2019 Volume 59 Issue 10 Pages 1735-1743
Contact angles of five common grades of molten steel on Al2O3, MgO and MgO·Al2O3 inclusion substrates have been measured precisely by a sessile drop technique at 1823/1873 K, and the wettability of each type of inclusions is discussed. Interfacial tension between the steels and the inclusions is calculated through the Young equation. Thus, coagulation coefficient, which is used to evaluate agglomeration tendency of inclusions, has been calculated based on the collision-coagulation theory combined with the interfacial tension data. The results of the coagulation coefficient show that the agglomeration tendency of the three typical oxide inclusions in molten steel is Al2O3 > MgO > MgO·Al2O3, and the relationship between coagulation coefficient αt of inclusions and contact angle θ between inclusions and molten steels can be expressed as αt = 4.923 × 10−5[μrI3(ρMε / μ)1/2 / (σI – σMcosθ)]−0.242. Furthermore, in order to verify the validity of the results obtained, three deoxidation experiments in resistance furnace were carried out. The resulting samples were quenched and then analyzed by SEM-EDS and ASPEX. The results of inclusions observed in the quenched samples show good agreement with the results of inclusion agglomeration tendency. Overall, the effects of wettability and interfacial tension on inclusion agglomeration behaviors are proved to be remarkable.
During steelmaking processes, large inclusions are favorable for removal via floating upward, however, compared with small inclusions, quality problems are more likely to occur once large-sized inclusions remain in steels.1,2,3,4,5) On the other hand, inclusions with fine sizes can be utilized as nucleation sites of acicular ferrite during phase transformation to improve steel strength according to the theory of oxide oetallurgy. Therefore, controlling the size of inclusion particles plays an essential role in improving product quality and mechanical properties of steel products.
A main factor affecting inclusion size is collision and agglomeration behaviors among inclusions in molten steel. Many researches6,7,8,9,10,11,12,13,14,15) have investigated the mechanism of inclusion agglomeration at the steel/Ar interface by confocal laser scanning microscope (CLSM). Yin considered that pronounced agglomeration force, especially between alumina inclusions, results from capillary force.6,7) Nakajima indicated that the agglomeration is significantly affected by inclusion characteristics (size, shape and phase of particles), and the contact angle of inclusion particles with steel based on capillary force theory.8,9,10,11) From another point of view, using physical and numerical simulation methods,16,17,18,19,20) fluid dynamic actions, such as Brownian agglomeration, turbulent agglomeration, and agglomeration by Stokes collision have been discussed deeply. In these studies, the agglomeration coefficient, which can be used to evaluate inclusion agglomeration tendency, was derived by taking the van der Waals force and the Hamaker constant into consideration. In addition, Cournil21,22,23,24) assumed that the cavity bridge force due to non-wetting property dominates when Al2O3 inclusions formed by Al deoxidation approach each other within a molten steel flow. According to the previous studies, the wettability of inclusion particles with liquid steel is a key point to reveal the mechanisms of inclusion agglomeration. Unfortunately, few studies have systematically investigated the inclusion agglomeration behavior in view of interfacial properties. The behaviors are vital to inclusion size control in steel.
In this work, 15 contact angles between molten steel droplets (pure iron, high sulphur steel, tire cord steel, die steel and stainless steel) and typical inclusions (Al2O3, MgO·Al2O3 and MgO) have been measured. This research aims to obtain the coagulation coefficient (or agglomeration tendency) of each inclusion in each steel based on the collision-coalescence theory,25,26,27,28) and clarify the agglomeration behaviors of the inclusions. In addition, three deoxidation treatment experiments in MoSi2 resistance furnace were carried out to verify the agglomeration tendency of inclusions by observing inclusion characteristics using SEM-EDS and ASPEX methods.
Polished metal samples of Φ5 × 5 mm and 0.7 ± 0.02 g are used in contact angle measuring experiments by the sessile drop technique. Chemical compositions of the metal samples are listed in Table 1. Sintered Al2O3, single crystal MgO and MgO·Al2O3 are used as substrates and shown in Fig. 1. Basic properties of the substrates are summarized in Table 2.
