Effect of in-plane biaxial strain paths on the variation of normal anisotropy and texture of steel sheet

Normal anisotropy is crucial for deep drawability of sheet metal. Texture of the sheet determines the normal anisotropy. It is often observed that the forming conditions remaining unchanged, sheets having similar tensile properties do not always show similar drawability in press working. It was therefore thought that strain and strain path dependent change in the normal anisotropy of two sheets could hold the key to this behaviour. Hence studies were undertaken to ascertain as to whether such a strain path dependent change in the anisotropy does occur. While in the literature, strain dependent change in anisotropy has been investigated, dependence on the strain path has not been reported. Studies on the strain dependent variation in texture of non-ferrous materials (Al alloys and brass in particular) have been widely reported. Literature on the strain and strain path dependence of texture and the anisotropy of steel sheet is not easily available. An experimental program was therefore undertaken to stretch the sheet in-plane to different strain levels for each of the five strain paths considered. Two strain paths characterised the negative minor strain and two characterised biaxial tension. The fifth one was close to plane strain. Special tooling with a 200 mm diameter flat (but relieved) bottom punch was fabricated. Specimens for R-value, texture and Modul-r̄ measurements were all cut from the flat, in-plane stretched (punch bottom) region of the sheet. Results indicated significant dependence of the R-value and texture on the strain and strain path. The R-values were found to decrease with increasing strain in all the three methods of testing. The rate of decrease was found to be strain path dependent. The inferences from the mechanical measurements of R-value based on the ASTM procedure and those predicted from texture measurements were in excellent agreement. Thus texture studies throw light on macroscopic mechanical behaviour (strain and strain path dependent variation in anisotropy).