High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition

Abstract

The present invention relates to a high-carbon, high-strength, and high-toughness spheroidal graphite cast iron in the as-cast condition, belonging to the field of ductile iron castings. The carbon content of the spheroidal graphite cast iron is ≥ 4.0%. The spheroidal graphite cast iron contains no or only small additions of expensive metal-reinforcing alloys. During the spheroidization of the molten iron during casting, an inhibitor to control the microstructure formation of the ductile iron and a sufficient quantity of spheroidizing agent are added in the casting vessel. Without rapid cooling or heat treatment of a casting, the mechanical properties of the ductile iron material in the as-cast condition reach QT700 or higher. The inhibitor is a mixture of the spheroidizing agent and weak carbide-forming elements.An added amount of the inhibitor is intended to ensure that minimal residues remain in the casting after casting. The spheroidal graphite cast iron product of the present invention achieves high strength and high toughness in the as-cast condition without heat treatment such as quenching or normalizing, thereby significantly reducing its production costs compared to alloyed, property-enhanced ductile iron products.

Description

Technical field

[0001] The present invention relates to a ductile cast iron material, in particular a high-strength and high-tough cast iron with spheroidal graphite in the as-cast condition and its casting process, which belongs to the field of iron casting. State of the art

[0002] In the fields of mechanical manufacturing and metalworking, iron-carbon alloys with a carbon content exceeding 2.11% are generally referred to as "iron," commonly known as cast iron. Ductile iron (abbreviated as spheroidal graphite cast iron) is a type of cast iron in which the free carbon, which has not formed carbides, is present primarily in a nearly spherical graphite form. After casting and heat treatment, its matrix structure consists predominantly of pearlitic microstructure or a mixture of pearlitic microstructure and ferrite, along with a small amount of carbides. Most of the excess carbon is distributed within the matrix structure in the form of spheroidal graphite bodies. Ductile iron may also contain small amounts of impurities. The pearlitic microstructure within the ductile iron matrix consists of layered cementite and ferrite.Ductile iron achieves both high strength and relative toughness, offering overall relatively favorable mechanical properties that have led to its widespread use.

[0003] In the ductile iron manufacturing industry, the carbon equivalent value of ductile iron products is typically below 4.3%, with the carbon content generally controlled between 3.5% and 3.9%. When the carbon equivalent value in cast iron exceeds 4.3%, it is referred to as hypereutectic cast iron. In this case, the metallographic structure consists mainly of cementite and ledeburite (a eutectoid of cementite and austenite). This makes the material relatively brittle and exhibits negligible elongation, thus failing to meet the property requirements for ductile iron. Therefore, professionals keep the carbon content of ductile iron below 4.0% to ensure that the carbon equivalent value of ductile iron products remains below 4.3%.To further improve the strength of ductile iron, alloying elements such as CuMo are added to form an alloy carbide within the ductile iron matrix.

[0004] Obtaining ductile iron with high strength and good toughness is a highly desirable goal. However, current methods for improving the strength of ductile cast iron typically involve adding alloying elements to the ductile iron, employing rapid quenching techniques, or heat-treating the casting after casting. For example, patent document CN115161539A discloses a planetary carrier blank made of isothermally quenched spheroidal graphite cast iron and its manufacturing process. The ductile iron casting contains 3.6% to 3.80% carbon by weight. During the melting of molten iron in the electric furnace, the carbon, silicon, copper, nickel, and molybdenum content is controlled based on raw materials such as pig iron and steel scrap to 1.5%–2.0% Ni, 0.8%–0.9% Cu, and 0.2%–0.25% Mo.Then, a specific quantity of carbon enrichment agent, spheroidizing agent, and inoculant for the spheroidization treatment is added to the ball casting vessel. After isothermal quenching, its mechanical properties reach a tensile strength of ≥ 820 MPa, a yield strength of ≥ 600 MPa, an elongation of ≥ 4.0%, a hardness of 280-310 HB, and a spheroidization rate of ≥ 90%.

[0005] Similarly, patent document CN101818297B discloses a method for producing a calender mold material from high-strength pearlitic spheroidal graphite cast iron. During casting, the following alloying elements are controlled by weight percent: 3.69%–4.04% C, 0.32%–0.65% Cu, 0.148%–0.171% Mo, 0.44%–0.54% Mn, 0.031%–0.035% Pb, etc. Alloy work hardening of ductile iron, followed by normalizing and tempering heat treatment, yields a casting with the following mechanical properties: yield strength σ0,2 = 655 - 680 MPa, tensile strength σ_ b = 965 - 1050 MPa, elongation at break = 3% - 5% and hardness = 264 - 272 HB.

[0006] Current ductile iron alloys are classified into numerous standard types or variants based on various properties, such as wear-resistant ductile iron, heat-resistant ductile iron, and friction-reducing ductile iron. The state of the graphite spheres in ductile iron exhibits standardized degrees of spheroidization, which include spheroidization rate, sphericity, number of spheroids, dispersion, and uniformity of sphere distribution. When casting ductile iron, spheroidizing agents and inoculants are typically added to promote carbon spheroidization. Ferrosilicon is commonly used as an inoculant, while magnesium-containing metal alloys are frequently used as spheroidizing agents. Rare-earth magnesium alloys are used, for example.Frequently used by experts as spheroidizing agents, the addition of rare-earth magnesium during the spheroidizing treatment typically does not exceed 1.4 wt% of the molten iron in the ball-casting kettle. Spheroidizing agents, inoculants, capping agents, and slag coagulants, etc., are placed in a cavity configured at the bottom of the ball-casting kettle. They are then compacted and covered with an iron plate or compacted iron chips to control the onset of the reaction based on contact between the spheroidizing agent and the molten iron (generally referred to as initiation) and the duration of the boiling reaction after initiation.

[0007] Graphite, as the primary structure and component of ductile iron, has properties such as a high melting point and low specific gravity, making it difficult to dissolve in large quantities in molten iron. Therefore, once the carbon content in ductile iron reaches a certain level, it becomes difficult to further increase the carbon content in the molten iron. Patent document CN106906333B discloses a method for increasing the carbon content in which the carbon enrichment agent and the molten iron are added to a melting furnace or a molten iron kettle. If the carbon enrichment agent does not dissolve directly in the molten iron and floats to its surface, the molten iron and the carbon enrichment agent are rapidly transferred from one molten iron container to another at least once (within 20 seconds).The temperature of the molten iron is maintained between 1450 and 1700 °C. Technical practice and research in the foundry sector show that an excessively high carbon content in ductile iron leads to the formation of significant amounts of free carbides. In spheroidal graphite cast iron, free carbides are generally considered detrimental structures because they fragment the matrix structure, thereby reducing the toughness of the ductile iron while simultaneously increasing its hardness and wear resistance. For example, if the cementite structure exhibits a net-like or nodular morphology, it degrades the overall properties of the ductile iron. If the carbon content in the intended ductile iron is too high, the carbon is difficult to dissolve in the molten iron, leading to graphite suspension and consequently to the scrapping of the molten iron in the entire furnace or of the castings.Furthermore, the chemical composition of ductile iron has a significant influence on its matrix structure, its spheroidization state, and its properties. Depending on the carbon content and the state of the carbon in the metal, the mechanical properties of ductile iron can vary considerably.

[0008] In foundries, experts often use strong carbide-forming elements such as chromium (Cr), tungsten (W), vanadium (V), molybdenum (Mo), nickel (Ni), copper (Cu), and manganese (Mn) to improve the mechanical strength of ductile iron. This approach increases the pearlitic content of the ductile iron. However, research shows that adding strong carbide-forming elements to cast iron, which significantly influence the formation and morphology of cementite, often leads to the formation of blocky carbides in the ductile iron. For example, adding approximately 0.4% to 0.8% Mo and Cu to ductile iron increases its strength and allows for a pearlitic content of up to 85%. However, the remaining structures consist predominantly of isolated, nodular carbides.Consequently, while the use of relatively expensive metals such as molybdenum and copper to reinforce cast iron significantly improves strength, it often leads to a considerable reduction in elongation.

