JP7737754B2 - Iron castings and methods for manufacturing same - Google Patents

Description

The present invention relates to iron castings and methods for manufacturing the same.

The abstract of Patent Document 1 describes a method for easily producing a cast iron material that has good castability and workability similar to those of general cast iron materials and even better low thermal expansion characteristics. The abstract also describes a method for producing a low thermal expansion cast iron material with a thermal expansion coefficient of 4×10 −6 /°C or less in the temperature range of 50°C to 200°C by holding a cast iron composition containing 2.5 mass% or less of carbon and 25 mass% to 40 mass% of nickel obtained by casting at a temperature of 550°C to 700°C for 3 hours or more, followed by natural cooling by furnace cooling to at least 200°C, and then holding the annealed casting at a temperature of 600°C to ¹¹⁵⁰ °C for 1.5 hours or more, followed by quenching by fan air cooling, water cooling, or oil cooling.

JP 2010-95747 A

We provide iron castings with reduced thermal expansion using austenitic casting materials.

One aspect of the present invention is a method for producing an iron casting by heat treating a heat-treatment object cast using an austenitic casting material. The casting material contains 26.0% to 50.0% by mass of Ni, 0.1% to 3.5% by mass of C, 0.1% to 3.5% by mass of Si, 0% to 8.0% by mass of Co, 0% to 3.0% by mass of Mn, and 0% to 0.2% by mass of Mg, with the balance being Fe and unavoidable elements. The heat treatment includes a first holding step in which the heat-treatment object is held at a first holding temperature of 850°C to 1250°C, and a first cooling step in which, after the first holding step, the heat-treatment object is cooled to a first cooling end temperature of -150°C to 150°C. The first holding step includes holding the heat treatment object for a first holding time of 0.25 hours or more and 100 hours or less.

In this method, in the first holding step, the heat-treatment object is held at a first holding temperature (850°C or higher and 1250°C or lower) for a first holding time (0.25 hours or higher and 100 hours or lower). Then, in the first cooling step following the first holding step, the heat-treatment object is cooled to a first cooling end temperature (-150°C or higher and 150°C or lower). This first holding step reduces solidification segregation of solute elements within the heat-treatment object and reduces the relative difference in crystal orientation (crystal orientation misorientation) within each crystal grain constituting the austenite phase. This allows for a more uniform crystal lattice arrangement within the austenite phase. As a result, the linear expansion coefficient of the iron casting can be reduced.

In this method, the first cooling step preferably includes cooling the heat treatment object at a first cooling rate of 0.01°C/min or more and 300°C/min or less. The first cooling rate is preferably 0.01°C/min or more and 20°C/min or less. The first holding time is preferably 2.5 hours or more and 25 hours or less. The first cooling end temperature is preferably 0°C or more and 100°C or less.

In this method, the first cooling step preferably includes a primary cooling step in which the heat treatment object is cooled at a primary cooling rate, and a secondary cooling step in which, after the primary cooling step, the heat treatment object is cooled at a secondary cooling rate that is higher than the primary cooling rate. The primary cooling step preferably includes cooling the heat treatment object to a primary cooling end temperature of 250°C or higher and 950°C or lower, and the secondary cooling step preferably includes cooling the heat treatment object to the first cooling end temperature.

In this method, the first cooling step following the first holding step includes a primary cooling step in which the heat-treatment object is cooled at a primary cooling rate, and a secondary cooling step in which, after the primary cooling step, the heat-treatment object is cooled at a secondary cooling rate greater than the primary cooling rate. Specifically, in the primary cooling step, the heat-treatment object is cooled at the primary cooling rate to a primary cooling end temperature (250°C or higher and 950°C or lower). This primary cooling step allows carbon in the austenite phase to diffuse toward the graphite. This reduces the amount of solute carbon in the austenite phase. This suppresses excessive distortion of the crystal lattice in the austenite phase. As a result, the linear expansion coefficient of the iron casting can be further reduced. Furthermore, in the secondary cooling step following the primary cooling step, the heat-treatment object is cooled at a secondary cooling rate greater than the primary cooling rate to a first cooling end temperature (-150°C or higher and 150°C or lower). This secondary cooling step tends to increase the amount of spontaneous volume magnetostriction that occurs with temperature changes below the Curie point. Therefore, when the temperature changes below the Curie point, the volume change due to spontaneous volume magnetostriction and the volume change due to crystal lattice vibrations tend to cancel each other out, which makes it easier to suppress volume fluctuations caused by temperature changes, and as a result, it is easier to further reduce the linear expansion coefficient of the iron casting.

In this method, the primary cooling rate is preferably 0.01°C/min or more and 20°C/min or less, and the secondary cooling rate is preferably 1°C/min or more and 40,000°C/min or less. The secondary cooling rate is preferably 100°C/min or more and 40,000°C/min or less. The first holding time is preferably 2.5 hours or more and 25 hours or less. The primary cooling end temperature is preferably 450°C or more and 850°C or less. The first cooling end temperature is preferably 0°C or more and 100°C or less.

In this method, the heat treatment may further include, after the first cooling step, a second holding step in which the heat treatment object is held at a second holding temperature of 250°C or more and 950°C or less, and, after the second holding step, a second cooling step in which the heat treatment object is cooled to a second cooling end temperature of -150°C or more and 150°C or less, and the second holding step may include holding the heat treatment object for a second holding time of 0.25 hours or more and 25 hours or less.

In this method, the first cooling step preferably includes cooling the heat-treatment object at a first cooling rate of 0.01°C/min or more and 300°C/min or less. The first cooling rate is preferably 1°C/min or more and 50°C/min or less. The second cooling step preferably includes cooling the heat-treatment object at a second cooling rate of 1°C/min or more and 40,000°C/min or less. The second cooling rate is preferably 100°C/min or more and 10,000°C/min or less. The first holding time is preferably 2.5 hours or more and 25 hours or less. The first cooling end temperature is preferably 0°C or more and 100°C or less. The second holding temperature is preferably 550°C or more and 950°C or less. The second cooling end temperature is preferably 0°C or more and 50°C or less.

In this method, the Co content of the casting material is preferably 0.1% by mass or more and 8.0% by mass or less. The Mn content of the casting material is preferably 0.01% by mass or more and 3.0% by mass or less. The Mg content of the casting material is preferably 0.01% by mass or more and 0.2% by mass or less.

A method according to another aspect of the present invention is a method for producing an iron casting by heat treating a heat treatment object cast using an austenitic casting material. The heat treatment includes a first holding step in which the heat treatment object is held at a first holding temperature of 850°C or more and 1250°C or less, and a first cooling step in which, after the first holding step, the heat treatment object is cooled to a first cooling end temperature of -150°C or more and 150°C or less. The first holding step includes holding the heat treatment object for a first holding time of 0.25 hours or more and 100 hours or less.

In this method, the first cooling step preferably includes a primary cooling step in which the heat treatment object is cooled at a primary cooling rate, and a secondary cooling step in which, after the primary cooling step, the heat treatment object is cooled at a secondary cooling rate that is higher than the primary cooling rate. The primary cooling step preferably includes cooling the heat treatment object to a primary cooling end temperature of 250°C or higher and 950°C or lower, and the secondary cooling step preferably includes cooling the heat treatment object to the first cooling end temperature.

In this method, the heat treatment may further include, after the first cooling step, a second holding step in which the heat treatment object is held at a second holding temperature of 250°C or more and 950°C or less, and, after the second holding step, a second cooling step in which the heat treatment object is cooled to a second cooling end temperature of -150°C or more and 150°C or less, and the second holding step may include holding the heat treatment object for a second holding time of 0.25 hours or more and 25 hours or less.

An iron casting according to one aspect of the present invention is an iron casting manufactured using a method according to one aspect of the present invention or a method according to another aspect of the present invention.

FIG. 1 is a diagram showing an example of an image of the microstructure of a heat treatment object cast using an austenitic casting material, observed with a microscope. FIG. 2 is a diagram showing the area of the image shown in FIG. 1 that is occupied by graphite. FIG. 3 is a diagram showing a region of the image shown in FIG. 1 that is occupied by an intermetallic compound mainly composed of iron (Fe). FIG. 4 is a diagram showing the region occupied by the austenite phase in the image shown in FIG.

Embodiments of iron castings according to the present disclosure will be described below with reference to the accompanying drawings. The present invention is not limited to the following embodiments, but includes those defined in the claims. In the following description, unless otherwise specified, terms such as "first" and "second" are used merely to distinguish components from one another and do not represent a specific order or position.

An iron casting according to an embodiment of the present invention can be manufactured by subjecting a heat-treatment object cast using an austenitic casting material to a predetermined heat treatment, as described below. "Austenitic casting material" refers to a material in which the main structure of the parent phase (iron matrix structure excluding graphite) of the cast heat-treatment object at room temperature is austenite. For example, the parent phase of the heat-treatment object comprises 50% or more of austenite. The parent phase of the heat-treatment object preferably comprises 70% or more of austenite, more preferably 80% or more, even more preferably 85% or more, even more preferably 90% or more, and even more preferably 95% or more.

