Effect of Na 2 O and Rb 2 O on Inclusion Removal in C96V Saw Wire Steels Using Low-Basicity LF (Ladle Furnace) Reﬁning Slags

: Inclusion removal and modiﬁcation of C96V saw wire steel using Na 2 O- and Rb 2 O-containing novel low-basicity LF (ladle furnace) Reﬁning Slags have been researched. The results indicated that the addition of Na 2 O deteriorates inclusion removal; by contrast, the addition of Rb 2 O seems to signiﬁcantly enhance inclusion removal. In detail, Rb 2 O can improve the cleanliness in the as-quenched C96V saw wire steel melts compared to preexisting synthetic LF reﬁning slag compositions: (i) The average inclusion diameter experienced a remarkable decrease after reaction between the liquid steel and the synthetic LF reﬁning slag; (ii) In addition, the number of inclusions also suffered from a dramatic decrease, with the reaction time increasing from 900 to 2700 s (15·to 45 min); (iii) Furthermore, both of the MnO-SiO 2 -Al 2 O 3 and CaO-SiO 2 -Al 2 O 3 inclusion system mainly concentrated in the low melting zone when the composition of Rb 2 O in synthetic reﬁning slag was ≥ 5.0 wt%. This is mainly because Na 2 O signiﬁcantly reduces the viscosity of reﬁning slag, while Rb 2 O increases it. Then, there are two remarkable inﬂuences causing the increase of viscosity of reﬁning slag with the addition of Rb 2 O: the inclusions can be sufﬁciently entrained within the slag once absorbed due to the signiﬁcant increase in the viscosity; and the slag entrapment during reﬁning process weakened dramatically.


Introduction
In order to obtain a high productivity, higher tensile properties and smaller diameters (60 × 10 -6 m (60 µm) to 80 × 10 -6 m (80 µm)) than tire cord are very for saw wire [1].Therefore, much a higher requirement for inclusion control should be put forward to prevent wire breakage caused by large size or hard inclusions which often initiate breakage during cord drawing and standing processes [2][3][4].To this end, a lot of technology has been developed by metallurgical workers, such as Si-Mn deoxidation [5], boron addition to modify inclusions [6], and low basicity top slag refining [7] and Na 2 CO 3 addition to modify inclusions [8].
On the other hand, there are still some problems that plagued metallurgical workers for many years, but which the subjects of new research and breakthroughs in recent years [9][10][11][12][13].For example, in terms of the formation mechanism and evolution of SiO 2 -type inclusions in Si-Mn killed steel wires, Wang et al. [9,10] found, in 2015, that the formation mechanism of SiO 2 -type inclusions in wire rods involves two main steps: First, solid SiO 2 phase precipitates form MnO-SiO 2 -Al 2 O 3 system inclusions during the casting process to form dual-phase inclusions in the as-cast bloom, and second, dual-phase inclusions experience phase separation during the rolling process for heterogeneity and great compression ratios [9,10].However, some of their results are not very persuasive.For example, they only analysed the inclusions in tundish and cast-bloom, but not in wire rod.As we know, dual-phase inclusions may experience phase separation during the drawing process for heterogeneity and great compression ratio.Thus, the formation mechanism and evolution of SiO 2 -type inclusions in Si-Mn killed steel wires should be studied further.With regard to the formation mechanism of CaO-SiO 2 -Al 2 O 3 -(MgO) inclusions, there were some controversies until Wang et al. [11,12] did some research in 2017 and came up with a very persuasive view.Wang et al. pointed out that it can be reasonably concluded that CaO-SiO 2 -type inclusions in saw wire were exogenous particles form entrapped/emulsified top slag, but not products of slag-steel-inclusion chemical reactions [11,12].However, this argument has not been confirmed by others or recognized by fellow researchers.
For the factors influencing oxide inclusion, deformability under high and low temperatures had not been noticed until Zhang et al. [13] did some research in 2018.They came up with a theory that the deformability of oxide inclusion under different temperatures could be scaled by Young's moduli.However, some of the results are not very persuasive.For example, the theory of the influence of temperature on the deformability of oxide inclusions has not been confirmed by experiments.
Few researchers have looked at changing the chemical composition of LF refining (ladle furnace) slag with new components to improve inclusions absorption.The influence of alkali oxides enhancing inclusion removal had not been noticed until Sohn et al. [14,15] did some research at 2014.The effect of alkali oxides (Li 2 O, Na 2 O, K 2 O, Rb 2 O, Cs 2 O) containing tundish fluxes on inclusion removal have been studied; the results indicated that K 2 O, Rb 2 O, and Cs 2 O addition seems to significantly enhance inclusions removal, but the influence of Li 2 O and Na 2 O has the opposite effect.In detail, not only the number of inclusions, but also the average inclusions diameter, increased with minimal improvement with Li 2 O and Na 2 O addition.Sohn et.al came up with an explanation that K + , Rb + and Cs + have a large cationic radius compared to Li + and Na + ; therefore, the inclusions can be sufficiently entrained within the slag once absorbed due to the significant increase in the viscosity.However, they did not give any more detailed explanation about how the increase in viscosity of slag can improve the absorption ability of inclusions.
On the other hand, the alkali oxides have not been applied in LF refining progress until now.The influence of alkali oxides on the number, size, morphology, and composition (especially for MnO-SiO 2 -Al 2 O 3 and CaO-SiO 2 -Al 2 O 3 inclusion systems) is still not completely understood.
Therefore, we have explored the influence of alkali oxides on enhancing inclusion removal by adding 2.5 wt%, 5.0 wt%, and 7.5 wt% Na 2 O and Rb 2 O into synthetic LF refining slag.The influence of alkali oxides (Na 2 O, Rb 2 O) on the number, average diameter, composition, and morphology of inclusions has been studied.Furthermore, we came up with a more detailed explanation about how the increase in viscosity of slag can improve the absorption ability of inclusions.

