A method for controlling nitrogen content of molten steel in a whole process of steelmaking

Abstract

The application discloses a kind of whole-process molten steel nitrogen content control methods of steelmaking, belong to steel metallurgy technical field, comprising: S1, in raw material control process, control the proportion of molten iron into furnace is not less than 78%, and the nitrogen content of scrap steel, auxiliary material and alloy is detected and screened;S2, in converter blowing process, nitrogen to argon bottom blowing gas switching is completed when blowing to 8 minutes, and control the end point of blowing is micro-positive pressure state in furnace;S3, in converter tapping process, before tapping, ladle is replaced by argon purging, and control the form of steel flow in the process of tapping;S4, in LF refining process, by controlling power supply mode, form foaming slag buried arc, and control ladle bottom argon flow and furnace pressure;S5, in continuous casting casting process, molten steel is fully sealed protection casting, block molten steel and atmosphere contact.The application is coordinated and optimized by multiple processes, and then solve the problem of abnormal nitrogen increase of molten steel in the prior art in whole process, nitrogen content exceeds the standard.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and in particular relates to a method for controlling the nitrogen content of molten steel throughout the entire steelmaking process. Background Technology

[0002] In the steelmaking process, nitrogen has a dual impact on steel properties. For most plain carbon steels and low-alloy steels, nitrogen is a harmful element, causing age embrittlement and reducing plasticity and toughness. In titanium-based microalloyed steels, nitrogen combines with titanium to form titanium nitride, weakening the solid solution strengthening effect of titanium and resulting in substandard steel performance. Therefore, precisely controlling the nitrogen content in molten steel is crucial for producing high-quality steel.

[0003] In existing technologies, multiple nitrogen-enhancing stages exist throughout the steelmaking process, leading to excessive nitrogen content in molten steel. The main problems include: During the converter blowing process, the timing of nitrogen-argon switching is unreasonable, and the subsequent blowing and supplementary blowing operations are frequent, which leads to nitrogen increase due to contact between molten steel and air.

[0004] During the tapping process, residual air in the ladle, the spillage of the tapping steel stream, and the long contact time between the molten steel and the atmosphere cause severe nitrogen absorption.

[0005] During the LF refining process, nitrogen atoms are generated by the high-temperature ionization of air by the electric arc, the excessive flow of bottom-blown argon blows open the slag layer, and air is drawn in by the negative pressure inside the furnace. These are all major sources of nitrogen increase.

[0006] During continuous casting, if protective measures are not in place, secondary oxidation and nitrogen absorption occur in the molten steel in the tundish and crystallizer.

[0007] Traditional nitrogen control methods often optimize single processes and lack a systematic control plan for the entire process, resulting in unstable nitrogen control effects and difficulty in meeting the production requirements of high-end steel. Summary of the Invention

[0008] To address the above-mentioned technical deficiencies, this invention provides a method for controlling the nitrogen content of molten steel throughout the entire steelmaking process. By coordinating and optimizing multiple processes, this method solves the problems of abnormal nitrogen increase and excessive nitrogen content in molten steel throughout the entire process in existing technologies.

[0009] To achieve the above-mentioned technical objectives, the present invention is implemented through the following technical solution: A method for controlling nitrogen content in molten steel throughout the entire steelmaking process includes: S1. During the raw material control process, the proportion of molten iron entering the furnace shall be controlled to be no less than 78%, and the nitrogen content of scrap steel, auxiliary materials and alloys shall be tested and screened. S2. During the converter blowing process, the bottom blowing gas switch from nitrogen to argon is completed at 8 minutes of blowing, and the furnace is controlled to be in a slightly positive pressure state at the end of the blowing. S3. During the converter tapping process, the ladle is purged and replaced with argon gas before tapping, and the shape of the steel flow is controlled during the tapping process. S4. During the LF refining process, foam slag submerged arc is formed by controlling the power supply mode, and the bottom blowing argon flow rate of the ladle and the pressure inside the furnace are controlled. S5. During the continuous casting process, the molten steel is fully sealed and protected during casting to prevent it from coming into contact with the atmosphere.

[0010] Preferably, in S1, when producing titanium-containing steel, TiFe30 is used instead of TiFe70 as the titanium source.

[0011] Preferably, in S1, the amount of scrap steel added per ton does not exceed 230 kg.

[0012] Preferably, in S2, the bottom blowing intensity of the converter is controlled between 0.042 m³ / min·t and 0.07 m³ / min·t.

[0013] Preferably, in S3, the argon purging time is not less than 1 minute.

[0014] Preferably, the steel flow during the tapping process is in a rounded and compact state, and the tapping time is controlled to be between 5 and 7 minutes.

