Thermodynamic Evaluation of Formation of Oxide–Sulfide Duplex Inclusions in Steel

Abstract

Calcium is widely used for improving the castability of liquid steel, as well as for improvement of steel cleanliness and inclusion modification for better quality steel. Calcium modifies solid alumina inclusions, arising out of deoxidation of liquid steel, into liquid calcium aluminate. Depending upon the steel composition, calcium sulfide (CaS) and/or various forms of calcium aluminates may form. Sulfides are often associated with the oxide phase, which is typically known as oxide-sulfide duplex inclusion. Formation of solid calcium sulfide must be avoided during the ladle treatment of liquid steel, since it is detrimental to the castability of steel. In the present work a thermodynamic model has been developed for predicting the formation of oxide-sulfide duplex inclusions arising out of competitive reactions between [O], [S] and [Ca] in Al-killed steel. The model predictions of the present work were compared with those reported in literature, as well as with the types of inclusions observed in steel samples collected from the plant. Reasonably good agreements amongst them were observed. The results indicated that in order to achieve completely liquid calcium aluminate without forming any sulfides the sulfur content of liquid steel must be sufficiently low. With increasing S content of liquid steel, complete modification of alumina inclusions into liquid calcium aluminate becomes difficult. The maximum sulfur content to avoid formation of CaS depends upon the steel composition, principally aluminum. The sulfide inclusions are often a solid solution of CaS and MnS. Thermodynamic analysis for this system was also carried out. Based on the analysis in the present work, it is possible to predict the influence of sulfide composition on formation of duplex inclusions.

Magnesium Non-metallic Inclusions in Non-oriented Electrical Steel Sheets

“…Calcium is generally used for the modification of alumina inclusions. 25,31) A typical MgO·Al2O3-CaO spinel is represented in Fig. 5.…”

“…These are harmful, not only for the mechanical properties of steel products, but also for the manufacturing process, because they cause the nozzle-blockage problems that are encountered in continuous casting. 25,30,31) If the dissolved magnesium in the melt reacts with the dissolved oxygen, it can be written as: 25,27) Mg ( Spinel (MgO·Al2O3) is known as one of the most harmful non-metallic inclusions in steel. Such inclusions have a high melting point and are hard, very harmful to the quality of the final products, especially the mechanical properties and the surface quality of steels.…”

Characteristics of Inclusions in Low Carbon Al-Killed Steel during Ladle Furnace Refining and Calcium Treatment

“…14(a)), however, actually the alumina was not fully changed to MgO·Al2O3 spinel in the plant experiment. [19][20][21][22][23][24][25][26][27] and some models have been proposed. [25][26][27] There are few thermodynamic calculation on the transformation of MgO·Al 2 O 3 spinel inclusions by calcium treatment.…”

Transient Evolution of Inclusions during Calcium Modification in Linepipe Steels

“…6) Aluminum is one of the most popular deoxidizers used in linepipe steels. Particularly, the generated Al2O3 inclusions lead to defects such as hydrogen induced cracking [7][8][9] and cause the clogging of submerged entry nozzle during continuous casting due to their high hardness and high melting temperature. [10][11][12][13][14][15][16] Calcium treatment is employed to modify Al2O3 inclusions.…”

“…However, insufficient or superfluous addition of calcium leads to incomplete or excessive modification of alumina inclusions and the formation of solid calcium aluminates or CaS, which are detrimental to the castability of linepipe steels. 8,28,29) There was a debate about the correct reaction between [Ca] and Al2O3 inclusions. Traditionally, the reactions of Al2O3 inclusions by calcium treatment were proposed as Eqs.…”