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 (X). In TBF steel, laser cutting resulted in much higher stretch-flangeability than mechanical punching. The hole-expanding ratio (X) 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 (TSX 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 TSXλ of 1100 MPa class TBF steel with fine bainitic ferrite matrix.