RU2605016C2 - Method of producing high-strength cast iron - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy and can be used for producing high-strength cast iron, suitable for production of articles with high ultimate strength. Method comprises melting and alloying of cast iron in induction furnace, graphitizing and inoculating modification, filling the shape with modified cast iron, obtaining casting with its further removal after crystallization from the mold at temperature of 900-1,000 °C, replacing the castings in the furnace with temperature of 950-1,000 °C and holding in the furnace for 10-30 minutes, with further isothermal hardening at temperature 300-320 °C for 60 minutes and air cooling, said alloying is performed with aluminium, silicon, nickel, copper, molybdenum and manganese, graphitizing and inoculating modification is carried out in the ladle, castings are produced in metal form in form of metal mold, after holding in the furnace cast iron castings are is produced with globular graphite and austenite-bainitic structure of the following chemical composition, wt%: carbon 2.5-3.2; silicon 1.5-2.5; aluminium 7.2-9.0; manganese 0.7-0.75; magnesium 0.04-0.06; molybdenum 0.7-1.2; nickel 0.48-0.52; copper 0.49-0.52; sulphur 0.01-0.012; phosphorus 0.06-0.08; iron-the rest, isothermal hardening of the produced casting is carried out in pseudo-boiling disperse carborundum layer in the area of austenite-bainitic conversion.

EFFECT: invention allows enlarging the application area of high-strength cast iron with globular graphite and austenite-bainitic structure with ultimate strength of σ_B≥1,200 MPa by bainitic hardening from cast state without additional thermal treatment.

1 cl, 1 tbl

Description

The invention relates to the field of metallurgy, to methods for producing ductile iron, can be used in the manufacture of products with a high tensile strength.

Known for high-strength cast irons with spherical and vermicular graphite [1], having a tensile strength σ _B = 600-700 MPa, ductility δ up to 8%, impact strength KS up to 60 J / cm ² .

The disadvantage of [1] is that cast irons have a low tensile strength that does not meet modern requirements and does not exceed σ _B = 600-700 MPa, which limits their scope.

A known method of producing cast iron with spherical graphite [2] the following chemical composition, mass. %: 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 (REM) 0.02-0.06; iron and impurities - the rest. Cast iron is melted in induction furnaces. During the modification, a cast-iron grating is used, which loads the modifying mixture containing the masses. %: silicobarium 9.0-10.5; fluorspar 12-15; ligature - the rest. Ligature (ligature in metallurgy - auxiliary alloys) contains, mass. %: silicon 45-55; magnesium 6-9; calcium 3-7; REM 3-8; aluminum 1-3; iron is the rest. The total amount of the mixture is 2.8-3.5% by weight of cast iron, and the mass of the lattice is 1.5-2.0% by weight of cast iron. During heat treatment of castings, homogenizing annealing is carried out at 950-1000 ° С for 3-5 hours, ferritizing annealing by cooling with an oven to 780-720 ° С, holding at this temperature for 2.5-3.0 hours and subsequent slow cooling with an oven up to 650-600 ° C. Then carry out rapid cooling in air or in water to room temperature and artificial aging by heating to 350-420 ° C, with an exposure of 2-3 hours. The technical result is to increase ductility, toughness while maintaining increased strength.

The disadvantage [2] is that in order to obtain half cast iron, it is necessary to perform complex heat treatment, which consists in preliminary graphitizing annealing of cast iron. In addition, cast iron according to [2] has low values of mechanical properties: tensile strength σ _B = 560-610 MPa; conditional yield strength σ _0.2 = 420-450 MPa; impact strength KCU2 ₂₀ ° _C = 150-179 J / cm ² ; Brinell hardness HB = 277-286. The disadvantages limit the scope of cast iron by the method [2].

