Steel plates and members, and their manufacturing methods

Abstract

The present invention provides a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range. The steel sheet has a specified chemical composition and 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 bainitic ferrite is 5% or more and less than 15%, and the prior γ grain boundary occupancy rate is 20% or more.

Description

[Technical field]

[0001] The present invention relates to a steel plate, a member made of the steel plate, and a method for manufacturing the same. [Background technology]

[0002] In order to achieve both reduced CO2 emissions by reducing the weight of automobile bodies and improved crashworthiness, efforts are being made to increase the strength of steel sheets, which are the raw material for automobile parts. New legal regulations are also being introduced one after another. As a result, the number of cases in which steel sheets with a tensile strength (TS) of 1180 MPa or more are being used in the main structural parts of automobiles (hereinafter referred to as automotive frame parts) is increasing.

[0003] Steel sheets used as materials for automobile parts are also required to have excellent dimensional accuracy (hereinafter, simply referred to as dimensional accuracy) when formed into parts. For example, in automobile frame parts such as bumpers, it is possible to suppress springback and improve dimensional accuracy by controlling the yield ratio (hereinafter, also referred to as YR) of the steel sheet within a certain range.

[0004] For example, Patent Document 1 describes a steel sheet used as a material for automobile parts as follows: "By mass%, C: 0.09% or more and 0.37% or less, Si: more than 0.70% and less than 2.00%, Mn: 2.60% or more and 3.60% or less, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% to 1.000% and N: 0.0100% or less, the balance being Fe and unavoidable impurities; The area fraction of martensite having a carbon concentration greater than 0.7×[%C] and less than 1.5×[%C] is 55% or more, The area ratio of tempered martensite having a carbon concentration of 0.7 × [%C] or less is 5% or more and 40% or less, The ratio of the carbon concentration in the retained austenite to the volume fraction of the retained austenite is 0.05 or more and 0.40 or less, The martensite and the tempered martensite have an average grain size of 5.3 μm or less, The steel structure further has a softened surface thickness of 10 μm or more and 100 μm or less, High-strength steel plate with a tensile strength of 1180MPa or more. In addition, [%C] indicates the content (mass%) of the component element C in the steel. has been disclosed.

[0005] In Patent Document 2, "The composition contains, by mass%, C: 0.15% or more and 0.40% or less, 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 satisfies the relationship of the following formula (1), with the balance being Fe and unavoidable impurities; The area ratio of tempered martensite and bainite to the entire structure is 95% or more and 100% or less in total, The inclusion particles are composed of one or more inclusion particles having a major axis of 0.3 μm or more, which are extended in the rolling direction and / or distributed in a dotted row pattern, and when the inclusion particles are composed of two or more particles, the distance between the inclusion particles is 30 μm or less, and the number of inclusion groups having a total length of more than 120 μm in the rolling direction is 0.8 particles / mm2 or less, The aspect ratio is 2.5 or less, and the major axis is 0.20 μm to 2 μm. The number of Fe-based carbides is 3,500 / mm 2 is as follows: The tempered martensite and / or the bainite have a diameter of 10 to 50 nm and are distributed within the tempered martensite and / or the bainite at a density of 0.7×107 particles / mm 2 That's all. The average grain size of prior γ grains is 18 μm or less. A cold-rolled steel sheet having a thickness of 0.5 to 2.6 mm, a tensile strength of 1320 MPa or more, and excellent resistance to delayed fracture. 5[%S]+[%N]<0.0115···(1) Here, [%S] and [%N] are the S and N contents (mass%) in the steel, respectively. Here, excellent resistance to delayed fracture means that the delayed fracture time when cold press forming involving shearing and punching is: Tensile strength: 1320MPa or more but less than 1530MPa: over 200hrs Tensile strength: 24 hours or more for 1530 MPa or more but less than 1550 MPa, Tensile strength: 12hr or more for 1550MPa or more but less than 1570MPa, Tensile strength: 9hr or more for 1570MPa or more but less than 1610MPa, Tensile strength: 1.0 hr or more for 1610 MPa or more but less than 1960 MPa, Tensile strength: 0.2hr or more at 1960MPa or more The delayed fracture time is the time from the start of immersion to the beginning of the development of microcracks when immersed in hydrochloric acid (hydrogen chloride aqueous solution) with a pH of 1 at an aqueous solution temperature of 20°C. has been disclosed.

[0006] In Patent Document 3, "By mass%, C: 0.12% or more and 0.40% or less, Si: 0.01% or more and 1.5% or less, Mn: over 1.7% and up to 3.5% P: 0.05% or less, S: 0.010% or less, Sol.Al: 1.00% or less, N: 0.010% or less, B: 0.0002% or more and 0.0050% or less, and A composition containing one or two of Nb and Ti in a total amount of 0.010% or more and 0.080% or less, with the balance being Fe and unavoidable impurities; A steel structure having an area ratio of martensite of 70% or more, an area ratio of bainite of 30% or less, and a total area ratio of ferrite and retained austenite of 10% or less, The density of carbides with a major axis of 0.5 μm or more at the 1 / 4 position of the steel plate thickness is 60,000 pieces / mm 2 is as follows: The number density of inclusion particles with a circular equivalent diameter of 4.0 μm or more in the range of 1 / 4 to 3 / 4 of the plate thickness of the steel plate is 10 particles / mm 2 More than 30 pieces / mm 2 is as follows: The density of inclusion particles with a circular equivalent diameter of 4.0 μm or more in the area from the surface of the steel plate to 1 / 4 of the plate thickness was 27 particles / mm 2 is as follows: Steel plate with a tensile strength of 1310 MPa or more. has been disclosed. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6747612 [Patent Document 2] Patent No. 6112261 [Patent Document 3] Patent No. 7001197 Summary of the Invention [Problem to be solved by the invention]

[0008] Meanwhile, automobile parts, particularly automobile frame parts, have many end faces (hereinafter also referred to as shear end faces) formed by shear processing. Therefore, the steel sheets used as the raw materials for automobile parts are also required to have excellent delayed fracture resistance properties after shear processing. Delayed fracture is a phenomenon that leads to destruction in the following manner. That is, when a part is placed in a hydrogen intrusion environment in a state where high stress is applied to the part by forming processing or the like, hydrogen penetrates into the part. The hydrogen that penetrates into the part causes a decrease in interatomic bonding strength and local deformation. This causes microcracks to form in the part, and the microcracks grow and lead to destruction.

