JPS6372856A - Low-alloy steel having satisfactory resistance to stress corrosion cracking - Google Patents

Abstract

PURPOSE:To obtain a low-alloy steel causing no stress corrosion cracking even in a severe environment by specifying the amounts of C, Mn, P, S, Ni, Cr, etc., in the compsn. of a low-alloy steel and adding a specified amount of at least one among Al, Ti, etc. CONSTITUTION:The compsn. of a low-alloy steel is composed of, by weight, <=0.40% C, >0.15-1.00% Si, <=0.01% P, <=0.030% S, >0.60-1.00% Mn, <=4.00% Ni, 0.50-2.50% Cr, 0.25-4.00% Mo, <=0.30% V, 0.001-0.50% at least one among Al, Ti, Nb, B, W and Ce and the balance Fe with impurities. The low-alloy steel has ASTM grain size No. >=4 with respect to the old austenite grain size. When the low-alloy steel is used, stress corrosion cracking can be prevented even in a severe environment.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to low alloy steel used as a material for steam turbines and the like.

<Conventional technology> Generally, high temperature and high pressure (approximately 300℃, 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 chromium-molybdenum steel and nickel-chromium-molybdenum steel, which are strong steels, are used as materials that meet these characteristics. ing. As is well known, this steel is sensitive to tempering brittleness, and is made by adding molybdenum and vanadium, which are elements that precipitate fine carbides, to ROM or nickel-chromium strong steel to suppress softening at high tempering temperatures, that is, to increase tempering resistance. This is a steel material suitable for the above uses.

<Problems to be solved by the invention> However, in recent years, vanadium-added chromium-molybdenum steel and nickel-chromium-molybdenum steel have been used in low-pressure steam turbines and their peripheral equipment, mainly in nuclear power plants and thermal power plants in Europe and the United States. It has become clear that stress corrosion cracking is occurring frequently and is becoming a major problem. For example, in the case of a disk, this stress corrosion cracking mainly occurs in the groove of the key that fixes the disk and shaft, and in the weld between the blade and the disk, and is caused by Na, an impurity in the steam, entering the gaps between these parts. This is said to be because it condenses as NaOH and, together with the high load stress during turbine operation, causes cracks along grain boundaries. Further, it has long been known that intergranular stress corrosion cracking occurs in carbon steel subjected to stress in a 011- environment. In view of this current situation, there is a strong desire to develop low alloy steel that exhibits excellent stress corrosion cracking resistance even under harsh usage environments.

Therefore, an object of the present invention is to provide a low-alloy steel that has additive elements and/or microstructures that reduce stress corrosion cracking susceptibility and can be used in harsh environments without cracking.

Means for Solving Problems〉 The inventors investigated various factors mentioned above in order to clarify the combination conditions of stress corrosion cracking susceptibility, micro-added elements, and microscopic structure of chromium-molybdenum steel and nickel-chromium-molybdenum steel. A stress corrosion cracking test was conducted on each of the changed test steels, and the present invention was constructed based on the test results.

The first low alloy steel with good stress corrosion cracking resistance of the present invention is
C: 60.40% by weight, Si: 60.15-1.00%
(excluding 0.15%), Mn: 0.60-1.00%
(excluding 0.60%), P: 50.010%, S: 6
0.030%, Ni:≦4.00%, Cr:0.50~
2.50%, Mo: 0.25-4.00%, V:≦0.
Contains 30% of A(, Ti, Nb, B.

The total amount of at least one of W and Ce is 0.001% to 0.00%.
It is characterized by containing 50% of Fe, with the remainder consisting of Fe and unavoidable impurities.

Further, the second low alloy steel having good stress corrosion cracking resistance of the present invention has a carbon content of 60.40% and a Si content of 60.15 to 1.00%.
% (excluding 0.15%), Mn: ≦0.60 to 1.0
0% (excluding 0.60%), P:≦0. oto%, S
:60.030%, Ni:54.00%, Cr:0.5
(1-2,50%, Mo: 0.25-4.00%, v:
≦0.30%, (i) ALTi, Nb, W.

At least one of B and Ce in total from 0.001 to 0.5
0%, (ii) Sn: 0.003-0.015%,
It is characterized by containing at least one of the two groups, with the remainder consisting of Fe and unavoidable impurities.

