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.