Welded member and method for manufacturing same

Abstract

The purpose of the present invention is to provide a welded member and a method for manufacturing the same. The present invention is a welded member obtained by resistance spot welding a plate group in which two or more steel plates are stacked, the total plate thickness (tall) of the plate group and the minimum thickness (tweld) of the resistance spot-welded portion satisfying formula (1), at least one of the two or more steel plates is a Si-containing galvanized steel plate containing 0.5-3.0 mass% of Si, and a specific region (A) in the Si-containing galvanized steel plate is formed below a zinc plating layer. The thickness (da) of the region (A) in the plate thickness direction satisfies formula (2). 0.5 < tweld / tall < 1.0... (1), 2 < da < 10... (2)

Description

Technical Field

[0001] The present invention relates to a welded member formed by resistance spot welding a steel plate, and in particular to a welded member having a resistance spot weld portion suitable as a structural member of an automobile or the like, and a method for manufacturing the welded member. Background Art

[0002] In recent years, due to the seriousness of environmental problems, CO2 emission restrictions have become stricter, and the lightweighting of the car body to improve fuel efficiency has become a problem in the automotive field. Therefore, the thinning brought about by the application of high-strength steel sheets to automobile parts is promoted, and the application of steel sheets with a tensile strength (TS) of 980 MPa or more is promoted. In addition, from the perspective of corrosion resistance, zinc-plated steel sheets with rust-proof properties are used in parts exposed to rainwater.

[0003] In addition, in the assembly of automobiles, from the perspective of cost and manufacturing efficiency, resistance spot welding is often used to combine press-formed automobile parts. Generally, a resistance spot welding method, which is a kind of overlapping resistance welding method, is used to join overlapping steel plates. This welding method is a method in which two or more overlapping steel plates are clamped from above and below by a pair of welding electrodes, pressurized by the welding electrodes, and a high current welding current is passed between the upper and lower electrodes for a short time to join. Figure 1 An example of resistance spot welding of two overlapping steel plates 1a and 1b by clamping them with welding electrodes 4 and 5 is shown. According to this method, a point-shaped weld 6 is obtained by utilizing resistance heat generated by a high-current welding current. This point-shaped weld 6 is called a nugget, and is a portion where the two steel plates 1a and 1b melt and solidify at the contact point of the steel plates when a current is passed through the overlapping steel plates, thereby the steel plates are joined to each other in a point shape. In order to ensure collision safety, it is required to increase the strength of the steel plates and increase the strength of the weld.

[0004] In the resistance spot welding of a plate group formed by overlapping a plurality of steel plates including a surface-treated steel plate, Figure 7 As shown, there is a problem that cracks sometimes occur in the welded portion. Here, the surface treated steel sheet refers to a steel sheet having a metal coating layer such as an electroplated zinc layer, a hot-dip galvanized layer (including an alloyed hot-dip galvanized layer), and a zinc alloy layer containing elements such as aluminum and magnesium in addition to zinc on the surface of the base material (base steel sheet). The melting point of the zinc coating layer and the zinc alloy coating layer is lower than the melting point of the base material of the surface treated steel sheet, so there is the following problem.

[0005] That is, it is believed that the cracks in the weld are cracks caused by the so-called liquid metal embrittlement (hereinafter referred to as "LME cracks"), which is caused by the melting of the low-melting-point metal coating on the surface of the steel plate during welding. When the electrode pressure and the tensile stress caused by the thermal expansion and contraction of the steel plate are applied to the weld, the molten low-melting-point metal intrudes into the grain boundaries of the base material of the surface-treated steel plate, thereby reducing the grain boundary strength and causing cracks. There are many locations where LME cracks occur, such as Figure 7 The surfaces of the steel plates 1a and 1b on the side in contact with the welding electrodes 4 and 5 and the surfaces of the steel plates 1a and 1b on the side in contact with each other are shown.

[0006] As a countermeasure for such LME cracks, for example, there are the technologies of Patent Documents 1 to 3. Patent Document 1 proposes a composition of a steel plate as a plate group within a specific range, specifically, a composition consisting of, in weight %, C: 0.003 to 0.01%, Mn: 0.05 to 0.5%, P: 0.02% or less, sol.Al: 0.1% or less, Ti: 48×(N / 14) to 48×{(N / 14)+(S / 32)}%, Nb: 93×(C / 12) to 0.1%, B: 0.0005 to 0.003%, N: 0.01% or less, Ni: 0.05% or less, and the remainder: Fe and inevitable impurities.

[0007] Patent Document 2 proposes a method for resistance spot welding high-strength plated steel sheets by setting the welding current application time and the holding time after welding current application so as to satisfy the following conditional expressions (A) and (B).

[0008] 0.25×(10×t+2) / 50≤WT≤0.50×(10×t+2) / 50…(A)

[0009] 300-500×t+250×t 2 ≤HT…(B)

[0010] In conditional expressions (A) and (B), t is the plate thickness (mm), WT is the welding power-on time (ms), and HT is the holding time after welding power-on (ms).

[0011] Patent Document 2 also proposes to appropriately set the energization time and the electrode holding time after energization according to the thickness of the steel sheet, and to perform welding using a high-tensile galvanized steel sheet having an alloy element content of a certain value or less.

[0012] Patent Document 3 proposes the following method: the power-on mode is set to a multi-stage power-on of 3 or more stages, welding conditions such as the power-on time and the welding current are adjusted so that the appropriate current range (ΔI) is 1.0 kA or more, preferably 2.0 kA or more, and cooling time is provided between each stage. The above-mentioned appropriate current range is a current range in which a nugget having a desired nugget diameter or more and a molten residual thickness of 0.05 mm or more can be stably formed.

[0013] Prior art literature

[0014] Patent Literature

[0015] Patent Document 1: Japanese Patent Application Laid-Open No. 10-195597

[0016] Patent Document 2: Japanese Patent Application Publication No. 2003-103377

[0017] Patent Document 3: Japanese Patent Application Laid-Open No. 2003-236676 Summary of the invention

[0018] However, Patent Document 1 has a problem that the use of steel sheets that meet the required performance is limited because the amount of alloying elements in the steel sheet must be limited. In particular, in the case of recent high-alloying accompanied by high strength in steel sheets, the application of the technology of Patent Document 1 is greatly limited.

[0019] Patent Document 2 only proposes a method for suppressing cracks when an excessively large welding current is set such that spatter is generated, and does not mention cracks in a state where spatter is not generated.

[0020] Patent Document 3 has the following problems: optimization of welding conditions requires a lot of man-hours, and cannot be applied to steel plates and plate groups for which it is difficult to ensure an appropriate current range. In addition, Patent Documents 2 and 3 do not study the influence of the electrode inclination angle, so considering the actual construction during automobile assembly, there is an insufficient countermeasure.

