Heat treatment oil

Abstract

To provide a heat treatment oil having both cooling properties and evaporation properties.SOLUTION: Provided is a heat treatment oil which includes a monoester (A) represented by general formula (1) as a base oil. (1): R1-COO-R2 (where R1 and R2 are each independently a hydrocarbon group optionally having an alkoxy group as a substituent, and the total carbon number of R1 and R2 is 15-30).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a heat treatment oil. [Background technology]

[0002] Heat treatment processes such as quenching of metal materials are usually carried out using a heat treatment liquid to impart a desired hardness to the metal material, and therefore the heat treatment liquid must have excellent cooling performance to increase the hardness of the metal material. Water is a liquid with excellent cooling capacity, but water-based heat treatment liquids have the risk of causing quenching cracks in metal materials due to their excessive cooling capacity, and they also cause significant quenching distortion. For this reason, oil-based heat treatment liquids, i.e., heat treatment oils, are generally used in heat treatment processes such as quenching of metal materials.

[0003] As an index of the cooling ability of heat treatment oil, the quench intensity (H value), calculated from the cooling time from 800°C to 300°C in the cooling curve specified in JIS K2242:2012, is widely used.

[0004] Regarding quenching of metal materials, when heated metal materials are immersed in heat treatment oil, the cooling rate is not constant, and they are usually cooled through the following three stages (1) to (3). (1) The first stage (vapor film stage) in which the metal material is enveloped in heat treatment oil vapor. (2) The second stage (boiling stage) occurs when the vapor film breaks and boiling occurs. (3) The third stage (convection stage) occurs when the temperature of the metal material drops below the boiling point of the heat treatment oil and heat is removed by convection. Of the three stages, the cooling rate is greatest in the second stage (boiling stage). If the time until the first stage (vapor film stage) ends (the "characteristic seconds" in the cooling test according to JIS K2242:2012) is long, the hardness of the resulting metal material may be insufficient, depending on the material, such as carbon steel.

[0005] On the other hand, although quenching metal materials improves their hardness, they may be subjected to a tempering process in which they are reheated to further impart toughness. In this case, since quenching oil adheres to the metal material after quenching, it is common to perform a cleaning process to remove this before performing the tempering process. However, from the perspective of improving productivity, studies are being conducted to reduce or omit the cleaning process. For example, Patent Document 1 discloses an invention relating to a heat treatment oil using multiple types of metal soaps, which has good aqueous cleanability after quenching. Patent Document 2 discloses a gas quenching method that uses gas as a coolant instead of heat treatment oil, thereby eliminating the need for cleaning. Patent Document 3 discloses an invention relating to an apparatus equipped with a reduced-pressure drying chamber for vaporizing heat treatment oil. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 9-176728 [Patent Document 2] Patent No. 5817173 [Patent Document 3] International Publication No. 2021 / 240718 Summary of the Invention [Problem to be solved by the invention]

[0007] However, even if the heat treatment oil described in Patent Document 1 is used, the cleaning treatment cannot be completely omitted, and problems remain from the viewpoint of productivity. Next, with the gas quenching method described in Patent Document 2, depending on the cooling gas blowing pressure, the H value may be insufficient, resulting in an insufficient hardness of the resulting metal material. Also, there is a risk of uneven cooling performance between areas with high and low cooling gas pressure. Furthermore, when using a quenching method using heat treatment oil, switching to a gas quenching method requires a complete replacement of the existing heat treatment equipment. The device described in Patent Document 3 also requires replacement of the existing heat treatment device, and there is a risk that the evaporation rate may be insufficient depending on the heat treatment oil used.

[0008] Therefore, the object of the present invention is to provide a heat treatment oil that can achieve both cooling properties and evaporativity, from the viewpoint of utilizing existing heat treatment equipment while eliminating the need for cleaning treatment after heat treatment. [Means for solving the problem]

[0009] The present inventors have found that a heat-treated oil containing a specific monoester as a base oil can solve the above problems, and have completed the present invention.

