A Dynamic Control Method for Segmented Vacuum Degree of VOD Furnace to Prevent Recarbonization and Deep Denitrification of CO2

Abstract

This application relates to the field of metallurgical vacuum refining technology, specifically a dynamic control method for segmented vacuum degree control in VOD furnaces to prevent back carbonization and achieve deep denitrification using CO2. This method divides the oxidation period into three stages based on the vacuum degree of the molten pool: a rapid denitrification stage, a continuous denitrification stage, and a deep denitrification stage. The CO2 injection flow rate and its mixing ratio with Ar are adjusted according to the vacuum degree, temperature, and steel composition. During the rapid denitrification stage, O2 is blown from the top while CO2 is blown from the bottom, activating the denitrification channel and accumulating a bubble base. During the continuous denitrification stage, when the carbon content is high, the volume multiplication effect of the CO2-carbon reaction is utilized to enhance stirring and denitrification. As the carbon content decreases, Ar is promptly introduced to assist the reaction and ensure its continuity. During the deep denitrification stage, CO2-CO microbubbles are used under high vacuum to break down interfacial barriers, and the CO2 flow rate is controlled within the limit to prevent back carbonization. Through three-stage synergistic control and model calculation, the kinetic limitations of traditional denitrification are overcome, achieving efficient and low-cost deep denitrification while preventing back carbonization in the molten steel.

Description

Technical Field

[0001] This application relates to the field of metallurgical vacuum refining technology, specifically a method for segmented control of vacuum degree in VOD furnace to prevent reverse carbonization and deep denitrification of CO2. Background Technology

[0002] The VOD (vacuum oxygen decarburization) furnace is the core ladle refining equipment for stainless steel production. It reduces the partial pressure of carbon monoxide by top-blowing oxygen decarburization and bottom-blowing argon stirring in a vacuum environment to promote the carbon-oxygen reaction. It can achieve deep decarburization while reducing the oxidation loss of chromium elements. It is a key process for producing high-end stainless steels such as ultra-low carbon and high chromium ferritic stainless steel.

[0003] The nitrogen content in molten steel directly affects the core properties of stainless steel, such as corrosion resistance and toughness, and is a key control indicator in VOD refining. Currently, VOD denitrification mainly relies on two pathways: vacuum precipitation and argon bubble carry-over. The former is limited by the nitrogen concentration boundary layer on the surface of the molten steel, resulting in a kinetic bottleneck in the denitrification rate; the latter, argon gas, is costly, and the bubble form is limited, with limited denitrification channels. Existing processes lack a refined matching mechanism between dynamic changes in vacuum degree and the denitrification medium, resulting in low overall denitrification efficiency and difficulty in achieving stable, low-cost deep denitrification.

[0004] In recent years, carbon dioxide has gradually gained attention as a green reaction medium in steelmaking. CO2 has weak oxidizing properties and can react with carbon in molten steel to produce carbon monoxide. This reaction is accompanied by a significant volume multiplication effect, possessing the potential to enhance molten pool stirring. Existing research has attempted to use CO2 for nitrogen control in smelting; for example, Chinese patent CN118835023A discloses a method for nitrogen control in the smelting of ultrapure ferritic stainless steel by injecting CO2 and carbon powder into a TSR-VOD furnace.

[0005] However, this existing technology requires additional carbon powder injection, which can easily lead to an increase in the carbon content of the molten pool, which is not conducive to the deep decarburization control of ultra-low carbon steel. At the same time, it does not recognize the dynamic regulation role of vacuum degree, a decisive parameter, on the direction of CO2 reaction and denitrification effect, and fails to propose a refined control strategy in coordination with vacuum degree. Summary of the Invention

[0006] This invention proposes a segmented dynamic control method for deep denitrification of VOD furnace vacuum degree to prevent back carbonization. The aim is to fully utilize the dynamic changes in vacuum degree to maximize the reaction stirring and denitrification channel opening effect of CO2 while suppressing back carbonization, achieving low-cost, high-efficiency deep denitrification. The method includes a rapid denitrification period, a continuous denitrification period, and a deep denitrification period. During each period, the bottom-blown CO2 injection flow rate and the mixing ratio of CO2 and Ar are dynamically adjusted based on the vacuum degree, furnace temperature, and steel composition. The method includes the following steps:

[0007] S1. Rapid denitrification period: Under the first vacuum, oxygen is blown from the top of the molten pool, while pure CO2 is blown from the bottom to remove impurities from the molten steel and activate the denitrification channel. When the carbon content in the furnace reaches the end carbon content of this stage, it enters the continuous denitrification period.

