Resistance spot welding joint and method for manufacturing resistance spot welding joint

Abstract

To provide a resistance spot welded joint whose LME split is suppressed, and a manufacturing method for the same.SOLUTION: A resistance spot welded joint includes a plurality of steel plates imposed on one another and a weld zone. One or more of the plurality of steel plates are high strength whose tensile strength is 980 MPa or more. The high-strength steel plate or a steel plate adjacent to the high-strength steel plate is a plated steel plate having a galvanized layer. In the high-strength steel plate, an average interval of a Mn concentration part in the direction orthogonal to a rolling direction is 300 μm or less at a 1 / 20 depth position of a base material and standard deviation of Mn concentration in a retained austenite is 0.40% or less, and Vickers hardness [Hvsur] at a 30 μm depth and Vickers hardness [Hvq] at a 1 / 4 depth position to be a position 1 / 4 plate thickness in the plate thickness direction from the surface of the base material satisfy [Hvsur] / [Hvq]≤0.80.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a resistance spot welding joint and a method for manufacturing the resistance spot welding joint.

Background Art

[0002] In order to reduce the weight of automobiles, improve fuel efficiency, reduce carbon dioxide emissions, and ensure the safety of passengers, high-strength steel sheets are used as automobile steel sheets. In recent years, in order to sufficiently ensure the corrosion resistance of vehicle bodies and parts, in addition to high-strength hot-dip galvanized steel sheets, high-strength alloyed hot-dip galvanized steel sheets are also used as automobile steel sheets.

[0003] In processes such as the assembly of automobile bodies and the attachment of parts, resistance spot welding is mainly used. Resistance spot welding is a resistance welding method in which the overlapped base materials are sandwiched between the tips of electrodes with appropriately shaped tips, current and pressure are concentrated on a relatively small part for local heating, and at the same time pressure is applied with the electrodes.

[0004] However, for the assembly of vehicle bodies and / or parts, when spot welding zinc-plated steel sheets (hot-dip galvanized steel sheets, electro-galvanized steel sheets, or alloyed hot-dip galvanized steel sheets) to each other, or when resistance spot welding a cold-rolled steel sheet and a zinc-plated steel sheet, cracks called liquid metal embrittlement (LME) cracks may occur at the spot welds. LME cracks are cracks that occur when the zinc in the zinc plating layer melts due to the heat generated during resistance spot welding, the molten zinc penetrates into the grain boundaries of the steel sheet structure at the weld, and tensile stress acts on that state. The requirements for crack generation are that the molten zinc during welding contacts the solid steel sheet, and tensile stress (strain) acts on that part. The higher the strength of the steel sheet, the higher the tendency for the susceptibility to LME cracks. Even if one is a cold-rolled steel sheet without zinc plating and the other is a zinc-plated steel sheet, LME cracks may occur when resistance spot welding due to the molten zinc in the zinc-plated steel sheet contacting the cold-rolled steel sheet.

[0005] When steel plates are joined by resistance spot welding to form a joint (resistance spot welding joint), if LME cracking occurs, the cracking will cause insufficient joint strength to be obtained compared to the joint strength assumed for the steel plates used for welding. Therefore, it is required to suppress LME cracking in resistance spot welding joints (parts including resistance spot welding joints).

[0006] In response to such problems, for example, Patent Document 1 discloses a joined structure in which a plurality of superimposed steel plates are resistance welded, and among the plurality of steel plates, at least one of the steel plates is a high-tensile steel plate having a chemical composition with a carbon equivalent Ceq of 0.53% or more and a tensile strength of 590 MPa or more. The high-tensile steel plate has a decarburized layer between the zinc-based plating layer formed on at least one surface of the superimposed surface side and the welding electrode side and the base material, or on the superimposed surface adjacent to the zinc-based plating layer of the superimposed zinc-based plated steel plate. The decarburized layer has a thickness of 5 μm or more and 200 μm or less. Patent Document 1 discloses that the presence of the decarburized layer suppresses embrittlement by zinc in which the melted zinc in the zinc-based plating layer disperses and penetrates into the grain boundaries of the HAZ during welding.

[0007] In addition, Patent Document 2 discloses a spot welding member in which received LME cracking in the welded portion is suppressed and can be manufactured without removing the plating layer. Patent Document 2 discloses that the occurrence of received LME cracking can be suppressed by controlling the surface layer Zn concentration inside the corona bond of the spot welded portion.

[0008] In addition, Patent Document 3 discloses a high-strength steel plate with a tensile strength of 980 MPa or more and a method for manufacturing the same, which are excellent in ductility, elongation flangibility, bendability, and LME resistance and can manufacture parts with high dimensional accuracy. In Patent Document 3, it is disclosed that by controlling the corresponding grain boundary frequency on the surface layer of the steel sheet after the high-temperature tensile test to 0.45 or less and the surface layer softening thickness to 5 μm or more and 150 μm or less, a high-strength steel sheet excellent in LME resistance can be realized.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] As described above, Patent Documents 1 to 3 disclose methods for improving LME resistance. However, as a result of investigations by the present inventors, although the techniques of Patent Documents 1 to 3 can obtain a certain LME resistance improvement effect, it has been found that there is room for improvement in that effect. Therefore, in the present invention, on the premise that one or more of a plurality of steel sheets to be resistance spot welded are high-strength steel sheets having a tensile strength of 980 MPa or more, and the high-strength steel sheet or the steel sheet adjacent to the high-strength steel sheet is a plated steel sheet having a base material and a zinc-based plating layer formed on the surface of the base material, an object is to provide a resistance spot welded joint in which LME cracking is suppressed and a method for manufacturing the same.

Means for Solving the Problems

[0011] The present inventors investigated a method for suppressing LME cracking in a resistance spot welded joint. The occurrence of LME cracks is affected by temperature and the magnitude of tensile stress. Therefore, in order to suppress LME cracks, it is important to suppress the increase in temperature and tensile stress. However, since sufficient heat input is required to obtain a sufficient nugget diameter, it can be said that it is difficult to suppress the temperature rise. Therefore, the inventors of the present invention have studied the reduction of tensile stress. In addition, the study was conducted focusing on LME susceptibility. As a result, it has been found that LME cracks can be suppressed by controlling the distribution of Mn concentration and hardness in the surface layer portion on the surface side in contact with the zinc-based plating layer in the base material of the high-strength steel sheet in contact with the zinc-based plating layer.

[0012] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] A welded joint including a plurality of steel plates overlapped with each other, a nugget joining the plurality of steel plates, and a corona bond and a heat-affected zone formed around the nugget, wherein one or more of the plurality of steel plates are high-strength steel plates having a tensile strength of 980 MPa or more, and the high-strength steel plate, or a steel plate adjacent to the high-strength steel plate, is a plated steel plate having a base material and a zinc-based plating layer formed on the surface of the base material, and the high-strength steel plate has an average interval of Mn enrichment portions in the direction orthogonal to the rolling direction at a position of 1 / 20 of the plate thickness in the plate thickness direction from the surface in contact with the zinc-based plating layer of the base material of 300 μm or less, and a standard deviation of the Mn concentration in retained austenite of 0.40% or less, and the Vickers hardness [Hv sur at a position of 30 μm in the plate thickness direction from the surface of the base material, and the Vickers hardness [Hv q at a position of 1 / 4 of the plate thickness in the plate thickness direction from the surface of the base material satisfy the following formula (1), a resistance spot welding joint. [Hv sur / [Hv q ≦ 0.80 (1) [2] The resistance spot welding joint according to [1], wherein the surface of the base material of the high-strength steel plate in contact with the zinc-based plating layer is located on the overlapping surface of the plurality of overlapped steel plates. [3] In the corona bond in the cross-section in the plate thickness direction passing through the center of the nugget, the ratio of the η-phase in the zinc-based plating layer is 20 area% or less, the resistance spot welding joint according to [2]. [4] In the cross-section in the plate thickness direction passing through the center of the nugget, the diameter of the heat-affected zone is 1.5 times or more the diameter of the nugget, and in the heat-affected zone, carbides having an equivalent circle diameter of 0.1 μm or more are distributed at a number density of 40 pieces / 100 μm 2 or more, the resistance spot welding joint according to [3]. [5] The high-strength steel sheet is the plated steel sheet, and the plated steel sheet is arranged such that the zinc-based plating layer becomes the outermost surface, the resistance spot welding joint according to any one of [1] to [4]. [6] The ratio of the η-phase of the zinc-based plating layer at the shoulder of the welded portion on the outermost surface is 20 area% or less, the resistance spot welding joint according to [5]. [7] The Vickers hardness [Hv sur at the 30 μm depth position and the Vickers hardness [Hv q at the 1 / 4 depth position satisfy the following formula (2), the resistance spot welding joint according to any one of [1] to [6]. 0.60 ≦ [Hv sur / [Hv q ≦ 0.80 (2) [8] A stacking step of stacking a plurality of steel plates in the thickness direction, and a current application step of forming a nugget and a corona bond by applying current between a pair of opposing electrodes while pressing the plurality of steel plates after the stacking step, and after the current application step, a current decrease step of decreasing the current value between the electrodes to 0 while maintaining the pressure, and among the plurality of steel plates, one or more are high-strength steel plates with a tensile strength of 980 MPa or more, and the high-strength steel plate, or the steel plate stacked on the high-strength steel plate, is a plated steel plate having a base material and a zinc-based plating layer formed on the surface of the base material, and the high-strength steel plate has an average interval of Mn enrichment portions in the direction orthogonal to the rolling direction at a 1 / 20 depth position, which is a position 1 / 20 of the plate thickness in the thickness direction from the surface of the base material, of 300 μm or less, and at the 1 / 20 depth position, the standard deviation of the Mn concentration in the retained austenite is 0.40% or less, and the Vickers hardness [Hv sur at a 30 μm depth position, which is a position 30 μm in the thickness direction from the surface of the base material, and the Vickers hardness [Hv q at a 1 / 4 depth position, which is a position 1 / 4 of the plate thickness in the thickness direction from the surface of the base material, satisfy the following formula (1), a method for manufacturing a resistance spot welding joint. [Hv sur / [Hv q ≦ 0.80 (1) [9] In the current decrease step, while maintaining the pressure, the down slope is adjusted so that the time for decreasing the current value between the electrodes to 0 is 420 msec or more, the method for manufacturing a resistance spot welding joint according to [8].