[0010] That is, the present invention provides the following [1] to [9]. [1] A heat-treated oil containing a monoester (A) represented by the following general formula (1) as a base oil. R 1 -COO-R 2 ···(1) (In the formula, R 1 and R 2 are each independently a hydrocarbon group which may have an alkoxy group as a substituent, and R 1 and R 2 The total number of carbon atoms is 15 to 30. [2] In the general formula (1), R 1 is a chain hydrocarbon group having 5 to 20 carbon atoms, and R 2 is a chain hydrocarbon group having 1 to 24 carbon atoms or an alkoxyalkyl group having 1 to 20 carbon atoms. [3] The heat treatment oil according to [1] or [2] above, wherein the content of the monoester (A) is 50 mass% or more based on the total amount of the heat treatment oil. [4] The heat-treated oil according to any one of [1] to [3] above, which has a flash point of 150°C or higher. [5] The heat treatment oil according to any one of [1] to [4], which is used as a quenching oil. [6] A method for producing a metal component, comprising a quenching step of immersing a heated metal component in the heat treatment oil according to any one of [1] to [5] and cooling it. [7] The method for producing a metal member according to [6], wherein the quenching temperature in the quenching step is 500 to 1400°C. [8] The method for manufacturing a metal component according to [6] or [7], further comprising a tempering step of reheating the metal component without cleaning it after the quenching step. [9] The method for producing a metal member according to [8], wherein the heating temperature in the tempering step is 150 to 650°C. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a heat treatment oil that has both cooling properties and evaporative properties. DETAILED DESCRIPTION OF THE INVENTION

[0012] The upper and lower limits of the ranges described herein can be combined in any way. For example, if the ranges are "A to B" and "C to D," the ranges "A to D" and "C to B" are also included in the scope of the present invention. Furthermore, unless otherwise specified, the numerical range "lower limit to upper limit" described in this specification means that the range is equal to or greater than the lower limit and equal to or less than the upper limit. In this specification, the numerical values in the examples are numerical values that can be used as upper or lower limit values.

[0013] [Heat treatment oil] The heat-treated oil of this embodiment contains a monoester (A) represented by the following general formula (1) as a base oil. R 1 -COO-R 2 ···(1) (In the formula, R 1 and R 2 are each independently a hydrocarbon group which may have an alkoxy group as a substituent, and R 1 and R 2 The total number of carbon atoms is 15 to 30.

[0014] As a result of intensive research conducted by the present inventors to solve the above problems, they have found that when a heat treatment oil contains the above-mentioned specific monoester (A) as a base oil, it ensures cooling properties during heat treatment such as quenching, and also exhibits volatility to such an extent that it can be evaporated during tempering without washing after the heat treatment, making washing after the heat treatment unnecessary.

[0015] The heat treatment oil of this embodiment may consist solely of the monoester (A), but may also contain other components in addition to the monoester (A) as long as the effects of the present invention are not impaired. When the heat-treated oil of this embodiment contains additives, the total content of the monoester (A) and the additives is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more, based on the total amount (100% by mass) of the heat-treated oil. Also, it is usually 100% by mass or less, preferably less than 100% by mass, more preferably 99% by mass or less, and even more preferably 98% by mass or less.

[0016] <Monoester (A)> The heat treatment oil of this embodiment contains a monoester (A) as a base oil. In this embodiment, the monoester (A) is represented by the following general formula (1). R 1 -COO-R 2 ···(1) (In the formula, R 1 and R 2 are each independently a hydrocarbon group which may have an alkoxy group as a substituent, and R 1 and R 2 The total number of carbon atoms is 15 to 30. R in the above general formula (1) 1 and R 2 When the total number of carbon atoms is 15 or more, the cooling property during heat treatment is good. When the total number of carbon atoms is 30 or less, the evaporation property of the heat treatment oil is good, and the cleaning treatment after the heat treatment can be omitted, resulting in excellent productivity. R in the above general formula (1)1 and R 2 From the viewpoint of the cooling property of the heat treatment oil, the total carbon number is preferably 18 or more, more preferably 22 or more, and from the viewpoint of the volatility of the heat treatment oil, the total carbon number is preferably 30 or less, more preferably 26 or less, specifically, 18 to 30 is preferred, and 22 to 26 is more preferred.

