A continuous casting protective slag for ultra-low-carbon high-aluminum steel

Abstract

The application discloses a continuous casting protective slag for ultra-low-carbon high-aluminum steel, and chemical component composition and mass percentage of the protective slag are as follows: CaO 28%-40%, BaO 10%-16%, SiO2 5%-10%, Al2O3 10%-20%, MgO 2%-5%, Na2O+CaF2 6%-12%, K2O+B2O3 3%-10%, C 2%-4%, Li2O and the balance of impurities; and (CaO+BaO) / SiO2=5-10 and (K2O+B2O3) / Li2O=1.6-2.5 are met. The protective slag adopts a CaO-BaO-Al2O3 non-reactive slag system, avoids the composition of the protective slag from changing sharply due to slag-metal reaction, and guarantees the stability of lubrication and heat transfer performance of the protective slag in a crystallizer during continuous casting. The high-alkalinity slag reduces the content of SiO2 in the slag, reduces the slag-metal reaction, and in order to reduce the melting point of the protective slag, K2O, B2O3 and Li2O are used in combination, and the fluxing effect is obviously improved. The protective slag can effectively inhibit the reaction between SiO2 in the protective slag and active element Al in the steel, avoids the composition and performance of the protective slag from changing greatly, can effectively avoid the deterioration of the protective slag, and guarantees the stability of the performance of the protective slag during continuous casting.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, and in particular to a continuous casting protective slag for ultra-low carbon high-aluminum steel. Background Technology

[0002] With the development of industrial technology, the requirements for steel are becoming increasingly specialized. High-alumina steel, with its advantages such as lightweight, oxidation resistance, and improved electromagnetic properties, can be widely used in the automotive, electrical, and aerospace industries. However, during continuous casting production of high-alumina steel, the SiO2 in the continuous casting flux reacts violently with the reactive element Al in the steel, leading to a rapid increase in Al2O3 content and a sharp decrease in SiO2 content in the flux. This alters the flux's fluidity and heat transfer properties, preventing it from effectively lubricating and transferring heat within the crystallizer. Consequently, quality problems such as indentations, longitudinal cracks, and slag inclusions in the cast billet increase, while the risk of leaks rises, making it impossible to guarantee smooth continuous casting. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a continuous casting protective slag for ultra-low carbon high-aluminum steel to improve the quality of the cast billet.

[0004] To solve the above-mentioned technical problems, the chemical composition and mass percentage of the protective slag of the present invention are as follows: CaO 28%~40%, BaO 10%~16%, SiO 2 5%~10%, Al 2O 3 10%~20%, MgO 2%~5%, Na 2O+CaF 2 6%~12%, K 2O+B 2O 3 3%~10%, C 2%~4%, with the balance being Li 2O and unavoidable impurities; and satisfying (CaO+BaO) / SiO 2=5~10, (K 2O+B 2O 3) / Li 2O=1.6~2.5.

[0005] Preferably, the chemical composition of the protective slag is as follows: CaO 30%–36%, BaO 12%–16%, and Al2O3 10%–15%.

[0006] Preferably, the chemical composition of the protective slag contains 7% to 10% Na2O + CaF2.

[0007] Preferably, the chemical composition of the protective slag contains 5-8% K2O+B2O3.

[0008] The protective slag described in this invention is suitable for steel grades with C ≤ 0.03% and Al 0.2% to 0.5%.

[0009] The design principle of this invention is as follows: To avoid violent slag-metal reactions, the protective slag used in this invention employs a CaO-BaO-Al2O3 slag system. The reaction between SiO2 in the slag and Al in the steel is suppressed by appropriately increasing the Al2O3 content and decreasing the SiO2 content. Simultaneously, to ensure suitable viscosity of the protective slag, a certain amount of BaO is added to lower the melting point and improve its fluidity; therefore, (CaO+BaO) / SiO2 = 5-10 is required. The MgO content in the protective slag can reduce its viscosity and melting point. CaF2 and Na2O in the protective slag promote ion migration and diffusion in the molten slag, enhancing the fluidity of the molten protective slag and improving the lubrication performance of the continuous casting process. However, excessive CaF2 and Na2O will promote nepheline precipitation, which is detrimental to the vitrification of the protective slag and affects its heat transfer stability in the crystallizer. The addition of Li2O to the protective slag ensures that the melting temperature remains relatively stable even with a slight increase in Al2O3 content, thus maintaining the slag's fluidity. Meanwhile, the fluxing effects of K2O and B2O3 are somewhat unique, exhibiting a stronger fluxing effect only when used in combination with Li2O. This protective slag is suitable for ultra-low carbon steel; therefore, the C content in the slag is controlled within the range of 2–4%.

[0010] The beneficial effects of adopting the above technical solution are as follows: This invention uses a CaO-BaO-Al2O3 non-reactive slag system, avoiding drastic changes in the composition of the protective slag caused by slag-metal reaction, and ensuring the stability of the lubrication and heat transfer performance of the protective slag in the crystallizer during continuous casting. This invention uses a high-basicity slag, reducing the SiO2 content in the slag and minimizing slag-metal reaction. Simultaneously, to lower the melting point of the protective slag, K2O, B2O3, and Li2O are used in combination, significantly improving the fluxing effect. This invention can effectively inhibit the reaction between SiO2 in the protective slag and the reactive element Al in the steel, avoiding significant changes in the composition and properties of the protective slag. It can effectively prevent the deterioration of the protective slag, ensuring the stability of the protective slag performance during continuous casting, thereby ensuring that the protective slag plays a stable lubrication and heat transfer role in the crystallizer, enabling smooth continuous casting and reducing the occurrence of billet quality problems. Detailed Implementation

[0011] The present invention will now be described in further detail with reference to specific embodiments.

