A protective slag and a preparation method and application thereof

Abstract

The present application relates to the technical field of metallurgical auxiliary materials, and particularly discloses a protective slag, a preparation method and application thereof. The protective slag comprises the following raw material components in mass percentage: 1.45-1.49% of carbon black, 9.81-9.85% of medium-carbon graphite, 5.88-5.92% of high-purity graphite, 0.96-1% of cryolite, 0.96-1% of fluorite, 3.42-3.46% of sodium carbonate, 0.96-1% of dextrin, 2.44-2.48% of sodium carboxymethyl cellulose, 4.9-4.94% of limestone, 6.86-6.9% of cement clinker, 10.01-10.05% of bauxite, 32.43-32.47% of phosphorous slag and 19.65-19.69% of wollastonite. The present application optimizes the formula and ratio of the protective slag, and the protective slag is used in straight-through water gap flat billets, thereby significantly improving the product surface qualification rate of the straight-through water gap flat billets, and avoiding the problem of cracks in the continuous casting process.

Description

Technical Field

[0001] This invention relates to the field of metallurgical auxiliary materials technology, and in particular to a protective slag, its preparation method and application. Background Technology

[0002] As the demand for special-performance steel grades continues to increase in the steel market, some steel mills have begun to develop steel grades with unique compositions and cross-sections. These steel grades are controlled according to the low-carbon steel system, but the actual carbon content is relatively high, and the composition is close to the peritectic reaction zone. During continuous casting, the initial billet shell is prone to unevenness due to peritectic phase transformation, significantly increasing the slab's susceptibility to cracking. The production cross-section is 165mm × (330~510)mm, belonging to the flat billet cross-section between traditional square billets, rectangular billets, and slabs. There is no mature continuous casting process that can be directly referenced.

[0003] In current continuous casting production, square and rectangular billets typically use straight-through nozzles, with molten steel flowing vertically downwards. This results in high turbulence, which, while meeting high casting speeds, leads to significant fluctuations in the molten steel surface within the crystallizer, making slag entrapment a common problem. Slabs, on the other hand, often use side-through nozzles, with molten steel injected horizontally, resulting in a stable flow field and a smooth liquid surface, which is more conducive to the melting of the protective slag and uniform shell growth. However, the special combination of straight-through nozzles and flat billet cross-sections does not conform to conventional process matching principles. This leads to poor molten steel flow, difficulty in controlling liquid surface stability, and a high susceptibility to surface and subsurface cracks in the cast billet. Conventional protective slags are unsuitable for the heat transfer and lubrication requirements of this condition, exhibiting poor crack suppression and severely impacting production stability and product yield. Therefore, there is an urgent need to develop a protective slag specifically for flat billets with straight-through nozzles to improve billet quality, solve the cracking problem, and achieve efficient and stable continuous casting. Summary of the Invention

[0004] In view of this, the present invention provides a protective slag, its preparation method, and its application. The present invention optimizes the raw material composition and proportions of the protective slag, resulting in a protective slag with specific basicity, melting point, and viscosity. When used in straight-through nozzle flat billets, it can significantly improve the quality of the cast billet and solve the cracking problem.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: This invention provides a protective slag, comprising the following raw material components in the following mass percentages: carbon black 1.45%~1.49%, medium carbon graphite 9.81%~9.85%, high purity graphite 5.88%~5.92%, cryolite 0.96%~1%, fluorite 0.96%~1%, sodium carbonate 3.42%~3.46%, dextrin 0.96%~1%, sodium carboxymethyl cellulose (CMC) 2.44%~2.48%, limestone 4.9%~4.94%, cement clinker 6.86%~6.9%, bauxite 10.01%~10.05%, phosphorus slag 32.43%~32.47%, and wollastonite 19.65%~19.69%.

[0006] Compared to existing technologies, the protective slag provided by this invention utilizes the synergistic effect of carbon black, medium-carbon graphite, and high-purity graphite to provide stable thermal insulation, reducing temperature drop and localized overcooling of the molten steel surface. Specific insulating materials, combined with cryolite, sodium carbonate, fluorite, and bauxite, form a composite flux system, further precisely controlling the melting point, viscosity, melting rate, and crystallization temperature of the protective slag. This results in a uniform and continuous liquid-slag film and optimizes the heat transfer uniformity of the protective slag, preventing stress concentration and cracking of the billet shell due to uneven heat flow. Cement clinker and limestone, through precise adjustment of the basicity of the protective slag, improve the lubricity and spreadability of the protective slag film, reducing the occurrence of surface cracks in the billet. Phosphorus slag and wollastonite, as the base materials of the protective slag, compensate for the defects of fluctuations in the composition of a single base material, improving the adaptability of the protective slag. Simultaneously, they can more stably control the melting state and crystallization of the protective slag, effectively suppressing cracking in the billet and synergistically improving the quality of the billet.

