Vacuum induction furnace for smelting steel
One important factor restricting the application of nitrogenous steel is its smelting problem. Since nitrogen is gas elements, low solubility in the steel, the high nitrogen steel smelting usually adopt pressurized electroslag remelting and counter pressure casting process to complete, there are other USES the hot isostatic pressing melting, pressurized induction melting and powder metallurgy method, the smelting process all need special equipment, the high cost. How to adopt economical and effective method to smelt high nitrogen steel has been a hot topic. The institute of metals of the Chinese academy of sciences recently proposed a technical route for the direct use of vacuum induction furnace in the smelting of high nitrogen steel. In this process, high nitrogen steel with nitrogen content of 0.5% or so can be smelted without positive pressure, and the impurity element content in steel can be effectively controlled. At present, the factors influencing the process of high nitrogen steel smelting in vacuum induction furnace are further studied.
A. Effect of alloy elements on nitrogen content. The solubility of nitrogen in ferroalloy solution is affected by other alloy components. Different alloy on the nitrogen solubility in the steel has the effect of different coefficient, Cr, Mn, Mo, V, Nb elements such as increase the solubility of nitrogen, and elements such as Ni, Cu, Si, C reduce nitrogen solubility. The influence of various elements on the solubility of nitrogen in molten steel can be effectively measured by the activity coefficient. According to the present experimental results, chromium or manganese in steel can effectively improve the solubility of nitrogen.
2. Effects of nitrogen addition on types and methods. The high nitrogen steel is smelted by vacuum induction furnace. Because the vacuum induction furnace smelting system is a low-pressure system, the large amount of nitride melting and dissolution in molten steel will inevitably lead to nitrogen overflow on the surface of molten steel. If nitrogen is added too early, the nitrogen dissolved in the molten steel will continuously participate in the overflow reaction on the surface of the molten steel, resulting in less and less nitrogen dissolved in the molten steel. In addition, if the molten nitrogen temperature exceeds the molten steel temperature, the melting time will be extended, and the higher melting temperature will cause the nitrogen to overflow from the molten steel. When the nitride melts at a low temperature (lower than the molten steel temperature), the nitride melts quickly when it is added to the molten steel and is stirred evenly. Due to the short time, a large amount of supersaturated nitrogen dissolved in the molten steel is left in the molten steel without overflow before the ingot is cast, resulting in a high nitrogen content. In nitrided manganese, chromium nitride, silicon nitride several nitride alloys, such as only the manganese nitride melting temperature is low, about 1200 ℃ or so, and chromium nitride, silicon nitride, the melting temperature above 1600 ℃. Therefore, when using vacuum induction furnace to smelt high nitrogen steel, it is advisable to use manganese nitride.
3. Influence of smelting parameters. The experiment shows that the solubility of nitrogen in steel decreases with the increase of temperature, which may be caused by the interaction of alloy elements in steel. Therefore, it is an important factor to control the temperature of molten steel and the temperature of molten steel during smelting. In addition, some specifications and parameters of vacuum furnace (such as the volume of furnace cavity and the specification of crucible for smelting, etc.) all influence nitrogen content in steel fluid to a certain extent. The furnace chamber is small, and the diffusion of nitrogen into the furnace chamber can be reduced under the condition of constant pressure. The smaller the inner diameter of the crucible, the smaller the specific surface area of the molten steel and the external contact, and the smaller the chance of nitrogen to participate in the overflow reaction, the higher the nitrogen content of the molten steel is. Moreover, when there is a large amount of oxygen in steel, the CO gas produced by the use of carbon deoxygenation can easily induce the nitrogen dissolved in the steel liquid to form bubbles and overflow, which is not conducive to obtaining high nitrogen content.