JPS6289842A - High-chromium ferritic steel for high temperature service - Google Patents

Abstract

PURPOSE:To obtain high-chromium ferritic steel with high Ac1 point capable of tempering at a temp. as high as >=600 deg.C, by providing a composition which contains each prescribed amount of C, Si, Mn, P, S, Ni, Cr, Mo, W, V, etc., and in which Ac1 value and Cr equivalent are specified, respectively. CONSTITUTION:The high-chromium ferritic steel contains, by weight, 0.05-0.15% C, <1% Si, 0.2-1% Mn, <0.025% P, 0.015% S, <0.8% Ni, 8-13% Cr, 0.5-3% Mo, 0.5-3% W, 0.1-0.5% V, 0.01-0.2% Nb, 0.005-0.04% Al and 0.003-0.07% Ni and satisfies the conditions represented by equations I and II. Moreover, when <=0.2% each of one or more elements among Ti, Zr, Y, La and Ce, <=0.01% B and <=0.2% Ca are added to the above basic system independently or in combination so as to satisfy the above equations I and II, properties can be greatly improved in respect of stabilization of carbide dispersion, weldability, and toughness.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is applicable to steels for heat exchangers such as steam generators and boilers used under high temperature and high pressure, especially in high temperature environments with metal chromatism of 600°C or higher. This article relates to high-chromium 7 elite steel for high temperature use.

(Prior art) Heat-resistant steel used for dζζ sill superheater tubes, reheater tubes, heat exchangers for nuclear power, chemical industry, etc. is a material that has excellent workability and weldability in addition to high-temperature strength, corrosion resistance, and toughness. is required. Steels used for these purposes include 1) austenitic stainless steel, 2) 2% Cr-I M
Low alloy steel such as o tA and 3)? There is high chromium ferrite (N) represented by Cr-lMo steel. Among them, high chromium ferrite pots have the advantages of being less prone to high temperature corrosion and stress corrosion, and being inexpensive.
60 because of its high content and improved oxidation resistance.
It is used in high-temperature parts in place of low-alloy steel, which is difficult to use at temperatures above 0°C. 9 Cr -I M is a high Cr steel with excellent high temperature strength.

Kfq (8TBA 26), improved 9 Cr iW
4 (ASTM A213T91), 12Cr-I
Mo'd4 (DINX20 CrMo WV 121
) are typical examples. In addition, 9-12Cr'A developed by the present inventors (Japanese Patent Publication No. 57-36341, Japanese Patent Application Publication No. 55-110758 and Japanese Patent Application Publication No. 58-181849)
There is. On the other hand, in recent years, boilers have become increasingly hot and pressurized.
l Metal @ 550 degrees to increase temperature and reduce cost
High Cr ferritic steel is being used for high temperature parts of ~650°C.

When used at such high temperatures, materials with long-term creep strength and stable structure are required.

Generally, normalizing and tempering are performed as heat treatments for ferritic steel, but in the case of materials for heat exchangers, the tempering treatment is usually about 30 to 50 degrees Celsius or more lower than 8C1 and about 150 degrees Celsius below the operating temperature. It has been carried out at higher temperatures. This is to make the structure a stable tempered martensite and stabilize the high-temperature/long-term creep strength. It is known that when the tempering temperature is low, although the creep strength increases on the short-term side, there are cases where the strength suddenly decreases due to recrystallization of the framework on the long-term side.

In order to stabilize the high-temperature strength of 12Cr%aIl, a tempering temperature of 800°C or higher is desirable, but in the case of conventional steel, the ACl transformation point is around 800°C, and in actual production lines, temperature fluctuations in the heating furnace cannot be controlled. Considering this, tempering at a high temperature of 800°C or higher is not possible.

Lowering the Ac1 point is due to the action of austenite stabilizing elements such as C, Mn, and Ni5N, but in the conventional -1, these elements are actively added to suppress δ ferrite, which is undesirable from the viewpoint of strength and toughness. It is often added.

Furthermore, in the past, component design taking Ac1 point into consideration has hardly been carried out.

