RO126400A2 - Lubricant powders for a crystallizer - Google Patents

Abstract

The invention relates to some lubricant powders with a grain size ranging from 2...0.06 mm which are applied on the surface of the liquid steel in a crystallizer during the casting process, they being refractory oxide materials with a certain content of carbon and are carried out in several compositional variants, depending on the quality of the cast steel by using two basic raw materials: indigenous wolastonite deposit and blast furnace slag, the powders having the following properties correlated with the casting parameters and the steel quality: average viscosity ranging between 5...15 poises, fusion temperature ranging between 1080...1250°C, and adjustable fusion rate depending on the carbon content in coke. The claimed lubricant powder based on wolastonite has the following composition: 60...85% wolastonite, 8...15% natural fluorine with a minimum 90% CaF, 10...16% calcined soda, 5...10% expanded pearlite and 10...25% residues of metallurgical coke. The claimed lubricant powder based on blast furnace slag has the following composition: 55...80% blast furnace slag, 5...10% fluorine, 4...8% calcined soda, 10...25% metallurgical coke, or 0...5% graphite and 5...15% pearlite.

Description

The invention relates to the compositions of some assortments of powders which are applied on the surface of the liquid steel of the crystallizer, during the casting process, powders called Zguralub, respectively Pulbwoll. The lubricating powder for the crystallizer is a refractory oxide material with carbon content, which has a complex role in the place of use, the main functions being the following: lubrication of the steel crust during continuous casting, thermal insulation of the liquid steel meniscus, its protection against oxidation, capture and dissolution of steel inclusions by liquid slag, formed by melting the powder. The correlation of the characteristics of the lubricating powder with the casting parameters but also with the type (brand) of steel, can ensure the steel quality increase, the significant reduction of the defects on the surface of the semi-manufactured, increasing the efficiency of the casting process.

At the present technical stage, no such products are manufactured in the country, they are imported from the European market from companies known in the field. The assortments of imported powders do not cover the whole range of steel produced in the country; for this reason it is necessary to produce powders for crystallizer depending on the type of casting plant and in accordance with the cast steel. The imported powders used in the molding of slabs and large slabs are powders obtained as a mechanical mixture and have the disadvantage of not matching their characteristics with the casting parameters. For this reason they have a high specific consumption, up to 1.3 kg / t of liquid steel, the molded semi-finished products have surface defects of up to 30%. Also, the powders used for low-temperature steels contain a fluorinated compound which decomposes at a low temperature below 1000 ° C, releases fluorine into the atmosphere and produces harmful effects on the personnel. The imported powders used in molding slabs are atomized, but not in accordance with the parameters of the casting plant and with the characteristics of the cast steel (temperature and speed of casting), leading to the appearance of surface defects in a high proportion. Their price is high, and the economic effects are minimal due to the shortcomings. under these conditions it is necessary to appear on the internal market of products that increase the yield and quality of cast steel.

The present invention relating to Zguralub, Pulbwoll products, obtained in different compositional variants, can be applied to the casting of various types of steels.

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9 -08- 2009 (slabs, slats, tags): microalloyed, alloyed, medium carbon or low carbon. The optimal hydrodynamic behavior of the molten powder on the liquid steel meniscus is consistent with its viscosity with the temperature and casting speed of the steel. The viscosity of the slag must be lower as the casting speed increases and the casting temperature decreases. For steels with high casting temperatures at low speeds below 2 m / min, the viscosity must be high. The regulation of the fluidity of the powder is made by the variation of the content of fluorinated compound, in this case the calcium fluoride, and the melting temperature is controlled within the desired range by using calcined soda. Fluorinated compounds and strongly basic oxides, in the case of Na2O, act as fluidizers and fluxes on the basic refractory mass, which is a silicate network in the system: CaO-SiO2-AI ₂ O3. The process of melting the lubricating powder, ie its melting rate, is inversely proportional to the viscosity of the slag and is regulated by the carbon content and its quality. Graphite flakes delay "melting because it is chemically inert, increasing the sintering-melting-flow interval; at the same time, it contributes to the increase of the melting temperature of the powder due to its refractory.

