High-tensile steel material for resistance welding, joint structure, and method for manufacturing joint structure

Abstract

To provide a high-tension steel material for resistance welding that can expand a proper range of a welding condition when manufacturing a joint structure by resistance-welding a member to be joined including a high-tension steel material with a tensile strength of 1700 MPa or more, and to provide a joint structure using the same and a method for manufacturing a joint structure.SOLUTION: A high-tension steel material for resistance welding is a steel sheet or a member obtained by processing a steel sheet and has a tensile strength of 1700 MPa or more and a nano-hardness of 5.0 GPa or less in a surface layer part to a depth of 100 μm from at least one surface thereof. The present invention also discloses a joint structure using the same and a method for manufacturing a joint structure.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present disclosure relates to a high-tensile steel material for resistance welding, a joint structure, and a method for manufacturing the joint structure. [Background technology]

[0002] In response to the need for even higher strength steel in the automotive field, the application of materials with high carbon and other alloying additives is progressing.

[0003] For example, Patent Document 1 discloses a joined structure that suppresses cracks at electrode indentations and the like when spot welding a sheet assembly including steel sheets having a zinc-plated layer, in which at least one steel sheet is a high-tensile steel sheet having a chemical composition such that the carbon equivalent Ceq, defined as C + Si / 24 + Mn / 6, is 0.53% or more and a tensile strength of 590 MPa or more, and the high-tensile steel sheet has a decarburized layer with a thickness of 5 μm or more and 200 μm or less between the base material and a zinc-based plating layer formed on at least one of the surfaces of the overlapping surface side and the welding electrode side, or on the overlapping surface adjacent to the zinc-based plating layer of the overlapping zinc-plated steel sheets. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2020 / 105325 Summary of the Invention [Problem to be solved by the invention]

[0005] In particular, joint strength in structures where multiple high-tensile steel plates are spot-welded tends to be low, which is an issue when using high-tensile steel. To solve this issue, welding methods aimed at increasing the nugget diameter at the weld or improving the weld quality are being investigated. Ultra-high tensile steel, which has a tensile strength of 1500 MPa or more, has high steel sheet strength, and when joining sheet assemblies including ultra-high tensile steel by spot welding, the electrode pressure must be higher than when spot welding sheet assemblies made of ordinary high tensile steel, for example, high tensile steel with a tensile strength of less than 1500 MPa. Furthermore, when spot welding using high-tensile steel with a tensile strength of over 2000 MPa, the appropriate current range for ensuring a certain nugget diameter or more is narrowed even when high pressure is applied. For example, when spot welding using ultra-high-tensile steel with a tensile strength of 2200 MPa, in order to ensure a nugget diameter in the appropriate range of 4√t or more (t: the thinner of the thicknesses of the two steel sheets that make up the sheet interface), it is necessary to increase the electrode pressure and change the current conditions.

[0006] The present disclosure aims to provide a high-tensile steel material for resistance welding that can expand the appropriate range of welding conditions when manufacturing a joined structure by resistance welding workpieces including high-tensile steel material with a tensile strength of 1700 MPa or more, as well as a joined structure using the same and a method for manufacturing the joined structure. [Means for solving the problem]

[0007] The gist of the present disclosure to achieve the above object is as follows. <1> A high-tensile steel material for resistance welding, which is a steel plate or a processed steel plate member, and has a tensile strength of 1700 MPa or more and a nano-hardness of 5.0 GPa or less in the surface layer extending from at least one surface to a depth of 100 μm. <2> A joint structure including a joint formed by resistance welding multiple overlapping steel materials, wherein at least one of the multiple steel materials is a steel plate or a processed steel plate member, and is a high-tensile steel material having a tensile strength of 1700 MPa or more and a nano-hardness of 5.0 GPa or less in a surface layer extending from at least one surface to a depth of 100 μm. <3> A nugget is formed at the welded portion, and when the thickness of the thinner of the two steel materials constituting the plate interface at the welded portion is t, the nugget diameter of the nugget at the plate interface is 4√t or more. <2> The bonded structure according to claim 1. <4> <2> or <3> A method for manufacturing the bonded structure according to claim 1, A method for manufacturing a joined structure, comprising a resistance welding step of joining, by resistance welding, a joined member formed by overlapping a plurality of the high-tensile steel materials, or a joined member formed by overlapping at least one of the high-tensile steel materials and at least one steel material having a tensile strength of less than 1700 MPa. [Effects of the Invention]

