Aluminum alloy brazing sheet having high strength, high corrosion resistance and high material elongation, and method of manufacturing heat exchanger

Abstract

An aluminum alloy brazing sheet has high strength, corrosion resistance and elongation, and includes an aluminum alloy clad material. The material includes a core material, one surface of which is clad with a sacrificial material and an other surface of which is clad with an AlSi-based or AlSiZn-based brazing filler metal. The core material has a composition containing 1.3 to 2.0% Mn, 0.6 to 1.3% Si, 0.1 to 0.5% Fe and 0.7 to 1.3% Cu, by mass, with the balance Al and impurities. The sacrificial material has a composition containing more than 4.0% to 8.0% Zn, 0.7 to 2.0% Mn, 0.3 to 1.0% Si, 0.3 to 1.0% Fe and 0.05 to 0.3% Ti, by mass, with the balance Al and impurities. At least the core material has a lamellar crystal grain structure. Elongation of material is at least 4% and a tensile strength after brazing is at least 170 MPa.

Claims

1. An aluminum alloy brazing sheet configured to be subjected to brazing, the brazing sheet comprising an aluminum alloy clad material comprising a core material having a first surface and a second surface, the first surface of the core material being clad with a sacrificial material, and the second surface of the core material being clad with a brazing filler metal which is AlSi-based or AlSiZn-based, wherein: the core material has a composition containing 1.3 to 2.0% Mn, 0.6 to 1.3% Si, 0.2 to 0.5% Fe and 0.85 to 1.3% Cu, by mass, with the balance comprising Al and unavoidable impurities; the sacrificial material has a composition containing more than 4.0% and up to and including 8.0% Zn, 0.7 to 2.0% Mn, 0.3 to 1.0% Si, 0.3 to 1.0% Fe and 0.05 to 0.3% Ti, by mass, with the balance comprising Al and unavoidable impurities; the core material has a lamellar crystal grain structure; the brazing sheet has an elongation of at least 4%; and the brazing sheet has a tensile strength of at least 170 MPa after a heat treatment corresponding to brazing, the heat treatment comprising heating the brazing sheet of 600 C. in about 7 minutes, maintaining the brazing sheet at 600 C. for 3 minutes, and then cooling the brazing sheet at a cooling rate of 100 C./min.

2. The aluminum alloy brazing sheet according to claim 1, wherein the crystal grain size of the core material in a longitudinal section parallel to a rolling direction after the heat treatment corresponding to brazing is in a range of 30 to 200 m.

3. The aluminum alloy brazing sheet according to claim 1, wherein the core material, before brazing, comprises second phase particles having a crystal grain size of 0.2 to 0.7 m in terms of an equivalent circle diameter, which have a density of 1 to 30 pieces/m.sup.2.

4. The aluminum alloy brazing sheet according to claim 1, wherein the aluminum alloy brazing sheet is a heat exchanger member.

5. The aluminum alloy brazing sheet according to claim 2, wherein the core material, before brazing, comprises second phase particles having a crystal grain size of 0.2 to 0.7 m in terms of an equivalent circle diameter, which have a density of 1 to 30 pieces/m.sup.2.

6. The aluminum alloy brazing sheet according to claim 2, wherein the aluminum alloy brazing sheet is a heat exchanger member.

7. The aluminum alloy brazing sheet according to claim 3, wherein the aluminum alloy brazing sheet is a heat exchanger member.

8. The aluminum alloy brazing sheet according to claim 5, wherein the aluminum alloy brazing sheet is a heat exchanger member.

9. A method of manufacturing a heat exchanger, the method comprising: forming the aluminum alloy brazing sheet according to claim 1 into a first heat exchanger member; and brazing said heat exchanger member and a second heat exchanger member.

Description

DESCRIPTION OF EMBODIMENT

(1) Hereinafter, one embodiment of the present invention will be described.

(2) An aluminum alloy for sacrificial material, an aluminum alloy for core material, and AlSi-based or AlSiZn-based brazing filler metal, which are in the composition range of the present invention, are prepared, respectively. These alloys and the like can be ingoted by a conventional method. The aluminum alloy for brazing filler metal is not particularly limited in the present invention as long as it is AlSi-based or AlSiZn-based, and examples thereof that can be used include JIS 4343 alloy, 4045 alloy, and 4047 alloy. Further, a brazing filler metal to which Zn is not added or a brazing filler metal in which the amount of Zn added is increased can also be used. Further, AlSi alloy and AlSiZn alloy which contain Mn, Fe, Zr, Ti, Cu, Li, and the like can also be used.