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

C is an element necessary to ensure strength, but it increases stress corrosion cracking susceptibility, and if its content exceeds 0.40%, it deteriorates toughness in relation to other alloying elements, so it should not be used in both cases. The upper limit was set at 0.040% for both low alloy steels.

Si is an element necessary for deoxidation during steel manufacturing, and 0.
It is desirable to add more than 15%, but if it is added more than 1.00%, the corrosion resistance of prior γ grain boundaries deteriorates, and the susceptibility to grain boundary stress corrosion cracking increases significantly. It was limited to 15 to 1.00% (excluding 0.15%).

P is an impurity element that segregates at prior γ grain boundaries, deteriorates its corrosion resistance, increases stress corrosion cracking susceptibility, and promotes temper brittleness. In JIS standard chromium molybdenum steel and nickel chromium molybdenum steel, 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 prevent stress corrosion cracking. . Therefore, the P content is 0 for both low alloy steels.
．． 0.010% or less.

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.

Mn is 0.6 for improving hardenability and deoxidizing and desulfurizing during steel manufacturing.
It is necessary to add more than 0%, but the content is 1.00%
If it exceeds the above, the grain boundary segregation of P will be promoted, and stress corrosion cracking susceptibility and temper embrittlement susceptibility will significantly increase. Therefore, the Mn-containing bond is 0.60 to 1.00X for both low alloy steels.
(excluding 0.60'%).

Ni and Cr are component elements that are effective in increasing strength, improving hardenability, and improving toughness. Particularly in the case of Cr, it is necessary to add 0.50% or more, but the content is 4.00% and 2.0%, respectively.
If it exceeds 50%, the transformation characteristics of the steel will change significantly and a long time will be required for heat treatment to obtain excellent toughness, which is not practical. Therefore, the Ni content of both paper alloy steels is 4.00%.
Hereinafter, the Cr content will be limited to a range of 0.50 to 2.50%, but especially when used as a material for equipment such as a low-pressure turbine, Ni: 3.25 to 4.00%, Cr: 1
．． It is desirable to set it as 25-2.50%.

Mo improves the corrosion resistance of prior γ grain boundaries and significantly reduces susceptibility to intergranular stress corrosion cracking, and precipitates within the grains as fine carbides during tempering, greatly contributing to prevention of temper embrittlement and strength improvement. To obtain such an effect, 0.
It is necessary to add 25% or more, but the content is 4.00%
If it exceeds this, the above effects will be saturated and the toughness will begin to deteriorate. Moreover, adding more than necessary is also uneconomical. Therefore, the Mo content for both paper alloy steels is 0.25% to 4.00%.
limited to the range of

■ is an effective element that increases the strength of steel through crystal grain refinement and precipitation hardening action, and is added as necessary, but its effect is saturated when the content exceeds 0.30%. The upper limit was 0.30% for both paper alloy steels.

A+2. Ti, Nb, B, W, and Ce are all essential additive elements that improve the corrosion resistance of prior γ grain boundaries and greatly contribute to reducing the susceptibility to grain boundary type stress corrosion cracking. In order to obtain such an effect, it is necessary to add one or more of these elements in a total amount of 0.001% or more, but if the total amount of addition exceeds 0.50%, the toughness will significantly deteriorate. Therefore, the total addition amount of these elements was limited to a range of 0.001 to 0.50% for both paper alloy steels. Here, T
In the case of i and Nb, it has been confirmed that sufficient effects can be obtained even when added in trace amounts of 0.03% or less.

On the other hand, in order to further increase the reliability of reducing stress corrosion cracking susceptibility, a detailed study of the effects of alloy composition and microstructure revealed that it is effective to add a small amount of Sn and/or to limit the grain size of prior austenite. It became clear that.

In the case of Sn, the reliability of reducing stress corrosion cracking susceptibility can be further increased by adding 0.003% or more, but if it exceeds 0.015%, tempering brittleness increases and toughness significantly deteriorates. Therefore, in the second low alloy steel, the Sn content was limited to a range of 0.003 to 0.015%.

In the case of grain size, as shown in the example, there is no effect if the ASTM grain size number (equivalent to JIS G 0511) is 3 or less, but by reducing the grain size to 4 or more, an effect equivalent to the addition of a small amount of Sn can be obtained. I found out what I could expect. Therefore, in the embodiment section of the present invention, the prior austenite grain size number is limited to 4 or more. Additionally, experiments and research have shown that if Sn is added and the grain size is restricted at the same time, as in the low alloy range of claim 4, further reliability in reducing stress corrosion cracking can be obtained. This is clear from the results.