[0008] According to the present disclosure, there are provided a high-tensile steel material for resistance welding that can expand the appropriate range of welding conditions when manufacturing a joined structure by resistance welding joined members including high-tensile steel material with a tensile strength of 1700 MPa or more, a joined structure using the same, and a method for manufacturing the joined structure. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing a current pattern in which spot welding is performed on a pair of high-tensile steel plates made of high-carbon hardened material. [Figure 2A] 10 is a graph showing the relationship between the set current value and the nugget diameter when spot welding is performed on a pair of high-tensile steel plates that are high-carbon hardened materials with different set current values. [Figure 2B] 1 is a graph showing the relationship between the set current value and nugget diameter in main current application when spot welding is performed on a pair of high-tensile steel plates made of high-carbon hardened material by adding an upslope or pre-current application and changing the set current value. [Figure 3] FIG. 2 is a schematic diagram illustrating the measurement positions of nano-hardness in the surface layer portion of the bonded structure. [Figure 4] 1 is a diagram schematically showing an example of a nugget and a heat-affected zone (HAZ) formed when resistance spot welding is performed on a sheet assembly in which two steel sheets are overlapped. FIG. [Figure 5]FIG. 2 is a schematic diagram showing an example of seam welding. [Figure 6] 10 is a graph showing the relationship between the set current value and the nugget diameter when spot welding is performed on a pair of high-tensile steel plates that have not been surface-softened by decarburization, with the set current value being changed. [Figure 7] 1 is a graph showing the relationship between the set current value and the nugget diameter when spot welding is performed on a pair of high-tensile steel plates whose surface layers have been softened by decarburization to a nano-hardness of 5.0 GPa or less, with the set current value changed. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment that is an example of the present disclosure will be described. In this disclosure, the "%" used to indicate the content of each element means "% by mass." In addition, in this disclosure, unless otherwise specified, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. In addition, when the numerical values ​​written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit. In the numerical ranges described in stages in this disclosure, the upper limit of a certain numerical range may be replaced with the upper limit of another numerical range described in stages or with a value shown in an Example. Also, in the numerical ranges described in stages in this disclosure, the lower limit of a certain numerical range may be replaced with the lower limit of another numerical range described in stages or with a value shown in an Example. Furthermore, the term "process" does not only refer to an independent process, but also includes processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0011] Prior to completing the invention of the high-tensile steel sheet for resistance welding according to the present disclosure, an investigation was conducted into the relationship between the welding conditions and the nugget diameter of the resulting nugget when spot welding a sheet assembly of high-tensile steel sheets. Specifically, a high-carbon, quenched high-tensile steel sheet (thickness: 1.6 mm) with a martensitic metal structure and chemical composition (unit: mass %, balance: Fe and impurities, Ceq = C + Si / 30 + Mn / 20 + 2P + 4S) shown in Table 1 was prepared. This high-tensile steel sheet was heated at 900 °C for 4 minutes in a gas furnace (air-fuel ratio: 0.85, reducing atmosphere) and then flat-pressed. The scale was then removed by shot blasting to prepare a sample. The elemental distribution in the surface layer was measured by high-frequency glow discharge optical emission spectroscopy (GDS), confirming that the surface layer was decarburized.

[0012] [Table 1]

[0013] The above samples were spot-welded as two identical overlapping plates. A single-phase AC servo-pressure spot welder (50 Hz) was used for welding, with a pressure of 500 to 800 kgf. The electrodes used were Cr-Cu electrodes with a tip diameter of 6 mm and a nominal diameter of 16 mm (sometimes referred to as "40R-16 mm" in this specification). Other welding conditions included a squeeze of 30 cycles, a current time of 18 cycles, and a hold time of 8 cycles, with an additional upslope of 5 cycles or a pre-current of 5 cycles and a cool time of 2 cycles. The welding conditions are shown in Table 2 and Figure 1 below.

[0014] [Table 2]

[0015] The current value was varied in 0.5kA increments from 5kA until expulsion occurred, and the nugget diameter was measured by cross-sectional observation.

[0016] Figures 2A and 2B show the relationship between the set current value and nugget diameter under each condition. Figure 2A shows the case where a single current was applied under condition A or B, while Figure 2B shows the case where an upslope current (condition C) or pre-current (condition D) was applied before the single current. In each figure, the open plots indicate the occurrence of expulsion. As shown in Fig. 2A, under single-current conditions A or B, expulsion occurred without the nugget diameter exceeding 5√t. On the other hand, as shown in Fig. 2B, when an upslope current (condition C) or pre-current (condition D) was applied, the nugget diameter sometimes exceeded 5√t before expulsion occurred, but this required special conditions compared to the usual conditions (e.g., an electrode tip diameter of 8 mm and a pressure of 800 kgf).

[0017] Additionally, cross sections were observed for welds containing the largest nugget before expulsion occurred under conditions A and B, and for welds containing a nugget with a nugget diameter of around 5√t and a current of -0.5kA at which expulsion occurred under conditions C and D. Additionally, cross sections of joints were also observed when two sheets of 1.5GPa-class cold-rolled high-tensile steel (plate thickness 1.6mm) and one sheet of 1.2GPa-class cold-rolled high-tensile steel (plate thickness 1.6mm) were spot welded with a pressure of 400kgf and an electrode tip diameter of 6mm. In a pair of 1.5 GPa-class cold-formed high-tensile steel sheets, expulsion occurred when the nugget diameter exceeded 6.9 mm. On the other hand, in a pair of high-carbon quenched high-tensile steel sheets, expulsion occurred earlier, occurring at a nugget diameter of around 6 mm. The generation of flash occurs when the fusion boundary exceeds the pressure welded area made by the electrode. However, in the case of high-tensile steel sheets made of high-carbon quenched materials, the high strength of the steel sheet makes it difficult to deform, and it is thought that this is due to the fact that the pressure welded area is difficult to form.

