Method for controlling nitrogen content of lf-vd duplex refining die steel

Abstract

The application discloses a nitrogen content control method for LF-VD duplex refining die steel, which comprises the following steps: LF furnace refining process: continuously inputting argon into the LF furnace to maintain a micro-positive pressure environment in the furnace, and executing a power supply strategy comprising initial low-power slagging and later high-power foaming submerged arc; VD furnace vacuum refining process: in the vacuum pumping process, first, degassing is performed at a first stirring intensity when a first vacuum degree is reached, then, after a second vacuum degree lower than the first vacuum degree is reached, the stirring intensity is increased to a second stirring intensity and the nitrogen removal is continuously performed for a second predetermined time. The LF-VD duplex refining die steel nitrogen content control method can stably control the LF nitrogen increase amount in the target range through the micro-positive pressure, dynamic foaming slag and refined stirring in the LF furnace refining process, effectively inhibits the nitrogen increase in the LF process, simultaneously strengthens the nitrogen removal efficiency in the VD process, and realizes the high-precision and high-stability control of the nitrogen content of the die steel through the systematic cooperation of the LF and the VD.

Description

Technical Field

[0001] This invention belongs to the field of metal smelting technology, specifically relating to a method for controlling the nitrogen content of LF-VD duplex refining die steel. Background Technology

[0002] Mold steel is the core material for manufacturing various molds in industrial production, and its performance directly determines the service life, machining accuracy, and stability of the mold. Among the many factors affecting the performance of mold steel, the content of gaseous elements, especially nitrogen (N), in the steel is a crucial control indicator.

[0003] Excessive nitrogen levels can cause a series of fatal damages to mold steel:

[0004] (1) Deterioration of toughness and induction of brittle fracture: Nitrogen atoms in solid solution and precipitated nitrides will severely deteriorate the impact toughness of steel, causing early failures such as chipping and cracking of the mold when subjected to impact load.

[0005] (2) Damage to polishing performance: Hard and brittle nitride inclusions (such as AlN, TiN) will be pulled out during mirror polishing, forming "pinholes" and "orange peel" defects, which cannot meet the stringent requirements of high-end plastic molds for surface finish.

[0006] (3) Reduced corrosion resistance: For chromium-containing corrosion-resistant mold steel, nitrogen will promote the precipitation of Cr2N at the grain boundary, creating a "chromium-depleted zone", making the mold susceptible to intergranular corrosion.

[0007] (4) Causes macroscopic segregation: exacerbates the uneven composition during the solidification process of the billet, leading to anisotropy of the mold and increasing the risk of deformation and cracking during heat treatment.

[0008] Therefore, maintaining the nitrogen content in mold steel at a very low level (e.g., ≤80ppm) is one of the key technologies for producing high-end mold steel.

[0009] Currently, the dual-process of LF (Ladle Refining Furnace) and VD (Vacuum Degassing Furnace) is commonly used in industrial production for refining die steel. However, this process has significant limitations in controlling nitrogen content. Die steel, due to its high content of alloying elements such as Cr, Mo, and V, requires a long LF refining process (usually exceeding 60 minutes) to ensure alloy melting, composition homogenization, and inclusion flotation. However, the long processing time in the LF furnace, under atmospheric pressure and electric arc heating conditions, greatly increases the risk of nitrogen absorption by the molten steel. The heating arc ionizes N2 molecules in the air, making them easily absorbed by the molten steel. Furthermore, prolonged argon blowing, slag formation, and alloying operations, any inadequate sealing or improper stirring can lead to exposed molten steel surfaces, contact with air, and increased nitrogen content. Conventional slag coverings, if unstable, cannot effectively prevent nitrogen transmission. Under current technology, the nitrogen gain in die steel during the LF process often reaches 50-60 ppm or even higher, with drastic fluctuations, placing a heavy burden on subsequent VD treatment.

