Phosphorus is one of the most harmful elements in steel, especially in stainless steel. It not only aggravates the composition segregation of stainless steel, but also has a very adverse effect on the resistance to corrosion, stress corrosion and welding of stainless steel. Reducing the phosphorus content of stainless steel has always been a concern for metallurgical workers. Until now, this problem has not been solved well and remains the largest research topic in refining technology. At present, the main use of stainless steel smelting furnace back to the oxygen blowing method and furnace refining method. Steel material is recycled again and again, making the phosphorus in steel more and more enriched. Coupled with the addition of ferrochrome and other parts into the phosphorus, molten steel is prone to phosphorus content exceeds the standard phenomenon. Therefore, the problem of dephosphorization of stainless steel is more and more prominent.
Research and development of stainless steel dephosphorization technology can be divided into two categories, that is, oxidative dephosphorization and dephosphorization. Oxidative dephosphorization requires BaO-based or CaO-based alkaline slag, while dephosphorization requires Ca or calcium alloy. No matter which route, the core issue is to minimize the phosphorus in the steel under the condition of keeping chromium. However, due to the high alloy composition required by modern industry, such as chromium and manganese, if such high-alloy steel is oxidized and dephosphorized, a large amount of alloying elements will be oxidized while dephosphorizing. On the other hand, phosphorus has always been hard to remove in ferroalloys, especially those produced under reducing conditions. In recent years, the production of low-phosphorus steels and ultra-low-phosphorus steels has become more stringent on the phosphorus content of ferroalloys. Therefore, it is hoped that a dephosphorization method that is effective under reducing conditions will be developed to achieve the goal of maintaining chromium and dephosphorization. In the process of steelmaking, there are two ways for phosphorus in molten steel to enter the slag, that is, oxidative dephosphorization and reductive dephosphorization. When the system oxygen potential is lower than the critical value, the phosphorus element will enter the slag mainly as P3-form (reductive dephosphorization), and the lower the system oxygen potential, the easier the process proceeds. People are most commonly used to restore dephosphorizer metal calcium and calcium alloy (including calcium carbide, calcium-calcium alloy), in addition to aluminum-magnesium, calcium-aluminum alloy.
Experiments show that the use of calcium-based slag system in a reducing atmosphere dephosphorization can achieve this goal. And the reduction of dephosphorization method can make the iron and steel enterprises using phosphorus containing high, low price of soft iron and return material as raw materials, but also greatly improve the recovery rate of the alloy, thereby reducing the cost of steelmaking. Therefore, the reduction of dephosphorization method has more research significance and economic value.
The impact of various factors on the dephosphorization rate
Dephosphorization rate and liquid steel carbon content is closely related. For example, in the case of dephosphorization with Ca, carbon high promotes the reaction and quickly consumes metallic calcium, resulting in a decrease in dephosphorization rate. When CaC2 is used as a dephosphorization agent, CaC2 is not sufficiently decomposed when carbon is high and the amount of decomposed metal is reduced Calcium, affecting the dephosphorization effect, therefore, the low carbon content in molten steel to speed up the dephosphorization reaction. Reducing dephosphorization must also consider the effect of oxygen content in steel on dephosphorization because oxygen in steel directly affects (Ca) utilization. The conditions for obtaining> 50% are: aC <0.92 and aO <4 x 10-4. In theory, the reduction of dephosphorization of chromium is not with calcium metal, do not participate in the reaction, so the liquid chromium should be no loss. In practice, however, chromium levels affect the rate of dephosphorization. The research shows that the reason why the chromium content affects the dephosphorization rate lies in its influence on the carbon activity. Dephosphorization with metallic calcium, the lower the temperature, the higher the dephosphorization rate. With CaC2 dephosphorization, high temperature dephosphorization speed, low temperature is the opposite. However, from the overall result, the temperature has little effect on the dephosphorization rate. Therefore, in the actual production to take into account the overall economic benefits, the general temperature may not make special requirements (except dephosphorization of pure calcium). Dephosphorization experiments with crucibles of different materials showed that almost no dephosphorization could be carried out by carbonation with graphite under the same experimental conditions; Dephosphorization was lower with Al2O3 crucible; dephosphorization was carried out in MgO and CaO and dolomite crucibles Higher rate. As a flux of CaF2 is not only cheap, and lower melting point, easy to form low-melting compounds with other ingredients, easy slag; can dissolve the dephosphorization agent Ca, CaC2, reduce the vapor pressure of calcium to reduce the volatilization loss; Calcium activity, to promote the smooth dephosphorization reaction, it is more widely used. However, when powder dephosphorization, due to good kinetic conditions, the reaction surface increases, so the role of slag CaF2 is not obvious.
With the recycling of scrap steel, phosphorus in steel can not be more effectively removed due to the oxidation process, resulting in the phosphorus content of some stainless steel return materials on the market being as high as 0.047 to 0.087%. Therefore, in-depth study, improvement and development of stainless steel dephosphorization has become an increasingly pressing issue.
With the increase of alloying elements such as Mn and Cr in the alloy, the activity of phosphorus decreases and the melting point of the alloy increases, so that the oxidative dephosphorization can hardly proceed. Especially in the neutral or reducing atmosphere of the vacuum induction furnace, the stainless steel , Superalloy and other special alloys can only be reduced dephosphorization method. The SiCa-CaF2 process is the only reductive dephosphorization with a small number of applications on an industrial scale, and it is easier to oxidize the added silicon. Ca-Al ,, Al-Mg reduction dephosphorization method is relatively simple operation process, a slight increase in dephosphorization rate, can be used in the smelting of the existing Al, Mg as beneficial elements.
Although the dephosphorization method is difficult to deal with dephosphorization products, easy to pollute the environment, complex processes before and after the device and the higher equipment defects, but because of the dephosphorization reduction method does not lose expensive alloying elements (such as Cr, Mn, etc.), high dephosphorization rate , So the reduction of dephosphorization can make the iron and steel enterprises with high phosphorus, low-cost soft iron and return material as raw materials, but also greatly improve the yield of the alloy, thereby reducing the cost of steelmaking. Therefore, the reduction of dephosphorization method has more research significance and economic value.