WO2012133574A1

WO2012133574A1 - Highly corrosion-resistant austenite stainless steel well-suited to brazing

Info

Publication number: WO2012133574A1
Application number: PCT/JP2012/058220
Authority: WO
Inventors: 透松橋; 純徳永
Original assignee: 新日鐵住金ステンレス株式会社
Priority date: 2011-03-29
Filing date: 2012-03-28
Publication date: 2012-10-04
Also published as: JP6016331B2; AU2012233539A1; CA2829874C; JP2012207259A; CN103380224B; TWI471427B; KR20130123443A; AU2012233539B2; CN103380224A; NZ614829A; TW201245468A; US20130336834A1; CA2829874A1

Abstract

The present invention provides an austenite stainless steel that not only is well-suited to brazing but is also highly resistant to corrosion in aqueous-solution environments containing chloride ions and environments in which low-pH condensed water containing nitrate ions and sulfate ions is produced via condensation of combustion exhaust gases. Said steel contains, by mass, up to 0.080% carbon, 1.2-3.0% silicon, 0.4-2.0% manganese, up to 0.03% phosphorus, up to 0.003% sulfur, 6.0-12.0% nickel, 16.0-20.0% chromium, 0.2-3.0% copper, 0.002-0.10% aluminum, 0.030-0.150% nitrogen, and 0.1-1.0% molybdenum, with the remainder comprising iron and unavoidable impurities. The mass percentages of copper, silicon, nickel, and molybdenum in said steel also satisfy relations (A) (1.6 ≤ [Cu]×[Si] ≤ 4.4) and (B) (0.16 ≤ 2[N]+[Mo] ≤ 1.0).

Description

Austenitic stainless steel with excellent corrosion resistance and brazing

The present invention relates to an austenitic stainless steel used for a structure joined with a brazing material such as nickel brazing or copper brazing. In particular, the present invention not only has excellent brazing properties, but also has corrosion resistance in an environment where condensed water having a low pH containing nitrate ions and sulfate ions is generated due to condensation of combustion exhaust gas. The present invention relates to an austenitic stainless steel having excellent corrosion resistance in an aqueous solution environment.

Brazing joining is a technique for joining materials by using a brazing material having a melting point lower than that of the structural material and heat-treating it at a temperature slightly higher than the melting point of the brazing material. Brazing joining is a joining method widely used also in stainless steel. The brazing material used for brazing stainless steel is a nickel or copper alloy.

In stainless steel brazing, the stainless steel passive film hinders brazing. Therefore, brazing joining is performed in a vacuum or in a hydrogen atmosphere in order to reduce and remove the passive film. The temperature of brazing joining is, for example, about 1100 ° C. when nickel brazing is used.

In brazing and joining, it is important that the brazing material sufficiently fills the gaps between the materials to be joined to ensure the strength of the joint. Therefore, the wettability of the brazing material with respect to the stainless steel to be joined becomes important. On the other hand, if the wettability of the brazing material is too good, the brazing material flows out from the gaps between the materials to be joined, the gap cannot be filled with the brazing material, and the bonding strength is reduced. For this reason, it is important to have appropriate wettability as stainless steel excellent in brazing joint.

Austenitic stainless steel is generally used as the stainless steel to be brazed. As austenitic stainless steel, JIS (Japan Industrial Standard) SUS304-based material and SUS316-based material (hereinafter referred to as SUS304-based material and SUS316-based material) are widely used. The SUS304-based material and the SUS316-based material have not only processability but also characteristics excellent in corrosion resistance in a general environment. However, the problem is that SUS316-based materials and SUS316-based materials are inferior in stress corrosion cracking resistance.

Stress corrosion cracking occurs when tensile stress remains in a material that is highly susceptible to stress corrosion cracking and exposed to an environment where corrosion occurs. When austenitic stainless steel is joined by brazing, there is no fear of stress corrosion cracking even if tensile stress remains in the material to be joined before brazing joining. This is because the austenitic stainless steel is brazed at a temperature at which the austenitic stainless steel is annealed, and residual stress is removed during brazing. For example, when nickel brazing is used, it is brazed at about 1100 ° C. as described above.