[0010] Vacuum denitrification (VD) is an effective method for nitrogen removal, but it still has limitations in practical applications. Because the nitrogen content of the incoming LF feed is already severely excessive and unstable, the VD process is often a "passive remedy" rather than an "active optimization." Traditional VD operations lack precise and sequential coordinated control of vacuum level and argon blowing / stirring. The denitrification rate is limited by liquid phase transport, resulting in low efficiency. Existing processes typically treat LF and VD as two independent processes, lacking the concept of using the end point of one process as the starting point for another for systematic linkage and target management.

[0011] When using the LF+VD duplex process to produce mold steel, there are three main systemic defects in controlling the nitrogen content of the steel:

[0012] 1. LF Refining Process: Proactive Nitrogen Increase Rather Than Passive Nitrogen Control

[0013] Die steels are highly alloyed and require long-term LF refining to ensure alloy melting and compositional uniformity. However, existing LF operations lack specific measures to prevent nitrogen absorption, which leads to a significant increase in the contact between molten steel and air under atmospheric pressure, arc heating, and prolonged stirring, making nitrogen absorption an almost inevitable and uncontrollable process.

[0014] The existing power supply system is not optimized to minimize the ionization of N2 in the air by the electric arc, thus failing to provide the driving force for nitrogen atom dissolution. The slightly positive pressure inside the furnace is unstable or not considered a key process parameter, making it impossible to effectively isolate the air. The argon blowing and stirring system fails to adhere to the core principle of "minimizing liquid surface exposure," often causing the molten steel to churn and "expose" to the light due to excessive stirring, resulting in direct nitrogen absorption. At the end of the existing LF process, the nitrogen increase in molten steel often reaches over 50 ppm, with a large fluctuation range, placing a heavy and unstable denitrification burden on the subsequent VD process.

[0015] 2. VD vacuum process: low denitrification efficiency and in a "passive remedial" state.

[0016] Because the nitrogen content of the incoming LF feedstock was already severely excessive, the VD process was in a passive remedial state from the outset due to inherent deficiencies, rather than undergoing deep purification on a good foundation. Existing VD operations typically employ fixed or empirical argon blowing patterns, failing to implement sequential stirring intensity control based on changes in vacuum (such as strong stirring after ultimate vacuum), resulting in insufficient gas-liquid interface renewal and low mass transfer efficiency for nitrogen removal. Vacuum holding time is often a fixed value, rather than dynamically adjusted based on the initial [N] content, leading to either insufficient nitrogen removal or wasted time and energy. Faced with high and fluctuating initial nitrogen content, the existing VD process exhibits unstable nitrogen removal efficiency, making it difficult to achieve stable and deep nitrogen removal effects.

[0017] 3. Between the LF and VD processes: they are isolated from each other and lack "systematic synergy".

[0018] This is the most fundamental flaw. Current technology treats LF and VD as two independent, single-function processes: LF handles heating, alloying, and desulfurization, while VD handles degassing (including denitrification). The endpoint nitrogen content of the LF process is not considered a key process control target to guide LF operation, nor is the denitrification efficiency of the VD process linked to the incoming feed conditions of the LF process through modeling. The entire refining process is fragmented and open-loop in terms of nitrogen control.

[0019] In summary, when using the existing LF+VD dual-process to produce mold steel, the nitrogen addition in the LF process is severe and uncontrollable, while the denitrification efficiency of the VD process is limited and passive. The lack of synergy between the two processes results in a low nitrogen content hit rate and large fluctuations in the final finished steel. This makes it impossible to stably produce high-purity, high-performance low-nitrogen mold steel, which seriously restricts the quality consistency and market competitiveness of high-end mold products.

[0020] This paper provides a method for controlling the nitrogen content of LF-VD double-refined mold steel, specifically how to effectively suppress nitrogen increase in the LF process and enhance the denitrification efficiency of the VD process, so as to achieve high-precision and high-stability control of nitrogen content in mold steel and stabilize the nitrogen hit rate to over 95%. Summary of the Invention

[0021] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention provides a method for controlling the nitrogen content of LF-VD duplex refining die steel, with the purpose of effectively suppressing nitrogen gain in the LF process and enhancing the denitrification efficiency of the VD process, thereby achieving high-precision and high-stability control of the nitrogen content of the die steel.

