A Thermodynamic Study on the Inclusion Formation in Ferritic Stainless Steel Melt

Recently there has been an increased demand for low carbon ferritic stainless steels due to their high performance in various applications at low production cost. However, the surface quality of ferritic stainless steels is very susceptible to non-metallic inclusions such as high melting point MgO·Al2O3 spinel crystallized in a slag-type inclusion as shown in Fig. 1. These inclusions are the major causes for inclusion line defects on both hot and cold rolled stainless steel sheets. For the production technology of clean ferritic stainless steels, it is important to know the origin and the formation mechanism of these non-metallic inclusions. In practice, the AOD stainless steelmaking process has a feature of tapping with slag, since there is no extra tapping hole in an AOD refining vessel. Therefore, some amount of slag particles can be entrapped in liquid steel, and they can provide the nucleation sites for the secondary deoxidation of Al with the decrease in molten steel temperature. This hypothesis can be confirmed by the close agreement between the compositions of AOD slag and inclusions in terms of CaO and SiO2 contents. Also, the Al2O3 content in inclusions found in the solidified steel product is high enough to form MgO·Al2O3 spinel phase despite the Al content in steel is very low. In the present study, a small amount of AOD-type slag was reacted with a type 430 stainless steel melt containing different Al levels to see how the slag composition changes by the reaction with Al. The equilibrium relations between Al and O contents in the stainless steel melt were examined as a function of Al2O3 activity in slag contained in an alumina or MgO crucible at different melt temperatures. A model calculation was performed to clarify the formation ISIJ International, Vol. 44 (2004), No. 7, pp. 1134–1139


Introduction
Recently there has been an increased demand for low carbon ferritic stainless steels due to their high performance in various applications at low production cost. However, the surface quality of ferritic stainless steels is very susceptible to non-metallic inclusions such as high melting point MgO · Al 2 O 3 spinel crystallized in a slag-type inclusion as shown in Fig. 1. These inclusions are the major causes for inclusion line defects on both hot and cold rolled stainless steel sheets. 1) For the production technology of clean ferritic stainless steels, it is important to know the origin and the formation mechanism of these non-metallic inclusions. In practice, the AOD stainless steelmaking process has a feature of tapping with slag, since there is no extra tapping hole in an AOD refining vessel. Therefore, some amount of slag parti-cles can be entrapped in liquid steel, and they can provide the nucleation sites for the secondary deoxidation of Al with the decrease in molten steel temperature. This hypothesis can be confirmed by the close agreement between the compositions of AOD slag and inclusions in terms of CaO and SiO 2 contents. Also, the Al 2 O 3 content in inclusions found in the solidified steel product is high enough to form MgO · Al 2 O 3 spinel phase despite the Al content in steel is very low.
In the present study, a small amount of AOD-type slag was reacted with a type 430 stainless steel melt containing different Al levels to see how the slag composition changes by the reaction with Al. The equilibrium relations between Al and O contents in the stainless steel melt were examined as a function of Al 2 O 3 activity in slag contained in an alumina or MgO crucible at different melt temperatures. A model calculation was performed to clarify the formation ISIJ International, Vol. 44 (2004), No. 7, pp. 1134-1139 mechanism of slag-type inclusions containing MgO · Al 2 O 3 spinel during the temperature drop along the stainless steelmaking process. The effect of refractory materials on how they affect the Al level in the melt and the MgO · Al 2 O 3 spinel formation was discussed.

Experimental Procedure
In order to check the composition change of entrapped slag by the reaction with Al in the melt, a small amount of AOD-type slag was equilibrated with a type 430 stainless steel melt containing different Al contents. One hundred and fifty grams of Fe-16% Cr-0.5% Mn-0.3% Si-0.04% C alloy, supplied from POSCO research center, was melted by a high frequency induction furnace shown in Fig. 2. The induction coil was specially designed to give a strong agitation of the melt. Strong agitation of melt would result in a faster attainment of reaction equilibrium and a good separation of nonmetallic inclusions from the steel melt. A detailed description of the furnace is given in the previous article. 2) After melting the metallic charge, pure Al (0.03 to 0.75 g) was added and 1 g of AOD-type slag with a composition of 44%CaO-49%SiO 2 -7%MgO was dropped onto the steel melt. In some experiments, stoichiometric MgO · Al 2 O 3 spinel powder (0.5 g) was used as an equilibrium oxide phase for the Al-O deoxidation reaction of the melt in MgO crucible. The method of obtaining stoichiometric MgO · Al 2 O 3 spinel powder is described elsewhere. 2) The slag and metal were equilibrated for 1 h in MgO or Al 2 O 3 crucible under a deoxidized Ar atmosphere at 1 873 and 1 923 K.
After each experiment, the melt was quenched rapidly by helium gas blowing in the chamber. The quenched metal sample was cross-sectioned and examined with an optical microscope for the presence of nonmetallic inclusions. The center part of metal sample was virtually clean without any noticeable inclusions, and was used for the chemical analyses. The Al content in metal was analyzed by inductively coupled plasma (ICP) spectroscopy. The metallic sample (0.5 g) was dissolved in 30 mL of HCl(1ϩ1) in a glass tube of 50 mL capacity heated in a block heater at 353 K for 12 h. Water and acids used were ultra pure grades. After dissolving the sample, the solution was transferred to the measuring flask (50 mL) using ultra pure water with filtration. Virtually no residue was remained in the filter paper, and it was confirmed by the fusion method using ultra pure Na 2 CO 3 for the analysis of acid-insoluble Al. Figure 3 shows the ICP intensities for Al in standard solutions containing 0 to 1 000 mass ppb of Al. The standard solution also contains 8 400 mass ppm of Fe and 1 600 mass ppm of Cr in order to have the same condition as the sample solution. At a wavelength of 396.152 nm, the ICP intensity shows a perfect correlation with Al content. The accuracy of Al analysis in stainless steel sample was 5Ϯ0.5 mass ppm. The oxygen content was measured with an accuracy of Ϯ1 mass ppm by the inert gas fusion-infrared absorptiometry, using the standard steel sample containing 10Ϯ1 mass ppm oxygen.

