JP7670970B2 - Steel - Google Patents

Description

This disclosure relates to steel materials, and more specifically, to steel materials suitable for use as materials for products such as steel pistons.

Engines, such as diesel engines, contain a piston. The piston is housed in a cylinder of the engine and moves back and forth within the cylinder. The piston is exposed to high temperatures during the combustion process while the engine is running.

Most conventional pistons are manufactured by casting aluminum. However, in recent years, there has been a demand for further improvements in engine combustion efficiency. With aluminum cast pistons, the surface temperature during use is around 240 to 330°C.

Recently, there has been research into using pistons in even higher combustion temperature ranges to improve combustion efficiency. For this reason, there is a demand for piston materials that can withstand surface temperatures of 400°C or higher, or even 500°C or higher, during use.

In order to meet such demands, steel pistons manufactured using steel material have begun to be proposed. A steel piston is proposed, for example, in Patent Document 1. Steel pistons have a higher melting point than pistons made of aluminum castings. Therefore, steel pistons can be used in a higher combustion temperature range than pistons made of aluminum castings.

Patent Document 2 proposes a technology to increase the lifespan of steel pistons. Specifically, Patent Document 2 points out the following points regarding the lifespan of steel pistons: During use of a steel piston in a high combustion temperature range, oxide scale forms on the surface of the piston crown of the steel piston. When the formed oxide scale peels off from the piston crown, scale scratches are formed on the piston crown. When these scale scratches (areas where the oxide scale has peeled off) spread, cracks occur in the piston crown of the steel piston. To solve this problem, Patent Document 2 forms a protective layer on the piston crown of the steel piston to suppress the formation of oxide scale.

In the above-mentioned Patent Document 2, a protective layer is formed on the steel piston to increase the lifespan of the steel piston. However, no particular consideration is given to the steel material from which the steel piston is made.

Patent Document 3 discloses a steel material that can be used as a material for steel pistons. In the steel material proposed in Patent Document 3, the Mo content and V content in the steel material are adjusted to satisfy a specific formula. As a result, when a steel piston is manufactured using the steel material as a material, a large number of composite carbides of V and Mo are aged and precipitated during the tempering treatment in the manufacturing process. These composite carbides increase the high-temperature strength. As a result, the high-temperature fatigue strength of the steel piston is increased.

JP 2004-181534 A JP 2015-078693 A International Publication No. 2019/230946

The steel material proposed in Patent Document 3 exhibits excellent high-temperature fatigue strength when it is subjected to a tempering treatment and made into a product (steel piston). However, it may also exhibit excellent high-temperature fatigue strength when it is subjected to a tempering treatment during the manufacturing process of the product as a raw material for the product by a means other than that proposed in Patent Document 3.

The objective of this disclosure is to provide a steel material that exhibits excellent high-temperature fatigue strength when subjected to thermal refining treatment during the manufacturing process of the product as a raw material for the product.

The steel material disclosed herein has the following configuration:

A steel material,
The chemical composition, in mass%, is
C: 0.15-0.50%,
Si: 0.01 to 0.50%,
Mn: 0.10-1.00%,
P: 0.020% or less,
S: 0.028% or less,
Cr: 0.50-2.00%,
Mo: 0.08-0.50%,
V: 0.05-0.50%,
Ti: 0.05-0.20%,
Al: 0.005-0.100%,
N: 0.0200% or less,
O: 0.0050% or less,
Cu: 0.00-0.50%,
Ni: 0.00 to 1.00%,
Nb: 0.000 to 0.100%, and
The balance: Fe and impurities,
It consists of:
The Ti content in mass% in the residue obtained by electrolytic extraction is defined as [Ti] _R ;
The V content in the residue in mass% is defined as [V] _R ,
The Ti content in mass% in the chemical composition of the steel material is defined as [Ti] _B ;
When the V content in mass% in the chemical composition of the steel material is defined as [V] _B ,
The solute Ti ratio R _Ti defined by the formula (1) is 0.30 or less,
The solute V ratio R _V defined by the formula (2) is 0.60 or more;
Steel.
R _Ti = ([Ti] _B - [Ti] _R )/[Ti] _B (1)
R _V = ([V] _B - [V] _R )/[V] _B (2)

The steel material disclosed herein exhibits excellent high-temperature fatigue strength when used as a raw material for products and is subjected to thermal refining treatment during the manufacturing process of the products.

The inventor first investigated and examined the high-temperature fatigue strength after tempering treatment of steel materials that are used as the raw material for products that undergo tempering treatment during the manufacturing process, such as steel pistons.

First, the inventors investigated steel materials that exhibit excellent high-temperature fatigue strength after tempering treatment from the standpoint of chemical composition. As a result, the inventors believed that a steel material with a chemical composition, in mass%, of C: 0.15-0.50%, Si: 0.01-0.50%, Mn: 0.10-1.00%, P: 0.020% or less, S: 0.028% or less, Cr: 0.50-2.00%, Mo: 0.08-0.50%, V: 0.05-0.50%, Al: 0.005-0.100%, N: 0.0200% or less, O: 0.0050% or less, Cu: 0.00-0.50%, Ni: 0.00-1.00%, Nb: 0.000-0.100%, and the balance: Fe and impurities could exhibit excellent high-temperature fatigue strength when used as a material for products such as steel pistons and subjected to thermal refining treatment during the manufacturing process of the product.

However, even when the ranges of each element in the chemical composition are within the above-mentioned ranges, there are cases in which sufficient high-temperature fatigue strength cannot be obtained in high-temperature bending fatigue tests at temperatures higher than 500°C. Therefore, the inventors further investigated means for obtaining high-temperature fatigue strength after thermal refining treatment. As a result, the inventors have come to the following findings.

When tempering treatment is performed on steel material whose chemical composition contains elements within the above-mentioned ranges, V precipitates will age and precipitate. Even in high-temperature environments of 500 to 600°C, V precipitates do not melt and remain in the steel material as precipitates. Therefore, V precipitates increase the high-temperature fatigue strength of the steel material. The higher the number density of V precipitates that age and precipitate in the steel material, the higher the high-temperature fatigue strength of the steel material.

