Resistance spot welding joint and resistance spot welding method thereof

Abstract

The invention aims to provide a resistance spot welding joint and a resistance spot welding method thereof. The present invention is a resistance spot welded joint obtained by resistance spot welding two or more steel sheets including at least one high-strength steel sheet, the high-strength steel sheet having a specific component composition, the average hardness (Hv) of a first region in a heat-affected region of a resistance spot welded portion satisfying the relationship 0.85 * Hvm > Hv with respect to the hardness (Hvm) of a nugget center portion, and the average hardness (Hv) of a second region in a heat-affected region of the resistance spot welded portion satisfying the relationship 0.85 * Hvm > Hv with respect to the hardness (Hvm) of the nugget center portion. The average number density of carbides having a particle diameter of 100 nm or more in the first region is 10 or more per 5 [mu] m2 in the cross section of the steel sheet.

Description

Technical Field

[0001] The present invention relates to a resistance spot welding joint and a resistance spot welding method thereof. Background Art

[0002] In recent years, for automobile bodies, from the viewpoints of weight reduction for improving fuel consumption and ensuring collision safety, the application of various high-strength steel sheets (also called high-tensile steel sheets) has been promoted. In addition, in the assembly line of automobiles, resistance spot welding is mainly used as a method for joining components.

[0003] As the steel sheet becomes higher in strength, the fusion zone and the HAZ (heat affected zone) tend to become embrittled structures, and thus the hydrogen embrittlement sensitivity of the welded portion becomes high. Furthermore, in resistance spot welding, due to the influence of rust preventive oil, plating, etc. on the steel sheet surface, hydrogen enters the resistance spot welding portion. Therefore, in a resistance spot welding joint using a high-strength steel sheet, delayed fracture is likely to occur.

[0004] As an index for evaluating the quality of the resistance spot welding portion, the fatigue strength can be cited. The fatigue strength does not increase as the steel sheet becomes higher in strength. Instead, as the high alloying accompanying the high strength of the steel sheet makes the fusion zone harder, the fatigue strength is reduced. In addition, it is considered that since a low load is applied to the resistance spot welding portion multiple times, in addition to the hardness of the fusion zone, the hardness, toughness, and structure from the heat affected zone or the end of the fusion zone to the sheet separation zone also affect the fatigue strength.

[0005] Therefore, in the present invention, attention is focused on the region near the sheet separation zone, and technologies for improving the delayed fracture resistance characteristics and the fatigue strength are studied.

[0006] As technologies for solving the above problems, for example, Patent Documents 1 to 3 can be cited. Patent Document 1 discloses: as a spot welding method for a high-strength steel sheet, using at least one high-strength steel sheet having a tensile strength of 750 to 1850 MPa, a single sheet thickness of 0.8 to 3.6 mm, and a carbon equivalent of 0.22 to 0.55 mass%, welding power-on and post-power-on are performed with a pressing force, a current value, and a power-on time that satisfy a specified relational expression, and then electrode holding is performed, thereby improving the cross tensile strength, fatigue strength, delayed fracture resistance, etc.

[0007] Patent Document 2 discloses a spot welding method that uses a high-strength steel sheet containing 0.15 mass% or more of carbon and having a tensile strength of 980 MPa or more and performs the spot welding process in the following three steps: a first power-on step for forming a fusion zone, a cooling step without power-on following the first power-on step, and a second power-on step for softening the fusion zone following the cooling step.

[0008] Patent Document 3 discloses a resistance spot welding method having a main energization process and a post-energization process. In the main energization process, while applying a first pressing force F1 (kN), a first current I1 (kA) is applied to form a nugget portion. Subsequently, in the post-energization process, a second current I2 (kA) is applied for an energization time t a (ms) to cool the nugget portion; this post-energization process includes: a first pressing process that maintains the first pressing force F1 (kN) for a pressing delay time t b (ms) from the start of the post-energization process, and a second pressing process that applies a second pressing force F2 (kN) after the first pressing process.

[0009] Prior Art Documents

[0010] Patent Documents

[0011] Patent Document 1: Japanese Patent Publication No. 6194765

[0012] Patent Document 2: Japanese Patent Publication No. 6107939

[0013] Patent Document 3: Japanese Patent Publication No. 6315161 Summary of the Invention

[0014] In Patent Document 1, due to extremely short-time cooling, the fatigue strength of the above-mentioned high carbon equivalent steel plate cannot be improved. Moreover, Patent Document 1 only describes that the toughness of the spot welded portion can be improved and the tensile residual stress can be reduced, but the mechanism thereof is not clarified. In contrast, according to the present invention described later, it is a technique using the mechanism of relaxing stress by softening the hardness of the sheet gap portion (i.e., the sheet separation zone portion), so the technical idea is different from that of Patent Document 1.

[0015] In Patent Document 2, in the second energization process, tempered martensite is formed by tempering the martensite structure formed in the first energization process, thereby softening the nugget. That is, in Patent Document 2, the structure inside the nugget is tempered. In contrast, according to the present invention described later, the structure inside the nugget is not tempered and is a hardened structure. Therefore, the technical ideas of the two are different.

[0016] In Patent Document 3, by combining the main energization process and the post-energization process, the amount of hydrogen entering the weld metal is reduced, thereby suppressing the reduction of the nugget due to the occurrence of spatter. However, the post-energization process of Patent Document 3 is a process of energizing and cooling the nugget portion, and does not include a cooling process other than the holding time during the energization process. Therefore, the hardness (i.e., softening) of the region near the sheet separation zone cannot be reduced. In contrast, the present invention described later suppresses the hydrogen entering the steel by softening the region near the sheet separation zone. Therefore, the technical ideas of the two are different.

[0017] Moreover, in a resistance spot welding method in which a steel sheet having a tensile strength of 780 MPa or more, particularly a high-strength steel sheet having a tensile strength of 780 MPa or more and a C content of 0.05 to 0.6 mass%, is welded by single energization only, a brittle fusion zone and HAZ are likely to occur. Therefore, there is a problem that high residual stress remains in the HAZ, resulting in delayed fracture. In addition, there is also a problem that the fatigue strength is not improved.

[0018] The present invention has been completed in view of the above problems, and an object thereof is to provide a resistance spot welded joint and a resistance spot welding method having improved delayed fracture resistance characteristics and fatigue strength, the resistance spot welded joint being formed by resistance spot welding a plurality of steel sheets including at least one high-strength steel sheet.

[0019] In the present invention, in order to solve the above problems, a sheet group including at least one high-strength steel sheet is used, and an in-depth study is made on the mechanism of reduction of delayed fracture resistance characteristics and the method of improving fatigue strength.

[0020] As described above, as the steel sheet becomes higher in strength, the delayed fracture resistance characteristics decrease. Regarding the cause of delayed fracture, it is considered to be the influence of the hardness at the end of the fusion zone as the fracture origin and segregation and inclusions present at the end of the fusion zone. In order to avoid these causes, it is effective to prevent hardening at the end of the fusion zone and reduce the residual stress at the end of the fusion zone.

