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 group of steel plates in which two or more steel plates are stacked, the average of the shortest distances from the center of a welding spot to the end surfaces of the steel plates being 3.0 mm or more, and the average distance between the centers of adjacent welding spots being 6.0 mm or more when a plurality of welding spots are present. At least one of the two or more steel sheets has a decarburized layer on the surface layer of the steel sheet, and the thickness of the decarburized layer between the base material portion and the welding heat-affected portion satisfies formula (1) on the steel sheet side having the decarburized layer. Tw / tb < 1.0... (1)

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 1 and 2 sandwiched between welding electrodes 4 and 5 is shown. According to this method, a spot-shaped weld 6 is obtained by utilizing resistance heat generated by a high-current welding current. This spot-shaped weld 6 is called a nugget, and is a portion where the two steel plates 1 and 2 melt and solidify at the contact point of the steel plates when a current is passed through the overlapping steel plates. The steel plates are thus joined to each other in a spot shape.

[0004] In order to ensure collision safety, it is required to improve the strength of the steel plate and the strength of the weld. There are many test methods for evaluating the strength of the resistance spot weld. As one of the common evaluation methods, the tensile shear test specified in JIS Z3136 can be cited. This is a test method for measuring the tensile shear strength (hereinafter referred to as "TSS") by applying a tensile load to the weld joint in the tensile shear direction.

[0005] In the resistance spot welding of a plate group formed by overlapping a plurality of steel plates including a surface-treated steel plate, Fig.10 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 a galvanized layer represented by an electroplated zinc layer and a hot-dip galvanized layer (including an alloyed hot-dip galvanized layer) on the surface of the base material (base steel sheet), and a galvanized alloy layer containing elements such as aluminum and magnesium in addition to zinc. The melting point of the galvanized layer and the galvanized alloy layer is lower than the melting point of the base material of the surface treated steel sheet, so there is the following problem.

[0006] 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 Fig.10 The surfaces of the steel plates 1 and 2 shown are in contact with the welding electrodes 4 and 5, and the surfaces of the steel plates 1 and 2 are in contact with each other.

[0007] As a countermeasure for such LME cracks, for example, there are the technologies of Patent Documents 1 to 4. 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.

[0008] 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).

[0009] 0.25×(10×t+2) / 50≤WT≤0.50×(10×t+2) / 50…(A)

[0010] 300-500×t+250×t 2 ≤HT…(B)

[0011] 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).

[0012] 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.

[0013] Patent Document 3 proposes a method in which 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.

[0014] In the joint structure and the method for manufacturing the joint structure disclosed in Patent Document 4, at least one of the plurality of steel plates to be resistance welded 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. It is proposed that the high-tensile steel plate has a decarburized layer between the zinc-based plating layer formed on the surface of at least one of the overlapping surface side and the welding electrode side and the base material, or on the overlapping surface adjacent to the zinc-based plating layer of the overlapping zinc-plated steel plate.

[0015] Prior art literature

[0016] Patent Literature

[0017] Patent Document 1: Japanese Patent Application Laid-Open No. 10-195597

[0018] Patent Document 2: Japanese Patent Application Publication No. 2003-103377

[0019] Patent Document 3: Japanese Patent Application Laid-Open No. 2003-236676

[0020] Patent Document 4: Japanese Patent Application Publication No. 2020-82102 Summary of the invention

[0021] 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.

[0022] 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.

[0023] 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.

[0024] Patent Document 4 suppresses LME cracks by providing a decarburized layer on the surface of the steel plate, but cracks are likely to occur during welding. The decarburized layer in the welded portion, which becomes a stress concentration portion during a tensile test, has not been studied.

[0025] Considering the viewpoint of joint strength and summarizing the common problems of Patent Documents 1 to 4, no technology is proposed for suppressing the occurrence of LME cracks while ensuring the strength of the welded portion of the high-strength steel plate.

[0026] 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 portion capable of achieving both ensuring the strength of the resistance spot weld portion in a high-strength steel plate and suppressing LME cracks, and a method for manufacturing the same.

[0027] The present inventors have conducted intensive studies to achieve the above-mentioned object.

[0028] Due to the complex influence of various factors, the effect of the present invention on cracks (LME cracks) generated during welding cannot be simply explained. However, 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. In particular, it is known that LME cracks are easily generated in the area with large local tensile stress when a pair of welding electrodes are opened after the power supply and pressurization of resistance spot welding are completed on the overlapping surface side of the steel plates where the steel plates are in contact with each other.

[0029] Therefore, by making the distance from the weld point to the end face of the steel plate and the interval between the weld points when there are multiple weld points both constant or above, the temperature and stress when the electrode is released are reduced. This is effective in suppressing the occurrence of LME cracks. In addition, it is believed that by using a steel plate having a decarburized layer on the surface of the steel plate, the tensile residual stress of the resistance spot weld can be reduced, and as a result, LME cracks can be suppressed.

[0030] However, even if there is a decarburized layer on the surface of the steel plate, if the decarburization of the surface of the steel plate on the side of the overlapping surface of the steel plates is significant, the strength of the resistance spot weld is also reduced. That is, the following idea was obtained: by controlling the thickness ratio of the decarburized layer of the steel plate surface of the base material part and the heat affected part within a certain range, it is possible to achieve both ensuring the strength of the resistance spot weld and suppressing LME cracks.

[0031] like Figure 3 (A)~ Figure 3 As shown in (C), a welded member formed by resistance spot welding a plate group including steel plates having decarburized layers on the surface of the steel plates has a decarburized layer on the surface of the lower plate on the steel plate overlap surface 7 side. Figure 3 (A)~ Figure 3In the example shown in (C), a steel plate having the above-mentioned decarburized layer on the lower plate is used. The decarburized layer exists in the area of ​​the weld heat affected zone (HAZ) and the base metal portion. Figure 3 As shown in (A), the decarburized layer in the region of the weld heat affected zone basically decreases as it approaches the nugget 6a. Figure 3 As shown in (B), part of the decarburized layer in the heat affected zone disappears completely due to the diffusion of elements such as C, or Figure 3 (C) Sometimes the decarburized layer of the weld heat affected zone hardly disappears but remains. Figure 3 (A)~ Figure 3 The decarburized layer existing in the weld heat affected zone region in the state shown in (C) is referred to as "the decarburized layer of the steel plate surface layer in the weld heat affected zone".

[0032] The present invention is based on the above findings, and the gist of the present invention is as follows.

[0033] [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,

[0034] The average value of the shortest distance from the center of the weld to the end surface of the steel plate is more than 3.0 mm.

[0035] When there are multiple solder joints, the average distance between the centers of adjacent solder joints is 6.0 mm or more.

