Study On The Fluid Flow In A Multi-heat Teeming Tundish For Heavy Steel Ingot

Generally, the weight of a heavy steel ingot is bigger than 400 tonne. Due to the limitation on the volume of smelting furnace, the multi-heat teeming is usually used as that in traditional continuous casting by flow rate 1.5 to 2.5 times. In the present study, the steel ingot is made for the 1000MW nuclear conventional island integral solid rotor forgings. It is one of the most important parts with the biggest weight of forging blank, which is the biggest of section size and the highest technical requirements in the world. Since the weight of rotor is 170 to 180 tonne, it requires the weight of ingot as large as 580 to 650 tonne. This kind of steel demands a super pure control in order to solve the problem of the temper brittleness. So, the multi-heat teeming of EAF—LF—VD—VCD, up-pour technology and vacuum casting are commonly used both at home and abroad. In the process of multi-heat teeming in heavy ingot, the tundish acts as an intermediate vessel which connecting the ladle and ingot mold, storing liquid steel to ensure the multi-heat teeming when ladle is changing. With the development of tundish metallurgical technology, the function of inclusions removal in a tundish of conventional continuous casting has turned into a key technology for the clean steel production of metallurgy industry. Large capacity tundish, high bath depth and flow control devices were used to enhance the residence time of liquid steel in a tundish, and to promote inclusions aggregation and floatation. In China, round tundish with large-capacity and deep bath is mainly used in the production of heavy ingot to meet the requirement of a high casting flow rate. However, the short residence time of liquid steel and the low inclusions floatation rate in the round tundish caused by the short distance from the zone of ladle injection to the outlet and large flow rate, weakened the inclusions removal. Therefore, it is quite significant to design a proper geometry size of tundish for the multi-heat teeming and the function of inclusions removal in heavy ingot casting processes, which will largely improve the cleanliness of low pressure rotor forging.In this paper, focused on the heavy steel ingot, physical modeling was performed on the research of the flow field pattern and inclusions removal efficiency in the different types of multi-heat teeming tundishs, the round tundishs with different bath depths (H) and the groove tundishs with different lengths (L). And a reasonable geometry size (L/H) of the multi-heat teeming with a large flow rate was obtained. A new ellipse tundish was designed by a comprehensive consideration of industrial field installation, walling.The trial results showed that the modified multi-heat teeming tundish of heavy ingot can effectively remove the nonmetal inclusions in liquid steel, improve the cleanliness of liquid steel and ensure the product quality of low pressure rotor. Study on the fluid flow in a multi-heat teeming tundish for the heavy steel ingot by using different turbulence models in numerical simulation.There are four main contents and results of the dissertation as follows:1,The evaluation of fluid flow in the round for multi-heat teeming tundishsThe flow condition of the molten steel and the inclusion floatation efficiency were evaluated in the round shape tundish for heavy steel ingot. The results showed that:①Without flow control devices, the residence time of molten steel was short, the dead volume was big and the inclusion floatation efficiency was low.②After installing the flow control devices, the mean residence time slightly increased,the inclusions has no time to float due to the short distance from the pouring point to the outlet of the tundish.Thus, the purpose of improving the molten steel cleanliness cannot be achieved.2,Study on the geometric dimension design in a multi-heat teeming tundish for heavy steel ingotEffect of fluid flow and inclusion motion on different bath depth and the ratio length and bath depth in a multi-heat teeming tundish for heavy steel ingot were firstly studied. The geometric dimension was afforded for a multi-heat teeming tundish of large flow rate. This is the first innovation in the dissertation.①Increasing the bath depth and tundish length was beneficial to the inclusions floatation. When the tundish volume was the same, prolonging the tundish length was more advantageous to inclusions floatation.②Considering the flow field patterns of liquid steel in the tundish and the inclusions floatation efficiency, the bath depth of the multi-heat teeming tundish for heavy steel ingot should be between 1500 mm and 2000 mm, and the L/H should be controlled between 2.0 to 2.7.3,Study on the flow control device in a multi-heat teeming tundish for heavy steel ingotConsidering the tundish hoisting, brickwork and placing, a new elliptical tundish of deep bath and large flow rate was designed. And the flow control device was optimized. This is the second innovation in the dissertation. ①According to the study of 18 design schemes of flow control devices and the residence time of molten steel, flow pattern and inclusion floatation efficiency, the optimal installation position of the flow control device is that, the distance from the weir to the bottom of tundish is 300mm, the height of the dam is 400mm,,the distance from pouring point to the weir is 1500 mm, and the distance from the weir to the dam is 490 mm.②Based on the research above and the site actual conditions, a new ellipse tundish with the ratio of length and bath depth is 2.0 was designed, and an optimization analysis was performed on the position of the weir exit, the results indicate that the scheme that the outlet locates on the axis of symmetry is the optimal one.③The comparison of simulation results between the two shapes of tundishs with the same capacity showed that, in the elliptical shape tundish with the flow control devices, the response time was 2.5 times longer than that in the round shape tundish, the mean residence time and the residence time of the plug increased by 6% and 24% respectively, the mixed volume increased by 30%, the dead volume decreased by 28%, the ratio of the plug volume to the dead volume increased by 68%, and no short -circuiting flow, the inclusion flotation efficiency increased 17%.4,Applicability of turbulence models in numerical simulation of flow field in a tundish for heavy steel ingotThe flow field of molten steel in a multi-heat teeming tundish for heavy steel ingot was simulated by using the standard k-εmodel and Realizable k-εmodel. And the accuracy of simulation was validated by the comparison between the results of physical modeling and numerical simulation. For the flow field simulation of the tundish with deep molten pool, large flow rate and vortex, the adoption of the Realizable k-εModel was more suitable than the Standard k-εModel. This is the three innovation in the dissertation The results indicate that:①Both two turbulence models can be applied in the simulation of flow conditions of molten steel inside the round and elliptical shape tundishs. In the Standard k-εModel, the viscosity coefficient was a constant, and it has a poor effect of vortex simulation inside the tundish, the maximum deviation of the round tundish reached 50% while the maximum deviation of the elliptical tundish reached 56.3% compared to the results of physical modeling. In the Realizable k-εModel, the viscosity coefficient was a variable, the flowing behavior of molten steel inside the tundish can be simulated more accurately, the maximum deviation of the round tundish was reduced to 9.6%, and the maximum deviation of the elliptical tundish was reduced to 16.5%.②It can be known from the comparison between the Realizable k-εModel and physical modeling that, as for the Realizable k-εModel, the simulation results of molten steel flowing condition inside the two tundishs with flow control device was even closer to that of physical modeling than that without flow control device. The maximum deviation was 50% in the round tundish without flow control device, and 9.6% in the round tundish with flow control device. The maximum deviation was 56.3% in the elliptical tundish without flow control device, and 16.5% in the elliptical tundishs with flow control device.5,The trial application of the elliptical tundishAccording to the research results mentioned above, the elliptical tundish was used in trial application. The trial results indicated that after using the elliptical tundish,①The content of the total oxygen in the elliptical was decreased by 25%～33%.②The equivalent diameter of single defect decreased from 2.5 mm to 1.6 mm, and no intensity defect observed.③The content of the total oxygen in the rotor was 21 ppm, which was decreased by 30%.Therefore, the quality of rotor forgings was improved greatly by using the elliptical shape tundishs.