JP7743971B2 - iron castings - Google Patents

Description

The present invention relates to iron castings produced using ferritic casting materials.

The abstract of Patent Document 1 states that a heat-resistant cast steel with excellent thermal fatigue resistance is provided. The abstract also states that the contents of C, Si, Mn, P, S, and Cr are 0.10 wt%≦C<0.30 wt%, 2.0 wt%≦Si≦4.0 wt%, 0.3 wt%≦Mn≦1.0 wt%, P≦0.04 wt%, S≦0.30 wt%, and 5.0 wt%≦Cr≦15.0 wt%, respectively, with the balance being substantially Fe, and that the cast structure of the matrix is a multiphase structure consisting of ferrite and pearlite phases, thereby extending the thermal fatigue life of the heat-resistant cast steel and improving its room-temperature elongation, thereby improving its thermal fatigue resistance.

Japanese Patent Application Publication No. 7-62500

We provide iron castings with improved elongation using ferritic casting materials.

One aspect of the present invention is an iron casting obtained by performing a first heat treatment on a heat treatment object cast using a ferritic casting material. The casting material contains 0.0001 to 0.03 mass% C, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, and the balance being Fe and unavoidable elements. Performing the first heat treatment includes holding the heat treatment object in a first temperature range of 750 to 1380°C.

In the above embodiment, a casting material containing 0.001 to 0.6% Ni by mass and 16.0 to 21.0% Cr by mass is used for casting, causing the ferrite phase to crystallize as the main phase within the heat-treated object. Furthermore, when the liquid phase solidifies during the ferrite phase crystallization process, a first heat treatment is performed on the solute elements dissolved with a non-uniform concentration distribution within the crystal grains that make up the ferrite phase (hereinafter referred to as "inside the crystal grains"), and the solute elements are uniformly dissolved within the crystal grains by maintaining the temperature within a first temperature range of 750 to 1380°C. This reduces the segregation of solute elements within the crystal grains. As a result, the elongation of the iron casting can be improved. Furthermore, in the above-mentioned aspect, by setting the C content in the casting material to 0.0001 to 0.03 mass%, the amount of solidification segregation of C can be reduced, and by performing the first heat treatment on the small amount of C that has solidified and segregated within the crystal grains, the segregation of C within the crystal grains can be reliably reduced. In the above-mentioned aspect, holding the heat treatment object in the first temperature range preferably includes holding the heat treatment object in a first time range of 0.5 to 6.0 hours.

The iron casting is preferably obtained by performing a second heat treatment on the heat-treatment object after the first heat treatment. The second heat treatment preferably includes holding the heat-treatment object at a second temperature range of 20 to 1080°C. By performing the second heat treatment on solute elements distributed within the ferrite phase grains or at the grain boundaries (hereinafter referred to as "grain boundaries") and holding the second temperature range of 20 to 1080°C, the segregation state of the solute elements within the grains or at the grain boundaries is controlled. This, for example, makes it easier to reduce the segregation of solute elements or change the segregation tendency of solute elements. As a result, it is easier to further improve the elongation of the iron casting. In the above aspect, holding the second temperature range preferably includes holding the heat-treatment object for a second time range of 0.3 to 48 hours.

It is preferable that the casting material further contains 0.0001 to 0.03 mass% N. By setting the N content in the casting material to 0.0001 to 0.03 mass%, the amount of N that solidifies and segregates is reduced, and by subjecting the small amount of N that has solidified and segregated within the crystal grains to the first heat treatment, it is possible to reliably reduce the segregation of N within the crystal grains.

The casting material preferably further contains 0.001 to 1.0 mass% Mn. By setting the Mn content in the casting material to 0.001 to 1.0 mass%, the precipitation of the austenite phase can be suppressed. This makes it easier to stabilize the ferrite phase from room temperature to high temperature, and to suppress the phase transformation associated with the first heat treatment. Therefore, the decrease in ductility associated with the phase transformation can be suppressed.

It is preferable that the casting material further contains 0.1 to 0.3 mass% Ti. By setting the Ti content in the casting material to 0.1 to 0.3 mass%, the crystal grains can be refined and the proportion of grain boundaries in the ferrite phase can be increased. This makes it easier to reduce the segregation concentration of solute elements at the grain boundaries, which in turn makes it easier to improve the elongation of the iron casting.

Another aspect of the present invention is an iron casting obtained by performing a first heat treatment on a heat treatment object cast using a ferritic casting material, wherein the first heat treatment includes holding the heat treatment object in a first temperature range of 750 to 1380°C. The iron casting is preferably obtained by performing a second heat treatment on the heat treatment object after the first heat treatment. The second heat treatment preferably includes holding the heat treatment object in a second temperature range of 20 to 1080°C.

