High-strength member and method for manufacturing high-strength member

Abstract

An object of the present invention is 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. [Solution] The high-strength member 10 of the present invention is a high-strength member 10 having a bending ridge portion 12 obtained using a steel plate 11, wherein the tensile strength of the member is 1470 MPa or more, the residual stress at the end face 13 of the bending ridge portion 12 is 800 MPa or less, and the longest crack length among those extending from the end face 13 of the bending ridge portion 12 in the bending ridge direction D1 is 10 μm or less.

Description

Technical Field

[0001] The present invention relates to a high-strength member used for automotive parts and the like, a method for manufacturing the high-strength member, and a method for manufacturing a steel sheet for the high-strength member. More specifically, the present invention relates to a high-strength member having excellent stress corrosion cracking resistance and a method for manufacturing the same. Further, the present invention relates to a method for manufacturing a steel sheet for the high-strength member.

Background Art

[0002] In recent years, the application of high-strength steel sheets with a tensile strength (TS) of 1320 to 1470 MPa has been progressing for body frame parts such as center pillar R / F (reinforcement) and parts such as bumpers and impact beam parts (hereinafter also referred to as parts). Furthermore, from the viewpoint of further weight reduction of the automobile body, the application of steel sheets having a strength of TS of 1800 MPa (1.8 GPa) or more to parts has also been studied.

[0003] With the increase in the strength of steel sheets, there is a concern about the occurrence of stress corrosion cracking. In recent years, stress corrosion cracking from the sheared end face of a sample processed into a part shape, particularly a bent portion where strain is concentrated, has been a concern, and it has become important to suppress stress corrosion cracking starting from such a sheared end face.

[0004] For example, in Patent Document 1, a steel sheet excellent in stress corrosion cracking resistance after forming is provided by specifying the particle size and density of oxides, sulfides, composite precipitates, and composite precipitates of Mg, the chemical components of which satisfy 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, and the balance being Fe and unavoidable impurities.

[0005] In Patent Document 2, a method for manufacturing a formed member excellent in stress corrosion cracking resistance is provided by subjecting the sheared end face of a steel sheet having a TS of 1180 MPa or more to shot peening to reduce the residual stress on the end face.

Prior Art Documents

Patent Documents

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

[0007] The technique disclosed in Patent Document 1 provides a steel plate excellent in stress corrosion cracking resistance by defining the chemical components and the particle size and density of precipitates in steel. However, since the amount of C added to the steel plate of Patent Document 1 is small, its strength is lower than that of the steel plate used for the high-strength member of the present invention, and the TS is less than 1470 MPa. Even if the strength is improved by increasing the amount of C in the steel plate of Patent Document 1, as the strength increases, the residual stress on the end face also increases, so the stress corrosion cracking resistance is considered to deteriorate.

[0008] The technique disclosed in Patent Document 2 provides a formed member excellent in stress corrosion cracking resistance by performing shot peening on the sheared end face to reduce the residual stress on the end face. However, stress corrosion cracking still occurs even at the residual stress on the end face of 800 MPa or less defined in the present invention, which is considered to be because the crack length on the end face is longer than the length defined in the present invention. Even if shot peening is performed, if the sheared end face remains as it is, the crack generated by shearing becomes more than 10 μm, and the effect of improving the stress corrosion cracking resistance is insufficient.

[0009] The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength member excellent in stress corrosion cracking resistance, a method for manufacturing the high-strength member, and a method for manufacturing a steel plate for the high-strength member. In the present invention, high strength means that the tensile strength (TS) is 1470 MPa or more.

[0010] In the present invention, excellent delayed fracture resistance means that, as described in the examples, when a member after bending a steel plate is 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 equal to or higher than the yield strength (YS).

Means for Solving the Problems

[0011] As a result of intensive studies to solve the above problems, the present inventors have found that a high-strength member having a bent ridge line portion obtained using a steel plate can be made into a high-strength member excellent in delayed fracture resistance by setting the tensile strength of the member to 1470 MPa or more, the residual stress at the end face of the bent ridge line portion to 800 MPa or less, and the length of the longest crack among the cracks extending in the bent ridge line direction from the end face of the bent ridge line portion to 10 μm or less, and thus the present invention has been achieved. The above problems are solved by the following means.

[0012] [1] A high-strength member having a bent ridge line portion obtained using a steel plate, wherein the tensile strength of the member is 1470 MPa or more, the residual stress at the end face of the bent ridge line portion is 800 MPa or less, and the length of the longest crack among the cracks extending in the bent ridge line direction from the end face of the bent ridge line portion is 10 μm or less, a high-strength member.

[0013] [2] The steel plate has, 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: 0.02% or less, S: 0.001% or less, Al: 0.01% or more and 0.2% or less, and N: 0.010% or less, and the balance consists of Fe and unavoidable impurities, and The high-strength member according to [1], having a microstructure in which the 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 is 90% or more with respect to the entire steel sheet structure.

[0014] [3] The steel sheet contains, by 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: 0.02% or less, S: 0.001% or less, Al: 0.01% or more and 0.2% or less, N: 0.010% or less, and Sb: 0.001% or more and 0.1% or less, and the balance consists of Fe and inevitable impurities, and The high-strength member according to [1], having a microstructure in which the 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 is 90% or more with respect to the entire steel sheet structure.

