JP2759222B2 - Austenitic stainless steel with excellent stress corrosion cracking resistance in chloride environment - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an austenitic stainless steel having excellent resistance to stress corrosion cracking and crevice corrosion and suitable for use in a high-temperature chloride environment.

(Conventional technology and problems) Austenitic stainless steels such as SUS304 and SOS316 have corrosion resistance in environments containing chloride ions such as tap water and tap water, and have excellent workability and weldability. From, various hot water equipment, heat exchange tubes,
Widely used as components for chemical plants. However, even in the presence of a small amount of Cl ^- ions, in a relatively high temperature environment, pitting or crevice corrosion occurs at a weld or the like, which may cause stress corrosion cracking.

Solving the problem of stress corrosion cracking in austenitic stainless steel has been studied and reported by many researchers, but the effect of alloying elements differs depending on the test conditions such as the type of test solution and method. In environments of relatively low concentrations of neutral chloride solutions, P, Mo and N
Is harmful and Cu is known to be effective against stress corrosion cracking. However, if P is to be reduced to a level that is harmless to stress corrosion cracking, a special refining method is required, and there is a problem that the production cost is significantly increased. Mo and N, which are harmful to stress corrosion cracking, are useful elements for improving local corrosion resistance such as crevice corrosion resistance and pitting corrosion resistance. When austenitic stainless steel is used in a warm water environment containing Cl ^- ions, since the stress corrosion cracking starts from local corrosion, it is required that the steel has both the corrosion corrosion resistance and the local corrosion resistance.

The present inventors have previously added the appropriate amounts of Cu and W to 18Cr-9Ni-based steel in Japanese Patent Publication No. 59-45751 as a steel having excellent stress corrosion cracking resistance without lowering P and also having crevice corrosion resistance. The present invention has disclosed a steel useful for hot water use at 100 ° C. or lower.

In addition, when used, such as an oil-fired hot water boiler, the temperature of the constituent materials of which slightly exceeds 100 ° C, is a suitable amount for hot water equipment as disclosed in Japanese Patent Application Nos. 62-217963 and 62-217964.
Austenitic stainless steel with added Cu and up to 4% Si to further increase the critical temperature of stress corrosion cracking was disclosed, providing steel suitable for high temperature water heater applications.

However, high concentrations of Cl ^- in the ionic environment, limit temperature of stress corrosion cracking even above the steel tends to decrease to 100 ° C. or less. Therefore, to use austenitic stainless steel safely in a wider environment, high concentration
The development of steel that has stress corrosion cracking resistance up to high temperatures even in Cl ^- ion environments is expected.

It is predicted from the steels disclosed in Japanese Patent Application Nos. 62-217963 and 62-217964 that it is effective to further increase the Si content in order to improve the stress corrosion cracking resistance.
%, There is a problem that the hot workability of steel is significantly reduced.

(Object of the present invention) Based on the above recognition, the present invention has a limit temperature of 100% for stress corrosion cracking even in a high-concentration Cl ^- ion environment.
It is an object of the present invention to provide an austenitic stainless steel excellent in stress corrosion cracking resistance at a temperature of not less than ° C and having sufficient crevice corrosion resistance and excellent manufacturability.

(Technical Means for Solving the Problems) The present inventors have examined in detail the hot workability in addition to the relationship between the stress corrosion cracking behavior and the alloying elements, and found that Cu, Si and
Al is effective for improving stress corrosion cracking resistance, especially 4% of Si
If added in excess of the above, the critical temperature of stress corrosion cracking becomes higher, the dependency of stress corrosion cracking resistance on Cl ^- concentration is small, stress corrosion cracking resistance at high temperatures can be maintained, and Mo is added. However, it was found that the decrease in stress corrosion cracking resistance was very small.

On the other hand, in terms of hot workability, adjusting the austenite stability of the steel so that it contains a certain amount of δ ferrite is effective in improving the limit temperature and workability of hot stress corrosion cracking. The hot workability of the steel contained in the composite decreases, and the degree becomes severe as the Si concentration increases. In the case of containing more than 4% of Si as in the steel of the present invention, it was found that adjustment of the amount of δ ferrite is not sufficient, and that it can be achieved by adding a trace amount of REM. Was.

