Control method for preventing duplex steel grade continuous casting pouring expansion stopper rod

Abstract

The invention provides a control method for preventing a double-steel-grade continuous casting pouring expansion stopper rod, and relates to the technical field of ferrous metallurgy. The calcium content and technological parameters are accurately regulated and controlled, the calcium element evaporation loss is reduced in duplex refining, it is ensured that the calcium content of the molten steel is maintained at the level of inhibiting inclusion accumulation during continuous casting, and the method has the advantages that calcium element loss is effectively controlled, it is ensured that the molten steel maintains enough calcium content during continuous casting so as to relieve the problem of stopper expansion, and the product quality is improved.

Description

Technical Field

[0001] This application relates to the field of iron and steel metallurgy technology, and in particular to a control method for preventing the expansion of the plug rod during continuous casting of double-strand steel grades. Background Technology

[0002] Thick-gauge hot-rolled automotive steel and other specific steel grades are prone to developing edge black line defects on the surface after rolling. These defects primarily originate from tiny air bubbles at the edges of the slab, which rupture or deform during rolling to form visible black lines. To suppress these defects, continuous casting processes commonly employ a combination of solid stopper rods and solid top nozzles, known as a double-solid scheme, to reduce gas entrapment and bubble formation. However, when using the double-solid scheme during the initial casting stage, the lack of through-hole structures in the stopper rod and nozzle allows inclusions in the molten steel to accumulate continuously at the head of the stopper rod, causing the stopper rod to rise abnormally—a phenomenon known as stopper rod expansion. When the accumulation of inclusions reaches a critical point, they may detach from the stopper rod and enter the crystallizer, resulting in large-sized inclusions inside the slab, severely damaging the surface quality and internal uniformity of the product. Furthermore, these steel grades typically employ a dual refining process, including LF refining and RH vacuum refining. During the RH refining process, a high vacuum environment must be maintained to achieve efficient denitrification, which leads to a significant evaporation loss of calcium in the molten steel under high temperature and vacuum conditions. After RH refining, the calcium content in the molten steel is often below 8 ppm. Calcium plays a crucial role in inhibiting the accumulation of inclusions at the stopper rod, and insufficient calcium content significantly exacerbates the risk of stopper rod bulging. Therefore, effectively controlling calcium loss in the duplex process and ensuring sufficient calcium content in the molten steel at the start of continuous casting to alleviate the stopper rod bulging problem has become a core challenge facing the current technology.

[0003] To address the aforementioned issues, existing technologies urgently need improvement. Summary of the Invention

[0004] The purpose of this application is to provide a method for controlling the expansion of the plug rod during the initial casting of twin-strand steel grades. This method has the advantages of effectively controlling calcium loss, ensuring that the molten steel maintains sufficient calcium content during the initial casting to alleviate the plug rod problem, and improving product quality.

[0005] This application provides a control method for preventing the expansion plug rod from opening during continuous casting of double-stranded steel grades. The technical solution is as follows: include: (1) LF refining process: In the early stage, calcium-containing desulfurizing agent is used to foam and improve the submerged arc effect; before calcium treatment, the aluminum content in the molten steel is adjusted to the upper limit of the finished aluminum content specified in the steel grade technical standard; during calcium treatment, the calcium treatment lifting guide is lowered to the preset range from the surface of the molten steel and aligned with the argon blowing hole, and the argon flow rate is adjusted after the calcium line is fed into the molten steel; when the LF refining station is exited, the calcium content in the molten steel is controlled to be ≥40ppm, and the nitrogen increase in the molten steel during the LF refining process is ≤10ppm. After calcium treatment, the soft blowing flow rate is 120-200NL / min, and the diameter of the soft blowing hole on the slag surface is 100-150mm to reduce calcium loss during the soft blowing stage; (2) RH refining process: No additional aluminum is added, and the required alloy is added to the molten steel at one time; the ultimate vacuum degree of RH refining is controlled to be ≤65Pa to achieve rapid denitrification. After sampling and testing the nitrogen content in the molten steel to ensure that it meets the technical standard requirements of the steel grade, the vacuum is broken; the total processing time of RH refining is ≤18 minutes, and the calcium content in the molten steel is above 11 when RH refining is completed. (3) Continuous casting process: The continuous casting process is carried out by a combination of solid stopper rod and solid top nozzle.

[0006] Furthermore, this application also proposes that the amount of nitrogen added to the molten steel in the LF refining process in step (1) be controlled to be ≤10ppm, and that the amount of nitrogen added caused by the arc ionization of air during the power supply and heating process is reduced by foaming the slag formed by the calcium-containing desulfurizing agent.

[0007] Furthermore, this application also proposes that, during calcium treatment in step (1), the calcium treatment lifting guide pipe is positioned within a preset range of 300-500 mm from the surface of the molten steel.

[0008] Furthermore, this application also proposes that, in step (1), after the calcium wire is fed into the molten steel, the argon flow rate is adjusted to 200-400 NL / min in order to improve the calcium element recovery rate.

[0009] Furthermore, this application also proposes that, in step (1), when the LF refining station is exiting, the calcium content in the molten steel is controlled at 40-60 ppm to ensure that the calcium content in the molten steel after RH refining still meets the requirements for preventing the expansion of the plug rod.

[0010] Furthermore, this application also proposes that the ultimate vacuum degree of RH refining in step (2) be controlled at 30-65 Pa, which can accelerate the denitrification rate while reducing the evaporation loss of calcium elements.

