Smelting method for controlling nitrogen content of low-alloy steel plate strip to be lower than 30 ppm

Abstract

The invention discloses a smelting method for controlling the nitrogen content of a low-alloy steel plate strip to be lower than 30 ppm, and belongs to the technical field of steel smelting. Comprising the following steps that S1, in the converter smelting process, the quality of raw materials is controlled, a dynamic oxygen supply strategy based on a low-nitrogen blowing model and whole-course argon bottom blowing cooperative control are adopted, reblowing operation is eradicated through a high-carbon-catch one-time tapping technology, the end-point carbon content is controlled to be 0.05-0.07%, the end-point temperature is controlled to be 1680-1700 DEG C, and the tapping time is smaller than or equal to 5 min; s2, in the RH vacuum treatment process, performing deep denitrification under a low-nitrogen condition based on an RH denitrification model; and S3, adopting a composite nitrogen resistance technology to prevent nitrogen increase in the process. Through a dynamic oxygen supply strategy based on a low-nitrogen blowing model, an RH nitrogen removal model, a composite nitrogen resistance technology and the like, production of the high-quality low-alloy steel plate strip with the finished product nitrogen content being smaller than or equal to 28 ppm and the casting blank defect rate being smaller than 0.15% is finally achieved.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, and particularly relates to a smelting method for controlling the nitrogen content of low alloy steel plates to be below 30 ppm. Background Technology

[0002] Nitrogen in low-alloy steel easily causes aging embrittlement, deterioration of processing performance, and surface defects in strips and plates. Under conventional process control conditions such as a high scrap ratio, impure furnace charge, and poor endpoint control, the nitrogen content at the converter endpoint is mostly between 20-30 ppm. After processing through the general LF refining process with poor slag formation, bottom blowing and heating, and poor matching of charging, as well as after conventional RH treatment and calcium treatment, the nitrogen content is mostly between 30-40 ppm. Finally, after casting with normal four-level protection from the ladle to the crystallizer (long nozzle + submerged nozzle + tundish covering agent + crystallizer protective slag), the nitrogen content of the finished product is generally between 36-46 ppm, and it is difficult to stably control it below 30 ppm. Summary of the Invention

[0003] The purpose of this invention is to provide a smelting method for controlling the nitrogen content of low alloy steel plates to be below 30 ppm, so as to solve the problems existing in the prior art.

[0004] The technical solution adopted by this invention to solve its technical problem is:

[0005] A smelting method for controlling the nitrogen content of low-alloy steel plates to be below 30 ppm includes the following steps:

[0006] S1. During the converter smelting process, control the quality of raw materials, adopt a dynamic oxygen supply strategy based on a low-nitrogen blowing model and a coordinated control of bottom blowing argon throughout the process, eliminate the need for supplementary blowing operation through high carbon one-time tapping technology, control the final carbon content to 0.05-0.07%, the final temperature to 1680-1700℃, and the tapping time to ≤5min.

[0007] In the S2 and RH vacuum treatment process, deep denitrification is carried out under low nitrogen conditions based on the RH denitrification model;

[0008] S3. Adopting composite nitrogen inhibition technology to prevent nitrogen increase during the process.

[0009] Furthermore, step S1, raw material quality control, includes:

[0010] Hot metal pretreatment: Dual-stage desulfurization and dephosphorization are adopted to control the sulfur content of hot metal to ≤0.002% and the phosphorus content to ≤0.010%, and the temperature to ≥1280℃;

[0011] Scrap steel screening: Use clean scrap steel with a nitrogen content ≤50ppm and a scrap steel weight percentage ≤12%;

[0012] Auxiliary material control: By mass percentage, TFe ≥ 60%, SiO2 ≤ 5.0%, and Al2O3 ≤ 1.2% in pellets or sinter.

