US20130336834A1

US20130336834A1 - Austenitic stainless steel excellent in corrosion resistance and brazeability

Info

Publication number: US20130336834A1
Application number: US14/002,932
Authority: US
Inventors: Tooru matsuhashi; Jun Tokunaga
Original assignee: Nippon Steel and Sumikin Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2011-03-29
Filing date: 2012-03-28
Publication date: 2013-12-19
Also published as: JP6016331B2; AU2012233539A1; CA2829874C; JP2012207259A; CN103380224B; TWI471427B; KR20130123443A; AU2012233539B2; CN103380224A; NZ614829A; WO2012133574A1; TW201245468A; CA2829874A1

Abstract

The present invention provides austenitic stainless steel which is not only excellent in brazeability, but is also excellent in corrosion resistance in an environment where condensation of combustion exhaust gas causes formation of condensed water which contains nitric acid ions or sulfuric acid ions and which is low in pH or in an environment of an aqueous solution which contains chloride ions, which contains, 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% 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%, has a balance of Fe and unavoidable impurities, and satisfies Formula (A): 1.6≦[Cu]×[Si≦]4.4 and Formula (B): 0.16≦2[N]+[Mo≦]1.0.

Description

TECHNICAL FIELD

The present invention relates to austenitic stainless steel which is used for a structure which is joined by a nickel braze filler metal, copper braze filler metal, or other braze filler metal. In particular, the present invention relates to austenitic stainless steel which is not only excellent in brazeability, but is also excellent in corrosion resistance in an environment in which condensation of combustion exhaust gas causes formation of condensed water which contains nitric acid ions and sulfuric acid ions and which is low in pH and in corrosion resistance in an environment of an aqueous solution which contains chloride ions.

BACKGROUND ART

Brazing is the art of joining materials by using a braze filler metal with a lower melting point than a structural material and performing heat treatment at a temperature somewhat higher than the melting point of the braze filler metal. Brazing is a joining method which is widely used even for stainless steel. The braze filler metal which is used for brazing stainless steel is an alloy of nickel or copper.
In brazing of stainless steel, the passivation film of the stainless steel impedes the brazeability. Therefore, brazing is performed in a vacuum or a hydrogen environment so as to remove the passivation film by reduction. The temperature of brazing is, for example, about 1100° C. when using the above nickel braze filler metal.
In brazing, it is important that the braze filler metal fully fill the clearances between joined materials and that the strength of the joint be secured. Therefore, the wettability of the braze filler metal to the joined material of stainless steel becomes important. On the other hand, if the braze filler metal is too good in wettability, the braze filler metal flows out from the clearances between joined materials, the clearances cannot be filled by the braze filler metal, and the joint strength is lowered. Therefore, as stainless steel which is excellent in brazeability, having suitable wettability becomes important.
As the stainless steel which is brazed, austenitic stainless steel is generally used. Further, as the austenitic stainless steel, a SUS304 material and a SUS316 material of JIS (Japan Industrial Standard) (below, referred to as a “SUS304 material” and a “SUS316 material”) are being broadly used. The SUS304 material and SUS316 material have not only workability, but also the characteristic of being excellent in corrosion resistance in a general environment. However, a SUS316-based material and SUS316-based material have the problem of being inferior in stress corrosion cracking resistance.
Stress corrosion cracking occurs when tensile stress remains in a material which is exposed to an environment where corrosion occurs and which is highly susceptible to stress corrosion cracking. When brazing austenitic stainless steel, even if tensile stress remains in the joined material at the stage before brazing, there is no concern over stress corrosion cracking. This is because austenitic stainless steel is brazed at a temperature where austenitic stainless steel is annealed and because the residual stress is removed during brazing. This is because, for example, when using a nickel braze filler metal, as explained above, the brazing is performed at about 1100° C.
However, depending on the part, sometimes parts are assembled by welding or screwing with other parts after brazing. In this case, tensile stress occurs at the parts after assembly and stress corrosion cracking is liable to be caused. For this reason, austenitic stainless steel which is brazed has to have stress corrosion resistance.
As the environments in which the braze filler metal of austenitic stainless steel is used, for example, there are exhaust system parts of automobiles and secondary heat exchangers of hot water heaters which are equipped with latent heat recovery devices. These materials are all used in an environment in which condensation of combustion exhaust gas forms condensed water which contains nitric acid ions and sulfuric acid ions and which is low in pH. This is because the air which is taken in for combustion contains a large amount of nitrogen and the fuel or the scented substances which are added to fuel contain sulfur compounds. Under such an environment, copper is corroded. Therefore, as materials which form the exhaust system parts of automobiles and secondary heat exchangers of hot water heaters which are equipped with latent heat recovery devices, copper cannot be used. Austenitic stainless steel becomes essential.
Accordingly, it is important that the austenitic stainless steel which is used for such a member achieve both corrosion resistance and brazeability even in an environment where condensed water which contains nitric acid ions and sulfuric acid ions and which is low in pH is formed.
Regarding the brazeability of stainless steel, PLT 1 proposes a braze filler metal-precoated metal sheet material which is obtained by spray coating a nickel-based braze filler metal which is suspended together with an organic binder on the surface of a sheet of stainless steel, then heating it. Further, PLT 2 proposes a method of production of nickel braze filler metal-coated stainless steel sheet which is excellent in self brazeability obtained by coating, by plasma spraying, a stainless steel sheet adjusted in surface roughness with a nickel-based braze filler metal. However, both PLTs 1 and 2 only study the conventional SUS304 materials and SUS316 materials as austenitic stainless steel materials to be coated with a braze filler metal.
PLT 3 proposes stainless steel which is reduced in Al and Ti and which is excellent in brazeability. Further, PLT 4 proposes stainless steel which has been adjusted to an M value, which is shown by M=−0.22T+34.5Ni+10.5Mn+13.5Cu−17.3Cr−17.3Si−18Mo+475.5, of 1 to 25. However, both PLT's 3 and 4 are studies of ferritic stainless steel. Austenitic stainless steel is not studied.
PLT 5 proposes an austenitic stainless steel material which has stress corrosion cracking resistance and crevice corrosion resistance. However, the steel sheet which is proposed in PLT 5 is applied for use in fuel system members of automobiles. The stress corrosion cracking resistance was studied but the brazeability is not described.
Further, when used for exhaust system parts of automobiles or secondary heat exchangers of hot water heaters which are equipped with latent heat recovery devices, since the atmosphere which is taken in includes chlorides, in particular when used in high salt damage regions near the coast, the corrosion resistance in environments which contain chloride ions also becomes an issue.