[10] Defining 1 / 2 of the total plate thickness of the plurality of steel plates in mm as tm, and when the current value between the electrodes at the time when the formation of the nugget is completed is I, in the current decrease step, the current value between the electrodes is within the range from I×0.9 to I×0.3, and is maintained at a constant value for 265×tm or more and 420 msec or more per unit msec, the method for manufacturing a resistance spot welding joint according to [8] or [9].

[11] Before the energization step, a preliminary energization step of energizing with a current smaller than the energization in the energization step is further provided. The method for manufacturing a resistance spot welding joint according to any one of [8] to

[10] .

[12] After the current reduction step, further, when the pressing force at the time when the formation of the nugget is completed is P, in a state where the current value between the electrodes is set to 0, for 200 msec or more and 400 msec or less, the pressing force is maintained at 0.8×P or more. The method for manufacturing a resistance spot welding joint according to any one of [8] to

[11] , which includes a pressure holding step.

Effect of the Invention

[0013] According to the present invention, it is possible to provide a resistance spot welding joint in which LME cracking is suppressed, and a method for manufacturing the same.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0015] Hereinafter, a resistance spot welding joint according to an embodiment of the present invention (the resistance spot welding joint according to this embodiment) and a method for manufacturing the resistance spot welding joint according to this embodiment will be described. The resistance spot welding joint according to this embodiment includes a plurality of steel plates superposed on each other, a nugget joining the plurality of steel plates, and a welded portion having a corona bond and a heat affected zone formed around the nugget. The plurality of steel plates may be two or three or more. In addition, in the resistance spot welding joint according to the present embodiment, one or more of the plurality of steel plates are high-strength steel plates with a tensile strength of 980 MPa or more, and the high-strength steel plate, or the steel plate adjacent to the high-strength steel plate, is a plated steel plate having a base material and a zinc-based plating layer formed on the surface of the base material. The zinc-based plating layer is not limited as long as it is a known plating layer containing zinc. Further, this high-strength steel plate has an average interval of Mn enrichment portions in the direction orthogonal to the rolling direction at a depth of 1 / 20 of 300 μm or less, and a standard deviation of the Mn concentration in the retained austenite of 0.40% or less at a depth of 30 μm, and the Vickers hardness [Hv sur at a depth of 1 / 4, which is a position of 1 / 4 of the plate thickness in the plate thickness direction from the surface of the base material, and the Vickers hardness [Hv q satisfy the following formula (1). [Hv sur / [Hv q ≦ 0.80 (1) In the resistance spot welding joint according to the present embodiment, all of the plurality of steel plates may be plated steel plates, or steel plates (including high-strength steel plates) that are not plated steel plates (made of a base material) may be included. Further, the surface of the base material of the high-strength steel plate that contacts the zinc-based plating layer may be located on the overlapping surface of the plurality of stacked steel plates. (That is, when a high-strength steel plate is stacked on the zinc-based plating layer of a plated steel plate, or when another steel plate is stacked on the zinc-based plating layer of a high-strength steel plate that is a plated steel plate, etc.) Further, the plated steel plate may be arranged such that the zinc-based plating layer becomes the outermost surface (the outermost surface). (For example, when the outermost side of the stacked steel plates is a high-strength plated steel plate and the zinc-based plating layer is formed on the outer surface thereof.) In the present embodiment, the surface of the base material that contacts the zinc-based plating layer means the interface between the base material of the zinc-based plated steel plate and the zinc-based plating layer, and the surface of the steel plate that is not a plated steel plate (made of a base material) and contacts the zinc plating layer of the adjacent plated steel plate. The following will be described for each. However, the specifications of the high-strength steel sheets are all specified at positions excluding the welded parts.

[0016] [Comprising a plurality of steel sheets overlapped with each other, a nugget joining the plurality of steel sheets, and a welded part having a corona bond and a heat-affected zone formed around the nugget] The resistance spot welding joint according to the present embodiment is a resistance spot welding joint obtained by overlapping a plurality of steel sheets, applying pressure using a pair of opposing electrodes, and energizing to form a nugget. That is, the plurality of steel sheets are joined by the nugget. When forming the nugget, the temperature of the steel sheet rises due to energization. The portion where the steel sheet melts due to the temperature rise becomes the nugget after solidification. On the other hand, even in the portion that does not melt, a ring-shaped portion where the steel sheets are solid-phase joined is formed around the nugget. This ring-shaped portion is called a corona bond. Also, a portion (heat-affected zone) affected by the heat of welding is generated around (outside) the corona bond. That is, the resistance spot welding joint according to the present embodiment includes a plurality of steel sheets overlapped with each other, a nugget joining the plurality of steel sheets, and a welded part having a corona bond and a heat-affected zone formed around the nugget.

[0017] LME cracking is a crack that occurs when the zinc in the zinc plating layer melts due to the heat generated during resistance spot welding, the molten zinc penetrates into the grain boundaries of the steel sheet structure of the welded part, and tensile stress acts in that state. The requirements for crack generation are that the molten zinc contacts the solid steel sheet during welding and tensile stress (strain) acts on that part. The higher the strength of the steel sheet, the higher the sensitivity to LME cracking tends to be. Therefore, in the resistance spot welding joint according to the present embodiment, one or more of the plurality of steel sheets are high-strength steel sheets with a tensile strength of 980 MPa or more, and the high-strength steel sheet, or the steel sheet adjacent to the high-strength steel sheet, is a plated steel sheet having a zinc-based plating layer formed on the surface of the base material, and this high-strength steel sheet has a predetermined Mn concentration distribution and a predetermined hardness distribution. For steel sheets with a tensile strength of less than 980 MPa, it is not necessary to satisfy the requirements for the Mn concentration distribution and hardness distribution shown below. Also, even for high-strength steel sheets, if they do not have a plating layer and do not come into contact with a zinc-based plating layer when stacked, it is not necessary to satisfy the requirements for the Mn concentration distribution and hardness distribution shown below.

