WO2025164631A1 - Heat treatment oil - Google Patents

Abstract

Provided is a heat treatment oil with which both cooling properties and evaporation properties can be achieved, the heat treatment oil comprising, as a base oil, a polyol ester (A) of a divalent or trivalent polyhydric alcohol having 1-10 carbon atoms and a monovalent fatty acid having 2-12 carbon atoms.

Description

Heat Treatment Oil

The present invention relates to heat treatment oil.

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.

The quench intensity (H value), calculated from the cooling time from 800°C to 300°C on the cooling curve specified in JIS K2242:2012, is widely used as an indicator of the cooling ability of heat treatment oil.

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) in which 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 boiling stage. If the time until the first vapor film stage ends (the "characteristic number of seconds" in the cooling test in accordance with JIS K2242:2012) is long, quench distortion is likely to occur.

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, and Patent Document 2 discloses an invention relating to an apparatus equipped with a reduced pressure drying chamber for vaporizing the heat treatment oil.

Japanese Patent Application Publication No. 9-176728 International Publication No. 2021/240718

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.
The device described in Patent Document 2 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.

The present invention therefore aims to provide a heat treatment oil that combines cooling and evaporative properties, allowing existing heat treatment equipment to be used while eliminating the need for post-heat treatment cleaning.

The inventors discovered that heat-treated oil containing a specific monoester as a base oil can solve the above problems, leading to the completion of the present invention.

That is, the present invention provides the following [1] to [10].
[1] A heat-treated oil containing a polyol ester (A) as a base oil, wherein the polyol ester (A) is a polyol ester of a dihydric or trihydric polyhydric alcohol having 1 to 10 carbon atoms and a monohydric fatty acid having 2 to 12 carbon atoms.
[2] The heat treatment oil according to [1] above, wherein the polyhydric alcohol has 2 to 6 carbon atoms.
[3] The heat-treated oil according to [1] or [2] above, wherein the fatty acid has 3 to 12 carbon atoms.
[4] The heat treatment oil according to any one of [1] to [3], wherein the content of the polyol ester (A) is 50 mass% or more based on the total amount of the heat treatment oil.
[5] The heat-treated oil according to any one of [1] to [4] above, having a flash point of 150°C or higher.
[6] The heat treatment oil according to any one of the above [1] to [5], which is used as a quenching oil.
[7] A method for manufacturing 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 [6] above and cooling the component.
[8] The method for producing a metal member according to the above [7], wherein the quenching temperature in the quenching step is 500 to 1400°C.
[9] The method for manufacturing a metal member according to the above [7] or [8], further comprising a tempering step of reheating the metal member without cleaning it after the quenching step.
[10] The method for producing a metal member according to the above [9], wherein the heating temperature in the tempering step is 150 to 600°C.

The present invention makes it possible to provide a heat treatment oil that has both cooling properties and evaporative properties.

The upper and lower limits of the ranges described herein can be combined in any way. For example, when ranges "A to B" and "C to D" are described, 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 "from the lower limit to the 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.

[Heat treatment oil]
The heat treatment oil of this embodiment is a heat treatment oil containing a polyol ester (A) as a base oil, and the polyol ester (A) is a polyol ester of a divalent or trivalent polyhydric alcohol having 1 to 10 carbon atoms and a monovalent fatty acid having 2 to 12 carbon atoms.

In order to solve the above problems, the inventors conducted extensive research and discovered that when a heat treatment oil contains the above-mentioned specific polyol ester (A) as a base oil, it ensures cooling properties during heat treatments such as quenching, while exhibiting volatility to the extent that it can be evaporated during tempering without washing after heat treatment, making washing after heat treatment unnecessary.

The heat treatment oil of this embodiment may consist solely of the above polyol ester (A), but may also contain other components other than the polyol ester (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 polyol ester (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.

