JPS62278251A - Low-alloy steel excellent in stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To develop a low-alloy steel excellent in stress corrosion cracking resistance and suitable for steam turbine material, by adding specific alloy contents to an Ni-Cr-Mo low-alloy steel and also by limiting respective contents of Si, Mn, and P to specific ranges. CONSTITUTION:The low-alloy steel used as a material for steam turbines driven by high-temp. and high-pressure steam has a composition containing, by weight, <=0.40% C, <=0.15% Si, <=0.60% Mn, <=0.010% P, <=0.030% S, 0.50-4.00% Ni 0.50-2.50% Cr, 0.25-4.00% Mo, and <=0.30% V, satisfying the compositional relations in Si+Mn+2P<=0.75%, and further containing either or both of 0.001-0.50%, in total, of at least one element among Al, Ti, Nb, W, B, and Ce and 0.003-0.015% Sn. Moreover, the above steel is an Ni-Cr-Mo low-alloy steel excellent in stress corrosion cracking resistance having a structure in which old austenite grain size is regulated to >=4 by ASTM grain size number.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention is used as a material for steam turbines, etc. ．． Concerning the 11th class of Gelcuronians.

<Conventional technology> Generally, high temperature and high pressure (approximately 300°C, 70kg/cm')
Materials for steam turbines that are driven by steam are required to have excellent strength and toughness over a wide temperature range, and vanadium-added nickel-chromium-molybdenum steel, which is a strong steel, is used as a material that achieves these characteristics. . This steel is
As is well known, fine carbide-producing elements such as molybdenum and vanadium are added to nickel-chromium strong steel, which is sensitive to tempering brittleness, to suppress softening at high tempering temperatures, that is, to increase tempering resistance, and it is suitable for the above applications. It is made of steel.

<Problems to be solved by the invention> However, in recent years, stress corrosion cracking has been occurring frequently in low-pressure steam turbines and their peripheral equipment that use vanadium-added nickel-chromium-molybdenum steel, mainly in nuclear power plants in Europe and the United States. It has become clear that this is the case, and it has become a big problem. This stress corrosion cracking occurs mainly in the groove of the key that secures the disc and shaft, and in the joint between the blade and disc.
The reason for this is said to be that Na, which is an impurity in the steam, concentrates as NaOH in the gaps between these parts, which, together with the high load stress during turbine operation, causes cracks along grain boundaries. Furthermore, it has long been known that intergranular stress corrosion cracking occurs in carbon steel subjected to stress in an OH environment. In view of this current situation, there is a strong desire to develop nickel-chromium molybdenum steel that has excellent stress corrosion cracking resistance even under harsh usage environments.

Therefore, an object of the present invention is to provide a steel that has an appropriate alloy composition, trace addition elements, and/or microscopic structure that reduces stress corrosion cracking susceptibility, and that can be used in harsh environments without cracking. .

Means for Solving the Problems> The inventors variously changed each of the above factors in order to clarify the combination conditions of the stress corrosion cracking susceptibility of nickel chromium molybdenum steel, alloy composition, trace addition elements, and microscopic structure. A stress corrosion cracking test was conducted on each sample steel.
The present invention was constructed by analyzing the test results in detail.

The first low alloy steel with stress corrosion cracking resistance of the present invention is
C50, 40%, Si 60.15%, Mn: 60.6
0%, P: 60.010%, S: 50.030%, Ni
:0.50~4゜00%, Cr:o, 50~2.50%
, Mo: 0.25 to 4.00%, ■ 50.30%, and the above Si, Mn, and P are (Si+Mn+20P)≦0
．． 75%, and (i) Al1. Ti
, Nb.

At least one of W, B, and Ce in total from 0.001 to 0
．． 50%, (ii) Sn:o, 003-0.015
%, and the remainder consists of Fe and unavoidable impurities.

In addition, the second low alloy steel with the stress corrosion cracking resistance of the present invention has C0≦040%, Si: 60.15%, Mn≦0.
．． 60%, P 60.010%, S, 50.030%,
Ni・0゜50-4.00%, Cr:0.50-2.5
0%, Mo: 0.25-4.00%, ■・50.30%
(i) Al, Ti, Nb, W, B.

At least one type of Ce in total from 0.001 to 0.50%
, (ii) containing at least one of the two groups of Sn 0.003-0015%, with the balance being Fe and unavoidable impurities) -1,1; is characterized by having an A8TM crystal grain size number of 4 or more.

The chemical components of the present invention and the reason for limiting the ASTM grain size number will be described below.

C is an element to ensure strength, but it increases stress corrosion cracking susceptibility, and if the content exceeds 0.4%, it deteriorates toughness in relation to other alloying elements, so both low alloy steels The upper limit for both was set at 0.40%.

S is an element that significantly deteriorates hot workability, and from the viewpoint of preventing cracking during hot forging, the upper limit was set at 0.030% for both low alloy steels.