(Constitution of the Invention) The present invention provides: 1. C: 0.08% or less Si: more than 4% and 6.0% or less Mn: 0.8% or less P: 0.045% or less S: 0.005% or less Ni: 10 to 25% Cr: 16 to 25% Cu: 1.5-4.0% N: 0.05% or less REM: Austenitic stainless steel containing 0.005-0.1% and the balance Fe and unavoidable impurities; 2. Si: more than 4% and 6.0% or less Mn: 0.8% or less P : 0.045% or less S: 0.005% or less Ni: 10 to 25% Cr: 16 to 25% Cu: 1.5 to 4.0% N: 0.05% or less REM: 0.005 to 0.1% Mo: More than 0.3% and less than 4.0% Austenitic stainless steel composed of Fe and inevitable impurities: 3. C: 0.08% or less Si: More than 4% and 6.0% or less Mn: 0.8% or less P: 0.045% or less S: 0.005% or less Ni: 10 to 25% Cr: 16-25% Cu: 1.5-4.0% N: 0.05% or less REM: 0.005-0.1% Al: 0.05% -3.0%, austenitic stainless steel consisting of balance Fe and unavoidable impurities; and 4.C: 0.08% or less Si: more than 4% and 6.0% or less Mn: 0.8% or less P: 0.045 S: 0.005% or less Ni: 10 to 25% Cr: 16 to 25% Cu: 1.5 to 4.0% N: 0.05% or less REM: 0.005 to 0.1% Mo: Over 0.3% to 4.0% Al: 0.05 to 3.0% Is an austenitic stainless steel containing Fe and unavoidable impurities, wherein Ni-balance = Ni (%) + 30 [C (%) + N (%)] + 0.5Mn (%) + 0.3Cu (%) −1.1 [Cr (%) + 1.2Si (%) + Mo (%) + 1.5Al (%)] + 8.2 The Ni-balance defined as +8.2 satisfies the relationship of −3.4 ≦ Ni-balance ≦ −0.6. A material having excellent resistance to stress corrosion cracking in a chloride environment characterized by having a composition prepared in the following manner.

 Next, the reason for limiting the steel composition in the present invention will be described.

C: C is a powerful element that stabilizes austenite,
Although the stress corrosion cracking resistance and crevice corrosion resistance are not significantly affected, the upper limit is set to 0.08% because the intergranular corrosion susceptibility in a welded portion is increased.

Si: Si is one of the essential and important elements in the steel of the present invention, and enhances stress corrosion cracking resistance in the presence of Cu, and when an appropriate amount of this element is present, it is resistant to Mo even when Mo is added. It is an extremely useful element that can utilize the effect of improving the crevice corrosion resistance of Mo without impairing the stress corrosion resistance. Si itself also has the effect of improving crevice corrosion resistance. 100 in a rich chloride environment
In order to maintain stress corrosion cracking resistance exceeding ℃, it is necessary to add more than about 4%. The upper limit is about 6% in consideration of hot workability.

Mn: Mn forms a sulfide which easily becomes a starting point of corrosion and impairs crevice corrosion resistance and pitting corrosion resistance. Therefore, the content of Mn is preferably small. However, in order to minimize Mn, the raw material cost becomes expensive, so the upper limit is set to about 0.8% as the amount that is inevitably mixed in steelmaking, but particularly high crevice corrosion resistance is required. In this case, it is desirable that the content be about 0.5% or less.

P: P does not need to be particularly reduced in the steel of the present invention, but it is clear that it is an element harmful to stress corrosion cracking resistance,
Higher values are not preferred and the upper limit is about 0.045%.

S: S forms sulfide with Mn in steel and is harmful to crevice corrosion resistance and pitting corrosion resistance.
0.005%.

Ni: Ni is the main element to maintain the austenite phase, so it requires a minimum of 10%, but about 25%
If it exceeds, it is disadvantageous in terms of cost, so the range is 10 to 25%. In this range, Ni is considered to have little effect on stress corrosion cracking resistance, but it is effective in improving crevice corrosion resistance. Addition is desirable.

Cr: Cr is an essential element in stainless steel,
For applications in high temperature water environments containing chlorides, additions of 16% or more are required. The more Cr, the better the corrosion resistance, but
Since the addition amount of Ni or the like for maintaining the austenite phase increases, and the manufacturability and workability are impaired, the upper limit is 25%.

Cu: Cu is an important element in the steel of the present invention. Effectively improves stress corrosion cracking resistance in a hot water environment containing NaCl. The effect increases as the amount of Cu increases. Since the steel of the present invention is intended for use at relatively high temperatures, it is added in an amount of 1.5% or more. However, when the addition exceeds 4%, the effect is saturated,
In addition, hot workability deteriorates, so 1.5-4.0%
Range.

