Super large section round billet low carbon peritectic alloy steel vertical semi-continuous casting protective slag and application

Abstract

This invention relates to the field of mold flux technology, specifically to a mold flux for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel and its application. The mold flux, by weight percentage, comprises: SiO2: 29-31%, CaO: 33-38%, MgO: 1-2.5%, Al2O3: 8-10%, (Na2O+K2O): 2-5%, F: 1-2.5%, Fe2O3: 1-3%, C: 17.5-20.5%, with the balance being unavoidable impurities. The binary basicity of the mold flux, CaO / SiO2, is 1.06-1.31. Using the mold flux provided by this invention, the requirements for liquid level preservation during the long static solidification stage of semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel can be met, the viscosity of the mold flux is matched with the casting speed, and heat transfer between the low-carbon peritectic alloy steel and the mold can be satisfied.

Description

Technical Field

[0001] This invention relates to the field of mold flux technology, and more specifically, to mold flux for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel and its application. Background Technology

[0002] Vertical semi-continuous casting technology for ultra-large cross-section round billets is a newly developed billet-making technology in recent years. The process of vertical semi-continuous casting for ultra-large cross-section round billets differs fundamentally from continuous casting technology, specifically in the following ways: First, after the billet is drawn, it is left to solidify online for an extended period, a much longer solidification stage than the drawing stage. This is the biggest difference from continuous casting. For example, for an ultra-large cross-section round billet with a diameter of 1600mm and a length of 10m, the solidification stage can last for more than 10 hours. This necessitates that the design of the protective slag for vertical semi-continuous casting must not only meet the functional requirements of preventing oxidation, heat preservation, lubrication, and heat transfer control during the drawing stage, but also meet the requirements of maintaining the liquid surface temperature during the long online solidification stage. This prevents solidification and crust formation on the liquid surface, reduces shrinkage depth, and improves billet yield. Second, the drawing speed is significantly slower than in continuous casting. Therefore, the protective slag selected for the vertical semi-continuous casting crystallizer must be matched to this speed; otherwise, it is difficult to obtain high-quality billets, and the process will not proceed smoothly. The viscosity of the mold flux in vertical semi-continuous casting is an important physicochemical property that matches the casting speed, controlling the flow of molten slag into the gap between the billet and the mold. Thirdly, low-carbon peritectic steel undergoes a δ→γ phase transformation during solidification, resulting in significant volume shrinkage and a high likelihood of surface longitudinal cracking. Furthermore, localized stress caused by uneven heat transfer during solidification in the mold can also lead to longitudinal cracking in these areas. This tendency for cracking due to volume shrinkage is particularly pronounced for ultra-large cross-section vertical semi-continuous casting round billets with diameters of Φ1000mm-Φ2000mm. Therefore, it is essential to control heat transfer between the low-carbon peritectic steel and the mold through the design of the mold flux in vertical semi-continuous casting. Summary of the Invention

[0003] The technical problem solved by this invention is to provide a protective slag to solve at least one of the following problems in the vertical semi-continuous casting process of ultra-large cross-section round billet low carbon peritectic alloy steel: heat preservation of liquid surface during long static solidification stage, matching the viscosity of the protective slag with the casting speed, and heat transfer between low carbon peritectic alloy steel and the crystallizer.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0005] A protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel, by weight percentage, comprises: SiO2: 29-31%, CaO: 33-38%, MgO: 1-2.5%, Al2O3: 8-10%, (Na2O+K2O): 2-5%, F: 1-2.5%, Fe2O3: 1-3%, C: 17.5-20.5%, with the balance being unavoidable impurities. The binary basicity CaO / SiO2 of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel is 1.06-1.31.

[0006] Preferably, the melting point of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low carbon peritectic alloy steel is 1150-1250℃, and the viscosity at 1300℃ is 0.6-0.8 Pa·s.

[0007] Preferably, the binary basicity CaO / SiO2 of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel is 1.1-1.2.

[0008] Preferably, the protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel is a hollow particle protective slag.

[0009] Preferably, the bulk density of the protective slag in the vertical semi-continuous casting of the ultra-large cross-section round billet low-carbon peritectic alloy steel is 0.7-1.0 kg / m³. 3 .

