Effect of YAG laser cutting on stretch-flangeability of ultra high strength TRIP sheet steel with bainitic ferrite matrix

Abstract

The effect of YAG laser cutting on stretch-flangeability of ultra high strength TRIP-aided steel sheets with bainitic ferrite matrix (TBF steel sheet) austempered at 375 or 450°C, was investigated for automotive applications. Holes of 5 mm diameter for hole-expanding test were produced by YAG laser cutting and mechanical punching and the stretch-flangeability was evaluated by measurement of the hole-expanding ratio (l ). In TBF steel sheet, laser cut specimens show higher stretch-flangeability than mechanically punched specimens. The hole-expanding ratio (λ) of TBF steel sheet austempered at 375°C in the case of laser cutting at powers between 50 and 100W was higher than those austempered at 450°C. Furthermore, the strength-stretch-flangeability balance (TS×λ ) of TBF steel sheet austempered at 375°C showed the highest value after laser cutting at 100W. Compared to mechanical punching, YAG laser cutting contributed to the improvement of the TS×λ to 1 100MPa with TBF steel sheet possessing fine bainitic ferrite matrix. © 2010 ISIJ.