[0017] The monoester (A) is represented by R in the general formula (1). 1 is a chain hydrocarbon group having 5 to 20 carbon atoms, and R 2 is preferably a chain hydrocarbon group having 1 to 24 carbon atoms or an alkoxyalkyl group having 1 to 20 carbon atoms. R in the general formula (1) 1 is preferably an alkyl group having 5 to 20 carbon atoms, and more preferably an alkyl group having 6 to 18 carbon atoms. R in the general formula (1) 2 is preferably an alkyl group having 1 to 20 carbon atoms or an alkoxyalkyl group having 1 to 20 carbon atoms, and more preferably an alkyl group having 1 to 16 carbon atoms or an alkoxyalkyl group having 1 to 16 carbon atoms.

[0018] In the heat treatment oil of this embodiment, the monoester (A) functions as a base oil. In the heat-treated oil of this embodiment, the content of the monoester (A) is preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 100% by mass, based on the total amount (100% by mass) of the heat-treated oil. The content of the monoester (A) may be 100% by mass or less, based on the total amount (100% by mass) of the heat-treated oil.

[0019] (Kinematic viscosity of monoester (A)) The kinematic viscosity at 40°C of the monoester (A) is preferably 1.0 mm from the viewpoint of achieving both cooling property and evaporative property. 2 / s or more 20mm 2 / s or less, preferably 2.5 mm 2 / s or more 10mm 2 / s or less, more preferably 3.0 mm 2 / s or more 8.5mm 2 / s or less. In this specification, the kinematic viscosity at 40°C of the monoester (A) means a value measured in accordance with JIS K2283:2000.

[0020] <Method for producing monoester (A)> The method for producing the monoester (A) is not particularly limited, and the monoester (A) can be produced, for example, by reacting an alcohol or an alkoxy alcohol with a fatty acid in a combination that results in a predetermined total carbon number, and then esterifying the alcohol or alkoxy alcohol with a fatty acid in a conventional manner.

[0021] <Base oils other than monoester (A)> The heat-treated oil of the present embodiment may or may not further contain a base oil other than the monoester (A). Examples of base oils other than the monoester (A) include one or more selected from the group consisting of synthetic oils that do not fall under the category of the monoester (A) and mineral oils.

[0022] Examples of synthetic oils that do not fall under the category of monoesters (A) include polyvinyl ethers; polyalkylene glycols; copolymers of polyalkylene glycols or their monoethers with polyvinyl ethers; polyol esters; polyesters; polycarbonates; hydrogenated α-olefin oligomers; alicyclic hydrocarbon compounds; alkylated aromatic hydrocarbon compounds; and GTL base oils produced by isomerizing GTL WAX (gas-to-liquid wax) produced by the Fischer-Tropsch process or the like. The synthetic oils may be used alone or in combination of two or more.

[0023] From the viewpoint of making it easier to exert the effects of the present invention, it is preferable that the content of synthetic oils that do not fall under the category of monoesters (A) is small. Specifically, the content of synthetic oils that do not fall under the category of monoesters (A) is preferably less than 3 parts by mass, more preferably less than 1 part by mass, even more preferably less than 0.1 parts by mass, and even more preferably no synthetic oils that do not fall under the category of monoesters (A) are contained, relative to 100 parts by mass of the monoesters (A).

[0024] Examples of mineral oils include atmospheric residues obtained by atmospheric distillation of crude oils such as paraffinic crude oil, intermediate crude oil, and naphthenic crude oil; distillates obtained by vacuum distillation of these atmospheric residues; mineral oils obtained by subjecting the distillates to one or more processes such as solvent deasphalting, solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing, and hydrorefining; and wax isomerized mineral oils. The mineral oils may be used alone or in combination of two or more.

[0025] From the viewpoint of more easily achieving the effects of the present invention, the mineral oil content is preferably low. Mineral oil has a wider molecular weight distribution than the monoester (A) of this embodiment, and therefore contains lower or higher molecular weight components. Low molecular weight components have high volatility, i.e., they evaporate easily, which lengthens the time (characteristic seconds) until the vapor film stage of the heat treatment oil ends, so a low content is preferable. Furthermore, high molecular weight components have low volatility, i.e., they are difficult to evaporate, so a low content is preferable from the viewpoint of improving the volatility of the heat treatment oil. From the above viewpoints, the content of the mineral oil is preferably less than 10 parts by mass, more preferably less than 1 part by mass, per 100 parts by mass of the monoester (A).

[0026] <Additives> The heat treatment oil of this embodiment may contain additives commonly used in heat treatment oils, such as glitter improvers, antioxidants, and cooling improvers, if desired. The additives may be used alone or in combination of two or more.

