A method and system for controlling low nitrogen content of molten steel at a converter smelting endpoint

Abstract

The application discloses a converter smelting endpoint molten steel low-nitrogen content control method and system, and belongs to the technical field of steel metallurgy, and comprises the following steps: S1, optimizing the converter charging system, and determining the charging ratio of molten iron and scrap steel; S2, dynamic slagging and foam slag maintaining: in the fierce carbon-oxygen reaction period of the converter blowing, sinter is added into the furnace; S3, stage top blowing oxygen supply: in the middle and late blowing period, the oxygen supply intensity is increased to 10%-20% higher than the conventional blowing intensity, the total blowing time is shortened, and the denitrification effect of the CO bubbles generated by the carbon-oxygen reaction on the molten steel is strengthened; S4, bottom blowing gas switching: bottom blowing stirring is implemented in the whole blowing process, nitrogen gas is used in the early and middle blowing periods, and before the late blowing period and the molten steel tapping stage, the bottom blowing gas is switched from nitrogen gas to argon gas, so that the molten steel is prevented from increasing nitrogen from the bottom blowing gas; and S5, endpoint control: the sub-lance is used to detect the molten pool parameters before the blowing endpoint, and the endpoint is accurately controlled according to the detection result and the molten steel is tapped.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy technology, and in particular relates to a method and system for controlling the low nitrogen content of molten steel at the end of converter smelting. Background Technology

[0002] Nitrogen is a common element in steel, usually existing in the form of nitrides. The precipitation of nitrides leads to "aging" and "blue brittleness" in steel, severely deteriorating its plasticity, impact toughness, hot workability, and weldability. Excessive nitrogen content in steel can also easily cause billet cracking and intergranular corrosion. Therefore, for most steel grades (except for special steel grades), nitrogen is considered a harmful element and requires strict control.

[0003] In modern manufacturing, the demand for high-grade steel products such as high-strength hot-rolled steel, high-strength pipeline steel, and IF cold-rolled steel plates is increasing. These steel products have extremely stringent requirements for nitrogen content control, requiring levels below 40 ppm and 30 ppm respectively. Therefore, controlling the nitrogen content in steel has become a key process technology in steelmaking.

[0004] Nitrogen content control in steel is crucial throughout the entire production process, from converter smelting and steel refining to continuous casting. From the perspective of denitrification efficiency and process optimization, converter smelting is the most important and effective step in denitrification. Subsequent processes such as tapping, refining, and casting primarily aim to prevent and reduce nitrogen accumulation in the molten steel. Therefore, developing efficient converter endpoint nitrogen content control technology is key and fundamental to achieving the development and mass production of low-nitrogen steel grades.

[0005] In existing technologies, conventional converter operations primarily focus on smelting ordinary steel grades, with relatively lenient requirements for nitrogen content control, typically between 45-60 ppm. Even for steel grades with specific requirements (such as ≤50 ppm), actual control levels fluctuate significantly. Maintaining stable nitrogen content at an extremely low level of ≤30 ppm has become a bottleneck restricting the production of high-end low-nitrogen steel. Current technologies lack systematic research and precise control methods regarding the behavior and variation patterns of nitrogen in the molten pool during converter blowing, as well as its correlation with various process parameters. Summary of the Invention

[0006] To address the above-mentioned technical deficiencies, this invention provides a method and system for controlling the low nitrogen content of molten steel at the end of converter smelting, which solves the technical problem that existing converter smelting processes cannot stably and reliably control the nitrogen content of molten steel at the end of the process below 30 ppm, thus meeting the production needs of high-end low-nitrogen steel grades.