A known method of producing austenitic-bainitic cast iron using double normalization of castings [3], the essence of this method lies in the fact that the heat treatment is subjected to spheroidal graphite iron, containing, mass. %: carbon 3.0-3.4; silicon 1.8-2.3; manganese 0.3-0.5; nickel 0.6-1.2; molybdenum 0.3-0.7; copper 0.3-0.7; magnesium 0.04-0.09; sulfur 0.01-0.02; phosphorus 0.06-0.08; iron - the rest; with a total content of alloying elements (Mn, Ni, Mo, Cu) - 1.7-2.4 mass. %, while the formation of a bainitic matrix is ensured by the first normalization in air from temperatures of 940-960 ° C after holding for 1.5-2.5 hours, the second normalization by heating to temperatures of 790-810 ° C at a speed of 100-200 ° C / h with a holding time of 1.5-2.0 h and cooling with a stream of compressed air and tempering at a temperature of 250-300 ° C with a holding time of 2-4 hours. The following mechanical properties are achieved: σ _B = 684-888 MPa; δ = 4.2-7.2%; KCU ₂₀ ° _C ≈ 110 J / cm ² ; HB = 311-388.

The disadvantage [3] is that in the production of austenitic-bainitic iron, a complex heat treatment technology is used, leading to a high cost of production. The disadvantage limits the scope of cast iron by the method [3].

Closest to the claimed technical solution, the prototype is a method for producing castings from half cast iron with austenitic-bainitic structure [4], including smelting, alloying, inoculating cast iron modification, casting in sand and clay form, removing it from the mold at a temperature of 900- 1000 ° C, moving to a furnace with a temperature of 950-1000 ° C and holding for 10-30 minutes, after holding it is quenched in an isothermal bath at a temperature of 300-320 ° C for 1.0-1.5 hours, while using cast iron following chemical composition (. In mass%): carbon (C) - 3.2-3.4; silicon (Si) - 3.0-3.3; manganese (Mn) - 0.3-0.4; magnesium (Mg) - 0.04-0.07; molybdenum (Mo) - 1.5-1.7; nickel (Ni) - 2.2-2.6; sulfur (S) 0.010-0.012; phosphorus (P) - 0.06-0.08; iron (Fe) - the rest.

The disadvantage of the prototype [4] is the complexity of the technical implementation of the processes for casting in sand-clay molds associated with the extraction of castings from molds at high temperatures (950-1000 ° C), removal (cleaning) from castings of the molding mixture and their subsequent transfer (castings) in heating furnaces. Other disadvantages [4] are: the use of environmentally harmful substances for hardening cast iron in an isothermal bath; poor machinability of parts due to the formation of carbides in the structure due to the increased content of molybdenum in the composition of cast iron; heterogeneity of the structure in castings obtained during crystallization in sand-clay forms; insufficiently high strength (σ _B <1000MPa). These disadvantages significantly limit the scope of the prototype [4].

The aim of the invention is to expand the scope of cast iron, the development of a method for producing high-strength cast iron with spherical graphite and austenitic-bainitic structure with a tensile strength σ _B > 1200 MPa by bainitic hardening from a cast state without additional heat treatment.

The goals are achieved by the fact that in order to obtain high-strength cast iron, it (cast iron) is smelted in an induction electric furnace, the melt is modified during pouring into a ladle to obtain globular form graphite inclusions, and when casting into a metal mold (chill mold), the melt in the ladle is additionally subjected to inoculant modification, castings after crystallization are removed from the mold at a temperature of plus 900-1000 ° C, transferred to a furnace with a temperature of 950-1000 ° C, kept in a furnace for 10-30 minutes, then isothermal hardening is used at the installation in a sevdokipelny dispersed carborundum layer in the region of austenite-bainitic transformation at a temperature of + 300 ° C for 60 min, followed by cooling in air; while using cast iron containing, mass. %: carbon (C) - 2.5-3.2; silicon (Si) - 1.5-2.5; aluminum (A1) - 7.2-9.0; manganese (Mn) - 0.7-0.75; magnesium (Mg) - 0.04-0.06; molybdenum (Mo) - 0.7-1.2; nickel (Ni) - 0.48-0.52; copper (Cu) - 0.49-0.52; sulfur (S) - 0.01-0.012; phosphorus (P) - 0.06-0.08; iron (Fe) - the rest.

