JPH05311343A - Ferritic heat resistant steel having high creep strength - Google Patents

Abstract

PURPOSE:To increase the high temp. strength of steel by incorporating specified amounts of C, Si, Mn, Cr, W, V, Nb, Al, N, B, Ni, Co, P, S and O therein. CONSTITUTION:The objective steel contains, by weight, 0.01 to 0.15% C, 0.01 to 0.8% Si, 0.05 to 1.5% Mn, 8 to 13% Cr, 0.05 to 4% W, 0.05 to 0.5% V, 0.02 to 0.15% Nb, 0.002 to 0.05% Al and 0.01 to 0.11% Ni, one or two kinds of 0.01 to 3% Ni and 0.01 to 5% Co and also, <=0.03% P, <=0.01% S and <=0.015% O and the balance substantial Fe as well as satisfying the relational inequality of 1.5W-4Ni-2Co<=40C+30N. This steel is the one in which the increase in high temp. strength corresponding to the conversion of the temp. and pressure of apparatus into high ones is attained compared with the conventional ferritic heat resistant steel, and it is excellent in toughness as well.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic heat-resistant steel, and more particularly to a ferritic Cr-containing steel for boiler steel pipe having excellent creep rupture properties and toughness at high temperatures.

[0002]

2. Description of the Related Art In recent years, in thermal power generation, high temperature and high pressure steam conditions have been promoted from the viewpoint of improving thermal efficiency, and a plan to raise the current supercritical pressure condition to an ultra supercritical pressure condition through an intermediate step has been promoted. .. Due to such trends in power generation conditions, 2 1 / 4C, which is currently used from the viewpoint of oxidation resistance and high temperature strength, in selecting materials for boiler tubes, etc.
It is difficult to apply the r-1Mo steel. On the other hand, application of austenitic heat resistant steel is considered, but there is a problem such as cost increase. Therefore, it is desired to develop a high-strength, high-toughness ferritic heat-resisting steel located between the two.

In view of such circumstances, new steel grades having a creep rupture strength significantly higher than that of conventional materials have been developed and proposed. So far, 9Cr-1Mo steel and 9Cr-
Although high Cr ferritic heat resistant steels such as 2Mo steel have been proposed, these are all difficult to apply from the viewpoint of creep rupture strength under the above super-supercritical pressure steam conditions. Steels having improved required characteristics have been developed, and it is known that the relationship between (Mo + W) and the amount of Nb should be determined to improve the creep characteristics and toughness.
It is disclosed in Japanese Patent No. 1139, Japanese Patent Laid-Open No. 62-297435 and Japanese Patent Laid-Open No. 62-297436. Further, it is disclosed in JP-A-63-89644 that the addition of W and Nb in the optimum range is effective for improving the creep strength.

For these steels, W is added to the conventional heat-resistant steel,
Although it is a steel whose creep strength is dramatically increased by solid solution strengthening and precipitation strengthening, it lacked consideration for toughness.

[0005]

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to provide a ferritic heat resistant steel having high strength and high toughness which can be used in an ultra-supercritical pressure boiler. ..

[0006]

In order to achieve the above-mentioned object, the present invention optimizes the alloy components, optimizes the amount of W added, and at the same time actively uses Ni and Co. It is intended to provide a ferritic heat resistant steel having excellent high temperature strength and toughness. That is, the gist of the present invention is as follows.

(1) C: 0.01 to 0.15%, Si: 0.01 to 0.80%, Mn: 0.05 to 1.50%, Cr: 8.00 to 13. 00%, W: 0.05-4.00%, V: 0.05-0.50%, Nb: 0.02-0.15%, Al: 0.002-0.050%, N: 0. 0.10% to 0.10%, Ni: 0.01 to 3.00%, Co: 0.01 to 5.00%, and 1 or 2 types, P: 0.030% or less , S: 0.010% or less, O: 0.015% or less, the balance being Fe and inevitable impurities,
Further, a ferritic heat-resistant steel excellent in high-temperature strength and toughness, characterized in that W, Ni, Co, C and N in the above component ranges satisfy the relational expression of 1.5W-4Ni-2Co≤40C + 30N.

(2) C: 0.01 to 0.15%, Si: 0.01 to 0.80%, Mn: 0.05 to 1.50%, Cr: 8.00 to 13. 00%, W: 0.05-4.00%, V: 0.05-0.50%, Nb: 0.02-0.15%, Al: 0.002-0.050%, N: 0. 0.010 to 0.110%, B: 0.001 to 0.030%, Ni: 0.01 to 3.00%, Co: 0.01 to 5.00%, or 2 Containing seeds, P: 0.030% or less, S: 0.010% or less, O: 0.015% or less, the balance being Fe and unavoidable impurities,
Further, a ferritic heat-resistant steel excellent in high-temperature strength and toughness, characterized in that W, Ni, Co, C and N in the above component ranges satisfy the relational expression of 1.5W-4Ni-2Co≤40C + 30N.

