Spheroidal graphite cast iron, manufacturing method for spheroidal graphite cast iron, and spheroidizing agent

Abstract

A spheroidal graphite cast iron comprising 2.8-3.3% of carbon, 2.5-4.0% of silicon, 0.32-0.40% of manganese, 0.020-0.030% of phosphorus, 0.020-0.035% of sulfur, 0.030-0.050% of magnesium, 0.010-0.050% of a total of lanthanum and cerium, and 0.0020-0.0050% of calcium, all in mass percentage, the remaining portion being iron and unavoidable impurities.

Description

[Technical Field]

[0001] The present invention relates to spheroidal graphite cast iron, a method for producing spheroidal graphite cast iron, and a spheroidizing agent suitable for use in the production method. [Background technology]

[0002] Cast iron is a general term for iron-carbon alloys suitable for casting. Cast iron can be classified into flake graphite cast iron, malleable cast iron, and spheroidal graphite cast iron, depending on the form of graphite present. Malleable cast iron can be further classified into whiteheart malleable cast iron, blackheart malleable cast iron, and pearlitic malleable cast iron. The carbon content of cast iron exceeds approximately 2.0% by mass, the carbon saturation solid solubility limit of austenite in the iron-carbon binary phase diagram, but does not significantly exceed approximately 4.3% by mass, the eutectic point. During the solidification process of cast iron, graphite and / or cementite crystallize or precipitate, first by a eutectic reaction and then by a eutectoid reaction with austenite.

[0003] Flake graphite cast iron, also known as gray cast iron, is a type of cast iron that has been used by humans since ancient times. The basic shape of the graphite in flake graphite cast iron is flaky. When tensile stress is applied to flake graphite cast iron, fracture tends to progress along the flake graphite, so the mechanical strength of flake graphite cast iron is weaker than, for example, carbon steel for machine structures. For this reason, attempts have been made to improve the shape of the graphite to make it more desirable in order to increase the mechanical strength of cast iron.

[0004] One such attempt involves adjusting the carbon content of the molten iron to, for example, 2.8% to 3.1% by mass, thereby producing white pig iron that does not crystallize graphite during casting. The resulting casting is then heat-treated to precipitate massive graphite liberated from cementite. The cast iron obtained using this method is called "blackheart malleable cast iron" or "malleable cast iron." Another attempt involves adjusting the carbon content of the molten iron to, for example, 3.4% to 3.9% by mass, while simultaneously reducing the sulfur content of the molten iron, thereby crystallizing spheroidal graphite during casting. The cast iron obtained using this method is called "spheroidal graphite cast iron" or "ductile cast iron." Both blackheart malleable cast iron and spheroidal graphite cast iron have superior mechanical strength compared to flake graphite cast iron, and are therefore widely used industrially. However, there are significant differences between their manufacturing methods.

[0005] For example, in the production of spheroidal graphite cast iron, additives are added to the molten metal poured into a ladle to promote the crystallization of spheroidal graphite. These additives are called "spheroidizing agents," and many of them are composed of alloys containing silicon, magnesium, cerium, calcium, and iron (see, for example, Patent Document 1). Known methods for adding spheroidizing agents include the "place-pour method" or "sandwich method," in which the molten metal is poured into a ladle whose bottom has already been filled with the spheroidizing agent, and the "wire method," in which an iron wire with the hollow center filled with the spheroidizing agent is gradually fed onto the surface of the molten metal and melted (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-182620 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-316331 Summary of the Invention [Problem to be solved by the invention]

[0007] In order to use the molten metal produced in a foundry without waste, it is desirable to be able to selectively cast cast irons of different properties using the same molten metal in response to fluctuations in demand. However, the preferred carbon content of blackheart malleable cast iron and spheroidal graphite cast iron according to conventional techniques is significantly different, and the difference in carbon content exceeds the range that can be adjusted by changing the production conditions of the molten metal or by recarburizing the ladle. For this reason, no method has yet been known that efficiently produces blackheart malleable cast iron and spheroidal graphite cast iron using molten metal continuously supplied from, for example, a cupola.

[0008] Furthermore, when producing molten metal using an acidic cupola, which uses acidic refractories as the furnace material and whose operation is relatively easy to manage compared to a basic cupola, sulfur derived from the coke fed into the cupola as a heat source dissolves in the molten metal. Because the presence of sulfur inhibits the crystallization of spheroidal graphite, conventional techniques require desulfurization of the molten metal. For example, desulfurization is performed by injecting calcium carbide powder into the molten metal using nitrogen gas. While this method is highly effective in desulfurization, it lowers the temperature of the molten metal, necessitating reheating. Introducing new desulfurization and reheating processes to produce spheroidal graphite cast iron using molten metal for blackheart malleable cast iron increases production costs.

[0009] The present disclosure has been made in view of the above-mentioned problems, and aims to provide spheroidal graphite cast iron that can be produced at low production costs using molten metal produced for casting blackheart malleable cast iron, for example, in an acid cupola, and a method for producing spheroidal graphite cast iron that uses the above molten metal and enables the production of spheroidal graphite cast iron at reduced production costs. [Means for solving the problem]

[0010] In a first embodiment, the present disclosure relates to spheroidal graphite cast iron containing, all by mass percentage, carbon of 2.8% or more and 3.3% or less, silicon of 2.5% or more and 4.0% or less, manganese of 0.32% or more and 0.40% or less, phosphorus of 0.020% or more and 0.030% or less, sulfur of 0.020% or more and 0.035% or less, magnesium of 0.030% or more and 0.050% or less, lanthanum and cerium in total of 0.010% or more and 0.050% or less, calcium of 0.0020% or more and 0.0050% or less, and the remainder being iron and unavoidable impurities.

[0011] The spheroidal graphite cast iron according to the present disclosure contains predetermined amounts of magnesium, lanthanum, cerium, and calcium, and these elements react with sulfur to form sulfides, thereby removing or rendering harmless the sulfur that inhibits the spheroidization of graphite. As a result, spheroidal graphite cast iron can be produced using a sulfur-containing molten metal for blackheart malleable cast iron produced, for example, using an acid cupola.

[0012] In a second embodiment, the present disclosure is an invention of a method for producing spheroidal graphite cast iron, the method including the steps of melting raw materials to produce a molten metal, adding a spheroidizing treatment agent to the molten metal, and pouring the molten metal to which the spheroidizing treatment agent has been added into a mold to cast spheroidal graphite cast iron, the composition of the spheroidal graphite cast iron being the same as the composition of the spheroidal graphite cast iron according to the present disclosure. In a third embodiment, the present disclosure is an invention of a spheroidizing treatment agent suitable for use in the method for producing spheroidal graphite cast iron according to the present disclosure. [Effects of the Invention]

[0013] According to the present disclosure, it is possible to selectively cast either spheroidal graphite cast iron or blackheart malleable cast iron using the same molten metal without adding costly processes, so that the molten metal produced in the foundry can be used up without waste in accordance with fluctuations in demand. Furthermore, according to the present disclosure, it is not necessary to separately prepare molten metals with different compositions to produce cast irons of different properties, which contributes to reducing total production costs and saving energy resources. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a flowchart showing a method for manufacturing spheroidal graphite cast iron according to the present embodiment. [Figure 2] FIG. 2 is a side view of a sample used in the examples. [Figure 3] FIG. 2 is a perspective view of a test material used in the examples. [Figure 4] FIG. 1 is a side view of a tensile test bar used in the examples. [Figure 5] 2 is an optical microscope photograph showing an example of the metal structure of spheroidal graphite cast iron after heat treatment (after annealing) according to the present embodiment. [Figure 6] 1 is an optical microscope photograph showing an example of the metal structure of blackheart malleable cast iron produced from the same molten metal as the spheroidal graphite cast iron according to the present embodiment. [Figure 7] 4 is an optical microscope photograph showing another example of the metal structure of spheroidal graphite cast iron before heat treatment (before annealing) according to the present embodiment. [Figure 8] 1 is an optical microscope photograph showing another example of the metal structure (before heat treatment) of the spheroidal graphite cast iron according to the present embodiment. [Figure 9] 10 is an optical microscope photograph showing another example of the metal structure (after heat treatment) of the spheroidal graphite cast iron according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] The following describes the modes for carrying out the present invention, divided into individual embodiments. Note that the embodiments and examples of the present invention shown here are merely illustrative examples of specific examples for carrying out the present invention, and the present disclosure is not limited to these embodiments. In this specification, when expressing the composition of a metal or alloy, it is expressed in mass percentage unless otherwise specified.

