JP5499300B2 - Copper alloy tube for heat exchanger - Google Patents

Description

  The present invention relates to a copper alloy tube for a heat exchanger. In the following description, this “copper alloy tube” is abbreviated and simply referred to as “copper tube”.

  As is well known, a heat exchanger such as an air conditioner is mainly composed of a U-shaped copper tube bent into a hairpin shape and fins made of aluminum or an aluminum alloy plate (hereinafter referred to as aluminum fins). That is, the heat transfer part of the heat exchanger is formed by passing a copper tube bent into a U-shape through a through hole of an aluminum fin, and inserting a jig into the U-shaped copper tube to expand the tube. Adhere the fins closely. Further, the open end of the U-shaped copper pipe is expanded (flared), and a bent copper pipe bent into a U-shape is inserted into the expanded open end, and a brazing material such as phosphor copper braze is used. The heat exchanger is manufactured by connecting the bend copper pipe to the open end of the copper pipe by brazing.

  For this reason, copper pipes used in heat exchangers are required to have good workability (bending, expansion / flare, contraction / drawing, etc.) and brazing. Accordingly, phosphorous deoxidized copper having good characteristics, good thermal conductivity, and appropriate strength is widely used.

  In recent years, refrigerants used in heat exchangers have changed greatly from the viewpoint of protecting the global environment. Until now, HCFC (hydrochlorofluorocarbon) chlorofluorocarbons such as R22, which have been used in heat exchangers such as air conditioners, have a large ozone depletion coefficient, so they are replaced by small HFC (hydrofluorocarbon) chlorofluorocarbons. It has come to be. In addition, carbon dioxide, which is a natural refrigerant, has come to be used in heat exchangers used in water heaters, automotive air conditioners, vending machines, and the like. The operating pressure of these newly adopted refrigerants has increased to about 1.6 to 6 times that of the conventional refrigerant R22.

  While these operating pressures are increasing, there is an increasing demand for thinner copper pipes to reduce the amount of copper used in order to suppress the increase in copper pipe costs associated with soaring copper bullion. On the other hand, the thickness can be reduced as the tensile strength of the copper pipe used is larger. However, since the conventional phosphorous-deoxidized copper pipe has a low tensile strength, it is necessary to increase the thickness of the pipe to cope with the increase in the operating pressure. The thickness cannot be reduced.

  Also, when assembling the heat exchanger, the brazed part is heated to a temperature of 800 ° C. or higher for several seconds to several tens of seconds, so that the crystal grains are coarsened and softened in the brazed part and its vicinity in comparison with other parts. Therefore, the thickness of the conventional phosphorous deoxidized copper pipe needs to be increased. As described above, phosphorous deoxidized copper cannot respond to the demand for increased operating pressure and copper pipe thinning, and instead of phosphorous deoxidized copper, a copper pipe capable of meeting these requirements is strongly desired. Become.

  In order to meet such a demand, a copper tube made of Sn-P-based copper alloy having higher strength instead of phosphorous deoxidized copper (hereinafter referred to as Sn-P-based copper tube or Sn-P-based copper alloy tube). Various proposals have been made in the past. This Sn-P-based copper tube basically contains Sn: 0.1 to 1.0%, P: 0.005 to 0.1%, regulates impurities such as O and H, and contains Zn. It consists of a copper alloy composition added selectively. In addition, the copper tube structure is basically composed of a fine crystal grain size, for example, an average crystal grain size of 30 μm or less (see Patent Documents 1, 2, 3, and 4).

  In this Sn-P-based copper tube, there is also disclosed a method for controlling the texture such as the Goss orientation accumulation rate to appropriately control the balance between the strength and elongation in the circumferential direction to improve the breaking pressure (patent) Reference 5). Furthermore, a copper tube that combines high fracture pressure and good bending workability by increasing the ratio of fracture strength and tensile strength (fracture strength / tensile strength) to that of phosphorous deoxidized copper has also been proposed. (See Patent Document 6).

  Further, in order to improve both the fracture strength and the bending workability of the Sn-P-based copper tube, Patent Document 7 proposes to regulate the number of coarse crystal grains that are twice or more the average crystal grain size. . In this patent document 7, when the Sn-P-based copper alloy tube is thinned to 0.5 mm or less, the coarse crystal grains that cause cracks in a severe U-shaped bending process with a small bending radius are regulated. It is.

  However, even Sn-P copper pipes, which have higher strength than phosphorous deoxidized copper pipes and respond to the demand for thinning, can be heated to a high temperature of 800 ° C. or higher by brazing during assembly of the heat exchanger. The problem of being exposed to coarsening of crystal grains and softening or strength reduction is also unavoidable, similarly to phosphorous deoxidized copper. Therefore, even with the Sn-P copper pipe, there is still room for improvement with respect to the problem of softening due to brazing.

  For this reason, Sn-P-based copper pipes that are subject to this softening suppression have also been proposed. In Patent Document 8 and the like, a fixed amount of P is dissolved in a copper alloy pipe matrix, and crystal grains are formed by brazing. Even if it becomes coarse, the strength reduction of the heat transfer tube is suppressed by the solid solution strengthening of P.

JP 2000-199023 A Japanese Patent No. 3794971 JP 2004-292917 A JP 2006-274313 A JP 2009-102690 A JP 2008-174785 A JP 2010-65270 A JP 2009-270166 A

  However, for improving the softening resistance at the time of brazing, it is difficult to make P dissolve in itself by means of solid solution strengthening of P as described in Patent Document 8. That is, in order to make P dissolve, each billet (ingot) in casting, rapid cooling after hot extrusion, rapid cooling after final annealing, etc. It is necessary to control the cooling rate (rapid cooling). When these cooling rates are slow, the precipitates of P increase and become coarse, and there is a high possibility that the definition of the P solid solution amount cannot be satisfied.

  Therefore, even if the softening resistance at the time of brazing of a copper pipe is improved metallurgically, there is a need for a practical improvement method that makes it possible to manufacture an improved copper pipe as much as possible.

  The present invention has been made in view of such problems, and provides an Sn-P-based copper alloy tube for a heat exchanger that is easy to manufacture, has improved softening resistance, and has excellent fracture strength and bending workability. The purpose is to do.

The gist of the copper alloy tube for a heat exchanger of the present invention for the above purpose is Sn: 0.1 to 3.0% by mass, P: 0.005 to 0.1% by mass, with the balance being Cu and A copper alloy tube having a composition composed of inevitable impurities, the copper alloy tube structure being a measurement result by the SEM-EBSP method, and an average grain size D1 in a cross section parallel to the axial direction of the copper alloy tube is 20 μm. The absolute value | D1-D2 | of the difference between D1 and the average crystal grain size D2 in the cross section perpendicular to the axial direction of the copper alloy tube is 3 μm or less (including 0 μm).