(3) These alloys are ingoted and then optionally subjected to homogenization treatment. The homogenization treatment of a core material can be performed, for example, by heating at 550 to 610 C. for 2 to 15 hours. A brazing filler metal is not subjected to homogenization treatment or heated at 400 to 580 C. for 2 to 10 hours. A sacrificial material is not subjected to homogenization treatment or heated at 400 to 500 C. for 2 to 10 hours.

(4) An ingot is formed into a sheet material through hot rolling. Further, an ingot may also be formed into a sheet material through continuous casting rolling.

(5) These sheet materials are clad at a suitable clad ratio in the state where a sacrificial material is arranged on one side of a core material; a brazing filler metal is arranged on the other side thereof; and these materials are superposed.

(6) The cladding is generally performed by hot rolling. Then, the hot-rolled sheet is further subjected to cold rolling to obtain an aluminum alloy brazing sheet having a desired thickness.

(7) The hot rolling is performed by rough rolling that is controlled to a starting temperature in the range of 450 to 530 C., a final sheet thickness in the range of 15 to 30 mm, and a final temperature in the range of 330 to 430 C., followed by reverse finish rolling that is controlled to a starting sheet thickness in the range of 15 to 30 mm, a starting temperature in the range of 320 to 420 C., an final sheet thickness in the range of 1 to 4 mm, and an end temperature in the range of 200 to 320 C.

(8) Metal structure is adjusted by adjusting the conditions of hot rolling to the above ranges, and an elongation of material of 4% or more can be obtained. For example, if rolling is performed at a higher temperature than the above, metal structure may be coarsened to reduce elongation. On the other hand, if rolling is performed at a lower temperature than the above, the rolling itself will be difficult.

(9) In the above production process, process annealing can be interposed in the cold-rolling. In order to make the crystal grains of material lamellar, it is necessary to prevent the recrystallization of materials in the heat treatment by process annealing. For this purpose, the process annealing is preferably performed at a temperature in the range of 150 to 250 C.2 to 10 hours. In the final cold rolling after the process annealing, the rolling is performed at a cold rolling rate of 5 to 25%, thus obtaining an H14 refined brazing sheet.

(10) Examples of the thickness of the final brazing sheet include, but are not particularly limited to, a thickness of 0.15 to 0.25 mm.

Examples

(11) The aluminum alloys having component compositions shown in Table 1 were subjected to semi-continuous casting to obtain aluminum alloy ingots for sacrificial material.

(12) The aluminum alloys having component compositions shown in Table 2 were subjected to semi-continuous casting to obtain aluminum alloy ingots for core material.

(13) Further, an alloy for brazing filler metal (4045 alloy) was cast by semi-continuous casting to obtain an aluminum alloy ingot for brazing filler metal.

(14) The above aluminum alloy ingots for core material were subjected to homogenization treatment under the conditions shown in Tables 3-1, 3-2, 4-1, and 4-2, and the aluminum alloy ingot for brazing filler metal were subjected to homogenization treatment under a condition of 400 C. for 5 hours. The aluminum alloy ingots for sacrificial material were not subjected to homogenization treatment.

(15) Hot rolling was performed by superposing the aluminum alloy ingot for sacrificial material on one side of the aluminum alloy ingot for core material and the aluminum alloy ingot for brazing filler metal on the other side thereof in combinations shown in Tables 3-1, 3-2, 4-1, and 4-2. In the above hot rolling, rough rolling was performed at a starting temperature of 500 C., a final sheet thickness of 20 mm, and a final temperature of 430 C. except for No. 29, 30, 31, 32, and 33, and finish rolling was performed under the conditions shown in Tables 3-1, 3-2, 4-1, and 4-2.

(16) Note that, in Nos. 29, 30, and 31, rough rolling was performed at a starting temperature of 560 C., a final sheet thickness of 30 mm, and a final temperature of 510 C., and finish rolling was performed under the conditions shown in Tables 4-1 and 4-2.

(17) Further, the ingot in No. 33 was rolled to 7 mm only by rough rolling.

(18) Further, the clad material was subjected to the above cold rolling, and process annealing which also serves as the adjustment of crystal grain structure was then performed at 220 C. for 5 hours followed by final cold rolling, thereby preparing H14 refined brazing sheets Nos. 1 to 33 each having a thickness of 0.20 mm as test specimens.