[0010] Compared with existing technologies, this invention designs the binary basicity (CaO / SiO2) of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel to be 1.06-1.31, thereby increasing the crystallization rate of the protective slag. The increased crystallization rate increases the thermal resistance of the protective slag, effectively slowing down heat transfer between the low-carbon peritectic alloy steel and the crystallizer, and reducing the risk of longitudinal cracks on the surface of the low-carbon peritectic alloy steel. The free carbon in the protective slag, heated by the heat transferred from the high-temperature molten steel, will burn, thus acting as a heating agent. Therefore, to ensure the heating effect, this invention designs the carbon content of the protective slag to be 17.5-20.5%, ensuring sufficient carbon in the protective slag to provide long-term heat preservation during the static solidification stage. Regarding viscosity, by adjusting the ratio of SiO2, CaO, and Al2O3, and designing the total content of Na2O and K2O to be 2-5% and the content of F to be 1-2.5%, the viscosity of the protective slag can be increased without affecting the basicity of the slag. The high-viscosity liquid protective slag enters the gap between the mold and the billet at a relatively slow speed, which can meet the requirement that the liquid protective slag needs to penetrate into the gap between the billet and the mold at a slow speed under ultra-slow casting conditions.

[0011] The present invention also provides the application of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel as described above. The protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is used for large round billets of low-carbon peritectic alloy steel with carbon content of less than 0.25%, casting speed of 0.02-0.1m / min, and cross-sectional diameter of 1000-2000mm.

[0012] Preferably, at the beginning of the billet pulling stage of the vertical semi-continuous casting of the low-carbon peritectic alloy steel, a protective slag of 50-60 mm thickness is added to the liquid surface of the crystallizer within 30 seconds.

[0013] Preferably, during the vertical semi-continuous casting process of the low-carbon peritectic alloy steel, protective slag is continuously added into the crystallizer to ensure that the thickness of the protective slag on the liquid surface of the crystallizer is 50-60mm.

[0014] Preferably, during the vertical semi-continuous casting process of the low-carbon peritectic alloy steel, the rate of addition of the protective slag is 0.8-1.5 kg / min.

[0015] Preferably, after the billet pulling stage, the billet is pulled out of the crystallizer and enters the static solidification stage. A protective slag of 30-40 mm thickness is then applied to the top liquid surface of the billet to make the protective slag thickness 80-100 mm.

[0016] The protective slag of this invention is used for vertical semi-continuous casting of large round billets of low-carbon peritectic alloy steel with a carbon content of less than 0.25%, a casting speed of 0.02-0.1 m / min, and a cross-sectional diameter of 1000-2000 mm. It can meet the requirements of liquid surface heat preservation during the long static solidification stage of the semi-continuous casting process of this ultra-large cross-section round billet of low-carbon peritectic alloy steel, the viscosity of the protective slag is matched with the casting speed, and the heat transfer between the low-carbon peritectic alloy steel and the crystallizer. Attached Figure Description

[0017] Figure 1 A partial photograph of the surface of the SA336 F22V large round billet obtained in Example 1;

[0018] Figure 2 This is a photograph of the tail end face of the SA336 F22V large round billet obtained in Example 1. Detailed Implementation

[0019] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0020] It should be noted that, unless otherwise specified, the features in the embodiments of this invention can be combined with each other. The terms "comprising," "including," "containing," and "having" are non-limiting, meaning that other steps and other components that do not affect the results can be added. The above terms cover the terms "composed of" and "substantially composed of." Unless otherwise specified, the materials, equipment, and reagents are commercially available.

[0021] A protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel, by weight percentage, comprises: SiO2: 29-31%, CaO: 33-38%, MgO: 1-2.5%, Al2O3: 8-10%, (Na2O+K2O): 2-5%, F: 1-2.5%, Fe2O3: 1-3%, C: 17.5-20.5%, with the balance being unavoidable impurities. The binary basicity CaO / SiO2 of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel is 1.06-1.31.

[0022] Compared with existing technologies, this invention designs the binary basicity (CaO / SiO2) of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel to be 1.3-1.5, thereby increasing the crystallization rate of the protective slag. The increased crystallization rate increases the thermal resistance of the protective slag, effectively slowing down heat transfer between the low-carbon peritectic alloy steel and the crystallizer, and reducing the risk of longitudinal cracks on the surface of the low-carbon peritectic alloy steel. The free carbon in the protective slag, heated by the heat transferred from the high-temperature molten steel, will burn, thus acting as a heating agent. Therefore, to ensure the heating effect, this invention designs the carbon content of the protective slag to be 17.5-20.5%, ensuring sufficient carbon to provide long-term heat preservation during the static solidification stage. Furthermore, the high carbon content also slows down the melting rate of the protective slag, reducing the rate at which liquid protective slag enters the gap between the billet and the crystallizer, thereby reducing the consumption of protective slag. Regarding viscosity, by adjusting the ratio of SiO2, CaO, and Al2O3, and designing the total content of Na2O and K2O to be 2-5% and the content of F to be 1-2.5%, the viscosity of the protective slag can be increased without affecting the slag basicity. The high-viscosity liquid protective slag enters the mold and the gap between the casting billet relatively slowly, which meets the requirement of slow penetration of the liquid protective slag into the gap between the casting billet and the mold under ultra-slow casting speed conditions. In the embodiments of this invention, the melting point of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel is 1150-1250℃, and the viscosity at 1300℃ is 0.6-0.8 Pa·s. Further controlling the melting point of the protective slag to 1150-1250℃ further reduces the solidification temperature of the protective slag, thereby further improving the lubrication performance of the protective slag and preventing sticking and leakage of steel. Controlling the viscosity of the protective slag at 1300℃ to 0.6-0.8 Pa·s can meet the requirement that the liquid protective slag needs to penetrate into the gap between the billet and the crystallizer at a relatively slow speed under ultra-slow casting conditions.

