Method of manufacturing welded joints

Abstract

To provide a manufacturing method of a weld joint with high strength of joint while suppressing dust generation.SOLUTION: A manufacturing method of a weld joint comprises the steps of: overlapping two or more steel plates while arranging first and second steel plates at one and the other outermost sides, respectively; pressurizing and holding the overlapped parts of the steel plates with first and second electrodes and a first pressurizing component arranged around the first electrode; and welding through conducting weld current between the first electrode and the second electrode. Carbon equivalents Ceq1, Ceq2 of the first and second steel plates satisfy the following expression (2): Ceq1<Ceq2 ...(2). The first electrode and the first pressurizing component are in contact with the first steel plate and the second electrode is in contact with the second steel plate in the pressurizing and holding step. The weld current is direct current which flows through the first electrode as a negative pole and the second electrode as a positive pole in the welding step.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This disclosure relates to a method for manufacturing welded joints. [Background technology]

[0002] The body of an automobile includes skeletal components for preventing deformation during collisions and absorbing collision energy, and panel components positioned on the outermost surface of the body so that the skeletal components are not exposed to the outside. Spot welding is used as a joining technique to connect the thick, high-strength steel plates that form the skeletal components with the thin, mild steel plates that form the panel components.

[0003] For example, Patent Document 1 discloses a technique for spot welding a thin steel plate onto the outermost layer of at least one of several overlapping steel plates. By providing a constricted portion on one of the spot welding electrodes that contacts the thin steel plate, the spread of the current path at the overlapping surface between the thin steel plate and the thick steel plate is controlled, forming a nugget at the overlapping surface and ensuring good joint characteristics. For example, good joint characteristics are ensured by spot welding two thick steel plates with a tensile strength of 590 MPa and a thickness of 1.4 mm to one steel plate with a tensile strength of 270 MPa and a thickness of 0.6 mm.

[0004] Patent Document 2 describes a method for overlapping and welding two or more steel plates, and an overlapping welded joint for steel plates. In the overlapping welding method of Patent Document 2, the two or more steel plates include a surface-side steel plate with a thickness in the range of 0.3 to 1 mm, and one or more high-thickness steel plates with a thickness greater than that of the surface-side steel plate. Furthermore, the ratio of the thickness of the surface-side steel plate to the total thickness of the surface-side steel plate and the one or more high-thickness steel plates (total thickness / thickness of surface-side steel plate) is 5 or more. Then, with the two or more steel plates overlapping so that the surface-side steel plate is positioned on the surface side, welding is performed using a combination of laser welding and spot welding. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2016-193451 [Patent Document 2] Japanese Patent Publication No. 2010-264503 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] To achieve both high strength (1180 MPa or higher) and ductility, or to ensure high strength (1470 MPa or higher), ultra-high-strength steel sheets containing large amounts of alloying elements such as C, Si, and Mn have been developed for use in automotive structural components. On the other hand, panel components placed on the outermost surface generally use mild steel sheets (e.g., galvanized steel sheets) with low alloying element content, low strength, and thinness (e.g., less than 1 mm thick). Therefore, it is necessary to join ultra-high-strength steel sheets with a high alloying element content to mild steel sheets with a low alloying element content and low strength.

[0007] In this type of steel plate assembly, ultra-high-strength steel plates have a higher electrical resistance and lower melting point compared to mild steel plates because they contain more alloying elements. Therefore, ultra-high-strength steel plates preferentially generate heat and melt. On the other hand, mild steel plates have a lower alloying element content, resulting in lower electrical resistance and a higher melting point, making them less prone to generating heat and melting. The outermost mild steel plate is prone to insufficient welding with the ultra-high-strength steel plate due to the cooling effect of the electrode tip, insufficient heat generation due to its relatively low electrical resistance, and its high melting point.

[0008] To facilitate the welding of mild steel plates, increasing the welding current in spot welding is effective. However, if the welding current becomes too high, the ultra-high-strength steel plate will melt excessively, and the mild steel plate will deform significantly, generating spatter. When spatter occurs, the amount of metal in the spot weld decreases, which may result in insufficient joint strength. Furthermore, the molten metal scattered by the spatter can contaminate the area around the weld and potentially damage the welding equipment and the worker. Therefore, the welding current can only be increased to a level where spatter does not occur, and it may not be possible to sufficiently improve the welding of mild steel plates.

