Effect of Rb₂O on Inclusion Removal in C96V Saw Wire Steels Using Low-Basicity LF Refining Slag

Abstract

A novel Rb₂O-containing synthetic LF refining slag has been developed in order to produce ultraclean C96V saw wire steel. In addition, the thermodynamic and kinetics mechanism of Rb₂O on inclusion removal have been discussed fully. The results indicated that (1) Rb₂O additions (≤10.00 wt%) seems to significantly enhance inclusion removal in steel melts. ① The average diameter of nonmetallic inclusions decreased significantly with the Rb₂O addition in synthetic LF refining slag increasing. In particular, the diameter of most of inclusions was less than 3.0 × 10⁻⁶ m (3.0 µm) when Rb₂O addition was 10.00 wt% ② The number of nonmetallic inclusions decreased sharply with the content of Rb₂O in synthetic LF refining slag raising. ③ Both of the MnO–SiO₂–Al₂O₃, CaO–SiO₂–Al₂O₃ inclusions system mainly concentrated in the low melting point zone. (2) For the 15.00 wt% Rb₂O-containing synthetic LF refining slag, not only the average diameter of inclusion increases slightly with reaction times increasing, but also the number. This is due to the fact that, the effect of Rb₂O on the ability of refining slag to absorb inclusions is a double-edged sword: On the one hand, Rb₂O addition would promote the thermodynamic conditions of inclusion removal, then, inclusions would enter the slag spontaneously easier. On the other hand, Rb₂O addition could also exacerbate the kinetic conditions of inclusion removal by increasing the viscosity of slag at the same time.

1. Introduction

Tire cord steel and saw wire steel have strict requirements due to its end use.¹⁾ The diameter of ultrafine saw wire is between 50 × 10⁻⁶ m (50 μm) and 80 × 10⁻⁶ m (80 μm), and it requires no more than once broken per 100000 meters during drawing and standing process. For this purpose, the brittle inclusions should be eliminated as far as possible^2,3) and the residual inclusions must have a good plasticity. There are three key points for controlling inclusions in tire cord steel and saw wire steel: remained inclusions must be less in count, smaller in size and better in deformability.⁴⁾

A lot of work about inclusion controlling^{5,6,7,8,9,10,11,12,13,14,15,16,17,18)} from secondary steelmaking to the continuous casting has been carried out. However, looking at existing technology, except for ways by Si–Mn deoxidation, low basicity R (CaO/SiO₂=0.6−1.0) and low Al₂O₃ containing (≤8 wt%) in refining slag, there are few ways of changing the chemical composition of LF refining slag for improving inclusion absorption.

Alkali oxide has been widely applied into tundish covering agent, mold flux and so on. However, the influence of alkali oxides on enhancing inclusion removal has not been noticed until I. Sohn et al.^19,20) and Y. Li et al.²¹⁾ did some researches. I. Sohn et al. founded that Li₂O, Na₂O do harmful to inclusion removal, on the contrary, K₂O, Rb₂O, Cs₂O can improve the absorption ability of tundish flux and was utilized to improve 321 stainless steels cleanliness. But even so, there are still some questions remaining to be solved: (i) Why does the influence between Li₂O, Na₂O and K₂O, Rb₂O, Cs₂O are opposite on inclusion removal ? (ii) The mechanism of K₂O, Rb₂O, Cs₂O enhancing inclusion removal has not been investigated compeletely (Including the mechanism of thermodynamics and kinetics). (iii) The reason why Rb₂O, Cs₂O would impede inclusion removal when their addition are too much has not been illustrated compeletely.

For “question (ii)”, some research focusing on the thermodynamic mechanism were carried out in our previous work.²¹⁾ Therefore, we attempt to explore the illustration of “question (ii)” (the mechanism of kinetics) and “question (iii)” through studying the effect of Rb₂O on inclusion removal in C96V saw wire steels by adding Rb₂O into typical low-basicity LF refining slag.

