A high-carbon, as-cast, high-strength, high-toughness ductile iron

Abstract

This invention discloses a high-carbon, as-cast, high-strength, high-toughness ductile iron, belonging to the field of ductile iron casting. The ductile iron has a carbon content ≥4.0% and contains little or no expensive metal strengthening alloys. During the spheroidization process after the molten iron is tapped from the furnace, an inhibitor to control the microstructure of the ductile iron and a sufficient amount of spheroidizing agent are added to the spheroidizing ladle. The inhibitor is a mixture of the spheroidizing agent and a weak carbide-forming element. The amount of inhibitor added should ensure that a small amount remains in the casting after pouring. The ductile iron product of this invention achieves high strength and high toughness in the as-cast state without quenching or normalizing heat treatment, and its production cost is far lower than that of ductile iron products whose performance is improved by alloying.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of ball mill cast iron materials, especially a kind of high strength, high toughness in as-cast state namely nodular cast iron and its casting method, belong to cast iron casting field. BACKGROUND

[0002] In the field of machinery manufacturing and metal processing, iron-carbon alloy with carbon content greater than 2.11% is generally referred to as "iron", commonly known as cast iron. As a kind of cast iron, the free carbon in nodular cast iron (referred to as nodular iron) mainly exists in the form of nearly spherical graphite. After casting and heat treatment, the matrix structure is mainly pearlite or a mixture of pearlite and ferrite, and a small amount of carbide. However, most of the excess carbon is distributed in the matrix structure in the form of spherical graphite. Nodular cast iron may also contain a small amount of impurities. The pearlite in the matrix of nodular cast iron is composed of lamellar cementite and ferrite. Nodular cast iron can achieve high strength and high toughness at the same time, and has good comprehensive mechanical properties, so it is widely used.

[0003] In the nodular cast iron production industry, the carbon equivalent of nodular cast 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 is greater than 4.3%, it is called peritectic cast iron. At this time, the metallographic structure of the cast iron is mainly cementite and ledeburite (ledeburite is a eutectic body composed of cementite and austenite). The material is brittle and has almost no elongation, which does not meet the performance requirements of nodular cast iron. Therefore, those skilled in the art will control the carbon content of nodular cast iron to be below 4.0% to make the carbon equivalent of nodular cast iron products less than 4.3%. When it is necessary to further improve the strength of nodular cast iron, alloying elements such as Cu and Mo are added to form alloy carbides in the matrix of nodular cast iron.

[0004] Obtaining nodular cast iron with high strength and good toughness is the goal pursued by people. However, the current method for improving the strength of nodular cast iron is to add alloying elements to the nodular cast iron. For example, patent document CN115161539A discloses an isothermal quenching nodular cast iron planetary gear blank and its preparation process. The nodular cast iron casting includes 3.6%-3.80% C by weight. When the molten iron is melted in an electric furnace, the carbon, silicon and copper, nickel, molybdenum alloy content in the raw materials such as pig iron and scrap steel is controlled at 1.5%-2.0% Ni, 0.8%-0.9% Cu, 0.2%-0.25% Mo. A certain amount of carbon additive, spheroidizing agent and inoculant is added to the spheroidizing package for spheroidizing treatment. After isothermal quenching, the mechanical properties can reach tensile strength ≥820 MPa, yield strength ≥600 MPa, elongation ≥4.0%, hardness 280-310 HB, spheroidizing rate ≥90%.

[0005] For example, patent document CN101818297B describes a method for manufacturing a high-strength pearlitic spheroidal graphite cast iron calender forming mold material, which controls the alloying elements C 3.69% to 4.04%, Cu 0.32% to 0.65%, Mo 0.148% to 0.171%, Mn 0.44% to 0.54%, Pb 0.031% to 0.035%, etc. by weight percentage during the casting process, and then performs alloy strengthening on the spheroidal graphite, and then performs normalizing and tempering heat treatment, so that the mechanical properties of the castings reach yield limit σ 0.2 = 655 to 680 MPa, tensile strength σ b = 965 to 1050 MPa, elongation after break = 3% to 5%, hardness = 264 to 272 HB.

[0006] The current spheroidal graphite cast iron can be divided into many standard grades or types according to different properties, such as wear-resistant spheroidal graphite cast iron, heat-resistant spheroidal graphite cast iron, and friction-reducing spheroidal graphite cast iron. The state of the graphite balls in the spheroidal graphite cast iron has a standardized spheroidization grade, including spheroidization rate, roundness, spheroidization quantity, dispersity, and uniformity of the distribution of the balls. During the casting of the spheroidal graphite cast iron, spheroidizing agents, inoculants, etc. are often added to promote the spheroidization of the C element. The general inoculant is silicon iron, and the spheroidizing agent is a metal alloy containing magnesium Mg, such as a rare earth magnesium alloy, which is commonly used by those skilled in the art as a spheroidizing agent. During spheroidization, the amount of the rare earth magnesium added is generally not more than 1.4% of the weight of the iron liquid in the spheroidizing ladle. The spheroidizing agent, the inoculant, and the covering agent, the slag collecting agent, etc. are placed in the recess provided at the bottom of the spheroidizing ladle, and then tamped and covered with an iron plate or tamped iron chips to control the boiling time of the reaction (commonly known as detonation) of the spheroidizing agent with the iron liquid.

[0007] As the main organization and constituent element of ductile iron, graphite has the characteristics of high melting point and light specific gravity, and it is difficult to dissolve into the molten iron in large quantities. Therefore, when the C content in ductile iron reaches a certain content, it is difficult to increase the C content in the molten iron. The patent document CN106906333B discloses a carbon addition method, which is to add carbon additive and molten iron into a smelting furnace or a ladle. When the carbon additive cannot be directly dissolved in the molten iron and floats on the surface of the molten iron, the molten iron and the carbon additive are poured from one molten iron container into another molten iron container at least once within 20 seconds, and the temperature of the molten iron is controlled at 1450-1700℃. Engineering practice and research in the field of casting believe that when the carbon content of ductile iron is too high, a large amount of free carbide will be produced. Free carbide is generally considered to be harmful organization in ductile iron, which will split the matrix organization and reduce the toughness of ductile iron, but it will increase the hardness and wear resistance of cast iron. If the morphology of cementite is network or block, the overall performance of ductile iron will decrease. When the designed carbon content of ductile iron is too high, C will be difficult to dissolve in the molten iron, and graphite will float, causing the whole molten iron or the casting to be scrapped. In addition, the chemical composition of ductile iron will have a great influence on the matrix organization, spheroidization state and performance of ductile iron. With different carbon contents and different states of C existing in the metal, the mechanical properties of ductile iron will also have great differences.

[0008] In the field of casting, in order to improve the mechanical strength of ductile iron, the skilled person in the art often adds strong carbide forming elements such as chromium Cr, tungsten W, vanadium V, molybdenum Mo, nickel Ni, copper Cu and manganese Mn to cast iron to increase the pearlite content of ductile iron. At the same time, it is also believed that after adding strong carbide forming elements which strongly affect the formation and morphology of cementite to cast iron, blocky carbides will be formed in ductile iron. For example, adding about 0.4%-0.8% Mo and Cu in ductile iron can improve the strength, which can make the content of pearlite reach 85%, but the remaining organization is basically composed of isolated and blocky carbides. Therefore, when using molybdenum and copper which are relatively expensive metals to strengthen cast iron, although the strength can be greatly improved, the elongation often decreases a lot.