[0008] S2. Continuous Denitrification Period: The molten pool maintains top-blown oxygen decarburization under the second vacuum level, divided into two stages based on the carbon-chromium-CO equilibrium carbon content at the current vacuum level: In the first stage, when the carbon content is not lower than the equilibrium carbon content, bottom-blown pure CO2 is used to enhance molten pool stirring and denitrification; in the second stage, when the carbon content is lower than the equilibrium carbon content, a mixture of CO2 and Ar is bottom-blown, and argon is used to reduce the partial pressure of carbon monoxide to maintain the reaction while inhibiting the back carbonization of the molten steel; when the carbon content drops to the target carbon content, the deep denitrification period begins.

[0009] The final carbon content during the rapid denitrification period and the carbon-chromium-CO equilibrium carbon content during the continuous denitrification period were both calculated using the carbon-chromium-CO equilibrium formula, as follows:

[0010]

[0011] in, The temperature inside the furnace is expressed in Kelvin (K). This represents the mass fraction of carbon in the molten steel. This represents the mass fraction of chromium in the molten steel. This represents the relative partial pressure of carbon monoxide.

[0012] S3, Deep Denitrification Period: Under the third vacuum level, top-blowing oxygen is stopped in the molten pool, and a mixture of CO2 and Ar gas is bottom-blown. The maximum CO2 injection rate is controlled to prevent steel molten carburization. The calculation formula is: ,in, For the target amount of liquid metal, The target carbon mass fraction. The minimum carbon mass fraction;

[0013] Deep denitrification is achieved by using CO microbubbles generated by the reaction to overcome the interface barrier of the molten steel. After the injection is completed, the steel enters the VD refining stage.

[0014] Preferably, the first vacuum degree satisfies 5kPa≤P1<15kPa, the second vacuum degree satisfies 1kPa≤P2<5kPa, and the third vacuum degree satisfies 20Pa≤P3≤150Pa.

[0015] Preferably, the volume of molten steel in the molten pool at the end of the rapid denitrification period is the final volume of molten steel for that stage. Simultaneously, as the initial molten steel volume for the first stage of the continuous denitrification period, the final molten steel volume for the rapid denitrification period... Calculated using the following material balance formula:

[0016]

[0017] in, This refers to the amount of molten metal entering the furnace. This refers to the amount of alloy added. The target amount of liquid metal; The final carbon mass fraction during the rapid denitrification period; For the molten metal entering the furnace , , , The sum of mass fractions; In molten steel , , The sum of mass fractions;

[0018] Total CO2 consumption during the rapid denitrification period Calculate using the following formula:

[0019]

[0020] in, These are the stoichiometric coefficients of each element in its reaction with CO2;

[0021] Total O2 consumption during the rapid denitrification period Calculate using the following formula:

[0022]

[0023] in, These are the stoichiometric coefficients of each element in its reaction with O2; express , , , element.

[0024] Preferably, the volume of molten steel in the molten pool at the end of the first stage of the continuous denitrification period is the volume of molten steel at the end of that stage. Simultaneously, as the initial molten steel volume for the second stage, the final molten steel volume for the first stage of the continuous denitrification period... The formula is as follows:

[0025]

[0026] in, The carbon mass fraction in the carbon-chromium-CO equilibrium of the first stage; Indicates the target carbon mass fraction;

[0027] Phase 1 Total CO2 Consumption Calculate using the following formula:

[0028] ;

[0029] Total O2 consumption in Phase 1 Calculate using the following formula:

[0030] .

[0031] Preferably, the total CO2 consumption in the second stage of the continuous denitrification period Calculate using the following formula:

[0032] ;

[0033] Total O2 consumption in the second phase Calculate using the following formula:

[0034] .

[0035] Preferably, the flow ratio coefficient of argon to CO2 is... Calculated using the following formula:

[0036]

[0037] in, The total pressure inside the furnace during the continuous denitrification period; Standard atmospheric pressure; This refers to the CO2 consumption in the second stage; This represents the O2 consumption in the second stage.

[0038] Preferably, the gas injection flow rate for the rapid denitrification period, the continuous denitrification period, and the deep denitrification period is the ratio of the total gas consumption for the corresponding stage to the set smelting time, calculated using the following formula:

[0039] Rapid denitrification period: , ;

[0040] Phase 1 of continuous denitrification: , ;

[0041] Phase 2 of continuous denitrification: , ; ;

[0042] Deep denitrification period: , ;

[0043] in, The smelting time for the rapid denitrification period is set at 25–35 minutes; The smelting time for the first stage of the continuous denitrification period is set at 1–5 minutes; The smelting time for the second stage of the continuous denitrification period is set at 8–12 minutes; The smelting time for the deep denitrification period is set at 10–15 minutes; This represents the flow ratio coefficient of Ar to CO2 during the deep denitrification period.