[0007] To achieve the above-mentioned technical objectives, the present invention is implemented through the following technical solution: The first objective of this invention is to provide a method for controlling the nitrogen content of molten steel at the end of converter smelting, thereby stably controlling the nitrogen content of the molten steel at the end of converter smelting to below 30 ppm; the method includes: S1. Optimize the converter charging system, determine the charging ratio of molten iron and scrap steel, balance the amount of nitrogen charged into the furnace and promote the carbon-oxygen reaction. S2. Dynamic slag formation and foam slag maintenance: During the intense carbon-oxygen reaction period of converter blowing, sintered ore is added into the furnace to maintain the foamed state of the molten slag and cover the surface of the molten steel. S3. Staged top-blown oxygen supply: In the middle and late stages of blowing, the oxygen supply intensity is increased to 10% to 20% higher than the conventional blowing intensity, which shortens the total blowing time and enhances the denitrification effect of CO bubbles generated by the carbon-oxygen reaction on the molten steel. S4. Bottom-blowing gas switching: Bottom-blowing stirring is implemented throughout the blowing process. In the early and middle stages of blowing, nitrogen is used for bottom blowing. In the later stage of blowing and before entering the tapping stage, the bottom-blowing gas is switched from nitrogen to argon to prevent nitrogen from being added to the molten steel from the bottom-blowing gas. S5. Endpoint Control: Before the blowing endpoint, the parameters of the molten pool are detected by a secondary lance, and the endpoint is precisely controlled and steel is tapped based on the detection results.

[0008] Preferably, the period of intense carbon-oxygen reaction refers to the time period before and / or after the point at which the decarbonization rate reaches its peak.

[0009] Preferably, the sinter is added in small batches, alternating multiple batches, to maintain the continuous foaming state of the slag before and after the intense carbon-oxygen reaction period.

[0010] Preferably, the later stage of blowing refers to the stage when the total blowing time has reached 70%-85%.

[0011] Preferably, the nitrogen supply intensity of the bottom-blown gas is 0.06.10 Nm³ / (min·t) to 0.10 Nm³ / (min·t) in the early and middle stages of blowing, and the argon supply intensity is 0.06.10 Nm³ / (min·t) to 0.10 Nm³ / (min·t) after switching to argon in the later stage of blowing.

[0012] Preferably, the middle and late stages of blowing refer to the stage where the total blowing time has reached more than 50%.

[0013] The second objective of this invention is to provide a control system for controlling the low nitrogen content of molten steel at the endpoint of converter smelting, for implementing the aforementioned method for controlling the low nitrogen content of molten steel at the endpoint of converter smelting, comprising: The process control module processes data based on the process model and issues control commands. The bottom blowing gas control unit is connected to the process control module and is used to control the bottom blowing flow rate and switch between nitrogen and argon according to the blowing process; The slag-forming auxiliary material addition control unit is connected to the process control module and is used to control the small-batch, multiple-batch alternating addition of sinter before and after the intense carbon-oxygen reaction period. The secondary gun detection device is used to detect the molten pool parameters before the end point and feed them back to the process control module.

[0014] Preferably, it further includes: a rapid endpoint nitrogen content analyzer, which analyzes the endpoint nitrogen content and feeds it back to the process control module.

[0015] The third objective of this invention is to provide a converter endpoint molten steel, which is obtained by smelting using the aforementioned converter smelting endpoint molten steel low nitrogen content control method, and its nitrogen content is ≤30ppm.

[0016] The fourth objective of this invention is to provide a steel product made from the aforementioned converter end-point molten steel through subsequent refining and rolling processes.

[0017] The advantages and technical effects of this invention are: This invention is the first to achieve a systematic and synergistic control of "reduced absorption, enhanced denitrification, and prevention of nitrogen accumulation" in a unified manner. It precisely coordinates three technologies—maintaining foamy slag, high-intensity oxygen supply in the mid-to-late stages, and switching bottom-blowing gas—on a timeline: maintaining the foamy slag barrier during the intense carbon-oxygen reaction period reduces nitrogen absorption from the furnace gas by the molten steel; enhancing oxygen supply in the mid-to-late stages promotes denitrification of the molten steel by CO bubbles and shortens exposure time; and switching to argon gas in the later stages eliminates the risk of nitrogen accumulation from bottom-blowing gas. The synergistic effect of these three technologies forms a complete nitrogen content control chain, overcoming the shortcomings of existing technologies where a single method is insufficient to stably achieve extremely low nitrogen content.

[0018] Using the technical solution of this invention, the nitrogen content of the molten steel at the converter endpoint can be stably controlled below 30 ppm, representing a qualitative leap compared to the 45 ppm to 60 ppm of conventional converter smelting. Examples demonstrate that the endpoint nitrogen content can be as low as 25 ppm, fully meeting the production requirements of high-end low-nitrogen steel grades. Detailed Implementation

[0019] 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.

