is the value obtained in the initial stage of the load-displacement curve obtained by nanoindentation, referring to the stress that migrates from the elastic region to the plastic region in a local area.

[0120] The plastic deformation initiation stress of the present invention is the stress corresponding to the generation / release of dislocations in a local region, which is completely different from the nanohardness obtained by nanoindentation test and the yield strength obtained by tensile test used in the past.

[0121] Here, the inventors conducted an in-depth study on the relationship between the initiation stress of plastic deformation obtained by nanoindentation and the resistance to delayed failure under atmospheric corrosion and in the coated state.

[0122] The results showed that by setting the average value of the initiation stress τq at 1 / 4 of the plate thickness of the steel plate to 2.50 GPa to 4.10 GPa, the resistance to delayed failure under atmospheric corrosion and in the coated state was improved.

[0123] This is because by setting the average value of the aforementioned plastic deformation initiation stress τq [τq] to an appropriate value as described above, the generation / release of dislocations at the delayed fracture crack tip is rationalized, thus suppressing the progression of delayed fracture cracks. As a result, it is inferred that the resistance to delayed fracture is improved under atmospheric corrosion and in the coated state.

[0124] That is, when the average value of the aforementioned plastic deformation initiation stress τq [τq] is less than 2.50 GPa, the generation / release of dislocations at the crack tip becomes significant, promoting the formation of voids and pores, resulting in delayed failure and crack progression. Particularly in steel plates with high strain dispersion capabilities, the resistance to delayed failure is reduced under atmospheric corrosion and in the coated state. Therefore, the average value of the plastic deformation initiation stress τq [τq] is 2.50 GPa or more. Preferably, the average value of the plastic deformation initiation stress τq [τq] is 2.65 GPa or more. More preferably, the average value of the plastic deformation initiation stress τq [τq] is 2.80 GPa or more.

[0125] On the other hand, when the average value of the aforementioned plastic deformation initiation stress τq exceeds 4.10 GPa, the generation / release of dislocations at the crack tip is suppressed, resulting in delayed fracture cracking that propagates brittlely along grain boundaries. Consequently, especially in the case of steel plates with high yield ratios, the resistance to delayed fracture under atmospheric corrosion and in the coated state is reduced.

[0126] Therefore, the average value of the initiation stress τq [τq] needs to be 4.10 GPa or less. Preferably, the average value of the initiation stress τq is 3.90 GPa or less. More preferably, the average value of the initiation stress τq [τq] is 3.80 GPa or less.

[0127] [The standard deviation σq of the stress at which plastic deformation begins is below 0.30 GPa]

[0128] The standard deviation σq of the plastic deformation initiation stress at 1 / 4 of the steel plate thickness is an important component in this invention. By keeping the standard deviation σq of the plastic deformation initiation stress below 0.30 GPa, especially in steel plates with high yield ratios, the resistance to delayed failure under atmospheric corrosion and in the coated state is improved. This is because, by keeping the standard deviation σq below 0.30 GPa, changes in the microscopic plastic deformation initiation stress are suppressed. That is, changes in the generation / release behavior of dislocations at the crack tip are suppressed, and the selective propagation of delayed failure cracks in weak parts of the microstructure is suppressed. As a result, the resistance to delayed failure under atmospheric corrosion and in the coated state is inferred to be improved.

[0129] When the standard deviation σq of the aforementioned plastic deformation initiation stress exceeds 0.30 GPa, the variation in the microscopic plastic deformation initiation stress becomes larger, and delayed failure cracking selectively propagates in the weaker parts of the microstructure. As a result, especially in the case of steel plates with high yield ratios, the resistance to delayed failure under atmospheric corrosion and in the coated state decreases. Therefore, the standard deviation σq of the plastic deformation initiation stress needs to be below 0.30 GPa. Preferably, the standard deviation σq of the plastic deformation initiation stress is below 0.26 GPa.

[0130] On the other hand, the smaller the standard deviation σq of the stress at the onset of plastic deformation, the better; it can be 0 GPa.

[0131] Next, the microstructure at a position 10 μm from the surface of the steel plate will be described.

[0132] [Area ratio of tempered martensite: below 40%]

[0133] If there is an excessive amount of tempered martensite in the microstructure at a distance of 10 μm from the surface of the steel plate, the initiation point of bending cracks increases, and the bending performance deteriorates. In addition, the tempered martensite present on the surface of the steel plate promotes hydrogen penetration caused by corrosion on the surface of the steel plate directly below the coating, which easily leads to delayed-failure cracking. Therefore, the resistance to delayed failure under atmospheric corrosion and in the coated state is reduced.

[0134] Therefore, in this invention, the area fraction of tempered martensite in the microstructure located at a position 10 μm from the surface of the steel plate is 40% or less. This area fraction of tempered martensite is preferably 35% or less, more preferably 30% or less. It should be noted that the effects of this invention can be obtained even if the area fraction of tempered martensite is 0%.

[0135] Tempered martensite is defined as martensite of carbides observed in SEM observations, as described later. This carbide comprises cementite (θ), epsilon (ε), eta (η), and chi (χ).

[0136] It should be noted that the observation position of the tempered martensite, as described below, is 10 μm from the surface of the steel plate.

[0137] [Pearlite area ratio: below 15%]

[0138] If there is an excessive amount of pearlite in the microstructure at a distance of 10 μm from the steel plate surface, the initiation point of bending cracks increases, and the bending performance deteriorates. In addition, stress concentration during bending occurs in the pearlite present on the surface, which easily leads to delayed failure cracking. Therefore, the resistance to delayed failure is reduced under atmospheric corrosion and in the coated state.

[0139] Therefore, in this invention, the area fraction of pearlite in the microstructure located at a position 10 μm from the surface of the steel plate is 15% or less. Preferably, this area fraction of pearlite is 10% or less. It should be noted that even if the area fraction of pearlite is 0%, the desired effect can still be achieved.

[0140] The pearlite was observed at a distance of 10 μm from the surface of the steel plate, as described below.

[0141] [Combined area ratio of ferrite and bainite: 60% or more]

[0142] By increasing the total area ratio of ferrite and bainite at a position 10 μm from the surface of the steel plate to more than 60%, the bending properties and resistance to delayed failure under atmospheric corrosion and in the coated state are improved.

[0143] That is, by increasing the combined area ratio of ferrite and bainite, which are soft phases, the initiation point of cracks during bending is reduced and the bending performance is improved.

