Inclusion Evolution after Calcium Addition in Low Carbon Al-Killed Steel with Ultra Low Sulfur Content

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1 , pp Inclusion Evolution after lcium Addition in Low rbon Al-Killed Steel with Ultra Low Sulfur Content Guangwei YANG* and Xinhua WANG School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, China. (Received on June 24, 2014; accepted on September 11, 2014) Inclusions in steel samples collected 0.5, 2, 5, 10 and 20 min after calcium addition were investigated in Low rbon Aluminum Killed steel (LCAK steel) with ultra low sulfur content ( % Al, 9 ppm S). It s found that inclusions change from O to O S and finally to O Al 2O 3 S with time. Al 2O 3 in inclusions linearly decreases by increasing T./T.O of the steel and disappears when T./T.O exceeds 3. S/O of the inclusions linearly increases by increasing S/T.O of the steel. Contents of O, Al 2O 3 and S can be estimated through T., S and T.O. O S clusters are found at 5 min. The mechanism of inclusion evolution and formation of O S clusters are discussed. KEY WORDS: O S; O Al 2O 3 S; clusters; calcium addition; LCAK Steel. 1. Introduction Al 2O 3 inclusions are considered to be harmful to steelmaking process and final product. 1 4) Proper calcium treatment can modify Al 2O 3 into low melting O Al 2O 3, through which clogging can be avoided. Excess or insufficient calcium addition can produce solid S or CA 6, CA 2 5 7) (where C is O and A is Al 2O 3), which are also harmful in terms of submerged entry nozzle (SEN) clogging. Thus, the composition of inclusion, such as O, S and Al 2O 3, should be well controlled in order to get desired properties of steel. There are two methods to predict the inclusion type after calcium treatment. One is through dissolved aluminum and sulfur contents. 8 12) However, activities of O, S and Al 2O 3 are needed in this method. Moreover, controversy exists in the literature over the choice of such data. 9,10) Different choice may lead to different results. Thus this method is not convenient. The other method is through T./T.O of the steel. 8,9,13) For example, if the ratio T./T.O is just larger than 0.6, inclusions will be a mixture of solid phase CA and liquid phase C 0.57A 0.43 at K. In this method, it s assumed that all calcium is bound only to oxygen in the inclusions. Only in this case, T./T.O ratio would give a realistic picture of inclusion type. But calcium can also react with dissolved sulfur to form solid S. For this case, T./T.O ratio could not well predict inclusions. Contents of T., T.O and S are very important in determining the inclusion type. The above methods, however, didn t consider them all. Moreover, they mainly focused on steel with relatively high sulfur content, 11 13) ppm. Most importantly, the two methods can only predict inclusion type but can t give the composition of inclusions. In present study, inclusions in steel samples collected 0.5, 2, 5, 10 and 20 min after calcium addition were investigated in LCAK steel with ultra low sulfur content ( % Al, 9 ppm S). Contents of O, S and Al 2O 3 are easily determined through T., T.O and S content in steel. The relation between steel and inclusion is obtained through the fitting of the experimental data. Dissolved calcium is calculated to explain the mechanism of evolution of inclusion. O S clusters were found and its formation is explained. 2. Experimental Methods 2.1. Experimental Procedures Five experiments were carried out in this study. Induction * Corresponding author: ustb_jack@hotmail.com DOI: Fig. 1. Schematic diagram of experimental apparatus ISIJ 126

