JPH11106871A - Heat resistant steel excellent in cold workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the heat resistance and cold workability of a steel by allowing it to have a compsn. contg. C, Si, Mn, Cr, Cr, Ni, Nb, Ta, Ti, Al and Cu, and the balance substantially Fe. SOLUTION: This steel is the one having a compsn. contg., by weight, 0.005 to 0.20% C, <=2.0% Si, <=2.0% Mn, 10.0 to 25.0% Cr and 20 to <25% Ni, furthermore contg. <=1.5% (Nb+Ta), 1.0 to <3.0% Ti, 0.7 to 2.0% Al and 0.1 to 5.0% Cu, and the balance Fe and withstands usage at >=700 deg.C. This alloy components may furthermore be incorporated with one or more kinds among <=3.0% W, <=3.0% Mo and <=1.0% V by the amt. of 0.5 W/%+Mo%+V% <=3, one ore more kinds of 0.001 to 0.02% B and 0.001 to 0.1% Zr and one or more kinds of Ca and Mg by 0.001 to 0.01%, and a part of Ni may be substituted with <=5.0% Co. It is preferable that the content of Nb+Ta+Ti+ Al is regulated to 5.2 to 7.0 atomic %, and the each content of P, S, O and N is regulated to prescribed value or below.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a heat-resistant steel having high high-temperature strength and high-temperature oxidation resistance and excellent cold workability.

[0002]

2. Description of the Related Art Materials used in engine parts, turbine parts, heat exchangers and heating furnaces, and nuclear facilities that require heat resistance and corrosion resistance include austenitic heat-resistant steel. SUH660 is frequently used. The upper limit temperature of SUH660 is about 700 ° C.
751 and NCF80A are used.

[0003] In recent years, efforts have been made to increase the output of the engine and the thermal efficiency of the turbine, and as a result, the exhaust gas temperature and the steam temperature tend to increase. For this reason, the conventional S
Parts and members made of UH660 often have insufficient heat resistance.

Therefore, the SUH660 was improved to 700
There is a demand to provide a material that can be used even at a high temperature exceeding ℃ without increasing the cost. In addition, cold working tends to be performed favorably for the purpose of suppressing costs in the process of manufacturing parts and members. So, SU
There is a demand for a heat-resistant steel having a higher cold workability than H660.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to satisfy such a demand for heat-resistant steel, and has higher heat resistance than SUH660 and can be used at a temperature exceeding 700 ° C.
Another object of the present invention is to provide a product having excellent cold workability without increasing the cost.

[0006]

Means for Solving the Problems The heat-resistant steel excellent in cold workability of the present invention has a basic alloy composition in terms of weight,
C: 0.005 to 0.20%, Si: 2.0% or less, M
n: 2.0% or less, Cr: 10.0 to 25.0% and N
i: In addition to 20% or more and less than 25%, Nb + Ta: 1.
5% or less, Ti: 1.0% or more to less than 3.0%, Al:
It has an alloy composition containing 0.7 to 2.0% and Cu: 0.1 to 5.0%, with the balance being substantially Fe.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The heat-resistant steel of the present invention may optionally contain, in addition to the above-mentioned basic alloy components, alloy components selected from one or more of the following groups.

1) One or more selected from W: 3.0% or less, Mo: 3.0% or less and V: 1.0% or less. However, it is contained so as to be 0.5 W% + Mo% + V%: 3 or less.

2) B: 0.001-0.02% and Z
r: One or two kinds of 0.001 to 0.1%.

3) One or two of Ca and Mg are
In the case of two types, the total amount is 0.001 to 0.01%.

In both the basic alloy composition and the alloy composition containing optional components, part of Ni is converted to C
o: It can be replaced by 5.0% or less. Also,
The total amount of Ti and Al to Nb + Ta is preferably 5.2 to 7.0 in atomic%. In the alloys of any compositions, the impurities are P: 0.05% or less, S:
0.01% or less, O: 0.01% or less, N: 0.01%
It is preferable to regulate as follows.

The function of each component constituting the above-mentioned alloy composition and the reason for limiting the composition range will be described as follows.

C: 0.005 to 0.20% C combines with Ti, Nb, and Cr to form carbides and enhances the high-temperature strength of the alloy. This effect is recognized in the presence of 0.005% or more. However, when the amount is too large, the amount of the precipitated carbide becomes excessive, thereby impairing the workability and lowering the corrosion resistance.
The upper limit is 0.2%.

Si: 2.0% or less Si is useful as a deoxidizing element, and the presence of an appropriate amount increases the oxidation resistance. When added in large amounts, the workability is reduced,
Since it does not meet the purpose of the invention, the content is set to 2.0% or less.

