REDUCING THE CONTENT OF NONMETALLIC INCLUSIONS IN STEEL FOR PLATES

Abstract

The content of nonmetallic inclusions in steel is one of the primary indicators of the quality of the finished product. Customers’ requirements on the overally quality of steel and on its nonmetallic inclusion content in particular have become more stringent. For example, products must conform to class 2‐3 under standard SEL072, class 0‐1 under standard GOST 22727, and class A under standard EN 160-85. To improve the quality of its flat-rolled products and retain its market reputation for high-quality steel alloyed with niobium, vanadium, chromium, nickel, molybdenum, and boron in different combinations, as well as for products destined for the domestic market (skelp for high-pressure gas-line pipe, plates and slabs for deep-sea drilling platforms, structural and bridge steel, ship plate, and high-strength boiler steel), the combine has had its specialists devise an integrated technology for obtaining continuous-cast semifinished products that keeps nonmetallic inclusions to a minimum. Stabilizing the carbon content of the steel within certain limits is the main way to lower the state of oxidation of the steel and prevent its contamination by nonmetallic inclusions. To this end, we modified the operating and design parameters of the nozzle block in the oxygen lances. This improved the blowing and slag-formation regimes in the converter process and stabilized carbon content at the sampling point (the carbon content now goes no lower than 0.05%), which in turn has allowed better control over the steelmaking operation as a whole. The quality of the deoxidizers used for the steel in large part determines the extent to which the steel will be contaminated by nonmetallic inclusions. The replacement of recycled aluminum of grade AV87 by primary aluminum reduced the amount of nonferrous-metal impurities that entered the steel. Those impurities tend to form low-melting eutectics along the boundaries of the dendrites during crystallization. The presence of these eutectics significantly weakens the boundaries in the cast steel and in the rolled product and ultimately increases the number of products rejected for defects during ultrasonic inspection. Blowing the entire melt with argon in the ladle with the use of a submersible lance creates the conditions necessary for the nonmetallic inclusions to be transferrd from the liquid metal to the slag and subsequently assimilated by the slag phase. When aluminum wire rod was added to the ladle instead of pig aluminum to correct the steel’s aluminum content, the degree of assimilation of this element increased from 45‐50% to 70‐75% and the total amount of aluminum used on the finishing unit decreased while the aluminum content of the finished steel remained stable (0.027‐0.035% Al). This ensured adequate deoxidation of the steel and attainment of the required set of mechanical properties. The reduction in aluminum consumption also decreases the degree of contamination of the steel by inclusions of alumina. The treatment of steel with calcium-silicon is known to improve the ductility properties of the rolled product. We increased the consumption of calcium-silicon of grade SK-25 or SK-30 to 1.5‐2.5 kg/ton steel by injecting it in a carrier gas (argon). In a variant, we also increased the consumption of calcium-silicon to 1.1‐1.6 kg/ton by introducing cored wire which