Alloy Design and Optimization of Heat Treatment Cycle to Develop Ultra High Strength with Superior Ductility Bainitic Steel

Abstract

Bainitic steel with YS> 600 MPa, UTS>1100 MPa and TE>20 % was developed in the present study with alloy design and optimization of heat treatment cycle. The steel with a chemical composition (wt.%) 0.2C-1.7%Si-2%Mn-0.9%Cr-0.2%C-0.18%Mo-0.07Nb-0.04Ti was induction melted and forged followed by slicing to 2mm thick sheets. The sheets were then subjected to inter critical annealing with 70% austenite and fully austenitized (above A3) followed by salt bath holding in the range of 350 to 480oC. YS, UTS and TE decreased in the case of inter critical annealing followed by increase in salt bath holding temperature due to the coarser lath structure of the bainites whereas the UTS increased with drastic decrease in the elongation and no significant change in the YS for the steel austenitized above A3 followed by increase in bainitic holding temperature due to the transformation of globular type bainite to lath type bainites and their further coarsening. It was found that the steel austenitized above A3 followed by salt bath holding at 350oC gives the best properties due to the presence of granular type bainite in the range of 50-55% along with fine precipitates in addition to the TRIP effect for the presence of 10% retained austenite. Presence of fine precipitates of micro alloying elements further improves the mechanical properties of the steel.