45 steel is a medium-carbon steel with a carbon content of 0.42%～0.50%, which combines high strength, good ductility and wear resistance. It is widely used in the manufacturing of axles, gears, drills and other mechanical parts. As a sulfur and aluminum containing steel, 45 steel adopts sulfur addition to improve strength, tensile and compression resistance, and machinability, while aluminum addition refines grains, reduces brittle transition temperature and enhances steel toughness.
However, abundant high-melting-point inclusions such as Al₂O₃ and CaS are easily formed during production. These inclusions reduce the purity of molten steel, accumulate on the inner wall of tundish submerged nozzles in continuous casting, and cause nozzle clogging and flow fluctuation. Severe clogging will lead to casting interruption and deteriorate steel quality.
Existing studies have analyzed nozzle clogging mechanisms and optimization measures for medium-carbon low-silicon aluminum-killed steel and E355S sulfur-containing steel. Relevant process improvements can modify rigid solid inclusions into low-melting calcium aluminate and improve castability. Nevertheless, targeted research on the clogging mechanism and control strategies for 45 steel submerged nozzles is still insufficient, and nozzle blockage frequently occurs in actual production.
Therefore, this paper analyzes the molten steel fluctuation and the composition of nozzle deposits, systematically investigates the nozzle clogging mechanism of 45 steel, and proposes targeted improvement measures. The research results can provide technical guidance for stable continuous casting and quality control of 45 steel.
1. Experimental Materials, Equipment and Methods
Ladle capacity: 80 t; tundish capacity: 80 t; casting section size: 160 mm × (800~1500) mm; vibration mode: sinusoidal vibration; amplitude: ±2 mm; vibration frequency: 0~300 C/min.
The production process of 45 steel is as follows: hot metal pretreatment → 80 t converter → 80 t LF refining furnace → VD vacuum degassing furnace → continuous casting.
During tapping, alloying and carburizing are carried out in the ladle for pre-deoxidation, with slag retaining operation adopted simultaneously. Bottom argon blowing stirring is applied to reduce the total oxygen content in molten steel. In the LF refining process, a low-basicity CaO-Al₂O₃ slag system is used, achieving a desulfurization rate of 50%～70%. This process reduces Class A inclusions and eliminates CaO-containing Class D inclusions. The main chemical composition of the 45 steel produced by this process is shown in Table 1.
Table 1 Chemical composition of 45 steel (mass fraction, %)
Unstable automatic control of the stopper rod, severe mold level fluctuation and submerged nozzle clogging in the tundish occurred during the production of 45 steel. To investigate the influence of stopper rod opening on casting and the formation mechanism of submerged nozzle clogging, a high-speed data acquisition system was adopted to monitor and plot the continuous casting curves. Two samples were taken from the clogged region of the tundish submerged nozzle, and their compositions were analyzed by EDX.
2 .Experimental Results and Analysis
2.1 Molten Steel Level Monitoring and Clogging Deposit Composition of 45 Steel
The first heat of each casting sequence for 45 steel is susceptible to oxidation due to tundish baking. Therefore, the first heat is produced according to the process for sulfur-uncontrolled and aluminum-controlled steel, while the subsequent heats adopt the conventional production process for sulfur‑ and aluminum‑containing steel. However, this process cannot effectively solve the problems of severe molten steel level fluctuation and poor castability. The corresponding casting curves are shown in Figure 1. From top to bottom, the three curves represent the mold level fluctuation, the stopper rod opening of Strand 1, and the stopper rod opening of Strand 2, respectively. When the clogging deposits peel off suddenly, the mold liquid level drops sharply and the stopper rod opening increases rapidly, resulting in turbulent flow and level drop in the casting process.
Figure 1 Continuous casting curves of 45 steel
It can be seen from the casting curves that the stopper rod opening and mold level fluctuation of the first heat in the tundish are relatively small without obvious fluctuations. However, starting from the second heat of the casting sequence, which is the first heat of 45 steel, the stopper rod opening increases significantly with casting time. Meanwhile, obvious mold level fluctuation occurs with a fluctuation range ≥3 mm, indicating clogging of the tundish submerged nozzle and gradual deterioration of molten steel castability. In the later stage of casting, the stopper rod opening reaches its maximum value, yet it still cannot meet the normal casting speed, eventually leading to casting interruption and unstable production.
