Medium-carbon high-aluminum steel low-reactivity continuous casting protective slag

Abstract

The present application relates to the technical field of steel metallurgy, and in particular, relates to a low-reactivity continuous casting protective slag for medium-carbon high-aluminum steel. The chemical composition of the protective slag is as follows in terms of mass percentage: MgO+Al2O3: 55%-60%, MgO / Al2O3=1.3-1.6, Na2O: 12%-15%, F: 7%-10%, TiO2: 7%-10%, Ti2O3: 6%-10%, C: 2%-6%, and the rest is impurities. The present application proposes a MgO-Al2O3 binary system protective slag, the viscosity and melting point of which are moderate, the crystallization temperature is high, and the precipitated MgTiO3 crystal ensures slow heat transfer of the casting blank and avoids longitudinal cracks of the casting blank, thereby solving the problems of slag-steel reaction and casting blank crack in the continuous casting process of medium-carbon high-aluminum steel.

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 for medium-carbon high-alumina 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 skeleton materials (carbon black, graphite and coke, etc.). It serves as a medium for interaction between the crystallizer and the billet shell. Its composition ratio is an important factor affecting the quality of the cast billet. The composition ratio directly affects the melting characteristics, viscosity, interfacial characteristics, and ability to absorb inclusions of the flux, thereby affecting the heat transfer and lubrication of the continuous casting process. However, the following problems exist in the continuous casting production of medium carbon high aluminum steel: (1) During the casting process, the reaction 4[Al]+3(SiO2)=3[Si]+2(Al2O3) will occur, that is, Al in the steel will react with SiO2 in the protective slag to generate Al2O3 which enters the slag, resulting in a continuous increase in the melting point and viscosity of the slag, and steel leakage will cause the casting to be unable to proceed smoothly; (2) During the solidification process, the phase transformation of ferrite to austenite brings about a large solidification shrinkage (peregrine reaction), and the unit volume shrinkage after the reaction reaches 4.7%, which causes uneven heat transfer in the primary billet shell formed in the crystallizer during the solidification process. Uneven heat transfer in the billet shell leads to stress concentration, and finally longitudinal cracks are formed.

[0003] Regarding the issue of aluminum reduction in the protective slag during casting, domestic and international solutions mainly include: 1. Increasing viscosity while maintaining basicity to reduce slag-steel reaction kinetics; 2. Increasing the SiO2 content in the slag system to control the basicity of the slag-steel reaction within a normal range; 3. Increasing basicity to weaken the reaction by reducing SiO2 activity; 4. Strictly controlling the SiO2 content in the protective slag.

[0004] Japanese Patent JP 2000000646 uses a CaO-Al2O3 system to replace the CaO-SiO2 system of protective slag, and uses CaTiO3 instead of gunmetal (3CaO·2SiO2·CaF2). However, CaTiO3 has a high melting point of 1975℃, which is not conducive to lubrication of the cast billet. Chinese Patent CN101332497A discloses a continuous casting protective slag for high-alumina steel and its manufacturing method. This invention effectively suppresses the changes in the composition and performance deterioration of the protective slag caused by slag-steel reaction by controlling the SiO2 content to within 2%. However, due to the high basicity and strong crystallinity of the protective slag, problems such as poor melting, easy slag ring formation, and low consumption may occur during application. Patent CN 113305274 A discloses a protective slag for medium carbon steel in continuous casting of thick plates, with CaO, SiO2, CaF2, and B2O3 as the main materials. The CaF2 content is 18±3%, the B2O3 content is 2±1%, and the CaO / (SiO2+B2O3) ratio is 0.95-1.15, achieving uniform and stable heat transfer within the crystallizer. This protective slag has a high fluorine content. Patent CN 114247860 A discloses a protective slag for continuous casting crystallizers used in casting medium carbon steel. By increasing the slag basicity (1.26-1.38), the crystallization rate of the protective slag is increased. The addition of fluxes such as Al2O3, R2O, F-, and MgO results in 5-15% glassy state in the protective slag. The overall basicity of the protective slag is relatively high, and Na2O inhibits the precipitation of gunpowder crystals, which affects the heat transfer effect. Summary of the Invention

[0005] To address the problems existing in the background technology, the purpose of this invention is to provide a low-reactivity continuous casting mold flux for medium-carbon high-alumina steel, solving the problem of slag-steel reaction during continuous casting, ensuring good lubricity while resolving the problem of longitudinal cracking in the billet caused by uneven heat transfer in the billet shell. It is particularly suitable for the continuous casting production of medium-carbon high-alumina steel (Al content > 0.55%).