[0011] Furthermore, this application also proposes that the total RH refining treatment time in step (2) is 12-18 minutes, and the average loss rate of calcium in the molten steel is ≤1.6ppm / min.

[0012] Furthermore, this application also proposes that, at the end of RH refining in step (2), the calcium content in the molten steel is precisely controlled to be above 11, so as to suppress the accumulation of inclusions at the solid stopper rod.

[0013] Furthermore, this application also proposes that the foaming degree of the calcium-containing desulfurizing agent in step (1) is slight foaming, which ensures the submerged arc effect and avoids excessive foaming causing foam slag to overflow from the ladle.

[0014] Furthermore, this application also proposes that the double-stranded steel grade is a thick-gauge hot-rolled automotive steel, which is prone to edge black line defects caused by small air bubbles at the edge of the slab after rolling.

[0015] As can be seen from the above, the method for controlling the expansion of the plug rod during the opening of continuous casting of twin-strand steel grades provided in this application includes the optimized control of the LF refining process, the RH refining process and the continuous casting process. By precisely controlling the calcium content and process parameters, the evaporation loss of calcium element is reduced in the twin-strand refining process, and the calcium content of the molten steel is maintained at a level that inhibits the accumulation of inclusions when the continuous casting starts. It has the advantages of effectively controlling the loss of calcium element, ensuring that the molten steel maintains a sufficient calcium content to alleviate the plug rod problem when the continuous casting starts, and improving product quality. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the first embodiment of the control method for preventing the expansion of the plug rod during continuous casting of dual-grade steel. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0018] This application provides a method for controlling the expansion of the stopper rod during continuous casting of double-stranded steel grades, referring to... Figure 1 , Figure 1 This is a flowchart illustrating the first embodiment of the control method for preventing the expansion of the plug rod during continuous casting of dual-grade steel.

[0019] In this embodiment, the control method for preventing the expansion plug rod during continuous casting of dual-grade steel includes the following steps: Step (1) LF refining process: In the early stage, calcium-containing desulfurizing agent is used to foam and improve the submerged arc effect; before calcium treatment, the aluminum content in the molten steel is adjusted to the upper limit of the finished aluminum content specified in the steel grade technical standard; during calcium treatment, the calcium treatment lifting guide is lowered to the preset range from the surface of the molten steel and aligned with the argon blowing hole, and the argon flow rate is adjusted after the calcium line is fed into the molten steel; when the LF refining station is exited, the calcium content in the molten steel is controlled to be ≥40ppm, and the nitrogen increase in the molten steel during the LF refining process is ≤10ppm. After calcium treatment, the soft blowing flow rate is 120-200NL / min, and the diameter of the soft blowing hole on the slag surface is 100-150mm to reduce calcium loss during the soft blowing stage.

[0020] Step (2) RH refining process: without adding aluminum, add the required alloy to the molten steel at once; control the ultimate vacuum degree of RH refining to ≤65Pa to achieve rapid denitrification, and break the vacuum after sampling and testing the nitrogen content in the molten steel to ensure that it meets the technical standard requirements of the steel grade; the total processing time of RH refining is ≤18 minutes, and ensure that the calcium content in the molten steel is above 11 when RH refining is completed.

[0021] Step (3) Continuous casting process: The continuous casting process is carried out by using a combination of solid stopper rods and solid top nozzles.

[0022] To facilitate understanding of this embodiment, some key terms involved are explained below: Duplex steel grades refer to steel grades that are refined using a combination of LF refining and RH refining processes.

[0023] "Stretching stopper rod" refers to the phenomenon in continuous casting where, due to the accumulation of inclusions in the molten steel at the interface between the stopper rod and the inlet nozzle, the stopper rod cannot descend or rise normally, thus affecting the control of molten steel flow.

[0024] The LF refining process refers to the refining steps in the ladle refining furnace where molten steel undergoes desulfurization, deoxidation, alloying, and temperature adjustment.

[0025] The RH refining process refers to the refining stage in which molten steel is degassed, denitrified, dehydrogenated, and its composition fine-tuned using a vacuum circulation degassing device.

[0026] Continuous casting refers to the production process of continuously casting refined molten steel into slabs, billets, or round billets.

[0027] Calcium-containing desulfurizers refer to additives containing calcium elements or calcium compounds, used for steel desulfurization and foam slag formation.

[0028] The submerged arc effect refers to the effect of covering the electric arc with slag foam during the LF refining process, reducing arc radiation and heat loss, and suppressing nitrogen addition in molten steel.

[0029] Calcium treatment refers to the process of feeding calcium wire or calcium-based alloy into molten steel to change the morphology of inclusions in the molten steel, improve the fluidity of the molten steel, and suppress the phenomenon of blockage rods.

[0030] Solid stopper rod and solid sprue nozzle refer to a combination scheme in which both the stopper rod and the sprue nozzle used to control the flow of molten steel during continuous casting are made of solid structure in order to prevent air bubble defects on the edge of the slab.

[0031] This embodiment provides a method for controlling the expansion of the stopper rod during continuous casting of double-stranded steel grades. The specific implementation method is as follows: In the LF refining process, a calcium-containing desulfurizing agent is used for foaming in the early stage to improve the submerged arc effect. This foaming can be achieved by adjusting the amount of desulfurizing agent added or by selecting desulfurizing agents with different foaming properties, thereby forming a layer of slag covering on the surface of the molten steel, reducing arc radiation and air entrapment.

