Resistance spot welding method

Abstract

The purpose of the present invention is to provide a resistance spot welding method. The present invention is a resistance spot welding method for joining two or more steel sheets including at least one zinc-plated steel sheet, the method comprising: forming a nugget having a nugget diameter of # imgabs0 # in a first energization step; in the present invention, an n-stage multi-stage step comprising a cooling step in which a non-energization state is maintained for a period of 10 ms or more and less than 160 ms and an energization step in which energization is performed for a period of 20 ms or more and less than 200 ms is provided, and furthermore, the current value of the energization step of each stage among the multi-stage steps satisfies expressions (1) and (2), and the current value I1 of the first stage is equal to or greater than the current value of the first energization step. I1 < = I2 < =... < = In (1) I1 < In (2).

Description

Technical Field

[0001] The invention relates to a resistance spot welding method. Background Art

[0002] Generally, the steel plates formed by overlapping two or more steel plates are joined together using a resistance spot welding method which is one of the overlapping resistance welding methods. Figure 1 The method shown is a method of joining the steel plates to each other by using two overlapping steel plates 1 and 2 as a plate group, clamping the upper and lower sides of the plate group with a pair of electrodes 3 and 4, applying pressure from the upper and lower sides of the plate group using a pair of electrodes 3 and 4, and simultaneously passing a welding current between the upper and lower electrodes. A point-shaped weld 5 is obtained by using resistance heat generated by the flow of welding current. The point-shaped weld 5 is called a nugget, and is the portion where the two steel plates 1 and 2 melt and solidify at the contact position of the steel plates when the welding current flows through the overlapping steel plates, thereby joining the steel plates to each other in a point-shaped manner. It should be noted that Figure 1 FIG. 2 shows a case where two steel plates are overlapped as an example.

[0003] However, in the resistance spot welding of a plate group formed by overlapping a plurality of steel plates including surface-treated steel plates, there is a problem of cracks in the welded portion. The surface-treated steel plate refers to a steel plate having a metal coating such as a zinc coating represented by electrogalvanizing, hot-dip galvanizing (including alloyed hot-dip galvanizing), or a zinc alloy coating containing elements such as aluminum and magnesium in addition to zinc on the surface of the base material (base steel plate). Since the melting point of the zinc coating and the zinc alloy coating is lower than the melting point of the base material, the following problems exist.

[0004] That is, it is believed that the cracks in the weld are cracks caused by the so-called liquid metal brittleness, that is, the low-melting-point metal coating on the surface of the steel plate melts during welding, and when the pressure brought by the electrode 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 invades the grain boundaries of the base material of the surface-treated steel plate and reduces the grain boundary strength, causing cracks.

[0005] Such cracks are likely to occur when a large deformation is applied to the welded portion, for example, when welding is performed under conditions that may cause scattering. Figure 8The surface of the steel plates 1 and 2 on the side where the electrodes 3 and 4 shown are in contact is prone to be generated. On the other hand, from the viewpoint of ensuring the strength of the joint, it is important to ensure a nugget with a large nugget diameter. In actual construction, during welding, due to the influence of construction interference such as shunting to welding points existing near the desired welding area and electrode loss caused by continuous points, there is a situation where sufficient current density cannot be ensured in the desired welding area. In this case, it is difficult to ensure a specified nugget diameter due to the reduction in heat input. Considering the above-mentioned reduction in heat input, the current value during welding can be set larger to ensure a nugget with a large nugget diameter. However, in this case, a large deformation of the weld portion represented by scattering will occur, and the risk of cracking will increase. As mentioned above, it is difficult to suppress the occurrence of cracks during welding and stably ensure a nugget with a large nugget diameter in actual construction, which has become a problem in the actual application of surface treated steel plates.

[0006] As a countermeasure to such a problem, for example, there are the techniques described in Patent Documents 1 to 4. Patent Document 1 proposes a method of suppressing cracking by appropriately adjusting the welding current application time and the holding time after welding current application in spot welding of a high-strength plated steel sheet.

[0007] In addition, Patent Document 2 proposes a method of suppressing the generation of cracks by setting the power-on mode to 3 or more levels, adjusting the power-on time, welding current and other welding conditions so that the appropriate current range is 1.0 kA or more, preferably 2.0 kA or more, and providing cooling time between each level. The above-mentioned "appropriate current range" refers to a current range that can stably form a nugget diameter of a desired nugget diameter or more and a molten residual pressure of 0.05 mm or more.

[0008] In addition, Patent Document 3 proposes a method of suppressing the occurrence of cracks by appropriately adjusting the holding time after energization.

[0009] In addition, Patent Document 4 proposes a method of improving the cross tensile strength of a joint by applying a short-time post-energization without the growth of the nugget after forming the nugget by the main energization.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Publication No. 2003-103377

[0013] Patent Document 2: Japanese Patent Application Laid-Open No. 2003-236676

[0014] Patent Document 3: Japanese Patent Application Publication No. 2017-47476

[0015] Patent Document 4: Japanese Patent Application Publication No. 2012-187639 Summary of the invention

[0016] However, Patent Documents 1 and 2 do not study the influence of construction interference, and in consideration of actual construction during automobile assembly, the countermeasures are sometimes insufficient.

