High-strength member, method of manufacturing high-strength member, and method of manufacturing steel plate for high-strength member

Abstract

To provide a high strength member excellent in delayed fracture resistance characteristics, a manufacturing method of the high strength member and a manufacturing method of a steel sheet for the high strength member.SOLUTION: A high strength member 10 of the present invention has a bent ridge part 12 obtained using a steel sheet 11, where a tensile strength of the member is 1470 MPa or more; a residual stress of an end face 13 of the bent ridge part 12 is 300 MPa or less; and a Vickers hardness (HV) of the end face 13 of the bent ridge part 12 is 200 or more and 450 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a high-strength member used in automobile parts, etc., a method for manufacturing a high-strength member, and a method for manufacturing a steel sheet for a high-strength member. More specifically, the present invention relates to a high-strength member having excellent delayed fracture resistance and a method for manufacturing the same, and also to a method for manufacturing a steel sheet for the high-strength member. [Background technology]

[0002] In recent years, the use of high-strength steel sheets with a tensile strength (TS) of 1320 to 1470 MPa has been increasing for body frame parts such as center pillar R / F (reinforcement), bumpers, impact beam parts, etc. (hereinafter referred to as parts.) Furthermore, from the perspective of further reducing the weight of automobile bodies, the use of steel sheets with a TS of 1800 MPa (1.8 GPa) or higher for parts is also being considered.

[0003] As the strength of steel sheets increases, there are concerns about the occurrence of delayed fracture. In recent years, there has been concern about delayed fracture originating from the shear edge of samples processed into part shapes, particularly in bent sections where strain is concentrated. It has become important to prevent delayed fracture originating from such shear edge.

[0004] For example, Patent Document 1 provides a thin steel sheet having excellent delayed fracture resistance after forming, which is made of steel having a chemical composition satisfying the following: C: 0.05 to 0.3%, Si: 3.0% or less, Mn: 0.01 to 3.0%, P: 0.02% or less, S: 0.02% or less, Al: 3.0% or less, N: 0.01% or less, with the balance being Fe and unavoidable impurities, and by specifying the grain size and density of Mg oxides, sulfides, complex crystallized products, and complex precipitates.

[0005] Patent Document 2 provides a method for manufacturing a formed part with excellent delayed fracture resistance by applying shot peening to the sheared end surface of a steel plate having a TS of 1180 MPa or more, thereby reducing residual stress at the end surface. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-166035 [Patent Document 2] Japanese Patent Application Publication No. 2017-125228 Summary of the Invention [Problem to be solved by the invention]

[0007] The technology disclosed in Patent Document 1 provides a steel sheet with excellent delayed fracture resistance by specifying the chemical composition and the grain size and density of precipitates in the steel. However, the steel sheet of Patent Document 1 has a low carbon content, so its strength is lower than that of the steel sheet used in the high-strength components of the present invention, with a TS of less than 1470 MPa. Even if the strength of the steel sheet of Patent Document 1 is improved by increasing the carbon content, the increased strength also increases the residual stress at the end face, which is thought to result in a deterioration in delayed fracture resistance.

[0008] The technology disclosed in Patent Document 2 provides a formed part with excellent delayed fracture resistance by applying shot peening to the sheared end face to reduce the residual stress at the end face. However, this is larger than the residual stress at the end face of 300 MPa or less specified in the present invention, and is not sufficient to improve delayed fracture resistance.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength member having excellent delayed fracture resistance and a method for manufacturing the same.

[0010] In the present invention, high strength means that the tensile strength (TS) is 1470 MPa or more.

[0011] In the present invention, "excellent delayed fracture resistance" means that, as described in the Examples, when a steel sheet is bent and then immersed in hydrochloric acid at pH 1 (25°C) and the maximum load stress at which delayed fracture does not occur is measured as the critical load stress, the critical load stress is 1.10 times or more the yield strength (YS). [Means for solving the problem]

[0012] The present inventors conducted extensive research to solve the above-mentioned problems. The inventors discovered that a high-strength component having a bend ridgeline obtained using a steel plate can be made to have excellent delayed fracture resistance by making the component have a tensile strength of 1470 MPa or more, a residual stress at the end face of the bend ridgeline of 300 MPa or less, and a Vickers hardness (HV) at the end face of the bend ridgeline of 200 to 450, and thus arrived at the present invention. The above-mentioned problems can be solved by the following means.

[0013] [1] A high-strength member having a bending ridge line portion obtained by using a steel plate, The tensile strength of the member is 1470 MPa or more, The residual stress at the end surface of the bent ridgeline is 300 MPa or less, and A high-strength member in which the end face of the bent ridge line portion has a Vickers hardness (HV) of 200 or more and 450 or less.

[0014] [2] The steel plate comprises, in mass%, C: 0.17% or more and 0.35% or less, Si: 0.001% or more and 1.2% or less, Mn: 0.9% or more and 3.2% or less, P: 0.020% or less, S: 0.0010% or less, Al: 0.010% or more and 0.20% or less, and N: 0.010% or less, the balance being iron and unavoidable impurities; [1] The high-strength member according to [1], having a microstructure in which the area ratio of one or both of bainite containing carbides having an average grain size of 50 nm or less and martensite containing carbides having an average grain size of 50 nm or less is 90% or more in total.

[0015] [3] The steel plate comprises, in mass%, C: 0.17% or more and 0.35% or less, Si: 0.001% or more and 1.2% or less, Mn: 0.9% or more and 3.2% or less, P: 0.020% or less, S: 0.0010% or less, Al: 0.010% or more and 0.20% or less, N: 0.010% or less, and Sb: 0.001% or more and 0.10% or less, the balance being iron and unavoidable impurities; [1] The high-strength member according to [1], having a microstructure in which the area ratio of one or both of bainite containing carbides having an average grain size of 50 nm or less and martensite containing carbides having an average grain size of 50 nm or less is 90% or more in total.

[0016] [4] The composition of the steel plate further comprises, in mass%, B: The high-strength member according to [2] or [3], containing 0.0002% or more and less than 0.0035%.

[0017] [5] The composition of the steel plate further comprises, in mass%, Nb: 0.002% or more and 0.08% or less; The high-strength member according to any one of [2] to [4], containing at least one selected from Ti: 0.002% to 0.12%.

[0018] [6] The composition of the steel plate further comprises, in mass%, Cu: 0.005% or more and 1% or less The high-strength member according to any one of [2] to [5], containing at least one selected from Ni: 0.005% to 1%.

[0019] [7] The composition of the steel plate further comprises, in mass%, Cr: 0.01% or more and 1.0% or less, Mo: 0.01% or more and less than 0.3% V: 0.003% or more and 0.5% or less, Zr: 0.005% or more and 0.20% or less, and The high-strength member according to any one of [2] to [6], containing at least one element selected from the group consisting of 0.005% to 0.20% of W.

[0020] [8] The composition of the steel plate further comprises, in mass%, Ca: 0.0002% or more and 0.0030% or less, Ce: 0.0002% or more and 0.0030% or less, La: 0.0002% or more and 0.0030% or less, and The high-strength member according to any one of [2] to [7], containing at least one selected from Mg: 0.0002% or more and 0.0030% or less.

[0021] [9] The composition of the steel plate further comprises, in mass%, The high-strength member according to any one of [2] to [8], containing Sn: 0.002% or more and 0.1% or less.

[0022]

[10] A bending process step of cutting out a steel plate having a tensile strength of 1470 MPa or more and bending the steel plate; and an end surface treatment step of heating the end surface produced by cutting at a temperature of 400°C or higher and 900°C or lower for more than 0 seconds and not more than 10 seconds after the bending process.

