Compacted vermicular graphite cast iron, and heat-treated product of compacted vermicular graphite cast iron and method for producing the same

Abstract

To provide CV graphite cast iron that is not expensive and exhibits high strength from the surface to the internal region, a heat-treated product of CV graphite cast iron, and a method for producing the same.SOLUTION: This CV graphite cast iron is composed of graphite, carbides, and a matrix, and contains, based on the total amount of the CV graphite cast iron, 3.50 to 3.90 mass% of C, 1.50 to 2.00 mass% of Si, 0.85 to 1.15 mass% of Mn, 0.025 mass% or less of P, 0.025 mass% or less of S, 0.006 to 0.013 mass% of Mg, 1.65 to 1.85 mass% of Cu, 0.60 to 0.80 mass% of Ni, 0.45 to 1.00 mass% of Mo, 0.20 to 0.40 mass% of V, 0.20 to 0.40 mass% of Cr, the balance being Fe and inevitable impurities. The carbon equivalent, defined as the total of C and one-third of Si, is 4.00 to 4.50 mass%.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to compacted vermicular graphite cast iron, as well as heat-treated compacted vermicular graphite cast iron and a method for producing the same. [Background technology]

[0002] Japanese Patent Application Laid-Open No. 2016-17208 (Patent Document 1) discloses a press die material having a nitride layer on the surface of a base made of CV graphite cast iron consisting of, by mass%, C: 3.0 to 3.9, Si: 1.5 to 2.5, Mn: 0.2 to 0.8, P: 0.02 or less, S: 0.02 or less, Mg: 0.01 to 0.02 or less, Cr: 0.1 to 1.5, Al: 0.05 to 1.0, unavoidable impurities, and the remainder iron (Fe).

[0003] Japanese Patent Laid-Open Publication No. 2006-104573 (Patent Document 2) discloses a martensitic cast material containing 5 to 15 mass% nickel (Ni), 0.05 to 5 mass% manganese (Mn), 0.07 to 7 mass% silicon (Si), 0.15 to 4 mass% carbon (C), with the balance being iron (Fe) and unavoidable impurities, and having a martensitic transformation completion temperature (Mf point) below freezing point.

[0004] JP 2014-214343 A (Patent Document 3) discloses a cast iron casting containing, by weight, 1.8-2.5% C, 1.0-2.0% Si, 0.2-1.5% Mn, 2.0-4.0% Ni, 1.5-2.5% Mo, 0.01-0.1% Mg, 0-0.8% Cr, and 0-0.3% Al, in which 60% or more of the matrix is ​​occupied by martensite phase and in which at least spheroidal graphite is dispersed in the matrix.

[0005] Japanese Patent Laid-Open Publication No. 2000-256776 (Patent Document 4) discloses a vehicle brake disc material made of CV graphite cast iron having a chemical composition containing 3.6-4.2% C, 1.8-2.4% Si, 0.5-0.7% Mn, 0.03% or less P, 0.03% or less S, and 0.004-0.015% Mg, which further contains 1.0-3.0% Ni and 0.3-0.7% Mo, with the Cr and Cu contents set to 0.03% or less, and which is subjected to a specified heat treatment to ferrite the base structure so that the ferrite ratio exceeds 70%. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2016-17208 A [Patent Document 2] JP 2006-104573 A [Patent Document 3] JP 2014-214343 A [Patent Document 4] JP 2000-256776 A Summary of the Invention [Problem to be solved by the invention]

[0007] The press die material disclosed in Patent Document 1 has a problem that the hardness of the CV graphite cast iron, which is the base part inside, is low. The martensitic cast material disclosed in Patent Document 2 has a problem that it is expensive because a large amount of expensive Ni is added, 5 to 15 mass %, and a complex heat treatment called sub-zero treatment is required for martensitic transformation of the base. The cast iron casting disclosed in Patent Document 3 has a problem that it is high in strength, but is expensive because a large amount of expensive Ni and Mo is added, 2.0 to 4.0 weight % and 1.5 to 2.5 weight %, respectively. In addition, the vehicle brake disc material disclosed in Patent Document 4 has a problem that the hardness is insufficient even if quenched because the Cu content is low, 0.03% or less.

