Effect Of Heat Treatment On Microstructures And Mechanical Properties Of Hot/Cold-rolled Medium Mn Steels

With the rapid development of the automobile industry,while bringing convenience to us,automobiles have also caused problems in energy,safety and environmental protection.The lightweight of automobiles is one of the main ways to solve these problems.Advanced high-strength steel is a cost-effective modern automobile manufacturing material that can not only ensure the lightweight of the car,but also improve and ensure the safety of the car.Therefore,it is of great practical significance to accelerate the development of advanced high-strength steel for automobiles.Medium-manganese steel(Mn:4?12 wt.%)still has high tensile plasticity(?30%)and work hardening ability when the tensile strength is up to 1000MPa,which has become one of the research hotspots in the field of steel materials in recent years.Based on thermodynamic software simulation and stacking fault energy calculation to optimize the alloy design,the reasonable chemical composition of the medium-manganese TRIP/TWIP steel was determined in this paper;Combined with the hot rolling,cold rolling and heat treatment processes,the microstructure evolution and mechanical property evolution of a typical medium manganese steel(0.3C-10Mn-4Al-0.5Si,wt.%)are systematically studied;By optimizing the process system,the coupling effect of TRIP+TWIP was realized in the steel with this composition,and the plastic deformation mechanism of the manganese TRIP/TWIP steel during room temperature deformation was discussed and analyzed.The main research contents and results are as follows:(1)For hot-rolled medium-manganese steel,as the annealing temperature increases,the volume fraction of retained austenite increases and then decreases.After annealing at 750?for 60 min,the volume fraction of retained austenite reached a maximum value of about 62.3%;The microstructure is mainly composed of acicular+strip-shaped residual austenite,long ?-ferrite and acicular ferrite.The comprehensive mechanical properties are the best:tensile strength>1000 MPa&elongation?30%.The holding time has little effect on the mechanical properties.(2)After the cryogenic treatment of the hot-rolled experimental steel,the grains were significantly refined,and part of the layered ferrite+austenite transformed into equiaxed.After the cryogenic treatment,the tensile curves of the experimental steels show continuous yield characteristics.Compared with conventional heat treatment,the elongation has nearly doubled,up to 50?60%,and the tensile strength is between 950?1000 MPa,PSE is between 45?59 GPa·%.(3)For the cold-rolled medium manganese steel,as the annealing temperature increases,the volume fraction of retained austenite increases and then decreases;After annealing at 800?for 10 minutes,the volume fraction of retained austenite reached a maximum value of about 61.8%,The microstructure is mainly composed of layered+equiaxed austenite and ferrite,The best comprehensive mechanical properties:tensile strength is close to 1000 MPa,elongation is about 50%,and strong plastic product is 48 GPa ·%.(4)The deformation mechanism of manganese steel in cold-rolled high-strength plastic under the typical process conditions(annealed at 800?)was studied.The results show that the dislocation density in the deformed structure increases under tensile deformation conditions,The retained austenite undergoes a martensitic transformation,inducing the TRIP effect;As the amount of deformation increases,deformed twins can be observed,confirming that the TWIP effect has occurred.This TRIP/TWIP coupling effect makes cold rolled medium manganese steel have both high strength and high plasticity.