Formability of austenitic and ferritic stainless steels at warm forming temperature

The warm temperature drawability of austenitic (STS 304) and ferritic stainless (STS 430) steel sheet samples was evaluated using the cup drawing test. The measured limiting drawing ratios (LDRs) increased about 44% and 25% for STS 304 and STS 430, respectively, compared to the isothermal case by introducing optimal temperature gradients between punch and die/blank holder. The drawability improvement was more drastic with STS 304 than with STS 430 using this temperature gradient approach. Fully coupled thermo-mechanical finite element (FE) analyses of the warm cup drawing tests were also carried out and validated using experimental data. For an accurate FE modeling, the mechanical properties measured in uniaxial and balanced biaxial tension tests were accounted for. For STS 304, an isotropic, temperature-dependent, yield function led to a reasonable prediction of the cup drawing performance at all temperature conditions. However, for STS 430, an anisotropic yield function was necessary to achieve good accuracy, in particular, for the prediction of the fracture location and cup height. Besides drawability, the earing profiles of the STS 430 cups were also simulated using three different yield functions, namely, von Mises, Hill48 and the non-quadratic anisotropic Yld2000-2d. Comparisons with experimental measurements indicated that the latter resulted in the best earing profile predictions for various temperature conditions.