A low-reactivity continuous casting protective slag suitable for high-rare earth weathering steels

Abstract

The present application relates to the technical field of steel metallurgy, and particularly relates to a low-reactivity continuous casting protective slag suitable for high-rare earth weathering steel. The protective slag is composed of the following components in percentage by mass: CaO 25%-40%, SiO2 20%-30%, Al2O3 5%-10%, Ce2O3 2%-10%, C 3%-10%, Na2O 5%-15%, CaF2 5%-10%, MgO 2%-10%, and the balance being inevitable impurities. The low-reactivity continuous casting protective slag is adapted to the characteristics of rare earth elements being easy to oxidize and easy to form high-melting-point substances, so that the defects such as cracks and adhesion of the cast slab are avoided.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and more particularly to a low-reactivity continuous casting protective slag suitable for high rare earth weathering steel. Background Technology

[0002] In the continuous casting process of rare earth weathering steel, the protective slag is a core functional material that determines the quality of the billet, the stability of the continuous casting process, and even the corrosion resistance of the subsequent steel. Weathering steel contains crack-sensitive elements such as Cu, Ti, Cr, and Ni, which easily lead to defects such as cracks and slag pits on the surface of the billet during continuous casting. At the same time, the high reactivity of rare earth elements amplifies the negative impact of the performance defects of the protective slag, directly affecting the smooth operation of production and the product qualification rate. In the traditional CaO-SiO2-Al2O3 system of protective slag, the highly active SiO2 and Al2O3 react with rare earth elements in the molten steel to generate high-melting-point phases, causing rare earth burn-off. After floating, the high-melting-point phases tend to accumulate on the meniscus of the crystallizer, forming hard slag strips or agglomerates, hindering the penetration of liquid slag into the gap between the billet shell and the copper plate, and disrupting the continuity of the slag film. Slag film rupture leads to insufficient lubrication, inducing longitudinal cracks and slag pits in the billet; unfloated rare earth oxide inclusions remain under the billet skin, reducing the corrosion resistance and mechanical properties of the weathering steel. Localized slag film loss can cause the billet shell to adhere to the copper plate, and in severe cases, it can lead to longitudinal cracks and steel leakage.

[0003] Patent CN202510390027.4 discloses a method for preparing rare earth steel using a pre-formed slag refining system based on a CaO-Al2O3-REO-MgO slag system. The protective slag contains 10%–20% rare earth oxides and 30%–50% Al2O3. When the Al2O3 content exceeds 10% (the critical threshold for rare earth steel protective slag), the protective slag struggles to form a stable liquid slag layer within the crystallizer, failing to achieve uniform spreading and lubrication. High-Al2O3 protective slag and rare earth inclusions easily deposit on the inner wall of the submerged entry nozzle, forming blockages and causing turbulent flow in the molten steel, requiring frequent nozzle replacement. Rare earth oxides are high-melting-point, highly chemically stable oxides; excessive mixing into the protective slag alters its compositional balance. When the rare earth oxide content in the protective slag itself is too high, the solubility of rare earth oxide inclusions in the slag system reaches saturation, preventing further adsorption and causing dissolved rare earth inclusions to precipitate back into the molten steel, forming new inclusions.

[0004] Patent CN202510245155.X discloses a rare earth weathering angle steel and its manufacturing method. The protective slag in this patent is suitable for smelting rare earth content La: 0.0034%~0.0038% or Ce: 0.0084%~0.0090%, but is not suitable for continuous casting with high rare earth content (RE≥0.01%).

[0005] Therefore, there is an urgent need to develop a protective slag suitable for high rare earth (RE≥0.01%) weathering steel, which can both adapt to the high rare earth elements in molten steel and solve the problem of defects such as easy cracking in weathering steel. Summary of the Invention

[0006] To address the performance issues caused by the consumption of SiO2 in the protective slag during the continuous casting process of rare earth weathering steel, which leads to billet quality problems, this invention provides a low-reactivity continuous casting protective slag suitable for high rare earth weathering steel. Its composition and performance are adapted to the characteristics of rare earth elements being easily oxidized and generating high-melting-point substances, thus avoiding defects such as billet cracks and adhesion.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] This invention provides a low-reactivity continuous casting protective slag suitable for high rare earth weathering steel, which is composed of the following components by mass percentage: CaO 25%~40%, SiO2 20%~30%, Al2O3 5%~10%, Ce2O3 2%~10%, C 3%~10%, Na2O 5%~15%, CaF 25%~10%, MgO 2%~10%, with the balance being unavoidable impurities.