[0022] To achieve the above objectives, the technical solution adopted by this invention is: a method for controlling the nitrogen content of LF-VD duplex refining die steel, comprising:

[0023] LF furnace refining process: Argon gas is continuously introduced into the LF furnace to maintain a slightly positive pressure environment inside the furnace, and a power supply strategy including low-power slag formation in the early stage and high-power foaming and submerged arc in the later stage is implemented.

[0024] VD furnace vacuum refining process: During the vacuuming process, degassing is first performed at a first stirring intensity when a first vacuum degree is reached. Then, after a second vacuum degree lower than the first vacuum degree is reached, the stirring intensity is increased to a second stirring intensity and denitrification is performed for a second predetermined time.

[0025] The power supply strategy specifically includes:

[0026] After adding slag-forming material into the LF furnace, the first electric power is used for a first predetermined time to form a protective slag layer covering the surface of the molten steel; then the electric power is increased to a second electric power higher than the first electric power to raise the temperature and promote the foaming of the protective slag layer to form a foamed slag layer.

[0027] The first scheduled time is 3 to 5 minutes.

[0028] In the LF furnace refining process, lime is periodically added to maintain the thickness of the foam slag layer;

[0029] The thickness of the foam residue is ≥200 mm.

[0030] In the LF furnace refining process, argon blowing and stirring include:

[0031] The baseline stirring mode uses the first argon flow rate;

[0032] The enhanced stirring mode is activated when materials are added, using a second argon flow rate higher than the first argon flow rate, and then returns to the baseline stirring mode after the material addition is completed.

[0033] The first argon gas flow rate is 200-500 L / min, and the second argon gas flow rate is 800-1000 L / min.

[0034] The first vacuum degree is ≤1000 Pa, and the second vacuum degree is ≤80 Pa, wherein the first vacuum degree is reached within 5 minutes after the start of vacuuming.

[0035] The first stirring intensity corresponds to an argon flow rate of 300 L / min, and the second stirring intensity corresponds to an argon flow rate of 400-450 L / min.

[0036] After the vacuum refining process in the VD furnace, a wire feeding process is also included, in which the argon flow rate is controlled to maintain the diameter of the bright ring on the molten steel surface at a third preset value.

[0037] Following the wire feeding process, a soft blowing process is also included, in which the argon flow rate is controlled to maintain the diameter of the bright ring on the molten steel surface at a fourth preset value that is less than the third preset value.

[0038] The nitrogen content control method for LF-VD dual-refining mold steel of the present invention can stably control the nitrogen increase in LF within the target range through micro-positive pressure, dynamic foam slag and fine stirring in the LF furnace refining process, effectively suppressing nitrogen increase in LF process, while enhancing the denitrification efficiency of VD process. Through the systematic synergy of LF and VD, high-precision and high-stability control of nitrogen content in mold steel is achieved, and the nitrogen hit rate is stably increased to over 95%. Detailed Implementation

[0039] The following description of the embodiments provides a more detailed explanation of the specific implementation of the present invention, with the aim of helping those skilled in the art to have a more complete, accurate, and in-depth understanding of the concept and technical solutions of the present invention, and facilitating its implementation.

[0040] This invention provides a method for controlling the nitrogen content of LF-VD double-refined die steel, including an LF furnace (ladle refining furnace) refining process and a VD furnace (vacuum degassing furnace) vacuum refining process.

[0041] The LF furnace refining process includes: continuously introducing argon gas into the LF furnace to maintain a slightly positive pressure environment inside the furnace, and implementing a power supply strategy that includes initial low-power slag formation and subsequent high-power foaming submerged arc.

[0042] The VD furnace vacuum refining process includes: during the vacuuming process, degassing is first performed at a first stirring intensity when a first vacuum degree is reached, and then, after a second vacuum degree lower than the first vacuum degree is reached, the stirring intensity is increased to a second stirring intensity and denitrification is performed for a second predetermined time.

