RU2138576C1 - cast iron - Google Patents

Abstract

FIELD: ferrous metallurgy. SUBSTANCE: cast iron, designed for manufacturing high-duty parts of motor cars, e.g. crankshafts, and stop valves in oil and gas production industry, is composed of, wt %: carbon 3.0-3.8, silicon 1.6-2.8, manganese 0.06-0.4, chromium 0.05-0.15, vanadium 0.04-0.2, titanium 0.01-0.1, magnesium 0.04-0.08, copper 0.05-1.8, sulfur ≤ 0.02, phosphorus ≤ 0.1, iron - the balance. Cast iron in cast state contains no structurally free cementite. Its tensile strength is 500 to 1000 MP, yield strength 330-550 MPa, elongation 5- 15%, and hardness 170-300 HB. EFFECT: improved working characteristics. 2 tbl

Description

 The invention relates to the field of metallurgy, in particular to the development of compositions of ductile iron for the manufacture of highly loaded parts of automobiles, for example crankshafts; shutoff valves in the oil and gas industry.

The following cast iron is known. composition, wt. %:
C-2.6-3.5; Si 1.2-1.8; Mn-0.3-0.8; Cr 0.2-0.5; Ti-0.1-0.4; Al-0.1-0.2; Cu 0.1-1.1; Ca-0.02-0.08, Sb, Te-0.01-0.07; B-0.001-0.02; Mo-0.1-0.9; V-0.1-0.25; REM 0.002-0.03; Fe-OCT. (ac N1068530, C 22 C 37/06, 03/21/84)
The high content of manganese, chromium and boron, belonging to the class of carbide-forming elements, leads to the production of a large amount of structurally free cementite, which sharply reduces the ductility of cast iron and increases its hardness in the cast state. A decrease in ductility leads to brittleness of cast iron, and an increase in hardness leads to a deterioration in its machinability.

Also known is cast iron of the following chemical composition, wt.%:
C-2.8-3.5; Si 0.5-1.8; Mn-0.15-0.60; Cr-0.5-1.5; Cu 0.5-2.5; Mo-0.1-1.0; Ni-2.0-4.5; REM 0.05-0.20; Fe- ost.

(ac N1701753, C 22 C 37/06, 12/30/89)
The significant content of chromium and molybdenum in this composition of cast iron also determines the production of a pearlitic carbide structure in castings, which is characterized by high hardness of more than 320 HB and low ductility (elongation of less than 2%). In addition, the absence of magnesium in the composition of cast iron will not ensure the production of spherical graphite in it, which is mainly responsible for obtaining the required properties and structure.

The closest in technical essence is the following chemical cast iron. composition, wt.%:
C-2.8-3.8; Si 0.2-1.2; Mn-0.1-0.8; Cr 0.1-0.6; Cu 0.02-0.30; Ti-0.01-0.05; N-0.07-0.30; V-0.15-0.80; Mg-0.003-0.08; Fe-ost.

(ac N1548246, C 22 C 37/10, 03/07/90)
The high manganese content in cast iron (up to 0.8%) reduces the ductility of ductile iron, because the latter, concentrating on the boundaries of eutectic grains, due to its direct elimination, promotes the formation of structures with a high manganese content in these places compared to the average amount in the metal matrix.

 The high chromium content leads to the formation of a large number of free carbides in the structure, which reduce the ductility of cast iron and impair its machinability. The carbide-forming effect of chromium increases the tendency of cast iron to micro-shrink processes, which causes a decrease in tensile strength, yield strength and elongation.

 The high content of vanadium also contributes to intensive carbide formation, which also reduces the complex of structural properties of cast iron.

 The low magnesium content in this composition leads to the production of imperfect graphite globules up to the vermicular form, in the presence of which it is impossible to achieve high properties in cast iron.

 The known composition does not provide the required complex of mechanical characteristics of cast iron in castings in a cast state.

