Residual Stress Analysis in Deep Drawn Twinning Induced Plasticity (TWIP) Steels Using Neutron Diffraction Method

In Twinning Induced Plasticity (TWIP) steels, delayed fracture occurs due to residual stresses induced during deep drawing. In order to investigate the relation between residual stresses and delayed fracture, in the present study, residual stresses of deep drawn TWIP steels (22Mn-0.6C and 18Mn-2Al-0.6C steels) were investigated using the finite element method (FEM) and neutron diffraction measurements. In addition, the delayed fracture properties were examined by dipping tests of cup specimens in the boiled water. In the FEM analysis, the hoop direction residual stress was highly tensile at cup edge, and the delayed fracture was initiated by the separation of hoop direction and propagated in an axial direction. According to the neutron diffraction analysis, residual stresses in 18Mn-2Al-0.6C steel were about half the residual stresses in 22Mn-0.6C steel. From the residual strain measurement using electron back-scatter diffraction, formation of deformation twins caused a lot of grain rotation and local strain at the grain boundaries and twin boundaries. These local residual strains induce residual stress at boundaries. Al addition in TWIP steels restrained the formation of deformation twins and dynamic strain aging, resulting in more homogeneous stress and strain distributions in cup specimens. Thus, in Al-added TWIP steels, residual stress of cup specimen considerably decreased, and delayed fracture resistance was remarkably improved by the addition of Al in TWIP steels.