How to reduce the phosphorus content of stainless steel

Phosphorus is one of the most harmful elements in steel, especially in stainless steel. It not only intensifies the composition segregation of stainless steel, but also has a very adverse effect on the resistance to pitting corrosion, stress corrosion resistance and welding performance of stainless steel. Reducing the phosphorus content in stainless steel has always been a concern of metallurgists. Until now, this problem has not been well resolved, and it is still the largest research topic in refining technology. At present, stainless steel smelting mainly adopts electric arc furnace return oxygen blowing method and out-of-furnace refining method. The steel material is recycled again and again, making the phosphorus in the steel more and more enriched. In addition, the addition of ferrochromium and the like brings in part of the phosphorus, and the molten steel is prone to the phenomenon that the phosphorus content exceeds the standard. Therefore, the problem of dephosphorization of stainless steel is becoming more and more prominent.
     The stainless steel dephosphorization technology in research and development can be roughly divided into two categories, namely oxidative dephosphorization and reductive dephosphorization. The oxidative dephosphorization process requires BaO-based or CaO-based alkaline slag, while the reductive dephosphorization requires Ca or calcium alloys. No matter which route it is, the core problem is to remove phosphorus in steel to the greatest extent under the condition of maintaining chromium. However, since the alloy steel required by modern industry contains high alloy components, such as chromium, manganese, etc., if such high alloy steel is dephosphorized by oxidation, a large amount of alloy elements will be oxidized while dephosphorizing; On the other hand, phosphorus in ferroalloys has always been difficult to remove, especially those ferroalloys produced under reducing conditions. In recent years, the production of low-phosphorus steel and ultra-low-phosphorus steel has increasingly stringent requirements on the phosphorus content in ferroalloys. Therefore, people hope to develop an effective dephosphorization method under reducing conditions to achieve the purpose of dephosphorization while preserving chromium. In the steelmaking process, there are two ways for phosphorus in molten steel to enter the slag, namely oxidative dephosphorization and reductive dephosphorization. When the oxygen potential of the system is lower than the critical value, the phosphorus element will mainly enter the slag in the form of P3- (reductive dephosphorization), and the lower the oxygen potential of the system, the easier this process will be. Calcium metal and calcium alloys (including calcium carbide, silicon-calcium alloys) are most commonly used as reductive dephosphorization agents, as well as aluminum-magnesium and calcium-aluminum alloys.

     Experiments have shown that this purpose can be achieved by using calcium-based slag system for dephosphorization under reducing atmosphere. Moreover, the reduction dephosphorization method can enable iron and steel enterprises to use soft iron with high phosphorus content and low price and recycled materials as raw materials, and greatly increase the yield of alloys, thereby reducing steelmaking costs. Therefore, the reduction dephosphorization method has more research significance and economic value.

Effects of Various Factors on Dephosphorization Rate
     The dephosphorization rate is closely related to the carbon content of molten steel. For example, when Ca is used for dephosphorization, high carbon will promote the reaction, thereby rapidly consuming metallic calcium, resulting in a decrease in dephosphorization rate. When CaC2 is used as a dephosphorization agent, if the carbon is high, CaC2 will be decomposed insufficiently, reducing the amount of decomposed metal. Calcium affects the effect of dephosphorization, so the low carbon content in molten steel can speed up the dephosphorization reaction. The effect of oxygen content in steel on dephosphorization must also be considered during reductive dephosphorization, because oxygen in steel directly affects the utilization rate of (Ca). The conditions for obtaining >50% are: aC<0.92 and aO<4×10-4. Theoretically speaking, chromium in reduction dephosphorization does not combine with metallic calcium and does not participate in the reaction, so there should be no loss of chromium in molten steel. But in fact, the chromium content will affect the dephosphorization rate. Studies have shown that the reason why chromium content affects dephosphorization rate is its effect on carbon activity. When using metallic calcium for dephosphorization, the lower the temperature, the higher the dephosphorization rate. When CaC2 is used for dephosphorization, the dephosphorization speed is fast at high temperature, and the opposite is true at low temperature. However, from the overall results, the temperature has little effect on the dephosphorization rate. Therefore, considering the overall economic benefits in actual production, there are generally no special requirements for temperature (except for pure calcium dephosphorization). When carrying out reductive dephosphorization experiments with crucibles of different materials, it was found that under the same experimental conditions, it was almost impossible to dephosphorize with graphite carburization; with Al2O3 crucibles, the dephosphorization rate was low; dephosphorization in MgO, CaO and dolomite crucibles The rate is higher. As a flux, CaF2 is not only cheap, but also has a low melting point, which is easy to form low-melting point compounds with other components, which is convenient for slagging; it can dissolve dephosphorization agents Ca, CaC2, reduce the vapor pressure of calcium, and reduce its volatilization loss; reduce phosphating The activity of calcium promotes the smooth progress of the dephosphorization reaction, so it is widely used. However, during dephosphorization by powder spraying, the effect of CaF2 in the slag is not obvious due to good kinetic conditions and enlarged reaction surface.
     With the recycling of scrap steel, the phosphorus in the steel will accumulate more and more because it cannot be effectively removed by oxidation, resulting in the phosphorus content of some stainless steel returns on the market as high as 0.047-0.087%. Therefore, in-depth research, improvement and development of stainless steel dephosphorization methods have become an increasingly urgent issue.
     With the increase of alloy elements such as Mn and Cr in the alloy, the activity of phosphorus decreases and the melting point of the alloy increases, so that oxidative dephosphorization can hardly be carried out, especially in the neutral or reducing atmosphere in the vacuum induction furnace, smelting stainless steel Special alloys such as superalloys and superalloys can only be reduced and dephosphorized. The SiCa-CaF2 method is the only reductive dephosphorization method that has been applied in a small amount on an industrial scale, and it is relatively easy to remove the increased silicon by oxidation. The Ca-Al, and A1-Mg reduction dephosphorization method is relatively simple in operation process, and the dephosphorization rate is slightly improved. It can be applied to the smelting of steel types in which A1 and Mg exist as beneficial elements.
     Although the reductive dephosphorization method has defects such as difficult to handle dephosphorization products, easy to pollute the environment, complex processes before and after, and high requirements for equipment, the dephosphorization rate is high because the reductive dephosphorization method does not lose expensive alloy elements (such as Cr, Mn, etc.) , so the reduction dephosphorization method can enable iron and steel enterprises to use high-phosphorus, low-cost soft iron and recycled materials as raw materials, and greatly increase the yield of alloys, thereby reducing steelmaking costs. Therefore, the reduction dephosphorization method has more research significance and economic value.