2. Experiment

2.1. Experimental Appratus and Procedure

The experiments were carried out in a silicon-molybdenum resistance furnace in order to simulate LF refining process, and the experimental apparatus were illustrated in Fig. 1. The silicon-molybdenum heating rods arranged symmetrically in the furnace body can provide a constant temperature zone of less than 2000 K and the temperature was continuously measured by a platinum-rhodium thermocouple. Argon gas flow of 3.0 L/min was used during experimental process all the time from the bottom of the reaction tube to the top to provide a non-oxidizing atmosphere. The experimental procedures were carried as follows. Firstly, 1.00 kg industrial pure iron was placed into a magnesia crucible with 60 × 10⁻³ m (60 mm) in inner diameter and 80 × 10⁻³ m (80 mm) in depth. Then, the crucible was placed in a graphite crucible to prevent any leakage of molten steel. The furnace was heated to the experimental temperature [1873 K (1600°C)] at a rate of 6 K/min.

Fig. 1.

Schematic diagram of experimental equipment (MoSi₂ furnace). (Online version in color.)

Alloys were added into the melts when the temperature reached 1873 K (1600°C). Sampling No. 0, which was 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. Samples No. 1 through No. 3 were taken from the molten metal after the slag has melted for 900, 1800 and 2700 s (15, 30, and 45 minutes), respectively, after the slag had melted. After each sampling, the steel liquid was stirred with a graphite rod for 120 s (2 minutes) to make the molten steel and the refining slag uniform. All of the samples were taken by quarts tube sampler and quenched immediately in water. There are 4 heats were taken by treating with different synthetic LF refining slag which were added 0 wt%, 5 wt%, 10 wt%, 15 wt% Rb₂O, respectively, as illustrated in Table 1.

Table 1. Chemical composition of synthesized refining slag.

	CaO (wt%)	SiO2 (wt%)	Al2O3 (wt%)	Rb2O (wt%)	time/min	CaO/SiO2
0#	42.22	52.78	5.00	–	0, 15, 30, 45	0.8
1#	40.11	50.14	4.75	5.00	0, 15, 30, 45	0.8
2#	38.00	47.50	4.50	10.00	0, 15, 30, 45	0.8
3#	35.89	44.86	4.25	15.00	0, 15, 30, 45	0.8

2.2. Analysis Methods

Direct reading spectrometer was utilized to detect the composition of Si, Mn, Cr, V, Mo, Ni, P and so on. Steel samples were also sent to Analysis and Testing Center (Chemical Laboratory) of Northeastern University to analysis Al content. For C and S, Infrared C/S analyzer was applied, and then, the LECO^® TC 500 O₂/N₂ analyzer has been chosen to detect O, N. The chemical composition of C96V saw wire steel were shown in Table 2.

Table 2. Chemical composition of C96V saw wire steels.

	C	Si	Cr	Mn	V	Al	O	N	P	S	Ni	Cu
0#	0.96	0.15	0.22	0.35	0.12	≤0.0003	0.0015	0.0028	0.0069	0.0028	0.0027	0.0025
1#	0.96	0.17	0.20	0.34	0.10	≤0.0003	0.0011	0.0029	0.0058	0.0026	0.0025	0.0036
2#	0.95	0.15	0.20	0.31	0.09	≤0.0003	0.0013	0.0037	0.0059	0.0031	0.0024	0.0039
3#	0.97	0.16	0.20	0.36	0.10	≤0.0003	0.0012	0.0026	0.0062	0.0023	0.0030	0.0027

All of these steel samples were treated by 100–2000 mesh sand papers and polished, after that, get enough photos (more than 50 pieces) around “S” route (On the surface of the sample, take pictures in order from right to left, followed by in order from left to right). The Image J software have been used in order to statistics the size and count of inclusions, and the Image J 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) and Energy Dispersion Spectrum (EDS) to determined the morphology and component of inclusions.

3. Results

3.1. Effect of Rb₂O on the Variation in Inclusion Diameter

Figure 2 shows the change of the average diameter of inclusion during refining process. For normal heat, the value decreased gently with refining time raising from 900 to 2700 s (15 to 45 minutes). Rb₂O addition seems to describe a general trend of decreasing average diameter of inclusion with reaction. With the refining time ascending from 900 to 2700 s (15 to 45 minutes), samples reacted with the synthetic LF refining slag containing 5.00 wt%, 10.00 wt% Rb₂O slightly lowered the average diameter of inclusion to approximately from 2 × 10⁻⁶ m to 3 × 10⁻⁶ m (2 μm to 3 μm). The minimum size of the observed inclusions is around 0.4 × 10⁻⁶ m (400 nm), it will be shown in Fig. 10(a).