[0015] [4] The component composition of the steel sheet further contains, by mass%, B: 0.0002% or more and less than 0.0035%, the high-strength member according to [2] or [3].

[0016] [5] The component composition of the steel sheet further contains, by mass%, Nb: 0.002% or more and 0.08% or less and Ti: at least one selected from 0.002% or more and 0.12% or less, the high-strength member according to any one of [2] to [4].

[0017] [6] The component composition of the steel sheet further contains, by mass%, Cu: 0.005% or more and 1% or less and The high-strength member according to any one of [2] to [5], containing at least one selected from among those having Ni of 0.005% or more and 1% or less.

[0018] [7] The component composition of the steel sheet 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 selected from among those having W of 0.005% or more and 0.20% or less.

[0019] [8] The component composition of the steel sheet 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 among those having Mg of 0.0002% or more and 0.0030% or less.

[0020] [9] The component composition of the steel sheet further comprises, in mass%, The high-strength member according to any one of [2] to [8], containing Sn of 0.002% or more and 0.1% or less.

[0021]

[10] A method for manufacturing a high-strength member, which has an end face treatment step of cutting out a steel sheet having a tensile strength of 1470 MPa or more, machining the end face generated by cutting before or after bending, and heating at a temperature of 270 °C or less after the bending and the machining.

[0022] A method for manufacturing a high-strength member, comprising: after cutting out a steel sheet according to any one of

[11] [2] to [9], machining the end face generated by cutting before or after bending, and heating at a temperature of 270°C or lower after the bending and the machining of the end face.

[0023]

[12] A method for manufacturing a steel sheet for a high-strength member for manufacturing a high-strength member according to any one of [2] to [9], comprising: a step of subjecting the steel having the above composition to hot rolling and cold rolling; a step of annealing: heating the cold-rolled steel sheet obtained by the cold rolling to an annealing temperature equal to or higher than the AC3 point, then cooling at an average cooling rate in the temperature range from the annealing temperature to 550°C of 3°C / second or more and a cooling stop temperature of 350°C or lower, and then holding for 20 seconds or more and 1500 seconds or less in a temperature range of 100°C or higher and 260°C or lower. A method for manufacturing a steel sheet for a high-strength member.

Advantages of the Invention

[0024] According to the present invention, it is possible to provide a high-strength member excellent in hydrogen embrittlement resistance, a method for manufacturing a high-strength member, and a method for manufacturing a steel sheet for a high-strength member. Further, 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 hydrogen embrittlement resistance of the steel sheet for automobiles. That is, the performance of the automobile body is improved by the present invention.

Brief Description of the Drawings

[0025]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0026] Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the following embodiments.

[0027] The high-strength member of 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 800 MPa or less, and among the cracks extending in the bent ridge line direction from the end face of the bent ridge line portion, the length of the longest crack is 10 μm or less.

[0028] If a high-strength member satisfying these conditions can be obtained, the steel sheet used for the high-strength member is not particularly limited. Hereinafter, preferred steel sheets 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 sheets described below.

[0029] A preferred steel sheet for obtaining a high-strength member preferably has a component structure and a microstructure, which will be described later. Note that if the high-strength member of the present invention can be obtained, it is not necessarily required to use a steel sheet having the component composition and microstructure described later.

[0030] First, the preferred component composition of a preferred steel sheet (material steel sheet) used for the high-strength member will be described. In the following description of the preferred component composition, the unit “%” of the content of the component means “mass %”.

[0031] <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 still more preferably 0.19% or more. On the other hand, when the C content exceeds 0.35%, even if the end face (plate thickness face) is machined by facing before or after bending and heated after bending, the residual stress at the end face of the bending ridge line portion exceeds 800 MPa, which may deteriorate the stress corrosion cracking resistance characteristics. Therefore, the C content is preferably 0.35% or less, more preferably 0.33% or less, and still more preferably 0.31% or less.

[0032] <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 the improvement of elongation by suppressing the excessive formation of coarse carbides. Furthermore, it also contributes to the reduction of Mn segregation at the center of the plate thickness and the suppression of the formation of MnS. To sufficiently obtain the above effects, the Si content is preferably 0.001% or more, more preferably 0.003% or more, and still more preferably 0.005% or more. On the other hand, if the Si content becomes too high, coarse MnS is likely to be generated in the plate thickness direction, promoting crack generation during bending and deteriorating the stress corrosion cracking resistance characteristics. Therefore, the Si content is preferably 1.2% or less, more preferably 1.1% or less, and still more preferably 1.0% or less.

[0033] <Mn: 0.9% or more and 3.2% or less> Mn is added 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 plate, which may reduce the strength. Therefore, the Mn content is preferably 0.9% or more, more preferably 1.0% or more, and even more preferably 1.1% or more. Also, in order to prevent an increase in MnS and promote crack generation during bending, the Mn content is preferably 3.2% or less, more preferably 3.1% or less, and even more preferably 3.0% or less.