[0023]

[11] A bending process step of cutting out the steel plate according to any one of [2] to [9] and bending the steel plate; and an end surface treatment step of heating the end surface produced by cutting at a temperature of 400°C or higher and 900°C or lower for more than 0 seconds and not more than 10 seconds after the bending process.

[0024]

[12] An edge treatment process in which, after cutting out a steel plate having a tensile strength of 1470 MPa or more, the edge created by the cutting is heated at a temperature of 400°C or more and 900°C or less for more than 0 seconds and less than 10 seconds; a bending process for bending the steel plate after the end face treatment process.

[0025]

[13] After cutting out the steel plate according to any one of [2] to [9], an end surface treatment process is performed in which the end surface generated by the cutting is heated at a temperature of 400°C or higher and 900°C or lower for more than 0 seconds and not more than 10 seconds; a bending process for bending the steel plate after the end face treatment process.

[0026]

[14] A method for producing a steel plate for high-strength members to be used in high-strength members, which is obtained by the method for producing a high-strength member according to any one of

[10] to

[13] , a hot rolling process for hot rolling a steel material; a cold rolling step of cold rolling the hot-rolled steel sheet obtained by the hot rolling; The cold-rolled steel sheet obtained by the cold rolling is called A C3 and an annealing step of heating the steel sheet to an annealing temperature of at least 500°C, cooling the steel sheet at an average cooling rate of at least 3°C / sec in a temperature range from the annealing temperature to 550°C and to a cooling stop temperature of at most 350°C, and then holding the steel sheet in a temperature range of at least 100°C and at most 260°C for at least 20 seconds and at most 1,500 seconds. [Effects of the Invention]

[0027] According to the present invention, it is possible to provide a high-strength member having excellent delayed fracture resistance, a method for manufacturing a high-strength member, and a method for manufacturing a steel plate for a high-strength member. Furthermore, by applying the high-strength member of the present invention to an automobile structural member, it is possible to achieve both high strength and improved delayed fracture resistance in the automobile steel plate. In other words, the present invention improves the performance of automobile bodies. [Brief explanation of the drawings]

[0028] [Figure 1] It is a perspective view showing an example of the high-strength member of the present invention. [Figure 2] In the example, it is a side view showing the state of the member tightened with bolts and nuts. [Figure 3] In the measurement of the residual stress on the end face of the example, it is an enlarged view of the end face showing the measurement position at the center of the plate thickness and the measurement direction.

Mode for Carrying Out the Invention

[0029] Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments.

[0030] The present invention is a high-strength member having a bent ridge line portion obtained using a steel sheet, wherein the tensile strength of the member is 1470 MPa or more, the residual stress on the end face of the bent ridge line portion is 300 MPa or less, and the Vickers hardness (HV) on the end face of the bent ridge line portion is 200 or more and 450 or less.

[0031] If a high-strength member satisfying these conditions is obtained, the steel sheet used for the high-strength member is not particularly limited. Hereinafter, a preferable steel sheet for obtaining the high-strength member of the present invention will be described, but the steel sheet used for the high-strength member of the present invention is not limited to the steel sheet described below.

[0032] A preferable steel sheet for obtaining the high-strength member preferably has a component structure and a microstructure described later. Note that if the high-strength member of the present invention is obtained, it is not necessarily required to use a steel sheet having the component composition and microstructure described later.

[0033] First, the preferable component composition of a preferable steel sheet (material steel sheet) used for the high-strength member will be described. In the description of the following preferable component composition, the unit of the content of the component, “%”, means “mass %”.

[0034] <C: 0.17% or more and 0.35% or less> C is an element that improves hardenability. From the perspective of ensuring a total area ratio of one or both of a predetermined martensite and bainite, increasing the strength of martensite and bainite, and ensuring TS ≥ 1470 MPa, the C content is preferably 0.17% or more, more preferably 0.18% or more, and even more preferably 0.19% or more. On the other hand, when the C content exceeds 0.35%, even if heated after bending, the residual stress at the end face of the bending ridge line portion may exceed 300 MPa, which may deteriorate the stress corrosion cracking resistance. Therefore, the C content is preferably 0.35% or less, more preferably 0.33% or less, and even more preferably 0.31% or less.

[0035] <Si: 0.001% or more and 1.2% or less> Si is a strengthening element by solid solution strengthening. Also, when the steel plate is held in a temperature range of 200°C or higher, Si contributes to improving elongation by suppressing the excessive formation of coarse carbides. Furthermore, it contributes to reducing Mn segregation in the center of the plate thickness and suppressing the formation of MnS, thereby improving the stress corrosion cracking resistance. To sufficiently obtain the above effects, the Si content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more. On the other hand, if the Si content becomes too high, coarse MnS is likely to be formed in the plate thickness direction, deteriorating the stress corrosion cracking resistance. Therefore, the Si content is preferably 1.2% or less, more preferably 1.1% or less, and even more preferably 1.0% or less.

[0036] <Mn: 0.9% or more and 3.2% or less> Mn is contained to improve the hardenability of the steel and ensure a total area ratio of one or both of a predetermined martensite and bainite. If the Mn content is less than 0.9%, ferrite may form in the surface layer of the steel sheet, which may reduce the strength. Therefore, the Mn content is preferably 0.9% or more, more preferably 1.0% or more, and still more preferably 1.1% or more. Also, in order not to increase MnS and deteriorate the stress corrosion cracking resistance, the Mn content is preferably 3.2% or less, more preferably 3.1% or less, and still more preferably 3.0% or less.

[0037] <P: 0.020% or less> P is an element that strengthens the steel, but if its content is high, it deteriorates the stress corrosion cracking resistance. Therefore, the P content is preferably 0.020% or less, more preferably 0.015% or less, and still more preferably 0.010% or less. The lower limit of the P content is not particularly limited, but currently, the industrially feasible lower limit is about 0.003%.

[0038] <S: 0.0010% or less> S forms inclusions such as MnS, TiS, Ti(C,S), etc. In order to suppress the deterioration of the stress corrosion cracking resistance due to these inclusions, the S content is preferably 0.0010% or less. The S content is more preferably 0.0009% or less, still more preferably 0.0007% or less, and particularly preferably 0.0005% or less. The lower limit of the S content is not particularly limited, but currently, the industrially feasible lower limit is about 0.0002%.

[0039] <Al: 0.010% or more and 0.20% or less> Al is added to sufficiently deoxidize and reduce coarse inclusions in the steel. To obtain its effect, the Al content is preferably 0.010% or more, more preferably 0.015% or more. On the other hand, when the Al content exceeds 0.20%, carbides mainly composed of Fe such as cementite generated during coiling after hot rolling are difficult to dissolve in the annealing process, and there is a possibility of generating coarse inclusions and carbides, which may deteriorate the delayed fracture resistance properties. Therefore, the Al content is preferably 0.20% or less, more preferably 0.17% or less, and even more preferably 0.15% or less.

[0040] <N: 0.010% or less> N is an element that forms nitride and carbonitride-based coarse inclusions such as TiN, (Nb,Ti)(C,N), and AlN in the steel, and deteriorates the delayed fracture resistance properties through the formation of these. To prevent the deterioration of the delayed fracture resistance properties, the N content is preferably 0.010% or less, more preferably 0.007% or less, and even more preferably 0.005% or less. Note that the lower limit of the N content is not particularly limited, but currently, the industrially feasible lower limit is about 0.0006%.

