JPH07286246A - High strength ferritic heat resistant steel - Google Patents

Abstract

PURPOSE:To produce a ferritic heat resistant steel having high strength and high toughness for boiler steel tube used under ultrahigh critical pressure conditions. CONSTITUTION:A boiler material improved in toughness after aging and steam oxidation characteristic can be obtained by the proper addition of Si and Al to a ferritic heat resistant steel excellent in high temp. strength, in which the additive quantities of Mo and W are properly regulated and Co and B are actively used. This boiler material has a chemical composition containing 0.21-0.50% Si, 0.002-0.050% Al, 0.02-<0.15% C, 0.05-1.50% Mn, 8.00-13.00% Cr, <=1.00% Ni, <0.50% Mo, 2.00-3.50% W, 0.10-0.30% V, 0.01-0.15% Nb, <=4.0% Co, and 0.01-0.10% N and satisfying Si+10Al<=0.80% and further containing, if necessary, 0.0010-0.0100% B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength ferritic heat-resistant steel having excellent high-temperature creep strength, sufficient toughness and oxidation resistance, and more particularly to steel for steel tubes for boilers and the like.

[0002]

^2. Description of the Related Art Recently, from the viewpoint of improving thermal efficiency, in the thermal power generation, high temperature and high pressure steam conditions have been promoted. For example, the steam conditions are the current 538 ° C./246 kgf / cm ²
To 593 ° C./316 kgf / cm ² , and even 649
A plan to raise the temperature to 352 kgf / cm ² is being promoted. Due to these trends, the currently used 2/4 Cr-Mo steel cannot be applied to the material selection of boiler tubes and the like from the viewpoint of oxidation resistance and high temperature strength. On the other hand, application of 18-8 austenitic heat resistant steel is considered, but there is a problem such as cost increase.
Therefore, it is desired to develop a ferritic heat-resisting steel having an oxidation resistance property and a high strength and a high toughness which are located between the two.

On the other hand, 9C has hitherto been used for such applications.
High-chromium ferritic heat resistant steels such as r-1Mo steel and 9Cr-2Mo steel have also been used, but none of them can be applied due to insufficient creep rupture strength under the above steam conditions. Incidentally, as other related arts, Japanese Patent Laid-Open Nos. 62-297435 and 62-297436 have already been disclosed.
And Japanese Patent Application Laid-Open No. 63-89644. Further, in Japanese Patent Laid-Open No. 4-371552, M
A technique for improving creep strength by the combined addition of o, W, Nb, V and B has been proposed.

[0004]

SUMMARY OF THE INVENTION In order to meet the above demands, the present invention has high strength and high toughness and can be used as a steel material for steel pipes for ultra-supercritical boilers, and has oxidation resistance. The object is to provide a ferritic heat resistant steel having excellent characteristics.

[0005]

The present invention has been made in order to achieve the above-mentioned object, by optimizing the alloying components, positively utilizing W rather than Mo, and adding Co and the like. A high-strength ferritic heat-resistant steel with improved high-temperature strength is obtained by appropriately adding Si and Al to obtain a steel having excellent toughness and oxidation resistance and maintaining high creep strength.

That is, the gist of the present invention is to
It is as follows. (1)% by weight, C: 0.02 to less than 0.15%, M
n: 0.05-1.50%, Cr: 8.00-13.0
0%, Ni: 1.00% or less, Mo: less than 0.50%,
W: 2.00 to 3.50%, V: 0.10 to 0.30
%, Nb: 0.01 to 0.15%, Co: 4.0% or less, N: 0.01 to 0.10%, and Si:
0.21-0.50%, Al: 0.002-0.05
%, And Si + 10Al 0.80% or less, the balance being Fe and unavoidable impurities, and the high temperature ferritic heat-resistant steel having excellent high temperature strength, sufficient toughness and oxidation resistance.

(2) B: 0.0010 to 0.0100%
A high-strength ferritic heat-resistant steel having the excellent high-temperature strength, sufficient toughness, and oxidation resistance as set forth in the preceding paragraph (1), characterized in that

[0008]

The reason for limiting each component of the present invention will be described below. C mainly precipitates as MC (M is an alloying element, the same applies below) and M ₂₃ C ₆ type carbides, and has a great influence on strength and toughness. When C is less than 0.02%, the amount of precipitation is small and it is insufficient for strengthening.
In the above case, the toughness is lowered, the coagulation and coarsening of carbides are promoted, and the creep rupture strength at the high temperature long time side is lowered, so the content is limited to less than 0.02 to 0.15%.

Mn has the effect of suppressing the formation of δ ferrite. Mn must be at least 0.05%, but 1.
If it exceeds 50%, the high temperature strength decreases, so the upper limit was made 1.50%. Cr is an essential element for ensuring high temperature oxidation resistance, and also has an effect of precipitating M ₂₃ C ₆ type carbide. If Cr is less than 8.00%, the oxidation resistance at high temperature is insufficient and the high temperature strength is also reduced. On the other hand, Cr is 1
If it exceeds 3.00%, it is difficult to suppress δ ferrite and the strength and toughness are impaired. Therefore, the Cr range is 8.00 to 13.
Limited to 00%.

Ni is an austenite forming element, and δ
It has the effect of suppressing ferrite and also has a beneficial effect on toughness. However, if Ni exceeds 1.00%, coarsening of precipitates will occur, and the creep rupture strength on the long-term side will decrease, so 1.00% was made the upper limit. Mo brings about solid solution strengthening and at the same time stabilizes M ₂₃ C ₆ and improves high temperature strength. Mo is 0.50 in relation to the added amount of W.
% Or more accelerates the formation of δ ferrite, and at the same time M
_The amount of Mo was set to less than 0.50% because it promotes precipitation of ₆ C and the Laves phase and coarsening of aggregation.