The inventive ductile iron is obtained, for example, in the following way.

High-strength cast iron is melted, for example, in an MGP-52 induction furnace with an acid lining, on a charge consisting of GOST 805-80 cast iron, steel scrap and graphitized electrode breakdown. Doping of cast iron Si (silicon), Al (aluminum), Mn (manganese), Mo (molybdenum), Ni (nickel), Cu (copper) is carried out with the appropriate ferroalloys in the furnace 10-15 minutes before the release of metal (M2 copper (99, 9 wt.% - copper (Cu), the rest is impurities), GOST 859-78, nickel, NC (nickel (Ni), wt.% More than 98.6%, the rest is cobalt (Co) - 0.7; carbon (C ) - 0.1; sulfur (S) - 0.03; copper (Cu) - 0.6;) GOST 849-70). Inoculant modification is carried out in a ladle at a temperature of 1450 - 1470 ° С with the ZhKMK-4P modifier of the following chemical composition of the masses. %: silicon (Si) - 49.6; calcium (Ca) - 9.5; magnesium (Mg) 8.6; rare earth metals (REM) - 4.7; iron (Fe) - the rest. Cryolite K2 (GOST 10561-73) is used as a flux. Graphitizing modification is carried out by FS-75 ferrosilicon (GOST 1415-78). Then, the ZhKMK-4P modifier is introduced into the bucket in a known manner [4] using a quartz “bell”. Obtaining castings is carried out in a metal form (chill mold), which (castings) are removed from the mold after crystallization at a temperature of 900-1000 ° C, then transferred to a furnace with a temperature of 950-1000 ° C and kept in an oven for 10-30 minutes. The result is castings of cast iron with globular graphite and the austenitic structure of the proposed chemical composition.

After aging in the chamber furnace of electrical resistance in the temperature range of 950-1000 ° C, castings of iron are transferred to the installation of the "pseudo-boiling" layer and isothermal hardening is carried out.

Depending on the conditions presented to the product (cast iron), isothermal hardening is carried out at various temperatures, for example, indicated below, with the results given in the Table.

Isothermal hardening of cast irons was carried out at temperatures of 300 ° C, 350 ° C and 380 ° C with an exposure of 30, 60 and 120 minutes. The heating of the working medium (carborundum) to the indicated temperatures is carried out by pumping compressed air through the heating elements of the electrical resistance (tube furnace). The temperature in the working space is controlled, for example, by chromel-alumel thermocouples.

In FIG. The table is presented, where the results of studies of the influence of the duration and temperature of isothermal holding within 300-380 ° C on the mechanical properties of austenite-bainitic cast irons are presented. When studying the influence of isothermal hardening conditions of cast iron in a pseudo-boiling layer, wedge-shaped samples 55 mm thick were cast (GOST 7293-85). It was found that an increase in the temperature of isothermal holding in the range of 300-380 ° C during hardening of cast iron leads to a decrease in strength by 21%; ductility by 3.5% and impact strength by 20%. Moreover, the hardness in cast irons practically does not change (Fig.). An increase in the isothermal holding during cooling in a pseudo-boiling layer for more than 60 minutes has practically no effect on the hardness and other mechanical properties of aluminum austenitic-bainitic cast irons. The maximum strength characteristics (σ _B , σ _0.2 ) are obtained by isothermal quenching in a pseudo-boiling layer of austenitic-bainitic iron at isothermal exposure at a temperature of 300 ° C for 60 min according to regime No. 1 (Fig.).

Mechanical tests of cast iron under static loads were carried out on a universal tensile testing machine. Hardness (HB) was determined, according to GOST 24805-81, on a Brinell device; impact strength, according to GOST 9454-78, on a pendulum head.

The inventive method allows to obtain aluminum austenitic-bainitic cast iron with globular graphite with the following maximum mechanical characteristics: tensile strength σ _B = 1220 MPa (for the prototype σ _B <1000 MPa); conditional yield strength σ _0.2 = 960MPa; ductility δ = 4.6%; hardness HB = 415; impact strength KS = 72 J / cm ² .

