Steel sheet, member and method for manufacturing same

Abstract

Provided is a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle universality. The steel sheet has a prescribed component composition and has a structure in which the area fraction of tempered martensite is 83% or more, the area fraction of retained austenite is less than 3%, the total area fraction of ferrite and bainite ferrite is 5% or more and less than 15%, and the original gamma grain boundary occupancy is 20% or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet and a member using the steel sheet as a base material, and a method for manufacturing the same. BACKGROUND

[0002] In order to reduce CO2 emission by lightening the body of an automobile and improve the crashworthiness, high-strengthening of a steel sheet as a base material of an automobile component is being promoted. In addition, new laws and regulations are being introduced. Therefore, in the main structural components of an automobile (hereinafter also referred to as automobile skeleton components), the application of a steel sheet having a tensile strength (hereinafter also referred to as TS): 1180 MPa or more is increasing.

[0003] The steel sheet as a base material of an automobile component is also required to have excellent dimensional accuracy (hereinafter also simply referred to as dimensional accuracy) when formed into a component. For example, in the automobile skeleton components such as bumpers, by controlling the yield ratio (hereinafter also referred to as YR) of the steel sheet to a certain range, springback can be suppressed and the dimensional accuracy can be improved.

[0004] As a steel sheet used as a base material of an automobile component, for example, Patent Literature 1 discloses:

[0005] "A high-strength steel sheet having the following composition consisting of, in mass %: C: 0.09 to 0.37%, Si: more than 0.70% and 2.00% or less, Mn: 2.60 to 3.60%, P: 0.001 to 0.100%, S: 0.0200% or less, Al: 0.010 to 1.000%, and N: 0.0100% or less, with the remainder consisting of Fe and unavoidable impurities;

[0006] having the following steel structure: martensite having a carbon concentration of more than 0.7 x [%C] and less than 1.5 x [%C] is 55% or more in area ratio, tempered martensite having a carbon concentration of 0.7 x [%C] or less is 5 to 40% in area ratio, the ratio of the carbon concentration in residual austenite to the volume ratio of residual austenite is 0.05 to 0.40, and the average grain diameter of the above-described martensite and the above-described tempered martensite is 5.3 μm or less;

[0007] and, in the above-described steel structure, the surface layer softening thickness is 10 μm to 100 μm, and the tensile strength is 1180 MPa or more.

[0008] Note that [%C] represents the content (mass %) of the component element C in the steel."

[0009] Patent Literature 2 discloses:

[0010] "A cold-rolled steel sheet having a composition consisting of, in mass%, C: 0.15 to 0.40%, Si: 1.5% or less, Mn: 0.9 to 1.7%, P: 0.03% or less, S: less than 0.0020%, sol. Al: 0.2% or less, N: less than 0.0055%, and O: 0.0025% or less, and satisfying the following relation of formula (1), and the remainder consisting of Fe and unavoidable impurities;

[0011] having a structure in which the area ratio of a total of a tempered martensite and a bainite to the entire structure is 95 to 100%, and a group of inclusions having a length of 0.3 μm or more in a rolling direction and / or being distributed in a string of dots, and having a length of 0.3 μm or more and a diameter of 0.1 μm or less, is 0.8 per mm 2 Hereinafter,

[0012] a carbide having a length of 0.20 μm to 2 μm and an aspect ratio of 2.5 or less and having Fe as a main component is 3500 per mm 2 Hereinafter,

[0013] a carbide having a diameter of 10 to 50 nm distributed in the tempered martensite and / or the bainite is 0.7 x 107 per mm 2 Hereinafter, the average grain size of the original γ grains is 18 μm or less;

[0014] the cold-rolled steel sheet has a sheet thickness of 0.5 to 2.6 mm, a tensile strength of 1320 MPa or more, and excellent delayed fracture resistance.

[0015] 5 [%S] + [%N] < 0.0115 (1)

[0016] Here, [%S] and [%N] are the contents of S and N in the steel (mass%).

[0017] Here, the excellent delayed fracture resistance means a delayed fracture time of 200 hr or more when cold press forming is performed with shearing and blanking.

[0018] 200 hr or more when the tensile strength is 1320 MPa or more and less than 1530 MPa,

[0019] 24 hr or more when the tensile strength is 1530 MPa or more and less than 1550 MPa,

[0020] 12 hr or more when the tensile strength is 1550 MPa or more and less than 1570 MPa,

[0021] 9 hr or more when the tensile strength is 1570 MPa or more and less than 1610 MPa,

[0022] 1.0 hr or more when the tensile strength is 1610 MPa or more and less than 1960 MPa,

[0023] 0.2 hr or more when the tensile strength is 1960 MPa or more.

[0024] In addition, the delayed fracture time refers to a time from the start of immersion in hydrochloric acid (hydrogen chloride aqueous solution) of pH 1 at a water solution temperature of 20°C to the start of generation of microcracks.

[0025] Patent Document 3 discloses:

[0026] "a steel sheet having a composition consisting of, by mass%, C: 0.12% to 0.40%, Si: 0.01% to 1.5%, Mn: more than 1.7% and 3.5% or less, P: 0.05% or less, S: 0.010% or less, sol. Al: 1.00% or less, N: 0.010% or less, B: 0.0002% to 0.0050%, and one or two of Nb and Ti in a total of 0.010% to 0.080%, with the remainder consisting of Fe and unavoidable impurities;

[0027] having a steel structure in which the area ratio of martensite is 70% or more, the area ratio of bainite is 30% or less, and the total of the area ratios of ferrite and residual austenite is 10% or less;

[0028] the number density of carbides having a length diameter of 0.5 μm or more at a position of 1 / 4 of the sheet thickness of the steel sheet is 60000 pieces / mm 2 Hereinafter,

[0029] the number density of inclusion particles having an equivalent circle diameter of 4.0 μm or more in the range of 1 / 4 to 3 / 4 of the sheet thickness of the steel sheet is 10 pieces / mm 2 to 30 pieces / mm 2 ,

[0030] the number density of inclusion particles having an equivalent circle diameter of 4.0 μm or more in the range of the surface to 1 / 4 of the sheet thickness of the steel sheet is 27 pieces / mm 2 Hereinafter,

[0031] the tensile strength of the above steel sheet is 1310 MPa or more."

[0032] Prior Art Documents

[0033] Patent Documents

[0034] Patent Document 1: Japanese Patent No. 6747612

[0035] Patent Document 2: Japanese Patent No. 6112261

[0036] Patent Document 3: Japanese Patent No. 7001197 SUMMARY

[0037] However, there are a large number of end faces (hereinafter also referred to as sheared end faces) formed by shearing processing in automobile parts, particularly, skeleton parts of automobiles. Therefore, a steel sheet as a material of an automobile part is also required to have excellent delayed fracture resistance after shearing processing. Delayed fracture refers to a phenomenon that causes a fracture. That is, when a part is placed in a hydrogen intrusion environment in a state where a high stress is applied to the part by forming processing or the like, hydrogen intrudes into the part. The hydrogen that intrudes into the part causes a decrease in interatomic binding force, local deformation. As a result, a micro crack is generated in the part, which progresses to cause a fracture.

[0038] The delayed fracture resistance is affected by the morphology of the sheared end face. In addition, the morphology of the sheared end face is affected by the shearing angle (hereinafter also simply referred to as the shearing angle) at the time of shearing processing. That is, the delayed fracture resistance is affected by the shearing angle. For example, even if parts that use the same steel sheet as a material, if the shearing angle is outside an appropriate range, the delayed fracture resistance decreases. Note that the shearing angle refers to the angle (opening angle) of the upper blade and the lower blade used in shearing processing.