[0018] As shown in Figure 2B, the current conditions are set to allow the sheets to blend together using an upslope current or pre-current, which is used when there is a sheet gap or other disturbance, and by increasing the pressure, the generation of spatter is suppressed and a nugget diameter of more than 5√t is obtained. However, when welding two 1.2 GPa-class steel plates (1.6 mm thick) at a pressure of 5.9 kN and an electrode tip diameter of 8 mm, a nugget diameter of 7.6 mm was obtained. In comparison, as shown in Figure 2B, even when upslope welding or pre-energization was performed on two high-carbon quenched steel plates, the largest nugget diameter was 7.05 mm, which is small.

[0019] The results of the above experiments and studies show that when spot welding sheet assemblies made of high-tensile steel sheets exceeding 1700 MPa, the range of appropriate spot welding is narrower than for cold-welded high-tensile steel sheets with a tensile strength lower than that, and special conditions are required.

[0020] Therefore, the inventors of the present disclosure conducted extensive research into a method for widening the range of suitability when resistance welding a sheet assembly of overlapping high-tensile steel sheets by spot welding, etc. As a result, they discovered that even if the high-tensile steel sheet has a strength of 1700 MPa or more, if the surface layer from the surface to a specific depth is softened, it becomes easier to form a pressure weld, and the range of suitability during resistance welding can be significantly widened.

[0021] [High tensile strength steel material for resistance joining] The high-tensile steel material for resistance welding according to the present disclosure (sometimes referred to in this specification as "high-tensile steel material" or simply as "steel material") is a steel plate or a member processed from a steel plate, and has a tensile strength of 1700 MPa or more, and a nano-hardness of 5.0 GPa or less in a surface layer extending from at least one surface to a depth of 100 μm. The high-tensile steel material for resistance joining according to the present disclosure may be a steel plate or a member obtained by processing a steel plate, and the shape of the high-tensile steel material is not limited as long as it can be joined by spot welding, seam welding, or the like, in which overlapping members to be joined are sandwiched between a pair of electrodes and pressure is applied while an electric current is passed through them to form a resistance heating weld. Examples include steel plate with a tensile strength of 1700 MPa or more, or a hot-stamped member in which a steel plate is formed into the shape of a part for an automobile or the like by hot stamping, and the portion to be joined is subjected to a high tension of 1700 MPa or more (in the present disclosure, high-tensile steel material for resistance joining in a shape other than a steel plate may be referred to as a "high-tensile member"). Hereinafter, as a representative example of the high-tensile steel material for resistance joining according to the present disclosure, a high-tensile steel plate for resistance joining (sometimes referred to in this specification as "high-tensile steel plate" or simply "steel plate") having a tensile strength of 1700 MPa or more and a nano-hardness of 5.0 GPa or less in a surface layer extending from at least one surface to a depth of 100 μm will be mainly described.

[0022] <Tensile strength> The high-tensile steel sheet according to the present disclosure has a tensile strength of 1700 MPa or more. This allows a high joint strength to be achieved when a joined structure is manufactured by spot welding using the high-tensile steel sheet according to the present disclosure, resulting in a high-strength joined structure. The tensile strength of the high-tensile steel plate according to the present disclosure is preferably 1900 MPa or more, more preferably 2000 MPa or more, and even more preferably 2200 MPa or more. The tensile strength of the high-tensile steel plate according to the present disclosure is a value measured by preparing a JIS No. 5 tensile test piece and conducting a tensile test in accordance with JIS Z 2241:2011.

[0023] <Surface hardness> The high-tensile steel sheet according to the present disclosure has a nano-hardness of 5.0 GPa or less in a surface layer portion extending from at least one surface to a depth of 100 μm. The nanohardness of the surface layer portion is measured using nanoindentation with a load of 10,000 μN. FIG. 3 is a diagram illustrating a method for measuring the nanohardness of the surface layer portion of a steel sheet. As shown in FIG. 3, nanohardness is measured in a cross section of the steel sheet in the thickness direction by striking points at a pitch of 10 μm in the thickness direction from the surface of the steel sheet and shifted by 20 μm in the plane direction. The high-tensile steel sheet according to the present disclosure has a nanohardness of 5.0 GPa or less at all 10 points measured in this manner from the steel sheet surface to a depth of 100 μm.

[0024] The high-tensile steel sheet according to the present disclosure may have a nano-hardness of 5.0 GPa or less in the surface layer from at least one surface (one side) to a depth of 100 μm, but in order to expand the appropriate range of electrode tip diameter, pressure, and current value in resistance welding, it is preferable that the nano-hardness in the surface layer on both sides of the steel sheet is 5.0 GPa or less.

[0025] From the viewpoint of suppressing the applied pressure from the electrodes when performing resistance welding, the nano-hardness of the surface layer is preferably 4.5 GPa or less, and more preferably 4.0 GPa or less. The lower limit of the nano-hardness in the surface layer is not particularly limited, but may be 1.0 GPa or more, or 2.0 GPa or more, from the viewpoint of avoiding a decrease in TS of the steel sheet.

[0026] The high-tensile steel sheet according to the present disclosure is not limited in chemical composition or metal structure as long as it has a tensile strength of 1700 MPa or more and a nano-hardness of 5.0 GPa or less in the surface layer portion extending from at least one surface to a depth of 100 μm. However, preferred chemical compositions and metal structures will be described below.