Fig. 2.

The average diameter of inclusion in experimental steel varies with refining time. (Online version in color.)

However, it should also be noted that for the 15.00 wt% Rb₂O-containing synthetic LF refining slag, a slightly increase in the average diameter of inclusion with reaction times increasing was observed. The similar results been founded in I. Sohn’s experiment.^19,20) K. Choi, Y. Kang and I. Sohn came up with a guess, after too much Rb₂O (or Cs₂O) were added, excessive increase in the viscosity of the flux may exacerbate the absorption ability of the inclusion momentum and mass transport of the flux may decrease slag/metal interface contact. However, they didn’t give any more detail explanation.

3.2. Effect of Rb₂O on Inclsion Distribution Overlayed on the Phase Diagram

The main kinds of inclusions in saw wire steel are MnO–SiO₂–Al₂O₃ and CaO–SiO₂–Al₂O₃ system. Figure 3 describes the distribution of two kinds of inclusion in ternary phase diagram in normal heat. It’s easily to get the information that most of MnO–SiO₂–Al₂O₃ inclusions are far away from low melting point zone (1300°C) and flock gather in the high melting area with the content of SiO₂ from 70 wt% to 90 wt%. On the other hand, majority of CaO–SiO₂–Al₂O₃ inclusions gather in the low melting zone (1400°C).

Fig. 3.

Inclusion distribution overlaid on phase diagram with refining time in 0^# steel sample (a) 900 s (15 minutes) (b) 1800 s (30 minutes) (c) 2700 s (45 minutes). (Online version in color.)

Figure 4 describes the distribution of inclusions in 10 wt% Rb₂O. The addition of Rb₂O makes the inclusion contents of SiO₂ and Al₂O₃ descend dramatically, especially for MnO–SiO₂–Al₂O₃ inclusion system, almost all inclusions concentrate in low melting point zone (1300°C). In details, contrasting normal heat steel sample or 2^# (10 wt%Rb₂O) steel sample, the content of SiO₂ decreased sharply from 80 wt% to 45 wt%, and the content of Al₂O₃ descend quickly from 20 wt% to below 10 wt% at the same time. Similar changes could be found in CaO–SiO₂–Al₂O₃ system.

Fig. 4.

Inclusion distribution overlaid on phase diagram with refining time in 2^# steel sample (a) 900 s (15 minutes) (b) 1800 s (30 minutes) (c) 2700 s (45 minutes). (Online version in color.)

Before-mentioned results vividly indicated that Rb₂O addition is likely to enhance inclusions removal and bring both MnO–SiO₂–Al₂O₃ and CaO–SiO₂–Al₂O₃ inclusions gather in low melting point zone. This would also be in turned in Fig. 5.

Fig. 5.

Average content of each component in inclusions during refining. (Online version in color.)

The changes of inclusions’ chemistry in MnO–SiO₂–Al₂O₃ and CaO–SiO₂–Al₂O₃ ternary phase diagram with varying amounts of Rb₂O additions from 5.00 wt% to 15.00 wt% for reacting 2700 s (45 minutes) were illustrated in Fig. 6. It’s obviously that, for all of the experimental heats, both MnO–SiO₂–Al₂O₃ and CaO–SiO₂–Al₂O₃ inclusions gather in low melting point zone. But even so, the changes of the composition of inclusions should be noted specially. In details, the content of SiO₂ and Al₂O₃ shows some fluctuation, it decreases in 10.00 wt% and 15.00 wt% Rb₂O addition. This would also be in turned in Fig. 7.

Fig. 6.

Inclusion distribution overlaid on phase diagram as a function of refining 2700 s (45 minutes) with the synthetic refining slags containing 5 to 15 weight percentage Rb₂O (a) 5.00 wt% Rb₂O (b) 10.00 wt% Rb₂O (c) 15.00 wt% Rb₂O. (Online version in color.)

Fig. 7.

Average content of each component in inclusions during refining. (Online version in color.)