[0009] The melting time of the protective slag in this invention is 35~40s.

[0010] In the above technical solution, the melting point of the protective slag is 1120~1230℃.

[0011] In the above technical solution, the viscosity of the protective slag at 1300℃ is further 0.6~1.0 Pa·s.

[0012] In the above technical solution, the particle size of the protective slag is further 0.1~2.4mm.

[0013] In the above technical solution, further, in the protective slag, the mass ratio of coarse powder is 10%~20%, the mass ratio of medium powder is 60%~80%, and the mass ratio of fine powder is 10%~20%, wherein the particle sizes of coarse powder, medium powder, and fine powder are d1, d2, and d3, respectively, 0.9 mm≤d1≤2.4 mm, 0.3 mm≤d2<0.9 mm, and 0.1 mm≤d3<0.3 mm.

[0014] In the above technical solution, C in the protective slag is added to the protective slag in the form of graphite and carbon black, wherein the mass percentage of graphite is 40%~60% and the mass percentage of carbon black is 40%~60%.

[0015] In the above technical solution, further, the graphite has a fixed carbon content of ≥98% and an ash content of ≤1%; The sulfur content in carbon black should be ≤0.5% to avoid introducing harmful impurities such as S and P that could contaminate the weathering steel.

[0016] In the above technical solution, the rare earth element content in the high rare earth weathering steel is ≥0.01%.

[0017] The beneficial effects of this invention are as follows: 1. The present invention introduces Ce2O3 rare earth oxide into the protective slag to reduce the activity of SiO2 and form a low reactive protective slag, which inhibits the burning loss of rare earth in steel, stabilizes the performance of the protective slag, reduces defects in the billet, and is suitable for the continuous casting requirements of weathering steel containing rare earth elements such as Ce and La.

[0018] 2. This invention utilizes graphite and carbon black to regulate the melting rate and slag layer thickness in the protective slag, avoiding localized overheating or uneven solidification. The total addition of graphite and carbon black should not exceed 10%, otherwise it will lead to excessive graphitization of the protective slag, resulting in slow heat transfer and bulging of the cast billet. If it is less than 3%, the melting rate will be too fast, making it impossible to form a stable slag film. Graphite can improve the lubrication performance of the protective slag, reduce the friction between the crystallizer and the cast billet, and reduce the risk of longitudinal cracks in weathering steel billets. The high reactivity of carbon black can undergo a weak reduction reaction with Na2O and SiO2 in the protective slag, reducing the probability of their combination with rare earth elements, reducing the amount of high-melting-point rare earth phases formed, and alleviating slag streaks and agglomeration problems.

[0019] 3. This invention introduces fluxing components such as Na2O and CaF2 to lower the melting point to 1120~1230℃, ensuring a continuous and uniform liquid slag layer.

[0020] 4. This invention regulates the slag structure by using MgO and Na2O, thereby suppressing the viscosity spike caused by the accumulation of Al2O3 in the protective slag during the smelting process, which affects the performance of the protective slag.

[0021] 5. In this invention, the protective slag adopts particle size classification to reduce stress concentration in the billet shell; fine powder is used to form slag quickly to prevent longitudinal cracks and subcutaneous bubbles; rare earth oxide agglomeration is reduced to lower the risk of slag strips and surface cracks; and the composite ratio ensures uniform penetration of liquid slag and avoids insufficient local lubrication.

[0022] 6. The protective slag of this invention enables the recovery rate of rare earth elements in high rare earth weathering steel to be over 70%, which is more than 30% higher than that of traditional CaO-SiO2-Al2O3 protective slag.

