Study On Transformation Kinetics And Properties Of Low Temperature Bainite In High Strength Steel

In this paper a kinds of medium carbon Si-Mn-Cr-Mo low temperature bainite high strength steel is designed and developed.The phase transition critical point and the TTT curve were determined by the expansion method and the metallographic method,and an isothermal transformation process was established based on the curves.The different isothermal times were hold in salt bath at the 260?to obtain the low temperature bainite transformation kinetics of the steel.By changing austempered temperature,the microstructures of low-temperature bainite with different sizes were obtained and its mechanical properties were tested.The test steel was subjected induction furnace to smelting-ingot-forging process in a20kg vacuum induction melting furnace to obtain a sheet having a thickness of 30mm.After annealing,the plates are cut and processed into tensile and heat treatment specimens.The TTT curves and critical point of the test steel were determined by the expansion method and the metallographic method.Ac3=918.9?,Ac1=715.5?,Ms=317?.The nose temperature in the bainite transformation zone is 300°C,the incubation period is 70s,and the end time of transformation is 2?10~4s.According to the TTT curves,two heat treatment processes were set up:the test steel is austenitized at 950?and then air-cooled to about 400?for austempering in salt bath.The salt bath temperatures were 220?,240?,260?,280?300?,and the salt bath time was 6h;The salt bath temperatures were 220?,and the salt bath time was 0s?40s?5min?30min?60min?90min?120min?180min?360min.The mechanical properties and microstructure of the specimens were observed.The first heat treatment process,when the isothermal temperature was 260?,the tensile strength,yield strength,strong plastic product and hardness of the test steel reached the maximum value,which were 1730MPa,1110MPa,52.1GPa%and 53.2HRC respectively.The laminated bainitic ferrite structure and the thin film retained austenite structure in the steel make the steel have good strength and toughness.The second heat treatment process,The maximum hardness reached53.6HRC at the salt bath time of 5min,and the minimum hardness reached 47.7HRC at90min.The test steel showed martensite structure at the early stage,and all of them changed into bainite structure at 90min.When the austempering temperature is increased from 220?to 260?,the impact energy is gradually increased and reaches 63.9J;When the isothermal temperature is 280?,the impact energy begins to decrease.At 300?,the impact energy decreases rapidly.When the transition temperature is increased from 300?to 400?,the impact energy is reduced from 40.2J to 10.8J,which is related to the deterioration of the mechanical properties of the upper bainite.Residual austenite content was determined by EBSD method.The test steel has the highest residual austenite content at 280?,which is 11.8%.The amount of retained austenite contained at 240?is the least,which is 7.18%.The shape of retained austenite also affects steel.