Simulation On Improving Of Molten Steel Cleanliness During Multiple-heat Teeming For Heavy Steel Ingot

At present, the middle container of continuous teeming heavy steel ingot was a round tundish in the First Heavy Machinery Plant. This tundish could realize multiple-heat teeming by storing some molten steel during changing the ladle. But, the distance between the pouring point of ladle stream and the export of the tundish was near, which resulted in the residence time of molten steel being short and the inclusion easily running into the mould. Also, the molten pool of the tundish was deep, which brought much difference in temperature between the both sides of the weir after installing the flow control devices, and could not keep the molten steel temperature uniformity. Besides, it was difficult to carry out protective teeming because of suspending casting of the ladle in practice. Hower, the requirement on the cleanliness of teeming super pure rotor steel was very high. So, aimed at the above problems, metallurgical effect of the former round tundish was evaluated through physical simulation in this article. Also, two tundishes with diffferent shapes were designed, theirs control flow devices were optimized through physical simulation, and the metallurgical effect of the former and the improved tundishes was compared. The quantity of absorbing air from the ladle stream also was calculated. What's more, the tundish covering flux and its refractory were comfirned by calculation, which could realize improving the molten steel cleanliness.Experimental results of physical simulation indicated that:①The responding time of tracer in the former tundish without control flow devices is 10.8s, the average residenc time is 212.8s,the dead volume fraction is 0.26, Vp/ Vd is 0.27 and the floatation rate of simulated inclusions is 73.0%; while after installed the control flow devices, the responding time becomes 15.7s, the average residence time is 235.4s, the dead volume fraction is 0.18, Vp/ Vd is 0.76 and the floatation rate of the simulated inclusions is 78.5%. These show that the flow field of the tundish with weir&dam is improved, but its residence time is still short and the inclusion in molten steel is difficult to be removed.②The responding time and average residence time of tracer in the improved trough tundish with the optimized weir&dam are 48.2s and 272.1s, which are 32.5s and 36.7s longer respectively than the former round tundish's. The Vp/ Vd is 1.00, which is 0.24 more than the round tundish's. The inclusion floatation rate gets to 95.1%. So, the residenc time of the improved trough tundish is longer and its flow field becomes more reasonable.③The responding time and average residence time in the improved oval tundish with the optimized weir&dam are 53.8s and 455.2s, which are 38.1s and 219.8s longer than the former round tundish's. Its dead volume fraction is only 0.13 and the Vp/ Vd reaches 1.25. Also, the floatation rate of the simulated inclusions can get to 96.0%. Thus,the flow field of this oval tundish's is quite reasonable and its inclusion floatation rate is much higher.④Comparied with the experimental results of the former and the improved tundishes, it can be found that the flow field of improved tundishes are more reasonable and that they can be more easily to remove the inclusions. Howere, the residece time of the impoved oval tundish is the longest, its flow pattern is the most reasonable and the effect of inclusions floatation is the best. Therefore, the molten steel cleanliness will be guaranteed when the improved oval tundish is applied in pratice.Aiming at teeming super pure rotor steel in the First Heavy Machinery Plant, the results of theoretical calculation showed that:①The increased [O] and [N] in molten steel from the ladle stream during open teeming are 17.9~20.3ppm and 6.8~7.7ppm; while the increased [O] and [N] in stable teeming are 11.3ppm and 4.3ppm. But, after taking some measures to protect the ladle stream, the increased [O] becomes less than 1.0ppm and the increased [N] is less than 0.5ppm when the O2 content of the protective atmosphere is kept at 1.0%. So, keeping the O2 content of protective atmosphere less than 1.0% can effectively protect the ladle stream against reoxidation from the air.②The main compositions of the tundish covering flux for teeming the 30Cr2Ni4MoV steel were comfirmed. The detailed results were as follows: SiO2 10%~15%,CaO 40%~45%,Al2O3 40%~45%,%FeO≤1.0%,MgO 6.0~8.0%. The above slag not only can protect the molten steel of the surface in the tundish against reoxidation, but also can effectively absorb the inclusions and not make the molten steel polluted.③The tundish refractory using by magnesium material or magnesium- calcium material can effectively improve the molten steel cleanliness, even can reduces the contents of [Si],[Al] and [O].