Control Of Microstructure And Mechanical Properties Of High Aluminum Low Silicon High Strength Automobile Steel

Silicon plays a very important role in various high strength steels.When Silicon is more than the content of 1%,the stable oxides is easy to produce in steel plate surface in heat treatment process,such as Mn2SiO4.It will reduce surface quality of hot-rolled and cold-rolled steel plate.Many domestic and foreign scholars and research institutions had demonstrated that high Aluminum and low silicon steel could improve the surface quality.Therefore,this paper focused on developing a new kind of high Aluminum low Silicon steel,and on the premise of alloy composition was constant,through different process designs changing the phase composition and phase ratio in micro structure,the microstructure and mechanical properties of tested steel were controlled.It could not only meet the requirements of high strength and advanced plastic,but also improve the surface quality and platability of steel plate.The main results of this study were as follows:(1)According to the principle "the substitution of Al for Si",about 1.2%Al(wt.%)was added in tested steel partial replacement of Silicon.Meanwhile,to make up for the loss of strength,some other alloy elements such as P,Cr,Nb etc.were added,in which element P played a role of solid solution strengthening and element Nb played a role of precipitation strengthening and fine-grain strengthening.Combined with Thermo-Calc thermodynamic simulation software,a new high A1 low Si advanced high strength steel was designed,which was composited of 0.16C-0.83Mn-0.06Si-1.18Al-0.075P-0.22Cr-0.027Nb.(2)Based on thermal simulation of hot-rolled TRIP steel,microstructure and mechanical property were studied variation with the bainite holding temperature and holding time.The best process was at 490 ℃ holding 100s,when bainite transformation had been basically completed and the C.partition from bainite to austenite transformation was at the stable stage,the volume fraction of residual austenite was the largest.(3)As for hot-rolled steel,through comparing microstructure and mechanical property of traditional Q&T(1#,2#)and online Q&T(3#),we could find that the microstructure of three kinds of hot-rolled processes were composed of ferrite,martenite or bainite and a small amount of retain austenite.But the mechanical property was different obviously.The engineering stress-strain curves of traditional Q&T were continuous yield state,showing higher strength and lower elongation.And the quenching microstructure was not stable,existing internal stress.Then tempering heat treatment was used in 1#and 2#hot-rolled processes.The microstructure and mechanical property were studied with variation tempering parameters.A significant yield phenomenon appeared after tempering heat treatment.Tensile strength decreased,yield strength increased before decreasing and elongation increases as the tempering temperature rised in heat treatment.When the tempering parameters was at 200℃ for 30min,the product of strength and elongation reached the highest,which was 24000MPa·%,the tensile strength and the total elongation were 950MPa and 25.92%respectively,achieving a better match of strength and plastic.Online Q&T(3#)could also achieve a better match of strength and plastic,in which the product of strength and elongation was 21729MPa·%,and the strength and the total elongation were 865MPa and 25.12%respectively,meanwhile it could achieve process reduction,energy conservation and emissions reduction.(4)As for cold-rolled steel,microstructure and mechanical property were studied in two different heat treatments.In the traditional TRIP process,tensile strength increased,yield strength decreased,the volume fraction of residual austenite and the product of strength and elongation increased before decreasing as the intercritical temperature rised,and residual austenite mainly existed in ferrite grain boundaries in one piece.The optimal intercritical annealing temperature was at 900 ℃,the mechanical property was the best,the product of strength and elongation,the tensile strength and the total elongation were 24534MPa·%,680MPa and 36.08%respectively.In the cold-rolled heat treatment of BF steel,the mechanical property at the quenching temperature of 490 ℃ was better than 450 ℃.When the quenching temperature was 490 ℃ holding 200s,the mechanical property achieved the best,the product of strength and elongation,the tensile strength and the total elongation were 21098MPa-%,770MPa and 27.4%respectively.