  The inventors of the present invention have once again studied the influence on the softening resistance of the crystal grains of the Sn-P-based copper alloy tube structure, in which the change in strength (strength reduction) before and after the brazing is small. As a result, not only the crystal grain size, which has been a problem in the past, but also the difference in crystal grain size depending on the copper tube part (or the difference in cross-sectional direction), that is, the anisotropy of the crystal grain size is greatly improved. It was found to affect the softening property.

  As a criterion for the anisotropy of the crystal grain size, the inventors have determined that the average crystal grain size D1 in the cross section (in the direction) parallel to the axial direction of the copper alloy tube is perpendicular to the axial direction of the copper alloy tube. The average crystal grain size D2 in the (direction) cross section was selected. The difference “D1−D2” between D1 and D2 of the copper alloy tube is not the ratio of the length and width of the same crystal grain, such as “aspect ratio, flatness or roundness”. In other words, the cross-section direction of the pipe is different from each other in the cross-section parallel to the axial direction of the copper alloy pipe and the cross-section perpendicular to the axial direction, which is parallel to the rolling or drawing direction and the crystal grains are easily elongated. Of the average crystal grain size. This is because it was found that the softening resistance is improved as the difference D1-D2 between D1 and D2 is small or small and the anisotropy of the average crystal grain size due to the copper tube part (cross-sectional direction) is small. . In contrast, the larger the difference between D1 and D2 and the greater the anisotropy of the average crystal grain size due to the copper tube part, the poorer the softening resistance, even if the crystal grains are finer.

  The cause of the anisotropy of the crystal grain size is greatly influenced by the manufacturing method (conditions) of the thinned Sn—P-based copper alloy tube. Usually, a copper alloy tube for a heat exchanger is reduced in thickness by rolling and drawing after hot extrusion, but when the copper tube is made thinner to, for example, 1.0 mm or less due to the above-described demand for thinning. The area reduction ratio in the rolling and drawing is 95% or more. With such a large area reduction rate, a large strain is introduced into the copper tube in these rolling processes and drawing processes.

  Moreover, from the viewpoint of production efficiency, it is common sense that intermediate annealing is not usually performed during the drawing process, and this large strain is introduced (the large strain is not released), and the final annealing step. Carried over to

  The Sn-P-based copper alloy tube is formed by the final annealing in the manufacturing process, and usually the crystal grains produced by continuous release of strain in the recovery by recovery (generated by the release of strain by recovery) and A structure in which two types of crystal grains having no distortion due to the formation of recrystallization nuclei are both generated (mixed) is obtained. The crystal grains free from distortion due to the formation of recrystallization nuclei are generated from the grain boundaries before the final annealing. For this reason, the smaller the crystal grain size before final annealing, the more crystal grains without distortion due to the formation of recrystallized nuclei after final annealing. Since the crystal grain size before the final annealing according to the related art corresponds to the crystal grain size after hot extrusion, it is very large, and the crystal grains generated by releasing the strain by the recovery tend to be the main structure.

  However, such crystal grains generated by releasing strain by recovery are crystal grains elongated in the rolling direction or drawing direction, i.e., the axial direction of the copper tube. Compared with no equiaxed crystal grains, the dislocation density is very high. For this reason, as in the conventional Sn-P-based copper alloy tube that has undergone final annealing, when the number of crystal grains generated by the release of strain by recovery increases, the number of crystal grains without distortion due to the formation of recrystallized nuclei decreases. It is considered that the density becomes high and the strength and workability are inferior.

  Based on such knowledge, the present invention reduces the number of crystal grains generated by releasing the strain by recovery, while increasing the number of undistorted crystal grains due to the formation of recrystallized nuclei. First, the dislocation density is lowered to improve the strength and workability.

  In addition, the crystal grains generated by the strain being released by recovery have the property of preferentially growing and coarsening over the crystal grains without distortion due to the formation of recrystallized nuclei during high-temperature heating during brazing. . For this reason, when there are many crystal grains generated by releasing strain due to this recovery in the copper tube structure, that is, when the anisotropy of the crystal grain size is large, the crystal grain size after brazing is remarkably coarsened, Strength after brazing and breaking pressure = softening resistance will decrease.

  On the other hand, in the present invention, the number of crystal grains generated by relieving strain by recovery is reduced, while the number of crystal grains without distortion due to the formation of recrystallization nuclei is increased to reduce the crystal grain anisotropy. And the growth of the crystal grain at the time of the high temperature heating at the time of the brazing is suppressed, and the softening resistance is improved.

  Thus, the conventional Sn-P-based copper alloy tube tends to have an anisotropy of crystal grain size in which the difference between the D1 and D2 specified in the present invention exceeds 3 μm because of its manufacturing method. Therefore, the softening resistance is particularly inferior. Incidentally, as in the present invention, in order to reduce the number of crystal grains generated by recovery and increase the number of undistorted crystal grains due to the formation of recrystallization nuclei, intermediate annealing may be added during the drawing process, etc. It is never difficult.

  Therefore, according to the present invention, the Sn-P-based copper alloy for heat exchangers, which is easy to manufacture and has improved softening resistance and excellent fracture strength and bending workability even when thinned to 1.0 mm or less. A tube can be provided.

It is explanatory drawing which shows the average crystal grain diameter D1 of the cross section parallel to the axial direction of a copper alloy pipe | tube, and the average crystal grain diameter D2 of a cross section perpendicular | vertical to an axial direction. 3 is a drawing-substituting photograph showing the crystal grain structure of Invention Example 1 in Example 1; 5 is a drawing-substituting photograph showing the crystal grain structure of Comparative Example 7 in Example Table 1. 3 is a drawing-substituting photograph showing the crystal grain structure of Comparative Example 2 in Example Table 1. 5 is a drawing substitute photograph showing a crystal grain structure of Comparative Example 1 in Example Table 1.

Hereinafter, the embodiments of the present invention will be specifically described in order for each requirement.
Copper alloy composition:
The copper alloy composition of the copper pipe in the present invention is Sn: 0.1-3. As a basic composition for improving various properties required for the copper alloy pipe, such as softening resistance, fracture strength and bending workability. The composition contains 0% by mass and P: 0.005 to 0.1% by mass with the balance being Cu and inevitable impurities. Next, the reason for addition of the additive element and the reason for limiting the composition will be described. All the described contents are mass%.