CITATIONS LIST

Patent Literature

PLT 1: Japanese Patent Publication No. 1-249294A
PLT 2: Japanese Patent Publication No. 2001-26855A
PLT 3: Japanese Patent Publication No. 2009-174046A
PLT 4: Japanese Patent Publication No. 2010-65278A
PLT 5: Japanese Patent Publication No. 2007-9314A

SUMMARY OF INVENTION

Technical Problem

The present invention has as its object the provision of austenitic stainless steel which is not only excellent in brazeability, but is also excellent in corrosion resistance in an environment where condensation of combustion exhaust gas causes formation of condensed water which contains nitric acid ions or sulfuric acid ions and which is low in pH or corrosion resistance in an environment of an aqueous solution which contains chloride ions.

Solution to Problem

The inventors engaged in intensive studies to obtain austenitic stainless steel which achieves both brazeability and corrosion resistance and as a result discovered the following:
(a) In the case of austenitic stainless steel, if Si and Cu are added in certain amounts or more, the wettability becomes excessively good and braze filler metal ends up flowing out from the clearances in the joined materials, so the joint becomes insufficient. To prevent this, it is important to prescribe not only the upper limits of the contents of Cu and Si but also the upper limit of the value of [Cu]×[Si]. Note that, in the following explanation, [Cu] and [Si] are made the contents of Cu and Si expressed by mass %.
(b) Brazed austenitic stainless steel suppresses stress corrosion cracking due to the synergistic effect of Cu and Si which are expressed by the value of [Cu]×[Si].
(c) The corrosion resistance in an environment where condensation of combustion exhaust gas causes the formation of condensed water which contains nitric acid ions and sulfuric acid ions and which is low in pH and further the corrosion resistance in an environment of an aqueous solution which contains chloride ions are improved by making the value of 2[N]+[Mo] a certain value or more. Note that, in the following explanation, [N] and [Mo] are the contents of N and Mo expressed by mass %.
The present invention was made based on the above findings and has as its gist the following:
(1) Austenitic stainless steel which is excellent in corrosion resistance and brazeability characterized by containing, 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% 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%, having a balance of Fe and unavoidable impurities, and satisfying the following Formula (A) and Formula (B):
1.6≦[Cu]×[Si≦]4.4 Formula (A)
0.16≦2[N]+[Mo≦]1.0 Formula (B)
where, [Cu], [Si], [N], and [Mo] are contents of elements expressed by mass %.
(2) Austenitic stainless steel which is excellent in corrosion resistance and brazeability as set forth in (1) characterized by further containing, by mass %, one 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%.

