JP3068868B2 - Heat resistant steel for engine valves - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to heat-resistant steel for engine valves used in internal combustion engines of automobiles and the like.

[0002]

2. Description of the Related Art Conventionally, exhaust valve steel has a high Mn which has moderately excellent high-temperature strength, corrosion resistance to lead and sulfur contained in gasoline, and oxidation resistance, and is inexpensive.
21-4N steel (0.55C-0.2Si-9)
Mn-4Ni-21Cr-0.4N) has been widely used. However, in recent years, with the increase in combustion temperature due to the high efficiency and high output of gasoline engines, the demand for 21-4N steel for valve heat-resistant steel having even higher high-temperature strength has been increased. Has been proposed (Japanese Patent Publication No. 61-27)
No. 41, JP-A-60-77964, JP-A-59-211557, JP-A-6
3-89645, JP-A-1-219147).

[0003]

The above-mentioned steels for the purpose of improving the high-temperature strength of 21-4N steels contain C of not less than 0.15% and contain alloying elements such as V, Nb, Mo and W. The amount added is increasing. The alloying elements of these known steels are added mainly for the purpose of strengthening the precipitation of carbides, as can be seen from the relatively high C content. However, such precipitation strengthening of carbide is not always a satisfactory strengthening mechanism for high-temperature strength of 850 ° C. or higher, and the development of a more excellent heat-resistant steel has been desired. Alloying elements such as V, Nb, and Mo added for the purpose of improving the high-temperature strength are instead of 21.
There is also a problem that the oxidation resistance is lower than that of -4N steel. In addition, the fatigue damage of actual engine valves often starts from corrosion due to lead oxide or sulfur, and the corrosion resistance, especially the corrosion resistance of mixed lead with lead oxide and sulfur lead, which can simultaneously evaluate the corrosion resistance to lead oxide and sulfur. Improving the value greatly contributes to improving the life of the valve.

[0004] Further, as a higher grade material than 21-4N steel, Inconel 751 (INCONEL is a trademark) of a Ni-based super heat-resistant alloy has been used as an alloy for exhaust engine valves. The problem with Inconel 751 is that the gamma prime phase (Ni ₃ (Al, Ti, Nb)), which is a precipitation strengthening element, becomes coarse during high-temperature and long-time heating, and the high-temperature strength decreases compared to after normal heat treatment. Inconel 751 has two problems that it is too large and that the Ni content is high, so that the weight loss in a corrosive environment containing S is large and therefore the mixed lead corrosion resistance is poor. It is an object of the present invention to provide an alloy having a basic composition of 21-4N high-Mn heat-resistant steel and being as close as possible to Inconel 751 alloy, which is a Ni-base super heat-resistant alloy, or having some properties exceeding Inconel 751. An object of the present invention is to provide a heat-resistant steel for an engine valve having both high-temperature strength, corrosion resistance, and oxidation resistance.

[0005]

In view of the above-mentioned problems, the present inventors have sought to strengthen the heat-resistant steel for engine valves by solid solution strengthening of various elements rather than the conventional precipitation strengthening of carbides. Tried. As a result, as a first feature of the present invention, C is added only to a minimum necessary amount, and among the substitutional solid solution strengthening elements, the degree of deterioration of oxidation resistance is small among the solid solution strengthening elements, and the creep strength is improved most. A steel having excellent oxidation resistance and high-temperature strength has been newly discovered by a strengthening mechanism combining the interaction between W and N, which is an interstitial solid solution strengthening element, which has been effective in the above. Further, the present inventors have as a second feature that the high M of the 21-4N system is
nIn Ni and Co where no significant difference has been found as an austenite forming element in heat resistant steels,
It has also been clarified that the proper addition of Co is very useful for improving the high temperature fatigue strength. A third feature of the heat-resistant steel of the present invention is that, unlike the above-described improved steel of 21-4N steel, V and Mo, which are elements that have a detrimental effect on oxidation resistance, are not added. . Further, a fourth feature of the heat-resistant steel of the present invention is that the optimum component ranges of Mn and Cr have been clarified with respect to the improvement of high-temperature strength and the improvement of mixed lead corrosion resistance and oxidation resistance.

