JPS6365059A - Heat resistant ferritic steel having superior strength at high temperature - Google Patents

Abstract

PURPOSE:To improve the strength of a heat resistant high Cr-Mo ferritic steel at high temp. and the resistance to oxidation at high temp. by adding specified amounts of V, W and Cu to the steel and specifying the quantitative relation between elements including C+N in the steel. CONSTITUTION:A heat resistant ferritic steel having a compsn. consisting of, by weight, 0.03-0.20% C, <1.0% Si, 0.1-1.5% Mn, 0.05-0.30% Cu, 0.1-1.0% Ni, 7.0-13.0% Cr, 0.4-2.5% Mo, 0.02-0.15% Nb, 0.02-0.35% V, 0.05-2.50% W, 0.005-0.080% N and the balance Fe is used as a heat resistant steel forming the members of an extra super critical pressure boiler or the like. In the compsn., W and Mo, V an Nb, Nb and (C+N), V and (Mo+W), and (Mo+W+2 V+2N) and (C+N) have separate specified quantitative relations. The heat resistant ferritic steel has improved strength at high temp. and improved resistance to oxidation at high temp.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to ■ evaporation tubes, superheater tubes, reheater tubes, main steam piping of boilers for supercritical pressure and ultra-supercritical pressure, ■ The present invention relates to a ferritic heat-resistant steel having excellent high-temperature strength and suitable for use in heater tubes, heat exchanger tubes, steam generator tubes and superheater tubes of fast breeder reactors, and fusion reactor parts - reactor wall materials.

[Prior Art] Recently, supercritical pressure boilers have been used as boilers for thermal power generation, but ultra-supercritical pressure boilers are required in order to improve thermal efficiency and save fuel. This type of boiler shuts down operations at night when electricity demand is low, so
The operation involves a heat cycle of high and low temperatures. Austenitic stainless steel, which has a large coefficient of thermal expansion, poses problems such as thermal fatigue and scale peeling. The exfoliated scale accumulates at the bends of steel pipes, causing localized high temperatures that can cause pipes to blow out, and scales that can even reach turbines, causing various problems.On the other hand, in the case of ferritic steel, the temperature is lower than that of austenitic steel. It not only has a low coefficient of thermal expansion, but also reduces stress corrosion cracking and intergranular corrosion at high temperatures, has high thermal conductivity, and has the advantages of being a device. For this reason, ferritic high chromium steel is used for evaporator tubes, superheater tubes, and reheater tubes in supercritical pressure and ultra-supercritical pressure boilers, heater tubes and heat exchanger tubes in various equipment for the chemical industry, and high-speed growth steels. It is also suitable as a material for steam generator tubes and superheater tubes in furnaces.

The boiler with the most severe steam conditions currently in practical use is the supercritical pressure boiler (246 atm, 586°C). Normally, the boiler with tube wall temperature up to 580°C is 2φ1cr-INo!
(STBA24), 9Cr-IMo up to 620℃
~2Mo steel (e.g. 5TBA2B and ■5TBA27)
Or 18Cr-8Nj stainless steel, 820°
When the temperature exceeds C, 18Cr-8Ni stainless steel is exclusively used as the pipe material.

[Problems to be solved by the invention] Conventional 9Cr-1111o to 2Mo steels are 20M
Although oxidation resistance is improved compared to Cr-INofIJ, its use is limited due to its low high temperature strength. 19Cr
-8Ni type has excellent high-temperature strength and high-temperature oxidation resistance, but has the disadvantages peculiar to austenitic type as mentioned above.
Moreover, since it contains large amounts of Cr and Ni, there is a problem in economic efficiency.

By the way, the first stage steam conditions (316 atm, 588 °C) as a supercritical pressure boiler will soon be put into practical use, but the second stage will have steam conditions of 318 atm and 583 °C, so the superheater tubes and The temperature of the tube wall of the heating tube is approximately 620°C, and long-term strength at this temperature is required. However, when comparing the allowable stress determined in accordance with the Ministry of International Trade and Industry's thermal power generation technical standards, the allowable stress of the already mentioned ferritic high chromium steel is 18C:r-8Ni stainless steel (5U93048〒B) at 820°C. It is approximately 475 or less.

