Steel piston

Abstract

To provide a steel piston that ensures high temperature strength even when used for an extended period in high temperature conditions.SOLUTION: A steel piston disclosed herein has an upper section with a chemical composition consisting of, in mass%, C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10-1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.10-0.80%, Mo: 0.80-2.50%, V: 0.01-0.30%, Ti: 0.010-0.049%, Al: 0.005-0.100%, N: 0.020% or less, and O: 0.0050% or less, with the balance being Fe and impurities, and satisfying the formula (1). The number density ND of MC carbides at the upper section of the piston is 2.0×1022 / m3 or more. The proportion RMo of Mo content in the metal elements constituting the MC carbides, excluding Fe, is 60 atom% or more. Mo / (Mo+V+Ti)≥0.89 (1).SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a piston, and more particularly to a steel piston used in an engine or the like.

Background Art

[0002] Engines represented by diesel engines and the like include pistons. The piston is housed in the cylinder of the engine and reciprocates within the cylinder. The piston is exposed to high-temperature heat during the combustion process in the engine operation.

[0003] Many conventional pistons are manufactured by casting aluminum. However, in recent years, further improvement in the combustion efficiency of engines has been demanded. In the case of a piston made of an aluminum casting, the surface temperature during use is about 240 to 330°C.

[0004] Recently, there has been a study on using a piston in an even higher combustion temperature range to increase the combustion efficiency. Therefore, a piston having high strength is required even when the surface temperature of the upper part of the piston during use is 400°C or higher, and further 500°C or higher. In order to meet such demands, steel pistons manufactured using steel materials have begun to be proposed. Steel pistons are proposed, for example, in Patent Document 1. Steel pistons have a higher melting point of the material compared to pistons made of aluminum castings. Therefore, steel pistons can be used in a higher combustion temperature range compared to pistons made of aluminum castings.

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Recently, further improvement in combustion efficiency has been demanded, and it is expected that the surface temperature of the upper part of the piston during use will further increase to more than 500°C. By the way, the piston is used for a long time in the above-mentioned high-temperature environment. Therefore, it is required to obtain excellent high-temperature strength even when used for a long time in a high-temperature environment.

[0007] An object of the present disclosure is to provide a steel piston that can obtain excellent high-temperature strength even when used for a long time in a high-temperature environment.

Means for Solving the Problems

[0008] The steel piston of the present embodiment includes an upper part of the piston including at least a top land, and a lower part of the piston disposed below the upper part of the piston. The upper part of the piston has a chemical composition in mass %, C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.10 to 0.80%, Mo: 0.80 to 2.50%, V: 0.01 to 0.30%, Ti: 0.010 to 0.049%, Al: 0.005 to 0.100%, N: 0.020% or less, and O: 0.0050% or less, and contains the balance consists of Fe and impurities, satisfies formula (1), the number density ND of MC-type carbides in the upper part of the piston is 2.0×10 , , , 3 , , , , Mo , 22 , pieces / m 3 or more, in the MC-type carbide, the ratio R of the Mo content among the metal elements excluding Fe constituting the MC-type carbide Mo is 60 atomic % or more. Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0009] The steel piston of this embodiment is At least the top of the piston, including the top land, The piston comprises a lower piston located below the upper piston, The upper part of the piston is, The chemical composition is expressed in mass percent. C: 0.30~0.50%, Si: 0.01~0.30%, Mn: 0.10~1.50%, P:0.030% or less, S: 0.030% or less, Cr: 0.10~0.80%, Mo: 0.80~2.50%, V: 0.01~0.30%, Ti: 0.010~0.049%, Al: 0.005~0.100%, N: 0.020% or less, and, Contains O: 0.0050% or less, Furthermore, it contains one or more elements selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities. Satisfying equation (1), The number density ND of MC-type carbides in the upper part of the piston is 2.0 × 10 22 pieces / m 3 That's all. In the aforementioned MC-type carbide, the ratio of Mo content among the metal elements excluding Fe that constitute the MC-type carbide is R Mo It is 60 atomic percent or more. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, and, B: One or more selected from the group consisting of 0.0100% or less. [Group 2] Nb: 0.100% or less, and, One or more selected from the group consisting of Zr: 0.300% or less. [Group 3] Sn: 0.100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, Bi: 0.300% or less, and, One or more selected from the group consisting of Te: 0.300% or less. Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element. [Effects of the Invention]

[0010] The steel piston of this embodiment provides excellent high-temperature strength even when used for extended periods in high-temperature environments. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is a cross-sectional view of a steel piston according to this embodiment, and is a cross-sectional view taken when the steel piston is cut by a plane including the central axis of the piston. [Modes for carrying out the invention]

[0012] The inventors initially investigated the chemical composition of the upper part of a steel piston that would provide sufficient high-temperature strength even when the surface temperature during use exceeds 500°C. As a result, the inventors considered incorporating V, Ti, and Mo to generate MC-type carbides of these elements in the steel material. These MC-type carbides precipitate more finely than other carbides such as cementite. Therefore, it is believed that these MC-type carbides exhibit precipitation strengthening and maintain excellent high-temperature strength in high-temperature environments. As a result of further consideration based on the above findings, if the chemical composition of the steel material constituting the upper part of the piston is, in mass %, C: 0.30 to 0.50%, Si: 0.01 to 0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.10 to 0.80%, Mo: 0.80 to 2.50%, V: 0.01 to 0.30%, Ti: 0.010 to 0.049%, Al: 0.005 to 0.100%, N: 0.020% or less, O: 0.0050% or less, Cu: 0 to 0.40%, Ni: 0 to 0.40%, B: 0 to 0.0100%, Nb: 0 to 0.100%, Zr: 0 to 0.300%, Sn: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Bi: 0 to 0.300%, and Te: 0 to 0.300%, and the balance consists of Fe and impurities, it is considered that excellent high-temperature strength may be obtained even when used for a long time in a high-temperature environment exceeding 500°C.

[0013] However, even in the upper part of the piston composed of a steel material having the above chemical composition, sufficient high-temperature strength may not be obtained when used for a long time in a high-temperature environment exceeding 500°C. Therefore, the inventors further investigated. Here, the inventors focused on the number density ND of MC-type carbides and the Mo content ratio R in the MC-type carbides. Mo If the number density ND of MC-type carbides is sufficiently large, excellent high-temperature strength can be maintained even when used for a long time in a high-temperature environment due to precipitation strengthening. Furthermore, the Mo content in the MC-type carbides suppresses the coarsening of the MC-type carbides when used for a long time in a high-temperature environment. When the MC-type carbides coarsen, they cannot contribute to strengthening in a high-temperature environment. Therefore, it is considered that by increasing the Mo content ratio R of the metal elements excluding Fe that constitute the MC-type carbides, the coarsening of the MC-type carbides when used for a long time in a high-temperature environment can be suppressed. Mo Based on the above findings, the inventors further investigated. As a result, in the above chemical composition, satisfying formula (1), the number density ND of MC-type carbides is 2.0×10 and above, and the ratio R of the Mo content among the metal elements excluding Fe that constitute the MC-type carbides 22 per m 3 is such that...Mo We found that if the content is 60 atomic percent or more, it can maintain excellent high-temperature strength even when used for extended periods in high-temperature environments exceeding 500°C. Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0014] Based on the above findings, the steel piston according to this embodiment has the following configuration.

