High strength low-carbon alloyed steel with good ductility by combining the retained austenite and nano-sized precipitates

A Cu-bearing Nb-microalloyed bainitic steel with high yield strength (>700 MPa), good elongation (>35%) and low temperature toughness was developed for heavy plates. An intercritical annealing plus tempering (L-T) heat treatment was applied for combining reversed transformation with the precipitation during the tempering stage. The enrichment of alloying elements during the annealing process leads to the drop of Ac1 temperature, which makes the reversed transformation and precipitation occur simultaneously in the subsequent tempering process. The secondary enrichment of Mn and Ni makes the reversed austenite stable enough to be retained at room temperature. About 29% retained austenite introduced by the L-T process contributes to excellent elongation more than 10% by the TRIP effect. The formation of Nb(C,N) and (Nb, Mo)C precipitates during the annealing and cooling stages inhibits the recovery of dislocations, while more Nb precipitates smaller than 10 nm and Cu precipitates with sizes of 20 nm play significant strengthening role on the matrix. The combination effects of retained austenite and nano-sized dual-precipitate increased yield strength, ductility and toughness remarkably.