[0009] One embodiment of the present invention solves the problem of spot welding two or more steel plates having different alloying element content, such as ultra-high-strength steel plates and mild steel plates, and provides a method for manufacturing a welded joint with high joint strength while suppressing the generation of dust. [Means for solving the problem]

[0010] One aspect of the present invention is: A method for manufacturing a welded joint by spot welding two or more steel plates, The process involves placing the first steel plate on the outermost edge of one side and the second steel plate on the outermost edge of the other side, thereby overlapping the two or more steel plates. The process involves pressing and clamping the overlapping portion of the two or more steel plates using a first electrode and a second electrode positioned opposite each other with respect to the overlapping portion, and a first pressing member positioned around the first electrode. The process includes a step of welding the overlapping portion by passing a welding current between the first electrode and the second electrode, The carbon equivalent of the first steel plate, calculated from the following equation (1), is C eq 1. The carbon equivalent of the second steel plate is C eq When set to 2, C eq 1 and C eq 2 satisfies the following equation (2): In the step of applying pressure and clamping, the first electrode and the first pressure member contact the first steel plate, and the second electrode contacts the second steel plate. In the step of welding, the welding current is a direct current passed with the first electrode as the negative electrode and the second electrode as the positive electrode. This is a method for manufacturing a welded joint. Carbon equivalent C eq (mass%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ···(1) Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], and [V] are the contents of C, Mn, Si, Ni, Cr, Mo, and V respectively, expressed in mass%. C eq 1 < C eq 2 ···(2)

[0011] Aspect 2 of the present invention is The C eq 1 is 0.40 mass% or less, and the C eq 2 is 0.60 mass% or more. This is the method for manufacturing a welded joint according to Aspect 1.

[0012] Aspect 3 of the present invention is The first electrode has an electrode tip that contacts the first steel plate. The first pressure member has a pressure tip that contacts the first steel plate around the electrode tip of the first electrode. The pressure tip is configured to apply pressure over a range of 10° or more in the circumferential direction centered on the electrode tip. The distance between the pressure tip and the electrode tip is 0.1 mm or more and 10.0 mm or less. This is the method for manufacturing a welded joint according to Aspect 1 or 2.

[0013] Aspect 4 of the present invention is In the step of applying pressure and clamping, the overlapping portion is pressurized by the first electrode and the second electrode with an electrode load of 1.5 kN or more, and is also pressurized by the first pressing member with an outer peripheral load of 0.1 to 1.0 times the electrode load, in a method for manufacturing a welded joint according to any one of embodiments 1 to 3.

[0014] Aspect 5 of the present invention is A method for manufacturing a welded joint according to any one of embodiments 1 to 4, wherein the tensile strength of the first steel plate is 500 MPa or less, and the tensile strength of the second steel plate is 1180 MPa or more.

[0015] Aspect 6 of the present invention is, A method for manufacturing a welded joint according to any one of embodiments 1 to 5, wherein the thickness of the first steel plate is less than 1.0 mm and the thickness of the second steel plate is 1.0 mm or more.

[0016] Aspect 7 of the present invention is The method for manufacturing a welded joint according to any one of embodiments 1 to 6, wherein the pressing and clamping of the overlapping portion in the pressing and clamping step further uses a second pressing member which is arranged around the second electrode and is positioned opposite the first pressing member with the overlapping portion in between. [Effects of the Invention]

[0017] According to a method for manufacturing a welded joint according to one embodiment of the present invention, it is possible to manufacture a welded joint that suppresses the generation of dust during welding and has high joint strength. [Brief explanation of the drawing]

[0018] [Figure 1] This is a schematic cross-sectional view illustrating a method for spot welding two steel plates. [Figure 2] This diagram shows the schematic configuration of the first electrode and first pressurizing member for spot welding; Figure 2(a) is a bottom view, and Figure 2(b) is a cross-sectional view along line AA in Figure 2(a). [Figure 3]These are schematic cross-sectional views of the first electrode and first pressurizing member for spot welding ((a), (b)). [Figure 4] These are schematic cross-sectional views of a welded joint fabricated by spot welding two steel plates in the example ((a), (b)). [Modes for carrying out the invention]

[0019] The inventors conducted diligent research to solve the problems that arise when welding steel plates with different alloying element content using conventional spot welding. These problems include the inability to obtain sufficient joint strength due to differences in electrical resistance and melting point, and the fact that steel plates with a lower carbon equivalent are relatively softer, more prone to deformation, and more likely to generate dust. Here, the effect of alloying elements on the strength and melting point of steel sheets differs depending on the type of element. Therefore, in the embodiments of the present invention, the content of alloying elements is set to the carbon equivalent C eq We investigated a method for manufacturing welded joints by spot welding steel plates with different carbon equivalents, after converting them to carbon equivalents.

[0020] The inventors discovered that when spot welding is performed using a direct current, the heat generation increases in the steel plate placed on the negative electrode side, while heat generation is suppressed in the steel plate placed on the positive electrode side. Based on this finding, by placing a steel plate with a low carbon equivalent on the negative electrode side and a steel plate with a high carbon equivalent on the positive electrode side and performing spot welding, it is possible to promote the melting of the steel plate with the low carbon equivalent and form a welded joint with sufficient joint strength.

[0021] However, steel plates with a low carbon equivalent are soft, and if melting is accelerated, they are prone to deformation, which can cause dust generation. Therefore, by placing a pressurizing member around the negative electrode and applying pressure to the negative electrode steel plate, deformation of the steel plate is suppressed, effectively preventing dust generation.