That is, the first invention of the present invention is characterized in that, by weight%, C is 0.02% or more and less than 0.15%, Si is 0.05-1.0%, and Mn is Mn.
4.5% or more and less than 7.5%, Ni 9.0-15.0%, Co 1.0-5.0%,
Cr 22.0-26.0%, W 4.0-8.0%, Nb 0.01-0.50%, N
A heat-resistant steel for an engine valve, comprising 0.40% or more and 0.70% or less, B 0.02% or less, and unavoidable impurities, the balance being Fe and having a composition of Fe. When the weight loss by oxidation after holding at 1000 ° C. for 100 hours is 2.0 mg / cm ² or less, and the weight loss by corrosion test in a mixed lead consisting of 60% PbO and 40% PbSO _{4 by} weight is 1 hour at 920 ° C. to 100mg / cm ² or less, the fatigue strength of 850 ℃ is 17kgf / mm
^The heat-resistant steel for an engine valve according to the first invention, wherein a tensile strength at 900 ° C is 24 kgf / mm ² or more, and a creep rupture life under a stress load of 6 kgf / mm ² at 900 ° C is 50 hours or more. The third invention is the heat-resistant steel for an engine valve according to the first invention, wherein a tensile strength at 900 ° C. after holding at 900 ° C. for 300 hours is 18 kgf / mm ² or more.

[0007]

First, the reasons for limiting numerical values in the present invention will be described. C is an extremely strong austenite-forming element, and is an element necessary for turning the matrix into austenite and increasing the strength, so that at least 0.02% is required. However, as the amount of C increases, the amount of carbides increases, and when it exceeds 0.15%, many of the added alloy elements generate carbides, resulting in 850%.
It does not help to strengthen the base above ℃. In order to improve the creep strength at high temperatures, it is desirable that the crystal grains grow to an appropriate size. Such an increase in carbides suppresses grain growth and is not effective for creep strength. Furthermore, since excessive addition of C lowers the solid solubility of N, which is the main strengthening element of the steel of the present invention, the range of C is set to 0.
Limited to 02% or more and less than 0.15%. One of the great features of the present alloy is that the amount of carbon is limited to such a low level in heat-resistant steel for valves.

Si is a deoxidizing agent at the time of dissolution and an element effective for imparting oxidation resistance at a high temperature.
Requires%. However, since Si exceeding 1.0% is not effective for high-temperature strength, the range of Si is set to 0.05 to 1.0%.

Mn stabilizes austenite in the matrix and acts as an alternative to expensive Ni and Co. Also,
Since Mn also increases the solubility of N, a minimum of 4.5% is required. However, 7.5% or more of Mn forms an oxide film having a spinel structure that is harmful to oxidation resistance and corrosion resistance, and lowers the high-temperature strength. Further, since a harmful sigma phase is easily precipitated by synergistic action with Cr, Mn is set to 4.5% or more and less than 7.5%.

[0010] Cr is an element indispensable for improving the corrosion resistance and oxidation resistance of heat-resisting steel for valves because it forms an oxide film with high adhesion, and has a greater effect of further increasing the solubility of N than Mn. Cr requires at least 22.0%.
However, if it exceeds 26%, a sigma phase is likely to be precipitated due to a synergistic effect with Mn, so Cr is limited to 22.0 to 26.0%.
By meeting the low Mn amount and the high Cr amount as described above,
One of the features of the present invention is that both high-temperature strength and corrosion resistance and oxidation resistance are improved.

Ni is an element necessary for stabilizing austenite of the matrix, and is required to be 9.0% or more in order to maintain strength, corrosion resistance and oxidation resistance. However, the addition of Ni in excess of 15% reduces the solid solubility of N, which is the main strengthening element of the steel of the present invention, and makes Ni expensive.
Is limited to 9.0-15.0%.

[0012] In conventional 21-4N high Mn heat-resistant steels, the difference between Ni and Ni, which is an austenite-forming element, has not been clarified except for improvement in lead oxide resistance.
The present inventors have sufficiently studied the influence of Co and found that Co lowers the stacking fault energy and clearly has an effect on the improvement of the fatigue strength. The necessary Co is at least 1.0% for this purpose, but excessive addition exceeding 5.0% does not contribute to the improvement of the fatigue strength so much that it lowers the solid solubility of N and unnecessarily increases the price of steel. To C
o is 1.0 to 5.0%.