P: 0.005 to 0.1% by mass:
In Sn-P copper pipes containing both Sn and P, if the P content exceeds 0.1% by mass, cracking is likely to occur during hot extrusion, the stress corrosion cracking sensitivity is increased, and the thermal conductivity is increased. The decrease in Further, if the P content is less than 0.005% by mass, the amount of oxygen is increased due to insufficient deoxidation, Sn oxide is generated, the soundness of the ingot is lowered, and the bending workability as a copper pipe is reduced. descend. Therefore, the range of P content shall be 0.005-0.1 mass%.

Sn: 0.1-3.0% by mass:
Sn has the effect of improving the tensile strength of the Sn-P-based copper tube and suppressing the coarsening of crystal grains. When Sn is contained in the copper alloy of the heat transfer tube using various refrigerants, phosphorus is removed. The thickness of the pipe can be reduced as compared with the acid copper pipe. In addition, Sn lowers the stacking fault energy, so that strain is likely to accumulate. Crystal grains that are not recovered by the formation of recrystallized nuclei are reduced by reducing the number of crystal grains caused by recovery during intermediate annealing or final annealing during drawing. Increases the roundness of the crystal grains.

  If the amount of Sn is too small, it becomes impossible to increase the roundness finely and obtain a crystal grain size after the final annealing, and it becomes impossible to obtain sufficient tensile strength and softening resistance after brazing. On the other hand, if the Sn content of the copper tube is too high, solidification segregation in the ingot becomes severe and segregation may not be completely eliminated by normal hot extrusion and / or processing heat treatment. Mechanical properties, bending workability, texture after brazing, and mechanical properties become non-uniform. Further, in order to perform extrusion molding at the same extrusion pressure as a copper alloy having an extrusion pressure increased and Sn content of 2% by mass or less, it is necessary to raise the extrusion temperature. Increases, resulting in decreased productivity and increased surface defects in the copper tube. Therefore, the range of Sn content shall be 0.1-3.0 mass%.

Zn: 0.01 to 1.0% by mass:
In the Sn-P-based copper tube and the P-based copper tube in common, by selectively containing Zn, the strength, softening resistance, and fatigue strength are reduced without greatly reducing the thermal conductivity of the copper tube. Can be improved. In addition, the inclusion of Zn can reduce the wear of tools used for cold rolling, drawing, rolling, etc., and has the effect of extending the life of drawing plugs, grooved plugs, etc., contributing to the reduction of production costs To do. If the Zn content is too small, these effects cannot be obtained sufficiently. On the other hand, when there is too much content of Zn, the tensile strength of the longitudinal direction of a pipe and the pipe circumference direction will fall on the contrary, and fracture strength will fall. In addition, the stress corrosion cracking sensitivity is increased. Therefore, the Zn content when selectively contained is 0.001 to 1.0 mass%.

Fe, Ni, Mn, Mg, Cr, Ti, Zr and Ag:
In the Sn-P-based copper pipe, by selectively containing one or more selected from the group consisting of Fe, Ni, Mn, Mg, Cr, Ti, Zr, and Ag, It is possible to improve the bending workability by improving the strength, pressure fracture strength, and softening resistance of the alloy, and refining the crystal grains. However, if the content of one or more elements selected from these elements exceeds 0.07% by mass in total, the extrusion pressure rises, so the same pressing as that without adding these elements. When extrusion is performed with output, it is necessary to increase the hot extrusion temperature. Thereby, since the surface oxidation of the extruded material increases, surface defects frequently occur in the copper tube of the present invention, and in particular, the fracture strength of the thinned copper tube as a heat transfer tube cannot be improved. For this reason, when it is selectively contained, one or more elements selected from the group consisting of Fe, Ni, Mn, Mg, Cr, Ti, Zr, Zr, and Ag are contained. The total content is less than 0.07% by mass (however, 0% by mass is not included). The content is desirably less than 0.05% by mass, and more desirably less than 0.03% by mass.

impurities:
The other elements are impurities, and the content is preferably as small as possible in order to improve the breaking strength of the Sn-P copper pipe, particularly the thinned copper pipe, as the heat transfer pipe. However, there is also a trade-off with the cost for reducing these impurities, and the practical allowable amounts (upper limit amounts) of typical impurity elements are shown below.

S:
In the Sn-P copper pipe, S forms a compound with Cu and exists in the parent phase. When the blending ratio of low-grade copper ingots and scraps used as raw materials increases, the S content increases. S promotes ingot cracking and hot extrusion cracking during ingots. Further, when the extruded material is cold-rolled or drawn, the Cu—S compound expands in the axial direction of the pipe, and cracks are likely to occur at the interface between the copper alloy matrix and the Cu—S compound. For this reason, it is easy to become a surface flaw, a crack, etc. in the half-finished product and the product after processing, and especially the fracture strength as a heat transfer tube of the thinned copper tube is reduced. Further, when the pipe is bent, it becomes a starting point of occurrence of cracks, and the frequency of occurrence of cracks at the bent portion increases. Therefore, the S content is 0.005% by mass or less, desirably 0.003% by mass or less, and more desirably 0.0015% by mass or less. In order to reduce the S content, reduce the amount of low-grade Cu ingots and scrap used, reduce the SOx gas in the melting atmosphere, select appropriate furnace materials, and have an affinity for S such as Mg and Ca. Measures such as adding trace amounts of strong elements to the molten metal are effective.

As, Bi, Sb, Pb, Se, Te etc .:
In the Sn-P based copper pipe, the soundness of the ingot, hot extruded material, and cold worked material is similarly reduced for impurity elements As, Bi, Sb, Pb, Se, Te, etc. other than S, In particular, the breaking strength of a thinned copper tube as a heat transfer tube is reduced. Therefore, the total content (total amount) of these elements is preferably 0.0015% by mass or less, desirably 0.0010% by mass or less, and more desirably 0.0005% by mass or less.

O:
In the Sn-P-based copper tube, when the O content exceeds 0.005 mass%, the Cu or Sn oxide is caught in the ingot, and the soundness of the ingot is lowered, and particularly the thinned copper Reduces the breaking strength of the tube as a heat transfer tube. For this reason, the content of O is preferably 0.005% by mass or less. In order to further improve the bending workability, the O content is desirably 0.003% by mass or less, and more desirably 0.0015% by mass or less.

