Heat treatment effects on the microstructure and mechanical properties of a medium manganese steel (0.2C–5Mn)

Abstract

Abstract Microstructures and mechanical properties of 0.2C–5Mn steel processed under different heat treatment conditions were examined by scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray diffraction (XRD). It was found that high temperature austenization (above Ac3) resulted in a full martensite structure after quenching, which gradually transformed into the ferrite/austenite duplex structure during the following annealing process. However, austenization in the intercritical region (between Ac1 and Ac3) gave a duplex structure after quenching, which was nearly not affected by followed annealing process. The ultrahigh strength ∼1000 MPa and total elongation ∼40% were only obtained in the specimens with 6 h annealing at 650 °C under both heat treatment conditions. However, the excellent mechanical properties could be obtained in the intercritically austenitized samples with only 10 min annealing at 650 °C. Based on the analysis on the relationship between microstructure and mechanical properties, it was found that the total elongation was strongly dependent on the austenite fraction, which was ascribed to the phase transformation induced plasticity of the large volume fraction of austenite. Otherwise, the dependence of the yield stress on the austenite grain size accords with Hall-Petch equation, which implies that the austenite is soft phase. It was concluded that 10 min annealing at 650 °C was enough to obtain a large volume fraction of austenite (∼30%) in 0.2C–5Mn steel when the specimens were austenitized at 675 °C.