Strain-induced martensite and its effect in 304 stainless steel sheet stamping forming

The results of X-ray diffraction analysis, magnetic needle analysis, mechanical properties and hardness testing show that the cold-rolled (solution-treated) 304 austenitic stainless steel sheet produced strain-induced martensite during the cold forming (deformation) process. Martensite, M), up to 35%M during reaming and cupping forming, and the unidirectional tensile deformation is less than 10%M. On the other hand, it is found that the metastable austenite (Austenite, A) of cold-rolled 304 steel must be driven by a certain degree of strain to be more obviously transformed into martensite. The threshold value is about 20 in the tensile test. %; The finite element analysis of 304 steel plate material expansion and forming shows that 22% martensite can be generated when the strain is about 10%; those places with higher hardness, that is, the parts with more strain-induced martensite, It is also the area with the largest amount of reaming strain. The analysis shows that the reaming and cupping forming of the two-way tensile stress state is more conducive to the A→M transformation with volume expansion effect, and can induce a large amount of martensite in the main deformation area (boss, cupping). The comprehensive hardening effect can reach Δ35 HRB. The martensite induced in the forming of 304 steel sheet is a very low carbon (wC<0.05%) body-centered cubic structure (α' martensite), which can increase the value of n, promote deformation propagation and strain uniformity, etc. , It is beneficial to the improvement of the elongation forming performance of stainless steel sheet and the high reaming value λ and cupping value IE, and it is also beneficial to improve the structural strength of 304 stainless steel stamping parts.