JPH04304337A - Heat resistant alloy for engine valve - Google Patents

Abstract

PURPOSE:To provide a low-cost heat resistant alloy for an engine valve having superior characteristics as compared with NCF751 used as an engine valve material for an automobile. CONSTITUTION:This heat resistant alloy for an engine valve consists of, by weight, 0.21-0.65% C, <=0.5% Si, <=1.0% Mn, 40-50% Ni, 15-23.5% Cr, <=6% Mo, 1.0-1.8% Al, 3.25-4.5% Ti, 1.0-2.5% Nb (5.3<=1.8Al+Ti+0.5Nb-4C<=6.5), 0.001-0.020% B and the balance Fe and contains 2-6 atomic % MC type carbide in the structure. This alloy may further contain one or more among Mg, Ca, Y and REM as required.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates primarily to engine valve materials for automobiles.

0002

[Prior Art] Conventionally, N was used as an engine valve material for automobiles.
NCF751, an i-based alloy, and JP-A-6, which lowered its price.
The Ni-based alloy disclosed in No. 0-70155 has been used. Furthermore, for the purpose of lowering the price of engine valve materials, we have developed Japanese Patent Publications No. 1-12827, No. 62-214149, and No. 2-47229 in which the Ni content is 50% or less.
No., JP-A-61-238942, JP-A-58-189
No. 359, JP 62-50542, JP 56-2
No. 0148 and the like have been proposed.

0003

The aforementioned NCF751 has been widely used as an alloy for high-grade exhaust engine valves. However, NCF751 is expensive because it contains more than 70% Ni, and its high-temperature fatigue strength and creep rupture strength in a corrosive environment containing sulfur are significantly lower than when used in the air. There was a problem.

On the other hand, Japanese Patent Publication No. 1-12827, Japanese Patent Application Publication No. 62-214149, Japanese Patent Application Publication No. 61-238942, which have been proposed as low-cost alloys of NCF751.
Japanese Patent Publication No. 58-189359, Publication No. 62-50542
Although alloys containing 50% Ni or less, such as those disclosed in No. 2014 and JP-A No. 56-20148, are slightly better than NCF751 in corrosive environments containing sulfur, they have the disadvantage that their hardness and high-temperature strength are not necessarily sufficient. On the other hand, the alloy disclosed in JP-A-2-47229 is a low-priced alloy with 50% Ni or less, but by dispersing an appropriate amount of TiC carbide, the austenite grain size is refined and toughness and ductility is improved. An alloy with increased hardness and high-temperature strength without deterioration.
It is characterized by having room temperature hardness superior to NCF751 and high temperature strength almost equivalent to NCF751. Such a technology that actively utilizes MC type carbides has not been used at all in conventional heat-resistant alloys for engine valves.

[0005] However, due to global environmental pollution problems,
Demand for lighter weight and higher efficiency automobile engines has become even stronger. The purpose of the present invention is to have a Ni content of 50% or less and to have superior high-temperature strength and corrosion resistance against corrosive environments containing sulfur (hereinafter referred to as The object of the present invention is to provide an alloy having a high resistance to sulfide corrosion.

[0006]

[Means for solving the problem] As mentioned above,
The alloy disclosed in No. 47229 exhibits high-temperature strength comparable to NCF751, but in order to further improve the high-temperature strength and sulfide corrosion resistance of this alloy, dispersion strengthening of MC-type carbides is used and gamma prime precipitation is applied. The optimal range of components for reinforcing elements and the optimal range of components for improving flow corrosion resistance were investigated. As a result, in order to obtain high-temperature strength superior to NCF751, it is necessary to disperse an appropriate amount of MC type primary carbides of Ti and Nb,
As a result, the austenite crystal grains become finer, ■ By limiting the content of Al, Ti, and Nb, which are gamma prime precipitation strengthening elements, to within the range of a specific relational expression, it is possible to forge the valve and achieve excellent strength. and ■Improvement of solid solution strengthening and sulfide corrosion resistance by Mo addition.
This led to the discovery of the alloy of the present invention.