The inventors therefore investigated means for age-precipitating a large number of V precipitates during tempering. As a result, they found that by further adding Ti to a steel material having the above chemical composition in place of a portion of the Fe, the number density of V precipitates after tempering increases.

As a result of the inventors' investigations, they discovered that if the above-mentioned chemical composition further contains 0.05 to 0.20% Ti instead of part of the Fe, the formation of V precipitates during tempering treatment is promoted, and excellent high-temperature fatigue strength can be obtained in the steel material (product) after tempering treatment.

However, even with steel materials with the above-mentioned chemical composition that contain Ti, there are cases where sufficient high-temperature fatigue strength cannot be obtained after tempering. Therefore, the inventors conducted further research and studies. As a result, they obtained the following findings.

The Ti content in mass% in the residue obtained by electrolytic extraction is defined as [Ti] _R . The V content in mass% in the residue is defined as [V] _R . Furthermore, the Ti content in mass% in the chemical composition of the steel is defined as [Ti] _B . The V content in mass% in the chemical composition of the steel is defined as [V] _B . At this time, the steel having the above-mentioned chemical composition is made to satisfy the following two requirements.
The solute Ti ratio R _Ti defined by the formula (1) is set to 0.30 or less.
The solute V ratio R _V defined by the formula (2) is 0.60 or more.
R _Ti = ([Ti] _B - [Ti] _R )/[Ti] _B (1)
R _V = ([V] _B - [V] _R )/[V] _B (2)

Steel that meets the above two requirements exhibits excellent high-temperature fatigue strength after tempering. The reason for this is unclear, but the following are thought to be the reasons:

Ti forms fine Ti precipitates in steel. Ti precipitates serve as nucleation sites for V precipitates during tempering. Therefore, in order to increase the number density of V precipitates after tempering, it is preferable that a large number of Ti precipitates are present in the steel before tempering.

On the other hand, if V precipitates are present in the steel before tempering, the amount of V present as dissolved V during tempering will be reduced. In this case, the number density of V precipitates that precipitate during aging will be low during tempering.

Therefore, in the steel material before the thermal refining treatment, Ti and V are preferably in the following state.
(A) Ti is preferably precipitated as Ti precipitates.
(B) V is made to exist as solid solution V as much as possible, and V precipitates in the steel material are reduced as much as possible.

In the steel material of this embodiment, the solute Ti ratio R _Ti is set to 0.30 or less, and the solute V ratio R _V is set to 0.60 or more. In this case, both of (A) and (B) above can be achieved. As a result, the number density of V precipitates can be increased in the tempering treatment, and the high-temperature fatigue strength of the steel material (product) after the tempering treatment is improved.

The steel material for steel pistons according to this embodiment, which was completed based on the above findings, has the following configuration.

[1]
A steel material,
The chemical composition, in mass%, is
C: 0.15-0.50%,
Si: 0.01 to 0.50%,
Mn: 0.10-1.00%,
P: 0.020% or less,
S: 0.028% or less,
Cr: 0.50-2.00%,
Mo: 0.08-0.50%,
V: 0.05-0.50%,
Ti: 0.05-0.20%,
Al: 0.005-0.100%,
N: 0.0200% or less,
O: 0.0050% or less,
Cu: 0.00-0.50%,
Ni: 0.00 to 1.00%,
Nb: 0.000 to 0.100%, and
The balance: Fe and impurities,
It consists of:
The Ti content in mass% in the residue obtained by electrolytic extraction is defined as [Ti] _R ;
The V content in the residue in mass% is defined as [V] _R ,
The Ti content in mass% in the chemical composition of the steel material is defined as [Ti] _B ;
When the V content in mass% in the chemical composition of the steel material is defined as [V] _B ,
The solute Ti ratio R _Ti defined by the formula (1) is 0.30 or less,
The solute V ratio R _V defined by the formula (2) is 0.60 or more;
Steel.
R _Ti = ([Ti] _B - [Ti] _R )/[Ti] _B (1)
R _V = ([V] _B - [V] _R )/[V] _B (2)

[2]
The steel material according to [1],
The chemical composition is
Cu: 0.01 to 0.50%,
Ni: 0.01 to 1.00%, and
Nb: 0.001 to 0.100%,
Contains one or more elements selected from the group consisting of
Steel.

The steel material according to this embodiment will be described in detail below. Unless otherwise specified, "%" for elements means mass %.

[Features of the steel material according to this embodiment]
The steel material of this embodiment has the following feature 1 and feature 2.
(Feature 1)
The content of each element in the chemical composition is within the range described below.
(Feature 2)
The Ti content in mass% in the residue obtained by electrolytic extraction is defined as [Ti] _R ;
The V content in the residue in mass% is defined as [V] _R ,
The Ti content in mass% in the chemical composition of the steel material is defined as [Ti] _B ,
When the V content in mass% in the chemical composition of the steel is defined as [V] _B ,
The solute Ti ratio R _Ti defined by the formula (1) is 0.30 or less,
The solute V ratio R _V defined by the formula (2) is 0.60 or more.
R _Ti = ([Ti] _B - [Ti] _R )/[Ti] _B (1)
R _V = ([V] _B - [V] _R )/[V] _B (2)

Features 1 and 2 are explained below.

[(Feature 1) Chemical composition]
The chemical composition of the steel material of this embodiment contains the following elements.

C: 0.15-0.50%
Carbon (C) increases the strength of steel. If the C content is less than 0.15%, this effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the C content exceeds 0.50%, the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the C content is 0.15 to 0.50%.
The lower limit of the C content is preferably 0.16%, more preferably 0.17%, further preferably 0.18%, and further preferably 0.19%.
The upper limit of the C content is preferably 0.48%, more preferably 0.46%, still more preferably 0.44%, still more preferably 0.42%, and still more preferably 0.40%.

Si: 0.01~0.50%
Silicon (Si) deoxidizes the steel. Si also increases the strength of ferrite. If the Si content is less than 0.01%, these effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Si content exceeds 0.50%, the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Si content is 0.01 to 0.50%.
The lower limit of the Si content is preferably 0.02%, more preferably 0.03%, more preferably 0.04%, more preferably 0.10%, more preferably 0.15%, and more preferably 0.20%.
The upper limit of the Si content is preferably 0.45%, more preferably 0.40%, further preferably 0.35%, and further preferably 0.30%.

