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. In this welded member, the total plate thickness of a plate group and the minimum thickness of a resistance spot welding section satisfy formula (1), at least one of the plate groups is a steel plate having an internal oxide layer on the surface layer of the steel plate, and the amount of oxygen per unit area in the internal oxide layer in a base material section on the steel plate side having the internal oxide layer satisfies formula (2). In the welding heat-affected portion on the steel plate side, the density of the Si-containing oxide in the first region satisfies formula (3), and the average Si concentration in the second region and the third region satisfies formula (4). 0.5 < twill / tall < 1.0... (1), 0.02 < Os < 1.0... (2), 0.01 < = Ns... (3), Ss / St < 1.0... (4)

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 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 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] Taking the joint strength into consideration and summarizing the common problems of Patent Documents 1 to 3, no technique is proposed for suppressing the occurrence of LME cracks while ensuring the strength of the 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 supply and pressurization of the resistance spot welding are completed, LME cracks are easily generated in the area with large local tensile stress.

[0026] In addition, as for the influence of the components in the steel plate, it is known that when the amount of Si increases, the LME crack resistance of the steel plate deteriorates as described in Reference 1. Therefore, it is considered effective to suppress LME cracks by reducing the amount of solid-solution Si by oxidizing Si inside the steel plate near the surface of the weld heat affected zone and fixing it in the form of oxides.

[0027] [Reference 1] WO2021 / 200412

[0028] However, even if internal oxidation of Si occurs near the surface of the steel plate, if the internal oxide of Si disappears particularly on the overlapping surface of the steel plates due to the heat effect during welding, the objective effect of the present invention cannot be obtained. That is, the following idea is obtained: by controlling the Si concentration ratio of the steel plate surface layer in the weld heat affected zone within a certain range, it is possible to achieve both ensuring the strength of the weld and suppressing LME cracks.

[0029] In order to allow the Si internal oxide layer to exist in the weld heat affected zone as described above, it is necessary to make the Si internal oxide amount of the base material to be above a certain level. In order to promote the formation of the Si internal oxide of the base material, it is effective to increase the annealing dew point of the steel plate, for example, as described later. The Si internal oxide amount of the base material is correlated with the oxygen amount of the steel plate surface layer. Therefore, by making the oxygen amount of the steel plate surface layer above a certain level, it is possible to make the Si internal oxide layer above a certain level exist in the base material, and it is possible to prevent the disappearance of the Si internal oxide in the weld heat affected zone.

[0030] 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 an internal oxide layer on the surface of the steel plates has an internal oxide layer on the surface of the lower plate on the steel plate overlap surface 7 side. Figure 3 (A)~ Figure 3 In the example shown in (C), a steel plate having the above-mentioned internal oxide layer on the lower plate is used. The internal oxide layer exists in the weld heat affected zone (HAZ) and the base metal area. Figure 3 As shown in (A), the internal oxide layer in the region of the weld heat affected zone basically decreases as it approaches the nugget 6a. Figure 3As shown in (B), a part of the internal oxide layer of the heat affected zone disappears completely due to the diffusion of elements such as C, or Figure 3 (C) Sometimes the internal oxide layer of the weld heat affected zone hardly disappears but remains. Figure 3 (A)~ Figure 3 The internal oxide layer existing in the weld heat affected zone region in the state shown in (C) is referred to as “the internal oxide layer of the steel plate surface layer in the weld heat affected zone”.

[0031] The present invention is based on the above findings, and the gist of the present invention is as follows.

[0032] [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,

[0033] The total thickness of the plate group is t all (mm), the minimum thickness of the resistance spot welding part is set as t weld (mm), t all (mm) and t weld (mm) satisfies formula (1),

[0034] At least one of the two or more steel plates is a steel plate having an internal oxide layer on the surface of the steel plate,

[0035] In the base material portion on the steel plate side having the above-mentioned internal oxide layer,

[0036] The amount of oxygen per unit area in the internal oxide layer on the overlapping surface of the steel plates is denoted as Os (g / m 2 ), Os satisfies formula (2),

[0037] In the weld heat affected zone on the steel plate side having the above-mentioned internal oxide layer,

[0038] The position on the overlapped surface of the steel sheets at the corona joint end is taken as the starting point, and the density of the oxide containing Si in the first region surrounded by the range of ±50 μm in the direction parallel to the overlapped surface of the steel sheets and the range of 10 μm from the starting point in the thickness direction is taken as Ns (pieces / μm 2 ), Ns satisfies formula (3),

[0039] Furthermore, when the average Si concentration in the second area surrounded by the range of ±50 μm in the direction parallel to the overlapping surface of the steel plate centered on the above-mentioned starting point and the range of 5 μm in the plate thickness direction from the above-mentioned starting point is set as Ss (mass %), and the average Si concentration in the third area of ​​100 μm square centered on a position 1 / 4 of the thickness of the plate away from the above-mentioned starting point in the plate thickness direction is set as St (mass %), Ss and St satisfy formula (4).

