A three-layer structure tundish covering agent for smelting rare earth steel grade and its preparation method and application

Abstract

The present application relates to the technical field of steel metallurgy, and particularly relates to a three-layer structure tundish covering agent for smelting rare earth steel, a preparation method and application thereof. The covering agent comprises, from bottom to top, a metallurgical functional layer, a reducing buffer layer and a low-sulfur holding layer. The metallurgical functional layer adopts a CaO-Al2O3-RE2O3-BaO slag system, and through high-alkalinity and low-oxidizing design, the burning loss of rare earth elements is inhibited, and at the same time, by using the activity balance principle of rare earth oxides and the fluxing effect of BaO, efficient adsorption of rare earth inclusions is realized; the reducing buffer layer is composed of fused magnesia particles and graphite powder, forming a physical and chemical double barrier to block the exchange of substances between the upper and lower layers; the low-sulfur holding layer is composed of expanded perlite and borosilicate glass, replacing the traditional carbonized rice husk, and completely solving the problem of sulfur increase.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and more particularly to a three-layer structure tundish covering agent for smelting rare earth steels, its preparation method, and its application. Background Technology

[0002] Rare earth elements play multiple roles in steel, including acting as inclusion modifiers, refining the microstructure, and microalloying, significantly improving the overall properties of steel such as strength, toughness, corrosion resistance, and wear resistance. However, due to their extremely reactive chemical properties, rare earth elements are prone to oxidation and burn-off during steelmaking and continuous casting, resulting in low and fluctuating rare earth yields, which seriously affects the production stability and product quality of rare earth steel.

[0003] The tundish is the transition container for molten steel from the ladle to the crystallizer during continuous casting, and it is also one of the main links in the loss of rare earth elements. In the existing technology, the tundish covering agent usually adopts a double-layer structure, with the lower layer being an alkaline pre-melted slag and the upper layer being a carbonized rice husk insulation layer. This structure can meet the basic requirements when smelting ordinary steel grades, but it has the following serious problems when smelting rare earth steel grades: (1) Severe loss of rare earth elements: Traditional covering agents contain a high content of oxidizing oxides such as SiO2, FeO, and MnO, which will react violently with the rare earth elements in the molten steel, resulting in a large loss of rare earth elements, and the yield is usually only about 30%. (2) Deterioration of covering agent performance: The high melting point rare earth calcium aluminate generated by the reaction of rare earth with the covering agent will significantly increase the melting point and viscosity of the covering agent, leading to frequent crusting and a significant decrease in the covering agent's heat preservation and inclusion adsorption capabilities. (3) Sulfurization problem in molten steel: The sulfur content in the upper carbonized rice husk is usually 0.3%-0.8%. At high temperature, sulfur will diffuse and convection into the lower slag, and then enter the molten steel through slag-metal reaction, causing sulfurization in the molten steel. This is particularly unfavorable for rare earth steel that requires ultra-low sulfur content. (4) Difficulty in removing rare earth inclusions: Traditional covering agents have limited adsorption capacity for inclusions such as rare earth oxides and rare earth sulfides. A large number of fine rare earth inclusions remain in the molten steel, which will affect the mechanical properties of the steel.

[0004] Chinese patent CN114700470B discloses a tundish covering agent for smelting rare earth steel. By increasing the basicity of the covering agent to 8-11, reducing the SiO2 content, and adding a small amount of rare earth oxides, the rare earth yield is increased to over 40%. However, this patent still uses a traditional single-layer covering agent structure, which cannot solve the problem of sulfur increase caused by the exchange of substances between the upper and lower layers, and the adsorption capacity for rare earth inclusions still needs to be improved.

[0005] Therefore, developing a tundish covering agent that can effectively suppress rare earth burn-off, efficiently adsorb rare earth inclusions, does not increase sulfur content, and has stable performance is of great significance for achieving large-scale and stable production of rare earth steel. Summary of the Invention

[0006] The purpose of this invention is to provide a three-layer tundish covering agent for smelting rare earth steel, its preparation method, and its application. The covering agent consists of, from bottom to top, a metallurgical functional layer, a reducing buffer layer, and a low-sulfur insulation layer. The metallurgical functional layer uses a CaO-Al2O3-RE2O3-BaO slag system, which inhibits the loss of rare earth elements through a high-alkalinity, low-oxidizing-activity design. Simultaneously, it utilizes the activity balance principle of rare earth oxides and the fluxing effect of BaO to achieve efficient adsorption of rare earth inclusions. The reducing buffer layer is composed of fused magnesia particles and graphite powder, forming a dual physical and chemical barrier to block the exchange of substances between the upper and lower layers. The low-sulfur insulation layer uses expanded perlite and borosilicate glass composites to replace traditional carbonized rice husks, completely solving the problem of sulfur increase.

