High carbon content as-cast high strength and high toughness ductile iron

Abstract

Provided is a high carbon content as-cast high strength and high toughness ductile iron, belonging to the field of ductile iron casting. Carbon content of the ductile iron is ≥4.0%, and the ductile iron contains no or little precious metal strengthening alloy. When molten iron is spheroidized out of a furnace, an inhibitor for controlling the morphology of the ductile iron and a sufficient amount of a spheroidizing agent are added to a spheroidizing bucket. The mechanical properties of ductile iron material in a cast state without rapid cooling casting or heat treatment of a casting can reach or exceed QT700. The inhibitor is a mixture of the spheroidizing agent and weak carbide forming elements. An amount of the inhibitor added should ensure that a small amount remains in the casting after pouring. Ductile iron products can achieve high strength and high toughness in the cast state without heat treatment such as quenching or normalizing, and the production costs thereof are much lower than those of ductile iron products that rely on alloys to improve performance.

Description

A high carbon content, high strength and high toughness ductile iron Technical Field

[0001] The invention relates to a ductile iron material, in particular to a ductile iron material having high strength and high toughness in the as-cast state and a casting method thereof, belonging to the field of cast iron casting. Background Art

[0002] In the fields of mechanical manufacturing and metal processing, iron-carbon alloys with a carbon content greater than 2.11% are generally referred to as "iron," commonly known as cast iron. Ductile iron (abbreviated as ductile iron), a type of cast iron, contains free carbon that has not yet formed carbides, primarily in the form of nearly spherical graphite. After casting and heat treatment, its matrix is ​​mostly pearlite or a mixture of pearlite and ferrite, along with a small amount of carbide. Most of the excess carbon is distributed in the matrix in the form of spherical graphite. Ductile iron may also contain a small amount of impurities. The pearlite in the ductile iron matrix is ​​composed of layered cementite and ferrite. Ductile iron can achieve both high strength and high toughness, and its comprehensive mechanical properties are excellent, leading to its widespread application.

[0003] In the ductile iron production industry, the carbon equivalent of ductile iron products is generally less than 4.3%, and the carbon content is generally controlled between 3.5% and 3.9%. When the carbon equivalent of cast iron exceeds 4.3%, it is called hypereutectic cast iron. At this time, the metallographic structure of the cast iron is mainly cementite and ledeburite (ledeburite is a eutectic composed of cementite and austenite). The material is brittle and has almost no elongation, which does not meet the performance requirements of ductile iron. Therefore, technicians in the field will control the carbon content of ductile iron to below 4.0% to keep the carbon equivalent of ductile iron products below 4.3%. When the strength of ductile iron needs to be further improved, alloying elements such as Cu and Mo are added to form alloy carbides in the ductile iron matrix.

[0004] Obtaining ductile iron with high strength and good toughness is a goal that people pursue. However, the current methods for improving the strength of ductile iron often involve adding alloying elements to the ductile iron, using a rapid cooling casting process, or heat treating the casting after casting. For example, patent document CN115161539A discloses an austempered ductile iron planetary carrier blank and its preparation process. The ductile iron casting contains 3.6% to 3.80% carbon by weight. When melting molten iron in an electric furnace, the carbon, silicon, and copper, nickel, and molybdenum alloy contents are controlled to 1.5% to 2.0% Ni, 0.8% to 0.9% Cu, and 0.2% to 0.25% Mo based on raw materials such as pig iron and scrap steel. A certain amount of recarburizer, spheroidizing agent, and inoculant are added to the spheroidizing bag for spheroidization. After austempering, its mechanical properties can reach tensile strength ≥820MPa, yield strength ≥600MPa, elongation ≥4.0%, hardness 280-310HB, and spheroidization rate ≥90%.

[0005] For example, patent document CN101818297B describes a method for manufacturing a high-strength pearlite ductile iron rolling forming mold material. During the casting process, the method controls the alloying elements such as 3.69% to 4.04% C, 0.32% to 0.65% Cu, 0.148% to 0.171% Mo, 0.44% to 0.54% Mn, and 0.031% to 0.035% Pb according to weight percentage to strengthen the ductile iron. The ductile iron is then subjected to normalizing and tempering heat treatment to achieve the mechanical properties of the casting reaching the yield limit σ 0.2 =655~680MPa, tensile strength σ b =965~1050MPa, elongation after fracture =3%~5%, hardness =264~272HB.

[0006] Current ductile iron can be divided into many standard grades or types according to different performances, such as wear-resistant ductile iron, heat-resistant ductile iron, and wear-reducing ductile iron. The state of the graphite balls in the ductile iron has standardized spheroidization grades, including spheroidization rate, roundness, number of spheroidizations, dispersion, and uniform distribution of the balls. The casting process of ductile iron often requires the addition of spheroidizing agents and inoculants to promote the spheroidization of the carbon element. Common inoculants often use ferrosilicon, while spheroidizing agents often use metal alloys containing magnesium. Rare earth magnesium alloys are commonly used spheroidizing agents by those skilled in the art. During spheroidization, the amount of rare earth magnesium added generally does not exceed 1.4% of the weight of the molten iron in the spheroidizing bag. The spheroidizing agent, inoculant, covering agent, slag-aggregating agent, etc. are placed in a pit provided at the bottom of the spheroidizing bag, and then tamped and covered with iron plates or tamped iron filings to control the time when the spheroidizing agent contacts the molten iron and begins to react (commonly known as detonation) and the time for the boiling reaction after detonation.

[0007] Graphite, as the main organization and constituent element of ductile iron, has a high melting point and a light specific gravity. It is difficult to be dissolved in large quantities into the metallic iron liquid. Therefore, when the C content in ductile iron reaches a certain content, it is difficult to increase the C content in the molten iron. Patent document CN106906333B discloses a carburizing method, which is to add a carburizer and molten iron to a smelting furnace or a ladle. When the carburizer cannot be directly dissolved in the molten iron and floats on the molten iron surface, the molten iron and the carburizer are quickly (within 20 seconds) poured into another molten iron container at least once from one molten iron container. At this time, the temperature of the molten iron is controlled at 1450-1700 ° C. The engineering practice and research in the casting field believe that when the carbon content of ductile iron is too high, a large amount of free carbides will be produced. Free carbides are generally considered to be harmful organizations in ductile iron. They will split the matrix organization and reduce the toughness of ductile iron, but they will improve the hardness and wear resistance of cast iron. For example, if the cementite structure is reticular or massive, the overall performance of ductile iron will be reduced. When the carbon content of the designed ductile iron is too high, carbon will have difficulty dissolving into the molten iron, resulting in graphite floating, causing the entire furnace of molten iron or castings to be scrapped. Furthermore, the chemical composition of ductile iron significantly affects its matrix structure, spheroidization state, and performance. The mechanical properties of ductile iron vary significantly depending on the carbon content and the state of carbon in the metal.

[0008] In the casting field, to improve the mechanical strength of ductile iron, those skilled in the art often add strong carbide-forming elements such as chromium (Cr), tungsten (W), vanadium (V), molybdenum (Mo), nickel (Ni), copper (Cu), and manganese (Mn) to the cast iron to increase the pearlite content of the ductile iron. At the same time, some studies have shown that the addition of strong carbide-forming elements, which strongly affect the formation and morphology of cementite, often results in the formation of massive carbides in the ductile iron. For example, adding approximately 0.4%-0.8% Mo and Cu to ductile iron can improve strength, increasing the pearlite content to 85%. However, the remaining structure is primarily composed of isolated, massive carbides. Therefore, when using more expensive metals such as molybdenum and copper to strengthen cast iron, while the strength can be significantly improved, the elongation often decreases significantly.