[0009]

The reason for limiting the content of each component of the present invention will be described below. C mainly precipitates as MC (M is an alloying element, the same applies hereinafter) and M ₂₃ C ₆ type carbides, and has a great influence on strength and toughness. If it is less than 0.01%, the amount of precipitation is small and it is insufficient for precipitation strengthening.
If the content exceeds the above range, the toughness is lowered, and the coagulation and coarsening of carbides are promoted, so that the creep rupture strength at high temperature and long time is lowered, so the content is limited to 0.01 to 0.15%.

Si is added for the purpose of deoxidizing effect, ensuring strength and resistance to oxidation, but is an element which adversely affects toughness. Therefore, the lower limit is 0.01% from the viewpoint of deoxidation, strength, and oxidation resistance, and the upper limit is 0.80% from the viewpoint of toughness. Mn is an element necessary not only for deoxidizing but also for improving strength, and it is necessary to add at least 0.05% or more.
However, excessive addition lowers the high temperature strength and toughness, so the upper limit was made 1.50%.

Cr is an essential element for ensuring high temperature oxidation resistance, has an effect of precipitating M ₂₃ C ₆ type carbide in the matrix, and enhances high temperature strength. 8.0
If it is less than 0%, the oxidation resistance at high temperature becomes insufficient and the high temperature strength also decreases. On the other hand, if it exceeds 13.00%, it becomes difficult to suppress δ-ferrite and strength and toughness are deteriorated.
The amount of r is limited to 8.00 to 13.00%.

[0012] W contributes to solid solution strengthening and fine precipitation of M ₂₃ C ₆ , and at the same time suppresses agglomeration and coarsening of carbides and significantly improves creep rupture strength at high temperature and long time. At least 0.
05% or more is required, but if it exceeds 4.00%, δ
Ferrite and coarse Laves phase are easily generated,
0.05 to 4.00 to reduce high temperature strength and toughness
%.

V precipitates fine carbonitrides as a precipitation strengthening element and enhances high temperature strength. If it is less than 0.05%, the effect is insufficient, and if it exceeds 0.50%, 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 high temperature strength is increased. Of 0.05 to 0.
Limited to 50%. Nb is precipitated as a carbonitride to increase the high temperature strength and to improve the toughness due to the effect of the refinement of the structure, so the minimum amount is 0.02%. However, 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. Therefore, the content is limited to 0.02 to 0.15%.

Al is used as a deoxidizer, and its content has a great influence on the crystal grain size and mechanical properties. 0.
If it is less than 002%, it is insufficient as a deoxidizer,
%, The creep rupture strength decreases, so 0.002
Limited to ~ 0.050%. N is one of the important elements that precipitates nitrides or carbonitrides and enhances high temperature strength. The effect is exhibited by adding 0.010% or more,
If it exceeds 0.110%, not only does the nitride become coarse and the toughness deteriorates, but also it becomes difficult to manufacture,
It is limited to 0.010 to 0.110%.

Ni is an austenite forming element, and δ
It has the effect of suppressing the formation of ferrite and is also effective in improving the toughness, and 0.01% is the minimum. But,
If it exceeds 3.00%, the coarsening of precipitates will occur, and the creep rupture strength on the long-term side will decrease, so the upper limit is 3.00%.
And The positive use of Co is one of the major features of the present invention. Co is an austenite-forming element, which suppresses the formation of δ-ferrite, stabilizes precipitates, and enhances high-temperature strength. If it is less than 0.01%, the effect is small, and if it exceeds 5.00%, the cost is high and embrittlement easily occurs, so the content is limited to 0.01 to 5.00%.

[0016] B brings about grain boundary strengthening and precipitation strengthening as M ₂₃ (C, B) ₆ etc., so that high temperature strength is improved.
If less than 0.001%, the effect is insufficient, and 0.0
If it exceeds 30%, a coarse B-containing phase is generated and embrittlement occurs, so the content is limited to 0.001 to 0.030%. P adversely affects temper embrittlement and reheat cracking susceptibility, so the upper limit was made 0.030%.

