RU2697134C1 - Cast iron and method for its production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly to compositions and methods of producing high-strength cast irons with spherical graphite, and can be used in production of cast items, for example rolls of rolling mills. Cast iron contains, wt%: carbon 2.8–3.6, silicon 0.8–1.5, manganese 0.6–1.5, chromium 0.8–2.0, nickel 3.5–5.0, niobium 0.1–2.0, tellurium 0.0002–0.001, boron 0.02–0.08, iron and unavoidable impurities – the rest. Method includes melting in melting unit of base melt containing iron, carbon, silicon, manganese, chromium and out-of-furnace treatment, including spheroidizing modification in ladle and casting into molds, wherein base melt is obtained with the following composition, wt%: carbon 2.8–3.6, silicon 0.8–1.5, manganese 0.6–1.5, chromium 0.8–2.0, nickel 3.5–5.0, niobium 0.1–2.0, iron and unavoidable impurities making the rest, solidus temperature of alloy 1,120–1,128 °C is provided, heating charge to 1,480–1,500 °C, held therein for 30 minutes, discharging into ladle with modification of ferroboron on chute at outlet from furnace in terms of obtaining in finished alloy 0.02–0.08 wt% boron, modifying in the ladle with tellurium in amount of 2–10 g/t of the melt and during pouring into the mold, modifying with ferrosilicium FS65 of fraction 0.5–2.0 mm in amount of 1.0–3.0 kg/t of the melt being poured.

EFFECT: high ultimate tensile strength, ultimate bending strength and impact strength of cast iron.

2 cl, 1 tbl

Description

The invention relates to the field of metallurgy, in particular to compositions and methods for the production of high-strength cast iron with spherical graphite, and can be used in the manufacture of cast products with high mechanical properties, including dynamic loading, such as rolls of rolling mills. In modern conditions of rolling production, the requirements for the surface quality of rolling products and, accordingly, the quality of rolling rolls are increasing. Indefinite rolls working in finishing stands of sheet rolling mills should have high wear resistance and resistance to the formation of surface microcracks. The properties of the working layer of indefinite rolls are determined by the structure of indefinite cast iron. The properties of indefinite cast iron can be improved by optimizing its chemical composition and structure.

From the description of RU 2403303 [1], an alloy is known that can be used for the manufacture of rolling rolls. The alloy contains, by weight. %: carbon 3.0-5.0; silicon 0.8-1.2; manganese 1.6-2.0; chrome 22.0-24.0; nickel 42.0-44.0; cobalt 21.2-25.2; iron 3.0-5.0. The disadvantages of the known alloy are the relatively high cost due to the high content of expensive metals (chromium, nickel, cobalt) and insufficiently high mechanical and operational characteristics.

From the description of RU 2153536 [2], the composition of wear-resistant cast iron is known for producing various cast billets of parts operating under conditions of increased wear, namely double-layer rolls with a working wear-resistant layer, rolling mills, crushing equipment, construction (crushers) and the food industry, flour milling, brewing (mill for grinding malt). Cast iron contains components in the following ratio by weight. %: carbon 3.0-3.3; silicon 0.3-0.6; manganese 0.3-0.6; phosphorus 0.3-0.8; vanadium 0.1-0.4; iron and impurities are the rest, while the ratio of the content of vanadium to the content of phosphorus is 0.3-0.5. Cast iron as impurities contains nickel and chromium up to 0.4% each and sulfur up to 0.02%. The disadvantages of the known alloy are the insufficiently high mechanical characteristics of the alloy — tensile strength (temporary resistance), flexural strength and impact strength of the working layer, which do not allow to achieve the resulting increase in productivity and operational reliability of the rolls of the finishing group of hot rolling mills.

Close to the claimed according to the list of chemical elements contained in it is chromium cast iron, known from the description to RU 2462527 [3]. Cast iron contains, by weight. %: carbon 3.6-4.0; silicon 1.0-1.2; manganese 0.2-0.3; chrome 18.0-20.0; nickel 0.3-0.5; niobium 0.3-0.5; zirconium 0.3-0.5; boron 0.9-1.1; tellurium 0.002-0.003; iron is the rest. The disadvantages of the known alloy are insufficiently high wear resistance due to the technology used to obtain it and relatively high cost due to the use of expensive components, as well as insufficiently high mechanical characteristics of the alloy — tensile strength (temporary resistance), flexural strength and impact strength of the working layer, not allowing to achieve the resulting increase in productivity and operational reliability of the finishing mill rolls hot rolling mills.

