Intercritical Annealing Processing and a New Type of Quenching and Partitioning Processing, Actualized by Combining Intercritical Quenching and Tempering, for Medium Manganese Lightweight Steel

The microstructure and its effects on mechanical properties of medium manganese lightweight steel are investigated under both intercritical annealing (IA) and a new type of quenching and partitioning processing, actualized by combining intercritical quenching and tempering (IQ-T-P). The result shows that the steel intercritically annealed at 800 degrees C exhibits a superior tensile ductility, which is related to long-term continued transformation-induced plasticity (TRIP) effect of austenite. Tempering has different effects on steels quenched at various temperatures. For the sample quenched at 800 degrees C, tempering has a negative effect on its mechanical properties due to a decrease in both volume fraction of austenite and carbon content in austenite. However, for the samples quenched at the range of 850-900 degrees C, the tempering heat treatment can remarkably improve mechanical properties, which is related to the improvement of austenite stability due to the partitioning of carbon from the carbon-supersaturated primary martensite into the austenite through the simple quenching and tempering treatment. Meanwhile, the tempering treatment can dramatically promote the yield strength. One of the important reasons is carbon in the primary martensite precipitation as carbides within tempered martensite.