iron castings

Abstract

An iron casting obtained by carrying out a first heat treatment of an object to be heat-treated which has been cast using a ferrite-based casting material, wherein: the casting material contains 0.0001 to 0.03 mass% of C, 0.001 to 0.6 mass% of Ni, 16.0 to 21.0 mass% of Cr, and the balance Fe and inevitable elements; and the first heat treatment includes holding of the object to be heat-treated in a first temperature range of 750 to 1,380 ˚C.

Description

[Technical Field]

[0001] The present invention relates to iron castings produced using ferritic casting materials. [Background technology]

[0002] The abstract of Patent Document 1 describes that a heat-resistant cast steel having excellent thermal fatigue resistance is provided. The abstract also describes 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%, with the balance being substantially Fe, and that the cast structure of the matrix is ​​a multiphase structure consisting of ferrite and pearlite, thereby extending the thermal fatigue life of the heat-resistant cast steel and improving its room-temperature elongation, thereby improving its thermal fatigue resistance. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 7-62500 Summary of the Invention [Problem to be solved by the invention]

[0004] To provide an iron casting with improved elongation using a ferritic casting material. [Means for solving the problem]

[0005] 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 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.

[0006] In the above-described embodiment, a casting material containing 0.001 to 0.6% by mass of Ni and 16.0 to 21.0% by mass of Cr is used for casting, thereby causing a ferrite phase to crystallize as a 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 (hereinafter referred to as "inside the crystal grains") that constitute the ferrite phase, 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 the 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 object in the first temperature range preferably includes holding the object for heat treatment in a first time range of 0.5 to 6.0 hours.

[0007] 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. The second heat treatment is performed on solute elements distributed within ferrite grains or at grain boundaries (hereinafter referred to as "grain boundaries"), and the solute elements are controlled by holding the second heat treatment at a second temperature range of 20 to 1080°C. This, for example, makes it easy to reduce the segregation of solute elements or change the segregation tendency of solute elements. As a result, the elongation of the iron casting is easily improved. 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.

[0008] The casting material preferably 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 solidification segregation of N is reduced, and by subjecting the small amount of N that has solidified and segregated within the crystal grains to the first heat treatment, the segregation of N within the crystal grains can be reliably reduced.

[0009] 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, it is possible to suppress the decrease in ductility associated with the phase transformation.

[0010] The casting material preferably 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 crystal grain boundaries in the ferrite phase can be increased. This makes it easier to reduce the segregation concentration of solute elements at the crystal grain boundaries. This, in turn, makes it easier to improve the elongation of the iron casting.

[0011] Another aspect of the present invention is an iron casting obtained by performing a first heat treatment on an object to be heat-treated that has been cast using a ferritic casting material, the first heat treatment comprising holding the object to be heat-treated in a first temperature range of 750 to 1380°C. The iron casting is preferably obtained by performing a second heat treatment on the object to be heat-treated after the first heat treatment. The second heat treatment preferably comprises holding the object to be heat-treated in a second temperature range of 20 to 1080°C. [Brief explanation of the drawings]

[0012] [Figure 1] 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. [Figure 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. [Figure 3A] FIG. 3A is a diagram showing the compositions of the third embodiment (Examples 9 to 43). [Figure 3B] FIG. 3B is a diagram showing the compositions of the third embodiment (Examples 44 to 77). [Figure 3C] 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. [Figure 4A] FIG. 4A is a diagram showing the heat treatment conditions and mechanical properties of the third embodiment (Examples 9 to 43). [Figure 4B] FIG. 4B is a diagram showing the heat treatment conditions and mechanical properties of the third embodiment (Examples 44 to 77). [Figure 4C] 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. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the iron casting disclosed in the present application will be described 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.

[0014] The material used to produce the iron casting according to the embodiment of the present invention is a ferritic casting material. "Ferritic casting material" refers to a casting material having a ferrite phase as the main phase. For example, the proportion of the ferrite phase in the entire structure is 50% or more. The proportion of the ferrite phase in the entire structure is preferably 70% or more, more preferably 80% or more, even more preferably 85% or more, even more preferably 90% or more, even more preferably 92.5% or more, even more preferably 95% or more, even more preferably 97.5% or more, and even more preferably 99% or more.

[0015] In the embodiments of the present invention, "casting" includes casting by various casting methods such as sand casting, metal 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 ferritic casting material. For example, the expression "X to Y mass % of elements" means that the mass % of the element is X% or more and Y% or less. The "balance" refers to components other than the listed elements that make up the ferritic casting material. Therefore, for example, the expression "including...C,...Ni,...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. Furthermore, the expression "X to Y °C" in the heat treatment conditions means that the holding temperature is X°C or more and Y°C or less, and the expression "X to Y hours" means that the holding time is X hours or more and Y hours or less.