[0040] 0.5<t weld / t all <1.0…(1)

[0041] 0.02<Os<1.0…(2)

[0042] 0.01≤Ns…(3)

[0043] Ss / St<1.0…(4)

[0044] [2] The welded member according to [1], wherein, when the thickness of the steel plate is t (mm), the tensile strength of the steel plate is TS (MPa), and the average Si content in the steel plate is Si (mass %),

[0045] The steel plate having the above-mentioned internal oxide layer satisfies the formula (5).

[0046] {(t+Si+TS / 1000)-1.4} / 66+0.01≤Ns…(5)

[0047] Among them, Ns (pieces / μm) in formula (5) 2 ) is the density of the Si-containing oxide in the first region.

[0048] [3] The welded member according to [1] or [2], wherein the Ss and St of the steel plate having the internal oxide layer satisfy equation (7).

[0049] {(t+Si+TS / 1000)-1.4} / 66<Ss / St<{(t+Si+TS / 1000)-1.4} / 66+0.9…(7)

[0050] In formula (7), t (mm) is the thickness of the steel plate having the internal oxide layer, TS (MPa) is the tensile strength of the steel plate having the internal oxide layer, and Si (mass %) is the average Si content in the steel plate having the internal oxide layer.

[0051] [4] The welded member according to any one of [1] to [3], wherein the steel plate having the internal oxide layer has a decarburized layer and / or a galvanized layer.

[0052] [5] The welded member according to [4], wherein an Fe-based pre-plating layer is provided below the galvanized layer.

[0053] [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,

[0054] 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.

[0055] [7] A method for manufacturing a welded member, the method for manufacturing a welded member according to any one of [1] to [6], comprising:

[0056] 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.

[0057] In the above welding process,

[0058] The plate group is clamped by a pair of welding electrodes and connected by applying pressure and electricity.

[0059] For at least one welding spot, immediately before the welding electrode is pressed, one or more of the conditions (a) to (e) are satisfied.

[0060] And it has: an electrification step, wherein the electrification is performed with a pressure of 2.0 to 10.0 kN, a welding current of 4.0 to 15.0 kA, and an electrification time of 0.1 to 2.0 s; and

[0061] 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 (8).

[0062] (a) A state in which the electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more.

[0063] (b) A state in which the electrode center deviation of a pair of welding electrodes is 0.1 mm or more.

[0064] (c) A state in which a gap of 0.5 mm or more exists between any one of the welding electrodes and the overlapping steel plates.

[0065] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0066] (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.

[0067] {(t+Si+TS / 1000)-1.4} / 132<Th<[-{(t+Si+TS / 1000)-1.4} / 66]+1.1…(8)

[0068] In the formula (8), t (mm) is the thickness of the steel plate having the internal oxide layer, TS (MPa) is the tensile strength of the steel plate having the internal oxide layer, and Si (mass %) is the average Si content in the steel plate having the internal oxide layer.

[0069] 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 an increase in Si content near the surface of a steel sheet and does not cause LME cracks even in resistance spot welding using a high-strength steel sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 This is a cross-sectional view schematically showing an example of the resistance spot welding of the present invention.

[0071] 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.

[0072] Figure 3 (A)~ Figure 3 (C) is an enlarged cross-sectional view illustrating the internal oxide layer of the steel plate surface layer in the weld heat affected zone.

[0073] Figure 4 This is a cross-sectional view in the plate thickness direction for explaining an example of a steel plate having an internal oxide layer on the surface layer of the steel plate used in the present invention.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] Figure 8 This is a cross-sectional view schematically showing an example of crack generation during conventional resistance spot welding. DETAILED DESCRIPTION

[0078] Below, refer to Figure 1 to Figure 6 , 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.

[0079] First, the welded member of the present invention will be described.

[0080] Figure 2 is a cross-sectional view in the plate thickness direction showing an example of a welded member of the present invention, and is also an enlarged cross-sectional view of a resistance spot welded portion and its surroundings in the welded member. The position shown in the 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 internal oxide layer of the heat-affected zone of the weld. 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 an internal oxide 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.

