Transient Evolution of Inclusions during Calcium Modification in Linepipe Steels

Abstract

The laboratory experiments of Al2O3 inclusions modified by calcium treatment in linepipe steels with the S content of 30 ppm and 310 ppm were performed at 1 873 K. Particularly, samples were taken before calcium treatment ( 1 minute before the calcium addition) and at various times (typically 1 minute, 10 minutes and 30 minutes) after calcium treatment to study the transient inclusions evolution during calcium modification. Traditional modification mechanism of Al2O3 inclusions is that Al2O3 inclusion is directly modified by dissolved calcium or indirectly modified via CaO formed from calcium and oxygen. In the current study, after Ca-Si alloy power addition, CaS outer layer were promptly formed on the angular Al2O3 inclusion. With the reaction of the CaS outer layer and the Al2O3 core or [O], the Al2O3 inclusion was modified to a spherical liquid calcium aluminate. Moreover, the equilibrium curve of Ca-O and Ca-S, the stability diagram of inclusions and equilibrium precipitation of inclusions during solidification in linepipe steels were calculated to study the formation of inclusions.

Effect of Cooling Rate on Oxide Inclusions During Solidification of 304 Stainless Steel

“…The ways to reduce the detrimental effects include the removal and modification of inclusions. The removal of inclusions is usually promoted by the control of fluid flow, while the modification of inclusions is usually carried out through slag refining and calcium treatment during secondary refining …”

“…The transformation of inclusions during the process induces the different natures of inclusions in steel products from those in the molten steel. The formation, removal, and modification of inclusions in molten steel have been widely investigated. Meanwhile, the changes of inclusions in solid steel during heat treatment have been paid more and more attention to, especially for stainless steels .…”

Theoretical Investigation on Deoxidation of Liquid Steel for Fe–Al–Si–O System

“…Control of non-metallic inclusion towards production of clean steel for various special applications has become a major concern in present days [1][2][3][4]. Deoxidizers, generally in the form of ferro-alloys, are added in the liquid bath to reduce the dissolved oxygen present in crude steel.…”

Thermodynamics of Complex Sulfide Inclusion Formation in Ca-Treated Al-Killed Structural Steel

“…[6][7][8][9] In Ca-treated Al-killed steel, complex sulfide inclusions are often observed; among these inclusions, single-phase (Mn,Ca)S solid solution and oxide-sulfide duplex, which consists of an oxide core and a sulfide ring, can be detected. [10][11][12][13] In the many theoretical investigations of inclusion formation in Ca-treated Al-killed steel, the activity of CaS associated with the oxide phase has often been assumed to be a constant, such as 1, [14] because of the very low solubility of CaS (2 to 5 wt pct) in calcium aluminate at 1823 K (1550°C) [15] and the lack of MnS inclusion in general structural steel above the liquidus temperature. However, MnS, resulting from the segregation and enrichment of sulfur element in the liquid portion, starts to precipitate in the interdendritic space; subsequently, CaS and MnS can dissolve each other to generate a solid solution of (Mn,Ca)S. [16] Therefore, the activity of CaS varies as a function of the molar fraction of CaS (X CaS ) in (Mn,Ca)S and cannot be treated as a constant.…”