[0032] Before calcium treatment, the aluminum content in the molten steel is adjusted to the upper limit of the finished product aluminum content specified in the steel grade's technical standards. This adjustment can be achieved by adding an appropriate amount of aluminum blocks or wires at the initial stage of LF refining, or by precisely controlling the aluminum content when tapping steel from the preceding converter, to ensure the effectiveness of subsequent calcium treatment.

[0033] During calcium treatment, the calcium treatment lifting guide is lowered to a preset distance from the molten steel surface and aligned with the argon blowing hole. After the calcium wire is fed into the molten steel, the argon flow rate is adjusted. This lifting guide can be lowered to a certain height above the molten steel surface, for example, by positioning using a robotic arm or hydraulic system, to ensure that the calcium wire can effectively enter the molten steel. After the calcium wire is fed in, the argon flow rate can be adjusted according to actual operating experience or preset parameters, for example, by manually adjusting valves or setting an automatic flow control system.

[0034] At the LF refining station, the calcium content in the molten steel is controlled to be no less than 40 ppm, and the nitrogen increase in the molten steel during the LF refining process is controlled to be no more than 10 ppm. After calcium treatment, the soft blowing flow rate is 120-200 NL / min, and the diameter of the soft blowing holes on the slag surface is 100-150 mm to reduce calcium loss during the soft blowing stage. The calcium content in the molten steel can be controlled by precisely controlling the feed rate and speed of the calcium wire during the calcium treatment process. The nitrogen increase in the molten steel can be controlled by optimizing slag coverage, controlling argon blowing intensity, and ensuring the sealing of the furnace cover.

[0035] During the RH refining process, no additional aluminum is added, and the required alloys are added to the molten steel all at once. This means that before the RH refining begins, all the alloying elements that need to be added, such as manganese, silicon, and chromium, are added to the molten steel all at once, reducing the RH smelting time.

[0036] The ultimate vacuum level in RH refining is controlled to be no higher than 65 Pa to achieve rapid denitrification. During the vacuum process, samples can be taken from the molten steel periodically, and the nitrogen content can be detected by methods such as spectral analysis. When the test results show that the nitrogen content in the molten steel meets the technical standards for the steel grade, the vacuum is released by filling the vacuum chamber with an inert gas (such as argon), i.e., the vacuum breaking operation is performed.

[0037] The total processing time for RH refining is controlled to be no more than 18 minutes to ensure that the calcium content in the molten steel is above 11% at the end of RH refining. The total processing time for RH refining can be set according to steel grade requirements and equipment capacity, for example, through precise control using a timer. After RH refining, sampling analysis is performed to confirm whether the calcium content in the molten steel meets the preset range to satisfy the calcium content requirements of subsequent continuous casting processes.

[0038] During continuous casting, a combination of solid stopper rods and solid sprue nozzles is used for initial casting. During the installation and commissioning of the continuous casting machine, solid stopper rods and sprue nozzles are selected and installed, and a rigorous inspection is conducted before casting begins to ensure that their fit clearance and concentricity meet the requirements.

[0039] This embodiment precisely controls the steel composition and process parameters during LF and RH refining processes, especially ensuring sufficient calcium content at the LF refining station and effectively controlling calcium loss and nitrogen content during RH refining. Ultimately, this ensures that the calcium content in the molten steel is within a suitable range to suppress inclusion accumulation at the start of continuous casting. This effectively solves the problem of block expansion caused by inclusion accumulation at the solid block during the start of continuous casting for thick-gauge hot-rolled automotive steel and other duplex steel grades, reducing the risk of large inclusions in the slab while also controlling edge black line defects.

[0040] In some of the embodiments described above in this application, the nitrogen addition in the LF refining process needs to be strictly controlled to ensure the performance of the final steel. However, during the LF refining process, contact between the molten steel and the outside air, as well as the refining operation, may cause nitrogen to enter the molten steel, making it difficult to effectively control the nitrogen addition and thus affecting the quality of the steel. This is especially true for duplex steel grades that are sensitive to nitrogen content, which may lead to problems such as plugging rods during subsequent continuous casting.

[0041] In response, this application further proposes that the nitrogen increase in molten steel during the LF refining process be controlled to ≤10ppm, and that foam slag be formed by foaming with calcium-containing desulfurizing agents to reduce nitrogen increase caused by arc ionization of air during the power supply and heating process.

[0042] Specifically, calcium-based desulfurizers typically refer to slag materials whose main components are calcium compounds such as calcium oxide and calcium fluoride. When added to the ladle during LF refining, these slags react with the molten steel or slag, or under argon stirring, to produce a certain degree of foaming. This foaming causes the slag layer to expand, forming a foamy slag layer with a certain thickness and viscosity. The submerged arc layer refers to the foamy slag layer formed by the foamed calcium-based desulfurizer, which effectively covers the surface of the molten steel and completely or partially buries the electric arc beneath the slag layer. This submerged arc layer not only provides insulation and reduces heat loss, but more importantly, it creates a physical barrier between the molten steel and the external atmosphere. The formation of the submerged arc layer significantly reduces the direct contact area between the molten steel surface and the air. Simultaneously, the foamed slag layer absorbs arc energy, stabilizes the arc, and reduces the impact and stirring effect of the arc on the molten steel surface, thereby suppressing surface fluctuations and further reducing the possibility of atmospheric nitrogen being entrained into the molten steel through the surface. In this way, the absorption of nitrogen in the molten steel during LF refining can be effectively controlled.