[0021] Specifically, according to this study, it is known that the delayed fracture resistance characteristics can be improved by reducing the hardness of a specific region near the sheet separation zone. As a reason therefor, it can be cited that by softening the hardness near the sheet separation zone, the residual stress in the region from the end of the fusion zone to the sheet separation zone is reduced, and thus, delayed fracture is less likely to occur.

[0022] In addition, it is known that the fatigue strength can also be improved by softening the hardness near the sheet separation zone. As a reason therefor, it can be cited that, as described above, by reducing the hardness near the sheet separation zone, the residual stress in the region from the end of the fusion zone to the sheet separation zone is reduced.

[0023] That is, it has been found that by reducing the hardness of a specific region near the sheet separation zone, the residual stress in the region from the end of the fusion zone to the sheet separation zone is reduced, and as a result, the delayed fracture resistance characteristics are improved, and further, the fatigue strength is improved.

[0024] Based on the above insights, the gist of the present invention is as follows.

[0025] [1] A resistance spot welded joint having: a resistance spot welded portion formed by resistance spot welding two or more steel sheets including at least one high-strength steel sheet,

[0026] The above high-strength steel sheet has the following composition: containing C: 0.05 to 0.6%, Si: 0.1 to 2.0%, Mn: 1.5 to 4.0%, P: 0.10% or less, S: 0.005% or less, N: 0.001 to 0.010%, and O: 0.03% or less by mass%, with the balance being Fe and inevitable impurities;

[0027] For the resistance spot welding part,

[0028] When the length of the line segment of the square with the separated end of the sheet as the center point is set as L (mm), the sheet thickness of the steel sheet with the smallest sheet thickness among the above high-strength steel sheets is set as t (mm), and the region of the above square within the heat-affected zone that satisfies the range of formula (1) with respect to the above sheet thickness is set as the first region,

[0029] The average hardness Hv of the above first region satisfies formula (2) with respect to the hardness Hvm of the nugget center part,

[0030] And, the average number density of carbides with a particle size of 100 nm or more in the above first region is 10 or more per 5 μm of the steel sheet cross-section 2 in 10.

[0031] 0.5×t≥L…(1)

[0032] 0.85×Hvm>Hv…(2)

[0033] [2] The resistance spot welded joint according to [1], wherein the above composition of the high-strength steel sheet further contains, by mass%, one or more selected from Cu: 0.8% or less, Ni: 1.0% or less, Mo: 1.0% or less, Cr: 1.0% or less, Nb: 0.080% or less, V: 0.50% or less, Ti: 0.20% or less, B: 0.005% or less, Al: 2.0% or less, and Ca: 0.005% or less.

[0034] [3] The resistance spot welded joint according to [1] or [2], wherein the above high-strength steel sheet has a coating on the steel sheet surface.

[0035] [4] A resistance spot welding method for a resistance spot welded joint, which is the resistance spot welding method for the resistance spot welded joint described in any one of [1] to [3],

[0036] When two or more steel sheets including at least one of the above high-strength steel sheets are stacked and joined by resistance spot welding,

[0037] As the above resistance spot welding, it has a main energization process and a post-temper heat treatment process,

[0038] The main energization process forms a nugget by energizing with a current value I1 (kA).

[0039] In the post-temper heat treatment process, the following process is carried out:

[0040] During the cooling time t c1 (ms) of the first cooling process, the non-energized state is maintained.

[0041] Next, the first heating process is carried out by energizing with the current value I2 (kA) shown in Equation (4) during the energization time t2 (ms) shown in Equation (5).

[0042] Next, during the cooling time t c2 (ms) of the second cooling process, the non-energized state is maintained.

[0043] Next, the second heating process is carried out by energizing with the current value I3 (kA) shown in Equation (7) during the energization time t3 (ms) shown in Equation (8).

[0044] 800≤t c1 …(3)

[0045] 1.01×I1≤I2≤1.4×I1…(4)

[0046] 100<t2≤500…(5)

[0047] 0<t c2 ≤300…(6)

[0048] I2<I3≤1.5×I2…(7)

[0049] 0<t3<500…(8)

[0050] According to the present invention, it is possible to prevent hardening of the end portion of the nugget in the resistance spot welding portion of the resistance spot welded joint formed by welding a plurality of steel sheets including high-strength steel sheets, and reduce the residual stress in the region from the end portion of the nugget to the sheet separation region. Thus, it is possible to improve the anti-delayed fracture property and fatigue strength of the resistance spot welded joint, and thus have a special effect in the industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 is a cross-sectional view schematically showing the periphery of the resistance spot welding portion of the resistance spot welded joint according to an embodiment of the present invention.

[0052] Figure 2 is schematically showing Figure 1 a cross-sectional view of an example of the peripheral region including the sheet separation region in the resistance spot welded joint shown.

[0053] Figure 3 Schematically shows Figure 1 A cross-sectional view of another example of a peripheral region including a sheet separation region in the resistance spot welding joint shown.

[0054] Figure 4 A diagram showing an example of a method for obtaining the average hardness (Hv) of the first region in the present invention.

[0055] Figure 5 A cross-sectional view showing an example of the resistance spot welding method of the present invention.

[0056] Figure 6 A diagram showing an example of the energization mode of the resistance spot welding method of the present invention. Detailed Description of the Invention

[0057] Hereinafter, the present invention will be described. It should be noted that the present invention is not limited to this embodiment.

[0058] [Resistance Spot Welding Joint]

[0059] First, with reference to Figures 1 to 3 the resistance spot welding joint of the present invention will be described. In Figures 1 to 3 , as an example, a cross-sectional view in the thickness direction of the resistance spot welding portion and its periphery in the resistance spot welding joint of the present invention is shown. In Figures 1 to 3 the example shown, the number of stacked steel plates is set to two.

[0060] The present invention is a resistance spot welding joint (hereinafter referred to as "welding joint") having a resistance spot welding portion formed by resistance spot welding a plurality of stacked steel plates. At least one high-strength steel plate is included in the stacked steel plates. The number of the above-mentioned plurality of steel plates is not particularly limited, and two or more are sufficient. It should be noted that the upper limit of the number of the above-mentioned plurality of steel plates is not particularly specified, and preferably 4 or less.

[0061] Figure 1 is a welding joint obtained by stacking two steel plates and performing resistance spot welding, and the steel plate 1 arranged on the lower side and / or the steel plate 2 arranged on the upper side uses a high-strength steel plate. In Figure 1 the example shown, the upper steel plate 2 uses a high-strength steel plate. It should be noted that, as described later, the high-strength steel plate sometimes has a coating, but in Figure 1 the illustration of the coating on the steel plate surface is omitted. A resistance spot welding portion 10 described below is formed on the steel plate joint surface (i.e., the overlapping surface) 7 of the steel plates 1 and 2.

[0062] [Resistance Spot Welding Portion]

[0063] The resistance spot welding portion in the welding joint of the present invention will be described in detail. As Figure 1As shown, the resistance spot welding portion (hereinafter referred to as the "welding portion") of the welded joint has a nugget 3 and a heat affected zone (HAZ) 6 formed around the nugget 3. There is a sheet separation zone 8 from the outer peripheral edge region within the heat affected zone 6 to the base material portion.