[0034] <P: 0.02% or less> P is an element that strengthens the steel, but if its content is high, it promotes crack generation and deteriorates the stress corrosion cracking resistance. Therefore, the P content is preferably 0.02% or less, more preferably 0.015% or less, and even more preferably 0.01% or less. Note that the lower limit of the P content is not particularly limited, but currently, the industrially feasible lower limit is about 0.003%.

[0035] <S: 0.001% or less> S forms inclusions such as MnS, TiS, and Ti(C,S). In order to suppress crack generation due to these inclusions, the S content is preferably 0.001% or less. The S content is more preferably 0.0009% or less, even more preferably 0.0007% or less, and particularly preferably 0.0005% or less. Note that the lower limit of the S content is not particularly limited, but currently, the industrially feasible lower limit is about 0.0002%.

[0036] <Al: 0.01% or more and 0.2% or less> Al is added to sufficiently deoxidize and reduce coarse inclusions in the steel. To obtain its effect, the Al content is preferably 0.01% or more, more preferably 0.015% or more. On the other hand, when the Al content exceeds 0.2%, carbides mainly composed of Fe such as cementite generated during coiling after hot rolling are less likely to dissolve in the annealing process, and there is a possibility of generating coarse inclusions and carbides, which may promote crack generation and deteriorate the stress corrosion cracking resistance. Therefore, the Al content is preferably 0.2% or less, more preferably 0.17% or less, and even more preferably 0.15% or less.

[0037] <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 promotes crack generation through the formation of these. To prevent deterioration of the stress corrosion cracking resistance, 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%.

[0038] <Sb: 0.001% or more and 0.1% 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 high strength. Furthermore, the improvement of the delayed fracture resistance property is also achieved by suppressing decarburization. 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 Sb is contained in an amount exceeding 0.1%, it segregates at the prior austenite (γ) grain boundary and promotes crack generation, so that the delayed fracture resistance property may be deteriorated. Therefore, the Sb content is preferably 0.1% or less, more preferably 0.08% or less, and still more preferably 0.06% or less. Although it is preferable to contain Sb, when the effects of high strength and improvement of the delayed fracture resistance property of the steel sheet can be sufficiently obtained without containing Sb, Sb may not be contained.

[0039] The steel preferably used for the high-strength member of the present invention preferably contains the above components basically, and the balance is iron and inevitable impurities, but the following allowable components (optional elements) can be contained within a range not impairing the action of the present invention.

[0040] <B: 0.0002% or more and less than 0.0035%> B is an element that improves the hardenability of steel, and has an advantage of generating martensite and bainite with a predetermined area ratio even when the Mn content is small. To obtain such an effect of B, the B content is preferably 0.0002% or more, more preferably 0.0005% or more, and still 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 solution 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, promoting crack generation and deteriorating the delayed fracture resistance property. Therefore, the B content is preferably less than 0.0035%, more preferably 0.0030% or less, and still more preferably 0.0025% or less.

[0041] <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 increasing the strength by refining the prior austenite (γ) grains. From this perspective, the Nb content and the Ti content are each preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more. On the other hand, when a large amount of Nb or Ti is contained, coarse Nb-based precipitates such as NbN, Nb(C,N), (Nb,Ti)(C,N), etc. and coarse Ti-based precipitates such as TiN, Ti(C,N), Ti(C,S), TiS, etc. that remain undissolved during slab heating in the hot rolling process increase, promoting crack generation and deteriorating the stress corrosion cracking resistance. Therefore, the Nb content is preferably 0.08% or less, more preferably 0.06% or less, and still more preferably 0.04% or less. Also, the Ti content is preferably 0.12% or less, more preferably 0.10% or less, and still more preferably 0.08% or less.

[0042] <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 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, it is preferable to contain Cu and Ni at 0.005% or more, more preferably 0.008% or more. However, if too much Cu or Ni is present, it will cause the generation of surface defects and deteriorate the plating property and chemical conversion treatment property. Therefore, the Cu content and the Ni content are each preferably 1% or less, more preferably 0.8% or less, and still more preferably 0.6% or less.

[0043] <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 each preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more. 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 these elements is present in excessive amounts, coarsening of carbides will promote crack generation and deteriorate the stress corrosion cracking resistance. Therefore, the Cr content is preferably 1.0% or less, more preferably 0.4% or less, and even more preferably 0.2% or less. The Mo content is preferably less than 0.3%, more preferably 0.2% or less, and even more preferably 0.1% or less. The V content is preferably 0.5% or less, more preferably 0.4% or less, and even more preferably 0.3% or less.

[0044] Zr and W contribute to increasing the strength by refining the 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 even more preferably 0.007% or more. However, if a large amount of Zr or W is contained, coarse precipitates remaining undissolved during slab heating in the hot rolling process will increase, promoting crack generation and deteriorating the stress corrosion cracking resistance. For this reason, the Zr content and the W content are each preferably 0.20% or less, more preferably 0.15% or less, and even more preferably 0.10% or less.