[0018] [Mn Concentration Distribution] LME cracks occur when liquid metal enters the grain boundaries. Si is known as an element that increases LME susceptibility, and LME cracks can be suppressed by preventing the concentration of Si in the surface layer of the steel sheet near the plating layer. However, qualitatively, although the above effects were known, it was difficult to quantitatively evaluate and control the distribution of the content etc. of Si during the manufacturing process, and no control of the Si distribution for suppressing LME cracks had been proposed conventionally. On the other hand, as a result of investigations by the present inventors, it has been found that the dispersion of Si can be evaluated by the Mn concentration distribution (degree of dispersion). This is because the degree of dispersion of Si mainly changes in the casting process and the annealing process, and in these processes, Mn shows the same behavior. Also, when one or more of a plurality of stacked steel sheets have a high-strength steel sheet with a tensile strength of 980 MPa or more, and the high-strength steel sheet, or the steel sheet adjacent to the high-strength steel sheet, is a plated steel sheet having a zinc-based plating layer formed on the surface of the base material, it has been found that LME cracks can be suppressed by controlling the Mn concentration distribution (degree of dispersion) in this high-strength steel sheet. Specifically, at the 1 / 20 depth position, which is the position of 1 / 20 of the sheet thickness in the thickness direction from the surface of the base material of the high-strength steel sheet, the average interval of the Mn-enriched portions in the direction orthogonal to the rolling direction is 300 μm or less, and at the 1 / 20 depth position, the standard deviation of the Mn concentration in the retained austenite is 0.40% or less by mass. Since the Mn-enriched portions and the Si-enriched portions almost coincide, if the average interval of the Mn-enriched portions exceeds 300 μm, portions with high LME susceptibility will be scattered in the steel sheet, and the LME resistance of the steel sheet will decrease. At the same time, the Mn-enriched portions remain, and the elongation and toughness of the base material deteriorate. The average interval of the Mn-enriched portions is preferably 270 μm or less from the viewpoint of ensuring the stability of the base material performance and LME resistance. On the other hand, when the standard deviation of the Mn concentration in the retained austenite exceeds 0.40%, a Si-enriched region with a large degree of Si enrichment is formed, and the LME susceptibility increases. (The LME resistance decreases.) The standard deviation of the Mn concentration of the retained austenite grains is preferably 0.36% or less in order to ensure the stability of the base material performance and the LME resistance as well.

[0019] As described above, the occurrence of LME cracks is affected by the temperature and the magnitude of the tensile stress. Therefore, in order to suppress LME cracks, it is important to suppress the increase in temperature and tensile stress. However, since sufficient heat input is required to obtain a sufficient nugget diameter, it can be said that it is difficult to suppress the temperature rise. The inventors of the present invention have found that by making the Vickers hardness at a position 30 μm deep in the plate thickness direction from the surface of the base material of the high-strength steel sheet in contact with the zinc-based plating layer smaller than the Vickers hardness at a position 1 / 4 deep in the plate thickness direction, which is 1 / 4 of the plate thickness, the strain generated by the tensile stress is dispersed and LME cracks are suppressed. Specifically, it has been found that LME cracks are suppressed when the Vickers hardness [Hv sur at the 30-μm depth position and the Vickers hardness [Hv q at the 1 / 4 depth position satisfy the following formula (1). [Hv sur / [Hv q ≦ 0.80 (1) The measurement position of the Vickers hardness was set at the 30-μm depth position in order to provide a sufficient hardness reduction region for strain dispersion. Even if the Vickers hardness at a position closer to the surface than the 30-μm depth position, for example, at the 20-μm depth position, is 0.80 times or less of the Vickers hardness at the 1 / 4 depth position, if the Vickers hardness at the 30-μm depth position exceeds 0.80 times of the Vickers hardness at the 1 / 4 depth position, the low-hardness region is too thin to sufficiently disperse the strain.

[0020] From the viewpoint of strain dispersion, [Hv sur / [Hv qIt is preferably lower, but if the hardness of the surface layer is made lower than necessary, the Vickers hardness inside also decreases, and the strength as a resistance spot weld joint decreases. Therefore, the Vickers hardness [Hv sur at the 30-μm depth position and the Vickers hardness [Hv q at the 1 / 4 depth position preferably satisfy the following formula (2). 0.60 ≦ [Hv sur / [Hv q ≦ 0.80 (2)

[0021] When there are three or more steel plates and there are a plurality of overlapping surfaces (joint surfaces) via a zinc-based plating layer, there will be a plurality of 1 / 20 depth positions. In the resistance spot weld joint according to the present embodiment, the above effects can be obtained if at least one of them satisfies the above conditions, but it is preferable that all 1 / 20 depth positions satisfy the above conditions. Similarly, [Hv sur / [Hv q also has an effect if it satisfies the above conditions at at least one position, but it is preferable that it satisfies the above conditions at all positions.

[0022] The average interval of the Mn enrichment part in the direction orthogonal to the rolling direction and the standard deviation of the Mn concentration in the retained austenite at the 1 / 20 depth position are obtained by the following method. Here, the 1 / 20 depth position is the position of 1 / 20 of the plate thickness in the plate thickness direction from the surface of the base material, but in measurement, it may be in the range from the depth of 1 / 40 of the plate thickness (1 / 40 thickness) from the surface to the depth of 3 / 40 of the plate thickness (3 / 40 thickness) from the surface. The rolling surface of the steel plate is ground and buffed to prepare an analysis sample that reveals the analysis surface at the 1 / 20 depth position, and the Mn distribution is investigated by EPMA. Since the average interval of the Mn enrichment part requires measurement in a relatively wide range, a line segment of 8 mm in the direction orthogonal to the rolling direction (plate width direction) is continuously measured at 1-μm intervals. The concentration at each measurement point is obtained by measuring at 1-μm points on a line segment of 500 μm in the rolling direction from each measurement point and taking the average value. The average value of the Mn concentration over all the measurement points of 8 mm in the plate width direction is defined as Mnave, the maximum as Mnmax. The region where the Mn concentration is equal to or higher than (Mnave + Mnmax) / 2 is defined as the Mn enrichment part, and the average of the intervals of the Mn enrichment parts is determined. Also, at each measurement point, the Si concentration is measured, and the Mn concentration in the region where Si is lower than the average Si concentration of the steel plate is regarded as the Mn concentration in the retained austenite, and the standard deviation thereof is determined.

[0023] [Hv sur and [Hv q are determined by the following method. At the 30-μm depth position and the 1 / 4 depth position of a sample in which the cross section is embedded such that the cross-sectional surface parallel to the rolling direction of the steel plate becomes the measurement surface, in accordance with JIS Z2244 (2009), the Vickers hardness is measured at three points with a load setting of 20 g, and from the respective average values, the Vickers hardness [Hv sur at the 30-μm depth position and the Vickers hardness [Hv] at the 1 / 4 depth position are determined. From these, [Hv sur / [Hv] can be calculated.

[0024] As described above, in the resistance spot welding joint according to the present embodiment, not only when the zinc-based plating layer is on the overlapping surface, but also when the zinc-based plating layer is on the outermost surface, based on the surface of the base material in contact with the zinc plating layer, at the 1 / 20 depth position, the average interval of the Mn enrichment parts in the direction orthogonal to the rolling direction is 300 μm or less, at the 1 / 20 depth position, the standard deviation of the Mn concentration in the retained austenite is 0.40 mass% or less, and the Vickers hardness [Hv sur at the 30-μm depth position and the Vickers hardness [Hv qIt is necessary to satisfy the above formula (1). This is because in resistance spot welding, a plurality of steel plates are energized between electrodes while being pressed using a pair of opposing electrodes. Therefore, even at the outermost surface rather than the overlapping surface, pressure is applied from the electrodes.

[0025] The thicknesses of the plurality of steel plates are not limited. However, considering their application to automotive parts, it is preferable that each thickness is in the range of 0.5 mm to 3.0 mm.

[0026] In the above description, parts other than the welded portions of the plurality of steel plates, which are particularly important in the resistance spot welding joint according to this embodiment, have been described. However, regarding the welded portions, the following is preferably the case.

[0027] [Corona bond] In the resistance spot welding joint according to this embodiment, in the corona bond of the cross-section in the plate thickness direction passing through the center of the nugget, the ratio of the η-phase in the zinc-based plating layer is preferably 20 area% or less. The η-phase in the zinc-based plating layer means a phase mainly composed of Zn and containing other elements such as Fe in a solid solution state. If the amount of the η-phase mainly composed of zinc in the zinc-based plating layer is 20 area% or less, Zn diffusing from the zinc-based plating layer of the corona bond and Fe diffusing from the steel plate are sufficiently alloyed by resistance spot welding. In this case, the melting start temperature of the plated alloy layer rises sufficiently, and it becomes possible to reduce the amount of Zn melting during energization. Tensile stress is generally generated during cooling, such as during electrode holding or after release. Also, under conditions where the heat input is sufficiently large, tensile stress may be generated in the welded portion during the latter stage of energization (during heating). However, as described above, if the ratio of the η-phase in the zinc-based plating layer is 20 area% or less, the amount of molten Zn required for the occurrence of LME cracks is reduced, so LME cracks can be suppressed. The area ratio of the η-phase is more preferably 18 area% or less, still more preferably 16 area% or less, and even more preferably 14 area% or less.

