Effect Of Heat Treatment On Microstructure And Mechanical Properties Of 20MnV High Strength Low Alloy Cryogenic Steel

With the rapid development of building structural steel, engineering machinery steel and increasingly vitality of marine exploitation activities in cold regions and considering a series of serious problems such as the lack of resources, environment and natural ecological overtaxed, developing the saving high strength low alloy cryogenic steel becomes the focus of steel industry gradually world over. The structural steel would bear impact load during the process of operation at cryogenic condition and cryogenic brittle fracture is the main form of damage, the development and research of high strength low alloy cryogenic steel is of great significance. The present research focused on 20 MnV high strength low alloy cryogenic steel which was developed by Baosteel by TMCP, and studied the effect of heat treatments on the microstructure and properties, especially the low temperature toughness of 20 MnV high strength low alloy cryogenic steel were studied by various analysis and testing methods, including optical microscope(OM), scanning electron microscope(SEM) and transmission electron microscope(TEM) with energy dispersive spectrometer(EDS), tensile testing and impact testing.The 20 MnV high strength low alloy cryogenic samples were subjected to normalizing treatment at different temperature and holding time. The results show that the normalizing temperature had significantly influence on microstructure and mechanical properties of hot-rolled steel compared with holding time. The heterogeneous microstructure of hot-rolled steel transform into fine and homogeneous equiaxed ferrite and massive pearlite after 860°C and 920°C normalizing treatment. The optimal cryogenic performance with impact energy values at-20°C and-50°C were 62 J and 40 J, respectively, which were twice of hot-rolled steel, was obtained at 860°C. While the optimal comprehensive mechanical property with yield strength, tensile strength and elongation of 481 MPa, 745 MPa and 20.5%, and the impact energy values at-20°C and-50°C were 45 J and 30 J, respectively, was obtained at 920°C.The 20 MnV high strength low alloy cryogenic samples were subjected to cold rolling followed by normalizing treatment at different temperature. The results show that the complete recrystallization temperature was determined above 780°C according to the variation of the micro-hardness as the normalizing temperature changed. The microstructure of 860°C as-normalized sample was composed of homogeneous equiaxed ferrite and pearlite and the dislocation featured the sparse dislocation wall. The optimal product of strength and elongation of cold-rolled steel with reduction 10% and 23% were 18816MPa% and 17472MPa%, repectively.The 20 MnV high strength low alloy cryogenic samples were subjected to normalizing and intercritical quenching treatment at different temperature followed by cold rolling and high-temperature tempering. The results show that the microstructure of the intercritical normalized sample was composed of ferrite, granular bainite and fine carbides, while the microstructure of the intercritical quenched sample was composed of ferrite and tempered sorbate. Compared with normalized samples followed by cold rolled and high-temperature tempering, the optimal cryogenic toughness and comprehensive mechanical property with yield and tensile strength exceeded 745 MPa, and the impact energy values at-20°C and-50°C were 70 J and 46 J, respectively, was obtained when intercritical quenching at 800°C.