[0041] <Sb: 0.001% or more and 0.10% or less> Sb suppresses oxidation and nitridation of the surface layer of the steel sheet, and suppresses decarburization due to oxidation and nitridation of the surface layer of the steel sheet. By suppressing decarburization, the formation of ferrite in the surface layer of the steel sheet is suppressed, contributing to the increase in strength. Furthermore, the suppression of decarburization also improves the hydrogen embrittlement resistance property. From such a viewpoint, the Sb content is preferably 0.001% or more, more preferably 0.002% or more, and still more preferably 0.003% or more. On the other hand, when the content of Sb exceeds 0.10%, it segregates at the prior austenite (γ) grain boundaries and promotes crack generation, which may deteriorate the hydrogen embrittlement resistance property. Therefore, the Sb content is preferably 0.10% or less, more preferably 0.08% or less, and still more preferably 0.06% or less. Although it is preferable to contain Sb, if the effects of increasing the strength of the steel sheet and improving the hydrogen embrittlement resistance property can be sufficiently obtained without containing Sb, Sb may not be contained.

[0042] The steel preferably used for the high-strength member of the present invention preferably basically contains the above components, and the balance is iron and inevitable impurities. The steel preferably used for the high-strength member of the present invention can contain the following optional elements within a range that does not impair the effects of the present invention. When the following optional elements are contained below the lower limit values described below, those optional elements are regarded as being contained as inevitable impurities.

[0043] <B: 0.0002% or more and less than 0.0035%> B is an element that improves the hardenability of steel and has the advantage of generating martensite and bainite with a predetermined area ratio even when the Mn content is low. To obtain such an effect of B, the B content is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0007% or more. Also, from the viewpoint of fixing N, it is preferable to add it in combination with 0.002% or more of Ti. On the other hand, when the B content becomes 0.0035% or more, the dissolution rate of cementite during annealing is delayed, and carbides mainly composed of Fe such as undissolved cementite remain. As a result, coarse inclusions and carbides are generated, deteriorating the stress corrosion cracking resistance. Therefore, when containing B, the B content is preferably less than 0.0035%, more preferably 0.0030% or less, and even more preferably 0.0025% or less.

[0044] <At least one selected from Nb: 0.002% or more and 0.08% or less and Ti: 0.002% or more and 0.12% or less> Nb and Ti contribute to strength improvement through the refinement of prior austenite (γ) grains. From such a viewpoint, the Nb content and the Ti content are each preferably 0.002% or more, more preferably 0.003% or more, and even more preferably 0.005% or more. On the other hand, when containing a large amount of Nb or Ti, the amount of coarse precipitates of Nb-based such as NbN, Nb(C,N), (Nb,Ti)(C,N) and Ti-based such as TiN, Ti(C,N), Ti(C,S), TiS that remain undissolved during slab heating in the hot rolling process increases, deteriorating the stress corrosion cracking resistance. For this reason, when containing Nb, the Nb content is preferably 0.08% or less, more preferably 0.06% or less, and even more preferably 0.04% or less. Also, when containing Ti, the Ti content is preferably 0.12% or less, more preferably 0.10% or less, and even more preferably 0.08% or less.

[0045] <At least one selected from Cu: 0.005% or more and 1% or less and Ni: 0.005% or more and 1% or less> Cu and Ni can improve the corrosion resistance in the usage environment of automobiles, and have the effect of suppressing hydrogen intrusion into the steel sheet by coating the surface of the steel sheet with corrosion products. Also, from the perspective of improving the stress corrosion cracking resistance characteristics, it is preferable to contain Cu and Ni at 0.005% or more, more preferably 0.008% or more, respectively. However, if too much Cu or Ni is contained, it will cause the generation of surface defects and deteriorate the plating property and chemical conversion treatment property. Therefore, when containing at least one of Cu and Ni, the Cu content and the Ni content are preferably 1% or less, more preferably 0.8% or less, and even more preferably 0.6% or less, respectively.

[0046] <At least one selected from Cr: 0.01% or more and 1.0% or less, Mo: 0.01% or more and less than 0.3%, V: 0.003% or more and 0.5% or less, Zr: 0.005% or more and 0.20% or less, and W: 0.005% or more and 0.20% or less> Cr, Mo, and V can be contained for the purpose of improving the hardenability of steel. To obtain such an effect, the Cr content and the Mo content are preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more, respectively. The V content is preferably 0.003% or more, more preferably 0.005% or more, and even more preferably 0.007% or more. However, if any of the elements is contained too much, the stress corrosion cracking resistance characteristics will be deteriorated due to the coarsening of carbides. Therefore, when containing Cr, the Cr content is preferably 1.0% or less, more preferably 0.4% or less, and even more preferably 0.2% or less. When containing Mo, the Mo content is preferably less than 0.3%, more preferably 0.2% or less, and even more preferably 0.1% or less. When containing V, the V content is preferably 0.5% or less, more preferably 0.4% or less, and even more preferably 0.3% or less.

[0047] Zr and W contribute to the increase in strength through the refinement of prior austenite (γ) grains. From this perspective, the Zr content and the W content are each preferably 0.005% or more, more preferably 0.006% or more, and still more preferably 0.007% or more. However, if a large amount of Zr or W is contained, the amount of coarse precipitates remaining undissolved during slab heating in the hot rolling process increases, deteriorating the delayed fracture resistance properties. Therefore, when at least one of Zr and W is contained, the Zr content and the W content are each preferably 0.20% or less, more preferably 0.15% or less, and still more preferably 0.10% or less.

[0048] <At least one selected from Ca: 0.0002% or more and 0.0030% or less, Ce: 0.0002% or more and 0.0030% or less, La: 0.0002% or more and 0.0030% or less, and Mg: 0.0002% or more and 0.0030% or less> Ca, Ce, and La contribute to the improvement of the delayed fracture resistance properties by fixing S as sulfides. Therefore, the content of each of these elements is preferably 0.0002% or more, more preferably 0.0003% or more, and still more preferably 0.0005% or more. On the other hand, if a large amount of these elements is added, the delayed fracture resistance properties deteriorate due to the coarsening of sulfides. Thus, when at least one of Ca, Ce, and La is contained, the content of each of these elements is preferably 0.0030% or less, more preferably 0.0020% or less, and still more preferably 0.0010% or less.

[0049] Mg fixes O as MgO and serves as a trap site for hydrogen in steel, thus contributing to the improvement of the delayed fracture resistance properties. Therefore, the Mg content is preferably 0.0002% or more, more preferably 0.0003% or more, and still more preferably 0.0005% or more. On the other hand, if a large amount of Mg is added, the delayed fracture resistance properties deteriorate due to the coarsening of MgO. Therefore, when Mg is contained, the Mg content is preferably 0.0030% or less, more preferably 0.0020% or less, and still more preferably 0.0010% or less.

[0050] <Sn: 0.002% or more and 0.1% or less> Sn suppresses oxidation and nitridation of the surface layer of the steel sheet, and suppresses decarburization due to oxidation and nitridation of the surface layer of the steel sheet. By suppressing decarburization, ferrite formation in the surface layer of the steel sheet is suppressed, contributing to high strength. From such a viewpoint, the Sn content is preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.004% or more. On the other hand, when Sn is contained in an amount exceeding 0.1%, it segregates at the prior austenite (γ) grain boundary and deteriorates the delayed fracture resistance characteristics. Therefore, when Sn is contained, the Sn content is preferably 0.1% or less, more preferably 0.08% or less, and still more preferably 0.06% or less.