(19) TABLE-US-00001 TABLE 1 Type Zn Mn Si Fe Ti Remarks Sacrificial a 4.0 1.1 0.6 0.6 0.15 Less than Zn lower limit material b 5.0 1.1 0.6 0.6 0.15 c 6.0 1.1 0.6 0.6 0.15 d 7.0 1.1 0.6 0.6 0.15 e 9.0 1.1 0.6 0.6 0.15 More than Zn upper limit f 6.0 0.5 0.6 0.6 0.15 Less than Mn lower limit g 6.0 0.8 0.6 0.6 0.15 h 6.0 1.3 0.6 0.6 0.15 i 6.0 2.2 0.6 0.6 0.15 More than Mn upper limit j 6.0 1.1 0.2 0.6 0.15 Less than Si lower limit k 6.0 1.1 0.4 0.6 0.15 l 6.0 1.1 0.9 0.6 0.15 m 6.0 1.1 1.3 0.6 0.15 More than Si upper limit n 6.0 1.1 0.6 0.2 0.15 Less than Fe lower limit o 6.0 1.1 0.6 0.4 0.15 p 6.0 1.1 0.6 0.9 0.15 q 6.0 1.1 0.6 1.3 0.15 More than Fe upper limit r 4.0 1.1 0.5 0.6 0.15 Chemical components of the precedent example s 5.5 1.1 0.5 0.6 0.15 Chemical components of the precedent example t 6.5 1.1 0.0 0.6 0.15 Chemical components of the precedent example

(20) TABLE-US-00002 TABLE 2 Type Mn Si Fe Cu Remarks Core material A 1.0 0.8 0.25 1.1 Less than Mn lower limit B 1.4 0.8 0.25 1.1 C 1.6 0.8 0.25 1.1 D 1.9 0.8 0.25 1.1 E 2.3 0.8 0.25 1.1 More than Mn upper limit F 1.6 0.5 0.25 1.1 Less than Si lower limit G 1.6 0.6 0.25 1.1 H 1.6 1.1 0.25 1.1 I 1.6 1.5 0.25 1.1 More than Si upper limit J 1.6 0.8 0.05 1.1 Less than Fe lower limit K 1.6 0.8 0.20 1.1 L 1.6 0.8 0.45 1.1 M 1.6 0.8 0.6 1.1 More than Fe upper limit N 1.6 0.8 0.25 0.6 Less than Cu lower limit O 1.6 0.8 0.25 0.85 P 1.6 0.8 0.25 1.3 Q 1.6 0.8 0.25 1.5 More than Cu upper limit

(21) The brazing sheets as test specimens were evaluated for the following characteristics under the following conditions, and the evaluation results were shown in Tables 3-1, 3-2, 4-1, and 4-2.

(22) (Elongation of Material)

(23) Samples were cut from the prepared brazing sheets in a direction parallel to a rolling direction to prepare test pieces each having a shape of JIS No. 13 B, which were subjected to a tensile test to measure the total elongation by a butt method. The stress rate was 3 mm/min. The results of measurement are shown in Tables 3-1, 3-2, 4-1, and 4-2.

(24) When the material elongation was 4.0% or more, the material was evaluated as good, and when the elongation of material was less than 4.0%, the material was evaluated as poor.

(25) (Crystal Structure of Material)

(26) Small pieces of samples were cut from the prepared brazing sheets, embedded in resin in a direction parallel to a rolling direction, and mirror-finished by emery polishing and buffing. Then, the crystal grain structure was revealed by the Barker's solution method, and the crystal structure was observed with a polarizing microscope. The observation magnitude was 100 times. The evaluation results are shown in Tables 3-1, 3-2, 4-1, and 4-2.

(27) (Strength after Brazing)

(28) The prepared brazing sheets were subjected to a heat-treatment corresponding to brazing. Specifically, the brazing sheets were heated to 600 C. in about 7 minutes, maintained at 600 C. for 3 minutes, and then cooled at a cooling rate of 100 C./min. Samples were cut from the prepared brazing sheets in a direction parallel to a rolling direction to prepare test pieces each having a shape of JIS No. 13 B, which were subjected to a tensile test to measure the tensile strength. The stress rate was 3 mm/min. The results of measurement are shown in Tables 3-1, 3-2, 4-1, and 4-2.

(29) When the strength after brazing was 175 MPa or more, the material was evaluated as excellent; when the strength after brazing was 170 MPa or more and less than 175 MPa, the material was evaluated as good; and when the strength was less than 170 MPa, the material was evaluated as poor.

