Study on Control of Inclusion Compositions in Tire Cord Steel by Low Basicity Top Slag

Chemical reactions between liquid steel and inclusions

As we known, the top slag composition has a great influence on inclusions. But its influence mechanism needs to be further studied. Table 1 is the chemical composition of LX72A steel. According to the interaction coefficient of each element (as shown in Table 2) and thermodynamics parameters provided by related literature at 1,600°C[7], activity coefficients of different solutes for LX72A steel can be calculated by eqns (1) and (2).

The Henry activity of solute in the liquid steel is suggested to be calculated as [8]:

where a_i is the activity of solute in steel; f_i the activity coefficient of solute in steel; ω_i the mass fraction of solute in steel.

The Henry activity coefficient of solute in the liquid steel is suggested to be calculated as eq. (2) [9]. And it neglects the two order interaction coefficient γ_i^j,k effects on f_i in this calculation:

where i, j and k, different solutes; e_i^j, the one order interaction coefficient; γ_i^j,k, the secondary interaction coefficient.

Combined with eqs (1) and (2), the results can be calculated that f_Si = 1.443, f_Mn = 0.891, and f_Al = 1.204.

During the deoxidizing stage, the deoxidization reaction may take place in liquid steel, as shown in eqs (3) and (4). It can be founded that A1₂O₃ content in inclusions is determined by a_[O] and a_[Al] in steel:

With the proceeding of reactions between liquid steel and inclusions, composition of inclusions changes inevitably. Thermodynamic phase diagram and equilibrium at 1,873 K was conducted by FactSage; iso-[Al]_sol lines of liquid steel equilibrated with CaO–Al₂O₃–SiO₂–10% MgO system inclusions in steel at 1,873 K are shown in Figure 1.

Figure 1:

Iso-[Al]_sol line in CaO–Al₂O₃–SiO₂–10% MgO phase diagram.

It can be seen that [Al]_sol content in steel has a great influence on Al₂O₃ content of inclusions. Al₂O₃ content of inclusions increases with acid-soluble aluminum in steel. As mentioned previously, one reason why inclusions cannot fall into the plastic area is that the basicity is not appropriate, while it can be seen from the diagram that [Al]_sol content in steel does have certain effect on alkalinity of inclusions. When Al₂O₃ content in inclusions is certain, the alkalinity of inclusions changes along with the change of acid-soluble aluminum content in steel. Therefore, taking alkalinity and the Al₂O₃ content of inclusion into consideration, to obtain the required inclusions, [Al]_sol content in steel must be controlled lower than 0.0002 wt% for this kind of steel.

Chemical reactions between top slag and liquid steel

After top slag is added during the refining process of tire cord steel, reaction as shown in eq. (5) [10] takes place at the steel/slag interface:

It can be seen that [Al]_sol content is controlled by the steel/slag reaction. As a result, a new steel/slag reaction equilibrium will be built with the change of chemical composition of top slag.

According to the ternary slag activity of CaO–Al₂O₃–SiO₂ and related thermodynamic data provided by literature [11], relationship between [Al]_sol content in steel and Al₂O₃ content of top slag with different basicity at 1,600°C is shown in Figure 2. It can be seen from the chart, when refining temperature remains unchanged, [Al]_sol content increases with the increase of Al₂O₃ content in top slag. And the higher Al₂O₃ content in slag and slag basicity are, the faster [Al]_sol content increases. This is mainly because the increase of basicity increases the activity of Al₂O₃ in slag, which is not conducive to reducing the acid-soluble aluminum in steel. In order to control [Al]_sol content under 0.0002 wt%, it will be more appropriate if the top slag composition meets the following conditions: R = 0.5, w(Al₂O₃)_s<20%; R = 0.8, w(Al₂O₃)_s<17%; R = 1.0, w(Al₂O₃)_s<13% or R = 1.2, w(Al₂O₃)_s<10%. It can be seen that the higher top slag basicity is, the larger controllable range of Al₂O₃ content in the top slag is. It is also one of the benefits of adopting low basicity top slag to produce tire cord steel. In addition, it suggests that [Al]_sol content in steel can be controlled by controlling the composition of top slag (basicity and Al₂O₃) to control, and then composition of inclusions can be also controlled.

Figure 2:

Relation between w(Al₂O₃)_s and [Al]_sol content by theoretical calculation.

Experimental method

Laboratory experiments were conducted in a silicon molybdenum resistance furnace, as shown in Figure 1. Silicon–molybdenum heating rods arranged symmetrically in the furnace body could provide a constant temperature zone of less than 1,650°C, and temperature deviation measured with a platinum–rhodium thermocouple is within ±5°C. The constant temperature region of the Al₂O₃ tube is 200 mm in length. Both ends of the corundum reaction tube are sealed to ensure the pressure tightness of the whole reaction system. To make a protective atmosphere, argon gas was used in experiments all the time, blowing from the bottom of the reaction tube to the top.

The LX72A steel billet casting produced by certain steel plant was used as the main raw material of the experiment. The main compositions of experimental slag are CaO, SiO₂, Al₂O₃ and MgO, and the content of each component is shown in Table 3 and analytically pure-grade reagents were used to guarantee the accuracy of each component. In order to prevent the slag-eroding magnesium oxide crucible, 10% MgO was added to the slag. A total of 25 groups of experiments were conducted by using the single factor experiment.

About 500 g steel was put into the MgO crucible and then heated to melt. Temperature should be kept at 1,600°C for a period of time to ensure the steel melting. When the solid steel turns into molten steel completely, 100 g slag previously melted was added into the molten steel. Every 5 min after the slag completely melted, the molten steel and slag were stirred by a molybdenum rod. Setting the time when slag completely melted as the zero point, six samples were taken at the times listed in Table 4. After sampling, the steel and slag in the crucible were then cooled down to room temperature in the furnace.

