JPS63157838A - Two-phase stainless steel excellent in crevice corrosion resistance - Google Patents

Abstract

PURPOSE:To improve crevice corrosion resistance, by providing a specific composition in which Al and Cu are incorporated besides Cr, Ni, Mo, and N as principal components of a two-phase stainless steel, respective contents of O and S are reduced, and Ca is incorporated. CONSTITUTION:The two-phase stainless steel has a composition consisting of, by weight, 0.005-0.10% C, 0.01-0.60% Si, 0.1-2.0% Mn, 21.0-27.0% Cr, 3.0-9.0% Ni, 1.0-5.0% Mo, 0.2-1.5% Cu, 0.03-0.30% Al, 0.0005-0.010% Ca, <=0.030% P, <=0.003% S, 0.1-0.5% N, <=0.0050% O, and the balance Fe. Moreover, one or more kinds among 0.01-0.50%, each, of W, Ti, and Nb may further by incorporated to the above composition. In this steel, the passivating capacity of the steel is remarkably improved by the incorporation of Al and Cu, MnS inclusions most liable to be the origin of corrosion are reduced by the reduction of O and S contents, and further, Ca spheroidizes the inclusions and reduces their surface areas. As the result, a steel excellent in crevice corrosion resistance can be obtained.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is used for heat exchangers that use seawater, chemical equipment or structures that require seawater resistance, oil country tubular goods, line pipes, etc. Regarding duplex stainless steel, that is, stainless steel consisting of an austenite phase and a ferrite phase,
In particular, it relates to duplex stainless steel with improved crevice corrosion resistance in seawater.

(Prior art) Duplex stainless steel, represented by SO5, 329Jl, is considered to be a promising material with excellent stress corrosion cracking resistance and general corrosion resistance, that is, properties intermediate between austenitic and ferritic systems. It is being

However, duplex stainless steel does not have sufficient resistance to so-called crevice corrosion, which occurs, for example, at the joints of equipment used in seawater.

Regarding duplex stainless steel, Japanese Patent Publication No. 59-5662 discloses steel containing Las Ce as a basic component of duplex stainless steel, and Japanese Patent Publication No. 59-14099 and Japanese Patent Publication No. 53-4
No. 3372 discloses steel containing W and (2) as basic components.

However, problems remain in that the alloy components of these steels are expensive and the improvement in crevice corrosion resistance is insufficient.

(Problems to be Solved by the Invention) As mentioned above, duplex stainless steel is known to have relatively excellent corrosion resistance even in seawater, and there is a track record of its use in this field. Crevice corrosion is likely to occur in the parts.

Therefore, it is an object of the present invention to provide a duplex stainless steel with higher reliability that can be used in a seawater environment by solving the problem of crevice corrosion.

(Means for Solving the Problems) The inventors investigated the crevice corrosion generation mechanism of duplex stainless steel using an electrochemical method, and investigated the effect of the alloy on the crevice corrosion initiation point and repassivation ability after crevice corrosion occurrence. A study focusing on the effects of elements revealed that in addition to the basic components of duplex stainless steel, such as Cr, Ni, Mo, and N, the repassivation ability of duplex stainless steel was improved by including both chuan and Cu. This results in significantly improved crevice corrosion resistance, and that the most likely starting points for crevice corrosion are MnS-based inclusions, followed by large oxide-based inclusions. I found out.

Therefore, for the purpose of reducing the inclusions mentioned above, 0 and S were reduced,
In addition, in order to reduce the surface area of remaining inclusions, we included Ca to make the inclusions spheroidal, and found that this inclusion reduction and spheroidization measures were very effective in improving crevice corrosion resistance. Ta.

This invention has been made based on the above findings.

That is, in this invention, C: 0.05 to 0.10 yt%
(Hereinafter, cars are indicated as %), Si: 0.01 to 0.6
0%, Mn: 0.1-2.0%, Cr: 21.
0-27.0%, Ni: 3.0-9.0%, Mo
: 1.0~5.0%, Cu: 0.2~1.5%,
Al: 0.03-0.30%, Ca: O, 0O
05-0.010%, P: 0.030% or less, S
: 0.003% or less, N: 0.1-0.5% and 0
: A duplex stainless steel with excellent crevice corrosion resistance characterized by containing 0.0050% or less, and the remainder consisting essentially of Fe and unavoidable impurities; 0.50%, Ti: 0.01
~0.50% and Nb: A duplex stainless steel containing one or more types selected from 0.01 to 0.50%.

By the way, several studies have been conducted on crevice corrosion of duplex stainless steel, but as there was no suitable accelerated test method, all of them could only qualitatively evaluate the effects of alloying elements, and could not specifically evaluate the crevice corrosion resistance. It was difficult to link this to measures to improve sexual performance.