[0022] In other words, the method for manufacturing a welded joint according to an embodiment of the present invention is a method for manufacturing a welded joint by spot welding two or more steel plates, The process involves placing the first steel plate on the outermost edge of one side and the second steel plate on the outermost edge of the other side, thereby overlapping the two or more steel plates. The process involves pressing and clamping the overlapping portion of the two or more steel plates using a first electrode and a second electrode positioned opposite each other with respect to the overlapping portion, and a first pressing member positioned around the first electrode. The process includes a step of welding the overlapping portion by passing a welding current between the first electrode and the second electrode, The carbon equivalent of the first steel plate, calculated from the following equation (1), is C eq 1. The carbon equivalent of the second steel plate is C eq When set to 2, C eq 1 and C eq 2 satisfies the following equation (2): In the pressing and clamping step, the first electrode and the first pressing member are in contact with the first steel plate, and the second electrode is in contact with the second steel plate. In the welding process described above, the welding current is a direct current that flows with the first electrode as the negative electrode and the second electrode as the positive electrode. carbon equivalent C eq (mass%)=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+ [V] / 14...(1) Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], and [V] are the mass percentages of C, Mn, Si, Ni, Cr, Mo, and V, respectively. C eq 1 <C eq twenty two)

[0023] This document details the manufacturing method for welded joints.

[0024] The method for manufacturing a welded joint according to this embodiment is as follows: The process involves placing a first steel plate P1 on the outermost edge of one side and a second steel plate P2 on the outermost edge of the other side, thereby overlapping two or more steel plates. A step of pressing and clamping the overlapping portion of two or more steel plates using a first electrode 11 and a second electrode 12 positioned opposite each other with the overlapping portion in between, and a first pressurizing member 2 positioned around the first electrode 11, The process includes a step of welding the overlapping portion by passing a welding current between the first electrode 11 and the second electrode 12.

[0025] (1. The process of overlapping) In the process of overlapping steel plates, as shown in Figure 1, the first steel plate P1 is placed on the outermost edge of one plate, and the second steel plate P2 is placed on the outermost edge of the other plate, thereby overlapping two or more steel plates.

[0026] The first steel plate P1 and the second steel plate P2 are steel plates with different carbon equivalents. The carbon equivalent of the first steel plate is calculated from the following formula (1) and C eq 1. The carbon equivalent of the second steel plate is C eq When set to 2, C eq 1 and C eq 2 satisfies the following equation (2).

[0027] carbon equivalent C eq (mass%)=[C]+[Mn] / 6+[Si] / 24+[Ni] / 40+[Cr] / 5+[Mo] / 4+ [V] / 14...(1) Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], and [V] are the mass percentages of C, Mn, Si, Ni, Cr, Mo, and V, respectively. Furthermore, the steel sheet does not need to contain one or more of the elements C, Mn, Si, Ni, Cr, Mo, and V. If it does not contain one or more of these elements, the content of those elements is treated as "0 mass%" when calculating formula (1).

[0028] C eq 1 <C eq twenty two)

[0029] C eq1 is preferably 0.40% by mass or less, C eq 2 is preferably 0.60% by mass or more. Conventional welding methods failed to produce sufficient joint strength when welding steel plates with carbon equivalents differing by 0.20 mass% or more. However, according to the manufacturing method of welded joints of this embodiment, welded joints with sufficient joint strength can be manufactured even with such steel plate assemblies. C eq 1 is more preferably 0.30% by mass or less, and particularly preferably 0.20% by mass or less. C eq 2 is more preferably 0.65% by mass or more, and particularly preferably 0.70% by mass or more.

[0030] It is preferable that the tensile strength (TS) of the first steel plate P1 is 500 MPa or less, and the tensile strength of the second steel plate P2 is 1180 MPa or more. In other words, it is preferable that the first steel plate P1 is a mild steel plate and the second steel plate P2 is a high-strength steel plate. The tensile strength of the first steel plate P1 is more preferably 400 MPa or less. The lower limit of the tensile strength of the first steel plate P1 is not particularly limited, but is, for example, 270 MPa or more. The tensile strength of the second steel plate P2 is more preferably 1470 MPa or higher. The upper limit of the tensile strength of the second steel plate P2 is not particularly limited, but for example, it is 1900 MPa or lower.

[0031] The thickness t1 of the first steel plate P1 is, for example, less than 1.0 mm, and the thickness t2 of the second steel plate P2 is, for example, 1.0 mm or more. For example, in the body of an automobile, the frame of the body is formed from thick (for example, 1.0 mm or more) steel plates to make it less prone to deformation during a collision, while the outer panels covering the outside of the body are formed from thin (for example, less than 1.0 mm) steel plates. In a plate assembly with such plate thicknesses, the C of the steel plate eqDue to the combined effect of differences in strength, a significant difference in heat generation occurs between steel plate P1 and steel plate P2 during spot welding. Furthermore, because steel plate P1 is easily deformed, spatter is likely to occur during spot welding, making it difficult to ensure joint strength. However, with the welding joint manufacturing method according to the embodiment, even plates of different thicknesses can be welded with sufficient welding strength.