[0007] That is, the first invention of the present invention is based on the weight percentage of C0.21 to 0.65%, Si0.5% or less,
Mn 1.0% or less, Ni 40-50%, Cr 15% or more and less than 23.5%, Mo 6.0% or less, Al 1.0-50%
1.8%, Ti over 3.25% and 4.5% or less, Nb1
．． More than 0% and less than 2.5% (5.3≦1.8A
l+Ti+0.5Nb-4C≦6.5), B0.001
~0.020%, the remainder is essentially F excluding impurities
2 to 6 MC carbides dispersed in the alloy structure.
The second invention is a heat-resistant alloy for engine valves characterized by containing atomic% of C0.21 to 0 by weight percentage.
．． 65%, Si 0.5% or less, Mn 1.0% or less, Ni
40-50%, Cr15% or more and less than 23.5%, Mo6
．． 0% or less, Al 1.0-1.8%, Ti3.25
% but not more than 4.5%, Nb more than 1.0% and not more than 2.5% (however, 5.3≦1.8Al+Ti+0.5Nb-
4C≦6.5), B0.001 to 0.020%,
Furthermore, Mg0.0005~0.02%, Ca0.000
5-0.02%, Y0.001-0.2% and REM
Contains one or more of 0.001 to 0.2%, the remainder essentially consists of Fe excluding impurities, and contains 2 to 6 at% of MC carbide dispersed in the alloy structure. It is a heat-resistant alloy for engine valves characterized by
The invention has a weight percentage of C0.21-0.65%, Si0.
5% or less, Mn 1.0% or less, Ni 40-50%, Cr
15% or more and less than 23.5%, Mo1.0-4.5%, A
l 1.0~1.8%, Ti over 3.25% 4.5
% or less, Nb over 1.0% and 2.5% or less (however, 5%
．． 3≦1.8Al+Ti+0.5Nb-4C≦6.5)
, B0.001~0.020%, the remainder is essentially Fe excluding impurities, and M dispersed in the alloy structure.
The fourth invention is a heat-resistant alloy for engine valves characterized by containing 2 to 6 at% of C carbide, and the fourth invention is a heat-resistant alloy for engine valves, which is characterized by containing C carbide in a weight percentage of 0.21 to 0.65%, Si 0.5% or less, Mn1
．． 0% or less, Ni40-50%, Cr15% or more23.
Less than 5%, Mo1.0-4.5%, Al 1.0-1
．． 8%, Ti over 3.25% and 4.5% or less, Nb1.
More than 0% and less than 2.5% (however, 5.3≦1.8Al
+Ti+0.5Nb-4C≦6.5), B0.001~
Contains 0.020% and further contains Mg0.0005-0.0
2%, Ca0.0005~0.02%, Y0.001~
0.2% and 1 of REM 0.001-0.2%
The remainder consists essentially of Fe excluding impurities, and the MC carbide dispersed in the alloy structure is
It is a heat-resistant alloy for engine valves characterized by containing ~6 at%.

[0008]

[Operation] In the present invention, C is an element necessary to combine with Ti and Nb to form a MC type primary carbide, and in order to have a MC carbide content of 2 at % or more, 0.
C is required to be 21% or more, but if it exceeds 0.65%, the amount of MC carbide exceeds 6 at %, which impairs forgeability and toughness, which is not preferable, so C is limited to 0.21 to 0.65%. .

[0009]Si and Mn are added as deoxidizing elements in the alloy of the present invention, but excessive addition of either leads to a decrease in high temperature strength, so Si is added at 0.5% or less and Mn is added at 1.0%.
Each is limited to the following.

[0010] Ni has austenite base and Ni3 (A
It is a basic element constituting the gamma prime precipitation strengthening phase of L, Ti, Nb), and is required to be at least 40% in order to precipitate a sufficient amount of gamma prime phase and still maintain a stable austenite base. However, if excessive addition exceeds 50%, the characteristic of an inexpensive alloy is lost, so Ni is limited to 40 to 50%.