[0013] Furthermore, the low-NOx blowing model for step S1 is as follows:

[0014] ;

[0015] in, The instantaneous rate of change of nitrogen content in liquid steel; This is the nitrogen addition term, representing the rate at which nitrogen in the furnace gas dissolves into the molten steel through the gas-liquid interface. k1 is the gas-liquid interface mass transfer coefficient, and A is the effective gas-liquid contact area. This refers to the nitrogen partial pressure inside the furnace; For the denitrification term, k2 represents the denitrification rate of CO bubbles carrying nitrogen out, k2 is the CO bubble denitrification efficiency coefficient, [C] and [O] are the activities of carbon and oxygen in the molten steel, respectively, and S bubble This represents the total surface area of ​​CO bubbles within a unit volume of molten steel. For the slag-gold interface term, k3 represents the mass transfer behavior of nitrogen at the slag-gold interface, k3 is the slag-gold interface mass transfer coefficient, and η is the mass transfer coefficient. slag [N] represents the adsorption or blocking efficiency of nitrogen by slag. slag These are nitrogen potential parameters related to slag.

[0016] Furthermore, the dynamic oxygen supply strategy in step S1 is based on the instantaneous change rate of nitrogen content in molten steel predicted by the low-nitrogen blowing model. The system automatically switches between at least three blowing modes, taking into account the carbon content and temperature of the molten steel.

[0017] Mode A: When the carbon content of molten steel is >0.3% and the temperature is <1550℃, if the model predicts... If the oxygen supply is greater than +0.5 ppm / min, switch to mode A, control the gun position height to 2.2-2.8 m, oxygen supply intensity to 3.0-3.5 Nm³ / (t·min), and bottom blowing argon intensity to 0.08-0.12 Nm³ / (t·min);

[0018] Mode B: When the carbon content of molten steel is between 0.1% and 0.3% and the temperature is between 1550 and 1650℃, if the model predicts... If the oxygen supply is >-0.8 ppm / min, switch to Mode B, which is divided into early and late stages: in the early stage, control the gun position height at 1.8-2.2 m and the oxygen supply intensity at 3.4-3.8 Nm³ / (t·min); in the late stage, control the gun position height at 2.2-2.5 m and the oxygen supply intensity at 2.8-3.2 Nm³ / (t·min); the bottom-blowing argon intensity throughout the process is 0.13-0.17 Nm³ / (t·min).

[0019] Mode C: When the carbon content of molten steel is <0.1% and the temperature is >1650℃, if the model predicts... If the oxygen supply is less than 0.3 ppm / min, switch to mode C, control the gun position height to 2.5-3.0 m, oxygen supply intensity to 1.5-2.5 Nm³ / (t·min), bottom blowing argon gas to use pulse mode, strong pulse period flow rate to 0.10-0.14 Nm³ / (t·min), weak pulse period flow rate to 0.04-0.08 Nm³ / (t·min), pulse period to 45-75 seconds.

[0020] Furthermore, the denitrification model for step S2RH is as follows:

[0021] ;

[0022] Where N(t) is the real-time denitrification rate; K is the comprehensive reaction coefficient, which is related to the steel temperature and oxygen content; [N] is the real-time nitrogen content of the steel; [N] is the nitrogen content of the steel. eg Q represents the nitrogen equilibrium solubility at the current vacuum level. gas To increase the gas flow rate, V represents the steel processing capacity, and P... vac P0 is the absolute pressure of the vacuum chamber and P0 is the pressure constant.

[0023] Furthermore, step S3, the composite nitrogen barrier technology, is as follows:

[0024] a. In the LF refining process, CaO-Al2O3-SiO2 system slag with an alkalinity of 4.5-5.5 is used, and the bottom blowing gas flow rate is controlled within the range of 200-800 NL / min;

[0025] b. During the RH vacuum treatment process, control the vacuum degree to ≤67Pa, increase the gas flow rate to 140-150NL / min, and the deep degassing time to ≥10min. When fine-tuning the alloy, use micro-alloys with N≤50ppm, V≤0.06%, and Nb≤0.04%. Nitriding alloys are prohibited.

[0026] c. During the calcium treatment process, Ca-Si wire is fed in using a double-layer sleeve protection method. Protective argon gas is introduced into the outer layer of the sleeve. After feeding the wire, argon gas is gently blown in for ≥15 minutes.