[0015] [1] It is a steel piston, At least the top of the piston, including the top land, The piston comprises a lower piston located below the upper piston, The upper part of the piston is, The chemical composition is expressed in mass percent. C: 0.30~0.50%, Si: 0.01~0.30%, Mn: 0.10~1.50%, P:0.030% or less, S: 0.030% or less, Cr: 0.10~0.80%, Mo: 0.80~2.50%, V: 0.01~0.30%, Ti: 0.010~0.049%, Al: 0.005~0.100%, N: 0.020% or less, and, Contains O: 0.0050% or less, The remainder consists of Fe and impurities. Satisfying equation (1), The number density ND of MC-type carbides in the upper part of the piston is 2.0 × 10 22 pieces / m 3 That's all. In the aforementioned MC-type carbide, the ratio of Mo content among the metal elements excluding Fe that constitute the MC-type carbide is R Mo It is 60 atomic percent or more. Steel piston. Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0016] [2] It is a steel piston, At least the top of the piston, including the top land, The piston comprises a lower piston located below the upper piston, The upper part of the piston is, The chemical composition is expressed in mass percent. C: 0.30~0.50%, Si: 0.01~0.30%, Mn: 0.10~1.50%, P:0.030% or less, S: 0.030% or less, Cr: 0.10~0.80%, Mo: 0.80~2.50%, V: 0.01~0.30%, Ti: 0.010~0.049%, Al: 0.005~0.100%, N: 0.020% or less, and, Contains O: 0.0050% or less, Furthermore, it contains one or more elements selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities. Satisfying equation (1), The number density ND of MC-type carbides in the upper part of the piston is 2.0 × 10 22 pieces / m 3 That's all. In the aforementioned MC-type carbide, the ratio of Mo content among the metal elements excluding Fe that constitute the MC-type carbide is R Mo It is 60 atomic percent or more. Steel piston. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, and, B: One or more selected from the group consisting of 0.0100% or less. [Group 2] Nb: 0.100% or less, and, One or more selected from the group consisting of Zr: 0.300% or less. [Group 3] Sn: 0.100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, Bi: 0.300% or less, and, One or more selected from the group consisting of Te: 0.300% or less. Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0017] [3] [2] The steel piston described above, The above group 1 contains, Steel piston.

[0018] [4] A steel piston as described in [2] or [3], The following include the second group: Steel piston.

[0019] [5] A steel piston as described in any one of items [2] to [4], The third group contains, Steel piston.

[0020] The steel piston according to this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass %.

[0021] [Steel piston configuration] Figure 1 is a cross-sectional view of the steel piston 1 according to this embodiment, and is a cross-sectional view taken when the steel piston 1 is cut by a plane including the central axis.

[0022] Referring to Figure 1, the steel piston 1 of this embodiment has a cylindrical shape. The steel piston 1 comprises a piston upper part 10 and a piston lower part 11. The upper piston portion 10 is the upper part of the steel piston 1 and is made of steel. The upper piston portion 10 includes at least the top land 16 of the crown portion 13.

[0023] The lower piston portion 11 is located below the upper piston portion 10. The lower piston portion 11 may be integrally formed with the upper piston portion 10, or it may be a separate component. If the lower piston portion 11 is separate from the upper piston portion 10, it is joined to the upper piston portion 10. In Figure 1, reference numeral 30 denotes the joint surface when the lower piston 11 is separate from the upper piston 10 and is joined to the upper piston 10. In this case, the lower piston 11 is friction-bonded, laser-bonded, or diffusion-bonded to the upper piston 10.

[0024] The lower part of the piston 11 includes at least a skirt portion 14 and a piston pin hole 15. The skirt portion 14 is located 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.

[0025] A pair of piston pin holes 15 are formed in the skirt portion 14, into which a piston pin (not shown) can be inserted. A gap 40 is formed between the pair of piston pin holes 15. The small end of a connecting rod (not shown) is positioned in the gap 40. The hole in the small end of the connecting rod and the pair of piston pin holes are arranged coaxially. A piston pin is inserted into the hole formed in the small end of the connecting rod and the pair of piston pin holes 15, thereby connecting the steel piston and the connecting rod.

[0026] In Figure 1, a cavity 50 is formed between the lower surface of the upper piston 10 and the upper surface of the lower piston 11. A cooling medium, for example, circulates through the cavity 50 to cool the steel piston 1 during use. In Figure 1, the steel piston 1 includes the cavity 50, but the shape of the cavity 50 is not limited to the shape shown in Figure 1. Furthermore, the steel piston 1 does not have to include the cavity 50. In other words, a cavity 50 does not have to be formed between the lower surface of the upper piston 10 and the upper surface of the lower piston 11.

[0027] In Figure 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 to 21. The top land 16 includes the uppermost top surface 161 of the steel piston 1. Land 17 is a circumferential surface of the crown portion 13 and is located below the top land 16, with a ring groove 19 formed between the top land 16 and land 17. Land 18 is a circumferential surface of the crown portion 13 and is located below land 17, with a ring groove 20 formed between land 17 and land 18. A skirt portion 14 is formed below land 18, and a ring groove 21 is formed between land 18 and the skirt portion 14. Piston rings (not shown) can be placed in each of the ring grooves 19 to 21.

[0028] In Figure 1, the upper part of the piston 10 does not include the entire crown portion 13, but includes the upper parts of the top land 16, land 17, and land 18 of the crown portion 13. However, the configuration of the upper part of the piston 10 is not limited to this. In pistons used in high-temperature environments, the surface temperature of the top land 16 is the highest. Therefore, it is sufficient for the upper part of the piston 10 to include at least the top land 16 of the crown portion 13. In other words, the upper part of the piston 10 may include the top land 16 of the crown portion 13, but may not include the portion below the land 17. In this case, the portion of the steel piston 1 that includes the land 17 and the portion below the land 17 becomes the lower part of the piston 11.

[0029] The upper part 10 of the piston may include the entire crown portion 13. In this case, the skirt portion 14 becomes the lower part 11 of the piston. In Figure 1, the crown portion 13 includes a top land 16, a plurality of lands 17 and 18, and a plurality of ring grooves 19 to 21. However, the crown portion 13 may consist of a top land 16, one land 17, and one ring groove 19.

[0030] [Features of Steel Piston 1] The steel piston 1 of this embodiment satisfies the following characteristics. (Feature 1) The chemical composition of the upper part 10 of the piston is as follows (in mass%): C: 0.30~0.50%, Si: 0.01~0.30%, Mn: 0.10~1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.10~0.80%, Mo: 0.80~2.50%, V: 0.01~0.30%, Ti: 0.010~0.049%, Al: 0.005~0.100%, N: 0.0 It contains 20% or less of the following: O: 0.0050% or less, Cu: 0-0.40%, Ni: 0-0.40%, B: 0-0.0100%, Nb: 0-0.100%, Zr: 0-0.300%, Sn: 0-0.100%, Ca: 0-0.0100%, Mg: 0-0.0100%, Bi: 0-0.300%, and Te: 0-0.300%, with the remainder being Fe and impurities. (Feature 2) The chemical composition of the upper part 10 of the piston further satisfies equation (1). Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element. (Feature 3) The number density ND of MC-type carbides in the upper part 10 of the piston is 2.0 × 10⁻⁶ 22 pieces / m 3 That's all. (Feature 4) R is the ratio of Mo content among the metal elements excluding Fe that make up MC-type carbides. Mo It is 60 atomic percent or more. Features 1 through 4 are explained below.

[0031] [(Feature 1) Regarding the chemical composition of the upper part 10 of the piston] The upper part of the piston 10 is made of steel. Specifically, the chemical composition of the upper part of the piston 10 contains the following elements.

[0032] C: 0.30~0.50% Carbon (C) increases the strength of steel. If the C content is less than 0.30%, this effect is not fully achieved. On the other hand, if the C content exceeds 0.50%, the machinability of the steel decreases. Therefore, the C content is 0.30-0.50%. The preferred lower limit for the C content is 0.31%, more preferably 0.32%, more preferably 0.33%, and still more preferably 0.34%. The preferred upper limit for the C content is 0.48%, more preferably 0.46%, more preferably 0.44%, more preferably 0.42%, and still more preferably 0.40%.

