Revealing the extra-high ductility and toughness of micro+duplex medium+Mn steel in a large temperature range from 200 ℃ to -196 ℃

A medium-Mn steel (0.2C5Mn) was processed by intercritical annealing at different temperature (625℃ and 650℃) after forging and hot rolling. The microstructures were characterized by transmission electron microscopy and the mechanical properties were measured by tensile tests and impact tests at different temperatures.

It was found that an ultrafine grained micro-duplex structure existed with austenite and ferrite laths formed by means of an austenite reverse transformation during intercritical annealing (shortly called ART-annealing). Ultrahigh ductility (total elongation larger than 30%) could be obtained in the temperature range from 200℃ to -196℃.

And significantly delayed transition from ductile to brittle and no less than 200J impact toughness at -40℃ could be obtained in the ART-annealed medium-Mn steel. Based on the analysis of microstructure and mechanical properties, the enhanced ductility in the full temperature range could be ascribed to the phase transformation effect of austenite (TRIP effects), while the delayed ductile to brittle transition could be attributed to the enhanced austenite stability.