JPH0959746A - High chromium ferritic steel excellent in high temperature strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high Cr ferritic steel excellent in high temp. strength by preparing a high Cr ferritic steel having a specific composition in which respective contents of C, V, W, Nb, and N are specified. SOLUTION: A high Cr ferritic steel, having a composition consisting of, by weight, 0.03-0.12% C, 0.1-0.7% Si, 0.1-1% Mn, 0.002-0.025% P, 0.001-0.015% S, 8-13% Cr, 0.1-1% Ni, 0.1-3% Mo, 0.01-0.5% V, 0.1-3% W, 0.01-0.2% Nb, 0.1-3% Co, 0.1-1.5% Re, 0.005-0.05% Al, 0.0001-0.01% B, 0.005-0.07% N, 0.01-1% Cu, and the balance iron with inevitable impurities, is prepared. By this method, the high Cr ferritic steel, remarkably improved in creep rupture strength at a temp. as high as >= about 600 deg.C, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steam generator used under high temperature and high pressure, a heat exchanger such as a boiler, etc.
The present invention relates to a high Cr ferritic steel suitable as a member used in a high temperature environment of 0 ° C. or higher.

[0002]

2. Description of the Related Art Austenitic stainless steel, high Cr ferritic steel having a Cr content of 9 to 12%, and high temperature heat and pressure resistant members for boilers, chemical industries, nuclear power, and the like.
-Low Cr ferritic steel represented by 1 / 4Cr-1Mo steel, carbon steel, etc. These are the operating temperature of the target member,
It is selected according to pressure, usage environment, etc., and in consideration of economy. Compared with austenitic stainless steel, 9-12% Cr ferritic steel is inexpensive, has a small coefficient of thermal expansion, does not cause stress corrosion cracking, and has excellent thermal conductivity. Further, it is excellent in high temperature corrosion and stress corrosion as compared with low Cr ferritic steel, and has high high temperature strength. Therefore, it is attracting attention as a substitute material for austenitic stainless steel.

[0003]

Therefore, the present invention is 600
A new high-Cr ferritic steel that significantly improves high-temperature creep strength at temperatures above ℃ and has toughness, workability, and weldability equivalent to or better than existing low-alloy steels, and can replace austenitic stainless steels. Is to provide.

[0004]

The present invention is a high Cr ferritic steel containing a Re element, which has not been used as an additive element in the past, and a Co element having an effect of suppressing the formation of δ ferrite. % By weight, C: 0.03 to
0.12%, Si: 0.1 to 0.7%, Mn: 0.1
1%, P: 0.002-0.025%, S: 0.001
~ 0.015%, Cr: 8-13%, Ni: 0.1-1
%, Mo: 0.1 to 3%, V: 0.01 to 0.5%,
W: 0.1 to 3%, Nb: 0.01 to 0.2%, Co:
0.1-3%, Re: 0.1-1.5%, Al: 0.0
05-0.05%, B: 0.0001-0.01%,
A high Cr ferritic steel which contains N: 0.005 to 0.07% and Cu: 0.01 to 1% and whose balance is iron and unavoidable impurities and has dramatically improved high temperature strength. The function of each component and the reason for selecting the content will be described below. In the following description, all% mean% by weight.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION

C: C combines with Cr, Fe, W, V, and Nb to form a carbide, which contributes to high-temperature strength and, by itself, stabilizes the structure as an austenite stabilizing element. If it is less than 0.03%, the precipitation of carbides is insufficient, and the amount of δ-ferrite increases, resulting in insufficient strength and toughness.
On the other hand, if it exceeds 0.12%, carbides are excessively precipitated and the steel is significantly hardened, resulting in poor workability. That is, the appropriate content of C is 0.03 to 0.12%.

Si: Si is an element that acts as a deoxidizing agent and enhances steam oxidation resistance, but if it exceeds 0.7%, the toughness is significantly lowered and it is also harmful to the strength. In particular, since large tempered steel products promote temper embrittlement, the Si content is set to 0.1 to 0.7%.

