JP2006009143A - Heat resistant alloy for use as material of engine valve - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low cost, economical and less resource-consuming heat resistant alloy for use as material of engine valve, while the alloy has strength at a high temperature and excellent toughness after heated for a long time that conventional heat resistant alloys have not had. <P>SOLUTION: The heat resistant alloy for use as material of engine valve consists essentially of, in mass percent, C of 0.01 to 0.15%, Si of 0.01 to 0.8%, Mn of 0.01 to 0.8, Cr of 14 to 17%, Mo of more than 3.0% but equal to or less than 5.0%, Al of 1.6 to 2.5%, Ti of 1.5 to 3.0%, Nb or Nb+Ta of 0.5 to 2.0%, Ni of 50 to 60%, B of 0.001 to 0.015%, at least one of Mg of 0.001 to 0.015% and Ca of 0.001 to 0.015%, and the balance substantially being Fe, wherein value A defined by 0.293[Ni]-0.513[Cr]-1.814[Mo]is 2.0 to 5.8, value B defined by [Al]/([Al]+[Ti]+[Nb]+[Ta]) is 0.45 to 0.65, and value C defined by [Al]+[Ti]+[Nb]+[Ta] is 6.2 to 7.6, wherein brackets mean atomic% of each element in the alloy. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a heat-resistant alloy for engine valves mainly used for automobile exhaust engine valves.

  Conventionally, Fe-based alloys (heat-resistant steel) such as SUH11 and SUH35 have been widely used for automobile engine valves. However, NCF751 (Ni— 15.5Cr-1Nb-2.3Ti-1.2Al-7Fe [mass%]) has come to be used.

However, NCF751 contains approximately 70% Ni and is therefore very expensive compared to Fe-based alloys. Therefore, the development of resource-saving alloys with high-temperature strength close to NCF751 and structural stability after long-time heating has been developed. Has been done. As a result, for example, an Fe-based heat-resistant alloy having a Ni content reduced to 30 to 35% by mass is disclosed in Patent Document 1, and an Fe-based heat-resistant alloy having a Ni content reduced to 30 to 49% by mass is disclosed in Patent Document 2. Patent Document 3 discloses an Fe-based heat-resistant alloy in which the Ni content is reduced to 35 to 45% by mass. Further, a high-Ni heat-resistant alloy having a higher temperature strength than NCF751 is disclosed in Patent Literature 4, Patent Literature 5, and the like.
JP-A-9-279309 JP-A-7-109539 JP 7-332035 A Japanese Patent Laid-Open No. 7-216482 Japanese Patent Laid-Open No. 11-229059

  In recent years, against the background of global environmental problems, high-temperature strength that is partially unsatisfactory even by NCF751 has been required for valve materials for the purpose of further increasing the efficiency of engines. On the other hand, in order to increase cost competitiveness with respect to globalization of the market, resource saving and cost reduction of parts are also desired.

  The alloys disclosed in Patent Documents 1 to 3 described above are excellent in terms of resource saving and low cost because the Ni content is 49% by mass or less, but the high-temperature strength does not greatly exceed NCF751. Further, the alloy disclosed in Patent Document 4 has a high temperature strength superior to that of NCF751, but is actually considered as an alloy having a Ni content exceeding 60% by mass, which means resource saving and cost reduction. Inadequate in terms.

  The alloy disclosed in Patent Document 5 proposed by the applicant of the present application that achieves both excellent high-temperature strength and low cost, which could not be achieved with these conventional alloys, has a Ni content of 50 to 60% by mass and is more than NCF751. It has been found that although it has excellent high-temperature strength at low cost, the structure stability at high temperatures is insufficient, and when used as a valve, toughness may be reduced by heating for a long time.

  It is an object of the present invention to provide a resource-saving engine valve heat-resistant alloy that has high-temperature strength and excellent toughness after long-time heating, which have not been achieved by the above-mentioned heat-resistant alloys, and that is economical and economical. Is to provide.

