FORMATION OF NON-METALLIC INCLUSIONS DURING THE PROCESS OF THEIR MODIFICATION BY CALCIUM

Abstract

The paper shows that in the process of pouring and crystallization of steel, it is necessary to control and reduce the content of non-metallic inclusions. It was determined that micro-inclusions are too small to be transported in the slag, while they can collide and agglomerate and lead to defects in the finished product. It was established that a potential solution for controlling inclusions in steel is their chemical and/or physical modification in such a way as to minimize potential harmful effects and enhance the beneficial effect on the properties of steel and cast structures. Using the ternary phase diagram Al2O3-CaO-MgO, it was established that when treated in a ladle with calcium, all inclusions are in a liquid state. Data are presented on the thermodynamic possibility of the course of the reactions studied, depending on the content of S and Al in the melt above which CaS precipitation can occur for low-carbon steels. It is shown that the activity coefficients were calculated using standard methods. The dependence of the solidification rate on temperature for a certain amount of S and Al is shown and the distribution of dissolved substances is estimated using the Scheele equation. The distribution coefficients of S and Al in binary Fe-X systems are established. The equations for calculating the Gibbs energies for the corresponding chemical processes occurring during the modification of inclusions are determined. It is found that the growth rate of CaS decreases with increasing solidification temperature, since at higher temperatures the crystallization rate is slower compared to lower temperatures, which leads to a low rate of distribution of S and Al in the melt. It is shown that a decrease in temperature by 10 °C (from 1525 °C to 1515 °C) significantly increases the growth rate of d – iron (from 7 to 19.4 mm/s) and, thus, significantly increases the rate of S and Al inflow into the melt. It was found that the rate and growth of CaS inclusions occur in the diffusion region and can transform into globular or liquid inclusions, and sometimes solid inclusions can be formed. It was shown that solid CaS inclusions are less ductile and undesirable for further processing. In addition, solid inclusions containing CaS agglomerate and form clusters, at least when free surfaces are present. It was found that from the point of view of product quality, the formation of CaS during crystallization can be avoided if the growth rate of d-iron is high enough for a given size of the inclusions, or the initial inclusion is large enough for a given growth rate of d-iron to ensure its early absorption. Using electron microscopy methods, it was shown that globular liquid or semi-liquid Al2O3-CaO inclusions did not show a tendency to agglomerate with each other on the melt surface, even at small distances from the phase boundary; During the crystallization process, the release of dissolved substances from solid d-iron initiates a reaction between Al2O3-CaO inclusions with dissolved S and Al, which leads to a change in the size and shape of the inclusions due to the formation of a solid CaS phase on their surface. The thermodynamic conditions of the modification processes have been determined. It has been established that at the beginning of the modification process the process occurs in the kinetic region, and then in the diffusion region.