Study On The Process For Controlling The Cleanness Of Molten Steel And Its Application For Medium-Thin Slab Continuous Casting

The technology of thin slab continuous casting and rolling has been paid high concern and attention by steel industry because of its shorter process, lower unit investment, lower energy consumption and higher productivity. Medium-thin slab continuous casting with 100-170 mm thickness is such a casting machine between conventional one and thin slab caster, and its production process and slab quality requirement is unique and complex. Therefore, how to optimize and control the medium-thin slab continuous casting process to ensure the product quality is particularly of importance. In view of the problems existing in current mediun-thin slab continuous casting practical production, the optimization and control of molten steel refining process, fluid flow in the tundish, flow field and steel/slag interface fluctuation behaivour in the mold, and the application of technologies for improving the cleanness of casting molten steel in the mediun-thin slab continuous casting process were investigated by combining the methods of on-line optimization test and mathematical and physical simulations in this paper. The main contents and results obtained are as follows:(1) Combining the medium-thin slab continuous casting practical production process, the on-line optimization test of the molten steel refining process was carried out. The refining technologies including the property modification of ladle top slag, LF furnace refining and wire-feeding process were studied and optimized. The test results show that the pre-melted top slag modifier mixing the white lime instead of the mixed type modifier in the ladle tapping process can significantly increase the basicity of top slag, decrease the content ofΣ(MnO+FeO), and the resulfurization can be avoided. The integrated quality of molten steel purified by LF refining furnace is relative higher, but the desulfurization efficiency of LF refining furnace is not high that needs to be further improved for deep-desulfurization process. The wire-feeding amount can be determined by the regressive linear relation between the wire-feeding amount and the [Als] content, which can control the oxygen content of molten steel and the morphology of inclusions effectively.(2) A water model for the medium-thin slab tundish was established according to similarity principle. With the consideration of fluid flow and resident time distribution in the tundish, the effects of turbulence inhibitor structure, casting speed and porous baffle on the process in the tundish were investigated, and an optimum combination of flow control devices in the tundish for fitting to the real production was proposed. Meanwhile, a three-dimensional mathematical model to simulate the flow of molten steel in the tundish was developed by the conservation equations for mass and momentum, turbulence model equations, and species transport equation of tracer, and the characteristics of flow field and resident time distribution of molten steel in the tundish with the optimum combination of flow control device proposed by water model and porous baffle were analyzed respectively. It provides a theoretical basis for further revealing the flow behaviour in the tundish and verifying the rationality of the optimum scheme of flow control device, as well as provides a guidance for real production and optimization of operating parameters. The results of mathematical and physical modelling show that the optimum struture of flow control device is the turbulence inhibitorwith square shape, the distance of dam and weir is 540 mm, the height of weir is 180 mm, and the distance of dam and bottom joint is 200 mm. For casting speed of 2.4 m/min, the dead volume ratio of fluid in the tundish is 11.83% and 9.1% respectively for the cases of without and with turbulence inhibitor and the plug flow volume ratio increases from 28.08% to 37.94%.(3) 3-D mathematical and physical models describing the fluid flow and steel (water)/slag (oil) interfacial behaviour in the mold were established by combining the water model experiments and mathematical simulations. The effects of submergence depth of 3-port submerged entry nozzle (SEN), side port angle and bottom port diameter of SEN, casting speed, mold width and argon gas injection on the fluid flow, interface fluctuation and slag entrapment behaviour in the mold were investigated. It provides the quantitative relations between the operating parameters and the flow as well as the interface fluctuation, and proposes an effective measure for optimizing and controlling the flow field and steel/slag interface fluctuation in the mold. The mathematical and physical simulation results show that increasing the submergence depth, side port angle and bottom port diameter of SEN all can reduce effectively the interface velocity of fluid and stabilize the level fluctuation in the mold for a given casting speed; increasing the casting speed worsens the steel/slag interface fluctuation significantly and increase the interface velocity in the mold; argon gas injection will aggravate the level fluctuation near the SEN. For casting speed of 2.5 m/min, the maximum interface velocity decreases from 0.53 m/s to 0.16 m/s and the maximum wave height reduces from the break state of interface to 13 mm with the submergence depth of SEN from 100 mm to 160 mm. Similarly, the maximum interface velocity decreases from 0.58 m/s to 0.17 m/s and the maximum wave height also reduces from the break state of interface to 10 mm with the side port angle of SEN from 10°to 25°. With the bottom port diameter of SEN increases fromΦ35×30 mm to 35×45 mm, the maximum interface velocity decreases from 0.44 m/s to 0.28 m/s and the maximum wave height also decreases from the break state of interface to 11 mm. Within the range of casting speed from 1.8 m/min to 3.0 m/min, the maximum interface velocity increases from 0.10 m/s to 0.51 m/s and the maximum wave height also increases from 3 mm to the break state of interface.(4) The on-line application tests by using tundish turbulence inhibitor and the mold nozzle improvement in real plant were carried based on the simulation results of flow behaviour in the tundish and mold. The quality of slab and product were analysed systemically in order to further clarify the slab quality and improvement effects of the process. Moreover, the casting process of peritectic steel was also studied and optimized. The results show that the main defects of slab include the central porosity, intermediate crack and triangular area crack. The slab defects of streak and peeling are caused by the complex incusions and mold powder entrapment into the molten steel, which can be eliminated effectively by improving the nozzle-seeling bowl and using the new type of tundish top flux. The time of mixed section for using the turbulence inhibitor in the tundish is 4.11 min, the removal rate of total oxygen is 24.06%, the free surface is stable, and the probability of slag entrapment, inclusions and gases contents are all reduced significantly. The phenomenon of nozzle clogging has been restrained and the lifespan of mold copper plate has been improved obviously, the total amount of molten steel casted in a mold has increased by 50% and the total content of inclusion in the slab has already dropped to 0.0041% by adjusting the bottom port diameter of SEN fromΦ35 mm toΦ35×38 mm. In addition, the qualified peritectic steel slab has reached to 98% by optimizing the secondary cooling water distribution and using the medium-carbon flux with low heat conductivity.