[0033] Si: 0.01~0.30% Silicon (Si) deoxidizes steel. Si also increases the strength of ferrite. If the Si content is less than 0.01%, these effects are not fully obtained. On the other hand, if the Si content exceeds 0.30%, the machinability of the steel decreases. Therefore, the Si content is 0.01 to 0.30%. The preferred lower limit for the Si content is 0.02%, more preferably 0.03%, more preferably 0.04%, and still more preferably 0.10%. The preferred upper limit for the Si content is 0.25%, more preferably 0.20%, even more preferably 0.18%, and even more preferably 0.15%.

[0034] Mn: 0.10~1.50% Manganese (Mn) enhances the hardenability of steel and increases its strength through solid solution strengthening. These effects are not fully achieved if the Mn content is less than 0.10%. On the other hand, if the Mn content exceeds 1.50%, the machinability of the steel decreases. Therefore, the Mn content is 0.10-1.50%. The preferred lower limit of the Mn content is 0.12%, more preferably 0.14%, even more preferably 0.16%, and even more preferably 0.18%. The preferred upper limit for the Mn content is 1.40%, more preferably 1.35%, even more preferably 1.30%, and even more preferably 1.25%.

[0035] P:0.030% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.030%, P segregates at the grain boundaries, reducing the strength of the steel. Therefore, the P content is 0.030% or less. A low phosphorus (P) content is preferable. However, excessively reducing the P content incurs manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the P content is 0.001%, and more preferably 0.002%. The preferred upper limit for the P content is 0.029%, more preferably 0.028%, more preferably 0.027%, and still more preferably 0.025%.

[0036] S: 0.030% or less Sulfur (S) is inevitably present. Therefore, the S content is greater than 0%. S combines with Mn to form Mn sulfides, improving the machinability of steel. Even a small amount of S can provide this effect to some extent. On the other hand, if the S content exceeds 0.030%, coarse Mn sulfides or excessive Mn sulfides may be formed. In this case, the high-temperature strength decreases. Therefore, the sulfur content is 0.030% or less. The preferred lower limit for the S content is 0.001%, more preferably 0.003%, more preferably 0.005%, and still more preferably 0.009%. The preferred upper limit for the S content is 0.028%, more preferably 0.025%, more preferably 0.023%, more preferably 0.020%, more preferably 0.018%, and more preferably 0.015%.

[0037] Cr: 0.10~0.80% Chromium (Cr) increases the strength of steel. If the Cr content is less than 0.10%, this effect is not fully achieved. On the other hand, if the Cr content exceeds 0.80%, Cr carbides are formed, and the high-temperature strength decreases. Furthermore, if the Cr content exceeds 0.80%, the machinability of the steel decreases. Therefore, the Cr content is 0.10-0.80%. The preferred lower limit for the Cr content is 0.15%, more preferably 0.20%, even more preferably 0.25%, and even more preferably 0.30%. The preferred upper limit for the Cr content is 0.75%, more preferably 0.70%, more preferably 0.65%, more preferably 0.60%, and still more preferably 0.50%.

[0038] Mo: 0.80~2.50% Molybdenum (Mo), along with V and Ti, forms MC-type carbides. MC-type carbides containing Mo are less prone to becoming coarse even when used for extended periods in high-temperature environments exceeding 500°C. Therefore, they can maintain high high-temperature strength even when used for extended periods in high-temperature environments, such as steel piston 1 used for long periods in engine operation. This effect is not fully obtained if the Mo content is less than 0.80%. On the other hand, if the Mo content exceeds 2.50%, the strength of the steel becomes excessively high, and its toughness decreases. Therefore, the Mo content is 0.80-2.50%. The preferred lower limit of the Mo content is 0.85%, more preferably 0.90%, more preferably 0.95%, more preferably 1.00%, more preferably 1.01%, and more preferably 1.05%. The preferred upper limit for the Mo content is 2.48%, more preferably 2.42%, more preferably 2.38%, more preferably 2.34%, and still more preferably 1.30%.

[0039] V: 0.01~0.30% Vanadium (V), together with Mo and Ti, forms MC-type carbides, maintaining high high-temperature strength even when the steel piston 1 is used for extended periods in high-temperature environments. If the V content is less than 0.01%, this effect is not fully achieved. On the other hand, if the V content exceeds 0.30%, the strength of the steel becomes excessively high, and its toughness decreases. Therefore, the V content is 0.01-0.30%. The preferred lower limit of the V content is 0.07%, more preferably 0.09%, even more preferably 0.11%, and even more preferably 0.13%. The preferred upper limit for the V content is 0.28%, more preferably 0.26%, more preferably 0.24%, more preferably 0.22%, and more preferably 0.20%.

[0040] Ti: 0.010~0.049% Titanium (Ti), together with Mo and V, forms MC-type carbides, maintaining high high-temperature strength even when the steel piston 1 is used for extended periods in high-temperature environments. If the Ti content is less than 0.010%, the above effect is not fully obtained. On the other hand, if the Ti content exceeds 0.049%, coarse Ti precipitates remain in the steel piston. In this case, the high-temperature strength of the steel piston actually decreases. Therefore, the Ti content is between 0.010% and 0.049%. The preferred lower limit of the Ti content is 0.015%, more preferably 0.020%, and even more preferably 0.025%. The preferred upper limit for the Ti content is 0.048%, more preferably 0.046%, and even more preferably 0.044%.

[0041] Al: 0.005~0.100% Aluminum (Al) deoxidizes steel. This effect is not achieved if the Al content is less than 0.005%. On the other hand, if the Al content exceeds 0.100%, excess oxides (inclusions) are formed, reducing the high-temperature strength of steel pistons containing HAZ. Therefore, the Al content is between 0.005% and 0.100%. The preferred lower limit of the Al content is 0.007%, more preferably 0.008%, more preferably 0.010%, more preferably 0.012%, and more preferably 0.014%. The preferred upper limit for the Al content is 0.090%, more preferably 0.080%, more preferably 0.070%, more preferably 0.060%, and more preferably 0.050%.

[0042] N: 0.020% or less Nitrogen (N) is an unavoidable impurity. Therefore, the N content is greater than 0%. If the N content exceeds 0.020%, the hot workability of the steel decreases. Therefore, the N content is 0.020% or less. The preferred upper limit for the N content is 0.019%, more preferably 0.018%, even more preferably 0.017%, and even more preferably 0.016%. It is preferable to have as low an N content as possible. However, excessively reducing the N content incurs manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the N content is 0.001%, more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.015%.

[0043] O: 0.0050% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. If the O content exceeds 0.0050%, excess oxides will be generated, reducing the high-temperature strength of the steel piston. Therefore, the O content is 0.0050% or less. The preferred upper limit for the O content is 0.0045%, more preferably 0.0040%, more preferably 0.0035%, more preferably 0.0030%, and more preferably 0.0020%. A low oxygen content is preferable. However, excessively reducing the oxygen content incurs manufacturing costs. Therefore, considering industrial production, the preferred lower limit for the oxygen content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%.

[0044] The remainder of the chemical composition of the upper piston 10 in this embodiment consists of Fe and impurities. Here, impurities refer to elements that are introduced during the industrial manufacturing of the steel material constituting the upper piston 10 from the raw materials such as ore, scrap, or the manufacturing environment, and are not intentionally included in the steel material.

[0045] [About Optional Elements] The chemical composition of the piston upper part 10 of this embodiment may further include, in place of a portion of Fe, one or more substances selected from the following groups 1 to 3. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, and, B: One or more selected from the group consisting of 0.0100% or less. [Group 2] Nb: 0.100% or less, and, One or more selected from the group consisting of Zr: 0.300% or less. [Group 3] Sn: 0.100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, Bi: 0.300% or less, and, One or more selected from the group consisting of Te: 0.300% or less. All of these elements are arbitrary elements. The following describes the arbitrary elements in groups 1 through 3.

[0046] [(Group 1: Cu, Ni, and B)] The chemical composition of the upper part 10 of the piston may include one or more elements selected from the group consisting of Cu, Ni, and B in place of some of the Fe. All of these elements increase the strength of the steel.