[0009] Furthermore, metals such as chromium (Cr), tungsten (W), vanadium (V), molybdenum (Mo), nickel (Ni), and copper (Cu) are expensive. These alloys are among the most costly metal-reinforcing alloys used in foundries and are often referred to as precious metals. Consequently, the use of alloying elements to reinforce cast iron significantly increases the cost of cast iron products. These expensive metals command considerably higher prices than carbon (C). For example, in June 2023, molybdenum-iron alloys with a molybdenum content of at least 55% (such as FeMo60) were trading at a market price of approximately 250,000 yuan per ton, which translates to a molybdenum cost of approximately 410,000 yuan per ton (410 yuan per kg). The market price for copper was around 70.000 yuan per ton (70 yuan per kg), while the market price for the rare-earth magnesium spheroidizing agent was around 10 yuan per kg. Using a conventional calculation with 0.6% molybdenum (Mo) and copper (Cu) per ton of casting, each ton of ductile iron requires 420 yuan worth of copper and 2400 yuan worth of Mo, increasing the casting cost per ton by 2820 yuan.

[0010] Furthermore, in the current production of ductile iron components, heat treatment processes such as quenching or normalizing are essential in addition to casting. Alternatively, rapid cooling casting techniques with accelerated cooling during the casting process must be employed to achieve the required tensile strength and toughness of ductile iron products. These accelerated cooling casting techniques and complex equipment result in high casting costs and can simultaneously lead to defects such as deformation or cracking in the castings. Heat treatment incurs additional costs of at least several hundred yuan per ton, consumes further energy, and requires waste disposal after heat treatment. Consequently, these methods increase overall product costs and lead to energy waste. Disclosure of the invention

[0011] The technical problem addressed by the present invention is to provide a cost-effective spheroidal graphite cast iron which, in its as-cast condition without heat treatments such as quenching or normalizing, exhibits high strength and high toughness.

[0012] To solve the aforementioned technical problem, the present invention provides the following technical solution: High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition, wherein the carbon content of the spheroidal graphite cast iron is ≥ 4.0%, wherein the spheroidal graphite cast iron contains no or only small additions of costly metal-reinforcing alloys, wherein during spheroidization of the molten iron during casting in the ball casting vessel an inhibitor to control the microstructure formation of the ductile iron and a sufficient amount of spheroidizing agent are added, wherein, without rapid cooling or heat treatment of a casting, a mechanical property of a ductile iron material in the as-cast condition QT700 or higher is achieved.

[0013] A further improvement of the present invention consists in the fact that the carbon content of the ductile iron is 4.05% to 6.0% and the inhibitor is a mixture of the spheroidizing agent and weak carbide-forming elements.

[0014] A further improvement of the technical solution of the present invention consists in the fact that the spheroidizing agent is a magnesium-containing alloy spheroidizing agent and the inhibitor is the spheroidizing agent and the metal tin or the spheroidizing agent and the metal antimony, wherein an added amount of the spheroidizing agent and the metal tin or the metal antimony is intended to ensure low residues of MgO, Sn or Sb in the casting after casting.

[0015] A further improvement of the above-mentioned technical solution of the present invention consists in the fact that the low residues of MgO, Sn or Sb refer to a residual magnesium oxide content of not less than 0.03%, a residual tin content of not less than 0.02% or a residual antimony content of not less than 0.03%, wherein the total amount added of the spheroidizing agent exceeds 1.6 wt% of the molten iron.

[0016] A further optimization of the above-mentioned technical solution of the present invention consists in the fact that the spheroidizing agent is a rare-earth magnesium spheroidizing agent, wherein, when rare-earth magnesium and tin are used as the inhibitor, the amount of inhibitor added is selected such that the residual content of magnesium oxide in the solidified casting is between 0.03% and 0.065% and the residual content of tin is between 0.02% and 0.06%, wherein, when rare-earth magnesium and antimony are used as the inhibitor, the amount of inhibitor added is selected such that the residual content of magnesium oxide in the solidified casting is between 0.03% and 0.065% and the residual content of antimony is between 0.03% and 0.05%.

[0017] A further improvement of the technical solution of the present invention consists in adding a portion of a carbon enrichment agent during the spheroidization of the molten iron when pouring it into the ball casting vessel, wherein, when filling the molten iron with the molten iron during pouring into the ball casting vessel, the carbon enrichment agent present in the ball casting vessel is first stirred and then reacts with the spheroidization agent for the spheroidization treatment, while at the same time the carbon enrichment agent in the ball casting vessel is stirred further during boiling, wherein a melting temperature and a pouring temperature of the molten iron are both at least 1550 °C.

[0018] A further improvement of the above-mentioned technical solution of the present invention consists in the fact that the addition of the carbon enrichment agent to the ball casting vessel relates to separating a portion of the carbon enrichment agent, which serves as the total carbon content of the casting, and adding it to the ball casting vessel, wherein the portion of the carbon enrichment agent is then dissolved in the molten iron during casting, wherein the carbon enrichment agent added to the ball casting vessel provides a carbon content of no more than 2.5% for the casting, wherein the remaining carbon content of the casting is provided by the carbon enrichment agent and other melting raw materials in the melting furnace.

[0019] A further improvement of the above-mentioned technical solution of the present invention consists in the fact that, during melting in the melting furnace, the added carbon enrichment agent is placed in the melting furnace together with steel scrap or recycled scrap, whereupon the melting begins until all the materials required for the addition have completely melted, wherein the carbon enrichment agent placed in the ball casting vessel provides a carbon content of 0.8% to 2.5% for the casting, wherein an inoculant, the spheroidizing agent, the inhibitor, a capping agent and a slag coagulation agent are placed in a spheroidizing cavity of the ball casting vessel, wherein an initiation time of the spheroidizing agent and a boiling spheroidizing time after initiation are controlled, wherein the temperature of the molten iron when pouring from the furnace into the ball casting vessel is 1550 to 1620 °C.

[0020] A further improvement of the above-mentioned technical solution of the present invention consists in the fact that, based on the weight of the molten iron that needs to be spheroidized in the ball casting vessel and the property requirements for the spheroidizing cast iron, one or two or more spheroidization cavities are configured within the ball casting vessel, wherein a time interval from the casting of the molten iron to the initiation of the spheroidizing agent, as well as an initiation sequence, the initiation time interval, and the spheroidization boiling reaction time for each spheroidization cavity are controlled.

[0021] A further improvement of the present invention consists in the fact that the expensive metal-reinforcing alloys are copper, molybdenum or other strongly carbide-forming elements, wherein the absence of the expensive metal-reinforcing alloys refers to an element content that does not exceed the permissible impurity content, and wherein the small additions of expensive metal-reinforcing alloys mean that the amount added of each expensive metal-reinforcing alloy does not exceed 0.3 wt.% of the molten iron.

[0022] The technical progress achieved by the present invention using the above-mentioned technical solution is as follows: The production of spheroidal graphite cast iron using the technical solution of the present invention results in low casting costs. Only inexpensive carbon is used to enhance the comprehensive mechanical properties of the ductile iron, with little or no addition of precious metals. Complex casting equipment or techniques such as accelerated cooling treatments are not required. Furthermore, high-strength, high-toughness ductile iron products can be obtained in the as-cast condition without heat treatments such as quenching or normalizing. No waste disposal is necessary, making it environmentally friendly. The ductile iron of the present invention is suitable for all casting processes. When using special casting methods such as rapid cooling, the mechanical properties of the ductile iron material, including tensile strength, are further improved.

[0023] The carbon content of the ductile iron of the present invention can easily exceed 4.0%, challenging the conventional view that the carbon content in the casting range of ductile iron must be kept low. Due to the natural presence of silicon (Si) and other carbon-equivalent elements in cast iron, the carbon equivalent value of the ductile iron of the present invention is typically above 4.3%, thus exceeding the carbon content at the eutectic point in the iron-carbon phase diagram. The present invention extends the carbon content range for ductile iron, thereby challenging the traditional view that an extremely high carbon content impairs the overall properties of ductile iron.By significantly increasing the carbon content in ductile iron through innovative carbon enhancement processes, and by controlling the structure and morphology of carbon within the matrix as well as the form, quantity, and distribution of free graphite, this process enables the production of cost-effective, inexpensive, high-strength, and high-tough ductile iron castings in the as-cast condition. By adjusting the chemical parameters of this material in conjunction with modernized processing techniques, the disadvantages associated with graphite in the conventional production of spheroidal graphite cast iron are overcome.

