| The rotor of supercritical steam turbine is an important part of modern ship and large power stations a steam turbine. Large castings production of high chromium alloy steel for rotor of supercritical steam turbine has strict requirements for quality:low content of impurities, gas and inclusions, homogeneous chemistry. However, the larger the diameter of steel ingot is, the more difficult the quality control is. Electroslag remelting (ESR) technology, which has excellent metallurgical reaction conditions and unique solidification crystallization ways, has incomparable superiority compared to other smelting methods in smelting large diameter high quality ingot. As the limitation of equipment conditions, there is some question in remelting large diameter high chromium alloy ingot using multi electrode electroslag furnace, such as great consumption of electricity and gas, shrinkage cavity, shrinkage porosity, and high hydrogen content in ingot, which may lead to high production cost and affect the quality of ingot. Therefore, this thesis carried out studies on remelting 120 tons of large diameter high chromium alloy ingot. This thesis studied the conductivity of the molten slag, solidification characteristics of the 13Cr steel and behavior of hydrogen diffusion during ESR process, developed the mathematical models of protective gas and studied the optimization of ESR process for a large three-phase bipolar series electroslag furnace of a factory, which provided a theoretical basis for reducing energy and resource consumption, and optimizing ESR process. On the basis, the optimization design of the electroslag furnace protective cover was carried out, which has important theoretical significance and practical value to control the hydrogen content in steel, reduce the consumption of shielding gas and improve the quality of ingot. In this thesis, the conductivity of CaF2-Al2O3-SiO2-CaO slag is measured by the four electrodes AC impedance method, and the result shows that:the conductivity of the slag increases with the temperature increases, and the change followed the Arrhenius curve. When the ratio of other components remain unchanged, the conductivity of the molten slag decreases with the amount of Al2O3 or SiO2 increases, but increases with the amount of CaO increases.The amount of Al2O3 or SiO2 has far more influence than CaO on the conductivity of molten slag.This thesis studied the liquidity, volume and linear shrinkage and thermal expansion performance of the 13Cr steel. The results show that:the 13Cr steel has a long solidification range; there are a wide variety of phase transformation occurred during solidification process, which may lead to heat stress and phase transformation stress; the 13Cr steel has poor liquidity, low malleability, large shrinkage ratio and holes rate, which may lead to large shrinkage and stresses during solidification. Therefore it most likely to generating the crack, porosity and shrinkage cavity in solidification process. Since the solidification ways of ESR is from the bottom up, the ESR ingots have denser crystallization structure compared to the traditional mould casting, which can effectively prevent the crack, porosity and shrinkage cavity.The diffusion of hydrogen among the atmosphere, slag and metal under the atmosphere with different water partial pressure during ESR process was studied by the high temperature experiments. And the equations describing the diffusion of hydrogen was derived base on the comparison analysis of experiment results. As the influence of water in the new slag, the hydrogen content in the molten steel cannot be controlled by controlling the atmosphere early in ESR process. However, the influence of water in the new slag on hydrogen content in the molten steel becomes much less in 3 h, then the water vapor pressure in the atmosphere should below 110 Pa to control the hydrogen content in the molten steel below 2×10-6.In order to control the water partial pressure in the furnace gas, the influence of protective cover structure and gas supplying parameters on the gases in the furnace were studied. Based on the theory of computational fluid dynamics, basic equations of the mathematical model were established, which were discreted into three-dimensional steady discrete equations. According to the flow characteristics of gas in the electroslag furnace, Realizable k-? two-equation model, DO radiation model and Transport model were introduced. Then the mathematical model of flow and heat transfer of gases in the large electroslag furnace was established.Based on the Fluent software, this thesis performed coupling numerical simulations in the flow field and temperature field of the protective gas in the electroslag furnace, the results show that:at the same condition, the percentage of air in furnace gas increases with the outlet cross-sectional area increases; the percentage of air in furnace gas decreases with the inlet cross-sectional area increase. In addition, the structure of cover is optimized and designed:it is divided into two parts and has three oval electrode pores; there is an outlet sized at 200×200 mm in upside and three inlets 160 mm in diameter.This thesis performed numerical simulations in the flow field of the protective gas in the electroslag furnace, the results show that:the percentage of air in furnace gas increases with the outlet pressure increases or the flux of the protective gas decrease; combining the results of the hydrogen diffusion experiments, the reasonable outlet pressure and protective gas flux are determined to be-20 Pa and 0.07 kg/s?The protective cover and protective gas flux determined in this thesis are adopted in industrial test, the increase of hydrogen content is effectively restrained during ESR process, and the ingot quality is improved. |
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