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

Ultrafine grained steel (grain size 2–3 µm) 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 (25 kg each) of different chemistries were made in a laboratory induction furnace and hot-rolled into 20 mm 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 4 mm 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 2.5–3.5 µm in the experimental steels. Both tensile and Charpy impact toughness properties were found to be good in these steels. Yield strength of 422 MPa, 472 MPa and 594 MPa 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 44 J, 68 J and 44 J at −20 °C respectively. Uniform through-thickness ultrafine grains resulted in the substantial improvement of strength and toughness of the experimental steels.