Effects of Na₂O, K₂O and B₂O₃ on Deformability of SiO₂-MnO-Al₂O₃ Inclusion in High-Carbon Steel

Abstract

Cord steel is used for making tire frames and wire saws for cutting silicon wafers. The diameter of mainstream cutting wire has been developed to be lower than 100 μm. The size and deformation ability of inclusions are very important to the wire breaking rate of cord steel during the drawing process. In order to improve the deformation ability of the inclusions in cord steel, alkali metal oxide was added into the molten steel to improve the inclusions in the steel so as to obtain good, plastic, low-melting-point inclusions. Mass fractions of 0.3%, 0.5% and 1.0% K₂CO₃, Na₂CO₃ and B₂O₃ were added into cord steel, which were melted in 10 furnaces (including 0% alkali metal oxides, mass fractions of 0.3%/0.5%/1.0% K₂CO₃, Na₂CO₃ and B₂O₃). The morphology and composition of inclusions were observed by SEM-EDS. Factsage phase diagram calculations and experimental results show that, with the increase in Na₂CO₃ content in cord steel, the aluminum content in the inclusions gradually decreased. When the mass fraction of Na₂CO₃ was 0.5% per ton, most of the inclusions in the steel fell in the low melting point region (less than 1300 °C). With the increase in K₂CO₃ content in cord steel, the silicon content in the inclusions decreased gradually. When the mass fraction of K₂CO₃ was 0.5% per ton, most of the inclusions in the steel fell in the low melting point region. The deformation ability of the inclusions added with 0.5% Na₂CO₃ in the steel during forging was better than that of the inclusions added with 0.5% K₂CO₃. After adding B₂O₃, the inclusions in the steel were SiO₂-MnO-Al₂O₃ inclusions or inclusions with SiO₂-MnO-Al₂O₃ as the core and BN wrapped around. Boron could not be dissolved into the inclusions for plastic modification.

1. Introduction

Cord steel wire is widely used in industry because of its excellent performance, a d is used in the bridge industry to make the main load-bearing component cables, in the tire industry to make tire skeletons [1,2] and in the photovoltaic industry for cutting silicon wafers [3,4]. The diameter of steel cord is 80~200 μm, which is for the main skeleton of tires, and the diameter of cutting steel wire is 50~80 μm, which is mainly used for cutting precious materials such as solar silicon wafers, gems and aviation semiconductors [5]. In the process of cutting silicon wafers, the diameter of the cutting steel wire determines the amount of debris loss of material. Therefore, in order to reduce the amount of debris loss as much as possible, the smaller the diameter of the cutting wire, the better. At present, the diameter of mainstream cutting steel wire in the market has reached lower than 100 μm. The most common reason for the fracture of steel cord and cutting steel wire is the presence of large hard inclusions in steel. Si-Mn combined deoxidization or Si-Ca combined deoxidization is usually used in the refining process of cord steel, and the deoxidization products are a MnO-Al₂O₃-SiO₂ system and a CaO-Al₂O₃-SiO₂ system [6,7,8,9,10]. Among them, Al₂O₃ inclusion is the most important component, which occupies a high content; it most affects the deformation of cord steel in the processing process and the wire breaking rate in the process of drawing and hot rolling. So, on the one hand, we want to reduce the content of Al₂O₃ in the inclusion because the high content of Al₂O₃ makes the inclusion harder. On the other hand, we also hope that the size of these kinds of hard, high-melting-point inclusions can be reduced, and the wire breaking rate of cord steel can be reduced to a certain extent. But neither of these two aspects can solve the final problem. We think the most important thing is to reduce the melting point of the inclusion so that it can be softened or even liquid at rolling temperature, with good deformability—that is, the wire of the cord steel can be drawn thinner and continuously [11].

