RU2267542C1 - Cast iron, method for producing the same and method for thermal processing of ingots cast from the same - Google Patents

Abstract

FIELD: metallurgy, in particular, production of high-strength cast irons with globular graphite for manufacture of high quality cast products.

SUBSTANCE: cast iron contains, wt%: carbon 3.23-4.08; silicon 2.76-3.89; manganese 0.20-0.47; molybdenum 0.15-0.48; aluminum 0.02-0.08; magnesium 0.02-0.05; barium 0.03-0.10; calcium 0.008-0.018; rare-earth metals 0.02-0.06; iron and admixtures the balance. Method involves melting cast iron in induction furnaces. In case of modifications, cast iron grid is used for overloading modified mixture containing, wt%: silicobarium 9.0-10.5; fluorspar 12-15; master alloy the balance. Master alloy contains, wt%: silicon 45-55; magnesium 6-9; calcium 3-7; rare-earth metals 3-8; aluminum 1-3; iron the balance. Total amount of mixture is 2.8-3.5% by weight of cast iron. Weight of grid is 1.5-2% by weight of cast iron. Method for thermal processing of ingots involves providing homogenizing annealing at temperature of 950-1000 C for 3-5 hours and ferritizing annealing by cooling in furnace to temperature of 780-720 C; holding at such temperature for 2.5-3 hours, followed by gradual cooling in furnace to temperature of 650-600 C; providing quick cooling in air or in water to room temperature and artificial aging by heating to temperature of 350-420 C, followed by holding for 2-3 hours.

EFFECT: increased plasticity, impact toughness and strength of products manufactured from cast iron of such composition.

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Description

The invention relates to the field of metallurgy, in particular to the production of high-strength nodular cast iron, and can be used in the manufacture of cast products, characterized by high strength, ductility and toughness.

A combination of high mechanical properties, including increased ductility and toughness, in nodular cast iron is obtained by selecting the necessary chemical composition, melting method and spheroidizing modification of cast iron and the method of heat treatment of castings.

Known for high-strength cast iron grade VCh 40 (GOST 7293-85), which has increased strength properties, ductility and toughness. The recommended chemical composition of this cast iron for castings with wall thicknesses up to 100 mm includes [1], wt.%:

Carbon 3.0-3.8, Silicon 1.2-2.9 Manganese 0.2-0.6.

As impurities in the composition of cast iron are contained, wt.%: Phosphorus up to 0.05, sulfur up to 0.02, chromium up to 0.1. Spheroidization of graphite is usually carried out by treating molten iron with magnesium-containing additives or complex modifiers. Ferritizing annealing at 680-800 ° C is used as a heat treatment.

The production of cast iron VCh 40 is provided, in particular, by the composition [2], including, wt.%:

Carbon 2.7-3.8 Magnesium 0.03-0.08 Silicon 2.1-2.9 Iron and impurities rest. Manganese 0.15-0.45

The disadvantages of this cast iron include insufficient stability of properties, which consists in too wide a scatter of the possible values of ductility and toughness, and a sharp drop in toughness at low temperatures.

Closest to the proposed is cast iron [3], containing, wt.%:

Carbon 2.7-3.2 Magnesium 0.005-0.05 Silicon 1.0-2.5 Calcium 0.001-0.004 Manganese 0.05-0.14 Rare earth Nickel 0.3-0.8 metals (REM) 0.008-0.09 Aluminum 0.005-0.02 Iron rest

This cast iron has a pearlitic structure of a metal base and has high strength properties.

The disadvantages of cast iron are determined by its pearlitic structure and are inadequate values of impact strength and ductility.

Closest to the proposed method is a method for producing cast iron [4], including smelting cast iron and processing of its melt in a casting ladle with a modifying mixture of components containing, wt.%:

Ligature 33-37 Silicobarium 4,5-6,0 Tin 1.8-2.2 Copper waste 20-25 Fluorspar 10-12 Steel and cast iron waste rest,

moreover, the total amount of the mixture is 4.5-6% by weight of cast iron, and the ligature contains, wt.%:

Silicon 18-32 Magnesium 4-9 Calcium 3-8 REM 3,5-10 Copper 27-50 Aluminum 1-3 Iron rest

This method provides a reduction in the pyroelectric effect upon modification, an increase in the degree of assimilation of magnesium, an increase in the stability of the process of modification, and stabilization of the hardness of cast iron in castings.