Examination of the tundish submerged nozzle after casting interruption reveals that non-metallic inclusions in molten steel gradually migrate and accumulate on the inner wall of the submerged nozzle during casting, forming deposits and resulting in nozzle clogging, as shown in Figure 2. The submerged nozzle was disassembled after shutdown, and two deposit samples inside the nozzle were taken for composition analysis. The results are listed in Table 2. The analysis shows that Sample 1 is mainly composed of Al₂O₃ and CaS. It is inferred that Al₂O₃ and [S] in molten steel form high-melting-point calcium aluminate and CaS inclusions due to improper calcium treatment. Sample 2 mainly consists of Al₂O₃, which indicates that Al₂O₃ in molten steel fails to form low-melting-point calcium aluminate owing to insufficient calcium addition, thus causing clogging of the tundish submerged nozzle.
(a) Nozzle (b) Deposits inside the nozzle
Figure 2 Morphology of tundish submerged nozzle
Table 2 Chemical composition of clogging deposits in tundish submerged nozzle (mass fraction)
Based on the above analysis, the general strategy to mitigate this type of nozzle clogging is to conduct appropriate calcium treatment on the aluminum deoxidation product Al₂O₃, so as to modify it into low-melting-point calcium aluminate. Meanwhile, excessive calcium treatment should be avoided to prevent the formation of high-melting-point CaS inclusions.
2.2 Analysis of Inclusion Formation
2.2.1 Al₂O₃ Inclusions
Al deoxidation is adopted during the smelting of 45 steel, and the deoxidation product is mainly solid Al₂O₃ with a high melting point. Excessive Al₂O₃ in molten steel or incomplete modification treatment will cause these inclusions to accumulate at the nozzle bowl and stopper rod tip of the tundish submerged nozzle, resulting in nozzle clogging and deteriorating the purity and castability of molten steel. To reduce the amount of Al₂O₃ inclusions, calcium treatment can be used to modify solid high-melting-point Al₂O₃ into low-melting-point liquid calcium aluminate.
After calcium treatment, Al₂O₃ can be transformed into compounds such as CaO, CaS, CaO·6Al₂O₃, CaO·2Al₂O₃, CaO·Al₂O₃, 12CaO·7Al₂O₃ and 3CaO·Al₂O₃. Among them, only 12CaO·7Al₂O₃ and 3CaO·Al₂O₃ are liquid at steelmaking temperatures. Therefore, the optimal calcium dosage lies in the range for forming these two phases, which is defined as the "liquid window", as shown in Figure 3. In the figure: \(w_\text{B}\) - mass fraction of component B, %; \(w_\text{Ca}\) - mass fraction of [Ca], %.
With the increase of Al₂O₃ content in molten steel, the required calcium content for calcium treatment rises, and the liquid window becomes wider. Increasing sulfur content in molten steel narrows the liquid window. Sulfur is only related to the formation of CaS and does not affect the modification of calcium aluminate\(^{8-10]}\). Therefore, desulfurization should be carried out prior to calcium treatment to improve calcium treatment efficiency and reduce the probability of tundish submerged nozzle clogging.
Figure 3 Precipitation phase diagram of inclusions during modification of Al₂O₃ by Ca
2.2.2 CaS Inclusions
During the smelting of 45 steel, [S] reacts with [Ca] in molten steel to form CaS. With a melting point of 2400 ℃, CaS exists as solid inclusions and tends to cause nozzle clogging during casting. As the sulfur content increases, the required calcium for CaS formation and the aluminum content for Al₂O₃ modification both decrease. The region for forming low-melting-point calcium aluminate becomes narrower, and Al₂O₃ is more likely to transform into high-melting-point CaO·Al₂O₃, which precipitates as solid phase with temperature drop and blocks the submerged nozzle. According to the thermodynamic equilibrium diagram of CaS, higher temperature requires higher calcium concentration to form CaS at a fixed sulfur level. Accordingly, high sulfur content promotes massive precipitation of solid CaS and CaO·Al₂O₃, resulting in nozzle clogging. In production, the contents of [S] and [Ca] in molten steel should be minimized, and the casting temperature can be moderately increased.
In summary, calcium treatment aims to modify high-melting-point Al₂O₃ deoxidation products into low-melting-point calcium aluminate. Calcium dosage dominates the modification effect: appropriate calcium addition produces liquid low-melting-point calcium aluminate; excessive calcium forms solid high-melting-point calcium aluminate and CaS; insufficient calcium fails to fully modify Al₂O₃, leaving high-melting solid inclusions. Hence, the "liquid window" for target inclusions is narrow and difficult to control.