[0008] The present disclosure aims to solve the above problems and provide compacted vermicular graphite cast iron that is inexpensive and has high strength from the surface to the interior, a heat-treated product of compacted vermicular graphite cast iron, and a manufacturing method thereof. [Means for solving the problem]

[0009] (1) According to an embodiment, the present disclosure provides a compacted vermicular graphite cast iron comprising graphite, carbides, and a matrix, the compacted vermicular graphite cast iron having, relative to a total amount of the compacted vermicular graphite cast iron, 3.50% by mass or more and 3.90% by mass or less of carbon, 1.50% by mass or more and 2.00% by mass or less of silicon, 0.85% by mass or more and 1.15% by mass or less of manganese, 0.025% by mass or less of phosphorus, 0.025% by mass or less of sulfur, 0.006% by mass or more and 0.013% by mass or less of magnesium, 1.65% by mass or less of copper, and 0.025% by mass or less of magnesium. % or more and 1.85% or less, nickel at 0.60% by mass or more and 0.80% by mass or less, molybdenum at 0.45% by mass or more and 1.00% by mass or less, vanadium at 0.20% by mass or more and 0.40% by mass or less, chromium at 0.20% by mass or more and 0.40% by mass or less, and the remainder being iron and unavoidable impurities, and the carbon equivalent, defined as the sum of carbon and 1 / 3 times the amount of silicon, is 4.00% by mass or more and 4.50% by mass or less.

[0010] (2) In the compacted vermicular graphite cast iron disclosed in (1) above, the spheroidization rate of the graphite can be set to 20% or more and 70% or less.

[0011] (3) According to another aspect, the present disclosure provides a heat-treated compacted vermicular graphite cast iron disclosed in (1) or (2) above, in which an area ratio of the carbides to the entire cross section (hereinafter also referred to as the carbide area ratio) in an arbitrarily specified cross section of the heat-treated product is 4.5% or more and 8.0% or less.

[0012] (4) According to another aspect, the present disclosure provides a heat-treated compacted vermicular graphite cast iron disclosed in (1) or (2) above, in which an area ratio of the retained austenite phase to the matrix in an arbitrarily specified cross section of the heat-treated product (hereinafter also referred to as the retained austenite phase area ratio) is 15% or less.

[0013] (5) According to another aspect of the present disclosure, there is provided a heat-treated compacted vermicular graphite cast iron as disclosed in (1) or (2) above, the heat-treated compacted vermicular graphite cast iron having a Brinell hardness of 550 HBW or more.

[0014] (6) According to still another aspect, the present disclosure provides a method for producing a heat-treated compacted vermicular graphite cast iron, comprising the steps of: preparing an ingot of compacted vermicular graphite cast iron as disclosed in (1) or (2) above; quenching the ingot by heating the ingot to a temperature of 810°C or higher and 830°C or lower, followed by water cooling or oil cooling; and tempering the quenched ingot by heating the ingot to a temperature of 325°C or lower, followed by air cooling or furnace cooling. Effect of the Invention

[0015] According to the present disclosure, it is possible to provide compacted vermicular graphite cast iron that is inexpensive and has high strength from the surface to the interior, a heat-treated product of compacted vermicular graphite cast iron, and a manufacturing method thereof. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is an optical microscope observation result showing an example of compacted vermicular graphite cast iron according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a flow chart showing an example of a method for producing a heat-treated compacted vermicular graphite cast iron product according to another embodiment of the present disclosure. [Diagram 3]FIG. 3 is a graph showing an example of the relationship between the Mo (molybdenum) content and the carbide area ratio in a heat-treated compacted vermicular graphite cast iron according to still another embodiment of the present disclosure. [Figure 4] FIG. 4 is a graph showing an example of the relationship between the Ni (nickel) content and the A3 transformation point in a heat-treated compacted vermicular graphite cast iron according to still another embodiment of the present disclosure. [Diagram 5] FIG. 5 is a graph showing an example of the relationship between the quenching temperature and the Brinell hardness in a heat-treated compacted vermicular graphite cast iron according to still another embodiment of the present disclosure. [Figure 6] FIG. 6 is a graph showing an example of the relationship between the quenching temperature and the retained austenite phase area fraction in a heat-treated compacted vermicular graphite cast iron according to still another embodiment of the present disclosure. [Figure 7] FIG. 7 is a graph showing an example of the relationship between the tempering temperature and the Brinell hardness in a heat-treated compacted vermicular graphite cast iron according to still another embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] <Embodiment 1> Compacted vermicular graphite cast iron (hereinafter also referred to as CV graphite cast iron) according to an embodiment of the present disclosure, referring to FIG. 1 , is CV graphite cast iron 10 consisting of graphite 12, carbide 14, and matrix 16, and the CV graphite cast iron 10 contains, with respect to the total amount of the CV graphite cast iron 10, 3.50 mass% or more of C (carbon) and 3.90 mass% or less, 1.50 mass% or more of Si (silicon) and 2.00 mass% or less, 0.85 mass% or more of Mn (manganese) and 0.025 mass% or less of P (phosphorus), 0.025 mass% or less of S (sulfur), and 0.025 mass% or less of Mg (magnesium). 0.006% by mass to 0.013% by mass, Cu (copper) is 1.65% by mass to 1.85% by mass, Ni (nickel) is 0.60% by mass to 0.80% by mass, Mo (molybdenum) is 0.45% by mass to 1.00% by mass, V (vanadium) is 0.20% by mass to 0.40% by mass, Cr (chromium) is 0.20% by mass to 0.40% by mass, the balance is Fe (iron) and inevitable impurities, and the carbon equivalent defined as the sum of C (carbon) and 1 / 3 times the amount of Si (silicon) is 4.00% by mass to 4.50% by mass. The CV graphite cast iron of this embodiment has the above structure and chemical composition, so it is not expensive and has high strength from the surface to the inside.