The control of inclusions is not only limited to the refining [12,13,14,15,16,17] and solidification processes [18,19,20], but also needs to pay attention to the evolution behavior of inclusions in the hot working process [21]. And in the study of steel samples, it was also found that there are hard, high-melting-point Al₂O₃ inclusions. The melting point of the inclusion decreases, and it softens at the rolling temperature, which makes it easier to deform. The increase in inclusion deformability will reduce the wire breaking rate of cord steel and make its diameter smaller. Although the inclusion elongates and becomes larger in the rolling direction after the melting point of the inclusion is reduced, the size of the inclusion on the cross-section becomes smaller, so the wire breaking rate of the cord steel can be reduced. In recent years, the method of adding alkali metals to molten steel to modify inclusions in steel has attracted the attention of metallurgical workers for the purpose of improving the deformability of inclusions in cord steel [22,23]. Chen et al. [24] studied the inclusions in modified low-carbon steel by boron treatment and on this basis, they applied it to the smelting of cord steel [25]. By adding boron to molten steel, the low melting point regions of the inclusions in SiO₂-MnO-Al₂O₃ and SiO₂-CaO-Al₂O₃ systems were significantly expanded, thus improving the deformability of inclusions. However, the hot workability was easy to deteriorate when the mass fraction of boron in the steel was too high, so it must be strictly controlled at 0.005%. Chen et al. [26] studied the modification mechanism of inclusions in cord steel treated with Na₂CO₃ and K₂CO₃, and pointed out that the addition of Na₂CO₃ to molten steel could significantly improve the deformability of inclusions [27]. The main mechanism was that Na₂O could sharply reduce the melting point of SiO₂-MnO-Al₂O₃ inclusions and significantly expanded the low melting point region of SiO₂-MnO-Al₂O₃ inclusions. Chen et al. [28] studied the effect of NaF treatment on inclusions in cord steel. The experimental results showed that after NaF treatment, the number of inclusions in the steel decreased, the average diameter decreased, a large number of inclusions containing Na appeared in the steel, and the melting point of the inclusions containing Na was lower, which was beneficial for improving the deformability of the inclusions. When smelting superfine steel wire [26,27,28,29], the effect of non-metallic inclusions in cord steel modified by alkali metals such as Na, K, B and their oxides was remarkable. In this paper, with the help of the Shagang Group, the effect of alkali metal carbonate on cord steel inclusions was studied.

To sum up, the influence of the types and contents of alkali metals on the inclusions in a SiO₂-MnO-Al₂O₃ system still needs to be further studied. We aim to study which type of alkali metal is more beneficial for improving the deformability of cord steel inclusions and what is the most suitable content. Alkali metal was added to cord steel to reduce the melting point of its inclusions so as to improve the deformability of the inclusions and reduce the wire breaking rate of cord steel. In this paper, the influence of Na₂CO₃, K₂CO₃ and B₂O₃ addition on the composition of SiO₂-MnO-Al₂O₃ inclusions in steel is studied, and the evolution law of the inclusions after adding alkali metals is pointed out, which provides research basis and theoretical guidance for the production of cord steel.

2. Experimental

The raw material of cord steel comes from the Shagang Iron and Steel Group, and its production process was KR (Kambara reactor) → BOF (basic oxygen furnace) → LF (ladle furnace) → CC (continuous casting). Among them, the KR process was desulfurized for molten iron and then transported to a converter for desilication, decarbonization and dephosphorization, and low titanium and low aluminum ferrosilicon were added in the converter tapping → silicomanganese → high-carbon ferrochrome → deoxidation and alloying of synthetic slag and slagging. Then, it was transported to the LF for refining, fine-tuning the composition of molten steel and slag, electrifying to adjust the temperature, and then it was transported to continuous casting and pouring after reaching the standard. The compositions of cord steel and refining slag are shown in

Table 1. Chemical composition of cord steel/mass%.

C	Si	Mn	P	S	Cr	Ni	Cu	Ti	Al
0.82	0.20	0.52	0.004	0.005	0.03	0.03	0.04	0.0008	0.0015

and

Table 2. Chemical composition of slag/mass%.

SiO2	Al2O3	CaO	MgO	T.Fe	MnO	R
41	5	42	6	2	4	1.02

, respectively.

The experiment of adding alkali metal carbonate to cord steel was carried out in a laboratory resistance furnace, and the device is shown in Figure 1. The test steps were as follows:

(1)

Mix and briquette the alkali metal carbonate and iron powder at a mass ratio of 1:1.

(2)

Put the cord steel sample with the mass of 600 g (the original sample comes from the hot-rolled bar produced by Shagang) into an MgO crucible, and put the outer graphite crucible into the MoSi₂ resistance furnace to heat up with electricity. The argon flow rate is 2.5 L/min.

(3)

Raise the temperature to 1540 °C, and keep the temperature constant for 10 min.

(4)

Add the mixed compact of alkali metal and iron powder into the molten steel for 20 min, and stop the power supply and cool after the test.

(5)

Use a scanning electron microscope SEM-EDS (Carl Zeiss Microscopy, Oberkochen, Germany) to analyze the composition and types of inclusions in the steel samples cut from a 600 g cord steel ingot.

(6)

Forge the solidified 600 g cord steel after holding at 1200 °C for 1 h, as shown in Figure 2b.

(7)

Analyze the deformability of inclusions during thermal deformation with a SEM-EDS.

2.1. Test Process of Adding Na₂CO₃ to Cord Steel

The mass ratio of Na₂CO₃ and iron powder was 1:1 for mixed briquetting, and the additional amounts of Na₂CO₃ were 0.3%, 0.5% and 1.0% of the steel sample. Totals of 1.8 g, 3.0 g and 6.0 g of Na₂CO₃ were added to the 600 g steel sample, that is, 3.6 g, 6.0 g and 12.0 g of Na₂CO₃ and iron powder-mixed compacts were added to the 600 g steel sample, and three groups of experiments were conducted to add Na₂CO₃ to the cord steel.

2.2. Test Process of Adding K₂CO₃ to Cord Steel

The mass ratio of K₂CO₃ and iron powder was 1:1 for mixed briquetting, and the additional amounts of K₂CO₃ were 0.3%, 0.5% and 1.0% of the steel sample. Totals of 1.8 g, 3.0 g and 6.0 g of K₂CO₃ were added to the 600 g steel sample, that is, 3.6 g, 6.0 g and 12.0 g of K₂CO₃ and iron powder-mixed compacts were added to the 600 g steel sample, and three groups of experiments were conducted to add K₂CO₃ to the cord steel.

2.3. Test Process of Adding B₂O₃ to Cord Steel

The mass ratio of B₂O₃ and iron powder was 1:1 for mixed briquetting, and the additional amounts of B₂O₃ were 0.3%, 0.5% and 1.0% of the steel sample. Totals of 1.8 g, 3.0 g and 6.0 g of B₂O₃ were added to the 600 g steel sample, that is, 3.6 g, 6.0 g and 12.0 g of B₂O₃ and iron powder-mixed compacts were added to the 600 g steel sample, and three groups of B₂O₃ added to the cord steel were tested.

3. Results and Discussion

3.1. Inclusion Types of Na₂CO₃, K₂CO₃ and B₂O₃ Added to Cord Steel

(1)

The influence of Na₂CO₃ addition on inclusion types of cord steel.

The 600 g steel sample without Na₂CO₃ added during the resistance furnace experiment process was analyzed by SEM-EDS, and the typical inclusion is shown in Figure 3. The scanning results of the inclusions in the steel sample with 0.3%, 0.5% and 1.0% Na₂CO₃ added are shown in Figure 4, Figure 5 and Figure 6, respectively. After adding alkali carbonate, the content of calcium in the inclusions in the sample decreased significantly (CaO from 15% to 8%). And with the increase in Na₂CO₃ content, the content of Na in the inclusions also increased significantly (Na₂O from 0% to 6%). After comparing the inclusions obtained by SEM analysis between the blank samples and the samples with a different alkali metal to sodium carbonate, it was found that the size of the inclusion did not change significantly with the increase in the amount of sodium carbonate, but only increased slightly.

(2)

The influence of K₂CO₃ addition on inclusion types of cord steel.

The 600 g steel sample without K₂CO₃ added during the resistance furnace experiment process was analyzed by SEM-EDS, and the typical inclusion is shown in Figure 7. The morphologies of the samples with 0.3%, 0.5% and 1.0% K₂CO₃ in the steel are shown in Figure 8, Figure 9 and Figure 10, respectively. After adding alkali carbonate, the calcium content in the inclusions in the sample decreased significantly (CaO from 15% to 8%). And with the increase in K₂CO₃ content, the content of K in the inclusions also increased significantly (from 0% to 6%). After comparing the inclusions obtained by SEM analysis between the blank samples and the samples with a different alkali metal to K₂CO₃, it was found that, similar to the addition of Na₂CO₃, the size of the inclusion in the sample with the addition of K₂CO₃ did not change significantly with the increase in potassium carbonate, but only increased slightly.

(3)

The influence of B₂O₃ addition on inclusion types of cord steel.

The 600 g steel sample with 0.3% and 1.0% B₂O₃ added during the resistance furnace experiment process was analyzed by SEM-EDS, and the typical inclusions are shown in Figure 11 and Figure 12, respectively. When the amount of B₂O₃ was small, the inclusions were boron-free inclusions in the SiO₂-MnO-Al₂O₃ system. When excessive B₂O₃ was added, the inclusions were composite inclusions with SiO₂-MnO-Al₂O₃ as the core and BN wrapped around. The boron element could not be incorporated into the original inclusion of SiO₂-MnO-Al₂O₃ when adding a small amount or an excessive amount B₂O₃, so adding B₂O₃ could not reduce the melting point of the inclusion and could not effectively modify it.

3.2. The Change in Inclusion Composition and Factsage Thermodynamic Calculation and Analysis after Na₂CO₃ and K₂CO₃ Are Added to Cord Steel

The FactPS and FToxid databases in Factsage thermodynamic software were used to calculate the phase diagrams of the SiO₂-Al₂O₃-MnO-Na₂O and SiO_2-Al₂O_3-MnO-K₂O systems. The inclusion components in the test results were counted into the phase diagram, as shown in Figure 13 and Figure 14. Because the addition of B₂CO₃ cannot be incorporated into the inclusions in the steel, the effect of B₂CO₃ on the low melting point phase diagram of the SiO₂-Al₂O₃-MnO inclusions is not carried out here. When Na₂CO₃/K₂CO₃ was added to the steel, the inclusion composition also tended to the low melting point area in the phase diagram, and the experimental results were in good agreement with the calculation of the Factsage phase diagram. In addition, due to the low content of sodium and potassium in the steel and the limitations of the detection methods, it was difficult to accurately detect the content of sodium and potassium in the steel. In the experiment of adding 1.0% Na₂CO₃ and K₂CO₃, the mass fractions of sodium and potassium in the steel were 0.0012% and 0.0014%, respectively.

(1)

Influence of Na₂CO₃ addition on inclusion composition of cord steel.

As can be seen from Figure 13, when Na₂CO₃ was added to steel, the mass fraction of Na₂O in the inclusions was about 6%, with little change. The differences were as follows: (1) When the mass fraction of Na₂CO₃ was 0%, the inclusions in the steel were brittle with a high melting point near SiO₂ and SiO₂-MnO-Al₂O₃ inclusions with high melting points of about 1500 °C. (2) With the increase in Na₂CO₃ content, the aluminum content in the inclusions decreased significantly. (3) When the mass fraction of Na₂CO₃ was 0.3%, the inclusions were completely modified, and there were still some high-alumina, high-melting-point, SiO₂-Al₂O₃-MnO inclusions. (4) When the mass fraction of Na₂CO₃ was 0.5%, the inclusions were completely modified, and almost all the inclusions fell in the low melting point region (less than 1300 °C). (5) When excessive Na₂CO₃ (the mass fraction was 1%) was added, the aluminum content in the inclusions further decreased, and the inclusions began to move away from the low melting point region.

