Nitrogen alloying of carbon and stainless steels by gas injection

摘要

The results of industrial trials of nitrogen alloying of carbon and stainless steels by bottom gas injection have been used to validate a global model of gas-metal reaction which couples a comprehensive description of gas and metal flows in the vessel (converter or ladle) and a description of local kinetics of nitrogen transfer at bubbles-metal interfaces. In contrast with the conditions prevailing for nitrogen removal in ladle vacuum treatments, in which the most active reaction zone is limited to the transit of flushing gas in the upper metal layers under the free surface, the active zones are in this case the plumes generated at the injection ports. A full description of dragged metal flow, nitrogen transfer from bubbles in the plumes and metal mixing out of the plumes is proposed. The effect of overlapping plumes is deduced from the results of the industrial trials. Similarly to the case of nitrogen removal, the local transfer kinetics across the gas-metal interfaces has to be described with a mixed control model combining interfacial kinetics and liquid phase mass transfer: 1. the simple surface site blockage model proposed by Belton was found adequate to describe the quantitative effect of the surface active elements, oxygen and sulfur, on the interfacial reaction rate. It was deduced, from a review of literature data, that the effect of chromium and nickel on the rate constant on uncontaminated interfaces can be adequately expressed by multiplying the rate for pure liquid iron by 1/f_N, where f_N represents the activity coefficient of dissolved nitrogen, 2. concerning the liquid phase mass transfer step, it was found that the very large increase of nitrogen diffusion coefficient with nickel content reported in the literature is consistent with the high initial yields obtained in the industrial data. The overall effect of reaction rate decrease due to surface active elements is somewhat less pronounced than for the reverse reaction of nitrogen removal. The decrease can still be quite large for drastically different condition (e.g. alloying in the converter during the oxygen blow, at 0.006%O, or during the post-blow stir, at 0.1 %O). It is rather limited when alloying killed steel of sulfur contents in the range 20-200 ppm in the ladle. In this range of sulfur contents, the detrimental effect of an increase in sulfur content on nitrogen removal during vacuum degassing had been found quite sizable. Application of the kinetic model provides, for both carbon and stainless steels, reliable guidelines on the effect of vessel geometry and treatment conditions (at atmospheric pressure or under vacuum), injectors arrangement, injected gas composition and flow rate and metal chemistry on the nitrogen alloying rates.