Processing of Ultrafine-Grained Steels by Warm Rolling and Annealing

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.