Therefore, in order to maintain strength at high temperatures for a long period of time, the wall thickness must be increased, which is not favorable in terms of heat exchange, and also increases material costs and construction costs.

Thus, boiler tubes for ultra-supercritical pressure are required to further improve their high-temperature strength, and at the same time, because they are used in high-temperature environments, they are required to have improved high-temperature corrosion resistance.

By the way, conventional 2ml Or-1No steel and STB
A2B (ICr-1No steel), ■5TBA27 (9Cr
5uper was developed mainly by Oak Ridge National Laboratory in the United States as a steel with improved high-temperature strength of
SCr steel or ASTMA213 Ta2 steel (9Cr
-IMo Nb, V steel) is known.

However, although the high-temperature strength of these steels has been improved to some extent compared to conventional steels, it is still insufficient, and further improvements are needed, and improvements in oxidation resistance have also been desired.

It is already known that high-temperature strength can be improved by adding W to 9-12Cr steel, and that high-temperature strength can be improved by adding Nb and V. No. 80-293092 proposed a steel with improved high temperature strength and further improved weldability. After conducting further detailed studies, we found that appropriate combinations of various elements are effective in improving high-temperature strength, and that (U and V) are effective in improving oxidation resistance at high temperatures. It was found that the effects were significantly improved by an appropriate combination of

In the present invention, the optimum values of No amount and V amount, the optimum value of V amount and Nb amount, the optimum value of Nb amount and C+N amount, the optimum value of V amount and MO◆W amount, and the optimum value of No, W, Wb, V amount and 04
By examining the optimal value for the amount of PN, we were able to achieve a significant improvement in high-temperature strength. Furthermore, in order to improve high-temperature oxidation resistance, we investigated the effects of component elements and found that by adding an appropriate amount of Cu along with V and - to high-0r-No steel, high-temperature oxidation resistance could be significantly improved. became usable.

The present invention was developed to satisfy the above-mentioned required characteristics, and an object of the present invention is to provide a relatively inexpensive high-temperature heat-resistant steel having the following characteristics.

■High temperature strength: Creep rupture strength of 850℃ x 5ooo h is 10Kg/mm2 or more, same <650℃ x 105
5Kg/m■2 or more in h.

■High-temperature oxidation resistance: Corrosion loss due to atmospheric oxidation at 850°C for 1000 hours is 1 mg/c12 or less.

Additionally, the present invention aims to improve the above items without causing deterioration of toughness, workability, and weldability, which are essential properties in the field of use of this material.

[Means for solving the problems] The ferritic heat-resistant steel with excellent high-temperature strength according to the present invention has the following features:
C: 0.03% to 0.20%, Si: 1.0% by weight. 0% or less Mn
: 0.1-1.5%, Cu: O, 05-0.
30%Xi: 0.1-1.0%, Or: 7.
0-13.0% No: 0.4-2.5%, Nb
: 0.02~0.15%V: 0.02~0.35
%, W: 0.05-2.50%N: 0.00
5 to 0.080% and the range surrounded by ABCD in Figure 1 where the relationship between - and the amount of No occupies the following coordinate points, and Figure 2 where the relationship between V and the amount of Nb occupies the following coordinate points The range surrounded by EFGHIJ, the range surrounded by KLMN in Figure 3 where the relationship between Nb and C0N occupies the following coordinate points, and the range surrounded by V and Mo
The relationship between the +W amount is in the range surrounded by 0PQR in Figure 4, which occupies the following coordinate points, and the relationship between No + W + 2V + Nb and C + N is in the range surrounded by STUV in Figure 5, which occupies the following coordinate points, and the rest. It consists of Fe and inevitable impurities.

