Spheroidal Graphite Cast Iron, Method for Manufacturing Spheroidal Graphite Cast Iron, and Spheroidizing Treatment Agent

Abstract

A spheroidal graphite cast iron comprising, in mass percentage: 2.8% or more and 3.3% or less of carbon; 2.5% or more and 4.0% or less of silicon; 0.32% or more and 0.40% or less of manganese; 0.020% or more and 0.030% or less of phosphorus; 0.020% or more and 0.035% or less of sulfur; 0.030% or more and 0.050% or less of magnesium; 0.010% or more and 0.050% or less in total of lanthanum and cerium; and 0.0020% or more and 0.0050% or less of calcium, with the balance being iron and inevitable impurities.

Description

TECHNICAL FIELD

The present invention relates to a spheroidal graphite cast iron, a method for manufacturing a spheroidal graphite cast iron, and a spheroidizing treatment agent suitable for use in the manufacturing method.

BACKGROUND ART

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 existence form of graphite. Malleable cast iron can be further classified into white heart malleable cast iron, black heart malleable cast iron, and pearlite malleable cast iron. The carbon content of the cast iron exceeds approximately 2.0 mass %, which is the saturation solubility limit of carbon in austenite in the iron-carbon binary equilibrium phase diagram, but it does not significantly exceed approximately 4.3 mass % at its eutectic point. During a solidification process of cast iron, initially, the eutectic reaction occurs, followed by the eutectoid reaction of austenite, resulting in the crystallization or precipitation of graphite and/or cementite.

Flake graphite cast iron, also called gray cast iron, is cast iron that has been used by humans for centuries. The basic shape of graphite in the flake graphite cast iron is a flake shape (flaky). When tensile stress is applied to flake graphite cast iron, fracture tends to progress along the flaky graphite. Because of this, the flake graphite cast iron has a weaker mechanical strength than carbon steel for mechanical structures, for example. For this reason, attempts have been made to improve the shape of graphite to a more favorable one for the purpose of enhancing the mechanical strength of cast iron.

One of such attempts is to perform a method which involves casting white pig iron, in which graphite does not crystallize during casting, by adjusting the carbon content in a molten metal to, for example, 2.8% or more and 3.1% or less in mass percentage, and precipitating lump graphite liberated from cementite by heat treatment on the resulting cast metal. The cast iron obtained by this method is called “black heart malleable cast iron” or “malleable cast iron”. Another attempt is to perform a method which involves adjusting the carbon content in the molten metal to, for example, 3.4% or more and 3.9% or less in mass percentage and reducing a sulfur content in the molten metal to crystallize spheroidal graphite during casting. The cast iron obtained by this method is called “spheroidal graphite cast iron” or “ductile cast iron”. Both black heart malleable cast iron and spheroidal graphite cast iron are widely used in industry because of their mechanical strength superior to flake graphite cast iron. However, manufacturing methods of both cast irons are significantly different in many ways.

For example, in the manufacturing of spheroidal graphite cast iron, an additive is added to the molten metal poured into a ladle so as to promote the crystallization of spheroidal graphite. This additive is called a “spheroidizing treatment agent” and is composed of an alloy containing silicon, magnesium, cerium, calcium, and iron in most cases (see, for example, Patent Document 1). Known methods for adding a spheroidizing treatment agent include a “pouring method” or “sandwich method,” in which molten metal is poured into a ladle, the bottom of which is filled in advance with the spheroidizing treatment agent, and a “wire method”, in which an iron wire that has its hollow portion filled with the spheroidizing treatment agent is gradually fed into the molten metal through its surface and melted therein (see, for example, Patent Document 2).

PRIOR ART DOCUMENT

Patent Document

• • Patent Document 1: JP 2007-182620 A
  • Patent Document 2: JP 2006-316331 A

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

In order to fully use the molten metal produced in foundries without waste, it is desirable to be able to selectively produce cast irons of different materials using the same molten metal in response to fluctuations in demand. However, there is a significant difference in the preferable carbon content between black heart malleable cast iron and spheroidal graphite cast iron in the related art. The difference in carbon content exceeds the range that can be adjusted by changing the manufacturing conditions of the molten metal or adding carbon in the ladle. Therefore, there is still no known method for separately making black heart malleable cast iron and spheroidal graphite cast iron efficiently, for example, using molten metal continuously supplied from a cupola.

When molten metal is produced using an acid cupola, which is considered to be relatively easy to manage its operation compared to a basic cupola and employs acid refractory as a furnace material, sulfur derived from coke introduced into the cupola as a heat source melts into the molten metal. Since the presence of sulfur interferes with the crystallization of spheroidal graphite, desulfurization treatment of molten metal is necessary in the prior art. As the desulfurization treatment, for example, a method which involves injecting calcium carbide powder into molten metal using nitrogen gas is used. This method is highly effective in desulfurization, but the temperature of the molten metal decreases, and thus reheating becomes necessary. Introducing a new process such as desulfurization treatment or reheating for the purpose of producing spheroidal graphite cast iron using molten metal for black heart malleable cast iron leads to an increase in the manufacturing costs.

The present disclosure has been made in view of the above-mentioned problems, and an object thereof is to provide a spheroidal graphite cast iron that can be produced without much manufacturing costs using molten metal produced for casting black heart malleable cast iron, for example, by an acid cupola, as well as a method for manufacturing a spheroidal graphite cast iron that enables the manufacturing of spheroidal graphite cast iron using the above-mentioned molten metal while suppressing the manufacturing costs.

Means for Solving the Problems

In a first aspect, the present disclosure relates to a spheroidal graphite cast iron comprising, in mass percentage: 2.8% or more and 3.3% or less of carbon; 2.5% or more and 4.0% or less of silicon; 0.32% or more and 0.40% or less of manganese; 0.020% or more and 0.030% or less of phosphorus; 0.020% or more and 0.035% or less of sulfur; 0.030% or more and 0.050% or less of magnesium; 0.010% or more and 0.050% or less in total of lanthanum and cerium; and 0.0020% or more and 0.0050% or less of calcium, with the balance being iron and inevitable impurities.

The spheroidal graphite cast iron according to the present disclosure contains predetermined amounts of magnesium, lanthanum, cerium, and calcium. Thus, these elements react with sulfur to form sulfides, thereby making it possible to remove or detoxify sulfur which may inhibit the spheroidization of graphite. As a result, the spheroidal graphite cast iron can be produced using the molten metal for black heart malleable cast iron containing sulfur and produced by an acid cupola, for example.

In a second aspect, the present disclosure is an invention of the method for manufacturing a spheroidal graphite cast iron, which includes the processes of: preparing a molten metal by melting raw materials; adding a spheroidizing treatment agent to the molten metal; and performing casting by pouring the molten metal to which the spheroidizing treatment agent is added, into a mold to produce spheroidal graphite cast iron, which has the same composition as the composition of the spheroidal graphite cast iron according to the present disclosure. In a third aspect, the present disclosure is an invention of a spheroidizing treatment agent suitable for use in the method for manufacturing a spheroidal graphite cast iron according to the present disclosure.

Effects of the Invention

According to the present disclosure, either spheroidal graphite cast iron or black heart malleable cast iron can be selectively produced by casting using the same molten metal without adding any costly process, so that the molten metal produced in foundries can be fully used without waste in response to fluctuations in demand. In addition, according to the present disclosure, it is not necessary to separately prepare molten metals of different compositions for the manufacturing of cast irons of different materials, thus contributing to the reduction in the total manufacturing costs and the saving of energy resources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method for manufacturing a spheroidal graphite cast iron according to the present embodiment.

FIG. 2 is a side view of a sample used in Examples.

FIG. 3 is a perspective view of a test material used in Examples.

FIG. 4 is a side view of a tensile test rod used in Examples.

FIG. 5 is an optical micrograph showing an example of a metallic microstructure of spheroidal graphite cast iron after heat treatment (after annealing) according to the present embodiment.

FIG. 6 is an optical micrograph showing an example of a metallic microstructure of black heart malleable cast iron produced from the same molten metal as the spheroidal graphite cast iron according to the present embodiment.

FIG. 7 is an optical micrograph showing another example of a metallic microstructure of spheroidal graphite cast iron before heat treatment (before annealing) according to the present embodiment.

FIG. 8 is an optical micrograph showing another example of a metallic microstructure (before heat treatment) of spheroidal graphite cast iron according to the present embodiment.

FIG. 9 is an optical micrograph showing another example of a metallic microstructure (after heat treatment) of spheroidal graphite cast iron according to the present embodiment.

MODE FOR CARRYING OUT THE INVENTION

Modes for carrying out the present invention will be described below by being classified into embodiments. The embodiments and examples of the present invention mentioned herein are merely illustrative examples for carrying out the present invention, and the present disclosure is not limited to these embodiments. In the present specification, the composition of a metal or an alloy is expressed in mass percentage, unless otherwise specified.

<Spheroidal Graphite Cast Iron>

In a first embodiment, the present disclosure relates to a spheroidal graphite cast iron. As used in the present specification, the term “spheroidal graphite cast iron” refers to cast iron having a metallic microstructure in which spheroidal graphite is dispersed and crystallized in a matrix composed of ferrite and/or pearlite. Regarding such features of the metallic microstructure, the spheroidal graphite cast iron according to the present disclosure has no difference from conventional spheroidal graphite cast iron. The spheroidal graphite cast iron according to the present disclosure can have a graphite spheroidization ratio of 70% or more as determined in accordance with JIS G 5502:2001 using an optical microscope image taken at a magnification of 100 times. The spheroidization ratio is preferably 80% or more, and more preferably 85% or more, and the upper limit thereof is not particularly limited, but the upper limit is approximately 95% in consideration of the chemical composition and the like. The spheroidal graphite cast iron according to the present disclosure has a composition different from that of a conventional general spheroidal graphite cast iron as mentioned later, but even in such a case, it is referred to as “spheroidal graphite cast iron” because it has the same metallic microstructure as the conventional spheroidal graphite cast iron as mentioned above.

