JPH0711366A - Alloy excellent in hot workability and corrosion resistance in high temperature water - Google Patents

Abstract

PURPOSE:To provide an alloy excellent in hot workability and corrosion resistance and to provide its producing method. CONSTITUTION:This alloy excellent in hot workability and corrosion resistance in high temp. water is a one having a compsn. contg. <=0.03% C, <=1.0% Si, <=1.0% Mn, 35 to 43% Cr, 40 to 57% Ni, <=0.5% Al, <=0.5% Ti, <=0.03% N, Ca and Mg by >=0.0004% independently and by 0.001 to 0.03% in total and 0.1 to 3.0% Nb, and in which O and S in impurities are regulated to <=0.002% and <=0.001% respectively. Moreover, hot working in which total 0.5 to 5.0% of one or >=two kinds among Mo, W and V are incorporated according to necessary is finished at least at >=1020 deg.C. This alloy is excellent in SCC resistance in Cl-contg. high temp. water and having good hot workability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to corrosion resistance, particularly stress corrosion cracking resistance (hereinafter referred to as stress corrosion cracking resistance), which is suitable as a material for materials used in the form of thick plates, round bars, pipes, containers, etc. in nuclear or chemical plants. , SCC resistance) and hot workability.

[0002]

2. Description of the Related Art As the above materials used in a nuclear power plant (boiling water type light water reactor) and a chemical plant exposed to high-temperature high-pressure water, Japanese Patent Publication No. 9186/1987 and Japanese Patent Laid-Open No.
Improvement of Nb addition as disclosed in Japanese Patent No. 232246, etc.
lloy600 alloy (trade name, 75% Ni-15% Cr-1.8% Nb-7
% Fe alloys, all wt%) and similarly improved Alloy 690 alloys (same as above, 60% Ni-30% Cr-1.8% Nb-7% Fe alloys, all wt%).

However, in such a Ni-base alloy containing Nb and having high Cr, the hot ductility is low and the hot workability is poor, so cracks and edge cracks occur during hot working, and a product of a predetermined thickness is obtained. There is a problem that it is hard to be caught. Further, in the welded portion of the above alloy, due to the heat effect, Nb is added to form solid solution C and N.
The effect of fixing and stabilizing as (C, N) disappears, and C re-dissolves in the crystal. Therefore, in the weld heat affected zone, a Cr deficient layer is formed in the vicinity of the grain boundaries due to the precipitation of Cr carbide grain boundaries, and a so-called sensitized state occurs, which causes a phenomenon in which the corrosion resistance of that portion deteriorates.

Japanese Unexamined Patent Publication No. 57-161043 discloses Ni50-60
%, An alloy containing 33 to 38% Cr and 0.010 to 0.10% Y is added to improve the hot workability. However, it is quite difficult to appropriately suppress O (oxygen) harmful to hot workability by adding only Y.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an alloy excellent in hot workability and corrosion resistance in high temperature water, especially SCC resistance, and a method for producing the alloy.

[0006]

The summary of the present invention is as follows.
The method of manufacturing the alloys of (1) and (2) and those of (3).

(1) C: 0.03% or less by weight%, Si: 1.0
% Or less, Mn: 1.0% or less, Cr: 35 to 43%, Ni: 40 to 57
%, Al: 0.5% or less, Ti: 0.5% or less, N: 0.03% or less, Ca, Mg: each independently 0.0004% or more, total 0.0
01-0.03% and Nb: 0.1-3.0%, balance Fe
And unavoidable impurities. O (oxygen) in the impurities is
An alloy with excellent hot workability and corrosion resistance in high temperature water, 0.002% or less and S 0.001% or less.

(2) In addition to the components described in (1) above,
An alloy excellent in hot workability and corrosion resistance in high temperature water according to (1) above, which contains 0.5 to 5.0% by weight of one or more of Mo, W and V in total.

(3) The above (1) or the above characterized in that the hot working is terminated at a temperature of at least 1020 ° C. or higher.
The method for producing the alloy according to (2).

The term "high temperature water" as used in the present invention means a temperature of 250 that contains Cl ^- ion which is an ionic species harmful to SCC resistance.
It means water at about 350 ° C.