[0144] Furthermore, ferrite and bainite have fewer lattice defects and fewer hydrogen trapping sites compared to tempered martensite and fresh martensite. This suppresses hydrogen penetration caused by corrosion on the steel surface directly beneath the coating, reducing the likelihood of delayed-crack damage. Therefore, the resistance to delayed-crack damage is improved under atmospheric corrosion and in the coated state.

[0145] Therefore, the combined area ratio of ferrite and bainite must be 60% or more, preferably 65% ​​or more, and more preferably 70% or more. On the other hand, there is no particular upper limit, and the above-mentioned effect can be obtained even if the combined area ratio of ferrite and bainite is 100%.

[0146] Ferrite is a soft BCC iron formed at high temperatures, which includes heteromorphic ferrite and euhedral ferrite.

[0147] Bainite is angular BCC iron containing fine carbides, formed at temperatures above Ms.

[0148] In addition, the ferrite and bainite were observed at a distance of 10 μm from the surface of the steel plate.

[0149] [Remaining parts of the organization]

[0150] The steel microstructure at a position 10 μm from the surface of the steel plate may have a microstructure other than the tempered martensite, pearlite, ferrite, and bainite described above (the remaining microstructure). However, for the sake of not impairing the effects of the present invention, the area ratio of the remaining microstructure is preferably 5% or less. Examples of the remaining microstructure include, for example, pearlite, alloyed carbonitrides precipitated in ferrite, and other microstructures known as steel plate microstructures.

[0151] The method for determining the area ratio of tempered martensite, fresh martensite, pearlite, ferrite, and bainite at a position of 1 / 4 of the plate thickness or 10 μm from the surface of the steel plate is as follows.

[0152] First, the sample was cut from the steel plate with the thickness section (L section) parallel to its rolling direction as the observation surface. The observation surface of the sample was mirror-polished using diamond polishing paste, then finely polished using colloidal silica, and further etched using 1 vol% nitric acid alcohol, thereby revealing the microstructure.

[0153] Next, a scanning electron microscope (SEM) with an accelerating voltage of 10 kV was used to observe three fields of view at a magnification of 3000x at a position of 1 / 4 of the thickness of the steel plate on the observation surface of the sample or at a position 10 μm away from the surface of the steel plate, and the SEM images of the three fields of view were obtained.

[0154] Based on the obtained SEM images, the area fraction of each tissue was calculated using Adobe Photoshop (Adobe Systems). Specifically, the area fraction of each tissue was obtained by dividing the area of ​​each tissue by the measured area. The area fraction of each tissue was calculated for three fields of view, and their average value was taken as the area fraction of each tissue.

[0155] In the SEM images above, tempered martensite is a microstructure with a fine, uneven internal layered structure and contains fine carbides with white contrast. Fresh martensite is a microstructure with a fine, uneven internal layered structure and does not contain fine carbides with white contrast. Pearlite is a layered microstructure composed of gray ferrite and white cementite. Ferrite is a flat microstructure that is gray and does not contain carbides. Additionally, bainite is a gray microstructure containing fine carbides.

[0156] In this way, tempered martensite, fresh martensite, pearlite, ferrite, and bainite can recognize each other.

[0157] [When the average value of the plastic deformation initiation stress τs determined by nanoindentation is set as [τs], the proportion of measurement points with a value less than 0.85 × [τs] is less than 25.0%.]

[0158] The proportion of measurement points defined by the average value of the plastic deformation initiation stress τs at a location 10 μm from the steel plate surface, determined by nanoindentation, is an important component in this invention. Specifically, when this average value is set to [τs], the proportion of measurement points with a value less than 0.85 × [τs] is 25.0% or less, thereby improving resistance to delayed failure, particularly in steel plates with high yield ratios, under atmospheric corrosion and in the coated state. This is because regions with a microstructure less than 0.85 × [τs] are areas prone to local dislocation generation and release, becoming the starting point for delayed failure cracking on the steel plate surface.

[0159] As described above, by reducing the proportion of measurement points with a value less than 0.85 × [τs] to 25.0% or less, it is possible to reduce the locations where local dislocations are generated and released on the surface of the curved portion, thereby reducing the initiation point of delayed failure. As a result, the resistance to delayed failure under atmospheric corrosion and in the coated state is improved.

[0160] Therefore, in this invention, the proportion of measurement points less than 0.85 × [τs] needs to be 25.0% or less. Preferably, the proportion of measurement points less than 0.85 × [τs] is 20.0% or less. On the other hand, there is no particular limitation on the lower limit, and the above-mentioned effect can be obtained even if the proportion of measurement points less than 0.85 × [τs] is 0%.

[0161] It should be noted that, in this invention, the position 10 μm from the surface of the steel plate refers to a position 10 μm deep in the thickness direction from the surface of the steel plate (the surface perpendicular to the thickness direction). Furthermore, both the aforementioned 1 / 4 thickness position and the 10 μm position from the surface of the steel plate must satisfy the above requirements at least on one side of the steel plate.

[0162] The following describes the method for determining the initiation stress of plastic deformation using the nanoindentation method.

[0163] The test specimen is prepared by cutting the specimen with the thickness section (L section) parallel to the rolling direction of the steel plate as the test surface. The test surface is then mirror-polished with diamond polishing paste and then finely polished with colloidal silica.

[0164] The stress at the onset of plastic deformation was determined using a nanoindentation device equipped with a glass indenter. The measurement location was set at 1 / 4 of the steel plate thickness or 10 μm from the steel plate surface. Under load control, with a loading and unloading rate of 50 μN / s, a maximum load of 500 μN, and a data acquisition interval of 5 ms, a nanoindentation test was conducted to obtain the load P (N) and the displacement h (nm) under that load. Forty nanoindentation tests were performed at various measurement locations. A distance of at least 2 μm was maintained between indentations during the measurement. Using the obtained load-displacement curves, the Hertz contact displacement hc (nm) was calculated under each load P (N) using the Hertz contact method shown in Equation 3.

[0165]

[0166] It should be noted that in Equation 3 above, h C (nm) represents the displacement of the assumed elastic deformation obtained from the Hertz contact equation, P(N) is the load, and E r (Pa) is the composite Young's modulus, and R(m) is the radius of curvature at the tip of the indenter. Additionally, the above E... r The average value (40 points) of the composite Young's modulus obtained from the unloading curve in each measurement.

[0167] The aforementioned R(m) varies depending on the wear condition of the glass indenter, and is therefore determined by fitting a load-displacement curve in the elastic region using standard samples such as FusedSilica.

