Internally linearly grooved extruded tube stock and internally helically grooved tube

Abstract

The present invention provides an internally linearly grooved extruded tube stock for manufacturing an internally helically grooved tube. The internally linearly grooved extruded tube stock is characterized by being formed of an aluminum alloy that has a composition which contains, in mass%, 0.05-0.35% of Cr, 0.05-0.5% of Fe, and 0.05-0.8% of Si, with the balance being made up of Al and unavoidable impurities, and is also characterized in that Fe and Si satisfy the relational expression 0.8Fe ≤ Si. In addition, in an observation field of view per 10,000 µm2 of the ED-TD parallel plane, it is preferable to have five or more Cr-containing intermetallic compounds that have a diameter of not less than 0.01 µm but less than 5.00 µm in terms of equivalent circle diameter in the observation field of view.

Description

Extruded tubes with internal straight grooves and internal spiral grooves

[0001] The present invention relates to an extruded tube with an internal straight groove and a tube with an internal spiral groove. This application claims priority based on Japanese Patent Application No. 2024-171079, filed in Japan on September 30, 2024, the contents of which are incorporated herein by reference.

[0002] Air conditioning heat exchangers generally consist of fins and heat transfer tubes, with the fins often made of aluminum and the heat transfer tubes of copper. In recent years, due to rising copper prices and the risk of resource depletion, there has been a growing need to substitute materials with aluminum. To ensure the heat conduction performance of the heat transfer tubes, technological development is underway to create aluminum tubes with internal spiral grooves.

[0003] Patent Document 1 discloses a method for manufacturing laminated metal materials, in which a surface treatment agent is applied to the surface of an aluminum alloy to form a corrosion-resistant oxide film, and then a resin film is laminated onto the oxide film to further improve corrosion resistance. Patent Document 2 discloses an extruded aluminum-zinc alloy, in which an anodic oxide film that exhibits excellent corrosion resistance even in environments with water vapor is formed by specifying the number of inclusions, which are oxide films. Patent Document 3 discloses an aluminum tube made of a hollow extruded material, in which a diffusion layer having a concentration distribution of metals less base than aluminum is formed on the tube surface, and a technique for obtaining corrosion resistance is described in which the base metal concentration distribution, surface concentration, and average surface concentration of the diffusion layer are specified.

[0004] WO2011 / 052520 Japanese Patent Publication No. 2014-037557 Japanese Patent Publication No. 2004-324998

[0005] However, conventional corrosion protection technology using Zn thermal spraying has problems such as reduced yield due to uneven spraying and increased costs, as well as reduced recyclability due to the excessive Zn content. Therefore, corrosion protection technology that does not rely on Zn thermal spraying was needed.

[0006] In view of the above-mentioned problems, the present invention aims to provide a technology that exhibits excellent corrosion resistance by controlling the composition ratio of materials containing Cr, Fe, and Si to minimize the cathode reaction during corrosion, and by generating a highly protective Cr-containing film when minor corrosion occurs in the material.

[0007] (1) An extruded tube with an inner linear groove according to one embodiment of the present invention is an extruded tube with an inner linear groove for manufacturing an inner spiral grooved tube, and is made of an aluminum alloy having a composition of Cr: 0.05 to 0.35%, Fe: 0.05 to 0.5%, Si: 0.05 to 0.8% by mass%, with the remainder being Al and unavoidable impurities, and is characterized in that Fe and Si satisfy the relationship 0.8Fe ≤ Si. (2) An extruded tube with an inner linear groove according to one embodiment of the present invention, wherein the ED-TD parallel surface is 10000 μm ２ In the observation field, it is preferable to have five or more intermetallic compounds containing Cr having a diameter of 0.01 μm or more and less than 5.00 μm in equivalent circular diameter. However, ED is the extrusion direction, TD is the transverse direction perpendicular to ED and the thickness direction, and the ED-TD parallel surface means a surface parallel to ED and TD that is formed by scraping the surface of the extruded tube.

[0008] (3) In the extruded tube with an inner surface straight groove according to one embodiment of the present invention, it is preferable that, in addition to the above composition, the aluminum alloy contains 0.01 to 0.2% by mass of Ti, with the remainder being Al and unavoidable impurities.

[0009] (4) In the extruded tube with an inner surface straight groove according to one embodiment of the present invention, it is preferable that in addition to the above composition, Cu is included, the Cu content is restricted to 0.05% or less by mass%, and the remainder is made of an aluminum alloy having a composition of Al and unavoidable impurities. (5) In the extruded tube with an inner surface straight groove according to one embodiment of the present invention, it is preferable that in addition to the above composition, Cu is included, the Cu content is restricted to 0.05% or less by mass%, and the remainder is made of an aluminum alloy having a composition of Al and unavoidable impurities.

[0010] (6) In the internally grooved extruded tube according to one embodiment of the present invention as described in (1) or (2), in addition to the above composition, it is preferable that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9 if the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9 if the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9 if the above composition is added, in addition that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 3.2 if the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 3.2 if the above composition is added, in addition that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 3.2

[0011] (8) In the internally grooved extruded tube described in (4) according to one embodiment of the present invention, in addition to the above composition, it is preferable to contain one or two of the following in mass%: Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and to contain Mn, and further contain Cr: 0.1% or more, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9. (9) In the internally grooved extruded tube described in (5) according to one embodiment of the present invention, in addition to the above composition, it is preferable to contain one or two of the following in mass%: Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and to contain Mn, and further contain Cr: 0.1% or more, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 3.2.

[0012] (10) In the internally grooved extruded tube described in (6) according to one embodiment of the present invention, it is preferable that, in addition to the above composition, one or more of the following are included: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, Zn: 0.01 to 1.5%. (11) In the internally grooved extruded tube described in (7) according to one embodiment of the present invention, it is preferable that, in addition to the above composition, one or more of the following are included: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, Zn: 0.01 to 1.5%.

