Formation of Nanocrystalline Structure in 301 Stainless Steel Produced by Martensite Treatment

The present work aims to obtain the optimum parameters of a thermomechanical treatment for developing a nanocrystalline structure in as-cast AISI 301 austenitic stainless steel. In the first step, the effect of homogenization times of 3 to 13 hours at 1200 °C on the grain size of hot-rolled specimens was studied. The results showed the minimum grain size obtained with homogenization at 1200 °C for 9 hours after hot rolling at the temperature range of 1000 °C to 1200 °C, strain of 0.8, and strain rate of 1.2 s⁻¹. In the second stage, the effects of strain, initial grain size, rolling temperature, and strain path were examined on martensite formation in a thermomechanical process consisting of cold rolling and subsequent annealing. Cold rolling was carried out at −196 °C, −10 °C, and 0 °C with a strain rate of 0.5 s⁻¹ and reductions of 10 to 95 pct, while annealing treatment was conducted at 600 °C to 850 °C for 1 to 50 minutes. The results showed that decreasing the rolling temperature and increasing the strain and initial grain size plus the use of cross rolling resulted in increased volume fraction of the strain-induced martensite during cold rolling and in reduced martensite saturating strain from 0.7 to 0.2, the overall result being enhanced grain refinement of the steel sheets produced. The nanocrystalline austenitic structure with a grain size of about 60 to 80 nm was produced by annealing at 850 °C for 1 minute from the strain-induced martensitic structure formed during cold rolling. It was found that the effect of grain size prior to cold rolling was more pronounced on grain refinement than thickness reduction.