[0023] In an embodiment of the present invention, preferably, the binary basicity CaO / SiO2 of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel is 1.1-1.2.

[0024] In embodiments of the present invention, the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is a hollow granular protective slag. Hollow granular protective slag has advantages such as good fluidity, uniform composition, excellent thermal insulation performance, good spreadability, and smooth melting behavior. For round billets with large cross-sections, the protective slag must spread rapidly on the large liquid surface to avoid localized oxidation of the molten steel; therefore, hollow granular protective slag is more effective in preventing localized oxidation. For example, the bulk density of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is 0.7-1.0 kg / m³. 3 .

[0025] The embodiments of the present invention also provide the application of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel as described above. The protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is used for large round billets of low-carbon peritectic alloy steel with carbon content of less than 0.25%, casting speed of 0.02-0.1m / min, and cross-sectional diameter of 1000-2000mm.

[0026] The protective slag of this invention is used for vertical semi-continuous casting of large round billets of low-carbon peritectic alloy steel with a carbon content of less than 0.25%, a casting speed of 0.02-0.1 m / min, and a cross-sectional diameter of 1000-2000 mm. It can meet the requirements of liquid surface heat preservation, slag consumption control, and lubrication between low-carbon peritectic alloy steel and crystallizer during the long-term static solidification stage of the semi-continuous casting process of this ultra-large cross-section round billet of low-carbon peritectic alloy steel.

[0027] In this embodiment of the invention, at the beginning of the billet-drawing stage of the vertical semi-continuous casting of the low-carbon peritectic alloy steel, a 50-60 mm thick protective slag is added to the surface of the molten steel in the crystallizer within 30 seconds. The thickness of this protective slag, set at 50-60 mm, ensures the effective formation of a three-layer structure on the molten steel surface: a powder slag layer, a sintered layer, and a molten layer. The bottom molten layer of protective slag continuously penetrates into the gap between the crystallizer and the billet, serving to lubricate and control heat transfer. The high content of free carbon in the top unmelted powder slag layer releases heat through slow combustion, ensuring the insulation and oxidation prevention of the molten steel surface in the crystallizer, thus guaranteeing the thermal insulation performance of the molten steel surface.

[0028] In this embodiment of the invention, protective slag is continuously added to the crystallizer during the vertical semi-continuous casting process of the low-carbon peritectic alloy steel to ensure that the thickness of the protective slag on the liquid surface of the crystallizer is 50-60 mm. During the vertical semi-continuous casting process of the low-carbon peritectic alloy steel, the addition rate of the protective slag is 0.8-1.5 kg / min.

[0029] In this embodiment of the invention, after the billet pulling stage, the billet is pulled out of the crystallizer and enters the static solidification stage. A 30-40mm thick protective slag is then applied to the top liquid surface of the billet, bringing the total thickness of the protective slag to 80-100mm. By adding a sufficiently thick protective slag, the carbon in the slag layer releases heat through slow combustion, ensuring that the liquid surface at the billet tail remains molten, preventing solidification and bonding, and thus improving the billet yield.

[0030] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0031] Example 1

[0032] This embodiment takes SA336 F22V hydrogenation reactor steel as an example. The main components of this material are C content 0.11-0.15%, Mn content 0.3-0.6%, Cr content 2-2.5%, Ni content below 0.2%, Mo content 0.9-1.1%, and V content 0.25-0.35%, making it a low-carbon, high-alloy peritectic steel. The vertical semi-continuous casting process parameters for SA336 F22V are: billet diameter φ1600mm, length 12m, and casting speed 0.08m / min. For the SA336 F22V steel and process conditions, the protective slag provided in the embodiments of the invention was used for the semi-continuous casting of large round billets of SA336 F22V low-carbon peritectic alloy steel. The protective slag, by weight percentage, comprises: SiO2: 30%, CaO: 37%, MgO: 1%, Al2O3: 8%, (Na2O+K2O): 2.5%, F: 1.5%, Fe2O3: 1%, C: 19%, with the balance being unavoidable impurities. The protective slag has a melting point of 1200℃, a binary basicity of CaO / SiO2 of 1.3, a viscosity of 0.75 Pa·s at 1300℃, a hollow granular structure, and a bulk density of 0.8 kg / m³. 3 .