[0051] Next, the preferred conditions of the microstructure of the steel sheet used for the high-strength member of the present invention will be described.

[0052] <The total area ratio of one or two of bainite containing carbides with an average particle size of 50 nm or less and martensite containing carbides with an average particle size of 50 nm or less is 90% or more> In order to obtain a high strength of TS≧1470 MPa, it is preferable that the total area ratio of one or two of bainite containing carbides with an average particle size of 50 nm or less and martensite containing carbides with an average particle size of 50 nm or less is 90% or more with respect to the entire steel sheet structure. If it is less than this, ferrite increases and the strength decreases. Also, from the viewpoint of increasing the strength, the total area ratio is more preferably 91% or more, still more preferably 92% or more, and particularly preferably 93% or more. The total area ratio may be 100% in total. Also, the area ratio of either one may be 90% or more, or the total area ratio of both may be 90% or more.

[0053] Martensite does not include as-quenched martensite, but refers to tempered martensite. In the present invention, martensite refers to a hard structure formed from austenite at low temperatures (below the martensitic transformation point), and tempered martensite refers to a structure that is tempered when martensite is reheated. Bainite refers to a hard structure formed from austenite at relatively low temperatures (above the martensitic transformation point) and in which fine carbides are dispersed in needle-like or plate-like ferrite.

[0054] The remaining structure other than martensite and bainite is ferrite, pearlite, and retained austenite, and the total amount thereof is permissible if it is less than 10%. It may even be 0%.

[0055] In the present invention, ferrite is a structure formed by transformation from austenite at relatively high temperatures and consisting of crystal grains with a bcc lattice. Pearlite is a structure formed by layered ferrite and cementite. Retained austenite is austenite that does not transform into martensite because the martensite transformation temperature is below room temperature.

[0056] In the present invention, the carbides having an average particle size of 50 nm or less refer to fine carbides that can be observed in bainite and martensite when observed with an SEM. Specific examples of the carbides include Fe carbide, Ti carbide, V carbide, Mo carbide, W carbide, Nb carbide, and Zr carbide.

[0057] The steel sheet may have a plating layer such as a hot-dip galvanized layer. Examples of such a plating layer include an electroplated layer, an electroless plated layer, and a hot-dip plated layer. Furthermore, the steel sheet may have an alloyed plated layer.

[0058] Next, the high-strength member will be described.

[0059] The high-strength member of the present invention is a high-strength member having a bending ridge obtained by using a steel plate, and the member has a tensile strength of 1470 MPa or more, a residual stress at the end face of the bending ridge of 300 MPa or less, and a Vickers hardness (HV) at the end face of the bending ridge of 200 or more and 450 or less.

[0060] The high-strength member of the present invention is obtained by using a steel plate and is a formed member obtained by performing processing such as forming and bending to obtain a predetermined shape. The high-strength member of the present invention can be suitably used, for example, for automobile parts.

[0061] The high-strength member of the present invention has a bend ridge. In the present invention, the term "bend ridge" refers to a region of a steel plate that is no longer flat due to bending. An example of a high-strength member 10 shown in FIG. 1 is a steel plate 11 that has been V-bent. The high-strength member 10 has a bend ridge 12 on the side of the steel plate 11 in the bent portion. An end face 13 of the bend ridge 12 is a plate thickness surface located on the side of the bend ridge 12. The bend ridge direction D1 in the present invention is a direction parallel to the bend ridge 12.

[0062] In the high-strength member of the present invention, the bending angle is not particularly limited as long as the residual stress at the end face of the bend ridge is 300 MPa or less and the Vickers hardness (HV) at the end face of the bend ridge is 200 or more and 450 or less.

[0063] The example of the high-strength member 10 shown in FIG. 1 is an example in which one portion is bent, but two or more portions may be bent to have two or more bent ridges.

[0064] <Tensile strength of component is 1470 MPa or more> The high-strength member has a tensile strength (TS) of 1470 MPa or more. In order to achieve a tensile strength (TS) of 1470 MPa or more, it is preferable to use the above steel plate. In the present invention, the tensile strength (TS) and yield strength (YS) are calculated by measuring the flat portion of the high-strength member that has not been bent. Furthermore, if the tensile strength (TS) and yield strength (YS) of an annealed steel sheet before bending (a steel sheet after an annealing process) are measured, these measured values ​​can be regarded as the measured values ​​of the tensile strength (TS) and yield strength (YS) of a high-strength member obtained using the annealed steel sheet. The strength of the member can be calculated by the method described in the Examples.

[0065] <Residual stress at the end face of the bent ridge is 300 MPa or less> The residual stress at the end face (plate thickness face) of the bend ridgeline of the high-strength member is 300 MPa or less. This makes it difficult for cracks to occur at the end face of the bend ridgeline, resulting in a member with excellent delayed fracture resistance. From the viewpoint of suppressing the occurrence of cracks due to delayed fracture, the residual stress is 300 MPa or less, preferably 250 MPa or less, and more preferably 200 MPa or less. There is no particular restriction on the lower limit, and it may be a compressive stress. The residual stress at the end face of the bend ridgeline can be calculated by the method described in the examples of this specification.

[0066] <Vickers hardness (HV) of the end face of the bent ridge is 200 to 450> The Vickers hardness (HV) of the end face (plate thickness face) of the bend ridgeline of the high-strength member is 200 or more and 450 or less. This makes it difficult for cracks to occur at the end face of the bend ridgeline, resulting in a member with excellent delayed fracture resistance. From the viewpoint of suppressing crack generation due to delayed fracture, the hardness is 450 or less, preferably 430 or less, and more preferably 400 or less. Furthermore, if the hardness of the end face of the bend ridgeline is low, the difference with the base material hardness becomes large, thereby promoting crack generation. Therefore, from the viewpoint of suppressing crack generation due to delayed fracture and obtaining strength of the member, the Vickers hardness (HV) of the end face is set to 200 or more, preferably 220 or more, and more preferably 250 or more. The Vickers hardness of the end face of the bend ridgeline can be calculated by the method described in the examples of this specification.

[0067] Next, an embodiment of the method for manufacturing a high-strength member according to the present invention will be described.

[0068] One example of an embodiment of the method for manufacturing a high-strength member of the present invention includes a bending process in which a steel plate having a tensile strength of 1470 MPa or more is cut out and bent, and an end surface treatment process in which the end surfaces resulting from the cutting are heated, after the bending process, at a temperature of 400°C or higher and 900°C or lower for more than 0 seconds and not more than 10 seconds.

[0069] Another example of an embodiment of the method for producing a high-strength member of the present invention includes a bending step of cutting out a steel plate having the above-described chemical composition and microstructure and bending the steel plate, and an end surface treatment step of heating the end surfaces resulting from the cutting at a temperature of 400°C or higher and 900°C or lower for more than 0 seconds and not more than 10 seconds after the bending step.

[0070] Another example of an embodiment of the method for manufacturing a high-strength member of the present invention includes an end surface treatment step in which, after cutting out a steel plate having a tensile strength of 1470 MPa or more, the end surface created by the cutting is heated at a temperature of 400°C or more and 900°C or less for more than 0 seconds and not more than 10 seconds, and a bending step in which the steel plate after the end surface treatment step is bent.

[0071] Another example of an embodiment of the method for producing a high-strength member of the present invention includes an end face treatment step in which, after cutting out a steel plate having the above-described chemical composition and microstructure, the end faces resulting from the cutting are heated at a temperature of 400°C or higher but not higher than 900°C for longer than 0 seconds but not longer than 10 seconds, and a bending step in which the steel plate after the end face treatment step is bent.