Mn: 0.10-1.00%
Manganese (Mn) improves the hardenability of steel and increases the strength of steel by solid solution strengthening. If the Mn content is less than 0.10%, these effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Mn content exceeds 1.00%, the machinability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Mn content is 0.10 to 1.00%.
The lower limit of the Mn content is preferably 0.12%, more preferably 0.14%, further preferably 0.16%, and further preferably 0.18%.
The upper limit of the Mn content is preferably 0.90%, more preferably 0.85%, further preferably 0.80%, and further preferably 0.75%.

P: 0.020% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is more than 0%.
If the P content exceeds 0.020%, even if the contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries, reducing the strength of the steel material.
Therefore, the P content is 0.020% or less.
The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of industrial production, the lower limit of the P content is preferably 0.001%, and more preferably 0.002%.
The upper limit of the P content is preferably 0.019%, more preferably 0.018%, further preferably 0.017%, and further preferably 0.015%.

S: 0.028% or less Sulfur (S) is unavoidably contained. In other words, the S content is more than 0%.
S combines with Mn to form Mn sulfides, which improve the machinability of steel. Even if only a small amount of S is contained, this effect can be obtained to some extent.
On the other hand, if the S content exceeds 0.028%, even if the contents of other elements are within the ranges of this embodiment, coarse Mn sulfides are formed or excessive Mn sulfides are formed, which reduces the high-temperature fatigue strength.
Therefore, the S content is 0.028% or less.
The lower limit of the S content is preferably 0.001%, more preferably 0.003%, further preferably 0.005%, and further preferably 0.009%.
The upper limit of the S content is preferably 0.025%, more preferably 0.023%, still more preferably 0.020%, still more preferably 0.019%, still more preferably 0.018%, and still more preferably 0.015%.

Cr:0.50~2.00%
Chromium (Cr) increases the strength of steel material. If the Cr content is less than 0.50%, this effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Cr content exceeds 2.00%, even if the contents of other elements are within the ranges of this embodiment, Cr carbides are formed, reducing the high-temperature fatigue strength. If the Cr content exceeds 2.00%, the machinability of the steel material also decreases.
Therefore, the Cr content is 0.50 to 2.00%.
The lower limit of the Cr content is preferably 0.55%, more preferably 0.60%, further preferably 0.65%, and further preferably 0.70%.
The upper limit of the Cr content is preferably 1.90%, more preferably 1.80%, further preferably 1.70%, further preferably 1.60%, and further preferably 1.50%.

Mo: 0.08~0.50%
Molybdenum (Mo) ages and precipitates together with V (described later) in the operating temperature range of steel pistons (500 to 600°C), forming precipitates. This enhances the high-temperature fatigue strength of the steel piston when the engine is in operation. If the Mo content is less than 0.08%, this effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Mo content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes excessively high and the toughness decreases.
Therefore, the Mo content is 0.08 to 0.50%.
The lower limit of the Mo content is preferably 0.09%, more preferably 0.10%, still more preferably 0.11%, still more preferably 0.12%, and still more preferably 0.13%.
The upper limit of the Mo content is preferably 0.46%, more preferably 0.42%, still more preferably 0.38%, still more preferably 0.34%, and still more preferably 0.30%.

V: 0.05-0.50%
Vanadium (V) ages and precipitates together with Mo in the operating temperature range of steel pistons (500-600°C), forming precipitates. This enhances the high-temperature fatigue strength of the steel piston when the engine is in operation. If the V content is less than 0.05%, this effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the V content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes excessively high and the toughness decreases.
Therefore, the V content is 0.05 to 0.50%.
The lower limit of the V content is preferably 0.07%, more preferably 0.09%, further preferably 0.11%, and further preferably 0.13%.
The upper limit of the V content is preferably 0.48%, more preferably 0.46%, still more preferably 0.44%, still more preferably 0.42%, and still more preferably 0.40%.

Ti: 0.05-0.20%
Titanium (Ti) promotes the formation of V precipitates in a steel piston when the steel piston is manufactured from steel material. As a result, the high-temperature fatigue strength of the steel piston during engine operation is increased. If the Ti content is less than 0.05%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Ti content exceeds 0.20%, even if the contents of other elements are within the ranges of this embodiment, coarse Ti precipitates remain in the steel piston, and in this case, the high temperature fatigue strength of the steel piston is rather reduced.
Therefore, the Ti content is 0.05 to 0.20%.
The lower limit of the Ti content is preferably 0.06%, more preferably 0.07%, and further preferably 0.08%.
The upper limit of the Ti content is preferably 0.18%, more preferably 0.16%, and further preferably 0.14%.

Al: 0.005-0.100%
Aluminum (Al) deoxidizes steel, and if the Al content is less than 0.005%, this effect cannot be obtained even if the contents of other elements are within the ranges of this embodiment.
On the other hand, if the Al content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, oxides (inclusions) are generated in excess, and the high-temperature fatigue strength of the steel piston including the HAZ decreases.
Therefore, the Al content is 0.005 to 0.100%.
The lower limit of the Al content is preferably 0.007%, more preferably 0.008%, further preferably 0.010%, further preferably 0.012%, and further preferably 0.014%.
The upper limit of the Al content is preferably 0.090%, more preferably 0.080%, further preferably 0.070%, further preferably 0.060%, and further preferably 0.050%.

N: 0.0200% or less Nitrogen (N) is an unavoidable impurity. In other words, the N content is more than 0%.
If the N content exceeds 0.0200%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the N content is 0.0200% or less.
The upper limit of the N content is preferably 0.0190%, more preferably 0.0180%, further preferably 0.0170%, and further preferably 0.0160%.
The N content is preferably as low as possible. However, excessive reduction in the N content increases production costs. Therefore, in consideration of industrial production, the preferred lower limit of the N content is 0.0001%, more preferably 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%.