[0024] The ductile iron produced by this invention uses only inexpensive and readily available carbon as the primary reinforcing element. It contains no costly metal-strengthening alloys such as Cu or Mo, or these are not actively added. The term "without costly metal-strengthening alloys" means that costly metal-strengthening alloying elements are not actively added. However, these may be impurities that have formed during the melting of ductile iron. These represent an unavoidable permissible level that should not impair the properties of the ductile iron and is not included in calculations of the alloy quantity. The content of these alloys does not exceed the permissible level for impurities. The content of elements occurring as impurities in ductile iron must not exceed a certain value.For example, titanium (Ti) significantly impairs graphite spheroidization in ductile iron. Excessive Ti content disrupts the spheroidization quality and thus reduces material toughness. Consequently, the Ti content in the ductile iron of the present invention must be strictly controlled. This invention utilizes metals such as tin and antimony, together with sufficient spheroidizing agents, as inhibitors within the spheroidization pot. This enables the complete dissolution of the extremely high carbon content of the molten iron and prevents graphite from floating to the surface. Furthermore, it allows for efficient control of the morphology, quantity, and distribution of the graphite spheres, as well as control of the cementite morphology within the matrix and the amount of pearlitic microstructure.Furthermore, it makes the lamellae of the pearlite-forming cementites and ferrites thin and uniform, thereby significantly improving both the tensile strength and the toughness of the ductile iron.

[0025] The present invention achieves high-strength, high-toughness ductile iron by adding the carbon enrichment agent and the inhibitor to the casting vessel, causing them to dissolve in the molten iron during spheroidization upon casting, thereby utilizing a significant amount of carbon to strengthen the ductile iron. The ductile iron of the present invention achieves a tensile strength of at least 600 MPa in the as-cast condition without the addition of cost-effective metals or subsequent heat treatment. Furthermore, despite this high tensile strength, it achieves an elongation of at least 4.0%. For example, if the tensile strength of the ductile iron product reaches 850 MPa, the elongation can still be more than 6.5%.The carbon content of the ductile iron of the present invention can be designed to be in the range of ≥ 4.05%, according to the requirements for the mechanical properties of the ductile iron product. Within the commonly used carbon content range of 4.5% to 6.0%, ductile iron products that meet the property requirements of QT800 or higher can be obtained in the as-cast condition without the addition of costly metal-reinforcing alloys and without subsequent heat treatment. The ductile iron of the present invention saves on costly metal alloys such as molybdenum and copper, eliminates the need for expensive casting plants with accelerated cooling, and dispenses with heat treatment processes such as quenching and normalizing. This significantly reduces casting costs and the price of ductile iron products.Based on current market prices for molybdenum, copper, and tin (tin at 200 yuan per kilogram, rare-earth magnesium at approximately 10 yuan per kilogram, with the carbon enrichment agent being inexpensive and therefore negligible in the cost calculation), the savings in raw materials alone amount to 2,600 yuan per ton of castings. Combined with the savings in heat treatment costs, this results in a production cost reduction of 3,000 yuan per ton of ductile iron castings.

[0026] The present invention involves adding a carbon enrichment agent to the ball casting vessel. Sufficiently hot molten iron is used in the ball casting vessel to dissolve the carbon enrichment agent in the molten iron. As the molten iron begins to fill the ball casting vessel, it directly impacts the carbon enrichment agent within a period prior to the initiation of the spheroidizing agent. This intense impact and stirring of the carbon enrichment agent by the molten iron can promote the dissolution of the carbon enrichment agent in the molten iron.Once the spheroidizing agent is initiated in the cavity of the sphere casting vessel and triggers the spheroidization reaction, the magnesium oxide produced by the spheroidizing agent causes the molten iron to boil. During the spheroidization process, the molten iron and the carbon enrichment agent are continuously stirred and mixed, thereby enabling further dissolution of the carbon enrichment agent in the molten iron within the sphere casting vessel. According to the present invention, the carbon dissolved in the molten iron in the sphere casting vessel also induces lattice distortion in the crystal structure of the ductile iron matrix. This results in an increase in strength through solid dissolution, further improving the mechanical strength of the casting in its as-cast condition.

[0027] The carbon enrichment process used in the present invention typically comprises dividing the carbon enrichment agent into two parts for dissolution in the molten iron: one part is added to the melting furnace during the melting of the molten iron, while the other part is added to the ball casting pot for dissolution during the spheroidization treatment. The amount of carbon from the carbon enrichment agent added to the ball casting pot should ideally be 0.8% to 2.5% of the carbon content of the casting. The remaining carbon content for the casting is provided by the carbon enrichment agent, the steel scrap, the recycled scrap, and other melting feedstocks in the melting furnace. This further facilitates the dissolution of the carbon from the carbon enrichment agent in the molten iron, resulting in the formation of a fine pearlite matrix and high-quality spheroidal graphite.The carbon increase process used in both the melting furnace and the ball casting kettle can be described as a composite carbon increase process. Adding a significant amount of carbon enrichment agent to the ball casting kettle can lead to improved results.

[0028] In the compound carbon enrichment process, the carbon enrichment agent is divided into two parts: one part added during melting in the furnace and one part added during spheroidization in the sphere casting vessel. The carbon enrichment agent for melting in the furnace is placed in the furnace along with steel scrap or recycled scrap. The carbon enrichment agent can be placed on the bottom of the furnace along with the recycled scrap and other melting feedstocks, or some of the carbon enrichment agent can be placed in the spaces between the steel scrap or recycled scrap. Melting then begins until all the materials required for the addition are completely melted. In this way, a significant portion of the carbon can be fully dissolved into the molten iron beforehand during the melting process.The remaining portion is the carbon content provided by the carbon enrichment agent added to the sphere casting vessel. During casting, a sufficient quantity of hot molten iron directly impacts the carbon enrichment agent in the sphere casting vessel. Through an interaction between inoculants, spheroidizing agents, inhibitors, capping agents, and slag coagulation agents added to the spheroidizing cavity of the sphere casting vessel and the molten iron, as well as simultaneous control of the spheroidizing agent initiation time, the boiling spheroidization time after initiation, and the initiation sequence, etc., the carbon from the carbon enrichment agent in the sphere casting vessel is completely dissolved in the molten iron, resulting in a further increase in the carbon content of the molten iron.As a result, the spheroidized molten iron achieves a remarkably high amount of carbon, easily exceeding 4.0% of the carbon content.

[0029] According to the present invention, two or more spheroidization cavities are provided in the ball casting vessel for storing inoculants, spheroidizing agents, inhibitors, capping agents, slag coagulants, and other materials, thereby enabling a multi-stage spheroidization treatment of the molten iron filled into the ball casting vessel. Based on the weight of the molten iron to be spheroidized in the ball casting vessel and the required properties of the ductile iron, the initiation time, initiation sequence, initiation time interval, and boiling reaction time are controlled. This increases and controls the spheroidization reaction time and boiling intensity during the magnesium oxide-induced boiling of the molten iron. Therefore, even if a significant amount of carbon enrichment agent is used in the ball casting vessel, the molten iron can readily dissolve it.When higher material and mechanical property requirements are needed for ductile iron products, multiple cavities are typically configured for a multi-stage treatment. Multi-stage treatment is particularly necessary when demanding toughness properties such as elongation and cross-sectional reduction rate are specified. Of course, if the use of carbon enrichment in the ball casting vessel is relatively low, satisfactory results can also be achieved with a single or continuous cavity for spheroidization treatment.

[0030] The inhibitor used in the present invention is a spheroidizing agent in combination with the metal tin or a spheroidizing agent in combination with the metal antimony. In the ductile iron casting sector, the spheroidizing agent is predominantly a magnesium-containing alloy spheroidizing agent; for example, rare-earth magnesium spheroidizing agents with varying magnesium contents can all serve as spheroidizing agents for the present invention. The spheroidizing agent in the present invention is added in a "sufficient amount," meaning that the total amount of spheroidizing agent added, which serves as both inhibitor and for spheroidization, exceeds the conventional amount of spheroidizing agent used in the production of ductile iron. Typically, this requires an addition of more than 1.6 wt.-% of the molten iron, whereas in the conventional production of ductile iron, no more than 1.4% of spheroidizing agent is added. The addition quantities of the various inhibitor compositions must ensure that a small residual amount of the added inhibitor elements remains in the casting after casting. This facilitates the complete dissolution of the carbon composition in the molten iron, resulting in a favorable matrix structure morphology and a favorable amount of pearlitic microstructure, in addition to the production of high-quality graphite spheres.