[0036] At least one of the two or more steel plates is a steel plate having a decarburized layer on the surface of the steel plate,

[0037] And on the steel plate side having the above-mentioned decarburized layer, the thickness of the above-mentioned decarburized layer in the plate thickness direction from the steel plate surface of the weld heat affected zone at a position 400 μm away from the weld nugget end of the above-mentioned resistance spot weld part in parallel with the steel plate overlap surface in the direction of the base metal is denoted as tw (mm), and the thickness of the above-mentioned decarburized layer in the plate thickness direction from the steel plate surface of the base metal portion at a position away from the weld nugget end in parallel with the steel plate overlap surface in the direction of the base metal is denoted as tb (mm),

[0038] tw and tb satisfy equation (1).

[0039] tw / tb<1.0…(1)

[0040] [2] The welded member according to [1], wherein in the steel plate having the decarburized layer,

[0041] When the hardness of the decarburized layer of the steel plate surface layer of the weld heat affected zone at a position 400 μm away from the end of the weld nugget in parallel with the overlapped surface of the steel plates in the direction of the base material is defined as Hws, and the hardness of the weld nugget at a position 200 μm away from the end of the weld nugget in parallel with the overlapped surface of the steel plates in the direction of the inner part of the weld nugget is defined as Hn,

[0042] Hws, Hn, the above tw and the above tb satisfy the equations (2) and (3).

[0043] tw / tb<{(Hws / Hn)-0.1} / 7+0.8…(2)

[0044] 0.1≤Hws / Hn≤1.5…(3)

[0045] Among them, tw (mm) in formula (2) is the thickness of the above-mentioned decarburized layer of the above-mentioned welding heat affected zone at a position 400 μm away from the above-mentioned weld core end in parallel with the overlapping surface of the steel plate in the direction of the base material, and tb (mm) is the thickness of the above-mentioned decarburized layer of the above-mentioned base material part at a position away from the above-mentioned weld core end in parallel with the overlapping surface of the steel plate.

[0046] [3] The welded member according to [1] or [2], wherein, when the thickness of the steel plate is t (mm) and the tensile strength of the steel plate is TS (MPa),

[0047] The steel plate having the above-mentioned decarburized layer satisfies the formula (4).

[0048] -2×{(1000×t / TS)-0.25} / 35+0.2<tw / tb<2×{(1000×t / TS)-0.25} / 35+0.8…(4)

[0049] Among them, tw (mm) in formula (4) is the thickness of the above-mentioned decarburized layer of the above-mentioned welding heat affected zone at a position 400 μm away from the above-mentioned weld core end in parallel with the overlapping surface of the steel plate in the direction of the base material, and tb (mm) is the thickness of the above-mentioned decarburized layer of the above-mentioned base material part at a position away from the above-mentioned weld core end in parallel with the overlapping surface of the steel plate.

[0050] [4] The welded member according to any one of [1] to [3], wherein the steel plate having the decarburized layer has an Fe-based pre-plating layer and / or a Si internal oxide layer.

[0051] [5] A method for manufacturing a welded member, the method for manufacturing a welded member according to any one of [1] to [4], comprising:

[0052] 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.

[0053] In the above welding process,

[0054] The plate group is clamped by a pair of welding electrodes and connected by applying pressure and electricity.

[0055] For at least one welding spot, immediately before the welding electrode is pressed, one or more of the conditions (a) to (e) are satisfied.

[0056] Furthermore, for the weld point, when the distance from the center of the weld point to the end surface of the steel plate is set to Xe, the state of Xe: 3 to 30 mm is satisfied, and when there is an existing weld point adjacent to the weld point, when the distance between the weld point and the existing weld point is set to Xn, the state of Xn: 6 mm or more is satisfied,

[0057] The method further comprises: an energizing step, wherein the energizing is performed with a pressure of 2.0 to 10.0 kN, a welding current of 4.0 to 15.0 kA, and an energizing time of 0.1 to 2.0 s, and

[0058] 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 of formula (6).

[0059] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more,

[0060] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more,

[0061] (c) A gap of 0.5 mm or more exists between any welding electrode and the overlapping steel plates.

[0062] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates,

[0063] (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,

[0064] -(tb / 15)+0.025<Th<(tb / 1.5)+1…(6)

[0065] Here, tb (mm) in the formula (6) is the thickness of the decarburized layer of the base material portion at a position spaced apart from the end of the nugget and parallel to the overlapped surface of the steel plates.

[0066] [6] The method for manufacturing a welded member according to [5], wherein in the energizing step,

[0067] The tensile strength of the steel plate with the highest strength among the steel plates constituting the plate group is denoted as TSm (MPa), and the total plate thickness of the plate group is denoted as t all (mm), the energizing time of the energizing step is Ts (S), the pressure of the energizing step is F (kN), and the average value of the flow rate of the cooling water flowing through the welding electrodes arranged on the upper and lower sides of the plate group is L (l / min),

[0068] Ts satisfies formula (8).

[0069] Ts≤{0.03×t all ×L×√(TSm)} / F…(8)

[0070] 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 strength of a resistance spot weld portion due to decarburization of the surface layer of a steel plate and does not cause LME cracks even in resistance spot welding using a high-strength steel plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0071] Figure 1 This is a cross-sectional view schematically showing an example of the resistance spot welding of the present invention.

[0072] Figure 2 This 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.

[0073] Figure 3 (A)~ Figure 3 (C) is an enlarged cross-sectional view illustrating a decarburized layer on the surface of a steel plate in a weld heat affected zone.

[0074] Figure 4 This is a cross-sectional view in the plate thickness direction for explaining an example of a steel plate having a decarburized layer on the surface of the steel plate used in the present invention.

[0075] Figure 5 It is an explanatory diagram for explaining the shortest distance (H1) from the center of the weld point to the end surface of the steel plate in the welded member of the present invention.

[0076] Figure 6 It is an explanatory diagram for explaining the distance (H2) between the centers of adjacent weld spots in the welded member of the present invention.

[0077] Figure 7 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.

[0078] Figure 8 It is an explanatory diagram for explaining the distance (Xn) between the centers of the welding spots (between the centers of the welding spots and the existing welding spots) in the welding step in the manufacturing method of the present invention.

[0079] Fig. 9 It is an explanatory diagram for explaining the distance (Xe) from the center of the weld spot to the end surface of the steel plate in the welding step in the production method of the present invention.

[0080] Fig.10 This is a cross-sectional view schematically showing an example of crack generation during conventional resistance spot welding. DETAILED DESCRIPTION

[0081] Below, refer to Figures 1 to 9 , a welded member and a method for manufacturing the same as one embodiment of the present invention will be described. It should be noted that the present invention is not limited to this embodiment.

[0082] First, the welded member of the present invention will be described.

