JPH0359135B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is applicable to supercritical pressure and ultra-supercritical pressure boiler evaporator tubes, superheater tubes, reheater tubes, main steam piping, and heaters for chemical industrial plants. tubes, heat exchanger tubes, fast breeder reactor steam generator tubes, superheater tubes,
The present invention relates to a ferritic heat-resistant steel with excellent high-temperature strength and suitable for use as a material for the first reactor wall of a nuclear fusion reactor.

[Prior Art] Recently, supercritical boilers have been used as boilers for thermal power generation, but ultra-supercritical pressure boilers are required in order to increase thermal efficiency and save fuel. This type of boiler shuts down its operation at night when electricity demand is low, so it operates with 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 peeled off scale accumulates at the bent portion of the steel pipe, causing local high temperatures that can cause the pipe to burst, and the scale may even reach the turbine, causing various problems. On the other hand, ferritic materials not only have a lower coefficient of thermal expansion than austenitic materials, but also have the advantage of reducing stress corrosion cracking and intergranular corrosion at high temperatures, high thermal conductivity, and low cost. . For this reason, ferritic high chromium steel is used for evaporator tubes, superheater tubes, and reheater tubes in supercritical pressure and ultrasupercritical pressure boilers, superheater tubes, heat exchanger tubes in various equipment for the chemical industry, and high-speed growth It is also suitable as a material for steam generator tubes and superheater tubes in furnaces.

The boiler currently in practical use with the most severe steam conditions is the supercritical pressure boiler (246 atm, 566°C). Normally 2 1/4 Cr when the pipe wall temperature is up to 580℃
−1Mo steel (STBA24), 9Cr−1Mo ~ up to 620℃
2Mo steel (for example, STBA or STBA27) or 18Cr-8Ni stainless steel; if the temperature exceeds 620°C, 18Cr-8Ni stainless steel is used as the pipe material.

[Problems to be solved by the invention] However, although conventional 9Cr-2Mo to 2Mo steels have improved oxidation resistance compared to 2-1/4Cr-1Mo steel, their high-temperature strength is relatively low, making them difficult to use. subject to restrictions. Although the 18Cr-8Ni system has excellent high-temperature strength and high-temperature oxidation resistance, it has the disadvantages peculiar to austenitic systems as mentioned above, and in addition, it contains large amounts of Cr and Ni, making it uneconomical. There's a problem.

By the way, the first stage steam conditions (316 atmospheres, 566 degrees Celsius) as an ultra-supercritical pressure boiler will soon be put into practical use, but the second stage will have steam conditions of 316 atmospheres, 593 degrees Celsius, so superheater tubes and The temperature of the tube wall of a heating tube is approximately 620℃, 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 technical standards for thermal power generation, the allowable stress of the already mentioned ferritic high chromium steel is
At 620℃, it is about 4/5 or less than 18Cr-8Ni stainless steel (SUS304HTB). 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 for heat exchange, and increases material costs and construction costs.

As described above, 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 also required to have improved high-temperature corrosion resistance.

By the way, conventional 2.1/4Cr-/Mo steel and
STBA26 (9Cr−1Mo steel), ○Fire STBA27 (9Cr−2Mo steel)
Oak Ridge National Laboratory in the United States led the development as a steel with improved high-temperature strength.
Super9Cr steel i.e. ASTMA213 T91 steel (9Cr−
1Mo, Nb, V steel) are 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 there has been a need for the development of materials that can cope with further increases in steam temperature.

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. The present inventors previously disclosed in Japanese Patent Application No. 60-293092,
We proposed a steel with improved high-temperature strength and further improved weldability. Subsequently, as a result of further detailed studies, we discovered that an appropriate combination of various elements is effective in improving high-temperature strength, and we have developed a heat-resistant steel with excellent high-temperature strength that has improved oxidation resistance at high temperatures. proposed.

The present invention was completed after continuing studies to improve the high-temperature strength of high-Cr ferrite steel, and finding that the combined addition of Nb, Ti, B, and W among various components had a significant effect. It is something.

