Effect of rolling reduction below γ non-recrystallization temperature on pancaked γ, microstructure, texture and low-temperature toughness for hot rolled steel

Abstract

To optimize rolling schedules for producing heavy plates with a limited slab-to-plate reduction ratio, we investigated the effect of rolling reduction below the non-recrystallization temperature (Tnr) of austenite (γ) on microstructure, texture, and low-temperature toughness for crack arrest heavy-gauge EH47 steel for shipbuilding. Four plates were hot rolled with 30%, 49%, 60% and 67% reduction below Tnr respectively, of which the total rolling reduction ratio was the same as 3.57. The microstructures and texture were characterized, and the low-temperature Charpy impact toughness was tested as well for each plate. Variations in toughness were significant among the steels, being highly correlated with the effective grain size (EGS), amount of high angle grain boundaries (HAGB), and texture. The steel with 60% reduction below Tnr showed the best toughness down to −80 °C with the impact absorbed energy of ~233 J for the minimum EGS of 3.4 ± 4.2 μm, the maximum HAGB proportion of 51.6%, {110} slip planes of 39.1%, and the strongest texture of {113}~{112}<110>, {112}<131>, {332}<113>. The theoretical calculation shows that the nucleation sites for austenite-ferrite transformation are mainly affected by the size of γ after rolling in the γ recrystallization region and the deformation substructure generated below Tnr. The changes of the pancaked γ can be quantified interpreted by the theory of effective interface area per unit volume. The relation of microstructures, texture, and low-temperature toughness to the rolling reduction below Tnr could be well interpreted by evaluating effective interfacial area per unit volume of austenite to ferrite during transformation.