Method for manufacturing welded joint and welded joint

Abstract

An object of the present invention is to provide a method for manufacturing a welded joint by resistance spot welding that can reduce the amount of hydrogen that penetrates in the usage environment after welding and suppress delayed fracture of the welded joint during use, and a welded joint. The steel plate includes at least one high-strength steel plate having a tensile strength of 980 MPa or more, and two or more of the steel plates are stacked and resistance spot welded to form a welded joint. The high-strength steel plate having a tensile strength of 980 MPa or more has a specific component, and the heat-affected zone of the welded joint contains carbides containing one or more of Mo and Cr and having a grain size of 100 nm or less in equivalent circle diameter, and the total number of carbides is 50 / μm 2 or more, and the average crystal grain size in the softened portion of the weld is 20 μm or less.

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a welded joint formed by resistance spot welding of overlapping steel plates, and to a welded joint. [Background technology]

[0002] In recent years, high-strength steel sheets have been used in the automotive industry to improve fuel efficiency by reducing the weight of vehicles and to improve vehicle safety in the event of a collision. However, as the strength of steel sheets increases, concerns have arisen about the deterioration of delayed fracture resistance in welds due to residual stress caused by welding and the presence of hydrogen that penetrates into welds during welding and in the operating environment.

[0003] More specifically, the welding method mainly used in the automobile manufacturing process is resistance spot welding, and the weld formed by this method has a martensite phase due to the molten part being rapidly cooled, and becomes a very hard structure. In addition, thermal contraction during the cooling process generates tensile residual stress in the weld.

[0004] During resistance spot welding, anti-rust oil or moisture on the surface of the steel sheet may penetrate into the weld, or hydrogen may penetrate into the weld from moisture that has adhered to the steel sheet surface from the outside under the operating environment. Since hydrogen derived from the penetrated oil or moisture affects the delayed fracture resistance, ensuring the delayed fracture resistance of resistance spot welds is an important issue.

[0005] As a method for preventing such delayed fracture of welds, Patent Document 1 discloses a technology in which a two-stage current is applied in which a post-heating current is applied after the main current (nugget formation) to soften the area from the steel plate pressure weld to the nugget end, thereby improving joint strength.

[0006] Furthermore, Patent Document 2 discloses a technology in which a post-current is applied after the main current (nugget formation) and the welding electrode is controlled so as to be continuously raised and lowered alternately, thereby reducing tensile residual stress and improving joint strength.

[0007] Furthermore, Patent Document 3 discloses a technology in which a post-current is applied after the main current (nugget formation) to maintain a high-temperature environment in which hydrogen easily diffuses, thereby discharging hydrogen that has entered during welding to the outside and preventing delayed fracture. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2014 / 171495 [Patent Document 2] JP 2018-144098 A [Patent Document 3] JP 2019-72764 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, the technologies of Patent Documents 1 and 2 aim to strengthen the pressure welded area and relieve residual stress by applying heat after the formation of the nugget through current flow, and do not take into account the effect of hydrogen, which is the main cause of delayed fracture, and there is a risk of delayed fracture due to hydrogen accumulation.

[0010] In addition, the technology of Patent Document 3 reduces the amount of hydrogen that penetrates into the welded joint during welding, but does not take into account hydrogen that penetrates from the outside in the usage environment after welding, so there is a concern that delayed fracture may occur in the usage environment after the welded joint is created.

[0011] However, these technologies do not take into account the effect of carbides made of Mo and Cr, which act as trapping sites for hydrogen, a factor in the occurrence of delayed fracture. By investigating the relationship between carbides made of these elements and delayed fracture properties, it may be possible to produce better resistance spot welds. Furthermore, by adding Mo and Cr, which are elements that improve hardenability, it is possible to prevent the coarsening of crystal grains and distribute the carbides finely and uniformly. This increases the area of ​​the carbide interfaces and crystal grain interfaces that act as trapping sites for hydrogen, which is expected to further improve delayed fracture properties.

[0012] An object of the present invention is to provide a method for manufacturing a welded joint by resistance spot welding that can reduce the amount of hydrogen that penetrates in the usage environment after welding and suppress delayed fracture of the welded joint during use, and a welded joint. [Means for solving the problem]

[0013] In order to suppress delayed fracture of welded joints obtained by resistance spot welding of high-strength steel sheets, the inventors investigated the behavior of hydrogen that penetrates under the usage environment, which is a cause of delayed fracture, and obtained the following findings.

[0014] First, hydrogen penetrates into the weld from the external environment. Also, resistance spot welds have a heat-affected zone around the nugget, which has different phases and structures depending on the distance. Hydrogen that penetrates from the outside must pass through the heat-affected zone to penetrate the nugget edge, which is the main fracture initiation point of the weld. Hydrogen that passes through the heat-affected zone accumulates at the nugget edge where residual stress is concentrated, causing delayed fracture.

[0015] Therefore, reducing the amount of hydrogen that passes through the heat-affected zone and enters the nugget, and reducing the amount of hydrogen that accumulates at the nugget edge, is effective in suppressing delayed fracture.

[0016] Therefore, the present inventors have studied a suitable method for manufacturing a welded joint that can reduce the amount of hydrogen that penetrates into the nugget end from the outside. First, it was found that hydrogen that penetrates from the outside can be trapped in advance by precipitating high-density carbides in the heat-affected zone of a resistance spot weld, and the delayed fracture resistance of the welded joint can be improved. In addition, it was found that when Mo and Cr are added to the steel plate components, resistance spot welding is performed, and Mo-based carbides and Cr-based carbides are precipitated in the heat-affected zone, the delayed fracture resistance can be improved with only a shorter post-current without reducing the strength of the welded joint. It was also found that the presence of the above-mentioned carbides makes the crystal grains of the nugget and heat-affected zone finer, which can further trap hydrogen that penetrates from the outside and increase the strength of the joint. It was also found that this welded joint can be manufactured by applying a post-current after welding current and tempering the heat-affected zone.

