Mechanical and Microstructural Properties of Autogenous Arc Welds of 2 GPa Strength Hot-Press-Forming Steel

초록

Currently, the strength of hot-press-forming steel has been increased to produce lightweight vehicles, and in this study, the first arc welding investigation on a 2.0 GPa-strength hot-press-forming steel was conducted. Hot-press-forming steel sheets with thicknesses of 1.1 mm were butt-welded by gas tungsten arc welding without a filler metal, with the heat input controlled in the range of 140-260 J/mm. The weld metal, coarse-grained heat-affected zone (HAZ), and fine-grained HAZ had martensitic microstructures. The softened zone was composed of an intercritical HAZ (with polygonal ferrite and martensite/bainite) and a subcritical HAZ (with tempered martensite). The intercritical HAZ was softened the most during welding and fractured during tensile testing. The strengths of the arc welds were in the range of 1141-1264 MPa, which were 57-63% strength of the base metal. The equiaxed austenite on martensite or ferrite matrix was developed at the intercritical HAZ of gas tungsten arc welds, therefore transforming into the coarse martensite during quenching. However, the intercritical HAZ of laser welds indicated acicular austenite transforming into the fine martensite due to its high cooling rate. The intercritical HAZ softening was revealed to originate from carbon diffusion during the martensite to austenite transformation, which is unavoidable in arc welding and should be considered in the design of a welded structure.

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