[0039] The steel sheet as a material of an automobile part is subjected to shearing processing at various shearing angles from the aspects of the required dimensional accuracy, productivity, constraints of the device, and the like. Therefore, it is also required that the appropriate range of the shearing angle with respect to delayed fracture (that is, the range of the shearing angle in which excellent delayed fracture resistance can be obtained in the steel sheet after shearing processing, hereinafter also referred to as shearing angle generality) be wide, that is, the shearing angle generality be excellent.

[0040] However, the steel sheets disclosed in Patent Documents 1 to 3 do not consider the shearing angle generality. Therefore, the current situation is that a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shearing angle generality needs to be developed.

[0041] The present application was developed in view of the above-described situation, and aims to provide a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shearing angle generality, and an advantageous manufacturing method thereof.

[0042] In addition, the object of the present application is to provide a member using the above-described steel sheet as a material and a manufacturing method thereof.

[0043] Here, the TS is measured by a tensile test in accordance with JIS Z 2241:2022.

[0044] The excellent dimensional accuracy refers to YR of 65 to 85%. Here, YR is calculated by the following formula.

[0045] YR = 100 x YS / TS

[0046] In the formula, YS is a yield stress, and TS is determined in the same manner as JIS Z 2241:2022 by a tensile test.

[0047] The excellent shear angle universality refers to a proper range of a shear angle in which no delayed fracture occurs when a load stress is 1000 MPa, being 0 to 0.5° or more.

[0048] Note that the details of the measurement methods are as described in the Examples described later.

[0049] The present inventors have made intensive studies in order to achieve the above object, and have arrived at the following insights.

[0050] (A) In order to obtain TS: 1180 MPa or more, it is important to make the area fraction of tempered martensite 83% or more, and to make the total area fraction of ferrite and bainite ferrite less than 15%. Thereby, the prescribed required characteristics can be ensured, and TS: 1180 MPa or more is obtained.

[0051] (B) In order to obtain excellent dimensional accuracy, it is important to make the total area fraction of ferrite and bainite ferrite 5% or more. Thereby, the prescribed required characteristics can be ensured, and excellent dimensional accuracy is obtained.

[0052] (C) In order to obtain excellent shear angle universality, it is important to make the area fraction of retained austenite less than 3%, and to make the occupancy ratio of ferrite and bainite ferrite to prior austenite grain boundaries 20% or more. Thereby, the prescribed required characteristics can be ensured, and excellent shear angle universality is obtained.

[0053] The present application is based on the above insights, and has been completed through further studies.

[0054] That is, the gist of the present application is as follows.

[0055] 1. A steel sheet having the following composition and structure:

[0056] The composition consists of, in mass%, C: 0.030 to 0.500%, Si: 0.010 to 2.500%, Mn: 0.10 to 5.00%, P: 0.100% or less, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, and Al: 1.000% or less, with the remainder consisting of Fe and unavoidable impurities;

[0057] In the structure, the area fraction of tempered martensite is 83% or more, the area fraction of residual austenite is less than 3%, the total area fraction of ferrite and bainite ferrite is 5% or more and less than 15%, and the occupancy ratio of the ferrite and the bainite ferrite to the prior austenite grain boundaries is 20% or more.

[0058] 2. The steel sheet according to the above 1, wherein the component composition further contains at least one selected from the group consisting of 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, in mass%.

[0059] 3. The steel sheet according to the above 1 or 2, wherein the surface has a plated layer.

[0060] 4. A member made of the steel sheet according to any one of the above 1 to 3.

[0061] 5. A method of manufacturing a steel sheet, which is a method of manufacturing the steel sheet according to any one of the above 1 to 3, comprising the following steps.

[0062] a preparation step of preparing a base steel sheet having the component composition according to the above 1 or 2;

[0063] a heating step of then heating the base steel sheet at an average heating rate of 5.0°C / s or less in a temperature range of 700°C to 750°C and a maximum reaching temperature Tl of 800°C to 900°C;

[0064] a first cooling step of then cooling the base steel sheet at an average cooling rate of 0.10°C / s to 5.00°C / s in a temperature range of the maximum reaching temperature Tl to an intermediate holding temperature T2;

[0065] an intermediate holding step of then holding the base steel sheet at an intermediate holding temperature T2 of 600°C to 750°C, an intermediate holding time t2 of 1.0 seconds to 2000.0 seconds, and a tension applied to the base steel sheet of 5 MPa or more;

[0066] second cooling step, next, the above-mentioned blank steel sheet is cooled to a second cooling end temperature under conditions of an average cooling rate in a temperature range of 300°C to 100°C: 300°C / s or more; and

[0067] tempering step, next, the above-mentioned blank steel sheet is tempered under conditions of a tempering temperature T3: 100°C to 400°C, a tempering time t3: 10 seconds to 10,000 seconds.

[0068] 6. The steel sheet manufacturing method according to the above 5, wherein a plating treatment step of performing plating treatment on the above-mentioned blank steel sheet is further provided between the above-mentioned intermediate holding step and the above-mentioned second cooling step or after the above-mentioned tempering step.

[0069] 7. A member manufacturing method comprising a step of manufacturing a member by performing at least one of a forming process and a joining process on the steel sheet according to any one of the above 1 to 3.

[0070] According to the present application, a steel sheet having TS of 1180 MPa or more and excellent in dimensional accuracy and shear angle universality can be obtained. In addition, the steel sheet of the present application can be more widely used as a blank for an automobile member, and thus can further improve fuel efficiency by lightening a vehicle body of an automobile, and can greatly contribute to reduction of CO2 emission. Therefore, the industrial value is extremely great. BRIEF DESCRIPTION OF DRAWINGS

[0071] Figure 1 is a schematic view for explaining the definition of the original γ grain boundary occupancy ratio. DETAILED DESCRIPTION

[0072] The present application will be described based on the following embodiments.

[0073] [1] Steel sheet

[0074] First, the composition of the steel sheet of one embodiment of the present application will be described. Note that the unit of the composition is "mass%" and, hereinafter, only "%" is used unless otherwise specified.

[0075] [C: 0.030% to 0.500%]

[0076] C is one of important essential components of the steel. In particular, in the steel sheet of one embodiment of the present application, C is an important element which affects the area fraction of tempered martensite. When the content of C is less than 0.030 %, the area fraction of tempered martensite is reduced, and it is difficult to achieve TS of 1180 MPa or higher. On the other hand, if the content of C is greater than 0.500 %, the tempered martensite is embrittled, and it is difficult to achieve excellent shear angle extensibility. Thus, the content of C is 0.030 % to 0.500 %. The content of C is preferably 0.050 % or higher, more preferably 0.100 % or higher. The content of C is preferably 0.400 % or lower, more preferably 0.350 % or lower.

[0077] [Si: 0.010 % to 2.500 %]

[0078] Si is one of important essential components of the steel. In particular, in the steel sheet of one embodiment of the present application, Si suppresses carbide generation in annealing and promotes generation of residual austenite. That is, Si is an important element which affects the area fraction of residual austenite. When the content of Si is less than 0.010 %, it is difficult to achieve TS of 1180 MPa or higher. On the other hand, if the content of Si is greater than 2.500 %, residual austenite is excessively increased, and it is difficult to achieve excellent shear angle extensibility. Thus, the content of Si is 0.010 % to 2.500 %. The content of Si is preferably 0.050 % or higher, more preferably 0.100 % or higher. The content of Si is preferably 2.000 % or lower, more preferably 1.200 % or lower.