[0045] <At least one selected from among 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 stress corrosion cracking resistance by fixing S as sulfide. Therefore, the content of each of these elements is preferably 0.0002% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more. On the other hand, if a large amount of these elements is added, coarsening of sulfide will promote crack generation and deteriorate the stress corrosion cracking resistance. Therefore, the content of each of these elements is preferably 0.0030% or less, more preferably 0.0020% or less, and even more preferably 0.0010% or less.

[0046] Mg fixes O as MgO and becomes a trap site for hydrogen in steel, thus contributing to the improvement of the stress corrosion cracking resistance. Therefore, the Mg content is preferably 0.0002% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more. On the other hand, if a large amount of Mg is added, coarsening of MgO will promote crack generation and deteriorate the stress corrosion cracking resistance, so the Mg content is preferably 0.0030% or less, more preferably 0.0020% or less, and even more preferably 0.0010% or less.

[0047] <Sn: 0.002% or more and 0.1% or less> Sn suppresses oxidation and nitridation of the steel sheet surface layer, and suppresses decarburization due to oxidation and nitridation of the steel sheet surface layer. By suppressing decarburization, ferrite formation in the steel sheet surface layer 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 even more preferably 0.004% or more. On the other hand, if Sn is contained in an amount exceeding 0.1%, it segregates at the prior austenite (γ) grain boundary and promotes crack generation, thus deteriorating the stress corrosion cracking resistance. Therefore, the Sn content is preferably 0.1% or less, more preferably 0.08% or less, and even more preferably 0.06% or less.

[0048] Next, the preferred microstructure of the preferred steel sheet used in the high-strength member of the present invention will be described.

[0049] <For the entire steel plate structure, 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> 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 plate structure. When it is less than 90%, ferrite increases and the strength decreases. Note that the total area ratio with respect to the entire structure of martensite and bainite may be 100%. Also, the area ratio of either one of martensite and bainite may be within the above range, or the total area ratio of both may be within the above range. Further, from the viewpoint of increasing the strength, the above area ratio is more preferably 91% or more, still more preferably 92% or more, and particularly preferably 93% or more.

[0050] The martensite is the sum of as-quenched martensite and tempered martensite after tempering. In the present invention, martensite refers to a hard structure formed from austenite at a low temperature (below the martensite 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 a relatively low temperature (above the martensite transformation point), in which fine carbides are dispersed in acicular or plate-like ferrite.

[0051] Note that the remaining structure other than martensite and bainite is ferrite, pearlite, and retained austenite, and the total amount thereof may be 10% or less. It may be 0%.

[0052] In the present invention, ferrite is a structure formed by transformation from austenite at a relatively high temperature and consists of crystal grains with a bcc lattice. Pearlite is a structure in which ferrite and cementite are formed in layers. Retained austenite is austenite that did not undergo martensitic transformation because the martensitic transformation temperature was below room temperature.

[0053] The carbide with an average particle size of 50 nm or less as referred to in the present invention is the fine carbide that can be observed in bainite and martensite when observed by SEM. Specifically, for example, Fe carbide, Ti carbide, V carbide, Mo carbide, W carbide, Nb carbide, Zr carbide can be mentioned.

[0054] Note that the steel sheet may be provided with a plating layer such as a hot-dip galvanized layer. Examples of such a plating layer include an electroplated layer, an electroless plating layer, a hot-dip plating layer, etc. Furthermore, an alloyed plating layer may also be used.

[0055] Next, the high-strength member will be described.

[0056] [High-strength member] The high-strength member of 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 at the end face of the bent ridge line portion is 800 MPa or less, and the length of the longest crack among the cracks extending in the bent ridge line direction from the end face of the bent ridge line portion is 10 μm or less.

[0057] The high-strength member of the present invention is obtained using a steel sheet 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, for example, automobile parts.

[0058] The high-strength member of the present invention has a bent ridge line portion. The "bent ridge line portion" as referred to in the present invention refers to a region that is no longer flat by performing bending on a steel plate. An example of the high-strength member 10 shown in FIG. 1 is obtained by performing V-bending on a steel plate 11. The high-strength member 10 has a bent ridge line portion 12 on the side surface of the steel plate 11 at the bent portion. The end face 13 of the bent ridge line portion 12 is a plate thickness plane located on the side surface of the bent ridge line portion 12. The bent ridge line direction D1 as referred to in the present invention is a direction parallel to the bent ridge line portion 12.

[0059] If the residual stress on the end face of the bent ridge line portion is 800 MPa or less and the length of the longest crack among the cracks extending in the bent ridge line direction from the end face of the bent ridge line portion is 10 μm or less, the bending angle is not particularly limited.

[0060] Although an example of the high-strength member 10 shown in FIG. 1 shows an example where there is one bent portion, it is also possible to perform bending at two or more locations to have two or more bent ridge line portions.

[0061] <The tensile strength of the member is 1470 MPa or more> The tensile strength (TS) of the high-strength member is 1470 MPa or more. In order to make the tensile strength (TS) 1470 MPa or more, it is preferable to use the above steel plate. The tensile strength (TS) and yield strength (YS) in the present invention are calculated by measuring at a flat portion which is a non-bent portion of the high-strength member. Also, if the tensile strength (TS) and yield strength (YS) of the annealed steel plate (the steel plate after the annealing process) before bending are measured, these measured values can be regarded as the measured values of the tensile strength (TS) and yield strength (YS) of the high-strength member obtained using the annealed steel plate. The strength of the member can be calculated by the method described in the examples.