2 vacuum furnace was employed as melting apparatus and its schematic diagram is shown in Fig. 1. In each experiment, g steel material, 0.3 g Al lump and 3 g Si (30%- 70%Si) were firstly placed into the furnace, then the chamber was evacuated with mechanical pumps and then backfilled with high-purity argon to atmospheric pressure. The Al lump was placed in the feed opening. Si alloy wrapped with steel was fixed in a molybdenum rod. The chemical composition of steel material used in the experiments is given in Table 1. After the steel had melted, the melt was held at K (1 600 C) for 3 minutes for homogenization. Then Al was added, followed by Si addition after 5 minutes. After different holding time, the steel was poured into an iron mold. The holding time was 0.5 min, 2 min, 5 min, 10 min and 20 min. It s the only difference between the five experiments. The flowchart of the procedure is shown in Fig. 2. The time after Si addition is set as 0 min Chemical Analysis and Characterization of Inclusions The total calcium (T.) and acid-soluble aluminum ([Al]) of the steel samples were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) method. Total oxygen (T.O) and sulfur contents of the steel were determined by infrared absorption method. ASPEX PSEM EXPLORER with Automated Feature Analysis system was employed to analyze inclusions, including chemistry, morphology, number and so on. Inclusions less than 1 μm were not detected in this study. The scan area was 20 mm 2, hundreds of particles were detected. It should be noted that the inclusion composition that ASPEX provides is the average composition of the whole inclusion, similar to average composition when selecting an area for EDX. 3. Experimental Results 3.1. Steel Composition The chemical compositions of steel samples are given in Table 2. Contents of [Al] and T.O are in the range of to and to pct. S content maintains 9 ppm regardless of holding time, the same as that in the steel material. T. content decreases from to pct from 0.5 min to 5 min, and maintains nearly the same afterwards Characterization of Inclusions Typical inclusions at 0.5 min and 2 min after calcium addition are shown in Fig. 3. At 0.5 min, the typical inclusions are mainly pure O. The mapping of such inclusion is shown in Fig. 3(a). Complex inclusions with O Al 2O 3 inside and pure O outside are also found as shown in Fig. 3(b). It indicates Al 2O 3 is gradually reduced by dissolved calcium from outside to inside. In the sample collected 2 min after calcium addition, O S type inclusions with O partially covered by S are observed. The mapping of such inclusion is shown in Fig. 3(c). In the sample collected 5 min after calcium addition, O S type inclusions are still the dominant inclusions. O S clusters are also found in this sample. The morphology and X-ray spectrums are shown in Figs. 4(a) 4(c). Inclusions change Table 1. Chemical compositions of steel material (mass pct). C Si Mn P S [Al] Table 2. Chemical compositions of steel samples (mass pct) Fig. 2. Flowchart of experimental procedure. Holding time, min [Al] S T. T.O Fig. 3. Typical inclusions at 0.5 min and 2 min after calcium addition ISIJ

3 Fig. 6. Change of inclusion composition with holding time. Fig. 5. Fig. 4. Typical inclusions at 5 min after calcium addition. Typical inclusions at (a) 10 min and (b) 20 min after calcium addition. to spherical O Al 2O 3 S for the sample collected 10 min and 20 min after calcium addition. The morphology and X-ray spectrums are shown in Fig. 5. peak at 10 min is higher than that at 20 min, which means O content in 10 min sample is more than that in 20 min sample. Verma et al. 14) observed similar evolution in Fe-0.1%Al %S low sulfur steel after calcium addition. They found inclusions in the sample collected 0.5 min after calcium treatment were mostly O. In some cases, S was found attached to O inclusions. In the sample collected 2 min after calcium treatment, S content increased and O decreased. In the sample collected 4 min after calcium treatment, the inclusions changed to liquid O Al 2O 3 with a low S content. Figure 6 shows the change of average composition of inclusions. With holding time, O content decreases. S content first increases and begins to decrease at 5 min. Al 2O 3 content begins to increase at 5 min. MgO gradually increases with time, but the content is low compared with others. Evolution of inclusions follows O O S O Al 2 O 3 S. Figure 7 shows distribution of inclusion