Closest to the claimed according to the list of chemical elements contained in its composition and their content in the alloy is cast iron, known from the description to RU 2306354 [4], which can be used for the manufacture of sand pumps, jet and ball mills requiring high wear resistance. Cast iron contains, by weight. %: carbon 3.0-3.8; silicon 0.8-1.2; manganese 1.5-2.5; chrome 15.0-20.0; nickel 1.0-2.0; niobium 2.0-3.0; zirconium 0.05-0.1; boron 0.1-0.2; tellurium 0.005-0.01; iron is the rest.

The disadvantages of the known alloy, which are caused not only by its composition, but also by the production method, are the insufficiently high mechanical characteristics of the alloy — tensile strength (temporary resistance), flexural strength and impact strength of the working layer, which do not allow achieving an increase in productivity and operational reliability of the rolls of the finishing group of hot rolling mills. In addition, the alloy has a relatively high cost due to the use of expensive components.

A known method for the production of graphitized pig iron of a foundry class, which includes introducing ferrosilicon into liquid cast iron before and during casting, the specific consumption of ferrosilicon per ton of cast iron is set depending on the silicon and manganese content in the cast iron. Moreover, 55-65% of the total consumption of ferrosilicon is introduced into cast iron before casting, and the remaining part after casting 40-50% of cast iron, ending 10-15 minutes before the end of casting (RU 2093586 [5]). The disadvantages of the known alloy are insufficiently high mechanical properties due to both the technology used to obtain it and the amount of some components introduced into the composition.

A known method of obtaining and processing alloyed casting material for the working part of the rolls containing elements of carbon, silicon, manganese, chromium, nickel, molybdenum, vanadium, if necessary, other elements of the V group of the Periodic system, aluminum, the rest is iron (RU 2221071 [6]). The method is characterized in that they obtain a melt with the following chemical composition, wt. %:

Carbon 2.0-3.5 Silicon 1.0-2.0 Manganese 0.5-2.0 Chromium 1.0-3.0 Nickel 3,5-4,9 Molybdenum 0.2-2.9 Iron and impurities rest.

Add more than 0.5 wt. % vanadium in an amount of up to 5.9 wt. %, it is dissolved in it and the melt composition is controlled by alloying by establishing carbon and silicon concentrations in the presence of nickel and the sum of the activities of carbide-forming elements with the possibility of forming a microstructure when it solidifies, after which the melt is poured into a mold, mainly in a centrifugal casting mold, and allowed to solidify with the formation of the casting of the working fluid of the roll.

The disadvantages of this method are not sufficiently high mechanical properties due to both the technology used to obtain it and the amount of some components introduced into the composition. In addition, the use of the known method to obtain the proposed alloy composition does not ensure the achievement of the desired performance characteristics of the alloy.

Closest to the claimed in the aggregate of its constituent operations and the sequence of their execution is the method known from RU 2109837 [7] for producing an alloy based on the iron-carbon system, which has increased hardness and wear resistance and is used for the manufacture of parts operating under conditions of increased shock loads and abrasive wear, such as grinding balls.

The method includes smelting in a smelting unit of a base melt containing iron, carbon, silicon, manganese, chromium, out-of-furnace treatment, including spheroidizing modification of magnesium-containing materials in the ladle and casting molds. In this case, aluminum, molybdenum and titanium are additionally introduced into the melt, modification in the ladle is carried out at a flow rate of magnesium of 0.007-0.2 wt. % and a modification temperature of 1380-1480 ° C and additionally carry out the microalloying of the melt in the ladle or on the trough with rare-earth elements (REM) at a flow rate of 0.02-0.3 wt. % to stabilize and grind the eutectic grain, and after removing the casting carry out its thermal hardening. The disadvantages of this method are not sufficiently high mechanical properties due to both the technology used to obtain it and the amount of some components introduced into the composition. In addition, the use of the known method to obtain the proposed alloy composition does not ensure the achievement of the desired performance characteristics of the alloy.

The claimed group of inventions is aimed at increasing the tensile strength (temporary resistance), tensile strength in bending and impact strength of cast iron used for the manufacture of the working layer of the rolls.

For the claimed cast iron, this result is achieved in that it contains the following chemical elements in the following ratio of components (wt.%):

Carbon 2.8-3.6 Silicon 0.8-1.5 Manganese 0.6-1.5 Chromium 0.8-2.0 Nickel 3,5-5,0 Niobium 0.1-2.0 Tellurium 0.0002-0.001 Boron 0.02-0.08 Iron and inevitable impurities rest.