W has the effects of solid solution strengthening and fine precipitation of M ₂₃ C ₆ , and at the same time suppresses coagulation and coarsening of carbides, and is extremely effective in improving creep rupture strength at high temperature and long time. W must be at least 2.00%, but if it exceeds 3.50%, δ ferrite and a coarse Laves phase are likely to be formed, and high temperature strength and toughness are reduced, so 2.
The range is from 00 to 3.50%.

V is deposited as a fine carbonitride and serves to enhance the high temperature strength. If V is less than 0.10%, the effect is small, and if it exceeds 0.30%, not only V (C, N) is coarsened, but also the amount of C that can be precipitated as M ₂₃ C ₆ is decreased, and conversely at high temperature. 0.10 to 0.3 because it reduces strength
Limit to 0% range. Nb is deposited as carbonitride,
It is an effective element for increasing strength. Nb is at least 0.0
1% is required, but if it is added in excess of 0.15%, it cannot be completely dissolved in the matrix at the normalizing temperature, and a sufficient strengthening effect cannot be obtained. Limited to

N is one of the important elements that combine with Nb, V, Cr and the like to precipitate nitrides or carbonitrides and enhance the high temperature strength. N is effective at a minimum of 0.01%, but if it exceeds 0.10%, not only does the nitride become coarse and the toughness deteriorates, but it also becomes difficult in manufacturing, so
It is limited to the range of 0.01 to 0.10%. Utilization of Co is one of the features of the present invention. Co is an austenite forming element and has an effect of suppressing the formation of δ ferrite which impairs mechanical properties. Further, since Co has a small effect of lowering the Ac ₁ transformation temperature of steel, it can be tempered at a high temperature for ensuring the toughness and ductility. On the other hand, if the amount of Co added exceeds 4.0%, it is disadvantageous in terms of cost.
It is limited to 4.0% or less.

B has the effect of strengthening the grain boundaries and precipitation strengthening as M ₂₃ (C, B) _6, etc., and improves the high temperature strength. B
Is less than 0.0010%, the effect is small, and 0.01
If it exceeds 00%, a coarse B-containing phase tends to occur,
In addition, embrittlement easily occurs, so 0.0010 to 0.
It is limited to the range of 0100%. Si is an element necessary for deoxidizing ferritic heat-resistant steel, and controlling the Si to a low level of 0.2% or less increases refining costs for ferritic heat-resistant steel, and the Si content should be increased above 0.2%. Since it became clear that the refining cost would be reduced, the minimum and maximum amounts of Si required from the viewpoint of the use performance of the ferritic heat-resistant steel were examined. When the effect of Si on creep strength, toughness, weldability, and steam oxidation resistance as a use performance of the boiler was investigated in detail, Si was 0.21%.
The use performance of the ferritic heat-resisting steel having the above components in the range of 0.50% or less is almost the same as the use performance of the ferritic heat-resisting steel having the component of Si in the range of 0.2% or less. It has been found that the necessary performance can be secured.

Al is used as a deoxidizing material, but the residual amount thereof has a great influence on the crystal grain size and mechanical properties. A
If 1 is less than 0.002%, it is insufficient for deoxidation, and if it exceeds 0.05%, the creep rupture strength decreases, so
It is limited to the range of 0.002 to 0.05%. Al is 0.
Even if the amount of Si is 0.5% or less and the amount of Si is 0.50% or less, the toughness after aging deteriorates when both are added in a large amount, so Si + 10Al was limited to 0.80% or less.

[0016]

EXAMPLES Steels of the present invention (N having the chemical composition shown in Table 1)
o. 8 to 11) and comparative steels (No. 1 to 7) were melted into ingots of 20 kg each in a vacuum induction melting furnace and hot rolled into a plate having a thickness of 15 mm, and then 1060 ° C. × 60 min
And normalizing at 780 ° C. × 60 min. The sample prepared in this manner was used at 600 ° C. and 25 kgf /
Creep rupture test was conducted under the condition of mm ²
After aging at 00 ° C. for 3000 hours, a Charpy impact test was performed at 20 ° C., and a steam oxidation test was performed at 650 ° C. for 500 hours. The results are shown in Table 2.

As is clear from Table 2, the steel of the present invention is 6
00 ° C, creep rupture time of 25 kgf / mm ² , 60
The balance between Charpy absorbed energy and steam oxidation property at 20 ° C. after aging at 0 ° C. for 3000 hours is better than that of the comparative steel.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

As described above, according to the present invention, it becomes possible to supply high-strength ferritic heat-resistant steel having excellent creep rupture strength and good toughness, and not only in the field of thermal power generation,
It is expected to be applied to many fields such as nuclear power generation and chemical industry, and it will make a great contribution to the industrial world.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Ogami 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd. Technology Development Division (72) Inventor Toshio Fujita 1-14-14, Mukooka, Bunkyo-ku, Tokyo

Claims (2)

[Claims] 1. By weight%, C: 0.02 to less than 0.15% Mn: 0.05 to 1.50% Cr: 8.00 to 13.00% Ni: 1.00% or less Mo: 0.0. Less than 50% W: 2.00 to 3.50% V: 0.10 to 0.30% Nb: 0.01 to 0.15% Co: 4.0% or less N: 0.01 to 0.10% And further contains Si: 0.21 to 0.50%, Al: 0.002 to 0.05%, and Si + 10Al: 0.80% or less, and the balance is excellent in high temperature strength consisting of Fe and unavoidable impurities. And high-strength ferritic heat resistant steel with sufficient toughness and oxidation resistance. 2. A high-strength ferritic heat-resistant steel having excellent high-temperature strength, sufficient toughness, and oxidation resistance according to claim 1, wherein B: 0.0010 to 0.0100% is added.