[0032] The thickness t1 of the first steel plate P1 is more preferably 0.9 mm or less. The lower limit of the thickness t1 of the first steel plate P1 is not particularly limited, but for example, it is 0.4 mm or more. The thickness t2 of the second steel plate P2 is more preferably 1.2 mm or more. The upper limit of the thickness t2 of the second steel plate P2 is not particularly limited, but for example, it is 2.4 mm or less.

[0033] Furthermore, the overlapping portion may include another steel plate. The other steel plate may be, for example, a high-strength steel plate. The other steel plate is placed between the first steel plate P1 and the second steel plate P2. There may be one or more of these other steel plates.

[0034] (2. Step of applying pressure and clamping the overlapping portion) As shown in Figure 1, the overlapping portion of two or more steel plates is compressed and clamped using a first electrode 11 and a second electrode 12 positioned opposite each other with the overlapping portion in between, and a first pressing member 2 positioned around the first electrode 11. At this time, the first electrode 11 and the first pressurizing member 2 are in contact with the first steel plate P1, and the second electrode 12 is in contact with the second steel plate P2.

[0035] For pressurizing the overlapping portion, it is preferable to further use a second pressurizing member 3 positioned around the second electrode 12. The second pressurizing member 3 is positioned opposite the first pressurizing member 2, with the overlapping portion in between. The second pressurizing member 3 is in contact with the second steel plate P2.

[0036] Two or more overlapping steel plates are subjected to pressure from above and below by a first electrode 11 and a second electrode 12, and then energized, causing the interface between the steel plates to melt due to Joule heating. As preparation, the steel plates are compressed and fixed inside the welding apparatus before energization. The steel plates are compressed and clamped between the first electrode 11, the second electrode 12, and the first pressing member 2. The first pressing member 2 compresses the area around the first electrode 11, suppressing deformation of the first steel plate P1 and reducing the generation of dust. The first pressing member 2 can also suppress the lateral growth of the molten pool formed in the first steel plate P1 and promote the growth of the molten pool in the thickness direction.

[0037] The first electrode 11, the second electrode 12, and the first pressurizing member 2 will be described in detail with reference to Figures 1 to 3.

[0038] As shown in Figure 1, the first electrode 11 has an electrode tip 11x that contacts the first steel plate P1, and the second electrode 12 has an electrode tip 12x that contacts the second steel plate P2. The first electrode 11 shown in Figures 1 and 2 is a cylindrical electrode with a smooth tip, and the diameter of the electrode tip 11x is D. The shape of the first electrode 11 can be any electrode shape commonly used in spot welding, such as the smooth-tip type shown in Figures 1 and 2(b), or the DR type shown in Figures 3(a) and (b). The shape and dimensions of the second electrode 12 are the same as those of the first electrode 11.

[0039] The first pressurizing member 2 is a member provided so as to surround the first electrode 11. The first pressurizing member 2 has a pressurizing tip 2x that contacts the first steel plate P1 around the electrode tip 11x of the first electrode 11. The pressurizing tip 2x of the first pressurizing member 2 contacts the overlapping steel plates and pressurizes them. The pressurizing tip 2x may be, for example, ring-shaped, or it may have another shape. When the steel plates soften during welding, the pressurizing tip 2x may be in a form in which a part of the ring shape is cut off to provide a non-pressurized portion, as long as the steel plates do not undergo local deformation. For example, the pressurizing tip 2x can be a C-shape with one part cut off when viewed from below, or a shape divided into multiple arcs by cutting off two or more parts (for example, a shape divided into three arcs as shown in Figure 2(a)).

[0040] The configuration of the first pressurizing member 2 shown in Figures 2(a) and 2(b) will be described in detail below. The first pressurizing member 2 shown in Figure 2(a) is composed of three pressurizing components 21, 22, and 23. The three pressurizing components 21, 22, and 23 are arranged to surround the electrode tip 11x of the first electrode 11. Each of the pressurizing components 21, 22, and 23 has a sector-shaped pressurizing tip 2x with a thickness t and circumferential central angles a1, a2, and a3, respectively, with the electrode tip 11x as the center. Adjacent pressurizing components 21, 22, and 23 are spaced apart by gaps with circumferential central angles b1, b2, or b3, with the electrode tip 1x as the center.

[0041] The first pressurizing member 2 is designed to pressurize a range of 10° or more in the circumferential direction centered on the electrode tip 11x. Here, "10° or more" means that, as shown in Figure 2(a), when the first pressurizing member 2 is divided into multiple pressurizing parts 21, 22, and 23, the sum of the central angles a1, a2, and a3 of the pressurizing tips 2x of each pressurizing part 21, 22, and 23 is 10° or more. In other words, the sum of the central angles b1, b2, and b3 of the gaps is 350° or less.