[0007] Preferably, the protective slag comprises the following chemical composition by mass percentage: SiO2: 20%~30%, CaO: 25%~35%, Al2O3 ≥ 10%, Na2O ≤ 5%, F - ≤3%, fixed carbon: 14%~18%, balance is impurities.

[0008] Preferably, the alkalinity of the protective slag is 1.137~1.143.

[0009] Preferably, the melting point of the protective slag is 1225~1255℃.

[0010] Preferably, the viscosity of the protective slag is 1.14 Pa·s to 1.16 Pa·s.

[0011] This invention further defines the basicity, melting point, and viscosity of the protective slag. During their research, the inventors discovered that when designing the protective slag according to conventional rectangular billet specifications, it is characterized by low basicity, high viscosity, and high melting point. However, when applied to flat billets with straight nozzles, this results in numerous slag streaks, crusting, poor lubrication, and surface depressions. Conversely, when designing the protective slag according to slab specifications and applying it to flat billets with straight nozzles, defects such as large central longitudinal cracks, cold steel buildup, and excessively thick liquid slag layers coupled with insufficient fine slag layers appear on the billets. Through extensive experimental research, the inventors found that by using specific raw material components and ensuring the basicity, melting point, and corresponding viscosity of the protective slag are within a specific range, the yield of the cast billets can be significantly improved, effectively suppressing cracking and synergistically enhancing billet quality.

[0012] Preferably, the protective slag is a hollow granular particle with a particle size of 0.1 mm to 1.5 mm.

[0013] More preferably, the hollow microspheres with a particle size of 0.1 mm to 1.5 mm account for ≥95%.

[0014] Preferably, the carbon black contains ≥99% C.

[0015] More preferably, the carbon black is carbon black 330.

[0016] Preferably, the medium-carbon graphite contains ≥91% C.

[0017] Preferably, the medium-carbon graphite is purchased from Jixi Xiwang Graphite Co., Ltd.

[0018] Preferably, the high-purity graphite contains ≥90% C.

[0019] For example, the cryolite is recycled cryolite.

[0020] Preferably, the F content in the cryolite is 45%~55%.

[0021] More preferably, the cryolite is mainly composed of sodium fluoroaluminate.

[0022] Preferably, the cement clinker contains 20% to 30% SiO2 and 60% to 70% CaO.

[0023] Preferably, the bauxite contains 78% to 88% Al2O3 and 1% to 10% SiO2.

[0024] Preferably, the phosphorus slag contains 37%~42% SiO2, 3.5%~8.5% Al2O3, 41%~46% CaO, MgO≤5%, and Fe2O3≤3%.

[0025] Preferably, the wollastonite contains 52.5%~57.5% SiO2, ≤4% Al2O3, 36%~41% CaO, ≤4% MgO, and ≤3% Fe2O3.

[0026] This invention provides a method for preparing the above-mentioned protective slag, comprising the following steps: Step 1: Weigh each raw material component according to the design ratio, and pulverize and sieve the other raw materials except for dextrin and sodium carboxymethyl cellulose, then mix them evenly to obtain a mixture. Step 2: Add dextrin and sodium carboxymethyl cellulose to water and mix evenly to obtain a primary mixed slurry with a mass concentration of 55wt%~65wt%. Then add the mixture to obtain a secondary mixed slurry. Step 3: Granulate the secondary mixed slurry using a spray method to obtain protective slag.

[0027] Preferably, in step 1, the sieving process uses a 300-mesh sieve, and the proportion of material passing through the sieve is ≥96%.

[0028] Preferably, in step 3, the pressure of the spray granulation is 10MPa~11MPa.

[0029] Preferably, in step 3, the spray granulation is carried out in a high-temperature spray tower, the temperature inside the high-temperature spray tower is 650~690℃, and the outlet air temperature of the high-temperature spray tower is 115~125℃.