[0044] Preferably, the , , , The stoichiometric coefficients of the elements reacting with CO2 are as follows: ;

[0045] The , , , The stoichiometric coefficients of the elements reacting with O2 are as follows: .

[0046] Preferably, the minimum carbon mass fraction is... The value range is 0.002% to 0.003%, and the flow ratio coefficient of Ar to CO2 during the deep denitrification period is... The value range is 1 to 3.

[0047] The VD refining stage that follows the deep denitrification period is as follows: maintaining bottom blowing argon gas, adding deoxidizer and slag-forming agent to the molten pool after temperature measurement and sampling to adjust the steel composition, and the smelting time of the VD stage is 10~15min; the deoxidizer is aluminum granules, and the slag-forming agent is one or a mixture of two of lime and fluorite.

[0048] The beneficial effects of this invention are as follows:

[0049] 1. Based on the evolution law of vacuum degree, this invention divides the denitrification process into three progressively functional stages, forming a denitrification mechanism of channel activation, continuous expansion, and interface breakthrough. In the rapid denitrification stage, the oxidation reaction of CO2 with impurities such as Si and Mn is used to generate a large number of initial bubbles while removing impurities, which activates the denitrification interface of the molten steel in advance and accumulates the bubble base, effectively improving the initial denitrification rate. In the continuous denitrification stage, the volume multiplication effect of the reaction between CO2 and carbon is used to increase the stirring intensity of the molten pool, break the mass transfer resistance of nitrogen elements inside the molten steel, and continuously expand the denitrification channel to achieve stable and efficient continuous denitrification. In the deep denitrification stage, under high vacuum environment, the diffuse CO microbubbles generated by the reaction of CO2 are used to break through the nitrogen concentration boundary layer on the surface of the molten steel, solving the problem of the sharp drop in denitrification rate under high vacuum in traditional processes.

[0050] 2. Using the carbon-chromium-CO thermodynamic equilibrium formula as the core criterion, precise boundary thresholds are set for carbon content at each stage, achieving dual control to prevent carbon backflow and preserve chromium. During the rapid and continuous denitrification periods, the critical point of carbon content under corresponding vacuum is quantitatively calculated using the equilibrium formula, thereby dividing the stage switching nodes to ensure that the reaction between CO2 and carbon is always in the thermodynamically positive range, avoiding a rise in carbon content. In the low-carbon range of the later stage of continuous denitrification, the partial pressure of carbon monoxide is precisely controlled by argon dilution to maintain the continuous CO2 reaction while preventing excessive oxidation of chromium. During the deep denitrification period, the maximum CO2 injection rate is constrained by the minimum carbon content limit, and the upper limit of gas injection is locked by the amount of consumable carbon, avoiding the risk of carbon backflow in the deep denitrification stage.

[0051] 3. Using low-cost CO2 as the denitrification and stirring medium replaces the expensive argon gas in traditional processes. Furthermore, the CO gas generated from the reaction of CO2 and carbon has a natural volume multiplication effect, resulting in significantly higher stirring intensity in the molten pool at the same gas flow rate compared to pure argon. This allows for superior stirring and denitrification effects with lower total gas consumption, further reducing operating costs. In addition, utilizing industrial by-product CO2 in the steelmaking denitrification process can reduce industrial carbon emissions, aligning with the steel industry's green and low-carbon transformation development direction.

[0052] 4. The gas flow rate and stage nodes throughout the entire process are quantitatively calculated using thermodynamic formulas and material balance models, eliminating the need for operator experience and ensuring effective replicability. Furthermore, the entire method can be directly implemented within existing steelmaking control systems: raw material data is automatically acquired by the data acquisition system, gas parameters are solved in real-time by the calculation system, and stage switching is determined by the linkage between waste gas analysis and temperature measurement systems, achieving fully automated dynamic control of the entire process. For different furnace compositions and steel grades, only the input parameters need to be adjusted to automatically match the optimal process scheme, effectively reducing composition fluctuations caused by manual intervention and improving the stability of the smelting cycle and product quality. Attached Figure Description

[0053] Figure 1 This is a flowchart of the segmented CO2 vacuum control process for the VOD furnace of the present invention; Detailed Implementation

[0054] The application principle of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0055] Example 1:

[0056] like Figure 1As shown, a method for segmented vacuum control of VOD furnace to prevent back carbonization and deep denitrification using CO2 is proposed. The VOD refining oxidation period is divided into rapid denitrification, continuous denitrification, and deep denitrification periods according to the vacuum level of the molten pool, from high to low. The bottom-blown CO2 injection flow rate and the mixing ratio of CO2 and Ar are dynamically adjusted based on the vacuum level, furnace temperature, and steel composition. The method includes the following steps:

[0057] S1. Rapid Denitrification Period: When the vacuum degree of the molten pool reaches 5kPa≤P1<15kPa, the rapid denitrification period begins. During this stage, top-blown oxygen is used for decarburization and impurity removal, while bottom-blown pure CO2 is used to react with the molten steel. , , The reaction of impurities and carbon activates the denitrification channels in advance and accumulates the bubble base, thus accelerating the initial denitrification rate.

