JP2003293090A - Duplex stainless steel having excellent pitting corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide duplex stainless steel which has excellent pitting corrosion resistance, and is suitable as the high corrosion resistant stock for piping for the chemical industry. <P>SOLUTION: The duplex stainless steel having excellent pitting corrosion resistance consists of steel having a composition containing, by mass, ≤0.05% C, 0.1 to 1.0% Si, 0.2 to 1.5% Mn, ≤0.035% P, ≤0.005% S, 3 to 7.5% Ni, 21 to 28% Cr, 1 to 5% Mo, ≤0.5% Cu, 0.005 to 0.04% Al and ≤0.25% N, and the balance Fe with inevitable impurities, and satisfying a γ ratio of 0.3 to 0.7, PRE (pitting resistant equivalent) of ≥32.5, and ΔPRE of -1.0 to 1.0. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a duplex stainless steel having excellent pitting corrosion resistance, which is particularly suitable as a highly corrosion-resistant material for piping for the chemical industry.

[0002]

2. Description of the Related Art Conventionally, as a high-strength duplex stainless steel having excellent pitting corrosion resistance, for example, JP-A-62-50444, JP-A-62-253755, and JP-A-11-111.
No. -80901 is known. Further, as a high corrosion-resistant duplex stainless steel excellent in hot workability, JP-A-2-2
Japanese Patent No. 58956 is known. These are γ-phase Cr
Setting + 3Mo + 16N to 34.5 or more (defining the corrosion resistance parameter of the γ phase) governs the presence or absence of pitting corrosion, and prevents the corrosion resistance contributing elements Cr, Mo, and N from being added excessively, and the appropriate pitting resistance It is said that stainless steel, which is edible, can be obtained.

[0003]

However, although it is very important to improve the corrosion resistance of the γ phase, simply defining the corrosion resistance parameter of the γ phase (improving the corrosion resistance of the γ phase) Even if it doesn't corrode,
On the contrary, it has been found that the preferential corrosion of the α phase may occur and the pitting corrosion resistance may be remarkably inferior, and a material having excellent pitting corrosion resistance cannot be obtained only by defining the corrosion resistance parameter of the γ phase. That is, in the duplex stainless steel, the amount of elements (Cr, Mo, N) that contribute to corrosion resistance is γ in the steel (= α
The amount of Cr and M in the steel
Even if the contents of o and N are the same, γ of Ni, Si, etc.
Corrosion resistance changes greatly depending on the content of α and α, and γ
A composition in which either the phase or the α phase is unevenly corroded is inferior in corrosion resistance. None of the above-mentioned patents have solved this point.

[0004]

[Means for Solving the Problems] As a result of intensive development to solve the above-mentioned problems, as a result, the pitting corrosion resistance of the material can be obtained without excessive addition of the elements Cr, Mo, N contributing to the corrosion resistance. In order to bring out the maximum value, the most important thing is to make the corrosion resistance of the γ phase and the corrosion resistance of the α phase as equal as possible (−1.0 ≦ ΔPRE ≦ 1.0, that is, −
If it is less than 1.0, the α phase is preferentially corroded, and conversely 1.
It was found that the γ-phase and the α-phase are corroded equally, and the maximum corrosion resistance can be obtained. Further, in order to make the corrosion resistance of the γ phase and the α phase equal, it is important to properly distribute Cr, Mo, and N in the γ phase and the α phase, and among them, the distribution of N is most important.

That is, in order to equalize the corrosion resistance of the γ phase and the α phase and suppress preferential corrosion of each phase,
Obtain the composition of each of the γ phase and α phase, and relate the composition of each of the γ phase and α phase to the matrix component,
By correlating the corrosion resistance of each phase with the obtained component composition of each phase and analyzing the relationship between the corrosion resistance and the parameters, the conditions under which the pitting corrosion resistance becomes equal are found.

The gist of the invention is as follows: (1) Mass%, C: 0.05% or less, Si: 0.1
1.0%, Mn: 0.2 to 1.5%, P: 0.035%
Hereinafter, S: 0.005% or less, Ni: 3 to 7.5%, C
r: 21 to 28%, Mo: 1 to 5%, Cu: 0.5% or less, Al: 0.005 to 0.04%, N: 0.25% or less, and the balance Fe and unavoidable. Impurity steel with a γ ratio of 0.3 to 0.7 and a PRE of 3
A duplex stainless steel having excellent pitting corrosion resistance, which is made of a steel having a value of 2.5 or more and a ΔPRE satisfying -1.0 to 1.0. γ rate = (149.7C-6.01Si + 2.
76Mn + 6.41Ni-5.62Cr-3.74Mo
+ 206.2N + 117.2) / 100, PRE = Cr
+ 3.3Mo + 16N, △ PRE = PRE (α) -PR
E (γ) = {Cr (α) +3.3 ・ Mo (α) +16 ・
N (α)}-{Cr (γ) + 3.3 · Mo (γ) +16
N (γ) = (0.08γ + 0.0824) Cr + 3.
3 (0.27γ + 0.2879) Mo-16 {N-0.
005 (0.21γ + 0.9563) Cr + 0.059
5} /γ-3.5 where Cr (α) represents the Cr concentration in the α phase and Mo (γ) represents the Mo concentration in the γ phase.