(Problems to be Solved and Resolved by the Invention) Therefore, the present invention has developed a high-chromium ferrite 3 with a high Ac1 point that is comparable in high-temperature strength to conventional steel and capable of high-temperature tempering treatment considering usage temperatures of 600°C or higher. It is an attempt to overcome inertia.

(Means for resolving the point) The present invention has c: o, os ~ 0.15%, Si: 1%
Hereinafter, Mn: Cl: 2 to 1%, P: 025% or less.

8; 0.015% or less, Ni; 8% or less, Cr: 8
~13%, Mo: α5~3%, W:0.5
~3%.

V; 0.1-0.5%, Nb; 0.01-[1,2
%, Nb; 0.04% or less, N: 0.003 to 0.0
7%, B as necessary; α001 to α02%, Ca,
Ti, Zr.

One or more of Y, La, Cp-〇, 01-0
．． 2%, the remainder consists of iron and unavoidable impurities,
ACl calculated by the following 0 formula (weight%) is 850
℃ or less, ACl-765-5000-450N+30Si-25
Mn+25Mo+15W+11Cr+50V+30Nb
−30Ni+30Nb −−1 ■And the following 0
Cr equivalent calculated by the formula is 17 or less, Cr equivalent - Cr-40C-30N+6Si-2Mn+4
Mo+1.5W+11'V+5Nb-4Ni+12Nb
−−−1−−−−− ■It is a high chromium ferrite steel for high temperature use.

In addition, in the above basic system, Ti, Zr, Y, La,
At least one of Ce is added at 0.2% or less, B at 101% or less, and Ca at 0.2% or less, each singly or in combination, to satisfy the above formulas (1) and (2). This is a preferred embodiment of the present invention because it significantly improves properties in terms of carbide dispersion stabilization, weldability, and toughness.

The reasons for limiting each condition of the steel of the present invention will be explained in detail below.

First, the limitation of kCta degrees, which is a feature of the present invention, has not been defined at all in the past.

The equation (2) for determining Acl was obtained through experiments by the inventors. In order to perform high-temperature tempering at 800° C. or higher without causing γ transformation, A+4 was set at 800° C. or higher in consideration of temperature fluctuations during heat treatment.

Next, the specification of Cr equivalent is the generated δ-ferrite (,
This is to limit 7. ■ According to the formula A (4 is 850℃
In the above-mentioned component system, the amount of δ-ferrite M may be considerably large. When δ-ferrite is present in an appropriate amount, weldability and workability are improved, but when a large amount is present, strength and toughness are impaired. Therefore, by limiting the Cr formula (2) to 17 or less, it was possible to obtain a component system that does not deteriorate strength and toughness.

Furthermore, the reasons for limiting the ingredients are shown below.

C: Combines with Cr, Mo, W, V, Nb to form carbides and increases high melting creep strength, but O, O
If it is less than S%, m'ft& becomes ferrite, resulting in insufficient toughness and strength. On the other hand, if it exceeds 115%, the A self will be significantly reduced, and the amount of carbides will increase and harden, resulting in poor workability and weldability. Therefore, it was set to 105 to LL15%.

Si: This element is added as a de-inertia agent and improves water vapor melting resistance, but if it exceeds 1%, the toughness decreases and it is also harmful to the creep strength, so it is set to 1% or less.

Mn: Effective for improving hot workability and stabilizing the structure, but if it is less than 12%, no effect will be obtained, and if it exceeds 1%, a hardened phase will be formed and the toughness will be impaired. ~1
%.

p, s: Both are impurity elements harmful to toughness, machinability, and weldability. The upper limit was set as an unavoidable content, and P was set to be 0.025% or less, and S was set to be α015% or less.

Ni stabilizes the martensite structure as a niostenite stabilizing element, but if it exceeds 1.8%, creep strength is impaired. Therefore, the content was set at 0.8% or less.

Cr: As one of the basic components, it is essential from the point of view of oxidation resistance, but if it is less than 8%, good oxidation resistance cannot be obtained, and if it is added in excess of 13%, δ-ferrite increases and strength is reduced. and impairs toughness. Therefore, it was set at 8 to 13%.

