Fine Grained Austenitic Stainless Steels: The Role of Strain Induced alpha ' Martensite and the Reversion Mechanism Limitations

The effect of the alpha'-strain induced martensite on the austenitic stainless steel grain size reduction is analyzed. Rolling with a 80% reduction factor at 200 degrees C and at 20 degrees C resulted respectively in a heavily deformed austenite and a mixed deformed austenite with alpha'-martensite bands. Further annealing treatments were carried out in the range of 20-880 degrees C. Fine grains of about 1 mu m can be obtained in the metastable austenitic stainless steel by a repetitive cold rolling and annealing thermomechanical process. In this case, reversion mechanism from deformation induced martensite alpha' to austenite gamma is responsible for grain refining. Non-conventional warm rolling and annealing process can also lead to fine grains of 2 mu m without a reversion stage and as a result of recrystallization process of the strain-hardened austenite. The cold rolling route gives rise to a bimodal grain size distribution with a lamellar structure resembling to a composite material.