[0013] (12) In the internally grooved extruded tube described in (8) according to one embodiment of the present invention, it is preferable that, in addition to the above composition, one or more of the following are included: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%. (13) In the internally grooved extruded tube described in (9) according to one embodiment of the present invention, it is preferable that, in addition to the above composition, one or more of the following are included: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

[0014] (14) An internally spiral grooved tube according to one embodiment of the present invention is made of an aluminum alloy having a composition of Cr: 0.05 to 0.35%, Fe: 0.05 to 0.5%, Si: 0.05 to 0.8% by mass%, with the remainder being Al and unavoidable impurities, and is characterized in that Fe and Si satisfy the relationship 0.8Fe ≤ Si. (15) An internally spiral grooved tube according to one embodiment of the present invention as described in (14), wherein the ED-TD parallel surface is 10000 μm ２ In the observation field, it is preferable to have five or more intermetallic compounds containing Cr having a diameter of 0.01 μm or more and less than 5.00 μm in equivalent circular diameter. However, ED is the extrusion direction, TD is the transverse direction perpendicular to ED and the thickness direction, and the ED-TD parallel surface means a surface parallel to ED and TD that is formed by grinding the surface of the internally grooved extruded tube. (16) In the internally spiral grooved tube according to one embodiment of the present invention, it is preferable that, in addition to the above composition, it contains 0.01 to 0.2% Ti by mass, with the remainder being Al and unavoidable impurities, and is made of an aluminum alloy.

[0015] (17) In the internally spiral grooved tube according to one embodiment of the present invention as described in (14) or (15), it is preferable that, in addition to the above composition, it contains Cu, the Cu content is restricted to 0.05% or less by mass%, and the remainder is made of an aluminum alloy having a composition of Al and unavoidable impurities. (18) In the internally spiral grooved tube according to one embodiment of the present invention as described in (16), it is preferable that, in addition to the above composition, it contains Cu, the Cu content is restricted to 0.05% or less by mass%, and the remainder is made of an aluminum alloy having a composition of Al and unavoidable impurities.

[0016] (19) In the internally spiral grooved tube according to one embodiment of the present invention as described in (14) or (15), in addition to the above composition, it is preferable that the tube contains one or two of the following in mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and that the tube contains Mn and Cr at a concentration of 0.1% or more, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9. (20) In the internally spiral grooved tube according to one embodiment of the present invention as described in (16), in addition to the above composition, it contains one or two of the following in mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and that the tube contains Mn and Cr at a concentration of 0.1% or more, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 3.2.

[0017] (21) In the internally spiral grooved tube described in (17) according to one embodiment of the present invention, in addition to the above composition, it is preferable that it contains one or two of the following in mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and that it contains Mn, and further contains Cr: 0.1% or more, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9. (22) In the internally spiral grooved tube described in (18) according to one embodiment of the present invention, in addition to the above composition, it is preferable that it contains one or two of the following in mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and further contains Mn, and further contains Cr: 0.1% or more, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 3.2.

[0018] (23) In the internally spiral grooved tube described in (19) according to one embodiment of the present invention, it is preferable that, in addition to the above composition, one or more of the following are included: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, Zn: 0.01 to 1.5%. (24) In the internally spiral grooved tube described in (20) according to one embodiment of the present invention, it is preferable that, in addition to the above composition, one or more of the following are included: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, Zn: 0.01 to 1.5%.

[0019] (25) In the internally spiral grooved tube described in (21) according to one embodiment of the present invention, it is preferable that, in addition to the above composition, one or more of the following are included: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, Zn: 0.01 to 1.5%. (26) In the internally spiral grooved tube described in (22) according to one embodiment of the present invention, it is preferable that, in addition to the above composition, one or more of the following are included: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, Zn: 0.01 to 1.5%.

[0020] The present invention provides an extruded tube with an inner surface straight groove and a tube with an inner surface spiral groove that exhibit excellent corrosion resistance by specifying the composition of an aluminum alloy mainly composed of Cr, Fe, and Si, without using Zn thermal spraying technology, and by densely forming a highly protective film containing Cr, driven by minor corrosion of the material.

[0021] A perspective view showing an example of an extruded tube with an internal straight groove according to the first embodiment of the present invention. A longitudinal cross-sectional view of the same extruded tube with an internal straight groove. A perspective view showing an example of a pipe with an internal spiral groove according to the first embodiment of the present invention. A longitudinal cross-sectional view of the pipe with an internal spiral groove.

[0022] An example of an embodiment will be described in detail below with reference to the attached drawings. Note that in the drawings used in the following description, characteristic parts may be enlarged for convenience in order to make the features easier to understand. Figures 1 and 2 show an internally straight-grooved extruded tube 14 used in the manufacture of the internally spiral-grooved tube 1 shown in Figures 3 and 4. Multiple straight grooves 12 are formed on the inner surface of this internally straight-grooved extruded tube 14 along the length of the tube at predetermined intervals in the inner circumference direction, and internal fins 13 are formed between adjacent straight grooves 12, 12 in the inner circumference direction of the tube.

[0023] The internally grooved extruded tube 14 shown in Figures 1 and 2 consists of a tube body 14A with a circular cross-sectional contour. The outer diameter of the tube body 14A (the diameter of the circle traced by the outer circumferential surface 14a of the tube body 14A) is, for example, 3 mm to 15 mm. On the inner circumferential surface 14b of the tube body 14A, as an example, multiple internal fins 13 are formed linearly along the length of the tube body 14A at predetermined intervals in the inner circumferential direction of the tube body 14A. In addition, linear grooves 12 of a predetermined width, for example, a constant width, are formed between adjacent linear internal fins 13, 13 in the inner circumferential direction of the tube body 14A. By twist drawing the internally grooved extruded tube 14 having linear internal fins 13, an internally spiral-grooved tube 1 equipped with spiral fins 3 and spiral grooves 4, as shown in Figures 3 and 4, can be obtained.