(30) (Crystal Grain Size after Brazing Heat Treatment)

(31) The prepared brazing sheets were subjected to a heat-treatment corresponding to brazing. Specifically, the brazing sheets were heated to 600 C. in about 7 minutes, maintained at 600 C. for 3 minutes, and then cooled at a cooling rate of 100 C./min. The section parallel to a rolling direction of the samples subjected to the heat treatment corresponding to brazing was embedded in resin and then mirror polished. Then, crystal grains were revealed with an etchant (for example, by immersing the samples in Keller's solution at ordinary temperature for 1 to 3 minutes), and a photograph of five places of each sample was taken at a magnification of 200 times with an optical microscope. The photograph taken was measured for crystal grain size by an intercept method in the rolling direction. The results of the measurement are shown in Tables 3-1, 3-2, 4-1, and 4-2.

(32) (Distribution State of Second Phase Particles of 0.7 m or Less in Core Material)

(33) The number density (pieces/m.sup.2) of second phase particles in the range of 0.2 to 0.7 m in terms of the equivalent circle diameter was measured with a transmission electron microscope (TEM).

(34) In the measuring method, material was subjected to salt bath annealing for 400 C.15 seconds to remove deformation strain to allow easy observation of second phase particles; then, a thin film was prepared from the central part of core material with mechanical polishing and electrolytic polishing by a common method; and a photograph of the thin film was taken at a magnification of 10000 times with a transmission electron microscope. Photographs of five visual fields (about 500 m.sup.2 in total) were taken and measured for the size and number density of second phase particles by image analysis. The results of the measurement are shown in Tables 3-1, 3-2, 4-1, and 4-2.

(35) The number density (pieces/m.sup.2) of second phase particles in the range of 0.2 to 0.7 m in terms of the equivalent circle diameter is shown in Tables 3-1, 3-2, 4-1, and 4-2.

(36) (Brazing Erosion Resistance (Erosion Depth))

(37) The prepared brazing sheets were subjected to a heat-treatment corresponding to brazing. Specifically, the brazing sheets were heated to 600 C. in about 7 minutes, maintained at 600 C. for 3 minutes, and then cooled at a cooling rate of 100 C./min. The samples subjected to the heat treatment corresponding to brazing were embedded in resin, and a section parallel to a rolling direction of the samples were mirror polished and revealed a structure with the Barker's solution. Then, the structure was observed with an optical microscope to measure the brazing erosion depth, which was evaluated as brazing resistance. The evaluation results are shown in Tables 3-1, 3-2, 4-1, and 4-2.

(38) When melting occurred, the sample was evaluated as poor; when melting did not occur and the erosion melting depth was less than 30 m, the sample was evaluated as excellent; and when the erosion melting depth was 30 m or more and less than 50 m, the sample was evaluated as good.

(39) (Internal Corrosion Resistance (Acidic))

(40) The prepared brazing sheets were subjected to a heat-treatment corresponding to brazing. Specifically, the brazing sheets were heated to 600 C. in about 7 minutes, maintained at 600 C. for 3 minutes, and then cooled at a cooling rate of 100 C./min. A sample having a size of 3040 mm was cut from the sample after brazing heat treatment, and the surfaces thereof other than a sacrificial material surface (ends and a brazing filler metal surface) were masked. The masked samples were subjected to immersion test for 8 weeks in a cycle of 80 C.8 hours.fwdarw.room temperature16 hours in an aqueous solution (OY water) containing 195 ppm of Cl, 60 ppm of SO.sub.4.sup.2, 1 ppm of Cu.sup.2+, and 30 ppm of Fe.sup.3+. The samples after corrosion test were immersed in a boiled mixed solution of phosphoric acid and chromic acid for 10 minutes to remove a corrosion product, and then the section of the maximum corrosion part was observed to measure the corrosion depth. The evaluation results are shown in Tables 3-1, 3-2, 4-1, and 4-2.

(41) When the corrosion depth was less than 50 m, the sample was evaluated as excellent; when the corrosion depth was 50 m or more and less than 80 m, the sample was evaluated as good; and when the corrosion depth was 80 m or more, the sample was evaluated as poor.