[0015] Preferably, in S3, the dissolved oxygen content during tapping is adjusted according to the steel grade requirements, specifically: The dissolved oxygen of ultra-low carbon steel is controlled at 600×10. -6 Up to 700×10 -6 ; The dissolved oxygen in cold-rolled silicon steel is controlled at 700×10⁻⁶. -6 Up to 900×10 -6 .

[0016] Preferably, in S4, the bottom-blown argon flow rate of the ladle is controlled between 0.19 Nm³ / min and 0.40 Nm³ / min, and the furnace is kept under a slightly positive pressure.

[0017] Preferably, in S5, the fully sealed protective casting includes: purging the tundish with argon gas before casting, covering the tundish with a cover agent, and sealing the long gate and immersion gate with argon gas.

[0018] Preferably, it is used for smelting plain carbon steel, low alloy steel, cold-drawn tube steel, cold-rolled silicon steel, and titanium-based microalloyed reinforced steel.

[0019] The advantages and technical effects of this invention are: After applying the above-mentioned technical solution of this invention, the overall nitrogen increase in the converter blowing and tapping processes is reduced by 7 ppm; the nitrogen increase in the LF refining process is reduced by 10 ppm. While the industry standard for tapping nitrogen content is controlled at 24 ppm, this invention can stably control the tapping nitrogen content below 21 ppm; the final nitrogen content at the converter tapping point is stably controlled below 15 ppm. Under mass production conditions, the nitrogen content of plain carbon steel and low-carbon steel is controlled below 40 ppm, and the nitrogen content of low-alloy steel is controlled below 50 ppm; the proportion of heats with nitrogen content > 50 ppm is less than 10%, and the proportion in the optimal production month is only 2%.

[0020] This invention effectively eliminates the aging defects and decreased plasticity and toughness caused by nitrogen, ensures that the strengthening effect of titanium in titanium-reinforced steel is fully exerted, significantly improves the stability of the mechanical properties of steel, greatly reduces the number of product downgrades, re-judgments, and scrapped batches caused by excessive gas content and unsuitable performance, and significantly improves the steel yield.

[0021] This invention effectively inhibits nitrogen absorption in molten steel, reduces the ineffective consumption of microalloyed alloys such as ferro-titanium, and lowers the cost of auxiliary materials procurement and use.

[0022] This invention is highly versatile and practical, and is compatible with a wide range of steel grades, including plain carbon steel Q235B, low alloy steel Q355B, Q345R, cold-rolled silicon steel DW60, weathering steel Q450NQR1, and B18 cold-drawn tube steel. The process modification can be completed using existing steelmaking equipment without the need for large-scale equipment upgrades. It is simple to operate, easy to standardize and promote, and can be directly applied to traditional converter-continuous casting long-process steel production lines.

[0023] The present invention features high process stability, standardized operation throughout the entire process, reduced nitrogen control fluctuations caused by human error, and reduced the number of abnormal nitrogen-increasing furnaces. At the same time, it optimizes basic processes such as slag, bottom blowing, and power supply, thereby improving the cleanliness of molten steel, steadily reducing the total oxygen content in the steel, and simultaneously enhancing the control effect of inclusions. Detailed Implementation

[0024] To make the above-mentioned objectives, control system design, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to 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.

[0025] This invention addresses the shortcomings of existing steelmaking processes, such as numerous potential nitrogen addition risks, low nitrogen control precision, unstable steel properties, and high alloy consumption. By optimizing process parameters, upgrading operating procedures, and strengthening equipment management in each stage of converter blowing, tapping deoxidation, LF refining, and continuous casting, this invention achieves segmented and coordinated nitrogen control throughout the entire process. This effectively reduces nitrogen addition at each stage of steelmaking, stabilizes the mechanical properties of steel, reduces product re-judgment, lowers alloy auxiliary material consumption, and ultimately reduces costs and increases efficiency.