However, some parts may be assembled with other parts by welding or screwing after brazing and joining, and in that case, tensile stress may be generated in the assembled parts and stress corrosion cracking may occur. Therefore, it is necessary that the austenitic stainless steel brazed and joined has stress corrosion resistance.

As an environment in which an austenitic stainless steel brazing joint material is used, for example, there are an exhaust system member of an automobile and a secondary heat exchanger of a water heater provided with a latent heat recovery unit. All of these members are used in an environment in which condensed water having a low pH containing nitrate ions and sulfate ions is generated because the combustion exhaust gas is condensed. This is because the atmosphere taken in for combustion contains a large amount of nitrogen, and the fuel or the odorizing substance added to the fuel contains a sulfur compound. Under such circumstances, copper is corroded. Therefore, copper cannot be used as a material constituting an automobile exhaust system member or a secondary heat exchanger of a water heater provided with a latent heat recovery device, and austenitic stainless steel is essential.

Therefore, it is important for the austenitic stainless steel used for such members to satisfy both corrosion resistance and brazing even in an environment where condensed water containing nitrate ions and sulfate ions and having a low pH is generated. .

Regarding the brazeability of stainless steel, Patent Document 1 proposes a pre-coated brazed metal sheet in which a nickel brazing material suspended together with an organic binder is spray-coated on a stainless steel plate surface and then heated. Patent Document 2 proposes a method for producing a nickel brazing-coated stainless steel sheet excellent in self-brazing property, in which a nickel-based brazing material is coated by plasma spraying on a stainless steel sheet having an adjusted surface roughness. ing. However, in both Patent Documents 1 and 2, only conventional SUS304-based materials and SUS316-based materials are examined as austenitic stainless steel materials to which a brazing material is applied.

Patent Document 3 proposes a stainless steel excellent in brazing property in which Al and Ti are reduced. Patent Document 4 proposes a stainless steel in which the M value represented by M = −0.22T + 34.5Ni + 10.5Mn + 13.5Cu-17.3Cr-17.3Si-18Mo + 475.5 is adjusted to 1 to 25. ing. However, both Patent Documents 3 and 4 are studies on ferritic stainless steel, and austenitic stainless steel has not been studied.

Patent Document 5 proposes an austenitic stainless steel material having stress corrosion cracking resistance and crevice corrosion resistance. However, the steel sheet proposed in Patent Document 5 is applied to automobile oil supply system members, and although stress corrosion cracking resistance has been studied, brazing performance is not described.

Also, when used for secondary heat exchangers of water heaters equipped with automobile exhaust system members or latent heat recovery devices, chlorides are contained in the intake air, so it is used especially in high salt damage areas near the coast. In addition, corrosion resistance in an environment containing chloride ions is also a problem.

JP-A-1-249294 JP 2001-26855 A JP 2009-174046 A JP 2010-65278 A JP 2007-9314 A

The present invention not only has excellent brazing properties, but also has corrosion resistance in an environment where condensed water having low pH containing nitrate ions and sulfate ions is generated by condensation of combustion exhaust gas, and further contains chloride ions. It aims at providing the austenitic stainless steel which is excellent also in the corrosion resistance in aqueous solution environment.

As a result of intensive studies to obtain an austenitic stainless steel having both brazing and corrosion resistance, the present inventors have found the following knowledge.

(A) In the case of austenitic stainless steel, when a certain amount or more of Si and Cu is added, the wettability becomes excessively good, and the brazing material flows out from the gaps between the joined materials, so that the joining is insufficient. Become. In order to prevent this, it is important to define not only the upper limits of the Cu and Si contents, but also the upper limit of the value of [Cu] × [Si]. In the following description, [Cu] and [Si] are the contents of Cu and Si expressed in mass%.

(B) The austenitic stainless steel that has been brazed and bonded can suppress stress corrosion cracking by the synergistic effect of Cu and Si expressed by the value of [Cu] × [Si].

