LU502587B1 - Low-cost, high-strength ferritic nodular cast iron, and preparation method and use thereof - Google Patents

Low-cost, high-strength ferritic nodular cast iron, and preparation method and use thereof Download PDF

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LU502587B1
LU502587B1 LU502587A LU502587A LU502587B1 LU 502587 B1 LU502587 B1 LU 502587B1 LU 502587 A LU502587 A LU 502587A LU 502587 A LU502587 A LU 502587A LU 502587 B1 LU502587 B1 LU 502587B1
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cast iron
nodular cast
iron
molten iron
cost
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LU502587A
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French (fr)
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Shibo Shao
Jianchen Cong
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Tianrun Industry Tech Co Ltd
Univ Shandong Technology
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D5/00—Heat treatments of cast-iron
    • C21D5/02—Heat treatments of cast-iron improving the malleability of grey cast-iron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C1/00—Refining of pig-iron; Cast iron
    • C21C1/10—Making spheroidal graphite cast-iron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C1/00—Refining of pig-iron; Cast iron
    • C21C1/10—Making spheroidal graphite cast-iron
    • C21C1/105—Nodularising additive agents
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D5/00—Heat treatments of cast-iron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/08—Making cast-iron alloys
    • C22C33/10—Making cast-iron alloys including procedures for adding magnesium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C37/00—Cast-iron alloys
    • C22C37/04—Cast-iron alloys containing spheroidal graphite
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C37/00—Cast-iron alloys
    • C22C37/10—Cast-iron alloys containing aluminium or silicon
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02F—CYLINDERS, PISTONS OR CASINGS, FOR COMBUSTION ENGINES; ARRANGEMENTS OF SEALINGS IN COMBUSTION ENGINES
    • F02F3/00—Pistons 
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21B—MANUFACTURE OF IRON OR STEEL
    • C21B2300/00—Process aspects
    • C21B2300/02—Particular sequence of the process steps
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02F—CYLINDERS, PISTONS OR CASINGS, FOR COMBUSTION ENGINES; ARRANGEMENTS OF SEALINGS IN COMBUSTION ENGINES
    • F02F2200/00—Manufacturing
    • F02F2200/06—Casting
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02F—CYLINDERS, PISTONS OR CASINGS, FOR COMBUSTION ENGINES; ARRANGEMENTS OF SEALINGS IN COMBUSTION ENGINES
    • F02F7/00—Casings, e.g. crankcases or frames
    • F02F7/0085—Materials for constructing engines or their parts
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

Abstract

The present invention discloses a low-cost, high-strength ferritic nodular cast iron, and a preparation method and use thereof. The ferritic nodular cast iron comprises the following elements: silicon of 2.6-3.2% by weight (wt%), carbon of 3-3.4 wt%, manganese of 0.3-0.5 wt%, copper of 0.1-0.2 wt%, tin of 0.008-0.017 wt%, magnesium of 0.04-0.06 wt%, aluminum and titanium in a total content of ≤ 0.035 wt%, sulfur of ≤0.02 wt%, and phosphorus of ≤0.03 wt%, with the rest being iron. In the preparation process of nodular cast iron, molten iron is sampled and analyzed periodically, and the contents of various elements are adjusted according to the test results, to prepare a nodular cast iron with excellent thermal and mechanical performances. The ferritic nodular cast iron prepared with the above composition and method has a thermal conductivity at 300°C of ≥40 W/(K.m), a linear expansion coefficient at 20-400°C of ≤11 μm (m.K), a tensile strength at 780°C of ≥75 MPa, and a yield strength of ≥40 MPa, thus meeting the performance requirement for an explosion pressure of 20-24 Mpa raised by a piston casting of an internal combustion engine.

Description

Low-cost, high-strength ferritic nodular cast iron, and preparation method and use LUS02587 thereof
FIELD OF THE INVENTION
The present invention relates to the technical field of metallurgy of cast iron, and particularly to a low-cost, high-strength ferritic nodular cast iron, and a preparation method and use thereof.