  First, the present inventors based on the alloy disclosed in Patent Document 5, in order to improve the defect of the alloy that the structural stability at high temperature is insufficient and the toughness is lowered by long-time heating. The examination was done. As a result, although this alloy has a high temperature strength of NCF751 or more, it actually contains Cr in excess of 17% by mass. To maintain the structural stability, its Ni content and Mo, which is a solid solution strengthening element, are included. It has been found that since Cr is excessive with respect to the W content, brittle phases such as σ phase and α ′ phase are produced after high temperature and long time heating, and the toughness is greatly reduced. As a result of diligent investigation to maintain the toughness after heating for a long time at a high level, the Cr content, the content of Ni, which is a constituent element of austenite, and the content of Mo, which is a solid solution strengthening element, are more optimal. It was found that a heat-resistant alloy having very high structure stability, excellent toughness after long-time heating, and high-temperature strength higher than that of NCF751 can be obtained.

That is, the heat-resistant alloy for engine valves of the present invention is C: 0.01 to 0.15%, Si: 0.01 to 0.8%, Mn: 0.01 to 0.8%, Cr: 14 in mass%. -17%, Mo: more than 3.0% and 5.0% or less, Al: 1.6-2.5%, Ti: 1.5-3.0%, Nb alone or Nb + Ta: 0.5- 2.0%, Ni: 50-60%, B: 0.001-0.015%, Mg: 0.001-0.015% and Ca: 0.001-0.015% Or two types are included, the remainder consists essentially of Fe, and the following A value is 2.0-5.8 in atomic%, B value is 0.45-0.65, C value is 6.2-7. .6 is satisfied.
A value 0.293 [Ni] -0.513 [Cr] -1.814 [Mo]
B value [Al] / ([Al] + [Ti] + [Nb] + [Ta])
C value [Al] + [Ti] + [Nb] + [Ta] [] represents atomic%.

  The heat-resistant alloy for engine valves of the present invention has Mo: 3.5 to 4.0%, A value of 2.4 to 4.0, B value of 0.5 to 0.6, and C value of 6.4. It is preferably ˜7.0.

  When Cr on the cross section of the engine valve heat resistant alloy is subjected to line analysis by EPMA, the maximum value and the minimum value of the Cr amount are preferably (maximum value) / (minimum value) ≦ 1.2.

Moreover, it is preferable that the ² mmU notch Charpy impact value in normal temperature after heating at 800 degreeC for 400 hours is 50 J / cm < ² > or more.

  More preferably, in the microstructure after heating at 800 ° C. for 400 hours, intermetallic compounds of σ phase, α ′ phase, η phase, and δ phase having a length of 3 μm or more are not substantially precipitated.

  The heat-resistant alloy for engine valves of the present invention has high-temperature strength and high toughness after long-time heating, which cannot be achieved by conventional heat-resistant alloys, and is excellent in economic efficiency with resource saving, low cost, When used as an engine valve material that requires high strength, it is possible to achieve higher engine efficiency, resource saving of the valve material, and cost reduction.

  An important feature of the present invention is that the structure stability after high-temperature and long-time heating is improved based on the alloy disclosed in Patent Document 5 proposed by the present applicant. In the alloy disclosed in Patent Document 5, the total amount of Cr, Mo, and W is regarded as important for ensuring the structural stability. However, even if the total amount of Cr, Mo and W is within the range specified in Patent Document 5, if the Ni content is low, the structure stability at high temperature is insufficient, and the σ phase and α are not heated by long-time heating. 'A brittle phase such as a phase was formed, and toughness decreased. Therefore, it is necessary to adjust Ni within a specific range. That is, an alloy excellent in high-temperature strength with a stable structure at high temperature can be obtained only by simultaneously managing not only the Cr, Mo and W amounts but also the Ni amount.

  Further, like Mo, W also has the effect of strengthening the austenite base in a solid solution and increasing the high temperature fatigue strength and the high temperature creep strength. However, W has a specific gravity nearly twice that of Mo, and in order to obtain the same effect as Mo, an addition amount almost twice that of Mo is required. Moreover, there is a demerit that the LAVES phase, which is an embrittlement phase, easily precipitates by adding W in an amount that can provide the same effect as Mo. Therefore, only Mo is essentially added to the alloy of the present invention aiming at low cost and structural stability. Furthermore, the effect of solid solution strengthening by Mo is stable compared to the fact that the effect of precipitation strengthening of γ 'phase gradually decreases as the γ' phase coarsens and aggregates during long-time heating. The alloy of the present invention that makes maximum use of strengthening while maintaining the structural stability has the advantage of high fatigue strength in the long-term region.

  In the heat-resistant alloy for engine valves of the present invention, the reason why each chemical composition is specified is as follows. Unless otherwise specified, the chemical composition is expressed in mass%.

