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 steel plate having a softened layer on the surface layer of the steel plate, and in the base material portion on the steel plate side having the softened layer, the thickness (tall) of the steel plate is less than 1. The nanoindentation hardness Hbs in the surface layer of the steel sheet and the nanoindentation hardness Hbt in the interior of the steel sheet satisfy formula (2). (1) 0.5 < tweld / tall < 1.0 (1); (2) 0.2 < Hbs / Hbt < 0.8 (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 1 and 2 by clamping them with 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, Figure 8 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 Figure 8 The surfaces of the steel plates 1 and 2 shown are those on which the welding electrodes 4 and 5 are in contact, and the surfaces of the steel plates 1 and 2 are those on which the steel plates 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 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.

[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] Prior art literature

[0015] Patent Literature

[0016] Patent Document 1: Japanese Patent Application Laid-Open No. 10-195597

[0017] Patent Document 2: Japanese Patent Application Publication No. 2003-103377

[0018] Patent Document 3: Japanese Patent Application Laid-Open No. 2003-236676 Summary of the invention

[0019] 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.

[0020] 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.

[0021] 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.

[0022] Considering the viewpoint of joint strength and summarizing the common problems of Patent Documents 1 to 3, 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.

[0023] 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.

[0024] The present inventors have conducted intensive studies to achieve the above-mentioned object.

[0025] 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 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-on and pressurization of the resistance spot welding are completed, LME cracks are easily generated in the area with large local tensile stress. Therefore, as described in the following reference 1, it is believed that by using a steel plate having a softened layer on the surface of the steel plate as an overlapping steel plate, the tensile residual stress of the resistance spot weld can be reduced, and as a result, the LME crack can be suppressed.

[0026] [Reference 1] International Publication No. 2021 / 019947

[0027] However, even if there is a softened layer on the surface of the steel plate, if the heat effect during welding causes the structure of the weld heat affected zone to change or alloy elements such as C to diffuse, and the softened layer on the surface of the steel plate in the weld heat affected zone disappears, the target effect of the present invention cannot be obtained. In addition, when the surface of the weld heat affected zone, especially the surface of the steel plate on the overlapping surface side of the steel plates, is significantly soft, the strength of the resistance spot weld is reduced. Therefore, the inventors of the present invention have the following idea: by controlling the hardness of the softened layer on the surface of the steel plate in the weld heat affected zone within a certain range, it is possible to ensure the strength of the resistance spot weld and suppress LME cracks.

[0028] Here, use Figure 3 The above-mentioned “softened layer on the surface of the steel plate in the weld heat affected zone” will be described. Figure 3 (A)~ Figure 3 (C) indicates the following Figure 2 The area enclosed by the rectangular frame shown in the figure is an enlarged cross-sectional view of the heat affected zone (HAZ) of the welded component and its surroundings. Figure 3 The figures are used to illustrate the softening layer, so the Figure 2 Records between the plates shown. Figure 3 In the examples shown in the figures of , the steel plate arranged on the lower side (ie, the lower plate) is the "steel plate having a softened layer on the surface of the steel plate" used in the present invention.

[0029] like Figure 3 (A)~ Figure 3 As shown in (C), the welded member formed by resistance spot welding a plate group including steel plates having a softened layer on the surface of the steel plates has a softened layer on the surface of the lower plate on the overlapping surface 7 side of the steel plates. The softened layer exists in the area of ​​the weld heat affected zone (HAZ) and the base material portion. Figure 3 As shown in (A), the softened layer in the region of the weld heat affected zone decreases substantially as it approaches the nugget 6a. Figure 3 As shown in (B), part of the softened layer of the heat affected zone disappears completely due to the diffusion of elements such as C, or Figure 3 (C) Sometimes the softened layer of the heat affected zone of welding hardly disappears but remains. Figure 3 (A)~ Figure 3 The softened layer existing in the weld heat affected zone region in the state shown in (C) is referred to as “the softened layer of the steel plate surface layer in the weld heat affected zone”.

[0030] The present invention is based on the above findings, and the gist of the present invention is as follows.

[0031] [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,

[0032] The total thickness of the plate group is t all , the minimum thickness of the resistance spot weld is set to t weld When t all and t weld Satisfying formula (1),

[0033] At least one of the two or more steel plates is a steel plate having a softened layer on the surface of the steel plate,

[0034] Furthermore, in the base material portion on the steel plate side having the softened layer, when the nanoindentation hardness at a position 20 μm away from the surface of the softened layer in the plate thickness direction is set to Hbs, and the nanoindentation hardness at a position 1 / 4 of the thickness away from the surface of the softened layer in the plate thickness direction is set to Hbt, Hbs and Hbt satisfy formula (2).