[0083] Figure 2 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 of its surroundings in the welded member. The position shown in this enlarged cross-sectional view is Figure 2 The area surrounded by a four-border in . Figure 3 (A)~ Figure 3 (C) is a cross-sectional view showing the decarburized layer in the heat affected zone of welding. Figure 2 The area surrounded by a four-border in . Figure 4 A cross-sectional view in the plate thickness direction of a steel plate having a decarburized layer used in the present invention is shown. Figure 5 and Figure 6 A plan view showing the periphery of a resistance spot weld portion of a welding member (ie, a view of the welding member as viewed from above) is shown.

[0084] like Figure 2 As shown, the present invention is a welded member 10 having a resistance spot welded portion (hereinafter referred to as a "welded portion") formed by resistance spot welding a plate group of two or more stacked steel plates.

[0085] The welding component 10 of the present invention is as follows Figure 5 and Figure 6 As shown, the average value (H1) of the shortest distance from the center of the weld spot of the weld to the end surface of the steel plate is 3.0 mm or more, and when there are multiple weld spots, the average distance (H2) between the centers of adjacent weld spots is 6.0 mm or more.

[0086] like Figure 5The above-mentioned "average value (H1) of the shortest distance 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".

[0087] When the average value (H1) of the shortest distance is less than 3.0 mm, the pressurization between the plates around the nugget on the end face of the steel plate becomes insufficient, and the molten metal easily splashes from the plate to the end face of the steel plate. Therefore, the spatter generation during welding becomes significant, and the nugget diameter easily deviates, thereby making the strength of the weld unstable. In order to enable welding with a general-shaped welding gun, the average value (H1) of the shortest distance is preferably 1000 mm or less.

[0088] The average value (H1) of the shortest distance is preferably 4.0 mm or more. In addition, the average value (H1) of the shortest distance is more preferably 100.0 mm or less, further preferably 30.0 mm or less, and further preferably 20.0 mm or less.

[0089] In addition, if Figure 6 The above-mentioned "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 6.0 mm, a shunt to the existing weld occurs during welding, and the current density of the weld portion is reduced, 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 likely to cause 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.

[0090] The average distance (H2) between the centers of the weld spots is preferably 10.0 mm or more. In addition, the average distance (H2) between the centers of the weld spots is more preferably 60.0 mm or less, and further preferably 40.0 mm or less.

[0091] 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 3.0 mm or more" among the above-mentioned weld point conditions is met.

[0092] In addition, in the welded member of the present invention, as described later, at least one of the two or more superimposed steel plates is a steel plate having a decarburized layer on the surface of the steel plate (see Figure 4 ). The number of the overlapping steel plates is not particularly limited, and may be 2 or more. Preferably, it is 3 or more. It should be noted that the upper limit of the number of the above-mentioned steel plates is not particularly specified, but is preferably 5 or less.

[0093] Figure 2 An example of a welded member 10 formed by welding two overlapping steel plates is shown. Both or either of the steel plate 2 (lower plate) disposed on the lower side and the steel plate 1 (upper plate) disposed on the upper side are steel plates having the above-mentioned decarburized layer. A welded portion 6 described below is formed on the overlapping surface 7 of the steel plates where the steel plates 1 and 2 are in contact.

[0094] exist Figure 2 In the case of the welded member of the example shown, only the lower plate 2 is a steel plate having a decarburized layer, and the upper plate 1 is a steel plate without a decarburized layer. In this case, the weld 6 of the present invention is formed only on the steel plate overlap surface 7 side of the lower plate 2. It should be noted that when the steel plate having a decarburized layer of the present invention is used for both the upper plate 1 and the lower plate 2, the weld 6 of the present invention is formed on both the upper plate 1 side and the lower plate 2 side of the steel plate overlap surface 7.

[0095] Although not shown in the figure, when three or more steel plates are overlapped and welded, all or at least one of the steel plate arranged at the bottom (i.e., the bottom plate), the steel plate arranged at the top (i.e., the top plate), and the steel plate arranged between them (i.e., the middle plate) is a steel plate having the above-mentioned decarburized layer. The welded portion of the present invention is formed in a manner including the overlapped surfaces of the steel plates between the steel plates.

[0096] For example, in the case of a plate set consisting of three steel plates, the welded portion of the present invention is formed in a manner including two overlapping surfaces of the steel plates where the lower plate contacts the middle plate and the middle plate contacts the upper plate. In the case where all three steel plates are steel plates having the above-mentioned decarburized layer, the thickness ratio of the decarburized layer of the steel plate surface layer on the upper and lower sides of the overlapping surface of each steel plate is controlled to be within a certain range.

[0097] <Welding Department>

[0098] 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 2 .

[0099] like Figure 2 As shown, the welded portion 6 of the present invention specifies the steel plate side having the decarburized layer (at Figure 2 In the example shown, it is the ratio of the thickness of the decarburized layer of the steel plate surface layer in the base material portion (on the lower plate 2 side) and the weld heat affected zone 6b.

[0100] Specifically, on the steel plate side having a decarburized layer, when the thickness of the decarburized layer in the heat affected zone in the plate thickness direction from the steel plate surface at a position 400 μm away from the weld nugget end of the resistance spot weld in parallel to the steel plate overlap surface in the parent material direction is denoted as tw (mm), and the thickness of the decarburized layer in the plate thickness direction from the steel plate surface of the parent material portion at a position away from the weld nugget end in parallel to the steel plate overlap surface in the parent material direction is denoted as tb (mm), the thickness of the decarburized layer in the heat affected zone (tw) and the thickness of the decarburized layer in the parent material portion (tb) satisfy the following formula (1).

[0101] tw / tb<1.0…(1)

[0102] When the ratio of the decarburized layer thickness of the heat affected zone and the base metal portion (tw / tb) shown in formula (1) is greater than 1.0, that is, when the decarburized layer thickness of the heat affected zone is greater than the decarburized layer thickness of the base metal portion, a coarse ferrite core structure is formed on the surface of the heat affected zone. As a result, the surface structure is prone to necking when a tensile load is applied, thereby reducing the joint strength. Therefore, the above-mentioned ratio of the decarburized layer thickness is less than 1.0.

[0103] On the other hand, if the decarburized layer of the weld heat affected zone disappears due to the diffusion of carbon, there is a concern that the LME suppression effect cannot be obtained. Therefore, the lower limit of the decarburized layer thickness ratio is preferably 0.1. It is more effective when the decarburized layer thickness ratio satisfies the relationship of 0.1<tw / tb<0.9, and it is more effective when the relationship of 0.2<tw / tb<0.8 is further preferably satisfied.

[0104] In addition, the above-mentioned decarburized layer thickness (tw, tb) can be measured by the method described in the Examples mentioned later.

[0105] With the above configuration, it is possible to achieve both ensuring the strength of the weld and suppressing LME cracks.