[0031] Both the spheroidizing agent and the inhibitor in the present invention utilize rare-earth magnesium spheroidizing agents, which are used in conventional ductile iron casting. Beyond its spheroidizing function, the magnesium element in the present invention also acts as a weak carbide-forming element. While the carbides it forms do not exhibit very high hardness and strength, they improve the formation and quantity of pearlitic structures and facilitate the ordered arrangement of cementite and ferrite within the pearlitic structure. The tin or antimony in the inhibitor in the present invention is also a weak carbide-forming element. It performs the same function as magnesium in ductile iron, controlling the quantity and structural morphology of the pearlitic structure and the graphite sphere.The amounts of rare-earth magnesium and tin added are such that the residual tin content in the solidified casting is not less than 0.02%, preferably between 0.02% and 0.06%. The residual magnesium oxide content is not less than 0.03%, preferably between 0.03% and 0.065%. When using rare-earth magnesium and antimony as an inhibitor, the amounts of rare-earth magnesium and antimony added are such that the residual antimony content in the solidified casting is not less than 0.03%, preferably between 0.03% and 0.05%. The residual magnesium oxide content is not less than 0.03%, preferably between 0.03% and 0.065%. This ensures that sufficient carbon is completely dissolved in the molten iron, forming a high-strength, high-toughness structure without wasting raw materials.

[0032] To further improve the strength of ductile iron, this invention allows the addition of small amounts of expensive metal-reinforcing alloys such as copper and molybdenum during casting. However, excessive alloying elements such as titanium must not be added to the ductile iron, as titanium impairs graphite spheroidization and significantly reduces the toughness of the ductile iron. The amount added of each expensive metal-reinforcing alloy does not exceed 0.3 wt% of the molten iron to minimize the increase in casting raw material costs. At a value of 0.3% of the maximum amount and current market prices, the increased raw material costs (using copper and molybdenum as examples) amount to 1473 yuan. The ductile iron material can achieve a tensile strength of over 960 MPa and an elongation of over 4% in the as-cast condition, making the ductile iron product suitable for special applications.Conversely, with conventional ductile iron grades, the casting fully meets the property requirements in the as-cast condition without the addition of costly metal-reinforcing alloys and without increasing the mechanical properties through processes such as heat treatment or casting with accelerated cooling, using only the method for increasing the carbon content in ductile iron. The production costs of the ductile iron product in the present invention are significantly lower than those of conventional ductile iron with equivalent mechanical properties. Brief description of the drawings Fig. Figure 1 shows a metallographic image (X100) of a graphite structure of a casting obtained from embodiment 1 of the present invention; Fig. Figure 2 shows a metallographic image (X400) of a matrix structure of a casting obtained from embodiment 1 of the present invention; Fig. Figure 3 shows a metallographic image of a graphite structure of a casting obtained from embodiment 2 of the present invention; Fig. Figure 4 shows a metallographic image of a matrix structure of a casting obtained from embodiment 2 of the present invention. Detailed descriptions

[0033] The present invention relates to a high-carbon, high-strength, high-toughness spheroidal graphite cast iron in the as-cast condition, with a carbon content of ≥ 4.0%. The ductile iron contains no costly metal-reinforcing alloys, where "without costly metal-reinforcing alloys" means that the elemental content does not exceed the permissible levels of impurities. Even if costly metal-reinforcing alloys are detected during the chemical elemental analysis of the ductile iron, these are present as impurities that likely originate from the smelting raw materials. The carbon content of the ductile iron commonly used in the present invention can be designed in the range of 4.05% to 6.0%, thereby fulfilling the property requirements of all ductile iron products.During the spheroidization of the molten iron during casting, sufficient quantities of spheroidizing agent and inhibitor are added to the ball casting vessel. The inhibitor is a mixture of a spheroidizing agent and weak carbide-forming elements, where the inhibitor is either a spheroidizing agent combined with the metal tin or a spheroidizing agent combined with the metal antimony. The spheroidizing agent is a magnesium-containing alloy spheroidizing agent; for example, a rare-earth magnesium spheroidizing agent with any desired magnesium content can be selected. The inhibitor can control the structural morphology of the pearlitic microstructure and graphite in the cast iron. The total amount of spheroidizing agent added, serving as both inhibitor and spheroidizing agent, must exceed the conventional amount used in the production of ductile iron.The amount of spheroidizing agent and the metal tin or antimony added must ensure that minimal residues remain in the casting after pouring. The total amount of rare-earth magnesium added as an inhibitor and spheroidizing agent must exceed 1.6 wt% of the molten iron. The amount of inhibitor and spheroidizing agent added must be "sufficient" and is generally controlled by the following residual amounts: the residual magnesium oxide content not less than 0.03%, the residual tin content not less than 0.02%, or the residual antimony content not less than 0.03%, if used.Example: When rare-earth magnesium and tin are used as the inhibitor, the amount of inhibitor added is preferably selected such that the residual tin content in the solidified casting is between 0.02% and 0.06% and the residual magnesium oxide content is between 0.03% and 0.065%. When rare-earth magnesium and antimony are used as the inhibitor, the amount of inhibitor added is preferably selected such that the residual antimony content in the solidified casting is between 0.03% and 0.05% and the residual magnesium oxide content is between 0.03% and 0.065%. Excessive amounts cause waste, while amounts that are too high or too low can also impair the structural morphology and properties of the ductile iron.

[0034] During the spheroidization of molten iron during casting according to the present invention, the carbon enrichment agent is added to the sphere casting vessel. When the molten iron is poured into the sphere casting vessel, it first strikes the carbon enrichment agent within the vessel and then reacts with the spheroidizing agent in the cavity for the spheroidization treatment. The magnesium oxide produced by the reaction between the spheroidizing agent and the molten iron during boiling further stirs the carbon enrichment agent within the sphere casting vessel. The melting and pouring temperatures of the molten iron are typically higher than those of conventional methods for melting ductile iron. Both the melting and pouring temperatures can be controlled to at least 1550 °C.For high carbon content, the melting temperature and the casting temperature should be increased accordingly.

[0035] The method for adding carbon enrichment agent to the ball casting vessel according to the present invention involves dividing the carbon enrichment agent into two parts: one part that is added during melting in the melting furnace and one part that is added to the ball casting vessel. The addition of the carbon enrichment agent to the ball casting vessel involves separating a portion of the carbon enrichment agent, which serves as the total carbon content of the casting, and adding it to the ball casting vessel. This portion of the carbon enrichment agent then dissolves into the molten iron during pouring. The carbon enrichment agent added to the ball casting vessel provides a carbon content of no more than 2.5% for the casting. The optimal range for the carbon content is controlled between 0.8% and 2.5%.This means that the carbon enrichment agent added to the ball casting kettle must be able to increase the carbon content of the molten iron by 0.8% to 2.5%. The remaining carbon content of the casting is provided by the carbon enrichment agent and other melting feedstocks in the furnace (such as steel scrap or recycled scrap). When adding the carbon enrichment agent to the ball casting kettle along with the steel scrap or recycled scrap, some or all of the carbon enrichment agent, along with some recycled scrap, is placed on the bottom of the furnace before being added with the steel scrap. Finally, the recycled scrap is added until the furnace is full.When a relatively large quantity of carbon enrichment agent is added, some of it can be placed in the spaces between the steel scrap and then fed into the furnace. The melting process then begins, with the remaining furnace materials being continuously added to the melting furnace as they melt until all the required materials have been melted. The carbon enrichment agent added to the ball casting pot is then fused with the molten iron. According to the present invention, inoculants, spheroidizing agents, inhibitors, capping agents, and slag coagulants are added to the spheroidizing cavity of the ball casting pot.Upon contact with the spheroidizing agent, the molten iron reacts to produce magnesium oxide. Both the initiation time of the spheroidizing agent and the boiling spheroidization time after initiation are controlled. Before pouring, the sphere casting vessel is heated, and then the required amount of carbon enrichment agent is added. After pouring, the molten iron first strikes the carbon enrichment agent at the bottom of the sphere casting vessel. Following the contact reaction between the molten iron and the spheroidizing agent, the magnesium oxide produced from rare-earth magnesium further stirs the molten iron and the carbon enrichment agent. The temperature of the molten iron poured into the sphere casting vessel after pouring is typically 1550 °C to 1620 °C, which is slightly higher than the conventional pouring temperature of ductile iron.