[0017] The method for manufacturing a welded joint of the present invention has been made based on the above findings, and its gist is as follows. [1] A method for manufacturing a welded joint in which two or more steel plates are stacked and resistance spot welded together, at least one of which is a high-strength steel plate having a tensile strength of 980 MPa or more, and a weld is formed, comprising the steps of: The high-strength steel plate having a tensile strength of 980 MPa or more comprises, in mass%, C: 0.100% or more and 0.400% or less, Si: 0.200% or more and 2.500% or less, Mn: 1.000% or more and 5.000% or less, P: 0.010% or less, S: 0.005% or less, Al: 0.100% or less, N: Contains 0.004% or less, moreover, Mo: 0.030~0.500%, Cr: Contains one or more of 0.030 to 0.300%. The balance is Fe and unavoidable impurities. The heat-affected zone of the weld contains 50 carbides / μm2 containing at least one of Mo and Cr with a diameter of 100 nm or less in equivalent circle. 2 or more, and the average crystal grain size in the softened portion of the weld is 20 μm or less. [2] The high-strength steel plate having a tensile strength of 980 MPa or more further contains, in addition to the above-mentioned composition, in mass%: Ti: 0.10% or less, Nb: 0.10% or less, V: 0.10% or less, Ca: 0.0200% or less, Sb: 0.200% or less, B: 0.0100% or less, Cu: 1.00% or less, Ni: 0.50% or less, A method for manufacturing a welded joint according to [1], which contains one or more selected from the following: [3] A method for manufacturing a welded joint as described in [1] or [2], wherein the maximum grain size of carbides containing one or more of Mo and Cr in the heat-affected zone of the weld is 500 nm or less in equivalent circle diameter. [4] The resistance spot welding current is a main current and a post-current. The method for manufacturing a welded joint according to any one of [1] to [3], wherein a current value I2 in the post-current satisfies the following formula (1) with respect to a current value I1 in the main current, and a current flow time t2 in the post-current satisfies the following formula (2). I1×(1.0-MC / m) / 0.9 <I2<I1×(1.3-MC / m)···(1) 1.8×(0.8-MC / m) / 0.7 <t2<2.5×(1-MC / m)···(2)

number

[0018] According to the method for manufacturing a welded joint of the present invention, the problem of delayed fracture of high-strength steel plates welded by resistance spot welding due to hydrogen entering the steel in the usage environment can be solved. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a cross-sectional view that typically illustrates an example of the resistance spot welding method of the present invention. [Diagram 2] FIG. 2 is a graph showing an example of a current pattern and a pressure pattern in the resistance spot welding method of the present invention. [Diagram 3] 3(a) and 3(b) are diagrams showing a test piece for resistance spot welding in an embodiment of the present invention, where FIG. 3(a) is a plan view and FIG. 3(b) is a side view. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The present invention provides a method for producing a welded joint in which sufficient Mo-based carbides and Cr-based carbides are precipitated in the heat-affected zone (hereinafter simply referred to as the heat-affected zone) of resistance spot welding (hereinafter simply referred to as the weld).

[0021] The resistance spot welding method of the present invention is a method in which two or more steel plates, including one or more high-strength steel plates having the chemical composition described below, are overlapped with each other, and the overlapped steel plates are clamped and pressed between a pair of welding electrodes while an electric current is passed through them, and the steel plates are heated by the resistance heat generated between the plates to form a nugget and join them.

[0022] [Component composition] The high-strength steel plate has a composition, in mass%, of C: 0.100% to 0.400%, Si: 0.200% to 2.500%, Mn: 1.000% to 5.000%, P: 0.010% or less, S: 0.005% or less, Al: 0.100% or less, N: 0.004% or less, and one or more of Mo: 0.030 to 0.500% and Cr: 0.030 to 0.300%, and has a tensile strength of 980 MPa or more. In the present invention, a steel plate having a tensile strength of 980 MPa or more is referred to as a high-strength steel plate.

[0023] The reasons for limiting the average composition of the high strength steel plate in the present invention will be explained below.

[0024] C: 0.100% or more and 0.400% or less C is used for controlling the structure to make the TS of the high strength steel plate 980 MPa or more and for controlling the hardness of the nugget. If the C content is less than 0.100%, it is difficult to make the TS 980 MPa or more. Therefore, the C content is set to 0.100% or more. The C content is preferably set to 0.130% or more. The C content is more preferably set to 0.140% or more, and further preferably set to 0.145% or more. On the other hand, if the C content exceeds 0.400%, the hardness in the nugget increases, making it difficult to suppress fracture in the nugget. Therefore, the C content is set to 0.400% or less. Moreover, the C content is preferably set to 0.380% or less. The C content is more preferably set to 0.375% or less. The C content is most preferably set to 0.370% or less.

[0025] Si: 0.200% or more and 2.500% or less Si is used to control the structure of high-strength steel sheets and the size and amount of inclusions. If the Si content is less than 0.200%, it is difficult to achieve a TS of 980 MPa or more. Therefore, the Si content is set to 0.200% or more. The Si content is preferably set to 0.210% or more. The Si content is more preferably set to 0.220% or more. The Si content is further preferably set to 0.240% or more. On the other hand, if the Si content exceeds 2.500%, the target amount of carbides is not generated in the heat-affected zone due to the carbide generation suppression effect of Si, and it becomes difficult to improve the delayed fracture properties of the heat-affected zone. Therefore, the Si content is set to 2.500% or less. The Si content is preferably set to 2.480% or less. The Si content is more preferably set to 2.460% or less. The Si content is most preferably set to 2.440% or less.

