Casting powder for continuous casting of medium carbon steel slab

Abstract

The invention relates to the technical field of ferrous metallurgy, in particular to casting powder for continuous casting of a medium carbon steel slab. The casting powder comprises the following components in percentage by weight: 30%-40% of CaO, 25%-30% of SiO2, 12%-15% of B2O3, 1%-3% of BaO, 2%-4% of Y2O3, 4%-7% of R2O, 3%-6% of C and less than or equal to 6% of inevitable impurities, wherein R is one of Li, Na, K, Rb and Cs. According to the invention, Y2O3 is added to promote crystal nucleus formation and control the component ratio, the casting powder is promoted to separate out a fine-size calcium borosilicate ore phase similar to gun spar in fluorine-containing slag in property under the fluoride-free condition, the crystallization inoculation time is short, the crystallization temperature is moderate, and the melting point, viscosity and the like of the casting powder are similar to those of traditional casting powder; stable heat transfer and lubrication are guaranteed, and the problems of continuous casting longitudinal cracks of medium carbon steel slabs and the like are effectively solved.

Description

Technical Field

[0001] The present invention relates to the technical field of iron and steel metallurgy, and in particular to a protective slag for continuous casting of medium carbon steel slabs. Background Art

[0002] Continuous casting protection slag is a functional material with CaO and SiO2 as the base material and added flux (alkali metal oxides and fluorides, etc.) and skeleton materials (carbon black, graphite and coke, etc.). As the medium for interaction between the crystallizer and the billet shell, the composition ratio of continuous casting protection slag is an important factor affecting the quality of the billet. The composition ratio will directly affect the melting characteristics, viscosity, interface characteristics, and ability to absorb inclusions of the protection slag, thereby affecting the heat transfer and lubrication of the continuous casting process. For medium carbon steel (C content of 0.14%-0.2%) slabs, the phase transformation from ferrite to austenite during solidification brings about a large solidification shrinkage (peritectic reaction), which leads to uneven heat transfer of the primary billet shell formed by the molten steel in the crystallizer during solidification. The uneven heat transfer of the billet shell leads to stress concentration, and finally forms longitudinal cracks.

[0003] From the perspective of mold slag design, the slag film formed by the mold slag in the gap between the crystallizer and the shell needs to: (1) have good lubricity to prevent continuous casting leakage; (2) have a large thermal resistance to prevent cracks through uniform and slow heat transfer. Therefore, fluorides such as fluorite (CaF2), sodium fluoride (NaF), and cryolite (Na3AlF6) are often added to the mold slag to form gunite to improve heat transfer. However, fluorides can corrode equipment, pollute water bodies, and endanger human health. Therefore, fluoride-free mold slag is the development trend. However, fluoride-free mold slag needs to ensure the precipitation of crystals with properties similar to gunite to ensure heat transfer, while ensuring a low melting point and viscosity. Current research ideas include adjusting the component content of conventional slag systems, replacing fluorine with B2O3, and adding Ti to the mold slag.

[0004] Patent CN104607607A discloses a fluorine-free protective slag for continuous casting of zirconium-containing medium-carbon steel. The protective slag is composed of the following oxides in percentage by mass: CaO 35%-45%, SiO2 30%-35%, Al2O3 2%-5%, MgO 1%-3%, B2O3 6%-10%, (Na2O+Li2O) 10-15%, ZrO2 1%-3%; and 1.3≤(Na2O+Li2O+ZrO2) / B2O3≤2. The addition of ZrO2 promotes the precipitation of calcium borosilicate, refines its grains and enhances its stability. The protective slag contains Al2O3, and Al2O3 inclusions are easily formed during the continuous casting process. In addition, ZrO2 is not stable during the continuous casting process, and its crystal form will change with temperature. ZrO2 is expensive, and its application in continuous casting protective slag is less economical, and is not suitable for promotion and use in industrial production. Patent CN107584090A discloses a fluorine-free protective slag for medium carbon steel with a composite nucleating agent added. The components and mass percentages of the protective slag are as follows: CaO 30%-45%, SiO2 25%-35%, Al2O3 4%-10%, MgO 1%-3%, B2O3 6%-10%, (Na2O+Li2O) 8%-12%, TiO2 2%-4%, Cr2O3 1%-2%; and 1≤CaO / SiO2≤1.5, 3.5≤CaO / B2O3≤7, 3≤SiO2 / B2O3≤5.5. By adding TiO2 and Cr2O3, calcium borosilicate is promoted to precipitate, its grains are refined and its stability is enhanced. The slag system has strict requirements on the content of Cr and Ti and has poor adaptability. Summary of the invention