[0106] In the present invention, from the viewpoint of more effectively improving the effect of ensuring the strength of the welded portion and suppressing LME cracks, the present invention may further include the following configuration in addition to the above configuration.

[0107] The strength and fracture position of the weld are affected by the hardness of the nugget and the heat-affected zone. Therefore, by appropriately controlling the ratio of the decarburized layer thickness (tw / tb) according to the surface hardness of the nugget and the heat-affected zone, the effect of the present invention can be more effectively obtained.

[0108] Specifically, if Figure 2As shown, in a steel plate having a decarburized layer, when the hardness of the decarburized layer of the steel plate surface layer at a position 400 μm away from the end of the weld nugget in parallel with the overlapping surface of the steel plates toward the base material is set as Hws, and the hardness of the weld nugget at a position 200 μm away from the end of the weld nugget in parallel with the overlapping surface of the steel plates toward the inside of the weld nugget is set as Hn, it is preferred that the surface hardnesses of the weld nugget and the weld heat affected zone (Hn and Hws) and the decarburized layer thicknesses (tw and tb) of the weld heat affected zone and the base material portion satisfy the following equations (2) and (3).

[0109] tw / tb<{(Hws / Hn)-0.1} / 7+0.8…(2)

[0110] 0.1≤Hws / Hn≤1.5…(3)

[0111] Here, if Figure 2 As shown, the above-mentioned "steel plate surface layer of the welding heat affected zone" refers to a position 400 μm away from the weld core end 6c and the steel plate overlapping surface 7 in the direction of the base material in parallel, and a position on the steel plate overlapping surface 7 is taken as the starting point A and a position 20 μm away from the starting point A in the plate thickness direction of the steel plate 2 side having the decarburized layer.

[0112] The greater the ratio of the hardness of the nugget and the surface layer of the weld heat affected zone (Hws / Hn), that is, the ratio of the hardness of the decarburized layer of the weld heat affected zone to the hardness of the nugget on the overlapping surface side of the steel plate, the greater the strain of the nugget is relatively, and the less the strain concentration on the weld heat affected zone is. Therefore, even if the decarburization of the weld heat affected zone is significant, fracture in the nugget is likely to occur. Therefore, the upper limit of the above-mentioned ratio of the decarburized layer thickness (tw / tb) becomes larger.

[0113] In addition, excessive softening of the decarburized layer causes a decrease in the strength of the weld, while excessive hardening of the decarburized layer causes LME cracks due to an increase in residual stress. Therefore, it is preferable to set appropriate upper and lower limits for the ratio of the surface layer hardness (Hws / Hn).

[0114] When the ratio of the surface hardness (Hws / Hn) shown in formula (3) is less than 0.1, the hardness of the heat-affected zone is excessively reduced relative to the nugget, thereby increasing the strain concentration on the heat-affected zone and reducing the strength of the weld. On the other hand, when the ratio of the surface hardness (Hws / Hn) exceeds 1.5, the high-temperature residual stress of the weld heat-affected zone becomes too large, thereby easily generating LME cracks. The ratio of the surface hardness is more preferably 0.3 or more, and more preferably 1.0 or less.

[0115] For the above reasons, the ratio of the surface hardness of the nugget to that of the heat-affected zone and the ratio of the decarburized layer thickness of the heat-affected zone to that of the base material preferably satisfy equations (2) and (3).

[0116] The lower limit of the formula (2) is not particularly specified. From the viewpoint of suppressing LME cracks by appropriately ensuring the thickness of the decarburized layer in the heat-affected zone, the lower limit of the formula (2) is preferably 0.15 or more.

[0117] The lower limit of the decarburized layer thickness ratio (tw / tb) represented by the formula (2) is more preferably 0.20 or more, and the upper limit of the decarburized layer thickness ratio is more preferably 0.80 or less.

[0118] It should be noted that the surface hardness (unit: GPa) can be measured by a nanoindentation hardness test as described in the examples described below. The nanoindentation hardness test method is not limited in the present invention, but can be performed using, for example, a nanoindentation device (TI-950 Tribo Indenter manufactured by Hysitron Corporation) in accordance with ISO 14577.

[0119] <Steel Plate>

[0120] [Board thickness, tensile strength]

[0121] The strength and residual stress of the weld are affected by the thickness and tensile strength of the overlapping steel plates. Therefore, in the present invention, it is preferred to appropriately control the ratio of the thickness of the decarburized layer of the weld heat affected zone and the base material portion (tw / tb) according to the plate thickness and tensile strength. In this way, the effect of the present invention can be further effectively obtained.

[0122] Specifically, when the plate thickness of the steel plate is t (mm) and the tensile strength of the steel plate is TS (MPa), the steel plate having a decarburized layer preferably satisfies the following formula (4).

[0123] -2×{(1000×t / TS)-0.25} / 35+0.2<tw / tb<2×{(1000×t / TS)-0.25} / 35+0.8…(4)

[0124] Among them, tw (mm) in formula (4) is the thickness of the decarburized layer of the weld heat affected zone at a position 400 μm away from the end of the weld nugget in parallel with the overlap surface of the steel plates in the direction of the base material, and tb (mm) is the thickness of the decarburized layer of the base material part at a position away from the end of the weld nugget in parallel with the overlap surface of the steel plates.

[0125] In addition, from the viewpoint of more appropriately controlling the above-mentioned decarburized layer hardness ratio (Hws / Hwt), in addition to the conditional expression ((Expression) 4), it is more preferable to satisfy the following Expression (5).

[0126] 0.25≤1000×t / TS≤3.75…(5)

[0127] When the thickness (t) of the steel plate having a decarburized layer is large, the cross-sectional area of ​​the weld heat affected zone becomes large, so when a tensile load is applied to the weld, fracture is more likely to occur in the nugget rather than in the weld heat affected zone. As a result, even if the surface layer of the weld heat affected zone is significantly decarburized, the strength of the weld is unlikely to decrease. Therefore, the larger the thickness (t) of the steel plate, the smaller the lower limit of tw / tb is preferably.

[0128] On the other hand, when the tensile strength (TS) of the steel plate with a decarburized layer is large, martensitic transformation occurs due to the heat effect during welding, so the weld heat affected zone is generally easy to harden. It is believed that this is due to the following differences: the weld heat affected zone is transformed into martensite inside the steel plate, while martensitic transformation is not easy to occur on the surface of the steel plate due to decarburization, or even if martensitic transformation occurs, the transfer density is low. Therefore, the difference in structure and / or hardness between the soft layer on the surface of the steel plate and the inside of the steel plate becomes larger, thereby the strain concentration to the soft part of the surface of the steel plate becomes significant, and the strength of the weld is easily reduced. That is, the greater the tensile strength (TS) of the steel plate, the greater the lower limit of the preferred tw / tb.

