Method for manufacturing spot welded joints and spot welded joints

Abstract

To provide a spot welded joint manufacturing method capable of effectively suppressing LME split, and a spot welded joint.SOLUTION: A manufacturing method for spot welded joint which spot welds two or more overlapped steel plates has one or more steel plates as high strength steel sheets, disposes a Zn system plating layer on a mating surface between a high strength steel sheet and a steel plate contacting the same. In main energization, welding current value I1(kA), a main energization time t1(sec), and Si content by unit mass % of a high strength steel sheet satisfy 28×Si+1111<58×I12×t1<30×Si+1250; in post-energization, a main energization time t1(kA), post-energization time t2(sec), and Si content by unit mass % of the high strength steel sheet satisfy {(-14.3×Si+34.3)2 / 3.7}×10-4<t2<0.5×t1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing spot-welded joints and to spot-welded joints. [Background technology]

[0002] The application of high-strength steel sheets to automotive parts is progressing with the aim of improving fuel efficiency (weight reduction) and collision safety. On the other hand, automobiles have thousands of spot welds. When zinc-based plating is applied to spot welds that join high-strength steel sheets to other steel sheets, there is a problem that liquid metal embrittlement (LME) cracking is likely to occur. LME cracking is particularly likely to occur when the tensile strength of the high-strength steel sheet is 780 MPa or higher. Therefore, measures to prevent LME cracking have been proposed.

[0003] Patent Document 1 discloses a spot welding method that can easily prevent liquid metal embrittlement cracking in spot welding of plated steel sheets, characterized in that, before spot welding, the plating is removed in an area centered on the location where the center of the nugget is to be formed, with the outer circumference including the circle inside the outer edge of the welding-affected zone, or in an area on the overlapping surface of the steel sheets to be welded, sharing a center with the location where the center of the nugget to be formed on the overlapping surface of the steel sheets is to be formed, with the outer circumference including the circle inside the outer edge of the welding-affected zone.

[0004] Patent Document 2 discloses a resistance spot welding method in which, when joining galvanized ultra-high-strength materials W1 and W2 by overlapping them, an adhesive A containing an element pre-selected to contribute to improving the toughness of the joint (at least one of Ti, Ni, V, Mo, Nb, and Al) is interposed between the ultra-high-strength materials W1 and W2, and then resistance spot welding is performed.

[0005] Patent Document 3 discloses a spot-welded member in which a plurality of steel plates are spot-welded, wherein at least one of the plurality of steel plates is a high-strength cold-rolled steel plate with a tensile strength of 780 MPa or more and has no plating layer on its surface, and at least one of the plurality of steel plates is a zinc-plated steel plate having a zinc-plated layer on its surface, and the surface Zn concentration inside the corona bond of the spot-welded portion is 1% by mass or more and less than 25% by mass.

[0006] Patent Document 4 discloses a resistance spot welding method for spot welding multiple steel plates, each containing C: 0.08 mass% or more, Si: 0.50 mass% or more, having a tensile strength of 980 MPa or more, and having at least one galvanized steel plate, wherein the welding is performed using a pair of electrodes comprising: a pair of electrode tips that sandwich and pressurize the multiple steel plates; and a pressure absorption mechanism provided on at least one of the pair of electrode tips, capable of absorbing the axial pressure applied to the electrode tip. The multiple steel plates are sandwiched between the pair of electrode tips, and current is passed through the pair of electrode tips while pressure is applied, thereby absorbing the fluctuating load of the pressure generated during current application with the pressure absorption mechanism. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2016-159169 [Patent Document 2] Japanese Patent Publication No. 2020-121334 [Patent Document 3] Japanese Patent Publication No. 2020-179413 [Patent Document 4] Japanese Patent Publication No. 2022-021770 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, the techniques disclosed in Patent Documents 1 to 4 have room for improvement in the following aspects.

[0009] In the technique described in Patent Document 1, a process of removing plating before spot welding is required. The plating removal process reduces the working efficiency of spot welding. Therefore, means for suppressing LME cracking without the plating removal process are desired.

[0010] In the technique of Patent Document 2, resistance spot welding is performed after interposing an adhesive between steel plates. The adhesive application process reduces the working efficiency of spot welding. Therefore, means for suppressing LME cracking without the adhesive application process are desired.

[0011] In the technique of Patent Document 3, the surface layer Zn concentration inside the corona bond is set to 1% by mass or more and less than 25% by mass. For this purpose, it is necessary to preferably adjust the composition of the zinc-based plating layer. However, from the viewpoint of increasing the degree of freedom in selecting the zinc-based plating layer, means for suppressing LME cracking other than the plating layer components are desired.

[0012] In the technique of Patent Document 4, a pressure absorption mechanism is provided in the electrode tip. However, it may be difficult to provide a pressure absorption mechanism in the spot welding machine. Means for suppressing LME cracking that can be implemented in various spot welding machines are desired.

[0013] Therefore, an object of the present invention is to provide a method for manufacturing a spot welded joint and a spot welded joint capable of more effectively suppressing LME cracking.