[0027] d. During continuous casting, five levels of protective pouring are adopted, including long nozzle, submerged nozzle, tundish covering agent, mold flux, and air curtain baffle wall set in the tundish.

[0028] e. The water volume in the secondary cooling zone is 0.45-0.55 L / kg, and the surface temperature gradient of the billet is ≤15℃ / min.

[0029] The present invention has the following beneficial effects:

[0030] 1. This invention integrates a coordinated nitrogen content control strategy across the entire process from molten iron pretreatment to continuous casting, achieving the system goal of a stable nitrogen content of ≤30ppm in the finished product.

[0031] 2. Through real-time prediction and feedback of the converter low-NOx blowing model and RH denitrification model, intelligent dynamic optimization of oxygen supply strategy and vacuum treatment parameters was achieved, overcoming the lag and instability of traditional empirical control.

[0032] 3. By using composite nitrogen inhibition technology, the key nitrogen-increasing pathways in each process are precisely cut off. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0034] A smelting method for controlling the nitrogen content of low-alloy steel sheet and strip to be less than 30 ppm, taking the production of low-alloy steel sheet and strip as an example, the steel grade is Q355B, and the target finished product nitrogen content is ≤30 ppm.

[0035] 1. Converter smelting:

[0036] After desulfurization and dephosphorization pretreatment using the double-stage hot metal process, the concentrations are: S = 0.0018%, P = 0.009%, and T = 1290℃. Scrap steel (nitrogen content 48 ppm) is added at a ratio of 11%. The quality indicators of the auxiliary feedstock pellets are: TFe = 62%, SiO2 = 4.5%, and Al2O3 = 1.0%. The bottom-blown argon intensity throughout the blowing process is 0.13 Nm³ / (t·min).

[0037] (1) During the early stage of blowing (approximately 0-4 minutes), at the 2nd minute, the real-time status was as follows: the secondary lance measurement showed a carbon content [C] ≈ 0.6% and a temperature T ≈ 1450℃. The furnace gas analyzer measured the nitrogen partial pressure inside the furnace. =0.40atm; Sonar monitoring showed that the molten pool was active, but the foamy slag had not completely covered it, and the effective gas-liquid contact area A was estimated to be 1.7m² / t; The oxygen activity [O] of the molten steel was measured at the same time by the secondary lance.

[0038] Based on the low-nitrogen blowing model:

[0039]

[0040] in, The instantaneous rate of change of nitrogen content in liquid steel;

[0041] The nitrogen addition term represents the rate at which nitrogen in the furnace gas dissolves into the molten steel through the gas-liquid interface. k1 is the gas-liquid interface mass transfer coefficient (related to the converter type, molten pool stirring intensity, and splashing state; it can be obtained through cold water model experiments combined with industrial data regression, typically ranging from 0.5 to 1.2, with a value of 0.85). A is the effective gas-liquid contact area (estimated in real time through furnace mouth cameras, sonar monitoring, or foam slag height models, 1.7 m² / t). The partial pressure of nitrogen inside the furnace (measured in real time by an online furnace gas analyzer (such as a mass spectrometer));

[0042] The term for denitrification of CO bubbles represents the rate at which nitrogen is carried away by CO bubbles. The CO bubble denitrification efficiency coefficient (related to molten pool temperature, bubble size distribution, and steel viscosity; it can be fitted based on the relationship between decarburization rate and bubble surface area, typically 0.08-0.20, with a value of 0.12), [C] and [O] represent the activities of carbon and oxygen in the molten steel, respectively, and S... bubble The total surface area of ​​CO bubbles per unit volume of molten steel (calculated based on decarburization rate, bottom-blown gas flow rate, and molten pool hydrodynamic model, valued at 40m²). -1 );

[0043] For the slag-gold interface term, it represents the mass transfer behavior of nitrogen at the slag-gold interface. The mass transfer coefficient at the slag-gold interface (related to slag basicity, fluidity, and oxidizing properties, and can be determined through slag-gold reaction experiments, typically ranging from 0.03 to 0.10, with a value taken over 0.05 min) is used. -1 ), η slag The adsorption or blocking efficiency of nitrogen by the slag (negatively correlated with the (FeO+MnO) content in the slag, which can be determined through slag sample analysis, is usually 0-1; since the slag is not fully formed in the early stage of the furnace, a value of 0.2 is used), [N] slag The nitrogen potential parameter related to the slag (calculated based on slag composition and temperature, measured at 15 ppm).