[0024] Regarding the twist drawing process, as an example, by using the internal spiral groove pipe manufacturing apparatus described in Figure 1 of Patent Document 1 (Japanese Patent No. 6169538) or the internal spiral groove pipe manufacturing apparatus described in Figure 1 of Patent Document 2 (Japanese Patent No. 6439222), the internal spiral groove pipe 1 shown in Figures 1 and 2 can be subjected to twist drawing to obtain the internal spiral groove pipe 1 shown in Figures 3 and 4. Therefore, the cross-sectional shape of the spiral fins 3 and spiral grooves 4 formed in the internal spiral groove pipe 1 is equivalent to the cross-sectional shape of the internal fins 13 and straight grooves 12 formed in the internal straight groove pipe 14. However, the difference is that the spiral fins 3 and spiral grooves 4 are formed spirally in the longitudinal direction of the pipe, while the internal fins 13 and straight grooves 12 are formed linearly in the longitudinal direction of the pipe.

[0025] The tip of the spiral fin 3 shown in the cross-section of Figure 3 is formed to be slightly tapered compared to the base end. The spiral fin 3 shown in Figure 3 is a convex fin in cross-sectional view of the internally spiral-grooved tube 1 shown in Figure 1, and has side walls 3A and 3B rising from the left and right spiral grooves 4, and a tip wall 3C formed on the tip side of the two side walls 3A and 3B and continuous with them. Because the spiral fin 3 is tapered, the side walls 3A and 3B are inclined. The internally straight-grooved extruded tube 14 shown in Figures 1 and 2 also has a convex internal fin 13 of the same shape. Note that the cross-sectional shape of the spiral fin 3 shown in Figure 3 is just one example, and the cross-sectional shape of the spiral fin is not limited to the shape shown in Figure 3, but can be various shapes such as rectangular or trapezoidal.

[0026] The extruded tube 14 with internal straight grooves is made of an aluminum alloy, which will be described later. For example, it is manufactured by placing a billet of an aluminum alloy of the desired composition in the container of an extruder and extruding the billet through the die of the extruder. Therefore, the straight grooves 12 and the internal fins 13 extend linearly in the longitudinal direction of the tube body 14A.

[0027] The internally spiral-grooved tube 1 consists of a tube body 1A with a circular cross-sectional shape. The diameter of the outer surface 1a of the tube body 1A is approximately 3 mm to 15 mm, for example, in the case of a small heat exchanger. Multiple spiral fins 3 are formed along the length direction on the inner surface 1b of the tube body 1A, and spiral grooves 4 are formed between adjacent spiral fins 3, 3, with a width larger than that of the tips of the spiral fins 3.

[0028] In this embodiment, the spiral fins 3 are intermittently arranged in the inner circumferential direction of the pipe body 1A, for example, with a number of 30 to 60 fins. The height of the spiral fins 3 (i.e., the radial dimension) is, for example, 0.1 mm to 0.4 mm. The bottom wall thickness c of the pipe body 1A (i.e., the thickness of the pipe body 1A corresponding to the bottom of the spiral groove 4) is, for example, 0.2 mm to 0.8 mm. The apex angle of the spiral fins 3 (the angle between the sides of the spiral fins 3) is, for example, 10° to 30°. The twist angle θ1 (twist angle) of the spiral fins 3 is, for example, 5° to 45°. Note that the twist angle θ1 does not necessarily have to be constant, and the configuration may have periodically different twist angles in the longitudinal direction of the internal spiral groove pipe 10. In this embodiment, by forming the spiral fins 3 on the inner circumferential surface 1b, the heat exchange efficiency between the internal spiral groove pipe 1 and the refrigerant liquid flowing inside it can be increased.

[0029] "Composition of Aluminum Alloy" The internally spiral-grooved tube 1 and the internally straight-grooved extruded tube 14 are, as an example, made of an aluminum alloy containing, by mass%, Cr: 0.05 to 0.35%, Fe: 0.05 to 0.5%, Si: 0.05 to 0.8%, with the remainder being Al and unavoidable impurities. In this specification, when the range of content of a specific element is indicated using "~", it refers to a range that includes both the lower and upper limits unless otherwise noted. Therefore, when it is written as Cr: 0.05 to 0.35%, it means that the Cr content is 0.05% or more and 0.35% or less.

[0030] Cr: 0.05-0.35% In this embodiment, the aluminum alloy can contain 0.05% to 0.35% Cr. Cr is either solid-dissolved in the matrix or densely distributed as a Cr-containing compound. As a result, Cr separated from the matrix by minor corrosion, or Cr generated by the decomposition of compounds, becomes concentrated on the surface of the aluminum alloy, forming a corrosion-resistant oxide film containing Cr in the corroded areas, thereby improving the corrosion resistance of the alloy. If the Cr content is less than 0.05%, sufficient corrosion resistance cannot be obtained, and if the Cr content exceeds 0.35%, the extrusion processability when extruding the raw pipe decreases.

[0031] Fe: 0.05-0.5% In the aluminum alloy of this embodiment, Fe can be contained in an amount of 0.05% to 0.5%. Fe forms intermetallic compounds such as Al-Fe and Al-Fe-Si and is distributed in the matrix. By devising the manufacturing conditions, Cr can be doped into the Fe sites of these Fe-containing intermetallic compounds, and the Cr-containing intermetallic compounds can be densely distributed. If the Fe content is less than 0.05%, it will affect the distribution of the intermetallic compounds described later and the manufacturing cost will also increase, and if the content exceeds 0.5%, the extrusion processability of the raw tube will decrease. Si: 0.05-0.8% In the aluminum alloy of this embodiment, Si can be contained in an amount of 0.05% to 0.8%. When Si is contained in the aluminum alloy together with Fe, the formation of Al-Fe-Si intermetallic compounds, which have a smaller cathode reaction compared to Al-Fe intermetallic compounds, becomes more advantageous. This can improve the corrosion resistance of the aluminum alloy. If the Si content is less than 0.05%, sufficient effects cannot be obtained, and if it exceeds 0.8%, the extrusion processability of the raw tube decreases.

[0032] "Relationship between Fe content and Si content" In the aluminum alloy of this embodiment, it is preferable that the relationship between Fe content and Si content be satisfied by the expression 0.8Fe ≤ Si. By satisfying the above relationship, the formation of Al-Fe-Si intermetallic compounds, which have a smaller cathode reaction than Al-Fe intermetallic compounds, can be made more advantageous, and the corrosion resistance of the aluminum alloy can be improved. If the above relationship cannot be satisfied, for example, if the Fe content is high, the formation of Al-Fe intermetallic compounds will be more advantageous, and the corrosion resistance will decrease. It is more preferable that the relationship between Fe content and Si content be Fe ≤ Si.

