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

Abstract

Controlling the morphology of the sulfide inclusion is of vital importance in enhancing the properties of structural steel. Long strip-shaped sulfides in hot-rolled steel can spherize when, instead of the inclusion of pure single-phase MnS, the guest is a complex sulfide, such as an oxide-sulfide duplex and a solid-solution sulfide particle. In this study, the inclusions in a commercial rolled structural steel were investigated. Spherical and elongated oxide-sulfide duplex as well as single-phase (Mn,Ca)S solid solution inclusions were observed in the steel. A thermodynamic equilibrium between the oxide and sulfide inclusions was proposed to understand the oxide-sulfide duplex inclusion formation. Based on the equilibrium solidification principle, thermodynamic discussions on inclusion precipitation during the solidification process were performed for both general and resulfurized structural steel. The predicted results of the present study agreed well with the experimental ones.

Formation of Complex Inclusions in Gear Steels for Modification of Manganese Sulphide

“…Normally, calcium addition is an effective way to alter the morphology and dispersion of non-metallic inclusions in the continuous casting process or in ingots. Calcium treatment is normally applied in Al-deoxidized steels to alter the solid alumina inclusions into liquid ones [11][12][13] which effortlessly pass through without spout impediment. In the current operations, the inclusions modification proficiency using Ca is usually not high; the surplus Ca may cause the development of high temperature stable inclusions such as CaS and Ca•Al 12 O 19 , leading to spout impediment [2,10,14].…”

Effect of Al on the Solid Reaction between 3CaO·Al2O3 Oxide and Fe–S–O–Al Alloy at 1373 K

“…During the production of Al-killed steels, alumina inclusions were modified to calcium aluminate by Ca treatment or slag-steel interaction [1][2][3] to reduce the adverse effect on the steel quality caused by the nozzle clogging or poor deformability of inclusions. [4][5][6][7] The formation of CaS occurred in both liquid [8,9] and solid steels, [10,11] resulting in defects of steel products such as pitting. [12] Shin et al verified the participation of Al during the formation of CaS in the liquid steel following the reaction of 2[Al] þ 3[S] þ 3(CaO) ¼ (Al 2 O 3 ) þ 3(CaS).…”

Inclusion Characteristics in 95CrMo Steels with Different Calcium and Sulfur Contents

“…Thermodynamic calculations also have been performed to predict the type of calcium aluminates and reliable stability phase diagram of various calcium aluminates were obtained [13]. However, the reactions in the molten steel are complex and inclusions containing CaS would form as the existence of S and CaS inclusions are undeformable [14,15]. Verma and Pistorius et al [16,17] investigated the composition evolution of inclusions in liquid steel modified by calcium and found that CaS may form as an intermediate reaction product, which can subsequently react with Al 2 O 3 to form calcium aluminates as reaction [1].…”

“…Oxide-sulfide duplex inclusions were also detected in the steel containing certain Mn, such as semi-free-cutting steels [26,27]. MnS precipitating during cooling and solidification can dissolve with CaS and generate a solid of (Mn, Ca) S [14]. The modification of inclusions by calcium treatment is more complex in this steel containing Al, Mn, and S. It is proposed Al 2 O 3 -CaS inclusions had a detrimental effect on the magnetic properties of a Ca-treated non-oriented electrical steel [28], but the complex inclusions mCaO.nAl 2 O 3 -(Mn, Ca) S with duplex phase can be a protective film on the rack face, which improves the durability of the tool [29].…”