[0011]

The present inventors have made the present invention based on the following findings.

More than the alloy (Cr: 15 to 35%, Nb: 0.10 to 2.0%) disclosed in the above-mentioned Japanese Patent Laid-Open No. 153763/58.
By increasing the Cr content (Cr: 35 to 43%), the sensitization of the weld heat affected zone can be significantly reduced. Therefore, local corrosion such as stress corrosion cracking does not occur, and the corrosion resistance does not deteriorate.

Further, the above effects are improved by making the Nb content and Nb / C proper.

In order to improve the hot workability, it is necessary to suppress the impurity components, O and S, to the proper contents at the same time.

It is difficult to properly suppress O and S by adding Y (rare earth element) as disclosed in the above-mentioned Japanese Patent Laid-Open No. 57-161043, and Mg and S which have a large affinity with O. It is more effective to add a proper amount of Ca, which has a high affinity for, in combination.

The hot working end temperature has a suitable temperature for reducing the occurrence of cracks.

The reasons why the chemical composition and the hot working finish temperature of the alloy of the present invention are limited as described above will be explained below. % Means% by weight.

C: 0.03% or less C is chrome carbide (Cr ₂₃
It is an element that precipitates C ₆ ) to form a Cr-deficient layer at the grain boundary, and causes deterioration of corrosion resistance. If the C content exceeds 0.03%, the corrosion resistance at grain boundaries deteriorates, so the content is 0.03%.
Below.

Si and Mn: 1.0% or less for each Si and Mn are elements that act as a deoxidizing agent for the alloy, and it is necessary to contain each of them to some extent.
However, in all cases, if the content exceeds 1.0%, the weldability and cleanliness of the alloy will deteriorate, so the content of each will be 1.0%.
Below.

Cr: 35 to 43% Cr is an essential element for maintaining SCC resistance. If this content is 35% or less, it is not possible to prevent the formation of a Cr-deficient layer in the heat-affected zone of the welding, and the sensitization becomes remarkable. Therefore, stress corrosion cracking resistance and corrosion resistance cannot be secured.

On the other hand, if it exceeds 43%, an intermetallic compound is formed and the hot workability is deteriorated. Therefore, the range of the Cr content is 35% or more and 43% or less.

Ni: 40 to 57% Ni is an element effective for improving the corrosion resistance, and particularly resistance to acid and S resistance in high temperature water containing chloride ion (Cl ⁻ ).
Improve CC property. To obtain this effect, the Ni content is
It is necessary to be 40% or more. On the other hand, it is not necessary to set an upper limit from the viewpoint of only corrosion resistance, but considering the contents of other elements such as Cr, it is sufficient to set the upper limit of the Ni content to 57%.

Al: 0.5% or less Al, like Si and Mn, is an element effective as a deoxidizing agent.
If its content exceeds 0.5%, the cleanliness of the alloy is reduced, so the content was made 0.5% or less.

Ti: 0.5% or less Ti combines with N to fix N as TiN or Ti (C, N),
It is an effective element for improving hot workability. The effect is saturated when the Ti content exceeds 0.5%, so the upper limit is 0.
It was set to 5%.

N: 0.03% or less When N content increases, TiN-based inclusions increase,
The N content must be suppressed to 0.03% or less because it deteriorates the cleanliness of the alloy and easily becomes the starting point of pitting corrosion.

Nb: In a 0.1 to 3.0% Ni-based alloy (Ni content of 40% or more), Nb has a larger effect of fixing C than Ti. Therefore, Nb fixes C
Cr is included to suppress the formation of carbides.

To obtain this effect, the Nb content should be 0.1-3.0.
Must be set to%.

When the Nb content is less than 0.1%, the effect of fixing C is insufficient, so that sensitization occurs and intergranular corrosion easily occurs. On the other hand, if it exceeds 3.0%, the fixing effect is saturated, and the hot ductility is greatly reduced. Therefore,
The range of Nb content was 0.1 to 3.0%.

