Corrosion Assessment for Boron Steel Weldments Prepared by Overlap Welding and Successive Hot Press Forming Processes

The hot press forming process for steel forming has been widely applied in the body-in-white structure of automobiles. Usibor™ 1500P is a representative hot press forming (HPF) steel which has a tensile strength of 1.5 GPa after the hot stamping process. The HPF steel undergoes austenization inside a heating oven at a high temperature of 900~950°C for a few minutes and then it is quenched for martensitic transformation. Therefore, the HPF steel sheet is often coated with an AlSi or Zn layer to avoid surface oxidation during the high- temperature process. For some cases, as like forming of the tailor-welded blanks or patched parts, the welding process is required prior to the HPF process and the coating layer is damaged near the welding heat source, which can detrimentally affect the corrosion resistance. In this study, corrosion characteristics were examined on the overlap-welded and subsequently hot pressed specimens. The welding processes selected were laser and resistance spot welding. Salt spray test was implemented up to 1000 hours. Tensile strength was measured and the cross-section was analyzed to investigate the effect of corrosion due to loss of the coating layer. Key words: Welding, Hot press forming, Boron steel, Corrosion, Salt spray test, Crevice corrosion