Therefore, we conducted research on crevice corrosion using electrochemical methods, and found that crevice corrosion has a process of occurrence and growth, and that different factors act on each process.

In other words, the amount of inclusions in steel and their morphology have the greatest influence on the occurrence of crevice corrosion, and the growth of crevice corrosion is
The rate of repassivation has an influence. Specifically, the starting points for crevice corrosion in duplex stainless steel are MnS-based inclusions and large SiO□/AhO3-based inclusions, and the morphology is A-based inclusions. It was found that this is the most likely source of occurrence.

In order to reduce this starting point of crevice corrosion, S should be set to 0.00.
3% or less, 0 to 0.005% or less, and at the same time Ca
The most effective measure was to make the remaining inclusions into spheroids by containing them in a range of 0.0005 to 0.010%.

In addition, as a result of investigating the influence of alloying elements on the repassivation rate of duplex stainless steel in seawater using the pinch electrode method, it was found that both 0.03% or more of AI and 0.2% or more of Cu were contained. was the most effective in accelerating the repassivation rate.

The alloy of the present invention may be manufactured by conventional means.

(Function) Next, the reason for limiting the chemical components in this invention will be explained.

C: 0.005-0.10% C is an element necessary to ensure strength, and 0.005%
If the content is less than 0.1%, the effect will be small, and if the content exceeds 0.1%, it will adversely affect the toughness, so the content was limited to the range of 0.005 to 0.1%.

Si: 0.01 to 0.60% Si is contained for the purpose of deoxidizing during steel manufacturing, but 0.0%
If the content is less than 1%, the effect will be small, and if the content exceeds 0.60%, it will adversely affect processability and toughness.
It was limited to a range of 0.01 to 0.60%.

Mn: 0.1 to 2.0% Mn is included for deoxidation and prevention of hot brittleness, but if it is less than 0.1%, its effect is small. At the same time, it also has the effect of stabilizing austenite, and can be used as a substitute element for expensive Ni. However, since a content exceeding 2.0% adversely affects corrosion resistance, the content is limited to a range of 0.1 to 2.0%.

Cr: 21.0 to 27.0% Cr is an important element that determines the phase ratio in duplex stainless steel, that is, the ratio of austenite phase to ferrite phase, and also controls its corrosion resistance. In order to obtain sufficient corrosion resistance as duplex stainless steel, at least 21.
It is necessary to have a content of 0% or more, but since a content exceeding 27.0% will have a negative effect on workability and weldability, 21.0% to
It was limited to a range of 27.0%.

Ni: 3.0 to 9.0% Ni is also an important element in determining the phase ratio of duplex stainless steel, and must contain at least 3.0% or more to form a duplex structure. Since a content exceeding 9.0% increases the price of steel and has a small effect on improving corrosion resistance, it is limited to a range of 3.0% to 9.0%.

Mo: 1.0 to 5.0% Mo is an element that improves corrosion resistance, and to improve crevice corrosion resistance, it is necessary to contain 1.0% or more, but 5.
．． Since a content exceeding 0% adversely affects weldability and toughness, the content was limited to a range of 1.0 to 5.0%.

Cu: 0.2 to 1.5% Cu significantly improves crevice corrosion resistance when coexisting with AI, but less than 0.2% has little effect;
If the content exceeds 0.2%, the hot workability will deteriorate.
It was limited to a range of ~1.5%.

Al 70.03-0.30% At coexists with Cu and is an effective element for improving crevice corrosion resistance, but if it is less than 0.03%, its effect is small, and on the other hand, from the viewpoint of crevice corrosion resistance The more the better, but from 0.
If the content exceeds 30%, the toughness deteriorates, so 0.0
It was limited to a range of 3 to 0.30%.

Ca: 0.0005 to 0.010% Ca has the effect of improving crevice corrosion resistance by spheroidizing sulfide inclusions, but below 0.0005%, the effect is small; If it exceeds 0%, it will actually increase oxide inclusions and deteriorate crevice corrosion resistance.
．． It was limited to a range of 0.0005% to 0.010%.

P: 0.030% or less P has a negative effect on corrosion resistance and hot workability, so 0.0
It was limited to 30% or less.

S: 0.003% or less S produces sulfide inclusions that become the starting point for crevice corrosion, and also has a negative effect on hot workability.
If it exceeds 3%, sulfide inclusions will increase and the crevice corrosion resistance will be significantly deteriorated, so it is limited to 0.003% or less.

N: 0.1-0.5% N is effective in improving corrosion resistance, but if the content is less than 0.1%, the effect will be small, and if the content exceeds 0.5%, hot workability will deteriorate. It was limited to a range of 0.1 to 0.5%.