The disadvantages of the method are the inability to provide high ductility and toughness of cast iron due to the perlitization of its structure, and due to insufficient cleaning of cast iron from impurities.

To ensure the ferrite structure of cast iron, the closest is the heat treatment method [5], including homogenization at 900-1000 ° C, cooling and aging at 700-760 ° C, tempering at 200 ° C.

This method provides an increase in ductility and toughness of cast iron, including at low temperatures. However, this method does not provide sufficiently high strength properties of cast iron.

The objective of the invention is to obtain cast iron with spherical graphite, homogenized and hardened ferrite.

The technical result is an increase in ductility and toughness, including at low temperatures, while maintaining the increased strength of cast iron.

This is achieved by the fact that:

1. Cast iron containing carbon, silicon, manganese, aluminum, magnesium, calcium, rare-earth metals, impurities and iron, additionally contains barium and molybdenum in the following ratio, wt.%:

Carbon 3.23-4.08 Magnesium 0.02-0.05 Silicon 2.76-3.89 Barium 0.03-0.10 Manganese 0.20-0.47 Calcium 0.008-0.018 Molybdenum 0.15-0.48 REM 0.02-0.06 Aluminum 0.02-0.08 Iron and impurities rest,

moreover, the total residual content of modifier elements must meet the condition P = Mg + Ba + Ca + REM = 0.112-0.188, wt.%.

As impurities are allowed, wt.%: Phosphorus up to 0.03, sulfur up to 0.015, chromium up to 0.05.

2. In the method for producing cast iron, including smelting cast iron and treating its melt in a casting ladle with a modifying mixture of components containing crushed ligature, silicobarium and fluorspar, cast iron is melted in induction furnaces; when modifying, a specially cast cast iron grate is used to load the mixture, and the casting itself the mixture contains components in the following ratio, wt.%:

Silicobarium 9.0-10.5 Fluorspar 12-15, Ligature rest,

moreover, the total amount of the mixture is from the weight of cast iron 2.8-3.5% with a mass of the lattice of 1.5-2%, and the ligature contains, wt.%:

Silicon 45-55 REM 3-8 Magnesium 6-9 Aluminum 1-3 Calcium 3-7 Iron rest.

3. In the method of heat treatment, including homogenization, cooling and aging, introduced an additional operation of artificial aging in the following combination of operations:

Homogenizing Annealing 950-1000 ° C, 3-5 hours Oven cooling 780-720 ° C Aging (ferritization) 780-720 ° C, 2.5-3 hours Oven cooling 650-600 ° C Air or water cooling to room temperature Artificial aging 350-420 ° C, 2-3 hours.

Changes in the chemical composition of smelted cast iron were introduced with the aim of stable production of non-bleached castings with different wall thicknesses, maximizing the ferritization of the cast iron structure in castings and ensuring the necessary properties of cast iron after heat treatment.

The content of silicon in iron is increased, which is the main element-ferritizer. With a silicon content of less than 2.76% in the structure of cast iron in thin-walled castings, significant perlitization occurs (more than 50%) and even a noticeable whitening appears. However, after heat treatment in this case, it is difficult to obtain increased strength properties at high values of ductility and toughness. With an increase in silicon content of more than 3.89%, ductility and toughness of heat-treated cast iron significantly decrease.

The manganese content should not exceed 0.47%, because with a higher content, the tendency of cast iron to perliterate the structure and bleach in castings increases, which complicates the homogenization process and reduces the ductility and toughness of cast iron. The minimum amount of manganese in cast iron, amounting to 0.2%, corresponds to its content as a technical impurity and can hardly be reduced when using conventional charge materials. The lower content of manganese in the known cast iron is associated with the use of special charge materials, which leads to an increase in the cost of cast iron.

Molybdenum in the composition of this cast iron is used to eliminate or reduce ferritic brittleness, which is especially important for structures with heterogeneous ferrite. With a content of less than 0.15 wt.% Molybdenum, this role is practically not manifested, and with a content of more than 0.48 wt.%, There is a significant rise in price of cast iron, additional components appear in the structure that increase its hardness.

The aluminum content in the composition of cast iron is increased in order to reduce the tendency of cast iron to bleach and increase the degree of ferritization of the structure. When the content is less than 0.02%, the effect of aluminum is not manifested. An aluminum content of more than 0.08% can lead to a decrease in the stability of properties, especially impact strength.