[0043] Through the above technical solution, in the LF refining process, the formation of foamed slag by a calcium-containing desulfurizing agent reduces nitrogen increase caused by air ionization during the power-on heating process. This effectively isolates the molten steel from the outside air and stabilizes the steel surface, thus significantly reducing the entrainment of atmospheric nitrogen into the molten steel. This allows the nitrogen increase in the molten steel during the LF refining process to be precisely controlled at a low level of ≤10ppm, effectively avoiding the deterioration of steel performance caused by excessive nitrogen content. It also provides stable steel quality for subsequent RH refining and continuous casting processes, thereby strongly ensuring the overall control effect against the expansion of the plug rod during continuous casting of dual-grade steels.

[0044] In some embodiments described above, a method for controlling the expansion of the plug rod during continuous casting of dual-grade steel is proposed. This method involves lowering the calcium treatment guide pipe to a preset distance from the molten steel surface and aligning it with the argon blowing hole during the calcium treatment stage of the LF refining process. After the calcium wire is fed into the molten steel, the argon flow rate is adjusted. However, if this preset distance is not properly controlled, it may lead to molten steel splashing, increased calcium oxidation and loss, or insufficient contact between the calcium wire and the molten steel, thereby affecting the calcium treatment effect and calcium yield. Consequently, it becomes difficult to accurately control the calcium content in the molten steel, adversely affecting the anti-expansion plug rod effect in subsequent RH refining and continuous casting processes.

[0045] In this regard, this application further proposes that in the calcium treatment of the LF refining process described in step (1), the calcium treatment lifting guide pipe is 300-500mm away from the surface of the molten steel.

[0046] The calcium treatment lifting guide is a device used to accurately and stably feed calcium wire into the molten steel in the LF refining furnace. This guide is typically made of high-temperature resistant materials and has a lifting mechanism to adjust its relative position to the molten steel surface during the calcium treatment process. The guide is designed to reduce oxidation and splashing during calcium wire feeding, ensuring that the calcium wire effectively penetrates deep into the molten steel. The preset range of 300-500mm from the molten steel surface refers to the vertical distance between the lower end of the calcium treatment lifting guide and the molten steel surface. Controlling the lower end of the guide within this specific range ensures that the calcium wire melts rapidly and reacts fully with the molten steel upon feeding, while avoiding uneven steel mixing or accelerated guide erosion due to excessive guide depth, and severe oxidation and burning of the calcium wire or splashing of the molten steel due to insufficient guide depth. This range is based on a comprehensive consideration of calcium wire feeding efficiency, calcium recovery rate, and molten steel surface stability.

[0047] By precisely controlling the distance between the calcium treatment lifting guide and the molten steel surface within a preset range of 300-500 mm, the contact conditions during calcium wire feeding into the molten steel can be effectively optimized. Within this range, the calcium wire can melt rapidly and disperse evenly in the molten steel, significantly reducing burn-off caused by oxidation before or immediately upon entering the molten steel. Simultaneously, it minimizes molten steel splashing, thereby improving calcium recovery. This precise distance control ensures the stability and consistency of the calcium treatment effect, providing a reliable guarantee that the calcium content in the molten steel reaches the preset target at the LF refining station. This, in turn, lays a solid foundation for calcium content control during subsequent RH refining and for preventing bridging during the final continuous casting pour.

[0048] In some embodiments of this application, during the calcium treatment stage of the LF refining process, the argon flow rate needs to be adjusted after the calcium wire is fed into the molten steel to promote the effective utilization of calcium. However, if the argon flow rate is not properly controlled, it may lead to uneven dispersion or excessive loss of calcium in the molten steel, thereby affecting the calcium treatment effect and making it difficult to ensure that the calcium content in the molten steel reaches the expected target. This, in turn, poses a potential risk to the control of the expansion rod in the subsequent continuous casting process.

[0049] In this regard, this application further proposes that after the calcium wire is fed into the molten steel in step (1), the argon flow rate be adjusted to 200-400 NL / min in order to improve the calcium element recovery rate.

[0050] Specifically, in the LF refining process, after the calcium wire is fed into the molten steel, the volumetric flow rate of inert argon gas blown into the bottom of the molten steel through the argon blowing port is precisely controlled at 200-400 NL / min. Here, argon flow rate refers to the volumetric flow rate of inert argon gas blown into the bottom of the molten steel through the argon blowing port when the calcium wire is fed into the molten steel. Precisely controlling the argon flow rate at 200-400 NL / min aims to provide adequate stirring intensity. Too low an argon flow rate may lead to uneven dispersion of the calcium wire in the molten steel, affecting the reaction efficiency between calcium and inclusions; while too high an argon flow rate may cause violent turbulence in the molten steel, increasing the contact area between the molten steel and air, accelerating the oxidation and evaporation loss of calcium, and potentially entraining air, leading to nitrogen accumulation in the molten steel. Therefore, 200-400 NL / min is an optimized flow rate that ensures sufficient and uniform diffusion of calcium in the molten steel, promotes effective reaction between calcium and oxide inclusions, and minimizes calcium loss, thereby significantly improving calcium yield.

[0051] By precisely adjusting the argon flow rate after calcium wire is fed into molten steel to 200-400 NL / min, this application effectively solves the problem of low calcium recovery caused by improper argon flow control. This precise flow control ensures sufficient and uniform dispersion of calcium in the molten steel and effective reaction with inclusions, while avoiding calcium oxidation and evaporation losses caused by excessive stirring, thus significantly improving the calcium recovery rate. The high calcium recovery rate ensures that the calcium content in the molten steel at the LF refining station can stably reach the preset target (e.g., ≥40 ppm), providing a basis for precise control of calcium content in the subsequent RH refining process. 11) This laid a solid foundation and ultimately effectively suppressed the accumulation of inclusions at the solid stopper rod, thereby preventing the occurrence of the stopper rod expansion during the continuous casting of double-strand steel grades.