[0030] This invention provides the application of the above-mentioned protective slag in straight-through nozzle flat billets.

[0031] This invention optimizes the composition and ratio of the protective slag. By preparing a protective slag with specific alkalinity, melting point and viscosity, it significantly improves the yield of flat billets with straight-through nozzles, effectively suppresses the problem of cracks in the cast billets, reduces the slag content and synergistically improves the quality of the cast billets. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] In the embodiments and comparative examples of this invention, the carbon black is Carbon Black 330 with a C content of 99.53%; the medium-carbon graphite has a C content of 93.93%; the high-purity graphite has a C content of 97.03%; the cryolite is recycled cryolite, wherein the F content is 47.56%, and the balance is impurities; the bentonite has a SiO2 content of 62.64%, an Al2O3 content of 14.71%, a CaO content of 3.52%, a MgO content of 4.90%, and a Fe2O3 content of 4.3%. 0%; Magnesia contains 2.53% SiO2, 0.92% Al2O3, 1.59% CaO, 92.93% MgO, 0.5% Fe2O3, and ≤4% SiO2; Bauxite contains 4.95% SiO2, 85.54% Al2O3, 1.17% CaO, 5.57% MgO, and 1.3% Fe2O3; Cement clinker contains 0% SiO2. The first type of blast furnace slag contained 22.88% SiO2, 5.46% CaO, 64.53% MgO, 2.46% Fe2O3, and 3.53% FeO, and was purchased from Jingxing Zhongshuocheng Trading Co., Ltd. The second type of blast furnace slag contained 38.88% SiO2, 40.87% CaO, 12.32% Al2O3, 5.79% MgO, and 0.96% Fe2O3, and was purchased from Xuanhua Haoxin, Zhangjiakou City. The phosphorus slag contains 39.5% SiO2, 6% Al2O3, 43.5% CaO, ≤5% MgO, ≤3% Fe2O3, with the remainder being impurities; the wollastonite contains 55% SiO2, ≤4% Al2O3, 38.5% CaO, ≤4% MgO, ≤3% Fe2O3, with the remainder being impurities; the silica contains 96.5% SiO2, 2.5% Al2O3, with the remainder being impurities.

[0034] Unless otherwise specified, the raw materials and reagents used in this invention are all conventional commercially available products; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0035] Example 1 This embodiment provides a protective slag, comprising the following raw material components in the following mass percentages: carbon black 1.47%, medium carbon graphite 9.83%, high-purity graphite 5.9%, cryolite 0.98%, fluorite 0.98%, sodium carbonate 3.44%, dextrin 0.98%, CMC 2.46%, limestone 4.92%, cement clinker 6.88%, bauxite 10.03%, phosphorus slag 32.45%, and wollastonite 19.68%. The protective slag comprises the following chemical components by mass percentage: SiO2: 26.09%, CaO: 29.75%, Al2O3: 11.92%, Fe2O3: 0.77%, Na2O: 2.58%, F - The composition of the product is 1.62% and the fixed carbon is 16.43%, with the remainder being impurities. The protective slag consists of hollow granular particles with a particle size of 0.1mm to 1.5mm, accounting for ≥95%.

[0036] This embodiment also provides a method for preparing the above-mentioned protective slag, including the following steps: Step 1: Weigh each raw material component according to the design ratio, crush all raw materials except dextrin and CMC, pass them through a 300-mesh sieve, and ensure that the mass of the material passing through the sieve accounts for 98%. Mix them evenly to obtain a mixture. Step 2: Add dextrin and CMC to water and mix well to obtain a primary mixed slurry with a mass concentration of 55wt%. Then add the mixed materials to obtain a secondary mixed slurry. Step 3: Determine the spray granulation pressure to be 10 MPa, the temperature inside the high-temperature spray tower to be 650℃, and the outlet air temperature to be 125℃. Granulate the secondary mixed slurry in the high-temperature spray tower to obtain protective slag.

[0037] Example 2 This embodiment provides a protective slag, comprising the following raw material components in the following mass percentages: carbon black 1.45%, medium carbon graphite 9.81%, high-purity graphite 5.92%, cryolite 0.96%, fluorite 0.99%, sodium carbonate 3.46%, dextrin 1%, CMC 2.44%, limestone 4.9%, cement clinker 6.9%, bauxite 10.05%, phosphorus slag 32.47%, and wollastonite 19.65%. The protective slag comprises the following chemical components by mass percentage: SiO2: 26.09%, CaO: 29.76%, Al2O3: 11.93%, Fe2O3: 0.77%, Na2O: 2.59%, F - The composition of the product is 1.61% and fixed carbon is 16.40%, with the remainder being impurities. The protective slag consists of hollow granular particles with a particle size of 0.1mm to 1.5mm, accounting for ≥95%.