3.3. Influence of Rb₂O on the Number of Inclusions

Figure 8 provides the effect of various concentrations of Rb₂O on the number of inclusions of the C96V saw wire steel melt. The number of inclusions decreases with the addition of Rb₂O increasing. Normally, the number of inclusions in the experimental heats which treated by Rb₂O were less than that in normal heat in any reaction time as shown in Fig. 8. Furthermore, with longer reaction time, the phenomenon that the number of inclusions decreasing sharply suggests Rb₂O additions and an increase in Rb₂O (below 10.00 wt%) would likely improve the cleanliness of C96V saw wire steels.

Fig. 8.

The count of inclusion in experimental steel varies with different refining time. (Online version in color.)

Even so, the abnormal changes of inclusion count when the Rb₂O addition increased to 15.00 wt% should be paid attention. Specifically, the number of inclusions suffered an slightly climb with the reaction time increasing from 900 to 2700 s (15 to 45 minutes) in 3^# (15.00 wt%Rb₂O addition). The similar results have been found in I Sohn’s experiments.^19,20) K Choi, Y Kang and I Sohn came up with a guess, after too much Rb₂O (or Cs₂O) were added, excessive increase in the viscosity of the flux may exacerbate the absorption ability of the inclusion momentum and mass transport of the flux may decrease during slag/metal interface contact. However, they didn’t give any more detailed explanation.

3.4. Morphology and Element Distribution of Typical Inclusions

Morphological observations of typical inclusions by SEM-EDS are shown in Figs. 9, 10. Majority of inclusions in all of the experimental heats are MnO–SiO₂–Al₂O₃ and CaO–SiO₂–Al₂O₃ system, the differences about inclusions’ component, size and so on have been discussed thereinafter.

Fig. 9.

Typical inclusions in the normal experimental heat. (Online version in color.)

Fig. 10.

Typical inclusions in steel which were treated by adding Rb₂O in synthetic refining slag. (Online version in color.)

As mentioned, complex inclusions were discovered in all of the experimental steel samples. In order to describe inclusions’ structure exactly, SEM mappings have been done as shown in Figs. 11 and 12. Obviously, both MnO–SiO₂–Al₂O₃ and CaO–SiO₂–Al₂O₃ inclusions are multi-layered composite structure in all of experimental steel samples. In details, there were SiO₂, MnO, CaO and Al₂O₃ homogeneous compositing in center and a periphery of MnS precipitating around it.

Fig. 11.

SEM mapping of typical inclusions in normal experiment heat. (Online version in color.)

Fig. 12.

SEM mapping of typical inclusions in steel treated with synthetic refining slag containing. (Online version in color.)

4. Discussion

The addition of alkali oxides to slag will significantly change the thermochemical and thermophysical properties of the slag, including their density,²²⁾ surface tension²³⁾ and viscosity.^23,24,25,26) Surface tension has a direct impact on the melt/slag reaction thermodynamics, and viscosity has a significant effect on the melt/slag reaction kinetics. According to literature²³⁾ the addition of Li₂O, Na₂O would decrease the surface tension of slag, but the influence of K₂O, Rb₂O and Cs₂O is completely opposite.

Furthermore, the influence of the addition of alkali oxides (Li₂O, Na₂O, K₂O, Rb₂O and Cs₂O) on the melt/slag reaction thermodynamics has been discussed in the previous work.²¹⁾ The results indicated that, K₂O addition would improve the thermodynamic conditions of inclusion removal, inclusions would enter the slag spontaneously easier. The effect of Rb₂O and Cs₂O on the structure and properties of refining slag is the same as that of K₂O, therefore, Rb₂O and Cs₂O addition would also promote the thermodynamic conditions of inclusion removal. The similar results of I Sohn et al.^19,20) also confirmed that.

However, too much Rb₂O (or Cs₂O) addition (≥15.00 wt%, in this study) will exacerbate the kinetic conditions of inclusion removal. (K Choi, Y Kang and I Sohn came up with a guess, after too much Rb₂O (or Cs₂O) were added, excessive increase in the viscosity of the flux may exacerbate the absorption ability of the inclusion momentum and mass transport of the flux may decrease during slag/metal interface contact. But even so, they didn’t give any more detail explanation.) Therefore, this study would focus on discussing the influence of viscosity on the melt/slag reaction kinetics after alkali oxides (Rb₂O) added.