[0129] In addition, if we focus on the upper limit of tw / tb, the larger the thickness (t) of the steel plate having the decarburized layer, the smaller the cooling rate after welding, and the less likely it is that the weld heat affected zone will become embrittled. Therefore, the larger the thickness (t) of the steel plate, the larger the upper limit of tw / tb is preferably.

[0130] On the other hand, when the tensile strength (TS) of the steel plate having a decarburized layer is large and there is interference in welding construction, the residual stress generated in the welded portion is likely to increase, resulting in the generation of LME cracks. Therefore, the greater the tensile strength (TS) of the steel plate, the smaller the upper limit of tw / tb is preferably.

[0131] For the above reasons, from the viewpoint of the plate thickness (t) and the tensile strength (TS) of the steel plate, tw / tb preferably satisfies the formula (4).

[0132] The lower limit of the formula (4) is preferably not less than -2×{(1000×t / TS)-0.25} / 35+0.3. The upper limit of the formula (4) is preferably not more than 2×{(1000×t / TS)-0.25} / 35+0.7.

[0133] In addition, for the above-mentioned reasons, it is more preferable to further satisfy the formula (5). The lower limit of the formula (5) is preferably 0.50 or more, and the upper limit of the formula (5) is preferably 3.00 or less.

[0134] As described above, generally, as the strength and alloy content of steel plates increase, LME cracks tend to occur more easily. Therefore, in the present invention, the tensile strength of the steel plate having a decarburized layer is preferably TS ≥ 980 (MPa). This is because in the case of a high-strength steel plate that satisfies this relationship, the effect of the present invention can be more effectively obtained.

[0135] [Steel plate with decarburized layer]

[0136] In the present invention, as described above, at least one of the two or more stacked steel plates is a steel plate having a decarburized layer on the surface of the steel plate. Figure 4 , a steel plate having a decarburized layer on the surface of the steel plate is described. Figure 4 An example of a steel sheet having a decarburized layer on the surface of the steel sheet is shown. Examples of the steel sheet having a decarburized layer include a galvanized steel sheet having a galvanized layer on the decarburized layer and a steel sheet having a Si internal oxide layer as the outermost layer in the decarburized layer.

[0137] Regarding the steel sheet used in the present invention, there is no particular limitation on the method of providing a decarburized layer on the surface of the steel sheet.

[0138] It should be explained that Figure 4 As an example, a steel sheet having a structure including a decarburized layer, a Si internal oxide layer, a zinc plating layer, and a Fe-based pre-plating layer described later is shown.

[0139] [Galvanized steel sheet]

[0140] LME cracks are a phenomenon caused by applying tensile stress when molten zinc is in contact with a steel sheet. Therefore, in the present invention, when the steel sheet having the above-mentioned decarburized layer is a galvanized steel sheet, the effect of the present invention can be obtained more effectively. In particular, considering the compressibility and continuous spot weldability of the steel sheet, the galvanized steel sheet is further preferably an alloyed hot-dip galvanized steel sheet (GA steel sheet) whose coating layer is alloyed hot-dip galvanizing, a hot-dip galvanized steel sheet (GI steel sheet) whose coating layer is hot-dip galvanizing, and an electrogalvanized steel sheet (EG steel sheet) whose coating layer is electrogalvanized.

[0141] like Figure 4 As shown, the galvanized steel sheet may have an Fe-based pre-plating layer between the decarburized layer and the galvanized layer. By forming a Fe-based pre-plating layer with a low C content, the C content of the steel sheet surface can be reduced. The C content in the Fe-based pre-plating layer is preferably 0.20% by mass or less. The C content is more preferably 0.10% by mass or less, and further preferably 0.05% by mass or less.

[0142] The pre-plated layer is preferably an Fe-based electroplated layer. As the Fe-based electroplated 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 electroplated layer is not particularly limited, but in the present invention, it is preferably composed of the following components: 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 electroplated layer at a low cost.

[0143] Furthermore, the deposition amount of the Fe-based electroplating layer on one side is preferably 0.5 g / m 2 The adhesion amount is more preferably 1.0 g / m 2 The upper limit of the amount of Fe-based electroplating layer deposited on one side is not particularly limited, but from the perspective of cost, it is preferred that the amount of Fe-based electroplating layer deposited on one side is 60 g / m 2 The adhesion amount is preferably 50 g / m 2 Below, more preferably 40g / m 2 Below, more preferably 30g / m 2 the following.

[0144] 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 (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. Then, the average value of the thickness of the three fields of view is multiplied by the specific gravity of iron, thereby converting it into the amount of adhesion on a single side of the Fe-based electroplating layer.

[0145] In the present invention, it is preferred to perform Fe-based electroplating treatment on the surface of the cold-rolled steel sheet before high-strength annealing to prepare the steel sheet treated with Fe-based electroplating 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 can also be subjected to Fe-based electroplating treatment without performing oxidation treatment in a preheating furnace or the like.

[0146] 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.

[0147] 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.

[0148] [Si internal oxide layer]

[0149] For example, it is known that LME cracks are more likely to occur when the amount of Si in steel increases. In the Fe-Zn phase diagram, Si lowers the solidus temperature at the same Zn concentration and moves the solidus concentration toward the Fe side. Therefore, during resistance spot welding, when the zinc coating on the surface of the steel plate melts and Zn diffuses into the steel plate, in steel plates with a high Si content, the steel plate and liquid Zn are in contact for a long time until they are in a lower temperature and low Zn concentration area, which is believed to be more likely to cause LME cracks. Figure 4 As shown, in the case of a steel plate in which an Si internal oxide layer is contained in the decarburized layer, by increasing the dew point during annealing in the production of the steel plate, an Si internal oxide layer can be formed on the outermost surface of the steel plate. By using such a steel plate, the effect of the present invention can be more effectively obtained. When Si internal oxidation occurs, Si-based oxides containing Si and O as main components are generated, and the amount of solid-dissolved Si near the surface of the steel plate is reduced. Therefore, it is effective in suppressing LME cracks.

[0150] 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.

[0151] Next, one embodiment of the method for producing a welded member according to the present invention will be described.

[0152] 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.

[0153] <Preparation process>

[0154] In this step, two or more steel plates including at least one steel plate having the decarburized layer are prepared, and the two or more steel plates are stacked to form a plate set. Figure 1 An example of resistance spot welding of two steel plates is shown. Figure 1 In the example shown, two steel plates 1 and 2 are stacked to form a plate set. It should be noted that the steel plates have been described above, so the description thereof is omitted. Next, a welding process is performed.