Advantageous Effects of Invention

According to the present invention, by establishing a suitable content of Cu and content of Si in austenitic stainless steel and controlling the content of N and content of Mo, it is possible to provide austenitic stainless steel which is excellent in corrosion resistance and brazeability.
Further, according to the present invention, it is possible to improve the corrosion resistance of heat recovery devices of combustion exhaust gas which is fueled by gasoline, LNG, LPG, oil, and other hydrocarbons and other heat exchangers and other structures which are obtained by brazing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view which shows the relationship between the contents of Cu and Si and the brazeability and corrosion resistance.

FIG. 2 is a view which shows the relationship between the value of 2[N]+[Mo] and the maximum depth of corrosion.

FIG. 3 is a view which shows the relationship between the value of 2[N]+[Mo] and value of [Cu]×[Si] and the brazeability and corrosion resistance.

DESCRIPTION OF EMBODIMENTS

The present invention will be explained in detail. In the following explanation, the % relating to the chemical composition means the mass % unless otherwise indicated.
First, tests run for obtaining a chemical composition realizing both brazeability and corrosion resistance and their results will be explained. Austenitic stainless steel changed in Si, Cu, Mo, and N were produced by vacuum melting. The other elements at this time were made ones in the range of chemical compositions of JIS SUS316.
Each of these austenitic stainless steels was hot rolled and heat treated at 1150° C.×1 minute, then was descaled by grinding off the scale and further was cold rolled to produce cold rolled sheet. This cold rolled sheet was heat treated under conditions of 1050 to 1150° C.×1 minute based on the recrystallization behavior, then was dipped and pickled in a nitric hydrofluoric acid aqueous solution until the scale was completely removed to thereby obtain a material for brazing. This material for brazing use was used to evaluate the brazeability and stress corrosion cracking.
(Evaluation of Brazeability)
The material for brazing was cut to 40×50 mm and 25×30 mm for use as a test material for evaluation of brazeability. The thickness of the test material for evaluation of the brazeability was 1 mm. The thus prepared test material was brazed by using a silver braze filler metal. The brazing was performed by superposing two test materials, inserting at the superposed portions 0.3 g of a braze filler metal comprised of a JIS BNi5 nickel braze filler metal in which an organic binder was mixed, and brazing. The brazing was performed using a hydrogen reduction furnace and in a 1100° C., hydrogen 100% atmosphere. The brazeability was evaluated by cutting the brazed test material and observing the cross-section visually.
The results of evaluation are shown in FIG. 1. At the cross-section of the brazed test material, the cases where the braze filler metal material is completely filled in the clearances are shown by the white circles or black circles, while the cases where clearances remain are shown by the x marks. Here, the white circles and the black circles show the results of evaluation of the stress corrosion cracking explained later differentiated. The good cases where no stress corrosion cracking occurs are shown by the white circles, while the defective cases where stress corrosion cracking occurs are shown by the black circles. Further, among the two curves which are shown in FIG. 1, the lower curve shows the case where the value of [Cu]×[Si] is 1.6, while the upper curve shows the case where the value of [Cu]×[Si] is 4.4. Note that, in FIG. 1, “SCC” means stress corrosion cracking. The same is true for FIG. 3.
As clear from FIG. 1, if Si is over 3.0%, Cu is over 3.0%, or the value of [Cu]×[Si] is over 4.4, clearances are formed at the cross-section of the brazed test materials. In the case of austenitic stainless steel, the addition of Si and Cu causes the wettability of the braze filler metal to become better. However, if Si and Cu are added in certain amounts or more, the wettability becomes excessively good and braze filler metal ends up flowing out from the clearances between the joint materials, so the joint becomes insufficient. Therefore, the upper limit of the value of [Cu]×[Si] is made 4.4. The more preferable upper limit is 4.0.
(Evaluation of Stress Corrosion Cracking)
Materials for brazing use were heated under the same conditions as when brazing but without brazing, that is, using a hydrogen reduction furnace, in a 1100° C., hydrogen 100% atmosphere. After this heating, the material for brazing use was cut to 30×30 mm and 15×15 mm sizes and polished at the cross-sectional end faces. Two materials of different sizes were superposed and spot welded at the center to impart clearance to the two materials and obtain a test material for evaluation of stress corrosion cracking. The test material for evaluation of stress corrosion cracking was dipped in an aqueous solution containing 200 ppm of Cl⁻ and held there at 100° C. for seven days. After the elapse of seven days, the spot welded part was drilled through to separate the materials and the presence of any cracks at the inside clearance surfaces was evaluated. The presence of any cracks was checked for by the dye penetration test detection test (color check test).