[0058] This stage first considers the current vacuum level P1 and the furnace temperature. The chromium content of the molten metal entering the furnace is calculated using the carbon-chromium-CO balance formula to obtain the final carbon content during the rapid denitrification period. Then, combining the composition and mass of the molten metal entering the furnace, the alloy composition and amount added, and the composition and mass of the target molten steel, the final amount of molten steel at this stage is calculated through material balance. The total consumption and injection flow rate of O2 and CO2, and the carbon content in the furnace after the gas injection is completed. At this time, it enters the continuous denitrification period;

[0059] The carbon-chromium-CO equilibrium formula is as follows:

[0060]

[0061] in, The temperature inside the furnace is expressed in Kelvin (K). This represents the mass fraction of carbon in the molten steel. This represents the mass fraction of chromium in the molten steel. The formula, which represents the relative partial pressure of carbon monoxide, is a quantitative expression of the carbon-chromium-CO thermodynamic equilibrium. It serves as the theoretical basis for determining the carbon content boundaries at each stage and achieving chromium conservation and anti-carbonization control.

[0062] The volume of molten steel in the molten pool at the end of the rapid denitrification period is the final volume of molten steel for this stage. Simultaneously, it serves as the initial molten steel volume for the first stage of the continuous denitrification period and the final molten steel volume for the rapid denitrification period. Calculated using the following material balance formula:

[0063]

[0064] in, This refers to the amount of molten metal entering the furnace; This refers to the amount of alloy added. The target amount of liquid metal; The final carbon mass fraction during the rapid denitrification period; For the molten metal entering the furnace , , , The sum of mass fractions; In molten steel , , The formula is based on the conservation of element mass. During the rapid denitrification period, Si, Mn, and P elements can be oxidized to the target content. Therefore, by subtracting the target remaining amount and carbon remaining amount from the total amount of elements brought in by the initial material, the total amount of elements consumed can be obtained, and then the total mass of molten steel at the end of the stage can be calculated.

[0065] Total CO2 consumption during the rapid denitrification period Calculate using the following formula:

[0066]

[0067] in, These are the stoichiometric coefficients of each element in its reaction with CO2;

[0068] Total O2 consumption during the rapid denitrification period Calculate using the following formula:

[0069]

[0070] in, These are the stoichiometric coefficients of each element in its reaction with O2; express , , , Elements; The above formula is derived based on the material balance of the oxidation reaction: it is assumed that 20% of the consumed elements react with CO2 and 80% react with O2. Combining the reaction stoichiometry of each element, the CO2 utilization rate (80%), and converting the molar mass to the standard molar volume, the coefficients are combined to obtain the above expression; the coefficients 127.4 and 560.4 in the formula are obtained by combining the above parameters.

[0071] S2. Continuous Denitrification Period: When the vacuum degree of the molten pool reaches 1kPa≤P2<5kPa, the continuous denitrification period begins. During this stage, top-blown oxygen continues to decarbonize, maintaining the carbon-chromium-CO balance carbon content at the current vacuum degree. Divided into two stages, with the boundary as the dividing line:

[0072] Phase 1: The carbon content of the molten pool is not lower than the equilibrium carbon content. At this stage, pure CO2 is blown in from the bottom. Due to the high carbon content at this stage, CO2 can directly and continuously react with the carbon in the molten pool. The volume multiplication effect of the CO generated by the reaction enhances the stirring of the molten pool, achieving stable and continuous denitrification. Based on the initial carbon content, the final equilibrium carbon content, and the corresponding steel volume, the total consumption of O2 and CO2 and the injection flow rate are calculated for this stage. Once the carbon content decreases to... Then, the second stage begins;

[0073] The volume of molten steel in the molten pool at the end of the first stage of the continuous denitrification period is the final volume of molten steel for that stage. Simultaneously, it serves as the initial molten steel volume for the second stage and the final molten steel volume for the first stage of the continuous denitrification period. The formula is as follows:

[0074]

[0075] in, The carbon mass fraction in the carbon-chromium-CO equilibrium of the first stage; This represents the target carbon mass fraction. After the rapid denitrification period, the Si, Mn, and P elements have basically reached the target content, and the oxidation loss in subsequent stages is negligible. Therefore, the final steel volume at the end of the first stage can be deduced from the difference between the target steel output and the carbon content. ;

[0076] Phase 1 Total CO2 Consumption Calculate using the following formula:

[0077]

[0078] Total O2 consumption in Phase 1 Calculate using the following formula:

[0079]

[0080] During the continuous denitrification period, only the oxidation consumption of carbon is considered. The coefficients 467.0 and 747.1 are obtained by combining the carbon reaction stoichiometry, reaction ratio, gas utilization rate and unit conversion.

