JP2833385B2 - Corrosion resistant austenitic Fe-based alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a corrosion-resistant austenitic Fe-based alloy having excellent corrosion resistance in an acidic aqueous environment containing chlorine and sulfur compounds.

[0002]

2. Description of the Related Art Many materials have been developed as corrosion-resistant austenitic Fe-based alloys and used in various corrosive environments. The representative and cheapest one is Cr
SUS 304 containing 18% and 8% Ni. SUS 304 has sufficient corrosion resistance in freshwater and indoor environments, but it is a corrosion-resistant material that causes local and general corrosion such as pitting and crevice corrosion in acidic aqueous solutions containing chlorine and sulfur compounds. Not really.

It is generally known that the corrosion resistance is improved by increasing the contents of Cr, Mo and N, and various corrosion resistant alloys have been developed by adding these alloying elements. For example, Japanese Patent Publication No. 55-8580 discloses that, in a heat exchanger used in a chlorine ion aqueous solution, it has excellent pitting corrosion resistance and stress corrosion cracking resistance and has a maximum value of Cr 22%, Mo 4%, N 0.5
Japanese Patent Publication No. 59-10426 discloses a crevice corrosion-resistant austenitic stainless steel containing a maximum value of 25% Cr, 8% Mo, and 0.35% N.
The publication discloses a method for producing an austenitic stainless steel having excellent seawater resistance containing Cr 35%, Mo 13% and N 0.5% at the highest values, respectively.

However, in these alloys, N in the alloy is used in a solid solution state by heat treatment, and therefore, the melting limit of N in the molten alloy during steelmaking under reduced pressure or solidification of ingot From the solid solution limit at which bubbles sometimes occur, N
The upper limit of the content is 0.5%.

Japanese Patent Publication No. 57-25633 discloses that the highest value of Cr
A high-strength austenitic stainless steel having pitting resistance, including 40%, Mo 5%, V 3%, and N 1% is shown. In this steel, the upper limit of the N content exceeds 0.5%. However, since the strength of the steel is increased by the precipitation effect of V nitride, all of the N contained is necessarily in a solid solution state to improve the corrosion resistance. Not what you use.

Further, not only Fe-based alloys but also alloys having excellent local corrosion resistance include Ni-based alloys such as Inconel 625 (trade name, 22% Cr, 9% Mo, remaining Ni) and Hastelloy C276 (trade name, 16% Cr, 16% Mo, and residual Ni). In these alloys, excellent corrosion resistance is achieved mainly due to a high Mo content without adding N. However, these alloys are very expensive because they contain a large amount of Ni.

[0007]

As described above, in a conventional austenitic Fe-based alloy having excellent local corrosion resistance, the contents of Cr and Mo are increased and N
Uses the effect of improving the corrosion resistance of
The content is at most 0.5% and does not have sufficient local corrosion resistance. Also contains a large amount of Cr and Mo
Ni-based alloys are extremely expensive.

An object of the present invention is to realize a corrosion-resistant austenitic alloy which is an inexpensive Fe-based alloy and maximizes the effect of improving the corrosion resistance of N.

[0009]

SUMMARY OF THE INVENTION The gist of the present invention is the following alloys (1) and (2).

(1) By weight%, Si: 0.1-0.5%, Mn: 3.
0 to 7.0%, Ni: 20.0 to 30.0%, Cu: 0.3 to 3.0%, C
r: 25.0-30.0%, Mo: 5.0-9.0%, W: 0.3-5.0
%, N: 0.5 to 1.0% and Al: 0.005 to 0.1%, and further contains at least one of Mg, Ca, B and a rare earth element (REM) in a total amount of 0.0005 to 0.02%, with the balance being Fe And unavoidable impurities, C in the impurities is 0.03% or less, P is 0.03% or less, S is 0.003% or less, and O is 0.01% or less.
% Or less, and an austenitic Fe-based alloy further satisfying the following formulas.

PI ≧ 50.0... 4.0 ≧ Ni-bal. ≧ 0... 0.005 ≧ HF ≧ −0.005 where PI = Cr (%) + 3Mo (%) + 1.5W (%) + 10N
(%) Ni-bal. = Ni eq. -1.2Cr eq. Ni eq. = Ni (%) + Cu (%) + 0.3Mn (%) + 30 C
(%) + N (%) Cr eq. = Cr (%) + 1.5Si (%) + Mo (%) + W (%) HF = 2S (%) + O (%)-B (%)-Ca (%)- Mg
(%)-REM (%) (2) In addition to the alloy of (1), one or more of Pd, Pt, and Ru are added in a total amount of 0.0%.
Austenitic Fe-based alloy containing 01-0.03%.

