KR20130123443A - Highly corrosion-resistant austenite stainless steel well-suited to brazing - Google Patents

Abstract

The present invention not only has excellent brazing properties, but also austenite having excellent corrosion resistance under an environment in which condensed water having a low pH including nitrate ions and sulfate ions is generated by condensation of combustion exhaust gas, but also in an aqueous solution environment containing chloride ions. By providing a system stainless steel, 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 0.150%, and Mo: 0.1 to 1.0%, the remainder being Fe and an unavoidable impurity. Formula (A): 1.6 ≦ [Cu] × [Si] ≦ 4.4 and (B) Formula: 0.16 ≦ 2 [N] + [Mo] ≦ 1.0.

Description

Austenitic stainless steel with excellent corrosion resistance and brazing {HIGHLY CORROSION-RESISTANT AUSTENITE STAINLESS STEEL WELL-SUITED TO BRAZING}

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

Brazing bonding is a technique of joining materials by using a brazing material having a lower melting point than that of a structural material and heat-processing 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 in brazing joining of stainless steel is an alloy of nickel and copper.

In brazing joining of stainless steel, the passivation film of stainless steel impairs brazing property. Therefore, brazing bonding is performed in vacuum or hydrogen atmosphere, in order to reduce | remove and remove a passivation film. The temperature of brazing junction is about 1100 degreeC, for example, when using nickel lead.

In brazing joining, it is important for the brazing material to sufficiently fill the gap between the joined materials and to secure the strength of the joining portion. Therefore, the wettability of the brazing material with respect to stainless steel which is a to-be-joined material becomes important. On the other hand, if the wettability of a brazing material is too good, a brazing material will flow out from the space | interval of to-be-joined materials, and a clearance gap will not be filled with a brazing material, and joining strength will fall. For this reason, as stainless steel which is excellent in brazing joining, it becomes important to have moderate wettability.

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

Stress corrosion cracking occurs when tensile stress remains in a material having high stress corrosion cracking sensitivity exposed to an environment where corrosion occurs. In brazing an austenitic stainless steel, there is no fear of stress corrosion cracking even if tensile stress remains in the joined material in the step before brazing bonding. This is because the brazing joining temperature of the austenitic stainless steel is performed at a temperature at which the austenitic stainless steel is annealed, and the residual stress is removed during the brazing joining. For example, when nickel lead is used, it is because it braze-bonds at about 1100 degreeC as mentioned above.

However, depending on the part, after brazing joining, assembly may be performed by welding, screwing, etc. with another part, and in that case, tensile stress may generate | occur | produce in the part after assembly, and there exists a possibility of stress corrosion cracking. Therefore, the austenitic stainless steel brazed is required to have stress corrosion resistance.

As an environment in which a brazing bonding material of austenitic stainless steel is used, for example, there is a secondary heat exchanger of a hot water heater provided with an exhaust system member of a vehicle and a latent heat recovery unit. All of these members are used in an environment in which combustion exhaust gas is condensed, so that condensed water having a low pH including nitrate ions and sulfate ions is generated. This is because a large amount of nitrogen is contained in the atmosphere introduced for combustion, and sulfur compounds are included in the fuel or odorous substances added to the fuel. Under these circumstances, copper corrodes. Therefore, copper cannot be used as a material constituting the exhaust system member of an automobile or the secondary heat exchanger of a hot 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 a member to achieve both corrosion resistance and brazing performance even in an environment in which condensed water having a low pH containing nitrate ions and sulfate ions is produced.

Regarding the brazing properties of the stainless steel, a precoat solder-coated metal sheet material is proposed in Patent Document 1 in which a nickel-based solder material suspended together with an organic binder is spray-coated and heated on a stainless steel sheet surface. Moreover, patent document 2 has proposed the manufacturing method of the nickel lead coating stainless steel plate excellent in the magnetic brazing property which coat | covered the nickel-type soldering material by the plasma spray on the stainless steel plate which adjusted the surface roughness. However, in both patent documents 1 and 2, only the conventional SUS304 material and the SUS316 material are examined as an austenitic stainless steel material to which a brazing material is apply | coated.