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 a 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, the carbon content of black heart malleable cast iron is adjusted to a lower composition range than the spheroidal graphite cast iron, e.g., to be 2.8% or more and 3.1% or less. This is because increasing the carbon content in the black heart malleable cast iron forms a microstructure called “mottle,” which is made up of graphite crystallized as primary crystals during casting and subsequent cooling in a mold, which may significantly impair the mechanical strength of the cast iron.

When the carbon content is 2.8% or more, the crystallization of spheroidal graphite is promoted during casting is performed to produce spheroidal graphite cast iron. When the carbon content is 3.3% or less, the formation of mottle can be prevented when the same molten metal is used to perform casting to produce black heart malleable cast iron. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 2.8% or more and 3.3% or less of carbon. A preferred range of the carbon content is 2.9% or more and 3.2% or less, and a more preferred range thereof is 3.0% or more and 3.1% or less. The “content” of carbon in the spheroidal graphite cast iron in the present specification refers to the averaged carbon content based on the total amount of carbon contained in the spheroidal graphite cast iron as the final product, regardless of the existence form of carbon. The same applies to the contents of other elements in spheroidal graphite cast iron.

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 the formation of graphite. When the silicon content is 2.5% or more, the crystallization of graphite is promoted during casting is performed to produce spheroidal graphite cast iron, resulting in the formation of spheroidal graphite. Furthermore, it is preferable that the silicon content is increased in the molten metal in which the carbon content is suppressed and which is used in the manufacturing of black heart malleable cast iron, for example, by adding a silicon-containing substance as a spheroidizing treatment agent to be mentioned later. This is because a carbon equivalent of the molten metal, which is to be mentioned later, can be enhanced, resulting in an improvement in the flowability of the molten metal and the promotion of the crystallization of graphite as mentioned above. As a result of the easier formation of spherical graphite, tensile strength easily increases. Moreover, the solid solution of silicon in a ferrite matrix can increase the tensile strength. On the other hand, if the silicon content is 4.0% or less, reduction in the elongation among mechanical strengths of the cast iron can be prevented. 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 reduction in the elongation, the silicon content is preferably 2.9% or less, and more preferably 2.75% or less. From the viewpoint of sufficiently preventing reduction in the elongation, a preferred range of silicon content is, for example, 2.55% or more and 2.75% or less. On the other hand, from the viewpoint of enhancing tensile strength, etc. as mentioned above, the silicon content is preferably 2.68% or more, and more preferably 2.70% or more. For example, to achieve a tensile strength of 450 MPa or higher which is required for spheroidal graphite cast iron FCD450 specified in the 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 higher, the silicon content is more preferably 3.0% or more and 3.3% or less.

The inclusion of silicon in a liquid or solid phase iron reduces the solubility of carbon in the iron, and the carbon content at the eutectic point becomes smaller than 4.3%. Based on this difference of the carbon content at the eutectic point, it is empirically thought that approximately one-third of the silicon content in the cast iron is equivalent to the carbon content. When the contents of carbon and silicon in the cast iron are defined as C and Si, respectively, a value obtained by a calculation formula of C+⅓Si (%) is called the “carbon equivalent”. The spheroidal graphite cast iron according to the present disclosure corresponds to a sub-eutectic composition when the carbon equivalent of this cast iron is 3.8% or more and 4.1% or less. A preferred range of the carbon equivalent is 3.6% or more and 4.2% or less.

It is noted that in the present disclosure, the silicon content in the spheroidal graphite cast iron includes not only the content of silicon originally contained in the molten metal, but also the content of silicon derived from an additive that is added to the molten metal, i.e., silicon derived from ferrosilicon and a spheroidizing treatment agent that can be added in the ladle as mentioned later as well as silicon derived from an inoculant. The same applies to the contents of other elements in these additives, such as magnesium, lanthanum, cerium, calcium, aluminum, and barium.

The spheroidal graphite cast iron according to the present disclosure contains 0.32% or more and 0.40% or less of manganese. Although a large content of manganese in the spheroidal graphite cast iron does not harm the spheroidization of graphite, the inclusion of manganese in the black heart malleable cast iron inhibits the formation of graphite. In addition, while manganese increases the hardness and strength of the spheroidal graphite cast iron and stabilizes a pearlite microstructure, the inclusion of manganese tends to reduce the elongation of the spheroidal graphite cast iron. Manganese is mixed into cast iron because manganese is slightly contained in iron ore, and manganese steel may be contained in scrap iron that is melted in cupolas furnace. When the manganese content is 0.32% or more, manganese combines with sulfur to form manganese sulfide and detoxifies the elemental sulfur which may inhibit the formation of graphite, thereby promoting the formation of graphite in the cast iron. When the manganese content is 0.40% or less, the formation of graphite is not inhibited due to excess manganese even when the same molten metal is used to manufacture a black heart malleable cast iron. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 0.32% or more and 0.40% or less of manganese. A preferred range of the manganese content is 0.33% or more and 0.39% or less.

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 spheroidization of graphite, but too much phosphorus may reduce the mechanical strength. When the phosphorus content is 0.020% or more, the crystallization of spheroidal graphite is promoted during casting in the manufacturing of spheroidal graphite cast iron. When the phosphorus content is 0.030% or less, the crystallization of mottle can be prevented during casting in the manufacturing of black heart malleable cast iron using the same molten metal as in the manufacturing of spheroidal graphite cast iron, and further the degradation of 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.

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 the formation and spheroidization of graphite. In the prior art, when the molten metal after spheroidizing treatment contains 0.020% or more of sulfur, graphite cannot be spheroidized completely. Therefore, as mentioned above, when molten metal is produced using an acid cupola, desulfurization treatment and reheating of the molten metal are necessary in order to remove sulfur trapped in the molten metal from the coke. However, in the present disclosure, graphite can be spheroidized because of the action of a spheroidizing treatment agent mentioned later without desulfurization of the molten metal even if the sulfur content is 0.020% or more. When the sulfur content is 0.035% or less, the spheroidization of graphite can be performed without desulfurization of the molten metal, and at the same time, graphitization can also be achieved when the same molten metal is used to manufacture a black heart malleable cast iron. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 0.020% or more and 0.035% or less of sulfur. A preferred range of the sulfur content is 0.025% or more and 0.033% or less.

Next, among the elements contained in the spheroidal graphite cast iron according to the present disclosure, elements derived from the spheroidizing treatment agent mentioned later will be described. The spheroidal graphite cast iron according to the present disclosure contains: 0.030% or more and 0.050% or less of magnesium; 0.010% or more and 0.050% or less in total of lanthanum and cerium; and 0.0020% or more and 0.0050% or less of calcium. All of these elements have a high affinity for oxygen and sulfur, and when added to sulfur-rich molten metal produced using an acid cupola, they act to reduce the concentration of elemental sulfur melted in the molten metal by forming sulfides. By using this action of the spheroidizing treatment agent, spheroidal graphite cast iron can be produced using molten metal that contains 0.020% or more and 0.035% or less of sulfur without performing the above-mentioned desulfurization treatment that uses calcium carbide or the like.

As mentioned above, a spheroidizing treatment agent composed of an alloy containing silicon, magnesium, cerium, calcium, and iron has been conventionally known (see, for example, Patent Document 1). However, there is no known spheroidal graphite cast iron that is produced by causing the spheroidizing treatment agent to act on molten metal having a carbon content of 2.8% or more and 3.3% or less, which is smaller than the conventional spheroidal graphite cast iron, and containing 0.020% or more and 0.035% or less of sulfur. In other words, one of the features of the spheroidal graphite cast iron according to the present disclosure is that two conditions unfavorable to the crystallization of spheroidal graphite, i.e., a low carbon content and a high sulfur content, are overcome by a single means, specifically, the addition of the spheroidizing treatment agent.

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 molten metal to detoxify sulfur which may inhibit the formation and spheroidization of graphite. Therefore, the inclusion of magnesium facilitates the formation of the microstructure of spheroidal graphite cast iron. In addition, magnesium has a high vapor pressure and tends to react violently with molten metal. When the magnesium content is 0.030% or more, the crystallization of spheroidal graphite is promoted when casting is performed to manufacture a spheroidal graphite cast iron. The magnesium content of 0.050% or less can prevent the addition of excess magnesium from increasing the amount of free magnesium which does not form a compound, in the molten metal, and promoting of 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. A preferred range of the magnesium content is 0.035% or more and 0.045% or less.

The spheroidal graphite cast iron according to the present disclosure contains 0.010% or more and 0.050% or less in total of lanthanum and cerium. Both lanthanum and cerium are rare earth elements that combine with oxygen and sulfur in the molten metal to detoxify sulfur which may inhibit the formation and spheroidization of graphite. Thus, the inclusion of lanthanum and cerium facilitates the formation of the microstructure of spheroidal graphite cast iron. When the total content of lanthanum and cerium is 0.010% or more, the crystallization of spheroidal graphite is promoted when casting is performed to manufacture a spheroidal graphite cast iron. When the total content of lanthanum and cerium is 0.050% or less, the reduction in the impact strength of the cast iron can be prevented. Therefore, the spheroidal graphite cast iron according to the present disclosure contains 0.010% or more and 0.050% or less in total of lanthanum and cerium. A preferred range of the total content of lanthanum and cerium is 0.025% or more and 0.045% or less.

Rare earth elements have similar chemical properties to each other and are found naturally in an unseparated state. For example, an alloy containing a plurality of light rare earth elements, called mischmetal, contains lanthanum, cerium, praseodymium, and neodymium, with smaller amounts of samarium, magnesium, aluminum, and iron. Of these elements, the most abundant element in the mischmetal is cerium, the content of which is approximately 50%, and the second most abundant element contained is lanthanum, the content of which is approximately 25%. Since the content of rare earth elements other than lanthanum and cerium in the mischmetal is less than the total content of lanthanum and cerium, only the total content of lanthanum and cerium is specified in the present embodiment, and the content of other rare earth elements is not specified. Rare earth elements other than lanthanum and cerium, such as praseodymium, neodymium, and samarium, can be included as inevitable impurities in the present embodiment. Because mischmetal is less expensive than pure rare earth sources that are elementally isolated, the mischmetal can reduce the manufacturing costs of cast iron when being used as a spheroidizing treatment agent. Since the ratio of the lanthanum content to the cerium content in the mischmetal is approximately 1 to 2, the total content of lanthanum and cerium can be estimated by calculation if the content of either lanthanum or cerium is known by chemical analysis or the like. In the present disclosure, other light rare earth alloys other than mischmetal may be used as long as the content of lanthanum and cerium is within the above range.

