JPH06108206A - Iron-base superalloy for heat resistant bolt - Google Patents

Abstract

PURPOSE:To provide an alloy for heat resistant bolt excellent, as a bolt, in cold header-workability and cold form-rollability and superior in relaxation characteristics at a temp. in the vicinity of 700 deg.C to A286. CONSTITUTION:This alloy is an iron-base superalloy for heat resistant bolt and has a composition which consists of, by weight, <=0.20% C, <=1.0% Si, <=2.0% Mn, >25-35% Ni, 10-<13.5% Cr, 0.70-1.5% Al, 2.5-4.0% Ti, <=0.020% B and/or <=0.20% Zr, and the balance essentially Fe except impurities and where the relations between Al and Ti satisfy 4.0<=1.8Al+Ti<=6.0 and 1.0<=Ti/1.8Al<=3.0. If necessary, specific amounts of Nb can be added to the above composition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a γ'precipitation type iron-base super heat resistant alloy mainly used for heat resistant bolts used at high temperatures.

[0002]

2. Description of the Related Art In general, it is desirable that a heat resistant bolt used at high temperature should have its fastening force not reduced as much as possible during use at high temperature for a long time. Such a phenomenon of stress relaxation is called relaxation. Further, it is required to have excellent oxidation resistance in the operating temperature range. As a material for bolts used at high temperatures near 600 ° C, A286 (JIS standard SU
H660) is commonly used as a γ'precipitation type iron-base super heat-resistant alloy (hereinafter referred to as A286). A2
In No. 86, good high-temperature strength and ductility were obtained in a temperature range of about 600 ° C., which is a conventional bolt use temperature. However, with the recent increase in the combustion temperature of automobile engines,
With A286, when the working temperature of the bolt rises up to about 700 ° C., there arises a problem that the fastening force of the bolt sharply decreases (reduction of relaxation characteristics).

According to the JIS standard, the composition range of the conventional alloy A286 is C0.08% or less, Si1.0% or less, Mn.
2.0% or less, P0.04% or less, S0.03% or less,
Ni 24.0 to 27.00%, Cr 13.50 to 1
6.00%, Mo 1.00 to 1.50%, V 0.10
0.50%, Al 0.35% or less, Ti 1.90-2.
35%, B 0.001 to 0.010%, and the balance Fe. On the other hand, as patents for alloys in which A286 is improved for applications such as bolts, there are JP-A-60-46353 and JP-A-62-199752, which all have a use temperature of A286 of 700 ° C. It is not an improvement aimed at raising it to a certain degree.

[0004]

The object of the present invention is to provide A2
Alloy composition that does not significantly increase the price of alloy than 86, excellent in cold header workability as bolts and thread rolling property, and relaxation characteristics in the state of being molded into heat resistant bolts are A286. Another object of the present invention is to provide a heat resistant bolt alloy characterized by being superior to the above.

[0005]

[Means for Solving the Problems] In the conventional Fe-based superheat-resistant alloy, in order to improve the strength up to about 600 ° C., a quasi-stable γ ′ phase (Ni ₃ (Al, Ti): L1 _{2 with} a high Ti / Al ratio is used. Alloy compositions with precipitation strengthened structure have been preferred and used (typical Fe such as V57 and A286).
The composition of the base super heat-resistant alloy is shown in Table 1 as the conventional alloy column Nos. 31 and 32.
Respectively). Such a high Ti / Al ratio is certainly advantageous for improving the tensile strength in the temperature range up to about 600 ° C, but when the operating temperature reaches about 700 ° C, the pseudo stable γ'phase is It transforms to the η phase (Ni ₃ Ti: hexagonal crystal), and the high temperature strength, that is, the relaxation property, suddenly decreases. Therefore, as a result of diligent studies, the present inventor has determined that the amount and composition of such a precipitation strengthening element is γ ′.
It was found that the relaxation characteristics can be improved by increasing the amount of the phase and changing the Ti / Al ratio in the γ ′ phase in the direction of decreasing (that is, increasing the amount of Al). Furthermore, it was clarified that the addition of Nb, which is a γ'precipitation strengthening element, is effective for enhancing the relaxation characteristics.

