Work hardening characteristics under high strain and their interaction with microstructure and formability of sheet steel

Abstract

The progress in measurement methods and equipment for stress and strain under large values of stress and strain has enabled the reliable determination of stage IV in addition to the well known stages I, II, and Ill in metal forming processes. Physical models based on cross slip and climb of dislocations or on cell reorganization and mobile dislocation evolution were developed for description of the entire strain range during the last years. The experimental data can be fitted well to both models. A simplification of the models leads to the classical model of Kocks. This is the basis for a physically reasonable description of the stress strain behaviour of metals. From this kind of physical constitutive equation the formability of sheet steels such as FLD and LDR can be calculated much more accurately. The strain hardening rate has a large influence on the formability besides the yield stress level. The strain hardening rate can, on the other hand, be changed by the microstructure of steels just as the yield stress. A quantitative relation between strain hardening rate and microstructure exists.