Cr is an essential element for imparting oxidation resistance to the alloy, and a minimum of 15% is required to ensure oxidation resistance for use in engine valves, but 23.5%.
C
r is 15% or more and less than 23.5%.

[0012] Mo has the effect of solid-solution strengthening the austenitic base and significantly increasing high-temperature fatigue strength and high-temperature creep rupture strength. Furthermore, it has become clear in the alloy of the present invention that Mo forms a stable MoS2 sulfide with sulfur, resulting in NC
Ni is the main cause of decreasing the sulfide corrosion resistance of F751.
-We have discovered the effect of preventing the low melting point eutectic reaction of NiS. Mo is indispensably added to obtain these effects, but excessive addition of more than 6.0% impairs hot workability and causes excessive precipitation of α phase, so Mo is added in an amount of 6.0%.
Limited to the following. Furthermore, the preferred range of Mo is 1.0 to 4
．． It is in the range of 5%.

Al is an essential element to precipitate a stable gamma prime phase and obtain the desired high-temperature strength, and requires a minimum content of 1.0%, but if it exceeds 1.8%, hot workability deteriorates. is limited to 1.0 to 1.8%.

[0014] In the alloy of the present invention, Ti, like Nb,
It combines with C to form MC-type primary carbide, while the remaining part combines with Ni together with Al and Nb to precipitate a gamma prime phase, which has the effect of increasing high-temperature strength.
%, but if it exceeds 4.5%, the gamma prime phase becomes unstable at high temperatures, making it easy to generate η phase, and impairing hot workability, so Ti should be added in an amount of 3.25%.
Exceeding 4.5% or less.

[0015] Nb is an essential element in the present invention because it imparts high-temperature strength that is much superior to that of the alloy disclosed in JP-A-2-47229, which has the same strength as the conventional alloy NCF751, and is an essential element in the present invention. It has an effect that cannot be obtained by the alloy composition of Kaihei 2-47229 alone. That is,
When strengthening the Al sites of the gamma prime phase with Ti only, the atomic ratio of Al and Ti is 4 for short-time tensile strength.
: About 6 is desirable, but if heated for a long time, a part of it will change to η.
Because it transforms into a phase, even if the amount of gamma prime is made higher than NCF751 as described later, the creep rupture strength will be about NCF751 at most.

On the other hand, by substituting a part of Ti with Nb, the gamma prime phase becomes more stable and the high temperature strength can be improved. The optimum atomic ratio of Al, Ti and Nb in the alloy of the present invention is 4.5:4.5:1, and N
If the atomic ratio of b becomes too high, a gamma double prime phase will precipitate, which will actually reduce the creep rupture strength at 900°C. In addition to the above-mentioned effect of strengthening the gamma prime phase, Nb also combines with C to form MC type primary carbide, similar to Ti. Due to these effects, in the present invention, Nb is limited to more than 1.0% and less than 2.5%.

In order to achieve the object of the present invention, the above-mentioned Al
, Ti, and Nb must not only individually satisfy the above-mentioned component ranges, but also it is important that the sum of each element be within an appropriate range as gamma prime constituent elements. According to the present invention, since a part of Ti and Nb forms MC carbides, the effective Ti equivalent involved in the precipitation of the gamma prime phase is 1.8Al+Ti+0.5Nb-4
It is represented by C. The amount of gamma prime phase increases in proportion to the height of the effective Ti equivalent value, but in the present invention, if the effective Ti equivalent value is less than 5.3, the target high temperature strength cannot be obtained; If it exceeds 5, the hot workability will be impaired and it will be difficult to forge the valve, so the effective Ti equivalent is 5.3.
-6.5.