FIG. 1 is a diagram showing the compositions of the first embodiment (Examples 1 to 7) and the second embodiment (Example 8), and the compositions of Comparative Examples 1 and 2. FIG. 2 is a diagram showing the heat treatment conditions and mechanical properties of the first embodiment (Examples 1 to 7) and the second embodiment (Example 8), and the heat treatment conditions and mechanical properties of Comparative Examples 1 and 2. FIG. 3A is a diagram showing the compositions of the third embodiment (Examples 9 to 43). FIG. 3B is a diagram showing the compositions of the third embodiment (Examples 44 to 77). FIG. 3C is a diagram showing the compositions of the fourth embodiment (Examples 78 to 105) and the compositions of Comparative Examples 3 to 5. FIG. 4A is a diagram showing the heat treatment conditions and mechanical properties of the third embodiment (Examples 9 to 43). FIG. 4B is a diagram showing the heat treatment conditions and mechanical properties of the third embodiment (Examples 44 to 77). FIG. 4C is a diagram showing the heat treatment conditions and mechanical properties of the fourth embodiment (Examples 78 to 105) and the heat treatment conditions and mechanical properties of Comparative Examples 3 to 5.

Embodiments of the iron castings disclosed in this application will be described below with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiments and includes those defined in the claims.

The material used to produce the iron castings according to the embodiments of the present invention is a ferritic casting material. "Ferritic casting material" refers to a casting material having ferrite as the main phase. For example, the proportion of ferrite in the entire structure is 50% or more. The proportion of ferrite in the entire structure is preferably 70% or more, more preferably 80% or more, more preferably 85% or more, more preferably 90% or more, more preferably 92.5% or more, more preferably 95% or more, more preferably 97.5% or more, and even more preferably 99% or more.

In the present invention, "casting" refers to various casting methods, such as sand casting, metal mold casting, die casting, and lost-wax casting. The "mass %" of an element refers to the mass percentage of the element relative to the mass of the ferritic casting material. For example, "X to Y mass % of elements" means that the mass % of the element is X to Y%. The "balance" refers to components other than the listed elements that make up the ferritic casting material. Therefore, for example, "including ...C, ...Ni, and ...Cr, with the balance being Fe and unavoidable elements" means that, of the components that make up the ferritic casting material, the components other than C, Ni, and Cr are Fe and unavoidable elements. In addition, the notation "X to Y°C" in the heat treatment conditions refers to a holding temperature of X to Y°C, and the notation "X to Y hours" refers to a holding time of X to Y hours.

<First embodiment of casting material>
A first form of the ferritic casting material (hereinafter referred to as "the material") contains 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Si, 0.001 to 1.0 mass% Mn, 0.0001 to 0.04 mass% P, 0.0001 to 0.027 mass% S, 0.3 to 0.8 mass% Cu, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.3 to 0.8 mass% Nb, and 0.0001 to 0.03 mass% N, with the remainder being Fe and unavoidable elements.

(C: carbon)
A first embodiment of the material contains 0.0001 to 0.03 mass% C. In the first embodiment of the material, setting the upper limit of the C content to 0.03 mass% allows C to be fixed together with N as Nb carbonitrides, thereby reducing the formation of Cr carbonitrides. This prevents a decrease in the Cr content in the ferrite phase. This, in turn, prevents a decrease in the corrosion resistance of the iron casting. Furthermore, setting the upper limit of the C content to 0.03 mass% prevents the precipitation of austenite phase at high temperatures. This reduces the proportion of austenite phase, which tends to slow the Cr diffusion rate, thereby ensuring a sufficient supply of Cr to the oxide film of the iron casting. This, in turn, improves the corrosion resistance of the iron casting. Furthermore, setting the upper limit of the C content to 0.03 mass% prevents excessive formation of Nb carbonitrides while still fixing C together with N as Nb carbonitrides. This prevents excessive precipitation of Nb carbonitrides within crystal grains and at grain boundaries. Therefore, it is possible to suppress the decrease in ductility of the iron casting and the occurrence of weld cracks. Furthermore, since it is possible to suppress the excessive generation of Nb carbonitrides, it is possible to suppress the decrease in the amount of Nb dissolved in the ferrite phase. Therefore, it is possible to improve the high-temperature strength of the iron casting. The upper limit of the C content is preferably 0.02 mass%. It is possible to further suppress the precipitation of Cr carbonitrides and Nb carbonitrides. Therefore, it is possible to further suppress the decrease in corrosion resistance and embrittlement of the iron casting. The same applies to the following embodiments of this material.

(Si: silicon)
A first embodiment of the material contains 0.001 to 1.0 mass% Si. In this first embodiment of the material, setting the upper limit of the Si content to 1.0 mass% can suppress an increase in the amount of Si dissolved in the ferrite phase. This can prevent a decrease in the ductility of the iron casting and suppress the occurrence of weld cracks. Furthermore, setting the upper limit of the Si content to 1.0 mass% can suppress the aggregation of non-metallic inclusions (oxides) such as silicon dioxide. This can suppress the occurrence of surface defects and internal defects in the iron casting. The lower limit of the Si content is preferably 0.4 mass%. Non-metallic inclusions such as silicon dioxide can act as nuclei for precipitation of Nb carbonitrides. This suppresses the aggregation of Nb carbonitrides and facilitates uniform dispersion of Nb carbonitrides within the crystal grains. Therefore, by suppressing excessive growth of Nb carbonitrides within the crystal grains, brittle fracture of the iron casting can be suppressed. The upper limit of the Si content is preferably 0.6 mass%. This makes it easier to disperse Nb carbonitrides more uniformly within the crystal grains, and makes it possible to further suppress excessive growth of Nb carbonitrides. The same applies to the following embodiments of the present material.