Photos of the substrates. (Online version in color.)
For the sessile drop measurements, a high temperature and vacuum contact angle measuring instrument (OCA25-HTV1800) provided by dataphysics was used. The apparatus consists of an image analyzing system and a MoSi2 resistance furnace, as shown in Fig. 2. An alumina reaction tube with an inner diameter of 40 mm and a length of 990 mm was placed horizontally in the furnace. Two quartz windows were fabricated on opposite sides of the furnace for observing the samples. Argon was introduced into the alumina tube to prevent the materials from oxidation. A high-resolution CCD camera (2048 × 1088 pixels) was used to capture a clear image of the metal samples on the substrates.
Schematic diagram of the sessile drop apparatus.
In the contact angle measurement experiments, a piece of polished metal sample was placed on the substrate, which was cautiously pushed to the center of the alumina reaction tube. Extreme care was taken to ensure the level of each substrate. After complete sealing, the system was evacuated to 10−3 Pa. High purity argon (99.999 vol%) was introduced to the alumina tube at a rate of 50 L/h to prevent the sample surface from oxidation (partial pressure of oxygen < 10−3 Pa). Then the power was switched on and the sample was heated to 1823 or 1873 K. The experimental temperatures of high sulphur steel, tire cord steel, die steel and stainless steel are set as 1823 K, and the temperature of pure iron experiment is set as 1873 K because of the melting point of pure iron is higher than the others. The images of the metal samples together with the substrates were captured by the CCD camera after the temperature was stabilized for 10 minutes. The same group of experiments was repeated for three times to ensure repeatability of results. After the experiments, the contact angle and surface tension of liquid metal drop can be extracted from these images by the post-processing software SCA20.
In order to analyze inclusion coagulation tendency from the aspect of the contact angle or other interfacial properties, a deoxidation treatment experiment was conducted in a MoSi2 resistance furnace. Schematic of the furnace is shown in Fig. 3. Before heating, the system was also evacuated to 10−3 Pa and then placed in a high purity argon atmosphere with a flow rate of 50 L/h to keep the oxygen potentials in both experiments as close as possible. An Al and a Ni–Mg alloy block were used as desoxidant to ensure generation of certain oxide inclusions (Al2O3, MgO and MgO·Al2O3) in pure iron and stainless steel separately. The experiment procedure is illustrated in Fig. 4 and quenched Samples 1–3 will be further analyzed by ASPEX and SEM-EDS.
Schematic diagram of the MoSi2 resistance furnace.
General melting and sampling procedure of the verification experiment.
During the experiments, the real-time contact angle was found to slightly vary with the change in experimental time and temperature, which has been also observed in previous works.29,30,31,32) In order to ensure the consistency, contact angle data used in subsequent discussion was always measured at 1823 K and 10 minutes as mentioned before. It is noted that all the contact angles measured are the apparent contact angles but not eigen contact angles because chemical reactions at the interface between the liquid steel droplets and the substrates are unavoidable.
Typical sessile drop images as well as the calculated average contact angle θ are shown in Fig. 5. The Laplace-Young fitting method and ellipse fitting method are used to analyze the Fig. 5(a) and Figs. 5(b)–5(o), respectively. As shown in Fig. 5(a), in super-nonwetting conditions (the contact angle is large than 120°), such as the case of the pure iron droplet on the Al2O3 substrate, measuring accuracy of the Laplace-Young fitting method is highest. The value of contact angle in Fig. 5(a) is 136.7°, which is in agreement with reported values ranging from 132° to 141° in previous studies.33,34,35,36,37,38) Differing from the Laplace-Young fitting, the ellipse fitting is suitable when the contact angle ranges from 90° to 120°, and therefore contact angles in Figs. 5(b)–5(o) are determined with the ellipse fitting. It is clear that the measured contact angles are slightly different on the left and right sides of the droplet. This is ascribed to the fact that the substrate can hardly remain absolute horizontal due to slight vibrations during the heating process. Hence an average value was used as the apparent contact angle in the following calculations.