H:
In the Sn-P-based copper pipe, when the amount of hydrogen (H) taken into the molten metal at the time of melting and casting increases, hydrogen whose solid solution amount has decreased at the time of solidification precipitates at the grain boundary of the ingot, forming a large number of pinholes, Cracks are generated during hot extrusion. In addition, when a rolled and drawn copper tube is annealed even after extrusion, H is concentrated at the grain boundary during annealing, and this tends to cause blistering, particularly as a heat transfer tube of a thinned copper tube. Reduces fracture strength. For this reason, it is preferable to make content of H 0.0002 mass% or less. In order to further improve the fracture strength including the product yield, the H content is preferably 0.0001% by mass or less. In order to reduce the H content, measures such as drying of the raw material during melting and casting, red hotness of the molten-coating charcoal, reduction of the dew point of the atmosphere in contact with the molten metal, and making the molten metal before the addition of phosphorus feel oxidized It is valid.

Copper tube grain structure:
The crystal grain structure of the copper alloy tube of the present invention is a measurement result by the SEM-EBSP method, and the average crystal grain size D1 in the cross section in the direction parallel to the axial direction of the copper alloy tube is 20 μm or less. The difference D1-D2 from the average crystal grain size D2 in the cross section perpendicular to the axial direction of the copper alloy tube is set to 3 μm or less (including 0 μm).

  The right side of FIG. 1 shows an example of the average crystal grain size D1 of a cross section (in the direction) parallel to the axial direction of the copper alloy pipe indicated by the alternate long and short dash line of the copper alloy pipe (copper pipe). The left side of FIG. 1 shows an example of the average crystal grain size D2 of the cross section perpendicular to (in the direction of) the axis of the copper alloy tube indicated by the alternate long and short dash line of the copper alloy tube (copper tube). The square frames described in each of these are not pinpoints according to the SEM-EBSP method described later, the width of the frame (short side) = thickness of copper tube mm × the length of the frame (long side) = 1. A wide measuring range (area) of 5 mm is shown. Therefore, the definition of the average crystal grain size of the present invention more accurately reflects the average crystal grain size characteristic of the copper tube than the local or pinpoint measurement using an optical microscope or the like.

(Average crystal grain size)
It is known that the smaller the crystal grain size in the copper tube structure, the better the fracture strength and the bending workability balance. Also in the present invention, by utilizing this mechanism, the above-mentioned D1 and D2 of the average crystal grain size are refined together with the anisotropy of crystal grains described later. That is, in the measurement result by the SEM-EBSP method to be described later, the average crystal grain size D1 in the cross section (direction) parallel to the axial direction of the copper alloy tube of the Sn—P based copper tube structure is 20 μm or less. To improve the balance between fracture strength and bending workability.

  Incidentally, when the thickness of the copper tube is relatively thick, the average crystal grain size has little influence on the fracture strength and the bending workability balance. However, when the thickness of the heat transfer tube is reduced to 1.0 mm or less due to demands for weight reduction and thinning, the influence of the crystal grain size on the fracture strength and bending workability balance is significant. growing. When the average crystal grain size is too large exceeding the upper limit, “strain concentration” when cracks are generated by the tensile force applied to the heat transfer tubes cannot be avoided, and cracks tend to occur in the heat transfer tubes. For this reason, the reliability when the copper tube for heat exchangers using the above-described alternative refrigerant having a high operating pressure is lowered. Moreover, when the crystal grain size becomes coarse, there is a problem that cracks are likely to occur in the bent portion when the copper tube is bent.

  Furthermore, when the copper tube is processed into a heat exchanger, the crystal grain size of the heated portion of the heat transfer tube is necessarily coarsened due to the heat effect of brazing exposed to a high temperature of 800 ° C. or higher. On the other hand, if the average crystal grain size of the copper tube is not refined in the above-mentioned range in advance, the average crystal grain size is likely to exceed 100 μm due to coarsening, or the pressure strength at the brazed portion is high. Decrease increases and softening resistance decreases.

(Anisotropy of crystal grains)
The difference D1-D2 between the average crystal grain size D1 in the cross section (direction) parallel to the axial direction of the copper alloy tube and the average crystal grain size D2 in the cross section (direction) perpendicular to the axial direction of the copper alloy tube is The smaller (0 μm) or 3 μm or less, the smaller the anisotropy of the average crystal grain size due to the copper tube part (cross-sectional direction), and the softening resistance improves. Therefore, in the present invention, the average crystal grain size D1 of the cross section parallel to the axial direction of the copper alloy tube is refined to 20 μm or less, and the difference D1-D2 between D1 and D2 is further set to 3 μm or less ( 0 μm).

  Here, the difference “D1−D2” between D1 and D2 of the copper alloy tube is, as described above, the vertical and horizontal directions of the same crystal grains such as “aspect ratio, flatness or roundness” that are usually used. It is not a ratio of length. In other words, the cross-section direction of the pipe is different from each other in the cross-section parallel to the axial direction of the copper alloy pipe and the cross-section perpendicular to the axial direction, which is parallel to the rolling or drawing direction and the crystal grains are easily elongated. It is a comparison of the average crystal grain size. Therefore, in the present invention, it is expressed as anisotropy of crystal grains due to the tube portion (or cross-sectional direction).

  In a normal manufacturing method, the average crystal grain size D1 that is parallel to the rolling or drawing direction and the crystal grains tend to extend is inevitably larger than the average crystal grain size D2, and the difference between D1 and D2 “D1-D2” never becomes negative. However, in the present invention, even when the difference “D1−D2” between D1 and D2 becomes negative (in the case of D1 <D2) as a copper tube having a small crystal grain anisotropy, 3 μm is defined. The following can be interpreted as absolute values and included in the range.

  As described above, the difference D1-D2 between D1 and D2 is the difference between the crystal grain size in the direction parallel to the axial direction of the copper alloy tube and the crystal grain size in the direction perpendicular to the axial direction of the copper alloy tube. That is, it represents the anisotropy of the crystal grain size. That is, as the difference D1-D2 is smaller, the structure of the copper alloy tube is a crystal structure mainly composed of equiaxed crystal grains free from distortion due to the formation of recrystallization nuclei. Such a crystal structure mainly composed of equiaxed crystal grains without distortion due to the formation of recrystallized nuclei has a very low dislocation density and is excellent in strength and workability. For this reason, even if the wall thickness of the copper pipe is reduced, it is possible to ensure a predetermined breaking strength without increasing the tensile strength so much, and bending the pipe by the margin of this tensile strength. Can be improved. Further, the more equiaxed crystal grains without distortion due to the formation of recrystallization nuclei, the more the growth of crystal grains of the copper alloy tube structure is suppressed during the brazing, and the softening resistance is excellent.

