Elimination of casting porosity in stainless steel butterfly valve body and control of steelmaking raw materials

(1) Proportion control of new material and returned material. The main chemical composition of SAF2507 super duplex stainless steel is shown in Table 1. The metal materials used in SAF2507 steelmaking mainly include new dual phase stainless steel and return alloy steel. The composition of new ingredients after smelting can meet the composition requirements of SAF2507 shown in the table below, but the returned steel includes alloy waste castings, pouring risers, pouring residues, waste materials of machining workshop, etc.

These materials are usually stored in the open air for more than two days or more, and SAF2507 is very easy to oxidize in the air and carry moisture easily. Although P, s and other harmful elements in the melted steel do not exceed the standard, they are very close to the standard content boundary value, which is extremely harmful to various properties of the steel. Therefore, there is an appropriate proportion between new material and returned material during smelting, and usually the new material is not less than 70%. In this way, the returned material is reasonably used in the smelting process, the element content does not exceed the standard, and the water hazard is greatly reduced.

(2) Control of slagging materials. There are two kinds of slagging materials used in the smelting process of SAF2507, one is fluorite (CAF) 2, and the other is stainless steel slagging degassing agent. The addition of fluorite improves the fluidity of slag and promotes the rapid chemical reaction between steel and slag. However, fluorite can not absorb harmful gases such as hydrogen in the charge, and when the air humidity is high, the content of hydrogen and free hydrogen in the charge is very large, which is very easy to form casting pores.

According to the strong suction capacity of SAF2507, professional stainless steel slagging degassing agent is used at present. Stainless steel slag degassing agent can not only effectively remove various oxidized inclusions and harmful impurities in molten steel, but also remove hydrogen. The main principle is to improve the surface tension and interfacial tension between liquid steel and molten slag, make it easier to separate liquid slag, and form a protective layer on the surface of liquid steel to play the role of thermal insulation and effectively prevent the oxidation and suction of liquid steel. The molten steel is purified by removing slag and hydrogen, which greatly reduces the incidence of casting porosity defects.