N: N is an element that has a remarkable effect of improving pitting corrosion resistance, but it is not necessarily an effective element for improving stress corrosion cracking resistance, especially when it coexists with Mo. Be impaired. Therefore, no adverse effect of N appears.
The upper limit is 05%.

Mo: Mo is an extremely effective element for improving crevice corrosion resistance and pitting corrosion resistance, but impairs stress corrosion cracking resistance. In the steel of the present invention, by adding Cu and Si exceeding 4%, the harmfulness of Mo to stress corrosion cracking resistance is suppressed. However, the addition of 4% or more increases cost greatly and is not economical. Upper limit. On the other hand, if the amount is 0.3% or less, the effect of improving the gap corrosion resistance does not appear.

Al: Al has an effect of improving stress corrosion cracking resistance in the coexistence of Cu and Si. In the steel of the present invention, stress corrosion cracking resistance is high, and the effect of improving stress corrosion cracking resistance by Al is not particularly remarkable, but the erosion depth in crevice corrosion resistance is improved, and the erosion depth due to crevice corrosion becomes shallower. However, when the amount of addition increases, hot workability and moldability deteriorate, so 0.05 to 3.
The range is 0%.

REM: REM is an effective element for improving the hot workability of steel. In particular, in the steel of the present invention, large edge cracks occur in the hot-rolled sheet without the addition of REM. In addition, 0.005% or more is added in order to make the effect of Al on stress corrosion cracking resistance and crevice corrosion resistance more effective. However, if the content exceeds 0.1%, inclusions in the steel increase, which impairs the surface properties of the product.

Ni-balance: In austenitic stainless steel, a small amount of δ ferrite is left in the ingot for the purpose of preventing defects in the slab and obtaining good hot workability.

The amount of δ ferrite is almost estimated from the Ni-balance calculated from the steel composition. In the steel of the present invention, although a large amount of Si and Cu is contained, the hot workability is lowered, but the hot workability is improved by leaving 2 to 9% of δ ferrite in the steel ingot. . In the composition range of the steel of the present invention, the amount of δ ferrite can be estimated by the following Ni-balance equation. By adjusting the value of Ni-balance from -0.6 to -3.4, 2 to 9% of δ ferrite is contained in the steel ingot. It has been confirmed by experiment that it remains.

Ni-balance is Ni−balance = Ni (%) + 30 [C (%) + N (%)] + 0.5Mn (%) + 0.3Cu (%) − 1.1 [Cr (%) + 1.2Si (%) + Mo (%) + 1. 5Al (%)] + 8.2.

This definition was obtained by adjusting the coefficient of the element so as to be applicable to the steel of the present invention based on the De-long equation.

(Specific disclosure of the invention) Example A steel having a composition shown in Table 1 was melted by a vacuum melting method and forged.
After hot rolling, a cold-rolled steel sheet having a thickness of 1 mm was prepared.

In Table 1, A1-A5 steels are comparative steels, A1-A3 steels are 4
% Si steel, A4 steel is 5% Si steel, and A5 steel is 3% Si steel. B1 ~
B6 steel is the present invention steel, B1 to B4 steels are 4% Si steels, and B5 and B6 steels are 5% Si steels.

FIG. 1 shows A3 and A5 in Table 1 subjected to solution heat treatment.
It shows the stress corrosion cracking resistance of steel and B1 to B6 steels.

The stress corrosion cracking resistance was determined by immersing a spot welding test piece in a high-temperature chloride solution and determining whether or not cracking occurred. That is, two large and small stainless steel sheets were overlapped and spot-welded, and then the temperature was changed using an autoclave container.
0PpmCl ^- solution was immersed 10 days to obtain the stress corrosion cracking limit temperature (SCC limit temperature) the presence or absence of cracks. In this test, stress corrosion cracking starts from crevice corrosion. Therefore,
The erosion depth due to crevice corrosion was also determined.