In patent document 3, the stainless steel excellent in the brazing property which reduced Al and Ti is proposed. In addition, 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. However, both patent documents 3 and 4 are examination in a ferritic stainless steel, and have not examined about an austenitic stainless steel.

Patent Document 5 proposes an austenitic stainless steel having stress corrosion cracking resistance and gap corrosion resistance. However, the steel sheet proposed in Patent Literature 5 is applied to automotive oil supply system members, and has been examined for stress corrosion cracking resistance, but it is not described for brazing resistance.

In addition, when used in a secondary heat exchanger of a hot water heater equipped with an exhaust system member of a vehicle or a latent heat recovery device, since chloride is contained in the air to be intaked, an environment containing chloride ions, especially when used in a coastal high salt area, Corrosion resistance also becomes a problem.

Japanese Patent Application Laid-open No. Hei 1-249294 Japanese Patent Application Laid-Open No. 2001-26855 Japanese Patent Application Publication No. 2009-174046 Japanese Patent Application Laid-open No. 2010-65278 Japanese Patent Application Publication No. 2007-9314

The present invention is not only excellent in brazing property, but also excellent in corrosion resistance under an environment in which condensed water having a low pH including nitrate ions and sulfate ions is generated by condensation of combustion exhaust gas, and also excellent in corrosion resistance in an aqueous solution environment containing chloride ions. It is an object to provide austenitic stainless steels.

MEANS TO SOLVE THE PROBLEM The present inventors discovered the following knowledge as a result of earnestly examining in order to obtain the austenitic stainless steel which makes brazing property and corrosion resistance compatible.

(a) In the case of an austenitic stainless steel, when a predetermined amount or more of Si and Cu are added, the wettability is excessively good, and the brazing material flows out from the gaps between the joined materials, resulting in insufficient joining. In order to prevent this, it is important to define not only the upper limit of each of content of Cu and Si, but an upper limit of the value of [Cu] x [Si]. In addition, in the following description, [Cu] and [Si] are taken as content of Cu and Si shown by the mass%.

(b) Brazing-bonded austenitic stainless steel is obtained by the synergistic effect of Cu and Si represented by the value of [Cu] x [Si] for suppressing stress corrosion cracking.

(c) Corrosion resistance under an environment where condensed water of low pH, including nitrate ions and sulfate ions, is generated by condensation of the combustion exhaust gas, and corrosion resistance under an aqueous solution environment containing chloride ions is 2 [N] + [Mo]. It improves by making a value more than a fixed value. In addition, in the following description, [N] and [Mo] are taken as content of N and Mo shown by the mass%.

This invention is made | formed based on said knowledge, The summary is as follows.

(1) 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 0.150%, and Mo: 0.1 to 1.0%, and the remainder is made of Fe and unavoidable impurities. Austenitic stainless steel excellent in corrosion resistance and brazing, characterized by satisfying the formulas (B) and (B).

(A) Formula: 1.6≤ [Cu] × [Si] ≤4.4

(B) Formula: 0.16≤2 [N] + [Mo] ≤1.0

Here, [Cu], [Si], [N] and [Mo] are taken as content of each element shown by the mass%.

(2) It further contains 1 type (s) or 2 or more types of Nb: 0.1-0.7%, Ti: 0.1-0.5%, V: 0.1-3.0%, and B: 0.0002% -0.003% by mass%, It is characterized by the above-mentioned. The austenitic stainless steel excellent in corrosion resistance and brazing property as described in said (1).

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

According to the present invention, the corrosion resistance of structures obtained by brazing and joining, such as heat recovery and recovery of combustion exhaust gas using hydrocarbons such as gasoline, LNG, LPG, and petroleum, and other heat exchangers, can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the relationship of content of Cu and Si, brazing property, and corrosion resistance.
2 is a diagram illustrating a relationship between a value of 2 [N] + [Mo] and a maximum corrosion depth.
Fig. 3 is a diagram showing the relationship between the values of 2 [N] + [Mo] and the values of [Cu] × [Si], and the brazing and corrosion resistance.

The present invention will be described in detail. In the following description,% concerning a component composition shall mean the mass% unless there is a notice in particular.