composition and size in O S Al 2 O 3 ternary system. Solid circles, open triangles and open squares represent inclusions with diameter in between 1 5 μm, 5 10 μm and μm, respectively. No inclusions larger than 20 μm are found in all samples. The solid line is the K liquid line, within which the inclusions are liquid. For samples collected at 0.5 min, 2 min and 5 min, inclusions are uniformly located in O S line and are small. No inclusions over 10 μm are found. For samples collected at 10 min and 20 min, inclusions are O Al 2 O 3 with different amounts of S. Big inclusions over 10 μm are found in these two samples, and the composition are mainly low melting O Al 2 O 3 with small amount of S. Figure 8 shows the change of inclusion number with holding time. The number suddenly picks up from 2.1/mm 2 to 9.5/mm 2 at 2 min, reaching 10/mm 2 at 5 min, then gradually decreases. Similarly, Zinngrebe et al. 15) found inclusion number increased by a large factor after calcium treatment in Al-killed steel. 4. Discussion 4.1. Mechanism on Composition Change of Inclusion with Time In the sample collected 0.5 min after calcium addition, the main inclusion is pure O. There are two sources. Pure Al 2 O 3 reacts with dissolved calcium and changes into pure O, as expressed by Eq. (1). In addition, dissolved oxygen reacts with dissolved calcium and becomes pure O, as expressed by Eq. (2). For steel with 0.05% Al, dissolved oxygen in equilibrium with pure Al 2O 3 and O are approximately 4.7 ppm and 0.6 ppm, as will be calculated later. Thus ( =4.1) ppm dissolved oxygen and (6 4.1=1.9) ppm undissolved oxygen (Al 2O 3) finally become O. 3[ ] + Al2O3( inclusion) = 3O( inclusion) + 2[ Al]... (1) Δ G θ 16, 17) = T [ ] + [ O] = ( O) inclusion ) Δ G θ = T (2) 2015 ISIJ 128

4 Fig. 7. Distribution of inclusion composition and size in O S Al 2O 3 ternary system. In order to analyze change of the calcium aluminates with time, dissolved calcium contents are estimated. T.O in Table 2 includes dissolved oxygen ([O]) in steel and undissolved oxygen in oxide inclusions, as expressed by Eq. (3). T. in Table 2 includes dissolved calcium ([]) in steel and undissolved calcium in S and O, as expressed by Eq. (4). T.O = [O] + O(oxide)... (3) T. = [] + (S) + (O)... (4) In an aluminum killed steel, the dissolved oxygen [O%] is determined by the equilibrium between [Al] in the steel and Al 2O 3 in the inclusions, as shown in Eq. (5). 2[ Al] + 3[ O] = Al2O3( inclusion)... (5) 16) Δ G θ = T The Al 2O 3 is either pure or Al 2O 3 in a calcium aluminate inclusion. [O%] is given by Eq. (6). [ O%] = exp 1 ΔG θ 3 aal 2O3 RT f[ Al] [ Al% ] f[ O]... (6) Activity of Al 2O 3 in O Al 2O 3 binary system was calculated with Thermo-lc, as shown in Fig. 9. Contents of dissolved oxygen are obtained assuming activity coefficients of Al and O as unity, as shown in Fig. 10. The [Al] ISIJ

5 in this figure is 0.05%, average of [Al] contents in Table 2. As can be seen, [O] is about 0.6 ppm when Al 2O 3 content is less than 40%. The average Al 2O 3 contents in all samples are around 40%, as can be read from Fig. 6. Thus [O] contents for all samples are 0.6 ppm. Fig. 9. Fig. 8. Change of inclusion number with holding time. Activity of Al 2O 3 and O in O Al 2O 3 binary system at K. For inclusions in samples collected 0.5 min, 2 min and 5 min after calcium addition, inclusions are mainly O and S, with the total contents over 95%. The normalized composition is shown in Table 3. The undissolved oxygen, calculated by Eq. (3), is in the form of O. Thus undissolved calcium in O is obtained. The undissolved calcium in S is also obtained based on the O and S contents in inclusions. The dissolved calcium [] finally is calculated by Eq. (4). The results are shown in Table 3. Figure 11 shows the relation between the natural logarithm of [%] and holding time. As can be seen, ln[%] linearly decreases with time. The relation is expressed as Eq. (7). Assuming [] at any time follows the same trend, [] contents at 10 min and 20 min are 0.37 ppm and ppm. ln[%] = 0.42 T 