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

[0008] The first aspect of this invention provides a three-layer structure tundish covering agent for smelting rare earth steel grades, wherein the covering agent comprises, from bottom to top: Metallurgical functional layer: Its chemical composition by mass percentage is: CaO 45%-55%, Al2O3 25%-35%, RE2O3 3%-8%, BaO 5%-12%, MgO 3%-7%, SiO2≤3%, and the remainder are unavoidable impurities; Reducing buffer layer: composed of fused magnesia particles and graphite powder, wherein the mass percentage of graphite powder is 15%-25%; Low-sulfur insulation layer: composed of expanded perlite and borosilicate glass, wherein the mass percentage of borosilicate glass is 20%-40%.

[0009] In the above technical solution, the basicity of the metallurgical functional layer CaO / SiO2≥15, the melting temperature is 1350-1420℃, and the viscosity at 1400℃ is 0.8-1.1Pa·s.

[0010] In the above technical solution, the RE2O3 is further described as Ce2O3, La2O3, or a mixture of Ce2O3 and La2O3; in the mixture of Ce2O3 and La2O3, the mass ratio of Ce2O3 to La2O3 is (2-4):1.

[0011] In the above technical solution, the purity of the fused magnesia particles is ≥98%, and the particle size is 0.5-2.0 mm; the carbon content of the graphite powder is ≥99%, and the particle size of the graphite powder is 0.1-0.5 mm.

[0012] In the above technical solution, further, the expanded perlite has a sulfur content ≤0.01wt% and a particle size of 0.1-1.0mm; the borosilicate glass has a density of 0.2-0.4g / cm³. 3 The particle size is 10-100μm.

[0013] In the above technical solution, the thickness ratio of the metallurgical functional layer, the reducing buffer layer and the low-sulfur insulation layer is (2-4):(1-2):(3-5); the total thickness of the covering layer is 60-110mm.

[0014] A second aspect of the present invention provides a method for preparing the above-mentioned three-layer structure intermediate packaging agent, comprising the following steps: Step 1: Preparation of metallurgical functional layer materials According to the composition ratio of the metallurgical functional layer, the raw materials are active lime, lightly calcined magnesia, corundum powder, barium oxide, rare earth oxide, silicon oxide and manganese oxide. The raw materials are mixed evenly, pre-melted at 1500-1550℃ for 2-3 hours, water quenched and dried, and then ground to a particle size ≤0.1mm to obtain the metallurgical functional layer material. Step 2: Preparation of reducing buffer layer material According to the composition ratio of the reducing buffer layer, fused magnesia particles and graphite powder are mixed for 10-15 minutes to obtain the reducing buffer layer material. Step 3: Preparation of low-sulfur insulation layer materials According to the composition ratio of the low-sulfur insulation layer, expandable perlite and borosilicate glass are mixed for 5-10 minutes to obtain the low-sulfur insulation layer material. The order of steps 1 to 3 is not limited; Step 4: Pack them separately The metallurgical functional layer material, the reducing buffer layer material, and the low-sulfur insulation layer material are packaged separately.

[0015] A third aspect of the present invention provides an application of the above-mentioned three-layer structure tundish covering agent for smelting rare earth steel.

[0016] In the above technical solution, further, in the rare earth steel, the rare earth elements are Ce and / or La, wherein the mass percentage content of the rare earth elements is ≥0.01%.

[0017] Furthermore, in the above technical solution, the method of using the covering agent includes the following steps: (1) The metallurgical functional layer material is brought into contact with molten steel to form a metallurgical functional layer. The reducing buffer layer material is covered on top of the metallurgical functional layer material to form a reducing buffer layer. The low-sulfur insulation layer material is covered on top of the reducing buffer layer material to form a low-sulfur insulation layer. (2) During the continuous casting process, the temperature of the molten steel in the tundish is controlled at 1530-1580℃, and argon gas is used for protection in the tundish with an argon gas flow rate of 5-15L / min.

