A route to produce toughened acicular ferrite with equivalent hardness as martensite: The combined effect of elements segregation and pre-transformed allotriomorphic ferrite

Acicular ferrite (AF) is a universal occurrence during the welding of steel. In this study, when heat treatment of intermediate annealing after austenitization was adopted, AF was obtained in the quenching process (−100 °C/s). The transformed AF has significantly higher toughness with no loss in hardness compared to martensite, which was also obtained at a cooling rate of −100 °C/s. The experimental and calculated results showed that film-like allotriomorphic ferrite are generated and high levels of boron (B) and molybdenum (Mo) segregation occur before transformation. These two effects inhibit the nucleation of bainitic ferrite (BF) and promote AF nucleation of inclusions. The segregation of B mainly occurred during the cooling process from austenitizing temperature to intermediate annealing temperature via the nonequilibrium mechanism, and the Mo segregation could be enhanced during intermediate annealing treatment by the generation of allotriomorphic ferrite.