Experimental Appratus and Procedure
With respect to simulating LF refining progress, the MoSi 2 furnace was utilized to fabricate the C96V saw wire steels in this experiment.A schematic diagram of the experimental equipment is shown in Figure 1.
In this experiment, the temperature of liquid metal is continuously measured by means of a B-type reference thermocouple.An argon atmosphere was kept in experiments all the time, blowing from the bottom to the top of the furnace tube.The experimental procedures were carried as follows.Firstly, 1.00 kg industrial pure iron is placed into a MgO crucible of 60 × 10 -3 m (60 mm) inner diameter and 80 × 10 -3 m (80 mm) depth.Secondly, the crucible is placed in a graphite crucible to prevent liquid metal from leaking.Finally, after the whole crucible is placed in the chamber, the power is switched on and the furnace is heated to the experimental temperature [1873 K (1600 Alloys were added into the melts when the temperature reached 1873 K (1600 • C).Sampling No. 0 is the original chemical composition in steel without interaction of slag and metal.After that, 0.05 kg synthetic LF refining slag was put onto the surface of the molten metal by corundum funnel (Shandong Hao Yang wear resistant material Co., Ltd., Zibo, China).The height of solid slag and liquid slag are about 2.0 × 10 -2 m (2.0 cm) and 1.5 × 10 -2 m (1.5 cm); it completely covered the surface of the metal.Samples No.1 to No. 3 are taken from the molten metal after the slag melted for 15, 30, and 45 min.After each sampling, the steel liquid is stirred with a graphite rod for 2 min to make the molten steel and the refining slag uniform.All of the samples are taken by quartz tube sampler and quenched immediately in water.
In total, there are 7 heats made by treating with different synthetic LF refining slags, to which were added 2.5 wt%, 5.0 wt%, and 7.5 wt% Na 2 O and Rb 2 O, as shown in Table 1.Samples were taken at 0, 15, 30, and 45 min during the refining progress.In this experiment, the temperature of liquid metal is continuously measured by means of a B-type reference thermocouple.An argon atmosphere was kept in experiments all the time, blowing from the bottom to the top of the furnace tube.The experimental procedures were carried as follows.Firstly, 1.00 kg industrial pure iron is placed into a MgO crucible of 60 × 10 -3 m (60 mm) inner diameter and 80 × 10 -3 m (80 mm) depth.Secondly, the crucible is placed in a graphite crucible to prevent liquid metal from leaking.Finally, after the whole crucible is placed in the chamber, the power is switched on and the furnace is heated to the experimental temperature [1873 K (1600 °C)].
Alloys were added into the melts when the temperature reached 1873 K (1600 °C).Sampling No. 0 is the original chemical composition in steel without interaction of slag and metal.After that, 0.05 kg synthetic LF refining slag was put onto the surface of the molten metal by corundum funnel (Shandong Hao Yang wear resistant material Co., Ltd., Zibo, China).The height of solid slag and liquid slag are about 2.0 × 10 -2 m (2.0 cm) and 1.5 × 10 -2 m (1.5 cm); it completely covered the surface of the metal.Samples No.1 to No. 3 are taken from the molten metal after the slag melted for 15, 30, and 45 min.After each sampling, the steel liquid is stirred with a graphite rod for 2 min to make the molten steel and the refining slag uniform.All of the samples are taken by quartz tube sampler and quenched immediately in water.
In total, there are 7 heats made by treating with different synthetic LF refining slags, to which were added 2.5 wt%, 5.0 wt%, and 7.5 wt% Na2O and Rb2O, as shown in Table 1.Samples were taken at 0, 15, 30, and 45 min during the refining progress.

Analysis Methods
A direct reading spectrometer was utilized to detect the composition of Si, Mn, P, Cr, V, Ni, and Cu.Steel samples were also sent to Analysis and Testing Center (Chemical Laboratory, Shenyang, China) of Northeastern University to analyse Al content by the ICP (Inductively Coupled Plasma) method.For C and S, an infrared C/S analyzer was applied.Then, the LECO ® TC 500 O 2 /N 2 analyzer (LECO, San Jose, MI, USA) was chosen to detect O and N. The chemical composition of C96V saw wire steel is shown in Table 2.All of these steel samples were treated by 100-2000 mesh sand papers and polished.After that, photos (more than 50 pieces) around "S" route were taken.(On the surface of the sample, photos were taken in order from right to left, followed by from left to right).The Image J software (Image J 1.48, National Institutes of Health, Bethesda, MD, USA, 2014) was used for statistics of the size and count of inclusions; this software is a public domain, Java-based image processing program developed at the National Institutes of Health (NIH).
Finally, the metal samples were analysed by scanning electron microscope (SEM; Carl Zeiss AG, Niedersachsen, Germany) and Energy Dispersion Spectrum (EDS; Carl Zeiss AG, Niedersachsen, Germany) to determine the morphology and component of inclusions.

Effect of Na 2 O and Rb 2 O on the Variation in Inclusion Diameter
Figure 2 shows the effect of reaction time on the average non-metallic inclusion diameter with the addition of Na 2 O or Rb 2 O.For normal heat, it is obvious that the count descends slowly with refining time being raised from 15 to 45 min.All of these steel samples were treated by 100-2000 mesh sand papers and polished.After that, photos (more than 50 pieces) around "S" route were taken.(On the surface of the sample, photos were taken in order from right to left, followed by from left to right).The Image J software (Image J 1.48, National Institutes of Health, Bethesda, MD, USA, 2014) was used for statistics of the size and count of inclusions; this software is a public domain, Java-based image processing program developed at the National Institutes of Health (NIH).
Finally, the metal samples were analysed by scanning electron microscope (SEM; Carl Zeiss AG, Niedersachsen, Germany) and Energy Dispersion Spectrum (EDS; Carl Zeiss AG, Niedersachsen, Germany) to determine the morphology and component of inclusions.

Effect of Na2O and Rb2O on the Variation in Inclusion Diameter
Figure 2 shows the effect of reaction time on the average non-metallic inclusion diameter with the addition of Na2O or Rb2O.For normal heat, it is obvious that the count descends slowly with refining time being raised from 15 to 45 min.
The addition of Na2O in Figure 2 shows that the inclusion diameter with the addition of Na2O gets larger than that at normal heat at all sampling times; what is worse, increments in Na2O seem to increase the inclusion diameter observed in steel samples, suggesting that Na2O is not at all a suitable candidate for enhancing inclusion removal in C96V saw wire steels.The addition of Na 2 O in Figure 2 shows that the inclusion diameter with the addition of Na Contrary to Na2O addition, Rb2O addition seems to yield a dramatic trend of decreased average inclusion diameter with increasing reaction time.(i) Under the same refining time, the average diameter of inclusions was reduced sharply with the content of Rb2O raised from 2.5% to 7.5%; (ii) On the other hand, the average diameter of inclusions decreased rapidly with the refining time ascending from 15 to 45 min when the weight percentage of Rb2O fixed.(iii) Furthermore, the average diameter of inclusions in steel samples which were treated by Rb2O (4 # , 5 # , 6 # ) was smaller than that of normal heat (0 # ).