Mn: 2.0% or less Mn has a deoxidizing effect like Si, but when contained in a large amount, not only impairs the workability and oxidation resistance of the alloy,
Since it promotes precipitation of η phase (Ni ₃ Ti) which impairs toughness,
The upper limit was set to 2.0%.

Cr: 10.0 to 25.0% Cr is an essential component for ensuring high-temperature oxidation resistance and corrosion resistance of the alloy, and requires addition of 10% or more. If the content exceeds 25.0%, the austenite phase becomes unstable, the σ phase (FeCr), which is an embrittlement phase, precipitates, and the toughness of the alloy decreases. A preferred range is 10-20%.

Ni: 20% or more and less than 25% Ni is an element forming austenite, which is a base material of the alloy, and has heat resistance and corrosion resistance. Further, it is an essential component for precipitating the γ 'phase as a strengthening phase. In order to fulfill such a role, addition of 20% or more is required. However, since Ni is a relatively expensive raw material, it is not desirable to add Ni in a large amount. Therefore, the upper limit is set to 25%. Part of Ni can be replaced by Co.
Co is preferred from the viewpoint of strength if Co is added, but Co is Ni
The use of a large amount is disadvantageous in terms of cost, since it is still more expensive than in the case of the above. The 5% limit was set primarily from this point of view.

Nb + Ta: 1.5% or less These are important precipitation phases together with Ni. The intermetallic compound γ '(gamma prime) phase Ni ₃ (Al, Ti, Nb,
Ta), and the precipitation of the γ 'phase effectively increases the high-temperature strength of the alloy. However, when the content of Nb + Ta exceeds 1.5%, a large amount of the Labus phase (Fe ₂ Nb) precipitates, and the toughness of the alloy decreases.

Ti: 1.0 to less than 3.0% Ti, together with Nb + Ta and Al described below, combines with Ni to form a γ 'phase which is useful for improving high-temperature strength. If the content does not reach 1.0%, the solid solution temperature of the γ 'phase will be low, so at least an additional amount is added. On the other hand, when the content exceeds 3.0%, the η phase (Ni ₃ T
i) precipitates and reduces high temperature strength and toughness,
This value is the upper limit. Preferably, 1.5 to 2.6%
It is.

Al: 0.7 to 2.0% Al is also the most important element in that it combines with Ni to form the γ 'phase. If the Al content does not reach 0.7%, the precipitation of the γ 'phase is insufficient and high temperature strength cannot be ensured. However, if the content exceeds 2.0%, the hot workability of the alloy decreases. Therefore, the addition amount within the above range is selected. A preferred range is 1.0-1.8%.

Nb + Ta + Ti + Al: 5.2-7.0
Atomic% Nb, Ta, Ti and Al are all elements constituting the γ 'phase as described above. When the Ni content is sufficient (not less than the lower limit of 20%), the precipitation amount of the γ 'phase is
It is proportional to the addition of the content of these elements. The high-temperature strength of the alloy increases with an increase in the precipitation amount of the γ 'phase. In order to sufficiently increase the high-temperature strength at 700 ° C. or higher, which is the object of the present invention, it is desirable that the total amount of these elements is 5.2 atomic% or higher. On the other hand, 7.
If it exceeds 0 atomic%, although the strength is still increased, there is a disadvantage that the hot workability is reduced.
The preferred range was chosen for this reason.

Cu: 0.1 to 5.0% Cu forms a solid solution in austenite to increase stacking fault energy and suppress work hardening, thereby improving cold workability of the alloy. Further, Cu has an effect of improving the adhesion of the oxide film at a high temperature of this alloy, which is considered to improve the high-temperature oxidation resistance. Since such an effect cannot be obtained unless the content is less than 0.1%, this value is set as the lower limit. On the other hand, even if the content exceeds 5%, the high-temperature oxidation resistance does not increase further. Since a large amount of Cu deteriorates hot workability,
The upper limit was 5.0%. The preferred range is 0.5 to
3.0%.

W: 3.0% or less, Mo: 3.0% or less,
V: 1.0% or less, provided that 0.5W% + Mo% + V%:
Addition of these elements is optional, but if added, the high-temperature strength is improved by solid solution strengthening. Addition of more than 3% of W and Mo and more than 1% of V does not increase the effect. In addition, cost increases and workability decreases. Therefore, the above-mentioned limit is set.

B: 0.001 to 0.02%, Zr: 0.
001 to 0.1% B and Zr segregate at crystal grain boundaries to strengthen the grain boundaries.
This effect is obtained because the content of each is 0.00
This is an area of 1% or more. However, B is 0.02%, Z
If the content of r exceeds 0.1%, the hot workability is impaired, so these were made the upper limits.