[0017] Furthermore, Patent Document 3 is a technology capable of suppressing cracks generated during the period from the end of energization to the opening of the electrodes, but does not mention cracks generated during energization, and it may be difficult to suppress such cracks.

[0018] In addition, Patent Document 4 does not mention cracks during welding. Under the condition that a large nugget diameter is formed by main current conduction, deformation of the steel plate surface increases, and cracks may occur.

[0019] It should be noted that the problem of difficulty in suppressing cracking during welding and stably securing a nugget with a large nugget diameter in such a surface-treated steel sheet is not limited to resistance spot welding of automobile steel sheets, but also exists in resistance spot welding of other steel sheets.

[0020] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a resistance spot welding method capable of suppressing cracking during welding of a surface-treated steel plate and stably securing a nugget with a large nugget diameter.

[0021] The present inventors have conducted intensive studies to achieve the above-mentioned object.

[0022] Cracks during welding are likely to occur when large deformation occurs in the weld. Therefore, if the nugget can be enlarged while suppressing deformation to a minimum, cracks during welding can be suppressed and a nugget with a large nugget diameter can be stably secured. In addition, it was found that one of the causes of deformation of the weld is the presence of scattering, and suppressing large scattering (i.e., scattering with a large amount of scattering liquid metal) is very effective in suppressing cracks during welding.

[0023] The process of resistance spot welding is divided into two processes: the first power-on process and the second power-on process. In the first power-on process, power is applied within a range that does not cause extreme deformation to form a nugget with a certain degree of nugget diameter. At this time, the current value is adjusted in order to suppress deformation, so a large nugget cannot be fully formed. For example, in the case of interference caused by a decrease in the nugget diameter, there is sometimes a problem that a nugget with a specified nugget diameter cannot be ensured. Therefore, in the second power-on process, the power-on conditions are adjusted in a manner that does not cause significant deformation in the welded portion to expand the nugget. As a result, cracks are suppressed during resistance spot welding, and a nugget with a large nugget diameter is stably ensured.

[0024] The energization conditions in the first energization step and the second energization step are shown below.

[0025] In the first energizing step, it is important to form a welded portion that does not deform significantly in the subsequent second energizing step. In this step, the heat input can be adjusted by adjusting the current value and the energizing pattern, and the crimping portion (plastic ring portion) around the nugget can be firmly joined. As a result, the scale of scattering (i.e., the amount of molten metal that is scattered) can be suppressed to a small level in the subsequent second energizing step, so that the deformation of the welded portion becomes small, and as a result, the generation of cracks can be suppressed.

[0026] In the second energizing process, the nugget can be enlarged by energizing with a current value higher than the current value in the first energizing process, but if the energization is performed for a long time, the heat input becomes too large, and a large deformation occurs in the weld. Therefore, by setting the energizing mode to repeat short-time energization and short-time cooling, the nugget can be expanded in stages, and the deformation of the weld can be suppressed, so that cracks during welding can be suppressed. In such a energizing mode, even if scattering occurs, its scale (that is, the amount of molten metal scattered due to scattering) can be suppressed to a small size by energizing for a short time. Therefore, the deformation of the weld becomes smaller, and as a result, cracks during welding can be suppressed. At this time, by gradually increasing the current value of repeated energization in the second energizing process, the nugget can be effectively enlarged, that is, the diameter of the nugget can be expanded while suppressing the scale of scattering.

[0027] The present invention has been accomplished based on the above findings, and the gist of the present invention is as follows.

[0028] [1] A resistance spot welding method, wherein two or more steel sheets, including at least one zinc-plated steel sheet, are overlapped and joined by resistance spot welding,

[0029] The resistance spot welding comprises a first energizing step and a second energizing step.

[0030] In the first energizing step, when the thickness of the thinnest steel plate among the stacked steel plates is t (mm), a nugget with a diameter of The molten core,

[0031] In the second energizing step,

[0032] A multi-stage process of n stages (n is an integer greater than 2) is formed by a cooling process in which the power is not supplied for a period of 10 ms to less than 160 ms and a power supply process in which the power is supplied for a period of 20 ms to less than 200 ms.

[0033] Furthermore, in the multiple stages, when the current value of the power-on process of the first stage is set to I1 (kA), and the current value of the power-on process of the nth stage is set to In (kA), the current value I1 of the first stage is greater than the current value of the first power-on process, and the current values ​​I1~In of each stage satisfy equations (1) and (2).

[0034] I1≤I2≤····≤In…(1)

[0035] I1<In…(2)

[0036] [2] The spot welding method according to [1], wherein in the first energizing step, the current value at the start of energizing is denoted as Is (kA), and the current value at the end of energizing is denoted as If (kA),

[0037] The energization is performed so that the current value is constant from the start of the energization to the end of the energization,

[0038] Alternatively, the energization is variably performed so that the current value Is at the start of the energization and the current value If at the end of the energization satisfy the equation (3).

[0039] If>Is…(3)

[0040] [3] The resistance spot welding method according to [2], wherein when the first energizing step is variably energized, the first energizing step is a multi-stage energization of m stages (m is an integer greater than or equal to 2),

[0041] When the current value of each stage in the multi-stage power-on is set to I1′~Im′ (kA), and the current value I1′ of the first stage is set to be the same as the current value Is at the beginning of the power-on, and the current value Im′ of the mth stage is set to be the same as the current value If at the end of the power-on, the current value I1′~Im′ of each stage in the multi-stage power-on satisfies formula (4).