[0009] Furthermore, metals such as chromium (Cr), tungsten (W), vanadium (V), molybdenum (Mo), nickel (Ni), and copper (Cu) are expensive. These alloys are considered precious metal strengthening alloys in the casting industry and are often referred to as noble metals. Therefore, using these alloying elements to strengthen cast iron significantly increases the cost of cast iron products. The prices of precious metals such as chromium (Cr), tungsten (W), vanadium (V), molybdenum (Mo), nickel (Ni), and copper (Cu) are much higher than those of carbon (C). For example, in June 2023, the market price of a molybdenum-iron alloy (such as FeMo60) containing at least 55% molybdenum was approximately 250,000 yuan per ton. This translates to approximately 410,000 yuan per ton (410 yuan per kilogram), copper at approximately 70,000 yuan per ton (70 yuan per kilogram), and the price of rare earth magnesium, a spheroidizing agent, at around 10 yuan per kilogram. Based on a conventional calculation of 0.6% Mo and Cu per ton of casting, each ton of ductile iron requires 420 yuan of Cu and 2,400 yuan of molybdenum, resulting in an additional 2,820 yuan in casting costs per ton.

[0010] Furthermore, the current production process for ductile iron parts requires, in addition to casting, heat treatment steps such as quenching or normalizing, or the use of a rapid supercooling casting process during casting to achieve the tensile strength and toughness requirements of ductile iron products. Rapid supercooling casting processes and equipment are complex, resulting in high casting costs and the potential for defects such as deformation or cracking in the castings. Heat treatment, on the other hand, incurs additional costs of several hundred yuan per ton, consumes additional energy, and requires waste disposal after heat treatment. Therefore, these methods increase product costs and waste energy.

[0011] Summary of the Invention

[0012] The technical problem to be solved by the present invention is to provide a low-cost ductile iron which can achieve high strength and high toughness in the cast state without undergoing heat treatment such as quenching or normalizing.

[0013] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0014] The invention relates to a high-carbon, as-cast, high-strength and high-toughness ductile iron. The carbon content of the ductile iron is ≥4.0%. The ductile iron contains no or little precious metal strengthening alloys. When the molten iron is spheroidized after being taken out of the furnace, an inhibitor for controlling the microstructure of the ductile iron and a sufficient amount of spheroidizing agent are added to the spheroidizing bag. The mechanical properties of the ductile iron material in the as-cast state reach QT700 or above without rapid cooling casting or heat treatment of the casting.

[0015] The further improvement of the present invention is that the carbon content of the ductile iron is 4.05%-6.0%; and the inhibitor is a mixture of the spheroidizing agent and a weak carbide-forming element.

[0016] A further improvement of the technical solution of the present invention is that the spheroidizer is a magnesium-containing alloy spheroidizer, the inhibitor is a spheroidizer and metallic tin, or a spheroidizer and metallic antimony, and the amount of the spheroidizer and metallic tin or metallic antimony added should ensure that a small amount of MgO, Sn or Sb remains in the casting after pouring is completed.

[0017] A further improvement of the above technical solution of the present invention is that: the remaining small amount of MgO, Sn or Sb refers to a residual amount of magnesium oxide of not less than 0.03%, a residual amount of tin of not less than 0.02%, or a residual amount of antimony of not less than 0.03%; and the total amount of spheroidizing agent added is greater than 1.6% of the weight of the molten iron.

[0018] The above technical solution of the present invention is further optimized in that: the spheroidizer is a rare earth magnesium spheroidizer; when rare earth magnesium and tin are used as inhibitors, the amount of inhibitor added is such that the residual amount of magnesium oxide in the solidified casting is between 0.03% and 0.065%, and the residual amount of tin is between 0.02% and 0.06%; when rare earth magnesium and antimony are used as inhibitors, the amount of inhibitor added is such that the residual amount of magnesium oxide in the solidified casting is between 0.03% and 0.065%, and the residual amount of antimony is between 0.03% and 0.05%.

[0019] A further improvement of the technical solution of the present invention is that: when the molten iron is spheroidized after being discharged from the furnace, a portion of the recarburizer is added to the spheroidizing bag; when the molten iron is injected into the spheroidizing bag, the recarburizer placed in the spheroidizing bag is first impacted, and then reacted with the spheroidizing agent to perform spheroidization treatment while further boiling and stirring the recarburizer in the spheroidizing bag; the melting temperature and the discharge temperature of the molten iron are not lower than 1550°C.

[0020] A further improvement of the above technical solution of the present invention is that the recarburizer is added to the spheroidizing bag by separating a portion of the recarburizer as the total carbon content of the casting and adding it to the spheroidizing bag, and then dissolving it into the molten iron when it is discharged from the furnace. The recarburizer added to the spheroidizing bag provides the casting with a carbon content of no more than 2.5%, and the remaining carbon content of the casting is provided by the recarburizer and other smelting raw materials in the smelting furnace.

[0021] A further improvement of the above technical solution of the present invention is that: the recarburizer added during smelting in the smelting furnace is placed together with the scrap steel or recycled material in the smelting furnace, and then smelting is started until all the required materials are melted; the recarburizer placed in the spheroidizing bag provides the casting with a carbon content of 0.8%-2.5%; the inoculant, spheroidizing agent, inhibitor, covering agent, and slag agglomerating agent are placed in the spheroidizing pits of the spheroidizing bag, and the time when the spheroidizing agent starts to detonate and the boiling spheroidization time after detonation is controlled; the temperature of the molten iron when it is discharged from the furnace and rushed into the spheroidizing bag is 1550-1620°C.

[0022] A further improvement of the above technical solution of the present invention is that: according to the weight of the molten iron to be spheroidized in the spheroidizing bag and the performance requirements of the ductile iron, one, two or more spheroidizing pits are set in the spheroidizing bag, and the time interval from the injection of the molten iron to the detonation of the spheroidizing agent, as well as the detonation sequence, detonation time interval and spheroidization boiling reaction time of each spheroidizing pit are controlled.

[0023] A further improvement of the present invention is that: the precious metal strengthening alloy is copper, molybdenum or other strong carbide-forming elements; the non-precious metal strengthening alloy means that the element content is not higher than the allowable content as an impurity; the low-precious metal strengthening alloy means that the addition amount of each precious metal strengthening alloy does not exceed 0.3% of the weight of the molten iron.

[0024] Due to the adoption of the above technical solution, the technical advancements achieved by the present invention are:

[0025] The technical solution of the present invention allows for the production of ductile iron products at low casting costs. Only inexpensive carbon is used to improve the comprehensive mechanical properties of the ductile iron, with no or minimal addition of precious metals. Complex casting equipment and processes, such as supercooling, are not required. Furthermore, high-strength, high-toughness ductile iron products can be obtained in the as-cast state without undergoing heat treatments such as quenching or normalizing, eliminating the need for waste disposal and being environmentally friendly. The ductile iron of the present invention is suitable for all casting methods, and when using specialized casting methods such as rapid cooling, the ductile iron material exhibits even higher mechanical properties, such as tensile strength.

[0026] The carbon content of the ductile iron of the present invention can easily exceed 4.0%, breaking the conventional wisdom in the field of ductile iron casting that carbon content cannot be too high. Because cast iron inevitably contains Si and other carbon-equivalent elements, the carbon equivalent of the ductile iron of the present invention is generally above 4.3%, exceeding the carbon content at the eutectic point in the iron-carbon phase diagram. The present invention broadens the carbon content range of ductile iron, breaking the conventional wisdom that ultra-high carbon is detrimental to the overall performance of ductile iron. By using a bold carbon addition method, the carbon content of the ductile iron is significantly increased. Furthermore, by controlling the structure and morphology of carbon in the matrix, as well as the shape, quantity, and distribution of free graphite, the present invention enables the production of ductile iron castings that exhibit high strength and toughness in the as-cast state, at low cost, and at low cost. The combination of chemical parameter adjustments and technological advancements in processing methods overcomes the drawbacks of graphite in conventional ductile iron production.

