TWI480114B - Can eliminate the cold-rolled dual-phase steel plate welding hole shrinkage defects in the resistance spot welding process - Google Patents

Description

Resistance spot welding process capable of eliminating weld core shrinkage defects of cold rolled duplex steel sheets

The invention relates to a resistance spot welding process, in particular to a resistance spot welding process capable of eliminating weld shrinkage defects of cold rolled dual phase steel plates.

Resistance spot welding is one of the main welding methods used to join steel plates in the automotive manufacturing process. On average, each small car contains about 3,600 solder joints. In recent years, due to the increasing demand for lightweight materials, the proportion of high-tension automotive steel sheets used in the body has gradually increased. However, the tolerance of such steel plates to weld defects is relatively low, especially the brittle fracture of the weld nuggets. The main reason - shrinkage defects.

When the cold-rolled double-phase steel plate is welded for resistance point welding, under the welding current that has not yet caused the flying explosion (sparking spark generation), the shrinkage defects are easily caused by the incomplete welding between the steel plates, especially when the thickness of the steel plate exceeds 2 mm. Such defects are more pronounced, and shrinkage defects cause a decrease in the strength of the weld nugget and cause brittle fracture. It has been known to use a circular flat-topped electrode tip to provide a uniformly distributed compressive stress to reduce the occurrence of weld nugget defects. However, this improvement can only reduce crack defects around the weld nugget because of its application to the central region of the weld nugget. The pressure capability is poor, so it is still impossible to avoid the disadvantage that the shrinkage cavity defects are concentrated in the center of the weld nugget after resistance spot welding.

In view of this, it is necessary to provide an innovative and progressive resistance spot welding process which can eliminate the shrinkage defects of cold-rolled dual-phase steel sheets to solve the above problems.

The invention provides a resistance spot welding process capable of eliminating weld core shrinkage defects of cold rolled duplex steel sheets, the process comprising the steps of: (a) providing at least one cold rolled duplex steel plate, the cold rolled duplex steel plate Having a thickness t; (b) performing a preheating step of preheating the cold rolled duplex steel sheet by a preheating current PC, the calculation formula of the preheating current PC being PC≧5t+4 ( Unit: 仟安培); and (c) performing a welding step of performing resistance spot welding on the cold-rolled dual-phase steel plate with a welding current WC, the calculation formula of the welding current WC is WC=flying current-0.2 (Unit: 仟安培), where the flying current is the current when the welding spark starts to occur.

The invention adds a preheating step in the resistance spot welding process, and uses the thickness of the cold rolled duplex steel plate as a known parameter to design the time and current required for the preheating and welding steps. The preheating step before welding of the invention can effectively utilize the contact resistance between the steel plates to generate sufficient resistance heat in the initial welding time to achieve good joint between the interfaces, and the effect can completely eliminate the cold rolled double phase steel plate. Weld hole shrinkage defects and improve weld joint strength.

1 shows a flow chart of a resistance spot welding process for eliminating shrinkage defects of a cold rolled dual-phase steel plate according to the present invention. Referring to step S11 of FIG. 1, at least one cold rolled duplex steel sheet is provided, and the cold rolled duplex steel sheet has a thickness t. In this embodiment, the thickness t of the cold-rolled dual-phase steel plate is between 0.6 mm and 2.5 mm, and the carbon equivalent of the cold-rolled dual-phase steel plate (Ceq=C+Si/30+Mn/20 The +2P+4S) range is between 0.18 and 0.28 weight percent (wt%).

Figure 2 is a graph showing the time-current relationship of the two-stage spot welding process of the preheating and welding of the present invention. Referring to step S12 and FIG. 2 of FIG. 1 , a preheating step is performed to preheat the cold rolled dual-phase steel plate by a preheating current PC, and the calculation formula of the preheating current PC is PC≧5t. +4 (unit: 仟安培, kA), where t is the thickness of the cold rolled duplex steel sheet. In this step, when the cold rolled duplex steel sheet is an ungalvanized steel sheet (CR steel), the preheating current PC is equal to (5t+4) 仟 amp; and when the cold rolled duplex steel sheet is hot When galvanized steel sheet (GI steel) or alloyed galvanized steel sheet (GA steel), the preheating current PC is equal to (5t+6) 仟 ampere. In addition, in the embodiment, when the thickness t of the cold-rolled dual-phase steel plate is not more than 1.4 mm, the preheating time of the cold-rolled dual-phase steel plate is 3 cycles; and when the cold rolling When the thickness t of the dual-phase steel plate is greater than 1.4 mm, the preheating time of the cold-rolled dual-phase steel plate is 4 cycles. In this embodiment, since resistance welding is performed by using 60 Hz alternating current, each cycle ( The cycle time is one-sixteenth of a second.

Referring to step S13 and FIG. 2 of FIG. 1 , a welding step is performed, and the cold-rolled dual-phase steel plate is subjected to resistance spot welding by a welding current WC, and the calculation formula of the welding current WC is WC=flying current. -0.2 (unit: 仟安培, kA), where the flying current is the current at which the welding spark begins to occur. In this step, the cold rolling duplex steel sheet has a welding time of no more than 10t cycles. In this embodiment, when the cold-rolled dual-phase steel plate is an ungalvanized steel plate (CR steel), the welding time of the cold-rolled dual-phase steel plate is equal to (10t-2) cycles; When the cold-rolled dual-phase steel plate is a hot-dip galvanized steel sheet (GI steel) or a galvanized steel sheet (GA steel), the welding time of the cold-rolled dual-phase steel sheet is equal to 10t cycles, and similarly The time per cycle is one-sixteenth of a second.