[0079] [Mn: 0.10 % to 5.00 %]

[0080] Mn is one of important essential components of the steel. In particular, in the steel sheet of one embodiment of the present application, Mn is an important element which affects the area fraction of tempered martensite and shear angle extensibility. When the content of Mn is less than 0.10 %, the area fraction of tempered martensite is reduced, and it is difficult to achieve TS of 1180 MPa or higher. On the other hand, if the content of Mn is greater than 5.00 %, the tempered martensite is embrittled, and it is difficult to achieve excellent shear angle extensibility. Thus, the content of Mn is 0.10 % to 5.00 %. The content of Mn is preferably 0.50 % or higher, more preferably 0.80 % or higher. The content of Mn is preferably 4.50 % or lower, more preferably 4.00 % or lower.

[0081] [P: 0.100 % or lower]

[0082] P is segregated at the prior austenite grain boundaries to embrittle the grain boundaries, and becomes a starting point of delayed fracture. Therefore, if the content of P is excessive, it is difficult to achieve excellent shear angle universality. Therefore, the content of P is 0.100% or less. The content of P is preferably 0.070% or less. It should be noted that the lower limit of the content of P is not particularly specified. However, P is a solid solution strengthening element, and can increase the strength of the steel sheet. Therefore, the content of P is preferably 0.001% or more.

[0083] [S: 0.0200% or less]

[0084] S exists in the form of sulfide, and becomes a starting point of delayed fracture. Therefore, if the content of S is excessive, it is difficult to achieve excellent shear angle universality. Therefore, the content of S is 0.0200% or less. The content of S is preferably 0.0050% or less. It should be noted that the lower limit of the content of S is not particularly specified. However, due to constraints in production technology, the content of S is preferably 0.0001% or more.

[0085] [N: 0.0100% or less]

[0086] N exists in the form of nitride, and becomes a starting point of delayed fracture. Therefore, if the content of N is excessive, it is difficult to achieve excellent shear angle universality. Therefore, the content of N is 0.0100% or less. The content of N is preferably 0.0050% or less. It should be noted that the lower limit of the content of N is not particularly specified. However, due to constraints in production technology, the content of N is preferably 0.0001% or more.

[0087] [O: 0.0100% or less]

[0088] O exists in the form of oxide, and becomes a starting point of delayed fracture. Therefore, if the content of O is excessive, it is difficult to achieve excellent shear angle universality. Therefore, the content of O is 0.0100% or less. The content of O is preferably 0.0050% or less. It should be noted that the lower limit of the content of O is not particularly specified. However, due to constraints in production technology, the content of O is preferably 0.0001% or more.

[0089] [Al: 1.000% or less]

[0090] Al exists in the form of oxide, and becomes a starting point of delayed fracture. Therefore, if the content of Al is excessive, it is difficult to achieve excellent shear angle universality. Therefore, the content of Al is 1.000% or less. The content of Al is preferably 0.500% or less. It should be noted that the lower limit of the content of Al is not particularly specified. However, due to constraints in production technology, the content of Al is preferably 0.001% or more.

[0091] The basic component composition of the steel sheet of one embodiment of the present application is described above, but the steel sheet of one embodiment of the present application has a composition containing the above basic components and the remainder other than the above basic components includes Fe (iron) and inevitable impurities. Here, the steel sheet of one embodiment of the present application preferably has a composition containing the above basic components and the remainder is composed of Fe and inevitable impurities. In the steel sheet of one embodiment of the present application, at least one element selected from the following can be contained as an optional additive element, alone or in combination, in addition to the above basic components.

[0092] 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

[0093] Note that the above optional additive element can provide the effect of the present application as long as it is contained in an amount of 0.0100% or less, and thus the lower limit is not particularly set. When the above optional additive element is contained in an amount less than the lower limit value described later, the element is contained as an inevitable impurity.

[0094] [Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less]

[0095] When Ti, Nb, and V are each 0.200% or less, coarse precipitates and inclusions are not generated in large amounts, and thus the starting points of delayed fracture are not formed. Thus, the decrease in the extent of the shear angle is not caused. Thus, when Ti, Nb, and V are contained, the content of each of them is preferably 0.200% or less. The content of each of Ti, Nb, and V is more preferably 0.100% or less. Note that the lower limit of the content of each of Ti, Nb, and V is not particularly set. However, Ti, Nb, and V form fine carbides, nitrides, or carbonitrides at the time of hot rolling or at the time of annealing, and thus the strength of the steel sheet is increased. Thus, the content of each of Ti, Nb, and V is preferably 0.001% or more.

[0096] [Ta: 0.10% or less, W: 0.10% or less]

[0097] Ta and W are each 0.10% or less, coarse precipitates and inclusions are not generated in large amounts, and the starting points of delayed fracture are not caused. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Ta and W are contained, the contents thereof are each preferably 0.10% or less. The contents of Ta and W are each more preferably 0.08% or less. The lower limit of the contents of Ta and W is not particularly specified. However, Ta and W form fine carbides, nitrides, or carbonitrides at the time of hot rolling or at the time of annealing, and increase the strength of the steel sheet. Therefore, the contents of Ta and W are each preferably 0.01% or more.

[0098] [B: 0.0100% or less]

[0099] B is 0.0100% or less, cracks are not generated in the steel sheet at the time of casting or at the time of hot rolling, and the starting points of delayed fracture are not caused. Therefore, a decrease in the shear angle is not caused widely. Therefore, when B is contained, the content thereof is preferably 0.0100% or less. The content of B is more preferably 0.0080% or less. Note that the lower limit of the content of B is not particularly specified. However, B is an element that segregates at the grain boundaries of austenite in annealing and increases the hardenability. Therefore, the content of B is preferably 0.0003% or more.

[0100] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less]

[0101] Cr, Mo, and Ni are each 1.00% or less, coarse precipitates and inclusions are not generated in large amounts, and the starting points of delayed fracture are not caused. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Cr, Mo, and Ni are contained, the contents thereof are each preferably 1.00% or less. The contents of Cr, Mo, and Ni are each more preferably 0.80% or less. Note that the lower limit of the contents of Cr, Mo, and Ni is not particularly specified. However, Cr, Mo, and Ni are elements that increase the hardenability. Therefore, the contents of Cr, Mo, and Ni are each preferably 0.01% or more.

[0102] [Co: 0.010% or less]

[0103] Co is 0.010% or less, coarse precipitates and inclusions are not generated in large amounts, and the starting points of delayed fracture are not caused. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Co is contained, the content thereof is preferably 0.010% or less. The content of Co is more preferably 0.008% or less. Note that the lower limit of the content of Co is not particularly specified. However, Co is an element that increases the hardenability. Therefore, the content of Co is preferably 0.001% or more.

[0104] [Cu: 1.00% or less]

[0105] Cu is 1.00% or less, coarse precipitates and inclusions are not generated in large amounts, and the inclusions do not become a starting point of delayed fracture. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Cu is contained, the content thereof is preferably 1.00% or less. The content of Cu is more preferably 0.80% or less. Note that the lower limit of the content of Cu is not particularly specified. However, Cu is an element that improves hardenability. Therefore, the content of Cu is preferably 0.01% or more.

[0106] [Sn: 0.200% or less]

[0107] Sn is 0.200% or less, cracks are not generated in the steel sheet at the time of casting or hot rolling, and the cracks do not become a starting point of delayed fracture. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Sn is contained, the content thereof is preferably 0.200% or less. The content of Sn is more preferably 0.100% or less. Note that the lower limit of the content of Sn is not particularly specified. However, Sn is an element that improves hardenability and is also an element that improves corrosion resistance in general. Therefore, the content of Sn is preferably 0.001% or more.