For the inventive method for producing cast iron, this result is achieved by the fact that the method provides for the smelting of a base melt containing iron, carbon, silicon, manganese, chromium and out-of-furnace treatment, including spheroidizing modification in a ladle and casting in a mold. For this, the base melt is obtained with the following content of components (wt.%):

Carbon 2.8-3.6 Silicon 0.8-1.5 Manganese 0.6-1.5 Chromium 0.8-2.0 Nickel 3,5-5,0 Niobium 0.1-2.0 Iron and inevitable impurities rest

At the same time, the composition of the charge is selected so as to ensure the solidus temperature of the alloy 1120-1128 ° С, the mixture is heated to a temperature of 1480-1500 ° С, maintained at this temperature for 30 minutes and is discharged into the bucket, modified by ferroboron on the chute when discharged from the furnace from the calculation of obtaining in the finished alloy 0.02-0.08%., modification in the ladle with tellurium in the amount of 2-10 g / t of melt and when pouring into the mold, modification with FS65 ferrosilicon fraction 0.5-2.0 mm in the amount of 1, 0-3.0 kg / t of molten melt.

A distinctive feature of the inventive alloy for the working layer of a two-layer rolling roll is the ratio of components given in the claims. Improving the mechanical properties of cast iron is achieved due to the complex effect of the components that make up its composition. Silicon and manganese are introduced as alloying agents. Moreover, with a silicon content of less than 0.80 wt. % and manganese less than 0.60 wt. % of their hardening properties are negligible compared to other alloying elements, and when their content is more than 1.5 wt. % decreases the viscosity of the alloy. The use of chromium as an alloying element increases the strength, hardness, wear resistance, heat resistance and corrosion resistance of cast iron, maintains the strength of the matrix during cyclic heating-cooling. These properties are best manifested when the chromium content in the metal in an amount of 0.8-2.0 wt. % A further increase in the chromium content leads to a decrease in heat resistance and ductility. The specified Nickel content in the composition of the proposed cast iron allows to obtain its martensitic metal base, resulting in increased strength, hardness and wear resistance of cast iron.

The addition of less than 3.5% nickel to the composition of the proposed cast iron does not ensure that a sufficient concentration of nickel in austenite is achieved, which contributes to the partial decomposition of austenite upon cooling into troostite, as a result of which the strength, hardness and required mechanical properties of cast iron are reduced. An increase in nickel content in excess of 5% contributes to an increase in the fraction of residual austenite in the metal base of cast iron, which leads to a decrease in crack resistance, hardness and wear resistance of cast iron.

The introduction of niobium in the composition of cast iron according to the invention contributes to the grinding of grain of the metal base and the formation of fine refractory carbides of the MS type, which have high hardness. Alloying with niobium increases the wear resistance, heat resistance, corrosion resistance of cast iron and improves the mechanical properties of cast iron. A niobium content of less than 0.1% does not provide effective grinding of the grain of the metal base and the formation of the required amount of refractory niobium carbides. The increase in the amount of niobium in excess of 2.0% does not contribute to a significant increase in wear resistance, heat resistance and corrosion resistance of cast iron, while the cost of manufacturing products increases.

The introduction of boron cast iron into the composition allows the formation of spherical graphite nucleation centers and contributes to the formation of finely dispersed carbides. The introduction of boron of less than 0.02% does not provide the formation of centers of nucleation of spherical graphite and the formation of finely dispersed carbides, which reduces the mechanical properties of cast iron; an increase in boron content in excess of 0.08% increases the amount of large eutectic carbides, which also reduces the mechanical properties of cast iron.

The introduction of tellurium into the composition of the alloy allows the creation of an adsorption layer around the centers of graphite nucleation, which, in turn, ensures the formation of finely dispersed spherical graphite inclusions. Moreover, when the tellurium content is less than 0.0002%, the condition for the modification and inclusion of graphite is not provided, irregular shapes are formed, and an excess of 0.001% leads to the strong carbide-forming effect of tellurium and the formation of large eutectic carbides that reduce the mechanical properties of cast iron.

