Study on refining slags targeting high cleanliness and lower melting temperature inclusions in Al killed steel

Slag–steel reaction experiments and sample preparation

Slag–steel reaction experiments were carried out in a vertical electrical resistance furnace with Si–Mo heating bars, and a total of 33 heats of experiments were carried out. During the experiment, 100 g of steel material and 50 g of slag material were put into an MgO crucible. Then, the crucible was afterward set in an even temperature zone of the furnace and heated to 1873 K. When held for 90 min at 1873 K, the crucible was picked out of the furnace and quenched rapidly in water, with steel samples and slag samples prepared for chemical analysis. Three high basicity slags were used in the experiments, namely, slags A, B and C respectively.

Chemistry analysis and inclusion detection

Acid soluble Al, Ca and Mg in steel were analysed by inductively coupled plasma–atomic emission spectrometry method and TO was determined by infrared absorption method. Composition of slag was analysed using an X-ray fluorescence spectrometer. At least 30 inclusions were randomly analysed in each steel sample by SEM–energy dispersive spectroscopy (EDS) (JEOL JSM-6701F), by which characteristics such as size, morphology and composition of inclusions were obtained.

Cleanliness of steel

Slag compositions are listed in Table 1. As can be seen, the basicity of slags A and B is of a very high level, and it varied in the range of 6·80–7·60 and 7·76–7·21 respectively. While alumina contents were about 40 and 30% respectively, by comparison, basicity and alumina of slag C were in the range of 3·3–3·4 and 23–25% respectively.

Total oxygen in steel samples refined by slags A and B were in the range of 0·0007–0·0010 and 0·0005–0·0010% respectively, while TO in steel melts refined by slag C are relatively higher and varied between 0·0008 and 0·0010%, as given in Table 2.

Relations between slag basicity and (FeO+MnO) content and TO in steel are listed in ^{Figure 1} Figs. 1 and 2 respectively. It was found that higher basicity in slag corresponded to lower TO in steel. Total oxygen in steel were about 0·0005–0·0010% when slags A and B were used, with basicity of about 6–8, while TO was about 0·0008–0·0011% when refined by slag C. However, influence of (FeO+MnO) in slag on TO was not very distinct because they are not >0·5%.

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Figure 1.

Relation of slag basicity and TO

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Figure 2.

Relation of (FeO+MnO) in slag and TO

Non-metallic inclusions in steel

A total of 1095 inclusions in the 33 steel samples were analysed by SEM–EDS method. Inclusions were all composed of CaO, MgO, Al₂O₃ and a very small part of SiO₂. In particular, some inclusions were of MgO based type. Typical inclusions under the three slags are shown in Fig. 3. As indicated, inclusions in steel were all in spherical under slags A and B, with sizes mainly <5 μm, while sharp inclusions were often found when slag C was used, and these inclusions were obviously >5 μm. Size distribution of inclusions is shown in Fig. 4. As can be read, inclusions were all <5 μm with slag B. However, ∼1 and 3·8% inclusions were >7 μm under slags A and C respectively.

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Figure 3.

Typical inclusions in steel samples refined by slags A, B and C respectively

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Figure 4.

Size distribution of inclusions

Energy dispersive spectroscopy results of inclusions were converted into mass percentage of CaO, MgO, Al₂O₃ and SiO₂ and were projected into the calculated CaO–MgO–Al₂O₃ pseudoternary system with certain percentage of SiO₂ (with FactSage software), as given in Fig. 5. As can be seen, majority of the inclusions concentrated in or around the lower melting point zone when slag A was used, although some MgO based inclusions were formed. While inclusion compositions became much more scattered and much more MgO based, inclusions were formed when refined by slag B. The same situation happened when slag C was applied, with a big part of inclusions located outside or far from the lower melting point region, and MgO based inclusions were formed on a very large scale.

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Figure 5.

Composition distribution of inclusions in steel refined by a slag A, b slag B and c slag C

Effects of slag system on TO in steel melts

Total oxygen in steel is often used to evaluate oxide cleanliness of steel, including soluble oxygen and oxygen contained in inclusions. Both aspects could be greatly influenced by variation of slag system during slag–steel reaction. For one thing, composition change brought variation in activities of Al₂O₃, SiO₂, etc., which would result in differences in soluble oxygen content. Furthermore, composition change of slag would also result in the change of physical properties such as viscosity, which influences the absorption ability of inclusion.