Ca and Mg: 0.0004% or more alone, and 0.001 to 0.03% in total Ca and Mg are elements having a large affinity for S and O, which are harmful to hot workability. To improve hot workability,
S and O existing in the alloy are reduced and Ca and
By including Mg in combination, S and O are respectively Ca
It is necessary to fix it as S and MgO. To obtain this effect, Ca and Mg alone should be 0.0004% or more,
The total amount of Ca and Mg must be 0.001% or more. On the other hand, if the total amount exceeds 0.03%, not only the effect is saturated, but also inclusions increase and the cleanliness of the alloy deteriorates, so the upper limit of the total content of these is set to 0.03%.

O (oxygen): 0.002% or less It is an element that deteriorates the hot workability when the amount thereof present in the alloy increases, and is suppressed to 0.002% or less.

S: 0.001% or less Like O, it is an element that deteriorates the hot workability when the amount present in the alloy is large, and is suppressed to 0.001% or less.

O that easily segregates at grain boundaries and causes grain boundary embrittlement
And S are simultaneously suppressed within the above range, and the above-mentioned Ca, Mg
By fixing S and O as CaS and MgO, respectively, it is possible to prevent cracks and edge cracks at the grain boundaries during hot working.

The alloy of the present invention may contain one or more selected from the following elements, if necessary.

Mo, W, V: These elements are effective for improving pitting corrosion resistance. Each of these elements is added so that the content of one kind or the total content of two or more kinds is 0.5 to 5.0%. If it is less than 0.5%, the passivation film on the surface is not strengthened, so that sufficient improvement in pitting corrosion resistance cannot be expected.
On the other hand, if it exceeds 5.0%, not only this effect is saturated, but also the hot workability is significantly deteriorated.

Hot working end temperature: 1020 ° C. or higher The high Cr, high Ni alloy has high deformation resistance even at high temperatures, and hot working is difficult. Therefore, hot workability is sensitive to the end temperature of processing. When the hot working finish temperature is less than 1020 ° C, the deformation resistance of the alloy base (inside the grain) is remarkably high, the intragranular strength becomes higher than the intergranular strength, and the intergranular strength relatively decreases, resulting in intergranular cracking. Is likely to occur.

Therefore, the end temperature of hot working is set to 1020 ° C. or higher. The upper limit is not particularly limited, but if the temperature is too high, the crystal grains are coarsened and the mechanical properties of the alloy are deteriorated.

The alloy of the present invention contains the above components. The desirable structure of the alloy of the present invention is Cr ₂₃ to the grain boundary.
Suppresses the precipitation of C ₆ and forms a Cr-deficient layer near the grain boundaries,
This is an organization that can prevent the occurrence of SCC due to this. Therefore, it is desirable to perform an appropriate heat treatment.

The heat treatment of the alloy of the present invention includes annealing for adjusting the mechanical properties and heat treatment for removing the Cr-deficient layer, which is subsequently performed as necessary.

The temperature for annealing the alloy of the present invention is 1000-
1200 ℃ is recommended. If the annealing temperature is less than 1000 ° C, the tensile strength, proof stress, hardness, etc. will be unnecessarily high. On the other hand, when the temperature exceeds 1200 ° C., the crystal grains are remarkably coarsened, and predetermined properties such as tensile strength, proof stress and hardness cannot be obtained.

The alloy of the present invention has excellent corrosion resistance even in such an annealed state. However, when heat treatment is further performed at a temperature of 800 ° C. or lower for 0.1 hour or longer, Cr is converted into grains from within the crystal grains. Since the Cr-deficient layer near the grain boundary is recovered by diffusion into the grain boundary, grain boundary type SCC in the presence of alkali at high temperature and high concentration does not occur even if grain boundary precipitation of Cr ₂₃ C ₆ occurs.

[0041]

[Examples] Alloys having the chemical compositions shown in Tables 1 (1) and 1 (2) were melted by a vacuum melting method to obtain steel ingots of 150 mm square and 1000 mm long. No. 1 to 5 in Table 1, No. 13 to 19, No. 24 and N
o.29 is the alloy of the present invention, No. 6 to 12, No. 20 to 23 and No. 2
5 to 28 are comparative alloys.