4.1. Influence of Rb₂O on the Viscosity of Metallurgical Slags

The effect of alkali oxides (Li₂O, Na₂O, K₂O, Rb₂O and Cs₂O) on the slag structure and viscosity have been discussed in depth by Y. Waseda and J. M. Toguri,^25,26) S. Sukenaga, S. Haruki and Y. Nomoto,²²⁾ I Sohn et al.^19,20) and so on. The conclusions could be summarized as follow: (i) The Li₂O, Na₂O addition would intensely depolymerize the slag network structure, but the addition of K₂O, Rb₂O and Cs₂O would swiftly polymerize the slag network structure; (ii) The Li₂O, Na₂O addition would sharply decrease the viscosity of slag, but the addition of K₂O, Rb₂O and Cs₂O would significantly increase the viscosity of slag.

4.2. Influence of Rb₂O on the Ability of Slags to Absorb Inclusions

Most authors report that inclusion absorption by slag occurs in three stages:^27,28,29,30)

(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 it is eliminated from steel when it is completely dissolved in the slag. According to M. Valdez et al.²⁸⁾ and B. H. Reis et al.,^31,32) the viscosity of slag would significantly influence on the ability of slag to absorb inclusions.

4.2.1. Influence of the Viscosity of Slag on the First Stage: Flotation in the Bath

It’s well known that inclusions would be caught easily by emulsified slag particle during the floatation process in the bath, especially when the steel liquid is stirred by graphite rod. The surface area of emulsified slag particle is the most important factor which determined the collision probability between inclusions and emulsified slag. As we all know, the changes of viscosity of slag will significantly influence the size of emulsified slag particle, and then change the surface area of emulsified slag particles, finally, influence the collision probability between inclusions and emulsified slag.

In this study, we established a mathematical model in order to analyze the relationship between the size of emulsified slag particle and the collision probability, the details are as follows:

The following general assumptions have been made in the formulation of the mathematical model:

• The total mass of the emulsified slag involved into molten steel is a constant, recorded as M.

• The shape of emulsified slag particle can be considered to be spherical.

• The size of all the emulsified slag particles is same, and the radius recorded as R.   

$$\mathrm{M}=\frac{4}{3}\pi R^3{\rho }_{\mathrm{s}}n$$(1)

  

$$S=4\pi R^{2}n=\frac{3\mathrm{M}}{R{\rho }_{\mathrm{s}}}$$(2)

Where ρ_s is the density of slag, n is the number of emulsified slag particles, S is the total surface area of emulsified slag particles.

For normal heat:   

$$S_0=4\pi R_0{}^2{n}_0=\frac{3\mathrm{M}}{R_0{\rho }_{\mathrm{s}}}$$(3)

For arbitrary experiment, we assumed that the radius of emulsified slag particles is R_i = k_i·R₀ (k_i > 0). Thus, S_i can be written as:   

$$S_i=4\pi R_i{}^2{n}_i=\frac{3\mathrm{M}}{R_i{\rho }_{\mathrm{s}}}=\frac{3\mathrm{M}}{k_i{R}_0{\rho }_{\mathrm{s}}}$$(4)

Thus:   

$$S_{\mathrm{i}}=\frac{S_0}{k_{\mathrm{i}}}$$(5)

Equation (5) could be illustrated in Fig. 13(a). It’s obviously that the value of S_i would descend with the R_i = k_i·R₀ (k_i > 0) increasing. According to previous statements, addition of Rb₂O would increase the viscosity of slag and the radius of emulsified slag particles (R_i = k_i·R₀ (k_i > 0)). Thus, the descending of S_i may decreases the collision probability significantly between inclusions and emulsified slag particles, which is visualized in Figs. 13(b), 13(c) and 13(d). Therefore, Rb₂O addition will exacerbate the kinetic conditions of inclusion removal during the flotation stage.

Fig. 13.

(a) The relationship between S_i and S₀; The collision probability between inclusions and emulsified slag (b) Stirring steel by graphite rod (c) Blank heat (d) Added Rb₂O. (Online version in color.)