Means for Solving the Problems

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

[0015] (1) The manufacturing method of a spot welding joint according to one aspect of the present invention includes a step of forming a spot welding portion that joins the steel plates by directly energizing two or more overlapped steel plates, and a step of post-energizing the spot welding portion. The manufacturing method of the spot welding joint is characterized in that one or more of the steel plates are high-strength steel plates with a tensile strength of 780 MPa or more, one or more of the high-strength steel plates are in contact with a Zn-based plating layer, the B content of the high-strength steel plate in contact with the Zn-based plating layer is 0.0010% by mass or more, in the direct energization, the welding current value I1 (kA), the direct energization time t1 (sec), and the Si content in the high-strength steel plate in contact with the Zn-based plating layer in terms of unit mass% satisfy 28×Si + 1111 < 58×I1 2 ×t1 < 30×Si + 1250, in the post-energization, the direct energization time t1 (kA), the post-energization time t2 (sec), and the Si content in the high-strength steel plate in contact with the Zn-based plating layer in terms of unit mass% satisfy {(-14.3×Si + 34.3) 2 / 3.7}×10 -4 < t2 < 0.5×t1, in the post-energization, the post-energization current value I2 (kA) satisfies 0.7×I1 < I2 < 1.5×I1. When the post-energization current value I2 (kA) in the post-energization satisfies 0.7×I1 < I2 < I1, the direct energization and the post-energization are performed continuously. When the post-energization current value I2 (kA) in the post-energization satisfies I1 ≦ I2 < 1.5×I1, the manufacturing method of the spot welding joint further includes a step of cooling the spot welding portion between the direct energization and the post-energization. In the cooling, the welding current value I1 (kA), the direct energization time t1 (sec), and the cooling time t S (sec) satisfy (4.07×I1 2 ×t1 - 19.30) / 1000 < t s < (4.07×I1 2 ×t1 - 17.15) / 1000. (2) In the manufacturing method of the spot welding joint described in (1) above, preferably, the manufacturing method of the spot welding joint further includes a step of maintaining the pressing force on the spot welding portion in a state where the energization to the spot welding portion is paused after the step of post-energizing, and the holding time for performing the holding is 0.4 seconds or more.

[0016] (3) Another aspect of the present invention is a spot welded joint comprising two or more overlapping steel plates, a spot weld having a nugget for joining the steel plates together, a pressure-welded portion formed around the nugget where the opposing steel plates are pressed together, and a plate gap formed on the outside of the pressure-welded portion, wherein one or more of the steel plates are high-strength steel plates with a tensile strength of 780 MPa or more, and a Zn-based plating layer is disposed on the mating surface between one or more of the high-strength steel plates and the steel plate in contact therewith, The B content of the high-strength steel sheet in contact with the Zn-based plating layer is 0.0010% by mass or more, and the number of BN particles with a major axis of 0.5 μm or more that are located inside the high-strength steel sheet in contact with the Zn-based plating layer in a cross section passing through the center of the nugget and parallel to the thickness direction of the spot-welded joint, having a square shape with sides of 100 μm, with one side of the square overlapping the pressure-welded portion, and the center of the one side of the square being included in the observation area where the boundary between the pressure-welded portion and the gap between the plates coincides, is 100 or less. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide a method for manufacturing a spot-welded joint and a spot-welded joint that can more effectively suppress LME cracking. [Brief explanation of the drawing]

[0018] [Figure 1] This is a cross-sectional view showing the schematic configuration of a spot welded joint according to this embodiment. [Figure 2] This is an enlarged cross-sectional view of the area near the boundary between the pressure-welded portion and the gap between the plates. [Figure 3] This is a cross-sectional photograph of an example of a spot welded joint where LME cracking occurred. [Modes for carrying out the invention]

[0019] It is known that boron (B) is effective in suppressing LME cracking that occurs directly outside the pressure-welded joint of high-strength steel plates. However, the inventors have found that the amount of boron in the heat-affected zone (HAZ) of high-strength steel plates and the LME resistance of the HAZ are not necessarily proportional. The inventors have confirmed that LME cracking can occur frequently even in the HAZ of spot-welded joints obtained from high-strength steel plates containing a large amount of boron.

[0020] Further investigation by the inventors revealed that when B is present as BN (boron nitride) in the HAZ of high-strength steel sheets, the LME crack suppression effect cannot be obtained. In order to suppress LME cracking using B, it is necessary to keep B in a solid solution state in the HAZ. This is thought to allow B to segregate at the grain boundaries, preventing molten zinc from penetrating the grain boundaries and effectively suppressing LME cracking.

[0021] In addition, the inventors have found that the optimal welding conditions for solid-solving B in the heat-activated zone (HAZ) vary depending on the composition of the high-strength steel sheet. Typical high-strength steel sheets contain Si. Si increases the activation energy of N, thereby stabilizing BN. To achieve a solid-solution state of B in the HAZ, it is necessary to sufficiently heat the HAZ to decompose the BN, and then segregate the B in a solid-solution state at the grain boundaries. However, the Si contained in high-strength steel sheets raises the temperature at which BN decomposes. Therefore, the inventors have found that the higher the Si content of the high-strength steel sheet, the higher the heat input to the HAZ needs to be.