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 molten metal to detoxify sulfur which may inhibit the formation and spheroidization of graphite. Thus, the inclusion of calcium facilitates the formation of the microstructure of spheroidal graphite cast iron. Calcium has a particularly strong affinity for oxygen compared to magnesium and rare earth elements. As in magnesium, calcium has a high vapor pressure and tends to react violently with molten metal. When the calcium content is 0.0020% or more, the crystallization of spheroidal graphite is promoted when casting is performed to manufacture a spheroidal graphite cast iron. When the calcium content is 0.0050% or less, sudden boiling of molten metal due to the addition of excess calcium can be prevented. Thus, 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.

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 as mentioned above. Some of the oxides and sulfides formed by the addition of a spheroidizing treatment agent are removed by making them float to the surface of the molten metal as slag. The remaining oxides and sulfides that are not removed as slag are trapped into and present in the matrix composed of ferrite and/or pearlite during the process of solidification of the molten metal. The oxides and sulfides of magnesium, lanthanum, cerium, and calcium are finely dispersed in the matrix and hardly affect the mechanical strength of the spheroidal graphite cast iron.

Next, the balance of the elements contained in the spheroidal graphite cast iron according to the present disclosure, other than the elements mentioned above, will be explained. The balance of the spheroidal graphite cast iron according to the present disclosure are iron and inevitable impurities. All iron and inevitable impurities correspond to the balance other than the above-mentioned elements. Iron is the most abundant element in the spheroidal graphite cast iron according to the present disclosure. In the present disclosure, in accordance with customary practice, the iron content is not specified. The iron content in the spheroidal graphite cast iron according to the present disclosure can be estimated as the balance of the other elements based on the total content of the other elements except for iron.

As used in the present specification, “inevitable impurity” means, in general, an impurity that is obvious to be present in the cast iron without being introduced intentionally during a producing process of obtaining the desired final cast iron product; the presence of the impurity is unnecessary but is left as it is because the impurity is contained in a trace amount and does not necessarily adversely affect the properties of the cast iron. Specific examples of inevitable impurities in the present disclosure include, but are not limited to, hydrogen, nitrogen, oxygen, titanium, vanadium, chromium, cobalt, nickel, and zinc. As mentioned above, inevitable impurities can also include rare earth elements other than lanthanum and cerium. The allowable amounts of inevitable impurities that do not affect the properties of the spheroidal graphite cast iron even when contained in trace amounts vary depending on each element, and it is difficult to set these amounts to the same value. However, if the content of one element is generally 0.1% or less, such an element corresponds to an inevitable impurity in the present disclosure, which is a trace element that does not affect the properties of spheroidal graphite cast iron.

In a preferred embodiment, in the spheroidal graphite cast iron according to the present disclosure, a desulfurization capacity factor DS expressed by the following equation (hereinafter referred to as “Equation 1”) is 0.055% or more and 0.085% or less when the contents of magnesium, lanthanum, cerium, and calcium expressed in mass percentage are Mg, La, Ce, and Ca, respectively.

$\left[\mathrm{Equation}1\right]$ $\mathrm{DS}=\frac{\mathrm{Mg}}{0.76}+\frac{\left(\mathrm{La}+\mathrm{Ce}\right)}{2.93}+\frac{\mathrm{Ca}}{1.25}$

As mentioned above, magnesium, lanthanum, cerium, and calcium are all elements contained in the spheroidizing treatment agent. These elements have a strong affinity for oxygen and sulfur contained in molten metal, and when sulfides are formed, they act to promote the crystallization of spheroidal graphite by reducing the concentration of elemental sulfur melted in the molten metal. The respective terms on the right side of Equation 1 represent the respective amounts of sulfur consumed, assuming that all of the magnesium, lanthanum, cerium, and calcium were used to form magnesium sulfide (MgS), rare earth sulfides (RE₂S₃, where RE is a rare earth element), and calcium sulfide (CaS), respectively, based on the stoichiometric compositions of the respective sulfides. A coefficient of 2.93 for the sum of lanthanum and cerium is a value calculated based on the atomic weight of cerium. Since the atomic weights of lanthanum and cerium do not differ significantly, an error in the coefficient at the second term can be ignored even when mischmetal is used as a rare earth source.

By adding up the values on the right side of Equation 1, the maximum value of sulfur that can be removed from molten metal by being combined with magnesium, lanthanum, cerium, and calcium contained in the spheroidizing treatment agent to form compounds is determined. Therefore, this value is defined as a desulfurization capacity factor DS (desulfurization). The unit of DS is %. When the DS is 0.055% or more, the elemental sulfur contained in the molten metal is detoxified in the form of a sulfide, which promotes spheroidization of graphite. When the DS is 0.085% or less, the temperature of the molten metal can be prevented from decreasing due to the addition of an excessive amount of the spheroidizing treatment agent. Therefore, in a preferred embodiment of the present disclosure, the desulfurization capacity factor DS is 0.055% or more and 0.085% or less.

In a preferred embodiment, in the spheroidal graphite cast iron according to the present disclosure, an excess magnesium content RM expressed by the following equation (hereinafter referred to as “Equation 2”) is 0.015% or more and 0.045% or less when the sulfur content expressed in mass percentage is S.

$\left[\mathrm{Equation}2\right]$ $\mathrm{RM}=\mathrm{Mg}-\left[S-\left\{\frac{\left(\mathrm{La}+\mathrm{Ce}\right)}{2.93}+\frac{\mathrm{Ca}}{1.25}\right\}\right]\times 0.76$

Magnesium, lanthanum, cerium, and calcium in the spheroidizing treatment agent all act as a deoxidizer and desulfurizer, but of these elements, the affinity of magnesium for oxygen and sulfur is not necessarily greatest. However, it is known empirically that the graphite spheroidizing capacity of magnesium is the greatest among these elements. Therefore, the spheroidization of graphite may be hindered if an elemental sulfur that does not form a sulfide is still melted and present in the molten metal obtained after all magnesium has been consumed by the formation of magnesium sulfide.

The two terms inside the medium brackets of Equation 2 represent the amounts of sulfur consumed by the formation of sulfides with lanthanum, cerium, and calcium, as mentioned above. The portion inside large brackets of Equation 2 represents the amount of sulfur remaining in the molten metal that is not consumed by lanthanum, cerium, and calcium. A value obtained by multiplying this by 0.76 represents a magnesium equivalent when all the remaining sulfur is consumed to form magnesium sulfide. The right side of Equation 2 is the value obtained by subtracting the magnesium equivalent from the actual magnesium content represented by the symbol Mg, and represents the amount of excess magnesium that remains in the molten metal without forming sulfides. Therefore, this value is defined as an excess magnesium amount RM (residual magnesium). The unit of RM is %. When RM is 0.015% or more, an elemental sulfur contained in the molten metal is removed or detoxified by the excess magnesium to form sulfides, which promotes spheroidization of graphite. When RM is 0.045% or less, the temperature of the molten metal can be prevented from being lowered due to the addition of an excess spheroidizing treatment agent. Therefore, in the more preferred embodiment of the present invention, the excess magnesium amount RM is 0.030% or more and 0.040% or less.

In a preferred embodiment, the spheroidal graphite cast iron according to the present disclosure contains 0.0020% or more and 0.0050% or less of aluminum in mass percentage. Aluminum combines with oxygen dissolved in molten metal to deoxidize the molten metal. Therefore, a small amount of aluminum acts to lower the surface tension at the interface between the graphite and molten metal, thereby making the graphite spherical in shape. When the aluminum content is 0.0020% or more, the crystallization of spheroidal graphite is promoted when casting is performed to manufacture a spheroidal graphite cast iron. When the aluminum content is 0.0050% or less, this can prevent the formation and spheroidization of graphite from being inhibited due to the addition of excess aluminum. Therefore, in a preferred embodiment of the present disclosure, the aluminum content is 0.0020% or more and 0.0050% or less.

<Method for Manufacturing Spheroidal Graphite Cast Iron>

In a second embodiment, the present disclosure is an invention of a method for manufacturing a spheroidal graphite cast iron. The kinds and composition ranges of the respective elements in the spheroidal graphite cast iron produced by the method for manufacturing a spheroidal graphite cast iron according to the present disclosure are the same as the kinds and composition ranges of the elements of the spheroidal graphite cast iron according to the present disclosure in the first embodiment. Therefore, regarding a description about the composition of spheroidal graphite cast iron obtained by this manufacturing method, parts of the description overlapping with the first embodiment will be omitted here, and a description will be given while focusing on each process included in the method for manufacturing the spheroidal graphite cast iron.

FIG. 1 is a flowchart illustrating the method for manufacturing a spheroidal graphite cast iron according to the present disclosure. The method for manufacturing a spheroidal graphite cast iron according to the present disclosure includes three processes, namely, steps 1 to 3, indicated by solid lines in FIG. 1. The method for manufacturing a spheroidal graphite cast iron according to the present disclosure includes a process of preparing molten metal by melting raw materials (Step 1 in FIG. 1). The raw materials usable for melting include pig iron produced in a blast furnace, as well as a mixture of the iron and other known raw materials such as scrap generated in foundries or scrap iron recovered from the market. For melting the raw materials, the known means, such as a continuous melting furnace, typified by a cupola, or a batch melting furnace, typified by an electric furnace, can be used. In the continuous melting furnace, it is generally more difficult to change the composition of molten metal than in the batch melting furnace. According to the present disclosure, spheroidal graphite cast iron or black heart malleable cast iron can be separately made from the same molten metal, so that the effect of the present disclosure can be demonstrated more effectively in the continuous melting furnace than in the batch melting furnace. However, the means of melting raw materials in the present disclosure is not limited to a continuous melting furnace.