[0047] Cu: 0.40% or less Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When included, Cu improves the hardenability and strength of the steel. Even a small amount of Cu will provide some of the above effects. However, if the Cu content exceeds 0.40%, the hot workability of the steel will decrease. Therefore, the Cu content is 0-0.40%, and if present, it is 0.40% or less. The preferred lower limit for the Cu content is 0.01%, more preferably 0.02%, more preferably 0.04%, and still more preferably 0.05%. The preferred upper limit for the Cu content is 0.38%, more preferably 0.36%, even more preferably 0.34%, and even more preferably 0.30%.

[0048] Ni: 0.40% or less Nickel (Ni) is an optional element and does not need to be included. In other words, the Ni content may be 0%. When present, Ni improves the hardenability and strength of the steel. Even a small amount of Ni will provide some of the above effects. However, if the Ni content exceeds 0.40%, the effect saturates, and the raw material cost also increases. Therefore, the Ni content is 0-0.40%, and if present, it is 0.40% or less. The preferred lower limit for the Ni content is 0.01%, more preferably 0.02%, more preferably 0.04%, and still more preferably 0.05%. The preferred upper limit for the Ni content is 0.38%, more preferably 0.36%, more preferably 0.34%, more preferably 0.32%, and still more preferably 0.30%.

[0049] B: 0.0100% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When present, B increases the hardenability and strength of the steel. Even a small amount of B will provide some of the above effects. However, if the B content exceeds 0.0100%, it may promote the segregation of a large amount of nitrides, potentially leading to cracking of the steel. Therefore, the B content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the B content is 0.0090%, more preferably 0.0070%, and even more preferably 0.0050%.

[0050] [(Group 2: Nb and Zr)] The chemical composition of the upper part 10 of the piston may include one or more elements selected from the group consisting of Nb and Zr in place of some of the Fe. All of these elements create fine precipitates in the steel, thereby increasing the strength of the steel.

[0051] Nb: 0.100% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. If Nb is included, it generates carbides, nitrides, or carbonitrides (hereinafter referred to as carbonitrides, etc.) in the steel, thereby increasing the strength of the steel. Even if only a small amount of Nb is included, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.100%, the strength of the steel becomes too high, and the machinability of the steel decreases. Therefore, the Nb content is between 0 and 0.100%, and if present, it is 0.100% or less. The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, more preferably 0.010%, more preferably 0.015%, and more preferably 0.020%. The preferred upper limit for the Nb content is 0.095%, more preferably 0.090%, more preferably 0.085%, more preferably 0.080%, and more preferably 0.070%.

[0052] Zr: 0.300% or less Zirconium (Zr) is an optional element and does not need to be included. In other words, the Zr content may be 0%. If it is included, Zr forms nitrides in the steel, increasing its strength. Even a small amount of Zr will provide some degree of the above effect. However, if the Zr content exceeds 0.300%, Zr will form coarse nitrides. Coarse nitrides reduce the hot workability of the steel. Therefore, the Zr content is between 0 and 0.300%, and if present, it is 0.300% or less. The preferred lower limit for the Zr content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Zr content is 0.250%, more preferably 0.200%, and even more preferably 0.150%.

[0053] [(Group 3: Sn, Ca, Mg, Bi, and Te)] The chemical composition of the upper part 10 of the piston may include one or more elements selected from the group consisting of Sn, Ca, Mg, Bi, and Te, in place of some of the Fe. All of these elements enhance the machinability of the steel material.

[0054] Sn: 0.100% or less Tin (Sn) is an optional element and does not need to be present. In other words, the Sn content may be 0%. When present, Sn improves the machinability of steel. Even a small amount of Sn will provide some of the above effect. However, if the Sn content exceeds 0.100%, the effect saturates, and the raw material cost also increases. Therefore, the Sn content is between 0 and 0.100%, and if present, it is less than 0.100%. The preferred lower limit for the Sn content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Sn content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0055] Ca:0.0100% or less Calcium (Ca) is an optional element and does not need to be present. In other words, the Ca content may be 0%. When present, Ca improves the machinability of steel. Even a small amount of Ca will provide some degree of the above effect. However, if the Ca content exceeds 0.0100%, coarse oxides will be formed. Coarse oxides reduce the high-temperature strength of steel. Therefore, the Ca content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit for the Ca content is 0.0010%, more preferably 0.0020%, and even more preferably 0.0030%. The preferred upper limit for the Ca content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0056] Mg: 0.0100% or less Magnesium (Mg) is an optional element and does not need to be present. In other words, the Mg content may be 0%. When present, Mg improves the machinability of the steel. Even a small amount of Mg will provide some degree of the above effect. However, if the Mg content exceeds 0.0100%, coarse oxides will be formed. Coarse oxides reduce the high-temperature strength of the steel. Therefore, the Mg content is between 0 and 0.0100%, and if present, it is 0.0100% or less. The preferred lower limit of the Mg content is 0.0010%, more preferably 0.0020%, and even more preferably 0.0030%. The preferred upper limit for the Mg content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0057] Bi:0.300% or less Bismuth (Bi) is an optional element and does not need to be present. In other words, the Bi content may be 0%. When present, Bi improves the machinability of steel. Even a small amount of Bi will provide some of the above effect. However, if the Bi content exceeds 0.300%, the hot workability of the steel will decrease. Therefore, the Bi content is between 0 and 0.300%, and if present, it is 0.300% or less. The preferred lower limit of the Bi content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Bi content is 0.250%, more preferably 0.200%, and even more preferably 0.150%.

[0058] Te: 0.300% or less Tellurium (Te) is an optional element and does not need to be present. In other words, the Te content may be 0%. When present, Te improves the machinability of steel. Even a small amount of Te will provide some of the above effect. However, if the Te content exceeds 0.300%, the hot workability of the steel will decrease. Therefore, the Te content is between 0 and 0.300%, and if present, it is 0.300% or less. The preferred lower limit for the Te content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Te content is 0.250%, more preferably 0.200%, and even more preferably 0.150%.

[0059] [Method for measuring the chemical composition of the upper part 10 of the piston] The chemical composition of the steel material constituting the upper piston 10 of this embodiment can be measured by a well-known component analysis method in accordance with JIS G0321:2017. 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. Elemental analysis of the chemical composition is performed on the solution using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The C and S content is determined by a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas melting-thermal conductivity method. The O content is determined using a well-known inert gas melting-infrared absorption method.

[0060] Furthermore, the content of each element is determined by rounding the measured value to the minimum number of digits specified in this embodiment, based on the significant figures defined in this embodiment. For example, the carbon content of the steel material in this embodiment is determined to two decimal places. Therefore, the carbon content is determined by rounding the measured value to two decimal places. Similarly, for the content of other elements in the steel material in this embodiment, the value obtained by rounding the measured value to the minimum number of digits specified in this embodiment is determined as the content of that element. Rounding means truncating if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0061] [Regarding (Feature 2) Equation (1)] The chemical composition of the steel material constituting the upper part 10 of the piston 1 in this embodiment further satisfies formula (1). Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

[0062] Here, we define F1 as follows: F1 is the left-hand side of equation (1). F1 = Mo / (Mo + V + Ti) F1 is one of the requirements for MC-type carbides to satisfy characteristic 4 in order to maintain high-temperature strength after prolonged use. If F1 is 0.89 or higher, the ratio of Mo content among the metal elements constituting the MC-type carbide, excluding Fe, is R Mo This amounts to 60 atomic percent or more.

[0063] A preferred lower limit for F1 is 0.90, more preferably 0.91, even more preferably 0.92, and even more preferably 0.93. The upper limit of F1 is not particularly limited. However, if the chemical composition of the upper part of the piston 10 satisfies characteristic 1, the upper limit of F1 is 0.99.