[0072] [End surface processing process] As described above, the method for manufacturing a high-strength member of the present invention includes an end surface treatment step in which, after cutting a steel sheet, the end surface resulting from the cutting is heated at a temperature of 400°C or higher and 900°C or lower for more than 0 seconds and not more than 10 seconds. Here, the steel sheet to be cut out is, for example, a steel sheet having a tensile strength of 1470 MPa or higher. Furthermore, the steel sheet to be cut out is, for example, a steel sheet having the above-mentioned chemical composition and microstructure.

[0073] Cutting in the present invention includes known cutting methods such as shear cutting (mechanical cutting), laser cutting, electrical cutting such as electric discharge machining, and gas cutting.

[0074] By performing the end surface treatment process, residual stress at the end surface of the steel sheet is reduced and the end surface is softened, making it difficult for cracks to occur at the end surface of the bending ridge, and a component with excellent delayed fracture resistance can be obtained. The method for heating the end surface is not particularly limited, and heating with a laser is an example.

[0075] In order to reduce residual stress at the end surface, the end surface of the formed part after bending the steel sheet is heated at a temperature of 400°C or higher and 900°C or lower. If the heating temperature exceeds 900°C, the formation and coarsening of ferrite becomes significant, reducing the strength of the formed part and causing excessive softening, which also deteriorates the delayed fracture resistance. Therefore, the heating temperature is 900°C or lower, preferably 870°C or lower. Also, if the temperature is lower than 400°C, the heating capacity is insufficient and the structure does not soften. Therefore, the heating temperature is 400°C or higher. Preferably, it is 450°C or higher, more preferably 500°C or higher, even more preferably above 600°C, and particularly preferably 700°C or higher. The heating time is 10 seconds or less. If the heating time exceeds 10 seconds, the structure coarsens, which deteriorates the delayed fracture resistance. Therefore, the heating time is 10 seconds or less, preferably 9 seconds or less, more preferably 8 seconds or less. The heating time is not particularly limited as long as the structure is softened and the Vickers hardness of the end face is 200 to 450. Therefore, the heating time is more than 0 seconds, preferably 1 second or more, and more preferably 2 seconds or more.

[0076] The heating range is not particularly limited, but is preferably about 5 mm from the end face of the bent ridge to ensure the strength of the formed part. The heating direction is also not particularly limited, but is preferably perpendicular to the plate thickness to eliminate temperature variations in the plate thickness direction.

[0077] [Bending process] The method for manufacturing a high-strength member of the present invention includes a bending step of bending a steel plate. The bending step may be performed before or after the edge treatment step.

[0078] The bending process of the present invention includes at least one of four deformation modes classified into bending deformation, deep drawing deformation, stretch flange deformation, and stretch flange deformation.

[0079] Next, an embodiment of a method for manufacturing a steel plate for high strength members used in the high strength members obtained by the method for manufacturing a high strength member will be described.

[0080] Further, one example of an embodiment of the method for producing a steel plate for high strength members of the present invention includes a hot rolling step of hot rolling steel (steel material), a cold rolling step of cold rolling the hot rolled steel plate obtained by the hot rolling, and a cold rolling step of cold rolling the cold rolled steel plate obtained by the cold rolling. C3 and an annealing step in which the steel sheet is heated to an annealing temperature of at least 550°C, and then cooled at an average cooling rate of at least 3°C / sec in the temperature range from the annealing temperature to 550°C, with the cooling stop temperature being 350°C or lower, followed by holding the steel sheet in a temperature range of at least 100°C and at most 260°C for at least 20 seconds and at most 1500 seconds.

[0081] These steps and a preferred casting step carried out before the hot rolling step will be described below. Note that the temperatures shown below refer to the surface temperatures of the steel material (slab), steel plate, etc. unless otherwise specified.

[0082] [Casting process] A steel having the above-described chemical composition is cast. The casting speed is not particularly limited, but in order to suppress the formation of the above-described inclusions and improve delayed fracture resistance, the casting speed is preferably 1.80 m / min or less, more preferably 1.75 m / min or less, and even more preferably 1.70 m / min or less. There is no particular lower limit, but from the viewpoint of productivity, it is preferably 1.25 m / min or more, and more preferably 1.30 m / min or more.

[0083] [Hot rolling process] In the hot rolling process, for example, a steel material (slab) having the aforementioned component composition is hot rolled. The slab heating temperature is not particularly limited, but setting the slab heating temperature to 1200°C or higher promotes the solid solution of sulfides and reduces Mn segregation, thereby reducing the amount of coarse inclusions described above and tending to improve delayed fracture resistance. For this reason, the slab heating temperature is preferably 1200°C or higher, more preferably 1220°C or higher. Furthermore, the heating rate during slab heating is preferably 5 to 15°C / min, and the slab soaking time is preferably 30 to 100 minutes.

[0084] The finish rolling end temperature is preferably 840°C or higher. If the finish rolling end temperature is less than 840°C, it takes a long time for the temperature to drop, and inclusions are formed, which not only deteriorates the delayed fracture resistance but also may degrade the internal quality of the steel sheet. Therefore, the finish rolling end temperature is preferably 840°C or higher, more preferably 860°C or higher. On the other hand, although there is no particular upper limit, since it becomes difficult to cool the steel sheet to the subsequent coiling temperature, the finish rolling end temperature is preferably 950°C or lower, more preferably 920°C or lower.

[0085] The cooled hot-rolled steel sheet is preferably coiled at a temperature of 630°C or lower. If the coiling temperature exceeds 630°C, the surface of the base steel may be decarburized, resulting in a difference in structure between the interior and surface of the steel sheet, which may cause uneven alloy concentration. Furthermore, decarburization of the surface layer reduces the area ratio of bainite and martensite, which have carbides in the surface layer of the steel, which tends to make it difficult to ensure the desired strength. Therefore, the coiling temperature is preferably 630°C or lower, more preferably 600°C or lower. There is no particular lower limit for the coiling temperature, but a temperature of 500°C or higher is preferred to prevent a decrease in cold rolling properties.

[0086] [Cold rolling process] In the cold rolling step, the hot-rolled steel sheet obtained by hot rolling is cold-rolled. In the cold rolling step, for example, the hot-rolled steel sheet coiled as described above is pickled and then cold-rolled to produce a cold-rolled steel sheet. The pickling conditions are not particularly limited. If the reduction is less than 20%, there is a risk that the surface flatness will be poor and the structure will become non-uniform. Therefore, the reduction is preferably 20% or more, more preferably 30% or more, and even more preferably 40% or more.

[0087] [Annealing process] The cold-rolled steel sheet obtained by cold rolling is called A C3 Heat to an annealing temperature above the annealing point. C3 Below this point, ferrite forms in the structure and the desired strength cannot be obtained. C3 points or more, preferably A C3 point +10℃ or higher, more preferably A C3 The upper limit of the annealing temperature is not particularly limited, but from the viewpoint of suppressing coarsening of austenite and preventing deterioration of delayed fracture resistance, the annealing temperature is preferably 900°C or less. C3 After heating to an annealing temperature equal to or higher than the annealing point, the steel sheet may be soaked at the annealing temperature.