[0028] The ratio of the η-phase in the zinc-based plating layer of the corona bond is determined by the following method. Using SEM-EDS, capture the Zn and Fe element distribution images of the corona bond in the cross-section of the welded joint (the cross-section in the plate thickness direction passing through the center of the nugget). Define the η-phase in this image as the region where the Zn concentration is 95 mass% or more and the Fe concentration is 5 mass% or less. Binarize the part that satisfies this definition and the other part using image analysis software, and calculate the area ratio of the η-phase in the plating layer within the corona bond. The measurement region may be, for example, a rectangle with a width of 100 μm in the horizontal direction and a height (thickness direction) of 10 μm. However, it is necessary to increase the height direction according to the thickness of the zinc-based plating layer within the corona bond. When the thickness of the zinc-based plating layer exceeds 10 μm, the height of the measurement region is set to a value exceeding 10 μm. The area ratio of the η-phase is obtained by dividing the area of the η-phase within this measurement region by the area of the region where Zn exists within the same region. The area ratio of the η-phase is measured in a rectangular range with a width of 100 μm in the horizontal direction and a height of, for example, 10 μm inside, based on the end on the side away from the nugget center of the corona bond.

[0029] [Shoulder] In the resistance spot welding joint according to this embodiment, when the zinc-based plating layer is on the outermost surface, it is preferable that the amount of the η-phase in the zinc-based plating layer at the shoulder of the welded joint is 20 area% or less. When alloying progresses between the zinc-based plating layer and the steel sheet due to the heat input of welding, not only within the corona bond described above but also within the heat-affected zone on the electrode side (the side in contact with the electrode), the proportion of the η-phase mainly composed of Zn decreases from the initial zinc-based plating layer. Therefore, by performing a welding method with more heat input compared to a normal welding method, alloying between the zinc-based plating layer and the steel sheet can be further promoted, so that the amount of the η-phase of the zinc-based plating layer within the heat-affected zone on the electrode side can be reduced to 20 area% or less. In the present embodiment, the amount of the η-phase of the zinc-based plating layer at the shoulder of the welded part is defined as representative of the heat-affected zone on the electrode side. Similar to what was described above, the melting point of the zinc-based plating layer on the electrode side increases due to the progress of alloying of the zinc-based plating layer on the electrode side by the heat input during spot welding. For this reason, compared with the case where the proportion of the η-phase is relatively high, the intrusion of molten zinc into the steel sheet can be suppressed or reduced, and the suppression effect of LME cracks on the surface of the electrode side can be further enhanced. From the viewpoint of enhancing the suppression effect of LME cracks, it is preferable that the amount of the η-phase of the zinc-based plating layer at the shoulder is smaller, and the η-phase may be, for example, 18 area% or less or 15 area% or less.

[0030] The measurement of the area ratio of the η-phase of the zinc-based plating layer at the shoulder is performed as follows. Using SEM-EDS, a lateral rectangular range of 100 μm and a height range of 10 μm centered on the outer boundary of the shoulder in the cross-section of the welded part (the part where the shape suddenly changes from the part with the curvature reflecting the electrode shape under electrode pressure during welding to the flat part not in contact with the electrode) is used to capture the distribution images of Zn and Fe elements. However, the height direction needs to be increased according to the thickness of the zinc-based plating layer at the shoulder. When the total thickness of the zinc-based plating layer exceeds 10 μm, the height of the measurement region is set to a value exceeding 10 μm. The η-phase in the image is defined as the region where the Zn concentration is 95 mass% or more and the Fe concentration is 5 mass% or less. The part satisfying this definition and the other part are binarized by image analysis software, and the area ratio of the η-phase occupying the zinc-based plating layer at the shoulder of the welded part is calculated. Specifically, the area ratio of the η-phase is obtained by dividing the area of the η-phase within this measurement region by the area of the region where Zn exists within the same region.

[0031] [Heat Affected Zone] In the resistance spot welding joint according to the present embodiment, in the cross-section in the plate thickness direction passing through the center of the nugget, the diameter of the heat affected zone is 1.5 times or more the diameter of the nugget, and in the heat affected zone, carbides with an equivalent circle diameter of 0.1 μm or more are distributed at a number density of 40 pieces / 100 μm 2 It is preferably distributed at the above number density or more.

[0032] The diameter of the heat affected zone is determined by the following method. Prepare a sample so that the cross-section in the plate thickness direction passing through the center of the nugget can be observed. After polishing the cross-section, corrode the polished surface using an aqueous picric acid solution. Observe this corroded surface with an optical microscope, visually determine the nugget, corona bond, and heat affected zone, and measure their respective diameters (diameters in the direction perpendicular to the plate thickness direction).

[0033] The number density of carbides with an equivalent circle diameter of 0.1 μm or more in the heat affected zone is determined by the following method. Prepare a sample so that the cross-section in the plate thickness direction passing through the center of the nugget can be observed. After polishing the cross-section, corrode the polished surface using an aqueous picric acid solution. Using a scanning electron microscope (SEM), select 10 regions of 5 μm × 5 μm in the heat affected zone on this corroded surface and photograph them at a magnification of 20,000 times. From the photographed images, use an image processing device to obtain the area of each carbide, and calculate the equivalent circle diameter from that value. Then, identify carbides with an equivalent circle diameter of 0.1 μm or more, divide the total number of these by the total area of the photographed region, and calculate the distribution density of the carbides.

[0034] [Manufacturing Method of Resistance Spot Welding Joint] A manufacturing method capable of manufacturing the resistance spot welding joint according to the above-described present embodiment will be described. The manufacturing method of the resistance spot welding joint according to the present embodiment is (I) A stacking step of stacking a plurality of steel plates in the thickness direction, (II) A energization step of forming a nugget and a corona bond by energizing between the pair of opposing electrodes while pressing the plurality of steel plates after the overlapping step; (III) A current reduction step of reducing the current value between the electrodes to 0 while maintaining the pressing after the energization step; It has. Regarding each step, preferable conditions will be described. For steps and conditions not described, known conditions can be applied.

[0035] [Overlapping step] In the overlapping step, prior to resistance spot welding, a plurality of steel plates are overlapped so that at least a part thereof overlaps in the thickness direction. One or more of the plurality of steel plates are high-strength steel plates having a tensile strength of 980 MPa or more, and the high-strength steel plates, or the steel plates to be overlapped on the high-strength steel plates, are plated steel plates having a zinc-based plating layer formed on the surface of the base material. All the steel plates may be plated steel plates, or all the steel plates may be high-strength steel plates. Further, the high-strength steel plate has an average interval of Mn enrichment portions in the direction orthogonal to the rolling direction at a 1 / 20 depth position, which is a position of 1 / 20 of the plate thickness in the plate thickness direction from the surface of the base material, of 300 μm or less, and at the 1 / 20 depth position, the standard deviation of the Mn concentration in the retained austenite is 0.40 mass% or less, and the Vickers hardness [Hv sur at a 30 μm depth position, which is a position 30 μm in the plate thickness direction from the surface of the base material, and the Vickers hardness [Hv q at a 1 / 4 depth position, which is a position 1 / 4 of the plate thickness in the plate thickness direction from the surface of the base material, satisfy the following formula (1), and the steel plate is used. [Hv sur / [Hv q ≦ 0.80 (1) The above characteristics are maintained for portions other than the welded portion even when a resistance spot weld joint is formed. For steel plates that are not high-strength steel plates, or even if they are high-strength steel plates, are not plated steel plates, and are not steel plates that are overlapped in contact with plated steel plates, there are no limitations, and known steel plates can be used.