O: 0.0050% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is more than 0%.
If the O content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, excessive oxides are formed, and the high-temperature fatigue strength of the steel piston including the HAZ decreases.
Therefore, the O content is 0.0050% or less.
The upper limit of the O content is preferably 0.0045%, more preferably 0.0040%, still more preferably 0.0035%, still more preferably 0.0030%, and still more preferably 0.0020%.
The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, in consideration of industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.

The remainder of the chemical composition of the steel material of this embodiment is composed of Fe and impurities. Here, impurities refer to components that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of steel material, and are not intentionally included in the steel material.

The impurities include any element other than the above-mentioned impurities. The impurities may be one element only, or two or more elements.
Impurities other than the above-mentioned impurities include, for example, Ca, B, Sb, Sn, W, Co, As, Pb, Bi, and H. These elements may have the following contents as impurities, for example.
Ca: 0.0000 to 0.0005%, B: 0.0000 to 0.0005%, Sb: 0.0000 to 0.0005%, Sn: 0.0000 to 0.0005%, W: 0.0000 to 0.0005%, Co: 0.0000 to 0.0005%, As: 0.0000 to 0.0005%, Pb: 0.0000 to 0.0005%, Bi: 0.0000 to 0.0005%, H: 0.0000 to 0.0005%.

[Regarding optional elements]
The steel material of this embodiment further contains, instead of a part of Fe,
Cu: 0.00-0.50%,
Ni: 0.00 to 1.00%, and
Nb: 0.000-0.100%,
Any of these elements increases the strength of a final product when the final product is manufactured using the steel material as a raw material.

Cu: 0.00-0.50%
Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0.00%.
When contained, Cu improves the hardenability and strength of the steel material. If the Cu content exceeds 0.00%, these effects can be obtained to some extent.
However, if the Cu content exceeds 0.50%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment.
Therefore, the Cu content is 0.00 to 0.50%.
The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, further preferably 0.04%, and further preferably 0.05%.
The upper limit of the Cu content is preferably 0.48%, more preferably 0.46%, further preferably 0.44%, and further preferably 0.40%.

Ni: 0.00~1.00%
Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0.00%.
When contained, Ni enhances the hardenability and strength of the steel material. If the Ni content exceeds 0.00%, these effects can be obtained to some extent.
However, if the Ni content exceeds 1.00%, even if the contents of other elements are within the ranges of this embodiment, the effect becomes saturated, and furthermore, the raw material cost becomes high.
Therefore, the Ni content is 0.00 to 1.00%.
The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, further preferably 0.04%, and further preferably 0.05%.
The upper limit of the Ni content is preferably 0.98%, more preferably 0.90%, more preferably 0.85%, more preferably 0.80%, more preferably 0.70%, and more preferably 0.60%.

Nb: 0.000-0.100%
Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0.000%.
When Nb is contained, it forms carbides, nitrides, or carbonitrides (hereinafter referred to as carbonitrides, etc.) in the steel material, thereby increasing the strength of the steel material. If the Nb content exceeds 0.000%, these effects can be obtained to a certain degree.
However, if the Nb content exceeds 0.100%, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel material becomes too high, and the machinability of the steel material decreases.
Therefore, the Nb content is 0.000 to 0.100%.
The lower limit of the Nb content is preferably 0.001%, more preferably 0.005%, further preferably 0.010%, further preferably 0.015%, and further preferably 0.020%.
The upper limit of the Nb content is preferably 0.095%, more preferably 0.090%, still more preferably 0.085%, still more preferably 0.080%, and still more preferably 0.070%.

[Method of measuring the chemical composition of steel]
The chemical composition of the steel material of this embodiment can be measured by a known component analysis method. Specifically, chips are collected from the inside of the steel material to a depth of 1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by a known inert gas melting-thermal conductivity method. The O content is determined by a known inert gas melting-infrared absorption method.

The content of each element is determined by rounding off the measured value based on the significant figures specified in this embodiment to the lowest digit of the content of each element specified in this embodiment. For example, the C content of the steel material in this embodiment is specified as a value to two decimal places. Therefore, the C content is determined as a value to two decimal places obtained by rounding off the measured value to two decimal places.

Similarly, for the contents of other elements other than the C content of the steel material of this embodiment, the measured value is rounded off to the smallest digit specified in this embodiment to obtain the content of that element.

Note that rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[(Feature 2) Regarding the solute Ti ratio R _Ti and the solute V ratio R _V in steel]
The steel material of this embodiment further has the following characteristics, provided that the content of each element in the chemical composition is within the range of this embodiment.
The Ti content in mass% in the residue obtained by electrolytic extraction is defined as [Ti] _R ;
The V content in the residue in mass% is defined as [V] _R ,
The Ti content in mass% in the chemical composition of the steel material is defined as [Ti] _B ,
When the V content in mass% in the chemical composition of the steel is defined as [V] _B ,
The solute Ti ratio R _Ti defined by the formula (1) is 0.30 or less,
The solute V ratio R _V defined by the formula (2) is 0.60 or more.
R _Ti = ([Ti] _B - [Ti] _R )/[Ti] _B (1)
R _V = ([V] _B - [V] _R )/[V] _B (2)
This point will be explained below.

In the steel material of this embodiment, Ti in the steel material is generated as Ti precipitates as much as possible, and V in the steel material is secured as solid solution V as much as possible. If many Ti precipitates are generated in the steel material and a large amount of V is dissolved, when a final product such as a steel piston is manufactured using the steel material as a raw material, many fine V precipitates are generated by the thermal refining treatment (quenching and tempering) during the manufacturing process. As a result, the high temperature fatigue strength of the product after the thermal refining treatment is significantly increased.
When the content of each element in the chemical composition of the steel material is within the range of this embodiment, if the solute Ti ratio R _Ti defined by formula (1) is 0.30 or less and the solute V ratio R _V defined by formula (2) is 0.60 or more, the high-temperature fatigue strength of the product after the thermal refining treatment is sufficiently increased.

Therefore, the solute Ti ratio R _Ti is 0.30 or less, and the solute V ratio _RV is 0.60 or more.