[0020] A first embodiment provides a method for controlling the low nitrogen content of molten steel at the end of converter smelting, which can stably control the nitrogen content of molten steel at the end of converter smelting to below 30 ppm, including: S1. Develop an optimized converter charging system: Determine the appropriate ratio of molten iron to scrap steel to promote the smooth progress of carbon-oxygen reaction during blowing and achieve an initial balance of the total nitrogen brought in by the molten iron and scrap steel, thus creating a favorable material basis for efficient denitrification.

[0021] S2. Implement a dynamic slag-forming system and foam slag maintenance technology: During the blowing process, based on the reaction status of the molten pool, especially during the intense carbon-oxygen reaction period, slag-steel is added to the furnace in small batches and alternately. This operation aims to avoid or reduce slag drying caused by drastic fluctuations in molten pool temperature and excessive consumption of FeO in the slag, thereby maintaining a good foaming state of the molten slag throughout the blowing period, especially in the critical middle and late stages of denitrification. Foam slag can effectively cover the molten steel, preventing contact between the molten steel and furnace gas, and reducing the probability of nitrogen accumulation.

[0022] S3. Implement a phased top-blown oxygen supply system, focusing on the application of high-intensity oxygen supply technology in the middle and late stages of blowing: Increase the oxygen supply intensity during the middle and late stages of blowing. This technology has dual benefits: First, it shortens the total blowing time, directly reducing the time the molten steel is exposed to the furnace gas, thereby reducing the probability of nitrogen absorption; second, the high oxygen supply intensity strongly promotes the carbon-oxygen reaction, maintaining a large and continuous generation of CO gas in the molten pool. The overflow of CO bubbles is the main kinetic channel for denitrification of molten steel, thus significantly improving the denitrification effect.

[0023] S4. Optimized bottom-blowing gas supply mode and N2 / Ar switching technology: Bottom-blowing stirring is implemented throughout the blowing process, and the bottom-blowing gas is scientifically switched according to the smelting progress. In the early and middle stages of blowing, low-cost nitrogen can be used for strong stirring to homogenize the molten pool composition and temperature and promote the reaction. In the later stage of blowing and before tapping, the bottom-blowing gas must be switched to inert argon to prevent nitrogen from directly dissolving into the molten steel through the ladle bottom-blowing system and causing nitrogen enrichment, ensuring that the final nitrogen content meets the standard.

[0024] S5. Enhance endpoint control and precise detection: Determine and strictly control appropriate endpoint parameters such as temperature, carbon content, and oxygen level. Near the endpoint, use secondary lance detection technology to dynamically monitor the molten pool, acquiring real-time information on steel temperature, carbon content, and oxygen activity. Based on this, make precise endpoint judgments and tapping control to prevent nitrogen absorption from the furnace gas by the molten steel during endpoint waiting or improper operation.

[0025] Furthermore, the method also includes the S0 process analysis and modeling stage: before implementing control, the behavior of nitrogen in the converter pool is analyzed, and the nitrogen absorption conditions and main influencing factors of the molten steel are clarified from a thermodynamic perspective; the denitrification conditions and limiting factors of the molten steel are analyzed from a kinetic perspective.

[0026] A second embodiment provides a method for controlling the low nitrogen content of molten steel at the end of converter smelting, which can stably control the nitrogen content of molten steel at the end of converter smelting to below 30 ppm, comprising the following steps: S1. Optimize the converter charging system: Determine the charging ratio of molten iron and scrap steel to balance the amount of nitrogen entering the furnace and promote the smooth progress of carbon-oxygen reaction, thus creating a favorable material basis for efficient denitrification.

[0027] S2. Dynamic Slag Formation and Foamed Slag Maintenance: During the intense carbon-oxygen reaction period in converter blowing, sinter is added to the furnace in small batches and alternately to maintain the foamed state of the molten slag, covering the surface of the molten steel. The foamed slag effectively blocks contact between the molten steel and the furnace gas, reducing the probability of the molten steel absorbing nitrogen from the furnace gas. The "intense carbon-oxygen reaction period" refers to the time period before and / or after the decarburization rate reaches its peak.

[0028] S3. Staged Top-Blow Oxygen Supply: In the later stages of blowing, the oxygen supply intensity is increased to 10%-20% higher than the conventional blowing intensity to shorten the total blowing time and enhance the carbon-oxygen reaction. Shortening the blowing time directly reduces the time the molten steel is exposed to the furnace gas, thereby reducing the probability of nitrogen absorption; high oxygen supply intensity strongly promotes the carbon-oxygen reaction, maintaining a large and continuous generation of CO gas in the molten pool. The overflow of CO bubbles is the main kinetic channel for denitrification of molten steel, thus contributing to a significant improvement in denitrification effect.