[0018] The beneficial effects of this invention are as follows: (1) In the three-layer structure tundish covering agent of the present invention, the metallurgical functional layer is prepared by pre-melting process, with uniform composition, suitable melting temperature and viscosity, and can quickly form a uniform liquid slag layer on the surface of molten steel. The reducing buffer layer and the low sulfur heat preservation layer have good heat insulation performance, which can effectively reduce the temperature drop of molten steel, prevent the liquid surface from forming a crust, and ensure the smooth operation of continuous casting production.

[0019] (2) In the three-layer tundish covering agent of the present invention, the metallurgical functional layer uses Al2O3 to stabilize the slag viscosity and enhance the adsorption capacity of inclusions, RE2O3 improves the rare earth yield through activity balance, BaO is used as a flux to improve slag performance and fluidity, and MgO is used to improve high-temperature stability and protect the lining; in the buffer layer, fused magnesia forms a physical barrier, and graphite powder provides a reducing atmosphere; the insulation layer uses a composite of low-sulfur perlite and borosilicate glass to achieve efficient heat preservation while completely avoiding sulfur increase. The covering agent of the present invention achieves the comprehensive effects of inhibiting rare earth burn-off, adsorbing rare earth inclusions, preventing sulfur increase in molten steel, and stabilizing continuous casting production.

[0020] (3) In the three-layer tundish covering agent of the present invention, the metallurgical functional layer adopts an ultra-high basicity design (CaO / SiO2≥15), controls the SiO2 content to below 3%, and strictly limits the content of FeO and MnO, significantly reducing the oxidizing properties of the covering agent and reducing the reaction between rare earth elements and oxidizing oxides; in addition, the addition of 3%-8% rare earth oxides increases the activity of rare earth oxides in the slag, and according to the principle of thermodynamic equilibrium, can effectively inhibit the transfer of rare earth elements from molten steel to the slag. Using the covering agent of the present invention, the rare earth recovery rate can be increased to over 55%.

[0021] (4) In the three-layer tundish covering agent of the present invention, the metallurgical functional layer adopts a CaO-Al2O3-RE2O3-BaO slag system, which has good wetting and dissolving ability for inclusions such as rare earth oxides and rare earth sulfides. The addition of BaO can not only reduce the melting point and viscosity of the slag and improve the slag fluidity, but also form a low-melting-point composite compound with rare earth inclusions, promoting the flotation and adsorption of inclusions. Using the covering agent of the present invention, the removal rate of rare earth inclusions larger than 10μm in the billet can reach more than 74%.

[0022] (5) In the three-layer tundish covering agent of the present invention, a composite of low-sulfur expanded perlite and borosilicate glass is used as the insulation layer, and its sulfur content is much lower than that of traditional carbonized rice husks; at the same time, the middle reducing buffer layer forms a dual physical and chemical barrier, blocking the exchange of substances between the upper and lower layers, and completely preventing sulfur in the insulation layer from entering the molten steel. Using the covering agent of the present invention, the sulfur increase in molten steel can be controlled below 0.0005%. Detailed Implementation

[0023] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0024] Example 1 This embodiment provides a three-layer intermediate package covering agent, and the composition ratio and parameters of each layer are as follows: Metallurgical functional layer (thickness 30mm): Chemical composition by mass percentage: CaO 50%, Al2O3 29%, Ce2O3 4%, La2O3 2%, BaO 8%, MgO 5%, SiO2 0.8%, the remainder being unavoidable impurities; basicity CaO / SiO2=62.5, melting temperature 1410℃, viscosity at 1400℃ 1.05Pa·s; The reducing buffer layer (15mm thick) is composed of fused magnesia particles (98.5% purity, 0.5-1.5mm particle size) and graphite powder (99.2% carbon content, 0.1-0.3mm particle size), with the graphite powder accounting for 20% by mass. Low-sulfur insulation layer (40mm thick): composed of low-sulfur expanded perlite (sulfur content 0.008%, particle size 0.1-0.8mm) and borosilicate glass (density 0.3g / cm³). 3 It consists of particles with a diameter of 20-80 μm, of which borosilicate glass accounts for 30% by mass.