Since S causes deterioration of toughness, anisotropy and increase in reheat cracking susceptibility, the upper limit was made 0.010%. Since O causes the formation of oxides which adversely affects toughness, the upper limit was made 0.015%. Further, according to the present invention, in order to improve creep rupture strength and toughness, δ
An appropriate balance of W, Ni, and Co contents is specified to obtain a martensite single-phase structure in which ferrite does not exist.
The inventors of the present invention can suppress the formation of δ ferrite and improve creep strength and toughness if W, Ni, Co, C and N within the above component ranges satisfy 1.5W-4Ni-2Co ≦ 40C + 30N. Found.

Since the present invention provides a heat resistant steel having high creep rupture strength, various manufacturing methods and heat treatments can be adopted for manufacturing the steel of the present invention depending on the purpose of use. However, the effects of the present invention are not hindered by adopting these methods. The steel of the present invention can be provided not only as a steel pipe but also in the form of a thick plate and a thin plate, and can be used in the form of various heat-resistant materials as hot-rolled or using a plate that has undergone the necessary heat treatment. However, the effect of the present invention is not affected at all.

The above-mentioned steel pipes, plates, and heat-resistant members of various shapes can be subjected to various heat treatments depending on the purpose and application, and are important for sufficiently exerting the effects of the present invention. Usually, the product is often subjected to a normalizing + tempering step, but in addition to this, quenching, tempering, and normalizing steps can be performed individually or in combination, and it is also useful. It is also possible to apply each of the above steps by repeating the steps a plurality of times within a range necessary for sufficiently expressing the material properties, and the effect of the present invention is not affected at all.

The above steps may be appropriately selected and applied to the steel manufacturing process of the present invention.

[0021]

EXAMPLES Steels of the present invention (N having the chemical composition shown in Table 1)
o. 1 to 7) and comparative steels (No. 8 to 10) are melted in a vacuum induction melting furnace, cast into ingots of 20 kg each, and hot rolled to form a plate having a thickness of 15 mm, and then 1080 ° C. × 60 minutes After normalizing and tempering at 780 ℃ for 60 minutes,
A creep rupture test under two conditions of 200 MPa at 00 ° C. and 150 MPa at 650 ° C. and a Charpy impact test at 0 ° C. after aging at 600 ° C. for 3000 hours were performed. The results are shown in Table 2. As is clear from Table 2, the steel of the present invention has a creep rupture time of 2 to 3 times or more that of the comparative steel under any condition, and the Charpy absorbed energy after aging at 600 ° C. for 3000 hours is equal to that of the comparative steel. It is considered that the above can be used at a higher temperature than conventional steel.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

INDUSTRIAL APPLICABILITY As described above, the steel of the present invention is a steel that achieves an increase in high-temperature strength that can withstand high temperatures and high pressures of equipment, as compared with conventional ferritic heat-resistant steel, and has excellent practical properties such as toughness Therefore, it can be applied to many fields such as ultra-supercritical thermal power generation, nuclear power generation, etc., and it has a great contribution to the industrial world.

Claims (2)

[Claims] 1. By weight%, C: 0.01 to 0.15%, Si: 0.01 to 0.80%, Mn: 0.05 to 1.50%, Cr: 8.00 to 13.00. %, W: 0.05 to 4.00%, V: 0.05 to 0.50%, Nb: 0.02 to 0.15%, Al: 0.002 to 0.050%, N: 0. 010 to 0.110%, Ni: 0.01 to 3.00%, Co: 0.01 to 5.00%, one or two kinds, P: 0.030% or less, S: 0.010% or less, O: 0.015% or less, the balance being Fe and inevitable impurities, and W, Ni in the above component range,
Co, C, N satisfy | fills the relational expression of 1.5W-4Ni-2Co <= 40C + 30N, The ferritic heat-resistant steel excellent in high temperature strength and toughness characterized by the above-mentioned. 2. C: 0.01 to 0.15% by weight, Si: 0.01 to 0.80%, Mn: 0.05 to 1.50%, Cr: 8.00 to 13.00. %, W: 0.05 to 4.00%, V: 0.05 to 0.50%, Nb: 0.02 to 0.15%, Al: 0.002 to 0.050%, N: 0. 010 to 0.110%, B: 0.001 to 0.030%, Ni: 0.01 to 3.00%, Co: 0.01 to 5.00%, one or two kinds P: 0.030% or less, S: 0.010% or less, O: 0.015% or less, the balance being Fe and unavoidable impurities, and W, Ni in the above component range,
Co, C, N satisfy | fills the relational expression of 1.5W-4Ni-2Co <= 40C + 30N, The ferritic heat-resistant steel excellent in high temperature strength and toughness characterized by the above-mentioned.