JPS59215469A - Austenitic stainless steel with superior stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To obtain an austenitic stainless steel with superior stress corrosion cracking resistance by specifying a composition consisting of C, Si, Mn, S, Ni, Cu, P and Fe and the relation between components in the composition. CONSTITUTION:This austenitic stainless steel consists of, by weight, <=0.08% C, <=1.0% Si, <=2.0% Mn, <=0.03% S, 6.0-20.0% Ni, 16.0-25.0% Cr, Cu and P by amounts satisfying equations 30P%+0.6<=Cu%<=3.0 and P%<=(Cu%+1.5)/200, and the balance Fe with inevitable impurities. The steel is provided with superior stress corrosion cracking resistance by allowing Cu and P among the components to satisfy said relation. The stress corrosion cracking resistance of the weld zone can be improved furthermore by adding 0.1-1.0% Ti and/or Nb.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an austenitic stainless steel having excellent stress corrosion cracking 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 C7- 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 in these accelerated test solutions vary depending on the type of solution. For example, the 4 standard in JISG[)576
Although the addition of Mo is harmful to a 2% magnesium chloride solution, the addition of Mo is effective to a 20% common salt solution to which 1% sodium dichromate is added as an oxidizing agent. Considering that the influence of component elements on stress corrosion cracking susceptibility differs depending on the type of test solution, etc., it is necessary to clarify the influence of component elements under test conditions close to the actual environment. In this case, it is desirable that the test method not only involve immersion in a solution with a constant Cl concentration, but also include factors that affect corrosion under actual usage conditions.

The present inventors used a spot welded specimen that had a structure with a gap in the welded part and had welding residual stress.

It was found that stress corrosion cracking in low-concentration salt solutions, which occurs in real environments, occurs from crevice corrosion.

Furthermore, under relatively high temperature conditions that cause stress corrosion cracking in austenitic stainless steel, the temperature is actually different on both sides of the stainless steel plate, meaning that heat transfer occurs across the stainless steel plate. I focused on the fact that In this case, if heat flows into the measurement surface, the corrosion conditions will be severe. Taking the above into consideration, the present inventors conducted nine different types of research in order to develop an austenitic stainless steel with excellent stress corrosion cracking resistance.

The present inventors have developed 5US3D for improving stress corrosion cracking and housing.
4 series stainless steel (eta), and at that time, the relationship with P contained in the steel was examined in detail.

It was discovered that there is a certain relationship between the relative amounts of Cu and P and corrosion, and a steel composition with excellent stress corrosion cracking resistance was discovered.

According to the present invention, at 2% by weight, C: 0.08% or less, Si: 1.0% or less, Mn:
2.0 or less, S: O, O or less, Ni: 6.0 ~
200 chi.

Cr: 16.0 to 25.0%, and an amount of Cu and P that satisfies the conditions of the following formula, and the balance is Fe and unavoidable impurities, and has excellent stress corrosion cracking resistance. Austenitic stainless steel is provided.

30P (%) + 0.6≦Cu (%)≦3.0 In order to further improve the stress corrosion cracking resistance of the welded part, one or both of Ti and Nb are added in the range of 0.1 to 1 to the above steel. .0% may be added.

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, if the C content is increased, Cr carbides tend to precipitate during welding, so the upper limit was set at 0.08%.

Although Si2Si is necessary for deoxidation during steel manufacturing, it impairs workability, so the upper limit is set to 1.0.

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

Since srs does not affect stress corrosion cracking susceptibility, it is sufficient if it is below the normally allowable 0.03%, but it is desirable to be low as it is harmful to the occurrence of corrosion.

Cr: Cr is an essential element 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.

Ni: Ni is an essential element for maintaining the austenite phase, and 6.0% or more is required to maintain acid resistance, but addition of more than 20% is economically expensive, so 6. It was limited to 0-20.0%.

Ti, Nb: Ti, Nl) has the effect of fixing C and N, so it improves the intergranular corrosion resistance of the weld zone.

Furthermore, it is effective in reducing the susceptibility to intergranular stress corrosion cracking, but if the coefficient is less than 0.1%, the effect is small, and if it exceeds 1.0%, the effect is saturated and becomes useless. .1 to 1.0%.

The present inventors have found that when Cu is added to 5US304 series steel, crevice corrosion spreads in a low concentration salt solution and stress corrosion cracking susceptibility decreases.

In other words, Cu has the effect of spreading corrosion.

It cancels out the action of P that concentrates corrosion and reduces 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 and the upper limit amount of P for preventing the occurrence of cracks can be defined by the following two experimentally derived equations as described in detail below.

C11(%)≦OPfchi+0.6 Cu has the effect of spreading corrosion in this way, so it may be added in large amounts, but! If it exceeds 1.0%, hot workability will be impaired, so the upper limit is set at 3.0%.