The upper limit of the solute Ti ratio R _Ti is preferably 0.28, more preferably 0.26, and further preferably 0.24.
It is preferable that the solute Ti ratio R _Ti is as low as possible. In other words, the solute Ti ratio R _Ti may be 0. However, there are cases where it is difficult to make the solute Ti ratio R _Ti 0. Therefore, a preferable lower limit of the solute Ti ratio R _Ti is 0.01, more preferably 0.03, and further preferably 0.05.

The lower limit of the solute V ratio R _V is preferably 0.62, more preferably 0.64, still more preferably 0.66, and still more preferably 0.68.
The solute V ratio R _V is preferably as high as possible. That is, the solute V ratio R _V may be 1.00. However, it may be difficult to make the solute V ratio R _V 1.00. Therefore, the preferred upper limit of the solute V ratio R _V is 0.98, more preferably 0.96, more preferably 0.94, more preferably 0.92, and even more preferably 0.90.

[Method of determining solute Ti ratio _RTi and solute V ratio _RV in steel material]
The solute Ti ratio _RTi and the solute V ratio _RV in a steel material can be determined by the following method.
A cylindrical test piece having a diameter of 10 mm and a length of 15 mm is taken from a position at a depth of 1 mm or more from the surface of the steel material.

Preliminary electrolysis is carried out to remove any deposits (surface scale, impurities, etc.) on the surface of the collected cylindrical test specimen. In preliminary electrolysis, a 10% AA-based solution (a solution containing, by volume, 10% acetylacetone, 1% tetramethylammonium chloride, and 89% methanol solution) is used to electrolyze approximately 50 μm from the surface of the cylindrical test specimen at room temperature (20±15°C) and a current of 1000 mA.

The cylindrical test piece after preliminary electrolytic polishing is immersed in an alcohol solution. The cylindrical test piece immersed in the alcohol solution is subjected to ultrasonic cleaning to remove any deposits on the surface of the cylindrical test piece. The mass of the cylindrical test piece after ultrasonic cleaning (cylindrical test piece before constant current electrolysis) is measured.

Next, constant current electrolysis is performed on the cylindrical test piece. Specifically, a new 10% AA-based solution is prepared. Then, using the new 10% AA-based solution, electrolysis is performed on a region of the cylindrical test piece from the surface to a depth of about 100 μm at room temperature while maintaining a current density of 20 mA/ ^cm2 .

After constant current electrolysis, the cylindrical test piece is immersed in an alcohol solution. Then, ultrasonic cleaning is performed to remove any deposits on the surface of the cylindrical test piece. After constant current electrolysis, the mass of the cylindrical test piece from which the deposits have been removed is measured.

The 10% AA solution used in the constant current electrolysis and the alcohol solution used in the subsequent ultrasonic cleaning are suction filtered through a filter with a mesh size of 0.2 μm to extract the residue.

The extracted residue is dissolved in acid to obtain a solution. Chemical elemental analysis using ICP-AES is performed on the solution to obtain the Ti mass in the residue and the V mass in the residue. Furthermore, the mass of the cylindrical test piece before and after the constant current electrolysis is subtracted to obtain the mass of the steel bar electrolyzed at constant current. Based on the Ti mass in the residue and the mass of the steel bar electrolyzed at constant current, the Ti content [Ti] _R in mass% in the residue is obtained. Based on the V mass in the residue and the mass of the steel bar electrolyzed at constant current, the V content [V] _R in mass% in the residue is obtained.

The solute Ti ratio R Ti defined by formula (1) is calculated using the Ti content [Ti] _B in the chemical composition of the steel material and the Ti content [Ti] _R in the residue, both of which are obtained by the above-mentioned well-known _composition analysis method. Similarly, the solute V ratio R _V defined by formula (2) is calculated using the V content [V] _B in the chemical composition of the steel material and the V content [V] _R in the residue.

[Effects of the steel material according to this embodiment]
The steel material of this embodiment has the above-mentioned features 1 and 2. Therefore, when a product such as a steel piston is manufactured using the steel material of this embodiment as a material, the high-temperature fatigue strength of the product can be improved.

[Use of the steel material according to this embodiment]
The steel material of this embodiment can be applied as a material for products manufactured by performing thermal refining treatment. The steel material of this embodiment is particularly suitable as a material for products that require high-temperature fatigue strength. One example of such a product is a steel piston. However, the steel material of this embodiment can be widely applied to applications other than steel pistons.

[Microstructure of the steel material in this embodiment]

The microstructure of the steel material of this embodiment is not particularly limited. When the steel material of this embodiment is used as a material for a product such as a steel piston, it is heated to the A _c3 transformation point or higher during the manufacturing process of the product. Therefore, the microstructure of the steel material of this embodiment is not particularly limited.

As an example of the microstructure of the steel material of this embodiment, at the R/2 position of a cross section (transverse section) perpendicular to the longitudinal direction of the steel material, the area ratio of bainite is 60% or more, and the remainder is one or more of ferrite, pearlite, and martensite. Here, the R/2 position means the center position of the line segment R (i.e., the radius) connecting the center of the steel material and the surface of the steel material in the transverse section. However, the microstructure of the steel material of this embodiment is not particularly limited to the above-mentioned microstructure.

[Production method]
An example of a method for manufacturing a steel material according to this embodiment will be described. The method for manufacturing a steel material described below is one example for manufacturing a steel material according to this embodiment. Therefore, a steel material having the above-mentioned configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel material according to this embodiment.

An example of a method for manufacturing a steel material according to this embodiment includes the following steps.
(Step 1) Material preparation step (Step 2) Hot working step The hot working step includes the following two steps.
(Step 21) Heating process 1st heating process:
Heating temperature T1: 900-1100℃
Holding time t1 at heating temperature T1: 20 minutes or more Second heating step:
Heating temperature T2: over 1100~1300℃
Holding time t2 at heating temperature T2: 10 minutes or more (Step 22) Working step Finishing rolling temperature FT: 900° C. or more Average cooling rate CR from finishing rolling temperature FT to 500° C.: 1.0° C./sec or more Each step will be described below.

