Transient Inclusion Evolution During Modification of Alumina Inclusions by Calcium in Liquid Steel: Part II. Results and Discussion

Abstract

The formation of intermediate reaction products after calcium addition to aluminum-killed steel was studied. Steel samples were taken from laboratory and industrial heats before and at various times after calcium treatment. Inclusions were characterized by automated and manual scanning electron microscopy and X-ray microanalysis of polished cross sections and inclusions extracted by dissolution of the steel. Industrial and laboratory melts containing more than 40 parts per million (by mass) of dissolved sulfur showed calcium sulfide as the main reaction product after calcium injection, with calcium aluminates appearing later. It is proposed that the calcium aluminates are formed by reaction between the calcium sulfide and the alumina. A laboratory heat containing 7 parts per million of sulfur showed calcium oxide as the main initial calcium reaction product. A simple mechanism is proposed for the modification of alumina inclusions by calcium, considering transient CaO and CaS formation.

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

“…5), which might be owing to the continuous modification reaction of Al 2 O 3 to calcium aluminate and the reaction between sulfur and calcium to form CaS. The current result is different from that of Verma et al 16,17) who claimed that immediately after calcium treatment CaS formed adhering to alumina or spinel, and longer after calcium treatment the CaS disappeared and spinels were fully or partially modified. After calcium treatment most of the inclusions were changed to spherical shape and with small size of < 5 μm.…”

“…[25][26][27] There are few thermodynamic calculation on the transformation of MgO·Al 2 O 3 spinel inclusions by calcium treatment. 14,16,17) Composite deoxidation by Mg, Al and Ca is considered. If the CaO-MgO-Al 2 O 3 inclusions are liquid in molten steel, they have been proved to be the most stable phase.…”

Transient Evolution of Inclusions during Calcium Modification in Linepipe Steels

“…[10][11][12][13][14][15][16] Calcium treatment is employed to modify Al2O3 inclusions. [17][18][19][20][21][22][23][24][25][26][27] The addition of calcium in the molten steel could modify oxide inclusions, desulphurization of steel and control shape of sulfide 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.…”

“…On the other hand, it is also reported that CaS inclusions are generated as transient phase immediately upon calcium addition, then decrease to disappearing. 17,21,22) Firstly, the [S] reacts with [Ca] to form CaS in the molten steel. Whereafter, CaS modified Al2O3 to calcium aluminates.…”

Thermodynamic Model for Prediction of Slag-Steel-Inclusion Reactions of 304 Stainless Steels

“…1,2) Non-metallic inclusions in steel have a detrimental effect on the performance of steels, such as their strength, toughness, fatigability, cleanliness, surface finishing, etc. 3) A wide range of operating approaches including deoxidation, 4,5) calcium treatment, [6][7][8][9][10][11] slag refining, [12][13][14][15][16][17] and prevention against reoxidation [18][19][20][21] are applied throughout the steelmaking processes of stainless steels. Inclusion composition is greatly influenced by the composition of the top slag and the refractory material.…”