[0033] In the aluminum alloy of this embodiment, in addition to the aforementioned Cr, Fe, and Si, one or more of Ti, Cu, and Mn may be included, as described below. Ti: 0.01 to 0.2% In the aluminum alloy of this embodiment, Ti can be included in an amount of 0.01 to 0.2%. Ti can reduce corrosion progression in the wall thickness depth direction by distributing layers with different Ti concentrations through the peritectic reaction during aluminum alloy casting and the extrusion process. Regarding the Ti content, a sufficient effect cannot be obtained if it is less than 0.01%, and if it exceeds 0.2%, the extrusion processability of the raw tube decreases. Cu: Restricted to 0.05% or less When Cu is included in the aluminum alloy of this embodiment, it is distributed as an Al-Cu compound. Alternatively, Cu ions dissolved from the matrix due to corrosion of the aluminum alloy of the above composition adhere to the material surface as metallic Cu. The Al-Cu compound or metallic Cu acts as a strong cathode, degrading the corrosion resistance of the aluminum alloy. Therefore, the copper content will be restricted to 0.05% or less.

[0034] Mn: 0.01-1.2% In this embodiment, the aluminum alloy can contain 0.01-1.2% of Mn. Mn improves material strength by being dissolved in the matrix of the aluminum alloy or by precipitating as intermetallic compounds such as Al-Mn, Al-Mn-Si, and Al-Mn-Si-Fe. If the Mn content is less than 0.01%, the effect is insufficient, and if it exceeds 1.2%, the extrusion processability of the raw tube decreases.

[0035] In the aluminum alloy of this embodiment, when Cr and Mn are contained, if Cr is contained at a concentration of 0.1% or more, it is preferable that the relationship between Mn content and Cr content satisfies the equation Mn ≤ -9 × Cr + 2.9. In the aluminum alloy of this embodiment, when Cr and Mn are contained, and Ti is also contained, if Cr is contained at a concentration of 0.1% or more and Ti is contained at a concentration of 0.01% or more, it is preferable that the relationship between Mn content and Cr content satisfies the equation Mn ≤ -9 × Cr + 3.2. When Mn is added while Cr is contained at a concentration of 0.1% or more, if the equation Mn ≤ -9 × Cr + 2.9 is not satisfied, a large amount of coarse intermetallic compounds may be formed, which may reduce extrusion processability. When Cr and Ti are contained as described above, and Mn is also contained, the relationship between Mn and Cr becomes Mn ≤ -9 × Cr + 3.2. If this relationship is not satisfied, a large amount of coarse intermetallic compounds will be formed, reducing extrusion processability.

[0036] Adding Mn to an aluminum alloy improves its strength, but may reduce its corrosion resistance. To improve corrosion resistance, 0.1% or more of Cr can be added to balance material strength and corrosion resistance. However, if the amount of Mn and Cr added increases, there is a risk of forming coarse intermetallic compounds, so it is preferable to maintain the relationship described in the aforementioned equation. Adding 0.01% or more of Ti in addition to Mn and Cr broadens the desirable range of Mn and Cr addition amounts, and when Ti is included at 0.01% or more, it is preferable to maintain the relationship described in the aforementioned equation.

[0037] Mg: 0.01-2.0% In the aluminum alloy of this embodiment, in addition to the elements mentioned above, Mg can be included in an amount of 0.01% to 2.0%. Mg is included to improve material strength by either solid-solubilizing in the matrix of the aluminum alloy or precipitating as an intermetallic compound such as Mg2Si. If the Mg content is less than 0.01%, the effect is insufficient, and if it exceeds 2.0%, the extrusion processability of the raw tube decreases.

[0038] In addition, regarding the aluminum alloy of the present embodiment, in the composition when containing Mn and Mg, one or more of Zr, V, Mo, Sr, Sc, Bi, Sn, and Zn shown below may be contained. The desirable content of each additive element is as shown below. Zr: 0.3% or less Zr precipitates as an intermetallic compound such as Al-Zr to improve the material strength. When the Zr content exceeds the upper limit of 0.3%, the extrudability of the bare tube decreases. V: 0.3% or less V precipitates as an intermetallic compound such as Al-V to improve the material strength. When the V content exceeds the upper limit of 0.3%, the extrudability of the bare tube decreases. Mo: 0.3% or less Mo precipitates as an intermetallic compound such as Al-Mo to improve the material strength. When the Mo content exceeds the upper limit of 0.3%, the extrudability of the bare tube decreases.

[0039] Sr: 1.0% or less Sr precipitates as an intermetallic compound such as Al-Sr or Al-Si-Sr to improve the material strength. When the Sr content exceeds the upper limit of 1.0%, the extrudability of the bare tube decreases. Sc: 0.3% or less Sc precipitates as an intermetallic compound such as Al-Sc to improve the material strength. When the Sc content exceeds the upper limit of 1.0%, the extrudability of the bare tube decreases. Bi: 0.5% or less Bi precipitates as an intermetallic compound such as Al-Mg-Bi to improve the material strength. When the Bi content exceeds the upper limit of 0.5%, the extrudability of the bare tube decreases. Sn: 0.5% or less Sn precipitates as an intermetallic compound such as Al-Sn or Al-Mg-Sn to improve the material strength. When the Sn content exceeds the upper limit of 0.5%, the extrudability of the bare tube decreases. Zn: 0.01% or more and 1.5% or less Zn can lower the potential of Al by dissolving in Al, increase the corrosion initiation points by destabilizing the surface oxide film, and lower the corrosion rate by making the corrosion form planar. If the Zn content is less than the lower limit, the effect is insufficient, and if it exceeds the upper limit, the self-corrosion resistance deteriorates significantly.

[0040] ・Inevitable impurities In addition, the aluminum alloy constituting the inner surface spiral groove tube 1 of the present embodiment may contain inevitable impurities other than those described above. It is desirable that the content of these impurities be 0.05% or less.

