Improvement and optimization of ultra-low carbon martensitic stainless steel smelting process
Ultra-low carbon martensitic stainless steel (06Cr13Ni46Mo and 06Cr16Ni46Mo) is an important material usually used for the upper crown, lower ring, blades, and guide vanes of important parts of hydraulic turbines. This series of stainless steel materials are difficult in smelting technology and have high requirements for product quality.
Especially when manufacturing the Three Gorges hydropower unit, the three major domestic hydropower equipment manufacturing companies have formulated domestic and foreign hydropower manufacturing technologies and standards. The Three Gorges standard for key components of hydropower equipment (the material requirements involved in this standard represent the highest level in the domestic industry). The composition requirements of molten steel are: ω(C)≤0.05%, ω(P)≤0.028%, ω(S) ≤0.015%, ω(Si)≤1.00%, ω(Mn)≤1.00%, ω(Cr)≥12.45%, ω(Ni)=3.5～4.5%, ω(Mo)=0.40～1.00%, ω( O)≤80×10-6, ω(N)≤150×10-6, ω(H)≤3×10-6.
In order to produce high-quality stainless steel castings, domestic and foreign manufacturers mainly use electric arc furnaces + refining (AOD, VOD, LF, etc.). Among them, the use of AOD to smelt stainless steel has become the leading technology in stainless steel refining. At present, when domestic and foreign manufacturers produce this series of stainless steel materials, cracks are more likely to occur during production and subsequent processing, and cracks occasionally occur during use. Therefore, in order to improve the quality of this series of material castings, it is necessary to improve the overall performance of the castings by improving the inherent quality of molten steel.
The technical staff of Dongfang Electric Group aimed at the problems of long decarburization time, large oxygen consumption, high molten pool temperature, and long high-temperature residence time in the company’s original stainless steel smelting process. Through process improvement and the use of an AOD refining furnace, the quality of molten steel was improved.
As we all know, the key to oxygen blowing and decarburization is to control the reaction of oxygen in molten steel with carbon, chromium, silicon, manganese, and other elements. Based on the principle of steel metallurgy, combined with the use of AOD, technicians have improved and optimized the main process parameters of the original stainless steel smelting.
The AOD method uses argon and oxygen gas to blow molten steel into the molten pool from the side of the furnace bottom in the form of mixed gas. In the blowing process, the partial pressure of CO is diluted by argon and is reduced, which greatly facilitates the decarburization and chromium preservation during stainless steel smelting.
In the reduction stage of AOD refining, due to the addition of ferrosilicon and limestone slag for deoxidation, and the intense stirring of argon, the molten steel can be deeply desulfurized; the intense stirring of argon during the AOD refining can promote the separation of oxides in the molten steel,
Ascending, the cleanliness of the molten steel is improved, so the content of inclusions in the molten steel is small, and there are almost no large-particle inclusions. The main form of inclusions has also changed from electric furnace method manganese silicate to calcium silicate.
In addition, the AOD method can also remove lead from steel during the converting process. Studies have shown that a certain amount of lead in stainless steel will affect its hot workability, and after AOD blowing, the ω (Pb) in the finished steel is less than 10×10-6, which will not affect the performance of the steel at all.
Through process improvement and the use of AOD, high-quality molten steel is obtained. Using Si-Ca and Si-Fe composite deoxidation, molten steel ω(O) is reduced to 80×10-6; activated lime is used to strengthen desulfurization, so that ω(S ) Is reduced to less than 0.015%, which greatly improves the cleanliness of molten steel, and meets the requirements of melting composition and various physical and chemical indicators; the content of harmful gas components and spherical oxide inclusions in molten steel (the spherical oxide rating does not exceed 2.0) is greatly reduced; The improvement of the purity of molten steel enhances the ability of the casting matrix to resist cracks.