[0036] The above high-strength steel sheet further has a chemical composition in mass% as follows: C: 0.150% to 0.400%, Si: 0.01% to 2.50%, Mn: 1.50% to 3.50%, P: 0.050% or less, S: 0.0100% or less, Al: 0.001% to 1.500%, Si and Al: 0.50% to 3.00% in total, N: 0.0100% or less, O: 0.0100% or less, Ti: 0% to 0.200%, V: 0% to 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Co: 0% to 1.00%, Mo: 0% to 1.00%, W: 0% to 1.00%, B: 0% to 0.0100%, Sn: 0% to 1.00%, Sb: 0% to 1.00%, Ca: 0% to 0.0100%, Mg: 0% to 0.0100%, Ce: 0% to 0.0100%, Zr: 0% to 0.0100%, La: 0% to 0.0100%, Hf: 0% to 0.0100%, Bi: 0% to 0.0100%, and REM other than Ce and La: 0% to 0.0100%. The balance consists of Fe and impurities. The microstructure at a depth of 1 / 4 of the plate thickness from the surface contains, in volume fraction, ferrite: 0% to 50%, retained austenite: 6% to 30%, bainite: 5% to 60%, tempered martensite: 5% to 50%, fresh martensite: 0% to 10%, pearlite: 0% to 5%. At a depth of 1 / 4 of the plate thickness from the surface, the ratio of the number of retained austenite with an aspect ratio of 2.0 or more to all the retained austenite is 50% or more, and the number density of inclusions and precipitates with a particle size of 1 μm or more is 30 pieces / mm 2 It is preferably as follows. The steel sheet having such a chemical composition and microstructure is excellent in press formability and, even after baking and painting after being formed into press parts, is excellent in toughness, so it is suitable as automotive parts.

[0037] [Electric current application process] In the electric current application process, a plurality of steel sheets after the overlapping process are pressed between a pair of opposing electrodes while applying an electric current between the electrodes to form nuggets and corona bonds. The energization time and current value are not particularly limited, and values corresponding to the plate thickness, number of sheets, and material of the steel plates joined by resistance spot welding may be appropriately set within a normal range. The current value may be set to the maximum value immediately after the start of energization, or may be gradually increased to reach the maximum value. (So-called upslope energization.)

[0038] Also, the pressing force is not particularly limited, and values corresponding to the plate thickness, number of sheets, and material of the steel plates to be joined may be appropriately set within a normal range. The pressing force may be constant, or may be appropriately changed within a range where a good nugget can be formed. Various preferable conditions for forming a nugget can be applied. Also, due to the accuracy of the resistance spot welding apparatus, it is assumed that the pressing force may fluctuate unintentionally, but such fluctuations in the pressing force are also allowed within a range where a good nugget can be formed.

[0039] [Pre-energization step] Before the energization step, a pre-energization step of energizing with a current smaller than the energization in the energization step may be further provided. In this case, alloying of the plating layer is promoted before the main energization step, and a further effect of suppressing LME cracking can be obtained.

[0040] [Current reduction step] In the current reduction step, after the energization step, while maintaining the pressing, the current value between the electrodes is reduced to 0. In this step, after forming a nugget by energization as in normal resistance spot welding, the current value between the electrodes may be immediately reduced to 0. On the other hand, while maintaining the pressing, the time for reducing the current value between the electrodes to 0 may be downsloped so as to be 21 cycles (420 msec) or more (in this embodiment, 1 cycle is 20 msec). In this way, when the current value is gradually decreased (downslope control is performed), compared with the case where the current value is rapidly decreased, the tensile stress generated in the welded portion during cooling is reduced, and the alloying of the plating layer further progresses due to the heat input of the downslope, making it easier to stably keep the area ratio of the η phase at 20% or less.

[0041] Also, when reducing the current value, a time for making the current value constant may be provided. Specifically, define 1 / 2 of the total plate thickness in mm of a plurality of steel plates as tm. At the time when the formation of the nugget is completed, that is, generally, at the energization time that changes according to tm, for example, at the time of 1000×tm (msec) (if it is 1 mm, multiply by 1000 after deleting the unit to get 1000×1 (msec)), or at the time when the nugget formation is sufficient as revealed by a preliminary test. When the current value between the electrodes is I, in the current reduction process, the current value between the electrodes may be held at a constant value within the range from I×0.9 to I×0.3 at 265×tm or more and 420 msec or more per unit msec. When performing the control as described above, the cooling rate of the welded portion decreases. Also, in this case, the heat extraction by the electrodes becomes smaller, and the heat transfer from the welded portion to the surrounding steel plates is promoted. As a result, when the temperature of the welded portion decreases, the shrinkage of the welded portion becomes gentle. On the other hand, the restraining force of the welded portion by the steel plate 11 around the welded portion becomes smaller. By such a mechanism, the tensile stress introduced into the corona bond decreases in the process of reducing the current value between the electrodes. The period during which the current value between the electrodes should be controlled is defined as the period from when the current value between the electrodes becomes I×0.9 until it decreases to I×0.3 because it is estimated that the temperature of the corona bond when the current value between the electrodes is I×0.9 generally coincides with the boiling point of zinc, and the temperature of the corona bond when the current value between the electrodes is I×0.3 generally coincides with the melting point of zinc. That is, based on this estimation, the period from when it becomes I×0.9 until it decreases to I×0.3 is the period during which the molten zinc that causes LME cracking exists around the nugget. However, if the holding time is less than 420 seconds or less than 265×tm, sufficient effects may not be obtained.

[0042] [Pressure Holding Process] After the current reduction process, a pressure holding process may be further provided in which the current value between the electrodes is set to 0 and the pressure is held at 0.8×P or more for 200 msec or more and 400 msec or less. This makes it possible to more reliably avoid the introduction of tensile stress due to the release of pressure while liquid zinc remains, and to suppress corona bonds and LME cracks on the outside thereof. Therefore, it is possible to more reliably prevent LME cracks. After setting the current value between the electrodes A to 0, it is preferable that the length of the period during which the applied pressure is maintained at 0.8×P or more is 200 msec or more. From the viewpoint of suppressing LME cracks, it is considered that the longer the holding time, the better. Therefore, the time during which the applied pressure is maintained at 0.8×P or more may be 260 msec or more, or 280 msec or more. However, if the holding time is made too long, while the LME crack suppression effect saturates, the welding efficiency decreases. Furthermore, if the time during which the applied pressure is maintained at 0.8×P or more exceeds 400 msec, after the electrode is released, the auto tempering (self-annealing) of the nugget that has been heat-treated does not proceed during the process of decreasing the current value, and there is a possibility that the joint strength and hydrogen embrittlement resistance characteristics may decrease. Therefore, in the pressure holding step, the time during which the applied pressure is maintained at 0.8×P or more may be 400 msec or less, or 300 msec or less. In addition to the above, a post-current application step may be separately performed.

[0043] <Method for manufacturing high-strength steel sheet as a material> Among the steel sheets used for resistance spot welding, the manufacturing method of the above-described high-strength steel sheet is not limited, but for example, it can be manufactured by the following method. A continuous casting step of casting molten steel having a predetermined chemical composition into a slab having a thickness of 200 to 300 mm; A hot rolling step of performing hot rolling on the slab at a finishing temperature of 850°C or higher to obtain a hot-rolled steel sheet; A winding step of winding the hot-rolled steel sheet in a temperature range of 25 to 450°C; If necessary, a cold rolling step of performing cold rolling on the hot-rolled steel sheet at a reduction ratio of 30% or less to obtain a cold-rolled steel sheet; An annealing step of annealing the hot-rolled steel sheet or the cold-rolled steel sheet; A soaking heat treatment step of holding the hot-rolled steel sheet or the cold-rolled steel sheet after the annealing step in a temperature range of 260 to 450°C for 10 to 1000 seconds; Method for manufacturing a steel sheet containing

[0044] <Continuous casting process> Details of preferred conditions for the continuous casting process will be described. Molten steel having a predetermined chemical composition (since the average chemical composition does not substantially change during the intermediate process, it is adjusted according to the chemical composition of the desired steel sheet) is melted by a known melting method such as a converter or an electric furnace, and the solidification rate at a depth of 10 mm from the slab surface is set to 100 to 1000 °C / min, and the amount of molten steel poured per unit time is set to 2.0 to 6.0 tons / min, and the average cooling rate between the liquidus temperature and the solidus temperature of the surface layer portion, which is a depth of 5 mm from the surface of the molten steel, is cooled at 4 °C / sec or more, and continuously cast into a slab having a thickness of 200 mm or more and 300 mm or less.

[0045] [Solidification rate: 100 °C / min or more and 1000 °C / min or less] In the continuous casting process, the solidification rate at a depth of 10 mm from the slab surface is preferably 100 °C / min or more and 1000 °C / min or less. If this solidification rate is less than 100 °C / min, it becomes difficult to make the dendrite primary arm spacing at a depth of (1 / 20) of the slab thickness from the slab surface 300 μm or less, and there may be cases where the standard deviation of the Mn concentration cannot be made 0.40% or less. On the other hand, if the solidification rate exceeds 1000 °C / min, surface cracking of the slab may be induced.