(Mn: Manganese)
A first embodiment of the material contains 0.001 to 1.0 mass% Mn. In this first embodiment of the material, setting the upper limit of the Mn content, an austenitizing element, to 1.0 mass% can suppress the precipitation of the austenite phase. This allows the ferrite phase to be stabilized from room temperature to high temperatures. Furthermore, setting the upper limit of the Mn content to 1.0 mass% can suppress the excessive formation of non-metallic inclusions (sulfides) such as manganese sulfide, which are formed together with S by Mn, which tends to segregate in the final solidification region. This can suppress the precipitation of Nb carbonitrides within crystal grains or at grain boundaries, using non-metallic inclusions such as manganese sulfide as precipitation nuclei. This can therefore suppress the occurrence of grain boundary embrittlement, particularly due to Nb carbonitrides precipitated at grain boundaries. The upper limit of the Mn content is preferably 0.2 mass%, more preferably 0.15 mass%. This can reduce the amount of non-metallic inclusions such as manganese sulfide that precipitate at grain boundaries. This makes it easier for non-metallic inclusions such as silicon dioxide to act preferentially as nuclei for precipitation of Nb carbonitrides, and makes it easier to uniformly disperse Nb carbonitrides within the crystal grains.

(P: Rin)
A first embodiment of the material contains 0.0001 to 0.04 mass% P. In the first embodiment of the material, by setting the upper limit of the P content to 0.04 mass%, it is possible to suppress an increase in the amount of P dissolved in the ferrite phase. This can therefore suppress a decrease in the ductility of the iron casting and the occurrence of weld cracks. The upper limit of the P content is preferably 0.03 mass%, which can further suppress embrittlement of the iron casting. The same applies to the following embodiments of the material.

(S: sulfur)
A first embodiment of the material contains 0.0001 to 0.027 mass% S. In this first embodiment of the material, setting the upper limit of the S content to 0.027 mass% can suppress the excessive formation of non-metallic inclusions (sulfides) such as manganese sulfide, which are formed together with Mn by S, which tends to segregate in the final solidification portion. This can suppress the excessive precipitation of Nb carbonitrides within crystal grains or at grain boundaries, using non-metallic inclusions such as manganese sulfide as precipitation nuclei. This can particularly suppress the occurrence of grain boundary embrittlement due to Nb carbonitrides precipitated at grain boundaries. The upper limit of the S content is preferably 0.025 mass%, more preferably 0.020 mass%, and even more preferably 0.015 mass%. The lower limit of the S content is preferably 0.001 mass%, more preferably 0.004 mass%. This can reduce the amount of non-metallic inclusions such as manganese sulfide that precipitate at grain boundaries. The same applies to the following embodiments of the material.

(Cu: Copper)
A first embodiment of the material contains 0.3 to 0.8 mass% Cu. In the first embodiment of the material, by setting the Cu content to 0.3 to 0.8 mass%, the corrosion resistance of the iron casting can be improved. The lower limit of the Cu content is preferably 0.4 mass%. This can prevent a decrease in the corrosion resistance of the iron casting. The upper limit of the Cu content is preferably 0.5 mass%. This can prevent Cu precipitation and prevent a decrease in the ductility of the iron casting. The same applies to the following embodiments of the material.

(Ni: Nickel)
A first embodiment of the material contains 0.001 to 0.6 mass% Ni. In this first embodiment of the material, by setting the upper limit of the content of Ni, an austenitizing element, to 0.6 mass%, the precipitation of the austenite phase can be suppressed. This makes it possible to stabilize the ferrite phase from room temperature to high temperatures. Furthermore, by setting the upper limit of the Ni content to 0.6 mass%, it is possible to suppress an increase in the thermal expansion coefficient due to the precipitation of the austenite phase and to suppress a decrease in oxidation resistance at high temperatures. The upper limit of the Ni content is preferably 0.4 mass%, more preferably 0.3 mass%. The corrosion resistance of the iron casting can be improved while suppressing an increase in alloy cost. The same applies to the following embodiments of the material.

(Cr: chromium)
A first embodiment of the material contains 16.0 to 21.0 mass% Cr. In this first embodiment of the material, by setting the content of Cr, a ferrite-forming element, to 16.0 to 21.0 mass%, the ferrite phase can be stabilized from room temperature to high temperature. Furthermore, by setting the Cr content to 16.0 to 21.0 mass%, the corrosion resistance of the iron casting can be improved. The lower limit of the Cr content is preferably 18.0 mass%. This can further improve the corrosion resistance of the iron casting. The upper limit of the Cr content is preferably 19.0 mass%. This can suppress the precipitation of embrittlement phases such as the σ phase, and suppress a decrease in the ductility of the iron casting. The same applies to the following embodiments of the material.