Effect of Na2O and Rb2O on Inclusion Distribution Overlayed on the Phase Diagram
Figure 3 describes the distribution of inclusions in the MnO-SiO2-Al2O3 and CaO-SiO2-Al2O3 phase diagram in normal heat.It is obvious that the contents of SiO2 and Al2O3 in inclusions increased gradually.In detail, MnO-SiO2-Al2O3 inclusion mainly concentrate in the area of 70-90 wt% SiO2, <18 wt% Al2O3, CaO-SiO2-Al2O3 inclusion mainly concentrate in the area of 40-60 wt% SiO2, 10-25 wt% Al2O3 after the steels are refined with 2700 s (45 min).In addition, some CaO-SiO2-Al2O3 inclusions have a small amount of mass fraction of CaO and Al2O3 below 10 wt%.The inclusion distribution overlaid in phase diagram with different refining time in 2 # (5.0 wt% Na2O) steel samples is shown in Figure 4. Obviously, with the 15 min reactions shown in Figure 4a, the distribution of inclusions seems to be dominant toward the SiO2-rich region.With longer reaction times, the distribution of inclusions are spread across the ternary phase diagram with the addition of Na2O.The inclusion distribution overlaid in phase diagram with different refining time in 2 # (5.0 wt% Na 2 O) steel samples is shown in Figure 4. Obviously, with the 15 min reactions shown in Figure 4a, the distribution of inclusions seems to be dominant toward the SiO 2 -rich region.With longer reaction times, the distribution of inclusions are spread across the ternary phase diagram with the addition of Na 2 O.
Contrary to the effect of Na 2 O, the Rb 2 O on the average inclusion diameter generally has a decreasing trend with longer reaction times.Figure 5 describes the distribution of inclusions with Rb 2 O content of 5.0 wt%.As the reaction time increased, there is a strong tendency for inclusions to be distributed toward the less SiO 2 and Al 2 O 3 region.In detail, the MnO-SiO 2 -Al 2 O 3 inclusion system mainly concentrate in the low melting area with 40-50 wt% SiO 2 , <20 wt% Al 2 O 3 , and the CaO-SiO 2 -Al 2 O 3 inclusion system mainly concentrate in the low melting area with 40-50 wt% SiO 2 , <20 wt% Al 2 O 3 at the same time.For instance, the addition of Rb 2 O seems to concentrate the distribution of inclusions toward the less SiO 2 , Al 2 O 3 phase, which is clearly shown in Figure 5. Contrary to the effect of Na2O, the Rb2O on the average inclusion diameter generally has a decreasing trend with longer reaction times.Figure 5 describes the distribution of inclusions with Rb2O content of 5.0 wt%.As the reaction time increased, there is a strong tendency for inclusions to be distributed toward the less SiO2 and Al2O3 region.In detail, the MnO-SiO2-Al2O3 inclusion system mainly concentrate in the low melting area with 40-50 wt% SiO2, <20 wt% Al2O3, and the CaO-SiO2-Al2O3 inclusion system mainly concentrate in the low melting area with 40-50 wt% SiO2, <20 wt% Al2O3 at the same time.For instance, the addition of Rb2O seems to concentrate the distribution of inclusions toward the less SiO2, Al2O3 phase, which is clearly shown in Figure 5.
In summary, at longer reaction times, the distribution of inclusions is spread across the ternary phase diagram, and dominant more toward the SiO2 and Al2O3 region, suggesting Na2O addition and an increase in Na2O would likely be detrimental to control inclusions systems concentrating in the low melting area.In contrast, Rb2O addition would likely be helpful to control both MnO-SiO2-Al2O3 and CaO-SiO2-Al2O3 inclusion system concentrating in low melting area.This is also shown in Figure 6.
Figure 6 shows the average content of each component in inclusions during refining.Obviously, the content of SiO2 and Al2O3 in inclusions decreases sharply when the steel samples are treated by Rb2O, as shown in Figure 6c.Contrary to the effect of Rb2O, the effect Na2O on the content of SiO2 and Al2O3 in inclusions yielded generally an increasing trend with longer reaction times, as shown in Figure 6b.In summary, at longer reaction times, the distribution of inclusions is spread across the ternary phase diagram, and dominant more toward the SiO 2 and Al 2 O 3 region, suggesting Na 2 O addition and an increase in Na 2 O would likely be detrimental to control inclusions systems concentrating in the low melting area.In contrast, Rb 2 O addition would likely be helpful to control both MnO-SiO 2 -Al 2 O 3 and CaO-SiO 2 -Al 2 O 3 inclusion system concentrating in low melting area.This is also shown in Figure 6.  Figure 7 compares the change in the inclusion chemistry within the MnO-SiO2-Al2O3, CaO-SiO2-Al2O3 ternary phase diagrams after 2700 s (45 min) with varying amounts of Rb2O additions from 2.5 wt% to 7.5 wt%.Additionally, he average content of each component in inclusions during refining process is shown in Figure 8.
It may be noted that the MnO-SiO2-Al2O3 and CaO-SiO2-Al2O3 inclusions system were distributed toward the less SiO2 and Al2O3 phase with Rb2O addition increased from 0 wt% to 7.5 wt%.

Influence of Na2O and Rb2O on the Number of Inclusions
Figure 9 shows the influence of reaction time on the number of inclusions with the addition of Na2O or Rb2O.For normal heat, it can be noted that the number of inclusions reduced gently when the reaction time increased from 900 to 2700 s (15 to 45 min).For heats with Na2O additions from 2.5 wt% to 7.5 wt%, obviously, the number of inclusions exceeds normal level for all the concentrations of Na2O.However, unlike Na2O, reactions of the C96V saw wire steel samples with Rb2O containing synthesized refining slag samples resulted in a significant decrease in the number of inclusions observed with both longer reaction times and higher concentrations of Rb2O, as observed in Figure 9.

Influence of Na 2 O and Rb 2 O on the Number of Inclusions
Figure 9 shows the influence of reaction time on the number of inclusions with the addition of Na 2 O or Rb 2 O.For normal heat, it can be noted that the number of inclusions reduced gently when the reaction time increased from 900 to 2700 s (15 to 45 min).For heats with Na 2 O additions from 2.5 wt% to 7.5 wt%, obviously, the number of inclusions exceeds normal level for all the concentrations of Na 2 O.However, unlike Na 2 O, reactions of the C96V saw wire steel samples with Rb 2 O containing synthesized refining slag samples resulted in a significant decrease in the number of inclusions observed with both longer reaction times and higher concentrations of Rb 2 O, as observed in Figure 9.

Morphology and Element Distribution of Typical Inclusions
Morphological observations of typical inclusions by SEM-EDS are shown in Figures 10-12.The majority of inclusions in all of the experimental heats are SiO2-MnO-Al2O3 and SiO2-CaO-Al2O3 systems, the differences about inclusion components, sizes and numbers have been discussed above.
It is obvious that the size of inclusions in normal heat is around 5 μm.as shown in Figure 10.However, this increased sharply after the steel samples were treated by Na2O; some are large than 20 μm, as shown in Figure 11.In contrast, inclusions became very small, with diameters of about only 1 μm after steel samples were treated with Rb2O, as shown in Figure 12.

Morphology and Element Distribution of Typical Inclusions
Morphological observations of typical inclusions by SEM-EDS are shown in Figures 10-12.The majority of inclusions in all of the experimental heats are SiO2-MnO-Al2O3 and SiO2-CaO-Al2O3 systems, the differences about inclusion components, sizes and numbers have been discussed above.
It is obvious that the size of inclusions in normal heat is around 5 μm.as shown in Figure 10.However, this increased sharply after the steel samples were treated by Na2O; some are large than 20 μm, as shown in Figure 11.In contrast, inclusions became very small, with diameters of about only 1 μm after steel samples were treated with Rb2O, as shown in Figure 12.