[0027] <Chemical composition> The chemical composition of the high-tensile steel sheet according to the present disclosure is not particularly limited, but from the viewpoint of achieving a tensile strength of 1700 MPa or more, it is preferable that the carbon equivalent (Ceq) represented by the following formula be 0.38 or more. Ceq=C+Si / 30+Mn / 20+2P+4S Each element symbol in the above formula represents the content of each element in mass% contained in the steel sheet. However, if the surface layer of the steel sheet is decarburized, the C in the formula representing Ceq refers to the C content in the part excluding the surface layer up to 100 μm from the surface.

[0028] The high-tensile steel plate according to the present disclosure has, for example, in mass%: C: 0.24~0.70%, Si: 3.50% or less, Mn: 5.00% or less, P:0.030% or less, S: 0.050% or less, Al: 3.000% or less, N: 0.010% or less, Ti: 0.300% or less, Nb: 0.300% or less, V: 0.30% or less, Cr:5.0% or less, Mo: 2.00% or less Cu: 2.00% or less, Ni: 10.0% or less, B: 0.020% or less, Ca: 0.003% or less, REM: 0.05% or less, Mg: 0.05% or less, and Zr: 0.05% or less and the balance being Fe and impurities. Of the above elements, any element other than the elements related to Ceq (C, Si, Mn, P, S) is an optional element.

[0029] C: 0.24 to 0.70% C is an element that improves the hardenability of steel and contributes to improving strength. If the C content is less than 0.24%, it is difficult to obtain high tensile strength, so the lower limit is preferably set to 0.24% so that the tensile strength is 1700 MPa or more. On the other hand, if the C content exceeds 0.70%, the strength improves too much, resulting in a decrease in workability and joint strength, so the upper limit is set to 0.70%. To ensure a good balance between strength and workability, the C content is more preferably 0.26 to 0.55%.

[0030] Si:3.50% or less Si is an element that increases the strength of steel through solid solution strengthening and microstructural strengthening. On the other hand, if the Si content exceeds 3.50%, workability and joint strength decrease, so the upper limit is preferably set to 3.50%. The upper limit of the Si content may be 2.50% or 2.00%. Although there is no particular lower limit for the Si content, it is preferably 0.10% or more from the viewpoint of increasing tensile strength. From the viewpoint of ensuring a good balance between strength and workability, the Si content is more preferably 0.50 to 2.00%.

[0031] Mn: 5.00% Mn is an element that increases the strength of steel. On the other hand, if the Mn content exceeds 5.00%, workability deteriorates and joint strength also decreases, so the upper limit is preferably set to 5.00%. Although there is no particular lower limit for the Mn content, it is preferably 0.50% or more from the viewpoint of increasing tensile strength. From the viewpoint of ensuring a good balance between strength and workability, the Mn content is more preferably 1.00 to 3.50%, and even more preferably 1.50 to 3.00%.

[0032] P:0.030% or less P is an impurity and an element that causes embrittlement. If the P content exceeds 0.030%, it becomes difficult to obtain sufficient joint strength, so the upper limit is preferably set to 0.030%.

[0033] S: 0.050% or less Like P, S is an impurity and an element that causes embrittlement. S also forms coarse MnS in steel, which reduces the workability of the steel and also reduces the joint strength. If the S content exceeds 0.050%, it becomes difficult to obtain the required joint strength and the workability of the steel decreases, so the upper limit is preferably set to 0.050%.

[0034] Al: 3.000% or less Al is an element that has a deoxidizing effect, stabilizes ferrite, and suppresses the precipitation of cementite. Al is added for deoxidation and to control the steel structure, but Al is easily oxidized. If the Al content exceeds 3.000%, the amount of inclusions increases, reducing workability and joint strength, so the upper limit is preferably set to 3.000%. From the viewpoint of ensuring workability, a more preferable upper limit is 1.200%. For deoxidation and to control the steel structure, the preferable lower limit of the Al content is 0.001%.

[0035] N: 0.010% or less N is an element that increases the strength of steel sheet, but it also forms coarse nitrides in steel, which deteriorates the formability of the steel. If the N content exceeds 0.010%, the formability of the steel and the joint strength will deteriorate significantly, so it is preferable to set the upper limit to 0.010%.

[0036] Ti: 0.300% or less Ti is an element that forms precipitates and refines the steel sheet structure. There is no particular lower limit for the Ti content, but in order to obtain the effects of containing Ti, it is preferably contained in an amount of 0.001% or more, and more preferably 0.01% or more. However, if Ti is contained in excess, not only does it reduce manufacturability and cause cracks during processing, but it also reduces joint strength. Therefore, if Ti is contained, the upper limit is preferably set to 0.300%, and more preferably 0.20% or less.

[0037] Nb: 0.300% or less Nb is an element that forms fine carbonitrides and suppresses the coarsening of crystal grains. There is no particular lower limit for the Nb content, but in order to obtain the effect of Nb inclusion, it is preferably contained in an amount of 0.001% or more, and more preferably 0.01% or more. However, if Nb is contained in excess, it not only impairs toughness and makes manufacturing difficult, but also causes a decrease in joint strength. Therefore, if Nb is contained, the upper limit is preferably set to 0.300%, and more preferably 0.20% or less.

