Microstructure evolution and precipitation of a low carbon low alloyed steel by the two-step intercritical treatment

Stable retained austenite and multi-scale nano-sized precipitates were obtained in a 0.08C-0.5Si-2.0Mn-0.5Ni-0.9Cu-0.14(Nb+V+Mo+Ti) (wt.%) steel by the two-step intercritical treatment. The first step of intercritical annealing creates a mixed microstructure of preliminary alloy-enriched martensite and lean alloyed intercritical ferrite. The second step of intercritical tempering is helpful for producing film-like stable reverted austenite along the reverted structure. The two-step austenite reverted transformation associated with intercritical partition of C, Mn and Ni is believed to be the underlying basis for stabilization of retained austenite during the two-step intercritical heat treatment. Fine precipitates of micro-alloying elements (Nb and V) of size ∼2-10 nm and copper precipitates of size ∼10-30 nm were obtained in the ferrite matrix. The combination of multiphase microstructure, the transformation-induced-plasticity effect of retained austenite and strengthening effect of nanometer-sized precipitates contributes to yield strength greater than 800 MPa, uniform elongation of ∼10% and excellent low temperature impact toughness.