[0043] Specifically, the goal of the LF furnace refining process is to strictly control the nitrogen increase during refining within a narrow range of 30 to 40 ppm, while meeting the requirements of conventional alloying and refining treatments. This lays a stable foundation for the subsequent VD furnace vacuum denitrification process and ensures the accuracy of the nitrogen content in the final mold steel product.

[0044] Furthermore, the LF furnace refining process includes the following steps:

[0045] First, a slightly positive pressure atmosphere is controlled. After the LF furnace lid is closed, high-purity argon gas is continuously blown into the LF furnace, and the opening of the dust removal system is adjusted to maintain a state of slight smoke at the gap of the furnace lid, thereby creating a slightly positive pressure environment inside the furnace and isolating nitrogen gas in the external atmosphere.

[0046] Secondly, a phased power supply and slag-forming operation is implemented. In the initial refining stage, slag-forming material is added to the ladle, followed by a period of low power supply to allow the slag to fully melt and cover the molten steel surface. Then, the power supply is increased for rapid heating, promoting slag foaming to form a foamed slag layer, thus achieving submerged arc operation. During the refining process, slag is periodically replenished to maintain the thickness and stability of the foamed slag layer.

[0047] Next, implement argon blowing and stirring in different modes. Set a baseline stirring mode using the first argon flow rate to maintain a slight creeping motion on the molten steel surface. When adding alloys or slag, switch to an enhanced stirring mode and temporarily use a second argon flow rate higher than the first argon flow rate for short-term strong stirring. After the material is evenly melted, return to the baseline stirring mode.

[0048] Finally, the composition and temperature of the molten steel are controlled. The acid-soluble aluminum content in the molten steel is controlled at the upper limit of the process target range, and the temperature of the molten steel is controlled in the range of 120~125℃ above the liquidus temperature to create conditions for subsequent processes.

[0049] In the LF furnace refining process, including step 1, a fully micro-positive pressure argon gas seal is maintained. The micro-positive pressure environment is constructed as follows: after the LF furnace lid is closed, high-purity argon gas is immediately introduced into the LF furnace, and the opening of the dust removal system is dynamically adjusted simultaneously. The pressure inside the LF refining furnace is maintained at a micro-positive pressure, with a slight amount of smoke visible above the furnace lid. This continuous micro-positive pressure environment, achieved through the above control, creates a protective gas curtain within the furnace, effectively preventing external air from intruding into the furnace and cutting off the diffusion channel of atmospheric nitrogen (N2) to the molten steel at its source, thereby suppressing nitrogen accumulation during the refining process.

[0050] In the LF furnace refining process, including step 2, the submerged arc power supply and dynamic foamed slag control are optimized. This step adopts a segmented power supply strategy of initial low-power slag formation and subsequent high-power foamed submerged arc, simultaneously coordinated with dynamic slag replenishment measures, to achieve stable slag layer construction and full-process submerged arc protection. The power supply strategy specifically includes:

[0051] After adding slag-forming material into the LF furnace, the first electric power is applied for a first predetermined time, which is 3 to 5 minutes, to form a protective slag layer covering the surface of the molten steel. Then, the electric power is increased to a second electric power higher than the first electric power to raise the temperature and promote the foaming of the protective slag layer to form a foamed slag layer.

[0052] After the LF furnace smelting is started, 800 kg of lime and 120 kg of fluorite are immediately added to the furnace as basic slag-forming materials. Then, the first power setting (low power level) is used for power supply for 3 to 5 minutes. The purpose of this stage is to allow the basic slag-forming materials to fully melt and evenly cover the molten steel surface during this period, forming a dense initial protective slag layer. At the same time, low-power arc stabilization can effectively avoid the risk of early nitrogen increase caused by the strong agitation and ionization of furnace gas components by the high-power arc when the slag layer is not fully formed.

