Method for improving the heating rate and cleanliness of molten steel in RH refining of IF steel

Abstract

The application discloses a method for improving the RH heating rate and liquid steel cleanliness of IF steel and belongs to the field of steel metallurgy, which comprises the following steps: conveying the molten steel after converter smelting to an RH station and measuring the temperature and determining the oxygen content; selecting a decarburization process according to the oxygen content of the molten steel entering the station to perform decarburization treatment, and simultaneously performing vacuum refining treatment; after a period of vacuum refining, spraying aluminum powder on the molten steel in batches by using a specific powder spraying device to heat the molten steel, and performing oxygen blowing by using an oxygen lance; and after the decarburization treatment, performing alloying treatment on the molten steel according to the requirements of the steel grade. The application effectively realizes low-cost online heating regulation and control of the molten steel by relying on the special powder spraying device, the synergistic matching of the molten steel flow field and the segmented intermittent spraying process, and has the advantages of excellent process stability, high steel purity and the like, and is particularly suitable for industrialized smelting application of high-quality ultra-low-carbon IF steel.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy, and specifically relates to a method for improving the RH heating rate and cleanliness of molten steel in IF steel. Background Technology

[0002] Interstitial-free steel (IF steel) is an advanced high-strength steel with excellent deep-drawing properties. It is produced by adding strong carbide-nitride-forming elements such as titanium (Ti) and niobium (Nb) to completely fix the interstitial atoms (carbon and nitrogen) in the steel. IF steel features a high plastic strain ratio, a high strain hardening index, and no aging properties, making it a key material in the automotive industry for manufacturing deep-drawn components such as body panels and structural parts. It plays a significant role in reducing vehicle weight, improving safety, and promoting energy conservation and environmental protection. With the continuous development of the automotive industry, the demand for IF steel is increasing, and users are placing increasingly higher requirements on it.

[0003] Currently, domestic technologies for improving the cleanliness of IF steel mainly focus on the smelting and refining stages, typically employing a process flow of "converter smelting (-LF refining) - vacuum circulation degassing (Ruhrstahl-Heraeus, RH) refining - continuous casting". Although existing technologies are quite mature, many challenges remain in pursuing high cleanliness.

[0004] Chinese patent (application number: 201910204725.5) discloses an RH oxygen blowing heating method that facilitates the control of IF steel cleanliness. The method includes: after molten steel enters the RH unit, when the steel temperature and carbon and oxygen content are lower than the RH process set values ​​of the steel plant, oxygen blowing heating is implemented. Oxygen blowing begins 5 minutes after RH decarburization, and aluminum shot is added in batches from the RH silo to the vacuum chamber according to the oxygen blowing volume and flow rate. After decarburization, aluminum shot and ferrotitanium are added sequentially, and alloying is completed within 5 minutes. After alloying, pure circulation for 10 minutes ends the RH refining process, and subsequent operations follow the conventional procedure. By controlling the oxygen blowing rate and the aluminum shot addition method, the method accelerates the aluminum-oxygen reaction, reduces steel over-oxidation during the oxygen blowing process, and ensures that the oxygen content of the molten steel does not increase significantly after decarburization, thereby reducing billet downgrading, saving costs, and improving yield. However, this patent does not provide a quantitative model for calculating the amount of aluminum added and the amount of oxygen blown.

[0005] Chinese patent (application number: 201910936957.X) discloses an RH refining method for reducing the tapping temperature of IF steel in a converter. This method involves adding aluminum granules during the RH process, utilizing the chemical heat generated by the aluminum-oxygen reaction to compensate for the steel temperature. It systematically optimizes key parameters such as oxygen blowing timing, oxygen volume, aluminum addition amount, aluminum addition timing, immersion tube insertion depth, gas flow rate, vacuum level, and steel circulation time. This results in a low-temperature tapping method for IF steel in a converter and a chemical heating method in the RH refining furnace, eliminating the need for excessively high tapping temperatures. The RH chemical heating process allows for stable mass production of IF steel without high-temperature tapping, only requiring temperature compensation in the RH stage. However, this patent only optimizes the RH refining process and lacks significant innovation.