[0027] The ductile iron of the present invention uses only cheap and abundant C as the main strengthening element of the ductile iron, and does not contain or actively add precious metal strengthening alloys such as Cu and Mo. The so-called "no precious metal strengthening alloy" means that precious metal strengthening alloy elements are not actively added, but may be impurities brought into the smelting raw materials of the ductile iron. These are unavoidable allowed contents and should not affect the performance of the ductile iron. They are not calculated as the alloy amount of the ductile iron, and the content of these alloys is not higher than the allowed content as impurities. The content of elements that appear as impurities in the ductile iron cannot be higher than a certain value. For example, titanium Ti has a greater interfering effect on the ball formation of graphite in ductile iron. When the Ti content is too high, it will interfere with the ballization quality, thereby reducing the toughness of the material. Therefore, the Ti content in the ductile iron of the present invention must be strictly controlled. The present invention adds metals such as tin and antimony and a sufficient amount of nodulizer as inhibitors in the nodulizing bag, so that an ultra-high amount of C in the molten iron can be completely dissolved into the molten iron, and graphite floating phenomenon is not generated. The shape, quantity and distribution of the graphite nodules can be reasonably controlled, the shape of the cementite and the quantity of the pearlite in the matrix can be controlled, and the lamellae of the cementite and ferrite constituting the pearlite can be made thin and uniform, thereby greatly improving the tensile strength and toughness of the ductile iron.

[0028] The present invention adds a recarburizer and an inhibitor to the spheroidizing ladle, dissolving them into the molten iron as it emerges from the furnace for spheroidization. This utilizes a large amount of carbon to strengthen the ductile iron, thereby producing high-strength and high-toughness ductile iron. The ductile iron of the present invention, without the addition of precious metals or subsequent heat treatment, can achieve a tensile strength of at least 600 MPa in the as-cast product. Furthermore, with very high tensile strength, it can achieve an elongation of at least 4.0%. For example, when the tensile strength of the ductile iron product reaches 850 MPa, the elongation can reach 6.5% or higher. The carbon content of the ductile iron of the present invention can be designed to be within a range of 4.05% or higher, depending on the mechanical performance requirements of the ductile iron product. Within the conventional range of 4.5%-6.0% carbon content, a ductile iron product with a QT of 800 or higher can be obtained in the as-cast state without the addition of precious metal strengthening alloys or heat treatment. The ductile iron of the present invention saves precious metal alloys such as molybdenum and copper, eliminates the need for expensive supercooling casting equipment, and eliminates heat treatment processes such as quenching and normalizing, significantly reducing casting costs and the price of ductile iron products. Based on current market prices for molybdenum, copper, and tin (tin is 200 yuan per kilogram, rare earth magnesium is about 10 yuan per kilogram, and the price of carburizer is very cheap and can be ignored), the raw material cost per ton of casting can be reduced by 2,600 yuan. If the saved heat treatment costs are added, the production cost of ductile iron castings can be reduced by 3,000 yuan per ton.

[0029] The present invention adds a recarburizer to a spheroidizing bag and uses a sufficient amount of high-temperature molten iron to dissolve the recarburizer into the molten iron in the spheroidizing bag. When the molten iron begins to be injected into the spheroidizing bag, the molten iron directly impacts the recarburizer for a period of time before the spheroidizing agent detonates. At this time, the molten iron strongly impacts and stirs the recarburizer, which can promote the dissolution of the recarburizer into the molten iron. After the spheroidizing agent in the pit of the spheroidizing bag detonates and the spheroidization reaction begins, the magnesium oxide produced by the spheroidizing agent causes the molten iron to boil. During the spheroidization process, the molten iron and the recarburizer are further stirred and mixed, so that the recarburizer in the spheroidizing bag is further dissolved into the molten iron. The carbon dissolved into the molten iron in the spheroidizing bag also causes lattice distortion in the crystal structure of the ductile iron matrix, achieving a solid solution strengthening effect, and further increasing the mechanical strength of the casting in the cast state.

[0030] The conventional method for recarburizing ductile iron of the present invention is to divide a recarburizer into two parts and dissolve them into the molten iron. One part is placed in a smelting furnace and added during smelting of the molten iron, and the other part is placed in a spheroidizing ladle and dissolved during spheroidizing. The amount of recarburizer added to the spheroidizing ladle is preferably sufficient to contain 0.8% to 2.5% of the carbon content of the casting. The remaining carbon content of the casting is provided by the recarburizer, scrap steel, recycled materials, and other smelting raw materials in the smelting furnace. This method is more conducive to the carbon in the recarburizer dissolving into the molten iron, forming a fine pearlite matrix and high-grade spheroidal graphite. The method of recarburizing in the smelting furnace and the spheroidizing ladle of the present invention can be called a combined recarburizing method. When a larger amount of recarburizer is added to the spheroidizing ladle, better results can be achieved.

[0031] The composite carburizing method is to divide the carburizer into two parts: one added during smelting in the smelting furnace and the other added during spheroidizing in the spheroidizing bag. The carburizer used for smelting in the furnace is placed in the smelting furnace together with scrap steel or recycled materials. The carburizer can be placed at the bottom of the smelting furnace together with the recycled materials and other smelting raw materials, or part of the carburizer can be placed in the gap between the scrap steel or recycled materials, and then smelting is started until all the materials to be added are melted, so that a part of the large amount of carbon can be fully dissolved in the molten iron in advance during smelting. The remaining carbon content is provided by the recarburizer placed in the spheroidizing bag. When the hot molten iron is discharged from the furnace, a sufficient amount of hot molten iron is directly impacted with the recarburizer placed in the spheroidizing bag. Furthermore, through the reaction of the inoculant, spheroidizing agent, inhibitor, covering agent, slag agglomerating agent, etc. placed in the spheroidizing pit of the spheroidizing bag with the molten iron, and by controlling the time when the spheroidizing agent starts to detonate, the boiling spheroidization time after detonation, and the detonation sequence, the carbon in the recarburizer in the spheroidizing bag is fully dissolved in the molten iron, further increasing the carbon content of the molten iron. In this way, the carbon content of the molten iron after spheroidization can be made very high, easily exceeding 4.0%.

[0032] The present invention provides two or more spheroidizing pits within a spheroidizing bag for storing materials such as inoculants, nodulizers, inhibitors, covering agents, and slag agglomerants, thereby subjecting the molten iron injected into the spheroidizing bag to a multi-stage spheroidizing treatment. The detonation time, detonation sequence, detonation interval, and boiling reaction time are controlled based on the weight of the molten iron to be spheroidized within the spheroidizing bag and the performance requirements of the ductile iron. This increases and controls the spheroidizing reaction time and boiling intensity of the magnesium oxide used to boil the molten iron, allowing the molten iron to be smoothly dissolved even when a large amount of recarburizer is used within the spheroidizing bag. When the material and mechanical properties of the ductile iron product are higher, multiple pits are generally required for multi-stage treatment, especially when high toughness requirements such as elongation and reduction of area are required. Of course, when the amount of recarburizer used within the spheroidizing bag is relatively small, using a single or continuous set of spheroidizing pits for spheroidization can also achieve the desired effect.

[0033] The inhibitors used in the present invention are a spheroidizer and metallic tin, or a spheroidizer and metallic antimony. Spheroidizers used in ductile iron casting are mostly magnesium-containing alloy spheroidizers. For example, rare earth magnesium spheroidizers with varying magnesium contents can all be used as the spheroidizers of the present invention. The spheroidizers of the present invention are added in sufficient quantities, meaning the total amount of spheroidizer used as an inhibitor and for spheroidization is greater than the amount used in conventional ductile iron production, typically requiring at least 1.6% of the molten iron weight. Conventional ductile iron production typically requires no more than 1.4% spheroidizer. The amounts of the various inhibitor components added must ensure that a small amount of the inhibitor elements remain in the casting after casting. This ensures that carbon fully dissolves in the molten iron, resulting in a good matrix microstructure and pearlite content, and high-grade graphite nodules.

