Abstract: Based on the actual production process of U20Mn2SiCrNiMo bainitic steel rails and the static CCT curves, a series of thermal simulation tests of rapid cooling + isothermal transformation were designed in the laboratory. The phase transformation laws and microstructure evolution of the bainitic steel during the isothermal cooling process at temperatures ranging from 475 ℃ to 280 ℃ were studied by using Formastor-F fully automatic phase transformation instrument, metallographic microscope, and scanning electron microscope. The results show that the undercooled austenite is relatively stable when isothermally held at 475 ℃, 450 ℃, and 400 ℃, and only a small amount of bainitic phase transformation occurs or no phase transformation takes place during the isothermal stage. When isothermally held at 425 ℃ and 375 ℃, a certain amount of granular bainite structure is formed. However, when isothermally held at 350 ℃, 325 ℃, and 300 ℃, the undercooled austenite transforms into a large amount of fine and uniform lath bainite. When the isothermal temperature is reduced to 280 ℃, almost no bainitic phase transformation occurs in the undercooled austenite. It is concluded that during the heat treatment process of U20Mn2SiCrNiMo bainitic steel rails, it is necessary to avoid staying in the temperature range above 375 ℃ to prevent the formation of coarse structures. The optimal bainitic transformation temperature range is from 350 ℃ to 300 ℃, and fine and uniform bainite+martensite structure can be obtained.

Key words: U20Mn2SiCrNiMo bainitic rail steel, isothermal transformation, microstructure evolution, CCT curve