[0168] Using h obtained from Equation 3 above under each load P (N) C Given the displacement h (nm) determined by nanoindentation test, find the minimum load P that satisfies the condition in Equation 4. min (N), the P min (N) is defined as the load at which plastic deformation begins.

[0169]

[0170] Then, the initial stress τ (GPa) of plastic deformation is calculated from the initial load of plastic deformation using the following formula 5.

[0171]

[0172] Use equations 3, 4, and 5 above to calculate the initial stress τ (GPa) at 40 points to determine the plastic deformation initiation stress, and then calculate their average value.

[0173] It should be noted that the average value of the initiation stress of plastic deformation at the 1 / 4 position of the plate thickness is set as [τq], and the average value of the initiation stress of plastic deformation at the position 10 μm from the steel plate surface is set as [τs]. Furthermore, a histogram is constructed from the values ​​of the initiation stress of plastic deformation at 40 points at the 1 / 4 position of the plate thickness, and the standard deviation σq is calculated. Then, the proportion of measurement points with an initiation stress of plastic deformation less than 0.85 × [τs] is calculated from the values ​​of the initiation stress of plastic deformation at the 40 points at the position 10 μm from the steel plate surface.

[0174] It should be noted that the thickness of the high-strength steel plate of the present invention is not particularly limited, but is generally preferred to be 0.3 mm or more, and preferably 2.8 mm or less.

[0175] [Coating]

[0176] The steel sheet of the present invention can have a coating on its surface. The coating is formed by the plating process described later. There is no particular limitation on the type of coating; examples include hot-dip galvanizing and electroplating. Furthermore, examples of the coating include a zinc coating (Zn coating) and an Al coating. It should be noted that a zinc coating is preferred as the coating. The zinc coating may contain elements such as Al and Mg. Additionally, the coating may be an alloyed coating (alloyed coating).

[0177] The composition of the coating is not particularly limited and can be a general composition.

[0178] For example, when the coating is a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, it generally has the following composition. That is, the following composition can be cited: 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 at least one of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM, and the remainder consists of Zn and unavoidable impurities.

[0179] Furthermore, when the coating is a hot-dip galvanized layer, the preferred adhesion amount per single side of the coating is 20 g / m². 2 That's all. On the other hand, the preferred coating thickness per single side is 80 g / m². 2 Furthermore, an alloyed hot-dip galvanized layer obtained by alloying the amount of hot-dip galvanized layer can also be used.

[0180] Furthermore, when the coating is a hot-dip galvanized layer, the Fe content in the coating is preferably less than 7% by mass. Additionally, when the coating is an alloyed hot-dip galvanized layer, the Fe content in the coating is preferably 7% by mass or more. On the other hand, the Fe content in the coating is preferably 20% by mass or less, more preferably 15% by mass or less.

[0181] [member]

[0182] The components of one embodiment of the present invention will be described below.

[0183] The component of the present invention is made using the steel sheet described in the embodiments of the present invention. This component is obtained, for example, by forming the steel sheet of the embodiments described above into the desired shape through forming or joining processes.

[0184] Furthermore, in one embodiment of the present invention, the component is preferably a component for a car frame structure or a component for a car reinforcement. That is, the steel sheet described above is a high-strength steel sheet that excels in component strength, elongation flange properties, bending properties, and resistance to delayed failure under atmospheric corrosion and in the painted state. Therefore, the component of the present invention is particularly suitable for all components for car frame structure or components for car reinforcement.

[0185] [part]

[0186] Hereinafter, the components of one embodiment of the present invention will be described.

[0187] The components of this invention are made using the aforementioned components of this invention. A component of one embodiment of this invention is preferably a frame structure component or a reinforcing component of an automobile. Here, the aforementioned components of this invention exhibit excellent properties in terms of component strength, extension flange capacity, flexural strength, and resistance to delayed failure under atmospheric corrosion and in a painted state. Therefore, a component of one embodiment of this invention made using these components is particularly suitable for all frame structure components or reinforcing components of an automobile.

[0188] [Methods for manufacturing steel plates]

[0189] Next, the method for manufacturing the steel plate of the present invention will be described.

[0190] First, a steel billet is produced by melting a steel billet having the above-mentioned composition. The method of melting the molten steel that will become the billet is not particularly limited, and known melting methods using converters, electric furnaces, etc., can be employed. To prevent macroscopic segregation, the billet is preferably manufactured by continuous casting, but it can also be manufactured by other methods such as ingot casting or slab casting.

[0191] It should be noted that the steel sheet of the present invention includes cold-rolled steel sheet manufactured by hot rolling, pickling, cold rolling and annealing, and steel sheet on which cold-rolled steel sheet has been plated.

[0192] Next, the steel billet is hot-rolled to produce a hot-rolled plate. In one example, the steel billet is temporarily cooled to room temperature and then reheated for hot rolling (roughing and finishing). It should be noted that the manufactured steel billet can also be loaded directly into the heating furnace as a hot sheet without cooling to room temperature, or it can be rough-rolled immediately after being slightly heated.

[0193] Rough rolling

[0194] A rough-rolled plate is obtained by rough rolling the above-mentioned steel billet under the following conditions.

[0195] From the viewpoint of reducing carbide melting and rolling load, the heating temperature of the steel billet (slab heating temperature) is preferably 1100°C or higher. On the other hand, to prevent increased oxide scale loss, the slab heating temperature is preferably 1300°C or lower. The slab heating temperature is based on the surface temperature of the steel billet. Next, the steel billet heated to the slab heating temperature is rough rolled under the following conditions.

[0196] [For temperature zones above 1000℃, the number of passes should be 4 or more.]

[0197] By increasing the number of passes in the temperature region above 1000°C to 4 or more, the proportion of measurement points with a stress less than 0.85 × [τs] can be reduced. Increasing the number of passes in the temperature region above 1000°C increases the number of strain introductions near the surface, promoting the diffusion of substitutional solid solutions of Si and Mn, resulting in greater uniformity. If the number of passes in the temperature region above 1000°C is less than 4, regions of Si and Mn thinning appear on the steel plate surface, reducing intracrystalline friction and increasing the number of measurement points where the stress at the start of plastic deformation significantly decreases. Therefore, it is necessary to increase the number of passes in the temperature region above 1000°C to 4 or more.

