The present invention relates to resistance spot welding method of the high strength steel sheet, in particular, it relates to resistance spot welding method in automobile parts and the vehicle body or the like.
BACKGROUND
[0002]
　In the field of motor vehicles, for environmental protection, together with improved fuel economy by reducing vehicle weight, the improvement of collision safety has been demanded. Therefore, along with thinning using high strength steel plate, by optimizing the body structure, in order to improve weight reduction and crashworthiness of the vehicle body, various efforts have been made so far.
[0003]
　The welding of parts manufacturing and vehicle assembly such as an automobile, resistance spot welding (hereinafter, sometimes referred to as "spot welding") is mainly used. The quality indicator of the welded joint formed by spot welding, the tensile strength and fatigue strength. The tensile strength of the welded joint, the tensile shear strength is measured by a load tensile shear direction (TSS), there is a cross tensile strength (CTS) is measured by a load tensile peeling direction. Further, the fatigue strength of the welded joint, the tensile shear fatigue strength measured by a load tensile shear direction, there is a cross tensile fatigue strength measured by a load tensile peeling direction.
[0004]
　On the other hand, in the case where a high-strength steel sheet was spot-welded, there is a delayed fracture (hydrogen embrittlement) problem. The delayed fracture is the hardness of the steel sheet, it is mainly governed by three factors of the hydrogen of the residual stress and steel sheet.
[0005]
　High-strength steel sheet in order to achieve its strength, Si in addition to C, which contains a large amount of quenching highly elements such as Mn, welding of the welded joints formed by spot welding a high strength steel plate Department enters is baked through the heating and cooling process of welding, it is martensitic structure, which is hard. Further, the welded portion, by transformation expansion and contraction of locally occurring, the tensile residual stress of the welded joint is increased.
[0006]
　Therefore, weld spot welding to form weld joints in high-strength steel sheet, high hardness, since the residual tensile stress is larger, If there 'hydrogen penetration is the site likely to cause delayed fracture . When such delayed fracture occurs, the tensile strength and the fatigue strength is the quality indicator of the welded joint described above, sufficient strength can be obtained and, if moisture penetrates into the portion (crack), corrosion problem occurs to the strength is further lowered. These problems are the factor that inhibits the weight of the vehicle body by the application of high-strength steel sheet (thin).
[0007]
　Under such circumstances, or subjected to tempering energization after the energization of the spot welding has passed a predetermined time completed, or by heating at a high frequency, the weld tempering, reducing the hardness of the weld technology has been known. However, in this technique, the welding process becomes long, and the productivity is lowered, weld softened by tempering, peeling breakage in the weld metal (nugget) were prone be like.
[0008]
　In contrast, Patent Document 1, than the initial pressure applied by the welding electrodes during nugget formation of spot welding, by increasing the late pressure applied by the welding electrodes during the retention time after completion of the energization time, the peripheral weld techniques for introducing compressive residual stress is disclosed.
CITATION
Patent Document
[0009]
Patent Document 1: JP 2010-110816 JP
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　The technique disclosed in Patent Document 1, it is possible to reduce the tensile residual stress of the weld, with respect to delayed fracture suppression, it is an effective technique, further, it has been desired to improve the suppression of delayed fracture.
[0011]
　In the present invention, in view of such circumstances, it can be formed by a welded joint stable with high delayed fracture resistance, and to provide a resistance spot welding process.
Means for Solving the Problems
[0012]
　The present inventors have intensively studied means for solving the above problems. The present inventors have, in order to improve the delayed fracture resistance of the spot welded joint, reducing the tensile residual stress near the weld is considered to be extremely important. Therefore, affecting the tensile residual stress near the weld was examined pressurized condition of the weld by the welding electrode after nugget formation.
[0013]
　As a result, while energized after nugget formation, processing of repeating the rising and falling of the pressure applied to the welding portion (hereinafter, referred to as "peening") by performing, impact while maintaining the temperature in the vicinity of the nugget in a high temperature, can promote plastic deformation in the vicinity nugget, as a result, reduce the tensile residual stress of the nugget end, or be changed to compressive stress, it was found to be improved delayed fracture resistance of the welded joint.