[0016] <Spheroidal graphite cast iron> In a first embodiment, the present disclosure relates to spheroidal graphite cast iron. In this specification, "spheroidal graphite cast iron" refers to cast iron having a metallurgical structure in which spheroidal graphite is dispersed and crystallized in a matrix of ferrite and / or pearlite. With regard to such metallurgical structure characteristics, the spheroidal graphite cast iron according to the present disclosure is no different from conventional spheroidal graphite cast iron. The spheroidal graphite cast iron according to the present disclosure may have a graphite spheroidization rate of 70% or more, as determined in accordance with JIS G 5502:2001 using an image taken under an optical microscope at 100x magnification. The spheroidal graphite cast iron is preferably 80% or more, more preferably 85% or more. While the upper limit is not particularly limited, taking into account the component composition, the upper limit is approximately 95%. As described below, the spheroidal graphite cast iron according to the present disclosure has a different composition from conventional general spheroidal graphite cast iron. However, as described above, the metallurgical structure is the same as that of conventional spheroidal graphite cast iron, and therefore the term "spheroidal graphite cast iron" is still used.

[0017] The spheroidal graphite cast iron according to the present disclosure contains 2.8% or more and 3.3% or less of carbon. As mentioned above, in conventional spheroidal graphite cast iron, the carbon content is adjusted to, for example, 3.4% or more and 3.9% or less. On the other hand, in blackheart malleable cast iron, the carbon content is adjusted to a lower composition range than that of spheroidal graphite cast iron, for example, 2.8% or more and 3.1% or less. This is because if the carbon content of blackheart malleable cast iron is increased, a structure called "mottle" is formed in which graphite crystallizes as primary crystals during casting and subsequent cooling in the mold, which may significantly impair the mechanical strength of the cast iron.

[0018] A carbon content of 2.8% or more promotes the crystallization of spheroidal graphite when casting spheroidal graphite cast iron. A carbon content of 3.3% or less can prevent the formation of mottle when casting blackheart malleable cast iron using the same molten metal. Therefore, the spheroidal graphite cast iron according to the present disclosure contains carbon of 2.8% or more and 3.3% or less. A preferred range of the carbon content is 2.9% or more and 3.2% or less, and a more preferred range is 3.0% or more and 3.1% or less. Note that, in this specification, the "carbon content" of spheroidal graphite cast iron refers to the average content based on the total amount of carbon contained in the spheroidal graphite cast iron as the final product, regardless of the form of carbon present. The same applies to the contents of other elements contained in the spheroidal graphite cast iron.

[0019] The spheroidal graphite cast iron according to the present disclosure contains 2.5% or more and 4.0% or less of silicon. Silicon is an element that promotes graphite formation. A silicon content of 2.5% or more promotes graphite crystallization during the casting of spheroidal graphite cast iron, resulting in the formation of spheroidal graphite. Furthermore, increasing the silicon content by, for example, adding a silicon-containing substance as a spheroidizing treatment agent (described below) to the molten metal with a reduced carbon content used in the production of blackheart malleable cast iron can increase the carbon equivalent (described below) of the molten metal, thereby improving the fluidity of the molten metal and promoting graphite crystallization as described above. This facilitates the formation of spheroidal graphite, thereby improving tensile strength. Furthermore, silicon dissolving in the ferrite matrix can increase tensile strength. On the other hand, a silicon content of 4.0% or less can prevent a decrease in elongation, one of the mechanical strengths. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 2.5% or more and 4.0% or less of silicon. From the viewpoint of further preventing a decrease in elongation, the silicon content is preferably 2.9% or less, more preferably 2.75% or less. From the viewpoint of sufficiently preventing a decrease in elongation, the preferred range of the silicon content is, for example, 2.55% or more and 2.75% or less. On the other hand, from the viewpoint of increasing the tensile strength described above, the silicon content is preferably 2.68% or more, more preferably 2.70% or more. For example, to achieve a tensile strength of 450 MPa or more required for spheroidal graphite cast iron FCD450 specified in JIS standard (JIS G 5502), the silicon content is preferably 2.68% or more and 3.3% or less. Furthermore, to achieve a tensile strength of 500 MPa or more, the silicon content is more preferably 3.0% or more and 3.3% or less.

[0020] When silicon is present in liquid or solid iron, the solubility of carbon in the iron decreases, causing the carbon content at the eutectic point to be less than 4.3%. Based on this shift in the eutectic point, it is empirically believed that approximately one-third of the silicon content in cast iron is equivalent to the carbon content. When the carbon and silicon contents of cast iron are C and Si, respectively, the value calculated using the formula C + 1 / 3Si (%) is called the "carbon equivalent." When the carbon equivalent of the spheroidal graphite cast iron according to the present disclosure is 3.8% or more and 4.1% or less, it corresponds to a hypoeutectic composition. The preferred range of carbon equivalent is 3.6% or more and 4.2% or less.

[0021] In the present disclosure, the content of silicon contained in spheroidal graphite cast iron includes not only the silicon originally contained in the molten metal, but also the silicon derived from additives added to the molten metal, i.e., silicon derived from ferrosilicon and spheroidizing agents that may be added in the ladle as described below, as well as silicon derived from inoculants, etc. The same applies to the contents of other elements contained in these additives, i.e., magnesium, lanthanum, cerium, calcium, aluminum, barium, etc.

[0022] The spheroidal graphite cast iron according to the present disclosure contains manganese at least 0.32% and not more than 0.40%. While a high manganese content in spheroidal graphite cast iron does not impair the spheroidization of graphite, manganese in blackheart malleable cast iron inhibits graphite formation. Manganese also increases the hardness and strength of spheroidal graphite cast iron and stabilizes the pearlite structure, but manganese content tends to reduce the elongation of spheroidal graphite cast iron. Manganese is found in small amounts in iron ore and is also mixed into cast iron when manganese steel is included in scrap iron melted in a cupola. Manganese at a manganese content of 0.32% or more combines with sulfur to form manganese sulfide, neutralizing elemental sulfur, which inhibits graphite formation, and promoting graphite formation in the cast iron. Manganese at a manganese content of 0.40% or less does not inhibit graphite formation due to excess manganese, even when blackheart malleable cast iron is produced using the same molten metal. Therefore, the spheroidal graphite cast iron according to the present disclosure contains manganese in an amount of 0.32% or more and 0.40% or less. The preferable range of the manganese content is 0.33% or more and 0.39% or less.

[0023] The spheroidal graphite cast iron according to the present disclosure contains 0.020% or more and 0.030% or less of phosphorus. Phosphorus does not inhibit the spheroidization of graphite, but too much phosphorus may reduce mechanical strength. If the phosphorus content is 0.020% or more, the crystallization of spheroidal graphite is promoted during casting in the production of spheroidal graphite cast iron. If the phosphorus content is 0.030% or less, the crystallization of mottle during casting can be prevented in the production of blackheart malleable cast iron using the same molten metal as that used to produce spheroidal graphite cast iron, and further, a decrease in the toughness of the cast iron can be prevented. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 0.020% or more and 0.030% or less of phosphorus.

[0024] The spheroidal graphite cast iron according to the present disclosure contains 0.020% or more and 0.035% or less of sulfur. Sulfur is an element that significantly inhibits graphite generation and spheroidization. In conventional techniques, if the molten metal after spheroidization contains 0.020% or more of sulfur, the graphite cannot be completely spheroidized. Therefore, as described above, when producing molten metal using an acidic cupola, desulfurization and reheating of the molten metal are required to remove sulfur absorbed from the coke. However, in the present disclosure, the action of the spheroidizing agent described below allows spheroidization of graphite even when the sulfur content is 0.020% or more without desulfurization of the molten metal. If the sulfur content is 0.035% or less, graphite can be spheroidized without desulfurization of the molten metal, and graphitization is also possible when producing blackheart malleable cast iron using the same molten metal. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 0.020% or more and 0.035% or less of sulfur. The preferred range of the sulfur content is 0.025% or more and 0.033% or less.

[0025] Next, among the elements contained in the spheroidal graphite cast iron according to the present disclosure, elements derived from the spheroidizing treatment agent described below will be described. The spheroidal graphite cast iron according to the present disclosure contains magnesium at 0.030% or more and 0.050% or less, lanthanum and cerium at a total content of 0.010% or more and 0.050% or less, and calcium at 0.0020% or more and 0.0050% or less. These elements all have a high affinity for oxygen and sulfur. When added to a sulfur-rich molten metal produced using an acidic cupola, they form sulfides, thereby reducing the concentration of elemental sulfur dissolved in the molten metal. By utilizing this effect of the spheroidizing treatment agent, it is possible to produce spheroidal graphite cast iron using a molten metal containing 0.020% or more and 0.035% or less of sulfur, without the need for desulfurization treatment using calcium carbide or the like.