[0108] [Sb: 0.200% or less]

[0109] Sb is 0.200% or less, coarse precipitates and inclusions are not generated in large amounts, and the inclusions do not become a starting point of delayed fracture. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Sb is contained, the content thereof is preferably 0.200% or less. The content of Sb is more preferably 0.100% or less. Note that the lower limit of the content of Sb is not particularly specified. However, Sb is an element that controls the thickness of surface layer softening and enables adjustment of strength. Therefore, the content of Sb is preferably 0.001% or more.

[0110] [Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less]

[0111] Ca, Mg, and REM are 0.0100% or less, respectively, coarse precipitates and inclusions are not generated in large amounts, and the inclusions do not become a starting point of delayed fracture. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Ca, Mg, and REM are contained, the contents thereof are preferably 0.0100% or less, respectively. The contents of Ca, Mg, and REM are more preferably 0.0050% or less, respectively. Note that the lower limit of the content of Ca, Mg, and REM is not particularly specified. However, Ca, Mg, and REM are elements that make the shape of nitrides and sulfides spherical and improve the limit deformation capacity of the steel sheet. Therefore, the contents of Ca, Mg, and REM are preferably 0.0005% or more, respectively.

[0112] [Zr: 0.100% or less, Te: 0.100% or less]

[0113] When Zr and Te are each 0.100% or less, coarse precipitates and inclusions are not generated in large amounts, and do not become the starting point of delayed fracture. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Zr and Te are contained, the content of each of Zr and Te is preferably 0.100% or less. The content of each of Zr and Te is more preferably 0.080% or less. Note that the lower limit of the content of each of Zr and Te is not particularly specified. However, Zr and Te are elements that make the shape of nitrides and sulfides spherical and improve the limit deformation capacity of the steel sheet. Therefore, the content of each of Zr and Te is preferably 0.001% or more.

[0114] [Hf: 0.10% or less]

[0115] When Hf is 0.10% or less, coarse precipitates and inclusions are not generated in large amounts, and do not become the starting point of delayed fracture. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Hf is contained, the content of Hf is 0.10% or less. The content of Hf is more preferably 0.08% or less. Note that the lower limit of the content of Hf is not particularly specified. However, Hf is an element that makes the shape of nitrides and sulfides spherical and improves the limit deformation capacity of the steel sheet. Therefore, the content of Hf is preferably 0.01% or more.

[0116] [Bi: 0.200% or less]

[0117] When Bi is 0.200% or less, coarse precipitates and inclusions are not generated in large amounts, and do not become the starting point of delayed fracture. Therefore, a decrease in the shear angle is not caused widely. Therefore, when Bi is contained, the content of Bi is 0.200% or less. The content of Bi is more preferably 0.100% or less. Note that the lower limit of the content of Bi is not particularly specified. However, Bi is an element that reduces segregation. Therefore, the content of Bi is preferably 0.001% or more.

[0118] The elements other than the above are Fe and inevitable impurities. As the inevitable impurities, for example, Zn, Pb, As, Ge, Sr, and Cs can be given. If the inevitable impurities are 0.100% or less in total, the inclusion of these is allowed.

[0119] Next, the structure of the steel sheet of one embodiment of the present application is described.

[0120] The structure of the steel sheet of one embodiment of the present application is the following structure:

[0121] The area fraction of the tempered martensite is 83% or more,

[0122] The area fraction of the residual austenite is less than 3%,

[0123] The total area fraction of ferrite and bainite ferrite: 5% or more and less than 15%,

[0124] The occupancy ratio of ferrite and bainite ferrite to prior austenite grain boundaries is 20% or more.

[0125] Hereinafter, each of the reasons for the limitation will be described. Note that the area fraction of each phase is the area ratio of each phase in the entire structure.

[0126] [Area fraction of tempered martensite: 83% or more]

[0127] In the steel sheet of one embodiment of the present application, it is extremely important that the area fraction of tempered martensite be 83% or more. That is, by making tempered martensite the main phase, particularly by making the area fraction of tempered martensite 83% or more, TS: 1180 MPa or more can be achieved. Thus, the area fraction of tempered martensite is 83% or more. The area fraction of tempered martensite is preferably 85% or more, further preferably 87% or more. There is no particular upper limit to the area fraction of tempered martensite. The area fraction of tempered martensite is preferably less than 95%, further preferably 94% or less, for example, and still further preferably 93% or less.

[0128] [Area fraction of residual austenite: less than 3%]

[0129] In the steel sheet of one embodiment of the present application, it is extremely important that the area fraction of residual austenite be less than 3%. That is, when the area fraction of residual austenite is 3% or more, it is difficult to achieve excellent shear angle universality. As one of the reasons for the decrease in shear angle universality, the residual austenite can be cited, which undergoes processing-induced martensite transformation to become high-hardness martensite at the time of shear processing, which becomes the starting point of fracture. Thus, the area fraction of residual austenite is less than 3%. The area fraction of residual austenite is preferably 1% or less. Note that there is no particular lower limit to the area fraction of residual austenite. The area fraction of residual austenite can be 0%.

[0130] The total area fraction of ferrite and bainite ferrite: 5% or more and less than 15%,

[0131] In the steel sheet of one embodiment of the present application, it is extremely important that the total area fraction of ferrite and bainitic ferrite be 5% or more and less than 15%. That is, when the total area fraction of ferrite and bainitic ferrite is 15% or more, it is difficult to achieve TS of 1180 MPa or more. On the other hand, if the total area fraction of ferrite and bainitic ferrite is less than 5%, it is difficult to achieve excellent dimensional accuracy. Thus, the total area fraction of ferrite and bainitic ferrite is 5% or more and less than 15%. The total area fraction of ferrite and bainitic ferrite is preferably 6% or more, further preferably 7% or more. In addition, the total area fraction of ferrite and bainitic ferrite is preferably 14% or less, further preferably 13% or less. Note that ferrite and bainitic ferrite can each be contained alone or both can be contained.

[0132] The area fraction of the remaining portion of the structure is preferably 5% or less. As the remaining portion of the structure, for example, pearlite, fresh martensite, acicular ferrite, and the like can be given. These remaining portions of the structure can be included as long as the area fraction is 5% or less, which does not affect the characteristics. Note that the area fraction of the remaining portion of the structure can be 0%.

[0133] Here, the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite are measured as follows, for example.

[0134] That is, a sample is cut out from a steel sheet in such a manner that a cross section of the thickness of the steel sheet (L cross section) parallel to the rolling direction of the steel sheet is an observation surface. Next, the observation surface of the sample is polished. Next, the observation surface of the sample is etched with 1 vol.% nitric acid ethanol to expose the structure. Next, the position of the thickness of the steel sheet 1 / 4 (the position corresponding to 1 / 4 of the thickness in the depth direction from the surface of the steel sheet) is observed at a magnification of 2000 times using an SEM. Note that in the observation image, tempered martensite is a structure in which fine irregularities are included in the structure and carbides are included in the structure. In addition, ferrite and bainitic ferrite are structures in which a concave portion is flat and no carbides are included in the structure. Then, for each field of view, the area of the region occupied by tempered martensite and the area of the region occupied by ferrite and bainitic ferrite are found, respectively. Next, the area of the region occupied by tempered martensite and the area of the region occupied by ferrite and bainitic ferrite are each divided by the area of the entire observation field and multiplied by 100. Then, the average values thereof are taken as the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite, respectively.

[0135] Note that the structure of the steel sheet is generally approximately symmetric in the thickness direction. Thus, it is sufficient to use any one of the surfaces (front surface and back surface) of the steel sheet as the starting point of the position of the thickness of the steel sheet (the position of the thickness 0) such as the position of the thickness 1 / 4 from the surface of the steel sheet at a depth of 100 μm.