[0014]
　The present invention has been made based on the above findings, it provides the followings.
[0015]
　A method of resistance spot welding a plurality of steel plates, at least the welding portions are overlapped, the plurality of steel plates comprises at least a tensile strength of at least one or more of the steel sheet 980 MPa, the method includes [A] a plurality while applying sheets of the welding electrode into the steel sheet the pressure P1 (kN), the step of energizing by electric current I1 (kA), while applying the pressure P1 [B], while the cooling time tc (s), a step of energizing by energizing current Ic (kA), [C] the welding while energized with electric current I2 (kA) to the electrodes, the inter plurality of the said welding electrode in steel pressure P2 (kN) a pressurization tf grant during (s), the step of repeating the rise and fall of the pressure twice or more to impart during the subsequent immediate pressure P3 to (kN) pressurizing time ti (s), the pressure to the [D] the pressure P2 grant between the pressure time between tf That step, the releasing the [E] pressure, comprising the step of terminating the energization, in this order, the conduction current Ic the following equation (1), the energizing current I2 the following equation (2), the pressure P2 is the following formula (3), the pressing time tf is the following equation (4), the pressure P3 is the following equation (5), the pressurizing time ti is resistance spot and satisfies the following formula (6), respectively welding method.
[0016]
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The invention's effect
[0017]
　According to the present invention, after the nugget formation, since subjected to peening the weld, it is possible to improve the hydrogen embrittlement resistance of the weld joint.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018]
FIG. 1 is a diagram showing an outline of the welding method of the present invention.
2 is a diagram showing an example of a pattern of pressure and electric current of the welding electrode.
[Figure 3] is a diagram schematically showing a change in current density when the pressure is changed welding electrode, (a) those according to the prior art, but is according to the invention (b).
DESCRIPTION OF THE INVENTION
[0019]
　Resistance spot welding method of the present invention (hereinafter, referred to as "welding method of the present invention") is a method in which at least a tensile strength is resistance spot welding a plurality of steel plates containing more than one of the above steel sheet 980 MPa, nugget formation later, a method of applying repeated peening the rise and fall of the pressure of the weld by the welding electrodes.
[0020]
　Then, the pressure applied during nugget formation P1 (kN), the electric current I1 (kA), and a cooling time tc (s), so as to satisfy the following equation (1), the energizing current in the cooling time Ic ( kA) and then, in peening, so as to satisfy the following (2) to (5), the electric current I2 (kA), the pressure applied during pressure rise P2 (kN), the pressing time tf ( s), the pressure applied during pressure lowering P3 (kN), in which the pressing time and ti (s).
[0021]
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[0022]
　Next, the description will be given of a flow of welding method of the present invention, a description will be given of the basic structure of the welding method of the present invention.
[0023]
　Figure 1 shows a schematic of a welding method of the present invention. Figure 1 shows a sectional view of the member to be welded in the thickness direction.
　First, in the process of the present invention, as the welding member, a plurality of steel plates (hereinafter, also referred to as "plate assembly") is prepared. The leaf assembly, at least a tensile strength of 980MPa or more of the steel sheet (hereinafter, "high-strength steel sheet" also referred to) is intended to include more than one.
[0024]
　Then, as shown in FIG. 1, superposing two steel plates 1, so as to sandwich the welded portion of the two steel plates from both sides, the welding electrode 2 made of a copper alloy or the like, the pressure P P1 (kN) while pressurized as the energizing current I is energized as I1 (kA), to form the molten metal. Thereafter, while applying a pressure P1, while the cooling time tc (s), as energizing current Ic (kA), the heat conduction by the water-cooled welding electrode 2 to the heat extraction and the steel plate, two steel plates 1 forming a weld metal (nugget) 3 elliptic cross section between the.
[0025]
　If the plate set comprises more than one sheet of high-strength steel sheet, the weld of the welded joint obtained by spot welding, nugget 3 and the heat-affected zone around the enters the baked during cooling, a martensitic structure. The heat shrinkage occurs in the cooling process, is in a state where tensile stress remaining in particular the end of the nugget 3.