[0034] The spheroidizer and inhibitor used in the present invention both utilize the rare earth magnesium spheroidizer commonly used in ductile iron casting. In addition to its spheroidizing function, magnesium is also a weak carbide-forming element in the present invention. While the carbides it forms are not very hard or strong, they can increase the formation and amount of pearlite and help organize the cementite and ferrite within the pearlite. Tin or antimony, also weak carbide-forming elements, serve the same function as magnesium in ductile iron, controlling the amount and microstructure of pearlite and graphite nodules. The amounts of rare earth magnesium and tin added are such that the residual tin content in the solidified casting is no less than 0.02%, preferably between 0.02% and 0.06%. The residual magnesium oxide content is no less than 0.03%, preferably between 0.03% and 0.065%. When using rare earth magnesium and antimony as inhibitors, the amounts of rare earth magnesium and antimony added are such that the residual antimony in the solidified casting is no less than 0.03%, preferably between 0.03% and 0.05%. The residual magnesium oxide content is no less than 0.03%, preferably between 0.03% and 0.065%. This ensures that sufficient carbon is fully dissolved in the molten iron, forming a high-strength, high-toughness structure without wasting raw materials.

[0035] To further enhance the strength of ductile iron, the present invention allows the addition of small amounts of precious metal strengthening alloys such as Cu and Mo during the casting process. However, excessive amounts of alloying elements such as Ti should not be added or introduced into the ductile iron, as Ti can affect graphite spheroidization and significantly reduce its toughness. The amount of each precious metal strengthening alloy added should not exceed 0.3% of the weight of the molten iron, thereby minimizing the cost of the raw materials used for casting. Based on the maximum amount of 0.3% and current market prices, the added raw material cost (using Cu and Mo as an example) is 1,473 yuan. This ductile iron material achieves tensile strength exceeding 960 MPa and elongation exceeding 4% in the as-cast state, making it suitable for special applications. Conventional ductile iron, without the addition of precious alloys, can achieve the desired performance in the as-cast state by simply increasing the carbon content of the ductile iron without heat treatment or supercooling to improve mechanical properties. The production cost of the ductile iron product of the present invention is significantly lower than that of conventional ductile iron with comparable mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG1 is a metallographic photograph (X100) of the graphite structure of the casting obtained in Example 1 of the present invention;

[0037] FIG2 is a metallographic photograph (X400) of the matrix structure of the casting obtained in Example 1 of the present invention;

[0038] FIG3 is a metallographic photograph of the graphite structure of the casting obtained in Example 2 of the present invention;

[0039] FIG4 is a metallographic photograph of the matrix structure of the casting obtained in Example 2 of the present invention. DETAILED DESCRIPTION

[0040] The present invention discloses a high-carbon, as-cast, high-strength, high-toughness ductile iron (ductile iron) with a carbon content of 4.0% or greater. The ductile iron contains no precious metal reinforcement alloys. This refers to the elemental content being no higher than the permitted impurity level. Even if precious metal reinforcement alloys are detected during chemical element analysis of the ductile iron, they are present as impurities, likely introduced from the smelting raw materials. The carbon content of the ductile iron commonly used in the present invention can be designed to be between 4.05% and 6.0%, meeting the performance requirements of all ductile iron products. A sufficient amount of spheroidizing agent and inhibitor is added to the spheroidizing bag during spheroidization of the molten iron. The inhibitor is a mixture of a spheroidizing agent and a weak carbide-forming element, such as a spheroidizing agent and metallic tin, or a spheroidizing agent and metallic antimony. The spheroidizing agent is a magnesium-containing alloy, such as a rare earth magnesium spheroidizing agent with any magnesium content. The inhibitor can control the microstructure of pearlite and graphite in the cast iron. The total amount of nodulizer used as an inhibitor and spheroidizer for spheroidization should be greater than the amount used in typical ductile iron production. The amount of nodulizer and metallic tin or antimony should ensure that a small amount remains in the casting after pouring. The total amount of rare earth magnesium used as an inhibitor and spheroidizer should be greater than 1.6% of the weight of the molten iron. The amount of inhibitor and spheroidizer added must be sufficient, generally controlled to the following residual amounts: residual magnesium oxide should be no less than 0.03%, residual tin should be no less than 0.02%, or, when using antimony, a residual of no less than 0.03%. For example, when using rare earth magnesium and tin as inhibitors, the best amount of inhibitor added is such that the residual tin content in the solidified casting is between 0.02% and 0.06%, and the residual magnesia content is between 0.03% and 0.065%. When using antimony and rare earth magnesium as inhibitors, the best amount of inhibitor added is such that the residual antimony content in the solidified casting is between 0.03% and 0.05%, and the residual magnesia content is between 0.03% and 0.065%. Too high an amount will result in waste, and too high or too low an amount will also affect the microstructure and performance of the ductile iron.

[0041] The present invention also adds a recarburizer to the spheroidizing ladle when the molten iron is discharged from the furnace for spheroidization. When the discharged molten iron is injected into the spheroidizing ladle, the molten iron first impacts the recarburizer placed in the spheroidizing ladle, and then reacts with the spheroidizing agent in the recesses to perform a spheroidization process. The magnesium oxide generated by the reaction of the spheroidizing agent with the molten iron further boils and stirs the recarburizer in the spheroidizing ladle. The smelting temperature and the discharge temperature of the molten iron are generally higher than those of conventional ductile iron smelting methods, and can be controlled to be no less than 1550°C. When the carbon content is high, the smelting temperature and the discharge temperature should be appropriately increased according to the situation.

[0042] The method of adding a recarburizer to a spheroidizing bag according to the present invention is to add the recarburizer in two parts: during smelting in a smelting furnace and in the spheroidizing bag. The recarburizer is added to the spheroidizing bag, wherein a portion of the recarburizer, which is used to make up the total carbon content of the casting, is added to the spheroidizing bag and then dissolved into the molten iron when the casting is discharged from the furnace. The recarburizer added to the spheroidizing bag provides the casting with a carbon content of no more than 2.5%, and the optimal range is controlled to be within the range of 0.8%-2.5%. That is, the recarburizer added to the spheroidizing bag can increase the carbon content of the molten iron by 0.8%-2.5%. The remaining carbon content of the casting is provided by the recarburizer and other smelting raw materials (such as scrap steel, recycled materials, etc.) in the smelting furnace. The recarburizer and scrap steel or return charge put into the smelting furnace are put into the smelting furnace together. Generally, part or all of the recarburizer and part of the return charge are put together at the bottom of the smelting furnace, and then the scrap steel is added, and finally the return charge is added until the furnace is full. When the amount of recarburizer added is large, the part of the recarburizer that needs to be put into the furnace can be added into the gap between the scrap steel; then start smelting, and as the charge melts, the remaining charge is continuously added into the smelting furnace until all the materials that need to be added are melted; and the recarburizer put into the spheroidizing bag is dissolved with the molten iron liquid. The present invention places an inoculant, a nodulizer, an inhibitor, a covering agent, and a slag agglomerating agent into the nodulizing pits of a nodulizing bag. Upon contact, the molten iron reacts with the nodulizer to produce magnesium oxide. The time for the nodulizer to start detonating and the boiling and spheroidizing time after detonation are controlled. Before being discharged from the furnace, the nodulizing bag is heated and a desired amount of recarburizer is added. The discharged molten iron is then used to directly impact the recarburizer at the bottom of the bag. After the molten iron reacts with the nodulizer, the magnesium oxide produced by the rare earth magnesium stirs the molten iron and the recarburizer. The temperature of the molten iron as it is discharged from the furnace and enters the nodulizing bag is generally 1550-1620°C, slightly higher than the discharge temperature of conventional ductile iron.

[0043] The present invention is to make the carbon content in the ductile iron exceed the carbon content of conventional ductile iron. And use scrap steel or ductile iron recycled materials that are convenient to control carbon content and S and P content as main raw materials, then try to utilize carburizer and carburizing method to make enough C be added in the ductile iron, so that the ductile iron product reaches the mechanical property requirements of design under cast condition. There are many types of carburizers in the foundry industry. For example, patent document CN106906333B discloses that one or more mixtures of pure graphite, graphite powder or graphite electrode in any proportion can be used as carburizers. In addition, materials containing carbon or graphite, such as coal, charcoal, coke, coke slag, etc. can be used as carburizers, and commercially available tar carburizers are also commonly used carburizers in the foundry industry. The C content in different carburizers varies greatly, and the absorption capacity of molten iron to the different carburizers used is also different. Therefore, when using different types of carburizers, the added weight of carburizer is also different. In the actual ductile iron production process, the weight of different types of recarburizers should be calculated based on the carbon content of different smelting raw materials of ductile iron and the carbon content and absorption amount of the recarburizer used, so that the casting products can finally meet the carbon content requirements.