In the present invention, B is effective in increasing high-temperature strength and ductility through grain boundary strengthening action, and has a minimum of 0.00
1% is required, but if it exceeds 0.020%, the initial melting temperature during heating will decrease and hot workability will deteriorate;
001% to 0.020%.

Mg, Ca, Y, and REM are strong deoxidizing and desulfurizing elements, and the inclusion of one or more of these elements helps improve the hot workability and ductility of the alloy of the present invention. The amounts of Mg, Ca, Y and REM required for this are 0.0005-0.02% and 0.0005-0.0005%, respectively.
0.02%, 0.001-0.2% and 0.001
~0.2%. Fe is an element that makes the alloy of the present invention inexpensive, and is left in the remainder because it stabilizes the austenite base together with Ni.

The alloy of the present invention contains P, S as impurities.
Although the following amounts may be included in the alloy of the present invention, there are no particular problems in terms of properties. P≦0.03%, S≦0.03% The appropriate amount of Ti and Nb MC type primary carbides contained in the present invention is uniformly dispersed in the matrix, refines the austenite grain size, and improves toughness and ductility. Hardness and high temperature strength can be increased without deterioration. For that purpose, M.C.
The carbide content is required to be at least 2 atomic %, but if it exceeds 6 atomic %, high temperature strength and toughness deteriorate, so it is limited to 2 to 6 atomic %.

[0021]

[Example] An alloy having the composition shown in Table 1 was made into a 10 kg ingot by vacuum induction melting, and then 2 kg was made by hot forging.
A 5 mm square bar material was created. This was subjected to solid solution treatment of water cooling after holding at 1050℃ for 1 hour and aging treatment of air cooling after 4 hours of 750℃.
Tensile properties at 0℃, rotating bending fatigue life at 800℃-35kgf/mm2 load, creep rupture life at 900℃-6kgf/mm2 load, and 55%CaSO4 +
30%BaSO4 +10%Na2 SO4 +5%C
The corrosion weight loss after 80 hours at 870°C in a mixed sulfurized corrosive salt was measured. These results are shown in Table 2.

[0022]

[Table 1]

[0023]

[Table 2]

[0024] Here, No. 1 to 10 are alloys of the present invention, N
o. Nos. 11 to 17 are comparative alloys, No. 21 and 22 are conventional alloys. Among the alloys of the present invention, No. 1 and 4 are the first invention alloys, No. 5 and 8 are the second invention alloys, No. 2 and 3 are the third invention alloy and No. 6, 7, 9, and 10 are the fourth invention alloys. Also, among conventional alloys, No. 21 is JP-A-2-
It is an alloy disclosed in No. 47229, and No. 22 is N
It is CF751. From Tables 1 and 2, the present invention alloy N
o. 1 to 10 are No. It can be seen that this material exhibits better properties than No. 22 NCF751 in hardness, tensile strength at room temperature and 800°C, fatigue life at 800°C, and creep rupture life at 900°C. Furthermore, the alloy of the present invention is the same as conventional alloy No. It can be seen that the sulfide corrosion resistance is greatly improved compared to No. 22.

[0025] Also, conventional alloy No. 21, the high temperature strength of the alloy of the present invention is superior to that of No. 21, and it is also No. 1 in terms of sulfide corrosion resistance. Better characteristics than No. 21 were obtained. Invention alloy No. The good tensile elongation at 800°C of 5 to 10 is mainly due to the addition of Mg, Ca, Y and REM. Among the comparative alloys, No. Since the Ti equivalent of No. 11 is lower than that of the alloy of the present invention, the tensile strength at 800° C. and fatigue life are inferior to the alloy of the present invention. No. No. 12 is an alloy with no addition of Mo and is inferior in high temperature strength and sulfide corrosion resistance compared to the alloy of the present invention, which shows how important the effect of Mo is on the alloy of the present invention.