[0064] [(Feature 3) Regarding the number density ND of MC-type carbides] In the steel piston 1 of this embodiment, the number density ND of MC-type carbides at the upper part 10 of the piston is 2.0 × 10 22 pieces / m 3 That concludes the report. The number density ND of MC-type carbides at the top 10 of the piston is 2.0 × 10⁻⁶. 22 pieces / m 3 If the above conditions are met, a sufficient amount of MC-type carbide has been generated. Therefore, high high-temperature strength is maintained even when used for extended periods in high-temperature environments.

[0065] The preferred lower limit of the number density ND of MC-type carbides is 2.2 × 10⁻⁶. 22 pieces / m 3 And more preferably 2.4 × 10 22 pieces / m 3 And more preferably 2.6 × 10 22pieces / m 3 That is the case. There is no particular upper limit to the number density ND of MC-type carbides. However, if the chemical composition of the upper piston 10 satisfies features 1 and 2, the upper limit to the number density ND of MC-type carbides is, for example, 200.0 × 10 22 pieces / m 3 Preferably 100.0 × 10 22 pieces / m 3 That is the case.

[0066] [Method for measuring the number density (ND) of MC-type carbides] The number density of MC-type carbides at the top 10 of the piston can be measured using a scanning transmission electron microscope (STEM) in the following way. First, a thin film sample for STEM is prepared using the following method. The diameter of the upper part of the piston 10 is D (mm). A disc is taken from the upper part of the piston 10, including a position at a depth of D / 4 radially from the outer surface, and at a depth of 2 mm or more axially from the top surface of the upper part of the piston 10. The thickness of the disc is approximately 2 mm. The thickness direction of the disc is parallel to the axial direction of the upper part of the piston 10. The surface of the disc (circular surface) is parallel to the surface perpendicular to the axial direction of the upper part of the piston 10. Both sides of the disc (front and back) are polished using emery paper. At this time, the front surface of the disc is polished so that it is parallel to the back surface. One of the front and back surfaces of the disc is defined as the observation surface. The observation surface is further polished to a mirror finish. The mirror-polished observation surface is further polished using colloidal silica as an abrasive.

[0067] MC-type carbides have a specific crystal orientation relationship with respect to the matrix (Fe). Specifically, MC-type carbides are plate-like particles that extend along the {100} plane of the matrix. Therefore, electron beam backscatter diffraction (EBSD) is performed on the observation surface after polishing to determine the crystal orientation of the matrix. Then, based on the determined crystal orientation of the matrix, the direction perpendicular to the observation surface of the thin film (observation direction) is determined to be the same as the matrix. <001> Thin film samples for STEM are prepared by performing focused ion beam (FIB) processing on a disk to achieve the desired crystal orientation.

[0068] The preparation of thin film samples for STEM by FIB processing can be carried out using well-known methods. For example, thin film samples for STEM can be prepared using the lift-out method with a gallium (Ga) ion beam at an accelerating voltage of 30 kV.

[0069] Thin film samples for STEM fabricated using an ion beam with an acceleration voltage of 30 kV have dislocation loops and amorphous regions on their surface, making them unsuitable for observing MC-type carbides of several nanometers. Therefore, the surface of the thin film sample for STEM is polished using an ion beam with a low acceleration voltage of 1 kV or less. Through this manufacturing process, thin film samples for STEM with a thickness of 100 nm or less are prepared.

[0070] The fabricated thin film samples are observed using a STEM optical system. Specifically, the martensite of the matrix phase in the thin film samples for STEM are observed. <001> The thin film sample for STEM is tilted so that the crystal orientation is incident on the crystal zone axis. The observation magnification is set to 320,000x and the acceleration voltage to 300kV. The detector is set to known high-angle annular dark-field (HAADF), known low-angle annular dark-field (LAADF), and known bright-field (BF) conditions, and 10 arbitrary observation fields are observed. Images are produced in each observation field under these observation conditions. Of the images generated in each observation field, the image in which the MC-type carbide is most clearly recognizable is selected.

[0071] Each observation field will have an area of ​​280 nm × 280 nm. The thickness of the STEM thin film will be measured in all observation fields using the Log-ratio method of electron energy loss spectroscopy (EELS).

[0072] In the images taken in each observation field, precipitates can be identified by contrast. Therefore, among the identified precipitates, those with a maximum length of 2 nm or more are selected. Here, "maximum length" in STEM observation refers to the maximum line segment length when any two points at the interface between the precipitate and the matrix are selected, and the entire line segment connecting those two points is included within the precipitate. Precipitates with a maximum length of less than 2 nm are extremely difficult to identify. Therefore, in this embodiment, precipitates with a maximum length of 2 nm or more are selected.

[0073] Among precipitates with a maximum length of 2 nm or more, MC-type carbides are identified by the following method: Precipitates with a maximum length of 2 nm or more are irradiated with an electron beam to obtain electron diffraction patterns. MC-type carbides and other precipitates have different electron diffraction patterns. Therefore, based on the obtained electron diffraction patterns, MC-type carbides are identified from among the identified precipitates.

[0074] Based on the total number of MC-type carbides identified in all observation fields using the method described above, and the total volume calculated from all observation fields and thicknesses, the number density of MC-type carbides (pieces / m³) is calculated. 3 )

[0075] Furthermore, when the content of each element in the chemical composition was within the range of this embodiment, STEM observation yielded electron diffraction patterns of precipitates, and precipitates with a maximum length of 10 nm or less were almost entirely MC-type carbides, with very few other precipitates present. The maximum lengths of other precipitates, such as M2C-type carbides and cementite, all significantly exceeded 10 nm. Therefore, instead of the electron diffraction pattern, precipitates with a maximum length of 10 nm or less (i.e., a maximum length of 2 to 10 nm) may be identified as MC-type carbides.

[0076] [(Feature 4) Mo content ratio R in MC type carbide Mo [About] In the steel piston 1 of this embodiment, the ratio of Mo content among the metal elements excluding Fe that constitute the MC-type carbide in the upper part 10 of the piston is R Mo It is 60 atomic percent or more.

[0077] Here, "the ratio of Mo content among the metal elements excluding Fe that make up MC-type carbides R" Mo "Mo content of 60 atomic percent or more" means the ratio (atomic percent) of the Mo content in the MC-type carbide to the total content of metal elements excluding Fe, when the total content of metal elements excluding Fe in the MC-type carbide is set to 100 atomic percent.

[0078] Ratio of Mo content in MC-type carbides Mo When the temperature is high, even when the steel piston 1 is used for a long time in a high-temperature environment exceeding 500°C, the MC-type carbides are less likely to undergo Ostwald growth and remain fine. Therefore, excellent high-temperature strength can be maintained even when used for a long time in a high-temperature environment.

[0079] Ratio of Mo content in MC-type carbides Mo The preferred lower limit is 62% in atomic percent, more preferably 64%, and even more preferably 66%.

[0080] [Mo content ratio R Mo [Measurement Method] The Mo ratio in MC-type carbides can be determined by the following method. Let D (mm) be the diameter of the upper part of the piston 10. A sample is cut from the upper part of the piston 10, including a position D / 4 in the radial direction from the outer surface, and from an interior depth of 1 mm or more from the surface in the axial direction from the top surface of the upper part of the piston 10. A well-known focused ion beam processing method is performed on the cut sample to produce a needle-shaped test piece with a tip radius of curvature of about 50 nm.

[0081] Three-dimensional atom probe analysis is performed on needle-shaped test specimens. Three-dimensional atom probe analysis identifies MC-type carbides in the needle-shaped test specimens using the following method. Three-dimensional atom probe analysis allows for the three-dimensional detection of precipitates present in the needle-shaped test specimens.

[0082] In three-dimensional atom probe measurement, the laser wavelength (λ) is set to 355 nm, the laser power to 30 pJ, and the temperature of the needle-shaped test specimen to 50 K. The apparatus used for three-dimensional atom probe measurement is not particularly limited. For example, the LEAP4000XHR, manufactured by AMETEK Corporation, is used as a three-dimensional atom probe measurement apparatus.