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

[0007] A low-reactivity continuous casting protective slag for medium-carbon high-alumina steel, wherein the chemical composition of the protective slag, by mass percentage, is: MgO+Al2O3: 55%-60%, MgO / Al2O3 = 1.3-1.6, Na2O: 12%-15%, F: 7%-10%, TiO2: 7%-10%, Ti2O3: 6%-10%, C: 2%-6%, with the remainder being impurities.

[0008] In the above technical solution, the chemical composition of the protective slag has MgO / TiO2 = 3.5-5.

[0009] In the above technical solution, the chemical composition of the protective slag is further defined as MgO / Ti2O3 = 3.5-5.8.

[0010] In the above technical solution, the melting point of the protective slag is 1120-1190℃.

[0011] In the above technical solution, the viscosity of the protective slag at 1300℃ is further 0.02-0.10 Pa·S.

[0012] In the above technical solution, further, the chemical composition of the protective slag contains ≤2% impurities.

[0013] In the above technical solution, the protective slag needs to be pre-melted at 1210-1250℃ for 15-30 minutes before use.

[0014] The mechanism of action of this invention is as follows:

[0015] Conventional (CaO-SiO2) slag systems undergo a slag-steel reaction during high-alumina steel continuous casting: 4[Al] + 3(SiO2) = 3[Si] + 2(Al2O3). This invention replaces SiO2 with Al2O3 and CaO with MgO, and prevents the slag-steel reaction by adding TiO2 and strictly controlling the ratios of MgO / Al2O3, MgO / TiO2, and MgO / Ti2O3. Simultaneously, it precipitates MgTiO3 crystals (melting point 1565℃), which can replace the role of gunmetal (3CaO·2SiO2·CaF2) in the CaO-SiO2 slag system, and has a much lower melting point than the currently widely accepted CaTiO3 (melting point 1975℃). Ti2O3 can act as a nucleation promoter, reducing the potential barrier that needs to be overcome for nucleation, thereby promoting the formation of the MgTiO3 mineral phase. Excessive TiO2 will make the slag crystallization ability too strong, which is not conducive to heat transfer control, while a small amount of TiO2 has little effect. Excessive Ti2O3 will lead to larger crystal size and reduced uniformity. Taking all factors into consideration, the TiO2 content should be 7%-10%, the Ti2O3 content should be 6%-10%, the MgO / TiO2 ratio should be 3.5-5, and the MgO / Ti2O3 ratio should be 3.5-5.8.

[0016] Na2O is a common flux in protective slag, which can adjust the melting point and viscosity of the protective slag. However, high Na2O content promotes excessive crystal precipitation in the slag. Therefore, this invention takes into account the viscosity and crystallization performance of the slag system suitable for medium carbon high aluminum steel and controls Na2O at 12%-15%.

[0017] Fluorides have a good effect on improving heat transfer and lubrication, but they can pollute the environment and corrode production equipment. Taking all factors into consideration, the content of fluoride is controlled at 7%-10%, which reduces the content of fluoride to a certain extent.

[0018] Carbon (C) is a necessary substance for regulating the melting rate and ensuring the thickness of the liquid slag layer. However, a high C content is not conducive to controlling heat transfer. Considering all factors, the C content is limited to 2%-6%.

[0019] The beneficial effects of this invention are as follows:

[0020] This invention proposes a binary protective slag system of MgO-Al2O3. The slag system has moderate viscosity and melting point, and high crystallization temperature. The precipitated MgTiO3 crystals ensure slow heat transfer in the billet, avoid longitudinal cracks in the billet, and solve the problems of slag-steel reaction and billet cracks that are prone to occur in the continuous casting process of medium carbon high aluminum steel.

