Effect of Annealing Process on Microstructures and Mechanical Properties of Cold-Rolled Martensitic Steels for Automotive Structural Parts

Recently, light-weight and energy-saving requirements for the automobile industry are extremely important in order to protect the environment by a reduction of the emission of CO 2 . Hence, high-strength steel (AHSS), even ultrahigh strength steels with tensile strength larger than 1GPa is used. Among AHSS, cold-rolled martensitic steels have attracted much attention due to their superior strength to improve crashworthiness. In this research, the influence of different annealing treatments, especially the auto-tempering, on the phase transformation further affecting the mechanical properties and microstructure was investigated. The result shows that the level of auto-tempering and strength in martensitic steel is dominated by the quenching/auto-tempering temperature. Furthermore, the auto-tempering carbides should be cementite which is fine enough to improve yield strength. The suitable chemical composition combined with auto-tempering method has been implemented to develop cold-rolled martensitic steels with a tensile strength of exceeding 1300MPa. These developed martensitic steels can meet the requirements of bumper reinforcement which has applied in a variety of automobiles.