[0026] Mn: 1.000% or more and 5.000% or less Mn is used to control the structure of high-strength steel plate and nugget, and to control the size and amount of inclusions. If the Mn content is less than 1.000%, it is difficult to make the TS 980 MPa or more. Therefore, the Mn content is set to 1.000% or more. The Mn content is preferably set to 1.200% or more. The Mn content is more preferably set to 1.220% or more, and further preferably set to 1.250% or more. On the other hand, if the Mn content exceeds 5.000%, the amount of coarse inclusions in the nugget increases, and the tendency of the nugget to brittle fracture increases. Therefore, the Mn content is set to 5.000% or less. The Mn content is preferably set to 4.950% or less, and more preferably set to 4.920% or less. The Mn content is further preferably set to 4.900% or less.

[0027] P:0.010% or less P exists in a solid solution state in the nugget and enhances the tendency of the nugget to brittle fracture. Therefore, if the P content exceeds 0.010%, fracture may occur in the nugget even if the hardness and structure of the nugget are controlled. Therefore, the P content is set to 0.010% or less. The P content is preferably set to 0.008% or less, more preferably set to 0.006% or less. Further preferably, the P content is set to 0.005% or less. Although there is no particular limit to the lower limit, the P content is preferably set to 0.0001% or more due to the economic cost associated with completely removing P.

[0028] S: 0.005% or less S exists in the nugget in a solid solution state or as sulfides, and enhances the tendency of the nugget to brittle fracture. If the S content exceeds 0.005%, fracture may occur in the nugget even if the hardness and structure of the nugget are controlled. Therefore, the S content is set to 0.005% or less. Preferably, the S content is set to 0.004% or less. More preferably, the S content is set to 0.003% or less. More preferably, the S content is set to 0.001% or less. There is no particular lower limit, but for reasons such as economic cost, the S content is preferably 0.0001% or more.

[0029] Al: 0.100% or less Al is an element that controls the size and amount of oxide-based inclusions formed by the reaction of the weld with the outside air. If the Al content exceeds 0.100%, the amount of coarse inclusions in the nugget increases, and the tendency of the nugget to be brittle fracture increases. Therefore, the Al content is set to 0.100% or less. The Al content is preferably set to 0.080% or less. The Al content is more preferably set to 0.070% or less. Further preferably set to 0.060% or less. Most preferably set to 0.050% or less. Although there is no lower limit for the Al content, due to constraints in production technology, the Al content is preferably set to 0.005% or more. The Al content is more preferably set to 0.006% or more. Further preferably set to 0.007% or more.

[0030] N: 0.004% or less N exists as nitrides in steel. If the N content exceeds 0.004%, the deformability of the high-strength steel plate is reduced, making it difficult to suppress brittle fracture in the nugget during or after welding. Therefore, the N content is set to 0.004% or less. The N content is preferably set to 0.003% or less. The N content is more preferably set to 0.002% or less. The N content is further preferably set to 0.001% or less. Note that there is no particular lower limit for the N content, but due to constraints in production technology, the N content is preferably set to 0.0005% or more.

[0031] Mo: 0.030% to 0.500% or Cr: 0.030% to 0.300% Mo and Cr are used to control the size and amount of carbides in the high-strength steel plate and the heat-affected zone. At least one of Mo and Cr must be contained, and the contents of Mo are 0.030% to 0.500%, and Cr is 0.030% to 0.300%. If the content of at least one of Mo and Cr exceeds the respective upper limits, excessive tempering will cause many coarse precipitates to form in the heat-affected zone, reducing the joint strength. Therefore, the Mo content is 0.500% or less. The Mo content is preferably 0.475% or less. The Mo content is more preferably 0.450% or less. The Mo content is even more preferably 0.420% or less. The Cr content is 0.300% or less. The Cr content is preferably 0.290% or less. The Cr content is more preferably 0.280% or less. The Cr content is even more preferably 0.270% or less. When both Mo and Cr are less than 0.030%, it becomes difficult to achieve the target hydrogen trapping effect of the heat affected zone. Therefore, the Mo content is set to 0.030% or more. The Mo content is preferably set to 0.035% or more, more preferably set to 0.040% or more. The Mo content is further preferably set to 0.050% or more. The Cr content is set to 0.030% or more. The Cr content is preferably set to 0.035% or more. The Cr content is further preferably set to 0.040% or more. The Cr content is further preferably set to 0.050% or more.

[0032] The high strength steel sheet of the present invention contains the above elements with the balance being Fe and unavoidable impurities.

[0033] The above-mentioned composition is the basic composition of the high-strength steel plate. In the present invention, in addition to the above-mentioned composition, one or more elements selected from Ti, Nb, V, Ca, Sb, B, Cu, and Ni may be added as necessary.

[0034] Ti: 0.10% or less, Nb: 0.10% or less, V: 0.10% or less Ti, Nb and V increase TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. On the other hand, when the content of at least one of Ti, Nb and V exceeds 0.10%, a large amount of coarse precipitates and inclusions are generated, and when diffusible hydrogen is contained in the steel sheet, these become the starting point of cracks during bending tests, and the bendability is reduced. Therefore, when at least one of Ti, Nb and V is contained, the content of each is 0.10% or less. The content of each is preferably 0.09% or less. The content of each is more preferably 0.08% or less. The content of each is most preferably 0.06% or less. Although the lower limit is not particularly limited, due to production technical constraints, the content of each is preferably 0.001% or more. The content of each is more preferably 0.002% or more.

