Repetitive Thermomechanical Processing towards Ultra Fine Grain Structure in 301, 304 and 304L Stainless Steels

Thermomechanical processing as a combination of cold rolling and annealing was performed on austenitic stainless steels 301, 304 and 304L. Two cold rolling steps each one up to a reduction of 75% were combined with an intermediate annealing at 800°C for 20 min. The final annealing was performed at the same temperature and time. Cold rolling contributed to martensite formation at the expense of metastable austenite in the studied materials. Austenite in 30l was found to be less stable than that in 304 and 304L. Hence, higher strength characteristics in the as-quenched 301 stainless steels were attributed to the higher volume fraction of martensite. Both α′-martensite and ɛ-martensite were found to form as induced by deformation. However, the intensity of ɛ-martensite increased as the stability of austenite decreased. Annealing after cold rolling led to the reversion of austenite with an ultra fine grained structure in the order of 0.5-1 μm from the strain induced martensite. The final grain size was found to be an inverse function of the amount of strain induced martensite. The thermomechanical processing considerably improved the strength characteristics while the simultaneous decrease of elongation was rather low.