Research And Development Of High Strength Ultra Low Carbon Bainitic Steel

With the development of building facilities in recent years, the steels which have more high strength and toughness are needed, so the concept of the third generation low alloy steel has been put forward by the third "973plan", and it demands to greatly improve the strength and toughness and obtain its production technology. Base on the requirement mentioned above, a composition of ultra-low carbon bainitic steel (ULCB) was designed in this paper. Through the thermal simulation experiment, the hot working characteristics of ULCB steel were studied during hot deformation and the phase change rule was also investigated during continuous cooling. The effects of heat treatment parameters on microstructure and properties of ULCB steel were also studied by heat treatment experiment. The main works involved are as follows:1. The recrystallization behaviors were investigated by single and double pass thermal simulation experiment with dynamic recrystallization activation energy Qd=402kJ/mol and static recrystallization activation energy Qrex=395kJ/mol, And the relations between critical strain, peak strain and peak stress and Zener-Hollomon parameter being determined and the functiones of dynamic recrystallization dynamics and static recrystallization dynamics were established. The curves of continuous cooling transformation of ULCB steel were drew by thermal simulation experiment of CCT.2. The effects of two-stage controlled rolling on offline microstructure and properties of ULCB steel were studied by double pass thermal simulation experiment. At the same rolling reduction, hardness first rises then falls with the increasing of rolling reduction in austenite non-recrystallization zone.3. The effects of heat treatment which contains Q-T process and offline-relaxation on microstructure and properties of ULCB steel were also investigated.(1) Through setting a series of different parameters of offline quenching, the effects of offline quenching on microstructure and properties of ULCB steel were discussed and the best quenching process is900℃,50min.(2) Residual austenite between bainitic lath decomposes gradually and the bainitic lath will gradually merge and granular bainite may appear with increasing tempering temperature. Coarse quasi-polygonal ferrite which has little dislocation is produced at about700℃. Tempered microstructure becomes more and more stable. The yield strength increases, then reduces sharply as tempering temperature rises, and the yield strength is only710MPa at700℃. The higher the tempering temperature is, the higher yield ratio is, and the better the plasticity is.(3) In offline relaxation process, tensile strength increases then slightly decreases with relaxation time, when the specimens are air cooled above two phase zone. And the tensile strength reaches a maximum1034MPa when the specimen relaxes about20seconds. When the specimens are air cooled into two phase zone, the yield and tensile strengths decline sharply, and elongation increases.