[0026] As mentioned above, spheroidizing agents composed of an alloy containing silicon, magnesium, cerium, calcium, and iron have been known for some time (see, for example, Patent Document 1). However, no spheroidal graphite cast iron is known that is produced by applying a spheroidizing agent to a molten metal that has a lower carbon content (2.8% or more and 3.3% or less) than conventional spheroidal graphite cast iron and a sulfur content of 0.020% or more and 0.035% or less. In other words, one of the features of the spheroidal graphite cast iron disclosed herein is that it overcomes two conditions unfavorable to the crystallization of spheroidal graphite, namely, a low carbon content and a high sulfur content, by a single means: the addition of a spheroidizing agent.

[0027] The spheroidal graphite cast iron according to the present disclosure contains 0.030% or more and 0.050% or less of magnesium. Magnesium is an element that combines with oxygen and sulfur in the molten metal and neutralizes sulfur, which inhibits graphite formation and spheroidization. Therefore, the inclusion of magnesium facilitates the formation of a spheroidal graphite cast iron structure. Magnesium also has a high vapor pressure and tends to react violently with the molten metal. A magnesium content of 0.030% or more promotes the crystallization of spheroidal graphite during the casting of the spheroidal graphite cast iron. A magnesium content of 0.050% or less prevents the addition of excessive magnesium from increasing free magnesium that does not form compounds in the molten metal, thereby promoting the formation of cementite. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 0.030% or more and 0.050% or less of magnesium. The preferred range of the magnesium content is 0.035% or more and 0.045% or less.

[0028] The spheroidal graphite cast iron according to the present disclosure contains lanthanum and cerium in a total amount of 0.010% or more and 0.050% or less. Both lanthanum and cerium are rare earth elements that combine with oxygen and sulfur in the molten metal to neutralize sulfur, which inhibits graphite formation and spheroidization. Therefore, the inclusion of lanthanum and cerium facilitates the formation of a spheroidal graphite cast iron structure. When the total content of lanthanum and cerium is 0.010% or more, the crystallization of spheroidal graphite is promoted during casting of the spheroidal graphite cast iron. When the total content of lanthanum and cerium is 0.050% or less, a decrease in the impact strength of the cast iron can be prevented. Therefore, the spheroidal graphite cast iron according to the present disclosure contains lanthanum and cerium in a total amount of 0.010% or more and 0.050% or less. The preferred range of the total content of lanthanum and cerium is 0.025% or more and 0.045% or less.

[0029] Rare earth elements have similar chemical properties and occur naturally in an unseparated state. For example, an alloy containing multiple light rare earth elements, known as misch metal, contains lanthanum, cerium, praseodymium, and neodymium, as well as trace amounts of samarium, magnesium, aluminum, and iron. Of these elements, cerium is the most abundant, at approximately 50%, followed by lanthanum, at approximately 25%. Because the content of rare earth elements other than lanthanum and cerium in misch metal is lower than the total content of lanthanum and cerium, this embodiment specifies only the total content of lanthanum and cerium, and does not specify the content of other rare earth elements. Rare earth elements other than lanthanum and cerium, such as praseodymium, neodymium, and samarium, may be included as unavoidable impurities in this embodiment. Because misch metal is less expensive than pure rare earth sources separated into individual elements, using misch metal as a spheroidizing agent can reduce the production cost of cast iron. Since the ratio of lanthanum to cerium in mischmetal is approximately 1:2, if the content of either one is known by chemical analysis, etc., the total content of lanthanum and cerium can be estimated by calculation. In the present disclosure, light rare earth alloys other than mischmetal may be used as long as the contents of lanthanum and cerium are within the above ranges.

[0030] The spheroidal graphite cast iron according to the present disclosure contains 0.0020% or more and 0.0050% or less of calcium. Calcium is an element that combines with oxygen and sulfur in the molten metal, neutralizing sulfur, which inhibits graphite formation and spheroidization. Therefore, the inclusion of calcium facilitates the formation of a spheroidal graphite cast iron structure. Calcium has a particularly strong affinity for oxygen compared to magnesium and rare earth elements. Furthermore, like magnesium, calcium has a high vapor pressure and tends to react violently with the molten metal. A calcium content of 0.0020% or more promotes the crystallization of spheroidal graphite during casting of the spheroidal graphite cast iron. A calcium content of 0.0050% or less can prevent bumping of the molten metal due to the addition of excessive calcium. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 0.0020% or more and 0.0050% or less of calcium. The calcium content is preferably 0.0025% or more, and is preferably 0.0040% or less, and more preferably 0.0035% or less.

[0031] As described above, the magnesium, lanthanum, cerium, and calcium contained in the spheroidal graphite cast iron according to the present disclosure are all elements that form oxides and sulfides. Some of the oxides and sulfides formed by the addition of the spheroidizing agent rise to the surface of the molten metal as slag and are removed. The remaining oxides and sulfides that are not removed as slag are incorporated into a matrix consisting of ferrite and / or pearlite during the solidification process of the molten metal. Because the oxides and sulfides of magnesium, lanthanum, cerium, and calcium are finely dispersed in the matrix, they have little effect on the mechanical strength of the spheroidal graphite cast iron.

[0032] Next, the remainder of the elements contained in the spheroidal graphite cast iron according to the present disclosure other than the aforementioned elements will be described. The remainder of the spheroidal graphite cast iron according to the present disclosure is iron and inevitable impurities. Both iron and inevitable impurities correspond to the remainder other than the aforementioned elements. Iron is the element contained in the spheroidal graphite cast iron according to the present disclosure in the greatest amount. In accordance with convention, the iron content is not specified in this disclosure. The iron content in the spheroidal graphite cast iron according to the present disclosure can be estimated as the remainder based on the total content of elements other than iron.

[0033] As used herein, the term "unavoidable impurities" generally refers to impurities that are present in cast iron without any intentional introduction during the manufacturing process leading up to the production of the desired final cast iron product, and whose presence is unnecessary but is so small that they are left present because they do not necessarily adversely affect the properties of the cast iron. Specific examples of unavoidable impurities in this disclosure include, but are not limited to, hydrogen, nitrogen, oxygen, titanium, vanadium, chromium, cobalt, nickel, and zinc. As mentioned above, unavoidable impurities may also include rare earth elements other than lanthanum and cerium. The acceptable amount of unavoidable impurities that do not affect the properties of spheroidal graphite cast iron varies depending on the element, making it difficult to uniformly define it. However, if the content of a single element is approximately 0.1% or less, such an element falls under the category of an unavoidable impurity in this disclosure, as it is a trace element that does not affect the properties of the spheroidal graphite cast iron.

[0034] In a preferred embodiment, the spheroidal graphite cast iron according to the present disclosure has a desulfurization capacity coefficient DS expressed by the following formula (hereinafter referred to as "Formula 1") of 0.055% or more and 0.085% or less, where the contents of magnesium, lanthanum, cerium, and calcium expressed in mass percentage are Mg, La, Ce, and Ca, respectively.

[0035]

number

[0036] As mentioned above, magnesium, lanthanum, cerium, and calcium are all elements contained in the spheroidizing agent. These elements have a strong affinity for oxygen and sulfur in the molten metal. When they form sulfides, they reduce the concentration of elemental sulfur dissolved in the molten metal and promote the crystallization of spheroidal graphite. The terms on the right side of Equation 1 represent the amount of sulfur consumed based on the stoichiometric composition of each sulfide, assuming that all of the magnesium, lanthanum, cerium, and calcium are used to produce magnesium sulfide (MgS), rare earth sulfides (RE2S3, where RE is the rare earth element), and calcium sulfide (CaS). The coefficient of 2.93 for the sum of lanthanum and cerium is calculated based on the atomic weight of cerium. Because the atomic weights of lanthanum and cerium are similar, the error in the coefficient of the second term can be ignored even when using misch metal as the rare earth source.

[0037] Adding up the right-hand sides of Equation 1 gives the maximum amount of sulfur that can be removed from the molten metal by forming compounds with the magnesium, lanthanum, cerium, and calcium contained in the spheroidizing agent. This value is therefore defined as the desulfurization coefficient (DS). The unit of DS is %. If DS is 0.055% or more, the elemental sulfur contained in the molten metal is converted into sulfides and rendered harmless, promoting the spheroidization of graphite. If DS is 0.085% or less, a decrease in the temperature of the molten metal due to the excessive addition of the spheroidizing agent can be prevented. Therefore, in a preferred embodiment of the present disclosure, the desulfurization coefficient DS is 0.055% or more and 0.085% or less.

[0038] In a preferred embodiment, the spheroidal graphite cast iron according to the present disclosure has an excess magnesium amount RM expressed by the following formula (hereinafter referred to as "Formula 2") of 0.015% or more and 0.045% or less, where S is the sulfur content expressed in mass percentage.

