A method for improving cleanliness of low carbon steel

Abstract

The application discloses a method for improving the cleanliness of low-carbon steel. The method comprises a process flow of a converter process-LF process-RH process-slab continuous casting process; at 1 / 8-1 / 6 of converter tapping, carbon structure alloy balls, manganese iron and silicon-manganese alloy are added into a ladle to deoxidize and alloy the molten steel, so that the control of carbon, manganese and silicon elements in the steel and the control of inclusion types in the steel are realized; the RH process promotes the floating and removal of inclusions in the steel by controlling process parameters, so that the cleanliness of the molten steel is improved; the carbon structure alloy ball is a double-layer composite structure, wherein the inner layer is an aluminum-magnesium-iron alloy, and the outer layer is a mixture of carbon powder and auxiliary materials. Compared with Al2O3 or CaO-Al2O3 hard inclusions generated in the original process, the method has less harm to the product, the application of the method effectively improves the cleanliness of the steel, and the deoxidizing alloy cost is reduced.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting and refining technology, and specifically relates to a method for improving the cleanliness of low-carbon steel. Background Technology

[0002] The converter steelmaking process is actually a complex metallurgical process involving dephosphorization and decarburization, which requires the injection of large amounts of oxygen into the steel. In recent decades, metallurgists both domestically and internationally have conducted extensive research on reducing the oxygen content at the converter's final stage. However, the oxygen activity at the converter's final stage is generally still 400–800 × 10⁻⁶. -4 For some companies, the percentage is even higher when producing low-carbon steel grades.

[0003] To deoxidize molten steel, the commonly used process involves adding aluminum or aluminum-iron alloys (aluminum-killed steel) or ferrosilicon alloys (silicon-killed steel) to the ladle during the converter tapping process. However, the biggest problem with metal deoxidation is that it leads to the formation of a large number of non-metallic oxide inclusions in the steel, thus degrading its quality. To address this, metallurgists have researched a process where carbonaceous materials are first added to the ladle for deoxidation, followed by the addition of other metal alloys for alloying. However, due to the low density of carbonaceous materials, they often float quickly to the ladle surface and become encapsulated by the ladle slag. This not only fails to effectively deoxidize the steel, but also results in the continuous dissolution of carbon from the ladle slag into the molten steel during the refining-continuous casting process, leading to excessive carbon content in the finished product. Summary of the Invention

[0004] The purpose of this invention is to provide a method for improving the cleanliness of low-carbon steel, which can be applied to high-quality low-carbon aluminum-killed steel such as TG22. This method is beneficial for the stable control of inclusions in the steel and for the low-cost cleanliness production of the steel.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A method for improving the cleanliness of low-carbon steel includes a process flow of converter process - LF process - RH process - slab continuous casting process; when the steel is tapped from the converter at 1 / 8 to 1 / 6, carbon structural alloy balls, ferromanganese and silicomanganese alloy are added to the ladle to deoxidize and alloy the molten steel, thereby controlling the carbon, manganese and silicon elements in the steel as well as the types of inclusions in the steel; the RH process promotes the flotation and removal of inclusions in the steel by controlling the processing parameters, thereby improving the cleanliness of the molten steel;

[0007] The carbon structural alloy ball has a double-layer composite structure, with the inner layer being an aluminum-magnesium-iron alloy and the outer layer being a mixture of carbon powder and auxiliary materials.

[0008] Furthermore, in the above technical solution, in the converter process, after deoxidation and alloying, the carbon content in the steel is controlled at 0.20wt% to 0.30wt%, and after tapping, the molten steel is weakly stirred by argon blowing. During the argon blowing process, the diameter of the exposed surface of the molten steel is 5cm to 19cm.