[0009] The delayed fracture resistance is affected by the shape of the sheared end surface. The shape of the sheared end surface is also affected by the shear angle used during shearing (hereinafter simply referred to as the shear angle). In other words, the delayed fracture resistance is affected by the shear angle. For example, even for parts made from the same steel plate, if the shear angle is outside the appropriate range, the delayed fracture resistance decreases. The shear angle is the angle (opening angle) between the upper blade and lower blade used in shearing.

[0010] Steel sheets used as raw materials for automotive parts are sheared at various shear angles in consideration of the required dimensional accuracy, productivity, equipment constraints, etc. Therefore, there is a demand for a wide range of appropriate shear angles for delayed fracture (i.e., the range of shear angles at which excellent delayed fracture resistance can be obtained in the steel sheet after shear processing, hereinafter also referred to as shear angle range), i.e., excellent shear angle range.

[0011] However, no consideration is given to the shear angle range in any of the steel sheets disclosed in Patent Documents 1 to 3. Therefore, there is currently a demand for the development of a steel sheet having a TS of 1180 MPa or more and also excellent in dimensional accuracy and shear angle range.

[0012] The present invention has been developed in view of the above-mentioned current situation, and has an object to provide a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range, together with an advantageous manufacturing method thereof. Another object of the present invention is to provide a member made of the above-mentioned steel plate and a method for manufacturing the same.

[0013] Here, TS is measured by a tensile test in accordance with JIS Z 2241:2022. Excellent dimensional accuracy means that YR is 65% or more and 85% or less, where YR is calculated by the following formula: YR=100×YS / TS In the formula, YS is the yield stress, which, like TS, is measured by a tensile test in accordance with JIS Z 2241:2022. The excellent shear angle range means that the appropriate range of the shear angle where delayed fracture does not occur when the load stress is 1000 MPa is 0 to 0.5° or more. Details of the measurement methods are as described in the Examples below. [Means for solving the problem]

[0014] In order to achieve the above object, the inventors have conducted extensive research and have obtained the following findings. (A) In order to obtain a TS of 1180 MPa or more, it is important that the area fraction of tempered martensite is 83% or more and the total area fraction of ferrite and bainitic ferrite is less than 15%. This makes it possible to obtain a TS of 1180 MPa or more while ensuring the required properties. (B) In order to obtain excellent dimensional accuracy, it is important that the total area fraction of ferrite and bainitic ferrite is 5% or more. This makes it possible to obtain excellent dimensional accuracy while ensuring the required properties. (C) In order to obtain an excellent shear angle range, it is important that the area fraction of retained austenite is less than 3% and the occupancy rate of prior austenite grain boundaries by ferrite and bainitic ferrite is 20% or more. This makes it possible to obtain an excellent shear angle range while ensuring the required properties. The present invention was completed based on the above findings and through further investigation.

[0015] That is, the gist and configuration of the present invention are as follows. 1. In mass percent, C: 0.030% or more and 0.500% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, 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 and the remainder being Fe and unavoidable impurities; Area fraction of tempered martensite: 83% or more, Area fraction of retained austenite: less than 3% Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%; and Occupancy rate of prior austenite grain boundaries by the ferrite and bainitic ferrite: 20% or more A steel plate having a structure in which

[0016] 2. The composition further comprises, in mass%, 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 The steel sheet according to 1 above, comprising at least one selected from the following:

[0017] 3. The steel sheet according to 1 or 2 above, having a plating layer on the surface.

[0018] 4. A member made using the steel plate according to any one of 1 to 3 above.

[0019] 5. A method for producing a steel sheet according to any one of 1 to 3 above, A preparation step of preparing a base steel sheet having the chemical composition according to 1 or 2 above; Next, the base steel sheet is Average heating rate in the temperature range of 700℃~750℃: 5.0℃ / s or less, Maximum temperature T1: 800℃ or more and 900℃ or less A heating step of heating under the conditions of Next, the base steel sheet is Average cooling rate in the temperature range from the maximum reaching temperature T1 to the intermediate holding temperature T2: 0.10°C / s or more and 5.00°C / s or less A first cooling step, Next, the base steel sheet is Intermediate holding temperature T2: 600℃ or more and 750℃ or less, Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less, Tension applied to the base steel plate: 5 MPa or more and an intermediate holding step of holding the mixture under the condition of Next, the base steel sheet is Average cooling rate in the temperature range of 300℃~100℃: 300℃ / s or more, A second cooling step of cooling the mixture to a second cooling end temperature under the above condition. Next, the base steel sheet is Tempering temperature T3: 100℃ or more and 400℃ or less, Tempering time t3: 10 seconds or more and 10000 seconds or less A tempering process, A manufacturing method for steel plates that:

[0020] 6. The method for producing steel plate as described in 5 above, further comprising a plating process step of plating the base steel plate between the intermediate holding process and the second cooling process, or after the tempering process.

[0021] 7. A method for producing a component, comprising the step of subjecting the steel plate according to any one of 1 to 3 above to at least one of forming and joining to form a component. Effect of the Invention

[0022] According to the present invention, a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range can be obtained. Furthermore, the steel sheet of the present invention can be applied to a wider range of materials for automobile parts, which can further improve fuel efficiency by reducing the weight of the automobile body and greatly contribute to reducing CO2 emissions. Therefore, the industrial utility value is extremely high. [Brief description of the drawings]

[0023] [Figure 1] FIG. 2 is a schematic diagram for explaining the definition of a prior γ grain boundary occupancy rate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The present invention will be described based on the following embodiments. [1] Steel plate First, the composition of the steel sheet according to one embodiment of the present invention will be described. Note that the unit of the composition is always "mass%", but hereinafter, unless otherwise specified, it will be simply represented as "%".

[0025] [C: 0.030% or more and 0.500% or less] C is one of the important basic components of steel. In particular, in a steel sheet according to an embodiment of the present invention, C is an important element that affects the area fraction of tempered martensite. If the C content is less than 0.030%, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, if the C content exceeds 0.500%, the tempered martensite becomes embrittled, making it difficult to achieve excellent shear angle range. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably set to 0.050% or more, more preferably 0.100% or more. The C content is preferably set to 0.400% or less, more preferably 0.350% or less.

[0026] [Si:0.010% or more and 2.500% or less] Si is one of the important basic components of steel. In particular, in the steel sheet according to one embodiment of the present invention, Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite. That is, Si is an important element that affects the area fraction of retained austenite. If the Si content is less than 0.010%, it is difficult to achieve a TS of 1180 MPa or more. On the other hand, if the Si content exceeds 2.500%, the retained austenite increases excessively, making it difficult to achieve excellent shear angle broadness. Therefore, the Si content is set to 0.010% or more and 2.500% or less. The Si content is preferably set to 0.050% or more, more preferably 0.100% or more. The Si content is preferably set to 2.000% or less, more preferably 1.200% or less.