Element Distribution in Complex Inclusion
As mentioned, complex inclusions were discovered in all of the experimental steel samples.In order to describe inclusion structures exactly, SEM mappings were done, as shown in Figures 13-15.Obviously, both MnO-SiO2-Al2O3 and CaO-SiO2-Al2O3 inclusions are multi-layered composite structures in all experimental steel samples.In detail, there was a SiO2, MnO, CaO, and Al2O3 homogeneous composite in the center, and a periphery of MnS precipitate around it.It is obvious that the size of inclusions in normal heat is around 5 µm.as shown in Figure 10.However, this increased sharply after the steel samples were treated by Na 2 O; some are large than 20 µm, as shown in Figure 11.In contrast, inclusions became very small, with diameters of about only 1 µm after steel samples were treated with Rb 2 O, as shown in Figure 12.

Element Distribution in Complex Inclusion
As mentioned, complex inclusions were discovered in all of the experimental steel samples.In order to describe inclusion structures exactly, SEM mappings were done, as shown in Figures 13-15

Element Distribution in Complex Inclusion
As mentioned, complex inclusions were discovered in all of the experimental steel samples.In order to describe inclusion structures exactly, SEM mappings were done, as shown in Figures 13-15.Obviously, both MnO-SiO2-Al2O3 and CaO-SiO2-Al2O3 inclusions are multi-layered composite structures in all experimental steel samples.In detail, there was a SiO2, MnO, CaO, and Al2O3 homogeneous composite in the center, and a periphery of MnS precipitate around it.

Discussion
The content of alkali oxides in metallurgical slag is usually low, but its influence is significant.This comprehensive change in the slag composition has a dramatic impact on the thermochemical and thermophysical properties of the slag, including its density, surface tension [16], and viscosity [17].In particular, surface tension and viscosity have a direct impact on melt/slag separation efficiency, melt/slag reaction kinetics, and the permeability of the intermediate gases used for reduction and heat transfer.

Influence of Na2O, Rb2O on the Structure and Viscosity of Metallurgical Slags
Studies that discuss the structure and properties of metallurgical slags have been reviewed by Waseda and Toguri [18,19].It is well known that the increase of Li2O, Na2O content in SiO2-CaO-Al2O3-R2O (R = Li, Na) slag system could decrease the viscosity.Sukenaga et al. [20] studied the effect of Na2O on the viscosity for CaO-SiO2-Al2O3-Na2O slag.The results indicated that the viscosity of this quaternary melts decreased sharply with increasing the additive content of Na2O.Similar results are also noted in the literature [21,22].However, the influence of K2O, Rb2O, and Cs2O is the opposite; this is due to the fact that the structure of SiO2-CaO-Al2O3-R2O (R = Li, Na, K, Rb, Cs) slag would like to change owing to the effects of alkali oxides (Li2O, Na2O, K2O, Rb2O, Cs2O) [18][19][20][21][22][23].
Based on the laws studied by past researchers, we calculated the viscosities for the slag compositions in our experiment by According to the literature [24,25], the structure of slag is very complex, including chains, rings, and three-dimensional network structures, as shown in Figure 16.

Discussion
The content of alkali oxides in metallurgical slag is usually low, but its influence is significant.This comprehensive change in the slag composition has a dramatic impact on the thermochemical and thermophysical properties of the slag, including its density, surface tension [16], and viscosity [17].In particular, surface tension and viscosity have a direct impact on melt/slag separation efficiency, melt/slag reaction kinetics, and the permeability of the intermediate gases used for reduction and heat transfer.According to the literature [24,25], the structure of slag is very complex, including chains, rings, and three-dimensional network structures, as shown in Figure 16.According to the literature [24,25], the structure of slag is very complex, including chains, rings, and three-dimensional network structures, as shown in Figure 16.(1) (i) The influence of Na2O on the viscosity of SiO2-CaO-Al2O3 slag can be understood when considering the change in the degree of ploymerization (DOP) of slag structure [26].where R + is the basic oxide cation such as Na + .Addition of Na2O would provide O 2− , which can modify the complex silicate structure into simple [Si2O7] 6− and [SiO4] 4− .In addition, according to Sohn [14], after treatment with Na2O, the [AlO4] 5− -tetrahedral structure will be depolymerized.We have provided the schematic diagram for Equations ( 2) and ( 3) in Figure 17.A schematic diagram for how Na2O can depolymerize the [AlO4] 5− -tetrahedral structure is shown in Figure 18.
Therefore, the viscosity of slag decreases sharply.(ii) The influence of Rb2O and Cs2O on the structure and properties of metallurgical slag have not been studied enough.Even so, some researchers [15,18,19] speculated that Rb2O and Cs2O have the same effect as K2O.Furthermore, Sohn et al. [14,15] did some experiments to research the effect of K2O, Rb2O and Cs2O on inclusion removal based on the above inference.The results indicated that Rb2O, Cs2O have the same effect as K2O.In detail, with the introduction of the large cationic radius of the K + , Rb + and Cs + compared to Na + and Li + , the inclusions can be sufficiently entrained within the alumino-silicate-based slag once absorbed due to the significant increase in the viscosity.
It is well known that K2O addition can catalyze the formation of [AlO4] 5− tetrahedral units, which act as network formers in the slag structure [27][28][29], as expressed by Equation ( 4) [27].A schematic diagram for Equation ( 4) is presented in Figure 19.(i) The influence of Na2O on the viscosity of SiO2-CaO-Al2O3 slag can be understood when considering the change in the degree of ploymerization (DOP) of slag structure [26].where R + is the basic oxide cation such as Na + .Addition of Na2O would provide O 2− , which can modify the complex silicate structure into simple [Si2O7] 6− and [SiO4] 4− .In addition, according to Sohn [14], after treatment with Na2O, the [AlO4] 5− -tetrahedral structure will be depolymerized.We have provided the schematic diagram for Equations ( 2) and ( 3) in Figure 17.A schematic diagram for how Na2O can depolymerize the [AlO4] 5− -tetrahedral structure is shown in Figure 18.
Therefore, the viscosity of slag decreases sharply.(ii) The influence of Rb2O and Cs2O on the structure and properties of metallurgical slag have not been studied enough.Even so, some researchers [15,18,19] speculated that Rb2O and Cs2O have the same effect as K2O.Furthermore, Sohn et al. [14,15] did some experiments to research the effect of K2O, Rb2O and Cs2O on inclusion removal based on the above inference.The results indicated that Rb2O, Cs2O have the same effect as K2O.In detail, with the introduction of the large cationic radius of the K + , Rb + and Cs + compared to Na + and Li + , the inclusions can be sufficiently entrained within the alumino-silicate-based slag once absorbed due to the significant increase in the viscosity.
It is well known that K2O addition can catalyze the formation of [AlO4] 5− tetrahedral units, which act as network formers in the slag structure [27][28][29], as expressed by Equation ( 4) [27].A schematic diagram for Equation ( 4) is presented in Figure 19.(ii) The influence of Rb 2 O and Cs 2 O on the structure and properties of metallurgical slag have not been studied enough.Even so, some researchers [15,18,19] speculated that Rb 2 O and Cs 2 O have the same effect as K 2 O. Furthermore, Sohn et al. [14,15] did some experiments to research the effect of K 2 O, Rb 2 O and Cs 2 O on inclusion removal based on the above inference.The results indicated that Rb 2 O, Cs 2 O have the same effect as K 2 O.In detail, with the introduction of the large cationic radius of the K + , Rb + and Cs + compared to Na + and Li + , the inclusions can be sufficiently entrained within the alumino-silicate-based slag once absorbed due to the significant increase in the viscosity.
It is well known that K 2 O addition can catalyze the formation of [AlO 4 ] 5− tetrahedral units, which act as network formers in the slag structure [27][28][29], as expressed by Equation ( 4) [27].A schematic diagram for Equation ( 4) is presented in Figure 19.
of K2O, Rb2O and Cs2O on inclusion removal based on the above inference.The results indicated that Rb2O, Cs2O have the same effect as K2O.In detail, with the introduction of the large cationic radius of the K + , Rb + and Cs + compared to Na + and Li + , the inclusions can be sufficiently entrained within the alumino-silicate-based slag once absorbed due to the significant increase in the viscosity.
It is well known that K2O addition can catalyze the formation of [AlO4] 5− tetrahedral units, which act as network formers in the slag structure [27][28][29], as expressed by Equation ( 4) [27].A schematic diagram for Equation ( 4) is presented in Figure 19.Furthermore, according to Sohn et al. [14,15], K 2 O addition could polymerize the slag to form complex aluminates and alumino-silicate structures.In order to help us understand this process, we provided the schematic diagram of how K 2 O, Rb 2 O and Cs 2 O polymerize the slag network structure (as shown in Figure 20).Thus, the viscosity of slag increases significantly with an increase in the content of Rb 2 O. Furthermore, according to Sohn et al. [14,15], K2O addition could polymerize the slag to form complex aluminates and alumino-silicate structures.In order to help us understand this process, we provided the schematic diagram of how K2O, Rb2O and Cs2O polymerize the slag network structure (as shown in Figure 20).Thus, the viscosity of slag increases significantly with an increase in the content of Rb2O.