[0035] Ca:0.0200% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, when diffusible hydrogen is contained in the high-strength steel plate, the inclusions become the starting points of cracks during bending tests, and the bendability is reduced. Therefore, when Ca is contained, the Ca content is set to 0.0200% or less. The Ca content is preferably set to 0.0180% or less. The Ca content is more preferably set to 0.0160% or less. The Ca content is further preferably set to 0.0150% or less. The lower limit of the Ca content may be 0.0000%, but due to restrictions in production technology, the Ca content is preferably set to 0.0001% or more. The Ca content is more preferably set to 0.0002% or more. The Ca content is further preferably set to 0.0003% or more.

[0036] Sb: 0.200% or less Sb is an element effective for suppressing oxidation of the steel sheet surface during annealing and controlling the thickness of the softened surface layer. In addition, Sb is an element that can reduce the frequency of the corresponding grain boundaries in the surface layer of the high-strength steel sheet by suppressing nitriding of the surface layer of the high-strength steel sheet during annealing. If the Sb content exceeds 0.200%, the softened surface portion cannot be formed, and the bendability and LME resistance properties are reduced. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. The Sb content is preferably set to 0.190% or less. The Sb content is more preferably set to 0.170% or less. The Sb content is further preferably set to 0.150% or less. The Sb content is most preferably set to 0.130% or less. The lower limit of the Sb content may be 0.000%, but in order to reduce the frequency of the corresponding grain boundaries and obtain better LME resistance properties, the Sb content is preferably set to 0.001% or more. The Sb content is more preferably 0.002% or more, and further preferably 0.005% or more.

[0037] B: 0.0100% or less B is an element that can improve hardenability by segregating at austenite grain boundaries, and adding B to steel can suppress the generation and grain growth of ferrite during annealing and cooling. In order to obtain such effects, when B is contained, the content of B is preferably 0.0001% or more. It is more preferable that the content of B is 0.0002% or more. On the other hand, when the content of B exceeds 0.0100%, cracks are generated inside the high-strength steel sheet during hot rolling, which reduces the ultimate deformability of the high-strength steel sheet, and the total void number density after punching increases and the stretch flangeability decreases. In addition, the bendability also decreases. Therefore, when B is contained, its content is 0.0100% or less. When B is contained, its content is preferably 0.0080% or less. The B content is more preferably 0.0070% or less. The B content is further preferably 0.0050% or less.

[0038] Cu:1.00% or less Cu is an element that increases hardenability, and is an effective element for setting the area ratio of the hard phase within a more suitable range, setting TS within a more suitable range, and further improving the dimensional accuracy during forming. In order to obtain such effects, when Cu is contained, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.02% or more. On the other hand, when the Cu content exceeds 1.00%, the area ratio of the hard phase increases, and the dimensional accuracy and ductility during forming decrease. In addition, when coarse precipitates and inclusions increase and diffusible hydrogen is contained in the steel sheet, the coarse precipitates and inclusions become the starting points of cracks during bending tests, and the bendability decreases. Therefore, when Cu is contained, its content is 1.00% or less. When Cu is contained, its content is preferably 0.95% or less. When Cu is contained, its content is more preferably 0.90% or less.

[0039] Ni: 0.50% or less Ni is an element that increases hardenability, and is an effective element for setting the area ratio of the hard phase within a more preferable range, setting TS within a more preferable range, and further improving the dimensional accuracy during forming. In order to obtain such effects, when Ni is contained, the Ni content is preferably 0.01% or more. It is more preferable that the Ni content is 0.02% or more. On the other hand, when the Ni content exceeds 0.50%, the area ratio of the hard phase increases, and the dimensional accuracy and ductility during forming decrease. In addition, when coarse precipitates and inclusions increase, and when diffusible hydrogen is contained in the high-strength steel sheet, the coarse precipitates and inclusions become the starting point of cracks during bending tests, and the bendability decreases. Therefore, when Ni is contained, its content is 0.50% or less. When Ni is contained, its content is preferably 0.45% or less. When Ni is contained, its content is more preferably 0.43% or less. When Ni is contained, its content is most preferably 0.40% or less.

[0040] [Tensile strength] The tensile strength of high-strength steel plate is said to be a strength at which delayed fracture becomes an issue, and must be 980 MPa or more. The tensile strength is more preferably 1180 MPa or more. Although there is no particular upper limit, the tensile strength is preferably 2000 MPa or less.

[0041] The thickness of the high-strength steel plate is not particularly limited, but is preferably 0.8 mm or more and 5.0 mm or less.

[0042] Furthermore, when performing resistance spot welding, it is necessary to use at least one of the above-mentioned high strength steel plates, but there are no particular limitations on the other steel plates.

[0043] A heat-affected zone that is affected by the heat of welding exists around the nugget, and the heat-affected zone includes a softened zone. The softened zone is a region that is softer than the base metal due to tempering, and is formed between the base metal and the nugget, and has a hardness lower than the average hardness (HV) of the base metal.

[0044] [Carbide in heat-affected zone] The contained Mo and Cr generate Mo-based carbides and Cr-based carbides (carbides containing at least one of Mo and Cr, including composite carbides of Mo and Cr) in the heat-affected zone around the nugget due to the influence of heat generated by resistance spot welding. The generated Mo-based carbides and Cr-based carbides can trap hydrogen that penetrates from the outside in the heat-affected zone, reducing the amount of hydrogen that accumulates at the nugget edge and suppressing delayed fracture. When the generated Mo-based carbides and Cr-based carbides have the same amount, the total surface area of ​​the carbides is smaller when the diameter of the circle equivalent is more than 100 nm than when the diameter of the circle equivalent is 100 nm or less. As a result, excessive tempering is required to precipitate an amount of carbides that can obtain a surface area that can sufficiently trap hydrogen that penetrates the nugget, and excessive tempering reduces the strength of the joint. Therefore, the diameter of the carbides is set to 100 nm or less in equivalent circle diameter. In addition, because the size of carbides effective for hydrogen trapping is 100 nm or less, we focused on carbides of this size. The particle size mentioned above is the equivalent circle diameter.