[0088] A C3 The points are calculated by the following formula: In the formula, (% element symbol) means the content (mass %) of each element. A C3 Point (℃)=910-203√(%C)+45(%Si)-30(%Mn)-20(%Cu)-15(%Ni)+11(%Cr)+32(%Mo)+104(%V)+400(%Ti)+460(%Al)

[0089] As mentioned above, cold rolled steel sheet A C3 After heating to an annealing temperature equal to or higher than the annealing temperature, the material is cooled at an average cooling rate of 3°C / sec or higher in the temperature range from the annealing temperature to 550°C, and the cooling stop temperature is 350°C or lower, and then the material is held in a temperature range of 100°C or higher and 260°C or lower for 20 seconds or longer and 1500 seconds or shorter.

[0090] If the average cooling rate in the temperature range from the annealing temperature to 550°C is less than 3°C / s, excessive ferrite formation occurs, making it difficult to obtain the desired strength. Furthermore, the formation of ferrite in the surface layer makes it difficult to obtain the fraction of bainite or martensite having carbides near the surface, deteriorating the delayed fracture resistance. Therefore, the average cooling rate in the temperature range from the annealing temperature to 550°C is 3°C / s or more, preferably 5°C / s or more, and more preferably 10°C / s or more. While there is no particular upper limit for the average cooling rate, if the rate is too fast, non-uniform martensitic transformation in the coil width direction is likely to occur, and shape deterioration may cause the steel sheet to come into contact with equipment. Therefore, from the viewpoint of obtaining the minimum shape, it is preferable to set the rate to 3000°C / s or less.

[0091] Unless otherwise specified, the average cooling rate in the temperature range from the annealing temperature to 550°C is "(annealing temperature - 550°C) / (cooling time from the annealing temperature to 550°C)".

[0092] The cooling stop temperature is 350°C or lower. If the cooling stop temperature exceeds 350°C, tempering will not proceed sufficiently, as-quenched martensite and retained austenite will form in the final structure, and the hardness of the end face of the bend ridge will increase, resulting in a deterioration of delayed fracture resistance. Therefore, in order to obtain excellent delayed fracture resistance, the cooling stop temperature is 350°C or lower, preferably 300°C or lower, and more preferably 250°C or lower. There is no particular restriction on the lower limit of the cooling stop temperature, but from the viewpoint of easily ensuring the temperature when subsequently reheating, 0°C or higher is preferred.

[0093] The carbides distributed within the bainite are formed during holding at low temperatures after quenching, and act as hydrogen trapping sites, capturing hydrogen and preventing a deterioration in delayed fracture resistance. If the holding temperature is less than 100°C or the holding time is less than 20 seconds, bainite does not form and as-quenched martensite containing no carbides forms, resulting in increased hardness at the end faces of the bend ridges and preventing the above-mentioned effects from being achieved.

[0094] Furthermore, if the holding temperature exceeds 260°C or the holding time exceeds 1500 seconds, decarburization occurs and coarse carbides are formed inside the bainite, which causes excessive softening and deteriorates the delayed fracture resistance.

[0095] Therefore, the holding temperature is 100° C. or higher and 260° C. or lower, and the holding time is 20 seconds or higher and 1500 seconds or lower. The holding temperature is preferably 130° C. or higher and 240° C. or lower, and the holding time is preferably 50 seconds or higher and 1000 seconds or lower.

[0096] In the present invention, "maintaining" does not only mean maintaining at a constant temperature, but also includes a case where the temperature changes within the range of the maintaining temperature of the present invention.

[0097] The hot-rolled steel sheet may be subjected to heat treatment to soften the structure after hot rolling. The surface of the steel sheet may be plated with Zn, Al, or the like. After annealing and cooling or after plating, the steel sheet may be subjected to temper rolling to adjust the shape. [Example]

[0098] The present invention will be specifically described with reference to examples, but the present invention is not limited to these.

[0099] [Example 1] Steel plates having the tensile strengths listed in Table 1 were sheared into small pieces of 30 mm x 110 mm. JIS No. 5 test pieces with a gauge length of 50 mm, gauge width of 25 mm, and thickness of 1.4 mm were taken from the rolling direction of the steel plates, and tensile tests were conducted in accordance with JIS Z 2241 at a tension speed of 10 mm / min. The measured tensile strength (TS) and yield strength (YS) are shown in Table 1. For some of the sheared steel sheets, the cut edges were subjected to edge treatment under the conditions shown in Table 1. Next, a steel sheet sample was placed on a die with a 90° angle and pressed with a punch with a 90° angle, thereby performing a V-bend. Next, as shown in the side view of Figure 2, the bent steel sheet (component) was tightened from both sides of the steel sheet surface 11 with bolts 20, nuts 21, and tapered washers 22. The relationship between the load stress and the tightening amount was calculated using CAE (Computer Aided Engineering) analysis, and the tightening amount was adjusted to match the critical load stress. The critical load stress was measured using the method described below. Next, for some of the bent steel sheets (components), the edge surfaces of the steel sheets were subjected to edge treatment under the conditions shown in Table 1. The edge treatment conditions are shown in Table 1. In the edge treatment section of Table 1, a "-" in the heat treatment temperature (°C) column indicates that no heat treatment was performed.

[0100] 2. Evaluation Method The delayed fracture resistance of the components obtained under various manufacturing conditions was evaluated using the critical load stress measured by a delayed fracture test. The residual stress and Vickers hardness of the end faces of the components were also measured as follows. The evaluation methods were as follows.

[0101] (Measurement of critical load stress) The critical load stress was measured by a delayed fracture test. Specifically, the components obtained under each manufacturing condition were immersed in hydrochloric acid with a pH of 1 (25°C), and the maximum load stress at which delayed fracture did not occur was evaluated as the critical load stress. Delayed fracture was determined by visual inspection and by using images magnified up to 20x with a stereomicroscope. If no cracks occurred after 96 hours of immersion, it was considered no fracture. "Cracks" here refers to the occurrence of cracks with a length of 200 μm or more.

[0102] (Measurement of residual stress on the end face) For the components obtained under each manufacturing condition, the residual stress at the end face was measured by X-ray diffraction. The residual stress was measured at the center of the plate thickness at the end face of the bend ridge, and the X-ray irradiation diameter was 150 μm. The measurement direction was perpendicular to the plate thickness direction and perpendicular to the bend ridge direction. Figure 3 is an enlarged view of the end face of the bend ridge, with the plate thickness center C1 and measurement direction D2 respectively indicated by symbols.

[0103] (Measurement of Vickers hardness on the end surface) The Vickers hardness (HV) of the end surface of the components obtained under each manufacturing condition was measured by a Vickers hardness test. The Vickers hardness was measured at a location 100 μm from the end surface at the center of the plate thickness of the cross section cut along the bend ridge direction D1 and mirror-polished. The measurement load was 1 kgf.

[0104] 3. Evaluation Results The evaluation results are shown in Table 1.

[0105] [Table 1]

[0106] Members with TS≧1470 MPa and critical load stress≧1.10×YS were deemed to have passed the test, and are shown in Table 1 as examples of the invention. Members with TS<1470 MPa or critical load stress<1.10×YS were deemed to have failed the test, and are shown in Table 1 as comparative examples. In Table 1, a "critical load stress / YS" ​​ratio of 1.10 or more means that critical load stress≧1.10×YS. As shown in Table 1, the members of the examples of the invention have high strength and excellent delayed fracture resistance.