Influence of Na2O, Rb2O on the Ability of Slags to Absorb Inclusions
Most authors report that inclusion absorption by slag occurs in three stages [30][31][32][33]: (i) Flotation in the bath-transport of the inclusion to the steel/slag interface.
(ii) Separation of liquid steel-movement of the inclusion to the interface, breaking the surface tension of steel.
(iii) Dissolution in slag-removal of the inclusion from the steel/slag interface for full incorporation into the slag.
An inclusion can only be considered that is eliminated from steel when it is completely dissolved in the slag.According to Valdez et al. [31] and Reis et al. [34,35], the viscosity of slag Added Rb2O The structure of refining slag after treated by Rb2O The structure of original refining slag Added Na2O The structure of refining slag after treated by Na2O The structure of original refining slag Furthermore, according to Sohn et al. [14,15], K2O addition could polymerize the slag to form complex aluminates and alumino-silicate structures.In order to help us understand this process, we provided the schematic diagram of how K2O, Rb2O and Cs2O polymerize the slag network structure (as shown in Figure 20).Thus, the viscosity of slag increases significantly with an increase in the content of Rb2O.

Influence of Na2O, Rb2O on the Ability of Slags to Absorb Inclusions
Most authors report that inclusion absorption by slag occurs in three stages [30][31][32][33]: (i) Flotation in the bath-transport of the inclusion to the steel/slag interface.
(ii) Separation of liquid steel-movement of the inclusion to the interface, breaking the surface tension of steel.
(iii) Dissolution in slag-removal of the inclusion from the steel/slag interface for full incorporation into the slag.
An inclusion can only be considered that is eliminated from steel when it is completely dissolved in the slag.According to Valdez et al. [31] and Reis et al. [34,35], the viscosity of slag Added Rb2O The structure of refining slag after treated by Rb2O The structure of original refining slag Added Na2O The structure of refining slag after treated by Na2O The structure of original refining slag

Influence of Na 2 O, Rb 2 O on the Ability of Slags to Absorb Inclusions
Most authors report that inclusion absorption by slag occurs in three stages [30][31][32][33]: (i) Flotation in the bath-transport of the inclusion to the steel/slag interface.(ii) Separation of liquid steel-movement of the inclusion to the interface, breaking the surface tension of steel.
(iii) Dissolution in slag-removal of the inclusion from the steel/slag interface for full incorporation into the slag.
An inclusion can only be considered that is eliminated from steel when it is completely dissolved in the slag.According to Valdez et al. [31] and Reis et al. [34,35], the viscosity of slag significantly influences the ability of slag to absorb inclusions.Stage (i) takes place in molten steel; thus, the effect of Na 2 O and Rb 2 O occurs mostly in stages (ii) and (iii).

The Slag Entrapment during Refining Process
The influence of alkali oxides (Li 2 O, Na 2 O, K 2 O, Rb 2 O, Cs 2 O) on the slag entrapment is an important factor that shouldn't be ignored.According to J. Strandh et al. [36], if the slag viscosity is too low, the risk of slag entrainment to the steel is promoted, resulting in more inclusions in the steel.
When In contrast, the effect of Rb 2 O is the opposite.When Rb 2 O was added into slag, the slag entrapment during refining process was significantly weakened.Thus, the number and average diameter of inclusions, and the content of SiO 2 and Al 2 O 3 contained in inclusions, decrease sharply compared to normal heat; this is described in Figure 22.
Metals 2018, 8, x FOR PEER REVIEW 14 of 20 significantly influences the ability of slag to absorb inclusions.Stage (i) takes place in molten steel; thus, the effect of Na2O and Rb2O occurs mostly in stages (ii) and (iii).

