Dev. Chem. Eng. Mineral Process. 14(3/4), pp. 439-447, 2006.

Effect of Refining Slag Composition on

Inclusions in Molten Steel treated by

Barium-bearing Alloy

Yang Li, Zhou-Hua Jiang* and Yang Liu Department of Ferrous Metallurgy, Northeastern University, PO Box 241, Shenyang 110004, P. R. China

The effect of the rejhing slag composition on the inclusions in molten steel was studied in MgO crucible by using MoSi2 furnace. The experimental steel was heavy rail steel and Si-Ca-Ba alloy was used as the deoxidizer. The results showed that the total oxygen content of steel, and the total quantities, total areas and average radius of inclusions decreased with the increased basiciry of rejhing slag. The total quantities and total areas of inclusions were the least when A1203 content in refining slag was between 13% and 20%, while the proportion of small inclusions increased when A1203 content was higher than 20%. The contents 05 SOl and A1203 in inclusions were affected directly by the Si02 and A1203 contents in refining slag, and it was very remarkble when the slag composition was CaO~A12O3.2SiO2. Therefore, it's feasible to control the composition of inclusions by controlling the composition of refining slag.

Introduction The increasing demands for product specification makes any nonmetallic inclusions present in steel an important issue during the steelmaking process. Non-metallic inclusions such as A1203 are known to cause cracking in steel products, and their removal from molten steel was studied by control of the slag composition [ 1,2].

Since A1 has some shortcomings such as low density, high price and causing nozzle clogging, various compound deoxidizers have been developed, especially the barium-bearing alloy. In 1969, Hilty et al. [3] used barium-bearing alloys to treat molten steel for the first time. In China, barium-bearing alloy was only adopted in the 1980's. The deoxidation mechanisms of barium-bearing alloys and the metamorphic mechanism of inclusions have also been studied [4,5].

There are two ways to reduce the disadvantageous effects of non-metallic inclusions on the steel properties. First, creating excellent conditions for inclusions to

* Author for correspondence (lyguaiguai@I 26.com).

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Yang Li, Zhou-Hua Jiang and Yang Liu

float up and dissolving in the slag. Second, monitoring the change in chemical composition of non-metallic inclusions during the steelmaking process. This can be achieved by controlling the reaction between molten steel, slag, flux, refractory and other materials. The present work focuses on modifying the refining slag in order to control the composition of non-metallic inclusions. Experiments to determine the variations of basicity (CaO%/Si02%) and the A1203 content in slag were performed in MoSi2 fimace, in order to evaluate the effect on the composition, quantity and size of non-metallic inclusions in molten steel treated by Si-Ca-Ba alloy.

Experimental Details The MoSiz furnace, MgO crucible and Pt-Rh thermocouple were used in deoxidation experiments with a flowing argon atmosphere of 3 Llmin at 1873 K. The composition (mass %) of heavy-rail steel (U71Mn) is carbon 0.67-0.75, silicon 0.20-0.30, and manganese 1.25-1.35. The composition (mass %) of deoxidizer (Si-Ca-Ba alloy) is silicon 50-55, calcium 12-14, and barium 13-15. The heats arrangement and the variation of the basicity and composition of the chosen C ! ~ O - A I Z ~ ~ - S ~ O ~ - M ~ O - C ~ F ~ refining slag are shown in Table I, and the refining slag of plagioclase (Ca0.Al2O3*2SiO2) was chosen for comparison.

The heavy-rail steel of 1 kg was charged into the MgO crucible, which was put into the MoSi2 furnace (see Figure 1). The dissolved oxygen was measured by an oxygen probe, and sampling was conducted at 1873 K. The top slag of 50 g was then added to molten steel. The initial steel sample was taken before the deoxidizer (Si-Ca-Ba alloy), FeSi, and FeMn were added to molten steel. The melting time was 40 minutes, during which dissolved oxygen was measured and samples were taken at 10 minutes, 20 minutes and 40 minutes.

Table 1. Heats arrangement.