[0018] [structure] As shown in FIG. 1, the CV graphite cast iron 10 of this embodiment is composed of graphite 12, carbides 14, and a matrix 16. As described later, the CV graphite cast iron of this embodiment has a C content of 3.50 mass% or more and 3.90 mass% or less, so that some C that cannot be dissolved in the matrix 16 together with other elements (for example, elements contained relatively in large amounts such as Si, Mn, Cu, Ni, Mo, V, and Cr in addition to the main component Fe) crystallizes or precipitates from the matrix 16 to form graphite 12 and carbides 14. The carbides 14 are carbides formed by combining C with the other elements. As shown in FIG. 1, the CV graphite cast iron 10 has graphite 12 and carbides 14 scattered within the continuous matrix 16. The distribution of the graphite 12 and carbides 14 scattered in an arbitrarily specified cross section of the CV graphite cast iron 10 is almost uniform.

[0019] [Chemical composition] The CV graphite cast iron of this embodiment contains predetermined amounts of C, Si, Mn, P, S, Mg, Cu, Ni, Mo, V, and Cr, with the balance being Fe and inevitable impurities, and has a carbon equivalent (CE value) defined as the sum of C and 1 / 3 times Si of 4.00% by mass to 4.50% by mass. The type and content of each element are measured by wet chemical analysis.

[0020] (C) The C content in the CV graphite cast iron of the present embodiment is 3.50 mass % or more and 3.90 mass % or less, from the viewpoint of forming the appropriate structure and amount of graphite 12 and carbide 14.

[0021] (Si) The Si content in the CV graphite cast iron of the present embodiment is 1.50 mass % or more and 2.00 mass % or less from the viewpoint of forming the appropriate structure and amount of graphite 12 and carbide 14.

[0022] (Carbon equivalent: CE value) The carbon equivalent (CE value) of the CV graphite cast iron of this embodiment is: CE value (mass%) = C (mass%) + Si (mass%) / 3 (1) and is 4.00% by mass or more and 4.50% by mass or less from the viewpoint of forming an appropriate structure and amount of graphite 12 and carbide 14. The CE value is sometimes defined by further adding the term P (% by mass) / 3 to the right side of the above formula (1), but in the CV graphite cast iron of this embodiment, the P content is a trace amount of 0.025% by mass or less as described later, so the term P (% by mass) / 3 can be omitted.

[0023] (Mn) The Mn content in the CV graphite cast iron of this embodiment fixes and renders S harmless, increasing the strength of the cast iron and increasing A. 3 The content is 0.85 mass % or more from the viewpoint of lowering the transformation point (austenitization temperature, unit: ° C.), and is 1.15 mass % or less from the viewpoint of reducing the residual austenite in the heat treatment described later.

[0024] (P) The P content in the CV graphite cast iron of the present embodiment is not particularly limited, but from the viewpoint of reducing impurities, it is, for example, 0.025 mass % or less.

[0025] (S) The S content in the CV graphite cast iron of this embodiment is 0.025% by mass or less, preferably 0.010% by mass or less, from the viewpoint of suppressing inhibition of spheroidization of graphite. This is because S combines with Mg to form MgS, thereby reducing the graphite spheroidization rate.

[0026] (Mg) The Mg content in the CV graphite cast iron of this embodiment is 0.006 mass% or more and 0.013 mass% or less, preferably 0.007 mass% or more and preferably 0.011 mass% or less, from the viewpoint of maintaining a graphite spheroidization rate suitable for CV graphite cast iron (for example, 20% or more and 70% or less).