[0022] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings. In this specification and the drawings, components having substantially the same functional configuration are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0023] <1. Method for manufacturing spot welded joints> First, the method for manufacturing spot-welded joints will be described. First, multiple steel plates 10 are prepared. Figure 1 shows an example of a spot-welded joint 1 including two steel plates 10, but the number of steel plates 10 may be three or more. Of these steel plates 10, one or more are high-strength steel plates 10H with a tensile strength of 780 MPa or higher. Furthermore, one or more of the high-strength steel plates 10H included in the plate assembly are in contact with a Zn-based plating layer 11. The concept of "in contact with the Zn-based plating layer 11" includes the following two cases. (1) High-strength steel sheet 10H is defined as Zn-plated steel sheet. (2) A high-strength steel sheet 10H without a Zn-based plating layer 11 and a Zn-based plated steel sheet are stacked in contact with each other (see Figure 1). In either case (1) or (2) above, molten zinc is formed on the surface of the heat-affected zone 123 of the high-strength steel plate 10H during spot welding. It is not necessary for all of the high-strength steel plates 10H to be in contact with the Zn-based plating layer 11. For example, if the plate assembly includes two or more high-strength steel plates 10H, it is sufficient if at least one of the high-strength steel plates 10H is in contact with the Zn-based plating layer 11.

[0024] LME cracking is prone to occur in the high-strength steel sheet 10H that is in contact with the Zn-based plating layer 11. Therefore, in the method for manufacturing the spot-welded joint according to this embodiment, the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is set to 0.0010 mass% (10 ppm) or more. Furthermore, using welding conditions described later, B is dissolved in the heat-affected zone 123 of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. This suppresses LME cracking.

[0025] All of the spot-welded steel plates 10 may be high-strength steel plates 10H, or any of them may be steel plates with a tensile strength of less than 780 MPa, i.e., low-strength steel plates 10L. The tensile strength of the high-strength steel plate 10H is preferably 980 MPa or higher, 1300 MPa or higher, 1500 MPa or higher, 1700 MPa or higher, or 1900 MPa or higher. There is no particular upper limit for the tensile strength of the high-strength steel plate 10H, but for example, its tensile strength may be 2700 MPa or lower, 2600 MPa or lower, or 2500 MPa or lower.

[0026] The high-strength steel sheet 10H in contact with the Zn-based plating layer 11 contains B. The B content is 0.0010 mass% or more, as described above. When the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is within this range, LME cracking can be effectively suppressed. If the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is less than 0.0010 mass%, there is too little solid-solution B segregated at the grain boundaries, and LME cracking cannot be effectively suppressed. The lower limit of the B content is preferably 0.0012 mass%, and more preferably 0.0015 mass%.

[0027] There is no particular upper limit specified for the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. From the viewpoint of suppressing LME cracking, a higher B content in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is preferable. On the other hand, it is even more preferable to set the B content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 to 0.0060 mass% or less. This suppresses the amount of B precipitates and further improves the joint strength of the spot-welded joint 1. The upper limit of the B content is preferably 0.0050 mass%, more preferably 0.0040 mass%, and even more preferably 0.0020 mass%.

[0028] Other chemical components are optional. Depending on the application of the spot welded joint 1, any chemical component can be selected and applied to the high-strength steel plate 10H in contact with the Zn-based plating layer 11.

[0029] For example, in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11, the Ti content may be 0.01 mass% or less. Ti forms TiN in the steel, reducing BN and increasing the amount of B in a solid solution state. Therefore, from the viewpoint of suppressing LME cracking, it is preferable for the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 to have a higher Ti content. On the other hand, by reducing the Ti content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11, the amount of Ti precipitates can be reduced, and the joint strength of the spot-welded joint 1 can be further increased. In the method for manufacturing the spot-welded joint according to this embodiment, the amount of B in a solid solution state can be secured in the heat-affected zone 123 of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 through welding conditions described later. Therefore, even if the Ti content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is 0.01 mass% or less, it is possible to suppress LME cracking using B.

[0030] Furthermore, the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 may contain Si. Si increases the activation energy of N and stabilizes BN. Therefore, Si reduces the amount of B in the solid solution state in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. On the other hand, Si enhances the mechanical properties such as strength of the high-strength steel sheet 10H. In the method for manufacturing spot-welded joints according to this embodiment, the welding conditions are adjusted according to the Si content. Therefore, the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 may contain Si. In the high-strength steel sheet 10H in contact with the Zn-based plating layer 11, the Si content may be, for example, 0.01 to 2.00 mass%.