[0009] In addition, the prices of metals such as chromium Cr, tungsten W, vanadium V, molybdenum Mo, nickel Ni, and copper Cu are high, these alloys are precious metal strengthening alloys in the casting field, and are often referred to as precious metals, so using alloy elements to strengthen cast iron can greatly increase 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 the price of carbon C, for example, in June 2023, the market price of molybdenum iron alloy containing not less than 55% of molybdenum (such as FeMo60) is about 250,000 yuan / ton, and the price of Mo element is about 410 yuan / Kg (41,000 yuan / ton), the market price of copper is about 70 yuan / Kg (7,000 yuan / ton), and the market price of rare earth magnesium as a spherulitic agent is about 10 yuan / Kg. According to the conventional calculation that each ton of castings contains 0.6% of Mo and Cu, each ton of ductile iron needs to use 420 yuan of Cu and 2,400 yuan of molybdenum, that is, the cost of each ton of cast iron will increase by 2,820 yuan.

[0010] In addition, in the current production process of ductile iron parts, in addition to casting, heat treatment processes such as quenching or normalizing must be performed, or a rapid undercooling casting process is used when pouring the castings, so that the tensile strength and toughness of the ductile iron products can meet the design requirements. The rapid undercooling casting process and equipment are complex, the cost of casting is high, and there is a risk of deformation or cracks in the castings; heat treatment will generate at least hundreds of yuan per ton of cost and consume energy again, so these methods will increase the cost of products. SUMMARY

[0011] The technical problem to be solved by the present application is to provide a low-cost nodular cast iron which can achieve high strength and high toughness in the as-cast state without quenching or normalizing heat treatment.

[0012] To solve the above technical problems, the technical scheme adopted by the present application is:

[0013] A high-carbon-content as-cast high-strength high-toughness nodular cast iron, the carbon content of the nodular cast iron is ≥4.0%, the nodular cast iron does not contain or contains less precious metal strengthening alloy, and a suppressor for controlling the morphology of the nodular cast iron and a sufficient amount of spherulitic agent are added into the spheroidizing ladle when the molten iron is spheroidized.

[0014] The further improvement of the present application is that the carbon content of the nodular cast iron is 4.05%-6.0%; and the suppressor is a mixture of the spherulitic agent and a weak carbide-forming element.

[0015] The further improvement of the technical scheme of the present application is that the spherulitic agent is a magnesium-containing alloy spherulitic agent, the suppressor is the spherulitic agent and metal tin, or the spherulitic agent and metal antimony, and the addition amount of the spherulitic agent and metal tin or metal antimony should ensure that there is a small amount of MgO, Sn or Sb remaining in the castings after pouring is completed.

[0016] The further improvement of the technical scheme of the present application is 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%; and the total amount of the added nodulizer is greater than 1.6% of the weight of the molten iron.

[0017] The further optimization of the technical scheme of the present application is that the nodulizer is a rare earth magnesium nodulizer; when the rare earth magnesium and tin are used as the inhibitor, the added amount of the inhibitor is such that the residual amount of tin in the solidified casting is between 0.02% and 0.06%, and the residual amount of magnesium oxide is between 0.03% and 0.065%; when the rare earth magnesium and antimony are used as the inhibitor, the added amount of the inhibitor is such that the residual amount of antimony in the solidified casting is between 0.03% and 0.05%, and the residual amount of magnesium oxide is between 0.03% and 0.065%.

[0018] The further improvement of the technical scheme of the present application is that part of the carbon additive is added into the nodulizing ladle when the molten iron is tapped and nodulized; when the tapped molten iron is injected into the nodulizing ladle, the carbon additive in the nodulizing ladle is first impacted and then reacts with the nodulizer to perform nodulizing treatment while the carbon additive in the nodulizing ladle is further boiled and stirred; the smelting temperature and the tapping temperature of the molten iron are both not less than 1550℃.

[0019] The further improvement of the technical scheme of the present application is that the carbon additive added into the nodulizing ladle is divided into two parts, one part is added into the smelting furnace during smelting, and the other part is added into the nodulizing ladle for solution; the carbon additive added into the nodulizing ladle provides a carbon content of not more than 2.5% for the casting, and the remaining carbon content of the casting is provided by the carbon additive and other smelting raw materials in the smelting furnace.

[0020] The further improvement of the technical scheme of the present application is that the carbon additive added into the smelting furnace is put into the smelting furnace together with the scrap steel or the recycled material, and then the smelting is started until all the required materials are smelted; the carbon additive put into the nodulizing ladle provides a carbon content of 0.8%-2.5% for the casting; the inoculant, the nodulizer, the inhibitor, the covering agent and the slag collecting agent are put into the nodulizing pit of the nodulizing ladle, and the time of the start of the explosion of the nodulizer and the boiling and nodulizing time after the explosion are controlled; the temperature of the molten iron tapped into the nodulizing ladle is 1550-1620℃.

[0021] The further improvement of the technical scheme of the present application is that according to the weight of the molten iron required to be nodulized in the nodulizing ladle and the performance requirements of the nodular cast iron, two or more nodulizing pits are arranged in the nodulizing ladle, and the time interval from the molten iron to the explosion of the nodulizer, the explosion sequence of each nodulizing pit, the explosion time interval and the boiling and nodulizing reaction time of each nodulizing pit are controlled.

[0022] The further improvement of the present application is that the noble metal strengthening alloy is one, two or three of copper, molybdenum or other strong carbide forming elements; the non-noble metal strengthening alloy refers to the element content not higher than the allowable content as impurities; the less noble metal strengthening alloy refers to the addition amount of each noble metal strengthening alloy not more than 0.3% of the weight of the iron liquid.

[0023] Thanks to the above technical solutions, the present application has the following technical progress:

[0024] When the present application is used to produce nodular cast iron products, the casting cost is low, only cheap C is used to improve the comprehensive mechanical properties of the nodular cast iron, no or less noble metal is added, no complex casting equipment and process such as undercooling treatment is needed, and high-strength and high-toughness nodular cast iron products can be obtained in the as-cast state without heat treatment such as quenching or normalizing. The nodular cast iron of the present application is suitable for all casting methods.

[0025] The carbon content of the nodular cast iron of the present application can easily break through 4.0%, breaking the conventional idea that the carbon content of nodular cast iron cannot be too high. Since the nodular cast iron necessarily contains Si and other carbon equivalent elements, the carbon equivalent of the nodular cast iron of the present application is generally above 4.3%, exceeding the carbon content of the eutectic point in the iron-carbon phase diagram. The present application widens the carbon content range of nodular cast iron, breaking the traditional idea that super-high carbon is harmful to the overall performance of nodular cast iron. By the method of boldly increasing carbon, the carbon content of the nodular cast iron is greatly increased, and by inhibiting the structure and morphology of C in the matrix, as well as the shape, quantity and distribution of free graphite, a nodular cast iron product with high strength and high toughness in the as-cast state, low cost and low price can be produced.

[0026] The nodular cast iron of the present application uses only cheap and abundant C as the main strengthening element of the nodular cast iron, and does not contain or actively add Cu, Mo and other precious metal strengthening alloys. The so-called non-precious metal strengthening alloy refers to the fact that the precious metal strengthening alloy element is not actively added, but may be an impurity brought in by the smelting raw material of the nodular cast iron. These are inevitable allowable contents and should not affect the performance of the nodular cast iron and should not be calculated as the alloy content of the nodular cast iron. The content of the element as an impurity in the nodular cast iron cannot be higher than a certain value. For example, titanium Ti has a great interference effect on graphite spheroidization. When the Ti content is too high, it will interfere with the spheroidization quality and thus reduce the toughness of the material. Therefore, the Ti content in the nodular cast iron of the present application must be strictly controlled. The present application adds tin, antimony and other metals and a sufficient amount of spheroidizing agent as inhibitors in the spheroidizing ladle, so that the super-high amount of C in the molten iron can be completely dissolved into the molten iron, without causing graphite floating phenomenon, and the shape, number and distribution of the graphite balls can be reasonably controlled, the morphology of the cementite in the matrix and the number of the pearlite can be controlled, and the lamella of the cementite and ferrite constituting the pearlite is thin and uniform, so that the tensile strength and toughness of the nodular cast iron are greatly improved.