On the other hand, since the bolt undergoes header processing and thread rolling in the cold, it is necessary to sufficiently lower the strength in the solution treatment state and to sufficiently enhance the adhesion of the lubricating coating. Usually, in the bolt manufacturing process of Fe-based super heat-resistant alloy, after the solution processing of the processing bar material at 900 to 1050 ℃, after pickling, lubricating film treatment and drawing direct drawing process, header processing and thread rolling are performed. Perform the aging treatment at the end. Therefore, in order to improve the adhesion of the lubricating film, it is necessary to sufficiently roughen the skin of the bar material by the solution treatment before the lubricating film treatment or the pickling. In the case of A286, since the contents of Cr and Mo are high, the corrosion resistance to the acidic aqueous solution is high, but since the content of Al is low, the oxidation resistance in the high temperature region around the solution treatment temperature is low, and the solid solution is formed. Since the skin becomes rough during the treatment, the adhesion of the lubricating film is good.

However, the alloy of the present invention has a composition in which Al is increased in terms of strength due to the above reason, so that the oxidation resistance at the solution treatment temperature is improved, and the solution treatment does not roughen the skin. With similar contents of Cr and Mo, the corrosion resistance to acidic aqueous solution is also high, so the adhesion of the lubricating film is reduced, and the problem of seizure occurs during cold forming of bolts, which impairs the formability of bolts. I understood. Therefore, it was found that if the content of Cr and Mo is made lower than that of A286 so that the skin is well roughened by pickling, the adhesion of the lubricating film is also improved. By improving the high temperature strength as described above and taking measures to prevent the adhesion of the lubricating coating from lowering as a result, it was possible to find an iron-based super heat resistant alloy for heat resistant bolts, which is superior to A286 in high temperature strength.

That is, the first aspect of the present invention is, in terms of weight percentage, C0.20% or less, Si1.0% or less, Mn.
2.0% or less, Ni more than 25% and 35% or less, Cr10
% Or more and less than 13.5%, Al 0.70 to 1.5%, Ti
2.5-4.0%, B0.020% or less and Zr
0.20% or less of one kind or two kinds, and the relationship between Al and Ti is 4.0 ≦ 1.8Al + Ti ≦ 6.0 and 1.0 ≦ Ti / 1.8Al ≦ 3.0. The balance is an iron-based super heat-resistant alloy for heat-resistant bolts, characterized in that the balance is essentially Fe except for impurities.
0% or less, Si 1.0% or less, Mn 2.0% or less, Ni
More than 25% and 35% or less, Cr 10% or more and less than 13.5%, Al 0.70 to 1.5%, Ti 2.5 to 4.0%,
And one or two of B 0.020% or less and Zr 0.20% or less, and Nb1% or less and Mo less than 1% 1
Or 2 types, and the relationship between Al, Ti and Nb is 4.0 ≦ 1.8Al + Ti + 0.5Nb ≦ 6.
0 and 1.0 ≦ (Ti + 0.5Nb) /1.8Al≦
It is 3.0, and the balance is essentially Fe except for impurities, which is an iron-based superheat-resistant alloy for heat-resistant bolts. Furthermore, the range of Ni is 25 in both the first invention and the second invention.
% And 30% or less, and the range of Cr is 10% or more and less than 13%, it is more preferable.

[0009]

The function limiting range of the alloy of the present invention will be described below. In the present invention, C combines with Ti to form MC type primary carbides and helps prevent coarsening of crystal grains during solution treatment, so a small amount is necessary, but C exceeding 0.2% is TiC. Since the precipitation strengthening ability of the alloy is reduced due to excessive formation of C, C is set to 0.2% or less. Si and Mn are added as deoxidizing elements in the alloy of the present invention, but excessive addition of both causes reduction in high temperature strength, so Si is limited to 1.0% or less and Mn is limited to 2.0% or less. .