[0062] <The residual stress on the end face of the bent ridge line portion is 800 MPa or less> The residual stress on the end face (plate thickness face) of the bending ridge line portion of the high-strength member is 800 MPa or less. As a result, cracks are less likely to occur on the end face of the bending ridge line portion, so that a member excellent in stress corrosion cracking resistance can be obtained. From the viewpoint of suppressing crack generation due to stress corrosion cracking, the residual stress is 800 MPa or less, preferably 700 MPa or less, more preferably 600 MPa or less, still more preferably 400 MPa or less, and most preferably 200 MPa or less. The residual stress on the end face of the bending ridge line portion can be calculated by the method described in the examples of this specification.

[0063] <Of the cracks extending in the bending ridge line direction from the end face of the bending ridge line portion, the length of the longest crack is 10 μm or less> Of the cracks extending in the bending ridge line direction from the end face of the bending ridge line portion, the length of the longest crack (hereinafter, also simply referred to as crack length) is 10 μm or less. By shortening the crack length, large cracks are less likely to occur on the end face of the bending ridge line portion, so that a member excellent in stress corrosion cracking resistance can be obtained. From the viewpoint of suppressing stress corrosion cracking by shortening the crack length, the crack length is 10 μm or less, preferably 8 μm or less, more preferably 5 μm or less. The crack length can be calculated by the method described in the examples of this specification.

[0064] Next, an embodiment of the method for manufacturing the high-strength member of the present invention will be described. An example of the embodiment of the method for manufacturing the high-strength member of the present invention includes cutting out a steel plate having a tensile strength of 1470 MPa or more, facing the end face generated by cutting before or after bending, and heating at a temperature of 270 ° C or less after the bending and the facing, which is an end face treatment step.

[0065] Another example of the embodiment of the method for manufacturing the high-strength member of the present invention includes cutting out a steel plate having the above component composition and the above microstructure, facing the end face generated by cutting before or after bending, and heating at a temperature of 270 ° C or less after bending and facing, which is an end face treatment step.

[0066] Also, an example of an embodiment of the method for manufacturing a high-strength member steel sheet of the present invention includes a step of subjecting steel (steel material) having the above-described component composition to hot rolling and cold rolling, and cold-rolled steel sheet obtained by the cold rolling is heated to an annealing temperature of A C3 After heating to a temperature of not less than the point, cooling is performed such that the average cooling rate in the temperature range from the annealing temperature to 550 ° C is 3 ° C / second or more and the cooling stop temperature is 350 ° C or less, and then, in the temperature range of 100 ° C or more and 260 ° C or less, it has an annealing step of staying for 20 seconds or more and 1500 seconds or less. Hereinafter, these steps and a preferable casting step performed before the hot rolling step will be described. The temperatures shown below mean the surface temperatures of the slab, steel sheet, etc.

[0067] [Casting Step] Steel having the above-described component composition is cast. The casting speed is not particularly limited, but in order to suppress the generation of the above inclusions and improve the stress corrosion cracking 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. The lower limit is not particularly limited, but from the viewpoint of productivity, it is preferably 1.25 m / min or more, and more preferably 1.30 m / min or more.

[0068] [Hot Rolling Step] Steel (steel slab) having the above-described component composition is subjected to hot rolling. The slab heating temperature is not particularly limited, but by setting the slab heating temperature to 1200 ° C or more, the solid solution promotion of sulfides and the reduction of Mn segregation can be achieved, the reduction of the above-described amount of coarse inclusions can be achieved, and the stress corrosion cracking resistance tends to be improved. For this reason, the slab heating temperature is preferably 1200 ° C or more. More preferably, it is 1220 ° C or more. Also, the heating rate during slab heating is preferably 5 to 15 ° C / min, and the soaking time of the slab is preferably 30 to 100 minutes.

[0069] The finish rolling end temperature is preferably 840°C or higher. If the finish rolling end temperature is less than 840°C, it takes time for the temperature to drop, and the formation of inclusions not only deteriorates the SCC resistance characteristics but also may reduce the internal quality of the steel plate. Therefore, the finish rolling end temperature is preferably 840°C or higher, more preferably 860°C or higher. On the other hand, although the upper limit is not particularly limited, since cooling to the subsequent coiling temperature becomes difficult, the finish rolling end temperature is preferably 950°C or lower, more preferably 920°C or lower.

[0070] The cooled hot-rolled steel plate is preferably coiled at a temperature of 630°C or lower. If the coiling temperature exceeds 630°C, there is a risk that the surface of the subway will decarburize, and a tissue difference may occur between the inside and the surface of the steel plate, which may cause alloy concentration unevenness. In addition, due to the decarburization of the surface layer, the area ratio of bainite and martensite having carbides on the surface layer of the steel plate decreases, so it tends to be difficult to ensure the desired strength. Therefore, the coiling temperature is preferably 630°C or lower, more preferably 600°C or lower. Although the lower limit of the coiling temperature is not particularly limited, it is preferably 500°C or higher in order to prevent a decrease in cold rolling property.

