Resistance spot weld joint, component for automobile, and method for manufacturing resistance spot weld joint

Abstract

To provide a resistance spot weld joint which is excellent in both tensile shear strength (TSS) and cross tensile strength (CTS), a component for an automobile, and a method for manufacturing a resistance spot weld joint.SOLUTION: A resistance spot weld joint includes two or more stacked steel plates, and a nugget for joining the steel plates, wherein area ratios of iron-based carbides on both ends of the nugget in a virtual line along a joint interface, which is measured in a cross section of the nugget, are 0.3% or more, and an area ratio of the iron-based carbide, in a central part of the nugget in the virtual line along the joint interface, which is measured in the cross section of the nugget, is 0.2% or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a resistance spot welded joint, an automotive part, and a method for manufacturing a resistance spot welded joint. [Background technology]

[0002] Resistance welding is a method of welding in which a large current is passed through the weld joint, heating it with the generated resistance heat and applying pressure. Resistance welding can be performed in a short time, so it is used to manufacture various machine parts.

[0003] A post-heating current may be applied to a weld formed by resistance welding. The post-heating current is a current that is applied to a weld after resistance welding for the purpose of performing heat treatment such as tempering, annealing, and segregation relief. The application of a post-heating current is also called post-current application.

[0004] An example of a material in which post-energization is used is high-strength steel plate. High-strength steel plate is applied in various technical fields to reduce the weight and improve the safety of machine parts. However, high-strength steel plate has a problem that resistance welds are easily embrittled. In a welded joint made of normal steel plate, the higher the strength of the steel plate, the higher the cross tensile strength (CTS). However, in a welded joint made of high-strength steel plate, the higher the strength of the steel plate, the lower the CTS. Note that cross tensile strength (CTS) is a value measured by a cross tensile test specified in JIS Z 3137:1999. CTS is an index of the peel strength of a welded joint.

[0005] Various techniques have been investigated to improve the properties of welds formed by resistance welding.

[0006] Patent Document 1 discloses a technique for providing a resistance spot welded component with excellent delayed fracture resistance by improving the structure of the nugget end and a manufacturing method thereof. According to this technique, there is provided a resistance spot welded component comprising two or more steel plates and a spot weld formed between the steel plates, in which at least one of the steel plates has a tensile strength of 980 MPa or more, and in which, when X of the steel plate having the largest coefficient X expressed by X=[C]+[Si] / 40+[Mn] / 200 is Xmax and Y of the steel plate having the smallest coefficient Y expressed by Y=[P]+3×[S] is Ymin, the Vickers hardness Hn(Hv) of the nugget end of the spot weld is equal to or less than Hob(Hv) expressed by Hob=(800×Xmax+300) / (0.7+20×Ymin), and the Vickers hardness Hmin(Hv) of the softest part of the weld heat affected zone of the spot weld satisfies 0.4×Hn≦Hmin≦0.9×Hn.

[0007] Patent Document 2 discloses that a high-strength resistance spot welded joint is provided without softening by reducing nugget segregation and making it difficult for cracks to occur for a combination of two or more sheets, thereby suppressing delamination fracture. According to this technology, a resistance spot welded joint of thin steel sheets is provided, characterized in that, when the diameter of the nugget made of the thin steel sheets is d, the distribution state of the amount of P present in the region in the nugget surrounded by a closed curve at a distance of d / 100 and a closed curve at a distance of d / 5 from the end of the fusion zone toward the inside of the nugget on the horizontal surface of the nugget surrounded by the corona bond is area analyzed, and the area ratio of the P concentration m (mass%) exceeding twice the P concentration M (mass%) of the base metal composition is 5% or less. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2020 / 036198 [Patent Document 2] JP 2013-151027 A Summary of the Invention [Problem to be solved by the invention]

[0009] The present inventors have found that the CTS of resistance welds of high-strength steel sheets is improved by tempering through post-current application. However, post-current application reduces the tensile shear strength (TSS) of resistance welds of high-strength steel sheets. The tensile shear strength (TSS) is a value measured by a tensile shear test specified in JIS Z 3136:1999. The TSS is an index of the shear strength of a weld.

[0010] In order to further improve the reliability of resistance spot welded joints, a resistance spot welding method capable of producing joints with excellent TSS and CTS is required. However, no technology that can meet this requirement has been reported yet.

[0011] The technology disclosed in Patent Document 1 aims to improve the CTS of automotive steel sheets with a tensile strength of 980 MPa or more. However, Patent Document 1 does not particularly consider TSS. Furthermore, Patent Document 1 does not disclose any technology capable of improving TSS.

[0012] Patent Document 2 discloses that the cross tensile strength hardly increases, but rather decreases, regardless of the increase in the tensile strength of the steel plate. Patent Document 2 also discloses that a decrease in hardness reduces the shear tensile strength. However, Patent Document 2 does not particularly consider specific means for improving the shear tensile strength. Patent Document 2 places emphasis on ensuring the cross tensile strength. In the examples of the resistance spot welded joints disclosed in Patent Document 2, the cross tensile strength is evaluated, but the tensile shear strength is not evaluated.

[0013] An object of the present invention is to provide a resistance spot welded joint having excellent both tensile shear strength (TSS) and cross tensile strength (CTS), an automobile part, and a method for manufacturing a resistance spot welded joint. [Means for solving the problem]

[0014] The gist of the present invention is as follows.

[0015] (1) A resistance spot welded joint according to one embodiment of the present invention comprises two or more stacked steel plates and a nugget joining the steel plates, wherein an area ratio of iron-based carbides at each end of the nugget along a virtual line along the joint interface, measured on a cross-section of the nugget, is 0.3% or more, and an area ratio of the iron-based carbides at a center of the nugget along the virtual line along the joint interface, measured on the cross-section of the nugget, is 0.2% or less. (2) In the resistance spot welded joint described in (1) above, preferably, one or more of the steel plates is a high-carbon steel plate having a carbon content of 0.20 mass % or more. (3) Preferably, in the resistance spot welded joint described in (2) above, the number of steel plates is three or more, and the area ratio of the iron-based carbides at each of the two end portions of the nugget along the imaginary line along the joint interface between the high carbon steel plate and the steel plate overlapping the high carbon steel plate is 0.3% or more, and the area ratio of the iron-based carbides at the central portion of the nugget is 0.2% or less.