Ni and Cr are essential elements for increasing strength, improving hardenability, and improving toughness, and both require addition of 0.50% or more.In order to obtain better performance in terms of hardenability and toughness, Ni should be added at 3. It is desirable that the content is 25% or more, and Cr is 125% or more, but the content is 4.00% and 2.50%, respectively.
%, it is not practical because the transformation characteristics of the steel change significantly and a long time is required for heat treatment to obtain excellent toughness. Therefore, the N1 content of both low alloy steels is 0.5 (1-4
, 00%, Cr content is limited to a range of 0.50 to 250%, respectively, but especially when used as a material for equipment such as a low-pressure fuel bottle, Ni: 3° 25 to 4.00%, Cr:
It is preferably 1.25 to 2.50%.

Mo improves the corrosion resistance of prior γ grain boundaries and significantly reduces susceptibility to intergranular stress corrosion cracking, and also precipitates in the grains as fine carbides during tempering, greatly contributing to preventing temper embrittlement and increasing strength. To obtain such an effect, 0.
It is necessary to add 25% or more, but if the content exceeds 400%, the above effects are saturated and the toughness begins to deteriorate. Moreover, adding more than necessary is also uneconomical. Therefore,
The Mo content of both low alloy steels was limited to a range of 0.25% to 4.00%.

■ is an effective element that increases the strength of the material through crystal grain refinement and precipitation hardening action, and is added as necessary, but its effect is saturated when the content exceeds 0.30%. Therefore, the upper limit was set at 0°30% for both low alloy steels.

Si, P and Mn are greatly involved in grain boundary type stress corrosion cracking susceptibility, and in this Q4m, grain size, Ti, A
(2, Nb, W, B, Ce, is an important element that should be limited in a complementary manner in connection with the addition of small amounts of Sn.

Si is an element necessary for deoxidation during steelmaking, but 0
If the content exceeds 15%, the corrosion resistance of prior γ grain boundaries deteriorates, and the susceptibility to grain boundary stress corrosion cracking increases significantly.
The upper limit was set to 0.15% for both low alloy steels.

P is an impurity element that segregates at prior γ grain boundaries, deteriorates its corrosion resistance, increases intergranular stress corrosion cracking susceptibility, and promotes temper brittleness. In chromium molybdenum steel and nickel chromium molybdenum steel according to JIS standards, the content is limited to 0.030% or less from the viewpoint of tempering brittleness, but it is necessary to further limit the content to reduce stress corrosion cracking. The P content in both alloy steels was 0.010% or less.

Furthermore, Mn is added for deoxidation and shoe removal during steel manufacturing, but if the content exceeds 0.60%, it promotes the above-mentioned polarization of P and significantly increases stress corrosion cracking susceptibility. It acts in combination with Si and P on stress corrosion cracking, and its appropriate range is greatly related to the grain size and the addition of small amounts of Ti, Af2°Nb, W, B, Ce, and Sn, which will be described later. This has become clear from the results of original research by the inventors. That is, from the standpoint of preventing stress corrosion cracking, the Mn content is limited to 0.60% or less for both low alloy steels, and the Si, P and Mn contents of the first low alloy steel, which is not subjected to grain size adjustment, are is (Si+
Is it necessary to strictly limit these contents so that the relationship of Mn + 20P) 5075% is satisfied, that is, the sum of the Mn weight % content, the SiN content %, and 20 times the content 2 weight % is 0.75% or less? be.

On the other hand, in order to further increase the reliability of reducing stress corrosion cracking susceptibility, we investigated in detail the influence of the microscopic structure, and found that stress corrosion cracking susceptibility also depends on prior austenite grain size.
ASTM grain size number (equivalent to JIS G 0511)
It was found that sufficient reliability could not be obtained with a value of 3 or less. Therefore, Si.

For the second low alloy steel in which the total amount of P and Mn was not adjusted, the prior austenite grain size number was limited to 4 or more.

ALTi, Nb, Ce, W, B, and Sn are all essential additive elements that improve the corrosion resistance of prior γ grain boundaries and greatly contribute to reducing the susceptibility to intergranular stress corrosion cracking.

In order to obtain such an effect, it is necessary to use the elements of the 6th century, ie A12. It is necessary to add one or more of these elements in Ti, Nb, Ce, W, and B in a total amount of 0.001% or more, but if the total amount of addition exceeds 0.050%, the toughness will significantly deteriorate. Therefore, the total addition amount of these elements was limited to a range of o,oot to 0.50%. On the other hand, in the case of the latter one, that is, Sn, the same effect as the former six can be obtained by adding 0.003% or more;
When it exceeds 5%, tempering brittleness is promoted and toughness is significantly deteriorated. Therefore, the content of Sn is 0.003 to 0.01
It was limited to a range of 5%.

However, in order to reduce stress corrosion cracking, in addition to adding these trace elements, it is necessary to limit the range of (Si + Mn'', 20P),
Alternatively, it is necessary to limit the grain size as in the first low alloy steel and the second low alloy steel described above.