The results of evaluation are shown together at FIG. 1. Case where stress corrosion cracking did not occur are shown by white circles, while cases where stress corrosion cracking occurred are shown by black circles. In FIG. 1, if investigating the test material where no stress corrosion cracking occurs, the value of [Cu]×[Si] was 1.6 or more. On the other hand, test materials with values of [Cu]×[Si] of less than 1.6 suffered from stress corrosion cracking. In general, there is the finding that for improvement of the stress corrosion cracking resistance of austenitic stainless steel, addition of Si and Cu is effective. In the present invention, it is clear that in brazed austenitic stainless steel as well, the effect of suppression of stress corrosion cracking is obtained by the synergistic effects of Cu and Si as expressed by the value of [Cu]×[Si]. Therefore, the lower limit of the value of [Cu]×[Si] is made 1.6. It is more preferably made 2.0.
Next, the method of evaluation of the corrosion resistance against the condensed water which is formed from the combustion exhaust gas and the results will be explained. As explained above, the brazed structures are used as exhaust system parts of an automobile or secondary heat exchangers of hot water heaters which are equipped with latent heat recovery devices etc. Therefore, it is not enough that the austenitic stainless steel forming a brazed structure be excellent in brazeability and stress corrosion cracking resistance.
(Evaluation of Corrosion Resistance Against Condensed Water Formed by Combustion Exhaust Gas)
The material which is used as the test material is austenitic stainless steel which is excellent in brazeability and stress corrosion resistance, that is, has a value of [Cu]×[Si] of 1.6 to 4.4 in range. The test solution was made one imitating the composition of condensed water which is formed by the combustion of general LNG or petroleum. Specifically, the test solution was made a composition which is adjusted to nitric acid ions: 100 ppm, sulfuric acid ions: 10 ppm, and pH2.5 and which has chloride ions added to in an amount of Cl⁻ of 100 ppm to accelerate the corrosion.
A material of austenitic stainless steel with a value of the above [Cu]×[Si] of 1.6 to 4.4 in range was heated under conditions the same as when brazing but without brazing, that is, using a hydrogen reduction furnace in a 100% hydrogen atmosphere. The heated material was cut to a size of 15×100 mm for use as a test material for evaluation of the corrosion resistance. This test material for evaluation of the corrosion resistance was immersed exactly half in the test solution in a test tube. Note that, the test solution in the test tube was made 10 ml. Further, this test tube was immersed in 80° C. warm water and held there for several hours until completely drying at the inside. After drying, another test tube was newly filled with test solution and the sample held there until completely drying. The repeated drying and wetting test was performed for 14 cycles. The corrosion resistance against condensed water formed by the combustion exhaust gas was evaluated by measuring the maximum depth of corrosion at the cross-section of the test material for evaluation of the corrosion resistance after the test.
The cases where the maximum depth of corrosion is less than 100 μm are shown by the white circles, while the cases where the depth is 100 μm or more are shown by the x marks. As clear from FIG. 2, when the value of 2[N]+[Mo] is 0.16 or more, the maximum depth of corrosion becomes less than 100 μm. This is believed to be because even in a low pH solution which contains Cl⁻, an effect of improvement of the pitting corrosion resistance by Mo and N is obtained.
Note that, in FIG. 2, even when the value of 2[N]+[Mo] is 0.16 or more, sometimes the maximum pitting depth is 100 μm or more. If investigating the test materials in this case, the content of Cu was outside the range explained later. This is because Cu dissolves and forms ions at the time of corrosion in a repeated wet and dry corrosive environment which contains an oxidizing agent such as nitric acid ions. Further, it was believed that in such an environment, Cu ions act as an oxidizing agent inside and outside pitting, so the depth of corrosion increases.
Further, along with an increase in the value of 2[N]+[Mo], the maximum depth of corrosion is reduced, but the drop in depth of corrosion becomes saturated at a certain value or more. This believed to be because if the contents of N and Mo exceed certain values, the effects of elements other than N and Mo on the depth of corrosion can no longer be ignored. In particular, if Cu is present, the Cu ions promote the corrosion. For the above such reason, the upper limit of 2[N]+[Mo] is made 1.0 or less. The preferable upper limit is 0.77, while the more preferable upper limit is 0.74. Note that, the lower limit of 2[N]+[Mo], as explained above, is 0.16, while the preferable lower limit is 0.20.
Summarizing the results which are shown in the above-mentioned FIG. 1 and FIG. 2 by the relationship of the value of [Cu]×[Si] and the value of 2[N]+[Mo], the relationship which is shown in FIG. 3 is obtained. As clear from FIG. 3, a test material with a value of [Cu]×[Si] of 1.6 to 4.4 in range and a value of 2[N]+[Mo] of 0.16 to 1.0 in range achieves both brazeability and corrosion resistance. Note that, in the present invention, the “corrosion resistance” means the stress corrosion cracking resistance, the corrosion resistance in an environment where condensation of combustion exhaust gas causes the formation of condensed water which contains nitric acid ions and sulfuric acid ions and which is low in pH, and the corrosion resistance in an environment of an aqueous solution which contains chloride ions.
Therefore, the austenitic stainless steel of the present invention has to satisfy the following formula (A) and formula (B) for Cu, Si, Mo, and N.
