Investigation of Tensile Properties and Bendability of CMnB Hot-Press Forming Steels

The tensile properties and bendability of CMnB hot press forming (HPF) steels with various carbon contents are investigated. The total elongation and yield ratio of hot-rolled steels significantly depend on increasing carbon content, suggesting that this result is due to the increasing pearlite phases. For the cold-rolled, annealed steels, the degenerated pearlites and cementites formed by higher cooling rate during continuous annealing process likely induce smaller reduction in total elongation and yield ratio compared to the hot-rolled steels. The increasing strength of HPF heated, die-quenching steels is considered to be due to a lot solute carbons and dislocation density induced during die-quenching. The bendability of cold-rolled, annealed steels is independent but that of HPF heated, die-quenched steels is significantly dependent on the increasing carbon content. For the die-quenched HPF steels the bendability has significantly relevance to the bending-induced work hardening affected likely by solute carbons and dislocation density produced during martensitic transformation. In comparison to cold rolled, annealed steels, the bendability of HPF heated, die-quenched steels is accelerated significantly by the bending-induced work hardening possessing the strong carbon dependence, although both materials have same alloying compositions.