Thermodynamic studies on oxysulfide containing CaO-MnO-Al2O3-SiO2-S systems, and their applications to inclusions engineering in sulfur bearing steels

Abstract

Thermodynamic study for inclusions in S-bearing steels by experiments and thermodynamic modelings and its application to inclusion engineering have been conducted. In the view of inclusion control and utilization, relevant inclusions systems were thermodynamically modeled and optimized. The newly developed databases were used to predict S-containing inclusions evolutions in steels.Phase equilibria of CaO-SiO2-CaS and CaO-Al2O3-CaS systems have been completely constructed at temperature of 1500°C to 1600°C using equilibration/quenching technique. Using present measurements and all available data from literature, the systems of CaO-Al2O3-CaS and CaO-SiO2-CaS, and CaO-CaS were modeled. A complete review, critical evaluation, and thermodynamic optimization of phase equilibrium and thermodynamic properties of the CaO-SiO2-Al2O3-CaS system at 1 bar pressure are presented. The molten oxysulfide phase was described by the Modified Quasichemical Model. Sets of optimized model parameters for all phases were obtained which reproduce all available thermodynamic and phase equilibrium data reliable within experimental error limits from 25°C to temperatures above the liquidus over the entire ranges of composition. The liquidus surface of the CaO-SiO2-CaS and CaO-Al2O3-CaS systems have been predicted. Complex phase relationships in these systems have been elucidated, and discrepancies among the data have been resolved. Compared to the ordinary thermodynamic modeling approach assuming random mixing of components, the important solution behavior by taking into account strong chemical short-range ordering in the liquid phases was analyzed. The database of model parameters can be used along with software for Gibbs energy minimization in order to calculate any phase diagram section or thermodynamic property. Activity measurements of MnS in CaS-MnS solid solution were conducted by using Gas/Pt/Sulfide Solid Solution equilibration and Gas/Pt/Sulfide Solid Solution/Flux equilibration methods. Using present thermodynamic measurements and all available data from literature, the system of CaS-MnS was modeled.A complete review, critical evaluation, and thermodynamic optimization of phase equilibrium and thermodynamic properties of the CaS-MnS systems at 1 bar pressure are presented. Solid solution phase was described by the Random Mixing Model with positive interaction while liquid phase was modeled using the Modified Quasichemical Model. Sets of optimized model parameters for all phases were obtained which reproduce all available thermodynamic and phase equilibrium data reliable within experimental error limits from 25°C to above the liquidus temperatures. The database of model parameters can be used along with software for Gibbs energy minimization in order to calculate any phase diagram section or thermodynamic property.By merging CaO-SiO2-Al2O3-CaS and MnS-CaS systems modeled in present study with CaO-Al2O3-MnO and MnO-SiO2-Al2O3-MnS system modeled previously, full sets of databases for CaO-SiO2-Al2O3-MnO-CaS-MnS-(SiS2-Al2S3) system could be constructed. The liquidus surface of MnO-Al2O3-MnS, and CaO-MnO-CaS-MnS been predicted and sulfide capacity of CaO-Al2O3-MnO was predicted. The predictions of phase equilibria between oxysulfide and sulfide solid solution phases in CaO-Al2O3-MnO-CaS-MnS system were compared with experimental data available in the literature and experimental data in present study. Based study on thermodynamic calculations using developed databases, predictions of inclusion engineering containing CaO-SiO2-Al2O3-MnO-CaS-MnS-(SiS2-Al2S3) in free cutting steels were conducted. Important factors of utilization of inclusions in free cutting steels are discussed and new steel grades containing Ca and Al were proposed. Formation behavior of inclusions in Ca/Al/Mn/Si deoxidized high S-bearing steel was studied from both ingot casting sample and thermodynamic calculation. Formation of MnS and oxysulfide inclusions in newly designed Ca/Al added steels at 1200°C (about machining temperature in free cutting steels) was confirmed and the new steel grades proposed in present study are expected to be applicable as new steel grades of free cutting steels.