[0026]
　Therefore, in the welding method of the present invention, after the nugget formed by spot welding, while energizing the energization current I as I2 (kA), the pressure P as P2 (kN), and applied during the pressing time tf (s) , then as soon as the pressure P P3 (kN), subjected to two or more repeated peening the rise and fall of imparting to pressure between the squeezing time ti (s).
　Next, a description will be given pattern of pressure and electric current of the welding electrode during spot welding及peening.
[0027]
　2 shows an example of a pattern of pressure and electric current of the welding electrode.
　First, a description will be given pattern of pressure and current applied during spot welding to form a nugget. Pressure by welding electrodes against the steel plate is pressed such that the P1. After the pressure reached P1, the current value and energizing current I1, the state was held for energization time t1 (s), to form the molten metal.
[0028]
　After a lapse of energization time t1, while applying pressurizing force P1, during the cooling time tc, the energizing current as Ic, the molten metal cooling, solidified to form a weld metal (nugget). At this time, the energizing current Ic is set so as to satisfy the above equation (1).
[0029]
　Next, a description will be given pattern of pressure and electric current during the peening process. After a lapse of the cooling time tc, the energizing current I2 the current value. Energizing current I2 is set so as to satisfy the above (2). Further, the energizing current I2, this state was held for energizing time t2 (s), carrying out the peening process.
[0030]
　Then, with the start of the energization by energization current I2, the pressure P2 is applied between the squeezing time tf by welding electrodes. At this time, pressure P2, the above (3), pressing time tf is set so as to satisfy the above expression (4).
[0031]
　After a lapse of squeezing time tf, immediately applied pressure P3 grant during the squeezing time ti. At this time, pressure P3, the above equation (5), pressing time ti is set so as to satisfy the above expression (6). The lowering of the rise and pressure applied by the pressing force P3 of pressure applied by the pressure P2 as one cycle, repeated two or more times.
[0032]
　After repeating the rise and fall of the pressure, after application between the pressure P2 squeezing time tf, while releasing the pressure, to terminate the energization. At this time, pressure P2, the above (3), pressing time tf is set so as to satisfy the above expression (4).
[0033]
　Thus, after the nugget formation, by carrying out the peening process, it improves the delayed fracture resistance of the welded joint, further can be improved fatigue strength. Although the delayed reasons fracture resistance improvement is not clear, under certain temperature welds, peening, i.e., by performing repetition of rise and fall of pressure, for example pressure or by the platen after welding it is considered that the tensile residual stress by adding plastic deformation can be reduced in the weld as ultrasonic striking treatment.
[0034]
　Figure 3 shows the material to be welded by welding electrodes when pressurized, the outline of the temperature change and the change nugget of current density as seen from the material. (A) If it is a constant pressure is a schematic of a case of repeating the (b) the rise and fall of the pressure. If a constant pressure as (a), increases the plastic deformation of the material to be welded over time, the current density decreases monotonously. As a result, as the temperature also the current density change in the nugget, gradually decreases.
[0035]
　On the other hand, if the repeated rise and fall of the pressure in as in (b), however, the current density is also vertically accordingly. As a result, the temperature of the nugget also changed with changes in the current density. Thereby the temperature of nugget can be kept within a certain range.
[0036]
　The current density in FIG. 3 (a) is an image view in the case of using the power of the direct current. If the AC power source, the current increases or decreases in power, although the current density as described in FIG. 3 (b) is obtained, pressure is for constant across, reducing tensile residual stress of the weld, or it can not be a compressive stress.
[0037]
　Further, In the case of (a), after the welding electrode is in contact with the workpieces, since the specific point is kept pressurized, distortion occurs in a specific location. On the other hand, in the case of (b), by the pressure up and down, is made uniform plastic deformation of the weld.
[0038]
　Resistance spot welding method of the present embodiment, since the flow flowing current I2 during peening, weld yield strength is reduced in a high temperature, plastic deformation is facilitated during peening. By raising or lowering the current density by repeating the rise and fall of the pressure in this, the temperature of the nugget even while the increase and decrease of the pressure applied to the non-welded material is repeated a plurality of times, and ease of plastic deformation it can be kept within a temperature range. Therefore, it is considered that the reduction of tensile residual stress is promoted.
