A Thermodynamic and Experimental Analysis of Inclusions Modification in AH36 Liquid Steel by Calcium and Magnesium Treatment

Abstract

The influence of calcium and magnesium treatment under different molten steel conditions, as well as that of the alloy proportion and addition sequence of calcium and magnesium in composite treatment, on the evolution of inclusions in AH36 liquid steel was analyzed systematically based on thermodynamic calculations. The results show that the inclusions in molten steel are mainly Al₂O₃, which gradually transform into a liquid phase after calcium treatment with a wide range of calcium contents, indicating that calcium treatment has a significant effect on inclusion modification. Magnesium treatment mainly converts Al₂O₃ into MgO·Al₂O₃ inclusions in molten steel; however, it is not suitable to modify inclusions with magnesium treatment alone since it does not produce a significant liquid phase. The effect of calcium and magnesium composite treatment varies with the alloy content composition and the order of alloy addition. The liquid phase range of inclusions follows the order of 80%Ca + 20%Mg composite treatment > calcium treatment > 50%Ca + 50%Mg composite treatment > 20%Ca + 80%Mg composite treatment. Combining the thermodynamic and experimental analysis results, it can be concluded that the composite treatment of magnesium followed by calcium is the best. Specifically, a small amount of magnesium should be added first as the nucleating particle to promote the fine dispersion of the inclusions, thus reducing their impact on steel performance. Then, calcium should be added to modify the surface of the inclusions into a liquid phase, which can effectively reduce nozzle clogging.

1. Introduction

With the continuous development of the steel industry, the influence of inclusions on steel properties has become increasingly prominent, and the control of inclusions has become an important research direction. The impact of non-metallic inclusions on reducing the fatigue resistance of steel can be ranked from strongest to weakest as follows: Al₂O₃ inclusions, spinel inclusions, CaO-Al₂O₃ series or MgO-Al₂O₃ series spherical non-deformable inclusions, large TiN inclusions, silicate inclusions, and manganese sulfide inclusions [1]. In addition to their impact on steel properties, the composition and form of non-metallic inclusions can also have adverse effects on continuous casting production; for example, Al₂O₃ and Al-Ti composite inclusions easily aggregate into clusters, aggravating the nodulation trend of the submerged nozzle and ultimately affecting the smooth running of continuous casting. In shipbuilding and marine engineering, steel is the most important material, accounting for 20% to 30% of the total manufacturing cost [2,3,4]. Among the different types, AH36 ship plate steel has excellent mechanical properties, high strength, good weldability, and good corrosion resistance and is widely used to manufacture various types of ships and marine engineering structures. In order to reduce or even avoid the impact of non-metallic inclusions on steel properties and continuous casting production, it is generally necessary to carry out calcium treatment, magnesium treatment, or calcium–magnesium composite treatment of liquid steel. However, these treatment methods have limitations, and better inclusion modification effects are expected.

Calcium treatment is a widely used technology for clean steel production. The process involves feeding calcium wire into molten steel to transform Al₂O₃ inclusions with a high melting point into calcium aluminate composite inclusions with a low melting point and low density, which promotes the flotation of inclusions and thereby purifies the molten steel. Thus, it can prevent nozzle blockage and improve the steel quality. However, the improvement effect of calcium treatment in actual production is uneven, and it is necessary to pay attention to details such as the calcium addition amount and smelting process parameters. When the amount of calcium is insufficient, the treatment will be ineffective, whereas, when the amount of calcium is too high, calcium sulfide and calcium oxide with a high melting point are easily generated, which can lead to nozzle blockage during continuous casting [5,6,7].

In recent years, a large number of magnesium treatment studies have been carried out in ship plate steel, pipeline steel, and other fields [8,9,10]. By adding magnesium, Al₂O₃ in steel can be transformed from clusters or bands into smaller, dispersed MgO·Al₂O₃ spinels, thereby improving the fatigue life of steel and presenting a new way to prepare ultra-clean steel. However, with the continuous progress of magnesium treatment, sulfides will gradually precipitate on the surface of the MgO·Al₂O₃ spinels, causing the overall particle size to increase. The resulting MgO·Al₂O₃ spinels with a high melting point could then block the submerged nozzle [11,12].