[0005] In view of the problems existing in the background technology, the purpose of the present invention is to provide a protective slag for continuous casting of medium carbon steel slabs, which has B2O3-CaO-SiO2 as the main components and a small amount of BaO and R2O. By controlling the component ratio and adding an appropriate amount of Yi2O3, the fluorine-free protective slag is promoted to precipitate small-sized calcium borosilicate to replace gunite, while ensuring suitable viscosity and melting point.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows:

[0007] The invention provides a protective slag for continuous casting of medium carbon steel slabs. The protective slag consists of the following components in weight percentage: CaO: 30%-40%, SiO2: 25%-30%, B2O3: 12%-15%, BaO: 1%-3%, Y2O3: 2%-4%, R2O: 4-7%, C: 3%-6%, inevitable impurities ≤6%, wherein R is one of Li, Na, K, Rb and Cs.

[0008] In the above technical solution, further, 1.35≤B2O3 / (R2O+Y2O3)≤1.70.

[0009] In the above technical solution, further, the melting point of the protective slag is 1040-1080°C.

[0010] In the above technical solution, further, the viscosity of the protective slag at 1300°C is 0.04-0.13 Pa·S.

[0011] In the above technical solution, further, the crystallization temperature of the protective slag is 1250-1290°C.

[0012] In the above technical solution, further, the crystallization incubation time of the protective slag is 15-18s.

[0013] In the above technical solution, further, the protective slag needs to be baked at 150-175° C. for 1.5-2 hours before use.

[0014] The principles for determining the components and contents of the mold slag for continuous casting of medium carbon steel slabs of the present invention are as follows:

[0015] BaO: A proper amount of BaO can form a low melting point phase in the slag system, which improves the lubrication performance of the protective slag, but will also deteriorate the heat transfer performance to a certain extent. As the BaO content continues to increase, the heat transfer performance is significantly improved, but at the same time the lubrication performance will be reduced. Based on comprehensive considerations, the present invention chooses to control the BaO content at 1%-3%.

[0016] R2O: An appropriate amount of R2O can reduce the melting point and viscosity, help promote the formation of fine crystals in the slag system, improve the lubrication and heat transfer of the protective slag, and stabilize the performance of the protective slag. Excessive R2O will reduce the glass properties of the slag film and deteriorate the lubrication ability. Taking all factors into consideration, the R2O content is controlled at 4%-7%.

[0017] CaO: Increasing the CaO content will increase the basicity, reduce the viscosity, and improve the ability to adsorb inclusions. The CaO content will also affect the melting temperature, crystallization temperature and other indicators. In order to control the slag viscosity and slag film thickness and comprehensively consider other indicators while ensuring a certain crystallization ability, the present invention controls the CaO content to 30%-40%.

[0018] SiO2, B2O3, Y2O3: In the present invention, B2O3 is used to partially replace CaF2. Adding B2O3 can form a fine phase with a relatively high melting point, which is helpful to adjust the melting characteristics of the slag system, improve lubrication, and reduce the crystallization temperature. Adding Y2O3 as a crystal nucleation promoter is beneficial to the incubation time of slag crystallization and the adjustment of crystallization temperature. By controlling the content of SiO2 and B2O3, and optimizing the ratio of flux R2O, promoter Y2O3 and B2O3 (Y2O3: 2%-4%, 1.35≤B2O3 / (R2O+Y2O3), B2O3: 12%-15%, SiO2: 25%-30%), the slag system can precipitate calcium borosilicate (Ca4Si2O7F2) with properties similar to that of gunite crystal (Ca4Si2O7F2) under the condition of no fluorine component. 11 Si4B2O 22 ), the crystals are small and uniform.

[0019] C: The main function of C is to play a skeleton effect in the protective slag, so that the molten slag droplets cannot gather together, which is conducive to controlling the molten slag to flow evenly and quickly into the gap between the billet shell and the crystallizer, ensuring that the thickness of the liquid slag layer is moderate. Therefore, the C content is limited to 3%-6%.

[0020] The beneficial effects of the present invention are:

[0021] The present invention promotes the formation of crystal nuclei and controls the ratio of each component by adding Y2O3, thereby promoting the precipitation of fine-sized calcium borosilicate ore phase with properties similar to those of lanceolite in fluorine-containing slag in the absence of fluorine. The crystallization incubation time is short and the crystallization temperature is moderate. The melting point and viscosity of the protective slag are similar to those of traditional protective slag, thereby ensuring stable heat transfer and lubrication, and effectively improving problems such as longitudinal cracking in continuous casting of medium carbon steel slabs.

