Study On The Control Of Non-metallic Inclusions During Wide And Thick Slab Continuous Casting Process

Heavy plate products are widely used in the realms of oil and gas transmission, ship building, bridge engineering, boiler of power station, pressure vessels, heavy equipment, offshore oil, and military industry. The inclusion control in continuous casting process is a key for the high-quality heavy plate production.The reasonable control of molten steel flow is crucial for the inclusion removal in the tundish, and the control of the steel/slag interface and slag entrapment behavior in the mold is a key for guaranteeing the smooth operation of continuous casting process and controlling the macro inclusions. Therefore, it has important scientific and practical guidance significances for developing the tundish flow control technology and understanding the flow characteristics and interfacial behavior in the mold. In this paper, the transport phenomena in the tundish and mold were studied by using mathematical and physical simulations according to the heavy plate practical production process in one domestic steel corporation, and one scheme of flow control devices for cleaning the molten steel in the tundish and the optimum submerged entry nozzle (SEN) parameters for restraining the slag entrapment effectively in the mold were proposed. In addition, the effectiveness of applying the optimal flow control devices and SEN to the actual performance was investigated. The main contents and results obtained are as follows:(1) With the combination of the wide and thick slab continuous casting tundish practical process, a physical model with the scale 1:2.5 was established based on the Froude similarity. By means of the measured results of the physical model, two combined models for characterizing the melt flow were compared and the optimal combined model was found out. The results show that the traditional model may make mistakes both in the estimation on the melt flow characterization and in the choice of optimal flow control devices. As a reasonable modification has been made on the calculation of dead volume fraction, the modified combined model is suitable for characterizing the melt flow in single-strand tundish.(2) The effect of different control devices on the characteristics of melt flow in the tundish was analyzed by the above-mentioned modified combined model presented by Sahai. The results show that the turbulence inhibitors with and without top extending lip both can improve obviously the plug flow volume and decrease the dead volume to some extent. The design parameters of the dam nearby the outlet combined with turbulence inhibitor with top extending lip have some effect on the melt flow in tundish. And the distance between the weir and dam combined with turbulence inhibitor with top extending lip has an optimal value for the melt flow. And a three-dimensional mathematical model to simulate the flow of molten steel in the tundish was developed to analyze the melt flow in tundish with typical control devices. One proposal for the best inclusion removal is presented based on the physical modeling and mathematical modelling.(3) With the combination of the wide and thick slab continuous casting mold practical process, the effects of different operation parameters on the fluid flow, interface fluctuation behaviour as well as the mechanism of mold powder entrapment in the mold were investigated by combining the water model experiments and mathematical simulations.(4) There are two new mechanisms of mold powder entrapment in wide and thick slab mold with argon blowing, viz. powder entrapment caused by the attack of bubbles stream and the vortex, and a flow reversing from the narrow face of the mold give rise to the powder entrapment was not found in the experimental casting speed range. With different slab widths and correspondent casting speed, the powder entrapment will not occure in the mold if the gas flowrate blowing into the SEN is less than 4 L/min.(5) The optimum process parameters for controlling the surface fluctuation in the mold with the section of 2200×280mm2 are that the argon gas flow rate of 4L/min, the nozzle side port angle of-15°and the nozzle submergence depth of 140 mm.(6) The results of application of continuous casting process parameters in practice shows that the presented process parameters are better than the original ones on the control of non-metallic incusions. With the presented process parameters, the inclusion content in the tundish and slab are decreased by 8% and 20% than those under the original process parameters, respectively. And the typical inclusion sizes in tundish and slab is 10μm and 5μm with the presented process parameters, wihle it is 30μm and 15μm with the original ones.