Aluminum Deoxidation Equilibria in a Type 430
Stainless Steel Experimental results of the Al-O deoxidation equilibria for a type 430 stainless steel melt are summarized in Table  1. Under the present experimental condition, the oxygen content in the stainless steel melt would be controlled by Al in the melt and Al 2 O 3 in slag. The deoxidation equilibrium of Al can be expressed by following equations: %Al  where i and j represent the elements other than O and Al, respectively, such as Cr, Mn, Si and C.
The values of e i j used in this study 3,5) are given in Table  2. Therefore, the equilibrium relation between Al and O contents in a type 430 stainless steel melt can be calculated from Eq.

Mechanism of Inclusion Formation
Based on the above thermodynamic study on Al deoxidation equilibrium in a type 430 stainless steel melt, a model can be derived for predicting the composition change of AOD slag particles entrapped in the melt along the manufacturing route. The following assumptions can be made for the calculation: (1) Thermodynamic equilibrium exists between slag and molten steel. (2) Entrapped slag in molten steel provides the nucleation site for the secondary deoxidation of Al on cooling. (3) Deoxidation products are mixed completely with entrapped slag particles. (4) Entrapped slag particles are not removed during the steelmaking process. For a model calculation, it is necessary to have information such as composition and weight of entrapped AOD slag and those of deoxidation products. In this calculation, the composition of entrapped slag is 55%CaO-30%SiO 2 -5%Al 2 O 3 -10%MgO, which is the average composition of AOD tapping slag. The weight of entrapped AOD slag in stainless steelmaking process is regarded as 65 to 100 g/ ton-steel. 1,8,9) In the present study, 80 g/ton-steel of entrapped slag was assumed. Figure 8 shows the equilibrium relations of Al-O in a type 430 stainless steel melt for two different temperatures of 1 923 and 1 773 K at AOD tapping and tundish (T/D), respectively. If the ladle is lined with alumina, the top slag after AOD tapping would be readily saturated with Al 2 O 3 , 10) and hence the steel melt would pick up a significant amount of Al from the slag. The oxygen content in stainless steel melt at AOD tapping is controlled by Si, and the level is about 65 ppm at 1 923 K. 11 If all of formed Al 2 O 3 (77 ppm) is mixed with entrapped slag particles, the Al 2 O 3 content in slag particles would increase significantly. When the amount of entrapped slag is regarded as 80 g/ton-steel, the calculation shows that the Al 2 O 3 content increases from 5 % up to 45 % where the MgO · Al 2 O 3 spinel can primarily crystallize in CaO-Al 2 O 3 -SiO 2 (-10%MgO) pseudo-ternary system as shown in Fig. 9. In general, the Al content in the melt would not readily reach the equilibrium level after the slag is saturated with alumina due to the kinetic factors. However, the calculated result assuming the equilibrium Al content of 53 ppm falls within the range of inclusion compositions observed in a type 430 stainless steel product treated in alumina ladle as shown in Fig. 9.
If the ladle is lined with MgO refractory, the activity of    Fig. 9. Therefore, the use of MgO refractory for ladle lining or at least at the slag line would help to keep Al level in the melt low during the secondary refining process.
In the present study, the effect of refractory, alumina or MgO, for ladle on the Al level in the stainless steel melt and hence the formation of MgO · Al 2 O 3 spinel inclusions was studied. In actual processes, to avoid the formation of this high melting point spinel phase, it is also recommended to use Al-free ferro-alloys and to reduce MgO content in AOD slag. 1,8)

Conclusions
In the present study, the equilibrium relations between Al and O contents in a type 430 stainless steel melt were examined as functions of Al 2 O 3 activity in slag and melt temperature. A model calculation was performed to clarify the composition change of entrapped slag particles during the temperature drop along the process. Aluminum pick up from the refractory or any other sources can increase the Al 2 O 3 content in slag particles significantly to a level where MgO · Al 2 O 3 spinel inclusion can form. The use of MgO refractory for ladle lining would help to keep Al level in the melt low during the process.