[0031] As described above, the B content is limited in the high-strength steel sheet 10H that is in contact with the Zn-based plating layer 11. On the other hand, the B content and chemical composition are not limited in the other steel sheets 10 included in the spot-welded joint 1. The risk of LME cracking is extremely low in the low-strength steel sheet 10L. LME cracking does not occur in the high-strength steel sheet 10H that is not in contact with the Zn-based plating layer 11. Therefore, the low-strength steel sheet 10L and the high-strength steel sheet 10H that is not in contact with the Zn-based plating layer 11 do not need to contain B. That is, in the low-strength steel sheet 10L with a tensile strength of less than 780 MPa, the B content may be less than 0.0010 mass%. Also, even in the case of high-strength steel sheet 10H with a tensile strength of 780 MPa or more, if it is not in contact with the Zn-based plating layer 11, its B content may be less than 0.0010 mass%.

[0032] The type of high-strength steel sheet 10H is not particularly limited. Examples of high-strength steel sheet 10H include DP steel sheet, TRIP steel sheet, composite structure steel sheet, martensitic steel sheet, and hot-stamped steel sheet. Furthermore, high-strength steel sheet 10H may be either cold-rolled or hot-rolled steel sheet.

[0033] Of the multiple steel sheets 10, at least one steel sheet is a Zn-plated steel sheet having a Zn-plated layer 11. The Zn-plated layer 11 is a plating layer whose main component is Zn. The high-strength steel sheet 10H may be a Zn-plated steel sheet, or the low-strength steel sheet 10L may be a Zn-plated steel sheet. Examples of Zn-plated steel sheets include GI-plated steel sheets, GA-plated steel sheets, EG-plated steel sheets, Zn-Ni-plated steel sheets, Zn-Al-plated steel sheets, Zn-Mg-plated steel sheets, and Zn-Mg-Al-plated steel sheets.

[0034] Next, multiple steel plates 10 are overlapped. The entire area of ​​the steel plates 10 may be overlapped, or only a portion of it may be overlapped. At this time, a gap (plate gap) may occur at the joint surface, but from the viewpoint of ensuring spot welding quality, it is desirable that the gap be 2.5 mm or less, and more preferably 1.5 mm or less.

[0035] Next, by applying current to multiple (two or more) overlapping steel plates 10, a spot weld 12 is formed to join the steel plates 10 together. Specifically, for example, multiple overlapping steel plates 10 are placed between a pair of opposing electrodes of a spot welding device, and a welding current is applied while applying pressure. Here, the spot weld 12 is a concept that includes a nugget 121 formed between the steel plates 10, a pressure-welded portion 122 formed around the nugget 121 where the opposing steel plates 10 are pressed together, and a gap 13 formed outside the pressure-welded portion 122. In addition, the base material around the nugget 121 is usually a heat-affected zone (HAZ) 123, and this heat-affected zone 123 is also included in the weld. The spot welding device is not particularly limited, and any known device can be used as appropriate. A preferred example of a spot welding device is shown below.

[0036] The spot welding equipment may be an inverter-type DC spot welding equipment or a single-phase AC spot welding equipment. The pressurization mechanism of the resistance spot welding equipment may be pressurized by a servo motor or by air. The shape of the gun may be stationary, C-type, or X-type.

[0037] There are no particular restrictions on the electrodes used for spot welding. A suitable example of an electrode is a DR-type electrode with a tip diameter of 5 to 9 mm. The upper and lower electrodes may be the same or different. The electrode material may be chromium copper, zirconium copper, or alumina-dispersed copper electrode. From the viewpoint of suppressing welding between the electrode and the steel plate 10, surface dust generation, and LME cracking, it is preferable to use alumina-dispersed copper as the electrode material.

[0038] The welding current value I1 (kA) during this energization, the energization time t1 (sec), and the Si content (Si) per unit mass % of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 satisfy the following equation 1. 28 × Si + 1111 < 58 × I1 2 ×t1<30×Si+1250 (Formula 1)

[0039] Equation 1 defines the heat input in this energization. By performing this energization so as to satisfy Equation 1, BN can be sufficiently decomposed in the heat-affected zone 123 of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. 58 × I1 2 When 58 × I1 2 × t1 is less than or equal to the lower limit value of Equation 1, the amount of BN becomes excessive in the heat-affected zone 123 of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. As a result, in the heat-affected zone 123 of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11, B does not sufficiently segregate at the grain boundaries, and LME cracking cannot be suppressed. On the other hand, 58 × I1

[0040] The reason why the Si content of the high-strength steel sheet 10H in contact with the Zn-based plating layer 11 is included in Equation 1 is that Si has an effect of stabilizing BN. The larger the Si content, the greater the energy required to decompose BN, so it is necessary to increase the heat input in this energization.

[0041] The sheet assembly may have two or more high-strength steel sheets 10H in contact with the Zn-based plating layer 11, and their Si contents may be different. In this case, Equation 1 needs to be satisfied with respect to the Si contents of all the high-strength steel sheets 10H in contact with the Zn-based plating layer 11. On the other hand, the Si content of the high-strength steel sheet 10H not in contact with the Zn-based plating layer 11 and the Si content of the low-strength steel sheet 10L do not affect the conditions of this energization. It is sufficient if Equation 1 is satisfied only for the steel sheet 10 where LME cracking is a concern.

