Effects of retained austenite thin films on mechanical properties of auto-tempered 0.15C-5Mn based medium Mn martensitic steel

Abstract

Effects of retained austenite thin films and auto-tempering on tensile properties in 0.15C-5Mn martensitic steel were studied. Specimens for uniaxial tensile tests were heat treated in the temperature range of 800~1000 °C for 10 minutes followed by air cooling. The results showed that both strength and ductility increased with decreasing heat treatment temperatures from 1000 to 800 °C. When the heat treatment was conducted at low temperature, it was expected that high Mn content region was existed by Mn partitioning due to cementite. Thus, large volume fraction of austenite could be retained at room temperature because of high austenite stability by Mn partitioning. It was identified by multiple tests, and the austenite volume fraction gradually decreased as deformation progressed in the specimen heat treated at 800 °C. Consequently, the retained austenite which had thin films shape made an increase in ductility by transformation-induced plasticity effect (TRIP). The effect of cooling rate after austenitizing was also studied. Auto-tempering occurred below 240 °C during air cooling (10°C s-1). Water quenched specimens which were not auto-tempered had higher strength but lower ductility than air cooled specimens. The auto-tempering affects strength and ductility by reducing tetragonality of martensite. Carbons released from martensite formed carbides and enhanced retained austenite stability.