[0038] V:0.30% or less V is an element that forms fine carbonitrides and suppresses the coarsening of crystal grains. When V is contained, in order to obtain the effect of containing V, it is preferably contained in an amount of 0.001% or more, and more preferably 0.03% or more. However, an excessive V content not only impairs toughness and makes manufacturing difficult, but also causes a decrease in joint strength, so the upper limit is preferably set to 0.30%, and more preferably 0.25% or less.

[0039] Cr:5.0% or less Cr is an element that contributes to improving the strength of steel. When adding Cr, in order to obtain the effect of adding Cr, the content is preferably 0.001% or more, and more preferably 0.05% or more. However, if the Cr content exceeds 5.0%, not only may problems occur during pickling and hot working, but also the strength of the joint may decrease. Therefore, the upper limit of the Cr content is preferably set to 5.0%, and more preferably to 3.0% or less.

[0040] Mo: 2.00% or less Mo is an element that contributes to improving the strength of steel. When Mo is contained, in order to obtain the effect of containing Mo, the content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Mo content exceeds 2.00%, problems may occur during pickling and hot working, and the joint strength may also decrease. Therefore, the upper limit of the Mo content is preferably set to 2.00%, and more preferably to 1.00%.

[0041] Cu:2.00% or less Cu is an element that contributes to improving the strength of steel. When Cu is contained, in order to obtain the effect of containing Cu, the content is preferably 0.001% or more, and more preferably 0.10% or more. However, if the Cu content exceeds 2.00%, problems may occur during pickling and hot working, and the strength of the joint may be reduced. Therefore, the upper limit of the Cu content is preferably set to 2.00%, and more preferably to 1.50% or less.

[0042] Ni: 10.0% or less Ni is an element that contributes to improving the strength of steel. When Ni is contained, in order to obtain the effect of containing Ni, the content is preferably 0.001% or more, and more preferably 0.10% or more. However, if the Ni content exceeds 10.0%, problems may occur during pickling and hot working, and the strength of the joint may be reduced. Therefore, the upper limit of the Ni content is preferably set to 10.0%, and more preferably 7.0% or less.

[0043] Ca: 0.003% or less REM0.05% or less Mg: 0.05% or less Zr: 0.05% or less

[0044] Ca, REM (rare earth metals), Mg, and Zr are elements that refine oxides after deoxidation and sulfides present in hot-rolled steel sheets, thereby contributing to improving formability. However, if the Ca content exceeds 0.003%, the REM content exceeds 0.05%, and each of the Mg and Zr contents exceeds 0.05%, the workability of the steel decreases. Therefore, it is preferable to set the upper limit of the Ca content to 0.003%, the upper limit of the REM content to 0.05%, and the upper limit of each of the Mg and Zr contents to 0.05%. When Ca, REM, Mg, and Zr are contained, in order to obtain the effects of containing them, it is preferable that Ca be 0.0005% or more, REM be 0.001% or more, Mg be 0.001% or more, and Zr be 0.001% or more.

[0045] "REM" is a general term for 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total content of one or more REM elements. REM is generally contained in misch metal. Therefore, for example, REM may be contained in the form of misch metal so that the REM content falls within the above range.

[0046] B: 0.020% or less B is an element that segregates at grain boundaries to increase grain boundary strength. When B is contained, in order to obtain the effect of containing B, the content is preferably 0.0001% or more, more preferably 0.0008% or more. On the other hand, if B is contained in excess, it not only reduces toughness and makes manufacturing difficult, but also causes a decrease in joint strength, so the upper limit is preferably set to 0.020%, more preferably 0.010% or less.

[0047] The high-tensile steel material according to the present disclosure may have a chemical composition containing, as an optional element, one or more elements selected from the group consisting of the following groups A to C: Group A: one or more of Ti, Nb, and V Group B: One or two of Cu and Ni Group C: One or more of B, Ca, REM, Mg, and Zr

[0048] <Metal structure> The metal structure of the high-tensile steel plate according to the present disclosure is not particularly limited, but from the viewpoint of achieving a tensile strength of 1700 MPa or more, a structure containing a single martensite phase or tempered martensite, bainite, ferrite, austenite, etc. is preferred. Similarly, the metal structure of the region 100 μm or less from the surface is not particularly limited. If there is no difference in the components between the region 100 μm or less from the surface and the region deeper than the surface 100 μm, it is difficult for martensite to achieve a nanohardness of 5.0 GPa or less, so tempered martensite or other multiphase structure is preferable. If there is a difference in the components (chemical composition) between the surface region within 100 μm from the surface and the region deeper than 100 μm, even martensite will suffice as long as it achieves a nanohardness of 5.0 GPa or less.

[0049] Examples of the high-tensile steel sheet according to the present disclosure include cold-rolled steel sheets, hot-rolled steel sheets, and hot-stamped steel sheets (hot-stamped members) after hot stamping (hot pressing). Furthermore, the high-tensile steel sheet according to the present disclosure may be a plated steel sheet that has been plated, or may be an unplated steel sheet that has not been plated. When plating is applied, the plating composition is not particularly limited, and examples thereof include Al plating, Al-Si plating, Ni plating, and Zn-based plating. The plating may be either hot-dip plating or electroplating, and alloying heat treatment may be performed.