In actual production, there may be different inclusions in the liquid steel, such as calcium, magnesium, aluminum, and oxygen complex inclusions. The chemical composition and size of these inclusions will have different degrees of influence on the performance of the steel. The introduction of calcium and magnesium composite treatment to modify inclusions by combining their respective advantages has become a research hotspot in recent years [13,14,15]. However, the mechanisms and effects of calcium and magnesium composite treatment on the inclusions of different types of steel are not the same. Only the correct combination and addition sequence of calcium and magnesium can reduce the number of inclusions and promote their transformation into a liquid phase to optimize the appearance of inclusions, thereby improving the quality and performance of steel [16]. However, the optimal proportion and order of addition vary for different steel grades.

In order to improve the properties of AH36 steel, this work systematically evaluates the effects of calcium and magnesium treatments. The effects of calcium and magnesium treatments under different conditions of molten steel, as well as the alloy proportion and alloy addition sequence, on the evolution of inclusions are analyzed. The results have important guiding significance in optimizing the content and form of inclusions of AH36 steel.

2. Methods and Procedure

2.1. Thermodynamic Calculation

The composition of AH36 ship plate steel is shown in

Table 1. Main composition content of AH36 ship plate steel (wt%).

Composition	C	Si	Mn	P	S	Ti	Als
Content	0.155	0.17	1.28	0.021	0.003	0.014	0.020

. Thereinto, Als represents the aluminum content that is dissolved into steel, called acid-soluble aluminum. During the thermodynamic calculation, in order to investigate the influence of the molten steel’s composition on the effects of calcium treatment, magnesium treatment, and calcium and magnesium composite treatment, the content of Al and O elements in molten steel, and the range of temperature change are determined according to the composition of AH36 ship plate steel, as shown in

Table 2. Composition and temperature ranges of AH36 ship plate steel for thermodynamic calculation.

Factor	Als (ppm)	O (ppm)	T (°C)
Range	100~400	5~25	1550~1650
Constant value	300	15	1600

.

In the calculation process, for example, when the influence of Als in molten steel is studied, the composition range of Als is set to 100~400 ppm, while the content of other components is set to O = 15 ppm and S = 30 ppm, and the temperature is 1600 °C. According to the calculation requirements of FactSage7.2 thermodynamic software, the total amount of calcium–magnesium alloy added is set as <A>, with a value range of 0~0.005, and the content of influencing factors is set as <B>, with 0.0001 as the step, and then the Fe content in the molten steel is obtained according to the composition of the molten steel under different influencing factors. When calcium and magnesium composite treatment is carried out, if 80%Ca + 20%Mg is added, the amount of Ca can be set to <0.8 A>, and the amount of Mg can be set to <0.2 A>. The FactSgae 7.2 databases of FToxide and FTmisc are chosen during calculation, and all inclusions that contain solid and solution phases are exported for analysis.

2.2. High-Temperature Experiment

The AH36 ship plate steel shown in Table 1 was used as the experimental raw material, and the alloy composition of calcium and magnesium treatment is shown in

Table 3. Chemical composition of calcium–magnesium-treated alloys (wt%).

Composition	Fe	Ni	Mg	C	Si	P	S	Al	Ca
Si-Ca alloy					61.03	0.022	0.041	1.27	30.45
Ni-Mg alloy	0.03	Trace	20.17	0.021	0.01	0.002	0.001

. A vacuum induction furnace (WZG-3 kg, Qingdao, China) and quartz tube (inner diameter 8 mm) were used to achieve charging and sampling in an inert atmosphere. AH36 ship plate steel with a polished surface was placed into a MgO crucible (inner diameter: 60 mm; height: 145 mm) using about 1000 g portions each time, and the total amount of alloys added in the experiments was 2 g.