[0021] Taking the viewpoint of joint strength into consideration and summarizing the common problems of Patent Documents 1 to 3, no technique is proposed for suppressing the occurrence of LME cracks while ensuring the strength of the resistance spot welded portion of the high-strength steel plate.

[0022] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a welded member having a resistance spot weld that can achieve both ensuring the strength of the weld in a high-strength steel plate and suppressing LME cracks, and a method for manufacturing the same.

[0023] The present inventors have conducted intensive studies to achieve the above-mentioned object.

[0024] Due to the complex influence of various factors, the effect of the present invention on cracks generated during welding (i.e., LME cracks) cannot be simply explained, but LME cracks in the resistance spot weld are easily generated when excessive tensile residual stress is generated in the resistance spot weld due to construction interference during welding, etc. In particular, it is known that on the overlapping surface side of the steel plates where the steel plates are in contact with each other, when a pair of welding electrodes are opened after the power supply and pressurization of the resistance spot welding are completed, LME cracks are easily generated in the area with large local tensile stress.

[0025] In addition, as for the influence of the components in the steel sheet, it is known that when the amount of Si increases, the LME crack resistance of the steel sheet deteriorates as described in Reference 1. Therefore, it is considered effective to suppress LME cracks by providing a solid-solution Si-deficient layer in which the amount of solid-solution Si is reduced near the surface of the base material on the overlapping surface side of the steel sheets where the steel sheets are in contact with each other, so that the layer having a certain amount of Si or more does not contact the zinc of the metal plating layer.

[0026] [Reference 1] WO2021-200412

[0027] However, when the thickness of the solid solution Si-deficient layer near the surface of the base material on the overlapping surface side of the steel plates is significant, there is a concern that the strength of the resistance spot weld will be reduced. That is, the following idea was obtained: by controlling the thickness of the solid solution Si-deficient layer near the surface of the base material in the heat affected zone to a certain range, it is possible to achieve both ensuring the strength of the resistance spot weld and suppressing LME cracks.

[0028] The present invention is based on the above findings, and the gist of the present invention is as follows.

[0029] [1] A welded component having a resistance spot weld portion formed by resistance spot welding a plate assembly of two or more stacked steel plates,

[0030] The total thickness of the plate group is t all (mm), the minimum thickness of the resistance spot welding part is set as t weld (mm), t all and t weld Satisfying formula (1),

[0031] At least one of the two or more steel sheets is a Si-containing galvanized steel sheet containing 0.5 to 3.0 mass % of Si,

[0032] Furthermore, a region in which the Zn concentration in the Si-containing galvanized steel sheet is 5.0 mass % or less and the Si concentration is less than 0.5 mass % is defined as region A, and a thickness in the plate thickness direction of the region A in the base material portion is defined as d a (μm), the above region A is formed under the zinc plating layer, and d a Satisfies formula (2).

[0033] 0.5<t weld / t all <1.0…(1)

[0034] 2<d a <10…(2)

[0035] [2] The welded member according to [1], wherein in the resistance spot weld portion on the Si-containing galvanized steel sheet side,

[0036] The thickness of the region A in the plate thickness direction in the corona welding at a position 200 μm away from the nugget end toward the base material portion is d wa (μm), d wa Satisfies formula (3).

[0037] 0<d wa <8…(3)

[0038] [3] The welded member according to [1] or [2], wherein the Si-containing galvanized steel sheet includes an Fe-based plating layer in the region A.

[0039] [4] The welded member according to any one of [1] to [3], wherein, when the tensile strength of the steel plate is TS (MPa),

[0040] The TS of the above Si-containing galvanized steel sheet is ≥980.

[0041] [5] The welded member according to any one of [1] to [4], wherein the average value of the shortest distance from the center of the weld spot of the resistance spot weld to the end surface of the steel plate is 3 mm or more,

[0042] Furthermore, when there are a plurality of weld points, the average distance between the weld point centers of adjacent weld points is 6 mm or more.

[0043] [6] A method for manufacturing a welded member, the method for manufacturing a welded member according to any one of [1] to [5], comprising:

[0044] The preparation step of stacking two or more steel plates to form a plate group and the welding step of performing resistance spot welding on the plate group are as follows.

[0045] In the above welding process,

[0046] The plate group is clamped by a pair of welding electrodes and connected by applying pressure and electricity.

[0047] For at least one welding spot, immediately before the welding electrode is pressed, one or more of the conditions (a) to (e) are satisfied.

[0048] And it has: an electrification step, wherein the electrification is performed with a pressure of 2.0 to 10.0 kN, a welding current of 4.0 to 15.0 kA, and an electrification time of 0.1 to 2.0 s; and

[0049] In the electrode holding step, when the time for holding the applied pressure after the end of the energization is defined as Th(S), Th satisfies the relationship between equations (5) and (6).

[0050] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more,

[0051] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more,

[0052] (c) A gap of 0.5 mm or more exists between any welding electrode and the overlapping steel plates.

[0053] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates,

[0054] (e) The shortest distance from the center of the weld point to the end surface of the overlapping steel plates is less than 10 mm,

[0055] (1 / d a +t+TS / 1000) / 110<Th<{-(1 / d a +1 / t+TS / 1000) / 65}+1…(5)

[0056] 0<Th…(6)

[0057] Among them, d in formula (5) a is the thickness in the plate thickness direction of the base material portion of the region A (μm), t is the plate thickness of the Si-containing galvanized steel sheet (mm), and TS is the tensile strength of the Si-containing galvanized steel sheet (MPa).

[0058] According to the present invention, a welded member and a method for manufacturing the welded member can be provided which can prevent a decrease in the strength of the resistance spot welded portion due to softening near the surface of the base material caused by the thickness of the solid solution Si deficiency layer even when resistance spot welding is performed using a high-strength steel plate, and does not cause LME cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 This is a cross-sectional view schematically showing an example of the resistance spot welding of the present invention.

[0060] Figure 2 This is a cross-sectional view in the plate thickness direction for explaining an example of a Si-containing galvanized steel plate used in the present invention.

[0061] Figure 3This is a cross-sectional view schematically showing a resistance spot weld portion and its surroundings in a welded member according to an embodiment of the present invention.

[0062] Figure 4 It is a plan view for explaining the shortest distance from the center of the weld point to the end surface of the steel plate in the welded member of the present invention.

[0063] Figure 5 It is a plan view for explaining the distance between the centers of the weld points in the welded member of the present invention.

[0064] Figure 6 This is a cross-sectional view for explaining an example of a tensile test in the case of a three-plate group in the embodiment of the present invention.

[0065] Figure 7 This is a cross-sectional view schematically showing an example of crack generation during conventional resistance spot welding. DETAILED DESCRIPTION

[0066] Hereinafter, a welded member and a method for manufacturing the same will be described as one embodiment of the present invention. However, the present invention is not limited to this embodiment.

[0067] First, refer to Figure 1 to Figure 3 The welded member of the present invention will be described.