[0023] 7. The design of the protective slag viscosity in this invention addresses the issue of grain boundary weakening in weathering steel containing elements such as P, Cu, and Cr, which is prone to occur in the high-temperature zone of the billet shell, thus achieving slow cooling and heat transfer. Detailed Implementation

[0024] The following detailed description of a low-reactivity continuous casting protective slag suitable for high rare earth weathering steel, based on specific embodiments of the present invention, will be provided in further detail.

[0025] In the following embodiments, the rare earth weathering steel requires a Ce content of 0.01% to 0.03%, and the smelting process adopts feeding rare earth cored wire into the continuous casting crystallizer to directly perform rare earth microalloying on the billet, thereby reducing the rare earth burn-off of the molten steel during the transportation process.

[0026] Example 1 A low-reactivity continuous casting protective slag suitable for high rare earth weathering steel is composed of the following components by mass percentage: CaO 30%, SiO2 27%, Al2O 36%, Ce2O 35%, C 5%, Na2O 8%, CaF 26%, MgO 7%, with the balance being unavoidable impurities. Specifically, coarse powder (0.9 ≤ particle size ≤ 2.4 mm) accounts for 20% by mass, medium powder (0.3 mm ≤ particle size < 0.9 mm) accounts for 60% by mass, and fine powder (0.1 mm ≤ particle size < 0.3 mm) accounts for 20% by mass. C is added to the protective slag in the form of graphite and carbon black, with a graphite to carbon black mass ratio of 1:1. The graphite contains ≥98% fixed carbon and ≤1% ash, while the carbon black contains ≤0.5% sulfur.

[0027] The aforementioned continuous casting protective slag has a melting point of 1150℃ and a viscosity of 0.8 Pa·s at 1300℃. After passing through the converter and refining processes, the molten steel reaches the continuous casting process. Using the protective slag crystallizer of this invention, the liquid slag layer is controlled at 8~12mm. When feeding rare earth cored wire into the crystallizer, the amount of protective slag added during the stable casting process is 0.2kg / t, compensating for the increased slag consumption caused by rare earth reactions. The continuously cast billet has no surface cracks or obvious defects, and the rare earth recovery rate reaches 76%.

[0028] The formula for calculating rare earth recovery rate is as follows:

[0029] Example 2 A low-reactivity continuous casting protective slag suitable for high rare earth weathering steel is composed of the following components by mass percentage: CaO 32%, SiO2 25%, Al2O 37%, Ce2O 36%, C 4%, Na2O 10%, CaF 25%, MgO 5%, with the balance being unavoidable impurities. Specifically, coarse powder (0.9 ≤ particle size ≤ 2.4 mm) accounts for 15% by mass, medium powder (0.3 mm ≤ particle size < 0.9 mm) accounts for 70% by mass, and fine powder (0.1 mm ≤ particle size < 0.3 mm) accounts for 15% by mass. C is added to the protective slag in the form of graphite and carbon black, with a graphite to carbon black mass ratio of 1:1. The graphite contains ≥98% fixed carbon and ≤1% ash, while the carbon black contains ≤0.5% sulfur.

[0030] The continuous casting protective slag has a melting point of 1160℃ and a viscosity of 0.85 Pa·s at 1300℃. After passing through the converter and refining processes, the molten steel reaches the continuous casting process. Using the protective slag crystallizer of this invention, the liquid slag layer is controlled at 8~12mm. When feeding rare earth cored wire into the crystallizer, the amount of protective slag added during the stable casting process is 0.2kg / t, compensating for the increased slag consumption caused by rare earth reactions. The continuously cast billet has no surface cracks or obvious defects, and the rare earth recovery rate reaches 79%.

[0031] Example 3 A low-reactivity continuous casting protective slag suitable for high rare earth weathering steel is composed of the following components by mass percentage: CaO 35%, SiO2 30%, Al2O 35%, Ce2O 38%, C 4%, Na2O 5%, CaF 25%, MgO 4%, with the balance being unavoidable impurities. Specifically, coarse powder (0.9 ≤ particle size ≤ 2.4 mm) accounts for 16% by mass, medium powder (0.3 mm ≤ particle size < 0.9 mm) accounts for 70% by mass, and fine powder (0.1 mm ≤ particle size < 0.3 mm) accounts for 14% by mass. C is added to the protective slag in the form of graphite and carbon black, with a graphite to carbon black mass ratio of 4:6. The graphite contains ≥98% fixed carbon and ≤1% ash, while the carbon black contains ≤0.5% sulfur.