[0027] (Cu) The Cu content in the CV graphite cast iron of this embodiment is 1.65 mass% or more from the viewpoint of suppressing chilling of the CV graphite cast iron and improving hardenability during heat treatment, and is 1.85 mass% or less from the viewpoint of taking into account solid solubility in austenite and suppressing a decrease in the carbide area ratio.

[0028] (Ni) The Ni content in the CV graphite cast iron of this embodiment improves the hardenability during heat treatment and also reduces the A content in the heat treatment described below. 3 From the viewpoint of lowering the transformation point (austenitization temperature), the Ni content is 0.60 mass % or more, and from the viewpoint of reducing the amount of expensive Ni used to reduce costs, the Ni content is, for example, 0.80 mass % or less.

[0029] (Mo) The Mo content in the CV graphite cast iron of this embodiment is 0.45 mass% or more from the viewpoint of increasing the carbide area ratio to 4.5% or more to enhance the wear resistance of the CV graphite cast iron, and is 1.00 mass% or less from the viewpoint of suppressing the carbide area ratio to 8.0% or less to prevent cracks from occurring inside the CV graphite cast iron. In addition, since the content of expensive Mo is 1.00 mass% or less, costs can be reduced.

[0030] (V) The V content in the CV graphite cast iron of this embodiment is 0.20% by mass or more from the viewpoint of refining the matrix structure. However, since excessive addition of V leads to a decrease in the toughness of the matrix, the V content is 0.40% by mass or less from the viewpoint of suppressing the decrease in the toughness of the CV graphite cast iron.

[0031] (Cr) The Cr content in the CV graphite cast iron of this embodiment is 0.20 mass% or more from the viewpoints of improving hardenability during heat treatment and promoting carbide formation, and is 0.40 mass% or less from the viewpoints of suppressing excessive carbide formation and preventing cracks from occurring inside the CV graphite cast iron.

[0032] [Graphite nodularity rate] The spheroidal ratio of the graphite 12 in the CV graphite cast iron 10 of this embodiment (hereinafter also referred to as the graphite spheroidal ratio) is preferably 20% or more and 70% or less. The graphite spheroidal ratio is preferably 20% or more from the viewpoint of increasing the strength of the CV graphite cast iron, and is preferably 70% or less from the viewpoint of increasing the castability of the CV graphite cast iron.

[0033] Here, the graphite spheroidization rate is the graphite spheroidization rate R described in "Appendix B (Regulation) Evaluation of the graphite spheroidization rate of CV graphite cast iron products" of JIS G5505:2013. sg Specifically, it is calculated as follows:

[0034] First, in the graphite 12 shown in FIG. 1, the major axis length L m Area A of a circle with a diameter of (the maximum distance between two points on the circumference of a graphite particle) mConsidering the area A of the graphite particle, the roundness factor R of the graphite particle is R=A / A m =(4×A) / (π×L m 2 ) ···(2) It is calculated as follows.

[0035] Next, all the target graphite particles N All The roundness coefficient of each of the graphite particles is calculated based on the above formula (2). Based on the classification of graphite particles by the roundness coefficient and the shape coefficient shown in Table 1, the graphite particles are classified into graphite types I to VI. The number of graphite particles classified into each type, N I ~N VI is calculated.

[0036] [Table 1]

[0037] Next, the graphite spheroidization rate R is calculated using the following formula (3). sg (%) is calculated. R sg (%)=(0.0N I +0.05N II +0.20N III +0.40N IV +0.90N V +1.0N VI ) / N All ×100 (3) In equation (3), N I : Number of graphite particles classified as type I, N II : Number of graphite particles classified as type II, N III : Number of graphite particles classified as type III, N IV : Number of graphite particles classified as type IV, N V : Number of graphite particles classified as type V, N VI : The number of graphite particles classified as type VI, and N All : represents the number of all graphite particles under consideration.

[0038] <Embodiment 2> A heat-treated product of compacted vermicular graphite cast iron according to another embodiment of the present disclosure (hereinafter also referred to as a heat-treated product of CV graphite cast iron) is the heat-treated product of the CV graphite cast iron of embodiment 1, and has the following (i) to (iii): (i) In an arbitrarily specified cross section of the heat-treated product, the area ratio of carbides to the entire cross section (carbide area ratio) is 4.5% or more and 8.0% or less. (ii) In any specified cross section of the heat-treated product, the area ratio of the retained austenite phase to the matrix (hereinafter also referred to as the retained austenite phase area ratio) is 15% or less. (iii) The Brinell hardness of the heat-treated product is 550 HBW or more. The heat-treated CV graphite cast iron of this embodiment has the above-mentioned structure and chemical composition of the CV graphite cast iron of embodiment 1, preferably further has the characteristics of the graphite spheroidization rate, and also has at least one of the characteristics (i) to (iii), so it is inexpensive and has high strength from the surface to the inside.

