JP2023060831A - steel piston - Google Patents

Abstract

To provide a steel piston having excellent high temperature fatigue strength at a joint part even if an upper member and a lower member are made of different materials.SOLUTION: The upper member includes a steel material containing, by mass%, 0.15-0.50% C, 0.01-0.50% Si, 0.10-1.00% Mn, 0.020% or less P, 0.028% or less S, 0.50-2.00% Cr, 0.08-0.50% Mo, 0.05-0.50% V, 0.05-0.20% Ti, 0.005-0.100% Al, 0.0200% or less N, 0.0050% or less O, 0.00-0.50% Cu, 0.00-1.00% Ni, 0.000-0.100% Nb, and a balance Fe with inevitable impurities. Fn1 expressed by an expression (1) is 0.410 or more: Fn1=(95Si+25Mn+25Cr+12.5Mo+5V+15Al)-0.2+10-5/Ti (1).SELECTED DRAWING: Figure 3

Description

TECHNICAL FIELD The present disclosure relates to pistons and, more particularly, to steel pistons utilized in engines and the like.

Engines, typified by diesel engines, include pistons. The piston is housed within a cylinder of the engine and reciprocates within the cylinder. Pistons are exposed to high temperatures during the combustion process during engine operation.

Most conventional pistons are made from cast aluminum. However, in recent years, further improvements in engine combustion efficiency have been demanded. The surface temperature of an aluminum casting piston is about 240 to 330°C during use.

Recently, studies have been made to improve combustion efficiency by using a piston in a higher combustion temperature range. Therefore, there is a demand for a piston that can endure even when the surface temperature of the upper portion of the piston during use reaches 400° C. or higher, or even 500° C. or higher. In order to meet such demands, steel pistons manufactured using steel have begun to be proposed. A steel piston is proposed, for example, in US Pat. Steel pistons have a higher melting point than aluminum casting pistons. Therefore, steel pistons can be used in a higher combustion temperature range than aluminum casting pistons.

Patent Literature 2 proposes a technique for increasing the service life of steel pistons. Specifically, Patent Document 2 points out the following points regarding the service life of the steel piston. Oxidation scale is formed on the surface of the piston crown of the steel piston during use of the steel piston in a high temperature range. Scale flaws are formed on the piston crown by exfoliation of the oxide scale. Cracks are generated in the piston crown of the steel piston due to the spread of these scale flaws (regions where the oxide scale is peeled off). In Patent Document 2, in order to solve this problem, a protective layer is formed on the piston crown of the steel piston to suppress the formation of oxide scale.

In Patent Document 2 mentioned above, the service life of the steel piston is increased by forming a protective layer on the steel piston. However, the steel material used for the steel piston has not been studied.

In the steel piston proposed in Patent Document 3, a steel material suitable for the properties required for the steel piston is proposed. The steel piston disclosed in Patent Document 3 consists of an upper member made of steel and a lower member. The upper member and the lower member are joined by friction welding or laser welding. In the steel material forming the upper member, the Mo content and V content are adjusted so as to satisfy specified parameter formulas. In this steel material, the number density of Mn sulfides and oxides is further adjusted.

In the steel piston disclosed in Patent Document 3, the high-temperature fatigue strength of the steel piston is increased by adjusting the Mo content and V content of the steel material forming the upper member. Furthermore, by adjusting the number density of Mn sulfides and oxides in the steel material constituting the upper member, the fatigue strength at high temperatures of the heat affected zone (HAZ) near the joint interface is increased.

JP 2004-181534 A JP 2015-078693 A WO2019/230938

As disclosed in Patent Document 3, there is a case in which a steel piston is divided into an upper member and a lower member, and the upper member and the lower member are joined by friction welding, laser welding, or the like. The upper member may be made of a material different from that of the lower member. However, if the upper member and the lower member are made of a material different from that of the lower member, and the upper member and the lower member are joined by friction welding, laser welding, or the like, the fatigue strength at the joint may be lowered at high temperatures.

An object of the present disclosure is a steel piston in which an upper member and a lower member are joined, and which has excellent high temperature fatigue strength at the joint even if the upper member and the lower member are made of different materials. , is to provide a steel piston.

The steel piston of the present disclosure has the following configuration.

a steel piston,
an upper member including at least a crown topland;
a lower member disposed below the upper member and joined to the upper member;
with
The upper member is
The chemical composition, in mass %,
C: 0.15 to 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 to 2.00%,
Mo: 0.08-0.50%,
V: 0.05 to 0.50%,
Ti: 0.05 to 0.20%,
Al: 0.005 to 0.100%,
N: 0.0200% or less, and
O: 0.0050% or less,
the balance consists of Fe and impurities,
Made of steel material whose Fn1 defined by formula (1) is 0.410 or more,
The lower member is
Made of aluminum alloy material,
steel piston.
Fn1=(95Si+25Mn+25Cr+12.5Mo+5V+15Al) ^−0.2 +10 ⁻⁵ /Ti (1)
Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula (1).

The steel piston according to the present disclosure has excellent high temperature fatigue strength at the joint even though the upper and lower members are made of different materials.

FIG. 1 is a cross-sectional view of the steel piston according to the present embodiment, taken along a plane including the central axis of the steel piston. FIG. 2 shows the friction welding of a steel material having a chemical composition in which the content of each element is within the range of the present embodiment and an aluminum alloy material having a chemical composition corresponding to AC8A specified in JIS H5202:2010. , and a schematic diagram showing the relationship between the distance from the joint interface and the hardness (HV) of the steel material and the aluminum alloy material. FIG. 3 shows the hardness reduction amount ΔHV in the region near the joint interface and Fn1 when a steel material having a chemical composition in which the content of each element is within the range of the present embodiment and an aluminum alloy material are friction-welded. is a diagram showing the relationship of FIG. 4 shows the amount of hardness reduction ΔHV in the region near the joint interface when friction welding is performed between a steel material having a chemical composition in which the content of each element is within the range of the present embodiment and an aluminum alloy material, and the and high temperature fatigue strength.

The present inventors first examined the mechanical properties required for the upper member and the lower member of the steel piston whose surface temperature during use is 400° C. or higher.

As described above, the upper member is expected to have a surface temperature of 400° C. or higher, and thus excellent high-temperature fatigue strength is required. When the upper member is made of steel, it is superior in high-temperature fatigue strength to conventional aluminum alloy materials. Therefore, it is suitable that the upper member of the steel piston is made of steel.

On the other hand, the lower member of the steel piston is not exposed to the high temperature environment as the upper member. Furthermore, the shape of the lower member is complicated. Therefore, the lower member is required to be made of a material that can be easily formed into a complicated shape. Therefore, it is suitable for the lower member to be made of a conventional aluminum alloy material.

As a result of the above studies, the inventors of the present invention thought that it would be suitable for a steel piston to join an upper member made of steel and a lower member made of aluminum alloy.

Therefore, the present inventors have investigated the steel material that constitutes the upper member. As described above, the upper member is exposed to high temperatures of 400° C. or higher during use. Therefore, the steel material forming the upper member is required to have excellent high-temperature fatigue strength. Therefore, the present inventors have investigated steel materials that are excellent in high-temperature fatigue strength from the viewpoint of chemical composition. As a result, the steel material constituting the upper member, in mass%, C: 0.15 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.10 to 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: 0.005% 00 to 1.00%, Nb: 0.000 to 0.100%, and the balance: Fe and impurities.

However, when a steel piston is manufactured by joining an upper member made of steel having the chemical composition described above and a lower member made of an aluminum alloy, the high-temperature fatigue strength at the joint of the steel piston is sufficient. It turned out that there are cases where it is not possible to obtain Therefore, the present inventors investigated and examined the cause of this. As a result, the inventors obtained the following knowledge.

