Evolution of Microstructure and Mechanical Behavior of a Lean Alloyed Continuously Cooled Bainitic Steel

The effect of continuous cooling on the development of the bainite transformation process and the mechanical properties of low-carbon (0.17% C) steel alloyed with 1.73% Mn, 1.35% Si and microalloyed with Nb (0.02%) and Ti (0.04%) is studied. The microstructure and phase analysis is carried out at various continuous cooling conditions. The work hardening behavior under various conditions is examined using the Hollomon equation and modified Crussard–Jaoul work hardening model. The steel is subjected to austenitization in the intercritical range and above the A3 temperature between 790 and 900°C for 5 and 30 min, followed by continuous cooling to form a dominantly bainitic microstructure, which makes it possible to achieve an ultrahigh strength (1100 MPa) at a high ductility (20%). A longer austenitization time contributes to an increase in the strength and ductility of the steel.