Optimisation of microstructures and texture of AISI 436 steel through simulated hot rolling for improved drawability and surface quality

Abstract

Ferritic stainless steels (FSS) are a versatile, cheaper alternative to austenitic stainless steel. This has resulted in a growing research interest in these steels. However, the downside of ferritic stainless steel is that they exhibit surface ridging, which contributes negatively to the final product aesthetics and requires machining and polishing to rectify this defect. The manifestation of ridging is known to be inherited from the casting and hot rolling of the FSS sheets. This study delved into the effects of the hot working parameters on the evolution of the microstructures and texture in 436 FSS through lab-based thermomechanical uniaxial, isothermal compression simulations. In a previous study on the optimisation of the roughing rolling process, it was shown that by varying the rolling parameters, it was possible to accumulate strain prior to the finishing rolling process. Therefore, in this work, the roughing rolling was simulated in such way to achieve a variation in the retained strain prior to the finishing rolling process with the intention of studying its effect on the recrystallisation process during the finishing rolling process. The simulated finishing rolling process parameters were kept constant. The industrial rolling process was simulated through multi-pass hot deformation, using the Gleeble 1500 and Bähr Dilatometer 850D thermomechanical simulators. The roughing and finishing rolling processes were simulated by three multi-pass deformation passes each. The strains were varied between 0.18 and 0.3 per pass with total strains per schedule ranging between 0.58 and 1.4. Strain rates were varied between 5 and 15 /s while inter-pass times were varied between 11 and 50 s per pass. The strain rates were consistent with the industrial roughing rolling but not the finishing rolling process because the latter was not achievable in the lab. The microstructures and hardness values were studied using the Olympus BX51M microscope, Jeol IT300LV SEM equipped with XMAXn EDS and Oxford Nordlys Nano EBSD detectors and a Struers Duramin-40 automated hardness tester. The Channel 5 software was used for texture characterisation and ImageJ was used for the microstructural analysis. The particle stimulated nucleation was present in all hot working schedules. As expected, the recrystallisation mechanism was by DDRX. This resulted in random grain orientations around the inclusions and precipitates. However, the recrystallised fraction was not adequate to significantly change the bulk of the cast structure and texture. The simulated roughing rolling schedule, R2, with increased total strain (from 0.68 to 0.8) and inter-pass times (from 43.3 to 60 s), exhibited less sub-grain refinement i.e., resulted in less retained strain than the other two schedules, CF and R1 after three passes. This resulted in CR and R1 schedules having increased driving force for recrystallisation during roughing but less retained strained for recrystallisation during finishing. The goal of accumulating strain during the simulated roughing rolling process to act as the driving for recrystallization during finishing rolling was not achieved, but this increased the potential for strain retention during finishing. Annealing of the finishing rolling schedules proved this by showing a higher reduction in hardness values of 14% compared to the simulated benchmark with a 2% reduction in hardness value. The accumulated strain per pass in all simulations could not reach the critical strain for recrystallisation during the simulated finishing rolling process to aid the breaking down of the cast columnar grains with predominantly Cube oriented grains. This was attributed to the limitations in strain rates and strains in laboratory simulations. However, industrial hot rolling with higher strain rates, and strains could possibly overcome these limitations i.e., the accumulated strain between passes could reach the critical strain for recrystallisation. Therefore, it would be worthwhile doing plant trials of higher strain per pass and increased inter-pass time during the roughing rolling or the early finishing passes to accumulate strain to aid the recrystallisation process during the latter finishing rolling passes, without a substantial drop in temperature to trigger mill stoppage due to increased mill load.