Effect of Alloying Elements on the Mechanical Properties of the Cold Rolled ULC High Strength Steel Sheets

To investigate the effect of alloying elements on the mechanical properties and cold work embrittlement of cold rolled and annealed Ti-added ULC high strength steel sheets the aging properties, mechanical properties and ductile to brittle transition temperature of steel sheets were measured. In addition, to investigate the microstructure variation in high Mn steel, γ→α transformation of steel was also studied. The addition of P to Ti-added ULC steel caused the segregation of P to grain boundaries and then formed coarse FeTiP precipitates along the grain boundaries, which might result in the brittle fracture of hot bands during the steel production processes and the cold work embrittlement of cold rolled sheets during the stamping processes. The addition of B to rephosphorized ULC steel improved the resistance to inter-granular fracture, but it deteriorated the formability of sheets sharply. The addition of Mn to rise strength decreased A3 temperature, which leads to the different morphologies of microstructure compared with low Mn steel, which increases strength and make yield point elongation disappear. The combined effect of the addition of Mn and low coiling temperature resulted in grain refinement in hot bands due to low temperature transformation.