[0027] The present application uses a large amount of carbon elements to strengthen the nodular cast iron by adding carbon additives and inhibitors in the spheroidizing ladle so that they are dissolved into the molten iron when the molten iron is spheroidized. The nodular cast iron of the present application can make the tensile strength of the as-cast product reach at least 600 MPa or more without adding precious metals and without subsequent heat treatment. In the case of high tensile strength, the elongation can reach at least 4.0% or more. For example, when the tensile strength of the nodular cast iron product reaches 850 MPa, the elongation can reach 6.5% or more. The carbon content of the nodular cast iron of the present application can be designed to be ≥4.05% according to the mechanical performance requirements of the nodular cast iron product. When the carbon content is in the commonly used range of 4.5%-6.0%, the nodular cast iron product with a performance requirement of QT800 or more can be obtained without adding precious metal strengthening alloys and without heat treatment. The nodular cast iron of the present application saves the precious metal alloys such as molybdenum and copper, does not need to use expensive supercooling casting equipment, and saves the heat treatment processes such as quenching and normalizing, thereby greatly reducing the casting cost and the price of the nodular cast iron product. According to the current market prices of molybdenum, copper and tin (tin is 200 yuan per kilogram, rare earth magnesium is about 10 yuan per kilogram, and the price of carbon additive is very cheap and can not be calculated), only the saving of raw materials can reduce the raw material cost of 2600 yuan per ton of castings. If the heat treatment cost is added, the production cost of 3000 yuan per ton of nodular cast iron castings can be saved.

[0028] The present application adds recarburizer into spheroidizing ladle, and dissolves the recarburizer into iron liquid in the spheroidizing ladle with enough high-temperature iron liquid, when the iron liquid starts to inject into the spheroidizing ladle, the iron liquid directly impacts the recarburizer before the spheroidizing agent explodes, at this time, the iron liquid strongly impacts and stirs the recarburizer, which can promote the recarburizer to dissolve into the iron liquid, after the spheroidizing agent in the spheroidizing ladle pit explodes and starts spheroidizing reaction, the magnesium oxide produced by the spheroidizing agent can make the iron liquid boil, and further stir and mix the iron liquid and the recarburizer in the spheroidizing process, so that the recarburizer in the spheroidizing ladle further dissolves into the iron liquid. The carbon dissolved into the iron liquid in the spheroidizing ladle can also make the crystal structure of the nodular iron matrix produce lattice distortion, and has the effect of solid solution strengthening, which further increases the mechanical strength of the castings in the as-cast state.

[0029] The present application usually adopts the method of adding the recarburizer into the iron liquid in two parts, one part is added into the smelting furnace when smelting the iron liquid, and the other part is added into the spheroidizing ladle when spheroidizing, the amount of the recarburizer added into the spheroidizing ladle is preferably 0.8%-2.5% of the carbon content of the castings, and the remaining carbon content of the castings is provided by the recarburizer, scrap steel, recycled material and other smelting raw materials in the smelting furnace. This is more conducive to the dissolution of carbon in the recarburizer into the iron liquid, forming fine pearlite matrix and high-grade spheroidal graphite. The method of adding recarburizer in the smelting furnace and the spheroidizing ladle can be called composite recarburization method, and when a larger amount of recarburizer is added into the spheroidizing ladle, better results can be achieved.

[0030] The composite recarburization method is to divide the recarburizer into two parts, one part is added into the smelting furnace when smelting, and the other part is added into the spheroidizing ladle when spheroidizing, the recarburizer for smelting in the furnace is placed in the smelting furnace together with scrap steel or recycled material, the recarburizer can be placed at the bottom of the smelting furnace together with the smelting raw materials such as recycled material, or part of the recarburizer can be placed in the gap of the scrap steel or recycled material, and then smelting is started, until all the materials to be added are smelted, so that part of the large amount of carbon can be fully dissolved into the iron liquid in advance during smelting; and the remaining carbon content is provided by the recarburizer placed in the spheroidizing ladle, and when the furnace is discharged, enough high-temperature iron liquid directly impacts the recarburizer placed in the spheroidizing ladle, and through the reaction of the inoculant, spheroidizing agent, inhibitor, covering agent, slag collecting agent and iron liquid placed in the spheroidizing pit of the spheroidizing ladle, the starting time and boiling spheroidizing time after explosion of the spheroidizing agent and the explosion sequence are controlled, so that the C in the recarburizer in the spheroidizing ladle is fully dissolved into the iron liquid, and the iron liquid is further recarburized. In this way, the iron liquid after spheroidizing can contain a very high carbon content, and easily break through the carbon content of 4.0%.

[0031] The present application sets two or more spheroidizing pits for storing inoculants, spheroidizing agents, inhibitors, covering agents, and other materials in the spheroidizing ladle, and performs multi-stage spheroidizing treatment on the molten iron poured into the spheroidizing ladle. According to the weight of the molten iron to be spheroidized and the performance requirements of the spheroidal graphite iron in the spheroidizing ladle, the initiation time, initiation sequence, initiation interval time, and boiling reaction time are controlled. In this way, the spheroidizing reaction time and boiling intensity of the molten iron when magnesium oxide is added are increased and controlled, and even when the amount of the carbon additive in the spheroidizing ladle is large, the molten iron can still be smoothly dissolved. When the material and mechanical performance requirements of the spheroidal graphite iron product are higher, generally, multiple pits are set to perform multi-stage treatment, and especially when the toughness requirements such as elongation and area reduction are high, multi-stage treatment is more required. Of course, when the amount of the carbon additive in the spheroidizing ladle is small, one or a continuous spheroidizing pit can also be used to perform spheroidizing treatment, and the effect can also be achieved.

[0032] The inhibitor used in the present application is a spheroidizing agent and metal tin, or a spheroidizing agent and metal antimony. The spheroidizing agent in the field of spheroidal graphite casting is mostly a magnesium-containing alloy spheroidizing agent, such as a rare earth magnesium spheroidizing agent with different magnesium contents, which can all be used as the spheroidizing agent of the present application. The total amount of the spheroidizing agent added as the inhibitor and the spheroidizing agent used for spheroidizing is greater than the amount of the spheroidizing agent used in conventional spheroidal graphite production, and generally needs to be added to more than 1.6% of the weight of the molten iron, while the amount of the spheroidizing agent added in conventional spheroidal graphite production does not exceed 1.4%. The amount of each component of the inhibitor needs to be added to ensure that a small amount of the added inhibitor elements remains in the castings after pouring is completed, so that C can be fully dissolved into the molten iron, and good matrix structure morphology and pearlite amount, as well as high-grade graphite spheres, can be obtained.

[0033] The spheroidizing agent and the spheroidizing agent used as the inhibitor in the present application are both rare earth magnesium spheroidizing agents used in conventional spheroidal graphite casting. In addition to having a spheroidizing function, magnesium is also a weak carbide-forming element in the present application, and the carbide formed thereby is not very high in hardness and strength, but it can increase the formation of pearlite and the amount of pearlite, and help the ordered arrangement of cementite and ferrite in the pearlite. The tin or antimony in the inhibitor of the present application is also a weak carbide-forming element, and has the same effect as magnesium in spheroidal graphite, and can control the amount and structure morphology of the pearlite and graphite spheres. The addition amount of rare earth magnesium and tin is such that the residual amount of tin in the solidified castings is not less than 0.02%, and is preferably between 0.02% and 0.06%; and the residual amount of magnesium oxide is not less than 0.03%, and is preferably between 0.03% and 0.065%. When rare earth magnesium + antimony is used as the inhibitor, the addition amount of rare earth magnesium and antimony is such that the residual amount of antimony in the solidified castings is not less than 0.03%, and is preferably between 0.03% and 0.05%; and the residual amount of magnesium oxide is not less than 0.03%, and is preferably between 0.03% and 0.065%. In this way, a sufficient amount of C can be fully dissolved into the molten iron to form a high-strength and high-toughness structure, and raw materials are not wasted.

