Submicronization Mechanism Of Ultra-low Carbon Medium Manganese Steel And The Control Of Strength And Plasticity

Energy conservation,emission reduction and safety improvement are the important development directions of modern automobile.It is required that a new generation of automotive steels have both high strength and high plasticity.Based on this background,many works on high strength and high plasticity steels have been carried out at home and abroad.For the issue that conventional steels appeared plasticity reduction when grain ultra-refinement and submicronization were performed,this paper proposed medium manganese alloying composition design,and the ultra-low carbon medium manganese steel was subjected to cold rolling and intercritical annealing.The study utilized Mn to increase the stability of austenite and finally obtained austenite-ferrite dual-phase microstructure at sub-micron grain size.Strength was improved by grain submicronization.Deformation-induced transformation of retained austenite to martensite has been developed,which improved the work hardening rate.Thus,the thin steel plates with combination of high strength and high ductility were obtained.In this paper,the main contents and main results are as follows:(1)The low-carbon medium-manganese alloying design of experimental steel composition was adopted.Ultra-low carbon design was preferred from the view point of weld ability.According to the results of phase diagram calculated by thermal-calc software,Mn and Ni can reduce the intercritical temperature range,which contributed to obtaining more metastable austenite.Since Ni was more expensive,adding Mn instead of Ni to enhance austenite stability.(2)The continuous cooling transformation(CCT)curve subjected to no deformation and 40%deformation of austenite at 900? was investigated by dilatometer and thermal simulation machine.Results show that during the cooling rate range of 0.1?60 ?/s,only martensite phase transformation and lath martensite was obtained.The hardness range of no deformation experiment was 345?369 HV and the hardness range of 40%deformation experiment was 349?369 HV,which was of only a smaller range of fluctuation.It was confirmed that medium-manganese steel have excellent hardenability.(3)Microstructures evolution of medium-manganese during hot-rolled and cold-rolled process was studied.Water-quenched steel and air-cooled steel after hot rolling both consisted of lath martensite,needle-shaped cementite,and a few retained austenite.Water-qunched steel had finer grain size.With the increasing of cold rolling reduction,martensite lath got refinement.When the reduction rate reached 80 percent,martensite lath was about 200?300 nm.(4)Effects of intercritical annealing time and temperature on medium-manganese steel were investigated.The microstructure of annealed steel consisted of retained austenite,ultra-fine grain ferrite and secondary martensite.With the increasing of annealing time and annealing temperature,the volume fraction of austenite first increased and then decreased,while the stability of austenite decreased.(5)Intercritical annealing time and temperature effects on the mechanical properties of medium-manganese steel were also investigated.The grain refinement strengthening,Mn partitioning and TRIP effect of metastable reversed austenite were studied to establish the mechanisms of strengthening and plasticity.The 650? annealing route for 10 minutes led to 14.60%volume fraction of austenite with combination of yield strength of 650 MPa,tensile strength of 1120 MPa.Moreover,the product of strength and ductility was 26.07 GPa%.