[0026] A method for controlling nitrogen content in molten steel throughout the entire steelmaking process includes: Raw material control procedures: The proportion of molten iron added to the furnace is controlled to 78% or higher, and the amount of scrap steel added per ton does not exceed 230kg; the nitrogen content of auxiliary materials and alloys is tested, and high-nitrogen materials are eliminated; when producing titanium-containing steel, TiFe30 is used instead of TiFe70; Converter blowing process: Nitrogen content is strictly controlled; nitrogen-argon switching is completed within 8 minutes of converter blowing for the steel grade; bottom blowing intensity of the converter is controlled at 0.042 m³ / min. t~0.07m³ / min t, reduce argon pressure in the early stage and increase stirring intensity in the middle and later stages; reduce post-blowing and supplementary blowing operations in the converter, create foamy slag in the later stage of smelting, and maintain a slight positive pressure in the converter furnace; Converter tapping process: Argon pre-blowing in the ladle for 1 minute before tapping for gas replacement; maintaining the tapping spout to ensure a smooth steel flow; tapping time controlled at 5-7 minutes; dissolved oxygen control at 600 × 10⁻⁶ for ultra-low carbon steel. -6 ~700×10 -6 Low-carbon, low-sulfur steel retains a reasonable sulfur content; a one-time complete deoxidation alloying process is adopted; LF refining process: Low voltage and low current arc stabilization and staged heating mode are adopted to ensure the submerged arc effect; foaming agent and fluorite are added to improve the foaming effect of top slag; the bottom blowing argon flow rate of the ladle is controlled at 0.19Nm³ / min~0.40Nm³ / min, and wire feeding and calcium treatment are carried out under soft blowing argon state; a slight positive pressure is maintained inside the LF furnace. Continuous casting process: Argon gas is used to purge the tundish before casting begins, the tundish is covered and a covering agent is added; long nozzles and submerged nozzles are sealed with argon gas, and a fully protected casting process is implemented throughout the entire process.

[0027] To better understand the technical solution of the present invention, further non-limiting explanations are provided below: In the converter blowing process, high nitrogen-controlled steel grades adopt the bottom blowing argon mode throughout the process. In the later stage of smelting, the bottom blowing intensity is appropriately increased without blowing away the slag surface. Steel grades with lower nitrogen content requirements adopt the early nitrogen blowing and later argon blowing mode.

[0028] During the converter blowing process, it is strictly forbidden to switch between nitrogen and argon after 11 minutes of converter blowing. The nitrogen content of the molten steel at the end of the converter process should be controlled within 15 ppm.

[0029] In the converter steelmaking process, for high-nitrogen-controlled steel grades, the pre-argon blowing time in the ladle is extended to 2-3 minutes; for cold-rolled silicon steel, no alloying is performed during tapping, and dissolved oxygen is maintained at 700×10⁻⁶. -6 ~900×10 -6 .

[0030] In the LF refining process, the power supply level is adjusted according to the slag layer thickness and the temperature rise, shortening the heating time and suppressing the generation of nitrogen atoms by electric arc ionization of air.

[0031] In the raw material control process, high-end cold-rolled silicon steel is smelted entirely with molten iron without adding scrap steel.

[0032] This invention comprises five core steps: raw material pretreatment, converter blowing, converter tapping, LF refining, and continuous casting. The specific control methods for each process are as follows: The specific methods for nitrogen control in raw material management processes include: Increase the proportion of molten iron fed into the furnace, controlling it to 78% or higher, and reasonably control the amount of scrap steel fed into the furnace; strictly control the amount of scrap steel added at one time, with the amount of scrap steel added not exceeding 230 kg of steel, and at the same time classify and manage the types of scrap steel to avoid the concentrated use of high-nitrogen scrap steel.

[0033] Nitrogen content is tested in smelting auxiliary materials and alloy materials, and high-nitrogen auxiliary materials are eliminated; when producing titanium-containing steel, TiFe30 is used instead of traditional TiFe70 to reduce the amount of nitrogen introduced into the alloy.

[0034] Strictly control the amount of easily nitrogen-fixing elements such as aluminum, vanadium, and niobium added as auxiliary materials, and accurately proportion them according to the steel grade composition requirements to reduce the amount of nitrogen in raw materials from the source.

[0035] Nitrogen control methods in the converter blowing process specifically include: Precise control of nitrogen-argon switching time is crucial for key steel grades with strict nitrogen content control. The nitrogen-argon switching must be completed within 8 minutes of converter blowing; switching after 11 minutes is prohibited, thus reducing the nitrogen content at the converter endpoint from the source. The nitrogen content of the molten steel at the converter endpoint can be controlled below 15 ppm.

[0036] The refined control of converter bottom blowing flow rate adopts a refined control model for converter composite bottom blowing, with bottom blowing intensity controlled between 0.042 m³ / min·t and 0.07 m³ / min·t. In the early stage of smelting, the argon pressure is reduced during the nitrogen stage, while the argon stirring intensity is increased in the middle and later stages to replace free nitrogen in the steel. For steel grades with low nitrogen control requirements, a "nitrogen blowing in the early stage, argon blowing in the later stage" mode can be adopted, with strict control over the gas switching flow rate. For steel grades with high nitrogen control requirements, argon blowing is used throughout the process, with the bottom blowing intensity appropriately increased in the later stages without blowing away the slag surface.