Fundamentally reducing the generation of Al₂O₃ inclusions can cut the required calcium dosage and restrain the reaction between excess [Ca] and [S] to form CaS. This strategy widens the "liquid window" and facilitates on-site continuous casting control of 45 steel.
3. Improvement Measures
3.1 Aluminum Control
Control concept: Reduce aluminum consumption and lower the generation of Al₂O₃ inclusions.
The converter endpoint carbon is controlled ≥0.08%. Aluminum ingot is adopted for deep deoxidation during tapping. Synthetic slag and lime are added along the steel stream for slag washing to emulsify slag, which promotes early slag formation in the LF furnace. A mixed deoxidizer of aluminum granules, carbon powder and silicon carbide is used in refining, and the deoxidizer is added in small batches and multiple times during smelting.
The relationship among aluminum content at LF departure, aluminum loss and aluminum loss ratio in the VD process is shown in Figure 4. The analysis shows that aluminum loss in VD is positively correlated with the aluminum content at LF departure. Higher aluminum content at LF departure leads to greater aluminum loss in VD, while no obvious correlation exists between LF outlet aluminum and VD aluminum loss ratio. Therefore, to mitigate aluminum oxidation loss in the VD stage, the aluminum content at LF departure should be reduced and controlled ≤0.015%.
Figure 4 Relationship curves of Al content at LF departure, Al loss and Al loss ratio during VD process
3.2 Sulfur Control
Control concept: The sulfur content shall be controllable during smelting, and sulfur alloying shall be implemented in advance.
Desulfurization in refining is realized mainly by adjusting slag dosage through top slag control. The composition of refining top slag is CaO: 42%–46%, SiO₂: 8%–12%, Al₂O₃: 34%–38%, MgO: 5%–7%, with binary basicity R=3.5–5. The sulfur content at LF departure is controlled within 0.028%–0.035%. Sulfur adjustment shall be completed before the VD process, which means advancing the sulfur alloying procedure.
3.3 Calcium Treatment Range
The principle of calcium treatment is precise calcium addition. It ensures that Al₂O₃ inclusions are fully modified into liquid calcium aluminate, while avoiding the formation of CaS inclusions. Proper control of [Ca]/[S] ratio can effectively prevent CaS generation. Production practice shows that no CaS is detected in nozzle deposits when the [Ca]/[S] ratio in molten steel is kept at 0.3–0.5. The timing of calcium treatment is determined according to composition test results after VD breaking. Aluminum alloying is carried out first, and calcium treatment is performed 2–3 min after Al alloying, with calcium content controlled at 0.0007%–0.0011%.
3.4 Superheat Control
Full-process protective casting is adopted in continuous casting, including monolithic tundish nozzle, tundish argon filling, shroud sealing and argon blowing for ladle, which reduces aluminum oxidation loss in tundish and limits it ≤0.004%. Based on the relationship between temperature and inclusions, a relatively high superheat is adopted. The molten steel superheat during casting is controlled at 25–35 ℃.
3.5 Application Performance
After process optimization and industrial application, stable casting of 45 steel for an entire casting sequence is achieved with good castability. The mold level curve remains stable, with single-point fluctuation ≤3 mm and continuous casting duration ≥500 min. The optimized casting curves are shown in Figure 5. From top to bottom are the mold level fluctuation curve and stopper rod opening curve, respectively.
Figure 5 Casting curves of 45 steel after process improvement
4 .Conclusions
(1) The clogging of tundish submerged nozzle originates from the accumulation of high-melting-point non-metallic Al₂O₃ inclusions and CaS formed during calcium treatment on the inner wall of the tundish submerged nozzle. The fundamental solution is to reduce the generation of Al₂O₃ inclusions, prevent the formation of CaS from excess [Ca] and [S], and cut the calcium dosage required to form liquid calcium aluminate.
(2) To fully modify Al₂O₃ inclusions into liquid calcium aluminate without generating CaS inclusions, improve the castability of molten steel and avoid tundish submerged nozzle clogging, the following control parameters shall be adopted in practical production: the final calcium content in molten steel is controlled within \(6\times10^{-6}～10\times10^{-6}\); the binary basicity R of refining slag is kept at 3.5–5 to realize controllable sulfur content and advance sulfur alloying before VD vacuum treatment; the aluminum content at LF departure is ≤0.015%; reduce the types of alloying operations after VD vacuum breaking, and perform calcium treatment 3–5 min after aluminum alloying; the superheat of molten steel is maintained at 25–35 ℃.