[0042] If the angle range in which the first pressurizing member 2 can apply pressure is 10° or more, the generation of dust during welding can be suppressed. The angle range in which the first pressurizing member 2 can apply pressure is preferably 15° or more, more preferably 30°, and particularly preferably 360° (corresponding to the case where the pressurizing tip 2x of the first pressurizing member 2 is ring-shaped).

[0043] The first pressurizing member 2 may be in a form that is tapered toward the pressurizing tip 2x, as shown in Figure 3(a), or it may be in a tubular form with a constant diameter, as shown in Figure 3(b).

[0044] As shown in Figures 2(b), 3(a), and 3(b), the pressurizing tips 2x of each pressurizing component 21, 22, and 23 and the electrode tip 11x of the first electrode 11 are separated by a distance (clearance) CL. The distance CL is preferably between 0.1 mm and 10.0 mm. If the separation distance CL between the pressurizing tip 2x and the electrode tip 1x is too narrow, the first electrode 11 and the first pressurizing member 2 may come into contact, causing current to flow from the first pressurizing member 2 to the first electrode 11, which may prevent the suppression of the lateral spread of the molten pool from being achieved. If the separation distance CL is too wide, the effect of pressurizing the overlapping portion around the first electrode 11 with the first pressurizing member 2 decreases, reducing the effect of suppressing the generation of dust. The lower limit of the separation distance CL is preferably 0.5 mm, more preferably 1.0 mm, and the upper limit of the separation distance CL is preferably 8.0 mm, more preferably 6.0 mm.

[0045] When using the second pressurizing member 3, it is preferable that the second pressurizing member 3 has the same form and characteristics as the first pressurizing member 2.

[0046] The overlapping portion of the steel plates is pressurized between the electrode tips 11x and 12x of the upper and lower pair of electrodes (first electrode 11 and second electrode 12). Furthermore, the area around the portion pressurized by electrode tip 11x is pressurized between the pressurizing tips 2x of the first pressurizing member 2. Preferably, the area around the portion pressurized by electrode tip 12x is also pressurized between the pressurizing tips 3x of the second pressurizing member 3.

[0047] A high load applied to the overlapping portion by the first electrode 11 and the second electrode 12 (referred to as the "electrode load") is effective in suppressing deformation of the steel plate caused by heating and melting during welding, and in suppressing the generation of spatter. However, if the electrode load is too high, the welded portion may be crushed and deformed during welding, potentially reducing the remaining plate thickness. If the electrode load is too low, the amount of heat dissipated through the first electrode 11 and the second electrode 12 from the heat generated in the welded portion during welding becomes small, making surface spatter more likely to occur.

[0048] The lower limit of the electrode load is preferably 1.5 kN, and more preferably 2.0 kN. The electrode load does not need to be excessively high, and its upper limit is preferably 7.0 kN, and more preferably 6.0 kN.

[0049] The load applied to the overlapping portion by the first pressurizing member 2 (referred to as the "peripheral load") is preferably 0.1 times or more and 1.0 times or less the electrode load. When the peripheral load is within this range, the effect of suppressing dust is high. However, due to the limitations of the welding equipment, there is an upper limit to the total load of the peripheral load and the electrode load. If the peripheral load is increased, the electrode load must be decreased, which reduces the amount of heat dissipated through the first electrode 11 and makes surface dust more likely to occur. Therefore, the peripheral load must be set appropriately so as to ensure a sufficient electrode load. The lower limit of the periphery load is preferably 0.2 times the electrode load, and more preferably 0.3 times. The upper limit of the periphery load is preferably 0.8 times the electrode load, and more preferably 0.5 times.

[0050] As materials for the first electrode 11 and the second electrode 12, electrode materials commonly used in spot welding, such as pure copper, chromium copper, and alumina-dispersed copper, can be used. The material of the first pressurizing member 2 may be either a non-conductor or a conductor. Furthermore, the material of the first pressurizing member 2 must have sufficient strength to prevent plastic deformation under pressure.

[0051] (3. The process of welding the overlapping parts) The overlapping portion is welded by passing a welding current between the first electrode 11 and the second electrode 12. The welding current is a direct current that passes through with the first electrode 11 as the negative electrode and the second electrode 12 as the positive electrode. Because it is a direct current, the current flows in one direction through the overlapping portion. As a result, the shape of the spot weld nugget exhibits anisotropy in the plate thickness direction. As can be seen from Figures 4(a) and (b), the nugget tends to spread towards the negative electrode side.

[0052] The reason why the nuggets spread to the negative electrode side is not entirely clear, but it is presumed to be due to the following reasons. When two metals with different properties are stacked and electrodes are brought into contact with them, and a direct current is passed through them, anisotropy occurs in the heating behavior at the metal interface depending on the direction of the current (polarity effect). In other words, the heating behavior at the metal interface changes depending on whether each stacked metal is located as the positive or negative electrode. In the case of steel plate assembly in the embodiment, the heating of the steel plate located on the negative electrode side of the welding electrode is accelerated, the molten pool expands to the negative electrode side, and the nugget formed by the solidification of the molten pool also expands to the negative electrode side. A thin plate is used on the negative electrode (first electrode 11) side. eq The first steel plate P1 has a low value, and the positive electrode (second electrode 12) side has a thick plate C eq When a second steel plate P2 with a high tangency is placed, the molten region of the first steel plate P1 on the negative electrode side expands. As a result, the concentration of alloy components in the molten pool and the nugget that solidifies afterward decreases, improving the toughness of the nugget and enhancing the joint strength, particularly the cross tensile strength, which is strongly influenced by the toughness of the weld metal.