[0034] If the strength of the ductile iron is to be further improved, the invention can add a small amount of Cu, Mo and other precious metal strengthening alloy during the casting process, but the Ti alloy element cannot be added or brought into the ductile iron too much, because Ti will affect the spheroidization of graphite, thereby greatly reducing the toughness of the ductile iron. The addition amount of each precious metal strengthening alloy does not exceed 0.3% of the weight of the iron liquid, so that the cost of the raw materials for casting will not increase too much. According to the maximum amount of 0.3% and the current market price, the increased raw material cost (taking Cu and Mo as examples) is 1473 yuan, and the performance of the as-cast ductile iron material can reach a tensile strength of 960 MPa or more and an elongation of 4% or more. This kind of ductile iron product can be used under special requirements. While the ordinary conventional grade of ductile iron does not add precious alloy, only uses the method of increasing the carbon content of the ductile iron, and does not use the process of improving the mechanical properties such as heat treatment and supercooling casting, the as-cast casting can fully meet the performance requirements. The production cost of the ductile iron product of the invention is much lower than that of the conventional ductile iron with the same mechanical properties. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 is the graphite organization metallographic photo (X100) of the casting obtained by the embodiment 1 of the invention;

[0036] Figure 2 is the base body organization metallographic photo (X400) of the casting obtained by the embodiment 1 of the invention;

[0037] Figure 3 is the graphite organization metallographic photo of the casting obtained by the embodiment 2 of the invention;

[0038] Figure 4 is the base body organization metallographic photo of the casting obtained by the embodiment 2 of the invention. DETAILED DESCRIPTION

[0039] The present application relates to a high-carbon-content high-strength high-toughness as-cast ductile iron, which contains carbon content of 4.0% or more, and does not contain precious metal strengthening alloy, wherein the precious metal strengthening alloy refers to the element content of no more than the allowable content as impurities, even if the precious metal strengthening alloy is found in the detection of chemical elements of the ductile iron, it is as impurities, which may be brought in the smelting raw materials. The carbon content of the commonly used ductile iron can be designed to be between 4.05% and 6.0%, which can meet the performance requirements of all ductile iron products. When the molten iron is tapped and spheroidized, a sufficient amount of spheroidizing agent and inhibitor is added in the spheroidizing ladle. The inhibitor is a mixture of spheroidizing agent and weak carbide forming element, the spheroidizing agent is magnesium-containing alloy spheroidizing agent, the inhibitor is spheroidizing agent and metal tin, or spheroidizing agent and metal antimony, such as any magnesium-containing rare earth magnesium spheroidizing agent. The inhibitor can control the morphology of the pearlite and graphite in the cast iron. The total amount of the spheroidizing agent and the spheroidizing spheroidizing agent as the inhibitor is greater than the amount used in general ductile iron production, the amount of spheroidizing agent and metal tin or metal antimony is ensured to be a small amount remaining in the castings after pouring, and the total amount of rare earth magnesium as the inhibitor and spheroidizing agent is greater than 1.6% of the weight of the molten iron. The amount of the inhibitor and the spheroidizing agent is generally controlled according to the following residual amount: 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% when antimony is used. For example, when rare earth magnesium and tin are used as the inhibitor, the amount of the inhibitor is preferably such that the residual amount of tin in the solidified castings is between 0.02% and 0.06%, and the residual amount of magnesium oxide is between 0.03% and 0.065%; when antimony + rare earth magnesium is used as the inhibitor, the amount of the inhibitor is preferably such that the residual amount of antimony in the solidified castings is between 0.03% and 0.05%, and the residual amount of magnesium oxide is between 0.03% and 0.065%. Too high will cause waste, and too high or too low will affect the morphology and performance of the ductile iron.

[0040] The present application also adds carbon additive in the spheroidizing ladle when the molten iron is tapped and spheroidized. When the molten iron is injected into the spheroidizing ladle, the molten iron first impacts the carbon additive placed in the spheroidizing ladle, and then reacts with the spheroidizing agent in the pit to perform spheroidizing treatment. The magnesium oxide generated by the reaction of the spheroidizing agent with the molten iron will further boil and stir the carbon additive in the spheroidizing ladle. The smelting temperature and tapping temperature of the molten iron are generally higher than the temperature of the conventional method of smelting ductile iron, which can be controlled to be not less than 1550℃. When the carbon content is high, the smelting temperature and tapping temperature can be appropriately increased according to the situation.

[0041] The method for adding the recarburizer into the spheroidizing ladle is to divide the recarburizer into two parts, one part is added into the smelting furnace during smelting, and the other part is added into the spheroidizing ladle, wherein the recarburizer added into the spheroidizing ladle provides the castings with a carbon content of not more than 2.5%, and the optimal range is controlled in the range of 0.8%-2.5% of the carbon content, that is, the recarburizer added into the spheroidizing ladle can increase 0.8%-2.5% of C in the molten iron; the remaining carbon content of the castings 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 recycled materials are put into the smelting furnace together, generally, part or all of the recarburizer and part of the recycled materials are put into the bottom of the smelting furnace, then the scrap steel is added, and finally the recycled materials are added until the furnace is full, when the amount of the recarburizer added is relatively large, part of the recarburizer needed to be put into the furnace can be added into the gap of the scrap steel; then the smelting is started, and with the melting of the furnace charge, the remaining furnace charge is continuously added into the smelting furnace until the smelting of all the materials to be added is completed; and the recarburizer put into the spheroidizing ladle is dissolved with the molten iron. The inoculant, spheroidizing agent, inhibitor, covering agent and slag collecting agent are put into the spheroidizing pit of the spheroidizing ladle, the molten iron reacts with the spheroidizing agent after contacting with the spheroidizing agent and produces magnesium oxide, and the time of starting the explosion of the spheroidizing agent and the boiling spheroidizing time after the explosion are controlled; before the furnace is discharged, the spheroidizing ladle is heated and the required amount of recarburizer is put into the spheroidizing ladle, then the molten iron is used to directly impact the recarburizer at the bottom of the spheroidizing ladle, after the molten iron reacts with the spheroidizing agent, the magnesium oxide produced by the rare earth magnesium stirs the molten iron and the recarburizer. The temperature of the molten iron discharged into the spheroidizing ladle is generally 1550-1620℃, which is slightly higher than the discharge temperature of the conventional ductile iron.

[0042] The present application is to make the carbon content in the ductile iron exceed the carbon content of the conventional ductile iron. And the scrap steel or recycled material of the ductile iron is used as the main raw material for the convenience of controlling the carbon content and the contents of S and P, then the recarburizer and the recarburizing method are used to add sufficient C into the ductile iron, so that the ductile iron product can meet the design mechanical property requirements in the as-cast state. There are many kinds of recarburizers in the casting industry, for example, the patent document CN106906333B discloses that any proportion of one or more than two kinds of pure graphite, graphite powder or graphite electrode can be used as a recarburizer. In addition, materials containing carbon or graphite, such as coal, charcoal, coke, coke slag, etc. can also be used as recarburizers, and commercially available tar recarburizer is also a commonly used recarburizer in the casting industry. The C content in different recarburizers is very different, and the absorption amount of the molten iron to different recarburizers is also different, so the added weight of different recarburizers is also different. In the actual production process of ductile iron, the added weight of different recarburizers should be calculated according to the carbon content of different smelting raw materials of the ductile iron and the carbon content and absorption amount of the used recarburizer, so that the final cast product can meet the carbon content requirements.