[0081] 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 of two or more stacked steel plates.

[0082] In the welded member of the present invention, as described later, at least one of the two or more superposed steel plates is a steel plate having an internal oxide 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. Preferably, it is 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.

[0083] 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 (i.e., the lower plate) disposed on the lower side and the steel plate 1 (i.e., the upper plate) disposed on the upper side are steel plates having the above-mentioned internal oxide 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.

[0084] exist Figure 2 In the case of the welded member of the example shown, only the lower plate 2 is a steel plate having an internal oxide layer, and the upper plate 1 is a steel plate having no internal oxide 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 an internal oxide 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.

[0085] 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 internal oxide layer. The welded portion of the present invention is formed so as to include the overlapping surfaces of the steel plates between the steel plates.

[0086] For example, in the case of a plate set consisting of three steel plates, the weld 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 internal oxide layer, the internal oxidation of the surface layers of each steel plate on the upper and lower sides of the overlapping surface of each steel plate is controlled respectively.

[0087] <Welding Department>

[0088] 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 .

[0089] In the welding portion 6 of the present invention, Figure 2 As shown, the total thickness of the plate group is set to t all (mm), the minimum thickness of the weld is set to t weld (mm), total plate thickness (t all ) and the minimum thickness of the weld (t weld ) satisfies the following formula (1).

[0090] 0.5<t weld / t all <1.0…(1)

[0091] The ratio of the total plate thickness to the minimum thickness of the weld (t weld / t all ) does not satisfy the above formula (1), especially when it is below the lower limit of formula (1), the thickness reduction of the weld portion 6 becomes significant, so the tensile residual stress generated in the weld portion 6 becomes too large, which easily causes LME cracks and leads to a decrease in joint strength. 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.

[0092] 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.

[0093] 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 later.

[0094] As described above, the purpose of the internal oxidation near the steel plate surface layer of the weld heat affected zone of the present invention is to reduce the amount of dissolved Si. Therefore, in the present invention, in addition to the conditional expression of the thickness ratio, on the steel plate side having the internal oxidation layer (at Figure 2 In the example shown, the following conditional expressions are also specified in the base material portion (on the lower plate 2 side) and the weld heat affected zone.

[0095] Specifically, in the base material portion on the steel sheet side having the internal oxide layer, the amount of oxygen per unit area in the internal oxide layer on the steel sheet overlapping surface side is denoted as Os (g / m 2 ), the oxygen (Os) satisfies the following formula (2).

[0096] 0.02<Os<1.0…(2)

[0097] When the above formula (2) is not satisfied, especially when Os is 0.02 or less, the amount of internal oxidation of Si is small, and the internal oxide layer near the surface of the steel plate almost disappears when affected by welding heat. As a result, it is difficult to reduce the amount of dissolved Si in the surface layer of the weld heat affected zone described later. When Os is 1.0 or more, oxidation becomes significant not only in the grains but also at the grain boundaries. As a result, the LME sensitivity increases due to oxidation at the grain boundaries, and the crack suppression effect cannot be obtained.

[0098] The lower limit of the formula (2) is preferably 0.08 or more, more preferably 0.1 or more. The upper limit of the formula (2) is preferably 0.8 or less.

[0099] In addition, the above-mentioned "the amount of oxygen per unit area (Os) in the internal oxide layer on the overlapping surface side of the steel plates" can be measured by the method described in the Examples described later.

[0100] In addition, in the weld heat affected zone on the steel sheet side having the internal oxide layer, the position on the overlapped surface of the steel sheets at the end of the corona joint is taken as the starting point ( Figure 2 The density of oxides containing Si in the region (first region 9a) enclosed by the starting point as the center in the direction parallel to the overlapped surface of the steel plate (i.e., the plate width direction) to ±50 μm and the starting point in the plate thickness direction (i.e., the direction perpendicular to the overlapped surface of the steel plate) to 10 μm is set to Ns (pieces / μm). 2 ), the density (Ns) of the oxide containing Si satisfies the formula (3).

[0101] Furthermore, when the average Si concentration of the position other than the oxide containing Si in the region (second region 9b) surrounded by the range of ±50 μm in the direction parallel to the overlapped surface of the steel plate and the range of 5 μm from the above starting point in the plate thickness direction is defined as Ss (mass %), and the average Si concentration in the region (third region 9c) of 100 μm square centered at a position 1 / 4 of the plate thickness away from the above starting point in the plate thickness direction is defined as St (mass %), the ratio (Ss / St) of the average Si concentration of the second region 9b to that of the third region 9c satisfies the formula (4) (refer to Figure 2 ).