Prior to the experiment, the pressure in the induction furnace was reduced to below 15 Pa using a mechanical pump, and then high-purity argon gas (>99.999%) was charged to near atmospheric pressure and the process was repeated three times to reduce the oxygen content in the furnace. The power of the induction furnace was gradually increased to heat the steel samples, and the temperature was measured using a dual-wavelength pyrometer (CIT-1MD1, Jing Yi (Beijing) Instrument Equipment Co., Ltd., Beijing, China) through the observation window. The 1600 °C steel sample was obtained after 5 min of alloying. The samples were cooled, wire-cut, ground, and polished to produce metallographic samples. The morphology and composition of inclusions were analyzed using a scanning electron microscope (SEM: JSM-6510LV, Japan Electronics Co., Ltd., Tokyo, Japan) equipped with an Oxford Electrochromic Spectrometer (EDS: INCA Feature X-Max20, INCA Energy, Oxford CO, Tokyo, Japan), and inclusions were statistically analyzed using its Feature function. The number of inclusions counted in each process sample was no less than 500.

3. Results and Discussion

3.1. Effect of Calcium Treatment on the Evolution of Inclusions with Various Molten Steel Conditions

Figure 1 shows the effect of the calcium treatment on the evolution of inclusions in molten steel with different aluminum content. As can be seen from the figure, when Al = 100 ppm, with the increase in Ca content, the transition trend of inclusions in molten steel is Al₂O₃ → CaO·6Al₂O₃ → CaO·2Al₂O₃ → Liquid slag → CaO. When the amount of Ca added is greater than 6 ppm, CaO·2Al₂O₃ disappears and all inclusions begin to be liquid; when the added Ca is greater than 10 ppm, the liquid phase disappears; and when the added Ca is greater than 45 ppm, CaO begins to be produced. At this time, the Ca content corresponding to the liquid phase of inclusions is 6–10 ppm. When Al = 200 ppm, the liquid phase zone begins to appear when the amount of Ca added is greater than 7 ppm; when the added Ca is greater than 15 ppm, 3CaO·Ti₂O₃ and 3CaO·2TiO₂ are produced, and the liquid phase zone disappears. At this time, the corresponding Ca content in the liquid phase of the inclusion is 7–15 ppm, and CaO is produced when the Ca content is greater than 44 ppm. When Al = 300 ppm, as the amount of Ca added is greater than 8 ppm, CaO·2Al₂O₃ disappears, and all inclusions begin to turn into liquid, whereas CaO is generated with Ca content greater than 46 ppm, as the liquid phase zone disappears, and the liquid inclusion content begins to decrease. At this time, the corresponding Ca content in the liquid phase zone of the inclusions is 8~46 ppm. When Al = 400 ppm, and when the amount of Ca added is greater than 5 ppm, liquid inclusions begin to appear; when the added Ca is greater than 8 ppm, CaO·2Al₂O₃ disappears, and all inclusions begin to be liquid; when the Ca amount is greater than 48 ppm, CaO occurs, and the liquid phase zone disappears. At this time, the corresponding Ca content in the liquid phase zone of inclusions is in the range of 8~48 ppm.

Similarly, the influence of calcium treatment on the evolution of inclusions under different conditions of molten steel is summarized, and the corresponding liquid phase region of inclusions is shown in Figure 2. It is revealed that with the increase in Al content in molten steel, the range of corresponding Ca content in the liquid phase of inclusion gradually increases. With the increase in O content in molten steel, the range of Ca content corresponding to the liquid phase of inclusions first increases and then becomes stable. With the increase in molten steel temperature, the corresponding range of Ca content in the liquid phase of inclusions fluctuates slightly. On the whole, with calcium treatment alone, inclusions easily fall into the liquid phase under different conditions of molten steel, which is conducive to the accumulation and floating of inclusions, thus improving the cleanliness of molten steel. However, it is necessary to pay attention to the increase in inclusion size, and if the upward floating is not timely, the steel performance will seriously deteriorate.