[(Process 1) Material preparation process]
In the material preparation process, molten steel having a chemical composition with each element content within the range of this embodiment is produced by a known steelmaking method. The produced molten steel is used to produce a slab by a continuous casting method. Here, the slab is, for example, a bloom or a billet. Instead of the slab, an ingot may be produced by an ingot casting method using the molten steel. The material (slab or ingot) is produced by the above production process.

[(Step 2) Hot working step]
In the hot working process, the produced material is hot worked to produce steel. In the hot working process, one or more hot working steps are usually performed. When multiple hot working steps are performed, the first hot working step (rough working step) is, for example, blooming or hot forging. The next hot working step (finishing working step) is, for example, finish rolling using a continuous rolling mill. In the continuous rolling mill, horizontal stands each having a pair of horizontal rolls and vertical stands each having a pair of vertical rolls are alternately arranged in a row.

The rough processing step may be omitted. In other words, the hot working step may only include the finish processing step. If the material prepared in the material preparation step is a bloom or an ingot, the rough processing step is carried out to produce a billet. If the material prepared in the material preparation step is a billet, the rough processing step is omitted and the finish processing step is carried out.

As described above, the finish processing step during the hot working step includes the following steps:
(Step 21) Heating step (Step 22) Processing step Each step will be described below.

[(Step 21) Heating step]
In the heating process, the billet is heated using a continuous heating furnace. The continuous heating furnace is, for example, a walking beam furnace. In the continuous heating furnace, a plurality of billets are arranged from the charging port toward the extraction port. The plurality of billets advance from the charging port to the extraction port over time. At this time, the billets are arranged so that the longitudinal direction of the billet is perpendicular to the billet advancement direction. The billet is charged into the heating furnace from the charging port of the heating furnace and moves within the heating furnace. Then, when the billet reaches the extraction port, the billet is transported from the extraction port to the outside of the heating furnace. The transported billet is transported to a continuous rolling mill that performs finish rolling by a transport mechanism such as a transport roller.

In the heating process, two-stage heating is performed in a heating furnace. Specifically, the first heating process is performed, and then the temperature is raised and the second heating process is performed. By performing two-stage heating, the Ti precipitates in the billet are sufficiently dissolved in the heating process. In this case, fine Ti precipitates are sufficiently generated in the steel material after the processing process, which is the next process, provided that other manufacturing conditions are satisfied. Therefore, the dissolved Ti ratio in the steel material can be set to 0.30 or less. The first heating process and the second heating process are described below.

[First heating step]
In the first heating step, the billet is held in the heating furnace at the next heating temperature T1 (° C.) for a holding time t1 (minutes).
Heating temperature T1: 900-1100℃
Holding time t1 at heating temperature T1: 20 minutes or more In short, in the first heating step, the billet is heated in a temperature range of 900 to 1100° C. for 20 minutes or more in the heating furnace.
When the heating temperature T1 is less than 900° C. or the holding time t1 is less than 20 minutes, Ti precipitates in the billet do not dissolve sufficiently. In this case, fine Ti precipitates are not sufficiently generated in the steel material after the processing step. Therefore, the dissolved Ti ratio in the steel material after production exceeds 0.30.
If the heating temperature T1 is set to 900 to 1100°C and the holding time is set to 20 minutes or more, the Ti precipitates in the billet will be sufficiently dissolved in the heating step, provided that other manufacturing conditions are satisfied. Therefore, fine Ti precipitates will be sufficiently formed in the steel material after the processing step, provided that other manufacturing conditions are satisfied.

The upper limit of the retention time t1 is not particularly limited. Considering the manufacturing cost, a preferable upper limit of the retention time t1 is, for example, 120 minutes.

[Second heating step]
In the heating furnace, the billet after the first heating step is heated and the second heating step is performed. In other words, the billet after the first heating step is not cooled and then the second heating step is performed, but the billet after the first heating step is further heated and the second heating step is performed.

In the second heating step, the billet is held in the heating furnace at the next heating temperature T2 (° C.) for a holding time t2 (minutes).
Heating temperature T2: over 1100~1300℃
Holding time t2 at heating temperature T2: 10 minutes or more In short, in the second heating step, the billet is heated in a temperature range of more than 1100 to 1300° C. for 10 minutes or more.
When the heating temperature T2 is more than 1100 to 1300°C and the holding time t2 is less than 10 minutes, even if the conditions in the first heating step are satisfied, the Ti precipitates in the billet do not form a solid solution sufficiently. In this case, fine Ti precipitates are not sufficiently formed in the steel material after the processing step. Therefore, the solid solution Ti ratio in the steel material after production exceeds 0.30.
If the heating temperature T2 is set to more than 1100 to 1300°C and the holding time is set to 10 minutes or more, the Ti precipitates in the billet will be sufficiently dissolved, provided that other manufacturing conditions are satisfied. Therefore, fine Ti precipitates will be sufficiently generated in the steel material after the processing step, provided that other manufacturing conditions are satisfied.

The upper limit of the heating temperature T2, 1300°C, is a well-known temperature. The upper limit of the holding time t2 is not particularly limited. If manufacturing costs are to be reduced, a preferable upper limit of the holding time t2 is, for example, 100 minutes.

As described above, by performing the two-stage heating (the first heating step and the second heating step) in the heating step, the Ti precipitates in the billet can be sufficiently dissolved. Therefore, fine Ti precipitates can be uniformly dispersed in the steel material in the processing step. As a result, the solute Ti ratio R _Ti in the steel material can be set to 0.30 or less.

[(Step 22) Processing step]
In the processing step, the billet after the heating step is subjected to hot processing (hot rolling) using a continuous rolling mill to produce a steel material. The continuous rolling mill includes a plurality of rolling stands. The plurality of rolling stands are arranged in a line. The billet is reduced (rolled) by the plurality of rolling stands to produce a steel material of a predetermined size.
The processing step satisfies the following conditions:
Finish rolling temperature FT: 900° C. or higher Average cooling rate CR from finish rolling temperature FT to 500° C.: 1.0° C./sec or higher The finish rolling temperature FT and the average cooling rate CR affect the solute V ratio R _V.