4.2.2. Influence of the Viscosity of Slag on the Second Stage: Separation of Liquid Steel

In terms of the second stage, D. Bouris et al.³³⁾ and K. Nakajima et al.³⁴⁾ have already done some correlative research, and described models 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 Fig. 14. The force, when the inclusion is in contact with both steel and slag, are:   

$$F_{\sigma \text{,Z}}=\left(-2\pi R_{\mathrm{i}}+2\pi Z\right){\sigma }_{\mathrm{M}\mathrm{S}}+2\pi R_{\mathrm{i}}{\mathrm{\sigma }}_{\mathrm{S}\mathrm{i}}-2\pi R_{\mathrm{i}}{\mathrm{\sigma }}_{\mathrm{M}\mathrm{i}}$$(6)

  

$$F_{\text{b}}=\frac{\text{4}}{3}\pi R_{\mathrm{i}}^3\left({\rho }_{\text{s}}{\tilde{\mathrm{\Delta }}}_{\text{b}}-{\rho }_{\mathrm{i}}\right)g$$(7)

  

$$F_{\text{d}}=\text{6}\pi R_{\mathrm{i}}{\eta }_{\mathrm{s}}{\tilde{B}}_2\frac{\mathrm{d}Z}{\mathrm{d}t}$$(8)

  

$$F_{\text{m}}=\frac{1}{2}\frac{\text{4}}{3}\pi R_{\mathrm{i}}^3{\rho }_{\mathrm{s}}{\tilde{\mathrm{\Delta }}}_{\text{b}}\frac{{\mathrm{d}}^{2}Z}{\mathrm{d}{t}^2}$$(9)

  

$${\tilde{\mathrm{\Delta }}}_{\text{b}}=\frac{1}{4}\left(\frac{{\rho }_{\mathrm{M}}}{{\rho }_{\mathrm{s}}}-1\right){\left(\frac{Z}{R_{\mathrm{i}}}\right)}^3-\frac{4}{3}\left(\frac{{\rho }_{\mathrm{M}}}{{\rho }_{\mathrm{s}}}-1\right){\left(\frac{Z}{R_{\mathrm{i}}}\right)}^2+\frac{{\rho }_{\mathrm{M}}}{{\rho }_{\mathrm{S}}}$$(10)

  

$${\tilde{B}}_2=1\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\frac{Z}{R_{\mathrm{i}}}\ge 1$$

  

$$\left(\frac{{\eta }_{\mathrm{M}}}{{\eta }_{\mathrm{S}}}-1\right){\left(\frac{Z}{R_{\mathrm{i}}}\right)}^2-2\left(\frac{{\eta }_{\mathrm{M}}}{{\eta }_{\mathrm{S}}}-1\right)\left(\frac{Z}{R_{\mathrm{i}}}\right)+\frac{{\eta }_{\mathrm{M}}}{{\eta }_{\mathrm{S}}},\mathrm{\hspace{0.17em} }\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{\hspace{0.17em} }0\le \frac{Z}{R_{\mathrm{i}}}\le 1$$(11)

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. While Z, dZ/dt and d²Z/dt² 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.

Fig. 14.

The schematic diagram of the force analysis of inclusions. (Online version in color.)

Based on D. Bouris/G. Bergeles and K. Nakajima/K. Okamura models, M. Valdez, George. S. Shannon and S. Sridhar²⁸⁾ studied the influence of viscosity of slag on the separation time, the results indicated that increasing viscosity causes the inclusions to require much more time to separate. Further, in 2014, H. B. Yang³⁵⁾ has researched the influence of viscosity of slag on the separation time in depth, and got the similar conclusions. Therefore, Rb₂O addition will exacerbate the kinetic conditions of inclusion removal during the separation stage.

4.2.3. Influence of the Viscosity of Slag on the Third Stage: Dissolution in Slag

J. Strandh et al.³⁶⁾ developed a mathematical model to study inclusion behavior at the interface, and came up with that there will be 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, H. B. Yang³⁵⁾ put forward a supplement that the forth type of inclusion behavior at the interface is “pass, and then return back”. We provided the schematic diagram of the four inclusion behavior in Fig. 15 in order to help us understand these theories.

Fig. 15.

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. (Online version in color.)