Mo: Effective in improving creep strength, but if it is less than 0.5%, sufficient effect will not be obtained, and if it exceeds 3%, it will not work at high temperatures.
Gold and interspace compounds precipitate, which not only reduces toughness, but also
It also has no effect on strength. Therefore, rl was set at 5 to 50%.

W: Like MO, it is effective in improving creep strength, but if α is less than 5%, a sufficient effect cannot be obtained, and if it is added in excess of 3%, it is no longer effective. Therefore, 0.5-1
It was set at 3%. Since W has a larger atomic size and slower diffusion than MO, it is an extremely effective solid solution strengthening element for high-temperature creep, and it also dissolves solidly in carbides to suppress coarsening and strengthen the IJ.
Although it contributes to stabilizing the strength of the
It is effective when added in combination with o. Furthermore, W has a lower tendency to form 6-ferrite than Mo.
Even if we consider the value of MO as A W + Mo, W composite addition is more advantageous in terms of strength and toughness because it suppresses δ-ferrite t lower than Moq alone.

V: Combines with C and N to form fine precipitates of V(C,N), and these precipitates are highly stable for long periods of time and mainly contribute to increasing long-term creep strength. If it is less than 11%, a sufficient effect will not be obtained, and if it exceeds 15%, the creep strength will actually decrease.

Therefore, it was set at 0.1 to 15%.

Nb: Like v, it combines with C and N, and Nb(C.

N) forms fine precipitates and contributes to creep strength.

Nl) is effective for short-term creep strength, but if added in excess, Nb (C, N) tends to become coarser on the long-term side, lowering the strength. Furthermore, is Nb unfixed ff? As Nbc, it is effective in refining austenite grains during normalizing and contributes to improving toughness, an effect that V does not have. If the CL content is less than 01%, no effect will be obtained, and if it exceeds 0.20%, a large amount of undissolved precipitates will remain in the normalizing treatment, resulting in a decrease in strength. Therefore, 0. Ol·-0.20%. Among these, α1
% or less, particularly about 0.05%.

Nb: Added as a deoxidizing agent, but 0. If it exceeds O4N, the creep strength will be impaired. Therefore, it was set at 104% or less.

N: Combines with V and Nb and contributes to improving creep strength as a carbonitride, but is ineffective at less than 3% o and oo; If O is 7% or more, workability and weldability deteriorate. Therefore, 1003~0. O was set at 7%.

In addition to these components, trace addition elements described as preferred embodiments are shown below.

B: Addition of a small amount has the effect of dispersing and stabilizing carbides. If it is less than 0.0001%, the effect is small, 0.01%
If it is more than that, weldability will be impaired, so a range of 1001% to 101% is preferable.

Ca, Ti, Zr, Y, La, Ce: These elements are added in trace amounts for the purpose of controlling the morphology of inclusions.

By adding 0.01% or more of at least one of these, impurities such as o, p, and s can be cleaned and strength and toughness can be improved. However, if it exceeds 0.2%, inclusions will increase and the toughness will be impaired.
~α2% range.

Each steel type whose chemical composition is shown in Table 1 was melted in a 50:9 vacuum heating furnace, and an ingot was forged at 1150 to 950°C to form a plate with a thickness of 20+W.

A, B'; % is normal treatment at 950℃ x 1h.
After AC tempering, AC tempering was performed at 750°C x 1 h. C', D, E, and F steels containing V and Ntl were heated at 1050°C for 1 h @780°C after AC normalization.
X 1 h -AC was tempered. G of invention steel
~○ steel was normalized at 1050°C x 1 h #AC and then tempered at 860°C xn, sh@Ac (DQ! Short time tempering treatment).

Tensile test results at room temperature and 650℃ and 650℃X 8
kgf/wa'' creep rupture test results are shown in Table 2. Also, the creep rupture time is shown in comparison in the 11th test.

Although the tensile strength at room temperature of the steel of the present invention is lower than that of comparative steels due to high-temperature tempering, it has good ductility and excellent workability at room temperature. Moreover, the tensile strength at 650°C was equivalent to that of the comparative steel.

