| In recent years, along with the development and optimization in continuous casting technologies, higher quality requirements for casting products have been put forward. The research shows that large size inclusions of casting products are mostly originated in the tundish. Surface and internal quality of casting steel are closely related to characteristics of liquid steel in the tundish, such as uneven temperature and chemical compositions. And also steel quality depends on flow patterns and solidification process of liquid steel in the mould, it's intensively related to several main factors, such as shell growth in the early stage of solidification, mould powder behaviors, immersion depth of SEN (submerged entry nozzle) and mould level fluctuation etc. Therefore, it is very important to know more about liquid steel's behaviors in the tundish and mould in order to ensure a steady casting operation and enhance quality of casting products. At present, by carrying out hydraulic simulation experiments, researchers involved in steel-making home and abroad have studied the influences of tundish internal shape and pouring flow on multi-strand casting; and they've learned flux entrapping mechanism and how level fluctuations are affected by SEN inner diameters, SEN immersion depth and casting speed. Some of the findings are successfully applied into practice and better results have been achieved.Although No.7 billet caster in the steel-making plant of KISC has been equipped with many advanced devices and quality control measures after modification, some shortcomings of the tundish are still obvious, for example, flow control devices are not suitable; SEN immersion depth isn't optimized; non-metallic inclusions couldn't float up to the surface efficiently; compositions of liquid steel is uneven and there's sharp temperature differences between different tapping holes etc. Thus some defects have showed up, particularly inclusions and crack have been found during 45, 60 and TS800U steel casting, which is becoming the restriction to further improve steel quality. Based on actual conditions of No.7 billet caster, hydraulic simulation models for the tundish and mould have been set up; by adopting the theory of reactors, this paper systematically analyzes the influences of different tundish flow control devices on average residence time, dead zone and stopper flow zone, and simulation results have been verified by floating rate of inclusions; By analyzing relations between level fluctuation and crack defect occurrence rate, and inspecting flux entrapment conditions and impact depth of stream, the optimization study of SEN immersion depth has been made.The optimum flow control devices and structures have been found by carrying out hydraulic simulation experiments in the tundish: the short circuit and by-pass breakdown flow occur in tundish without barricade, turbulence flow will result in worse cleanliness of steel due to uneven stirring of fluids; in order to improve flow field, it is necessary to install flow control devices in the tundish. In case of simple barricade, option 2# with the barricade installed 710mm away from the tapping hole is the best option; In case of multihole barricade, option 3 with multihole barricade installed 710mm away from the tapping hole is the best one. A range of 100-125mm SEN immersion depth is found reasonable by hydraulic simulation experiments.The flow control devices of the tundish and mould for No.7 billet caster are optimized and adjusted based on hydraulic simulation experiment findings: option 2# with simple barricade is applied for carbon steel casting; option 3 with multihole barricade is applied for quality steel casting; SEN immersion depth is to be controlled at 100-115mm during casting. Obvious progress has been made after applying experimental findings into practice: casting process is becoming steady and reliable, the casting success rate of billet increases from 99.22% to 99.65%, the breakout rate reduces from 0.09% to 0.04%; the quality of billet improves greatly, the qualified rate of quality steel billet increases from 97.5% to 99.2%; great economical and social benefits are achieved, the loss of defective products decreases 10,795t, therefore 26.67 million yuan profit has been created; No.7 billet caster is able to produce additional 12,500t annually after optimization of flow control devices, so it can save more ore and coal resources, as well as water and electricity. It's good for environmental protection and ecological balance. |
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