DESCRIPTION OF THE RELATED ART
In the art of internal combustion engines, a piston of a traditional internal combustion engine is usually made of aluminum. As the explosion pressure on the piston rises to 20 MPa or higher, the aluminum piston does not meet the requirements of use due to its low performance. More and more original equipment manufacturers (OEMs) require the development of forged steel pistons, but forged steel pistons are high in production cost. The cost and performance of nodular cast iron are between those of aluminum and forged steel. Pistons made of nodular cast iron need to be adapt to work at high temperatures, that is, they need to have smaller coefficient of thermal expansion, and higher thermal conductivity and mechanical performance. However, the existing nodular cast iron is usually a low-temperature ferritic nodular cast iron adapted to the low temperature environment. Its highest performance only meets the QT400-18AL index; or the pearlitic nodular cast iron suitable for normal-temperature work although has a room- temperature performance that is as high as the QT900-5 index, the coefficient of thermal expansion is large, and the thermal conductivity is poor. In addition, noble metals are usually added to the nodular cast iron in the prior art to enhance the overall performance of the nodular cast iron. This will greatly increase the production cost. For example, the Patent No.
CN106011609B discloses a medium silicon molybdenum niobium nodular cast iron material, where the eutectic clusters are refined by adding the molybdenum element, to strengthen the ferrite matrix and improve the casting strength. Moreover, the high-temperature strength of the casting is enhanced by adding the niobium element. The prepared nodular cast iron has a tensile strength reaching 71-74 Mpa at a high temperature of 780°C. The nodular cast iron material contains rare metal elements such as molybdenum, niobium, and vanadium to improve the mechanical strength of cast iron at high temperatures, which greatly increases the production cost. To fully smelt each component, the preparation process is complex and energy-intensive, thus being not suitable for industrial production.
Therefore, there is an urgent need to develop a low-cost, ferritic nodular cast iron with good mechanical performance in normal- or high-temperature environments.
SUMMARY OF THE INVENTION
To solve the above technical problems, the present invention provides a low-cost ferritic nodular cast iron with good thermal conductivity, and high mechanical strength at high temperatures.
To solve the above technical problems, the present invention provides the following technical solutions.
A first aspect of the present invention provides a low-cost, high-strength ferritic nodular cast iron, which comprises the following elements: silicon of 2.6-3.2% by weight (wt%), carbon of 3-3.4 wt%, manganese of 0.3-0.5 wt%, copper of 0.1-0.2 wt%, tin of 0.008-0.017 wt%, magnesium of 0.04-0.06 wt%, aluminum and titanium in a total content of < 0.035 wt%, and sulfur of <0.02 wt%, with the rest being iron, wherein the weight percentages of various elements add up to 100%.
Silicon is an element that promotes the nodularization of graphite, which is solid-dissolved in the ferrite lattice and forms a silicon-containing ferrite by covalently bonding with iron atoms, to effectively improve the tensile strength of nodular cast iron. However, the increase of silicon content increases the ductile-brittle transition temperature of nodular cast iron, and reduces the impact strength of cast iron. The silicon content in the nodular cast iron provided in the present invention is preferably 2.6% to 3.2%.
Preferably, the carbon content in the nodular cast iron is (4.2%-silicon content/3).
Carbon is one of the essential elements in nodular cast iron that promotes graphitization. The increase in carbon content helps to increase the graphitization expansion during the solidification of cast iron, thereby improving the self-feeding ability of molten iron. However, a too high carbon content will cause graphite floatation. When the carbon content is 4.2%, the fluidity of molten iron is the most optimum, and less shrinkage and porosity defects are present.
However, since the graphitization of silicon in cast iron is equivalent to 1/3 of that of carbon, the carbon content is preferably (4.2%-silicon content/3).
Manganese can lower the eutectoid transformation temperature, to stabilize and refine the pearlite, and increase the strength and hardness of nodular cast iron. Moreover, manganese can bind with sulfur in molten iron to form MnS, which is removed together with the slag, so as to have a desulfurization effect. However, every 0.1% increase in manganese content in ferritic nodular cast iron causes an increase of 10-12°C in the brittle transition temperature, and increase in shrinkage tendency. Therefore, the manganese content is preferably controlled to 0.3% to 0.5%.
Copper can promote the formation of pearlite and strengthen the ferrite. During the LU502587 solidification of molten iron, copper is solid dissolved in the ferrite, to increase the hardness of the ferrite. However, excessive copper causes the large eduction of pearlite. Therefore, the copper content is preferably 0.1% to 0.2%.
Preferably, the tin content in the nodular cast iron is preferably (0.3%-copper content)/12.
Tin is easily segregated around the graphite ball during the solidification of molten iron, to reduce the diffusion of carbon to graphite balls, so as to refine the graphite. However, excessive tin causes the large eduction of pearlite. The effect of tin is about one-twelfth of that of copper, and preferably, the tin content is (0.3%-copper content)/12. By comprehensively controlling the contents of copper and tin, the pearlite eduction is reduced, and the performance of nodular cast iron is improved while a ferrite matrix is obtained.
