On the strain distribution during the stretch-forming of low- and high-strength sheet steels

Abstract

Strain distributions during stretch-forming with a hemispherical punch have been studied for six steels in cold-rolled gauges: a deep-drawing quality steel, a re-phosphorized steel, three dual-phase steels and a HSLA steel. Strain distributions are presented for radial strain, tangential strain and thickness strain. A number of different characteristics of the strain distributions have been evaluated such as the pole-to-peak distance, the width of the strain distributions and the peak- and polar-strains. Significant differences have been observed between, on one hand, the deep-drawing quality steel and, on the other hand, the dual-phase steels. The deep-drawing quality steel gives widely separated strain peaks and thus wide strain distributions, with the state of strain at the peaks being close to plane strain, whilst the dual-phase steels have rather closely situated strain peaks and thus narrow strain distributions, with the state of strain at the peaks being close to equi-biaxial straining. The influence of material parameters such as the work-hardening exponent n and the plastic anisotropy index r on the different characteristics of the strain distribution has been explored. Differences in punch heights to failure between the six materials have been investigated: it has been found that the dual-phase steels have a favourable combination of punch height at failure and tensile strength. Previously it had been thought that this is due to there being wide smooth strain distributions for dual-phase steels but the dual-phase steels do not, however, produce wider strain distributions than, for example, HSLA steels of comparable strength. Instead, the dual-phase steels have the ability to generate a state of strain close to equi-biaxial straining over a large proportion of the cup which, combined with a high forming-limit diagram close to equi-biaxial straining, produces a large punch height at failure.