Abstract
In this study, investigations on stretch flangeability were carried out on as-received DP 600 steel which had 22 volume % of martensite. Stretch flangeability is normally characterized by the hole expansion test and refers to the ability to avoid cracking during stretch flanging or hole expansion. Two intercritical annealing treatments (soaking temperatures of 760 °C and 800 °C followed by water quenching) were selected in order to obtain lower and higher volume fractions of martensite respectively. The influences of four different hole preparation techniques, electrical discharge machining, drilling, drilling + reaming, and punching, on the hole expansion ratios were investigated. Higher surface roughness at the surface of the hole showed inverse correlation with HER values. The highest hole expansion ratio of 156% was obtained for the as-received DP 600 steel with the hole prepared by electrical discharge machining. The HER values observed for various hole preparation conditions can be normalized with respect to the electrical discharge condition so that the HER values can be estimated when one of the HER values is known.
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