A method for controlling converter end point of ultra-low carbon steel based on gradient deoxidation of composite carbon source

Abstract

The application discloses a method for controlling the end point of ultra-low carbon steel based on composite carbon source gradient deoxidation, and belongs to the technical field of steelmaking converter steelmaking process. The method constructs a three-stage gradient deoxidation system of "converter-outlet of tundish-steel ladle", and matches accurate carbon source design and adding strategy, so that the oxygen content is efficiently and stably reduced in the converter tapping process, the risk of carbon increase is avoided, and the foundation is laid for subsequent production of ultra-high cleanliness molten steel.

Description

Technical Field

[0001] This invention belongs to the field of steelmaking process technology in iron and steel metallurgy converters, and particularly relates to an endpoint control method for ultra-low carbon steel converters based on gradient deoxidation using composite carbon sources. Background Technology

[0002] In the smelting process of ultra-low carbon steel and IF steel, the oxygen content control of the molten steel at the converter endpoint is a key factor in determining the cleanliness of the final product. As the carbon content decreases (C≤0.0030%), the oxygen content in the molten steel increases sharply (usually reaching 800-1200ppm), which becomes the main source of subsequent endogenous oxide inclusions, seriously affecting the deep drawing performance, surface quality, and fatigue life of the steel.

[0003] Currently, the industry mainly uses two methods to control endpoint oxygen:

[0004] 1. Thermodynamic equilibrium control: The oxygen content is reduced by forcibly lowering the final temperature. However, due to the temperature reduction requirements of subsequent processes, the final temperature usually needs to be maintained at 1680-1710℃, and the oxygen content remains high.

[0005] 2. Post-smelting treatment: During the ladle refining stage, oxygen content is reduced through aluminum deoxidation or vacuum carbon deoxidation (VCD). However, this has obvious drawbacks: aluminum deoxidation produces solid Al2O3 inclusions, which are difficult to remove; while VCD produces gaseous products, the reaction occurs inside the ladle, where stirring conditions are limited, resulting in low deoxidation efficiency and potentially causing excessive carbon increase in the molten steel.

[0006] Patent searches and literature reviews revealed that existing technologies (such as CN113215406A) disclose methods for pre-deoxidation by adding a carburizing agent during the steel tapping process, but these methods have three core problems:

[0007] ① The deoxidation mechanism is too simple: it relies solely on the carbon-oxygen reaction, which is insufficient for controlling the overall oxygen potential of complex steel slag systems;

[0008] ② Low efficiency and instability: The temperature drop during the tapping process is large, the mixing of steel and slag is intense, the carbon-oxygen reaction kinetics are poor, the measured deoxidation efficiency is far lower than the theoretical value, and the fluctuation is huge;

[0009] ③ Uncontrollable carbon increase risk: The dissolution rate of a single type of carbon source (such as coke) does not match the deoxidation reaction, which can easily lead to local carbon increase or excessive carbon content, which does not meet the precise control requirements of ultra-low carbon steel.

[0010] Therefore, there is an urgent need for a systematic method that can efficiently, stably, and with low risk reduce the final oxygen content in a converter. Summary of the Invention

[0011] The purpose of this invention is to provide a method for controlling the endpoint of ultra-low carbon steel converter deoxidation based on a composite carbon source gradient deoxidation. This method constructs a three-stage gradient deoxidation system ("in-furnace-tapping-ladle") and matches it with a precise carbon source design and addition strategy. This achieves efficient and stable reduction of oxygen content during converter tapping while avoiding the risk of carbon increase, laying the foundation for subsequent production of ultra-high purity steel.

[0012] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0013] This invention discloses a method for controlling the endpoint of ultra-low carbon steel converter based on gradient deoxidation using a composite carbon source, comprising the following steps:

[0014] 1) Endpoint target control and preparation

[0015] The final target for converter blowing is: carbon content controlled at 0.025%-0.040%, and temperature controlled at 1685-1700℃.

[0016] Prepare a composite carbon source: including a highly reactive carbon source A and a slow-release carbon source B, wherein the highly reactive carbon source A has a particle size of 0.5-1.0 mm and a fixed carbon content of ≥98%, and the slow-release carbon source B has a particle size of 5-10 mm and a fixed carbon content of ≥96%, and the mass ratio of A to B is (6:4) to (7:3).

[0017] 2) In-furnace pre-deoxidation - primary and secondary deoxidation

[0018] After raising the lance and before tapping the steel from the furnace, add 30%-40% of the total weight of the composite carbon source to the surface of the molten pool through the auxiliary lance hole or furnace opening;

[0019] After addition, maintain weak stirring with bottom-blown argon gas at a flow rate of 0.03-0.05 Nm³ / min·t for 1.5-2.5 minutes to perform preliminary deoxidation using residual heat from the molten pool and the slag-gold interface reaction.