[0064] In the present invention, the structure and properties in a specific region near the sheet separation zone are defined as follows.

[0065] As Figure 1 shown, the line segment length of the square with the end of the sheet separation zone 8 of the welding portion 10 as the center point is set to L (mm), the plate thickness of the high-strength steel sheet (the upper steel sheet 2 in the Figure 1 example shown) is set to t (mm), and the region of the square within the heat affected zone 6 that is surrounded by the range where the length (L) of this line segment satisfies the formula (1) with respect to the plate thickness (t) of the high-strength steel sheet is set as the first region.

[0066] The average hardness (Hv) of this first region satisfies the formula (2) with respect to the hardness (Hvm) of the center portion of the nugget 3, and the average number density of carbides with a particle size of 100 nm or more in this first region is 10 or more per 5 μm in the steel sheet cross-section 2 in.

[0067] 0.5×t≥L…(1)

[0068] 0.85×Hvm>Hv…(2)

[0069] Here, the "first region" refers to the square region surrounded by the line segments L1 and L2 that are at equal intervals from the upper and lower directions with the center point A as the origin and are horizontal to the steel sheet joint surface 7, and the line segments L3 and L4 that are at equal intervals from the right and left directions with the center point A as the origin and are perpendicular to the steel sheet joint surface 7 (refer to Figure 2 , 3 ). In addition, the "end of the sheet separation zone 8" refers to one end of the sheet separation zone 8 on the heat affected zone 6 side. The sheet separation end may be Figure 2 in the shape of a U as shown in the example or Figure 3 in the shape of a V as shown in the example. In addition, the "center point" refers to the intersection point of the sheet separation end and the steel sheet joint surface 7 ( Figure 1 the point A shown).

[0070] The "plate thickness (t) of the high-strength steel sheet" refers to the minimum plate thickness. For example, in the case of laminating two or more high-strength steel sheets with different plate thicknesses, the plate thickness of the high-strength steel sheet with the smallest plate thickness is set as t.

[0071] There is a change in hardness from the center of the fusion zone to the heat-affected zone. In addition, the sizes of the fusion zone and the heat-affected zone vary depending on the plate thickness. Therefore, when the relationship of Equation (1) is not satisfied, it may be a hardness measurement performed at a non-target position (i.e., a non-target area). Therefore, the length (L) of the above-mentioned line segment is set to 0.5×t mm or less. For the reason of conducting the test described in the embodiments of the present invention, the length (L) of the line segment in the first region is preferably more than 0 mm, and more preferably 1.0 mm or more.

[0072] The first region of the present invention is provided at a position away from the end of the fusion zone. Since this is the position where the crack first propagates when the crack propagates, it is a position that has a great influence on the crack propagation in fatigue strength, delayed fracture, etc.

[0073] In the present invention, as described below, by appropriately tempering the first region, the delayed fracture resistance property and fatigue strength are improved.

[0074] [Ratio of hardness]

[0075] It is important that the average hardness (Hv) of the first region satisfies Equation (2) with respect to the hardness Hvm of the center part of the fusion zone. When Equation (2) is not satisfied, the residual stress in the first region cannot be reduced, and as a result, delayed fracture is likely to occur. Therefore, the average hardness of the first region is set to be less than 0.85 times the hardness of the center part of the fusion zone. Preferably, the average hardness of the first region is set to 0.80 times or less the hardness of the center part of the fusion zone.

[0076] It should be noted that there is no particular limitation on the upper limit value of the average hardness of the first region. However, from the viewpoint of appropriately performing tempering, the average hardness of the first region is preferably 0.50 times or more, and more preferably 0.55 times or more the hardness of the center part of the fusion zone.

[0077] Each of the above hardnesses refers to the Vickers hardness measured according to JIS Z2244 (2020).

[0078] The above-mentioned "average hardness (Hv) of the first region" can be measured by the following method. Specifically, according to the measurement method described in the embodiments below, at positions spaced 0.2 mm apart in the plate thickness direction of the high-strength steel plate side from the steel plate joint surface 7 and on a straight line parallel to the steel plate joint surface 7, measurements are made at intervals of 0.2 mm. The measurement region is the range from the line segment L3 to the line segment L4 of the first region. The average value of the hardness in this measurement region is obtained as the "average hardness of the first region".

[0079] For example, as Figure 1 shown, when the upper steel plate 2 uses a high-strength steel plate, the measurement is performed in the above-mentioned measurement region at a position 0.2 mm above in the plate thickness direction of the steel plate 2 side from the steel plate joint surface 7.

[0080] Further, for example, although not shown in the drawings, when both of the upper and lower steel plates 1 and 2 are made of high-strength steel plates, the same method as described above is used for measurement on the side of the high-strength steel plate having a low strength (low tensile strength).

[0081] In addition, the "hardness (Hvm) at the center of the nugget" refers to the hardness at the center position within the nugget 3. Specifically, as described in the measurement method in the embodiments described later, the hardness at the position on the steel plate joint surface 7 and the center position of the nugget 3 is measured, and the obtained value is used as the "hardness at the center of the nugget".

[0082] [Carbides in the first region]

[0083] In the present invention, as an index indicating tempering, the average number density of carbides in the first region is defined. Specifically, the average number density of carbides having a particle size of 100 nm or more in the first region is 10 or more per 5 μm of the steel plate cross-section 2 . It should be noted that in the embodiments described later, Figure 4 is used for description. In the present invention, as long as the average number density of the above-mentioned carbides is satisfied at at least one place in a 5-μm square field of view in the first region, the effects described later can be obtained.

[0084] The reason why the particle size of the above-mentioned carbides is 100 nm or more is that it has been confirmed that coarse carbides are formed by sufficient tempering. However, if the particle size of the carbides is large, there may be precipitates other than the carbides generated during tempering. Therefore, the particle size of the carbides is preferably 500 nm or less.

[0085] In addition, in the first region, when the average number density of carbides having the above-mentioned particle size is less than 10 in a 5-μm square field of view on the steel plate cross-section, the tempering is insufficient. As a result, the residual stress near the first region becomes high, and cracks easily invade the welded portion, so the delayed fracture resistance and fatigue strength are reduced. Therefore, the above-mentioned average number density is set to 10 or more per 5 μm 2 of the steel plate cross-section. The above-mentioned average number density is more preferably 15 or more per 5 μm 2 of the steel plate cross-section. It should be noted that the upper limit of the above-mentioned average number density is not particularly limited. Among them, considering the particle size of the carbides, the above-mentioned average number density is preferably 450 or less per 5 μm 2 of the steel plate cross-section, more preferably 400 or less per 5 μm 2 of the steel plate cross-section, and further preferably 200 or less per 5 μm 2 of the steel plate cross-section.

[0086] It should be noted that in the present invention, the particle size of the carbide and the average number density of the carbide can be measured by the method described in the following examples.