[0016] (4) An automobile part according to another aspect of the present invention includes the resistance spot welded joint according to any one of (1) to (3) above.

[0017] (5) A method for manufacturing a resistance spot welded joint according to another embodiment of the present invention includes a step of spot welding two or more stacked steel plates together, and a step of post-current passing through the nugget formed by the spot welding, and reducing the current passing area during the post-current passing. (6) Preferably, in the method for manufacturing a resistance spot welded joint described in (5) above, the current value in the post-current is controlled to a constant value, the time from the start to the end of the post-current is set to T seconds, and the current area is reduced during a period within a range of T / 2 seconds to 3T / 2 seconds from the start of the post-current. (7) Preferably, in the method for manufacturing a resistance spot welded joint described in (5) or (6) above, the current flow area is reduced by reducing the pressure from F1 to F2 during the post-current flow, and F2 / F1 is 0.40 or more and 0.80 or less. (8) Preferably, in the method for manufacturing a resistance spot welded joint described in (5) or (6) above, the first electrode for the post-current passage is replaced with a second electrode having a smaller tip diameter than the first electrode during the post-current passage, thereby reducing the current passage area. (9) In the method for manufacturing a resistance spot welded joint described above in (8), preferably, a ratio d2 / d1 of a tip diameter d1 of the first electrode to a tip diameter d2 of the second electrode is 0.40 or greater and 0.80 or less. Effect of the Invention

[0018] According to the present invention, it is possible to provide a resistance spot welded joint, an automobile part, and a method for manufacturing a resistance spot welded joint, which are excellent in both tensile shear strength (TSS) and cross tensile strength (CTS). [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a schematic cross-sectional view of an example of a resistance spot welded joint. [Diagram 2] FIG. 1 is a schematic diagram of the tensile shear test for measuring TSS. [Diagram 3] FIG. 1 is a schematic diagram of the stress applied to a nugget during a tensile shear test to measure TSS. [Figure 4] FIG. 1 is a schematic diagram of a cross tensile test for measuring CTS. [Diagram 5] FIG. 1 is a schematic diagram of the stress applied to a nugget during a cross tensile test to measure CTS. [Figure 6] FIG. 13 is a schematic diagram of a current path in the early stage of post-current conduction. [Figure 7] FIG. 13 is a schematic diagram of a current path in a later stage of post-current application when the pressure force is changed. [Figure 8] FIG. 13 is a schematic diagram of a current path in a later stage of post-current application when the tip diameter of the electrode is changed. [Figure 9] 13 is a schematic diagram of a current flow pattern when a current flow area is reduced by changing a pressure force. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] (1. Resistance spot welded joints 1) First, a resistance spot welded joint 1 according to this embodiment will be described. As shown in the cross-sectional view of Fig. 1, the resistance spot welded joint 1 according to this embodiment includes two or more stacked steel plates 11 and a nugget 12 that joins the steel plates 11, and the area ratio of iron-based carbides at both ends 12E of the nugget 12 on a virtual line 13L along the joint interface 13 measured on the cross section of the nugget 12 is 0.3% or more, and the area ratio of iron-based carbides at a center portion 12C of the nugget 12 on the virtual line 13L along the joint interface 13 measured on the cross section of the nugget 12 is 0.2% or less.

[0021] Hereinafter, a resistance spot welded joint 1 according to this embodiment will be described with reference to a cross-sectional view of the resistance spot welded joint 1 illustrated in Fig. 1. In this embodiment, the term "cross section" refers to a cross section that passes through the center or its vicinity in a plan view of the indentation 14 and is approximately perpendicular to the surface of the steel sheet 11. The indentation 14 refers to a depression on the surface of the steel sheet 11 caused by the electrode tip as a result of spot welding to form the nugget 12.

[0022] (Steel plate 11 and nugget 12) The resistance spot welded joint 1 has a plurality of overlapping steel plates 11. The steel plates 11 can have any shape suitable for spot welding. In other words, the term "steel plate" is a concept that includes not only flat steel but also flat regions of steel members having a three-dimensional shape. For example, the flange portion of a hat-shaped member is also considered to be a steel plate 11.

[0023] The number of steel plates 11 is an arbitrary value of 2 or more. The plurality of steel plates 11 are joined by a nugget 12. The nugget 12 is a molten and solidified portion that occurs in a welded portion during spot welding.

[0024] (Joint interface 13 and virtual line 13L along it) In the resistance spot welded joint 1 according to this embodiment, the configuration of the nugget 12 is defined by using a virtual line 13L along the joint interface 13. As shown in FIG. 1, the joint interface 13 is a surface where the steel sheets 11 contact and face each other. The joint interface 13 is sometimes called a mating surface. However, the joint interface 13 disappears inside the nugget 12. In this embodiment, the virtual line 13L along the joint interface 13 is used to evaluate the nugget 12. When there are two steel sheets 11, there is one joint interface 13 and one virtual line 13L. When there are three or more steel sheets 11, there are two or more joint interfaces 13 and two or more virtual lines 13L.

[0025] (End 12E and center 12C of nugget 12 on virtual line 13L) When a cross section of the nugget 12 is observed, it can be seen that iron-based carbides are contained in both ends 12E of the nugget 12 at the imaginary line 13L along the joint interface 13. In the resistance spot welded joint 1 according to this embodiment, (1) The area ratio of iron-based carbides at each of the two end portions 12E of the nugget 12 on the imaginary line 13L along the joint interface 13, and (2) Area ratio of iron-based carbides in the center 12C of the nugget 12 on the virtual line 13L along the joint interface 13 is prescribed.

[0026] The end 12E of the nugget 12 on the virtual line 13L along the joint interface 13 is a region near the intersection of the outer edge of the nugget 12 (i.e., the fusion boundary) and the virtual line 13L, as shown in Fig. 1. The central portion 12C of the nugget 12 on the virtual line 13L along the joint interface 13 is a region located midway between the two ends 12E of the nugget 12 described above. There are two ends 12E and one central portion 12C on one virtual line 13L. When there are two or more virtual lines 13L, there are two ends 12E and one central portion 12C on each of the multiple virtual lines 13L.

