On balanced strength and ductility synergy in low alloy steels through multiphase heterostructure involving cumulative process of hot rolling, coiling and tempering

We present here a new pathway of combining hot rolling, coiling and tempering for multiphase heterostructure to achieve a balanced combination of strength and ductility in low alloy steels. The multiphase heterostructure was composed of bainite/martensite and retained austenite (RA), which was tuned by coiling and tempering. The steel coiled at 450 °C exhibited superior mechanical properties compared with the steels coiled at 350 and 550 °C. After tempering at 300 °C, the steel achieved excellent combination of strength and ductility, with twice the elongation and a corresponding product of strength and elongation (PSE) of 25.1 GPa·%. After coiling, 17 vol% of RA was retained. The RA fraction decreased slightly to 14 vol% by tempering, the steel still sustained considerable austenite. Tempering further enabled carbon partitioning and improved the mechanical stability of RA. The continuous transformation induced plasticity (TRIP) effect from stable RA over the entire strain range resulted in good ductility of the steel.