To determine the optimal ratio of components that ensure the achievement of the stated result, the corresponding experiments were conducted on smelting cast irons with variations in chemical composition. The working layers of the rolls were made from the obtained alloys and their mechanical properties and operational characteristics were investigated. In order to study the effect of chemical composition on the structure and hardness of the working layer of centrifugally cast indefinite rolls, a neural network processing of the obtained database was performed. As a result of processing, a neuromodel was obtained that predicts properties with a relative error of up to 10%. The effect of each element was studied at constant concentrations of the remaining elements. Ultimately, the optimal composition of cast iron is selected according to the results of processing.

The proposed composition of cast iron provides in the molten state obtaining a fine-grained structure of the metal base and the carbide phase with a small amount of residual austenite and the optimal amount of spherical graphite, which is achieved due to the fact that the alloy is obtained by the proposed method. As a result, the tensile strength, bending strength and toughness of cast iron under conditions of impact-abrasive wear are increased and high casting properties are maintained.

Distinctive features of the proposed method are:

- obtaining a base melt containing the components listed in paragraph 2 of the claims;

- selection of the composition of the charge providing the temperature of the solidus alloy 1120-1128 ° C;

- the mixture is heated to a temperature of 1480-1500 ° C, kept with it for 30 minutes;

- produce the release in the bucket, carrying out the modification of ferroboron on the gutter when released from the furnace based on the receipt of boron in the finished alloy of 0.02-0.08%;

- carry out the modification in the bucket tellurium in the amount of 2-10 g / t of melt;

- carry out the modification of ferrosilicon FS65 fractions of 0.5-2.0 mm in an amount of 1.0-3.0 kg / t of molten melt when casting in the mold.

Obtaining a base melt containing the components listed in paragraph 2 of the claims, allows for subsequent out-of-furnace processing provided by the proposed method, to obtain cast iron with spherical graphite for the manufacture of a working layer of a centrifugally cast two-layer rolling roll with increased mechanical properties - tensile strength, tensile bending strength and impact strength of cast iron

The selection of the composition of the mixture, ensuring the solidus temperature of the alloy 1120-1128 ° C, also affects the increase of the listed mechanical properties of the resulting cast iron. The solidus temperature of cast iron within the specified limits is a prerequisite for obtaining spherical graphite.

In order to obtain spherical graphite in cast iron by the proposed method, it is necessary to provide a condition for nucleation of secondary graphite in the melt, which is subsequently modified. This condition is the solidus temperature of the alloy 1120-1128 ° C. The indicated temperature range is chosen empirically.

Heating the melt to a temperature of 1480-1500 ° C and holding it for 30 minutes is necessary in order to ensure a uniform distribution of all the components of the alloy in volume, since local heterogeneity of the elements in the melt can lead to an inhomogeneous distribution of structural components in the finished casting , and hence the mechanical properties of cast iron, and this, in turn, can adversely affect the operational characteristics of the rolls.

Carrying out the modification by ferroboron on the gutter when discharged from the furnace also affects the formation of spherical graphite and carbides of the resulting cast iron. Boron in indefinite cast iron forms various compounds, some of which are centers of spherical graphite nucleation, others contribute to the formation of finely dispersed carbides. It is important to perform ferroboron treatment upon discharge from the furnace, until the next stage of modification, so that the necessary number of graphitization centers and carbide formation centers are formed in cast iron.

In this case, ferroboron is introduced based on the preparation of boron in the finished alloy 0.02-0.08%. The introduction of boron of less than 0.02% does not provide the formation of centers of nucleation of spherical graphite and the formation of finely dispersed carbides, which reduces the mechanical properties of cast iron; an increase in boron content in excess of 0.08% increases the amount of large eutectic carbides, which also reduces the mechanical properties of cast iron.

Modification in the ladle with tellurium in the amount of 2-10 g / t of melt is necessary so that tellurium, as a surface-active element, creates an adsorption layer around the centers of graphite nucleation, which ensures the formation of finely dispersed spherical graphite inclusions. When the tellurium content is less than 0.0002%, the condition for the modification and inclusion of graphite is not provided, irregular shapes are formed, and an excess of 0.001% leads to the strong carbide-forming effect of tellurium and the formation of large eutectic carbides that reduce the mechanical properties of cast iron. Modification with tellurium should be carried out only after the formation of the required number of spherical graphite nucleation centers (after treatment with ferroboron).

Modification of the working layer with FS65 ferrosilicon of a fraction of 0.5-2.0 mm in an amount of 1.0-3.0 kg / t of molten melt is necessary for additional graphitization and to ensure the amount of graphite in the working layer of the rolls of 2-3%. It is most advisable to use FS65, because when FS75 is used, silicon flotation occurs due to a lower modifier density. The selected ferrosilicon fraction is most suitable for modification to a jet when casting into a mold.