3.2. Effect of Magnesium Treatment on the Evolution of Inclusions with Various Molten Steel Conditions

Figure 3 shows the influence of magnesium treatment on the evolution of inclusions in molten steel with different aluminum content. It can be seen that when Al = 100 ppm, there are Al₂O₃ inclusions in molten steel without magnesium treatment. With the addition of Mg, the content of Al₂O₃ inclusions decreases continuously until it disappears when Mg content is greater than 20 ppm; when Mg content is greater than 10 ppm, Titania spinel (MgO Al₂O₃-MgO·Ti₂O₃) is produced and the content of MgO·Al₂O₃ begins to decrease. When Al = 200 ppm, no liquid inclusions appear in the process and Al₂O₃ disappears when Mg content is greater than 31 ppm. MgO·Al₂O₃ content begins to decline when Mg content is greater than 41 ppm and MgO·Al₂O₃-MgO·Ti₂O₃ begins to be produced. When the content of Al is 300 ppm and 400 ppm, the composition of inclusions changes little with the increase in Mg content. In summary, under the condition of different Al content in molten steel, the liquid phase of inclusions does not appear in the magnesium treatment process, and the Al₂O₃ inclusion is mainly transformed into a MgO·Al₂O₃ inclusion.

By analyzing the influence of magnesium treatment on the evolution of inclusions under different conditions of molten steel and temperature conditions, it is found that magnesium treatment mainly converts Al₂O₃ into MgO·Al₂O₃ inclusions in the steel. Overall, it is not suitable to modify inclusions with magnesium treatment alone since a liquid inclusion phase is not generated, and the formation of MgO·Al₂O₃ spinel inclusions is prone to causing nozzle blockage.

3.3. Effect of Alloy Ratio on Evolution Pattern of Inclusions During Calcium–Magnesium Composite Treatment

(1) 80%Ca + 20%Mg composite treatment

Figure 4 shows the effect of 80%Ca + 20%Mg composite treatment on the evolution of inclusions in molten steel with various aluminum contents. As can be seen from the figure, when Al = 100 ppm and the amount of Ca and Mg added is greater than 3 ppm, inclusions begin to be liquid, whereas 3CaO·Ti₂O₃ and 3CaO·2TiO₂ begin to be produced as the amount of Ca and Mg added is greater than 15 ppm. At this time, the liquid phase area of inclusions corresponds to a Ca and Mg content range of 5~13 ppm. When Al = 200 ppm and the amount of Ca and Mg added is greater than 8 ppm, all inclusions begin to be liquid. But it should be noted that the Ca and Mg content of 20~26 ppm is within the interval of the production of 3CaO·Ti₂O₃ and 3CaO·2TiO₂. At this time, the liquid phase area corresponding to Ca and Mg content can be approximated as 8~50 ppm. When the Al content is 300 ppm, 400 ppm, and 500 ppm, the inclusions are of the same type and composition. The inclusions subsequently stabilize to the liquid state when the amount of Ca and Mg added is greater than 8 ppm.

The effect of 80%Ca + 20%Mg composite treatment on the evolution of inclusions under different molten steel conditions is summarized, and the corresponding liquid phase region of inclusions is obtained, as shown in Figure 5. It is revealed that with the increase in Al and O content in molten steel, the corresponding Ca and Mg content range of the inclusions liquid phase area first increases rapidly and then tends to stabilize; with the increase in molten steel temperature, the corresponding range of Ca and Mg content in the liquid phase of inclusions slightly fluctuates. Overall, similar to calcium treatment, with 80%Ca + 20%Mg composite treatment, it is easy for inclusions to fall into the liquid phase under different molten steel conditions, which is conducive to the accumulation and floating of inclusions and thus improves the cleanliness of molten steel.