Figure 1 shows an example of an image, observed with a microscope, of the microstructure of a heat-treated object cast using an austenitic casting material. Figure 2 shows the region occupied by graphite in the image shown in Figure 1. Figure 3 shows the region occupied by iron (Fe)-based intermetallic compounds in the image shown in Figure 1. Figure 4 shows the region occupied by the austenite phase in the image shown in Figure 1. For example, the "area of the parent phase of the heat-treated object" can be calculated by subtracting the "area of the region occupied by graphite" shown in Figure 2 from the "area of the entire microstructure of the heat-treated object" shown in Figure 1. The "area of the austenite phase" can be calculated by subtracting the "area of the region occupied by iron (Fe)-based intermetallic compounds" shown in Figure 3 from the "area of the parent phase of the heat-treated object" calculated above. Therefore, the "proportion of the austenite phase in the matrix of the heat treatment object" can be calculated by dividing the "area of the austenite phase (see Figure 4)" calculated as above by the "area of the matrix of the heat treatment object." In this example, the number of pixels in the regions corresponding to each area shown in Figures 1 to 4 was calculated using general-purpose image processing software, and as a result, the proportion of the austenite phase in the matrix of the heat treatment object was 84.6% (approximately 85%). Note that if no region occupied by an intermetallic compound mainly composed of iron (Fe) is formed in Figure 1, the proportion of the austenite phase in the matrix of the heat treatment object would be 100%.

In this disclosure, "casting" includes casting using various casting methods, such as sand casting, permanent mold casting, die casting, and lost wax casting. Furthermore, the "mass %" of an element refers to the mass percentage of the element relative to the mass of the austenitic casting material. For example, the expression "X mass % to Y mass % of an element" means that the mass % of the element is X mass % to Y mass %. For example, the expression "0 mass % to Y mass % of an element" means that the element is not included or that the mass % of the element is Y mass % or less. The "balance" refers to components other than the listed elements that make up the austenitic casting material.

<First embodiment of casting material>
A first embodiment of an austenitic casting material (hereinafter referred to as "the material") contains 26.0 mass % to 50.0 mass % Ni, with the remainder being Fe and unavoidable elements. Hereinafter, the first embodiment of the material may be referred to as "the material (Ni-Fe composition)."

(Ni: Nickel)
A first embodiment of the material contains 26.0% by mass or more and 50.0% by mass or less of Ni. In this first embodiment of the material, by setting the Ni content to 26.0% by mass or more and 50.0% by mass or less, Ni segregates around the graphite. In other words, by concentrating Ni in the region around the graphite, austenite is stabilized. By setting the lower limit of the Ni content to 26.0% by mass, austenite can be stabilized and martensite formation can be suppressed. This can prevent a decrease in the ductility of the iron casting and improve the machinability of the iron casting. Furthermore, by setting the upper limit of the Ni content to 50.0% by mass, an increase in the linear expansion coefficient can be suppressed. The same applies to the following embodiments of the material.

(Fe: iron, an unavoidable element)
The balance in the first embodiment of this material is Fe and unavoidable elements. Examples of unavoidable elements contained in the balance include P (phosphorus), S (sulfur), Cu (copper), Al (aluminum), Cr (chromium), Mo (molybdenum), V (vanadium), Ti (titanium), and Zn (zinc). The content of unavoidable elements is, for example, preferably 10.0% by mass or less in total, more preferably 5.0% by mass or less in total, even more preferably 3.0% by mass or less in total, and even more preferably 1.0% by mass or less in total. The same applies to the following embodiments of this material.

In the first form of this material, the lower limit of the Ni content is preferably 26.5% by mass, more preferably 27.0% by mass, more preferably 27.5% by mass, more preferably 28.0% by mass, more preferably 28.5% by mass, more preferably 29.0% by mass, more preferably 29.5% by mass, more preferably 30.0% by mass, more preferably 30.5% by mass, more preferably 31.0% by mass, more preferably 31.5% by mass, and even more preferably 32.0% by mass. Furthermore, the upper limit of the Ni content is preferably 45.0% by mass, more preferably 42.0% by mass, more preferably 41.0% by mass, more preferably 40.0% by mass, more preferably 39.5% by mass, more preferably 39.0% by mass, more preferably 38.5% by mass, more preferably 38.0% by mass, more preferably 37.5% by mass, and even more preferably 37.0% by mass. The same applies to the following forms of this material.

<Second embodiment of casting material>
The second form of the material contains 26.0 mass % to 50.0 mass % Ni, 0.1 mass % to 3.5 mass % C, and the remainder being Fe and unavoidable elements. Hereinafter, the second form of the material may be referred to as "the second form of the material (Ni-C-Fe composition)."

(C: carbon)
A second embodiment of the material contains 0.1% by mass or more and 3.5% by mass or less of carbon. In this second embodiment, setting the lower limit of the carbon content to 0.1% by mass lowers the liquidus temperature of the material. This improves the flowability of the material. Setting the lower limit of the carbon content to 0.1% by mass increases the amount of graphite crystallization or precipitation. This improves the machinability of the iron casting. Setting the upper limit of the carbon content to 3.5% by mass suppresses carbon flotation. This prevents a decrease in the strength and ductility of the iron casting. Setting the upper limit of the carbon content to 3.5% by mass and the upper limit of the nickel content, which acts as a graphitization-promoting element, to 50.0% by mass suppresses excessive graphitization of carbon. This prevents the formation of chunky graphite. This improves the elongation of the iron casting. The same applies to the following embodiments of the material.

In the second form of this material, the lower limit of the C content is preferably 0.15% by mass, more preferably 0.2% by mass, more preferably 0.4% by mass, more preferably 0.7% by mass, more preferably 1.0% by mass, more preferably 1.25% by mass, more preferably 1.5% by mass, and even more preferably 1.75% by mass. By setting the lower limit of the C content to 0.7% by mass, the tendency of graphite crystallizing during solidification to form a eutectic structure is increased, thereby increasing the amount of expansion of graphite and suppressing the occurrence of shrinkage cavities. The upper limit of the C content is preferably 3.3% by mass, more preferably 3.1% by mass, more preferably 3.0% by mass, more preferably 2.95% by mass, more preferably 2.9% by mass, more preferably 2.85% by mass, more preferably 2.8% by mass, more preferably 2.75% by mass, more preferably 2.7% by mass, more preferably 2.65% by mass, more preferably 2.6% by mass, more preferably 2.55% by mass, and even more preferably 2.5% by mass. The same applies to the following embodiments of the material.

<Third embodiment of casting material>
The third form of this material contains 26.0% by mass to 50.0% by mass of Ni, 0.1% by mass to 3.5% by mass of C, and 0.1% by mass to 3.5% by mass of Si, with the remainder being Fe and unavoidable elements. Hereinafter, this third form of this material may be referred to as "the third form of this material (Ni-C-Si-Fe composition)."

(Si: silicon)
A third embodiment of the material contains 0.1% by mass or more and 3.5% by mass or less of Si. In this third embodiment, the Ni content is set to 26.0% by mass or more and 50.0% by mass or less, and the Si content is set to 0.1% by mass or more and 3.5% by mass or less, thereby causing Ni to segregate around the graphite, resulting in Si segregation in the final solidification region. In other words, Ni is concentrated in the region around the graphite to stabilize austenite, and Si is concentrated in the final solidification region, which is the residual liquid side. Setting the lower limit of the Si content to 0.1% by mass facilitates lowering the liquidus temperature of the material. This facilitates improving the fluidity of the material. Furthermore, setting the lower limit of the Si content to 0.1% by mass increases the ratio of Si content to C content. This suppresses the formation of CO gas. This reduces gas defects on the surface of iron castings. Furthermore, setting the upper limit of the Si content to 3.5% by mass reduces the amount of Si dissolved in the Fe (iron matrix). Therefore, an increase in the linear expansion coefficient can be suppressed. Furthermore, by setting the upper limit of the content of Si, which acts as a graphitization-promoting element, to 3.5 mass %, excessive graphitization of C can be suppressed. This makes it possible to suppress the generation of chunky graphite. Therefore, the elongation of the iron casting can be improved. The same applies to the following embodiments of this material.

In the third embodiment of this material, the lower limit of the Si content is preferably 0.25% by mass, more preferably 0.5% by mass, more preferably 0.75% by mass, preferably 1.0% by mass, more preferably 1.2% by mass, more preferably 1.3% by mass, and even more preferably 1.4% by mass. Furthermore, the upper limit of the Si content is preferably 3.3% by mass, more preferably 3.1% by mass, more preferably 2.9% by mass, more preferably 2.7% by mass, more preferably 2.5% by mass, more preferably 2.3% by mass, and even more preferably 2.1% by mass. The same applies to the following embodiments of this material.

<Fourth embodiment of casting material>
A fourth embodiment of the material contains 26.0% by mass to 50.0% by mass of Ni, 0.1% by mass to 3.5% by mass of C, 0.1% by mass to 3.5% by mass of Si, and 0.1% by mass to 8.0% by mass of Co, with the remainder being Fe and unavoidable elements. Hereinafter, this fourth embodiment of the material may be referred to as "fourth embodiment of the material (Ni-C-Si-Co-Fe composition)."