[0087] [Microstructure of the first region] (Optimal conditions)

[0088] In order to satisfy the above hardness ratio, the hardness of the first region is preferably controlled such that the microstructure in the first region has a tempered martensite microstructure. Thereby, the residual stress from the end of the fusion zone to the sheet separation region can be reduced more effectively. In order to obtain such an effect, the area ratio of the tempered martensite in the first region to the entire first region is preferably 50% or more. The area ratio of the tempered martensite in the first region is more preferably 55% or more.

[0089] It should be noted that there is no particular limitation on the upper limit of the tempered martensite in the first region. As described above, this is because even when the microstructure of the first region is composed of tempered martensite with an area ratio of 100%, the effect of reducing the residual stress can be expected. The tempered martensite in the first region is preferably 100% or less, and more preferably 98% or less.

[0090] The remaining microstructure other than the tempered martensite in the first region is martensite and / or ferrite. Among them, if the area ratio of the microstructure other than the tempered martensite (the remaining microstructure) is large, it is difficult to reduce the residual stress. For this reason, the total area ratio of the above remaining microstructure is preferably 50% or less, more preferably 45% or less, further preferably 40% or less, and even more preferably 30% or less. It should be noted that the total area ratio of the above remaining microstructure can also be 0%.

[0091] It should be noted that in the present invention, by appropriately controlling the "average number density of carbide" which is an index representing tempering, it is possible to judge that the tempered martensite is within the above area ratio range.

[0092] Although not shown in the figure, when welding three or more steel sheets, the first region exists at each steel sheet joint surface. As long as at least one of the first regions in each steel sheet joint surface has the above microstructure and properties, the effects of the present invention can be obtained.

[0093] [High-strength steel sheet]

[0094] The reasons for limiting the composition of the base material of the high-strength steel sheet in the welded joint of the present invention will be described. It should be noted that in the following description, the "%" mark of the composition refers to "mass%" unless otherwise specified.

[0095] C: 0.05 - 0.6%

[0096] C is an element that contributes to the strengthening of steel. When the C content is less than 0.05%, the strength of the steel decreases, and it is extremely difficult to produce a steel plate with a tensile strength of 780 MPa or more. On the other hand, if the C content exceeds 0.6%, although the strength of the steel plate becomes high, the amount of hard martensite is excessive and the microvoids increase. In addition, in the welded joint, the fusion zone and its surrounding HAZ are excessively hardened, and embrittlement also develops, so it is difficult to improve the fatigue strength. Furthermore, cracks are likely to appear in the embrittled molten part, so delayed fracture is likely to occur. Therefore, the C content is set to 0.05 - 0.6%. The C content is preferably 0.10% or more. The C content is preferably 0.50% or less, more preferably 0.45% or less.

[0097] Si: 0.1 - 2.0%

[0098] If the Si content is 0.1% or more, it effectively acts on the strengthening of steel. In addition, since Si is a ferrite-forming element, it plays a dominant role in the formation of ferrite at the end of the fusion zone. On the other hand, if the Si content exceeds 2.0%, although the steel is strengthened, it has an adverse effect on toughness. Therefore, the Si content is set to 0.1 - 2.0%. The Si content is preferably 0.2% or more. The Si content is preferably 1.8% or less.

[0099] Mn: 1.5 - 4.0%

[0100] If the Mn content is less than 1.5%, it is impossible to obtain a high fatigue strength for the welded joint without long-time cooling of the welded part as in the present invention. On the other hand, if the Mn content exceeds 4.0%, embrittlement of the welded part or cracks accompanied by embrittlement are significantly observed, making it difficult to improve the fatigue strength and the characteristics of resistance to delayed fracture. Therefore, the Mn content is set to 1.5 - 4.0%. The Mn content is preferably 2.0% or more. The Mn content is preferably 3.5% or less, more preferably 2.9% or less.

[0101] P: 0.10% or less

[0102] P is an inevitable impurity. If the P content exceeds 0.10%, it is difficult to improve the joint strength due to strong segregation at the end of the fusion zone of the welded part. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.05% or less, more preferably 0.02% or less. It should be noted that the lower limit of the P content is not particularly limited. However, excessive reduction will lead to an increase in cost. Therefore, the P content is preferably 0.005% or more.

[0103] S: 0.005% or less

[0104] S is an element that segregates at grain boundaries and embrittles steel. S is an inevitably contained element. Furthermore, S reduces the local deformation ability of sulfides and steel plates. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.004% or less, and more preferably 0.003% or less. It should be noted that the lower limit of the S content is not particularly limited. However, excessive reduction will lead to an increase in cost. Therefore, the S content is preferably 0.001% or more.

[0105] N: 0.001 - 0.010%

[0106] N is an element that deteriorates the aging resistance of steel. N is an inevitably contained element. Therefore, the N content is set to 0.001 - 0.010%. The N content is preferably 0.008% or less.

[0107] O: 0.03% or less

[0108] O (oxygen) is an element that deteriorates the cleanliness and toughness of steel due to the formation of non-metallic inclusions. Therefore, the O content is set to 0.03% or less. The O content is preferably 0.02% or less. Additionally, the O content is preferably 0.005% or more.

[0109] The high-strength steel plate used in the present invention contains the above elements, and the remaining part is Fe and inevitable impurities.

[0110] In the present invention, the above composition is the basic composition of the high-strength steel plate. In the present invention, in addition to the above composition, one or more elements selected from Cu, Ni, Mo, Cr, Nb, V, Ti, B, Al, and Ca can be added as needed. It should be noted that since these elements can be contained as needed, these elements can also be 0%.

[0111] Cu: 0.8% or less, Ni: 1.0% or less, Mo: 1.0% or less

[0112] Cu, Ni, and Mo are elements that can help improve the strength of steel. However, for Cu, Ni, and Mo, if added in large amounts, the toughness deteriorates. Therefore, when these elements are contained, the Cu content is set to 0.8% or less, the Ni content is set to 1.0% or less, and the Mo content is set to 1.0% or less, respectively. The Cu content is more preferably 0.6% or less. The Cu content is preferably 0.005% or more, and more preferably 0.006% or more. The Ni content is more preferably 0.8% or less. The Ni content is preferably 0.01% or more. The Mo content is more preferably 0.8% or less. The Mo content is preferably 0.005% or more, and more preferably 0.006% or more.

[0113] Cr: 1.0% or less

[0114] Cr is an element that can increase strength by improving hardenability. However, if the Cr content exceeds 1.0% excessively, the toughness of the HAZ may deteriorate. Therefore, when Cr is contained, the Cr content is set to 1.0% or less. The Cr content is more preferably 0.8% or less. The Cr content is preferably 0.01% or more.

[0115] Nb: 0.080% or less

[0116] Nb improves the cross-tensile strength and the delayed fracture resistance characteristics after resistance spot welding by forming fine carbonitrides. To obtain this effect, a Nb content of 0.005% or more is preferably contained. On the other hand, if a large amount of Nb is added, not only the elongation rate is significantly reduced, but also the toughness is significantly impaired. Thus, when Nb is contained, the Nb content is set to 0.080% or less. The Nb content is more preferably 0.070% or less, and still more preferably 0.060% or less. The Nb content is preferably 0.005% or more, and more preferably 0.006% or more.