First, the experiment and the result which were performed in order to acquire the component composition which make brazing property and corrosion resistance compatible are demonstrated. Austenitic stainless steels with varying Si, Cu, Mo and N were prepared by vacuum melting. At this time, other elements were made into the component range of JIS SUS316.

After hot-rolling this austenitic stainless steel and performing heat treatment at 1150 ° C for 1 minute, the scale was ground and cold-rolled to obtain a cold rolled sheet. The cold rolled sheet was heat-treated under conditions of 1050 to 1150 캜 for 1 minute based on the recrystallization behavior, and then immersion pickling was performed until the scale was completely removed in an aqueous nitric acid solution to obtain a brazing joint material. . Using this brazing joining material, the brazing properties and stress corrosion cracking were evaluated.

(Brassability Evaluation)

The raw material for brazing joining was cut into 40x50 mm and 25x30 mm, and it was set as the test material for brazing property evaluation. The plate | board thickness of this test material for brazing property evaluation is 1 mm. Thus, the brazing joining was performed to the test material produced using silver lead. In brazing joining, 0.3 g of nickel-lead of JIS BNi5 which mixed the organic binder was arrange | positioned to the overlapping site | part which laminated two test materials as a brazing material, and brazing was carried out. Brazing bonding was performed in 1100 degreeC and 100% of hydrogen atmosphere using the hydrogen reduction furnace. Brazing property was evaluated by cutting the brazing bonded specimen and visually observing the cross section.

An evaluation result is shown in FIG. In the cross section of the brazing bonded specimens, white gaps (○) or black circles (●) were indicated by scissors tables (×) when the gap was completely filled with the brazing material. Here, the white circle (○) and the black circle (●) are distinguished with respect to the evaluation results of the stress corrosion cracks described later, and stress corrosion cracks do not occur, and white circles (○) and stress corrosion cracks are generated in good cases. In the case of failure, black circles (●) are indicated. In addition, of the two curves shown in FIG. 1, the lower curve shows that the value of [Cu] × [Si] is 1.6, and the upper curve shows that the value of [Cu] × [Si] is 4.4. In addition, SCC in FIG. 1 means stress corrosion cracking. The same applies to FIG. 3.

As apparent from FIG. 1, when Si was more than 3.0%, Cu was more than 3.0%, or the value of [Cu] × [Si] was more than 4.4, a gap was generated in the cross section of the brazing bonded specimen. In the case of the austenitic stainless steel, the wettability of the brazing material is improved by addition of Si and Cu. However, when Si and Cu are added in a fixed amount or more, the wettability is excessively good, and the brazing material flows out from the gaps between the joined materials, resulting in insufficient bonding. Therefore, the upper limit of the value of [Cu] x [Si] is set to 4.4. A more preferable upper limit is 4.0.

(Stress Corrosion Crack Evaluation)

The raw material for brazing joining was heated in 1100 degreeC and 100% of hydrogen using the same conditions as brazing joining, ie, a hydrogen reduction furnace, without brazing joining. After this heating, the raw material for brazing joining was cut out to the size of 30x30 mm and 15x15 mm, and the cut end surface was polished. Two raw materials of different sizes were overlapped and spot welded at the center portion, and a gap was provided between the two raw materials to prepare a specimen for stress corrosion cracking evaluation. This test specimen for stress corrosion cracking evaluation was immersed in an aqueous solution containing 200 ppm of Cl ⁻ , and maintained at 100 ° C. for 7 days. After 7 days had elapsed, the spot welds were drilled and separated, and the presence or absence of cracks in the inner gap surface was evaluated. The presence or absence of a crack was confirmed by the dye penetration flaw test (color check method).