Next, from the results of the 650°C x 8 kgf/-tension creep rupture test, it can be seen that the steel of the present invention has a strength equal to or higher than that of the comparative steel.

The advantages of the steel of the present invention having an Ac1 point of 850°C or higher are shown below.

1) High-temperature tempering at 800°C or higher is possible to stabilize the strength of the hammer when used in a high-temperature environment of 6 on°C or higher.

2) It can be treated at higher temperatures and in a shorter time than conventional steel tempering treatments.

3) Hot working by untransformed heat softening treatment at about 830° C. is possible, and SXC (stress relief annealing) can be omitted. In addition, since the strength at room temperature is low and the chemotaxis is high, it has excellent workability at room temperature, such as bending workability.

4) In the case of conventional steel, it was necessary to restore the joint for a long time of 1 hour or more at a temperature of 800°C or higher to stabilize the structure (tempering of martensite), but for the steel of the present invention, a short time of 1 hour or less at a temperature of 800°C or higher was required. Heat treatment is possible.

5) In the case of conventional steel, high temperature tempering above 800℃μ was not possible due to low Ac1, and the structure and strength were unstable when used for long periods of time at 600℃ or higher, but the present invention In the case of steel, its structure and strength are stable even when used for long periods of time at high temperatures of 600°C or higher.

[Brief explanation of drawings]