[0035] 0.5<t weld / t all <1.0…(1)

[0036] 0.2<Hbs / Hbt<0.8…(2)

[0037] [2] The welded member according to [1], wherein, in the weld heat affected zone on the steel plate side having the softened layer, the nanoindentation hardness at a position 400 μm away from the nugget end of the resistance spot weld in the direction of the base material and on the overlapping surface of the steel plates is taken as the starting point, the nanoindentation hardness at a position 20 μm away from the starting point in the plate thickness direction is taken as Hws, and the nanoindentation hardness at a position 1 / 4 of the plate thickness away from the starting point in the plate thickness direction is taken as Hwt,

[0038] Hws, Hwt and the above Hbs satisfy equations (3) and (4).

[0039] 0.2<Hws / Hwt<1.0…(3)

[0040] 0.8<Hws / Hbs<7.0…(4)

[0041] [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),

[0042] The steel plate having the above-mentioned softened layer satisfies the formula (5).

[0043] -{(1000×t / TS)-0.25} / 35+0.2<(Hws / Hwt)<1.0…(5)

[0044] Here, Hws in formula (5) is the nanoindentation hardness at a position 20 μm away from the starting point in the plate thickness direction, and Hwt is the nanoindentation hardness at a position 1 / 4 of the plate thickness away from the starting point in the plate thickness direction.

[0045] [4] The welded member according to any one of [1] to [3], wherein the steel plate having the softened layer has a Si internal oxide layer and / or a zinc plating layer.

[0046] [5] The welded member according to [4], wherein an Fe-based pre-plating layer is provided below the galvanized layer.

[0047] [6] The welded member according to any one of [1] to [5], 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,

[0048] 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.

[0049] [7] A method for manufacturing a welded member, the method for manufacturing a welded member according to any one of [1] to [6], comprising:

[0050] 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.

[0051] In the welding process, the plate group is clamped by a pair of welding electrodes and connected by applying pressure and electricity.

[0052] The above-mentioned power-on is carried out 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.

[0053] Furthermore, there is an electrode holding step, and when the pressing force holding time after the end of the energization is defined as Th(s), Th(s) satisfies the relationship of formula (7).

[0054] {(Hbs / Hbt)-0.2} / 30<Th<[-{(Hbs / Hbt)-0.2} / 3]+1.2…(7)

[0055] Among them, Hbs in formula (7) is the nanoindentation hardness at a position 20 μm away from the softening layer surface of the above-mentioned base material part in the plate thickness direction, and Hbt is the nanoindentation hardness at a position 1 / 4 of the thickness away from the softening layer surface of the above-mentioned base material part in the plate thickness direction.

[0056] [8] The method for manufacturing a welded member according to [7], wherein the plate group is clamped by a pair of welding electrodes and joined while applying pressure and passing electricity, and at this time,

[0057] At least one weld point satisfies one or two or more of the conditions (a) to (e) immediately before pressurization is performed by the welding electrode.

[0058] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more,

[0059] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more,

[0060] (c) A gap of 0.5 mm or more exists between any welding electrode and the overlapping steel plates.

[0061] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates,

[0062] (e) The shortest distance from the center of the weld point to the end surface of the overlapping steel plates is 10 mm or less.

[0063] 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 softening of a steel sheet surface layer even in resistance spot welding using a high-strength steel sheet and does not cause LME cracks. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Figure 1 This is a cross-sectional view schematically showing an example of the resistance spot welding of the present invention.

[0065] 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.

[0066] Figure 3 (A)~ Figure 3 (C) is an enlarged cross-sectional view illustrating a softened layer of the surface layer of a steel plate in a weld heat affected zone.

[0067] Figure 4 This is a cross-sectional view in the plate thickness direction for explaining an example of a steel plate having a softened layer on the surface layer of the steel plate used in the present invention.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] Figure 8 This is a cross-sectional view schematically showing an example of crack generation during conventional resistance spot welding. DETAILED DESCRIPTION

[0072] 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.

[0073] First, refer to Figure 1 and Figure 2 The welded member of the present invention will be described.

[0074] Figure 1 An example of resistance spot welding of two steel plates is shown. Figure 2 1 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 its surroundings in the welded member.

[0075] like Figure 1 and Figure 2 As shown, the present invention is a welded member having a resistance spot welded portion (hereinafter referred to as a "welded portion") formed by resistance spot welding a plate group in which two or more steel plates are stacked.