Sessile drop images and average contact angle θ of five kinds of liquid steel droplets upon three different oxide substrates at (a)–(c) 1873 K/(d)–(o) 1823 K. (Online version in color.)
The values of each average contact angle measured in this experiment are plotted in Fig. 6. It is found that all the contact angles are larger than 90°, which means neither the liquid iron nor the steels is wetted on the three kinds of substrates. For all the five liquid steel matrices, the order of unwettablity is Al2O3 > MgO > MgO·Al2O3, and this difference of unwettability is most outstanding in pure iron but unapparent in stainless steel. Considering molten steels, their wettability on the substrates approximately increases with the increase of in alloy content except for high sulphur steel. Compared with the other four steel grades, the wettability of high sulphur steel is best on each substrate because sulphur, a typical strong surface active element, is able to remarkably reduce the surface tension of steel. Notice that the contact angle between liquid pure iron and Al2O3 substrate is much larger than the ones measured for the other 14 combinations. The strong agglomeration tendency of Al2O3 inclusion particles in molten steel is assumed to be related to this phenomenon, and this topic will be discussed further in following sections.
Contact angles of five types of molten steel droplets on Al2O3, MgO and MgO·Al2O3 substrates. (Online version in color.)
Surface tension calculations were performed by using the Profile Fitting Method.33) This method is based on the shape of droplet naturally formed on a horizontal substrate in sessile drop technique. A schematic of a liquid droplet stably staying on a substrate is shown in Fig. 7. According to the Young-Laplace equation, a formula can be established at any point P on the surface of the droplet as follows:
Shape of liquid droplet on substrate. (Online version in color.)
At vertex O, z = 0, R1 = R2 = R0, so p0 = 2σ/R0. Substituting this into Eq. (1), the equation turns to be:
The shape factor β is defined as:
And then Eq. (2) can be transformed to:
The profile of each metal droplet could be precisely extracted from the high-resolution image of melted sample by the post-processing software SCA20 as mentioned before. And surface tension calculation program had been built in this software. The results are shown in Fig. 8.
Surface tension of each kind of molten steel.
The surface tension σM of pure iron at 1873 K was calculated to be 1.787 ± 0.035 N/m, which is close to the average value (1.772 N·m−1) measured in previous works.29,30,31,32,33,34,35) Hence this profile fitting method for surface tension calculation is feasible. Due to the existence of surface active element sulphur mentioned in Section 3.1, the surface tension of liquid high sulphur steel at 1823 K (0.962 ± 0.022 N/m) is lowest among the five kinds of steels, while the surface tensions of the rest three steels are always around 1.7 N/m.
If assuming that the surface tension data of the Al2O3, MgO and MgO·Al2O3 substrates are 0.75, 0.71 and 0.55 N/m obtained from reference,36) the interfacial tension between the molten steels and the substrates can be calculated through the Young equation shown as follows.
It is well known that the collision-coagulation process among the inclusions in molten steel occurs due to the following combined collision mechanisms: (i) the Brownian collisions as a result of random movements of inclusions in the melt, ; (ii) the Stokes collisions as a result of floating of inclusions and clusters in the liquid steel, ; and (iii) the turbulent collisions as a result of movement of inclusions and clusters together with melt flow, . The total collision volume 25,26,27,28) is expressed as:
In summary of the results concluded in references,25,26,27,28) the effect of the Brownian collision can be entirely neglected, while the turbulent collision plays a much more important role than the Stokes collision since the size difference of pratical inclusions can be ignored in most conditions. Therefore, the total collision volume is dominantly influenced by the turbulent collision. In the case of the turbulent collisions, the inclusion agglomeration potency in liquid steel is related to the coagulation coefficient, which can be expressed using Eq. (10).27,28,43,44,45)
Hamaker constant is originally utilized in the field of colloid chemistry.44) Subsequently, Taniguchi et al.16) applied this concept to particle coagulation phenomenon in liquid iron/steel by conducting a physical simulation experiment based on the coagulation of Polystyrene-latex, Al2O3 and SiO2 particles. They considered these particles as rigid spheres, and proposed the Hamaker constant of Al2O3 and SiO2 in liquid iron. Then Nakajima et al.46) proposed a method to calculate the Hamaker constant as well as coagulation coefficient of various inclusion particles using the physical properties including contact angle and interfacial energy between inclusions and liquid metal. Their method is based on the original equation by Frenkel et al.47) shown in Eq. (13). The present work applied the same method to investigate the agglomeration potency of three kinds of inclusion particles in the five types of steel matrix mentioned above.
