JPS61136662A - Austenitic stainless steel having superior resistance to stress corrosion cracking - Google Patents

Abstract

PURPOSE:To provide superior resistance to stress corrosion cracking of an austenitic stainless steel as a material for a storage tank contg. tap water or treated water for recycling, contg. a trace of chlorine by specifying the composition and adding a specified amount of La. CONSTITUTION:The composition of an austenitic stainless steel is composed of, by weight, <0.6% C, <1% Si, <2% Mn, 16-20% Cr, 7-13% Ni, 0.1-0.5% La, <0.35% P, <0.01% S and the balance Fe with inevitable impurities. In the composition, La hinders stress corrosion cracking because it forms a compound with P which deteriorates the resistance to chloride stress corrosion cracking.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to austenitic stainless steel with excellent stress corrosion cracking resistance, and is suitable for use in environments containing trace amounts of chlorine ions, such as hot water tanks, water storage tanks, and heat exchangers. used.

[Conventional technology]

Austenitic stainless steels such as 5US3o4.316 exhibit excellent corrosion resistance due to their main alloy components, Cr5Ni and MO, as well as excellent weldability and workability, so they are used in many applications that require corrosion resistance. has been done. However, the biggest weakness of austenitic stainless steel is that stress corrosion cracking may occur even in relatively high-temperature water supply, gray water, or industrial water environments that contain trace amounts of chlorine ions, and it is difficult to prevent this from occurring. ing. Therefore, for these uses,
Since ferritic stainless steel is immune to stress corrosion cracking, the use of high-purity ferritic stainless steel (SUS444) with added Mo to improve corrosion resistance is being considered.

However, this stainless steel is expensive and has poor workability and weldability, so it has not become widely used. Therefore, there is a strong demand for austenitic stainless steels that are resistant to chloride stress corrosion cracking.

Many studies have been reported on chloride stress corrosion cracking in austenitic stainless steel, and among them, the influence of constituent elements in steel has been studied, for example, in ``Metal Corrosion Damage and Corrosion Prevention Technology'' by Masamichi Kowaka (Agne Publishing Co., Ltd.). It is summarized on page 339 of the publication).

It is a well-known fact that if the content of P, which is included as an impurity element among these component elements, is reduced, chloride stress corrosion cracking in austenitic stainless steel becomes significantly less likely to occur. Dephosphorization is extremely difficult, and dephosphorization until stainless steel becomes immune to chloride stress corrosion cracking would result in a significant increase in cost and is not practical at the current technological level. Moreover, there is still a strong demand for stainless steel that has excellent resistance to chloride stress corrosion cracking.

[Problem that the invention seeks to solve]

The purpose of the present invention is to meet the above-mentioned demands for the prior art,
An object of the present invention is to provide an austenitic stainless steel having excellent stress corrosion cracking resistance.

[Means and operations for solving the problems] The gist of the present invention is as follows.

That is, C: 0.06% or less, Si: 1. 0% or less Mn: 2%
Below, Cr: 16-20% Ni near ~1.3
%, La: o, 1-0.5%P: 0.035
% or less, including S: 0.01% or less, the remainder being Fe
It is an austenitic stainless steel with excellent stress corrosion cracking resistance and is characterized by consisting of unavoidable impurities.

The present inventors have repeatedly investigated various methods for rendering P harmless rather than removing P, which inhibits chloride stress corrosion cracking resistance. That is, YSLa that forms a compound with P
It was investigated whether it was possible to make P harmless by adding a third element such as , Ce, In, or Se. As a test for chloride stress corrosion cracking, the cracking test in a high-concentration chloride environment that has been commonly used in the past is far from the actual environment, so we first conducted a test using JP-A-57-47850. The test was carried out using the method described in the publication, in which a three-point spot test piece was immersed at the gas-liquid interface in a low-concentration chloride aqueous solution.

As a result of the study, it was found that I and a form a compound with P and significantly inhibit stress corrosion cracking, and the present invention was made based on this knowledge.

The reason for limiting the components in the present invention will be explained.

C: It is said that the higher the amount of C, the greater the inhibiting effect on stress corrosion cracking, but if it exceeds O,OS%, the susceptibility to intergranular corrosion due to the sensitization of the weld becomes significant, and the strength increases and the workability decreases. The upper limit was set at 0.06% because of the deterioration of the content.

