Development of anisotropy and strain hardening in damaged stamped parts made of IF steel

Abstract

The high formability of interstitial-free (IF) steel for the automotive industry is conditioned by anisotropy for delayed necking. The processes leading to the formation of the crack under manufacturing conditions, driven by localized deformation in the critical stamped parts, are investigated in the present research. The development of anisotropy along with intra-granular misorientation is analyzed with the increasing necking up to the fracture. The real failure of the stamped part in the stage of the crack formed during the forming vs limited necking is compared with standard tensile and Nakazima samples to involve the different strain paths. The limiting phase of primary misorientation degradation is defined as the critical decrease of γ-fibre in the thickness direction (ND//〈111〉). The low angle grain boundary (LAGB) formation is expressed as the decisive parameter in the sense of anisotropy development during necking up to the point of failure. Strain-path affects the limit value of LAGB and indicates the non-proportional strain in the fracture process zone. Furthermore, the depletion of plasticity is expressed by local yield stress. The limiting necking of 48 % is found to be correlated with hardening over 90 %, i.e. a local yield strength above 300 MPa leads to a critical decrease in plasticity.