[0029] S4. Bottom-blowing gas switching: Bottom-blowing agitation is implemented throughout the blowing process. Nitrogen is used in the early and middle stages of blowing, while argon is switched to the bottom-blowing gas in the later stages of blowing, before the tapping stage. Nitrogen bottom-blowing in the early and middle stages of blowing is cost-effective and meets agitation requirements; switching to argon in the later stages prevents nitrogen from directly dissolving into the molten steel through the ladle bottom-blowing system, thus ensuring the final nitrogen content meets the standard. The "late stage of blowing" is preferably the stage when the total blowing time has reached 70%-85%.

[0030] S5. Endpoint Control: Before the blowing endpoint, the parameters of the molten pool are detected by the auxiliary lance to obtain information such as steel temperature, carbon content and oxygen activity in real time. The endpoint is precisely controlled and steel is tapped based on the detection results to prevent the molten steel from absorbing nitrogen from the furnace gas when waiting at the endpoint or when the operation is improper.

[0031] Furthermore, the method also includes the S0 process analysis and modeling stage: before implementing control, the behavior of nitrogen in the converter pool is analyzed, the nitrogen absorption conditions and main influencing factors of the molten steel are clarified from a thermodynamic perspective, and the denitrification conditions and limiting factors of the molten steel are analyzed from a kinetic perspective.

[0032] In S4, the nitrogen supply intensity of the bottom-blown gas is 0.06 Nm³ / (min·t) to 0.10 Nm³ / (min·t) in the early and middle stages of blowing, and after switching to argon in the later stage of blowing, the argon supply intensity is 0.06 Nm³ / (min·t) to 0.10 Nm³ / (min·t).

[0033] In S3, the mid-to-late stage of refining refers to the stage where the total refining time has reached more than 50%.

[0034] A converter smelting system for implementing the above control method includes: The process control module is used to process data based on the process model and issue control commands. The bottom blowing gas control unit is connected to the process control module and is used to control the bottom blowing flow rate and switch between nitrogen and argon according to the blowing process; The slag-forming auxiliary material addition control unit is connected to the process control module and is used to control the small-batch, multiple-batch alternating addition of sinter before and after the intense carbon-oxygen reaction period. A secondary gun detection device is used to detect molten pool parameters before the endpoint and feed them back to the process control module; A rapid endpoint nitrogen content analyzer is used to analyze the endpoint nitrogen content and feed it back to the process control module.

[0035] A converter final steel, obtained by the above method, has a nitrogen content ≤30ppm.

[0036] A type of steel produced from the aforementioned converter end-point molten steel through subsequent refining and rolling processes.

[0037] Application examples: In our factory's 180-ton top-and-bottom combined blowing converter, we applied the method described in this invention to smelt low-nitrogen automotive steel (requiring [N] ≤ 30ppm).

[0038] Loading procedure: The loading scheme adopts a 90% molten iron ratio and 10% high-quality low-nitrogen scrap steel.

[0039] Refining process: After the blowing process begins, depending on the temperature rise of the molten pool and the slag formation, small batches of slag-forming materials such as lime and dolomite are added in the early stage of blowing (3-5 minutes after the blowing process begins).

[0040] When the blowing process enters the middle stage (about halfway through), and the carbon-oxygen reaction begins to intensify, about 2-3 tons of slag-forming auxiliary materials are added to the furnace in 3-5 batches through the slag-forming auxiliary material addition system to maintain good foaming of the molten slag and cover the molten steel.

[0041] When the blowing time reaches about 80% of the total blowing time, the bottom blowing gas control unit automatically switches the bottom blowing gas from N2 to Ar, and the gas flow rate is maintained at 0.06 Nm³ / (min·t) to 0.10 Nm³ / (min·t).

[0042] Endpoint control: Two minutes before the end of the blowing process, the secondary lance is used for inspection. The measured molten pool temperature is 1630℃, carbon content is 0.06%, and oxygen activity is 650ppm. Before tapping, ensure bottom blowing with Ar for 2 minutes.