[0042] Other conditions of this energization are not particularly limited and can be appropriately selected according to the configuration of the plate assembly. For example, the pressing force during this energization is not particularly limited and may be appropriately adjusted so that the nugget 121 is formed between the steel plates 10. As an example, the pressing force may be set to satisfy the following formula 2. In formula 2, FE is the pressing force (N), and h is the average plate thickness (the sum of the plate thicknesses of the plurality of steel plates 10 divided by the number of steel plates 10) (mm). The pressing force may be constant during this energization or may be appropriately changed. When the pressing force is increased, the tendency for LME cracks to occur is reduced. 1960×h≦FE≦3920×h (Formula 2)

[0043] After this energization, post-energization is performed on the spot weld portion 12. Generally, post-energization is performed for the purpose of annealing the hardened nugget 121 and relaxing the segregation of elements such as P, etc. However, in the method for manufacturing a spot weld joint according to this embodiment, the main purpose of performing post-energization is to segregate B at the grain boundaries in a solid solution state in the heat-affected zone 123 of the high-strength steel plate 10H that contacts the Zn-based plating layer 11. In this energization, the heat-affected zone 123 of the high-strength steel plate 10H that contacts the Zn-based plating layer 11 is heated to such an extent that BN can be decomposed. In post-energization, the heat-affected zone 123 of the high-strength steel plate 10H that contacts the Zn-based plating layer 11 is maintained in a temperature range for segregating B at the grain boundaries in a solid solution state.

[0044] In post-energization, the energization time t1 (sec), the post-energization time t2 (sec), and the Si content (Si) in terms of unit mass% of the high-strength steel plate 10H that contacts the Zn-based plating layer 11 satisfy the following formula 3. Also, the welding current value I1 (kA) and the post-energization current value I2 (kA) satisfy the following formula 4. {(-14.3×Si + 34.3) 2 / 3.7}×10 -4 <t2 < 0.5×t1 (Formula 3) 0.7×I1 < I2 < 1.5×I1 (Formula 4)

[0045] When the post - energization time t2 is less than or equal to the lower limit value of Equation 3, and / or when the post - energization current value I2 is less than or equal to the lower limit value of Equation 4, it is difficult to keep B in a solid - solution state in the heat - affected zone 123 of the high - strength steel sheet 10H in contact with the Zn - based plating layer 11. On the other hand, when t2 is greater than or equal to the upper limit value of Equation 3, and / or when the post - energization current value I2 is greater than or equal to the upper limit value of Equation 4, the temperature of the spot - welded part 12 becomes excessively high, and LME is likely to occur.

[0046] The timing to start the post - energization is set according to the post - energization current value (kA). Specifically, when the post - energization current value I2 (kA) satisfies the following Equation 5, the post - energization is started immediately after the main energization ends. That is, the main energization and the post - energization are carried out continuously. When the post - energization current value I2 satisfies the following Equation 6, after cooling only the spot - welded joint 1 by the cooling time t s shown in Equation 7, the post - energization is started. In Equations 5 and 6, I1 (kA) represents the welding current value, and I2 (kA) represents the post - energization current value. In Equation 7, I1 (kA) is the welding current value I1 (kA), t1 (sec) is the main energization time, and t S (sec) represents the cooling time. 0.7×I1 < I2 < I1 (Equation 5) I1 ≤ I2 < 1.5×I1 (Equation 6) (4.07×I1 2 ×t1 - 19.30) / 1000 < t s <(4.07×I1 2 ×t1 - 17.15) / 1000 (Equation 7) When I2 satisfies Equation 5, the main energization and the post - energization are carried out continuously. Thereby, the temperature drop in the heat - affected zone can be prevented, and the segregation of B can be further promoted. On the other hand, when I2 satisfies Equation 6, cooling is performed between the main energization and the post - energization. Such cooling is performed to prevent the temperature of the spot - welded part 12 from becoming excessively high.

[0047] Furthermore, cooling of the spot weld 12 can be achieved by stopping the current while the spot weld 12 is still held between the pair of electrodes used for the current application. Conventional spot welding electrodes have a coolant circulating inside them, and their tips are always kept cool. Therefore, when the current is stopped while the spot weld 12 is still held between the pair of electrodes, heat transfer occurs from the spot weld 12 to the electrodes, and the spot weld 12 is cooled.

[0048] The pressure applied during post-energization and cooling is not particularly limited. For example, the pressure applied during post-energization and cooling may be determined in the same way as the pressure applied during initial energization, or it may be the same as the pressure applied during initial energization.

[0049] After the post-energization, the pressure applied to the spot weld 12 may be maintained while the current is stopped from flowing to the spot weld 12. During post-energization, the Zn-based plating layer 11 may be in a molten state. By solidifying the Zn-based plating layer 11 through the holding process and ending the state in which LME cracking is a concern before releasing the electrodes, LME cracking can be further suppressed.