[0020] 3) Enhanced deoxidation during steel tapping - secondary deoxidation

[0021] When the steel begins to be tapped, when 1 / 4 of the total amount of molten steel has flowed out, 60%-70% of the remaining total weight of the composite carbon source is evenly added to the steel flow through a special chute above the steel flow impact zone.

[0022] 4) Final deoxidation and slag layer protection in the ladle (three-stage deoxidation)

[0023] When the steel is tapped to 2 / 3 full, pre-melted refining slag, CaO-AL2O3 based, is added with the steel stream at a rate of 3.5-4.5 kg / t steel to quickly form a liquid covering slag layer.

[0024] The liquid slag layer can isolate air, while providing an escape channel for CO bubbles rising from the molten steel, and adsorbing fine inclusions of deoxidation products that rise to the surface.

[0025] 5) Immediate processing after tapping

[0026] After tapping, a small amount of aluminum granules (0.1-0.2 kg / t steel) is immediately added to the slag surface of the ladle to perform final reduction of the top slag, rapidly reducing the FeO+MnO content in the slag to below 1.0% and preventing oxygen from the slag from returning to the molten steel.

[0027] Furthermore, the smelting of ultra-low carbon IF steel aims for a carbon content of ≤0.0020%.

[0028] The composite carbon source consists of 2.4 tons of highly reactive activated carbon powder and 1.6 tons of slow-release small-particle coke, mixed evenly.

[0029] Further, in-furnace pre-deoxidation: After lifting the lance, add 1.2 tons of composite carbon source (30% of the total) to the slag surface through the furnace opening feeding system. Start bottom blowing argon at a flow rate of 8 Nm³ / min and gently stir for 2 minutes.

[0030] Further, the steel tapping process involves enhanced deoxidation: Steel tapping begins. When approximately 60 tons of molten steel have flowed out, the vibrating feeder of the tapping chute is activated to evenly add the remaining 2.8 tons of composite carbon source to the steel flow impact zone. The tapping time is controlled within 7-8 minutes.

[0031] Further, final deoxidation and slag formation in the ladle: When about 160 tons of steel have been tapped, pre-melted refining slag is added, totaling 9.6 tons. By the time the tapping is complete, the slag layer has basically melted.

[0032] Furthermore, the pre-melted refining slag comprises, by mass percentage, 50% CaO and 40% Al₂O₃.

[0033] Furthermore, immediately add 48 kg of aluminum granules to the slag surface of the ladle.

[0034] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0035] 1. Significantly Improved Deoxidation Efficiency and Stability: The "three-stage gradient deoxidation" design overcomes the limitations of single-stage deoxidation. Highly active carbon source A reacts rapidly in the furnace and tapping stream, reducing the initial oxygen potential; slow-release carbon source B reacts continuously in the ladle, extending the effective deoxidation time. Actual measurements show that the oxygen content reduction from the endpoint to tapping can stably reach 250-350 ppm, with deoxidation efficiency more than 80% higher than the single tapping carbon addition method, and fluctuations reduced by 60%.

[0036] 2. Precise control of carbon gain risk: Through the design and phased addition of composite carbon sources, carbon is released "on demand." This avoids uneven carbon gain caused by the local dissolution of a single large carbon source. The carbon gain from the final carbon source to the steel tapping can be precisely controlled within the range of 0.0015%-0.0025%, fully meeting the composition control requirements of ultra-low carbon steel.

[0037] 3. Significantly Improved Steel Cleanliness: The deoxidation products of this method are mainly CO gas, with virtually no new solid inclusions. Simultaneously, the rapidly formed liquid refining slag effectively adsorbs the original deoxidation products in the steel, reducing the number of large oxide inclusions in the molten steel by approximately 40% after tapping. 4. Significant Economic Benefits: Based on a 240-ton converter, this method can reduce deoxidation aluminum consumption in the subsequent RH process by approximately 0.3 kg / t of steel, saving approximately 10 yuan per ton of steel. Furthermore, due to the cleaner molten steel, the risk of nozzle clogging in continuous casting is reduced, improving production smoothness. Detailed Implementation

[0038] Example

[0039] A converter endpoint control method for ultra-low carbon steel based on gradient deoxidation using a composite carbon source, for smelting ultra-low carbon IF steel (target C≤0.0020%):

[0040] 1. Endpoint Control: After converter blowing ends, the endpoint carbon content (C) at the auxiliary lance is measured to be ≤0.040%, and the temperature is 1685-1690℃. The calculated target oxygen reduction at tapping is 300-500ppm.