[0136] Further, the area fraction of the residual austenite is determined, for example, as follows.

[0137] That is, the steel sheet is mechanically ground to a depth of 1 / 4 - 0.1 mm of the sheet thickness with the observation position being at the 1 / 4 position of the sheet thickness, and then further ground by 0.1 mm by chemical polishing. The ground surface is used as the observation surface, and the ratio of the integrated intensity of the diffraction peaks of each of the {200}, {220}, and {311} faces of fcc iron (austenite) to the integrated intensity of the diffraction peaks of each of the {200}, {211}, and {220} faces of bcc iron is found using a Co Kα ray by an X-ray diffractometer. Then, the volume fraction of the residual austenite is calculated from the ratio of the integrated intensities of the faces. Then, the residual austenite is regarded as being three-dimensionally uniform, and the volume fraction of the residual austenite is used as the area fraction of the residual austenite.

[0138] Further, the area fraction of the remaining portion is found by subtracting the area fraction of the tempered martensite, the total area fraction of the ferrite and the bainitic ferrite, and the area fraction of the residual austenite, which are found as described above, from 100%.

[0139] [Area fraction of the remaining portion (%)]=100-[Area fraction of the tempered martensite (%)]-[Total area fraction of the ferrite and the bainitic ferrite (%)]-[Area fraction of the residual austenite (%)]

[0140] [Occupancy ratio of the ferrite and the bainitic ferrite to the prior austenite grain boundaries (hereinafter also referred to as prior γ grain boundary occupancy ratio): 20% or more]

[0141] In the steel sheet of one embodiment of the present application, making the prior γ grain boundary occupancy ratio 20% or more is extremely important to achieve excellent shear angle universality. The prior austenite grain boundaries (hereinafter also referred to as prior γ grain boundaries) are the starting points of delayed fracture. Here, it is important that the prior γ grain boundaries are occupied by soft ferrite and bainitic ferrite, and in particular, that the prior γ grain boundary occupancy ratio is 20% or more. Thus, the influence of the shear angle at the time of shear working can be reduced as much as possible and the occurrence of delayed fracture can be suppressed, leading to excellent shear angle universality. Therefore, the prior γ grain boundary occupancy ratio is 20% or more. The prior γ grain boundary occupancy ratio is preferably 22% or more, further preferably 24% or more. Note that the upper limit of the prior γ grain boundary occupancy ratio is not particularly specified. The prior γ grain boundary occupancy ratio can be 100%.

[0142] Here, the prior γ grain boundary occupancy ratio is found, for example, as follows (see Figure 1 ).

[0143] In one prior γ grain (hereinafter also referred to as this prior γ grain) observed in the observation image in the determination of the area fraction of the tempered martensite and the total area fraction of the ferrite and the bainitic ferrite described above, the total length of this prior γ grain is determined.Figure 1 The perimeter of the original γ grains, represented by the solid line (the original austenite grain boundaries not occupied by ferrite and bainitic ferrite) and the dashed line, is hereinafter also referred to as L. T ), and the length of the interface between the original γ grain and the ferrite and bainitic ferrite adjacent to the original γ grain ( Figure 1 The sum of the dashed lines represents the original γ grain boundary length, hereinafter also referred to as L. F Then, the occupancy rate of the original γ grain boundary in the original γ grain is calculated using the following formula.

[0144] The occupancy rate (%) of the original γ grain boundary in the original γ grain = (L F / L T )×100

[0145] Starting from the original γ grain closest to the original γ grain, the measurement was performed on 30 original γ grains in sequence. The average value of the original γ grain boundary occupancy measured in each original γ grain was set as the original γ grain boundary occupancy of the steel plate being measured.

[0146] In addition, L T and L F For example, the following measurements were taken.

[0147] The specimen was cut from the steel plate with the thickness section (L section) parallel to the rolling direction of the steel plate as the observation plane. Next, the observation plane of the specimen was ground. Then, the observation plane of the specimen was etched with 1 vol.% nitric acid ethanol to expose the microstructure. Next, the microstructure was observed at 2000x magnification using SEM at a position of 1 / 4 of the plate thickness (equivalent to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate). The L section was determined using the object function of Adobe Illustrator from the obtained microstructure images. T and L F .

[0148] It should be noted that the mechanical properties of the steel plate in one embodiment of the present invention are as described above.

[0149] Furthermore, in one embodiment of the present invention, the steel sheet may have a coating on its surface. The coating may be provided on only one surface of the steel sheet or on both surfaces. The coating is not particularly limited. Examples of coatings include a zinc coating with Zn as the main component (Zn content of 50.0% by mass or more). Examples of zinc coatings include hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electro-galvanized layers. It should be noted that a steel sheet with a zinc coating may also be called a galvanized steel sheet. Furthermore, steel sheets having the above-mentioned hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electro-galvanized layers may also be called hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), and electro-galvanized steel sheet (EG), respectively.

[0150] As the plated layer other than the zinc plated layer, an aluminum plated layer, an alloy plated layer can be exemplified. As the alloy plated layer, for example, a hot-dip zinc-aluminum-magnesium alloy plated layer, an electroplated Zn-Ni alloy plated layer can be exemplified.

[0151] Further, the plated layer is not particularly limited in plated adhesion amount per one side surface, and is preferably 20 g / m 2 ~ 80 g / m 2 .

[0152] Note that the steel sheet of one embodiment of the present application is not particularly limited in thickness, and is preferably 0.50 mm to 2.50 mm.

[0153] [2] Member

[0154] Next, a member of one embodiment of the present application is described.

[0155] The member of one embodiment of the present application is a member (as a base material) formed using the above-described steel sheet. For example, the steel sheet as a base material is subjected to at least one of forming processing and joining processing to be a member.

[0156] Here, the above-described steel sheet is TS: 1180 MPa or higher and is excellent in dimensional accuracy and shear angle universality. Thus, the member of one embodiment of the present application is particularly suitable for a base material of an automobile part, for example. Thus, fuel efficiency can be improved by lightening of a vehicle body of an automobile, which greatly contributes to reduction in CO2 emission.

[0157] [3] Manufacturing method of steel sheet

[0158] Next, a manufacturing method of a steel sheet of one embodiment of the present application is described.

[0159] The manufacturing method of a steel sheet of one embodiment of the present application includes:

[0160] a preparation step of preparing a base material steel sheet having the above-described composition;

[0161] a heating step of then heating the base material steel sheet at an average heating rate of 5.0 °C / s or lower and at a maximum reaching temperature T1 of 800 °C to 900 °C in a temperature range from T1 to T2;

[0162] a first cooling step of then cooling the base material steel sheet at an average cooling rate of 0.10 °C / s to 5.00 °C / s in a temperature range from T1 to T2;

[0163] the intermediate holding step, then the base material steel sheet is held at an intermediate holding temperature T2: 600°C to 750°C, an intermediate holding time t2: 1.0 second to 2000.0 seconds, and a tension applied to the base material steel sheet of 5 MPa or more;

[0164] the second cooling step, then the base material steel sheet is cooled to a second cooling end temperature at an average cooling rate in the temperature range of 300°C to 100°C of 300°C / s or more; and

[0165] the tempering step, then the base material steel sheet is tempered at a tempering temperature T3: 100°C to 400°C, a tempering time t3: 10 seconds to 10000 seconds.

[0166] Note that the above temperatures are surface temperatures of the steel sheet unless otherwise specified. Also, the average heating rate and the average cooling rate are based on the surface temperature of the steel sheet unless otherwise specified.