[0076] As described later, at least one of the two or more stacked steel plates is a steel plate having a softened layer on the surface of the steel plate (see Figure 4 ). The number of steel plates is not particularly limited, and may be 2 or more. The number of steel plates is preferably 3 or more. It should be noted that the upper limit of the number of steel plates is not particularly specified, but is preferably 5 or less.

[0077] Figure 2An 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 softened layer. The 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.

[0078] exist Figure 2 In the case of the welded member of the example shown, only the lower plate 2 is a steel plate having the above-mentioned softening layer (not shown), and the upper plate 1 is a steel plate without a softening layer. In this case, the weld 6 of the present invention is formed only on the overlapping surface 7 side of the lower plate 2. It should be noted that when the steel plate having the above-mentioned softening 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 overlapping surface 7.

[0079] 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 softened layer. The welded portion of the present invention is formed in a manner including the overlapped surfaces between the steel plates.

[0080] 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 including two overlapping surfaces 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 softened layer, the hardness of the softened layer of the steel plate surface layer on the upper and lower sides of each overlapping surface is controlled respectively.

[0081] <Welding Department>

[0082] 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 .

[0083] like Figure 2 As shown, in the welded portion 6 of the present invention, the total plate thickness of the plate group is set to t all , the minimum thickness of the weld is set to t weld When the total plate thickness (t all ) and the minimum thickness of the weld (t weld ) satisfies the following formula (1).

[0084] In addition, in addition to this condition, in the base material portion 9 on the steel plate side having the above-mentioned softening layer, when the nanoindentation hardness at a position 20 μm away from the surface of the softening layer in the plate thickness direction is set to Hbs, and the nanoindentation hardness at a position 1 / 4 of the thickness of the plate away from the surface of the softening layer in the plate thickness direction is set to Hbt, the nanoindentation hardnesses (Hbs and Hbt) at the above two positions satisfy the following formula (2).

[0085] 0.5<t weld / t all <1.0…(1)

[0086] 0.2<Hbs / Hbt<0.8…(2)

[0087] 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), that is, when it is below the lower limit of formula (1), the thickness reduction of the weld 6 becomes significant, so the tensile residual stress generated in the weld 6 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.

[0088] 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.

[0089] Here, if Figure 2 As shown in the figure, the minimum thickness of the weld (t weld )" refers to the minimum thickness in the plate thickness direction within the plate 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 below.

[0090] In addition, when the hardness ratio (Hbs / Hbt) of the base material part shown in the above formula (2) is 0.8 or more, the tensile residual stress of the steel plate surface layer becomes too large when the electrode is released after resistance spot welding, resulting in LME cracks. On the other hand, when the hardness ratio of the base material part is 0.2 or less, cracks are likely to occur in the steel plate surface layer, resulting in a decrease in joint strength.

[0091] Due to these factors, there is a risk that it will be difficult to achieve the softening of the surface layer of the weld heat affected zone described later (ie, to control the hardness of the softened layer of the steel plate surface layer in the weld heat affected zone within a predetermined range).

[0092] The lower limit of the formula (2) is preferably 0.22 or more, and the upper limit of the formula (2) is preferably 0.75 or less.

[0093] With the above configuration, it is possible to achieve both ensuring the strength of the weld and suppressing LME cracks.

[0094] In the present invention, from the viewpoint of more effectively improving 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.

[0095] like Figure 2 As shown, in the weld heat affected zone 6b on the steel plate side having a softened layer, the starting point is a position 400 μm away from the nugget end 6c of the weld toward the parent material and on the overlapped surface 7 of the steel plate (i.e. Figure 2 As shown in point A), when the nanoindentation hardness at a position 20 μm away from the starting point in the direction of the plate thickness is set to Hws, and the nanoindentation hardness at a position 1 / 4 of the thickness of the plate away from the starting point in the direction of the plate thickness is set to Hwt, the nanoindentation hardnesses (Hws, Hwt and Hbs) at the above three positions satisfy the following equations (3) and (4) to be valid.

[0096] 0.2<Hws / Hwt<1.0…(3)

[0097] 0.8<Hws / Hbs<7.0…(4)

[0098] In the above formula (3), the difference in hardness between the steel plate surface layer in the weld heat affected zone 6b (i.e., the position 20 μm away from the starting point A in the plate thickness direction) and the steel plate interior in the weld heat affected zone 6b (i.e., the position 1 / 4 of the plate thickness away from the starting point A in the plate thickness direction) is taken into consideration. When the hardness ratio (Hws / Hwt) in the weld heat affected zone 6b shown in formula (3) is less than 0.2, that is, when the softening in the steel plate surface layer of the weld heat affected zone 6b on the overlapping surface 7 side of the steel plates where stress concentration becomes large becomes significant relative to the steel plate interior of the weld heat affected zone 6b, the ductile cracking resistance in the steel plate surface layer of the weld heat affected zone is excessively reduced. As a result, the strength of the weld is reduced.