[0032] The continuous casting protective slag has a melting point of 1180℃ and a viscosity of 1.0 Pa·s at 1300℃. After passing through the converter and refining processes, the molten steel reaches the continuous casting process. Using the protective slag crystallizer of this invention, the liquid slag layer is controlled at 8~12mm. When feeding rare earth cored wire into the crystallizer, the amount of protective slag added during stable casting is 0.3kg / t, compensating for the increased slag consumption caused by rare earth reactions. The continuously cast billet has no surface cracks or obvious defects, and the rare earth recovery rate reaches 80%.

[0033] Comparative Example 1 A traditional CaO-SiO2-Al2O3 protective slag system is used, composed of the following mass percentages: CaO 30%, SiO2 25%, Al2O3 7%, MgO 4%, C 7%, CaF 25%, MgO 4%. The slag layer in the molded slag is controlled at 8-12 mm. After passing through the converter and refining processes, the molten steel reaches the continuous casting process. Rare earth cored wire is fed into the continuous casting mold. When feeding the rare earth cored wire into the mold, the amount of protective slag added during the stable casting process is 0.2 kg / t.

[0034] In Comparative Example 1, the continuously cast billet exhibited numerous surface cracks, with a rare earth recovery rate reaching 40%. This is because Ce, a rare earth element in rare earth weathering steel, is extremely chemically reactive, reacting with SiO2 and Al2O3 in the protective slag to form a high-melting-point phase. After floating to the surface, this high-melting-point phase tends to accumulate on the meniscus of the crystallizer, forming hard slag strips or agglomerates. This hinders the penetration of liquid slag into the gap between the billet shell and the copper plate, disrupting the continuity of the slag film.

[0035] 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 low-reactivity continuous casting protective slag suitable for high rare earth weathering steel, characterized in that, It is composed of the following components by mass percentage: CaO 25%~40%, SiO2 20%~30%, Al2O3 5%~10%, Ce2O3 2%~10%, C 3%~10%, Na2O 5%~15%, CaF 25%~10%, MgO 2%~10%, with the balance being unavoidable impurities.

2. The low-reactivity continuous casting protective slag suitable for high rare earth weathering steel according to claim 1, characterized in that, The melting point of the protective slag is 1120~1230℃.

3. The low-reactivity continuous casting protective slag suitable for high rare earth weathering steel according to claim 1, characterized in that, The viscosity of the protective slag at 1300℃ is 0.6~1.0 Pa·s.

4. The low-reactivity continuous casting protective slag suitable for high rare earth weathering steel according to claim 1, characterized in that, The particle size of the protective slag is 0.1~2.4mm.

5. The low-reactivity continuous casting protective slag suitable for high rare earth weathering steel according to claim 4, characterized in that, The protective slag contains 10% to 20% coarse powder, 60% to 80% medium powder, and 10% to 20% fine powder. The particle sizes of the coarse, medium, and fine powders are d1, d2, and d3, respectively, with 0.9 mm ≤ d1 ≤ 2.4 mm, 0.3 mm ≤ d2 < 0.9 mm, and 0.1 mm ≤ d3 < 0.3 mm.

6. The low-reactivity continuous casting protective slag suitable for high rare earth weathering steel according to claim 1, characterized in that, In the protective slag, C is added in the form of graphite and carbon black, wherein the mass percentage of graphite is 40%~60% and the mass percentage of carbon black is 40%~60%.

7. The low-reactivity continuous casting protective slag suitable for high rare earth weathering steel according to claim 6, characterized in that, The graphite has a fixed carbon content of ≥98% and an ash content of ≤1%. The sulfur content in the carbon black is ≤0.5%.

8. The low-reactivity continuous casting protective slag suitable for high rare earth weathering steel according to claim 1, characterized in that, The rare earth element content in the high rare earth weathering steel is ≥0.01%.

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