JPH0665689A - Ferritic heat resistant steel minimal in softening in weld heat-affected zone - Google Patents

Abstract

PURPOSE:To produce a Cr heat resistant steel reduced in the softening of creep strength in a weld heat-affected zone based on the creep strength of a base material by controlling carbonitride in a high Cr ferritic stainless steel, dispersing Ta2O5, and adding specific amounts of Mo and W. CONSTITUTION:This steel is a high Cr ferritic heat resistant steel which has a composition containing, by weight, 5-13% Cr, 0.05-1% Si, 0.1-1% Mn, 0.5-3% Mo and/or 1-6% W, and <=0.05% (C+N), containing Ta2O5 grains of <=1mum average grain size by 0.25-2.5%, having 0.20-2.2% total Ta content including Ta2O5, and having the balance Fe. The precipitation of carbonitride is actively inhibited by controlling C and N contents and creep strength is improved by means of dispersion strengthening by Ta2O5 grains and solid solution strengthening of Mo and W, by which the high Cr ferritic heat resistant steel extremely reduced in the softening ratio of creep strength in a weld heat- affected zone based on creep strength of a base material can be produced.

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 that is less likely to soften during use at high temperatures, and more particularly to a Cr-based heat-resistant steel in which the creep strength softening of the weld heat affected zone with respect to the base material creep strength is extremely small. Is.

[0002]

2. Description of the Related Art Welding structural members for boilers and steam generators that are used for a long time at high temperatures, are inexpensive, have no fear of stress corrosion cracking, and have a weld heat affected zone (hereinafter referred to as HAZ) for creep strength of a base metal. There is a strong demand for ferritic heat resistant steels with low creep strength and softening ratio. As an example of steel for this type of application, American ASTM standard A213 T
So-called 9% Cr steel represented by 91 steel is known. Further, Japanese Patent Laid-Open No. 61-69948 discloses a steel in which Mo and W are added in a good balance to 9Cr steel to improve the strength of the base material. A high Cr steel having improved creep strength by oxide dispersion is disclosed in Japanese Patent Application No. 2-105217.

[0003]

However, in the conventional 9% Cr steel, HA caused by coarsening of a trace amount of carbonitride is generated.
It has a drawback that the creep strength of Z is significantly lowered.
In order to suppress the coarsening of carbonitrides in HAZ, C, N
Alternatively, it is necessary to reduce the Cr, Mo, Nb, and V contents, but the strength of the base material is also reduced by this method alone. Therefore, it is possible to supplement the strengths of the base material and HAZ by strengthening the oxide dispersion. However, the conventional oxide-dispersed steel is expensive because it is manufactured by powder metallurgy, and its toughness is not sufficient. Therefore, it is necessary to secure the strength by using an oxide dispersed in steel by the melting method.

[0004]

The present invention has such a circumstance as described above.
It was invented in consideration of the above, and is a C in high Cr ferritic steel.
Actively precipitate carbonitrides by limiting the amount of N and N added
Suppressed. Even if added to molten steel instead, it will be dispersed uniformly and
Ta stably present in HAZ₂O_FiveDisperse
By strengthening the dispersion and solid solution of Mo and W
Improves creep strength. As a result, H for the base metal
Ferrite-based resistance with extremely low creep strength softening ratio of AZ
It succeeded in obtaining hot steel. Summary of the invention
Is a weight ratio of Cr: 5 to 13%, Si: 0.05 to 1
%, Mn: 0.1 to 1%, added in the range of Mo, W
About Mo: 0.5 to 3%, W: 1 to 6%, in the range
And one or two of them, and C and N are limited to C + N ≦ 0.05%, and the average particle diameter is 1 μm or less.
₂O_Five) Particle is Ta₂O _Five: 0.25 to 2.5% range
And Ta₂O_FiveIncluding Ta that exists as
Total Ta amount of 0.20 to 2.2%, balance Fe and unavoidable
For base material creep strength, which is characterized by being composed of impurities
On the other hand, HAZ has less creep strength softening ferrite
It relates to heat-resistant steel.

[0005]

The function of the present invention will be described below. Cr is very important to secure high temperature corrosion resistance, and at least 5% is necessary, but if it exceeds 13%, solution treatment in the austenite region cannot be performed, a uniform structure cannot be obtained, and two-phase separation occurs. In addition, spinodal transformation is caused and the creep strength is lowered. Therefore, the Cr content is 5
13%. Si is important as a deoxidizer and has a minimum of 0.1.
Requires 05%. However, if added in a large amount, it has an adverse effect on the creep strength, and if added in an amount of 1% or more, the creep strength is significantly reduced. Therefore, the amount of Si is set to 0.05% to 1%. Mn is important for the deoxidizing effect and it is necessary to secure at least 0.05%, but if it exceeds 1%, the creep strength decreases. Therefore, the amount of Mn is set to 0.05 to 1%.