(Co: Cobalt)
A fourth embodiment of the material contains 0.1 mass% or more and 8.0 mass% or less of Co. In the fourth embodiment of the material, by setting the Co content to 0.1 mass% or more and 8.0 mass% or less, the linear expansion coefficient can be further reduced by a synergistic effect with Ni. By setting the lower limit of the Co content to 0.1 mass%, the minimum value of the linear expansion coefficient can be reduced by a synergistic effect with Ni. Furthermore, by setting the upper limit of the Co content to 8.0 mass%, it is possible to suppress an increase in the linear expansion coefficient after reaching a minimum value due to excessive addition of Co. The same applies to the following embodiments of the material.

In the fourth embodiment of this material, the lower limit of the Co content is preferably 0.5 mass%, more preferably 1.0 mass%, more preferably 1.5 mass%, more preferably 2.0 mass%, more preferably 2.5 mass%, more preferably 3.0 mass%, more preferably 3.5 mass%, and even more preferably 4.0 mass%. The upper limit of the Co content is preferably 7.5 mass%, more preferably 7.0 mass%, more preferably 6.5 mass%, more preferably 6.25 mass%, and even more preferably 6.0 mass%. The Ni content is preferably 31.0 mass% to 34.0 mass%, and the Co content is preferably 4.0 mass% to 5.5 mass%. When the lower limit of the Ni content is 28.5 mass%, the Co content is preferably 5.0 mass% to 8.0 mass%. By setting the upper and lower limits of the Co content as described above, the linear expansion coefficient can be further reduced due to a synergistic effect with Ni. The same applies to the following embodiments of the present material.

<Fifth embodiment of casting material>
A fifth embodiment of the material contains 26.0% by mass to 50.0% by mass of Ni, 0.1% by mass to 3.5% by mass of C, 0.1% by mass to 3.5% by mass of Si, 0.1% by mass to 8.0% by mass of Co, and 0.01% by mass to 3.0% by mass of Mn, with the remainder being Fe and unavoidable elements. Hereinafter, this fifth embodiment of the material may be referred to as "fifth embodiment of the material (Ni-C-Si-Co-Mn-Fe composition)."

(Mn: Manganese)
A fifth embodiment of the material contains 0.01% by mass or more and 3.0% by mass or less of Mn. In the fifth embodiment of the material, by setting the Mn content to 0.01% by mass or more and 3.0% by mass or less, a synergistic effect with Ni stabilizes austenite and suppresses the formation of martensite. This improves the machinability of the iron casting. Setting the lower limit of the Mn content to 0.01% by mass allows austenite to be stabilized even at room temperature. Furthermore, setting the upper limit of the Mn content to 3.0% by mass reduces the amount of Mn dissolved in the Fe (iron matrix). This suppresses an increase in the linear expansion coefficient.

In the fifth embodiment of this material, the lower limit of the Mn content is preferably 0.05% by mass, more preferably 0.07% by mass, more preferably 0.08% by mass, more preferably 0.09% by mass, and even more preferably 0.1% by mass. Furthermore, the upper limit of the Mn content is preferably 2.5% by mass, more preferably 2.0% by mass, more preferably 1.5% by mass, more preferably 1.0% by mass, more preferably 0.85% by mass, and even more preferably 0.7% by mass. The same applies to the following embodiments of this material.

<Sixth embodiment of casting material>
A sixth embodiment of the material contains 26.0% by mass to 50.0% by mass of Ni, 0.1% by mass to 3.5% by mass of C, 0.1% by mass to 3.5% by mass of Si, 0.1% by mass to 8.0% by mass of Co, and 0.01% by mass to 0.2% by mass of Mg, with the remainder being Fe and unavoidable elements. Hereinafter, this sixth embodiment of the material may be referred to as "sixth embodiment of the material (Ni-C-Si-Co-Mg-Fe composition)."

(Mg: magnesium)
A sixth embodiment of the material contains 0.01% by mass or more and 0.2% by mass or less of Mg. In the sixth embodiment of the material, by setting the Mg content to 0.01% by mass or more and 0.2% by mass or less, the spheroidizing effect of graphite can be enhanced and Mg can be segregated in the final solidification portion. By setting the lower limit of the Mg content to 0.01% by mass, the spheroidizing effect of graphite can be enhanced. Furthermore, by setting the upper limit of the Mg content to 0.2% by mass, the formation of Mg oxides or sulfides can be suppressed. Therefore, the deterioration of the flowability of the material can be suppressed. Furthermore, casting defects in iron castings can be reduced.

In the sixth embodiment of this material, the lower limit of the Mg content is preferably 0.02% by mass, more preferably 0.03% by mass, and even more preferably 0.04% by mass. The upper limit of the Mg content is preferably 0.15% by mass, more preferably 0.1% by mass, and even more preferably 0.08% by mass. The same applies to the following embodiments of this material.

<Seventh embodiment of casting material>
A seventh form of the material contains 26.0 to 50.0 mass% Ni, 0.1 to 3.5 mass% C, 0.1 to 3.5 mass% Si, 0.1 to 8.0 mass% Co, 0.01 to 3.0 mass% Mn, and 0.01 to 0.2 mass% Mg, with the remainder being Fe and unavoidable elements. Hereinafter, this seventh form of the material may be referred to as "seventh form of the material (Ni-C-Si-Co-Mn-Mg-Fe composition)."

<Eighth embodiment of casting material>
An eighth embodiment of the material contains 26.0% by mass to 50.0% by mass of Ni, 0.1% by mass to 3.5% by mass of C, 0.1% by mass to 3.5% by mass of Si, and 0.01% by mass to 3.0% by mass of Mn, with the remainder being Fe and unavoidable elements. Hereinafter, this eighth embodiment of the material may be referred to as "eighth embodiment of the material (Ni-C-Si-Mn-Fe composition)."

<Ninth embodiment of casting material>
A ninth form of the material contains 26.0 to 50.0 mass% Ni, 0.1 to 3.5 mass% C, 0.1 to 3.5 mass% Si, 0.01 to 3.0 mass% Mn, and 0.01 to 0.2 mass% Mg, with the remainder being Fe and unavoidable elements. Hereinafter, this ninth form of the material may be referred to as "ninth form of the material (Ni-C-Si-Mn-Mg-Fe composition)."

<Tenth embodiment of casting material>
A tenth embodiment of the material contains 26.0% by mass or more and 50.0% by mass or less of Ni, 0.1% by mass or more and 3.5% by mass or less of C, 0.1% by mass or more and 3.5% by mass or less of Si, 0.01% by mass or more and 0.2% by mass or less of Mg, and the balance being Fe and unavoidable elements. Hereinafter, this tenth embodiment of the material may be referred to as "tenth embodiment of the material (Ni-C-Si-Mg-Fe composition)."

<First form of heat treatment>
A first embodiment of the heat treatment (hereinafter referred to as "this heat treatment") performed on a heat-treated object cast using this material includes a first holding step in which the heat-treated object is held at a first holding temperature of 850°C or higher and 1250°C or lower, and a first cooling step in which, after the first holding step, the heat-treated object is cooled to a first cooling end temperature of -150°C or higher and 150°C or lower. In this first embodiment of the heat treatment, the first holding step includes holding the heat-treated object for a first holding time of 0.25 hours or higher and 100 hours or lower. Note that this first embodiment of the heat treatment may include other heat treatment steps before and after each of the first holding step and the first cooling step. In the first embodiment of the heat treatment, if no other heat treatment steps are included before or after each of the above steps, unexpected thermal effects on the heat-treated object can be prevented by performing only the first holding step and the first cooling step, in that order, on the heat-treated object cast using this material. Therefore, the linear expansion coefficient of the iron casting can be more reliably reduced.

In the first form of this heat treatment, in the first holding step, the heat treatment object is held at a first holding temperature (850°C or higher and 1250°C or lower) for a first holding time (0.25 hours or higher and 100 hours or lower). Then, in the first cooling step following the first holding step, the heat treatment object is cooled to a first cooling end temperature (-150°C or higher and 150°C or lower). This first holding step reduces solidification segregation of solute elements within the heat treatment object and reduces the relative difference in crystal orientation (crystal orientation misorientation) within each crystal grain constituting the austenite phase. This allows for a more uniform crystal lattice arrangement within the austenite phase. As a result, the linear expansion coefficient of the iron casting can be reduced.

In the first form of this heat treatment, the first cooling step includes cooling the heat treatment object at a first cooling rate of 0.01°C/min or more and 300°C/min or less. The first cooling rate is preferably 0.01°C/min or more and 20°C/min or less. The first holding time is preferably 2.5 hours or more and 25 hours or less. The first cooling end temperature is preferably 0°C or more and 100°C or less.