[0046] [Amount of molten steel poured per unit time: 2.0 to 6.0 tons / min] If the amount of molten steel poured per unit time is less than 2.0 tons / min, the amount of heat supplied to the mold decreases, and the claw length at the upper part of the solidified shell becomes longer, so that it becomes easier to capture mold powder on the slab surface layer, and the number density of inclusions and precipitates having a particle size of 1 μm or more present on the steel sheet surface is 30 pieces / mm 2It may exceed. Therefore, the amount of molten steel poured per unit time is preferably 2.0 tons / minute or more. Mold powder is generally composed of a component system of Al2O3, SiO2, and CaO. On the other hand, when the amount of molten steel poured per unit time exceeds 6.0 tons / minute, the flow of the molten steel becomes too large, and it is likely to cause the capture of alumina-based inclusions on the slab surface due to the entrainment of the mold powder, and the number density of inclusions and precipitates with a particle size of 1 μm or more existing on the steel plate surface is 30 pieces / mm 2 It may exceed. Therefore, it is preferable that the amount of molten steel poured per unit time is 6.0 tons / minute or less.

[0047] [The average cooling rate between the liquidus temperature and the solidus temperature in the surface layer part, which is the depth position 5 mm from the surface of the molten steel, is 4°C / second or more] Since the range between the liquidus temperature and the solidus temperature is an intermediate process of solidification, Ti, Al, N, etc. segregate in the molten steel, and the precipitation and growth of TiN and AlN start at the solidification interface. Therefore, when the average cooling rate between the liquidus temperature and the solidus temperature in the surface layer part at a depth of 5 mm from the surface of the slab is less than 4°C / second, due to the low cooling rate, the segregation of the molten steel progresses, promoting the formation of precipitation nuclei and also promoting the grain growth of the precipitates after precipitation, and the number density of inclusions and precipitates with a particle size of 1 μm or more existing on the steel plate surface is 30 pieces / mm 2 It may exceed. Therefore, it is preferable that the average cooling rate between the liquidus temperature and the solidus temperature in the surface layer part at a depth of 5 mm from the surface of the slab is 4°C / second or more. From the viewpoint of suppressing the formation of precipitates on the steel plate surface, the larger the above average cooling rate is, the better. Therefore, there is no particular need to specify the upper limit of the above average cooling rate. However, if the cooling rate is too large, the slab may crack. Therefore, the average cooling rate is preferably 100°C / second or less.

[0048] [Slab thickness: 200 mm or more and 300 mm or less] The slab thickness is preferably 200 mm or more and 300 mm or less. If the slab thickness is less than 200 mm, it becomes difficult to obtain a desired structure. On the other hand, if the slab thickness exceeds 300 mm, it becomes difficult to make the average interval in the direction orthogonal to the rolling direction of the Mn enrichment part extending in the rolling direction 300 μm or less at a depth position of (1 / 20) of the plate thickness from the steel plate surface.

[0049] <Hot rolling process> [Average heating rate between Ac1 and Ac1 + 30°C: 2 to 50°C / min] In this method, the average heating rate of the slab between Ac1 and Ac1 + 30°C during slab heating prior to hot rolling is controlled to be 2 to 50°C / min. In the two-phase (austenite and ferrite) temperature range just above Ac1, the distribution of alloy elements particularly easily proceeds between austenite and ferrite. Therefore, when reheating the slab, it is heated at a relatively fast average rate of 2°C / min or more in the above temperature range. When the average heating rate is less than 2°C / min, Mn is distributed between austenite and ferrite during heating, making it difficult to make the standard deviation of the Mn concentration 0.40% or less. For example, the above average heating rate may be 4°C / min or more. On the other hand, when rapid heating is carried out such that the average heating rate exceeds 50°C / min, the temperature distribution in the thickness direction of the slab becomes non-uniform and thermal stress is generated. In this case, problems such as thermal deformation of the slab may occur. For example, the above average heating rate may be 40°C / min or less, 30°C / min or less, 20°C / min or less, or 10°C / min or less. The Ac1 point is calculated by the following formula. Substitute the mass% of the element into the element symbol in the following formula. For elements not contained, substitute 0 (mass%). Ac1 (°C) = 723 - 10.7×Mn - 16.9×Ni + 29.1×Si + 16.9×Cr

[0050] [The slab is heated at 1200°C or more for 20 minutes or more] To mitigate Mn segregation, holding at a high temperature for a long time is effective. Since Mn is a substitutional element, its diffusion rate is extremely slow, and diffusion only proceeds when heated to a high temperature of 1200 °C or higher. By heating to 1200 °C or higher and holding in that temperature range for 20 minutes or more, the standard deviation of the Mn concentration is reduced, and it becomes possible to achieve 0.40% or less.

[0051] [Rough rolling] In this method, for example, the heated slab is subjected to rough rolling before finish rolling for the purpose of adjusting the plate thickness and the like. Such rough rolling is not particularly limited, but it is preferably carried out so that the total reduction ratio at 1050 °C or higher is 60% or more. If the total reduction ratio is less than 60%, recrystallization during hot rolling becomes insufficient, which may lead to non-uniformity of the structure of the hot-rolled steel sheet. The above total reduction ratio may be, for example, 90% or less.

[0052] [Finish rolling by multiple rolling stands] The conditions for finish rolling are not particularly limited, but in a facility configuration consisting of multiple rolling mills commonly used by those skilled in the art, it is preferable to set the final pass exit side temperature (finish temperature) to 850 °C or higher. When the finish temperature is less than 850 °C, the phase transformation from worked austenite is accelerated, and high-temperature transformation product phases such as ferrite are generated during cooling, and the entire surface cannot be made into a lath structure. Thereafter, it is preferable to cool so that the average cooling rate up to 600 °C is 10 °C / second or more.

[0053] [Coiling process] In the coiling process, it is preferable to coil the hot-rolled steel sheet in the temperature range of 25 to 450 °C. The coiling temperature is set to 450 °C or less in order to make the microstructure of the hot-rolled steel sheet mainly composed of bainite or martensite. When coiling at a temperature higher than 450 °C, a microstructure mainly composed of bainite cannot be obtained. Also, setting the coiling temperature to less than 25 °C requires special equipment. Therefore, the coiling temperature is set to 25 °C or higher.

[0054] [Cold rolling process] For the hot-rolled steel sheet after the coiling process, cold rolling may be performed to improve the sheet thickness accuracy and flatness. However, if the reduction ratio is too high, excessive strain is introduced, promoting the recrystallization of ferrite during annealing heating and making it impossible to maintain the acicular structure derived from bainite. Therefore, the reduction ratio in cold rolling is preferably 30% or less. Also, as the cold rolling ratio increases, the workability of the product deteriorates. Thus, the reduction ratio is more preferably 10% or less, and even more preferably 5% or less. Since cold rolling may not be performed, the lower limit of the reduction ratio is 0%.

[0055] <Annealing process> As described above, in the coiling process, the microstructure is mainly bainite or martensite. Therefore, after the coiling process, heat treatment in the annealing process is performed on the hot-rolled steel sheet without cold rolling or the cold-rolled steel sheet with a cold rolling reduction ratio of 30% or less.

[0056] [Held at a maximum heating temperature of Ac1 + 20°C or more and less than Ac3 (°C) for 1 second to 1000 seconds and then cooled to 250°C or less] Due to the phase transformation in the coiling process, the microstructure mainly consists of bainite and martensite. Therefore, by holding at a maximum heating temperature of Ac1 + 20°C or more and less than Ac3 (°C) for 1 second to 1000 seconds in the annealing process, the desired austenite volume fraction can be achieved. If the maximum heating temperature is less than Ac1 + 20°C, the volume fraction of austenite that undergoes reverse transformation during heating is insufficient, and the volume fractions of bainite, tempered martensite, and retained austenite generated in the subsequent cooling and soaking heat treatment processes cannot meet the desired range. On the other hand, when heating to a temperature of Ac3 (°C) or higher, the lath-like structure formed in the annealing process is completely lost, making it impossible to control the aspect ratio of the retained austenite to 2.0 or more. The holding time in this temperature range requires at least 1 second, and the upper limit is 1000 seconds, which is industrially realistic.

[0057] The steel plate heated to the maximum heating temperature needs to undergo bainite transformation or martensite transformation in order to obtain the retained austenite volume fraction within the scope of the present disclosure. Therefore, the steel plate heated to the maximum heating temperature needs to be cooled to 250°C or lower. If the cooling stop temperature exceeds 250°C, during the holding in the subsequent soaking heat treatment process, the bainite transformation will not proceed sufficiently, and the tempering of the martensite obtained by cooling to 250°C or lower will not be possible, and the desired volume fraction in the microstructure cannot be obtained.