[0037] To optimize the final carbon and temperature accuracy of the converter, it is necessary to eliminate post-blowing and multiple supplementary blowing operations. Post-blowing in the converter will cause an average increase of 7 ppm in nitrogen in the molten steel, with a single heat increase ranging from 1 to 18 ppm, so the post-blowing rate must be strictly reduced. The longer the supplementary blowing time and the lower the final carbon content, the higher the nitrogen content in the steel. Therefore, the supplementary blowing time should be strictly limited, and the final carbon content should be reasonably increased.

[0038] Foamed slag and furnace pressure control are achieved through the addition of steel slag, calcined dolomite, and other foaming agents during the later stages of smelting. The entire process employs a foamed slag production technique to ensure the slag-forming materials are fully melted, resulting in a uniform slag coating of the molten steel. The use of nitrogen to suppress slag is strictly prohibited. Adjusting the dust collector fan valves and lowering the furnace hood maintains a slight positive pressure inside the converter, preventing the entrainment of outside air and nitrogen absorption.

[0039] Nitrogen control methods in the converter tapping process specifically include: The tapping process is the most severe nitrogen increase stage in the entire process. Based on the surface activity characteristics of oxygen and sulfur in molten steel and the operating conditions of the ladle, multiple nitrogen control measures are implemented: Before tapping steel from the converter, the empty ladle is purged with argon gas for 1 minute to completely replace any residual air inside. Increasing the ladle lining temperature effectively reduces nitrogen buildup caused by residual air. For high-requirement steel grades, the pre-purging time can be extended to 2-3 minutes.

[0040] The tapping spout and the tapping flow are regularly inspected, repaired, and replaced. The service life of the tapping spout is strictly controlled to ensure that the tapping flow is smooth and compact, and to prevent the flow from being scattered or thin. The total tapping time of a single furnace is controlled to be 5 to 7 minutes to shorten the contact time between the molten steel and the atmosphere.

[0041] Precise control of dissolved oxygen and sulfur in steel: Both oxygen and sulfur are surface-active elements that can occupy active sites on the surface of molten steel, inhibiting the adsorption and dissolution of nitrogen atoms. Ultra-low carbon steel grades: Controlling the dissolved oxygen content at converter tapping to 600 × 10⁻⁶. -6 ~700×10 -6 High dissolved oxygen is used to suppress nitrogen absorption; for low carbon and low sulfur steel grades, there is no need to deliberately reduce the sulfur content of the steel output. Desulfurization is completed through the LF refining and slag-making process, avoiding the problem of aggravated nitrogen absorption caused by the superposition of low sulfur and low oxygen.

[0042] The deoxidation and alloying control of steel tapping adopts a one-time complete deoxidation process, reducing the need for secondary deoxidation operations during the tapping process; deoxidation and alloying are carried out simultaneously, shortening the exposure time of the molten steel to the low oxygen state after deoxidation, and the low oxygen environment will greatly improve the nitrogen absorption capacity of the molten steel.

[0043] Nitrogen control methods in the refining process specifically include: The LF refining process has multiple potential risks of nitrogen increase, including arc ionization of air, exposed slag layer, excessive bottom blowing, and negative pressure inside the furnace. These risks are controlled from four dimensions: power supply, slag formation, bottom blowing, and furnace pressure. The power supply heating and submerged arc nitrogen control are matched with the heating mode according to the slag layer thickness, heating range and refining cycle: in the early stage of power-on, low voltage and low current are used to stabilize the arc, low gear for slag formation and high gear for rapid heating, shortening the power-on heating time and reducing the generation of nitrogen atoms from nitrogen gas in the air by high temperature arc ionization; ensuring good submerged arc effect and avoiding exposure of the molten pool surface.

[0044] Top slag foaming and slag optimization involve adding fluorite and molten slag foaming agents to improve the slag formation effect, maximize the foaming performance of top slag, form a thick and stable foamed slag layer, fully cover the molten steel surface, and isolate it from the atmosphere; rationally control the slag basicity, desulfurization and deoxidation depth, reduce the use of strong deoxidizers, control the timing of white slag formation and desulfurization, and take into account both desulfurization effect and nitrogen control requirements.

[0045] Standardized control of bottom-blown argon flow rate: The bottom-blown argon flow rate of the LF ladle is strictly controlled at 0.19~0.40 Nm³ / min. High-flow-rate argon blowing is prohibited to avoid blowing open the slag surface and exposing the molten steel. Operations such as wire feeding and calcium treatment are carried out under soft argon blowing to avoid disturbing the slag layer and causing secondary nitrogen absorption. The bottom-blown flow rate is dynamically fine-tuned according to the furnace condition and steel grade to ensure stirring effect while preventing nitrogen absorption from exposed areas.