(C) Corrosion resistance in an environment where condensed water having low pH containing nitrate ions and sulfate ions is generated by condensation of combustion exhaust gas, and further in an aqueous solution environment containing chloride ions, the corrosion resistance is 2 [N ] + [Mo] is improved by setting the value to a certain value or more. In the following description, [N] and [Mo] are the contents of N and Mo expressed in mass%.

The present invention has been made on the basis of the above findings, and the gist thereof is as follows.

(1) By mass%, C: 0.080% or less, Si: 1.2 to 3.0%, Mn: 0.4 to 2.0%, P: 0.03% or less, S: 0.003 %: Ni: 6.0 to 12.0%, Cr: 16.0 to 20.0%, Cu: 0.2% to 3.0%, Al: 0.002 to 0.10%, N: 0.030 to 0.150%, and Mo: 0.1 to 1.0%, the balance is made of Fe and inevitable impurities, and satisfies the following formulas (A) and (B) Austenitic stainless steel with excellent corrosion resistance and brazing characteristics.
(A) Formula: 1.6 ≦ [Cu] × [Si] ≦ 4.4
(B) Formula: 0.16 ≦ 2 [N] + [Mo] ≦ 1.0
Here, [Cu], [Si], [N], and [Mo] are the contents of each element expressed in mass%.

(2) Further, in terms of mass%, Nb: 0.1 to 0.7%, Ti: 0.1 to 0.5%, V: 0.1 to 3.0%, and B: 0.0002% to The austenitic stainless steel excellent in corrosion resistance and brazing as described in (1) above, containing one or more of 0.003%.

According to the present invention, an austenitic stainless steel excellent in corrosion resistance and brazing is provided by optimizing the Cu content and the Si content in the austenitic stainless steel and further controlling the N content and the Mo content. Is possible.

And according to this invention, the structure obtained by brazing joining, such as a waste heat recovery device of combustion exhaust gas which used hydrocarbons, such as gasoline, LNG, LPG, and petroleum, and other heat exchangers Corrosion resistance can be improved.

It is a figure which shows the relationship between content of Cu and Si, brazing property, and corrosion resistance. It is a figure which shows the relationship between the value of 2 [N] + [Mo], and the maximum corrosion depth. It is a figure which shows the relationship between the value of 2 [N] + [Mo] and the value of [Cu] x [Si], brazing property, and corrosion resistance.

The present invention will be described in detail. In the following description, “%” relating to the component composition means “% by mass” unless otherwise specified.

First, an experiment conducted to obtain a component composition that achieves both brazability and corrosion resistance and the results thereof will be described. Austenitic stainless steel in which Si, Cu, Mo, and N were changed was manufactured by vacuum melting. At this time, other elements were within the component range of JIS SUS316.

The austenitic stainless steel was hot-rolled and heat-treated at 1150 ° C. for 1 minute, and then the scale was ground and removed, and further cold-rolled to obtain a cold-rolled sheet. This cold-rolled sheet is heat-treated at 1050 to 1150 ° C. for 1 minute based on the recrystallization behavior, and then subjected to immersion pickling treatment until the scale is completely removed in an aqueous nitric hydrofluoric acid solution. It was made a material for. Using this brazing material, brazing properties and stress corrosion cracking were evaluated.

(Brassability evaluation)
The brazing joint material was cut into 40 × 50 mm and 25 × 30 mm, and used as test materials for brazing evaluation. The thickness of this brazing material evaluation test material is 1 mm. The specimen material thus produced was brazed using silver brazing. In the brazing joining, 0.3 g of JIS BNi5 nickel brazing mixed with an organic binder was placed as a brazing material in the overlapping portion where two test materials were stacked, and brazing joined. The brazing joining was performed in an atmosphere of 1100 ° C. and 100% hydrogen using a hydrogen reduction furnace. Brazing property was evaluated by cutting a brazed specimen and visually observing a cross section.