[0039]
　Furthermore, to increase or decrease the pressure in the peening time, so that the contact area of ​​the electrode and the weld is increased or decreased. Load per contact area, that is, the stress current density or temperature is increased or decreased in a local state within the proper range, plastic deformation is likely to further proceed. Additionally, organizations and refinement of the weld occurs solidification segregation split such embrittlement elements, delayed fracture resistance is presumed to be enhanced.
[0040]
　The present invention having the basic configuration described above, for such the present invention, further, sequentially described necessary requirements and preferred features.
[0041]

(tensile strength of the steel sheet)
　steel sheet is welded member for spot welding at least one sheet, the tensile strength is greater than or equal to the high-strength steel sheet 980 MPa. If the tensile strength is less than 980MPa, since lower values of tensile residual stress generated in the welded portion hardly occurs in delayed fracture problems. Therefore, the tensile strength is subject to the welding process of the present invention the plate set including more than one or more of the steel sheet 980 MPa. The upper limit of the tensile strength of high-strength steel sheet is not particularly limited.
[0042]
　Plate set is not only those of higher tensile strength 980MPa all steel, is intended to include the case where only one of at least one has the tensile strength. For example, a steel sheet having a tensile strength of not less than 980 MPa, or even when welding the steel sheet having a tensile strength of less than 980 MPa.
[0043]
(Steel grade steel, chemical composition)
　steel type and chemical composition of the steel sheet is not particularly limited. Chemical composition of the steel sheet, in the high-strength steel sheet described above, it may be selected component composition Tensile strength (more than 980 MPa) can be ensured. Further, the carbon equivalent Ceq of the steel sheet is not particularly limited, it is exemplified at least 0.20 mass%. Here, a Ceq = C + Si / 24 + Mn / 6. This was a reference to the Ceq of WES.
[0044]
(Thickness of steel)
　plate thickness of the steel sheet is not particularly limited, it may be in the range of 0.5 ~ 3.2 mm. Even thickness is less than 0.5 mm, the effect of improving the delayed fracture characteristics of the welded portion is obtained, lower stress load on the welded portion during a tensile test, also the tensile residual stress generated in the weld for low value hardly occurs delayed fracture. Further, even if the plate thickness is a 3.2mm greater, the effect is obtained to improve the delayed fracture properties of the weld, it may be difficult to reduce the weight of the member.
[0045]
(Surface treatment film of the steel sheet)
　a plurality of steel sheets, the steel sheet to form a surface treatment film on both surfaces or one surface of at least the welding portions may include more than one. Surface treatment film are those containing a plating film can be further intended to include paint films. The plating film, for example, zinc plating, aluminum plating, zinc-nickel plating, zinc-iron plating, a zinc-aluminum-magnesium-plated or the like, the kind of plating, hot dipping, an electric plating.
[0046]
(Steel sheet form)
　form of the steel sheet may be at least welds plate it may not be entirely plate shape. For example, those comprising a flange portion of the members that are press-molded to a particular shape of the cross section a hat-shaped. Number of steel plates superimposed is not limited to two, it may be three or more. Further, each steel sheet steels, chemical composition and thickness all be the same, may be different from each other. Further, not limited to those composed of separate steel plate, by forming a single steel plate into a predetermined shape, such as tubular, or may be superimposed ends.
[0047]

　spot welding performed on a plurality of steel plates, so as to sandwich the welded portion of the plurality of steel plates, are energized while pressing the electrodes, to form a molten metal, disconnect by water-cooled electrode after completion of the energization by heat conduction to the heat and the steel plate, allowed to solidify by rapid cooling of molten metal, between the steel plates to form a nugget oval cross section.
[0048]
　Conditions of the spot welding is not particularly limited, for example, the electrode assumes the Domurajiasu type tip diameter 6 ~ 8 mm, pressure 1.5 ~ 6.0kN, energizing time 0.1 to
1 . 0s (5 ~ 50 cycles, power supply frequency 50 Hz), it is possible to supply current 4 ~ 15 kA. Nugget diameter, the thickness of the thinnest steel sheet When t (mm), can be 3.0√t ~ 8.0√t.
