Mechanical Properties Of Medium Manganese High Aluminium Steel With IQPT Heat Treatment

The rapid development of the automotive industry has put tremendous pressure on resources and environment,which makes people eagerly demand for energy-saving and environmentally friendly vehicles,and weight reduction of vehicle is an effective means to achieve this.Currently,the most viable material to reduce weight of vehicle is advanced high-strength automotive steel.This thesis uses hot rolled steel plates containing 0.25C-3.81Mn-1.22Al-trace of Nb and Mo component as experimental materials,and methods of OM,SEM,TEM and XRD are used to discuss the effect of different annealing temperature,quenching temperature and carbon partitioning-tempering time on the microstructure and mechanical properties of the experimental steel under the new type of IQPT heat treatment.The microstructure and mechanical properties of the steel under ART and QPT heat treatment are compared as well.The main conclusions are as follows:(1)With the annealing temperature increasing,the grain size of lath martensite and the content of massive martensite increases obviously;the optimum annealing temperature is 760?.(2)Based on Speer's constrained carbon equilibrium(CCE)equation,the theoretical optimum quenching temperature is 220?.But with the dual phase Mn partition,the thermal stability of the original austenite increases and makes it keep stable at the lower quench temperature without decomposing.The optimum quenching temperature shows a significant downward shift at 180?.(3)With carbon partition tempering time increasing,the lath martensite changes as follows: At 60 s and 120 s,boundaries of lath martensite are sharp,straight and clear.At 180 s,boundaries become curved and blurred.At 360 s,boundaries dissolve and disappear.At 360 s,a large number of lamellar temperature-tempering groups are formed.(4)Under the optimal IQPT heat treatment process,the tensile strength of the experimental steel reaches 1231 MPa and the elongation reaches 24.8%,the product of strength and elongation up to 30.53 GPa·%..(5)The microstructures of the test steel under IQPT,ART and QPT heat treatment show obvious differences.The microstructures of IQPT steel mainly consists of lath martensite and composite lath M-A phase.The microstructure of ART steel mainly consists of ?-ferrite,martensite and film-like residual austenite.The microstructure of QPT steel mainly consists of bainite,lath martensite and film-like residual austenite.(6)The experimental steel can reach the demands of the third advanced high-strength automotive steel after the optimal IQPT or ART heat treatment.However,the ART steel shows the dynamic strain aging effect,which is not conducive to the material processing and forming,so the overall performance of IQPT steel is better.