Microstructure Evolution And Mechanical Properties Of Vanadium-Titanium Microalloyed Medium Manganese Steel

Advanced high-strength steel has been widely used in automotive field due to its'excellent mechanical properties.Medium manganese steel belongs to the third generation advanced high strength steel and it has became a hot point in the research field of high strength steel currently.The excellent mechanical properties of medium manganese steel are attributed to its TRIP effect during deformation,that is,the martensite transformation of metastable austenite can significantly improve the work hardening rate and plasticity.The decisive factor affecting the TRIP effect is the content of retained austenite and its stability.More and stable retained austenite can be obtained at room temperature by austenitic reverse transformation annealing.In addition,the addition of Nb,V,Ti and other microalloying elements to the medium manganese steel could act as pinning grain boundaries and refining grains,at the same time,it has precipitation strengthening,fine grain strengthening and solid solution strengthening.In this paper,V-Ti microalloyed 5%Mn medium manganese steel was studied,which aims to achieve solid solution strengthening and precipitation strengthening by V-Ti microalloying technology,revealing the effects of V-Ti microalloying on the microstructure evolution and mechanical properties and the influence of the austenite reverse transformation annealing process on the microstructure evolution,elemental distribution behavior and mechanical properties,Establish the relationship among process,organization and performance.The main research contents and research results are as follows:(1)The precipitation behavior of V and Ti carbides in experimental steel was studied.It is observed that the spherical VC,(V-Ti)C,and TiC precipitate particles are mainly dispersed in the ferrite grain boundaries and grain interior,and the size fluctuation is large,and most of them are between 10 to 300nm.There were no significant differences in the types,sizes and shapes of precipitates between hot-rolled and cold-rolled experimental steels.(2)The elemental partitioning behavior of experimental steel during austenite reverse transformation annealing was studied.Significant element partitioning of C and Mn were observed in ferrite and austenite,and the diffusion rate of C was obviously faster than that of Mn.The partitioning behavior of V and Ti was not observed.(3)The effect of austenite reverse transformation annealing process on microstructure,elemental partitioning behavior and mechanical properties in hot-rolled experimental steel was studied.After austenite reverse transformation annealing,the duplex lamellar structure of austenite and ferrite was obtained.With the increases of annealing temperature and prolongs of annealing time,the cementite dissolves gradually,the austenite and ferrite laths coarses gradually,the yield strength decreases gradually,while the tensile strength increases gradually,and the fracture elongation increased firstly and then decreased.It gets excellent mechanical properties after annealing at 640? for 6h,tensile strength was 1320.51 MPa,fracture elongation was up to 36.41%,and the product of strength and ductility was up to 48.05GPa·%.With the increases of annealing temperature,the austenite content increases continuously.After annealing at 680? for 1h,the austenite content was up to 67.13%,but the elongation was only 9.41%.(4)The effects of austenite reverse transformation annealing process on micro structure,elemental partitioning behavior and mechanical properties of cold-rolled experimental steel was studied.After austenite reverse transformation annealing,the equiaxed duplex structure of ultrafine ferrite and austenite was obtained.With the increases of annealing temperature and prolongs of annealing time,the grain size of austenite and ferrite increases continuously,the amount of cementite decreases gradually,and the retained austenite content increases gradually.There were long yield platforms for the room temperature tensile curves of the experimental steel under different annealing conditions.Excellent mechanical properties were obtained after annealing at 660? for 10min.The tensile strength was 1387.97MPa,the fracture elongation was up to 33.68%,and the product of strength and ductility was up to 46.74GPa·%.Experimental steel has achieving great combination of strength and plasticity.(5)The influence of quenching-austenite reverse transformation annealing process on microstructure evolution,elemental partitioning behavior and mechanical properties of cold-rolled experimental steel was studied.After quenching,lots of bluk martensite and a small amount of island austenite was obtained.Afterwards,irregular austenite and ferrite were obtained by austenite reverse transformation annealing.After a short time reverse transformation annealing,the grains were finer,the austenite content was lower,and the yield strength was enhanced,but the fracture elongation was less than 21%.After a long time reverse transformation annealing,relatively large microstructure was obtained,and the yield strength was low.The tensile strength of the sample annealed at 640? for 1h was 1248.20MPa,the fracture elongation was 32.14%,and the product of strength and ductility was 40.12GPa·%.(6)This work shows that V-Ti microalloying technology could promote solid solution strengthening,precipitation strengthening and fine-grain strengthening of medium manganese steel,achieve a good match between the strength and plasticity,and make the product of strength and ductility significantly superior to traditional medium manganese steel.