High temperature mechanical properties of triple phase steels

A 0.15% C–1.2% Si–1.7% Mn steel was intercritically annealed at 780 °C for 5 min and then isothermally held at 400 °C for 4 min followed by oil quenching to room temperature and the annealed microstructure consist of 75% ferrite , 15% bainite and 10% retained austenite was produced. Samples of this steel with triple phase structure were tensile tested at temperature range of 25–450 °C. Stress–strain curves showed serration flow at temperature range of 120–400 °C and smooth flow at the other temperatures. All of the stress–strain curves showed discontinuous yielding at all testing temperatures. Both yield and ultimate tensile strength decreased with increasing temperature, but there exists a temperature region (120–400 °C) where a reduction of strength with increasing temperature is retarded or even slightly increased. The variation in the mechanical properties with temperature was related to the effects of dynamic strain aging, high temperature softening, bainite tempering and austenite to martensite transformation during deformation.