[0042] I1′<I2′<I3′<···<Im′…(4)

[0043] [4] The resistance spot welding method according to [2], wherein when the first energizing step is variably energized,

[0044] The energization is performed with an upward slope in which the current value continuously increases from the current value Is at the start of the energization to the current value If at the end of the energization.

[0045] [5] The resistance spot welding method according to any one of [1] to [4], wherein the diameter of the nugget formed after the first energizing step is set to N1 (mm), and the diameter of the nugget formed after the first energizing step and the second energizing step are set to N2 (mm),

[0046] The amount of expansion of the nugget diameter in the second energization step represented by (N2-N1) is above.

[0047] [6] The resistance spot welding method according to any one of [1] to [5], wherein the zinc-plated steel sheet is a high-strength steel sheet having a Ceq represented by formula (5) of 0.20% or more and a tensile strength of 780 MPa or more.

[0048] Ceq(%)=C+Si / 30+Mn / 20+2P+4S…(5)

[0049] The element symbols in formula (5) represent the content (mass %) of each element.

[0050] According to the present invention, cracking during welding of a surface-treated steel plate can be suppressed and a nugget with a large nugget diameter can be stably formed, thus exerting a remarkable effect industrially. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a cross-sectional view schematically showing an example of resistance spot welding of the present invention.

[0052] Figure 2 It is a diagram for explaining a first embodiment of a current-carrying pattern of the resistance spot welding method of the present invention.

[0053] Figure 3 It is a diagram for explaining a second embodiment of the energization pattern of the resistance spot welding method of the present invention.

[0054] Figure 4 It is a diagram for explaining a third embodiment of the current-carrying pattern of the resistance spot welding method of the present invention.

[0055] Figure 5 The figures show a test piece for resistance spot welding according to an example of the present invention, with the upper side being a plan view and the lower side being a side view.

[0056] Figure 6 The figures show a test piece for resistance spot welding according to an example of the present invention, with the upper side being a plan view and the lower side being a side view.

[0057] Figure 7 The figures show a test piece for resistance spot welding according to an example of the present invention, with the upper side being a plan view and the lower side being a side view.

[0058] Figure 8 This is a cross-sectional view schematically showing an example of crack generation during conventional resistance spot welding. DETAILED DESCRIPTION

[0059] Hereinafter, one embodiment of the present invention will be described with reference to the drawings. However, the present invention is not limited to this embodiment.

[0060] The resistance spot welding method of the present invention overlaps two or more steel plates including at least one zinc-plated steel plate as a plate group, clamps the plate group with a pair of welding electrodes, and applies electricity under the welding conditions described below while applying pressure in this state. Thus, a nugget is formed between the steel plates by resistance heating, and the overlapping steel plates are joined. By this resistance spot welding method, even if it is a plate group including a zinc-plated steel plate, it is possible to manufacture a weld joint that can suppress the generation of cracks during resistance spot welding and stably form a nugget with a large nugget diameter.

[0061] First, the welding conditions in the resistance spot welding method of the present invention will be described.

[0062] In the present invention, the resistance spot welding process includes a first energizing process and a second energizing process. In the first energizing process, a nugget with a diameter of In the second energizing step, a multi-step process consisting of a cooling step of maintaining a non-energized state and an energizing step of energizing in a specific pattern is performed, thereby expanding the nugget formed in the first energizing step.

[0063] <First energization step>

[0064] In the first energizing step, when the thickness of the thinnest steel plate among the stacked steel plates is t (mm), at least the current value Ip (kA) and the energizing time tp (ms) are set to energize to form a nugget with a diameter of At this time, the energizing condition is set to prevent the formation of scattered nuggets. The target nugget diameter for setting the current value Ip (kA) and the energizing time tp refers to the nugget diameter when welding is performed without interference.

[0065] The diameter of the nugget is smaller than When the diameter of the nugget formed in the first energizing step is too small, it is difficult to effectively and sufficiently expand the nugget in the subsequent second energizing step. As a result, it is difficult to ensure a nugget with a large nugget diameter. In addition, when the diameter of the formed nugget exceeds 4.5√t (mm), the heat input caused by energization becomes too large, the weld part deforms, and cracks are easily generated. Therefore, the diameter of the nugget formed in the first energizing step is set to

[0066] The diameter of the nugget formed in the first energization step is preferably If range, the effect of enlarging the nugget diameter without generating cracks in the subsequent second energization step can be more significantly obtained.

[0067] Here, the above "t" refers to the thickness of the steel plate (mm). Specifically, the thickness of the thinnest steel plate among the overlapping steel plates constituting the plate group is set to t. For example, when the thicknesses of the plurality of steel plates constituting the plate group are different, the thickness of the thinnest steel plate among them is set to t, and the smallest nugget diameter among the plurality of nugget diameters formed on the overlapping surfaces of the overlapping steel plates is taken as the object, and the smallest nugget diameter is taken as The current value and the energizing time of the first energizing step are set in a manner to obtain a desired nugget diameter.