 The objective of the proposed invention is to increase the mechanical characteristics of cast iron without structurally free cementite, as well as increasing its machinability in a cast state.

The solution of this problem is achieved by choosing the boundary limits of the content of components in cast iron, wt.%:
Carbon - 3.0-3.8
Silicon - 1.6-2.8
Manganese - 0.06-0.4
Copper - 0.05-1.8
Chrome - 0.05-0.15
Vanadium - 0.04-0.2
Titanium - 0.01-0.1
Magnesium - 0.04-0.08
Sulfur - ≤0.02
Phosphorus - ≤0.1
Iron - Else
The technical result obtained by carrying out the invention is that as a result of its use, cast iron is obtained with such a set of mechanical characteristics in the cast state: tensile strength 500-1000 MPa, yield strength 330-550 MPa, elongation 5-15 %, hardness 170-300 HB.

 Depending on the properties, the structure is characterized by a high ferrite content for cast iron with high elongation and strength up to 500 MPa and with a 100% highly dispersed ferrite-cementite mixture (perlite, sorbitol) for cast iron with a tensile strength of 850-1000 MPa and a relative elongation of at least 5% . Only the presence of all components without exception in the specified range makes it possible to obtain the above technical result.

 The limits of carbon content (3.0-3.8%) are selected based on the conditions for obtaining a given high complex of properties in castings that crystallize at different cooling rates. The specified carbon content provides optimal conditions for the formation of graphite inclusions in the correct spherical shape in castings of various sections. A carbon content below 3.0% will lead to a deterioration in the spherical shape of graphite, the appearance of structurally free cementite and shrinkage in castings with wall thicknesses of 10 mm or less.

 A high carbon content (more than 3.8%) leads to an abundant release of graphite inclusions already in the liquid melt, during crystallization of which in the casting the latter float and form defects, known as graphite composites. An increase in carbon over the above limit also contributes to the enlargement of the structural components of the ferrite-cement mixture (perlite) and the formation of free ferrite, which reduces the level of the maximum mechanical properties of cast iron indicated above.

 The silicon content in the range of 1.6-2.8% is selected from the conditions of the intense influence of this element both on the process of graphitization and the formation of the correct spherical shape of graphite, and on the mechanism of structure formation of the metal matrix of cast iron. The specified limits of the silicon content in cast iron create favorable conditions for the formation of the required forms of graphite and the necessary ratio of the main structural components in the metal matrix of cast irons with a different set of mechanical properties. Low silicon content (less than 1.6%) leads to the formation of structurally-free cementite, bleaching and shrinkage defects in castings with wall thickness less than 10 mm. The high silicon content in the castings contributes to abundant graphite formation, the enlargement of graphite globules and the formation of free ferrite in castings with a cross section of more than 60 mm. These factors do not allow to obtain maximum mechanical properties in these castings.

 Manganese, as an element with direct segregation, is localized mainly in those places that crystallize last during the eutectic transformation and form the boundaries of the eutectic grain. An increase in the amount of manganese above the specified limit leads to a thickening of the boundaries of the eutectic grains, which leads to a sharp drop in ductility (relative elongation) and strength of cast iron in castings, regardless of their wall thickness.

 Chromium within the stated limits of 0.05-0.15% in the metal matrix of cast iron is in a solid solution of iron, strengthens it and helps disperse the ferrite-cement mixture (perlite). Such an effect of chromium within the indicated limits promotes the formation of a high-strength metal matrix in castings that crystallize at different cooling rates. A lower chromium content (below 0.05%) will not make a positive contribution to the formation of a finely dispersed structure and high properties, and a larger one (above 0.15%) leads to the formation of free cementite and a decrease in ductility of cast iron.

 Vanadium, as a highly active element, in amounts of 0.04-0.2% promotes the refinement of pig iron from oxygen, strengthens the metal matrix by dissolving iron in a solid solution. A lower content (less than 0.04%) loses the positive effect of vanadium, and a larger content (more than 0.2%) leads to the formation of highly hard carbides, which reduce the ductility of cast iron.