Magnesium mainly functions for nodularization, and blocks graphitization. If the magnesium content is low, the graphite ball is not round, If the magnesium content is high, a large amount of cementite is likely to be formed Preferably, the magnesium content is controlled to 0.04- 0.06%.
The contents of unavoidable harmful elements in nodular cast iron, including phosphorus, sulfur, aluminum, and titanium, are controlled.
Phosphorus tends to form phosphorus eutectic in cast iron, reducing the strength and toughness of cast iron. Especially when the phosphorus content exceeds 0.06%, the plasticity, and toughness drops sharply. Preferably, the phosphorus content is controlled to be below 0.03%.
Sulfur is an anti-nodularizing element, and the presence of sulfur in nodular cast iron reduces the nodularization rate, affecting the performance of cast iron. Although the addition of a nodularizing agent can reduce the harmful effect of sulfur, a high sulfur content causes increased inclusions, which is not good for the performance of nodular cast iron. Preferably, the sulfur content is controlled to be below 0.02%.
Aluminum and Titanium: Aluminum can easily cause blow hole defects in cast iron. Generally, when the aluminum content in nodular cast iron is 0.05-0.2%, the blow hole defect is the most serious. Titanium also has a similar effect as aluminum in cast iron. They work together to make the blow hole defects to increase linearly approximately. Therefore, the total content of aluminum and titanium in molten iron is controlled to be <0.035% in the present invention to reduce blow holes in a thin-walled member, eliminate hard spots in the structure, and improve the workability.
The second aspect of the present invention provides a method for preparing the ferritic nodular cast iron described in the first aspect, which includes heating and melting a mixture of scrap steel, a compounding agent, and a recarburizer, to obtain molten iron, heating to 1180-1220 LU502587 °C, then adding ferromanganese, copper, and tin alloy elements, heating, and removing the slag, wherein the molten iron was sampled and analyzed in real time, and the contents of various elements in the molten iron are adjusted according to the test results.
Preferably, according to the test results, the silicon and copper contents are controlled within a preferred range. The carbon content is calculated and determined from the silicon content, the carbon content in molten iron is adjusted according to the calculated value, the carbon content is calculated and determined according to the copper content in molten iron, and the tin content in molten iron is adjusted according to the calculated value.
Preferably, wherein the scrap steel is carbon steel, specifically is selected from the group consisting of 45# carbon steel, Q235# carbon steel, 20# carbon steel and any combination thereof .
Carbon steel is used as an iron source, to reduce the heredity of pig iron, and improve the purity of molten iron.
Preferably , the compounding agent is silicon carbide.
Preferably , the recarburizer is a graphitized recarburizer or a high-temperature-calcined graphitized coke recarburizer, and more preferably graphitized recarburizer. The graphitized recarburizer is produced with petroleum coke in a graphitized smelting furnace at a temperature of about 3000°C, and has a low sulfur content (generally 0.03-0.06%), and a high absorption rate (generally 90-95%). Compared with a high-temperature-calcined graphitized coke recarburizer, the use of a graphitized recarburizer is beneficial to the reduction of sulfur content in molten iron.
Preferably , the heating and removing the slag specifically comprises heating to 1400-1440°C and removing the slag primarily; further heating to 1500-1540°C and holding for 3-5 min; and then reducing the temperature by 20-40 °C and removing the slag secondarily.
After the primary slag removal, the temperature is further raised and the system is let to stand for a period of time, which is beneficial to the rising of sulfides, sulfur oxides, and other residues in molten iron. Then the secondary slag removal is carried out to remove residues in molten iron, to avoid the resulfurization due to oxidation, causing problems such as poor nodularization of graphite.
Preferably, the preparation method further comprises nodularizing, casting and annealing.
Preferably, the nodularizing comprises specifically adding a nodularizing agent and the compounding agent to the molten iron after the secondary slag removal .
Preferably, the nodularization time is 60-90 s, and the temperature of the nodularized molten iron is controlled to 1380-1400 °C. LU502587
Preferably, the nodularizing agent is a manganese-magnesium-based rare earth-free nodularizing agent.
