Research On MAG Welding Procedure Of High Strength Steel For Automobile

Dual-phase steel and micro-alloyed steel both have the comprehensive benefits, such as high strength and good ductility. They have a lot of advantages over traditional mild steel and low alloy high strength steel. So recent years they are applied widely in automobile industry. During manufacturing and forming RCL540 steel plate and SPFH540 steel plate into wheel rims, a major problem was cracking of wheel rims. To solve the problem, this paper made a systematic research on chemical composition, mechanical properties, and microstructures of RCL540 steel and SPFH540 steel. And comprehensive analysis of weldability differences was made between RCL540 steel and SPFH540 steel during resistance butt welding. Besides, utilizing the method of orthogonal design and univariate variable control, the process of metal active welding was optimized.Aiming to 2mm RCL540 and SPFH540 steel plate, experiments of chemical composition measuring, microstructures observation and mechanical properties tests were done. The results proved that, comparing to RCL540 steel, SPFH540 had the lower sulfur content and Lower carbide forming content. Microstructures of them were mainly ferrite and pearlite. However, SPFH540 had severe banded structure. The tensile strength of RCL540 differed obviously in the parallel rolling direction and the perpendicular rolling direction. High strength and higher sulfur content were the main reason that RCL540 steel plate has the larger cracking ratio.Orthogonal design was applied to get the better parameters for metal active welding. The influence of main factors (welding current, arc voltage, welding speed)on shaping ability, mechanical properties and microstructure were investigated. The investigation showed that welding current and welding speed were the main factors which affected depth of penetration. Because of the differences in oxide, SPFH540 steel welded joints had 4.5J larger impact energy than RCL540 steel welded joints. While RCL540 steel welded joints had the oxide of Al, SPFH540 steel welded joints had the oxide of Mg, Ca, S, Al. Besides, the excess height was high, the impact strength was cut down.In addition, the effects of protective gas flow and mixing ratio on formation of joints and protective efficiency were studied. It revealed that when the gas flow was too small, protective efficiency would be influenced. It would be easy to produce gas cavity. Protective gas mixing ratio would also affected the shape of weld. As the fraction of CO2 got larger, the depth of penetration of 2mm steel plate would turn deeper. However, the surface was not so beautiful and the impact energy would decrease.At last, when using ER50-6 welding wire rod, matching mode of deposited metal and base metal was researched. The matching was under strength matching. After learned the shortcomings of ER50-6 welding wire rod, at the principle of"low cost, high efficiency", two low alloy welding wire rod were chosen. At the same welding condition, the joints which used the ER70S-6 welding wire rod and ER70S-G welding wire rod had a little higher impact energy.