[0044] Calculations show: nitrogen increase = +0.60 ppm / min; bubble denitrification = -0.08 ppm / min; the early stage of blowing is characterized by low temperature and rapid slagging, resulting in weak decarburization reaction; the slag-metal interface term is temporarily ignored; the low-nitrogen blowing model outputs predicted values. =+0.52ppm / min. If this is higher than the threshold of 0.5ppm, then mode A is triggered, and parameters biased towards strong nitrogen suppression are executed: gun position height 2.7m, oxygen supply intensity 3.0Nm³ / (t·min), bottom blowing intensity 0.12Nm³ / (t·min), in order to quickly form foam slag to cover the molten steel and suppress nitrogen increase.

[0045] (2) Mid-stage of blowing (4-12 minutes): This period is mainly for decarburization and enhanced denitrification. At the 6th minute, the real-time status is: [C]≈0.25%, T≈1580℃. Foamy slag has fully formed and provides good coverage (A drops to 0.9 m² / t), and the furnace gas... Due to good sealing, the temperature dropped to 0.18 atm, [O] = 0.06%, S bubble =300m -1 Furthermore, the slag has already formed and has a strong adsorption capacity, η slag =0.8. The carbon-oxygen reaction is intense; sonar and furnace gas flow rate back-calculation show a huge amount of CO bubble generation. Calculations yield: Nitrogen increase term = +0.35 ppm / min, bubble denitrification term = 0.54 ppm / min, slag-metal interface term = 0.6 ppm / min. Model output predicted values: = -0.815ppm / min.

[0046] Predicted The trigger threshold of Mode B is reached at -0.8, and [C] and T are within the corresponding ranges. The system immediately switches to Mode B. Based on the carbon content (0.25% > 0.2%), it is determined to be in the early stage, and the corresponding parameters are executed: gun position height 2.0m, oxygen supply intensity 3.6Nm³ / (t·min), bottom blowing intensity 0.15Nm³ / (t·min).

[0047] State tracking and adjustment at the 10th minute: [C]≈0.15%, T≈1650℃. Model calculation. =-1.1ppm / min, at which point the denitrification efficiency is high. The system determines that it has entered the later stage and automatically adjusts the parameters to: gun position height 2.3m, oxygen supply intensity 3.0Nm³ / (t·min), and bottom blowing intensity maintained at 0.15Nm³ / (t·min).

[0048] (3) Late stage of refining / before the end point (12-15 minutes): This period is for fine-tuning and stabilization control at the end point. Real-time status at 14 minutes: [C]≈0.06%, T≈1690℃, [O]≈0.08%. =0.18atm, A=0.8m² / t, S bubble =20m -1 Due to the weakening of slag adsorption capacity at the final stage η slag A value of 0.5 significantly slows down the decarbonization reaction. Real-time calculations using a low-NOx blowing model show: NOx increase = +0.288 ppm / min, bubble denitrification = -0.0115 ppm / min, and slag-metal interface = 0.375 ppm / min. Model output predictions: =-0.0985ppm / min. (Predicted) The threshold for triggering mode C is less than 0.3, and [C] and T satisfy the endpoint conditions. The system switches to mode C, the gun position height is 2.8m, the oxygen supply intensity is 2.0Nm³ / (t·min), and the bottom blowing is switched to pulse mode (strong pulse 0.12Nm³ / (t·min), weak pulse 0.06Nm³ / (t·min), cycle 60 seconds).