Soluble oxygen in steel melts depended on [Al] in steel and activity of alumina in slag with Al deoxidation, as expressed by equations (1) and (2). Activity of elements in steel melts can be estimated by equation (3). During the calculation, [C], [Si], [Mn], [S] and [Cr] in steel melts were taken as 0·60, 1·70, 0·80, 0·0010 and 0·2 respectively. Interaction coefficients used in the calculation are listed in Table 3, while activities of alumina in slags were estimated by empirical equation (4) given by Ohta and Suito,12 and were estimated to be about 0·009–0·013, 0·005–0·008 and 0·023–0·026 in slags A, B and C respectively. With the content of [Al] listed in Table 2, soluble oxygen content were estimated to be about 0·0001–0·0002 and 0·0001–0·0002% respectively when slags A and B were used, while it was 0·0001–0·0003% when refined by slag C (1) (2) (3) (4) Removal of inclusions in steel melts depends on their flotation behaviour in the steel melt and absorption of them by the slag phase. As no stirring was used in experiments, flotation of inclusions in steel melts could be approximately expressed by equation (5), where V is the velocity of inclusion particle, ρ _m is the steel density at 1873 K and taken as 7000 kg m^–3, ρ _in is the estimated average density of inclusion at 1873 K and taken as 3500 kg m^–3, μ is the viscosity of steel at 1873 K and taken as 0·005 Pa s, g is the acceleration of gravity and taken as 9·80 kg m s^–2 and d _in is the diameter of inclusion (5) During the experiments, the inner diameter of the MgO crucible was 0·03 m. As 100 g of steel material was used, the depth of steel melts can be estimated to be ∼0·02 m. The flotation time for inclusions with varied size from the bottom of steel melts to steel/slag interface is plotted in Fig. 6. As indicated, 90 min is enough for inclusion >4 μm to float to steel/slag interface. However, inclusions >5 μm were still found in the steel, especially in slag C, as stated above in Fig. 4. In all the experiments, composition, depth, etc. of steel melt were nearly the same, which means that the influence of steel on flotation of inclusions would be the same. As a result, it is considered that differences in size distributions of inclusions were caused by the absorption ability of the slag.

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Figure 6.

Relation of flotation time and inclusion size

Valdez et al. 15 investigated the effect of CaO–Al₂O₃–SiO₂ ternary system slag on inclusions removal and divided the liquid phase diagram at 1873 K into several regions by their absorption ability on inclusions, as indicated in Fig. 7. The dark colour marked the poor absorption ability on inclusions. Compositions of slags A, B and C were projected into this ternary system. As can be seen, compositions of slags A and B were in the same region with stronger absorption ability on inclusions, while slag C was located outside the liquid region at 1873 K and was not beneficial for the absorption of inclusions. It is considered as an important reason for higher TO.

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Figure 7.

Absorption ability of inclusions of slags A–C

Effects of slag system on MgO content in inclusions

During the reaction of slag/steel/inclusions, equilibrium between slag/steel and steel/inclusions could be reached, as they were in contact with each other directly. Slag phase affected inclusions indirectly by influencing steel compositions, as they did not directly react with each other on a large scale. As a result, total equilibrium among slag, steel and inclusions phase is very hard to realise. However, local equilibrium is possible, that is why slag system strongly affects inclusions.

It is noticed in Fig. 5 that MgO weight percentage in average inclusions composition is much higher when slags B and C were used. Beskow and co-workers. ¹⁶ 16,17 have found that MgO based inclusions could also be formed as chemical reaction products and as metallurgical vessel lining refractory erosion. However, this phenomenon was analysed here with emphasis on slag system. During the experiments, erosion of initial CaO–Al₂O₃–SiO₂ slag would happen, which resulted in the entry of MgO into the slag, and the slag system changed into (CaO+MgO)–SiO₂–Al₂O₃ pseudoternary system. The stronger the solubility of MgO in the slag, the more MgO is need for the slag to reach saturation. Ohta and Suitio12 investigated the saturation behaviour of MgO in (CaO+MgO)–SiO₂–Al₂O₃ pseudoternary system and measured the saturation solubility lines of MgO, as given in Fig. 8. Compositions of slags A, B and C in experiments of the present study were projected into this pseudoternary system. It is found that saturation content of MgO in slag A is ∼15%, while it is ∼10% in slags B and C. During the experiments, MgO in slags A, B and C are in the range of 9·68–10·88, 9·22–9·82 and 7·00–8·56% respectively. It means that MgO in slag A did not reach saturation, while MgO was saturated in slag B and nearly saturated in slag C. It is considered as a very important reason why much more MgO based inclusions were formed in steel when slags B and C were used.

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Figure 8.

Solubility behaviour of MgO in slags A–C