LF furnace refining process for aluminum-killed clean steel

Abstract

This invention provides an LF furnace refining process suitable for aluminum deoxidized pure steel, belonging to the field of iron and steel metallurgical technology. This invention ensures stable aluminum content by adding aluminum only once during converter tapping and eliminating aluminum replenishment throughout the LF process. It employs a pre-melted refining slag with specific composition, combined with a four-stage precise bottom-blowing argon system, achieving efficient deoxidation and inclusion removal. No calcium treatment is performed throughout the process, eliminating the formation of Ds-type spherical inclusions at the source. This invention solves the problems of unstable aluminum content, reliance on calcium treatment, insufficient purity, and poor castability in existing technologies. The total oxygen content of the molten steel is reduced to below 3.5 ppm, Ds-type inclusions are undetectable, and the number of consecutive casting heats is increased to 12-18, significantly improving the fatigue life and production efficiency of the steel. It is suitable for the industrial production of high-purity aluminum deoxidized steels such as bearing steel and spring steel.

Description

Technical Field

[0001] This invention relates to the field of iron and steel smelting technology, and in particular to an LF furnace refining process suitable for aluminum deoxidation and purification of steel. Background Technology

[0002] With the increasing demands for fatigue life, purity, processing performance, and stability in steel from high-end equipment manufacturing, energy, transportation, aerospace, and other industries, steel materials are upgrading from ordinary quality to high purity, high uniformity, and ultra-fine grains. Aluminum-deoxidized steel, due to its strong deoxidizing ability, low cost, and mature operation, is widely used in the production of key steel grades such as bearing steel, spring steel, gear steel, high-strength structural steel, and cold heading steel.

[0003] In current industrial production, the conventional LF refining process for aluminum deoxidized steel generally suffers from the following four core defects: 1. Aluminum content control is unstable, resulting in large fluctuations in recovery rate. Conventional processes involve adding aluminum granules, aluminum wires, or aluminum ash multiple times during LF refining for deoxidation. The aluminum recovery rate is greatly affected by the oxidizing properties of the slag, the intensity of argon blowing, temperature fluctuations, and slag composition. This results in fluctuating acid-soluble aluminum (Als) content in the steel, low component hit rate, and problems such as nozzle blockage and turbulence during continuous casting, limiting the number of consecutive casting furnaces.

[0004] 2. It must rely on calcium treatment, which introduces harmful impurities. To address the issue of high-melting-point Al2O3 inclusions causing nozzle blockage during aluminum deoxidation, mainstream industrial aluminum deoxidation steelmaking processes currently employ calcium treatment (feeding silicon-calcium wire, calcium-iron wire, etc.) to convert Al2O3 into low-melting-point calcium aluminate. However, calcium treatment has serious drawbacks: The recovery rate of calcium is extremely low (only 10%–30%) and fluctuates greatly. Calcium aluminate inclusions have high hardness and poor plasticity. Although they are not easy to clog the gate, they will form large spherical oxide particles of type Ds, which will seriously reduce the fatigue life and processing performance of steel. Calcium treatment causes a violent reaction, which can easily lead to secondary oxidation and nitrogen accumulation in molten steel. Calcium accelerates the corrosion of refractory materials and increases the risk of foreign inclusions.

[0005] 3. The slag composition is unstable, and the removal efficiency of inclusions is low. Conventional LF refining slag mostly uses a mixed slag system of lime + fluorite + aluminum ash. Its composition fluctuates greatly, its alkalinity is unstable, its fluidity is poor, and its adsorption capacity for Al2O3 inclusions is weak. A large amount of fine Al2O3 remains in the steel, resulting in insufficient purity of the molten steel and high oxygen content (usually ≥6ppm).

[0006] 4. An unreasonable bottom-blowing argon system affects the flotation of inclusions and the castability of molten steel. Conventional bottom-blowing argon flow control is crude: strong stirring in the early stage, arbitrary adjustment in the middle stage, and insufficient soft blowing in the later stage can easily lead to exposed molten steel, secondary oxidation, and inclusions that are difficult to collide, grow and float; or excessive stirring can lead to slag entrapment, which in turn increases the number of inclusions.

[0007] According to the search, relevant domestic and foreign patents are mainly divided into three categories: Aluminum deoxidation + calcium treatment is the mainstream: such as CN113718085A and CN101597666A, both of which adopt the LF multiple aluminum replenishment + calcium treatment mode, which cannot eradicate the Ds type inclusion problem.