[0049] (4) Endpoint hit: After blowing ends, high-carbon steel is tapped in one go without supplementary blowing. Actual endpoint measurement: carbon content 0.06%, temperature 1690℃. Based on the comprehensive calculation of furnace gas analysis and auxiliary lance detection, the endpoint nitrogen content is 17ppm. Tapping time 4.5 minutes.

[0050] 2. Refining and continuous casting processes

[0051] a. In LF refining, a CaO-Al2O3-SiO2 system slag is produced, with the basicity controlled at 5.0. The bottom-blown argon flow rate is dynamically matched to approximately 500 NL / min based on the heating power.

[0052] b. The RH vacuum treatment process is based on the RH denitrification model:

[0053] ;

[0054] Where N(t) is the real-time denitrification rate; K is the comprehensive reaction coefficient, which is related to the steel temperature and oxygen content; [N] is the real-time nitrogen content of the steel; [N] is the nitrogen content of the steel. eg Q represents the nitrogen equilibrium solubility at the current vacuum level. gas To increase the gas flow rate, V represents the steel processing capacity, and P... vac P0 is the absolute pressure of the vacuum chamber and P0 is the pressure constant.

[0055] In the 2nd minute, P vac The pressure has dropped to 500 Pa, increasing the gas flow rate Q. gas The flow rate is 130 NL / min, the real-time nitrogen content [N] in the molten steel is 23.8 ppm, the steel processing capacity is 250 t, and [N]... eg ≈2ppm. Calculated N(t)≈-1.05ppm / min, the denitrification rate is good, the system maintains the current parameters, and the goal is to reduce the vacuum level to the target value within 4 minutes.

[0056] In the 5th minute, P vac The pressure has dropped to 80 Pa, so the gas flow rate Q has been increased. gas The flow rate is 145 NL / min, the real-time nitrogen content [N] in the molten steel is 22.5 ppm, the steel processing capacity is 250 t, and [N]... eg The nitrogen concentration was approximately 0.7 ppm. Calculations showed N(t) ≈ -0.7 ppm / min, a decrease in rate compared to the second minute, indicating that the denitrification driving force weakens as the nitrogen content decreases. To maintain denitrification efficiency, the gas flow rate Q was automatically increased.gas Increase the flow rate to 150 NL / min to enhance circulation and increase the gas-liquid reaction interface.

[0057] In the 10th minute, P vac The pressure has been reduced to 65 Pa (target vacuum level has been reached and is being maintained), and the gas flow rate Q has been increased. gas The flow rate is 150 NL / min, the real-time nitrogen content [N] in the molten steel is 21.8 ppm, the steel processing capacity is 250 t, and [N]... eg The nitrogen content is approximately 0.6 ppm. Calculations show N(t) ≈ -0.52 ppm / min, indicating a further decrease in the denitrification rate. Maintaining the current parameters would result in very slow subsequent denitrification. Based on the minimum guaranteed deep degassing time logic, from this point onward, the current deep vacuum (≤67 Pa) must be maintained for at least 10 minutes (i.e., total deep degassing time ≥10 min) to overcome the mass transfer limitations in the low-nitrogen zone.

[0058] At the 13th minute, the real-time nitrogen content [N] in the molten steel was 21.3 ppm, which met the standard but was not optimal, and the denitrification rate continued to decline. Intervention was needed at this point. While ensuring no slag entrapment, the booster gas flow rate was finely adjusted to 148 NL / min, and the vacuum pump was used to remove P... vac It instantly drops to 60 Pa for 2 minutes to provide additional driving force.

[0059] Deep degassing was completed at the 16th minute. The calculated [N] was 20.9 ppm. Low-nitrogen ferrovanadium with a nitrogen content of ≤50 ppm was added for alloying fine-tuning.

[0060] At the 18th minute, the final soft measurement of molten steel [N] was 20.8 ppm. The vacuum was slowly broken, and the process was completed.