[0044] To increase the duration of the spheroidization reaction and boiling agitation, two or more recesses can be provided within the spheroidizing ladle. Based on the weight of the molten iron to be spheroidized and the performance requirements of the ductile iron, the time interval from the injection of the molten iron to the start of the spheroidizing agent detonation reaction, the detonation sequence of each spheroidizing recess, the detonation interval between recesses, and the boiling reaction time after detonation are controlled. This not only impacts and agitates the recarburizer with the molten iron, but also the magnesium oxide gas generated by the reaction between the spheroidizing agent and the molten iron, which in turn agitates the molten iron through boiling. This extends the agitation time and ensures more complete contact between the inoculant, spheroidizing agent, and the molten iron, allowing for a large amount of carbon to dissolve in the molten iron, thus facilitating the formation of lamellar pearlite and fine, dispersed graphite nodules.

[0045] If the strength or other mechanical properties of the ductile iron product need to be further improved, a small amount of precious metals may be added to the ductile iron. The precious metal strengthening alloy may be one, two, or more of copper, molybdenum, vanadium, or other strong carbide-forming elements. The amount of each precious metal strengthening alloy added is not more than 0.3% of the weight of the molten iron. The amount of precious metal strengthening alloy added is lower than that added to other ductile irons other than those of the present invention. In particular, titanium should not be added, as this will affect the toughness of the ductile iron.

[0046] The present invention is further described in detail below with reference to the embodiments.

[0047] Example 1

[0048] The molten iron smelting equipment used in this embodiment is a 1.5T induction furnace, which smelts 2T of molten iron in one furnace.

[0049] The main charges used include: 25% recycled materials, totaling 500kg; 71% scrap steel with a carbon content of 0.3%-0.45%, totaling 1420kg; discarded graphite electrodes for carburizer, totaling 87.5kg; 0.5% ferrosilicon (FeSi75), totaling 10kg.

[0050] Distribution of recarburizer: In this embodiment, the recarburizer added to the furnace and the recarburizer added to the spheroidizing bag are distributed in a ratio such that the carbon content of the molten iron in the furnace reaches 3.5% C and the recarburizer placed in the spheroidizing bag increases the carbon content of the molten iron by another 0.9%.

[0051] Charging into the smelting furnace: first add 40kg of recarburizer and about 100kg of small pieces of recycled material to the bottom of the smelting furnace. The recarburizer can be placed at the very bottom or mixed with the small pieces of recycled material and added to the bottom of the furnace; then add scrap steel to fill the furnace, and at the same time add the remaining recarburizer to the gaps between the scrap steel.

[0052] After adding the materials, the furnace is started for smelting. As the materials melt, the remaining 1420 kg of scrap steel is continuously added to the furnace to be melted into molten iron. Finally, the remaining 400 kg of recycled material is added to the furnace for melting. Once all the materials have melted, 10 kg of ferrosilicon (FeSi75) is added to the furnace ten minutes before the materials are removed from the furnace.

[0053] The melting temperature of this embodiment is 1550°C and the furnace temperature is 1610°C.

[0054] The spheroidization process is carried out in a 1.5-ton spheroidization bag, with each bag processing 1 ton of molten iron. 10 kg of waste graphite electrode blocks are added to each spheroidization bag as a recarburizer. This embodiment employs a single-stage spheroidization process, namely, a spheroidization pit is provided within the spheroidization bag. The pit contains an inoculant, spheroidizing agent, inhibitor, covering agent, and slag agglomerating agent. After being compacted, the inoculant, spheroidizing agent, and inhibitor, covering agent, and slag agglomerating agent are placed in the pit. These are then tightly covered with iron filings and iron plates. The recarburizer placed within the spheroidization bag is placed directly at the bottom of the bag. When molten iron is injected, the molten iron directly impacts the recarburizer placed at the bottom of the bag. This allows some of the recarburizer to dissolve into the molten iron due to the impact and agitation of the high-temperature molten iron. After the iron plates covering the pit melt, the molten iron comes into contact with the inoculant, spheroidizing agent, inhibitor, covering agent, and slag agglomerating agent. The spheroidizing agent reacts with the molten iron to produce a large amount of magnesium oxide, further boiling and agitating the molten iron, causing the carbon in the recarburizer to dissolve further.

[0055] The inoculant in the spheroidizing bag in this embodiment is ferrosilicon (FeSi-75), and the amount of FeSi75 added is 0.8% of the weight of the molten iron in the spheroidizing bag, that is, 8 kg is added to each bag.

[0056] The spheroidizing agent used in this embodiment is a rare earth magnesium alloy spheroidizing agent, and the amount of the spheroidizing agent added is 1.7% of the weight of the molten iron in the spheroidizing bag, that is, 17 kg is added to each bag.

[0057] This embodiment also adds metallic tin to the spheroidizing pits of the spheroidizing bag. Together with rare earth magnesium, the metallic tin acts as an inhibitor, controlling the morphology of pearlite and graphite. This increases the pearlite content in the ductile iron matrix and the order of cementite and ferrite within the pearlite, enhancing the strength and toughness of the ductile iron matrix. It also promotes the formation of rounded graphite and the dispersion of graphite nodules. The amount of metallic tin added is 0.04% of the weight of the molten iron in the spheroidizing bag, or 0.4 kg per bag.

[0058] The covering agent, slag-aggregating agent, etc. added to the bag in this embodiment are all commercially available products, and the amount added can be appropriate according to the casting site conditions and experience.

[0059] In this embodiment, the detonation of the nodulizer is controlled as follows: After the molten iron injected into the nodulizer bag is impact-stirred and dissolved with the recarburizer for 30-40 seconds, the iron plate covering the pit is melted, allowing the molten iron to come into contact with the inoculant, nodulizer, inhibitor, covering agent, slag agglomerating agent, and other additional materials in the pit, causing a reaction. The nodulizer reacts with the molten iron to form magnesium oxide gas, causing the molten iron to boil (this phenomenon is generally referred to as detonation in the casting field). In this embodiment, the target time for controlling the reaction between the nodulizer and the molten iron is 40-50 seconds. The actual detonation time in this embodiment is 33 seconds, and the spheroidization reaction time after detonation is 41 seconds. After the molten iron stops boiling, slag removal and other work are carried out to prepare for casting. The pouring temperature is controlled at 1460±10°C.

[0060] This embodiment utilizes the lost foam casting method to cast both the casting and the standard wedge-shaped test blocks. The test blocks are poured approximately 7 minutes after the start of the casting, taking the middle of the casting process. After the test blocks cool, samples are taken from the test blocks for chemical composition testing and processed into tensile test bars for physical and chemical testing. The castings of this embodiment can be turned over and cooled 40 minutes after pouring.

[0061] The chemical composition of the test piece after the furnace test in this embodiment is: 4.33% C, 1.97% Si, 0.31% Mn, 0.034% S, 0.036% P, 0.038% Sn, 0.042% MgO, 0.019% Re, and the rest is Fe and other inevitable impurities.

[0062] The mechanical properties of the test bars formed during the casting process of this example were: tensile strength of 670 MPa and elongation of 11%. These test bars and mechanical properties are as-cast, without any heat treatment such as quenching or normalizing. The metallographic structure of the product obtained in this example was characterized by the predominance of lamellar pearlite, accounting for 68%, and the predominance of graphite, spherical graphite, of grade 2. Photographs of the metallographic structure of the test blocks obtained in this example are shown in Figures 1 and 2.