One or two types of Ca and Mg (in the case of two types, in total): 0.001 to 0.01% These elements may be added as deoxidizing / desulfurizing agents during melting of the alloy. , Helps to improve the hot workability of the alloy. This effect is observed even when the amount of addition is as small as 0.001%, but when it exceeds 0.01%, the hot workability tends to be rather reduced.

P: 0.05% or less, S: 0.01% or less, O: 0.01% or less, N: 0.01% or less These are impurities, and P and S are heat of the alloy. O and N form non-metallic inclusions and degrade the mechanical properties of the alloy. The above values are defined as limits at which such effects do not substantially appear for each element.

[0027]

【Example】

Example 1 50 kg of each alloy having the composition (% by weight, balance Fe) shown in Table 1 was melted in a high frequency induction furnace and cast into an ingot. After soaking the ingot at 1100 ° C for 6 hours, it was forged and rolled in a temperature range of 1100 to 900 ° C to obtain a round bar having a diameter of 16 mm. Insert this round bar at 975 ° C x 3
After heating for 0 minutes, solution heat treatment was performed under oil-cooling conditions. A test piece having a diameter of 15 mm and a height of 22.5 mm was cut out from the heat-treated round bar.

Table 1 No. C Si Mn Cr Ni Nb + Ta Ti Al Cu Other Ti etc. * Example 1 0.0050 0.20 0.21 16.0 23.9 0.81 2.45 0.98 2.02-5.30 2 0.049 0.21 0.20 16.1 24.0 0.80 2.46 1.15 0.99-5.64 3 0.030 0.21 0.20 15.0 22.2 0.47 2.88 1.47 2.97-6.56 4 0.031 0.20 0.20 14.2 24.0 0.24 2.60 1.24 3.02 Mo 0.5 5.67 5 0.010 0.11 0.10 12.1 23.9 0.09 2.90 1.25 2.00 W 1.0 5.98 B 0.003 6 0.010 0.09 0.09 12.0 24.1 0.06 2.87 1.20 1.98 W 0.5 5.82 Mo 0.3 B 0.02 7 0.031 0.20 0.21 15.1 21.2 0.51 2.33 1.12 2.01 Co 2.5 5.77 Zr 0.004 Mg + Ca 0.001 8 0.029 0.19 0.20 14.8 24.0 0.80 2.79 1.02 2.01 V 0.1 5.77 Zr 0.003 Mg + Ca 0.002 9 0.030 0.20 0.20 15.1 24.3 1.02 1.95 1.29 1.99 B 0.003 5.48 10 0.030 0.20 0.20 15.0 24.1 1.20 1.89 1.21 2.02 B 0.005 5.37 11 0.033 0.22 0.21 14.9 23.9 0.81 2.64 0.95 2.00 B 0.002 5.47 Comparative example 1 ** 0.040 0.19 0.20 13.8 24.4-2.30 0.18-M 01.1 3.03 V 0.2 B 0.002 2 0.057 1.69 0.42 18.5 24.6-2.03 0.56-B 0.003 3.42 3 0.031 0.20 0.23 16.3 27.2 0.31 4.27 0.12 - - 5.36 * Ti + Al + Nb + Ta, atomic% ** SUH660.

Each of the test pieces was subjected to an end face restraint compression test at room temperature, and the presence or absence of cracks at an upsetting ratio of 75% was examined. Next, the heat-treated round bar was further subjected to an aging heat treatment at 750 ° C. for 4 hours, and a material having a diameter of 10 mm and a thickness of 5.5 was obtained from the heat-treated material.
mm test pieces were cut out. For these test pieces, 8
Vickers hardness (10 kgf) at 00 ° C. was measured. Separately, a test piece having a diameter of 7 mm and a length of 15 mm was cut out, placed in an alumina crucible, heated at 850 ° C. in the atmosphere for 400 hours, air-cooled, and measured for oxidation weight gain. Table 2 shows the results.

Table 2 Vickers hardness at the time of upsetting Oxidation weight increase No. Status of cracking 800 ° C. (Hv) (mg / cm ² ) Example 1 ○ 229 1.3 2 ○ 232 2.1 3 ○ 228 14 * 254 1.15 * 250 1.46 * 258 1.37 * 239 1.58 * 247 1.89 * 2211.4 10 * 222 1.6 11 * 2481.7 Comparison Example 1 ○ 181 5.6 2 ○ 192 5.2 3 × 288 6.1 * Classification is based on the rate of occurrence of cracks at an upsetting rate of 75%. did.