[0027] (gloss improver) When the heat treatment oil of this embodiment contains a glitter improver, the glitter of the appearance can be improved. Examples of glitter improvers include fats and oils; complete esters of alkylsuccinic acid, alkylsuccinimides and derivatives thereof; complete esters of alkenylsuccinic acid, alkenylsuccinimides and derivatives thereof; substituted hydroxyaromatic carboxylic acid esters (complete esters) and derivatives thereof. Specific examples include polybutenyl succinimide, polyisobutenyl succinimide, and pentadecenyl succinic acid. These glitter improvers may be used alone or in combination of two or more. The content of the glitter improver is preferably 0.1 to 5.0 mass %, more preferably 0.3 to 3.0 mass %, and even more preferably 0.4 to 2.5 mass %, based on the total amount of the heat treatment oil.

[0028] (antioxidant) Examples of the antioxidant include phenol-based antioxidants and amine-based antioxidants. Examples of phenolic antioxidants include 2,6-di-tert-butyl-para-cresol, 2,6-di-tert-butyl-4-ethylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-hydroxymethylphenol, 2,6-di-tert-butylphenol, 2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-butyl-4-(N,N-dimethylaminomethyl)phenol, 2,6-di-tert-amyl-4-methylphenol, and n-octadecyl-3-(4-hydroxy-3,5-di-tert-butylphenyl) monocyclic phenols such as propionate; and polycyclic phenols such as 4,4'-methylenebis(2,6-di-tert-butylphenol), 4,4'-isopropylidenebis(2,6-di-tert-butylphenol), 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 4,4'-bis(2,6-di-tert-butylphenol), 4,4'-bis(2-methyl-6-tert-butylphenol), 2,2'-methylenebis(4-ethyl-6-tert-butylphenol), and 4,4'-butylidenebis(3-methyl-6-tert-butylphenol). Examples of the amine-based antioxidant include diphenylamine-based antioxidants and naphthylamine-based antioxidants. Examples of diphenylamine antioxidants include alkylated diphenylamines having an alkyl group having 3 to 20 carbon atoms, and specific examples thereof include diphenylamine, monooctyldiphenylamine, monononyldiphenylamine, 4,4'-dibutyldiphenylamine, 4,4'-dihexyldiphenylamine, 4,4'-dioctyldiphenylamine, 4,4'-dinonyldiphenylamine, tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenylamine, and tetranonyldiphenylamine. Examples of naphthylamine antioxidants include alkyl-substituted phenyl-α-naphthylamines having 3 to 20 carbon atoms, and specific examples include α-naphthylamine, phenyl-α-naphthylamine, butylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine, octylphenyl-α-naphthylamine, and nonylphenyl-α-naphthylamine. These antioxidants may be used alone or in combination of two or more. The content of the antioxidant is preferably 0.01 to 5.0% by mass, more preferably 0.02 to 3.0% by mass, and even more preferably 0.05 to 2.0% by mass, based on the total amount of the heat-treated oil.

[0029] (cooling agent) Examples of the cooling improver include imide-based dispersants such as boron-containing alkenyl succinimides, and mono- or di-carboxylic acid amides typified by fatty acids or succinic acid. These cooling improvers may be used alone or in combination of two or more. The content of the coolability improver is preferably 0.05 to 5.0 mass %, more preferably 0.1 to 3.0 mass %, and even more preferably 0.3 to 2.0 mass %, based on the total amount of the heat treatment oil.

[0030] Furthermore, from the viewpoint of the volatility of the heat treatment oil, the content of the vapor film breaker in the heat treatment oil of this embodiment is preferably less than 3 mass%, more preferably less than 1 mass%, even more preferably less than 0.1 mass%, even more preferably less than 0.01 mass%, and even more preferably no vapor film breaker is contained, based on the total amount of the heat treatment oil.