[0052] In some embodiments of this application, the calcium content in the molten steel is controlled to be no less than 40 ppm when leaving the LF refining station. However, during the subsequent RH refining process, the calcium in the molten steel is easily lost through evaporation and oxidation due to factors such as vacuum degassing and stirring. If the calcium content control range at the LF refining station is too wide or too low, the calcium content in the molten steel after RH refining may be insufficient to effectively suppress the accumulation of inclusions at the continuous casting stopper rod, thereby affecting the effect of preventing the stopper rod from expanding.

[0053] In response, this application further proposes that the calcium (Ca) content in the molten steel be controlled at 40-60 ppm during the LF refining process in step (1) to ensure that the calcium (Ca) content in the molten steel after RH refining still meets the requirements for preventing stopper blockage. "Controlling the calcium (Ca) content in the molten steel at 40-60 ppm" means that at the end of the LF refining process, when the molten steel is ready to leave the station for the next refining process, the mass fraction of calcium in the molten steel is maintained within the range of 40 to 60 ppm through sampling analysis. The main role of calcium in molten steel is to react with non-metallic inclusions such as alumina, modifying them into low-melting-point, highly fluid calcium aluminate inclusions, thereby preventing these inclusions from adhering to the stopper rod or nozzle wall during continuous casting and causing blockage. Controlling the calcium content within this specific range of 40-60 ppm aims to reserve sufficient margin for the unavoidable calcium loss in subsequent refining processes, while avoiding other metallurgical problems that may be caused by excessively high calcium content. Achieving this control objective typically requires precise calculation of the calcium wire feed rate and feed speed based on factors such as steel grade requirements, initial calcium content, and calcium wire yield. During calcium treatment, optimizing operating parameters such as argon blowing intensity and slag layer coverage can effectively reduce calcium oxidation and evaporation losses, thereby improving calcium yield. Furthermore, rapid composition analysis before LF refining allows for real-time monitoring and necessary fine-tuning of the calcium content to ensure it falls precisely within the target range.

[0054] By precisely controlling the calcium content in the molten steel at the LF refining station within a specific range of 40-60 ppm, this application effectively solves the problem of excessive calcium loss due to evaporation and oxidation during subsequent RH refining. This control range provides a sufficient calcium margin for the RH refining stage, ensuring that adequate calcium content is maintained in the molten steel even under vacuum degassing and stirring during RH refining. This allows inclusions in the molten steel to be fully modified, forming low-melting-point, highly fluid calcium aluminates, thereby significantly inhibiting the accumulation of these inclusions at the solid stopper rod in continuous casting. Ultimately, this ensures the smooth progress of the continuous casting process and effectively prevents the occurrence of stopper rod bulging.

[0055] In some embodiments described above in this application, the RH refining process requires control of the ultimate vacuum level to achieve rapid denitrification. However, in actual production, if only the ultimate denitrification rate is pursued without refined management of the vacuum level, excessive evaporation loss of calcium in the molten steel may occur, thereby affecting the stability of the calcium content in the molten steel at the end of RH refining, and consequently adversely affecting the effect of preventing stopper rod expansion during subsequent continuous casting.

[0056] To address this, this application further proposes controlling the ultimate vacuum level of RH refining to 30-65 Pa during the RH refining process. Ultimate vacuum refers to the lowest pressure value that the RH refining unit can achieve under vacuum operation. To achieve this precise control, a vacuum system consisting of multi-stage vacuum pumps (e.g., steam jet pumps, mechanical pumps, or dry pumps) is typically used. By adjusting the operating parameters of the vacuum pumps, such as the number of pump stages and the pumping rate, the pressure inside the RH furnace can be finely adjusted. Simultaneously, a high-precision vacuum pressure sensor is installed inside the furnace to monitor pressure changes in real time and feed the data back to the central control system (such as a PLC or DCS). The control system automatically or semi-automatically adjusts the operating status of the vacuum pumps according to the preset target range of 30-65 Pa to ensure that the vacuum level inside the furnace is stably maintained within this range. For example, in the early stages of RH refining, a lower pressure can be rapidly evacuated to accelerate denitrification, while after calcium treatment or in the later stages of refining, more precise vacuum control is required to balance the needs of denitrification and calcium retention.

[0057] Controlling the ultimate vacuum level in RH refining within the range of 30-65 Pa aims to achieve the dual goals of accelerating denitrification while minimizing calcium evaporation loss. From a denitrification perspective, a lower vacuum level (e.g., 30-65 Pa) significantly reduces the nitrogen partial pressure on the molten steel surface. According to Sieverts' law, this promotes the diffusion and removal of dissolved nitrogen from the molten steel into the gas phase, thereby accelerating the denitrification rate and ensuring that the nitrogen content in the molten steel meets the technical standards for the steel grade. From the perspective of reducing calcium evaporation loss, calcium evaporation loss is positively correlated with vacuum level; that is, the lower the vacuum level, the more intense the calcium evaporation. By controlling the ultimate vacuum level within the range of 30-65 Pa, rather than lower levels (e.g., below 30 Pa), sufficient denitrification motive power can be ensured while avoiding a significant increase in the saturated vapor pressure of calcium due to excessively low vacuum, thus effectively suppressing excessive calcium evaporation. This specific vacuum range represents an optimized balance point that satisfies the requirements of rapid denitrification while maximizing the retention of valuable calcium in the molten steel.