[0068] It should be noted that the current value, the power-on time, and the pressure in the first power-on process can be appropriately selected as welding conditions that can ensure the above-mentioned nugget diameter. For example, from the viewpoint of not generating a nugget with the above-mentioned nugget diameter that is formed in a scattered manner, the resistance spot welding welding conditions in the first power-on process are preferably appropriately selected from the range of the current value Ip of 4 to 10 kA, the power-on time tp of 100 to 500 ms, and the pressure of 1.5 kN to 8.0 kN according to the plate group.

[0069] In the present invention, the first energizing process can be controlled to the following energizing mode. Specifically, in addition to the welding conditions of the first energizing process and the second energizing process, the energizing can be performed in a manner that the current value from the start of the energizing process to the end of the energizing process is constant, or the energizing can be performed variably in a manner that the current value from the start of the energizing process to the end of the energizing process satisfies the formula (3) described later.

[0070] Here, use Figure 2 to Figure 4 As a preferred embodiment of the first energizing process, three energizing modes are described. It should be noted that the description of the second energizing process will be given later and is therefore omitted.

[0071] [First power-on mode] (preferred condition)

[0072] Reference Figure 2 An example in which the current value Ip of the first energizing step is made constant (that is, constant current) will be described as the first energizing mode.

[0073] like Figure 2 As shown, in the first energizing step, when the current value at the start of energizing is set to Is (kA) and the current value at the end of energizing is set to If (kA), energizing can be performed in such a manner that the current value Is at the start of energizing and the current value If at the end of energizing are constant. By this energizing, a nugget with a diameter of The above-mentioned “keeping the current value constant” means that the current value is set constant without changing from the start of energization to the end of energization.

[0074] [Second and third energizing modes] (preferred conditions)

[0075] Reference Figure 3 and Figure 4 An example in which the current value Ip of the first energization step is made variable will be described as the second and third energization patterns.

[0076] In the first energizing step, the current value at the start of energizing is set to Is (kA), and the current value at the end of energizing is set to If (kA). The energizing can be variably performed in such a manner that the current value Is at the start of energizing and the current value If at the end of energizing satisfy the formula (3). By this energizing, a nugget with a diameter of The welding part.

[0077] If>Is…(3)

[0078] In the present invention, it is important to form a welded portion in the first energizing step as described above so as not to cause significant deformation in the subsequent second energizing step. As a method for this, the current during energizing in the first energizing step may be controlled so as to satisfy equation (3) and set as a variable current.

[0079] As a method for controlling the current to be variable, for example, "multi-stage energization (see Figure 3 )”, “Upward slope power-on (refer to Figure 4 )". By variably controlling in such a way that the current increases, the crimping portion around the nugget can be more firmly joined in the first energizing step, the occurrence of scattering can be suppressed in the subsequent second energizing step, and even if scattering occurs, the scale of scattering can be suppressed to a small level. As a result, the deformation of the weld portion is reduced, and as a result, the occurrence of cracks can be suppressed.

[0080] A specific example will be described below.

[0081] [Second power-on mode: multi-stage power-on]

[0082] like Figure 3 As shown, when the first energization process is variably energized, the energization of the first energization process can be a multi-stage energization of m stages (m is an integer greater than 2). When the current value of each stage in the multi-stage energization is set to I1′~Im′(kA), and the current value I1′(kA) of the first stage is set to be the same as the current value Is(kA) at the beginning of the above energization, and the current value Im′(kA) of the mth stage is set to be the same as the current value If(kA) at the end of the above energization, the current values ​​I1′~Im′ of each stage in the multi-stage energization of the first energization process are controlled to satisfy formula (4).

[0083] I1′<I2′<I3′<····<Im′ (4)

[0084] When the current value I1′(kA) of the first stage, the current value I2′(kA) of the second stage, the current value I3′(kA) of the third stage, ..., and the current value Im′(kA) of the mth stage (final stage) do not satisfy the formula (4), the heat input to the crimping portion is insufficient. As a result, the crimping portion around the nugget cannot be firmly joined in the first energizing step, and the effect of suppressing the scale of scattering that may occur in the subsequent second energizing step cannot be obtained. In addition, it is difficult to make the nugget grow sufficiently, and the nugget diameter in the above range may not be obtained.

[0085] In the case of multi-stage energization, the energization time in the first energization step is preferably controlled so that the total of the energization time in each stage is within the range of the energization time tp (100 to 500 ms).

[0086] "m" indicating the multi-stage is an integer greater than or equal to 2. From the viewpoint of preventing the management of welding conditions during construction from becoming too complicated, m is preferably 5 or less.

[0087] [Third power-on mode: Upward slope power-on]

[0088] like Figure 4 As shown, the first energization process may be energized with an upward slope that continuously increases the current value from the current value Is at the start of the energization to the current value If at the end of the energization. As a result, the crimping portion around the nugget can be firmly joined in the first energization process, and the scale of scattering can be suppressed to a small extent in the subsequent second energization process. Therefore, the deformation of the welded portion is reduced, and as a result, the generation of cracks can be suppressed.

[0089] <Second energization step>

[0090] After the first energizing step, the second energizing step is performed. At this time, it is important to gradually increase the current value of repeated energizing in the second energizing step. By gradually increasing the current value, the nugget can be effectively enlarged.

