Large steel castings in the fields of nuclear power, hydropower, thermal power and metallurgical equipment have strict service conditions and high performance parameters. They are required to have excellent room temperature performance, high temperature performance, manufacturing performance and heat treatment performance. Precise control technology of composition, removal technology of impurities and harmful elements, vacuum purification and degassing technology in metal smelting of large steel castings play an important role in ensuring the performance of large steel castings. At present, with the development of manufacturing technology of large steel castings, both at home and abroad are developing to high-purity liquid steel. In the production of large steel castings, the open hearth equipment is basically eliminated in the smelting of liquid steel, the electric furnace is mainly developed, especially the high-power DC electric arc furnace, and the secondary refining is actively promoted and applied by out of furnace refining and vacuum treatment technology.
Refining operation is the key process to improve the high-quality liquid steel of castings. At the same time, the scientific control of refining process is the key link to ensure the smooth pouring. Its main purpose is to ensure that the composition, temperature and time nodes of pouring liquid steel meet the pouring requirements. At present, a series of out of furnace refining technologies widely used in metallurgical industry, such as LF (ladle) furnace method, RH furnace method and VD furnace method, are widely used in some large heavy machinery plants. These equipment can effectively decarbonize, desulfurize and remove [H], [O], [n] and other gas impurities in molten steel. Practice shows that the desulfurization rate of ladle furnace can reach 60%, the sulfur content of finished products can be reduced to less than 0.005%, and the hydrogen content can be reduced to 2 × Below 10 – 6, the oxygen content decreases to (30 ~ 40) × 10- 6。
For example, in the production process of hot-rolled and cold-rolled double plate mill stands, the molten steel is refined by converter + LF or SKF furnace. By determining a reasonable refining process, sufficient refined molten steel is provided at one time in the smelting process, so as to finally ensure that the composition and temperature of molten steel, especially the [S] and total aluminum content in steel, meet the requirements, and the hot-rolled stand meets the German din1681 standard, The cold rolling stand meets the Japanese jisg5101 standard. Stainless steel widely used in hydraulic turbine castings often needs to reduce the carbon content as much as possible in the smelting process. Due to the high Cr content in stainless steel, decarburization is very difficult. Therefore, special methods are usually used for the refining of this kind of alloy. At present, VOD (vacuum oxygen decarburization) method and AOD (argon oxygen decarburization) method have become the most common stainless steel refining process. These refining processes can better achieve the purpose of reducing carbon and maintaining chromium in stainless steel smelting. By properly controlling the process, it can smelt [Cr] 20% ~ 30% and [C + n] ≤ 100 μ G / g ultra low carbon stainless steel.
Since some of the above liquid steel refining methods have been widely used in the production of continuous casting and rolling and slab continuous casting in the iron and steel metallurgical industry, the metallurgical industry has advantages over the general machinery industry in refining liquid steel for large castings and controlling the composition of materials. At present, China’s heavy machinery industry also has a number of off furnace refining equipment, which has played an important role in production; However, from the overall situation, a number of small and medium-sized heavy machinery plants do not have refining equipment. If they have refining equipment, the equipment utilization rate is not high, and some important refining processes and related technologies are still in the development stage.
Electroslagis a casting process that combines electroslag remelting refining with casting solidification forming (metal mold) to complete casting forming at one time. The castings produced not only have good internal quality, but also have high material properties. The main advantages of electroslag casting guide vane are:
① It reduces the sulfur content (0.007% ~ 0.015%) and impurity content in the casting, and makes the structure compact, so it has high toughness, fatigue resistance and welding performance;
② Ultrasonic flaw detection meets cch70-3 and ASTM A609 standards! 3. Equivalent level 1 requirements;
③ The cavitation resistance is 30% higher than that of ordinary;
④ Small blank machining allowance and high material utilization rate, up to more than 78%;
⑤ Electroslag casting process can produce electroslag casting special castings with a single weight of 0.1 ~ 20 t.
In recent years, Shenyang Foundry Research Institute has provided tens of thousands of tons of high-quality electroslag casting products such as turbine guide vanes for dozens of large and medium-sized power stations at home and abroad, such as the Three Gorges. However, there are still many technical problems to be solved in the production of larger electroslag casting castings.