When an upper member made of steel and a lower member made of aluminum alloy are joined by friction welding, laser welding, or diffusion welding, the hardness of the aluminum alloy material decreases in the vicinity of the joint interface. Aluminum alloys are less hard than steel and have a lower melting point. Therefore, it is considered that the hardness of the region near the joint interface is likely to be reduced as a result of being easily affected by heat generated during welding.

The present inventors considered that the high-temperature fatigue strength of the joint portion of the steel piston is reduced due to the decrease in hardness in the region near the joint interface of the aluminum alloy material. Therefore, the present inventors have investigated a means for suppressing a decrease in hardness in the region near the joint interface of the lower member made of an aluminum alloy material.

As a result of investigation, the present inventors found that Si, Mn, Cr, Mo, V, Al and Ti, among the chemical compositions of the steel material of the upper member, are present in the region near the joint interface of the lower member made of an aluminum alloy material. It was found that it affects the hardness of Therefore, the content of these elements in the steel material of the upper member and the decrease in hardness of the aluminum alloy material of the lower member in the vicinity of the joint interface were further investigated and examined. As a result, if Fn1 defined by formula (1) is 0.410 or more, the decrease in hardness in the vicinity of the joint interface of the lower member made of aluminum alloy material is remarkably suppressed. It was found that excellent high-temperature fatigue strength was obtained in the part.
Fn1=(95Si+25Mn+25Cr+12.5Mo+5V+15Al) ^−0.2 +10 ⁻⁵ /Ti (1)
Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula (1).

The steel piston according to this embodiment completed based on the above findings has the following configuration.

[1]
a steel piston,
an upper member including at least a crown topland;
a lower member disposed below the upper member and joined to the upper member;
with
The upper member is
The chemical composition, in mass %,
C: 0.15 to 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 to 2.00%,
Mo: 0.08-0.50%,
V: 0.05 to 0.50%,
Ti: 0.05 to 0.20%,
Al: 0.005 to 0.100%,
N: 0.0200% or less, and
O: 0.0050% or less,
the balance consists of Fe and impurities,
Made of steel material whose Fn1 defined by formula (1) is 0.410 or more,
The lower member is
Made of aluminum alloy material,
steel piston.
Fn1=(95Si+25Mn+25Cr+12.5Mo+5V+15Al) ^−0.2 +10 ⁻⁵ /Ti (1)
Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula (1).

[2]
a steel piston,
an upper member including at least a crown topland;
a lower member disposed below the upper member and joined to the upper member;
with
The upper member is
The chemical composition, in mass %,
C: 0.15 to 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 to 2.00%,
Mo: 0.08-0.50%,
V: 0.05 to 0.50%,
Ti: 0.05 to 0.20%,
Al: 0.005 to 0.100%,
N: 0.0200% or less, and
O: 0.0050% or less,
moreover,
Cu: 0.50% or less,
Ni: 1.00% or less, and
Nb: 0.100% or less, containing one or more selected from the group consisting of
the balance consists of Fe and impurities,
Made of steel material whose Fn1 defined by formula (1) is 0.410 or more,
The lower member is
Made of aluminum alloy material,
steel piston.
Fn1=(95Si+25Mn+25Cr+12.5Mo+5V+15Al) ^−0.2 +10 ⁻⁵ /Ti (1)
Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula (1).

[3]
The steel piston according to [1],
Further, in the chemical composition of the steel material,
Fn2 defined by formula (2) is 3.56 to 4.20,
steel piston.
Fn2=100×(92C+357Si+200Mn+67Ni+100Cr+44Mo+4) ^−0.54 (2)
Here, the content in mass % of the corresponding element is substituted for each element symbol in the formula (2). When an element is not contained, "0" is substituted for the corresponding element symbol.

[4]
The steel piston according to [2],
Further, in the chemical composition of the steel material,
Fn2 defined by formula (2) is 3.56 to 4.20,
steel piston.
Fn2=100×(92C+357Si+200Mn+67Ni+100Cr+44Mo+4) ^−0.54 (2)
Here, the content in mass % of the corresponding element is substituted for each element symbol in the formula (2). When an element is not contained, "0" is substituted for the corresponding element symbol.

[5]
The steel piston according to any one of [1] to [4],
Further, in the chemical composition of the steel material,
Fn3 defined by formula (3) is 1.07 or more,
steel piston.
Fn3=100×(35C+161Si+200Mn+84Cr+5Mo) ^−0.8 (3)
Here, the content in mass % of the corresponding element is substituted for each element symbol in the formula (3).

The steel piston according to this embodiment will be described in detail below. "%" for elements means % by weight unless otherwise specified.

[Structure of Steel Piston]
FIG. 1 is a cross-sectional view of a steel piston 1 according to this embodiment, taken along a plane including the central axis of the steel piston 1. FIG.

Referring to FIG. 1, a steel piston 1 of this embodiment has a cylindrical shape. A steel piston 1 comprises an upper member 10 and a lower member 11 .
The upper member 10 is an upper portion of the steel piston 1 and is made of steel. Top member 10 includes at least a top land 16 of crown portion 13 .

The lower member 11 is arranged below the upper member 10 . The lower member 11 is joined with the upper member 10 . In FIG. 1, the lower member 11 is joined to the upper member 10 at the joint surface 30 . More specifically, the lower member 11 is friction-bonded, laser-bonded, or diffusion-bonded to the upper member 10 . Preferably, the diameter of the upper end surface of the lower member 11 is the same as the diameter of the lower end surface of the upper member 10 .

Lower member 11 includes at least a skirt portion 14 and a piston pin hole 15 . The skirt portion 14 is arranged below the crown portion 13 , and the upper end of the skirt portion 14 is connected to the lower end of the crown portion 13 .

A pair of piston pin holes 15 are formed in the skirt portion 14 into which piston pins (not shown) can be inserted. A gap 40 is formed between the pair of piston pin holes 15 . A small end of a connecting rod (not shown) is arranged in the gap 40 . The small end hole of the connecting rod and the pair of piston pin holes are coaxially arranged. A piston pin is inserted into a hole formed in the small end of the connecting rod and a pair of piston pin holes 15 to connect the steel piston and the connecting rod.

In FIG. 1 , the cavity 50 is defined by the lower surface of the upper member 10 and the upper surface of the lower member 11 . A cooling medium, for example, circulates in the cavity 50 to cool the steel piston 1 during use. Although the steel piston 1 includes a cavity 50 in FIG. 1, the shape of the cavity 50 is not limited to the shape shown in FIG. Also, the steel piston 1 may not include the cavity 50 . That is, the cavity 50 does not have to be formed between the lower surface of the upper member 10 and the upper surface of the lower member 11 .

In FIG. 1, the crown portion 13 of the steel piston 1 includes a top land 16, a plurality of lands 17 and 18, and a plurality of ring grooves 19-21. The top land 16 includes the top surface 161 of the uppermost end of the steel piston 1 . The land 17 is the peripheral surface of the crown portion 13 and is arranged below the top land 16 , and a ring groove 19 is formed between the top land 16 and the land 17 . A land 18 is a peripheral surface of the crown portion 13 and is arranged below the land 17 , and a ring groove 20 is formed between the lands 17 and 18 . A skirt portion 14 is formed below the land 18 , and a ring groove 21 is formed between the land 18 and the skirt portion 14 . A piston ring (not shown) can be arranged in each of the ring grooves 19-21.

In FIG. 1 , the upper member 10 does not include the entire crown portion 13 , but includes the upper portions of the top land 16 , the land 17 , and the land 18 of the crown portion 13 . However, the configuration of the upper member 10 is not limited to this. The surface temperature of the top land 16 is the highest in pistons for high temperature use environments. Therefore, it is sufficient for the upper member 10 to include at least the top land 16 of the crown portion 13 . That is, the upper member 10 may include the top land 16 of the crown portion 13 and not include the portion below the land 17 . In this case, the land 17 and the portion below the land 17 of the steel piston 1 serve as the lower member 11 .