The evaluation result was written together in FIG. The case where a stress corrosion crack did not generate | occur | produce was shown by the white circle ((circle)), and the case where a stress corrosion cracking generate | occur | produced is shown by the black circle (●). In FIG. 1, when the test material which stress stress cracking did not generate | occur | produce was examined, the value of [Cu] x [Si] was 1.6 or more. On the other hand, the stress corrosion cracking generate | occur | produced the test material whose value of [Cu] x [Si] is less than 1.6. In general, there is a knowledge that the addition of Si and Cu is effective for improving the stress corrosion cracking resistance of austenitic stainless steels. In this invention, also in the austenitic stainless steel brazed, it was made clear that the suppression effect of stress corrosion cracking is obtained by the synergistic effect of Cu and Si represented by the value of [Cu] x [Si]. Therefore, the lower limit of the value of [Cu] x [Si] is 1.6. More preferably, it is 2.0.

Next, a method for evaluating corrosion resistance to condensate generated by combustion exhaust gas and the result thereof will be described. As described above, the structure to be brazed is used as a secondary heat exchanger of a hot water heater provided with an exhaust system member of an automobile or a latent heat recovery device. Therefore, the austenitic stainless steel constituting the brazed bonded structure is insufficient only in being excellent in brazing property and stress corrosion cracking property.

(Evaluation of Corrosion Resistance to Condensate Generated by Combustion Exhaust Gas)

As the material used as the test material, an austenitic stainless steel having excellent brazing properties and stress corrosion resistance, that is, a value of [Cu] × [Si] in the range of 1.6 or more and 4.4 or less was used. The test liquid was supposed to be able to simulate the composition of condensate generated by combustion of general LNG or petroleum. Specifically, the test solution was adjusted to 100 ppm of nitrate ions, 10 ppm of sulfate ions, and pH 2.5, and had a composition in which chloride ions were added in an amount of Cl ^{− in an} amount of 100 ppm to accelerate corrosion.

1100 using the austenitic stainless steel material in which the above-mentioned value of [Cu] × [Si] is in the range of 1.6 or more and 4.4 or less, without brazing bonding, using the same conditions as that for brazing bonding, that is, a hydrogen reduction furnace, It heated in the atmosphere of 100 degreeC and hydrogen 100%. The raw material after heating was cut | disconnected to the magnitude | size of 15 * 100 mm, and it was set as the corrosion resistance evaluation specimen. This corrosion resistance test specimen was only half immersed in the test solution in the test tube. In addition, the test liquid in a test tube was 10 ml. Then, the test tube was immersed in a hot water bath at 80 ° C., and maintained until it was completely dried for several hours, and after drying, 14 cycles of the wet and dry test were carried out in which another test tube was filled with fresh test liquid and maintained until it was completely dried. . The evaluation of the corrosion resistance with respect to the condensate produced by the combustion exhaust gas was performed by measuring the maximum corrosion depth on the surface of the corrosion resistance evaluation test material after a test.

An evaluation result is shown in FIG. The case where the maximum erosion depth was less than 100 micrometers was represented by the white circle ((circle)) and the case where it is 100 micrometers or more by the scissors table (x). As apparent from Fig. 2, when the value of 2 [N] + [Mo] was 0.16 or more, the maximum corrosion depth became less than 100 µm. This is presumably because the effect of improving the formal resistance by Mo and N is obtained even in a low pH solution containing Cl ⁻ .

In addition, even if the value of 2 [N] + [Mo] is 0.16 or more in FIG. 2, the maximum formula depth may be 100 micrometers or more. When the test material in this case was examined, Cu content was out of the range mentioned later. This elutes and ionizes Cu at the time of corrosion in a wet and dry cyclic corrosion environment containing an oxidizing agent such as nitrate ions. In this environment, the corrosion depth is estimated to increase because Cu ions act as oxidants in and out of the corrosion holes.

In addition, with the increase of the value of 2 [N] + [Mo], although the maximum corrosion depth was reduced, the fall of corrosion depth was saturated more than a certain value. This is presumably because when the content of N and Mo exceeds a fixed value, the corrosion depth cannot be ignored by the influence of elements other than N and Mo. In particular, when Cu is present, corrosion is promoted by Cu ions. For this reason, the upper limit of 2 [N] + [Mo] is made 1.0 or less. A preferable upper limit is 0.77, and a more preferable upper limit is 0.74. In addition, as mentioned above, the minimum of 2 [N] + [Mo] is 0.16, and a preferable minimum is 0.20.