The roundness of graphite balls in nodular iron treated with a rare earth-containing nodularizing agent is poor, and graphite blooming tends to occur. The density of the resulting rare earth sulfide (5.01 g/cm?) and the density of rare earth oxide (7.13 g/cm?) is equivalent to that of molten iron (6.6-7.4g/cm?), and unlikely to float out of the molten iron surface, and form inclusions inside the casting, affecting the toughness of the casting. The direct removal of rare earths in the nodularizing agent will lead to a violent reaction of magnesium, causing instable nodularization. Based on the fact that manganese has a relatively large affinity to oxygen and sulfur in cast iron, the manganese-magnesium-based rare earth-free nodularizing agent is prepared to reduce the hazards of sulfur and oxygen, and strengthen the nodularizing effect of magnesium. Manganese sulfide (3.99 g/cm?) produced by the reaction of manganese in molten iron and sulfur and manganese oxide (4.8 g/cm?) produced by the reaction with oxygen both have a density less than the density of molten iron, and can easily float out of the molten iron surface, and be transferred to the slag and removed.
Preferably, the casting comprises specifically removing the slag from the molten iron after nodularization, and casting the treated molten iron into a casting mold to obtain a roughcast.
Preferably, the annealing comprises specifically heating the roughcast at a rate of 80-100 °C/h to 890-910°C, and holding for 2-3 hrs.
Preferably, in the preparation method, the contents in parts by weight of various raw materials are preferably: 100 parts of scrap steel, 2.7-3.6 parts of compounding agent, 1.5-2.0 parts of recarburizer, 0.4-0.7 parts of ferromanganese, 0.1-0.2 parts of copper, and 0.008-0.017 parts of tin.
A third aspect of the present invention provides use of the ferritic nodular cast iron described in the first aspect in the materials for preparing a piston of an internal combustion engine.
Preferably, the piston of internal combustion engine has an explosion pressure of 24 Mpa or less.
Compared with related art, the present invention has the following beneficial effects. 1. In the present invention, the contents of various elements in molten iron are accurately controlled, and the contents of silicon and carbon in molten iron and the contents of copper and tin are synergistically controlled, to improve the controllability of the performances and preparation process of the material, and a ferritic nodular cast iron with excellent high- temperature mechanical performance and high thermal conductivity is obtained. In addition,
molybdenum niobium, vanadium, and other rare metal elements have no need to be added in LU502587 the nodular cast iron prepared in the present invention, greatly reducing the production cost.
Therefore, the nodular cast iron is suitable for large-scale production in industry. 2. The low-cost, and high-strength ferritic nodular cast iron prepared in the present invention has a tensile strength of not less than 70 MPa at 780°C, and a yield strength of not less than 40
Mpa, which meet the requirements for tensile strength and yield strength raised by a piston of an internal combustion engine with an explosion pressure of 20-24 MPa. The ferritic nodular cast iron can be used as a good material for making the piston of internal combustion engine.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be further described below in connection with specific examples, so that those skilled in the art can better understand and implement the present invention; however, the present invention 1s not limited thereto.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by persons skilled in the art to which the present invention pertains.
The terms used in the descriptions of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and/or” as used herein includes any and all combinations of one or more of the listed related items.
Example 1
This example provides a ferritic nodular cast iron, which mainly comprises the following elements: silicon of 2.6% by weight (wt%), carbon of 3.33 wt%, manganese of 0.3 wt%, copper of 0.1 wt%, tin of 0.017 wt%, magnesium of 0.04 wt%, aluminum and titanium in a total content of not higher than 0.035 wt%, sulfur in a content of not higher than 0.02 wt%, phosphorus in a content of not higher than 0.03 wt%, with the balance being iron, wherein the weight percentages of various element add up to 100%.
This example provides a method for preparing a ferritic nodular cast iron, which comprises the following steps:
S1: Melting treatment: 100 parts of 45# steel scraps, 2.6 parts of silicon carbide and 2.0 parts of graphitized recarburizer were added to an electric furnace and melted by heating to obtain molten iron. After the temperature reached 1200°C, 0.4 part of ferromanganese, 0.1 part of copper and 0.017 part of a tin alloying element were added, and the proportions of various components in the molten iron were controlled. The temperature was further raised to 1420°C and a primary slag removal was carried out. The system was further heated to 1520°C, allowed to stand at high temperature for 3 min, and then cooled to 1480 °C, at which a secondary slag LUS02587 removal was performed. Then, the molten iron was quantitatively fed to a transfer ladle; and the molten iron was sampled and analyzed periodically. The preset proportions of various components in the molten iron were controlled according to the test results.
S2: Nodularization treatment: The transfer ladle in S1 was transferred to a nodularizing procedure, and the molten iron was poured into a sealed nodularizing casting system for nodularization treatment. That is, 0.8 parts of manganese-magnesium-based rare earth-free nodularizing agent was added to molten iron for nodularization treatment. Also, 0.1 part of silicon carbide was added to strengthen inoculation. The nodularization time was 60 s, and the temperature was measured after nodularization, and the temperature was controlled at 1380- 1400°C.