[Means for Solving the Problems] The ferritic heat-resistant steel having excellent high-temperature strength according to the first aspect of the present invention has a weight percentage of C: 0.03 to 0.20%, Si: 1.0% or less, Mn: 0.1 to 1.5%, 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% Ti: 0.01-0.40%, W: 0.05-2.50% N: 0.005-0.080 %, B: 0.0008 to 0.0100%, with the remainder consisting of Fe and inevitable impurities.

Further, the ferritic heat-resistant steel having excellent high-temperature strength according to the second aspect of the present invention has, in weight percent, C: 0.03 to 0.20%, Si: 1.0% or less, Mn: 0.1 to 1.5%, Ni: 0.1 to 1.0% Cr: 7.0-13.0%, Mo: 0.4-2.5% Nb: 0.02-0.15%, V: 0.02-0.35% Ti: 0.01-0.40%, W: 0.05-2.50% N: 0.005-0.080%, B: 0.0008-0.0100% In addition, it contains Cu: 0.3% or less, with the balance consisting of Fe and inevitable impurities.

[Function] In order to achieve the present invention, as a result of detailed studies on the effects of the addition of various elements on high-temperature strength, it was found that in high Cr steel containing appropriate amounts of Mo, Nb, and V, additionally appropriate amounts of W, Ti, It has been found that high temperature strength, particularly high temperature creep rupture strength, is significantly improved by adding B. Both Mo and W 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 amounts of Mo and W added are small, the strength improvement effect will be diminished. Based on this idea, we investigated the creep rupture strength at 650°C for 5000 hours across many combinations of Mo and W addition amounts, and found that heat-resistant steel with excellent creep rupture strength was obtained when an appropriate combination of Mo and W amounts was used. It was developed and presented in Japanese Patent Application No. 60-293092.

The present invention is a continuation of that, and includes Ti, B,
Creep strength is significantly improved by adding W in combination. Figure 1 shows 0.09%C-91%Cr-
1.1%Mo-0.11%Nb-0.21%V-0.11%Ti-0.88
%W-B in steel containing 0.03%N (S steel)
This shows the effect of As shown in the figure,
In the case of S steel, it is a steel containing Ti and W, and by adding B in combination to this steel at a rate of 0.0008% or more, the rupture time when a stress of 10Kg/ ^mm2 is applied at 650℃ is significantly improved. It is shown that on the other hand,
It can be seen that in the same type of steel that does not contain Ti (T steel) and the same type of steel that does not contain W (U steel), the creep strength is not improved as much as in S steel even if B is added.

Next, Figure 2 shows 0.10%C-8.9%Cr-1.2%Mo-
0.12%Nb-0.20%V-0.85%W-0.03%N-0.003
% B-containing steel (X steel). As shown in the figure, in the case of steel X, it is a steel containing W and B, and Ti is added to this steel.
By adding 0.01% or more in combination, at 650℃
It is shown that the rupture time is significantly improved when a stress of 10 Kg/mm ² is applied. On the other hand, in the same type of steel that does not contain B (Y steel) and the same type of steel that does not contain W (Z steel), even if Ti is added, only a significantly smaller effect can be obtained compared to X steel. In this way, an appropriate amount of Nb,
It has been found that in a high Cr-Mo steel containing V, N, etc., the creep rupture strength is significantly improved by further adding appropriate amounts of Ti, B, and W in combination. The present invention was completed based on the knowledge of the synergistic effect of Ti, B, and W explained above.

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

C is an inexpensive element that contributes to improving creep rupture strength through the formation of low-temperature transformation products and the precipitation of carbides. If it exceeds 0.20%, the hardenability increases significantly,
Although the strength increases, weldability and workability deteriorate, so the C content was 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.
It was set at 0.03% or more.

Si is added as a deoxidizing agent, but if used in large amounts, the toughness of the steel will deteriorate, so the upper limit was set at 1.00%.
In order to improve long-term strength and toughness at high temperatures, it is better to lower the Si content, so no lower limit is specified.

Mn is a useful element as a deoxidizing and desulfurizing agent, and for obtaining a suitable structure with improved strength and hot workability, but if it is less than 0.1%, it has no useful effect.
If it exceeds 1.5%, hardenability becomes high and strength increases, but it causes deterioration in workability such as bending and toughness, so it was set at 0.1 to 1.5%.