[0053] As described above, the first electrode 11 is in contact with the first steel plate P1, and the second electrode 12 is in contact with the second steel plate P2. In other words, the first steel plate P1, which is difficult to melt, is positioned on the negative electrode (first electrode 11) side. By biasing the nugget towards the first steel plate P1 side, the first steel plate P1 and the second steel plate P2 can be welded with sufficient joint strength. Furthermore, by placing the easily meltable second steel plate P2 on the positive electrode (second electrode 12) side, the nugget separates from the surface of the second steel plate P2, thereby suppressing the generation of surface dust from the second steel plate P2 side.

[0054] The appropriate range of welding current varies depending on the composition and strength of the steel plate being welded, so it is preferable to adjust it according to the steel plate being welded. For example, the C of the first steel plate P1 eq The pressure is 0.03, the TS is 371 MPa, and the plate thickness is 0.8 mm. The second steel plate P2 is C eq In an example where the pressure is 0.83, the TS is 1572 MPa, and the plate thickness is 1.4 mm, it is preferable to set the welding current within the range of 6 kA to 8 kA. [Examples]

[0055] • Welded test specimen Two steel plates were stacked according to the plate assembly shown in Table 1 to prepare a welded test specimen (a laminate of two steel plates). In Table 1 and Figure 4 of the example, the steel plate in contact with the positive electrode is referred to as "first steel plate P1," and the steel plate in contact with the negative electrode is referred to as "second steel plate P2." Table 1 shows the steel grade, thickness, and carbon equivalent of the steel plates used. Regarding the steel grade of the steel plates, "1.5G grade steel" has a tensile strength (TS) of 1572 MPa and C eq The steel plate has a thickness of 1.4 mm and a tensile strength (TS) of 0.83. "SGCC" is a steel plate with a tensile strength (TS) of 330 MPa and C eq The steel plate has a 0.03 ion and a thickness of 0.8 mm. The chemical composition of each steel type is shown in Table 2. In Table 2, a line (-) indicates that the element was not detected. For each example, a total of five welded test specimens were prepared using three different specimen shapes: two specimens measuring 40 mm wide x 125 mm long for shear tensile testing, two specimens measuring 50 mm wide x 150 mm long for cruciate tensile testing, and one specimen measuring 40 mm x 40 mm for cross-sectional observation.

[0056] • Welding method A DC inverter spot welding machine with an air cylinder pressurization mechanism was used for the welding process. The first electrode 11 and the second electrode 12 used chromium copper electrode tips with a DR shape (electrode diameter 13.0 mm, tip diameter 6.0 mm) as shown in Figure 3(b). As the first pressurizing member 2, a pipe-shaped pressurizing member (inner diameter 14.0 mm, outer diameter 16.0 mm) that surrounds the first electrode 11, as shown in Figure 3(b), was used. By using the pipe-shaped pressurizing member, an outer peripheral load was applied in a 360° range in the circumferential direction centered on the electrode tip 11x of the first electrode 11. In addition to the pressurizing mechanisms for the first electrode 11 and the second electrode 12, an air cylinder was prepared to pressurize the first pressurizing member 2.

[0057] The fabricated welded specimen was clamped under pressure by the first electrode 11, the second electrode 12, and the first pressurizing member 2. The electrode load from the first electrode 11 and the second electrode 12 was 2.5 kN, and the outer circumferential load from the first pressurizing member 2 was 1.5 kN. The clearance CL between the pressurizing tip 2x and the electrode tip 11x was 4.0 mm. In addition, in the welding of the test specimens in Examples No. 1 to 8, external pressure was not applied by the first pressure member 2.

[0058] Subsequently, spot welding was performed by applying the welding current (DC current) shown in Table 1 between the electrodes. In the welding of the specimens in Examples No. 1 to 8, no external pressure was applied, so the welding current was set to a relatively low range of 5kA to 8kA. In the welding of the specimens in Examples No. 9 to 14, external pressure was applied, which suppressed spatter, so the welding current was set to a relatively high range of 7kA to 9kA. The energizing time during welding was set to 320 ms (milliseconds).

[0059] In each embodiment, welding was performed on five test specimens to be welded. In the five welding trials, cases where no spatter was generated were classified as "OK," and cases where spatter was generated once or more were classified as "NG," as shown in Table 3.