Evaluation results are shown in FIG. When the brazing filler metal is completely filled in the cross section of the brazed joint, the white circle (○) or black circle (●), and if there is a gap, the cross mark (×) displayed. Here, the white circle mark (○) and the black circle mark (●) distinguish the evaluation results of stress corrosion cracks, which will be described later. A black circle mark (●) indicates a case where a defect occurs. In addition, among the two curves shown in FIG. 1, the lower curve indicates that the value of [Cu] × [Si] is 1.6, and the upper curve indicates [Cu] × [Si]. Indicates that the value of is 4.4. Note that SCC in FIG. 1 means stress corrosion cracking. The same applies to FIG.

As is apparent from FIG. 1, when Si is more than 3.0%, Cu is more than 3.0%, or the value of [Cu] × [Si] is more than 4.4, the specimens brazed and joined. There was a gap in the cross section. In the case of austenitic stainless steel, the addition of Si and Cu improves the wettability of the brazing material. However, when a certain amount or more of Si and Cu is added, the wettability becomes excessively good, and the brazing material flows out from the gaps between the materials to be joined, so that the joining becomes insufficient. Therefore, the upper limit of the value of [Cu] × [Si] is 4.4. A more preferred upper limit is 4.0.

(Stress corrosion crack evaluation)
The brazing material was heated in the atmosphere of 1100 ° C. and 100% hydrogen using the same conditions as when brazing, ie, using a hydrogen reduction furnace, without brazing. After this heating, the brazing joint material was cut into a size of 30 × 30 mm and 15 × 15 mm and the cut end face was polished. The two materials having different sizes were overlapped and spot welded at the center, and a gap was provided between the two materials to obtain a test material for stress corrosion cracking evaluation. The stress corrosion cracking evaluation test material, 200 ppm of Cl ^- is immersed in an aqueous solution containing was held at 100 ° C. 7 days. After 7 days, the spot weld was punched and separated, and the presence or absence of cracks in the inner clearance surface was evaluated. The presence or absence of cracks was confirmed by a dye penetration test (color check method).

The evaluation results are shown in FIG. The case where stress corrosion cracking did not occur is indicated by a white circle (◯), and the case where stress corrosion cracking occurred is indicated by a black circle (●). In FIG. 1, when the test material in which stress corrosion cracking did not occur was examined, the value of [Cu] × [Si] was 1.6 or more. On the other hand, stress corrosion cracking occurred in the test material having a value of [Cu] × [Si] of less than 1.6. In general, it is known that the addition of Si and Cu is effective in improving the stress corrosion cracking resistance of austenitic stainless steel. In the present invention, it is clear that the effect of suppressing the stress corrosion cracking can be obtained by the synergistic effect of Cu and Si represented by the value of [Cu] × [Si] even in the brazed austenitic stainless steel. did. Therefore, the lower limit of the value of [Cu] × [Si] is set to 1.6. More preferably, it is set to 2.0.

Next, a method for evaluating the corrosion resistance against the condensed water generated by the combustion exhaust gas and the result thereof will be described. As described above, the structure to be brazed is used as an exhaust system member of an automobile, a secondary heat exchanger of a water heater provided with a latent heat recovery device, or the like. Therefore, it is not sufficient for the austenitic stainless steel constituting the brazed joint structure to be excellent only in brazing and stress corrosion cracking properties.

(Evaluation of corrosion resistance against condensed water generated by combustion exhaust gas)
The material used as the test material is excellent in brazeability and stress corrosion resistance, that is, an austenitic stainless steel having a value of [Cu] × [Si] in the range of 1.6 to 4.4. Using. The test liquid can simulate the composition of condensed water generated by combustion of general LNG or petroleum. Specifically, the test solution was adjusted to nitrate ion 100 ppm, sulfate ion 10 ppm, pH 2.5, and had a composition in which chloride ions were added at 100 ppm in terms of Cl 2 ⁻ to accelerate corrosion.