[0043] In order to increase the time of spheroidizing reaction and boiling stirring, two or more than two pits can be arranged in the spheroidizing ladle, according to the weight of the molten iron to be spheroidized in the spheroidizing ladle and the performance requirements of the ductile cast iron, the time interval from the molten iron to the spheroidizing agent to start the reaction, the spheroidizing pit starting sequence, the spheroidizing pit starting time interval, and the boiling reaction time after starting. In addition to the impact stirring of the tapping molten iron on the recarburizer, the magnesium oxide gas generated by the reaction of the spheroidizing agent with the molten iron will perform boiling stirring on the molten iron, prolong the stirring time, and make the inoculant, spheroidizing agent and other materials contact with the molten iron more fully, so that C can be dissolved into the molten iron in large quantities, which is beneficial to the formation of flake pearlite and fine dispersed graphite balls.

[0044] If it is necessary to further improve the strength or other mechanical properties of the ductile iron product, a small amount of precious metal strengthening alloy can be added to the above-mentioned ductile iron, and the precious metal strengthening alloy is one, two or three of copper, molybdenum, vanadium or other strong carbide forming elements. The addition amount of the precious metal strengthening alloy is not more than 0.3% of the weight of the molten iron for each precious metal strengthening alloy. The addition amount of the precious metal strengthening alloy is lower than that in other ductile irons other than the present application, and Ti cannot be added, otherwise it will affect the toughness of the ductile iron.

[0045] The present application will be further described in detail below in conjunction with examples.

[0046] Example 1

[0047] The molten iron smelting equipment used in this example is a 1.5T induction furnace, and 2T of molten iron is smelted in one furnace.

[0048] The main furnace charge used includes: 25% of the recycled material, a total of 500Kg; 71% of the scrap steel with a carbon content of 0.3%-0.45%, a total of 1420Kg; 0.5% of ferrosilicon (FeSi75), a total of 10Kg.

[0049] The distribution of the recarburizer: in this example, the recarburizer added to the furnace and the recarburizer added to the spheroidizing ladle are distributed according to the proportion that the carbon content of the molten iron in the furnace reaches 3.5%C, and the recarburizer placed in the spheroidizing ladle increases the carbon content of the molten iron by 0.9%.

[0050] The charging in the smelting furnace: first, 40Kg of recarburizer and about 100Kg of small pieces of recycled material are added to the bottom of the smelting furnace, and the recarburizer can be placed at the bottom or mixed with the small pieces of recycled material and added to the bottom of the furnace; then the scrap steel is added to the full furnace, and the remaining recarburizer to be added to the smelting furnace is added to the gaps between the scrap steel.

[0051] After the materials are added, the furnace is started to melt. With the melting of the materials, the remaining 1420 Kg of scrap steel is continuously added to the furnace to melt into molten iron; finally, the remaining 400 Kg of return material is also added to the melting furnace to melt. When the materials are completely melted, 10 Kg of ferrosilicon (FeSi75) is added to the melting furnace ten minutes before tapping.

[0052] The melting temperature of this embodiment is 1550°C, and the tapping temperature is 1610°C.

[0053] The spheroidizing treatment is carried out in a 1.5T spheroidizing ladle, and 1T of molten iron is treated in each spheroidizing ladle. 10 Kg of scrap graphite electrode block is added as a recarburizer in each spheroidizing ladle. This embodiment adopts a one-stage spheroidizing treatment mode, that is, a spheroidizing pit is arranged in the spheroidizing ladle, and inoculants, spheroidizing agents, inhibitors, covering agents, and slag collecting agents are placed in the spheroidizing pit and tamped, and then covered with iron filings and iron plates. The recarburizer placed in the spheroidizing ladle is directly placed at the bottom of the spheroidizing ladle. When the molten iron is poured, the molten iron directly impacts the recarburizer placed at the bottom of the spheroidizing ladle, so that part of the recarburizer is dissolved into the molten iron due to the high-temperature molten iron impact and stirring. After the iron plate covering the pit is melted, the molten iron contacts the inoculants, spheroidizing agents, inhibitors, covering agents, and slag collecting agents, the spheroidizing agents react with the molten iron and generate a large amount of magnesium oxide, thereby further boiling and stirring the molten iron, and the C in the recarburizer is further dissolved into the molten iron.

[0054] The inoculants in the spheroidizing ladle in this embodiment use ferrosilicon (FeSi-75), and the addition amount of FeSi75 is 0.8% of the weight of the molten iron in the spheroidizing ladle, that is, 8 Kg is added per ladle.

[0055] The spheroidizing agent of this embodiment uses a rare earth magnesium alloy spheroidizing agent, and the addition amount of the spheroidizing agent is 1.7% of the weight of the molten iron in the spheroidizing ladle, that is, 17 Kg is added per ladle.

[0056] Metal tin is also added to the spheroidizing pit of the spheroidizing ladle in this embodiment, and the metal tin and the rare earth magnesium together act as an inhibitor. They can control the morphology of pearlite and graphite, increase the content of pearlite in the nodular iron matrix and the arrangement order of cementite and ferrite in the pearlite, improve the strength and toughness of the nodular iron matrix, promote the formation of round graphite spheres and the dispersion distribution of graphite spheres. The addition amount of the metal tin is 0.04% of the weight of the molten iron in the spheroidizing ladle, that is, 0.4 Kg is added per ladle.

[0057] The covering agents, slag collecting agents, and the like added to the ladle in this embodiment are all commercially available products, and the addition amount can be appropriately adjusted according to the casting site conditions and experience.

[0058] The control of the detonation of the spheroidizing agent in this embodiment is performed as follows: after the molten iron in the spheroidizing ladle is mixed with the recarburizer for 30-40 seconds, the iron plate covering the pit is melted, the molten iron contacts the additional materials in the pit, such as the inoculant, the spheroidizing agent, the inhibitor, the covering agent, the slag collecting agent, and the like, and a reaction occurs. At this time, the spheroidizing agent reacts with the molten iron to form magnesium oxide gas, and the molten iron boils (this phenomenon is generally referred to as detonation in the casting field). The target time for controlling the reaction of the spheroidizing agent with the molten iron in this embodiment is 40-50 seconds. The actual detonation time in this embodiment is 33 seconds, and the spheroidizing reaction time after detonation is 41 seconds. After the molten iron stops boiling, slagging and the like is performed, and the casting of the castings is prepared. The pouring temperature is controlled at 1460±10°C.

[0059] In this embodiment, the lost foam casting method is used to cast the castings and the standard wedge-shaped test blocks. The time for casting the test blocks is the middle time for casting the castings, and the test blocks can be cast at about 7 minutes after the casting of the castings is started. After the test blocks are cooled, the chemical composition detection samples are taken from the test blocks, and the test blocks are processed into tensile test bars for physical and chemical test. The castings in this embodiment can be turned over and cooled after 40 minutes of casting.

[0060] The chemical composition of the test blocks for post-furnace analysis 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 balance of Fe and other unavoidable impurities.

[0061] The mechanical properties of the test bars formed during the casting process in this embodiment are: tensile strength 670 MPa, elongation 11%. The test bars and the mechanical properties are as-cast test bars and as-cast properties, and have not been subjected to quenching or normalizing and the like. In the microstructure of the product obtained in this embodiment, the pearlite is mainly in the form of lamellar pearlite, and the amount of the lamellar pearlite is 68%. The graphite is mainly in the form of nodular graphite, and the grade is 2. The microstructure photograph of the test blocks obtained in this embodiment is as follows: Figure 1 and Figure 2 .

