RU2343202C2 - Charge for cast iron smelting in cupola furnace - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: charge contains pig, recycled material, broken pieces, ferroalloys, limestone and coke, barium carbonate and strontium carbonate or barium carbonate at following ratio of components, wt %: pig 3.0-70.0; recycled material 2.0-65.0; broken pieces 0.5-30.0; ferroalloys 1.0-10.0, but limestone, coke and barium carbonate and strontium carbonate or barium carbonate, wt % from weight of charge metal part: limestone 1.5-6.0; coke 10-15.0; barium carbonate and strontium carbonate or barium carbonate 0.01-5.0.

EFFECT: increasing of cast iron casting characteristics and performance attributes of ingots made from it.

2 dwg, 2 ex

Description

The invention relates to ferrous metallurgy, and in particular to compositions of mixtures for smelting cast iron in cupola furnaces.

Known charge for smelting cast iron in a cupola (Ananyin A.A., Brilakh M.M., Chernobrovkin V.P. "Wagon worker" - M .: Gostekhizdat, 1964), taken as a prototype containing pig iron, return in the form of sprues and underfilling, pig-iron scrap, steel scrap, in the following ratio of components, wt.%:

Pig iron 3.0-70.0 Return 2.0-65.0 Scrap iron 0.5-30.0 Steel scrap 10.0-50 Ferroalloys 1.0-10.0

limestone and coke wt.% by weight of the obtained metal mixture:

Limestone 1,5-6,0 Coke 10.0-15.0

The classical technology of iron smelting involves the use of foundry and pig iron, steel scrap, ferroalloys. But in reality, in enterprises, in order to utilize their own return, reduce costs and adjust the chemical composition of cast iron, 30-60% of pig iron scrap is added to the charge, in some cases, pig iron is completely replaced by cast iron scrap. This adversely affects the structure and quality of the resulting cast iron. Cast iron scrap makes unfavorable heredity in cast iron, as well as various non-metallic inclusions contained in it. To adjust the chemical composition according to the carbon content, steel scrap is added.

The main disadvantage of the prototype is the disadvantages inherent in cupola melting, namely, a significant dispersion of the chemical composition of the metal, due to fluctuations in the chemical composition and the ratio of the components of the charge fed to the filling, which are the cause of the low quality of the resulting cast iron and castings from it.

An object of the invention is to improve the quality of cast iron melted in a cupola.

According to the invention, a new technical result is achieved due to the fact that barium and strontium carbonates or barium carbonates are introduced into the charge for smelting cast iron in a cupola containing pig iron, return, scrap, ferroalloys, limestone and coke in the following ratio of components, wt.%:

Pig iron 0.1-70.0 Return 2.0-65.0 Scrap 0.5-40.0 Ferroalloys 0.1-10.0

and limestone, coke, barium and strontium carbonates or barium carbonate wt.% by weight of the metal part of the mixture:

Limestone 1,5-6,0 Coke 10-15,0 Barium and strontium carbonates or barium carbonate 0.01-5.0

In this case, the content of barium and strontium carbonates or barium carbonate can vary within the range, wt.%:

BaCO ₃ 0.01-99.9 Srco ₃ 0.01-99.9

The technical result is an increase in the casting properties of cast iron and operational properties of castings from it, in particular, an increase in the durability of cast iron molds.

The introduction of barium and strontium carbonates or barium carbonate in the composition of the mixture for smelting cast iron in a cupola made it possible to increase the casting properties of cast iron by increasing its fluidity, reducing the number of non-metallic inclusions, and operating properties by grinding the graphite component, converting graphite from a plate form to swirl graphite shape, reducing the cementite mesh in the structure of cast iron and reducing the number of non-metallic inclusions.

The effect of barium and strontium carbonates or barium carbonate is due to their ability to partially be reduced by silicon and carbon during cupola melting. As a result of the reduction reaction, silicon or carbon monoxide and active elements, such as Ba and Sr, are obtained, which in turn have a positive effect on cast iron as a whole (reactions occurring in the metal and slag after reduction of barium and strontium are not known in detail in connection with the inability to conduct a study of the processes occurring in the cupola). It can be theoretically and indirectly suggested that the active elements of barium and strontium bind nonmetallic inclusions dissolved in liquid cast iron to strong chemically stable compounds and bring them to the surface of the metal in slag. This, in turn, contributes to the formation of a cleaner from non-metallic inclusions, finely dispersed, respectively, more fluid-moving structure of the metal in the liquid phase. This effect contributes to a more uniform and dense structure of the metal in the liquid phase, respectively, and casting from it has a greater density and uniformity in both chemical and structural composition.

