Effect Of Continuous Annealing Process On Microstructure-Properties And Study On Strength Plasticization Mechanism Of Cold Rolled High Strength Steel

It is very effective to use retained austenite and bainite transformation to improve the strength and plasticity of high-strength steel.Based on the study of the influence of microstructure on mechanical properties of traditional CMn cold rolled dual phase steel DP780,this paper focuses on optimizing chemical composition,improving phase microstructure and different continuous annealing processes.Through systematic research on high strength plastic cold rolled dual phase steel from the perspectives of multi-phase,multi-scale,meta-stable austenite and morphology,innovative results have been obtained,and the strength plastic product of tensile strength of 800-1200MPa has been increased to 15-20GPa%.(1)The main factors of strength and plasticity of cold rolled dual phase steel are chemical composition,continuous annealing process and microstructure composition of each phase.Under the same annealing process,the mechanical properties of CMn steel are improved by adding Cr and Nb elements.The process window of CMn steel prototype is narrow,the mechanical properties are low,and the strength plastic product is 13.2GPa%.MnCr steel can be selected for large-scale industrial application.Cr is conducive to the formation of island martensite,and the process window is obviously widened.The strength plastic product is about 13.1?15.1 GPa%.MnNb steel is conducive to the formation of retained austenite,fine carbides and NbC precipitates of 1-10nm,which can improve the mechanical properties and stability,and the process window is wider and the strength plastic product is about 16?18.5 GPa%.(2)The plasticizing process of MnNb dual phase steel should be considered from three aspects:composition design,flexible continuous annealing process and microstructure control.The main composition system is 0.15%C+1.8%Mn+0.04%Nb.The microstructure of F?+F?+M+R.A can be obtained by appropriately reducing the critical annealing temperature of dual phase zone,increasing the rapid cooling temperature,increasing the cooling rate and reducing the overaging temperature.The comprehensive effect of fine grain structure,dispersed precipitation and a small amount of thin-film retained austenite distributed parallel to lath martensite during deformation is transformation induced plastic TRIP effect,which further improves the strength plasticity.(3)The strength plastic product with 980MPa cold-rolled multiphase steels of MnCr and MnCrNb are both over 15GPa%in this paper,and the strength plastic matching is good.By adjusting the continuous annealing process parameters,the average mechanical properties of MnCr multiphase steel are Rel=454MPa,Rm=992MPa,A80=16.0%,YS/TS=0.46;The average mechanical properties of MnCrNb multiphase steel in laboratory are Rel=495-538MPa,Rm?995-1022MPa,YS/TS=0.48-0.53,A50=13.5-15.5%,Agt=9.0-11.8%,n=0.15-0.18,r=0.63-0.90.(4)The strengthening mechanism model of cold rolled multiphase steel containing bainite is as follows:?=241+67.7[P]+86[Si]+45[Mn]+11[Al]+9.3×D-1/2+387×D0-12× B%+(229.63-2.8990×C%)×M%;D is the ferrite grain size,D0 is the effective grain size of bainite,C%is carbon content,B%is bainite content,and M%is martensite content,which can well reflect the contribution value and proportion of each strengthening mode to the yield strength of cold rolled multiphase steel containing bainite.The formation of bainite can adjust the mechanical properties of cold rolled multiphase steel,F+M+B+R.A can obtain good elongation,and F+M+R.A can obtain ultra-low strength yield ratio.(5)The effects of different continuous annealing process paths(DP,TRIP,QP)on microstructure and mechanical properties under the same MnCrNb composition were systematically elucidated.By controlling different processes,different multiphase,metastable,multi-scale,morphology microstructures and multiphase microstructure control processes were obtained.The best strength plasticity match of DP980 process was obtained,and the strength plastic product was over 15GPa%,the best of the TRIP is strength plastic product over 18GPa%when the tensile strength is 800MPa,and the QP process line strength level reaches 1200MPa,but the elongation is greatly reduced,and the strength plastic product is 8GPa%.