<Polyol ester (A)>
The heat treatment oil of this embodiment contains a polyol ester (A) as a base oil.
In this embodiment, the polyol ester (A) is a polyol ester of a dihydric or trihydric polyhydric alcohol having 1 to 10 carbon atoms and a monohydric fatty acid having 2 to 12 carbon atoms.
When the monovalent fatty acid has two or more carbon atoms, the resulting polyol ester (A) has good cooling properties during heat treatment. When the monovalent fatty acid has 12 or less carbon atoms and the polyhydric alcohol is a dihydric or trihydric alcohol, the resulting polyol ester (A) has good volatility and can omit a washing treatment after heat treatment, resulting in excellent productivity.
Furthermore, since the heat treatment oil of this embodiment contains the polyol ester (A), it has superior volatility compared to one containing a diester of a dibasic acid and an alcohol.

(Polyhydric alcohol)
In this embodiment, the polyhydric alcohol is an alcohol component constituting the polyol ester (A), and may be dihydric or trihydric and have 1 to 10 carbon atoms.
The polyhydric alcohol may be linear, branched, or cyclic, and may be saturated or unsaturated, but is preferably linear or branched, and more preferably linear or branched and saturated.
The polyhydric alcohol preferably has 2 to 8 carbon atoms, more preferably 2 to 6 carbon atoms, and even more preferably 3 to 6 carbon atoms.

Specific examples of the polyhydric alcohol preferably include dihydric alcohols such as ethylene glycol, propylene glycol, butylene glycol, 2-methyl-1,3-propanediol, pentanediol, neopentyl glycol, hexanediol, 2-ethyl-2-methyl-1,3-propanediol, heptanediol, 2-methyl-2-propyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, octanediol, nonanediol, and decanediol; and trihydric alcohols such as glycerin, trimethylolethane, ditrimethylolethane, and trimethylolpropane.
Among these, it is more preferable to use at least one selected from neopentyl glycol and glycerin, from the viewpoint of achieving both cooling properties and evaporation properties.
The polyhydric alcohols may be used alone or in combination of two or more.

(fatty acid)
In this embodiment, the monovalent fatty acid having 2 to 12 carbon atoms is an acid component constituting the polyol ester (A).
The monovalent fatty acid preferably has 3 to 12 carbon atoms, more preferably 5 to 12 carbon atoms, and even more preferably 5 to 10 carbon atoms.

The fatty acids constituting the polyol ester (A) may be used singly or in combination of two or more.
The fatty acid may be a linear saturated fatty acid, a linear unsaturated fatty acid, a branched saturated fatty acid, or a branched unsaturated fatty acid. Therefore, the fatty acid may be one or more selected from a linear saturated fatty acid, a linear unsaturated fatty acid, a branched saturated fatty acid, and a branched unsaturated fatty acid, and is preferably one or more selected from a linear saturated fatty acid and a branched saturated fatty acid.

Specific examples of straight-chain saturated fatty acids include acetic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, and dodecanoic acid.
These may be used alone or in combination of two or more.

Specific examples of branched saturated fatty acids include 2-ethylhexanoic acid, isodecanoic acid, and 2-methyldecanoic acid.
Of these, 2-ethylhexanoic acid is preferred.

In the heat treatment oil of this embodiment, the polyol ester (A) functions as a base oil.
In the heat treatment oil of this embodiment, the content of the polyol ester (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 treatment oil. Note that the content of the polyol ester (A) may be 100% by mass or less, based on the total amount (100% by mass) of the heat treatment oil.

The polyol ester (A) may be a partial ester or a complete ester, but preferably contains a complete ester from the viewpoint of improving evaporation and cooling properties.
In the present embodiment, the content of complete esters in the polyol ester (A) is preferably 50% by mass to 100% by mass, more preferably 60% by mass to 100% by mass, even more preferably 70% by mass to 100% by mass, still more preferably 80% by mass to 100% by mass, even more preferably 90% by mass to 100% by mass, and even more preferably 95% by mass to 100% by mass.