The Slag Entrapment during Refining Process
The influence of alkali oxides (Li2O, Na2O, K2O, Rb2O, Cs2O) on the slag entrapment is an important factor that shouldn't be ignored.According to J. Strandh et al. [36], if the slag viscosity is too low, the risk of slag entrainment to the steel is promoted, resulting in more inclusions in the steel.
In contrast, the effect of Rb2O is the opposite.When Rb2O was added into slag, the slag entrapment during refining process was significantly weakened.Thus, the number and average diameter of inclusions, and the content of SiO2 and Al2O3 contained in inclusions, decrease sharply compared to normal heat; this is described in Figure 22.In terms of the second stage, Bouris et al. [37] and Nakajima et al. [38] have already done some correlative research, and described models for predicting the separation times for spherical, rigid, and chemically inert particles.During this process, the inclusion's motion is decided by a force balance between a capillary force ( Z ,  F ), a buoyancy force ( b F ), a drag force ( d F ) and a fluid force ( m F ), which is visualized in Figure 23.The force, when the inclusion is in contact with both steel and slag, are: In terms of the second stage, Bouris et al. [37] and Nakajima et al. [38] have already done some correlative research, and described models for predicting the separation times for spherical, rigid, and chemically inert particles.During this process, the inclusion's motion is decided by a force balance between a capillary force (F σ,Z ), a buoyancy force (F b ), a drag force (F d ) and a fluid force (F m ), which is visualized in Figure 23.The force, when the inclusion is in contact with both steel and slag, are: (5) Metals 2018, 8, 691 where R i is the inclusion's radius, g is acceleration due to gravity, ρ s , ρ i and ρ M are the density of slag, inclusion and the metal, respectively.Z, dZ/dt and d 2 Z/dt 2 is the inclusion's position, speed and acceleration, η M , η s are the viscosity of metal and slag, respectively.σ MS , σ Si and σ Mi are the interfacial tensions between metal and slag, and slag and inclusion, and metal and inclusion, respectively.The equation of motion is given by: where F a is the force from the acceleration of the inclusions, given as: Obviously, viscosity is an important factor on the stress of inclusions during the separation process, as shows in Equation (11).According to Valdez et al. [31], decreasing viscosity causes the inclusions easier enter into the slag.By contrast, that increasing viscosity causes the inclusions to require much more time to separate.Furthermore, in 2014, Yang [39] researched the influence of viscosity of slag on the separation time in depth, and observed the similar conclusions. Therefore, where a F is the force from the acceleration of the inclusions, given as: Obviously, viscosity is an important factor on the stress of inclusions during the separation process, as shows in Equation (11).According to Valdez et al. [31], decreasing viscosity causes the inclusions easier enter into the slag.By contrast, that increasing viscosity causes the inclusions to require much more time to separate.Furthermore, in 2014, Yang [39] researched the influence of viscosity of slag on the separation time in depth, and observed the similar conclusions.Strandh et al. [36] developed a mathematical model to study inclusion behavior at the interface, and came up three types (remain, oscillating and pass) of inclusion behavior at the interface depending on the inclusion size, the velocity of the inclusion, and the interfacial properties of the system.Furthermore, Yang [39] put forward a supplement that a forth type of inclusion behavior at the interface is "pass, and then return back".We provided the schematic diagram of the four inclusion behaviors in Figure 24 in order to help the reader to understand these theories.
In Na 2 O addition experiment heats, inclusions (especially for SiO 2 -containing and Al 2 O 3containing inclusion) are difficult to dissolute into slag due to the depolymerization by Na 2 O.In detail, inclusions (especially for SiO 2 -containing and Al 2 O 3 -containing inclusion) will be depolymerized into smaller unites with more simple structure, but not combine with slag units.As a result, inclusions may return back into molten steel, which is visualized in Figure 24d.Thus, the number and SiO 2 and Al 2 O 3 content of inclusions increased remarkably with increasing not only reaction time, but also the content of Na 2 O.
In contrast, in Rb 2 O addition experiment heats, it is easy for inclusions (especially for SiO 2 -containing and Al 2 O 3 -containing inclusion) to dissolute into slag due to the polymerization by Rb 2 O.In detail, inclusions (especially for SiO 2 -containing and Al 2 O 3 -containing inclusion) will be polymerized with slag units into larger unites of more complex structure, and they will stay in the molten slag, which is visualized in Figure 24c.Thus, the number, average diameter, and SiO 2 and Al 2 O 3 content of inclusions decreased significantly, not only with the reaction time but also the content of Rb 2 O. inclusion behaviors in Figure 24 in order to help the reader to understand these theories.
In Na2O addition experiment heats, inclusions (especially for SiO2-containing and Al2O3containing inclusion) are difficult to dissolute into slag due to the depolymerization by Na2O.In detail, inclusions (especially for SiO2-containing and Al2O3-containing inclusion) will be depolymerized into smaller unites with more simple structure, but not combine with slag units.As a result, inclusions may return back into molten steel, which is visualized in Figure 24d.Thus, the number and SiO2 and Al2O3 content of inclusions increased remarkably with increasing not only reaction time, but also the content of Na2O.
In contrast, in Rb2O addition experiment heats, it is easy for inclusions (especially for SiO2containing and Al2O3-containing inclusion) to dissolute into slag due to the polymerization by Rb2O.In detail, inclusions (especially for SiO2-containing and Al2O3-containing inclusion) will be polymerized with slag units into larger unites of more complex structure, and they will stay in the molten slag, which is visualized in Figure 24c.Thus, the number, average diameter, and SiO2 and Al2O3 content of inclusions decreased significantly, not only with the reaction time but also the content of Rb2O.In summary, in order to understand the influence of Na2O and Rb2O on the ability of slag to absorb inclusions, its comprehensive effect on all of stages should be considered.
The schematic diagram of the dissolution model of inclusions in Na2O-containing or Rb2Ocontaining refining slag is shown in Figure 25.For the sake of understanding, we simplified the structure of refining and inclusions: only the [AlO4] 5− -tetrahedral structure, the Si-O-Al structure and Si-O-structure are shown.
Therefore, the number and average diameter of inclusion increased rapidly, and the content of SiO2 and Al2O3 contained in inclusions increased at the same time.
(ii) For Rb2O addition experiment heats: When the quantity of Rb2O addition was low, most of the inclusions could pass through the molten-slag interface.It is easy for inclusions (especially for SiO2-containing and Al2O3-containing  3) might be dissolved into molten steel, as shown in Figure 25a.
Therefore, the number and average diameter of inclusion increased rapidly, and the content of SiO 2 and Al 2 O 3 contained in inclusions increased at the same time.
(ii) For Rb 2 O addition experiment heats: When the quantity of Rb 2 O addition was low, most of the inclusions could pass through the molten-slag interface.It is easy for inclusions (especially for SiO 2 -containing and Al 2 O 3 -containing inclusion) to be polymerized with slag units into larger unites with more complex structure, and then remain in the molten slag.Additionally, slag entrapment during refining process will be significantly weakened.Therefore, the number and average diameter of inclusion decreased rapidly, and the content of SiO 2 and Al 2 O 3 contained in inclusions decreased at the same time, as shown in Figure 25b.inclusion) to be polymerized with slag units into larger unites with more complex structure, and then remain in the molten slag.Additionally, slag entrapment during refining process will be significantly weakened.Therefore, the number and average diameter of inclusion decreased rapidly, and the content of SiO2 and Al2O3 contained in inclusions decreased at the same time, as shown in Figure 25b.