[0083] A three-dimensional atomic map is obtained by reconstructing the acquired measurement data. Specifically, by using the detection efficiency of the instrument and adjusting the {110} atomic plane spacing to 0.2 nm in the measurement of iron (Fe), the measurement data is reconstructed to obtain a three-dimensional atomic map.

[0084] In a three-dimensional atomic map, the region of a needle-shaped specimen analyzed using three-dimensional atom probes is divided into tiny cubes called voxels. Each side of a voxel is 1.0 nm long. The elemental concentration (atomic %) within a voxel is defined as the number of atoms of that element contained within the voxel divided by the total number of atoms of all elements contained within the voxel.

[0085] The sum of the C, Mo, V, and Ti concentrations in each voxel is defined as the concentration of a specific element (atomic %). An isoconcentration surface is created by connecting voxels where the concentration of the specific element is 20%. The region enclosed by the isoconcentration surface has a high concentration of the specific element. The region enclosed by the isoconcentration surface is identified as a precipitate.

[0086] Among the identified precipitates, those with a maximum length of 10 nm or less are identified. Here, a line segment connecting any two points on the surface of the precipitate detected in three dimensions (i.e., the interface between the precipitate and the steel matrix) that is entirely contained within the precipitate is defined as a "specific line segment." The maximum length of this specific line segment of the precipitate is defined as the maximum length of the precipitate. Precipitates with a maximum length of 10 nm or less are identified as MC-type carbides.

[0087] When the elemental content in the chemical composition is within the range of this embodiment, the precipitates in the upper part 10 of the piston with a maximum length of 10 nm or less are almost entirely MC-type carbides, and there are almost no other precipitates besides MC-type carbides. The maximum lengths of other precipitates besides MC-type carbides, such as M2C-type carbides and cementite, all greatly exceed 10 nm. This point will be explained below.

[0088] A sample was taken from the interior of the upper piston 10 of the steel piston 1 in this embodiment, at a depth of 1 mm or more, to prepare a thin film sample with a thickness of 50 nm. The prepared thin film sample was observed with a transmission electron microscope (TEM), and precipitates were identified by contrast. The maximum length of the identified precipitates was determined. Furthermore, crystal structure analysis was performed on the identified precipitates to determine their crystal structure and identify the type of precipitate. As a result, in the upper piston 10 having the chemical composition of this embodiment, precipitates with a maximum length of 10 nm or less were almost entirely MC-type carbides, and there were hardly any other precipitates. The maximum length of other precipitates (such as M2C-type carbides and cementite) was significantly greater than 10 nm.

[0089] Based on these results, precipitates with a maximum length of 10 nm or less in the three-dimensional atomic map obtained by three-dimensional atom probe analysis are identified as MC-type carbides.

[0090] From the identified set of MC-type carbides, select any 20 MC-type carbides. Then, determine the total number of atoms of metallic elements excluding Fe and the number of atoms of Mo in each selected MC-type carbide. Finally, define the ratio of the number of Mo atoms to the total number of atoms of metallic elements excluding Fe as the Mo content ratio (atomic %). Metallic elements are elements that, in their elemental form, form metals. Therefore, Si, B, P, S, C, N, and O are not included in the definition of metallic elements.

[0091] The arithmetic mean of the Mo content ratios obtained for each of the 20 selected MC-type carbides is used to determine the Mo content ratio among the metal elements excluding Fe in the MC-type carbides of the piston upper part 10. Mo Defined as (atomic %).

[0092] [Microstructure of the upper part of the piston 10] The microstructure of the piston upper part 10 in this embodiment contains a hard phase that accounts for 90% or more of the area. The microstructure of the piston upper part 10 referred to here means the microstructure of the piston upper part 10 manufactured by quenching and tempering in the manufacturing process described later. The hard phase consists of martensite and / or bainite. If the microstructure of the piston upper part 10 contains other phases besides the hard phase, the remainder of the microstructure of the piston upper part 10 consists of one or more selected from the group consisting of retained austenite, ferrite, and pearlite. Preferably, the microstructure contains a hard phase that accounts for 90% or more of the area, with the remainder consisting of retained austenite. The yield strength of the piston upper part 10 correlates with the microstructure.

[0093] [Method for measuring the area ratio of the hard phase in the upper part 10 of the piston] The area ratio of the hard phase in the upper part 10 of the piston can be determined by the following method. A sample is taken from the top surface 161 at the uppermost end of the upper part 10 of the piston, including a depth of 5 mm. Of the surface of the sample, the surface that is parallel to the axial direction of the upper part 10 of the piston and includes a depth of 5 mm from the top surface 161 is defined as the observation surface. The observation surface of the sample is mirror polished. The mirror polished observation surface is etched using 2% nitric acid alcohol (Nital etching solution). Of the etched observation surface, five arbitrary observation fields (100 μm × 100 μm) near a depth of 5 mm from the upper surface 161 are observed with a 500x optical microscope. In the observation field, the hard phase (bainite and / or martensite) and other phases (protoprecipitation ferrite, pearlite, etc.) can be easily distinguished by contrast. Ferrite is observed as a white region. Pearlite is observed as a phase with a lamellar structure. The hard phase is observed as a region with lower brightness than ferrite. The area percentage (%) of the hard phase is determined based on the total area of ​​the hard phase in the five observation fields and the total area of ​​the five observation fields.

[0094] [Regarding the lower part of the piston 11] The lower piston section 11 may be made of steel or other metal materials. During engine operation, the upper piston section 10 faces the combustion chamber. Therefore, as mentioned above, the upper piston section 10 requires high high-temperature strength. On the other hand, the lower piston section 11 does not face the combustion chamber. Therefore, the lower piston section 11 does not require the same high high-temperature strength as the upper piston section 10. For this reason, the lower piston section 11 may be made of steel or other metal materials.

[0095] Preferably, the lower part of the piston 11 is made of either an aluminum alloy or steel. The aluminum alloy is one or more selected from the group consisting of, for example, Al-Cu alloys, Al-Si alloys, Al-Cu-Mg-Ni alloys, and Al-Si-Cu-Mg-Ni alloys. The aluminum alloy may be a wrought material or a cast material.

[0096] If the lower piston portion 11 is made of steel, that steel may be the same as the steel that makes up the upper piston portion 10, or it may be a different steel. The lower piston portion 11 may also be integrally formed with the upper piston portion 10. In this case, the lower piston portion 11 is made of the same steel as the upper piston portion 10.

[0097] [Regarding the effects of Steel Piston 1] As described above, the steel piston 1 of this embodiment satisfies features 1 to 4. Therefore, the steel piston 1 of this embodiment can maintain excellent high-temperature strength even when used for a long time in a high-temperature environment. Specifically, the steel piston 1 of this embodiment achieves a yield strength of 380 MPa or higher at 600°C after being held at 600°C for 1000 hours.

[0098] [Manufacturing method] An example of a manufacturing method for the steel piston 1 according to this embodiment will be described. The steel piston 1 satisfying features 1 to 4 of this embodiment may be manufactured by other manufacturing methods other than those described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the steel piston 1 according to this embodiment.

[0099] The manufacturing method of the steel piston 1 of this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Upset forging process (Step 3) Quenching and tempering process (Process 4) Bonding process (Process 5) Machining process The following describes each step.

[0100] [(Process 1) Material preparation process] In the material preparation process, the material for the steel piston 1 is prepared. If the upper piston 10 and the lower piston 11 are a single unit, the material shall be steel. If the upper piston 10 and the lower piston 11 are separate, the material of the upper piston 10 shall be steel, and the material of the lower piston 11 may be a metal other than steel, or it may be the same material as the upper piston 10.

[0101] When manufacturing steel material for the steel piston 1 or the piston upper part 10, for example, it is manufactured by the following method. First, molten steel is produced by a well-known steelmaking method in which the content of each element in the chemical composition is within the range of this embodiment and satisfies features 1 and 2. Using the manufactured molten steel, a cast material (bloom, ingot, or billet) is produced by continuous casting or ingot making. A well-known hot working method is performed on the cast material to produce steel material. Hot working methods include, for example, hot rolling and hot forging. Through the above manufacturing process, steel material to be used for the piston upper part 10 is produced.

[0102] [(Process 2) Upsetting forging process] An intermediate product of the steel piston 1 or piston upper part 10 is manufactured by hot upsetting forging of the steel material prepared in the material preparation process. The diameter of the steel piston 1 or piston upper part 10 material is smaller than the diameter of the piston upper part 10. Therefore, an intermediate product including at least the piston upper part 10 is manufactured by hot upsetting forging of the steel material. The hot upsetting forging satisfies the following conditions 1 to 4. (Condition 1) The steel material temperature T1 immediately before the start of hot upsetting forging shall be 1000°C or higher. (Condition 2) The fixed rate e is set to 30-55%. (Condition 3) The average cooling rate CR1 from upsetting to 450°C shall be FA1 (°C / sec) or higher. FA1 = -5.68 × 10 -2 ×T1+0.20×e+7.32×10 Here, T1 in the formula is replaced with the steel temperature T1 (°C), and e is replaced with the upsetting rate (%). (Condition 4) The average cooling rate CR2 at 450-400°C shall be FA2 (°C / sec) or less. FA2 = -1.20 × 10 -3 ×T1+4.20×10 -3 ×e+1.85 Here, T1 in the formula is replaced with the steel temperature T1 (°C), and e is replaced with the upsetting rate (%). Conditions 1 through 4 will be explained below.

[0103] [(Condition 1) Regarding steel material temperature T1] If the steel temperature T1 immediately before the start of hot upsetting forging is less than 1000°C, Mo will not be sufficiently dissolved in the intermediate product after the upsetting forging process. In this case, the number density ND of MC-type carbides at the top 10 of the piston will not be sufficiently obtained. Therefore, the steel temperature T1 should be 1000°C or higher. There is no particular upper limit to the steel temperature T1, but considering the normal equipment capacity, for example, it is 1300°C.

[0104] [(Condition 2) Regarding the fixed rate e] The adjustment rate e can be calculated using the following formula. The rate of adjustment e = (1 - Ds 2 / Df 2 ) × 100 Here, Ds in the formula is substituted with the diameter (mm) of the material before upsetting and forging, specifically the portion corresponding to the upper part 10 of the piston. Df is substituted with the diameter (mm) of the intermediate product after upsetting and forging, specifically the portion corresponding to the upper part 10 of the piston. If the upsetting rate e is less than 30%, Mo segregation in the steel material will not be sufficiently resolved. In this case, the Mo content ratio R of the MC type carbide at the top 10 of the piston is... Mo Insufficient results are obtained. On the other hand, if the upsetting rate e exceeds 55%, the amount of strain applied to the steel becomes excessively large. In this case, the number density ND of MC-type carbides at the top of the piston 10 cannot be sufficiently obtained. If the upsetting rate e is between 30% and 55%, appropriate strain is applied to the steel by upsetting forging. Therefore, the number density ND of MC-type carbides and the Mo content ratio R at the top of the piston 10 are not obtained. Mo This would be an appropriate range.

[0105] [(Condition 3) Regarding the average cooling rate CR1] After hot upsetting forging, a two-stage cooling process is carried out. Specifically, the average cooling rate CR1 during the period from upsetting forging until the steel material temperature reaches 450°C is set to FA1 (°C / second) or higher. FA1 = -5.68 × 10 -2 ×T1+0.20×e+7.32×10 Here, T1 in the formula is replaced with the steel temperature T1 (°C), and e is replaced with the upsetting rate (%). If the average cooling rate CR1 is less than FA1, Mo will not be sufficiently dissolved in the intermediate product after the upsetting forging process. In this case, the number density ND of MC-type carbides at the top 10 of the piston will not be sufficiently obtained. Therefore, the average cooling rate CR1 should be set to FA1 (°C / sec) or higher.

[0106] [(Condition 4) Regarding the average cooling rate CR2] Furthermore, the average cooling rate CR2 during the period when the steel material temperature is between 450 and 400°C should be less than or equal to FA2. FA2 = -1.20 × 10 -3×T1+4.20×10 -3 ×e+1.85 Here, T1 in the formula is replaced with the steel temperature T1 (°C), and e is replaced with the upsetting rate (%). If the average cooling rate CR2 is faster than FA2, excess Mo will dissolve in the intermediate product after the upsetting forging process. In this case, the ratio R of the Mo content among the metal elements excluding Fe that constitute the MC type carbide at the top of the piston 10 Mo This cannot be obtained sufficiently. Therefore, the average cooling rate CR2 should be set to FA2 (°C / sec) or less.

[0107] [(Step 3) Quenching and tempering process] Among the intermediate materials after upsetting and forging, the intermediate material consisting of steel that constitutes the upper part 10 of the piston is subjected to quenching and tempering.

[0108] [Hardening] The hardening is carried out by a well-known method. The hardening temperature and the holding time at the hardening temperature are not particularly limited. For example, the hardening temperature is 840 to 970°C. The holding time at the hardening temperature is, for example, 15 minutes to 360 minutes (6 hours). After the holding time, the intermediate product is rapidly cooled. Specifically, the intermediate product is water-cooled or oil-cooled.

[0109] [Tempering] Tempering is performed on the intermediate product after quenching. The tempering temperature T2 is set to 600-680°C. The holding time t2 at tempering temperature T2 is set to 0.5-6.0 hours. Furthermore, the tempering process satisfies the following condition 5. (Condition 5) The FB defined by the following formula shall be 3500 or greater. FB = T² × t² × [Mo] / [V] Here, T2 in FB is substituted with the tempering temperature T2 (°C). t2 is substituted with the holding time t2 (hours) at tempering temperature T2. [Mo] is substituted with the Mo content (mass%) in the steel. [V] is substituted with the V content (mass%) in the steel. Condition 5 will be explained below.

[0110] [(Condition 5) Regarding Facebook] FB is the ratio of Mo content in MC-type carbides R Mo This is a parameter related to the following: If FB is less than 3500, the MC type carbide does not contain enough Mo. Therefore, the Mo content ratio R of the MC type carbide in the upper part 10 of the piston is... Mo The amount is less than 60 atomic percent. Therefore, FB is 3500 or higher. Note that there is no particular upper limit to FB. However, if FB is too high, the high-temperature strength of steel piston 1 will decrease. Therefore, the upper limit of FB is, for example, 155000.

[0111] The preferred lower limit of FB is 4000, more preferably 5000, more preferably 6000, more preferably 7000, more preferably 8000, and more preferably 10000. The preferred upper limit of FB is 100000, more preferably 80000, more preferably 60000, and more preferably 50000.

[0112] [(Process 4) Bonding process] The joining process is optional and does not need to be performed. The joining process is performed when the upper piston part 10 and the lower piston part 11 are manufactured separately. Therefore, if the upper piston part 10 and the lower piston part 11 are formed as a single unit, the joining process does not need to be performed. When performing the joining process, the upper piston 10 and the lower piston 11 are joined by friction bonding, laser bonding, or diffusion bonding, etc.

[0113] [(Process 5) Machining process] The steel piston 1, which is the final product, is manufactured by performing machining such as cutting on the intermediate product after the hardening and tempering process, or on the intermediate product after the joining process. The steel piston 1 of this embodiment can be manufactured through the above manufacturing process. [Examples]

[0114] A simulated piston top, which mimics the upper part of a steel piston, was manufactured, and its high-temperature strength was investigated. Specifically, steel materials with the chemical compositions shown in Tables 1-1 and 1-2 were prepared.

[0115] [Table 1-1]

[0116] [Table 1-2]

[0117] The steel materials for each test number were manufactured using the following method: Blooms were produced by continuous casting using the molten steel for each test number. Hot working was performed on the manufactured blooms. Specifically, billets were produced by bloc rolling of the blooms. The heating temperature of the blooms before bloc rolling was 1000-1200°C. After bloc rolling, finish rolling was performed to produce steel materials (round bars). The heating temperature of the billets during finish rolling was 1000-1200°C. Through the above manufacturing process, steel materials (round bars) with a diameter of 100 mm were produced, which will be used as the material for the piston upper part dummy.

[0118] Intermediate products were manufactured by performing upsetting forging on steel materials. The steel material temperature T1 (°C), upsetting rate e (%), average cooling rate CR1 (°C / sec), and average cooling rate CR2 (°C / sec) immediately before the start of the upsetting forging process are shown in Table 2.

[0119] [Table 2]

[0120] Quenching and tempering processes were performed on the intermediate products after the upsetting forging process. In the quenching process, the quenching temperature was set to 920°C for each test number, and the holding time at the quenching temperature was 60 minutes (1 hour). After the holding time, the intermediate products were water-cooled. The tempering temperature T2 (°C), the holding time t2 (hours) at tempering temperature T2, and FB in the tempering process are shown in Table 2. A piston upper dummy was manufactured using the above process. The area ratio of the hard phase of the piston upper dummy was determined based on the method described in [Method for measuring the area ratio of the hard phase of the piston upper 10]. At this time, the end face of the piston upper dummy (the surface that the die contacted during upsetting forging) was considered as the top surface 161, and a sample was taken. As a result, in all test numbers, the area ratio of the hard phase of the piston upper dummy was 90% or more.

[0121] [Evaluation Test] The following evaluation tests were performed on the piston upper replicas for each test number. (Test 1) MC-type carbide number density ND measurement test (Test 2) Ratio of Mo content in MC type carbide Mo Measurement test (Test 3) High-temperature strength evaluation test The following describes each test.

[0122] [(Test 1) MC-type carbide number density ND measurement test] In accordance with the method described in the above-mentioned [Method for measuring the number density ND of MC-type carbides], the number density ND (pieces / m³) of the piston upper simulated product for each test number was measured. 3 The number density ND of the obtained MC-type carbides was determined. The number densities ND are shown in Table 3.

[0123] [Table 3]

[0124] [(Test 2) Ratio of Mo content in MC type carbide Mo [Measurement Test] The above-mentioned [Mo content ratio R MoIn accordance with the method described in [Measurement Method], the Mo content ratio R in the MC type carbide of the piston upper dummy for each test number is measured. Mo The atomic percentage (%) was calculated. The obtained Mo content ratio R Mo This is shown in Table 3.

[0125] [(Test 3) High-temperature strength evaluation test] To simulate the long-term use of steel piston 1, a heat treatment was performed on the piston upper dummy for each test number, holding it at 600°C for 1000 hours. A round bar tensile test specimen was taken from the outer circumferential surface of the piston upper dummy at a depth of D / 4, extending axially from the piston upper dummy after heat treatment. The length of the round bar tensile test specimen, which corresponds to a flanged bar-shaped test specimen in accordance with JIS G0567:2020, was set to 78 mm. Using the taken round bar tensile test specimen, a tensile test was performed in air at 600°C in accordance with JIS G0567:2020 to obtain the yield strength (MPa). The obtained yield strengths at 600°C are shown in Table 3.

[0126] [Test Results] The test results are shown in Tables 1-1, 1-2, 2, and 3.

[0127] Tests 1 through 29 met the requirements for features 1 through 4. As a result, the yield strength at 600°C after holding it at 600°C for 1000 hours was 380 MPa or higher, demonstrating excellent high-temperature strength even when used for extended periods in high-temperature environments.

[0128] In tests 30 and 31, F1 did not satisfy formula (1). Therefore, the Mo content ratio R Mo The yield strength was low. As a result, the yield strength at 600°C after holding it at 600°C for 1000 hours was less than 380 MPa.

[0129] Tests 32 and 33 did not meet condition 1. As a result, the MC-type carbide number density (ND) was low. Consequently, the yield strength at 600°C after holding at 600°C for 1000 hours was less than 380 MPa.

[0130] In tests 34 and 35, the incorporation rate e under condition 2 was too low. Therefore, the Mo content ratio R Mo The yield strength was low. As a result, the yield strength at 600°C after holding it at 600°C for 1000 hours was less than 380 MPa.

[0131] In tests 36 and 37, the upsetting rate e under condition 2 was too high. As a result, the MC-type carbide number density ND was low. Consequently, the yield strength at 600°C after holding at 600°C for 1000 hours was less than 380 MPa.

[0132] Tests 38 and 39 did not meet condition 3. As a result, the MC-type carbide number density (ND) was low. Consequently, the yield strength at 600°C after holding at 600°C for 1000 hours was less than 380 MPa.

[0133] Test numbers 40 and 41 did not meet condition 4. Therefore, the Mo content ratio R Mo The yield strength was low. As a result, the yield strength at 600°C after holding it at 600°C for 1000 hours was less than 380 MPa.

[0134] Test numbers 42 and 43 did not meet condition 5. Therefore, the Mo content ratio R Mo The yield strength was low. As a result, the yield strength at 600°C after holding it at 600°C for 1000 hours was less than 380 MPa.

[0135] Embodiments of the present invention have been described above. However, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of the invention.

Claims

1. It is a steel piston, At least the top of the piston, including the top land, The piston comprises a lower piston located below the upper piston, The upper part of the piston is, The chemical composition is expressed in mass percent. C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.10-0.80%, Mo: 0.80-2.50%, V: 0.01-0.30%, Ti: 0.010 to 0.049%, Al: 0.005-0.100%, N: 0.020% or less, and O: Contains 0.0050% or less, The remainder consists of Fe and impurities. Satisfying equation (1), The number density ND of MC-type carbides in the upper part of the piston is 2.0 × 10 ２２ pieces / m ３ That's all. In the aforementioned MC-type carbide, the ratio of Mo content among the metal elements excluding Fe that constitute the MC-type carbide is R Ｍｏ It is 60 atomic percent or more. Steel piston. Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

2. It is a steel piston, At least the top of the piston, including the top land, The piston comprises a lower piston located below the upper piston, The upper part of the piston is, The chemical composition is expressed in mass percent. C: 0.30-0.50%, Si: 0.01-0.30%, Mn: 0.10 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: 0.10-0.80%, Mo: 0.80-2.50%, V: 0.01-0.30%, Ti: 0.010 to 0.049%, Al: 0.005-0.100%, N: 0.020% or less, and O: Contains 0.0050% or less, Furthermore, it contains one or more elements selected from the groups consisting of Group 1 to Group 3, with the remainder being Fe and impurities. Satisfying equation (1), The number density ND of MC-type carbides in the upper part of the piston is 2.0 × 10 ２２ pieces / m ３ That's all. In the aforementioned MC-type carbide, the ratio of Mo content among the metal elements excluding Fe that constitute the MC-type carbide is R Ｍｏ It is 60 atomic percent or more. Steel piston. [Group 1] Cu: 0.40% or less, Ni: 0.40% or less, B: One or more selected from the group consisting of 0.0100% or less. [Group 2] Nb: 0.100% or less, One or more selected from the group consisting of Zr: 0.300% or less. [Group 3] Sn: 0.100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, Bi: 0.300% or less, One or more selected from the group consisting of Te: 0.300% or less. Mo / (Mo+V+Ti)≧0.89 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element.

3. A steel piston according to claim 2, The above group 1 contains, Steel piston.

4. A steel piston according to claim 2, The following include the second group: Steel piston.

5. A steel piston according to claim 2, The following contain the third group: Steel piston.

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