In the method for manufacturing a 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 spheroidal graphite cast iron as the final product. However, the composition of the molten metal poured from the melting furnace needs to be adjusted in advance by taking into account variations in the composition caused by additives added later, such as a spheroidizing treatment agent or an inoculant added as necessary. In the manufacturing of molten metal, additives that are solely intended to adjust the component composition of the molten metal may be added to the molten metal that has been taken from the melting furnace into the ladle, apart from the spheroidizing treatment agent and inoculant. For example, the spheroidal graphite cast iron according to the present disclosure is a cast iron that contains more silicon than black heart malleable cast iron. In the manufacturing of this spheroidal graphite cast iron, when molten metal that can be shared with black heart malleable cast iron (for example, molten metal obtained during melting in Step 1 of FIG. 1) is poured into the ladle, an in-ladle composition adjusting agent is added to the molten metal. For example, ferrosilicon may be pre-installed in the ladle as the in-ladle component adjusting agent and melted in the molten metal to adjust the silicon content.

For example, the composition of the molten metal prepared for the spheroidal graphite cast iron by the above method includes 3.1% of carbon, 2.0% of silicon, 0.30% of manganese, 0.035% of phosphorus, and 0.10% of sulfur, with the balance being iron and inevitable impurities. The molten metal given as the example is produced by being melted in an acid cupola, and thus it contains a large amount of sulfur derived from coke. The silicon content is adjusted to be less than the silicon content in the spheroidal graphite cast iron as the final product by taking into account variations caused by a spheroidizing treatment agent further added later and an inoculant added as needed. The composition regarding silicon can be adjusted in the ladle immediately before casting in this way.

On the other hand, as mentioned above, with regard to carbon, the preferred carbon content differs greatly between spheroidal graphite cast iron and black heart malleable cast iron. Thus, it is not realistic to afterwards increase the carbon content in the molten metal produced for black heart malleable cast iron. Specifically, even if a large amount of ferro carbon or the like is added to the molten metal in the ladle for the purpose of increasing the carbon content, the added carbon cannot be dissolved in the molten metal, and thus the object cannot be achieved. Therefore, it is preferable that the carbon content in the molten metal is adjusted in advance to 2.8% or more and 3.3% or less, which is the range of carbon content in the spheroidal graphite cast iron according to the present disclosure, at the stage of producing the molten metal in the melting furnace. However, fine adjustment of the carbon content by adding a small amount of ferro carbon or the like into the ladle is permissible in the present embodiment. The same applies to elements other than carbon and silicon.

The method for manufacturing a spheroidal graphite cast iron according to the present disclosure includes a process of adding the spheroidizing treatment agent to the molten metal. This process is sometimes called “spheroidizing treatment” (Step 2 in FIG. 1). The term “spheroidizing treatment agent” in the present disclosure refers to an agent that has the effect of promoting spheroidization of graphite in the spheroidal graphite cast iron by being added to and melted in the molten metal. One of the actions of the spheroidizing treatment agent is thought to be the formation of sulfides by reacting with sulfur which may inhibit spheroidization of graphite. The addition of the spheroidizing treatment agent to the molten metal is performed before pouring the molten metal into the mold. Known methods such as the above-mentioned pouring method, sandwich method, and wire method can be adopted as the method of addition. The spheroidizing treatment agent is used only in the method for manufacturing a spheroidal graphite cast iron. As mentioned later, the spheroidizing treatment agent is not added to the molten metal when black heart malleable cast iron is produced using the molten metal obtained by the melting in Step 1 of FIG. 1.

In the specific embodiment of the spheroidizing treatment agent used in the present disclosure, in addition to the above-mentioned magnesium, lanthanum, cerium, and calcium, silicon can be included as an element that promotes the crystallization of graphite. The spheroidizing treatment agent containing these five kinds of elements may be produced by mixing the respective elements individually or by producing one or more alloys containing iron and then mixing these alloys. Preferred composition ranges of the elements contained in the spheroidizing treatment agent will be mentioned later. The size of the spheroidizing treatment agent can be selected as appropriate according to the addition method. For example, when the sandwich method is used as the addition method, the spheroidizing treatment agent is preferably a relatively large lump, whereas when the wire method is used as the addition method, the spheroidizing treatment agent is preferably added in the form of finely ground powder particles that can be easily melted when added to the molten metal.

In the process of adding the spheroidizing treatment agent to the molten metal, the molten metal is agitated, causing the molten metal and the spheroidizing treatment agent to react with each other violently. Thus, carbon contained in the molten metal combines with oxygen in the atmosphere and is discharged as gas, which may promote decarburization of the molten metal. A decrease in the carbon content in the molten metal due to decarburization may reach appropriately 0.1%. In such cases, it is preferable to adjust the carbon content in the molten metal in advance so that the carbon content in the spheroidal graphite cast iron as the final product is within the range specified by the present disclosure in anticipation of decarburization in the spheroidizing treatment.

The method for manufacturing a spheroidal graphite cast iron according to the present disclosure includes a process of performing casting by pouring the molten metal to which the spheroidizing treatment agent is added, into a mold to manufacture a spheroidal graphite cast iron (Step 3 in FIG. 1). Pouring the molten metal into the mold may be performed using a ladle that is used to perform spheroidizing treatment, or may be performed after the molten metal is transferred from the ladle to another container for further pouring (e.g., a ladle for pouring). As the mold used for casting, known molds such as sand molds or metal molds can be used. The molten metal filled in a cavity of the mold is cooled there and solidified while spherical graphite is being crystallized during a cooling process. As a result, spheroidal graphite cast iron having the same shape as that of the mold is completed.

In a preferred embodiment, the method for manufacturing a spheroidal graphite cast iron according to the present disclosure may include a process of performing 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 manufacturing a spheroidal graphite cast iron according to the present disclosure, melting, spheroidizing treatment and casting (Steps 1 to 3) are essential processs, but the heat treatment (Step 4) is not an essential process. The heat treatment includes, but is not limited to, one performed for the purpose of removing stress to guarantee predetermined load performance and high dimensional accuracy, and one performed for the purpose of decomposing cementite and pearlite into ferrite and graphite to enhance the mechanical strength. Various conditions including temperature, treatment time, and atmosphere in the heat treatment can be selected as appropriate in accordance with the known methods. For example, heat treatment (annealing) can be performed in two stages. The first stage of annealing can be preferably performed at a temperature range of 850° C. or higher and 1,000° C. or lower for a holding time of 30 minutes or longer and 3 hours or shorter. The first stage of annealing can be performed to further decompose the remaining cementite into austenite and graphite. The temperature is preferably 850° C. or higher because the decomposition of cementite easily progresses in a short time. It is preferably 1,000° C. or lower because decarburization and distortion are less likely to occur. The more preferred temperature range is 900° C. or higher and 980° C. or lower. For example, after the first stage of annealing, a second stage of annealing is performed. By performing the second stage of annealing, the remaining austenite can be further separated into ferrite and graphite, allowing more graphite to precipitate. The conditions for the second stage of annealing vary depend on whether the matrix of the spheroidal graphite cast iron is to be transformed into ferrite or pearlite. When the matrix is to be transformed into ferrite, it is cooled slowly to a temperature slightly lower than the temperature of the Al transformation point (723° C.). On the other hand, when the matrix is to be transformed into pearlite, it is cooled to a temperature higher than the temperature of the Al transformation point, followed by furnace cooling or air cooling.

The spheroidal graphite cast iron obtained by performing the method for manufacturing a spheroidal graphite cast iron according to the present disclosure has the same composition as the spheroidal graphite cast iron according to the first embodiment mentioned above. That is, the spheroidal graphite cast iron has a composition comprising, in mass percentage, 2.8% or more and 3.3% or less of carbon, 2.5% or more and 4.0% or less of silicon, 0.32% or more and 0.40% or less of manganese, 0.020% or more and 0.030% or less of phosphorus, 0.020% or more and 0.035% or less of sulfur, 0.030% or more and 0.050% or less of magnesium, 0.010% or more and 0.050% or less in total of lanthanum and cerium, 0.0020% or more and 0.0050% or less of calcium, with the balance being iron and inevitable impurities.

Using the same molten metal as that used in the manufacturing of the spheroidal graphite cast iron according to the present disclosure, black heart malleable cast iron can be produced by the processes indicated by the dashed lines in FIG. 1. That is, the molten metal produced in Step 1 of FIG. 1 is cast in a mold, and the resulting cast metal is subjected to heat treatment, called graphitization treatment, whereby black heart malleable cast iron can be produced. However, when producing black heart malleable cast iron, the adjustment of the silicon content by the addition of ferrosilicon to the molten metal as well as the addition of the spheroidizing treatment are not performed. Thus, the manufacturing method according to the present disclosure involves selectively selecting the process of producing the spheroidal graphite cast iron indicated by the solid lines in FIG. 1 or the process of producing the black heart malleable cast iron indicated by the broken lines in FIG. 1 for the same molten metal. In the case of producing spheroidal graphite, by adjusting the composition of the molten metal to the minimum necessary, the spheroidal graphite cast iron can be made separately from the black heart malleable cast iron. Thus, two kinds of cast iron materials can be easily made separately from each other in response to market demand without stopping the operation of a melting furnace even when molten metal is produced using a continuous melting furnace such as a cupola, for example. This is economical because the molten metal and thermal energy required for melting are not wasted.

