Effect of Hot-stamping Heat Treatment on the Microstructure of Al-Segregated Zone in TWB Laser Joints of Al-Si-coated Boron Steel and Zn-coated DP Steel

摘要

Al-Si coated boron steel and Zn coated DP steel plates were laser-welded to manufacture a Tailor Welded Blank (TWB) for a car body frame. Hot-stamping heat treatment (900 °C, 5 min) was applied to the TWB weld, and the microstructural change and transformation mechanism were investigated in the Al-rich area near the bond line of the Al-Si coated steel side. There was Al-rich area with a single phase, Fe_3(Al,Si), which was transformed to a-Fe (Ferrite) after the heat treatment. It could be explained that the Fe_3(Al,Si) phase was transformed to a-Fe during heat treatment at 900 °C for 5 min and the resultant a-Fe phase was not transformed by rapid cooling. Before the heat treatment, the microstructures around the Fe_3(Al,Si) phase consisted of martensite, bainite and a-Fe while they were transformed to martensite and 8-Fe after the heat treatment. Due to the heat treatment, Al was diffused to the Fe_3(Al,Si) and this resulted in an increase of Al content to 0.7 wt% around the Al-rich area. If the weld was held at 900°C for 5 min it was transformed to a mixture of austenite (y) and 8-Fe, and only y was transformed to the martensite by water cooling while the 5-Fe was remained unchanged.