If the finish rolling temperature FT is less than 900° C., V precipitates are generated during the finish rolling, and therefore the solute V ratio R _V of the steel material becomes less than 0.60.
Therefore, the finish rolling temperature FT is set to 900° C. or higher.
The lower limit of the finish rolling temperature FT is preferably 920°C, more preferably 940°C, and further preferably 960°C.
The upper limit of the finish rolling temperature FT is not particularly limited. The finish rolling temperature FT depends on the heating temperature T2 in the heating step. When the heating temperature T2 is 1100 to 1300°C, the upper limit of the finish rolling temperature FT is, for example, 1100°C.

If the average cooling rate CR is less than 1.0° C./sec, V precipitates will precipitate at the phase interface during the cooling process, and the solute V ratio R _V of the steel material will be less than 0.60. Therefore, the average cooling rate CR is 1.0° C./sec or more.
The lower limit of the average cooling rate CR is preferably 2.0° C./sec, and more preferably 3.0° C./sec.
The upper limit of the average cooling rate CR is not particularly limited. The upper limit of the average cooling rate CR is, for example, 15.0° C./sec, for example, 12.0° C./sec.

The above steps allow the steel material of this embodiment to be manufactured.

[Method of manufacturing steel piston]
An example of a method for manufacturing a steel piston using the steel material of the present embodiment described above will be described.

A method for manufacturing the steel piston of this embodiment is, for example, as follows.
Hot forging process → Thermal treatment process → Joining process → Machining process

Specifically, hot forging is performed on the steel material of this embodiment, which is the raw material, to manufacture the upper and lower intermediate members (hot forging process). The heating temperature of the steel material during hot forging is 1100 to 1250°C. Here, the heating temperature refers to the furnace temperature of the heating furnace.

As described above, the heating temperature of the steel material in the hot forging process is 1100 to 1250° C., but the holding time at the heating temperature is short. Therefore, the solute Ti ratio R _Ti is almost maintained in the intermediate product after hot forging.

The manufactured upper and lower members are subjected to a known thermal refining treatment (quenching and tempering) (thermal refining treatment step). The quenching treatment is performed at a known quenching temperature ( _A3 transformation point or higher), followed by quenching. The quenching is, for example, water cooling or oil cooling. The tempering treatment is also performed at a known tempering temperature (A _C1 transformation point or lower).

After the thermal refining process, the upper and lower parts are subjected to well-known friction welding or laser welding to produce a joined product in which the upper and lower parts are joined (joining process). The joined product is then subjected to machining such as cutting (machining process) to produce the final product, a steel piston.

After the hot forging process and before the tempering process, cutting may be performed on the upper and/or lower members. Also, after the tempering process and before the joining process, cutting may be performed on the upper and/or lower members.

A steel piston manufactured using the steel material of this embodiment has excellent high-temperature fatigue strength.
In addition, among steel pistons, the upper member may be manufactured using the steel material of this embodiment as a material, and the lower member may be manufactured using an alloy other than the steel material of this embodiment. Also, the lower member may be manufactured using the steel material of this embodiment as a material, and the upper member may be manufactured using an alloy other than the steel material of this embodiment.
In short, the steel material of this embodiment may be used for at least one of the upper and lower members constituting a steel piston.

Steel material with the chemical composition shown in Table 1 was manufactured.

In Table 1, "-" means that the content of the corresponding element is 0% in the significant figures (numbers to the least significant digits) specified in the embodiment. In other words, it means that the content of the corresponding element is 0% when the fraction in the significant figures (numbers to the least significant digits) specified in the above embodiment is rounded off.

For example, the Cu content is specified in this embodiment as a value up to the second decimal place. Therefore, the Cu content of test number 1 is rounded off to the third decimal place, which means that the result is 0.00%. Similarly, the Nb content is specified in this embodiment as a value up to the third decimal place. Therefore, the Nb content of test number 1 is rounded off to the fourth decimal place, which means that the result is 0.000%.

Blooms were produced by continuous casting using the molten steel of each test number. The produced blooms were subjected to a rough processing process to produce billets. Specifically, the blooms were subjected to blooming to produce billets. The heating temperature of the blooms for each test number before blooming in the rough processing process was 1000 to 1200°C. The billets produced in the rough processing process were air-cooled to room temperature.

Next, a finishing process was carried out on the billets. Specifically, a heating process (first heating process, second heating process) was carried out on the billets of each test number. Table 2 shows the heating temperature T1 (°C) and holding time t1 (min) in the first heating process, and the heating temperature T2 (°C) and holding time t2 (min) in the second heating process.

In test number 30, the first heating step was not performed, and only the second heating step was performed. In other words, in test number 30, one-stage heating was performed in the finishing process. In test numbers other than test number 30, two-stage heating (first and second heating steps) was performed.

The billets after the heating process were subjected to finish rolling (processing process) using a continuous rolling mill. The finish rolling temperature FT (°C) for each test number was as shown in Table 2. Furthermore, the average cooling rate CR (°C/sec) from the finish rolling temperature FT to 500°C was as shown in Table 2. Through the above manufacturing process, a steel bar having a diameter of 40 mm was produced.

[Evaluation test]
The following evaluation tests were carried out using the steel materials (steel bars) having each test number thus produced.
(Test 1) Test for measuring the chemical composition of steel material (Test 2) Test for determining the solute Ti ratio _RTi and the solute V ratio _RV in steel material (Test 3) High-temperature fatigue strength evaluation test Each test will be described below.

[(Test 1) Steel Chemical Composition Measurement Test]
The chemical composition of the steel material of each test number was analyzed based on the method described in the above [Method of measuring chemical composition of steel material]. As a result, the chemical composition of the steel material of each test number was as shown in Table 1.

[(Test 2) Determination test of solute Ti ratio R _Ti and solute V ratio R _V in steel]
Based on the above-mentioned [Method for determining the solute Ti ratio _RTi and the solute V ratio _RV in steel material], the Ti content [Ti] _R in the residue and the V content [V] _R in the residue were determined.
Furthermore, the solute Ti ratio R Ti defined by formula (1) was calculated using the Ti content [Ti] _B in the chemical composition of the steel material obtained in Test 1 and the Ti content [Ti] _R in the residue. Similarly, the solute V ratio R _V defined by formula (2) was calculated using the V content [V] _B in the chemical composition of the steel material and the V content [ _V ] _R in the residue.
The obtained Ti content [Ti] _R , V content [V] _R , solute Ti ratio R _Ti , and solute V ratio R _V are shown in Table 2.