[0099] On the other hand, when the hardness ratio (Hws / Hwt) is 1.0 or more, the steel plate surface layer of the weld heat affected portion 6b is not softened relative to the steel plate interior of the weld heat affected portion 6b, so the tensile residual stress reduction effect of the weld cannot be obtained, resulting in difficulty in suppressing LME cracks. Therefore, it is effective to satisfy the relationship of 0.2<Hws / Hwt<1.0.

[0100] The hardness ratio in the weld heat affected zone 6 b preferably satisfies the relationship of 0.25<Hws / Hwt<0.9, and more preferably satisfies the relationship of 0.30<Hws / Hwt<0.8.

[0101] In addition, in the above formula (4), the difference between the base material part and the weld heat affected zone in the steel plate surface layer is focused. In the steel plate surface layer of the weld heat affected zone 6b, the hardness changes due to the change of the structure caused by the heat effect during welding and the diffusion of alloy elements. Therefore, satisfying the relationship of 0.8<Hws / Hbs<7.0 is very important for controlling the softening layer of the steel plate surface layer in the weld heat affected zone (refer to Figure 3 ) hardness is effective.

[0102] Similar to formula (1), when the ratio of the hardness of the steel plate surface layer (Hws / Hbs) shown in formula (4) is less than 0.8, the resistance to ductile cracking in the steel plate surface layer of the weld heat affected zone is excessively reduced, resulting in a decrease in the strength of the weld. On the other hand, when the ratio of the hardness (Hws / Hwt) is 7.0 or more, the hardness of the steel plate surface layer of the weld heat affected zone becomes too high. The greater the hardness, the greater the tensile residual stress when the electrode is released, and therefore the effect of reducing the tensile residual stress of the weld cannot be obtained.

[0103] The ratio of the hardness of the steel plate surface preferably satisfies the relationship of 1.0<Hws / Hbs<6.5, and more preferably satisfies the relationship of 1.1<Hws / Hbs<6.0. In the steel plate surface, the hardness of the weld heat affected portion is greater than the hardness of the base material portion, thereby suppressing excessive strain concentration on the weld heat affected portion when a tensile load is applied to the weld, which is beneficial to avoiding fracture in the weld heat affected portion.

[0104] It should be noted that the nanoindentation hardness (unit: GPa) can be measured by a nanoindentation hardness test as described in the examples 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 TriboIndenter manufactured by Hysitron) in accordance with ISO 14577.

[0105] <Steel Plate>

[0106] [Board thickness, tensile strength]

[0107] The strength of the weld is affected by the thickness and tensile strength of the overlapping steel plates. Therefore, in the present invention, it is preferred to appropriately control the hardness ratio (Hws / Hwt) of the steel plate surface layer and the steel plate interior in the weld heat affected zone according to the plate thickness and tensile strength. Thus, the effect of the present invention can be more effectively obtained.

[0108] 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 softened layer preferably satisfies the following formula (5).

[0109] -{(1000×t / TS)-0.25} / 35+0.2<(Hws / Hwt)<1.0…(5)

[0110] Here, Hws in formula (5) is the nanoindentation hardness at a position 20 μm away from the starting point in the plate thickness direction, and Hwt is the nanoindentation hardness at a position 1 / 4 of the plate thickness away from the starting point in the plate thickness direction.

[0111] In addition, from the viewpoint of more appropriately controlling the above-mentioned hardness ratio (Hws / Hwt), in addition to the conditional expression ((Expression) 5), it is more preferable to satisfy the following Expression (6).

[0112] 0.25≤1000×t / TS≤3.75…(6)

[0113] When the thickness (t) of the steel plate having a softened 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 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 softens significantly, 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 the preferred Hws / Hwt.

[0114] On the other hand, when the tensile strength (TS) of the steel plate having a softened layer is large, the heat affected portion of the weld is generally easy to harden due to the martensitic transformation caused by the heat during welding. Therefore, the surface softening of the weld heat affected portion occurs due to the large hardness difference between the softened layer on the surface of the steel plate and the inside of the steel plate, 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 Hws / Hwt.

[0115] Therefore, from the viewpoint of the plate thickness (t) and the tensile strength (TS) of the steel plate, Hws / Hwt preferably satisfies the formula (5).

[0116] The lower limit of the formula (5) is preferably not less than -{(1000×t / TS)-0.25} / 35+0.3. The upper limit of the formula (5) is preferably not more than 0.9.