[0050] <Thickness> The thickness of the high-tensile steel plate according to the present disclosure is not particularly limited, and is, for example, 0.5 to 3.5 mm, although it depends on the application of the joined structure after resistance welding.

[0051] [Method of manufacturing high-tensile steel for resistance joining] The method for producing the high-tensile steel material according to the present disclosure is not particularly limited as long as it has a tensile strength of 1700 MPa or more and the nano-hardness of the surface layer portion 100 μm deep from the surface can be 5.0 GPa or less. Examples of methods for making the nano-hardness of the surface layer portion 100 μm deep from the surface of high-tensile steel material having a tensile strength of 1700 MPa or more 5.0 GPa or less include decarburization, tempering, bonding, and combinations thereof.

[0052] First, steel adjusted to the specified composition is melted in a converter and formed into a slab by continuous casting, and the slab is rolled while still hot, or after cooling to room temperature, it is inserted into a heating furnace and heated to a temperature range of 1100 to 1300°C and rolled, and then finish-rolled in a temperature range of 800 to 950°C. It is water-cooled from the finish-rolling temperature to 200 to 700°C, and then air-cooled. Next, the steel sheet is cold-rolled to a desired thickness, for example, 0.5 to 3.5 mm, and then annealed or hot-pressed to obtain a high-tensile steel sheet or high-tensile member with a tensile strength of 1700 MPa or more.

[0053] <Decarbonization> After cold rolling, the surface layer is decarburized. The conditions for the decarburization treatment are not particularly limited as long as the nano-hardness in the surface layer from the surface to a depth of 100 μm can be reduced to 5.0 GPa or less. For example, the decarburization conditions are preferably such that the steel sheet is held in an environment with a dew point of 5°C and a temperature of 800°C for 3 minutes, and then air-cooled.

[0054] Furthermore, when microstructural control within the high-tensile steel sheet is required depending on the application of the joined structure, the desired internal microstructure can be obtained with a low C content in the surface layer by performing microstructural control by heat treatment or the like after decarburization. Alternatively, the desired internal microstructural control and decarburization may be performed simultaneously by controlling the microstructure in the temperature range or atmosphere where decarburization occurs.

[0055] The manufacturing method of the high-tensile steel sheet according to the present disclosure is not limited, and the above manufacturing method is merely an example. For example, the surface layer of the hot-rolled steel sheet may be decarburized without performing cold rolling, or decarburization may be performed simultaneously with hot rolling.

[0056] <Tempering> The nano-hardness of the surface layer may be reduced to 5.0 GPa or less by tempering. The conditions are not particularly limited as long as the nano-hardness of the surface layer from the surface to a depth of 100 μm can be reduced to 5.0 GPa or less. For example, only the surface layer may be tempered using a high-frequency heating device.

[0057] <Clad steel> The high-tensile steel sheet according to the present disclosure may be manufactured by laminating a low-carbon steel sheet to one or both sides of a high-tensile steel sheet of 1700 MPa or more, and hot-rolling the clad steel, followed by cold rolling and annealing. In this case, the thickness of the low-carbon steel sheet that constitutes the surface layer after laminating and rolling the two together is set to 100 μm or more.

[0058] [Method of manufacturing a bonded structure] The method for manufacturing a joined structure according to the present disclosure includes a resistance welding step of joining, by resistance welding, a joined member formed by overlapping a plurality of high-tensile steel materials according to the present disclosure, or a joined member formed by overlapping at least one high-tensile steel material according to the present disclosure and at least one steel material having a tensile strength of less than 1700 MPa. As the resistance welding, either spot welding or seam welding can be applied.

[0059] <Spot welding> When manufacturing a joint structure according to the present disclosure by spot welding, the energization conditions are not particularly limited, but from the viewpoint of obtaining high joint strength, it is preferable to perform spot welding under conditions that form a nugget with a nugget diameter of preferably 4√t or more, more preferably 5√t or more, at the plate interface of a plate assembly, where t is the thickness of the thinner of the two steel plates that make up the plate interface. When welding two or more plates, it is preferable to use conditions that satisfy the above-mentioned nugget diameter at each plate interface.

[0060] Fig. 4 shows a schematic diagram of an example of a cross section of a weld (joint) when spot welding is performed on a sheet assembly in which two steel sheets are stacked. As shown in Fig. 4, electrodes 2A and 2B are pressed against each other so as to sandwich the sheet assembly in which steel sheets 1A and 1B are stacked in the sheet thickness direction, and current is passed between electrodes 2A and 2B while applying pressure in the sheet thickness direction. As a result, a nugget 13 and a heat-affected zone (so-called HAZ) 14 are formed in the current-carrying area between steel sheets 1A and 1B, and the two steel sheets are joined.