(Nb: niobium)
A first embodiment of the material contains 0.3 to 0.8 mass% Nb. In the first embodiment of the material, by setting the Nb content to 0.3 to 0.8 mass%, Nb is dissolved in the ferrite phase, improving the high-temperature strength of the iron casting. Furthermore, C and N are fixed as Nb carbonitrides, and the formation of Cr carbonitrides is suppressed, improving oxidation resistance. The upper limit of the Nb content is preferably 0.6 mass%, more preferably 0.4 mass%. This suppresses the decrease in ductility of the iron casting due to excessive solid-solution strengthening and excessive formation of Nb carbonitrides, and also suppresses the occurrence of weld cracks. The same applies to the following embodiments of the material.

(N: nitrogen)
A first embodiment of the material contains 0.0001 to 0.03 mass% N. In this first embodiment of the material, setting the upper limit of the N content to 0.03 mass% reduces the formation of Cr carbonitrides formed by N together with C. This prevents a decrease in the Cr content in the ferrite phase. This prevents a decrease in the corrosion resistance of the iron casting. Furthermore, setting the upper limit of the N content to 0.03 mass% prevents the precipitation of austenite phase at high temperatures. This reduces the proportion of austenite phase, which tends to reduce the diffusion rate of Cr, thereby ensuring a sufficient supply of Cr to the oxide film of the iron casting. This improves the corrosion resistance of the iron casting. Furthermore, excessive formation of Nb carbonitrides formed by N together with C can be suppressed. This prevents excessive precipitation of Nb carbonitrides within grains and at grain boundaries. This prevents a decrease in the ductility of the iron casting and also prevents the occurrence of weld cracks. Furthermore, since excessive generation of Nb carbonitrides can be suppressed, the amount of Nb dissolved in the ferrite phase can be suppressed from decreasing. Therefore, the high-temperature strength of the iron casting can be improved. The upper limit of the N content is preferably 0.02 mass%. Precipitation of Cr carbonitrides and Nb carbonitrides can be further suppressed. Therefore, the decrease in corrosion resistance and embrittlement of the iron casting can be further suppressed. The same applies to the following embodiments of this 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 elements such as Al (aluminum), Mo (molybdenum), V (vanadium), Co (cobalt), Sn (tin), Ce (cerium), Te (tellurium), La (lanthanum), Bi (bismuth), and Zn (zinc). The content of unavoidable elements is, for example, preferably 5.0% by mass or less in total, more preferably 3.0% by mass or less in total, more preferably 1.0% by mass or less in total, more preferably 0.5% by mass or less in total, more preferably 0.2% by mass or less in total, and even more preferably 0.1% by mass or less in total. The same applies to the following embodiments of this material.

<Second embodiment of casting material>
A second form of the material contains 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Si, 0.001 to 1.0 mass% Mn, 0.0001 to 0.04 mass% P, 0.0001 to 0.027 mass% S, 0.3 to 0.8 mass% Cu, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.3 to 0.8 mass% Nb, 0.0001 to 0.03 mass% N, 0.1 to 0.3 mass% Ti, and the balance being Fe and unavoidable elements.

(Ti: Titanium)
A second embodiment of the material contains 0.1 to 0.3 mass% Ti. In this second embodiment, the Ti content of 0.1 to 0.3 mass% refines the grains and increases the proportion of grain boundaries in the ferrite phase. This reduces the segregation concentration of solute elements at the grain boundaries, thereby improving the elongation of the iron casting. Setting the lower limit of the Ti content to 0.1 mass% promotes grain refinement. Setting the upper limit of the Ti content to 0.3 mass% suppresses the reduction in ductility of the iron casting due to excessive solid solution strengthening and excessive formation of Ti oxides, and also suppresses the occurrence of weld cracks. Furthermore, setting the upper limit of the C content to 0.03 mass%, the upper limit of the N content to 0.03 mass%, and the Ti content to 0.1 to 0.3 mass% suppresses the excessive formation of Ti nitrides and Ti carbonitrides. This makes it possible to suppress excessive precipitation of Ti nitrides and Ti carbonitrides within crystal grains and at grain boundaries. This, in turn, makes it possible to suppress a decrease in the ductility of the iron casting and to suppress the occurrence of weld cracks. Furthermore, because excessive formation of Ti nitrides and Ti carbonitrides can be suppressed, it is possible to suppress a decrease in the amount of Ti dissolved in the ferrite phase. This, in turn, makes it possible to improve the high-temperature strength of the iron casting.

Note that this material is not limited to the above-mentioned form. For example, this material may contain one or more of the above-mentioned elements C, Si, Mn, P, S, Cu, Ni, Cr, Nb, N, and Ti, with the balance being Fe and unavoidable elements. Another example of this material contains 0.0001 to 0.03 mass% C, 0.001 to 0.6 mass% Ni, and 16.0 to 21.0 mass% Cr, with the balance being Fe and unavoidable elements. Another example of this material contains 0.0001 to 0.03 mass% C, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, and 0.1 to 0.3 mass% Ti, with the balance being Fe and unavoidable elements. Another example of this material contains 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Mn, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.0001 to 0.03 mass% N, with the balance being Fe and unavoidable elements. Another example of this material contains 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Mn, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.0001 to 0.03 mass% N, and 0.1 to 0.3 mass% Ti, with the balance being Fe and unavoidable elements.