[0058] By precisely controlling the ultimate vacuum level in the RH refining process within a specific range of 30-65 Pa, this application effectively balances the relationship between the denitrification rate of molten steel and the evaporation loss of calcium. Under this vacuum level, the diffusion and removal of dissolved nitrogen in the molten steel are accelerated, ensuring a rapid and efficient denitrification effect and meeting the nitrogen content requirements of the steel grade technical standards. Simultaneously, compared to lower vacuum levels, this range significantly inhibits excessive evaporation of calcium, thereby maximizing the retention of calcium added during the LF refining stage. This ensures that the calcium content in the molten steel remains within the target range at the end of RH refining, providing a stable calcium treatment basis for suppressing inclusion accumulation and preventing plugging during subsequent continuous casting. This refined vacuum control avoids calcium loss due to excessive pursuit of denitrification, thus guaranteeing the overall effect of preventing plugging during the initial casting of twin-strand steel grades.

[0059] In some embodiments of this application, the RH refining process requires control of the total processing time to ensure that the calcium content in the molten steel reaches a preset range at the end of refining. However, in actual production, if only an upper limit is set for the total RH refining processing time, the calcium element may be lost too quickly or too slowly due to evaporation during vacuum treatment, making it difficult to accurately control the calcium content in the molten steel at the end of RH refining and affecting the anti-bulging rod effect in the subsequent continuous casting process.

[0060] In this regard, this application further proposes that in step (2), the total RH refining treatment time is 12-18 minutes, and the average loss rate of calcium in the molten steel is ≤1.6ppm / min.

[0061] Specifically, the total RH refining treatment time refers to the duration of refining operations in the molten steel within the RH vacuum treatment unit. During this time, the molten steel undergoes a series of reactions, including degassing, decarburization, denitrification, and alloying. Limiting the total treatment time to 12-18 minutes aims to balance refining efficiency with calcium loss. Too short a time may result in poor degassing and denitrification effects; too long a time may lead to excessive calcium evaporation and loss, making it difficult to meet the calcium content requirements of subsequent continuous casting. This time period is typically achieved by precisely controlling the operating cycle of the RH vacuum treatment unit, for example, starting the timer from when the molten steel enters the RH bath until when the vacuum is broken and the molten steel leaves the RH bath.

[0062] Meanwhile, the average calcium loss rate in molten steel refers to the average rate at which the calcium content in molten steel decreases during the total RH refining process. Calcium is easily lost through evaporation under vacuum conditions due to its high vapor pressure during RH vacuum refining. Limiting this rate aims to ensure that the total calcium loss in molten steel remains within a controllable range during the total RH refining process, thereby guaranteeing that the calcium content in the molten steel stabilizes within the target range at the end of RH refining. The average calcium loss rate can be controlled in various ways, such as optimizing the vacuum curve of RH refining to avoid prolonged periods of extremely high vacuum; adjusting the argon blowing intensity and mode to reduce surface disturbance in the molten steel and lower the calcium evaporation interface; or by supplementing a small amount of calcium flux during the refining process.

[0063] The above technical solution limits the total RH refining processing time to an optimized range of 12-18 minutes, while simultaneously strictly controlling the average calcium loss rate in the molten steel to no more than 1.6 ppm / min. This dual control mechanism more precisely balances the refining effects such as degassing and denitrification during the RH refining process with the evaporation loss of calcium, avoiding excessive calcium evaporation due to excessive processing time and insufficient refining due to insufficient processing time. In particular, by directly limiting the calcium loss rate, even within the total RH refining processing time, it effectively suppresses the unexpected rapid loss of calcium, thus ensuring that the calcium content in the molten steel can be stably and accurately controlled within the target range above 11% at the end of RH refining. This allows inclusions such as alumina in the molten steel to be more effectively modified, forming low-melting-point, non-adherent calcium aluminate inclusions, significantly inhibiting the accumulation of these inclusions at the solid stopper rod in continuous casting, thereby effectively preventing the occurrence of stopper rod expansion and improving the stability of continuous casting production.

[0064] In some of the embodiments described above in this application, although the calcium content in molten steel is initially controlled through the LF refining and RH refining processes, and a target range for the calcium content in molten steel at the end of RH refining is set, in actual production, due to various uncertainties in the refining process, such as the evaporation loss of calcium elements and the reaction with inclusions, the final calcium content may fluctuate greatly and it is difficult to accurately reach the ideal range. This affects the effective control of the morphology of inclusions in molten steel, and may lead to the accumulation of inclusions at the solid stopper rod in continuous casting, causing the stopper rod expansion problem.

[0065] In this regard, this application further proposes that the calcium content in the molten steel be precisely controlled to be above 11 when the RH refining is completed in step (2) in order to suppress the accumulation of inclusions at the solid stopper rod.

[0066] Specifically, precisely controlling the calcium content in molten steel to above 11 ppm means ensuring that the mass fraction of dissolved calcium in the molten steel is stably maintained above 11 ppm at the end of the RH refining process through precise control and real-time monitoring of refining parameters. This control objective aims to optimize the morphology and distribution of inclusions in the molten steel, making them less likely to adhere to the refractory material surface during continuous casting. Achieving precise control typically requires synergistic optimization of multiple parameters, including the calcium treatment rate in the LF refining stage, the vacuum level, treatment time, molten steel temperature, and argon blowing intensity during the RH refining process. For example, a calcium consumption model can be established during the RH refining process, and refining parameters can be dynamically adjusted based on online monitoring data to compensate for calcium loss and ensure that the target range is ultimately achieved. Inhibiting the accumulation of inclusions at the solid stopper rod refers to precisely controlling the calcium content in the molten steel to transform high-melting-point inclusions such as alumina into low-melting-point, spherical, or liquid calcium aluminate inclusions. These modified inclusions have lower wetting angles and smaller surface energies, making them less prone to physical or chemical adsorption with refractory materials such as solid stoppers, thus reducing their deposition and growth on the stopper surface. Furthermore, the spherical inclusions exhibit better fluidity in molten steel and are less prone to agglomeration, further reducing the risk of blockage.