[0025] The preparation method of the above-mentioned covering agent includes the following steps: (1) According to the metallurgical functional layer composition ratio, the raw materials are active lime, lightly calcined magnesia, corundum powder, barium oxide, lanthanum oxide, cerium oxide, silicon oxide and manganese oxide. The raw materials are mixed evenly, pre-melted at 1520℃ for 2.5 hours, water quenched and dried at 120℃ for 2 hours, and ground to a particle size ≤0.1mm to obtain the metallurgical functional layer material. (2) Mix the fused magnesia particles and graphite powder in a mixer for 12 minutes according to the specified ratio to obtain a reducing buffer layer material; (3) Mix expanded perlite and borosilicate glass in a mixer for 8 minutes according to the specified ratio to obtain low-sulfur insulation material; The order of steps (1) to (3) is not limited; (4) Pack the above three materials separately in moisture-proof packaging bags.

[0026] Instructions for using the above-mentioned covering agent: (1) The metallurgical functional layer material is brought into contact with molten steel to form a metallurgical functional layer with a thickness of 30 mm. The reducing buffer layer material is covered on top of the metallurgical functional layer material to form a reducing buffer layer with a thickness of 15 mm. The low sulfur insulation layer material is covered on top of the reducing buffer layer material to form a low sulfur insulation layer with a thickness of 40 mm. During the continuous casting process, each layer of covering agent is added for every 10 t of molten steel cast to maintain a total thickness of about 85 mm. (2) Argon gas is used for protection in the tundish, with an argon gas flow rate of 10L / min and the molten steel temperature controlled at 1540-1560℃.

[0027] An industrial test was conducted on the tundish of a steel plant. The steel grade being smelted was rare earth weathering steel Q355RE, with a target rare earth content of Ce+La=0.03%. The test results are as follows: The rare earth recovery rate was 58.2%, which was 26.5% higher than that of the conventional covering agent used in Comparative Example 1; the number density of rare earth inclusions larger than 10 μm in the billet was reduced to 2.8 inclusions / mm. 2 The removal rate was 76.7%; the sulfur increase in molten steel was 0.0003%; no crusting or nozzle blockage occurred during continuous casting, and production proceeded smoothly.

[0028] Example 2 This embodiment provides a three-layer intermediate package covering agent, and the composition ratio and parameters of each layer are as follows: Metallurgical functional layer (thickness 25mm): Chemical composition by mass percentage: CaO 48%, Al2O3 32%, Ce2O3 35%, BaO 8.5%, MgO 4%, SiO2 1.2%, the remainder being unavoidable impurities; The basicity CaO / SiO2=40, the melting temperature is 1395℃, and the viscosity at 1400℃ is 0.98 Pa·s; The reducing buffer layer (12mm thick) is composed of fused magnesia particles (98.2% purity, 0.8-2.0mm particle size) and graphite powder (99.0% carbon content, 0.2-0.5mm particle size), with the graphite powder accounting for 18% by mass. Low-sulfur insulation layer (35mm thick): composed of low-sulfur expanded perlite (sulfur content 0.007%, particle size 0.2-1.0mm) and borosilicate glass (density 0.25g / cm³). 3 It consists of particles with a diameter of 10-60 μm, of which borosilicate glass accounts for 25% by mass.

[0029] The preparation method and usage method are the same as in Example 1.

[0030] An experiment was conducted on rare earth wear-resistant steel NM450RE smelted in the tundish of a steel plant, with a target rare earth content of Ce+La=0.05%. The experimental results are as follows: The rare earth recovery rate was 56.7%; the removal rate of rare earth inclusions larger than 10μm in the billet was 74.2%; the sulfur increase in molten steel was 0.0004%; and the production process was stable.

[0031] Comparative Example 1 A comparative test was conducted using a traditional double-layer structure intermediate ladle covering agent. The lower layer was ordinary alkaline pre-melted slag (40 mm thick), and its chemical composition by mass percentage was: CaO 42%, SiO2 28%, Al2O3 20%, MgO 8%. The upper layer was carbonized rice husk (45 mm thick).

[0032] An industrial test was conducted on a tundish at a steel plant. The tundish parameters were the same as in Example 1. The steel grade being smelted was rare earth weathering steel Q355RE, with a target rare earth content of Ce+La=0.03%. The test results are as follows: The rare earth recovery rate was 31.7%; the number density of rare earth inclusions larger than 10 μm in the billet was 11.5 inclusions / mm. 2 The sulfur content in the molten steel was 0.0021%; two instances of slight crust formation occurred during the continuous casting process.

[0033] As can be seen from the comparison of the above embodiments and comparative examples, the three-layer structure tundish covering agent of the present invention has significant advantages in improving rare earth yield, removing rare earth inclusions and controlling sulfur increase in molten steel, and can effectively improve the production stability and product quality of rare earth steel.

[0034] 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, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A three-layer structure tundish covering agent for smelting rare earth steel grades, characterized in that, The covering agent comprises, from bottom to top: Metallurgical functional layer: Its chemical composition by mass percentage is: CaO 45%-55%, Al2O3 25%-35%, RE2O3 3%-8%, BaO 5%-12%, MgO 3%-7%, SiO2≤3%, and the remainder are unavoidable impurities; Reducing buffer layer: composed of fused magnesia particles and graphite powder, wherein the mass percentage of graphite powder is 15%-25%; Low-sulfur insulation layer: composed of expanded perlite and borosilicate glass, wherein the mass percentage of borosilicate glass is 20%-40%.

2. The three-layer intermediate packing covering agent according to claim 1, characterized in that, The basicity of the metallurgical functional layer is CaO / SiO2≥15, the melting temperature is 1350-1420℃, and the viscosity at 1400℃ is 0.8-1.1 Pa·s.

3. The three-layer intermediate packing covering agent according to claim 1, characterized in that, The RE2O3 is Ce2O3, La2O3, or a mixture of Ce2O3 and La2O3; In the mixture of Ce2O3 and La2O3, the mass ratio of Ce2O3 to La2O3 is (2-4):

1.

4. The three-layer intermediate packing covering agent according to claim 1, characterized in that, The purity of the fused magnesia particles is ≥98%, and the particle size is 0.5-2.0 mm; The graphite powder has a carbon content of ≥99% and a particle size of 0.1-0.5 mm.

5. The three-layer intermediate packing covering agent according to claim 1, characterized in that, The expanded perlite has a sulfur content ≤0.01wt% and a particle size of 0.1-1.0mm; The density of the borosilicate glass is 0.2-0.4 g / cm³. 3 The particle size is 10-100μm.

6. The three-layer intermediate packaging covering agent according to claim 1, characterized in that, The thickness ratio of the metallurgical functional layer, the reducing buffer layer and the low-sulfur insulation layer is (2-4):(1-2):(3-5); The total thickness of the covering layer is 60-110 mm.

7. A method for preparing a three-layer intermediate packaging covering agent according to any one of claims 1-6, characterized in that, Includes the following steps: Step 1: Preparation of metallurgical functional layer materials According to the composition ratio of the metallurgical functional layer, the raw materials are active lime, lightly calcined magnesia, corundum powder, barium oxide, rare earth oxide, silicon oxide and manganese oxide. The raw materials are mixed evenly, pre-melted at 1500-1550℃ for 2-3 hours, water quenched and dried, and then ground to a particle size ≤0.1mm to obtain the metallurgical functional layer material. Step 2: Preparation of reducing buffer layer material According to the composition ratio of the reducing buffer layer, fused magnesia particles and graphite powder are mixed for 10-15 minutes to obtain the reducing buffer layer material. Step 3: Preparation of low-sulfur insulation layer materials According to the composition ratio of the low-sulfur insulation layer, expandable perlite and borosilicate glass are mixed for 5-10 minutes to obtain the low-sulfur insulation layer material. The order of steps 1 to 3 is not limited; Step 4: Pack them separately The metallurgical functional layer material, the reducing buffer layer material, and the low-sulfur insulation layer material are packaged separately.

8. The application of a three-layer intermediate packaging covering agent according to any one of claims 1-6, characterized in that, Used in the smelting of rare earth steel.

9. The application according to claim 8, characterized in that, In the rare earth steel, the rare earth elements are Ce and / or La, wherein the mass percentage content of the rare earth elements is ≥0.01%.

10. The application according to claim 8, characterized in that, The method of using the covering agent includes the following steps: (1) The metallurgical functional layer material is brought into contact with molten steel to form a metallurgical functional layer. The reducing buffer layer material is covered on top of the metallurgical functional layer material to form a reducing buffer layer. The low-sulfur insulation layer material is covered on top of the reducing buffer layer material to form a low-sulfur insulation layer. (2) During the continuous casting process, the temperature of the molten steel in the tundish is controlled at 1530-1580℃, and argon gas is used for protection in the tundish with an argon gas flow rate of 5-15L / min.