[0062] In the present application, since a sufficient amount of carbon is used, the chances of Fe and C forming carbides increase, Sn and rare earth magnesium as inhibitors can make more C form sufficient pearlite (cementite + ferrite), and the inhibitors can prevent the cementite from forming free blocks, so that the cementite is mainly in the form of lamellar pearlite, thereby improving the comprehensive mechanical properties of the ductile iron, such as the tensile strength and the toughness. At the same time, since the present application does not use precious metal alloys and does not go through quenching or normalizing and the like to improve the mechanical properties, and does not have to use the casting production equipment with rapid cooling, the production cost of the ductile iron product is greatly reduced.

[0063] Example 2

[0064] The iron liquid smelting equipment used in this embodiment is a 1T induction furnace, and 1.2T of iron liquid is smelted in one furnace.

[0065] The ingredients in the smelting furnace are as follows: 25% of the total 300Kg of recycled materials; 71% of the total 852Kg of scrap steel with a carbon content of 0.3%-0.5%; 0.7% of 8.4Kg of ferrosilicon (FeSi75) used as the carbon additive.

[0066] The carbon additive is distributed according to the proportion that the carbon content of the iron liquid in the furnace reaches 4.0% and the carbon content of the iron liquid in the spheroidizing ladle increases by 1.4% due to the carbon additive placed in the spheroidizing ladle. In this embodiment, 18Kg of carbon additive is added to the spheroidizing ladle.

[0067] The order of adding materials in the furnace is as follows: ① about 100Kg of small pieces of recycled materials and 25Kg of carbon additive are added to the bottom of the furnace; ② then the scrap steel is added to the furnace until it is full, and the remaining carbon additive to be added to the furnace is added to the gaps between the scrap steel; ③ then the furnace is started to smelt, and the remaining scrap steel is continuously added to the furnace as the materials in the furnace melt, and then the remaining 200Kg of recycled materials is also added to the furnace to melt; ④ 8.4Kg of ferrosilicon is added to the furnace 10 minutes before the furnace is stopped.

[0068] The smelting temperature in this embodiment is 1580℃, and the temperature when the furnace is stopped is 1620℃.

[0069] The spheroidizing treatment is carried out in a 1.5T spheroidizing ladle, and the spheroidizing treatment capacity of a single ladle is 1.2T of iron liquid.

[0070] The spheroidizing treatment in this embodiment adopts the method of two-stage spheroidizing treatment by pouring into the bottom of the ladle, that is, two spheroidizing pits for storing materials are set in the bottom of the spheroidizing ladle, the materials (inoculants, spheroidizing agents, inhibitors, covering agents, and slag collecting agents, etc.) required for spheroidizing treatment in the ladle are divided into two parts and placed in the two spheroidizing pits, respectively, and the materials in the spheroidizing pits are tamped and covered with iron chips or iron plates of different thicknesses according to the design that the two spheroidizing pits are detonated in sequence, so that the detonation time of the first pit (first-stage detonation) is controlled to be detonated at 30-40 seconds from the start of pouring the iron liquid, and the detonation time of the second pit is delayed by 10-15 seconds compared with the first-stage detonation. The two pits successively and continuously spheroidize the iron liquid in the spheroidizing ladle.

[0071] The silicon iron FeSi75 is used as the inoculant in this embodiment, and the total amount of the inoculant added is 0.8%, i.e. 9.6 Kg. The spherulizing agent used in this embodiment is rare earth magnesium, and the inhibitor used is rare earth magnesium and metal tin Sn, the rare earth magnesium being one of the inhibitors and the spherulizing agent, and the total amount added is 1.9% of the weight of the molten iron, i.e. 22.8 Kg; the amount of the metal tin added as one of the inhibitors is 0.06% of the weight of the molten iron in the ladle, i.e. 0.72 Kg. The inoculant, spherulizing agent, inhibitor, etc. are placed in the two spherulizing pits of the spherulizing ladle according to the design of the detonation time, detonation interval time and reaction time, and an appropriate amount of covering agent and slag collecting agent is placed in each pit. Before the molten iron is poured into the spherulizing ladle, the recarburizer to be placed in the spherulizing ladle is put into the bottom of the ladle, and the molten iron is poured so as to directly impact 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 spherulizing reaction time is 59 seconds in total.

[0072] After the molten iron in the spherulizing ladle stops boiling and reaches the pouring condition, the casting of the castings is started. The slag stopping rock wool is used at the nozzle of the molten iron ladle during pouring or ladle splitting. The wedge-shaped test block is poured at the 6th minute after the start of the pouring of the castings, and after the test block is cooled, the chemical test sample is taken from the test block, and the test block is processed into the as-cast test bar for detecting mechanical properties and the metallographic test block for observing the metallographic structure.

[0073] The mold is turned over 35 minutes after the pouring of the castings is completed, and the castings are cooled in air.

[0074] The chemical composition of the post-furnace test block of this embodiment is: 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 balance is iron and other unavoidable impurities.

[0075] The metal tensile test is performed on the test bar, and the tensile strength of the test bar obtained in this embodiment is 842 MPa, the elongation is 6.5%, and the reduction of area is 5%. The pearlite content in the metallographic structure is greater than 99%, and the morphology of the pearlite is mainly in the form of flakes; the graphite grade is grade 2, and the spherical graphite is more than 90%. The metallographic structure of the metallographic test block obtained in this embodiment is shown in Figure 3 Figure 4 The tensile test bar and the metallographic test block are both in the as-cast state and have not been subjected to heat treatment such as quenching or normalizing, the metallographic structure is the as-cast structure, and the mechanical properties are the as-cast properties.

[0076] Example 3

[0077] ​The present example is based on example 2, and 0.3% copper and 0.3% molybdenum are added to the total amount of molten iron in the spheroidizing ladle. The chemical composition of the castings obtained after the furnace 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.

[0078] In the present example, because of the addition of strong carbide-forming elements Cu and Mo, and the Cr and Mo brought in by the charge due to the lack of strict detection, blocky carbides appear in the pearlite matrix, the strength of the ductile iron is increased, and the toughness is obviously decreased. The mechanical properties of the test bars obtained (as-cast) are: tensile strength 961 MPa, elongation 4.2%, and the pearlite in the matrix structure accounts for 99%.

[0079] Examples 4-9 below only illustrate the main technical solutions and features of the present application. The spheroidizing agents of examples 4-9 all use rare earth magnesium 3-8 spheroidizing agent. The main differences between examples 4-9 and examples 1 and 2 are: the amount of carbon additive added is more or less, the total amount of spheroidizing agent added and the amount of tin or antimony added as one of the inhibitors differ in each example, the number of spheroidizing pits (the number of spheroidizing stages), the start ignition time of the spheroidizing agent, the ignition time interval in different spheroidizing pits, and the total ignition reaction time are different, and finally the carbon content and other main chemical compositions of the ductile iron castings obtained, the residual amount of the inhibitors (magnesium oxide, rare earth, Sn or Sb), and the mechanical properties are different.

[0080] Example 4

[0081] The iron melt smelting equipment and the main materials used in the present example are the same as those of examples 1 and 2. The difference between the present example and examples 1 and 2 is that the amount of carbon additive added to the smelting furnace in the present example is added according to the requirement that the carbon content of the molten iron in the smelting furnace reaches 3.2%, that is, the carbon content of the carbon additive added and the carbon content of the scrap steel and the carbon content of the recycled material together make the carbon content of the molten iron in the smelting furnace reach 3.2%; the carbon content of the molten iron is increased by 0.8% by the carbon additive placed in the spheroidizing ladle. The carbon additive added in the smelting furnace is mixed with the scrap steel and the recycled material and put into the smelting furnace, the carbon additive is placed in the gap between the scrap steel or the recycled material, and then the smelting is started. As the charge is melted, the remaining scrap steel and recycled material are continuously added to the smelting, and the smelting of all the materials to be added to the smelting furnace is completed.