6... (7) Activities of [Al] and [] can be calculated by Eq. (8). Contents of C, Si, Mn and P are shown in Table 1. [Al], [O] and [] contents are shown in Tables 2 and 3. The first and second order interaction coefficients are shown in Table 4. lg a = lg[ i%] + lg f i i j jk, = lg[ i%] + ei [ j%] + r ( ) i [ j%][ k%] ( j k) j j k... (8) To analyze evolution of calcium aluminates, the stability phase diagram of various calcium aluminates inclusion was drawn as shown in Fig. 12. It s based on Eq. (9). Activities of O and Al 2O 3 are given in Fig. 9. The solid circles denote present results. As seen in Fig. 6, the calcium aluminates at 0.5 min, 2 min and 5 min are O. The calcium aluminate at 10 min are O-39%Al 2O 3, nearly the same with C 3A which contains 38%Al 2O 3. Thus calcium aluminates at 0.5 min, 2 min, 5 min and 10 min are O and C 3A, which is in accordance with the location in Fig. 12. The calcium aluminate at 20 min are O-43%Al 2O 3, locating between Fig. 10. Dissolved oxygen in steel. Fig. 11. Change of [] with time. Table 3. Total, dissolved and undissolved oxygen and calcium of steel samples (ppm). Time/min O S T. T.O [O] O(O) (O) (S) [] ISIJ 130

6 Table 4. First and second order interaction coefficients e j i, r j i. j i C Si Mn P S Al O Al ) O ) All data are at K (1 600 C) and the ones without notation are from Ref. 19). r = )., O, r = )., O r = ).,, O r = ) O O Fig. 12. Stability phase diagram of various calcium aluminates. Fig. 13. Change of Gibbs free energies of reactions with dissolved calcium. C 3A and C 12A 7 in O Al 2O 3 binary system, which is also in accordance with its location in Fig. 12. K 3[ ] + Al O = 3O + 2[ Al] θ K 2 3( inclusion) ( inclusion) Δ G θ = T 3 2 O Al 3 Al a 2O 3 16, 17) θ G a a = exp Δ = = RT a a = a a 3 O θ Al K 2O K a 3 Al... (9) The formation of S is discussed below. There are two sources of S. One is reaction product of O and sulfur, as shown in Fig. 3(c). The reaction can be expressed by Eq. (10). The other is product of dissolved calcium and sulfur, as expressed by Eq. (11). In fact, Eq. (11) happens at 0.5 min after calcium addition, but most of these S inclusions are too small (less than 1 μm) that couldn t be detected by the SEM. They grow into bigger ones and can be detected with time. It can be verified by the increase of inclusions near the S corner of the O S Al 2O 3 ternary shown in Figs. 7(a) 7(c).... (10)... (11) Figure 13 shows the change of Gibbs free energies of reactions with dissolved calcium. Gibbs free energy of reaction, ΔG r, can be obtained by Eq. (12). ΔG r is often used to ( O) + [ S] = ( S) + [ O] inclusion inclusion Δ G θ = T [ ] + [ S] = ( S) 17, 20) inclusion Δ G θ = T 20 ) describe whether one reaction is favored or not. If ΔG r is negative, this reaction can proceed spontaneously. ΔG = ΔG θ + RT lnq r... (12) Q r is the reaction quotient. For example, reaction quotient of Eq. (11) can be expressed as as Qr = a a Activities of [S] and [] can be calculated by Eq. (8). The steel composition used in this calculation is shown in Table 5. Contents of C, Si, Mn and P are from Table 1. Content of [Al] is 0.05%, average of [Al] contents in Table 2. Content of [O] is from Table 3. The calcium aluminates in all samples are O or near C 3 A (Al 2 O 3 content less than 38%), thus the activities of O is unity based on Fig. 9. Activities of S are taken as unity due to low solubility of S in the calcium aluminates. 21) The [] contents are 21.9, 9.5 and 3.2 ppm for samples collected 0.5, 2 and 5 min after calcium addition. As can be seen in Fig. 13 that Gibbs free energies of Eqs. (10) and (11) are negative, thus S can form, but the driving force of the reactions decreases with time. Thus S content increases as shown in Fig. 6. Because the formation of S by Eq. (10) consumes previous formed O, thus O content decreases. For samples collected 10 min after calcium addition, the [] content is 0.37 ppm, Gibbs free energy of Eq. (11) is negative, thus S can form. However, the Gibbs free energy of Eq. (10) is positive, thus the reaction proceeds in the reverse direction. Previous formed S through Eq. (10) changes to O, which further changes to Al 2 O 3 through Eq. (1) and decomposes to dissolved calcium and oxygen S r ISIJ