Buttocks and No amount V and Nb amount WX Mo$ VX Nb! A
(0,05,2,50) E(0,02,0
,15) B(0,05,0,85) F(0
,02,0,04)C(0,35,0,40)
G(0,0fl, 0.02)D(2,50,0,
40) H(0,35,0,02)I(0,
35,0,75) J (0,20,0,15) Nb and C ten N amount V)! o+W amount Nb$
C◆NZ VX Mo+W%K(
0,02,0,15) O(0,02,2,
5) L(0,02,0,05) P(0,0
? , 0.5) M(0,15,0,05) Q
(0,35,0,8)N(0,15,0,28)
R(0,30,2,9)Mo+W+2V+Nb and C1N Quantity Mo+W+2V+NbX C◆N! S(0,7,0,19)? (0,7,0,05) U (3,8,0,08) V (3, El, 0.28) [Function] In order to achieve the present invention, first, the addition of MO and lily increases high temperature strength. As a result of a detailed study of its influence, it was found that the addition of W significantly improves high-temperature strength, especially creep rupture strength at temperatures exceeding 800°C. Both Mo and Mo are ferrite-forming elements, and if they are added in too large a quantity, the volume fraction of delta ferrite will increase and the high temperature strength will decrease. On the other hand, if the amount of N01W added is small, the strength improvement effect will be diminished. Based on this idea, we investigated the creep rupture strength at 850°C for 5000 hours across many combinations of No. and W addition amounts, and found that the creep rupture stress σr≧1
The shaded area in FIG. 1 was obtained as the 0 kg/m2 area. That is, excellent creep rupture strength can be obtained when the resistance and MO are within the shaded area in FIG.

Here, the straight line AB is a line showing a V content of 0.05%, and below this, the effect of improving creep strength is extremely weak, and the straight line CD is a straight line showing a Mo content of 0.4%.
Below this, the effect of improving creep strength is weak, straight line BC
In the lower region, the amount of beam and Mo is too small, resulting in a decrease in creep strength. On the other hand, in the region above the straight line AD, the amount of Mo is too large, so the volume of delta ferrite increases and the creep strength decreases.

Next, as a result of a detailed study of the effect of the addition of V and Wb on high-temperature strength, it was found that high-temperature strength can be significantly improved by adding an appropriate amount of a combination of V and Nb.

The optimum range obtained as a result is the shaded area in FIG.

Here, the straight line EF is a straight line in which the V amount is 0.02%, the straight line GH is a straight line in which the Nb amount is 0.02%, and the V
If both Nb and Nb are less than this, the effect of improving creep strength is extremely weak.The straight line IH is a straight line indicating a V content of 0.35%, and the straight line EJ is a straight line indicating a Nb content of 0.15%. If Nb is greater than the respective values, the carbides will become coarse and the creep strength will decrease, as well as notch toughness and weldability. In the region below straight line FC, the amount of Wb and V is too small, so the effect of improving creep strength is weak, and in the region above straight line IJ, carbides become coarse and creep strength decreases.

Next, as a result of a detailed study of the effects of Nb content and C and N content on high-temperature strength, it was found that excellent high-temperature strength can be obtained when the relationship between C+N content and Nb content is within the diagonal line in Figure 3. It was discovered that

Here, the straight line KL is a straight line indicating an Nb content of 0.02%; below this, the effect of improving creep strength is extremely weak; the straight line LM is a straight line indicating a C◆N content of 0.05%; If the amount is less than this, carbonitrides with Nb will not be effectively generated, so there is no effect of improving creep strength.The straight line > IN is a straight line that indicates the amount of Nb is 0.15%, and if it exceeds this value, carbonitrides will not form effectively. becomes coarse and the creep strength decreases, as well as notch toughness and weldability. The straight line KN indicates that the upper limit of the CAN content increases with an increase in the Nb content, and in the region above this straight line, coarse carbonitrides are formed, resulting in a decrease in creep strength.