[0033] The method for using the protective slag in this embodiment is as follows:

[0034] At the beginning of the casting stage, a 50mm thick protective slag needs to be added to the liquid surface of the crystallizer within 30 seconds, and it must be spread evenly.

[0035] During the continuous casting process, protective slag needs to be continuously added to the crystallizer at a rate of 1.3 kg / min.

[0036] After the billet pulling stage, the billet is pulled out of the crystallizer and enters the static solidification stage. At this time, a 40mm thick protective slag is placed on the top of the billet liquid surface, and the total thickness of the protective slag is about 80mm.

[0037] During the production process, the measured dynamic slag layer thickness fluctuated between 48-53mm during the billet pulling stage, and the average slag consumption per ton of steel was 1.01kg, which effectively controlled the slag consumption, ensured a stable billet pulling process, and ensured uniform melting of the protective slag. Figure 1 The image shows a partial surface photograph of the SA336 F22V large round billet obtained in Example 1. The billet surface exhibits regular vibration marks, no slag inclusions, and no quality defects such as cracks or subcutaneous bubbles. During the static solidification stage, the measured thickness of the protective slag was 93 mm, indicating good liquid surface insulation and no solidified shell formed. Figure 2 The image shows the tail end face of the SA336 F22V large round billet obtained in Example 1. The measured shrinkage cavity depth after complete solidification is approximately 300 mm.

[0038] It is evident that using the protective slag provided in this embodiment of the invention for vertical semi-continuous casting of large round billets of low-carbon peritectic alloy steel with a carbon content of 0.25-0.75%, a total alloy element content of 5-15%, a billet pulling speed of 0.02-0.1 m / min, and a cross-sectional diameter of 1000-2000 mm can meet the requirements of liquid surface heat preservation during the long-term static solidification stage of the semi-continuous casting process of this ultra-large cross-section round billet of low-carbon peritectic alloy steel, the viscosity of the protective slag being matched with the pulling speed, and heat transfer between the low-carbon peritectic alloy steel and the crystallizer.

[0039] Furthermore, it should be noted that although the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel, characterized in that, The composition, by weight percentage, includes: SiO2: 29-31%, CaO: 33-38%, MgO: 1-2.5%, Al2O3: 8-10%, (Na2O+K2O): 2-5%, F: 1-2.5%, Fe2O3: 1-3%, C: 17.5-20.5%, with the balance being unavoidable impurities. The binary basicity CaO / SiO2 of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is... 1.06-1.31; The melting point of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is 1150-1250℃, and the viscosity at 1300℃ is 0.6-0.8 Pa·s; The protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is used for large round billets of low-carbon peritectic alloy steel with carbon content of less than 0.25%, casting speed of 0.02-0.1 m / min, and cross-sectional diameter of 1000-2000 mm.

2. The protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel according to claim 1, characterized in that, The binary basicity (CaO / SiO2) of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel is 1.1-1.

2.

3. The protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel according to claim 1, characterized in that, The protective slag for vertical semi-continuous casting of ultra-large cross-section round billet low-carbon peritectic alloy steel is a hollow particle protective slag.

4. The protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel according to claim 3, characterized in that, The bulk density of the protective slag in the vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is 0.7-1.0 kg / m³. 3 .

5. An application of a protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel as described in any one of claims 1-4, characterized in that, The protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel is used for large round billets of low-carbon peritectic alloy steel with a carbon content of less than 0.25%, a casting speed of 0.02-0.1 m / min, and a cross-sectional diameter of 1000-2000 mm. At the beginning of the casting stage of vertical semi-continuous casting of low-carbon peritectic alloy steel, a protective slag of 50-60 mm thickness is added to the liquid surface of the crystallizer within 30 seconds.

6. The application of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets and low-carbon peritectic alloy steel according to claim 5, characterized in that, During the vertical semi-continuous casting process of low-carbon peritectic alloy steel, protective slag is continuously added into the crystallizer to ensure that the thickness of the protective slag on the liquid surface of the crystallizer is 50-60mm.

7. The application of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets and low-carbon peritectic alloy steel according to claim 6, characterized in that, During the vertical semi-continuous casting process of low-carbon peritectic alloy steel, the flux is added at a rate of 0.8-1.5 kg / min.

8. The application of the protective slag for vertical semi-continuous casting of ultra-large cross-section round billets of low-carbon peritectic alloy steel according to claim 5 or 6, characterized in that, After the billet pulling stage, the billet is pulled out of the crystallizer and enters the static solidification stage. A protective slag of 30-40 mm thickness is then applied to the top liquid surface of the billet to make the protective slag thickness 80-100 mm.

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