[0050] The pressure applied during holding is not particularly limited. For example, the pressure applied during holding may be determined in the same way as the pressure applied during main energization, or it may be the same as the pressure applied during main energization. The holding time is also not particularly limited, but from the viewpoint of more effectively suppressing LME cracking, the holding time is preferably 0.04 seconds or longer, or 0.4 seconds or longer.

[0051] On the other hand, in the method for manufacturing spot-welded joints according to this embodiment, the heat-affected zone 123 of the high-strength steel plate 10H in contact with the Zn-based plating layer 11 is sufficiently modified at the stage when post-energization is completed. Therefore, the holding step may be omitted. Even if a holding step is provided, the holding time can be set to a much smaller value than usual (for example, 0.04 seconds or more as described above).

[0052] Through the above process, a spot-welded joint 1 in which LME cracking is suppressed can be manufactured.

[0053] <2. Configuration of Spot Welded Joint 1> Next, the configuration of the spot-welded joint 1 according to this embodiment will be described based on Figure 1. Figure 1 is a cross-sectional view showing a cross section that passes through the center of the nugget 121 and is parallel to the thickness direction of the spot-welded joint 1.

[0054] A spot-welded joint 1 comprises two or more overlapping steel plates 10 and a spot-welded area 12. The spot-welded area 12 has a nugget 121 that joins multiple steel plates 10, a pressure-welded area 122 formed around the nugget 121 where opposing steel plates 10 are pressed together, and a gap 13 formed on the outside of the nugget 121. In the example in Figure 1, two steel plates 10 are spot-welded, but three or more steel plates 10 may be spot-welded. In addition, a heat-affected zone (HAZ) 123 is usually formed around the nugget 121. The heat-affected zone 123 is also included in the spot-welded area 12.

[0055] The multiple steel plates 10 are the steel plates 10 described in <1. Method for manufacturing spot-welded joints>. That is, one or more of the multiple steel plates 10 are high-strength steel plates 10H with a tensile strength of 780 MPa or higher. All of the steel plates 10 may be high-strength steel plates 10H, or, as shown in Figure 1, one or more of the steel plates 10 may be low-strength steel plates 10L.

[0056] One or more high-strength steel sheets 10H are in contact with the Zn-based plating layer 11. That is, one or more high-strength steel sheets 10H are Zn-based plated steel sheets, or one or more high-strength steel sheets 10H are stacked in contact with Zn-based plated steel sheets. The Zn-based plating layer 11 is the Zn-based plating layer 11 described in <1. Method for manufacturing spot-welded joints>. In the example in Figure 1, the Zn-based plating layer 11 is applied to the lower surface (joint surface with the lower steel sheet 10) of the lower steel sheet 10, which is the low-strength steel sheet 10L. The Zn-based plating layer 11 is in contact with the upper surface of the high-strength steel sheet 10H.

[0057] The spot welded joint 12 comprises a nugget 121 and a pressure-welded joint 122. The nugget 121 is formed between the steel plates 10 and joins them together. The pressure-welded joint 122 is formed around the nugget 121 and is the portion where two opposing steel plates 10 are pressed together. The pressure-welded joint 122 is sometimes referred to as a corona bond. A gap 13 is formed on the outside of the pressure-welded joint 122.

[0058] Normally, the pressure-welded portion 122 is formed inside the heat-affected zone 123, and LME cracks are likely to occur in the pressure-welded portion 122. However, in the spot-welded joint 1 according to this embodiment, the amount of BN is reduced in the vicinity of the pressure-welded portion 122. More specifically, in the observation area A shown in the cross-section of Figure 2 (a cross-section passing through the center of the nugget 121 and parallel to the thickness direction of the spot-welded joint 1), the number of BN particles with a major axis of 0.5 μm or more is set to 100 or less.

[0059] As shown in Figure 2, observation area A is provided inside the high-strength steel plate 10H in contact with the Zn-based plating layer 11, has a square shape with sides of 100 μm, one side of the square overlaps with the pressure-welded portion 122, and the center of one side of the square is set to coincide with the boundary B between the pressure-welded portion 122 and the gap portion 13. If there are two or more high-strength steel plates 10H in contact with the Zn-based plating layer 11, observation area A is provided in each of the high-strength steel plates 10H in contact with the Zn-based plating layer 11. In each of the two or more observation areas A, the number of BNs with a major axis of 0.5 μm or more is set to 100 or less.

[0060] Observation region A is provided on the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. As described above, a B content of 0.0010 mass% or more is ensured in the high-strength steel sheet 10H in contact with the Zn-based plating layer 11. Therefore, when the number of BN particles with a major axis of 0.5 μm or more in observation region A is 100 or less, a sufficient amount of B exists in a solid solution state in observation region A. The B in a solid solution state naturally segregates at the grain boundaries, preventing the penetration of molten zinc into the grain boundaries. Observation region A is the region where LME cracking is most likely to occur, but by sufficiently reducing BN in this region, LME cracking is suppressed very effectively.