[0036] According to the present invention, the carbon content in ductile iron exceeds that of conventional ductile iron. Steel scrap or recycled ductile iron scrap is used as the main raw material, which facilitates control of the carbon, sulfur, and phosphorus content. Subsequently, the carbon enrichment agent and the carbon enhancement process are employed to add sufficient carbon to the ductile iron, enabling the cast product to meet the intended mechanical property requirements in its as-cast condition. Numerous types of carbon enrichment agents exist in the foundry industry. For example, patent document CN106906333B discloses that a mixture of one or more than two of these substances (i.e., pure graphite, graphite powder, or graphite electrode) in any proportion can serve as a carbon enrichment agent.Furthermore, materials containing carbon or graphite, such as coal, charcoal, coke, coke oven slag, etc., can act as carbon enrichment agents. Commercially available tar-based carbon enrichment agents are also commonly used in the foundry industry. The carbon content varies considerably between different carbon enrichment agents, and the absorption rate of molten iron also differs depending on the specific agent used. Consequently, the weight of carbon enrichment added varies depending on the type of carbon enrichment agent used.In the actual production of ductile iron, the addition weights of different types of carbon enrichment agents should be calculated based on the carbon content of the various melting raw materials, the carbon content of the carbon enrichment agent used, and its absorption rate. This ensures that the casting ultimately meets the carbon content requirements.

[0037] To extend the duration of the spheroidization reaction and boiling agitation, two or more cavities can be configured within the spheroidization kettle. Based on the weight of the molten iron to be spheroidized and the property requirements of the spheroidizing cast iron, the time interval from the pouring of the molten iron to the start of the spheroidizing agent initiation reaction, the initiation sequence of each spheroidization cavity, the initiation time interval between spheroidization cavities, and the boiling reaction time after initiation should be controlled. In addition to the impact and agitation of the molten iron against the carbon enrichment agent during pouring, the magnesium oxide gas generated by the reaction between the spheroidizing agent and the molten iron causes a boiling reaction of the molten iron.This extends the stirring time and ensures more thorough contact between the inoculant, the spheroidizing agent, and the molten iron. As a result, carbon can largely dissolve in the molten iron, facilitating the formation of lamellar pearlitic structures and finely dispersed graphite spheres.

[0038] Should a further improvement in the strength or other mechanical properties of the ductile iron product be required, small amounts of expensive metal-strengthening alloys may be added to the aforementioned ductile iron. These alloys may consist of one, two, or more of the following: copper, molybdenum, vanadium, or other strong carbide-forming elements. The amount of expensive metal-strengthening alloys added is such that the amount added for each expensive metal-strengthening alloy does not exceed 0.3 wt.% of the molten iron. The amount of expensive metal-strengthening alloys added is less than that added to other ductile irons not covered by the scope of the present invention. In particular, titanium must not be added, as this impairs the toughness of the ductile iron.

[0039] The present invention will be explained in more detail below with reference to the exemplary embodiments. Example 1

[0040] The molten iron melting plant used in this embodiment comprises a 1.5-ton induction furnace that melts 2 tons of molten iron per batch.

[0041] The main furnace materials comprise the following: 25% recycled scrap, totaling 500 kg; 71% steel scrap with a carbon content of 0.3% to 0.45%, totaling 1420 kg; carbon enrichment agent from discarded graphite electrodes, totaling 87.5 kg; 0.5% ferrosilicon (FeSi75), totaling 10 kg.

[0042] The distribution of the carbon enrichment agent is as follows: in this embodiment, the carbon enrichment agent added to the furnace and the carbon enrichment agent added to the ball casting vessel are distributed in the following proportions: the carbon content of the molten iron in the furnace reaches 3.5%, while the carbon enrichment agent added to the ball casting vessel increases the carbon content of the molten iron by a further 0.9%.

[0043] Charging the melting furnace is as follows: first, 40 kg of carbon enrichment agent and approximately 100 kg of small scrap metal are added to the bottom of the furnace. The carbon enrichment agent can be placed at the very bottom or mixed with the small scrap metal and then added to the furnace floor. It is then added along with the steel scrap until the furnace is full, while simultaneously adding the remaining carbon enrichment agent, which must be added to the furnace, into the spaces between the steel scrap.

[0044] After the materials are loaded, the melting process begins in the furnace. While the material is melting, the remaining 1420 kg of steel scrap are continuously added to the furnace to melt it into molten iron. Finally, the remaining 400 kg of recycled scrap are added to the furnace. Once the material is completely melted, 10 kg of ferrosilicon (FeSi75) are added to the furnace ten minutes before casting.

[0045] The melting temperature in this embodiment is 1550 °C, and the casting temperature is 1610 °C.

[0046] The spheroidization treatment is carried out in 1.5-ton ball casting kettles, with 1 ton of molten iron being processed per kettle. 10 kg of graphite electrode scrap are added to each kettle as a carbon enrichment agent. In this embodiment, a one-stage spheroidization treatment process is used, in which a spheroidization cavity is provided within the ball casting kettle. This cavity contains inoculant, spheroidizing agent, inhibitors, capping agent, and slag coagulant. After compaction of these materials, they are securely covered with iron chips and iron plates. The carbon enrichment agent placed in the ball casting kettle is positioned directly at the bottom of the kettle. During the filling of the kettle with molten iron, the molten iron strikes the carbon enrichment agent placed at the bottom of the kettle.This process, through the agitation and stirring of hot, molten iron, leads to a partial dissolution of the carbon enrichment agent in the molten iron. After the cavity's iron cover plate melts, the molten iron comes into contact with the inoculant, the spheroidizing agent, the inhibitor, the cover agent, the slag coagulant, and other materials. The spheroidizing agent reacts with the molten iron, producing a significant amount of magnesium oxide, which further stirs the molten iron as it boils. This facilitates the dissolution of carbon from the carbon enrichment agent into the molten iron.

[0047] In this embodiment, ferrosilicon (FeSi-75) is used as the inoculant placed in the ball casting vessel. The amount of FeSi-75 added is 0.8 wt% of the molten iron in the ball casting vessel, which corresponds to 8 kg of inoculant per ball casting vessel.

[0048] In this embodiment, a rare-earth magnesium alloy spheroidizing agent is used. The amount of spheroidizing agent added is 1.7 wt% of the molten iron in the ball casting pot, which corresponds to 17 kg of additives per ball casting pot.

[0049] In this embodiment, the metal tin is added to the spheroidization cavity of the sphere casting vessel. In conjunction with the rare-earth magnesium, the tin acts as an inhibitor. Together, they can control the morphology of the pearlitic microstructure and graphite, increase the pearlitic content within the ductile iron matrix, and improve the ordered arrangement of cementite and ferrite within the pearlitic microstructure. This increases the strength and toughness of the ductile iron matrix and promotes the formation of well-rounded graphite spheres and their dispersed distribution. The amount of tin added is 0.04 wt% of the molten iron in the sphere casting vessel, which corresponds to 0.4 kg of tin per vessel.

[0050] The covering agent, slag coagulant, etc., added to the ball casting vessel in this exemplary embodiment are commercially available products. Their quantities should be adjusted appropriately according to the local foundry conditions and experience.

[0051] In this embodiment, the initiation control for the spheroidizing agent is carried out as follows: after the molten iron in the ball casting vessel has been stirred by impact for 30 to 40 seconds and dissolved with the carbon enrichment agent, the iron plate covering the cavity is melted. This brings the molten iron into contact with the additional materials in the cavity—namely, the inoculant, the spheroidizing agent, the inhibitor, the capping agent, and the slag coagulant—and triggers a reaction. The spheroidizing agent reacts with the molten iron, forming magnesium oxide gas, which causes the molten iron to boil (a phenomenon commonly referred to as "initiation" in foundry work). In this embodiment, the desired reaction time between the spheroidizing agent and the molten iron is controlled for 40 to 50 seconds.In this embodiment, the actual initiation time is 33 seconds, with the spheroidization reaction lasting 41 seconds after initiation. Once the boiling of the molten iron has ceased, the slag is removed and other preparatory work is carried out before casting the part. The casting temperature is controlled at 1460 ± 10 °C.

[0052] In this embodiment, the casting and a standard wedge-shaped test block are produced using the lost-foam casting process. The timing for casting the test block is determined by the mean time of the casting, with the test block being cast approximately 7 minutes after the start of the casting process. After the test block has cooled, samples of its chemical composition are taken, and the block is then processed into a tensile test specimen for physical and chemical testing. In this embodiment, the casting can be removed from the mold for cooling 40 minutes after casting.

[0053] The chemical composition of the test block analyzed after melting in this embodiment is as follows: 4.33% C, 1.97% Si, 0.31% Mn, 0.034% S, 0.036% P, 0.038% Sn, 0.042% MgO, 0.019% Re, the remainder consisting of Fe and unavoidable impurities.

[0054] The mechanical properties of the test specimen formed during casting in this embodiment are as follows: tensile strength 670 MPa, elongation 11%. The test specimen and its mechanical properties are in the as-cast condition, without any heat treatment such as quenching or normalizing. In the metallographic structure of the product obtained in this embodiment, the pearlitic microstructure consists predominantly of lamellar pearlite, which comprises 68% of the structure.