[0053] After the protective slag layer is formed, the power supply is rapidly increased to the second power level (a higher level than the first power level) for rapid heating, with the heating rate controlled at 5℃ / min. This high-power heating strategy shortens the overall refining cycle and reduces the exposure time of molten steel in the LF furnace, thus reducing the probability of nitrogen accumulation over time. Furthermore, the sufficient heat generated by the high-power supply, combined with the foaming components in the slag-forming material, promotes rapid foaming of the initial protective slag layer, forming a foamed slag layer with a thickness ≥200mm. This achieves deep submerged arc – even if the arc is completely enveloped by the foamed slag layer, it significantly inhibits the ionization and dissociation of nitrogen (N2) in the air by the arc, further blocking the path of nitrogen intrusion into the molten steel.

[0054] In step 2, the ratio of 800 kg lime to 120 kg fluorite is the basic slag-forming ratio determined through process optimization. Lime, as the main slag-forming agent, can provide sufficient CaO to achieve slag alkalinity control, laying the compositional foundation for subsequent desulfurization, dephosphorization, and foam slag formation. Fluorite, as a fluxing agent, can reduce the melting temperature of the slag-forming material, improve the fluidity of the molten slag, accelerate the melting and slag formation process of lime, and ensure that the slag-forming material can melt quickly and form an initial protective slag layer covering the surface of the molten steel during the subsequent low-power power supply stage.

[0055] In the LF furnace refining process, lime is periodically replenished to maintain the thickness of the foamed slag layer; the thickness of the foamed slag is ≥200 mm. In step 2, the performance of the foamed slag layer is maintained by periodically replenishing slag-forming material: every 10–12 minutes, 100–150 kg of lime is added to the ladle. This replenishment operation can compensate for the amount of slag-forming material consumed by chemical reactions and lost due to molten steel scouring, dynamically maintaining the thickness stability of the foamed slag layer and the submerged arc effect, ensuring that from power-on start-up to the end of LF furnace refining, the electric arc is always in an effectively shielded state by the foamed slag layer.

[0056] The LF furnace refining process also includes step 3, refined argon blowing and stirring. This step achieves refined control by dividing the molten steel into differentiated argon blowing and stirring modes and combining them with real-time monitoring. This ensures the refining effect of the molten steel while minimizing the contact area between the molten steel and the furnace gas, thereby suppressing nitrogen accumulation. Argon blowing and stirring includes:

[0057] The baseline stirring mode uses the first argon flow rate;

[0058] The enhanced stirring mode is activated when materials are added, using a second argon flow rate higher than the first argon flow rate, and then returns to the baseline stirring mode after the material addition is completed.

[0059] Argon blowing stirring adopts a dual-mode control strategy that combines baseline stirring and enhanced stirring. The furnace camera is used to observe the slag turning status in real time, thereby dynamically monitoring the steel liquid level online to ensure that the stirring intensity matches the refining conditions.

[0060] The baseline stirring mode is used throughout the entire LF furnace refining process. Argon gas is introduced into the LF furnace, with the initial argon gas flow rate controlled at 200–500 L / min. This flow rate control ensures that only slight creep occurs on the molten steel surface, and the diameter of the bright ring on the molten steel surface is stably controlled within the range of 50 cm ± 5 cm. This stirring intensity can minimize the contact area between the molten steel and the furnace gas while ensuring that inclusions in the molten steel float sufficiently, thus blocking the intrusion path of nitrogen elements at the contact interface.

[0061] The enhanced stirring mode is only activated when large batches of alloy or slag are being added: At the moment of material addition, the argon flow rate is temporarily increased to 800–1000 L / min (i.e., the second argon flow rate), and maintained at this flow rate for 2–3 minutes to ensure rapid melting and uniform mixing of the added material with the molten steel. Once the material is uniformly melted, the argon flow rate is immediately returned to the baseline stirring mode corresponding to the first argon flow rate. This short-term enhanced stirring design effectively avoids problems such as excessively low local temperatures and uneven composition and temperature in the molten steel caused by batch feeding, while also avoiding the risk of excessive nitrogen accumulation caused by prolonged high-intensity stirring.