[0045] The Mo-based carbides with a grain size of 100 nm or less and the Cr-based carbides with a grain size of 100 nm or less were formed at 50 pieces / μm. 2 If the density is less than 50 / μm, the generated Mo-based carbides and Cr-based carbides cannot sufficiently reduce the amount of hydrogen that penetrates into the nugget, which is the cause of delayed fracture. Therefore, the total density of Mo-based carbides with a grain size of 100 nm or less in equivalent circle diameter and Cr-based carbides with a grain size of 100 nm or less in equivalent circle diameter is set to 50 / μm. 2 The preferred total density of Mo-based carbides and Cr-based carbides is 80 particles / μm 2 Although there is no particular upper limit, the total density of Mo-based carbides and Cr-based carbides is 1000 particles / μm 2 It is preferable to keep the density to 900 / μm or less. 2 It is more preferable to set it as follows: The Mo-based carbide and the Cr-based carbide preferably have a particle size of 80 nm or less in terms of equivalent circle diameter, and although there is no particular lower limit, the Mo-based carbide and the Cr-based carbide preferably have a particle size of 5 nm or more.

[0046] When the maximum grain size of the carbide containing one or more of Mo and Cr in the heat-affected zone of the weld is reduced, the surface area of ​​the carbide interface, which is a hydrogen trapping site, is increased, and by trapping additional hydrogen at the carbide interface, the amount of hydrogen that reaches the nugget from the hydrogen that penetrates from the outside can be reduced. The maximum grain size of the carbide is a circle equivalent diameter. When the maximum grain size of the carbide containing one or more of Mo and Cr in the heat-affected zone of the weld exceeds 500 nm in circle equivalent diameter, the surface area of ​​the carbide interface cannot be secured to sufficiently reduce the amount of hydrogen at the same carbide density. In this case, more carbide precipitation is required to reduce the amount of hydrogen to prevent delayed fracture, and excessive tempering makes it impossible to ensure the strength of the joint. Therefore, it is preferable that the maximum grain size of the carbide containing one or more of Mo and Cr in the heat-affected zone of the weld is 500 nm or less in circle equivalent diameter. More preferably, the maximum grain size of the carbide is 450 nm or less in circle equivalent diameter. Even more preferably, the maximum grain size of the carbide is 400 nm or less in circle equivalent diameter. Although there is no particular lower limit, it is preferable that the maximum grain size of the carbide be 5 nm or more in terms of circle equivalent diameter in order to ensure strength.

[0047] [Average grain size in the softened part of the weld] When the prior γ grains in the softened portion are refined to reduce the average grain size, the area of ​​the grain boundaries is increased, and in addition to the effect of reducing the amount of hydrogen entering from the outside due to the carbides described above, the amount of hydrogen entering from the outside can be reduced by trapping additional hydrogen at the grain boundaries. Here, the softened portion means a region formed between the base material and the nugget and having a hardness lower than the average hardness (HV) of the base material. When the average grain size of the softened portion exceeds 20 μm, the area of ​​the grain boundaries is small and the amount of hydrogen cannot be reduced sufficiently. In this case, more carbide precipitation is required to reduce the amount of hydrogen to prevent delayed fracture, and the joint strength cannot be guaranteed. Therefore, the average grain size of the softened portion is set to 20 μm or less. Preferably, the average grain size is set to 18 μm or less. More preferably, the average grain size is set to 15 μm or less. Even more preferably, the average grain size is set to 14 μm or less. Although there is no particular limit to the lower limit, it is preferable that the average grain size is set to 1 μm or more for reasons such as ensuring strength.

[0048] [Plating] In the present invention, the high-strength steel sheet may have a plating layer on the surface of the high-strength steel sheet as a base material, if necessary. The plating layer is preferably a zinc plating layer or a zinc alloy plating layer. In particular, for automotive applications, a hot-dip galvanizing process may be performed to obtain a hot-dip galvanized steel sheet (GI) having a zinc plating layer on the surface of the high-strength steel sheet as a base material. In addition, after the hot-dip galvanizing process, an alloying process may be performed to obtain a hot-dip galvanized steel sheet (GA) having an alloyed zinc plating layer on the surface of the high-strength steel sheet as a base material. When the plating layer is 0.1 mm or less, the volume of the plating that melts during nugget formation is very small, so it does not affect delayed fracture. Therefore, the plating layer is preferably 0.1 mm or less. Although the lower limit is not particularly limited, it is preferably 0.05 mm or more from the viewpoint of ensuring plating properties.

[0049] [Welding method] Fig. 1 is a cross-sectional view showing a schematic example of a resistance spot welding method, and shows an example of resistance spot welding of two steel sheets. Hereinafter, the resistance spot welding method of the present invention will be described with reference to Fig. 1.

[0050] First, two or more steel plates are overlapped. In Fig. 1, a steel plate arranged on the lower side (hereinafter referred to as lower steel plate 1) and a steel plate arranged on the upper side (hereinafter referred to as upper steel plate 2) are overlapped.