Barium and strontium carbonates or barium carbonate in the composition of the proposed mixture for smelting cast iron in a cupola play the role of a flux-modifying additive. Barium and strontium or barium, introduced into the mixture in carbonate form, have a positive effect on both slag and liquid metal, increasing the capacity of the slag for non-metallic inclusions and harmful impurities and exerting a modifying effect on cast iron. Barium and strontium carbonates, in addition to increasing the slag capacity, contribute to a decrease in the content of iron and silicon oxides in the slag and contribute to their reduction into metal, which reduces the consumption of expensive ferroalloys.

Figure 1 shows a photo of the shape of graphite modified cast iron (× 100). Figure 2 shows a photo of the shape and distribution of ferrite and perlite in modified cast iron (× 100).

Requirements for the charge

The charge materials entering the charge yard of the smelting department should be placed in separate bins and bins and separated by chemical composition. Only in this case, it is possible to make a mixture that provides cast iron of a given chemical composition.

At present, pig iron is produced in low mass (about 25 kg) and therefore can be used in a charge without preliminary preparation. Scrap and return, if they have a large mass and dimensions, are broken under the copra to the required size. The return is usually contaminated by the molding earth burned to it and should be cleaned. Steel scrap is cut into pieces of a certain mass and size using fire cutting. The mass of a piece of the mixture is desirable to have no more than 40 kg. The smaller the mass of the pieces of the mixture, the greater the productivity of the cupola and the higher the temperature of cast iron. In all cases, in order to avoid hang-charge in the shaft during the smelting of scrap pieces of size and return must be less than ^{_one third} the internal diameter of the cupola.

Before loading the mixture into the cupola, the charge is calculated. Calculation of the charge is the selection of charge materials, such as pig iron (foundry and steelmaking), scrap (steel and cast iron), having a certain chemical composition in such quantity that, taking into account the fumes of the elements, to obtain cast iron of a given chemical composition.

The proposed composition of the mixture for smelting cast iron in a cupola is obtained as follows. On the bream, cupolas are filled with a single head (coke) and kindled with gas or firewood. After ignition of the blank spike, a lime spike is fed to it, then a metal spike (pig iron - foundry and pig iron), scrap (steel and cast iron), return, ferroalloys, followed by a fuel spike (coke), then the cycle repeats, i.e. lime-metal-fuel spikes. The supply of barium and strontium carbonates or barium carbon can be carried out in metal, limestone or fuel spikes or between them.

The granulometric composition of barium and strontium carbonates for use in the cupola process is desirable from 20 to 80 mm in diameter, since it provides the highest result and does not impair the gas permeability of the charge.

Steel scrap is added, if necessary, to a metal spike to adjust the carbon content of cast iron.

The content of barium and strontium carbonates in various initial products (minerals) can significantly differ from each other. The higher the content of barium and strontium carbonates or barium carbonate in the used starting material, the less it is necessary for smelting cast iron.

The cupola cup speed is controlled by the size of the fuel spikes and the intensity of the blast.

The components for smelting cast iron in a cupola are taken in the following ratio, wt.%:

Pig iron 0.1-7.0 Return 2.0-65.0 Scrap 0.5-40.0 Ferroalloys 0.1-10.0

and limestone, coke, barium and strontium carbonates or barium carbonate wt.% by weight of the metal part of the mixture:

Limestone 1,5-6,0 Coke 10-15,0 Barium and strontium carbonates or barium carbonite 0.01-5.0

Example 1

Melting was carried out on a coke cupola of a shaft type with a capacity of 5 t / h with chamotte lining. The metal charge consisted of: 70% pig iron (50% foundry cast iron, 20% pig iron), pig iron scrap and 29% recovery, ferroalloys (FeSi 45%) 1.0%, barium and strontium carbonates (barium-strontium modifier BSK-2 according to TU 1717-001-75073896-2005) in the amount of 10 kg per ton of metal charge, which is 1.0% of the metal part of the charge, as well as limestone in the amount of 15 kg per ton, which is 1.5% of the metal part of the charge, and coke in an amount of 10% by weight of the metal part of the charge. The temperature in the ladle ranged from 1250 to 1320 ° C. In the course of the work, samples were cast to determine the chemical composition and mechanical properties from each ladle, and therefore, to each mold. Samples for determining the mechanical properties were cast on the first, middle and last molds. 128 modified molds were installed on the open-hearth pouring open-hearth ditch.