Subsequently, as a result of a detailed study of the effects of the amount of V and the amount of No + W on high temperature strength, it was found that the addition of ■ was effective, and this Vfi
It was found that there is a close relationship with the amount of o+W, and that when it is within the diagonal line in FIG. 4, excellent high-temperature strength can be obtained.

Here, the straight line OP indicates that it is necessary to increase the V amount in order to achieve high temperature strength as the No◆W amount decreases, and the straight line RQ indicates that the V amount increases as the No◆- amount increases. This shows that it is necessary to reduce the amount of V in order to achieve strength, and in the region to the left of the straight line OP, the formation of carbonitrides of V is insufficient, so the effect of improving high-temperature strength is weak. In the region to the right of straight line RQ, carbides become coarse and reduce creep strength, as well as notch toughness and weldability. Linear OR and PQ indicate that in order to achieve high temperature strength, it is necessary to increase the amount of Mo + W as the amount of V increases, and in the region above the line OR, No + W
Since i is too large, the amount of ferrite increases and the creep strength decreases. In the area below the straight line PQ, MO◆w3
is too small, so the effect of improving high-temperature strength is small.

Furthermore, as a result of a detailed study of the influence of the relationship between the amounts of No, W, V, and Nb and the amount of CAN on high-temperature strength,
It has been found that when the amount of C+N relative to Mo+W◆2v◆Nbfl is within the shaded range in FIG. 5, excellent high-temperature strength can be obtained.

Here, the straight line ST has No+W+2V+Nb amount of 0.7%
Below this, the amount of carbonitride produced is small, so the effect of improving creep strength is weak.The straight line vU is a straight line that indicates the amount of No+W◆2V+Nb is 3.6%; Creep strength decreases due to coarsening of carbides. The straight line Sv and Tυ are Mo+W+2V
◆This shows that it is necessary to increase the amount of C+% as Nbi increases, and in the region below the straight line TO, the amount of carbonitride produced is small, so the effect of improving creep strength is weak, and the straight line SV In the upper region, the creep strength decreases because the carbonitrides become coarser.

Furthermore, it has been found that adding an appropriate amount of Cu along with V to high Cr steel has a significant effect on improving oxidation resistance at high temperatures. The results are shown in Figure 6.
In other words, to show the effect of adding Cu on the change in corrosion loss after heating in the atmosphere at 750°C for 500 hours, in high Cr with combined addition of V and W, Cu was added by 0.05
By adding more than %, the corrosion weight loss decreases rapidly.

On the other hand, in high Cr steel without the addition of V or W, no significant effect is observed compared to the former.

The present invention was completed based on the above six findings.

The reasons for limiting the components of the present invention will be explained below.

C is an essential element that contributes to the formation of low-temperature transformation products and the precipitation of carbides to improve the creep rupture strength. 0-2
If it exceeds 0%, the hardenability and strength will increase significantly, but the weldability and workability will deteriorate, so the C content is set to 0.20% or less. If it is less than 0.03%, it is difficult to ensure high temperature strength, the amount of delta ferrite increases, and the notch toughness deteriorates, so the C content is set to be 0.03% or more.

Si is added as a deoxidizing agent, but if used in large amounts, the toughness of the steel deteriorates, so the upper limit was set at 1.00%.

xn is an element useful as a deoxidizing and desulfurizing agent, as well as for improving strength and hot workability and obtaining an appropriate structure, but 0.
If it is less than 1%, there will be no useful effect, and if it exceeds 1.5%, the hardenability will increase, and although the strength will increase, it will cause deterioration of workability such as bending and toughness, so it was set at 0.1 to 1.5%. .

When added to high Cr steel, Cu gives viscosity to the oxide film containing Or, further improves the adhesion of the film, and helps the oxidation resistance of the Cr steel. Especially for improving oxidation resistance at high temperatures.
It has been found that the combined addition of Cu and V to high Cr-No steel has a significant effect as shown in FIG. 6. In order to exhibit this effect, Cu needs to be added in an amount of 0.05% or more. On the other hand, since adding too much leads to deterioration of toughness, the upper limit was set at 0.3%.