[0071] [Cold rolling process] In the cold rolling process, the coiled hot-rolled steel plate is pickled and then cold-rolled to produce a cold-rolled steel plate. The pickling conditions are not particularly limited. When the reduction ratio is less than 20%, the surface flatness may be poor and the structure may be non-uniform. Therefore, the reduction ratio is preferably 20% or higher, more preferably 30% or higher, and still more preferably 40% or higher.

[0072] [Annealing process] The steel plate after cold rolling is heated to an annealing temperature of point A or higher. If the annealing temperature is less than point A, ferrite is generated in the structure and the desired strength cannot be obtained. Therefore, the annealing temperature is point A or higher, preferably point A + 10°C or higher, and more preferably point A + C3 point A C3 point less than, ferrite is generated in the structure and the desired strength cannot be obtained. Therefore, the annealing temperature is point A C3 point or higher, preferably C3 point A + 10°C or higher, more preferably C3The temperature is above +20°C. Although the upper limit of the annealing temperature is not particularly limited, from the viewpoint of suppressing the coarsening of austenite and preventing the deterioration of the delayed fracture resistance property, the annealing temperature is preferably 900°C or lower. Note that A C3 After heating to an annealing temperature above the A

[0073] A C3 point, soaking may be performed at the annealing temperature. A C3 point (°C) = 910 - 203√(%C) + 45(%Si) - 30(%Mn) - 20(%Cu) - 15(%Ni) + 11(%Cr) + 32(%Mo) + 104(%V) + 400(%Ti) + 460(%Al)

[0074] As described above, after heating the cold-rolled steel sheet to an annealing temperature above the A C3 point, cooling is performed such that the average cooling rate in the temperature range from the annealing temperature to 550°C is 3°C / second or more, and the cooling stop temperature is 350°C or lower. Then, it is retained in the temperature range of 100°C or higher and 260°C or lower for 20 seconds or more and 1500 seconds or less.

[0075] If the average cooling rate in the temperature range from the annealing temperature to 550°C is less than 3°C / second, excessive formation of ferrite occurs, making it difficult to obtain the desired strength. Also, the formation of ferrite on the surface makes it difficult to obtain the bainite and martensite fractions having carbides near the surface, deteriorating the delayed fracture resistance property. Therefore, the average cooling rate in the temperature range from the annealing temperature to 550°C is 3°C / second or more, preferably 5°C / second or more, and more preferably 10°C / second or more. Note that the upper limit of the average cooling rate is not particularly specified, but if it becomes too fast, non-uniformity of martensite transformation is likely to occur in the coil width direction, and there is a risk that the steel sheet may come into contact with the equipment due to shape deterioration. Therefore, from the viewpoint of obtaining the minimum shape, it is preferably 3000°C / s or lower.

[0076] 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)".

[0077] The cooling stop temperature is 350°C or lower. When the cooling stop temperature exceeds 350°C, tempering does not proceed sufficiently, and martensite as quenched without carbide and retained austenite are excessively generated in the final structure, and the amount of fine carbides on the steel plate surface decreases, resulting in deterioration of the delayed fracture resistance property. Therefore, in order to obtain excellent delayed fracture resistance property, the cooling stop temperature is 350°C or lower, preferably 300°C or lower, more preferably 250°C or lower.

[0078] The carbides distributed inside bainite are carbides generated during holding in the low temperature range after quenching. By serving as hydrogen trap sites, they can capture hydrogen and prevent deterioration of the delayed fracture resistance property. When the holding temperature is less than 100°C or the holding time is less than 20 seconds, bainite is not generated, and martensite as quenched without carbide is generated, so the amount of fine carbides on the steel plate surface decreases and the above effects cannot be obtained.

[0079] Also, when the holding temperature exceeds 260°C or the holding time exceeds 1500 seconds, decarburization occurs and coarse carbides are generated inside bainite, deteriorating the delayed fracture resistance property. Therefore, the holding temperature is 100°C or higher and 260°C or lower, and the holding time is 20 seconds or longer and 1500 seconds or shorter. Also, the holding temperature is preferably 130°C or higher and 240°C or lower, and the holding time is preferably 50 seconds or longer and 1000 seconds or shorter.

[0080] In addition, the hot-rolled steel plate after hot rolling may be subjected to heat treatment for tissue softening, or may be plated with Zn, Al, etc. on the steel plate surface. Also, temper rolling for shape adjustment may be performed after annealing cooling or after plating treatment.

[0081] [End face treatment process] One embodiment of the method for manufacturing a high-strength member of the present invention includes, after cutting out a steel plate, facing the end face generated by cutting before or after bending, and having an end face treatment step of heating at a temperature of 270°C or lower after the bending and the facing. The cutting referred to in the present invention means shear cutting (mechanical cutting), electrical cutting such as laser cutting and electrical discharge machining, and known cutting such as gas cutting.