The dissolution process of inclusions on the interface of steel slag was deeply analyzed by M. Valdez, George. S. Shannon and S. Sridhar,²⁸⁾ and the mathematical formula for the time needed for complete dissolution of inclusions was derived:   

$$\tau =\frac{{\rho }_{\text{i}}{R}_{0}3\pi a{\eta }_{\mathrm{s}}}{2kT\mathrm{\Delta }C}$$(12)

where ρ_i is the inclusion density, ΔC is the driving force for the dissolution, k is the Boltzmann constant, T is the temperature, a is the ionic diameter and η_s is the viscosity of the slag.

According to Eq. (12): (i) the value of τ would increase with the η_s raising, that means the rate of inclusion dissolution would descend. (ii) In addition, with the decrease of inclusion dissolution rate, the ΔC value will decrease, and then the value of τ would increase sharply. Thus, undissolved large-sized inclusions would return back to steel when the viscosity of the slag reach a large value, then, the number, average diameter of inclusions will increase, which is visualized in Fig. 15(d).

In one word, Rb₂O addition will exacerbate the kinetic conditions of inclusion removal during the dissolution stage.

4.3. Influence of Rb₂O on the Number, Average Diameter and Content of Inclusions

The following analysis can illustrate the change of the number, average diameter and content of inclusions in this experiment.

When the amount of Rb₂O is less (less than 5.00 wt%), the influence of Rb₂O on inclusion removal is mainly from the thermodynamics conditions, which improve the adsorption ability of refining slag to inclusions. Thus, inclusions would enter the slag spontaneously easier, the number, average diameter of inclusions decreased (It can be seen from the diagram that the slope of the related fold line is very steep), and the content of Si₂O and Al₂O₃ of inclusions decreased sharply too, as described in Fig. 16.

Fig. 16.

The kinds and number of inclusions in 1^#, 2^# steel samples with refining time 0, 900, 1800, 2700 s (0, 15, 30, 45 minutes). (Online version in color.)

When the Rb₂O addition is too much (more than 10.00 wt%), the adsorption ability of refining slag will decrease because the effect of Rb₂O on the kinetics of refining slag adsorption capacity exacerbated. Thus, the velocity of inclusions adsorbed by refining slag will decrease. Therefore, the rate of reduction is obviously reduced although the number, size of inclusion decreased (It can be seen from the diagram that the slope of the related fold line is slightly). Particularly, when the Rb₂O addition increased to 15.00 wt%, undissolved large-sized inclusions would return back to steel because the viscosity of the slag reach to a large value. And then, the number, average diameter of inclusions will increase and the content of Si₂O and Al₂O₃ of inclusions increased with the refining time increasing, as described in Fig. 17.

Fig. 17.

The kinds and number of inclusions in 3^# steel samples with refining time 0, 900, 1800, 2700 s (0, 15, 30, 45 minutes). (Online version in color.)

5. Conclusions

The influence of Rb₂O containing synthetic LF refining slag on the absorption ability of inclusions for C96V saw wire steels has been studied. In addition, the kinetics mechanism of Rb₂O on inclusion removal have been discussed in depth, it can be concluded that:

(1) Rb₂O additions (≤10.00 wt%) seems to significantly enhance inclusion removal in steel melts. ① The average diameter of nonmetallic inclusions decreased significantly with the Rb₂O addition in synthetic LF refining slag increasing. In particular, the diameter of most of inclusions was less than 3.0 × 10⁻⁶ m (3.0 μm) when Rb₂O addition was 10.00 wt% ② The number of nonmetallic inclusions decreased sharply with the content of Rb₂O in synthetic LF refining slag raising. ③ Both of the MnO–SiO₂–Al₂O₃, CaO–SiO₂–Al₂O₃ inclusions system mainly concentrated in the low melting point zone.

(2) For the 15.00 wt% Rb₂O-containing synthetic LF refining slag, both the average diameter and the number of inclusion suffered a slightly increase with reaction times increasing.

In summary, utilization of Rb₂O-containing (≤10.00 wt%) basic refining slag could possibly be an alternative method to improve the cleanliness of C96V saw wire steels.

Acknowledgments

The authors are greatful for the support from the National Key Research and Development Program of China (Grant Nos. 2016YFB0300105), and the Transformation Project of Major Scientific and Technological Achievements in Shenyang (Grant Number. Z17-5-003), and the Fundamental Research Funds for the Central Universities (N172507002).

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