[0041] 2. Carbon source preparation: Based on theoretical calculations and empirical coefficients, the total carbon requirement is approximately 0.8 kg / t of steel. Prepare a composite carbon source: 2.4 tons of highly reactive activated carbon powder (A) and 1.6 tons of slow-release small-particle coke (B), mix them evenly.

[0042] 3. In-furnace pre-deoxidation: After lifting the lance, add 1.2 tons of composite carbon source (30% of the total) to the slag surface through the furnace opening feeding system. Start bottom blowing argon at a flow rate of 8 Nm³ / min and gently stir for 2 minutes.

[0043] 4. Steel tapping and deoxidation: Begin tapping. When approximately 60 tons of molten steel have flowed out, activate the vibrating feeder in the tapping chute to evenly add the remaining 2.8 tons of composite carbon source to the steel flow impact zone. Tapping time should be controlled within 7-8 minutes.

[0044] 5. Final deoxidation and slag formation in the ladle: When approximately 160 tons of steel have been tapped, pre-melted refining slag (CaO 50%, Al2O3 40%) is added, totaling approximately 9.6 tons (4 kg / t). By the time tapping is complete, the slag layer has essentially melted.

[0045] 6. Final reduction of top slag: After tapping, immediately add about 48 kg of aluminum granules (0.2 kg / t) to the slag surface of the ladle.

[0046] Comparative example (traditional method)

[0047] Traditional methods for controlling the endpoint of ultra-low carbon IF steel in converters: Ultra-low carbon IF steel (target C≤0.0020%)

[0048] 1. Endpoint control: After the converter blowing is completed, the carbon C at the endpoint of the secondary lance test is ≤0.040%, the temperature is 1685~1690℃, and no pre-deoxidation treatment is performed in the furnace.

[0049] 2. Carbon source preparation: A single carbon source, ordinary coke granules, is used, with a total addition of 0.8 kg / t steel, without gradation or gradient control.

[0050] 3. Furnace operation: No pre-deoxidation is performed after lifting the lance, no composite carbon source is added, bottom blowing argon is maintained at a normal flow rate of 3 Nm³ / min, and there is no weak stirring enhancement.

[0051] 4. Steel tapping deoxidation: All carbon sources are added to the steel stream at once throughout the entire tapping process, without gradients or segmented additions; the tapping time is 6-9 minutes, with large fluctuations, and precise feeding control without impact zones. 5. Ladle slag formation: Pre-melted refining slag (CaO 50%, Al2O3 40%) is added simultaneously during the tapping process, with an addition amount of 9.6 tons (4 kg / t). It is added all at once and the melting is insufficient.

[0052] 6. Top slag reduction: After tapping, 48 kg of aluminum granules (0.2 kg / t) are added at once. There is no gradient reduction and the slag system has weak reducibility.

[0053] The differences between the method of this invention and the traditional method are shown in the table below:

[0054] Process Example (Composite carbon source gradient deoxygenation method) Comparative example (traditional converter endpoint control method) Core differences Applicable steel grades Ultra-low carbon IF steel (target C≤0.0020%) Ultra-low carbon IF steel (target C≤0.0020%) Consistent Converter end point C≤0.040%, temperature 1685-1690℃ C≤0.040%, temperature 1685-1690℃ Consistent Carbon source type Composite carbon source: 2.4t of highly active carbon powder + 1.6t of slow-release pyroxene. Single carbon source: ordinary coke particles Composite / Single Total carbon 0.8kg / t steel 0.8kg / t steel Consistent In-furnace pre-deoxidation After lifting the lance, add 1.2t (30%) of composite carbon source; bottom blow argon at 8 Nm³ / min, and gently stir for 2 min. No in-furnace pre-deoxidation; bottom-blown argon 3 Nm³ / min, no enhanced stirring. With / without pre-deoxygenation Steel deoxidation Starting from 60 tons of steel produced, the remaining 2.8 tons are evenly added to the steel flow impact zone. All steel is added at once during the entire tapping process, without segmentation or fixed points. Gradient / One-time Steel tapping time Stable control for 7-8 minutes 6-9 minutes, large fluctuations Precise / Extensive Slag production system When the steel output reaches 160 tons, 9.6 tons (4 kg / t) of pre-melted slag are added. Add steel at the same time during tapping Step-by-step / synchronous Top slag reduction 48 kg of steel-grade Pica aluminum granules (0.2 kg / t) were produced. 48 kg of steel-grade Pica aluminum granules (0.2 kg / t) were produced. Consistent Deoxygenation mode Gradient-step deoxidation, in-furnace + tapping dual-stage deoxidation Single-point one-time deoxygenation Gradient / Traditional

[0055] The experimental data for the examples and comparative examples are shown in the table below:

[0056] Project (data from 20 furnaces) Example Traditional method (comparative example) endpoint oxygen content (ppm) 892 888 Oxygen content (ppm) after tapping 625 733 Oxygen content decreased by ppm 267 155 Decrease in value fluctuation range (ppm) ±25 ±65 Carbon increase % 0.0021 0.0028 Carbon increase fluctuation range % ±0.0005 ±0.0015

[0057] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from its spirit should fall within the protection defined by the claims.