[0091] In order to suppress the scale of scattering in the second energizing process, it is effective to expand the nugget in stages in each stage of repeated energizing. When the current value of repeated energizing is kept constant, the effect of expanding the nugget is greater in the first half of the repetition and smaller in the second half of the repetition. On the other hand, by gradually increasing the current value of repeated energizing, the effect of increasing the nugget in stages can be significantly obtained regardless of the first half or the second half of repeated energizing.

[0092] Specifically, in the second energizing step, a multi-stage step of n stages (n is an integer greater than 2) is set, which consists of a cooling step of maintaining a non-energized state for a period of more than 10 ms and less than 160 ms and a energizing step of energizing for a period of more than 20 ms and less than 200 ms. Furthermore, in the multi-stage, when the current value of the first stage energizing step is set to I1 (kA) and the current value of the nth stage (final stage) energizing step is set to In (kA), the current value I1 of the first stage is adjusted to be greater than the current value of the first energizing step, and the current values ​​I1 to In of each stage satisfy equations (1) and (2).

[0093] I1≤I2≤I3≤····≤In…(1)

[0094] I1<In…(2)

[0095] Here, the "current value of the first energizing step" in the above "the current value I1 of the first stage is equal to or greater than the current value of the first energizing step" is equal to the constant current (refer to Figure 2 ) refers to the "current value Ip", in the case of variable current (refer to Figure 3 , 4 ) refers to the "current value If at the end of energization".

[0096] [Cooling process]

[0097] When the non-power-on time in the cooling process is less than 10ms, sufficient cooling effect cannot be obtained, and large deformation of the weld portion is likely to occur, so cracks are likely to occur. When the non-power-on time is more than 160ms, the effect of setting it as a multi-stage, that is, the effect of expanding the molten metal nugget due to repeated power-on, cannot be fully obtained. Therefore, the non-power-on time in the cooling process in each stage is more than 10ms and less than 160ms. The non-power-on time is preferably 10 to 120ms.

[0098] [Electrification process]

[0099] When the power-on time in the power-on process is less than 20 ms, the nugget expansion effect cannot be fully obtained due to insufficient heat input. When the power-on time is more than 200 ms, excessive heat input is applied by one power-on, which easily causes large deformation of the weld part and easily causes cracks. Therefore, the power-on time of the power-on process in each stage is more than 20 ms and less than 200 ms.

[0100] In the present invention, the nugget is enlarged in the second energizing process as described above. In the case where it is desired to more significantly suppress the deformation of the weld and stably enlarge the nugget diameter, it is preferred that the non-energizing time of the cooling process in each stage of the second energizing process is set to 10 to 80 ms, and the energizing time of the energizing process is set to 20 to 100 ms. The non-energizing time of the cooling process is more preferably 10 ms or more and less than 80 ms, more preferably 10 to 60 ms, and even more preferably 10 to 20 ms.

[0101] It should be noted that, when the current value in the energization step does not satisfy the equations (1) and (2), the effect of effectively expanding the nugget cannot be obtained.

[0102] In the present invention, the current values ​​of each stage of the second energizing step preferably satisfy the following conditions in addition to equation (1).

[0103] Specifically, when the current value during the energization of the kth stage is set to Ik (kA), and the current value during the energization of the (k+1)th stage is set to I (k+1) (kA), it is preferably I (k+1) ≤ 1.3 × Ik. Here, k is an integer from 1 to n-1. If it is within this range, it is possible to suppress the rapid increase in heat input, and it is easier to suppress the deformation of the weld caused by the generation of large scattering.

[0104] The number of steps (stages) performed in multiple stages in the second energization step is preferably 2 to 10. When the number of stages exceeds 10, the effect of expanding the nugget is saturated, so it is difficult to obtain a large effect, and the total time of the entire welding process becomes longer. Therefore, from the perspective of construction efficiency, the number of stages is preferably 10 or less.

[0105] <Expansion of nugget diameter> (preferred conditions)

[0106] In the present invention, from the viewpoint of ensuring a large diameter nugget, when the diameter of the nugget formed after the first energizing step is set to N1 (mm), and the diameter of the nugget formed after the first energizing step and the second energizing step are set to N2 (mm), the value of the expansion amount of the nugget diameter in the second energizing step, i.e., (N2-N1), is preferably The value of (N2-N1) is more preferably The above, more preferably above.

[0107] In addition, when the expansion of the nugget diameter in the second energization step is extremely large, the deformation of the weld portion becomes large due to excessive heat input, and cracks may occur. Therefore, the above expansion amount is preferably set to The value of (N2-N1) is more preferably The following is more preferably the following.

[0108] <Steel Plate>

[0109] The present invention is applied to a resistance spot welding method for a plate group in which one of the two surfaces or one surface of the plate group in contact with each welding electrode (upper electrode, lower electrode) arranged on the upper side and the lower side of the plate group is a surface-treated steel plate having a metal coating. Here, the above-mentioned "two surfaces of the plate group in contact with each welding electrode" refers to the two outermost steel plates arranged in the plate group consisting of a plurality of steel plates in contact with the upper electrode and the lower electrode. In addition, the above-mentioned "one of the single surfaces of the plate group in contact with each welding electrode" refers to any one of the two outermost steel plates arranged in the plate group consisting of a plurality of steel plates in contact with the upper electrode or the lower electrode. It should be noted that the melting point of the metal coating is preferably a melting point lower than the melting point of the base material of the surface-treated steel plate.