[0039] [Carbide area ratio] In the present embodiment, the heat-treated CV graphite cast iron has a carbide area ratio of 4.5% or more and 8.0% or less, so that the strength is high from the surface to the inside, and the wear resistance can be expected to be improved. Here, referring to FIG. 1, the carbide area ratio refers to the area ratio of carbides to the entire cross section in an arbitrarily specified cross section of the heat-treated CV graphite cast iron. Carbides are identified by performing microstructural observation using an optical microscope on a cross section that has been buffed and then chemically etched using nital. The carbide area ratio is calculated as the ratio of the area of ​​carbides to the entire area of ​​the cross section by image analysis of a cross-sectional photograph taken by a digital camera attached to an optical microscope. The carbide area ratio is 4.5% or more from the viewpoint of increasing the strength and wear resistance of the heat-treated product, and 8.0% or less from the viewpoint of preventing the occurrence of cracks inside the heat-treated product.

[0040] [Retained austenite phase area ratio] Another heat-treated CV graphite cast iron in this embodiment has a retained austenite phase area ratio of 15% or less, so that the hardness from the surface to the inside is high and the workability is also high. Here, the retained austenite phase area ratio refers to the area ratio of the retained austenite phase to the matrix in an arbitrarily specified cross section of the heat-treated CV graphite cast iron. The area of ​​the retained austenite phase is calculated from the integrated intensity of each crystal such as austenite and martensite obtained by X-ray diffraction. Specifically, the retained austenite phase area ratio is calculated as a percentage of the ratio of the integrated intensity (austenite integrated intensity) at a diffraction angle of 120° to 140° to the integrated intensity at a diffraction angle of 120° to 170°. From the viewpoint of increasing the strength and workability of the heat-treated product, the retained austenite phase area ratio is preferably 15% or less, more preferably 10% or less.

[0041] [Brinell hardness] Another heat-treated CV graphite cast iron product in this embodiment has a Brinell hardness of 550 HBW or more, and therefore has high hardness from the surface to the inside. Here, the Brinell hardness refers to the hardness measured by the method described in JIS Z2243:2018. The Brinell hardness is 550 HBW or more from the viewpoint of increasing the strength of the heat-treated product.

[0042] <Embodiment 3> A manufacturing method of a heat-treated compacted vermicular graphite cast iron (CV graphite cast iron) according to yet another embodiment of the present disclosure includes the steps of preparing an ingot of the CV graphite cast iron of embodiment 1 (S10) as a heat treatment S20, performing a quenching treatment S21 of heating the ingot at 810°C or higher and 830°C or lower, followed by water or oil cooling, and performing a tempering treatment S22 of heating the quenched ingot at 325°C or lower, followed by air or furnace cooling. The manufacturing method of the heat-treated CV graphite cast iron of this embodiment includes the above steps, thereby making it possible to manufacture the heat-treated CV graphite cast iron of embodiment 2, which is inexpensive and has high strength from the surface to the inside.

[0043] [Preparation of CV graphite cast iron ingots] In the step S10 of preparing an ingot of CV graphite cast iron, an ingot of CV graphite cast iron according to the first embodiment is prepared. Here, "preparation" refers to purchasing or manufacturing the CV graphite cast iron. The method of manufacturing the ingot of CV graphite cast iron according to the first embodiment is not particularly limited, but is, for example, as follows.

[0044] (Manufacturing method for CV graphite cast iron ingots) First, various raw materials are blended and melted so that C is 3.50 mass% or more and 3.90 mass% or less, Si is 1.50 mass% or more and 2.00 mass% or less, Mn is 0.85 mass% or more and 1.15 mass% or less, P is 0.025 mass% or less, S is 0.025 mass% or less, Mg is 0.006 mass% or more and 0.013 mass% or less, Cu is 1.65 mass% or more and 1.85 mass% or less, Ni is 0.60 mass% or more and 0.80 mass% or less, Mo is 0.45 mass% or more and 1.00 mass% or less, V is 0.20 mass% or more and 0.40 mass% or less, Cr is 0.20 mass% or more and 0.40 mass% or less, and the balance is Fe and inevitable impurities. Next, the molten liquid is poured into a mold and slowly cooled to obtain an ingot of CV graphite cast iron. This manufacturing method is called casting. As shown in Fig. 1, the ingot of CV graphite cast iron 10 obtained in this manner has graphite 12 and carbides 14 scattered within a continuous matrix 16. The graphite 12 and carbides 14 scattered in an arbitrarily specified cross section of the CV graphite cast iron are distributed almost uniformly. Therefore, by the heat treatment described below, a heat-treated CV graphite cast iron product with high strength from the surface to the inside can be obtained.