[0027] [Mn:0.10% or more and 5.00% or less] Mn is one of the important basic components of steel. In particular, in the steel sheet according to one embodiment of the present invention, Mn is an important element that affects the area fraction of tempered martensite and the shear angle range. If the Mn content is less than 0.10%, the area fraction of tempered martensite decreases, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, if the Mn content exceeds 5.00%, the tempered martensite becomes embrittled, making it difficult to achieve an excellent shear angle range. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably 0.50% or more, more preferably 0.80% or more. The Mn content is preferably 4.50% or less, more preferably 4.00% or less.

[0028] [P:0.100% or less] P segregates at prior austenite grain boundaries, embrittling the grain boundaries and becoming the starting point of delayed fracture. Therefore, if the P content is excessive, it becomes difficult to achieve excellent shear angle range. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.070% or less. There is no particular lower limit for the P content. However, P is a solid solution strengthening element and can increase the strength of the steel sheet. Therefore, the P content is preferably set to 0.001% or more.

[0029] [S:0.0200% or less] S exists as sulfides and becomes the starting point of delayed fracture. Therefore, if the S content is excessive, it becomes difficult to achieve excellent shear angle range. Therefore, the S content is set to 0.0200% or less. The S content is preferably set to 0.0050% or less. There is no particular lower limit for the S content. However, due to constraints in production technology, the S content is preferably set to 0.0001% or more.

[0030] [N:0.0100% or less] N exists as nitrides and becomes the starting point of delayed fracture. Therefore, if the N content is excessive, it becomes difficult to achieve an excellent shear angle range. Therefore, the N content is set to 0.0100% or less. The N content is preferably set to 0.0050% or less. There is no particular lower limit for the N content. However, due to constraints in production technology, the N content is preferably set to 0.0001% or more.

[0031] [O:0.0100% or less] O exists as an oxide and becomes the starting point of delayed fracture. Therefore, if the O content is excessive, it becomes difficult to achieve excellent shear angle range. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0050% or less. There is no particular lower limit for the O content. However, due to constraints in production technology, the O content is preferably set to 0.0001% or more.

[0032] [Al: 1.000% or less] Al exists as an oxide and becomes the starting point of delayed fracture. Therefore, if the Al content is excessive, it becomes difficult to achieve an excellent shear angle range. Therefore, the Al content is set to 1.000% or less. The Al content is preferably set to 0.500% or less. There is no particular lower limit for the Al content. However, due to constraints in production technology, the Al content is preferably set to 0.001% or more.

[0033] The basic composition of the steel sheet according to one embodiment of the present invention has been described above, but the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities. Here, it is preferable that the steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance consisting of Fe and unavoidable impurities. In addition to the above basic components, the steel sheet according to one embodiment of the present invention may contain at least one element selected from the following, either alone or in combination, as an optional added element. 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 addition, the above-mentioned optional elements are not limited to a lower limit because the effects of the present invention can be obtained as long as they are contained in an amount not exceeding the upper limit. In addition, when the above-mentioned optional elements are contained in an amount less than the preferable lower limit described later, the elements are considered to be contained as inevitable impurities.

[0034] [Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less] If the contents of Ti, Nb and V are each 0.200% or less, large amounts of coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, when Ti, Nb and V are contained, the contents are preferably 0.200% or less. The contents of Ti, Nb and V are more preferably 0.100% or less. The lower limits of the contents of Ti, Nb and V are not particularly specified. However, Ti, Nb and V increase the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. Therefore, the contents of Ti, Nb and V are preferably 0.001% or more.

[0035] [Ta: 0.10% or less, W: 0.10% or less] If the Ta and W contents are each 0.10% or less, large amounts of coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, when Ta and W are contained, the contents are preferably 0.10% or less. The contents of Ta and W are more preferably 0.08% or less. There is no particular lower limit for the contents of Ta and W. However, Ta and W increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Therefore, the contents of Ta and W are preferably 0.01% or more.

[0036] [B:0.0100% or less] If B is 0.0100% or less, cracks will not form inside the steel sheet during casting or hot rolling, and will not become the starting point of delayed fracture. Therefore, the shear angle range will not be reduced. Therefore, when B is contained, its content is preferably 0.0100% or less. The B content is more preferably 0.0080% or less. There is no particular lower limit for the B content. However, B is an element that segregates to the austenite grain boundaries during annealing and improves hardenability. Therefore, the B content is preferably 0.0003% or more.

[0037] [Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less] If the contents of Cr, Mo and Ni are each 1.00% or less, large amounts of coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, when Cr, Mo and Ni are contained, the contents are preferably 1.00% or less. The contents of Cr, Mo and Ni are more preferably 0.80% or less. There is no particular lower limit for the contents of Cr, Mo and Ni. However, Cr, Mo and Ni are elements that improve hardenability. Therefore, the contents of Cr, Mo and Ni are preferably 0.01% or more.

[0038] [Co:0.010% or less] If the Co content is 0.010% or less, large coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, when Co is contained, the content is preferably 0.010% or less. The Co content is more preferably 0.008% or less. There is no particular lower limit for the Co content. However, Co is an element that improves hardenability. Therefore, the Co content is preferably 0.001% or more.

[0039] [Cu:1.00% or less] If the Cu content is 1.00% or less, large amounts of coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, it does not lead to a decrease in the shear angle range. Therefore, when Cu is contained, its content is preferably 1.00% or less. The Cu content is more preferably 0.80% or less. There is no particular lower limit for the Cu content. However, Cu is an element that improves hardenability. Therefore, the Cu content is preferably 0.01% or more.

[0040] [Sn:0.200% or less] If the Sn content is 0.200% or less, cracks will not form inside the steel sheet during casting or hot rolling, and will not become the starting point of delayed fracture. Therefore, the shear angle range will not be reduced. Therefore, when Sn is contained, its content is preferably 0.200% or less. The Sn content is more preferably 0.100% or less. There is no particular lower limit for the Sn content. However, Sn is an element that improves hardenability, and generally also improves corrosion resistance. Therefore, the Sn content is preferably 0.001% or more.

[0041] [Sb:0.200% or less] If the Sb content is 0.200% or less, large coarse precipitates and inclusions are not generated in large quantities and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, when Sb is contained, the content is preferably 0.200% or less. The Sb content is more preferably 0.100% or less. There is no particular lower limit for the Sb content. However, Sb is an element that controls the surface softening thickness and enables strength adjustment. Therefore, the Sb content is preferably 0.001% or more.