  On the other hand, as the difference D1-D2 increases beyond 3 μm, the structure of the copper alloy tube is a crystal grain formed by releasing the strain by recovery, that is, the axis of the copper tube in the rolling direction or the drawing direction. This indicates that the crystal structure is mainly composed of crystal grains elongated in the direction. When this difference D1-D2 exceeds 3 μm, the number of crystal grains generated by the recovery of strain increases, and conversely, evidence that the number of crystal grains without distortion due to the formation of recrystallization nuclei decreases. It is.

  In such a structure, the dislocation density of the copper alloy tube structure is high, local strain is likely to occur, the ratio of fracture strength to tensile strength is lowered, and the balance between fracture strength and bending workability is reduced. End up. Further, the crystal grains whose strain is continuously released by recovery have the property of preferentially growing over the crystal grains without distortion due to the formation of recrystallized nuclei during the high temperature heating during brazing. For this reason, as the number of crystal grains increases and the difference D1-D2 exceeds 3 μm, the crystal grain size after brazing becomes remarkably large, and softening resistance such as strength and breaking pressure after brazing. Will be reduced.

  As described above, not only the refinement of the crystal grain size but also the origin and shape of the fine crystal grains themselves or the anisotropy of the fine crystal grains greatly affect the softening resistance. The main component is crystal grains that are generated by releasing strain due to recovery, that is, crystal grains that are elongated in the axial direction of the copper tube in the rolling direction or drawing direction, for example, even if they are fine crystal grains. In particular, the softening resistance is not improved.

Grain control method:
In order to control the crystal grains of the copper tube structure as defined in the present invention, it is necessary to reduce the crystal grains generated by the recovery and increase the crystal grains without distortion due to the formation of recrystallization nuclei. For this purpose, intermediate annealing, which is not normally performed, is performed during the drawing process, and recrystallization is performed once by this intermediate annealing to reduce the crystal grain size, and then the final annealing is performed. According to the present invention, a Sn-P-based copper alloy tube for a heat exchanger that is improved in softening resistance and excellent in fracture strength and bending workability even when thinned to 1.0 mm or less is thus obtained. There is also an advantage that it is easy to manufacture.

  In contrast, thin copper alloy tubes for ordinary heat exchangers are processed with a reduction in area of 95% or more in the rolling and drawing processes after hot extrusion when thinned to 1.0 mm or less. Therefore, a large strain is introduced in these rolling processes and drawing processes. In addition, from the viewpoint of production efficiency, normally, intermediate annealing is not performed during the drawing process, so that the final annealing process is carried over while the crystal grain size remains large. For this reason, the crystal grains produced in the final annealing step of the conventional Sn-P-based copper alloy tube are inevitably produced more easily because the crystal grains produced by the continuous release of strain are generated. Therefore, in the conventional Sn-P-based copper alloy tube, the D1-D2 is likely to exceed 3 μm, and therefore the softening resistance is particularly inferior.

Method for measuring average grain size:
The average crystal grain size D1 and D2 is a crystal orientation analysis method in which a field emission scanning electron microscope (FESEM) is equipped with a backscattered electron diffraction image (EBSP: Electron Back Scattering (Scattered) Pattern) system. To measure each. At this time, as described above, a wide measurement range (region) within each square frame illustrated in FIG. 1 is measured. However, the measurement positions of these D1 and D2 do not necessarily need to be measured at the same length position (site) in the copper pipe, even if the positions (sites) in the longitudinal direction of the copper pipe are different from each other, or Even if they are far apart from each other, the material of the copper tube is uniform, so it does not matter. However, it is preferable to avoid only the both ends in the longitudinal direction of the copper pipe to be manufactured.

  In the EBSP method, an electron beam is irradiated onto a sample set in a FESEM column and the EBSP is projected onto a screen. This is taken with a high-sensitivity camera and captured as an image on a computer. In the computer, the orientation of the crystal is determined by analyzing this image and comparing it with a pattern obtained by simulation using a known crystal system. The calculated crystal orientation is recorded as a three-dimensional Euler angle together with position coordinates (x, y) and the like. Since this process is automatically performed for all measurement points, tens of thousands to hundreds of thousands of crystal orientation data can be obtained at the end of measurement.

  Here, in the case of a normal copper alloy plate, it mainly forms a texture composed of many orientation factors called Cube orientation, Goss orientation, Brass orientation, Copper orientation, S orientation, etc., and a crystal plane corresponding to them Exists. These facts are described in, for example, edited by Shinichi Nagashima, “Aggregate” (published by Maruzen Co., Ltd.) and “Light Metal”, Vol. 43, 1993, P285-293, published by the Japan Institute of Light Metals. The copper pipe of the present invention is manufactured by extrusion / rolling / drawing. In this case as well, as in the case of the texture of the plate material by rolling, the extrusion surface and the extrusion direction of the extrusion element pipe (the extrusion element tube is rolled) In this case, the extrusion surface is expressed by {ABC}, and the extrusion direction is expressed by <DEF>.

  In the present invention, basically, those whose orientations deviate within ± 15 ° from these crystal planes belong to the same crystal plane (orientation factor), and the orientation difference between adjacent crystal grains is 5 °. The above crystal grain boundary is defined as a crystal grain boundary.

  In addition, in the present invention, an electron beam is irradiated at a pitch of 1.0 μm with respect to a measurement area, tube axis direction 1000 × tube circumferential direction 800 μm, and a wide area within each square frame shown in FIG. Each measuring range (area). Then, when the number of crystal grains measured by the crystal orientation analysis method is n and each measured crystal grain size is x, the average crystal grain size is calculated as (Σx) / n.

Copper tube manufacturing method:
The method for producing a copper tube of the present invention will be described below by taking a smooth tube as an example. The Sn-P-based copper pipe of the present invention can be manufactured by a conventional method in the basic process itself, but in order to make the crystal grain structure of the copper pipe into the above-mentioned perfect circular crystal grains, in the drawing process It is particularly necessary to perform intermediate annealing. Below, each process is demonstrated concretely.

  First, the raw electrolytic copper is dissolved in a charcoal-coated state, and after the copper is dissolved, a predetermined amount of alloy element is added so that a predetermined Sn-P-based copper alloy composition is obtained. At this time, it is preferable to add P as a Cu-15 mass% P intermediate alloy also for deoxidation. In addition, in the Sn—P based copper alloy, a Cu—Sn—P master alloy can be used instead of the Sn and Cu—P master alloy. After these component adjustments are completed, billets having predetermined dimensions are produced by semi-continuous casting. The obtained billet is heated in a heating furnace and homogenized. Before hot extrusion, it is desirable to improve segregation by homogenization by holding the billet at 750 to 950 ° C. for about 1 minute to 2 hours.