[0009] Furthermore, in the above technical solution, the aluminum-magnesium-iron alloy has a particle size of 15mm to 45mm and a composition of: Al: 50wt% to 80wt%, Mg: 0.5wt% to 1.9wt%, with the remainder being iron and unavoidable impurities; the carbon powder has a particle size of 0.5 to 10mm, the auxiliary material has a particle size of 300nm to 100μm, and the composition of the mixture of auxiliary material and carbon powder is: CaO: 3wt% to 9wt%, SiO2: 2wt% to 6wt%, C: 80wt% to 93wt%, and CaO / SiO2: 0.9 to 1.9, with the remainder being unavoidable impurities.

[0010] Furthermore, in the above technical solution, manganese iron and silicon manganese alloy are added 45 to 70 seconds after the carbon structural alloy balls are added.

[0011] Furthermore, in the above technical solution, the argon flow rate of the weak argon blowing stirring is 155NL / min to 180NL / min, and the argon blowing time is 5min to 10min.

[0012] Furthermore, in the above technical solution, the RH process parameters are as follows: at the beginning of vacuum treatment, the gas flow rate is increased from 1610 NL / min to 1790 NL / min. When the vacuum level drops to less than 100 Pa, the gas flow rate is reduced to 1200 NL / min to 1380 NL / min, and the cycle is repeated for 8 min to 15 min until the vacuum process ends.

[0013] Furthermore, in the above technical solution, the inner diameter of the carbon structural alloy ball is 15-45 mm, and the outer diameter is 25-60 mm.

[0014] Furthermore, in the above technical solution, the mass ratio of the inner layer to the outer layer of the carbon structural alloy ball is 2.1:1 to 1:1.8.

[0015] The beneficial effects of this invention are as follows:

[0016] This invention involves adding a double-layered composite carbon alloy sphere to molten steel. Because the inner layer is composed of alloying elements, the sphere is added to the molten steel early in the tapping process, allowing the outer layer sufficient time to react with the steel. The carbon effectively deoxidizes the steel, and small amounts of ultrafine CaO and SiO2 from the outer layer enter the molten steel. Only after the outer carbon has fully reacted with or melted into the molten steel does the inner silicon-manganese alloy begin to melt. This carbon deoxidation process, during the converter tapping process, can reduce the oxygen content in the steel to 100 × 10⁻⁶.-6 This process significantly reduces the content of oxide inclusions in the steel. Furthermore, residual oxygen reacts with aluminum (the aluminum content is much higher than the magnesium content, so the amount of Mg involved in deoxidation is negligible) to form Al2O3. Al2O3 then reacts with CaO and SiO2 in the steel to form CaO-SiO2-Al2O3 inclusions. These inclusions are ductile and pose less of a threat to the product than the hard Al2O3 or CaO-Al2O3 inclusions generated under the original process. The application of this process effectively improves the cleanliness of the steel while reducing the cost of deoxidizing alloys. Attached Figure Description

[0017] Figure 1 The image shows the morphology of inclusions in the RH steel leaving the station in Comparative Example 1. Figure 2 This is a morphological image of inclusions in the RH steel leaving the station in Example 1;

[0018] Figure 3 This is a morphological image of inclusions in the RH steel leaving the station in Example 2;

[0019] Figure 4 This is a schematic diagram of the carbon structural alloy sphere structure in this invention;

[0020] In the diagram: ① is an aluminum-magnesium-iron alloy; ② is a mixture of carbon powder and auxiliary materials. Detailed Implementation

[0021] The following detailed embodiments further illustrate the above-mentioned content of the present invention. Given that those skilled in the art have conducted extensive research in the field of inclusions and accumulated a wealth of theoretical and practical experience, after carefully reading these embodiments and their corresponding analyses, one can certainly, based on other specific conditions, and within the range of the process scheme and alloy composition design ratio proposed in this invention, at most conduct a few limited conventional experiments to specifically select several sets of process technology schemes that meet other conditions, thereby achieving the technical effects described in this invention. Therefore, only some embodiments are given below. However, this should not be construed as limiting the scope of the above-mentioned subject matter of this invention to the following examples; all technologies implemented based on the above-mentioned content of this invention fall within the scope of this invention.