[0006] Chinese patent (application number: 201711313398.4) discloses a temperature-compensated refining process for producing IF steel in an RH refining furnace. The specific steps include: after the molten steel is lifted upon arrival at the refining station, measuring the inlet temperature and oxygen content; determining whether temperature compensation is needed and setting the compensation value, while simultaneously vacuuming the molten steel; determining whether oxygen blowing is needed and setting the oxygen blowing amount; adding heated aluminum granules in batches; increasing the gas flow rate after adding the heated aluminum granules and measuring the oxygen content after decarburization; adding the amounts of deoxidized aluminum granules and alloyed aluminum granules; adding alloying materials to alloy the molten steel; breaking the vacuum after circulating the molten steel; and purging with argon for cleaning and stirring after breaking the vacuum. This invention adds aluminum for temperature compensation in the early stage of decarburization, while simultaneously adjusting the vacuum level, increasing the gas flow rate, and adding argon blowing to the bottom of the ladle after breaking the vacuum, ensuring the complete removal of Al2O3, guaranteeing the quality of the continuously cast billet, and preventing Al2O3 from clogging the casting nozzle, resulting in significant economic benefits. However, this patent involves adding aluminum particles at the top of the vacuum chamber, resulting in a long contact path with the molten steel, low reaction efficiency, and slow heating rate. Summary of the Invention

[0007] To address the aforementioned problems, this invention provides a method for improving the RH refining efficiency and molten steel cleanliness of IF steel.

[0008] The technical solution of the present invention is as follows: A method for improving the RH heating rate and molten steel cleanliness of IF steel includes the following steps: The molten steel after converter smelting is transported to the RH station, and the temperature and oxygen content of the molten steel are measured to obtain the temperature and oxygen content of the molten steel entering the station. The decarburization process is selected based on the oxygen content of the molten steel entering the station, and vacuum refining is carried out simultaneously. During the vacuum refining process for 7 minutes, aluminum powder was sprayed onto the molten steel in batches using a powder spraying device to raise the temperature, and oxygen was blown into the steel using an oxygen lance. After decarburization, the molten steel is alloyed according to the requirements of the steel grade. The molten steel is ultra-low carbon molten steel; the powder spraying device includes 2 to 4 spraying channels evenly distributed along the circumference of the immersion tube, and the spraying direction is at an angle of 15° to 45° with the molten steel flow direction.

[0009] This invention relies on a dedicated powder spraying device, coordinated matching of molten steel flow field, and segmented intermittent spraying process to effectively achieve low-cost vacuum online temperature control of molten steel. It also has the advantages of excellent process stability and high steel purity, and is particularly suitable for the industrial smelting application of high-quality ultra-low carbon IF steel.

[0010] To achieve precise heating within ±5℃, the heating process is made predictable and precisely controllable, rather than relying on trial and error, thus significantly improving the accuracy of heating.

[0011] Preferably, the method of using a powder spraying device to spray aluminum powder into the molten steel in the ladle in batches for heating includes: the calculation formula for the total amount of aluminum powder sprayed (Q) is as follows: Q = 0.03 × W × △T / η In the formula, 0.03 is an empirical coefficient, representing the theoretical total amount of aluminum powder required per ton of molten steel to increase its temperature by 1°C, expressed in kgAl / (t). W is the weight of molten steel in tons (t); ΔT is the required temperature, i.e., the difference between the target temperature and the actual temperature, in degrees Celsius (°C); η is the aluminum powder reaction efficiency coefficient, which is 0.5~1.