S3: Casting: A secondary slag removal was performed on the molten iron after nodularization treatment in S2, then an insulation material was covered and the molten iron was transferred to a casting procedure. The molten iron was cast into a mold to form a roughcast.
S4: Annealing treatment: The roughcast obtained in S3 was annealed, where the temperature was maintained at 900°C, and held for 2 hrs. After cooling, a nodular cast iron casting was obtained, and the ferrite content in the casting was detected to be 90%.
Example 2
This example provides a ferritic nodular cast iron, which mainly comprises the following elements: silicon of 2.6%, carbon of 3.33%, manganese of 0.5%, copper of 0.2%, tin of 0.008%, magnesium of 0.06%, aluminum and titanium in a total content of not higher than 0.035%, sulfur in a content of not higher than 0.02%, and phosphorus in a content of not higher than 0.03%, with the balance being iron, wherein the weight percentages of various element add up to 100%.
This example provides a method for preparing a ferritic nodular cast iron. The method comprises the following steps:
S1: Melting treatment: 100 parts of 45# steel scraps, 2.6 parts of silicon carbide and 2.0 parts of graphitized recarburizer were added to an electric furnace and melted by heating to obtain molten iron. After the temperature reached 1200°C, 0.7 part of ferromanganese, 0.2 part of copper and 0.008 part of a tin alloying element were added, and the proportions of various components in the molten iron were controlled. The temperature was further raised to 1420°C and a primary slag removal was carried out. The system was further heated to 1520°C, allowed to stand at high temperature for 3 min, and then cooled to 1480 °C, at which a secondary slag removal was performed. Then, the molten iron was quantitatively fed to a transfer ladle; and the molten iron was sampled and analyzed periodically. The preset proportions of various LUS02587 components in the molten iron were controlled according to the test results.
S2: Nodularization treatment: The transfer ladle in S1 was transferred to a nodularizing procedure, and the molten iron was poured into a sealed nodularizing casting system for nodularization treatment. That is, 0.8 part of manganese-magnesium-based rare earth-free nodularizing agent was added to molten iron for nodularization treatment. Also, 0.1 part of silicon carbide was added to strengthen inoculation. The nodularization time was 70 s, and the temperature was measured after nodularization, and the temperature was controlled at 1380- 1400°C.
S3: Casting: À secondary slag removal was performed on the molten iron after nodularization treatment in S2, then an insulation material was covered and the molten iron was transferred to a casting procedure. The molten iron was casted into a mold to form a roughcast.
S4: Annealing treatment: The roughcast obtained in S3 was annealed, where the temperature was maintained at 890 °C, and held for 3 hrs. After cooling, a nodular cast iron casting was obtained, and the ferrite content in the casting was detected to be 95%.
Example 3
This example provides a ferritic nodular cast iron, which mainly comprises the following elements: silicon of 3.2% by weight (wt%), carbon of 3.13 wt%, manganese of 0.3 wt%, copper of 0.1 wt%, tin of 0.017 wt%, magnesium of 0.055 wt%, aluminum and titanium in a total content of not higher than 0.035 wt%, sulfur in a content of not higher than 0.02 wt%, and phosphorus in a content of not higher than 0.03 wt%, with the balance being iron, wherein the weight percentages of various element add up to 100%.
This example provides a method for preparing a ferritic nodular cast iron. The method comprises the following steps:
S1: Melting treatment: 100 parts of Q235# steel scraps, 3.4 parts of silicon carbide and 1.5 parts of graphitized recarburizer were added to an electric furnace and melted by heating to obtain molten iron. After the temperature reached 1200°C, 0.4 part of ferromanganese, 0.1 part of copper and 0.017 part of a tin alloying element were added, and the proportions of various components in the molten iron were controlled. The temperature was further raised to 1420°C and a primary slag removal was carried out. The system was further heated to 1520°C, allowed to stand at high temperature for 3 min, and then cooled to 1480 °C, at which a secondary slag removal was performed. Then, the molten iron was quantitatively fed to a transfer ladle; and the molten iron was sampled and analyzed periodically. The preset proportions of various components in the molten iron were controlled according to the test results.
S2: Nodularization treatment: The transfer ladle in S1 was transferred to a nodularizing LUS02587 procedure, and the molten iron was poured into a sealed nodularizing casting system for nodularization treatment. That is, 0.8 part of manganese-magnesium-based rare earth-free nodularizing agent was added to molten iron for nodularization treatment. Also, 0.1 part of silicon carbide was added to strengthen inoculation. The nodularization time was 90 s; and the temperature was measured after nodularization, and the temperature was controlled at 1380- 1400°C.