Mn: Mn improves the hot workability of steel and is effective in stabilizing the structure, but if it is less than 0.1%, a sufficient effect cannot be obtained, and if it exceeds 1%, the steel is hardened. Not only does it impair the workability, but it also enhances the temper embrittlement susceptibility like Si. Therefore, the Mn content is 0.1 to 1%.

P, S: P and S are all elements that are harmful to toughness and workability. Even if the amount of S is extremely small, grain boundaries and Cr ₂ O ₃
The scale film becomes unstable and causes deterioration of strength, toughness, and workability. Therefore, it is preferable that the amount is as small as possible even within the above allowable range. As an unavoidable content, P is 0.002
.About.0.025% and S was 0.001 to 0.015%.

Cr: Cr is an essential element from the viewpoint of oxidation resistance and high temperature corrosion resistance of heat resistant steel, and if the content thereof is less than 8%, sufficient oxidation resistance and high temperature corrosion resistance cannot be obtained. on the other hand,
If added in excess of 13%, δ-ferrite is likely to be formed, resulting in deterioration of strength and toughness. Therefore, the content of Cr is 8
To 13%.

Ni: Ni is an austenite stabilizing element and contributes to the improvement of toughness, but if its content exceeds 1%, the high temperature creep strength is impaired. Also, in view of economy, large-scale addition is disadvantageous. Therefore, the Ni content is 0.
1 to 1%.

Mo: Mo is effective for improving the creep strength, but if it is less than 0.1%, a sufficient effect cannot be obtained and 3%
If it exceeds, not only the intermetallic compound precipitates at high temperature, the toughness decreases, but also the effect on the strength disappears. Therefore, it is set to 0.1 to 3%.

V: V combines with C and N to form fine precipitates such as V (C, N). This precipitate greatly contributes to the improvement of long-term creep strength at high temperature, but 0.01%
If it is less than 0.5%, a sufficient effect cannot be obtained, and if it exceeds 0.5%, the creep strength is rather deteriorated. Therefore, the appropriate content of V is 0.01 to 0.5%.

W: W is a solid solution strengthening and fine carbide precipitation strengthening element and is effective for improving creep strength.
If it is less than 3%, it is not effective, and if it exceeds 3%, the steel is hardened and the workability is impaired.

Nb: Nb is the same as V and is bonded to C and N to form N
It forms b (C, N) and contributes to the creep strength. Especially 6
At a relatively low temperature of 00 ° C. or lower, a remarkable strength improving effect is exhibited. If it is less than 0.01%, the above effects cannot be obtained, and if it exceeds 0.2%, undissolved NbC increases, and creep strength and toughness are impaired. Therefore, the Nb content is 0.01
To 0.2%.

Co: Co, like Ni, is an austenite stabilizing element and has the effect of suppressing the formation of δ-ferrite. Further, compared to Ni, there is less decrease in Ac ₁ temperature with respect to the added amount, and there is an advantage that the tempering temperature can be set higher. Therefore, the Co content is 0.1 to 3%.

Re: Re increases the creep strength in proportion to the amount added, but is 0.1 to 1.5% in consideration of economy.

Al: Al is essential as a deoxidizing element, and if the content is less than 0.005%, it has no effect.
If it exceeds 0.05%, the creep strength and workability are impaired, so the Al content is made 0.005 to 0.05%.

B: B has the effect of dispersing and stabilizing carbides by adding a very small amount. If it is less than 0.0001%, its effect is small, and if it exceeds 0.01%, the workability is impaired. Therefore, the addition of B makes the content 0.0001 to 0.0
Make it within the range of 1%.

N: N is necessary for forming carbonitrides with V and Nb, and if it is less than 0.005%, it has no effect. However, if it exceeds 0.07%, the structure becomes finer and the nitride coarsens, and the strength, toughness, and workability are impaired. Therefore, the content of N is set to 0.07% or less, and 0.005 to
It is set to 0.07%.

Cu: Cu can be expected to be solid solution strengthened and precipitation strengthened, but when a large amount of Cu is added, not only the strength is rather lowered, but also the toughness, hot workability and weldability are remarkably lowered. Therefore, Cu is 0.01 to 1%.

