UDC/UDK 669.041:669.14 Professional article/Strokovni članek ISSN 1580-2949 MTAEC9, 41(4)185(2007) INVESTIGATION OF THE INFLUENCE OF THE MELT SLAG REGIME IN A LADLE FURNACE ON THE CLEANLINESS OF THE STEEL RAZISKAVA VPLIVA REŽIMA ŽLINDRE V PONOVČNI PEČI NA ČISTOST JEKLA Zdenčk Adolf, Ivo Husar, Petr Suchánek VŠB-Technical University of Ostrava, Department of Metallurgy, 17. listopadu 15/2172, Ostrava-Poruba, 708 33, Czech Republic zdenek.adolfŽvsb.cz Prejem rokopisa – received: 2006-10-16; sprejem za objavo - accepted for publication: 2007-01-15 An experimental investigation of the influence of the ladle slag’s chemical composition, the ladle slag’s mass and the furnace slag’s mass (during the tapping flow in the ladle) on the sulphur content in steel was carried out. The parameters of the slag and the steel were obtained in operational conditions of oxygen LD converter melts and from the subsequent ladle furnace LF processing. Key words: ladle furnace, steel processing, effect of slag composition, refining effect Eksperimentalna raziskava vpliva kemične sestave, količine (pri izlivu jekla v ponovco) na vsebnost žvepla v jeklu. Parametri jekla in žlindre so določeni pri izdelavi jekla v kisikovem konvertorju in obdelavi taline v ponovčni peči. Ključne besede: ponovčna peč, procesiranje jekla, sestava žlindre, rafinacijski učinek 1 INTRODUCTION Steel refined in a ladle furnace should have the following characteristics prior to casting: • a chemical composition within the prescribed interval of alloying-element content and limited amounts of impurities, • the required metallographic purity in terms of composition, magnitude, number and density of inclusions, • the required casting temperature, depending on the steel’s liquidus temperature. In order to achieve these parameters the steelmaker usually has at his or her disposal a reheating ladle furnace, aluminium wire and a cored wire feeder, the mixing of steel with argon blowing or with induction stirring and an oxygen activity measurement system. The slag is the most important factor for ensuring the quality of molten steel in the ladle. Ladle slag is formed from the products of de-oxidation of the steel, the added mixtures and from the corrosion products of the ladle lining, particularly at the slag line. Admixtures added intentionally to the slag ensure the required chemical composition of the slag, its fluidity and its ability to refine steel, i.e., for the absorption of inclusions and unwanted elements from the steel. The admixtures consist mostly of lime, fluorspar, calcium carbide and fireclay. Also, synthetic slags usually containing Al2O3, CaO, MgO, SiO2, and a minimum of iron oxides, MnO and sulphur are used more and more often. Slags are often prepared from waste materials, either by simple mixing or by sintering, while the most expensive slags of the highest quality are manufactured by re-melting the input raw materials. These slags are usually used as a replacement for fluorspar and for the preservation of the required fluidity and for obtaining a sufficient refining effect in the ladle. In this article the slag regime in a ladle furnace during steel refining, partly under fluorspar and partly under alumina slags, is compared. The slag regime of heats was evaluated with respect to the type and mass of the additions to the ladle, the ladle slag mass, the mass of converter slag overflowed to the ladle, the mass of corroded ladle lining and the slag desulphurisation capacity. 2 CHARACTERISTICS OF THE HEATS The investigated steel I (intended for rail production) has an increased content of carbon (0.70-0.76 %), manganese (0.85-0.85 %) and silicon (0.30-0.40 %), controlled amounts of sulphur (0.01-0.02 %) and limited amounts of aluminium (0.003 %). It is, therefore, produced according to a technology without the use of aluminium and the deoxidation of steel in the ladle with coke, FeSi and MnSi. At the same time, during the tapping from the LD converter slag-forming materials are added to the ladle, e.g., lime (1200 kg), fluorspar (300 kg) and fireclay (150 kg), or lime, fluorspar and synthetic slag CCA with high contents of Al2O3 (see Table 1). Materiali in tehnologije / Materials and technology 41 (2007) 4, 185-188 185 Z. ADOLF ET AL.: INVESTIGATION OF THE INFLUENCE OF THE MELT SLAG REGIME ... Table 1: Chemical composition of the synthetic slag CCA Tabela 1: Kemična sestava sintetične žlindre CCA w/% Al2O3 CaO MgO SiO2 S Granularity average 55.05 26.60 6.56 3.21 0.0295 5-50 mm min 53.60 26.10 6.21 3.21 0.0290 max 56.50 27.10 6.91 3.21 0.0300 The refining of the rail steel was compared to that of the structural steel II with 0.18–0.20 % C, 0.41–0.51 % Mn, 0.20–0.30 % Si, with limited sulphur content (0.020 %) and a specified aluminium content (0.015–0.030 %). During tapping to the ladle this steel is deoxidised by aluminium and alloyed with FeMn and FeSi. At the same time, lime (1200 kg) and fluorspar (200 kg), or lime (900 kg) and synthetic slag CCA (700 kg), replacing the fluorspar, are added to the ladle. The average chemical composition of ladle slags based on lime and fluorspar prior to the exit from the LF stand is given in Table 2. Alumina slags were formed during the making of steel I for rails with synthetic slag CCA (700 kg) with 800 kg of lime (variant A), or with 600 kg of lime (variant B), and during the making of structural steel II with the synthetic slag CCA (700 kg) with 900 kg of lime. Table 3 gives their average chemical compositions. The average content of FeO in the fluorspar slags was 0.55 %, and in the alumina slags it was 2.0 %. If we compare both types of slags, then fluorspar slags have approximately 8 % to 11 % more CaO, higher basicity and a significantly higher content of CaF2, which is added to the ladle during tapping in the form of fluorspar. On the other hand, alumina slags have higher contents of Al2O3 (by 9 % to 13 %) and MgO (by 1.5 % to 3.5 %). The synthetic slag CCA is, apart from converter slag and lime, also a source of MgO and Al2O3. 3 RESULTS AND DISCUSSION The mass of the slag in the ladle was calculated from the balance of CaF2 (Steel I - fluorspar slags) and from the balance of Al2O3 (Steel I and II - alumina slags). The mass of converter slag that overflowed into the ladle was calculated from the balance of CaO, and the extent of the wear of the ladle from the balance of MgO in the ladle slag. These data are for fluorspar slags and for two variants of alumina slags (rail steel I or structural steel II) summarised in Table 4. The data in Table 4 shows that ladle slags differ primarily in terms of their mass. Steel I fluorspar slags are formed, apart from SiO2, from deoxidising silicon, lime, fluorspar and fireclay. The sum of the masses of these three components in the charge was approximately 1720 kg. The converter slag that overflowed into the ladle during the tapping affects the mass and the Table 2: Chemical composition of the fluorspar slags (CaF2) Tabela 2: Kemična sestava fluoridnih (CaF2) žlinder w/% CaO SiO2 Al2O3 MgO CaF2 S Basicity Steel I - rails average 52.9 23.5 4.2 4.9 13.29 0.64 2.25 min 50.9 21.9 3.5 4.5 11.74 0.42 2.12 max 55.2 25.2 5.1 5.7 14.48 0.75 2.52 Steel II - structural average 56.41 19.34 12.94 7.02 4.22 0.41 2.92 min 50.30 17.40 10.90 5.30 2.84 0.28 2.52 max 59.80 21.70 14.60 14.00 6.11 0.66 3.23 Table 3: Chemical compositions of alumina slags (CCA) Tabela 3: Kemična sestava aluminatnih žlinder (CCA) w/% CaO SiO2 Al2O3 MgO CaF2 S Basicity Steel I - rails Variant A average 45.2 22.3 15.4 8.11 1.0 0.23 2.02 min 45.0 20.9 15.0 7.88 0.6 0.19 1.93 max 45.5 23.6 15.7 8.50 1.8 0.20 2.15 Variant B average 44.7 20.1 17.9 8.5 0.9 0.23 2.22 min 44.0 18.9 17.4 7.7 0.2 0.14 2.13 max 45.3 21.3 18.3 9.3 1.5 0.31 2.44 Steel II - structural average 45.23 17.89 22.05 8.51 0.10 0.20 2.53 min 41.10 15.50 18.00 6.80 0.00 0.13 2.12 max 48.30 21.10 25.30 11.00 0.27 0.31 2.95 186 Materiali in tehnologije / Materials and technology 41 (2007) 4, 185–188 Z. ADOLF ET AL.: INVESTIGATION OF THE INFLUENCE OF THE MELT SLAG REGIME ... Table 4: Parameters of ladle slags Tabela 4: Parametri ponovčne žlindre ra/kg LS CS MgO lining Steel I - rails Fluorspar slags average 2411 325 71.5 min 2319 291 52.6 max 2508 380 82.9 Alumina slags - variant A average 2569 564 99.1 min 2519 431 93.4 max 2637 745 111.0 Alumina slags - variant B average 2210 590 69.0 min 2161 521 64.9 max 2273 719 75.6 Steel II - structural Fluorspar slags average 3374 1495 88.4 min 2632 153 57.3 max 4092 2800 99.8 Alumina slags average 3650 1693 86.9 min 2743 323 59.3 max 4645 3308 101.0 Note: mass LS = mass of ladle slag, m/kg mass CS = mass of converter slag that overflowed into the ladle, m/kg MgO lining = wear of ladle lining, m/kg chemical composition of the ladle slag. The mass of the slag in the heats with fluorspar slag was approximately 250 kg lower than the average for heats with alumina slag, for which a slightly lower overall mass of ladle slag was used. Alumina slags are formed of lime and synthetic slag CCA with an average mass of 1510 kg in variant A, and 1310 kg in variant B (again, apart from the SiO2 from the deoxidising silicon). For steel II the fluorspar slags are formed, apart from SiO2 and Al2O3, from deoxidising silicon and aluminium, of lime and fluorspar with a total mass of 1400 kg; for alumina slags fluorspar is