Planar Variations in Normal Anisotropy in Mild Steel Strip

Abstract

After a short discussion on the significance of planar variations of normal anisotropy, ('R' value) and associated earing in mild steels, the published literature on the subject has been reviewed. Previous work on earing formation in deep drawing was broadly divided into the macroscopic and crystallographic approaches. The control of earing potential in mild steels was studied with reference to process variables such as hot finishing temperature, coiling temperature, cold reduction, annealing conditions and composition. Finally, the testing methods available for earing measurement were assessed. Several important points emerged from the literature survey to form the basis for subsequent experimental work. Firstly, it was apparent that there was a need for an accurate earing test, simulative of the deep drawing operation. A test originally proposed for aluminium, was examined as to its suitability for mild steels, and various parameters, such as test piece dimensions, tool geometry, lubrication, were evaluated. A test specification was developed and used in subsequent experimental work. Secondly, the relationship between planar variations of normal anisotropy and earing behaviour required a quantitative assessment. Thirdly, there was a need for a statistical evaluation of earing tendency across the width, and along the length of commercially produced mild steel coils. Material from three sources was examined and the main conclusions were:- (a) the planar variations of 'R' value correlated well with the profiles of deep drawn cups. However, the correlation became much more significant when 'R' values determined in tension in a radial direction within the sheet, were considered in relation to the circumferential hoop stress that creates earing in a typical deep drawn blank. This was particularly important for rimming steels. (b) Although there were certain trends of variation, earing was relatively uniform across the width and along the length of typical coils, except at the ends, and, in the case of rimming steels, the extreme edges. (c) The variation in earing was greater between coils of nominally the same composition and process history, than within a given coil.