The upper member 10 may include the entire crown portion 13 . In this case, the skirt portion 14 becomes the lower member 11 . In FIG. 1, the crown portion 13 includes a top land 16, a plurality of lands 17 and 18, and a plurality of ring grooves 19-21. However, the crown portion 13 may be composed of the top land 16 , one land 17 and one ring groove 19 .

[Chemical Composition of Steel Constituting Upper Member 10]
The upper member 10 of the steel piston 1 is made of steel. Specifically, the chemical composition of the steel material forming the upper member 10 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 content of other elements is within the range of the present embodiment.
On the other hand, if the C content exceeds 0.50%, the machinability of the steel deteriorates even if the content of other elements is within the range of the present embodiment.
Therefore, the C content is 0.15-0.50%.
The lower limit of the C content is preferably 0.16%, more preferably 0.17%, still more preferably 0.18%, still more 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%, still more preferably 0.40 %.

Si: 0.01-0.50%
Silicon (Si) deoxidizes 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 content of other elements is within the range of the present embodiment.
On the other hand, if the Si content exceeds 0.50%, the machinability of the steel deteriorates even if the content of other elements is within the range of the present embodiment.
Therefore, the Si content is 0.01-0.50%.
The lower limit of the Si content is preferably 0.02%, more preferably 0.03%, still more preferably 0.04%, still more preferably 0.10%, still more preferably 0.15 %, more preferably 0.20%.
A preferable upper limit of the Si content is 0.45%, more preferably 0.40%, still more preferably 0.35%, still more preferably 0.30%.

Mn: 0.10-1.00%
Manganese (Mn) enhances the hardenability of the steel material, and enhances the strength of the steel material by solid-solution strengthening. If the Mn content is less than 0.10%, these effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Mn content exceeds 1.00%, the machinability of the steel material deteriorates even if the content of other elements is within the range of the present embodiment.
Therefore, the Mn content is 0.10-1.00%.
A preferred lower limit for the Mn content is 0.12%, more preferably 0.14%, still more preferably 0.16%, and still more preferably 0.18%.
A preferred upper limit of the Mn content is 0.90%, more preferably 0.85%, still more preferably 0.80%, still more preferably 0.75%.

P: 0.020% or less Phosphorus (P) is an unavoidable impurity. That is, the P content is over 0%.
If the P content exceeds 0.020%, even if the content of other elements is within the range of the present embodiment, P segregates at grain boundaries and the strength of the steel decreases.
Therefore, the P content is 0.020% or less.
The lower the P content is, the better. However, reducing the P content excessively increases manufacturing costs. Therefore, considering industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%.
A preferable upper limit of the P content is 0.019%, more preferably 0.018%, still more preferably 0.017%, and still more preferably 0.015%.

S: 0.028% or less Sulfur (S) is inevitably contained. That is, the S content is over 0%.
S combines with Mn to form Mn sulfide and enhances the machinability of the steel material. This effect can be obtained to some extent if even a small amount of S is contained.
On the other hand, if the S content exceeds 0.028%, coarse Mn sulfides are generated or excessive Mn sulfides are generated even if the content of other elements is within the range of the present embodiment. do. In this case, the high temperature fatigue strength is lowered.
Therefore, the S content is 0.028% or less.
The lower limit of the S content is preferably 0.001%, more preferably 0.003%, still more preferably 0.005%, still more preferably 0.009%.
The preferred upper limit of the S content is 0.025%, more preferably 0.023%, still more preferably 0.020%, still more preferably 0.019%, still more preferably 0.018 %, more preferably 0.015%.

Cr: 0.50-2.00%
Chromium (Cr) increases the strength of steel. If the Cr content is less than 0.50%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Cr content exceeds 2.00%, Cr carbides are formed and the high-temperature fatigue strength is lowered even if the content of other elements is within the range of the present embodiment. If the Cr content exceeds 2.00%, the machinability of the steel further deteriorates.
Therefore, the Cr content is 0.50-2.00%.
A preferable lower limit of the Cr content is 0.55%, more preferably 0.60%, still more preferably 0.65%, still more preferably 0.70%.
The preferred upper limit of the Cr content is 1.90%, more preferably 1.80%, still more preferably 1.70%, still more preferably 1.60%, still more preferably 1.50 %.

Mo: 0.08-0.50%
Molybdenum (Mo) precipitates during aging together with V, which will be described later, in the operating temperature range (500 to 600° C.) of the steel piston to form precipitates. This increases the high temperature fatigue strength of the steel piston under engine operating conditions. If the Mo content is less than 0.08%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Mo content exceeds 0.50%, the strength of the steel material becomes excessively high and the toughness decreases even if the content of other elements is within the range of the present embodiment.
Therefore, the Mo content is 0.08-0.50%.
The preferred lower limit of the Mo content is 0.09%, more preferably 0.10%, more preferably 0.11%, more preferably 0.12%, still more preferably 0.13 %.
A preferable upper limit of Mo content is 0.46%, more preferably 0.42%, more preferably 0.38%, more preferably 0.34%, more preferably 0.30 %.

V: 0.05-0.50%
Vanadium (V) precipitates during aging together with Mo in the temperature range (500 to 600° C.) in which steel pistons are used, forming precipitates. This increases the high temperature fatigue strength of the steel piston under engine operating conditions. If the V content is less than 0.05%, this effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the V content exceeds 0.50%, the strength of the steel material becomes excessively high and the toughness decreases even if the content of other elements is within the range of the present embodiment.
Therefore, the V content is 0.05-0.50%.
A preferable lower limit of the V content is 0.07%, more preferably 0.09%, still more preferably 0.11%, and still more 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%, still more preferably 0.40 %.

Ti: 0.05-0.20%
Titanium (Ti) promotes the formation of V precipitates in the steel piston when steel is used as the raw material to manufacture the steel piston. As a result, the high temperature fatigue strength of the steel piston is enhanced under engine operating conditions. If the Ti content is less than 0.05%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Ti content exceeds 0.20%, coarse Ti precipitates remain in the steel piston even if the content of other elements is within the range of the present embodiment. In this case, the high-temperature fatigue strength of the steel piston rather decreases.
Therefore, the Ti content is 0.05-0.20%.
A preferable lower limit of the Ti content is 0.06%, more preferably 0.07%, and still more preferably 0.08%.
A preferable upper limit of the Ti content is 0.18%, more preferably 0.16%, and still more preferably 0.14%.

Al: 0.005-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, this effect cannot be obtained even if the content of other elements is within the range of the present embodiment.
On the other hand, if the Al content exceeds 0.100%, even if the content of other elements is within the range of the present embodiment, oxides (inclusions) are excessively generated, resulting in a steel piston containing HAZ. High temperature fatigue strength is reduced.
Therefore, the Al content is 0.005-0.100%.
The preferred lower limit of the Al content is 0.007%, more preferably 0.008%, still more preferably 0.010%, still more preferably 0.012%, still more preferably 0.014 %.
The preferred upper limit of the Al content is 0.090%, more preferably 0.080%, still more preferably 0.070%, still more preferably 0.060%, still more preferably 0.050 %.

N: 0.0200% or less Nitrogen (N) is an unavoidable impurity. That is, the N content is over 0%.
If the N content exceeds 0.0200%, the hot workability of the steel deteriorates even if the content of other elements is within the range of the present embodiment.
Therefore, the N content is 0.0200% or less.
A preferable upper limit of the N content is 0.0190%, more preferably 0.0180%, still more preferably 0.0170%, still more preferably 0.0160%.
N content is preferably as low as possible. However, reducing the N content excessively increases manufacturing costs. Therefore, when considering industrial production, the preferable lower limit of the N content is 0.0001%, more preferably 0.0005%, still more preferably 0.0010%, still more preferably 0.0015% is.