[0082] By performing the end face treatment step, microcracks generated when the steel plate is cut out can be removed, residual stress can be reduced, cracks are less likely to occur on the end face of the bent ridge portion, and a member excellent in stress corrosion cracking resistance can be obtained. If the longest crack length among the cracks extending from the end face of the bent ridge portion in the bent ridge direction can be made 10 μm or less, the facing amount of the end face is not particularly limited, but in order to reduce the residual stress, it is preferably removed by 200 μm or more from the surface, and more preferably removed by 250 μm or more. Also, the method for facing the end face is not particularly limited, and for example, any of laser, grinding, and coining treatment methods may be used. The bending and the facing of the end face may be performed in either order, the end face may be faced after bending, or the bending may be performed after facing the end face.

[0083] In order to reduce the residual stress of the end face, the formed member after the bending and the facing of the steel plate is heated at a temperature of 270°C or lower. When the heating temperature exceeds 270°C, tempering of the martensite structure progresses, making it difficult to obtain the desired TS. Therefore, the heating temperature is 270°C or lower, preferably 250°C or lower. Also, if the residual stress of the end face of the bent ridge portion can be made 800 MPa or less, the lower limit of the heating temperature and the heating time are not particularly limited. Note that the heating at a temperature of 270°C or lower may be substituted by heating in paint baking.

[0084] Also, this heating may heat at least the faced end face portion, or the entire steel plate may be heated.

Example

[0085] The present invention will be specifically described with reference to examples, but the present invention is not limited thereto.

[0086] 1. Manufacture of members for evaluation Steel having the component composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in a vacuum melting furnace and then slab-rolled to obtain a slab-rolled material with a thickness of 27 mm. The obtained slab-rolled material was hot-rolled to a thickness of 4.0 to 2.8 mm to manufacture a hot-rolled steel sheet. Next, the hot-rolled steel sheet was cold-rolled to a thickness of 1.4 mm to manufacture a cold-rolled steel sheet. Next, the cold-rolled steel sheet obtained as described above was heat-treated under the conditions shown in Tables 2 to 4 (annealing process). Note that the blanks in the component composition of Table 1 indicate that the components are not intentionally added, including not only the case where they are not contained (0% by mass) but also the case where they are inevitably contained. The details of the conditions for each of the hot-rolling process, cold-rolling process, and annealing process are shown in Tables 2 to 4.

[0087] The heat-treated steel sheet was sheared into small pieces of 30 mm × 110 mm. In some samples, the end faces generated by shearing were machined by laser or grinding before bending. Next, a sample of the steel sheet was placed on a die having an angle of 90°, and the steel sheet was pressed with a punch having an angle of 90° to perform V-bending. Next, as shown in the side view in Figure 2, using bolts 20, nuts 21, and tapered washers 22, the bent steel sheet (member) was tightened with bolts 20 from both sides of the plate surface of the steel sheet 11. By CAE (Computer Aided Engineering) analysis, the relationship between the load stress and the tightening amount was calculated so that the tightening amount and the critical load stress were made to coincide. The critical load stress was measured by the method described later.

[0088] A part of the samples whose end faces were not machined before bending was, after bending, tightened with bolts 20 as shown in Figure 2 above with tightening amounts corresponding to various critical load stresses, and then the end faces were removed (machined) by laser or grinding.

[0089] After bending and facing machining, some samples were heat-treated at various heating temperatures. Each condition of the end face treatment is shown in Tables 2 to 4. In the end face treatment of Tables 2 to 4, those with "-" described in the column of facing machining mean that facing machining was not performed, and those with "-" described in the column of heat treatment temperature (°C) mean that heat treatment was not performed.

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3]

[0093] [Table 4]

[0094] 2. Evaluation method For the members obtained under various manufacturing conditions, the tissue fraction was investigated by analyzing the steel structure (microstructure), the tensile properties such as tensile strength were evaluated by conducting a tensile test, and the stress corrosion cracking resistance was evaluated by the critical load stress measured by a stress corrosion cracking test. The methods for each evaluation are as follows.

[0095] (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 with respect to the entire steel plate structure) A test piece was sampled from the annealed steel sheet (hereinafter referred to as the annealed steel sheet) in a direction perpendicular to it, the plate thickness L cross-section parallel to the rolling direction was mirror-polished, the structure was revealed with nital solution, and then observed using a scanning electron microscope. On an SEM image at a magnification of 1500 times, a 16 mm × 15 mm grid with an interval of 4.8 μm was placed on a region with an actual length of 82 μm × 57 μm, and the number of points on each phase was counted. By the point counting method, the area ratios 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, and the total area ratio of them was calculated. The area ratio was taken as the average value of three area ratios obtained from separate SEM images at a magnification of 1500 times. Martensite exhibits a white structure, and bainite has fine carbides precipitated inside the black structure. The average grain size of the carbides was calculated as follows. Also, the area ratio is the area ratio with respect to the entire observation range, and this was regarded as the area ratio with respect to the entire steel sheet structure.

[0096] (Average Grain Size of Carbides in Bainite and Martensite) A test piece was sampled from a direction perpendicular to the rolling direction of the annealed steel sheet, the plate thickness L cross-section parallel to the rolling direction was mirror-polished, the structure was revealed with nital solution, and then observed using a scanning electron microscope. The total area of the carbides on an SEM image at a magnification of 5000 times was measured by image analysis by binarization, and the area per carbide was calculated by averaging the total area. The equivalent circle diameter obtained from the area per carbide was taken as the average grain size.