To sum up, the aluminum content in the inclusions gradually decreased with the increase in Na₂CO₃. When the mass fraction of Na₂CO₃ was 0.5%, most inclusions in the steel fell in the low melting point region, which was beneficial to the deformation of inclusions during the hot rolling process.

(2)

Influence of K₂CO₃ addition on inclusion composition of cord steel.

As can be seen from Figure 14, when K₂CO₃ was added to steel, the mass percentage of K₂O in the inclusions was about 3%. With the increase in K₂CO₃ content, the content of potassium in the inclusions did not increase significantly. The differences in the influence of K₂CO₃ addition on the inclusions were as follows: (1) When K₂CO₃ addition was 0%, the inclusions in the steel were divided into brittle inclusions with a high melting point near SiO₂ and SiO₂-MnO-Al₂O₃ inclusions with a high melting point of about 1500 °C. (2) With the increase in K₂CO₃ content, the silicon content in the inclusions decreased significantly. (3) When the mass fraction of K₂CO₃ was 0.3%, the inclusion modification was incomplete, and some high-silicon SiO₂-Al₂O₃-MnO inclusions still existed. (4) When the mass fraction of K₂CO₃ was 0.5%, the inclusions were completely modified, and almost all the inclusions fell in the low melting point region. (5) When excessive (1% by mass) K₂CO₃ was added, the silicon content in the inclusions decreased slightly, but the aluminum content in some inclusions increased, and some inclusions were far away from the low melting point area.

To sum up, with the increase in K₂CO₃, the silicon content in the inclusions gradually decreased. When the mass fraction of K₂CO₃ was 0.5%, most of the inclusions in the steel fell in the low melting point region, which was beneficial to the deformation of inclusions during the hot rolling process.

3.3. Thermodynamic Calculation and Reaction Mechanism of Inclusion Transformation in Molten Steel

Based on the steel composition in Table 1 and the activity interaction coefficients [30] $e_{\mathrm{C}}^j$ of various elements in molten steel at 1600 °C in

Table 3. Activity interaction coefficient of each element for carbon in molten steel.

Element	C	Si	Mn	V	Nb	N	P	S
$e_{\mathrm{C}}^j$	−0.45	−0.131	−0.021	−0.3	−0.14	0.057	0.07	−0.133

, the activity coefficient of C in steel was calculated to be 0.445 according to Equation (1):

$$\lg f_i={\displaystyle \sum e_i^{j}w[\%j]}.f_{\left[\mathrm{C}\right]}=0.445$$

(1)

In order to reveal the modification mechanism of inclusions in steel by adding alkali metal carbonate into molten steel, thermodynamic analysis was carried out [27].

Na₂CO₃ is added to the molten steel and decomposed into Na₂O at a high temperature, which reacts with C in the molten steel to generate Na in the molten steel, as shown in Equations (2)–(5). The thermodynamic calculation for the generation of gaseous Na(g) is shown in Equations (6)–(9).

$${\mathrm{Na}}_2{\mathrm{O}}_{\left(\mathrm{L}\right)}+\left[\mathrm{C}\right]=2{\mathrm{Na}}_{\left(\mathrm{L}\right)}+\mathrm{CO}{\Delta }_{\mathrm{r}}{G}_m^{\theta }=-26186.2\mathsf{J}/\mathrm{mol}$$

(2)

$$\ln K=\lg \frac{a_{\mathrm{Na}}^2\cdot a_{\mathrm{CO}}^{}}{a_{\left[\mathrm{C}\right]}^{}\cdot a_{{\mathrm{Na}}_2\mathrm{O}}^{}}=\ln \frac{w_{\left[\mathrm{Na}\right]}^2\cdot f_{\mathrm{Na}}^2\cdot a_{\mathrm{CO}}^{}}{w_{\left[\mathrm{C}\right]}^{}\cdot f_{\mathrm{C}}^{}}=\frac{-\Delta G^{\theta }}{RT}=\frac{26186.2}{8.314\times 1873}=1.68161$$

