Study On Microstructure And Mechanical Properties Of Hot-Rolled Medium Carbon Nanobainite Steel

With the rapidly development of car industry,the manufacture of automobile is getting more and more focus on the lightweight and safety.The market demand in motorcar of third generation advanced high strength steels(AHSS)combined superor strength with extraordinary ductility is hotter than before.The Nano-Bainite which was first put forward by H.K.D.H Bhadeshia research group from Cambridge University.The Nano-bainite steel can not only reduce the weight of car but increase the safety of the auto structure as well.Base on this it’s has bright market prospects.However,it still stays in the period of basic research about changing the components of traditional Nano-Baintie which keep the fantastic mechanical properties while suit for large scale production.General research in Nano-Bainite involve the calculation by Thermo-calc、measureing the continuous cooling transformation cct curves、multi heat treatment with salt bath furnaces and tensile tested with observing microstructure.On this basis,the hot-rolled online direct quenching and isothermal heat treatment is tried to simulate industry production.(1)After two steps heat treatment,the steel yield stress is 814MPa,tensile stress is 1030MPa,and total elongation is 39.5%.(2)After one step heat treatment,the thinkness of bainite ferrite plate decrease with the temperature decline,resulting in the increase of yield stress and tensile stress.With the increase in the isothermal temperature,the content of retained austenite rises.(3)During different hold time in same isothermal heat treatment temperature,the tensile stress increase with short hold time,on the opposite the elongation drop with transient hold time.The bainite plate grow with the carbon diffusion,the stable of retained austinte increase accompanied with hold time.The unstable retained austenite which formed in short hold time will transform into martensite during air cooling process reduce the elongation.(4)During two steps transformation,there will generate more bainite ferrite and divid block retained austenite for lower size.After the second steps,more carbon will spread into retained austenite,which will have good mechanical stablility and continuously heppend TRIP effect for better elongation.(5)The crack propagation is easily spread around the original austenite grain boundary and block austenite which is residual after one steps heat treatment.The smaller size retained austenite which obtained from two steps heat treatment will reduce the possibility of crack propagation and improve the total elongation.