[0110] As described above, the surface treated steel sheet refers to a steel sheet having a metal coating such as a zinc coating represented by electrogalvanizing, hot dip galvanizing (including alloyed hot dip galvanizing), or a zinc alloy coating containing elements such as aluminum and magnesium in addition to zinc on the surface of the base material (base steel sheet). Here, such a surface treated steel sheet is referred to as a "zinc-plated steel sheet". Therefore, in the present invention, at least one of the plurality of steel sheets constituting the above-mentioned sheet group is a zinc-plated steel sheet.

[0111] The zinc-plated steel sheet is preferably a high-strength steel sheet (high-strength zinc-plated steel sheet) having a carbon equivalent (Ceq) (%) of 0.20% or more and a tensile strength of 780 MPa or more, as represented by the following formula (5).

[0112] Ceq(%)=C+Si / 30+Mn / 20+2P+4S…(5)

[0113] The element symbols in formula (5) represent the content (mass %) of each element, and are set to 0 when the element is not contained.

[0114] High-strength steel sheets are highly sensitive to cracks, and cracking during welding is likely to be a problem. Therefore, when the carbon equivalent is 0.20% or more, the effect of the present invention can be further exerted.

[0115] The upper limit of the carbon equivalent is not particularly specified. Even if the energization mode of the present invention is applied to a steel plate with an extremely high carbon equivalent, i.e., a steel plate with an extremely high sensitivity to cracks, sometimes the crack suppression effect cannot be fully obtained. Therefore, the carbon equivalent (Ceq) is preferably 0.60% or less, and more preferably 0.50% or less.

[0116] For example, when using a plate group formed by overlapping three or more steel plates including at least one zinc-plated steel plate, the carbon equivalent of any one of the zinc-plated steel plates in contact with any one of the welding electrodes (upper electrode and lower electrode) arranged on the upper side and lower side of the plate group can be 0.20% or more.

[0117] In addition, the component composition of the high-strength zinc-based plated steel sheet used in the present invention may be appropriately adjusted so as to obtain the above-mentioned carbon equivalent (Ceq) range and tensile strength.

[0118] In the present invention, the thickness of the steel sheet to be resistance spot welded is not particularly limited. For example, it is preferably in the range of 0.5 mm to 3.0 mm. Steel sheets having a thickness in this range can be suitably used as automobile parts.

[0119] The two or more steel plates to be resistance spot welded may be of the same type and shape, or of different types and shapes. In addition, a surface treated steel plate with a metal coating and a steel plate without a metal coating may be overlapped.

[0120] As an example, Figure 1 The figure shows the state in which a plate set consisting of two overlapping steel plates is joined by the resistance spot welding method of the present invention. Figure 1 As shown, a pair of welding electrodes, namely, a welding electrode 4 (lower electrode) disposed on the lower side of the plate group and a welding electrode 3 (upper electrode) disposed on the upper side of the plate group, are used to clamp the overlapping steel plates (lower steel plate 2 and upper steel plate 1), and electricity is applied under the above welding conditions while applying pressure. The number of overlapping steel plates is preferably 4 or less.

[0121] In addition, as the device and welding electrode for carrying out the resistance spot welding method of the present invention, the following devices and welding electrodes can be used.

[0122] The device for implementing the resistance spot welding method of the present invention can be any device that uses a lower electrode and an upper electrode to apply pressure and control the pressure, and the structure is not particularly limited. For example, a cylinder, a servo motor and other devices can be used. In addition, the structure that supplies current and controls the current value when power is turned on is also not particularly limited, and both direct current and alternating current can be applied to the present invention. In the case of alternating current, "current" refers to "effective current".

[0123] In addition, the shape of the front end of the lower electrode and the upper electrode is not particularly limited. For example, the DR shape (dome radius shape), R shape (radius shape), D shape (dome shape) and the like described in JIS C 9304:1999 can be cited. The front end diameter of each electrode is, for example, 4 mm to 16 mm. The radius of curvature is, for example, 10 mm to 400 mm, and a flat-top electrode with a flat front end can also be made.

[0124] In the above description, the case of resistance spot welding of two overlapping steel plates is mainly described. The present invention can also be applied to the case of resistance spot welding of three or more overlapping steel plates, and the above-mentioned effects can also be obtained.

[0125] Example

[0126] 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.

[0127] In this embodiment, two steel plates including at least one zinc-plated steel plate are used, and the steel plates are overlapped to form a plate set (see Table 1). Here, as the upper steel plate and the lower steel plate, an alloyed hot-dip galvanized (GA) steel plate and a cold-rolled steel plate whose carbon equivalent (Ceq) represented by the above formula (5), tensile strength, and plate thickness are in the conditions shown in Table 1 are used. The plate set is subjected to resistance spot welding under the conditions shown in Tables 2-1 to 4-2 to produce a welded joint.