The present inventors have prepared alloys having various chemical compositions and investigated their performance in order to achieve the above object. As a result, alloys containing appropriate amounts of Mn, Ni and Cr can contain high N exceeding 0.5%, and at the same time provide excellent local corrosion resistance. To improve the corrosion resistance of Pt, P
It became clear that the inclusion of trace amounts of d and Ru was effective.
Furthermore, for hot workability, which is a problem in high Cr, Mo austenitic alloys, Ni-bal.
It has been found that optimization of the values and the inclusion of Mg, Ca, B and rare earth elements are effective.

[0013]

The reason why the chemical composition of the austenitic Fe-based alloy of the present invention is limited as described above will be described below.

Si: Si is necessary as a deoxidizing element for reducing the O (oxygen) content in the alloy. For this purpose, a content of at least 0.1% must be ensured. On the other hand, 0.
If it exceeds 5%, precipitation of carbides and intermetallic compounds is promoted in hot working and in welds, and the corrosion resistance and toughness are reduced. Therefore, the Si content is set in the range of 0.1 to 0.5%.

Mn: Mn has an effect of increasing the amount of N dissolved in the alloy, and is an essential component for realizing high N. To obtain this effect, a content of 3.0% or more is required. But 7.
If it exceeds 0%, the corrosion resistance and the hot workability are reduced.
Therefore, the Mn content is set in the range of 3.0 to 7.0%. Preferred is a range of 3.0 to 5.0%.

Ni: Ni is important as an austenite forming element, and also improves corrosion resistance in an acidic aqueous solution. For this purpose, a content of 20.0% or more is required.
However, if it is contained in excess of 30.0%, the solid solution amount of N decreases. Therefore, the Ni content is in the range of 20.0 to 30.0%. Preferably it is in the range of 20.0 to 25.0%.

Cu: Like Ni, Cu improves corrosion resistance in an acidic aqueous solution. In the case of Cu, this effect appears above 0.3%. On the other hand, if it exceeds 3.0%, the local corrosion resistance deteriorates. Therefore, the Cu content is set in the range of 0.3 to 3.0%.

Cr: Cr is effective for improving corrosion resistance and increasing the solid solubility of N. In order to obtain an excellent local corrosion resistance by setting the N content described below to 0.5% or more, the content of Cr must be 25.0% or more. On the other hand, if it exceeds 30.0%, the precipitation of nitrides and intermetallic compounds is accelerated, and toughness and corrosion resistance deteriorate. Therefore, the upper limit is set to 30% .0. Therefore, the Cr content is 25.0-3
A range of 0.0% is appropriate.

Mo: Mo is extremely effective for improving the local corrosion resistance. To obtain the excellent corrosion resistance, which is the object of the present invention, 5.0%
% Or more is required. However, if the content exceeds 9.0%, the toughness and the corrosion resistance deteriorate as in the case of excessive Cr. Therefore, the Mo content is set in the range of 5.0 to 9.0%.

W: Like Mo, W is also effective in improving the local corrosion resistance, and for that purpose, a content of 0.3% or more is required. However, if it exceeds 5.0%, the effect is saturated, and the toughness and corrosion resistance are further deteriorated. Therefore, the Mo content is 0.
The range was 3 to 5.0%. Preferably, it is in the range of 0.5 to 3.0%.

N: N is effective for stabilizing austenite and improving local corrosion resistance. The alloy of the present invention achieves a drastic improvement in local corrosion resistance by containing N higher than before.

If less than 0.5%, excellent local corrosion resistance cannot be obtained. On the other hand, if the content exceeds 1.0%, even in the alloy of the present invention, N cannot be dissolved completely and bubbles are generated during solidification or precipitate as nitride in steel, so that toughness and corrosion resistance deteriorate. Therefore, the proper content of N is in the range of 0.5 to 1.0%.

Al: In the alloy of the present invention containing extremely high N, O (oxygen) in the alloy adversely affects corrosion resistance and hot workability. By setting the Al content in an appropriate range in addition to the Si, it is possible to remove the adverse effects of O. Al is an essential component for this, at least 0.
A content of 005% is required. On the other hand, when the content exceeds 0.1%, Al nitride is generated, and the local corrosion resistance is reduced. Therefore, the Al content is set in the range of 0.005 to 0.1%. More preferably, it is in the range of 0.01 to 0.07%.