[0012] Preferably, oxygen blowing using the lower oxygen lance includes an oxygen flow rate of 1800~2000 Nm³. 3 / h, the formula for calculating the total oxygen blowing volume is: G = 0.74 × Q In the formula, G represents the total amount of oxygen blown, in Nm³. 3 0.74 is an empirical coefficient representing the volume of oxygen required for the reaction of one kilogram of aluminum powder, in Nm³. 3 / kg; Q is the total amount of aluminum powder sprayed, in kg.

[0013] More preferably, the batch size is 2 to 3, with an interval of 30 to 70 seconds between each batch of aluminum powder spraying; the powder spraying ratio is 6 to 12 kg / Nm³. 3 .

[0014] Further preferably, the time interval between each batch of aluminum powder spraying is 40–60 seconds; the powder spraying ratio is 8–10 kg / Nm³. 3 .

[0015] Preferably, the carrier gas flow rate for heating the molten steel by spraying aluminum powder in batches using a powder spraying device is 1800~2400 Nm³. 3 / h.

[0016] More preferably, the carrier gas flow rate is 2000~2200 Nm³.3 / h.

[0017] More preferably, the carrier gas is argon.

[0018] Preferably, the decarburization process is selected based on the oxygen content of the molten steel entering the station, including: when the oxygen content of the molten steel entering the station is <350×10⁻⁶. -6 Forced decarbonization is used for decarbonization treatment; conversely, natural decarbonization is used for decarbonization treatment.

[0019] Preferably, the vacuum refining process includes: starting the molten steel circulation under vacuum conditions.

[0020] Preferably, the decarbonization treatment time is 15-20 minutes.

[0021] More preferably, after the decarburization treatment, the carbon content of the molten steel is ≤15ppm.

[0022] Preferably, before transporting the molten steel after converter smelting to the RH station, the process further includes: Control the final temperature of the converter and add active lime to the ladle during the tapping process; After the converter tapping is completed, the top slag of the ladle is modified.

[0023] More preferably, the converter endpoint temperature is 1640~1660℃.

[0024] More preferably, the mass of the active lime is 450~550 kg.

[0025] More preferably, the modification of ladle top slag includes adding a high-alumina slag conditioner and aluminum particles to the ladle slag surface. More preferably, the mass of the high-alumina slag conditioner is 300-400 kg; the mass of the aluminum particles is 50 kg.

[0026] Preferably, if the temperature of the molten steel entering the station is ≤1620℃, an aluminum heating process is used to heat the molten steel.

[0027] Preferably, the alloying time for molten steel is 5 to 10 minutes, depending on the steel grade requirements.

[0028] Preferably, after alloying the molten steel, the steel is tapped using conventional operations.

[0029] The beneficial effects of this invention are as follows: This invention, relying on a dedicated powder spraying device, coordinated matching of molten steel flow field, and segmented intermittent spraying process, effectively achieves low-cost vacuum online temperature control of molten steel. It also boasts advantages such as excellent process stability and high steel purity, making it particularly suitable for the industrial smelting of high-quality ultra-low carbon IF steel. Specifically: (1) By using the special powder spraying device provided by the present invention and its steel liquid flow field to coordinate and match, aluminum powder can be instantly dispersed and quickly participate in the metallurgical reaction, the aluminum powder reaction efficiency can be increased by more than 10%, and the heating rate can be increased by more than 20%.

[0030] (2) The segmented intermittent spraying process provided by this invention can avoid local overheating, splashing, or the formation of large Al2O3 inclusions caused by a single large-volume aluminum spraying, while providing sufficient time for the reaction products to float and be removed, thereby ensuring the cleanliness of the molten steel. After RH refining, the number density of inclusions can be reduced to 0.7~2.9 inclusions / mm. 2 .