The results shown in FIG. 1 and FIG. 2 described above are summarized in the relationship between the values of [Cu] × [Si] and the values of 2 [N] + [Mo]. As is apparent from FIG. 3, the test material having a value of [Cu] × [Si] in the range of 1.6 to 4.4 and a value of 2 [N] + [Mo] in the range of 0.16 to 1.0 has brazing and corrosion resistance. Compatible. In addition, in this invention, corrosion resistance means stress corrosion cracking property, corrosion resistance in the environment in which the low pH condensed water containing nitrate ion and sulfate ion is produced | generated by condensation of combustion exhaust gas, and the aqueous solution containing chloride ion. Corrosion resistance under the environment.

Therefore, the austenitic stainless steel of this invention needs to satisfy following formula (A) and (B) with respect to Cu, Si, Mo, and N.

(A) Formula: 1.6≤ [Cu] × [Si] ≤4.4

(B) Formula: 0.16≤2 [N] + [Mo] ≤1.0

Next, the reason for limitation to each element contained in the austenitic stainless steel of this invention is demonstrated.

Since C reduces grain boundary corrosion resistance and workability, it is necessary to reduce the content, and therefore it is necessary to make an upper limit into 0.080%. However, excessively reducing the C content deteriorates the refining cost. Therefore, preferable C content is 0.005 to 0.060% of range.

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

Although Mn is an important element as a deoxidation element, when it adds excessively, it will become easy to produce MnS which becomes a starting point of corrosion. Therefore, Mn content needs to be 0.4 to 2.0% of range. More preferably, it is 0.5 to 1.2% of range.

P not only lowers the weldability and workability but also makes it easier to generate grain boundary corrosion. Therefore, P needs to be suppressed as low as possible. For this reason, the upper limit of content of P needs to be 0.03%. Preferable P content is 0.001 to 0.025% of range.

S produces | generates the water-soluble inclusion which becomes a starting point of corrosion, such as MnS mentioned above, and needs to reduce as much as possible. Therefore, S content rate is made into 0.003% or less. However, since excessive cost reduction of S takes cost, it is preferable to make S content into 0.0002 to 0.002% of range.

Ni does not affect stress corrosion cracking in the quantity of the grade prescribed | regulated to JIS SUS316L. However, under the environment of being exposed to exhaust gas when LNG or petroleum is combusted, it is feared that the stress corrosion cracking resistance is lowered. In addition, it is necessary to maintain the austenite phase and ensure workability. Therefore, it is necessary to make Ni content into 6.0 to 12.0% of range. Preferably, it is 6.5 to 11.0% of range.

Cr is the most important element in securing the corrosion resistance of stainless steel. Therefore, the lower limit of the Cr content is 16.0%. However, if Cr is increased, the corrosion resistance is improved, but workability including workability is lowered, so the upper limit of the Cr content is 20.0%. Preferable Cr content is 16.5 to 19.0% of range.

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

Al is important as a deoxidation element, and also controls the composition of nonmetallic inclusions to refine the structure. However, excessive addition may cause coarsening of nonmetallic inclusions and may cause scratches of the product. Therefore, Al content needs to be 0.002 to 0.10% of range. Preferably it is 0.005 to 0.08% of range.

N improves formula resistance, but excessive addition decreases grain boundary corrosion resistance and workability similarly to C. Therefore, N content needs to be 0.030 to 0.150% of range. Preferably, it is 0.037 to 0.10% of range.

Mo is effective in repairing a passivation film and is a very effective element for improving corrosion resistance. In addition, in an environment containing nitrate ions and chloride ions, there is an effect of improving the formula resistance in combination with N. Therefore, it is necessary to contain Mo at least 0.1%. On the other hand, although increasing corrosion resistance improves Mo, excessive addition reduces workability and raises a cost. Therefore, the upper limit of Mo content needs to be 1.0%. Preferable Mo content is 0.2 to 0.8% of range.

In the present invention, one or two or more kinds may be contained from Nb, Ti, V, and B in addition to the essential elements described so far.

Nb produces | generates carbonitride by the addition, and since there exists an effect which suppresses the sharpening of the vicinity of a weld part, or increases high temperature strength, it can add it as needed. However, excessive addition causes cost increase. Therefore, it is preferable to make Nb content into 0.1 to 0.7% of range.