[0042] [Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less] If the contents of Ca, Mg and REM are each 0.0100% or less, large amounts of coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, when Ca, Mg and REM are contained, the contents are preferably 0.0100% or less. The contents of Ca, Mg and REM are more preferably 0.0050% or less. The lower limits of the contents of Ca, Mg and REM are not particularly specified. However, Ca, Mg and REM are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the contents of Ca, Mg and REM are preferably 0.0005% or more.

[0043] [Zr: 0.100% or less, Te: 0.100% or less] If the content of Zr and Te is 0.100% or less, large amounts of coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, when Zr and Te are contained, the content is preferably 0.100% or less. The content of Zr and Te is more preferably 0.080% or less. The lower limit of the content of Zr and Te is not particularly specified. However, Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet. Therefore, the content of Zr and Te is preferably 0.001% or more.

[0044] [Hf:0.10% or less] If the Hf content is 0.10% or less, large coarse precipitates and inclusions are not generated in large quantities and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, if Hf is contained, its content is set to 0.10% or less. The Hf content is more preferably set to 0.08% or less. There is no particular lower limit for the Hf content. However, Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet. Therefore, the Hf content is preferably set to 0.01% or more.

[0045] [Bi:0.200% or less] If the Bi content is 0.200% or less, large coarse precipitates and inclusions are not generated and do not become the starting point of delayed fracture. Therefore, the shear angle range is not reduced. Therefore, if Bi is contained, its content is set to 0.200% or less. The Bi content is more preferably set to 0.100% or less. There is no particular lower limit for the Bi content. However, Bi is an element that reduces segregation. Therefore, the Bi content is preferably set to 0.001% or more.

[0046] The elements other than those mentioned above are Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. These unavoidable impurities are permitted to be contained in a total amount of 0.100% or less.

[0047] Next, the structure of a steel sheet according to one embodiment of the present invention will be described. The structure of the steel plate according to one embodiment of the present invention is Area fraction of tempered martensite: 83% or more, Area fraction of retained austenite: less than 3% Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%; and This is a structure in which the occupancy rate of prior austenite grain boundaries by ferrite and bainitic ferrite is 20% or more. The reasons for each of the limitations will be explained below. The area fraction of each phase is the area ratio that each phase occupies with respect to the entire structure.

[0048] [Area fraction of tempered martensite: 83% or more] In a steel sheet according to an embodiment of the present invention, it is extremely important that the area fraction of tempered martensite is 83% or more. That is, by making tempered martensite the main phase, and in particular by making its area fraction 83% or more, it becomes possible to realize TS: 1180 MPa or more. Therefore, the area fraction of tempered martensite is 83% or more. The area fraction of tempered martensite is preferably 85% or more, more preferably 87% or more. There is no particular upper limit for the area fraction of tempered martensite. The area fraction of tempered martensite is, for example, preferably less than 95%, more preferably 94% or less, and further preferably 93% or less.

[0049] [Area fraction of retained austenite: less than 3%] In the steel sheet according to one embodiment of the present invention, it is extremely important that the area fraction of the retained austenite is less than 3%. That is, if the area fraction of the retained austenite is 3% or more, it becomes difficult to realize an excellent shear angle wide range. One of the causes of the deterioration of the shear angle wide range is that the retained austenite is transformed into high-hardness martensite during shear processing due to processing induced martensite, which becomes the starting point of fracture. Therefore, the area fraction of the retained austenite is less than 3%. The area fraction of the retained austenite is preferably 1% or less. The lower limit of the area fraction of the retained austenite is not particularly specified. The area fraction of the retained austenite may be 0%.

[0050] [Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%] In the steel sheet according to one embodiment of the present invention, it is extremely important that the total area fraction of ferrite and bainitic ferrite is 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 realize a TS of 1180 MPa or more. On the other hand, when the total area fraction of ferrite and bainitic ferrite is less than 5%, it is difficult to realize excellent dimensional accuracy. Therefore, 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, more preferably 7% or more. In addition, the total area fraction of ferrite and bainitic ferrite is preferably 14% or less, more preferably 13% or less. In addition, ferrite and bainitic ferrite may be contained alone, or both of them may be contained.

[0051] The area fraction of the remaining structure other than the above is preferably 5% or less. Examples of the remaining structure include pearlite, fresh martensite, and acicular ferrite. These remaining structures may be contained as long as they are 5% or less, since they do not affect the properties. The area fraction of the remaining structure may be 0%.

[0052] Here, the area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite are measured, for example, as follows. That is, a sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. Next, the observation surface of the sample is polished. Next, the observation surface of the sample is corroded with 1 vol.% nital to reveal the structure. Next, the 1 / 4 plate thickness position of the steel plate (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) is observed by SEM at a magnification of 2000 times in 10 fields of view. In the observed images, the tempered martensite has fine irregularities inside the structure and has carbides inside the structure. In addition, the ferrite and bainitic ferrite are structures in which the inside of the structure is flat at the recesses and does not have carbides inside. Then, the areas of the regions occupied by the tempered martensite, ferrite, and bainitic ferrite are calculated for each field of view. Next, the areas of the regions occupied by tempered martensite, and ferrite and bainitic ferrite are each divided by the area of ​​the entire observation field, and multiplied by 100. The average values ​​are then taken as the area fraction of tempered martensite, and the total area fraction of ferrite and bainitic ferrite, respectively. In addition, the structure of a steel sheet is generally symmetrical from top to bottom in the sheet thickness direction. Therefore, any one of the surfaces (front and back) of the steel sheet may be set as the starting point of the sheet thickness position (sheet thickness 0 position), such as the 1 / 4 sheet thickness position or a position 100 μm deep from the steel sheet surface.

[0053] The area fraction of retained austenite is measured, for example, as follows. That is, the steel plate is mechanically ground to a depth of 1 / 4-0.1 mm of the plate thickness so that the observation position is 1 / 4 of the plate thickness, and then chemically polished for an additional 0.1 mm. Using the polished surface as the observation surface, the ratio of the integrated intensity of the diffraction peaks of the {200}, {211}, and {220} planes of fcc iron (austenite) to the integrated intensity of the diffraction peaks of the {200}, {220}, and {311} planes of bcc iron is calculated using CoKα radiation in an X-ray diffractometer. The volume fraction of the retained austenite is then calculated from the ratio of the integrated intensities of the diffraction peaks of the {200}, {211}, and {220} planes of bcc iron. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.

[0054] Furthermore, the area fraction of the remaining structure is determined by subtracting the area fraction of tempered martensite, the total area fraction of ferrite and bainitic ferrite, and the area fraction of retained austenite determined as described above from 100%. [Area fraction of residual structure (%)] = 100 - [Area fraction of tempered martensite (%)] - [Total area fraction of ferrite and bainitic ferrite (%)] - [Area fraction of retained austenite (%)]

[0055] [Occupation rate of prior austenite grain boundaries by ferrite and bainitic ferrite (hereinafter also referred to as prior γ grain boundary occupation rate): 20% or more] In the steel sheet according to one embodiment of the present invention, it is extremely important to set the prior γ grain boundary occupancy rate to 20% or more in order to realize an excellent shear angle range. Prior austenite grain (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 rate is set to 20% or more. This makes it possible to minimize the influence of the shear angle during shear processing and suppress the occurrence of delayed fracture, thereby realizing an excellent shear angle range. Therefore, the prior γ grain boundary occupancy rate is set to 20% or more. The prior γ grain boundary occupancy rate is preferably 22% or more, more preferably 24% or more. The upper limit of the prior γ grain boundary occupancy rate is not particularly specified. The prior γ grain boundary occupancy rate may be 100%.

[0056] Here, the prior γ grain boundary occupancy rate is determined, for example, as follows (see FIG. 1). In one prior γ grain (hereinafter also referred to as the prior γ grain) observed in the observation image in the measurement of the above-mentioned area fraction of tempered martensite and the total area fraction of ferrite and bainitic ferrite, the total length of the prior γ grain (the perimeter of the prior γ grain, which is the sum of the solid line (prior austenite grain boundary not occupied by ferrite and bainitic ferrite) and the dotted line in FIG. 1, hereinafter referred to as L T (also called the prior γ grain boundary length), and the length of the interface between the prior γ grain and the ferrite and bainitic ferrite adjacent to the prior γ grain (the total prior γ grain boundary length of the dotted lines in Fig. 1, hereafter referred to as L F Then, the prior γ grain boundary occupancy rate of the prior γ grain is calculated using the following formula: Prior γ grain boundary occupancy rate (%) of the prior γ grain = (L F / L T ) x 100 This measurement is performed on 30 prior γ grains in order from the closest to the prior γ grain in question, and the average of the prior γ grain boundary occupancy ratios measured for each prior γ grain is regarded as the prior γ grain boundary occupancy ratio of the steel plate to be measured.

[0057] Also, L T and L F is measured, for example, as follows. A sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate becomes the observation surface. The observation surface of the sample is then polished. The observation surface of the sample is then corroded with 1 vol.% nital to reveal the structure. The structure is then observed at a position 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate) with an SEM at a magnification of 2000x. From the obtained structure image, the L cross section is created using the object function of Adobe Illustrator. T and L F Measure.

[0058] The mechanical properties of the steel plate according to one embodiment of the present invention are as described above.

[0059] Moreover, the steel sheet according to an embodiment of the present invention may have a plating layer on the surface. The plating layer may be provided on only one surface of the steel sheet, or on both surfaces. The plating layer is not particularly limited. An example of the plating layer is a zinc plating layer containing Zn as a main component (Zn content is 50.0 mass% or more). Also, an example of the zinc plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer. Note that a steel sheet having a zinc plating layer can also be called a zinc-plated steel sheet. Also, the above-mentioned steel sheets having a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer can also be called a hot-dip galvanized steel sheet (GI), an alloyed hot-dip galvanized steel sheet (GA), and an electrogalvanized steel sheet (EG), respectively.

[0060] Examples of the plating layer other than the zinc plating layer include an aluminum plating layer and an alloy plating layer. Examples of the alloy plating layer include a hot-dip zinc-aluminum-magnesium alloy plating layer and a Zn-Ni electric alloy plating layer.

[0061] In addition, the plating weight per side of the plating layer is not particularly limited, but is preferably 20 g / m 2 More than 80g / m 2 It is preferable that:

[0062] The thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.50 mm or more and 2.50 mm or less.

[0063] [2] Materials Next, a member according to one embodiment of the present invention will be described. A member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material). For example, the raw material steel plate is subjected to at least one of forming and joining to form a member. Here, the above steel sheet has a TS of 1180 MPa or more, and is also excellent in dimensional accuracy and shear angle range. Therefore, the member according to one embodiment of the present invention is particularly suitable for use as a material for automobile parts, for example. This allows for improved fuel efficiency by reducing the weight of the automobile body, which contributes greatly to reducing CO2 emissions.

[0064] [3] Manufacturing method of steel sheets Next, a method for manufacturing a steel sheet according to one embodiment of the present invention will be described.

[0065] A method for producing a steel sheet according to one embodiment of the present invention includes the steps of: A preparation step of preparing a base steel sheet having the above-mentioned composition; Next, the base steel sheet is Average heating rate in the temperature range of 700℃~750℃: 5.0℃ / s or less, Maximum temperature T1: 800℃ or more and 900℃ or less A heating step of heating under the conditions of Next, the base steel sheet is Average cooling rate in the temperature range from the maximum reaching temperature T1 to the intermediate holding temperature T2: 0.10°C / s or more and 5.00°C / s or less A first cooling step, Next, the base steel sheet is Intermediate holding temperature T2: 600℃ or higher and 750℃ or lower, Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less, Tension applied to the base steel plate: 5 MPa or more and an intermediate holding step of holding the mixture under the condition of Next, the base steel sheet is Average cooling rate in the temperature range of 300℃~100℃: 300℃ / s or more, A second cooling step of cooling the mixture to a second cooling end temperature under the above condition. Next, the base steel sheet is Tempering temperature T3: 100℃ or more and 400℃ or less, Tempering time t3: 10 seconds or more and 10000 seconds or less A tempering process, It has the following. Unless otherwise specified, the above temperatures are the surface temperatures of the steel sheet. Furthermore, unless otherwise specified, the average heating rate and the average cooling rate are also based on the surface temperatures of the steel sheet.

[0066] ·Preparation process First, a base steel sheet having the above-mentioned composition is prepared. For example, the base steel sheet can be prepared by hot rolling a steel slab to obtain a hot-rolled steel sheet, optionally pickling and heat treating the hot-rolled steel sheet, and then cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet. The conditions for these steps are not particularly limited, and may be in accordance with conventional methods.

[0067] For example, the method of smelting steel slabs (steel material) may be any known smelting method such as a converter or an electric furnace, etc. In order to prevent macrosegregation, it is preferable to smelt steel slabs by a continuous casting method.

[0068] Examples of hot rolling include a method of rolling a steel slab after heating, a method of directly rolling a steel slab after continuous casting without heating it, and a method of rolling a steel slab after continuous casting by subjecting it to a short-term heat treatment. The slab heating temperature, slab soaking time, and coiling temperature in hot rolling are also 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 time is preferably 30 minutes or more. The slab soaking time is preferably 250 minutes or less. 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. The Ar3 transformation point is calculated by the following formula. Ar3 transformation point (℃) = 868-396 x [%C] + 24.6 x [%Si] - 68.1 x [%Mn] - 36.1 x [%Ni] - 20.7 x [%Cu] - 24.8 x [%Cr] In the above formula, the symbol [% element symbol] represents the content (mass %) of the corresponding element in the above composition.

[0069] Pickling can remove oxides from the surface of the hot-rolled steel sheet, and is preferably performed to ensure good chemical conversion treatability and plating quality in the final steel sheet product. Pickling may be performed once or multiple times. The hot-rolled steel sheet after pickling may be subjected to a heat treatment.

[0070] The total reduction in cold rolling is preferably 30% or more. The total reduction in cold rolling is preferably 80% or less. The number of rolling passes and the reduction in each pass are not particularly limited, and a desired effect can be obtained.

[0071] ·Heating process Next, the base steel sheet prepared in the preparation step is heated to a maximum temperature T1 under conditions of an average heating rate of 5.0°C / s or less in a temperature range of 700°C to 750°C.

[0072] [Average heating rate in the temperature range of 700℃~750℃: 5.0℃ / s or less] The inventors have conducted extensive research and found that the average heating rate in the temperature range of 700°C to 750°C (hereinafter, simply referred to as the average heating rate) affects the prior γ grain boundary occupancy rate. That is, by setting the average heating rate to 5.0°C / s or less, the dissolution of carbides is promoted. This refines the prior γ grains, contributing to an increase in the prior γ grain boundary occupancy rate. As a result, the shear angle range is improved. Therefore, the average heating rate is set to 5.0°C / s or less. The average heating rate is preferably 3.0°C / s or less. There is no particular upper limit to the average heating rate. For example, the average heating rate is preferably 0.1°C / s or more.

[0073] [Maximum temperature T1: 800℃ or higher and 900℃ or lower] If the maximum temperature T1 is less than 800°C, the total area fraction of ferrite and bainitic ferrite will be 15% or more, making it difficult to achieve a TS of 1180 MPa or more. On the other hand, if the maximum temperature T1 exceeds 900°C, the total area fraction of ferrite and bainitic ferrite will be less than 5%, making it difficult to achieve excellent dimensional accuracy of parts. Therefore, the maximum temperature T1 is set to 800°C or more and 900°C or less. The maximum temperature T1 is preferably 810°C or more. The maximum temperature T1 is preferably 890°C or less.

[0074] After the maximum temperature T1 is reached, the process may immediately proceed to the cooling step described later, or the maximum temperature T1 may be held for a certain period of time, for example, 1.0 to 5.0 seconds, before proceeding to the cooling step described later.

[0075] ·First cooling process Next, the base steel sheet is cooled under the condition of an average cooling rate in the temperature range from the maximum reachable temperature T1 to the intermediate holding temperature T2 of 0.10° C. / s or more and 5.00° C. / s or less.

[0076] [Average cooling rate in the temperature range from the maximum reaching temperature T1 to the intermediate holding temperature T2 (hereinafter also referred to as the first average cooling rate): 0.10°C / s or more and 5.00°C / s or less] As a result of intensive research, the inventors have found that the first average cooling rate affects the prior γ grain boundary occupancy rate. That is, by setting the first average cooling rate to 5.00°C / s or less, the nucleation of ferrite from the prior γ grain boundary is promoted, which contributes to an increase in the prior γ grain boundary occupancy rate. As a result, the shear angle wideness is 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 bainitic ferrite becomes 15% or more, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the first average cooling rate is set to 0.10°C / s or more and 5.00°C / s or less. 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.

[0077] The first cooling end temperature may be set to 600° C. or higher and 750° C. or lower. For example, the first cooling end temperature may be set to the intermediate holding temperature T2.

[0078] ·Intermediate holding process Next, the base steel plate is Intermediate holding temperature T2: 600℃ or more and 750℃ or less, Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less, Tension applied to steel plate: 5MPa or more The condition is maintained as follows.

[0079] [Intermediate holding temperature T2: 600℃ or higher and 750℃ or lower] When the intermediate holding temperature T2 is less than 600°C, the transformation of ferrite and bainitic ferrite from other than the prior γ grain boundary may be promoted. Therefore, it becomes difficult to make the prior γ grain boundary occupancy rate 20% or more, and it becomes difficult to realize an excellent shear angle range. On the other hand, when the intermediate holding temperature T2 exceeds 750°C, the total area fraction of ferrite and bainitic ferrite becomes less than 5%, and it becomes difficult to realize excellent dimensional accuracy of the part. Therefore, the intermediate holding temperature T2 is set to 600°C or more and 750°C or less. 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. The intermediate holding temperature may be constant during holding. In addition, the intermediate holding temperature does not have to be constant during holding as long as it is in the temperature range of 600°C or more and 750°C or less, and the temperature fluctuation is within the set temperature ±10°C.

[0080] [Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less] When the intermediate holding time t2 is less than 1.0 second (including the case where no intermediate holding is performed), the prior γ grain boundary occupancy rate is less than 20%, and excellent shear angle wideness cannot be realized. On the other hand, when the intermediate holding time t2 exceeds 2000.0 seconds, the total area fraction of ferrite and bainitic ferrite is 15% or more, and it becomes difficult to realize a TS of 1180 MPa or more. Therefore, the intermediate holding time t2 is set to 1.0 second or more and 2000.0 seconds or less. The intermediate holding time t2 is preferably 10.0 seconds or more. The intermediate holding time t2 is preferably 1500.0 seconds or less. The intermediate holding time t2 is the holding time at the intermediate holding temperature T2.

[0081] [Tension applied to steel plate: 5MPa or more] As a result of intensive research, the inventors have found that applying tension to the material steel sheet during intermediate holding affects the prior γ grain boundary occupancy rate. In this case, by applying a tension to the material steel sheet (hereinafter also simply referred to as the applied tension) of 5 MPa or more, the nucleation of ferrite from the prior γ grain boundaries is promoted, which contributes to an increase in the prior γ grain boundary occupancy rate. This makes it possible to make the prior γ grain boundary occupancy rate 20% or more, and to realize an excellent shear angle range. Therefore, the applied tension is set to 5 MPa or more. The applied tension is preferably 10 MPa or more. The upper limit of the applied tension is not particularly specified. The applied tension is preferably, for example, 100 MPa or less.