In the first embodiment of this heat treatment, the lower limit of the first holding temperature is preferably 875°C, more preferably 900°C, more preferably 925°C, more preferably 950°C, more preferably 975°C, more preferably 1000°C, and even more preferably 1025°C. Furthermore, the upper limit of the first holding temperature is preferably 1225°C, more preferably 1200°C, more preferably 1175°C, more preferably 1150°C, and even more preferably 1125°C. The same applies to the following embodiments of this heat treatment.

In the first embodiment of this heat treatment, the lower limit of the first holding time is preferably 0.5 hours, more preferably 1.0 hours, more preferably 1.5 hours, more preferably 2.0 hours, more preferably 2.5 hours, more preferably 3.0 hours, more preferably 3.5 hours, and even more preferably 4.0 hours. The upper limit of the first holding time is preferably 90 hours, more preferably 80 hours, more preferably 70 hours, more preferably 60 hours, more preferably 50 hours, more preferably 40 hours, more preferably 30 hours, more preferably 25 hours, more preferably 20 hours, more preferably 15 hours, and even more preferably 10 hours. The same applies to the following embodiments of this heat treatment.

In the first embodiment of this heat treatment, the lower limit of the first cooling end temperature is preferably -125°C, more preferably -100°C, more preferably -75°C, more preferably -50°C, more preferably -25°C, and even more preferably 0°C. Furthermore, the upper limit of the first cooling end temperature is preferably 125°C, more preferably 100°C, more preferably 75°C, and even more preferably 50°C. The same applies to the following embodiments of this heat treatment.

In the first embodiment of this heat treatment, the lower limit of the first cooling rate is preferably 0.1°C/min, more preferably 0.2°C/min, more preferably 0.3°C/min, more preferably 0.4°C/min, more preferably 0.5°C/min, more preferably 0.6°C/min, more preferably 0.7°C/min, and even more preferably 0.75°C/min. Furthermore, the upper limit of the first cooling rate is preferably 250°C/min, more preferably 200°C/min, more preferably 150°C/min, more preferably 100°C/min, more preferably 50°C/min, more preferably 25°C/min, more preferably 10°C/min, and even more preferably 5°C/min.

<Second form of heat treatment>
A second embodiment of the present heat treatment includes a first holding step in which the heat-treatment object is held at a first holding temperature of 850°C or higher and 1250°C or lower, and a first cooling step in which, after the first holding step, the heat-treatment object is cooled to a first cooling end temperature of -150°C or higher and 150°C or lower. In the second embodiment of the present heat treatment, the first holding step includes holding the heat-treatment object for a first holding time of 0.25 hours or higher and 100 hours or lower. The first cooling step includes a primary cooling step in which the heat-treatment object is cooled at a primary cooling rate, and a secondary cooling step in which, after the primary cooling step, the heat-treatment object is cooled at a secondary cooling rate that is higher than the primary cooling rate. In the first cooling step, the primary cooling step includes cooling the heat-treatment object to a primary cooling end temperature of 250°C or higher and 950°C or lower. The secondary cooling step includes cooling the heat-treatment object to the first cooling end temperature (-150°C or higher and 150°C or lower). The second embodiment of the present heat treatment may include other heat treatment steps before and after each of the first holding step and the first cooling step. In the second embodiment of the present heat treatment, if no other heat treatment steps are included before and after each of the above steps, the heat treatment object cast using the present material can be subjected to only the first holding step and the first cooling step in that order, thereby preventing unexpected thermal effects on the heat treatment object. This allows the linear expansion coefficient of the iron casting to be more reliably reduced.

In a second form of this heat treatment, in the first holding step, the heat-treated object is held at a first holding temperature (850°C or higher and 1250°C or lower) for a first holding time (0.25 hours or higher and 100 hours or lower). Then, in the first cooling step following the first holding step, the heat-treated object is cooled to a first cooling end temperature (-150°C or higher and 150°C or lower). This first holding step reduces solidification segregation of solute elements within the heat-treated object and reduces the relative difference in crystal orientation (crystal orientation misorientation) within each crystal grain constituting the austenite phase. This allows for the alignment of the crystal lattice within the austenite phase. As a result, the linear expansion coefficient of the iron casting can be reduced.

Furthermore, in a second embodiment of this heat treatment, the first cooling step following the first holding step includes a primary cooling step in which the heat-treated object is cooled at a primary cooling rate, and a secondary cooling step in which, after the primary cooling step, the heat-treated object is cooled at a secondary cooling rate greater than the primary cooling rate. Specifically, in the primary cooling step, the heat-treated object is cooled at the primary cooling rate to a primary cooling end temperature (250°C or higher and 950°C or lower). This primary cooling step allows carbon in the austenite phase to diffuse toward the graphite. This reduces the amount of solute carbon in the austenite phase. This suppresses excessive distortion of the crystal lattice in the austenite phase. As a result, the linear expansion coefficient of the iron casting can be further reduced. Furthermore, in the secondary cooling step following the primary cooling step, the heat-treated object is cooled at a secondary cooling rate greater than the primary cooling rate to a first cooling end temperature (-150°C or higher and 150°C or lower). This secondary cooling step tends to increase the amount of change in spontaneous volume magnetostriction associated with temperature changes below the Curie point. Therefore, when the temperature changes below the Curie point, the volume change due to spontaneous volume magnetostriction and the volume change due to crystal lattice vibrations tend to cancel each other out, which makes it easier to suppress volume fluctuations caused by temperature changes, and as a result, it is easier to further reduce the linear expansion coefficient of the iron casting.

In the second form of this heat treatment, the primary cooling rate is preferably 0.01°C/min or more and 20°C/min or less, and the secondary cooling rate is preferably 1°C/min or more and 40,000°C/min or less. The secondary cooling rate is preferably 100°C/min or more and 40,000°C/min or less. The first holding time is preferably 2.5 hours or more and 25 hours or less. The primary cooling end temperature is preferably 450°C or more and 850°C or less. The first cooling end temperature is preferably 0°C or more and 100°C or less.

In the second form of this heat treatment, the lower limit of the primary cooling end temperature is preferably 275°C, more preferably 300°C, more preferably 325°C, more preferably 350°C, more preferably 375°C, more preferably 400°C, more preferably 425°C, more preferably 450°C, more preferably 475°C, more preferably 500°C, more preferably 525°C, more preferably 550°C, more preferably 575°C, and even more preferably 600°C. Furthermore, the upper limit of the primary cooling end temperature is preferably 925°C, more preferably 900°C, more preferably 875°C, more preferably 850°C, more preferably 825°C, and even more preferably 800°C.

In the second form of this heat treatment, the lower limit of the primary cooling rate is preferably 0.1°C/min, more preferably 0.5°C/min, more preferably 0.75°C/min, more preferably 1.0°C/min, more preferably 1.25°C/min, more preferably 1.5°C/min, and even more preferably 1.75°C/min. Furthermore, the upper limit of the primary cooling rate is preferably 17.5°C/min, more preferably 15.0°C/min, more preferably 12.5°C/min, more preferably 10.0°C/min, and even more preferably 7.5°C/min.

In the second form of this heat treatment, the lower limit of the secondary cooling rate is preferably 2.5°C/min, more preferably 5°C/min, more preferably 7.5°C/min, more preferably 10°C/min, more preferably 50°C/min, more preferably 100°C/min, more preferably 200°C/min, more preferably 300°C/min, more preferably 400°C/min, more preferably 500°C/min, and even more preferably 600°C/min. Furthermore, the upper limit of the secondary cooling rate is preferably 37,500°C/min, more preferably 35,000°C/min, more preferably 32,500°C/min, more preferably 30,000°C/min, more preferably 27,500°C/min, and even more preferably 25,000°C/min.

<Third form of heat treatment>
A third embodiment of the present heat treatment includes a first holding step of holding the heat-treatment object at a first holding temperature of 850°C or more and 1250°C or less, a first cooling step of cooling the heat-treatment object to a first cooling end temperature of -150°C or more and 150°C or less after the first holding step, a second holding step of holding the heat-treatment object at a second holding temperature of 250°C or more and 950°C or less after the first cooling step, and a second cooling step of cooling the heat-treatment object to a second cooling end temperature of -150°C or more and 150°C or less after the second holding step. In the third embodiment of the present heat treatment, the first holding step includes holding the heat-treatment object for a first holding time of 0.25 hours or more and 100 hours or less. The second holding step includes holding the heat-treatment object for a second holding time of 0.25 hours or more and 25 hours or less. The third embodiment of the present heat treatment may include other heat treatment steps before or after each of the first holding step, first cooling step, second holding step, and second cooling step. In the third embodiment of the present heat treatment, if no other heat treatment steps are included before or after each of the above steps, the heat treatment object cast using the present material can be subjected to only the first holding step, first cooling step, second holding step, and second cooling step in that order, thereby preventing unexpected thermal effects on the heat treatment object. This allows the linear expansion coefficient of the iron casting to be more reliably reduced.