7 Table 5. Chemical compositions of steel for calculation (mass pct). C Si Mn P S [Al] [] [O] variable through Eq. (2) due to the decrease of dissolved calcium. Thus S and O decreases, while Al 2 O 3 increases, as shown in Fig. 6. For samples collected 20 min after calcium addition, the [] content is ppm, Gibbs free energies of Eqs. (10) and (11) are positive, thus S can t form. The two reactions proceeds in the reverse direction. Previous formed S either decomposes into dissolved calcium and sulfur through Eq. (11) or changes into O through Eq. (10). Also, O will change to Al 2 O 3 through Eq. (1) and decompose to dissloved calcium and oxygen through Eq. (2) due to the decrease of dissolved calcium, thus O decreases, while Al 2 O 3 increases, as shown in Fig. 6. Nevertheless, S content is slightly higher than that at 10 min. It s mainly due to the higher S/T.O in this sample. And a higher S/T.O represents a higher S/O in inclusion. The relation is discussed later. Fig. 14. Al 2O 3 in inclusions and T./T.O in steel Relationship between Steel and Inclusions Figure 14 shows the relation between T./T.O in the steel and Al 2 O 3 in O S Al 2 O 3 inclusions. Results from Numata and Higuchi 22,23) are also included in this figure. It can be seen that Al 2 O 3 decreases with T./T.O, becoming trace when T./T.O is over 3. Inclusions change from O Al 2 O 3 with small amount of S to O S. When T./T.O is less than 3, a significant linear dependence is observed between Al 2 O 3 in the inclusions and T./T.O in steel. The best-fit line equation is expressed by Eq. (13). The correlation coefficient R 2 is nearly unity. Equation (13) is mainly based on the results by Numata et al. Accordingly, results from Fig. 14 are in reasonable agreement with Numata et al. s. When T. / T. O 3, 2 ( Al2O3%) = 30 T. / T. O + 84 R = (13) Figure 15 shows the relation between S/O of inclusions and S/T.O in steel. As can be seen, S/O of inclusion linearly increases with S/T.O in steel except the sample collected 0.5 min after calcium addition. There may be two reasons for the low S/O of inclusion. One is that 0.5 min is too short for the system to reach equilibrium. The other is that the formed S particles, which are product of dissolved calcium and sulfur, are too small (less than 1 μm) that couldn t be detected by the SEM, leading to an underestimate of S. The best-fit line equation is expressed by Eq. (14). The correlation coefficient R 2 is nearly unity. 2 S / O = S / T. O R = (14) For LCAK steel with Al among % and S less than %, contents of O, Al 2 O 3 and S in inclusions after calcium treatment can roughly obtained through Eqs. (13) and (14). It s useful in many ways. Fig. 15. S/O of inclusions and S/T.O in steel. Solid Al 2O 3 can cause SEN clogging. lcium treatment are always employed to solve this issue. Excess calcium treatment can produce solid S, which can also give rise to SEN clogging. Insufficient calcium treatment can produce CA 6 or CA 2, which is more harmful than Al 2O 3 in terms of SEN clogging. Proper calcium treatment can modify Al 2O 3 into low melting O Al 2O 3, through which clogging can be avoided. As can be read from Fig. 14, when controlling T./T.O around 1.0, Al 2O 3 can be fully modified to low melting O Al 2O 3 with composition near C 12A 7. Deng and Zhu 13) found good castability is obtained when /T.O is controlled between 0.91 and 1.25, which is in consistent with present study. It s found that type B inclusions, which are disconnected oxides containing, Al and S, can lead to hydrogen induced cracking. In order to prevent the formation of type B inclusions in plate rolling, composition of O, Al 2O 3 and S in inclusions must be well controlled. 24) Through adjusting T., S and T.O content in steel, the target composition of O, Al 2O 3 and S can be hit based on Eqs. (13) and (14) Formation of O S Clusters At 5 min, many O S clusters forms. There are two reasons that possibly explain the formation. The first reason is huge number of O S type inclusions, and the second 2015 ISIJ 132