[0081] Phase 2: Molten pool carbon content is lower than equilibrium carbon content At this stage, a mixture of CO2 and Ar is blown in from the bottom. The carbon content is low at this stage, and the equilibrium partial pressure of CO in a pure CO2 atmosphere is insufficient to maintain the forward reaction, easily leading to carbonization. By introducing argon to dilute the gas phase and reduce the partial pressure of carbon monoxide, the reaction between CO2 and carbon can be sustained, while simultaneously inhibiting carbonization in the molten steel and ensuring chromium retention. Based on the initial carbon content, target carbon content, and corresponding molten steel volume, the total consumption of O2 and CO2 and the argon mixing ratio for this stage are calculated. Once the carbon content drops to the target carbon content... At this time, it enters the deep denitrification period;

[0082] Total CO2 consumption during the second stage of continuous denitrification Calculate using the following formula:

[0083] ;

[0084] Total O2 consumption in the second phase Calculate using the following formula:

[0085] .

[0086] S3, Deep Denitrification Period: When the vacuum level of the molten pool reaches 20Pa≤P3<150Pa, the deep denitrification period begins. At this time, top-blowing oxygen is stopped, and a mixture of CO2 and Ar is blown from the bottom. Under high vacuum, the CO microbubbles generated by the reaction of CO2 and carbon can overcome the nitrogen concentration boundary layer barrier at the steel interface, achieving deep denitrification. Simultaneously, to prevent steel recarburization, the maximum allowable CO2 injection rate is calculated based on the difference between the target carbon content and the minimum allowable carbon content. By controlling the CO2 injection intensity, the carbon content of the molten steel is prevented from rising excessively. After gas injection is completed, the subsequent VD refining stage begins. The formula for calculating the maximum allowable CO2 consumption is:

[0087]

[0088] in, This represents the maximum permissible CO2 consumption during the deep denitrification period, expressed in Nm³. 3 , For the target amount of liquid metal, The target carbon mass fraction. The minimum carbon mass fraction is defined as 0.0025%. This formula, by limiting the maximum amount of carbon that can be consumed, constrains the upper limit of CO2 injection at the source and serves as the calculation basis for preventing carbon reversion during deep denitrification.

[0089] In this embodiment, the flow rate ratio coefficient of argon to CO2 is... Based on Dalton's law of voltage division, the calculation formula is as follows:

[0090]

[0091] in, The total pressure inside the furnace during the continuous denitrification period; Standard atmospheric pressure, taken as 100 kPa; This represents the CO2 consumption in the second stage. This represents the O2 consumption in the second stage. This formula, through the quantitative relationship between gas phase composition and partial pressure, calculates the argon dilution ratio required to maintain the CO2 reaction. It serves as the basis for calculations to ensure the continuous CO2 reaction under low carbon content and to inhibit carbon regeneration.

[0092] In this embodiment, the gas injection flow rate during the rapid denitrification period, the continuous denitrification period, and the deep denitrification period is the ratio of the total gas consumption in the corresponding stage to the set smelting time, and the calculation formula is as follows:

[0093] Rapid denitrification period: , ;

[0094] Phase 1 of continuous denitrification: , ;

[0095] Phase 2 of continuous denitrification: , ; ;

[0096] Deep denitrification period: , ;

[0097] in, The smelting time for the rapid denitrification period is set at 30 minutes; The smelting time for the first stage of the continuous denitrification period is set at 3 minutes; The smelting time for the second stage of the continuous denitrification period is set at 10 minutes; The smelting time for the deep denitrification period is set at 13 minutes; The flow ratio coefficient of Ar to CO2 during the deep denitrification period is 2.

[0098] In this embodiment, the stoichiometric coefficients of each element reacting with CO2 are as follows: stoichiometric coefficient of carbon reacting with CO2 ; stoichiometric coefficient of the reaction between silicon and CO2 The stoichiometric coefficient of the reaction between manganese and CO2 ; Stoichiometric coefficient of the reaction between phosphorus and CO2 ;

[0099] The stoichiometric coefficients for the O2 reaction are as follows: Stoichiometric coefficients for the reaction between carbon and O2 ; stoichiometric coefficient of the reaction between silicon and O2 The stoichiometric coefficient of the reaction between manganese and O2 ; Stoichiometric coefficient of phosphorus reaction with O2 .