[0117] V: 0.50% or less

[0118] V is an element that can strengthen steel by controlling the structure through precipitation hardening. However, if a large amount of V is added, it causes deterioration of the HAZ toughness. Therefore, when V is contained, the V content is set to 0.50% or less. The V content is more preferably 0.30% or less. The V content is preferably 0.005% or more, and more preferably 0.02% or more.

[0119] Ti: 0.20% or less

[0120] Ti is an element that can improve hardenability to strengthen steel. However, if a large amount of Ti is added, carbides are formed, and due to the precipitation hardening of these carbides, the toughness deteriorates significantly. Therefore, when Ti is contained, the Ti content is set to 0.20% or less. The Ti content is more preferably 0.15% or less. The Ti content is preferably 0.003% or more, and more preferably 0.004% or more.

[0121] B: 0.005% or less

[0122] B is an element that can improve hardenability to strengthen steel. Therefore, when B is contained, the B content is preferably 0.0005% or more, and more preferably 0.0007% or more. However, even if a large amount of B is added, the above effects are saturated. Therefore, when B is contained, the B content is set to 0.005% or less. The B content is more preferably 0.0020% or less, and still more preferably 0.0010% or less.

[0123] Al: 2.0% or less

[0124] Al is an element capable of controlling the microstructure for austenite grain refinement. If added in a large amount, the toughness deteriorates. Therefore, when Al is contained, the Al content is set to 2.0% or less. The Al content is more preferably 1.5% or less, further preferably 1.3% or less, and even more preferably 1.2% or less. The Al content is preferably 0.01% or more.

[0125] Ca: 0.005% or less

[0126] Ca is an element that can contribute to improving the workability of steel. However, if Ca is added in a large amount, the toughness deteriorates. Therefore, when Ca is contained, the Ca content is set to 0.005% or less. The Ca content is more preferably 0.004% or less. The Ca content is preferably 0.001% or more.

[0127] The tensile strength of the high-strength steel sheet having the above composition is preferably 780 MPa or more. The tensile strength of the high-strength steel sheet is more preferably 1180 MPa or more. As described above, especially when the tensile strength of the base material is 780 MPa or more, the fatigue strength and the delayed fracture resistance may decrease. According to the present invention, even for a high-strength steel sheet with a tensile strength of 780 MPa or more, by reducing the hardness of a specific region near the sheet separation zone and reducing the residual stress in the region from the end of the fusion core to the sheet separation zone, the reduction of the fatigue strength and the delayed fracture resistance of the welded portion can be suppressed. It should be noted that a high-strength steel sheet with a tensile strength of less than 780 MPa can of course also achieve the above effects.

[0128] [Coating type of high-strength steel sheet]

[0129] Even for the high-strength steel sheet of the present invention, which is a steel sheet having a galvanized layer on the steel sheet surface after galvanizing treatment (i.e., a galvanized steel sheet), the above effects can be obtained. The galvanized layer refers to a coating mainly composed of zinc. The coating mainly composed of zinc may include, for example, a hot-dip galvanized layer, an electro-galvanized layer, a Zn-Al coating, and a Zn-Ni layer. In addition, the high-strength steel sheet of the present invention may also be an alloyed galvanized steel sheet having an alloyed galvanized layer on the surface of the base material after the above galvanizing treatment and then an alloying treatment.

[0130] It should be noted that in the present invention, the stacked steel sheets may be multiple stacked same steel sheets, or may also be multiple stacked different steel sheets. It is also possible to stack a steel sheet having a galvanized layer on the steel sheet surface (here a galvanized steel sheet) and a steel sheet not having a galvanized layer on the steel sheet surface (here a cold-rolled steel sheet). It does not matter whether the plate thicknesses of the respective steel sheets are the same or different. From the viewpoint of targeting ordinary automotive steel sheets, for example, the plate thickness of the steel sheet is preferably 0.4 mm to 2.2 mm.

[0131] [Resistance spot welding method]

[0132] Next, one embodiment of a resistance spot welding method for producing a welded joint of the present invention having the above-described welded portion will be described.

[0133] The welded joint of the present invention can be manufactured by resistance spot welding as follows: a plate group consisting of two or more superimposed steel plates including at least one of the above-mentioned high-strength steel plates is clamped by a pair of welding electrodes and joined while applying pressure and applying electricity by the pair of welding electrodes.

[0134] For example Figure 5 As shown in FIG. 1 , two steel plates 1 and 2 are stacked to form a plate group. Next, the plate group is clamped by a pair of welding electrodes 4 and 5 disposed on the lower and upper sides of the plate group, and current is applied while being pressurized and controlled to a predetermined welding condition. Thus, the above-mentioned welded portion (see FIG. 1 ) can be formed by joining the plates as the steel plate joint surface 7. Figures 1 to 3 ). It should be noted that when a high-strength cold-rolled steel sheet and a high-strength galvanized steel sheet are stacked to form a sheet group, the plurality of steel sheets may be stacked in such a manner that the surface of the high-strength galvanized steel sheet having the galvanized layer faces the high-strength cold-rolled steel sheet.

[0135] In the present invention, as a process of applying electricity to the superimposed steel plates 1 and 2 clamped by the welding electrodes 4 and 5, as shown in FIG. Figure 6 As shown, there are a main energization process and a post-tempering heat treatment process. In addition, the post-tempering heat treatment process has a first cooling process, a first heating process, a second cooling process, and a second heating process. Hereinafter, each process of the present invention will be described in detail.

[0136] <Main energization process>

[0137] The main energization step is a step of melting the steel plate joint surface 7 of the steel plates 1 and 2 to form a nugget 3 of a desired size (see Figure 5 In the main energizing step, a current of I1 (kA) is passed to form a nugget.

[0138] The diameter of the nugget used in the resistance spot welding (welding) of automobile steel plates is usually 3.0√t sheet ~6.0√t sheet (Here, t sheet (mm) is the plate thickness). In the present invention, this numerical range is referred to as the "target nugget diameter (mm)". In the main energizing step of the present invention, the energizing conditions and pressurizing conditions for forming the nugget 3 are not particularly limited as long as the nugget 3 having the target nugget diameter can be obtained.

[0139] The above “t sheet "t" in "(mm) plate thickness" sheet " refers to the thickness of the smallest steel plate among the steel plates used in the plate group.

[0140] From the viewpoint of stably forming a nugget 3 having a target nugget diameter at the joint surface of the steel plates when using the high-strength steel plate of the present invention in the superposed steel plates, it is preferable to control the energization conditions and the pressing conditions in the main energization process as follows.

[0141] The current value I1 (kA) in the main energization process is preferably 3.0 kA to 8.0 kA. If the current value I1 is too small, it may not be possible to stably obtain the target nugget diameter. On the other hand, if the current value I1 is too large, the nugget diameter may become too large, or the melting degree of the steel plate may increase, resulting in splashing and the melted welded portion overflowing from between the plates, and the nugget diameter may become smaller. For these reasons, the current value I1 is set to 3.0 kA to 8.0 kA. The current value I1 is more preferably 3.5 kA or more. The current value I1 is more preferably 7.5 kA or less.

