Effect of YAG Laser Cutting on Stretch-flangeability of Ultra High Strength TRIP Sheet Steel with Bainitic Ferrite Matrix

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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 (λ).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 100 W 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 100 W. Compared to mechanical punching, YAG laser cutting contributed to the improvement of the TS×λ to 1100 MPa with TBF steel sheet possessing fine bainitic ferrite matrix.