When Mo is less than 0.5%, the solid solution strengthening is insufficient, so Mo is required to be at least 0.5%. On the other hand, 3%
If it exceeds, the two-phase separation and spinodal decomposition will be caused and the creep strength will be reduced. Therefore, the Mo addition range is 1 to 3%. When W is less than 1%, solid solution strengthening is insufficient, so at least 1% is required. On the other hand, 6
If it exceeds%, two-phase separation and spinodal decomposition are caused, and the creep strength is reduced. Therefore, the addition range of W is set to 2 to 6%. Mo and W partially precipitate as an intermetallic compound during creep deformation, and the creep strength is also improved by increasing the precipitation strength. On the other hand, C
Since N and N consume Fe, Cr, Mo, W, etc. and precipitate as carbonitrides, their addition must be limited. In particular, when the sum of the amounts of C and N added exceeds 0.05%, coarse carbonitrides are formed during creep deformation of the HAZ. As a result, the HAZ creep strength is significantly reduced with respect to the base material. Therefore, the addition range of C and N is C + N ≦
0.05%.

Ta ₂ O ₅ acts as a dispersion strengthening source and significantly improves creep rupture strength. The effect is 0.25%
Above, it is remarkably saturated at about 2.5%, and the addition beyond that causes a decrease in strength due to a decrease in ductility. Further, Ta ₂ O ₅ is relatively expensive and it is not preferable to add a large amount. Therefore, it is limited to 0.25% to 2.5%. Ta ₂
The average particle size of O ₅ is set to 1 μm or less because the average particle size is 1
When it exceeds μm, Ta ₂ O is added even if the added amount is within the proper range.
_{This is because 5} is likely to be the starting point of brittle fracture and the toughness is significantly reduced. The smaller the thickness is 1 μm or less, the higher the creep rupture strength. Note that Ta substantially exists as Ta ₂ O ₅ and does not positively add metal Ta. The content was limited to 0.20 to 2.2% in accordance with the upper limit and the lower limit of Ta ₂ O ₅ .

[0008]

[Examples] Sample steels with composition ranges shown in Table 1 (sheet thickness 20 m
m) was used to prepare a latent welded joint (heat input 20 kJ / cm), and then a creep test piece was prepared so that the weld melt line portion crossed the central portion of the test piece. Creep test pieces were also prepared from the test steel base material that did not include joints. Using these, a creep rupture test was performed under the conditions of 650 ° C. and 120 MPa. Of the steels shown in Table 1, No. 1-No. No. 5 is a steel in the composition range of the present invention, and No. 6-No. 13 is a comparative steel outside the scope of the present invention. Comparative steel No. In No. 6, since the amount of Cr is less than the components of the present invention, the hardenability is low, resulting in HAZ,
Both base materials have low creep strength. Comparative steel No. 7 is Mo,
Since both W are below the lower limit, the HAZ creep strength is low. Comparative steel No. In No. 8, since Mo exceeds the upper limit of 3%, a large amount of intermetallic compounds are precipitated and the HAZ creep strength is extremely low. Comparative steel No. 9 has C + N of 0.05%
The creep strength of the HAZ is lowered because it exceeds the above.

Comparative steel No. No. 10 has an excessive amount of Si, so the creep ductility is lowered, and the HAZ and the base metal both have low creep strength. Comparative steel No. No. 11 does not have Ta ₂ O ₅ added, and therefore HAZ has a particularly low creep strength. Comparative steel No.
In No. 12, since the Ta ₂ O ₅ was added excessively, the creep ductility was lowered and both the HAZ and the base metal had low creep strength. Comparative steel No. In No. 13, the amount of Ta ₂ O ₅ added was within the limited range, but the average grain size was too large, so the creep ductility was lowered and the creep strength was low for both HAZ and the base metal. On the other hand, the steel No. 1-No. No. 5 has sufficient creep strength.

[0010]

[Table 1]

[0011]

[Table 2]

[0012]

As described above, according to the present invention, the amounts of Mo, W and C + N added to the Cr-based heat-resisting steel are optimized and Ta ₂ O
Dispersing ₅ provides heat resistant steel with less HAZ creep strength softening, and will play an extremely important role in the future industry.

Claims (1)

[Claims] 1. A weight ratio of Cr: 5 to 13%, Si: 0.05 to 1%, Mn: 0.1 to 1% is added, and Mo and W are added.
Mo: 0.5 to 3%, W: 1 to 6%, containing 1 or 2 types, and
C and N are limited to C + N ≦ 0.05%, and the average particle size is 1 μm or less.
Tantalum oxide (Ta₂O_Five) Particle is Ta₂O _Five: 0.2
5 to 2.5% in range, and Ta₂O_FiveAs
Total Ta amount including existing Ta: 0.20 to 2.2%, balance
Part Fe and unavoidable impurities,
Creep strength of weld heat affected zone with respect to base material creep strength
Ferrite heat resistant steel with little softening.