Ultra-low carbon steel slag modification method

Abstract

The invention discloses a method for modifying ultra-low carbon steel slag. The invention provides an ultra-low carbon steel slag modification method aiming at the technical problem that clean molten steel after smelting of an RH refining furnace is easily subjected to secondary oxidation due to high oxidability of ultra-low carbon steel slag. The method comprises the following steps: step 1, after molten steel enters a treatment position of the RH refining furnace and before an RH refining furnace dip pipe is inserted into molten steel, adding an aluminum slag surface deoxidizer into ladle slag in a range on one side of a dip pipe downcomer to carry out primary slag modification; secondly, after decarburization of the RH refining furnace is finished, an aluminum slag surface deoxidizing agent is added into a steel ladle slag area without preliminary modification for secondary slag modification; and thirdly, after vacuum treatment of the RH refining furnace is finished and vacuum breaking is conducted, an aluminum slag surface deoxidizing agent is added into a vortex core area formed by insertion of the dip pipe, and third-time slag modification is conducted.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgical refining technology, and relates to a method for modifying slag in ultra-low carbon steel. More specifically, it relates to a control method for improving the cleanliness of ultra-low carbon steel through staged and regional slag modification in the RH vacuum refining process, which is particularly suitable for the production of high-end ultra-low carbon steel grades such as automotive steel, electrical steel, and ultra-thin strip deep-drawing steel. Background Technology

[0002] In the production of ultra-low carbon steel (C≤0.0030%), deep decarburization treatment is generally required through the RH process. The RH refining furnace achieves decarburization through the carbon-oxygen reaction under vacuum. Therefore, the molten steel entering the RH refining furnace must be under high-oxygen conditions (generally, the oxygen content of the molten steel is ≥400ppm). At this time, the top slag of the ladle usually has a high oxygen potential (FeO+MnO≥10%). Although the molten steel has a very high cleanliness after RH vacuum smelting (the number of inclusions larger than 1 micrometer is about 10 / mm²), after the RH vacuum breaking process, the high-cleanliness molten steel inevitably comes into contact with the highly oxidizing slag and air, triggering a severe secondary oxidation reaction. This causes the number of inclusions in the steel to surge by more than 20% (reaching 12 / mm²). This secondary oxidation not only deteriorates the quality of the cast billet and affects the performance of the product, but in severe cases, it can also cause blockage of the tundish nozzle in continuous casting, resulting in a casting stoppage and causing significant economic losses to steel companies.

[0003] In existing technologies, traditional slag surface modification methods either modify the slag before the RH process, which interferes with the decarburization reaction kinetics in the early stages of RH, or modify the slag after RH, by which time secondary oxidation has already occurred, making it impossible to recover the loss of cleanliness. Publication number CN120818731A developed a top slag modification process for ultra-low carbon single-stage slag modification, namely, top slag modification in the converter process + top slag modification in the RH process. Although this reduces the oxidizability of the slag and can reduce the degree of secondary oxidation to a certain extent, this modification has a certain negative impact on the oxygen decarburization process of molten steel and cannot improve the secondary oxidation caused by contact with air during the air breaking process.

[0004] Therefore, there is an urgent need in this field to develop a new method for precise slag modification in stages and regions to overcome the technical barriers to secondary oxidation of clean molten steel in the refining process. Summary of the Invention

[0005] Based on the above background technology, the purpose of this invention is to provide a phased and regional RH refining slag surface modification method. This method involves adding a specific aluminous slag surface deoxidizer to different areas of the ladle slag surface at three specific time points during the RH refining furnace treatment stage. This ensures both the smooth progress of the RH decarburization reaction and the improvement of secondary oxidation problems in clean molten steel, ultimately achieving an increase in the cleanliness of ultra-low carbon molten steel.

[0006] More specifically, the present invention adopts the following technical solution.