In a third form of this heat treatment, in the first holding step, the heat-treated object is held at a first holding temperature (850°C or higher and 1250°C or lower) for a first holding time (0.25 hours or higher and 100 hours or lower). Then, in the first cooling step following the first holding step, the heat-treated object is cooled to a first cooling end temperature (-150°C or higher and 150°C or lower). This first holding step reduces solidification segregation of solute elements within the heat-treated object and reduces the relative difference in crystal orientation (crystal orientation misorientation) within each crystal grain constituting the austenite phase. This allows for a more uniform crystal lattice arrangement within the austenite phase. As a result, the linear expansion coefficient of the iron casting can be reduced.

Furthermore, in a third form of this heat treatment, in a second holding step after the first cooling step, the heat treatment object is held at a second holding temperature (250°C or higher and 950°C or lower). Then, in a second cooling step after the second holding step, the heat treatment object is cooled to a second cooling end temperature (-150°C or higher and 150°C or lower). This second cooling step tends to increase the amount of change in spontaneous volume magnetostriction that accompanies temperature changes below the Curie point. Therefore, with temperature changes below the Curie point, the volume change due to spontaneous volume magnetostriction and the volume change due to crystal lattice vibrations tend to cancel each other out. Therefore, volume fluctuations associated with temperature changes are easily suppressed. As a result, the linear expansion coefficient of the iron casting is easily reduced even further.

In the third form of this heat treatment, the first cooling step preferably includes cooling the heat treatment object at a first cooling rate of 0.01°C/min or more and 300°C/min or less. The first cooling rate is preferably 1°C/min or more and 50°C/min or less. The second cooling step preferably includes cooling the heat treatment object at a second cooling rate of 1°C/min or more and 40,000°C/min or less. The second cooling rate is preferably 100°C/min or more and 10,000°C/min or less. The first holding time is preferably 2.5 hours or more and 25 hours or less. The first cooling end temperature is preferably 0°C or more and 100°C or less. The second holding temperature is preferably 550°C or more and 950°C or less. The second cooling end temperature is preferably 0°C or more and 50°C or less.

In the third form of this heat treatment, the lower limit of the first cooling rate is preferably 1.0°C/min, more preferably 5.0°C/min, more preferably 7.5°C/min, more preferably 10.0°C/min, more preferably 12.0°C/min, and even more preferably 14.0°C/min. Furthermore, the upper limit of the first cooling rate is preferably 250°C/min, more preferably 200°C/min, more preferably 150°C/min, more preferably 100°C/min, more preferably 75°C/min, more preferably 50°C/min, more preferably 45°C/min, and even more preferably 40°C/min.

In the third form of this heat treatment, the lower limit of the second holding temperature is preferably 275°C, more preferably 300°C, more preferably 325°C, more preferably 350°C, more preferably 375°C, more preferably 400°C, more preferably 425°C, more preferably 450°C, more preferably 475°C, more preferably 500°C, more preferably 525°C, more preferably 550°C, more preferably 575°C, and even more preferably 600°C. Furthermore, the upper limit of the second holding temperature is preferably 925°C, more preferably 900°C, more preferably 875°C, more preferably 850°C, more preferably 825°C, and even more preferably 800°C.

In the third form of this heat treatment, the lower limit of the second holding time is preferably 0.3 hours, more preferably 0.4 hours, more preferably 0.5 hours, more preferably 0.6 hours, more preferably 0.7 hours, more preferably 0.8 hours, more preferably 0.9 hours, and even more preferably 1.0 hours. Furthermore, the upper limit of the second holding time is preferably 20 hours, more preferably 15 hours, more preferably 10 hours, more preferably 9 hours, more preferably 8 hours, more preferably 7 hours, more preferably 6 hours, and even more preferably 5 hours.

In the third form of this heat treatment, the lower limit of the second cooling end temperature is preferably -125°C, more preferably -100°C, more preferably -75°C, more preferably -50°C, more preferably -25°C, and even more preferably 0°C. Furthermore, the upper limit of the second cooling end temperature is preferably 125°C, more preferably 100°C, more preferably 75°C, more preferably 50°C, more preferably 40°C, and even more preferably 30°C.

In the third form of this heat treatment, the lower limit of the second cooling rate is preferably 25°C/min, more preferably 50°C/min, more preferably 75°C/min, more preferably 100°C/min, more preferably 200°C/min, more preferably 300°C/min, and even more preferably 400°C/min. The upper limit of the second cooling rate is preferably 35,000°C/min, more preferably 30,000°C/min, more preferably 25,000°C/min, more preferably 20,000°C/min, more preferably 15,000°C/min, more preferably 10,000°C/min, more preferably 9,000°C/min, more preferably 8,000°C/min, more preferably 7,000°C/min, and even more preferably 6,000°C/min.

First Embodiment
The iron casting of the first embodiment is an iron casting obtained by subjecting a heat treatment object cast using the fifth form (Ni-C-Si-Co-Mn-Fe composition), the seventh form (Ni-C-Si-Co-Mn-Mg-Fe composition), the eighth form (Ni-C-Si-Mn-Fe composition), or the ninth form (Ni-C-Si-Mn-Mg-Fe composition) of the present material to the second form of the present heat treatment. In the iron casting of the first embodiment, by subjecting it to the second form of the present heat treatment, the linear expansion coefficient of the iron casting can be reduced, as described above.

Furthermore, in the iron casting of the first embodiment, the heat-treated object is cast using an austenitic casting material containing 26.0% or more Ni by mass, which makes it easy to lower the Ms point (the temperature at which the transformation from austenite to martensite begins). Therefore, it is easy to provide an iron casting in which martensite formation is suppressed. Furthermore, in the first holding step of the second embodiment of this heat treatment, solidification segregation of solute elements in the heat-treated object is reduced, thereby increasing the concentration of Ni, which is distributed at low concentrations in the final solidification portion of the heat-treated object. This further lowers the Ms point in the final solidification portion. Therefore, in the first cooling step after the first holding step, when the heat-treated object is cooled to the first cooling end temperature (-150°C or higher and 150°C or lower), martensite formation can be further suppressed. Therefore, it is possible to provide an iron casting in which thermal expansion is reduced and martensite formation is suppressed. The same applies to the following embodiments.

Second Embodiment
The iron casting of the second embodiment is an iron casting obtained by subjecting a heat treatment object cast using the fifth form (Ni-C-Si-Co-Mn-Fe composition), the seventh form (Ni-C-Si-Co-Mn-Mg-Fe composition), or the ninth form (Ni-C-Si-Mn-Mg-Fe composition) of the present material to the third form of the present heat treatment. In the iron casting of the second embodiment, by subjecting it to the third form of the present heat treatment, the linear expansion coefficient of the iron casting can be reduced, as described above.

Third Embodiment
The iron casting of the third embodiment is an iron casting obtained by subjecting a heat treatment object cast using the fifth form (Ni-C-Si-Co-Mn-Fe composition), the seventh form (Ni-C-Si-Co-Mn-Mg-Fe composition), or the ninth form (Ni-C-Si-Mn-Mg-Fe composition) of the present material to the first form of the present heat treatment. In the iron casting of the third embodiment, by subjecting it to the first form of the present heat treatment, the linear expansion coefficient of the iron casting can be reduced, as described above.

According to the above embodiment, an iron casting with reduced thermal expansion can be provided. Therefore, the iron casting of the above embodiment is suitable for a wide variety of applications requiring a low thermal expansion (coefficient). Examples of applications of the iron casting of the above embodiment include components for semiconductor manufacturing equipment, electronic component manufacturing equipment, machine tools, etc. For example, examples of applications in operating environments of approximately 20°C to 50°C include spindle holders for dicers and calibration gauges (gauge blocks, etc.) in semiconductor manufacturing equipment, and work stages and wire holding members for wire electric discharge machines in machine tools. Furthermore, examples of applications in operating environments of approximately 20°C to 100°C (or 150°C) include dry vacuum pump parts and card holders for probers in semiconductor manufacturing equipment.

<Example>
Table 1 shows the compositions of examples in the first embodiment and the compositions of comparative examples. Table 2 shows the compositions of examples in the second embodiment and the compositions of comparative examples. Table 3 shows the compositions of examples in the third embodiment and the compositions of comparative examples. In Tables 1 to 3, the C (carbon) content (mass%) is a value measured by combustion-infrared absorption method using a material carbon/sulfur analyzer "EMIA-Expert" manufactured by Horiba, Ltd. The contents (mass%) of Si (silicon), Ni (nickel), Mg (magnesium), and Co (cobalt) are values measured by inductively coupled plasma optical emission spectroscopy using an optical emission spectrometer "SPS3520UV" manufactured by Hitachi High-Tech Science Corporation. The contents (mass%) of other elements are values measured by optical emission spectroscopy using an optical emission spectrometer "PDA-8000" manufactured by Shimadzu Corporation.