1.6≦[Cu]×[Si≦]4.4 Formula (A)
0.16≦2[N]+[Mo≦]1.0 Formula (B)
Next, the reasons for limitation of the elements which are contained in the austenitic stainless steel of the present invention alone will be explained.
C causes the intergranular corrosion resistance and the workability to fall, so the content has to be reduced, therefore the upper limit has to be made 0.080%. However, excessive reduction of the C content causes the refining costs to deteriorate. Therefore, the preferable C content is 0.005 to 0.060% in range.
Si, as explained above, like Cu, is added to improve the wettability and prevent stress corrosion cracking. If the Si content is less than 1.2%, these effects are not manifested. On the other hand, if the Si content exceeds 3.0%, the wettability is excessively increased and the brazeability falls. Therefore, the Si content has to be 1.2 to 3.0% in range. Preferably, it is 1.4 to 2.5% in range.
Mn is an element which is important as a deoxidizing element, but if added in excess, it easily forms MnS which acts as starting points of corrosion. Therefore, the content of Mn has to be 0.4 to 2.0% in range. More preferably, it is 0.5 to 1.2% in range.
P not only causes the weldability and workability to drop, but also facilitates intergranular corrosion, so must be kept as low as possible. For this reason, the upper limit of the content of P has to be 0.03%. The preferable content of P is 0.001 to 0.025% in range.
S causes the formation of the above-mentioned MnS and other aqueous inclusions which form starting points of corrosion, so have to be reduced as much as possible. For this reason, the S content is made 0.003% or less. However, excessive reduction of S is costly, so the S content is preferably 0.0002 to 0.002% in range.
Ni has no effect on stress corrosion cracking resistance in the amount prescribed in JIS SUS316L. However, in an environment in which the steel is exposed to the exhaust gas when LNG or oil burns, there is a concern that the stress corrosion cracking resistance will fall. Further, it is necessary to maintain the austenite phase and also secure the workability. Therefore, the Ni content has to be 6.0 to 12.0% in range. Preferably, it is 6.5 to 11.0% in range.
Cr is the most important element in securing the corrosion resistance of stainless steel. Therefore, the lower limit of Cr content is made 16.0%. However, if increasing the Cr, the corrosion resistance is also improved, but the workability and other manufacturing properties are lowered, so the upper limit of the content of Cr is made 20.0%. The preferable content of Cr is 16.5 to 19.0% in range.
Cu, along with Si, causes a drop in brazeability due to its addition, but acts to suppress stress corrosion cracking. On the other hand, excessive addition of Cu causes a drop in the corrosion resistance in a solution which contains nitric acid ions. Therefore, the content of Cu has to be 0.2 to 3.0% in range. Preferably, it is 0.5 to 2.5% in range.
Al is important as a deoxidizing element. Further, it controls the composition of the nonmetallic inclusions and refines the structure. However, if excessively added, it invites coarsening of the nonmetallic inclusions and is liable to form starting points for the formation of defects in the product. Therefore, the content of Al has to be 0.002 to 0.10% in range. Preferably it is 0.005 to 0.08% in range.
N improves the pitting corrosion resistance, but excessive addition, like C, causes a drop in the intergranular corrosion resistance and workability. Therefore, the content of N has to be 0.030 to 0.150% in range. Preferably, it is 0.037 to 0.10% in range.
Mo has an effect in repairing the passivation film and is an element which is extremely effective for improving the corrosion resistance. Furthermore, in an environment which contains nitric acid ions and chloride ions, in combination with N, it has an effect in improving the pitting corrosion resistance. Therefore, Mo has to be included in at least an amount of 0.1%. On the other hand, if increasing Mo, the corrosion resistance is improved, but excessive addition causes a drop in workability and invites a rise in costs. Accordingly, the upper limit of the content of Mo has to be made 1.0%. The preferable content of Mo is 0.2 to 0.8% in range.
In the present invention, in addition to the essential elements which have been explained up to here, in accordance with need, it is possible to include one or more elements from Nb, Ti, V, and B.
Nb, by addition, forms carbonitrides and suppresses the sensitization near the weld zone so has the effect of increasing the high temperature strength, therefore can be added in accordance with need. However, excessive addition invites a rise in cost. Therefore, the content of Nb is preferably 0.1 to 0.7% in range.
Ti has effects similar to Nb, but excessive addition invites an increase in surface defects due to nitrides of titanium. Therefore, the content of Ti is preferably made 0.1 to 0.5% in range.
V improves the corrosion resistance and crevice corrosion resistance, so if keeping down the use of Cr and Mo and adding V, excellent workability can be secured. Therefore, V can be added in accordance with need. However, excessive addition invites a drop in workability. Accordingly, the content of V is preferably 0.1 to 3.0% in range.
B is a grain boundary strengthening element which is effective for improvement of the hot workability, so can be added in accordance with need. However, excessive addition becomes a cause of a drop in workability. Therefore, the lower limit of the content of B is preferably 0.0002% and the upper limit is preferably 0.003%.