[0022] Comparative Example 1

[0023] The steelmaking process adopts the flow of "converter process - RH process - billet continuous casting process". The product composition is shown in Table 1:

[0024] Table 1 Steel Grade Composition Requirements / wt%

[0025]

[0026]

[0027] Converter process: When the converter has tapped 1 / 2 of the steel, a recarburizing agent is added to the ladle. After circulating for 20 seconds, aluminum-iron, silicon-iron, and manganese-iron alloys are added to alloy the molten steel. After tapping, the molten steel is stirred by weak argon blowing at a flow rate of 80 NL / min for 5 min. After argon blowing, the concentration of a[O] in the steel is 35 × 10⁻⁶. -4 wt%, [C]0.24wt%, [Si]0.19wt%, [Mn]1.25wt%, [P]0.017wt%, [S]0.009wt%;

[0028] RH process: The gas flow rate is increased to 1400 NL / min throughout the vacuum treatment process, and the cycle lasts for 10 minutes before the vacuum process ends.

[0029] The morphology of inclusions in the RH produced in this batch is as follows: Figure 1 As shown.

[0030] Example 1

[0031] The steelmaking process adopts the flow of "converter process - LF process - RH process - slab continuous casting process". The product composition is shown in Table 2:

[0032] Table 2 Steel Grade Composition Requirements / wt%

[0033] C Si Mn P S Al 0.24～0.30 0.15～0.30 1.2～1.4 ≤0.025 ≤0.015 0.01～0.04

[0034] Converter process: When 1 / 8 of the steel is tapped from the converter, carbon structural alloy balls are added to the ladle. 48 seconds after the addition, ferromanganese alloy and ferrosilicon alloy are added to alloy the molten steel. After tapping, the molten steel is stirred by weak argon blowing at a flow rate of 157 NL / min for 8 minutes. After argon blowing, the concentration of a[O] in the steel is 26 × 10⁻⁶. -4 wt%, [C]0.26wt%, [Si]0.22wt%, [Mn]1.23wt%, [P]0.010wt%, [S]0.007wt%;

[0035] RH process: The vacuum treatment begins with a gas flow rate of 1620 NL / min. When the vacuum level drops below 100 Pa, the gas flow rate is reduced to 1360 NL / min. The cycle is repeated for 10 minutes, and the vacuum process ends.

[0036] The composition parameters of the carbon structural alloy balls added in the converter process are shown in Table 3.

[0037] Table 3. Component Indicators of Carbon Structural Alloy Spheres

[0038]

[0039]

[0040] The carbon structural alloy spheres added in the converter process are wrapped with low carbon steel of 0.1 to 0.3 mm thickness between the inner and outer layers and on the surface of the outer layer. The mass ratio of the inner and outer layers is 2.1:1 to 1:1.8.

[0041] The morphology of inclusions in the RH produced in this batch is as follows: Figure 2 As shown.

[0042] Example 2

[0043] The steelmaking process adopts the flow of "converter process - RH process - billet continuous casting process". The product composition is shown in Table 4:

[0044] Table 4 Steel Grade Composition Requirements / wt%

[0045] C Si Mn P S Al 0.24～0.30 0.15～0.30 1.2～1.4 ≤0.025 ≤0.015 0.01～0.04

[0046] Converter process: When 1 / 6 of the steel has been tapped from the converter, carbon structural alloy balls are added to the ladle. 64 seconds after this addition, ferromanganese alloy and ferrosilicon alloy are added to alloy the molten steel. After tapping, the molten steel is gently stirred with argon at a flow rate of 175 NL / min for 8 minutes. At the end of argon blowing, the a[O] content in the steel is 22 × 10⁻⁶. -4 wt%, [C] 0.27wt%, [Si] 0.25wt%, [Mn] 1.25wt%, [P] 0.010wt%, [S] 0.006wt%.