[0046] Adjusting the speed of the dust removal fan, the opening of the damper, and the dust removal baffle plate in the LF furnace atmosphere pressure control creates a slightly positive pressure environment inside the refining furnace, preventing outside air from being drawn in by negative pressure and suppressing nitrogen increase from the furnace gas level.

[0047] The process connection is optimized to improve the top slag condition of the converter argon station, improve the slag system performance in advance, and reduce the secondary oxidation and nitrogen absorption of molten steel caused by shell breaking and stirring after LF enters the station.

[0048] Nitrogen control methods in the continuous casting process specifically include: Continuous casting is the process with the smallest nitrogen increase in the entire process. There are no denitrification conditions in this stage. The core idea is to provide full-process sealed protection to prevent the molten steel from contacting the atmosphere. The equipment sealing was modified and maintained by adopting a long nozzle argon sealing and immersion nozzle sealing structure, and replacing the sealing ring with a high-performance one; ensuring that the inner diameter of the long nozzle is completely filled with molten steel to eliminate the phenomenon of turbulence during injection; and adding a sealing cover to the tundish to enhance the overall sealing performance.

[0049] Gas replacement and covering protection: Before continuous casting begins, argon gas is used to purge and replace the tundish as a whole; sufficient covering agent is added to the tundish to ensure that the molten steel surface is completely covered and there are no exposed areas.

[0050] Operational procedures must be strictly controlled to ensure the normal operation of the long nozzle, immersion nozzle, and argon gas curtain seal in the tundish. The entire process must be carried out with full protection during casting, and interruption of the protection system is prohibited.

[0051] Overall process linkage logic This invention focuses on nitrogen control at the source, nitrogen suppression during processes, and nitrogen prevention at the end of the process: at the raw material stage, nitrogen sources are reduced; at the converter stage, initial nitrogen reduction is achieved through optimized blowing, gas switching, and endpoint control; at the tapping stage, concentrated nitrogen accumulation is blocked through gas replacement, flow control, and composition regulation; at the LF refining stage, nitrogen absorption during refining is suppressed by controlling the slag layer, power supply, bottom blowing, and pressure; and at the continuous casting stage, nitrogen accumulation in the later stages is prevented through full-sealing protection. The process parameters of each stage are matched, and the operating standards are unified, forming a closed-loop nitrogen control system for the entire steelmaking process.

[0052] Example 1: Nitrogen Control Throughout the Smelting Process of Ordinary Carbon Steel Q235B Raw material control: the proportion of molten iron added to the furnace is controlled at 80%, and the amount of scrap steel added per ton is 200kg, using low-nitrogen clean scrap steel; auxiliary materials and ordinary alloys are used in a conventional manner, and no additional high-nitrogen additives are added.

[0053] During converter blowing, the nitrogen and argon gas switching should be completed after 8 minutes of converter blowing; delayed switching is strictly prohibited. The bottom blowing intensity of the converter is set to 0.055 m³ / min. In the early stage, the argon pressure is low, and the argon stirring intensity is gradually increased in the middle and later stages; the endpoint control is optimized, and the post-blowing and supplementary blowing operations are eliminated. The carbon content at the converter endpoint is controlled within a reasonable range, and a slight positive pressure is maintained in the furnace; in the later stage of smelting, iron oxide scale is added to prepare foam slag, and the slag completely covers the surface of the molten steel. Nitrogen gas is not used to pressurize the slag.

[0054] Before tapping from the converter, the ladle is pre-blown with argon for 1 minute to replace the internal air; the ladle lining temperature is maintained at the normal production temperature; the taphole is inspected in advance to ensure a smooth steel flow; the tapping time is controlled at 6 minutes; the dissolved oxygen at tapping is controlled at 650 × 10⁻⁶. -6 It does not deliberately suppress sulfur content; it adopts one-time aluminum-manganese-iron deoxidation alloying, and there is no secondary deoxidation throughout the process.

[0055] Refining involves maintaining a stable arc with low voltage and current during the initial energization phase, followed by rapid slag formation and increased temperature to ensure effective arc submersion and shorten energization time. Fluorite and foaming agents are added to enhance the foaming effect of the top slag, ensuring the slag layer thickness meets the standard and fully covers the molten steel surface. The bottom blowing argon flow rate of the ladle is set to 0.28 Nm³ / min, and calcium treatment and wire feeding operations are completed under soft blowing conditions. The dust removal system is adjusted to maintain a slight positive pressure inside the LF furnace to prevent nitrogen absorption under negative pressure.