[0063] In the present invention, since sufficient carbon is used, the opportunity for Fe and C to form carbides is increased. Sn and rare earth magnesium act as inhibitors, allowing more C to form sufficient pearlite (cementite + ferrite). The inhibitors prevent the cementite from forming free blocks, allowing the cementite to primarily form lamellar pearlite, thereby improving the overall mechanical properties of the ductile iron, such as tensile strength and toughness. Furthermore, since the present invention does not use precious metal alloys and does not undergo heat treatment methods such as quenching or normalizing to improve mechanical properties, it does not require the use of rapid cooling casting production equipment, thereby significantly reducing the production cost of ductile iron products.

[0064] Example 2

[0065] The molten iron smelting equipment used in this embodiment is a 1T induction furnace, which smelts 1.2T of molten iron in one furnace.

[0066] The ingredients in the smelting furnace are: 25% recycled materials, totaling 300Kg; 71% scrap steel with a carbon content of 0.3%-0.5%, totaling 852Kg; the recarburizer used is waste carbon slag, with a total addition amount of 53Kg; 0.7% ferrosilicon (FeSi75), totaling 8.4Kg.

[0067] Recarburizer distribution: The recarburizer added to the furnace and the spheroidizing ladle is distributed in such a ratio that the carbon content of the molten iron in the furnace reaches 4.0% and the recarburizer placed in the spheroidizing ladle increases the carbon content of the molten iron by another 1.4%. In this embodiment, 18 kg of recarburizer is added to the spheroidizing ladle.

[0068] The order of charging into the furnace is as follows: ① Add about 100 kg of small pieces of recycled material and 25 kg of recarburizer to the bottom of the furnace; ② Then add scrap steel to fill the furnace, and at the same time add the remaining recarburizer to the gaps between the scrap steel in the furnace; ③ Then open the furnace for smelting, and as the material in the furnace melts, continuously add the remaining scrap steel into the furnace, and then add the remaining 200 kg of recycled material into the furnace for melting; ④ Add 8.4 kg of ferrosilicon 10 minutes before taking out of the furnace.

[0069] The melting temperature of this embodiment is 1580°C and the furnace temperature is 1620°C.

[0070] The spheroidizing process is carried out in a 1.5T spheroidizing bag, and the spheroidizing processing capacity of a single bag is 1.2T of molten iron.

[0071] The spheroidization process in this embodiment adopts a two-stage spheroidization method with bottom-injection spheroidization. Specifically, two storage pits are provided at the bottom of the spheroidization ladle. The materials required for spheroidization (inoculant, spheroidizing agent, inhibitor, covering agent, slag agglomerating agent, etc.) are divided into two portions and placed in the two spheroidization pits, respectively. The materials in the spheroidization pits are then compacted and tightly covered with iron filings or iron plates of varying thicknesses, with the first pit detonating (the first stage) at 30-40 seconds from the start of iron injection. The second pit detonates 10-15 seconds later. The two pits sequentially and continuously spheroidize the molten iron in the spheroidization ladle.

[0072] This example uses ferrosilicon (FeSi75) as the inoculant, with a total addition amount of 0.8%, or 9.6 kg. Rare earth magnesium is used as the spheroidizer, and the inhibitors used are rare earth magnesium and metallic tin (Sn). Rare earth magnesium, as both an inhibitor and a spheroidizer, is added in a total amount of 1.9%, or 22.8 kg, of the molten iron weight. Tin, as an inhibitor, is added in an amount of 0.06%, or 0.72 kg, of the molten iron weight. The inoculant, spheroidizer, and inhibitor are placed in two spheroidizing pits of the spheroidizing bag, based on the design of the detonation time, detonation interval, and reaction time. Appropriate amounts of covering agent and slag agglomerating agent are also added. Before the molten iron is injected into the spheroidizing bag, a recarburizer is placed at the bottom of the bag. During injection, the molten iron is directly impacted with the recarburizer. In this embodiment, the first-stage detonation time is actually 30 seconds, the start time of the second-stage detonation is actually the 43rd second (the time interval between the second-stage detonation and the first-stage detonation is 13 seconds), and the spheroidization reaction time is 59 seconds in total.

[0073] After the molten iron in the spheroidizing ladle stops boiling and reaches pouring conditions, castings begin. Rock wool is used at the ladle spout to prevent slag during pouring or sub-laundry. Wedge-shaped test blocks are poured six minutes after the casting begins. After the blocks cool, chemical test samples are taken from them and processed into as-cast test bars for testing mechanical properties and metallographic test blocks for observing the metallographic structure.

[0074] 35 minutes after the casting is completed, the box is turned over and cooled in the air.

[0075] The chemical composition of the post-furnace test piece of this embodiment is as follows: 5.26% C, 2.1% Si, 0.27% Mn, 0.024% S, 0.031% P, 0.057% Sn, 0.054% MgO, 0.028% Re, and the remainder is iron and other inevitable impurities.

[0076] Tensile testing of the test bars in this example revealed a tensile strength of 842 MPa, an elongation of 6.5%, and a reduction of area of ​​5%. The metallographic structure showed a pearlite content exceeding 99%, with the pearlite primarily being flake-shaped. The graphite grade was 2, with spherical graphite accounting for over 90%. The metallographic structure of the metallographic test blocks obtained in this example is shown in Figures 3 and 4. Both the tensile test bars and metallographic test blocks were in the as-cast state, without undergoing heat treatment such as quenching or normalizing. Their metallographic structure and mechanical properties were as-cast.

[0077] Example 3

[0078] This example is based on Example 2, but further adds 0.3% copper and 0.3% molybdenum to the spheroidizing ladle, based on the total amount of molten iron. The chemical composition of the resulting casting is: 5.4% C, 2.2% Si, 0.34% Mn, 0.025% S, 0.033% P, 0.21% Cr, 0.27% Cu, 0.36% Mo, 0.056% Sn, 0.061% MgO, and 0.029% Re.

[0079] In this example, the addition of strong carbide-forming elements Cu and Mo, along with Cr and Mo introduced into the charge due to lack of rigorous testing, resulted in the formation of massive carbides within the pearlite matrix. This increased the strength of the ductile iron while significantly decreasing its toughness. The resulting test bar exhibited mechanical properties (as-cast): tensile strength of 961 MPa, elongation of 4.2%, and a matrix composition of 99% pearlite.

[0080] The following Examples 4-9 illustrate only the main technical solutions and features of the present invention. Examples 4-9 all used rare earth magnesium 3-8 as a spheroidizer. The main differences from Examples 1 and 2 are: the amount of recarburizer added varied; the total amount of spheroidizer added, the amount of tin or antimony added as an inhibitor, and the amount of addition varied between Examples; the number of spheroidization stages (the number of spheroidization pits), the start time of spheroidizer detonation, the detonation intervals between different spheroidization pits, and the total detonation reaction time varied; and the resulting ductile iron castings differed in carbon content, other major chemical components, residual inhibitor content (magnesium oxide, rare earth, Sn, or Sb), and mechanical properties.

[0081] Example 4

[0082] The molten iron smelting equipment and main materials used in this embodiment are the same as those in Examples 1 and 2. The difference from Examples 1 and 2 is that the amount of recarburizer added to the smelting furnace in this embodiment is required to achieve a carbon content of 3.2% for the molten iron in the smelting furnace. That is, the amount of recarburizer added, the carbon content of the scrap steel, and the carbon content of the recycled material together bring the carbon content of the molten iron in the smelting furnace to 3.2%. The recarburizer placed in the spheroidizing bag increases the carbon content of the molten iron by an additional 0.8%. The recarburizer added to the smelting furnace is mixed with the scrap steel and recycled material and placed in the smelting furnace. The recarburizer is placed in the gaps between the scrap steel and recycled material. Smelting then begins. As the material melts, the remaining scrap steel and recycled material are continuously added to the smelting furnace until all the materials required for the smelting furnace are melted.

[0083] The inhibitors are metallic tin and rare earth magnesium. The total amount of rare earth magnesium (one of the spheroidizer and inhibitor) added is 1.65% of the weight of the molten iron in the spheroidizing bag, and the amount of metallic tin added is 0.025% of the weight of the molten iron in the spheroidizing bag, that is, 0.25 kg of Sn is added to every ton of molten iron.