(42) (Internal Corrosion Resistance (Alkaline))

(43) The prepared brazing sheets were subjected to a heat-treatment corresponding to brazing. Specifically, the brazing sheets were heated to 600 C. in about 7 minutes, maintained at 600 C. for 3 minutes, and then cooled at a cooling rate of 100 C./min. A sample having a size of 3040 mm was cut from the sample after brazing heat treatment, and the surfaces thereof other than a sacrificial material surface (ends and a brazing filler metal surface) were masked. An aqueous solution (OY water), in which Cl was adjusted to 195 ppm; SO.sub.4.sup.2 was adjusted to 60 ppm; Cu.sup.2+ was adjusted to 1 ppm; and Fe.sup.3+ was adjusted to 30 ppm, was further adjusted to a pH of 11 with caustic soda. The masked samples were subjected to immersion test for 8 weeks in a cycle of 80 C.8 hours.fwdarw.room temperature16 hours in the adjusted solution. The samples after corrosion test were immersed in a boiled mixed solution of phosphoric acid and chromic acid for 10 minutes to remove a corrosion product, and then the section of the maximum corrosion part was observed to measure the corrosion depth. The evaluation results are shown in Tables 3-1, 3-2, 4-1, and 4-2.

(44) When the corrosion depth was less than 80 m, the sample was evaluated as good; and when the corrosion depth was 80 m or more, the sample was evaluated as poor.

(45) As the overall evaluation, when all the evaluations were good or more, the samples were evaluated as good; and when all the evaluations are good or more and the strength after brazing, corrosion resistance (acid), and brazing resistance were excellent, the samples were evaluated as excellent.

(46) As shown in Tables 3-1, 3-2, 4-1, and 4-2, in Inventive Examples, material elongation, tensile strength, and corrosion resistance were excellent, and the overall evaluation was good or excellent.

(47) On the other hand, in Comparative Examples, at least one of material elongation, tensile strength, and corrosion resistance was poor. Note that, in both Comparative Examples 13 and 18, it was unable to produce clad material.

(48) TABLE-US-00003 TABLE 3 Core Final Sacrificial Core material Starting End sheet Sacrificial Core material material homogenization No. temperature temperature thickness material material structure structure treatment Comparative 1 400 C. 250 C. 3 mm a N Lamellar Lamellar 550 C. 10 h example 2 400 C. 250 C. 3 mm e N Lamellar Lamellar 550 C. 10 h 3 400 C. 250 C. 3 mm f N Lamellar Lamellar 550 C. 10 h 5 400 C. 250 C. 3 mm i N Lamellar Lamellar 550 C. 10 h 5 400 C. 250 C. 3 mm j N Lamellar Lamellar 550 C. 10 h 6 400 C. 250 C. 3 mm m N Lamellar Lamellar 550 C. 10 h 7 400 C. 250 C. 3 mm n N Lamellar Lamellar 550 C. 10 h 8 400 C. 250 C. 3 mm q N Lamellar Lamellar 550 C. 10 h 9 400 C. 250 C. 3 mm c A Lamellar Lamellar 550 C. 10 h Present 10 400 C. 250 C. 3 mm c B Lamellar Lamellar 550 C. 10 h invention 11 400 C. 250 C. 3 mm c C Lamellar Lamellar 550 C. 10 h 12 400 C. 250 C. 3 mm c D Lamellar Lamellar 550 C. 10 h Comparative 13 400 C. 250 C. 3 mm c E Lamellar Lamellar Not example manufacturable 14 400 C. 250 C. 3 mm c F Lamellar Lamellar 550 C. 10 h Present 15 400 C. 250 C. 3 mm c G Lamellar Lamellar 550 C. 10 h invention 16 400 C. 250 C. 3 mm c H Lamellar Lamellar 550 C. 10 h Comparative 17 400 C. 250 C. 3 mm c I Lamellar Lamellar 550 C. 10 h example 18 400 C. 250 C. 3 mm c J Lamellar Lamellar Difficult in manufacturing Present 19 400 C. 250 C. 3 mm c K Lamellar Lamellar 550 C. 10 h invention 20 400 C. 250 C. 3 mm c L Lamellar Lamellar 550 C. 10 h Comparative 21 400 C. 250 C. 3 mm c M Lamellar Lamellar 550 C. 10 h example 22 400 C. 250 C. 3 mm c N Lamellar Lamellar 550 C. 10 h Present 23 400 C. 250 C. 3 mm c P Lamellar Lamellar 550 C. 10 h invention Comparative 24 400 C. 250 C. 3 mm c Q Lamellar Lamellar 550 C. 10 h example Corrosion resistance Elongation TS (acidic) Second of base after brazing 80 or more X phase material less than 170 X Less than 80 particles Crystal grain Less than 4.0 X 170 or more Less than No. (pieces/m.sup.2) size (m) 4.0 or more 175 or more 50 Comparative 1 21 220 3.8X 174 X example 2 21 220 6.0 174 3 21 220 5.5 174 5 21 220 3.5X 179 5 21 220 5.5 168X 6 21 200 3.6X 183 7 21 220 4.5 173 8 21 220 4.5 178 X 9 23 210 5.5 168X Present 10 25 215 5.5 177 invention 11 25 220 5.5 180 12 29 240 5.5 186 Comparative 13 Not Not Not Not Not example manufacturable manufacturable manufacturable manufacturable manufacturable 14 35 210 5.5 165X Present 15 28 220 5.5 175 invention 16 29 240 5.5 190 Comparative 17 30 250 4.5 Melting Melting example 18 Difficult in Difficult in Difficult in Difficult in Difficult in manufacturing manufacturing manufacturing manufacturing manufacturing Present 19 25 220 5.5 179 invention 20 25 215 5.5 184 Comparative 21 25 205 3.4X example 22 25 220 3.8X 160X Present 23 25 220 5.5 188 invention Comparative 24 25 220 5.7 196 Melting example Overall evaluation Brazing All or more Corrosion resistance All or more and resistance melting X TS, corrosion (alkaline) Less than 50 m resistance(acidic) 80 or more X Less than and brazing resistance No. Less than 80 30 m : Remarks Comparative 1 X X Zn lower Short of example limit potential difference 2 X X Zn upper limit 3 X X Mn lower limit 5 X X Mn upper limit 5 X X Si lower limit 6 X X Si upper limit 7 X X Fe lower limit 8 X X Fe upper limit 9 X Mn lower limit Present 10 invention 11 12 Comparative 13 Not Not Not Mn upper Not example manufacturable manufacturable manufacturable limit manufacturable 14 X Si lower limit Present 15 invention 16 Comparative 17 Melting X X Si upper Melting when example limit brazing 18 Difficult in Difficult in Difficult in Fe lower Cast crack manufacturing manufacturing manufacturing limit Present 19 invention 20 Comparative 21 X Fe upper Not example limit manufacturable 22 X Cu lower limit Present 23 invention Comparative 24 Melting X X Cu Melting when example upper brazing limit