The above-mentioned [Cu] × [Si] values in the range of 1.6 to 4.4 are the same conditions as when brazing and joining the austenitic stainless steel material without brazing. Then, it was heated in an atmosphere of 1100 ° C. and 100% hydrogen using a hydrogen reduction furnace. The heated material was cut into a size of 15 × 100 mm, and used as a test material for evaluating corrosion resistance. This corrosion resistance evaluation specimen was immersed in the test solution by half in a test tube. The test solution in the test tube was 10 ml. Then, the test tube is immersed in hot water at 80 ° C. and held for several hours until it is completely dried. After drying, another test tube is filled with the test solution and held until it is completely dried. 14 cycles were performed. The evaluation of the corrosion resistance against the condensed water generated by the combustion exhaust gas was performed by measuring the maximum corrosion depth on the surface of the test material for the corrosion resistance evaluation after the test.

The evaluation results are shown in FIG. A case where the maximum erosion depth is less than 100 μm is indicated by a white circle mark (◯), and a case where the maximum erosion depth is 100 μm or more is indicated by a cross mark (×). As is apparent from FIG. 2, when the value of 2 [N] + [Mo] is 0.16 or more, the maximum corrosion depth is less than 100 μm. This, Cl ^- even at a low pH solution containing, pitting corrosion resistance improving effect by Mo and N are estimated because obtained.

In FIG. 2, even when the value of 2 [N] + [Mo] is 0.16 or more, the maximum pitting corrosion depth may be 100 μm or more. When examining the test material in such a case, the Cu content was outside the range described below. This is because Cu is eluted and ionized during corrosion in a wet and dry repeated corrosive environment containing an oxidizing agent such as nitrate ions. And in such an environment, since Cu ion works as an oxidizing agent inside and outside the corrosion hole, it is estimated that the corrosion depth has increased.

Also, as the value of 2 [N] + [Mo] increases, the maximum corrosion depth decreases, but the decrease in the corrosion depth is saturated above a certain value. This is presumably because when the N and Mo contents exceed a certain value, the corrosion depth is such that the influence of elements other than N and Mo cannot be ignored. In particular, when Cu is present, corrosion is accelerated by Cu ions. For this reason, the upper limit of 2 [N] + [Mo] is 1.0 or less. A preferable upper limit is 0.77, and a more preferable upper limit is 0.74. The lower limit of 2 [N] + [Mo] is 0.16 as described above, and the preferable lower limit is 0.20.

When the results shown in FIGS. 1 and 2 described above are summarized by the relationship between the value of [Cu] × [Si] and the value of 2 [N] + [Mo], the relationship shown in FIG. 3 is obtained. As is apparent from FIG. 3, the value of [Cu] × [Si] is in the range of 1.6 to 4.4, and the value of 2 [N] + [Mo] is 0.16 to 1.0. Test materials in the range have both brazing and corrosion resistance. In the present invention, the corrosion resistance refers to the stress corrosion cracking resistance, the corrosion resistance in an environment where condensed water having a low pH containing nitrate ions and sulfate ions is generated due to condensation of combustion exhaust gas, and further chloride ions. Means corrosion resistance in an aqueous solution environment containing

Therefore, the austenitic stainless steel of the present invention needs to satisfy the following formulas (A) and (B) for Cu, Si, Mo, and N.
(A) Formula: 1.6 ≦ [Cu] × [Si] ≦ 4.4
(B) Formula: 0.16 ≦ 2 [N] + [Mo] ≦ 1.0

Next, the reasons for limitation of each element contained in the austenitic stainless steel of the present invention will be described.

C lowers the intergranular corrosion resistance and workability, so its content needs to be reduced, so the upper limit needs to be 0.080%. However, excessively reducing the C content deteriorates the refining cost. Therefore, the preferable C content is in the range of 0.005 to 0.060%.

As described above, Si is added to improve wettability and prevent stress corrosion cracking, as with Cu. If the Si content is less than 1.2%, these effects are not exhibited. On the other hand, when the Si content exceeds 3.0%, the wettability is excessively improved and the brazing property is lowered. Therefore, the Si content needs to be in the range of 1.2 to 3.0%. Preferably, it is in the range of 1.4 to 2.5%.