[0084] This example uses a single-stage spheroidization process, placing the inoculant, spheroidizing agent, inhibitor, slag agglomerating agent, and covering agent into a single spheroidization pit. The time from injection of the molten iron to detonation of the spheroidizing agent is 34 seconds, and the spheroidization reaction after detonation lasts 39 seconds.

[0085] The chemical composition of the test piece obtained in this example is: 4.05% C, 1.96% Si, 0.35% Mn, 0.025% S, 0.026% P, 0.020% Sn, 0.030% MgO, 0.018% Re, with the remainder being Fe and other unavoidable impurities. The ductile iron product obtained in this example has a tensile strength of 703 MPa, an elongation of 10.8%, and a pearlite content of 62% in the matrix.

[0086] Example 5

[0087] This embodiment differs from Example 4 in that, in accordance with the requirement that the carbon content of the molten iron reach 4.2% by combining the in-furnace recarburizer and the smelting raw materials, a recarburizer is added to the smelting furnace; the recarburizer placed in the spheroidizing ladle further increases the carbon content of the molten iron by 0.8%. The smelting temperature in this embodiment is 1570°C, and the discharge temperature is 1550°C.

[0088] The inhibitors are metallic tin and rare earth magnesium. The total amount of spheroidizing agent added is 1.9% of the weight of the molten iron in the spheroidizing bag, and the amount of Sn added is 0.06% of the weight of the molten iron, that is, 0.6 kg of tin is added to every ton of molten iron.

[0089] This example uses a two-stage spheroidization process, distributing the inoculant, spheroidizing agent, inhibitor, slag agglomerating agent, and covering agent into two spheroidization pits. In this example, the time from injection of molten iron to the first-stage detonation of the spheroidizing agent is 37 seconds, with the interval between the second and first detonations being 22 seconds. The total spheroidization reaction time from the first detonation is 60 seconds.

[0090] The chemical composition of the test piece obtained in this example is: 4.95% C, 2.07% Si, 0.32% Mn, 0.024% S, 0.031% P, 0.053% Sn, 0.051% MgO, 0.031% Re. The remaining elements are Fe and other unavoidable impurities. The ductile iron product obtained in this example has a tensile strength of 798 MPa, an elongation of 7.2%, and a pearlite content of 99% in the matrix.

[0091] Example 6

[0092] This embodiment differs from Example 4 in that a recarburizer is added to the smelting furnace to bring the carbon content of the molten iron to 3.5%. An additional 1.0% carbon is added to the nodulizing bag. Metallic Sb and rare earth magnesium are used as inhibitors. The total amount of rare earth magnesium nodulizer added is 1.7% of the weight of the molten iron, and the amount of antimony added is 0.06%, meaning 0.6 kg of Sb is added per ton of molten iron. The smelting temperature in this embodiment is 1560°C, and the discharge temperature is 1580°C.

[0093] This embodiment uses a two-stage detonation spheroidization process, with the inoculant, spheroidizing agent, inhibitor, slag agglomerating agent, and covering agent placed separately in two spheroidization pits. In this embodiment, the time from injection of the molten iron to the first-stage detonation of the spheroidizing agent is 28 seconds, with a 25-second interval between the second and first detonations. The total spheroidization reaction time after the first-stage detonation is 53 seconds.

[0094] The chemical composition of the test block obtained in this example is: 4.48% C, 2.0% Si, 0.29% Mn, 0.027% S, 0.034% P, 0.042% Sb, 0.046% MgO, 0.022% Re. The remaining chemical components are Fe and other unavoidable impurities. The mechanical properties of the ductile iron product obtained in this example are: tensile strength of 735 MPa, elongation of 8.7%, and a pearlite content of 90% in the matrix.

[0095] Example 7

[0096] The difference between this embodiment and embodiment 4 is that the amount of recarburizer added to the smelting furnace in this embodiment is such that the sum of the carbon content of the recarburizer and the carbon content of the smelting raw materials reaches a carbon content of 4.3% in the molten iron in the smelting furnace, and the weight of the recarburizer placed in the spheroidizing bag is such that the carbon content of the molten iron is further increased by 1.3%.

[0097] The inhibitors are metallic tin and rare earth magnesium. The total amount of rare earth magnesium added is 1.9% of the weight of the molten iron, and the amount of Sn added is 0.07% of the weight of the molten iron in the spheroidizing bag, that is, 0.7 kg of tin is added to every ton of molten iron.

[0098] This embodiment adopts two-stage spheroidization treatment. The time from the injection of molten iron to the first-stage detonation of the spheroidizer is 31 seconds. The time interval between the second-stage detonation and the first-stage detonation is 23 seconds. The total spheroidization reaction time after the first-stage detonation is 63 seconds.

[0099] The chemical composition of the test piece obtained in this example is: 5.5% C, 2.04% Si, 0.33% Mn, 0.026% S, 0.035% P, 0.060% Sn, 0.062% MgO, 0.029% Re, with the remainder being Fe and other unavoidable impurities. The ductile iron product obtained in this example has a tensile strength of 872 MPa, an elongation of 4.1%, and a matrix pearlite content of 99%.

[0100] Example 8

[0101] This embodiment differs from the previous embodiment in that a recarburizer is added to the smelting furnace at a ratio calculated to bring the carbon content of the molten iron in the smelting furnace to 4.5%, and a recarburizer is added to the spheroidizing ladle at a ratio calculated to increase the carbon content of the molten iron in the spheroidizing ladle by another 1.7%. The smelting temperature in this embodiment is 1590°C, and the tapping temperature is 1600°C.

[0102] The inhibitors are Sn and rare earth magnesium. The total amount of rare earth magnesium added is 2.1% of the weight of the molten iron, and the amount of tin added is 0.9% of the weight of the molten iron in the spheroidizing bag, that is, 0.9 kg of tin is added to every ton of molten iron.

[0103] This embodiment uses a three-stage spheroidization process, distributing the inoculant, spheroidizing agent, inhibitor, slag agglomerating agent, and covering agent into three spheroidization pits. The detonation and spheroidization reaction times for this embodiment are as follows: the first-stage detonation time is 35 seconds, the interval between the second and first-stage detonations is 21 seconds, and the interval between the third and second-stage detonations is 18 seconds. The total spheroidization reaction time after the first-stage detonation is 81 seconds.

[0104] The chemical composition of the test piece of this example is: 6.0% C, 1.99% Si, 0.34% Mn, 0.030% S, 0.033% P, 0.083% Sn, 0.067% MgO, 0.032% Re. The remaining chemical components are Fe and other unavoidable impurities. The ductile iron product obtained in this example has a tensile strength of 907 MPa, an elongation of 4.3%, and a pearlite content in the matrix of 99%.

[0105] Example 9

[0106] This embodiment differs from the previous embodiment in that the recarburizers added to the furnace and the spheroidizing ladle are distributed in such a manner that the carbon content of the molten iron reaches 4.1% with the recarburizer and smelting raw materials, and the carbon content of the molten iron is increased by an additional 2.5% with the recarburizer placed in the spheroidizing ladle. The smelting temperature in this embodiment is 1580°C, and the discharge temperature is 1620°C.

[0107] The inhibitors are metallic antimony and rare earth magnesium. The total amount of rare earth magnesium added as the spheroidizing agent is 2.0% of the weight of the molten iron in the spheroidizing bag, and the amount of metallic antimony added is 0.06% of the weight of the molten iron, that is, 0.6 kg of antimony is added to every ton of molten iron.

[0108] This embodiment uses a three-stage spheroidization process, distributing the inoculant, spheroidizing agent, inhibitor, slag agglomerating agent, and covering agent into three spheroidization pits. The detonation and spheroidization reaction times for this embodiment are: the first-stage detonation time is 32 seconds, the interval between the second and first detonations is 24 seconds, and the interval between the third and second detonations is 25 seconds. The total spheroidization reaction time after detonation is 88 seconds.