[0016] <First form 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.

[0017] (C: carbon) A first embodiment of the present material contains 0.0001 to 0.03 mass% C. In the first embodiment of the present material, setting the upper limit of the C content to 0.03 mass% allows C and N to be fixed 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. Setting the upper limit of the C content to 0.03 mass% also prevents the precipitation of austenite at high temperatures. This reduces the proportion of austenite, which tends to slow the diffusion rate of Cr, 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. Setting the upper limit of the C content to 0.03 mass% also prevents excessive formation of Nb carbonitrides while fixing C and N as Nb carbonitrides. This prevents excessive precipitation of Nb carbonitrides within grains and at grain boundaries. Therefore, it is possible to suppress a decrease in the ductility of the iron casting and to suppress the occurrence of weld cracks. Furthermore, it is possible to suppress the excessive generation of Nb carbonitrides, thereby suppressing a 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 a decrease in the corrosion resistance and embrittlement of the iron casting. The same applies to the following embodiments of this material.

[0018] (Si: Silicon) A first embodiment of the present material contains 0.001 to 1.0 mass% Si. In the first embodiment of the present 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. Setting the upper limit of the Si content to 1.0 mass% can also 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%. This allows non-metallic inclusions such as silicon dioxide to 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. This suppresses excessive growth of Nb carbonitrides within the crystal grains, thereby suppressing brittle fracture of the iron casting. 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.

[0019] (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 content of Mn, an austenitizing element, to 1.0 mass% can suppress the precipitation of the austenite phase. This can stabilize the ferrite phase 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 portion. 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 the Nb carbonitrides within the crystal grains.

[0020] (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 therefore prevents a decrease in the ductility of the iron casting and also prevents 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.

[0021] (S: sulfur) A first embodiment of the material contains 0.0001 to 0.027 mass% S. In the first embodiment of the material, setting the upper limit of the S content to 0.027 mass% can suppress the excessive formation of nonmetallic 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 nonmetallic 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 nonmetallic inclusions such as manganese sulfide that precipitate at grain boundaries. The same applies to the following embodiments of the material.

[0022] (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.

[0023] (Ni: Nickel) A first embodiment of the material contains 0.001 to 0.6 mass% Ni. In this first embodiment of the material, the precipitation of austenite phase can be suppressed by setting the upper limit of the content of Ni, an austenitizing element, to 0.6 mass%. 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 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%. This makes it possible to improve the corrosion resistance of iron castings while suppressing an increase in alloy cost. The same applies to the following embodiments of the material.

[0024] (Cr: chromium) A first embodiment of the material contains 16.0 to 21.0 mass% Cr. In the first embodiment of the material, by setting the content of Cr, a ferrite 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.

[0025] (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 a decrease in the 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.

[0026] (N: nitrogen) A first embodiment of the material contains 0.0001 to 0.03 mass% N. In the 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. Setting the upper limit of the N content to 0.03 mass% also prevents the precipitation of austenite at high temperatures. This reduces the proportion of austenite, which tends to slow the diffusion rate of Cr, 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 the occurrence of weld cracks. Furthermore, suppressing the excessive formation of Nb carbonitrides also prevents a decrease in the amount of Nb dissolved in the ferrite phase. 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, deterioration in corrosion resistance and embrittlement of the iron casting can be further suppressed. The same applies to the following embodiments of the material.

[0027] (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.

[0028] <Second form 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, with the remainder being Fe and unavoidable elements.

[0029] (Ti: Titanium) A second embodiment of the material contains 0.1 to 0.3 mass% Ti. In this second embodiment, the Ti content is 0.1 to 0.3 mass% to refine the grains and increase the proportion of grain boundaries in the ferrite phase. This facilitates a reduction in the segregation concentration of solute elements at the grain boundaries. This in turn facilitates improving the elongation of the iron casting. Setting the lower limit of the Ti content to 0.1 mass% facilitates grain refinement. Setting the upper limit of the Ti content to 0.3 mass% prevents the reduction in ductility of the iron casting due to excessive solid solution strengthening and excessive formation of Ti oxides, and also prevents the occurrence of weld cracks. Furthermore, setting the upper limit of the C content to 0.03 mass% and the upper limit of the N content to 0.03 mass%, while also limiting the Ti content to 0.1 to 0.3 mass%, prevents the excessive formation of Ti nitrides and Ti carbonitrides. This in turn prevents excessive precipitation of Ti nitrides and Ti carbonitrides within the grains and at the grain boundaries. Therefore, it is possible to prevent the decrease in ductility of iron castings and the occurrence of weld cracks. In addition, it is possible to prevent the excessive formation of Ti nitrides and Ti carbonitrides, which prevents the decrease in the amount of Ti dissolved in the ferrite phase. Therefore, it is possible to improve the high-temperature strength of iron castings.