<First form of heat treatment>
A first form of heat treatment (hereinafter referred to as "this heat treatment") performed on a heat-treatment object cast using this material includes performing a first heat treatment (solution treatment, primary heat treatment) on the heat-treatment object. Performing the first heat treatment includes holding the heat-treatment object in a first temperature range of 750 to 1380°C. Holding the heat-treatment object in the first temperature range includes holding the heat-treatment object for a first time range of 0.5 to 6.0 hours.

First, before performing the first heat treatment, a casting material containing 0.001 to 0.6% Ni by mass and 16.0 to 21.0% Cr by mass is cast, causing the ferrite phase to crystallize as the main phase within the heat-treated object. Next, in a first embodiment of this heat treatment, when the liquid phase solidifies during the ferrite phase crystallization process, the first heat treatment is performed on the solute elements dissolved with a non-uniform concentration distribution within the crystal grains that make up the ferrite phase. Performing the first heat treatment involves holding the heat-treated object within a first temperature range of 750 to 1380°C, allowing the solute elements to be dissolved uniformly within the crystal grains. This reduces segregation of the solute elements within the crystal grains, resulting in improved elongation of the iron casting. The same applies to the following embodiments of this heat treatment.

Furthermore, by setting the upper limit of the content of Ni, an austenitizing element, in the casting material to 0.6 mass %, and the content of Cr, a ferrite-forming element, to 16.0 to 21.0 mass %, the ferrite phase can be stabilized from room temperature to high temperature. This makes it easier to suppress the phase transformation that occurs during the first heat treatment. Therefore, it is possible to suppress the decrease in ductility that occurs during phase transformation. The same applies to the following forms of this heat treatment.

Furthermore, by casting a casting material with a C content of 0.0001-0.03% by mass and an N content of 0.0001-0.03% by mass before performing the first heat treatment, the amount of solidification segregation of C and N within the heat-treated object can be reduced. Next, in the first form of this heat treatment, the first heat treatment is performed on the small amounts of C and N that have solidified and segregated within the crystal grains. This makes it possible to reliably reduce the segregation of C and N within the crystal grains. The same applies to the following forms of this heat treatment.

The upper limit of the first temperature range is preferably 1350°C, more preferably 1300°C, more preferably 1250°C, more preferably 1200°C, more preferably 1150°C, and even more preferably 1125°C. The lower limit of the first temperature range is preferably 775°C, more preferably 800°C, more preferably 850°C, more preferably 900°C, more preferably 950°C, more preferably 1000°C, more preferably 1050°C, and even more preferably 1075°C. The same applies to the following forms of this heat treatment.

The upper limit of the first time range is preferably 5.5 hours, more preferably 5.0 hours, more preferably 4.5 hours, more preferably 4.0 hours, and even more preferably 3.5 hours. The lower limit of the first time range is preferably 1.0 hour, more preferably 1.5 hours, more preferably 2.0 hours, and even more preferably 2.5 hours. The same applies to the following forms of this heat treatment.

<Second form of heat treatment>
A second form of the present heat treatment includes performing a second heat treatment (aging treatment, secondary heat treatment) on the heat treatment object after performing the first heat treatment. Performing the second heat treatment includes holding the heat treatment object at a second temperature range of 20 to 1080°C. Holding at the second temperature range includes holding the heat treatment object for a second time range of 0.3 to 48 hours.

In a second form of this heat treatment, a second heat treatment is performed on solute elements distributed within the crystal grains or at the grain boundaries of the ferrite phase. The second heat treatment involves maintaining the temperature in a second temperature range of 20 to 1080°C, which allows for control of the segregation state of the solute elements within the crystal grains or at the grain boundaries. This makes it possible, for example, to reduce the segregation of the solute elements or change the segregation tendency of the solute elements. As a result, the elongation of the iron casting can be further improved.

In addition, by setting the upper limit of the content of Ni, an austenitizing element, in the casting material to 0.6 mass %, and the content of Cr, a ferrite-forming element, to 16.0 to 21.0 mass %, the ferrite phase can be stabilized from room temperature to high temperature. This makes it easier to suppress the phase transformation that occurs during the second heat treatment. Therefore, it is possible to suppress the decrease in ductility that occurs due to the phase transformation.

When the second heat treatment is performed, there is a possibility that the C and N dissolved in the crystal grains may be reprecipitated as Nb carbonitrides. However, by setting the upper limit of the C content to 0.03 mass% and the upper limit of the N content to 0.03 mass%, it is possible to prevent an increase in the amount of reprecipitation of Nb carbonitrides that occurs during the second heat treatment. This makes it possible to prevent excessive reprecipitation of Nb carbonitrides within crystal grains and at grain boundaries. This in turn prevents a decrease in the ductility of the iron casting and also prevents the occurrence of weld cracks.

The upper limit of the second temperature range is preferably 1050°C, more preferably 1000°C, more preferably 950°C, more preferably 900°C, more preferably 850°C, more preferably 800°C, more preferably 750°C, more preferably 700°C, more preferably 650°C, more preferably 600°C, more preferably 550°C, and even more preferably 525°C. The lower limit of the second temperature range is preferably 50°C, more preferably 100°C, more preferably 150°C, more preferably 200°C, more preferably 250°C, more preferably 300°C, more preferably 350°C, more preferably 400°C, more preferably 450°C, and even more preferably 475°C.