Si: Si has no improvement effect on stress corrosion cracking. However, although a small amount is required for deoxidizing purposes during steel manufacturing, if the amount added is large, workability deteriorates, so the upper limit was set at 1.0%.

Mn: Mn is used as a deoxidizing agent and a desulfurizing agent during steel manufacturing, and has little to do with stress corrosion cracking, so it was limited to the normally used 2% or less.

Cr: Cr is an element that determines the corrosion resistance of stainless steel, and the higher the amount, the better, but if it exceeds 20%, there is a risk of precipitation of intermetallic compounds such as α phase, and workability will deteriorate.
The upper limit was 0%. Furthermore, since at least 16% is required to obtain a stable austenite phase, this was set as the lower limit.

N I: Ni is a main element for maintaining the austenite phase, and in consideration of the amount of Cr, it is required to be at least 7%. Further, since Ni improves corrosion resistance, particularly acid resistance, the amount of Ni is preferably as large as possible, but from an economical point of view, the upper limit was set at 13%.

La: La is the most important element in the present invention, forming a compound with P and having a great effect of inhibiting stress corrosion cracking.
If it is less than 0.5%, there is no effect, and if it exceeds 0.5%, the effect is saturated, so from an economic point of view, this is the upper limit.
．． It was limited to a range of 1 to 0.5.

P: Since P is extremely harmful to stress corrosion cracking, it is preferable to reduce it as much as possible, but based on the current theory of P technology, the upper limit is set at 0.03.
It was set at 5%.

S: S significantly impairs the corrosion resistance, especially the pitting corrosion resistance, of stainless steel, and in particular, if its amount exceeds 0.01%, the pitting corrosion resistance rapidly deteriorates, so the upper limit was set at 0.01%.

As mentioned above, the present invention relates to austenitic stainless steel y, tM.
By limiting C, Si, Mn, Cr1NiSP1S, L
By adding 0.1 to 0.5% of a to render P harmless, the resistance to chloride stress corrosion cracking could be significantly improved.

〔Example〕

In a vacuum induction furnace, 100 kg each of the invention steel and 14 types of comparative tfI7t having the components shown in Table 1 were melted. These steels were hot rolled to a thickness of 5 mm, softened and annealed at 1050° C. for 15 minutes, and then cold rolled to a thickness of 2 mm. As a final heat treatment, solution treatment was performed at 1050° C. for 5 minutes, and the sample was subjected to a test.

As shown in FIG. 1 (Al, (B), two test pieces 2 were spot welded 4 at three points to make a welded test piece, and as shown in FIG. After being immersed in the gas-liquid interface of .5% saline solution 8 for two months, the spot welds were cut and the presence or absence of stress corrosion cracking was investigated.Three tests were conducted for each steel type. They are also shown in the table.

In the stress corrosion cracking test results shown in Table 1, the mark 0 indicates no cracking, and the mark X indicates cracking.

In Table 1, cracks were found in all three comparative steels No. 5, which is ordinary 5US304 stainless steel. Comparative steel No. 7 has a higher Si content than the present invention, and comparative steel No. 7
Although the La content was less than 0.1%, cracks were found in two out of three of them. On the other hand, no stress corrosion cracking was observed in any of the three steels of the present invention Nos. 1 to 4, indicating that they had excellent resistance to chloride stress corrosion cracking.

〔Effect of the invention〕

As is clear from the above examples, the present invention limits the components of the austenitic stainless steel, and in particular, by adding a limited amount of L&, stress corrosion can occur in environments containing trace amounts of chlorine, such as storage tanks for waterworks and gray water. It was possible to achieve the effect of exhibiting good corrosion resistance without any corrosion.

[Brief explanation of the drawing]

Figure 1 (Al, (Bl indicates a 3-point spot test piece, (
Al is a side view, (B) is a plan view, and FIG. 2 is a cross-sectional view showing a corrosion test of a three-point spot test piece immersed in a 3.5% saline solution at the gas-liquid interface.

Claims (1)

[Claims] (1) Weight ratio C: 0.06% or less, Si: 1.0% or less Mn: 2% or less, Cr: 16-20% Ni: 7-13%, La: 0.1-0.5 %P: 0.0
An austenitic stainless steel with excellent stress corrosion cracking resistance, characterized in that it contains S: 35% or less, S: 0.01% or less, and the remainder consists of Fe and inevitable impurities.