W is an element belonging to the same family as Mo and, like Mo, forms a solid solution in the matrix as a substitutional atom, and at the same time, partially forms carbide to maintain high-temperature strength. However, since W has an atomic weight twice that of Mo, the diffusion rate at high temperatures is small, and as a result, the effect of improving the creep rupture strength is large. Further, W, which is a substitution type solid solution strengthening element, further contributes to improvement in high temperature strength by interaction with N which is an interstitial type solid solution strengthening element, as compared with the case of adding each alone. Further, unlike Mo, W hardly reduces the oxidation resistance of steel.
For the above reasons, W is an essential additive element of the steel of the present invention, and if it is less than 4.0%, sufficient high-temperature strength cannot be obtained, and 8.
The addition of W in excess of 0% produces nitrides of W, does not have a sufficient effect on the solid solution strength, and merely unnecessarily increases the specific gravity and price of the steel. Therefore, W is limited to 4.0 to 8.0%. Another feature of the present invention is to clarify the difference between the homologous elements W and Mo and to include only W as an alloying element.

Nb forms fine primary carbides that are stable up to high temperatures, prevents austenite from coarsening, and provides an appropriate crystal grain size. As a result, good high-temperature tensile strength and creep rupture strength are obtained. can get. Therefore, the necessary amount of Nb is 0.01% or more, but the addition exceeding 0.50% remarkably lowers the oxidation resistance, so that the Nb content is 0.01 to 0.5%.
0%.

N is a strong austenite forming element in line with C, but in the steel of the present invention, Nb, W,
It hardly forms compounds with alloying elements such as Cr and works as an interstitial solid solution strengthening element. Therefore, the steel of the present invention works very effectively together with the above-mentioned substitutional solid solution strengthening element for improving the high-temperature strength of 850 ° C. or higher as intended. More specifically, the solute nitrogen after solid solution treatment and aging treatment forms fine nitrides in the matrix when heated at a high temperature for a long time, but the amount of the precipitate and the growth rate are small, so that the nitrogen is exposed to the high temperature for a long time. Even if it does, the deterioration of the characteristics is relatively small. On the other hand, Inconel 751, which is used for valves as a high-grade material, has higher high-temperature strength immediately after regular heat treatment than the steel of the present invention, but when heated for a long time at high temperature, the gamma prime phase, which is the precipitation strengthening phase, is agglomerated. This is equivalent to the high-temperature strength after the steel of the present invention has been exposed to a high temperature for a long time. In the composition range of the steel of the present invention, when N is 0.40% or less, the above effects cannot be obtained. On the other hand, since the maximum solid solubility of N is 0.70%, N is limited to more than 0.40% and 0.70% or less. I do.

B segregates at the crystal grain boundaries by adding a small amount,
It is useful for improving creep rupture strength and hot workability, but its effective amount is 0.02% or less. The heat-resistant steel for an engine valve according to the present invention is an iron-based alloy composed of the above-mentioned main elements, the following unavoidable impurities, and the balance Fe. P ≦ 0.04% V ≦ 0.1% Ca ≦ 0.02% S ≦ 0.03% Ta ≦ 0.1% Cu ≦ 0.30% Mg ≦ 0.02%

Next, the reason for the numerical limitation of the second invention of the present invention will be described. In the present invention, the steel having the above composition is formed into a desired shape by ingot casting, forging or rolling after melting and refining. Next, it is quenched after a solution treatment of 15 to 60 minutes in a temperature range of 1050 to 1150 ° C. which is a standard solution treatment temperature of 21-4N steel. And heat it again and it will be 1-4 at around 750 ℃
Use with time aging.