The upper limit of the second time range is preferably 42 hours, more preferably 36 hours, more preferably 32 hours, more preferably 24 hours, more preferably 22 hours, more preferably 20 hours, more preferably 18 hours, more preferably 16 hours, more preferably 14 hours, more preferably 12 hours, and even more preferably 10 hours. The lower limit of the second time range 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, more preferably 4.0 hours, more preferably 4.5 hours, more preferably 5.0 hours, more preferably 5.5 hours, more preferably 6.0 hours, more preferably 6.5 hours, more preferably 7.0 hours, and even more preferably 7.5 hours.

By subjecting a heat-treatment object cast using this material to this heat treatment, an iron casting with improved elongation can be produced. Therefore, this iron casting is suitable for a wide variety of applications requiring excellent elongation properties. For example, in the case of exhaust system components (exhaust manifolds, turbine housings, etc.) for automobiles and other vehicles, it is necessary to suppress cracking (weld cracking) during welding between the cast components and steel components that make up the exhaust system. Therefore, this iron casting is suitable for the production of cast exhaust system components. Furthermore, this iron casting is suitable for a wide variety of applications requiring complex shapes and low thermal expansion and/or high ductility to withstand high-temperature strength and thermal fatigue. For example, it is suitable for the production of engine parts for aircraft and ships, and boilers and turbines for thermal power plants.

First Embodiment
The iron casting of the first embodiment is an iron casting obtained by subjecting a heat treatment object cast using the first embodiment of the present material to the first embodiment of the present heat treatment. That is, the iron casting of the first embodiment is obtained by subjecting a heat treatment object cast using the present material containing 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Si, 0.001 to 1.0 mass% Mn, 0.0001 to 0.04 mass% P, 0.0001 to 0.027 mass% S, 0.3 to 0.8 mass% Cu, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.3 to 0.8 mass% Nb, and 0.0001 to 0.03 mass% N, with the remainder being Fe and unavoidable elements, to a first heat treatment.

Second Embodiment
The iron casting of the second embodiment is an iron casting obtained by subjecting an object to heat treatment that has been cast using the second embodiment of the material to the first embodiment of the heat treatment. That is, the iron casting of the second embodiment is obtained by performing a first heat treatment on a heat treatment object that is cast using the material containing 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Si, 0.001 to 1.0 mass% Mn, 0.0001 to 0.04 mass% P, 0.0001 to 0.027 mass% S, 0.3 to 0.8 mass% Cu, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.3 to 0.8 mass% Nb, 0.0001 to 0.03 mass% N, and 0.1 to 0.3 mass% Ti, with the remainder being Fe and unavoidable elements.

Third Embodiment
An iron casting according to a third embodiment is an iron casting obtained by subjecting a heat treatment object cast using the first embodiment of the present material to the second embodiment of the present heat treatment. That is, the iron casting according to the third embodiment is obtained by subjecting a heat treatment object cast using the present material containing 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Si, 0.001 to 1.0 mass% Mn, 0.0001 to 0.04 mass% P, 0.0001 to 0.027 mass% S, 0.3 to 0.8 mass% Cu, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.3 to 0.8 mass% Nb, and 0.0001 to 0.03 mass% N, with the remainder being Fe and unavoidable elements, to a first heat treatment and then to a second heat treatment.

<Fourth embodiment>
The iron casting of the fourth embodiment is an iron casting obtained by subjecting an object to heat treatment that has been cast using the second embodiment of the material to the second embodiment of the present heat treatment. That is, the iron casting of the fourth embodiment is obtained by performing a first heat treatment on a heat treatment object cast using the material containing 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Si, 0.001 to 1.0 mass% Mn, 0.0001 to 0.04 mass% P, 0.0001 to 0.027 mass% S, 0.3 to 0.8 mass% Cu, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.3 to 0.8 mass% Nb, 0.0001 to 0.03 mass% N, and 0.1 to 0.3 mass% Ti, with the remainder being Fe and unavoidable elements, and then performing a second heat treatment on the heat treatment object.

In the first and second embodiments described above, in which the second heat treatment is not performed after the first heat treatment, the "heat-treated object" refers to the "iron casting after casting and before the first heat treatment." Furthermore, in the third and fourth embodiments, in which the second heat treatment is performed after the first heat treatment, the "heat-treated object" refers to the "iron casting after casting and before the first heat treatment" before the first heat treatment, and to the "iron casting after the first heat treatment and before the second heat treatment" before the second heat treatment. The heat-treated object may be an iron casting before or after machining. Note that in the examples described below of the above embodiments, the heat-treated object was not machined, nor was it subjected to plastic processing (hot processing, cold processing, etc.) such as forging or rolling.

<Example>
FIG. 1 shows the compositions of Examples 1 to 7 in the first embodiment and Example 8 in the second embodiment, and the compositions of Comparative Examples 1 and 2. FIG. 3A shows the compositions of Examples 9 to 43 in the third embodiment. FIG. 3B shows the compositions of Examples 44 to 77 in the third embodiment. FIG. 3C shows the compositions of Examples 78 to 105 in the fourth embodiment and the compositions of Comparative Examples 3 to 5. In FIGS. 1, 3A, 3B, and 3C, the content (mass%) of each element is a value measured by optical emission spectroscopy using an optical emission spectrometer "PDA-8000" manufactured by Shimadzu Corporation.