Xi is added to control the amount of delta ferrite to an appropriate value, maintain high temperature strength, and improve toughness, but 0
．． If it is less than 10%, there is no effect, and if it exceeds 1.0%, hot deformation resistance increases and it is not favorable for hot processing. Since it is an expensive element, the upper limit was set at 1.0%.

Cr is added to improve high-temperature oxidation resistance and high-temperature long-term strength.The high-temperature long-term strength of 800℃ or higher is high when Cr is 7.0 to 13.0%, and exceeds 13.0%. teC
As r increases, delta ferrite increases and high temperature long-term strength decreases. On the other hand, if it is less than 7.0%, precipitation strengthening due to Cr carbides and solid solution strengthening of Cr cannot be expected, and high-temperature long-term strength decreases, and if it is less than 7.0%, high-temperature oxidation resistance is poor. The component range of Cr is 7.0 to 13.
It was set to 0%.

Mo and W are indispensable elements for heat-resistant steel because they significantly increase high-temperature, long-term strength, and the combined addition of No and W has been found to have a significant effect in high-Cr steel.

Both elements dissolve in solid solution in steel and strengthen it, as well as precipitate carbides and improve creep strength.
At less than 0.05% -, this effect is absent, and in the range below the straight line BC shown in Fig. 1, the high temperature strength is not sufficiently improved.
It has been found that the addition of V and V can significantly improve high temperature oxidation resistance. In order to exhibit this effect, W must be 0.05% or more; on the other hand, in the range above 2.5% (No, 2.5% or more, or straight line AI), the amount of delta ferrite is In addition, since Mo and W are expensive elements, the cost increases, which is not preferable from an economic point of view.

Nb and V precipitate as carbides or carbonitrides and suppress the decline in high temperature strength over a long period of time. The solubility product of Nb carbide or Nb carbonitride is smaller than that of 7th oxide or 7th nitride, and it is easy to precipitate, so it significantly increases the high temperature short-time strength, but when added alone, Nb carbide and Nb carbonitride coagulate. , it tends to become coarse and it is difficult to maintain high temperature strength for a long time. N for long-term strength improvement
Combined addition of b and V is effective, and Nb precipitated during the manufacturing process
During use at high temperatures (C, N), carbides of X, 3C, and M6C precipitate, and V is in a solid solution state in addition to carbides of V and C3. 1. , C5, and M2C, and suppresses coarsening of these carbides. Precipitates of Wb and V that are finely precipitated by the combined addition of Nb and ■, as well as M, lG, and intrinsic V (M≦C1) that finely precipitate after a long period of time improve the high-temperature long-term strength. Therefore, even with V alone, fine carbides >4. , C≦, C cannot be obtained and long-term strength cannot be improved.Furthermore, when V is added with an appropriate amount of Cu, it has a remarkable effect on improving high-temperature oxidation resistance. It was discovered that. Nb
, V are less than 0.02%, the above effects are insufficient. Further, if too much Nb or V is present, the carbides will become coarse and the creep rupture strength will be reduced, and the notch toughness and weldability will also be reduced. Therefore, the Nb content was set to 0.15% or less, and the V content was set to 0.35% or less.

N improves high-temperature long-term strength through the formation of nitrides or carbonitrides and the residual inherent N, but if it is less than 0.005%, it has no effect, and if it exceeds 0.080%, blowholes will form during welding. Since this significantly deteriorates weldability, the content of N was set to 0.005% to o, oso%.