replaced with the slag CCA and the new mass of lime and slag CCA is 1600 kg. The wear of the lining is expressed as the mass of MgO released from the ladle wear into the slag. The smallest lining wear for steel I was observed in the variant B, while the greatest wear was observed for the variant A of alumina slag. This is apparently related to the considerable differences in the total mass of slag in the ladle. The fluorspar slag shows - contrary to expectations - on average a low wear of lining; however, with a large scatter of values of approximately 30 kg of MgO. The mass of converter slag overflowed into the ladle was, for the structural steel, three to five times greater then for the steel I. The share of converter slag in approximately 35 % of the total slag mass in the ladle is the source of the relatively high content of MgO in the ladle slag. The wear of the lining is practically the same for both types of ladle slags, and it is more influenced by the slag’s mass then by the slag’s chemical composition. The parameters of the steel II desulphurisation with fluorspar and alumina slags are given in Table 5. Table 5: Parameters of steel II desulphurisation by fluorspar and alumina slags Tabela 5: Parametri razžveplanja jekla II s fluoridno in z aluminatno žlindro c s \wŠS]BOF/%\wŠS]LF/%\ ti/% Ls Fluorspar slag average 0.009945 0.026 0.011 68.769 40.24 min 0.006744 0.016 0.006 47.619 15.56 max 0.015171 0.043 0.018 80.000 55.00 Alumina slag average 0.002481 0.030 0.019 36.131 11.42 min 0.001426 0.018 0.011 20.000 4.48 max 0.003582 0.038 0.029 52.778 20.71 Note: e's= sulphide capacity of slag, 1 ŠS]bof = sulphur content in steel during tapping from the converter, w/% ŠS]lf = sulphur content in steel after treatment in the ladle furnace, w/% rj = rate of steel desulphurisation after treatment in the ladle furnace, % Ls = distributing coefficient of sulphur (Sslag /Ssteel), 1 It can be concluded from Table 5 that fluorspar slags have a greater desulphurisation effect than alumina slags (see tj, Ls), due to their higher basicity and their higher value of sulphide capacity given by the CaO content, which is on average higher by 10 %. It was observed that fluorspar slags had a greater fluidity than alumina slags. This means that a small content of CaF2 (up to 5 %) in the fluorspar slag increases the fluidity of this slag in a wide range of chemical compositions, especially of the CaO content. In contrast to this, the liquidity of the alumina slags can only approach that of the fluorspar slags in a relatively narrow range of content of the more active oxides (CaO, Al2O3, SiO2, MgO). It is necessary to increase the CaO content to at least 50 %, the Al2O3 content to 25-30 %, and preserve the SiO2 content at the level of 15-20 % and the MgO content at the level of 7 % in order to increase the sulphide capacity of the alumina slag to the level of the fluorspar slag. 4 CONCLUSION The slag regime of heats during the making of rail steel I with a limited content of aluminium, increased amounts of C, Mn and Si and with controlled amounts of sulphur and structural steel II deoxidised by aluminium with a limited sulphur content are compared. The original fluorspar slag with low amounts of Al2O3 was substituted with an alumina slag in which the fluorspar was replaced by synthetic slag CCA. Materiali in tehnologije / Materials and technology 41 (2007) 4, 185-188 187 Z. ADOLF ET AL.: INVESTIGATION OF THE INFLUENCE OF THE MELT SLAG REGIME ... The following conclusions from the comparison of both slag regimes of these heats (based on 200-tonne heats) are proposed: a) Alumina slags formed by lime, synthetic slag CCA and the products of steel deoxidisation have a lower desulphurisation capacity then the fluorspar slags due to their lower CaO content. b) The main advantage of fluorspar slags is in their great desulphurisation effect over a wide range of slag chemical compositions, even for a low CaF2 content. c) The liquidity of alumina slags is limited to a narrow range of CaO, Al2O3, SiO2 and MgO content. d) The mass of ladle slags is markedly affected by the volume of furnace slag overflowed into the ladle that has to be minimised. e) The wear of the lining depends on the mass of the ladle slag. The effect of the slag’s chemical composition is of minor importance. This investigation was carried out in the frame of Grant project Reg. No. 106/04/0029 with the financial support of the Grant Agency of the Czech Republic and the project EUREKA Reg. No. OE214, with the financial support of the Ministry of Education, Youth and Sports of the Czech Republic. 188 Materiali in tehnologije / Materials and technology 41 (2007) 4, 185–188