O: 0.0050% or less Oxygen (O) is an unavoidable impurity. That is, the O content is over 0%.
If the O content exceeds 0.0050%, oxides are excessively generated even if the content of other elements is within the range of the present embodiment, and the high-temperature fatigue strength of the steel piston containing the HAZ is reduced. .
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%, still more preferably 0.0020 %.
It is preferable that the O content is as low as possible. However, excessively reducing the O content increases production costs. Therefore, considering industrial production, the lower limit of the O content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%.

The rest of the chemical composition of the steel material that constitutes the upper member 10 of this embodiment consists of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the steel material constituting the upper member 10 is industrially manufactured, and are intentionally contained in the steel material. means an ingredient that is not made from

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

[About Optional Elements]
The chemical composition of the steel material constituting the upper member 10 of the present embodiment further includes Cu: 0.50% or less, Ni: 1.00% or less, and Nb: 0.100% or less instead of part of Fe. may contain one or more selected from the group consisting of All of these elements enhance the strength of the steel material forming the upper member 10 . These arbitrary elements are described below.

Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0.00%.
When contained, Cu enhances the hardenability of the steel material and enhances the 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 deteriorates even if the content of other elements is within the range of the present embodiment.
Therefore, the Cu content is between 0.00 and 0.50% and, if included, is less than or equal to 0.50%.
A preferable lower limit of the Cu content is 0.01%, more preferably 0.02%, still more preferably 0.04%, still more preferably 0.05%.
A preferable upper limit of the Cu content is 0.48%, more preferably 0.46%, more preferably 0.44%, still more preferably 0.40%.

Ni: 1.00% or less 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 of the steel material and enhances the 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 content of the other elements is within the range of the present embodiment, the effect is saturated, and the raw material cost increases.
Therefore, the Ni content is 0.00-1.00%, and if included, it is 1.00% or less.
The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, still more preferably 0.04%, still more preferably 0.05%.
The upper limit of the Ni content is preferably 0.98%, more preferably 0.90%, still more preferably 0.85%, still more preferably 0.80%, still more preferably 0.70 %, more preferably 0.60%.

Nb: 0.100% or less 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 to increase the strength of the steel. If the Nb content exceeds 0.000%, these effects can be obtained to some extent.
However, if the Nb content exceeds 0.100%, the strength of the steel material becomes too high and the machinability of the steel material deteriorates even if the contents of other elements are within the ranges of the present embodiment.
Therefore, the Nb content is between 0.000 and 0.100% and, if included, is 0.100% or less.
The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.015%, still more preferably 0.020 %.
The preferred upper limit of the Nb content is 0.095%, more preferably 0.090%, still more preferably 0.085%, still more preferably 0.080%, still more preferably 0.070 %.

[Method for measuring chemical composition of steel]
The chemical composition of the steel material forming the upper member of this embodiment can be measured by a well-known component analysis method. Specifically, using a drill, chips are collected from the inside at a depth of 1 mm or more from the surface of the steel material. 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 obtained by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using the well-known inert gas fusion-thermal conductivity method. The O content is determined using a well-known inert gas dissolution-infrared absorption method.

It should be noted that each element content, based on the significant digits defined in this embodiment, rounded off the measured numerical value, the numerical value up to the minimum digit of each element content defined in this embodiment do. For example, the C content of the steel material of this embodiment is defined by a numerical value up to the second decimal place. Therefore, the C content is a numerical value to the second decimal place obtained by rounding the measured numerical value to the second decimal place.

Similarly, the content of elements other than the C content of the steel material of the present embodiment is a value obtained by rounding off the numerical value to the minimum digit specified in the present embodiment with respect to the measured value. is the content of the element.

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

[About Fn1]
In the chemical composition of the steel material constituting the upper member 10 of the steel piston 1 of the present embodiment, on the premise that the content of each element is within the range of the present embodiment, Fn1 defined by the formula (1) is 0.410 or more.
Fn1=(95Si+25Mn+25Cr+12.5Mo+5V+15Al) ^−0.2 +10 ⁻⁵ /Ti (1)
Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula (1).

The steel piston 1 is made up of different alloys for the upper member 10 and the lower member 11 . Specifically, the upper member 10 is made of steel having a chemical composition in which the content of each element is within the range of the present embodiment. The lower member 11 is made of an aluminum alloy material.

Aluminum alloy materials have a lower melting point and lower strength than steel materials. Therefore, when joining the upper member 10 made of steel and the lower member 11 made of aluminum alloy, the hardness of the lower member 11 near the joint interface decreases due to heat generated during the joining process. there is a possibility.

In the steel piston 1 of the present embodiment, when the upper member 10 is made of steel and the lower member 11 is made of aluminum alloy, the decrease in hardness in the region near the joint interface of the lower member 11 due to joining is prevented. Suppress. Specifically, in the steel material forming the upper member 10, Fn1 defined by the formula (1) is set to 0.410 or more. This sufficiently suppresses a decrease in hardness in the vicinity of the joint interface of the lower member 11 . As a result, the high temperature fatigue strength of the steel piston joint is significantly increased. This point will be described below.

FIG. 2 shows the friction welding of a steel material having a chemical composition in which the content of each element is within the range of the present embodiment and an aluminum alloy material having a chemical composition corresponding to AC8A specified in JIS H5202:2010. , and a schematic diagram showing the relationship between the distance (mm) from the joint interface and the hardness (HV) of the steel material and the aluminum alloy material. The horizontal axis in FIG. 2 indicates the distance from the bonding interface. This horizontal axis corresponds to the axial direction of the steel piston 1 . The vertical axis in FIG. 2 indicates hardness (HV).

The dashed-dotted line in FIG. 2 is an example of hardness distribution when the content of each element in the chemical composition is within the range of the present embodiment, but Fn1 is less than 0.410 (comparative example). . The solid line in FIG. 2 is an example of hardness distribution when the content of each element in the chemical composition is within the range of the present embodiment and Fn1 is 0.410 or more (example of the present invention).

Referring to FIG. 2, the internal hardness HVL of the aluminum alloy material forming the lower member 11 is lower than the internal hardness HVU of the steel material forming the upper member 10 . In this case, if the content of each element in the chemical composition is within the range of the present embodiment, but Fn1 is less than 0.410 (one-dotted dashed line), the hardness of the region near the joint interface of the lower member 11 will increase. is lower than the hardness HVL. The area where the hardness is lower than the HVL is wide.

On the other hand, if the content of each element in the chemical composition is within the range of the present embodiment and the Fn1 is 0.410 or more (solid line), the hardness in the region near the joint interface of the lower member 11 will increase. Although the hardness is slightly lower than the internal hardness HVL, the hardness rapidly recovers as the distance from the bonding interface increases. Therefore, compared to the case where Fn1 is less than 0.410, the region where the hardness is lower than HVL becomes narrower.

Here, with reference to FIG. 2, a Vickers hardness test was performed in accordance with JIS Z2244:2009 from the joint interface of the lower member 11 to a depth of 5.0 mm at a pitch of 0.5 mm in the axial direction. Ask for The test force at this time is set to 9.8N. The arithmetic mean value of the obtained Vickers hardnesses is defined as the bonding interface vicinity region hardness HVI (HV). Further, the hardness obtained by executing the Vickers hardness test at a position 10.0 mm deep from the joint interface of the lower member 11 is defined as internal hardness HVB (HV). At this time, the amount of decrease in hardness ΔHV (%) in the vicinity of the joint interface of the lower member 11 is defined by the following formula (A).
ΔHV=(HVB−HVI)/HVB×100 (A)

FIG. 3 shows the hardness reduction amount ΔHV in the region near the joint interface and Fn1 when a steel material having a chemical composition in which the content of each element is within the range of the present embodiment and an aluminum alloy material are friction-welded. is a diagram showing the relationship of FIG. 3 was created based on the results of Examples described later.