[0056] For continuous casting, the tundish is purged with argon gas before pouring, and the tundish is covered with a sufficient amount of covering agent; the long nozzle is argon-sealed throughout the process, the immersion nozzle is properly sealed, and full protective casting is performed; the flow state is controlled to prevent churning and exposure.

[0057] In this embodiment, 30 heats of Q235B steel were continuously tracked: the average nitrogen content at the converter endpoint was 13.2 ppm, and the average nitrogen content after tapping was 20.8 ppm; the average nitrogen increase during the LF refining process was 9.2 ppm, and the total nitrogen increase throughout the process was reduced by more than 7 ppm compared with the original process; the nitrogen content of the finished steel was ≤38 ppm, there were no heats with excessive nitrogen content, the mechanical properties of the steel were all qualified, and there were no batches that were reclassified.

[0058] Example 2: Nitrogen Control Throughout the Craftsmanship Process of Low-Alloy Steel Q355B Raw material control: the proportion of molten iron entering the furnace is 79%, the amount of scrap steel added per ton is 208kg, and low-nitrogen scrap steel is screened; the amount of alloys such as vanadium and niobium added is strictly controlled, the nitrogen content of auxiliary materials is tested, and high-nitrogen materials are eliminated.

[0059] Converter blowing, nitrogen-argon switching completed in 8 minutes; bottom blowing intensity 0.06 m³ / min Argon is used as the main bottom blowing medium throughout the process; the carbon temperature at the converter endpoint is strictly controlled, and short-term supplementary blowing is only carried out when the index deviation is extremely small, eliminating post-blowing; foam slag is generated, and the furnace operates under slight positive pressure.

[0060] The converter tapping process involves argon pre-blowing in the ladle for 1 minute, tapping time of 5.5 minutes, and a complete steel stream without any spillage. The dissolved oxygen level at tapping is controlled at 680 × 10⁻⁶. -6 It retains a reasonable sulfur content; deoxidation and alloying are completed in one step, without secondary deoxidation operations.

[0061] LF refining utilizes staged power supply for heating to enhance the submerged arc effect; it optimizes slag alkalinity to thicken the foamed slag layer while ensuring desulfurization efficiency; bottom-blowing argon flow rate is 0.32 Nm³ / min, with soft blowing as the main method throughout to avoid slag layer disturbance; the LF furnace operates under slightly positive pressure to prevent air intake.

[0062] Continuous casting employs fully protected casting, with double argon sealing of the tundish and long nozzle, and a covering agent completely covering the molten steel surface.

[0063] Continuous tracking of 30 heats of Q355B: the average nitrogen content at the converter endpoint was 14.1 ppm, and the average nitrogen content at tapped steel was 21.0 ppm; the average nitrogen increase in the LF process was 9.5 ppm, and the nitrogen control throughout the process met the standards; the nitrogen content of the finished steel was ≤48 ppm, which fully met the technical requirements for nitrogen content of low alloy steel, and the plasticity and toughness of the steel were stable.

[0064] Example 3: Nitrogen Control Throughout the Cold-Rolled Silicon Steel DW60 Smelting Process Raw material control involves using molten iron for smelting without adding scrap steel; all auxiliary materials and alloys are low-nitrogen, eliminating nitrogen from the raw materials at the source.

[0065] The converter blowing process involves switching between nitrogen and argon gas in 8 minutes, with argon gas being blown from the bottom throughout the process; the bottom blowing intensity is 0.048 m³ / min. t; zero post-blowing and zero supplementary blowing, strictly ensuring that the converter's final indicators meet the requirements; creating a thick layer of foamy slag, with slight positive pressure inside the furnace.

[0066] After tapping from the converter, the ladle is pre-blown with argon for 2 minutes to completely replace the air; the tapping time is 5 minutes, and the steel stream is compacted; this steel grade is not alloyed during tapping, and the dissolved oxygen is maintained at 800 × 10⁻⁶. -6 The process utilizes high oxygen levels to suppress nitrogen absorption; the nitrogen content at the converter endpoint is 13.5 ppm, and the nitrogen content after tapping is 15.5 ppm, with only a 2 ppm increase in nitrogen during the tapping process.

[0067] LF refining and continuous casting follow the standard refining and nitrogen control process of this invention, strengthening slag layer protection and sealed casting.

[0068] DW60 cold-rolled silicon steel has extremely low nitrogen addition throughout the entire process, and the nitrogen content of the finished product fully meets the standards for high-end silicon steel. It has no aging defects and the product quality reaches the high-end level in the industry.

