Improvement to the vacuum treatment of steel

Abstract

centration in a degassed metal was insufficiently low (0.004‐0.009%), and the long treatment time (up to 40 min), which was longer than the heat cycle in a continuous caster (about 30 min), restricted the series of casting to three heats. Therefore, we had to modernize the CDU. To improve the steel quality at OAO NLMK, in 2004, we cooperated with the VAI FUCHS Company (Austria) to modernize the circulating degassing unit equipment and, thus, widened the range of steels. We began to make automobile body sheet IF steels with an ultralow carbon content; electrical isotropic sheet steels (EISSs) with up to 3.25% Si and less than 0.005% C; and pipe, low-alloy, and structural steels with specified contents of dissolved gases and nonmetallic inclusions. The main efforts made to shorten the vacuum treatment cycle of a heat and to decrease the carbon concentration in a finished product were as follows: an increase in the capacity of the vacuum pump system, an increase in the steel circulation velocity through the vacuum chamber through the use of large-diameter branch pipes and increasing the carrier-gas flow rate, and the application of forced decarburization. Upon modernization, we installed a new steam-jet ejector pump, which provided a rarefaction of less than 100 Pa in the vacuum chamber at the time of reaching the working residual pressure of 4 min, and installed a new high-speed ferroalloy feed system. The chamber was equipped with a lateral multifunctional T-COB (Technometal Combined Blowing) tuyere, which substantially broadened the possibilities of the unit. The installed technical equipment permits heating of the vacuum-chamber lining, chemical heating of the steel during treatment (lateral T-COB tuyere), control of the residual concentration of dissolved oxygen at the end of the decarburization stage (MultiLab Celox system), and precise alloying of steels in a narrow range. Figures 1 and 2 show the mass fraction of carbon in a metal at various technological stages of steelmaking before and after the modernization of the CDU.