Referring to FIG. 3, when Fn1 is less than 0.410, the decrease in hardness .DELTA.HV in the region near the junction interface remarkably decreases as Fn1 increases. On the other hand, when Fn1 is 0.410 or more, the amount of decrease in hardness ΔHV is substantially constant even if Fn1 increases. That is, in FIG. 3, there is an inflection point at Fn1=0.410.

FIG. 4 shows the amount of hardness reduction ΔHV in the region near the joint interface when friction welding is performed between a steel material having a chemical composition in which the content of each element is within the range of the present embodiment and an aluminum alloy material, and the and high temperature fatigue strength. FIG. 4 was created based on the results of Examples described later.

Referring to FIG. 4, the amount of decrease in hardness ΔHV in the region near the joint interface has a negative correlation with the high-temperature fatigue strength of the joint. When the hardness reduction amount ΔHV in the region near the joint interface is 7.5% or less, the joint high-temperature fatigue strength is 85 MPa or more, and excellent high-temperature fatigue strength is obtained in the joint.

From the above, when a steel material having a chemical composition in which the content of each element is within the range of the present embodiment and an aluminum alloy material are friction-welded, if Fn1 is 0.410 or more, the hardness in the region near the joint interface is increased. The reduction amount ΔHV can be suppressed to 7.5% or less. Therefore, the high-temperature fatigue strength of the joint is remarkably increased. Therefore, Fn1 is 0.410 or more.

A preferable lower limit of Fn1 is 0.412, more preferably 0.414, still more preferably 0.416, still more preferably 0.418. The upper limit of Fn1 is not particularly limited.

[Chemical Composition of Aluminum Alloy Constituting Lower Member 11]
The lower member 11 is made of an aluminum alloy material.
The aluminum alloy material is one selected from the group consisting of Al--Cu alloys, Al--Si alloys, Al--Cu--Mg--Ni alloys, and Al--Si--Cu--Mg--Ni alloys. That's it. The aluminum alloy material may be a wrought material or a cast material.

Preferably, the aluminum alloy material is a cast material. Aluminum alloy materials are, for example, A2218, A2618, A4032 specified in JIS H4140: 1998, and AC5A, AC8A, AC8B, AC8C, AC9A, AC9B, 2000 series (Al-Cu), 4000 series (Al-Cu) specified in JIS H5202: 2010. Al—Si) and the like.

Preferably, the chemical composition of the aluminum alloy material constituting the lower member 11 is Si: 0 to 25.0%, Fe: 0 to 1.5%, Cu: 0.2 to 5.5%, Mn: 0 to 0.70%, Mg: 0.2-2.0%, Cr: 0-0.30%, Zn: 0-0.60%, Pb: 0-0.20%, Sn: 0-0.20 %, Ni: 0.1 to 3.0%, Ti: 0 to 0.30%, and the balance consists of Al and impurities.

As described above, the steel piston 1 of this embodiment has the following features.
(Feature 1)
An upper member 10 made of a steel material having a chemical composition within the range of the present embodiment is joined to a lower member 11 made of an aluminum alloy material.
(Feature 2)
In the steel material forming the upper member, Fn1 defined by the formula (1) is 0.410 or more.
In the steel piston 1 having the features 1 and 2 described above, the amount of decrease in hardness ΔHV of the aluminum alloy material in the vicinity of the joint interface can be sufficiently suppressed. Therefore, the high-temperature fatigue strength of the joint is remarkably increased.

[Preferred form 1 of the steel piston 1 of the present embodiment]
Preferably, the steel piston 1 of this embodiment further satisfies the following feature 3 in addition to features 1 and 2 described above.
(Feature 3)
In the chemical composition of the steel material forming the upper member, Fn2 defined by formula (2) is 3.56 to 4.20.
Fn2=100×(92C+357Si+200Mn+67Ni+100Cr+44Mo+4) ^−0.54 (2)
Here, the content in mass % of the corresponding element is substituted for each element symbol in the formula (2). When an element is not contained, "0" is substituted for the corresponding element symbol.

As mentioned above, using a piston in a high combustion temperature range can increase the combustion efficiency of the engine. In a running engine, the upper member 10 of the steel piston 1 contacts hot gases in the combustion chamber. Therefore, the thermal conductivity of the steel material forming the upper member 10 affects the combustion efficiency of the engine during operation.

If the thermal conductivity of the steel material is too high at the temperature (for example, 100° C.) at the beginning of engine operation, the upper member 10 excessively removes heat from the combustion chamber. Therefore, the combustion temperature is difficult to rise, and the combustion efficiency is lowered. On the other hand, if the thermal conductivity of the steel material forming the upper member 10 is too low, excessive heat tends to remain in the combustion chamber. Therefore, it becomes difficult to control the temperature in the combustion chamber, and the combustion efficiency decreases.

Therefore, in order to increase the combustion efficiency of the engine, it is preferable to set the thermal conductivity of the steel material constituting the upper member 10 within an appropriate range at the initial stage of engine operation. Specifically, the thermal conductivity of the steel material at 100° C. is preferably about 40.0 to 48.0 W/m·K.

Fn2 is an index relating to the thermal conductivity of the steel material when the temperature of the steel material is about 100°C. When Fn2 is 3.56 or more, the thermal conductivity of the steel material at 100°C is 40.0 W/m·K or more. Therefore, when the steel piston 1 is used, the temperature of the combustion chamber of the engine can be easily controlled, and the combustion efficiency of the engine can be enhanced. Furthermore, if Fn2 is 4.20 or less, the thermal conductivity of the steel material at 100° C. is 48.0 W/m·K or less. Therefore, excessive heat removal from the combustion chamber is suppressed. Therefore, the combustion efficiency of the engine can be enhanced.

Therefore, preferably, in the chemical composition of the steel material of the upper member 10, Fn2 is 3.56 to 4.20.

A preferable lower limit of Fn2 is 3.60, more preferably 3.70, and still more preferably 3.80.
A preferable upper limit of Fn2 is 4.10, more preferably 4.00, and still more preferably 3.90.

[Preferred form 2 of the steel piston 1 of the present embodiment]
Preferably, the steel piston 1 of this embodiment further satisfies the following feature 4 in addition to features 1 and 2 described above.
(Feature 4)
In the chemical composition of the steel material forming the upper member, Fn3 defined by the formula (3) is 1.07 or more.
Fn3=100×(35C+161Si+200Mn+84Cr+5Mo) ^−0.8 (3)
Here, the content in mass % of the corresponding element is substituted for each element symbol in the formula (3).

As mentioned above, using a piston in a high combustion temperature range can increase the combustion efficiency of the engine. However, when the temperature of the upper member 10 is in an excessive temperature range near 600° C., the formation of an oxide film on the steel material forming the upper member 10 is promoted. As a result, engine life is reduced.

Therefore, it is preferable that the steel material has a high thermal conductivity in the above-described excessive temperature range. Specifically, the thermal conductivity of the steel material at 600° C. is preferably 34.0 W/m·K or more. In this case, it is possible to suppress the deterioration of the service life of the engine.

Fn3 is an index relating to the thermal conductivity of the steel material when the temperature of the steel material is about 600°C. When Fn3 is 1.07 or more, the thermal conductivity of the steel material at 600°C is 34.0 W/m·K or more.

Therefore, preferably, in the chemical composition of the steel material of the upper member 10, Fn3 is 1.07 or more.