[0082] The inhibitors are selected to be metallic tin and rare earth magnesium. The total amount of rare earth magnesium (one of the spheroidizing agents and inhibitors) added is 1.65% of the weight of the molten iron in the spheroidizing ladle, and the amount of metallic tin added is 0.025% of the weight of the molten iron in the spheroidizing ladle, that is, 0.25 kg of Sn is added per ton of molten iron.

[0083] The present example adopts one-stage spheroidizing treatment, and puts inoculants, spheroidizing agents and inhibitors, slag-collecting agents, covering agents and the like into one spheroidizing pit. The time from the start of injecting the iron liquid to the explosion of the spheroidizing agent is 34 seconds, and the spheroidizing reaction time after the explosion lasts for 39 seconds.

[0084] The chemical composition of the test block obtained in the present 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, and the rest is Fe and other inevitable impurities. The tensile strength of the nodular cast iron product obtained in the present example is 703 MPa, the elongation is 10.8%, and the amount of pearlite in the matrix structure is 62%.

[0085] Example 5

[0086] The difference between the present example and Example 4 is that, in the present example, the carbon content of the iron liquid is required to reach 4.2% by adding carbon-increasing agents into the smelting furnace, and the carbon content of the iron liquid is increased by 0.8% by placing carbon-increasing agents in the spheroidizing bag. The smelting temperature of the present example is 1570°C, and the tapping temperature is 1550°C.

[0087] The inhibitors are selected from metallic tin and rare earth magnesium, and the total amount of the spheroidizing agent added is 1.9% of the weight of the iron liquid in the spheroidizing bag. The amount of Sn added is 0.06% of the weight of the iron liquid, i.e. 0.6 Kg of tin is added per ton of iron liquid.

[0088] The present example adopts two-stage spheroidizing treatment, and distributes inoculants, spheroidizing agents and inhibitors, slag-collecting agents, covering agents and the like into two spheroidizing pits. The time from the start of injecting the iron liquid to the explosion of the spheroidizing agent in the first stage is 37 seconds, the time interval between the explosion in the first stage and the explosion in the second stage is 22 seconds, and the total spheroidizing reaction time after the explosion in the first stage is 60 seconds.

[0089] The chemical composition of the test block obtained in the present 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, and the rest is Fe and other inevitable impurities. The tensile strength of the nodular cast iron product obtained in the present example is 798 MPa, the elongation is 7.2%, and the amount of pearlite in the matrix structure is 99%.

[0090] Example 6

[0091] The difference between this example and example 4 is that the carbon content of the molten iron in the smelting furnace is 3.5% by adding carbon additive into the smelting furnace, and the carbon content of the molten iron is increased by 1.0% in the spheroidizing ladle. The inhibitor is selected from metallic Sb and rare earth magnesium. The total amount of the rare earth magnesium spheroidizing agent is 1.7% by weight of the molten iron. The amount of Sb is 0.06%, i.e. 0.6 Kg of Sb is added per ton of molten iron. The smelting temperature of this example is 1560°C, and the tapping temperature is 1580°C.

[0092] This example adopts two-stage initiation spheroidizing process. The inoculant, spheroidizing agent and inhibitor, slag collecting agent, covering agent, etc. are respectively put into two spheroidizing pits. The time from the start of the injection of the molten iron to the first-stage initiation of the spheroidizing agent is 28 seconds. The time interval between the second-stage initiation and the first-stage initiation is 25 seconds. The total spheroidizing reaction time after the first-stage initiation is 53 seconds.

[0093] 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, and the rest is Fe and other unavoidable impurities. The mechanical properties of the spheroidal graphite cast iron product obtained in this example are: tensile strength 735 MPa, elongation 8.7%, and the amount of pearlite in the matrix structure is 90%.

[0094] Example 7

[0095] The difference between this example and example 4 is that the amount of the carbon additive added into the smelting furnace is such that the sum of the carbon content of the carbon additive and the carbon content of the smelting raw material is 4.3% of the carbon content of the molten iron in the smelting furnace. The weight of the carbon additive placed in the spheroidizing ladle is such that the carbon content of the molten iron is increased by 1.3%.

[0096] The inhibitor is selected from metallic Sn and rare earth magnesium. The total amount of the rare earth magnesium is 1.9% by weight of the molten iron. The amount of Sn is 0.07% by weight of the molten iron in the spheroidizing ladle, i.e. 0.7 Kg of Sn is added per ton of molten iron.

[0097] This example adopts two-stage spheroidizing process. The time from the start of the injection of the molten iron to the first-stage initiation of the spheroidizing agent is 31 seconds. The time interval between the second-stage initiation and the first-stage initiation is 23 seconds. The total spheroidizing reaction time after the first-stage initiation is 63 seconds.

[0098] The chemical composition of the test block 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, and the rest is Fe and other unavoidable impurities. The mechanical properties of the spheroidal graphite cast iron product obtained in this example are: tensile strength 872 MPa, elongation 4.1%, and the amount of pearlite in the matrix is 99%.

[0099] Example 8

[0100] The difference between this example and the above examples is that the carbon additive is added to the melting furnace in a proportion to make the carbon content of the molten iron in the melting furnace 4.5%, and the carbon additive is placed in the spheroidizing ladle in a proportion to make the carbon content of the molten iron in the spheroidizing ladle increase by 1.7%. The melting temperature of this example is 1590°C, and the tapping temperature is 1600°C.

[0101] The inhibitor is selected from Sn and rare earth magnesium, and the total amount of the rare earth magnesium added is 2.1% by weight of the molten iron, and the amount of tin added is 0.9% by weight of the molten iron in the spheroidizing ladle, i.e. 0.9 Kg of tin is added per ton of molten iron.

[0102] This example adopts three-stage spheroidizing treatment, and the inoculant, spheroidizing agent and inhibitor, slag collecting agent, covering agent, etc. are distributed into three spheroidizing pits. The ignition time and spheroidizing reaction time of this example are: the first-stage ignition time is 35 seconds, the time interval between the second-stage ignition and the first-stage ignition is 21 seconds, the time interval between the third-stage ignition and the second-stage ignition is 18 seconds, and the total spheroidizing reaction time after the first-stage ignition is 81 seconds.

[0103] The chemical composition of the test block 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, and the rest is Fe and other unavoidable impurities. The tensile strength of the nodular iron product obtained in this example is 907 MPa, the elongation is 4.3%, and the pearlite content in the matrix structure is 99%.

[0104] Example 9

[0105] The difference between this example and the above examples is that the carbon additive is added to the melting furnace and the spheroidizing ladle in a proportion to make the carbon content of the molten iron in the melting furnace 4.1% and the carbon content of the molten iron in the spheroidizing ladle increase by 2.5%. The melting temperature of this example is 1580°C, and the tapping temperature is 1620°C.

[0106] The inhibitor is selected from metallic antimony and rare earth magnesium, and the total amount of the rare earth magnesium added is 2.0% by weight of the molten iron in the spheroidizing ladle, and the amount of metallic antimony added is 0.06% by weight of the molten iron, i.e. 0.6 Kg of antimony is added per ton of molten iron.

[0107] The present example adopts three-stage spheroidizing treatment, and inoculants, spheroidizing agents, inhibitors, slag-collecting agents, covering agents and the like are distributed into three spheroidizing pits. The detonation time and spheroidizing reaction time of the present example are as follows: the first-stage detonation time is 32 seconds, the time interval between the second-stage detonation and the first-stage detonation is 24 seconds, the time interval between the third-stage detonation and the second-stage detonation is 25 seconds, and the total spheroidizing reaction time after detonation is 88 seconds.

[0108] The chemical composition of the test block obtained in the present example is as follows: 6.2% C, 1.48% Si, 0.27% Mn, 0.029% S, 0.036% P, 0.05% Sb, 0.065% MgO, 0.037% Re, and other chemical elements are Fe and other unavoidable impurities. The tensile strength of the nodular cast iron product obtained in the present example is 1012 MPa, the elongation is 4.0%, and the amount of pearlite in the matrix structure is 99%. The nodular cast iron product of the present example is mainly used for products of friction pairs.