(2) In addition to the component composition described in (1) above, Ti: 0.1 to 0.4%, V: 0.1 to 0.4%,
Nb: Duplex stainless steel excellent in pitting corrosion resistance, characterized in that it is 0.1% to 0.4% or more, and is 0.4% or less in total. (3) In addition to the component composition described in (1) or (2) above, Ca: 0.0005 to 0.01%, B: 0.000
It is a duplex stainless steel excellent in pitting corrosion resistance, characterized by containing 5 to 0.003% of one or two kinds.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition of the present invention will be described below. C: 0.05% or less C is a typical solid solution strengthening element, but a large amount of C is contained.
The upper limit was set to 0.05% because the r-carbide precipitates on the grain boundaries and deteriorates the corrosion resistance. Si: 0.1 to 1.0% Si is necessary for deoxidizing steel. However, if less than 0.1%, the effect is not obtained, and addition of a large amount causes embrittlement due to σ phase precipitation, so the upper limit is 1.0%.
And

Mn: 0.2-1.5% Mn, like Si, is necessary for deoxidizing steel. However, if less than 0.2%, the effect cannot be obtained, and addition of a large amount leads to an economical disadvantage, so the upper limit is set to 1.5.
%. P: 0.035% or less P has a maximum content of 0.035% because a large amount of P increases the weld cracking susceptibility.

S: 0.005% or less S is contained in a large amount, which causes deterioration in hot workability, so the upper limit was made 0.005%. Ni: 3 to 7.5% Ni is an element necessary for improving workability and corrosion resistance as a two-phase structure. However, if it is less than 3%, the effect cannot be obtained, and if it exceeds 7.5%, the effect is saturated and the cost becomes high, so the upper limit was made 7.5%.

Cr: 21 to 28% Cr is a necessary element for improving pitting corrosion resistance and obtaining desired corrosion resistance. However, if it is less than 21%, the effect cannot be obtained, and if a large amount is added, the σ phase tends to precipitate and the manufacturability deteriorates. Therefore, the upper limit was made 28%. Mo: 1 to 5% Mo is an element necessary for improving pitting corrosion resistance and obtaining desired corrosion resistance. However, if it is less than 1%, the effect cannot be obtained, and if a large amount is added, the σ phase tends to precipitate and the manufacturability deteriorates. Therefore, the upper limit was made 5%.

Cu: 0.5% or less If Cu is added in a large amount, hot workability deteriorates.
The upper limit was 0.5%. Al: 0.005-0.04% Al is an element necessary for deoxidation. But 0.0
If it is less than 05%, the effect cannot be obtained, and addition of a large amount causes deterioration of workability due to precipitation of AlN, so the upper limit was made 0.04%.

N: 0.25% or less N is an element that improves pitting corrosion resistance and obtains desired corrosion resistance. However, addition of a large amount causes deterioration of hot workability, so the upper limit was made 0.25%. Ti: 0.1-0.4%, V: 0.1-0.4%, N
b: 0.1 to 0.4% of one kind or two or more kinds in total of 0.
4% or less Ti, V, and Nb are elements that further increase the strength,
However, if less than 0.1%, the effect cannot be obtained, and
Since addition of more than 0.4% deteriorates hot workability,
The upper limit was 0.4%.

Ca: 0.0005 to 0.01% Ca is an element that improves hot workability. But,
If it is less than 0.0005%, there is no effect, and if it exceeds 0.01%, the hot workability is deteriorated. Therefore, the upper limit was made 0.01%. B: 0.0005 to 0.003% B, like Ca, is an element that improves hot workability. However, if less than 0.0005%, there is no effect,
On the contrary, the addition of more than 0.003% deteriorates the hot workability, so the upper limit was made 0.003%.

Γ ratio: 0.3 to 0.7 The γ ratio is an equivalent formula showing the toughness of a product, and is 0.3.
If it is less than 100%, the toughness of the product deteriorates. On the other hand, if it exceeds 0.7, the σ phase tends to precipitate in the α phase, which causes embrittlement. Therefore, the upper limit is set to 0.7. PRE: 32.5 or more PRE is a value indicating the relationship between Cr, Mo and N, and 3
If it is less than 2.5, the desired corrosion resistance cannot be obtained, so the lower limit was made 32.5.