[0067] By employing the aforementioned technical solution, the calcium content in the molten steel is precisely controlled within a specific range of 11% or higher at the end of RH refining. This application effectively solves the problem of poor inclusion morphology control caused by fluctuations in calcium content, leading to inclusion accumulation at the solid stopper rod. This precise control ensures that high-melting-point inclusions such as alumina in the molten steel can be fully converted into low-melting-point, highly fluid calcium aluminate inclusions. These modified inclusions are less likely to adhere to the refractory surface of the solid stopper rod, significantly reducing the tendency for inclusions to deposit and grow at the stopper rod. Compared to simply setting a calcium content range, precise control can more stably achieve the desired inclusion modification effect, thereby effectively suppressing the blockage phenomenon of the solid stopper rod during continuous casting, ensuring smooth continuous casting production and stable product quality.

[0068] In some embodiments of this application, calcium-containing desulfurizers are used to foam the LF refining process (1) in the early stage to improve the submerged arc effect, which is of great significance for stabilizing the refining process and reducing secondary oxidation of molten steel. However, in actual operation, if the foaming degree of the calcium-containing desulfurizer is not properly controlled, such as excessive foaming, it may lead to slag liquefaction, which will eventually cause foamed slag to overflow from the ladle, affecting the subsequent refining effect and the quality of the final product.

[0069] In response, this application further proposes a scheme where the foaming degree of the calcium-based desulfurizing agent in the LF refining process (1) is slightly foamed, which ensures the submerged arc effect while avoiding excessive foaming that causes foamy slag to overflow from the ladle. "Slight foaming" refers to the formation of a stable and moderately foamed slag layer on the surface of molten steel during the LF refining process through the action of the calcium-based desulfurizing agent. This foamed slag layer effectively covers the surface of the molten steel, stabilizes the electric arc, reduces arc radiation and heat loss, and inhibits air entrainment into the molten steel, thereby improving the submerged arc effect. Compared with vigorous foaming or no foaming, the slag layer in the slightly foamed state has good fluidity and coverage, but does not cause violent turbulence in the molten steel. Slight foaming can be achieved in various ways, such as precisely controlling the amount and rate of addition of the calcium-based desulfurizing agent to react with oxygen and sulfur in the molten steel to generate an appropriate amount of bubbles; or adjusting the argon blowing flow rate to ensure that argon is evenly distributed in the slag layer, forming stable small bubbles and avoiding local bubble accumulation or rupture. In addition, the composition (such as alkalinity and MgO content) and viscosity of the slag layer also affect the foaming performance. By optimizing the slag composition ratio, a stable, slightly foamed slag layer can be formed.

[0070] In some embodiments described above, a method for controlling the expansion of the stopper rod during continuous casting of dual-grade steel is proposed. Through refined control of the LF refining, RH refining, and continuous casting processes, the accumulation of inclusions at the stopper rod is effectively suppressed, thereby avoiding the expansion of the stopper rod and ensuring smooth continuous casting. However, in actual production, even if the problem of the expansion of the stopper rod is solved for certain specific steel grades, other quality defects may still occur. For example, for steel grades with extremely high product quality requirements, such as thick-gauge hot-rolled automotive steel, black line defects caused by small air bubbles at the edge of the slab are prone to appear after rolling, seriously affecting product performance and production efficiency.

[0071] In this regard, this application further proposes that the double-stranded steel grade is a thick-gauge hot-rolled automotive steel, which is prone to edge black line defects caused by small air bubbles at the edge of the slab after rolling.

[0072] The term "double-strand steel" refers to steel produced through a dual-strand or multi-strand casting process during continuous casting. This production method is typically used to improve production efficiency but places high demands on steel quality and casting stability. The term "thick-gauge hot-rolled automotive steel" refers to steel used in specific applications, primarily for manufacturing automotive structural components such as frames and chassis. It is characterized by its large thickness and hot-rolling process. This type of steel has stringent requirements for mechanical properties, surface quality, and internal microstructure uniformity. During production, due to its large thickness and long solidification time, defects are more likely to form within the slab, especially in the edge areas. The term "edge black line defect" caused by small air bubbles at the slab edge, which is prone to appear after rolling, describes a quality problem that occurs in subsequent processing (hot rolling) of this specific steel grade. Small air bubbles at the slab edge are usually caused by gases trapped in the molten steel during casting, the accumulation of protective slag or deoxidation products at the edge that are not completely expelled, or uneven solidification shrinkage. These small air bubbles are flattened and elongated during hot rolling, forming linear defects that appear as "black lines," which seriously affect the surface quality and performance of the steel.

