Stabilization Of Ultra-low Carbon Bake Hardening Steel Bake-hardening Properties

As a new generation steel sheet for deep drawing applications, ultra low carbon bake hardening (ULC-BH) steel has become one of the main materials of automotive body panels. At present, only a few steel companies in China can produce ULC-BH steels, but the output is low and the bake hardening property is unstable. For ULC-BH steels, the main problem is how to obtain stable bake hardening property. The solute carbon content is a crucial factor influencing the bake hardening property. When producing ULC-BH steels, the key to obtain stable bake hardening property is to strictly control steel compositions and processing parameters and to ensure the solid solution carbon content within a certain range.This work based on summary of domestic and foreign ULC-BH steel production and research status, focusing on Nb-Ti treated ULC-BH steel, the main factors of bake hardening property was studied, and some of the important microcosmic mechanism was deeply discussed in order to provide important theory and technology support for production. The main conclusions include:Thermodynamic calculations indicate that, the total C content is the key factor which affects solute C content control. When the total carbon content in ULC-BH steel is constant, the solute C content decrease with increasing the Ti/N ratio, so Ti/N atom ratio is always≤1. When Ti/N atom ratio is always<1, the solute C content is controlled by Nb content; When the total carbon content in ULC-BH steel is constant, solute C content is controlled by Nb content; The higher Nb content, the lower solute carbon content.When annealing temperature is 830℃, the cooling rate of slow cooling section and fast cooling section are 50℃/s, the cooling rate after averaging stage is 15℃/s, the relationship of unstabilized carbon content and BH2 value is BH2=1.74×w(C)unstablized+7.64. In this case unstabilized carbon contents should be in the range of 12-24ppm which ensure a typical bake hardening response is between 30-50 MPa.When unstabilized carbon content meets the design requirements, it shows that continuous annealing process is the main factor affects the stability of bake hardening property during the studies of the effect of production process on BH2 value. Increasing annealing temperature, cooling rate of slow cooling section (annealing temperature-600℃) and cooling rate after over-aging stage can improve the BH2 value at the continuous annealing process. It is damaged to the bake hardening property when the temper rolling elongation is too high or too low. When temper rolling elongation being controlled at a range of 0.5-1.5%, the bake hardening property of the steel will be stability controlled at the high value. The bake hardening property can be improved by reducing the coiling temperature.Re-dissolving of NbC when annealing heating, precipitation of NbC when cooling at slow cooling section and grain boundary segregation of carbon atoms after aging stage are the important reason which influence the stability of bake hardening property at annealing process. Increasing the annealing temperature will make the NbC re-dissolved content increased. In this case the bake hardening property will be improved. Increasing cooling rate of slow cooling section will reduce the precipitates of NbC and benefits improving the bake hardening value. The more unstabilized carbon content, the less re-dissolved NbC, and the less influence of annealing temperature on bake hardening property; The more unstabilized carbon content, the less NbC re-precipitated, and the less influence of cooling rate on bake hardening property. In this case, when unstabilized carbon content meets the design requirements, increase the solute carbon content is beneficial to improving stability of bake hardening property. The bake hardening property of ultra-low carbon bake hardening steel will be reduced, because a lot of carbon atoms segregations to grain boundary when cooling at low temperature cooling section after over aging-stage. Increasing the low temperature (room temperature-400℃) cooling rate and grain size can decrease the grain boundary segregation of carbon atoms and benefits stabilized control of bake hardening property.The phase analysis and TEM analysis results show that the major precipitates are MnS and (Ti,Nb)(C,N), excluded TiS and Ti4C2S2 which always appear in Ti-IF steel. After coiling, the elements Nb and Ti are almost completely precipitated; a small amount of carbides dissolve during annealing process.Phase analysis and thermodynamic calculation showed that:when Ti/N atomic ratio of steel was<1, TiN and NbN are precipitated in austenite region, Nb(C,N) are precipitated in ferrite region, when Ti/N atomic ratio of steel was> 1, Ti(C,N) are precipitated in austenite region and (Nb,Ti)C are precipitated in ferrite region. When the Ti/N atom ratio is close to 1, N is almost fully combined with Ti precipitated as TiN and C is only stabilized by Nb precipitated as NbC, so the solute carbon content of the steels is easily estimated because of the simple precipitation order.It is an important method of controlling the bake hardening stability property through controlling the ferrite grain size of annealed cold rolling sheet. The ferrite grain size of annealed cold rolling sheet can be controlled by composition and parameters adjustment. From the chemical composition point of view, decreasing Nb, Ti contents (especial Nb content) help get larger ferrite grains. From the technology point of view, increasing annealing temperature or decrease cold-rolling reduction benefits increasing grain size. Improving finishing temperature also benefit increasing the grain size, but coiling temperature have less impact on grain size. In this case the ferrite grain size is mainly affected by four factors:①Nb, Ti contents;②annealing temperature;③cold rolling reduction;④finishing temperature. The ferrite grain size can be adjusted by control these 4 factors.