In addition, as in the case of the low-alloy steel of claim 2, if both of the above limitations are satisfied, better stress corrosion cracking resistance can be obtained, and as in the case of the low-alloy steel of claim 3, It has been found from experimental and research results that if the former restriction is made stricter, reliability in reducing stress corrosion cracking can be obtained even further.

<Effects of the Invention> The low alloy steel of the present invention contains optimal alloying elements in an optimal component ratio range in order to have excellent stress corrosion cracking resistance, and/or has an appropriate microscopic structure (crystal grain size). Therefore, the possibility of stress corrosion cracking occurring is small even when used in a member that is subjected to high load stress in a corrosive environment such as NaOH or OH-.

<Example> Hereinafter, the effectiveness of the present invention will be explained in detail with reference to Examples.

Table 1 listed at the end shows the chemical composition and prior γ crystal grain size of the test steel subjected to the stress corrosion cracking test. These test steels were melted in a high-frequency induction electric furnace after adjusting their composition, then formed into ingots, hot forged to a thickness of 25 mm, heated to an austenitizing temperature, water quenched, and then 620 mm thick.
It was manufactured by subjecting it to a tempering process in which it was heated to a temperature of 10°C, held for 1 hour, and then cooled at a rate of 4°C/minute. The grain size was varied by adjusting the quenching temperature (heating temperature) and the holding time. The sample steel manufactured in this way was machined to a thickness of 1.5 m as shown in Figure 1.
A strip-shaped test piece T having a width of 15 mm and a length of 65+++ m was manufactured.

In the stress corrosion cracking test, a test piece made of the above test steel was
It is attached to a four-point bending constant load test jig J made of S316, and the yield strength of 0.2% of the test steel is 60% (test I) or 100%.
% (test ■) is applied by screwing bolt B, and the bending stress corresponding to After that, the cross section of the test piece was observed with an optical microscope to determine the presence or absence of cracks and the depth of the cracks.

The results of the above stress corrosion cracking test are also shown in Table 1. As is clear from the results of test ■ in the table, claim i
X The steel of the present invention (No.
, 1 to 44) did not cause stress corrosion cracking at all.

In contrast, comparative steel (No.
, 45 to 59) all caused intergranular stress corrosion cracking. From this, it can be seen that Mn, S as intended by the present invention
It can be seen that limiting i, P; J, limiting (Si+Mn+20P')I, or limiting crystal grain size is extremely effective in preventing stress corrosion cracking.

In addition, in test (2) in which the conditions of test T were made stricter regarding stress, stress corrosion cracking did not occur for jll (No. 16 to No. 37) corresponding to claims 2 and 3. First, it can be seen that limiting the amount of (Mr++Si+20P) and limiting the grain size at the same time further increases the reliability of stress corrosion cracking.

Furthermore, in the present example, in test (3), which is the most severe test condition, no force corrosion cracking corresponding to No. 35 to No. 044, that is, (Si+Mn+20P)≦050, occurred. That is, it is clear that this condition is the most effective limit for preventing stress corrosion cracking found in the present invention.

-Margin below-

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the stress load situation on the test piece in the stress corrosion cracking test of the example, and FIG. 2 is a diagram showing the immersion situation of the test piece in FIG. 1.

Claims (4)

[Claims] (1) C:≦0.40% by weight (hereinafter referred to as “weight%”), Si:≦
0.15%, Mn:≦060%, P:≦0.010%,
S:≦0.030%, Ni:0.50-4.00%, C
r: 0.50-2.50%, Mo: 0.25-4.00
%, V: ≦0.30%, and the above Si, Mn, P are (
Si+Mn+20P)≦0.75%,
Furthermore, it contains at least one of the following groups (i) and (ii), (i) at least one of Al, Ti, Nb, W, B, and Ce in a total of 0.001 to 0.50% (ii) Sn: 0.003 to 0.015% A low alloy steel with excellent stress corrosion cracking resistance characterized by the balance being Fe and unavoidable impurities. (2) The low alloy steel according to claim 1, which has excellent stress corrosion cracking resistance, characterized in that the prior austenite grain size is ASTM grain size number 4 or more. (3) In the low alloy steel according to claim 1 or 2, Si, Mn, and P are (Si+Mn+2
A low alloy steel with excellent stress corrosion cracking resistance, characterized by satisfying the relationship: 0P)≦0.50%. (4) C:≦0.40%, Si:≦0, 15%, Mn:
≦0.60%, P:≦0.010%, S:≦0.030
%, Ni: 0.50-4.00%, Cr: 0.50-2
．． 50%, Mo: 0.25-4.00%, V: ≦0.3
0%, and further contains at least one of the following groups (i) and (ii), and (i) at least one of Al, Ti, Nb, W, B, and Ce in total from 0.001 to 0. .50% (ii) Sn: 0.003 to 0.015% The balance consists of Fe and unavoidable impurities, and the stress corrosion cracking resistance is characterized by that the prior austenite grain size is ASTM grain size number 4 or higher. Superior low alloy steel.