[0031] [Physical properties of heat treatment oil] <Flash point> The properties of the heat-treated oil of this embodiment are not particularly limited, but it is preferable that the flash point be 130°C or higher, more preferably 140°C or higher, even more preferably 150°C or higher, and particularly preferably 160°C or higher. In this specification, the flash point of the monoester (A) refers to a value measured by the Cleveland Open Chamber (COC) method in accordance with K2265-4:2007. <Kinematic viscosity> The kinematic viscosity at 40°C of the heat treatment oil of this embodiment is preferably 1.0 mm from the viewpoint of achieving both cooling properties and evaporative properties. 2 / s or more 20mm 2 / s or less, preferably 2.5 mm 2 / s or more 10mm 2 / s or less, more preferably 3.0 mm 2 / s or more 9.0mm 2 / s or less. In this specification, the kinematic viscosity at 40°C of the monoester (A) means a value measured in accordance with JIS K2283:2000.

[0032] <Cooling property> The coolability of the heat treatment oil of this embodiment can be evaluated using the quench intensity (H value) according to the method described in the examples below. The H value can be calculated from the cooling time from 800°C to 300°C in the cooling curve specified in JIS K2242:2012. The H value of the heat treatment oil of this embodiment is preferably 0.100 cm -1 More than 0.105cm, preferably 0.105cm -1 More preferably, 0.110 cm -1 More than 0.120 cm, especially preferred -1 That's all.

[0033] <Characteristic seconds> The heat-treated oil of this embodiment can be evaluated for its characteristic number of seconds by the method described in the examples below. From the viewpoint of shortening the time in the vapor film stage and suppressing quenching distortion, the characteristic number of seconds is preferably 10 seconds or less, more preferably 8.0 seconds or less, even more preferably 7.0 seconds or less, still more preferably 5.5 seconds or less, and most preferably 4.5 seconds or less.

[0034] [Metal component manufacturing method] The method for manufacturing a metal component of this embodiment includes a quenching step in which a heated metal component is immersed in the heat treatment oil and cooled. The quenching temperature in the quenching step is preferably 400 to 1500°C, more preferably 500 to 1400°C, and even more preferably 600 to 1200°C. Furthermore, the method for manufacturing a metal component of this embodiment preferably includes a tempering step in which the metal component is reheated after the quenching step without being washed. By subjecting the quenched metal component to the tempering step as is in this way, the washing step can be omitted, improving productivity. Furthermore, the heat treatment oil of this embodiment has excellent volatility, particularly under the temperature conditions of the tempering step, and can therefore be evaporated from the metal component during the tempering step. The heating temperature in the tempering step can be appropriately set depending on the desired hardness and toughness of the metal member, and is, for example, about 150 to 600°C.

[0035] [Uses of heat treatment oil] The heat treatment oil of this embodiment can be used for heat treatments such as quenching, tempering, annealing, and normalizing. Furthermore, since the heat treatment oil of this embodiment has excellent cooling and evaporating properties, the heat treatment oil of this embodiment can be suitably used in heat treatments that do not require cleaning after the heat treatment, and can be particularly suitably used as a quenching oil that does not require cleaning after quenching. [Example]

[0036] The present invention will be specifically described with reference to the following examples, but the present invention is not limited to these examples.

[0037] [Method of measuring physical properties] The physical properties were measured as follows. (1)Kinematic viscosity at 40℃ Measurements were made in accordance with JIS K2283:2000.

[0038] <Flash point measurement> Measurement was performed using the Cleveland Open Chamber (COC) method in accordance with JIS K2265-4:2007.

[0039] [Examples 1 to 10 and Comparative Examples 1 to 5] Using heat-treated oils containing the following components in the amounts shown in Table 1, the flash point and kinematic viscosity at 40°C were measured, and the volatility, characteristic seconds and H value were also evaluated.

[0040] <Base oil> Monoester (A'1): Butyl hexanoate (R 1 and R 2 Total carbon number: 10) Monoester (A'2): Methyl laurate (R 1 and R 2 Total carbon number: 13) Monoester (A1): Butyl laurate (R 1 and R 2 Total carbon number: 16) Monoester (A2): Isopropyl myristate (R 1 and R 2 Total carbon number: 17) Monoester (A3): Isononyl isononanoate (R 1 and R 2 Total carbon number: 18) Monoester (A4): 2-ethylhexyl laurate (R 1 and R 2 Total carbon number: 18) Monoester (A5): Methyl oleate (R 1 and R 2 Total carbon number: 19) Monoester (A6): Butyl oleate (R 1 and R 2 Total carbon number: 22) Monoester (A7): Cetyl 2-ethylhexanoate (R 1and R 2 Total carbon number: 24) Monoester (A8): 2-ethylhexyl palmitate (R 1 and R 2 Total carbon number: 24) Monoester (A9): 2-butoxyethyl oleate (R 1 and R 2 Total carbon number: 24) Monoester (A10): 2-ethylhexyl oleate (R 1 and R 2 Total carbon number: 26) Monoester (A'3): Tridecyl stearate (R 1 and R 2 Total carbon number: 31) Air: 5kg / cm instead of heat treatment oil 2 Air is injected at