Mn is an important element as a deoxidizing element, but if it is added excessively, MnS that becomes a starting point of corrosion is likely to be generated. Therefore, the Mn content needs to be in the range of 0.4 to 2.0%. More preferably, it is in the range of 0.5 to 1.2%.

ＰP not only reduces weldability and workability, but also tends to cause intergranular corrosion, so P should be kept as low as possible. For this reason, the upper limit of the content of P needs to be 0.03%. A preferable P content is in the range of 0.001 to 0.025%.

It is necessary to reduce S as much as possible in order to generate water-soluble inclusions that become the starting point of corrosion such as MnS described above. Therefore, the S content is set to 0.003% or less. However, since excessive reduction of S is costly, the S content is preferably in the range of 0.0002 to 0.002%.

Ni does not affect stress corrosion cracking resistance in the amount specified by JIS SUS316L. However, there is a concern that the stress corrosion cracking resistance may deteriorate in an environment where the LNG or petroleum is exposed to exhaust gas when burned. Moreover, it is necessary to maintain an austenite phase and to ensure workability. Therefore, the Ni content needs to be in the range of 6.0 to 12.0%. Preferably, it is in the range of 6.5 to 11.0%.

Cr is the most important element for ensuring the corrosion resistance of stainless steel. Therefore, the lower limit of the Cr content is 16.0%. However, when Cr is increased, the corrosion resistance is also improved, but the manufacturability and other manufacturability are lowered, so the upper limit of the Cr content is 20.0%. A preferable Cr content is in the range of 16.5 to 19.0%.

Cu, together with Si, reduces brazing by its addition, but has the function of suppressing stress corrosion cracking. On the other hand, excessive addition of Cu reduces the corrosion resistance in a solution containing nitrate ions. Therefore, the Cu content needs to be in the range of 0.2 to 3.0%. Preferably, it is in the range of 0.5 to 2.5%.

Al is important as a deoxidizing element, and also refines the structure by controlling the composition of non-metallic inclusions. However, if it is added excessively, the non-metallic inclusions become coarse, and there is a possibility that it may become a starting point for product wrinkles. Therefore, the Al content needs to be in the range of 0.002 to 0.10%. Preferably it is 0.005 to 0.08% of range.

N improves pitting corrosion resistance, but excessive addition reduces the intergranular corrosion resistance and workability in the same manner as C. Therefore, the N content needs to be in the range of 0.030 to 0.150%. Preferably, it is 0.037 to 0.10% of range.

Mo is an element that is effective in repairing the passive film and is very effective in improving corrosion resistance. Furthermore, in an environment containing nitrate ions and chloride ions, there is an effect of improving pitting corrosion resistance in combination with N. Therefore, it is necessary to contain Mo at least 0.1%. On the other hand, when Mo is increased, the corrosion resistance is improved, but excessive addition reduces workability and causes an increase in cost. Therefore, the upper limit of the Mo content needs to be 1.0%. A preferable Mo content is in the range of 0.2 to 0.8%.

In the present invention, in addition to the essential elements described so far, Nb, Ti, V, and B may be contained alone or in combination as required.

Nb can be added as needed because it has the effect of generating carbonitrides to suppress sensitization in the vicinity of the weld and increasing the high-temperature strength. However, excessive addition causes cost increase. Therefore, the Nb content is preferably in the range of 0.1 to 0.7%.

Ti has the same effect as Nb, but excessive addition causes an increase in surface defects due to titanium nitride. Therefore, the Ti content is preferably in the range of 0.1 to 0.5%.

Since V improves weather resistance and crevice corrosion resistance, excellent workability can be secured if V is added while suppressing the use of Cr and Mo. Therefore, V can be added as necessary. However, excessive addition causes deterioration of workability. Therefore, the V content is preferably in the range of 0.1 to 3.0%.

