RU2138578C1 - Cast iron - Google Patents

Abstract

FIELD: ferrous metallurgy. SUBSTANCE: invention is designed for production of cast irons which can be used for manufacturing critical parts, for example, crankshafts, couplings, guides, and bushings of rolling mills. Cast iron is composed of, wt %: carbon 3.7- 4.0, silicon 0.06-1.1, manganese 0.05-1.4, nickel 1.8-2.4, copper 0.6-2.0, chromium 0.1-0.8, vanadium 0.03-0.08, titanium 0.01-0.04, magnesium 0.03-0.1, iron - the balance. Cast iron is characterized by bainite structure in cast state, high strength and plasticity, stretching strength in cast state 800-1000 MPa, yield strength 7-9%, and Brinell hardness 255-300 N. 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 critical parts of mechanical engineering and metallurgy, for example crankshafts, couplings, wiring and bushings for rolling mills.

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-ost.

 (a.c. N1068530, C 22 C 37/06, 03/21/84).

 The high content of manganese, antimony, tellurium and boron embrittle the metal matrix of cast iron and increases the whitening in castings. The absence of magnesium does not allow graphite to crystallize in spherical form. The absence of nickel in the composition accelerates the decomposition of austenite in the pearlite region, which makes it impossible to obtain a bainitic structure in the molten state.

Cast iron is also known 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.

 (A.S. N1701753, C 22 C 37/06, 12/30/89).

 A high silicon content reduces the hardenability of castings when cooled in molds and in air, and increases the amount of primary ferrite in the cast iron structure. A small amount of carbon reduces the stability of austenite against decomposition, as a result of which its transformation begins and ends in the pearlite region without going into the intermediate, where the bainitic structure is forming.

 The high nickel content significantly expands the austenitic region and leads to a large amount of residual austenite and reduces the mechanical properties of cast iron.

 The absence of magnesium causes the crystallization of graphite in cast iron in a plate form, which sharply reduces the mechanical properties of castings.

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. 1548246, March 7, 90.

 The high content of manganese and vanadium embrittle the metal matrix of cast iron and makes it relatively weak and non-ductile. The absence of nickel in the composition does not allow the cooling of castings to go into the intermediate region, where a bainitic structure is formed.

 The known composition does not provide a bainitic structure in a molten state and the required values of strength and ductility.

 The objective of the invention is to obtain cast iron with bainitic structure in a molten state, with high strength and ductility, for the manufacture of critical parts of mechanical engineering, metallurgy, for example crankshafts, couplings, wiring and bushings for rolling mills.

The solution of this problem is achieved by choosing the boundary limits of the content of components in cast iron wt.%:
Carbon - 13.7-4.0
Silicon - 0.06-1.1
Manganese - 0.05-0.4
Nickel - 1.8-2.4
Copper - 0.6-2.0
Chrome - 0.1-0.8
Vanadium - 0.03-0.08
Titanium - 0.01-0.04
Magnesium - 0.03-0.1
Iron - the rest
Sulfur and phosphorus may be present as impurities in cast iron.

 This choice of components provides increased strength and ductility, while cast iron has a bainitic structure. Only the presence of all components without exception in the indicated range makes it possible to obtain the above technical result. The high carbon content by weight in the range of 3.7-4.0% provides complete graphitization, increases the stability of austenite and increases the hardenability of castings of various thicknesses. Thus, the selected carbon content creates the prerequisites for the formation of a bainitic structure with continuous cooling of cast iron in castings.

 A carbon content below 3.7% will lead to the formation of primary cementite during crystallization of cast iron in thin sections of castings, and reduce the hardenability and stability of austenite in the pearlite region, which can lead to the formation of non-neynitic structures. A high carbon content (more than 4.0%) will lead to the formation of large accumulations of graphite in the structure, which form defects such as graphite spell.

 The silicon content is selected in the range of 0.06-1.1% by weight, based on the conditions of its minimal effect on hardenability and the formation of free ferrite in a metal matrix. The latter is especially undesirable in the formation of a bainitic structure.

 A low silicon content (below 0.06%) is difficult and not economical to obtain in the initial charge materials. A high content (above 1.1%) will increase the ferrite content in the structure and make it difficult to transform austenite in the intermediate (bainitic) region.

 Manganese increases the stability of austenite and improves the hardenability of cast iron. At the same time, manganese, concentrating on the boundary of the eutectic grains, embrittle the metal matrix. Therefore, its limits are selected at the minimum acceptable level, equal to 0.05-0.4% by weight. A manganese content below 0.05% is not economically feasible. Content above 0.4% leads to embrittlement and a decrease in ductility and strength of cast iron.