[0117] In addition, for the above-mentioned reasons, it is more preferable to further satisfy the formula (6). The lower limit of the formula (6) is preferably 0.5 or more, and the upper limit of the formula (6) is preferably 2.5 or less.

[0118] As described above, generally, as the strength and alloy content of steel plates increase, LME cracks tend to occur. Therefore, in the present invention, the tensile strength of the steel plate having a softened 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 improved.

[0119] [Steel plate with softened layer]

[0120] In the present invention, as described above, at least one of the two or more stacked steel plates is a steel plate having a softened layer on the surface of the steel plate. Figure 4 , a steel plate having a softened layer on the surface of the steel plate is described. Figure 4 An example of a steel sheet having a softened layer on the surface of the steel sheet is shown. Examples of the steel sheet having a softened layer include a galvanized steel sheet having a galvanized layer on the softened layer and a steel sheet having a Si internal oxide layer as the outermost layer in the softened layer.

[0121] Regarding the steel sheet used in the present invention, there is no particular limitation on the method of providing a softened layer on the surface of the steel sheet.

[0122] It should be explained that Figure 4 As an example, a steel sheet having a structure including a softened layer, a Si internal oxide layer, a zinc plating layer, and a Fe-based pre-plating layer described later is shown.

[0123] [Galvanized steel sheet]

[0124] 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, in the case of a galvanized steel sheet, a softening layer is preferably provided on the surface of the steel sheet that is in contact with the galvanized layer. In the case of the 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 more preferably an alloyed hot-dip galvanized steel sheet (GA steel sheet), a hot-dip galvanized steel sheet (GI steel sheet), or an electrogalvanized steel sheet (EG steel sheet).

[0125] like Figure 4 As shown, the galvanized steel sheet may have an Fe-based pre-plating layer between the softened layer and the galvanized layer. By forming the Fe-based pre-plating layer as a soft layer, the surface layer of the steel sheet can be further softened.

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] [Si internal oxide layer]

[0133] like Figure 4 As shown, for example, in the case of a steel sheet in which an Si internal oxide layer is contained in the softened layer, by increasing the dew point during annealing in the manufacture of the steel sheet, an Si internal oxide layer is formed in the outermost layer of the steel sheet, and the behavior of other alloy elements generated therewith is also controlled. Thus, a softened layer can be formed. By using such a steel sheet, the effect of the present invention can be more effectively obtained.

[0134] 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.

[0135] <Welding points of welded components>

[0136] use Figure 5 and Figure 6 The weld points of welded components will be described. Figure 5 and Figure 6 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).

[0137] like Figure 5 and Figure 6 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.

[0138] Here, if Figure 5 The "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 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".

[0139] 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 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. There is no upper limit for the average value of the shortest distance. In order to enable welding with a general shape welding gun, the average value (H1) of the shortest distance is preferably 1000 mm or less.

[0140] In addition, if Figure 6 The "average distance (H2) between the centers of adjacent welds" is obtained by measuring the distances between the centers of adjacent welds. When the average distance (H2) between the centers of the welds is less than 6 mm, current is diverted to the existing welds during welding, and the current density of the weld is reduced, so the diameter of the nugget is reduced, and the strength of the weld is easily reduced. In addition, the weld is constrained by the existing welds, and the tensile residual stress increases, which makes LME cracks more likely to occur. 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.

[0141] Next, one embodiment of the method for producing a welded member according to the present invention will be described.

[0142] 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.

[0143] <Preparation process>

[0144] In this process, two or more steel plates including at least one steel plate having the softened layer are prepared, and the two or more steel plates are stacked to form a plate group. Figure 1 As 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.

[0145] <Welding process>

[0146] 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 overlapping surfaces 7 of the steel plates 1 and 2, the steel plates can be joined to each other (see FIG. 1 ). Figure 2 ).

[0147] It should be noted that when a steel plate having a softened layer and a steel plate without a softened layer are used to form a plate set, the plates are overlapped in such a manner that the surface side having the softened layer is the overlapped surface of the steel plates. When the steel plate having a softened 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 overlapped surface of the steel plates. It should be noted that there is no problem if the steel plate surface on the side in contact with the electrode also has a softened layer and / or a galvanized layer.

[0148] 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.

[0149] Next, welding conditions in the welding process of the present invention will be described.

[0150] 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 pressurization is applied by the welding electrode, one or more of the following conditions (a) to (e) are satisfied, and the process comprises: an electricity-applying process, in which the electricity-applying is carried out under welding conditions of 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 when the pressure-applying holding time after the end of the electricity-applying is set to Th (s), Th satisfies the relationship of formula (7).

[0151] [Electrification process]

[0152] [Pressure, welding current, power-on time]

[0153] 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.

[0154] 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.

[0155] If the power-on time is less than 0.1s, the diffusion of alloy elements in the heat affected zone of the weld becomes insufficient sometimes, and the hardness of the softened layer of the heat affected zone of the weld is almost unchanged from the hardness of the base material. Since the shape strain concentration of the heat affected zone of the weld is greater than that of the base material, when the hardness of the softened layer of the heat affected zone of the weld is almost unchanged from the hardness of the base material, the strain concentration caused by the softening of the surface layer of the steel plate in the heat affected zone of the weld overlaps with the shape strain concentration, and TSS is reduced. 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 is reduced. The power-on time is preferably 0.10s or more. In addition, the power-on time is preferably 0.12s or more, preferably 1.5s or less.

[0156] The more preferable energization time is (0.13×√(t all ×H bs / H bt ))s or more, more preferably (0.19×√(t all ×H bs / H bt ))s or more. This is because, as the total plate thickness increases, the energization time required to ensure the nugget diameter increases; in addition, H, which is the ratio of the hardness of the above-mentioned base material part bs / H bt The larger the value of is, the easier it is to generate LME cracks, so the power-on time required to remove the Zn plating from the corona joint and promote alloying increases. The power-on time is more preferably 0.30 s or more.

[0157] It should be noted that resistance spot welding can be performed in a specific mode with multiple stages of pressure and current value or in combination with power on and no power on. For example, when the pressure during power on is set to multiple stages, the minimum pressure during power on is set to 2.0 kN or more, and the maximum pressure during power on is set to 10.0 kN or less. When the current value during power on is set to multiple stages, the minimum current value during power on except during the no power on 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 on time except during the no power on period is set to 0.1s to 2.0s.

[0158] [Welding construction interference]

[0159] 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.

[0160] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more

[0161] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more

[0162] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0163] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0164] (e) The shortest distance from the center of the weld to the end surface of the overlapping steel plates is less than 10 mm.

[0165] 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 is a detailed description of each process disturbance.

[0166] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more

[0167] 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.

[0168] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more

[0169] 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.

[0170] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0171] 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.

[0172] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0173] 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.

[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] If the shortest distance from the center of the weld to the end face of the steel plate is small, 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.

[0176] [Electrode holding process]

[0177] 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.

[0178] 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 to Th(s), the pressure holding time (Th(s)) satisfies the relationship of the following formula (7). Thus, the effect of the present invention can be effectively obtained.

[0179] {(Hbs / Hbt)-0.2} / 30<Th<[-{(Hbs / Hbt)-0.2} / 3]+1.2…(7)

[0180] Among them, Hbs in formula (7) is the nanoindentation hardness at a position 20 μm away from the softening layer surface of the above-mentioned base material part in the plate thickness direction, and Hbt is the nanoindentation hardness at a position 1 / 4 of the thickness away from the softening layer surface of the above-mentioned base material part in the plate thickness direction.

[0181] In addition, from the viewpoint of more appropriately controlling the temperature, in addition to the conditional expression ((Expression) 7), it is more preferable to satisfy the following Expression (8).

[0182] 0<Th…(8)

[0183] If the pressure holding time (Th) is too small, 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 large, the cycle time for each weld point 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 easily causing delayed failure of the weld. In particular, the larger the hardness ratio (Hbs / Hbt) of the base material part (that is, the degree of softening of the steel plate surface is smaller than that of the steel plate inside the base material part), the smaller the LME crack suppression effect brought by the softening layer of the steel plate surface, so the lower limit of the pressure holding time (Th) needs to be increased. In addition, the larger the Hbs / Hbt, the easier it is for the structure of the steel plate surface of the weld to become brittle, so the upper limit of Th needs to be reduced.

[0184] For this reason, the pressure holding time is a time that satisfies the formula (7). The lower limit of the formula (7) is preferably not less than {(Hbs / Hbt)-0.2} / 30+0.02. The upper limit of the formula (7) is preferably not more than [-{(Hbs / Hbt)-0.2} / 3]+1.0.

[0185] In addition, for the above reasons, it is more preferable to further satisfy the formula (8). The lower limit of the formula (8) is preferably 0.02 or more, and more preferably 0.17 or more. The upper limit of the formula (8) is preferably 1.0 or less.

[0186] Example

[0187] 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.

