Abstract
The effect of continuous annealing atmosphere on material properties of ultra low carbon Ti-added cold-rolled steels has been investigated. By replacing the conventional annealing atmosphere with the carburizing atmosphere, the improvement of formability and bake hardenability of ultra low carbon Ti-added cold-rolled steel are obtained as compared with those of steels annealed in conventional atmosphere. By carburizing for a short time during continuous annealing, the extremely small amount of carbon is added without changes in the microstructure. The resistance to cold-work embrittlement and fatigue property are improved due to the increase in grain boundary strength. Furthermore, bake hardenability exceeding 30 MPa is achieved through the control of carburizing condition and chemical composition of steel. The excellent combination of bake hardenability with r-value is attainable by carburizing after recrystallization. A part of added carbon atoms exist in solution, and the others are combined with the excess Ti of steel. Furthermore, the solute carbon is distributed nonuniformly in the direction of thickness due to the diffusion of carbon and the combination of added carbon with excess Ti. Consequently, the amount of solute carbon is controlled by the carburizing condition and the amount of excess Ti.