Table 4 shows the heat treatment conditions and average linear expansion coefficients of the examples of the first embodiment, and the heat treatment conditions and average linear expansion coefficients of the comparative examples. Table 5 shows the heat treatment conditions and average linear expansion coefficients of the examples of the second embodiment, and the heat treatment conditions and average linear expansion coefficients of the comparative examples. Table 6 shows the heat treatment conditions and average linear expansion coefficients of the examples of the third embodiment, and the heat treatment conditions and average linear expansion coefficients of the comparative examples. In Tables 4 to 6, the holding temperature (°C) indicates the temperature at which the heat treatment object is held in the heat treatment furnace (heat treatment device). The heat treatment furnace used was a muffle furnace "QUICK TEMPER" manufactured by TEC Corporation. The holding time (hours) indicates the time the heat treatment object is held in the heat treatment furnace at the holding temperature. The cooling method refers to a method for cooling the heat-treatment object. Furnace cooling refers to gradually cooling the heat-treatment object within a heat treatment furnace. Natural air cooling refers to cooling the heat-treatment object in the air outside the heat treatment furnace. Rapid cooling refers to rapidly cooling the heat-treatment object by immersing it in a coolant such as water, oil, or dry ice, or liquid nitrogen. The cooling end temperature (°C) refers to the temperature at which the cooling of the heat-treatment object by the cooling method is completed. In the case of furnace cooling, the cooling end temperature refers to the surface temperature of the heat-treatment object measured inside the heat treatment furnace by contacting the temperature measuring part of a thermocouple with the heat-treatment object inside the heat treatment furnace. In the case of natural air cooling and rapid cooling, the cooling end temperature refers to the surface temperature of the heat-treatment object measured outside the heat treatment furnace by contacting the temperature measuring part of a thermocouple with the heat-treatment object outside the heat treatment furnace. The cooling rate (°C/min) or (°C/sec) refers to the amount of temperature change over time from the start to the end of cooling the heat-treatment object. The average linear expansion coefficient (×10 ⁻⁶ /°C) is a value measured in accordance with the ASTM standard (ASTM E228-17) using a thermal expansion meter "DIL 402 Expedis Supreme" manufactured by NETZSCH Japan Co., Ltd., for linear expansion coefficient measurement test pieces (diameter 6 mm, length 25 mm) taken from iron castings (Y-type B specimens) after a predetermined heat treatment was performed on heat treatment objects cast by sand casting, and shows the average linear expansion coefficient up to each temperature (50°C, 100°C, 150°C) in Tables 4 to 6, with 20°C as the base temperature.

(Comparison of Examples 1-1 to 1-30 with Comparative Example 1-1)
As shown in Table 4, the primary cooling end temperatures of Examples 1-1 to 1-30 are 950°C or lower, while the primary cooling end temperature of Comparative Example 1-1 is 1000°C. As shown in Table 4, in Examples 1-1 to 1-30, the average linear expansion coefficient at 20°C or higher and 50°C or lower, the average linear expansion coefficient at 20°C or higher and 100°C or lower, and the average linear expansion coefficient at 20°C or higher and 150°C or lower are all reduced compared to Comparative Example 1-1. Specifically, in Examples 1-1 to 1-30, the average linear expansion coefficient at 20°C or higher and 50°C or lower is 3.51 x ^10-6 /°C or lower (3.63 x ^10-6 /°C in Comparative Example 1-1), the average linear expansion coefficient at 20°C or higher and 100°C or lower is 3.58 x ^10-6 /°C or lower (3.90 x ^10-6 /°C in Comparative Example 1-1), and the average linear expansion coefficient at 20°C or higher and 150°C or lower is 4.09 x ^10-6 /°C or lower (4.28 x ^10-6 /°C in Comparative Example 1-1). Thus, in Examples 1-1 to 1-30, the primary cooling end temperature is set to 950°C or lower (particularly 900°C or lower), thereby making it possible to reduce the thermal expansion of the iron castings.

(Comparison of Examples 1-1 to 1-30 with Comparative Example 1-2)
As shown in Table 4, the primary cooling end temperatures of Examples 1-1 to 1-30 are 250° C. or higher, while the primary cooling end temperature of Comparative Example 1-2 is 200° C. As shown in Table 4, in Examples 1-1 to 1-30, the average linear expansion coefficient at 20° C. or higher and 50° C. or lower, the average linear expansion coefficient at 20° C. or higher and 100° C. or lower, and the average linear expansion coefficient at 20° C. or higher and 150° C. or lower are all reduced compared to Comparative Example 1-2. Specifically, in Examples 1-1 to 1-30, the average linear expansion coefficient at 20°C or higher and 50°C or lower is 3.51 x ^10-6 /°C or lower (4.98 x ^10-6 /°C in Comparative Example 1-2), the average linear expansion coefficient at 20°C or higher and 100°C or lower is 3.58 x ^10-6 /°C or lower (4.87 x ^10-6 /°C in Comparative Example 1-2), and the average linear expansion coefficient at 20°C or higher and 150°C or lower is 4.09 x ^10-6 /°C or lower (4.87 x ^10-6 /°C in Comparative Example 1-2). Thus, in Examples 1-1 to 1-30, the primary cooling end temperature is set to 250°C or higher (particularly 300°C or higher), thereby making it possible to reduce the thermal expansion of the iron castings.

(Comparison of Examples 2-1 to 2-26 with Comparative Example 2-1)
As shown in Table 5, the second holding temperature in Examples 2-1 to 2-26 is 950° C. or lower, while the second holding temperature in Comparative Example 2-1 is 1000° C. As shown in Table 5, in Examples 2-1 to 2-26, the average linear expansion coefficient at 20° C. or higher and 50° C. or lower, the average linear expansion coefficient at 20° C. or higher and 100° C. or lower, and the average linear expansion coefficient at 20° C. or higher and 150° C. or lower are all reduced compared to Comparative Example 2-1. Specifically, in Examples 2-1 to 2-26, the average linear expansion coefficient at 20°C or higher and 50°C or lower is 3.30 × 10 ^-6 /°C or lower (3.54 × 10 ^-6 /°C in Comparative Example 2-1), the average linear expansion coefficient at 20°C or higher and 100°C or lower is 3.55 × 10 ^-6 /°C or lower (3.78 × 10 ^-6 /°C in Comparative Example 2-1), and the average linear expansion coefficient at 20°C or higher and 150°C or lower is 4.05 × 10 ^-6 /°C or lower (4.15 × 10 ^-6 /°C in Comparative Example 2-1). Thus, in Examples 2-1 to 2-26, the second holding temperature is set to 950°C or lower (particularly 900°C or lower), thereby making it possible to reduce the thermal expansion of the iron castings.

(Comparison of Examples 2-1 to 2-26 with Comparative Example 2-2)
As shown in Table 5, the second holding temperature in Examples 2-1 to 2-26 is 250° C. or higher, while the second holding temperature in Comparative Example 2-2 is 200° C. As shown in Table 5, in Examples 2-1 to 2-26, the average linear expansion coefficient at 20° C. or higher and 50° C. or lower, the average linear expansion coefficient at 20° C. or higher and 100° C. or lower, and the average linear expansion coefficient at 20° C. or higher and 150° C. or lower are all reduced compared to Comparative Example 2-2. Specifically, in Examples 2-1 to 2-26, the average linear expansion coefficient at 20°C or higher and 50°C or lower is 3.30 x ^10-6 /°C or lower (5.57 x ^10-6 /°C in Comparative Example 2-2), the average linear expansion coefficient at 20°C or higher and 100°C or lower is 3.55 x ^10-6 /°C or lower (5.47 x ^10-6 /°C in Comparative Example 2-2), and the average linear expansion coefficient at 20°C or higher and 150°C or lower is 4.05 x ^10-6 /°C or lower (5.40 x ^10-6 /°C in Comparative Example 2-2). Thus, in Examples 2-1 to 2-26, the second holding temperature is set to 250°C or higher (particularly 300°C or higher), thereby making it possible to reduce the thermal expansion of the iron castings.

(Comparison of Examples 3-1 to 3-13 with Comparative Example 3-1)
As shown in Table 6, the first holding temperature in Examples 3-1 to 3-13 is 850° C. or higher, while the first holding temperature in Comparative Example 3-1 is 800° C. As shown in Table 6, in Examples 3-1 to 3-13, the average linear expansion coefficient at 20° C. or higher and 50° C. or lower, the average linear expansion coefficient at 20° C. or higher and 100° C. or lower, and the average linear expansion coefficient at 20° C. or higher and 150° C. or lower are all reduced compared to Comparative Example 3-1. Specifically, in Examples 3-1 to 3-13, the average linear expansion coefficient at 20°C or higher and 50°C or lower is 3.80 x ^10-6 /°C or lower (4.53 x ^10-6 /°C in Comparative Example 3-1), the average linear expansion coefficient at 20°C or higher and 100°C or lower is 4.16 x ^10-6 /°C or lower (5.02 x ^10-6 /°C in Comparative Example 3-1), and the average linear expansion coefficient at 20°C or higher and 150°C or lower is 4.84 x ^10-6 /°C or lower (5.58 x ^10-6 /°C in Comparative Example 3-1). Thus, in Examples 3-1 to 3-13, the thermal expansion of the iron casting can be reduced by setting the first holding temperature to 850°C or higher (particularly 950°C or higher).