B is a grain boundary strengthening element effective for improving hot workability, and can be added as necessary. However, excessive addition causes a decrease in workability. Therefore, the B content is preferably 0.0002% at the lower limit and 0.003% at the upper limit.

Next, the present invention will be further described with reference to examples. Conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is examples of these one condition. It is not limited to. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steel having the chemical composition shown in Table 1 was produced by a normal austenitic stainless steel production method. First, an ingot having a thickness of 40 mm was manufactured after vacuum melting, and this was hot rolled to a thickness of 4.0 mm. Thereafter, a heat treatment was performed at 1150 ° C. for 1 minute, the scale was removed by grinding, and a steel plate having a thickness of 1.0 mm was manufactured by cold rolling. This was heat-treated at 1050 to 1150 ° C. for 1 minute based on each recrystallization behavior, and then subjected to immersion pickling treatment until the scale was completely removed in an aqueous nitric hydrofluoric acid solution. It used for the test.

(Brassability test)
Silver brazing was made by cutting various stainless steels with a thickness of 1 mm into 40 x 50 mm and 25 x 30 mm and wet-polishing the entire surface using # 600 water-resistant emery paper (water-resistant abrasive paper). A brazeability test using was conducted.

Brazing joining was performed by stacking two test materials in the same manner as described above. Specifically, 0.3 g of JIS BNi5 silver brazing mixed with an organic binder was placed on the overlapping part of the test material and brazed and joined. The brazing joining was performed in an atmosphere of 1100 ° C. and 100% hydrogen using a hydrogen reduction furnace. In the evaluation method, the cross section of the brazed specimen was evaluated by visual observation as being good when the gap was completely filled with the gap and poor when the gap remained.

(Corrosion resistance test)
Next, a wet and dry repeated test method performed in a test solution simulating condensed water generated by LNG or petroleum combustion will be described. As the test material, various stainless steels were heated in an atmosphere of 1100 ° C. and 100% hydrogen using the same conditions as when brazing without using brazing, that is, using a hydrogen reduction furnace. Then, it cut | disconnected to the magnitude | size of 15 * 100 mm and tested. In addition, the plate | board thickness of a test material is 1 mm. As described above, the composition of the test solution was adjusted to nitrate ion 100 ppm, sulfate ion 10 ppm, pH 2.5 by simulating the composition of the condensed water generated by the combustion of general LNG or petroleum, and concentrated salt As a simulation, the chloride ion was set to 100 ppm. The test material was immersed in a test tube containing 10 ml of the test solution and placed in a warm bath at 80 ° C. This test solution was held until it completely dried, and after drying, the sample was transferred to a new test tube filled with the test solution and dried again. The maximum corrosion depth after the test after 14 times of this drying was measured.

(Stress corrosion cracking evaluation test)
In the stress corrosion cracking evaluation test, the same material as that used in the brazeability test was subjected to the same conditions as when brazing without brazing, that is, using a hydrogen reduction furnace at 1100 ° C. and hydrogen. The heating was performed in a 100% atmosphere. From this material, it was cut into a size of 30 × 30 mm and 15 × 15 mm and wet-polished on the entire surface, and then the two sheets were overlapped and spot-welded to provide a gap. The specimen thus provided with the gap was immersed in distilled water containing 200 ppm of Cl 2 ⁻ and continuously treated at 100 ° C. for 7 days. After the spot welded portion of the test material after the treatment was drilled and separated, the presence or absence of cracks was observed by a dye penetration test (color check method). Here, the case where a crack did not occur was determined to be good, and the case where a crack occurred was determined to be defective.

These test results are also shown in Table 1. In addition, about the brazing property test result and the stress corrosion cracking evaluation test result, “A” indicates good and “B” indicates failure.

As is clear from Table 1, No. In Examples 1 to 13, it was confirmed that the brazing test, the maximum corrosion depth in the corrosion resistance test (dry and wet repetition test), and the stress corrosion cracking evaluation test were all good results.