(3)

$$\frac{w_{\left[\mathrm{Na}\right]}^2\cdot f_{\mathrm{Na}}^2\cdot P_{\mathrm{CO}}^{}/P_{}^{\theta }}{w_{\left[\mathrm{C}\right]}^{}\cdot f_{\mathrm{C}}^{}}=5.3742$$

(4)

$$w_{\left[\mathrm{Na}\right]}^2\cdot f_{\mathrm{Na}}^2\cdot P_{\mathrm{CO}}^{}/P_{}^{\theta }=5.3742\times 0.3=1.61$$

(5)

$${\mathrm{Na}}_2{\mathrm{O}}_{\left(\mathrm{L}\right)}+\left[\mathrm{C}\right]=2{\mathrm{Na}}_{\left(\mathrm{g}\right)}+\mathrm{CO}{\Delta }_{\mathrm{r}}{G}_m^{\theta }=-139154.9\mathsf{J}/\mathrm{mol}$$

(6)

$$\ln K=\lg \frac{a_{\mathrm{Na}}^2\cdot a_{\mathrm{CO}}^{}}{a_{\left[\mathrm{C}\right]}^{}\cdot a_{{\mathrm{Na}}_2\mathrm{O}}^{}}=\ln \frac{a_{\mathrm{Na}}^2\cdot a_{\mathrm{CO}}^{}}{w_{\left[\mathrm{C}\right]}^{}\cdot f_{\mathrm{C}}^{}}=\frac{-\Delta G^{\theta }}{RT}=\frac{139155}{8.314\times 1873}=8.94$$

(7)

$$\frac{{\left(P_{\mathrm{Na}}^{}/P_{}^{\theta }\right)}^2\cdot P_{\mathrm{CO}}^{}/P_{}^{\theta }}{w_{\left[\mathrm{C}\right]}^{}\cdot f_{\mathrm{C}}^{}}=7601$$

(8)

$${\left(P_{\mathrm{Na}}^{}/P_{}^{\theta }\right)}^2\cdot P_{\mathrm{CO}}^{}/P_{}^{\theta }=2280$$

(9)

K₂CO₃ is added to the molten steel and decomposed into K₂O at a high temperature, which reacts with C in the molten steel to generate K in the molten steel, as shown in Equations (10)–(13). The thermodynamic calculation for the generation of gaseous K(g) is shown in Equations (14)–(17).

$${\mathrm{K}}_2{\mathrm{O}}_{\left(\mathrm{L}\right)}+\left[\mathrm{C}\right]=2{\mathrm{K}}_{\left(\mathrm{L}\right)}+\mathrm{CO}{\Delta }_{\mathrm{r}}{G}_m^{\theta }=-81707.6\mathsf{J}/\mathrm{mol}$$

(10)

$$\ln K=\lg \frac{a_{\mathrm{K}}^2\cdot a_{\mathrm{CO}}^{}}{a_{\left[\mathrm{C}\right]}^{}\cdot a_{{\mathrm{K}}_2\mathrm{O}}^{}}=\ln \frac{w_{\left[\mathrm{K}\right]}^2\cdot f_{\mathrm{K}}^2\cdot a_{\mathrm{CO}}^{}}{w_{\left[\mathrm{C}\right]}^{}\cdot f_{\mathrm{C}}^{}}=\frac{-\Delta G^{\theta }}{RT}=\frac{81707.6}{8.314\times 1873}=5.25$$

(11)

$$\frac{w_{\left[\mathrm{K}\right]}^2\cdot f_{\mathrm{K}}^2\cdot P_{\mathrm{CO}}^{}/P_{}^{\theta }}{w_{\left[\mathrm{C}\right]}^{}\cdot f_{\mathrm{C}}^{}}=190$$