[0043] Results: Samples were taken after tapping the steel and tested with a rapid analyzer. The final nitrogen content of the molten steel was 25 ppm, which stably met the target of ≤30 ppm, fully satisfying the smelting requirements of this low-nitrogen automotive steel.

[0044] The above embodiments demonstrate that the control method and system provided by the present invention can effectively and stably control the nitrogen content at the converter endpoint at an extremely low level, providing a reliable technical guarantee for the production of high-grade low-nitrogen steel.

[0045] 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 the low nitrogen content in molten steel at the end point of converter smelting, characterized in that, The nitrogen content of the final molten steel from the converter is stably controlled below 30 ppm; the method includes: S1. Optimize the converter charging system, determine the charging ratio of molten iron and scrap steel, balance the amount of nitrogen charged into the furnace and promote the carbon-oxygen reaction. S2. Dynamic slag formation and foam slag maintenance: During the intense carbon-oxygen reaction period of converter blowing, sintered ore is added into the furnace to maintain the foamed state of the molten slag and cover the surface of the molten steel. S3. Staged top-blown oxygen supply: In the middle and late stages of blowing, the oxygen supply intensity is increased to 10% to 20% higher than the conventional blowing intensity, which shortens the total blowing time and enhances the denitrification effect of CO bubbles generated by the carbon-oxygen reaction on the molten steel. S4. Bottom-blowing gas switching: Bottom-blowing stirring is implemented throughout the blowing process. In the early and middle stages of blowing, nitrogen is used for bottom blowing. In the later stage of blowing and before entering the tapping stage, the bottom-blowing gas is switched from nitrogen to argon to prevent nitrogen from being added to the molten steel from the bottom-blowing gas. S5. Endpoint Control: Before the blowing endpoint, the parameters of the molten pool are detected by a secondary lance, and the endpoint is precisely controlled and steel is tapped based on the detection results.

2. The method for controlling low nitrogen content in molten steel at the end point of converter smelting according to claim 1, characterized in that: The intense carbon-oxygen reaction period refers to the time period before and / or after the point when the decarbonization rate reaches its peak.

3. The method for controlling low nitrogen content in molten steel at the end point of converter smelting according to claim 1, characterized in that: The sinter is added in small batches, alternating multiple times, to maintain the continuous foaming state of the slag before and after the intense carbon-oxygen reaction period.

4. The method for controlling low nitrogen content in molten steel at the end point of converter smelting according to claim 1, characterized in that: The later stage of the blowing process refers to the stage when the total blowing time has reached 70%-85%.

5. The method for controlling low nitrogen content in molten steel at the end point of converter smelting according to claim 1, characterized in that: The nitrogen supply intensity of the bottom-blown gas is 0.06.10 Nm³ / (min·t) to 0.10 Nm³ / (min·t) in the early and middle stages of blowing, and the argon supply intensity is 0.06.10 Nm³ / (min·t) to 0.10 Nm³ / (min·t) after switching to argon in the later stage of blowing.

6. The method for controlling low nitrogen content in molten steel at the end point of converter smelting according to claim 1, characterized in that: The mid-to-late stage of refining refers to the stage where more than 50% of the total refining time has been completed.

7. A control system for low nitrogen content in molten steel at the end point of converter smelting, characterized in that: The method for controlling the low nitrogen content of molten steel at the endpoint of converter smelting as described in any one of claims 1-6 includes: The process control module processes data based on the process model and issues control commands. Bottom-blown gas control unit, connected to the process control module, is used to control the bottom-blown flow rate and switch between nitrogen and argon according to the blowing process; The slag-forming auxiliary material addition control unit is connected to the process control module and is used to control the small-batch, multiple-batch alternating addition of sinter before and after the intense carbon-oxygen reaction period. The secondary gun detection device is used to detect the molten pool parameters before the end point and feed them back to the process control module.

8. The control system for low nitrogen content in molten steel at the end point of converter smelting according to claim 7, characterized in that, Also includes: The endpoint nitrogen content rapid analyzer analyzes the endpoint nitrogen content and feeds it back to the process control module.

9. A converter final stage molten steel, characterized in that, The steel is obtained by smelting using the method for controlling the low nitrogen content of molten steel at the end of converter smelting as described in any one of claims 1-6, and its nitrogen content is ≤30ppm.

10. A type of steel, characterized in that, It is made from the final molten steel of the converter as described in claim 9 through subsequent refining and rolling processes.