Phase Transformation Behavior,Microstructure And Mechanical Properties Of TRIP Steels With Different Initial Structures

Two pretreatments were conducted on the tested steel to obtain two precursor microstructures,i.e.martensite and pearlite,then the samples were treated by intercritical annealing followed by austempering.The phase transformation behavior of two types of samples was studied by DIL thermal expansion instrument and Gleeble 3500 thermal simulation test machine.The two types of samples were treated by intercritical annealing with different temperatures and holding times,followed by austempering,the microstructures and properties of which were studied by optical microscope,scanning electron microscope,transmission electron microscope,X-ray diffraction techniques and MST tensile testing machine.The results show that there was only one deflection on the expansion curves of the pearlite sample in two-phase region,showing that only austenite transformation occured.But for the martensitic sample,there was another deflection before austenite transformation in two-phase region,showing the appearance of carbide precipitation on heating up.The Ac₁ and Ac₃ points of the pearlite sample are higher than that of the martensitic sample.The Ac₁,Ac₃ points of the two samples and T_?1,T_?22 of the martensitic sample all increase with the`increase of the heating rate.When holding at 780?,the formation rate of austenite for the martensitic sample is faster than that of the pearlite sample.With the holding time increasing,the Ms points of the martensitic sample remains unchanged,but that of the pearlite sample first rises followed by decreasing.When holding for 1h at 780?,the Ms points of the two samples are the same.When austempering after holding different times at 780?,The rates of bainite transformation of the martensitic sample are relatively low,and don,t change with intercritical holding time.However the rate of bainite transformation of the 10min is more faster for the pearlite sample;With the time increasing,the rate of bainite transformation becomes slow,and when holding 1h,the rates of bainite transformation of the two samples are approximately the same.Treated by intercritical annealing with different temperatures or different holding times,followed by austempering,the microstructures of the two types of samples are composed of ferrite,bainite,retained austenite and martensite.For the martensitic sample,the ferrite and the mixture of bainite/retained austenite/martensite is striped,but there are also large blocky ferrites for the pearlite sample,besides the similar striped microstructure.With the intercritical temperature or the holding time increasing,the volume of the of the striped microstructure of the two samples increases and then decreases.When holding at 780?,with the holdimng time increasing,the yield strength and tensile strength of the two types of samples first rise and then reduce,but the elongations show different variations:for the martensitic sample,the elongation increases;but for the pearlite sample,the elongation first decrease and then increases.When holding at 780?for different time,the yield strength,tensile strength,elongation and the product of strength and ductility of the martensitic sample are higher than that of the pearlite sample.When holding at different intercritical temperatures for 1h,with the intercritical temperature increasing,for the martensitic sample,the yield strength and tensile strength increase,and elongation decreases;but for the pearlite sample,the tensile strength increases,and the yield strength and elongation first decrease and then increase.When holding at different intercritical temperatures,the yield strength and tensile strength of the two samples are approximately the same,but the elongation and the product of strength and ductility of the martensitic sample are higher.This show that the martensitic sample has more excellent comprehensive mechanical properties.