[0022] The protective slag of the present invention does not contain fluorine and is more environmentally friendly. DETAILED DESCRIPTION

[0023] The present invention is further described below in conjunction with examples, which are only used to illustrate the present invention and are not intended to limit the present invention in any form.

[0024] Examples 1-4

[0025] The composition of the protective slag for continuous casting in Examples 1-4 of the present invention is shown in Table 1.

[0026] Table 1 Composition of continuous casting mold slag of Examples 1-4 (wt%)

[0027] Group CaO <![CDATA[SiO2]]> <![CDATA[B2O3]]> BaO <![CDATA[Y2O3]]> <![CDATA[R2O]]> C <![CDATA[B2O3 / (R2O+Y2O3)]]> Impurities <![CDATA[Alkalinity CaO / SiO2]]> Example 1 30 30 15 3 4 R=Li,7 6 1.36 5 1 Example 2 40 28 12 1 2 R=K,6 5 1.5 6 1.43 Example 3 40 25 14 3 4 R=Rb,5 3 1.56 6 1.6 Example 4 38 28 13 2.5 2.5 R=Cs,5 5 1.7 6 1.36

[0028] The physical and chemical properties of the above-mentioned continuous casting protection slag are shown in Table 2.

[0029] Table 2 Physical and chemical properties of continuous casting mold slag of Examples 1-4

[0030] Performance Indicators Melting point(℃) Viscosity at 1300℃(Pa·S) Crystallization temperature(℃) Crystallization incubation time (s) Example 1 1040 0.04 1262 15.1 Example 2 1071 0.06 1250 15.9 Example 3 1062 0.11 1290 18.0 Example 4 1080 0.13 1278 17.4

[0031] As can be seen from Table 2, the melting point of the protective slag ranges from 1040 to 1080°C, the viscosity at 1300°C is 0.04 to 0.13 Pa·S, the crystallization temperature is between 1250 and 1290°C, and the crystallization incubation time is between 15 and 18s. The protective slag system has moderate viscosity and melting point, and low crystallization temperature; the protective slag of this system uses Y2O3 as a crystal nucleation promoter to form a calcium borosilicate phase. The slag system effectively improves the heat transfer properties while ensuring lubricity, thereby improving the longitudinal cracking problem caused by uneven solidification during the continuous casting of medium carbon steel slabs.

[0032] The continuous casting protection slag of Examples 1-4 was baked at 150-175°C for 1.5-2h, and a casting test of medium carbon steel was carried out on a thick slab continuous casting machine with a casting section of 300mm×1500-2000mm. The pulling speed was controlled to be less than 1.3m / min, the liquid level fluctuation was less than 5mm, the liquid slag layer thickness range was 14.5±1mm, the average slag consumption was 0.60kg / t steel, and the secondary cooling form was a combination of steam mist + dynamic control. At the same time, secondary cooling electromagnetic stirring was used. The test results show that the protection slag has good lubrication and filling properties, is non-sticky and has small slag strips, and the slab has no surface longitudinal cracks and other surface defects.

[0033] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A mold slag for continuous casting of medium carbon steel slab, characterized in that: The protective slag is composed of the following components in weight percentage: composition: CaO: 30%-40%, SiO2: 25%-30%, B2O3: 12%-15%, BaO: 1%-3%, Y2O3: 2%-4%, R2O: 4%-7%, C: 3%-6%, unavoidable impurities ≤ 6%, wherein R is one of Li, Na, K, Rb, Cs.

2. The mold slag for continuous casting of medium carbon steel slab according to claim 1, characterized in that: 1.35≤B2O3 / (R2O+Y2O3)≤1.

70.

3. The mold slag for continuous casting of medium carbon steel slab according to claim 1, characterized in that: The melting point of the protective slag is 1040-1080°C.

4. The mold slag for continuous casting of medium carbon steel slab according to claim 1, characterized in that: The viscosity of the protective slag at 1300° C. is 0.04-0.13 Pa·S.

5. The mold slag for continuous casting of medium carbon steel slab according to claim 1, characterized in that: The crystallization temperature of the protective slag is 1250-1290°C.

6. The mold slag for continuous casting of medium carbon steel slab according to claim 1, characterized in that: The crystallization incubation time of the protective slag is 15-18s.

7. The mold slag for continuous casting of medium carbon steel slab according to claim 1, characterized in that: The protective slag needs to be baked at 150-175° C. for 1.5-2 hours before use.