Ti has the same effect as Nb, but excessive addition causes an increase in surface scratches by the nitride of titanium. Therefore, it is preferable to make Ti content into 0.1 to 0.5% of range.

Since V improves rust resistance and gap corrosion resistance, it is possible to secure excellent workability by suppressing the use of Cr and Mo and adding V. Therefore, V can be added as needed. However, it is because excessive addition will cause workability fall. Therefore, it is preferable to make V content into 0.1 to 3.0% of range.

Since B is a grain boundary strengthening element effective for hot workability improvement, it can be added as necessary. However, excessive addition becomes a cause of workability fall. Therefore, as for B content, it is preferable to make a minimum into 0.0002% and an upper limit to 0.003%.

Example

Next, although an Example further demonstrates this invention, the conditions in an Example are one condition example employ | adopted in order to confirm the feasibility and effect of this invention, and this invention is limited to this one condition example. no. 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 conventional method for producing austenitic stainless steel. First, a 40-mm-thick ingot was produced after the vacuum solvent, and this was rolled to 4.0-mm thickness by hot rolling. Then, after heat-processing at 1150 degreeC x 1 minute, the scale was ground and removed, and the steel plate of 1.0 mm thickness was manufactured by cold rolling. This is heat-treated under the conditions of 1050 to 1150 ° C for 1 minute based on each recrystallization behavior, and then immersion pickling treatment is performed until the scale is completely removed in an aqueous nitric acid solution, and the following three tests are provided. It was.

(Brazing test)

Various stainless steels having a thickness of 1 mm were cut into 40 × 50 mm and 25 × 30 mm, and wet polished on the whole surface using # 600 water-resistant emery paper (water-resistant abrasive paper). Brazing test was conducted.

Brazing bonding was carried out by overlapping two test materials in the same manner as described above. Specifically, 0.3 g of silver lead of JIS BNi5 in which an organic binder was mixed was placed in the overlapping portions of the test materials and brazed. Brazing bonding was performed in 1100 degreeC and 100% of hydrogen atmosphere using the hydrogen reduction furnace. In the cross section of the test material brazed and bonded, the evaluation method was good when the brazing material was completely filled in the clearance by visual observation, and was bad when the clearance remained.

(Corrosion resistance test)

Next, the wet and dry repetition test method which performs in the test liquid which simulated the condensed water produced by the combustion of LNG or petroleum is demonstrated. The test material was heated in 1100 degreeC and the atmosphere of 100% of hydrogen using the same conditions as the case of brazing joining, ie, a hydrogen reduction furnace, without brazing joining various stainless steels. Then, it cut | disconnected and tested to the magnitude | size of 15 * 100 mm. In addition, the plate | board thickness of a test material is 1 mm. As described above, the composition of the test solution simulates the composition of condensate generated by the combustion of general LNG or petroleum, adjusts it to 100 ppm of nitrate ions, 10 ppm of sulfate ions, and pH 2.5, simulates the concentration of salts, and simulates chloride ions. Was 100 ppm. In the test tube which put 10 ml of this test liquid, the test material was immersed half and put in the 80 degreeC warm bath. This test solution was maintained until it was 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 performing this drying 14 times was measured.

(Stress Corrosion Crack Evaluation Test)

In the stress corrosion cracking evaluation test, the same material as that provided for the brazing test is heated in an atmosphere of 1100 ° C. and 100% hydrogen using the same conditions as when brazing, without brazing bonding, that is, a hydrogen reduction furnace. Was carried out. From this material, it cut out to the size of 30x30mm and 15x15mm, wet surface-washing whole surface, two sheets were overlapped, the spot welding was performed, and the clearance gap was provided. Thus, the test material which gave the clearance gap was immersed in distilled water containing 200 ppm Cl <^-> , and it processed continuously at 100 degreeC for 7 days. After the spot welded portion of the test specimen after the treatment was drilled and separated, the presence or absence of a crack was observed by a dye penetration flaw test (color check method). Here, the case where a crack did not generate | occur | produced it as favorable, and the case where a crack generate | occur | produced it as defect.