[0061] The pressure applied by the electrodes 2A, 2B to the sheet assembly is, for example, 200 to 800 kgf so as to suppress the generation of spatter and to stably obtain nuggets. The pressure may be constant or may be varied during welding. The high-tensile steel sheet according to the present disclosure has a tensile strength of 1700 MPa or more, while the nano-hardness of the surface layer from the surface to 100 μm is 5.0 GPa or less. Therefore, when spot welding high-tensile steel sheet assemblies is performed under typical current conditions, for example, by spot welding with a single current using an electrode tip diameter of 6 mm and a pressure of 400 kgf, nuggets with diameters of 4√t or more, 5√t or more, or even 6√t or more can be formed at the sheet interface without generating spatter.

[0062] The current value I1 in the current application process for forming the nugget is set to a current value that will give the desired nugget diameter, taking into consideration the total thickness t of the sheet assembly, etc., and is set to the thinnest steel sheet t of the sheet assembly. min (mm), the current flow time t1 is 10t min -5 to 10t minIt may be set to +50 cycles (in this disclosure, the unit of time is the number of cycles at 50 Hz).

[0063] An upslope and / or pre-energization corresponding to a preheating process for promoting the formation of the welding portion may be performed. For example, an upslope of 1 cycle to 80 cycles may be set before the main energization for forming the nugget. Also, before the main energization, pre-energization may be performed at a current value lower than that of the main energization for 2 to 80 cycles.

[0064] In addition, when only one side of the high-strength steel sheet according to the present disclosure has a nano-hardness of 5.0 GPa or less in the surface layer portion, the surface layer portion having a nano-hardness of 5.0 GPa or less may be located on the electrode side or on the overlapping surface side. However, from the viewpoint of forming a nugget having a larger nugget diameter, it is preferably located on the overlapping surface side.

[0065] Also, the steel sheets joined by spot welding may be a set of two sheets or a set of three or more sheets. For example, when joining a set of three steel sheets stacked in the order of plate thicknesses t1 < t2, t1, t2, and t2 by spot welding, the nugget diameter at the plate interface of the two steel sheets with plate thicknesses t1 and t2 is 4√t1 or more, and it is preferable to form a nugget having a nugget diameter of 4√t2 or more at the plate interface of the two steel sheets both having a plate thickness of t2.

[0066] <Seam Welding> FIG. 5 is a schematic view showing an example of a process for manufacturing a joined structure by seam welding as resistance welding. As shown in FIG. 5, a set of two stacked steel sheets 10 and 20 is sandwiched between roller electrodes 50 and 60, pressure is applied to the set of sheets, and while rotating the roller electrodes 50 and 60 in the direction R, energization is performed, so that the set of sheets is continuously joined by heating due to electrical resistance. When seam welding is performed on a set of sheets including the high-tensile steel sheet according to the present disclosure, compared with the case where the nano-hardness of the surface layer portion of the high-tensile steel sheet exceeds 5.0 GPa, a welding portion is easily formed by the pressurization of the roller electrodes, and the appropriate range of the energization conditions can be widened.

Examples

[0067] Hereinafter, the high-tensile steel material, the joint structure, and the manufacturing method of the joint structure according to the present disclosure will be described with reference to examples. Note that the high-tensile steel material according to the present disclosure is not limited to these examples.

[0068] Example 1 <Manufacturing of high-tensile steel plates> A slab having the chemical composition shown in Table 1 was prepared, and a high-tensile steel plate (plate thickness 1.6 mm) was manufactured under the following conditions. The slab was reheated and held at 1250°C for 1 hour, then rolled to a thickness of 3.5 mm at a rolling end temperature of 900°C. It was then cooled and coiled at a hot rolling temperature of 650°C, followed by cold rolling to a thickness of 1.6 mm. Samples were cut from the manufactured high-tensile steel sheets, and some of the samples were decarburized to soften the surface. The decarburization method involved annealing at a temperature range of 700°C to 900°C with a dew point of 0°C and holding the temperature for 150 seconds. For comparison, a sample was also produced by holding the temperature range at a dew point of -30°C for 150 seconds.

[0069] The tensile strength and nano-hardness of the surface layer of the high-tensile steel sheets before and after decarburization were measured using the methods described above. The results are shown in Table 3.

[0070] [Table 3]

[0071] <Manufacturing of bonded structures> Spot welding was carried out using the steel sheet samples and conditions A to E in combination as shown in Table 4 below.

[0072] [Table 4]

[0073] A single-phase AC servo pressure spot welding machine (50 Hz) was used, and the electrodes used were Cr-Cu 40R-16mm electrodes with tip diameters of 6mm and 8mm. The current value for the single current (main current) was varied from 5kA in 0.5kA increments. Other basic welding conditions were a squeeze of 30 cycles, a current time of 18 cycles, and a hold time of 8 cycles. Note that under condition C, an upslope of 5 cycles was applied, followed by a pre-current of 5 cycles (current value: 70% of the main current value) and a cool time of 2 cycles, followed by the main current.

[0074] After spot welding, each joint structure was cut in the thickness direction through the center of the joint, the cross section was observed, and the nugget diameter (nugget length at the plate interface) was measured. The relationship between the set current value and nugget diameter under each condition is shown in Figures 6 and 7. The open plots indicate the occurrence of expulsion.