[0142] The energization time t1 (ms) in the main energization process is preferably 120 ms to 400 ms. This is the time for stably forming the nugget 3 having the target nugget diameter, similar to the current value I1. When the energization time t1 is less than 120 ms, it may be difficult to generate a nugget. On the other hand, if the energization time t1 exceeds 400 ms, there is a concern that the formed nugget diameter may be larger than the target nugget diameter and the workability may be reduced. The energization time t1 is preferably 200 ms or more. The energization time t1 is preferably 300 ms or less. However, as long as the required nugget diameter can be obtained, the energization time t1 can be set shorter or longer than the above numerical range.

[0143] In the pressing condition in the main energization process, it is preferable to set the pressing force to 2.0 kN to 7.0 kN. If the pressing force is too large, the energized diameter expands, so it is likely to be difficult to ensure the nugget diameter. On the other hand, if the pressing force is too small, the energized diameter decreases and splashing is likely to occur. For these reasons, the pressing force is set to 2.0 kN to 7.0 kN. The pressing force is more preferably 3.0 kN or more and more preferably 6.5 kN or less. The pressing force is sometimes limited by the capacity of the device used. As long as the pressing force for obtaining the required nugget diameter can be obtained, the pressing force can be set lower or higher than the above numerical range.

[0144] <Post-tempering heat treatment process>

[0145] The post-temper heat treatment process refers to a post-heat treatment process in which the first region in the welded portion formed in the main energization process is tempered to reduce the residual stress at the time of delayed fracture. Specifically, it is a process of effectively tempering to make the structure of the first region tempered martensite. In the post-temper heat treatment process, after the main energization process, the first cooling process, the first heating process, the second cooling process, and the second heating process are sequentially performed on the welded portion. In order to reduce the residual stress in the region from the end of the fusion zone to the sheet separation zone by tempering the first region, it is important to control the welding conditions of each process in the post-temper heat treatment process as follows.

[0146] [First Cooling Process]

[0147] First, after the main energization process, cooling is performed to lower the end of the fusion zone to the temperature at which martensite transformation occurs. This process is called the first cooling process. In this first cooling process, in order to fully obtain the subsequent tempering effect, during the cooling time t shown in Equation (3) c1 (ms), the welding portion is cooled while maintaining a non-energized state.

[0148] 800 ≤ t c1 …(3)

[0149] When the cooling time t of the first cooling process c1 (ms) is less than 800 ms, martensite transformation does not occur sufficiently, and since martensite does not appear, the structure remains as retained austenite. Therefore, even if the subsequent processes (specifically, the first heating process, the second cooling process, and the second heating process) are performed, austenite remains as it is and finally becomes martensite structure (i.e., quenched martensite structure). As a result, the first region becomes a brittle structure, and thus the delayed fracture resistance property is not improved. Therefore, the cooling time t c1 (ms) is set to 800 ms or more. The cooling time t c1 is preferably 850 ms or more, more preferably 900 ms or more, and further preferably 1000 ms or more. In the first cooling process, in the subsequent heating process, in order to further generate carbides and increase the average number density of the above carbides, sufficient martensite transformation below the Ms point is performed.

[0150] The upper limit of the cooling time t of the first cooling process c1 (ms) is not particularly limited. Since the steel sheet targeted in the present invention is an automotive steel sheet, if the welding time is too long, the implementation efficiency decreases. Therefore, the cooling time t c1 (ms) is preferably 2200 ms or less, more preferably 2000 ms or less.

[0151] [First Heating Process]

[0152] After the first cooling process, the first heating process is carried out. During the first heating process, in order to temper the martensite structure formed due to cooling in the previous process, energization (i.e., post-energization) is carried out to raise the temperature to an appropriate temperature range. The above-mentioned "appropriate temperature range" refers to the tempering temperature range for softening the hardness of the first region.

[0153] Specifically, during the first heating process, the welded portion is energized with a current value I2 (kA) shown in Formula (4) for a energization time t2 (ms) shown in Formula (5).

[0154] 1.01×I1≤I2≤1.4×I1…(4)

[0155] 100<t2≤500…(5)

[0156] Generally, tempering energization is carried out with a current value lower than this energization to temper the vicinity of the end of the fusion zone. However, in the present invention, in order to temper the first region located away from the fusion zone, post-energization at a high temperature is carried out. Here, it is particularly important to rapidly raise the temperature to an appropriate temperature for tempering the first region, that is, a temperature above the Ac1 point, in a short time. Thereby, the structure of the first region can be made into tempered martensite, and tempering can be effectively carried out.

[0157] If the current value I2 in this process is too low, the tempering effect is weak. On the other hand, if the current value I2 in this process is too high, since it exceeds the Ac3 point, the structure at the end of the fusion zone cannot be made into tempered martensite. Or sometimes spatter occurs and the molten metal overflows outward, resulting in an inability to obtain the required fusion zone diameter.

[0158] For such reasons, the current value I2 (kA) in the first heating process satisfies the relationship of Formula (4). When the current value I2 in the first heating process is less than (1.01×I1) (kA), the temperature becomes lower than the Ac1 point, and the end of the fusion zone cannot be effectively tempered. The current value I2 is preferably (1.12×I1) (kA) or more.

[0159] On the other hand, when the current value I2 in the first heating process exceeds (1.4×I1) (kA), the possibility of exceeding the Ac3 point is high, and in the subsequent process, austenite phase transformation occurs again, and finally it becomes a martensite structure and becomes brittle. As a result, the toughness of the end of the fusion zone cannot be obtained. The current value I2 is preferably (1.35×I1) (kA) or less.

[0160] As described above, in the first heating process, in order to rapidly increase the temperature in a short time, the energization time t2 (ms) of the first heating process satisfies formula (5). The energization time t2 is preferably 120 ms or more. The energization time t2 is preferably 400 ms or less.

[0161] By increasing post-heating within an appropriate temperature range in this way, tempering is promoted, the average number density of carbides can be increased, and as a result, the hardness is also reduced. Therefore, the hardness ratio with respect to the center part of the fusion nucleus also becomes smaller.

[0162] [Second cooling process]

[0163] The second cooling process is performed after the first heating process.

[0164] In the first region, the heating rate in the first heating process is increased to form tempered martensite structure. However, since the tempering time in the first heating process alone is short, retained austenite remains in a part of the structure in the first region. However, if the post-energization time is directly delayed to avoid this, or post-energization is performed without setting a cooling process, the possibility of spatter occurring during this post-energization becomes high. Therefore, in the present invention, a second cooling process for temporarily reducing the temperature of the welded part is provided after the first heating process.

[0165] Specifically, as the second cooling process, cooling is performed while maintaining a non-energized state during the cooling time t c2 (ms) shown in formula (6).

[0166] 0 < t c2 ≤ 300…(6)

[0167] In the second cooling process, the purpose is to temporarily slightly reduce the temperature of the welded part, and the energization time t c2 is greater than 0 ms and 300 ms or less. To avoid spatter and effectively perform tempering, the cooling time t c2 of the second cooling process is preferably 20 ms or more. The cooling time t c2 is preferably 250 ms or less, more preferably 200 ms or less.