[0049]
　Basic pressurization and energization pattern when spot welding is not particularly limited, the above-mentioned pressure, weld time, after the range of energizing current, is adjusted to an appropriate optimal conditions according to the workpiece to be welded Bayoi. Basic pressurization and energization patterns nugget diameter required can be realized, it is sufficient that the condition dust is not generated, the optimum conditions vary depending steel strength and plate thickness and the like. The current value at the start of the energization, immediately without flowing current, until the current value becomes current flowing, a current value 0 may be (zero) or 0 increasing from low currents greater (up-slope).
[0050]

　elapsed time after energization of the spot welding, while maintaining the pressure of the welding electrodes, lowering the energizing current. A low degree of solidification of the molten metal progresses current to cool the weld. The cooling time tc is not limited in particular, may be made of the weld metal (nugget) is, but also depends on the plate thickness of the steel sheet, 0.04 ~ 0.4 s are exemplified.
[0051]
　Energizing current Ic in the cooling time, it is necessary to satisfy 0 ≦ Ic
　peening treatment performed after the spot welding, while energized in energizing current I2 to the welding electrodes, the pressure P2 by welding electrodes steel imparted between squeezing time tf, then immediately applied pressure P3 pressurizing time the rise and fall of the pressure imparting between ti repeatedly performed twice or more, then, after applying between the pressure P2 squeezing time tf, while releasing the pressure, but to terminate the energization.
[0053]
(Electric current I2, conduction time t2)
　during peening, a state of being energized welding electrodes. As electric current I2 when it shall satisfy the above (2). Temperature of the weld in the peening treatment is appropriate, the plastic deformation of the weld is facilitated, the tensile residual stress of the weld is reduced.
[0054]
　Further, the energization current I2 With electric current less than I1 during spot welding, to suppress the expansion of the nugget in the peening process. Running an enlarged allowed while peened nuggets, so solidification of the nugget unstable, scattering or cause, Dari recessed welds, energy may or wasted.
[0055]
　In order to maintain the yield strength of the steel sheet in conferring pressure enough, I2 is preferably set to more than 0.8 times the I1, further preferably not more than 0.7 times the I1. By the I2 such a value, ensuring the temperature of the welded portion in conferring pressure can be set to 800 ° C. or less. When the temperature of the weld is equal to or greater than 800 ° C., the temperature range, the yield strength of the steel sheet is lowered. Because the absolute value of the residual stress has a strong correlation with the yield strength, that yield strength of the steel sheet is decreased, compressive stresses can be introduced during the striking decreases. As a result, large thermal shrinkage of the welds in the subsequent cooling process, high tensile stress in the weld is likely to be introduced. Furthermore, it is easy to plastic deformation of the steel sheet surface layer, dent becomes excessive due blow process, the joint strength may be lowered.
[0056]
　In addition, I2 is set to 0.3 or more of I1. By the I2 such a range, it is possible to obtain the residual stress reduction effect tensile by peening sufficiently.
[0057]
　Also, conduction time t2 during the peening process, from the start of application of pressure P2, and to the open. Until the current value becomes to be conducted current I2, also be gradually increased current value from Ic (up-slope) may be immediately energized current I2 the current value.
[0058]
(Pressure P2, pressing time tf)
　pressure P2 during peening, in order to reduce the tensile residual stress, and 1.2 times the pressure P1 at the time of spot welding. Preferably, it is 1.3 times or more. The upper limit is not particularly limited, is preferably 2.5 times or less in order to avoid excessive pressure to the welding unit. The upper limit of the squeezing time tf is to reduce the tensile residual stress of the weld, and 0.4 s. Preferably is 0.2s. The lower limit is, 0.02s is preferable.
[0059]
　Well pressure P2 during the peening be constant, for example, such as stepwise increased, may be changed. The value of P2 is a Cu used for normal welding electrodes, it can be appropriately determined by the balance of strength between the steel workpieces having a high temperature.
[0060]
(Pressure P3, pressing time ti)
　pressure P3 during the peening process is required to be P3