According to current practice, the desulphurisation of steel melts is mainly performed in the ladle with lime saturated top slags. The injection of argon gas into liquid steel provides for intensive mixing and accelerated sulphur transfer. During gas stirring treatment, an emulsification of top slag takes place. A numerical model describing the course of the desulphurisation reaction has been developed, which takes all relevant process parameters into account, including those of the emulsification effect. To check the validity of the model, operational tests have been performed on 185‐t‐heats. Results obtained from model calculations and operational tests show an excellent agreement.

The desulphurisation of liquid steels with a refining ladle top slag is one of the most important processes of secondary metallurgy. But the process control is even now based on empirical rules. Deviations from the sulphur contents aimed at can often be observed in practice. An improvement of the process control supported by knowledge of the thermodynamics and kinetics of the desulphurisation reaction is of great importance, especially for the production of steels with very low sulphur contents. To investigate the removal of sulphur, operational trials were carried out in 185 t steel ladles. In the trials the stirring gas flow rate, the pressure on the bath surface and the steel composition were varied as process parameters. The obtained results show that ladle slags saturated with lime have a high sulphide capacity. The desulphurisation rate of steel melts increases if the specific stirring energy is enhanced by increasing the gas flow rate or reducing the pressure on the bath surface. In the case of production of Al/Si‐killed steel melts, a desulphurisation degree above 90 % can be obtained by a vacuum treatment within 10 minutes.

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within the iron matrix can diffuse to cause a rise in hydrogen pressure at matrix defects. Consequently, internal cracking Hydrogen in steels causes various types of cracking, and flaking may occur in heavy sections of crack sensitive which occur when the amount of hydrogen in a steels if the hydrogen content is too high. steel reaches a critical level. This phenomenonIn general there is a trend of higher diffusivity with results from excessive internal hydrogen pressure, increasing temperature in solid iron. The interstitial elements, and is associated with the formation of cracks at i.e. oxygen, nitrogen, carbon, boron, and hydrogen have material imperfections, for example non-metallic diffusivities that are around several orders of magnitude inclusions. Consequently, cracking can be prevented greater than those of the substitutional elements. Because either by avoiding an excessive amount of of larger interatomic spacing, the diffusivities of interstitials hydrogen or by increasing the critical level for in body centred cubic iron (a-, d-Fe) are greater than those cracking. These two cases are demonstrated from in face centred cubic iron (c-Fe). On account of this solubility the points of view of steelmaking and plate behaviour, the hydrogen attempts to escape from solid steel production. In certain applications, the hydrogen by means of diffusion. The diffusion coefficient and, hence, content in a steel may be increased by the the diffusion velocity are strongly temperature dependent. absorption and diffusion of atomic hydrogenThe diffusion of hydrogen can occur both to the surface produced on the metal surface by a corrosion with subsequent escape from the steel and in the direction reaction, for example in a wet H 2 S environment. of the slab or plate centre (Fig. 2). During diffusion into This phenomenon can lead to hydrogen induced the slab or plate centre, hydrogen atoms react with each cracking (HIC). The damage mechanism as well as other and accumulate as molecular hydrogen gas, both in the main strategy to prevent hydrogen induced lattice defects and at inclusion interfaces. In some cases, cracking is described in detail.I&S/1508 the resulting hydrogen pressure can exceed the strength of the steel and lead to material cleavage. Whether such

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