The heat-resistant steel for an engine valve thus obtained has the following properties in order to combine corrosion resistance and oxidation resistance higher than 21-4N steel and high-temperature strength higher than the above-mentioned 21-4N improved steel. It is desirable to satisfy the characteristics shown at the same time. That is, the steel of the present invention is
Oxidation weight loss when held for 0 hours is 2.0 mg / cm ² or less, weight%
In 60% PbO + 40% corrosion weight loss of mixed lead corrosion test of PbSO ₄ is 920 ° C. for 1 hour (h) 100mg / cm ² or less when held at 850
The fatigue strength at 17 ° C is 17 kgf / mm ² or more, the tensile strength at 900 ° C is 24 kgf / mm ² or more, and the creep rupture life under the stress load of 6 kgf / mm ² at 900 ° C is 50 hours or more. If at least one of the above high temperature characteristics is not achieved, the value is 2.0 mg / cm ² because it is insufficient as heat resistant steel for engine valves.
Hereinafter, it is limited to 100 mg / cm ² or less, 17 kgf / mm ² or more, 24 kgf / mm ² or more, and 50 hours or more. Further, the numerical limitation of the third invention of the present invention will be described. The steel of the present invention is the same as that of the aforementioned 2
It is desirable that the high-temperature strength after long-term exposure to a high temperature after the heat treatment besides the characteristics of the 1-4N steel in the state of being subjected to the standard solution treatment and aging treatment. More specifically, 18 kgf / mm ^2, which is approximately equivalent to the tensile strength at 900 ° C. after holding at 900 ° C. for 300 hours after regular heat treatment of Inconel 751 known as an Ni-based alloy for engine valves.
The above strength is desirable. Accordingly, in the present invention, the tensile strength at 900 ° C after holding at 900 ° C for 300 hours is 1
Limited to 8 kgf / mm ² or more.

[0019]

EXAMPLES The steels of the present invention, comparative steels and conventional alloys having the compositions shown in Table 1 were melted in an air induction furnace, turned into 10 kg ingots, and then forged into 30 mm square bars by heating at 1100 ° C. . Solution treatment of the steel of the present invention and the comparative steel, after holding at 1150 ° C. for 30 minutes,
Air cooling was carried out, and the solution treatment of the conventional alloy was kept at 1050 ° C. for 30 minutes, followed by air cooling. Furthermore, the steels of the present invention, comparative steels and conventional alloys
After holding at 750 ° C. for 4 hours, an air-cooling aging treatment was performed. Then, it processed into a predetermined test piece shape, and was used for the experiment. In Table 1, Comparative Steel No. 15 had a composition without B added, and only this steel had many flaws during forging and had a problem in hot workability. Since there was no problem in the hot workability of the steel of the present invention, it is clear that B is an essential element for the present invention. Accuracy test items were 850 ° C-107 rotating bending fatigue strength of ⁷ times, 900
° C. tensile strength, creep rupture life at ^{900 ℃ -6kgf / mm 2, 1000} ℃ × 100 hours (h) oxidation weight loss after heating (in air),
And weight loss after heating at 920 ° C. for 1 hour (h) in a mixed lead corrosive of 60% by weight PbO + 40% by weight PbSO ₄ .
Further, for the steels Nos. 1 to 4 of the present invention and the conventional alloys Nos. 21 and 22, the tensile strength was measured at 900 ° C. after heating at 900 ° C. × 300 hours (h). Tables 2 and 3 show the results of these experiments.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

Samples Nos. 1 to 4 are steels of the present invention, Nos. 11 to 16 are comparative steels, and Nos. 21 and 22 are conventional alloys. Among conventional alloys,
No. 21 is 21-4N steel, No. 22 is Inconel 751
It is. From Table 2, all the steels of the present invention satisfy all the limit values described in the second invention. In contrast, comparative steel No. 11
Has high temperature strength equivalent to that of the steel of the present invention, but is inferior to the steel of the present invention in oxidation resistance and mixed lead corrosion resistance. This is because the steel of the present invention has a high Mn of No. 11 and a low Cr, and how the contents of Mn and Cr affect the corrosion resistance and oxidation resistance. Recognize. Comparative steel No. 12 has a higher C and lower N composition than the steel of the present invention, but this steel is inferior in all high-temperature strengths to the steel of the present invention. Therefore, high C such as No. 12
It can be seen that the conventional high-temperature precipitation-hardened steel of the carbide cannot obtain the high-temperature strength as high as the steel of the present invention which is the solid-solution-strengthened type. Further, the comparative steel No. 13 changed Ni of the present invention steel No. 1 to Ni.
The composition of the steel is compared with the comparative steel No. 13
The fatigue strength of the steel of the present invention is clearly low, and it is clear how Co contributes to the improvement of the fatigue strength in the steel of the present invention.
Further, it can be seen that a high Nb-containing steel such as the comparative steel No. 14 exhibits a high-temperature strength comparable to that of the steel of the present invention, but has extremely poor oxidation resistance and mixed lead corrosion resistance. Comparative steel No. 15 has a composition without B added to the steel of the present invention. However, since this steel does not contain B that contributes to grain boundary strengthening, the hot workability is reduced as described above; There is a disadvantage that the creep rupture life is reduced. Comparative steel No. 16 is the steel N of the present invention.
O.1 is a steel in which part of W is replaced by Mo of equivalent weight (W 2% is equivalent to Mo 1% in terms of weight%), but this steel also has lower high-temperature strength than No. 1, In particular, the difference in creep rupture life is large. This is clearly due to the diffusion rates of W and Mo. In addition, the oxidation weight loss and the mixed lead corrosion weight loss of No. 16 are larger than those of No. 1, and it can be seen that Mo deteriorates the oxidation resistance and corrosion resistance of the steel of the present invention.

