Development of ultrafine grains in C–Mn steel plates through hot-rolling and air-cooling

Abstract

Ultrafine grained steel (grain size 2–3µm) that has a numerous futuristic applications in automobile and engineering industry has been developed in laboratory scale with plain C–Mn and C–Mn–Nb, C–Mn–V compositions. Three heats (25kg each) of different chemistries were made in a laboratory induction furnace and hot-rolled into 20mm thick plates initially in a 2 high/4 high experimental rolling mill. Dilatometry was carried out to determine the transformation temperatures. Subsequently, hot-compression tests were performed in a thermo-mechanical simulator with designed parameters and optimum processing window for obtaining fine and uniform through-thickness ferrite grains. The experimental steels were further hot-rolled to 4mm plates in 3 passes and then air-cooled. Optical microscopy, SEM and TEM studies confirmed the presence of uniform through-thickness ultrafine ferrite grains in the range of 2.5–3.5µm in the experimental steels. Both tensile and Charpy impact toughness properties were found to be good in these steels. Yield strength of 422MPa, 472MPa and 594MPa with 22–23% elongation could be achieved in plain C–Mn, C–Mn–Nb and C–Mn–V steels respectively. Charpy V-notch energies of these steels were 44J, 68J and 44J at −20°C respectively. Uniform through-thickness ultrafine grains resulted in the substantial improvement of strength and toughness of the experimental steels.