C: 0.01 to 0.15%
C combines with Ti and Nb to form MC carbide, which is useful for preventing coarsening of crystal grains and improving creep rupture ductility. If C is less than 0.01%, those effects due to MC carbide are not sufficient. On the other hand, if it exceeds 0.15%, a large amount of decomposition reaction from MC carbide to M ₂₃ C ₆ carbide occurs during long-time heating, resulting in a problem that the ductility of the grain boundary at normal temperature is lowered. It was made into the range of -0.15%. The preferable range of C is 0.01 to 0.08%.

Si: 0.01 to 0.8%
Si needs to be added in an amount of 0.01% or more in order to obtain a deoxidizing effect. However, if it exceeds 0.8%, there is a problem that the high-temperature strength is lowered. Stipulate. A preferable range of Si is 0.01 to 0.6%.

Mn: 0.01 to 0.8%
Mn also needs to be added in an amount of 0.01% or more in order to obtain a deoxidizing effect as in the case of Si. However, if it exceeds 0.8%, there is a problem that the high-temperature strength is lowered. .8% is specified. A preferable range of Mn is 0.01 to 0.6%.

Cr: 14-17%
Cr is an indispensable element for imparting oxidation resistance to an alloy used at a high temperature, and is an important element having a role of forming a solid solution in austenite and strengthening the solution. In order to ensure the oxidation resistance and heat resistance required for automotive engine valve materials, a minimum of 14% is required. However, if the upper limit of 17% is exceeded, the structure becomes unstable due to heating at high temperatures for a long time, resulting in Cr A harmful embrittlement phase such as a rich α ′ phase or σ phase is generated, which reduces fatigue strength and creep rupture strength, and also causes a decrease in normal temperature ductility. An important feature of the present invention is the stability of the structure after high-temperature and long-time heating, so the Cr content is kept to a minimum level that can maintain oxidation resistance. Therefore, Cr is specified to be 14 to 17%. The preferable range of Cr is 14.5 to 16.5%. A more preferable range is 15 to 16.5%, and more preferably 15.5 to 16.5%.

Mo: more than 3.0% to 5.0% or less Mo is one of the most important elements in the alloy of the present invention, which has the effect of solid solution strengthening the austenite base and remarkably increasing the high temperature fatigue strength and the high temperature creep strength. is there. If the Mo content is 3.0% or less, the solid solution strengthening does not work sufficiently, so that the high temperature strength that the present invention can achieve cannot be achieved. Conversely, excessive addition of Mo exceeding 5.0% impairs hot workability, and a harmful phase such as an α ′ phase or a σ phase is precipitated by heating at a high temperature for a long time, as in the case of containing excessive Cr. This causes a problem. Therefore, Mo is specified to be more than 3.0% and 5.0% or less. A preferable range is 3.3 to 4.8%. Furthermore, the upper limit of preferable Mo content is 4.6% or less, and 4.0% or less is more preferable. Moreover, the minimum of more preferable Mo content is 3.5% or more.

Al: 1.6-2.5%
Al is an indispensable element for obtaining a desired high-temperature strength by precipitating a stable γ 'phase, and at least 1.6% is required. However, when it exceeds 2.5%, hot workability deteriorates. Therefore, it is defined as 1.6 to 2.5%. A preferable range of Al is 1.6 to 2.1%, and a more preferable range is 1.6 to 1.9%.

Ti: 1.5-3.0%
Ti combines with C to form MC carbide to refine the austenite crystal grains, and together with Al, Nb, and Ta to form Ni 'phase by forming Ni and has the effect of improving the high temperature strength. Addition of 5% or more is necessary. However, if Ti is added in excess of 3.0%, transformation from the γ 'phase to the η phase is likely to occur at high temperatures and the high-temperature strength is lowered. Further, excessive addition of Ti excessively increases the γ' phase. This causes a problem of reducing hot workability. Therefore, the composition range of Ti is defined as 1.5 to 3.0%. A preferable range of Ti is 1.5 to 2.5%. A more preferable upper limit of the Ti content is 2.3% or less, and a further preferable lower limit of the Ti content is 1.8% or more.

Nb alone or Nb + Ta: 0.5 to 2.0%
Nb and Ta are combined with C in the same way as Ti to form MC carbide and refine the austenite crystal grains, and also have the effect of improving the high temperature strength by forming the γ 'phase. This has the effect of further stabilizing the γ 'phase, and therefore suppresses the decrease in high temperature strength after high temperature and long time heating. Therefore, it is necessary to add Nb alone or Nb and Ta in total at least 0.5% or more, but excessive addition exceeding 2.0% causes transformation from the γ ′ phase to the δ phase at a high temperature. Since it becomes easy and the problem that high temperature intensity | strength falls arises, Nb and Ta are prescribed | regulated to 0.5 to 2.0% in total. A preferable range is 0.5 to 1.8%. Furthermore, the upper limit of preferable Nb + Ta content is 1.6% or less, and 1.4% or less is more preferable. Moreover, the minimum of more preferable Nb + Ta content is 0.8% or more, and 1.0% or more is more preferable.

Ni: 50-60%
Ni is a very important element that stabilizes the austenite base to increase the high-temperature strength and is also a constituent element of the γ ′ phase that contributes to precipitation strengthening. If Ni is less than 50%, precipitation of the γ ′ phase becomes insufficient, and Mo, which is a solid solution strengthening element, cannot be dissolved in a sufficient amount for strengthening while maintaining the structural stability. The problem that high temperature strength falls arises. On the other hand, when it exceeds 60%, the hot workability is lowered, and further, the problem that the merit as a low cost material is lost occurs. Therefore, Ni is specified in the range of 50 to 60%. A preferable range of Ni is 50 to 58%. A more preferable upper limit of the Ni content is 56% or less. Further, the lower limit of the Ni content is more preferably 52% or more, and more preferably 54% or more.

B: 0.001 to 0.015%
B is effective for increasing the high-temperature strength and ductility by the grain boundary strengthening action by adding an appropriate amount. The effect starts to occur with a small amount of addition of 0.001% or more, but when it exceeds 0.015%, the melting point of the part where B segregates is lowered and the hot ductility is partially lowered, thereby causing hot workability. Therefore, B is specified to be 0.001 to 0.015%.

Mg: 0.001 to 0.015%, Ca: 0.001 to 0.015% Mg and Ca are powerful deoxidation / desulfurization elements that increase the cleanliness of the alloy, as well as high-temperature tension and creep deformation. In order to help improve the ductility during hot working, at least one kind is added in an appropriate amount. The effect begins to occur with addition of a small amount of 0.001% or more, but when Mg and Ca are added in excess of 0.015%, a low melting point compound is formed and the high temperature ductility is lowered. Therefore, Mg and Ca are respectively specified to be 0.001 to 0.015%.

The balance is substantially Fe
The balance is Fe, but can contain inevitable impurities. Further, the following elements can be contained in the ranges shown below.
P ≦ 0.04%, S ≦ 0.02%, O ≦ 0.02%, N ≦ 0.05%
In the present invention, in order to obtain a high-temperature strength of NCF751 or higher and a structure stability at a high temperature, not only the contents of the elements are individually defined as described above, but also Ni and Cr which are constituent elements of austenite of the base material. The content of Mo, which is a solid solution strengthening element, needs to be specified in a more optimal range as a relational expression.

  The structural stability is determined by the balance between Ni and Cr, which are constituent elements of austenite, and Mo, which is a solid solution strengthening element. In order to increase the high-temperature strength, it is necessary to contain Mo as a solid solution strengthening element while maintaining the structural stability in the range of Ni content of 50-60 mass% and Cr content of 14-17 mass%. It is important to be able to do it. A value expressed in atomic%: 0.293 [Ni] -0.513 [Cr] -1.814 [Mo] High temperature tissue stability can be achieved simultaneously. When the A value is less than 2.0, the σ phase and α ′ phase, which are embrittled layers, are precipitated after high temperature and long time heating, and the toughness of the material is deteriorated. On the other hand, when the A value exceeds 5.8, the solid solution strengthening becomes insufficient, so that the high-temperature strength decreases. A more preferable range of the A value is 2.2 to 5.6. A more preferable range is 2.4 to 5.0, and a more preferable range is 2.4 to 4.0.

Furthermore, in the present invention, the Al ratio in the γ ′ phase and the total amount of Al, Ti, Nb, Ta as the γ ′ phase forming elements are also defined as follows.
B value [Al] / ([Al] + [Ti] + [Nb] + [Ta])
C value [Al] + [Ti] + [Nb] + [Ta]
[] Represents atomic%.

  The B value indicates the ratio of Al in the γ ′ phase. If the Al ratio is low and the B value is less than 0.45, the high temperature strength decreases due to the transformation of the γ 'phase to the η phase or δ phase after high temperature heating for a long time. Conversely, the Al ratio is high and the B value is high. If it exceeds 0.65, the lattice constant of the γ ′ phase is decreased, the effect of precipitation strengthening is reduced, the high temperature strength is lowered, and the hot workability is lowered. Therefore, it is necessary to define the B value to 0.45 to 0.65. A more preferable range of the B value is 0.5 to 0.6.

  Moreover, in order to obtain sufficient high temperature strength, C value of 6.2 or more is required. However, if the C value exceeds 7.6, the γ ′ phase becomes excessive and the hot deformation resistance increases, so that the hot workability is lowered, and it is difficult to manufacture the valve. Therefore, the C value is defined as 6.2 to 7.6. A more preferable range of the C value is 6.2 to 7.4. A more preferable range is 6.4 to 7.2, and a more preferable range is 6.4 to 7.0.

  In the present invention, the segregation of elements in the alloy was also examined in order to further improve the structural stability after high-temperature and long-time heating.

  In the alloy of the present invention, in order to improve the high-temperature strength when the Ni content is in the range of 50 to 60% by mass, Mo, which is a solid solution strengthening element, is added in an amount close to the limit where the structural stability is maintained. If there is segregation of elements in the alloy in this state, the structure may become partially unstable. Therefore, as a result of investigating the magnitude of the influence of segregation of each element on the structure stability, it was found that the influence of Cr, which is a constituent element of austenite, is the largest. If there is segregation of Cr in the alloy, embrittled phases such as σ phase and α ′ phase are very likely to be formed in a portion where the Cr concentration is high, and the strength tends to be lowered. Therefore, as a result of various studies on the segregation of Cr, the maximum value and the minimum value of the Cr amount analyzed on one cross section of the alloy of the present invention by EPMA are managed as (maximum value) / (minimum value) ≦ 1.2. It was found that the structural stability of the alloy can be secured. Preferably, (maximum value) / (minimum value) ≦ 1.1.

  In a heat-resistant alloy having a Ni content of 50 to 60% by mass, ensuring the structural stability after high-temperature and long-time heating is essential for improving high-temperature strength and maintaining toughness after long-time heating. is there. The segregation of Cr can be reduced by subjecting the steel ingot after melting or the steel ingot subjected to remelting by VAR, ESR or the like to homogenization heat treatment at 1150 to 1220 ° C. for 10 hours or more. Since the alloy of the present invention has a composition in the vicinity of the boundary of the region where the austenite of the parent phase is stable, particularly when segregation of Cr occurs, an embrittled phase that easily falls out of the austenite stable region and lowers the toughness of the alloy is likely to be generated. Therefore, by performing homogenization heat treatment on the alloy of the present invention, it is possible to reduce the segregation of Cr and improve the structural stability after high-temperature and long-time heating, so that stable high toughness can be ensured. it can.

In recent years, the service life of automobiles has been extended, and as a result, durability of each component material has been required more than ever. For heat-resistant alloys for automotive engine valves, if the Charpy impact value at room temperature after heating at 800 ° C. for 400 hours does not satisfy 50 J / cm ² or more, the toughness required for the valves will be insufficient after long-term use. Sometimes. Therefore, in the alloy of the present invention, the ² mm U notch Charpy impact value at room temperature after heating at 800 ° C. for 400 hours is defined as 50 J / cm ² or more as necessary.

Furthermore, it is specified that intermetallic compounds of σ phase, α ′ phase, η phase, and δ phase having a length of 3 μm or more are not substantially precipitated in the microstructure of the alloy of the present invention heated at 800 ° C. for 400 hours. By doing, the toughness after a high-temperature and long-time heating can be made more reliable. Intermetallic compounds of σ phase, α ′ phase, η phase, and δ phase reduce the toughness of the alloy. However, in an alloy in which these intermetallic compounds are not substantially precipitated, room temperature after heating at 800 ° C. for 400 hours. 2 mmU notch Charpy impact value of 50 J / cm ² or more is satisfied, and preferably 70 J / cm ² or more can be obtained.

  If intermetallic compounds of σ phase, α ′ phase, η phase, and δ phase having a length of 3 μm or more are precipitated in the alloy, the toughness may be greatly reduced. Here, the fact that the intermetallic compounds of σ phase, α ′ phase, η phase, and δ phase are not substantially precipitated means that the above intermetallic compounds are not observed when the SEM structure is observed at 1000 times. Say. Moreover, in order to ensure the toughness after heating at high temperature for a long time, it is preferable that those intermetallic compounds having a length of 2 μm or more are not deposited, and those intermetallic compounds having a length of 1 μm or more are deposited. More preferably not.

  The alloy of the present invention and the comparative alloy were melted in a vacuum induction furnace to produce a 10 kg steel ingot, then subjected to homogenization heat treatment at 1180 ° C. for 20 hours, heated to 1150 ° C. and forged into 30 mm square bars. Table 1 shows the alloy No. of the present invention. 1-9, conventional alloy no. 21, Comparative Alloy No. The chemical composition of 22-27 is shown. Here, the conventional alloy no. 21 is an alloy equivalent to NCF751. Comparative Alloy No. Nos. 22, 23 and 27 are alloys disclosed in Patent Document 5, and in particular, comparative alloy Nos. No. 27 in Example No. 5 of Patent Document 5. 6 equivalent alloy. Comparative alloy No. No. 23 was not subjected to the homogenization heat treatment. Furthermore, in order to evaluate a sample having a shape close to an actual valve material, the alloy No. 1 of the present invention is used. In No. 9, the steel ingot was forged into a round bar at 1150 ° C. hot, and then finished into a 6 mm diameter bar by cold drawing.

  In order to evaluate the segregation of Cr, a sample was taken so that the longitudinal section of the forged material and the bar became a measurement surface, and using EPMA, a measurement distance of 3 mm and a beam diameter of 7 were perpendicular to the length direction. A Cr line analysis was performed at 5 μm. The maximum value and the minimum value were read, and the value of (maximum value) / (minimum value) indicating the degree of segregation was calculated. Further, the forged material and the bar were subjected to an aqueous aging treatment at 750 ° C. for 4 hours after a solid solution treatment at 1050 ° C. for 30 minutes and water cooling. For the forged material, after this heat treatment, a round bar test piece having a parallel part diameter of 6.35 mm and a distance between gauge points of 25.4 mm was collected and subjected to a tensile test at 800 ° C. by the ASTM method.

In addition, a round bar test piece having a parallel part diameter of 8 mm was collected after the same heat treatment, and a rotating bending fatigue test was performed at a test temperature of 800 ° C. and a rotational speed of 3600 rpm in accordance with JIS Z2274, and fatigue strength at 10 ⁷ times was obtained. . Furthermore, after heating this aging-treated material at 800 ° C. for 400 hours, a 2 mm U-notch JIS No. 3 test piece was collected and subjected to a Charpy impact test at room temperature in accordance with JIS Z2242.

Moreover, in order to check the structure stability at high temperature, the microscope structure observation by SEM at 1000 times was performed on an area of 150 mm ² after heating at 800 ° C. for 400 hours. These results are shown in Table 2. Here, the present alloy No. Since No. 9 is a 6 mm diameter bar, it is difficult to process the test piece from the bar, so only the segregation measurement of Cr, the tensile test using the reduced test piece, and the microscopic observation were performed.

  In FIG. 3 and FIG. 22 was heated at 800 ° C. for 400 hours, then corroded with a solution in which hydrochloric acid, nitric acid, and glycerin were mixed at a volume ratio of 3: 1: 1, and an SEM structure photograph observed at a magnification of 1000 times was shown together with a schematic diagram thereof. . Invention alloy No. 3, in the austenite of the base material, 1. Although only MC carbide was observed, comparative alloy No. 1 with low structural stability after prolonged heating was used. 22: 1. It is an embrittlement layer together with MC carbides. The σ phase is precipitated.

  From Table 2, the conventional alloy No. In comparison with the NCF751-equivalent alloy of No. 21, the present alloy No. 1 to 8 are far superior in mechanical properties and fatigue strength at 800 ° C. Further, the alloy of the present invention has little segregation of Cr, a high Charpy impact value after heating at 800 ° C. for 400 hours, and a structure observation by SEM at 1000 times, a σ phase having a length of 3 μm or more, an α ′ phase, η It can be seen that the structure stability at high temperature is very high because no intermetallic compound of δ phase and δ phase is confirmed. Furthermore, even when the structure was observed by SEM at a high magnification of 4000 times, no intermetallic compound exceeding 1 μm in length was found in the alloy of the present invention. In addition, the alloy No. 1 of the present invention having a 6 mm diameter bar. Similarly, No. 9 has excellent mechanical properties at 800 ° C. and very little segregation of Cr.

  Comparative Alloy No. disclosed in Patent Document 5 No. 22 has high mechanical properties and fatigue strength at 800 ° C., but the Cr content is high and the structural stability at high temperature is low. And the Charpy impact value is low. Comparative Alloy No. Similarly, No. 23 has low structure stability at high temperature due to segregation of Cr, and thus the Charpy impact value after heating at 800 ° C. for 400 hours is low. Comparative alloy No. No. 24 has a high Al ratio in the γ 'phase. No. 25 has a low Al ratio in the γ ′ phase, and the γ ′ phase transforms into an embrittled η phase by high temperature and long time heating. Since the amount of γ ′ phase does not sufficiently contribute to precipitation strengthening, the mechanical properties and fatigue strength at 800 ° C. are lower than that of the alloy of the present invention.

Comparative alloy No. corresponding to the example alloy of Patent Document 5 27, the fatigue strength of the long-range 10 ⁷ times amount of Mo is low low and is low structural stability at high temperatures lower A values, sigma phase embrittlement phase is precipitated after heating 800 ° C. × 400 hours However, it becomes brittle and has a low Charpy impact value. In other words, it has a sufficient amount of Mo for material reinforcement, and only the invention alloy which satisfies the A value indicating the structural stability at high temperature, long-time heating and high fatigue strength at long range 10 ⁷ times It has high toughness at the same time.

  As described above, the alloy of the present invention has a high-temperature strength higher than that of NCF751, has excellent structure stability at high temperatures, exhibits high toughness even after long-time heating, and is low-cost and resource-saving engine with excellent economy. It turns out that it is a heat-resistant alloy for valves.

  The heat-resistant alloy for engine valves of the present invention has high-temperature strength and high toughness after long-time heating, which cannot be achieved by conventional heat-resistant alloys, and is excellent in economic efficiency with resource saving, low cost, When used as an engine valve material that requires high strength, it is possible to achieve higher engine efficiency, resource saving of the valve material, and cost reduction.

Invention alloy No. It is the electron microscopic structure photograph after heating 3 for 800 degreeC x 400 hours, and its schematic diagram. Comparative Alloy No. It is the electron microscopic structure photograph after heating 22 for 800 degreeC x 400 hours, and its schematic diagram.

Explanation of symbols

1. MC carbide, σ phase

Claims (5)

C: 0.01 to 0.15% by mass%, Si: 0.01 to 0.8%, Mn: 0.01 to 0.8%, Cr: 14 to 17%, Mo: 3.0% More than 5.0%, Al: 1.6-2.5%, Ti: 1.5-3.0%, Nb alone or Nb + Ta: 0.5-2.0%, Ni: 50-60% , B: 0.001 to 0.015%, Mg: 0.001 to 0.015% and Ca: 0.001 to 0.015%, one or two of them, the balance is substantially An engine comprising Fe and having an atomic percent satisfying the following A value of 2.0 to 5.8, B value of 0.45 to 0.65, and C value of 6.2 to 7.6. Heat resistant alloy for valves.
A value 0.293 [Ni] -0.513 [Cr] -1.814 [Mo]
B value [Al] / ([Al] + [Ti] + [Nb] + [Ta])
C value [Al] + [Ti] + [Nb] + [Ta] [] represents atomic%. The Mo value is 3.5 to 4.0%, the A value is 2.4 to 4.0, the B value is 0.5 to 0.6, and the C value is 6.4 to 7.0. Heat resistant alloy for engine valves. The maximum and minimum values of Cr amount are (maximum value) / (minimum value) ≦ 1.2 when the Cr segregation is subjected to line analysis by EPMA on the cross section. Heat resistant alloy. The heat resistant alloy for engine valves according to any one of claims 1 to 3, wherein a ² mmU notch Charpy impact value at normal temperature after heating at 800 ° C for 400 hours is 50 J / cm ² or more. 5. The microscopic structure after heating at 800 ° C. for 400 hours is substantially free of intermetallic compounds of σ phase, α ′ phase, η phase, and δ phase having a length of 3 μm or more. The heat-resistant alloy for engine valves as described.

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