[0167] • preparation step

[0168] First, a base material steel sheet having the above composition is prepared. For example, the base material steel sheet can be prepared by hot-rolling a steel billet to produce a hot-rolled steel sheet, optionally performing pickling and heat treatment on the hot-rolled steel sheet, and then cold-rolling to obtain a cold-rolled steel sheet. The conditions of these steps are not particularly limited and can be performed according to conventional methods.

[0169] For example, as a method of melting the steel billet (steel base material), a publicly known melting method such as a converter or an electric furnace is suitable. In order to prevent macro-segregation, it is preferable that the steel billet be melted by a continuous casting method.

[0170] As the hot-rolling, a method in which the steel billet is heated and then rolled, a method in which the steel billet after continuous casting is directly rolled without heating, a method in which the steel billet after continuous casting is heated for a short time and then rolled, and the like can be given. Also, the slab heating temperature, the slab soaking holding time, and the coiling temperature in the hot-rolling are not particularly limited. The slab heating temperature is preferably 1100°C or higher. The slab heating temperature is preferably 1300°C or lower. The slab soaking holding time is preferably 30 minutes or longer. The slab soaking holding time is preferably 250 minutes or shorter. The finish rolling temperature is preferably the Ar3 transformation point or higher. The coiling temperature is preferably 350°C or higher. The coiling temperature is preferably 650°C or lower. Note that the Ar3 transformation point is calculated by the following formula.

[0171] Ar3 transformation point (°C) = 868 - 396 [%C] + 24.6 [%Si] - 68.1 [%Mn] - 36.1 [%Ni] - 20.7 [%Cu] - 24.8 [%Cr]

[0172] Note that [% element symbol] in the above formula indicates the content (mass %) of the element in the above component composition.

[0173] The pickling can remove oxides on the surface of the hot-rolled steel sheet, and is preferably performed in order to ensure good chemical conversion treatment properties and plating quality of the steel sheet of the final product. The pickling can be performed once or can be divided into a plurality of times. In addition, the hot-rolled steel sheet after the pickling can be subjected to heat treatment.

[0174] The total reduction rate in the cold rolling is preferably 30% or more. The total reduction rate in the cold rolling is preferably 80% or less. Note that the number of rolling passes and the reduction rate of each pass can be set without particular limitation to obtain the prescribed effects.

[0175] • Heating step

[0176] Next, the base material steel sheet prepared in the preparation step is heated to a maximum reaching temperature T1 at an average heating rate of 5.0°C / s or less in a temperature range of 700°C to 750°C.

[0177] [average heating rate in a temperature range of 700°C to 750°C: 5.0°C / s or less]

[0178] The inventors and others have conducted intensive research, and as a result, have found that the average heating rate in a temperature range of 700°C to 750°C (hereinafter also simply referred to as the average heating rate) affects the original γ grain boundary occupancy. That is, by making the average heating rate 5.0°C / s or less, dissolution of carbides is promoted. As a result of this, the original γ grains are refined, which contributes to an increase in the original γ grain boundary occupancy. As a result, the shear angle universality is improved. Therefore, the average heating rate is 5.0°C / s or less. The average heating rate is preferably 3.0°C / s or less. The upper limit of the average heating rate is not particularly specified. For example, the average heating rate is preferably 0.1°C / s or more.

[0179] [maximum reaching temperature T1: 800°C to 900°C]

[0180] When the maximum reaching temperature T1 is less than 800°C, the total area fraction of ferrite and bainite ferrite is 15% or more, and it is difficult to achieve a TS of 1180 MPa or more. On the other hand, when the maximum reaching temperature T1 is more than 900°C, the total area fraction of ferrite and bainite ferrite is less than 5%, and it is difficult to achieve excellent dimensional accuracy of the member. Therefore, the maximum reaching temperature T1 is 800°C to 900°C. The maximum reaching temperature T1 is preferably 810°C or more. The maximum reaching temperature T1 is preferably 890°C or less.

[0181] It should be noted that after reaching the maximum temperature T1, the process can be immediately shifted to the cooling process described later, or can be shifted to the cooling process described later after maintaining the maximum temperature T1 for a certain period of time, for example, 1.0 to 5.0 seconds.

[0182] • First cooling process

[0183] Next, the base material steel sheet is cooled at an average cooling rate in the temperature range from the maximum temperature T1 to the intermediate holding temperature T2: 0.10°C / s to 5.00°C / s.

[0184] [The average cooling rate in the temperature range from the maximum temperature T1 to the intermediate holding temperature T2 (hereinafter also referred to as the first average cooling rate): 0.10°C / s to 5.00°C / s]

[0185] The inventors and others have conducted intensive research, and as a result, it has been found that the first average cooling rate affects the original γ grain boundary occupancy rate. That is, by making the first average cooling rate 5.00°C / s or less, the nucleation of ferrite from the original γ grain boundary is promoted, which helps to increase the original γ grain boundary occupancy rate. As a result, the shear angle is widely improved. On the other hand, when the first average cooling rate is less than 0.10°C / s, the total area fraction of ferrite and bainite ferrite is 15% or more, and it is difficult to achieve a TS of 1180 MPa or more. Therefore, the first average cooling rate is 0.10°C / s to 5.00°C / s. The first average cooling rate is preferably 0.20°C / s or more. The first average cooling rate is preferably 3.00°C / s or less.

[0186] It should be noted that the first cooling end temperature can be 600°C to 750°C. For example, the first cooling end temperature can be the intermediate holding temperature T2.

[0187] • Intermediate holding process

[0188] Next, the base material steel sheet is maintained at the intermediate holding temperature T2: 600°C to 750°C, the intermediate holding time t2: 1.0 seconds to 2000.0 seconds, and the tension applied to the base material steel sheet: 5 MPa or more.

[0189] [Intermediate holding temperature T2: 600°C to 750°C]

[0190] When the intermediate holding temperature T2 is less than 600°C, transformation of ferrite and bainite ferrite from phases other than the original γ grain boundaries is promoted. Therefore, it is difficult to make the original γ grain boundary occupancy 20% or more, and it is also difficult to achieve excellent shear angle universality. On the other hand, when the intermediate holding temperature T2 is greater than 750°C, the total area fraction of ferrite and bainite ferrite is less than 5%, and it is difficult to achieve excellent dimensional accuracy of the member. Therefore, the intermediate holding temperature T2 is 600°C to 750°C. The intermediate holding temperature T2 is preferably 610°C or more. The intermediate holding temperature T2 is preferably 740°C or less. The intermediate holding temperature here is the holding temperature in the intermediate holding step. Note that the intermediate holding temperature can not always be constant during the holding, if the intermediate holding temperature is in the temperature range of 600°C to 750°C and the temperature variation is within ±10°C of the set temperature.

[0191] [Intermediate holding time t2: 1.0 seconds to 2000.0 seconds]

[0192] When the intermediate holding time t2 is less than 1.0 second (including the case where intermediate holding is not performed), the original γ grain boundary occupancy is less than 20%, and excellent shear angle universality cannot be achieved. On the other hand, when the intermediate holding time t2 is greater than 2000.0 seconds, the total area fraction of ferrite and bainite ferrite is 15% or more, and it is difficult to achieve a TS of 1180 MPa or more. Therefore, the intermediate holding time t2 is 1.0 second to 2000.0 seconds. The intermediate holding time t2 is preferably 10.0 seconds or more. The intermediate holding time t2 is preferably 1500.0 seconds or less. Note that the intermediate holding time t2 is the holding time at the intermediate holding temperature T2.