[0061] The number of BN molecules is measured, for example, by SIMS (Secondary Ion Mass Spectrometry). The measurement conditions are as follows: • Irradiation ions: Bi1 + • Acceleration voltage: 300 keV The number of BNs with a major axis of 0.5 μm or larger included in the image of observation region A obtained in this way is counted. The major axis of a BN is the diameter of the smallest circle that can enclose the BN. As shown in Figure 1, two observation regions A can be defined for one high-strength steel plate 10H, but the above evaluation can be performed in either observation region A. [Examples]

[0062] The effects of one aspect of the present invention will be further explained in detail by the examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention.

[0063] Steel plate 1 (tensile strength approximately 2 GPa) shown in Table 1 and steel plate 2 (GA980DP steel) shown in Table 2 were used as the base material for spot-welded joints. Spot welding was performed on the plate assembly composed of steel plate 1 and steel plate 2 under the conditions shown in Tables 3 to 5 to obtain the spot-welded joints shown in Table 6. In these spot-welded joints, both steel plate 1 and steel plate 2 fall under the category of "high-strength steel plates in contact with a Zn-based plating layer."

[0064] The following conditions are not shown in the table. • Electrodes: Both upper and lower electrodes are made of CrCu, DR type, φ6 R40. • Pressure applied: 400gf from the start of the current application to the end of the holding process. ·Holding time: 0.1sec • Board thickness: 1.6mm for both top and bottom boards • Basis weight of the Zn-based plating layer on steel plate 2: 45 g / m² per side2 • Plating of steel plate 1: None Furthermore, the underlined values ​​in the table are outside the scope of the present invention.

[0065] [Table 1]

[0066] [Table 2]

[0067] [Table 3]

[0068] The "I1-t1-Steel Plate 1" column in Table 3 indicates the suitability of the current value I1 and energizing time t1 for steel plate 1, and the "I1-t1-Steel Plate 2" column indicates the suitability of the current value I1 and energizing time t1 for steel plate 2. That is, regarding the Si content of steel plate 1, "28 × Si + 1111 < 58 × I1 2 If the relationship "×t1 < 30 × Si + 1250" is satisfied, "Good" is written in the "I1-t1-Steel Plate 1" column; otherwise, "Poor" is written. Similarly, regarding the Si content of steel plate 2, "28 × Si + 1111 < 58 × I1 2 If the relationship "×t1 < 30 × Si + 1250" is satisfied, "Good" is written in the "I1-t1-Steel Plate 2" column; otherwise, "Poor" is written.

[0069] [Table 4]

[0070] [Table 5]

[0071] In the column of "Post - energization time t2 for Steel Plate 1" described in Table 5, the suitability of the post - energization time t2 for Steel Plate 1 is indicated, and in the column of "Post - energization time t2 for Steel Plate 1", the suitability of the post - energization time t2 for Steel Plate 2 is indicated. That is, regarding the Si content of Steel Plate 1, when the relationship "{(-14.3×Si + 34.3) 2 / 3.7}×10 -4 <t2" is satisfied, "good" is described in the column of "Post - energization time t2 for Steel Plate 1", and in other cases, "bad" is described. Similarly, regarding the Si content of Steel Plate 2, when the relationship "{(-14.3×Si + 34.3) 2 / 3.7}×10 -4 <t2" is satisfied, "good" is described in the column of "Post - energization time t2 for Steel Plate 2", and in other cases, "bad" is described.

[0072]

Table 6

[0073] The "Number of BN" described in Table 6 is the number of BN with a major diameter of 0.5 μm or more included in the above - mentioned observation region A. The measurement of the number of BN with a major diameter of 0.5 μm or more was carried out according to the above - mentioned measurement method.

[0074] In the column of "Crack length" in Table 6, the observation results of LME cracks confirmed in the cross - section passing through the center of the nugget and parallel to the thickness direction of the spot - welded joint are described. When observing the cross - section of the spot - welded joint where LME cracks occurred, as shown in the photograph of Figure 3, cracks generated near the boundary between the welded part and the plate gap part can be clearly recognized. When the length of this crack is measured and the length is 0.25 mm or less, it is judged that LME cracks are suppressed, and "qualified" is described in the column of "Crack length". In other cases, "unqualified" is described in the column of "Crack length".

[0075] In the spot-welded joints of Comparative Examples 2, 5, and 6, the crack length was unacceptable. In the spot-welded joints of Comparative Examples 2, 5, and 6, a large amount of BN was present in the HAZ of the high-strength steel plate in contact with the Zn-based plating layer. Therefore, it is presumed that the LME crack suppression effect using solid-solution B did not manifest in the spot-welded joints of Comparative Examples 2, 5, and 6.

[0076] In the spot welded joint of Comparative Example 2, 28 × Si + 1111 < 58 × I1 2 ×t1<30×Si+1250, and (4.07×I1 2 ×t1-19.30) / 1000 <t s <(4.07×I1 2 The formula ×t1-17.15) / 1000 was not satisfied with respect to steel plate 1. Furthermore, in the spot welded joint of Comparative Example 2, the post-energizing current value I2 was inappropriate. In addition, in the spot welded joint of Comparative Example 2, the post-energizing time was inappropriate with respect to steel plate 2. The reason for the high amount of BN in the spot welded joint of Comparative Example 2 is thought to be that the temperature of the weld during the main energizing did not reach a temperature at which BN could be sufficiently decomposed, and furthermore, the cooling time was long, causing the temperature to drop too low during post-energizing, preventing solid solution of B.