EXAMPLES

Next, the present invention will be further explained by examples, but the conditions in the examples are just illustrations which have been employed for confirming the workability and effects of the present invention. The present invention is not limited to these illustrations. The present invention can employ various conditions so long as not deviating from the gist of the present invention and achieving the object of the present invention.
Steel of each of the chemical compositions which are shown in Table 1 was produced by the method of production of usual austenitic stainless steel. First, the steel was vacuum smelted, then cast into a 40 mm thick ingot. This was hot rolled to a 4.0 mm thickness. After this, the steel was heat treated at 1150° C.×1 minute, then was descaled by grinding off the scale and further was cold rolled to produce a 1.0 mm thick steel sheet. This was heat treated under conditions of 1050 to 1150° C.×1 minute based on various recrystallization behaviors, then was dipped and pickled in a nitric hydrofluoric acid aqueous solution until the scale was completely removed. This was used for the following three tests.

TABLE 1

(mass %)

No.	C	Si	Mn	P	S	Ni	Cr	Mo	Cu	Nb	Ti

1	0.064	1.3	0.7	0.028	0.0007	7.9	18.0	0.10	2.9	—	—
2	0.055	1.6	1.1	0.028	0.0007	6.4	17.1	0.15	2.1	—	—
3	0.033	2.9	1.3	0.020	0.0010	11.2	19.3	0.36	0.6	—	—
4	0.040	1.8	0.6	0.022	0.0006	10.2	17.9	0.60	1.8	—	—
5	0.066	2.6	0.4	0.019	0.0002	8.0	19.5	0.31	0.7	—	—
6	0.039	1.5	0.6	0.020	0.0003	9.5	17.9	0.52	1.5	—	—
7	0.044	1.9	0.7	0.022	0.0006	9.1	18.0	0.89	2.3	—	—
8	0.035	2.0	0.5	0.028	0.0007	7.5	18.9	0.50	0.9	0.54	—
9	0.051	2.7	0.5	0.020	0.0005	7.0	16.9	0.62	1.5	—	0.25
10	0.047	1.3	0.4	0.027	0.0005	7.4	18.0	0.30	2.2	—	—
11	0.061	2.2	0.9	0.024	0.0009	8.8	19.5	0.35	1.4	—	—
12	0.015	1.3	0.6	0.028	0.0003	7.0	16.4	0.13	1.4	—	—
13	0.030	1.6	0.3	0.028	0.0005	6.9	17.1	0.77	2.6	—	—
14	0.036	1.5	0.6	0.002	0.0007	10.5	18.4	0.19	3.5	—	—
15	0.040	1.9	0.5	0.020	0.0010	7.9	17.6	0.28	3.2	0.50	—
16	0.054	2.1	1.5	0.018	0.0007	5.5	17.1	0.41	4.2	—	0.31
17	0.020	3.0	0.4	0.020	0.0010	11.0	16.1	0.70	3.1	—	—
18	0.027	2.2	0.5	0.022	0.0006	10.2	17.9	0.63	3.3	—	—
19	0.025	1.4	0.9	0.002	0.0014	7.2	16.5	0.40	0.8	—	—
20	0.025	1.8	0.9	0.002	0.0014	7.2	16.2	0.12	1.5	—	—
21	0.015	3.2	0.3	0.023	0.0009	6.3	17.0	0.00	1.5	—	—
22	0.051	1.4	1.5	0.022	0.0030	7.8	15.1	0.90	2.8	—	—
23	0.015	0.8	0.6	0.023	0.0014	6.8	16.6	0.10	1.5	—	—
24	0.045	0.6	0.8	0.020	0.0012	8.1	18.1	0.05	0.1	—	—
25	0.010	0.2	0.2	0.020	0.0006	12.1	17.3	2.10	0.2	—	—

									Max.
								Stress	corrosion
					2 [N] +	[Cu] ×	Braze-	corrosion	depth
No.	Al	V	B	N	[Mo]	[Si]	ability	cracking	(μm)	Remarks

