CF8C material is generally selected as valve casting material because of its small deformation, high strength, wear resistance and corrosion resistance when used in high temperature and high pressure environment. The standard cf8c material composition requires sulfur and phosphorus content to be s ≤ 0.040% and P ≤ 0.040% respectively, with high sulfur and phosphorus content. With high sulfur content, sulfur and hydrogen form hydrogen sulfide in the use of valve castings. Hydrogen sulfide can cause stress corrosion cracking of metal materials at room temperature and rapid and uniform corrosion of metal materials at high temperature and high pressure, which can not meet the requirements of high temperature and high pressure environment of valve castings. In order to meet the application requirements of high temperature and high pressure valve castings, the sulfur content of harmful elements is strictly controlled below 0.015% and the phosphorus content of harmful elements is controlled below 0.020%.
The alloy element content of cf8c material is high, the carbon content is low, and the sulfur and phosphorus content is low. The following problems mainly exist in the smelting process of electric arc furnace return oxygen blowing method: the alloy elements are burned seriously in the smelting process; Long smelting time; The furnace lining is seriously burned. Casting defects such as chromium oxide film, cold barrier, surface wrinkle and inclusion are easy to occur on the surface of liquid steel during pouring.
In order to solve the problems existing in the smelting process of cf8c material and obtain qualified chemical composition, the following measures are mainly adopted.
Proportioning: 90% of the furnace charge adopts the high alloy steel return material of this steel type. Before charging, FeSi with the weight of 1.0% ~ 1.5% of liquid steel is added to the furnace bottom, and Ni is added to the upper middle line.
Melting period: when the temperature of molten steel is ≥ 1600 ℃ after the furnace charge is melted, judge whether to discharge slag according to the slag condition, and ensure that oxygen blowing decarbonization is carried out under thin slag.
Oxidation period: take samples to analyze the composition of molten steel, adjust and control the composition of Si to 1.0% ~ 1.5% of the weight of molten steel, and the composition of Ni to 9.5% ~ 10.5% of the weight of molten steel. Calculate the chromium carbon mass ratio of molten steel composition and determine the oxygen blowing temperature (1705 ~ 1835 ℃). When the temperature of molten steel in the furnace meets the requirements, increase the oxygen blowing pressure to 1.5 ~ 1.8 MPa, and continuously blow oxygen with two oxygen blowing tubes for 18 ~ 22 minutes, Observe the furnace atmosphere and the color of oxygen blowing flame, take samples again to analyze the composition of liquid steel, and adjust the composition of C to ≤ 0.03% of the weight of liquid steel.
Reduction period: add Fe Cr alloy in the case of white slag with good deoxidation, stir the molten steel in time, push the solid Fe Cr alloy block exposed on the slag surface into the molten steel, add reducing agent in batches to recover Cr in the slag, and use the power supply mode of voltage 210 V and current 10 Ka when the added alloy is exposed on the slag surface; If the alloy is buried under the slag surface, the power supply mode with voltage of 180 V and current of 8 ka shall be adopted. Short arc operation is strictly prohibited in the reduction period, and the electrode drop shall be synchronous.
In order to solve the casting defects such as cold lap, surface wrinkle, inclusion and chromium oxide film on the surface of liquid steel due to the poor fluidity of liquid steel, the pouring temperature should be appropriately increased, the pouring system of valve casting should be reasonably designed to shorten the pouring time, and argon protection pouring should be implemented at the same time.