Conclusions
The effect of Na2O and Rb2O containing synthetic LF refining slag on the absorption ability of inclusions for C96V saw wire steels has been studied.Kinetic studies using a MoSi2 furnace at 1873

Conclusions
The effect of Na 2 O and Rb 2 O containing synthetic LF refining slag on the absorption ability of inclusions for C96V saw wire steels has been studied.Kinetic studies using a MoSi 2 furnace at 1873 K (1600 • C) on the reaction of synthetic LF refining slag containing alkali oxides (Na 2 O, Rb 2 O) with C96V saw wire steels suggested minimal improvement with Na 2 O additions would deteriorate inclusion removal.In contrast, Rb 2 O additions seems to remarkably enhance inclusion removal for to two reasons: after Rb 2 O additions (i) the inclusions can be sufficiently absorbed into the refining slag due to the significant increase in the viscosity; (ii) the slag entrapment during refining process will be weakened dramatically.In detail, with Rb 2 O additions: (1) Not only did the number of inclusions, but also the average inclusion diameter, decrease in the steel samples; (2) On the other hand, both of the MnO-SiO 2 -Al 2 O 3 and CaO-SiO 2 -Al 2 O 3 inclusion systems mainly concentrated in the low melting zone when the composition of Rb 2 O in synthetic refining slag was ≥5.0 wt%.
In summary, Rb 2 O additions seem to significantly improve the cleanliness in the as-quenched C96V saw wire steel melts, compared with preexisting synthetic refining slag compositions.

Future Work
It should be noted that much work needs to be done in future: (i) the influence of stirring on slag entrainment that supported by rod.Obviously, there are many factors that should be considered in order to explore the influence of stirring on slag entrainment, such as the time of stirring, the velocity of stirring, the shape of the crucible, the diameter of the crucible, the diameter of the rod, the temperature of molten slag.(Other factors may also have important effects on the process, we can not describe all of them at present.) (ii) The viscosities of CaO-SiO 2 -Al 2 O 3 -R 2 O (R = Na or Rb) should be measured.We got the viscosities of slag after Na 2 O addition by calculating with Factsage 7.2, but it must be measured in experiments, although the calculated results are in good agreement with the expected outcomes.Furthermore, we don't have any value about the viscosities of slag after Rb 2 O addition due to Factsage 7.2 lack of enough data base.
(iii) The influence of Na 2 O, Rb 2 O addition on the structure of slag (CaO-SiO 2 -Al 2 O 3 ) should be researched further, especially for Rb 2 O addition.This is mainly because there is hardly any literature to have noticed this topic.

Figure 2 .
Figure 2. The average diameter of inclusion in experimental steel varies with refining time.

Figure 7
Figure7compares the change in the inclusion chemistry within the MnO-SiO2-Al2O3, CaO-SiO2-Al2O3 ternary phase diagrams after 2700 s (45 min) with varying amounts of Rb2O additions from 2.5 wt% to 7.5 wt%.Additionally, he average content of each component in

Figure 6
Figure6shows the average content of each component in inclusions during refining.Obviously, the content of SiO 2 and Al 2 O 3 in inclusions decreases sharply when the steel samples are treated by Rb 2 O, as shown in Figure6c.Contrary to the effect of Rb 2 O, the effect Na 2 O on the content of SiO 2 and Al 2 O 3 in inclusions yielded generally an increasing trend with longer reaction times, as shown in Figure6b.Figure7compares the change in the inclusion chemistry within the MnO-SiO 2 -Al 2 O 3 , CaO-SiO 2 -Al 2 O 3 ternary phase diagrams after 2700 s (45 min) with varying amounts of Rb 2 O additions from 2.5 wt% to 7.5 wt%.Additionally, he average content of each component in inclusions during refining process is shown in Figure8.It may be noted that the MnO-SiO 2 -Al 2 O 3 and CaO-SiO 2 -Al 2 O 3 inclusions system were distributed toward the less SiO 2 and Al 2 O 3 phase with Rb 2 O addition increased from 0 wt% to 7.5 wt%.

Figure 7 Figure 7 .Figure 8 .
Figure 6 shows the average content of each component in inclusions during refining.Obviously, the content of SiO 2 and Al 2 O 3 in inclusions decreases sharply when the steel samples are treated by Rb 2 O, as shown in Figure 6c.Contrary to the effect of Rb 2 O, the effect Na 2 O on the content of SiO 2 and Al 2 O 3 in inclusions yielded generally an increasing trend with longer reaction times, as shown in Figure 6b.Figure 7 compares the change in the inclusion chemistry within the MnO-SiO 2 -Al 2 O 3 , CaO-SiO 2 -Al 2 O 3 ternary phase diagrams after 2700 s (45 min) with varying amounts of Rb 2 O additions from 2.5 wt% to 7.5 wt%.Additionally, he average content of each component in inclusions during refining process is shown in Figure 8.It may be noted that the MnO-SiO 2 -Al 2 O 3 and CaO-SiO 2 -Al 2 O 3 inclusions system were distributed toward the less SiO 2 and Al 2 O 3 phase with Rb 2 O addition increased from 0 wt% to 7.5 wt%.Metals 2018, 8, x FOR PEER REVIEW 8 of 20

Figure 9 .
Figure 9.The count of inclusion in experimental steel varies with different refining time.

Figure 10 .
Figure 10.Typical inclusions at the normal experimental heat.

Figure 9 .
Figure 9.The count of inclusion in experimental steel varies with different refining time.

3. 4 .Figure 9 .
Figure 9.The count of inclusion in experimental steel varies with different refining time.

Figure 10 .
Figure 10.Typical inclusions at the normal experimental heat.

Figure 11 .
Figure 11.Typical inclusions in steel which were treated by adding Na2O in synthetic refining slag.

Figure 11 .Figure 12 .
Figure 11.Typical inclusions in steel which were treated by adding Na 2 O in synthetic refining slag.Metals 2018, 8, x FOR PEER REVIEW 10 of 20

Figure 13 .
Figure 13.SEM mapping of typical inclusions in normal experiment heat.

Figure 14 .Figure 12 .
Figure 14.SEM mapping of typical inclusions in steel treated with synthetic refining slag containing Na2O.

Figure 12 .
Figure 12.Typical inclusions in steel which were treated by adding Rb2O in synthetic refining slag.

Figure 13 .Figure 13 .
Figure 13.SEM mapping of typical inclusions in normal experiment heat.

Figure 13 .
Figure 13.SEM mapping of typical inclusions in normal experiment heat.

Figure 15 .
Figure 15.SEM mapping of typical inclusions in steel treated with synthetic refining slag containing Rb 2 O.