[0168] [Second heating process]

[0169] From the viewpoint of performing appropriate temperature control to effectively perform tempering in the first region, the second heating process is performed after the second cooling process. Specifically, in the second heating process, the welded part is energized with the current value I3 (kA) shown in formula (7) during the energization time t3 (ms) shown in formula (8).

[0170] I2 < I3 ≤ 1.5 × I2…(7)

[0171] 0 < t3 < 500…(8)

[0172] The current value I3 (kA) in the second heating process needs to exceed the current value I2 (kA) in the first heating process. When the current value I3 in the second heating process is equal to or less than the current value I2 (kA) in the first heating process, there is a possibility that the tempering temperature is too low to promote tempering in the first region. As a result, the structure in the first region remains in a hardened state, and there is a concern about an increase in residual stress. The current value I3 in the second heating process is preferably (1.1 × I2) (kA) or more.

[0173] On the other hand, when the current value in the second heating process is too high, there is a possibility of spatter occurring and a possibility that the first region becomes a hard structure due to the re-transformation of tempered martensite into martensite. For these reasons, the current value I3 in the second heating process is set to be (1.5 × I2) (kA) or less. The current value I3 is preferably (1.45 × I2) (kA) or less, and more preferably (1.40 × I2) (kA) or less.

[0174] In addition, in order to maintain the tempering temperature in the first heating process, the energization time t3 in the second heating process satisfies Equation (8). The energization time t3 is preferably 40 ms or more, and more preferably 80 ms or more. The energization time t3 is selected to be 250 ms or less.

[0175] It should be noted that in the present invention, the post-tempering heat treatment process is divided into two stages: the first cooling process and the first heating process, and the second cooling process and the second heating process. By controlling each process as described above, there is no need to set other processes after the post-tempering heat treatment process.

[0176] As described above, in the resistance spot welding method of the present invention, by appropriately controlling the welding conditions of the post-tempering heat treatment process, the first region near the sheet separation zone can be made into tempered martensite, and the residual stress from the end of the fusion core to the sheet separation zone can be reduced. That is, the welded joint obtained by this welding method can improve the delayed fracture resistance characteristic and further improve the fatigue strength due to the reduction of residual stress.

[0177] Examples

[0178] Hereinafter, the functions and effects of the present invention will be described using examples. It should be noted that the present invention is not limited to the following examples.

[0179] Test pieces were made of steel plates (Steel plates A - L) with a tensile strength of 780 MPa to 1800 MPa and a plate thickness of 0.8 to 1.2 mm as shown in Table 1 and Table 2. The dimensions of the test pieces were: long side: 100 mm, short side: 30 mm. Table 1 shows the component compositions of Steel plates A - L. It should be noted that "-" in Table 1 indicates that the element was not deliberately added, including not only the case of not containing the element (0%), but also the case of unavoidably containing it. "GA steel plate" shown in Table 2 represents the above - mentioned alloy - coated galvanized steel plate.

[0180] First, the obtained test pieces were overlapped and arranged as shown in Table 2 to form a plate group. It should be noted that the "overlapping position of steel plates" in Table 2 was counted as "the first", "the second" in sequence from the lower - side steel plate. Then, using each plate group, resistance spot welding was carried out under the welding conditions shown in Table 3 to form a nugget 3 of the required size between the plates and produce a resistance spot - welded joint. Some of the plate groups were composed of three overlapping steel plates. It should be noted that "-" in Table 3 indicates that this process was not carried out.

[0181] In this embodiment, as Figure 5 shown, for a plate group formed by overlapping multiple steel plates ( Figure 5 in the example shown, the lower - side steel plate 1 and the upper - side steel plate 2), resistance spot welding was carried out using a C - type gun servo - pressurized DC resistance welder.

[0182] It should be noted that other welding conditions were carried out according to the conditions shown below. The applied pressure during energization was set to be constant, and here it was 3.5 kN. For both the lower - side welding electrode 4 and the upper - side welding electrode 5 with respect to the plate group, the diameter of the tip was set to 6 mm and the radius of curvature of the tip was set to 40 mm, and DR - type electrodes made of chromium - copper were used. The applied pressure was controlled by the lower - side welding electrode 4 and the upper - side welding electrode 5, and welding was carried out using a DC power source. When the nugget diameter was set to the plate thickness: t sheet (mm), it was formed in such a way that it was 5.5√t sheet (mm) or less.

[0183] Using the obtained resistance spot - welded joints (welded joints), the delayed fracture test and the fatigue strength test were carried out and evaluated according to the methods described below. In addition, according to the methods described below, the evaluation of the welded part, the determination of the hardness of the first region, the measurement of the particle size of the carbides in the first region, and the measurement of the average number density of these carbides were carried out respectively.

[0184] [Evaluation of resistance to delayed fracture characteristics]

[0185] The evaluation of the resistance to delayed fracture characteristics was carried out as follows. Using the fabricated welded joints, after 24 hours in the atmosphere, it was confirmed whether there was delayed fracture. The phenomenon that the nugget cracked in half at the bonding interface in the welded part was defined as the occurrence of delayed fracture.

[0186] After leaving the welded joints stationary for 24 hours, those with delayed fracture are marked with the symbol "×", while those without delayed fracture are marked with the symbol "〇". It should be noted that in this embodiment, the case of the symbol "○" is evaluated as good, and the case of the symbol "×" is evaluated as poor. The measurement results are shown in Table 4.

[0187] [Evaluation of Fatigue Strength]

[0188] The evaluation of fatigue strength is carried out according to the fatigue strength test of JIS Z3138. Specifically, test pieces for tensile-shear fatigue test of the shape specified in JIS Z3138 are made using the produced welded joints, a load of 2 kN is continuously applied to the shear tensile test pieces, and the number of times of applying the above load until fracture is counted.

[0189] On the other hand, for comparison, resistance spot welding of the plate group is carried out with the above welding conditions only as the main energization to produce welded joints for comparison. The same fatigue strength test as above is carried out using the welded joints for comparison, and the number of times of load application (hereinafter referred to as "comparison repetition number") until the shear tensile test piece fractures is counted.

[0190] Then, the obtained counts are compared with the above comparison repetition number. The case where the ratio of each count to the above comparison repetition number is less than 1.2 times is marked with the symbol "×", and the case where the ratio is 1.2 times or more is marked with the symbol "〇".

[0191] It should be noted that in this embodiment, the case of the symbol "○" is evaluated as good, and the case of the symbol "×" is evaluated as poor. The measurement results are shown in Table 4.

[0192] [Evaluation of Welded Part]

[0193] In this embodiment, the evaluation of the welded part of the welded joint is carried out using the above evaluations of resistance to delayed fracture characteristics and fatigue strength. In Table 4, when the evaluations of both the resistance to delayed fracture characteristics and fatigue strength are "〇", the welded part is evaluated as "〇 (qualified)".

[0194] On the other hand, when any one of the evaluations of the resistance to delayed fracture characteristics and fatigue strength is "×", or when the evaluations of both the resistance to delayed fracture characteristics and fatigue strength are "×", the welded part is evaluated as "× (unqualified)".