[0155] <Welding process>

[0156] The welding process includes a power supply process and an electrode holding process described later. In this welding process, the plate group prepared in the preparation process is joined. In this process, for example, Figure 1 As 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 1 and 2, the steel plates can be joined to each other (see FIG. 1 ). Figure 2 ).

[0157] It should be noted that when a steel plate having a decarburized layer and a steel plate without a decarburized layer are used to form a plate set, the plates are overlapped in such a manner that the surface side having the decarburized layer is the overlapping surface of the steel plates. When the steel plate having a decarburized layer is a galvanized steel plate (GI steel plate, GA steel plate, EG steel plate) having a galvanized layer, the plates are overlapped in such a manner that the surface side having the galvanized layer is the overlapping surface of the steel plates. It should be noted that there is no problem even if the surface of the steel plate on the side in contact with the electrode also has a decarburized layer and / or a galvanized layer.

[0158] 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 electrodes and can arbitrarily control the pressure and welding current during welding can be used. In addition, the present invention can be applied to both direct current and alternating current welding power sources. In the case of alternating current, "current" means "effective current". 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), etc. 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.

[0159] Next, welding conditions in the welding process of the present invention will be described.

[0160] In the welding process, when the plate group is clamped by a pair of welding electrodes and joined while applying pressure and electricity, for at least one weld point, before the welding electrode is used to apply pressure, one or more of the following conditions (a) to (e) are satisfied, and when the distance from the center of the weld point to the end face of the steel plate is set to Xe, the condition of Xe: 3 to 30 mm is satisfied, and when there is a weld point and an existing weld point adjacent to the weld point, when the distance between the weld point and the existing weld point is set to Xn, the condition of Xn: 6 mm or more is satisfied, and there are: an electricity-applying process, in which the electricity is applied with a pressure of 2.0 to 10.0 kN, a welding current of 4.0 to 15.0 kA, and an electricity-applying time of 0.1 to 2.0 S; and an electrode holding process, in which the pressure holding time after the end of the electricity is set to Th (S), Th satisfies the relationship of formula (6).

[0161] [Electrification process]

[0162] [Pressure, welding current, power-on time]

[0163] If the pressure 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 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 part becomes significant, so LME cracks occur or the joint strength decreases. The pressure is preferably 3.0 kN or more, and preferably 7.0 kN or less.

[0164] If the welding current 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 easily generated, so LME cracks are easily generated. The welding current is preferably 5.0 kA or more and preferably 12.0 kA or less.

[0165] If the energization time is less than 0.1S, the diffusion of alloy elements in the heat affected zone may become insufficient, and the thickness of the decarburized layer in the heat affected zone may be almost the same as that of the base material. In this case, the structure of the coarse ferrite core is formed on the surface of the heat affected zone. As a result, the surface structure is prone to necking when a tensile load is applied, so TSS is reduced.

[0166] On the other hand, if the power-on time exceeds 2.0 seconds, the cycle time of the automobile assembly process becomes longer and productivity decreases. The power-on time is preferably 0.10 seconds or more. The power-on time is more preferably 0.12 seconds or more, and preferably 1.5 seconds or less.

[0167] 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.

[0168] [Welding construction interference]

[0169] 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.

[0170] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more

[0171] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more

[0172] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0173] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0174] (e) The shortest distance from the center of the weld to the end surface of the overlapping steel plates is less than 10 mm

[0175] These welding process 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 process disturbances, thereby increasing the margin for process disturbance management during the manufacture of welded components. The following describes the details of each process disturbance.

[0176] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more

[0177] 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. As a result, the tensile stress after the electrode is released increases, and LME cracks are easily generated. 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.

[0178] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more

[0179] 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.

[0180] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0181] When there is a gap between any welding electrode and the steel plate just before the start of pressurization, for example, when one welding electrode is movable (hereinafter referred to as the movable side electrode) and the other welding 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 portion, 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 metal nugget becomes unstable, which becomes a cause of spatter generation, so the gap amount is preferably 5 mm or less. The gap amount is more preferably 1 mm or more, and more preferably 3 mm or less.

[0182] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0183] 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.

[0184] (e) The shortest distance from the center of the weld to the end surface of the overlapping steel plates is less than 10 mm.

[0185] 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.

[0186] [Soldering points]

[0187] Furthermore, in the present invention, in addition to the above conditions, it is also necessary to Figure 8 and Fig. 9 As shown, when the distance from the center of the weld to the end face of the steel plate is set to Xe, the state of Xe: 3 to 30 mm is satisfied, and when there is a weld and an existing weld adjacent to the weld, when the distance between the weld and the existing weld is set to Xn, the state of Xn: greater than 6 mm is satisfied.

[0188] The above-mentioned “distance from the center of the weld to the end face of the steel plate (Xe)” is as follows Fig. 9 The distance between the end face of the steel plate closest to the center of the welding spot 8 and the distance between the end face of the steel plate and the center of the welding spot 8 is shown in FIG. Figure 8 The figure shows the distance between the centers of adjacent solder joints (solder joint 8 and existing solder joint 9).

[0189] It should be explained that Figure 8 and Fig. 9 It is a plan view showing the vicinity of a welded portion of a member (a view of the member viewed from above).

[0190] If the distance (Xe) from the center of the weld point to the end surface of the steel plate is less than 3 mm, spatter generation becomes significant and LME is likely to occur. On the other hand, if the distance (Xe) exceeds 30 mm, welding itself becomes difficult using a small welding gun, requiring a large welding gun, thereby causing equipment restrictions. The distance (Xe) is preferably 3.0 mm or more, and more preferably 4.0 mm or more.

[0191] If the distance (Xn) between the weld and the existing weld adjacent to the weld is less than 6 mm, the weld is constrained by the existing weld and the residual stress increases, which makes LME cracks more likely to occur. Although there is no upper limit for the distance (Xn), in order to ensure the strength and rigidity of the component, the distance (Xn) is preferably 200 mm or less. The distance (Xn) is preferably 6.0 mm or more, and more preferably 10.0 mm or more. In addition, the distance (Xn) is more preferably 40.0 mm or less.

[0192] In the present invention, in addition to the above-mentioned conditions of the energization step, the following conditions may be further met.

[0193] [Power-on time] (preferred condition)

[0194] Specifically, in the current-applying step of the welding process, the tensile strength of the steel plate with the highest strength among the steel plates constituting the plate set is denoted as TSm (MPa), and the total plate thickness of the plate set is denoted as t all (mm), the energizing time of the energizing process is Ts (S), the pressure of the energizing process is F (kN), and the average value of the flow rate of cooling water flowing through the welding electrodes arranged on the upper and lower sides of the plate group is L (l / min), preferably Ts satisfies the following formula (8). Thereby, the effect of the present invention can be obtained more effectively.

[0195] Ts≤{0.03×t all ×L×√(TSm)} / F…(8)

[0196] When the formula (8) is not satisfied, the current-on time (Ts) during welding is too long, which easily causes softening of the weld heat-affected zone and disappearance of the decarburized layer due to carbon diffusion.

[0197] The power-on time (Ts) preferably satisfies Ts≤{0.02×t all The relationship of ×L×√(TSm)} / F is more effective, and it is more preferable to satisfy Ts≤{0.014×t all The relationship of} ×L×√(TSm) / F is further valid.

[0198] The energization time is more preferably [0.13×√{t all×(1-tb)}]S or more, more preferably [0.17×√{t all ×(1-tb)}]S or more. This is because, as the total plate thickness increases, the power-on time required to ensure the nugget diameter increases; in addition, as tb, which is the thickness of the decarburized layer of the parent material portion, decreases, the LME cracks are more likely to occur, and thus the power-on time required to discharge the Zn plating from the corona joint and promote alloying increases. The power-on time is more preferably more than 0.30S.

[0199] [Electrode holding process]

[0200] 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.

[0201] 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 energization is set as Th(S), the pressure holding time (Th) satisfies the relationship of the following formula (6). Thus, the effect of the present invention can be effectively obtained.

[0202] -(tb / 15)+0.025<Th<(tb / 1.5)+1…(6)

[0203] Here, tb (mm) in the formula (6) is the thickness of the decarburized layer of the base material portion at a position spaced apart from the end of the nugget and parallel to the overlapped surface of the steel plates.

[0204] In addition, from the viewpoint of more appropriately controlling the temperature, in addition to the conditional expression (ie, (Expression 6), it is more preferable to satisfy the following Expression (7).

[0205] 0<Th…(7)

[0206] 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.

[0207] In particular, as the thickness of the decarburized layer (tb) of the base material portion decreases, the sensitivity to LME and delayed fracture increases, and therefore the range that satisfies the upper and lower limits of the pressure holding time (Th) becomes narrower.

[0208] For such reasons, the pressure holding time is a time that satisfies the formula (6). The lower limit of the formula (6) is more preferably not less than -(tb / 15)+0.050. The upper limit of the formula (6) is more preferably not more than (tb / 1.5)+0.8.

[0209] In addition, for the above reasons, it is more preferable to further satisfy the formula (7). The lower limit of the formula (7) is more preferably 0.02 or more, and more preferably 0.17 or more. The lower limit of the formula (7) is more preferably 1.0 or less.

[0210] Example

[0211] Hereinafter, the functions and effects of the present invention will be described using examples. However, the present invention is not limited to the following examples.

[0212] Using the plate group shown in Table 1, a welded joint (welded component) was made under the welding conditions shown in Table 3. 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. "None" shown in the "Type of galvanizing" column of Table 1 refers to a steel plate (cold-rolled steel plate) without a coating. In addition, the symbols shown in the "State just 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.

[0213] 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.

[0214] The TSS and the presence of LME cracks in the welded joint were evaluated by the following method. In addition, the thicknesses (tb, tw, t all ) and surface hardness (Hn, Hws).

[0215] [Measurement of each thickness]

[0216] 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 sum as the total plate thickness.

[0217] In addition, the thickness of the decarburized layer (tb, tw) of the weld heat affected zone and the base metal portion was measured using the cross section. Specifically, the carbon content was measured by electron beam microanalysis described in JIS G 0558, and the thickness of the decarburized layer was measured in the base metal portion and the weld heat affected zone at a position 400 μm away from the end of the nugget of the resistance spot weld in parallel to the overlapped surface of the steel plate in the base metal direction. In addition, the measurement position of the base metal portion was performed at a position 5000 μm away from the intersection of the outer edge of the weld heat affected zone and the overlapped surface 7 of the steel plate in the base metal direction.

[0218] Using the obtained values, the "tw / tb value" was determined as a value indicating the decarburized layer thickness ratio in each weld joint. Here, the "tw / tb" values ​​of the steel plates listed in the "Evaluated Steel Plate" column of Table 2 are listed in Table 2 as representative values.

[0219] [Measurement of surface hardness]

[0220] The welded joint was cut with a micro-cutter in a manner that passed through the center of the welded part of the welded joint, and the hardness test of the weld heat affected zone and the nugget was performed. The hardness test was performed by a nanoindentation hardness test. In the nanoindentation hardness test, a triangular pyramidal diamond indenter was pressed into the measurement position with an indentation load of 500μN, and a load-displacement curve was obtained from the start of loading to the completion of unloading, and then the hardness was calculated using the Oliver-Pharr analysis method.

[0221] The hardness of the nugget is determined as follows. Specifically, Figure 2 As shown, on the steel plate side with decarburized layer ( Figure 2 In the example shown, the lower plate 2 side) is measured for the hardness (Hn) of the nugget at a position 200 μm away from the nugget end 6 c of the weld and parallel to the steel plate overlap surface 7 toward the inside of the nugget.

[0222] In addition, the surface hardness of the weld heat affected zone is measured as follows. Figure 2 As shown, the steel plate side with decarburized layer (at Figure 2 In the example shown, the hardness (Hws) is measured at a position on the lower plate 2 side) that is 400 μm away from the weld core end 6c of the weld in the direction of the base material in parallel with the overlapping surface 7 of the steel plates, and at a position on the overlapping surface 7 of the steel plates as the starting point A and 20 μm away from the starting point A in the direction of the plate thickness.

[0223] The obtained values ​​were used to determine the value of "Hws / Hn" as a value indicating the hardness difference in each welded joint. Here, the values ​​of "Hws / Hn" of the steel plates listed in the "Evaluated Steel Plate" column of Table 2 are listed in Table 2 as representative values.

[0224] [TSS's evaluation]

[0225] The tensile shear strength (TSS) was evaluated 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 and a welded joint (test piece) was prepared by resistance spot welding under the plate set shown in Table 1 and the welding conditions shown in Table 3.

[0226] In addition, in order to compare with each test piece obtained (welded joint produced by the welding method of the present invention), a comparative joint in which the surface properties of the welded portion were not controlled (i.e., a welded joint in which the decarburized layer thickness ratio was not controlled) was also produced. The comparative joint was also evaluated for TSS based on the same tensile shear test method.

[0227] The test piece shape for TSS evaluation is based on JIS Z3136, but in the case of three overlapping plate groups (plate group No. H in Table 1, etc.), Figure 7 As shown in FIG. 1 , the evaluation was performed by applying a tensile load between the steel plates 2 and 3. Figure 7 The direction of the thick arrow shown in FIG. 1 is referred to as the “tensile load direction”.

[0228] In the preparation of the welded joint for TSS evaluation, only one weld point was used, and welding operation disturbances such as the electrode inclination angle and plate gap shown in Tables 2 and 3 were not set.

[0229] Then, TSS was evaluated according to the following criteria.

[0230] <Evaluation Criteria>

[0231] A: (TSS of the welded joint produced by the welding method of the present invention) / (TSS of the comparative joint) ≥ 0.9

[0232] B: 0.9>(TSS of welded joint produced by the welding method of the present invention) / (TSS of comparative joint)≥0.8

[0233] F: 0.8 > (TSS of the welded joint produced by the welding method of the present invention) / (TSS of the comparative joint)

[0234] Here, when the evaluation results were A or B, the evaluation was acceptable (having excellent shear tensile strength).

[0235] [Evaluation of LME cracks]

[0236] Welding is performed in a state with one or more of the welding construction interferences (a) to (e) above, and the obtained welded joint is cut with a micro-cutter in a manner passing through the center of the welded portion, and then the cross-section of the welded portion in the plate thickness direction is observed. 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-section observations are performed on the overlapping surface side of the steel plates between the steel plates to confirm LME cracks. It should be noted that cross-section observations and evaluations are performed in the welded portion of the steel plate described in the "Evaluation Steel Plate" column of Table 2.

[0237] In particular, in the welding tests where the column "Existing welds" in Table 3 is "Yes", in addition to the welding interferences (a) to (e) above, Figure 8 As shown, in a state where there are already two weld points 9 around the weld point (weld point 8), welding is performed with the weld point interval (Xn (mm)) between the two adjacent points being the value shown in Table 3. In this welding test, the number of weld points on the test piece is 1 or 3, but it can be considered that the same is true when a plurality of weld points are performed on a welded component such as an automobile component.

[0238] <Evaluation Criteria>

[0239] A: All 10 pieces without cracks

[0240] 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.

[0241] Here, when the evaluation results are A and B, the evaluation is passed.

[0242] The obtained values ​​and evaluation results are shown in Tables 2 and 3, respectively.

[0243]

[0244]

[0245]

[0246] 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.

[0247] Explanation of symbols

[0248] 1, 2, 3 Steel Plate

[0249] 4.5 Welding electrodes

[0250] 6 Welding Department

[0251] 6a Nugget

[0252] 6b Heat affected zone

[0253] 6c End of nugget

[0254] 7. Overlapping surface of steel plates

[0255] 8 Soldering points

[0256] 9 Existing solder joints

[0257] 10 Welded components

Claims

1. A welded component having a resistance spot welded portion formed by resistance spot welding a plate group of two or more stacked steel plates, The average value of the shortest distance from the center of the weld to the end surface of the steel plate is 3.0 mm or more. When there are multiple solder joints, the average distance between the centers of adjacent solder joints is 6.0 mm or more. At least one of the two or more steel plates is a steel plate having a decarburized layer on the surface of the steel plate, And on the steel plate side having the decarburized layer, the thickness of the decarburized layer in the heat affected zone of the resistance spot welded portion in the plate thickness direction from the steel plate surface at a position 400 μm away from the weld nugget end of the resistance spot welded portion in parallel to the steel plate overlap surface in the parent material direction is denoted as tw, and the thickness of the decarburized layer in the plate thickness direction from the steel plate surface of the parent material portion in the position away from the weld nugget end in parallel to the steel plate overlap surface in the parent material direction is denoted as tb, tw and tb satisfy formula (1), the units of tw and tb are mm, tw / tb<1.0…(1).

2. The welding component according to claim 1, wherein: In the steel plate having the decarburized layer, When the hardness of the decarburized layer of the steel plate surface layer of the weld heat affected zone at a position 400 μm away from the end of the weld nugget in parallel with the steel plate overlap surface in the direction of the base material is defined as Hws, and the hardness of the weld nugget at a position 200 μm away from the end of the weld nugget in parallel with the steel plate overlap surface in the direction of the inner part of the weld nugget is defined as Hn, Hws, Hn, the tw and the tb satisfy equations (2) and (3), tw / tb<{(Hws / Hn)-0.1} / 7+0.8…(2) 0.1≤Hws / Hn≤1.5…(3) Among them, tw in formula (2) is the thickness of the decarburized layer in the heat affected zone of the weld at a position 400 μm away from the end of the weld core in parallel with the overlapping surface of the steel plate in the direction of the base material, and the unit is mm; tb is the thickness of the decarburized layer in the base material part at a position away from the end of the weld core in parallel with the overlapping surface of the steel plate, and the unit is mm.

3. The welding member according to claim 1 or 2, wherein: When the plate thickness of the steel plate is t and the tensile strength of the steel plate is TS, The steel plate having the decarburized layer satisfies the formula (4), the unit of t is mm, the unit of TS is MPa, -2×{(1000×t / TS)-0.25} / 35+0.2<tw / tb<2×{(1000×t / TS)-0.25} / 35+0.8…(4) Among them, tw in formula (4) is the thickness of the decarburized layer in the heat affected zone of the weld at a position 400 μm away from the end of the weld nugget in the direction of the parent material in parallel with the overlapping surface of the steel plate, and the unit is mm; tb is the thickness of the decarburized layer in the parent material part at a position away from the end of the weld nugget in parallel with the overlapping surface of the steel plate, and the unit is mm.

4. The welded member according to any one of claims 1 to 3, wherein The steel sheet having the decarburized layer has a Fe-based pre-plating layer and / or a Si internal oxide layer.

5. A method for manufacturing a welded member, the method for manufacturing a welded member according to any one of claims 1 to 4, 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 spot, immediately before the welding electrode is pressed, one or more of the conditions (a) to (e) are satisfied. Furthermore, for the weld point, when the distance from the center of the weld point to the end surface of the steel plate is set to Xe, the state of Xe: 3 to 30 mm is satisfied, and when there is an existing weld point adjacent to the weld point, when the distance between the weld point and the existing weld point is set to Xn, the state of Xn: 6 mm or more is 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 of formula (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, -(tb / 15)+0.025<Th<(tb / 1.5)+1…(6) in, In the formula (6), tb is the thickness of the decarburized layer of the base material portion at a position spaced apart from the end of the nugget in parallel with the overlapped surface of the steel plates, and the unit is mm.

6. The method for manufacturing a welded member according to claim 5, wherein: In the energizing process, The tensile strength of the steel plate with the highest strength among the steel plates constituting the plate group is denoted as TSm, and the total plate thickness of the plate group is denoted as t all , when the energization time of the energization process is set to Ts, the pressure of the energization process is set to F, and the average value of the flow rate of the cooling water flowing through the welding electrodes arranged on the upper and lower sides of the plate group is set to L, Ts satisfies formula (8), the unit of TSm is MPa, and the t all The unit of is mm, the unit of Ts is S, the unit of F is kN, the unit of L is l / min, Ts≤{0.03×t all ×L×√(TSm)} / F…(8).

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