Study On Microstructure Evolution Mechanism And Mechanical Properties Of Nb Micro-alloyed Q&P Steel

With the increasingly fierce competition in the automotive market,environmental protection issues are becoming more and more serious.The third-generation automobile steel as the main force to solve these two problemsis expanding.The third-generation automobile steel has the high strength and excellent plasticity of the first and secondgeneration automobile steels,and at the same time reduces the production cost.The third-generation automotive steel uses microalloying elements to refine the matrix structure to increase the strength and control the metastable phase transformation induced plasticity to improve the forming properties.The third generation of highstrength steel-Q&P steel concept was proposed by Professor Speer et al.in 2003.It is a multi-phase structure with martensite as the matrix and residual austenite to produce TRIP effect,which can obtain high PSE.This paper focuses on the effect of microalloying element Nb on the microstructure evolution and mechanical properties of Q&P steel.The hardness of the steel after tested was measured using a microhardness tester(MHT),and the microstructure of the steel was observed and analyzed using an optical microscope(OM)and a scanning electron microscope(SEM),the retained austenite volume fraction and its carbon content was calculated based on the X-ray diffraction(XRD)test results.In this paper,three kinds of component test steels were designed by adding different Nb elements to the CSiMn steel.The ferritic static recrystallization model and the austenite formation kinetics model were established.The effects of retained austenite volume fraction and its carbon content on the plasticity of Q&P steel under different processes were studied.The test results are as follows:(1)The static recrystallization activation energies of ferrites of 1.68Si-0.052 Nb steel and 2.2Si-0.012 Nb steel were 135.7KJ/mol and 123.7KJ/mol,respectively.The higher the activation energy,the more obvious the effect of inhibiting the static recrystallization of ferrite.The holding temperature and time are 660 ° C,740 s,the ferrite static recrystallization volume fraction of 1.68Si-0.052 Nb steel and 2.2Si-0.012 Nb steel.(SRX)were 57.86% and 96.9%,respectively.Nb is more effective than Si in suppressing the static recrystallization of ferrite.The three component steels reached the maximum rate when the volume fraction of ferrite static recrystallization reached 50%.(2)The Avrami exponent n in the ferritic static recrystallization model and the austenite formation kinetic model are both less than 2.The former n value is 0.897 to 1.813,and the latter n value is 0.083 to 0.405.The temperature is different and the value of n is different.In the static recrystallization of ferrite,the value of n increases with increasing temperature,while in the austenite formation model,the value of n is inversely proportional to the annealing temperature.Avrami appears as a non-integer,indicating the presence of homogeneous nucleation and heterogeneous nucleation.The smaller the value of n,the greater the probability of heterogeneous nucleation.(3)In the austenitizing phase transformation test,the austenite formation rate of 1.68Si-0.052 Nb steel is higher than that of other steels at 820 °C,because at this temperature,the austenite solid solution state Nb precipitates to form Nb(C,N),and coarsening at this temperature with the increase of holding time,the ability of the coarsened Nb(C,N)particles to pin the austenite grain boundary is weakened,thereby increasing the austenitizing At the rate,the effect of Si on suppressing austenite growth is not obvious.The higher the temperature,the longer the holding time,and the ability of Nb to inhibit austenite grain growth is further weakened.Therefore,in the continuous annealing test,when the annealing temperature is 900 ° C,the plasticity is generally deviated,and the coarse lath martensite of 2.2Si-0.012 Nb steel is most obvious.(4)Full austenitizing annealing,high strength,critical zone annealing,plasticity has been greatly improved,1.68Si-0.052 Nb steel is particularly prominent,and has a higher strong plastic product,up to 21.4GPa·%.(5)The plasticity of Q&P steel at partitioning of 400 °C is generally better than 350 °C.the partitioning temperature is raised from 350 ° C to 400 ° C,and the retained austenite has better chemical stability due to carbon enrichment.Nbcan not increase the retained austenite volume fraction,but by refining the size of the retained austenite,making its carbon content easier to reach saturation,thus becoming more stable and plastic.