Abstract: The results of XRD diffraction analysis, magnetic needle analysis, mechanical properties and hardness tests show that the 304 austenitic stainless steel sheet in the cold-rolled (solution treated) condition produces strain-induced martensite(M) during cold forming(deformation) processes, which can reach 35% M for holes expansion and cup protrusion forming, but uni-directional tensile deformation being less than 10% M. On the other hand, it is found that the meta-stable austenite(A) of cold-rolled 304 steel must be driven by a certain degree of strain to be more obviously transformed into martensite. This threshold is about 20% ε during the tension test. The finite element analysis of 304 steel sheet expansion holes forming shows that 22% martensite can be formed at strain of about 10%，those locations with higher hardness, that is, areas with a lot of strain-induced martensite, are also the areas where the expansion holes strain is the largest. The analysis shows that the bi-directional tensile stress state of expansion holes and cupping forming is more conducive to the A→M transformation with volume expansion effect, which can induce a large amount of martensites in the main deformation area (boss, cupping) and the comprehensive hardening effect up to Δ35HRB. The martensite induced in the forming of 304 steel sheet is a very low carbon(w_C<0.05%) body-centered cubic tough structure(α'-martensite), which has the effects of increasing n value, promotes deformation propagation, strain homogenization and others, it is conducive to the improvement of the elongation-type forming properties of the stainless steel sheet, the attainment of a high λ value of the expansion holes and the IE value of the cupping，and the promotion of structural strength of 304 stainless steel sheet stamping parts.

Key words: 304 austenitic stainless steel, strain induced martensite, expansion holes forming, cupping forming, tensile deformation, FEM