Embrittlement mechanism of ferrite-martensite dual-phase steel during strain-baking

Abstract

Press forming and paint curing are necessary in automobile production. However, strain and baking effects induced by these processes cannot be ignored in steel plates. Previously researchers have focused on the baking hardening effect but neglected the embrittlement issue. Baking embrittlement occurs when the elongation of ferrite-martensite dual-phase (DP) steel decreases significantly after strain-baking. In this work, the baking embrittlement value (ΔE) was defined for the first time as an evaluation index, and the baking embrittlement mechanism of DP steels was clarified. The baking embrittlement process of DP steels was divided into two stages. The first stage consisted of dislocations pinned by C clusters or low-temperature carbide, while the second stage was the tempering effect of martensite. Furthermore, the volume contraction of martensite led to the release of internal stress, resulting in an increase in the critical stress for dislocation movement. Both processes reduced dislocation mobility. In addition, by calculating the geometrically necessary dislocation (GND) densities before and after strain-baking, we found that the multiplication ability of dislocations was weakened.