(12)

$$w_{\left[\mathrm{K}\right]}^2\cdot f_{\mathrm{K}}^2\cdot P_{\mathrm{CO}}^{}/P_{}^{\theta }=57$$

(13)

$${\mathrm{K}}_2{\mathrm{O}}_{\left(\mathrm{L}\right)}+\left[\mathrm{C}\right]=2{\mathrm{K}}_{\left(\mathrm{g}\right)}+\mathrm{CO}{\Delta }_{\mathrm{r}}{G}_m^{\theta }=-203500.5\mathsf{J}/\mathrm{mol}$$

(14)

$$\ln K=\lg \frac{a_{\mathrm{K}}^2\cdot a_{\mathrm{CO}}^{}}{a_{\left[\mathrm{C}\right]}^{}\cdot a_{{\mathrm{K}}_2\mathrm{O}}^{}}=\ln \frac{a_{\mathrm{K}}^2\cdot a_{\mathrm{CO}}^{}}{w_{\left[\mathrm{C}\right]}^{}\cdot f_{\mathrm{C}}^{}}=\frac{-\Delta G^{\theta }}{RT}=\frac{203501}{8.314\times 1873}=13.06$$

(15)

$$\frac{{\left(P_{\mathrm{K}}^{}/P_{}^{\theta }\right)}^2\cdot P_{\mathrm{CO}}^{}/P_{}^{\theta }}{w_{\left[\mathrm{C}\right]}^{}\cdot f_{\mathrm{C}}^{}}=473665$$

(16)

$${\left(P_{\mathrm{K}}^{}/P_{}^{\theta }\right)}^2\cdot P_{\mathrm{CO}}^{}/P_{}^{\theta }=142100$$

(17)

In Equations (1)–(17), $a$: activity; $P$: partial pressure; $P_{}^{\theta }$: standard atmosphere; $w$: mass fraction; $f_{}^{}$: activity coefficient.

The above calculation results show that Na₂CO₃ and K₂CO₃ added to the molten steel decomposed into Na₂O and K₂O, and may theoretically be reduced by C in the steel and enter the molten steel in the form of Na and K. Since Na and K elements are very reactive, they will react quickly with a small amount of free oxygen in the surrounding molten steel or react with oxide inclusion: 2[M] + [O] = M₂O or y[M] + NxOz = MyOz + xN, M: Na, K; N: Mn, Si, Al.

High-carbon steel wire such as cord steel and spring steel and low-titanium and low-aluminum ferrosilicon and manganese are used for deoxidizing and alloying, and the aluminum content in the alloy is very low so the deoxidized inclusions produced in the molten steel are mainly SiO₂-MnO-Al₂O₃ clips. When Na and K elements first react with free oxygen, part of the oxide is formed to float into the slag, and the other part may collide and fuse with the SiO₂-MnO-Al₂O₃ inclusion to form a SiO₂-MnO-Al₂O₃-M₂O complex inclusion. When Na and K elements react directly with oxide inclusion to form a SiO₂-MnO-Al₂O₃-M₂O complex inclusion, the contents of Na₂O and K₂O in the complex inclusion increase, and the contents of one or more components of SiO₂, MnO and Al₂O₃ decrease. The modification and deformation mechanisms of Na₂CO₃ and K₂CO₃ on SiO₂-MnO-Al₂O₃ inclusions are shown in Figure 15.

3.4. Effect of Na₂CO₃/K₂CO₃ Addition on Low Melting Point Region of Inclusions in Cord Steel

The influences of different K₂O and Na₂O contents on the low melting point region of SiO₂-MnO-Al₂O₃ inclusions were calculated by Factsage 8.1, as shown in Figure 16. It can be seen that the low melting point region of SiO₂-MnO-Al₂O₃ inclusions expands in varying degrees with the increase in alkali metal content. Among the inclusions with the same alkali metal content, Na₂O had a better expansion effect on the low melting point region of SiO₂-MnO-Al₂O₃ inclusions than K₂O. At the same time, with the increase in Na₂O, the melting point of SiO₂-MnO-Al₂O₃ inclusions was lower. When the mass fraction of K₂O in inclusions increased from 6% to 9%, the melting point region of inclusions below 1200 °C was divided into two parts, and the composition control region was narrowed, which was not conducive to the control of the low melting point region of the inclusions.