All the parameters used for the calculation of coagulation coefficient are listed in Table 4. By applying the data in Table 4 to Eqs. (5), (10) and (13), the coagulation coefficient of Al2O3, MgO and MgO·Al2O3 inclusions in steel matrix mentioned above has been calculated. The radius of inclusion particles is assumed to be 10 μm in this calculation. Since the coagulation coefficient is affected by the density and dynamic viscosity of liquid steel, inclusion particle size and turbulent energy dissipation rate to a large extent, relative value of is plotted in Fig. 9 to highlight the influence of interfacial tension as well as the Hamaker constant during coagulation process. The maximum value, coagulation coefficient of Al2O3 inclusions in liquid pure iron, is defined to be one as a reference.
Comparison of relative coagulation coefficients of three types of inclusions in five kinds of steel matrix. (Online version in color.)
As can be seen from Fig. 9, the order of coagulation potency of these three inclusions follows a trend of Al2O3 > MgO > MgO·Al2O3 in all of the five kinds of steel matrices. When the steel matrix remains unchanged, the surface tension of steel is a constant so the coagulation coefficient mainly depends on the contact angle. This result is consistent with the results reported by Wangzhong Mu.28) For the condition that a same kind of inclusion in different steel matrix, both surface tension of molten steel and interfacial interaction vary with chemical composition of the steels so the difference in coagulation coefficient is caused by combined action of surface tension and contact angle. In brief, the tendency of coagulation coefficient shows some similarity with the contact angle data illustrated in Fig. 6 and the relationship between them can be reduced to be Eq. (14) according to Eqs. (5), (10) and (13).
The coagulation behaviors of alumina in pure iron, magnesium oxide in pure iron and alumina in stainless steel are investigated in this section. An area of around 5 × 5 mm on the fine polished surface of samples 1–3 was scanned by Thermo Scientific Explorer 4 Analyzer separately. Detailed information of chemical composition, profile and average size etc. of each inclusion particle exerted on the sample surface was precisely identified. The results of samples 1–3 are summarized in Figs. 10, 11, 12.
Chemical composition and size distribution of inclusions in sample 1. (Online version in color.)
Chemical composition and size distribution of inclusions in sample 2. (Online version in color.)
Chemical composition and size distribution of inclusions in sample 3. (Online version in color.)
Figure 10 shows the chemical composition and size distribution of inclusions in sample 1. The actual scanning area is 28.444 mm2, and the total number of inclusion particles is 1459. It can be found that almost all the inclusions are alumina as expected, and the average size Dav of them was calculated to be 1.405 μm in diameter.
The information of sample 2 is given in Fig. 11. Actual scanning area on the surface of this sample is 26.122 mm2 on account of instrumental error. Total inclusion number N is counted to be 615 and the Dav is 1.323 μm, slightly smaller than those of sample 1. It can be seen that after magnesium addition, most alumina inclusions transformed to alumina rich spinel containing magnesium oxide, with their size smaller than several calcium aluminate type inclusions obviously.
Chemical composition of inclusions in stainless steel is complicated. Few pure alumina inclusions were generated. Instead, most of the inclusions are with a mixture of oxides like CaO, MgO, SiO2 and MnO, while a few calcium sulfide inclusions are not included in the discussion as common. Generally, the sizes of alumina inclusions (around 2–5 μm) containing CaO and MgO are slightly larger than those containing SiO2 and MnO (most < 2 μm) as shown in Fig. 12. Thus the average size Dav increases to 1.769 μm and the total number of inclusions in this area decreases to 566 correspondingly.
