Simulation Calculation And Experimental Study On Quenching And Partitioning Of QP980 Steel

QP steel,as one of the third generation advanced high strength automotive steels,has excellent strength and plasticity.Compared with the traditional hot continuous rolling line,CSP line has the advantages of short process and low energy consumption in the production of thin steel products.The production of low carbon micro alloy automobile steel by Q&P process combined with CSP technology can not only ensure the safety of automobiles,but also save energy and reduce emission,which is in line with the development trend of green manufacturing of steel materials.In this study,Fe-0.193C-1.702Si-1.953Mn was used as the experimental object,and CCE model by Speer et al.was taken as the theoretical guidance,and the optimal quenching temperature and maximum residual austenite under Q&P process were calculated to be 232 ? and 18.54%,respectively.The conventional CCE model was improved by modifying the martensitic transformation kinetics model.The optimal quenching temperature calculated by the improved model was 292 ?,which is consistent with the experimental results.The maximum amount of residual austenite is obtained at this temperature.In this study,a simplified model containing only general algebraic expressions was established based on the calculation data of CCE model.This model can replace CCE model to calculate the optimal quenching temperature and maximum of residual austenite.The carbon partitioning from martensite to austenite in QP steel with two compositions(Fe-0.21C-4.0Mn-1.6Si-1.0Cr and Fe-0.193C-1.953Mn-1.702Si(wt.%))was calculated by one-dimensional finite difference method.The results show that it takes only 10 s for the two different chemical compositions to complete the carbon partitioning at 450 ?,while it takes 2700 s and 150 s for QP steel with Fe-0.193C-1.953Mn-1.702Si to complete carbon partition at 292 ? and 370 ?,respectively.Temperature plays a decisive role in the partitioning process.One-step Q&P treatment for of Fe-0.193C-1.953Mn-1.702Si(wt.%)steel at 202 ?,232 ?,262 ?,292 ? and 330 ? for 1 min,2 min and 5 min respectively was carried out by using Gleeble3500 thermal simulation testing machine.The results show that the maximum amount of residual austenite is 9%at 292 ? for 2 min,and the maximum strength plastic product is 34.5 GPa*%at 292 ? and 232 ? for 1 min.The change of residual austenite content of the steel at quenching temperature 292 ? for 1min,2min and 5min was analyzed in combination with carbon partition.The results showed that the amount of retained austenite for 5 min of partition time was less than 2 min.The reason may be that carbide formation and bainite transformation promote carbon consumption,and carbon-poor austenite transforms into martensite in the second quenching.