[0068] Figure 1 An example of resistance spot welding of two steel plates is shown. Figure 2 This is a cross-sectional view in the plate thickness direction showing an example of a Si-containing galvanized steel plate used in the present invention. Figure 3 This is a cross-sectional view in the plate thickness direction showing an example of a welded member of the present invention, and is an enlarged cross-sectional view of a resistance spot welded portion and a portion thereof in the welded member. The position shown in the enlarged cross-sectional view is Figure 3 Each area surrounded by a four-border in .

[0069] The present invention is a welded member having a resistance spot welded portion (hereinafter sometimes referred to as a "welded portion") formed by resistance spot welding a plate group of two or more stacked steel plates.

[0070] As described later, at least one of the two or more stacked steel sheets is a Si-containing galvanized steel sheet containing 0.5 to 3.0 mass % Si (see Figure 2 ).like Figure 2 As shown, the Si-containing galvanized steel sheet 2 of the present invention is a galvanized steel sheet with a Si-containing steel sheet 2c having a region A as a base material. The Si-containing galvanized steel sheet 2 has a galvanized layer 2a formed on at least one side (at least one base material surface) and a region A2b formed in the lower layer of the galvanized layer 2a. That is, the region A2b is formed on the surface layer of the Si-containing steel sheet (base material) 2c. It should be noted that the detailed description of the Si-containing galvanized steel sheet will be described later, so it is omitted here.

[0071] The number of steel plates used in the present invention is not particularly limited, and may be 2 or more, preferably 3 or more. It should be noted that the upper limit of the number of steel plates is not particularly limited, but is preferably 5 or less.

[0072] Figure 3 The example shown is a welded component 10 formed by resistance spot welding two overlapping steel plates. The steel plate (lower plate) 2 arranged on the lower side is set as a Si-containing galvanized steel plate, and the steel plate (upper plate) 1a arranged on the upper side is set as a cold-rolled steel plate. The welded portion 6 described below is formed on the overlapping surface 7 of the steel plates 1a and 2 where the steel plates 1a and 2 are in contact.

[0073] exist Figure 3 In the case of the welded member of the example shown, the weld 6 of the present invention in which the thickness of the solid solution Si-deficient layer in the weld heat-affected zone is controlled within a certain range is formed only on the side of the lower plate 2. It should be noted that when the Si-containing galvanized steel sheet of the present invention is used for both the upper plate and the lower plate, the weld 6 of the present invention is formed on both the upper plate side and the lower plate side of the steel plate overlap surface 7.

[0074] Although the illustration is omitted, when more than three steel plates are overlapped and welded, all or at least one of the steel plate (lower plate) arranged at the bottom, the steel plate (upper plate) arranged at the top, and the steel plate (middle plate) arranged therebetween is a Si-containing galvanized steel plate. The weld of the present invention is formed in a manner that includes the overlapping surfaces of the steel plates between the plates. For example, in the case of a plate group consisting of three steel plates, the weld of the present invention is formed in a manner that includes the overlapping surfaces of the two steel plates in contact with the lower plate and the middle plate, and the middle plate and the upper plate. In the case where the above-mentioned Si-containing galvanized steel plates are used for all three steel plates, the thickness of the solid solution Si-deficient layer on the surface of the steel plates on the upper and lower sides of the overlapping surfaces of each steel plate is controlled respectively.

[0075] <Welding Department>

[0076] The welded portion of the present invention will be described in detail. It should be noted that the same welded portion is formed in the case of a plate group in which two steel plates are stacked and in the case of a plate group in which three or more steel plates are stacked, so the following description uses Figure 3 .

[0077] like Figure 3 As shown, in the welded portion 6 of the present invention, the total plate thickness of the plate group is set to t all (mm), the minimum thickness of the weld is set to t weld (mm), total plate thickness (t all ) and the minimum thickness of the weld (t weld ) satisfies the following formula (1).

[0078] In addition to this condition, if Figure 3As shown in FIG. 1 , the region where the Zn concentration in the Si-containing galvanized steel sheet 2 is 5.0 mass % or less and the Si concentration is less than 0.5 mass % is defined as region A, and the thickness of region A in the base material portion in the plate thickness direction is defined as d a (μm), the region A is formed in the lower layer of the zinc-plated layer 2a, and the thickness of the region A in the base material portion (d a ) satisfies the following formula (2).

[0079] 0.5<t weld / t all <1.0…(1)

[0080] 2<d a <10…(2)

[0081] The ratio of the total plate thickness to the minimum thickness of the weld (t weld / t all ) does not satisfy the above formula (1), especially when it is below the lower limit of formula (1), the thickness reduction of the weld becomes significant, so the tensile residual stress generated in the weld becomes too large, and LME cracks are easily generated. At the same time, the joint strength is reduced. It should be noted that the thickness of the plate group is reduced due to the pressure and heat generated by the pair of welding electrodes during welding, so t weld / t all The upper limit is less than 1.0.

[0082] The lower limit of the formula (1) is preferably 0.55 or more. The upper limit of the formula (1) is preferably 0.95 or less.

[0083] Here, if Figure 3 As shown in the figure, the minimum thickness of the weld (t weld )" refers to the minimum thickness (mm) in the plate thickness direction within the width direction of the nugget 6a. It should be noted that the minimum thickness (t weld ) can be measured by the method described in the examples described later.

[0084] In addition, if Figure 2 As shown in the figure, region A2b is the lower layer of the galvanized layer 2a, which is formed on the surface layer of the Si-containing steel plate 2c. In the weld, due to the heat during welding, Zn diffuses into the inside of the contacting steel plates. However, by forming a region A with low Zn concentration and Si concentration on the surface layer of the Si-containing steel plate with a galvanized layer, the region A functions as a barrier layer to inhibit the intrusion of Zn into the Si-containing steel plate. Furthermore, the region A also acts as a solid solution Si-deficient layer. Through such a function, LME cracks in the weld can be suppressed. The above-mentioned "solid solution Si-deficient layer" refers to a layer in which the amount of Si dissolved in the steel is less than the amount of Si in the base material, that is, it is deficient.

[0085] The thickness of region A in the base material portion (da ) is insufficient, it becomes difficult to form a solid solution Si-deficient layer in the weld heat affected zone described later, so d a More than 2μm.

[0086] In addition, as a method for forming region A on the surface of a Si-containing steel sheet having a galvanized layer, there is, for example, Fe-based electroplating described later, but a method is also required to reduce the concentration of alloy elements other than Si in the surface of the Si-containing steel sheet. Therefore, if the thickness of region A is too large, there is a concern that the surface of the steel sheet in the weld heat affected zone will be greatly softened. In particular, excessive softening of the overlapping surfaces of the steel sheets will reduce the strength of the weld, so d a Less than 10μm.