[0128] Table 2-1 shows welding conditions when the first energizing process is energized at a constant current value, and the second energizing process is energized in a multi-stage process consisting of a cooling process and an energizing process. Table 3-1 shows welding conditions when the first energizing process is energized at a variable current value (multi-stage energization), and the second energizing process is energized in a multi-stage process consisting of a cooling process and an energizing process. Table 4-1 shows welding conditions when the first energizing process is energized at a variable current value (energization with an upward slope), and the second energizing process is energized in a multi-stage process consisting of a cooling process and an energizing process.

[0129] It should be noted that the "second energizing step" shown in Table 2-1, Table 3-1, and Table 4-1 is a multi-stage step of n stages (n is an integer greater than or equal to 2) consisting of a cooling step and a energizing step. In addition, in the multi-stage energizing of the "first energizing step" shown in Table 3-1, the "current value I1' (kA) of the first stage" refers to the "current value Is (kA) at the start of energizing", and the "current value Im' (kA) of the final stage (mth stage)" refers to the "current value If (kA) at the end of energizing". That is, in the case of multi-stage energizing with m = 3, the current value of the third stage, which is the final stage, is the current value at the end of energizing.

[0130] Resistance spot welding is performed at room temperature with the welding electrodes (lower electrode, upper electrode) continuously water-cooled. Both the lower electrode and the upper electrode are 6mm in diameter at the tip and 40mm in radius of curvature, and are DR-shaped electrodes made of chrome copper. In addition, the pressure is controlled by driving the upper electrode with a servo motor, and a DC power supply is supplied when power is turned on.

[0131] For one welding condition, the welded joints were made under three interference conditions, namely, "no interference", "with plate gap", and "with spotting", and the nugget diameter and the presence of cracks (LME cracks) were observed using each of the obtained welded joints.

[0132] Here, refer to Figure 5 to Figure 7 The case of making a welded joint using the above three interference conditions will be described. Each figure shows a test body in which resistance spot welded steel plates are overlapped, the upper figure is a top view thereof, and the lower figure is a side view thereof.

[0133] For the welding joint with interference condition of "no interference", it is made as follows. Figure 5 As shown, two steel plates (upper steel plate 1 and lower steel plate 2) of 30 mm × 100 mm (short side × long side) made of the above steel plates were prepared, and the steel plates were overlapped to make a test body, and the welded portion 6 in the center of the test body was welded under the conditions shown in Table 2-1, Table 3-1, and Table 4-1 to make a welded joint. Using the welded portion 6 of the obtained welded joint, the nugget diameter and the presence or absence of cracks were observed.

[0134] For the welding joint with interference condition of "with plate gap", it is made as follows. Figure 6 As shown, two steel plates (upper steel plate 1 and lower steel plate 2) of 30 mm × 100 mm (short side × long side) made of the above steel plates were prepared, and spacers 7 and 8 of 1.6 mm thickness and 30 mm × 25 mm (long side × short side) were sandwiched between the two steel plates at both ends to make a test body. The welded portion 6 in the center of the test body was welded under the conditions shown in Table 2-1, Table 3-1 and Table 4-1 to make a welded joint. The welded portion 6 of the obtained welded joint was used to observe the nugget diameter and the presence or absence of cracks.

[0135] For the test body with interference condition "existing dot", the following is made. Figure 7 As shown, two steel plates (upper steel plate 1 and lower steel plate 2) of 30 mm × 100 mm (short side × long side) made of the above steel plates were prepared, and the steel plates were overlapped to make a test body. Welded points 9 and 10 with a nugget diameter of 5 mm were respectively arranged at positions deviated 20 mm from the center of the test body in the longitudinal direction, and then the welded portion 6 in the center of the test body was welded under the conditions shown in Table 2-1, Table 3-1 and Table 4-1 to make a welded joint. Using the welded portion 6 of the obtained welded joint, the nugget diameter and the presence or absence of cracks were observed.

[0136] The evaluation and determination of the nugget diameter, the amount of expansion, and the presence or absence of cracking were performed as follows.

[0137] [Evaluation of nugget diameter and the above-mentioned enlargement amount]

[0138] Regarding the nugget diameter (N1) after the first energizing step, in the tests under various interference conditions, a welding test only in the first energizing step was performed in advance, and the nugget diameter was measured by cross-sectional observation.

[0139] Here, the cross section obtained by cutting the welded portion was etched and observed with an optical microscope to measure the nugget diameter between the steel plates. The obtained measured values ​​are shown in the column "Nugage diameter N1 after the first current-applying step" in Table 2-2, Table 3-2, and Table 4-2.

[0140] The nugget diameter (N2) after the second energizing step was measured by the same method as above using each weld joint obtained by the welding test under the above three interference conditions. The obtained measured values ​​are shown in the "Nugget diameter N2 after the second energizing step" column in Table 2-2, Table 3-2, and Table 4-2.

[0141] In addition, the value calculated from (N2-N1) indicating the amount of expansion of the nugget diameter in the second current-carrying step is shown in the "N2-N1" column in Table 2-2, Table 3-2, and Table 4-2.