[0061] After RH vacuum treatment, the nitrogen content [N] decreased from 24.0 ppm to 20.8 ppm, a net removal of 3.2 ppm, and a denitrification efficiency of 13.3%. This successfully reduced the nitrogen content in the molten steel to below the target of ≤23 ppm, leaving a buffer for subsequent processes. Compared with traditional operations: at the same initial nitrogen content (24 ppm), the denitrification efficiency of traditional fixed-parameter operations (vacuum degree ≤100 Pa, fixed flow rate 140 NL / min, treatment time 15 min) is typically 8-10%, with an outlet nitrogen content of approximately 22.0-22.5 ppm. Within a similar treatment time, this embodiment improved the denitrification efficiency by more than 30% and achieved a lower final nitrogen content.

[0062] c. During the calcium treatment process, Ca-Si wire is fed in using a double-layer sleeve protection method. Protective argon gas is introduced into the outer layer of the sleeve. After feeding the wire, argon gas is gently blown in for 16 minutes.

[0063] d. During continuous casting, five levels of protective pouring are adopted, including long nozzle, submerged nozzle, tundish covering agent, mold flux, and air curtain baffle wall set in the tundish.

[0064] e. The water volume in the secondary cooling zone is 0.5 L / kg, and the surface temperature gradient of the billet is ≤15℃ / min.

[0065] 3. Through the above-mentioned whole-process system control, the nitrogen content of the continuous casting billet is 26ppm. After the final rolling, the nitrogen content of the finished low alloy steel strip is stable at 27ppm, which meets the quality requirement of ≤30ppm, and the surface defect rate of the billet is significantly reduced to below 0.12%.

[0066] The above embodiments are merely descriptions of preferred embodiments of the present invention and are not intended to limit the concept and scope of the present invention. Various modifications and improvements made to the technical solutions of the present invention by those skilled in the art without departing from the design concept of the present invention should fall within the protection scope of the present invention.

[0067] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A smelting method for controlling the nitrogen content of low-alloy steel plates to be below 30 ppm, characterized in that, Includes the following steps: S1. During the converter smelting process, the quality of raw materials is controlled. A dynamic oxygen supply strategy based on a low-nitrogen blowing model and full-process bottom blowing argon are used for coordinated control. High carbon pulling and one-time tapping technology is used to eliminate the need for supplementary blowing operations. The final carbon content is controlled at 0.05-0.07%, the final temperature is 1680-1700℃, and the tapping time is ≤5min. In the S2 and RH vacuum treatment process, deep denitrification is carried out under low nitrogen conditions based on the RH denitrification model; S3. Adopting composite nitrogen inhibition technology to prevent nitrogen increase during the process.

2. The smelting method for controlling the nitrogen content of low-alloy steel plates to be below 30 ppm according to claim 1, characterized in that, The raw material quality control in step S1 includes: Hot metal pretreatment: Dual-stage desulfurization and dephosphorization are adopted to control the sulfur content of hot metal to ≤0.002% and the phosphorus content to ≤0.010%, and the temperature to ≥1280℃; Scrap steel screening: Use clean scrap steel with a nitrogen content ≤50ppm and a scrap steel weight percentage ≤12%; Auxiliary material control: By mass percentage, TFe ≥ 60%, SiO2 ≤ 5.0%, and Al2O3 ≤ 1.2% in pellets or sinter.

3. The smelting method for controlling the nitrogen content of low-alloy steel plates to be below 30 ppm according to claim 1, characterized in that, The low-NOx blowing model mentioned in step S1 is as follows: ； in, The instantaneous rate of change of nitrogen content in liquid steel; This is the nitrogen addition term, representing the rate at which nitrogen in the furnace gas dissolves into the molten steel through the gas-liquid interface. k1 is the gas-liquid interface mass transfer coefficient, and A is the effective gas-liquid contact area. This refers to the nitrogen partial pressure inside the furnace; For the denitrification term, k2 represents the denitrification rate of CO bubbles carrying nitrogen out, k2 is the CO bubble denitrification efficiency coefficient, [C] and [O] are the activities of carbon and oxygen in the molten steel, respectively, and S bubble This represents the total surface area of ​​CO bubbles within a unit volume of molten steel. For the slag-gold interface term, k3 represents the mass transfer behavior of nitrogen at the slag-gold interface, k3 is the slag-gold interface mass transfer coefficient, and η is the mass transfer coefficient. slag [N] represents the adsorption or blocking efficiency of nitrogen by slag. slag These are nitrogen potential parameters related to slag.