[0075] In sheet combinations using high-tensile steel sheets with a nano-hardness of the surface layer exceeding 5.0 GPa without surface softening (decarburization), expulsion occurred when the set current value exceeded 7.5 kA under all of conditions A to C, as shown in Figure 6. Furthermore, even under condition C, in which the electrode tip diameter was set to 8 mm and an upslope and pre-current were added, the optimum current value for forming a nugget with a nugget diameter of 5√t or more was in the range of 0.5 kA between 7.0 and 7.5 kA.

[0076] On the other hand, in the case of high-tensile steel sheet assemblies in which the surface was softened (decarburized) to achieve a nano-hardness of 5.0 GPa or less, no spatter occurred under conditions D or E as long as the set current was 8.5 kA or less, as shown in Figure 7. The optimum current value for forming a nugget with a diameter of 5√t or more was in the range of 2.0 kA under condition D and 2.5 kA under condition E.

[0077] Example 2 <Manufacturing of high-strength steel plates> Slabs having the chemical compositions shown in Table 5 below (balance: Fe and impurities) were prepared, and high-strength steel plates (plate thickness 1.6 mm) were produced under the conditions shown in Table 6 below.

[0078] [Table 5]

[0079] For steel type A, the slab was reheated and held at 1250°C for 1 hour, then rolled to a thickness of 3.5 mm at a rolling end temperature of 900°C, and then cooled and coiled to a hot rolling process at a temperature of 650°C, followed by cold rolling to a thickness of 1.6 mm. The treatment shown in Table 6 was then carried out. For steel type B, the slab was reheated and held at 1250°C for 1 hour, then rolled to a rolling finish temperature of 950°C to a plate thickness of 2.3 mm, water-cooled, and coiled. Thereafter, the treatments shown in Table 6 were carried out. Using these methods, samples in Table 6 were obtained. The tensile strength (TS) and nanohardness of the surface layer of the high-tensile steel sheet or hot-stamped member after the treatments shown in Table 6 were measured using the methods described above, and the depth from the surface where the nanohardness was within 5.0 GPa is shown in Table 6.

[0080] [Table 6]

[0081] <Manufacturing of bonded structures> The steel plates or hot stamped components listed in Table 6 were welded together as two overlapping sheets of the same type. A single-phase AC servo pressure spot welding machine (50Hz) was used, and the electrodes used were Cr-Cu 40R-16mm electrodes with tip diameters of 6mm and 8mm. The current value for the single current (main current) was set to 0.3kA in increments from the start of the weld. Other welding conditions were: for 1.6mm steel plate, the basic conditions were a squeeze of 30 cycles, a current time of 18 cycles, and a hold time of 8 cycles. For 2.3mm thick steel plate, the basic conditions were a squeeze of 30 cycles, a current time of 25 cycles, and a hold time of 8 cycles.

[0082] After spot welding, each joint structure was cut in the thickness direction through the center of the joint, the cross section was observed, the nugget diameter (nugget length at the plate interface) was measured, and the width of the current range in which a nugget diameter of 4√t or more could be obtained and no expulsion occurred was determined. In the sheet assemblies (comparison examples: test numbers 1 and 5) of high-tensile steel sheets having a tensile strength of 1700 MPa or more, no surface softening, a nano-hardness of the surface layer of 5.0 GPa or less, and a surface layer depth of less than 100 μm, the current range in which a nugget diameter of 4√t or more could be obtained without generating flash was within 0.5 kA. In contrast, in the combination of high-strength steel plates with a tensile strength of 1700 MPa or more, a surface layer depth of more than 100 μm, and a nano-hardness of the surface layer of 5.0 GPa or less (Examples: Test Nos. 2, 3, 6-8), the current range in which a nugget diameter of 4√t or more can be obtained without generating flash is 1.5 kA or more. In addition, for test number 4, the current range in which a nugget diameter of 4√t or more could be obtained without generating flash was 2.5 kA, but since the steel sheets used in the test had a tensile strength of less than 1700 MPa, which is outside the scope of this disclosure, this test number is a reference example. [Explanation of symbols]

[0083] 1A, 1B steel plate 2A, 2B electrode 13. Nuggets 14 Heat Affected Zone (HAZ)

Claims

1. A high-tensile steel material for resistance welding, which is a steel plate or a member processed from a steel plate, and has a tensile strength of 1700 MPa or more and a nano-hardness of 5.0 GPa or less in a surface layer portion extending from at least one surface to a depth of 100 μm.

2. A joint structure including a joint formed by resistance welding of multiple overlapping steel materials, wherein at least one of the multiple steel materials is a steel plate or a processed steel plate member, and is a high-tensile steel material having a tensile strength of 1700 MPa or more and a nano-hardness of 5.0 GPa or less in a surface layer extending from at least one surface to a depth of 100 μm.

3. A joint structure as described in claim 2, wherein a nugget is formed at the joint, and when the thickness of the thinner of the two steel materials constituting the plate interface at the joint is t, the nugget diameter of the nugget at the plate interface is 4√t or more.

4. A method for manufacturing the bonded structure according to claim 2 or 3, comprising: A method for manufacturing a joined structure, comprising a resistance welding step of joining, by resistance welding, a joined member formed by overlapping a plurality of the high-tensile steel materials, or a joined member formed by overlapping at least one of the high-tensile steel materials and at least one steel material having a tensile strength of less than 1700 MPa.

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