[0031] Implementation methods The specific embodiments listed in this invention are merely examples, and the invention is not limited to the specific embodiments described below. For those skilled in the art, any equivalent modifications and substitutions to the embodiments described below are also within the scope of this invention. Therefore, all equivalent transformations and modifications made without departing from the spirit and scope of this invention should be covered within its scope. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. All reagents or instruments whose manufacturers are not specified are commercially available conventional products. To better illustrate this invention, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this invention can be practiced even without certain specific details. In other embodiments, methods, means, equipment, and steps well known to those skilled in the art are not described in detail in order to highlight the main points of this invention.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Unless otherwise specified, all units used in this specification are International Standard Units (SI), and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors unavoidable in industrial production.

[0033] Example 1 This embodiment provides a method for improving the RH heating rate and molten steel cleanliness of IF steel, taking the production of IF steel in a 200t ladle at a steel plant as the implementation object, including the following steps: S1. Control the converter's final temperature to 1645℃ and add 500kg of active lime to the ladle during the converter tapping process.

[0034] S2. After the converter tapping is completed, add 350 kg of high-alumina slag conditioner and 50 kg of aluminum granules to the slag surface of the ladle.

[0035] S3. The ladle is transported to the RH station, where the temperature and oxygen content of the molten steel are measured. The measured steel volume is 210t, the steel inlet temperature is 1600℃, and the inlet oxygen content is 400×10⁻⁶. -6 The aluminum heating process was used to raise the temperature of the molten steel entering the station to 1620℃.

[0036] S4. Decarburization is carried out using a natural decarburization process, and vacuum refining is performed simultaneously (the immersion tube is inserted into the molten steel to below the molten steel surface, a vacuum is drawn, and argon gas is introduced to start the molten steel circulation).

[0037] S5. During the vacuum refining process for 7 minutes, a powder spraying device was used, with argon gas of ≥99.99% purity as the carrier gas, to spray aluminum powder into the molten steel in the ladle in 3 batches to raise the temperature, and then oxygen was blown in with an oxygen lance. The powder spraying device consists of two spraying channels evenly distributed around the immersion tube, with the spraying direction at a 20° angle to the direction of molten steel flow; the 3 batches of aluminum powder spraying to raise the temperature of the molten steel include: the total amount of aluminum powder (Q) is 126 kg, calculated as shown in formula (1), the time interval between each batch of aluminum powder spraying is 60 s, and the powder spraying ratio is 10 kg / Nm 3 The carrier gas flow rate is 2200 Nm³. 3 / h; oxygen blowing flow rate is 2000 Nm 3 / h, the total oxygen blowing volume (G) is approximately 94 Nm³. 3 The calculation formula is shown in equation (2).

[0038] Q = 0.03 × W × △T / η (1) In the formula, 0.03 is an empirical coefficient, representing the theoretical amount of aluminum powder required per ton of molten steel to increase its temperature by 1°C, expressed in kgAl / (t). W is the weight of molten steel in tons (t), ΔT is the required heating temperature in ℃; η is the aluminum powder reaction efficiency coefficient (considering the influence of vacuum), which is taken as 1.

[0039] G = 0.74 × Q (2) In the formula, 0.74 is an empirical coefficient, representing the volume of oxygen required for the reaction of one kilogram of aluminum powder, in Nm³. 3 / kg; Q is the total amount of aluminum powder, in kg.

[0040] S6. After decarburization, aluminum shot and ferrotitanium are added sequentially for alloying, and the uniform alloying time is 6 minutes.

[0041] After S7.RH refining is completed, it is hoisted to the continuous casting process for pouring according to conventional operations.

[0042] Example 2 This embodiment provides a method for improving the RH heating rate and molten steel cleanliness of IF steel, taking the production of IF steel in a 200t ladle at a steel plant as the implementation object, including the following steps: S1. Control the final temperature of the converter to 1650℃, and add 550kg of active lime to the ladle during the tapping of the converter.

[0043] S2. After the converter tapping is completed, add 350 kg of high-alumina slag conditioner and 50 kg of aluminum granules to the slag surface of the ladle.