<Effects of the Invention> The low alloy steel of the present invention contains optimal alloying elements in an optimal component ratio in order to have excellent stress corrosion cracking resistance, and/or has an appropriate microstructure (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 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 heating it to ℃, holding it for 1 hour, and then cooling it at a rate of 4 ℃/min and performing a tempering treatment. The grain size was varied by adjusting the quenching temperature (heating temperature) and the holding time. By machining the test steel manufactured in this way, a thickness of 1.5 of as shown in Fig. 1 was obtained.
A strip-shaped test piece T having f+X width of 15 mm and length of 65 mm was manufactured. In Table 1, test steels No. 1 to 19 are inventive steels within the scope of the claims of the present invention, and test samples No. 9 No. 0.20 to 25 are comparative steels for the above invention steels.

In the stress corrosion cracking test, a test piece made of the above-mentioned test steel is mounted on a 4-point bending constant load test jig J (see Figure 1), and the test piece is 60% or 100% of the 0.2% proof stress (σy) of the test steel. A bending stress equivalent to % is applied by screwing bolt B, and
5 30℃ at 120℃ in autoclave A manufactured by US310S
% NaOH aqueous solution for one week or three weeks (see FIG. 2), and then the presence or absence of cracking and the crack depth were measured by observing the cross section of the test piece with an optical microscope.

The results of the above stress corrosion cracking test are shown in Table 2 at the end. As is clear from the results of Test I in the table, the comparative steels all suffered from extremely large cracks with a depth of 0.84 mm or more, but the steel of the present invention did not show any stress corrosion cracking. That is, Al! as intended by the present invention! , Ti, Nb.

It is clear that the addition of B, W, and Ce is extremely effective in reducing the stress corrosion cracking susceptibility of low alloy steel.

Furthermore, in Test H, which made the conditions of Test I stricter in terms of stress, stress corrosion cracking occurred only in II (N0.8 to !9) corresponding to claims 2, 3, and 4. Not occurring, ALTi, Nb, B.

It can be seen that in addition to the addition of W and Ce, the addition of a small amount of Sn or the restriction of grain size further increases the reliability against stress corrosion cracking. Furthermore, in test (2), which is the most severe test condition in this example, M (No.

16-19), namely Al, Ti, Nb, B, W, Ce
In addition to the addition of a small amount of Sn, only those with a grain size number of 4 or more did not generate any cracks. That is, it is clear that this condition is the most effective in preventing stress corrosion cracking found in the present invention.

Table 2 *: Through-thickness cracking

[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 wt% (hereinafter referred to as wt%), Si: 0
．． 15-1.00% (not including 0.15%), Mn: 0
．． 60-1.00% (not including 0.60%), P:≦0
．． 010%, S:≦0.030%, Ni:≦4.00%
, Cr: 0.50-2.50%, Mo: 0.25-4.
00%, V:≦0.30%, and further contains Al, Ti
, Nb, B, W, and Ce in total of 0.0
A low alloy steel having good stress corrosion cracking resistance, characterized by containing 0.01% to 0.50%, with the remainder consisting of Fe and unavoidable impurities. (2) The low alloy steel according to claim 1, which has good stress corrosion cracking resistance, characterized in that the prior austenite grain size is ASTM grain size number 4 or more. (3) C: ≦0.40%, Si: 0.15-1.00%
(excluding 0.15%), Mn: ≦0.60-1.00
% (excluding 0.60%), P:≦0.010%, S:
≦0.030%, Ni:≦4.00%, Cr:0.50
~2.50%, Mo: 0.25~4.00%, V:≦0
．． 30%, and further contains the following groups (i) and (ii)
); (i) at least one of Al, Ti, Nb, W, B, Ce;
A low alloy steel having good stress corrosion cracking resistance, characterized in that the total amount of seeds is 0.001 to 0.50% (ii) Sn: 0.003 to 0.015%, the balance being Fe and unavoidable impurities. (4) The low alloy steel according to claim 3, which has good stress corrosion cracking resistance, characterized in that the prior austenite grain size is ASTM grain size number 4 or more.