[0007] According to one aspect of the present invention, a method for modifying ultra-low carbon steel slag is provided, characterized in that the method comprises the following steps:

[0008] S1. Preliminary slag modification: In the preliminary slag modification stage, an aluminum slag surface deoxidizer is added to the area of ​​the preliminary slag modification to carry out the preliminary slag modification.

[0009] S2, Secondary slag modification: In the secondary slag modification stage, the aluminous slag surface deoxidizer is added to the area of ​​the secondary slag modification to carry out the secondary slag modification.

[0010] S3. Three-stage slag modification: In the three-stage slag modification phase, the aluminum slag surface deoxidizer is added to the area of ​​the three-stage slag modification to carry out the three-stage slag modification.

[0011] According to the method of the present invention, preferably, the chemical composition of the aluminum slag surface deoxidizer is as follows by mass percentage: metallic aluminum: 10-55%, CaO: 10-30%, CaF2: 5-20%, Al2O3: 5-25%, SiO2: 2-7%, and other unavoidable impurities.

[0012] According to the method of the present invention, preferably, in step S1, the preliminary slag modification stage is after the molten steel enters the RH refining furnace treatment position and before the RH refining furnace immersion tube is inserted into the molten steel.

[0013] According to the method of the present invention, preferably, in step S1, the area of ​​preliminary slag modification is the ladle slag within the range of the downcomer side of the impregnation tube.

[0014] According to the method of the present invention, preferably, in step S1, the amount of the aluminous slag surface deoxidizer used in the preliminary slag modification is 0.2-1.0 kg / ton of steel.

[0015] According to the method of the present invention, preferably, in step S2, the secondary slag modification stage is after the decarburization of the RH refining furnace is completed.

[0016] According to the method of the present invention, preferably, in step S2, the area of ​​secondary slag modification is the other area of ​​ladle slag that has not undergone preliminary modification.

[0017] According to the method of the present invention, preferably, in step S2, the amount of the aluminous slag surface deoxidizer used in the secondary slag modification is 0.2-1.0 kg / ton of steel.

[0018] According to the method of the present invention, preferably, in step S3, the three-stage slag upgrading is performed after the vacuum treatment of the RH refining furnace is completed and the vacuum is broken.

[0019] According to the method of the present invention, preferably, in step S3, the region of the three-stage slag modification is the vortex region formed by the insertion of the impregnation tube.

[0020] According to the method of the present invention, preferably, in step S3, the amount of aluminous slag surface deoxidizer used in the three-stage slag modification is 0.1-1.0 kg / ton of steel.

[0021] The core method in this step is to add a specific aluminous slag surface deoxidizer to different areas of the ladle slag surface at three specific time points during the RH refining furnace treatment stage. The first stage ensures the smooth progress of the RH decarburization reaction and reduces the slag oxidizability; the second stage further reduces the slag oxidizability; and the third stage ensures that the slag oxidizability is further reduced and prevents the exposed molten steel after the cavitation is broken from coming into contact with air and being oxidized again.

[0022] This invention, through precise slag surface modification in stages and regions, effectively reduces secondary oxidation caused by the contact between clean molten steel and high-oxygen-potential slag and air during the RH decarburization stage of the refining furnace, while ensuring the normal progress of the RH decarburization reaction. This reduces the increase in the number of inclusions in the molten steel after RH decarburization from more than 20% in the traditional stage to less than 5%, significantly improving the cleanliness level of ultra-low carbon steel and solving the billet quality problems and tundish nozzle blockage accidents caused by secondary oxidation of molten steel during the refining process.

[0023] Beneficial technical effects

[0024] Compared with existing technologies, the technical concept and corresponding technical solution of this invention can achieve at least the following beneficial technical effects:

[0025] 1. Precise step-by-step control: By using a three-step slag modification method, the decoupling of decarbonization and anti-oxidation processes is achieved, solving the problem of the contradiction between the two in traditional methods.