[0102] 0.01≤Ns…(3)

[0103] Ss / St<1.0…(4)

[0104] The density (Ns) of the above oxides is less than 0.01 (pieces / μm 2 ), the internal oxidation amount of Si is insufficient, and the amount of dissolved Si near the surface of the steel plate in the weld heat affected zone increases, resulting in the formation of LME cracks. Therefore, it is effective for the first region to satisfy the relationship of formula (3) (0.01≤Ns). It is more effective that the density of the oxide preferably satisfies the relationship of 0.02≤Ns, and it is more effective that it satisfies the relationship of 0.05≤Ns.

[0105] It should be noted that the upper limit of the density of the oxides is not particularly specified. However, from the viewpoint of suppressing excessive oxidation of the grain boundaries, the density of the oxides (Ns) is preferably 100 (pieces / μm 2 The density (Ns) of the oxide is more preferably 20 (pieces / μm 2 )the following.

[0106] In addition, when the average Si concentration ratio (Ss / St) between the second and third regions is 1.0 or more, the amount of dissolved Si near the surface of the steel plate in the weld heat affected zone increases, and as a result, the LME crack suppression effect cannot be obtained. The average Si concentration ratio preferably satisfies the relationship of Ss / St<0.9, and more preferably satisfies the relationship of Ss / St<0.7.

[0107] It should be noted that the lower limit of the average Si concentration ratio is not particularly specified. Even if internal oxidation is promoted, it is difficult to reduce the amount of dissolved Si to zero, so the average Si concentration ratio (Ss / St) is preferably 0.05 or more, more preferably 0.10 or more.

[0108] The density (Ns) of the oxide and the ratio (Ss, St) of the average Si concentration can be measured by the method described in the examples described later.

[0109] With the above configuration, it is possible to achieve both ensuring the strength of the weld and suppressing LME cracks.

[0110] <Steel Plate>

[0111] [Board thickness, tensile strength]

[0112] It is believed that LME cracks are likely to occur when the thickness, Si content, and tensile strength of the overlapped steel plates increase. Specifically, when the Si content and tensile strength of the base material increase, the LME crack resistance of the steel plate deteriorates. Therefore, if the internal oxidation amount is not further increased to reduce the amount of dissolved Si, LME cracks cannot be suppressed. That is, it is believed that the amount of internal oxides that are effective in suppressing LME cracks in the weld heat affected zone increases with the increase in these parameters.

[0113] Therefore, in the present invention, it is preferable to appropriately control the density (Ns) of the oxide containing Si near the surface layer of the steel sheet based on these parameters. This can more effectively achieve the effects of the present invention.

[0114] Specifically, when the thickness of the steel plate is t (mm), the tensile strength of the steel plate is TS (MPa), and the average Si content in the steel plate is Si (mass %), the steel plate having an internal oxide layer preferably satisfies the formula (5).

[0115] {(t+Si+TS / 1000)-1.4} / 66+0.01≤Ns…(5)

[0116] Here, Ns in the formula (5) is the density of the oxide containing Si in the first region.

[0117] In addition, from the viewpoint of more appropriately controlling the density (Ns) of the oxide containing Si, it is more preferable to satisfy the following formula (6) in addition to the conditional formula ((formula) 5).

[0118] 1.4≤t+Si+TS / 1000≤8.0…(6)

[0119] When the thickness (t) of the steel plate having the internal oxide layer is large, when the weld heat affected zone is significantly deformed due to welding interference, the local thermal stress increases and LME cracks are easily generated. Therefore, the larger the thickness (t) of the steel plate, the larger the preferred lower limit of Ns.

[0120] On the other hand, when the tensile strength (TS) of the steel plate having the internal oxide 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 layer of the weld heat affected portion softens due to the difference in Si concentration between the internal oxide 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 Ns.

[0121] In addition, when the average Si content (Si) in the steel sheet having an internal oxide layer is large, it is necessary to further increase the internal oxide amount to reduce the amount of dissolved Si.

[0122] Therefore, from the viewpoints of the sheet thickness (t) and tensile strength (TS) of the steel sheet and the average Si content (Si) in the steel sheet, Ns preferably satisfies the formula (5).