[0027] The end portion 12E and the central portion 12C of the nugget 12 on the imaginary line 13L along the joint interface 13 are extremely important regions for improving the joint strength of the nugget 12. This is because the imaginary line 13L along the joint interface 13 includes a stress concentration portion in both the tensile shear test and the cross tension test.

[0028] The tensile shear test and the cross tension test are outlined as shown in FIG. 2 to FIG. 5. The tensile shear test of the resistance spot welded joint 1 is performed using a test piece shown in FIG. 2. The two steel plates 11 are pulled in the left-right direction of the paper to break the nugget 12. In this tensile shear test, as shown in FIG. 3, a shear stress along the imaginary line 13L is applied to the nugget 12. The cross tension test of the resistance spot welded joint 1 is performed using a test piece shown in FIG. 4. The steel plate 11 arranged on the upper side of the paper is pulled toward the upper side of the paper, and the steel plate 11 arranged on the lower side of the paper is pulled toward the lower side of the paper. In the cross tension test, as shown in FIG. 5, a peeling stress is applied to the end 12E of the nugget 12 at the imaginary line 13L.

[0029] (Area ratio of iron-based carbides at both ends 12E of the nugget 12) In the resistance spot welded joint 1 according to this embodiment, the area ratio of iron-based carbides is 0.3% or more at both ends 12E of the nugget 12 at the imaginary line 13L along the joint interface 13. In other words, the area ratio of iron-based carbides is 0.3% or more at both ends 12E of the nugget 12 at the imaginary line 13L. Preferably, the area ratio of iron-based carbides at both ends 12E of the nugget 12 is 0.4% or more, 0.5% or more, 0.6% or more, or 0.8% or more.

[0030] (Area ratio of iron-based carbides in the central portion 12C of the nugget 12) In the resistance spot welded joint 1 according to this embodiment, the area ratio of iron-based carbides in the central portion 12C of the nugget 12 on the imaginary line 13L along the joint interface 13 is 0.2% or less. Preferably, the area ratio of iron-based carbides in the central portion 12C of the nugget 12 is 0.15% or less, 0.1% or less, 0.05% or less, or 0%.

[0031] Iron-based carbides are compounds that are mainly composed of iron (Fe) and carbon (C). One example of an iron-based carbide is cementite (Fe 3 C). The alloy elements contained in the steel plate 11, such as Si and Mn, may be contained in the iron-based carbides.

[0032] Iron-based carbides are structures that precipitate from the parent phase when high-temperature steel is cooled. The carbon concentration of iron-based carbides is higher than that of the parent phase. Therefore, the more iron-based carbides precipitate from the parent phase, the lower the carbon concentration of the parent phase. When steel is quenched, the amount of iron-based carbides decreases. On the other hand, when steel after quenching is tempered, the amount of iron-based carbides increases.

[0033] The area ratio of iron-based carbides can be used as an index value of the carbon content of the parent phase of the nugget 12 and the hardening degree of the nugget 12. The larger the area ratio of iron-based carbides, the smaller the carbon content of the parent phase. Also, the larger the area ratio of iron-based carbides, the smaller the degree of hardening or the more tempered the nugget 12 is. In the nugget 12 of the resistance spot welded joint according to this embodiment, the degree of hardening of the center portion 12C is greater than the degree of hardening of the end portion 12E.

[0034] As illustrated in FIG. 1, when there are two or more virtual lines 13L, the area ratio of the iron-based carbide at both ends 12E and the center 12C of the nugget 12 in at least one virtual line 13L may be within the above-mentioned range. Preferably, the area ratio of the iron-based carbide at both ends 12E and the center 12C of the nugget 12 in at least the virtual line 13L along the joint interface 13 that contributes most to the joint strength of the resistance spot welded joint 1 is within the above-mentioned range. For example, in the resistance spot welded joint 1 manufactured from a thick-thick-thin plate assembly (i.e., a plate assembly consisting of two overlapping thick steel plates 11 and one thin steel plate 11) illustrated in FIG. 1, the joint interface 13 that contributes most to the joint strength of the resistance spot welded joint 1 is the thick-thick interface (i.e., the joint interface 13 between the two thick steel plates 11). For example, in the resistance spot welded joint in FIG. 1, the joint interface 13 on the lower side of the paper contributes most to the joint strength. More preferably, the area ratio of the iron-based carbide in both ends 12E and the center 12C of the nugget 12 is within the above-mentioned range in all imaginary lines 13L. For example, in the resistance spot welded joint in Fig. 1, it is more preferable that all the requirements for the area ratio of the iron-based carbide are satisfied in the imaginary line 13L along the joint interface 13 on the upper side of the page, and that all the requirements for the area ratio of the iron-based carbide are also satisfied in the imaginary line 13L along the joint interface 13 on the lower side of the page.

[0035] (Action and effect) The following describes the effects of the resistance spot welded joint 1 according to this embodiment and how the inventors discovered these effects.

[0036] It is generally known that the CTS of the resistance spot welded joint 1 is improved by tempering the nugget 12. Post-energization, which will be described later, is known as a means for tempering the nugget 12. On the other hand, tempering the nugget 12 reduces the TSS of the resistance spot welded joint 1.

[0037] The present inventors have conducted extensive research into a method for obtaining a resistance spot welded joint 1 having excellent CTS and TSS. As a result, the present inventors have found that while the CTS depends on the amount of iron-based carbides at the end 12E of the nugget 12, the CTS is hardly affected by the amount of iron-based carbides at the center 12C of the nugget 12.

[0038] CTS is measured using a cross tensile test piece shown in FIG. 4. The end 12E of the steel plate 11 arranged on the upper side of the paper is pulled toward the upper side of the paper, and the end 12E of the steel plate 11 arranged on the lower side of the paper is pulled toward the lower side of the paper, thereby peeling off the weld. FIG. 5 shows a schematic cross-sectional view of the nugget 12 at this time. As shown in FIG. 5, the end 12E of the nugget 12 becomes a stress concentration part in the cross tensile test for evaluating the CTS. Therefore, the end 12E of the nugget 12 needs to be tempered.

[0039] On the other hand, according to the experimental results of the present inventors, it is not necessary for the central portion 12C of the nugget 12 to be tempered. It is believed that the mechanical properties of the central portion 12C of the nugget 12 also have a slight effect on the CTS. However, it is believed that the effect of the central portion 12C on the CTS is smaller than that of the end portion 12E. The relationship between the mechanical properties of the central portion 12C of the nugget 12 and the CTS of the resistance spot welded joint 1 has not been reported in the prior art.