[0030] Note that the present material is not limited to the above-mentioned configuration. For example, the present 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 the present 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 the present 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 the 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 the 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, 0.1 to 0.3 mass% Ti, with the balance being Fe and unavoidable elements.

[0031] <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 at a first temperature range of 750 to 1380°C. Holding the heat-treatment object at the first temperature range includes holding the heat-treatment object for a first time range of 0.5 to 6.0 hours.

[0032] First, before performing the first heat treatment, a casting material containing 0.001 to 0.6 mass% Ni and 16.0 to 21.0 mass% Cr is cast to crystallize a ferrite phase as a main phase in the heat-treated object. Next, in a first embodiment of this heat treatment, when the liquid phase solidifies during the ferrite phase crystallization process, a first heat treatment is performed on solute elements dissolved in a non-uniform concentration distribution within the crystal grains that constitute the ferrite phase. The first heat treatment involves maintaining the heat-treated object at 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. As a result, the elongation of the iron casting can be improved. The same applies to the following embodiments of this heat treatment.

[0033] Furthermore, in the casting material, by setting the upper limit of the content of Ni, an austenitizing element, to 0.6 mass %, and the content of Cr, a ferrite element, to 16.0 to 21.0 mass %, the ferrite phase can be stabilized from room temperature to high temperature. This makes it easy 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 the phase transformation. The same applies to the following embodiment of this heat treatment.

[0034] Furthermore, by casting a casting material with a C content of 0.0001 to 0.03 mass% and an N content of 0.0001 to 0.03 mass% before the first heat treatment, the amount of solidification segregation of C and N in the heat-treated object can be reduced. Next, in a first embodiment of this heat treatment, the first heat treatment is performed on the C and N that have solidified and segregated within the crystal grains, even if in small amounts. This makes it possible to reliably reduce the segregation of C and N within the crystal grains. The same applies to the following embodiments of this heat treatment.

[0035] 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 embodiments of this heat treatment.

[0036] 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 embodiments of this heat treatment.

[0037] <Second form of heat treatment> A second form of this heat treatment involves carrying out a second heat treatment (aging treatment, secondary heat treatment) on the heat treatment object after carrying out the first heat treatment. Carrying out 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.

[0038] In a second embodiment 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 range of 20 to 1080°C, thereby controlling the segregation state of the solute elements within the crystal grains or at the grain boundaries. This can, for example, 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.

[0039] Furthermore, in the casting material, by setting the upper limit of the content of Ni, an austenitizing element, 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 phase transformation associated with the second heat treatment. Therefore, it is possible to suppress the decrease in ductility associated with phase transformation.

[0040] During the second heat treatment, C and N dissolved in the 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 due to the second heat treatment. This makes it possible to prevent excessive reprecipitation of Nb carbonitrides within the grains and at the grain boundaries. This in turn makes it possible to prevent a decrease in the ductility of the iron casting and to suppress the occurrence of weld cracks.

[0041] 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.

[0042] 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.

[0043] By subjecting a heat treatment object cast using this material to this heat treatment, an iron casting with improved elongation can be provided. Therefore, this iron casting is suitable for a wide variety of applications requiring excellent elongation properties. For example, in exhaust system components (exhaust manifolds, turbine housings, etc.) for automobiles and the like, it is necessary to suppress cracking (weld cracking) during welding between the cast parts and steel parts that make up the exhaust system. Therefore, this iron casting is suitable for manufacturing 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 also suitable for manufacturing engine parts for aircraft and ships, and boilers and turbines for thermal power plants.

[0044] 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.

[0045] <Second embodiment> The iron casting of the second embodiment is an iron casting obtained by subjecting a heat treatment object cast using the second embodiment of the present material to the first embodiment of the present heat treatment. That is, the iron casting of the second 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, 0.0001 to 0.03 mass% N, and 0.1 to 0.3 mass% Ti, with the remainder being Fe and unavoidable elements, to a first heat treatment.

[0046] <Third embodiment> The iron casting of the 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 of 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.

[0047] <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 of the present invention 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-treated object cast using the present material, which 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, with the remainder being Fe and unavoidable elements, and then performing a second heat treatment on the heat-treated object.