  Thereafter, the billet is perforated by piercing and hot extruded at 750 to 950 ° C. At this time, it is necessary to eliminate Sn segregation in the Sn—P based copper pipe and to refine the structure of the product pipe. For this purpose, the reduction rate of the cross-section of the Sn-P-based copper tube by hot extrusion ([perforated billet donut area-cross-sectional area of the base tube after hot extrusion] / [perforated billet donut shape] Area] × 100%) is 88% or more, desirably 93% or more, and further, the cooling rate until the surface temperature reaches 300 ° C. is 10 ° C./second or more by a method such as water cooling. The cooling is preferably performed at 15 ° C./second or more, more preferably 20 ° C./second or more.

  Next, the extruded element tube is rolled to reduce the outer diameter and thickness. By setting the processing rate at this time to 92% or less in terms of the cross-sectional reduction rate, product defects during rolling can be reduced. The rolling blank is subjected to drawing to produce a blank having a predetermined size and thickness. When the thickness is reduced to 1.0 mm or less during the drawing process, the total processing rate is set to 95% or more in terms of the cross-sectional reduction rate. At this time, the drawing process is usually performed using a plurality of drawing machines. By setting the processing rate (cross-sectional reduction rate) by each drawing machine to 35% or less, surface defects and internal cracks in the raw pipe can be reduced.

  During or after the drawing process, intermediate annealing is performed at a temperature range of 400 ° C. to 700 ° C. for 2 minutes to 1 hour. When the intermediate annealing temperature is lower than 300 ° C., recrystallization does not occur in the intermediate annealing step, and the final annealing step is carried over with the crystal grain size being large. For this reason, as with conventional Sn-P-based copper alloy tubes, the crystal grains produced in the final annealing step are more likely to be produced by the continuous release of strain, and the D1-D2 is It tends to be elongated grains exceeding 3 μm and is inferior in softening resistance.

  On the other hand, when the intermediate annealing temperature is 700 ° C. or higher, the crystal grain size becomes coarse, and the converted stress of the fracture pressure is too low at 230 MPa or less.

  After this intermediate annealing, drawing is further performed to manufacture a smooth tube. The cross-sectional area reduction ratio after this intermediate annealing is set to 35% or more and 80% or less. If the area reduction rate is lower than 35%, the accumulated strain amount is too small, and the driving force necessary for generating recrystallized nuclei cannot be increased. For this reason, in the subsequent final annealing, it becomes difficult to produce equiaxed crystal grains without distortion due to the formation of recrystallization nuclei, and the D1-D2 tends to be elongated grains exceeding 3 μm, and the softening resistance is poor. It becomes. On the other hand, if the area reduction rate exceeds 80% and is too high, the outer diameter of the final copper tube becomes too small, and the thickness of the copper tube becomes too thin, so that it cannot withstand the internal pressure of the refrigerant.

  Moreover, it is preferable that the temperature increase rate in the range of 300 to 400 ° C. is increased to 150 ° C./min or more during the temperature increase of the intermediate annealing. In this temperature range, the reduction in dislocation density due to recovery is significant, and it is important to increase the rate of temperature rise so that recovery does not occur as much as possible. On the other hand, since recrystallization nuclei start to be generated at temperatures higher than 400 ° C., the temperature increase rate has little influence on the variation of crystal grains at temperatures higher than this, and it is not necessary to increase the temperature. The heating furnace used for the intermediate annealing can use an induction heater, increase the set temperature to a high temperature, and shorten the holding time to increase the heating rate in the range of 300 to 400 ° C.

  After this drawing process, a final annealing process is performed on the drawing element tube. In order to continuously anneal a copper pipe, heating by a high-frequency induction coil is performed by passing a drawing element pipe through the coil while energizing a roller hearth furnace or a high-frequency induction coil normally used for annealing a copper pipe coil or the like. Can be used.

  In order to manufacture the copper tube of the present invention using a roller hearth furnace, annealing is performed so that the actual temperature of the drawn element tube is 350 to 700 ° C., and the drawn element tube is heated for about 1 to 120 minutes at that temperature. Is desirable. When the body temperature of the drawn element tube is lower than 350 ° C., a complete recrystallized structure is not formed, and a fibrous processed structure remains, which makes it difficult for a customer to perform bending. Moreover, when the temperature exceeds 700 ° C., the crystal grains become coarse, and the bending workability of the tube is lowered. Therefore, it is desirable to anneal the drawing tube at a solid temperature of 350 to 700 ° C.

  In addition, when the heating time in this temperature range is shorter than 1 minute, a complete recrystallized structure is not obtained, and thus the above-described problem occurs. Further, even if annealing is performed for more than 120 minutes, the crystal grain size does not change, and the effect of annealing is saturated. For this reason, the heating time in the temperature range is suitably 1 minute to 120 minutes.

  The smooth tube manufacturing method has been described above. However, the smooth annealed smooth tube may be subjected to drawing processing at various processing rates as necessary to obtain a processed tube with improved tensile strength. In the case of an internally grooved tube, the smooth tube is subjected to grooved rolling to produce an internally grooved tube, followed by final annealing. Further, the annealed inner surface grooved tube may be subjected to a drawing process at a light processing rate as necessary to improve the tensile strength.

  Examples of the present invention will be described below. Various Sn-P-based copper tubes with different crystal grain structures were manufactured as smooth tubes with various chemical compositions shown in Table 1 and production conditions shown in Table 2 (whether or not intermediate annealing in drawing).

  An average crystal grain size D1 in a cross section parallel to the axial direction of the copper alloy tube and an average crystal grain size D2 in a cross section perpendicular to the axial direction were measured by the SEM-EBSP method, respectively, and the difference D1 between D1 and D2 -D2 (μm) was also determined. The tensile strength, fracture strength, and bending workability of these copper tubes were also measured and evaluated. These results are also shown in Table 1. Specific manufacturing methods, measurements, and evaluation methods for these Sn-P-based copper tubes (smooth tubes) are as follows.

(Smooth tube manufacturing conditions)
(A) Using electrolytic copper as a raw material, Sn—P-based copper pipe is to add a predetermined Sn to the molten metal, and optionally add additional additive elements, and then add a Cu—P master alloy. Thus, a molten metal having a predetermined composition was produced. The component composition of these molten copper alloys was defined as the component composition of the copper tube.