[0123] The lower limit of the formula (5) is preferably not less than {(t+Si+TS / 1000)-1.4} / 66+0.03. The upper limit of the formula (5) is preferably not more than 100.

[0124] 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 2.5 or more, and the upper limit of the formula (6) is preferably 7.0 or less.

[0125] In the present invention, similarly to the above, it is considered that the reduction rate of the solute Si concentration near the surface of the steel plate, which is effective for suppressing LME cracking in the weld heat affected zone, changes with the increase in the plate thickness, Si content, and tensile strength of the steel plate.

[0126] Specifically, when the plate thickness of the steel plate is t (mm), the tensile strength of the steel plate is TS (MPa), and the average Si content in the steel plate is Si (mass %), the ratio of the average Si concentration (Ss / St) of the steel plate having the internal oxide layer preferably satisfies the formula (7). Thus, the effect of the present invention can be more effectively obtained.

[0127] {(t+Si+TS / 1000)-1.4} / 66<Ss / St<{(t+Si+TS / 1000)-1.4} / 66+0.9…(7)

[0128] The reason for setting a lower limit value for the average Si concentration ratio (Ss / St) in formula (7) is as follows. In order to significantly reduce the solid solution Si concentration near the surface of the weld heat affected zone, it is necessary to have a method for manufacturing a steel plate that significantly reduces not only Si but also the concentration of other alloying elements near the surface of the steel plate, so there is a concern that the surface of the steel plate at the weld heat affected zone will be significantly softened. In particular, excessive softening of the overlapping surfaces of the steel plates will reduce the strength of the weld, so it is preferable to set a lower limit value for the average Si concentration ratio (Ss / St).

[0129] The reason for setting an upper limit value in formula (7) is that if the average Si concentration ratio (Ss / St) is too large, it means that the amount of dissolved Si in the surface layer of the weld heat affected zone is large, and the effect of suppressing LME cracks cannot be obtained.

[0130] 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. 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.

[0131] [Steel plate with internal oxide layer]

[0132] In the present invention, as described above, at least one of the two or more stacked steel plates is a steel plate having an internal oxide layer on the surface of the steel plate. Figure 4 , a steel plate having an internal oxide layer on the surface of the steel plate is described. Figure 4 An example of a steel sheet having an internal oxide layer on the surface of the steel sheet is shown. Examples of the steel sheet having the internal oxide layer include a galvanized steel sheet having a galvanized layer on the internal oxide layer and a steel sheet having a decarburized layer including the internal oxide layer on the outermost surface.

[0133] Regarding the steel sheet used in the present invention, there is no particular limitation on the method of providing the internal oxide layer on the surface layer of the steel sheet.

[0134] 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.

[0135] [Galvanized steel sheet]

[0136] 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, the steel sheet having an internal oxide layer is preferably a galvanized steel sheet. This is because the effect of the present invention can be more effectively obtained in the case of a galvanized steel sheet. 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).

[0137] like Figure 4 As shown, the galvanized steel sheet may have an Fe-based pre-plating layer under the galvanized layer. By forming the Fe-based pre-plating layer, the amount of dissolved Si in the surface layer of the steel sheet can be reduced.

[0138] [Fe-based pre-plating layer]

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] [Decarburized layer]

[0146] like Figure 4 As shown, for example, in the case of a steel sheet in which an Si internal oxide layer is contained in a decarburized layer, by increasing the dew point during annealing during the production 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. By using such a steel sheet, the effect of the present invention can be more effectively obtained.

[0147] 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.

[0148] <Welding points of welded components>

[0149] use Figure 5 and Figure 6 The weld points in the welded components will be described.

[0150] 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 member viewed from above).

[0151] 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.

[0152] 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 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".

[0153] When the average value (H1) of the shortest distance is less than 3 mm, the pressurization between the plates around the nugget on the end face side of the steel plate becomes insufficient, and the molten metal easily splashes from the plate to the end face side of the steel plate. 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, and more preferably 30 mm or less.

[0154] In addition, if Figure 6 The "average distance (H2) between the centers of adjacent welds" is obtained by measuring the distance between the centers of each adjacent weld 8. When the average distance (H2) between the centers of the welds is less than 6 mm, a shunt to the existing weld occurs during welding, and the current density of the weld portion decreases, so the diameter of the nugget is reduced, and the strength of the weld portion is easily reduced. In addition, the weld portion is constrained by the existing weld, and the tensile residual stress increases, which also 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 less than 100 mm, and more preferably less than 60 mm. The average distance (H2) between the centers of the welds is preferably greater than 10.0 mm.

