Nb-ti Composite Ultra-low Carbon Bake Hardening Steel Sheet Composition Of The Optimization Study

As a new general sheet for deep drawing applications,Ultra-Low Carbon bake hardening (ULC-BH) steels have become the main materials of automotive body outside plate. At present, we can product the Nb-Ti treated ULC-BH steels,but the production's pass percent is low and the bake hardening property is untable.The solution carbon content is the main factor impact the bake hardening property.It is the key point to stabilize the bake hardening property by controlling the solution carbon content.The main research of this article is composition and processing parameter,which provided important theory and technology support.The present status of Nb-Ti treated ULC-BH steel development was generalized after document retrieval and studying,then the work as following was done:(1)Study on the phase transformation rule, recrystallization rule and grain growth rule of cold rolling plate for Composition Design and hot rolling,annealing technology formulation; (2) Study on finding the second-phase particles form,precipitation rule,calculation expressions of solution carbon content for Composition Design and annealing technology formulation;(3) Study on effect of process parameter on the bake hardening property of Nb-Ti treated ULC-BH steels;The following conclusions were obtained:(1) Procated the main reason why it was better for NbC dissolution and grain growth to improve annealing temperature and the reason why increasing the cooling rate can improve bake hardening property is to prevent reprecipitation of MC.So improwing annealing temperature and cooling rate can improve solution carbon content and bake hardening property. (2) When the Ti/N atom ratio is 1, it's simple to use Ti to stabilize the N and Nb to stabilize the C,so it's available to forecast and stably control solution carbon content and BH2 value. It's better for decreasing MC phase content to decrease the Nb,Ti,C.It's useful to control solution carbon content and bake hardening property to decrease Nb,Ti and C content,because the M(C,N) phase can be decreased by decreasing Nb,Ti,C content,so it is available for controlling solution carbon content and bake hardening property.(3)The grain size can be improved by improving the annealing temperature and decrease the M(C,N) precipitation which can be decreased by decreasing the Nb,Ti,C content.