[0055] The graphite is predominantly spherical and is classified as grade 2. The metallographic structure images of the test block obtained in this embodiment are shown in Fig. 1 and Fig. 2 shown.

[0056] In the present invention, the use of sufficient carbon increases the likelihood of iron and carbon forming carbides. The tin and rare-earth magnesium act as inhibitors, allowing more carbon to form sufficient pearlitic microstructures (cementite + ferrite). The inhibitor prevents cementite from forming free, block-like structures and promotes the formation of predominantly lamellar pearlite. Consequently, the overall mechanical properties of the ductile iron, including tensile strength and toughness, are improved. Since the present invention does not use expensive metal alloys, does not require heat treatment processes such as quenching or normalizing to improve mechanical properties, and does not necessitate rapid cooling casting equipment, the production costs for ductile iron products are significantly reduced. Example 2

[0057] The molten iron melting plant used in this embodiment comprises a 1-ton induction furnace that melts 1.2 tons of molten iron per batch.

[0058] The batch composition for the melting furnace is as follows: 25% recycled scrap, total 300 kg; 71% steel scrap with a carbon content of 0.3% to 0.5%, total 852 kg; carbon enrichment agent from carbon slag waste, total addition 53 kg; 0.7% ferrosilicon (FeSi75), 8.4 kg.

[0059] The distribution of the carbon enrichment agent is as follows: the carbon enrichment agent added to the furnace and the carbon enrichment agent added to the ball casting vessel are distributed in the following proportions: the carbon content of the molten iron in the furnace reaches 4.0%, while the carbon enrichment agent added to the ball casting vessel increases the carbon content of the molten iron by a further 1.4%. In this embodiment, 18 kg of carbon enrichment agent are added to the ball casting vessel.

[0060] The furnace loading sequence is as follows: ① approximately 100 kg of small recycled scrap and 25 kg of carbon enrichment agent are added to the furnace floor; ② then the steel scrap is added until the furnace is full, with the remaining carbon enrichment agent being added to the spaces between the steel scrap in the furnace; ③ the melting process in the furnace is started, with the remaining steel scrap being continuously added while the materials in the furnace are melting, followed by the addition of the remaining 200 kg of recycled scrap to be melted in the furnace; ④ 10 minutes before pouring, 8.4 kg of ferrosilicon are added.

[0061] The melting temperature in this embodiment is 1580 °C, and the casting temperature is 1620 °C.

[0062] The spheroidization treatment is carried out in 1.5-ton ball casting kettles, with 1.2 tonnes of molten iron being processed per ball casting kettle during the spheroidization treatment.

[0063] In this embodiment, a two-stage spheroidization treatment process is applied by bottom-pouring into the ball casting vessel: two spheroidization cavities are provided at the bottom of the vessel for storage. The materials required for the spheroidization treatment (inoculation agent, spheroidizing agent, inhibitor, capping agent, slag coagulant, etc.) in the ball casting vessel are divided into two parts and each is placed into one of the two spheroidization cavities. Following a predefined sequence for the sequential initiation of the two spheroidization cavities, the materials in the cavities are compacted and securely covered with iron chips or iron plates of varying thicknesses. The initiation time for the first cavity (primary initiation) is controlled so that initiation occurs between 30 and 40 seconds after the start of the molten iron filling process.The initiation time for the second cavity is delayed by 10 to 15 seconds compared to the primary initiation. The molten iron inside the ball casting pot undergoes a sequential and continuous spheroidization treatment in both cavities.

[0064] In this embodiment, ferrosilicon FeSi75 is used as the inoculant, with a total inoculant addition of 0.8%, corresponding to 9.6 kg. The spheroidizing agent used in this embodiment is rare-earth magnesium, while rare-earth magnesium and the metal tin (Sn) are used as inhibitors. Rare-earth magnesium serves as both an inhibitor and a spheroidizing agent, with a total addition of 1.9 wt% of the molten iron, corresponding to 22.8 kg. The addition of the metal tin, acting as an inhibitor, is 0.06 wt% of the molten iron in the sphere casting vessel, corresponding to 0.72 kg. The inoculant, the spheroidizing agent, and the inhibitor, etc., are added to the two spheroidizing cavities of the sphere casting vessel according to their intended initiation time, initiation time interval, and reaction time.In addition, appropriate amounts of capping agent and slag coagulant are added. Before filling the sphere casting vessel with molten iron, the carbon enrichment agent is placed on the bottom of the vessel, which must be positioned inside. During the filling of the molten iron, the molten iron is positioned so that it strikes the carbon enrichment agent directly. In this embodiment, the primary initiation time is 30 seconds, with the secondary initiation starting at the 43rd second (an interval of 13 seconds between the primary and secondary initiations). The total duration of the spheroidization reaction is 59 seconds.

[0065] Once the boiling of the molten iron in the ball casting kettle has ceased and the casting conditions are met, the casting of the part begins. During casting or when transferring the molten iron between kettles, slag-blocking rock wool is used at the nozzle of the molten iron kettle to prevent slag ingress. The wedge-shaped test block is cast 6 minutes after the start of the casting process. After cooling, chemical test samples are taken from this test block, which is then processed into a test bar in its as-cast state for testing mechanical properties and a metallographic test block for examining the metallographic structure.

[0066] The casting is removed from the mold 35 minutes after the casting process is complete, so that it can cool in the air.

[0067] The chemical composition of the test block analyzed after melting in 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, the remainder consisting of Fe and unavoidable impurities.

[0068] When performing metallic tensile tests on the aforementioned test specimen, the specimen in this embodiment exhibits a tensile strength of 842 MPa, an elongation of 6.5%, and a cross-sectional reduction rate of 5%. The pearlitic content in the metallographic structure is over 99%, and the pearlitic structure predominantly exhibits a lamellar morphology. The graphite grade is classified as Grade 2, with the proportion of globular graphite exceeding 90%. The metallographic structure of the metallographic test block obtained in this embodiment is as follows: Fig. 3 and Fig.Figure 4 illustrates this. Both the tensile test bar and the metallographic test block are in their as-cast condition and are not subjected to any heat treatment such as quenching or normalizing. The metallographic structure corresponds to the as-cast structure, and the mechanical properties correspond to those of the as-cast condition. Example 3

[0069] This embodiment builds upon embodiment 2, wherein an additional 0.3 wt.% copper and 0.3 wt.% molybdenum are added to all the molten iron within the sphere casting pot. The chemical composition of the casting obtained after melting is as follows: 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, 0.029% Re.

[0070] In this embodiment, the addition of the strongly carbide-forming elements Cu and Mo, in conjunction with Cr and Mo, which entered the furnace batch due to insufficient testing, leads to the formation of block-like carbides within the pearlite matrix. This increases the strength of the ductile iron but significantly reduces its toughness. The mechanical properties of the test bar (in the as-cast condition) are as follows: tensile strength 961 MPa, elongation 4.2%, with the pearlitic microstructure comprising 99% of the matrix structure.

[0071] The following embodiments 4 to 9 illustrate only the most important technical solutions and features of the present invention. In all embodiments 4 to 9, rare-earth magnesium 3-8 spheroidizing agents are used.The main differences from embodiments 1 and 2 are as follows: different amounts of carbon enrichment agent added; differences in the total amount of spheroidizing agent added and the tin or antimony added as one of the inhibitors, as well as its amount added in the different embodiments; differences in the number of spheroidization treatment stages (number of spheroidization cavities), the initiation time of the spheroidizing agent, the initiation time interval in different spheroidization cavities, and the overall initiation reaction time; consequently, the final ductile iron castings obtained differ in terms of carbon content, other primary chemical compositions, residual inhibitor content (magnesium oxide, rare earth elements, Sn or Sb), and mechanical properties. Example 4

[0072] The melting plants for molten iron and primary materials used in this embodiment are identical to those in embodiments 1 and 2. The differences from embodiments 1 and 2 are as follows: in this embodiment, the amount of carbon enrichment added to the melting furnace is determined such that the carbon enrichment is added when a carbon content of 3.2% is reached in the molten iron in the melting furnace. This means that the carbon content provided by the added carbon enrichment, together with the carbon content of the steel scrap and the carbon content of the recycled scrap, reaches a carbon content of 3.2% in the molten iron in the melting furnace. The carbon enrichment placed in the ball casting pot increases the carbon content of the molten iron by a further 0.8%.The carbon enrichment agent to be added to the melting furnace is mixed with the steel scrap and recycled scrap and added to the furnace. The carbon enrichment agent is placed in the spaces between the steel scrap or recycled scrap. The melting process then begins. While the furnace materials melt, the remaining steel scrap and recycled scrap are continuously added to the melting furnace until all the materials to be added have completely melted.