[0045] [Ingot heat treatment] The heat treatment S20 of the CV graphite cast iron ingot includes the following quenching treatment S21 and tempering treatment S22. This quenching treatment can increase the strength by martensitic transformation of the matrix. Furthermore, this tempering treatment can improve the toughness of the CV graphite cast iron and improve the workability by reducing the residual austenite phase in the matrix.

[0046] [Hardening treatment] In the quenching treatment S21, the ingot of CV graphite cast iron is heated to 810°C or more and 830°C or less, and then cooled with water or oil. The quenching treatment temperature is preferably 810°C or more and 830°C or less from the viewpoint of increasing the hardness of the ingot. The quenching treatment temperature is set to the estimated A of CV graphite cast iron from the viewpoint of maintaining a high temperature and stably forming an austenite phase. 3 The transformation point (austenitization temperature) is 50°C higher than 810°C. Here, the estimated A of the CV graphite cast iron of embodiment 1 3 The transformation point is calculated to be about 760°C using JMatPro, a material simulation software developed by Sente Software. The quenching temperature is 830°C or less in order to suppress the excess heat of the CV graphite cast iron and enable rapid cooling. Even if a tempering process, which will be described later, is performed after the quenching process, the hardness is reduced by the tempering process. For this reason, it is necessary to impart sufficient hardness to the ingot by the quenching process.

[0047] The holding time of the heat treatment during quenching is not particularly limited, but from the viewpoint of maintaining a high temperature by heating and stably forming an austenite phase in the CV graphite cast iron, it is preferable to hold it for 1 hour or more per 25 mm of product thickness. Water cooling means that the ingot is quenched by putting it in water during quenching, and oil cooling means that the ingot is quenched by putting it in oil during quenching. There are no particular limitations on the cooling medium, water or oil, but since cast products generally have complex shapes, it is preferable to use oil from the viewpoint of preventing deformation and quenching cracks.

[0048] [Tempering treatment] In the tempering treatment S22, the quenched ingot is heated to 325°C or less and then air-cooled or furnace-cooled. The tempering treatment temperature is preferably 325°C or less from the viewpoint of not affecting the graphite spheroidization rate and carbide area rate of the quenched ingot and reducing the decrease in hardness associated with the tempering treatment. The tempering treatment temperature is preferably 160°C or more from the viewpoint of improving the toughness of the cast product, and more preferably 275°C or more from the viewpoint of improving the workability.

[0049] The holding time for the heat treatment during tempering is not particularly limited, but taking into consideration the heat transfer to the inside of the product, it is preferable to hold the product for at least 1 hour per 25 mm of wall thickness. Air cooling refers to removing the ingot from the heat treatment device and cooling it by directly exposing it to the outside air, while furnace cooling refers to cooling the ingot inside the heat treatment device. EXAMPLES

[0050] [Example I] 1. Preparation of CV graphite cast iron ingot For Examples I-1 to I-5 and Comparative Examples I-1 to I-3, various raw materials were melted so that the contents of C, Si, Mn, P, S, Mg, Cu, Ni, Mo, V, and C were as shown in Tables 2 and 3, respectively, with the remainder being Fe and unavoidable impurities. The molten liquid was poured into a Y-shaped specimen B (Examples I-1 to I-3 and Comparative Examples I-1 and I-2) or a cylindrical mold with a volume of 30 cm in diameter and 10 cm in thickness (Examples I-4 and I-5 and Comparative Example I-3)) specified in JIS G5505:2013, and slowly cooled to cast an ingot of CV graphite cast iron. For Examples I-1 to I-3 and Comparative Examples I-1 and I-2, a part of the ingot was cut, and a metal structure photograph was taken of the cross section of the cut part using an optical microscope (Nikon ECLIPSE LV150N) and a digital camera attached to the optical microscope, and the graphite spheroidization rate was calculated using an image analyzer (Innotek Quick Grain) in accordance with the above-mentioned JIS G5505:2013 "Appendix B (Regulation) Evaluation of Graphite Spheroidization Rate of CV Graphite Cast Iron Products". The results are summarized in Table 2.