[0022]

EXAMPLES The effects of the present invention will be clarified by giving concrete examples of the ferritic steel of the present invention. Table 1 shows the chemical composition of the test steel. In Table 1, A to D are comparative steels, and E to P are inventive steels. Steels A, B, and C are fire STBA27 and fire ST, respectively, which are stipulated in the technical standards for thermal power plants for power generation.
BA28, Fire SUS410J2TB equivalent material,
D steel is a material equivalent to DIN standard X20CrMoV121.

Each of these steels was melted in a 50 kg vacuum melting furnace, and the ingot was forged at 1150 ° C to 950 ° C to form a plate having a thickness of 20 mm. Next, the A steel and the B steel were subjected to normal heat treatment at 1050 ° C. × 1 h · AC, and then tempered at 770 ° C. × 1 h · AC. For C steel and D steel, after normalizing at 1100 ° C x 1h.AC, 7
Tempering was performed at 60 ° C. × 1 h · AC. The present invention steel is 1
050 ℃ × 1h ・ After normalizing AC, 790 ℃ × 1h
-Tempered in AC.

In order to compare the mechanical properties, a tensile test, a Charpy impact test and a creep rupture test were conducted on the comparative steel and the steel of the present invention. In the tensile test and creep rupture test, the diameter of the plate is 6 mm and the gauge length is 30 m in the longitudinal direction.
The test piece of m was sampled, the tensile test was carried out at room temperature and 600 ° C., and the creep rupture test was carried out at 600 ° C., 650 ° C. and 700 ° C. for a long time of about 10 ⁴ h.
The creep rupture strength at 50 ° C. × 10 ⁵ h was determined by interpolation and compared. In the Charpy impact test, the No. 4 test piece was sampled in the longitudinal direction of the plate in accordance with JIS Z2202, and the ductility-
The brittle fracture transition temperature was determined.

Table 2 shows the test results. As is clear from Table 2, the steels of the present invention show considerably higher values in tensile strength and 0.2% proof stress than the comparative steels.
The same applies to the results of the high temperature tensile test at ° C. 650 ° C
It can be seen that the creep rupture strength at 1 is much higher than that of existing steel. Usually, a material having a high tensile strength has a high creep rupture strength in a short time, but many materials have a large decrease in strength over a long time. However, the steel of the present invention suppresses the formation of δ-ferrite, which lowers the long-term creep rupture strength by adding Ni and Co, and exhibits excellent characteristics at high temperature and long-time creep rupture strength. Further, it has a toughness equivalent to that of the existing steel used as the comparative steel from the ductility-brittleness transition temperature, and it can be seen that there is no problem in practical use. As concretely shown above, the steel of the present invention is a material having high temperature strength and toughness equivalent to that of conventional steel.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

The steel of the present invention has a stable structure at high temperature due to the components shown in Table 1, and significantly improves the creep rupture strength at high temperatures of 600 ° C. or higher at which the conventional high Cr ferritic steel is difficult to use. The present invention provides a ferritic steel. The creep rupture strength at high temperature is SU of austenitic steel SUS347HTB and SUS321HTB.
It has a strength equal to or higher than that of S316HTB. This steel is a material that combines the advantages of ferritic steel, such as toughness, workability, and economic efficiency.As a heat and pressure resistant member used in industrial fields such as boilers, chemical industry, and nuclear power, it replaces conventional austenitic steel with pipes and plates. In addition, it can be widely applied to forged products of various shapes.

Claims (1)

[Claims] C .: 0.03 to 0.12% by weight,
Si: 0.1 to 0.7%, Mn: 0.1 to 1%, P:
0.002 to 0.025%, S: 0.001 to 0.01
5%, Cr: 8 to 13%, Ni: 0.1 to 1%, Mo:
0.1-3%, V: 0.01-0.5%, W: 0.1
3%, Nb: 0.01 to 0.2%, Co: 0.1 to 3
%, Re: 0.1 to 1.5%, Al: 0.005 to 0.
05%, B: 0.0001 to 0.01%, N: 0.00
A high Cr ferritic steel characterized by being excellent in high temperature strength composed of 5 to 0.07% and Cu: 0.01 to 1% and the balance being iron and inevitable impurities.