[0040] Based on the above findings, in the resistance spot welded joint 1 according to the embodiment, the amounts of iron-based carbides in the end 12E and the central portion 12C of the nugget 12 are specified on the imaginary line 13L along the joint interface 13. The area ratio of the iron-based carbides in the end 12E of the nugget 12 is larger than that in the central portion 12C of the nugget 12. That is, the end 12E of the nugget 12 is tempered, while the central portion 12C of the nugget 12 is quenched.

[0041] By setting the area ratio of the iron-based carbide in the end 12E of the nugget 12 within the above-mentioned range, it is possible to increase the peel strength of the end 12E of the nugget 12. And, the CTS of the resistance spot welded joint 1 is increased.

[0042] Furthermore, by setting the area ratio of iron-based carbides in the central portion 12C of the nugget 12 within the above-mentioned range, it is possible to increase the carbon concentration of the parent phase in the central portion 12C of the nugget 12. By increasing the carbon concentration of the parent phase in the central portion 12C of the nugget 12, it is possible to increase the TSS of the resistance spot welded joint 1.

[0043] According to the conventional technology, it was thought that if the nugget 12 is not fully tempered overall and the carbon concentration of the parent phase of the nugget 12 is high, the CTS of the resistance spot welded joint 1 would be impaired. However, to improve the CTS of the resistance spot welded joint 1, it is sufficient to temper at least the end portion 12E of the nugget 12. Then, by increasing the carbon concentration of the parent phase in the central portion 12C of the nugget 12 while the end portion 12E is tempered, it is possible to ensure the TSS without impairing the CTS of the resistance spot welded joint 1.

[0044] In addition, by setting the area ratio of iron-based carbides at the end 12E of the nugget 12 within the above-mentioned range, it is believed that the hydrogen embrittlement resistance of the resistance spot welded joint 1 is improved. This is because the lower the carbon content of the steel parent phase, the higher the hydrogen embrittlement resistance of the steel. It is believed that the area ratio of iron-based carbides at the center 12C of the nugget 12 does not affect the hydrogen embrittlement resistance of the resistance spot welded joint 1. This is because the center 12C of the nugget 12 is away from the external environment.

[0045] The above has described the most basic aspect of the resistance spot welded joint 1 according to this embodiment. Next, a more preferred aspect of the resistance spot welded joint 1 according to this embodiment will be described.

[0046] (Carbon content of steel plate 11) It is preferable that one or more of the steel plates 11 be high carbon steel plates 11 having a carbon content of 0.20 mass% or more. This dramatically increases the strength of the resistance spot welded joint 1 and a mechanical component having the same. More preferably, the carbon content of the one or more high carbon steel plates 11 is 0.22 mass% or more, 0.25 mass% or more, 0.28 mass% or more, or 0.30 mass% or more. There is no particular upper limit to the carbon content of the high carbon steel plate 11, but it is preferable that the carbon content of the high carbon steel plate 11 be 0.40 mass% or less, 0.35 mass% or less, or 0.32 mass% or less, for example.

[0047] Generally, when the carbon content of the steel plate 11 exceeds 0.20 mass %, the CTS of the resistance spot welded joint 1 decreases. However, in the resistance spot welded joint 1 according to this embodiment, there is no need to worry about a decrease in CTS when increasing the carbon content of the steel plate 11. This is because, in the resistance spot welded joint 1 according to this embodiment, the area ratio of iron-based carbides in both end portions 12E and the central portion 12C of the nugget 12 is set within the above-mentioned range, thereby increasing the CTS.

[0048] As described above, the number of steel plates 11 may be three or more. In addition, one or more of the three or more steel plates 11 may be a high carbon steel plate 11. In this case, it is preferable that the area ratio of the iron-based carbide at each of the two end portions 12E of the nugget 12 is 0.3% or more and the area ratio of the iron-based carbide at the center portion 12C of the nugget 12 is 0.2% or less on the imaginary line 13L along the joint interface 13 between the high carbon steel plate 11 and the steel plate 11 superposed on the high carbon steel plate 11. In other words, it is preferable that the area ratio of the iron-based carbide at each of the two end portions 12E and the center portion 12C of the nugget 12 is within the above-mentioned range at the joint interface 13 of the high carbon steel plate 11. This is because the high carbon steel plate 11 has a role of ensuring the strength of the resistance spot welded joint 1, and the joint interface 13 of the high carbon steel plate 11 greatly contributes to the joint strength of the resistance spot welded joint 1.

[0049] (2. Automotive parts) Next, an automobile part according to this embodiment will be described. The automobile part according to this embodiment includes the resistance spot welded joint 1 according to this embodiment. Therefore, the automobile part according to this embodiment is excellent in both tensile shear strength (TSS) and cross tensile strength (CTS).

[0050] Examples of automotive parts include bumpers, A-pillars, B-pillars, side sills, roof rails, floor members, front side members, front side member kick parts, rear side members, front suspension towers, tunnel reinforcements, dash panels, torque boxes, seat frameworks, seat rails, battery case frames, and their joints. Examples of joints include the joint between the B-pillar and side sill, the joint between the B-pillar and roof rail, the joint between the roof cross member and roof rail, the joint between the side sill and A-pillar, the joint between the dash panel and the tunnel, and the base of the front side member.

[0051] (3. Manufacturing method of resistance spot welded joint 1) Next, a method for manufacturing the resistance spot welded joint 1 according to this embodiment will be described. As shown in Fig. 6 etc., the method for manufacturing the resistance spot welded joint 1 according to this embodiment includes a step of spot welding two or more overlapping steel plates 2 and a step of post-energizing the nugget 12 formed by the spot welding, and reduces the current passing area during the post-energization.

[0052] (Spot welding) First, two or more stacked steel sheets 2 are spot-welded. The steel sheets 2 are melted and solidified by spot welding to form a nugget 12. The nugget 12 joins the steel sheets 2. Note that the application of current to form the nugget 12 may be referred to as a main current application.

[0053] (After energization) Next, post-current is applied to the nugget 12. Post-current means that after spot welding, a post-heating current is applied to the nugget 12 for the purpose of tempering the hardened nugget 12. The post-heating current is also called a tempering current.

