Processing of Ultrafine-Grained Steels by Warm Rolling and Annealing

Abstract

Low-carbon microalloyed steel was subjected to warm rolling followed by rapid transformation annealing (RTA) at 800-850 C and subcritical annealing (SCA) at 600 C to develop ultrafine ferrite grain structures (UFFG) with grain size less than 3 lm. The present study investigated the influence of light (40%) and heavy (80%) warm rolling deformation (LWR and HWR) applied during the finishing pass of two-pass rolling schedules on the microstructural evolution after rolling and subsequent annealing treatments. RTA treatment of HWR sample at a lower intercritical temperature for an optimum duration (800 C, 30 s) developed UFFG-martensite dual-phase structure that offered the best combination of strength (YS 900 MPa and UTS 1200 MPa) and ductility (25% elongation). The SCA treatment provided sufficient time to achieve a uniform distribution of carbide particles throughout the ferrite matrix. SCA treatment of HWR at 600 C for 4 h developed UFFG-carbide structure achieving YS of 800 MPa with 20% ductility. The SCA of LWR resulted in coarser ferrite grain structures (grain size > 5 lm) having higher ductility (more than 30%) but lower strength (UTS of 400-550 MPa) as compared to RTA.