[0012] Examples 1-8: The continuous casting protective slag for ultra-low carbon high-aluminum steel adopts the following component ratio.

[0013] (1) The chemical composition and mass percentage of the protective slag are as follows: CaO 28%–40%, BaO 10%–16%, SiO2 5%–10%, Al2O3 10%–20%, MgO 2%–5%, Na2O+CaF2 6%–12%, K2O+B2O3 3%–10%, C 2%–4%, with the balance being Li2O and unavoidable impurities; and satisfying (CaO+BaO) / SiO2=5–10, (K2O+B2O3) / Li2O=1.6–2.5. In the preferred composition: CaO 30%–36%, BaO 12%–16%, Al2O3 10%–15%, Na2O+CaF2 7%–10%, K2O+B2O3 5–8%. The composition ratio of the protective slag in each embodiment is shown in Table 1.

[0014] Table 1: Composition ratio of the protective slag in each embodiment

[0015]

[0016] (2) The protective slag is suitable for continuous casting of steel grades with C ≤ 0.03wt% and Al 0.2wt%~0.5wt%. During continuous casting, the melting point of the protective slag is 997~1138℃, the viscosity is 0.192~0.397Pa·s, the casting speed is 1.2~1.4m / min, the slag consumption is 0.33~0.43kg / t, and the thickness of the molten slag layer on the molten steel in the crystallizer is 9.4~15.8mm. This ensures the stability of the composition and performance of the protective slag during continuous casting. The protective slag has good melting and inclusion absorption properties and spreads evenly on the surface of the molten steel in the crystallizer, which can play a good role in lubrication and heat transfer. Defects such as longitudinal cracks and depressions on the surface of the continuously cast billet are significantly reduced. Through low-magnification inspection and statistics of the billet surface, the defect rate of the billet has been reduced from about 10% to less than 2.5%. The performance of this protective slag meets the continuous casting requirements of this steel grade.

[0017] Comparative Example 1: The protective slag used in this comparative example had the following composition and mass percentage: CaO 7%, BaO 14.5%, Al2O3 31%, B2O3 8%, SiO2 5%, CaF2 11.3%, Na2O 12.8%, Li2O 1.5%, MnO 2.5%, and C 5%. Its melting temperature was 988℃ and its viscosity was 0.24 Pa·s. Using this protective slag, one sticking and leakage alarm occurred during continuous casting, while no leakage alarms occurred with the protective slag used in the example. This is because the protective slag used in this example employs a high Al2O3 content and low SiO2 content design to suppress the reaction between SiO2 in the slag and Al in the steel. To lower the melting point of the protective slag, a large amount of fluxing agents CaF2 and Na2O were added. However, a large amount of CaF2 and Na2O promotes the precipitation of nepheline and Na-Al-O crystals, leading to poor heat transfer uniformity of the protective slag and an increased risk of sticking and leakage. Table 2 shows a comparison of the application effects of the comparative example and the embodiment.

[0018] Comparative Example 2: This comparative example still uses the traditional CaO-SiO2 system protective slag, with the following composition and mass percentage: CaO 25.41%, SiO2 33.15%, Al2O3 9.9%, Na2O 6.6%, Li2O 4.97%, MnO 5.5%, MgO 5.4%, F - The content of the protective slag is 8.8%, with a melting temperature range of 1157–1266℃ and a viscosity of 0.19 ± 0.05 Pa·s. This protective slag, by adding MnO, preferentially reacts with SiO2 to slow down the reaction with Al in the steel. However, due to the high SiO2 content in the slag, it still reacts with Al in the steel, resulting in a relatively drastic change in composition after the protective slag is added to the crystallizer. The Al2O3 content in the slag increases rapidly. As Al2O3 accumulates in the slag, the adsorption conditions for inclusions deteriorate, easily leading to defects such as subsurface inclusions in the cast billet. Therefore, this slag is not suitable for long continuous casting times. A comparison of the application effects of this comparative example and the embodiment is shown in Table 2.

[0019] Table 2: Application Effects of Examples and Comparative Examples

[0020] .

Claims

1. A continuous casting protective slag for ultra-low carbon high-alumina steel, characterized in that, The chemical composition and mass percentage of the protective slag are as follows: CaO 30%–36%, BaO 12%–16%, SiO2 5%–10%, Al2O3 10%–15%, MgO 2%–5%, Na2O+CaF2 6%–12%, K2O+B2O3 3%–10%, C 2%–4%, with the balance being Li2O and unavoidable impurities; and satisfying (CaO+BaO) / SiO2=5–10, (K2O+B2O3) / Li2O=1.6–2.5; The protective slag is suitable for steel grades with C ≤ 0.03% and Al 0.2% to 0.5%; The protective slag has a melting point of 997–1138℃ and a viscosity of 0.192–0.397 Pa·s; The protective slag is suitable for the following continuous casting production processes: the casting speed is 1.2 to 1.4 m / min, the slag consumption is 0.33 to 0.43 kg / t, and the thickness of the molten slag layer on the molten steel in the crystallizer is 9.4 to 15.8 mm.

2. The continuous casting protective slag for ultra-low carbon high-alumina steel according to claim 1, characterized in that: The chemical composition of the protective slag is 7%–10% Na2O + CaF2.

3. A continuous casting protective slag for ultra-low carbon high-alumina steel according to claim 1 or 2, characterized in that: The chemical composition of the protective slag contains 5-8% K2O and B2O3.

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