3% Si steel (A5 steel) is, 50ppmCl ^- the exhibit stress corrosion cracking resistance of up to 120 ° C. (stress corrosion cracking limit temperature), Cl ^-
When the concentration increases, the stress corrosion cracking resistance decreases, and 500 ppm Cl
At the temperature, the stress corrosion cracking limit temperature becomes 100 ° C or less. A3
Steel comprises 3.87% of the Si, 50ppmCl ^- the stress corrosion cracking limit temperature at has risen to 140 ℃, 500ppmCl ^- cracks occur in the test of the 140 ° C., stress corrosion cracking resistance Cl ^- concentration Depends. On the other hand, the B1-B4 steel containing more than 4% of Si has excellent stress corrosion cracking resistance, and the stress corrosion cracking limit temperature at 50-500ppmCl ^- concentration is 140 ℃ -150 ℃, and the Cl ^- concentration becomes high. No significant decrease in stress corrosion cracking resistance was observed, and the dependency of stress corrosion cracking resistance on Cl ^- concentration was very small. From the comparison between B1 and B3 steels, no decrease in stress corrosion cracking resistance is observed even when 2% Mo is added. The stress corrosion cracking resistance of B5 and B6 steels to which 5% Si is added is further excellent, and in the range of 50 to 500 ppm Cl ^- concentration, no cracks are observed in the test up to 150 ° C,
The limit temperature for stress corrosion cracking is 150 ° C or higher.

As described above, by adding Si in excess of 4%, the stress corrosion cracking resistance of Cu-containing austenitic stainless steel is dramatically improved, the dependency on Cl ^- concentration is small, and Mo is added. It became clear that the stress corrosion cracking resistance was not impaired. Therefore, since there is no restriction on the addition of Mo, it is possible to further improve the crevice corrosion resistance.

Next, the effect of alloying elements on crevice corrosion resistance will be described based on the measurement results of the erosion depth of crevice corrosion generated in spot welding test pieces in an autoclave test. Table 2
0.99 ° C. of 200PpmCl ^- in test at solution 240 hours, showing the corrosion depth by crevice corrosion of the present invention steels and comparative steels. A5
(2.98% Si), A3 (3.87% Si) and B5 (5.10% Si) steels show that as the Si content increases from 3% to 5%, both the maximum and average pit depths decrease. You can see that there is. This is thought to be due to the fact that, in addition to the effect of Si, the amount of Ni is increased together with Si to adjust the Ni balance. A3 (3.87% Si) with almost the same Si level,
From comparison of B1 (4.26% Si) and B3 (4.34% Si) steel, A3
There is almost no difference in the erosion depth between steel (0.8% Mo) and B1 steel (1.1% Mo), but the erosion depth of B3 steel (2.14% Mo) is shallower than these steels and 2%. Improvement of crevice corrosion resistance was clearly observed by adding Mo. A3, B1 and B4 steel (0.147% A
Comparison of l) shows that B4 steel has the shallowest erosion depth, and that the addition of Al is effective in improving the erosion depth in crevice corrosion. Compared to B3 steel containing 2% Mo, the average erosion depth is slightly deeper, but the maximum erosion depth is 0.8% Mo.
B3 steel to which Al is added is shallower, albeit in a small amount. The effect of such Al is 5% Si steel (B5 and B6 (0.209% Al
Steel)) was also observed.

FIG. 2 shows the degree of edge cracks (edge cracks) after hot rolling performed on 4 to 5% Si steel excluding the 3% Si steel (A5 steel) among the steels shown in Table 1. Ni-balance and δ
It is shown in relation to the amount of ferrite (measured value in ingots).

Ni−balance = Ni (%) + 30 [C (%) + N (%)] + 0.5Mn (%) + 0.3Cu (%) − 1.1 [Cr (%) + 1.2Si (%) + Mo (%) + 1. 5Al (%)] + 8.2 In hot rolling, after heating a slab with a thickness of 40 mm at 1100 to 1150 ° C, adjust the rolling pass schedule or apply intermediate heating appropriately so that the final rolling temperature does not fall below 850 ° C. In addition, a hot rolled sheet of 5 mm was prepared.

The austenitic steel without δ-ferrite (A2 steel) and the steel with 12% δ-ferrite (Al steel) have ear cracks exceeding 5 mm, resulting in poor hot workability. Steel in which the amount of δ ferrite in the ingot is adjusted to 2 to 9% shows relatively good hot workability. However, even when the amount of δ ferrite is 2 to 9%, the A3 and A4 steels have a maximum 5 mm edge crack.
On the other hand, it can be seen that the hot-rolled sheet of the steel of the present invention (B1 to B6 steel) to which REM is added has no or very small edge cracks and exhibits good hot workability.