FIG. 1 is a chart showing a comparative comparison of the creep rupture times of the invention steel and comparative steel. Sub-agents 1) Meifuku agent Ryo Hagiwara - Sub-agent Atsuo Anzai Procedures Namasho Tatamiaki, TLJ November 1961 / Japan Patent Office Commissioner Kuro 1) Akio Tono 1, Indication of the case 1985 Patent No. M226995 ) Sanen Industrial Co., Ltd. (and 1 other person) 4, @! Agent address: Toranomon Chiyode Building Denbo (504) 1894, No. 2 Toranomon-16-chome, Higashijuto Minato-ku Name: Patent attorney (7179) 1) Ming (2 persons) Date of five amendment orders Voluntary action Number of inventions increased by amendment & amendment None 7, Subject of amendment (1) Scope of claims of specification (2) Detailed explanation of Ming#l's invention +31 E!
A brief explanation of the drawings of the A subspecies (Drawing 41 Contents of the amendment to surface a + 11 The claims on pages 1-4 of the specification are corrected as shown in the attached sheet. (2) 8th line 4-5 cv r Nb; 0.04
% or less" is corrected to r Nb;003~(104%".
℃ or more,” he corrected. (4) “raooc” on page 9, line 12, “850℃”
I am corrected. (5) On page 113, lines 11-12 of lt, change ``Therefore, it was set below @...'' to ``If it is less than 1003%, the explanation will be insufficient, and not only will strength and toughness be impaired, but weldability and creep strength will decrease. Therefore, it was set as α00350.O4%.'' (6) Insert the following statement after Table 1 on page 15 @(7
) Correct "G~0WI4" in the negative 6th line of direction 16 to "G~Z Steel". (8) "830℃x[1L5h11" on page 16, line 7
AC" to "810-b." (9) "Compare and show" on page 16, lines 11-12'.
The second factor is the creep rupture time, ACl.
Pervert pervert point water. In addition, the B hook and E steel in Figure 2 are as shown in Table 1.
Bm, E steel. ” he corrected. 00 Correct "6217. I J" in the creep rupture time of comparative steel C in Table 2 on page 17 to "2617. I J." αυ Below Table 2 on page 17 and above the "Notes" column. Insert the following statement. (2) "8RCJ" on page 18, line 17 is corrected to "SR". o3 In the bottom line of page 19, change "Figures and tables."
This is a diagram showing the metamorphosis. ” he corrected. 0.4 Figure 1 will be corrected as shown in the attached sheet. (2) Add Figure 2 as attached. Claim m N amount % Te, C; 0.05~G, 15%, Si
: 1% or less, Mn: 112-1%, P: α025% or less, S: α015% or less, Ni: 0.8% or less. Cr; 8-13%, Mo; 0.5-3%, W
;0.5-6%, v: (11-[lL5%,
Nb: α0 1 ~ 0.2 attack, Nb; [1003 ~
[104%, N; 0. Contains OO3~0.07%, the remainder consists of iron and unavoidable impurities, and the following formula 0 (wt%)
) is 850℃ or higher, Ac1=765-5000-450N4-1si-25
Mn+25Mo+15W+11 Or+50V+50N
b-30Ni+30Nb...■ And the Cr equivalent calculated by the following formula 0 is 17 or less, Cr = @ji = cr-40C-3[IN+6Si-2
Mn+4Mo+1.5W+ 11V+5Nb-4Ni
A high-temperature chromium ferrite steel characterized by +12Al Fist@ * -II ■. (2) N arm, C; α05 to α15%, 31; 0. Below, Mn: 0.2-1%, P: 0. O25% or less
S;0. O: 15% or less, Ni: 0.8% or less. Cr: 8-13%, Mo: 0.5-3%,
W; 0.5-3%, v; α 1-α 5%, N
b: 0. O1~α2chi, Nb; (11,003~0
．． o 4%, N; 0. O03~0. o 7%, B;
α001~0.02%, the remainder consists of iron and unavoidable impurities, kQl calculated by the following 0 formula (weight 1t%) VC is 850°C or higher, AO1 = 76.5-50 DC-450N + 30Si-
25Mn+25Mo+15W+11Cr+50V+30
Nb-30Ni+50Nb...φ■ And the Cr equivalent calculated by the following formula is 17 or less, Cr,% = Cr-40C-30N+6Si-2Mn+
4Mo+1.5W+11V+5Nb-4Ni+12N
A high-temperature chromium ferrite steel characterized by b・---■. +31 heavy ii%T, C; CLO5~(115%,
Si: 1 or less, Mn: 2-1%, P: l10
2596 or less, S: 015% or less, Ni: α8% or less. Cr; 8-13%, Mo; 0.5-3%, W; α5-5
%, V: 0. 1 to CL 5%,
Nl): 0. Ol ~ CL 2 heads, Nb; 0. OO5
~104%, N:0. OO3~a07%, Ca,
1m or 2 or more of Ti, Zr, Y, La, Ce; 0.01 to 12%, the remainder consists of iron and unavoidable impurities, ACl calculated by the following formula (wt%) is 850 ℃ or higher, Act=765-5000-450N+30Si-25
Mn+25Mo+15W+11Or+50V+30Nb
-30Ni+30Nb"■And the Cr ship calculated by the following formula 0 is 17 or less, 1 per Cr = cr-40C-30N+6Si-2Mn+4
MO+1.5W+11V+5Nb-4Ni+12Nb”
・・Rom ferrite for use in Karasuon characterized by being ■. (at 41Mt%, C: α05 to α15%, Si: 0.chi or less 1Ml1ll; 2 to 1%, P: Q-025'To or less, s: a, ois% or less, Ni-08% or less. Cr: 8-13%, MO: [shi 5-5
%, W: CL 5-5%, v;
α1~CL5%, Nb; 0.
0l-Cl3, Nb; 0.003-α04%, N: 0
．． OO3~0. o7%, B:0. OOl ~0
．． o2%, Ca. Ti, Zr, Y, La, Ce 1 & also U2a1
Contains 0.01 to α2%, the remainder consists of iron and unavoidable impurities, ACl calculated by the following 0 formula (34% by weight) is 850°C or more, Act = 765-500
0-450N+50Si-25Mn+25Mo+15W
+11Cr+50V+30Nb-30Ni+50Nb・
0 and the Cr equivalent calculated by the following 0 formula is 17 or less, Cr equivalent = cr-40C-30N+6Si-2Mn+4
Mo+1.5W+11V+5Nb-4Ni+12Nb・
・High-temperature chromium ferrite steel characterized by -■■.