1	0.048	—	—	0.041	0.18	3.77	A	A	99	Inv. ex.
2	0.025	—	—	0.075	0.30	3.26	A	A	85	Inv. ex.
3	0.020	—	—	0.051	0.46	1.68	A	A	55	Inv. ex.
4	0.045	—	—	0.065	0.73	3.24	A	A	49	Inv. ex.
5	0.034	—	—	0.101	0.51	1.81	A	A	35	Inv. ex.
6	0.036	—	—	0.051	0.62	2.26	A	A	58	Inv. ex.
7	0.013	—	—	0.038	0.97	4.30	A	A	51	Inv. ex.
8	0.018	—	—	0.049	0.60	1.82	A	A	39	Inv. ex.
9	0.032	—	—	0.075	0.77	4.05	A	A	40	Inv. ex.
10	0.038	0.41	—	0.075	0.45	2.86	A	A	78	Inv. ex.
11	0.055	—	0.0006	0.035	0.42	3.08	A	A	40	Inv. ex.
12	0.042	0.18	0.0003	0.039	0.21	1.75	A	A	88	Inv. ex.
13	0.035	0.15	0.0004	0.041	0.85	4.16	A	A	49	Inv. ex.
14	0.040	—	—	0.063	0.32	5.25	B	A	187	Comp. ex.
15	0.061	—	—	0.030	0.34	6.08	B	A	174	Comp. ex.
16	0.015	—	—	0.048	0.51	8.82	B	A	199	Comp. ex.
17	0.005	0.21	—	0.030	0.76	9.30	B	A	120	Comp. ex.
18	0.100	—	0.0003	0.038	0.71	5.72	B	A	104	Comp. ex.
19	0.003	—	—	0.010	0.42	1.12	A	B	94	Comp. ex.
20	0.003	—	—	0.010	0.14	2.70	A	B	121	Comp. ex.
21	0.054	—	—	0.041	0.08	4.80	B	A	168	Comp. ex.
22	0.005	—	—	0.040	0.98	3.92	A	A	110	Comp. ex.
23	0.003	—	—	0.005	0.11	1.20	A	B	189	Comp. ex.
24	0.003	—	—	0.034	0.12	0.06	A	B	151	Comp. ex.
25	0.002	—	—	0.011	2.12	0.03	A	B	30	Comp. ex.

Note)
Underlines indicate outside range of present invention. “—” mean not contained.

(Brazeability Test)
Various types of stainless steel with a thickness of 1 mm were cut into 40×50 mm and 25×30 mm pieces which were wet polished over their entire surfaces using No. #600 waterproof emery paper (waterproof polishing paper) for use as test materials. These were subjected to a brazeability test using silver braze filler metal.
The brazing was performed by superposing two test materials by the same method as explained above. Specifically, the superposed parts of the test materials were filled with 0.3 g of silver braze filler metal of JIS BNi5 mixed with an organic binder and then brazed. The brazing was performed using a hydrogen reduction furnace in a 1100° C., hydrogen 100% atmosphere. In the method of evaluation, the cases where visual observation showed that the clearances in the cross-sections of the brazed test materials were completely filled were judged as good, while the cases where clearances remained were judged as poor.
(Corrosion Resistance Test)
Next, the method of a repeated drying and wetting test which is performed in a test solution imitating the condensed water which is formed by combustion of LNG or oil will be explained. For the test material, various types of stainless steel were heated under the same conditions as when brazing but without brazing, that is, using a hydrogen reduction furnace in a 1100° C., hydrogen 100% atmosphere. After this, the steel was cut into a 15×100 mm size and tested. Note that, the thickness of the test material was 1 mm. The composition of the test solution, as explained above, imitated the composition of the condensed water which is formed by general LNG or oil. It was adjusted to nitric acid ions: 100 ppm and sulfuric acid ions: 10 ppm and pH2.5, imitated the concentration of the salt content, and was given 100 ppm of chloride ions. 10 ml of this test solution was placed in a test tube. The test material was immersed half into this and the test tube was placed in a 80° C. warm bath. The test tube was held until the test solution completely dried. After drying, the sample was transferred to a new test tube filled with the test solution and again dried. This drying was performed 14 times, then the maximum depth of corrosion after the test was measured.
(Stress Corrosion Cracking Evaluation Test)
The stress corrosion cracking evaluation test was performed by heating a material the same as that used for the brazeability test under the same conditions as when brazing but without brazing, that is, using a hydrogen reduction furnace, in a 1100° C., hydrogen 100% atmosphere. This material was cut into 30×30 mm and 15×15 mm sizes and was wet polished over its entire surface, then two sheets were superposed and spot welded to impart clearances. The test material given clearances in this way was immersed in distilled water containing 200 ppm of Cl⁻ and was treated continuously at 100° C. for seven days. The spot weld of the treated test material was removed by a drill and the material separated, then the presence of any cracks was checked for by the dye penetration test detection test (color check test). Here, cases where no cracks occurred were judged as “good” while cases where cracks occurred were judged as “poor”.
These test results are listed together in Table 1. Note that, in the results of the brazeability test and the results of the stress corrosion cracking evaluation test, good is indicated as “A” and poor as “B”.
As clear from Table 1, it was confirmed that the invention examples of Nos. 1 to 13 were excellent in all of the brazeability test, the maximum depth of corrosion in the corrosion resistance test (repeated drying and wetting test), and the test for evaluation of the stress corrosion cracking.
As opposed to this, it was confirmed that Nos. 14 to 18, and 21 with values of [Cu]×[Si] of over 4.4 did not give sufficient brazeability. Further, it was confirmed that Nos. 19, 23, 24, and 25 with values of [Cu]×[Si] of less than 1.6 were excellent in brazeability, but cracked in the stress corrosion cracking evaluation test. Furthermore, Nos. 20, 21, 23, and 24 with values of 2[N]+[Mo] below the lower limit of the present invention had maximum depths of pitting of 100 μm or more in the corrosion resistance test (repeated drying and wetting test). No. 22 has a value of [Cu]×[Si] and a value of 2[N]+[Mo] in the range of the present invention, but has Cr below the lower limit of the range of the present invention, so had a maximum depth of corrosion of over 100 μm in the corrosion resistance test (repeated drying and wetting test). Note that, in Nos. 14 to 18, even if the values of 2[N]+[Mo] were in the range of the present invention, the maximum depths of corrosion exceeded 100 μm in the corrosion resistance test (repeated drying and wetting test) because the Cu was outside the present invention in range, so it was judged that the effect of acceleration of corrosion due to the eluted Cu ions was in action.
From the above, it could be confirmed that the austenitic stainless steel of the present invention is excellent in brazeability and did not suffer from stress corrosion cracking even in an environment inside of a heat exchanger which is exposed to combustion gas of a hydrocarbon fuel. Further, simultaneously with this, it was confirmed that the austenitic stainless steel of the present invention is excellent in corrosion resistance in an environment in which condensed water which contains nitric acid ions and sulfuric acid ions and is low in pH is formed and in an environment of an aqueous solution which contains chloride ions.