(2) 50%Ca + 50%Mg composite treatment

The effect of 50%Ca + 50%Mg composite treatment on the evolution of inclusions at different aluminum contents in molten steel is shown in Figure 6. It shows that when Al = 100 ppm, with the addition of Ca and Mg, the type and composition of inclusions generated at the beginning are very complex, while all inclusions began to turn into liquid with the disappearance of 3CaO·Ti₂O₃ and 3CaO·2TiO₂, as the amount of Ca and Mg added is more than 12 ppm. Then, MgO begins to appear, and the liquid phase region disappears when the amount of added Ca and Mg is greater than 22 ppm. At this time, the Ca and Mg content corresponding to the liquid phase of inclusions is 12~22 ppm. When Al = 200 ppm and the amount of Ca and Mg added is more than 14 ppm, the liquid phase begins to appear, whereas the liquid phase area disappears when the amount of added Ca and Mg is more than 22 ppm. When Al = 300 ppm, 3CaO·Ti₂O₃ and 3CaO·2TiO₂ disappear when the amount of Ca and Mg is greater than 14 ppm, and the liquid phase zone begins to appear, until MgO is produced when the amount is greater than 23 ppm, and the liquid phase zone disappears. When Al is 400 ppm and 500 ppm, all inclusions begin to turn into liquid as the amount of Ca and Mg added is greater than 15 ppm, whereas MgO begins to be generated and the liquid phase disappears as the amount of added Ca and Mg is greater than 23 ppm. When the amount of Ca and Mg added is greater than 41 ppm, there are no liquid inclusions. At this time, the Ca and Mg content corresponding to the liquid phase of inclusions ranges from 15 to 23 ppm.

The effect of 50%Ca + 50%Mg composite treatment on the evolution of inclusions under different conditions in molten steel is summarized, and the corresponding liquid phase area of inclusions is obtained, as shown in Figure 7. As can be seen from the figure, with the increase in aluminum content in molten steel, the corresponding range of Ca and Mg content in the liquid phase of inclusions is relatively stable. When the oxygen content of molten steel reaches 5~10 ppm, the corresponding range of Ca and Mg content in the liquid phase of inclusions gradually increases, whereas when the oxygen content is greater than 10 ppm, the liquid phase of inclusions is gradually reduced. When the temperature of molten steel is greater than 1600 °C, the liquid phase of inclusions will appear and increase with the increase in temperature. Overall, under different conditions in molten steel, the liquid phase of inclusions is narrower and attention needs to be paid to the amount of alloy added during 50%Ca + 50%Mg composite treatment.

(3) 20%Ca + 80%Mg composite treatment

The effect of 20%Ca + 80%Mg composite treatment on the evolution pattern of steel inclusions at different aluminum contents in molten steel is shown in Figure 8. The figure shows that when Al = 100 ppm, the transformation trend of inclusions is Al₂O₃ → CaO·2MgO-8Al₂O₃ → MgO·Al₂O₃ → MgO·Al₂O₃-TiO₂ → Liquid slag → MgO. With Ca and Mg additions greater than 3 ppm, the liquid inclusions begin to appear until the additions are greater than 24 ppm, at which point the liquid inclusions disappear. After that, the inclusions are MgO, and there is no liquid phase area during this time. When the Al content is 200 ppm, 300 ppm, and 400 ppm, the situation is essentially the same as that of Al = 100 ppm, and only the content of inclusions increases. In sum, under different aluminum contents in molten steel, there is no liquid phase of inclusions, and the composition and content of inclusions change during 20%Ca + 80%Mg composite treatment.

To sum up, it was found that there is no liquid phase of inclusions in the process of 20%Ca + 80%Mg composite treatment under different compositions and temperature conditions of molten steel. The type and content of inclusions increased with the increase in aluminum and oxygen content in molten steel, whereas with the increase in temperature, the content of inclusions gradually decreased. On the whole, in the composite treatment process of 20%Ca + 80%Mg, when the amount of its addition is small, a variety of inclusions will be generated, and when the amount of its addition is too great, MgO inclusions will be generated, which is unfavorable to the cleanliness of molten steel and continuous casting production.