[0193] [Imparting tension to the base material steel sheet: 5 MPa or more]

[0194] The inventors and others have repeatedly conducted intensive research, and as a result, it has been found that imparting tension to the base material steel sheet in the intermediate holding affects the original γ grain boundary occupancy. Also at this time, by making the imparting tension to the base material steel sheet (hereinafter also simply referred to as imparting tension) 5 MPa or more, nucleation of ferrite from the original γ grain boundaries is promoted, and this contributes to an increase in the original γ grain boundary occupancy. As a result, it is possible to make the original γ grain boundary occupancy 20% or more, and it is possible to achieve excellent shear angle universality. Therefore, the imparting tension is 5 MPa or more. The imparting tension is preferably 10 MPa or more. The upper limit of the imparting tension is not particularly specified. The imparting tension is preferably, for example, 100 MPa or less.

[0195] In addition, plating treatment can be performed on the base material steel sheet between the intermediate holding step and the second cooling step described later. The plating treatment is as described later.

[0196] • Second cooling step

[0197] Next, the blank steel sheet is cooled to a second cooling end temperature at an average cooling rate in a temperature range of 300°C to 100°C: 300°C / s or more.

[0198] [average cooling rate in a temperature range of 300°C to 100°C (hereinafter also referred to as second average cooling rate): 300°C / s or more]

[0199] When the second average cooling rate is less than 300°C / s, the area fraction of residual austenite is 3% or more, and it is difficult to achieve excellent shear angle universality. Therefore, the second average cooling rate is 300°C / s or more. The second average cooling rate is preferably 800°C / s or more. The upper limit of the second average cooling rate is not particularly specified. For example, the second average cooling rate is preferably 2000°C / s or less.

[0200] Note that the second cooling end temperature may, for example, be less than 100°C. In addition, the second cooling end temperature may, for example, be around room temperature.

[0201] • Tempering Step

[0202] Next, the blank steel sheet is tempered at a tempering temperature T3: 100°C to 400°C, for a tempering time t3: 10 seconds to 10000 seconds.

[0203] [tempering temperature T3: 100°C to 400°C]

[0204] The tempered martensite is generated by tempering the martensite by the tempering treatment. Here, if the tempering temperature T3 is less than 100°C, the martensite is not sufficiently tempered, and becomes a structure in which a quenched state martensite is the main component. In such a structure in which the quenched state martensite is the main component, excellent shear angle universality cannot be obtained. On the other hand, if the tempering temperature T3 exceeds 400°C, the tempering of the martensite is excessively performed, and it is difficult to achieve a TS of 1180 MPa or more. Therefore, the tempering temperature T3 is 100°C to 400°C. The tempering temperature T3 is preferably 150°C or more. The tempering temperature T3 is preferably 350°C or less. The tempering temperature referred to here is the holding temperature in the tempering step. The tempering temperature can not always be constant in the holding. Alternatively, if the tempering temperature is in a temperature range of 100°C to 400°C and the temperature variation is within ±10°C of the set temperature, the tempering temperature can not always be constant in the holding.

[0205] [tempering time t3: 10 seconds to 10000 seconds]

[0206] As described above, the tempered martensite is generated by tempering the martensite by the tempering treatment. Here, if the tempering time t3 is less than 10 seconds, the martensite is not sufficiently tempered, and becomes a structure of a main body of the quenched state martensite. In such a structure of the main body of the quenched state martensite, excellent shear angle universality cannot be obtained. On the other hand, if the tempering time t3 is more than 10000 seconds, the tempering of the martensite excessively proceeds, and it is difficult to achieve the TS of 1180 MPa or more. Therefore, the tempering time t3 is 10 seconds to 10000 seconds. The tempering time t3 is preferably 50 seconds or more. The tempering time t3 is preferably 5000 seconds or less. Note that the tempering time t3 here is the holding time at the tempering temperature T3.

[0207] Note that the cooling after the tempering is not particularly limited. For example, the cooling can be performed by an arbitrary method according to a conventional method. Note that the cooling end temperature after the tempering can be, for example, room temperature or the like.

[0208] In addition, the base steel sheet can be subjected to working under a condition that an equivalent plastic strain amount of 0.10% to 5.00% is obtained after the tempering step. Further, the base steel sheet can be subjected to reheating at 100°C to 400°C after the working.

[0209] Further, the base steel sheet can be subjected to plating treatment after the tempering step. The plating treatment is described later.

[0210] • Plating treatment step

[0211] In addition, the base steel sheet can be subjected to plating treatment arbitrarily. The plating treatment is not particularly limited. As the plating treatment, for example, zinc plating treatments such as hot-dip galvanizing treatment, galvannealing treatment, and electroplating zinc treatment can be exemplified. In addition, as the plating treatment other than the zinc plating treatment, aluminum plating treatment and alloy plating treatment can be exemplified. As the alloy plating treatment, for example, hot-dip zinc-aluminum-magnesium alloy treatment and electroplating Zn-Ni alloy treatment can be exemplified. The treatment conditions can be performed according to a conventional method. Note that, as described above, the plating treatment is preferably performed between the intermediate holding step and the second cooling step or after the tempering step. For example, the hot-dip galvanizing treatment and the galvannealing treatment are preferably performed between the intermediate holding step and the second cooling step. In addition, the electroplating zinc treatment and the electroplating Zn-Ni alloy treatment are preferably performed after the tempering step.

[0212] Note that, in the case of the hot-dip galvanizing treatment and the galvannealing treatment, from the viewpoint of productivity, it is preferable that the series of the above-described heating step, plating treatment step, and the like are performed in a CGL (Continuous Galvanizing Line) as a hot-dip galvanizing line. After the hot-dip galvanizing, in order to adjust the unit area weight of the plating, wiping can be performed.

[0213] In addition, the base material steel sheet can be subjected to working under conditions that become an equivalent plastic strain of 0.10% to 5.00% after the plating treatment step. Furthermore, the base material steel sheet (plated steel sheet) can be subjected to reheating under conditions of 100°C to 400°C after the working.

[0214] The conditions other than the above are not particularly limited and can be performed according to a conventional method. According to the method for manufacturing a steel sheet of one embodiment of the present application, a steel sheet having TS of 1180 MPa or more and excellent dimensional accuracy and shear angle universality can be obtained. The obtained steel sheet can be used as a base material for automobile parts, for example. Note that when a steel sheet is to be processed, the steel sheet is usually processed after being cooled to room temperature.

[0215] [4] Method for manufacturing a member

[0216] Next, a method for manufacturing a member of one embodiment of the present application is described.

[0217] The method for manufacturing a member of one embodiment of the present application includes a step of manufacturing a member by performing at least one of forming working and joining working on the above-described steel sheet.

[0218] Here, the forming working method is not particularly limited and can be a general working method such as press forming. In addition, the joining working method is not particularly limited and can be a general welding method such as spot welding, laser welding, arc welding, rivet joining, pressure bonding, or the like. Note that the forming conditions and the joining conditions are not particularly limited and can be set according to a conventional method.

[0219] Example

[0220] A steel having a composition shown in Table 1 (the remainder is Fe and inevitable impurities) was melted in a converter, and a steel slab was manufactured by a continuous casting method. Next, the steel slab was heated. Next, the steel slab was subjected to hot rolling, and a hot-rolled steel sheet was manufactured. Next, the hot-rolled steel sheet was subjected to pickling treatment. Next, the hot-rolled steel sheet was subjected to cold rolling, and a cold-rolled steel sheet was manufactured. Thus, a base material steel sheet was prepared. Next, the prepared base material steel sheet was subjected to a heating step, a first cooling step, an intermediate holding step, a second cooling step, and a tempering step under the conditions shown in Table 2, and a steel sheet (sheet thickness: 0.6 to 2.2 mm) that became a final product was obtained. In addition, a plating treatment was performed on a part of the steel sheets (the steel sheets in which the kind column in Table 2 is GI, GA, and EG). The steel sheets in which the kind column in Table 2 is GI and GA were subjected to the plating treatment between the intermediate holding step and the second cooling step. In addition, the steel sheet in which the kind column in Table 2 is EG was subjected to the plating treatment after the tempering step. The conditions not specifically specified were set according to a conventional method.