Figure 2 shows the heat treatment conditions and mechanical properties of Examples 1 to 7 of the first embodiment and Example 8 of the second embodiment, as well as the heat treatment conditions and mechanical properties of Comparative Examples 1 and 2. Figure 4A shows the heat treatment conditions and mechanical properties of Examples 9 to 43 of the third embodiment. Figure 4B shows the heat treatment conditions and mechanical properties of Examples 44 to 77 of the third embodiment. Figure 4C shows the heat treatment conditions and mechanical properties of Examples 78 to 105 of the fourth embodiment and the heat treatment conditions and mechanical properties of Comparative Examples 3 to 5. In Figures 2, 4A, 4B, and 4C, 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 at the corresponding holding temperature in the heat treatment furnace. The cooling method indicates the method for cooling the heat treatment object. For example, natural air cooling is a method in which a heat-treatment object is cooled naturally in air without using a cooling fan or the like after being removed from a heat treatment furnace. Forced air cooling is a method in which a heat-treatment object is quickly cooled in air using a cooling fan or the like after being removed from a heat treatment furnace. Furnace cooling is a method in which a heat-treatment object is gradually cooled in a heat treatment furnace. Oil cooling is a method in which a heat-treatment object is quickly cooled in oil. The tensile strength (MPa), 0.2% proof stress (MPa), and fracture elongation (%) were measured in accordance with JIS Z 2241 (Method of Tensile Testing for Metallic Materials) for test pieces of heat-treated iron castings. The test pieces used were 14A test pieces with a diameter of 8 mm and a parallel length of 48 mm, taken from Y-shaped B test pieces or knock-off test pieces cast by sand casting.

(Comparison between Examples 1 to 8 and Comparative Examples 1 and 2)
As shown in FIG. 2 , the holding temperature of the first heat treatment in Comparative Example 1 is below 750°C, while the holding temperatures of Examples 1 to 7 in the first embodiment and Example 8 in the second embodiment are 750°C or higher. Here, the fracture elongation of Comparative Example 1 is 8.42%, while the fracture elongation of Examples 1 to 8 is 12.03 to 17.95%. Therefore, the fracture elongation of Examples 1 to 5 is improved by approximately 1.43 to 2.13 times compared to Comparative Example 1. Thus, in Examples 1 to 8, the fracture elongation of the iron castings can be improved by setting the lower limit of the holding temperature of the first heat treatment to 750°C and the holding time to 0.5 to 6.0 hours. Furthermore, the holding temperature of the first heat treatment in Comparative Example 2 exceeds 1380°C, while the holding temperature of the first heat treatment in Examples 1 to 8 is 1380°C or lower. Here, the fracture elongation of Comparative Example 2 was 10.18%, while the fracture elongation of Examples 1 to 8 was 12.03 to 17.95%. Therefore, in Examples 1 to 8, the fracture elongation was improved by about 1.18 to 1.76 times compared to Comparative Example 2. In this way, in Examples 1 to 8, the fracture elongation of the iron castings was improved by setting the upper limit of the holding temperature of the first heat treatment to 1380°C and the holding time to 0.5 to 6.0 hours.

(Comparison of Examples 9 to 105 with Examples 1 to 8)
As shown in Figures 2, 4A, 4B, and 4C, Examples 1 to 8 did not undergo a second heat treatment after the first heat treatment (see Figure 2), whereas Examples 9 to 77 of the third embodiment and Examples 78 to 105 of the fourth embodiment underwent a second heat treatment after the first heat treatment (see Figures 4A, 4B, and 4C). The holding temperature for the second heat treatment in Examples 9 to 105 was 20 to 1080°C, and the holding time was 0.3 to 48 hours. The breaking elongations of Examples 1 to 8 were 12.03 to 17.95%, while the breaking elongations of Examples 9 to 77 were 18.50 to 38.95%. Therefore, the breaking elongations of Examples 9 to 77 were improved by a maximum of approximately 3.24 times (at least approximately 1.04 times) compared to Examples 1 to 8. In this way, in Examples 9 to 77, the holding temperature of the second heat treatment was set to 20 to 1080°C and the holding time was set to 0.3 to 48 hours, thereby further improving the fracture elongation of the iron castings.

(Comparison of Examples 9 to 77 with Comparative Example 3)
As shown in Figures 3A, 3B, and 3C, the S content of Comparative Example 3 is 0.029% by mass, while the upper limit of the S content is 0.027% by mass in Examples 9 to 77. As shown in Figures 4A, 4B, and 4C, the breaking elongation of Comparative Example 3 is 8.85%, while the breaking elongation of Examples 9 to 77 is 18.50 to 29.75%. Therefore, the breaking elongation of Examples 9 to 77 is improved by approximately 2.09 to 3.36 times compared to Comparative Example 3. In this way, by setting the upper limit of the S content to 0.027% by mass in Examples 9 to 77, the breaking elongation of the iron castings can be improved.