[0039]

number

[0040] The magnesium, lanthanum, cerium, and calcium contained in the spheroidizing agent all act as deoxidizers and desulfurizers, but among these elements, magnesium does not necessarily have the greatest affinity for oxygen and sulfur. However, it is empirically known that magnesium has the greatest graphite spheroidizing ability among these elements. Therefore, if elemental sulfur that does not form sulfides remains dissolved in the molten metal after all magnesium has been consumed by the formation of magnesium sulfide, there is a risk that the spheroidization of graphite will be hindered.

[0041] As mentioned above, the two terms in curly brackets in Equation 2 represent the amount of sulfur consumed by the formation of sulfides by lanthanum, cerium, and calcium. The part in square brackets in Equation 2 represents the amount of sulfur remaining in the molten metal without being consumed by lanthanum, cerium, and calcium. Multiplying this by 0.76 represents the magnesium equivalent when all of the remaining sulfur is consumed by forming magnesium sulfide. The right-hand side of Equation 2 is the actual magnesium content, represented by the symbol Mg, minus the magnesium equivalent, and represents the amount of excess magnesium remaining in the molten metal without forming sulfides. Therefore, this value is defined as the excess magnesium amount RM (residual magnesium). RM is expressed in %. If RM is 0.015% or more, the excess magnesium converts the elemental sulfur in the molten metal into sulfides, which are removed or rendered harmless, promoting the spheroidization of graphite. If RM is 0.045% or less, it is possible to prevent a drop in the temperature of the molten metal due to the excessive addition of the spheroidizing agent. Therefore, in a more preferred embodiment of the present invention, the excess magnesium amount RM is 0.030% or more and 0.040% or less.

[0042] In a preferred embodiment, the spheroidal graphite cast iron according to the present disclosure contains, by mass percentage, 0.0020% or more and 0.0050% or less of aluminum. Aluminum combines with oxygen dissolved in the molten metal to deoxidize the molten metal. Therefore, a small amount of aluminum has the effect of reducing the surface tension at the interface between the graphite and the molten metal and making the shape of the graphite spherical. If the aluminum content is 0.0020% or more, the crystallization of spheroidal graphite is promoted when the spheroidal graphite cast iron is cast. If the aluminum content is 0.0050% or less, the inhibition of graphite generation and spheroidization due to the addition of excessive aluminum can be prevented. Therefore, in a preferred embodiment of the present disclosure, the aluminum content is 0.0020% or more and 0.0050% or less.

[0043] <Manufacturing method of spheroidal graphite cast iron> In a second embodiment, the present disclosure relates to a method for producing spheroidal graphite cast iron. The types of elements contained in the spheroidal graphite cast iron produced by the method for producing spheroidal graphite cast iron according to the present disclosure and the composition ranges of each element are the same as the types of elements and composition ranges of the spheroidal graphite cast iron according to the first embodiment. Therefore, in the description of the composition of the spheroidal graphite cast iron obtained by carrying out the production method, the description overlaps with the first embodiment, and the description focuses on each step included in the method for producing spheroidal graphite cast iron.

[0044] FIG. 1 is a flowchart showing a method for producing spheroidal graphite cast iron according to the present disclosure. The method for producing spheroidal graphite cast iron according to the present disclosure includes three steps, Step 1 to Step 3, indicated by solid lines in FIG. 1 . The method for producing spheroidal graphite cast iron according to the present disclosure includes a step of melting raw materials to produce molten metal (Step 1 in FIG. 1 ). The raw materials used for melting can be a mixture of known raw materials, such as pig iron produced in a blast furnace or scrap generated in a foundry or scrap iron collected from the market. Known methods, such as a continuous melting furnace, typified by a cupola, or a batch-type melting furnace, typified by an electric furnace, can be used to melt the raw materials. It is generally more difficult to change the composition of the molten metal in a continuous melting furnace than in a batch-type melting furnace. According to the present disclosure, spheroidal graphite cast iron and blackheart malleable cast iron can be produced from the same molten metal. Therefore, the effects of the present disclosure can be more effectively achieved in a continuous melting furnace than in a batch-type melting furnace. However, the method for melting the raw materials according to the present disclosure is not limited to a continuous melting furnace.

[0045] In the method for producing spheroidal graphite cast iron according to the present disclosure, the composition of the molten metal produced by melting raw materials is adjusted by known means to a composition close to that of the final product, spheroidal graphite cast iron. However, the composition of the molten metal tapped from the melting furnace must be adjusted in advance, taking into consideration changes in the composition due to additives added later, such as a spheroidizing agent and an inoculant, if necessary. In producing the molten metal, additives solely for adjusting the chemical composition may be added to the molten metal poured from the melting furnace into a ladle, in addition to the spheroidizing agent and inoculant. For example, the spheroidal graphite cast iron according to the present disclosure is a cast iron containing more silicon than blackheart malleable cast iron. In producing this spheroidal graphite cast iron, when pouring a molten metal compatible with blackheart malleable cast iron (e.g., the molten metal obtained in step 1 of FIG. 1 ) into a ladle, a ladle composition adjuster may be added. For example, ferrosilicon may be added as a ladle composition adjuster and dissolved in the molten metal to adjust the silicon content.

[0046] An example of the composition of molten metal prepared for spheroidal graphite cast iron using the above method is 3.1% carbon, 2.0% silicon, 0.30% manganese, 0.035% phosphorus, 0.10% sulfur, and the remainder being iron and unavoidable impurities. Because the molten metal shown here is produced by melting in an acidic cupola, it contains a large amount of sulfur derived from coke. The silicon content is adjusted to be lower than the silicon content in the final spheroidal graphite cast iron product, taking into account fluctuations due to the addition of a spheroidizing agent and an inoculant added as needed. In this way, the silicon composition can be adjusted in the ladle immediately before casting.

[0047] On the other hand, as mentioned above, the preferred carbon content differs significantly between spheroidal graphite cast iron and blackheart malleable cast iron, so it is not practical to increase the carbon content of the molten metal produced for blackheart malleable cast iron after the fact. Specifically, even if a large amount of ferrocarbon or the like is added to the molten metal in the ladle to increase the carbon content, the carbon content will not be dissolved in the molten metal and the purpose will not be achieved. Therefore, when producing the molten metal in the melting furnace, it is preferable to adjust the carbon content of the molten metal to a range of 2.8% to 3.3%, which is the carbon content range for spheroidal graphite cast iron according to the present disclosure. However, fine adjustment of the carbon content by adding a small amount of ferrocarbon or the like in the ladle is permitted in this embodiment. The same applies to elements other than carbon and silicon.

[0048] The method for producing spheroidal graphite cast iron according to the present disclosure includes a step of adding a spheroidizing agent to molten metal. This step is sometimes referred to as "spheroidizing treatment" (step 2 in FIG. 1). In this disclosure, a "spheroidizing agent" refers to a substance that, when added to and dissolved in molten metal, promotes the spheroidization of graphite in spheroidal graphite cast iron. One of the functions of the spheroidizing agent is thought to be to react with sulfur, which inhibits the spheroidization of graphite, to form sulfides. The spheroidizing agent is added to the molten metal before pouring the molten metal into a mold. Known methods for addition, such as the aforementioned pouring method, sandwich method, or wire method, can be used. Note that the spheroidizing agent is used only in the method for producing spheroidal graphite cast iron. As described below, when blackheart malleable cast iron is produced using the molten metal obtained by melting in step 1 in FIG. 1, a spheroidizing agent is not added to the molten metal.

[0049] In a specific embodiment of the spheroidizing agent used in the present disclosure, in addition to the aforementioned magnesium, lanthanum, cerium, and calcium, silicon may be included as an element that promotes graphite crystallization. A spheroidizing agent containing these five elements may be produced by mixing each element individually, or by producing one or more alloys containing iron and then mixing these alloys. A preferred composition range for the elements contained in the spheroidizing agent will be described later. The size of the spheroidizing agent can be appropriately selected depending on the addition method. For example, when the sandwich method is used as the addition method, relatively large chunks are preferred. When the wire method is used as the addition method, it is preferred to add the agent in the form of finely pulverized powder particles that can be easily dissolved in the molten metal.

[0050] In the process of adding the spheroidizing agent to the molten metal, the molten metal is stirred due to the violent reaction between the spheroidizing agent and the molten metal. This may cause the carbon contained in the molten metal to combine with oxygen in the atmosphere and be emitted as gas, accelerating decarburization of the molten metal. The decrease in carbon content in the molten metal due to decarburization may reach approximately 0.1%. In such cases, it is preferable to adjust the carbon content of the molten metal in advance, anticipating decarburization during the spheroidizing treatment, so that the carbon content of the final spheroidal graphite cast iron falls within the range specified in the present disclosure.