[0087] The lower limit of formula (2) is preferably 3 μm or more, more preferably 3.0 μm or more. The upper limit of formula (2) is preferably 8 μm or less, more preferably 8.0 μm or less, and further preferably 6.0 μm or less.

[0088] In the present invention, in addition to the above conditional expressions, the following conditions are preferably met.

[0089] [Thickness of region A in the corona bonding region in the plate thickness direction] (preferable condition)

[0090] Specifically, if Figure 3 As shown, the Si-containing galvanized steel sheet 2 has a region A on the steel sheet surface on the steel sheet overlap surface 7 side of the overlapped steel sheets. In the welded portion on the Si-containing galvanized steel sheet side, the thickness of the region A in the plate thickness direction in the corona bonding at a position 200 μm away from the nugget end toward the parent material portion is set to d wa When the thickness (d wa It is effective when (μm) satisfies the following formula (3).

[0091] 0<d wa <8…(3)

[0092] The thickness of the region A passing through the corona bonding region in the plate thickness direction (d wa ) satisfies the formula (3). Even in the heat affected zone where LME cracks occur, the region containing more than 5 mass % Zn and the region containing more than 0.5 mass % Si do not contact each other, and LME cracks are not easily generated.

[0093] In the case where the lower limit of equation (3) is not satisfied (i.e., in d wa When the Zn concentration is 0 μm, the above-mentioned high-Zn concentration and high-Si concentration regions contact each other, promoting LME cracks, so d wa Satisfying a thickness of more than 0 μm is effective in suppressing LME cracks. wa) more preferably exceeds 1 μm, and further preferably exceeds 2 μm, in which case the above-mentioned effect can be more effectively obtained.

[0094] In addition, as described above, if the thickness (d wa ) is too large, there is a concern that the surface layer of the steel plate at the heat-affected zone may soften significantly and the joint strength may decrease. wa ) is preferably less than 8 μm.

[0095] <Si-containing galvanized steel sheet>

[0096] In the present invention, as described above, two or more steel sheets including at least one Si-containing plated steel sheet are stacked to form a sheet set. Figure 2 The Si-containing galvanized steel sheet of the present invention will be described.

[0097] like Figure 2 As shown, the Si-containing galvanized steel sheet is a galvanized steel sheet having a Si-containing steel sheet 2c having a region A2b on the surface as a base material. Region A is a region where the Zn concentration is 5.0 mass % or less and the Si concentration is less than 0.5 mass %. The Si-containing galvanized steel sheet 2 can be manufactured, for example, by the following method.

[0098] First, a cold-rolled steel sheet containing 0.5 mass % to 3.0 mass % of Si is manufactured. As an example, the cold-rolled steel sheet is manufactured by hot-rolling a steel slab having the above-mentioned composition to obtain a hot-rolled sheet, then pickling the hot-rolled sheet, and then cold-rolling the hot-rolled sheet to obtain a cold-rolled steel sheet.

[0099] Next, the cold-rolled steel sheet is subjected to an annealing step in a reducing atmosphere and then subjected to galvanizing, thereby obtaining a Si-containing steel sheet having a galvanized layer (Si-containing galvanized steel sheet).

[0100] [Area A]

[0101] The Si-containing galvanized steel sheet of the present invention may have a Fe-based plating layer as the region A. Alternatively, it may have a Si internal oxide layer as the region A. The method for forming the region A in the Si-containing galvanized steel sheet is not particularly limited.

[0102] [Fe-based pre-plating layer]

[0103] As the Fe-based plating layer, there is a method of forming an Fe-based pre-plating layer, for example.

[0104] Specifically, the Si-containing galvanized steel sheet preferably has an adhesion amount of more than 20.0 g / m on one side of the lower layer of the galvanized layer. 2 That is, the surface layer below the zinc plating layer and in the region A includes the Fe-based plating layer.

[0105] The above-mentioned pre-plating layer is preferably an Fe-based electroplating layer. As the Fe-based electroplating layer, in addition to pure Fe, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can also be used. The composition of the Fe-based electroplating layer is not particularly limited, but in the present invention, it is preferably the following composition: containing a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, and the remainder is composed of Fe and unavoidable impurities. By making the amount of elements other than Fe less than 10% by mass in total, it is possible to prevent the reduction of electrolysis efficiency and form the Fe-based electroplating layer at a low cost.

[0106] Furthermore, the deposition amount of the Fe-based electroplating layer on one side is preferably 20.0 g / m 2 The adhesion amount is more preferably 25.0 g / m 2 More preferably, 30.0 g / m 2 More preferably, 35.0 g / m 2 The upper limit of the amount of Fe-based electroplating layer on one side is not particularly limited. From the perspective of preventing the area A from being formed too large and from the perspective of cost, the amount of Fe-based electroplating layer on one side is preferably 60 g / m 2 the following.

[0107] It should be noted that the thickness of the Fe-based electroplating layer is measured as follows. A sample of 10×15 mm in size is taken from a high-strength hot-dip galvanized steel sheet alloyed after hot-dip galvanizing and embedded in a resin to make a cross-sectional embedded sample. Using a scanning electron microscope (SEM), observe any three locations of the cross section at an accelerating voltage of 15 kV and a magnification of 2000 to 10000 times according to the thickness of the Fe-based electroplating layer, and multiply the average value of the thickness of the three fields of view by the specific gravity of iron, thereby converting it into the adhesion amount on a single side of the Fe-based electroplating layer.

[0108] It is preferred that the surface of the cold-rolled steel sheet before high-strength annealing is subjected to Fe-based electroplating treatment to prepare the steel sheet before annealing. The Fe-based electroplating treatment method is not particularly limited. For example, as the Fe-based electroplating bath, a sulfuric acid bath, a hydrochloric acid bath or a mixture of the two can be used. It should be noted that the cold-rolled high-strength annealing cold-rolled steel sheet before high-strength annealing can also be subjected to Fe-based electroplating treatment without being subjected to oxidation treatment in a preheating furnace or the like.

[0109] The Fe ion content in the Fe-based electroplating bath before the start of energization is preferably Fe 2+If the Fe ion content in the Fe-based electroplating bath is in the form of Fe 2+ When the content of Fe is 1.0 mol / L or more, a sufficient amount of Fe deposition can be obtained.

[0110] By using such a steel sheet, the effects of the present invention can be more effectively obtained, and in particular, in the case of an alloyed hot-dip galvanized steel sheet in which a pre-galvanizing process is performed before forming a galvanized layer and an alloying process is performed after hot-dip galvanizing.

[0111] [Internal oxide layer]

[0112] In addition to the above-mentioned pre-plating layer, there is a method of forming a Si internal oxide layer on the surface of the steel sheet as a method of forming the region A on the surface of the steel sheet. For example, by increasing the dew point during annealing during the manufacturing process of the Si-containing steel sheet, the Si internal oxide layer is formed on the surface of the steel sheet.