(Comparison of Examples 9 to 77 with Comparative Example 4)
As shown in Figures 3A, 3B, and 3C, Comparative Example 4 and Examples 9 to 77 all share the common feature of not containing Ti. On the other hand, as shown in Figures 4A, 4B, and 4C, the holding temperature of the second heat treatment in Comparative Example 4 exceeds 1080°C, while the holding temperature of the second heat treatment in Examples 9 to 77 is 1080°C or lower. The fracture elongation of Comparative Example 4 is 9.88%, while the fracture elongation of Examples 9 to 77 is 18.50 to 29.75%. Therefore, the fracture elongation of Examples 9 to 77 is improved by approximately 1.87 to 3.01 times compared to Comparative Example 4. Thus, in Examples 9 to 77, the fracture elongation of the iron castings can be improved by setting the upper limit of the holding temperature of the second heat treatment to 1080°C and the holding time to 0.3 to 48 hours.

(Comparison of Examples 78 to 105 with Comparative Example 5)
As shown in Figure 4C, the holding temperature of the second heat treatment in Comparative Example 5 exceeds 1080°C, while the holding temperature of the second heat treatment in Examples 78 to 105 is 1080°C or lower. Here, the breaking elongation of Comparative Example 5 is 18.50%, while the breaking elongation of Examples 78 to 105 is 21.98 to 38.95%. Therefore, the breaking elongation of Examples 78 to 105 is improved by approximately 1.19 to 2.11 times compared to Comparative Example 5. In this way, in Examples 78 to 105, the breaking elongation of the iron castings can be improved by setting the upper limit of the holding temperature of the second heat treatment to 1080°C and the holding time to 0.3 to 48 hours.

(Comparison of Examples 78 to 105 with Examples 9 to 77)
As shown in Figures 3A, 3B, and 3C, Examples 9 to 77 do not contain Ti, while Examples 78 to 105 contain Ti in an amount of 0.1 to 0.3 mass%. As shown in Figures 4A, 4B, and 4C, Examples 9 to 77 and Examples 78 to 105 share the same second heat treatment temperatures of 20 to 1080°C and holding times of 0.3 to 48 hours. The fracture elongations of Examples 9 to 77 ranged from 18.50 to 29.75%, while the fracture elongations of Examples 78 to 105 ranged from 21.98 to 38.95%. Therefore, in Examples 78 to 105, the maximum fracture elongation was approximately 1.31 times higher and the minimum fracture elongation was approximately 1.19 times higher than in Examples 9 to 77. In this way, in Examples 78 to 105, the Ti content was set to 0.1 to 0.3 mass%, the holding temperature of the second heat treatment was set to 20 to 1080°C, and the holding time was set to 0.3 to 48 hours, thereby making it possible to improve the maximum and minimum values of the fracture elongation of the iron castings.

As described above, the above embodiment can provide iron castings suitable for a wide variety of applications requiring excellent elongation properties. One aspect of the method for producing the iron casting of the above embodiment includes casting a heat-treatment object using a ferritic casting material, and performing a first heat treatment on the heat-treatment object after casting the heat-treatment object. Another aspect of the method for producing the iron casting of the above embodiment includes casting a heat-treatment object using a ferritic casting material, performing a first heat treatment on the heat-treatment object after casting the heat-treatment object, and performing a second heat treatment on the heat-treatment object after performing the first heat treatment. Performing the first heat treatment includes holding the heat-treatment object in a first temperature range of 750 to 1380°C. Holding in the first temperature range includes holding the heat-treatment object for a first time range of 0.5 to 6.0 hours. Performing the second heat treatment includes holding the heat-treatment object in a second temperature range of 20 to 1080°C. Holding the heat treatment object in the second temperature range includes holding the heat treatment object for a second time range of 0.3 to 48 hours.

Claims (4)

A method for producing an iron casting, comprising: performing a first heat treatment on a heat treatment object cast using a ferritic casting material,
The casting material contains 0.0001 to 0.03 mass% C, 0.001 to 1.0 mass% Si, 0.001 to 1.0 mass% Mn, 0.0001 to 0.04 mass% P, 0.0001 to 0.027 mass% S, 0.3 to 0.8 mass% Cu, 0.001 to 0.6 mass% Ni, 16.0 to 21.0 mass% Cr, 0.25 to 0.8 mass% Nb, and 0.0001 to 0.03 mass% N, with the remainder being Fe and unavoidable elements;
performing the first heat treatment includes holding the heat treatment object in a first temperature range of 750 to 1380°C;
The method for manufacturing an iron casting, wherein holding the heat treatment object in the first temperature range includes holding the heat treatment object in a first time range of 0.5 to 6.0 hours. In claim 1,
The method further includes performing a second heat treatment on the heat treatment object after performing the first heat treatment,
The method for manufacturing an iron casting, wherein the second heat treatment comprises holding the heat treatment object at a second temperature range of 50 to 1080°C. In claim 2,
The method for manufacturing an iron casting, wherein holding the heat treatment object in the second temperature range includes holding the heat treatment object in a second time range of 0.3 to 48 hours. In any one of claims 1 to 3,
The method for producing an iron casting, wherein the casting material further contains 0.1 to 0.3 mass % of Ti.