[0073] The metals tin and rare-earth magnesium are selected and used as inhibitors. The total amount of rare-earth magnesium added (serving both as a spheroidizing agent and as one of the inhibitors) is 1.65 wt% of the molten iron in the shot mold. The amount of tin added is 0.025 wt% of the molten iron in the shot mold, which corresponds to 0.25 kg of tin per ton of molten iron added.

[0074] This embodiment uses a one-stage spheroidization treatment in which the inoculant, spheroidizing agent, inhibitor, slag coagulant, and capping agent, etc., are placed in a spheroidization cavity. The time from the start of filling with molten iron to the initiation of the spheroidizing agent is 34 seconds in this embodiment, with the spheroidization reaction itself lasting 39 seconds after initiation.

[0075] The chemical composition of the test block obtained in this embodiment is as follows: 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 remaining components consisting of Fe and unavoidable impurities. The ductile cast iron product obtained in this embodiment exhibits a tensile strength of 703 MPa, an elongation of 10.8%, and a pearlitic content of 62% within the matrix structure. Example 5

[0076] The differences between this embodiment and embodiment 4 are as follows: in this embodiment, if a carbon content of 4.2% in the molten iron is achieved in the furnace by the carbon enrichment agent and the raw materials for melting, the carbon enrichment agent is added to the melting furnace. The carbon enrichment agent placed in the ball casting pot increases the carbon content of the molten iron by a further 0.8%. The melting temperature in this embodiment is 1570 °C, and the casting temperature is 1550 °C.

[0077] The inhibitors used are the metal tin and rare-earth magnesium. The total amount of spheroidizing agent added is 1.9 wt% of the molten iron in the ball casting pot. The amount of tin added is 0.06 wt% of the molten iron, which corresponds to 0.6 kg of tin per ton of molten iron added.

[0078] This embodiment employs a two-stage spheroidization treatment in which the inoculant, spheroidizing agent, inhibitor, slag coagulant, and capping agent, etc., are distributed in two spheroidization cavities. The time from the start of filling the molten iron to the primary initiation of the spheroidizing agent is 37 seconds in this embodiment, with the time interval between the secondary and primary initiation being 22 seconds, and the entire spheroidization reaction lasting 60 seconds since the primary initiation.

[0079] The chemical composition of the test block obtained in this embodiment is as follows: 4.95% C, 2.07% Si, 0.32% Mn, 0.024% S, 0.031% P, 0.053% Sn, 0.051% MgO, 0.031% Re, with the remaining elements consisting of Fe and unavoidable impurities. The ductile cast iron product obtained in this embodiment exhibits a tensile strength of 798 MPa, an elongation of 7.2%, and a pearlitic content of 99% within the matrix structure. Example 6

[0080] The differences between this embodiment and embodiment 4 are as follows: the carbon enrichment agent added to the melting furnace in this embodiment enables a carbon content of 3.5% in the molten iron. Additionally, the carbon content is increased by a further 1.0% in the casting kettle. The inhibitors used are the metals Sb and rare-earth magnesium. The total amount of rare-earth magnesium added as a spheroidizing agent is 1.7 wt% of the molten iron. The amount of antimony added is 0.06 wt%, which corresponds to 0.6 kg Sb per ton of molten iron. The melting temperature in this embodiment is 1560 °C, and the casting temperature is 1580 °C.

[0081] This embodiment uses a two-stage initiation spheroidization treatment method in which the inoculant, spheroidizing agent, inhibitor, slag coagulant, and capping agent, etc., are each introduced into two spheroidization cavities. The time from the start of filling the molten iron cavities to the primary initiation of the spheroidizing agent is 28 seconds in this embodiment, with the time interval between the secondary and primary initiation being 25 seconds, and the entire spheroidization reaction after the primary initiation lasting 53 seconds.

[0082] The chemical composition of the test block obtained in this embodiment is as follows: 4.48% C, 2.0% Si, 0.29% Mn, 0.027% S, 0.034% P, 0.042% Sn, 0.046% MgO, 0.022% Re, with the remaining chemical composition consisting of Fe and unavoidable impurities. The ductile cast iron product obtained in this embodiment exhibits the following mechanical properties: a tensile strength of 735 MPa, an elongation of 8.7%, and a pearlitic content of 90% within the matrix structure. Example 7

[0083] The differences between this embodiment and embodiment 4 are as follows: in this embodiment, the quantity of carbon enrichment agent added to the melting furnace is such that the sum of the carbon content of the carbon enrichment agent and the carbon content of the raw materials to be melted reaches a carbon content of 4.3% in the molten iron in the melting furnace. The weight of the carbon enrichment agent placed in the ball casting pot increases the carbon content of the molten iron by a further 1.3%.

[0084] The inhibitors used are the metal tin and rare-earth magnesium. The total amount of rare-earth magnesium added is 1.9 wt% of the molten iron. The amount of tin added is 0.07 wt% of the molten iron in the shot casting kettle, which corresponds to 0.7 kg of tin per ton of molten iron added.

[0085] This embodiment uses a two-stage spheroidization treatment. The time from the start of filling the molten iron to the primary initiation of the spheroidizing agent is 31 seconds, with the time interval between the secondary and primary initiation being 23 seconds, and the entire spheroidization reaction after the primary initiation taking 63 seconds.

[0086] The chemical composition of the test block obtained in this embodiment is as follows: 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 remaining components consisting of Fe and unavoidable impurities. The ductile cast iron product obtained in this embodiment exhibits a tensile strength of 872 MPa, an elongation of 4.1%, and a pearlitic content of 99% within the matrix. Example 8

[0087] The differences between this embodiment and the embodiments mentioned above are as follows: in this embodiment, when a carbon content of 4.5% is reached in the molten iron in the furnace, the amount of carbon enrichment agent to be added to the furnace is calculated. When the carbon enrichment agent increases the carbon content of the molten iron in the casting pot by a further 1.7%, it is placed in the casting pot. The melting temperature in this embodiment is 1590 °C, and the casting temperature is 1600 °C.

[0088] Tin and rare-earth magnesium are selected and used as inhibitors. The total amount of rare-earth magnesium added is 2.1 wt% of the molten iron. The amount of tin added is 0.9 wt% of the molten iron in the shot casting pot, which corresponds to 0.9 kg of tin per ton of molten iron added.

[0089] This embodiment uses a three-stage spheroidization treatment in which the inoculant, spheroidizing agent, inhibitor, slag coagulant, and capping agent, etc., are distributed into three spheroidization cavities. The initiation time and spheroidization reaction time in this embodiment are as follows: the primary initiation time is 35 seconds, the time interval between the secondary and primary initiation is 21 seconds, the time interval between the tertiary and secondary initiation is 18 seconds, and the entire spheroidization reaction after primary initiation lasts 81 seconds.

[0090] The chemical composition of the test block in this embodiment is as follows: 6.0% C, 1.99% Si, 0.34% Mn, 0.030% S, 0.033% P, 0.083% Sn, 0.067% MgO, 0.032% Re, with the remaining chemical composition consisting of Fe and unavoidable impurities. The ductile cast iron product obtained in this embodiment exhibits a tensile strength of 907 MPa, an elongation of 4.3%, and a pearlitic content of 99% within the matrix structure. Example 9

[0091] The differences between this embodiment and the embodiments mentioned above are as follows: if a carbon content of 4.1% in the molten iron is achieved in the melting furnace by the carbon enrichment agent and the raw materials for melting, and the carbon enrichment agent placed in the ball casting pot increases the carbon content of the molten iron by a further 2.5%, the carbon enrichment agent is distributed in the furnace and the ball casting pot in such a proportion. The melting temperature in this embodiment is 1580 °C, and the casting temperature is 1620 °C.

[0092] The inhibitors used are the metal antimony and rare-earth magnesium. The total amount of rare-earth magnesium added as a spheroidizing agent is 2.0 wt% of the molten iron in the sphere casting vessel. The amount of antimony added is 0.06 wt% of the molten iron, which corresponds to 0.6 kg of antimony per ton of molten iron added.