[0198] [The reduction rate for each pass is 15% or more]

[0199] By ensuring a reduction rate of 15% or more in each pass, the standard deviation σq of the initiation stress for plastic deformation can be reduced. A reduction rate of 15% or more in each pass promotes the dynamic recrystallization of γ-grains within the steel sheet, resulting in a more uniform grain size and element concentration distribution. It should be noted that if the reduction rate in each pass is less than 15%, the dynamic recrystallization of γ-grains is not promoted to the fullest extent, leading to a non-uniform distribution of grain size and element concentration, resulting in a greater variation in the initiation stress for plastic deformation in the final microstructure and an increase in the standard deviation σq of the initiation stress for plastic deformation. Therefore, a reduction rate of 15% or more in each pass is necessary.

[0200] The average strain rate is 9 × 10⁻⁶. －4 / s~1×10 －2 / s]

[0201] By setting the average strain rate in rough rolling to 9 × 10 －4 / s~1×10 －2The range of / s can reduce the standard deviation σq of the stress at the onset of plastic deformation. It should be noted that the average strain rate in roughing is defined as the rolling rate ε(-) from the first to the last mill in roughing divided by the time t required for the roughing to complete rolling from the start of rolling using the first mill to the completion of rolling using the last mill. R The value obtained from (s) (ε / t) R ).

[0202] The average strain rate in rough rolling exceeds 1×10⁻⁶. －2 Under conditions of / s, the diffusion of solute atoms such as Si and Mn during the plastic deformation and dynamic recrystallization of austenite grains becomes insufficient, resulting in regions of low Si and Mn concentration and regions of high Si and Mn concentration within the steel plate. Moreover, differences in intracrystalline friction occur between these regions, thus increasing the variation in stress at the onset of plastic deformation and raising the standard deviation σq of the stress at the onset of plastic deformation.

[0203] On the other hand, when the average strain rate in rough rolling is less than 9×10 －4 At a temperature of / s, dynamic recovery of dislocations in austenite grains is promoted. Consequently, the dislocation density decreases, inhibiting the diffusion of solute atoms such as Si and Mn through dislocation tubes. This diffusion becomes insufficient, resulting in regions of low Si and Mn concentration and regions of high Si and Mn concentration within the steel plate. Furthermore, a difference in intracrystalline friction occurs between these two regions, leading to a greater change in the stress at which plastic deformation begins, and an increase in the standard deviation σq of the stress at which plastic deformation begins.

[0204] Therefore, the average strain rate mentioned above is 9 × 10⁻⁶. －4 / s~1×10 －2 The range is / s. The preferred average strain rate is 1×10⁻⁶. －3 / s or higher. Furthermore, the preferred average strain rate is 9 × 10⁻⁶. －3 / s and below.

[0205] <Precision Rolling>

[0206] Next, the rough-rolled plate is finished rolled to produce a hot-rolled plate (hot rolling process). The hot-rolled plate is then appropriately coiled. When the slab heating temperature is set relatively low, from the viewpoint of preventing defects during hot rolling, it is preferable to heat the rough-rolled plate using a bar heater or the like before finish rolling.

[0207] The finishing temperature is preferably 700°C or higher. This reduces the rolling load. Consequently, the reduction rate of austenite in its non-recrystallized state is reduced, suppressing the development of abnormal structures that elongate in the rolling direction, resulting in steel sheets with excellent workability.

[0208] It should be noted that the finishing rolling can be performed continuously by joining the rough-rolled plates together. Alternatively, the rough-rolled plates can be temporarily coiled before the finishing rolling is performed.

[0209] To reduce rolling load, some or all of the finishing rolling can be lubricated rolling. Lubricated rolling is also preferred from the viewpoint of making the shape and material of the steel sheet more uniform. The coefficient of friction during lubricated rolling is preferably 0.10 or higher, and on the other hand, it is more preferably 0.25 or lower.

[0210] From the viewpoint of ensuring good sheet flowability during cold rolling and annealing (described later), the coiling temperature after hot rolling is preferably 300°C or higher, and more preferably 700°C or lower.

[0211] Next, the hot-rolled sheet obtained from hot rolling is appropriately pickled. Pickling removes oxides from the surface of the hot-rolled sheet, resulting in high-strength steel sheets with excellent chemical conversion properties and coating quality in the final product. Pickling can be performed in one step or in multiple steps.

[0212] After pickling, the hot-rolled sheet is subjected to any softening heat treatment, followed by cold rolling to obtain a cold-rolled sheet. The conditions for this cold rolling are not particularly limited and can be performed using conventional methods, but the cumulative reduction rate is preferably in the range of 20% to 75%. It should be noted that the number of rolling passes and the reduction rate for each pass are not particularly limited and can be performed using conventional methods.

[0213] The cold-rolled sheet obtained in this way is then subjected to annealing as described below and cooling to below 150°C, followed by reheating.

[0214] <annealing>

[0215] [Heating temperature is above 800℃]

[0216] If the heating temperature in the annealing process is too low, the reverse transformation to austenite will not proceed sufficiently, resulting in an increased ferrite area ratio in the first quarter of the steel plate and a decreased tempered martensite area ratio in the same quarter. Therefore, the heating temperature should be 800°C or higher. Preferably, this heating temperature is 830°C or higher. On the other hand, there is no particular upper limit to the heating temperature, but from the viewpoint of workability and minimizing damage to the furnace, a heating temperature of 1000°C or lower is preferred. It should be noted that the heating temperature is based on the surface of the steel plate.

[0217] [Dew point is above -25℃]

[0218] If the dew point of the atmosphere in the heating temperature zone T1 (above 800°C) during the annealing process is low, decarburization will not occur on the surface, and the total area fraction of ferrite and bainite at a distance of 10 μm from the steel plate surface will become too low. Furthermore, if the dew point is too low, the decarburization distribution at a distance of 10 μm from the steel plate surface will become uneven, resulting in areas with lower stress at the onset of plastic deformation, and an increase in the proportion of measurement points at a distance of 10 μm from the steel plate surface with a value less than 0.85 × [τs]. Therefore, the dew point should be -25°C or higher. This dew point is preferably -20°C or higher. On the other hand, while there is no particular upper limit to the dew point, from the viewpoint of workability and minimizing damage to the furnace, a dew point of +15°C or lower is preferred.

[0219] [Annealing is performed under the condition that Equation 1 is satisfied]

[0220] Equation 1: 2.0 ≤ K ≤ 60.0

[0221] It should be noted that K (mm) in Equation 1 above 2 () is defined by the following formula.

[0222]

[0223] In Equation 1 above, the time when the temperature of the cold-rolled plate first reaches 800°C during the annealing heating is defined as t = 0 (s), and t = tE (s) is the time when the annealing is completed and the temperature of the cold-rolled plate reaches 800°C again.