[0008] No calcium treatment but relying on rare earth / titanium alloy substitutes: such as CN117758132A and CN108330389B, calcium treatment is replaced by rare earth or titanium treatment, but rare earth is extremely expensive and scarce, and titanium alloy is prone to forming TiN and TiC inclusions, which affect the performance of steel.

[0009] Single optimization of bottom blowing or slag system: such as ZL202511691003.9 and CN113718085A, only adjust the bottom blowing flow rate or slag system composition, without solving the systemic problem of "one-time aluminum addition to stabilize composition + calcium-free treatment + precise bottom blowing + special pre-melted slag".

[0010] In summary, none of the existing technologies can simultaneously achieve: precise control of aluminum content in a single operation, calcium-free treatment throughout the entire process, a dedicated pre-melted slag system, and a segmented precise bottom-blowing argon system. Therefore, it is impossible to significantly reduce Ds-type inclusions, improve the castability of molten steel, and increase the number of consecutive castings while ensuring high purity. Summary of the Invention

[0011] This invention proposes an LF furnace refining process suitable for aluminum deoxidized pure steel, overcoming the shortcomings of existing aluminum deoxidized steel LF refining processes, such as unstable aluminum content, dependence on calcium treatment, large amount of Ds-type inclusions, insufficient purity, poor castability, and low number of consecutive castings.

[0012] To achieve the above objectives, the present invention adopts the following technical solution: an LF furnace refining process suitable for aluminum deoxidation and pure steel, wherein the method is as follows: 1) Precipitation deoxidation: Based on the final C content, sufficient aluminum iron, aluminum ingots or other aluminum-containing alloys are added during the tapping process in the converter. To improve the stability of Al element recovery rate, the aluminum iron, aluminum ingots or other aluminum-containing alloys can be added after the other alloys are added to ensure that the Al content in the initial LF smelting is in the range of 0.020-0.050%, and no further adjustment of Al content is made. 2) Slag-forming material: Pre-melted refining slag is added to the LF furnace, and an appropriate amount of lime is added according to the liquidus temperature of the steel grade to ensure the viscosity and fluidity of the slag; 3) Ladle bottom blowing: The bottom blowing mode needs to be used in conjunction with the pre-melted refining slag. After the LF furnace refining starts, for the first 5-10 minutes, the ladle bottom blowing argon flow rate is ≥500NL / min, and the steel-slag interface is fully mixed to promote the rapid melting of the pre-melted refining slag. After refining sampling, for 10-20 minutes, the ladle bottom blowing argon flow rate is adjusted to 200-400NL / min for rapid deoxidation, and exposed molten steel is prohibited. Then, during the 15-25 minute refining period, the ladle bottom blowing argon flow rate is adjusted to 50-100NL / min. Then, during the refining period until the end, the ladle bottom blowing argon flow rate is adjusted to 20-50NL / min. During the LF refining period, the deoxidation intensity decreases with the bottom blowing intensity, and the power supply current also decreases continuously. In the later stage, only the slag fluidity (to maintain the adsorption capacity of inclusions) and the molten steel temperature are maintained. The initial strong stirring accelerates the melting of the slag, and the subsequent bottom blowing pressure is gradually reduced. Under the premise of ensuring that the molten steel is not exposed, the inclusions collide and grow, and are subsequently removed in large quantities and absorbed by the slag.

[0013] 4) No calcium treatment is performed during the entire LF refining process.

[0014] Preferably, the composition of the pre-melted refining slag and the weight percentage of each component are as follows: CaO: 42%-55%, Al2O3: 30%-36%, MgO: 2-7%, SiO2≤2%, FeO+MnO≤0.5%, S≤0.1%, P≤0.1%, TiO≤0.30%, H2O≤0.5%, and the amount of the pre-melted refining slag used per ton of steel is 10kg-15kg.

[0015] Preferably, the lime added in step 2 requires a CaO content of ≥90%.

[0016] Preferably, the C content at the converter endpoint in step 1 is ≥0.15%, and the tapping temperature is ≥1630℃.

[0017] Preferably, after LF refining, RH vacuum circulation treatment is performed, and the vacuum holding time is 15min-25min; or the ladle is left to stand and softly blow argon for 15min-20min.

[0018] Preferably, for steel grades that do not have requirements for finished aluminum content, the content of acid-soluble aluminum (Als) in the finished product should be controlled to be ≤0.015%.