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Eflect of Refining Slag Composition on Inclusions in Molten Steel

Results and Discussion (i) Variation of oxygen content The variation of total oxygen content with the basicity of refining slag is given in Figure 2. It can be seen that the total oxygen content reduces significantly between 0 to 10 minutes, and then the total oxygen content decreases slowly with time and reaches equilibrium in about 20 minutes. The terminal total oxygen content is between 0.0013% and 0.0038% after 40 minutes.

From the experimental data and the terminal condition, it can be seen that the total oxygen content of molten steel decreases as the refining slag basicity increases. This indicates that increasing the basicity of refining slag is beneficial to the floatation of the nonmetallic inclusions, and their dissolution in slag, in the molten steel treated by barium-bearing alloys.

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Figure I . Schematic diagram of experimental device, the MoSi, furnace.

Figure 2. Variation of total oxygen content versus basicity ofrefining slag

441

Yang Li, Zhou-Hua Jiang and Yang Liu

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The variation of total oxygen content with AI2O3 content in refining slag is shown in Figure 3. The terminal total oxygen content is between 0.0022% and 0.0035% when the experiment time is 40 minutes. Figure 3 shows that the total oxygen content of molten steel decreases as the AL203 content in refining slag increases. However, the effect of A1203 content in refining slag on the inclusions is not as significant as the influence of the basicity of refining slag.

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Figure 3. Variation of total oxygen content versus AlzO3 contents in refining slag,

(si) Size and distribution of non-metallic inclusion After the terminal steel samples were milled and polished, 30 fields of view with 500 magnifications were observed with a LEICA Q550lW image device and DMRME microscope. Then automatic and semiautomatic processes were used to treat the fields of view containing inclusions by the image processing system.

Table 2 shows the size distribution of inclusions when the A1203 content in refining slag is 10% and the basicity is between 0.8 and 4.0. The data indicate that the total quantity, total area and average radius of inclusions decrease as refining slag basicity increases, except for few deviations. The proportion of inclusions smaller than 2 pm increases significantly as the refining slag basicity increases, which indicates that increasing refining slag basicity is beneficial to the generation of small inclusions or the removal of large inclusions.

Table 3 shows the size distribution of inclusions when the basicity is 1 .O and the AI2O3 content in refining slag is between 5% and 37.8% (Ca0.Al2O3.2SiO2). Table 3 indicates that the total quantity and the total area of inclusions are the largest when the AI2O3 content in refining slag is 5% and 37.8%, and they are the smallest when the A1203 content in refining slag is 15%. The proportion of inclusions smaller than 2pm increases when the A1203 content in refining slag is more than lo%, which indicates

442

Effect of Refining Slag Composition on Inclusions in Molten Steel

that a correct Al2O3 content in refining slag is beneficial in order to obtain small sized inclusions. The proportion of inclusions smaller than 10pm is similar (about 98%) in all the experiments, hence the effect of A1203 content on the removal of large sized inclusions is not significant.

Figure 4 shows the effect of A1203 content in refining slag on the total quantity and the total area of inclusions. These values total first decrease, and then increase as the A1203 content in refining slag increases, with minimum values when the A1203 content in refining slag is 17%. Therefore, considering both parameters, the optimal range of A1203 content in refining slag is 13% to 20%.

Table 2. Distribution of terminal inclusions (%) at diflerent basicities of refining slag.

Table 3. Dktribution of terminal inclusions (?A) at dflerent AI2O3 contents in refining slag.

(iii) Analysis of composition and photograph of typical inclusions The morphology and composition of terminal steel samples were analyzed by scanning electron microscope (SEM) and energy spectrometer. The spectrum and stereoscan photograph of terminal inclusions are shown in Figure 5 . All the inclusions are approximately globular in shape, and their size is small. The CaO, A1203 and SiOz complex inclusions are found in the inclusions with a small amount of MnO and sulfide. In all of the inclusions studied, only 2 inclusions containing BaO were found and their content is less than 0.1%.