[0041] "Number of Cr-containing intermetallic compounds" In the aluminum alloy applied to this embodiment, in the observation field of 10,000 μm per ２ in the ED-TD parallel plane, it is preferable to have 5 or more intermetallic compounds containing Cr having a diameter equivalent to a circle of 0.01 μm or more and less than 5.00 μm. ED is the extrusion direction, TD is the lateral direction orthogonal to the ED and the thickness direction (ND), and the ED-TD parallel plane is a plane parallel to ED and TD formed by shaving the surface of the extruded material. Whether it is an intermetallic compound containing Cr can be determined by, for example, compositional analysis using EPMA (electron probe microanalyzer) for coarse intermetallic compounds with a diameter equivalent to a circle of more than 1.0 μm (diameter equivalent to a circle: more than 1 μm to less than 5.00 μm). Also, for fine intermetallic compounds with a diameter equivalent to a circle of 1.0 μm or less (intermetallic compounds with a diameter equivalent to a circle of 0.01 μm to 1.0 μm), the presence or absence of Cr content can be determined by compositional analysis using EDS (energy dispersive X-ray spectroscopy).

[0042] By densely distributing the intermetallic compounds containing Cr, when the compound is decomposed by slight corrosion, Cr is concentrated on the surface layer of the aluminum alloy to form a corrosion-resistant oxide film containing Cr in the corroded part, improving the corrosion resistance of the aluminum alloy. That is, even if a slight corrosion part occurs, Cr seeps out from the intermetallic compound, and an oxide film containing Cr is formed to cover the corroded part to improve the corrosion resistance. When the Cr content is less than 0.05%, a film with sufficient corrosion resistance cannot be formed. Regarding the intermetallic compounds containing Cr, more preferably, there are 10 or more in the observation field of 10,000 μm ２ per.

[0043] In order to distribute a predetermined number of intermetallic compounds containing Cr with a predetermined size, it can be achieved by making the aluminum alloy contain appropriate amounts of Cr, Fe, and Si as described above, and then selecting an appropriate homogenization treatment temperature and an appropriate extrusion temperature for the melted aluminum alloy ingot. For the aluminum alloy with the target number of intermetallic compounds, appropriate solution treatment, aging treatment, etc. can be carried out as necessary.

[0044] "Homogenization Treatment Temperature" It is preferable to perform homogenization treatment on aluminum alloy ingots at a temperature of 400°C or higher and less than 600°C. Homogenization treatment can be carried out by heat treatment at a predetermined temperature within the above range for a period of 1 to 12 hours. Heat treatment at the predetermined temperature causes Al-Fe intermetallic compounds and Al-Fe-Si intermetallic compounds to precipitate, and further promotes Cr doping at the Fe sites. If the homogenization treatment temperature is below 400°C, sufficient effects cannot be obtained, and if the homogenization treatment temperature is above 600°C, the precipitates become coarser or re-dissolve, resulting in a sparse distribution of intermetallic compounds. For similar reasons, a treatment temperature of 430°C or higher and less than 580°C is more desirable for homogenization treatment.

[0045] "Extrusion Temperature" It is preferable to use an aluminum alloy with the aforementioned composition and homogenization treatment, and to extrude it at a maximum temperature of less than 620°C during extrusion to produce the extruded raw tube 14 with an inner straight groove. During extrusion, processing heat is generated by friction between the aluminum alloy and the die, causing the material temperature to rise. If the material temperature during extrusion exceeds 620°C, the distribution of the desired intermetallic compounds becomes sparse due to coarsening and re-solubilization of intermetallic compounds. For the same reason, it is preferable that the extrusion temperature be less than 580°C. "Extrusion Speed" It is preferable that the extrusion speed be 0.5 m / s or more. By setting the predetermined extrusion speed to 0.5 m / s or more, processing stress is applied to the material, crushing the coarse intermetallic compounds and allowing a state in which uniform intermetallic compounds are precipitated.

[0046] The internally grooved extruded tube 14 of this embodiment is made of an aluminum alloy having the composition described above, and contains the aforementioned amounts of Cr, Fe, and Si. It also contains one or more of the aforementioned amounts of Ti, Cu, Mn, and Mg as needed. If the internally grooved tube 1 is manufactured from this internally grooved extruded tube 14 by twist drawing, it is made of an aluminum alloy having the same composition as the internally grooved extruded tube 14. If this internally grooved tube 1 is used as a heat transfer tube for a heat exchanger, it has spiral fins 3 and spiral grooves 4 on its inner surface, so good heat exchange characteristics can be obtained. That is, it can obtain better heat exchange characteristics compared to a heat transfer tube that does not have spiral fins 3 and spiral grooves 4 on its inner surface. Therefore, a heat exchanger using the internally grooved tube 1 as a heat transfer tube can exhibit excellent heat exchange characteristics.

[0047] Furthermore, if the internal spiral grooved tube 1 of this embodiment contains a suitable amount of Cr, a heat transfer tube can be constructed that exhibits excellent corrosion resistance due to the formation of an oxide film with excellent corrosion resistance resulting from the suitable Cr content and the dense dispersion of intermetallic compounds containing Cr. In addition, by maintaining the relationship 0.8Fe ≤ Si, the formation of Al-Fe-Si intermetallic compounds, which have a smaller cathode reaction than Al-Fe intermetallic compounds, is made more favorable, thereby improving the corrosion resistance of the aluminum alloy itself. Therefore, a heat exchanger with excellent corrosion resistance can be provided without using Zn thermal spraying technology.

[0048] In this embodiment, the application of Zn thermal spraying technology is not excluded. An appropriate amount of Zn thermal spray layer may be formed on the outer surface of the internally spiral-grooved pipe 1, and further corrosion resistance may be improved by forming the Zn thermal spray layer. Furthermore, the method for manufacturing the internally spiral-grooved pipe 1 shown in Figures 3 and 4 is not limited to the method of manufacturing from the internally straight-grooved extruded pipe 14 shown in Figures 1 and 2. For example, a round pipe can be manufactured by extrusion as an extruded pipe without grooves on the inside, and a straight groove can be formed on the inner surface of this round pipe by a grooving method to produce an internally straight-grooved extruded pipe with internal fins. This internally straight-grooved extruded pipe can also be processed by the twist drawing process described above to manufacture the internally spiral-grooved pipe shown in Figures 3 and 4.