[0113] Specifically, the above-mentioned “Si internal oxide layer” refers to a region where Si oxide is formed in a part of a crystal grain and / or a grain boundary.

[0114] In the case where the Si-containing galvanized steel sheet has a Si internal oxide layer as region A, the lower limit of the thickness of the Si internal oxide layer in the plate thickness direction is preferably 2.0 μm or more. The amount of dissolved Si in the steel in the Si internal oxide layer is less than 0.5% by mass, so the thickness of region A can be 2.0 μm or more. On the other hand, in order to make the thickness of the Si internal oxide layer 6.0 μm or more, it is necessary to greatly increase the dew point during annealing, and there is a concern about corrosion in the annealing furnace. Therefore, the upper limit of the thickness in the plate thickness direction is preferably less than 6.0 μm.

[0115] The above thickness of the Si internal oxide layer is measured as follows. First, a sample of 10×15 mm in size is taken from the alloyed high-strength hot-dip galvanized steel sheet after hot-dip galvanizing and embedded in a resin to make a cross-sectional embedded sample. Use a scanning electron microscope (Scanning Electron Microscope: SEM) to observe any three locations of the cross section at an accelerating voltage of 10 kV and a magnification of 3000 times. The thickness of the Si internal oxide layer in the plate thickness direction is measured from each of the three observed photographs, and the average value of the three measured locations is taken as the thickness of the Si internal oxide layer in the plate thickness direction.

[0116] [Galvanized layer]

[0117] In the present invention, from the viewpoint of manufacturability, the Si-containing galvanized steel sheet is preferably a hot-dip galvanized steel sheet (GI steel sheet) having a hot-dip galvanized layer. In particular, in consideration of the pressability and continuous spot weldability of the steel sheet, the Si-containing galvanized steel sheet is preferably an alloyed hot-dip galvanized steel sheet (GA steel sheet) having an alloyed hot-dip galvanized layer.

[0118] [Tensile strength]

[0119] As described above, generally, as the steel sheet becomes higher in strength and alloy composition, LME cracks tend to be easily generated. Therefore, in the present invention, when the tensile strength of the steel sheet is set to TS (MPa), the Si-containing galvanized steel sheet preferably has TS ≥ 980. This is because in the case of a high-strength steel sheet that satisfies this relationship, the effect of the present invention can be more effectively improved.

[0120] <Welding points of welded components>

[0121] Next, use Figure 4 and Figure 5 The weld point in the welded member of the present invention will be described.

[0122] Figure 4 and Figure 5 It is a plan view showing the periphery of a welded portion of a welded member (a view of the welded member as viewed from above).

[0123] like Figure 4 and Figure 5 As shown, the welded component 10 of the present invention preferably has an average value of the shortest distance (H1) from the center of the weld of the weld to the end surface of the steel plate of more than 3 mm, and when there are multiple welds, the average distance (H2) between the centers of adjacent welds is more than 6 mm.

[0124] It should be noted that the welded component of the present invention also includes the case where the number of weld points is only one. In this case, it is sufficient as long as the "average value of the shortest distance (H1) from the weld point center of the weld to the end face of the steel plate is 3 mm or more" among the above-mentioned weld point conditions is met.

[0125] Here, if Figure 4 The "average value of the shortest distance (H1) from the center of the weld to the end face of the steel plate" is obtained by setting the end face of the steel plate closest to the center of the weld 8 and measuring the distance for each weld. When the number of welds in the welded component is 5 or more, the average value of 5 welds selected appropriately is taken as the "average value of the shortest distance", and when the number of welds in the welded component is less than 5, the average value of all welds is taken as the "average value of the shortest distance".

[0126] When the average value (H1) of the shortest distance is less than 3 mm, the pressurization between the plates around the nugget on the end face side of the steel plate becomes insufficient, and the molten metal easily splashes from the plate to the end face side of the steel plate. As a result, the spatter generation during welding becomes significant, and the nugget diameter easily deviates, thereby the strength of the weld becomes unstable. There is no upper limit for the average value of the shortest distance. In order to be able to weld with a general shape welding gun, the average value (H1) of the shortest distance is preferably less than 1000 mm. The average value (H1) of the shortest distance is more preferably less than 30.0 mm, and further preferably less than 20.0 mm.

[0127] In addition, if Figure 5 The "average distance (H2) between the centers of adjacent welds" is obtained by measuring the distance between the centers of each adjacent weld 8. When the average distance (H2) between the centers of the welds is less than 6 mm, a shunt to the existing weld occurs during welding, and the current density of the weld portion decreases, so the diameter of the nugget is reduced, and the strength of the weld portion is easily reduced. In addition, the weld portion is constrained by the existing weld, and the tensile residual stress increases, which is also prone to LME cracks. There is no upper limit for the average distance between the centers of the welds. From the viewpoint of ensuring the strength and rigidity of the welded component, the average distance (H2) between the centers of the welds is preferably 100 mm or less. The average distance (H2) between the centers of the welds is more preferably 60.0 mm or less, and further preferably 40.0 mm or less.

[0128] Next, one embodiment of the method for producing a welded member according to the present invention will be described.

[0129] The welded member of the present invention is manufactured through a manufacturing process including a preparation process of stacking two or more steel plates to form a plate group and a welding process of resistance spot welding the plate group.

[0130] <Preparation process>

[0131] In this process, two or more steel plates are stacked to form a plate group. At least one of the two or more steel plates is a Si-containing galvanized steel plate having the above-mentioned region A. For example, Figure 1 As shown, two steel plates 1a and 1b are stacked to form a plate set. It should be noted that the steel plates used in the plate set have been described above, so the description thereof is omitted. Next, a welding process is performed.

[0132] <Welding process>

[0133] The welding process includes a power supply process and an electrode holding process described later. In the welding process, the plate group prepared in the preparation process is joined. In this process, for example, Figure 1As shown in FIG. 1 , the plate set is clamped by a pair of welding electrodes 4 and 5 disposed on the lower and upper sides of the plate set, and current is applied while being controlled to a predetermined welding condition. Thus, by forming the welded portion 6 of the present invention between the steel plates that become the steel plate overlap surfaces 7 of the steel plates 1a and 1b, the steel plates can be joined to each other (see FIG. 1 ). Figure 3 ).

[0134] In addition, when a plate set is made using a Si-containing galvanized steel plate having region A and a steel plate without region A, the plates are overlapped so that the surface side having region A (i.e., the surface side having the galvanized layer) is the overlapping surface of the steel plates. Even if the steel plate surface of the Si-containing galvanized steel plate on the side in contact with the welding electrode also has region A and a galvanized layer, there is no problem.