[0038] This embodiment also provides a method for preparing the above-mentioned protective slag, including the following steps: Step 1: Weigh each raw material component according to the design ratio, crush all raw materials except dextrin and CMC, pass them through a 300-mesh sieve, and ensure that the mass percentage of the material passing through the sieve is 99%. Mix them evenly to obtain a mixture. Step 2: Add dextrin and CMC to water and mix well to obtain a primary mixed slurry with a mass concentration of 65wt%. Then add the mixed materials to obtain a secondary mixed slurry. Step 3: Determine the spray granulation pressure to be 10 MPa, the temperature inside the high-temperature spray tower to be 660℃, and the outlet air temperature to be 120℃. Granulate the secondary mixed slurry in the high-temperature spray tower to obtain protective slag.

[0039] Example 3 This embodiment provides a protective slag, comprising the following raw material components in the following mass percentages: carbon black 1.49%, medium carbon graphite 9.84%, high-purity graphite 5.91%, cryolite 0.97%, fluorite 0.96%, sodium carbonate 3.43%, dextrin 0.96%, CMC 2.48%, limestone 4.94%, cement clinker 6.9%, bauxite 10.01%, phosphorus slag 32.43%, and wollastonite 19.68%. The protective slag comprises the following chemical components by mass percentage: SiO2: 26.09%, CaO: 29.76%, Al2O3: 11.90%, Fe2O3: 0.77%, Na2O: 2.57%, F - The composition of the product is 1.6% and the fixed carbon is 16.46%, with the remainder being impurities. The protective slag consists of hollow granular particles with a particle size of 0.1mm to 1.5mm, accounting for ≥95%.

[0040] This embodiment also provides a method for preparing the above-mentioned protective slag, including the following steps: Step 1: Weigh each raw material component according to the design ratio, crush all raw materials except dextrin and CMC, pass them through a 300-mesh sieve, and ensure that the mass percentage of the material passing through the sieve is 99%. Mix them evenly to obtain a mixture. Step 2: Add dextrin and CMC to water and mix well to obtain a primary mixed slurry with a mass concentration of 65wt%. Then add the mixed materials to obtain a secondary mixed slurry. Step 3: Determine the spray granulation pressure to be 10 MPa, the temperature inside the high-temperature spray tower to be 690℃, and the outlet air temperature to be 125℃. Granulate the secondary mixed slurry in the high-temperature spray tower to obtain protective slag.

[0041] Comparative Example 1 This comparative example provides a protective slag comprising the following raw material components in the following mass percentages: carbon black 1%, medium carbon graphite 5.5%, high purity graphite 11%, recycled cryolite 2%, industrial soda ash 8.5%, dextrin 1.5%, bentonite 5%, silica 3.5%, phosphorus slag 18%, and blast furnace slag 44%. The preparation method for this comparative example is the same as that for Example 1, and will not be repeated here.

[0042] Comparative Example 2 This comparative example provides a protective slag comprising the following raw material components in the following mass percentages: carbon black 1%, medium carbon graphite 8%, high purity graphite 8%, recycled cryolite 1%, industrial soda ash 7%, dextrin 1.5%, bentonite 2.5%, phosphorus slag 13%, and blast furnace slag 58%. The preparation method for this comparative example is the same as that for Example 1, and will not be repeated here.

[0043] Comparative Example 3 This comparative example provides a protective slag comprising the following raw material components in the following mass percentages: carbon black 1%, medium carbon graphite 8%, high purity graphite 8%, recycled cryolite 2%, industrial soda ash 11%, dextrin 1.5%, limestone 8%, bauxite 8%, phosphorus slag 10%, and blast furnace slag 42.5%. The preparation method for this comparative example is the same as that for Example 1, and will not be repeated here.