[0048] Among the above embodiments, the "heat treatment object" in the first and second embodiments in which the second heat treatment is not performed after the first heat treatment means "an iron casting after casting and before the first heat treatment." Furthermore, the "heat treatment object" in the third and fourth embodiments in which the second heat treatment is performed after the first heat treatment means "an iron casting after casting and before the first heat treatment" before the first heat treatment, and means "an iron casting after the first heat treatment and before the second heat treatment" before the second heat treatment. The heat treatment object may be an iron casting before or after machining. Note that in each example of the above embodiments described later, the heat treatment object was not subjected to machining, nor was it subjected to plastic processing (hot processing, cold processing, etc.) such as forging or rolling.

[0049] <Example> FIG. 1 shows the compositions of Examples 1 to 7 in the first embodiment and Example 8 in the second embodiment, as well as 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.

[0050] FIG. 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. FIG. 4A shows the heat treatment conditions and mechanical properties of Examples 9 to 43 of the third embodiment. FIG. 4B shows the heat treatment conditions and mechanical properties of Examples 44 to 77 of the third embodiment. FIG. 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 FIGS. 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 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-treated object is cooled naturally in the air without the use of a cooling fan after being removed from a heat treatment furnace. Forced air cooling is a method in which a heat-treated object is quickly cooled in the air using a cooling fan after being removed from a heat treatment furnace. Furnace cooling is a method in which a heat-treated object is gradually cooled in the heat treatment furnace. Oil cooling is a method in which a heat-treated object is quickly cooled in oil. The tensile strength (MPa), 0.2% yield strength (MPa), and fracture elongation (%) were measured in accordance with JIS Z 2241 (Method of tensile testing for metallic materials) for heat-treated iron casting test pieces. The test pieces used were No. 14A test pieces with a diameter of 8 mm and a parallel length of 48 mm, taken from Y-type No. B test pieces or knock-off type test pieces cast by sand casting.

[0051] (Comparison of Examples 1 to 8 with Comparative Examples 1 and 2) As shown in FIG. 2, the holding temperature of the first heat treatment in Comparative Example 1 is less than 750°C, whereas the holding temperatures of the first heat treatment in Examples 1 to 7 of the first embodiment and Example 8 of the second embodiment are 750°C or higher. Here, the breaking elongation of Comparative Example 1 is 8.42%, whereas the breaking elongation of Examples 1 to 8 is 12.03 to 17.95%. Therefore, the breaking 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 breaking 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, whereas the holding temperature of the first heat treatment in Examples 1 to 8 is 1380°C or lower. Here, the breaking elongation of Comparative Example 2 was 10.18%, while the breaking elongation of Examples 1 to 8 was 12.03 to 17.95%. Therefore, the breaking elongation of Examples 1 to 8 was improved by about 1.18 to 1.76 times compared to Comparative Example 2. In this way, in Examples 1 to 8, the breaking elongation of the iron castings could be 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.

[0052] (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 those 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. Thus, 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 making it possible to further improve the fracture elongation of the iron castings.

[0053] (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.

[0054] (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 have in common the lack of 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 approximately 1.87 to 3.01 times higher than that of 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.

[0055] (Comparison of Examples 78 to 105 with Comparative Example 5) As shown in FIG. 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 upper limit of the holding temperature of the second heat treatment is set to 1080°C and the holding time is set to 0.3 to 48 hours, thereby improving the breaking elongation of the iron castings.

[0056] (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 a range 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 elongations at break for Examples 9 to 77 ranged from 18.50 to 29.75%, while those for Examples 78 to 105 ranged from 21.98 to 38.95%. Therefore, the maximum elongations at break for Examples 78 to 105 were approximately 1.31 times higher and the minimum elongations at break were approximately 1.19 times higher than those for Examples 9 to 77. Thus, in Examples 78 to 105, the maximum and minimum values ​​of the fracture elongation of the iron castings can be improved by setting the Ti content to 0.1 to 0.3 mass%, setting the holding temperature of the second heat treatment to 20 to 1080°C, and setting the holding time to 0.3 to 48 hours.

[0057] As described above, the above-described embodiment can provide iron castings suitable for a wide variety of applications requiring excellent elongation characteristics. One aspect of the method for producing the iron casting of the above-described 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-described 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 at a first temperature range of 750 to 1380°C. Holding the heat-treatment object at 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 at 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 in a second time range of 0.3 to 48 hours.

Claims

1. 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.

2. 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 performing the second heat treatment includes holding the heat treatment object at a second temperature range of 50 to 1080°C.

3. 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.

4. 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.

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