As another indicator, size distribution of the inclusions was analyzed in the different samples as illustrated in Fig. 13. It can be found that the number of inclusions whose average particle size larger than 2 μm in sample 2 is much smaller than those in samples 1 and 3. The overall inclusion size is therefore extremely reduced by Mg addition, indicating that magnesium contained inclusions tend to disperse in steel matrix. This result is consistent with the coagulation coefficient data in Fig. 9. Noticed that the number of inclusions smaller than 2 μm in sample 1 is several times greater than the ones in samples 2 and 3. This seems to be inconsistent with the opinion proposed above. Actually, clusters formed by agglomeration of alumina inclusions are three-dimensional in steel matrix, while the size of them can only be measured on the two-dimensional surface by ASPEX. Therefore, an integral alumina cluster was usually observed to be uncontinuous on the cross section of sample 1 and the size of each part would be measured one by one, causing the abnormally raising of the number of inclusions of small sizes.
Size distribution of inclusion particles in different samples.
Furthermore, the SEM-EDS method was employed for observation of inclusions as a supplement of ASPEX. Morphology, size and chemical composition of typical inclusions in Sample 1–3 are shown in Fig. 14. Figures 14(a)–14(d) show the observed alumina clusters and single alumina inclusion with large size (around 5–10 μm in diameter). Hence the explanation for exceptionally high number of small inclusions (d < 2 μm) is proved to be right incidentally. Figures 14(e)–14(h) show that spinel rich inclusions have formed and the size of them, mostly around 1–2 μm, is much smaller than the one of pure alumina inclusions, accounting for the remarkable difference in coagulation coefficient calculated above. As for inclusions in stainless steel, the composition of them is complicated so it may be inappropriate to analyze the inclusion size in the same way. Despite this, it’s summarized that few coagulation phenomena occurred in Sample 3.
Morphology and chemical composition of typical inclusions in a)–d) sample 1, e)–h) sample 2 and i)–l) sample 3 observed by SEM-EDS.
In the present work, the effects of interfacial properties on inclusion agglomeration behavior has been studied. Other conditions, for example, number density of inclusions, effect of steel flow and Ar bubbling etc., which cannot be ignored in actual steelmaking process, are not considered. If taking all of these potential affecting factors into consideration, the question that how much contribution to agglomeration of an inclusion pair is made by the effect of wettability is still in doubt. Hence further work needs to be conducted from this point of view.
Experimental and theoretical studies of inclusion agglomeration behavior in molten steel matrix have been performed in this research. Conclusions can be drawn as follows.
(1) The contact angles between three typical kinds of oxide inclusions and five common types of molten steels have been measured by the sessile drop technique in high purity Ar atmosphere at 1823/1873 K. The interfacial tensions between substrates and steel melts were calculated according to the Young equation. The results indicate that the order of unwettablity is Al2O3 > MgO > MgO·Al2O3.
(2) Based on the collision-coagulation theory, the relationship between wettability and agglomeration tendency of inclusions in molten steel matrix has been obtained by calculating the relative coagulation coefficient. The expression of coagulation coefficient can be summarized as , and the agglomeration tendency of Al2O3, MgO and MgO·Al2O3 inclusions in each steel is Al2O3 > MgO > MgO·Al2O3.
(3) A series of deoxidation treatments for the steels was conducted. Through analyzing the generated oxide inclusions, it is found that coagulation tendency of them shows consistence with the relative coagulation coefficient. In the condition of ignoring the influence of molten steel flow, the effect of wettability and interfacial tension on inclusion agglomeration behaviors is therefore proved to be remarkable.
This project is supported by National Nature Science Foundation of China with grant No. 51674172, U1860205 and 51874203. This project is also supported by China Postdoctoral Science Foundation with grant No. 7131704818, and Jiangsu Science and Technology Foundation with grant No. 18KJB450002.