The complexity of correlating the characteristics of the powder with the casting parameters is given by the existence of the special conditions of "operation" of the material, sudden melting on the surface of the liquid steel, then the infiltration of the slag film formed in the crystalline-crustal intersection of steel, where the slag cooling occurs. it solidifies partially, as the crust advances to the bottom of the crystallizer. The infiltrated slag film is the lubricant between the crystallizer and the steel "wire", and the thermal transfer through this oxide "resistance" determines the quality of the surface of the molded blank. The appearance of cracks or cracks is caused by rapid and inhomogeneous heat transfer through the slag film. The solidification structure (chemical and phase) of the film, as well as the ratio between its liquid and solid slag ratio, must cause a consistent, slow and slow thermal transfer from the steel crust (approx. 1500 ° C) to the walls. crystallizer (approx. 500 ° C). The composition of the powder must satisfy all these requirements, without which the efficiency of its use would be low.

The Zguralub and Pulbwoll products applied in the steel industry, by their adjustable characteristics according to the needs of the technology, can solve the qualitative problems of the castings, by significantly reducing the surface defects. at the same time they are less harmful due to the fact that the fluoride, present in the sub composition

cv-2 0 0 9 - 0 0 6 4 ⁶ -1 9 08- 2009 form of CaF ₂ , composed with high temperature stability, has a low weight compared to the imported powders, thus reducing the environmental damage from the casting. In conducting the research to obtain Zguralub and Pulbwoll products, compositional variations were made which led to the selection of powders with the best characteristics correlated with the technological parameters of the casting. The advantages of using the investigated powders are:

- Increasing the efficiency of the casting process by reducing surface defects by adjusting the main characteristics of the powder (viscosity, temperature and melting speed) with the casting parameters;

- Reducing the specific consumption of lubricating powder by a minimum of 0.2 kg / tonne liquid steel, by correlating the speed of melting of the powder with the speed and the temperature of casting;

- The costs / selling price of indigenous powders is much lower, given that Zguralub has as the basic raw material of furnace slag, and Pulbwoll is a powder based on indigenous Wolastonite deposits; all other raw materials are available in the country at affordable prices;

- The manufacturing technologies are in accordance with the sustainable development due to the use of disposable waste (pollutants for the environment) and due to the compositions of the produced products (fluoride content lower than the imported products), the harmfulness of the working environment in the molding station will be lower;

- Zguralub powder with graphite or electrographite has a low melting rate, ensuring very high thermal insulation capacity for continuous casting of large section formats (in slabs, slabs, blues); it is used for alloy and medium carbon steels, reducing their incidence at cracking.

Two examples of embodiment of the invention will be given below, according to the data in table 1.

Table 1

Raw materials, [%] Pulbwoll Zguralub Crude Wolastonite deposit 60-85 - Blast furnace slag - 55-80 Fluorine, CaF ₂ 8-15 5-10 Calcined soda 10-16 4-8 Metallurgical coke residue / Graphite residue (electrographed, flakes) 10-25 / 5-10 10-25 / 0-5 pearl stone 5-10 5-15

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Each material in the composition of the powder plays an important role, namely:

- The raw Wolastonite deposit and furnace slag are the basic materials, which contain the main refractory oxides: CaO, SiO ₂ , AI ₂ O ₃ and small quantities of other oxides that accompany them. The composition of the deposit has a lower variability than that of the furnace slag, and its basicity (CaO / SiO ₂ ) is close to value 1.

- Fluorine is the fluidizing component of the main mass, and soda, through Na ₂ O, is the flux component.

- Metallurgical coke and graphite play the role of moderator of the powder melting rate and at the same time it contributes to increasing the thermal insulation capacity of the powder by forming an adequate stratification (powder layer, sintered layer, molten layer = liquid slag). The coke due to its calorific power contributes to the reduction of heat losses through slow combustion upon contact with the steel meniscus.

- The pearlite is added in the composition to decrease the volumetric mass, and the spherical shape of its granules helps to increase the dust spreading surface.

Example 1

The raw materials are prepared according to the technology, that is: the wolastonitic deposit, the fluorine and the furnace slag are dried and then ground (100% under 1 mm) to a fineness of max. 30% <0.06, and 1.0 mm <20%> 0.5 mm. prior to the Wolastonite deposit, it is crushed to a maximum size of 5 mm. The calcined soda is dried and sieved, and the metallurgical coke is dried with special precautions, under ventilation, at low temperature (below 200 ° C), avoiding the direct flame because it can ignite. The coke is not ground but is crushed to the desired size. , using a 2 mm mesh screen, because it is friable. The electrograph is milled and dried under normal conditions. The pearlite dries if necessary, but its fixing should be avoided by improper storage. All materials are dried up to max. 0.8% water, so that even the finished product does not exceed this limit.