[0109] The chemical composition of the test block obtained in this example is: 6.2% C, 1.48% Si, 0.27% Mn, 0.029% S, 0.036% P, 0.05% Sb, 0.065% MgO, 0.037% Re. Other chemical elements include Fe and other unavoidable impurities. The resulting ductile iron product has a tensile strength of 1012 MPa, an elongation of 4.0%, and a matrix pearlite content of 99%. The ductile iron product of this example is primarily used in friction pair products.

[0110] Example 10

[0111] This embodiment differs from Example 4 in that a recarburizer is added to the smelting furnace to ensure that the carbon content of the molten iron reaches 4.0% based on the recarburizer and smelting raw materials. No recarburizer is placed in the spheroidizing ladle. The smelting temperature in this embodiment is 1590°C, and the discharge temperature is 1550°C.

[0112] The inhibitors are metallic antimony and rare earth magnesium. The total amount of the spheroidizing agent rare earth magnesium added is 1.6% of the weight of the molten iron in the spheroidizing bag, and the amount of metallic antimony added is 0.04% of the weight of the molten iron.

[0113] This embodiment adopts a one-stage spheroidization process, and controls the time interval from the injection of molten iron to the detonation of the spheroidizing agent to be 25 seconds, and the spheroidization reaction time after detonation is 47 seconds.

[0114] The chemical composition of the test piece obtained in this example is: 4.0% C, 2.08% Si, 0.32% Mn, 0.024% S, 0.025% P, 0.03% Sb, 0.035% MgO, 0.021% Re. The remaining chemical elements are Fe and other unavoidable impurities. The ductile iron product obtained in this example has a tensile strength of 583 MPa, an elongation of 12%, and a pearlite content of 60% in the matrix. As can be seen from Examples 1-10, as the material requirements for ductile iron (e.g., tensile strength, toughness), the carbon content of the ductile iron product tends to increase, and the amount of inhibitors (spheroidizing agents such as rare earth magnesium and Sn or Sb) added accordingly tends to increase. Furthermore, the tapping temperature or smelting temperature of the molten iron should be increased accordingly (the smelting temperature should be controlled to ensure that the recarburizer added to the smelting furnace is completely dissolved into the molten iron, generally controlled to be no less than 1550°C, and the tapping temperature should also be increased to no less than 1550°C, generally controlled to be above 1600°C), or the smelting time and the duration of impact stirring and boiling stirring should be extended accordingly. When the carbon content of the ductile iron product of the present invention is high, while the tensile strength is increased, the toughness is reduced. The present invention uses Sn, Sb, and rare earth magnesium as carbon inhibitors, which can achieve a pearlite content of over 99% in the ductile iron matrix in the as-cast state, with the majority being lamellar pearlite. Without the addition of Cu, Mo, or other strong carbide-forming elements, the process and inhibitors of the present invention rarely produce network or large carbides. Consequently, the tensile strength of ductile iron products is increased while also achieving high toughness. Furthermore, because the ductile iron of the present invention does not require the use of supercooled casting equipment or quenching or normalizing heat treatment, it can achieve the aforementioned high strength and toughness in the as-cast state, significantly reducing the cost of producing ductile iron products.

Claims

1. A high-carbon-content, high-strength, high-toughness as-cast ductile iron, wherein the carbon content is ≥4.0%, characterized in that: Ductile iron contains no or little precious metal strengthening alloys. When the molten iron is spheroidized, inhibitors for controlling the morphology of the ductile iron and sufficient spheroidizing agents are added to the spheroidizing bag. The mechanical properties of the ductile iron material in the cast state reach above QT700 without rapid cooling casting or heat treatment of the casting.

2. The high-carbon-content, high-strength, high-toughness as-cast ductile iron according to claim 1, characterized in that: The carbon content of ductile iron is 4.05%-6.0%; the inhibitor is a mixture of the spheroidizer and weak carbide forming elements.

3. A high-carbon-content, high-strength, high-toughness as-cast ductile iron according to any one of claims 1-2, characterized in that: The spheroidizer is a magnesium-containing alloy spheroidizer, the inhibitor is a spheroidizer and metal tin, or a spheroidizer and metal antimony, and the amount of the spheroidizer and metal tin or metal antimony added should ensure that a small amount of MgO, Sn or Sb remains in the casting after pouring is completed.

4. The high-carbon-content, high-strength, high-toughness as-cast ductile iron according to claim 3, characterized in that: The retention of a small amount of MgO, Sn or Sb means that the residual amount of magnesium oxide is not less than 0.03%, the residual amount of tin is not less than 0.02%, or the residual amount of antimony is not less than 0.03%; the total amount of spheroidizing agent added is greater than 1.6% of the weight of the molten iron.

5. The high-carbon-content, high-strength, high-toughness as-cast ductile iron according to claim 4, characterized in that: The spheroidizer is a rare earth magnesium spheroidizer; when rare earth magnesium and tin are used as inhibitors, the inhibitors are added in an amount such that the residual amount of magnesium oxide in the solidified casting is between 0.03% and 0.065%, and the residual amount of tin is between 0.02% and 0.06%; when rare earth magnesium and antimony are used as inhibitors, the inhibitors are added in an amount such that the residual amount of magnesium oxide in the solidified casting is between 0.03% and 0.065%, and the residual amount of antimony is between 0.03% and 0.05%.

6. The high-carbon-content, high-strength, high-toughness as-cast ductile iron according to any one of claims 1-2, characterized in that: When the molten iron is taken out of the furnace for spheroidization, part of the recarburizer is added into the spheroidizing bag; when the molten iron is injected into the spheroidizing bag, it first impacts the recarburizer in the spheroidizing bag, and then reacts with the spheroidizing agent for spheroidization treatment. At the same time, the recarburizer in the spheroidizing bag is further boiled and stirred; the melting temperature and the temperature of the molten iron out of the furnace are not lower than 1550℃.

7. The high-carbon-content, high-strength, high-toughness as-cast ductile iron according to claim 6, characterized in that: The method of adding the recarburizer into the spheroidizing bag is to separate a part of the recarburizer as the total carbon content of the casting into the spheroidizing bag, and then dissolve it into the molten iron when it is discharged from the furnace, wherein the recarburizer added into the spheroidizing bag provides the casting with a carbon content of no more than 2.5%, and the remaining carbon content of the casting is provided by the recarburizer and other smelting raw materials in the smelting furnace.

8. The high-carbon-content, high-strength, high-toughness as-cast ductile iron according to claim 7, characterized in that: The recarburizer added during smelting in the smelting furnace is put into the smelting furnace together with scrap steel or returned materials, and then smelting begins until all the materials to be added are smelted; the recarburizer put into the spheroidizing bag provides 0.8%-2.5% carbon content for the casting; the inoculant, spheroidizing agent, inhibitor, covering agent, and slag gathering agent are put into the spheroidizing pit of the spheroidizing bag, and the time when the spheroidizing agent starts to detonate and the boiling spheroidizing time after detonation is controlled; the temperature of the molten iron rushing into the spheroidizing bag out of the furnace is 1550-1620℃.

9. The high-carbon-content, high-strength, high-toughness as-cast ductile iron according to claim 8, characterized in that: According to the weight of molten iron to be spheroidized in the spheroidizing bag and the performance requirements of ductile iron, one, two or more spheroidizing pits are set in the spheroidizing bag, and the time interval from the injection of molten iron to the detonation of the spheroidizing agent, as well as the detonation sequence, detonation time interval and spheroidization boiling reaction time of each spheroidizing pit are controlled.

10. The high-carbon-content, high-strength, high-toughness as-cast ductile iron according to any one of claims 1-2, characterized in that: The precious metal strengthening alloy is copper, molybdenum or other strong carbide forming elements; the alloy without precious metal strengthening refers to an element content not higher than the allowable content as an impurity; the alloy with less precious metal strengthening is an alloy in which the addition amount of each precious metal strengthening alloy does not exceed 0.3% of the weight of the molten iron.

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