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

Abstract

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 (~8 vol%) with 1.3 wt% C. In the austempering sample, the microstructure contained martensite/bainite laths and adequate amount of retained austenite (~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 (~10 vol%) and C concentration (~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 (~15%) after austempering treatment. Also, the impact toughness increases from ~12.5 J (quenching–tempering) to ~16 J (austempering) and ~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 ~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.