 Titanium, as an element with a high affinity for oxygen, refines molten iron and contributes to the highly efficient subsequent absorption of magnesium in the process of spherodizing modification of cast iron with this element. The optimum titanium content is 0.01-0.1%. A lower titanium content (less than 0.01%) is ineffective, a larger one (above 0.1%) leads to a distortion of the spherical shape of graphite.

 Copper with a content of 0.5-1.8% contributes to the hardening of the metal matrix through the solid-solution hardening mechanism and due to the allocation of fine particles in a ferrite-cement mixture (perlite). In addition, copper in these quantities plays the role of a graphitizing element in the transition of cast iron from liquid to solid.

 A lower copper content (below 0.5%) is inefficient because hardening mechanisms cease to work in the claimed chemical composition of cast iron, and a higher content (above 1.8%) is impractical from an economic point of view.

 The residual magnesium content in cast iron in an amount of 0.04-0.08% ensures the obtaining of the correct spherical form of graphite in cast iron in castings that crystallize at different cooling rates. A decrease in the residual magnesium content (below 0.04%) leads to the formation of a mixed form of graphite in spherical and vermicular cast iron. An increase of more than 0.08% coarsens (distorts) the spherical shape of graphite, increases the tendency of cast iron to bleach. In both cases, the structure of the metal matrix deteriorates and the mechanical properties decrease.

 In the proposed cast iron, an increase in the content of carbon, silicon, copper and magnesium, a decrease in the content of manganese, chromium and vanadium and the exclusion of nitrogen from the composition of the known cast iron make it possible to obtain a high complex of mechanical properties of cast iron in castings.

Cast iron is smelted in an arc, induction or other furnace, ensuring efficient charge remelting and overheating of the melt before discharge to a temperature of 1500-1550 ^o C.

 As charge materials, steel scrap is used, the return of own production (gates, profits) of high-strength cast iron, electrode battle, pig iron or a similar iron-carbon alloy containing vanadium, chromium, and titanium. Electrode battle, steel scrap, cast iron, return are sequentially loaded into an arc (or other) furnace. The estimated amount of copper is introduced by melting. Magnesium in the form of FSMG 7K03 ligature or any other, containing not less than 6-8% magnesium, is introduced by the method of intraform modification.

To obtain 0.04-0.088% residual magnesium in finished castings, the calculated amount of modifier is 0.8-1.2% of the metal content of the mold. Cast iron is poured into molds at a temperature of 1350-1460 ^o C.

 Castings are knocked out of molds after 30-40 minutes or, based on the operating conditions of a particular foundry, after 1-2 hours, or the next day, or shift. The mechanical properties of cast iron are determined either on samples prepared from separately cast wedge-shaped samples, or samples are cut directly from the castings. The shape and dimensions of the samples for mechanical testing comply with the requirements of the relevant standards. The structure of cast iron is determined on the samples subjected to mechanical tests, from the side opposite to the plane of the gap. The chemical compositions and mechanical properties of the known and proposed cast irons are given in tables 1, 2, respectively.

 As follows from tables 1 and 2, the claimed invention allows to increase the tensile strength in 1.4-2.5 times, the yield strength of 1.2-1.9 times, the relative elongation of 1.2-1.9 compared to the known cast iron times.

Claims (1)

Cast iron containing carbon, silicon, manganese, copper, chromium, vanadium, titanium, magnesium, sulfur, phosphorus and iron, characterized in that it contains components in the following ratio, wt.%:
Carbon - 3.0 - 3.8
Silicon - 1.6 - 2.8
Manganese - 0.06 - 0.4
Chrome - 0.05 - 0.15
Vanadium - 0.04 - 0.2
Titanium - 0.01 - 0.1
Magnesium - 0.04 - 0.08
Copper - 0.05 - 1.8
Sulfur - ≤ 0.02
Phosphorus - ≤ 0.1
Iron - Else

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