[0224] T t (°C) represents the average temperature of the above-mentioned cold-rolled sheet during time t: t-1 to t (s).

[0225] Additionally, [%C] represents the carbon content in the steel plate.

[0226] In steel sheets annealed to 800°C, a reverse transformation to austenite occurs at the 1 / 4 position of the sheet, resulting in reduced microsegregation and grain growth through diffusion. Additionally, decarburization occurs on the steel sheet surface, where carbon (C) detaches from the surface. These phenomena are defined as parameter K, based on the diffusion of C in the austenitic region.

[0227] The parameter K is calculated as described above from the amount of carbon in the steel plate, the temperature and time in the annealing process. By appropriately controlling the value of parameter K as described above, the standard deviation σq of the stress at the 1 / 4 position of the steel plate, the total area ratio of ferrite and bainite at a position 10 μm from the surface of the steel plate, and the proportion of measurement points less than 0.85 × [τs] can be controlled within the specified range.

[0228] That is, if the above K (mm) 2If K(mm) is too small, the diffusion of solute atoms in the austenite at the 1 / 4 position of the steel plate becomes insufficient, microsegregation remains, and the standard deviation σq of the stress at the start of plastic deformation at the 1 / 4 position of the steel plate becomes too high. Furthermore, decarburization at a position 10 μm from the steel plate surface becomes insufficient, and the combined area fraction of ferrite and bainite at this position becomes too low. Consequently, the diffusion of solute atoms in the austenite at the steel plate surface also becomes insufficient, microsegregation remains, and the proportion of measurement points with a value less than 0.85 × [τs] at a position 10 μm from the steel plate surface becomes too high. Therefore, K(mm) 2 The value is 2.0 or higher. K (mm) 2 Preferably, the value is 3.0 or higher, and more preferably 4.0 or higher.

[0229] On the other hand, if K(mm) 2 If the value of K (mm) is too large, excessive grain growth will occur in a portion of the austenite at the 1 / 4 position of the steel plate, resulting in the formation of coarse proto-γ grains. If the proto-γ grains become larger, the Ms point within each proto-γ grain unit will increase, creating localized areas where martensitic transformation occurs at higher temperatures. This leads to microstructure inhomogeneity, and the standard deviation σq of the stress at the onset of plastic deformation becomes excessively high. Therefore, K (mm) 2 () is below 60.0. K (mm) 2 Preferably, the value is 45.0 or less, and more preferably 30.0 or less.

[0230] It should be noted that the temperature history in the annealing process is only required to be within the above range, and there are no special restrictions.

[0231] [The average cooling rate v2 in the temperature range T2 of 600℃ to 750℃ is 1.0℃ / s to 15.0℃ / s]

[0232] The temperature range T2, from 600°C to 750°C, is the temperature region where ferrite phase transformation occurs at the 1 / 4 position of the steel plate and at a position 10 μm from the surface of the steel plate. If the average cooling rate v2 in this temperature range T2 is too low, excessive ferrite phase transformation occurs at the 1 / 4 position of the steel plate, resulting in a high ferrite area fraction at that position. Therefore, the average cooling rate v2 is 1.0°C / s or higher. Preferably, the average cooling rate v2 is 2.0°C / s or higher.

[0233] On the other hand, if the average cooling rate v2 is too high, ferrite phase transformation is difficult to occur at a position 10 μm from the steel plate surface, and the area fraction of ferrite at this position decreases. Therefore, the average cooling rate v2 is 15.0 °C / s or less. Preferably, the average cooling rate v2 is 13.0 °C / s or less.

[0234] [The average cooling rate in the temperature range above 500°C and below 600°C exceeds v2]

[0235] The temperature range of 500°C to 600°C is the temperature region where the pearlitic phase transformation occurs at a position 10 μm from the steel plate surface. That is, if the average cooling rate in this temperature range is less than or equal to v2, the pearlitic phase transformation occurs at the ferrite-austenite interface, resulting in an excessive increase in the area fraction of pearlite at a position 10 μm from the steel plate surface. Therefore, the average cooling rate in this temperature range exceeds v2. Preferably, it exceeds (v2 + 2°C / s). It should be noted that there is no specific upper limit to the average cooling rate in this temperature range, but it is approximately 1000°C / s or less on the equipment.

[0236] [The residence time in temperature range T3, above 400℃ but below 500℃, is 10s to 150s.]

[0237] The temperature range T3, above 400°C but below 500°C, is the temperature region where bainitic phase transformation occurs at the 1 / 4 position of the steel plate and at a distance of 10 μm from the steel plate surface. If the residence time in this temperature range T3 is too short, bainitic phase transformation is difficult to occur at the 10 μm position from the steel plate surface, and the area fraction of bainite at this position decreases. Therefore, the residence time in the aforementioned temperature range T3 is 10 s or more. Preferably, the residence time in this temperature range T3 is 15 s or more.

[0238] On the other hand, if the residence time in the aforementioned temperature region T3 is too long, excessive bainitic phase transformation will occur at the 1 / 4 position of the steel plate, resulting in a higher bainite area fraction at that position. Therefore, the residence time in the aforementioned temperature region T3 is 150 s or less. Preferably, the residence time in this temperature region T3 is 130 s or less.

[0239] [The average cooling rate v4 in the temperature range T4 from Ms -100℃ to Ms℃ is above 3.0℃ / s.]

[0240] The temperature range T4, from Ms -100℃ to Ms℃, is the temperature region where martensitic transformation occurs, self-tempering occurs in the generated martensite, and carbon is distributed from martensite to untransformed austenite. If the average cooling rate v4 in this temperature range T4 is too slow, it significantly promotes self-tempering in the generated martensite and the distribution of carbon from martensite to untransformed austenite, leading to microstructure inhomogeneity and an excessively high standard deviation σq of the stress at the onset of plastic deformation. Therefore, the average cooling rate v4 is 3.0℃ / s or higher. Preferably, this average cooling rate v4 is 4.0℃ / s or higher.

[0241] On the other hand, there is no particular upper limit to the above-mentioned average cooling rate v4, but from the viewpoint of reducing the burden of equipment investment, it is preferably around 1000°C / s or less.

[0242] It should be noted that point Ms is obtained by Equation 6 below.

[0243] Ms=499-308[%C]-10.8[%Si]-32.4[%Mn]-27[%Cr]-10.8[%Mo]...Equation 6

[0244] Wherein, [%M] represents the content of M in steel (mass %).