One or more of Mg, Ca, B and a rare earth element (REM): As described later, S segregates as an impurity at a grain boundary and adversely affects corrosion resistance and hot workability. M
Rare earth elements such as g, Ca and La and Ce are elements that easily combine with S, and B segregates preferentially at the grain boundaries to prevent and suppress the segregation of S to the grain boundaries. By including one or more selected from the above, it is possible to suppress the adverse effect of S. Rare earth elements such as Mg, Ca, B and La, Ce, etc., when their total amount is less than 0.005%,
The effect is not enough. On the other hand, if it exceeds 0.02%, on the contrary, the local corrosion resistance deteriorates. Therefore, the total content was limited to the range of 0.005 to 0.02%.

One or more of Pd, Pt and Ru: In the alloy of the present invention, one or more of Pd, Pt and Ru are further selectively contained in order to improve the corrosion resistance in an acidic aqueous solution. Is effective. The total content of these components is 0.0
If it is less than 05%, the effect is not exhibited, while if it exceeds 0.03%, the effect is saturated. Since these precious metal elements are extremely expensive, it is not economical to add more than the required amount. Therefore, the total content was limited to the range of 0.005 to 0.03%.

The alloy of the present invention has excellent corrosion resistance and toughness,
In order to achieve both hot workability, the content of impurities in the alloy is further limited as follows.

C: Since C mainly forms Cr carbide during hot working and in a welded portion and deteriorates corrosion resistance, C is set to 0.03% or less.

P: P is set to 0.03% or less because P deteriorates hot workability and resistance to hot cracking during welding.

Since S degrades hot workability and local corrosion resistance, S is 0.00%.
3% or less.

O: O is limited to 0.01% or less because O combines with rare earth elements such as Ca, Mg and La and Ce to suppress the adverse effect of S and reduces its effect. Preferably, it is 0.006% or less.

In the alloy of the present invention, in order to obtain the local corrosion resistance and the hot workability, the following restrictions are further given to the alloy components.

PI value, Ni-bal. Value, HF value: PI value: PI = Cr (%) + 3Mo (%) + 1.5W (%) + 10N
It is an index related to corrosion resistance expressed in (%). Here, Cr is a basic constituent element of the passivation film and strengthens the passivation.
In addition, although Mo, W and N do not contribute to the passive film itself, the corrosion of the passive film which is locally destroyed, such as local corrosion, accelerates the repair of the passive film and reduces local corrosion. Has the effect of suppressing.

Ni-bal. Value is Ni-bal. = Ni eq.-1.2 Cr eq. Ni eq. = Ni (%) + Cu (%) + 0.3Mn (%) + 30 C
(%) + N (%) Cr eq. = Cr (%) + 1.5Si (%) + Mo (%) + W (%), and the HF value is HF = 2S (%) + O (%)-B (%) −Ca (%) −Mg
(%)-REM (%), each of which is an index related to hot workability.
In the alloy of the present invention, these indices are used to simultaneously obtain local corrosion resistance and hot workability.

If the PI value is less than 50.0, the desired local corrosion resistance cannot be obtained. On the other hand, if the Ni-bal. Value exceeds 4.0, internal cracks occur in the steel ingot, which tends to promote cracking during hot working such as forging and hot rolling. On the other hand, if it is less than 0, a uniform austenite structure cannot be obtained. Furthermore, the HF value is 0.005
If S exceeds 400, the suppression of grain boundary segregation of S is insufficient, and cracks are likely to occur during hot working. On the other hand, if it is less than -0.005, Mg, C
a, REM and B become excessive, thereby deteriorating the corrosion resistance.

Therefore, the PI value, the Ni-bal. Value and the HF value were set within the ranges defined by the following formulas (1) and (2), and were satisfied at the same time.

PI ≧ 50.0 ・ ・ ・ ・ ・ ・ 4.0 ≧ Ni-bal. ≧ 0 ・ ・ ・ 0.005 ≧ HF ≧ −0.005 ・ ・ ・

[0037]

EXAMPLES Table 1-1, Table 1-2, Table 2-1 and Table 2-
An alloy having the chemical composition shown in No. 2 was melted by a vacuum induction heating furnace to prepare a 25 kg steel ingot. After heating this ingot to 1250 ° C, it was hot forged to a width of 100 mm x a thickness of 30 mm,
The sheet was heated to 50 ° C. and hot-rolled into a sheet having a thickness of 9 mm. The plate thus manufactured was subjected to a solution treatment by heating to 1150 ° C. for 1 hour and then cooling with water.