[0054] In the manufacturing method of the resistance spot welded joint 1 according to this embodiment, the current flow area is reduced during the post-current flow. That is, the current flow area A2 (see FIGS. 7 and 8) in the later period of the post-current flow is made smaller than the current flow area A1 (see FIG. 6) in the early period of the post-current flow. The current flow area refers to the cross-sectional area of ​​the current path C. The current flow area is roughly proportional to the contact area between the tip of the electrode and the steel sheet 2.

[0055] The method for reducing the current-carrying area is not particularly limited. For example, as shown in FIG. 7, the current-carrying area can be reduced by reducing the pressing force. By reducing the pressing force, the contact area between the electrode 3 and the steel sheet 2 is reduced, and the current-carrying area is reduced. Also, as shown in FIG. 8, the current-carrying area can be reduced by changing the electrode 3 to an electrode 4 having a smaller tip diameter than the electrode 3. By reducing the tip diameter of the electrode, the contact area between the electrode and the steel sheet 2 is reduced, and the current-carrying area is reduced.

[0056] (Action and effect) In the manufacturing method of the resistance spot welded joint 1 according to this embodiment, the current passing area A1 is increased in the early period of the post-current passing. As a result, the entire nugget 12 is heated uniformly in the early period of the post-current passing. On the other hand, the current passing area A2 is decreased in the later period of the post-current passing. As a result, the heat input to the center portion 12C of the nugget 12 is increased while the heat input to the end portion 12E of the nugget 12 is decreased in the later period of the post-current passing.

[0057] As a result, in the early stage of post-current, the entire nugget 12 is heated uniformly. In the later stage of post-current, the central portion 12C of the nugget 12 is preferentially heated. As a result, during post-current, only the central portion 12C of the nugget 12 can be heated to the re-hardening temperature. According to the manufacturing method for a resistance spot welded joint according to this embodiment, the central portion 12C of the nugget 12 can be made into a re-hardened structure with an area ratio of iron-based carbides of 0.2% or less, and the end portion 12E of the nugget 12 can be made into a tempered structure with an area ratio of iron-based carbides of 0.3% or more.

[0058] A method for re-hardening only the center portion of the nugget 12 will be specifically described with reference to Fig. 9. Fig. 9 shows an example of a current pattern in which the current passing area is reduced by reducing the pressing force. The dashed line in Fig. 9 indicates the pressing force. The solid line in Fig. 9 indicates the current value.

[0059] At the start of post-energization (t1 in FIG. 9), the structure of the nugget 12 is a hardened structure (e.g., martensite). This is because the nugget 12 formed by spot welding is rapidly cooled from a high temperature near the melting point. A coolant flows inside the spot welding electrodes 3 that hold the welded portion. The welded portion is rapidly cooled by the heat transfer from the welded portion to the electrodes 3.

[0060] In the early period of post-current, the pressing force is large, and therefore the current passing area is large. Therefore, in the early period of post-current, the entire nugget 12 is uniformly heated. If post-current is terminated before the pressing force is reduced (t2 in FIG. 9), the entire nugget 12 is tempered. The structure of the nugget 12 becomes a tempered structure (e.g., tempered martensite) as a whole. And, the area ratio of iron-based carbides becomes 0.3% or more in both end portions 12E and a central portion 12C of the nugget 12.

[0061] In the later stage of the post-current, the pressing force is small, so the current area is small. Therefore, in the later stage of the post-current, the central portion of the nugget 12 is heated preferentially. As a result, only the central portion 12C of the nugget 12 is heated to the re-quenching temperature, and the end portion 12E of the nugget 12 can be heated to a tempering temperature lower than the re-quenching temperature. When the post-current is terminated at the time when a temperature difference is generated inside the nugget 12 (t3 in FIG. 9), the central portion 12C of the nugget 12 is re-quenched, and the end portion 12E of the nugget 12 is tempered. The structure of the end portion 12E of the nugget 12 becomes a tempered structure, and the area ratio of iron-based carbides becomes 0.2% or less. The structure of the central portion 12C of the nugget 12 mainly becomes a re-quenched structure (e.g., re-quenched martensite), and the area ratio of iron-based carbides becomes 0.3% or more.

[0062] The above has described the most basic aspect of the method for manufacturing the resistance spot welded joint 1 according to this embodiment. Next, a more preferred aspect of the method for manufacturing the resistance spot welded joint 1 according to this embodiment will be described.

[0063] (The best time to reduce the current carrying area) The time point at which the current passing area is reduced is not particularly limited, but a suitable example will be described below. When the current value in the post-current is controlled to a constant value, it is preferable to reduce the current passing area in a period within a range of T / 2 seconds to 3T / 2 seconds from the start of the post-current, as shown in Fig. 9. "T" means the time from the start to the end of the post-current. This ensures that the temperature of both end portions 12E of the nugget 12 can be reliably raised to the tempering temperature, and that the temperature of the center portion 12C of the nugget 12 can be reliably raised to the re-quenching temperature.

[0064] The current value in the post-current does not have to be a constant value. For example, up-slope current in which the current value increases continuously, or down-slope current in which the current value decreases continuously may be applied to the post-current. Multi-stage current in which the current value increases and decreases stepwise may be applied to the post-current. In this case, the time point at which the current area is reduced can be determined based on the heat input. The time integral of the current value can be used as an index of the heat input. For example, if the time integral of the current value during the period from the start to the end of the post-current is defined as I, and the time integral of the current value during the period from the start of the post-current to the time t at which the current area is switched is defined as It, I and It should satisfy the following relationship. I / 2≦It≦2I / 3 This makes it possible to reliably raise the temperature of both end portions 12E of the nugget 12 to the tempering temperature, and also to reliably raise the temperature of the central portion 12C of the nugget 12 to the re-hardening temperature.

[0065] (A good example of a method for reducing the current carrying area: Reducing the applied pressure) A preferred example of a method for reducing the current-carrying area is to reduce the pressure. Although the specific amount of reduction in the pressure is not particularly specified, it is preferred to reduce the pressure by, for example, reducing the pressure from F1 to F2. It is preferred that F1 and F2 are values ​​that satisfy the following formula: 0.40≦F2 / F1≦0.80 This makes it possible to reliably raise the temperature of both end portions 12E of the nugget 12 to the tempering temperature, and also to reliably raise the temperature of the central portion 12C of the nugget 12 to the re-hardening temperature.

