Causes of Surface Imprints of Laser Welded DP780 High-Aluminum Dual-Phase Strip Steel after Cold Rolling

Imprints appeared on the surface of DP780 high-aluminum (aluminum mass fraction of 0.55%-0.70%) dual-phase strip steel by hot rolling, head-to-tail laser welding, pickling and cold rolling. Causes of the imprints were studied by the microstructure, chemical composition and hardness analysis. The results show that during welding of DP780 dual-phase steel, the aluminum element in the weld metal was oxidized and alumina formed. The alumina was not effectively removed by pickling. During cold rolling, the hard alumina caused damage of the work roll, leading to the imprints on the surface of the strip steel. The surface imprint of the strip steel was completely eliminated by increasing the welding speed, the concentration of acid solution and the pickling bath temperature.