[0026]No. In No. 13, the amount of MC carbides is smaller than that of the alloy of the present invention, so the crystal grains become coarse and the creep rupture life is good, but the fatigue life is significantly reduced. No.
Since No. 14 contains an excessive amount of MC carbide, the elongation at room temperature and the high temperature strength are generally lowered. No. Since No. 15 has a higher Cr content than the present alloy, a large amount of α phase precipitates at the grain boundaries, and No. 8
In short-time tensile strength such as 00℃ tensile strength,
Although the degree is slightly lower than that of the alloy of the present invention, the reduction in high-temperature long-term strength such as high-temperature fatigue life and creep rupture life is significant.

[0027] Also, No. In No. 16, cracks occurred during hot working because the Ti equivalent was too high, and although the hardness was very high, other confirmation tests could not be performed. No. No. 17 is an alloy with a higher amount of Mo than the alloy of the present invention. Similar to No. 16, cracks occurred during forging, making it impossible to conduct an accuracy test.

[0028]

Effects of the Invention According to the present invention, it is suitable as an alloy for automobile engine valves because it has mechanical properties at room temperature and high temperature and sulfide corrosion resistance that are superior to NCF751 containing 70% or more Ni. As a result, it becomes possible to manufacture low-cost, high-performance engines.

Claims (4)

[Claims] Claim 1: C0.21-0.65% by weight percentage
, Si0.5% or less, Mn1.0% or less, Ni40~5
0%, Cr 15% or more and less than 23.5%, Mo 6.0% or less, Al 1.0-1.8%, Ti more than 3.25% and less than 4.5%, Nb more than 1.0% and 2.5% Below (5.3≦1.8Al+Ti+0.5Nb-4C≦6
．． 5), for engine valves, containing 0.001 to 0.020% of B, the remainder essentially consisting of Fe excluding impurities, and containing 2 to 6 at.% of MC carbide dispersed in the alloy structure. Heat resistant alloy. Claim 2: C0.21-0.65% by weight percentage
, Si0.5% or less, Mn1.0% or less, Ni40~5
0%, Cr 15% or more and less than 23.5%, Mo 6.0% or less, Al 1.0-1.8%, Ti more than 3.25% and less than 4.5%, Nb more than 1.0% and 2.5% Below (5.3≦1.8Al+Ti+0.5Nb-4C≦6
．． 5), contains 0.001 to 0.020% of B, and further contains M
g0.0005-0.02%, Ca0.0005-0.
02%, Y0.001-0.2% and REM0.00
It is characterized by containing one or more of 1 to 0.2%, the remainder essentially consisting of Fe excluding impurities, and containing 2 to 6 at% of MC carbide dispersed in the alloy structure. Heat-resistant alloy for engine valves. Claim 3: C0.21-0.65% by weight percentage
, Si0.5% or less, Mn1.0% or less, Ni40~5
0%, Cr15% or more and less than 23.5%, Mo1.0-4
．． 5%, Al 1.0-1.8%, Ti over 3.25% and up to 4.5%, Nb over 1.0% and up to 2.5% (
However, 5.3≦1.8Al+Ti+0.5Nb-4C
≦6.5), contains 0.001 to 0.020% of B, the remainder essentially consists of Fe excluding impurities, and contains 2 to 6 at.% of MC carbide dispersed in the alloy structure. Heat-resistant alloy for engine valves. Claim 4: C0.21-0.65% by weight percentage
, Si0.5% or less, Mn1.0% or less, Ni40~5
0%, Cr15% or more and less than 23.5%, Mo1.0-4
．． 5%, Al 1.0-1.8%, Ti over 3.25% and up to 4.5%, Nb over 1.0% and up to 2.5% (
However, 5.3≦1.8Al+Ti+0.5Nb-4C
≦6.5), B0.001~0.020%, further Mg0.0005~0.02%, Ca0.0005~
0.02%, Y0.001-0.2% and REM0.
001 to 0.2%, the remainder essentially consists of Fe excluding impurities, and contains 2 to 6 atomic percent of MC carbide dispersed in the alloy structure. Heat-resistant alloy for engine valves.