[0066] (A good example of a method for reducing the current carrying area: changing the tip diameter of the electrode) Another preferred example of a method for reducing the current-carrying area is to change the tip diameter of the electrode, as shown in Figures 6 and 8. Specifically, it is preferred to replace the first electrode 3 used for the post-current-carrying with a second electrode 4 having a smaller tip diameter during the post-current-carrying. The current-carrying area is determined according to the tip diameter of the electrode. Therefore, the current-carrying area can be reduced by reducing the tip diameter of the electrode.

[0067] Although the specific tip diameters of the electrodes are not particularly limited, for example, it is preferable that the tip diameter d1 of the first electrode 3 and the tip diameter d2 of the second electrode 4 satisfy the following formula. 0.40≦d2 / d1≦0.80 By setting the ratio d2 / d1 of the tip diameter d1 of the first electrode 3 and the tip diameter d2 of the second electrode 4 to be 0.40 or more and 0.80 or less, the temperature of both end portions 12E of the nugget 12 can be reliably raised to the tempering temperature, and the temperature of the central portion 12C of the nugget 12 can be reliably raised to the re-hardening temperature.

[0068] The first electrodes 3 are a pair of electrodes for spot welding. The tip diameters of the pair of first electrodes 3 illustrated in FIG. 6 are the same. On the other hand, the tip diameters of the pair of first electrodes 3 may be different. In this case, the tip diameter d1 of the first electrode 3 mentioned above means the larger of the tip diameters of the pair of first electrodes 3. It is presumed that the electrode with the larger tip diameter is dominant in expanding the current path.

[0069] The second electrodes 4 are also a pair of electrodes for spot welding. The tip diameters of the pair of second electrodes 4 illustrated in Fig. 8 are the same. On the other hand, the tip diameters of the pair of second electrodes 4 may be different. In this case, the tip diameter d2 of the second electrode 4 described above means the larger value of the tip diameters of the pair of second electrodes 4.

[0070] (Other configurations) Although the embodiment of the present invention has been described above, the present invention is not limited thereto and can be modified as appropriate without departing from the technical idea of ​​the invention. More preferred examples of the resistance spot welded joint 1, automotive parts, and manufacturing method of the resistance spot welded joint 1 according to the present embodiment will be described below. Note that, unless otherwise specified, the preferred aspects described below can be applied to any of the resistance spot welded joint 1, automotive parts, and manufacturing method of the resistance spot welded joint 1.

[0071] (Tensile strength, thickness, and surface treatment of steel plate 11) The tensile strength of the steel plate 11 is not particularly limited. From the viewpoint of improving the rigidity and fracture resistance of the resistance spot welded joint 1, it is preferable that at least one of the multiple steel plates 11 is a high carbon steel plate 11 having a tensile strength of 980 MPa or more. It is more preferable that the tensile strength of the high carbon steel plate 11 is 1000 MPa or more, 1200 MPa or more, or 1500 MPa or more. On the other hand, at least one of the multiple steel plates 11 may be a mild steel having a tensile strength of 500 MPa or less.

[0072] The thickness of the steel plate 11 is not particularly limited. The plate thickness ratio of the plate assembly is also not particularly limited. The plate thickness ratio of the plate assembly is a value obtained by dividing the total plate thickness of the plate assembly by the thinner plate thickness of the steel plates 11 arranged on the surface of the plate assembly. For example, when the resistance spot welded joint 1 is an automobile part, it is preferable that the steel plates 11 are two thick high carbon steel plates 11 and one thin mild steel plate 11. The high carbon steel plate 11 is a frame member of the automobile. The mild steel plate 11 is an exterior member of the automobile. The plate thickness of the high carbon steel plate 11 is preferably, for example, 1.0 to 2.5 mm. The plate thickness of the mild steel plate 11 is preferably, for example, 0.4 to 1.2 mm.

[0073] In order to improve corrosion resistance and aesthetics, it is preferable that a plating is provided on the surface of the steel sheet 11. Examples of the types of plating include Al-based plating, Al-based alloy plating, Zn-based plating, and Zn-based alloy plating.

[0074] (Diameter of nugget 12) The diameter of the nugget 12 is not particularly limited and can be appropriately selected within a range that does not cause poor bonding. For example, the diameter of the nugget 12 may be within a range of 2.5√t to 5.0√t. From the viewpoint of preventing poor bonding, it is more preferable that the diameter of the nugget 12 is 2.8√t or more, 3.0√t or more, or 3.2√t or more. Also, from the viewpoint of suppressing the amount of heat input and preventing the occurrence of expulsion, it is more preferable that the diameter of the nugget 12 is 4.8√t or more, 4.5√t or more, or 4.2√t or more.

[0075] The diameter of the nugget 12 refers to the diameter of the nugget 12 measured at the joint interface 13 by a cross-sectional test of the welded portion. t refers to the average value of the sheet thicknesses of the two steel sheets 11 constituting the joint interface 13 at which the diameter of the nugget 12 was measured. When the number of steel sheets 11 is three or more and the number of joint interfaces 13 is two or more, it is preferable that the diameter of the nugget 12 at one or more joint interfaces 13 is within the above-mentioned range, and it is even more preferable that the diameter of the nugget 12 at all joint interfaces 13 is within the above-mentioned range.

[0076] (Measurement method) The method for evaluating a resistance spot welded joint according to this embodiment will be described below.

[0077] The joint interface 13 and the imaginary line 13L along the joint interface 13 are specified by the following procedure. First, the resistance spot welded joint is cut along a plane that passes through the center or the vicinity of the indentation 14 in a plan view and is approximately perpendicular to the surface of the steel sheet 11. The cross section is polished and etched as necessary. As shown in FIG. 1, a gap called a sheet separation 15 is usually generated between the steel sheets 11 around the nugget 12. The bottom of the sheet separation 15 is an area where stress is concentrated during a cross tensile test. Therefore, the line connecting the bottoms of the two sheet separations 15 on both sides of the nugget 12 is regarded as the joint interface 13 and the imaginary line 13L along the joint interface 13.