The adopted carbon content provides the necessary structure and properties of cast iron in a cast state. When the carbon content is less than 3.23 wt.%, The degree of ferritization of the structure decreases, it becomes possible to form perlite and increase hardness. If cast iron contains more than 4.08 wt.% Carbon, the amount of graphite increases in its structure, and the likelihood of the formation of graphite inclusions of an unfavorable shape (with an insufficient degree of spheroidization) increases, which can manifest itself in a decrease in all the mechanical properties of cast iron.

Parameter P, characterizing the total content of complex modifier elements in cast iron, should be at least 0.112%. Otherwise, the degree of refining of cast iron and spheroidization of graphite is insufficient. When P is more than 0.188%, increased consumption of the modifier, increasing the cost of cast iron, does not lead to an increase in its properties, on the contrary, it is even possible to overmodify cast iron with a deterioration in the shape of graphite inclusions.

The magnesium content is recommended in the range of 0.02-0.05 wt.%. If the residual magnesium content is less than 0.02 wt.%, The results of the modification are unstable. An increase in magnesium content of more than 0.05 wt.% Is impractical, since this does not increase the properties of cast iron.

Calcium plays the role of desulfurizer and deoxidizer, significantly reducing the consumption of magnesium and rare-earth metals. Content less than 0.008 wt. % calcium corresponds to cast iron not modified with calcium. Too much calcium consumption, corresponding to a residual content of more than 0.018 wt.%, Increases the number of non-metallic inclusions, impairs the absorption of the modifier and reduces the properties of cast iron.

REMs are introduced in order to neutralize elements that have a desferoidizing effect on graphite (for example, aluminum and various trace elements). When the content is less than 0.02 wt.% REM, complete spheroidization of graphite is not provided. An increase in the content of rare-earth metals more than 0.06 wt.% Is impractical, since it does not have a positive effect, but the cost of cast iron.

Additionally, barium was introduced into the composition of cast iron (in the form of silicobarium as part of a complex modifier). Together with other components of the complex modifier, it provides deep refining of cast iron, complete elimination of bleached and ferritization of the structure of cast iron even in thin-walled castings. For this, the barium content in the claimed limits is sufficient. With a residual barium content of more than 0.10%, its modifying effect is not enhanced, but the cost of cast iron increases. When the barium content is less than 0.03%, its refining and ferritizing effect is not significant.

In the method of producing cast iron, induction electric melting is used, which provides the most accurate control of the chemical composition and good mixing of molten cast iron. Silicobarium is included in the composition of the modifying mixture for the purpose of the most profound metal refining. To protect the modifier from floating and burning in the atmosphere, a cast-iron grate is used.

Complex processing of molten iron is carried out in a casting ladle at a temperature of 1390-1430 ° C, and the mixture is laid on the bottom of the ladle layer-by-layer in the form of a “sandwich” (a mixture of ligature and silicobarium, then fluorspar, and loading from above with a cast-iron grate).

Heat treatment of castings consists of three stages. The first stage is carried out with the aim of homogenizing cast iron in the austenitic state, which is ensured by heating to 950-1000 ° C and holding for 3 to 5 hours. The second stage consists in ferritizing annealing, for which the castings are slowly cooled (together with the furnace) from the temperature of the first stage to 780-720 ° C, kept at these temperatures for 2.5-3 hours and slowly cooled to 650-600 ° C, after which cooling to room temperature is carried out quickly (in air for thin-walled castings, in water for castings with a wall thickness of more than 20 mm) in order to prevent ferritic embrittlement. The third stage of the heat treatment is carried out in order to harden the ferrite by spinodal separation during artificial aging. For this, the castings are heated to 350-420 ° C, incubated for 2-3 hours and cooled in air.

Cast iron was melted in induction crucible furnaces with a capacity of 50 and 150 kg with an acid lining. Used a mixture consisting of cast iron and ferroalloys (ferrosilicon and ferromolybdenum). The modification was carried out in casting ladles with a capacity of 50 to 100 kg. For each variant of the chemical composition of cast iron, standard wedge samples 30 mm thick were cast into dry sand-clay molds. From the wedge samples after their heat treatment, standard samples were cut for mechanical tests.

The chemical compositions of cast irons for all options are given in Table 1, and the results of mechanical tests are shown in Table 2.

It can be seen that the proposed combination of the chemical composition of cast iron, the method of its production and the method of heat treatment provides, in comparison with the prototype, significantly higher values of ductility and impact strength, including at a negative temperature. When the chemical composition of cast iron goes beyond the proposed limits (alloys Nos. 5 and 6), the properties of cast iron deteriorate significantly. Deviation of the heat treatment method from claim 3 of the invention (i.e., during heat treatment without aging) also reduces the properties of cast iron, especially the tensile strength.