[0021] The Si-free design of this invention avoids the problem of drastic changes in the physicochemical properties of the slag system caused by the reaction between slag and steel, thus enabling the slag system to have stable lubricity and fluidity. Detailed Implementation

[0022] The present invention will be further illustrated below with reference to the embodiments. The embodiments are only used to illustrate the present invention and are not intended to limit the present invention in any way.

[0023] Examples 1-4

[0024] The composition of the continuous casting protective slag in Examples 1-4 of this invention is shown in Table 1.

[0025] Table 1. Composition (wt%) of the continuous casting protective slag in Examples 1-4

[0026] Group MgO <![CDATA[Al2O3]]> <![CDATA[Na2O]]> F <![CDATA[TiO2]]> <![CDATA[Ti2O3]]> C <![CDATA[MgO / Al2O3]]> <![CDATA[MgO / TiO2]]> <![CDATA[MgO / Ti2O3 <!-- 2 -->]]> Example 1 31.1 23.9 15.0 10.0 8.5 7.5 3.1 1.3 3.7 4.1 Example 2 35.0 25.0 12.0 7.0 7.0 6 6.0 1.4 5 5.8 Example 3 34.8 23.2 12.4 7.5 9.0 10.0 2.0 1.5 3.9 3.5 Example 4 35.1 21.9 13.0 8.0 10.0 6.6 4.5 1.6 3.5 5.3

[0027] The physicochemical properties of the above-mentioned continuous casting protective slag are shown in Table 2.

[0028] Table 2. Physicochemical properties of the continuous casting protective slags in Examples 1-4

[0029] index Example 1 Example 2 Example 3 Example 4 Melting point (°C) 1120 1190 1145 1157 Viscosity (Pa·S) 0.02 0.1 0.05 0.07

[0030] As shown in Table 2, the melting point of the protective slag is in the range of 1120-1190℃, and the viscosity at 1300℃ is 0.02-0.10 Pa·S. The protective slag system of the present invention has moderate viscosity and melting point, and the high crystallization temperature ensures the fluidity and good lubrication effect of the slag system. The precipitated MgTiO3 crystals ensure slow heat transfer of the billet and avoid longitudinal cracking of the billet. The Si-free design avoids the problem of drastic changes in the physicochemical properties of the slag system due to the reaction between slag and steel.

[0031] The continuous casting protective slags from Examples 1-4 were pre-melted at 1210-1250℃ for 15-30 minutes. Casting tests of medium-carbon high-alumina steel grades were conducted on a square billet continuous casting machine with a casting cross-section of 160mm×160mm. The casting speed was controlled to be less than 2.8-3.0m / min, the liquid level fluctuation was less than 6mm, the slag layer thickness ranged from 14±2mm, and the average slag consumption was 0.47kg / t steel. Simultaneously, electromagnetic stirring was applied to the crystallizer. The test results showed that slag strip formation in the crystallizer was rare, the protective slag had good spreading and lubrication properties, and it could effectively avoid slag-steel reaction during application. Quality defects such as longitudinal cracks in the billet were significantly improved.

[0032] 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 for medium-carbon high-alumina steel, characterized in that, The chemical composition of the protective slag, by mass percentage, is as follows: MgO + Al2O3: 55%-60%, MgO / Al2O3 = 1.3-1.6, Na2O: 12%-15%, F: 7%-10%, TiO2: 7%-10%, Ti2O3: 6%-10%, C: 2%-6%, with the remainder being impurities; The chemical composition of the protective slag has MgO / TiO2 = 3.5-5; The chemical composition of the protective slag has an MgO / Ti2O3 ratio of 3.5-5.

8. The melting point of the protective slag is 1120-1190 ℃; The viscosity of the protective slag at 1300℃ is 0.02-0.10 Pa·S.

2. The low-reactivity continuous casting protective slag for medium-carbon high-alumina steel according to claim 1, characterized in that, The chemical composition of the protective slag contains ≤2% impurities.

3. The low-reactivity continuous casting protective slag for medium-carbon high-alumina steel according to claim 1, characterized in that, The protective slag needs to be pre-melted at 1210-1250 ℃ for 15-30 min before use.