A protective slag for continuous casting of a medium carbon steel slab

Abstract

The present application relates to the technical field of steel metallurgy, and in particular, relates to a kind of middle carbon steel slab for continuous casting with protective slag.The protective slag is composed of the following components by weight percentage: 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 and Cs.The present application promotes the formation of crystal nucleus by adding Y2O3 and controls the proportion of each component, promotes the protective slag to precipitate similar size small calcium borosilicate phase with the nature of fluorine-free and fluorine-containing slag, the crystallization incubation time is short, the crystallization temperature is moderate, the melting point, viscosity and the like of the protective slag are similar to the traditional protective slag, which ensures stable heat transfer and lubrication, and effectively improves the problems of middle carbon steel slab continuous casting longitudinal crack and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel metallurgy, and particularly relates to a protective slag for medium-carbon steel slab continuous casting. BACKGROUND

[0002] The continuous casting protective slag is a functional material taking CaO and SiO2 as base materials and adding flux (alkali metal oxide and fluoride, etc.) and skeleton material (carbon black, graphite and coke, etc.). The continuous casting protective slag is a medium for interaction between the crystallizer and the shell, and the component proportion is an important factor affecting the quality of the cast slab. The component proportion directly affects the melting characteristics, viscosity, interface characteristics, inclusion absorption capacity and the like of the protective slag, thereby affecting the heat transfer and lubrication and the like in the continuous casting process. For the medium-carbon steel (C content is 0.14%-0.2%) slab, the solidification process brings a large solidification shrinkage (peritectic reaction) due to the phase transition of ferrite to austenite, which causes uneven heat transfer of the primary shell formed by the molten steel in the crystallizer during the solidification process. The uneven heat transfer of the shell causes stress concentration, and finally forms longitudinal cracks.

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

[0004] Patent CN104607607A discloses a kind of zirconium-containing medium carbon steel continuous casting fluoride-free protective slag, protective slag is made of following mass percentage of oxide: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, by adding ZrO2 promote calcium borosilicate precipitate and refine its grain and enhance its stability, the protective slag contains Al2O3, Al2O3 inclusion is easy to form in continuous casting process, in addition ZrO2 is not stable in continuous casting process, its crystal form will change with temperature, and ZrO2 is expensive, economy is poor in continuous casting protective slag, not suitable for industrial production and popularization and application.Patent CN107584090A discloses a kind of medium carbon steel fluoride-free protective slag added with composite nucleating agent, the components of protective slag and its mass percentage 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 promote calcium borosilicate precipitate and refine its grain and enhance its stability, the slag system is strict for Cr, Ti content, and the adaptability is poor. SUMMARY

[0005] In view of the problems in the background art, the present application aims to provide a kind of medium carbon steel slab continuous casting protective slag, with B2O3-CaO-SiO2 as main component, with a small amount of BaO, R2O, by controlling component ratio, and adding appropriate amount of Yi2O3, promote fluoride-free protective slag to precipitate small size calcium borosilicate to replace leucite, while ensuring appropriate viscosity, melting point.

[0006] In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0007] The present application provides a kind of medium carbon steel slab continuous casting protective slag, the protective slag is made of following weight percentage of components: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.

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

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

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

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

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

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

[0014] The components and content of the protective slag for medium carbon steel slab continuous casting are determined according to the following principles:

[0015] BaO: appropriate BaO can form a low-melting point phase in the slag system, so that the lubricating property of the protective slag is good, but it will also deteriorate the heat transfer performance to some extent, and with the continuous increase of BaO content, the improvement of the heat transfer performance is obvious, at the same time, the lubricating property is reduced, and the BaO content is controlled at 1%-3% according to the comprehensive consideration.

[0016] R2O: appropriate R2O can reduce the melting point and viscosity, help to promote the formation of fine crystals in the slag system, improve the lubricating property and heat transfer of the protective slag, and make the performance of the protective slag stable, and excessive R2O will reduce the glass performance of the slag film and deteriorate the lubricating ability, and the R2O content is controlled at 4%-7% according to the comprehensive consideration.

[0017] CaO: increasing the CaO content will increase the basicity, reduce the viscosity, improve the ability to absorb inclusions, and the CaO content will also affect the melting temperature, crystallization temperature and other indexes, in order to control the viscosity of the slag system and the thickness of the slag film and consider other indexes comprehensively, and ensure a certain crystallization ability, the CaO content is controlled at 30%-40% according to the present application.