[0100] After the deep denitrification period, the VD refining stage begins: bottom blowing argon is maintained, and after temperature measurement and sampling, deoxidizer and slag-forming agent are added according to the composition of the molten pool to adjust the composition of the molten steel and the slag system; the smelting time of the VD stage is 13 minutes; the deoxidizer is aluminum granules, and the slag-forming agent is one or a mixture of two of lime and fluorite.

[0101] This method relies on the steelmaking control system to achieve automated operation: the composition and quality of the molten metal entering the furnace, and the composition and amount of alloy added are obtained by the raw material acquisition system of the steelmaking control system; the gas consumption and injection flow rate at each stage are calculated by the data calculation system of the steelmaking control system; the temperature of the molten pool is measured in real time by the temperature measurement and sampling system of the steelmaking control system; and the carbon content in the furnace is monitored online by the waste gas analysis system, serving as the basis for determining the stage switching.

[0102] Example 2:

[0103] This invention is applied to the refining process of an 80t VOD furnace, producing 409 stainless steel. The raw materials are primary stainless steel molten steel and ordinary ferrosilicon. The nitrogen content of the 409 stainless steel before refining is 0.03%, calculated first... , The vacuum degree during the rapid denitrification period was set to 12 kPa, and the smelting time t1 was set to 30 min. The carbon content in the furnace reached... At this point, the continuous denitrification period begins, with the vacuum level set at 3.5 kPa. The first stage of the continuous denitrification period has a smelting time t2 of 1 minute, during which the carbon content in the furnace reaches... The process then enters the second stage of the continuous denitrification period, where the smelting time t3 is set at 12 minutes, until the carbon content reaches the target carbon content. At this time, the deep denitrification period begins, with the vacuum level set at 100 Pa and the smelting time t4 set at 10 min. The carbon content... It is 0.002%. The value is 2. The specific smelting steps are as follows:

[0104] The composition of primary molten steel and ordinary ferrosilicon was obtained by the raw material acquisition system in the steelmaking control system, as shown in Tables 1 and 2. The weight of primary molten steel was 71.6t, and the weight of ordinary ferrosilicon was 0.4t. The preset target composition is shown in Table 3, and the target steel output was 70.947t.

[0105] Table 1 Composition of primary steel

[0106] Element C Si Mn P Cr Fe content 0.3% 0.12% 0.28% 0.012% 11.4% 88.168%

[0107] Table 2 Composition of ordinary ferrosilicon

[0108] Element C Si Mn P Fe content 0.1% 75% 0.5% 0.004% 24.396%

[0109] Table 3 Target Components

[0110] Element C Si Mn P Cr Fe content 0.004% 0.001% 0.21% 0.01% 11.4% 88.375%

[0111] S1, 0~30min, rapid denitrification period, the O2 flow rate during this stage was calculated to be 13.50 Nm³ by the data calculation system.3 / min, CO2 flow rate is 8.54 Nm 3 / min, after the carbon dioxide and oxygen injection is completed, the carbon content in the furnace reaches 0.023%, and the continuous denitrification period begins.

[0112] S21, 30~31min, the first stage of continuous denitrification, the O2 flow rate of the first stage of continuous denitrification calculated by the data calculation system is 8.57 Nm³. 3 / min, CO2 flow rate is 5.35 Nm 3 / min, after the carbon dioxide and oxygen injection is completed, the carbon content in the furnace reaches 0.007%, and enters the second stage of continuous denitrification;

[0113] During the continuous denitrification period (S22, 31~43 min), the O2 flow rate during stage D2 was calculated to be 0.12 Nm³. 3 / min, CO2 flow rate is 0.08 Nm 3 / min, Ar flow rate is 0.23 Nm 3 After the carbon dioxide, oxygen, and argon gases are injected, the carbon content in the furnace reaches 0.004% per minute, and the furnace enters the deep denitrification period.

[0114] S3, 43~53 min, deep denitrification period, the calculated CO2 flow rate for this stage is 0.07 Nm³. 3 / min, Ar flow rate is 0.13Nm 3 / min, after the carbon dioxide and argon gas are completely injected, the VD period begins.

[0115] After 53 minutes, the VD period begins. After temperature measurement and sampling, the nitrogen content is 0.002%, and the denitrification rate reaches 93.3%, achieving stable deep denitrification, which significantly improves the low-temperature toughness and intergranular corrosion resistance of 409 stainless steel.

[0116] Finally, 100 kg of aluminum granules, 2000 kg of lime and 600 kg of fluorite were added, and pure Ar was injected for smelting.