4. 1 .
Influence of Na 2 O, Rb 2 O on the Structure and Viscosity of Metallurgical Slags Studies that discuss the structure and properties of metallurgical slags have been reviewed by Waseda and Toguri [18,19].It is well known that the increase of Li 2 O, Na 2 O content in SiO 2 -CaO-Al 2 O 3 -R 2 O (R = Li, Na) slag system could decrease the viscosity.Sukenaga et al. [20] studied the effect of Na 2 O on the viscosity for CaO-SiO 2 -Al 2 O 3 -Na 2 O slag.The results indicated that the viscosity of this quaternary melts decreased sharply with increasing the additive content of Na 2 O. Similar results are also noted in the literature [21,22].However, the influence of K 2 O, Rb 2 O, and Cs 2 O is the opposite; this is due to the fact that the structure of SiO 2 -CaO-Al 2 O 3 -R 2 O (R = Li, Na, K, Rb, Cs) slag would like to change owing to the effects of alkali oxides (Li 2 O, Na 2 O, K 2 O, Rb 2 O, Cs 2 O) [18-23].Based on the laws studied by past researchers, we calculated the viscosities for the slag compositions in our experiment by Factsage 7.2 (Bale, C.W.; Pelton, A.D.; Thompson, W.T.; Eriksson, G.; Hack, K.; Chartand, P.; Decterov, S.; Jung, I.H.; Melanson, J.; Petersen, S. Thermfact/CRCT (Montreal, QC, Canada), GTT-Technologies (Aachen, Germany), 2017) at 1873 K (1600 • C).In detail, the viscosities are 0.351 Pa•s (0 # , normal heat), 0.343 Pa•s (1 # , 2.5 wt% Na 2 O), 0.318 Pa•s (2 # , 5.0 wt% Na 2 O), 0.273 Pa•s (3 # , 7.5 wt% Na 2 O).The calculated results are in good agreement with the expected law.
)where R + is the basic oxide cation such as Na + .Addition of Na 2 O would provide O 2− , which can modify the complex silicate structure into simple [Si 2 O 7 ] 6− and [SiO 4 ] 4− .In addition, according to Sohn[14], after treatment with Na 2 O, the [AlO 4 ] 5− -tetrahedral structure will be depolymerized.We have provided the schematic diagram for Equations (2) and (3) in Figure17.A schematic diagram for how Na 2 O can depolymerize the [AlO 4 ] 5− -tetrahedral structure is shown in Figure18.Therefore, the viscosity of slag decreases sharply.Metals 2018, 8, x FOR PEER REVIEW 12 of 20

Figure 20 .
Figure 20.The schematic diagram of Rb2O polymerize the slag network.The schematic diagram of how Na2O and Rb2O change the structure of refining slag is shown in Figure 21.For the sake of understanding, we simplified the structure of refining, showing only the [AlO4] 5− -tetrahedral structure, the Si-O-Al structure, and the Si-O-structure.

Figure 20 .
Figure 20.The schematic diagram of Rb 2 O polymerize the slag network.

Figure 20 .
Figure 20.The schematic diagram of Rb2O polymerize the slag network.The schematic diagram of how Na2O and Rb2O change the structure of refining slag is shown in Figure 21.For the sake of understanding, we simplified the structure of refining, showing only the [AlO4] 5− -tetrahedral structure, the Si-O-Al structure, and the Si-O-structure.

Figure 21 .
Figure 21.The structure of refining slag treated by alkali oxides (a) Na 2 O addition, (b) Rb 2 O addition.

Figure 22 .
Figure 22.The schematic diagram of slag entrainment to the steel: (a) stirring steel by graphite rod; (b) normal heat; (c) added Na2O; (d) added Rb2O.The blue and orange part of the picture shows molten slag and molten steel respectively; Dots and trangles shows inclusions in steel; The arrows in (a) indicates the direction of the stirring of the graphite rod; The arrows in (b-d) indicates the flow direction of molten steel when slag entrapment happens during refining process.4.2.2.Influence of Na2O, Rb2O on the Second Stage: Separation of Liquid Steel

Figure 22 .
Figure 22.The schematic diagram of slag entrainment to the steel: (a) stirring steel by graphite rod; (b) normal heat; (c) added Na 2 O; (d) added Rb 2 O.The blue and orange part of the picture shows molten slag and molten steel respectively; Dots and trangles shows inclusions in steel; The arrows in (a) indicates the direction of the stirring of the graphite rod; The arrows in (b-d) indicates the flow direction of molten steel when slag entrapment happens during refining process.
gradually with the viscosity of slag increasing caused by Rb 2 O addition.In other words, Rb 2 O addition will exacerbate the kinetic conditions of inclusion removal during the separation stage due to Rb 2 O addition decrease the viscosity of slag.By contrast, the effect of Na 2 O addition is opposite.Namely, F a = F b − F d − F σ,Z − F m increased gradually with the viscosity of slag decreasing caused by Rb 2 O addition.

FFigure 23 .Figure 23 .
Figure 23.The schematic diagram of the force analysis of inclusions.(a) Force analysis of inclusions passing through steel slag interface; (b) Three dimensional simulation of inclusions passing through steel slag interface.

Figure 24 .
Figure 24.The four types of inclusion behavior at the steel-slag interface (a) Remain; (b) Oscillate; (c) Pass and dissolve; (d) Pass and then return back.

Figure 24 .
Figure 24.The four types of inclusion behavior at the steel-slag interface (a) Remain; (b) Oscillate; (c) Pass and dissolve; (d) Pass and then return back.

Figure 25 .
Figure 25.The schematic diagram of the dissolution model of inclusions in refining slag (a) containing Na2O; (b) containing Rb2O.

Figure 25 .
Figure 25.The schematic diagram of the dissolution model of inclusions in refining slag (a) containing Na 2 O; (b) containing Rb 2 O.

Table 1 .
Chemical composition of synthesized refining slag.

Table 1 .
Chemical composition of synthesized refining slag.
China) of Northeastern University to analyse Al content by the ICP (Inductively Coupled Plasma) method.For C and S, an infrared C/S analyzer was applied.Then, the LECO ® TC 500 O2/N2 analyzer (LECO, San Jose, MI, USA) was chosen to detect O and N. The chemical composition of C96V saw wire steel is shown in Table2.
2O gets larger than that at normal heat at all sampling times; what is worse, increments in Na 2 O seem to increase the inclusion diameter observed in steel samples, suggesting that Na 2 O is not at all a suitable candidate for enhancing inclusion removal in C96V saw wire steels.Contrary to Na 2 O addition, Rb 2 O addition seems to yield a dramatic trend of decreased average inclusion diameter with increasing reaction time.(i)Under the same refining time, the average diameter of inclusions was reduced sharply with the content of Rb 2 O raised from 2.5% to 7.5%; (ii) On the other , the average diameter of inclusions decreased rapidly with the refining time ascending from 15 to 45 min when the weight percentage of Rb 2 O fixed.(iii)Furthermore, the average diameter of inclusions in steel samples which were treated by Rb 2 O (4 # , 5 # , 6 # ) was smaller than that of normal heat (0 # ).3.2.Effect ofNa 2 O and Rb 2 O on Inclusion Distribution Overlayed on the Phase Diagram Figure 3 describes the distribution of inclusions in the MnO-SiO 2 -Al 2 O 3 and CaO-SiO 2 -Al 2 O 3 phase diagram in normal heat.It is obvious that the contents of SiO 2 and Al 2 O 3 in inclusions increased gradually.In detail, MnO-SiO 2 -Al 2 O 3 inclusion mainly concentrate in the area of 70-90 wt% SiO 2 , <18 wt% Al 2 O 3 , CaO-SiO 2 -Al 2 O 3 inclusion mainly concentrate in the area of 40-60 wt% SiO 2 , 10-25 wt% Al 2 O 3 after the steels are refined with 2700 s (45 min).In addition, some CaO-SiO 2 -Al 2 O 3 inclusions have a small amount of mass fraction of CaO and Al 2 O 3 below 10 wt%. hand