Using these alloys, a hot workability test and an intergranular stress corrosion cracking resistance test in high temperature water containing Cl ⁻ were carried out.

(1) Hot workability test The above steel ingot was heated at 1200 ° C. for 5 hours, and then hot-worked to a thickness of 50 mm.
Finished in the plate. Table 2 shows the end temperature of the hot working of each alloy at this time.

The presence of cracks on the surface of the thick plate thus obtained is examined by PT (penetration flaw detection method), and those with cracks are cut and polished, and then the crack depth is measured with an optical microscope. The hot workability was evaluated by the method. The results of these tests are shown in Table 2 and FIG.

(2) Grain boundary stress corrosion cracking resistance (SCC resistance)
Test The 50 mm thick plate obtained in (1) above was reheated to 1200 ° C. and then hot rolled to a thickness of 7 mm. Further, after annealing in an argon atmosphere at 1100 ° C. for 30 minutes in air, a sensitization treatment was performed at 700 ° C. for 1 hour. The SCC resistance test piece was made from this sensitized plate material, thickness 2 mm x width 10 mm x length 75 mm.
The strip-shaped test piece was used.

The SCC resistance was determined by polishing these test pieces with No. 320 emery paper, bending them into a U-shape, restraining them with bolts and nuts, and then, in an autoclave container, 3% NaCl at 350 ° C.
After immersing in high temperature water containing TiO2 for 1000 hours, the maximum crack depth was evaluated by a method of measuring with an optical microscope. The results are shown in Table 2.

FIG. 1 shows the effect of S and O on the hot crack depth.
It is a figure which shows the influence of content. The numbers in the figure are alloys in Table 1.
Corresponding to No., the vertical axis and horizontal axis are displayed in Wtppm.

As can be seen from Table 2 and FIG. 1, the S content is 10 ppm or less, the O content is 20 ppm or less, the (Ca + Mg) content is 10 ppm or more, and the hot working finish temperature is 1020 ° C. or more. Invention alloys (No.1-5, No.13-19, No.24,
No. 29) shows no hot cracking and good SCC resistance in high temperature water.

On the other hand, comparative alloys (Nos. 6-12, No. 20-) in which any one or more of the contents of S, O, (Ca + Mg) and Nb are out of the range defined by the present invention. 23, N
In o.28), hot cracking occurs even when the hot working finish temperature is 1020 ° C or higher. Even if the chemical composition is within the range defined in the present invention, the comparative alloy (N
25 to 27), hot cracking similarly occurs.

In the comparative alloy (No. 28) whose Nb content is lower than the lower limit of 0.1% defined by the present invention, the contents of S, O, (Ca + Mg) and the hot working finish temperature are within the ranges defined by the present invention. If so, hot cracking does not occur, but the corrosion resistance deteriorates under the influence of sensitization treatment, and stress corrosion cracking occurs in Cl ^- containing high temperature water.

[0051]

[Table 1 (1)]

[0052]

[Table 1 (2)]

[0053]

[Table 2]

[0054]

The alloys of the present invention exhibits, Cl ^- excellent in SCC resistance in containing high-temperature water, have good hot workability. This alloy is suitable as a structural member such as a core of a chemical plant or a nuclear plant (boiling water type light water reactor) exposed to high temperature and high pressure water.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of S and O contents on hot cracking depth.

Claims (3)

[Claims] 1. By weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, Cr: 35 to 43%, Ni: 40 to 57%, A
l: 0.5% or less, Ti: 0.5% or less, N: 0.03% or less, C
a, Mg: 0.0004% or more for each, 0.001-
0.03% and Nb: 0.1 to 3.0%, the balance consisting of Fe and unavoidable impurities, and O (oxygen) in the impurities is 0.0
02% or less, S is 0.001% or less, an alloy with excellent hot workability and corrosion resistance in high temperature water. 2. In addition to the components according to claim 1, further, in weight%, one or more of Mo, W and V in total.
An alloy excellent in hot workability and corrosion resistance in high temperature water according to claim 1, containing 0.5 to 5.0%. 3. The method for producing an alloy according to claim 1, wherein the hot working is terminated at a temperature of at least 1020 ° C. or higher.