[0188] Using the plate group shown in Table 1, a welded joint (welded member) was produced under the welding conditions shown in Table 2. It should be noted that the plate group was arranged from the top in the order of steel plate 1, steel plate 2, and steel plate 3 shown in Table 1 and overlapped. The "None" shown in the coating column of Table 1 refers to a steel plate (cold-rolled steel plate) without a coating. Here, all Figure 4 As shown in , "softened layer on the surface of the steel plate" means the state of having a softened layer on the surface of the steel plate, "internal oxide layer" means the state of having an Si internal oxide layer on the surface of the steel plate, and "Fe-based pre-plating" means the state of having Fe-based pre-plating on the surface of the steel plate. 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 the 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.

[0189] 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.

[0190] 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 ) and nanoindentation hardness (Hbs, Hbt, Hws, Hst) at each position.

[0191] [Measurement of each thickness]

[0192] 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 nugget in the plate width direction is measured at intervals of 100 μm, and the minimum value is taken as the "minimum thickness" (refer to Figure 2 ). Use the obtained values ​​to find "t weld / t all ” value.

[0193] [Measurement of each nanoindentation hardness]

[0194] The welded joint was cut with a micro-cutter in a way that passed through the center of the welded part, and the nanoindentation hardness test of the weld heat affected zone and the base material was performed. In the nanoindentation hardness test, a triangular pyramidal diamond indenter was pressed into the measurement position with an indentation load of 500μN, and the load-displacement curve from the start of loading to the completion of unloading was obtained, and then the hardness was calculated using the Oliver-Pharr analysis method.

[0195] The nanoindentation hardness of the base material is measured as follows. Figure 2 As shown, on the steel plate side with softened layer ( Figure 2 In the example shown, the nanoindentation hardness (Hbs) at a position 20 μm away from the softening layer surface in the plate thickness direction and the nanoindentation hardness (Hbt) at a position 1 / 4 of the thickness away from the softening layer surface in the plate thickness direction are measured in the base material part of the lower plate 2 side. In addition, the measurement position of the base material part is carried out at a position 5000 μm away from the intersection of the outer edge of the weld heat affected zone and the overlap surface 7 of the steel plate in the base material direction.

[0196] In addition, the nanoindentation hardness in the weld heat affected zone is measured as follows. Specifically, Figure 2As shown, the steel plate side with softened layer (at Figure 2 In the example shown, the nanoindentation hardness (Hws) of a position 400 μm away from the weld core end of the weld in the direction of the base material on the lower plate 2 side, with the position on the overlapping surface of the steel plates as starting point A and 20 μm away from starting point A in the direction of the plate thickness, and the nanoindentation hardness (Hwt) of a position 1 / 4 of the plate thickness away from starting point A in the direction of the plate thickness.

[0197] Using the obtained values, the values ​​of "Hbs / Hbt", "Hws / Hwt" and "Hws / Hbs" were obtained as the values ​​representing the ratio of hardness in each welded joint. In addition, the values ​​of "Hbs / Hbt", "Hws / Hwt" and "Hws / Hbs" of the steel plates described in the "Evaluated Steel Plate" column of Table 2 are shown in Table 2 as representative values.

[0198] [TSS's evaluation]

[0199] 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.

[0200] In addition, in order to compare with the obtained test pieces (welded joints produced by the welding method of the present invention), comparative joints in which the surface properties of the steel plate were not controlled (i.e., welded joints in which the hardness difference of the obtained welded joints was Hbs / Hbt≈1.0) were also produced. The comparative joints were also evaluated for TSS based on the same tensile shear test method.

[0201] In addition, in the welded joint of condition No. 4 (plate set No. D) shown in Table 3, since all the overlapped steel plates did not have the above-mentioned softened layer, TSS evaluation was not performed.

[0202] 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, evaluation was performed by applying a tensile load between the steel plates 2 and 3 .

[0203] It should be noted that in the preparation of the welded joint for TSS evaluation, only one weld point was used, and welding operation interferences such as the electrode inclination angle and plate gap shown in Tables 2 and 3 were not set.

[0204] Then, TSS was evaluated according to the following criteria.

[0205] <Evaluation Criteria>

[0206] A: (TSS of the welded joint produced by the welding method of the present invention) / (TSS of the comparative joint) ≥ 0.9

[0207] B: 0.9>(TSS of welded joint produced by the welding method of the present invention) / (TSS of comparative joint)≥0.8

[0208] F: 0.8 > (TSS of the welded joint produced by the welding method of the present invention) / (TSS of the comparative joint)

[0209] Here, when the evaluation results were A or B, the evaluation was acceptable (having excellent shear tensile strength).

[0210] [Evaluation of LME cracks]

[0211] 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.