[0078] The area ratio of the iron-based carbide at both ends 12E of the nugget 12 on the virtual line 13L along the joint interface 13 is specified by the following procedure. The above-mentioned cross section is appropriately adjusted to make the iron-based carbide visible. The area ratio of the iron-based carbide at both ends 12E of the nugget 12 is measured in an observation field that satisfies the following requirements. - Rectangular shape of 400μm square The center is on the imaginary line 13L along the joint interface 13. One vertex coincides with the outer edge (fusion boundary) of nugget 12 The two sides are parallel to the imaginary line 13L along the joint interface 13. A backscattered electron image is taken of each of the observation fields that satisfy the above-mentioned requirements at each of the end portions 12E of the nugget 12. In the backscattered electron image, iron-based carbides can be easily identified. Then, the area ratio of the iron-based carbides in the observation field is calculated for each of the end portions 12E. If the area ratio of the iron-based carbides at each of the end portions 12E is equal to or greater than a predetermined value, the area ratio of the iron-based carbides at the end portions 12E is deemed to be acceptable.

[0079] The area ratio of iron-based carbides in the central portion 12C of the nugget 12 on the imaginary line 13L along the joint interface 13 is measured in an observation field that satisfies the following requirements. - Rectangular shape of 400μm square The center is located midway between the centers of the observation fields of both ends of the nugget 12 described above. The center is on the imaginary line 13L along the mating surface of the steel plate 11. The two sides are parallel to the imaginary line 13L along the mating surface of the steel plate 11. By capturing a reflected electron image in the observation field, it becomes possible to measure the area ratio of iron-based carbides in the central portion 12C of the nugget 12.

[0080] The carbon content of the steel plate 11 is specified in accordance with JIS G 1201:2014 "Iron and steel - General rules for analytical methods". The tensile strength of the steel plate 11 is specified in accordance with JIS Z 2241:2011 "Method of tensile test for metallic materials". In either case, the location of the sample is sufficiently far from the welded part. The shape of the test piece for the tensile test can be appropriately selected according to the shape of the steel plate 11. If the resistance spot welded joint does not have a sufficiently wide flat part and a test piece for the tensile test cannot be taken, the Vickers hardness of the steel plate may be measured instead of measuring the tensile strength of the steel plate. The tensile strength of the steel plate can be estimated from the hardness of the steel plate using a known conversion table. The Vickers hardness is measured in accordance with JIS Z 2244:2009 "Vickers hardness test - Test method". The test force is 300 gf.

[0081] The diameter of the nugget 12 is measured based on an imaginary line 13L along the joint interface 13, which is specified by the above-mentioned method. The distance between the outer edge of the nugget 12 and two intersections of the imaginary line 13L is the diameter of the nugget 12 measured along the imaginary line 13L. EXAMPLES

[0082] The effects of one embodiment of the present invention will be explained more specifically with reference to an example. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to this example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0083] (Example of pressure control) Various resistance spot welded joints were created by spot welding (main current) two overlapping steel plates. Post-current was applied to the nuggets of some of the resistance spot welded joints. In addition, the pressure was reduced in the post-current application to the nuggets of some of the resistance spot welded joints.

[0084] The specific manufacturing conditions are as follows: Carbon content of steel plate: As shown in Table 1 · Thickness of steel plate t: 2.0mm for both steel plate 1 and steel plate 2 Actual weld conditions: Current value was changed so that the nugget diameter was 5√t (weld time 18cyc (=360ms)) Post-heating current value in post-heating: 0.6 to 0.65 times the main heating current value Pressure F1 during the first period of post-current application: 3.9kN - Pressure F2 at the later stage of post-current application: 2.0kN Time from start to end of post-current application T: 1.98s Timing for changing pressure from F1 to F2: 1.20s

[0085] The cross tensile strength (CTS) of the resistance spot welded joints was measured by a cross tensile test specified in JIS Z 3137: 1999. The tensile shear strength (TSS) of the resistance spot welded joints was measured by a tensile shear test specified in JIS Z 3136: 1999. The CTS and TSS are shown in Table 1.

[0086] In addition, the area ratio of iron-based carbides at the end of the nugget on a virtual line along the joint interface, measured on the cross section of the nugget, is listed in Table 1. The area ratio was measured at both ends. The smaller of the measured values ​​at each end is listed in Table 1. When the value listed in Table 1 is 0.3% or more, the area ratio of iron-based carbides at each end is 0.3% or more. The area ratio of iron-based carbides at the center of the nugget on a virtual line along the joint interface, measured on the cross section of the nugget, is also listed in Table 1. In addition, in examples where post-current was not applied, the measurement of the area ratio of iron-based carbides was omitted.

[0087] [Table 1]

[0088] All of Examples 1 to 3 were resistance spot welded joints in which the carbon content of both steel plates was 0.20 mass%. In Example 1, no post-energization was performed. The CTS and TSS of Example 1 were used as evaluation criteria for the resistance spot welded joints of Examples 2 and 3. Resistance spot welded joints with a CTS greater than that of Example 1 were determined to have excellent CTS. In addition, resistance spot welded joints with a TSS higher than "TSS-3kN of Example 1" were determined to have excellent TSS.

[0089] In Example 2, current was applied after the pressing force was kept constant. In the nugget of Example 2, the area ratio of iron-based carbides in the center of the nugget was excessive. The CTS of Example 2 was higher than that of Example 1. However, the TSS of Example 2 was significantly lower than that of Example 1.

[0090] In Example 3, post-current was applied to reduce the pressure midway. In the nugget of Example 3, the area ratio of iron-based carbides was appropriate in the center and both ends of the nugget. The CTS of Example 3 was higher than that of Example 1. Furthermore, the TSS of Example 3 was almost the same as that of Example 1. In the resistance spot welded joint of Example 3, both the TSS and CTS were excellent.

[0091] All of Examples 4 to 6 were resistance spot welded joints in which the carbon content of both steel plates was 0.35 mass%. In Example 4, no post-energization was performed. The CTS and TSS of Example 4 were used as evaluation criteria for the resistance spot welded joints of Examples 5 and 6. Resistance spot welded joints with a CTS greater than that of Example 4 were determined to have excellent CTS. In addition, resistance spot welded joints with a TSS higher than "TSS-3kN of Example 4" were determined to have excellent TSS. The CTS of Example 4 was significantly lower than that of Example 1.