8 is the high aggregation tendency of S in inclusions. As seen in Fig. 8, the inclusion number suddenly picks up at 2 min, reaching the maximum at 5 min, nearly 5 times that at 0.5 min. At 0.5 min after calcium treatment, Eqs. (1), (2), (10) and (11) happen. Equations (1) and (10) are displacement reactions, not involving the increase of inclusion number. Equations (2) and (11) generate lots of tiny O and S particles, these inclusions are so small that they are beyond the scope of the detection limit, 1 μm at least. At 2 min to 5 min after treatment, previous formed tiny inclusions grow into larger ones through agglomeration and become detectable, thus the inclusion number increases. And O S clusters form with further agglomeration. Wang et al. 25) investigated effect of S formation on the agglomerate ability of Al 2O 3 O inclusions in lowcarbon and -treated steel through high-temperature confocal scanning laser microscopy (CSLM). They found liquid-globular Al 2O 3 O inclusions don t agglomerate with each other, but when S forms outside Al 2O 3 O, these inclusions begin to agglomerate with each other, and finally becomes clusters. It can be inferred S has a strong tendency to promote inclusions to agglomerate. In present study, S also exists in inclusions, thus it could be another reason for the formation of O S clusters. Both Ikeda et al. 26) and Nafisi et al. 27) found O S clusters in the HIC fracture surface of pipeline steel. It indicates that O S clusters are harmful for HIC resistance property. Thus O S clusters should be minimized. From above discussion, it can be reasonably concluded that the formation of tiny O S particles can be prevented through lowering dissolved calcium content. And O Al 2O 3 S type inclusions may be a good choice in terms of HIC resistance. The composition of O, Al 2O 3 and S can be controlled through adjust steel composition based on Eqs. (13) and (14). 5. Conclusions Inclusions in steel samples collected 0.5, 2, 5, 10 and 20 min after calcium addition are investigated in ultra low sulfur LCAK steel ( % Al, 9 ppm S). The following conclusions were obtained: (1) At 0.5 min after calcium addition, pure O inclusions are the main inclusions. They are the reaction product of dissolved calcium with i) Al 2O 3 in inclusions and ii) dissolved oxygen. With time, S content increases and inclusions change to O S. S content reaches the most at 5 min. They are the reaction product of dissolved sulfur with i) O in inclusions and ii) dissolved calcium. Because dissolved calcium content decreases with time, previous formed S changes to O or decomposes into dissolved calcium and sulfur. With further decrease of dissolved calcium, O changes to Al 2O 3 and decomposes to dissloved calcium and oxygen. And the inclusions change to O Al 2O 3 S at 10 min after calcium treatment. (2) Contents of O, Al 2O 3 and S of inclusions can be estimated through T., S and T.O in the steel based on following equations. 30 T. / TO T. / TO. 3 ( Al2O3% ) = 0 T. / TO. > 3 S / O = S/ TO. (3) O S clusters are found at 5 min after calcium addition. There are two reasons that possibly explain the formation. Firstly, dissolved calcium reacts with dissolved oxygen and sulfur in the first 5 minutes, thus a huge number of tiny O S type inclusions are generated. Initially, they are very small and not detectable. Later, they grow into larger ones through agglomeration and become detectable, thus the inclusion number increases. O S clusters form with further agglomeration. Secondly, S has a strong tendency to promote inclusions to agglomerate. Acknowledgments The authors gratefully express their appreciation to the National Basic Research Program of China (No. 2010CB630806) and National Natural Science Foundation of China (No and No ) for sponsoring this work. REFERENCES 1) M. Burty, P. 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