Ni controls the amount of delta ferrite to an appropriate value,
It is added to maintain and improve high-temperature strength, but if it is less than 0.10%, it has no effect, and if it exceeds 1.0%, the hot deformation resistance increases, making it unfavorable for hot processing. However, since no further toughness improvement effect was observed, and Ni 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.High-temperature long-term strength above 650℃ is high when Cr is 7.0 to 13.0%, and Cr is 13.0%.
If the amount is higher, delta ferrite will increase and the high temperature long-term strength will decrease. 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.
High-temperature long-term strength decreases, and if it is less than 7.0%, high-temperature oxidation resistance decreases. Therefore, the Cr component range was set to 7.0 to 13.0%.

Mo and W are indispensable elements for heat-resistant steel because they significantly increase high-temperature, long-term strength. Both elements dissolve in solid solution in steel and strengthen it, as well as precipitate carbides and improve creep strength.
If the amount of W is less than %, this effect will not be obtained, and the high temperature strength will not be sufficiently improved in the range below the straight line BC shown in FIG. On the other hand, Mo exceeding 2.5%, 2.5%
In a range where W exceeds 100% or above the straight line AD, the amount of delta ferrite increases, lowering the high-temperature strength, and since Mo and W are expensive elements, the cost increases, which is also unfavorable from an economic point of view. Also Mo
Since the creep strength at temperatures exceeding 600° C. is insufficient when added alone, it is necessary to add at least 0.05% of W.

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 the solubility product of V carbide or V carbonitride, 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 aggregate, It tends to coarsen and it is difficult to maintain high temperature strength for a long time. Nb, V to improve long-term strength
The combined addition of Nb is effective, and Nb precipitated during the manufacturing process is
During use at high temperatures (C, N), carbides of M ₂₃ C ₆ and M ₆ C precipitate, and V is in a solid solution state in addition to _the carbides of _{V 4} C ₃ . ₆ C and suppresses the coarsening of these carbides. Precipitates of Nb and V that are finely precipitated by the combined addition of Nb and V, as well as M ₂₃ C ₆ , M ₆ C, and intrinsic V that are finely precipitated after a long period of time improve the high-temperature long-term strength. Therefore, even if V is used alone, fine carbides M ₂₃ C ₆ and M ₆ C cannot be obtained, and long-term strength cannot be improved. Nb,
If V is less than 0.02%, the above effects are insufficient. Also, if too much Nb or V is present, the carbides will become coarser and the creep rupture strength will be reduced, as well as notch toughness and weldability.
0.15% or less, and V 0.35% or less.

Ti is based on the knowledge that high-temperature creep rupture strength is significantly increased by adding Ti in combination with appropriate amounts of W and B to high Cr steel containing appropriate amounts of Nb and V. This effect cannot be obtained with Ti alone, but is noticeable when Ti, W, and B are added in combination, and in order to obtain this effect, it is necessary to add 0.01% or more. On the other hand, if it is added in excess of 0.4%, the effect will be saturated and the carbides will tend to coarsen, so the content must be 0.4%.

N improves high-temperature long-term strength due to the formation of nitrides or carbonitrides and the residual inherent N.
If it is less than 0.005%, it has no effect, and if it exceeds 0.08%, blowholes are formed during welding and weldability is significantly deteriorated, so the N content is set to 0.05% to 0.080%.

B is based on the knowledge that high-temperature creep strength is significantly increased by adding B in combination with appropriate amounts of W and Ti to high Cr steel containing appropriate amounts of Nb and V. This effect cannot be obtained with B alone, but is noticeable when B, W, and Ti are added in combination. To obtain this effect, B must be added in an amount of 0.0008% or more. On the other hand, if added in excess of 0.01%, the effect will be saturated and hot workability will decrease, so the content must be 0.01% or less.

When added to high Cr steel, Cu gives viscosity to the Cr-containing oxide film, improves the adhesion of the film, and helps the oxidation resistance of the Cr steel. 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%.