3.4. Effect of Alloy Addition Sequence on Evolution Pattern of Inclusions During Calcium–Magnesium Composite Treatment

Figure 9 shows the effect of the composite treatment of calcium followed by magnesium on the evolution of inclusions. As shown in Figure 9a, when 0~10 ppm Ca is added first, the inclusion transformation trend is Al₂O₃ → CaO-6Al₂O₃ → CaO-2Al₂O₃ → Liquid slag, and when 0~20 ppm Mg is added later to interact with liquid-phase aluminate, the inclusions still remain in the liquid phase. As shown in Figure 9b, when Ca content over 15 ppm is added first, the liquid phase of inclusions gradually changes to 3CaO-2TiO₂ until the liquid-phase inclusions disappear when Ca is 19 ppm. After adding Mg, 3CaO-2TiO₂ is reduced and the liquid-phase inclusions increase and then fall into the all-liquid-phase region when Mg increases to 5 ppm.

The effect of composite treatment of magnesium followed by calcium on the evolution of inclusions is shown in Figure 10. As can be seen from Figure 10a, when 0~10 ppm of Mg is added first, the evolution trend of inclusions is Al₂O₃ → MgO-Al₂O₃ → MgO-Al₂O₃-TiO₂, and when Ca is added at a later stage, the content of MgO-Al₂O₃-TiO₂ inclusions is gradually reduced. When the amount of Ca is 2 ppm, a liquid phase of inclusions appears; when Ca is increased to 6 ppm, the inclusion components enter into the full-liquid-phase zone. As shown in Figure 10b, when the content of Mg added first exceeds 12 ppm, MgO-Al₂O₃-TiO₂ is gradually transformed into MgO inclusions, and with the subsequent addition of Ca, it remains a solid-phase MgO or CaO inclusion.

The effect of simultaneous treatment of calcium and magnesium on the evolution of inclusions is shown in Figure 11. As shown in Figure 11a, when the mass ratio of simultaneously added Ca/Mg is 1/2, the inclusion evolution is more complex and there is no liquid phase. As shown in Figure 11b, when the mass ratio of simultaneously added Ca/Mg is 2/1, the inclusions are still more varied but a larger range of liquid phases appears. Inclusions of CaS appear when the amount of Ca and Mg content reaches 22 ppm. Overall, compared with composite treatments of calcium followed by magnesium or magnesium followed by calcium, when calcium and magnesium are added at the same time, the number of inclusions significantly increases in molten steel. In addition, with the increase in the proportion of Mg, the number of inclusions is more complex, and the all-liquid window becomes narrower until it disappears.

3.5. Experimental Validation of Inclusion Modification in AH36 Liquid Steel by Calcium and Magnesium Treatment

Comparing the inclusion distribution diagrams of different calcium–magnesium composite treatments shown in Figure 12, in the composite treatment of calcium followed by magnesium, the thermodynamic calculations show that the calcium added in the early stage modifies the Al₂O₃ inclusion into liquid aluminate, and the magnesium added later interacts with liquid aluminate and remains in the liquid phase. The experimental results show that the inclusions fall into the liquid phase when the Si-Ca alloy is added; then, Ni-Mg alloy is added and magnesium acts with the outer aluminate to cause the inclusions to accumulate and grow continuously. However, due to the increase in magnesium content, the composition of the inclusions deviates from the liquid phase. In general, the proportion of inclusions falling into the liquid phase is very low when treated with calcium followed by magnesium, and the size of inclusions is very large, which is not conducive to the modification of inclusions.

When the steel is treated with magnesium followed by calcium, the thermodynamic calculations show that the magnesium added in the early stage modifies the Al₂O₃ inclusion into MgO·Al₂O₃ spinel, and the calcium added later modifies the surface MgO·Al₂O₃ spinel into liquid aluminate. The experimental results show that, after the addition of Ni-Mg alloy, the inclusion becomes fine-dispersed MgO·Al₂O₃ spinel, and then, with the addition of Si-Ca alloy, the MgO·Al₂O₃ spinel on the surface of the inclusion is modified by calcium into liquid aluminate, the inclusion components gradually fall into the liquid phase, and the size of the inclusion slightly increases. In summary, in the treatment method of magnesium before calcium, magnesium in the early stage causes inclusions to finely disperse, and calcium added in the later stage can make the surface components of inclusions fall into the liquid phase. Therefore, the modification effect of inclusions is remarkable, and the treatment method with more calcium and less magnesium is better.

