Al-Si용융 도금된 보론강과 500MPa급 고강도강의 레이저 TWB용접재의 핫스탬핑 열처리에 따른 미세조직과 기계적 특성

Abstract

One of the most important challenges for the automotive industry is to meet the demand of reducing the fuel consumption with a contemporaneously increase of the safety properties. Hot Stamping using the Boron steel is employed for fabricating automobile structural members having a strength of 1,500MPa. Boron steels have been widely used in automobile industries due to their attractive properties such as small spring-back and superior mechanical properties. Recently For increasing the impact absorption, the combination of the advantage of tailor-welded blank(TWB) with laser welding and hot stamping have been investigated. In previous work, During laser welding of Al-Si coated boron steel, the coating layer is diluted into weld zone, which results in Fe3(Al,Si) intermetallic phase formation near the bond line. These phase transformed Fe3(Al, Si) to ferrite during hot-stamping heat treatment. So fracture occurred at the welds zone and tensile properties are decreased. In this study, we fabricated a TWB by laser welding using Al-Si coated boron steel(1.2mm) and Al-Si coated 500MPa steel plates(1.4mm). The joint was heat treated and the tensile properties of the joints were examined before and after the hot-stamping. An intermetallic compound, Fe3(Al,Si), appeared near the bond line of joint on the TWB. Since intermetallic compound had a higher hardness than the base metals and the tensile load of B-Steel was lower than that of 500-steel, we concluded that the fracture occurred at the base metal of the B-Steel. After hot-stamping, in spite of Al-segregated zone transformed into ferrite and ferrite phase observed at weld zone, fracture occurred at the base metal of 500 steel. This is expected to be caused by the difference in the microstructure of the welds and base metal. Therefore, we examined the hot-stamping heat treatment on the microstructure of the TWB weld. The micro-structural change was correlated with the change in the tensile properties.