Enabling bainitic transformation acceleration and mechanical properties improvement in ultrafine bainitic steel

A novel approach involving the fast heating of cold-rolled pearlite followed by low-temperature isothermal transformation (FBT) was devised to address the technical challenges of long processing times and strength-ductility trade-off in ultrafine bainitic steels. Compared to conventional treatment (CBT), the FBT process yielded chemically heterogeneous bainite, along with refined austenite grain size, decreased retained austenite fraction and reduced dimension of blocky retained austenite. This heterogeneity was primarily manifested in the form of coarse filmy retained austenite with significant compositional fluctuations. The FBT process accelerated the bainitic transformation kinetics, which is attributed to the increased nucleation sites from prior austenite grain refinement and preferential nucleation at Mn-/Cr-depleted regions within the chemically heterogeneous austenite. Consequently, the FBT sample exhibited superior tensile properties, which are rationalized by the distinct retained austenite transformation behaviors. Interrupted tensile tests revealed that the CBT sample underwent a rapid transformation of unstable blocky retained austenite at low strains, while the FBT sample exhibited a progressive strain-induced transformation of its coarse filmy retained austenite, enabling a sustained transformation-induced plasticity effect. A key commonality was the exceptional stability of the fine filmy retained austenite in both samples, which persisted even after tensile fracture.