Claims (4)

[Claims] (1) In weight%, C: 0.05-0.15%, Si: 1
% or less, Mn; 0.2 to 1%, P; 0.025% or less,
S: 0.015% or less, Ni: 0.8 or less, Cr: 8~
13%, Mo; 0.5-3%, W; 0.5-3%, V;
0.1-0.5%, Nb; 0.01-0.2%, Al;
0.04% or less, N; containing 0.003 to 0.07%,
The remainder consists of iron and unavoidable impurities, and Ac_1 calculated by the following formula [1] (wt%) is 850°C or less, Ac_1 = 765-500C-450N+30Si-2
5Mn+25Mo+15W+11Cr+50V+30N
b-30Ni+30Al...[1] and the following [2]
Cr equivalent calculated by the formula is 17 or less, Cr=equivalent=Cr-40C-30N+6Si-2Mn+
4Mo+1.5W+11V+5Nb-4Ni+12Al
...[2] A high-temperature chromium ferrite steel characterized by... (2) In weight%, C: 0.05-0.15%, Si: 1
% or less, Mn; 0.2 to 1%, P; 0.025% or less,
S; 0.015% or less, Ni; 0.8% or less, Cr; 8
~13%, Mo; 0.5-3%, W; 0.5-3%, V
;0.1-0.5%, Nb;0.01-0.2%, Al
; 0.04% or less, N; 0.003 to 0.07%, B;
0.001 to 0.02%, the remainder consists of iron and unavoidable impurities, Ac_1 calculated by the following formula [1] (weight%) is 850°C or less, Ac_1 = 765-500C-450N+30Si- 2
5Mn+25Mo+15W+11Cr+50V+30N
b-30Ni+30Al...[1] and the following [2]
Cr equivalent calculated by the formula is 17 or less, Cr equivalent = Cr-40C-30N+6Si-2Mn+4
Mo+1.5W+11V+5Nb-4Ni+12Al・
...[2] A high-temperature chromium ferrite steel characterized by: (3) In weight%, C: 0.05-0.15%, Si: 1
% or less, Mn; 0.2 to 1%, P; 0.025% or less,
S; 0.015% or less, Ni; 0.8% or less, Cr; 8
~13%, Mo; 0.5-3%, W; 0.5-3%, V
;0.1-0.5%, Nb;0.01-0.2%, Al
; 0.04% or less, N; 0.003 to 0.07%, Ca
, Ti, Zr, Y, La, Ce or more; 0
．． 01 to 0.2%, the remainder consists of iron and unavoidable impurities, Ac_1 calculated by the following formula [1] (weight%) is 850 ° C or less, Ac_1 = 765-500
C-450N+30Si-25Mn+25Mo+45W
+11Cr+50V+30Nb-30Ni+30Al・
... [1] and the Cr equivalent calculated by the following formula [2] is 17 or less, Cr equivalent = Cr-40C-30N + 6Si-2Mn + 4
Mo+1.5W+11V+5Nb-4Ni+12Al・
...[2] A high-temperature chromium ferrite steel characterized by: (4) In weight%, C: 0.05-0.15%, Si: 1
% or less, Mn; 0.2 to 1%, P; 0.025% or less,
S: 0.015% or less, Ni: 0.8 or less, Cr: 8~
13%2Mo; 0.5-3%, W; 0.5-3%, V;
0.1-0.5%, Nb; 0.01-0.2%, Al;
0.04% or less, N; 0.003 to 0.07%, B; 0
．． 001-0.02%, Ca, Ti, Zr, Y, La,
Contains one or more types of Ce; 0.01 to 0.2%,
The remainder consists of iron and unavoidable impurities, and Ac_1 calculated by the following formula [1] (wt%) is 850°C or less, Ac_1 = 765-500C-450N+30Si-2
5Mn+25Mo+45W+11Cr+50V+30N
b-30Ni+30Al...[1] and the following [2]
Cr equivalent calculated by the formula is 17 or less, Cr equivalent = Cr-40C-30N+6Si-2Mn+4
Mo+1.5W+11V+5Nb-4Ni+12Al・
...[2] A high-temperature chromium ferrite steel characterized by: