Steel sheet, member, and method for manufacturing the same

Abstract

The present invention provides a high-strength steel sheet excellent in formability, delayed fracture resistance, and LME resistance, and a method for manufacturing the same. A steel sheet having a composition consisting of, in mass%, C: 0.24% or more and 0.40% or less, Si: 0.2% or more and 1.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol. Al: 0.50% or less (excluding 0%), N: 0.0006% or more and 0.01% or less, and the balance consisting of Fe and inevitable impurities, and having a steel structure containing, in area ratio, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and residual austenite: 2% or more, the average particle diameter of carbides in the tempered martensite in the martensite being 0.40 μm or less, the average C amount in the residual austenite being 0.5 mass% or more, the Si concentration in the steel sheet from the surface to 100 μm in the thickness direction being 1.3 mass% or less, and the tensile strength being 1470 MPa or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a high-strength steel sheet for cold press forming use suitable for use after a cold press forming process in automobiles, home electric appliances, and the like, a member using the high-strength steel sheet as a raw material, and a manufacturing method thereof. BACKGROUND

[0002] In recent years, as the demand for weight reduction of automobile bodies has further increased, the application of high-strength steel sheets of TS (Tensile Strength) : 1320 to 1470 MPa class to automobile body frame members such as center pillars R / F (reinforcing members), bumpers, and impact beam members, and the like is being promoted. In addition, from the viewpoint of further weight reduction, high-strength of 1.8 GPa class or more has also been studied. In the past, active research has been conducted on high-strength based on hot press of hot press forming, but recently, from the viewpoints of cost and productivity, research on the application of high-strength steel in cold press has been restarted.

[0003] However, in the case where a high-strength steel sheet of TS: 1470 MPa class or more is to be formed by cold press forming, press cracking is easily caused due to a decrease in ductility, and is limited to application to members of a relatively simple shape. Therefore, high-strength steel sheets are required to have better formability than in the past. In addition, in the case where a member is to be manufactured by cold press forming of a high-strength steel sheet of TS: 1470 MPa class or more, delay fracture becomes significant due to an increase in residual stress in the member and deterioration of delay fracture resistance characteristics caused by the raw material itself. Delay fracture is a phenomenon in which, in a state where a member is subjected to high stress, when the member is left to stand in a hydrogen intrusion environment, hydrogen intrudes into the steel sheet, the interatomic binding force is reduced or local deformation occurs, thereby causing a minute crack, and the minute crack progresses to cause fracture.

[0004] Further, in recent years, in the case where a high-strength hot-dip galvanized steel sheet and a high-strength alloyed hot-dip galvanized steel sheet are subjected to spot welding in assembling an automobile body or a member, or in the case where a high-strength cold-rolled steel sheet and a galvanized steel sheet are subjected to spot welding, liquid metal embrittlement cracking (LMEC: hereinafter also referred to as LME cracking) is confirmed to occur at the welded portion. LME cracking is cracking that occurs due to the melting of zinc of the galvanized layer at the time of spot welding, intrusion of the molten zinc into the grain boundaries of the steel structure of the welded portion, and stress generated at the time of opening of the welding electrode. Even in the case of a high-strength cold-rolled steel sheet that is not subjected to galvanizing, LME cracking sometimes occurs due to contact of the molten zinc in the galvanized steel sheet with the high-strength cold-rolled steel sheet in the case where the high-strength cold-rolled steel sheet is subjected to spot welding with the galvanized steel sheet. In a high-strength steel sheet of TS: 780 MPa or more, the Si content is high, and thus there is a concern about the risk of occurrence of LME cracking.

[0005] In addition, various high-strength steel sheets have been developed for use in automobile parts. For example, Patent Literature 1 discloses a high-strength steel sheet in which alloy components are specified, the steel structure is mainly composed of bainite and tempered martensite, and the steel shape is excellent.

[0006] Patent Literature 2 discloses a high-strength steel sheet having excellent formability.

[0007] Patent Literature 3 discloses a high-strength steel sheet in which the surface layer structure is mainly composed of ferrite and bainite, and in which formability and hydrogen embrittlement resistance can be balanced at a high level.

[0008] Prior Art Documents

[0009] Patent Literature

[0010] Patent Literature 1: Japanese Patent No. 6291289

[0011] Patent Literature 2: Japanese Patent No. 6288394

[0012] Patent Literature 3: Japanese Patent No. 6635236 SUMMARY

[0013] Problems to be Solved by the Invention

[0014] However, in the technology of Patent Literature 1, the amount of C is low, and a strength of TS: 1470 MPa or more can not be obtained. In addition, the amount of residual austenite is small, and the average amount of C in the residual austenite is low, and thus ductility can also be deteriorated.

[0015] In the technology of Patent Literature 2, the cooling rate is low, and self-tempered martensite tempered at a high temperature is contained, and thus the carbides in the self-tempered martensite are coarse, and the resistance to delayed fracture can be deteriorated. In addition, the tempering temperature of the martensite is high, and a strength of TS: 1470 MPa or more can not be obtained. In addition, the carbides in the tempered martensite are coarse, and the resistance to delayed fracture can be deteriorated.

[0016] In the technology of Patent Literature 3, the amount of martensite in the surface layer is as small as 30% or less, and it is presumed that this martensite becomes hard martensite in which C is enriched in γ during ferrite transformation. Thus, this martensite easily becomes a starting point of cracking, and the resistance to delayed fracture can be deteriorated.

[0017] The present application has been achieved in order to solve such problems, and aims to provide a steel sheet, a member, and a method for manufacturing the same, which have a tensile strength (TS) of 1470 MPa or more (TS ≥ 1470 MPa), excellent formability, excellent resistance to delayed fracture, and excellent resistance to LME.

[0018] Note that the tensile strength is the tensile strength TS obtained by cutting JIS No. 5 tensile test pieces in the rolling direction as the length direction and performing a tensile test according to JIS Z2241.

[0019] In addition, the excellent formability means that the elongation El obtained by cutting JIS No. 5 tensile test pieces in the rolling direction as the length direction and performing a tensile test according to JIS Z2241 is 11% or more.

[0020] In addition, the excellent delayed fracture resistance means that no fracture occurs by the following evaluation.

[0021] (1) A long test piece of 100 mm in the rolling right angle direction and 30 mm in the rolling direction was cut from the 1 / 4 position from the coil width of the steel sheet.

[0022] (2) The cutting processing of the 100 mm length end surface was set to shearing processing, and in the state where the shearing processing was maintained (without performing mechanical processing for removing burrs), bending processing was performed in such a manner that the burrs were on the curved outer peripheral side, the test piece shape when the bending was maintained, and the test piece was fixed with a bolt.

[0023] The clearance rate of the shearing processing was set to 15%, and the rake angle was set to 0 degrees.

[0024] The bending processing was performed in such a manner that the tip end inner angle was 90 degrees (V-bending) with a bending radius (R) of R / t = 4 when the tip end bending radius R and the plate thickness t were set (for example, when the plate thickness t was 2.0 mm, the bending was performed using a punch with a tip end radius of 8.0 mm).

[0025] The punch was a punch with the above-mentioned radius of the tip end and a U-shaped shape (the tip end R portion was a semicircular shape, and the thickness of the punch main body portion was 2R), and the corner R of the die was 30 mm.

[0026] The depth of the punch pressed into the steel sheet was adjusted to form in such a manner that the bending angle of the tip end was 90 degrees (V-shaped shape).

[0027] In the bolt fastening, the test piece was locked by a hydraulic jack in such a manner that the distance between the flange end portions of the straight piece portion when the bending was formed was the same distance as when the bending was formed (in such a manner that the opening of the straight piece portion based on the springback was offset), and the bolt fastening was performed in this state.

[0028] The bolt was passed through the hole of an elliptical shape (short axis 10 mm, long axis 15 mm) provided inside 10 mm from the short side edge of the long test piece to fix.

[0029] (3) The bolt-fastened test piece obtained is immersed in 1 L or more of hydrochloric acid (hydrogen chloride aqueous solution) having a pH of 3 per one, and the test is performed under conditions where the pH is managed to be constant at a temperature of 25°C of the aqueous solution.

[0030] The presence or absence of a minute crack (an initial state of delayed fracture) of a level (1 mm or more in length) that can be visually confirmed is confirmed visually or by a camera at any time, and the time from the start of the immersion until the start of the generation of the minute crack is measured as the delayed fracture time.

[0031] (4) The case where no fracture occurs even after 10 (-0.0055×(TS-1760)+0.3) hours (10 to the power of (-0.0055 x (TS - 1760) + 0.3) hours) after the start of the immersion is judged to be no fracture.

[0032] In addition, the excellent LME resistance means that no crack of 0.1 mm or more is observed by the resistance welding cracking test.

[0033] (1) One test piece cut to 30 mm x 100 mm with the direction at right angles to the rolling direction of the steel sheet as the long side is used, and the other test piece is a 980 MPa-grade hot-dip galvanized steel sheet, and resistance welding (spot welding) is performed.

[0034] The plate set obtained by overlapping two steel sheets in a welding machine is subjected to resistance spot welding in a state where the plate set is inclined at 5° using a single-phase alternating current (50 Hz) resistance welding machine of a servo motor pressurization type installed in a welding gun.

[0035] As for the welding conditions, the pressurizing force is set to 3.8 kN, and the holding time is set to 0.2 seconds. In addition, the welding current is set to 5.7 to 6.2 kA, the power feeding time is set to 21 cycles, and the holding time is set to 5 cycles.

[0036] (2) After the welding, the test piece is cut into two halves, and the cross section is observed using an optical microscope.

[0037] (3) The test piece in which no crack of 0.1 mm or more is observed is set to have excellent LME resistance.

[0038] Method for solving the problem

[0039] The present inventors have repeatedly conducted intensive research in order to solve the above problem, and as a result, the following insight has been obtained.

[0040] i) It was found that high strength is achieved using tempered martensite, and by increasing the C content to 0.24 mass% or more after the bainite transformation, residual austenite can be ensured, and thus a high-strength steel sheet having a tensile strength of 1470 MPa or more (TS≥1470 MPa) and excellent formability can be obtained.

[0041] ii) It was found that by suppressing the coarsening of carbides in the tempered martensite in the martensite, the resistance to delayed fracture characteristics can be improved in a high-strength steel sheet having TS≥1470 MPa.

[0042] iii) It was found that in order to appropriately control the particle size of the carbides, optimization of the contents of C, Si, and Mn, the cooling rate after the bainite transformation, and the tempering temperature is required.

[0043] iv) It was found that by suppressing the enrichment of Si near the surface, liquid metal embrittlement is not easily caused, and as a result, excellent resistance welding properties (cracking is not easily caused at the time of resistance welding) are shown.

[0044] The present application is based on the above insights, and specifically, the following invention is provided.

[0045] [1] A steel sheet having a composition consisting of, in mass%, C: 0.24% or more and 0.40% or less, Si: 0.2% or more and 1.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol. Al: 0.50% or less (excluding 0%), N: 0.0006% or more and 0.01% or less, and the balance consisting of Fe and unavoidable impurities,

[0046] and having a steel structure containing, in area ratio, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and residual austenite: 2% or more,

[0047] the average particle size of carbides in the tempered martensite in the above-described martensite is 0.40 μm or less,

[0048] the average C amount in the above-described residual austenite is 0.5 mass% or more,

[0049] the Si concentration from the surface of the steel sheet to 100 μm in the sheet thickness direction is 1.3 mass% or less,

[0050] the tensile strength of the steel sheet is 1470 MPa or more.

[0051] [2] The steel sheet according to the above [1], wherein the hardness difference of the microhardness of the above-described bainite and the above-described martensite is 1.5 GPa or more.

[0052] [3] The steel sheet according to any one of the above [1] or [2], wherein as the above component composition, further contains one or two or more selected from the group consisting of Nb: 0.1% or less, Ti: 0.10% or less, B: 0.0050% or less, Cu: 1% or less, and Ni: 1% or less, in mass%.

[0053] [4] The steel sheet according to any one of the above [1] to [3], wherein as the above component composition, further contains one or two or more selected from the group consisting of Cr: 1.0% or less, Mo: less than 0.3%, V: 0.45% or less, Zr: 0.2% or less, and W: 0.2% or less, in mass%.

[0054] [5] The steel sheet according to any one of the above [1] to [4], wherein as the above component composition, further contains one or both of Sb: 0.1% or less and Sn: 0.1% or less, in mass.

[0055] [6] The steel sheet according to any one of the above [1] to [5], wherein as the above component composition, further contains one or two or more selected from the group consisting of Ca: 0.0050% or less, Mg: 0.01% or less, and REM: 0.01% or less, in mass.

[0056] [7] The steel sheet according to any one of the above [1] to [6], wherein a plated layer is provided on the surface of the steel sheet.

[0057] [8] A member obtained by at least one of forming processing and joining processing of the steel sheet according to any one of the above [1] to [7].

[0058] [9] 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 [6], comprising:

[0059] a hot rolling step of hot-rolling a steel billet to obtain a hot-rolled steel sheet;

[0060] a cold rolling step of cold-rolling the hot-rolled steel sheet after the hot rolling step to obtain a cold-rolled steel sheet;

[0061] a continuous annealing step of, after the cold rolling step, annealing the cold-rolled steel sheet in an atmosphere having a dew point of -60°C or higher and -20°C or lower, at an annealing temperature of Ac3 or higher, with a soaking time of 15 seconds or longer, cooling to a holding temperature of Ms or higher and (Ms + 200°C) or lower at a first average cooling rate of 5°C / second or higher, holding at the holding temperature with a holding time of 1 second or longer and 1000 seconds or shorter, and cooling to a cooling stop temperature of 250°C or lower at a second average cooling rate of 5°C / second or higher; and

[0062] overaging treatment step of maintaining at a temperature range of 150°C or higher and 250°C or lower for 30 seconds or more and 1500 seconds or less after the continuous annealing step.

[0063]

[10] The method of manufacturing a steel sheet according to the above-mentioned [9], wherein a plating step of performing plating treatment on the surface of the steel sheet is included before or after the overaging treatment step.

[0064]

[11] A method of manufacturing a member, comprising a step of performing at least one of forming processing and joining processing on a steel sheet manufactured by the method of manufacturing a steel sheet according to the above-mentioned [9] or

[10] .

[0065] Effects of the Invention

[0066] According to the present application, a high-strength steel sheet having excellent formability, excellent delayed fracture resistance, and excellent LME resistance can be obtained. Through the improvement of these characteristics, the high-strength steel sheet can be applied to difficult-to-form parts in cold press forming applications, contributing to the improvement of part strength and weight reduction. DETAILED DESCRIPTION

[0067] Hereinafter, embodiments of the present application will be described. Note that the present application is not limited to the following embodiments.

[0068] The steel sheet of the present application has a composition consisting of, in mass%, C: 0.24% or more and 0.40% or less, Si: 0.2% or more and 1.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol. Al: 0.50% or less (excluding 0%), N: 0.0006% or more and 0.01% or less, and the balance consisting of Fe and unavoidable impurities, contains, in area ratio, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and residual austenite: 2% or more, the particle diameter of carbides in tempered martensite in the above-mentioned martensite is 0.40 pm or less, the average C amount in the residual austenite is 0.5 mass% or more, the Si concentration (the concentration of the enriched portion of Si in the surface layer) in the range of 100 pm from the surface of the steel sheet is 1.3 mass% or less in the thickness direction, and the tensile strength is 1470 MPa or more.

[0069] Composition

[0070] First, the content of each component will be described. The "%" indicating the content of a component means "mass%" unless otherwise specified.

[0071] C: 0.24% or more and 0.40% or less

[0072] C increases the strength of the tempered martensite or the bainite, and is contained from the viewpoint of ensuring TS ≥ 1470 MPa. In addition, C is contained from the viewpoint of generating fine carbides in the tempered martensite or the bainite, which become hydrogen trapping sites. When the C content is less than 0.24%, excellent delayed fracture resistance and the prescribed strength cannot be maintained. From the viewpoint of maintaining excellent delayed fracture resistance and obtaining TS ≥ 1470 MPa, the C content is set to 0.24% or more. When C exceeds 0.40%, the strength becomes excessively high, and it is difficult to obtain sufficient delayed fracture resistance. Therefore, C is set to 0.24% or more and 0.40% or less. The C content is preferably 0.25% or more, more preferably 0.26% or more, and further preferably 0.28% or more. In addition, the C content is preferably 0.37% or less, more preferably 0.35% or less, and further preferably 0.33% or less.

[0073] Si: 0.2% or more and 1.0% or less

[0074] Si is a strengthening element based on solid solution strengthening, and Si is contained from the viewpoint of obtaining retained austenite, and suppressing the precipitation of carbides in the bainite transformation. In addition, when the Si content is reduced, the melting point of zinc increases, and thus the grain boundary corrosion of zinc at the time of spot welding is suppressed, and the LME resistance can be improved. When the Si content is less than 0.2%, the amount of precipitation of carbides in the bainite transformation increases, the amount of retained austenite is small, and the formability deteriorates. The Si content is preferably 0.3% or more, and more preferably 0.4% or more. On the other hand, from the viewpoint of the safety of welding, and the viewpoint of controlling the surface layer Si concentration to a prescribed range, the Si content is set to 1.0% or less, preferably 0.8% or less, and more preferably 0.7% or less.

[0075] Mn: 1.5% or more and 3.5% or less

[0076] Mn improves the hardenability of the steel, and is contained in order to set the total area ratio of the martensite and the bainite to a prescribed range. In addition, Mn is contained in order to fix S in the steel as MnS, and to reduce the hot brittleness. Mn is an element that particularly promotes the generation and coarsening of MnS in the central portion of the plate thickness, and is complexed with the inclusion particles such as AI2O3, (Nb, Ti)(C, N), TiN, TiS, and the like, but by controlling the segregation state of Mn, this can be avoided. In addition, Mn is contained at 1.5% or more in order to stabilize the residual austenite and obtain residual austenite. The upper limit of the Mn content is set to 3.5% in consideration of the stability of the weldability. In addition, the upper limit of the Mn content is also set to 3.5% from the viewpoint of obtaining sufficient delayed fracture resistance. Therefore, the Mn content is set to 1.5% or more and 3.5% or less. The Mn content is preferably 1.8% or more, more preferably 2.1% or more, and further preferably 2.3% or more. In addition, the Mn content is preferably 3.3% or less, more preferably 3.1% or less, and further preferably 3.0% or less.

[0077] P: 0.002% or more and 0.010% or less

[0078] P is an element that strengthens the steel, but when the content thereof is large, the delayed fracture resistance and the spot weldability are significantly deteriorated. Therefore, the P content is set to 0.010% or less. In addition, the P content is preferably set to 0.006% or less from the above viewpoint. On the other hand, the lower limit that is currently industrially implementable is 0.002%. Therefore, the P content is set to 0.002% or more and 0.010% or less.

[0079] S: 0.0002% or more and 0.0020% or less

[0080] S has a large effect on the delayed fracture resistance through the formation of MnS, TiS, Ti(C, S), and the like, and therefore needs to be precisely controlled. In order to reduce the disadvantages caused by the inclusion group, the S content needs to be set to 0.0020% or less at least. From the viewpoint of improving the delayed fracture resistance, S is preferably set to 0.0010% or less. On the other hand, the lower limit that is currently industrially implementable is 0.0002%. Therefore, the S content is set to 0.0002% or more and 0.0020% or less.

[0081] sol. Al: 0.50% or less (excluding 0%)

[0082] Al is contained in order to sufficiently deoxidize and reduce inclusions in the steel. In order to stably perform deoxidization, sol. Al is preferably set to 0.005% or more, and more preferably set to 0.01% or more. On the other hand, when sol. Al exceeds 0.50%, cementite generated at the time of coiling is not easily solid-solved in the annealing process, and the delayed fracture resistance characteristics deteriorate. Therefore, the sol. Al content is set to 0.50% or less, and preferably set to 0.45% or less.

[0083] N: 0.0006% or more and 0.01% or less

[0084] N is an element that forms nitrides, carbonitrides, and the like, such as TiN, (Nb, Ti)(C, N), and AlN, in the steel, and the delayed fracture resistance characteristics deteriorate by the generation thereof. These inclusions hinder the achievement of the steel structure prescribed in the present application, and adversely affect the delayed fracture resistance characteristics. In order to reduce such adverse effects, N needs to be set to 0.01% or less at least. The N content is preferably 0.0055% or less, and more preferably 0.0050% or less. The lower limit that can be implemented industrially at present is 0.0006%.

[0085] The balance other than the above has a composition containing Fe (iron) and inevitable impurities. Here, the steel sheet of the present application preferably has a composition containing the above essential components, and the balance consisting of Fe and inevitable impurities. Note that the following optional elements can be contained. In the case where the following optional elements are contained at less than the preferable lower limit value, the optional elements can be contained as inevitable impurities.

[0086] Nb: 0.1% or less

[0087] Nb contributes to high strength by the refinement of the internal structure of martensite and bainite, and improves the delayed fracture resistance characteristics as described above. From such a viewpoint, it is preferable to contain 0.001% or more of Nb, and more preferable to contain 0.005% or more. When the content of Nb exceeds 0.1%, Nb-based inclusion groups distributed in a dot column shape in the rolling direction are generated in a large amount, and it is considered that this adversely affects the delayed fracture resistance characteristics. In order to alleviate such adverse effects, in the case where Nb is contained, the content of Nb is set to 0.1% or less. The Nb content is preferably 0.08% or less, and more preferably 0.06% or less.

[0088] Ti: 0.10% or less

[0089] Ti contributes to high strength by the refinement of the internal structure of martensite and bainite. In addition, the formation of fine Ti-based carbides and carbonitrides that become hydrogen trapping sites improves the delayed fracture resistance. In addition, the castability is improved. From this viewpoint, the Ti content is preferably set to 0.002% or more, and more preferably 0.005% or more. When the Ti content becomes excessive, a large number of Ti-based inclusion particles distributed in a dot columnar shape in the rolling direction are generated, and it is considered that this adversely affects the delayed fracture resistance. In order to alleviate such adverse effects, in the case where Ti is contained, the Ti content is set to 0.10% or less. The Ti content is preferably 0.07% or less, and more preferably 0.05% or less.

[0090] B: 0.0050% or less

[0091] B is an element that improves the quenching property of the steel, and has the advantage that martensite and bainite are generated at a prescribed area ratio even with a small Mn content. In order to obtain the effect of B, the B content is preferably set to 0.0001% or more, and more preferably 0.0005% or more. From the viewpoint of fixing N, it is preferable that B be complexly contained with 0.002% or more of Ti. On the other hand, when B is contained in excess of 0.0050%, not only is the effect saturated, but also the solution rate of cementite at the time of annealing is delayed, and residual unsolved cementite deteriorates the delayed fracture resistance. Therefore, in the case where B is contained, the B content is set to 0.0050% or less, and preferably less than 0.0035%.

[0092] Cu: 1% or less

[0093] Cu improves the corrosion resistance in the use environment of the automobile. In addition, by containing Cu, there is an effect that corrosion products coat the surface of the steel sheet, and the intrusion of hydrogen into the steel sheet is suppressed. In addition, Cu is an element that is mixed when scrap is used as a raw material, and by allowing the mixing of Cu, it is possible to use recycled materials as raw material resources, and it is possible to reduce the manufacturing cost. From the above viewpoint, it is preferable to contain 0.01% or more of Cu, and further from the viewpoint of improving the delayed fracture resistance, it is preferable to contain 0.05% or more of Cu. However, when the Cu content exceeds 1%, it becomes a cause of surface defects. Therefore, in the case where Cu is contained, the Cu content is set to 1% or less. The Cu content is preferably 0.40% or less, and more preferably 0.30% or less.

[0094] Ni: 1% or less

[0095] Ni is also an element that improves corrosion resistance. In addition, Ni has the effect of reducing surface defects that are easily generated when Cu is contained. Therefore, from the above-mentioned viewpoint, it is preferable to contain 0.01% or more of Ni, and more preferably 0.02% or more. However, when the Ni content exceeds 1%, the scale formation in the heating furnace becomes uneven, resulting in surface defects, and the cost significantly increases. Therefore, in the case where Ni is contained, the Ni content is set to 1% or less. It is preferable that the Ni content be 0.20% or less, and more preferably 0.10% or less.

[0096] Cr: 1.0% or less

[0097] Cr can be added in order to obtain the effect of improving the quenching property of the steel. In order to obtain the effect thereof, it is preferable to contain 0.01% or more of Cr, and more preferably 0.02% or more. However, when the Cr content exceeds 1.0%, the solid solution speed of cementite at the time of annealing is delayed, and residual unsolved cementite is generated, thereby deteriorating the delayed fracture resistance. In addition, the pitting corrosion resistance is also deteriorated. Furthermore, the chemical conversion treatment property is also deteriorated. Therefore, in the case where Cr is contained, the Cr content is set to 1.0% or less. The delayed fracture resistance, the pitting corrosion resistance, and the chemical conversion treatment property all have a tendency to start deteriorating when the Cr content exceeds 0.8%, and therefore, from the viewpoint of preventing these phenomena, the Cr content is preferably 0.8% or less. It is further preferable that the Cr content be 0.6% or less, and further more preferably 0.4% or less.

[0098] Mo: less than 0.3%

[0099] Mo can be added in order to obtain the effect of improving the quenching property of the steel, the effect of generating fine Mo-containing carbides that become hydrogen trapping sites, and the effect of improving the delayed fracture resistance by refining the martensite. When Nb and Ti are added in large amounts, coarse precipitates thereof are generated, and the delayed fracture resistance is deteriorated. However, the solid solution limit amount of Mo is larger than that of Nb and Ti. When Nb and Ti are added in combination, fine precipitates thereof in combination with Mo are formed, and have the effect of refining the structure. Therefore, by adding Mo in combination with a small amount of Nb and Ti, it is possible to refine the structure without leaving coarse precipitates, and to improve the delayed fracture resistance by dispersing fine carbides in large amounts. In order to obtain the effect thereof, it is preferable to contain 0.01% or more of Mo, and more preferably 0.02% or more. However, when Mo is contained in an amount of 0.3% or more, the chemical conversion treatment property is deteriorated. Therefore, in the case where Mo is contained, Mo is set to less than 0.3%. It is preferable that the Mo content be 0.2% or less, and more preferably 0.1% or less.

[0100] V: 0.45% or less

[0101] V can be added for the effect of improving the quenching property of the steel, the effect of generating fine V-containing carbides that become hydrogen trapping sites, and the effect of refining the martensite to improve the delayed fracture resistance. To obtain these effects, the V content is preferably set to 0.003% or more, and more preferably 0.005% or more. However, when V is contained at more than 0.45%, the castability is significantly deteriorated. Therefore, in the case where V is contained, the V content is set to 0.45% or less. The V content is preferably 0.2% or less, and more preferably 0.1% or less.

[0102] Zr: 0.2% or less

[0103] Zr contributes to high strength and improves the delayed fracture resistance by the refinement of the original γ grain diameter, the reduction of the internal structure units of the martensite and the bainite, i.e., the block size, the Bain grain diameter, and the like, and the formation of fine Zr-based carbides and carbonitrides that become hydrogen trapping sites. In addition, the castability is improved. From this viewpoint, the Zr content is preferably set to 0.001% or more, and more preferably 0.005% or more. However, when Zr is added in a large amount, the coarse precipitates of ZrN and ZrS that remain unsolved at the time of heating of the billet in the hot rolling process increase, the delayed fracture resistance is deteriorated. Therefore, in the case where Zr is contained, the Zr content is set to 0.2% or less. The Zr content is preferably 0.05% or less, and more preferably 0.01% or less.

[0104] W: 0.2% or less

[0105] W contributes to high strength and improves the delayed fracture resistance by the formation of fine W-based carbides and carbonitrides that become hydrogen trapping sites. From this viewpoint, it is preferable to contain 0.005% or more of W, and more preferably 0.01% or more. However, when W is contained at more than 0.2%, coarse precipitates that remain unsolved at the time of heating of the billet in the hot rolling process increase, the delayed fracture resistance is deteriorated. Therefore, in the case where W is contained, the W content is set to 0.2% or less. The W content is preferably 0.1% or less, and more preferably 0.05% or less.

[0106] Sb: 0.1% or less

[0107] Sb suppresses oxidation and nitridation of the surface layer, thereby suppressing the reduction of C and B caused thereby. By suppressing the reduction of C and B, ferrite generation in the surface layer is suppressed, which contributes to the improvement of high strength and delayed fracture resistance. From this viewpoint, the Sb content is preferably set to 0.002% or more, and more preferably 0.005% or more. However, when the Sb content exceeds 0.1%, castability deteriorates, and in addition, Sb segregates at the original γ grain boundaries to deteriorate the delayed fracture resistance. Therefore, in the case where Sb is contained, the Sb content is set to 0.1% or less. The Sb content is preferably 0.06% or less, and more preferably 0.04% or less.

[0108] Sn: 0.1% or less

[0109] Sn suppresses oxidation and nitridation of the surface layer, thereby suppressing the reduction of the content of C and B in the surface layer caused thereby. By suppressing the reduction of C and B, ferrite generation in the surface layer is suppressed, which contributes to the improvement of high strength and delayed fracture resistance. From this viewpoint, the Sn content is preferably 0.002% or more, and more preferably 0.004% or more. However, when the Sn content exceeds 0.1%, castability deteriorates, and in addition, Sn segregates at the original γ grain boundaries to deteriorate the delayed fracture resistance. Therefore, in the case where Sn is contained, the Sn content is set to 0.1% or less. The Sn content is preferably 0.04% or less, and more preferably 0.02% or less.

[0110] Ca: 0.0050% or less

[0111] Ca fixes S as CaS to improve the delayed fracture resistance. In order to obtain this effect, it is preferable to contain 0.0001% or more of Ca. More preferably, the Ca content is 0.0005% or more. However, when Ca exceeding 0.0050% is contained, the surface quality and bendability deteriorate. Therefore, in the case where Ca is contained, the Ca content is set to 0.0050% or less.

[0112] Mg: 0.01% or less

[0113] Mg fixes O as MgO to improve the delayed fracture resistance. In order to obtain this effect, it is preferable to contain 0.0001% or more of Mg. However, when Mg exceeding 0.01% is contained, the surface quality and bendability deteriorate. Therefore, in the case where Mg is contained, the Mg content is set to 0.01% or less. The Mg content is preferably 0.005% or less, and more preferably 0.001% or less.

[0114] REM: 0.01% or less

[0115] The REM refines the inclusions and reduces the starting point of fracture, thereby improving the bendability and the delayed fracture resistance. In order to obtain this effect, it is preferable to contain 0.0001% or more of REM. However, when REM is contained in an amount exceeding 0.01%, the inclusions are coarsened, and the bendability and the delayed fracture resistance are deteriorated. Therefore, in the case where REM is contained, the content of REM is set to 0.01% or less. It is preferable that the content of REM be 0.004% or less, and more preferably 0.002% or less.

[0116] Steel structure

[0117] The steel structure of the steel sheet of the present application has the following configuration.

[0118] (Composition 1) Containing, in terms of area ratio, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and residual austenite: 2% or more.

[0119] (Composition 2) The average C amount in the residual austenite is 0.5 mass% or more.

[0120] (Composition 3) The average particle diameter of carbides in the tempered martensite in the martensite is 0.40 μm or less.

[0121] (Composition 4 (Preferred Condition)) The hardness difference of the microhardness of the bainite and the martensite is 1.5 GPa or more.

[0122] (Composition 1) Containing, in terms of area ratio, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and residual austenite: 2% or more.

[0123] In order to obtain high strength of TS ≥ 1470 MPa, the area ratio of the martensite in the steel structure is set to 40% or more. When it is less than 40%, the bainite and the residual austenite increase, and the strength decreases. In order to obtain higher TS, it is preferable that it be 50% or more. On the other hand, when the martensite is more than 78%, the bainite and the residual austenite are insufficient, and the formability decreases. In order to obtain higher formability, the area ratio of the martensite is preferably 70% or less. Note that, in the present application, the martensite includes tempered martensite in which carbides are precipitated.

[0124] The bainite is a structure excellent in strength and formability. In order to obtain high formability, the area ratio of the bainite is set to 20% or more. When it is less than 20%, the martensite increases, and the formability decreases. The residual austenite increases along with the generation of the bainite, and therefore, the area ratio of the bainite is preferably 30% or more. On the other hand, when the bainite is more than 58%, the martensite decreases, and the strength decreases. Therefore, the bainite is 58% or less, and in order to obtain more excellent strength, it is preferable that it be 50% or less.

[0125] The balance between strength and ductility is improved by the residual austenite. When the residual austenite is less than 2%, the balance between strength and ductility is not improved. Therefore, the area ratio of the residual austenite is 2% or more, and in order to improve the balance between strength and ductility, the area ratio of the residual austenite is preferably 3% or more, and more preferably 4% or more. There is no upper limit, but when the residual austenite is excessive, martensite transformation occurs at the time of forming, and the starting point of delayed fracture increases. The area ratio of the residual austenite is preferably 20% or less, and more preferably 15% or less. In addition, in the present application, the residual austenite satisfies the following condition (2).

[0126] When ferrite is present, the ferrite is very soft, and thus the strength is reduced. In addition, the difference in hardness from the martensite is large, and thus, at the time of deformation, strain is concentrated at the interface between the martensite and the ferrite, which becomes the starting point of fracture, and the delayed fracture resistance can be deteriorated. Therefore, the area ratio of the ferrite is preferably 3% or less, and more preferably 0%. The pearlite is a structure composed of layered ferrite and cementite, and when it is generated, the amount of C in the martensite is reduced, and the strength can be reduced. The area ratio of the pearlite is preferably 3% or less, and more preferably 0%. That is, in the present application, the total area ratio of the ferrite and the pearlite is preferably 6% or less, and more preferably 2% or less, and further preferably 0%.

[0127] (2) The average amount of C in the residual austenite is 0.5% by mass or more.

[0128] In the present application, the average amount of C in the residual austenite is 0.5% by mass or more. The higher the average amount of C in the residual austenite, the higher the stability of the residual austenite, and thus a good balance between strength and ductility can be obtained. When the average amount of C in the residual austenite is less than 0.5% by mass, a good balance between strength and ductility is not obtained. In addition, the stability is low, and the residual austenite that undergoes martensite transformation at the time of forming increases, which becomes the starting point of delayed fracture, and the delayed fracture resistance can be deteriorated. Therefore, the average amount of C in the residual austenite is 0.5% by mass or more, and preferably 0.7% by mass or more. There is no upper limit to the average amount of C in the residual austenite, but when the average amount of C in the residual austenite is too high, the martensite transformation of the residual austenite that occurs with tensile deformation does not sufficiently proceed, and thus sufficient work hardening ability is not obtained. The average amount of C in the residual austenite is preferably 2.0% by mass or less.

[0129] (3) The average particle diameter of the carbide in the tempered martensite in the martensite is 0.40 μm or less.

[0130] In the steel structure of the present invention, composition 3 is important for improving the delayed fracture resistance of the steel plate. The average particle size of the carbides in the tempered martensite is 0.40 μm or less. By setting the average particle size of the carbides to 0.40 μm or less, the delayed fracture resistance can be improved. If the carbides are coarser than this, the delayed fracture resistance may deteriorate. The average particle size is preferably 0.38 μm or less, more preferably 0.36 μm or less. There is no particular lower limit, but to improve toughness, the average particle size of the carbides is preferably set to 0.001 μm or more, more preferably 0.01 μm or more.

[0131] (Configuration 4 (Preferred Condition)) The difference in microhardness between bainite and martensite is 1.5 GPa or more.

[0132] In the steel structure of the present invention, composition 4 is important for obtaining high formability. The greater the hardness difference between bainite and martensite, the greater the plastic deformation gradient formed during plastic deformation, and it is believed that the GN dislocation density accumulated in the bainite increases. Therefore, the greater the hardness difference, the greater the amount of work hardening of the bainite based on the GN dislocation density, and high elongation can be obtained. Therefore, the hardness difference between bainite and martensite is preferably greater than 1.5 GPa. There is no particular upper limit, but when the hardness difference is too large, there is a tendency for the hole expandability to deteriorate. Therefore, the hardness difference between bainite and martensite is preferably set to 15 GPa or less, and more preferably set to 13 GPa or less.

[0133] (Measurement conditions of tissue)

[0134] In the quantification of the metal structure, the L section of the steel plate (the section parallel to the rolling direction and perpendicular to the surface of the steel plate) is ground and then corroded with nitric acid solution. At a position 1 / 4 of the thickness from the surface of the steel plate, four fields of view are observed using an SEM at a magnification of 2000 times, and the image analysis of the taken tissue photographs is performed for measurement. Here, martensite refers to a structure that appears gray in the SEM. Among them, tempered martensite refers to a structure containing fine carbides in the martensite. On the other hand, bainite and ferrite refer to areas that appear black in the SEM. It should be noted that the interior of tempered martensite and bainite contains trace amounts of carbides, nitrides, sulfides, and oxides, but it is difficult to exclude them. Therefore, the area ratio of the area including them is set to its area ratio. In addition, ferrite is a structure composed of grains of bcc lattice and generated by phase transformation from austenite at relatively high temperature. Bainite is a structure formed from austenite at relatively low temperatures (above the martensite transformation point) and consists of spherical carbides dispersed within acicular or plate-like ferrite. Pearlite is a structure in which cementite is precipitated in layers within ferrite.

[0135] Further, in the measurement of residual austenite, the surface layer 200 μm of the steel sheet was chemically polished with oxalic acid, and the surface was subjected to X-ray diffraction intensity method. The integral intensity of the (200)α, (211)α, (220)α, (200)γ, (220)γ, (311)γ diffraction plane peaks measured by Mo-K α ray was calculated.

[0136] The area ratio of the residual austenite obtained in the above-described manner was subtracted from 100%, and the ratio of the martensite, bainite, ferrite, and pearlite obtained by the point counting method using the observation of the microstructure photograph based on the SEM was allocated to the remaining area ratio, whereby the area ratios of the martensite, bainite, ferrite, pearlite, and residual austenite could be determined.

[0137] Regarding the average C amount in the residual austenite, the lattice constant (a α ) of γ was calculated from the {220} peak angle of γ using a Co-K γ ray source, and the amount of the contained alloy elements was substituted into the following equation, whereby the average C amount was calculated.

[0138] α γ = 3.578 + 0.00095 (%Mn) + 0.022 (%N) + 0.0056 (%Al) + 0.033 (%C)

[0139] In the equation, (%Mn), (%N), and (%Al) are each the content (mass %) of Mn, N, and Al. Further, (%C) is the average C amount (mass %) in the residual austenite.

[0140] In the quantification of the size of the carbide in the interior of the tempered martensite in the martensite, the L section (vertical section parallel to the rolling direction) of the steel sheet was polished and then etched with a nitric acid ethanol solution, and two fields of view were observed at a magnification of 10,000 times using an SEM at a position 1 / 4 of the thickness from the surface of the steel sheet. Then, the white microstructure finely dispersed in the tempered martensite microstructure was taken as the carbide, and the long axis length of the carbide was measured by appropriate magnification. The carbide was assumed to be elliptical or needle-shaped, and the long axis was determined. Specifically, the long axis length of five carbides was measured from each of three martensite lath blocks, and the average was taken as the average particle diameter of the carbide in the tempered martensite.

[0141] Regarding the microhardness, the nanometer indentation method was used to measure the surface of the 1 / 4 position of the thickness after polishing at a load of 100 μN. Five points were measured in each of the bainite and martensite microstructures, and the average was taken, and the difference was taken as the hardness difference. The maximum load was set to 500 μN, and a Berkovich type having an angle of 115 degrees was used as the indenter.

[0142] The Si concentration (surface layer Si concentration) from the surface of the steel sheet to 100 μm in the sheet thickness direction is 1.3% or less

[0143] In the present application, the Si concentration (surface layer Si concentration) from the surface of the steel sheet to 100 μm in the sheet thickness direction is 1.3% or less. In order to obtain excellent resistance welding properties, the Si concentration from the surface of the steel sheet to 100 μm needs to be 1.3% or less by mass. When the concentration of Si is within this range, it is considered that liquid metal embrittlement is not easily caused, and as a result, excellent resistance welding properties are exhibited. The surface layer Si concentration is preferably 1.2% or less by mass, and more preferably 1.1% or less by mass.

[0144] Note that, in the present application, with respect to the Si concentration in a region within 100 μm from the surface of the steel sheet in the sheet thickness direction, a field emission electron probe micro analyzer (FE-EPMA) is used to analyze a range of 100 μm x 100 μm with an electron beam diameter of 1 μm, 10,000 points are extracted, and the average value of the concentrations of the first 10% is set.

[0145] The steel sheet of the present application described above can have a plated layer on the surface. The type of the plated layer is not particularly limited, and can be any one of a Zn plated layer (zinc plated layer), a plated layer of a metal other than Zn. In addition, the plated layer can contain components other than the main component such as Zn. The zinc plated layer is, for example, a hot-dipped zinc layer, an electroplated zinc layer.

[0146] Next, the manufacturing method of the steel sheet of the present application will be described.

[0147] The manufacturing method of the steel sheet of the present application is a manufacturing method of a steel sheet including: a hot rolling step of hot-rolling a steel billet having the above-described component composition to obtain a hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet after the hot rolling step; a continuous annealing step of, after the cold rolling step, annealing the cold-rolled steel sheet in an atmosphere having a dew point of -60°C or higher and -20°C or lower at an annealing temperature of Ac3 or higher under a soaking time of 15 seconds or more, cooling to a holding temperature of Ms or higher and (Ms + 200°C) or lower at a first average cooling rate of 5°C / second or more, holding at the holding temperature under a holding time of 1 second or more and 1000 seconds or less, and cooling to a cooling stop temperature of 250°C or lower at a second average cooling rate of 5°C / second or more; and an overaging treatment step of, after the continuous annealing step, maintaining at a temperature range of 150°C or higher and 250°C or lower for 30 seconds or more and 1500 seconds or less.

[0148] Hot rolling step

[0149] As a method of hot-rolling the billet, there are a method of heating the billet and then rolling, a method of directly rolling the billet without heating after continuous casting, a method of performing short-time heating treatment to the billet after continuous casting and then rolling, and the like. In the hot-rolling, the average heating rate when the billet is heated like a conventional method is set to 5 to 15°C / min, the finish rolling temperature FT is set to 840 to 950°C, and the coiling temperature CT is set to 400 to 700°C.

[0150] In order to remove the primary and secondary oxide films generated on the surface of the steel sheet, descaling can be appropriately performed. It is preferable that pickling be sufficiently performed to reduce the remaining of the oxide film before cold-rolling the hot-rolled coil. In addition, from the viewpoint of reducing the load of cold-rolling, annealing can be performed to the hot-rolled steel sheet as necessary.

[0151] Cold-rolling step

[0152] In the cold-rolling, the reduction (cold-rolling rate) is set to 40% or more, and the recrystallization behavior and texture orientation in the subsequent continuous annealing can be stabilized. When the reduction is less than 40%, a part of the austenite grains at the time of annealing becomes coarse, and the strength can be reduced.

[0153] Continuous annealing step

[0154] The steel sheet after the cold-rolling is subjected to annealing and tempering treatment in a continuous annealing line (CAL).

[0155] In the present application, in order to obtain the prescribed martensite and bainite, the annealing temperature needs to be Ac3 or more, and the soaking time needs to be 15 seconds or more. When the annealing temperature is less than Ac3 or the soaking time is less than 15 seconds, sufficient austenite is not generated at the time of annealing, and the prescribed martensite and / or bainite is not obtained in the final product, and the tensile strength of 1470 MPa or more is not obtained. The upper limit of the annealing temperature and the soaking time is not particularly limited, and when the annealing temperature and the soaking time are a certain amount or more, the austenite grain size becomes coarse, and the resistance to delayed fracture can be deteriorated, and therefore, it is preferable that the annealing temperature be 950°C or less and the soaking time be 900 seconds or less.

[0156] Further, in the present application, the dew point of the atmosphere during the annealing is set to -60°C or higher and -20°C or lower. By setting the dew point to -60°C or higher and -20°C or lower, the Si concentration in the surface layer of the steel sheet can be made 1.3 mass% or less, and excellent LME resistance can be obtained. When the dew point is lower than -60°C, the equipment cost and manufacturing cost can increase. Further, the Si concentration in the surface layer of the steel sheet cannot be made 1.3 mass% or less, and the desired delayed fracture resistance and LME resistance cannot be obtained. On the other hand, when the dew point is higher than -20°C, decarburization in the surface layer is promoted, the strength decreases, and a strength of 1470 MPa can not be obtained. Further, the desired delayed fracture resistance cannot be obtained.

[0157] Therefore, in the present application, the annealing is performed in an atmosphere having a dew point of -60°C or higher and -20°C or lower. The dew point is preferably -55°C or higher. Further, the dew point is preferably -25°C or lower.

[0158] Then, in order to reduce ferrite, the temperature needs to be cooled to a holding temperature of Ms point or higher and (Ms point + 200°C) or lower at a first average cooling rate of 5°C / s or higher. When the first average cooling rate is less than 5°C / s, ferrite is generated in large amounts. Therefore, the first average cooling rate is 5°C / s or higher, preferably 7°C / s or higher, and more preferably 10°C / s or higher.

[0159] Then, in order to obtain prescribed bainite, the isothermal holding at a holding temperature of (Ms point + 200°C) or lower and Ms point or higher needs to be 1 second or longer and 1000 seconds or shorter. When the holding temperature is higher than (Ms point + 200°C), residual austenite cannot be obtained, or ferrite is generated in large amounts. On the other hand, when the holding temperature is lower than Ms point, tempered martensite (self-tempered martensite) is generated at a temperature higher than 250°C during the isothermal holding, the coarsening of carbides in the grain and at the lath grain boundaries becomes significant, and the delayed fracture resistance can deteriorate.

[0160] The holding temperature is preferably (Ms point + 20°C) or higher, and more preferably (Ms point + 30°C) or higher. Further, the lower the holding temperature, the easier it is to obtain residual austenite, and therefore the holding temperature is preferably (Ms point + 150°C) or lower, and more preferably (Ms point + 100°C) or lower.

[0161] When the holding time is less than 1 second, bainite and residual austenite are small, and there is a problem in that formability deteriorates. Therefore, the holding time is 1 second or longer, preferably 15 seconds or longer, and more preferably 30 seconds or longer. On the other hand, when the holding time is more than 1000 seconds, martensite becomes small, and there is a problem in that a tensile strength of 1470 MPa or higher cannot be obtained. Therefore, the holding time is 1000 seconds or shorter, preferably 500 seconds or shorter, and more preferably 300 seconds or shorter.

[0162] Then, in order to obtain tempered martensite having excellent delayed fracture resistance, it is necessary to cool to a cooling stop temperature of 250°C or lower at a second average cooling rate of 5°C / sec or more. When the cooling stop temperature exceeds 250°C or the second average cooling rate is less than 5°C / sec, coarse self-tempered martensite is generated in the grains, and the delayed fracture resistance can deteriorate. Therefore, in the present application, the cooling stop temperature is 250°C or lower at a second average cooling rate of 5°C / sec or more. The second average cooling rate is preferably 50°C / sec or more, and more preferably 100°C / sec or more. In addition, the cooling stop temperature is preferably 150°C or lower, and more preferably 50°C or lower.

[0163] overaging treatment step

[0164] The tempered martensite contains carbides generated during the low-temperature range holding after quenching, and appropriate control is necessary in order to ensure the delayed fracture resistance and TS ≥ 1470 MPa. That is, it is necessary to perform reheating holding after quenching to 250°C or lower, or to set the holding temperature to 150°C or higher and 250°C or lower, and to control the holding time to 30 seconds or more and 1500 seconds or less.

[0165] When it is less than 150°C or less than 30 seconds, the distribution density of carbides becomes insufficient, and the toughness can deteriorate. On the other hand, when it exceeds 250°C or exceeds 1500 seconds, the coarsening of carbides in the grains and at the boundaries of the lath blocks becomes significant, and the delayed fracture resistance can deteriorate.

[0166] plating treatment step

[0167] In addition, plating treatment can be performed on the surface of the obtained steel sheet before or after the above-described overaging treatment step. By performing the plating treatment, a steel sheet having a plating layer on the surface can be obtained. The type of plating treatment is not particularly limited, and plating using a plating layer imparting technique based on sputtering, or electroplating can be used. Note that, in the case where plating treatment is performed, the skin pass rolling described above is performed after the plating treatment.

[0168] The above-described Ac3 point and Ms point are calculated according to the following (1) and (2) described in "Leslie Steel Material Science" (published by Maruzen Co., Ltd., 1985, P. 273, P. 231). Note that, [M%] is set to the content (mass%) of each element M.

[0169] Ac3 (°C) = 910 - 203 x [C%] 1 / 2+ 44.7 x [Si%] - 30 x [Mn%] + 700 x [P%] + 130 x [Al%] - 15.2 x [Ni%] - 11 x [Cr%] - 20 x [Cu%] + 31.5 x [Mo%] + 104 x [V%] + 400 x [Ti%]... (1)

[0170] Ms (°C) = 561 - 474 x [C%] - 33 x [Mn%] - 17 x [Ni%] - 17 x [Cr%] - 21 x [Mo%]... (2)

[0171] The steel sheet of the present application obtained by the above production method is preferably a sheet thickness of 0.5 mm or more. Also, the sheet thickness of the steel sheet is preferably 2.5 mm or less.