From FIG. 2, the amount of δ ferrite in the ingot was
-Corresponds well to the balance equation, and the Ni-balance value is-
0.6 to -3.4, the δ ferrite content of the ingot is 2 to 9%
It can be.

(Effect of the Invention) It is clear that the steel of the present invention has both excellent resistance to stress corrosion cracking and resistance to crevice corrosion, and is excellent in manufacturability, cost increase can be minimized and economical. It is steel. Therefore, it is suitable as a material for an apparatus for handling a neutral chloride solution in a state of being heated to a high temperature.

[Brief description of the drawings]

FIG. 1 is a diagram showing stress corrosion cracking resistance of A3 and A5 steels and B1 to B6 steels subjected to solution heat treatment in relation to Cl ⁻ (ppm) and stress corrosion cracking limit temperature. FIG. 2 is a diagram showing edge cracks at the edge degree after hot rolling of 4-5% Si steel in relation to the Ni-balance and the amount of δ ferrite in the ingot.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-1-159351 (JP, A) JP-A-63-65058 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] [Claim 1] C: 0.08% or less Si: more than 4% and 6.0% or less Mn: 0.8% or less P: 0.045% or less S: 0.005% or less Ni: 10 to 25% Cr: 16 to 25% Cu: 1.5 to 4.0% N: 0.05% or less REM: 0.005 to 0.1%, with the balance being Fe and unavoidable impurities and having a composition adjusted to satisfy the following Ni-balance, resistant to chloride environment. Austenitic stainless steel with excellent stress corrosion cracking properties. −3.4 ≦ Ni−balance ≦ −0.6, where Ni−balance = Ni (%) + 30 [C (%) + N (%)] + 0.5Mn (%) + 0.3Cu (%) −1.1 [Cr (%) + 1 .2Si (%) + Mo (%) + 1.5Al (%)] + 8.2 C: 0.08% or less Si: more than 4% and 6.0% or less Mn: 0.8% or less P: 0.045% or less S: 0.005% or less Ni: 10 to 25% Cr: 16 to 25% Cu: 1.5 to 4.0% N: 0.05% or less REM: 0.005 to 0.1%, Mo: more than 0.3% and 4.0% or less, balance Fe and unavoidable impurities, and composition to satisfy the following Ni-balance Austenitic stainless steel with excellent resistance to stress corrosion cracking in chloride environments, characterized by the following: −3.4 ≦ Ni−balance ≦ −0.6, where Ni−balance = Ni (%) + 30 [C (%) + N (%)] + 0.5Mn (%) + 0.3Cu (%) −1.1 [Cr (%) + 1 .2Si (%) + Mo (%) + 1.5Al (%)] + 8.2 C: 0.08% or less Si: more than 4% and 6.0% or less Mn: 0.8% or less P: 0.045% or less S: 0.005% or less Ni: 10 to 25% Cr: 16 to 25% Cu: 1.5 to 4.0% N: 0.05% or less REM: 0.005 to 0.1%, Al: 0.05 to 3.0%, the balance is made up of Fe and unavoidable impurities, and the composition was adjusted to satisfy the following Ni-balance. Austenitic stainless steel with excellent resistance to stress corrosion cracking in chloride environments. −3.4 ≦ Ni−balance ≦ −0.6, where Ni−balance = Ni (%) + 30 [C (%) + N (%)] + 0.5Mn (%) + 0.3Cu (%) −1.1 [Cr (%) + 1 .2Si (%) + Mo (%) + 1.5Al (%)] + 8.2 C: 0.08% or less Si: more than 4% and 6.0% or less Mn: 0.8% or less P: 0.045% or less S: 0.005% or less Ni: 10 to 25% Cr: 16 to 25% Cu: 1.5 to 4.0% N: 0.05% or less REM: 0.005 to 0.1%, Mo: more than 0.3% and 4.0% or less Al: 0.05 to 3.0%, the balance being Fe and unavoidable impurities, and the following Ni-balance An austenitic stainless steel excellent in stress corrosion cracking resistance in a chloride environment, characterized by having a composition adjusted to satisfy the following. −3.4 ≦ Ni−balance ≦ −0.6, where Ni−balance = Ni (%) + 30 [C (%) + N (%)] + 0.5Mn (%) + 0.3Cu (%) −1.1 [Cr (%) + 1 .2Si (%) + Mo (%) + 1.5Al (%)] + 8.2