[0135] As a welding device that can be used in the resistance spot welding method of the present invention, a welding device that has a pair of upper and lower welding electrodes and can arbitrarily control the pressure and welding current during welding can be used. The pressurizing mechanism (cylinder, servo motor, etc.), form (fixed type, robot welding gun, etc.), electrode shape, etc. of the welding device are not particularly limited. As the form of the tip of the electrode, for example, DR shape (dome arc shape), R diameter (arc shape), D shape (dome shape) and the like described in JIS C 9304:1999 can be cited. In addition, the tip diameter of the electrode is, for example, 4 mm to 16 mm.

[0136] Next, welding conditions in the welding process of the present invention will be described.

[0137] In the welding process, when a plate group is clamped by a pair of welding electrodes and joined while being pressurized and powered on, for at least one weld point, before being pressurized by the welding electrode, one or more of the following conditions (a) to (e) are satisfied, and the process comprises: a power-on process, in which the power-on is performed under welding conditions of a pressure of 2.0 to 10.0 kN, a welding current of 4.0 to 15.0 kA, and a power-on time of 0.1 to 2.0 S; and an electrode holding process, in which when the pressurization holding time after the end of the power-on is set to Th (S), Th satisfies the relationship between equations (5) and (6).

[0138] [Electrification process]

[0139] [Pressure, welding current, power-on time]

[0140] If the pressure applied during the energization process is less than 2.0 kN, the pressurization between the steel plates becomes insufficient, spatter is likely to occur, and thus LME cracks are likely to occur. On the other hand, if the pressure applied exceeds 10.0 kN, a special welding gun with high pressurization specifications is required, which greatly limits the equipment. In addition, the thickness reduction of the welded portion also becomes significant, thus causing LME cracks or reducing the joint strength. The pressure applied is preferably 3.0 kN or more, and preferably 7.0 kN or less.

[0141] If the welding current in the power-on process is less than 4.0 kA, the heat input is insufficient and the nugget diameter cannot be ensured. On the other hand, if the welding current exceeds 15.0 kA, the heat input is too large and spatter is likely to occur, so LME cracks are likely to occur. The welding current is preferably 5.0 kA or more and preferably 12.0 kA or less.

[0142] If the power-on time in the power-on process is less than 0.1S, the diffusion of alloy elements in the heat-affected zone of the weld may become insufficient, and the hardness of the softened layer in the heat-affected zone of the weld may hardly change from the hardness of the base material. Therefore, the softening of the surface layer of the steel plate in the heat-affected zone becomes significant, and TSS decreases. On the other hand, if the power-on time exceeds 2.0S, the cycle time of the automobile assembly process becomes longer and the productivity decreases. The power-on time is preferably 0.12S or more, and preferably 1.5S or less.

[0143] It should be noted that resistance spot welding can be performed in multiple stages of pressure and current value or in a specific mode with and without power supply. For example, when the pressure during power supply is set to multiple stages, the minimum pressure during power supply is set to 2.0 kN or more, and the maximum pressure is set to 10.0 kN or less. When the current value during power supply is set to multiple stages, the minimum current value during power supply except for the non-power supply period is set to 4.0 kA or more, and the maximum current value is set to 15.0 kA or less. In addition, the total power supply time except for the non-power supply period is set to 0.1S to 2.0S.

[0144] [Welding construction interference]

[0145] In the present invention, in addition to the above conditions, when manufacturing a welded component, for at least one weld point, immediately before pressurization by a welding electrode, one or more of the following conditions (a) to (e) are satisfied, and the effect of the present invention can be obtained more effectively.

[0146] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more

[0147] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more

[0148] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0149] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0150] (e) The shortest distance from the center of the weld to the end surface of the overlapping steel plates is less than 10 mm.

[0151] These welding construction disturbances will locally increase the temperature and / or tensile stress of the weld when the electrode is released, thus making it easy for LME cracks to occur. However, by performing surface layer control of the weld of the present invention, LME cracks can be suppressed even in the presence of these welding construction disturbances, thereby increasing the margin for construction disturbance management during component manufacturing. The following is a detailed description of each construction disturbance.

[0152] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more

[0153] The electrode inclination angle refers to the angle at which the electrode is inclined relative to the steel plate, that is, "the angle formed by the direction of the electrode pressure and the thickness direction of the steel plate". If the electrode inclination angle is large, bending stress is applied to the weld, and large compressive plastic deformation is locally generated, thereby increasing the tensile stress after the electrode is released. When the electrode inclination angle is greater than 0.2 degrees, the effect of the present invention can be effectively obtained. When the electrode inclination angle is too large, the formation of the molten core becomes unstable, which becomes the cause of spatter, so the electrode inclination angle is preferably less than 10 degrees. The electrode inclination angle is more preferably greater than 1 degree, and more preferably less than 8 degrees.

[0154] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more

[0155] Electrode center offset refers to a state in which the central axes of a pair of welding electrodes are not aligned. As with the electrode inclination angle described above, if the electrode center offset is large, bending stress is applied to the weld, which makes it easy to generate LME cracks. When the electrode center offset is 0.1 mm or more, the effect of the present invention can be effectively obtained. When the electrode center offset is too large, the nugget formation becomes unstable, which becomes a cause of spatter generation, so the electrode center offset is preferably 5 mm or less. The electrode center offset is more preferably 0.2 mm or more, and more preferably 3 mm or less.

[0156] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0157] When there is a gap between any electrode and the steel plate just before the start of pressurization, for example, when one electrode is movable (hereinafter referred to as the movable side electrode) and the other electrode is fixed (hereinafter referred to as the fixed side electrode), pressurization is started using the movable side electrode in a state where there is a gap between the fixed side electrode and the steel plate. As a result, the steel plate is bent and deformed, so bending stress is applied to the weld, which makes it easy to generate LME cracks. When the gap amount is 0.5 mm or more, the effect of the present invention can be effectively obtained. When the gap amount is too large, the formation of the molten core becomes unstable, which becomes the cause of spatter, so the gap amount is preferably less than 5 mm. The gap amount is more preferably more than 1 mm, and more preferably less than 3 mm.

[0158] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0159] As in (c) above, when there is a gap between any of the steel plates just before pressurization begins, the steel plate is bent and deformed, so bending stress is applied to the weld, which makes it easy to generate LME cracks. When the gap amount is 0.5 mm or more, the effect of the present invention can be effectively obtained. When the gap amount is too large, the formation of the molten core becomes unstable, which becomes the cause of spatter, so the gap amount is preferably less than 4 mm. The gap amount is further preferably more than 1 mm, and further preferably less than 3 mm. It should be noted that the above-mentioned "gap between more than one group of steel plates" refers to the presence of a gap between more than one group of steel plates when two steel plates arranged in the upper and lower directions are set as one group among two or more overlapping steel plates.

[0160] (e) The shortest distance from the center of the weld to the end surface of the overlapping steel plates is less than 10 mm.