[0019] Compared with the prior art, the advantages and positive effects of the present invention are as follows: Stable and controllable aluminum content, high recovery rate: aluminum is added only once and no additional aluminum is needed throughout the process. The Al content in the initial LF refining is stable at 0.020%–0.050%, and the Al content in the finished product can be stably ≤0.015%. The aluminum recovery rate fluctuates by ≤5%, completely solving the problem of fluctuating aluminum content in conventional processes.

[0020] Completely calcium-free treatment to eliminate Ds-type inclusions: No calcium treatment is performed to avoid the formation of Ds-type spherical inclusions of calcium aluminate. The Ds-type inclusion rating in the steel is reduced from level 2.0 in conventional processes to level 0 or 0.5. The maximum inclusion diameter is ≤10μm, and the fatigue life of the steel is increased by more than 30%.

[0021] Special pre-melted slag + precise bottom blowing significantly improves purity: The pre-melted slag has a strong ability to adsorb inclusions. Combined with four-stage precise bottom blowing, the total oxygen content in the steel is reduced from 7ppm-8ppm in conventional processes to below 3.5ppm. The B and D class inclusion ratings are significantly reduced, and the purity of the molten steel reaches the level of ultra-high purity steel.

[0022] The steel exhibits excellent pourability, significantly increasing the number of consecutive heats: the finished product has a low Al content (≤0.015%), requires no calcium treatment, and contains small and easily removed inclusions, completely solving the problems of nozzle blockage and turbulence. The number of consecutive heats has increased from 8–10 heats in the conventional process to 12–18 heats, greatly improving production efficiency.

[0023] Cost reduction and process simplification: Eliminating calcium treatment saves on silicon-calcium line costs; LF process requires no aluminum replenishment, reducing aluminum particle consumption; Simplified process steps and strong operational stability make it suitable for large-scale industrial production. Detailed Implementation

[0024] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described below through embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0025] Numerous specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways than those described herein, and therefore the invention is not limited to the specific embodiments disclosed in the following specification. Example 1

[0026] This embodiment provides an LF furnace refining process suitable for aluminum deoxidation and pure steel production of 52CrMoV4 spring steel (high purity requirement). The process flow is as follows: 100t converter → tapping with aluminum addition → LF refining (pre-melted slag + four-stage bottom blowing + calcium-free treatment) → RH vacuum circulation → static soft blowing → continuous casting.

[0027] Process parameter control Converter tapping: final C=0.17%, tapping temperature=1635℃; 244kg of ferroaluminum added in the later stage of tapping, Als=0.032% in the initial LF smelting.

[0028] LF refining slag: Pre-melted slag (CaO 48%, Al2O3 33%, MgO 4.5%, SiO2 1.2%, FeO+MnO 0.35%), add 12kg per ton of steel, and supplement with 120kg of lime.

[0029] Four-stage bottom-blown argon: 0–6 min: 523 NL / min; 6–19 min: 364 NL / min; 19–38 min: 92 NL / min; 38 min to end: 43 NL / min.

[0030] Calcium treatment: Do not feed calcium supplements or add calcium-containing materials throughout the entire process.

[0031] Subsequent processing: RH vacuum maintained for 24 min, then allowed to stand and soft blown for 15 min; finished product Als=0.008%.

[0032] Performance test results Total oxygen content: 2.2 ppm; Inclusion rating: Class A fine series 0.5, Class B fine series 0.5, Class D fine series 0.5, Ds fine series inclusions not detected; Number of consecutive casting furnaces: 12; There was no sprue blockage or turbulence during the continuous casting process. Example 2

[0033] This embodiment provides an LF furnace refining process suitable for aluminum deoxidation and pure steel production, producing high-standard 51CrV4 spring steel. The process flow is as follows: 100t converter → tapping with aluminum addition → LF refining (pre-melted slag + four-stage bottom blowing + calcium-free treatment) → RH vacuum circulation → static soft blowing → continuous casting.

[0034] Process parameter control Converter tapping: final C=0.16%, tapping temperature=1632℃; 256kg of ferroaluminum added in the later stage of tapping, Als=0.038% in the initial LF smelting.

[0035] LF refining slag: Pre-melted slag (CaO 51%, Al2O3 32%, MgO 3.8%, SiO2 1.5%, FeO+MnO 0.4%), add 14kg per ton of steel, and supplement with 110kg of lime.