[0051] Next, the overlapped steel sheets (lower steel sheet 1 and upper steel sheet 2) are sandwiched between a pair of welding electrodes, i.e., an electrode arranged on the lower side (hereinafter referred to as lower electrode 4) and an electrode arranged on the upper side (hereinafter referred to as upper electrode 5), and electricity is applied while applying pressure. The configuration for applying pressure with the lower electrode 4 and the upper electrode 5 and controlling the pressure is not particularly limited, and conventionally known devices such as air cylinders and servo motors can be used. The configuration for supplying current during current application and controlling the current value is also not particularly limited, and conventionally known devices can be used. The present invention can be applied to both direct current and alternating current. In the case of alternating current, "current" means "effective current". The type of the tip of the lower electrode 4 and the upper electrode 5 is also not particularly limited, and examples thereof include DR type (dome radius type), R type (radius type), and D type (dome type) described in JIS C 9304:1999. The tip diameter of the electrode may be, for example, 4 mm or more. The tip diameter of the electrode may be, for example, 16 mm or less. It is preferable to perform resistance spot welding while keeping the electrodes constantly water-cooled.

[0052] The overlapping steel sheets are sandwiched between a pair of welding electrodes and pressurized while current is passed through them to form a nugget 3 through resistance heating and join the overlapping steel sheets to obtain a welded joint. In the present invention, the pressurization and current passing are performed in a specific pattern.

[0053] Fig. 2 is a graph showing an example of a current pattern and a pressure pattern of the resistance spot welding method according to the present invention. In Fig. 2, the vertical axis represents the current value or the pressure, and both the current value and the pressure increase toward the upper side of the vertical axis on the graph.

[0054] The main current application process (hereinafter also referred to as main current application) is a process for forming a nugget portion that becomes a nugget 3 (see FIG. 1) when solidified. The current application conditions and pressure application conditions for forming the nugget portion are not particularly limited, and conventionally used welding conditions can be adopted. For example, the current I1 in the main current application process may be 1.0 kA or more. The current I1 in the main current application process may be 15.0 kA or less. The pressure F1 may be 2.0 kN or more. The pressure F1 may be 7.0 kN or less. The time t1 of the main current application process is also not particularly limited, and may be, for example, 0.10 sec or more. The time t1 of the main current application process may be 1.00 sec or less. The "nugget" is a molten and solidified portion that occurs in a welded portion in resistance welding. The main current application process may be a multi-stage current application and multi-stage pressure application process in which the current value and pressure change during the current application process. The nugget diameter is not particularly limited, but is preferably 3√t or more when the plate thickness is t mm, and is preferably 6√t or less.

[0055] After the main current application step, a cooling step may be performed.

[0056] The cooling process is performed after the molten part formed in the main current application process is solidified to become the nugget 3 and then continues for a cooling time t c This is a process in which the electrode is in contact with the steel plate for t (sec) to cool the weld. This process lowers the temperature of the nugget 3 before the post-current process, and prevents the nugget 3 from being overheated during the post-current process, causing remelting or splashing. c If the cooling time t (sec) is too large, the temperature of the nugget 3 drops too much, so that the weld cannot be reheated sufficiently and the required delayed fracture properties may not be obtained. c The cooling time t (sec) is preferably 1.00 sec or less. cMore preferably, the cooling time t (sec) is 0.80 sec or less. There is no particular lower limit, but the cooling time t c It is preferable that (sec) is 0.05 sec or more.

[0057] After the cooling process, a post-current process is performed. In the post-current process, the value of the current supplied is changed in order to heat the heat-affected zone without increasing the diameter of the nugget.

[0058] First, the current value in the post-current application step will be described.

[0059] The post-current step is preferably carried out by applying a current I2 (kA) that satisfies the following formula (1) and for a current time t2 (sec) that satisfies the following formula (2).

[0060] In the present invention, after the main current flow process is completed, a current value I2 (kA) is applied in the post-current flow process to reheat the inside of the nugget. In other words, the nugget 3 formed in the main current flow process is tempered by reheating the welded portion in the post-current flow process, and a state in which more carbides can be precipitated during the post-current flow is maintained. The overlapped steel plates are counted from 1st to mth in order of thinnest plate thickness, and the Mo content (mass%) of the nth steel plate is defined as [Mo] n The Cr content (mass%) is [Cr] n Let us assume that.

[0061] The post-current process is a process of reheating the heat-affected zone around the solidified nugget. If the current value I2 (kA) is I1×(1.0-MC / m) / 0.9 or less, the weld is not sufficiently reheated, and sufficient carbides are not precipitated to trap hydrogen entering from the outside. Therefore, it is preferable to set the current value I2 (kA) to more than I1×(1.0-MC / m) / 0.9 with respect to I1 set in the main current process. It is more preferable to set the current value I2 (kA) to more than I1×(1.0-MC / m) / 0.88. It is even more preferable to set the current value I2 (kA) to more than I1×(1.0-MC / m) / 0.85. It is most preferable to set the current value I2 (kA) to more than I1×(1.0-MC / m) / 0.82. Also, if the current value I2 (kA) is too high, the heat-affected zone will overheat and sufficient carbide will not be obtained, and if the current value is even higher, defects such as splattering due to melting are likely to occur. Therefore, the current value I2 (kA) is set to less than I1 x (1.3-MC / m) with respect to I1 set in the main current application process. It is preferable to set the current value I2 (kA) to less than I1 x (1.3-MC / m) x 0.98. It is more preferable to set the current value I2 (kA) to less than I1 x (1.3-MC / m) x 0.95.