[0051] [Table 2]

[0052] 2. Heat treatment of ingots (1) Quenching The cast ingots were quenched by heating at 820° C. (Example I-5) or 830° C. (Examples I-1 to I-4 and Comparative Examples I-1 to I-3) for 2.5 hours and then oil-cooling.

[0053] (2) Tempering treatment The ingot after the quenching treatment was further heated at 160° C. for 5 hours and then air-cooled to perform a tempering treatment.

[0054] Of the heat-treated products after the above-mentioned quenching and tempering treatments, for Examples I-1 to I-3 and Comparative Examples I-1 and I-2, a portion of the heat-treated product was further cut, and the graphite spheroidization rate was calculated in the cross section of the further cut portion in the same manner as described above.The graphite spheroidization rate was found to be equivalent to the graphite spheroidization rate of the ingot before the heat treatment (quenching and tempering treatments).

[0055] Among the ingots after the above-mentioned tempering treatment, the ingots of Examples I-4 and I-5 and Comparative Example I-3 were cut at their ingot entry portions. Metallographic photographs of the cross sections of the cut portions were taken using an optical microscope (Nikon Corporation ECLIPSE LV150N) and a digital camera attached to the optical microscope. From the metallographic photographs, the carbide area ratio was calculated using an image analyzer (Mitani Corporation WinROOF). The results are summarized in Table 3 and FIG. 3.

[0056] [Table 3]

[0057] 3.Results (1) Relationship between Mg content and graphite spheroidization rate Referring to Table 2, there is a strong positive correlation between the Mg content and the graphite spheroidization rate, and by setting the Mg content relative to the total amount of the CV graphite cast iron and its heat-treated products to 0.006 mass% or more and 0.013 mass% or less, the graphite spheroidization rate can be set to 20% or more and 70% or less.

[0058] (2) Relationship between Mo content and carbide area ratio Referring to Table 3 and Figure 3, there was a strong positive correlation between the Mo content and the carbide area ratio, so by setting the Mo content to 0.45 mass% or more and 1.00 mass% or less relative to the total amount of the heat-treated CV graphite cast iron, the carbide area ratio could be set to 4.5% or more and 8.0% or less.

[0059] [Example II] The Ni content and A of the CV graphite cast iron of embodiment 1 3 The relationship with the transformation point (austenitization temperature) was simulated using JMatPro, a material simulation software developed by Sente Software. In this simulation, the contents of each element were C: 3.70 mass%, Si: 1.70 mass%, Mn: 0.95 mass%, Cu: 1.75 mass%, Mo: 0.75 mass%, V: 0.30 mass%, P and S were omitted because of their small amounts, Ni was 0 mass%, 0.4 mass%, 0.7 mass%, or 1.0 mass%, and the balance was Fe. The results are summarized in Figure 4.

[0060] Referring to Figure 4, the relationship between Ni content and A 3 Since there was a strong negative correlation with the transformation point, by setting the Ni content to 0.6 mass% or more and 0.8 mass% or less with respect to the total amount of CV graphite cast iron, 3 It was found that the transformation point could be lowered to approximately 760°C.

[0061] [Example III] Three ingots of CV graphite cast iron of Example I-2 (see Table 2) were heated at 810 ° C, 820 ° C, or 830 ° C, respectively, and four ingots of CV graphite cast iron of Example I-3 (see Table 2) were heated at 840 ° C, 850 ° C, 880 ° C, or 890 ° C, respectively, for 2.5 hours, then oil-cooled to perform quenching treatment, and further heated at 160 ° C for 5 hours, then air-cooled to perform tempering treatment. For the heat-treated CV graphite cast iron thus obtained, a part of the heat-treated product was cut, and the Brinell hardness was measured on the cross section of the cut part in accordance with JIS Z2243:2018. In order to evaluate the hardening of the base material due to the martensitic transformation accompanying quenching, measurements were performed with a test force of 7.355 kN using a carbide ball with a diameter of 5 mm. The relationship between the quenching temperature and the Brinell hardness is shown in FIG. 5. In addition, the area ratio of the retained austenite phase was calculated from a region of φ1 mm arbitrarily selected from the above cross section using an X-ray residual stress measurement device (μ-X360s manufactured by Pulstec Corporation). The relationship between the quenching temperature and the area ratio of the retained austenite phase is shown in Figure 6.