[0195] [Determination of Hardness in the First Region]

[0196] The measurement of the hardness at the center of the fusion zone and in the first region is carried out as follows.

[0197] Cut the fabricated welded joint at the position passing through the center of the nugget formed in a circular shape as a test piece. After ultrasonically cleaning the test piece, grind the plate thickness cross-section of the sample filled with resin, etch it using a picric acid solution, and prepare the sample. The hardness is measured by a Vickers hardness tester according to the method specified in JIS Z2244. The measurement load is applied under the condition of loading a 300 gf indenter for 15 seconds.

[0198] Specifically, for the Vickers hardness, the upper left of the first region is numbered 1, and each measurement is taken 0.2 mm to the right until the right end of the first region, and then at a position 0.2 mm below the number 1. Repeat this, and take the average value measured at the lower right of the first region as the average hardness of the first region. Here, the high-strength steel plate side with lower strength (i.e., lower tensile strength) in the laminated steel plates is used to evaluate the steel plate.

[0199] It should be noted that in the "Hardness Judgment" column in Table 4, the judgment result of whether the hardness ratio (Hv / Hvm) satisfies less than 0.85 is shown. The symbol "〇" indicates a qualified case (i.e., Hv / Hvm is less than 0.85), and the symbol "×" indicates an unqualified case (i.e., Hv / Hvm is 0.85 or more).

[0200] [Particle size and average number density of carbides in the first region]

[0201] As Figure 1 shown, observe the steel plate structure in the first region. Cut out this region from the obtained resistance spot welding component, grind the plate thickness cross-section, then etch it with 3% nitric acid ethanol, and observe it using a TEM (transmission electron microscope) at a magnification of 10,000 times. Further, use Image-Pro, set the lower limit to 0.005 μm, calculate the equivalent circle diameter of cementite, and obtain the particle size of cementite.

[0202] For the average number density (number / 5 μm 2 ) of cementite with a particle size of 100 nm or more, observe it using a TEM (transmission electron microscope) at a magnification of 10,000 times, and obtain the number density per 5 μm 2 in the steel plate cross-section at 5 positions. Take the average value of the obtained values as the average number density of carbides with a particle size of 100 nm or more per 5 μm 2 in the steel plate cross-section. Show this average number density in Table 4. It should be noted that if the particle size of the carbide becomes larger, there may be precipitates other than the carbides generated during tempering, so the particle size of the carbide is set to 500 nm or less. Here, the high-strength steel plate side with lower strength (i.e., lower tensile strength) in the laminated steel plates is used to evaluate the steel plate.

[0203] Figure 4 The order of the above observations is shown. As Figure 4As shown, within the range of distance L (i.e., within the first region), as long as the average number density (number / 5μm 2 ) is satisfied at one position. For example, as Figure 4 shown, the first region is subdivided into a checkerboard grid pattern, with the upper left of the first region being numbered 1. Starting from the position indicated by "1", observe continuously while moving the observation position until the observation result satisfies the above average number density (i.e., reaches qualified). The first observation is from the position of point 1 to a position (0.2×L) mm away from the base material part (here in the direction of the right side of the paper surface of Figure 4 ) to observe the average number density. When the observation result does not satisfy the above average number density (i.e., in the case of unqualified), the second observation is made at the position indicated by "2". The second observation is also made at a position (0.2×L) mm away from the position of point 2 to the base material part. Continue observing in sequence until the observation result is "qualified".

[0204]

[0205] [Table 2]

[0206]

[0207]

[0208]

[0209] According to Tables 3 and 4, in the examples of the present invention, a welded joint formed by resistance spot welding of a plurality of steel sheets including at least one high-strength steel sheet is a good welded joint having excellent shear tensile strength. In contrast, in the comparative examples, good welded joints were not obtained.

[0210] Symbol Explanation

[0211] 1, 2 Steel sheets

[0212] 3 Nugget

[0213] 4, 5 Welding electrodes

[0214] 6 Heat-affected zone

[0215] 7 Steel sheet joint surface

[0216] 8 Sheet separation zone

[0217] 9 First region

[0218] 10 Resistance spot welding part

Claims

1. A resistance spot welding joint, comprising: a resistance spot welding portion formed by resistance spot welding two or more steel plates including at least one high-strength steel plate, The high-strength steel plate has the following composition: containing C: 0.05 - 0.6% by mass, Si: 0.1 - 2.0% by mass, Mn: 1.5 - 4.0% by mass, P: 0.10% or less, S: 0.005% or less, N: 0.001 - 0.010% by mass, and O: 0.03% or less, with the balance being Fe and unavoidable impurities; For the resistance spot welding portion, When the length of the line segment of the square with the separated end of the plate as the center point is set as L mm, the thickness of the steel plate with the smallest thickness among the high-strength steel plates is set as t mm, and the area of the square in the heat-affected zone that is surrounded by the length of the line segment satisfying the range of formula (1) with respect to the plate thickness is set as the first region, The average hardness Hv of the first region satisfies formula (2) with respect to the hardness Hvm of the nugget center portion, Further, the average number density of carbides having a particle size of 100 nm or more in the first region is 10 or more per 5 μm of the steel plate cross-section 2 and 0.5×t≥L…(1) 0.85×Hvm>Hv…(2).

2. The resistance spot welding joint according to claim 1, wherein, The composition of the high-strength steel plate further contains, by mass%, one or more selected from Cu: 0.8% or less, Ni: 1.0% or less, Mo: 1.0% or less, Cr: 1.0% or less, Nb: 0.080% or less, V: 0.50% or less, Ti: 0.20% or less, B: 0.005% or less, Al: 2.0% or less, and Ca: 0.005% or less.

3. The resistance spot welding joint according to claim 1, wherein, The high-strength steel plate has a coating on the steel plate surface.

4. The resistance spot welding joint according to claim 2, wherein, The high-strength steel plate has a coating on the steel plate surface.

5. A resistance spot welding method for a resistance spot welding joint, which is the resistance spot welding method for the resistance spot welding joint according to any one of claims 1 to 4, When two or more steel plates including at least one of the high-strength steel plates are stacked and joined by resistance spot welding, As the resistance spot welding, it has a main energization process and a post-temper heat treatment process, In the main energization process, energization is performed at a current value I1 to form a nugget, In the post-temper heat treatment process, the following process is carried out: During the cooling time t shown in formula (3) c1 a first cooling process in which no power is applied Then, a first heating process in which energization is performed at a current value I2 shown in formula (4) for a energization time t2 shown in formula (5), Next, in the cooling time t shown in formula (6) c2 a second cooling process that maintains a non-energized state during Then, a second heating process in which energization is performed at a current value I3 shown in formula (7) for a energization time t3 shown in formula (8), Among them, The units of I1, I2, and I3 are kA, and the units of t c1 , t2, and t3 are ms. 800≤t c1 …(3) 1.01×I1≤I2≤1.4×I1…(4) 100＜t2≤500…(5) 0＜t c2 ≤300…(6) I2<I3≤1.5×I2…(7) 0＜t3＜500…(8)。

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