JPS5970749A - Austenitic stainless steel with superior stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To improve the stress corrosion cracking resistance of an austenitic stainless steel, by adding Cu to the steel by an amount related to the amount of P in the steel. CONSTITUTION:This austenitic stainless steel with superior stress corrosion cracking 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, Cu by an amount satisfying the equation and the balance essentially Fe. 0.1-1.0% Ti and/or Nb 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.

Austenitic stainless steel, typified by 5US304, has excellent corrosion resistance, weldability, and workability, and is therefore used in a wide range of applications.

Stress corrosion cracking may occur in a usage environment that contains Cu2- 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 JISGO576
Addition of Mo is harmful to magnesium dichloride solution, but addition of Mo is effective to 20% common salt solution to which 1t4 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 clearly examine the influence of component elements under test conditions close to the actual environment.

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

Low-concentration food corrosion i (stress corrosion in ζ7;
)] was found to occur from crevice corrosion.

Various studies were conducted to develop austenitic stainless steel with excellent stress corrosion cracking resistance.

The present inventors have developed 5t) S3 to improve stress corrosion cracking resistance.
When adding Co to 04 series stainless steel, the relationship with P contained in the steel was investigated in detail 1.

It was discovered that there is a certain relationship between the relative amount of C11 and P content, and corrosion, and a steel composition with excellent stress corrosion cracking resistance was discovered.

According to the present invention (at 1% by weight, C: 0.08% or less, Si: 1.0% or less, Mn:
2.0 inch or less, P: 0.045 inch or less, S:
0.0 '5 or less.

Ni: 6.0-20.0%, Cr: 16.0-2
5.0%.

and contains an amount of Co that satisfies the conditions of the following formula, with the remainder being Fe, ? The present invention provides an austenitic stainless steel with excellent stress corrosion cracking resistance and characterized by being composed of carbon dioxide and unavoidable impurities.

30 [M] + 0.6% ≦ [cuqb] ≦ 3.0% To the above steel, in order to further improve the stress corrosion cracking resistance of the welded part, one or two types of T+ * Nb 01 to i, D
% 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%.

Si: Sr is necessary for deoxidation in fJI steel, but since it impairs workability, the upper limit was set at 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%.

S: Since S does not affect stress corrosion cracking susceptibility, it may be less than the normally allowed value of 0.031, but since the occurrence of corrosion is harmful, a low value is desirable.

Cr2Cr is an essential element to maintain corrosion resistance.
If it is less than 16%, sufficient corrosion resistance cannot be obtained.

On the other hand, if it exceeds 25 inches, workability will deteriorate, so 16.0~2
It was limited to 5.0%.

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. Limited to 0-200 staff.

T i r Nb: Since T r e N b has the effect of fixing C and N, it improves the intergranular corrosion resistance of the weld zone and further reduces the susceptibility to intergranular stress corrosion cracking; Although they are effective, if they are less than 0.1%, the effect is small, and if they exceed 1% or 0%, the effect becomes saturated and useless, so we limited them to 0.1% to 1.0%.

As mentioned earlier, the present inventors added C to 808304 series steel.
It was found that the addition of u spreads crevice corrosion in low-concentration salt solutions and reduces stress corrosion cracking susceptibility.

That is, since C11 has the effect of spreading corrosion, it cancels out the effect of P that concentrates the corrosion described above and reduces 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.

(Cub)≧30 [i)+0.696Co has the effect of spreading corrosion in this way, so it may be added in large amounts, but if it exceeds 3.0 inches, it impairs hot workability, so the upper limit is 3.
．． Set to 0%. P represents weldability as tJA, so the second limit is 0.045%.

The steel of the present invention will be specifically explained with reference to Examples.

Table 1 shows the compositions of the present invention@i1 (example steel) and comparative steel.

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 mm and length 31 mm is made of the same material and has a width of j4 mm.
A test piece made by stacking and spot welding plates with a length of 16 mm was
50ppmC! A 30-day immersion test was conducted to determine the concentration of NaC. The presence or absence of stress corrosion chipping was determined by cross-sectional observation.

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

The first thing to ensure stress corrosion cracking resistance is the amount of P and Cu.
It can be seen that there is a need to maintain the correlation between the quantities in the relationship expressed by the above equation: l+1. As P decreases, stress corrosion cracking susceptibility decreases, but if an appropriate amount of Cu is not included, P: 0.
．． Even in 0.005% 'a4 (steel Na5), cracks occur from the bottom of the corrosion hole due to crevice corrosion. In this case, the depth of the corrosion hole reaches 0.3-0.4 mm. In steel with low P content, the growth of crevice corrosion is large at first, but after 9 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.

Since the steel of the present invention has excellent stress corrosion cracking resistance, stress corrosion cracking is currently occurring using 5tJ8304.

It is suitable as a material for water storage equipment (e.g. electric water heaters, tight water boilers) and hot water supply piping, and since it contains Cu, it is excellent even in environments with non-oxidizing acids such as sulfuric acid and hydrochloric acid. Corrosion resistance is expected.

[Brief explanation of drawings]

Figure 1 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 50 ppm C-free α at 0° C. for 30 days. Patent applicant Nisshin Steel Co., Ltd.

Claims (1)

[Claims] 1. At wound 0, C: 0.08% or less, Si: 1.0% or less, Mn:
2.0chi or less, P: 0.045% or less, S: 0.03
Below, Ni: 6.0-20.0%, Cr
: An austenitic stainless steel with excellent stress corrosion cracking resistance characterized by containing C11 of sulfur having a ratio of 16.0 to 25.0 and satisfying the conditions of the following formula, with the remainder consisting of Fe and inevitable impurities. steel. 30[:P%]+0.6%≦(Cu%]≦3.0%2
, C: 0.08% or less, Si: 1.0% or less, Mn:
2.0 ratio or less, P: 0.045% or less, 8: 0.03
% or less]”, Ni: 6.0 to 20.0%, C
r: 16.0 = 25.0 ratio, contains 0.1 to 1.0% of one or two of Ti and Nh and an amount of C+1 that satisfies the conditions of the following formula, the balance being Fe and unavoidable Austenitic stainless steel with excellent stress corrosion cracking resistance due to impurities. 30[P%)+0.6%≦(-Cu%'J≦6.0%