[0044] S3. The ladle containing molten steel was transported to the RH station, and the temperature and oxygen content of the molten steel were measured. The measured molten steel volume was 220t, the molten steel temperature upon entry was 1603℃, and the oxygen content upon entry was 415×10⁻⁶. -6 The aluminum heating process was used to raise the temperature of the molten steel entering the station to 1620℃.

[0045] S4. Decarburization is carried out using a natural decarburization process, and vacuum refining is performed simultaneously (the immersion tube is inserted into the molten steel to below the molten steel surface, a vacuum is drawn, and argon gas is introduced to start the molten steel circulation).

[0046] S5. During the vacuum refining process for 7 minutes, a powder spraying device was used, with argon gas of ≥99.99% purity as the carrier gas, to spray aluminum powder into the molten steel in the ladle in 3 batches to raise the temperature, and then oxygen was blown in by the oxygen lance. The powder spraying device consists of two spraying channels evenly distributed around the immersion tube, with the spraying direction at a 25° angle to the direction of molten steel flow; the aluminum powder spraying into the molten steel in the ladle in 3 batches to raise the temperature includes: the total amount of aluminum powder (Q) is 113 kg, calculated as shown in formula (1), the time interval between each batch of aluminum powder spraying is 60 s, and the powder spraying ratio is 8 kg / Nm 3 The carrier gas flow rate is 2000 Nm³. 3 / h; oxygen blowing flow rate is 1800 Nm 3 / h, total oxygen blowing (G) is 84Nm 3 The calculation formula is shown in equation (2).

[0047] S6. After the decarburization treatment is completed, aluminum shot and ferrotitanium are added in sequence for alloying. The uniform alloying time is 8 minutes.

[0048] After S7.RH refining is completed, it is hoisted to the continuous casting process for pouring according to conventional operations.

[0049] Experimental results show that the method provided by this invention allows aluminum powder to come into full contact with molten steel, effectively accelerating the heating rate of the molten steel. By spraying aluminum powder in batches, the problems of local overheating of molten steel, splashing of molten steel, or formation of large Al2O3 particles caused by spraying a large amount of aluminum at once can be eliminated. At the same time, sufficient reaction time is reserved to ensure that deoxidation products are fully floated and discharged, significantly improving the cleanliness of molten steel.

[0050] Compared to conventional aluminum heating processes, the inclusion density in the molten steel at the RH refining endpoint is 3.7~5.8 inclusions / mm. 2After using the method provided by this invention, the number density of inclusions is reduced to 0.7~2.9 inclusions / mm. 2 The cleanliness of molten steel is significantly improved.

[0051] Comparative Example 1 Compared with Example 1, Comparative Example 1 differs only in the powder spraying device—aluminum powder is sprayed into the molten steel flow in the riser pipe through a spray gun on the side wall of the immersion pipe.

[0052] Experimental results show that, compared with Example 1, the powder spraying device provided in Comparative Example 1 makes it difficult for aluminum powder to disperse instantly, and it is easy to aggregate or escape with the airflow, resulting in a reduction of more than 10% in aluminum powder reaction efficiency; at the same time, the aluminum powder does not come into sufficient contact with molten steel, the metallurgical reaction is delayed, and the heating rate decreases by more than 20%; it is also easy to cause excessive oxidation of aluminum powder in some areas or an increase in inclusions, affecting the purity of molten steel.

[0053] Comparative Example 2 Compared with Example 1, Comparative Example 2 differs only in its quantitative model, namely, it predicts the total amount of aluminum powder sprayed and the total amount of oxygen blown based solely on experience.

[0054] Experimental results show that, compared with Example 1, the quantitative model provided by Comparative Example 2 has poor heating accuracy, and the actual heating value fluctuation range is usually far greater than ±5℃; the heating process is unpredictable and requires repeated trial and error adjustments based on the operator's experience; at the same time, it is impossible to achieve precise control, and underheating or overheating is prone to occur, affecting subsequent alloying and molten steel quality.

[0055] Comparative Example 3 Compared to Example 1, Comparative Example 3 used only one batch of aluminum spraying for heating.