[0049] Using aluminum alloys of any composition shown in Material Grades No. 1 to 43 in Table 1, extruded raw tubes with internal straight grooves in the shape shown in Figure 1 were manufactured by extrusion. Using each of these internal straight grooved extruded raw tubes, internal spiral grooved tubes were manufactured by twist drawing at room temperature using the internal spiral grooved tube manufacturing apparatus described in Figures 1 and 2 of Patent Document 2 (Japanese Patent No. 6439222). The aluminum alloys used were subjected to homogenization treatment for 8 hours at any of the temperatures shown in Manufacturing Methods A to F in Table 2 to adjust the microstructure. When manufacturing the internal straight grooved extruded raw tubes, the extrusion temperature and extrusion speed shown in Manufacturing Methods A to F in Table 2 were adopted and extrusion was carried out. Internal spiral grooved tubes of samples No. 1 to No. 63, to which the Material Grade No. and manufacturing method shown in Tables 3 and 4 were applied were manufactured, and the extrudeability, distribution number of intermetallic compounds, corrosion loss, and material strength were measured as follows.

[0050] "Extrudeability" The extrudeability was evaluated by measuring the shape of the internal fins formed on the inner surface of the extruded tube with an internal straight groove obtained by extrusion. The method for measuring the shape of the internal fins involved cutting the extruded tube to obtain a cross-section, embedding it in resin, and observing the polished surface, then measuring the tips of 50 internal fins. When the aluminum alloy has poor extrudeability, the flow of aluminum into the fin portion during the extrusion of the tube becomes insufficient, which easily leads to defects such as the internal fins becoming too narrow. If the measured internal fin tip width was 70% or less of the design value, it was judged as a poor extrusion and was rated C; if it was 90% or less, it was judged as somewhat good and was rated B; and if it exceeded 90%, it was judged as good and was rated A. The results of each judgment are shown in Table 3.

[0051] "Distribution of Intermetallic Compounds" Thin films were prepared by mechanically polishing and electrolytically polishing sample pieces cut from internally spiral-grooved tubes No. 1 to No. 63 shown in Tables 3 and 4, and then measured on a 10,000 μm ED-TD parallel plane using a TEM (transmission electron microscope). ２Taking the observation field of view as (100 μm angle), the number of fine intermetallic compounds and coarse intermetallic compounds was measured. The presence or absence of Cr in the intermetallic compounds was determined by compositional analysis using EPMA (electron probe microanalyzer) for coarse intermetallic compounds with an equivalent circle diameter greater than 1.0 μm (intermetallic compounds with an equivalent circle diameter: greater than 1 μm and less than 5.00 μm). Also, for fine intermetallic compounds with an equivalent circle diameter of 1.0 μm or less (intermetallic compounds with an equivalent circle diameter of 0.01 μm to 1.0 μm), the presence or absence of Cr was determined by compositional analysis using EDS (energy dispersive X-ray spectroscopy). The total number of those containing Cr in the fine intermetallic compounds and coarse intermetallic compounds was listed in Table 3 as the distribution number.

[0052] "Corrosion weight loss" Specimens were cut from the inner spiral groove pipes numbered No. 1 to No. 63 shown in Tables 3 and 4 with a length of 150 mm, both ends were masked, and the specimens were subjected to a corrosion test in a state where the inner surface was protected. The corrosion test was carried out for 2000 h using the SWAAT (standard ASTM) test, and it was evaluated based on the weight change before and after the corrosion test. When the corrosion weight loss was less than 5 mg / cm ２ it was judged that the corrosion resistance was good and determined as A, and when the corrosion weight loss was 5 mg / cm ２ or more and less than 10 mg / cm ２ it was judged that the corrosion resistance was slightly good and determined as B, and when the corrosion weight loss was 10 mg / cm ２ or more it was judged that the corrosion resistance was poor and determined as C, and the respective determination results were listed in Table 3.

[0053] "Material strength" Specimens were cut from the inner spiral groove pipes numbered No. 1 to No. 63 shown in Tables 3 and 4 with a length of 160 mm, and a tensile test was carried out on the pipes as they were. The tensile test conformed to JIS Z2241. In order to convert the stress, the weight of the test piece was measured to calculate the cross-sectional area. When the strength was 92 MPa or more, it was judged to be good and determined as A, when the strength was 85 MPa or more and less than 92 MPa, it was judged to be slightly good and determined as B, and when it was less than 85 MPa, it was judged to be poor and determined as C, and the respective determination results were listed in Table 3.

[0054]

[0055]

[0056]

[0057]

[0058] As shown in Table 3, samples No. 1 to 20 are aluminum alloys containing Cr: 0.05 to 0.35% by mass, Fe: 0.05 to 0.5%, Si: 0.05 to 0.8%, with the remainder being Al and unavoidable impurities. These are internally spiral-grooved tubes satisfying the relationship 0.8Fe ≤ Si between Fe and Si. Specifically, in samples No. 1 to 20, the relationship between Fe and Si is in the range of 0.8 to 2.3. These internally spiral-grooved tubes corresponding to the examples have a 10,000 μm ED-TD parallel surface. ２ Within the observation field, five or more intermetallic compounds containing Cr with a diameter of 0.01 μm or more and less than 5.00 μm in equivalent circular diameter were present. Specifically, the number of intermetallic compounds ranged from 7 to 22. Furthermore, these internally spiral-grooved tubes exhibited minimal corrosion loss and sufficient strength. In addition, some of these samples contained appropriate amounts of Ti, Mn, and Mg, satisfying the aforementioned relationship. In samples No. 1 to 20, the Ti-containing samples contained 0.01 to 0.2 mass% Ti. In samples No. 1 to 20, all Cu-containing samples had Cu suppressed to 0.05 mass% or less. More specifically, Cu was suppressed to 0.02% or less. In samples No. 1 to 20, the Mn-containing samples contained 0.01 to 1.2 mass% Mn. In samples No. 1 to 20, those containing Mg contained 0.01 to 2.0% by mass of Mg.