[0026] 2. Significant Improvement in Cleanliness: The method of this invention minimizes the degradation of molten steel cleanliness after RH decontamination. Industrial test data shows that after using this method, the number of inclusions larger than 1μm in the molten steel before RH decontamination is approximately 10 / mm², and after decontamination, it only slightly increases to approximately 10.5 / mm²; while without this method, the number of inclusions after decontamination surges to over 12 / mm². This invention significantly reduces the increase in inclusions caused by secondary oxidation from over 20% to approximately 5%.

[0027] 3. Strong process adaptability: The method has clear steps and specific process parameters (timing, location, type, dosage), which makes it easy to achieve standardized operation in industrial production and ensures stable reproduction of the technology.

[0028] 4. High economic benefits: By stabilizing the cleanliness of molten steel, it significantly reduces casting stoppages caused by nozzle blockage during continuous casting, increases the number of continuous casting furnaces and production efficiency, resulting in huge economic benefits. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0030] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0031] The following are embodiments of the present invention. The described embodiments are only a part of the embodiments of the present invention. All other embodiments that can be obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0032] The technical solution of the present invention will be further described in detail below through specific embodiments and comparative examples.

[0033] Example 1

[0034] Taking a steel mill's production of ultra-low carbon steel (grade DC06) for automotive sheet as an example:

[0035] Molten steel conditions: C=0.035%, [O]=650ppm, top slag FeO+MnO=18%;

[0036] Step 1: After the molten steel enters the RH refining furnace treatment area and before the immersion tube of the RH refining furnace is inserted into the molten steel, an aluminous slag surface deoxidizer is added to the ladle slag on one side of the immersion tube downcomer for preliminary slag modification. The aluminous slag surface deoxidizer is added at a rate of 0.6 kg / ton of steel. The deoxidizer composition is: Al 41%, CaO 26%, CaF2 7%, Al2O3 20%, SiO2 3%, and other unavoidable impurities.

[0037] Step 2: Decarburization was performed for 13 minutes under RH vacuum conditions ≤ 67 Pa, with an endpoint [C] = 0.0014%. After decarburization in the RH refining furnace, aluminous slag surface deoxidizer was added to other areas of the ladle slag that had not undergone initial modification for secondary slag modification. The dosage of aluminous slag surface deoxidizer for secondary slag modification was 0.9 kg / ton of steel.

[0038] The third step: After the vacuum treatment and degassing of the RH refining furnace are completed, an aluminous slag surface deoxidizer is added to the vortex area formed by the insertion of the impregnation tube to perform a third slag modification. The dosage of the aluminous slag surface deoxidizer for the third slag modification is 0.5 kg / ton of steel.

[0039] Treatment results: The final slag of refining has FeO+MnO=1.9%, the number of inclusions before RH void breaking is 10.1 / mm², and the number of inclusions after void breaking is 10.5 / mm². The continuous casting is stable and the quality of the finished billet is good.

[0040] Example 2

[0041] Taking the production of ultra-thin strip deep-drawing steel by a certain steel mill as an example:

[0042] Molten steel conditions: C=0.030%, [O]=490ppm, top slag FeO=17%;

[0043] Step 1: After the molten steel enters the RH refining furnace treatment position and before the immersion tube of the RH refining furnace is inserted into the molten steel, an aluminous slag surface deoxidizer is added to the ladle slag on one side of the immersion tube downcomer to perform preliminary slag modification. The aluminous slag surface deoxidizer is added at a rate of 0.5 kg / ton of steel. The deoxidizer composition is: Al 43%, CaO 22%, CaF2: 6%, Al2O3: 21%, SiO2: 4%, and other unavoidable impurities.

[0044] Step 2: Decarburization was performed for 14 minutes under RH vacuum conditions ≤ 67 Pa, with an endpoint [C] = 0.0016%. After decarburization in the RH refining furnace, aluminous slag surface deoxidizer was added to other areas of the ladle slag that had not undergone initial modification for secondary slag modification. The dosage of aluminous slag surface deoxidizer for secondary slag modification was 1.0 kg / ton of steel.