[0245] [Cool down to below 150°C]

[0246] By cooling the steel plate, which will become temperature region T4, to below 150°C, the martensitic transformation proceeds fully. If the cooling completion temperature is above 150°C, the martensitic transformation cannot be completed, and tempering does not occur during subsequent reheating, resulting in an excess of fresh martensite. Therefore, the cooling completion temperature is below 150°C. Preferably, the cooling completion temperature is below 100°C.

[0247] [Reheating is carried out under the condition that the highest reheating temperature, i.e., temperature X, and the holding time Y above temperature X-10°C satisfy the following equation 2]

[0248] 8000≤(273+X)×(20+Log(Y / 3600))≤12000...Equation 2

[0249] The unit of temperature X is °C, and the unit of holding time Y is seconds.

[0250] As described above, by appropriately reheating the cold-rolled sheet after it has been cooled to below 150°C, carbide precipitation and segregation on C-direction dislocations are generated in the resulting tempered martensite.

[0251] Here, if the variable part of Equation 2 [(273+X)×(20+Log(Y / 3600))], which is expressed by the highest reheating temperature X and the holding time Y in the temperature range below X and above X-10°C, is too small, the precipitation of carbides and segregation on C-direction dislocations in tempered martensite become insufficient, YS decreases, and the strength of the component decreases. In addition, the migration rate of dislocations in tempered martensite increases, and the average value of the stress τq at the start of plastic deformation [τq] becomes lower. Therefore, the variable part is 8000 or more. Preferably, the variable part is 8500 or more.

[0252] On the other hand, if the aforementioned variable value is too large, excessive carbide precipitation and segregation on C-direction dislocations will occur in the tempered martensite, leading to dislocation recovery and thus reducing TS. Furthermore, dislocations in the tempered martensite are pinned, and the average value of the stress τq at the start of plastic deformation [τq] becomes higher. Therefore, the aforementioned variable value is 12000 or less. Preferably, the variable value is 11500 or less.

[0253] The cold-rolled sheet that has undergone such heat treatment is then cooled to room temperature. This yields the high-strength steel sheet (cold-rolled steel sheet) of the present invention.

[0254] In the manufacturing method of the present invention, when the plating process described later is carried out, the high-strength steel sheet obtained is a plated steel sheet with a coating.

[0255] Furthermore, as long as the heat treatment process of the manufacturing method of the present invention meets the above-mentioned conditions, other conditions are not particularly limited, and the equipment for performing the heat treatment is not particularly limited.

[0256] <Plating Treatment>

[0257] In the manufacturing method of the present invention, the cold-rolled sheet can be coated.

[0258] Examples of such plating processes include hot-dip galvanizing (a process that forms a hot-dip galvanized layer) and alloyed hot-dip galvanizing (a process that forms an alloyed hot-dip galvanized layer by performing an alloying process after hot-dip galvanizing). Alternatively, an electroplated layer can be formed by electroplating.

[0259] For example, in the case of hot-dip galvanizing, it is preferable to immerse the cold-rolled sheet in a galvanizing bath, and then adjust the coating adhesion by means of gas wiping or the like. The bath temperature of the galvanizing bath is not particularly limited, but is preferably 440°C or higher, and even more preferably 500°C or lower. The Al content of the galvanizing bath is preferably 0.10% by mass or higher, and even more preferably 0.23% by mass or lower.

[0260] Here, the galvanizing process is preferably carried out after a stay in the temperature range T3 of 400°C or higher and less than 500°C during the cooling process following the annealing.

[0261] Next, to optimize the Zn-Fe alloying speed and productivity, the alloying temperature is preferably 470°C or higher. On the other hand, to appropriately prevent the transformation of untransformed austenite to pearlite and to optimize the total sulfide (TS), the alloying temperature is preferably 600°C or lower, more preferably 560°C or lower. It should be noted that the alloying temperature is based on 530°C.

[0262] Skin rolling can be performed on the steel sheet after it has been cooled to the aforementioned room temperature. From the viewpoint of stabilizing the shape, the reduction rate of skin rolling is preferably 0.01% or more. On the other hand, there is no particular upper limit to the reduction rate, but from the viewpoint of productivity, it is preferably 1.50% or less.

[0263] Furthermore, surface finishing can be performed online or offline. It can be performed in one go to achieve the target reduction rate, or it can be done in multiple stages.

[0264] From a productivity standpoint, the aforementioned annealing and plating processes are preferably carried out in CAL (Continuous Annealing Line) or CGL (Continuous Galvanizing Line).

[0265] It should be noted that manufacturing conditions other than those mentioned above can be handled using conventional methods.

[0266] [Method for manufacturing components]

[0267] The component of the present invention can be manufactured by performing at least one of forming or joining processes on the aforementioned high-strength steel plate. It should be noted that the forming and joining processes can be performed using conventional methods.

[0268] In the manufacturing methods of the steel plates, components and parts of the present invention, items not described in this specification can be manufactured using conventional methods.

[0269] Example

[0270] The present invention will now be specifically described with reference to the embodiments described below. However, the present invention is not limited to the embodiments described below.

[0271] <Steel Plate Manufacturing>

[0272] Molten steel with the composition shown in Table 1 (the remainder consists of Fe and unavoidable impurities) is melted in a converter and billets are obtained by continuous casting.

[0273]

[0274] The steel billet obtained in this way is hot-rolled to obtain a hot-rolled plate. Specifically, the steel billet is heated to 1250°C and rough-rolled in the temperature range above 1000°C, with the number of passes, reduction rate of each pass, and average strain rate shown in Table 2. Then, it is finish-rolled at a finishing temperature of 900°C, and finally coiled at 500°C. After coiling, it is cooled to room temperature to obtain a hot-rolled plate. The obtained hot-rolled plate is pickled, then subjected to a softening heat treatment at 500°C, and then cold-rolled at a rolling rate of 50%. This yields a cold-rolled plate with a thickness of 1.4 mm. It should be noted that in Table 2, a value of 0 is assigned when all reduction rates in each pass of the rough rolling are 15% or higher, and a value of × is assigned when even one pass's reduction rate is less than 15%.

[0275] By subjecting the cold-rolled sheet to annealing, cooling, and reheating under the conditions shown in Table 2 below, the high-strength steel sheet (cold-rolled steel sheet) of the present invention is obtained.

[0276] Plating Treatment

[0277] For a portion of cold-rolled sheets, after being held in the temperature range of T3 above 400℃ and below 500℃, hot-dip galvanizing is performed to form a coating (hot-dip galvanized layer) on both sides, resulting in hot-dip galvanized steel sheet (GI).