[0051] The method for producing spheroidal graphite cast iron according to the present disclosure includes a step of pouring molten metal containing a spheroidizing treatment agent into a mold to cast spheroidal graphite cast iron (step 3 in FIG. 1 ). Pouring the molten metal into the mold may be performed using a ladle that has been subjected to spheroidizing treatment, or the molten metal may be transferred from the ladle to another container for pouring (e.g., a pouring ladle) beforehand. Known casting molds, such as sand molds and metal molds, can be used for casting. The molten metal filled into the mold cavity is cooled there, and solidifies while crystallizing spheroidal graphite during the cooling process. As a result, spheroidal graphite cast iron having the same shape as the mold is completed.

[0052] In a preferred embodiment, the method for producing spheroidal graphite cast iron according to the present disclosure may include a step of heat treatment (annealing) for the purpose of improving the performance of the spheroidal graphite cast iron (step 4 in FIG. 1). In the method for producing spheroidal graphite cast iron according to the present disclosure, melting, spheroidizing, and casting (steps 1 to 3) are essential steps, while heat treatment (step 4) is not. Heat treatments include, but are not limited to, those performed to remove stress to ensure predetermined load performance and high dimensional accuracy, and those performed to decompose cementite and pearlite into ferrite and graphite to increase mechanical strength. The heat treatment conditions, such as temperature, treatment time, and atmosphere, can be appropriately selected according to known methods. For example, the heat treatment (annealing) can be performed in two stages, and the first stage annealing can be preferably performed at a temperature range of 850°C to 1000°C for a holding time of 30 minutes to 3 hours. This first-stage annealing can further decompose the remaining cementite into austenite and graphite. A temperature of 850°C or higher is preferable because the decomposition of cementite can proceed quickly, and a temperature of 1000°C or lower is preferable because decarburization and distortion are less likely to occur. A more preferable temperature range is 900°C or higher and 980°C or lower. After the first-stage annealing, a second-stage annealing can be performed. The second-stage annealing can further separate the remaining austenite into ferrite and graphite, allowing for more graphite to precipitate. The conditions for the second-stage annealing vary depending on whether the matrix of the spheroidal graphite cast iron is to be ferrite or pearlite. To convert the matrix to ferrite, the cast iron is slowly cooled to a temperature slightly lower than the A1 transformation point (723°C). To convert the matrix to pearlite, the cast iron is cooled to a temperature higher than the A1 transformation point, followed by furnace cooling or air cooling.

[0053] The spheroidal graphite cast iron obtained by carrying out the method for producing spheroidal graphite cast iron according to the present disclosure has the same composition as the spheroidal graphite cast iron according to the first embodiment described above. That is, the composition of the spheroidal graphite cast iron contains, in mass percentages, 2.8% to 3.3% carbon, 2.5% to 4.0% silicon, 0.32% to 0.40% manganese, 0.020% to 0.030% phosphorus, 0.020% to 0.035% sulfur, 0.030% to 0.050% magnesium, 0.010% to 0.050% lanthanum and cerium in total, 0.0020% to 0.0050% calcium, and the remainder being iron and unavoidable impurities.

[0054] Blackheart malleable cast iron can be produced by the process shown by the dashed line in FIG. 1 using the same molten metal as that used to produce the spheroidal graphite cast iron according to the present disclosure. That is, blackheart malleable cast iron can be produced by casting the molten metal produced in step 1 of FIG. 1 into a mold and subjecting the resulting casting to a heat treatment called graphitization. However, when producing blackheart malleable cast iron, the silicon content is not adjusted by adding ferrosilicon to the molten metal, and spheroidization is not performed. Thus, according to the production method according to the present disclosure, spheroidal graphite cast iron and blackheart malleable cast iron can be produced separately by appropriately selecting, using the same molten metal, the process for producing spheroidal graphite cast iron shown by the solid line in FIG. 1 and the process for producing blackheart malleable cast iron shown by the dashed line in FIG. 1, and adjusting the composition of the molten metal to the minimum necessary when producing spheroidal graphite. This means that even when molten metal is produced using a continuous melting furnace such as a cupola, two types of cast iron can be easily produced according to market demand without stopping the operation of the melting furnace, so the molten metal and the thermal energy required for melting are not wasted, making it economical.

[0055] In a preferred embodiment, in the method for producing spheroidal graphite cast iron according to the present disclosure, a spheroidizing agent is filled into an iron wire, and the step of adding the spheroidizing agent is carried out by immersing the wire filled with the spheroidizing agent in a sealed space into the molten metal. This embodiment corresponds to the wire method described above. In the above-mentioned pouring method and sandwich method, adding the spheroidizing agent to the molten metal remaining in the ladle later can cause a dangerous explosive reaction. For this reason, it is necessary to previously load an empty ladle with an amount of spheroidizing agent corresponding to the amount of molten metal to be poured, and then pour the molten metal into the ladle. In contrast, in the preferred method for producing spheroidal graphite cast iron according to the present disclosure, the spheroidizing agent filled into the iron wire can be added little by little to the molten metal tapped into the ladle, reducing the possibility of an explosive reaction. Furthermore, since the weight of the molten metal after tapping into the ladle can be measured and an amount of spheroidizing agent appropriate for that weight can be added, the amount of spheroidizing agent added can be adjusted appropriately even if the amount of molten metal tapped from the melting furnace changes. The step of adding the spheroidizing agent can be carried out inside a sealed space, for example, by placing a lid on the ladle and conveying a wire toward the inside through a hole drilled in the lid. This allows for safe operation even if the molten metal bumps. Magnesium and calcium vapors that do not react with the molten metal and accumulate in the space between the surface of the molten metal and the ladle lid can be forcibly vented to the outside by an exhaust means.

[0056] The iron wire can be constructed as a hollow tube with a wall thickness of approximately 0.35 mm and a diameter of approximately 13 mm. The spheroidizing agent can be constructed by filling the interior of this tube with powdered spheroidizing agent that has been melted and crushed in advance. When the spheroidizing agent thus constructed is immersed in the molten metal from the surface toward the interior, it takes some time for the iron wire to dissolve in the molten metal. Therefore, the position at which the wire completely dissolves and the spheroidizing agent is released into the molten metal is below the surface of the molten metal. In this case, the reaction between the spheroidizing agent and the molten metal occurs inside the molten metal, and therefore, the molten metal is subjected to molten metal pressure compared to when the reaction occurs near the surface of the molten metal, resulting in an improved yield of the spheroidizing agent. The amount of spheroidizing agent added can be adjusted by the length of the wire immersed in the molten metal. The amount of spheroidizing agent to be added can be determined appropriately, for example, within the range of 1.0 to 2.0 kg per 100 kg of molten metal (excluding the mass of the iron shell), depending on the component composition of the molten metal before addition, particularly the amount of sulfur, the component composition of the spheroidizing agent, etc.

[0057] In a preferred embodiment, the method for producing spheroidal graphite cast iron according to the present disclosure includes a step of adding an inoculant to a molten metal to which a spheroidizing treatment agent has been added. In this specification, "inoculation" in the production of spheroidal graphite cast iron refers to the addition of an inoculant to the molten metal, primarily for the purpose of affecting graphitization and preventing the formation of white cast iron (white pig iron). In this specification, "inoculant" refers to a type of additive added to promote the crystallization of graphite in spheroidal graphite cast iron or to regulate the shape and number of spheroidal graphite particles. Even in small amounts, the inoculant exhibits a more pronounced inoculation effect than simple alloying elements. While the exact function of inoculants in the production of spheroidal graphite cast iron is unclear, it is believed that they promote nucleation, which triggers the crystallization of spheroidal graphite, rather than acting on specific elements contained in the molten metal like spheroidizing treatment agents. The inoculant may be, for example, ferrosilicon or an alloy of ferrosilicon and one or more of calcium, aluminum, barium, potassium, bismuth, and zirconium. The inoculant differs from the spheroidizing agent in that it does not contain magnesium. To fully demonstrate the inoculant's effectiveness, the inoculant is preferably added after the spheroidizing agent has been added and immediately before casting, for example, within one minute of pouring into the mold. Examples of methods for adding an inoculant include (1) adding the inoculant to the molten metal in a ladle. Specifically, when a spheroidizing ladle and a pouring ladle for pouring into the mold are prepared, the inoculant is placed in the pouring ladle beforehand, and the molten metal is poured from the spheroidizing ladle into the pouring ladle. (2) When pouring the molten metal from the pouring ladle into the mold, the inoculant, for example, in powder form, is added to the molten metal so that it comes into contact with the molten metal. (3) The inoculant is placed in a mold runner or the like beforehand, and the inoculant is added to the molten metal flowing through the runner. The amount of inoculant added may be any amount sufficient to achieve the above-mentioned objective. For example, the amount may be approximately 300 g per 100 kg of molten metal, i.e., a mass percentage preferably in the range of 0.10% to 0.50%.