(B) An ingot having a diameter of 300 mm and a length of 6500 mm was semi-continuously cast at a casting temperature of 1200 ° C., and a billet having a length of 450 mm was cut out from the obtained ingot.

(C) After heating the billet to 650 ° C. with a billet heater, the billet is heated to 950 ° C. with a heating furnace (induction heater). After reaching 950 ° C., after 2 minutes, the billet is taken out from the heating furnace, Immediately after the piercing process with a diameter of 80 mm at the center of the billet (without delay), an extruded element tube having an outer diameter of 96 mm and a wall thickness of 9.5 mm was produced with the same hot extruder (cross-sectional reduction rate: 96.6). %). The average cooling rate to 300 ° C. of the extruded tube after hot extrusion was 40 ° C./second.

(D) The extruded element tube is rolled to produce a rolled element tube having an outer diameter of 35 mm and a wall thickness of 2.3 mm, and the rolling element tube has a cross-sectional reduction rate of 35% or less in one drawing process. Then, drawing and drawing were performed to obtain an outer diameter of 22 mm, a wall thickness of 1.2 mm to an outer diameter of 12 mm, and a wall thickness of 0.95 mm.

(E) After that, as intermediate annealing, heating to a temperature shown in Table 1 in a heating furnace (induction heater), holding at this temperature for 30 minutes, passing through a cooling zone and gradually cooling to room temperature, test A material was used. In the case of the temperature increase of the intermediate annealing, the temperature increase rate in the range of 300 to 400 ° C. is commonly increased to about 200 ° C./min in the range of 300 to 400 ° C. in each example subjected to the intermediate annealing. .

(F) Thereafter, drawing and drawing were performed, and copper tubes having an outer diameter of 9.52 mm and a wall thickness of 0.80 mm and various cross-sectional area reduction rates were produced. Table 1 shows the cross-sectional area reduction ratio (%).

(G) As final annealing, in the reducing gas atmosphere in an annealing furnace, the drawing tube was annealed at the temperature shown in Table 1 (average heating rate is 12 ° C./min) and holding time, The sample was allowed to pass through a cooling zone and cooled to room temperature at a cooling rate to room temperature shown in Table 1 and a cooling rate between 300 to 400 ° C. to obtain a test material.

(H) The average crystal grain diameters D1 and D2 of these manufactured copper tubes (outer diameter 9.52 mm, wall thickness 0.80 mm), copper tube structure, tensile strength, fracture strength, bending workability of this difference D1-D2 Table 2 shows the properties of copper pipes. In Table 1, the S content of the copper tube is 0.005 mass% or less in common with each of the examples of the invention and the comparative example, and the total content of As, Bi, Sb, Pb, Se, Te ( The total amount) was 0.0005 mass% or less, the O content was 0.003 mass% or less, and the H content was 0.0001 mass% or less.

(Grain structure)
The average crystal grain diameters D1 and D2 in the crystal grain structure of the manufactured copper tube were measured in the square frames shown in FIG. 1 by the crystal orientation analysis method in which the EBSP system was mounted on the SEM. The measurement range of these square frames was a measurement range (region) of frame width (short side) = copper tube thickness 0.80 mm × frame length (long side) = 1.5 mm.
Of these measurement results, FIG. 2 shows Invention Example 1 in Table 1, FIG. 3 shows Comparative Example 7 in Table 1, FIG. 4 shows Comparative Example 2, and FIG. Each is shown. In these FIGS. 2-5, the average crystal grain size D1 was measured, the cross section parallel to the axial direction of the copper tube (described as parallel D1) is on the right side, and the average crystal grain size D2 was measured in the axial direction of the copper tube. Vertical sections (denoted as vertical D2) are shown on the left, respectively.

(Tensile test)
A tensile test (MPa) was performed on the specimen under the conditions of room temperature, test speed 10.0 mm / min, GL = 50 mm, using a JIS No. 11 test piece and a 5882 type Instron universal testing machine. ) Was measured. Three test pieces under the same conditions were tested, and the average value thereof was adopted.

(destruction strength)
A copper tube having a length of 500 mm was taken from the manufactured copper tube for testing, and one end of the copper tube was closed in a pressure-resistant manner with a metal jig (bolt). Then, from the other open side end, the water pressure loaded into the pipe by the pump is gradually increased (pressure increase rate: about 1.5 MPa / second), and the water pressure (MPa) when the pipe completely ruptures. Was read with a Bourdon tube pressure gauge and used as the breaking strength (pressure resistance, pressure resistance, breaking pressure) of the heat transfer tube. This test was performed five times on the same copper tube (for five test tubes), and the average value of each water pressure (MPa) was defined as the fracture strength at room temperature. Moreover, the conversion stress which remove | eliminated the influence of the thickness and outer diameter of a copper pipe from fracture strength was calculated | required as fracture pressure. Here, the converted stress σ was obtained from the following equation when the fracture strength was P, the outer diameter of the copper tube was D, and the thickness of the copper tube was t.
σ = P × (D−0.8t) / (2 × t)

(Softening resistance)
Simulating that a copper alloy tube is brazed as a heat exchanger tube for a heat exchanger, a 500 mm long copper alloy tube taken for testing from the manufactured copper alloy tube was heated to 800 ° C. for 10 minutes. The breaking strength (breaking pressure) was determined as the breaking pressure after brazing equivalent heating by the same test method described above.

(Bending test)
Simulating the heat transfer part of the heat exchanger, the manufactured copper alloy tubes were bent into 10 U-shapes with a pitch of 40 mm and into U-shapes with a pitch of 30 mm for each example. At this time, the occurrence of cracks and cracks in the bent portion of the copper alloy tube was visually examined, and all the ten pieces that could be bent without any cracks or cracks were evaluated as ◯ having good bending workability. In addition, there are no cracks or cracks in all of the ten pieces, but wrinkles are generated, the bending radius is smaller, and when bending conditions are strict, bending and cracking may occur. It evaluated as (triangle | delta) inferior in property. Furthermore, out of 10 bent pieces, those in which even one crack or crack occurred were evaluated as x with poor bending workability.

(Invention example)
As shown in Table 1, since the inventive compositions 1 to 13 are within the scope of the present invention and the drawing (intermediate annealing) conditions are appropriate, this copper alloy tube structure is a measurement result by the SEM-EBSP method. The average crystal grain size D1 in the cross section parallel to the axial direction of the alloy tube is 20 μm or less, and the difference D1-D2 between this D1 and the average crystal grain size D2 in the cross section perpendicular to the axial direction of the copper alloy tube is 3 μm. It is as follows. For this reason, the copper tube has good bending workability despite its high tensile strength, and also has a low resistance to breakage after heating equivalent to brazing and has excellent softening resistance. .