The results of these tests are listed in Table 1. In addition, about the brazing test result and the stress corrosion cracking evaluation test result, A was shown as good and A was bad.

As apparent from Table 1, it was confirmed that the invention examples Nos. 1 to 13 were satisfactory results for both the maximum corrosion depth and stress corrosion cracking evaluation test in the brazing test and the corrosion resistance test (dry and dry repeated test).

On the other hand, Nos. 14-18 and 21 whose value of [Cu] x [Si] exceed 4.4 confirmed that sufficient brazing property was not obtained. Moreover, although No.19, 23, 24, 25 whose value of [Cu] x [Si] is less than 1.6, although the brazing property was favorable, a crack was confirmed by the stress corrosion cracking evaluation test. In addition, No. 20, 21, 23, 24 whose value of 2 [N] + [Mo] is outside the lower limit of this invention resulted in the maximum formula depth in a corrosion resistance test (wet-dry test) being 100 micrometers or more. . In No. 22, the value of [Cu] × [Si] and the value of 2 [N] + [Mo] are within the scope of the present invention. However, since Cr is outside the lower limit of the scope of the present invention, the corrosion resistance test (wet and dry repeat test) ), The maximum formula depth exceeds 100 μm. In addition, in Nos. 14 to 18, even if the value of 2 [N] + [Mo] was within the range of the present invention, it was found that the maximum corrosion depth exceeded 100 µm in the corrosion resistance test (dry and dry repeated test). Is outside the scope of the present invention, it is considered that the corrosion promoting effect by the eluted Cu ion acted.

As mentioned above, it was confirmed that the austenitic stainless steel of the present invention is excellent in brazing property and does not generate stress corrosion cracking even in an environment in a heat exchanger exposed to combustion gas of a hydrocarbon fuel. At the same time, it was confirmed that the austenitic stainless steel of the present invention was excellent in corrosion resistance under an environment in which condensed water having a low pH containing nitrate ions and sulfate ions was produced, but in an aqueous solution environment containing chloride ions.

The present invention relates to a structure for brazing and joining austenitic stainless steels, which is suitable for all applications requiring corrosion resistance in an environment where low pH condensed water containing nitrate ions and sulfate ions are generated, and corrosion resistance in an aqueous solution containing chloride ions. Application is possible. Specifically, the austenitic stainless steel of the present invention is particularly suitable for use in materials for heat exchangers, particularly for secondary heat exchanger materials in latent heat recovery type hot water heaters using kerosene or LNG as fuel. In this case, the austenitic stainless steel of the present invention can be applied not only to a heat exchanger pipe but also to any material such as a case or a partition plate. In addition, the austenitic stainless steel of the present invention is similarly suitable for use as a heat recovery component from exhaust gas such as EGR mounted on a vehicle in an automobile having a gasoline and a diesel engine.

In addition, the austenitic stainless steel of the present invention is particularly suitable for use in an environment in which wet and dry repetition is exposed to a low pH solution containing nitrate ions and sulfate ions. Specifically, it is an outdoor exterior material, a building material, a roofing material, outdoor equipment, etc. in which an acid rain environment is assumed. In addition, the austenitic stainless steel sheet of the present invention is applied to equipments used in general water supply equipment that is concerned about stress corrosion cracking, specifically, water storage tanks, home appliances, bathtubs, kitchen appliances, and other outdoor and indoor equipment. Suitable for use As described above, the present invention has high industrial value.

Claims (2)

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 0.150%, and Mo: 0.1% to 1.0%, and the remainder is made of Fe and unavoidable impurities, and further formulas (A) and ( B) Austenitic stainless steel excellent in corrosion resistance and brazing, characterized by satisfying the formula.
(A) Formula: 1.6≤ [Cu] × [Si] ≤4.4
(B) Formula: 0.16≤2 [N] + [Mo] ≤1.0
Here, [Cu], [Si], [N], and [Mo] are taken as the content of each element expressed in mass%. The mass% according to claim 1, which further contains one or two or more of 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%. Austenitic stainless steel having excellent corrosion resistance and brazing resistance.

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