Effect of silicon content and annealing temperature on formation of retained austenite and mechanical properties in multi-phase steels

Abstract

Cold-rolled and annealed ultra-high strength sheet steels with good ductility accompanied by TRIP of retained austenite have received considerable attention in recent years. This paper discusses the effect of silicon content and annealing temperature on the formation of retained austenite and the mechanical properties in Fe-0.34%C-1.7% Mn steels whose structure consists of ferrite, bainite and retained austenite. Silicon inhibited the cementite formation in bainite during isothermal holding and partitioned carbon from bainite to austenite, resulting in an increase in retained austenite content. When the silicon content was increased to 1.0 wt.% or higher, the amount of retained austenite markedly increased leading to good mechanical properties. 0.34%C-1.03%Si-1.7%Mn steel showed a high tensile strength of 1,030 MPa and a total elongation of 34.5% when annealed at 780°C for 5 min followed by isothermal holding at 400°C for 5 min. In this case, the amount of retained austenite was about 25%. The variation in tensile strength-elongation combination had good correlation with that in the amount of retained austenite with both annealing temperature and silicon content. The most retained austenite was obtained in the steel annealed at just above AC1 temperature. The annealing temperature which gives the most retained austenite was decreased with decreasing the silicon content.