[0069] Example 4: Nitrogen Control Throughout the Smelting Process of Titanium-Reinforced Steel (Including Titanium Steel) Raw material control: 82% of the iron is fed into the furnace, with a small amount of scrap steel added; TiFe30 is used instead of TiFe70 to reduce the nitrogen element introduced by the titanium alloy; nitrogen content of all auxiliary materials is tested.

[0070] In the converter, tapping, LF, and continuous casting processes, the nitrogen control parameters of this invention are strictly implemented throughout the entire process, with a focus on strengthening the pre-blowing of argon in the ladle, one-time deoxidation during tapping, foam slag protection in LF, and fully sealed casting.

[0071] The nitrogen content in the steel is effectively reduced, the nitrogen fixation phenomenon of titanium is greatly weakened, the strengthening effect of titanium is fully exerted, the strength index of the steel meets the standard and the performance is stable, the consumption of titanium-iron alloy is significantly reduced, and the cost reduction effect is obvious.

[0072] Traditional process, using the same furnace type and steel grade Q235B, employing traditional smelting techniques: switching to nitrogen-argon after 11 minutes of converter blowing, pre-blowing argon without a ladle, post-blowing and supplementary blowing in the converter, high-flow bottom blowing in the LF (sulfurized oil refining) system, and simplified continuous casting sealing. Test results: average nitrogen content at the converter endpoint was 27.3 ppm, average nitrogen content at tapped steel was 28 ppm; average nitrogen increase during LF refining was 17 ppm; finished product nitrogen content was generally >50 ppm, with some furnace runs failing performance tests, resulting in downgrading and reassessment; alloy consumption was high.

[0073] The comparison shows that the technology of this invention significantly improves the nitrogen control capability throughout the entire process compared to the traditional process, and has outstanding comprehensive benefits.

[0074] The following section will verify the process parameters and analyze the mechanism: Analysis of the impact mechanism of nitrogen-argon switching time in converters: Experimental data shows that switching from nitrogen to argon at 8 minutes during converter blowing results in an average nitrogen content of 13.5 ppm at the endpoint; switching at 11 minutes results in an average nitrogen content of 27.3 ppm at the endpoint, a difference of 13.8 ppm. In the early stages of smelting, the carbon-oxygen reaction in the furnace is intense, leading to a high denitrification rate. Switching to argon earlier allows for the removal of nitrogen from the steel through strong stirring; delaying the switch results in prolonged contact between nitrogen and molten steel, causing nitrogen solubility to increase continuously with rising temperature, leading to severe nitrogen accumulation. Therefore, for high-nitrogen-controlled steel grades, the nitrogen-argon switching time must be completed within a fixed 8 minutes.

[0075] Analysis of the nitrogen uptake mechanism after converter blowing: The average nitrogen content of Q235B steel before blowing was 11 ppm, and after blowing, it was 18 ppm, with a nitrogen uptake of 1-18 ppm per furnace. During the blowing process, slag protection fails, the molten steel is repeatedly exposed to the atmosphere, and the air from the supplementary blowing is entrained, directly aggravating nitrogen uptake; moreover, the blowing process leads to a decrease in carbon at the endpoint, weakens the carbon-oxygen reaction, and reduces the denitrification rate, further increasing the nitrogen content.

[0076] Analysis of the inhibitory mechanism of oxygen and sulfur in steel on nitrogen absorption reveals that oxygen and sulfur, as typical surface-active elements, preferentially adsorb onto active sites on the surface of molten steel, crowding out nitrogen atom adsorption sites and reducing the steel's ability to dissolve nitrogen. High dissolved oxygen levels (700~900×10⁻⁶) are observed in cold-rolled silicon steel DW60 at the tapping stage. -6 The nitrogen increase after tapping of Q450NQR1 steel is only 2 ppm; however, after deoxidation, the dissolved oxygen level drops significantly, surface active sites are released, and combined with the nitrogen carried by the alloy, the nitrogen increase after tapping reaches 14 ppm. Therefore, controlling the dissolved oxygen and sulfur content according to the differences in steel grades is the core method for controlling nitrogen during tapping.

[0077] The core factors contributing to nitrogen increase in LF refining include High-temperature electric arc (>2300℃) ionizes nitrogen gas in the furnace to generate nitrogen atoms. Poor arc submersion and excessively thin slag layer will lead to exposed molten steel, and nitrogen atoms will quickly dissolve into the molten steel. Excessive bottom-blowing argon flow rate can blow away the protective slag, allowing molten steel to directly absorb nitrogen from the atmosphere; bottom-blowing flow rate is positively correlated with nitrogen increase. The negative pressure environment of the LF furnace will draw in a large amount of outside air, further aggravating nitrogen enrichment; Slag and alloy additives themselves carry nitrogen, which is also a source of slight nitrogen increase.