BSK-2 grade barium-strontium carbonate according to TU 1717-001-75073896-2005 contains the following components, wt.%: BaO - 16.0, CaO - 21.5, SrO - 5.5, CO ₂ - 18.0. Accordingly, the BSK-2 modifier contains 61% of barium, strontium and calcium carbonates in the form of BaSrCa (CO) ₃ .

In the process of research, the following results were obtained:

1. The fluidity of the obtained modified gray cast iron is 10-50% higher than the fluidity of cast iron according to the prototype, which allows pouring molds at lower temperatures.

2. The structure of plate graphite has a more swirling and evenly distributed shape, which positively affects the mechanical properties of the casting (see figures 1 and 2).

The average value of temporary tensile strength is 15 kg / mm ² , which is 15% higher than on samples cast using the existing technology.

3. A batch of experienced molds of 128 pcs. was subjected to natural aging for 60 days and then set in production. After rejecting the molds and obtaining the results, we obtain an average of 76 fillings in one mold, while the expenditure coefficient is

P _outl = 785 / (76 × 0.65) = 15.9 kg / t,

where 785 is the weight of the mold, kg;

76 - the average number of fillings;

0.65 - ingot weight, t.

4. The chemical composition of the experimental molds corresponds to the chemical composition of gray cast iron grade SCH10 in accordance with GOST.

Findings:

- When using barium and strontium carbonates for the modification of cast iron, additional costs for technical equipment are not required, since carbonates are fed directly to the cupola.

- The use of barium and strontium carbonates to modify cast iron improves the casting properties of cast iron. The increase in fluidity ensures a longer exposure of cast iron in the ladle before pouring (up to 7 minutes), which made it possible to lower the pouring temperature, thereby reducing the burning on the molds. As a result of casting with high fluidity cast iron, visible defects (voids, shells) in the bottom part of the experimental molds were not.

- The consumption of experimental molds made of cast iron modified with barium and (or) strontium carbonates averaged 15.9 kg / t, which is 4.22% lower than the standard coefficient and 15% lower than the actual consumption. The actual expenditure coefficient for the first half of 2006 amounted to 18.7 kg / t of steel. This indicates a significantly better heat resistance of ingot cast iron modified with barium and / or strontium carbonates.

- Barium and strontium carbonate was set in the filling with different specific consumption: 8, 7, 10 and 15 kg / t. The optimal consumption of the modifier was 10 kg per ton of the metal part of the charge.

- Due to the more uniform structure of cast iron and the absence of a cementite component, the mechanical processing of the molds was facilitated.

Example 2. The use of a modifier based on barium carbonates for the modification of cast iron in a cupola in order to increase the durability of cast iron molds

In order to master the technology of modifying cast iron with barium during its smelting and verify the effectiveness of its influence on increasing the life of molds, in April 2007 a batch (60 pcs.) Of experimental molds was cast: 14 molds (square 570 mm) for ingots weighing 2.9 t and 10 molds (square 590 mm) for ingots weighing 3.6 tons were tested during steel casting at ESPC 1, 36 molds (square 670 mm) for ingots weighing 5.85 tons were tested during steel casting in the open-hearth workshop.

Modified cast iron was smelted according to TI 01-L-2005 with the following features: a barium carbonate modifier based on 10 kg / t of metal charge was introduced into the metal galosh so that it was filled into the cupola after filling cast iron scrap, but not later than filling foundry or pig iron. In the process of smelting cast iron and casting molds, samples of slag and metal were taken, including samples cast in the form of tides to the molds to determine the chemical composition of slag and cast iron, determine the mechanical properties and evaluate the microstructure of cast iron. A metallographic analysis of the structure of the metal base (the amount of perlite and ferrite,%) and the shape of the graphite inclusions, their sizes and amounts were carried out according to the method and scales of GOST 3443-87. The data obtained were compared with the results of a similar study of the microstructure of ordinary cast iron molds.