In a preferred embodiment, in the method for manufacturing a spheroidal graphite cast iron according to the present disclosure, a steel wire is filled with the spheroidizing treatment agent, and the spheroidizing treatment agent filled in the wire is immersed in the molten metal within a sealed space in the process of adding the spheroidizing treatment agent. This embodiment corresponds to the above-mentioned wire method. In the above-mentioned pouring method and sandwich method, if the spheroidizing treatment agent is added later to the molten metal remaining in the ladle, an explosive reaction may occur, which is dangerous. Thus, it is necessary to set the spheroidizing treatment agent in advance in an amount that is appropriate for the amount of the molten metal poured into an empty ladle, and then to pour the molten metal into the ladle. In contrast, in the preferred method for manufacturing a spheroidal graphite cast iron according to the present disclosure, the spheroidizing treatment agent filled in the iron wire can be added later little by little to the molten metal poured into the ladle. Thus, there is little possibility of an explosive reaction occurring. In addition, since the weight of the molten metal that has been poured to the ladle can be measured and the spheroidizing treatment agent can be added to the molten metal in accordance with this weight, the amount of the spheroidizing treatment agent added can be adjusted to an appropriate amount even when the amount of the molten metal poured from the melting furnace has changed. The process of adding the spheroidizing treatment agent inside the sealed space can be performed, for example, by placing a lid on the ladle, and conveying the wire from a hole formed in the lid to the inside of the ladle. Thus, it is possible to operate the processes safely even in the event of a sudden boiling of the molten metal. Vapor of magnesium and calcium stored in a space between the molten metal surface and the ladle lid without reacting with the molten metal can be forcibly exhausted to the outside by exhaust means.

The iron wire can be composed, for example, of a hollow tube with a wall thickness of 0.35 mm and a diameter of approximately 13 mm. The spheroidizing treatment agent can be formed by filling the interior of this tube with powder particles of the spheroidizing treatment agent that have been melted and pulverized in advance. When the spheroidizing treatment agent thus composed is immersed from the surface to the interior of the molten metal, it takes a little time for the iron wire to melt in the molten metal, so that the position where the wire completely melts to release the spheroidizing treatment agent into the molten metal is lower than the molten metal surface. In such a case, the reaction between the spheroidizing treatment agent and the molten metal occurs inside the molten metal, whereby a higher molten metal pressure is applied than that when the reaction occurs near the molten metal surface, leading to an improvement in the yield of the spheroidizing treatment agent. The amount of the spheroidizing treatment agent added can be adjusted by the length of the wire immersed in the molten metal. The amount of the spheroidizing treatment agent added can be determined as appropriate, for example, within a range of 1.0 to 2.0 kg per 100 kg of the molten metal (excluding the mass of the iron skin), depending on the component composition of the molten metal before addition, especially the sulfur content, the component composition of the spheroidizing treatment agent, or the like.

In a preferred embodiment, the method for manufacturing a spheroidal graphite cast iron according to the present disclosure includes a process of adding an inoculant to the molten metal to which the spheroidizing treatment agent is added. In the present specification, “inoculation” in the manufacturing of the spheroidal graphite cast iron refers to the addition of the following inoculant to the molten metal, mainly for the purpose of mainly acting on graphitization and preventing the formation of white cast iron (white pig iron). In the present specification, the “inoculant” is one kind of additive added for the purpose of promoting the crystallization of graphite in the spheroidal graphite cast iron or adjusting the shape and number of particles of spheroidal graphite. A small amount of the above inoculant exhibits a significantly greater action and effect of the above inoculation than a mere alloying element. Although the details of the action of the inoculant in the manufacturing of the spheroidal graphite cast iron are unknown, it is thought that the inoculant promotes nucleation which triggers the crystallization of spheroidal graphite, and does not act on specific elements in the molten metal, unlike a spheroidizing treatment agent. The inoculant suitable for use can be, for example, ferrosilicon or an alloy of ferrosilicon with one or more of calcium, aluminum, barium, potassium, bismuth, and zirconium. The inoculant differs from the spheroidizing treatment agent in that it does not contain magnesium. From the viewpoint of sufficiently exhibiting the action and effect of the above inoculation, the time when the inoculant is added is preferably a time after the addition of the spheroidizing treatment agent and immediately before casting, for example, within one minute from the time of injection into the mold. Embodiments suitable for use in the addition of the inoculant include, for example, (1) a method of adding it to the molten metal in the ladle, specifically, in which when preparing a ladle for spheroidizing treatment and a ladle for pouring into the mold, the inoculant is placed in advance in the ladle for pouring and the molten metal is poured from the ladle for spheroidizing treatment into the ladle for pouring, (2) a method of adding an inoculant, for example in powder form, to the molten metal so that it comes in contact with the molten metal when pouring the molten metal from the ladle for pouring into the mold, and (3) a method in which the inoculant is placed in advance in a mold channel of the mold or the like, and added to the molten metal flowing through the mold channel. The amount of the inoculant added only needs to be an amount that can achieve the above object. For example, it can be approximately 300 g per 100 kg of the molten metal, i.e., preferably in the range of 0.10% or more and 0.50% or less in mass percentage, for example.

<Spheroidizing Treatment Agent>

In a third embodiment, the present disclosure is an invention of a spheroidizing treatment agent. The spheroidizing treatment agent according to the present disclosure contains, in mass percentage, 45% or more and 47% or less of silicon, 14% or more and 16% or less of magnesium, 4.5% or more and 8.0% or less in total of lanthanum and cerium, and 4.5% or more and 10% or less of calcium, with the balance being iron and inevitable impurities. The spheroidizing treatment agent according to the present disclosure is suitable for use in the method for manufacturing a spheroidal graphite cast iron according to the second embodiment. As mentioned above, silicon is an element that promotes the crystallization of graphite, whereas magnesium, lanthanum, cerium, and calcium are elements that have the action of promoting the formation and spheroidization of graphite in the spheroidal graphite cast iron.

Silicon contained in the spheroidizing treatment agent according to the present disclosure not only promotes the crystallization of graphite when melted in the molten metal, but also forms an alloy that has a low melting point and is easy to pulverize, together with iron and other elements, thereby facilitating the manufacturing of the spheroidizing treatment agent. When the silicon content is 45% or more, the crystallization of graphite in the spheroidal graphite cast iron is promoted, which facilitates the manufacturing of the spheroidizing treatment agent. When the silicon content is 47% or less, an excessive reaction with the molten metal and a decrease in the temperature of the molten metal are suppressed. Therefore, the spheroidizing treatment agent according to the present disclosure contains 45% or more and 47% or less of silicon.

The magnesium content in the spheroidizing treatment agent according to the present disclosure is approximately two to three times the magnesium content in a conventional spheroidizing treatment agent mentioned in, for example, Patent Document 1. The total content of lanthanum and cerium and the calcium content are also slightly greater than those in the conventional spheroidizing treatment agent. It is thought that with such compositional features, the spheroidizing treatment agent according to the present disclosure forms sulfides to consume sulfur even when the molten metal contains a large amount of sulfur, thereby promoting the crystallization of spheroidal graphite, whereby this spheroidizing treatment agent can omit the desulfurization process of the molten metal, which is performed in the manufacturing of a conventional spheroidal graphite cast iron. Therefore, the spheroidizing treatment agent according to the present disclosure is said to be suitable for use in the method for manufacturing a spheroidal graphite cast iron according to the second embodiment, which does not include desulfurization treatment of the molten metal.

In a preferred embodiment, the spheroidizing treatment agent according to the present disclosure contains 0.30% or more and 0.80% or less of aluminum in mass percentage. As mentioned above, aluminum combines with oxygen dissolved in the molten metal to deoxidize the molten metal. Therefore, a small amount of aluminum acts to lower the surface tension at the interface between the graphite and molten metal, thereby making the graphite spherical in shape. When the aluminum content in the spheroidizing treatment agent is 0.30% or more, the crystallization of spheroidal graphite is promoted when casting is performed to manufacture a spheroidal graphite cast iron. When the aluminum content in the spheroidizing treatment agent is 0.80% or less, the formation and spheroidization of graphite can be prevented from being inhibited due to the addition of excess aluminum. Therefore, in a preferred embodiment of the present disclosure, the spheroidizing treatment agent contains 0.30% or more and 0.80% or less of aluminum.

In a preferred embodiment, the spheroidizing treatment agent according to the present disclosure is filled in an iron wire. By using the wire filled with the spheroidizing treatment agent, unlike the above-mentioned pouring method and sandwich method, the spheroidizing treatment agent can be added later to the molten metal that has been poured into the ladle, whereby the amount of the spheroidizing treatment agent added can be adjusted to an appropriate amount even when the amount of the molten metal poured from the melting furnace changes. As mentioned above, the iron wire can be composed, for example, of a hollow tube with a wall thickness of 0.35 mm and a diameter of approximately 13 mm. The spheroidizing treatment agent can be formed by filling the interior of this tube with powder particles of the spheroidizing treatment agent that have been melted and pulverized in advance. The amount of the spheroidizing treatment agent added can be adjusted by the length of the wire immersed in the molten metal.

EXAMPLES

First Example

(Preparation of Spheroidizing Treatment Agent)

Ferrosilicon (an alloy of iron and silicon), calcium silicide, a rare earth silicide, and magnesium were blended and melted to fabricate several mother alloys for spheroidizing treatment agents having different compositions. The resulting mother alloys were pulverized to fabricate powders, and these powders were blended and mixed to have the compositions shown in Table 1 to manufacture six different kinds of spheroidizing treatment agents with different compositions. The powders were blended such that the contents of the rare earth elements and calcium, among the elements contained in the spheroidizing treatment agents, increase in order from the top row to the bottom row of Table 1. The six resulting powders were individually filled into iron wires, each having a wall thickness of 0.35 mm and a diameter of 13 mm. The weights of the spheroidizing treatment agents filled in the wires varied depending on the composition, but ranged from approximately 260 g to 300 g per meter of the wire.

TABLE 1
	Si	Mg	RE	Ca	Al
	(%)	(%)	(%)	(%)	(%)	Valance
Comparative	43.8	16.0	4.14	3.23	0.65	Iron and
Example 1						inevitable
Example 1	45.6	15.0	4.96	4.96	0.33	impurities
Example 2	46.0	14.7	7.04	5.00	0.79
Example 3	46.2	15.2	7.13	9.91	0.52
Comparative	44.8	14.6	11.7	9.62	0.49
Example 2
Comparative	44.4	15.9	14.0	12.4	0.72
Example 3
Note)
RE represents the sum of the contents of La and Ce.