(Kinematic Viscosity of Polyol Ester (A))
From the viewpoint of achieving both cooling properties and evaporativity, the kinematic viscosity at 40°C of the polyol ester (A) is preferably 1.0 mm ² /s or more and 40 mm ² /s or less, more preferably 1.0 mm ² /s or more and 30 mm ² /s or less, even more preferably 2.5 mm ² /s or more and 30 mm ² /s or less, and particularly preferably 2.5 mm ² /s or more and 25 mm ² /s or less.
In this specification, the kinematic viscosity at 40°C of the polyol ester (A) means a value measured in accordance with JIS K2283:2000.

<Method for producing polyol ester (A)>
The method for producing the polyol ester (A) is not particularly limited, and the polyol ester (A) can be produced, for example, by combining and reacting the polyhydric alcohol with a monohydric fatty acid, and then esterifying the resulting mixture by a conventional method.

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

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

In order to more easily achieve the effects of the present invention, it is preferable that the content of synthetic oils that do not fall under the category of polyol ester (A) is small. Specifically, the content of synthetic oils that do not fall under the category of polyol ester (A) is preferably less than 40 parts by mass, more preferably less than 10 parts by mass, even more preferably less than 1 part by mass, per 100 parts by mass of polyol ester (A), and even more preferably no synthetic oils that do not fall under the category of polyol ester (A).

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 treatments 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.

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 polyol ester (A) of this embodiment, and therefore contains low molecular weight components. Low molecular weight components are highly volatile and therefore easily evaporate. As a result, the time (characteristic seconds) until the vapor film stage of the heat treatment oil is completed becomes longer, so a low content is preferable.
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, even more preferably less than 0.1 parts by mass, and even more preferably no mineral oil is contained, relative to 100 parts by mass of the polyol ester (A).

<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.

(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% by mass, more preferably 0.3 to 3.0% by mass, and even more preferably 0.4 to 2.5% by mass, based on the total amount of the heat treatment oil.

(antioxidant)
Examples of the antioxidant include phenol-based antioxidants and amine-based antioxidants.
Examples of phenolic antioxidants include 2,6-di-tert-butyl-paracresol, 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 thereof 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 treatment oil.

(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% by mass to 5.0% by mass, more preferably 0.1% by mass to 3.0% by mass, and even more preferably 0.3% by mass to 2.0% by mass, based on the total amount of the heat treatment oil.

In addition, 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.

[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 even more preferably 160°C or higher.
In this specification, the flash point of the polyol ester (A) means a value measured by the Cleveland Open Chamber method (C.O.C. method) in accordance with K2265-4:2007.
<40℃ kinematic viscosity>
From the viewpoint of achieving both cooling properties and evaporativity, the 40°C kinematic viscosity of the heat treatment oil of this embodiment is preferably 1.0 mm ² /s or more and 40 mm ² /s or less, more preferably 2.5 mm ² /s or more and 30 mm ² /s or less, and even more preferably 3.0 mm ² /s or more and 20 mm ² /s or less.
In this specification, the kinematic viscosity at 40°C of the polyol ester (A) means a value measured in accordance with JIS K2283:2000.

<Volatility>
The volatility of the heat-treated oil of this embodiment can be evaluated by the method described in the examples below.
<Cooling property>
The cooling ability of the heat treatment oil of this embodiment can be evaluated by 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 0.095 cm ⁻¹ or more, more preferably 0.100 cm ⁻¹ or more, even more preferably 0.105 cm ⁻¹ or more, still more preferably 0.110 cm ⁻¹ or more, and even more preferably 0.120 cm ⁻¹ or more.

<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 6.0 seconds or less, and even more preferably 5.0 seconds or less.

[Metal member 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 hardness and toughness of the target metal member, but is, for example, about 150 to 600°C.

[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 in which cleaning of metal components is not required after the heat treatment, and can be particularly suitably used as a quenching oil in which cleaning of metal components is not required after quenching.

The present invention will be explained in more detail using the following examples, but the present invention is not limited to these examples.

[Method for measuring physical properties]
The physical properties were measured as follows.
(1) Kinematic Viscosity at 40°C: Measured in accordance with JIS K2283:2000.