[0062] 5, when the quenching temperature exceeds 830°C, the heat capacity of the ingot increases, the cooling rate decreases, and high hardness cannot be obtained. For this reason, it was found that the quenching temperature is preferably 830°C or less. In addition, the estimated A of the CV graphite cast iron of the embodiment 1 by the simulation in Example II was 3 Since the transformation point is about 760°C, the quenching temperature was set to 810°C or higher in order to stably generate austenite by quenching. In order to improve the hardness and workability of the obtained heat-treated CV graphite cast iron, the quenching temperature was set to 830°C or lower in order to make the area ratio of the retained austenite phase 15% or less, as shown in Figure 6.

[0063] [Example IV] Six ingots of CV graphite cast iron of Example I-2 (see Table 2) were quenched by heating at 820 ° C for 2.5 hours and then cooled with oil, and further tempered by heating at 160 ° C, 250 ° C, 275 ° C, 300 ° C, 325 ° C, or 350 ° C for 5 hours and then cooled in air. For the heat-treated CV graphite cast iron obtained, a part of the heat-treated product was cut, and the Brinell hardness was measured on the cross section of the cut part in accordance with JIS Z2243:2018. This was a hardness measurement after quenching and tempering, and in order to evaluate it in a form close to the product, the measurement was performed with a test force of 29.42 kN using a cemented carbide ball with a diameter of 10 mm. The relationship between the tempering temperature and the Brinell hardness is shown in FIG. 7.

[0064] 7, when the tempering temperature exceeds 325°C, it is not possible to obtain a heat-treated CV graphite cast iron product having a Brinell hardness of 550HBW or more, and the hardness of the heat-treated product cannot be increased. Therefore, it was found that a tempering temperature of 325°C or less is suitable for making the Brinell hardness of the heat-treated product 550HBW or more.

[0065] The embodiments and examples disclosed herein are illustrative in all respects and should not be considered as limiting. The scope of the present invention is indicated by the claims, not by the embodiments and examples described above, and is intended to include the meaning equivalent to the claims and all modifications within the scope. [Explanation of symbols]

[0066] 10 CV graphite cast iron, 12 graphite, 14 carbide, 16 matrix, S10 preparation of CV graphite cast iron ingots, S20 heat treatment, S21 quenching treatment, S22 tempering treatment.

Claims

1. A compacted vermicular graphite cast iron consisting of graphite, carbide, and matrix, With respect to the total amount of the compacted vermicular graphite cast iron, carbon is 3.50 mass% or more and 3.90 mass% or less, silicon is 1.50 mass% or more and 2.00 mass% or less, manganese is 0.85 mass% or more and 1.15 mass% or less, phosphorus is 0.025 mass% or less, sulfur is 0.025 mass% or less, magnesium is 0.006 mass% or more and 0.013 mass% or less, copper is 1.65 mass% or more and 1.85 mass% or less, nickel is 0.60 mass% or more and 0.80 mass% or less, The composition is composed of 0.45% by mass or more and 1.00% by mass or less of molybdenum, 0.20% by mass or more and 0.40% by mass or less of vanadium, 0.20% by mass or more and 0.40% by mass or less of chromium, and the balance being iron and inevitable impurities, A compacted vermicular graphite cast iron having a carbon equivalent, defined as the sum of carbon and 1 / 3 amount of silicon, of 4.00% by mass or more and 4.50% by mass or less.

2. 2. The compacted vermicular graphite cast iron according to claim 1, wherein the spheroidization rate of the graphite is 20% or more and 70% or less.

3. A heat-treated compacted vermicular graphite cast iron according to claim 1 or 2, A heat-treated compacted vermicular graphite cast iron product, wherein in an arbitrarily specified cross section of the heat-treated product, the area ratio of the carbides to the entire cross section is 4.5% or more and 8.0% or less.

4. A heat-treated compacted vermicular graphite cast iron according to claim 1 or 2, A heat-treated compacted vermicular graphite cast iron product, in which an area ratio of a retained austenite phase to the base metal in an arbitrarily specified cross section of the heat-treated product is 15% or less.

5. A heat-treated compacted vermicular graphite cast iron according to claim 1 or 2, A heat-treated compacted vermicular graphite cast iron product with a Brinell hardness of 550 HBW or more.

6. A method for producing an ingot of compacted vermicular graphite cast iron according to claim 1 or 2, The ingot is subjected to a quenching treatment in which the ingot is heated to 810° C. or more and 830° C. or less, and then cooled in water or oil; The method for producing a heat-treated compacted vermicular graphite cast iron product includes subjecting the quenched ingot to a tempering treatment in which the ingot is heated to 325°C or less and then air-cooled or furnace-cooled.

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