JPS5970750A - Austenitic stainless steel with superior stress corrosion cracking resistance and crevice corrosion resistance - Google Patents

Abstract

PURPOSE:To improve the stress corrosion cracking resistance and crevice corrosion resistance of an austenitic stainless steel by adding Cu to the steel by an amount related to the amount of P in the steel as well as N. CONSTITUTION:This austenitic stainless steel with superior stress corrosion cracking resistance and crevice corrosion resistance has a composition consisting of, by weight, <=0.08% C, <=1.0% Si, <=2.0% Mn, <=0.045% P, <=0.03% S, 6.0- 20.0% Ni, 16.0-25.0% Cr, 0.03-0.30% N, Cu by an amount satisfying the equation and the balance essentially Fe. 0.2-1.0% Mo may be added to the composition

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an austenitic stainless steel having excellent stress corrosion cracking resistance and crevice corrosion resistance.

Austenitic stainless steels, such as 5US304, are used in a wide range of applications because of their excellent corrosion resistance, weldability, and workability.

Stress corrosion cracking may occur in a usage environment that contains Cj2- ions and has a relatively high temperature.

Concentrated magnesium chloride solutions or concentrated salt solutions have mainly been used to study stress corrosion cracking in stainless steel. The effects of component elements on stress corrosion cracking susceptibility during melting for accelerated testing differ depending on the type of solution. For example, the 42 standard of JISGO57 Otsu
% magnesium chloride solution, although the addition of Mo is detrimental.

20 with 1% sodium dichromate added as an oxidizing agent
% salt bath solution, addition of Mo is effective.

Considering that the influence of component elements on stress corrosion cracking susceptibility differs depending on the type of test solution, it is necessary to clarify the influence of component elements under test conditions close to actual conditions.

The present inventors used spot welded specimens with a welded part having a structure with gaps and welding residual stress, and found that stress corrosion cracking in a low concentration salt solution, as in a real environment, occurs from the crevice corrosion area. Various studies were conducted to develop an austenitic stainless steel with excellent stress corrosion cracking resistance and crevice corrosion resistance.

The present inventors used 5O830 to improve stress corrosion cracking resistance.
When Cu was added to 4-series stainless steel, the relationship with P contained in the steel was examined in detail.

It was found that there is a certain relationship between the relative amount of Co-q and P content and corrosion, and that stress corrosion cracking occurs from crevice corrosion areas. We have discovered a steel composition that has the most significant feature in that it has improved properties. According to the invention.

1. At Shigeharu Chi.

c: o, oa or less, Si: 1.0 or less, Mn: 2.
0% or less, P: 0.045 inches or less, S: 0.03 inches or less, Ni: 6.0-15.0%, Cr:
16.0 to 25.0%, N: 0.03 to 0.30%, and an amount of Cu that satisfies the conditions of the following formula, with the balance being F.
There is provided an austenitic stainless steel 6 (4) which has excellent stress corrosion cracking resistance and crevice corrosion resistance, and is characterized by comprising e and unavoidable impurities.

60 [P%] + 0.6%≦[Cu%]≦3.0% and according to the invention, 2% by weight.

c: o, os% or less, Si: 1.0% or less, Mn:
2.0% or less, P: 0.045% or less, 8: 0.03 or less, Ni: 6.0-20.0%, Cr
: 16.0-25.0%, N: 0.03-0.
Stress corrosion cracking resistance characterized by containing 30%, Mn: 0.20 to 1.0 chi, and an amount of CIJ that adds 1 to the conditions of the above formula, with the balance consisting of Fe and unavoidable impurities. An austenitic stainless steel with excellent crevice corrosion resistance is provided.

The reason for limiting the composition of the steel of the present invention will be explained below.

C: C does not significantly affect stress corrosion cracking resistance. However, when C is increased, it is necessary to improve crevice corrosion resistance without impairing stress corrosion cracking resistance due to Cr carbide when welded.

The present invention ensures stress corrosion cracking resistance by limiting the amounts of CufiH and P, and improves crevice corrosion resistance by adding N or N and Mo. As is clear from Table 1, commercially available steel and comparative steel have a concentration of 50 ppm.
Cracks occur in the immersion test with a C-free solution, or the crevice corrosion loss in the immersion test with a 6.5% NaC solution is 5 Wu1 or more.

