Developing Efficient Charges for Making Ultra-Clean Steel

Abstract

Structural steels used in products that are subjected to high temperatures and alternating loads in service must meet strict standards on their contents of phosphorus, sulfur, and nonferrous-metal impurities. It has been established that an increase in the steelmaking charge’s content of metallized materials directly affects the physico-mechanical properties of the product. In particular, such an increase improves the resistance of the steel to brittle fracture. Charges of different compositions have been developed and commercially tested to make ultra-clean steels for machine-building and special-metallurgy plants. Use of the charges is making it possible to reduce the total content of nonferrous-metal impurities in steel by a factor of 1.5 without significantly increasing the cost of the charge materials. Structural steel ‐ a material used in products that are subjected to large alternating loads and high temperatures ‐ must meet stringent requirements on its contents of sulfur and phosphorus (no more than 0.005% of each) and nonferrous-metal impurities (Cu  0.06%; As, Sn, Sb  0.005%). The steel’s contents of zinc, lead, and bismuth are also determined and included in the certification data. To prevent the rejection of continuous-cast semifinished products due to unsatisfactory ultrasonic inspection results or surface defects, the maximum allowable contents of Pb, Sb, Sn, Zn, and Bi are 0.0046% and 0.0065%, respectively [1]. However, the content of nonferrous metals in the steel of heats that were investigated reached 0.0112%. We made a study of the effect of the combined content of Cu, Sb, Sn, and As on property anisotropy and the coldshortness threshold for steel 20Kh2N4A, which is used to make shells. The contents of the other elements in the steel did not exceed the narrow prescribed ranges and all of the products were subjected to the same heat treatment. The test group of 40 heats was divided into two samples based on their combined contents of Cu, Sb, Sn, and As. The metal in each sample was characterized by a mechanical property distribution that was close to a normal distribution, which allowed us to use the methods of mathematical statistics with a 95% confidence level. The results of our analysis showed that a 30% increase in the combined content of the nonferrous-metal impurities increases the anisotropy of impact toughness 12%. Here, the absolute values of impact toughness in the longitudinal and transverse directions decrease by 10 and 21%, respectively. There is also a 25% drop in the cold-shortness threshold (Table 1). The main reason harmful impurities enter the steel is the scrap metal used in the charge. The scrap can be divided into three categories, depending on its origin: home scrap (metallurgical and foundry scrap); chips from machining; dormant scrap. The amount of dormant scrap in the scrap total and the steelmaking charge is continually increasing, which is accompanied by an increase in the content of contaminating elements in the finished steel and, accordingly, an increase in the amount of home scrap (HS) created. The technology traditionally used to make quality steel in the shops of special metallurgy plants and machine factories involves the use of a cold charge containing conversion pig iron, HS, and dormant scrap.