Austenite stabilization and precipitation of carbides during quenching and partitioning (Q&P) of low-alloyed Si–Mn steels with different carbon content

Abstract

Three silicon-manganese steels with carbon content of 0.25, 0.33, and 0.44 wt% were subjected to quenching and partitioning (Q&P) treatment to obtain the microstructures consisting of primary martensite (PM) and significant fraction of retained austenite (RA). The microstructural changes during Q&P were systematically characterized by various techniques while tensile and impact tests were performed to investigate the mechanical response of the steel samples. The carbon partitioning between martensite and austenite at 350 °C was accompanied by the formation of carbide-free bainite and resulted in the stabilization of the irregular layers and thin films of RA. Moreover, the precipitation of carbides in PM before partitioning and their subsequent growth affected the carbon partitioning from PM to RA. The volume fraction and width of RA islands in the final microstructure increased with increasing the carbon content. The transformation of significant fraction of RA in 0.44C steel before the strain localization improved uniform elongation, resulting in superior combination of strength and ductility (σ0.2 = 1225 MPa, UTS = 1580 MPa, δ = 16.8 %). On the other hand, the transformation of more stable RA in lower carbon steels occurred mainly in the neck portions of tensile specimens and its contribution to uniform elongation was negligibly small. The prevalence of the dimple pattern on the fracture surfaces of the specimens after tensile and impact tests indicated ductile fracture mode. The effect of carbon content on the precipitation and growth of carbides in PM and its influence on the impact toughness are discussed.