The essence of the claimed cast iron and method for its production is illustrated by examples of implementation.

In the most general case, the proposed method for producing cast iron is implemented as follows. A charge containing steel scrap, scrap iron, return of own production, nickel, ferrochrome, ferrosilicon, ferromanganese, ferroniobium, and carburizer is loaded into a melting unit, which can be used as an induction crucible furnace of medium frequency. The components of the charge are loaded in such an amount as to ensure that the content of the resulting base melt contains carbon 2.8-8.3.6%, silicon 0.8-1.5%, manganese 0.6-1.5%, chromium 0.8- 2.0%, nickel 3.5-5.0%, niobium 0.1-2.0%. The scrap steel is introduced into the charge in an amount of 68-80% to ensure the temperature of the solidus alloy 1120-1128 °. Solidus temperature is controlled by a Quik-Lab E. Thermographic analysis device. The mixture is melted and held at a temperature of 1480-1500 ° C for 30 minutes. When the melt is discharged from the furnace to the ladle, it is modified with ferroboron on the trench in the amount of 0.02-0.08% of boron in the finished alloy. In the bucket, tellurium is modified in the amount of 2-10 g / t of melt by immersing a sample in the melt using a rod. When pouring the melt, the FS65 grade ferrosilicon is modified with a fraction of 0.5-2.0 mm in an amount of 1.0-3.0 kg / t of molten melt. With direct use of the melt for the manufacture of the working layer of a two-layer rolling roll, casting is carried out by a centrifugal method. Samples for studying its mechanical properties were made from the obtained cast iron of the proposed composition obtained by the proposed method. Determination of tensile strength (temporary resistance) was carried out in accordance with GOST 1497 -84 “Metals. Tensile test methods. " For the experiments, samples of a cylindrical shape — type 3 No. 6 according to GOST 1497-84 — were made from templates selected from the roll body. The tests were carried out using a tensile testing machine brand MUP-50. The determination of the flexural strength was carried out in accordance with GOST 14019-2003 “Metals. Bending test methods "and GOST 27208-87" Castings of cast iron. Methods of mechanical testing. " For the experiments, rectangular samples 10 × 10 × 61 mm in size were made from templates selected from the roll body. The tests were carried out using a universal testing machine brand MUP-50.

The determination of impact strength was carried out in accordance with GOST 9454-70 “Metals. Impact bending test method at low, room and high temperatures. " For the experiments, rectangular samples of 10 × 10 × 61 mm in size were made. from templates selected from the roll body. The tests were carried out on a pendulum headstock MK-30A. Research was conducted on 81 samples of various chemical composition. The results of comparative tests for some samples of known cast irons and the proposed composition obtained by the proposed method are shown in the table. The table shows that the cast iron of the proposed composition obtained by the proposed method has a higher tensile strength, bending strength and impact strength while maintaining high casting properties of cast iron.

Claims (11)

1. Cast iron containing carbon, silicon, manganese, chromium, nickel, niobium, boron, tellurium and iron, characterized in that it contains the listed chemical elements in the following ratio of components, wt. %: carbon 2.8-3.6 silicon 0.8-1.5 manganese 0.6-1.5 chromium 0.8-2.0 nickel 3,5-5,0 niobium 0.1-2.0 tellurium 0.0002-0.001 boron 0.02-0.08 iron and inevitable impurities rest. 2. A method of producing cast iron, including the smelting of a base melt containing iron, carbon, silicon, manganese, chromium, out-of-furnace treatment, including spheroidizing modification in a ladle and casting into casting molds, characterized in that the base melt is melted with the following components wt. %: carbon 2.8-3.6 silicon 0.8-1.5 manganese 0.6-1.5 chromium 0.8-2.0 nickel 3,5-5,0 niobium 0.1-2.0 iron and inevitable impurities rest, at the same time, the composition of the mixture is selected that ensures the solidus temperature of the alloy 1120-1128 ° C, the mixture is heated to a temperature of 1480-1500 ° C, maintained for 30 minutes and discharged into the bucket, modified by ferroboron in the trough when discharged from the furnace, taking into account the alloy 0.02-0.08 wt.% boron, modification in the ladle with tellurium in the amount of 2-10 g / t of melt and when casting in the form, modification with ferrosilicon FS65 fraction of 0.5-2.0 mm in the amount of 1.0-3 , 0 kg / t of molten melt.