Vacuum annealing furnace smelting high nitrogen steel
The effects of various factors on the process of smelting high nitrogen steel in vacuum annealing furnace:
An important factor limiting the application of nitrogen-containing steel is its smelting problem. Since nitrogen is a gaseous element, the solubility in steel is low. At present, the smelting of high-nitrogen steel is usually carried out by processes such as pressurized electroslag remelting and counter-pressure casting, and others are also subjected to hot isostatic smelting and pressure induction. Smelting and powder metallurgy methods, these smelting processes require special equipment and high cost. How to adopt a cost-effective method to smelt high-nitrogen steel has always been a concern. The Institute of Metals of the Chinese Academy of Sciences has recently proposed a technical route to directly use a vacuum induction furnace to smelt medium and high nitrogen steel. With this process, high-nitrogen steel with a nitrogen content of about 0.5% can be smelted without positive pressure, and the content of impurity elements in the steel can be effectively controlled. Further research is being conducted on various influencing factors in the process of smelting high nitrogen steel in vacuum induction furnace.
First, the effect of alloying elements on nitrogen content. The solubility of nitrogen in iron alloy solutions is greatly affected by other alloy compositions. Different alloys have different coefficients of action on the solubility of nitrogen in steel. Elements such as Cr, Mn, Mo, V, and Nb increase the solubility of nitrogen, while elements such as Ni, Cu, Si, and C reduce the solubility of nitrogen. The activity coefficient can be used to effectively measure the effect of various elements on the solubility of nitrogen in molten steel. According to the current experimental results, chromium or manganese added to steel can effectively increase the solubility of nitrogen.
Second, the impact of the addition of nitride types and methods. The high-nitrogen steel is smelted by a vacuum induction furnace, and nitrogen is alloyed mainly by adding a nitride. Since the vacuum induction furnace smelting system is a low pressure system, the large amount of melting and dissolution of nitride in the molten steel is bound to cause nitrogen to overflow on the molten steel surface. If the nitride is added prematurely, the nitrogen dissolved in the molten steel will continuously participate in the overflow reaction on the surface of the molten steel, so that the dissolved nitrogen in the molten steel is less and less. In addition, if the melting temperature of the nitride exceeds the temperature of the molten steel, the melting time is inevitably prolonged, and the higher melting temperature causes the nitrogen to escape from the molten steel. When the melting temperature of the nitride is low (below the temperature of the molten steel), the nitride is rapidly melted and added to the molten steel, and is uniformly stirred; due to the short time, a large amount of supersaturated nitrogen dissolved in the molten steel is cast into The steel ingot was left in the molten steel before it was too large to overflow, resulting in a higher nitrogen content. Among the nitrided alloys such as manganese nitride, chromium nitride, and silicon nitride, only the melting temperature of manganese nitride is relatively low, about 1200 ° C, and the melting temperatures of chromium nitride and silicon nitride are all at 1600 ° C. the above. Therefore, when smelting high-nitrogen steel using a vacuum induction furnace, it is preferable to use manganese nitride.
Third, the impact of smelting parameters. Experiments show that as the temperature increases, the solubility of nitrogen in steel decreases, which may be due to the interaction of alloying elements in steel. Therefore, controlling the molten steel temperature and tapping temperature during smelting is an important factor to ensure the nitrogen content in the molten steel. In addition, some specifications and parameters of the vacuum furnace (such as the volume of the furnace chamber, the specifications of the smelting crucible, etc.) all affect the content of nitrogen in the molten steel to some extent. The furnace cavity is small, and the diffusion of nitrogen to the furnace cavity 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 that nitrogen participates in the overflow reaction, which is beneficial to Maintain a high nitrogen content in the molten steel. Further, when the steel contains a large amount of oxygen, the CO gas generated by carbon deoxidation easily induces the nitrogen dissolved in the molten steel to form bubbles and overflow, which is disadvantageous for obtaining a higher nitrogen content.