S3: Casting: À secondary slag removal was performed on the molten iron after nodularization treatment in S2, then an insulation material was covered and the molten iron was transferred to a casting procedure. The molten iron was cast into a mold to form a roughcast.
S4: Annealing treatment: The roughcast obtained in S3 was annealed, where the temperature was maintained at 910 °C, and held for 2.5 hrs. After cooling, a nodular cast iron casting was obtained, and the ferrite content in the casting was detected to be 95%.
Example 4
This example provides a ferritic nodular cast iron, which mainly comprises the following elements: silicon of 3.2% by weight (wt%), carbon of 3.13 wt%, manganese of 0.5 wt%, copper of 0.2 wt%, tin of 0.008 wt%, magnesium of 0.06 wt%, aluminum and titanium in a total content of not higher than 0.035 wt%, sulfur in a content of not higher than 0.02 wt%, and phosphorus in a content of not higher than 0.03 wt%, with the balance being iron, wherein the weight percentages of various element add up to 100%.
This example provides a method for preparing a ferritic nodular cast iron. The method comprises the following steps:
S1: Melting treatment: 100 parts of 20# steel scraps, 3.4 parts of silicon carbide and 1.5 parts of graphitized recarburizer were added to an electric furnace and melted by heating to obtain molten iron. After the temperature reached 1200°C, 0.7 part of ferromanganese, 0.2 part of copper and 0.008 part of a tin alloying element were added, and the proportions of various components in the molten iron were controlled. The temperature was further raised to 1420°C and a primary slag removal was carried out. The system was further heated to 1520°C, allowed to stand at high temperature for 3 min, and then cooled to 1480 °C, at which a secondary slag removal was performed. Then, the molten iron was quantitatively fed to a transfer ladle; and the molten iron was sampled and analyzed periodically. The preset proportions of various components in the molten iron were controlled according to the test results.
S2: Nodularization treatment: The transfer ladle in S1 was transferred to a nodularizing procedure, and the molten iron was poured into a sealed nodularizing casting system for nodularization treatment. That is, 0.8 part of manganese-magnesium-based rare earth-free LUS02587 nodularizing agent was added to molten iron for nodularization treatment. Also, 0.1 part of silicon carbide was added to strengthen inoculation. The nodularization time was 90 s, and the temperature was measured after nodularization, and the temperature was controlled at 1380- 1400°C.
S3: Casting: À secondary slag removal was performed on the molten iron after nodularization treatment in S2, then an insulation material was covered and the molten iron was transferred to a casting procedure. The molten iron was casted into a mold to form a roughcast.
S4: Annealing treatment: The roughcast obtained in S3 was annealed, where the temperature was maintained at 900°C, and held for 3 hrs. After cooling, a nodular cast iron casting was obtained, and the ferrite content in the casting was detected to be 95%.
Comparative Example 1
This comparative example provides a ferritic nodular cast iron, which mainly comprises the following elements: silicon of 2.4% by weight (wt%), carbon of 3.6 wt%, manganese of 0.4 wt%, magnesium of 0.04 wt%, sulfur in a content of not higher than 0.03 wt%, and phosphorus in a content of not higher than 0.03 wt%, with the balance being iron, wherein the weight percentages of various element add up to 100%.
In this comparative example, ferritic nodular cast iron was produced according to an existing conventional technology. That is, pig iron, a small amount of scrap steel and recycled scrap were used as raw materials, nodularized, and cast into a roughcast. It should be noted that this comparative example mainly differs from the examples in that in this comparative example, pig iron and recycled scrap are used as raw materials, which are not alloyed; and in the examples, scraps of 45# steel, Q235# steel and other carbon structural steels are used as raw materials, with which the hereditary hazards of pig iron are avoided.
The carbon content and silicon content in the comparative example are not adjusted as long as they are within a set range; however, in the examples, after the silicon content falls a set range, the carbon content is adjusted by (4.2%-silicon content/3) according to the determined silicon content. The reason for the above adjustment is that scrap steel is used as the raw material, and the scrap steel may be scrap steel of 45# steel, Q235# steel and other carbon steels, or a mixture thereof. The carbon and silicon contents in the scrap steel vary greatly from batch to batch, if the carbon content and silicon content in each batch of molten iron are controlled within the process range according to the existing process, the actual carbon equivalent varies greatly. For the same reason, the alloying process in the examples is also precisely controlled by combined control of copper and tin in real time.
