Research On Key Steelmaking Technologies For Ultra-low Carbon Clean Steel

In order to satisfy the requirement for the properties of ultra-low carbon steel, and solve the problems of ultra-low carbon content, high cleanliness, the controlling of inclusions and castability of molten steel in practice, the key and common technologies for ultra-low carbon steel, such as converter steelmaking, RH refining, Ca treatment, castability of molten steel in CSP and so on, were studied systematically in the thesis with the methods of model simulation, industrial experiments, physical and chemical inspection and so on. The main contents are as followings.?1? The transition tempreture of the selective oxidation for carbon and phosphorous in the earlier stage during steelmaking and the relationship between [C] and [P] content were estabished. Dephosphorization was priority to decarburization by suppress oxygen decarburization in the earlier stage during steelmaking. And the [P] content in molten steel can be less than 0.01wt% by slagging off in earlier stage and slag-less steelmaking at later stage. It provided a method for low phosphorous steel-making in common converter. A parameter FC which can characterize oxidizing reactions of molten steel in converter was established by experimental results. The parameter was related to top and bottom blowing intensity, carbon content in molten steel, partial pressure of CO and converter lifetime. The oxygen distribution between slag and melt can be improved by process optimization.The end point contents of [O], [P] and [S] decreased obviously by the improvement of oxygen lance nozzle and bottom blowing component, and by the optimization of oxygen blowing process and bottom blowing intensity. The efficiency of oxygen decarburization was improved, and oxygen blowing time was reduced by 1.5 min.?2? The characterstics of mixing, circulation and decarburization for 150 t RH system were studied by water model and industry experiments. The mixing time for RH unit was mesured by Si and Mn which replaced Cu first time. The equation on the relationship between mixing time and stirring power was built. And relationship between mixing time and circulation flow rate was built. A new modified equation for circulation flow rate was built, which considered the combined action of mixing time and stirring power. The equation considered the factors of the weight of molten steel, the immersed depth of nozzle and the depth of molten steel in cacumm vessel. The model of decarburization reaction rate in RH was established with the factors of stagnant carbon content and the combined mass-transfer by diffusion and circulation. The experiment results agree well with the calculation results by the model.The experiment results show that decarburization can be improved by process optimization, such as optimize oxygen blowing in earlier stage, reduce vacuum pressure rapidly, control the argon blowing rate by section, increase the reaction area, decrease the stagnant carbon content and so on. And most carbon contents for ultra-low carbon steel are less than 15×10^-6 in practice.?3? The effects of decarbonizing by oxygen, increasing temperature with Al addition, argon blowing rate and slagging on cleanliness of molten steel are studied. A deoxidization rate model which describes the relationship between T.O and treatment time after deoxidization was established by considering the effects of circulation and diffusion of molten steel and oxidizing slag. The experiment results were agreed well with the calculation results by model. The predition of T.O in RH refining was built based on the experiment results.The research results showed that the total oxygen in molten steel can be less than 10×10^-6 in mass production with the process as following: the content of?TFe+MnO? in slag is less than 5%, the slag was adjusted by CaO-Al₂O₃-Al or CaO-Ca F2 flux, and the vacuum treatment time was 10-12 min after deoxidization by Al.?4? The thermodynamic equilibrium between [Ca], [Al], [S] and [O] for ultra-low carbon and high Al steel was built based on calcium treatment. And the thermodynamic model was verified by experiment results. The castability of molten steel for ultra-low carbon steel in CSP was solved by process of RH refining, slag modification by Al addition and Ca treatment. And the sequence heats were more than 10 heats.The mechanism of inclusion modification by calcium treatment for ultra-low carbon steel was studied. Most inclusions before Ca treatment are CaO-Mg O-Al₂O₃ and CaO-SiO₂-Al₂O₃ with low CaO content. The inclusions after Ca treatment were modified into CaO-Al₂O₃-Mg O inclusions with high CaO content. Then the content of CaO in compound inclusions decreased because of the reduction of calcium and the homogenization of CaO in core and outer layer of the inclusion.