[0172] Next, the member of the present application and the production method thereof will be described.

[0173] The member of the present application is a member obtained by subjecting the steel sheet of the present application to at least one of forming processing and joining processing. Also, the production method of the member of the present application has a step of subjecting the steel sheet produced by the production method of the steel sheet of the present application to at least one of forming processing and joining processing.

[0174] The steel sheet of the present application has a tensile strength of 1470 MPa or more, and has excellent formability, delayed fracture resistance, and LME resistance. Therefore, the member obtained using the steel sheet of the present application is also high in strength, and has excellent formability, delayed fracture resistance, and LME resistance compared to conventional high-strength members. Also, when the member of the present application is used, weight reduction can be achieved. Therefore, the member of the present application can be suitably used, for example, for complex-shaped members used in the automobile field such as a vehicle body frame member.

[0175] The forming processing can be performed without limitation using a general processing method such as press processing. The joining processing method is also not particularly limited, and a general welding such as spot welding, laser welding, arc welding, rivet joining, fastening joining, and the like can be used. Note that the forming conditions and the joining conditions are not particularly limited, and can be performed according to a conventional method.

[0176] Example

[0177] [Example 1]

[0178] Examples of the present application will be described below.

[0179] A cold-rolled steel sheet having a sheet thickness of 1.2 mm and having the composition shown in Table 1 was subjected to heat treatment under the annealing conditions and overaging treatment conditions shown in Table 2.

[0180] The soaking time at the annealing temperature was set to 300 seconds. In addition, with No. 20, plating treatment was performed on the surface of the steel sheet after the overaging treatment. The conditions of the plating treatment were set to electrogalvanizing.

[0181] Note that the cold-rolled steel sheet was obtained by subjecting a steel billet having the composition shown in Table 1 to hot rolling (average heating rate at heating: 10°C / min, finish rolling temperature FT: 900°C, coiling temperature CT: 500°C), and then to cold rolling (reduction: 55%).

[0182]

[0183] [Table 2]

[0184]

[0185] For the obtained steel sheets, quantification of the metal structure was performed, and further, tensile tests, and delayed fracture resistance evaluation tests were performed.

[0186] In the quantification of the metal structure, after polishing the L section (section parallel to the rolling direction, and perpendicular to the surface of the steel sheet) of the steel sheet, etching was performed with a nitric acid ethanol solution, and at a position 1 / 4 thickness from the surface of the steel sheet, 4 fields of view were observed at a magnification of 2000 times using an SEM, and measurement was performed by image analysis of the photographed metal structure. Here, martensite refers to a structure that appears gray in the SEM. Among them, tempered martensite refers to a structure that contains fine carbides in the martensite.

[0187] On the other hand, bainite, ferrite refer to regions that appear black in the SEM. Note that a small amount of carbides, nitrides, sulfides, oxides are contained in the interior of the martensite, bainite, but it is difficult to exclude them, and thus the area ratio of the regions including them was set to the area ratio.

[0188] Note that among the metal structures observed above, ferrite is a structure that is generated by phase transformation from austenite at a relatively high temperature, and is composed of grains of bcc lattice. Bainite is a structure that is generated from austenite at a relatively low temperature (above the martensite transformation point), and has globular carbides dispersed in acicular or platy ferrite. Pearlite is a structure in which cementite is precipitated in layers within the ferrite.

[0189] In addition, in the measurement of the retained austenite, the surface layer 1 / 4 thickness of the steel sheet was chemically polished with oxalic acid, and the plate surface was subjected to X-ray diffraction intensity method. The integral intensity of the (200) a, (211) a, (220) a, (200) g, (220) g, (311) g diffraction plane peaks measured using Mo-K α rays was calculated.

[0190] The area ratio of the residual austenite obtained in the above-described manner is subtracted from 100%, and for the remaining area ratio, the ratio of the martensite, bainite, ferrite, and pearlite obtained by the point counting method using the observation of the microstructure photograph based on the SEM is allocated, whereby the area ratios of the martensite, bainite, ferrite, pearlite, and residual austenite are determined.

[0191] Regarding the average C amount in the residual austenite, the Co-K α ray source, the lattice constant of γ is calculated from the {220} peak angle of γ γ , and the amount of the contained alloy element is substituted into the following equation, whereby it is solved.

[0192] α γ = 3.578 + 0.00095 (%Mn) + 0.022 (%N) + 0.0056 (%Al) + 0.033 (%C)

[0193] In the equation, (%Mn), (%N), and (%Al) are each the content (mass %) of Mn, N, and Al. In addition, (%C) is the average C amount (mass %) in the residual austenite.

[0194] In the quantification of the size of the carbide in the interior of the tempered martensite in the martensite, the L section (vertical section parallel to the rolling direction) of the steel sheet is polished and then etched with a nitric acid ethanol solution, and at the position of 1 / 4 thickness from the surface of the steel sheet, 2 fields of view are observed at a magnification of 10000 times using an SEM. Then, the white-colored structure finely dispersed in the tempered martensite structure in the observed photograph is taken as the carbide, and the long axis length of the carbide is measured by appropriate magnification. The carbide is assumed to be elliptical or needle-shaped, and the long axis is determined. Specifically, the long axis length of 5 carbides is measured from each of 3 martensite lath blocks, and the average thereof is taken as the average particle diameter of the carbide in the tempered martensite.

[0195] In the tensile test, a JIS No. 5 tensile test piece is cut out in such a manner that the rolling direction is the length direction, and a tensile test (in accordance with JIS Z2241) is performed, and TS and El are evaluated.

[0196] Regarding the micro hardness, the surface of the sheet at the position of 1 / 4 thickness after polishing is measured using a nano indentation method at a load of 100 μN. 5 points are each measured in each of the bainite and martensite structures, and the average value is taken, and the difference is taken as the hardness difference. As the nano indentation device, a Triboindenter manufactured by Hysitron, Inc. is used, the maximum load is set to 500 μN, and a Berkovich type having an angle of 115 degrees for the corners is used as the indenter.

[0197] As for the Si concentration (surface layer Si concentration) from the surface of the steel sheet to 100 μm in the thickness direction of the steel sheet, the L section was used as the evaluation surface. Specifically, using a field emission electron probe micro analyzer (FE-EPMA), a range of 100 μm x 100 μm was analyzed with an electron beam diameter of 1 μm, 10,000 points were extracted, and the average value of the top 10% of the concentrations was determined.

[0198] In the evaluation of the delayed fracture resistance of the steel sheet, the delayed fracture of the steel sheet base material was evaluated.

[0199] The delayed fracture evaluation of the steel sheet base material was performed by cutting a long test piece of 100 mm in the rolling right angle direction and 30 mm in the rolling direction from the obtained steel sheet at the 1 / 4 position of the coil width. The cutting processing of the 100 mm long end surface was set to shearing processing, and in the state where the shearing processing was maintained (without performing mechanical processing for removing burrs), bending processing was performed in a manner such that the burrs were on the curved outer peripheral side, the test piece shape was maintained in the bent shape, and the test piece was fixed with a bolt. The clearance rate of the shearing processing was set to 15%, and the rake angle was set to 0 degrees. The bending processing was performed in a manner such that the inner angle of the tip was 90 degrees (V-bending) at a bending radius (steel sheet thickness t: 1.2 mm, bending into a shape using a punch with a tip radius of 5.0 mm) set to R / t = 4 when the tip bending radius R and the steel sheet thickness t were set. A punch with a tip of the above-mentioned radius and a U-shaped shape (the tip R portion was a semicircular shape, and the thickness of the punch main body portion was 2R) was used, and the corner R of the die was 30 mm. The depth at which the punch was pressed into the steel sheet was adjusted in a manner such that the bending angle of the tip was 90 degrees (V-shaped shape). In the bolt fastening, the test piece was locked by being sandwiched by a hydraulic jack in a manner such that the distance between the flange end portions of the straight piece portion at the time of bending into a shape was the same distance as at the time of bending into a shape (in a manner such that the opening of the straight piece portion based on springback was offset), and the bolt fastening was performed in this state. The bolt was passed through the hole provided in advance in the oval shape (short axis 10 mm, long axis 15 mm) inside the short side edge of the long test piece by 10 mm, and was fixed. The obtained test piece after the bolt fastening was immersed in hydrochloric acid (hydrogen chloride aqueous solution) with a pH of 3 of 1 L or more per 1 piece, and the test was performed under conditions where the pH was managed to be constant at an aqueous solution temperature of 25°C. The presence or absence of a fine crack (initial state of delayed fracture) that could be visually confirmed was confirmed at all times by visual observation or a camera, the time from the start of the immersion until the start of the generation of the fine crack was measured as the delayed fracture time, and the test was performed for up to a maximum of 96 hours.

