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

Effect of YAG laser cutting on stretch-flangeability of 0.2C–1.5Si–1.5Mn (mass%) ultra high-strength TRIP-aided sheet steel with bainitic ferrite matrix (TBF steel) having different martensite start temperatures, which were austempered at 375 or 450°C, was investigated for automotive applications. Hole notches of 5 mm diameter were produced by YAG laser cutting or mechanical punching for hole-expanding test. The stretch-flangeability was evaluated using hole-expanding ratio (λ).In TBF steel, laser cutting resulted in much higher stretch-flangeability than mechanical punching. The hole-expanding ratio (λ) of TBF steel austempered at 375°C in the case of laser cutting at average powers between 50 and 100 W was higher than that at 450°C, and also higher than mechanical punching. Furthermore, the strength–stretch-flangeability balance (TS×λ) of TBF steel austempered at 375°C showed the highest stretch-flangeability by laser cutting at 100 W. We found that compared with mechanical punching, YAG laser cutting contributed to the improvement of the TS×λ of 1100 MPa class TBF steel with fine bainitic ferrite matrix.