Effects of deep cryogenic treatment on the microstructure and mechanical properties of an ultrahigh-strength TRIP-aided bainitic steel

Abstract

Deep cryogenic treatment (DCT) provides an effective and convenient pathway to improve the comprehensive mechanical properties of multiphase steels. However, the transformation characteristics of retained austenite (RA) during the DCT and its effect on the precipitation behavior and bainitic transformation at the subsequent tempering stage are still unclear. In the present work, DCT is applied to a 1500 MPa grade ultrahigh-strength bainitic steel subjected to quenching and austempering treatment (QAT). The same region before and after DCT is selected and characterized by electron backscatter diffraction (EBSD), and the observations show that blocky RA located at the boundaries of primary martensite and prior austenite undergoes martensitic transformation. Dilatometry and transmission electron microscopy (TEM) characterization methods were used to elucidate the effects of tempering temperature and DCT on the competitive relationship between bainitic transformation and nanoscale carbide precipitation during tempering. Compared with QAT steel, the yield strength of QAT-C-T steels is significantly enhanced by 199–292 MPa, depending on the tempering temperature, due to the precipitation of nanoscale carbides and the elimination of unstable RA. EBSD characterization of RA at different tensile strains reveals that large blocky RA at prior austenite grain boundaries and fine RA between the bainite laths transform to martensite during uniform deformation and the necking stage, respectively. The evolution of RA during DCT and tensile strain demonstrates the correlation of its thermal and mechanical stabilities. • Effects of DCT and tempering on structure-properties of bainitic steel are studied. • DCT eliminates 5.5% blocky RA distributed at PAGB as observing same region via EBSD. • DCT affects both carbide precipitation and bainitic transformation during tempering. • DCT increases yield strength by eliminating unstable RA and promoting precipitation. • Thermal and mechanical stabilities of RA are interdependent in bainitic steel.