[0058] <Spheroidizing agent> In a third embodiment, the present disclosure relates to a spheroidizing treatment agent. The spheroidizing treatment agent according to the present disclosure contains, by mass percentage, 45% to 47% silicon, 14% to 16% magnesium, a total of 4.5% to 8.0% lanthanum and cerium, and 4.5% to 10% calcium, with the remainder being iron and unavoidable impurities. The spheroidizing treatment agent according to the present disclosure is suitable for use in the method for producing spheroidal graphite cast iron according to the second embodiment. As described above, silicon is an element that promotes graphite crystallization, and magnesium, lanthanum, cerium, and calcium are elements that promote the formation and spheroidization of graphite in spheroidal graphite cast iron.

[0059] The silicon contained in the spheroidizing agent according to the present disclosure not only promotes the crystallization of graphite when dissolved in molten metal, but also forms an alloy with iron and other elements that has a low melting point and is easily pulverizable, facilitating the production of the spheroidizing agent. A silicon content of 45% or more promotes the crystallization of graphite in spheroidal graphite cast iron, facilitating the production of the spheroidizing agent. A silicon content of 47% or less suppresses excessive reaction with the molten metal and a decrease in the temperature of the molten metal. Therefore, the spheroidizing agent according to the present disclosure contains silicon in an amount of 45% or more and 47% or less.

[0060] The magnesium content of the spheroidizing agent according to the present disclosure is approximately two to three times the magnesium content of the conventional spheroidizing agent described in, for example, Patent Document 1. Furthermore, the total lanthanum and cerium content and the calcium content are also slightly higher than those of conventional spheroidizing agents. Due to these compositional characteristics, the spheroidizing agent according to the present disclosure forms sulfides, consuming sulfur even when the molten metal contains a large amount of sulfur, thereby promoting the crystallization of spheroidal graphite. Therefore, it is believed that the desulfurization treatment of the molten metal, which is conventionally performed in the production of spheroidal graphite cast iron, can be omitted. Therefore, the spheroidizing agent according to the present disclosure can be said to be suitable for use in the production method of spheroidal graphite cast iron according to the second embodiment, which does not include desulfurization of the molten metal.

[0061] In a preferred embodiment, the spheroidizing agent according to the present disclosure contains, by mass percentage, 0.30% or more and 0.80% or less of aluminum. As described above, aluminum combines with oxygen dissolved in the molten metal to deoxidize the molten metal. Therefore, even a small amount of aluminum has the effect of reducing the surface tension at the interface between graphite and the molten metal and making the shape of the graphite spherical. If the aluminum content in the spheroidizing agent is 0.30% or more, the crystallization of spheroidal graphite is promoted when casting spheroidal graphite cast iron. If the aluminum content in the spheroidizing agent is 0.80% or less, the inhibition of graphite generation and spheroidization due to the addition of excessive aluminum can be prevented. Therefore, in a preferred embodiment of the present disclosure, the spheroidizing agent contains 0.30% or more and 0.80% or less of aluminum.

[0062] In a preferred embodiment, the spheroidizing agent according to the present disclosure is filled into an iron wire. Unlike the previously described pouring method or sandwich method, the use of a wire filled with the spheroidizing agent allows the spheroidizing agent to be added to the molten metal after it has been tapped into the ladle. Therefore, even if the amount of molten metal tapped from the melting furnace changes, the amount of spheroidizing agent added can be adjusted to an appropriate amount. As described above, the iron wire can be formed, for example, into a hollow tube with a wall thickness of approximately 0.35 mm and a diameter of approximately 13 mm. The spheroidizing agent can be formed by filling the inside of this tube with powder of the spheroidizing agent, which has been melted and pulverized in advance. The amount of spheroidizing agent added can be adjusted by the length of the wire immersed in the molten metal. [Example]

[0063] <First Example> (Preparation of spheroidizing agent) Ferrosilicon (an alloy of iron and silicon), calcium silicide, rare earth silicide, and magnesium were blended and melted to produce master alloys for spheroidizing agents with different compositions. The resulting master alloys were then pulverized to produce powders, which were then blended and mixed to produce six types of spheroidizing agents with different compositions, as shown in Table 1. The powders were blended so that the rare earth elements and calcium contained in the spheroidizing agents increased in order from the top row to the bottom row in Table 1. The six resulting powders were individually packed into iron wires with a wall thickness of 0.35 mm and a diameter of 13 mm. The weight of the spheroidizing agent packed into the wire varied depending on the composition, but ranged from approximately 260 g to 300 g per meter of wire.

[0064] [Table 1]

[0065] (Manufacturing of spheroidal graphite cast iron) Raw materials and coke were alternately charged and stacked into the acidic cupola furnace, which was constructed with acidic refractory materials. Hot air was blown into the furnace to combust the coke and continuously melt the raw materials. The resulting molten metal was tapped into a ladle at regular intervals. The weight of the molten metal tapped into the ladle at one time was approximately 700 kg. The temperature of the molten metal at the time of tapping was approximately 1500°C. 5.5 kg of ferrosilicon with a silicon content of 75% was placed in the ladle in advance as a ladle composition adjuster, and this was dissolved in the molten metal (approximately 700 kg). The composition of the molten metal in the ladle was analyzed and the results are shown in Table 2. The molten metal contained 0.10% sulfur derived from the coke. Furthermore, as a result of the composition adjustment using ferrosilicon, the molten metal contained 2.0% silicon.

[0066] [Table 2]

[0067] (Addition of spheroidizing agent) Next, a lid was placed on top of the ladle containing the molten metal to create a sealed space, and a wire filled with a spheroidizing agent was gradually inserted through a hole in the lid and immersed in the molten metal, where the spheroidizing agent was added to and mixed with the molten metal. The length of the wire inserted into the molten metal was approximately 30 to 40 meters, and the amount of spheroidizing agent added relative to the amount of molten metal was as shown in Table 3 below. While the spheroidizing agent was being added to and mixed with the molten metal, magnesium and calcium vapors generated inside the ladle were forcibly vented to the outside through an exhaust port in the lid. Spheroidizing was performed using one type of spheroidizing agent per ladle, and a total of six types of molten metal were produced.

[0068] [Table 3]

[0069] Next, each of the six molten irons containing the spheroidizing agent was poured into a mold to cast spheroidal graphite cast iron test pieces. Test pieces for composition analysis were prepared by rapidly solidifying the molten iron in a 5 mm-thick mold to prevent segregation of the components. The compositional analysis results for the six test pieces are shown in Table 4. The test piece compositions were analyzed using photoelectric emission spectrometry. The RE in Table 4 was calculated by determining the Ce content using the photoelectric emission spectrometry method, estimating the La content from the Ce content and the Ce:La ratio (Ce:La = 2:1) of the misch metal contained in the wire, and then summing these Ce and La contents. The desulfurization capacity coefficient DS and the excess magnesium content RM calculated using Equation 1 and Equation 2 based on the composition values ​​in Table 4 are also shown in Table 5.

[0070] [Table 4]

[0071] [Table 5]

[0072] Six types of molten metals to which a spheroidizing treatment agent had been added were used to obtain the samples shown in Figure 2 for observation of the metal structure before heat treatment. The six obtained samples were cut as shown in Figure 2, the cut surfaces were polished, and the metal structure before heat treatment was observed using an optical microscope. The observation showed that spheroidal graphite was formed in the samples of Examples 1 to 3, and almost no cementite was observed, whereas a tendency for the amount of cementite to increase was observed in Comparative Example 1, which used a spheroidizing treatment agent with low calcium and rare earth elements, and Comparative Examples 2 and 3, which used spheroidizing treatment agents with high calcium and rare earth elements.

[0073] Molten metal was poured into a sand mold (inverted when pouring) as shown in Figure 3 to obtain test materials for metallographic observation and strength evaluation. Tensile test bars were taken from the bold-framed area A of the shaded area in Figure 3. Specifically, JIS Z 2241 No. 4 test pieces were taken. Furthermore, as samples for metallographic observation after heat treatment, the cut surface of the tensile test bar was observed as shown in Figure 4, or a test piece for metallographic observation was obtained from the convex portion B in Figure 3. The interior of the cut surface was observed, avoiding surfaces that were strongly affected by heat treatment and the solidification rate. It was separately confirmed that there was almost no difference in the metallographic structure between the cut surface of the tensile test bar and the convex portion B in Figure 3.