The difference in the values of the service life of the molds cast from modified and ordinary cast iron, revealed during operation, indicates the positive effect of the modifier based on barium carbonate on their resistance. So, the actual consumption of experienced molds sq. 570 and 590 mm amounted to 23.07 kg / t against 24.4 kg / t - the actual consumption of molds in April 2007 by ESPTs No. 1, i.e. decreased by 5.5%. Service life of experienced molds sq. 670 mm amounted to 43.6 heats compared to 39.6 heats for its size for ordinary molds operated simultaneously with them in the open-hearth workshop in April, i.e. increased by 10.14%. At the same time, the consumption of experienced molds decreased by 1.7 kg / t of steel in suitable ingots or by 8.3%.

It should be emphasized that the difference in the chemical composition of the experimental and comparative melts cannot explain the aforementioned increase in the service life of molds cast from cast iron modified with barium and strontium during its smelting.

From a comparison of the data on the microstructure of cast iron, it follows that both during normal smelting and with modification, the structure of cast iron contains mainly lamellar (PGF 1) graphite of a straight shape with individual inclusions of vortex graphite (PGF 2) with a length of 15 to 250 μm and individual inclusions up to 500 microns long (PGD 15 - 180, 350 det.). The number of graphite inclusions has also not changed. It is equal to 12% for all swimming trunks. The distribution of graphite inclusions in ordinary cast iron is generally uniform (PGR 1) with the presence of interdendritic graphite colonies (PGR 4). In the cast iron of individual experimental molds, there is a uniform distribution of graphite (PGI 1), graphite colonies with dendritic arrangement (PGI 4), rosette distribution of graphite (PGI 7) and point graphite with dendritic arrangement (PGI 9). However, it is known that point graphite with a dendritic arrangement facilitates the production of a metal with the least elastic properties and hardness. Such cast iron well resists crack formation during sudden heating of products [1].

The most significantly ordinary and modified cast iron differ in the content of structural components. In ordinary cast iron, the metal base is: up to ~ 80% perlite and up to ~ 20% ferrite. Modified cast iron contains from 40 to 70 (P 55) perlite and from 30 to 60% (F45) ferrite. The increase in the ferrite content in the test cast iron is in good agreement with the results of metal strength testing:

Number and option of melting Temporary resistance to a gap (about _in ), MPa Number and option of melting Temporary resistance to a gap (about _in ), MPa 23 ordinary 109.11 (3 arr.) 22, with modification 105.35 24 ordinary 116.62 (2 arr.) 24, with modification 108.78 27, with the modification 108.78

Perlite has a higher strength and brittleness than ferrite, therefore, the more it is in the structure of cast iron, the latter is stronger. At the same time, according to the author of [2], for products subjected to heating, a ferrite-pearlite structure with large deposits of lamellar graphite is preferable. In this case, the likelihood of cracking at the beginning of work is less.

Thus, an increase in the ferrite content in the metal matrix of the modified cast iron by a factor of ~ 2 compared with ordinary cast iron apparently caused a later formation of cracks in the experimental molds and a longer service life. This was partly facilitated by a more favorable distribution of graphite inclusions in the structure of cast iron of individual molds.

Thus, the use of a modifier based on barium carbonates in the smelting of pig iron 1.5 times or more increases the content of ferrite in it and, thereby, reduces the likelihood of cracking at the beginning of operation. Due to the fact that molds in steelmaking shops are rejected mainly due to the formation and development of longitudinal cracks in the process of their application, a decrease in the crack sensitivity of cast iron increases their service life by ~ 10% and reduces the consumption of molds by 5.5-8.3% when casting steel.

List of references

1. Bidul P.N. Foundry. - M.: Gostekhizdat, 1953, p. 427, 310 p.

2. Lvovsky P.G. Reference manual mechanics metallurgical plant. - Sverdlovsk: Gostekhizdat, 1962, 1100 p.

Claims (1)

A mixture for smelting cast iron in a cupola containing pig iron, return, scrap, ferroalloys, limestone and coke, characterized in that it additionally contains barium and strontium carbonates or barium carbonate in the following ratio, wt.%:
pig iron 3.0-70.0 return 2.0-65.0 scrap 0.5-30.0 ferroalloys 1.0-10.0
and limestone, coke and barium and strontium carbonates or barium carbonate, wt.% from the weight of the metal part of the charge:
limestone 1,5-6,0 coke 10.0-15.0 barium and strontium carbonates or barium carbonate 0.01-5.0

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