The influence of primary cooling and rebound temperature at C–Mn slab corner surfaces during continuous casting on ferrite film transformation and AlN precipitation was investigated. Laboratory simulations included primary cooling to minimum temperature, T_min, rebounding to various maximum temperatures, T_max, followed by secondary cooling. The negative effect of a low T_min on hot ductility could not be readily reversed, even at relatively high temperatures. Quantitative metallography was employed to study the evolution of the microstructure during rebounding and secondary cooling. Following primary cooling to temperatures just above the Ar₃, thin films of allotriomorphic ferrite formed on the austenite grain boundaries. These films did not completely transform to austenite during the rebound at 3 °C/s up to temperatures as high as 1130 °C and persisted during slow secondary cooling up to the simulated straightening operation. Whilst dilatometry did not indicate the presence of ferrite after high rebound temperatures, metallography provided clear evidence of its existence, albeit in very small quantities. Coincident with the ferrite at these high temperatures was the predicted (TC-PRISMA) grain boundary precipitation of AlN in bcc iron during the rebound from a T_min of 730 °C. Importantly no thin ferrite films were observed, and AlN precipitation was not predicted to occur when T_min was restricted to 830 °C. Cooling below this temperature promotes austenite grain boundary ferrite films and AlN precipitation, which both increase the risk of corner cracking in C–Mn steels.

The influence of roughing strain on the extent of austenite recrystallisation in plain carbon steel and Nb-Ti-V microalloyed steel was investigated. Reheating and roughing simulations were conducted on a deformation dilatometer using industrial heating, soaking times, strain and strain rates. The roughing schedules comprised of varying the pass strain magnitude within a typical roughing temperature range. The double stroke method was used to determine the austenite softening fraction. The austenite grain size, prior to and after rolling, was measured on quenched specimens. Grain refinement was achieved in both steels after all rolling schedules. An applied pass strain of 0.15 was sufficient to completely soften the austenite after the first pass and produced the finest recrystallised grain size. This was attributed to sufficient nucleation sites and driving force for recrystallisation compared to lower strains. Partial recrystallisation occurred after the first pass due to the relatively coarse initial grain size. The steel chemistry played an insignificant role in controlling the recrystallisation kinetics at high roughing temperatures. The through-thickness strain distribution calculated from FEM simulations showed that, for a given applied strain, a similar magnitude of deformation is achieved at the centre of a hot-rolled plate.

The influence of the strain sequence during slab hot rolling (also known as “roughing”) on the evolution of austenite in plain carbon, C-Mn-V and C-Mn-Nb-Ti-V steels was investigated. Reheating and roughing simulations were conducted in a Bähr deformation dilatometer using a constant austenitising temperature, constant soaking time and various heating rates and roughing strain sequences. Stress analysis was used to quantify the austenite softening behaviour and the prior austenite grain size was measured from quenched specimens. The austenite grains of the plain carbon steel were coarser than those of both microalloyed steels, with the C-Mn-Nb-Ti-V grade being the finest due to effective pinning of the grain boundaries. Pass strains greater than 0.2 were sufficient for initiation of dynamic recrystallisation (DRX) for the C-Mn and C-Mn-V steels and led to uniform austenite microstructure with austenite grain sizes less than 40µm after the roughing stage.

The effect of thermomechanical controlled processing (TMCP) strain and finishing temperature on the microstructure of as-rolled and air-cooled 50mm thick Nb-Ti-V-Ni microalloyed steel plate was investigated through laboratory simulations, incorporating variations in reheating, roughing and finishing practices. Laboratory simulations produced microstructures similar to those observed in industrial rolling owing to comparable total strain in the TMCP region. Larger total strains in the TMCP region promoted sub-grain formation and increased nucleation site density, leading to grain refinement. Recrystallisation was completely suppressed at the commencement of finishing in all TMCP schedules due to sufficiently lower starting temperatures. The extent of recrystallisation during finishing depended on the finishing temperature: i) partial softening after finishing above 910°C and ii) complete recrystallisation below 910°C due to substantial accumulated strain. Finishing below 910°C produced finer polygonal ferrite and pearlite microstructures. Microstructure and mechanical properties were fairly consistent when the finishing temperature was between 925 and 950°C. However, the sub-zero impact toughness can be significantly improved by employing lower finish temperatures or applying larger total TMCP strains. Interrupted accelerated cooling at 5°C/s after finishing significantly refined the microstructure.