[0097] (Tensile Test) A JIS No. 5 test piece with a gauge length of 50 mm, a gauge width of 25 mm, and a plate thickness of 1.4 mm was sampled from the rolling direction of the annealed steel sheet, and a tensile test was conducted in accordance with JIS Z2241 (2011) at a tensile speed of 10 mm / min to measure the tensile strength (TS) and the yield strength (YS).

[0098] (Measurement of Critical Load Stress) The critical load stress was measured by a delayed fracture test. Specifically, the members obtained under each manufacturing condition were immersed in hydrochloric acid at pH = 1 (25°C), and the maximum load stress at which delayed fracture did not occur was evaluated as the critical load stress. The determination of delayed fracture was performed visually and by observing an image magnified up to ×20 with a stereomicroscope. When no cracks occurred after 96 hours of immersion, it was regarded as no fracture. Here, the crack refers to a case where a crack with a crack length of 200 μm or more occurred.

[0099] (Measurement of residual stress on the end face) For the members obtained under each manufacturing condition, the residual stress on the end face was measured by X-ray diffraction. The measurement location of the residual stress was the center of the plate thickness on the end face of the bending ridge line portion, and the X-ray irradiation diameter was 150 μm. The measurement direction was perpendicular to the plate thickness direction and perpendicular to the bending ridge line direction. Fig. 3 is an enlarged view of the end face of the bending ridge line portion, and the center of the plate thickness C1 and the measurement direction D2 are respectively marked with symbols.

[0100] (Measurement of crack length on the end face) For the members obtained under each manufacturing condition, the length of the crack extending in the bending ridge line direction from the end face of the bending ridge line portion was measured by magnifying 50 times with a stereomicroscope. The longest crack length among the cracks extending in the bending ridge line direction from the end face of the bending ridge line portion is shown in Tables 5 to 7.

[0101] 3. Evaluation results The above evaluation results are shown in Tables 5 to 7.

[0102]

Table 5

[0103]

Table 6

[0104]

Table 7

[0105] In this embodiment, a member with TS ≥ 1470 MPa and a critical load stress ≥ YS is regarded as qualified and shown as an inventive example in Tables 5 to 7. Also, a member with TS < 1470 MPa or a critical load stress < YS is regarded as unqualified and shown as a comparative example in Tables 5 to 7. In Tables 5 to 7, "critical load stress / YS" being 1.00 or more means that the critical load stress ≥ YS.

[0106] As shown in Tables 5 to 7, the members of the inventive examples are high-strength and excellent in stress corrosion cracking resistance.

Explanation of Reference Signs

[0107] 10 High-strength member 11 Steel plate 12 Bent ridge line part 13 End face of bent ridge line part 20 Bolt 21 Nut 22 Taper washer C1 Plate thickness center D1 Bent ridge line direction D2 Measuring direction

Claims

1. A high-strength member having a bent ridge line portion obtained using a steel plate, 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 800 MPa or less, and the length of the longest crack among the cracks extending in the bent ridge line direction from the end face of the bent ridge line portion is 10 μm or less, (critical load stress) / (yield strength) is 1.00 or more, the steel plate has, 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: 0.02% or less, Al: 0.01% or more and 0.2% or less, and N: 0.010% or less and has a component composition containing these, a high-strength member for an automobile structural member.

2. A microstructure having an area ratio of a total of 90% or more of one or two 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 with respect to the entire steel plate structure, the high-strength member for an automobile structural member according to Claim 1.

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

4. The component composition of the steel plate further contains, in mass%, Nb: 0.002% or more and 0.08% or less and Ti: at least one selected from 0.002% or more and 0.12% or less, the high-strength member for an automobile structural member according to any one of Claims 1 to 3.

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

6. The component composition of the steel plate further contains, 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 W: at least one selected from 0.005% or more and 0.20% or less, the high-strength member for an automobile structural member according to any one of Claims 1 to 5.

7. The component composition of the steel plate further contains, 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 an automotive structural member according to any one of claims 1 to 6, containing at least one selected from among Mg: 0.0002% or more and 0.0030% or less.

8. The component composition of the steel sheet further comprises, in mass%, The high-strength member for an automotive structural member 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 member for an automotive structural member, the high-strength member being according to any one of claims 1 to 8, the method comprising: a step of subjecting the steel to hot rolling and cold rolling; The cold-rolled steel sheet obtained by the cold rolling is heated to an annealing temperature of A or higher, and then cooled such that the average cooling rate in the temperature range from the annealing temperature to 550°C is 3°C / second or higher and the cooling stop temperature is 350°C or lower. After that, the steel sheet is manufactured by a process having an annealing step of retaining the steel sheet in the temperature range of 100°C or higher and 260°C or lower for 20 seconds or longer and 1500 seconds or shorter. Ｃ３ ​ a step of manufacturing a high-strength member by a process having an end face treatment step of, after cutting out the steel sheet, machining the end face generated by cutting before or after bending, and heating at a temperature of 270°C or lower after the bending and the machining of the end face.

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