[Example] Table 1 shows the chemical composition of the invention steel and Table 2 shows the chemical composition of the comparative steel.
A steel ingot with these chemical components is manufactured in a high-frequency melting furnace, and the steel ingot is hot rolled into a 20 mm thick steel plate at 1050℃.
It was normalized at 780° C. and tempered for 1 hour. After that, a 6■■Φ round bar clip test piece and a 3mm thick X
A 30×30 atmospheric high temperature oxidation test piece and a weld hardness test piece were taken. Creep test was conducted at 650℃ and stress 10K.
Breaking time in g/am2, high temperature oxidation test in air,
Weight loss after holding at 700°C for 1000 hours (mg/a
The weldability test was conducted using a 18(r-8N+ stainless steel welding rod), followed by post-heat treatment at 75Q°C for 1 hour, and the maximum hardness of the heat-affected zone was determined.

The steel types in the examples of the present invention include C, Si, and Mn.

Cu, old, Cr, No, Nb, V, W,
N c7) Steel types with different weight % are shown. Among the comparison steels, the steel is 5TBA26, and the tone is ■5TBA27 (7).
Conventional steel, He-ho steel 11Nb and V are added singly or in combination, but W and Cu are not added; He steel is added with So, but Nb, V and Cu are not added. , G steel is one to which Cu is not added, and Z steel is one to which Cu is not added and the amount of C exceeds that of the steel of the present invention. The steels are ASTM Ta2 (Super9Cr) steels to which Cu is added, but the steels to which no V and V are added.

As shown in the right column of Table 1, the test results for unexplored seeds show that they have excellent high-temperature strength and a creep rupture time of 7,000 hours or more. High-temperature oxidation resistance is also all 0 in the case of the steel of the present invention.
．． The value is as low as 8 tag/c 2 or less, indicating excellent oxidation resistance. Although the maximum hardness (Hma) is excellent, there is no large increase, and all Hma<320, and the weldability is also excellent.

On the other hand, as shown in the right column of Table 2, the comparative steel is A, and the tone is Wb.
, V, W wo-free, 650℃, 10Kg
/ms+' The fracture time under stress is extremely short.The fracture time of C steel containing Nb and the double chain containing ■ is improved, but it is significantly shorter than the steel of the present invention.Combined addition of Hb and V Although the fracture time of the steel E steel is further improved, it is shorter and inferior to the steel of the present invention because it does not contain W. N
b. The steel containing - but not containing V has an improved rupture time to the same extent as steel E, but it is shorter than the steel of the present invention.
G and H steels have a composite addition of Nb, V, and W, and their breaking strength is significantly improved over a long period of time. However, in the case of the comparative steels, none of them contain Cu, and their high-temperature oxidation resistance is significantly inferior to that of the steel of the present invention. Furthermore, since the weight percent of C in steel is higher than the range specified in the present invention, there is a drawback that the maximum hardness is significantly increased. The steel has a short creep rupture time because it does not contain V or V, and its high-temperature oxidation resistance is inferior to that of the steel of the present invention.

As described above, the steel of the present invention is superior in high-temperature long-term strength to the currently used Cr-Not I4, and has a long-term creep rupture strength of 5OS304 or higher in the temperature range of 800 to 850°C. Moreover, it is a steel with extremely excellent high-temperature oxidation resistance, and considering weldability, the maximum hardness (H) is 320 or less, which is a range that can be used without any problems in practical use. Therefore, it is possible to use the steel as an alternative to the expensive 18Cr-8Ni austenitic stainless steel, and the steel of the present invention is extremely useful as an economical heat-resistant steel for heat exchange equipment and the like.

The steel of the present invention is suitable for use as heat-resistant steel pipes such as boiler pipes, heat-resistant steel pipes for chemical plants, steam generator pipes for fast breeder reactors, and superheater pipes. It can also be widely used.

