## 1 Introduction  
The calcium treatment for aluminum-killed steels is commonly used to modify non-metallic inclusion (NMI) such as Al\({}_{2}\)O\({}_{3}\) to less detrimental inclusions [1; 2]. The main aim of the modification of inclusions is the control and transformation of the solid inclusions into fully or partially liquid inclusions by changing their chemical or phase composition. Calcium treatment of Al-killed steels can be used to obtain calcium aluminate inclusions with lower liquidus temperature compared to the steel melt. The use of calcium treatment to achieve more liquid inclusions has the potential to control castability challenges such as submerged entry nozzle (SEN) clogging [3; 4] at casting temperatures. The preferred inclusions are either low-melting calcium aluminate inclusions such as C12A7 or C3A and CA, which may be present in partial liquid inclusions [5; 6]. However, the occurrence of solid CA2 and CA6 phases should be avoided [5]. Although calcium treatment plays a role in modifying high melting point inclusions, with low Al\({}_{2}\)O\({}_{3}\) activity in the inclusions, elemental sulphur can react with calcium to form CaS inclusions. They are solid at casting temperatures and can be very detrimental to the steel quality [7; 8]. For example,studies conducted by researchers [7; 8; 9] have demonstrated that CaS can be found in steel samples from billet [8] and hot-rolled plates [7]. Additionally, [PERSON] et al. in 2009 [9] suggested that the formation of CaS may be largely associated with the addition of Ca to liquid steel. The amount of calcium aluminate phases, such as C12A7, CA, C3A, and calcium sulphides, generally depends on the calcium and sulphur contents, the activity of the aluminum, and the temperature of the molten steel [6]. Therefore, the study of calcium aluminate and sulphide inclusions has received much attention, with various characterization techniques, such as SEM-EDS (scanning electron microscopy-energy dispersive X-ray spectroscopy [7; 8], confocal scanning laser microscopy (CSLM) [10], and others [3], used to understand the transformation and control measures for inclusions. Using Raman spectroscopy presents a potential characterization technique for inclusion studies. The utilization of Raman spectroscopy as a characterization technique may perform better than conventional methods when comparing the duration for sample preparation and the acquisition of measurement results. Additionally, phases such as Al\({}_{2}\)O\({}_{3}\), MgO-Al\({}_{2}\)O\({}_{3}\), C12A7, CA, C3A, and CaS have been measured and demonstrated to be Raman active [11; 12; 13; 14; 15]. Furthermore, Raman spectroscopy is found to be applicable in various research fields for quantitative and qualitative analysis [16], including some studies already done on synthetic [17; 18] and actual [19] inclusion phase studies. However, spectroscopic techniques like Raman spectroscopy have not been used in detail to study duplex oxide-sulphide inclusions like C12A7-CaS, C3A-CaS, and CA-CaS. These duplex oxide-sulphides have been demonstrated by researchers [20; 21; 22] to be present in some steel samples. Other inclusions could also be possible, for example, Al\({}_{2}\)O\({}_{3}\)-CaS and MgAl\({}_{2}\)O\({}_{4}\)-CaS, which could be the intermediate product during calcium treatment [23; 24]. The authors' previous studies focused on the use of Raman spectroscopy for characterizing calcium-aluminate (CaO-Al\({}_{2}\)O\({}_{3}\)) inclusions [17] and calcium aluminate-spinel (MgAl\({}_{2}\)O\({}_{4}\)) [18] in a binary system. However, these studies were limited to only oxides, and the application of Raman spectroscopy for characterizing potential detrimental sulphide inclusions such as CaS associated with oxides were not studied. Furthermore, the literature [25] has suggested that free CaS is rarely observed compared with CaS associated with modified calcium aluminate inclusions that may be partly liquid by characterizing steel samples from plant trials with varying calcium additions.