[0074] Next, the six types of test materials shown in Figure 3 were subjected to heat treatment to obtain heat-treated (annealed) test materials. The heat treatment involved holding the first annealing at 980°C for 1 hour, followed by cooling from 980°C to 760°C, and then slowly cooling the second annealing from a starting temperature of 760°C to a completion temperature of 700°C over 1.5 hours. In this example, all heat treatments were performed under the above conditions. Samples for metallographic observation and tensile test bars were taken from the heat-treated test materials. The metallographic structure of the samples for metallographic observation was observed using an optical microscope. The spheroidization rate and graphite grain count were evaluated according to JIS G 5502:2001 using images of the metallographic structure taken at 100x magnification using the image processing software QuickGrain Pad+FilePro manufactured by Innotec Co., Ltd., and average values ​​were calculated from images taken at five locations on the test specimen. The results are shown in Table 6. The results of measuring the tensile strength and elongation using a tensile test bar are also shown in Table 6. Furthermore, as an example of the optical microscope image, an optical microscope image of the test piece of Example 1 after the heat treatment is shown in Figure 5.

[0075] [Table 6]

[0076] Next, the composition image of the test piece of Example 1 was observed by an electron probe microanalyzer, and it was found that magnesium and cerium were concentrated in the same locations as the sulfur-concentrated areas. These sulfides were present as finely dispersed particles in the matrix of the spheroidal graphite cast iron.

[0077] From the above results, it can be seen that in the spheroidal graphite cast irons of Examples 1 to 3 having the spheroidal graphite cast iron composition according to the present disclosure, a portion of the sulfur contained in the molten metal produced in an acidic cupola was removed as slag by the action of the spheroidizing treatment agent, and the remainder was finely dispersed as sulfides in the matrix of the spheroidal graphite cast iron. As a result, it is believed that the sulfur contained in the spheroidal graphite cast iron at 0.020% or more and 0.035% or less was rendered harmless, resulting in a spheroidal graphite structure. On the other hand, in Comparative Example 1, which used a spheroidizing treatment agent with low calcium and rare earth elements, the remaining sulfur prevented the spheroidization of graphite. For example, if a spheroidization rate of 70.0% or more is considered acceptable, Comparative Example 1 would be considered to have insufficient spheroidization. Furthermore, in Comparative Examples 2 and 3, which used a spheroidizing treatment agent with high calcium and rare earth elements, the sulfur content was reduced, but the amount of cementite increased, the number of graphite nodules decreased, and elongation decreased. Therefore, it is clear that there are appropriate ranges for the contents of magnesium, lanthanum, cerium, and calcium in the spheroidal graphite cast iron according to the present disclosure, and that the spheroidizing treatment agent according to the present disclosure is suitable for use in the method for producing spheroidal graphite cast iron according to the present disclosure.

[0078] Next, approximately 700 kg of the molten metal removed from the cupola was poured into a ladle separate from the ladle used to produce spheroidal graphite cast iron. The results of analyzing the composition of the molten metal in the ladle are shown in Table 7. As the silicon composition of the molten metal in this ladle was not adjusted using ferrosilicon, the silicon content of the molten metal was 1.4%.

[0079] [Table 7]

[0080] Next, the molten metal in the ladle was poured into a mold to cast white pig iron, and the resulting casting was graphitized under specified conditions to produce blackheart malleable cast iron. The cross section of the resulting blackheart malleable cast iron was polished, and its structure was observed using an optical microscope. Figure 6 shows an optical microscope photograph showing the metal structure of the blackheart malleable cast iron. The metal structure of the blackheart malleable cast iron was observed, with massive graphite precipitated in a ferrite matrix. These results demonstrate that the manufacturing method according to the present disclosure makes it possible to selectively produce spheroidal graphite cast iron and blackheart malleable cast iron using the same molten metal produced in an acid cupola without desulfurization.

[0081] <Second Example> In the second example, a spheroidal graphite cast iron test piece was prepared with a lower carbon content than in the first example. Specifically, the same spheroidizing agent as used in Example 2 of the first example was used, and the spheroidal graphite cast iron test piece was prepared in the same manner as in the first example, except as described below. To adjust the composition in the ladle, 4.5 kg of ferrosilicon with a silicon content of 75% was placed in the ladle beforehand as a ladle composition adjuster, and molten metal (approximately 700 kg) was poured into the ladle and melted. The molten metal used for the spheroidizing treatment had a lower carbon content than the composition shown in Table 2. The amount of spheroidizing agent added to the molten metal was as shown in Table 8 below. Test pieces for composition analysis were prepared by rapidly solidifying the molten metal using a 5 mm-thick mold to prevent segregation of the components. The composition of the obtained test piece was analyzed using the same method as in the first example, and the results are shown in Table 9. Table 10 also shows the desulfurization capacity coefficient DS and the amount of excess magnesium RM calculated using Equation 1 and Equation 2 based on the composition values ​​in Table 9.

[0082] [Table 8]

[0083] [Table 9]

[0084] [Table 10]

[0085] The test specimen for metallographic observation was prepared as described in Example 1, and the metallographic structure before heat treatment (before annealing) was observed using an optical microscope. An optical microscope photograph showing the metallographic structure of the test specimen of Example 4 (before heat treatment (before annealing)) is shown in Figure 7. The spheroidization rate of this test specimen was 76.9%, and the number of graphite grains was 128 grains / mm 2 7 has a large amount of residual pearlite, but spheroidal graphite has crystallized, and the spheroidization rate and the number of graphite nodules are comparable to those of the first example. From these results, it can be seen that the manufacturing method according to the present disclosure can manufacture spheroidal graphite cast iron even when the carbon content is as low as 2.84%.

[0086] <Third Example> In the third example, an inoculant is added during the manufacturing process.

[0087] (Preparation of spheroidizing agent) First, in the same manner as in Example 1, powders prepared by pulverizing the master alloy were blended and mixed to obtain the compositions shown in Table 11 to produce two types of spheroidizing agents with different compositions (the spheroidizing agent used in Example 5 and the spheroidizing agent used in Examples 6 to 8). The two types of powder obtained were individually packed into iron wires with a wall thickness of 0.35 mm and a diameter of 13 mm. The weight of the spheroidizing agent packed into the wire varied depending on the composition, but ranged from approximately 260 g to 300 g per meter of wire. The spheroidizing agents used in Examples 6 to 8 contained a higher amount of Ca than the spheroidizing agent used in Example 5 in order to further reduce the amount of sulfur in the steel.

[0088] [Table 11]

[0089] (Manufacturing of spheroidal graphite cast iron) Raw materials and coke were alternately charged and stacked into the furnace from above an acidic cupola, which used acidic refractory materials for the furnace lining. Hot air was blown into the furnace to combust the coke, and the raw materials were continuously melted. The molten metal obtained by melting was tapped into a ladle at regular intervals. In Examples 5 and 6, 4.5 kg of ferrosilicon with a silicon content of 75% was placed in the ladle as a ladle composition adjuster, and the molten metal (approximately 700 kg) was poured into the ladle for melting. In Examples 7 and 8, 4.5 kg of ferrosilicon with a silicon content of 75% and 2.0 kg of calcium silicon with a silicon content of 59% were placed in the ladle as ladle composition adjusters, and the molten metal (approximately 700 kg) was poured into the ladle for melting. The weight of the molten metal tapped into the ladle in one tapping run was approximately 700 kg. The temperature of the molten metal at the time of tapping was approximately 1500°C.

[0090] (Addition of spheroidizing agent) Next, a lid was placed on top of the ladle containing the molten metal to form a sealed space, and a wire filled with a spheroidizing agent was gradually inserted through a hole in the lid and immersed in the molten metal, where the spheroidizing agent was added to and mixed with the molten metal. The length of the wire inserted into the molten metal was approximately 34 to 46 meters, and the amount of spheroidizing agent added to the molten metal was as shown in Table 12 below. While the spheroidizing agent was being added to and mixed with the molten metal, magnesium and calcium vapors generated inside the ladle were forcibly vented to the outside through an exhaust port in the lid.

[0091] [Table 12]

[0092] The composition in Table 13 is the composition after the composition adjustment of the molten metal and the spheroidization treatment, but before inoculation. The composition was determined by photoelectric photometric emission spectrometry. The method for determining RE was the same as in Example 1.

[0093] In Examples 7 and 8, the composition was adjusted using ferrosilicon and calcium silicon, resulting in a silicon content of approximately 3.2% in the molten iron. The composition shown in Table 13 below is the composition after the composition adjustment and spheroidization treatment of the molten iron, but before inoculation, as described above. However, even after the addition of the inoculant, the composition is considered to be within the range of the composition of the spheroidal graphite cast iron disclosed herein. Furthermore, the desulfurization capacity coefficient DS and the amount of excess magnesium RM calculated using Equation 1 and Equation 2 based on the composition values ​​in Table 13 are shown in Table 14.

[0094] [Table 13]

[0095] [Table 14]

[0096] (Addition of inoculant) After the spheroidizing treatment agent was added to the molten metal, an inoculant was further added. In Examples 5 to 8, an alloy having a composition containing 69.9% Si, 1.5% Ca, 1.4% Al, and 0.3% Ba, with the remainder being iron and unavoidable impurities, was used as the inoculant. The amount of inoculant added to the molten metal, expressed as a mass percentage relative to the molten metal, was 0.3% in Example 5 and 0.5% in Examples 6 to 8. In all of Examples 5 to 8, a powdered inoculant was added so as to come into contact with the molten metal when the molten metal was poured from the ladle into the mold.

[0097] The obtained test pieces were subjected to heat treatment in the same manner as in Example 1, and samples for metallographic observation and tensile test bars were taken from the heat-treated test pieces in the same manner as in Example 1. Using the samples for metallographic observation, the spheroidization rate and graphite nodule count were evaluated in the same manner as in Example 1. The results are shown in Table 15. In Table 15, the graphite nodule count is shown in parentheses because it is data before heat treatment. The results of measuring tensile strength and elongation using the tensile test bar are also shown in Table 15. Furthermore, optical microscope photographs showing the metallographic structures of the test pieces before and after the heat treatment of Example 7 are shown in Figures 8 and 9. A comparison of Figures 8 and 9 reveals that the cementite and pearlite were sufficiently decomposed by the heat treatment, resulting in the formation of ferrite (gray areas) and graphite (black areas) as shown in Figure 9.

[0098] [Table 15]

[0099] The results show that the use of an inoculant and an increased Ca content in the spheroidizing treatment agent promotes desulfurization, increasing the number of graphite nodules and further improving the spheroidization rate. Furthermore, based on the analysis results in Table 13, Examples 5 to 8 are estimated to have a silicon content in the spheroidal graphite cast iron within a preferred range of 2.68% to 3.3%, and satisfy the tensile strength of 450 MPa or more required for spheroidal graphite cast iron FCD450 specified in JIS standard (JIS G 5502). Furthermore, among Examples 5 to 8, Examples 7 and 8 are estimated to have a silicon content in the spheroidal graphite cast iron within a more preferred range of 3.0% to 3.3%, and have a higher tensile strength of 500 MPa or more, and also satisfy the elongation requirement of 10% or more for spheroidal graphite cast iron FCD450.

[0100] The disclosure of the present disclosure may include the following aspects. (Aspect 1) All are mass percentages Carbon: 2.8% or more, 3.3% or less, Silicon: 2.5% or more, 2.9% or less, Manganese: 0.32% or more, 0.40% or less, Phosphorus: 0.020% or more, 0.030% or less, Sulfur: 0.020% or more, 0.035% or less Magnesium: 0.030% or more, 0.050% or less, Lanthanum and cerium total not less than 0.010% and not more than 0.050% Contains calcium at least 0.0020% and not more than 0.0040%. The balance is iron and unavoidable impurities Spheroidal graphite cast iron. (Aspect 2) When the contents of magnesium, lanthanum, cerium, and calcium expressed in mass percentage are Mg, La, Ce, and Ca, respectively, the desulfurization capacity coefficient DS expressed by the following formula is 0.055% or more and 0.085% or less. 2. The spheroidal graphite cast iron according to embodiment 1.

[0101]

number

[0102] (Aspect 3) When the sulfur content expressed in mass percentage is S, the excess magnesium amount RM expressed by the following formula is 0.008 or more and 0.031 or less. 3. The spheroidal graphite cast iron according to claim 1 or 2.

[0103]

number

[0104] (Aspect 4) Furthermore, it contains aluminum by mass percentage of 0.0020% or more and 0.0050% or less. Aspects 4. The spheroidal graphite cast iron according to any one of aspects 1 to 3. (Aspect 5) a step of melting raw materials to produce a molten metal; adding a spheroidizing agent to the molten metal; and a step of pouring the molten metal to which the spheroidizing treatment agent has been added into a mold to cast a casting made of spheroidal graphite cast iron, The composition of the castings is as follows: Carbon: 2.8% or more, 3.3% or less, Silicon: 2.5% or more, 2.9% or less, Manganese: 0.32% or more, 0.40% or less, Phosphorus: 0.020% or more, 0.030% or less, Sulfur: 0.020% or more, 0.035% or less Magnesium: 0.030% or more, 0.050% or less, Lanthanum and cerium total not less than 0.010% and not more than 0.050% Contains calcium at least 0.0020% and not more than 0.0040%. The balance is iron and unavoidable impurities Manufacturing method of spheroidal graphite cast iron. (Aspect 6) The spheroidizing agent is filled into an iron wire, and the step of adding the spheroidizing agent is carried out by immersing the wire filled with the spheroidizing agent into the molten metal in a sealed space. A method for producing spheroidal graphite cast iron according to aspect 5. (Aspect 7) adding an inoculant to the molten metal to which the spheroidizing agent has been added. A method for producing spheroidal graphite cast iron according to any one of aspects 5 and 6. (Aspect 8) All are mass percentages Silicon content: 45% or more, 47% or less, Magnesium: 14% or more, 16% or less Lanthanum and cerium total not less than 4.5% and not more than 8.0% Contains more than 4.5% and less than 10% calcium, The balance is iron and unavoidable impurities Spheroidizing agent. (Aspect 9) Furthermore, it contains aluminum by mass percentage of 0.30% or more and 0.80% or less. A spheronizing treatment agent according to aspect 8. (Aspect 10) 10. The spheroidizing agent according to claim 8 or 9, wherein the spheroidizing agent is filled into an iron wire.

[0105] This application claims priority from Japanese Patent Application No. 2021-050635, which is incorporated herein by reference.

Claims

1. All are mass percentages, Carbon: 2.8% or more, 3.3% or less, Silicon: 2.5% or more and 4.0% or less; Manganese: 0.32% or more, 0.40% or less; Phosphorus: 0.020% or more, 0.030% or less; Sulfur: 0.020% or more, 0.035% or less; Magnesium: 0.030% or more and 0.050% or less; Lanthanum and cerium total not less than 0.010% and not more than 0.050%; Contains calcium at least 0.0020% and not more than 0.0050%; The balance is iron and unavoidable impurities Spheroidal graphite cast iron.

2. When the contents of magnesium, lanthanum, cerium, and calcium expressed in mass percentage are Mg, La, Ce, and Ca, respectively, the desulfurization capacity coefficient DS expressed by the following formula is 0.055% or more and 0.085% or less. The spheroidal graphite cast iron according to claim 1. [Equation 1]

3. When the sulfur content expressed in mass percentage is S, the excess magnesium amount RM expressed by the following formula is 0.015% or more and 0.045% or less. The spheroidal graphite cast iron according to claim 1 or 2. [Equation 2]

4. Furthermore, the mass percentage of aluminum is 0.0020% or more and 0.0050% or less. The spheroidal graphite cast iron according to any one of claims 1 to 3.

5. a step of melting raw materials to produce a molten metal; adding a spheroidizing agent to the molten metal; and a step of pouring the molten metal to which the spheroidizing treatment agent has been added into a mold to cast spheroidal graphite cast iron, The composition of the spheroidal graphite cast iron is as follows in mass percentage: Carbon: 2.8% or more, 3.3% or less, Silicon: 2.5% or more and 4.0% or less; Manganese: 0.32% or more, 0.40% or less; Phosphorus: 0.020% or more, 0.030% or less; Sulfur: 0.020% or more, 0.035% or less; Magnesium: 0.030% or more and 0.050% or less; Lanthanum and cerium total not less than 0.010% and not more than 0.050%; Contains calcium at least 0.0020% and not more than 0.0050%; The balance is iron and unavoidable impurities Manufacturing method of spheroidal graphite cast iron.

6. The spheroidizing agent is filled into an iron wire, and the step of adding the spheroidizing agent is carried out by immersing the wire filled with the spheroidizing agent into the molten metal in a sealed space. The method for producing spheroidal graphite cast iron according to claim 5.

7. adding an inoculant to the molten metal to which the spheroidizing agent has been added. The method for producing spheroidal graphite cast iron according to claim 5 or 6.

8. After the step of casting the spheroidal graphite cast iron, a step of heat treating the spheroidal graphite cast iron is included. A method for producing spheroidal graphite cast iron according to any one of claims 5 to 7.

9. All are mass percentages Silicon content: 45% or more, 47% or less, Magnesium: 14% or more, 16% or less Lanthanum and cerium total not less than 4.5% and not more than 8.0%; Contains calcium at least 4.5% and no more than 10%. The balance is iron and unavoidable impurities Spheroidizing agent.

10. Furthermore, the mass percentage of aluminum is 0.30% or more and 0.80% or less. The spheroidizing agent according to claim 9.

11. 11. The spheroidizing agent according to claim 9 or 10, wherein the spheroidizing agent is filled into an iron wire.

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