The grinding of all materials must lead to optimum granulation of the lubricating powders, the advanced fineness (70 <0.04 mm) leading to defective handling, the compaction of the material in the intermediate technological phases (storage, before dosing). This leads to the defective dispersion of the powder on the liquid steel meniscus, causing the thermal insulation capacity to decrease.

In order to obtain the Pulbwoll product, the following components are dosed according to the manufacturing recipe: wolastonitic deposit, fluorine, calcined soda and metallurgical coke. To make a powder with high melting temperature,

1200 ° C, low melting speed and high viscosity, over 10 poise, CaF ₂ + Na ₂ CC> 3 are max. 15%, and the coke is replaced by graphite, with a maximum weight of 10%. To obtain a fluid powder, it is used fluidizer and flux in a larger proportion, up to 25%, and the weight of metallurgical coke will be maximum 10%. The components, with the exception of the perlite, are mixed in an Eirich mixer, or mixer with a capacity of 500 I, possibly in ball mill. The mixing time is maximum 15 minutes, then the pearlite is added and the mixture is stirred for another 5 minutes. In the end, the product is hermetically packaged in paper bags with a plastic intermediate layer to avoid humidification before use.

The main features of the Pulbwoll product, for 4-6 compositions, are the following:

- Melting temperature: 1080-1250 ° C;

- Basicity index: 0.9-1.1;

- Viscosity: 5-15 poise;

- Free carbon content: max. 15%;

- Granulometry: 2.0 mm <max.5%> 1.0 mm; 1.0 mm <20%> 0.5 mm;

max. 30% <0.06.

Example 2.

In order to obtain the product Zguralub, the procedure is analogous to Example 1, but as a basic material, the blast furnace slag is used (see Table 1). This has a much lower melting temperature than wolastonite, which requires lower amounts of sodium and calcium fluoride to regulate melting and fluidity. The conditions of preparation of the raw materials are the same. Zguralub product, made in 2-4 technological variants, has the following main characteristics:

- Melting temperature: 1000-1150 ° C;

- Basicity index: 1.2-1.5;

- Viscosity: 0.1 -10 poise;

- Free carbon content: max. 15%;

- Granulometry: 2.0 mm <max.5%> 1.0 mm; 1.0 mm <20%> 0.5 mm;

max. 30% <0.06.

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Claims (4)

1. Lubricant powder for crystallizer, type Pulbwoll, which is a carbon-refractory oxide refractory material, based on indigenous Wolastonite deposits, characterized by having a composition with controlled refractivity, melting point and viscosity, made from: Wolastonite 60 -85%, natural fluorine (min. 90% CaF2) or CaF ₂ 8-15%, calcined soda 10-16%, expanded pearlite 5-10%, metallurgical coke residue 10-25%, made in several compositional variants , with the granulation (2-4), 06) mm, the properties being correlated with the casting parameters and the quality of the cast steel, characterized by: average viscosity 5-10 poise, melting temperature 10801150 ° C, the melting speed being adjustable by the content carbon coke. Pulbwoll according to claim 1, wherein the carbon carrier material is graphite or electrograph in a proportion of 5-10%, characterized in that the powder has high viscosity (10-15 poise), high melting temperature, 1150-1250 ° C. , low melting rate, regulated by graphite / electrographed carbon, intended for the casting of very low carbon steels, alloyed and microalloyed with medium carbon. 3. Lubricant powder for crystallizer, Zguralub, based on blast furnace slag (5580%), which has the additives present in claim 1, but in different amounts, namely: fluorine 5-10%; calcined soda 4-8%; metallurgical coke 10-25% or graphite 0-5%, pearlite 5-15%, made in maximum 3 compositional variants, characterized in that it is intended for high-speed casting of medium carbon steels type C45, and alloy steels with Mn, powder having low melting temperature, high melting speed, relatively low viscosity, variable depending on the content of fluidizer (fluorine). The scraper according to claim 3, which contains graphite or electrograph which modifies (decreases) the melting rate of the powder ensuring a very high thermal insulation capacity, characterized in that it can be used for continuous casting (in slabs, slabs, blues) of a powder. wide ranges of alloy and medium carbon steels, which are prone to cracking due to the strong thermal flux to which the steel crust is subjected to solidification.