[0109] Example 10

[0110] The difference between the present example and Example 4 is that, in the present example, the carbon-increasing agent is added into the smelting furnace according to the requirement that the carbon content of the molten iron reaches 4.0% by the carbon-increasing agent and the smelting raw materials in the smelting furnace; and the present example does not place the carbon-increasing agent in the spheroidizing ladle. The smelting temperature of the present example is 1590°C, and the tapping temperature is 1550°C.

[0111] The inhibitors are selected from metallic antimony and rare earth magnesium, and the total amount of the added rare earth magnesium as spheroidizing agent is 1.6% of the weight of the molten iron in the spheroidizing ladle, and the amount of the added metallic antimony is 0.04% of the weight of the molten iron.

[0112] The present example adopts one-stage spheroidizing treatment, and the time interval from the start of the injection of the molten iron to the detonation of the spheroidizing agent is controlled to be 25 seconds, and the spheroidizing reaction time after detonation is 47 seconds.

[0113] The chemical composition of the test block obtained in the present example is as follows: 4.0% C, 2.08% Si, 0.32% Mn, 0.024% S, 0.025% P, 0.03% Sb, 0.035% MgO, 0.021% Re, and other chemical elements are Fe and other unavoidable impurities. The tensile strength of the nodular cast iron product obtained in the present example is 583 MPa, the elongation is 12%, and the amount of pearlite in the matrix structure is 60%.

[0114] As can be seen from the examples 1-10, when the material quality (such as tensile strength, toughness) of the ductile iron is required to be higher, the carbon content of the ductile iron product has a higher tendency, and the amount of the added inhibitors (spheroidizing agent rare earth magnesium and Sn or Sb) also has a higher tendency, and the tapping temperature or smelting temperature of the molten iron should also be increased accordingly (the smelting temperature should be controlled at a temperature at which the carbon additive added into the smelting furnace is completely dissolved into the molten iron, generally controlled at not less than 1550℃, and the tapping temperature is also increased, not less than 1550℃, generally controlled at more than 1600℃) or the smelting time and the impact stirring and boiling stirring time are correspondingly prolonged. When the carbon content of the ductile iron product of the present application is high, the tensile strength is increased, while the toughness is reduced. The Sn, Sb and rare earth magnesium are used as the carbon element inhibitors in the present application, which can make the pearlite content of the ductile iron matrix reach more than 99% in the as-cast state, and most of the pearlite is lamellar. Without adding Cu, Mo and other strong carbide forming elements, under the action of the process and inhibitors of the present application, there are rarely net-like or massive carbides, thus the tensile strength of the ductile iron product is increased, and also a very high toughness is obtained. At the same time, the ductile iron of the present application does not need to use undercooling casting equipment, and does not need to be heat treated by quenching or normalizing, and can reach the above high strength and high toughness in the as-cast state, thus the cost of producing the ductile iron product can be greatly saved.

Claims

1. A high-carbon-content as-cast high-strength high-toughness ductile cast iron, characterized by: The carbon content is greater than or equal to 4.05%, the spheroidal graphite cast iron does not contain or contains less than 0.3% of each precious metal alloy by weight of the molten iron, and the comprehensive mechanical properties of the spheroidal graphite cast iron are improved to QT800 or above by using sufficient C without using supercooling casting, quenching or normalizing heat treatment, and the less than 0.3% of each precious metal alloy is added into the spheroidal graphite cast iron; an inhibitor and sufficient spheroidal agent are added into the spheroidal graphite package when the molten iron is tapped and spheroidized, the adding amount of the inhibitor and the spheroidal agent is controlled to ensure that a small amount of the inhibitor and the spheroidal agent remains in the castings after pouring is completed; the inhibitor is a mixture of the spheroidal agent and weak carbide forming elements, and the small amount of the inhibitor refers to the residual amount of magnesium oxide being not less than 0.03%, the residual amount of tin being not less than 0.02%, or the residual amount of antimony being not less than 0.03% when the spheroidal agent is a magnesium-containing alloy spheroidal agent, the inhibitor is the spheroidal agent and tin, or the inhibitor is the spheroidal agent and antimony.

2. The high-carbon-content as-cast high-strength high-toughness ductile cast iron according to claim 1, characterized by: The carbon content of the spheroidal graphite cast iron is 4.05%-6.0%.

3. A high carbon content as-cast high strength high toughness ductile cast iron according to any one of claims 1-2, characterized in that: The total adding amount of the spheroidal agent is greater than 1.6% by weight of the molten iron.

4. The high-carbon-content as-cast high-strength high-toughness ductile cast iron according to claim 3, characterized in that: The spheroidal agent is a rare earth magnesium spheroidal agent; when the rare earth magnesium and tin are used as the inhibitor, the adding amount of the inhibitor is controlled to ensure that the residual amount of tin in the solidified castings is between 0.02% and 0.06%, and the residual amount of magnesium oxide is between 0.03% and 0.065%; when the rare earth magnesium and antimony are used as the inhibitor, the adding amount of the inhibitor is controlled to ensure that the residual amount of antimony in the solidified castings is between 0.03% and 0.05%, and the residual amount of magnesium oxide is between 0.03% and 0.065%.

5. The high carbon content as-cast high strength high toughness ductile cast iron according to any one of claims 1-2, characterized in that: Part of the carbon additive is added into the spheroidal graphite package when the molten iron is tapped and spheroidized; when the tapped molten iron is poured into the spheroidal graphite package, the carbon additive in the spheroidal graphite package is first impacted and then reacts with the spheroidal agent to perform spheroidization treatment while the carbon additive in the spheroidal graphite package is further boiled and stirred; the smelting temperature and the tapping temperature of the molten iron are both not less than 1550℃.

6. The high carbon content as-cast high strength high toughness ductile cast iron according to claim 5, characterized in that: The carbon additive is divided into two parts, one part is added into the smelting furnace during smelting, and the other part is added into the spheroidal graphite package for dissolving; the carbon additive added into the spheroidal graphite package provides a carbon content of not more than 2.5% for the castings, and the remaining carbon content of the castings is provided by the carbon additive and other smelting raw materials in the smelting furnace.

7. The high-carbon-content as-cast high-strength high-toughness ductile cast iron according to claim 6, characterized in that: The carbon additive and the scrap steel or the recycled material are put into the smelting furnace together during smelting, and then the smelting is started until all the required materials are smelted; the carbon additive added into the spheroidal graphite package provides a carbon content of 0.8%-2.5% for the castings; the inoculant, the spheroidal agent, the inhibitor, the covering agent and the slag collecting agent are put into the spheroidal pit of the spheroidal graphite package, and the time for the spheroidal agent to start detonation and the boiling spheroidization time after detonation are controlled; the temperature of the molten iron tapped into the spheroidal graphite package is 1550-1620℃.

8. The high-carbon-content as-cast high-strength high-toughness ductile cast iron according to claim 7, characterized in that: According to the weight of the molten iron to be spheroidized in the spheroidal graphite package and the performance requirements of the spheroidal graphite cast iron, two or more spheroidal pits are arranged in the spheroidal graphite package, and the time interval from the molten iron being poured into the spheroidal agent to detonation, the detonation sequence of each spheroidal pit, the detonation time interval and the spheroidal boiling reaction time are controlled.

9. A high carbon content as-cast high strength high toughness ductile cast iron according to any one of claims 1-2, characterized in that: The noble metal strengthening alloy is one, two or three of copper, molybdenum or other strong carbide forming elements; the non-noble metal strengthening alloy means that the element content is not higher than the allowable content as impurities.

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