[0142] 〔Evaluation of cracks〕

[0143] The evaluation of cracks (LME cracks) was performed using each weld joint obtained by performing welding tests under the above three interference conditions. The cross section of the weld was observed using an optical microscope. When cracks of 200 μm or more were observed on the surface of the weld, it was evaluated as "cracks were present". On the other hand, when cracks of 200 μm or more were not observed, it was evaluated as "no cracks". In the case of "cracks were present", it was indicated as "present" in Table 2-2, Table 3-2 and Table 4-2, and in the case of "no cracks were present", it was indicated as "no" in Table 2-2, Table 3-2 and Table 4-2.

[0144] 〔determination〕

[0145] For the determination, the diameter of the nugget (N2) after welding under all the above three interference conditions is The case where the above conditions were met and no cracks were generated was evaluated as “○ (acceptable)”. On the other hand, the cases other than these conditions were evaluated as “× (unacceptable)”. The evaluation results are shown in Table 2-2, Table 3-2, and Table 4-2.

[0146] In Test No. 11 shown in Tables 2-1 and 2-2, as described above, the welding conditions in the first energizing step were such that an excessively large nugget was produced for the plate set C. Therefore, the nugget diameter (N1) after the first energizing step may deviate from the prescribed range.

[0147]

[0148]

[0149]

[0150]

[0151] [Table 3-2]

[0152]

[0153] *4.“↓” means the diameter of the weld nugget after welding is smaller than

[0154]

[0155]

[0156] As can be seen from Tables 2-1 to 4-2, in the examples of the present invention, no cracks occurred regardless of the interference conditions, and a good weld joint with a target nugget diameter was obtained. In contrast, a good weld joint was not obtained in the comparative examples.

[0157] Explanation of symbols

[0158] 1 Upper steel plate

[0159] 2 Lower steel plate

[0160] 3 Upper electrode

[0161] 4 Lower electrode

[0162] 5 Nugget

[0163] 6 Welding Department

[0164] 7.8 Isolation

[0165] 9, 10 Welded points

Claims

1. A resistance spot welding method, which is a method of overlapping two or more steel plates including at least one zinc-plated steel plate and joining them by resistance spot welding, The resistance spot welding comprises a first energizing step and a second energizing step. In the first energizing step, when the thickness of the thinnest steel plate among the overlapped steel plates is t, a nugget having a diameter of 3.0√t mm to 4.5√t mm is formed by setting the current value and the energizing time, wherein the unit of t is mm. In the second energizing step, The process is a multi-stage process of n stages, which consists of a cooling process in which the non-energized state is maintained for a period of 10 ms to less than 160 ms and an energizing process in which the energized state is energized for a period of 20 ms to less than 200 ms. n is an integer greater than or equal to 2, Furthermore, in the multiple stages, when the current value of the first stage power-on process is set to I1 and the current value of the nth stage power-on process is set to In, the current value I1 of the first stage is greater than the current value of the first power-on process, and the current values ​​I1 to In of each stage satisfy equations (1) and (2), and the units of I1 and In are kA. I1≤I2≤····≤In…(1) I1<In…(2).

2. The spot welding method according to claim 1, wherein: In the first energizing step, the current value at the start of energizing is defined as Is, and the current value at the end of energizing is defined as If. The energization is performed so that the current value is constant from the start of the energization to the end of the energization, Alternatively, the current is variably supplied so that the current value Is at the start of the supply and the current value If at the end of the supply satisfy equation (3), wherein the units of Is and If are kA. If>Is…(3).

3. The resistance spot welding method according to claim 2, wherein: When the first energizing step is variably energized, the first energizing step is m-stage multi-stage energization, where m is an integer greater than or equal to 2. When the current value of each stage in the multi-stage energization is set to I1′~Im′, and the current value I1′ of the first stage is set to be the same as the current value Is at the beginning of the energization and the current value Im′ of the mth stage is the same as the current value If at the end of the energization, the current values ​​I1′~Im′ of each stage in the multi-stage energization satisfy equation (4), and the unit of I1′~Im′ is kA. I1′<I2′<I3′<···<Im′…(4).

4. The resistance spot welding method according to claim 2, wherein: When the first energizing step is variably energized, The energization is performed with an upward slope in which the current value continuously increases from the current value Is at the start of the energization to the current value If at the end of the energization.

5. The resistance spot welding method according to any one of claims 1 to 4, wherein: When the diameter of the nugget formed after the first energizing step is defined as N1, and the diameter of the nugget formed after the first energizing step and the second energizing step are defined as N2, the units of N1 and N2 are mm. The amount of increase in the nugget diameter in the second current-applying step represented by (N2-N1) is 0.1√t mm or more.

6. The resistance spot welding method according to any one of claims 1 to 4, wherein: The zinc-plated steel sheet is a high-strength steel sheet having a Ceq of 0.20% or more and a tensile strength of 780 MPa or more represented by formula (5). Ceq(%)=C+Si / 30+Mn / 20+2P+4S…(5) The element symbols in formula (5) represent the content of each element, and the unit is mass %.

7. The resistance spot welding method according to claim 5, wherein: The zinc-plated steel sheet is a high-strength steel sheet having a Ceq of 0.20% or more and a tensile strength of 780 MPa or more represented by formula (5). Ceq(%)=C+Si / 30+Mn / 20+2P+4S…(5) The element symbols in formula (5) represent the content of each element, and the unit is mass %.

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