Samples of 15 mm × 15 mm × 10 mm were analyzed by ZEISS URTRA 55 scanning electron microscope (SEM) and energy-dispersive spectroscopy (EDS) to determine the morphology, the size and the composition of inclusions. Acid-soluble aluminum was analyzed by plasma mass spectrometry (ICP MS)

Experimental results and discussion

Figure 4 is the relationship between the [Al]_sol content and refining time. It can been seen that the [Al]_sol content remains unchanged after 35 min. The top slag refining time was 45 min in this experiment, so the reaction reached equilibrium.

The analysis results of inclusions in samples by SEM/EDS indicate that non-metallic composite oxide inclusions in steel are mainly Al₂O₃–SiO₂–CaO–MgO type, as shown in Figure 3 (show the morphology and spectrums of typical inclusions). This type of inclusions, which are in big numbers and have a size within 10 μm, is the product of slag–metal reaction and presents as spherical or ellipsoidal.

Figure 3:

Experimental equipment used in laboratory experiments.

There is a certain relationship between the inclusion plasticity and its melting point. Low melting point inclusions have better deformation performance, and produce a certain amount of deformation in the process of drawing with the direction of rolling, So that, they have less impact on the fracture of wire [12, 13]. Area ① in Figure 4 is low melting point area (temperature is less than 1,400℃ in the Al₂O₃–SiO₂–CaO–10% MgO ternary system, also called the plastic area), which is calculated by thermodynamic software FactSage. Due to the applications of slag splashing technology and the erosion of ladle lining, there exists MgO in inclusions. According to the practical situation, content of MgO was set to 10%. It can be seen from Figure 4 that in the CaO–Al₂O₃–SiO₂–10% MgO phase diagram, the composition of inclusion is as follows: CaO = 8–48%, SiO₂ = 35–75%, Al₂O₃ = 0–32%, the inclusion is in low melting point area (Figures 5 and 6).

Figure 4:

The relationship between the [Al]sol content and refining time.

Figure 5:

SEM images and spectrums for Al₂O₃–SiO₂–CaO–MgO inclusions.

Figure 6:

Aim of chemical composition of inclusions.

Figures 7–11 show the distribution of inclusions in Al₂O₃–SiO₂–CaO–10% MgO phase diagram after top-slag refining processing with different slag compositions. It can be seen that when the basicity of top slag is certain, Al₂O₃ content in inclusions increases with Al₂O₃ content in the slag, and the distribution of inclusion composition disperses with the increase of the Al₂O₃ content in the slag. When basicity of top slag is 0.6–0.8, and w(Al₂O₃)_s is 2%, parts of inclusions with high SiO₂ content are not in the plastic area, as shown in Figures 3 and 4; this type of inclusion breaks in the process of rolling, so it will not cause the fracture of wire [14]. For R = 0.6, 5–15% Al₂O₃ and R = 0.8, 2–10% Al₂O₃ slags, the majority of inclusions are in the plastic area. When slag basicity increased to 1.4, the distribution of inclusions tends to be scattered. Inclusions fall into the plastic area only when Al₂O₃ content in the slag is 2%. On the basis of analysis above, there are two reasons why inclusions don’t fall into the plastic area. The first reason is that Al₂O₃ content of inclusions is too high, and the second is that the basicity of inclusions is too high or too low. Therefore, plastic inclusions can be obtained through controlling the composition of top slags.

Figure 7:

Composition of inclusions after slag (R = 0.6) treatment.

Figure 8:

Composition of inclusions after slag (R = 0.8) treatment.

Figure 9:

Composition of inclusions after slag (R = 1.0) treatment.

Figure 10:

Composition of inclusions after slag (R = 1.2) treatment.

Figure 11:

Composition of inclusions after slag (R = 1.4) treatment.

Experimental method

The industrial production processes of LX72A of certain steel mill are as follows: Molten iron is firstly pretreated to make sure S < 0.005, high-quality self-produced steel scrap is used in the BOF process. Catching carbon practice is used before tapping, then ferrosilicon and silicon–manganese alloys are added during tapping process, active lime is also added along with the flow of liquid steel. Lime quartz sand and fluorite are used for slagging in LF refining process, and protective casting technology is used during the whole process of continuous casting, as well as M-EMS. Two industrial experiments were conducted, and the composition of top slag used in industrial experiments is shown in Table 5. Then casting billet samples of two industrial experiments were taken, and inclusions in the samples were analyzed by SEM/EDS.

Results and discussion

It can be seen from Figure 12(a) that the distribution of inclusions in samples of the first industrial experiment is scattered, and the main reason for inclusions’ deviating from plastic area is that Al₂O₃ content in inclusions is high. According to laboratory experiment results, appropriate basicity of top slag and a certain low level of Al₂O₃ content are beneficial for plasticity of inclusions in tire cord steel. Therefore, the second industrial experiment was carried out. During LF refining process, basicity of top slag is controlled in the range of 0.67–0.9, and Al₂O₃ content in slag is controlled lower than 10%, different types of inclusion in casting billet samples were observed, and results are shown in Figure 12(b). Compared with results of the first industrial experiment that are shown in Figure 12(a), it can be seen that the distribution of inclusions in Figure 12(b) is comparatively centralized, Al₂O₃ content in inclusions decreases obviously, most of inclusions are in the plastic area. Results of two industrial experiments prove the importance and feasibility of controlling top slag for the control of inclusion type in steel.

Figure 12:

Distribution of inclusions in steel slab during two industrial experiments.