The solubility of barium is very low and it is possible that only a small amount of BaO is generated in early deoxidation when Si-Ca-Ba alloys are added to the molten

443

Yang Li, Zhou-Hua Jiang and Yang Liu

steel. Simple deoxidation products were generated by the other deoxidation elements in the deoxidizer, however, the complex deoxidation products were generated following the aggregation, impaction and growth of the simple deoxidation products. The radius of BaO (deoxidation product of barium) is large due to the large radius of barium. Therefore, the probability of generating complex deoxidation products after impaction and growth with other deoxidation products is high, and the radius of barium-bearing complex deoxidation products is also large. According to fundamental dynamics, the rise velocity of inclusions is proportional to the inclusion radius. The quantity of inclusions is reduced at the end of smelting operations when deoxidation products of alloys containing barium easily float upwards. Therefore, the inclusions containing BaO were only found in small amounts in the experiments.

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400

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0 5 10 15 20 25 30 35 40

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Figure 4. Effect of A1203 content in rejhing slag on quantity and area of inclusions,

444

Effect of Refining Slag Composition on Inclusions in Molten Steel

Figure 5. Stereoscan photograph and spectral line pattern of QpicaI terminal inclusions.

AAer statistical analysis of about 10 inclusions in each sample, it was found that the A1203 content in the complex inclusions increases as the A1203 content in refining slag increases, and the SiOz content in the complex inclusions decreases as the slag basicity increases, as shown in Figure 6. This indicates that the slag composition has a direct effect on the composition of inclusions. Analysis of the terminal inclusions in the plagioclase experiment (No. 10 heat) show equal mole percentages of A1 and Si, and that of Ca is about half of A1 and Si. Therefore, the composition of CaO, A1203 and Si02 in inclusions is very similar to that of the Ca0.AI20y2SiO2, and the desired composition of the inclusions can be obtained by controlling the slag composition.

Conclusions Under the present experimental conditions, as the refining slag basicity increases then the total oxygen content of molten steel, the total number, the total area and average radius of inclusions decreases, and the proportion of inclusions less than 2pm increases. This indicates that increasing the basicity of the refining slag is beneficial to the generation of small sized inclusions, or the floating up and dissolution in slag of larger sized inclusions in the molten steel when treated by barium-bearing alloys.

445

Yang Li, Zhou-Hua Jiang and Yang Liu

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Figure 6. Eflect of refining slag properties on the properties of inclusions

The total number and the total area of inclusions in molten steel treated by Si-Ca-Ba alloys are the minimum when the A1203 content in the refining slag is 13% to 20%. The proportion of inclusions smaller than 2pm increases when the Alz03 content in the refining slag exceeds lo%, which indicates that controlling the A1203 content in refining slag is beneficial in order to obtaingain small sized inclusions.

Analysis of the composition of the final inclusions shows that the desired composition of inclusions can be achieved by controlling slag composition, due to the direct effect of the slag composition on the composition of inclusions.

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Effect of Refining Slag Composition on Inclusions in Molten Steel

Acknowledgments The authors acknowledge National Natural Science Foundation of China (NSFC) and Shanghai Baoshan Iron & Steel Corporation of China (No. 50174012) for their financial support.

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CaO-Al~OrSiOrMgO slags, Ironmaking & Steelmaking, 29( I), 47-52. 2. Yoon, B.H., Heo, K.H., Kim, J.S., et al. 2002. Improvement of steel cleanliness by controlling slag

composition, Ironmaking & Steelmaking, 29(3), 215-2 18. 3 . Hilty, D.C., and Popp, V.T. 1969. Improving influence of calcium on inclusion control. ISS Electric

Furnace Proc., 52-66. 4. Han, Q.Y., Tang, L., and Wang, Q.K. 1992. Application of barium-containing alloys in steelmaking,

J. Iron Steel Res. (in Chinese), 4(3), 98-106. 5 . Li, Y., Jiang, Z.H., Jiang, M.F., et al. 2003. Deoxidation behavior of alloys bearing barium in molten

steel, J. Iron Steel Res. In!., 10(4), 13-17.

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