 Nickel expands the existence of austenite, increases its stability and improves hardenability. Based on this, the limits of its content are taken as follows: 1.8-2.4% by weight.

 A low nickel content (below 1.8%) will reduce the stability of austenite and will cause the onset of pearlite transformation. A high nickel content (above 2.4%) will expand the existence of the austenitic region, as a result of which, at the end of the transformation, residual austenite forms in the structure, which reduces the strength of the metal base in the cast state.

 Copper, like nickel, increases the stability of austenite and improves the hardenability of cast iron in castings. Its content in cast iron is selected in the range of 0.6 - 2.0% by weight.

 Less than its value (below 0.6%) is inefficient, since the effect of increasing the stability of austenite and hardenability of cast iron is reduced. A higher content (above 2.0%) is not economically feasible.

 Chromium increases the stability of austenite and increases the hardenability of cast iron. Its content in the proposed embodiment is 0.1-0.8% by weight.

 The lower limit of chromium (0.1%) is selected based on its effectiveness in thin-walled castings, and a lower chromium content in such castings is inefficient. A high chromium content (more than 0.8%) leads to complete inhibition of graphitization of structurally-free cementite, the presence of which in the metal matrix is unacceptable, since it reduces ductility.

 Vanadium is introduced into the proposed composition of cast iron in an amount of 0.03-0.08% by weight, which is due to solid-solution hardening of the metal matrix and purification of the melt from impurities such as oxygen. A decrease in the content of vanadium below 0.03% leads to its low efficiency, and an increase of more than 0.08% is not economically feasible, since the indicated amounts enter the charge with naturally-alloyed cast iron and therefore there is no need to use expensive ferrovanadium.

 Titanium is the most active element in the proposed composition of cast iron with respect to oxygen and nitrogen dissolved in liquid cast iron and its concentration is taken in an amount of 0.01-0.04% by weight.

 These amounts are calculated for the deactivation of the above gas impurities. Content below 0.01% of titanium is inefficient, and above 0.04% is not economically feasible.

 Magnesium is one of the main elements in the claimed composition and is in the range of 0.03-0.1%. The role of magnesium is the spheroidization of graphite inclusions and the cleaning of grain boundaries of a ferrite-cement mixture (in this case, bainitic grains) from sulfides, phosphides, and oxides.

 With a content of less than 0.03% magnesium in cast iron, it does not fulfill the indicated role. Exceeding more than 0.1% of magnesium leads to embrittlement of the metal matrix, coarsens graphite inclusions and initiates the formation of undesirable bleached in the structure.

 In the proposed cast iron, an increase in the limits of carbon, copper, magnesium, nickel input and the elimination of nitrogen allows to obtain high-strength bainitic cast iron in a cast state with a sufficient level of tensile strength up to 900 MPa, yield strength up to 550 MPa and a relative elongation of at least 8% in cast condition.

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, chromium-containing steel scrap is used, return of own production (sprues, profits) of bainitic iron, electrode battle, pig iron or a similar iron-carbon alloy containing vanadium, chromium, and titanium. Electrode battle, steel scrap, cast iron, and return from the calculation of obtaining the average number of declared elements in cast iron are sequentially loaded into an arc (or other) furnace. By melting, the calculated amount of silicon, copper, and nickel is introduced. Magnesium in the form of the FSMG 7KOZ ligature or any other, containing at least 6-8% magnesium, is introduced by the method of intraform modification.

To obtain 0.03-0.1% 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 1350-1470 ^o C.

 Knocking out of castings from molds is carried out after 20-50 minutes or, based on the working 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 destruction of the sample during the test. The chemical compositions and mechanical properties of the known and proposed cast irons are shown in tables 1 and 2, respectively.

 As follows from tables 1 and 2, the claimed invention allows to increase in comparison with the known cast iron strength of 2-2.5 times, ductility of 3.5-4.5 times.

Claims (1)

Cast iron containing carbon, silicon, manganese, copper, chromium, vanadium, titanium, magnesium and iron, characterized in that it additionally contains nickel, and the components are taken in the following ratio, wt.%:
Carbon - 3.7 - 4.0
Silicon - 0.06 - 1.1
Manganese - 0.05 - 0.4
Nickel - 1.8 - 2.4
Copper - 0.6 - 2.0
Chrome - 0.1 - 0.8
Vanadium - 0.03 - 0.08
Titanium - 0.01 - 0.04
Magnesium - 0.03 - 0.1
Iron - Else

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