[0056] Experimental results show that, compared with Example 1, the method provided in Comparative Example 3 is more prone to local overheating, causing molten steel splashing and generating large Al2O3 inclusions; the reaction products have insufficient flotation time, making them difficult to remove effectively, and the cleanliness of the molten steel deteriorates; after RH refining, the inclusion number density increases by 3 particles / mm. 2 above.

Claims

1. A method for improving the RH heating rate and molten steel cleanliness of IF steel, characterized in that, Includes the following steps: The molten steel after converter smelting is transported to the RH station, and the temperature and oxygen content of the molten steel are measured to obtain the temperature and oxygen content of the molten steel entering the station. The decarburization process is selected based on the oxygen content of the molten steel entering the station, and vacuum refining is performed simultaneously. During the vacuum refining process for 7 minutes, aluminum powder is sprayed onto the molten steel in batches using a powder spraying device to raise the temperature, and oxygen is blown in using an oxygen lance. After the decarburization process is completed, the molten steel is alloyed according to the requirements of the steel grade. The molten steel is ultra-low carbon molten steel; the powder spraying device includes 2 to 4 spraying channels evenly distributed along the circumference of the immersion tube, and the spraying direction is at an angle of 15° to 45° with the flow direction of the molten steel.

2. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 1, characterized in that, The step of using a powder spraying device to spray aluminum powder onto the molten steel in batches for heating includes: the formula for calculating the total amount of aluminum powder sprayed is as follows: Q = 0.03 × W × △T / η In the formula, 0.03 is an empirical coefficient, representing the theoretical total amount of aluminum powder required per ton of molten steel to increase its temperature by 1°C, expressed in kgAl / (t). W is the weight of molten steel in tons (t); ΔT is the required heating temperature in degrees Celsius (°C); η is the aluminum powder reaction efficiency coefficient, which is 0.5 to 1.

3. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 1, characterized in that, The oxygen blowing process performed by the lower oxygen lance includes an oxygen flow rate of 1800~2000 Nm. 3 / h, the formula for calculating the total oxygen blowing volume is: G = 0.74 × Q In the formula, G represents the total amount of oxygen blown, in Nm³. 3 0.74 is an empirical coefficient representing the volume of oxygen required for the reaction of one kilogram of aluminum powder, in Nm³. 3 / kg; Q is the total amount of aluminum powder sprayed, in kg.

4. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 1, characterized in that, The batch size is 2-3, with an interval of 30-70 seconds between each batch of aluminum powder spraying; the powder spraying ratio is 6-12 kg / Nm³. 3 .

5. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 4, characterized in that, The time interval between each batch of aluminum powder spraying is 40-60 seconds; the powder spraying ratio is 8-10 kg / Nm³. 3 .

6. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 1, characterized in that, The carrier gas flow rate for heating the molten steel by spraying aluminum powder in batches using a powder spraying device is 1800~2400 Nm. 3 / h.

7. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 1, characterized in that, The step of selecting a decarburization process based on the oxygen content of the molten steel entering the station includes: when the oxygen content of the molten steel entering the station is <350×10 -6 Forced decarbonization is used for decarbonization treatment; conversely, natural decarbonization is used for decarbonization treatment.

8. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 1, characterized in that, Before transporting the molten steel from the converter to the RH station, the process also includes: Control the final temperature of the converter and add active lime to the ladle during the tapping process; After the converter tapping is completed, the top slag of the ladle is modified.

9. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 8, characterized in that, The final temperature of the converter is 1640~1660℃; the mass of the active lime is 450~550kg.

10. The method for improving the RH heating rate and molten steel cleanliness of IF steel according to claim 8, characterized in that, The modification of ladle top slag includes: adding a high-alumina slag conditioner and aluminum particles to the ladle slag surface; the mass of the high-alumina slag conditioner is 300~400 kg, and the mass of the aluminum particles is 50 kg.