<Flash point measurement>
Measurement was carried out by the Cleveland Open Crack Method (C.O.C. method) in accordance with JIS K2265-4:2007.

[Examples 1 to 4 and Comparative Examples 1 to 6]
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.

<Base oil>
Polyol ester (A1): Complete ester of neopentyl glycol and 2-ethylhexanoic acid Polyol ester (A2): Complete ester of glycerin and acetic acid (purity of complete ester: 95% by mass or more)
Polyol ester (A3): Complete ester of glycerin and propionic acid (purity of complete ester: 95% by mass or more)
Polyol ester (A4): Complete ester of glycerin and 2-ethylhexanoic acid (purity of complete ester: 95% by mass or more)
Polyol ester (A'1): a complete ester of glycerin and oleic acid (purity of the complete ester: 95% by mass or more)
Polyol ester (A'2): a complete ester of pentaerythritol and a fatty acid having 5 to 10 carbon atoms (purity of the complete ester: 95% by mass or more)
Dibasic acid ester 1: ditridecyl adipate Dibasic acid ester 2: bis(2-ethylhexyl) dodecanedioate
Dibasic acid ester 3: bis(2-ethylhexyl) sebacate
Monoester: Butyl hexanoate

[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.

[Evaluation of cooling performance]
In accordance with the cooling property test method specified in JIS K 2242:2012, a cooling curve was created by recording the temperature change from 800°C for 60 seconds, and the characteristic number of seconds was calculated from the time required to reach the characteristic temperature. In addition, the H value was calculated using the time required to reach 300°C from 800°C on the created cooling curve, according to the Osaka University cooling capacity evaluation method.

The compositions and evaluation results of the heat-treated oils of Examples 1 to 4 and Comparative Examples 1 to 6 are shown in Table 1.

As shown in Table 1, the heat-treated oils of Examples 1 to 4, which used as base oils polyol esters (A) of dihydric or trihydric polyhydric alcohols having 1 to 10 carbon atoms and monohydric fatty acids having 2 to 12 carbon atoms, had excellent volatility, a relatively short characteristic time in seconds, and a relatively high H value.
In contrast, the heat-treated oil of Comparative Example 1, which used a polyol ester (A'1) of a polyhydric alcohol and a fatty acid having 18 carbon atoms as the base oil, the heat-treated oil of Comparative Example 2, which used a polyol ester (A'2) of a tetrahydric polyhydric alcohol and a fatty acid, and the heat-treated oils of Comparative Examples 3 to 5, which used dibasic acid esters, had low volatility and required cleaning after heat treatment.
The heat-treated oil of Comparative Example 6, which used a monoester as the base oil, also had a long characteristic time and a low H value.

Claims (10)

A heat-treated oil containing, as a base oil, a polyol ester (A) of a dihydric or trihydric polyhydric alcohol having 1 to 10 carbon atoms and a monohydric fatty acid having 2 to 12 carbon atoms. The heat treatment oil according to claim 1, wherein the polyhydric alcohol has 2 to 6 carbon atoms. The heat-treated oil according to claim 1 or 2, wherein the fatty acid has 3 to 12 carbon atoms. The heat treatment oil according to any one of claims 1 to 3, wherein the content of the polyol ester (A) is 50 mass% or more based on the total amount of the heat treatment oil. The heat-treated oil according to any one of claims 1 to 4, having a flash point of 150°C or higher. The heat treatment oil described in any one of claims 1 to 5, used as a quenching oil. A method for manufacturing a metal component, comprising a quenching step in which a heated metal component is immersed in the heat treatment oil described in any one of claims 1 to 6 and cooled. The method for manufacturing a metal component according to claim 7, wherein the quenching temperature in the quenching step is 500 to 1400°C. The method for manufacturing a metal component according to claim 7 or 8, further comprising a tempering step in which the metal component is reheated without being cleaned after the quenching step. The method for producing a metal member according to claim 9, wherein the heating temperature in the tempering step is 150 to 600°C.