4. The smelting method for controlling the nitrogen content of low-alloy steel plates to be below 30 ppm according to claim 3, characterized in that, The dynamic oxygen supply strategy described in step S1 is based on the instantaneous rate of change of nitrogen content in molten steel predicted by a low-nitrogen blowing model. The system automatically switches between at least three blowing modes, taking into account the carbon content and temperature of the molten steel. Model A: When the carbon content of molten steel is >0.3% and the temperature is <1550℃, if the model predicts... If the oxygen supply is greater than +0.5 ppm / min, switch to mode A, control the gun position height to 2.2-2.8 m, oxygen supply intensity to 3.0-3.5 Nm³ / (t·min), and bottom blowing argon intensity to 0.08-0.12 Nm³ / (t·min); Mode B: When the carbon content of molten steel is between 0.1% and 0.3% and the temperature is between 1550 and 1650℃, if the model predicts... If the oxygen supply is >-0.8 ppm / min, switch to Mode B, which is divided into early and late stages: in the early stage, control the gun position height at 1.8-2.2 m and the oxygen supply intensity at 3.4-3.8 Nm³ / (t·min); in the late stage, control the gun position height at 2.2-2.5 m and the oxygen supply intensity at 2.8-3.2 Nm³ / (t·min); the bottom-blowing argon intensity throughout the process is 0.13-0.17 Nm³ / (t·min). Mode C: When the carbon content of molten steel is <0.1% and the temperature is >1650℃, if the model predicts... If the oxygen supply is less than 0.3 ppm / min, switch to mode C, control the gun position height to 2.5-3.0 m, oxygen supply intensity to 1.5-2.5 Nm³ / (t·min), bottom blowing argon gas to use pulse mode, strong pulse flow rate to 0.10-0.14 Nm³ / (t·min), weak pulse flow rate to 0.04-0.08 Nm³ / (t·min), pulse period to 45-75 seconds.

5. The smelting method for controlling the nitrogen content of low-alloy steel plates to be below 30 ppm according to claim 1, characterized in that, The RH denitrification model described in step S2 is as follows: ； Where N(t) is the real-time denitrification rate; K is the comprehensive reaction coefficient, which is related to the steel temperature and oxygen content; [N] is the real-time nitrogen content of the steel; [N] is the nitrogen content of the steel. eg Q represents the nitrogen equilibrium solubility at the current vacuum level. gas To increase the gas flow rate, V represents the steel processing capacity, and P... vac P0 is the absolute pressure of the vacuum chamber and P0 is the pressure constant.

6. The smelting method for controlling the nitrogen content of low-alloy steel plates to be below 30 ppm according to claim 1, characterized in that, The composite nitrogen barrier technology mentioned in step S3 is as follows: a. In the LF refining process, CaO-Al2O3-SiO2 system slag with an alkalinity of 4.5-5.5 is used, and the bottom blowing gas flow rate is controlled within the range of 200-800 NL / min; b. During the RH vacuum treatment process, control the vacuum degree to ≤67Pa, increase the gas flow rate to 140-150NL / min, and the deep degassing time to ≥10min. When fine-tuning the alloy, use micro-alloys with N≤50ppm, V≤0.06%, and Nb≤0.04%. Nitriding alloys are prohibited. c. During the calcium treatment process, Ca-Si wire is fed in using a double-layer sleeve protection method. Protective argon gas is introduced into the outer layer of the sleeve. After feeding the wire, argon gas is gently blown in for ≥15 minutes. d. During continuous casting, five levels of protective pouring are adopted, including long nozzle, submerged nozzle, tundish covering agent, mold flux, and air curtain baffle wall set in the tundish. e. The water volume in the secondary cooling zone is 0.45-0.55 L / kg, and the surface temperature gradient of the billet is ≤15℃ / min.

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