Effect Of Heat Treatment On Microstructure And Mechanical Properties Of Rolled Fe-Mn-Al-C Steel

Fe-Mn-Al-C steel has become an important research direction for the development of automotive steel in the future because of its excellent strong plasticity,high strain hardening rate,excellent impact absorption energy,good corrosion resistance and low density.At present,the research on the heat treatment conditions of Fe-Mn-A1-C steel mainly focuses on the high temperature and long time solution treatment or annealing treatment of deformed specimens,but there are few reports on the effects of medium and high temperature short-time heat treatment and aging treatment after short-time heat treatment on the microstructure and properties of Fe-Mn-A1-C steel.In this paper,the hot rolled Fe-27.34Mn-8.63Al-1.03 c steel and cold rolled Fe-26.5Mn-10.1Al-0.98 C steel were taken as the research object,and the hot rolled and cold rolled high-strength and low-density steel with tensile strength greater than1000 MPa and elongation greater than 20% were successfully developed by medium and high temperature short-time heat treatment process.The effects of short-time heat treatment at different temperatures on the microstructure and properties of rolled FeMn-Al-C steel were analyzed by combining related auxiliary tests and characterization methods.The main research contents are as follows:With the increase of annealing temperature,austenite grains grow up,strip ferrite decompose and VC particles precipitate in various states.The tensile strength of low density steel decreased from 1230 MPa to 960 MPa,and the elongation increased from18% to 46%.When annealed at 800 and 900℃,the relative mass fraction of austenite decreases and a γ→α transition occurs.Short-term solution in different temperature of cold rolled steel processing,solid solution treatment makes ferrite broken decomposition,with the increase of solution temperature,austenite grain coarsening,and broken ferrite test steel yield strength and tensile strength,break elongation and strong plastic product increases,at 1000 ℃tensile strength of 916 ma,break elongation is 56%,The maximum strong plastic product is 51296 MPa·%.Under these conditions,the comprehensive mechanical properties are the best.After solution treatment at 700℃ and 800℃,the quantity of second phase B2 and B2+κ carbides increases gradually.The strain hardening rate of cold rolled steel gradually changes from a rapid decline to three stages of "rapid decline--steady--decline again".The second stage tends to be stable with the increase of solution temperature,and the strain hardening ability increases gradually.At the same time,the fracture mode changes from brittle to ductile transgranular.The aging treatment of cold rolled steel with solution treatment at 1000℃ for 90 min at different temperatures did not cause the obvious growth of austenite grains,but mainly provided energy for the growth of κ carbides in the matrix and the precipitation of B2 at grain boundaries.With the increase of aging temperature,nano-κ carbides grow up gradually,B2 phase and κ carbides precipitate at grain boundary and increase along grain boundary.It is found that the deformed structure has obvious plane slip characteristics,and the strain hardening rate is in three stages of "rapid decline,tending to be stable,and falling again".After aging at 550℃,the yield strength of the test steel increases 329 MPa to 1039 MPa.This increase was mainly caused by the growth of κ carbides and the increase of B2,but the plasticity was worsened.The grain boundary of B2 phase in austenite increases with aging temperature,resulting in the transition from ductile transgranular fracture to transgranular brittle fracture.