[0161] If the shortest distance from the center of the weld to the end face of the steel plate is short, heat conduction from the weld is hindered at the end face of the steel plate, so the cooling rate of the weld is reduced. Therefore, the temperature increases when the electrode is released, and LME cracks are likely to occur. When the shortest distance from the center of the weld to the end face of the steel plate is less than 10 mm, the effect of the present invention can be effectively obtained. In addition, as described above, when the shortest distance is less than 3 mm, spatter generation during welding becomes significant, and the nugget diameter is prone to deviation, thereby making the strength of the weld unstable. Therefore, the shortest distance is preferably greater than 3 mm. The shortest distance is preferably greater than 4 mm, and more preferably less than 8 mm.

[0162] [Electrode holding process]

[0163] After the above-mentioned energizing step, an electrode holding step is performed. The electrode holding step is a step of suppressing the generation of pores by holding the welding electrode at a certain pressure after the energizing step is completed.

[0164] In the electrode holding step, from the viewpoint of temperature control when the electrode is released, when the pressure holding time after the end of the energization is set as Th(S), the pressure holding time (Th) satisfies the relationship between the following equations (5) and (6). Thus, the effect of the present invention can be effectively obtained.

[0165] (1 / d a +t+TS / 1000) / 110<Th<{-(1 / d a +1 / t+TS / 1000) / 65}+1…(5)

[0166] 0<Th…(6)

[0167] Among them, d in formula (5) a is the thickness in the plate thickness direction of the base material portion of the region A (μm), t is the plate thickness of the Si-containing galvanized steel sheet (mm), and TS is the tensile strength of the Si-containing galvanized steel sheet (MPa).

[0168] If the pressure holding time (Th) is too short, the temperature at the time of electrode release increases and LME cracks are likely to occur. On the other hand, if the pressure holding time (Th) is too long, the takt time per weld increases, thereby reducing productivity. At the same time, the cooling rate of the weld increases, thereby making the structure of the weld brittle, thereby reducing the joint strength or causing delayed failure of the weld.

[0169] The thickness of the base material portion in the region A in the plate thickness direction (d a ) is thinner, the function of the barrier layer to inhibit the intrusion of liquid Zn is reduced, and the LME crack inhibition effect is smaller. Therefore, it is necessary to increase the lower limit of the pressure holding time (Th). In addition, as the plate thickness (t) of the Si-containing galvanized steel sheet increases, the cooling rate of the weld decreases, so the lower limit of Th needs to be increased. Furthermore, as the tensile strength (TS) of the Si-containing galvanized steel sheet increases, the structure of the weld tends to become brittle and LME cracks are prone to occur, so the lower limit of Th needs to be increased. On the other hand, d a It is a layer softer than the base steel plate. a The thinner the steel sheet is, the easier it is to make the structure of the steel sheet surface at the weld part brittle, so the upper limit of Th needs to be reduced. In addition, when the plate thickness (t) is thin and the cooling rate of the weld part is high, or when the TS of the Si-containing galvanized steel sheet is high, the structure of the weld part is easy to become brittle. Therefore, the upper limit of Th needs to be reduced.

[0170] For this reason, the pressure holding time is a time that satisfies the equations (5) and (6). The lower limit of the equation (5) is preferably (1 / d a+t+TS / 1000) / 110+0.02 or more. The upper limit of formula (5) is preferably {-(1 / d a +1 / t+TS / 1000) / 65}+0.9 or less.

[0171] In addition, the lower limit of formula (6) is preferably 0.02 or more. The upper limit of formula (6) is preferably 1.0 or less. It should be noted that in the case of a plate group formed by overlapping two or more Si-containing galvanized steel plates having the above-mentioned region A, any one of them may satisfy formulas (5) and (6).

[0172] Example

[0173] Hereinafter, the function and effect of the present invention will be described using examples. However, the present invention is not limited to the following examples.

[0174] Using the plate group shown in Table 1, a welded joint (welded component) was made under the welding conditions shown in Table 2. It should be noted that the plate group is arranged and overlapped from the upper side in the order of steel plate 1, steel plate 2, and steel plate 3 shown in Table 1. "GA" shown in the coating column of Table 1 refers to a steel plate having an alloyed hot-dip galvanized layer, and "GI" refers to a steel plate having a hot-dip galvanized layer. In addition, the symbols shown in the "State immediately before pressurization" column of Table 3 correspond to (a) to (e) shown in the above-mentioned welding construction interference. In addition, the welding device uses a servo motor pressurized single-phase AC (50Hz) resistance welding machine installed on a welding gun. The pair of electrode chips used are DR-type electrodes of chromium copper with a front end radius of curvature R40mm and a front end diameter of 6mm.

[0175] In addition, the column "Tensile Strength" in Table 1 shows the tensile strength (MPa) measured by taking a JIS No. 5 tensile test piece from each steel plate in the rolling direction and conducting a tensile test in accordance with JIS Z 2241.

[0176] The TSS and the presence of LME cracks in the welded joint were evaluated by the following method. In addition, the thickness (t all ,t weld ), the thickness of the plate in the thickness direction of the above-mentioned region A (d a and d wa ).

[0177] <Measurement of each thickness>

[0178] [t all ,t weld ]

[0179] For the welded joints, a micro-cutter was used to cut through the center of the welded part and observe the cross section in the thickness direction. all), measure the thickness of the steel plate before welding, and calculate the total thickness. In addition, for the minimum thickness of the weld (t weld ), the size of the plate thickness direction in the area of ​​the width direction of the nugget was measured at intervals of 100 μm, and the minimum value was taken as the "minimum thickness" (refer to Figure 3 ). Use the obtained values ​​to find "t weld / t all ” value.

[0180] [d a ,d wa ]

[0181] The center of the welded joint was cut out, and the surface of the steel plate in the base metal part and the weld heat affected zone was analyzed using a scanning electron microscope (SEM) and an energy dispersive X-ray detector (EDX) to obtain d a and d wa .

[0182] Specifically, first, the welded joint is cut with a micro-cutter in a manner passing through the center of the welded portion of the welded joint to be filled with resin. The cross section of the welded joint after resin filling is ground and processed for observation. Then, the cross section of the sample is subjected to X-ray analysis at an observation magnification of 10,000 times and an accelerating voltage of 5 kV, and the line distribution of the Zn concentration and Si concentration in the plate thickness direction is obtained from the surface layer of the steel plate in the parent material part and the weld heat affected part. From the obtained concentration line distribution, the region A where the above-mentioned Zn concentration and Si concentration are obtained is obtained, and the thickness of the region A in the parent material part and the weld heat affected part is calculated.

[0183] Here, each value in the steel plate described in the column of "Evaluated Steel Plate" in Table 2 is described in Table 2 as a representative value.

[0184] [TSS's evaluation]

[0185] The evaluation of tensile shear strength (TSS) was performed based on the tensile shear test method (JIS Z3136). In the tensile shear test, a shear tensile test piece was cut out from each steel plate shown in Table 1. The size of the shear tensile test piece was a length of 50 mm in the short side direction and a length of 150 mm in the long side direction. Next, a welded joint (test piece) was produced by resistance spot welding using the plate group shown in Table 1 and the welding conditions shown in Table 3.