INDUSTRIAL APPLICABILITY

The present invention can be applied in structures obtained by brazing austenitic stainless steel in all applications requiring corrosion resistance in an environment where condensed water which contains nitric acid ions and sulfuric acid ions and which is low in pH and corrosion resistance in an aqueous solution which contains chloride ions. Specifically, the austenitic stainless steel of the present invention is particularly suitable when used as a material for heat exchanger use, in particular a material for use for a secondary heat exchanger of a latent heat type water heater fueled by kerosene or LNG. In this case, the austenitic stainless steel of the present invention may be applied to not only heat exchanger pipes, but also cases, partition plates, and other materials. Further, the austenitic stainless steel of the present invention is similarly suitable even if used as a part for recovery of heat from exhaust gas such as EGR which is installed in an automobile which has a gasoline or diesel engine.
In addition, the austenitic stainless steel of the present invention is particularly suitable when used in an environment of repeated drying and wetting in which the steel is exposed to a solution which contains nitric acid ions and sulfuric acid ions and which is low in pH. Specifically, this includes outdoor panels, building materials, roofing materials, outdoor equipment, etc. which are envisioned as being exposed to an acid rain environment. Further, the austenitic stainless steel of the present invention is suitable when used as equipment which is generally used around water and thereby stress corrosion cracking is feared, specifically a cold water or hot water storage tank, household electric appliance, bath tub, kitchen equipment, and other outdoor and indoor equipment. In this way, the present invention has a high value of utilization in industry.

Claims

1-3. (canceled)

4. Austenitic stainless steel which is excellent in corrosion resistance and brazeability characterized by containing, 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% 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%, having a balance of Fe and unavoidable impurities, and satisfying the following Formula (A) and Formula (B):

1.6≦[Cu]×[Si<]4.4 Formula (A)

0.16≦2[N]+[Mo≦]1.0 Formula (B)

where, [Cu], [Si], [N], and [Mo] are contents of elements expressed by mass %.

5. Austenitic stainless steel which is excellent in corrosion resistance and brazeability characterized by containing, 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% 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%, having a balance of Fe and unavoidable impurities, and satisfying the following Formula (A) and Formula (B):

1.6≦[Cu]×[Si≦]4.4 Formula (A)

0.16≦2[N]+[Mo≦]1.0 Formula (B)

where, [Cu], [Si], [N], and [Mo] are contents of elements expressed by mass %.

6. Austenitic stainless steel which is excellent in corrosion resistance and brazeability as set forth in claim 4 characterized by further containing, by mass %, one 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%.

7. Austenitic stainless steel which is excellent in corrosion resistance and brazeability as set forth in claim 5 characterized by further containing, by mass %, one 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%.