A preferable lower limit of Fn3 is 1.10, more preferably 1.15, and still more preferably 1.25.
Although the upper limit of Fn3 is not particularly limited, it is, for example, 3.37. If Fn3 is 3.37 or less, the thermal conductivity of the steel material at 600° C. is 57.0 W/m·K or less.

[Preferred form 3 of the steel piston 1 of the present embodiment]
Preferably, the steel piston 1 of this embodiment further satisfies the features 1 to 4 described above. That is, in the chemical composition of the steel material of the upper member 10, Fn1 is 0.410 or more, Fn2 is 3.56 to 4.20, and Fn3 is 1.07 or more. In this case, the steel piston 1 of this embodiment can be used even when the temperature of the upper member 10 is in a low temperature range of about 100° C. or a high temperature range of about 600° C. Steel has good thermal conductivity. Therefore, it is possible to suppress the deterioration of the life of the engine while increasing the combustion efficiency of the engine.

[Production method]
An example of a method for manufacturing a steel piston according to this embodiment will be described. The steel piston manufacturing method described below is an example for manufacturing the steel material according to the present embodiment. Therefore, the steel piston having features 1 and 2 described above 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 the manufacturing method of the steel piston according to this embodiment.

For example, the method for manufacturing the steel piston of this embodiment is as follows.
(Step 1) Material preparation step (Step 2) Intermediate upper member and intermediate lower member forming step (Step 3) Heat treatment step (Step 4) Joining step (Step 5) Machining step Each step will be described below.

[(Step 1) Material preparation step]
In the material preparation step, a steel material that will be the material of the upper member 10 and an aluminum alloy material that will be the material of the lower member 11 are prepared. The steel material that is the material of the upper member 10 and the aluminum alloy material that is the material of the lower member 11 may be supplied by a third party.

When manufacturing the steel material which comprises the upper member 10, it manufactures by the following method, for example.
First, the content of each element in the chemical composition is within the range of the present embodiment, and molten steel that satisfies feature 2 is produced by a well-known steelmaking method. The content of each element in the chemical composition is within the range of the present embodiment, and molten steel that satisfies feature 2 and feature 3 may be produced. The content of each element in the chemical composition is within the range of the present embodiment, and molten steel that satisfies feature 2 and feature 4 may be produced. The content of each element in the chemical composition is within the range of the present embodiment, and molten steel satisfying features 2 to 4 may be produced. Using the produced molten steel, a cast material (bloom, ingot, or billet) is produced by continuous casting or ingot casting.

A known hot working is performed on the cast material to produce a steel material. Hot working includes, for example, hot rolling and hot forging. Through the manufacturing process described above, the steel material that is the material of the upper member 10 is manufactured.

The aluminum alloy material used as the material for the lower member 11 is manufactured by a well-known manufacturing method.

[(Step 2) Intermediate Upper Member and Intermediate Lower Member Forming Step]
In the intermediate upper member and intermediate lower member forming step, the intermediate upper member, which is an intermediate product of the upper member 10, is manufactured using the steel material prepared in the material preparing step as a raw material. Further, an intermediate lower member, which is an intermediate product of the lower member 11, is manufactured using the aluminum alloy material prepared in the material preparation step. The manufacture of the intermediate upper member and the manufacture of the intermediate lower member will be described below.

[Manufacture of intermediate upper member]
An intermediate upper member, which is an intermediate product, is manufactured by performing hot forging on a steel material, which is the material of the upper member 10, under well-known conditions. The heating temperature of the steel material during hot forging of the intermediate upper member is not particularly limited, but is, for example, 1000 to 1250°C. Here, the heating temperature means the furnace temperature of the heating furnace.

[Manufacture of intermediate lower member]
An intermediate lower member having the shape of the lower member 11 is manufactured by a well-known casting method or a well-known machining process using an aluminum alloy material that is the material of the lower member 11 .

[(Step 3) heat treatment step]
In the heat treatment step, the intermediate upper member and the intermediate lower member thus manufactured are subjected to the following heat treatment.

[Heat Treatment for Middle Upper Member]
The intermediate upper member is subjected to thermal refining treatment (quenching and tempering) under well-known conditions. The quenching treatment is performed at a well-known quenching temperature (above the _Ac3 transformation point) and then quenched. Quenching is, for example, water cooling or oil cooling. The tempering treatment is also carried out at a well-known tempering temperature (below the _AC1 transformation point).

[Heat Treatment for Intermediate Lower Member]
The intermediate lower member is subjected to solution treatment and artificial aging treatment under well-known conditions. Specifically, the aluminum alloy is subjected to T6 treatment (artificial aging treatment after solution treatment) specified in JIS H 0001:1998 or JIS H 4202:2011.
[(Step 4) Joining step]
In the bonding step, the intermediate upper member and the intermediate lower member after the heat treatment step are joined by friction bonding, laser bonding, or diffusion bonding to manufacture a bonded product. Specifically, the central axis of the intermediate upper member is arranged coaxially with the central axis of the intermediate lower member. The lower end surface of the intermediate upper member and the upper end surface of the intermediate lower member are brought into contact with each other to perform friction bonding, laser bonding, or diffusion bonding.

When the lower end surface of the intermediate upper member and the upper end surface of the intermediate lower member are brought into contact with each other to perform well-known friction welding, for example, the friction pressure is set to 50 to 200 MPa and the friction time is set to 2 to 20 seconds. An upset pressure (pressure from both ends of the intermediate upper member and the intermediate lower member to the joint) is set to 100 to 400 MPa. Set the upset time to 2 to 20 seconds. The number of revolutions during friction welding is set to 1500 to 2500 rpm. However, the friction welding conditions are not limited to this.

[(Step 5) Machining step]
Machining such as cutting is performed on the joined product after the joining step to manufacture a steel piston as a final product. The steel piston of this embodiment can be manufactured by the manufacturing process described above.

[Microstructure of Steel Constituting Upper Member 10 of Steel Piston]
The microstructure of the steel material forming the upper member 10 of the steel piston is a structure mainly composed of bainite. Here, the structure mainly composed of bainite means a structure in which the total area ratio of ferrite and pearlite is 10.0% or more and the balance is bainite.
The lower limit of the area ratio of bainite is 70.0%, preferably 80.0%, more preferably 85.0%.
The upper limit of the total area ratio of ferrite and pearlite is 30.0%, preferably 25.0%, still more preferably 20.0%, still more preferably 15.0%.
In the microstructure, regions other than bainite, ferrite, and pearlite are, for example, retained austenite, precipitates (including cementite), and inclusions. The area fraction of retained austenite is so small that it can be ignored.

[Method for measuring bainite area ratio]
The total area ratio (%) of ferrite and pearlite and the area ratio (%) of bainite in the microstructure of the steel material constituting the upper member 10 of the steel piston of this embodiment are measured by the following method. A sample is taken from the R/2 position of the top member 10 . After the surface (observation surface) of the collected sample is mirror-polished, the observation surface is etched using 2% nitric acid alcohol (nital etchant). The etched viewing surface is viewed using a 500x optical microscope to generate photographic images of any 20 fields. The size of each field of view is 100 μm×100 μm.

In each field of view, each phase such as bainite, ferrite, pearlite, etc. has a different contrast for each phase. Therefore, each phase is identified based on the contrast. Among the specified phases, the total area (μm ² ) of ferrite and the total area (μm ² ) of pearlite in each field of view are obtained. The ratio of the total area of the total area of ferrite and the total area of pearlite in all fields of view to the total area of all fields of view is defined as the total area ratio (%) of ferrite and pearlite. Using the total area ratio of ferrite and pearlite, the area ratio (%) of bainite is obtained by the following method.
Bainite area ratio=100.0-Total area ratio of ferrite and pearlite The total area ratio (%) of ferrite and pearlite is a value obtained by rounding off to the second decimal place.