[0062] The LF furnace refining process also includes step 4, composition and temperature control. This step, by adjusting the acid-soluble aluminum (Als) content and temperature parameters of the molten steel exiting the LF furnace, creates suitable thermodynamic and kinetic conditions for the subsequent VD furnace vacuum denitrification process, ensuring denitrification efficiency and stability. Specific control requirements are as follows:

[0063] When tapping steel from the LF furnace refining process, the acid-soluble aluminum content in the molten steel is controlled to the upper limit of the process target.

[0064] Regarding composition control, when tapping steel after refining in the LF furnace, the content of acid-soluble aluminum (Als) in the molten steel is controlled according to the upper limit of the process target + 0.005%. As a strong deoxidizing element, the precise control of the content of acid-soluble aluminum can ensure that the molten steel has sufficient deoxidizing capacity, reduce the interference of oxidizing components in the molten steel on the subsequent denitrification process, and provide basic conditions for stable denitrification reaction.

[0065] Regarding temperature control, the temperature of the molten steel exiting the LF furnace is set within the range of +120 to +125°C of the liquidus temperature of the corresponding mold steel. This temperature range is designed to prevent the kinetics of the denitrification reaction in the VD furnace from deteriorating due to excessively low molten steel temperature, while also preventing excessive oxidation or other metallurgical defects caused by excessively high temperature, thus ensuring that the subsequent vacuum denitrification process in the VD furnace can be carried out efficiently and stably.

[0066] The goal of the VD furnace vacuum refining process is to achieve efficient and stable denitrification of approximately 20 ppm by optimizing denitrification kinetics, based on the favorable conditions provided by the LF furnace refining process.

[0067] In the vacuum refining process of the VD furnace, during the vacuuming process, degassing is first performed at a first stirring intensity when a first vacuum degree is reached. Then, after a second vacuum degree lower than the first vacuum degree is reached, the stirring intensity is increased to a second stirring intensity and maintained for a second predetermined time for denitrification.

[0068] This process employs a synergistic strategy of staged vacuum control and time-sequential stirring enhancement. Specifically, during the vacuuming process, degassing is first performed at a first stirring intensity when a first vacuum level is reached. Once a second vacuum level, lower than the first, is reached, the stirring intensity is increased to a second stirring intensity and maintained for a second predetermined time to enhance denitrification. The specific operation is as follows:

[0069] Step 1: Staged vacuum denitrification operation

[0070] The VD furnace treatment process is divided into two consecutive stages: a rapid degassing stage and an enhanced denitrification stage. The parameters and operating requirements for each stage are as follows:

[0071] Phase 1 (Rapid Degassing Period): Start the vacuum pump system and control the vacuuming rate to ensure that the vacuum level inside the furnace is evacuated to ≤1000Pa (i.e., the first vacuum level) within ≤5 minutes. During this phase, argon blowing is performed at the first stirring intensity, corresponding to an argon flow rate of 300L / min. This stirring intensity can achieve stable degassing, effectively avoiding the risk of splashing caused by violent disturbance of molten steel in the early stage of vacuuming. At the same time, it initially removes gaseous impurities such as hydrogen from the molten steel, creating a stable environment for subsequent high-vacuum denitrification.

[0072] Phase Two (Enhanced Denitrification Period): Continuously evacuate the furnace until the ultimate vacuum level of ≤80Pa is reached (i.e., the second vacuum level), and maintain this vacuum level stably for 5 minutes. Combined with real-time observation of the molten steel surface by the in-furnace camera, immediately increase the argon flow rate by 100-150L / min, so that the total argon flow rate reaches 400-450L / min (i.e., the second stirring intensity), and maintain this condition for 15-18 minutes (i.e., the second predetermined time) to carry out enhanced denitrification.