0: 0.0050% or less Since 0 forms oxide inclusions and promotes crevice corrosion, it is desirable that the content be as low as possible.

Further, if the content exceeds 0.0050%, inclusions cannot be sufficiently spheroidized by addition of Ca, so the content is limited to 0.0050% or less.

Further, one or two selected from W, Ti, and Nb.
It is advantageous to contain more than one species, WlTt and N
Both b have the same function and effect of improving corrosion resistance and crevice corrosion resistance, and each content ranges from 0.01 to 0.
The range is 50%.

In other words, W, Ti, and Nb are effective in improving corrosion resistance and crevice corrosion resistance, but if the content is less than 0.01%, the effect is small, while if the content exceeds 0.5%, it deteriorates weldability and toughness. , limited to a range of 0.01 to 0.50%.

(Example) The stainless steels shown in Table 1 as the invention steel and comparative steel were
After melting in a 0 kg vacuum melting furnace and hot rolling to a thickness of 5 mm, it was solution treated at 1050°C and processed into the crevice corrosion test piece shown in Figure 1. The test piece was tightened with a washer and subjected to a crevice corrosion test.

The crevice corrosion test was carried out in artificial seawater at 30°C (A37M011
41) for 90 days, and the crevice corrosion resistance was evaluated based on the maximum crevice corrosion depth.

There were 15 test pieces for each steel type, and the maximum erosion depth among them was used as the evaluation value.

As shown in Table 2, the crevice corrosion test results show that invention steels 9 to 17
Both exhibit excellent crevice corrosion resistance, with a maximum corrosion depth of 0.
．． It was 0.02 mm or less.

On the other hand, comparative steels 1, 2.3, and 6 have N, Mo, and 8! Also, the content of Cu is small and sufficient crevice corrosion resistance cannot be obtained. This is N,M. This is thought to be due to the insufficient repassivation ability of the steel due to the insufficient content of , AI and Cu.

Furthermore, Comparative Steel 4 did not contain enough Ca and the inclusions were not sufficiently spheroidized, and the occurrence of crevice corrosion could not be suppressed, so that sufficient crevice corrosion resistance could not be obtained.

It is thought that Comparative Steels 5, 7, and 8 had excessive amounts of Ca, ○, and S, respectively, which increased the amount of inclusions and promoted the occurrence of crevice corrosion, so that sufficient crevice corrosion resistance could not be obtained. It will be done.

Table 2 Crevice Corrosion Test Results (Effects of the Invention) According to the present invention, a duplex stainless steel with good crevice corrosion resistance in seawater can be provided.

For example, the maximum crevice corrosion rate of the invented steel in the above example is 0.08 mm/year or less, and if the invented steel is used as a corrosion-proofing plate for offshore structures, the plate Jt3+nm can be guaranteed for 30 years. It is possible to develop new demand. It is also advantageously suitable as a member for seawater valves, flanges, and the like.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing a test piece to be subjected to a crevice corrosion test. Patent applicant Kawasaki Steel Corporation Daiichi P4RNM

Claims (1)

[Claims] 1. C: 0.005 to 0.10 wt%, Si: 0.01 to 0.60 wt%, Mn: 0.1 to 2.0 wt%, Cr: 21.0 to 27.0 wt% %, Ni: 3.0 to 9.0 wt%, Mo: 1.0 to 5.0 wt%, Cu: 0.2 to 1.5 wt%, Al: 0.03 to 0.30 wt%, Ca: 0. 0005 to 0.010 wt%, P: 0.030 wt% or less, S: 0.003 wt% or less, N: 0.1 to 0.5 wt%, and O: 0.0050 wt% or less, and the remainder is substantially Fe. and unavoidable impurities. 2. C: 0.005 to 0.10 wt%, Si: 0.01 to 0.60 wt%, Mn: 0.1 to 2.0 wt%, Cr: 21.0 to 27.0 wt%, Ni: 3. 0 to 9.0 wt%, Mo: 1.0 to 5.0 wt%, Cu: 0.2 to 1.5 wt%, Al: 0.03 to 0.30 wt%, Ca: 0.0005 to 0.010 wt% , P: 0.030 wt% or less, S: 0.003 wt% or less, N: 0.1 to 0.5 wt%, and O: 0.0050 wt% or less, further W: 0.01 to 0.50 wt%, Ti:
0.01-0.50wt% and Nb: 0.01-0.5
A duplex stainless steel having excellent crevice corrosion resistance, characterized in that it contains one or more selected from 0wt%, and the remainder consists essentially of Fe and unavoidable impurities.