[0060] Furthermore, tensile shear tests and cross-tensile tests were conducted to evaluate the joint strength of the welded joints. The tensile shear tests and cross-tensile tests were performed in accordance with JIS Z3140:2017. Two tensile shear tests and two cross-tensile tests were performed, and the average values ​​of the tensile shear strength and cross-tensile strength are shown in Table 2. The tensile shear strength (TSS) was evaluated as follows: an average tensile shear strength (Table 2) of 4.8 kN or higher was judged as "○", and a value below 4.8 kN was judged as "×". The cross-tensile strength (CTS) was evaluated as follows: an average cross-tensile strength of 3.2 kN or higher (Table 2) was judged as "○", and a value below 3.2 kN was judged as "×".

[0061] Furthermore, the evaluation standard for tensile shear strength (TSS) (4.8kN) was set at 80% of the standard value for tensile shear strength (6.0kN), and the evaluation standard for cross tensile strength (CTS) (3.2kN) was set at 80% of the standard value for cross tensile strength (4.0kN). The "reference values" for tensile shear strength and cross-tensile strength refer to the maximum values ​​obtained by spot welding two 0.8mm thick SGCC steel plates together while varying the welding current, and then measuring the cross-tensile strength of the welded joints that were welded without generating spatter.

[0062] For Examples No. 9 and No. 12, the welded joints were cut in the thickness direction and the cross-sections were observed. Schematic diagrams of the cross-sections are shown in Figures 4(a) and (b). In the figures, the sign "-" is placed on the upper side of the first steel plate P1 and the sign "+" is placed on the lower side of the second steel plate. This means that during welding, the negative electrode was placed on the upper side and the positive electrode on the lower side of the overlapping portion of the two steel plates P1 and P2.

[0063] Table 3 shows the measurement and evaluation results.

[0064] [Table 1]

[0065] [Table 2]

[0066] [Table 3]

[0067] Examine the results in Table 3. In Examples No. 1 to 4, during welding, a thick high-strength steel plate (1.5G class steel plate: 1.4 mm thick) was placed on the negative electrode side, and a thin mild steel plate (SGCC steel plate: 0.8 mm thick) was placed on the positive electrode side, and outer peripheral pressure was not applied by the first pressure member 2. In Examples No. 1 to 3, the current value was 7 kA or less, and a sufficient melting area could not be secured, resulting in low cross-tensile strength. In No. 4, the current value was 8 kA, and spatter was generated.

[0068] In Examples No. 5 to 8, a thin mild steel plate was placed on the negative electrode side and a thick high-strength steel plate was placed on the positive electrode side, and outer peripheral pressure was not applied by the first pressurizing member 2. In Examples No. 5 to 6, the current value was 6kA or less, and a sufficient melting area could not be secured, resulting in low cross-tensile strength. In Examples No. 7 and 8, the current values ​​were 7kA and 8kA, respectively, and dust was generated in both cases.

[0069] In Examples No. 9 to 11, a thick high-strength steel plate was placed on the negative electrode side and a thin mild steel plate on the positive electrode side, and outer periphery pressure was applied by the first pressure member 2. With outer periphery pressure, welding was possible without spatter even when the current value was increased up to 8kA, but spatter occurred at 9kA. In all of Examples No. 9 to 11, the spread of the molten area to the mild steel plate side was small, and the cross tensile strength was low.

[0070] In Examples No. 12-14, a thin mild steel plate was placed on the negative electrode side and a thick high-strength steel plate on the positive electrode side, and outer periphery pressure was applied by the first pressure member 2. The appropriate arrangement of the steel plates relative to the direction of current flow, combined with the outer periphery pressure, caused the molten pool to spread to the thin mild steel plate on the negative electrode side. As a result, the nugget formed by the solidification of the molten pool had reduced strength and improved toughness, leading to increased cross-tensile strength. Furthermore, the resulting welded joint also exhibited high tensile shear strength. Moreover, welding was possible without spatter even when the current was increased to 9kA.

[0071] Thus, it was found that by placing a thin mild steel plate on the negative electrode side and applying pressure to the outer circumference, the appropriate current range can be expanded, and good cross-tensile strength can be ensured.

[0072] Figures 4(a) and 4(b) are schematic diagrams of cross-sectional images created based on micrographs of welded joints obtained in Examples No. 9 and No. 12, respectively. Figure 4(a) shows a welded joint in which a thick high-strength steel plate is placed on the negative electrode side and a thin mild steel plate is placed on the positive electrode side and welded together. Figure 4(b) shows a welded joint in which a thin mild steel plate is placed on the negative electrode side and a thick high-strength steel plate is placed on the positive electrode side and welded together. In both welded joints, a nugget Ng was formed at the interface between the two steel plates.

[0073] The extent of the weld pool (mainly in the thickness direction) was determined as follows: The welded joint was cut in the thickness direction, the cross-section was observed under a microscope, and a photograph of the cross-section was taken. In the cross-sectional photograph, the shape of the nugget Ng formed by the solidification of the molten pool was identified, and a center line C passing through the center of the maximum width W of the nugget Ng was defined. At the position of center line C, the thickness of the thin mild steel plate (the thickness of the steel plate remaining without welding) was measured. In Figure 4(a), the thin mild steel plate P2 has a thickness of Pt2, and in Figure 4(b), the thin mild steel plate P1 has a thickness of Pt1.