[0047] RH process: The vacuum treatment begins with a gas flow rate of 1780 NL / min. When the vacuum level drops below 100 Pa, the gas flow rate is reduced to 1200 NL / min. The cycle is repeated for 13 minutes, and the vacuum process ends.

[0048] The composition parameters of the carbon structural alloy balls added in the converter process are shown in Table 5.

[0049] Table 5. Component Indicators of Carbon Structural Alloy Spheres

[0050]

[0051] The carbon structural alloy spheres added in the converter process are wrapped with low carbon steel of 0.1 to 0.3 mm thickness between the inner and outer layers and on the surface of the outer layer. The mass ratio of the inner and outer layers is 2.1:1 to 1:1.8.

[0052] The morphology of inclusions in the RH produced in this batch is as follows: Figure 3 As shown.

[0053] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for improving the cleanliness of low-carbon steel, characterized in that: The process flow includes the converter process, the LF process, the RH process, and the slab continuous casting process. When the steel is tapped from the converter at 1 / 8 to 1 / 6 of its length, carbon structural alloy balls, ferromanganese alloy, and silicomanganese alloy are added to the ladle to deoxidize and alloy the molten steel, thereby controlling the carbon, manganese, and silicon elements in the steel as well as the types of inclusions. The RH process promotes the flotation and removal of inclusions in the steel by controlling the process parameters, thereby improving the cleanliness of the molten steel. The carbon structure alloy ball has a double-layer composite structure, wherein the inner layer is an aluminum-magnesium-iron alloy and the outer layer is a mixture of carbon powder and auxiliary materials; The toner particle size is 0.5mm~10mm, the auxiliary material particle size is 300nm~100μm, and the composition of the mixture of auxiliary material and toner is: CaO: 3wt%~9wt%, SiO2: 2wt%~6wt%, C: 80wt%~93wt%, and CaO / SiO2: 0.9~1.9, with the remainder being unavoidable impurities.

2. The method for improving the cleanliness of low-carbon steel according to claim 1, characterized in that: In the converter process, after deoxidation and alloying, the carbon content in the steel is controlled at 0.20wt%~0.30wt%. After tapping, the molten steel is stirred by weak argon blowing. During the argon blowing process, the diameter of the exposed surface of the molten steel is 5~19cm.

3. The method for improving the cleanliness of low-carbon steel according to claim 1, characterized in that: The aluminum-magnesium-iron alloy has a particle size of 15mm to 45mm and a composition of: Al: 50wt% to 80wt%, Mg: 0.5wt% to 1.9wt%, with the remainder being iron and unavoidable impurities.

4. The method for improving the cleanliness of low-carbon steel according to claim 1, characterized in that: After the carbon structural alloy spheres are added, add the ferromanganese alloy and the silicon-manganese alloy 45 to 70 seconds later.

5. The method for improving the cleanliness of low-carbon steel according to claim 2, characterized in that: The argon flow rate for the weak argon blowing stirring is 155 NL / min to 180 NL / min, and the argon blowing time is 5 min to 10 min.

6. The method for improving the cleanliness of low-carbon steel according to claim 1, characterized in that: The RH process parameters are as follows: at the start of vacuum treatment, the gas flow rate is 1610NL / min~1790NL / min. When the vacuum level drops to less than 100Pa, the gas flow rate is reduced to 1200NL / min~1380NL / min. The cycle is repeated for 8min~15min, and the vacuum process ends.

7. The method for improving the cleanliness of low-carbon steel according to claim 1, characterized in that: The inner diameter of the carbon structural alloy sphere is 15mm~45mm, and the outer diameter is 25mm~60mm.

8. The method for improving the cleanliness of low-carbon steel according to claim 1, characterized in that: The mass ratio of the inner and outer layers of the carbon structural alloy sphere is 2.1:1 to 1:1.8.

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