[0082] Furthermore, between the intermediate holding step and the second cooling step described below, the base steel sheet may be subjected to a plating treatment, as described below.

[0083] ·Second cooling process Next, the base steel plate is Average cooling rate in the temperature range of 300℃~100℃: 300℃ / s or more, The mixture is cooled to the second cooling end temperature under the above conditions.

[0084] [Average cooling rate in the temperature range from 300℃ to 100℃ (hereinafter referred to as the second average cooling rate): 300℃ / s or more] If the second average cooling rate is less than 300°C / s, the area fraction of the retained austenite becomes 3% or more, making it difficult to realize an excellent shear angle range. Therefore, the second average cooling rate is set to 300°C / s or more. The second average cooling rate is preferably 800°C / s or more. There is no particular upper limit to the second average cooling rate. For example, the second average cooling rate is preferably 2000°C / s or less.

[0085] The second cooling end temperature may be, for example, less than 100° C. The second cooling end temperature may be, for example, about room temperature.

[0086] ·Tempering process Next, the base steel sheet is tempered under the conditions of a tempering temperature T3 of 100° C. or more and 400° C. or less, and a tempering time t3 of 10 seconds or more and 10,000 seconds or less.

[0087] [Tempering temperature T3: 100℃ or higher and 400℃ or lower] Tempered martensite is generated by tempering martensite through a tempering process. Here, if the tempering temperature T3 is less than 100°C, the martensite is not sufficiently tempered, and the structure is mainly composed of as-quenched martensite. With such a structure mainly composed of as-quenched martensite, excellent shear angle range cannot be obtained. On the other hand, if the tempering temperature T3 exceeds 400°C, the tempering of martensite proceeds excessively, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the tempering temperature T3 is set to 100°C or more and 400°C or less. The tempering temperature T3 is preferably set to 150°C or more. The tempering temperature T3 is preferably set to 350°C or less. The tempering temperature here is the holding temperature in the tempering process. The tempering temperature may be constant during holding. In addition, the tempering temperature does not have to be constant during holding as long as it is in a temperature range of 100°C or more and 400°C or less, and the temperature fluctuation is within ±10°C of the set temperature.

[0088] [Tempering time t3: 10 seconds or more and 10000 seconds or less] As described above, tempered martensite is generated by tempering martensite through a tempering process. Here, if the tempering time t3 is less than 10 seconds, the martensite is not sufficiently tempered, and the structure is mainly composed of as-quenched martensite. With such a structure mainly composed of as-quenched martensite, excellent shear angle range cannot be obtained. On the other hand, if the tempering time t3 exceeds 10,000 seconds, the tempering of martensite progresses excessively, making it difficult to achieve a TS of 1180 MPa or more. Therefore, the tempering time t3 is set to 10 seconds or more and 10,000 seconds or less. The tempering time t3 is preferably set to 50 seconds or more. The tempering time t3 is preferably set to 5,000 seconds or less. The tempering time t3 here is the holding time at the tempering temperature T3.

[0089] The cooling after tempering is not particularly limited. For example, the cooling may be performed by any method according to a conventional method. The cooling end temperature after tempering may be, for example, about room temperature.

[0090] After the tempering step, the steel sheet may be processed to have an equivalent plastic strain of 0.10% to 5.00%. After the processing, the steel sheet may be reheated at a temperature of 100°C to 400°C.

[0091] Furthermore, after the tempering step, the base steel sheet may be subjected to a plating treatment, which will be described later.

[0092] Plating process In addition, the base steel sheet may be optionally subjected to a plating treatment. The plating treatment is not particularly limited. Examples of the plating treatment include galvanizing treatment such as hot-dip galvanizing treatment, alloyed hot-dip galvanizing treatment, and electric galvanizing treatment. Examples of plating treatment other than galvanizing treatment include aluminum plating treatment and alloy plating treatment. Examples of alloy plating treatment include hot-dip zinc-aluminum-magnesium alloy plating treatment and Zn-Ni electric alloy plating treatment. All of the treatment conditions may be in accordance with conventional methods. 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 alloyed hot-dip galvanizing treatment are preferably performed between the intermediate holding step and the second cooling step. In addition, the electric galvanizing treatment and the Zn-Ni electric alloy plating treatment are preferably performed after the tempering step.

[0093] In the case of hot-dip galvanizing and galvannealed hot-dip galvanizing, from the viewpoint of productivity, it is preferable to carry out a series of processes such as the heating process and the plating process in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping is possible to adjust the coating weight of the plating.

[0094] After the plating process, the base steel sheet may be processed under conditions that result in an equivalent plastic strain of 0.10% to 5.00%. After the processing, the base steel sheet (plated steel sheet) may be reheated under conditions of 100°C to 400°C.

[0095] The conditions other than those mentioned above are not particularly limited and may be the same as those described above. According to the method for producing a steel sheet according to one embodiment of the present invention, a steel sheet having a TS of 1180 MPa or more and excellent dimensional accuracy and shear angle range can be obtained. The obtained steel sheet can be suitably used as a material for automobile parts, for example. When the steel sheet is to be traded, it is usually cooled to room temperature before being traded.

[0096] [4] Manufacturing methods for components Next, a method for manufacturing a member according to one embodiment of the present invention will be described. A method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form a component. Here, the molding method is not particularly limited, and for example, a general processing method such as press molding can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, crimp joining, etc. The molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods. EXAMPLES

[0097] Steel having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by a continuous casting method. The steel slab was then heated. The steel slab was then hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then pickled. The hot-rolled steel sheet was then cold-rolled to obtain a cold-rolled steel sheet. In this manner, a base steel sheet was prepared. The prepared base steel sheet was then subjected to a heating process, a first cooling process, an intermediate holding process, a second cooling process, and a tempering process under the conditions shown in Table 2, to obtain a steel sheet (sheet thickness: 0.6 to 2.2 mm) as a final product. In addition, some of the steel sheets (those in the type column of Table 2, which are GI, GA, and EG) were subjected to a plating process. Of these, those in the type column of Table 2, which are GI and GA, were subjected to a plating process between the intermediate holding process and the second cooling process. In addition, those in the type column of Table 2, which are EG, were subjected to a plating process after the tempering process. Conditions not specified were those according to standard practice.

