| Currently, the top and bottom blowing steelmaking process has become mainstream technology for converter steelmaking, and the number, arrangement and gas flow rate of bottom blowing have significant impact on the flow field and mixing efficiency of the converter molten bath. Therefore, it is necessary to reveal clearly the fundamental behavior of gas-liquid two phases in converter, and to optimize the parameters of bottom and top combined blowing process. In view of the present research work for achieving numerical simulation on the top and bottom combined blowing processes in converter that is still rarely reported systematically. Combined with the top and bottom combined blowing reconstruction process for converter in a steel plant in China, a three-dimensional mathematical model has been established to describe the the gas-liquid two phases flow joint action of the supersonic jet of top blowing and bubbly plume of bottom blowing. The effects of number, arrangement and gas flow rate of bottom blowing and parameters of top blowing on the gas-liquid two phase flow and mixing efficiency in converter with top and bottom combined blowing are investigated by mathematical and physical simulations in this paper. The main contents and results obtained are as follows:(1) The physical model for the 50 ton converter with top and bottom combined blowing has been established in 1:4.56 scale according to similarity principle. Mixing time is measured by monitoring the concentration curves of the two points of different regions after adding the tracers in the bath, and the effects of number, arrangement and gas flow rate of bottom blowing and parameters of top blowing on the mixing time in converter with top and bottom combined blowing are investigated. The results of physical model are greatly essential to validate and perfect the numerical model.(2) The supersonic jet behavior of four hole top lance is described with the compressible model, and through the combination of the Volume of Fluid (VOF), Lagrange Discrete Particle Model (DPM) and Species Transport Model, the gas-liquid two phase flow, mixing efficiency and stress filed of lining impacted by steel in the top and bottom combined converter are described. On the basis of good agreement of the predicted and measured supersonic jet of multi-hole top lance and mixing time of converter with top and bottom combined blowing, the effect of different numbers, location, gas flow rate of the bottom blowing, top lance height and gas flow rate of top blowing on the gas and liquid two-phase flow, mixing time and stress filed of lining are investigated, and the proper parameters have been proposed. The results show that:① There are fluctuations of supersonic jet near the top lance exit, and the compressibility of the fluid have a great impact on the attenuation behavior of the supersonic jet. Under the only top-blowing, multiple independent penetration zone (pots) are formed due to the multi-jets impacting the molten steel surface, and the molten steel moves from the penetration zone towards the wall and a large re-circulation loop is formed in the center of the bath (between the axis and the wall). The mixing time of the only top-blown converter is about 523 s because of the large dead zone appearing at the bottom of the converter. But after joining bottom blowing, the molten steel flow velocity at the bottom of the converter increases obviously and becomes more uniform in the converter bath, and the mixing time_is about 93 s.② With the numbers of bottom tuyeres increasing from 2 to 4, the mixing time firstly decreases, and then increases. The mixing efficiency using three bottom tuyeres performs better than the case of two or four bottom tuyeres. With increasing distance from the bottom tuyeres to the center of converter, the mixing time decreases firstly, then increases. The optimum location of the bottom nozzle away from the axis is 0.4D. With angle θ (θ is the angle between bottom tuyeres and trunnion) reducing from 45°to 15°, the mixing time decreases firstly, then increases. The mixing efficiency with θ=30° performs the worst and θ=45° is suggested. The flow field and mixing efficiency in converter using the non-equidistant arrangement of bottom tuyeres (0.35D,0.35D,0.4D), preform better than that using three nozzles equally spaced. With the increase of gas flow rate of bottom blowing, the mixing efficiency increases significantly, but the lance height and gas flow rate of top blowing have small influence on the mixing efficiency for the combined blowing converter, and it is beneficial for mixing efficiency to improve the lance height or reduce the gas flow rate of top blowing within the allowable supply of oxygen.③ Maximum stress of furnace lining appears below the liquid level where is about 80% of the height of the liquid steel in converter. The stress of furnace lining decreases with the increase of number of bottom tuyeres and distance from the bottom tuyeres to the center of converter, but that increases with the increase of gas flow rate of top blowing.Using the optimized blowing mode, the pure top blowing converter has been reconstructed with top and bottom combined blowing technology in a steel plant in China. Comparing pre and post reconstruction, the difference of the smelting end [C] and [O] plot, slag composition, petrographic and economic benefits between combined blowing and pure top blowing was investigated.① The homogenization of composition and temperature of the melt bath have been enhanced due to superior dynamic conditions. The whole blowing process has become steadier, and slag melting rate and decarburization rate are improved with the reduced splashing. Additionally unreacted lime in the slag has disappeared, the oxygen blowing time and converter melting cycle are reduced by 30-60 s and 90-120 s, respectively.② Ideal end point of blowing can be obtained and controlled, Steel-slag contact sufficiently and slag forming conditions are ideal. C3S content in the form of long strip and board in top and bottom combined blowing converter is 25%～30% more than that in pure top blowing converter.③ The smelting end [C] and [O] plot is less than 0.003, and the cost of per ton of steel has reduced by about ten yuan RMB, and the life-span of bottom nozzle is as long as campaign of converter by using slag splashing and slag-metal mushroom nozzle technology. |
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