Steel materials having chemical compositions shown in Tables 1-1 and 1-2 and aluminum alloy materials having chemical compositions shown in Table 2 were prepared.

Steel materials of each test number were manufactured by the following method. A bloom was produced by a continuous casting method using molten steel of each test number. Hot working was performed on the blooms produced. Specifically, the bloom was subjected to blooming to produce a billet. The bloom heating temperature before blooming was 1000 to 1200°C. After blooming, finish rolling was performed to produce a steel material (round bar). The heating temperature of the billet during finish rolling was 1000 to 1200°C. A steel material (round bar) having a diameter of 40 mm was manufactured by the manufacturing process described above.

The aluminum alloy material of each test number (aluminum alloy material number) was a round bar with a diameter of 40 mm.

[Evaluation test]
A test piece simulating a steel piston was produced using the steel material and aluminum alloy material of each test number, and the following evaluation tests were carried out.
(Test 1) Bainite area ratio measurement test (Test 2) Hardness reduction amount ΔHV measurement test in the vicinity of the joint interface (Test 3) Joint high-temperature fatigue strength test Each test will be described below.

[(Test 1) Bainite area ratio measurement test]
A test piece simulating the upper member of a steel piston was produced for each test number steel material.
Specifically, a steel material (round bar) with a diameter of 40 mm of each test number was heated at a heating temperature of 1200° C. for 30 minutes. Hot forging was performed on the heated steel material to produce a round bar with a diameter of 30 mm. The finishing temperature in hot forging was 950° C. or higher in any test number.

A refining treatment was performed on the manufactured round bar. Specifically, the round bar was heated at a heating temperature of 950° C. for 1 hour, and then quenched by being immersed in an oil bath having an oil temperature of 80° C. A tempering treatment was performed on the round bar after the quenching treatment. In the tempering treatment, the round bar after the quenching treatment was held at a heating temperature of 600° C. for 1 hour and then allowed to cool in the atmosphere.

A test piece (simulated upper member) having a diameter of 20 mm and a length of 40 mm was produced by machining the round bar (simulated intermediate upper member) after the tempering treatment described above. The center axis of the test piece substantially coincided with the center axis of the round bar after heat treatment.

[Measurement test of bainite area ratio]
Using the above test piece (simulated upper member) of each test number, the total area ratio (%) of ferrite and pearlite in the microstructure and the area ratio (%) of bainite Measurement method]. As a result, in any test number, the bainite area ratio was 70.0% or more, and the balance was ferrite and/or pearlite.

[(Test 2) Hardness decrease amount ΔHV measurement test in the vicinity of the bonding interface]
The amount of decrease in hardness ΔHV in the region near the joint interface for each test number was obtained by the following method.
A simulated steel piston manufacturing process was carried out on steel materials and aluminum alloy materials to prepare bonded round bar test pieces (simulated upper member and simulated lower member).

[Manufacture of simulated upper member]
Specifically, a steel material (round bar) with a diameter of 40 mm of each test number was heated at a heating temperature of 1200° C. for 30 minutes. Hot forging was performed on the heated steel material to produce a round bar with a diameter of 30 mm. The finishing temperature in hot forging was 950° C. or higher in any test number.

A thermal refining treatment was performed on the round bar after hot forging. Specifically, the round bar was heated at a heating temperature of 950° C. for 1 hour, and then quenched by being immersed in an oil bath having an oil temperature of 80° C. A tempering treatment was performed on the round bar after the quenching treatment. In the tempering treatment, the round bar after the quenching treatment was held at a heating temperature of 600° C. for 1 hour and then allowed to cool in the atmosphere. A plurality of simulated upper members (round bars) having a diameter of 20 mm and a length of 150 mm were produced by machining the round bar after the refining treatment. The center axis of the manufactured simulated upper member substantially coincided with the center axis of the round bar after thermal refining.

[Manufacture of Simulated Lower Member]
An aluminum alloy material having a diameter of 40 mm and having an aluminum alloy material number corresponding to each test number shown in Table 3 was prepared. The aluminum alloy material was subjected to solution treatment and artificial aging treatment in accordance with JIS H 0001:1998. In the solution treatment, the aluminum alloy material was held at 510° C. (783 K) for 4 hours and then cooled with water. Artificial aging treatment was performed on the aluminum alloy material after the solution treatment. In the artificial aging treatment, the aluminum alloy material was held at 170°C (443K) for 20 hours.

A plurality of simulated lower members (round bars) having a diameter of 20 mm and a length of 150 mm were produced by machining the heat-treated aluminum alloy material. The central axis of the fabricated simulated lower member substantially coincided with the central axis of the aluminum alloy material (a round bar with a diameter of 40 mm) as the raw material.

The end portions of the simulated upper member and the simulated lower member were butted against each other and friction-bonded to prepare a bonded test piece (corresponding to a simulated steel piston). In the friction bonding, the friction pressure was set to 150 MPa, and the friction time was set to 3 seconds. An upset pressure (applied pressure from both ends of the intermediate upper member and the intermediate lower member to the joint) was set to 300 MPa, and the upset time was set to 20 seconds. The number of revolutions during friction welding was set to 2000 rpm.

[Vickers hardness measurement test]
The bonded test piece was cut along a cross section (longitudinal cross section) containing the central axis. The cut surface was used as the observation surface. The observation surface was mirror-polished. In terms of observation, the joint interface between the simulated upper member (steel material) and the simulated lower member (aluminum alloy material) could be easily identified by the contrast.

Among the observation surfaces, at the central axis of the simulated lower member (aluminum alloy material), from the joint interface to a depth of 5.0 mm at a pitch of 0.5 mm (a distance of 5.0 mm along the central axis from the joint interface), JIS Z2244 : 2009, the Vickers hardness was measured. The test force was 9.8N. The arithmetic mean value of the obtained Vickers hardness was defined as the bonding interface vicinity region hardness HVI (HV). Furthermore, the Vickers hardness (HV) was obtained by the above-described test method at a position 10.0 mm deep from the joint interface on the center axis of the lower member. The obtained Vickers hardness was defined as internal hardness HVB (HV).
Using the hardness HVI of the region near the joint interface and the internal hardness HVB, the amount of decrease in hardness ΔHV (%) in the region near the joint interface was obtained by the following formula (A).
ΔHV=(HVB−HVI)/HVB×100 (A)
Table 3 shows the amount of decrease in hardness ΔHV (%) in the vicinity of the bonding interface thus obtained.

[(Test 3) Joint high temperature fatigue strength test]
Machining (turning) was performed on the joint test piece (corresponding to the simulated steel piston) of each test number prepared in (Test 2), and from the center of the cross section perpendicular to the longitudinal direction of the joint test piece, A high-temperature Ono-type rotating bending fatigue test piece was prepared. The center axis of the high-temperature Ono type rotating bending fatigue test piece coincided with the center axis of the bonded round bar test piece. The diameter of the parallel portion of the high-temperature Ono-type rotating bending fatigue test piece was 8 mm, and the length of the parallel portion was 15.0 mm. The central position of the parallel portion in the axial direction (longitudinal direction) corresponded to the joining position.

Using the prepared high-temperature Ono-type rotary bending fatigue test pieces, a high-temperature Ono-type rotary bending fatigue test was performed under the following conditions. The evaluation temperature was 250°C. After mounting the test piece on the testing machine in the heating furnace, the temperature of the heating furnace was started to rise while rotating at 2500 rpm. After the indicated value of the furnace thermometer of the heating furnace reached 250°C, the test piece was soaked at 250°C for 30 minutes. After soaking, a load was applied to start the fatigue test. The stress ratio was set to -1, and the maximum number of repetitions was set to 1×10 ⁷ times. The endurance stress at the maximum number of repetitions (1×10 ⁷ times) was defined as fatigue strength (MPa). Table 3 shows the obtained fatigue strength (MPa) of each test number. If the fatigue strength was 85 MPa or more, it was determined that excellent high-temperature fatigue strength was obtained.