In the comparative example, a method for preparing a ferritic nodular cast iron is provided. The LUS02587 method comprises the following steps:
S1: Melting treatment: Pig iron, scrap steel and recycled scrap were added to an electric furnace and melted at a weight ratio of 5:2:3, ferrosilicon and ferromanganese were added during the melting process. The proportions of various components in the molten iron were controlled to give a carbon content of 3.6%, a silicon content of 1.6%, and a manganese content of 0.4%.
The system was further heated up to 1500°C, allowed to stand at high temperature for 4 min, and then cooled to 1400 °C, at which a secondary slag removal was performed. Then, the molten iron was quantitatively fed to a transfer ladle; and the molten iron was sampled and analyzed periodically. The preset proportions of various components in the molten iron were controlled according to the test results.
S2: Nodularization treatment: The transfer ladle in S1 was transferred to a nodularizing procedure, and the molten iron was poured into a sealed nodularizing casting system for nodularization treatment. That is, a nodularizing agent was added to molten iron for nodularization treatment, ferrosilicon was added to increase the silicon content, and the silicon content in molten iron was controlled to 2.4%.
S3: Casting: A secondary slag removal was performed on the molten iron after nodularization treatment in S2, then an insulation material was covered and the molten iron was transferred to a casting procedure. The molten iron was casted into a mold to form a roughcast.
S4: Annealing treatment: The roughcast obtained in S3 was annealed, where the temperature was maintained at 900°C, and held for 2 hrs. After cooling, a nodular cast iron casting was obtained, and the ferrite content in the casting was detected to be 95%.
Comparative Example 2
This comparative example provides a ferritic nodular cast iron, which mainly comprises the following elements: silicon of 2.7%, carbon of 3.2%, manganese of 0.6%, magnesium of 0.07%, sulfur in a content of not higher than 0.03 wt%, and phosphorus in a content of not higher than 0.03 wt%, with the balance being iron, wherein the contents in weight percentages of various elements add up to 100%.
In this comparative example, ferritic nodular cast iron was produced according to an existing conventional technology. That is, pig iron, a small amount of scrap steel and recycled scrap were used as raw materials, nodularized, and cast into a roughcast. For steps that are the same as those in Comparative Example 1, descriptions are repeated here.
In the comparative example, a method for preparing a ferritic nodular cast iron is provided. The method comprises the following steps:
S1: Melting treatment: Pig iron, scrap steel and recycled scrap were added to an electric furnace LUS02587 and melted at a weight ratio of 4:3:3, ferrosilicon and ferromanganese were added during the melting process. The proportions of various components in the molten iron were controlled to give a carbon content of 3.2%, a silicon content of 2.3%, and a manganese content of 0.6%.
The system was further heated up to 1500°C, allowed to stand at high temperature for 4 min, and then cooled to 1400 °C, at which a secondary slag removal was performed. Then, the molten iron was quantitatively fed to a transfer ladle; and the molten iron was sampled and analyzed periodically. The preset proportions of various components in the molten iron were controlled according to the test results.
S2: Nodularization treatment: The transfer ladle in S1 was transferred to a nodularizing procedure, and the molten iron was poured into a sealed nodularizing casting system for nodularization treatment. That is, a nodularizing agent was added to molten iron for nodularization treatment, ferrosilicon was added to increase the silicon content, and the silicon content in molten iron was controlled to 2.7%.
S3: Casting: À secondary slag removal was performed on the molten iron after nodularization treatment in S2, then an insulation material was covered and the molten iron was transferred to a casting procedure. The molten iron was casted into a mold to form a roughcast.
S4: Annealing treatment: The roughcast obtained in S3 was annealed, where the temperature was maintained at 900°C, and held for 3 hrs. After cooling, a nodular cast iron casting was obtained, and the ferrite content in the casting was detected to be 90%.