[0117] Example 3:

[0118] This invention is applied to the refining process of an 80t VOD furnace, producing 439 stainless steel. The raw materials are primary stainless steel molten steel and ordinary ferrosilicon. The nitrogen content of the 439 stainless steel before refining is 0.02%. First, the nitrogen content is calculated... , The vacuum level during the rapid denitrification period was set to 15 kPa, and the smelting time t1 was set to 27 min. The carbon content in the furnace reached... At this point, the continuous denitrification period begins, with the vacuum level set at 1.5 kPa. The first stage of the continuous denitrification period has a smelting time t2 of 3 minutes, during which the carbon content in the furnace reaches... The process then enters the second stage of the continuous denitrification period, where the smelting time t3 is set to 10 minutes, until the carbon content reaches the target carbon content. At this time, the deep denitrification period begins, with the vacuum level set at 150 Pa and the smelting time t4 set at 15 min. The carbon content... It is 0.003%. The value is 3. The specific smelting steps are as follows:

[0119] The composition of primary molten steel and ordinary ferrosilicon was obtained by the raw material acquisition system in the steelmaking control system, as shown in Tables 1 and 2. The weight of primary molten steel was 75.7t, and the weight of ordinary ferrosilicon was 0.4t. The preset target composition is shown in Table 3, and the target steel output was 75.1t.

[0120] Table 1 Composition of primary steel

[0121] Element C Si Mn P Cr Fe content 0.3% 0.06% 0.24% 0.014% 17.5% 81.886%

[0122] Table 2 Composition of ordinary ferrosilicon

[0123] Element C Si Mn P Fe content 0.1% 75% 0.5% 0.04% 24.36%

[0124] Table 3 Target Components

[0125] Element C Si Mn P Cr Fe content 0.005% 0.01% 0.19% 0.013% 17.5% 82.295%

[0126] S1, 0~27min, rapid denitrification period, the O2 flow rate during this stage was calculated to be 13.14 Nm³ by the data calculation system. 3 / min, CO2 flow rate is 8.30 Nm 3 The carbon content in the furnace reaches 0.077% per minute, and the furnace enters the continuous denitrification period.

[0127] During the continuous denitrification period (S21, 27-30 min), stage D1, the O2 flow rate for the first stage of continuous denitrification, calculated by the data calculation system, was 12.93 Nm³. 3 / min, CO2 flow rate is 8.08 Nm 3 / min;

[0128] S22, 30~40min, the carbon content in the furnace reaches 0.008%, entering the second stage of continuous denitrification. During this second stage, the O2 flow rate, calculated by the data calculation system, is 0.17 Nm³. 3 / min, CO2 flow rate is 0.10 Nm 3 / min, Ar flow rate is 0.28 Nm 3The carbon content in the furnace reaches 0.005% at a rate of 0.005% per minute, indicating the start of the deep denitrification period.

[0129] S3, 40~55min, deep denitrification period, the calculated CO2 flow rate for this stage is 0.05Nm³. 3 / min, Ar flow rate is 0.14 Nm 3 / min, after the carbon dioxide and argon gas are completely injected, the VD period begins.

[0130] After 55 minutes, the VD period begins. After temperature measurement and sampling, the nitrogen content is 0.001%, and the denitrification rate reaches 95%, achieving stable deep denitrification, which significantly improves the low-temperature toughness and intergranular corrosion resistance of 439 stainless steel.

[0131] Add 150 kg of aluminum granules, 2100 kg of lime and 620 kg of fluorite, and inject pure Ar for smelting.

Claims

1. A method for segmented vacuum degree control in a VOD furnace to prevent reverse carbonization and deep denitrification of CO2, characterized in that: This includes a rapid denitrification period, a continuous denitrification period, and a deep denitrification period. During each period, the bottom-blown CO2 injection flow rate and the mixing ratio of CO2 and Ar are dynamically adjusted based on the vacuum level, furnace temperature, and molten steel composition. The process includes the following steps: S1. Rapid denitrification period: Under the first vacuum, oxygen is blown from the top of the molten pool, while pure CO2 is blown from the bottom to remove impurities from the molten steel and activate the denitrification channel. When the carbon content in the furnace reaches the end carbon content of this stage, it enters the continuous denitrification period. S2. Continuous Denitrification Period: The molten pool maintains top-blown oxygen decarburization under the second vacuum level, divided into two stages based on the carbon-chromium-CO equilibrium carbon content at the current vacuum level: In the first stage, when the carbon content is not lower than the equilibrium carbon content, bottom-blown pure CO2 is used to enhance molten pool stirring and denitrification; in the second stage, when the carbon content is lower than the equilibrium carbon content, a mixture of CO2 and Ar is bottom-blown, and argon is used to reduce the partial pressure of carbon monoxide to maintain the reaction while inhibiting the back carbonization of the molten steel; when the carbon content drops to the target carbon content, the deep denitrification period begins. The final carbon content during the rapid denitrification period and the carbon-chromium-CO equilibrium carbon content during the continuous denitrification period were both calculated using the carbon-chromium-CO equilibrium formula, as follows: in, The temperature inside the furnace is expressed in Kelvin (K). This represents the mass fraction of carbon in the molten steel. This represents the mass fraction of chromium in the molten steel. This represents the relative partial pressure of carbon monoxide. S3, Deep Denitrification Period: Under the third vacuum level, top-blowing oxygen is stopped in the molten pool, and a mixture of CO2 and Ar gas is blown from the bottom. The maximum CO2 injection rate is controlled to prevent steel molten carburization. The calculation formula is: ,in, For the target amount of liquid metal, The target carbon mass fraction. The minimum carbon mass fraction; Deep denitrification is achieved by using CO microbubbles generated by the reaction to overcome the interface barrier of the molten steel. After the injection is completed, the steel enters the VD refining stage.

2. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 1, characterized in that: The first vacuum degree satisfies 5kPa≤P1<15kPa, the second vacuum degree satisfies 1kPa≤P2<5kPa, and the third vacuum degree satisfies 20Pa≤P3≤150Pa.

3. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 1, characterized in that: The volume of molten steel in the molten pool at the end of the rapid denitrification period is the final volume of molten steel for that stage. Simultaneously, as the initial molten steel volume for the first stage of the continuous denitrification period, the final molten steel volume for the rapid denitrification period... Calculated using the following material balance formula: in, This refers to the amount of molten metal entering the furnace. This refers to the amount of alloy added. The target amount of liquid metal; The final carbon mass fraction during the rapid denitrification period; For the molten metal entering the furnace , , , The sum of mass fractions; In molten steel , , The sum of mass fractions; Total CO2 consumption during the rapid denitrification period Calculate using the following formula: in, These are the stoichiometric coefficients of each element in its reaction with CO2; Total O2 consumption during the rapid denitrification period Calculate using the following formula: in, These are the stoichiometric coefficients of each element in its reaction with O2; express , , , element.

4. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 1, characterized in that: The volume of molten steel in the molten pool at the end of the first stage of the continuous denitrification period is the volume of molten steel at the end of that stage. Simultaneously serving as the initial and final molten steel volume for the second stage. The formula is as follows: in, The carbon mass fraction in the carbon-chromium-CO equilibrium of the first stage; Indicates the target carbon mass fraction; Phase 1 Total CO2 Consumption Calculate using the following formula: ； Total O2 consumption in Phase 1 Calculate using the following formula: 。 5. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 4, characterized in that: Total CO2 consumption during the second stage of the continuous denitrification period Calculate using the following formula: ； Total O2 consumption in the second phase Calculate using the following formula: 。 6. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 5, characterized in that: The flow ratio coefficient of argon to CO2 Calculated using the following formula: in, The total pressure inside the furnace during the continuous denitrification period; Standard atmospheric pressure; This represents the CO2 consumption in the second stage. This represents the O2 consumption in the second stage.

7. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 1, characterized in that: The gas injection flow rates for the rapid denitrification period, continuous denitrification period, and deep denitrification period are the ratios of the total gas consumption for the corresponding stage to the set smelting time, calculated using the following formula: Rapid denitrification period: , ; Phase 1 of continuous denitrification: , ; Phase 2 of continuous denitrification: , ; ; Deep denitrification period: , ; in, The smelting time for the rapid denitrification period is set at 25–35 minutes; The smelting time for the first stage of the continuous denitrification period is set at 1–5 minutes; The smelting time for the second stage of the continuous denitrification period is set at 8–12 minutes; The smelting time for the deep denitrification period is set at 10–15 minutes; This represents the flow ratio coefficient of Ar to CO2 during the deep denitrification period.

8. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 3, characterized in that: The , , , The stoichiometric coefficients of the elements reacting with CO2 are as follows: ; The , , , The stoichiometric coefficients of the elements reacting with O2 are as follows: .

9. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 7, characterized in that: The minimum carbon mass fraction The value range is 0.002% to 0.003%, and the flow ratio coefficient of Ar to CO2 during the deep denitrification period is... The value range is 1 to 3.

10. The method for segmented vacuum degree control of VOD furnace to prevent reverse carbonization and deep denitrification of CO2 according to claim 1, characterized in that: The VD refining stage that follows the deep denitrification period is as follows: maintaining bottom blowing argon gas, adding deoxidizer and slag-forming agent to the molten pool after temperature measurement and sampling to adjust the steel composition, and the smelting time of the VD stage is 10~15min; the deoxidizer is aluminum granules, and the slag-forming agent is one or a mixture of two of lime and fluorite.

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