[0073] Through the above technical solution, this application clarifies that the controlled double-strand steel grade is thick-gauge hot-rolled automotive steel, and points out the specific problem of edge black line defects caused by small air bubbles at the edge of the slab after rolling. Thick-gauge hot-rolled automotive steel, as a key automotive structural material, has extremely stringent requirements for its internal cleanliness and surface quality. The formation of small air bubbles at the edge of the slab is often closely related to non-metallic inclusions, gas content, and defects during solidification in the molten steel. The control method proposed in this application ensures high cleanliness of the molten steel by precisely controlling the aluminum and calcium content in the LF refining process and further optimizing the calcium content and degassing effect in the RH refining process. This strict control of molten steel cleanliness, combined with the combination of solid stopper rods and solid top nozzles in the continuous casting process, not only effectively prevents stopper rod expansion, but more importantly, reduces potential factors leading to the formation of small air bubbles at the edge of the slab from the source, such as oxide inclusions and nitrogen bubbles caused by excessive nitrogen content. Therefore, this method can be more effectively applied to the production of steel with extremely high quality requirements. While ensuring smooth continuous casting, it significantly reduces the occurrence of black line defects on the edge after rolling, thereby improving the product quality and yield of thick-gauge hot-rolled automotive steel.

[0074] It should be noted that the workflow described above is merely illustrative and does not limit the scope of protection of this application. In practical applications, those skilled in the art can select some or all of it to achieve the purpose of this embodiment according to actual needs, and no restrictions are imposed here.

[0075] In addition, for technical details not described in detail in this embodiment, please refer to the method for controlling the expansion of the plug rod in continuous casting of double-strand steel grades provided in any embodiment of this application, which will not be repeated here.

[0076] Furthermore, it should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or system that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or system. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or system that includes that element.

[0077] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0078] Through the above description of the embodiments, those skilled in the art can clearly understand that the methods of the above embodiments can be implemented by means of software plus necessary general-purpose hardware platforms. Of course, they can also be implemented by hardware, but in many cases the former is a better implementation method. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product is stored in a storage medium (such as read-only memory (ROM) / RAM, magnetic disk, optical disk), and includes several instructions to cause a terminal device (which may be a mobile phone, computer, server, or network device, etc.) to execute the methods of the various embodiments of this application. The above are only preferred embodiments of this application and do not limit the patent scope of this application. All equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method for controlling the expansion of the plug rod during continuous casting of double-strand steel grades, characterized in that, include: (1) LF refining process: In the early stage, calcium-containing desulfurizing agent is used to foam and improve the submerged arc effect; before calcium treatment, the aluminum content in the molten steel is adjusted to the upper limit of the finished aluminum content specified in the steel grade technical standard; during calcium treatment, the calcium treatment lifting guide is lowered to the preset range from the surface of the molten steel and aligned with the argon blowing hole, and the argon flow rate is adjusted after the calcium line is fed into the molten steel; when the LF refining station is exited, the calcium content in the molten steel is controlled to be ≥40ppm, and the nitrogen increase in the molten steel during the LF refining process is ≤10ppm. After calcium treatment, the soft blowing flow rate is 120-200NL / min, and the diameter of the soft blowing hole on the slag surface is 100-150mm to reduce calcium loss during the soft blowing stage; (2) RH refining process: No additional aluminum is added, and the required alloy is added to the molten steel at one time; the ultimate vacuum degree of RH refining is controlled to be ≤65Pa to achieve rapid denitrification. After sampling and testing the nitrogen content in the molten steel to ensure that it meets the technical standard requirements of the steel grade, the vacuum is broken; the total processing time of RH refining is ≤18 minutes, and the calcium content in the molten steel is above 11 when RH refining is completed. (3) Continuous casting process: The continuous casting process is carried out by a combination of solid stopper rod and solid top nozzle.

2. The control method according to claim 1, characterized in that, In step (1), the amount of nitrogen added to the molten steel in the LF refining process is controlled to be ≤10ppm. The amount of nitrogen added is reduced by the formation of foam slag through the foaming of calcium-containing desulfurizing agent to reduce the amount of nitrogen added caused by the ionization of air by electric arc during the power supply and heating process.

3. The control method according to claim 1, characterized in that, In step (1), during calcium treatment, the calcium treatment lifting guide pipe is positioned within a preset range of 300-500 mm from the surface of the molten steel.

4. The control method according to claim 1, characterized in that, In step (1), after the calcium wire is fed into the molten steel, the argon flow rate is adjusted to 200-400 NL / min to improve the calcium recovery rate.

5. The control method according to claim 1, characterized in that, In step (1), when the LF refining station is opened, the calcium content in the molten steel is controlled at 40-60 ppm to ensure that the calcium content in the molten steel after RH refining still meets the requirements for preventing the expansion of the plug rod.

6. The control method according to claim 1, characterized in that, In step (2), the ultimate vacuum degree of RH refining is controlled at 30-65 Pa, which accelerates the denitrification rate while reducing the evaporation loss of calcium.

7. The control method according to claim 1, characterized in that, In step (2), the total RH refining treatment time is 12-18 minutes, and the average loss rate of calcium in the molten steel is ≤1.6ppm / min.

8. The control method according to claim 1, characterized in that, When the RH refining process in step (2) ends, the calcium content in the molten steel is precisely controlled to be above 11 to inhibit the accumulation of inclusions at the solid stopper rod.

9. The control method according to claim 1, characterized in that, In step (1), the calcium-containing desulfurizing agent is foamed only slightly, which ensures the submerged arc effect and avoids excessive foaming that causes foam slag to overflow from the ladle.

10. The control method according to claim 1, characterized in that, The double-strand steel grade is a thick-gauge hot-rolled automotive steel. This type of steel is prone to edge black line defects caused by small air bubbles at the edge of the slab after rolling.

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