[0155] It should be noted that the welded component of the present invention also includes the case where the number of weld points is only one. In this case, it is sufficient as long as the "average value of the shortest distance (H1) from the weld point center of the weld to the end face of the steel plate is 3 mm or more" among the above-mentioned weld point conditions is met.

[0156] Next, one embodiment of the method for producing a welded member according to the present invention will be described.

[0157] 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.

[0158] <Preparation process>

[0159] In this step, two or more steel plates including at least one steel plate having the internal oxide 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.

[0160] <Welding process>

[0161] 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 ).

[0162] In the case where a steel plate having an internal oxide layer and a steel plate having no internal oxide layer are used to form a plate set, the plates are overlapped so that the surface having the internal oxide layer is the overlapping surface of the steel plates. In the case where the steel plate having the internal oxide layer is a galvanized steel plate having a galvanized layer (GI steel plate, GA steel plate, EG steel plate), the plates are overlapped so that the surface 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 welding electrode also has an internal oxide layer and / or a galvanized layer.

[0163] As a welding device that can be used in the resistance spot welding method of the present invention, a welding device that has a pair of upper and lower welding electrodes and can arbitrarily control the pressure and welding current during welding can be used. 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.

[0164] Next, welding conditions in the welding process of the present invention will be described.

[0165] 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 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 application is set to Th (S), and Th satisfies the relationship of formula (8).

[0166] [Electrification process]

[0167] [Pressure, welding current, power-on time]

[0168] 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.

[0169] 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.

[0170] If the power-on time is less than 0.1S, the diffusion of alloy elements in the heat affected zone of the weld is sometimes insufficient, and the thickness of the decarburized layer formed together with the solid solution Si layer in the heat affected zone of the weld is almost the same as that of the base material. At this time, the structure of the coarse ferrite center is formed on the surface of the heat affected zone of the weld. As a result, the surface structure is prone to necking when a tensile load is applied, so 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 0.12S or more, preferably 1.5S or less.

[0171] 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.

[0172] [Welding construction interference]

[0173] 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.

[0174] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more

[0175] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more

[0176] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0177] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0178] (e) The shortest distance from the center of the weld to the end surface of the overlapping steel plates is less than 10 mm.

[0179] 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.

[0180] (a) The electrode inclination angle between the welding electrode and the overlapping steel plates is 0.2 degrees or more

[0181] 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.

[0182] (b) The electrode center deviation of a pair of welding electrodes is 0.1 mm or more

[0183] 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.

[0184] (c) A state where there is a gap of 0.5 mm or more between any welding electrode and the overlapping steel plates

[0185] When there is a gap between any electrode and the steel plate just before the start of pressurization, for example, when one electrode is movable (hereinafter referred to as the movable side electrode) and the other electrode is fixed (hereinafter referred to as the fixed side electrode), pressurization is started using the movable side electrode in a state where there is a gap between the fixed side electrode and the steel plate. As a result, the steel plate is bent and deformed, so bending stress is applied to the weld, which makes it easy to generate LME cracks. When the gap amount is 0.5 mm or more, the effect of the present invention can be effectively obtained. When the gap amount is too large, the formation of the molten core becomes unstable, which becomes the cause of spatter, so the gap amount is preferably less than 5 mm. The gap amount is more preferably more than 1 mm, and more preferably less than 3 mm.

[0186] (d) A state in which a gap of 0.5 mm or more exists between at least one set of the stacked steel plates.

[0187] 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.

[0188] (e) The shortest distance from the center of the weld to the end surface of the overlapping steel plates is less than 10 mm.

[0189] 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.

[0190] [Electrode holding process]

[0191] The electrode holding step is a step for suppressing the generation of pores by holding the welding electrode at a certain pressure after the energization is completed.

[0192] 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 (8). Thus, the effect of the present invention can be effectively obtained.

[0193] {(t+Si+TS / 1000)-1.4} / 132<Th<[-{(t+Si+TS / 1000)-1.4} / 66]+1.1…(8)

[0194] In the formula (8), t is the thickness of the steel plate having the internal oxide layer, TS is the tensile strength of the steel plate having the internal oxide layer, and Si is the average Si content in the steel plate having the internal oxide layer.

[0195] In addition, from the viewpoint of more appropriately controlling the temperature, in addition to the conditional expression ((Expression) 8), it is more preferable to satisfy the following Expression (9).