No. 21 (21-4N) of the steel of the present invention and the conventional alloy
In all respects, the steel of the present invention is No. 21
It can be seen that the characteristics are higher than the above. Also, the steel of the present invention has a creep rupture life of more than that of No. 22 compared with the conventional alloy No. 22 (Inconel 751). It can be seen that it exceeds the properties of steel and is quite close to Inconel 751. Further, with regard to the mixed lead corrosion resistance, No. 22 shows a markedly worse value than the steel of the present invention. No. 22 is an alloy based on Ni, which is generated because the base Ni and the sulfide of Ni produced by the mixed lead corrosive agent cause a eutectic reaction and melt during the mixed lead corrosion test. This is the most problematic in the characteristics of Inconel 751 conventionally. Therefore, when the fatigue damage of the valve starts from a crack generated by a corrosion reaction on the surface, the steel of the present invention may have a life of the actual valve exceeding that of Inconel 751 in some cases. According to Table 3, the tensile strength at 900 ° C after holding at 900 ° C for 300 hours shows almost no difference between the steel of the present invention and the conventional alloy No. 22. It can be seen that the steel has excellent properties.

[0025]

According to the present invention, high-temperature strength and oxidation resistance, which are significantly better than conventional 21-4N high-Mn heat-resistant steel,
A steel having corrosion resistance can be provided. Moreover, the high-temperature strength of the steel of the present invention after prolonged heating is maintained at the same level as that of the current exhaust valve material, Inconel 751, which is a Ni-based super heat-resistant alloy.
The steel of the present invention has significantly improved characteristics against mixed lead corrosion containing lead oxide and lead sulfate as compared with Inconel 751. Therefore, if the steel of the present invention is used, the conventional 21
Since the operating temperature of an automobile engine valve can be raised as compared with the -4N series and Inconel 751, and the characteristics at high temperature and long time are stable, an engine with high output and high efficiency can be manufactured.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Chiyoshi Toshima 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (56) References JP-A-63-210260 (JP, A) JP-A-56 JP-A-84445 (JP, A) JP-A-60-77964 (JP, A) JP-A-64-79351 (JP, A) JP-A-1-219147 (JP, A) JP-A-51-40321 (JP, A) JP-A-3-166342 (JP, A) JP-A-58-224153 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 302 C22C 38/58 F01L 3 / 02

Claims (3)

(57) [Claims] (1) In weight%, C is 0.02% or more and less than 0.15%;
0.05-1.0%, Mn 4.5% or more and less than 7.5%, Ni 9.0-15.0%,
Co 1.0 to 5.0%, Cr 22.0 to 26.0%, W 4.0 to 8.0%, Nb
A heat-resistant steel for an engine valve, comprising a steel having a composition of 0.01 to 0.50%, N exceeding 0.40%, 0.70% or less, B 0.02% or less, and unavoidable impurities, and a balance of Fe. 2. An oxidative weight loss of not more than 2.0 mg / cm ² when held at 1000 ° C. for 100 hours in the atmosphere, and 60% by weight of PbO 2
If when the corrosion loss of the corrosion test in the mixed lead consisting of 40% PbSO ₄ was held 1 hour at 920 ℃ 100mg / cm ² or less, 850 fatigue strength ° C. is 17 kgf / mm ² or more, the tensile strength at 900 ° C. of Is 24
kgf / mm ² or more, and 900 ° C. engine heat resistant steel valve of claim 1 creep rupture life at a stress load of 6 kgf / mm ² for at least 50 hours at. 3. The heat-resistant steel for an engine valve according to claim 1, wherein the tensile strength at 900 ° C. after holding at 900 ° C. for 300 hours is 18 kgf / mm ² or more.