| In order to solve the environmental problems and energy crisis in the process of rapid economic development,the steel industry and the automotive steel industry have been making transformation in recent years,lightweight and security have become the main target of the current research and application in automotive steel industry.Transformation-induced plasticity aided annealed martensitic steel(TAM steel)has great application prospect in automotive industry due to its high strength,high plasticity and excellent tensile flange performance.This paper focuses on the influence of bainite partitioning on microstructure and mechanical properties of TAM steel,with a view to obtain the high strength-plastic TAM steel with optimized performance.This research was carried out mainly from the following contents:The traditional cold rolled TRIP steel was treated by austenitizing heat treatment process,two-phase region heat treatment and the bainite transformation region under different temperatures and different times.The microstructure evolution and retained austenite contents,morphology and sizes of TAM steel under different partitioning processes were studied by means of optical microscope(OM),scanning electron microscope(SEM)and X-ray diffraction(XRD),and the mechanical properties were characterized by tensile curves.The test steel was treated by austenitizing heat treatment process,two-phase region heat treatment and the one-step and two-step processes under bainite transformation region.The microstructure and mechanical properties of TAM steel were studied by means of OM,SEM,XRD,electron back scattering diffraction(EBSD),transmission electron microscope(TEM)and tensile test.The main conclusions are as follows:The test steel was treated by different bainite partitioning temperatures and times.With the increase of the partitioning temperature from 380℃ to 460℃,the content of retained austenite in TAM steel increased firstly and then decreased,and the size of retained austenite reduced.The plates were relatively coarser and the number of plates decreased.With the extension of the partitioning time from 120 s to 360 s,the retained austenite in TAM steel had similar trend,and the blocky retained austenite gradually evolved into the film retained austenite and the granulated retained austenite.The number of laths increased and the laths were elongated,and the arrangement of them was gradually neat and tight.After the two-step process in the bainite region,this method obtained with a large amount of film retained austenite and granulated retained austenite.The retained austenite distribution by two-step treatment was more uniform and the plates were elongated and aligned tightly.The test steel was treated by different bainite partitioning temperatures and times.With the increase of the partitioning temperature from 380℃ to 460℃,tensile strength of the test steel slightly decreased,yield strength slightly increased,elongation and the product of strength and elongation firstly rised and then reduced.After the partition time from 120 s to 360 s,mechanical performance indicators of test steel changed in accordance with the above basic.TAM steel achieved the best comprehensive mechanical property under 420℃x300 s partitioning technology.The elongation rate and the product of strength and elongation reached 31%and 26040 MPa·%respectively.After the two-step process in the bainite region,compared with the one-step process,the tensile strength and yield strength of TAM steel in two-step process did not change obviously,the elongation rate and the product of strength and elongation greatly improved.When the test steel was treated by 400℃×240 s(one-step process)and 480℃×20 s+400℃×220 s(two-step process),The elongation rate of steel increased from 23%by one-step process to 30%by two-step process.The product of strength and elongation rised from 20424 MPa·%by one-step process to 25560 MPa·%by two-step process.The comprehensive mechanical properties of TAM steel have been optimized by two-step process. |
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