From the results shown in Table 2, it can be seen from Example No. 1
No. 11 to 50% crack generation at upsetting rate of 75%
And the high-temperature hardness at 800 ° C. is less than Hv20.
It turns out that it is 0 or more. Comparative Example No. 1 and 2
Can avoid cracks, but has a high-temperature hardness of Hv200
Does not reach. No. No. 3 has high hardness at high temperature but low workability. In the oxidation test at 850 ° C. × 400 hours,
The increase in oxidation of the alloy of the present invention is smaller than that of the comparative example, and it can be seen that the alloy has excellent oxidation resistance.

Example 2 The alloy composition in Table 3 (% by weight,
P, S, O and N are ppm, and the steel having the balance Fe) is melted and heat-treated in the same manner as in Example 1 to prepare a test piece, an upsetting test, a measurement of high-temperature hardness, and an increase in oxidation weight. A measurement was made.

Table 3 Nb + No. C Si Mn Cr Ni Ta Ti Al Cu P SON Example 12 0.030 0.10 0.10 15.2 23.8 0.51 2.36 1.26 2.00 20 10 20 30 13 0.028 0.11 0.09 14.8 24.2 0.52 2.76 1.30 2.04 10 10 10 40 14 0.032 0.19 0.20 15.1 24.0 0.80 1.93 1.32 1.99 20 20 10 30 15 0.030 0.20 0.21 15.1 24.0 0.79 2.67 1.10 2.02 20 10 10 20 Comparative Example 4 0.410 0.31 0.28 15.9 26.8-4.88 0.48-9 10--5 0.020 0.13 0.09 23.4 41.2 1.23 2.67 1.33-12 8-- Table 4 shows the test results.

Table 4 Vickers hardness at the time of upsetting Oxidation weight increase No. State of cracking 800 ° C. (Hv) (mg / cm ² ) Example 12 ○ 233 1.5 13 ○ 254 1.1 14 ○ 218 1.3 15 ○ 255 1.5 Comparative Example 4 × 263 5.9 5 × 280 4.0 In Comparative Example, the high-temperature hardness was higher than Hv200, but the workability was low and the corrosion resistance was not good.

[0035]

The heat-resistant steel of the present invention is made of a known heat-resistant steel SUH.
It has improved high-temperature hardness compared to 660, shows heat resistance enough to be used at 700 ° C. or more, and has excellent oxidation resistance. First, the difference between the high-temperature strengths is that, when compared with the alloy composition of both alloys, the alloy contains not only an appropriate amount of Nb + Ta but also a larger amount of Al. As a result, Nb + Ta + Ti
This is considered to be due to the fact that the total amount of + Al became higher and a sufficient amount of γ 'phase was precipitated. The oxidation resistance may have been brought about by the inclusion of Cu or an increase in the amount of Al. Another advantage is that the cold workability is high, and as described above, this often depends on the addition of Cu.

While having such high performance, the heat resistant steel of the present invention has a low Ni content (SUH66).
0 to 24% to 27%, the present invention is 20% or more.
(Less than 25%), and the material price can be kept low. This, together with the fact that components can be manufactured by cold working and the cost of the molding process can be suppressed, the manufacturing cost of heat-resistant components can be reduced.

Claims (7)

[Claims] 1. C: 0.005 to 0.20% by weight,
Si: 2.0% or less, Mn: 2.0% or less, Cr: 1
0.0-25.0% and Ni: 20% or more and less than 25%, Nb + Ta: 1.5% or less, Ti: 1.0%
Or more to less than 3.0%, Al: 0.7 to 2.0% and C
u: heat-resistant steel excellent in cold workability, characterized by containing 0.1 to 5.0% and having an alloy composition substantially consisting of Fe in the balance. 2. In addition to the alloy component according to claim 1,
W: 3.0% or less, Mo: 3.0% or less, and V: 1.
0% or less of one or more of 0.5 W% + Mo%
+ V%: heat resistant steel containing 3 or less. 3. In addition to the alloy component according to claim 1 or 2, B: 0.001 to 0.02% and Zr: 0.0
A heat-resistant steel containing one or two kinds of 01 to 0.1%. 4. In addition to the alloy component according to claim 1, one or two of Ca and Mg (2
(In the case of seeds in total): heat-resistant steel containing 0.001 to 0.01%. 5. The heat-resistant steel having the alloy composition according to claim 1, wherein a part of Ni is Co: 5.
Heat resistant steel having an alloy composition replaced by 0% or less. 6. The heat-resistant steel according to claim 1, wherein Nb + Ta + Ti + Al: 5.2-7.
Heat resistant steel with 0 atomic%. 7. The heat-resistant steel according to claim 1, wherein P: 0.05% or less, S: 0.01% or less, O: 0.01% or less, and N: 0.01. % Heat-resistant steel.