Ni is an indispensable element for making the parent phase of the alloy stable austenite, and is an important element contributing to precipitation strengthening of the alloy by forming a γ'phase in combination with Al, Ti and Nb. Is. Particularly, in the alloy of the present invention, the amount of the γ ′ phase is higher than that of A286, and
The amount of Ni required is equal to or more than. Further, increasing the amount of Ni also helps improve the oxidation resistance during the solution treatment. Therefore, Ni needs to be added at least in excess of 25%, but excessive addition in excess of 35% unnecessarily increases the price of the alloy more than improving the properties, and cannot maintain a price comparable to A286. Is limited to a range of more than 25.0% and 35.0% or less. The more preferable Ni range is 25.0.
% And 30.0% or less.

Since Cr is an essential element for imparting oxidation resistance and corrosion resistance to the alloy, at least 10.0% or more is necessary, but addition of 13.5% or more is a lubricating film for valve molding. Since it reduces the adhesiveness of Cr, 10.0% of Cr
It is above 13.5%. The more preferable range of Cr is 1
It is 0.0% or more and less than 13.0%. Al is an essential element for precipitating a stable gamma prime phase to obtain strength in a high temperature range of about 700 ° C., and 900 to 1
Improves oxidation resistance during high temperature solution treatment at about 050 ° C. Therefore, Al needs to be at least 0.70%, but if it exceeds 1.5%, the hot workability deteriorates.
l is limited to 0.70 to 1.5%.

In the alloy of the present invention, Ti combines with C to form MC type primary carbides, while the balance combines with Al or Nb with Ni to precipitate the γ'phase and enhance the high temperature strength. It is necessary to add 0.5% or more, but if it exceeds 4.0%, the γ'phase becomes unstable at high temperature, the η phase is formed, the relaxation property is deteriorated, and the hot workability is impaired. , Ti is 2.5 to 4.
Limited to 0%. The desired properties of the alloy of the present invention as an iron-based super heat-resistant alloy for heat-resistant bolts can be achieved without Nb, but by adding Nb, higher high-temperature strength can be obtained. Addition of 1.0% or less of Nb contributes to solid solution strengthening of the γ'phase and contributes to improvement of relaxation characteristics at 700 ° C. However, in a matrix composition of Fe, Ni, Cr such as the alloy of the present invention, when Nb exceeds 1%, a Laves phase having a basic composition of Fe ₂ Nb is precipitated and it does not work effectively for improving the high temperature strength. Is limited to 1.0% or less.

The desired properties of the alloy of the present invention as an iron-based super heat-resistant alloy for heat-resistant bolts can be achieved without Mo.
Mo in the range of less than 10% has the effect of solid-solution strengthening the austenite matrix and increasing the high temperature strength, so it can be added.
However, addition of 1% or more of Mo lowers the adhesion of the lubricating film and deteriorates the formability of the bolt, so Mo is 1.
Limited to less than 0%.

To achieve the object of the present invention, Al and Ti
Further, it is not only necessary for each Nb to independently satisfy the above-mentioned component range, but it is also important that the sum of each element as a gamma prime constituent element falls within an appropriate range. According to the present invention, when Al and Ti and Nb amounts involved in the precipitation of the γ ′ phase are represented as Ti equivalents in atomic ratio,
The Ti equivalent is 1.8Al + Ti + 0.5Nb. The amount of the γ'phase increases in proportion to the Ti equivalent value, but in the present invention, when the Ti equivalent is less than 4.0, the target high temperature strength cannot be obtained. Since the inter-workability is impaired and the valve is difficult to forge, the Ti equivalent is limited to the range of 4.0 to 6.0.