[0107] [Example 2] 1. Manufacturing of evaluation components Steel having the chemical composition shown in Table 2, with the balance being Fe and unavoidable impurities, was melted in a vacuum melting furnace and then bloomed to obtain a 27 mm thick bloomed material. The obtained bloomed material was hot rolled to a thickness of 4.2 mm to produce a hot-rolled steel sheet. The hot-rolled steel sheet was then ground to a thickness of 3.2 mm, and then cold-rolled to a thickness of 2.4 to 1.12 mm to produce a cold-rolled steel sheet. The cold-rolled steel sheet obtained above was then heat-treated (annealed) under the conditions shown in Tables 3 and 4. Note that blank columns in the chemical composition in Table 2 indicate that the corresponding element was not intentionally added, and include not only cases where the element is not contained (0 mass%), but also cases where the element is unavoidably contained. Note that the details of the conditions for the hot-rolling process, cold-rolling process, and annealing process are shown in Tables 3 and 4.

[0108] The heat-treated steel sheet was sheared into small pieces measuring 30 mm x 110 mm, and a V-shaped bending process was performed by placing the steel sheet sample on a die with a 90° angle and pressing the steel sheet with a punch with a 90° angle. Next, as shown in the side view of Figure 2, the bent steel sheet (member) was tightened from both sides of the steel sheet surface 11 with bolts 20, nuts 21, and tapered washers 22. The relationship between the load stress and the tightening amount was calculated using CAE (Computer Aided Engineering) analysis, and the tightening amount was adjusted to match the critical load stress. The critical load stress was measured using the method described below.

[0109] For Nos. 1 to 72 in Tables 3 and 4, the end faces of the bent ridges were heated at various temperatures after bending. For No. 73 in Table 4, the steel plate was sheared into small pieces, and then the end faces resulting from the cutting were heated before the bending. The end face treatment conditions are shown in Tables 3 and 4. For end face treatment in Tables 3 and 4, a "-" in the heat treatment temperature (°C) column means that no heat treatment was performed.

[0110] [Table 2]

[0111] [Table 3]

[0112] [Table 4]

[0113] 2. Evaluation Method The microstructure of the components obtained under various manufacturing conditions was analyzed to investigate the microstructure fraction. Tensile tests were also conducted to evaluate tensile properties such as tensile strength, and delayed fracture resistance was evaluated using the critical load stress measured in a delayed fracture test. Residual stress and Vickers hardness of the end faces of the components were also measured as follows. The methods for each evaluation are as follows:

[0114] (Total area ratio of one or both of bainite containing carbides with an average grain size of 50 nm or less and martensite containing carbides with an average grain size of 50 nm or less) Test specimens were taken perpendicular to the steel sheet obtained in the annealing process (hereinafter referred to as "annealed steel sheet"). The cross section parallel to the rolling direction was mirror-polished. The structure was then revealed with nital solution and observed using a scanning electron microscope. A 16 mm × 15 mm grid with 4.8 μm intervals was placed over an area of ​​82 μm × 57 μm in actual length on the SEM image at 1500x magnification. The area fractions of martensite containing carbides with an average grain size of 50 nm or less and bainite containing carbides with an average grain size of 50 nm or less were calculated using the point counting method, and the total area fraction was calculated. The area fraction was calculated as the average of three area fractions obtained from separate 1500x magnification SEM images. Martensite exhibits a white structure, while bainite exhibits fine carbides precipitated within a black structure. The average grain size of carbides was calculated as follows: The area ratio is the area ratio relative to the entire observation range, and this was considered to be the area ratio relative to the entire steel sheet structure.

[0115] (Average grain size of carbides in bainite and martensite) Test specimens were taken from the annealed steel sheet perpendicular to the rolling direction, and the cross section parallel to the rolling direction (L thickness) was mirror-polished. The structure was revealed with nital solution and then observed using a scanning electron microscope. The total area of ​​carbides on the SEM image at 5000x magnification was measured by binarization image analysis, and the average area per carbide was calculated by averaging the total area by number. The circle-equivalent diameter calculated from the average area per carbide was taken as the average grain size.

[0116] (Tensile test) JIS No. 5 test pieces with a gauge length of 50 mm, a gauge width of 25 mm, and a plate thickness of 1.4 mm were taken from the rolling direction of the annealed steel sheet, and tensile tests were conducted in accordance with JIS Z2241 at a tension speed of 10 mm / min to measure the tensile strength (TS) and yield strength (YS).

[0117] (Measurement of critical load stress) The critical load stress was measured by a delayed fracture test. Specifically, the components obtained under each manufacturing condition were immersed in hydrochloric acid with a pH of 1 (25°C), and the maximum load stress at which delayed fracture did not occur was evaluated as the critical load stress. Delayed fracture was determined by visual inspection and by using images magnified up to 20x with a stereomicroscope. If no cracks occurred after 96 hours of immersion, it was considered no fracture. "Cracks" here refers to the occurrence of cracks with a length of 200 μm or more.

[0118] (Measurement of residual stress on the end face) For the components obtained under each manufacturing condition, the residual stress at the end face was measured by X-ray diffraction. The residual stress was measured at the center of the plate thickness at the end face of the bend ridge, and the X-ray irradiation diameter was 150 μm. The measurement direction was perpendicular to the plate thickness direction and perpendicular to the bend ridge direction. Figure 3 is an enlarged view of the end face of the bend ridge, with the plate thickness center C1 and measurement direction D2 respectively indicated by symbols.

[0119] (Measurement of Vickers hardness on the end surface) The Vickers hardness (HV) of the end surface of the components obtained under each manufacturing condition was measured by a Vickers hardness test. The Vickers hardness was measured at a location 100 μm from the end surface at the center of the plate thickness of the cross section cut along the bend ridge direction D1 and mirror-polished. The measurement load was 1 kgf.

[0120] 3. Evaluation Results The evaluation results are shown in Tables 5 and 6.

[0121] [Table 5]

[0122] [Table 6]

[0123] In this example, members with TS≧1470 MPa and critical load stress≧1.10×YS were deemed to have passed, and are shown as invention examples in Tables 5 and 6. Members with TS<1470 MPa or critical load stress<1.10×YS were deemed to have failed, and are shown as comparative examples in Tables 5 and 6. In Tables 5 and 6, a "critical load stress / YS" ​​ratio of 1.10 or more means that critical load stress≧1.10×YS. As shown in Tables 5 and 6, the members of the invention examples have high strength and excellent delayed fracture resistance.

[0124] [Example 3] In Example 3, a member was manufactured using a steel type that did not contain Sb and was evaluated. 1. Manufacturing of evaluation components Steel having the chemical composition shown in Table 7, with the balance being Fe and unavoidable impurities, was melted in a vacuum melting furnace and then bloomed to obtain a 27 mm thick bloomed material. The obtained bloomed material was hot rolled to a thickness of 4.2 mm to produce a hot-rolled steel sheet. The hot-rolled steel sheet was then ground to a thickness of 3.2 mm, and then cold-rolled to a thickness of 2.4 to 1.12 mm to produce a cold-rolled steel sheet. The cold-rolled steel sheet obtained above was then heat-treated under the conditions shown in Table 8 (annealing step). Note that blank columns in the chemical composition in Table 7 indicate that the element was not intentionally added, and include not only cases where the element is not contained (0 mass%), but also cases where the element is unavoidably contained. Note that details of the conditions for the hot-rolling step, cold-rolling step, and annealing step are shown in Table 8.

[0125] The heat-treated steel sheet was sheared into small pieces measuring 30 mm x 110 mm, and a sample of the steel sheet was placed on a die with a 90° angle. The steel sheet was then pressed with a punch with a 90° angle to perform a V-bend. Next, as shown in the side view of FIG. 2, the bent steel sheet (member) was tightened with bolts 20 from both sides of the steel sheet surface 11 using bolts 20, nuts 21, and tapered washers 22. The relationship between the load stress and the tightening amount was calculated using CAE (Computer Aided Engineering) analysis, and the tightening amount was adjusted to match the critical load stress. The critical load stress was measured using the method described in Example 2.