(Comparison of Examples 3-1 to 3-13 with Comparative Example 3-2)
As shown in Table 6, the first holding temperature in Examples 3-1 to 3-13 is 850°C or higher, while the first holding temperature in Comparative Example 3-2 is 800°C. As shown in Table 6, in Examples 3-1 to 3-13, the average linear expansion coefficient at 20°C or higher and 50°C or lower and the average linear expansion coefficient at 20°C or higher and 100°C or lower are each reduced compared to Comparative Example 3-2. Specifically, in Examples 3-1 to 3-13, the average linear expansion coefficient at 20°C or higher and 50°C or lower is 3.80× ^10-6 /°C or lower (3.89× ^10-6 /°C in Comparative Example 3-2), and the average linear expansion coefficient at 20°C or higher and 100°C or lower is 4.16× ^10-6 /°C or lower (4.17× ^10-6 /°C in Comparative Example 3-2). In this way, in Examples 3-1 to 3-13, the thermal expansion of the iron casting can be reduced by setting the first holding temperature to 850° C. or higher (particularly 950° C. or higher).

(Comparison of Examples 3-1 to 3-13 with Comparative Example 3-3)
As shown in Table 6, the first holding temperatures of Examples 3-1 to 3-13 are 1250° C. or lower, whereas the first holding temperature of Comparative Example 3-3 is 1300° C. As shown in Table 6, in Examples 3-1 to 3-13, the average linear expansion coefficient at 20° C. or higher and 50° C. or lower, the average linear expansion coefficient at 20° C. or higher and 100° C. or lower, and the average linear expansion coefficient at 20° C. or higher and 150° C. or lower are all reduced compared to Comparative Example 3-3. Specifically, in Examples 3-1 to 3-13, the average linear expansion coefficient at 20°C or higher and 50°C or lower is 3.80 x ^10-6 /°C or lower (5.87 x ^10-6 /°C in Comparative Example 3-3), the average linear expansion coefficient at 20°C or higher and 100°C or lower is 4.16 x ^10-6 /°C or lower (5.72 x ^10-6 /°C in Comparative Example 3-3), and the average linear expansion coefficient at 20°C or higher and 150°C or lower is 4.84 x ^10-6 /°C or lower (5.67 x ^10-6 /°C in Comparative Example 3-3). Thus, in Examples 3-1 to 3-13, the thermal expansion of the iron casting can be reduced by setting the first holding temperature to 1250°C or lower (particularly 1200°C or lower).

(Comparison of Example 3-9 with Comparative Examples 3-1 and 3-3)
As shown in Table 3, the casting materials of Example 3-9, Comparative Example 3-1, and Comparative Example 3-3 do not contain Co. Furthermore, as shown in Table 6, the first holding temperature of Example 3-9 is 850°C or higher and 1250°C or lower, whereas the first holding temperature of Comparative Example 3-1 is 800°C and the first holding temperature of Comparative Example 3-3 is 1300°C. As shown in Table 6, when Example 3-9 is compared with Comparative Examples 3-1 and 3-3, none of which contain Co, Example 3-9 has a lower average linear expansion coefficient at 20°C or higher and 150°C or lower than Comparative Examples 3-1 and 3-3. Specifically, in Example 3-9, the average linear expansion coefficient at 20°C or higher and 150°C or lower is 4.84 × 10 ^-6 /°C or lower (5.58 × 10 ^-6 /°C for Comparative Example 3-1 and 5.67 × 10 ^-6 /°C for Comparative Example 3-3). As described above, in Example 3-9, by not including Co and setting the first holding temperature to 850° C. or higher and 1250° C. or lower, it is possible to reduce the thermal expansion of the iron casting at high temperatures.

(Comparison of Examples 3-1 to 3-8 and Examples 3-10 to 3-13 with Comparative Example 3-2)
As shown in Table 3, the casting materials of Examples 3-1 to 3-8, Examples 3-10 to 3-13, and Comparative Example 3-2 contain Co. Also, as shown in Table 6, the first holding temperature of Examples 3-1 to 3-8 and Examples 3-10 to 3-13 is 850°C or higher and 1250°C or lower, whereas the first holding temperature of Comparative Example 3-2 is 800°C. As shown in Table 6, when Examples 3-1 to 3-8 and Examples 3-10 to 3-13, all of which contain Co, are compared with Comparative Example 3-2, and it is found that Examples 3-1 to 3-8 and Examples 3-10 to 3-13 have a reduced average linear expansion coefficient at 20°C or higher and 150°C or lower compared to Comparative Example 3-2. Specifically, in Examples 3-1 to 3-8 and Examples 3-10 to 3-13, the average linear expansion coefficient at 20°C or higher and 150°C or lower is 4.49 × 10 ^-6 /°C or lower (4.53 × 10 ^-6 /°C in Comparative Example 3-2). Thus, in Examples 3-1 to 3-8 and Examples 3-10 to 3-13, by containing Co and setting the first holding temperature to 850°C or higher and 1250°C or lower, it is possible to reduce the thermal expansion of the iron castings at high temperatures.

The present disclosure may also have the following configurations.
(1)
An iron casting obtained by performing a first heat treatment on a heat treatment object cast using an austenitic casting material,
The iron casting, wherein performing the first heat treatment includes holding the heat treatment object in a first temperature range of 950°C or higher and 1200°C or lower.
(2)
The iron casting according to (1), wherein holding the heat treatment object in the first temperature range includes holding the heat treatment object for a time range of 1 hour to 100 hours.
(3)
The method further includes performing a second heat treatment on the heat treatment object after performing the first heat treatment,
The iron casting according to (1) or (2), wherein performing the second heat treatment includes cooling the heat treatment object to a second temperature range of 300°C or higher and 900°C or lower.
(4)
The iron casting according to (3), wherein cooling to the second temperature range includes cooling the heat treatment object at a cooling rate range of 0.01°C/min or more and 20°C/min or less.
(5)
The method further includes performing a third heat treatment on the heat treatment object after performing the second heat treatment,
The iron casting according to (3) or (4), wherein performing the third heat treatment includes cooling the heat treatment object to a third temperature range of 0°C or higher and 100°C or lower.
(6)
The iron casting according to (5), wherein cooling to the third temperature range includes cooling the heat treatment object at a cooling rate range of 1°C/sec or more and 1000°C/sec or less.
(7)
The iron casting according to any one of (1) to (6), wherein the casting material contains 26.0 mass% or more and 42.0 mass% or less of Ni, with the remainder being Fe and unavoidable elements.
(8)
The iron casting according to (7), wherein the casting material further contains 0.3 mass % or more and 3.5 mass % or less of C.
(9)
The iron casting according to (7) or (8), wherein the casting material further contains 0.1 mass % or more and 3.0 mass % or less of Si.
(10)
The iron casting according to any one of (7) to (9), wherein the casting material further contains 0.001 mass% or more and 8.0 mass% or less of Co.
(11)
A method for producing an iron casting, comprising: performing a first heat treatment on an object to be heat-treated that is cast using an austenitic casting material,
The manufacturing method, wherein performing the first heat treatment includes holding the heat treatment object in a first temperature range of 950°C or higher and 1200°C or lower.
(12)
The manufacturing method according to (11), wherein maintaining the heat treatment object in the first temperature range includes maintaining the heat treatment object for a time range of 1 hour to 100 hours.
(13)
The method further includes performing a second heat treatment on the heat treatment object after performing the first heat treatment,
The manufacturing method according to (11) or (12), wherein the second heat treatment includes cooling the heat treatment object to a second temperature range of 300°C or higher and 900°C or lower.
(14)
The manufacturing method according to (13), wherein cooling to the second temperature range includes cooling the heat treatment object at a cooling rate range of 0.01°C/min or more and 20°C/min or less.
(15)
The method further includes performing a third heat treatment on the heat treatment object after performing the second heat treatment,
The manufacturing method according to (13) or (14), wherein performing the third heat treatment includes cooling the heat treatment object to a third temperature range of 0°C or higher and 100°C or lower.
(16)
The manufacturing method according to (15), wherein cooling to the third temperature range includes cooling the heat treatment object at a cooling rate range of 1°C/sec or more and 1000°C/sec or less.