[0044] Comparative Example 4 This comparative example provides a protective slag comprising the following raw material components in the following mass percentages: carbon black 2.67%, medium carbon graphite 2.47%, cryolite 3.96%, fluorite 7.91%, sodium carbonate 10.88%, magnesia 1.98%, sodium fluoride 3.46%, dextrin 0.99%, carboxymethyl cellulose 1.68%, limestone 5.93%, bauxite 3.96%, cement clinker 4.65%, blast furnace slag 9.89%, and wollastonite 39.57%. The preparation method for this comparative example is the same as that for Example 1, and will not be repeated here.

[0045] Example of effect The protective slag provided in the above embodiments and comparative examples was used to prepare a flat billet continuous casting crystallizer with a size of 165mm*410mm, and the billet casting condition was observed. The melting point was determined using an RDS-05 fully automatic slag melting point and melting rate tester, and the viscosity and inflection curve were determined using an RTW-10 melt physical property tester. The specific test results are shown in Table 1: Table 1

[0046] As shown in Table 1, the protective slag provided in Example 1 of this invention resulted in no cracks on the surface of the cast billet during use. However, the protective slags provided in Comparative Examples 1 and 2 were both acidic slags. During use, the protective slag provided in Comparative Example 1 caused a large longitudinal crack in the middle of the cast billet; the protective slag provided in Comparative Example 2 caused a central crack on the surface of the cast billet; the protective slag provided in Comparative Example 3 was extremely unstable during use, resulting in both central and corner cracks; and the protective slag provided in Comparative Example 4 caused pits during use, affecting the quality of the cast billet. In summary, the protective slag provided in the embodiments of this invention, by adjusting its basicity, melting point, and viscosity to an appropriate range, can significantly improve the crack situation and improve product quality when used on flat billets with straight-through nozzles.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A protective slag, characterized in that, The raw material components include the following components by mass percentage: carbon black 1.45%~1.49%, medium carbon graphite 9.81%~9.85%, high purity graphite 5.88%~5.92%, cryolite 0.96%~1%, fluorite 0.96%~1%, sodium carbonate 3.42%~3.46%, dextrin 0.96%~1%, sodium carboxymethyl cellulose 2.44%~2.48%, limestone 4.9%~4.94%, cement clinker 6.86%~6.9%, bauxite 10.01%~10.05%, phosphorus slag 32.43%~32.47%, and wollastonite 19.65%~19.69%.

2. The protective slag as described in claim 1, characterized in that, The protective slag comprises the following chemical components by mass percentage: SiO2: 20%~30%, CaO: 25%~35%, Al2O3 ≥10%, Na2O ≤5%, F - ≤3%, fixed carbon: 14%~18%, balance is impurities.

3. The protective slag as described in claim 1, characterized in that, The alkalinity of the protective slag is 1.137~1.

143.

4. The protective slag as described in claim 1, characterized in that, The melting point of the protective slag is 1225~1255℃.

5. The protective slag as described in claim 1, characterized in that, The viscosity of the protective slag is 1.14 Pa·S to 1.16 Pa·S.

6. The protective slag as described in claim 1, characterized in that, The protective slag is composed of hollow granular particles with a particle size of 0.1 mm to 1.5 mm; The cryolite contains 45% to 55% F.

7. The protective slag as described in claim 1, characterized in that, The cement clinker contains 20% to 30% SiO2 and 60% to 70% CaO. The bauxite contains 78% to 88% Al2O3 and 1% to 10% SiO2.

8. The protective slag as described in claim 1, characterized in that, The phosphorus slag contains 37%~42% SiO2, 3.5%~8.5% Al2O3, 41%~46% CaO, ≤5% MgO, and ≤3% Fe2O3. The wollastonite contains 52.5%~57.5% SiO2, ≤4% Al2O3, 36%~41% CaO, ≤4% MgO, and ≤3% Fe2O3.

9. A method for preparing the protective slag according to any one of claims 1 to 8, characterized in that, Includes the following steps: Step 1: Weigh each raw material component according to the design ratio, and pulverize and sieve the other raw materials except for dextrin and sodium carboxymethyl cellulose, then mix them evenly to obtain a mixture. Step 2: Add dextrin and sodium carboxymethyl cellulose to water and mix evenly to obtain a primary mixed slurry with a mass concentration of 55wt%~65wt%. Then add the mixture to obtain a secondary mixed slurry. Step 3: Granulate the secondary mixed slurry using a spray method to obtain protective slag.

10. The application of the protective slag according to any one of claims 1 to 8 in a straight-through nozzle flat billet.

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