On the other hand, in the steel of the present invention, stress corrosion cracking does not occur, and crevice corrosion loss is small.

The reason for limiting the amounts of Cu and P is as described above, but since they tend to precipitate, the upper limit was set to 0.08%.

Si: Si is necessary for H adjustment and deoxidation, but since it impairs processability, the upper limit was set at 1.0%.

Mn DiMn is necessary for deoxidizing, desulfurizing, and improving hot workability during steel manufacturing, but since it deteriorates corrosion resistance, the upper limit was set at 2.0%.

Since SO8 does not affect stress corrosion cracking susceptibility, it can be kept below the normally allowed 0.03%, but it is desirable to have a low content since it is harmful to the occurrence of corrosion.

Cr: Cr is arsenic, which is essential for maintaining corrosion resistance, and if it is less than 16%, sufficient corrosion resistance cannot be obtained.

On the other hand, if it exceeds 25%, the workability will deteriorate, so 16.0~2
Limited to 5.0 staff.

Nj: Ni is an essential element for maintaining the austenite phase, and 6.0% or more is required to maintain acid resistance, but adding more than 20% is economically expensive6. It was limited to 0 to 20.0%.

N: N is an element effective in improving crevice corrosion resistance.
If it is less than 0.03%, no effect will be obtained, and if it exceeds 0.30% (!: 0.03 to 0.0%, since a sound steel ingot cannot be obtained during manufacturing
．． Limited to 30%.

Mo: Mo increases stress corrosion cracking susceptibility when Co is not added. When C+s is added, without increasing stress corrosion cracking susceptibility.

Improves crevice corrosion resistance. Addition of less than 0.2% will not produce any effect, and addition of more than 1.0 will be economically expensive, so it is limited to 0.2 to 1.0%.

As mentioned above, the present inventors used SU! containing Mo! 93
It has been found that when Cu is added to 04 series steel, crevice corrosion spreads in low concentration salt solutions and stress corrosion cracking susceptibility decreases. That is, since Co has the effect of spreading corrosion, it cancels out the effect of P that concentrates the corrosion described above, thereby reducing the stress corrosion cracking susceptibility. The amount of Cu required to prevent stress corrosion cracking increases as the amount of P increases. The lower limit amount of Cu for preventing the occurrence of cracks can be defined by the following equation, which was experimentally derived as described in detail below.

CCII section] ≧ 30 [PLl)] + 0.6% Co has the effect of spreading corrosion, so it may be added in large amounts, but if it exceeds 3.0%, it impairs hot workability, so the upper limit is 6%.
．． Set to 0%. Since P impairs weldability, the upper limit is 0.04.
5%.

Next, the steel of the present invention will be specifically explained using examples.

The compositions of the invention steel (example steel) and comparative steel are shown in Table 1.

These steels are made into steel plates with a thickness of 1, and are solution-treated to have a width of 2.
9 fights, on a board of length 61 m + n, width j4 m of the same material
m, a specimen with a length of 16 mm stacked and in contact with a spot m,
A test was conducted in which the specimens were immersed in a NaC solution with a concentration of 50 ppm CJ-' at 80° C. for 3 days, and the presence or absence of stress corrosion cracking was determined by cross-sectional observation. Crevice corrosion was evaluated by immersing a similar specimen in 3.5% NaC at 80° C. for 10 days and determining the change in weight before and after the test. The results are shown in Table 1 and in FIG.

To ensure stress corrosion cracking resistance from the first stage.

It can be seen that it is necessary to maintain the correlation between the amount of P and the amount of Co as expressed by the above equation.

As P is lowered, stress corrosion cracking susceptibility decreases, but even in @I4 (steel Na4) that does not contain an appropriate amount of Cu (!:P: 0.002%), cracking occurs from the bottom of the corrosion hole due to crevice corrosion. occurs.In this case, the depth of the corrosion hole is 03
~0.4 tnx is reached. In steel with low P, crevice corrosion initially grows large, but after 1 hour, P is leached into the corrosion pores.
will suppress dissolution inside the corrosion hole.

However, the tip of the corrosion hole remains active because the stress is greater than in other parts of the corrosion hole. It is thought that stress corrosion cracking occurs in these areas because this type of corrosion is concentrated locally.

