Effects of Mechanical Properties on Biaxial Stretchability of Low Carbon Steels 680094

The response of low carbon steel sheets to biaxial tension forces was studied in stretch cup tests of 3 and 8 in. diameter. Three levels of lubrication were obtained by the use of mineral oil, drawing compound, and hydraulic bulge pressure. The mineral oil and drawing compound were applied to hemispherical steel punches and a 0.004 in. thick polythene sheet was placed between these and the metal under test. Hydraulic bulge tests were relatively insensitive to plastic anisotropy changes in sheet metal. It was found that improving the lubrication in mechanical punch tests or lowering the plastic strain ratio r̄ had similar effects in increasing the height of the cup at fracture, transferring the maximum strains closer to the pole, and increasing the amount of strain which could be developed before fracture. Material with high strain hardening capacity distributed the strains more uniformly and therefore had better biaxial stretchability. Too little temper rolling applied to the sheets caused premature failure due to localized thinning and too much temper rolling caused a decrease in biaxial stretchability. It is concluded that interactions of material properties and test conditions are such that many factors must be considered in assessing sheet metal formability.