[0200] With the high strength of the steel sheet, the concern of delayed fracture increases, and thus, even at high strength, a difference in delayed fracture characteristics occurs, and thus, a case where the fracture time is 10 (-0.0055×(TS-1760)+0.3) hours (10 of (-0.0055 x (TS - 1760) + 0.3) power hours) or more is set as excellent in delayed fracture characteristics, and is marked as "O" (pass), and a case where the fracture time is less than the above time is set as "X" (fail).

[0201] The LME resistance characteristics were judged by a resistance welding cracking test. One test piece of 30 mm x 100 mm cut in a direction perpendicular to the rolling direction of the obtained steel sheet was used, and another test piece of a 980 MPa grade hot-dip galvanized steel sheet was used, and resistance welding (spot welding) was performed. With respect to the sheet set obtained by overlapping the two steel sheets in a welding machine, using a servomotor pressurization type installed in a welding gun, a single-phase alternating current (50 Hz) resistance welding machine was used, and resistance spot welding was performed in a state where the sheet set was inclined at 5°. With respect to the welding conditions, the pressurization force was set to 3.8 kN, and the holding time was set to 0.2 seconds. In addition, the welding current was set to 5.7 to 6.2 kA, the power application time was set to 21 cycles, and the holding time was set to 5 cycles. After welding, the test piece was cut in half, and the cross section was observed with an optical microscope, and a test piece in which no cracks of 0.1 mm or more were observed was set as good in LME cracking (O), and a test piece in which cracks of 0.1 mm or more were observed was set as poor in LME cracking (X).

[0202] [Table 3]

[0203]

[0204] (*1): Average C amount in residual austenite (mass%)

[0205] (*2): Average particle diameter of carbide in tempered martensite (μm)

[0206] (*3): Si concentration in the steel sheet in the thickness direction from the surface to 100 μm (mass%)

[0207] (*4): Hardness difference of microhardness of the bainite and the martensite (GPa)

[0208] With respect to the steel in which the composition, the hot-rolling conditions, and the annealing conditions were optimized, a TS of 1470 MPa or more was obtained. In addition, the residual austenite in which the average C amount in the residual austenite was 0.5 mass% or more was 2% or more in area ratio, and an elongation of 11% or more was obtained.

[0209] With respect to the delayed fracture resistance characteristics of the steel sheet base material, the fracture time was 10 (-0.0055×(TS-1760)+0.3)hours or more, excellent properties with respect to delayed fracture were obtained. The steel sheet of the present application has a tensile strength of 1470 MPa or more, an elongation of 11% or more, and a fracture time of 10 (-0.0055×(TS-1760)+0.3) hours or more, excellent properties with respect to LME resistance. For the steel sheets of the comparative examples, one of these conditions was not satisfied.

[0210] [Example 2]

[0211] A galvanized steel sheet subjected to galvanizing treatment for Manufacturing Condition No. 1 (suitable steel) of Table 2 of Example 1 was subjected to press forming, and a member of the present application example was manufactured. Further, a galvanized steel sheet subjected to galvanizing treatment for Manufacturing Condition No. 1 (suitable steel) of Table 2 of Example 1 and a galvanized steel sheet subjected to galvanizing treatment for Manufacturing Condition No. 2 (suitable steel) of Table 2 of Example 1 were joined by spot welding, and a member of the present application example was manufactured.

[0212] The tensile strength TS of these members of the present application example was 1470 MPa or more, and the formability, delayed fracture resistance, and LME resistance were excellent, and thus it was found that these members were suitable for use in automobile parts and the like.

[0213] Similarly, a member of the present application example was manufactured by subjecting a steel sheet based on Manufacturing Condition No. 1 (suitable example) of Table 2 of Example 1 to press forming. Further, a member of the present application example was manufactured by joining a steel sheet based on Manufacturing Condition No. 1 (suitable example) of Table 2 of Example 1 and a steel sheet based on Manufacturing Condition No. 2 (suitable example) of Table 2 of Example 1 by spot welding. The tensile strength TS of these members of the present application example was 1470 MPa or more, and the formability, delayed fracture resistance, and LME resistance were excellent, and thus it was found that these members were suitable for use in automobile parts and the like.

[0214] Industrial applicability

[0215] According to the present application, a high-strength steel sheet having excellent formability, delayed fracture resistance, and LME resistance can be obtained. By improving these properties, a high-strength steel sheet can be applied to difficult-to-form members in cold press forming applications, contributing to an increase in member strength and weight reduction.

Claims

1. A steel plate, The invention has a composition containing, in mass%, C: 0.24% to 0.40%, Si: 0.2% to 1.0%, Mn: 1.5% to 3.5%, P: 0.002% to 0.010%, S: 0.0002% to 0.0020%, sol. Al: 0.50% to 0%, excluding 0%, N: 0.0006% to 0.01%, with the balance being Fe and unavoidable impurities. The steel structure comprises, by area ratio, not less than 40% and not more than 78% of martensite, not less than 20% and not more than 58% of bainite, and not less than 2% of retained austenite. The average particle size of carbides in the tempered martensite in the martensite is 0.40 μm or less, The average amount of C in the retained austenite is 0.5 mass % or more, The Si concentration in the depth from the steel plate surface to 100 μm in the plate thickness direction is 1.3 mass % or less. The tensile strength of the steel plate is greater than or equal to 1470 MPa.

2. The steel plate according to claim 1, wherein The difference in microhardness between the bainite and the martensite is 1.5 GPa or more.

3. The steel plate according to claim 1 or 2, wherein: The above-mentioned component composition further contains, in terms of mass %, one or more members selected from the following groups A to D: Group A: one or more selected from the group consisting of Nb: 0.1% or less, Ti: 0.10% or less, B: 0.0050% or less, Cu: 1% or less, and Ni: 1% or less; Group B: one or more selected from the group consisting of Cr: 1.0% or less, Mo: less than 0.3%, V: 0.45% or less, Zr: 0.2% or less, and W: 0.2% or less; Group C: one or two selected from Sb: 0.1% or less and Sn: 0.1% or less; Group D: one or more selected from the group consisting of Ca: 0.0050% or less, Mg: 0.01% or less, and REM: 0.01% or less.

4. The steel plate according to claim 1 or 2, wherein: There is a coating on the surface of the steel plate.

5. The steel plate according to claim 3, wherein There is a coating on the surface of the steel plate. 6 . A member obtained by subjecting the steel sheet according to claim 1 to at least one of forming and joining.

7. A method for manufacturing a steel plate, which is a method for manufacturing the steel plate according to any one of claims 1 to 3, comprising: Hot rolling process, hot rolling the steel slab to obtain hot rolled steel plate; a cold rolling step, after the hot rolling step, cold rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; a continuous annealing step, after the cold rolling step, annealing the cold-rolled steel sheet in an atmosphere having a dew point of -60°C to -20°C, at an annealing temperature of Ac3 point or higher, and for a soaking time of 15 seconds or longer, cooling the cold-rolled steel sheet at a first average cooling rate of 5°C / s to 18°C / s to a holding temperature of Ms point or higher and (Ms point + 200°C) or lower, holding the cold-rolled steel sheet at the holding temperature for a holding time of 1 second to 1000 seconds, and finally cooling the cold-rolled steel sheet at a second average cooling rate of 5°C / s to a cooling stop temperature of 250°C or lower; and The overaging treatment step is to maintain the temperature within a range of 150° C. to 250° C. for 30 seconds to 1500 seconds after the continuous annealing step.

8. The method for manufacturing a steel plate according to claim 7, wherein: The method comprises the following plating step: performing plating treatment on the surface of the steel sheet before or after the over-aging treatment step. 9 . A method for manufacturing a member, comprising the step of performing at least one of forming and joining on a steel plate manufactured by the method for manufacturing a steel plate according to claim 7 .

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