Information sources

1. Cast Iron: Ref. ed. / Ed. A.D.Sherman and A.A. Zhukov. - M.: Metallurgy, 1991 .-- 576 p.

2. RF patent No. 2112073, class. C 22 C 37/10.

3. RF patent No. 2098508, cl. C 22 C 37/10.

4. RF patent No. 2198227, cl. C 22 C 37/10.

5. Cold-resistant spheroidal graphite cast iron / Lekov AT, Ivancheva Ts.R., Iliev Z.M., Dafinova R.I. // Crystallization and properties of high-strength cast iron in castings. - Kiev: Publishing House of the IPL Academy of Sciences of the Ukrainian SSR, 1990. - P.97-102.

Table 1
Chemical compositions of cast irons Cast iron The content of elements, wt. % Parameter P, wt.% FROM Si Mn Mo Al Mg Ba Sa REM one 3.23 3.73 0.31 0.48 0.02 0.02 0,03 0.012 0.05 0,112 2 3.48 3.42 0.20 0.36 0.08 0,03 0.08 0.018 0.06 0.188 3 3.61 3.89 0.39 0.30 0.05 0,03 0.05 0.011 0,03 0.121 four 4.08 2.76 0.47 0.15 0,03 0.05 0.10 0.008 0.02 0.178 5 3.02 2.14 0.66 0.61 0.01 0.06 0.11 0.004 0.08 0.254 6 4.30 3.96 0.20 0,03 0.12 0.01 0.02 0,020 0.01 0,060 Known * [3] ° C 2.86 1.78 0.14 thirty% 0.01 0,03 0.004 0,03 0,064 ^*) 0.30% Ni is also contained

table 2
Mechanical properties of cast irons (average values Cast iron σ _V , MPa δ,% HB KCU, J / cm ² at Note ^*) + 20 ° С -40 ° C one 510 23 163 125 41 T.O. one 466 twenty 160 121 38 T.O. 2 2 508 24 159 127 44 T.O. one 3 515 twenty 167 109 40 T.O. one four 512 25 161 142 43 T.O. one 480 21 159 137 39 T.O. 2 5 664 7 217 fifty 8 T.O. one 6 240 one 149 12 2 T.O. one Famous [3] 596 12 183 68 21 T.O. one 602 fourteen 187 70 twenty T.O. 2 ^*) Thus 1 - heat treatment in accordance with paragraph 3 of the invention (aging); thus 2 - heat treatment without aging.

Claims (3)

1. Cast iron containing carbon, silicon, manganese, aluminum, magnesium, calcium, rare-earth metals, iron and impurities, characterized in that it additionally contains barium and molybdenum in the following ratio, wt.%: Carbon 3.23-4.08 Silicon 2.76-3.89 Manganese 0.20-0.47 Molybdenum 0.15-0.48 Aluminum 0.02-0.08 Magnesium 0.02-0.05 Barium 0.03-0.10 Calcium 0.008-0.018 REM 0.02-0.06 Iron and impurities Rest 2. A method of producing cast iron, including smelting cast iron and treating its melt in a casting ladle with a modifying mixture of components containing crushed ligature, silicobarium and fluorspar, characterized in that the cast iron is produced according to claim 1, while cast iron is melted in induction furnaces, modification use a cast-iron grate, loading the mixture, and the melt is treated with a modifying mixture containing components in the following ratio, wt.%: Silicobarium 9.0-10.5 Fluorspar 12-15 Ligature Rest moreover, the total amount of the mixture is 2.8-3.5% by weight of cast iron with a mass of the lattice of 1.5-2% by weight of cast iron, and the ligature contains, wt.%: Silicon 45-55 Magnesium 6-9 Calcium 3-7 REM 3-8 Aluminum 1-3 Iron Rest 3. The method of heat treatment of castings of cast iron, including annealing, heating, cooling and tempering, characterized in that the heat treatment is subjected to castings of cast iron according to claim 1, while conducting homogenizing annealing at 950-1000 ° C with an exposure of 3-5 hours ferritizing annealing by cooling with an oven to 780-720 ° C, holding at this temperature for 2.5-3 hours and then slowly cooling with an oven to 650 ÷ 600 ° C, after which quick cooling in air or water to room temperature is carried out and carry out artificial aging by heating to 350-420 ° C delay 2-3 hours.