[0045] The third step: After the vacuum treatment and degassing of the RH refining furnace are completed, an aluminous slag surface deoxidizer is added to the vortex area formed by the insertion of the impregnation tube to perform a third slag modification. The dosage of the aluminous slag surface deoxidizer for the third slag modification is 0.7 kg / ton of steel.

[0046] Treatment results: The final slag of refining has FeO+MnO=1.7%, the number of inclusions before RH void breaking is 10.3 / mm², and the number of inclusions after void breaking is 10.7 / mm². The continuous casting is stable and the quality of the finished billet is good.

[0047] Comparative Example 1

[0048] The traditional ultra-low carbon RH smelting process was adopted without refining slag modification. Steel conditions: C=0.033%, [O]=612ppm, top slag FeO=17%. After RH refining furnace treatment, the results were: final slag FeO+MnO=6%, the number of inclusions before RH cavitation was 12 / mm², and the number of inclusions after cavitation was 14.5 / mm². The continuous casting crystallizer experienced significant molten liquid level fluctuations, resulting in poor production stability.

[0049] The comparison shows that the slag modification method for ultra-low carbon steel provided by this invention significantly improves the cleanliness level of ultra-low carbon steel and solves the billet quality problems and tundish nozzle blockage accidents caused by secondary oxidation of molten steel during the refining process.

[0050] The above description is only a specific embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for modifying ultra-low carbon steel slag, characterized in that, The method includes the following steps: S1. Preliminary slag modification: In the preliminary slag modification stage, an aluminum slag surface deoxidizer is added to the area of ​​the preliminary slag modification to carry out the preliminary slag modification. S2, Secondary slag modification: In the secondary slag modification stage, the aluminous slag surface deoxidizer is added to the area of ​​the secondary slag modification to carry out the secondary slag modification. S3. Three-stage slag modification: In the three-stage slag modification phase, the aluminum slag surface deoxidizer is added to the area of ​​the three-stage slag modification to carry out the three-stage slag modification.

2. The method according to claim 1, characterized in that, The chemical composition of the aluminum slag surface deoxidizer, by mass percentage, is as follows: metallic aluminum: 10-55%, CaO: 10-30%, CaF2: 5-20%, Al2O3: 5-25%, SiO2: 2-7%, and other unavoidable impurities.

3. The method according to claim 1, characterized in that, In step S1, the initial slag modification stage is after the molten steel enters the RH refining furnace treatment position and before the RH refining furnace immersion tube is inserted into the molten steel.

4. The method according to claim 1, characterized in that, In step S1, the area of ​​preliminary slag modification is the ladle slag within the range of the downcomer side of the impregnation tube.

5. The method according to claim 1, characterized in that, In step S1, the amount of aluminous slag surface deoxidizer used in the preliminary slag modification is 0.2-1.0 kg / ton of steel.

6. The method according to claim 1, characterized in that, In step S2, the secondary slag modification stage is after the decarburization of the RH refining furnace is completed.

7. The method according to claim 1, characterized in that, In step S2, the area for secondary slag modification is the other areas of ladle slag that have not undergone initial modification.

8. The method according to claim 1, characterized in that, In step S2, the amount of deoxidizer used in the secondary slag modification of aluminous slag surface is 0.2-1.0 kg / ton of steel.

9. The method according to claim 1, characterized in that, In step S3, the three stages of slag upgrading are after the vacuum treatment of the RH refining furnace ends and the vacuum is broken.

10. The method according to claim 1, characterized in that, In step S3, the region where the slag is modified three times is the vortex region formed by inserting the impregnation tube.

11. The method according to claim 1, characterized in that, In step S3, the amount of aluminous slag surface deoxidizer used in the three-stage slag modification is 0.1-1.0 kg / ton of steel.