The steel of the present invention has excellent stress corrosion cracking resistance and crevice corrosion resistance.Currently, austenitic stainless steels are used in hot water equipment (e.g. electric hot water It is suitable as a material for water heaters, hot water wheelers) and hot water supply piping, and because it contains Co as a component, it is expected to have excellent corrosion resistance even when exposed to non-oxidizing acids such as sulfuric acid and hydrochloric acid. Ru.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the influence of P and Cu on the stress corrosion cracking resistance of spot welded specimens of the steel of the present invention and comparative steel after being immersed in a 50 ppm C-free solution at 80° C. for 30 days. Patent Applicant: Nisshin Steel Co., Ltd. Agent, Patent Attorney: Masahiro Matsui (2 others), 0 people, 40% of the total amount) L Mochi, Shi, Kurikoku, J. Tenfuso Proceedings Amendment 1982 May 24th, Mr. Kazuo Wakasugi, Commissioner of the Japan Patent Office, 1. Indication of the case, Patent Application No. 179125 of 1982, 3. Relationship with the person making the amendment. Patent applicant: 11 if, N 3-chome Marunouchi, Chiyoda-ku, Tokyo. 1
Mr. 14 No. 1 Name) (458) Nisshin Steel Co., Ltd. 4
, Agent 5, Date of amendment order Voluntary 6, Number of inventions increased by amendment None 7, Application subject to amendment (furigana of the title of the invention). 1. The scope of claims is amended as follows. "13% by weight, C: 0.08% or less, Si: 1.0% or less, Mn +
2.0% or less, P: 0.045% or less, S: 0.03% or less, Ni: 6.0-20.0%, Cr
: 16.0~25.0%, N2 0.03~030% and Cu in an amount that satisfies the following conditions, the balance being Fe.
and an austenitic stainless steel with stress corrosion cracking resistance and crevice corrosion resistance, characterized by the presence of unavoidable impurities. ro01) (φ) -1-[-1,65Co 6 voices・)≦
Filter D2: C: 0.08 or less, Si: 1.0% or more, Mn: 2 by weight
．． 0% or less, P: 0.045 inches or less, S: LiO filtration, Ni: 6. O~20. ．． 0%, Cr:1
60-25.0%, N: 0.03-0.30t, Mo
: 0.2~1.0ch and 1 that satisfies the following conditions
11: Containing noniu 5. An austenitic stainless steel with stress corrosion cracking resistance and crevice corrosion resistance, characterized by comprising the remainder Fe and unavoidable impurities. 30F(chi)+06≦Ct+(chi)≦6.02, "6.0-15.Ochi" on page 5, line 1 of the specification is changed to "60-20.
Corrected to 0%. 6. Specification page 5 line 7 2r 30 (:Pchi) +〇,
6chi≦[C11chi]≦6.0%” j 30F(%l+
0.6≦Cu(%)≦6.0”. 4. Specification, page 6, lines 2-3 [Stress Corrosion Resistance] There is a need to improve crevice corrosion resistance without impairing properties. ”
``Because it is easy to precipitate'' - the limit was set to 0.08%. ” is corrected. 5. Specification page 6 lines 4-12 [The present invention is (':u sleeve
...6 "C11 and..." on line 13-14 of specification 6
... Medium 0.08%. ” to be deleted. Z Table 1 of Specification 9 is amended as follows. 253

Claims (1)

[Claims] 1. C in weight%: 0.08% or less r 8 r: 1.0% or less,
Mn: 2.0% or less, P: 0.1145% or less,
S: 0.03 inch or less, Ni: 6.0-20.0
%, Cr: 16.0-25.0%, N: 0.0
An austenitic system with excellent stress corrosion cracking resistance and crevice corrosion resistance, characterized by containing 3 to 0.60% of Cu in an amount that satisfies the following conditions, with the remainder consisting of 1'e and unavoidable impurities. stainless steel. '50CP%] +〇, 6chi≦[011%]≦3.0%2
, C: 0.08% or less, Si: 1.0% or less, Mn in weight%
: 2.0% or less, P: 0.045% or less, S: 0.03
Below, Ni: 6.0-20.0%, Cr
: 16.0~25.0%, N:0.05~[130
%, Mo: 0.2 to 1.0%, and an amount of Cu that satisfies the following conditions, with the balance consisting of Fe and inevitable impurities.It has stress corrosion cracking resistance and crevice corrosion resistance. Excellent austenitic stainless steel. 30 [Pchi) +0.6%≦(Co%]≦5.O%