[0186] In addition, for comparison with the obtained test pieces (welded joints produced by the welding method of the present invention), a comparative joint in which the region A was not controlled (i.e., the region A in the above-mentioned base material portion in which the surface layer of the Si-containing steel plate did not have the region A and the thickness was d a =0). The comparative joints were also evaluated for TSS based on the same tensile shear test method.

[0187] The test piece shape for TSS evaluation is based on JIS Z3136, but in the three overlapping plate sets (plate set No. H in Table 1), Figure 6 As shown, the evaluation was performed by applying a tensile load between the steel plates 1 b and 1 c.

[0188] It should be noted that in the preparation of the welded joint for TSS evaluation, there was only one weld point, and no welding construction interferences such as the electrode inclination angle and plate gap shown in Tables 2 and 3 were set.

[0189] Then, TSS was evaluated according to the following criteria.

[0190] <Evaluation Criteria>

[0191] A: (TSS of the welded joint produced by the welding method of the present invention) / (TSS of the comparative joint) ≥ 0.9

[0192] B: 0.9>(TSS of welded joint produced by the welding method of the present invention) / (TSS of comparative joint)≥0.8

[0193] F: 0.8 > (TSS of the welded joint produced by the welding method of the present invention) / (TSS of the comparative joint)

[0194] Here, when the evaluation results were "A" and "B", it was evaluated as passing (having excellent shear tensile strength).

[0195] [Evaluation of LME cracks]

[0196] Welding is performed in a state with one or more of the welding construction interferences (a) to (e) above, and the welded joint is cut with a micro-cutter in a manner passing through the center of the welded portion of the obtained welded joint, and then a cross-sectional observation of the welded portion in the plate thickness direction is performed. Based on the observation results, the presence or absence of LME cracks is evaluated according to the following criteria. Specifically, 10 welded joints are made under each welding condition shown in Table 3, and cross-sectional observations are performed on the overlapping surface side between the steel plates to confirm LME cracks. It should be noted that in the welded portion of the steel plate recorded in the "Evaluation Steel Plate" column of Table 2, cross-sectional observations on the overlapping surface side between the steel plates are performed and evaluated.

[0197] <Evaluation Criteria>

[0198] A: All 10 pieces without cracks

[0199] B: There are 2 or fewer welded joints with cracks, and the maximum crack depth is less than 100 μm. F: There are 3 or more welded joints with cracks, or the maximum crack depth is 100 μm or more.

[0200] Here, the evaluation was considered acceptable when the evaluation results were A and B. The obtained values ​​and evaluation results are shown in Tables 2 and 3, respectively.

[0201]

[0202]

[0203]

[0204] As is clear from Table 3, the evaluation results of the welded joints (welded members) as the inventive examples were all A or B. According to the present invention, it is found that both ensuring the strength of the welded portion and suppressing the LME crack can be achieved.

[0205] Explanation of symbols

[0206] 1a, 1b, 1c Steel Plate

[0207] 2. Si-containing galvanized steel sheet

[0208] 2a Galvanized layer

[0209] 2b Area A

[0210] 2c Si-containing steel plate (base material)

[0211] 4.5 Welding electrodes

[0212] 6 Welding Department

[0213] 6a Nugget

[0214] 6b Heat affected zone

[0215] 7 Overlapping surfaces of steel plates

[0216] 8 Soldering points

[0217] 10 Welded components

Claims

1. A welded component having a resistance spot welded portion formed by resistance spot welding a plate group in which two or more steel plates are stacked, The total thickness of the plate group is set as t all , the minimum thickness of the resistance spot welding portion is set to t weld When t all and t weld Satisfying formula (1), the t all ,t weld The unit is mm, At least one of the two or more steel sheets is a Si-containing galvanized steel sheet containing 0.5 to 3.0 mass % of Si, Furthermore, a region in which the Zn concentration in the Si-containing galvanized steel sheet is 5.0 mass % or less and the Si concentration is less than 0.5 mass % is defined as region A, and a thickness in the plate thickness direction of the region A in the base material portion is defined as d a When the area A is formed under the zinc coating, and d a Satisfying formula (2), the d a The unit is μm, 0.5<t weld / t all <1.0…(1) 2＜d a ＜10…(2)。 2. The welding component according to claim 1, wherein: In the resistance spot welded portion on the Si-containing galvanized steel sheet side, The thickness of the region A in the plate thickness direction in the corona welding at a position 200 μm away from the nugget end toward the base material portion is d wa When wa Satisfying formula (3), the d wa The unit is μm, 0＜d wa ＜8…(3)。 3. The welding member according to claim 1 or 2, wherein: The Si-containing galvanized steel sheet includes an Fe-based plating layer in the region A.

4. The welded member according to any one of claims 1 to 3, wherein When the tensile strength of the steel plate is defined as TS, The TS of the Si-containing galvanized steel sheet is ≥980, and the unit of the TS is MPa.

5. The welded member according to any one of claims 1 to 4, wherein The average value of the shortest distance from the center of the welding spot of the resistance spot welding part to the end surface of the steel plate is more than 3 mm. Furthermore, when there are a plurality of weld points, the average distance between the weld point centers of adjacent weld points is 6 mm or more.

6. A method for manufacturing a welded member, the method for manufacturing a welded member according to any one of claims 1 to 5, comprising: a preparation step of stacking two or more steel plates to form a plate group and a welding step of performing resistance spot welding on the plate group, In the welding process, The plate group is clamped by a pair of welding electrodes and joined while applying pressure and electricity. For at least one welding point, immediately before the welding electrode is pressed, one or more of the conditions (a) to (e) are satisfied. The invention also comprises a power-on process and an electrode holding process. In the power-on process, the power-on is performed with a pressure of 2.0 to 10.0 kN, a welding current of 4.0 to 15.0 kA, and a power-on time of 0.1 to 2.0 s. In the electrode holding process, when the pressure holding time after the power-on is set to Th, Th satisfies the relationship between equations (5) and (6), and the unit of Th is S. (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more, (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more, (c) A gap of 0.5 mm or more exists between any welding electrode and the overlapping steel plates. (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates, (e) The shortest distance from the center of the weld point to the end surface of the overlapping steel plates is less than 10 mm, (1 / d a +t+TS / 1000) / 110<Th<{-(1 / d a +1 / t+TS / 1000) / 65}+1…(5) 0<Th…(6) in, In formula (5), d a is the thickness in the plate thickness direction of the base material portion of the region A, and its unit is μm; t is the plate thickness of the Si-containing galvanized steel sheet, and its unit is mm; TS is the tensile strength of the Si-containing galvanized steel sheet, and its unit is MPa.

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