(Manufacturing of Spheroidal Graphite Cast Iron)

Raw materials and coke were alternately introduced into and stacked in the furnace from above an acid cupola, which was composed of acid refractory used as a furnace material, and hot air was blown into the furnace to burn the coke to continuously melt the raw materials. The molten metal obtained from the melting was poured into the ladle at certain intervals. The weight of the molten metal poured into the ladle at one time was approximately 700 kg. The temperature of the molten metal at the time of pouring was approximately 1,500° C. In the ladle, 5.5 kg of ferrosilicon containing 75% of silicon was placed as the in-ladle composition adjusting agent in advance and melted in the molten metal (approximately 700 kg). The results of analysis of the composition of the molten metal in the ladle are shown in Table 2. The molten metal contained 0.10% of sulfur derived from coke. In addition, as a result of composition adjustment using ferrosilicon, the molten metal contained 2.0% of silicon.

TABLE 2
C (%)	Si (%)	Mn (%)	P (%)	S (%)	Valance
3.1	2.0	0.30	0.035	0.10	Iron and inevitable
					impurities

(Addition of Spheroidizing Treatment Agent)

Next, a lid was placed on top of the ladle containing the molten metal to create a sealed space. A wire filled with the spheroidizing treatment agent was inserted little by little through a hole provided in the lid and immersed in the molten metal to add and mix the spheroidizing treatment agent into the molten metal. The length of the wire inserted into the molten metal was approximately 30 to 40 meters, and the amount of the spheroidizing treatment agent with respect to the molten metal was as shown in Table 3 below. While the spheroidizing treatment agent was added and mixed into the molten metal, vapor of magnesium and calcium generated in the ladle was forcibly discharged outward through an exhaust port provided in the lid. The spheroidizing treatment was performed using one kind of spheroidizing treatment agent per ladle, whereby a total of six kinds of molten metals were fabricated.

	TABLE 3
				Weight of
				spheroidizing
	Weight of		Weight of	treatment agent
	molten	Wire	spheroidizing	per 100 kg of
	metal	length	treatment agent*	molten metal
	(kg)	(m)	(kg)	(kg)
Comparative	755	31.8	9.45	1.25
Example 1
Example 1	763	34.4	9.37	1.23
Example 2	703	33.8	9.17	1.30
Example 3	710	38.3	9.89	1.39
Comparative	708	38.2	10.31	1.46
Example 2
Comparative	692	39.4	9.76	1.41
Example 3
*The weight of the spheroidizing treatment agent indicates the total weight of only the spheroidizing treatment agent excluding an iron skin (hoop material) of the wire.

Next, for each of the six kinds of molten metals to which the respective spheroidizing treatment agents were added, the molten metal was poured into a mold, thereby performing casting to fabricate a specimen of spheroidal graphite cast iron. The specimens for composition analysis were prepared by rapidly solidifying the respective molten metals using a mold of 5 mm in thickness for the purpose of preventing segregation of the components. The results of the analysis of the compositions of the resulting six specimens are shown in Table 4. The compositions of the specimens were analyzed by the photoelectric photometric emission analysis method. RE in Table 4 was obtained by determining a Ce content in the above photoelectric photometric emission analysis method, estimating a La content from the obtained Ce content and the ratio of the Ce content to the La content in the mischmetal contained in the wire (Ce:La=2:1), and summing these Ce and La contents. The desulfurization capacity factor DS and excess magnesium content RM, calculated by Equations 1 and 2 based on the values of the compositions in Table 4, are shown in Table 5.

TABLE 4
	C	Si	Mn	P	S	Mg	RE	Ca
	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	Balance
Comparative	3.07	2.59	0.348	0.023	0.037	0.033	0.0216	0.0025	Iron and
Example 1									inevitable
Example 1	3.10	2.55	0.333	0.021	0.031	0.035	0.0294	0.0027	impurities
Example 2	3.08	2.66	0.358	0.023	0.031	0.043	0.0390	0.0030
Example 3	3.12	2.73	0.363	0.028	0.025	0.049	0.0448	0.0037
Comparative	3.06	2.56	0.363	0.027	0.025	0.048	0.0619	0.0039
Example 2
Comparative	3.06	2.69	0.356	0.024	0.016	0.037	0.0738	0.0040
Example 3
Note)
RE represents the sum of the contents of La and Ce.

	TABLE 5
	Desulfurization	Excess magnesium
	capacity factor	content
	DS (%)	RM (%)
Comparative Example 1	0.053	0.012
Example 1	0.058	0.021
Example 2	0.072	0.031
Example 3	0.083	0.044
Comparative Example 2	0.087	0.047
Comparative Example 3	0.077	0.046

Samples shown in FIG. 2 for observation of the metallic microstructure before heat treatment were obtained using six kinds of molten metals to which the respective spheroidizing treatment agents were added. The resulting six samples each were cut as shown in FIG. 2, and the cut surfaces were polished. Then, the metallic microstructure of these cut surfaces before heat treatment were observed with an optical microscope. As a result of the observation, spheroidal graphite was formed and almost no cementite was observed in the samples of Examples 1 to 3. On the other hand, Comparative Example 1 using a spheroidizing treatment agent in which the contents of calcium and a rare earth element were small and Comparative Examples 2 and 3 using a spheroidizing treatment agent in which the contents of calcium and a rare earth element were large showed the tendency that the amount of cementite increased.

For observation of the metallic microstructure and evaluation of the strength after heat treatment, casting was performed by injecting the molten metal into a sand mold of the shape shown in FIG. 3 (the vertical direction being reversed when molten metal is injected) to thereby produce a test material. A part A of the test material enclosed by a thick line in a shaded portion shown in FIG. 3 was taken from the test material to obtain a tensile test rod. In detail, a test piece No. 4 of JIS Z 2241 was taken. As a sample for observation of the metallic microstructure after heat treatment, the cut surface of the above-mentioned tensile test rod was observed as shown in FIG. 4, or a specimen for observation of the metallic microstructure was obtained from a convex portion, denoted by B in FIG. 3. In the above cut surface, the interior of the specimen was observed while avoiding its surface strongly affected by heat treatment or solidification rate. It was separately confirmed that there was almost no difference in the metallic microstructure between the cut surface of the above-mentioned tensile test rod and the convex portion B in FIG. 3.

Next, heat treatment was applied to six kinds of test materials shown in FIG. 3 mentioned above, thereby producing test materials subjected to the heat treatment (annealing). The heat treatment was carrying out by performing and holding the first stage of annealing at 980° C. for 1 hour, followed by cooling from 980° C. to 760° C., and then performing the second stage of annealing through slow cooling 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 observation of the metallic microstructure after the heat treatment and tensile test rods were taken from the test materials subjected to the heat treatment. The metallic microstructure of the above sample for observation of the metallic microstructure was observed with the optical microscope, and the spheroidization ratio and the number of graphite particles were evaluated according to the standard of JIS G 5502:2001 using images taken by the optical microscope with a magnification of 100 times and showing the metallic microstructure. For the evaluation, by using QuickGrain Pad+FilePro, i.e., image processing software manufactured by Innotech Corporation, the average value of the spheroidization ratio and the average value of the number of graphite particles were obtained from images taken at five locations on the specimen. The results obtained are shown in Table 6. The results of the tensile strength and elongation measured using the tensile test rod are also shown in Table 6. Furthermore, as an example of the above optical microscope images, optical microscope images of the specimen after the heat treatment in Example 1 are shown in FIG. 5.

	TABLE 6
		Number of
	Spheroidization	graphite	Tensile
	ratio	particles	strength	Elongation
	(% )	(pieces/mm2)	(MPa)	(%)
Comparative	69.5	65	443	29.8
Example 1
Example 1	74.8	151	428	27.1
Example 2	81.7	92	426	28.1
Example 3	76.0	72	435	28.3
Comparative	72.8	48	430	27.0
Example 2
Comparative	79.9	49	438	25.5
Example 3

Next, a compositional image of the specimen of Example 1 was observed with an electron beam microanalyzer, and it was found that magnesium and cerium were enriched in the locations that are the same as those where sulfur was concentrated. These sulfides were finely dispersed and present in the matrix of the spheroidal graphite cast iron.

The above results show that in the spheroidal graphite cast irons of Examples 1 to 3, which had the compositions of the spheroidal graphite cast irons according to the present disclosure, sulfur, which was contained in an amount of 0.10% in the molten metal produced in an acid cupola, was partly removed as slag due to the action of the spheroidizing treatment agent, and the remaining sulfur was finely dispersed and present as sulfides in the matrix of the spheroidal graphite cast iron. As a result, it is considered that the sulfur contained in an amount of 0.020% or more and 0.035% or less in the spheroidal graphite cast iron was detoxified, thereby producing the microstructure of the spheroidal graphite. On the other hand, in Comparative Example 1, where a spheroidizing treatment agent with small contents of calcium and a rare earth element was used, the remaining sulfur prevented the spheroidization of the graphite. Assuming that samples with a spheroidization ratio of 70.0% or more were acceptable, Comparative Example 1 is determined to lack the spheroidization. In Comparative Examples 2 and 3, where spheroidizing treatment agents with large contents of calcium and a rare earth element were used, the sulfur content decreased, but the cementite content increased, the number of graphite particles decreased, and the elongation was reduced. Therefore, it can be seen that in the spheroidal graphite cast iron according to the present disclosure, the contents of magnesium, lanthanum, cerium, and calcium should be within appropriate ranges. It can also be seen that the spheroidizing treatment agent according to the present disclosure is suitable for use in the method for manufacturing a spheroidal graphite cast iron according to the present disclosure.

Then, only approximately 700 kg of the molten metal taken from the cupola was poured into another ladle different from the ladle used to manufacture the spheroidal graphite cast iron. The analysis result of the composition of the molten metal in the ladle are shown in Table 7. As a result of not having adjusted the content of silicon in the composition of the molten metal in the ladle using ferrosilicon, the silicon content in the molten metal was 1.4%.