[0126] After bending, the end faces of the bent ridges were heated at various temperatures. The end face treatment conditions are shown in Table 8.

[0127] [Table 7]

[0128] [Table 8]

[0129] 2. Evaluation Method The members obtained under various manufacturing conditions were measured and evaluated in the same manner as in Example 2.

[0130] 3. Evaluation Results The evaluation results are shown in Table 9.

[0131] [Table 9]

[0132] In this example, members with TS≧1470 MPa and critical load stress≧1.10×YS were deemed to have passed, and are shown as inventive examples in Table 9. Members with TS<1470 MPa or critical load stress<1.10×YS were deemed to have failed, and are shown as comparative examples in Table 9. In Table 9, a "critical load stress / YS" ​​ratio of 1.10 or more means that critical load stress≧1.10×YS. As shown in Table 9, the members of the inventive examples have high strength and excellent delayed fracture resistance. [Explanation of symbols]

[0133] 10 High-strength components 11 Steel plate 12 Bending ridge 13 End face of bent ridge 20 volts 21 Nut 22 Tapered washer C1 center of plate thickness D1 Bending ridge direction D2 Measuring direction

Claims

1. A high-strength member having a bending ridge line portion obtained by using a steel plate, The steel plate comprises, in mass%, C: 0.17% or more and 0.35% or less, Si: 0.001% or more and 1.2% or less, Mn: 0.9% or more and 3.2% or less, P: 0.020% or less, Al: 0.010% or more and 0.20% or less, and N: 0.010% or less, The steel plate has a microstructure in which the area ratio of one or both of bainite containing carbides having an average grain size of 50 nm or less and martensite containing carbides having an average grain size of 50 nm or less is 90% or more in total, The tensile strength of the member is 1470 MPa or more, The residual stress at the end surface of the bent ridgeline is 300 MPa or less, and The Vickers hardness (HV) of the end surface of the bending ridge line portion is 200 or more and 450 or less, A high-strength component for an automobile structural member, in which the critical load stress of the component is 1.10 times or more the yield strength when immersed in hydrochloric acid of pH = 1 (25°C) and the maximum load stress at which delayed fracture does not occur is measured as the critical load stress.

2. A high-strength member having a bending ridge line portion obtained by using a steel plate, The steel plate comprises, in mass%, C: 0.17% or more and 0.35% or less, Si: 0.001% or more and 1.2% or less, Mn: 0.9% or more and 3.2% or less, P: 0.020% or less, Al: 0.010% or more and 0.20% or less, N: 0.010% or less, and Sb: 0.001% or more and 0.10% or less, The steel plate has a microstructure in which the area ratio of one or both of bainite containing carbides having an average grain size of 50 nm or less and martensite containing carbides having an average grain size of 50 nm or less is 90% or more in total, The tensile strength of the member is 1470 MPa or more, The residual stress at the end surface of the bent ridgeline is 300 MPa or less, and The Vickers hardness (HV) of the end surface of the bending ridge line portion is 200 or more and 450 or less, A high-strength component for an automobile structural member, in which the critical load stress of the component is 1.10 times or more the yield strength when immersed in hydrochloric acid of pH = 1 (25°C) and the maximum load stress at which delayed fracture does not occur is measured as the critical load stress.

3. The composition of the steel plate further comprises, in mass%, 3. The high-strength member for automobile structural members according to claim 1 or 2, containing B: 0.0002% or more and less than 0.0035%.

4. The composition of the steel plate further comprises, in mass%, Nb: 0.002% or more and 0.08% or less; The high-strength member for automobile structural members according to any one of claims 1 to 3, containing at least one element selected from the group consisting of Ti: 0.002% or more and 0.12% or less.

5. The composition of the steel plate further comprises, in mass%, Cu: 0.005% or more and 1% or less; The high-strength member for automobile structural members according to any one of claims 1 to 4, containing at least one selected from Ni: 0.005% to 1%.

6. The composition of the steel plate further comprises, in mass%, Cr: 0.01% or more and 1.0% or less, Mo: 0.01% or more and less than 0.3% V: 0.003% or more and 0.5% or less, Zr: 0.005% or more and 0.20% or less, and The high-strength member for automobile structural members according to any one of claims 1 to 5, containing at least one selected from W: 0.005% or more and 0.20% or less.

7. The composition of the steel plate further comprises, in mass%, Ca: 0.0002% or more and 0.0030% or less, Ce: 0.0002% or more and 0.0030% or less, La: 0.0002% or more and 0.0030% or less, and The high-strength member for automobile structural members according to any one of claims 1 to 6, containing at least one selected from Mg: 0.0002% or more and 0.0030% or less.

8. The composition of the steel plate further comprises, in mass%, The high-strength member for automobile structural members according to any one of claims 1 to 7, containing Sn: 0.002% or more and 0.1% or less.

9. A method for manufacturing a high-strength component for an automobile structural member having a bend ridge line portion obtained by using a steel plate, comprising: A bending process step of cutting out the steel plate according to any one of claims 1 to 8 and bending the steel plate; and an end surface treatment step of heating the end surface generated by cutting after the bending process at a temperature of 400°C or higher and 900°C or lower for a period of more than 0 seconds and not more than 10 seconds, The tensile strength of the member is 1470 MPa or more, The residual stress at the end surface of the bent ridgeline is 300 MPa or less, and The Vickers hardness (HV) of the end surface of the bending ridge line portion is 200 or more and 450 or less, A method for manufacturing a high-strength component for an automobile structural member, in which the critical load stress of the component is 1.10 times or more the yield strength when the component is immersed in hydrochloric acid of pH = 1 (25°C) and the maximum load stress at which delayed fracture does not occur is measured as the critical load stress.

10. A method for manufacturing a high-strength component for an automobile structural member having a bend ridge line portion obtained by using a steel plate, comprising: An end surface treatment process in which the steel plate according to any one of claims 1 to 8 is cut out and the end surface generated by cutting is heated at a temperature of 400 ° C. or higher and 900 ° C. or lower for more than 0 seconds and 10 seconds or less; A bending process is performed on the steel plate after the end face treatment process, The tensile strength of the member is 1470 MPa or more, The residual stress at the end surface of the bent ridgeline is 300 MPa or less, and The Vickers hardness (HV) of the end surface of the bending ridge line portion is 200 or more and 450 or less, A method for manufacturing a high-strength component for an automobile structural member, in which the critical load stress of the component is 1.10 times or more the yield strength when the component is immersed in hydrochloric acid of pH = 1 (25°C) and the maximum load stress at which delayed fracture does not occur is measured as the critical load stress.

11. The method for producing a steel plate according to any one of claims 1 to 8, which is used for a high-strength member obtained by the method for producing a high-strength member for an automobile structural member according to claim 9 or 10, a hot rolling process for hot rolling a steel material; a cold rolling step of cold rolling the hot-rolled steel sheet obtained by the hot rolling; The cold-rolled steel sheet obtained by the cold rolling is Ｃ３ and an annealing step of heating the steel sheet to an annealing temperature of at least 500°C, cooling the steel sheet at an average cooling rate of at least 3°C / sec in a temperature range from the annealing temperature to 550°C and to a cooling stop temperature of at most 350°C, and then holding the steel sheet in a temperature range of at least 100°C and at most 260°C for at least 20 seconds and at most 1,500 seconds.

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