[0058] [When holding at the maximum heating temperature for 1 second to 1000 seconds, the atmosphere in the heating furnace satisfies -1.1 ≦ log(PH2O / PH2) ≦ -0.07] When decarburizing the surface layer of the steel plate, the oxygen potential log(PH2O / PH2) in the annealing process is increased. If the oxygen potential is less than -1.1, the oxygen potential is insufficient and decarburization does not proceed, and the Vickers hardness [Hv sur at a depth of 30 μm from the surface cannot be made sufficiently small (the surface layer cannot be softened). On the other hand, when the oxygen potential is higher than -0.07, oxidation of iron itself starts and it cannot be made into a product, so this is set as the upper limit. log(PH2O / PH2) may be within the above range even before heating up to the maximum heating temperature.

[0059] <Soaking heat treatment process> In the soaking heat treatment step, in order to obtain a predetermined volume fraction of bainite and retained austenite in the hot-rolled steel sheet or the cold-rolled steel sheet after the annealing step, it is preferable to heat the steel sheet cooled to 250°C or lower in the second annealing step and hold it in the temperature range of 260 to 450°C. By holding in this temperature range, the carbon atoms discharged by the bainite transformation are concentrated in the untransformed austenite, improving the stability of the untransformed austenite, so that the desired amount of retained austenite can be ensured. If the holding temperature in the soaking heat treatment step is lower than 260°C, the progress of the bainite transformation becomes slow, and finally not only the volume fraction of bainite but also the volume fraction of retained austenite becomes insufficient. On the other hand, when held at a temperature higher than 450°C, not only does the strength of bainite decrease, but carbides are formed in the austenite, and the C concentration in the austenite decreases, so that finally the volume fraction of retained austenite is insufficient when cooled. The holding time in the same temperature range needs to be at least 10 seconds. If it is shorter than this, the bainite transformation becomes insufficient. Also, holding for more than 1000 seconds is industrially difficult, so this is set as the upper limit. The cooling after holding in the soaking heat treatment step is not limited.

[0060] [Plating process] When using a high-strength steel sheet as the plating layer, a plating process of immersing the steel sheet in a plating bath may be further provided. The plating process is preferably a hot-dip galvanizing process of immersing the steel sheet in a hot-dip galvanizing bath. When forming a hot-dip galvanized layer, the immersion in the hot-dip galvanizing bath may be carried out during the cooling from the maximum heating temperature in the annealing process before the homogenization heat treatment process to 250°C or lower, or after the homogenization heat treatment process. Although the influence of the steel sheet temperature at this time on the steel sheet performance is small, if the difference between the steel sheet temperature and the plating bath temperature is too large, the plating bath temperature may change and cause problems in operation. Therefore, it is desirable to provide a process of reheating or cooling the steel sheet within the range of plating bath temperature - 20°C to plating bath temperature + 20°C. The hot-dip galvanizing may be carried out according to a conventional method. For example, the plating bath temperature may be 440 - 470°C and the immersion time may be 5 seconds or less. The plating bath preferably contains 0.08 - 0.2 mass% of Al and the balance is Zn, but may also contain Fe, Si, Mg, Mn, Cr, Ti, Pb as impurities. Also, it is preferable to control the coating weight by a known method such as gas wiping. The coating weight is preferably 25 - 75 g / m per side. 2 is preferable.

Example

[0061] <Example 1> Steel sheets (Steel sheet Nos. a - c and A) having the tensile strength described in Table 1 and with an alloyed hot-dip galvanized layer formed on the surface (both sides) were prepared. The coating weight of the plating layer was 45 g / m. 2 was used.

[0062] Two sheets of the same type of these steel plates were overlapped, and while applying pressure to the overlapping portion of the steel plates using a pair of opposing electrodes, an electric current was passed between the electrodes to form a nugget and a corona bond. After the nugget was formed, the current value was decreased to 0 by various methods, and while the current value between the electrodes was 0, the pressure was maintained at 0.8×P or more for the time shown in Table 2. When forming the nugget, the two steel plates were each arranged substantially horizontally, and the pair of electrodes were arranged so as to sandwich the steel plates. The electrode arranged above the steel plate was used as the movable electrode, and the electrode arranged below the steel plate was used as the fixed electrode, and the steel plates were pressed by moving the upper electrode toward the lower electrode. At the start of energization (this energization), the current value was instantaneously increased to a predetermined value, and then the current value was maintained constant until the nugget was completed. The conditions for this energization were as shown in Table 2. Also, for Tests No. 1 to 4 and 6, preliminary energization was performed under the conditions shown in Table 2 before this energization. For Test No. 7, after the completion of this energization, while decreasing the current value to 0 while applying pressure, when the current value reached 7.2 kA, it was maintained for 200 msec so that the current value would be constant. For Test No. 8, after the completion of this energization, while applying pressure, a time of not energizing for 80 msec was provided once, then the current value was maintained at 7.2 kA for 200 msec, and then the current value was decreased to 0. In Test No. 9, after the completion of this energization, while applying pressure, the time for decreasing the current value between the electrodes to 0 was downsloped so as to be 30 cycles (600 msec). The other welding conditions were as shown below. · Welding machine: Servo pressure fixed type welding machine, single-phase alternating current (frequency 50 kHz) · Electrode: Dome radius (DR) Cr-Cu · Shape of the electrode tip: φ6 mm R40 mm · Pressure P during energization and holding: 4 kN · Current value I at the time when the formation of the nugget was completed: 8 kA (Conditions for forming a nugget of 5.5 to 6.5 mm) · Energization time during this energization (when forming the nugget): 18 cycles (360 ms) · Holding time: 5 cycles (100 ms) or 15 cycles (300 ms) · Welding angle: 5° · Clearance: 0.3 mm Here, the welding angle is the angle formed between the axial direction of the movable electrode and the direction perpendicular to the surface of the steel plate. The clearance is the distance between the steel plate surface and the electrode, and here it is defined as the distance at the location where the tip of the lower electrode is closest to the steel plate surface before welding. The welding angle and clearance are disturbance factors in resistance spot welding and are factors that cause LME cracking. By setting the welding angle and clearance to the above conditions, it is made easier for LME cracking to occur. The pressing force was held during the above period after the nugget was formed. Under the above conditions, resistance spot welding was carried out 20 times for each condition (forming nuggets at 20 locations) to create resistance spot weld joints.

[0063] For the high-strength steel plate of the obtained resistance spot weld joint, the average interval of the Mn enrichment part in the direction orthogonal to the rolling direction at the 1 / 20 depth position, the standard deviation of the Mn concentration in the retained austenite at the 1 / 20 depth position, and the Vickers hardness [Hv sur and the Vickers hardness [Hv q at the 1 / 4 depth position are equivalent to those of the supplied steel plate, so the measurement was omitted.

[0064] <LME resistance> For each of the 20 locations of the obtained resistance spot weld joint, a cut was made along a plane passing through the center of the nugget and perpendicular to the steel plate surface, and a cross-section (cross-section in the plate thickness direction) was appropriately prepared, and the presence or absence of cracks on both the overlapping surface side and the surface (the outermost surface) in contact with the electrode was confirmed using an optical microscope. If there is even one crack, that condition was judged to have cracks. The 20 resistance spot welds were observed, and if the total number of crack conditions was 4 or less, it was judged to have excellent LME resistance.

[0065] <Ratio of η phase (area ratio)> Using the above method, the ratio of the η-phase in the zinc-based plating layer (plated alloy layer) in the corona bond and the ratio of the η-phase in the zinc-based plating layer (plated alloy layer) at the shoulder of the welded part on the outermost surface of the plurality of stacked steel plates were measured.

[0066] As can be seen from Tables 1 and 2, the average interval of the Mn enrichment part in the direction orthogonal to the rolling direction at the 1 / 20 depth position, the standard deviation of the Mn concentration in the retained austenite at the 1 / 20 depth position, and the Vickers hardness [Hv sur at the 30 μm depth position and the Vickers hardness [Hv q at the 1 / 4 depth position, in the examples (No. 5 to 10) within the scope of the present invention, the LME resistance was excellent. In addition, when the ratio of the η-phase in the zinc-based plating layer at the corona bond and the shoulder of the welded part was 20 area% or less, the LME resistance was even more excellent.