[0278] This hot-dip galvanizing process uses a hot-dip galvanizing bath (bath temperature: 470°C) containing 0.20% Al by mass, with the remainder consisting of Zn and unavoidable impurities. Furthermore, the coating weight per side of the hot-dip galvanized layer is 45–72 g / m². 2 about.

[0279] The hot-dip galvanized layer consists of Fe: 0.1-1.0% by mass and Al: 0.2-1.0% by mass, with the remainder being Zn and unavoidable impurities.

[0280] For another portion of the cold-rolled sheets, after being held in a temperature range of T3 above 400°C but below 500°C, alloyed hot-dip galvanizing is performed to form a coating (alloyed hot-dip galvanized layer) on both sides. That is, alloyed hot-dip galvanized steel sheet (GA) is obtained.

[0281] The hot-dip galvanizing process uses a hot-dip galvanizing bath (bath temperature: 470°C) containing 0.14% Al by mass, with the remainder consisting of Zn and unavoidable impurities. Additionally, an alloying treatment is performed at 550°C. The alloyed hot-dip galvanized layer has an adhesion weight of 45 g / m² per side. 2 about.

[0282] The resulting alloyed hot-dip galvanized layer consists of Fe: 7-15% by mass and Al: 0.1-1.0% by mass, with the remainder being Zn and unavoidable impurities.

[0283] In the "Coating Type" column of Table 2 below, it is recorded as "GI" when a hot-dip galvanized layer is formed, as "GA" when an alloyed hot-dip galvanized layer is formed, and as "CR" when no coating is formed.

[0284] Table 2

[0285]

[0286]

[0287] <Observations on Steel Structure>

[0288] The area ratios of tempered martensite, fresh martensite, ferrite, bainite, retained austenite, and the remaining microstructure were measured at a position 1 / 4 of the thickness of the obtained steel plate using the method described above. Additionally, the area ratios of tempered martensite, pearlite, ferrite, bainite, and the remaining microstructure were measured at a position 10 μm from the surface of the obtained steel plate using the same method.

[0289] The measurement results are shown in Table 3 below.

[0290] <Nanoindentation Test>

[0291] The average value [τq] and standard deviation σq of the initiation stress τq of plastic deformation were measured at a position 1 / 4 of the thickness of the obtained steel plate using the method described above. Furthermore, the proportion of measurement points with a value less than 0.85 × [τs] at a position 10 μm from the surface of the obtained steel plate, when the average value of the initiation stress τs measured by the nanoindentation method is set as [τs], was determined using the method described above.

[0292] The measurement results are shown in Table 3 below.

[0293] <evaluate>

[0294] The obtained steel plate was subjected to the following tests to evaluate various properties. The results are recorded in Table 3 below.

[0295] Tensile Testing

[0296] The tensile test was performed in accordance with JIS Z 2241:2021.

[0297] Specifically, JIS 5 test pieces are taken from the obtained steel sheet with the long side perpendicular to the rolling direction of the steel sheet as the long side. Using the taken test pieces, at a crosshead speed of 1.67 × 10⁻⁶... －1 Tensile tests were conducted under conditions of mm / s to determine the yield strength (YS) [MPa] and tensile strength (TS) [MPa].

[0298] In addition, the yield ratio (YR) (=100×YS / TS) [%] is calculated from the yield strength and tensile strength.

[0299] It should be noted that in this embodiment, a tensile strength (TS) of 1180 MPa or higher is considered high strength. Furthermore, a yield ratio (YR) exceeding 75% is considered excellent component strength.

[0300] Hole Enlargement Test

[0301] The hole enlargement test shall be carried out in accordance with JIS Z 2256.

[0302] Specifically, the steel plate obtained from shearing is used to make test pieces with dimensions of 100mm × 100mm. Holes with a diameter of 10mm are punched into the test pieces with a clearance of 12.5%. Then, using a die with an inner diameter of 75mm, a conical punch with a 60° apex angle is pressed into the hole under a restraining blank holder force of 9ton (88.26kN), and the hole diameter Df [mm] at the limit of crack initiation is measured. The initial hole diameter is set as D0 [mm], and the hole expansion rate λ [%] is calculated using Equation 7.

[0303] λ={(Df-D0) / D0}×100...Equation 7

[0304] In this embodiment, when the expansion ratio (λ) is 30% or more, it is judged to have excellent extension flange properties.

[0305] Bending Test

[0306] The bending test was conducted in accordance with JIS Z 2248:2022.

[0307] Specifically, a rectangular test piece with a width of 30 mm and a length of 100 mm is taken from the obtained steel plate, with the direction parallel to the rolling direction of the steel plate as the axis of the bending test. It should be noted that the end face of the long side of the test piece is used as the grinding end face.

[0308] Using the acquired test pieces, a 90°V bending test was conducted under an indentation load of 100 kN and a holding time of 5 seconds. Specifically, five test pieces were subjected to a 90°V bending test with an appropriate bending radius R. Next, it was confirmed whether any cracks were formed at the edge of the bending apex.

[0309] The presence or absence of cracks was confirmed by observing the ridge of the curved apex at 40x magnification using a digital microscope (RH-2000, manufactured by HIROX).

[0310] The minimum bending radius R that did not produce cracks among the five test pieces was determined, and the value obtained by dividing it by the plate thickness t (R / t) was taken as the ultimate bending radius. In this embodiment, when the ultimate bending radius (R / t) is below 5.0, it is judged to have excellent bending performance.

[0311] Resistance to delayed failure under atmospheric corrosion and in coated state

[0312] The delayed failure test, which confirms the resistance to delayed failure under atmospheric corrosion and in the coated state, is carried out by chemical conversion electrodeposition coating on a test piece with shear end face after bending and stress loading, followed by repeated dry and wet corrosion cycle tests.

[0313] Specifically, a rectangular test piece with a width of 30 mm and a length of 100 mm is taken from the obtained steel plate, with the direction parallel to the rolling direction of the steel plate as the axis of the bending test. It should be noted that the end face of the long side of the test piece is the shear end face (gap: 15%, shear angle: 0°).

[0314] The test piece was subjected to a 90°V bending process with an R / t of 5.0, and then bolted in such a way that the load stress at the apex outside the bend was 1000 MPa.