[0077] In the spot welded joint of Comparative Example 5, 28 × Si + 1111 < 58 × I1 2 The condition ×t1 < 30 × Si + 1250 was not satisfied for both steel plate 1 and steel plate 2. Furthermore, the post-energization time t2 was also inappropriate for the spot welded joint in Comparative Example 5. The reason for the high amount of BN in the spot welded joint of Comparative Example 5 is thought to be that the temperature of the weld during the main energization did not reach a temperature at which the BN could be sufficiently decomposed.

[0078] In the spot welded joint of Comparative Example 6, 28 × Si + 1111 < 58 × I1 2The condition ×t1 < 30 × Si + 1250 was not satisfied for both steel plate 1 and steel plate 2. Furthermore, the post-current value I2 was also inappropriate for the spot-welded joint in Comparative Example 6. The reason for the high amount of BN in the spot-welded joint of Comparative Example 6 is thought to be that the temperature of the weld during the main current application did not reach a temperature at which the BN could be sufficiently decomposed. [Explanation of symbols]

[0079] 1. Spot welded joint 10 steel plate 10L low strength steel plate 10H high strength steel plate 11 Zn-based plating layer 12 Spot welds 121 Nuggets 122 Pressure-welded section 123 Heat Affected Zone (HAZ) 13 Gap area A Observation Area B Boundary between the pressure-welded area and the gap between the plates

Claims

1. A step of forming spot welds that join two or more overlapping steel plates by applying current to them, The process of applying current to the spot welded area afterwards, A method for manufacturing a spot welded joint comprising, One or more of the aforementioned steel plates are high-strength steel plates with a tensile strength of 780 MPa or more. One or more of the aforementioned high-strength steel plates are in contact with a Zn-based plating layer. The B content of the high-strength steel sheet in contact with the Zn-based plating layer is 0.0010% by mass or more. In the above energization, the welding current value I １ (kA), main energization time t １ (sec), and the Si content in unit mass% of the high-strength steel sheet in contact with the Zn-based plating layer is 28 × Si + 1111 < 58 × I １ ２ ×t １ <30 × Si + 1250 satisfies, In the subsequent energization, the main energization time t １ (kA), the subsequent energization time t ２ (sec), and the Si content in terms of unit mass % of the high-strength steel sheet in contact with the Zn-based plating layer is {(-14.3 × Si + 34.3) ２ / 3.7} × 10 －４ < t ２ < 0.5 × t １ is satisfied, In the aforementioned post-energization, the post-energization current value I ２ (kA) is 0.7 × I １ <I ２ <1.5 × I １ Satisfying the conditions, The value of the post-energization current I ２ (kA) is 0.7 × I １ <I ２ <I １ If the conditions are met, the initial energization and the subsequent energization will be performed in succession. The value of the post-energization current I ２ (kA) is I １ ≤I ２ <1.5 × I １ If the condition is met, the method for manufacturing the spot welded joint further comprises a step of cooling the spot welded portion between the initial energization and the post-energization, In the cooling process, the welding current value I １ (kA), the above energizing time t １ (sec), and cooling time t Ｓ (sec) is (4.07 × I １ ２ ×t １ -19.30) / 1000<t ｓ <(4.07 x I １ ２ ×t １ (-17.15) / 1000 A method for manufacturing spot-welded joints.

2. The method for manufacturing the spot welded joint further comprises a step of maintaining pressure applied to the spot weld while suspending the supply of current to the spot weld after the step of applying current, The holding time for the aforementioned holding is set to 0.4 seconds or more. A method for manufacturing a spot welded joint according to claim 1, characterized in that

3. Two or more overlapping steel plates, A spot weld having a nugget for joining the steel plates together, a pressure-welded portion formed around the nugget where the opposing steel plates are pressed together, and a gap portion formed on the outside of the pressure-welded portion, A spot welded joint comprising, One or more of the aforementioned steel plates are high-strength steel plates with a tensile strength of 780 MPa or more. A Zn-based plating layer is provided on the joint surface between one or more of the aforementioned high-strength steel plates and the steel plates in contact with them. The B content of the high-strength steel sheet in contact with the Zn-based plating layer is 0.0010% by mass or more. The BN particles, which have a major axis of 0.5 μm or more, are provided inside the high-strength steel plate in contact with the Zn-based plating layer in a cross-section that passes through the center of the nugget and is parallel to the thickness direction of the spot welded joint, and have a square shape with sides of 100 μm, one side of the square overlapping with the pressure-welded portion, and the observation area in which the center of the one side of the square coincides with the boundary between the pressure-welded portion and the gap between the plates, and number 100 or less. Spot welded joint.

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