TABLE 7
C (%)	Si (%)	Mn (%)	P (%)	S (%)	Balance
3.1	1.4	0.30	0.035	0.10	Iron and inevitable
					impurities

Next, the molten metal in the ladle was poured into a mold to cast white pig iron, and the resulting cast metal was subjected to graphitization treatment under predetermined conditions to manufacture a black heart malleable cast iron. The cross section of the resulting black heart malleable cast iron was polished, and its microstructure was observed with an optical microscope. FIG. 6 shows an optical micrograph of the metallic microstructure of the black heart malleable cast iron. The metallic microstructure of the black heart malleable cast iron in which lump graphite precipitated in a matrix composed of ferrite was observed. This result shows that according to the manufacturing method of the present disclosure, spheroidal graphite cast iron or black heart malleable cast iron can be selectively produced without desulfurization treatment by using the same molten metal produced in an acid cupola.

Second Example

In a second example, a spheroidal graphite cast iron specimen with a smaller carbon content than in Example 1 was prepared. In detail, a specimen of spheroidal graphite cast iron was prepared by using the spheroidizing treatment agent, which was the same as that used in Example 2 of the first example, in the same manner as in the first example except for the following. To adjust the composition in the ladle, 4.5 kg of ferrosilicon in which the silicon content was 75% was placed as the in-ladle composition adjusting agent in the ladle in advance, and melted by pouring the molten metal (approximately 700 kg). As the molten metal to be used for the spheroidizing treatment, one with a smaller carbon content than the composition shown in Table 2 was used. The amount of the spheroidizing treatment agent added to the molten metal was as shown in Table 8 below. The specimen for the composition analysis was prepared by rapidly solidifying the molten metal using a mold of 5 mm in thickness for the purpose of preventing segregation of the components. The composition of the resulting specimen was analyzed by the same method as in the first example, and the results are shown in Table 9. The desulfurization capacity factor DS and excess magnesium content RM, calculated by Equations 1 and 2 based on the values of the compositions in Table 9, are shown in Table 10.

	TABLE 8
				Weight of
				spheroidizing
	Weight of		Weight of	treatment agent
	molten	Wire	spheroidizing	per 100 kg of
	metal	length	treatment agent*	molten metal
	(kg)	(m)	(kg)	(kg)
Example 4	616	38.1	10.57	1.72
*The weight of the spheroidizing treatment agent indicates the total weight of only the spheroidizing treatment agent excluding an iron skin (hoop material) of the wire.

TABLE 9
	C	Si	Mn	P	S	Mg	RE	Ca
	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	Balance
Example 4	2.84	2.82	0.392	0.028	0.033	0.042	0.0291	0.0029	Iron and
									inevitable
									impurities
Note)
RE represents the sum of the contents of La and Ce.

	TABLE 10
	Desulfurization	Excess magnesium
	capacity factor	content
	DS (%)	RM (%)
	Example 4	0.068	0.026

Specimens for observation of the metallic microstructure were prepared as mentioned in the first example, and the metallic microstructure before heat treatment (before annealing) of the specimen was observed with an optical microscope. FIG. 7 shows an optical micrograph of the metallic microstructure of the specimen in Example 4 (before heat treatment (before annealing)). The spheroidization ratio of this specimen was 76.9%, and the number of graphite particles was 128 pieces/mm². Although the metallic microstructure shown in FIG. 7 has a large content of residual pearlite, and spherical graphite was crystallized, the spheroidization ratio and the number of graphite particles were comparable to those in the first example. From these results, it can be seen that the method for manufacturing a spheroidal graphite cast iron can be produced even at a low carbon content of 2.84% according to the manufacturing method of the present disclosure.

Third Example

A third example shows an example in which an inoculant was added in a manufacturing procedure.

(Preparation of Spheroidizing Treatment Agent)

First, in the same manner as in the first example, mother alloys were pulverized to fabricate powders, and these powders were blended and mixed to have the compositions shown in Table 11 to manufacture two different kinds of spheroidizing treatment agents with different compositions (a spheroidizing treatment agent used in Example 5 and spheroidizing treatment agents used in Examples 6 to 8). The two kinds of resulting powders were individually filled into iron wires, each having a wall thickness of 0.35 mm and a diameter of 13 mm. The weights of the spheroidizing treatment agents filled in the wires varied depending on the composition, but ranged from approximately 260 g to 300 g per meter of the wire. The spheroidizing treatment agents used in Examples 6 to 8 has a higher Ca content than the spheroidizing treatment agent used in Example 5 in order to further reduce the sulfur content in the steel.

TABLE 11
	Si	Mg	RE	Ca	Al
	(%)	(%)	(%)	(%)	(%)	Balance
Example 5	45.6	15.0	4.96	4.96	0.33	Iron and
Example 6	46.9	15.0	5.02	7.26	0.68	inevitable
Example 7	46.9	15.0	5.02	7.26	0.68	impurities
Example 8	46.9	15.0	5.02	7.26	0.68
Note)
RE represents the sum of the contents of La and Ce.

(Manufacturing of Spheroidal Graphite Cast Iron)

Raw materials and coke were alternately introduced into and stacked in the furnace from above an acid cupola, which was composed of acid refractory used as a furnace material, and hot air was blown into the furnace to burn the coke to continuously melt the raw materials. The molten metal obtained from the melting was poured into the ladle at certain intervals. To adjust the composition in the ladle, in Examples 5 and 6, 4.5 kg of ferrosilicon in which the silicon content was 75% was placed as the in-ladle composition adjusting agent in the ladle in advance, and melted by pouring the molten metal (approximately 700 kg). In Examples 7 and 8, 4.5 kg of ferrosilicon in which the silicon content was 75% and 2.0 kg of calcium silicon in which the silicon content was 59% were placed as the in-ladle composition adjusting agent in the ladle in advance, and melted by pouring the molten metal (approximately 700 kg). The weight of the molten metal poured into the ladle at one time was approximately 700 kg. The temperature of the molten metal at the time of pouring was approximately 1500° C.

(Addition of Spheroidizing Treatment Agent)

Next, a lid was placed on top of the ladle containing the molten metal to create a sealed space. A wire filled with the spheroidizing treatment agent was inserted little by little through a hole provided in the lid and immersed in the molten metal to add and mix the spheroidizing treatment agent into the molten metal. The length of the wire inserted into the molten metal was approximately 34 to 46 meters, and the amount of the spheroidizing treatment agent added to the molten metal was as shown in Table 12 below. While the spheroidizing treatment agent was added and mixed into the molten metal, vapor of the magnesium and calcium generated in the ladle was forcibly discharged outward through an exhaust port provided in the lid.

	TABLE 12
				Weight of
				spheroidizing
	Weight of		Weight of	treatment agent
	molten	Wire	spheroidizing	per 100 kg of
	metal	length	treatment agent*	molten metal
	(kg)	(m)	(kg)	(kg)
Example 5	598	36.4	9.91	1.66
Example 6	617	33.9	8.89	1.44
Example 7	739	46.4	12.17	1.65
Example 8	661	40.2	10.54	1.60
*The weight of the spheroidizing treatment agent indicates the total weight of only the spheroidizing treatment agent excluding an iron skin (hoop material) of the wire.

The component compositions in Table 13 are those of the molten metal after the adjustment of the components of the molten metal and the spheroidizing treatment, but before inoculation. The component compositions were determined by the analysis in the photoelectric photometric emission analysis method. The determination way of RE was the same as in the first example.

In Examples 7 and 8, as a result of the adjustment of the composition using ferrosilicon and calcium silicon, the molten metal contained approximately 3.2% of silicon. The component compositions shown in Table 13 below are the component compositions of molten metals after the adjustment of the components of the molten metal and the spheroidizing treatment as mentioned above, but before inoculation. However, these component compositions are thought to fall within the range of the component compositions of the spheroidal graphite cast iron of the present disclosure even after the addition of the inoculant. The desulfurization capacity factor DS and excess magnesium content RM, calculated by Equations 1 and 2 based on the values of the compositions in Table 13, are shown in Table 14.

TABLE 13
	C	Si	Mn	P	S	Mg	RE	Ca
	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	Balance
Example 5	2.95	2.70	0.369	0.031	0.034	0.043	0.0297	0.0030	Iron and
Example 6	2.90	2.68	0.340	0.029	0.032	0.046	0.0352	0.0033	inevitable
Example 7	2.95	3.19	0.397	0.026	0.025	0.047	0.0351	0.0042	impurities
Example 8	2.88	3.24	0.385	0.028	0.032	0.045	0.0357	0.0034
Note)
RE represents the sum of the contents of La and Ce.

	TABLE 14
	Desulfurization	Excess magnesium
	capacity factor	content
	DS (%)	RM (%)
	Example 5	0.069	0.027
	Example 6	0.075	0.033
	Example 7	0.077	0.040
	Example 8	0.074	0.035

(Addition of Inoculant)

After the above spheroidizing treatment agent was added to the molten metal, an inoculant was further added. As the inoculant, Examples 5 to 8 each used an alloy which had a component composition containing: 69.9% of Si, 1.5% of Ca, 1.4% of Al, and 0.3% of Ba with the balance being iron and inevitable impurities. The amount of the inoculant added to the molten metal was 0.3% in Example 5 and 0.5% in Examples 6 to 8, respectively, in mass percentage with respect to the molten metal. In each of Examples 5 to 8, the inoculant was added in the form of powder such that the inoculant was in contact with the molten metal when the molten metal was poured from the ladle into the mold.

The resulting specimen was subjected to the heat treatment in the same manner as in the first example, and a sample for observation of the metallic microstructure and a tensile test rod were taken from the specimen subjected to the heat treatment in the same manner as in the first example. The spheroidization ratio and the number of graphite particles were evaluated using the sample for observation of the metallic microstructure in the same manner as in the first example. The results obtained are shown in Table 15. In Table 15, the number of graphite particles is shown inside brackets because this data was before the heat treatment. The results of the tensile strength and elongation measured using a tensile test rod are also shown in Table 15. Furthermore, FIGS. 8 and 9 show optical micrographs of the metallic microstructures of the respective specimens of Example 7 before and after the above heat treatment. From the comparison of FIGS. 8 and 9, it can be seen that the heat treatment sufficiently decomposed cementite and pearlite to form ferrite in gray areas and graphite in black areas, as shown in FIG. 9.