[0059] Next, samples No. 21 to 30 were prepared by adding an appropriate amount of one of the following elements—Zr, V, Mo, Sr, Sc, Bi, Sn, or Zn—to the aluminum alloy with the aforementioned composition, and all samples obtained properties equivalent to those of the previously described samples. Samples No. 21 to 30 contained one or more of the following elements: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

[0060] The samples No. 31 to 43 shown in Tables 3 and 4 are internally spiral-grooved tubes using aluminum alloys of grades No. 31 to 43 shown in Table 1. Grade No. 31 is an aluminum alloy with a Cr content of 0.4%, and grade No. 32 is an aluminum alloy with a Cr content of 0.03%. However, the extrudeability of the aluminum alloy No. 31 was rated as C. The internally spiral-grooved tube using the aluminum alloy No. 32 had a small number of intermetallic compound particles, resulting in a corrosion loss rating of C. The Cr content of the aluminum alloy used in this example is considered to be preferably in the range of 0.05 to 0.35%, taking into account the Cr content of the equivalent samples in the aforementioned example.

[0061] Material No. 33 is an aluminum alloy with an Fe content of 0.6%, and No. 34 is an aluminum alloy with an Fe content of 0.03%. However, when using aluminum alloy No. 33, the extrudeability was rated as C, as shown in Table 3. Aluminum alloy No. 34 requires a lower Fe content, which is disadvantageous in terms of material cost. Fe is an element that is present to some extent in aluminum ingots, and in order to reduce the Fe content to about 0.03%, it is necessary to use high-purity ingots, which increases material costs. The Fe content of the aluminum alloy used in this example is considered to be preferably in the range of 0.05 to 0.5%, taking into account the Fe content of the sample equivalent to the example described above.

[0062] Material No. 35 is an aluminum alloy with a Si content of 1.0%, and No. 36 is an aluminum alloy with a Si content of 0.04%. The extrusion processability of aluminum alloy No. 35 was rated as C. The corrosion weight loss of the internally spiral-grooved tube using aluminum alloy No. 36 was also rated as C. It is considered desirable for the Si content of the aluminum alloy used in this embodiment to be in the range of 0.05 to 0.8%.

[0063] Aluminum alloy No. 37 is within the desirable range for Cr, Fe, Si, and Cu content as described above, but it does not satisfy the relationship 0.8Fe ≤ Si (0.8 ≤ Si / Fe) regarding the ratio of Fe to Si content. Sample No. 25, a pipe with an internal spiral groove using aluminum alloy No. 37, exhibits significant corrosion weight loss. Therefore, it is considered necessary to satisfy the relationship 0.8Fe ≤ Si regarding the Fe to Si content.

[0064] The aluminum alloy of grade No. 38 is a sample containing more Ti than the aforementioned range (Ti: 0.3 mass%), the aluminum alloy of grade No. 39 is a sample containing more Cu than the aforementioned range (Cu: 0.1 mass%), and the aluminum alloy of grade No. 40 is a sample containing more Mn than the aforementioned range (Mn: 1.5 mass%). The internal spiral grooved tube of sample No. 38 using aluminum alloy No. 38 had problems with extrusion processability, the internal spiral grooved tube of sample No. 39 using aluminum alloy No. 39 had a lot of corrosion loss, and the internal spiral grooved tube of sample No. 40 using aluminum alloy No. 40 also had problems with extrusion processability. Regarding the Ti content, the internally spiral-grooved tubes of samples 8-10, 14, and 20, using aluminum alloys of grades 8-10, 14, and 20, contained 0.01-0.2% Ti and did not cause any problems. Regarding the Mn content, the internally spiral-grooved tubes of grades 11-14, using grades 11-14 which contained 0.01-1.2% Mn, did not cause any problems. Therefore, it is considered that when the aluminum alloy used in this embodiment contains Ti, the range is preferably 0.01-0.2%, when containing Cu, it is preferably 0.05% or less, and when containing Mn, it is preferably 0.01-1.2%.

[0065] Aluminum alloy No. 41 is an alloy that does not satisfy the relationship Mn ≤ -9 × Cr + 2.9, and aluminum alloy No. 42 is an alloy that does not satisfy the relationship Mn ≤ -9 × Cr + 3.2. Aluminum alloys No. 41 and 42 caused problems during extrusion processing.

[0066] Aluminum alloy sample No. 43 contains a high amount of Mg. Aluminum alloy sample No. 43 exhibited problems with extrusion processability. Internally spiral grooved tubes of samples No. 46-48, 51-53, 56-58, and 61-63 were manufactured using manufacturing methods D, E, and F shown in Table 2. Manufacturing method D is a manufacturing method in which the homogenization treatment temperature is higher than the aforementioned desirable temperature range, manufacturing method E is a manufacturing method in which the extrusion temperature is higher than the aforementioned desirable temperature range, and manufacturing method F is a manufacturing method in which the extrusion speed is too slow. When these manufacturing methods D, E, and F are adopted, even if the desired amounts of Cr, Fe, and Si are used as in material samples No. 1, 8, 11, and 14, the number of intermetallic compound particles in the distribution becomes small, ranging from 1 to 4, and corrosion loss increases.

[0067] Samples 44, 45, 49, 50, 54, 55, 59, and 60 were produced using manufacturing methods B and C shown in Table 2. Manufacturing method B, shown in Table 2, involves a homogenization treatment at a temperature of 420°C, while manufacturing method C involves an extrusion temperature of 600°C. Manufacturing methods B and C can also be used to obtain internally spiral-grooved tubes with five or more intermetallic compound particles, and internally spiral-grooved tubes with minimal corrosion loss.

[0068] 1...Tube with internal spiral grooves, 3...Spiral fins, 4...Spiral grooves, 12...Straight grooves, 13...Internal fins, 14...Extruded tube with internal straight grooves.