[(Test 3) High-Temperature Fatigue Strength Evaluation Test]
The manufacturing process of a steel piston was simulated, and the following manufacturing process was carried out using the steel material with each test number to prepare a high-temperature Ono-type rotating bending fatigue test specimen simulating a steel piston.

Specifically, a steel bar with a diameter of 40 mm for each test number was heated at a heating temperature of 1200°C for 30 minutes. After heating, the steel bar was hot forged to produce a round bar with a diameter of 30 mm. The finishing temperature for hot forging was 950°C or higher for all test numbers.

After hot forging, the round bar was subjected to a tempering treatment. Specifically, the round bar was heated at a heating temperature of 950°C for one hour, and then quenched by immersing it in an oil bath with an oil temperature of 80°C. After quenching, the round bar was subjected to a tempering treatment. In the tempering treatment, the round bar after quenching was held at a heating temperature of 600°C for one hour, and then allowed to cool in the air.

A high-temperature Ono-type rotating bending fatigue test specimen was prepared from the center of a cross section perpendicular to the axial direction (longitudinal direction) of the round bar after the tempering treatment. The central axis of the high-temperature Ono-type rotating bending fatigue test specimen was approximately the same as the central axis of the round bar after the tempering treatment. In addition, the diameter of the parallel part of the high-temperature Ono-type rotating bending fatigue test specimen was 8 mm, and the length of the parallel part was 15.0 mm.

Using the produced high-temperature Ono-type rotating bending fatigue test pieces, high-temperature Ono-type rotating bending fatigue tests were carried out under the following conditions. The evaluation temperature was 550°C.
After the test piece was mounted in the test machine in the heating furnace, the heating furnace was started to heat up while rotating at 2500 rpm. After the furnace temperature gauge reading of the heating furnace reached 550°C, the test piece was soaked at 550°C for 30 minutes. After soaking, a load was applied and the fatigue test was started.
The stress ratio was set to -1, and the maximum number of repetitions was set to 1 x ^107. The endurance stress at the maximum number of repetitions (1 x ¹⁰⁷ ) was defined as the fatigue strength (MPa).
The fatigue strength (MPa) obtained for each test number is shown in Table 2. If the fatigue strength was 250 MPa or more, it was determined that excellent high-temperature fatigue strength was obtained.

[Test Results]
The test results are shown in Tables 1 and 2.

The steel materials with test numbers 1 to 20 met characteristics 1 and 2. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was 250 MPa or more, and excellent high-temperature fatigue strength was obtained in the test pieces simulating steel pistons.

On the other hand, the V content of the steel material of test number 21 was too low. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test piece simulating the steel piston was low.

The Ti content of the steel material of test number 22 was too low. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test piece simulating the steel piston was low.

The Ti content of the steel material of test number 23 was too high. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test piece simulating the steel piston was low.

In the steels of test numbers 24 and 25, the heating temperature T1 in the first heating step in the finish processing step was low. Therefore, the solute Ti ratio R _Ti was too high. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test pieces simulating the steel piston was low.

In the steel material of test number 26, the holding time t1 at the heating temperature T1 in the first heating step of the finish processing step was short. Therefore, the solute Ti ratio R _Ti was too high. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test piece simulating the steel piston was low.

In the steels of test numbers 27 and 28, the heating temperature T2 in the second heating step in the finish processing step was low. Therefore, the solute Ti ratio R _Ti was too high. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test pieces simulating the steel piston was low.

In the steel material of test number 29, the holding time t2 at the heating temperature T2 in the second heating step of the finish processing step was short. Therefore, the solute Ti ratio R _Ti was too high. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test piece simulating the steel piston was low.

For the steel material of test number 30, the first heating step of the finishing process was not performed, and only the second heating step was performed. Therefore, the solute Ti ratio R _Ti was too high. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test piece simulating a steel piston was low.

In the steels of test numbers 31 and 32, the finishing temperature FT in the finishing process was low. Therefore, the solid solution V ratio R _V was too low. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test pieces simulating the steel piston was low.

In the steels of test numbers 33 and 34, the average cooling rate CR in the finishing process was slow. Therefore, the solute V ratio _RV was too low. As a result, the fatigue strength obtained in the high-temperature fatigue strength evaluation test was less than 250 MPa, and the high-temperature fatigue strength of the test pieces simulating the steel piston was low.

The above describes an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be modified as appropriate within the scope of the spirit of the present invention.

Claims (2)

A steel material,
The chemical composition, in mass%, is
C: 0.15-0.50%,
Si: 0.01 to 0.50%,
Mn: 0.10-1.00%,
P: 0.020% or less,
S: 0.028% or less,
Cr: 0.50-2.00%,
Mo: 0.08-0.50%,
V: 0.05-0.50%,
Ti: 0.05-0.20%,
Al: 0.005-0.100%,
N: 0.0200% or less,
O: 0.0050% or less,
Cu: 0.00-0.50%,
Ni: 0.00 to 1.00%,
Nb: 0.000 to 0.100%, and
The balance: Fe and impurities,
It consists of:
The Ti content in mass% in the residue obtained by electrolytic extraction is defined as [Ti] _R ;
The V content in the residue in mass% is defined as [V] _R ,
The Ti content in mass% in the chemical composition of the steel material is defined as [Ti] _B ;
When the V content in mass% in the chemical composition of the steel material is defined as [V] _B ,
The solute Ti ratio R _Ti defined by the formula (1) is 0.30 or less,
The solute V ratio R _V defined by the formula (2) is 0.60 or more;
Steel.
R _Ti = ([Ti] _B - [Ti] _R )/[Ti] _B (1)
R _V = ([V] _B - [V] _R )/[V] _B (2) The steel material according to claim 1,
The chemical composition is
Cu: 0.01 to 0.50%,
Ni: 0.01 to 1.00%, and
Nb: 0.001 to 0.100%,
Contains one or more elements selected from the group consisting of
Steel.