Study On Microstructure And Properties Of Fe-11Mn-xAl-xSi Steel

Steel,because of its excellent mechanical properties and relatively low cost,stable and reliable supply,and easy recycling,is the dominant material in the automotive industry.The automotive industry’s continued demand for body weight reduction,fuel efficiency and safety improvements over the past few decades has driven the development of advanced high-strength steel(AHSS).Medium Mn steel,which has achieved a good balance between production costs and mechanical properties,is widely favored as a typical representative of the third generation of AHSS.At present,the main production processes of Medium Mn steel are hot rolling and cold rolling and corresponding heat treatment.In this thesis,the Fe-11Mn-xAl-xSi series of manganese steel is mainly used to study the microstructure evolution and mechanical properties of hot-rolled and cold-rolled mediummanganese steel.The effect of retained austenite content in the steel was analyzed,and the effect of alloying elements(Al and Si)on retained austenite value and stability was analyzed.The main experimental results and conclusions of this paper are as follows:(1)Hot rolled 2Al,2Al-2Si and 4Al steels,after heat treatment,showed an excellent combination of high strength(900～1700MPa)and adequate ductility(34～48%)with a PSE(production of strength and elongation)of 40～48GPa%.As the Si content increased,the ultimate tensile strength of the investigated steels increased approximate 100 MPa,while the total elongation changed little.As the Al content increased,the total elongation of the investigated steels increased significantly,while the ultimate tensile strength decreased.(2)After heat treatment,cold rolled 2Al,2Al-2Si and 4A1 steels showed an excellent combination of high strength(950～1549MPa)and adequate ductility(40～61%)with a PSE(production of strength and elongation)of 59～61.8GPa%.With the increase of Si content,the ultimate tensile strength and total elongation of cold-rolled steels did not change much.As the increased of Al element,the ultimate tensile strength of steels decreased significantly,while the elongation increased by 20%.(3)For hot-rolled or cold-rolled 2Al,2Al-2Si,and 4A1 steels,the retained austenite volume fraction increased first and then decreases with the increase of annealing temperatures and the retained austenite volume fraction ranged from 71.1%to 75.7.%.Accompanying the increase of Si and Al elements,the heat treatment temperatures to obtain the maximum retained austenite volume fraction had been increasing.(4)Cold rolled steel had low total elongation and high ultimate tensile strength at lower annealing temperatures,which was related to both the microstructure of the ultrafine grain size and the too stable retained austenite.The former factors caused the insufficient work hardening ability during the deformation process,while the latter factor caused the lack of the TRIP effect,which caused the instability of the shaping to cause the experimental steel fractured early in the deformation process.