The obtained high values of the strength properties of austenitic-bainitic iron by the present method are achieved due to the rational content of alloying elements: aluminum, silicon, nickel and copper, as well as carbide-forming molybdenum in combination with manganese. Performing isothermal hardening of cast iron in a moving "pseudo-boiling" layer provides uniform cooling of cast iron, contributing to an increase in the uniformity of its (cast iron) structure. The implementation of the proposed method allows to obtain cast iron with significantly better, compared with the prototype, characteristics, to produce products with the best consumer properties.

The examples of the invention show its usefulness for producing ductile iron. The application of the proposed method contributes to the production of ductile iron with spherical graphite. The method can be used in the manufacture of products with a high tensile strength.

In addition, compared with the prototype, the process of producing ductile iron by the present method has a significantly better environmental property. In the prototype [4] isothermal hardening of cast iron products is carried out in salt baths, the use of which (baths) is associated with the use of environmentally harmful salt melts, which subsequently (after quenching) requires the operation of removing salts from the surface of the products. In the claimed method for isothermal hardening, instead of salt baths, plants with a pseudo-boiling dispersed carborundum layer are used, which provides a more advanced technological process for producing high-strength austenitic-bainitic cast iron and significantly contributes to the preservation of the environment, improvement of the ecological situation.

The present invention satisfies the criteria of novelty, since when determining the level of technology no means were found that are inherent in signs identical (matching in the function performed by them and in the form of performing these signs) to all the signs listed in the claims, including the purpose of the application.

The inventive method has an inventive step, because it has not been identified technical solutions having features that match the distinguishing features of this invention, and the popularity of the influence of distinctive features on the specified technical result is not established.

The claimed technical solution can be implemented in the production environment of machine-building enterprises. This meets the criterion of "industrial applicability" presented to the invention.

Information sources

1. Gorshkov A.A. Handbook for the manufacture of castings of ductile iron. - Kiev: Mashgiz, 1960 .-- 302 p .; Alexandrov N.N. High strength cast iron for castings. - M. Mechanical Engineering, 1982.- 222 p.

2. RF patent No. 2267542, IPC (2006.01) C21C 1/10, C21D 5/00. Priority from 07/09/2004. Published on October 1, 2006. Patent Description

3. RF patent No. 2449043, IPC (2006.01) C22C 37/10, C21D 5/00. Priority from 04/12/2010. Published on April 27th, 2012. Patent Description

4. RF patent No. 2250268, IPC7 C21C 1/10, C22C 37/04, C21D 5/00. Priority from 07.24.2003. Published on April 20, 2005. Patent Description

Claims (1)

A method of producing high-strength cast iron, including smelting and alloying cast iron in an induction furnace, graphitizing and inoculating modification, pouring modified cast iron into a mold, casting with its subsequent extraction after crystallization from a mold at a temperature of 900-1000 ° C, moving the casting to a furnace with a temperature of 950 -1000 ° C and holding in the furnace for 10-30 minutes, followed by isothermal hardening at a temperature of 300-320 ° C for 60 minutes and cooling in air, characterized in that the alloying is carried out by aluminum iem, silicon, nickel, copper, molybdenum and manganese, graphitized and inoculated modification is carried out in a ladle, using a metal mold in a mold, after aging in an oven prepared casting spheroidal graphite cast iron and austenitic-bainitic structure of the following chemical composition, wt.%:
carbon (C) 2.5-3.2
silicon (Si) 1.5-2.5
aluminum (Al) 7.2-9.0
Manganese (Mn) 0.70-0.75
magnesium (Mg) 0.04-0.06
molybdenum (Mo) 0.7-1.2
nickel (Ni) 0.48-0.52
copper (Cu) 0.49-0.52
sulfur (S) 0.010-0.012
phosphorus (P) 0.06-0.08
iron (Fe) the rest,
wherein the isothermal hardening of the obtained casting is carried out in a pseudo-boiling dispersed carborundum layer in the region of austenitic-bainitic transformation.

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