Further, Al for stabilizing the γ'phase,
Between Ti and Nb, which strengthen γ'as a solid solution but destabilize it, there is a range for optimizing the strength in a high temperature region of about 700 ° C. That is, the atomic ratio of both is (Ti + 0.5N
b) /1.8Al. If this value is less than 1.0, the γ'phase is not sufficiently solid-solution strengthened, while if this value exceeds 3.0, γ'during long-time use at high temperature. The phases become unstable, and harmful phases such as η phase are precipitated, and in either case, the high temperature relaxation characteristics are deteriorated. Therefore, the (Ti + 0.5Nb) /1.8Al value is 1.
It is limited to the range of 0 to 3.0.

In the present invention, B and Zr are effective in strengthening the grain boundaries independently and enhancing the strength and ductility at high temperature, but when added excessively, a eutectic having a low melting point is formed and hot workability is increased. Deteriorates, so B and Zr are 0.020% each.
Or less and 0.20% or less. The impurities contained in the alloy of the present invention have no problem in terms of performance within the following range. Further, except for these elements, the balance of the alloy of the present invention is F
e.

[0017]

EXAMPLES Alloys of the present invention, comparative alloys and conventional alloys (V5
7 and A286), ingots were melted in a vacuum, followed by hot forging and cold drawing to prepare rods having a diameter of 7.4 mm. Table 1 shows the chemical composition of the sample. Where N
o. 1 to 10 are alloys of the present invention, No. 21 to 23 are comparative alloys, No. 31 and 32 are conventional alloys. No. 1 among the conventional alloys. No. 31 is an alloy corresponding to V57 and is No. 31. 32 is A2
86 equivalent alloy.

[0018]

[Table 1]

This was heated at 980 ° C. for 1 hour and then subjected to a solution treatment of water cooling, followed by a lubrication film treatment to obtain a bar material of 90
The film adhesion was investigated by the peeling condition of the film by the ° bending test and the film weight per unit surface area. Furthermore, the sample in this state was machined to φ7 mm × 15 mm1,
The oxidation resistance and microstructure stability were investigated. The heating conditions were heating in air at 800 ° C. for 200 hours, and the structural stability was investigated by increasing the amount of oxidation before and after heating and observing the cross-sectional microstructure after heating (inventive alloy No. 3 and comparative alloy N.
o. 21 and conventional alloy No. 31 (V57) only, the results of cross-sectional microstructure observation after long-term heating are shown in FIG. 1).

Further, the bar material subjected to the lubrication film treatment
% By cold drawing, cold header processing and thread rolling to form M8 hexagon bolts, which were heated at 730 ° C. for 16 hours, air-cooled for aging, and subjected to relaxation tests. In the relaxation test, fix both ends of M8 bolts with nuts in a tensile tester to a jig, heat to 700 ° C in a resistance heating furnace, and then load 1350kgf (35kgf / 35kgf / mm ² ), the displacement was controlled to be constant, the load after holding for 50 hours was read from the chart, and the axial force holding ratio (axial force after holding for 50 h / initial load × 100) was obtained. Table 2 shows the evaluation results of the adhesion of the lubricating coating, the axial force retention, the increase in oxidation, and the structural stability. The microstructure stability shown in Table 2 is indicated by a circle in the structure where the γ'phase and carbides are precipitated in the matrix of the γ phase, and in addition to the above structures, harmful phases such as η phase and α phase are precipitated. The tissues that have been evaluated were each marked with X.

[0021]

[Table 2]

From Table 2 and FIG. 1
All of Nos. 11 to 11 are excellent in the adhesion of the lubricating film, the axial force retention, the oxidation resistance and the structural stability, and all have good properties as a heat resistant bolt. Comparative alloy No. Two
1 and the conventional alloy No. 31 (V57) and No. 31. 32
In each case of (A286), the content of Al is lower than that of the alloy of the present invention and the (Ti + 0.5Nb) /1.8Al ratio is too high, so that the η phase precipitates after heating for a long time and the structure becomes unstable. The axial force retention rate is also reduced (the acicular precipitation phase seen in FIG. 1 is the η phase). Also, the oxidation resistance is inferior to that of the alloy of the present invention because of low Al. No. No. 22 has a Ni content that is too low, so that the oxidation resistance after heating at 800 ° C. for a long time is lower than that of the alloy of the present invention, and a part of the γ phase is transformed into the α phase, and the structure becomes unstable, and The axial force retention rate also decreases. No.
In No. 23, since Cr and Mo were higher than those of the alloy of the present invention, the adhesion of the lubricating film was deteriorated and seizure occurred during bolt forming, so the test piece processing was interrupted and the relaxation test was not performed.