[0196] 0<Th…(9)

[0197] 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 cycle time for each 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 easily causing delayed failure of the weld. In particular, the greater the thickness, Si content, and tensile strength of the steel plate, the greater the sensitivity to LME and delayed failure, and therefore the narrower the range that satisfies the upper and lower limits of the pressure holding time (Th).

[0198] For this reason, the pressure holding time is a time that satisfies the formula (8). The upper limit of the formula (8) is preferably [-{(t+Si+TS / 1000)-1.4} / 66]+0.9 or less. The lower limit of the formula (8) is preferably {(t+Si+TS / 1000)-1.4} / 110 or more.

[0199] It should be noted that, for the above reasons, it is more preferable to further satisfy the formula (9). The lower limit of the formula (9) is preferably 0.02 or more, and more preferably 0.1 or more. The upper limit of the formula (9) is preferably 1.0 or less, and more preferably 0.8 or less.

[0200] Example

[0201] 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.

[0202] Using the plate group shown in Table 1, a welded joint (welded component) was made under the welding conditions shown in Tables 2 and 3. It should be noted that the plate group is arranged from the upper side and overlapped in the order of steel plate 1, steel plate 2, and steel plate 3 shown in Table 1. 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.

[0203] 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.

[0204] The TSS and the presence of LME cracks in the welded joint were evaluated by the following method. In addition, the thickness (t all ,t weld ), the amount of oxygen per unit area in the decarburized layer (Os), the density of Si-containing oxides (Ns) and the average Si concentration (Ss, St).

[0205] [Measurement of each thickness]

[0206] 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.

[0207] [Amount of oxygen per unit area of ​​the inner oxide layer]

[0208] In the plate group shown in Table 1, at least one steel plate has an internal oxide layer on the surface of the steel plate. Here, the oxygen content (Os) per unit area (g / m 2 ).

[0209] The same steel type as that used for welding was prepared. For the steel plate without galvanizing, pickling treatment was performed in 5 mass% HCl to remove the external oxide on the surface of the steel plate. For the steel plate with galvanizing, the galvanizing layer was dissolved in a 20 mass% NaOH aqueous solution containing triethanolamine. Then, Os, which is the amount of oxygen per unit area on one side of the steel plate, was measured by the melting infrared absorption method.

[0210] In addition, Os of the steel plate described in the column of "Evaluated Steel Plate" in Table 2 is described in Table 2 as a representative value.

[0211] [Oxide density (Ns) and average Si concentration (Ss, St)]

[0212] The Ns, Ss, and St of each joint were measured using an electron probe microanalyzer (EPMA). Specifically, for Ns, the position on the overlapped surface of the steel sheets in the corona joint end was measured from the mapping image of Si and O obtained by EPMA as the starting point ( Figure 2 The number of oxides containing Si in the first region enclosed by the range of ±50 μm in the direction parallel to the overlapped surface of the steel sheet and the range of 10 μm in the thickness direction from the starting point is calculated by dividing the obtained number by the area (100×10=1000(μm 2 Here, a region where both Si and O are enriched relative to the surrounding area is defined as an oxide containing Si.

[0213] For Ss, the Si concentration of any five points containing no Si oxide in a second region surrounded by a range of ±50 μm in a direction parallel to the overlapped surface of the steel sheet and a range of 5 μm in the sheet thickness direction from the starting point as the center was determined by point analysis using EPMA. The average value was taken as Ss.

[0214] For St, the Si concentration at five arbitrary points containing no Si oxide in a third region of 100 μm square centered at a position 1 / 4 of the thickness away from the starting point in the thickness direction was determined by point analysis using EPMA, and the average value was taken as St.

[0215] The value of "Ss / St" in each welded joint was determined using the obtained values. Here, the values ​​in the steel plates described in the "Evaluated Steel Plate" column of Table 2 are described in Table 2 as representative values.

[0216] [TSS's evaluation]

[0217] 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.

[0218] In addition, for comparison 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 welded portion were not controlled (i.e., welded joints in which the average Si concentration ratio of the obtained welded joint was Ss / St≈1.0) were also produced. TSS was also evaluated for the comparative joints based on the same tensile shear test method.

[0219] The test piece shape for TSS evaluation is based on JIS Z3136, but in the three overlapping plate sets (plate set No. H in Table 1), Figure 7 As shown, evaluation was performed by applying a tensile load between the steel plates 2 and 3 .

[0220] 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.

[0221] Then, TSS was evaluated according to the following criteria.