[0062] When current is applied at a current value I2 (kA), the current application time t2 that satisfies the range of formula (2) forms carbides by the targeted tempering of the welded portion. If the current application time t2 of the post-current application process is equal to or less than the left side of formula (2), the heat-affected zone is not heated sufficiently, and carbides are not sufficiently precipitated, so that the targeted delayed fracture properties cannot be obtained. Therefore, the current application time of the post-current application process is preferably 1.8×(0.8-MC / m) / 0.7 or more. It is more preferable that the current application time of the post-current application process is 1.8×(0.8-MC / m) / 0.65 or more. It is even more preferable that the current application time of the post-current application process is 1.8×(0.8-MC / m) / 0.6 or more. In addition, if the current application time t2 of the post-current application process is equal to or more than the right side of formula (2), excessive tempering occurs, and the welded joint strength cannot be ensured. Therefore, the current application time t2 of the post-current application process is preferably 2.5×(1-MC / m) or less. The current application time of the post-current application process is more preferably 2.4×(1-MC / m) or less. The current application time of the post-current application process is further preferably 2.3×(1-MC / m) or less. I1×(1.0-MC / m) / 0.9 <I2<I1×(1.3-MC / m)···(1) 1.8×(0.8-MC / m) / 0.7 <t2<2.5×(1-MC / m)···(2)

number

[0063] As shown in FIG. 1, a lower steel sheet 1 and an upper steel sheet 2 were overlapped and subjected to resistance spot welding.

[0064] Resistance spot welding was performed in a room temperature atmosphere, with the electrodes constantly water-cooled. The lower electrode 4 and the upper electrode 5 were both DR-type electrodes made of chromium copper, with a tip diameter (tip diameter) of 6 mm and a curvature radius of 40 mm. The pressure was controlled by driving the lower electrode 4 and the upper electrode 5 with a servo motor, and a single-phase AC with a frequency of 50 Hz was supplied during current application. The lower steel plate 1 and the upper steel plate 2 both had a tensile strength of 980 MPa or more, a long side of 100 mm, a short side of 30 mm, and a plate thickness of 1.4 mm. The strength, presence or absence of a plating layer, and chemical composition of the steel plates used for each steel plate were as shown in Table 1. The tensile strength was determined by preparing a JIS No. 5 tensile test piece from each steel plate in a direction parallel to the rolling direction and conducting a tensile test in accordance with the provisions of JIS Z 2241:2011.

[0065] [Table 1]

[0066] As shown in Figure 3, resistance spot welding was performed by sandwiching a 1.5 mm thick, 30 mm square spacer 6 between two of the above steel plates (long side: 100 mm, short side: 30 mm) on both sides and temporarily welding them, and then welding the center of the plate assembly in which the two steel plates were overlapped under the conditions listed in Table 2. Figure 3 shows a plan view (Figure 3(a)) and a side view (Figure 3(b)) of a test piece for resistance spot welding, where 7 is the welding point and 8 is the temporary welding point. Note that this current flow process was performed for all samples at a current value and current flow time such that the nugget diameter (mm) relative to the plate thickness t (mm) was 4√t. Note that "√t" means the square root of t.

[0067] [Table 2]

[0068] The welded joints obtained were left in the air at room temperature (20°C) and cross-sectional observations were performed using an optical microscope and a scanning electron microscope (SEM), and delayed fracture properties were evaluated using an immersion test. The nugget diameter was measured by cutting the center of the weld, etching the cross section in the thickness direction with picric acid, and observing the cross section of the target surface at 40x magnification using an optical microscope. Next, the target surface was mirror-polished until it was no longer affected by the etching, and then nital etched. The Mo-based carbides and Cr-based carbides contained in the heat-affected zone of the target surface were confirmed using a SEM, and the Mo-based carbides and Cr-based carbides were identified using an EPMA (Electron Probe Micro Analyzer). The measurement conditions for the EPMA were an acceleration voltage of 15 kV and a probe current of 2 × 10 -7 A, observation magnification: 10,000 times, beam diameter: 1.4 μm, time: 30 ms, number of fields of view: 5. The concentration was the average value of the 5 fields of view. Then, the particle size of Mo-based carbides and Cr-based carbides was measured in the photographs taken by SEM observation, and the density of Mo-based carbides and Cr-based carbides with a circle equivalent diameter of 100 nm or less was measured. The SEM observation conditions were an acceleration voltage of 5 kv and a magnification of 5000 times, and the number of observation fields was 5, and the average value of the 5 fields was calculated.

[0069] The grain size of the carbide in the heat-affected zone was measured by converting the diameter of the carbide into the equivalent circle diameter through SEM observation at an accelerating voltage of 5 kv and a magnification of 10,000. The number of observation fields was 5, and the maximum value among them was taken as the maximum grain size of the carbide.

[0070] Using the samples measured above, the average crystal grain size was measured by randomly selecting five locations in the softened portion, enlarging the sample 1000 times, and measuring and averaging the grain sizes based on JIS G 0551.

[0071] For the delayed fracture property evaluation by immersion test, samples that were left in the air for 24 hours or more after welding were immersed in a hydrochloric acid solution of pH 2.5 for 96 hours, and samples that did not experience nugget delamination (phenomenon in which the nugget peels into two at the joint interface) were rated as ◯, samples that had cracks in the nugget but no delamination were rated as △, and samples that experienced delamination were rated as ×. Here, the symbols "◯" and "△" were used to evaluate the specimen as having excellent delayed fracture resistance.

[0072] Table 3 shows the nugget diameter obtained from cross-sectional observation, the carbide density in the heat-affected zone, the maximum grain size of the carbides, the average grain size in the softened zone, and the results of the delayed fracture evaluation.