[0067]

Table 1

[0068]

Table 2

[0069] <Example 2> A steel plate (Steel plate No. a-2) with a tensile strength of less than 980 MPa described in Table 3 and an alloyed hot-dip zinc plating layer formed on the surface (both sides) and a high-strength steel plate (Steel plate No. A-2) with a tensile strength of 980 MPa or more and no plating layer on the surface were prepared. These steel plates were stacked in combinations as shown in Table 4, either two or three at a time. While applying pressure to the overlapping portions of the steel plates using a pair of opposing electrodes, an electric current was passed between the electrodes to form nuggets and corona bonds. After forming the nuggets, the current value was decreased to zero in various ways, and the pressure was maintained at 0.8×P or more for 100 msec with the current value between the electrodes being zero. When forming the nuggets, the two steel plates were each arranged substantially horizontally, and a pair of electrodes was arranged so as to sandwich these steel plates. The electrode arranged above the steel plate was used as the movable electrode, and the electrode arranged below the steel plate was used as the fixed electrode. The steel plates were pressed by moving the upper electrode toward the lower electrode. At the start of energization (this energization), the current value was instantaneously increased to a predetermined value, and thereafter the current value was maintained constant until the nuggets were completed. The conditions for this energization were as shown in Table 4. Also, for Test No. 12, after completion of this energization, while decreasing the current value to zero under pressure, when the current value reached 7.2 kA, it was held constant for 200 seconds. Other welding conditions were the same as in Example 1.

[0070] <Ratio (area ratio) of η-phase> Using the method described above, the ratio of the η-phase in the zinc-based plating layer in the corona bond was measured.

[0071] <LME resistance> For each of 20 locations of the obtained resistance spot weld joints, a plane perpendicular to the steel plate surface passing through the center of the nugget was cut, and a cross-section (cross-section in the plate thickness direction) was appropriately prepared. The presence or absence of cracks on both the steel plate overlapping surface side and the surface in contact with the electrodes was confirmed using an optical microscope. If there was even one crack, the condition was determined to have cracks. By observing 20 resistance spot weld portions, if the total number of cracks was 4 or less, it was judged to have excellent LME resistance.

[0072]

Table 3

[0073]

Table 4

[0074] As can be seen from Table 3 and Table 4, based on the surface of the base material in contact with the zinc plating layer, the average interval of the Mn enrichment part in the direction orthogonal to the rolling direction at the 1 / 20 depth position, the standard deviation of the Mn concentration in the retained austenite at the 1 / 20 depth position, and the Vickers hardness [Hv sur at the 30 μm depth position and the Vickers hardness [Hv q at the 1 / 4 depth position, if the ratio is within the range of the present invention, excellent liquid metal embrittlement resistance can be obtained even in the case of combination with other steel plates.

Explanation of Signs

[0075] 1 Resistance spot welding joint 11 Steel plate (base material) 11’ High-strength steel plate (base material) 12 Zinc-based plating layer 13 Nugget 14 Corona bond 15 Overlap surface (superposed surface) 16 Heat-affected zone (HAZ) 17 Weld zone A Electrode

Claims

1. A plurality of steel plates overlapped with each other, a nugget joining the plurality of steel plates, and a welded portion having a corona bond and a heat-affected zone formed around the nugget, comprising: one or more of the plurality of steel plates being high-strength steel plates with a tensile strength of 980 MPa or more, the high-strength steel plate, or a steel plate adjacent to the high-strength steel plate, being a plated steel plate having a base material and a zinc-based plating layer formed on the surface of the base material, at a position 1 / 20 of the plate thickness in the plate thickness direction from the surface of the base material in contact with the zinc-based plating layer of the high-strength steel plate, in a direction orthogonal to the rolling direction, the average interval of the Mn enrichment portions is 300 μm or less, and the standard deviation of the Mn concentration in the retained austenite is 0.40% or less, The Vickers hardness [Hv ｓｕｒ at a depth of 30 μm, which is the position 30 μm in the thickness direction from the surface of the base material, and the Vickers hardness [Hv ｑ at a depth of 1 / 4 of the thickness, which is the position 1 / 4 of the thickness in the thickness direction from the surface of the base material, satisfy the following formula (1): characterized in that it is a resistance spot welding joint. [Hv ｓｕｒ / [Hv ｑ ≤ 0.80 (1)

2. the surface of the base material of the high-strength steel plate in contact with the zinc-based plating layer is located on the overlapping surface of the plurality of overlapped steel plates, characterized in that it is the resistance spot welding joint according to Claim 1.

3. in the corona bond in the cross-section in the plate thickness direction passing through the center of the nugget, the ratio of the η phase in the zinc-based plating layer is 20 area% or less, characterized in that it is the resistance spot welding joint according to Claim 2.

4. In the cross-section in the plate thickness direction passing through the center of the projectile, the diameter of the heat-affected portion is 1.5 times or more the diameter of the projectile, and in the heat-affected portion, carbides having an equivalent circle diameter of 0.1 μm or more are distributed at a number density of 40 pieces / 100 μm ２ or more, characterized in that it is the resistance spot welding joint according to Claim 3.

5. the high-strength steel plate is the plated steel plate, and the plated steel plate is arranged such that the zinc-based plating layer becomes the outermost surface, characterized in that it is the resistance spot welding joint according to any one of Claims 1 to 4.

6. the ratio of the η phase of the zinc-based plating layer at the shoulder of the welded portion on the outermost surface is 20 area% or less, characterized in that it is the resistance spot welding joint according to Claim 5.

7. The Vickers hardness [Hv ｓｕｒ at the 30-μm depth position and the Vickers hardness [Hv ｑ at the 1 / 4 depth position satisfy the following formula (2): characterized in that it is the resistance spot welding joint according to any one of Claims 1 to 6. 0.60 ≤ [Hv ｓｕｒ / [Hv ｑ ≤ 0.80 (2)

8. a stacking step of stacking a plurality of steel plates in the thickness direction, a energization step of forming a nugget and a corona bond by energizing between a pair of opposing electrodes while pressing the plurality of steel plates after the stacking step, a current reduction step of reducing the current value between the electrodes to 0 while maintaining the pressure after the energization step, having: one or more of the plurality of steel plates being high-strength steel plates with a tensile strength of 980 MPa or more, The high-strength steel sheet, or the steel sheet superposed on the high-strength steel sheet, is a plated steel sheet having a base material and a zinc-based plating layer formed on the surface of the base material, The high-strength steel sheet is, at a position of 1 / 20 of the plate thickness in the plate thickness direction from the surface of the base material, that is, at a 1 / 20 depth position, the average interval of the Mn enrichment portions in the direction orthogonal to the rolling direction is 300 μm or less, at the 1 / 20 depth position, the standard deviation of the Mn concentration in the retained austenite is 0.40% or less, The Vickers hardness [Hv ｓｕｒ at a position 30 μm deep in the thickness direction from the surface of the base material, i.e., at a 30-μm depth position, and the Vickers hardness [Hv ｑ at a position 1 / 4 of the thickness in the thickness direction from the surface of the base material, i.e., at a 1 / 4-depth position, satisfy the following formula (1): A method for manufacturing a resistance spot welding joint, characterized by this. [Hv ｓｕｒ / [Hv ｑ ≤ 0.80 (1)

9. In the current reduction step, while maintaining the pressurization, the current value between the electrodes is downsloped so that the time to decrease to 0 is 420 msec or more, A method for manufacturing a resistance spot welding joint according to claim 8, characterized by this.

10. When 1 / 2 of the total plate thickness of the plurality of steel sheets in units of mm is defined as tm, and the current value between the electrodes at the time when the formation of the nugget is completed is defined as I, In the current reduction step, the current value between the electrodes is maintained at a constant value within the range from I×0.9 to I×0.3, at 265×tm or more and 420 msec or more per unit msec, A method for manufacturing a resistance spot welding joint according to claim 8 or 9, characterized by this.

11. Before the energization step, further includes a preliminary energization step of energizing with a current smaller than the energization in the energization step, A method for manufacturing a resistance spot welding joint according to any one of claims 8 to 10, characterized by this.

12. After the current reduction step, further, When the pressing force at the time when the formation of the nugget is completed is defined as P, in a state where the current value between the electrodes is 0, a pressure holding step of holding the pressing force at 0.8×P or more for 200 msec or more and 400 msec or less is provided, A method for manufacturing a resistance spot welding joint according to any one of claims 8 to 11, characterized by this.

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