[0098] Using the steel sheets thus obtained, the area fraction of tempered martensite, the area fraction of retained austenite, the total area fraction of ferrite and bainitic ferrite, and the prior γ grain boundary occupancy rate were determined in the manner described above. The results are shown in Table 3.

[0099] In addition, each evaluation was performed according to the following procedures. The evaluation results are shown in Table 3.

[0100] (TS rating) From the obtained steel plate, a JIS No. 5 test piece (gauge length: 50 mm, parallel part width: 25 mm) was taken so that the direction perpendicular to the rolling direction of the steel plate was the longitudinal direction of the test piece. Next, a tensile test according to JIS Z 2241:2022 was performed using the taken test piece, and TS and YS were measured. The crosshead speed was 1.67 × 10 -1 The condition was set to mm / sec. TS was evaluated according to the following criteria. Good (passed, excellent): TS is 1180MPa or more Bad (failed): TS is less than 1180MPa

[0101] (Evaluation of dimensional accuracy) From the TS and YS measured in the above TS evaluation, YR was calculated according to the following formula. YR=100×YS / TS The dimensional accuracy was evaluated according to the following criteria. Good (pass, excellent): YR is between 65% and 85% Defective (failed): YR less than 65% or YR greater than 85%

[0102] (Evaluation of shear angle range) The obtained steel plate was sheared to a size of 16 mm x 75 mm with the direction perpendicular to the rolling direction as the longitudinal direction to prepare a test piece. The clearance during shearing was 15% in all cases. The shear angle during shearing was changed in a range of 0° to 2.0° at 0.25° intervals. Next, a four-point bending test was performed according to ASTM (G39-99), and a stress of 1000 MPa was applied to the bending apex of the test piece. Next, while the stress was applied, the test piece was immersed in hydrochloric acid at 25°C and pH: 3 for 100 hours. After immersion, the presence or absence of cracks was visually confirmed for each test piece. Then, the shear angle range was evaluated according to the following criteria. Excellent (passed, especially excellent): The appropriate range of shear angle for delayed fracture is 0° to 1.0° or more. Good (pass, excellent): The appropriate range of shear angle for delayed fracture is 0°~0.5° or more and less than 1.0°. Poor (rejected): The appropriate range of shear angle for delayed fracture is less than 0° to 0.5°. The appropriate range of shear angles for delayed fracture is the range of shear angles in which no cracks are observed in the test pieces in the above test. For example, if no cracks are observed in any of the test pieces prepared with a shear angle of 0° to 0.75° during shearing, and cracks are observed in the test pieces prepared with a shear angle of 1.00° or more during shearing, the appropriate range of shear angles for delayed fracture is "0° to 0.75°", and the test pieces are evaluated as "good (pass, excellent)". In addition, if no cracks are observed in any of the test pieces prepared with a shear angle of 0° to 0.25° during shearing, and cracks are observed in the test pieces prepared with a shear angle of 0.50° or more during shearing, the appropriate range of shear angles for delayed fracture is "0° to 0.25°", and the test pieces are evaluated as "poor (fail)".

[0103] [Table 1]

[0104] [Table 2] TIFF0007597273000003.tif233168

[0105] [Table 3] TIFF0007597273000005.tif232170

[0106] As shown in Table 3, all of the inventive examples passed the test for TS, dimensional accuracy, and shear angle range. In addition, all of the components obtained by forming or joining using the steel plates of the inventive examples had the desired shape without any cracks. Furthermore, no delayed fracture occurred in these components, even when the shear angle of the shear process was changed. Dimensional accuracy was also good. On the other hand, in the comparative example, at least one of TS, dimensional accuracy, and shear angle range was unacceptable.

Claims

1. In mass percent, C: 0.030% or more and 0.500% or less, Si: 0.010% or more and 2.500% or less, Mn: 0.10% or more and 5.00% or less, 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 and the balance being Fe and unavoidable impurities; Area fraction of tempered martensite: 83% or more, Area fraction of retained austenite: less than 3%; Total area fraction of ferrite and bainitic ferrite: 5% or more and less than 15%; and Occupancy rate of the prior austenite grain boundary by the ferrite and the bainitic ferrite: 20% or more A steel plate having a structure in which Here, the occupancy rate of the prior austenite grain boundary by the ferrite and the bainitic ferrite is an average value of the occupancy rates of the prior austenite grain boundary by ferrite and bainitic ferrite in prior austenite grains calculated by the following formula. Occupancy rate (%) of prior austenite grain boundaries by ferrite and bainitic ferrite in prior austenite grains = (L F / L T ) x 100 In the formula, L T is the total length of the prior austenite grain, and L F is the length of the interface between the prior austenite grain and the ferrite and bainitic ferrite in contact with the prior austenite grain.

2. The composition further comprises, in mass%, 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; Bi: 0.200% or less The steel sheet according to claim 1, comprising at least one selected from the following:

3. The steel sheet according to claim 1 , which has a plating layer on a surface thereof.

4. The steel sheet according to claim 2 , which has a plating layer on a surface thereof.

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

6. A method for producing a steel sheet according to any one of claims 1 to 4, comprising the steps of: A preparation step of preparing a base steel sheet having the component composition according to claim 1 or 2; Next, the base steel sheet is Average heating rate in the temperature range of 700°C to 750°C: 5.0°C / s or less; Maximum temperature T1: 800℃ or more and 900℃ or less A heating step of heating under the conditions of Next, the base steel sheet is Average cooling rate in the temperature range from the maximum reaching temperature T1 to the intermediate holding temperature T2: 0.10°C / s or more and 5.00°C / s or less A first cooling step, Next, the base steel sheet is Intermediate holding temperature T2: 600°C or more and 750°C or less, Intermediate holding time t2: 1.0 seconds or more and 2000.0 seconds or less, Tension applied to the base steel plate: 5 MPa or more and an intermediate holding step of holding the mixture under the condition of Next, the base steel sheet is Average cooling rate in the temperature range of 300°C to 100°C: 300°C / s or more; A second cooling step of cooling the mixture to a second cooling end temperature under the conditions of Next, the base steel sheet is Tempering temperature T3: 100°C or more and 400°C or less, Tempering time t3: 10 seconds or more and 10,000 seconds or less A tempering process, A manufacturing method for steel plates that:

7. 7. The method for producing a steel sheet according to claim 6, further comprising a plating step of plating the base steel sheet between the intermediate holding step and the second cooling step, or after the tempering step.

8. A method for producing a component, comprising the step of subjecting the steel plate according to any one of claims 1 to 4 to at least one of forming and joining to form a component.

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