	TABLE 15
		Number of
	Spheroidization	graphite	Tensile
	ratio	particles	strength	Elongation
	(%)	(pieces/mm2)	(MPa)	(%)
Example 5	88.0	(215)	458	28.8
Example 6	83.0	(207)	452	26.5
Example 7	90.8	(187)	503	25.2
Example 8	89.1	(223)	504	24.6

The obtained results show that desulfurization is promoted by using the inoculant and increasing the Ca content in the spheroidizing treatment agent, thereby increasing the number of graphite particles and further enhancing the spheroidization ratio. From the analysis results in Table 13, in Examples 5 to 8, the silicon content in the spheroidal graphite cast iron is estimated to be within a preferred range of 2.68% or more and 3.3% or less, and thus the tensile strength of 450 MPa or more which is required for spheroidal graphite cast iron FCD 450 specified in the JIS standard (JIS G 5502) is satisfied. Furthermore, in Examples 7 and 8 of Examples 5 to 8, the silicon content is estimated to be within a more preferred range of 3.0% or more and 3.3% or less of the spheroidal graphite cast iron, the tensile strength is higher, specifically, 500 MPa or more, and the elongation satisfies a value of not less than 10%, which is the standard value specified by spheroidal graphite cast iron FCD450.

The disclosed contents of the present disclosure can include the following aspects.

(Aspect 1)

A spheroidal graphite cast iron including, in mass percentage:

• • 2.8% or more and 3.3% or less of carbon;
  • 2.5% or more and 2.9% or less of silicon;
  • 0.32% or more and 0.40% or less of manganese;
  • 0.020% or more and 0.030% or less of phosphorus;
  • 0.020% or more and 0.035% or less of sulfur;
  • 0.030% or more and 0.050% or less of magnesium;
  • 0.010% or more and 0.050% or less in total of lanthanum and cerium; and
  • 0.0020% or more and 0.0040% or less of calcium, with the balance being iron and inevitable impurities.

(Aspect 2)

The spheroidal graphite cast iron according to aspect 1, wherein, when contents of magnesium, lanthanum, cerium, and calcium expressed in mass percentage are Mg, La, Ce, and Ca, respectively, a desulfurization capacity factor DS expressed by the following equation is 0.055% or more and 0.085% or less.

$\left[\mathrm{Equation}3\right]$ $\mathrm{DS}=\frac{\mathrm{Mg}}{0.76}+\frac{\left(\mathrm{La}+\mathrm{Ce}\right)}{2.93}+\frac{\mathrm{Ca}}{1.25}$

(Aspect 3)

The spheroidal graphite cast iron according to aspect 1 or 2, wherein, when a sulfur content expressed in mass percentage is S, an excess magnesium content RM expressed by the following equation is 0.008 or more and 0.031 or less.

$\left[\mathrm{Equation}4\right]$ $\mathrm{RM}=\mathrm{Mg}-\left[S-\left\{\frac{\left(\mathrm{La}+\mathrm{Ce}\right)}{2.93}+\frac{\mathrm{Ca}}{1.25}\right\}\right]\times 0.76$

(Aspect 4)

The spheroidal graphite cast iron according to any one of aspects 1 to 3, further including, in mass percentage, 0.0020% or more and 0.0050% or less of aluminum.

(Aspect 5)

A method for manufacturing a spheroidal graphite cast iron, which includes the processes of:

• • preparing a molten metal by melting raw materials;
  • adding a spheroidizing treatment agent to the molten metal; and
  • performing casting by pouring the molten metal to which the spheroidizing treatment agent is added, into a mold to produce a cast metal made of spheroidal graphite cast iron,
  • the spheroidal graphite cast iron having the composition comprising, in mass percentage:
  • 2.8% or more and 3.3% or less of carbon;
  • 2.5% or more and 2.9% or less of silicon;
  • 0.32% or more and 0.40% or less of manganese;
  • 0.020% or more and 0.030% or less of phosphorus;
  • 0.020% or more and 0.035% or less of sulfur;
  • 0.030% or more and 0.050% or less of magnesium;
  • 0.010% or more and 0.050% or less in total of lanthanum and cerium; and
  • 0.0020% or more and 0.0040% or less of calcium, with the balance being iron and inevitable impurities.

(Aspect 6)

The method for manufacturing a spheroidal graphite cast iron according to aspect 5, wherein the spheroidizing treatment agent is filled in an iron wire, and wherein the process of adding the spheroidizing treatment agent is performed by immersing the spheroidizing treatment agent filled in the wire, in the molten metal within a sealed space.

(Aspect 7)

The method for manufacturing a spheroidal graphite cast iron according to aspect 5 or 6, further including a process of adding an inoculant to the molten metal to which the spheroidizing treatment agent is added.

(Aspect 8)

A spheroidizing treatment agent including, in mass percentage;

• • 45% or more and 47% or less of silicon;
  • 14% or more and 16% or less of magnesium;
  • 4.5% or more and 8.0% or less in total of lanthanum and cerium; and
  • 4.5% or more and 10% or less of calcium, with the balance being iron and inevitable impurities.

(Aspect 9)

The spheroidizing treatment agent according to Aspect 8, further comprising, in mass percentage, 0.30% or more and 0.80% or less of aluminum.

(Aspect 10)

The spheroidizing treatment agent according to aspect 8 or 9, wherein the spheroidizing treatment agent is filled in an iron wire.

This application claims priority based on Japanese Patent Application No. 2021-050635, the disclosure of which is incorporated by reference herein.

Claims

1. A spheroidal graphite cast iron comprising, in mass percentage:

2.8% or more and 3.3% or less of carbon;

2.5% or more and 4.0% or less of silicon;

0.32% or more and 0.40% or less of manganese;

0.020% or more and 0.030% or less of phosphorus;

0.020% or more and 0.035% or less of sulfur;

0.030% or more and 0.050% or less of magnesium;

0.010% or more and 0.050% or less in total of lanthanum and cerium; and

0.0020% or more and 0.0050% or less of calcium, with the balance being iron and inevitable impurities.

2. The spheroidal graphite cast iron according to claim 1, wherein, when contents of magnesium, lanthanum, cerium, and calcium expressed in mass percentage are Mg, La, Ce, and Ca, respectively, a desulfurization capacity factor DS expressed by the following equation is 0.055% or more and 0.085% or less [ Equation ⁢ 1 ] DS = Mg 0.76 + ( La + Ce ) 2.93 + Ca 1.25.

3. The spheroidal graphite cast iron according to claim 1, wherein, when a sulfur content expressed in mass percentage is S, an excess magnesium content RM expressed by the following equation is 0.015% or more and 0.045% or less [ Equation ⁢ 2 ] RM = Mg - [ S - { ( La + Ce ) 2.93 + Ca 1.25 } ] × 0.76.

4. The spheroidal graphite cast iron according to claim 1, further comprising, in mass percentage, 0.0020% or more and 0.0050% or less of aluminum.

5. A method for manufacturing a spheroidal graphite cast iron, which comprises the processes of:

preparing a molten metal by melting raw materials;

adding a spheroidizing treatment agent to the molten metal; and

performing casting by pouring the molten metal to which the spheroidizing treatment agent is added, into a mold to produce spheroidal graphite cast iron,

the spheroidal graphite cast iron having a composition comprising, in mass percentage:

2.8% or more and 3.3% or less of carbon;

2.5% or more and 4.0% or less of silicon;

0.32% or more and 0.40% or less of manganese;

0.020% or more and 0.030% or less of phosphorus;

0.020% or more and 0.035% or less of sulfur;

0.030% or more and 0.050% or less of magnesium;

0.010% or more and 0.050% or less in total of lanthanum and cerium; and

0.0020% or more and 0.0050% or less of calcium, with the balance being iron and inevitable impurities.

6. The method for manufacturing a spheroidal graphite cast iron according to claim 5,

wherein the spheroidizing treatment agent is filled in an iron wire, and

wherein the process of adding the spheroidizing treatment agent is performed by immersing the spheroidizing treatment agent filled in the wire, in the molten metal within a sealed space.

7. The method for manufacturing a spheroidal graphite cast iron according to claim 5, which further comprises a process of adding an inoculant to the molten metal to which the spheroidizing treatment agent is added.

8. The method for manufacturing a spheroidal graphite cast iron according to claim 5, which further comprises a process of performing heat treatment on the spheroidal graphite cast iron after the process of performing the casting to produce the spheroidal graphite cast iron.

9. A spheroidizing treatment agent comprising, in mass percentage;

45% or more and 47% or less of silicon;

14% or more and 16% or less of magnesium;

4.5% or more and 8.0% or less in total of lanthanum and cerium; and

4.5% or more and 10% or less of calcium, with the balance being iron and inevitable impurities.

10. The spheroidizing treatment agent according to claim 9, further comprising, in mass percentage, 0.30% or more and 0.80% or less of aluminum.

11. The spheroidizing treatment agent according to claim 9, wherein the spheroidizing treatment agent is filled in an iron wire.

12. The spheroidal graphite cast iron according to claim 2, wherein, when a sulfur content expressed in mass percentage is S, an excess magnesium content RM expressed by the following equation is 0.015% or more and 0.045% or less [ Equation ⁢ 2 ] RM = Mg - [ S - { ( La + Ce ) 2.93 + Ca 1.25 } ] × 0.76.

13. The spheroidal graphite cast iron according to claim 2, further comprising, in mass percentage, 0.0020% or more and 0.0050% or less of aluminum.

14. The spheroidal graphite cast iron according to claim 3, further comprising, in mass percentage, 0.0020% or more and 0.0050% or less of aluminum.

15. The method for manufacturing a spheroidal graphite cast iron according to claim 6, which further comprises a process of adding an inoculant to the molten metal to which the spheroidizing treatment agent is added.

16. The method for manufacturing a spheroidal graphite cast iron according to claim 6, which further comprises a process of performing heat treatment on the spheroidal graphite cast iron after the process of performing the casting to produce the spheroidal graphite cast iron.

17. The method for manufacturing a spheroidal graphite cast iron according to claim 7, which further comprises a process of performing heat treatment on the spheroidal graphite cast iron after the process of performing the casting to produce the spheroidal graphite cast iron.

18. The spheroidizing treatment agent according to claim 10, wherein the spheroidizing treatment agent is filled in an iron wire.