(49) TABLE-US-00004 TABLE 4 Core Final Sacrificial Core material Starting End sheet Sacrificial Core material material homogenization No. temperature temperature thickness material material structure structure treatment Comparative 25 400 C. 250 C. 3 mm r N Lamellar Lamellar 550 C. 10 h example 26 400 C. 250 C. 3 mm s M Lamellar Lamellar 550 C. 10 h 27 400 C. 250 C. 3 mm t M Lamellar Lamellar 550 C. 10 h 28 400 C. 250 C. 3 mm c N Lamellar Recrystallization 550 C. 10 h 29 480 C. 300 C. 3 mm j N Lamellar Lamellar 550 C. 10 h 30 510 C. 300 C. 3 mm j N Lamellar Lamellar 550 C. 10 h 31 510 C. 350 C. 3 mm j N Lamellar Lamellar 550 C. 10 h 32 420 C. 330 C. 4.2 mm j N Lamellar Lamellar 550 C. 10 h 33 Rolled to 7 mm only j N Lamellar Lamellar 550 C. 10 h by rough rolling Corrosion resistance Elongation TS (acidic) Second of base after brasing 80 or moreX phase Crystal material Less than 170X Less than 80 particles grain Less than 4.0X 170 or more Less than No. (pieces/m.sup.2) size(m) 4.0 or more 175 or more 50 Comparative 25 21 220 3.8X 174 X example 26 21 220 3.8X 160X 27 21 220 3.5X 160X 28 21 220 2.3X 174 29 20 200 4.8 168X 30 19 190 4.4 168X 31 19 185 4.1 168X 32 20 210 4.6 168X 33 20 210 4.5 168X Overall evaluation Brazing All or more Corrosion resistance All or more, and resistance melting X TS, corrosion (alkaline) Less than 50 m Resistance (acidic) 80 or moreX Less than and brazing resistance No. Less than 80 30 m : Remarks Comparative 25 X X example 26 X 27 X 28 X Core material Recrystallized structure 29 X X Si lower limit 30 X X Si lower limit 31 X X Si lower limit 32 X X Si lower limit 33 X X Si lower limit