[0078] This invention addresses the main nitrogen increase problem in LF refining by implementing four measures: graded power supply, foam slag formation, standardized bottom blowing flow rate, and micro-positive pressure inside the furnace.

[0079] Analysis of the nitrogen absorption mechanism in continuous casting reveals that there are no denitrification thermodynamic conditions in continuous casting, so nitrogen can only be prevented. Failure of the long nozzle seal, lack of tundish coverage, and exposure of the molten steel during pouring can all cause secondary nitrogen absorption by the molten steel. The integrity of the seal directly determines the amount of nitrogen added in this process; therefore, full protective casting is the only control method.

[0080] Synergistic relationship between total oxygen and nitrogen content, total oxygen variation throughout the B18 steel process: LF inlet 160×10 -6 →LF exit 36×10 -6 →VD exit 22.4×10 -6 →Intermediate package 24.2×10 -6The refining process achieved excellent deoxidation, resulting in a continuous decrease in total oxygen. Secondary oxidation in the tundish led to a slight increase in total oxygen, accompanied by a rise in nitrogen content. The oxidation state of the molten steel is negatively correlated with its nitrogen absorption capacity; therefore, while improving cleanliness (reducing total oxygen), it is necessary to simultaneously strengthen measures to prevent nitrogen absorption.

[0081] This invention is applicable to long-process steel production lines including converter, LF refining, and continuous casting. Applicable steel grades include: plain carbon steel (Q235A, Q235B), low alloy steel (Q345R, Q355B, Q450NQR1), cold-drawn tube steel (B18), cold-rolled silicon steel (DW60), titanium-based microalloyed reinforced steel, and other hot-rolled steel products.

[0082] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling nitrogen content in molten steel throughout the entire steelmaking process, characterized in that, include: S1. During the raw material control process, the proportion of molten iron entering the furnace shall be controlled to be no less than 78%, and the nitrogen content of scrap steel, auxiliary materials and alloys shall be tested and screened. S2. During the converter blowing process, the bottom blowing gas switch from nitrogen to argon is completed at 8 minutes of blowing, and the furnace is controlled to be in a slightly positive pressure state at the end of the blowing. S3. During the converter tapping process, the ladle is purged and replaced with argon gas before tapping, and the shape of the steel flow is controlled during the tapping process. S4. During the LF refining process, foam slag submerged arc is formed by controlling the power supply mode, and the bottom blowing argon flow rate of the ladle and the pressure inside the furnace are controlled. S5. During the continuous casting process, the molten steel is cast in a fully sealed environment to prevent it from coming into contact with the atmosphere.

2. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 1, characterized in that, In S1, when producing titanium-containing steel, TiFe30 is used instead of TiFe70 as the titanium source.

3. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 1, characterized in that, In S1, the amount of scrap steel added per ton shall not exceed 230 kg.

4. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 1, characterized in that, In S2, the bottom blowing intensity of the converter is controlled between 0.042 m³ / min·t and 0.07 m³ / min·t.

5. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 1, characterized in that, In S3, the argon purging time shall not be less than 1 minute.

6. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 5, characterized in that, The steel flow during the tapping process is in a rounded and compact state, and the tapping time is controlled between 5 and 7 minutes.

7. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 1, characterized in that, In S3, the dissolved oxygen content during tapping is adjusted according to the requirements of the steel grade. Specifically: The dissolved oxygen of ultra-low carbon steel is controlled at 600×10. -6 Up to 700×10 -6 ; The dissolved oxygen in cold-rolled silicon steel is controlled at 700×10⁻⁶. -6 Up to 900×10 -6 .

8. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 1, characterized in that: In S4, the bottom-blown argon flow rate of the ladle is controlled between 0.19 Nm³ / min and 0.40 Nm³ / min, and the furnace is kept under a slightly positive pressure.

9. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 1, characterized in that, In S5, fully sealed protective casting includes: purging the tundish with argon before pouring, covering the tundish with a cover agent, and sealing the long gate and immersion gate with argon.

10. The method for controlling nitrogen content in molten steel throughout the entire steelmaking process according to claim 9, characterized in that: Used in the smelting of plain carbon steel, low alloy steel, cold-drawn tube steel, cold-rolled silicon steel, and titanium-based microalloyed reinforced steel.