[0221] Using the thus obtained steel sheet, the area fraction of tempered martensite, the area fraction of residual austenite, the total area fraction of ferrite and bainite ferrite, and the occupancy ratio of prior γ grain boundaries were found in accordance with the above-mentioned protocols. The results are shown in Table 3.

[0222] In addition, each evaluation was performed in accordance with the following protocols. The evaluation results are collectively shown in Table 3.

[0223] (Evaluation of TS)

[0224] A JIS No. 5 test piece (punctured distance: 50 mm, parallel portion width: 25 mm) was taken from the obtained steel sheet in a manner such that the direction perpendicular to the rolling direction of the steel sheet was the long side direction of the test piece. Subsequently, using the taken test piece, a tensile test was performed in accordance with JIS Z2241:2022 to measure TS and YS. The crosshead speed was 1.67 x 10 －1 mm / sec. Then, TS was evaluated in accordance with the following criteria.

[0225] Good (pass, excellent): TS was 1180 MPa or more

[0226] Poor (fail): TS was less than 1180 MPa

[0227] (Evaluation of Dimensional Precision)

[0228] TS and YS measured in the above-mentioned evaluation of TS were used to find YR by the following formula.

[0229] YR = 100 x YS / TS

[0230] Then, dimensional precision was evaluated in accordance with the following criteria.

[0231] Good (pass, excellent): YR was 65% to 85%

[0232] Poor (fail): YR was less than 65% or YR was more than 85%

[0233] (Evaluation of Shear Angle Extensibility)

[0234] The obtained steel sheet was cut into a size of 16 mm x 75 mm in the direction perpendicular to the rolling direction to make test pieces. The gap at the time of cutting was 15% in each case. In addition, the shear angle at the time of cutting was varied in the range of 0° to 2.0° at an interval of 0.25°. Subsequently, a 4-point bending test was performed in accordance with ASTM (G39-99) to load a stress of 1000 MPa on the bending apex portion of the test piece. Subsequently, the test piece was immersed in a hydrochloric acid having a pH of 3 at 25°C for 100 hours in a state where the stress was loaded. After the immersion, it was visually confirmed whether or not there was a crack in each test piece. Then, shear angle extensibility was evaluated in accordance with the following criteria.

[0235] Good (pass, excellent): the proper range of the shear angle for delayed fracture is 0° to 0.5° or more and less than 1.0°

[0236] Good (pass, excellent): the proper range of the shear angle for delayed fracture is 0° to 0.5° or more and less than 1.0°

[0237] Poor (fail): the proper range of the shear angle for delayed fracture is less than 0° to 0.5°

[0238] The proper range of the shear angle for delayed fracture means the range of the shear angle in which no cracks are observed in the test piece in the above test. For example, when no cracks are observed in the test pieces in which the shear angle at the time of shearing is 0° to 0.75°, and cracks are observed in the test pieces in which the shear angle at the time of shearing is 1.00° or more, the proper range of the shear angle for delayed fracture is "0° to 0.75°", and the evaluation is "Good (pass, excellent)". Further, when no cracks are observed in the test pieces in which the shear angle at the time of shearing is 0° to 0.25°, and cracks are observed in the test pieces in which the shear angle at the time of shearing is 0.50° or more, the proper range of the shear angle for delayed fracture is "0° to 0.25°", and the evaluation is "Poor (fail)".

[0239] [Table 1]

[0240] Table 1

[0241] Underline: indicates the range outside the present invention.

[0242] [Table 2]

[0243] Table 2

[0244]

[0245] Underline: indicates the range outside the present invention.

[0246] (*) CR: cold rolled steel sheet (non-plated), GI: hot-dip galvanized steel sheet (non-zinc-plated alloying treatment)

[0247] GA: galvannealed steel sheet, EG: electrogalvanized steel sheet

[0248] Table 2 (continued)

[0249]

[0250] Underline: indicates the range outside the present invention.

[0251] (*) CR: cold rolled steel sheet (non-plated), GI: hot-dip galvanized steel sheet (non-zinc-plated alloying treatment)

[0252] GA: Galvannealed steel sheet, EG: Electro-galvanized steel sheet [Table 3]

[0253] Table 3

[0254] Underline: Indicates out of the scope of the present application.

[0255] Table 3 (continued)

[0256]

[0257] Underline: Indicates out of the scope of the present application.

[0258] As shown in Table 3, in all of the inventive examples, TS, dimensional accuracy, and shear angle universality were all satisfactory. In addition, members obtained by performing forming processing using the steel sheets of the inventive examples or members obtained by performing joining processing did not have cracks and had target shapes. In addition, in the members, even if the shear angle of the shearing processing was changed, delayed fracture did not occur. The dimensional accuracy was also good.

[0259] On the other hand, in the comparative examples, at least one of TS, dimensional accuracy, and shear angle universality was not satisfactory.

Claims

1. A steel sheet having the following composition and structure: the composition consisting of, in mass%, C: 0.030% to 0.500%, Si: 0.010% to 2.500%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, and Al: 1.000% or less, with the remainder consisting of Fe and unavoidable impurities; in the structure, the area fraction of tempered martensite is 83% or more, the area fraction of retained austenite is less than 3%, the total area fraction of ferrite and bainite ferrite is 5% or more and less than 15%, and the occupancy ratio of the ferrite and the bainite ferrite to prior austenite grain boundaries is 20% or more.

2. The steel sheet according to claim 1, wherein, the composition further contains, in mass%, at least one selected from the group consisting of 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.

3. The steel sheet according to claim 1, wherein, the surface has a plated layer.

4. The steel sheet according to claim 2, wherein the surface has a plated layer.

5. A member made using the steel sheet according to any one of claims 1 to 4.

6. A method for manufacturing a steel sheet according to any one of claims 1 to 4, comprising: a preparation step of preparing a base steel sheet having the composition according to claim 1 or 2; a heating step of then heating the base steel sheet at an average heating rate of 5.0°C / s or less in a temperature range of 700°C to 750°C and with a maximum reaching temperature T1 of 800°C to 900°C; a first cooling step of then cooling the base steel sheet at an average cooling rate of 0.10°C / s to 5.00°C / s in a temperature range of the maximum reaching temperature T1 to an intermediate holding temperature T2; an intermediate holding step of then holding the base steel sheet at an intermediate holding temperature T2 of 600°C to 750°C, an intermediate holding time t2 of 1.0 seconds to 2000.0 seconds, and a tension applied to the base steel sheet of 5 MPa or more; a second cooling step of then cooling the base steel sheet to a second cooling end temperature at an average cooling rate of 300°C / s or more in a temperature range of 300°C to 100°C; and a tempering step of then tempering the base steel sheet at a tempering temperature T3 of 100°C to 400°C and a tempering time t3 of 10 seconds to 10000 seconds. ​ 7. The method of producing a steel sheet according to claim 6, wherein Between the intermediate holding step and the second cooling step or after the tempering step, a plating treatment step of performing plating treatment on the blank steel sheet is further provided.

8. A method of manufacturing a member, comprising a step of manufacturing a member by at least one of forming processing and joining processing of the steel sheet according to any one of claims 1 to 4.

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