Phase equilibria of the Al₂O₃–CaO–SiO₂-(0%, 5%, 10%) MgO slag system for non-metallic inclusions control

Abstract

Al₂O₃–CaO-(MgO–SiO₂) inclusions are one of the dominant inclusions in Al-deoxidized spring steel, the compositions changes of which are closely related to refining slags and deoxidization process. The Al₂O₃–CaO–SiO₂–MgO system can represent the primary ingredients of the Al₂O₃–CaO inclusions. According to analyzed compositions and predicted liquidus temperature ranges of inclusions and refining slag, equilibra experiments under high temperature, water quenching technique and subsquent electron probe X-ray microanalysis (EPMA) has been conducted to ascertain detailed thermodynamic database for inclusions control. Liquidus temperatures within the dominant phase fields of Ca₃SiO₅, Ca₂SiO₄, CaAl₂O₄, Ca₃Al₄O₉, spinel and MgO with the intervals of 20 °C from 1350 to 1560°C were identified. To further promote inclusions control, the influences of mass ratios of Mass(Al2O3)Mass(Al2O3+SiO2+CaO) and MgO contents on equilibrated phases and liquidus temperature changes have been explored. To further enhance modification levels of Al₂O₃–CaO-(MgO–SiO₂) system inclusions, it is suggested that refining time could be suitably prolonged.

Introduction

Supreme wear resistance and fatigue strength properties are notably preferred for hardened spring steel working with high-frequency dynamics stresses, making it possible for applications in automobile production, serving as valve, clutch, suspensions and plate springs [[1], [2], [3], [4]]. However, these properties could be severely impaired by stress concentration gradient and small voids surrounding inclusions related to thermal expansion coefficients differences and detachment between inclusions and surrounding matrix [[5], [6], [7]]. Owing to the unfeasibility of thorough removal of non-metallic inside molten steel, inclusions with excellent deformability and low liquidus temperatures were generally favoured [8,9]. To enhance overall steel cleanliness levels, ladle furnace (LF) refining and aluminium deoxidation process was commonly employed, the unstable operations of which could result into severe inclusion contamination caused by slag entrapment and spinel generation [[10], [11], [12]]. To further promote steel cleanliness levels and inclusions compositions optimization, reliable thermodynamic properties of liquidus temperature and equilibrated phases of Al₂O₃–CaO–SiO₂–MgO system related to refining slag should be investigated in detail [13,14]. Numerous preceding research had been conducted concerning the Al₂O₃–CaO–SiO₂–MgO system. Mao et al. [15] optimized cordierite and sapphirine ternary compounds for the Al₂O₃–MgO–SiO₂ system and illustrated the liquid phases utilizing the ionic two-sublattice model. Fabrichnaya et al. [16] established the thermodynamic properties consistent with calorimetric measurements and phase equilibria for the liquid phase in the system CaSiO₃–CaAl₂Si₂O₇–Ca₂Al₂SiO₇ utilizing molecular regular solution models. Gran et al. [17] confirmed the glassy phases in the regions of high basicity for the Al₂O₃–CaO–MgO–SiO₂ system at 1500 and 1600°C utilizing quenching methods and subsequent EPMA microanalysis. Heulens et al. [18] confirmed the wollastonite crystal growth for CaO–Al₂O₃–SiO₂ melt from 1320 °C to 1370 °C utilizing high-temperature confocal scanning laser microscope. Valdez et al. [19] studied dissolution of alumina in ternary slags with high SiO₂ contents, a high silica quaternary slag and a low silica ternary slag close to ladle slag composition within the temperature range 1470 and 1530°C with CSLM. The liquidus temperature and phase equilibrium of CaO–Al₂O₃–SiO₂–MgO system had been explored within previous research [[20], [21], [22]]. Kim et al. [23] found viscosities of CaO–SiO₂-20 wt%Al₂O₃–MgO slags (CaO/SiO₂ = 1.0–1.2) decreased with increased MgO contents. Shi et al. [24] constructed liquidus lines and phase diagram for the CaO–SiO₂-5wt%MgO-20wt%Al₂O₃–TiO₂ phase diagram system between 1300 °C and 1400°C. Shi et al. [25] also established the 1400 °C and 1450 °C isotherms and phase relations for CaO–SiO₂-5wt%MgO-30 wt%Al₂O₃–TiO₂ system relevant to high Al₂O₃ Ti-bearing slag system. Wang et al. [26] confirmed initial crystallization temperature of CaO–SiO₂–MgO–Al₂O₃ slag system decreased with increasing Al₂O₃/SiO₂ ratio. Shen et al. [27] found the viscosity of CaO–MgO–SiO₂–Al₂O₃–FeO slag system increased with increased ratios of (Al₂O₃)/(Al₂O₃+FeO). Sun et al. [28] found minimum viscosity of CaO–SiO₂-10mass% MgO–Al₂O₃ slags was achieved at 16 mass%, which improved the amphiprotic properties of Al₂O₃. Nevertheless, the experimental isotherm intervals of 100 K in part of these research could not provide precise thermodynamic data concerning liquidus and equilibrated phases related to refining slag compositions. Furthermore, the dashed lines in predicted phase diagrams indicated the isotherm distribution inaccuracy. Meanwhile, owing to the obvious incompatability of analyzed solid compositions with X-ray examination and EMPA techniques, dependable thermodynamic properties of CaO–Al₂O₃–SiO₂–MgO quaternary system are indispensable for industrial operations [29,30].

As clarified in our precedent investigation of Al₂O₃-containing inclusions in spring steel deoxidized by aluminium, the Al₂O₃–CaO–SiO₂-(MgO) inclusions inside Al-killed spring steel primarily originated from entrapped high-basicity slags, the SiO₂ of which was reduced by dissolved [Al] [31]. Meanwhile, according to our previous research of pseudo-ternary diagram of CaO–Al₂O₃–SiO₂–MgO quaternary slag related to Al₂O₃–SiO₂–CaO inclusions originating fromcollision between Al₂O₃–SiO₂ and CaO–SiO₂ inclusions in Si-deoxidized spring steel, significant incompatibility of liquidus temperature and equilibrated phases distribution between FactSage calculations, previous experimental results and our results were determined [32]. The Al₂O₃–CaO–SiO₂–MgO pseudo-ternary system with basicity of 1.1, 1.3, and 1.5 have been determined in the proceeding research [30,33,34]. Fig. 1 illustrated the comparison results of preceding experimental results from Ma et al. [30], Cavalier et al. [35] and predictions of FactSage 7.3. Apparent differences of liquidus temperatures and equilibrated phases for system of CaO–Al₂O₃–SiO₂–5%MgO between previous and current experimental results were confirmed by Ma et al. [24], Cavalier et al. [35] and FactSage predictions, especially when CaO contents were between 35wt% and 55wt%. For example, based on experimental data of Cavalier et al. [35], the liquidus temperatures of points with CaO contents around 45% and SiO₂ contents almost 35% were near 1400 °C, much higher than that determined by FactSage prediction being almost 1470 °C.

During the refining process of Al-deoxidized spring steel, high-basicity slag was generally utilized for the sake of inclusions removal and control, but inaccurate thermodynamic data of predetermined phase diagram of Al₂O₃–CaO–SiO₂–MgO system related to high-basicity slag is still a problem. In this study, the principal phases and liquidus temperatures of Al₂O₃–CaO–SiO₂–MgO system with magnesium oxide mass percent being 0, 5 and 10 predicted by FactSage 7.3 were compared with experimental analysis results both from previous publications [30,33,34] and present authors.