[0073] The "stabilize first, strengthen later" sequential control strategy adopted in this process is as follows: In the first stage, low-intensity stirring and appropriate vacuum are matched to achieve stable degassing and build a high vacuum basic environment; in the second stage, the stirring intensity is increased under the premise of high vacuum, which can drastically increase the steel-bubble renewal interface, break the barrier of the liquid phase boundary layer of the steel, and promote the rapid diffusion of nitrogen atoms deep inside the steel to the bubble interface, significantly improving the mass transfer efficiency of the denitrification reaction, and finally achieving a high-efficiency and stable denitrification effect.

[0074] Furthermore, the VD furnace vacuum refining process also includes a wire feeding process, during which the argon flow rate is controlled to maintain the diameter of the bright ring on the molten steel surface at a third preset value.

[0075] In the wire feeding process, the diameter of the bright ring on the molten steel surface is maintained at a third preset value (50cm ± 5cm) by adjusting the argon flow rate. Simultaneously, calcium wire is continuously fed into the molten steel at a rate of 100-150 meters. The calcium wire reacts with non-metallic inclusions in the molten steel, modifying their morphology and refining their particle size, thus improving their flotation and removal efficiency. Maintaining the diameter of the bright ring on the molten steel surface within the aforementioned third preset value ensures uniform stirring of the molten steel during the wire feeding process and guarantees sufficient reaction between the calcium wire and the molten steel, while minimizing the contact area between the molten steel and the furnace gas. This effectively suppresses the risk of secondary nitrogen accumulation in the molten steel during the wire feeding process.

[0076] Furthermore, after the wire feeding process, the VD furnace vacuum refining process also includes a soft blowing process, in which the argon flow rate is controlled to keep the diameter of the bright ring on the molten steel surface at a fourth preset value that is less than the third preset value.

[0077] During the soft blowing process, the argon flow rate is further reduced to maintain the diameter of the bright ring on the molten steel surface at the fourth preset value, which is 25cm ± 5cm and is less than the aforementioned third preset value. This soft blowing agitation intensity is low, which provides sufficient time for the aggregation and flotation of fine inclusions in the molten steel, while avoiding secondary nitrogen enrichment caused by violent disturbances. This promotes further aggregation, growth, and detachment of inclusions from the molten steel, achieving deep purification. Simultaneously, the low-intensity soft blowing maintains the uniformity of the molten steel temperature, providing high-quality molten steel with uniform composition and temperature for the smooth operation of subsequent continuous casting processes.

[0078] The nitrogen content control method for LF-VD duplex refining die steel according to embodiments of the present invention has the following advantages:

[0079] (1) Excellent nitrogen inhibition effect of LF furnace refining process: Through micro positive pressure, dynamic foam slag and fine stirring, the nitrogen increase of LF is stably controlled at 30-40ppm, solving the problem from the source.

[0080] (2) The denitrification efficiency of the vacuum refining process in the VD furnace is doubled: the denitrification amount is stabilized at around 20ppm by adopting the "time-intensified denitrification" strategy, which is highly efficient and has good reproducibility.

[0081] (3) The hit rate has been greatly improved: through the systematic synergy of the LF furnace refining process and the VD furnace vacuum refining process, the hit rate of nitrogen element in mold steel has been greatly improved from the general level in the industry to more than 95%.

[0082] (4) Win-win for product quality and economic benefits: The finished steel has high purity and stable performance. At the same time, the improved hit rate leads to a significant increase in yield and a decrease in quality loss, resulting in huge economic benefits.

