Research On The Hot Deformation Behavior And Heat Treatment Process Of High Strength Cryogenic Structural Steel

With the rapid development of steel materials used in the alpine region,requirements on the excellent cryogenic toughness and safety were raised,and high strength cryogenic structural steels have become the world's important development direction.High strength cryogenic structural steel exhibits outstanding mechanical properties and has been got successful application in cold area pressure vessel,building structural and railway transportation etc..In this paper,hot deformation behavior and cryogenic toughness of a high strength cryogenic structural steel were investigated in terms of 0.1%Co addition and heat treatment.Based on a Gleeble 3800 thermo-simulation machine,the CCT curves of the high strength cryogenic structural steel were measured,and dynamic recrystallization behavior were conducted over the temperature range of 850~1200°C and a strain rate of 5s-1.Values of Ac1,Ac3 and Ms of high strength cryogenic structural steel were 719°C,849°C and 390°C,respectively.True stress-strain curves exhibited no obvious peak value at temperatures range of 850~1200°C.The size of recrystallized grains and the X were about 50?m and 67.4% at 850°C,respectively,while that of 1100? were about 9?m and 100%,respectively.The improvement of cryogenic toughness of the tested steel was evaluated based on a heat treatment(normalizing + 20% cold rolling + 650°C tempering).Compared with the hot rolled high strength cryogenic structural steel,the strength of the tested steels after normalizing,20% cold rolling and 650°C tempering was slightly decreased,while the toughness was improved significantly.The tested samples after the heat treatment(800°C normalizing + 20% cold rolling + 650°C tempering)obtained the optimum of strength and toughness,the microstructure was composed of ferrite and tempered granular bainite,the yield strength and tensile strength were 637 MPa,706MPa,respectively,and the KV2 at-20°C and-50°C were 63 J,40J,respectively.The fracture of the tested samples after the heat treatment(800°C normalizing + 20% cold rolling + 650°C tempering),which was characterized by bending propagation path of the secondary cracks,also shows the excellent cryogenic toughness.Hot deformation behavior and cryogenic toughness of a fine grain cryogenic structural steel were investigated in terms of 0.1%Co addition.Hot compression experiment was conducted at temperatures range of 850~1200°C and stain rate of 5s-1,and the effect of 0.1%Co addition on cryogenic toughness was evaluated based on a heat treatment(800°C quenching + 20% cold rolling + 650°C tempering).The addition of 0.1%Co has the trend to increase the flow stress and delay the occurrence of DRX.DRX fraction of the steels with 0 and 0.1%Co was about 67.4% and 43.9% at 850°C,respectively,then all increased to 100% at 1,100°C.Compared with that of the steels without Co addition,cementite particles in tempered sorbite of the steel with 0.1%Co addition became smaller in size but more numerous in quantity,the yield strength increased from 756 MPa to 787 MPa,and the Charpy V-notch energy at-20°C improved from 69J to 102J.