Performance study
The thermal and mechanical performances of ferritic nodular cast iron prepared in Examples 1-4 and Comparative Examples 1 and 2 were tested. The test items and standards were as follows:
Thermal conductivity: The thermal conductivity was tested according to GB/T3651-2008 "Measuring method for thermal conductivity of metal at high temperature";
Linear expansion coefficient: The linear expansion coefficient was tested according to GB-
T4339-2008 "Testing method for thermal expansion characteristic parameters of metallic materials":
High-temperature tensile strength and yield strength: The high-temperature tensile strength and yield strength was tested according to GB/T4338-2006 "Metallic materials - Tensile testing -
Method of test at elevated temperature". The test results are shown in Table 1 below:
Table 1. Test results of thermal and mechanical performances in Examples 1-4 and
Comparative Examples 1 and 2
Tensile Yield LU502587
Thermal Linear expansion
Co strength at | strength
Sample conductivity at | coefficient at 20-400°C, 780°C (MPa) at 300°C (W/m-°C) | um (mK) (MPa) 780°C
Comparative 36.1 12.6 53 30
Example 1
Comparative 36.0 12.8 58 32
Example 2
From the test results in Table 1, it can be seen that Through the combined control of the carbon and silicon contents, as well as the copper and tin contents, a ferritic nodular cast iron is prepared. Compared with the uncontrolled products in Comparative Examples 1 and 2, the thermal and mechanical performance of the ferritic nodular cast iron are significantly improved. The ferritic nodular cast iron shows a high thermal conductivity, a low expansion coefficient, and a high tensile strength (775 MPa) and yield strength (740 MPa) at 780°C, and meets the performance requirements raised by a piston of an internal combustion engines with an explosion pressure of 20-24 MPa (the tensile strength at 780 °C is not less than 70 MPa, and the yield strength is not less than 35 MPa).
The above-described embodiments are merely preferred embodiments for the purpose of fully illustrating the present invention, and the scope of the present invention is not limited thereto.
Equivalent substitutions or modifications can be made by those skilled in the art based on the present invention, which are within the scope of the present invention as defined by the claims.
The scope of the present invention is defined by the appended claims.

Claims (10)

WHAT IS CLAIMED IS: LU502587
1. A low-cost, high-strength ferritic nodular cast iron, comprising elements by weight of : silicon of 2.6-3.2% wt%, carbon of 3-3.4 wt% , manganese of 0.3-0.5 wt%, copper of 0.1-0.2 wt%, tin of 0.008-0.017 wt%, magnesium of 0.04-0.06 wt%, aluminum and titanium in a total content of <0.035 wt%, sulfur of <0.02 wt%, and phosphorus of <0.03 wt%, with the rest being iron, wherein the weight percentages of various elements add up to 100%.
2. The low-cost, high-strength ferritic nodular cast iron, wherein the carbon content in the nodular cast iron is (4.2%-silicon content/3).
3. The low-cost, high-strength ferritic nodular cast iron, wherein the tin content in the nodular cast iron is (0.3%-copper content)/12.
4. A method for preparing a low-cost, high-strength ferritic nodular cast iron, comprising: heating and melting a mixture of scrap steel, a compounding agent, and a recarburizer, to obtain molten iron, heating to 1180-1220°C, then adding ferromanganese, copper, and a tin alloying element, heating, and removing the slag, wherein the molten iron was sampled and analyzed in real time, and the contents of various elements in the molten iron are adjusted according to the test results.
5. The method for preparing a low-cost, high-strength ferritic nodular cast iron according to claim 4, wherein the scrap steel is carbon steel, specifically is selected from the group consisting of 45# carbon steel, Q235# carbon steel, 20# carbon steel and any combination thereof, the compounding agent is silicon carbide; and the recarburizer is a graphitized recarburizer or a high-temperature-calcined graphitized coke recarburizer.
6. The method for preparing a low-cost, high-strength ferritic nodular cast iron according to claim 4, wherein the heating and removing the slag specifically comprises heating to 1400- 1440°C and removing the slag primarily; further heating to 1500-1540°C and holding for 3-5 min; and then reducing the temperature by 20-40°C and removing the slag secondarily.
7. The method for preparing a low-cost, high-strength ferritic nodular cast iron according to claim 4, further comprising nodularizing, casting and annealing.
8. The method for preparing a low-cost, high-strength ferritic nodular cast iron according to claim 7, wherein the nodularizing comprises specifically adding a nodularizing agent, the compounding agent to the molten iron after the secondary slag removal ; the casting comprises specifically removing the slag from the molten iron after nodularization, and casting the treated molten iron into a casting mold to obtain a roughcast; and the annealing comprises specifically heating the roughcast at a rate of 80-100 °C/h to 890-910°C, and holding for 2-3 hrs. LU502587
9. The method for preparing a low-cost, high-strength ferritic nodular cast iron according to claim 8, wherein the nodularizing agent is a manganese-magnesium-based rare earth-free nodularizing agent, nodularization time is 60-90 s, and a temperature of the nodularized molten iron is controlled to 1380-1400°C.
10. Use of the ferritic nodular cast iron according to any one of claims 1 to 3 in the material for preparing a piston of an internal combustion engine.
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