Enhanced mechanical properties by retained austenite in medium-carbon Si-rich microalloyed steel treated by quenching-tempering, austempering and austempering-tempering processes

The microstructure evolution and mechanical properties of a designed 0.56C-1.48Si-0.70Mn-0.71Cr-0.148V-0.011Nb (wt.%) steel subjected to three heat treatments including quenching-tempering, austempering, and austempering-tempering have been investigated. In the quenching-tempering sample, the microstructure of the steel consisted of tempered martensite and a small amount of retained austenite (similar to 8 vol%) with 1.3 wt% C. In the austempering sample, the microstructure contained martensite/bainite laths and adequate amount of retained austenite (similar to 15 vol%) with 1.33 wt% C. Subsequent tempering treatment promoted MC carbides to form and retained austenite to decompose, respectively. Resulting in decrease in the retained austenite amount (similar to 10 vol%) and C concentration (similar to 1.24 wt%). The carbon distribution indicated the stability of the C-rich area along prior austenite grain boundary is higher than that between the martensite/bainite laths. As a result, the present steel shown an ultrahigh ultimate tensile strength (>2200 MPa) and an excellent total elongation (similar to 15%) after austempering treatment. Also, the impact toughness increases from similar to 12.5 J (quenching-tempering) to similar to 16 J (austempering) and similar to 17.5 J (austempering-tempering). As a result of transformation-induced plasticity (TRIP) effect, the austempering sample had the highest values of strain hardening rate and strain hardening exponent in the uniform strain stage and necking stage, thus it obtained a maximum uniform true strain of similar to 0.087. In addition, the crack source analysis of the three samples all shown ductile dimpled fracture while crack propagation presented as a quasi-cleavage fracture.