ΔPRE is -1.0 to 1.0 ΔPRE is a value indicating the difference between the α phase and the γ phase in PRE. If this value is less than -1.0, preferential corrosion of the α phase occurs. ,
Since the corrosion resistance corresponding to the amount of the added alloy cannot be obtained, the lower limit is set to -1.0. Further, if it exceeds 1.0, preferential corrosion of the γ phase occurs, and corrosion resistance commensurate with the amount of added alloy cannot be obtained, so the upper limit was made 1.0.

FIG. 1 is a diagram showing the relationship between ΔPRE and corrosion weight loss. That is, FIG. 1 shows an alloy having a PRE of about 32.6 (invention examples No. 1 to 11 and comparative examples No. 13 to 23).
The results of pitting corrosion tests are summarized by ΔPRE. PRE: 32.5 or more, as shown in this figure, ΔPRE is -1.0 to 1.
In the range of 0, the corrosion resistance is good, and those outside this range have pitting corrosion. Therefore, it can be seen that, for the material having PRE: 32.5 or more, if the parameter for calculating ΔPRE is used, the material exhibits good pitting corrosion resistance. Further, it can be seen that the material having a PRE of 32.5 or more is a material with good economy in which the added element effectively functions.

[0018]

[Example] 100 kg of the test material having the chemical composition shown in Table 1 was melted in a vacuum induction melting furnace and forged into a 10 mm thick plate, and 1
After being held at 000 to 1100 ° C. for 20 minutes and subjected to solution heat treatment by water cooling, it was processed into a test piece of 20 mm × 30 mm × 3.5 mm, wet-polished to # 600 and degreased to obtain a test piece. As a corrosion test condition, the product was immersed in a 6% ferric chloride solution at 50 ° C for 24 hours to remove the corrosion products attached after the test, washed and dried, and then weighed. It asked and made it the corrosion evaluation.

[0019]

[Table 1]

As shown in Table 1, No. Nos. 1 to 12 are examples of the present invention. 13 to 25 are comparative examples. Comparative Example No. In Nos. 13, 14, and 18 to 21, since the value of ΔPRE is low, no. 15 to 17 and 23 are all ΔP
Since the value of RE is high, the pitting corrosion resistance is inferior.
No. 22 has a high Ni content and a high ΔPRE value, and therefore has poor pitting corrosion resistance. In addition, Comparative Example No. Even if the value of PRE is within the range of -1.0 to 1.0 as in the case of 24 to 25, the pitting resistance is poor when the value of ΔPRE is low. On the other hand, the case of No. It can be seen that all of 1 to 12 have excellent pitting corrosion resistance.

[0021]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, it has become possible to obtain a duplex stainless steel suitable as a highly corrosion-resistant material for piping for the chemical industry. It plays.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between ΔPRE and corrosion weight loss.

Claims (3)

[Claims] 1. In mass%, C: 0.05% or less, Si: 0.1 to 1.0%, Mn: 0.2 to 1.5%, P: 0.035% or less, S: 0. 0.005% or less, Ni: 3 to 7.5%, Cr: 21 to 28%, Mo: 1 to 5%, Cu: 0.5% or less, Al: 0.005 to 0.04%, N: 0 .25% or less, and a balance of Fe and unavoidable impurities, and a γ ratio shown in the following formula of 0.3 to 0.7 and a PRE of 3
A duplex stainless steel having excellent pitting corrosion resistance, which is made of a steel having a value of 2.5 or more and a ΔPRE satisfying -1.0 to 1.0. γ rate = (149.7C-6.01Si + 2.76Mn +
6.41Ni-5.62Cr-3.74Mo + 206.
2N + 117.2) / 100 PRE = Cr + 3.3Mo + 16N ΔPRE = PRE (α) -PRE (γ) = {Cr (α)
+ 3.3 · Mo (α) + 16 · N (α)}-{Cr
(Γ) + 3.3 · Mo (γ) + 16 · N (γ) = (0.
08γ + 0.0824) Cr + 3.3 (0.27γ +
0.2879) Mo-16 {N-0.005 (0.21
γ + 0.9563) Cr + 0.0595} /γ-3.5 where Cr (α) represents the Cr concentration in the α phase and Mo (γ) represents the Mo concentration in the γ phase. 2. In addition to the component composition according to claim 1, T
i: 0.1 to 0.4%, V: 0.1 to 0.4%, Nb:
0.1% to 0.4%, 1 type or 2 types or more, 0.4% in total
Duplex stainless steel with excellent pitting corrosion resistance, characterized in that: 3. In addition to the component composition according to claim 1 or 2, Ca: 0.0005 to 0.01%, B: 0.00
A duplex stainless steel having excellent pitting corrosion resistance, characterized by containing 05 to 0.003% of 1 type or 2 types.