[0212] <Evaluation Criteria>

[0213] A: All 10 pieces without cracks

[0214] B: There are less than 2 welded joints with cracks, and the maximum crack depth is less than 100 μm

[0215] F: There are more than 3 welded joints with cracks, or the maximum crack depth is more than 100μm

[0216] Here, when the evaluation results are A and B, the evaluation is passed.

[0217] The obtained values ​​and evaluation results are shown in Tables 2 and 3, respectively.

[0218]

[0219] [Table 2]

[0220]

[0221] *1:-{(1000xt / TS)-0.25] / 35+0.2<(Hws / Hwt)<1.0…(5)

[0222] [Table 3]

[0223]

[0224] *1:{(Hbs / Hbt)-0.2}30<Th<[-{(Hbs / Hbt)-0.2} / 3]+1.2…(7)

[0225] 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.

[0226] Explanation of symbols

[0227] 1, 2, 3 Steel Plate

[0228] 4.5 Welding electrodes

[0229] 6 Welding Department

[0230] 6a Nugget

[0231] 6b Heat affected zone

[0232] 6c Nugget end

[0233] 7 Overlapping surfaces of steel plates

[0234] 8 Soldering points

[0235] 9. Parent material department

[0236] 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 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), At least one of the two or more steel plates is a steel plate having a softened layer on the surface of the steel plate. And in the base material portion on the steel plate side having the softened layer, when the nanoindentation hardness at a position 20 μm away from the surface of the softened layer in the plate thickness direction is set as Hbs, and the nanoindentation hardness at a position 1 / 4 of the plate thickness away from the surface of the softened layer in the plate thickness direction is set as Hbt, Hbs and Hbt satisfy the formula (2), 0.5<t weld / t all <1.0…(1) 0.2<Hbs / Hbt<0.8…(2).

2. The welding component according to claim 1, wherein: In the weld heat affected zone on the steel plate side having the softened layer, a position 400 μm away from the nugget end of the resistance spot weld in the direction of the base material and on the overlapping surface of the steel plates is taken as a starting point, and the nanoindentation hardness at a position 20 μm away from the starting point in the plate thickness direction is set as Hws, and the nanoindentation hardness at a position 1 / 4 of the plate thickness away from the starting point in the plate thickness direction is set as Hwt, Hws, Hwt and Hbs satisfy equations (3) and (4), 0.2<Hws / Hwt<1.0…(3) 0.8<Hws / Hbs<7.0…(4).

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 softened layer satisfies the formula (5), the unit of t is mm, the unit of TS is MPa, -{(1000×t / TS)-0.25} / 35+0.2<(Hws / Hwt)<1.0…(5) Here, Hws in formula (5) is the nanoindentation hardness at a position 20 μm away from the starting point in the plate thickness direction, and Hwt is the nanoindentation hardness at a position 1 / 4 of the plate thickness away from the starting point in the plate thickness direction.

4. The welding member according to claim 1 or 2, wherein: The steel sheet having the softened layer has a Si internal oxide layer and / or a zinc plating layer.

5. The welding component according to claim 4, wherein: An Fe-based pre-plating layer is provided under the zinc plating layer.

6. The welding member according to claim 1 or 2, 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.

7. A method for manufacturing a welded member, the method for manufacturing a welded member according to any one of claims 1 to 6, 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 connected by applying pressure and electricity. 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. The method further comprises an electrode holding step in which, when the pressing force holding time after the end of the energization is set to Th, Th satisfies the relationship of formula (7), wherein the unit of Th is s, {(Hbs / Hbt)-0.2} / 30<Th<[-{(Hbs / Hbt)-0.2} / 3]+1.2…(7) in, Hbs in formula (7) is the nanoindentation hardness at a position 20 μm away from the softening layer surface of the base material in the plate thickness direction, and Hbt is the nanoindentation hardness at a position 1 / 4 of the plate thickness away from the softening layer surface of the base material in the plate thickness direction.

8. A method for manufacturing a welded member, the method for manufacturing a welded member according to any one of claims 1 to 6, 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. Furthermore, there are an energizing step and an electrode holding step. In the energizing step, 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 a energizing time of 0.1 to 2.0 s. In the electrode holding step, when the pressure holding time after the energizing is completed is set to Th, Th satisfies the relationship of formula (7), 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, {(Hbs / Hbt)-0.2} / 30<Th<[-{(Hbs / Hbt)-0.2} / 3]+1.2…(7) in, Hbs in formula (7) is the nanoindentation hardness at a position 20 μm away from the softening layer surface of the base material in the plate thickness direction, and Hbt is the nanoindentation hardness at a position 1 / 4 of the plate thickness away from the softening layer surface of the base material in the plate thickness direction.

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