In contrast, No. with a value of [Cu] × [Si] exceeding 4.4. 14 to 18 and 21 confirmed that sufficient brazability could not be obtained. In addition, the value of [Cu] × [Si] is less than 1.6. Nos. 19, 23, 24, and 25 were confirmed to have cracks in the stress corrosion cracking evaluation test even though the brazing property was good. Furthermore, No. 2 in which the value of 2 [N] + [Mo] is outside the lower limit of the present invention. Nos. 20, 21, 23, and 24 resulted in a maximum pitting corrosion depth of 100 μm or more in the corrosion resistance test (dry and wet repetition test). No. 22, the value of [Cu] × [Si] and the value of 2 [N] + [Mo] are within the range of the present invention, but Cr is outside the lower limit of the range of the present invention, so the corrosion resistance test (wet and dry In the repeated test), the maximum pitting corrosion depth exceeded 100 μm. In addition, No. 14 to 18, even when the value of 2 [N] + [Mo] was within the range of the present invention, the maximum corrosion depth was more than 100 μm in the corrosion resistance test (dry and wet repeated test). Is outside the scope of the present invention, it is judged that the corrosion promotion effect by the eluted Cu ions worked.

From the above, it was confirmed that the austenitic stainless steel of the present invention is excellent in brazing property and does not cause stress corrosion cracking even in an environment in a heat exchanger exposed to combustion gas of hydrocarbon fuel. At the same time, the austenitic stainless steel of the present invention is excellent in corrosion resistance in an environment where condensed water containing nitrate ions and sulfate ions is generated at a low pH and in an aqueous solution environment containing chloride ions. It could be confirmed.

The present invention is a structure for brazing austenitic stainless steel, corrosion resistance in an environment where condensed water containing nitrate ions and sulfate ions and having a low pH is generated, and in an aqueous solution containing chloride ions. It can be applied to all uses that require corrosion resistance. Specifically, the austenitic stainless steel of the present invention is particularly suitable for use as a heat exchanger material, particularly as a secondary heat exchanger material for a latent heat recovery type hot water heater using kerosene or LNG as fuel. In that case, the austenitic stainless steel of the present invention can be applied not only to heat exchanger pipes but also to any materials such as cases and partition plates. Further, the austenitic stainless steel of the present invention is also suitable when used as a heat recovery component from exhaust gas such as EGR mounted on an automobile having gasoline and diesel engines.

In addition, the austenitic stainless steel of the present invention is particularly suitable for use in an environment of repeated wet and dry exposure to a low pH solution containing nitrate ions and sulfate ions. Specifically, outdoor exterior materials, building materials, roofing materials, outdoor equipment, etc. that are assumed to have an acid rain environment. In addition, the austenitic stainless steel sheet of the present invention is generally used for water-related equipment where stress corrosion cracking is a concern, specifically, water storage / hot water storage tanks, home appliances, bathtubs, kitchen equipment, and other outdoor / indoor use. It is suitable for use in equipment. Thus, the present invention has a high utility value in the industry.

Claims

In mass%, C: 0.080% or less, Si: 1.2 to 3.0%, Mn: 0.4 to 2.0%, P: 0.03% or less, S: 0.003% or less, Ni: 6.0 to 12.0%, Cr: 16.0 to 20.0%, Cu: 0.2 to 3.0%, Al: 0.002 to 0.10%, N: 0.030 to Corrosion resistance characterized by containing 0.150% and Mo: 0.1-1.0%, the balance being Fe and inevitable impurities and satisfying the following formulas (A) and (B) Austenitic stainless steel with excellent brazing properties.
(A) Formula: 1.6 ≦ [Cu] × [Si] ≦ 4.4
(B) Formula: 0.16 ≦ 2 [N] + [Mo] ≦ 1.0
Here, [Cu], [Si], [N], and [Mo] are the contents of each element expressed in mass%.
Further, in terms of mass%, Nb: 0.1 to 0.7%, Ti: 0.1 to 0.5%, V: 0.1 to 3.0%, and B: 0.0002% to 0.003 The austenitic stainless steel excellent in corrosion resistance and brazing properties according to claim 1, comprising one or more of the following: