A continuous casting protective slag to avoid slag inclusions on the surface of low-carbon steel

Abstract

This invention relates to the field of iron and steel metallurgy, and more particularly to a continuous casting protective slag that avoids slag inclusions on the surface of low-carbon steel. The continuous casting protective slag is composed of the following components by weight percentage: CaO: 24%-26%, SiO2: 28%-30%, La2O3: 12%-15%, MgO: 1%-2.5%, Na2O: 2.5%-4.5%, Li2O: 1.5%-2%, C: 2.5%-3.0%, Fe2O3: 3.5%-4%, B2O3: 1%-3%, BaO: 8%-10%, F ≤ 1%, with the balance being unavoidable impurities. The slag system of this invention is designed with suitable viscosity and melting point, high surface tension of the liquid slag, and good fluidity, allowing the liquid slag to flow evenly into the meniscus and the gap between the crystallizer. This results in a uniform slag film, moderate slag consumption, and effectively improves slag inclusions and adhesion phenomena on the surface of the continuously cast billet.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, and more particularly to a continuous casting protective slag that avoids slag inclusions on the surface of low-carbon steel. Background Technology

[0002] Continuous casting flux is a functional material based on CaO and SiO2, supplemented with fluxes (alkali metal oxides and fluorides, etc.) and framework materials (carbon black, graphite, and coke, etc.). As the medium for interaction between the mold and the billet shell, the composition ratio of the flux is a crucial factor affecting the quality of the cast billet. The composition ratio directly influences the flux's melting characteristics, viscosity, interfacial properties, and ability to absorb inclusions, thus affecting heat transfer and lubrication during the continuous casting process.

[0003] For low-carbon steel, continuous casting billets are prone to surface defects such as subcutaneous slag inclusions and slag entrapment. This is because protective slag in the crystallizer is carried into the molten steel for various reasons during the continuous casting process. Some of it is captured by the growing solidified billet shell, which is called slag entrapment. This forms surface (subcutaneous) slag inclusion defects, which eventually lead to defects such as voids and cracks in the product during subsequent processing, resulting in an increased scrap rate.

[0004] To address the slag inclusion defects on the surface of cast billets and improve the yield of low-carbon steel products, it is necessary to improve the performance of continuous casting protective slag. Patent CN103128240 A discloses a protective slag for a low-carbon steel continuous casting crystallizer. By controlling the Na2O content and adding Ce2O3, the protective slag exhibits low binary basicity and high viscosity. However, this invention has a high F content, which can cause environmental pollution, erosion of the sprue, corrosion of equipment, and harm to human health. Patent CN102019384 A discloses a high-viscosity continuous casting protective slag for solving the problem of slag inclusions on the surface of low-carbon steel. This slag system increases viscosity and surface tension by adding lithium carbonate, fluorite, and white alkali. However, the high Al2O3 content in this slag system easily leads to an increase in inclusions in the cast billet, resulting in inclusion aggregation on the billet surface (0-5mm). Summary of the Invention

[0005] In view of the problems existing in the background technology, the purpose of this invention is to provide a continuous casting protective slag that avoids slag inclusions on the surface of low carbon steel. The slag system has suitable viscosity and melting point, which ensures slag consumption and slag layer thickness. It not only solves the problems of slag inclusions and slag entanglement in continuous casting, but also effectively avoids carbon increase and has a relatively low fluorine content.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] This invention provides a continuous casting protective slag to avoid slag inclusions on the surface of low-carbon steel, wherein the continuous casting protective slag is composed of the following components by weight percentage:

[0008] CaO: 24%-26%, SiO2: 28%-30%, La2O3: 12%-15%, MgO: 1%-2.5%, Na2O: 2.5%-4.5%, Li2O: 1.5%-2%, C: 2.5%-3.0%, Fe2O3: 3.5%-4%, B2O3: 1%-3%, BaO: 8%-10%, F≤1%, balance being unavoidable impurities.

[0009] The principles for determining the components and contents of the continuous casting protective slag used in this invention to avoid slag inclusions on the surface of low-carbon steel are as follows:

[0010] CaO: The CaO content is related to the crystallization temperature. Increasing the CaO content will increase the alkalinity, which will first decrease and then increase the slag film thickness, reduce the viscosity, and improve the ability to adsorb inclusions. The CaO content will also affect indicators such as melting temperature and crystallization temperature. In order to control the viscosity and slag film thickness of the slag system and take into account other indicators, while ensuring a certain crystallization ability, this invention controls the CaO content at 24%-26%.

[0011] SiO2: Increased SiO2 content will increase the viscosity of the slag system and the generated glass phase will improve the lubrication performance between the protective slag and the billet. However, excessive SiO2 will cause the mineral phase to form chains, affecting the lubrication effect. If it is too low, it will not meet the lubrication requirements of the billet. Taking all factors into consideration, this invention controls the SiO2 content at 28%-30%.

[0012] La2O3: La2O3 can increase the viscosity of the slag system. By partially replacing Al2O3 and other components in the slag system with La2O3, it helps reduce the formation of Al2O3 inclusions during continuous casting. While increasing the amount of Al2O3 in the slag system within a certain range does not significantly improve lubrication, it can actually worsen heat transfer. In comparison, La2O3 offers a better overall effect. Taking all factors into consideration, this invention controls the La2O3 content at 12%-15%.

[0013] MgO: Adding an appropriate amount of MgO can improve slag consumption, but excessive MgO will deteriorate the melting performance of the slag. Taking all factors into consideration, this invention limits the MgO content to 1%-2.5%.

[0014] Na2O: Na2O can disrupt the silicate network structure, reduce the surface tension of liquid slag, and lower viscosity and melting point, but it is not conducive to improving lubricity. This invention takes into account both improving viscosity and lubricity, and limits the Na2O content to 2.5%-4.5%.

[0015] F: Fluorides have a good effect on improving heat transfer and lubrication, but they can pollute the environment, corrode production equipment, and harm human health. Taking all factors into consideration, F≤1%.

[0016] Li2O: A small amount of Li2O can ensure the melting point, viscosity and glassiness of the protective slag, but Li2O is relatively expensive. Taking all factors into consideration, the Li2O content is limited to 1.5%-2%.

[0017] C: The main function of C is to regulate the melting rate and ensure the thickness of the slag layer. However, high C content may lead to carbon increase in continuously cast low-carbon steel. The C content is limited to 2.5%-3.0%.

[0018] B2O3: Partially replacing CaF2 with B2O3 can form fine, high-melting-point phases, which helps to adjust the melting characteristics of the slag system. Taking all factors into consideration, the B2O3 content is controlled at 1%-3%.

[0019] BaO: BaO partially replaces CaF2 to form a low-melting-point phase with a larger size. A certain amount of BaO improves the lubrication performance of the protective slag but worsens the heat transfer. However, if it exceeds a certain range, it will deteriorate the lubrication and improve the heat transfer. Taking all factors into consideration, the BaO content is controlled at 8%-10%.

[0020] Fe2O3: Fe2O3 is a common component in continuous casting protective slag, and its content generally does not exceed 7%. When the Fe2O3 content in the slag is high, the slag viscosity is relatively low. In addition, Fe2O3 is easy to form low melting point phases with SiO2 and CaO. In order to make the protective slag viscosity relatively high and the melting point relatively low, the Fe2O3 content is controlled at 3.5%-4% after comprehensive consideration.

[0021] In the above technical solution, the melting point of the continuous casting protective slag is 1080-1100℃.

[0022] In the above technical solution, the viscosity of the continuous casting protective slag at 1300℃ is further 0.42-0.47 Pa·S.

[0023] In the above technical solution, the basicity of the continuous casting protective slag (CaO / SiO2) is further 0.83-0.92.

[0024] In the above technical solution, the C in the continuous casting protective slag is added to the protective slag in the form of a carbonaceous material composed of carbon black and flake graphite, wherein the mass ratio of carbon black to flake graphite is 1:0.5-1:1.

[0025] In the above technical solution, the protective slag needs to be baked at 180-220℃ for 1.5-2 hours before use.

[0026] The beneficial effects of this invention are as follows:

[0027] The slag system design of this invention has suitable viscosity and melting point, high surface tension and good fluidity of liquid slag, which allows the liquid slag to flow evenly into the meniscus and the gap between the crystallizer, resulting in a uniform slag film and moderate slag consumption, effectively improving the phenomena of slag inclusion and slag adhesion on the surface of the continuously cast billet.

[0028] The slag system of this invention partially replaces Al2O3 with La2O3 and controls the fluorine and carbon content while ensuring the physicochemical properties of the slag system, effectively avoiding the problems of increased carbon content and inclusions in molten steel, and reducing environmental pollution.

[0029] In summary, this invention controls the basicity of the protective slag to be 0.83-0.92, the melting point to be 1080-1100℃, and the viscosity at 1300℃ to be 0.42-0.47 Pa·S. The slag viscosity is relatively high, the slag consumption and liquid slag layer thickness are moderate, the lubrication and heat transfer performance are good, the slag entrapment and inclusion are avoided to a limited extent, and the invention is environmentally friendly with minimal carbon increase. Detailed Implementation

[0030] 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.

[0031] Examples 1-3

[0032] The composition of the continuous casting protective slag in Examples 1-3 of this invention is shown in Table 1.

[0033] Table 1. Composition (wt%) of the continuous casting protective slag in Examples 1-3

[0034] Group CaO <![CDATA[SiO2]]> <![CDATA[La2O3]]> MgO <![CDATA[Na2O]]> F <![CDATA[Li2O]]> C <![CDATA[Fe2O3]]> <![CDATA[B2O3]]> BaO <![CDATA[Alkalinity CaO / SiO2]]> Example 1 25 30 12 2.5 2.5 0.2 2 3.0 3.5 1 10 0.83 Example 2 26 28.5 15 1 3.2 0.6 1.8 2.7 3.7 2 9 0.91 Example 3 25.5 29.1 13.5 2 4.5 1 1.5 2.5 4 3 8 0.88

[0035] Wherein: C is added to the protective slag in the form of a carbonaceous material composed of carbon black and flake graphite, and the mass ratio of carbon black to flake graphite is 1:0.5-1:1.

[0036] The physicochemical properties of the above-mentioned continuous casting protective slag are shown in Table 2.

[0037] Table 2. Physicochemical properties of the continuous casting fluxes in Examples 1-3

[0038] Performance indicators Melting point (°C) Viscosity at 1300℃ (Pa·S) Example 1 1088 0.44 Example 2 1080 0.47 Example 3 1098 0.42

[0039] The continuous casting protective slags of Examples 1-3 were baked at 180-220℃ for 1.5-2 hours. Casting tests of low-carbon steel grades were carried out on a medium-thin slab continuous casting machine with a casting cross-section of 170mm×900-2010mm. The casting speed was controlled at 0.62-2.00m / min, the average slag consumption was 0.54kg / t steel, the thickness of the liquid slag layer was 13-15mm, and the total thickness of the slag film was 1.5-2.5mm.

[0040] The test results show that the protective slag has good lubrication and filling properties, the liquid slag flows into the meniscus and the gap between the crystallizer has a significant effect, the slag film is uniform, the slag consumption is moderate, there is no adhesion and the slag strips are small, the continuous casting process basically does not increase carbon, and no surface defects were found in the subsequent rolling of the steel grade.

[0041] 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 continuous casting protective slag to avoid slag inclusions on the surface of low-carbon steel, characterized in that, The continuous casting protective slag consists of the following components by weight percentage. composition: CaO: 24%-26%, SiO2: 28%-30%, La2O3: 12%-15%, MgO: 1%-2.5%, Na2O: 2.5%-4.5%, Li2O: 1.5%-2%, C: 2.5%-3.0%, Fe2O3: 3.5%-4%, B2O3: 1%-3%, BaO: 8%-10%, F≤1%, balance being unavoidable impurities.

2. The continuous casting protective slag for avoiding slag inclusions on the surface of low-carbon steel according to claim 1, characterized in that, The melting point of the continuous casting protective slag is 1080-1100℃.

3. The continuous casting protective slag for avoiding slag inclusions on the surface of low-carbon steel according to claim 1, characterized in that, The viscosity of the continuous casting protective slag at 1300℃ is 0.42-0.47 Pa·S.

4. The continuous casting protective slag for avoiding slag inclusions on the surface of low-carbon steel according to claim 1, characterized in that, The basicity of the continuous casting protective slag, CaO / SiO2, is 0.83-0.

92.

5. The continuous casting protective slag for avoiding slag inclusions on the surface of low-carbon steel according to claim 1, characterized in that, The carbon in the continuous casting protective slag is added to the protective slag in the form of a carbonaceous material composed of carbon black and flake graphite, wherein the mass ratio of carbon black to flake graphite is 1:0.5-1:

1.

6. The continuous casting protective slag for avoiding slag inclusions on the surface of low-carbon steel according to claim 1, characterized in that, The protective slag needs to be baked at 180-220℃ before use, and the baking time is 1.5-2 hours.