The steel of the present invention will be specifically explained with reference to 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.
A board of 9 myn and 31 m long, made of the same material, 14 m wide,
A test piece made by overlapping and spot welding plates with a length of 16 mm was
A test was conducted in which the sample was immersed in a NaCl solution with a concentration of 50 ppm Cl-' at 0°C for 60 days. The presence or absence of stress corrosion cracking was determined by cross-sectional observation. , (first test).

Furthermore, as mentioned earlier, tests were conducted in the presence of heat transfer. 9 width 25 in the center of a disk with a diameter of 701n'11t
Layer the ends of the veneers with a length of 160mm and 6mm on this part.
Using a test piece that was spot welded and bent 9 times at a right angle 25 mm from the end face of the spot welded side, a test under heat transfer was conducted for 50 minutes using the testing apparatus shown in Figure 1.
It was carried out for 10 days using ppm Cl- solution.

In FIG. 1, the test apparatus has a water inlet and an outlet fixed to a disc part 6 of a specimen 10 with a bolt 5, bolts, and nuts.

A copper rod 2 is contained in a quartz tube 6 wrapped around a heater 1 protected by a refractory material 4.

One end of the copper rod is in contact with the disk portion 6 of the specimen 10.

The number of cracks was determined by drilling out the welded area, opening the specimen, and observing it. (Second test).

The results are listed in Table 1 and shown in FIG.

Figure 2 shows the influence of P and CLI on stress corrosion cracking.
The number of cracks in the second test is 50 or less. (The test mark indicates that there were no cracks in the first test, and the number of cracks was 50 or more in the second test.) The mark indicates that there were cracks in the first test.

From Fig. 2, the amount of P and Cu are required to ensure stress corrosion cracking resistance.
It can be seen that it is necessary to maintain the correlation between the quantities as expressed in the above equation. In the first test, Cu (%) was 30 P (%l+
[If the corrosion rate is 1.6 or more, stress corrosion cracking has not occurred.

However, the second test has more severe corrosion conditions than the first test, and the region with low stress corrosion cracking susceptibility becomes narrower.

Stress corrosion cracking resistance Cu t@+1.5 To maintain P (%) -7. -It is necessary to do the following.

As P is lowered, stress corrosion cracking susceptibility decreases, but if an appropriate amount of Cu is not included, cracks will occur from the bottom of the corrosion hole due to crevice corrosion even in steel with P: 0.005% (Steel No. 2). Occur. In this case, the depth of the corrosion hole is 0.6~
It reaches 0.4 m71. In steel with low P content, the growth of crevice corrosion is large at first, but after 2 hours, the P dissolved into the corrosion holes suppresses dissolution inside the corrosion holes. 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 the corrosion is concentrated in these areas.

The steel of the present invention has excellent stress corrosion cracking resistance.Currently, stress corrosion cracking has occurred using 5US304. It is suitable as a material for hot water equipment (e.g. electric water heaters, hot water boilers) and hot water supply piping, and since it contains + Cu, it has excellent corrosion resistance even in environments with non-oxidizing acids such as sulfuric acid and hydrochloric acid. Be expected.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the apparatus for the second test. Figure 2 shows spot welded specimens of the invention steel and comparative steel.
FIG. 3 is a diagram showing the influence of P and Cu on the presence or absence of stress corrosion cracking when immersed in a 50 ppm C11=solution at 0° C. for 30 days. Patent applicant: Nisshin Steel Co., Ltd. Agent: Patent attorney Masahiro Matsui (1 other person) Figure 1 Figure 2 P C'/) Procedural amendment August 18, 1981 Commissioner of the Japan Patent Office Kazuo Wakasugi 1 , Indication of the case Patent Application No. 087673 No. 3 of 1987, Person making the amendment Relationship to the case Patent applicant address 3-4-1-4 Marunouchi, Chiyoda-ku, Tokyo, Agent 5 Date of amendment order Voluntary action 6 Number of inventions increased by amendment None Ordinance 8 (The formula % is corrected in the second line of page 7 of the specification.)

Claims (1)

[Claims] 1. C: 0.08% or less, Si: 1.0% or less, Mn:
2.0 chi or less, S: 0.03% or less, Ni: 6
．． 0-20.0Li6+Cr:16. O~25.0%,
and an austenitic stainless steel with excellent stress corrosion cracking resistance, which is characterized by containing Cu and P in amounts that satisfy the conditions of the following formula, with the remainder consisting of Fe and unavoidable impurities. 60P(%)+0.6≦Cu(%)≦3. [lP (%)
<Intermediate (%) + 1.5-00 2, C: 0.08% or less, Si: 1.0% or less, Mn:
2.0 inches or less, S: 0.03 inches or less, Ni: 6
．． 0-20.0%. Cr: 16.0-25.0%, Ti, Nb
Austenite with excellent stress corrosion cracking resistance, containing Cu and P in an amount of 0.1 to 1.0 and satisfying the conditions of the following formula, with the remainder being Fe and unavoidable impurities. stainless steel. RoOF(%i+0.6≦Cu(@≦6.0