Claims (26)

A method for producing an iron casting by performing heat treatment on a heat treatment object cast using an austenitic casting material, comprising:
The casting material contains 30.5% by mass or more and 50.0% by mass or less of Ni, 1.75% by mass or more and 3.5% by mass or less of C, 0.1% by mass or more and 3.5% by mass or less of Si, 0% by mass or more and 8.0% by mass or less of Co, 0% by mass or more and 3.0% by mass or less of Mn, and 0% by mass or more and 0.2% by mass or less of Mg, with the remainder being Fe and unavoidable elements;
The heat treatment is
a first holding step of holding the heat treatment object at a first holding temperature of 1025°C or higher and 1250°C or lower;
a first cooling step of cooling the heat-treatment object to a first cooling end temperature of −150° C. or higher and 150° C. or lower after the first holding step;
Including,
The method, wherein the first holding step includes holding the heat-treatment object for a first holding time of 0.25 hours or more and 100 hours or less. A method for producing an iron casting by performing heat treatment on a heat treatment object cast using an austenitic casting material, comprising:
The casting material contains 30.5% by mass or more and 50.0% by mass or less of Ni, 0.1% by mass or more and 3.5% by mass or less of C, 1.2% by mass or more and 3.5% by mass or less of Si, 0% by mass or more and 8.0% by mass or less of Co, 0% by mass or more and 3.0% by mass or less of Mn, and 0% by mass or more and 0.2% by mass or less of Mg, with the remainder being Fe and unavoidable elements;
The heat treatment is
a first holding step of holding the heat treatment object at a first holding temperature of 1025°C or higher and 1250°C or lower;
a first cooling step of cooling the heat-treatment object to a first cooling end temperature of −150° C. or higher and 150° C. or lower after the first holding step;
Including,
The method, wherein the first holding step includes holding the heat-treatment object for a first holding time of 0.25 hours or more and 100 hours or less. 3. The method according to claim 1 or 2,
The method, wherein the first cooling step includes cooling the heat-treatment object at a first cooling rate of 0.01°C/min or more and 300°C/min or less. 4. The method of claim 3,
The method, wherein the first cooling rate is greater than or equal to 0.01° C./min and less than or equal to 20° C./min. 3. The method according to claim 1 or 2,
The method, wherein the first holding time is greater than or equal to 2.5 hours and less than or equal to 25 hours. 3. The method according to claim 1 or 2,
The method, wherein the first cooling end temperature is 0°C or higher and 100°C or lower. A method for producing an iron casting by performing heat treatment on a heat treatment object cast using an austenitic casting material, comprising:
The casting material contains 26.0% by mass or more and 50.0% by mass or less of Ni, 1.75% by mass or more and 3.5% by mass or less of C, 0.1% by mass or more and 3.5% by mass or less of Si, 0% by mass or more and 8.0% by mass or less of Co, 0% by mass or more and 3.0% by mass or less of Mn, and 0% by mass or more and 0.2% by mass or less of Mg, with the remainder being Fe and unavoidable elements;
The heat treatment is
a first holding step of holding the heat treatment object at a first holding temperature of 850°C or higher and 1250°C or lower;
a first cooling step of cooling the heat-treatment object to a first cooling end temperature of −150° C. or higher and 150° C. or lower after the first holding step;
Including,
the first holding step includes holding the heat treatment object for a first holding time of 0.25 hours or more and 100 hours or less,
The first cooling step includes:
a primary cooling step of cooling the heat treatment object at a primary cooling rate;
a secondary cooling step of cooling the heat treatment object at a secondary cooling rate higher than the primary cooling rate after the primary cooling step;
Including,
the primary cooling step includes cooling the heat treatment object to a primary cooling end temperature of 250°C or higher and 950°C or lower,
The method, wherein the secondary cooling step includes cooling the heat-treatment object to the first cooling end temperature. A method for producing an iron casting by performing heat treatment on a heat treatment object cast using an austenitic casting material, comprising:
The casting material contains 26.0% by mass or more and 50.0% by mass or less of Ni, 0.1% by mass or more and 3.5% by mass or less of C, 1.2% by mass or more and 3.5% by mass or less of Si, 0% by mass or more and 8.0% by mass or less of Co, 0% by mass or more and 3.0% by mass or less of Mn, and 0% by mass or more and 0.2% by mass or less of Mg, with the remainder being Fe and unavoidable elements;
The heat treatment is
a first holding step of holding the heat treatment object at a first holding temperature of 850°C or higher and 1250°C or lower;
a first cooling step of cooling the heat-treatment object to a first cooling end temperature of −150° C. or higher and 150° C. or lower after the first holding step;
Including,
the first holding step includes holding the heat treatment object for a first holding time of 0.25 hours or more and 100 hours or less,
The first cooling step includes:
a primary cooling step of cooling the heat treatment object at a primary cooling rate;
a secondary cooling step of cooling the heat treatment object at a secondary cooling rate higher than the primary cooling rate after the primary cooling step;
Including,
the primary cooling step includes cooling the heat treatment object to a primary cooling end temperature of 250°C or higher and 950°C or lower,
The method, wherein the secondary cooling step includes cooling the heat-treatment object to the first cooling end temperature. 9. The method according to claim 7 or 8,
the primary cooling rate is 0.01°C/min or more and 20°C/min or less,
The method, wherein the secondary cooling rate is 1°C/min or more and 40,000°C/min or less. 10. The method of claim 9,
The method, wherein the secondary cooling rate is 100°C/min or more and 40,000°C/min or less. 9. The method according to claim 7 or 8,
The method, wherein the first holding time is greater than or equal to 2.5 hours and less than or equal to 25 hours. 9. The method according to claim 7 or 8,
The method, wherein the primary cooling end temperature is 450°C or higher and 850°C or lower. 9. The method according to claim 7 or 8,
The method, wherein the first cooling end temperature is 0°C or higher and 100°C or lower. A method for producing an iron casting by performing heat treatment on a heat treatment object cast using an austenitic casting material, comprising:
The casting material contains 26.0% by mass or more and 50.0% by mass or less of Ni, 1.75% by mass or more and 3.5% by mass or less of C, 0.1% by mass or more and 3.5% by mass or less of Si, 0% by mass or more and 8.0% by mass or less of Co, 0% by mass or more and 3.0% by mass or less of Mn, and 0% by mass or more and 0.2% by mass or less of Mg, with the remainder being Fe and unavoidable elements;
The heat treatment is
a first holding step of holding the heat treatment object at a first holding temperature of 850°C or higher and 1250°C or lower;
a first cooling step of cooling the heat-treatment object to a first cooling end temperature of −150° C. or higher and 150° C. or lower after the first holding step;
a second holding step of holding the heat treatment object at a second holding temperature of 250°C or higher and 950°C or lower after the first cooling step;
a second cooling step of cooling the heat-treatment object to a second cooling end temperature of −150° C. or higher and 150° C. or lower after the second holding step;
Including,
the first holding step includes holding the heat treatment object for a first holding time of 0.25 hours or more and 100 hours or less,
The method, wherein the second holding step includes holding the heat-treatment object for a second holding time of 0.25 hours or more and 25 hours or less. A method for producing an iron casting by performing heat treatment on a heat treatment object cast using an austenitic casting material, comprising:
The casting material contains 26.0% by mass or more and 50.0% by mass or less of Ni, 0.1% by mass or more and 3.5% by mass or less of C, 1.2% by mass or more and 3.5% by mass or less of Si, 0% by mass or more and 8.0% by mass or less of Co, 0% by mass or more and 3.0% by mass or less of Mn, and 0% by mass or more and 0.2% by mass or less of Mg, with the remainder being Fe and unavoidable elements;
The heat treatment is
a first holding step of holding the heat treatment object at a first holding temperature of 850°C or higher and 1250°C or lower;
a first cooling step of cooling the heat-treatment object to a first cooling end temperature of −150° C. or higher and 150° C. or lower after the first holding step;
a second holding step of holding the heat treatment object at a second holding temperature of 250°C or higher and 950°C or lower after the first cooling step;
a second cooling step of cooling the heat-treatment object to a second cooling end temperature of −150° C. or higher and 150° C. or lower after the second holding step;
Including,
the first holding step includes holding the heat treatment object for a first holding time of 0.25 hours or more and 100 hours or less,
The method, wherein the second holding step includes holding the heat-treatment object for a second holding time of 0.25 hours or more and 25 hours or less. 16. The method of claim 14 or 15,
The method, wherein the first cooling step includes cooling the heat-treatment object at a first cooling rate of 0.01°C/min or more and 300°C/min or less. 17. The method of claim 16,
The method, wherein the first cooling rate is greater than or equal to 1° C./min and less than or equal to 50° C./min. 16. The method of claim 14 or 15,
The method, wherein the second cooling step includes cooling the heat-treatment object at a second cooling rate of 1°C/min or more and 40,000°C/min or less. 20. The method of claim 18,
The method, wherein the second cooling rate is 100°C/min or more and 10,000°C/min or less. 16. The method of claim 14 or 15,
The method, wherein the first holding time is greater than or equal to 2.5 hours and less than or equal to 25 hours. 16. The method of claim 14 or 15,
The method, wherein the first cooling end temperature is 0°C or higher and 100°C or lower. 16. The method of claim 14 or 15,
The method, wherein the second holding temperature is 550°C or higher and 950°C or lower. 16. The method of claim 14 or 15,
The method, wherein the second cooling end temperature is 0°C or higher and 50°C or lower. 16. The method of any one of claims 1, 2, 7, 8, 14 and 15,
The method, wherein the Co content of the casting material is 0.1% by mass or more and 8.0% by mass or less. 16. The method of any one of claims 1, 2, 7, 8, 14 and 15,
The method, wherein the Mn content of the casting material is 0.01% by mass or more and 3.0% by mass or less. 16. The method of any one of claims 1, 2, 7, 8, 14 and 15,
The method, wherein the Mg content of the casting material is 0.01% by mass or more and 0.2% by mass or less.