[Example] Table 1 shows the chemical composition of the steel of the present invention, and Table 2 shows the chemical composition of the comparative steel. A steel ingot with these chemical components was manufactured in a high-frequency melting furnace, and the steel ingot was hot rolled into a 20 mm thick steel plate, normalized at 1050°C, and tempered at 760°C for 1 hour. Thereafter, a 6 mmφ round bar creep test piece and a weld hardness test piece were taken. The creep test is the rupture time at 650℃ and a stress of 10Kg/ ^mm2 , and the weldability test is the thermal effect after one-layer overlay welding using a 18Cr-8Ni stainless steel welding rod, followed by post-heat treatment at 750℃ for 1 hour. The maximum hardness of the part was investigated.

The steel types used in the present invention include C, Si, Ni,
Steel types with varying Cr, Mo, Nb, V, W, Ti, B, and Cu are shown. Among the comparative steels, steel is STBA26,
B steel is a conventional steel with STBA27, C steel is a steel with Nb and V added, D steel is a steel with Nb, V, and W added, E steel is a steel with Ti added in addition to Nb, V, and W. , F steel is steel with B added in addition to Nb, V, and W;
It is a steel to which Ti and B are added in addition to Nb and V, and the steel is an ASTM T91 (Super9Cr) steel.

The test results of the present invention are as shown in the right column of Table 1.
Has excellent high temperature strength and creep rupture time of 9000
It's been over an hour. The maximum hardness (Hv) did not increase significantly despite the excellent maximum strength, and all were below 300.

On the other hand, as shown in the right column of Table 2, comparative steels A and B do not contain Nb, V, W, Ti, or B, and their creep rupture times are extremely short. Although the creep rupture time of steel C and D is improved by the addition of Nb, V and W, the creep rupture time is shorter than that of the steel of the present invention. Ho,
Although the creep rupture time of F steel is improved by adding Ti and B in addition to Nb, V, and W, it is inferior to the steel of the present invention. Since steel is not added with W, its rupture time is inferior. Note that this is the conventional standard steel.
Super9Cr steel has significantly inferior creep properties compared to the steel of the present invention.

As mentioned above, the steel of the present invention has superior pipe strength at high temperatures than the currently used Cr-Mo steel, and has long-term strength equal to or higher than SUS304 in the temperature range of 600 to 650°C. Moreover, considering weldability, the maximum hardness (Hv
(10)) is 300 or less, which is a range that can be used without any problem in practical use. Therefore, expensive
It is possible to use the steel as an alternative to 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 higher strength (especially creep rupture strength) at high temperatures, particularly at temperatures above 600°C, than conventional ferritic heat-resistant steels. It is a steel with significantly improved weldability and workability.For example, it can be used as a material for ultra-supercritical pressure boilers used in high-temperature, high-pressure environments, steam generator tubes for FBR, superheater tubes, etc. This can contribute to thinner walls and longer lifespans.

[Brief explanation of drawings]

Figure 1 is a diagram showing the influence of B on creep rupture time, Figure 2 is a diagram showing the influence of Ti on creep rupture time, and Figure 3 is a diagram showing the relationship between Mo content, W content, and creep rupture strength. FIG.

【table】

【table】

【table】

【table】

Claims (1)

[Claims] 1. C: 0.03-0.20%, Si: 1.0% or less Mn: 0.1-1.5%, Ni: 0.1-1.0% Cr: 7.0-13.0%, Mo: 0.4-2.5% Nb: Contains 0.02 to 0.15%, V: 0.02 to 0.35%, Ti: 0.01 to 0.40%, W: 0.05 to 2.50%, N: 0.005 to 0.080%, B: 0.0008 to 0.0100%, with the remainder consisting of Fe and inevitable impurities. A ferritic heat-resistant steel with excellent high-temperature strength. 2 Weight% C: 0.03-0.20%, Si: 1.0% or less Mn: 0.1-1.5%, 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% Ti: 0.01~0.40%, W: 0.05~2.50%, N: 0.005~0.080%, B: 0.0008~0.0100%, and contains Cu: 0.3% or less, with the balance containing Fe and unavoidable impurities. A ferritic heat-resistant steel with excellent high-temperature strength.