[0083] Example

[0084] This embodiment is used to verify the practical application effect of the nitrogen content control method for LF-VD double-stage refining die steel described in this invention. By detecting the nitrogen content of the molten steel at different process nodes, the changes in indicators before and after using the nitrogen control method of this invention are compared. The specific detection and results are as follows:

[0085] In this embodiment, the detection nodes for nitrogen content in molten steel are as follows: converter exit (initial nitrogen content), LF furnace refining process exit (nitrogen content after LF refining), VD furnace vacuum refining process breaking-through moment (nitrogen content during VD vacuum treatment), and VD furnace vacuum refining process exit (final nitrogen content). The nitrogen content detection results at each node are shown in the table below:

[0086]

[0087] The data in the table above shows that when the method of the present invention is not used, the nitrogen increase in the LF furnace refining process is relatively high, and the denitrification effect in the VD furnace vacuum refining process is unstable. However, when the method of the present invention is used, the nitrogen increase in the LF furnace refining process is effectively suppressed. Combined with the enhanced denitrification strategy in the VD furnace vacuum refining process, the nitrogen content in the final liquid steel is accurately and stably controlled, which verifies the effectiveness of the method of the present invention.

[0088] The present invention has been described above by way of example. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention; or the direct application of the inventive concept and technical solution to other situations without modification, are all within the protection scope of the present invention.

Claims

1. A method for controlling the nitrogen content of LF-VD duplex refining die steel, characterized in that, include: LF furnace refining process: Argon gas is continuously introduced into the LF furnace to maintain a slightly positive pressure environment inside the furnace, and a power supply strategy including low-power slag formation in the early stage and high-power foaming and submerged arc in the later stage is implemented. VD furnace vacuum refining process: During the vacuuming process, degassing is first performed at a first stirring intensity when a first vacuum degree is reached. Then, after a second vacuum degree lower than the first vacuum degree is reached, the stirring intensity is increased to a second stirring intensity and denitrification is performed for a second predetermined time.

2. The method for controlling nitrogen content in LF-VD duplex refining die steel according to claim 1, characterized in that, The power supply strategy specifically includes: After adding slag-forming material into the LF furnace, the first electric power is used for a first predetermined time to form a protective slag layer covering the surface of the molten steel; then the electric power is increased to a second electric power higher than the first electric power to raise the temperature and promote the foaming of the protective slag layer to form a foamed slag layer.

3. The method for controlling nitrogen content in LF-VD duplex refining die steel according to claim 2, characterized in that, The first scheduled time is 3 to 5 minutes.

4. The method for controlling nitrogen content in LF-VD duplex refining die steel according to claim 2, characterized in that, In the LF furnace refining process, lime is periodically added to maintain the thickness of the foam slag layer; The thickness of the foam residue is ≥200 mm.

5. The method for controlling the nitrogen content of LF-VD duplex refining die steel according to any one of claims 1 to 4, characterized in that, In the LF furnace refining process, argon blowing and stirring include: The baseline stirring mode uses the first argon flow rate; The enhanced stirring mode is activated when materials are added, using a second argon flow rate higher than the first argon flow rate, and then returns to the baseline stirring mode after the material addition is completed.

6. The method for controlling the nitrogen content of LF-VD duplex refining die steel according to claim 5, characterized in that, The first argon gas flow rate is 200-500 L / min, and the second argon gas flow rate is 800-1000 L / min.

7. The method for controlling the nitrogen content of LF-VD duplex refining die steel according to any one of claims 1 to 6, characterized in that, The first vacuum degree is ≤1000 Pa, and the second vacuum degree is ≤80 Pa, wherein the first vacuum degree is reached within 5 minutes after the start of vacuuming.

8. The method for controlling the nitrogen content of LF-VD duplex refining die steel according to any one of claims 1 to 6, characterized in that, The first stirring intensity corresponds to an argon flow rate of 300 L / min, and the second stirring intensity corresponds to an argon flow rate of 400-450 L / min.

9. The method for controlling the nitrogen content of LF-VD duplex refining die steel according to any one of claims 1 to 8, characterized in that, After the vacuum refining process in the VD furnace, a wire feeding process is also included, in which the argon flow rate is controlled to maintain the diameter of the bright ring on the molten steel surface at a third preset value.

10. The method for controlling the nitrogen content of LF-VD duplex refining die steel according to claim 9, characterized in that, Following the wire feeding process, a soft blowing process is also included, in which the argon flow rate is controlled to maintain the diameter of the bright ring on the molten steel surface at a fourth preset value that is less than the third preset value.

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