The invention is illustrated by the following examples, which are not intended to be limited to the scope of the invention.

Examples and comparative examples:

The embodiment and the comparative example of the present invention take the resistance spot welding butt joint of the cold-rolled dual-phase steel plate with the carbon equivalent range of 0.20-0.25 wt% and the thickness range of 0.8-2.0 mm as an example, and use an AC spot welder with conductivity LACS. : 80~90% of the copper electrodes are welding equipment, and the joint welding of the two-layer automobile parts of the same thickness is carried out.

Before spot welding, the thickness t of the single-layer cold-rolled dual-phase steel plate is taken as a known parameter, and the current and time of preheating and welding are calculated according to the welding parameter conversion relationship of the present invention, and the composition is as shown in FIG. The heat and welding two-stage resistance spot welding process is performed by the welding machine controller.

Then, the actual spot welding is performed, and the flying current is subtracted by 0.2 ampere as the operating current value of the welding current WC, and gradually increased from the low current (70% of the operating current value) to the flying current (100% operation). The current value range is spot-welded, and the non-destructive ray detection and destructive metallographic profile of the weld nugget are observed after spot welding.

Figure 3 is a cross-sectional view showing the non-destructive ray detection and destructive metallographic phase of the weld nugget of the comparative example (b) of the preheating step of the present invention and the non-preheating step (a). Fig. 4 is a view showing the non-destructive ray detection results of the weld nugget in the butt welding of the cold-rolled dual-phase steel sheets having thicknesses of 0.8, 1.2, 1.6 and 2.0 mm in the examples of the present invention and the comparative examples. From the results of Fig. 3, it was found that the inside of the weld nugget where the preheating step was not performed had significant shrinkage cavity defects, and the shrinkage cavity defects inside the weld nugget where the preheating step was performed were completely eliminated. In addition, the non-destructive ray detection result of the weld nugget of FIG. 4 also shows that the preheating step and the preheating condition of the present invention are for different thicknesses of cold-rolled dual-phase steel plates of 0.8, 1.2, 1.6 and 2.0 mm, and different sizes of welding. The core has a good effect of eliminating shrinkage defects.

Fig. 5 is a view showing the test results of the weld nugget tensile strength of the cold-rolled double-phase steel plate having a thickness of 2.0 mm in the embodiment and the comparative example of the present invention. From the results of Fig. 5, it can be found that the shear strength of the weld nugget of the embodiment having the preheating step can be increased by about 3 to 8%, which will contribute to the improvement of the joint strength of the automobile structural member.

The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the scope of the present invention. The scope of the invention should be as set forth in the appended claims.

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1 shows a flow chart of a resistance spot welding process for eliminating shrinkage defects of a welded core of a cold rolled dual phase steel sheet according to the present invention;

2 is a graph showing the time-current relationship of the two-stage spot welding process of the preheating and welding of the present invention;

Figure 3 is a cross-sectional view showing the non-destructive ray detection and destructive metallographic phase of the weld nugget of the comparative example (b) of the preheating step of the present invention and the non-preheating step;

4 is a view showing non-destructive ray detection results of weld nuggets in the case of butt welding of cold-rolled dual-phase steel sheets having thicknesses of 0.8, 1.2, 1.6, and 2.0 mm according to the embodiment of the present invention; and

Fig. 5 is a view showing the test results of the weld nugget tensile strength of the cold-rolled double-phase steel plate having a thickness of 2.0 mm in the embodiment and the comparative example of the present invention.

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Claims (8)

A resistance spot welding process capable of eliminating weld core shrinkage defects of cold rolled dual phase steel sheets, comprising the steps of: (a) providing at least one cold rolled duplex steel sheet having a thickness t (b) performing a preheating step of preheating the cold rolled duplex steel sheet by a preheating current PC, the calculation formula of the preheating current PC being PC≧5t+4 (unit: 仟安培) And (c) performing a welding step of performing resistance spot welding on the cold-rolled dual-phase steel sheet by a welding current WC, and the calculation formula of the welding current WC is WC=flying current-0.2 (unit: 仟安培), wherein the flying current is the current when the welding spark starts to be generated. The process of claim 1, wherein in step (a), the thickness t of the cold rolled duplex steel sheet is between 0.6 mm and 2.5 mm. The process of claim 1, wherein in the step (a), the cold-rolled dual-phase steel sheet has a carbon equivalent range of between 0.18 and 0.28 weight percent (wt%). The process of claim 1, wherein in the step (b), when the thickness t of the cold-rolled dual-phase steel plate is not more than 1.4 mm, the preheating time of the cold-rolled dual-phase steel plate is 3 cycles (cycle) ), the time per cycle is one-sixtieth of a second. The process of claim 1, wherein in the step (b), when the thickness t of the cold-rolled dual-phase steel plate is greater than 1.4 mm, the preheating time of the cold-rolled dual-phase steel plate is 4 cycles. , the time of each cycle is sixty One second. The process of claim 1, wherein the preheating current PC is equal to (5t + 6) ampere. The process of claim 1, wherein in the step (c), the cold rolling dual-phase steel plate has a welding time of not more than 10t cycles, and the cycle time per one cycle is one-sixtieth. second. The process of claim 7, wherein the cold rolling duplex steel sheet has a welding time equal to (10t-2) cycles, and the cycle time per one cycle is one-sixteenth of a second.