[Effects of the Invention] As described above, the ferritic heat-resistant steel with excellent high-temperature strength according to the present invention has C: 0.03% to 0.20%, Si: 1.0% or less, and Mn in weight percent.
: 0.1~1.5%, Cu: 0.05~0.3
0%Xi: 0.1-1.0%, Or: 7.0
~13.0%Mo: 0.4~2.5%, Nb:0.0
2~0.15%V: 0.02~0.35%, W
: 0.05~2.50%N: 0.005~0.0
80% And the relationship between the amount of V and the amount of Nb occupies the following coordinate points in Figure 1 AB (D), and the relationship between the amount of V and the amount of Nb occupies the following coordinate points in Figure 2 EFGHI J Also, the range surrounded by KLMN in Figure 3, where the relationship between Hb and the amount of C0N occupies the following coordinate points, and the range surrounded by V and Mo
Figure 4 0PQR where the relationship between +W amount occupies the following coordinate points
Also, No+W+2V+Nb and CAM
The quantity relationship is in the range surrounded by STUV in FIG. 5, which occupies the following coordinate points, and the remainder consists of Fe and unavoidable impurities.

W and No amount V and Nb amount W% No amount VX Nb$A
(0,05,2,50) E(0,02,0
,15) B(0,05,0,65) F(0
,02,0,04)C(0,35,0,40)
G(0,0B, 0.02)D(2,50,0,4
0) H(0,35,0,02)1(0,35
,0,75) J(0,20,0,15) Hb and 04M amount V and Mo+W amount NbX
C+N% VX Mo◆W%K(0,
02,0,15) O(0,02,2,5)
L(0,02,0,05) PCo, 0? ,
0.5))! (0,15,0,05) Q
(0,35,0,8)N(0,15,0,28)
R(0,30,2,9)No+W+2V+N
b Quantity Mo+W+2V+Nb$ C+N$ S(0,7,0,19) t(o,7,0.05) U(3,8,0,08) V(3,8,0,28) Therefore, Compared to conventional ferritic heat-resistant steels, this steel has significantly improved strength (especially creep rupture strength) at high temperatures, particularly above 600°C, as well as improved high-temperature oxidation resistance, as well as improved weldability and processability. For example, ferrite can be used as a material for ultra-supercritical pressure boilers used in high-temperature, high-pressure environments, as well as for FBR steam generation tubes, superheater tubes, etc., contributing to thinner walls and longer life. It becomes possible to obtain heat-resistant steel.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between No. and W content and creep rupture strength. Figure 2 is a diagram showing the relationship between Nb and V content and creep rupture strength. Figure 3 is a diagram showing the relationship between CAM and Nll content and creep rupture strength. Figure 4 is a diagram showing the relationship between No+W, V amount and creep rupture strength, Figure 5 is CAM
, No+W+2V◆A diagram showing the relationship between the amount of Nb and creep rupture strength. Figure 6 is a diagram showing the influence of the amount of Cu on the corrosion weight loss after an oxidation test in 750" CX 500 h atmosphere. Agent Patent Attorney Shuji Shiokawa 10th Fig. 2 Fig. 4 V (wt%) Fig. 5 MO+Step 2V+Nb, C1N S (0,7,0,19)

Claims (1)

[Claims] (1) C: 0.03 to 0.20%, Si: 1.0% or less, Mn:
0.1-1.5%, Cu: 0.05-0.30% Ni:
0.1-1.0%, Cr: 7.0-13.0% Mo: 0
．． 4-2.5%, Nb: 0.02-0.15% V: 0.
02-0.35%, W: 0.05-2.50% N: 0.
005 to 0.080% and the range surrounded by ABCD in Figure 1 where the relationship between W and the amount of Mo occupies the following coordinate points, and Figure 2 where the relationship between V and the amount of Nb occupies the following coordinate points The range surrounded by EFGHIJ, the range surrounded by KLMN in Figure 3 where the relationship between Hb and C+H amount occupies the following coordinate points, and the range surrounded by V and Mo+
The relationship between the amounts of W is in the range surrounded by OPQR in Figure 4, which occupies the following coordinate points, and the relationship between Mo+W+2V+Nb and C+N is in the range surrounded by STUV in Figure 5, which occupies the following coordinate points, and the remainder is Fe and A ferritic heat-resistant steel with excellent high-temperature strength, characterized by the presence of unavoidable impurities. ▲Contains mathematical formulas, chemical formulas, tables, etc.▼