Study On RefiningTechnology And Mechanism Of Cast Stainless Steel

With the fast economic and social development in China, the stainless steel (SS) is more and more widely used. The yield of the SS castings is also increasing rapidly. The major smelting method used in SS production in China is the non-oxidizable process in an induction furnace. The advantage of this process is that it is simple and cost-effective. However, it has numerous disadvantages too, for example, the large amount of non-metal inclusions in molten steel and the poor mechanical properties and poor corrosive resistance because of the lack of carbon decreasing ability and the refining process for the melt. Consequently, it would be of great valuable to improve the SS properties through refining and modification by trace elements under the conditions of non-oxidizable process. Moreover, as a most popular kind of SS——18-8 austenitic stainless steel, the solution heat treatment is usually necessary in most cases to improve its mechanical properties and corrosive resistance. Meanwhile, the temperature of the solution treatment is high and the corresponding energy consumption is large. At the same time, the castings tend to be deformed severely, which is prohibitive in some castings, such as a marine propeller. As a result, it is necessary to control the formation of the as-cast intergranular Cr23C6 carbide and to improve the as-cast SS properties, which can widen SS applications in relevant fields. Furthermore, it is an important to decrease the inclusions in SS and to improve the mechanical properties and corrosive resistance of SS to more effectively remove the surface slag in the later stage of the smelting process and to cover molten steel in a good manner in order to prevent its oxidation. It is, therefore, urgent for the improvement of SS casting quality to study mechanisms of the collection and coverage of the slag-remover and to develop suitable processing methods to improve its properties. The studies related to refining, modifying, and slag-removing on 18-8 austenitic stainless steel, which is necessary to improve the mechanical properties and corrosive resistance of SS, were carried out in this paper. The main contents are the following: 1) The effects of alloying elements such as rare earth (RE), calcium, barium, magnesium etc. on the mechanical properties and corrosive resistance of cast SS and the action mechanism were studied in detail through experiments. 2) The effects of trace elements, such as zirconium, vanadium, niobium, which are strong carbide forming elements, on as-cast mechanical properties and corrosive resistance of SS were studied systematically through orthogonal experiments. 3) The refining effect of the cored-wire feeding method on the SS was studied under the conditions of actual production. 4) The correlation between the moisture content of perlite and its swelling performances was studied, the process to control the moisture content was developed, and a controllable light swelling technology was developed. The main conclusions are the following: 1) The complex silicate-based refining agent with rare earth (RE), calcium, barium, magnesium etc. exhibited good refining effects on the cast SS. It can reduce the residual amount of inclusions and change their morphology and distribution and improve the mechanical properties and corrosive resistance. 2) Ttrace elements, such as zirconium, vanadium, niobium etc., which are strong carbide producer, clearly influenced the mechanical properties and corrosive resistance of as-cast SS. The as-cast mechanical performance of SS with these trace elements increased to a point higher than that required by the national standard (heat-treated state). The corrosive resistance of this steel was improved, too. The corrosion rate of as-cast samples in 65% HNO3 was much smaller than the value required by the national standard (forging state). Meanwhile, the pit corrosion rate of anti-FeCl3 was slightly larger than that in the national standard (forging state). 3) The trace elements, such as zirconium, vanadium, niobium etc., can hinder the formation of intragranular Cr23C6 carbide and refine SS microstructure through nucleus formation. Furthermore, the element of zirconium can improve the morphology and distribution of inclusions in SS by virtue of the formation of complex inclusions. 4) The trace elements, such as zirconium, vanadium, niobium etc., have interaction effects with carbon, which reduce the disadvantageous effect of high carbon content on the mechanical properties, especially for corrosive resistance. The causes lay in that these strong carbide forming elements reacted with carbon superior to chromium, which solve the problem of intergranular depleted chromium in high carbon SS. This has the great significance in actual application. 5) Under the conditions of real production with large bulk smelting, the cored-wire feeding can ensure the absorption of the refining agent by sending it into the interior area of the melt, and preventing its losses from oxidation at the interface, which will promote the absorption of elements with high vapor pressure, such as calcium, magnesium etc. Therefore the cored-wire feeding can acquire higher absorption efficiency than other methods. Under the conditions of cored-wire feeding, the SS refining agent had great influences on changing the character of inclusion. The inclusions in the SS processed by the complex refining agent were generally in a complex state. The total amount of inclusions became fewer, the morphology appeared circular and their distribution changed from a nested style to a dispersed one, which has advantages to improve the mechanicalproperties and the corrosive resistance. 6) The real cause for slag-remover swelling was related to the moisture content of perlite. The different moistures had different effects on the swelling character of perlite and experienced different volatilization processes when heated gradually with low temperature conditions. Based on this result, a pretreatment process for slag-remover, which can control the moisture content of perlite and further control its swelling character, was developed and licensed as a national patent of invention. 7) A controllable slight swelling process of perlite was discovered through experiments. By virtue of the operational parameter regulations, such as process temperature, treatment time etc., the swelling extent of perlite can be controlled within a certain range. The lightweight slag-remover with a micro-pore structure is more effective.