The performance evaluation was performed by the following method.

(1) Hot workability The maximum crack length at the end of the hot-rolled sheet was measured.

(2) Pitting Corrosion Resistance The pitting potential of the test specimen was measured in a degassed 20% NaCl + 0.005 mol / l aqueous potassium thiosulfate solution at 100 ° C. according to the method specified in JIS G 0577.

(3) Acid Resistance The test piece was immersed in 20% HCl at 60 ° C. for 2 hours, and the corrosion rate was determined from the weight loss.

The results of the above evaluation tests are shown in Tables 3-1 and 3-2.
Shown in

As can be seen from the table, in the test materials Nos. 1 to 11 in the range of the chemical composition specified in the present invention, the pitting potential is all 0.8 V or more, the corrosion rate is low, and Also showed good performance of 2 mm or less. On the other hand, outside the range of the chemical composition defined by the present invention, high Mn (No. 12), high S (No. 13),
High Ni (No. 15), low N (No. 16) and low Ni (No. 14) were inferior in any of pitting potential, corrosion rate, and edge crack length. Further, any of the test materials (Nos. 17 to 21) whose PI value, Ni-bal. Value or HF value was out of the range defined by the present invention was clearly inferior in any of the above properties.

[0044]

[Table 1-1]

[0045]

[Table 1-2]

[0046]

[Table 2-1]

[0047]

[Table 2-2]

[0048]

[Table 3-1]

[0049]

[Table 3-1]

[0050]

According to the present invention, an inexpensive austenitic Fe-based alloy having excellent pitting corrosion resistance and acid resistance can be obtained.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] (1) Si: 0.1-0.5%, Mn: 3.0-% by weight.
7.0%, Ni: 20.0-30.0%, Cu: 0.3-3.0%, Cr: 2
5.0-30.0%, Mo: 5.0-9.0%, W: 0.3-5.0%,
N: 0.5 to 1.0% and Al: 0.005 to 0.1%, and M
g, one or more of Ca, B and rare earth elements (REM) are contained in a total amount of 0.0005 to 0.02%, and the balance consists of Fe and unavoidable impurities, and C in the impurities is 0.03% or less;
P is 0.03% or less, S is 0.003% or less, and O is 0.01% or less.
Fe-based alloy. PI ≧ 50.0 ・ ・ ・ ・ ・ ・ ・ ・ ・ 4.0 ≧ Ni-bal. ≧ 0 ・ ・ ・ ・ ・ ・ 0.005 ≧ HF ≧ −0.005 ・ ・ ・ PI = Cr (%) + 3Mo (%) + 1.5W ( %) + 10N
(%) Ni-bal. = Ni eq. -1.2Cr eq. Ni eq. = Ni (%) + Cu (%) + 0.3Mn (%) + 30 C
(%) + N (%) Cr eq. = Cr (%) + 1.5Si (%) + Mo (%) + W (%) HF = 2S (%) + O (%)-B (%)-Ca (%)- Mg
(%)-REM (%) 2. Si: 0.1-0.5%, Mn: 3.0-% by weight.
7.0%, Ni: 20.0-30.0%, Cu: 0.3-3.0%, Cr: 2
5.0-30.0%, Mo: 5.0-9.0%, W: 0.3-5.0%,
N: 0.5 to 1.0% and Al: 0.005 to 0.1%, and M
g, Ca, B and one or more of the rare earth elements (REMs) in total 0.0005-0.02%, and Pd, Pt and
One or more of Ru are contained in a total amount of 0.001 to 0.03%, the balance being Fe and unavoidable impurities, C in the impurities is 0.03% or less, P is 0.03% or less, S is 0.003% or less and O Is an austenitic Fe-based alloy that is 0.01% or less and further satisfies the following formulas. PI ≧ 50.0 ・ ・ ・ ・ ・ ・ ・ ・ ・ 4.0 ≧ Ni-bal. ≧ 0 ・ ・ ・ ・ ・ ・ 0.005 ≧ HF ≧ −0.005 ・ ・ ・ PI = Cr (%) + 3Mo (%) + 1.5W ( %) + 10N
(%) Ni-bal. = Ni eq. -1.2Cr eq. Ni eq. = Ni (%) + Cu (%) + 0.3Mn (%) + 30 C
(%) + N (%) Cr eq. = Cr (%) + 1.5Si (%) + Mo (%) + W (%) HF = 2S (%) + O (%)-B (%)-Ca (%)- Mg
(%)-REM (%)