[0023]

EFFECTS OF THE INVENTION According to the present invention, the adhesion of the lubricating coating is good and the cold workability is excellent. Further, in comparison with A286 which has been conventionally used for bolts, the relaxation at a high temperature, particularly around 700 ° C. It is possible to manufacture heat resistant bolts with excellent properties, structural stability and oxidation resistance.

[Brief description of drawings]

FIG. 1 is a solution treatment of rods of the present invention alloy No. 3, comparative alloy No. 21 and conventional alloy No. 31 (V57), and then 8
It is a photograph of a micro metal structure after long-time heating at 00 ° C for 200 hours.

Claims (4)

[Claims] 1. A weight percentage of C 0.20% or less, Si
1.0% or less, Mn 2.0% or less, Ni over 25% 3
5% or less, Cr 10% or more and less than 13.5%, Al 0.7
0-1.5%, Ti 2.5-4.0%, and B0.0
20% or less and Zr 0.20% or less of 1 type or 2 types, and the relationship between Al and Ti is 4.0 ≦ 1.8Al +
Ti ≦ 6.0 and 1.0 ≦ Ti / 1.8Al ≦ 3.0
And the balance is essentially Fe except for impurities, which is an iron-based superheat-resistant alloy for heat-resistant bolts. 2. C0.20% or less by weight percentage, Si
1.0% or less, Mn 2.0% or less, Ni over 25% 3
5% or less, Cr 10% or more and less than 13.5%, Al 0.7
0-1.5%, Ti 2.5-4.0%, and B0.0
20% or less and Zr 0.20% or less 1 type or 2 types, and Nb 1% or less and Mo 1% or less 1 type or 2 types, and the relationship of Al, Ti, and Nb is 4.0 ≦.
1.8Al + Ti + 0.5Nb ≦ 6.0 and 1.0 ≦
(Ti + 0.5Nb) /1.8Al≦3.0, and the balance is essentially Fe except for impurities, which is an iron-base superheat-resistant alloy for heat-resistant bolts. 3. A weight percentage of C 0.20% or less, Si
1.0% or less, Mn 2.0% or less, Ni over 25% 3
0% or less, Cr 10% or more and less than 13%, Al 0.70
1.5%, Ti 2.5-4.0%, and B0.020
% Or less and Zr 0.20% or less 1 type or 2 types,
Furthermore, the relationship between Al and Ti is 4.0 ≦ 1.8Al + Ti
An iron-based superheat-resistant alloy for heat-resistant bolts, wherein ≦ 6.0 and 1.0 ≦ Ti / 1.8Al ≦ 3.0, and the balance is essentially Fe except impurities. 4. A weight percentage of C 0.20% or less, Si
1.0% or less, Mn 2.0% or less, Ni over 25% 3
0% or less, Cr 10% or more and less than 13%, Al 0.70
1.5%, Ti 2.5-4.0%, and B0.020
% Or less and Zr 0.20% or less 1 type or 2 types, and Nb 1% or less and Mo 1% or less 1 type or 2 types, and the relationship of Al, Ti and Nb is 4.0 ≦.
1.8Al + Ti + 0.5Nb ≦ 6.0 and 1.0 ≦
(Ti + 0.5Nb) /1.8Al≦3.0, and the balance is essentially Fe except for impurities, which is an iron-base superheat-resistant alloy for heat-resistant bolts.