[0093] This embodiment uses a three-stage spheroidization treatment in which the inoculant, spheroidizing agent, inhibitor, slag coagulant, and capping agent, etc., are distributed into three spheroidization cavities. The initiation time and spheroidization reaction time in this embodiment are as follows: the primary initiation time is 32 seconds, the time interval between the secondary and primary initiation is 24 seconds, the time interval between the tertiary and secondary initiation is 25 seconds, and the entire spheroidization reaction after initiation lasts 88 seconds.

[0094] The chemical composition of the test block obtained in this embodiment is as follows: 6.2% C, 1.48% Si, 0.27% Mn, 0.029% S, 0.036% P, 0.05% Sn, 0.065% MgO, 0.037% Re, with the remaining chemical elements consisting of Fe and unavoidable impurities. The ductile cast iron product obtained in this embodiment exhibits a tensile strength of 1012 MPa, an elongation of 4.0%, and a pearlitic content of 99% within the matrix structure. The ductile iron product in this embodiment is primarily used in friction pair products. Example 10

[0095] The differences between this embodiment and embodiment 4 are as follows: in this embodiment, if a carbon content of 4.0% in the molten iron is achieved in the melting furnace by the carbon enrichment agent and the raw materials, the carbon enrichment agent is added to the melting furnace. In this embodiment, the carbon enrichment agent is not placed in the casting pot. The melting temperature in this embodiment is 1590 °C, and the casting temperature is 1550 °C.

[0096] The inhibitors used are the metal antimony and rare-earth magnesium. The total amount of rare-earth magnesium added as a spheroidizing agent is 1.6 wt% of the molten iron in the sphere casting vessel. The amount of antimony added is 0.04 wt% of the molten iron.

[0097] This embodiment uses a single-stage spheroidization treatment. The time interval from the start of filling with molten iron until the initiation of the spheroidizing agent is controlled to 25 seconds, with the spheroidization reaction lasting 47 seconds after initiation.

[0098] The chemical composition of the test block obtained in this embodiment is as follows: 4.0% C, 2.08% Si, 0.32% Mn, 0.024% S, 0.025% P, 0.03% Sn, 0.035% MgO, 0.021% Re, with the remaining chemical elements consisting of Fe and unavoidable impurities. The ductile cast iron product obtained in this embodiment exhibits a tensile strength of 583 MPa, an elongation of 12%, and a pearlitic content of 60% within the matrix structure.

[0099] From Exemplary Works 1 to 10, it can be seen that as the requirements for the properties of ductile iron increase (such as tensile strength and toughness), the carbon content in the ductile iron product also tends to increase. Correspondingly, the amount of added inhibitors (spheroidizing agents such as rare-earth magnesium and tin or sulfur) also increases. Furthermore, the pouring or melting temperature of the molten iron should be increased accordingly. (The melting temperature should be maintained to ensure complete dissolution of the carbon enrichment agent added to the furnace in the molten iron, typically at no less than 1550 °C. The pouring temperature is also increased, does not fall below 1550 °C, and is generally controlled above 1600 °C.) Alternatively, the melting time and the duration of stirring during beating and stirring during boiling can be extended accordingly.When the carbon content of the ductile iron product of the present invention is high, the tensile strength increases while the toughness decreases. The present invention uses Sn, Sb, and rare-earth magnesium as carbon element inhibitors, enabling the ductile iron matrix to have a pearlitic microstructure content of over 99% in the as-cast condition, consisting predominantly of lamellar pearlite. Without the addition of Cu, Mo, or other strongly carbide-forming elements, the process and inhibitors of the present invention rarely produce network-like or large nodular carbides. Consequently, high toughness is achieved while simultaneously improving the tensile strength of ductile iron products. Since the ductile iron produced by the present invention requires neither casting equipment with accelerated cooling nor heat treatments such as quenching or normalizing, it achieves the aforementioned high strength and toughness in the as-cast condition.This significantly reduces the production costs for ductile iron products. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] CN 115161539A

[0004] CN 101818297B

[0005] CN 106906333B [0007, 0036]

Claims

[1] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition, with a carbon content ≥ 4.0%, characterized by , that the spheroidal graphite cast iron contains no or only small additions of costly metal-reinforcing alloys, wherein during spheroidization of the molten iron during casting in the ball casting vessel an inhibitor to control the microstructure of the ductile iron and a sufficient amount of spheroidizing agent are added, wherein, without rapid cooling or heat treatment of a casting, a mechanical property of a ductile iron material in the as-cast condition QT700 or higher is achieved. [2] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to claim 1, characterized by, that the carbon content of ductile iron is 4.05% to 6.0%, where the inhibitor is a mixture of the spheroidizing agent and the weak carbide-forming elements. [3] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to one of claims 1 to 2, characterized by , that the spheroidizing agent is a magnesium-containing alloy spheroidizing agent and the inhibitor is the spheroidizing agent and the metal tin or the spheroidizing agent and the metal antimony, wherein an added amount of the spheroidizing agent and the metal tin or the metal antimony is intended to ensure low residues of MgO, Sn or Sb in the casting after casting. [4] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to claim 3, characterized by, that the small residues of MgO, Sn or Sb refer to a residual magnesium oxide content of not less than 0.03%, a residual tin content of not less than 0.02% or a residual antimony content of not less than 0.03%, where the total amount of spheroidizing agent added exceeds 1.6 wt% of the molten iron. [5] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to claim 4, characterized by, that the spheroidizing agent is a rare-earth magnesium spheroidizing agent, wherein, when rare-earth magnesium and tin are used as the inhibitor, the amount of inhibitor added is selected such that the residual magnesium oxide content in the solidified casting is between 0.03% and 0.065% and the residual tin content is between 0.02% and 0.06%, wherein, when rare-earth magnesium and antimony are used as the inhibitor, the amount of inhibitor added is selected such that the residual magnesium oxide content in the solidified casting is between 0.03% and 0.065% and the residual antimony content is between 0.03% and 0.05%. [6] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to one of claims 1 to 2, characterized by, that during the spheroidization of the molten iron, a portion of a carbon enrichment agent is added when pouring it into the ball casting vessel, wherein, when filling the molten iron with the molten iron during pouring into the ball casting vessel, the carbon enrichment agent present in the ball casting vessel is first stirred and then reacts with the spheroidizing agent for the spheroidization treatment, while at the same time the carbon enrichment agent in the ball casting vessel is stirred further during boiling, wherein a melting temperature and a pouring temperature of the molten iron are both at least 1550 °C. [7] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to claim 6, characterized by, that the addition of the carbon enrichment agent to the ball casting vessel relates to separating a portion of the carbon enrichment agent, which serves as the total carbon content of the casting, and adding it to the ball casting vessel, wherein the portion of the carbon enrichment agent is then dissolved in the molten iron during pouring, wherein the carbon enrichment agent added to the ball casting vessel provides a carbon content of no more than 2.5% for the casting, wherein the remaining carbon content of the casting is provided by the carbon enrichment agent and other melting raw materials in the melting furnace. [8] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to claim 7, characterized by, that during melting in the melting furnace the added carbon enrichment agent is placed in the melting furnace together with steel scrap or recycled scrap, whereupon melting begins until all materials required for the addition are completely melted, wherein the carbon enrichment agent placed in the ball casting pot provides a carbon content of 0.8% to 2.5% for the casting, wherein an inoculant, the spheroidizing agent, the inhibitor, a capping agent and a slag coagulation agent are placed in a spheroidizing cavity of the ball casting pot, wherein an initiation time of the spheroidizing agent and a boiling spheroidizing time after initiation are controlled, wherein the temperature of the molten iron when poured from the furnace into the ball casting pot is 1550 to 1620 °C. [9] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to claim 8, characterized by , that based on the weight of the molten iron to be spheroidized in the ball casting vessel and the property requirements for the spheroidal graphite cast iron, one or two or more spheroidization cavities are configured within the ball casting vessel, controlling a time interval from the pouring of the molten iron to the initiation of the spheroidizing agent, as well as an initiation sequence, the initiation time interval, and the spheroidization boiling reaction time for each spheroidization cavity. [10] High-carbon, high-strength and high-toughness spheroidal graphite cast iron in the as-cast condition according to one of claims 1 to 2, characterized by, that the costly metal-reinforcing alloys are copper, molybdenum or other strongly carbide-forming elements, wherein the absence of costly metal-reinforcing alloys refers to an element content that does not exceed the permissible impurity level, wherein the low additions of costly metal-reinforcing alloys mean that the addition amount of each costly metal-reinforcing alloy does not exceed 0.3 wt.% of the molten iron.

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