3.5. Effect of 0.5%Na₂CO₃/K₂CO₃ Addition on Inclusion Deformability of Cord Steel after Forging

(1)

Analysis of inclusion deformation test results of cord steel samples with 0.5% Na₂CO₃.

Inclusions were observed in the longitudinal section of a 600 g forged bar in Figure 2. The deformation of the inclusions after forging is shown in Figure 17, and the distribution of each element in the inclusions is shown in Figure 18, which shows obvious deformation behavior. It can be seen from Figure 17 that when Na₂CO₃ was added to the cord steel, after forging the inclusion changed from spherical to a long strip as cast, and has good deformability. Adding sodium carbonate to cord steel could significantly reduce the melting point of the inclusions, and almost all inclusions were deformed during hot forging. Among the thirty inclusions observed, twenty-seven inclusions had obvious deformation, and three inclusions had no obvious deformation. It could be seen from the distribution of inclusions in Figure 13c and Figure 17 that almost all inclusions in the phase diagram of MnO-Al₂O₃-SiO₂-Na₂O were located in the low melting point region. Adding sodium carbonate could significantly improve the deformation behavior of inclusions during hot forging.

(2)

Analysis of inclusion deformation test results of cord steel samples with 0.5% K₂CO₃.

Inclusions were observed in the longitudinal section of a 600 g forged bar in Figure 2, and the deformation of the inclusions after forging is shown in Figure 19 and Figure 20, which shows obvious deformation behavior. It can be seen from Figure 19 that after the addition of K₂CO₃ to the cord steel, the inclusion changed from spherical to a strip after forging. However, compared with the addition of Na₂CO₃, the deformation ability of the inclusion with K₂CO₃ was lower, and the deformation was not obvious. Adding potassium carbonate to cord steel could reduce the melting point of inclusions, and most inclusions were deformed during hot forging, but there were still a few inclusions that were not easy to deform. Among the 30 inclusions observed, 20 inclusions had obvious deformation and 10 inclusions had no obvious deformation. It could be seen from the distribution of inclusions in Figure 14c and Figure 19 that most of the inclusions in the phase diagram of MnO-Al₂O₃-SiO₂-K₂O were located in the low melting point region, but there were still a few inclusions in the high melting point region. Adding potassium carbonate could improve the deformation behavior of inclusions during hot forging to some extent; most inclusions were easy to deform and a few were not easy to deform.

Therefore, after adding Na₂CO₃ and K₂CO₃ to the cord steel, the samples were forged. After SEM analysis, it was found that most of the inclusions had good deformability.

4. Conclusions

In summary, the effects of different contents of three kinds of alkali metal oxides on the inclusions of cord steel were studied in this paper. The following conclusions were drawn.

After the experiment of alkali metal carbonate treatment of inclusions in cord steel, it was found that the size of inclusions did not change significantly after Na and K treatment. However, the composition of the inclusion changed, such as the decrease in Al content. After forging, the inclusions in the longitudinal section of steel were long, and the deformability of inclusions had been greatly improved. Moreover, B had no effect on the melting point because it could not be integrated into the inclusions.

What is more, the phase diagram of FactSage (FactPS and FToxid databases) showed that Na₂O had a better expansion effect on the low melting point region of SiO₂-MnO-Al₂O₃ inclusions than K₂O-treated samples. After adding K₂CO₃ and Na₂CO₃, most of the inclusions observed fell in the low melting point region (less than 1300 °C). The decrease in the melting point of the inclusions made them deform more easily. The inclusion analysis of the forged samples showed that the inclusion deformation ability of cord steel after adding Na₂CO₃ was better than that of K₂CO₃ treatment.