[Test results]
Test results are shown in Tables 1-1, 1-2, 2 and 3.

With reference to Tables 1-1, 1-2, 2 and 3, the chemical compositions of the upper members of Test Nos. 1-24 were appropriate and Fn1 was 0.410 or more. Furthermore, the lower member is made of an aluminum alloy material. Therefore, in these test numbers, the amount of decrease in hardness ΔHV in the region near the joint interface was 7.5% or less. As a result, the high-temperature fatigue strength of the joint was 85 MPa or more, indicating that the high-temperature fatigue strength of the joint was high.

On the other hand, in Test No. 25, the V content was low. Therefore, the high-temperature fatigue strength of the joint was less than 85 MPa, and the high-temperature fatigue strength of the joint was low.

In test number 26, the Ti content was low. Therefore, the high-temperature fatigue strength of the joint was less than 85 MPa, and the high-temperature fatigue strength of the joint was low.

In test number 27, the Ti content was high. Therefore, the high-temperature fatigue strength of the joint was less than 85 MPa, and the high-temperature fatigue strength of the joint was low.

In Test Nos. 28-38, Fn1 was less than 0.410, although the chemical composition of the top member was appropriate. Therefore, in these test numbers, the amount of decrease in hardness ΔHV in the region near the joint interface exceeded 7.5%. As a result, the high-temperature fatigue strength of the joint was less than 85 MPa, indicating that the high-temperature fatigue strength of the joint was low.

The following evaluation test was performed using simulated upper members (20 mm in diameter and 40 mm in length) of test numbers 1 to 24 in Tables 1-1 and 1-2.
(Test 4) Thermal conductivity measurement test Test 4 will be described below.

[(Test 4) Thermal conductivity measurement test]
The thermal conductivity of the simulated upper member for each test number was obtained based on the laser flash method. Specifically, a disc-shaped test piece having a diameter of 10 mm and a thickness of 2 mm was taken from the simulated upper member of each test number. A test piece was taken from an internal position at a depth of 1 mm or more from the surface of the simulated upper member. The sampled specimen was coated with graphite spray. A specimen of the coating was loaded into the furnace of the thermal diffusivity measurement device. The atmosphere of the heating furnace was _N2 gas. The test piece was heated in a heating furnace. At this time, the temperature increase rate was 10° C./min. A temperature rise curve of the test piece was obtained using a thermal diffusivity measuring device. Thermal diffusivity α ₁₀₀ (m ² /sec) at 100° C. and thermal diffusivity α ₆₀₀ (m ² /sec) at 600° C. were determined using the obtained temperature rise curves. Using the specific heat capacity C (J · kg ^-1 · K ^-1 ) and the density ρ (kg/m ³ ) of the steel material of the simulated upper member, the thermal conductivity λ ₁₀₀ at 100 ° C. and , and the thermal conductivity λ ₆₀₀ (W/m·K) at 600°C.
λ ₁₀₀ = α ₁₀₀ ρ C
λ ₆₀₀ = α ₆₀₀ ρ C

As the graphite spray, GB396352 (trade name) manufactured by NETSCH Corporation was used. LFA457MicroFlash (trade name) manufactured by NETSCH Corporation was used as a thermal diffusivity measurement device. Table 4 shows the obtained thermal conductivity λ ₁₀₀ at 100°C and the thermal conductivity λ ₆₀₀ at 600°C.

[Test results]
With reference to Tables 1-1, 1-2, and 4, in test numbers 2, 4, 6, 8 to 15, 17, 18 and 20 of test numbers 1 to 24, which are examples of the present invention, Fn2 was in the range of 3.56-4.20. Therefore, the thermal conductivity λ ₁₀₀ at 100° C. is within the range of 40.0 to 48.0, and it was expected that the combustion efficiency of the engine would be improved.

Also, in test numbers 1 to 5, 7, 8, 10, 12, and 15 to 20, Fn3 was 1.07 or more. Therefore, the thermal conductivity λ ₆₀₀ at 600° C. is 34.0 or more, and it was expected that the reduction in the service life of the engine could be suppressed.

Furthermore, in test numbers 2, 4, 8, 10, 12, 15, 17, 18 and 20, Fn2 was within the range of 3.56 to 4.20 and Fn3 was 1.07 or more. Therefore, the thermal conductivity λ ₁₀₀ at 100°C was within the range of 40.0 to 48.0, and the thermal conductivity λ ₆₀₀ at 600°C was 34.0 or higher. Therefore, it was expected that the deterioration of the life of the engine could be suppressed while improving the thermal efficiency of the engine.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be modified as appropriate without departing from the scope of the invention.

Claims (5)

a steel piston,
an upper member including at least a crown topland;
a lower member disposed below the upper member and joined to the upper member;
with
The upper member is
The chemical composition, in mass %,
C: 0.15 to 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 to 2.00%,
Mo: 0.08-0.50%,
V: 0.05 to 0.50%,
Ti: 0.05 to 0.20%,
Al: 0.005 to 0.100%,
N: 0.0200% or less, and
O: 0.0050% or less,
the balance consists of Fe and impurities,
Made of steel material whose Fn1 defined by formula (1) is 0.410 or more,
The lower member is
Made of aluminum alloy material,
steel piston.
Fn1=(95Si+25Mn+25Cr+12.5Mo+5V+15Al) ^−0.2 +10 ⁻⁵ /Ti (1)
Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula (1). a steel piston,
an upper member including at least a crown topland;
a lower member disposed below the upper member and joined to the upper member;
with
The upper member is
The chemical composition, in mass %,
C: 0.15 to 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 to 2.00%,
Mo: 0.08-0.50%,
V: 0.05 to 0.50%,
Ti: 0.05 to 0.20%,
Al: 0.005 to 0.100%,
N: 0.0200% or less, and
O: 0.0050% or less,
moreover,
Cu: 0.50% or less,
Ni: 1.00% or less, and
Nb: 0.100% or less, containing one or more selected from the group consisting of
the balance consists of Fe and impurities,
Made of steel material whose Fn1 defined by formula (1) is 0.410 or more,
The lower member is
Made of aluminum alloy material,
steel piston.
Fn1=(95Si+25Mn+25Cr+12.5Mo+5V+15Al) ^−0.2 +10 ⁻⁵ /Ti (1)
Here, the content of the corresponding element in mass % is substituted for each element symbol in the formula (1). A steel piston according to claim 1,
Further, in the chemical composition of the steel material,
Fn2 defined by formula (2) is 3.56 to 4.20,
steel piston.
Fn2=100×(92C+357Si+200Mn+67Ni+100Cr+44Mo+4) ^−0.54 (2)
Here, the content in mass % of the corresponding element is substituted for each element symbol in the formula (2). When an element is not contained, "0" is substituted for the corresponding element symbol. A steel piston according to claim 2,
Further, in the chemical composition of the steel material,
Fn2 defined by formula (2) is 3.56 to 4.20,
steel piston.
Fn2=100×(92C+357Si+200Mn+67Ni+100Cr+44Mo+4) ^−0.54 (2)
Here, the content in mass % of the corresponding element is substituted for each element symbol in the formula (2). When an element is not contained, "0" is substituted for the corresponding element symbol. The steel piston according to any one of claims 1 to 4,
Further, in the chemical composition of the steel material,
Fn3 defined by formula (3) is 1.07 or more,
steel piston.
Fn3=100×(35C+161Si+200Mn+84Cr+5Mo) ^−0.8 (3)
Here, the content in mass % of the corresponding element is substituted for each element symbol in the formula (3).

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