[0222] <Evaluation Criteria>

[0223] A: (TSS of the welded joint produced by the welding method of the present invention) / (TSS of the comparative joint) ≥ 0.9

[0224] B: 0.9>(TSS of welded joint produced by the welding method of the present invention) / (TSS of comparative joint)≥0.8

[0225] F: 0.8 > (TSS of the welded joint produced by the welding method of the present invention) / (TSS of the comparative joint)

[0226] Here, when the evaluation results were A or B, the evaluation was acceptable (having excellent shear tensile strength).

[0227] [Evaluation of LME cracks]

[0228] 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.

[0229] <Evaluation Criteria>

[0230] A: All 10 pieces without cracks

[0231] 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.

[0232] Here, when the evaluation results are A and B, the evaluation is passed.

[0233] The obtained values ​​and evaluation results are shown in Tables 2 and 3, respectively.

[0234]

[0235]

[0236]

[0237] 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.

[0238] Explanation of symbols

[0239] 1, 2, 3 Steel Plate

[0240] 4.5 Welding electrodes

[0241] 6 Welding Department

[0242] 6a Nugget

[0243] 6b Heat affected zone

[0244] 7. Overlapping surface of steel plates

[0245] 8 Soldering points

[0246] 9a First Area

[0247] 9b Second Area

[0248] 9c Third Area

[0249] 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), the t all ,t weld The unit is mm, Furthermore, at least one of the two or more steel plates is a steel plate having an internal oxide layer on the surface of the steel plate. In the base material portion on the steel plate side having the internal oxide layer, When the amount of oxygen per unit area in the inner oxide layer on the overlapping surface side of the steel plates is denoted as Os, Os satisfies the formula (2), and the unit of Os is g / m 2 , In the weld heat affected zone on the steel plate side having the internal oxide layer, When the position on the overlapped surface of the steel sheets in the corona joint end is taken as the starting point, and the density of the silicon-containing oxide in the first region surrounded by the range of ±50 μm in the direction parallel to the overlapped surface of the steel sheets and the range of 10 μm from the starting point in the thickness direction is taken as Ns, Ns satisfies the formula (3), and the unit of Ns is pieces / μm 2 , Furthermore, when the average silicon concentration in the second region surrounded by the range of ±50 μm in the direction parallel to the overlapped surface of the steel plate and the range of 5 μm in the plate thickness direction from the starting point is defined as Ss, and the average silicon concentration in the third region of 100 μm square centered at a position 1 / 4 of the thickness of the plate from the starting point in the plate thickness direction is defined as St, Ss and St satisfy the formula (4), and the units of Ss and St are mass %. 0.5<t weld / t all <1.0…(1) 0.02<Os<1.0…(2) 0.01≤Ns…(3) Ss / St<1.0…(4).

2. The welding component according to claim 1, wherein: When the plate thickness of the steel plate is t, the tensile strength of the steel plate is TS, and the average silicon content in the steel plate is Si, The steel plate having the internal oxide layer satisfies the formula (5), wherein the unit of t is mm, the unit of TS is MPa, and the unit of Si is mass %. {(t+Si+TS / 1000)-1.4} / 66+0.01≤Ns…(5) Wherein, Ns in formula (5) is the density of silicon-containing oxide in the first region, in units of pieces / μm 2 .

3. The welding member according to claim 1 or 2, wherein: The Ss and St of the steel plate having the internal oxide layer satisfy formula (7); {(t+Si+TS / 1000)-1.4} / 66<Ss / St<{(t+Si+TS / 1000)-1.4} / 66+0.9…(7) Among them, t in formula (7) is the thickness of the steel plate having the internal oxide layer, the unit is mm, TS is the tensile strength of the steel plate having the internal oxide layer, the unit is MPa, and Si is the average silicon content in the steel plate having the internal oxide layer, the unit is mass %.

4. The welded member according to any one of claims 1 to 3, wherein The steel sheet having the inner oxide layer has a decarburized layer and / or a galvanized 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 welded member according to any one of claims 1 to 5, wherein The average value of the shortest distance from the center of the resistance spot weld to the end surface of the steel plate is greater 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 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, the invention has 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 (8), 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, {(t+Si+TS / 1000)-1.4} / 132<Th<[-{(t+Si+TS / 1000)-1.4} / 66]+1.1…(8) in, In formula (8), t is the thickness of the steel plate having the internal oxide layer, in units of mm, TS is the tensile strength of the steel plate having the internal oxide layer, in units of MPa, and Si is the average silicon content in the steel plate having the internal oxide layer, in units of mass %.

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