When calcium and magnesium are added at the same time, the thermodynamic calculations show that the inclusions in the liquid steel significantly increase, the inclusions become more complex, and the liquid window area narrows with the increase in magnesium alloy proportion. The experimental results show that some of the inclusions enter the liquid phase after the addition of the alloy, and the components of the inclusions are basically MgO·Al₂O₃ spinel and calcium aluminate. In addition, the proportion of inclusions falling into the liquid phase decreases with the increase in magnesium proportion. In summary, when calcium and magnesium are added at the same time, magnesium alloy helps to finely disperse the inclusions, and the addition of calcium and magnesium at the same time can cause the surface components of the inclusions to fall into the liquid phase. However, due to the simultaneous addition of calcium and magnesium, the reaction is violent and there are many inclusions, with uncontrollable factors increasing during the reaction process.

On the whole, the inclusion size follows the sequence of calcium treatment > composite treatment of calcium followed by magnesium > composite treatment of calcium and magnesium added simultaneously > composite treatment of magnesium followed by calcium > magnesium treatment. The proportion of inclusions falling into the liquid phase area follows the order of calcium treatment > composite treatment of magnesium followed by calcium > composite treatment of calcium and magnesium added simultaneously > composite treatment of calcium followed by magnesium > magnesium treatment. It can thus be concluded that the composite treatment of magnesium followed by calcium is the best; that is, a small amount of magnesium is added first as the nucleating particle, causing inclusions to finely disperse and thus reducing the impact on steel performance, and then calcium is added to transform the surface of inclusions into a liquid phase, which can effectively reduce nozzle clogging.

4. Conclusions

By thermodynamically calculating the effects of calcium treatment, magnesium treatment, and calcium and magnesium composite treatment on the evolution of inclusions in AH36 steel with varying composition and temperature, the following conclusions are drawn.

• The initial inclusion in AH36 molten steel is Al₂O₃, which is gradually modified into a liquid phase after calcium treatment. The calcium treatment has a significant effect on the inclusion modification to the liquid phase under different molten steel conditions; however, attention should be paid to avoid CaO inclusions when the calcium content is too high.
• Magnesium treatment mainly converts Al₂O₃ in molten steel into MgO·Al₂O₃ inclusions. When the O content is high, a high Mg content will promote the further denaturation of MgO·Al₂O₃ into MgO. Due to magnesium treatment, a liquid inclusion phase will not be generated; thus, magnesium treatment is not suitable for inclusion modification.
• The effect of the composite treatment of calcium and magnesium varies with the content of the alloy. The liquid phase range of inclusions follows the order of 80%Ca + 20%Mg composite treatment > calcium treatment > 50%Ca + 50%Mg composite treatment > 20%Ca + 80%Mg composite treatment, which means that the 80%Ca + 20%Mg composite treatment has a better effect on inclusion modification.
• The effect of the combined treatment of calcium and magnesium also varies according to the addition sequence of alloys. The liquid phase of the inclusions treated with calcium followed by magnesium is wider and widens with the decrease in calcium content. The inclusion can be modified into the liquid phase only when magnesium content is low and calcium is added later. The simultaneous addition of calcium and magnesium corresponds to a wide variety of inclusion species, and the liquid phase of the inclusion can be formed only when the content of calcium is high.
• Combining thermodynamic and experimental analysis, it can be concluded that the composite treatment of magnesium followed by calcium is the best; that is, a small amount of magnesium is added first as the nucleating particle, causing inclusions to finely disperse and thus reducing the impact on steel performance, and then calcium is added to transform the surface of inclusions into a liquid phase, which can effectively reduce nozzle clogging.