Effects of Bainitic Phase on Mechanical Properties of Bainite-Aided Multiphase Steels

Advanced high strength steels play an important role in the automotive industry. However, they still show inferior formability such as low stretch-flangeability. Therefore, in this work, various heat treatment processes are performed for generating multiphase steels with enhanced mechanical and forming behavior. The bainitic phase with different amounts is included in dual phase (DP) steels by controlling oil-quenching temperatures. In other steels, varying bainitic transformation temperatures are applied. Microstructures and phase fractions of the investigated multiphase (MP) steels are characterized by optical and scanning electron microscope. Tensile test, hole expansion test, and micro-hardness measurement are carried out for the steel sheets. It is found that increased oil temperature leads to larger amount of bainite occurred in the DP steels. The bainite-aided DP steels exhibit lower strength, but considerably higher elongation, improved hole expansion ratio, and larger damage tolerance value. In case of MP steels with retained austenite, increasing the bainitic transformation temperature causes ferrite in the steels to be replaced by bainite and retained austenite with various amounts. It seems that the MP steels with higher bainitic phase fraction show deteriorated edge-flangeability and fracture resistance.