Thermo-mechanical treatment with multi-direction upsetting for improvement of superplasticity in SUS304

Various thermo-mechanical treatments have been tried in order to improve the structural superplasticity in conventionally produced JIS SUS304. The attempts of thermo-mechanical treatment consisting of three or four cycles including cold working by roll-mill and subsequent annealing. By adopting the cold multi-direction upsetting method, which is suitable for the accumlation of enough volume of martensite phase, the grain size decreases into submicrons in diameter. Especially by thermo-mechanical treatment of four cycles, the grain size of similar to200 nm is achieved. The upper limit of strain rate to obtain significant superplastic behaviour is improved up to 1 x 10(-2)/s at 973K by the grain refinement.