[0041] <Additives> Calcium salicylate: Calcium salicylate (calcium atom content: 8.0% by mass, base number (perchloric acid method): 225 mg KOH / g, density: 1.051 g / cm 3 ) Na sulfonate: Sodium sulfonate (sodium atom content: 3.4 mass%, density: 1.02 g / cm 3 )

[0042] [Evaluation of volatility] The evaluation was carried out using a Tg-DTA device according to JIS K0129:2005. 5.0 mg of sample oil was weighed and placed in the sample measurement position of the heating furnace. The heating furnace was closed, and while flowing nitrogen at 200 mL / min, the temperature was raised to 200°C at 25°C / min, and after reaching 200°C, it was held for 2 hours. Thereafter, the mass of the residual oil after holding was measured, and the ratio (mass%) to the mass before holding was calculated.

[0043] [Cooling performance evaluation] In accordance with the cooling test method specified in JIS K 2242:2012, a cooling curve was created recording the temperature change from 800°C for 60 seconds, and the characteristic number of seconds was calculated from the time it took to reach the characteristic temperature. In addition, the H value was calculated using the time it took to reach 300°C from 800°C on the created cooling curve, using the Osaka University cooling capacity evaluation method. However, in Comparative Example 5, in which air was used as the coolant, air at 25°C was used at 5 kg / cm 2 The cooling curve was created.

[0044] The compositions and evaluation results of the heat-treated oils of Examples 1 to 10 and Comparative Examples 1 to 5 are shown in Table 1.

[0045] [Table 1]

[0046] As shown in Table 1, R 1 and R 2 The heat-treated oils of Examples 1 to 10, which used monoester (A) having a total carbon number of 15 to 30 as the base oil, had excellent volatility, a relatively short characteristic time, and a relatively high H value. In contrast, the heat-treated oil of Comparative Example 1, which used mineral oil as the base oil, and the R 1 and R 2 The heat-treated oil of Comparative Example 4, which used the monoester (A'3) having a total carbon number of 31, had low volatility and required washing after the heat treatment. On the other hand, R 1 and R 2 The heat-treated oils of Comparative Examples 2 and 3, which used monoesters (A'1) and (A'2) having a total carbon number of 10 to 13, had a long characteristic time and a relatively low H value. Furthermore, in Comparative Example 5, in which air was used as the coolant, the H value was low.

Claims

1. A heat-treated oil containing a monoester (A) represented by the following general formula (1) as a base oil: R １ -COO-R ２ ・・・(1) (In the formula, R １ and R ２ are each independently a hydrocarbon group which may have an alkoxy group as a substituent, and R １ and R ２ The total number of carbon atoms is 15 to 30.

2. In the general formula (1), R １ is a chain hydrocarbon group having 5 to 20 carbon atoms, and R ２ The heat treatment oil according to claim 1, wherein is a chain hydrocarbon group having 1 to 24 carbon atoms or an alkoxyalkyl group having 1 to 20 carbon atoms.

3. 3. The heat treatment oil according to claim 1, wherein the content of the monoester (A) is 50 mass% or more based on the total amount of the heat treatment oil.

4. The heat-treated oil according to any one of claims 1 to 3, having a flash point of 150°C or higher.

5. The heat treatment oil according to any one of claims 1 to 4, which is used as a quenching oil.

6. A method for manufacturing a metal member, comprising a quenching step of immersing a heated metal member in the heat treatment oil according to any one of claims 1 to 5 and cooling the metal member.

7. The method for manufacturing a metal member according to claim 6, wherein the quenching temperature in the quenching step is 500 to 1400°C.

8. 8. The method for manufacturing a metal member according to claim 6, further comprising a tempering step of reheating the metal member without cleaning it after the quenching step.

9. The method for producing a metal member according to claim 8, wherein the heating temperature in the tempering step is 150 to 650°C.

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