[0036] Four-stage bottom-blown argon: 0–7 min: 514 NL / min; 7–19 min: 325 NL / min; 19–40 min: 87 NL / min; 40 min to end: 39 NL / min.

[0037] Calcium treatment: Do not feed calcium supplements or add calcium-containing materials throughout the entire process.

[0038] Subsequent processing: RH vacuum maintained for 17 min, then allowed to stand and soft blown for 20 min; finished product Als=0.012%.

[0039] Performance test results Total oxygen content: 3.4 ppm; Inclusion rating: Class A fine series 0.5, Class B fine series 0.5, Class D fine series 0.5, Ds fine series inclusions not detected; Number of consecutive casting furnaces: 18; There was no sprue blockage or turbulence during the continuous casting process.

[0040] Comparative example: The conventional process for producing 51CrV4 spring steel has the following process parameters: Converter tapping: final C=0.14%, tapping temperature=1635℃; add 114kg of aluminum iron first.

[0041] LF refining: Synthetic slag + lime slag formation, aluminum particle deoxidation, initial bottom blowing 551 NL / min, later adjusted according to power supply, without deliberate flow control; 100m of silicon-calcium wire is fed. Subsequent treatment: RH vacuum for 21min, static soft blowing for 20min; finished product Als=0.021%.

[0042] Performance Results Total oxygen content: 7.4 ppm; Ds type inclusions: Grade 2.0, maximum diameter 48 μm; Number of consecutive casting furnaces: 8; Slight nozzle clogging occurred during the continuous casting process.

[0043] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A LF furnace refining process suitable for aluminum killed clean steel, characterized in that, The method is as follows: 1) Precipitation deoxidation: Based on the final C content, sufficient aluminum iron, aluminum ingots or other aluminum-containing alloys are added during the tapping process in the converter. To improve the stability of Al element recovery rate, the aluminum iron, aluminum ingots or other aluminum-containing alloys can be added after the other alloys are added to ensure that the Al content in the initial LF smelting is in the range of 0.020-0.050%, and no further adjustment of Al content is made. 2) Slag-forming material: Pre-melted refining slag is added to the LF furnace, and an appropriate amount of lime is added according to the liquidus temperature of the steel grade to ensure the viscosity and fluidity of the slag; 3) Ladle bottom blowing: The bottom blowing mode must be used in conjunction with the pre-melted refining slag. After the LF furnace refining begins, for the first 5-10 minutes, the ladle bottom blowing argon flow rate should be ≥500 NL / min to ensure thorough mixing of the steel and slag interface and promote rapid melting of the pre-melted refining slag. After refining sampling, for 10-20 minutes, the ladle bottom blowing argon flow rate should be adjusted to 200-400 NL / min for rapid deoxidation, and exposed molten steel should be prohibited. Then, for the next 15-25 minutes of refining, the ladle bottom blowing argon flow rate should be adjusted to 50-100 NL / min. Finally, for the period until the end of refining, the ladle bottom blowing argon flow rate should be adjusted to 20-50 NL / min. 4) No calcium treatment is performed during the entire LF refining process.

2. The LF furnace refining process for aluminum deoxidation and pure steel according to claim 1, characterized in that: The composition of the pre-melted refining slag and the weight percentage of each component are as follows: CaO: 42%-55%, Al2O3: 30%-36%, MgO: 2-7%, SiO2≤2%, FeO+MnO≤0.5%, S≤0.1%, P≤0.1%, TiO≤0.30%, H2O≤0.5%, and the amount of the pre-melted refining slag used per ton of steel is 10kg-15kg.

3. The LF furnace refining process for aluminum deoxidized pure steel according to claim 1, characterized in that: The lime added in step 2 must have a CaO content of ≥90%.

4. The LF furnace refining process for aluminum deoxidized pure steel according to claim 1, characterized in that: The C content at the converter endpoint in step 1 is ≥0.15%, and the tapping temperature is ≥1630℃.

5. The LF furnace refining process for aluminum deoxidized pure steel according to claim 1, characterized in that, After LF refining, perform RH vacuum circulation treatment for 15-25 minutes; or perform argon blowing into the ladle for 15-20 minutes.

6. The LF furnace refining process for aluminum deoxidized pure steel according to claim 1, characterized in that: For steel grades that do not have requirements for finished aluminum content, the content of acid-soluble aluminum (Als) in the finished product should be controlled to be ≤0.015%.