[0018] SiO2, B2O3, Y2O3: The application partially replaces CaF2 with B2O3, and the addition of B2O3 can form a small and high melting point phase, which helps to adjust the melting characteristics of the slag system, improve lubrication and reduce the crystallization temperature; the addition of Y2O3 as a crystal nucleation promoter is beneficial to the incubation time and adjustment of the crystallization temperature of the molten slag, and by controlling the content of SiO2, 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 (Ca 11 Si4B2O 22 ) similar to the properties of gun crystal (Ca4Si2O7F2) under the condition of no fluorine composition, and the crystal has the characteristics of small and uniform size.

[0019] C: The main role of C is to play a skeleton effect in the protective slag, so that the molten slag drops cannot aggregate with each other, which is beneficial to control the uniform and rapid flow of the molten slag into the gap between the shell and the crystallizer, and ensure that the liquid slag layer has a moderate thickness, so the content of C is limited to 3%-6%.

[0020] The beneficial effects of the application are:

[0021] The application promotes the formation of crystal nuclei by adding Y2O3 and controls the ratio of each component, promotes the precipitation of small-sized calcium borosilicate mineral phase similar to the properties of gun crystal in fluorine-containing slag under the condition of no fluorine, has a short incubation time and a moderate crystallization temperature, and the melting point and viscosity of the protective slag are similar to those of the traditional protective slag, which ensures stable heat transfer and lubrication and effectively improves the problems of longitudinal cracks of medium carbon steel slab continuous casting.

[0022] The protective slag of the application does not contain fluorine and is more friendly to the environment. DETAILED DESCRIPTION

[0023] The application will be further described below in combination with examples, and the examples are only used to illustrate the application, but not to limit the application in any form.

[0024] Examples 1-4

[0025] The components of the protective slag for continuous casting in Examples 1-4 of the application are shown in Table 1.

[0026] Table 1 Components of the protective slag for continuous casting in Examples 1-4 (wt%)

[0027] Group CaO SiO2 B2O3 BaO [Y2O3] [R2O] C B203 / (R20+Y203) Impurities 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 performance indexes of the above protective slag for continuous casting are shown in Table 2.

[0029] Table 2 Physical and chemical performance indexes of the protective slag for continuous casting in Examples 1-4

[0030] Performance Index Melting point (°C) Viscosity at 1300°C (Pa-S) Crystallization temperature (°C) 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 seen from Table 2, the melting point range of the protective slag is 1040-1080℃, the viscosity at 1300℃ is 0.04-0.13Pa·S, the crystallization temperature is between 1250-1290℃, and the crystallization incubation time is between 15-18s. The protective slag system has moderate viscosity and melting point, and low crystallization temperature. The system uses Y2O3 as a crystal nucleus forming promoter to form calcium borosilicate phase, which effectively improves the heat transfer property while ensuring lubricity, and further improves the longitudinal crack problem caused by uneven solidification in the process of continuous casting of medium carbon steel slab.

[0032] The continuous casting protective slags of Examples 1-4 are baked at 150-175℃ for 1.5-2h, and casting tests of medium carbon steel are carried out on a thick slab caster with a casting section of 300mm×1500-2000mm, the withdrawal speed is controlled to be less than 1.3m / min, the liquid surface fluctuation is less than 5mm, the liquid slag layer thickness is in the range of 14.5±1mm, the average slag consumption is 0.60kg / t steel, the secondary cooling form is a combination of steam mist and dynamic control, and the secondary cooling electromagnetic stirring is applied simultaneously. The test results show that the lubrication and filling properties of the protective slag are good, there is no sticking and the slag strip is small, and the slab has no surface longitudinal crack and other surface defects.

[0033] The above examples are only preferred examples of the present application, and are not a limitation on the embodiments. The protection scope of the present application should be limited by the scope defined by the claims. Other different forms of changes or variations can be made on the basis of the above description. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A protective slag for continuous casting of medium carbon steel slabs, characterized in that, The protective slag consists of the following components by weight percentage Composition: CaO: 30%-40%, SiO2: 25%-30%, B2O3: 12%-15%, BaO: 1%-3%, Y2O3: 2%-4%, R2O: 4%-7%, C: 3%-6%, and 1.35≤B2O3 / (R2O+Y2O3)≤1.70, unavoidable impurities≤6%, where R is one of Li, Na, K, Rb, and Cs.

2. The protective slag for continuous casting of medium carbon steel slabs according to claim 1, characterized in that, The melting point of the protective slag is 1040-1080 ℃.

3. The protective slag for continuous casting of medium carbon steel slabs according to claim 1, characterized in that, The viscosity of the protective slag at 1300℃ is 0.04-0.13 Pa·S.

4. The protective slag for continuous casting of medium carbon steel slabs according to claim 1, characterized in that, The crystallization temperature of the protective slag is 1250-1290 ℃.

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

6. The protective slag for continuous casting of medium carbon steel slabs according to claim 1, characterized in that, The protective slag needs to be baked at 150-175 ℃ for 1.5-2 h before use.