Claims

1. An extruded tube with an inner straight groove for manufacturing an inner spiral grooved tube, characterized in that it contains, by mass%, Cr: 0.05 to 0.35%, Fe: 0.05 to 0.5%, Si: 0.05 to 0.8%, with the remainder being an aluminum alloy having a composition of Al and unavoidable impurities, and the relationship between Fe and Si is 0.8Fe ≤ Si.

2. 10,000 μm of ED-TD parallel surface ２ The extruded tube with an inner surface straight groove according to claim 1, characterized in that it has five or more intermetallic compounds containing Cr having a diameter of 0.01 μm or more and less than 5.00 μm in the equivalent diameter of a circle within the observation field. However, ED is the extrusion direction, TD is the transverse direction perpendicular to ED and the thickness direction, and the ED-TD parallel surface means a surface parallel to ED and TD that is formed by grinding the surface of the extruded tube with an inner surface straight groove.

3. The extruded tube with an inner surface straight groove according to claim 1 or 2, characterized in that, in addition to the above composition, it contains 0.01 to 0.2% by mass of Ti, with the remainder being Al and unavoidable impurities, and is made of an aluminum alloy.

4. The extruded tube with an inner surface straight groove according to claim 1 or 2, characterized in that, in addition to the above composition, it contains Cu, the Cu content is restricted to 0.05% or less by mass, and the remainder is an aluminum alloy having a composition of Al and unavoidable impurities.

5. The extruded tube with an inner surface straight groove according to claim 3, characterized in that, in addition to the above composition, it contains Cu, the Cu content is restricted to 0.05% or less by mass, and the remainder is an aluminum alloy having a composition of Al and unavoidable impurities.

6. In addition to the above composition, the extruded tube with an inner surface straight groove according to claim 1 or 2, characterized in that, if it contains Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0% by mass%, and if it contains Mn and further contains 0.1% or more of Cr, the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.

9.

7. The extruded tube with an inner surface straight groove according to claim 3, characterized in that, in addition to the above composition, it contains one or two of the following in mass percent: Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and if it contains Mn and further contains Cr: 0.1% or more, the amount of added Mn and Cr satisfies the relationship Mn ≤ -9 × Cr + 3.

2.

8. In addition to the above composition, the extruded tube with an inner surface straight groove according to claim 4, characterized in that it contains one or two of the following in mass percent: Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and if it contains Mn and further contains 0.1% or more of Cr, the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.

9.

9. In addition to the above composition, the extruded tube with an inner surface straight groove according to claim 5, characterized in that, if it contains Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0% by mass%, and if it contains Mn and further contains Cr: 0.1% or more, the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 3.

2.

10. The extruded tube with an inner surface straight groove according to claim 6, characterized in that, in addition to the above composition, it contains one or more of the following: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

11. The extruded tube with an inner surface straight groove according to claim 7, characterized in that, in addition to the above composition, it contains one or more of the following: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

12. The extruded tube with an inner surface straight groove according to claim 8, characterized in that, in addition to the above composition, it contains one or more of the following: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

13. The extruded tube with an inner surface straight groove according to claim 9, characterized in that, in addition to the above composition, it contains one or more of the following: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

14. An internally spiral-grooved tube characterized by being made of an aluminum alloy containing Cr: 0.05-0.35% by mass, Fe: 0.05-0.5%, Si: 0.05-0.8%, with the remainder being Al and unavoidable impurities, and satisfying the relationship 0.8Fe ≤ Si between Fe and Si.

15. 10,000 μm of ED-TD parallel surface ２ The internal spiral grooved tube according to claim 14, characterized in that it has five or more intermetallic compounds containing Cr having a diameter of 0.01 μm or more and less than 5.00 μm in terms of equivalent circular diameter within the observation field. However, ED is the extrusion direction, TD is the transverse direction perpendicular to ED and the thickness direction, and the ED-TD parallel surface means a surface parallel to ED and TD that is formed by scraping the surface of the internal straight grooved extruded tube.

16. The internal spiral grooved tube according to claim 14 or 15, characterized in that, in addition to the above composition, it contains 0.01 to 0.2% by mass of Ti, with the remainder being an aluminum alloy having a composition of Al and unavoidable impurities.

17. The internal spiral grooved tube according to claim 14 or 15, characterized in that, in addition to the above composition, it contains Cu, the Cu content is restricted to 0.05% or less by mass, and the remainder is an aluminum alloy having a composition of Al and unavoidable impurities.

18. The internal spiral grooved tube according to 16, characterized in that, in addition to the above composition, it contains Cu, the Cu content is restricted to 0.05% or less by mass, and the remainder is an aluminum alloy having a composition of Al and unavoidable impurities.

19. In addition to the above composition, the tube contains one or two of the following by mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and if it contains Mn and further contains 0.1% or more of Cr, the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9, as described in claim 14 or 15.

20. The internal spiral grooved tube according to 16, characterized in that, in addition to the above composition, it contains one or two of the following in mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and if it contains Mn and further contains Cr: 0.1% or more, the amount of added Mn and Cr satisfies the relationship Mn ≤ -9 × Cr + 3.

2.

21. In addition to the above composition, the tube contains one or two of the following by mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and if it contains Mn and further contains 0.1% or more of Cr, the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9, as described in 17.

22. The internal spiral grooved tube according to 18, characterized in that, in addition to the above composition, it contains one or two of the following by mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and if it contains Mn and further contains Cr: 0.1% or more, the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 3.

2.

23. The internal spiral grooved tube according to 19, characterized in that, in addition to the above composition, it contains one or more of the following: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

24. The internal spiral grooved tube according to 20, characterized in that, in addition to the above composition, it contains one or more of the following: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

25. The internal spiral grooved tube according to 21, characterized in that, in addition to the above composition, it contains one or more of the following: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

26. The internal spiral grooved tube according to 22, characterized in that, in addition to the above composition, it contains one or more of the following: Zr: 0.3% or less, V: 0.3% or less, Mo: 0.3% or less, Sr: 0.3% or less, Sc: 0.3% or less, Bi: 0.5% or less, Sn: 0.5% or less, and Zn: 0.01 to 1.5%.

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