[0074] Figure 4 shows that the thickness Pt1 when a thin mild steel plate (P1 in Figure 4(b)) is placed on the negative electrode side and welded is thinner than the thickness Pt2 when a thin mild steel plate (P2 in Figure 4(a)) is placed on the positive electrode side and welded. In other words, as shown in Figure 4(b), it was confirmed that by placing a thin mild steel plate on the negative electrode side and welding it, the nugget expands in the thickness direction of the thin mild steel plate. [Explanation of Symbols]

[0075] 11 First electrode 12. Second electrode 2. First pressurizing member 3. Second pressurizing member P1 First steel plate P2 Second steel plate

Claims

1. A method for determining the polarity of electrodes in spot welding of two or more steel plates, When the carbon equivalent of the first steel plate placed on the outermost side, obtained from the following equation (1), is C eq 1, and the carbon equivalent of the second steel plate placed on the outermost side, is C eq 2, then C eq 1 and C eq 2 satisfy the following equation (2), When welding the overlapping portion of the two or more steel plates by passing a direct current between a pair of electrodes, the electrode in contact with the first steel plate is the negative electrode, and the electrode in contact with the second steel plate is the positive electrode. A method for determining the polarity of an electrode, comprising arranging a first pressurizing member around the negative electrode. Carbon equivalent C eq (mass%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (1) Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], and [V] represent the content of C, Mn, Si, Ni, Cr, Mo, and V, respectively, expressed in mass percent. C eq 1<C eq 2...(2)

2. A method for manufacturing a welded joint by spot welding two or more steel plates, The process involves placing a first steel plate on the outermost edge of one side and a second steel plate on the outermost edge of the other side, thereby overlapping the two or more steel plates. The process involves pressing and clamping the overlapping portion of the two or more steel plates using a first electrode and a second electrode positioned opposite each other with respect to the overlapping portion, and a first pressing member positioned around the first electrode. The process includes a step of welding the overlapping portion by passing a welding current between the first electrode and the second electrode, The carbon equivalent of the first steel plate, calculated from the following formula (1), is C ｅｑ 1. The carbon equivalent of the second steel plate is C ｅｑ When set to 2, C ｅｑ 1 and C ｅｑ 2 satisfies the following equation (2): In the pressing and clamping step, the first electrode and the first pressing member are in contact with the first steel plate, and the second electrode is in contact with the second steel plate. A method for manufacturing a welded joint, wherein in the welding step, the welding current is a direct current that is passed through the first electrode as the negative electrode and the second electrode as the positive electrode, according to the electrode polarity determination method described in claim 1. Carbon equivalent C ｅｑ (mass%) = [C] + [Mn] / 6 + [Si] / 24 + [Ni] / 40 + [Cr] / 5 + [Mo] / 4 + [V] / 14 ... (1) Here, [C], [Mn], [Si], [Ni], [Cr], [Mo], and [V] represent the content of C, Mn, Si, Ni, Cr, Mo, and V, respectively, expressed in mass percent. C ｅｑ 1<C ｅｑ 2 ・・・(2)

3. The above-mentioned C ｅｑ 1 is 0.40% by mass or less, and the above-mentioned C ｅｑ 2 is 0.60% by mass or more. The method for manufacturing a welded joint according to claim 2.

4. The first electrode has an electrode tip that contacts the first steel plate, The first pressurizing member has a pressurizing tip that contacts the first steel plate around the electrode tip of the first electrode, The aforementioned pressurizing tip is configured to pressurize a range of 10° or more in the circumferential direction centered on the electrode tip. The method for manufacturing a welded joint according to claim 2 or 3, wherein the distance between the pressurizing tip and the electrode tip is 0.1 mm or more and 10.0 mm or less.

5. A method for manufacturing a welded joint according to any one of claims 2 to 4, wherein in the step of pressurizing and clamping, the overlapping portion is pressurized by the first electrode and the second electrode with an electrode load of 1.5 kN or more, and is also pressurized by the first pressurizing member with an outer peripheral load of 0.1 to 1.0 times the electrode load.

6. A method for manufacturing a welded joint according to any one of claims 2 to 5, wherein the tensile strength of the first steel plate is 500 MPa or less, and the tensile strength of the second steel plate is 1180 MPa or more.

7. A method for manufacturing a welded joint according to any one of claims 2 to 6, wherein the thickness of the first steel plate is less than 1.0 mm and the thickness of the second steel plate is 1.0 mm or more.

8. A method for manufacturing a welded joint according to any one of claims 2 to 7, wherein, in the step of applying pressure and clamping the overlapping portion, a second pressurizing member is further used, which is arranged around the second electrode and positioned opposite the first pressurizing member across the overlapping portion.

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