Control Of Microstructure And Mechanical Properties Of Low Carbon Transformation Induced Plasticity Steel

Transformation induced plasticity steel is a typical advanced high strength steel.Silicon plays a very important role in various advanced high-strength steels,when Silicon is more than the content of 1%,the steel plate surface tends to produce stable oxides 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 Aluminum instead of silicon could improve the surface quality of steel.Therefore,this paper focused on developing a new kind of low carbon transformation induced plasticity steel.And on the premise of alloy composition was constant,the study obtained different microstructures by different process designs,which includebainite+ferrite+retained austenite TRIP steel and bainitic bainite+retained austenite TRIP-aided BF steel.Based on thermal simulation and hot-rolled online heat treatment,studying the effect of process parameters on the microstructure and properties of the two phase transformation plastic steels.The main results were as follows:(1)According to the principle "the substitution of Al for Si",designed a new low-carbon Fe-Mn-Al-Si series of advanced high strength steel by the Al element partially replaces the Si element,which is composited of 0.15C-0.83Mn-0.99Si-0.5Al-0.08P-0.23Cr-0.18Mo.Studying the continuous cooling phase transformation of the experimental steels,bainite phase transformation temperature range and static CCT curve were obtained.(2)By the thermal simulation experiment of TRIP steel,the effect on the microstructure and properties of TRIP steel with bainite holding temperature and holding time was studied.The best process was at 450? holding 120s,when bainite transformation had been basically completed with the regular distribution of bainitic lath and the C partition from bainite to austenite transformation was at the stable stage,the volume fraction of residual austenite was the largest.The retained austenite volume fraction and carbon content reached a maximum value of 11.4%and 1.13 wt.%,respectively.Based on the thermal simulation,hot-rolled on-line heat treatmen of TRIP steel was conducted.The best process was at 450? holding 10min,when Tensile strength is above 809MPa,total elongation is above 29.5%,strong plastic product reached 23865MPa%,then the strength and plasticity of TRIP steel is excellent.(3)By the thermal simulation experiment of BF steel,the effect on the microstructure and properties of BF steel with bainite holding temperature and holding time was studied.hot-rolled on-line heat treatmen of TRIP steel was conducted by the thermal simulation.The process was at 450? holding 10min,when tensile strength is above 1024MPa,total elongation is above 17.2%.when holding temperature of the bainite increased to 500?,tensile strength is above 875Pa,total elongation is above 22.8%,strong plastic product reached 19950MPa%,then the strength and plasticity of BF steel is excellent.(4)As for cold-rolled steel,microstructure and mechanical property were studied in two different heat treatments.In the traditional TRIP process,tensile strength decreased,the volume fraction of residual austenite and the product of strength and elongation increased before decreasing as the intercritical temperature rised.The optimal intercritical annealing temperature was at 900?,the retained austenite volume fraction a reached a maximum value of 12.3%,the mechanical property was the best,that the tensile strength,the total elongation and the product of strength and elongation were 745MPa MPa,30.9%and 23065MPa%,respectively.In the cold-rolled heat treatment of BF steel,when the quenching temperature was 490 ?,the mechanical property achieved the best,the tensile strength,the total elongation and the product of strength and elongation were 785MPa,24.5%and 19233MPa%respectively.When holding time in the 120s-300s,bainite transformation had been basically completed with the regular distribution of bainitic lath and the C partition from bainite to austenite transformation was at the stable stage.