[0315] For the stress-loaded test pieces, chemical conversion treatment was carried out by immersion in "Palbond" manufactured by Nihon Parkerizing Co., Ltd. under standard conditions (35°C, 120 seconds). This was followed by electrodeposition coating using "GT-100" electrodeposition coating manufactured by Kansai Paint Co., Ltd., and baking to form a coating film. The thickness of this electrodeposited coating was 15 μm, which was confirmed by measuring the film thickness using an electromagnetic thickness gauge.

[0316] The delayed failure test piece with a shear end face of the bent part, which was thus produced in the coated state, was used for corrosion cycle testing.

[0317] It should be noted that the corrosion cycle test was conducted at 50°C in a constant temperature and humidity bath, with drying (30% RH, 2h), humidity transfer (30% ⇒ 90% RH, 2h), wetting (90% RH, 2h), and humidity transfer (90% ⇒ 30% RH, 2h) constituting one cycle. Additionally, this was performed twice a week, with a NaCl adhesion concentration of 3 g / m³. 2 Salt water was sprayed onto the surface of the delayed-destruction test piece.

[0318] The corrosion cycle test was conducted for 30 days. If no cracks were observed after 30 days, the resistance to delayed failure under atmospheric corrosion and in the coated state was considered good, indicated by ○ in Table 3. Furthermore, if no cracks were observed after 35 days, the resistance to delayed failure under atmospheric corrosion and in the coated state was considered exceptionally good, indicated by ◎ in Table 3. Conversely, if cracks were observed before 30 days, they were indicated by × in Table 3.

[0319] Table 3

[0320]

[0321]

[0322] As shown in Table 3, the inventive examples of the present invention exhibit high strength, and excellent component strength, elongation flange properties, flexural properties, and resistance to delayed failure under atmospheric corrosion and in the coated state. On the other hand, in the comparative examples, any one or more of the following are poor: strength, component strength, elongation flange properties, flexural properties, and resistance to delayed failure under atmospheric corrosion and in the coated state.

[0323] The embodiments of the present invention have been described above, but the present invention is not limited to the descriptions that constitute a part of the disclosure of the present invention based on these embodiments. That is, all other embodiments, examples, and applications of technology made by those skilled in the art based on these embodiments are included within the scope of the present invention. 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 thermal process conditions are met.

[0324] Industrial availability

[0325] According to the present invention, high-strength steel sheets with excellent component strength, elongation flange properties, bending properties, and resistance to delayed failure under atmospheric corrosion and in a painted state can be manufactured. Furthermore, by applying the steel sheets obtained according to the method of the present invention, for example, to automotive structural components, fuel efficiency can be improved based on vehicle body lightweighting, resulting in significant industrial applications.

Claims

1. A steel plate comprising, by mass percent: C: 0.090%–0.390%, Si: 0.01%–2.00%, Mn: 2.00%–4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 1.000%, N: less than 0.0100%, and O: less than 0.0100%, with the remainder consisting of Fe and unavoidable impurities. The microstructure at 1 / 4 of the steel plate thickness is as follows: The area ratio of tempered martensite is above 75%, the area ratio of fresh martensite is below 15%, the combined area ratio of ferrite and bainite is below 15%, and the area ratio of retained austenite is below 15%. Furthermore, the average value of the plastic deformation initiation stress τq at the 1 / 4 position of the plate thickness, measured by nanoindentation, is 2.50 GPa to 4.10 GPa, and the standard deviation σq of the plastic deformation initiation stress τq is less than 0.30 GPa. The microstructure at a distance of 10 μm from the surface of the steel plate is as follows: The area ratio of tempered martensite is below 40%, the area ratio of pearlite is below 15%, and the combined area ratio of ferrite and bainite is above 60%. When the average value of the plastic deformation initiation stress τs at the 10μm position measured by nanoindentation is set as [τs], the proportion of measurement points with a value less than 0.85 × [τs] is less than 25.0%.

2. The steel plate according to claim 1, wherein, The composition further contains, by mass percent, at least one element selected from the following: Ti: less than 0.200%, Nb: less than 0.200%, V: less than 0.200%, Ta: less than 0.10%, W: less than 0.10%, B: less than 0.0100%, 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%.

3. The steel plate according to claim 1 or 2, wherein, The steel plate has a coating on its surface.

4. A component comprising a steel plate as described in any one of claims 1 to 3.

5. A frame structure component or reinforcing component of an automobile, comprising the component described in claim 4.

6. A method for manufacturing a steel plate, comprising performing a hot rolling process on a steel billet having the composition described in claim 1 or 2, wherein the number of passes in a temperature region of 1000°C or higher is 4 or more, the reduction rate of each pass is 15% or more, and the average strain rate is 9 × 10⁻⁶. －4 / s~1×10 －2 After rough rolling under conditions within the range of / s, finish rolling is performed, followed by coiling to obtain a hot-rolled plate; the hot-rolled plate is then further subjected to pickling and cold rolling to obtain a cold-rolled plate. After the pickling and cold rolling processes, the cold-rolled plate is annealed and cooled to below 150°C, and then further reheated. The annealing is performed under the following conditions: the heating temperature is above 800°C, and within the heating temperature range T1 above 800°C, the dew point is above -25°C, and the following equation 1 is satisfied. It should be explained that in, [%C] indicates the carbon content in the steel plate. The time when the temperature of the cold-rolled sheet first reaches 800°C during the annealing heating is defined as t = 0 (s), and t = tE (s) is the time when the annealing is completed and the temperature of the cold-rolled sheet reaches 800°C again. t (°C) represents the average temperature of the cold-rolled sheet during time t: t-1 to t (s). Furthermore, during the cooling process, The average cooling rate v2 in the temperature range T2 of 600℃~750℃ is 1.0℃ / s~15.0℃ / s. The average cooling rate in the temperature range above 500°C and below 600°C exceeds v2. The residence time in temperature range T3 (above 400℃ but below 500℃) is set to 10s–150s. The average cooling rate v4 in the temperature range T4 from Ms -100℃ to Ms℃ is made to be greater than 3.0℃ / s. Furthermore, the reheating is carried out under the condition that the highest reheated temperature, i.e., temperature X, and the holding time Y above temperature X - 10°C satisfy the following formula 2. 8000≤(273+X)×(20+Log(Y / 3600))≤12000…Equation 2 Wherein, the temperature X is in °C, and the holding time Y is in seconds.

7. The method for manufacturing a steel plate according to claim 6, wherein, A plating process is further performed during the cooling process.

8. A method for manufacturing a component, comprising the following steps: forming or joining a steel plate according to any one of claims 1 to 3 to produce the component.

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