[0058] Within the range.

Claims

1. A method for controlling the endpoint of ultra-low carbon steel converter based on gradient deoxidation using a composite carbon source, characterized in that, Includes the following steps: 1) Endpoint target control and preparation The final target for converter blowing is: carbon content controlled at 0.025%-0.040%, and temperature controlled at 1685-1700℃. Prepare a composite carbon source: including a highly reactive carbon source A and a slow-release carbon source B, wherein the highly reactive carbon source A has a particle size of 0.5-1.0 mm and a fixed carbon content of ≥98%, and the slow-release carbon source B has a particle size of 5-10 mm and a fixed carbon content of ≥96%, and the mass ratio of A to B is (6:4) to (7:3). 2) In-furnace pre-deoxidation - primary and secondary deoxidation After raising the lance and before tapping the steel from the furnace, add 30%-40% of the total weight of the composite carbon source to the surface of the molten pool through the auxiliary lance hole or furnace opening; After addition, maintain weak stirring with bottom-blown argon gas at a flow rate of 0.03-0.05 Nm³ / min·t for 1.5-2.5 minutes to perform preliminary deoxidation using residual heat from the molten pool and the slag-gold interface reaction. 3) Enhanced deoxidation during steel tapping - secondary deoxidation When the steel begins to be tapped, when 1 / 4 of the total amount of molten steel has flowed out, 60%-70% of the remaining total weight of the composite carbon source is evenly added to the steel flow through a special chute above the steel flow impact zone. 4) Final deoxidation and slag layer protection in the ladle - three-stage deoxidation When the steel is tapped to 2 / 3 full, pre-melted refining slag, CaO-AL2O3 based, is added with the steel stream at a rate of 3.5-4.5 kg / t steel to quickly form a liquid covering slag layer. The liquid slag layer can isolate air, while providing an escape channel for CO bubbles rising from the molten steel, and adsorbing fine inclusions of deoxidation products that rise to the surface. 5) Immediate processing after tapping After tapping, a small amount of aluminum granules (0.1-0.2 kg / t steel) is immediately added to the slag surface of the ladle to perform final reduction of the top slag, rapidly reducing the FeO+MnO content in the slag to below 1.0% and preventing oxygen from the slag from returning to the molten steel.

2. The method for controlling the endpoint of ultra-low carbon steel converter based on gradient deoxidation using a composite carbon source according to claim 1, characterized in that, Smelting ultra-low carbon IF steel, with a target C ≤ 0.0020%: The composite carbon source consists of 2.4 tons of highly reactive activated carbon powder and 1.6 tons of slow-release small-particle coke, mixed evenly.

3. The method for controlling the endpoint of ultra-low carbon steel converter based on gradient deoxidation using a composite carbon source according to claim 2, characterized in that, Pre-deoxidation in the furnace: After lifting the lance, add 1.2 tons of composite carbon source to the slag surface through the furnace opening feeding system; start bottom blowing argon at a flow rate of 8 Nm³ / min and stir weakly for 2 minutes.

4. The method for controlling the endpoint of ultra-low carbon steel converter based on gradient deoxidation using a composite carbon source according to claim 3, characterized in that, Strengthening deoxidation during tapping: Start tapping; when 60 tons of molten steel have flowed out, start the vibrating feeder of the tapping chute to evenly add the remaining 2.8 tons of composite carbon source into the steel flow impact zone; the tapping time is controlled at 7-8 minutes.

5. The method for controlling the endpoint of ultra-low carbon steel converter based on gradient deoxidation using a composite carbon source according to claim 4, characterized in that, Final deoxidation and slag formation in the ladle: When the steel reaches 160 tons, pre-melted refining slag is added, totaling 9.6 tons. By the time the steel is finished being tapped, the slag layer has basically melted.

6. The method for controlling the endpoint of ultra-low carbon steel converter based on gradient deoxidation using a composite carbon source according to claim 1, characterized in that, The pre-melted refining slag comprises, by mass percentage, 50% CaO and 40% Al2O3.

7. The method for controlling the endpoint of ultra-low carbon steel converter based on gradient deoxidation using a composite carbon source according to claim 5, characterized in that, Immediately after tapping, add 48 kg of aluminum granules to the slag surface of the ladle.