[0092] In Example 5, current was applied after the pressing force was kept constant. In the nugget of Example 5, the area ratio of iron-based carbides in the center of the nugget was excessive. The CTS of Example 5 was significantly higher than that of Example 4. However, the TSS of Example 5 was lower than that of Example 4.

[0093] In Example 6, post-current was applied to reduce the pressure midway. In the nugget of Example 6, the area ratio of iron-based carbides was appropriate in the center and both ends of the nugget. The CTS of Example 6 was higher than that of Example 4. Furthermore, the TSS of Example 6 was almost the same as that of Example 4. In the resistance spot welded joint of Example 6, both the TSS and CTS were excellent.

[0094] (Example of changing electrodes) Various resistance spot welded joints were created by spot welding (main current) two overlapping steel plates. Post-current was performed on the nuggets of some of the resistance spot welded joints. In addition, the electrode was changed in the post-current of some of the nuggets of the resistance spot welded joints. Specifically, in the early stage of the post-current, the electrode used in the main current was used as is. In the later stage of the post-current, an electrode with a smaller electrode tip diameter than the electrode used in the main current was used.

[0095] The specific manufacturing conditions are as follows: Carbon content of steel plate: As shown in Table 2 · Thickness of steel plate t: 2.0mm for both steel plate 1 and steel plate 2 Actual weld conditions: Current value is changed so that the nugget diameter is 5√t (weld time 18 cycles (=360 ms)) Post-heating current value in post-heating: 0.6 to 0.65 times the main heating current value Pressure F1 during the first period of post-current application: 3.9kN - Pressure F2 at the later stage of post-current application: 3.0kN Time from start to end of post-current application T: 1.98s Timing for changing electrodes: 1.20s

[0096] Then, using the same procedures as in the experiment in Table 1, the CTS, TSS, area ratio of iron-based carbides at the end of the nugget, and area ratio of iron-based carbides at the center of the nugget of the resistance spot welded joint were measured and the results are listed in Table 2.

[0097] [Table 2]

[0098] Both Examples 7 and 8 were resistance spot welded joints in which the carbon content of both steel plates was 0.20 mass %, similar to Example 1. The CTS and TSS of Example 1 were used as evaluation criteria for the resistance spot welded joints of Examples 7 and 8.

[0099] In Example 7, current was passed after the tip diameter of the electrode was kept constant. In the nugget of Example 7, the area ratio of iron-based carbides in the center of the nugget was excessive. The CTS of Example 7 was higher than that of Example 1. However, the TSS of Example 7 was significantly lower than that of Example 1.

[0100] In Example 8, post-current was applied to reduce the tip diameter of the electrode midway. In the nugget of Example 8, the area ratio of iron-based carbides was appropriate in the center and both ends of the nugget. The CTS of Example 8 was higher than that of Example 1. Furthermore, the TSS of Example 8 was almost the same as that of Example 1. In the resistance spot welded joint of Example 8, both the TSS and CTS were excellent.

[0101] Both Examples 9 and 10 were resistance spot welded joints in which the carbon content of both steel plates was 0.35 mass %, similar to Example 4. The CTS and TSS of Example 4 were used as evaluation criteria for the resistance spot welded joints of Examples 9 and 10.

[0102] In Example 9, current was passed after the tip diameter of the electrode was kept constant. In the nugget of Example 9, the area ratio of iron-based carbides in the center of the nugget was excessive. The CTS of Example 9 was higher than that of Example 4. However, the TSS of Example 9 was significantly lower than that of Example 4.

[0103] In Example 10, post-current was applied to reduce the tip diameter of the electrode midway. In the nugget of Example 10, the area ratio of iron-based carbides was appropriate in the center and both ends of the nugget. The CTS of Example 10 was higher than that of Example 4. Furthermore, the TSS of Example 10 was almost the same as that of Example 4. In the resistance spot welded joint of Example 10, both the TSS and CTS were excellent. [Explanation of symbols]

[0104] 1 Resistance spot welded joints 11 Steel plate 12. Nugget 12E End 12C central part 13 Joint interface 13L Virtual Line 14 Indentation 15 Seat separation 2. Steel plate (before welding) 3 electrodes (first electrode) 4 Second electrode C Current Path

Claims

1. Two or more stacked steel plates; A nugget that joins the steel plates; Equipped with an area ratio of iron-based carbides at each of both ends of the nugget along a virtual line along the joint interface, which is measured on a cross section of the nugget, is 0.3% or more; The area ratio of the iron-based carbides in the center of the nugget on the virtual line along the joint interface, as measured in the cross section of the nugget, is 0.2% or less. Resistance spot welded joints.

2. At least one of the steel plates is a high carbon steel plate having a carbon content of 0.20 mass% or more.

2. The resistance spot welded joint of claim 1.

3. The number of the steel plates is three or more, In the imaginary line along the joint interface between the high carbon steel plate and the steel plate overlapped on the high carbon steel plate, the area ratio of the iron-based carbides at each of the two end portions of the nugget is 0.3% or more, and the area ratio of the iron-based carbides at the central portion of the nugget is 0.2% or less.

3. The resistance spot welded joint of claim 2.

4. An automobile part comprising the resistance spot welded joint according to any one of claims 1 to 3.

5. A step of spot welding two or more stacked steel plates; a step of post-energizing the nugget formed by the spot welding; Equipped with The current passing area is reduced during the post-current passing. A method for manufacturing resistance spot welded joints.

6. The current value in the post-energization is controlled to a constant value, The time from the start to the end of the post-energization is T seconds. The current application area is reduced during a period within a range of T / 2 seconds to 3T / 2 seconds from the start of the post-current application. The method for manufacturing a resistance spot welded joint according to claim 5 .

7. In the middle of the post-current application, the pressure is reduced from F1 to F2 to reduce the current application area, F2 / F1 is set to 0.40 or more and 0.80 or less.

7. The method for manufacturing a resistance spot welded joint according to claim 5 or 6.

8. During the post-current application, the first electrode for the post-current application is replaced with a second electrode having a tip diameter smaller than that of the first electrode, thereby reducing the current application area.

7. The method for manufacturing a resistance spot welded joint according to claim 5 or 6.

9. A ratio d2 / d1 of a tip diameter d1 of the first electrode to a tip diameter d2 of the second electrode is set to 0.40 or more and 0.80 or less. The method for manufacturing a resistance spot welded joint according to claim 8.

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