  As a representative of these invention examples, FIG. 2 shows the copper tube crystal grain structure of Invention Example 1. As shown in FIG. 2, a cross section parallel to the axial direction of the copper tube in which the right average crystal grain size D1 was measured (parallel D1), and a cross section perpendicular to the axial direction of the copper tube in which the left average crystal grain size D2 was measured. Both (vertical D2) have the same fine and equiaxed crystal grains. Therefore, it can be seen that the example of the invention has a small crystal grain anisotropy depending on the tube part or the cross-sectional direction of the tube, and the average crystal grain size D1 of the present invention or the difference D1 between the D1 and the average crystal grain size D2 The significance of the definition of -D2 is supported from FIG.

(Comparative example)
On the other hand, as shown in Table 1, Comparative Examples 7 to 12 are alloys within the composition range of the present invention, but the drawing (intermediate annealing) conditions and the final annealing conditions are not in an appropriate range.

  In Comparative Example 7, intermediate annealing in the middle of drawing is not performed, and in Comparative Example 8, the intermediate annealing temperature in the middle of drawing is too low. For this reason, said D1-D2 exceeds 3 micrometers, intensity | strength is low, the fall of the fracture strength after the heating equivalent to brazing is also large, and softening resistance is inferior.

  In Comparative Example 8, the intermediate annealing temperature during drawing is too low. For this reason, as in Comparative Example 7, the D1-D2 exceeds 3 μm, the strength is low, the fracture strength after heating corresponding to brazing is greatly reduced, and the softening resistance is inferior.

  In Comparative Example 9, the intermediate annealing temperature during drawing is too high. For this reason, although D1-D2 is 3 μm or less, the average crystal grain size D1 is too large, the strength is low, and the breaking pressure is too low.

  In Comparative Example 10, the final annealing temperature is too low. For this reason, said D1-D2 exceeds 3 micrometers, the fall of the fracture strength after the heating equivalent to brazing is also large, and softening resistance is inferior.

  In Comparative Example 11, the final annealing temperature is too high. For this reason, although D1-D2 is 3 μm or less, the average crystal grain size D1 is too large, the strength is low, and the breaking pressure is too low.

  In Comparative Example 12, the area reduction rate (working rate) of drawing after intermediate annealing in the middle of drawing is too small. For this reason, the amount of accumulated strain is too small, and the driving force required for the formation of recrystallized nuclei cannot be increased. For this reason, in the subsequent final annealing, it becomes difficult to generate equiaxed crystal grains without distortion due to the formation of recrystallization nuclei, the D1-D2 exceeds 3 μm, the average crystal grain size D1 is too large, and is equivalent to brazing. The fracture strength after heating is greatly reduced, and the softening resistance is inferior.

  In Comparative Examples 1 to 6, as shown in Table 1, the drawing (intermediate annealing) conditions and the final annealing conditions are in an appropriate range, but the composition of the copper tube is outside the scope of the present invention.

  In Comparative Examples 1 and 2, since the Sn content is too much less than the specified range, the D1-D2 exceeds 3 μm, the strength is low, regardless of the presence or absence of intermediate annealing in the middle of drawing, after heating corresponding to brazing. The decrease in fracture strength is also large and the softening resistance is poor.

  In Comparative Example 3, since the Sn content was too high, extrusion processing could not be performed and a copper tube could not be manufactured. In Comparative Example 4, since the P content was too high, cracks occurred after extrusion, and a copper tube could not be produced.

  In Comparative Example 5, since the P content is too much less than the specified range, the D1-D2 is 3 μm or less, but the strength is low and the breaking pressure is too low.

  In Comparative Example 6, the Zn content is too high, and thus D1-D2 is 3 μm or less, but the bending workability is low.

  Among these comparative examples, FIG. 3 shows the comparative example 7 of Table 1, FIG. 4 shows the comparative example 2, and FIG. As shown in these figures, both the cross section parallel to the axial direction of the copper pipe on the right side (parallel D1) and the cross section perpendicular to the axial direction of the left side (vertical D2) are crystal grains as compared to FIG. The diameter is coarse. In particular, the crystal grains of the cross section (parallel D1) parallel to the axial direction of the right copper tube are not only coarse, but also extend in the axial direction of the copper tube. Therefore, it can be seen that these comparative examples have large crystal grain anisotropy depending on the tube portion or the cross-sectional direction of the tube. Therefore, together with the above-described FIG. 1, the significance of the definition of the average crystal grain size D1 of the present invention and the difference D1-D2 between the D1 and the average crystal grain size D2 is supported.

  From the above results, it is easy to manufacture, and even if the thickness is reduced to 1.0 mm or less, the softening resistance is improved, and at the same time, an Sn-P-based copper alloy tube for a heat exchanger excellent in fracture strength and bending workability is obtained. Therefore, the component composition of the present invention, the definition of the crystal grain structure, and the significance of preferable production conditions for obtaining such a structure are supported.

  As described above, according to the present invention, it is easy to manufacture, excellent in softening resistance even when thinned to 1.0 mm or less, excellent in bending workability, thinned and strengthened Sn- A P-based copper alloy tube can be provided. As a result, it can be suitably applied to a heat exchanger tube for a heat exchanger that is used after being thinned to a high operating pressure by a new alternative refrigerant.

Claims (3)

A copper alloy tube containing Sn: 0.1 to 3.0% by mass, P: 0.005 to 0.1% by mass, with the balance being composed of Cu and inevitable impurities, The structure is a measurement result by the SEM-EBSP method, and the average crystal grain size D1 in the cross section parallel to the axial direction of the copper alloy tube is 20 μm or less, and the cross section perpendicular to the axial direction of D1 and the copper alloy tube A copper alloy tube for a heat exchanger, characterized in that the absolute value | D1-D2 | of the difference from the average crystal grain size D2 is 3 μm or less (including 0 μm).   The copper alloy tube for a heat exchanger according to claim 1, wherein the copper alloy tube further contains Zn: 0.01 to 1.0 mass%.   The copper alloy tube further contains one or more elements selected from the group consisting of Fe, Ni, Mn, Mg, Cr, Ti, Co, Zr and Ag in total less than 0.07% by mass (provided that The copper alloy tube for a heat exchanger according to claim 1 or 2, which contains (excluding 0%).

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