[0073] [Table 3]

[0074] As is clear from Table 3, the inventive examples were able to reduce the amount of hydrogen that penetrated under the operating environment, and good welded joints with excellent delayed fracture resistance were obtained. In contrast, the comparative examples did not provide good welded joints. [Explanation of symbols]

[0075] 1: Lower steel plate 2: Upper steel plate 3: Nugget 4: Lower electrode 5: Upper electrode 6: Spacer 7: Welding points 8: Temporary welding points

Claims

1. A method for manufacturing a welded joint comprising the steps of: stacking two or more steel plates and resistance spot welding the stacked steel plates together; at least one of the two or more steel plates being a high-strength steel plate having a tensile strength of 980 MPa or more; and forming a welded portion, The high-strength steel plate having a tensile strength of 980 MPa or more comprises, in mass%, C: 0.100% or more and 0.400% or less, Si: 0.200% or more and 2.500% or less, Mn: 1.000% or more and 5.000% or less, P: 0.010% or less, S: 0.005% or less, Al: 0.100% or less, N: 0.004% or less; moreover, Mo: 0.030-0.500%, Cr: 0.030 to 0.300%; The balance is Fe and unavoidable impurities, The heat-affected zone of the welded joint contains carbides containing at least one of Mo and Cr, each having a diameter of 100 nm or less in equivalent circle. ２ and the average crystal grain size in the softened portion of the weld is 20 μm or less.

2. The high-strength steel plate having a tensile strength of 980 MPa or more further contains, in addition to the above-mentioned composition, in mass%: Ti: 0.10% or less, Nb: 0.10% or less, V: 0.10% or less, Ca: 0.0200% or less, Sb: 0.200% or less, B: 0.0100% or less, Cu: 1.00% or less, Ni: 0.50% or less, The method for manufacturing a welded joint according to claim 1, comprising one or more selected from the following:

3. 3. The method for manufacturing a welded joint according to claim 1, wherein the maximum grain size of carbides containing at least one of Mo and Cr in the heat affected zone of the weld is 500 nm or less in terms of circle equivalent diameter.

4. The resistance spot welding current application includes a main current application and a post-current application, The current value I ２ is the current value I １ The following formula (1) is satisfied, and the current application time t ２ The method for manufacturing a welded joint according to claim 1 or 2, wherein the following formula (2) is satisfied: I １ ×(1.0-MC / m) / 0.9<I ２ <I １ ×(1.3-MC / m)・・・(1) 1.8×(0.8-MC / m) / 0.7<t ２ <2.5×(1-MC / m)・・・(2) [0010] Where: I １ : current value (kA) in this current application process, I ２ : current value (kA) in post-current application step, t ２ : Post-power-on time (sec), [Mo] ｎ : Mo content (mass%) of the nth steel plate in the order of thinnest thickness among the stacked steel plates, [Cr] ｎ : Cr content (mass%) of the nth steel plate in the stacked steel plates in ascending order of thickness, dn: thickness (mm) of the nth thinnest steel plate among the stacked steel plates, d1: thickness (mm) of the thinnest steel plate among the overlapping steel plates, m: total number of steel plates, The stacked steel plates are arranged in order of thickness from 1st to mth. It is.

5. The resistance spot welding current flow comprises a main current flow and a post-current flow, 4. The method for manufacturing a welded joint according to claim 3, wherein a current value I2 in the post-current passage satisfies the following formula (1) with respect to a current value I1 in the main current passage, and a current passage time t2 in the post-current passage satisfies the following formula (2). I 1 × (1.0-MC / m) / 0.9<I 2 <I 1 × (1.3-MC / m) (1) 1.8×(0.8-MC / m) / 0.7<t 2 <2.5×(1-MC / m)...(2) [0010] Where: I1: current value (kA) in this current application step, I2: current value in the post-current application step (kA), t2: Post-energization time (sec), [Mo] n : Mo content (mass%) of the n-th steel plate in ascending order of thickness among the stacked steel plates; [Cr] n : Cr content (mass%) of the nth steel plate in ascending order of thickness among the stacked steel plates, dn: thickness (mm) of the nth thinnest steel plate among the stacked steel plates, d1: thickness (mm) of the thinnest steel plate among the overlapping steel plates, m: total number of steel plates, The stacked steel plates are arranged in order of thickness from 1st to mth. It is.

6. A welded joint having two or more steel plates and a weld, at least one of the two or more steel plates being a high-strength steel plate having a tensile strength of 980 MPa or more, The high-strength steel plate having a tensile strength of 980 MPa or more comprises, in mass%, C: 0.100% or more and 0.400% or less, Si: 0.200% or more and 2.500% or less, Mn: 1.000% or more and 5.000% or less, P: 0.010% or less, S: 0.005% or less, Al: 0.100% or less, N: 0.004% or less; moreover, Mo: 0.030-0.500%, Cr: 0.030 to 0.300%; The balance is Fe and unavoidable impurities, The heat-affected zone of the welded joint contains carbides containing at least one of Mo and Cr, each having a diameter of 100 nm or less in equivalent circle. ２ or more, and the average crystal grain size in the softened portion of the weld is 20 μm or less.

7. The high-strength steel plate having a tensile strength of 980 MPa or more further contains, in addition to the above-mentioned composition, in mass%: Ti: 0.10% or less, Nb: 0.10% or less, V: 0.10% or less, Ca: 0.0200% or less, Sb: 0.200% or less, B: 0.0100% or less, Cu: 1.00% or less, Ni: 0.50% or less, The welded joint according to claim 6, comprising one or more selected from the following:

8. 8. The welded joint according to claim 6, wherein the maximum grain size of carbides containing at least one of Mo and Cr in the heat-affected zone of the weld is 500 nm or less in terms of circle equivalent diameter.

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