Extruded tubes with internal straight grooves and internal spiral grooves

Abstract

The present invention aims to provide an extruded tube with an internal straight groove and a tube with an internal spiral groove. [Solution] The present invention relates to an internally straight-grooved extruded tube for manufacturing internally spiral-grooved tubes, characterized in that it is made of an aluminum alloy containing Cr: 0.05~0.35%, Fe: 0.05~0.5%, and Si: 0.05~0.8% by mass%, with the remainder being Al and unavoidable impurities, and that the relationship between Fe and Si is 0.8Fe≦Si. Furthermore, the ED-TD parallel surface is 10000μm 2 It is preferable that the observation field contains five or more intermetallic compounds containing Cr, each having a diameter of 0.01 μm or more and less than 5.00 μm in equivalent circular diameter.

Description

[Technical Field]

[0001] The present invention relates to an extruded tube with an internal straight groove and a tube with an internal spiral groove. [Background technology]

[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, against the backdrop of rising copper prices and the increasing need for material substitution with aluminum due to resource depletion risks, technological development of aluminum tubes with internal spiral grooves is underway to ensure the thermal conductivity performance of heat transfer tubes.

[0003] The technical requirements for heat transfer tubes in air conditioning heat exchangers include not only thermal conductivity but also corrosion resistance that prevents corrosion that would create through-holes during long-term use. Therefore, heat transfer tubes that can withstand long-term use are typically made by applying Zn thermal spraying and heat diffusion treatment to extruded tubes to control the corrosion pattern in a planar manner.

[0004] 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 is formed that exhibits excellent corrosion resistance even in environments where water vapor is present, 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 a metal 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 defined. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] WO2011 / 052520 [Patent Document 2] Japanese Patent Publication No. 2014-037557 [Patent Document 3] Japanese Patent Publication No. 2004-324998 [Overview of the project] [Problems that the invention aims to solve]

[0006] 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 inclusion of excessive Zn. Therefore, corrosion protection technology that does not rely on Zn thermal spraying was needed.

[0007] 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. [Means for solving the problem]

[0008] (1) One embodiment of the present invention is an extruded tube with an internal straight groove for manufacturing an internal spiral grooved tube, characterized in that it is made of an aluminum alloy containing Cr: 0.05~0.35%, Fe: 0.05~0.5%, and Si: 0.05~0.8% by mass, with the remainder being Al and unavoidable impurities, and that the relationship between Fe and Si is 0.8Fe≦Si. (2) In the extruded tube with an inner surface straight groove described in (1) according to one embodiment of the present invention, the ED-TD parallel surface is 10,000 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. Here, ED is the extrusion direction, TD is the transverse direction perpendicular to the ED and the thickness direction, and the ED-TD parallel surface means a surface parallel to the ED and TD formed by scraping the surface of the extruded tube. (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 an 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, 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. (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, 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 the composition of Al and unavoidable impurities.

[0010] (6) In the extruded tube with an inner surface straight groove 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 by mass%, Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0%, and that it contains Mn, and further contains 0.1% or more of Cr, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9. (7) In the extruded tube with an inner surface straight groove described in (3) 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 + 3.2.

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

[0012] (10) In the extruded tube with an inner surface straight groove according to one embodiment of the present invention as described in (6), 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%. (11) In the extruded tube with an inner surface straight groove according to one embodiment of the present invention as described in (7), it is preferable 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) In the extruded tube with an inner surface straight groove according to one embodiment of the present invention as described in (8), 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 extruded tube with an inner surface straight groove according to one embodiment of the present invention as described in (9), it is preferable 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%.

[0013] (14) One embodiment of the present invention is an aluminum alloy having a composition of Cr: 0.05~0.35%, Fe: 0.05~0.5%, Si: 0.05~0.8% by mass, with the remainder being Al and unavoidable impurities, and is characterized in that the relationship between Fe and Si is 0.8Fe ≤ Si. (15) In the internal spiral grooved tube described in (14) according to one embodiment of the present invention, the ED-TD parallel surface is 10,000 μm 2 It is preferable that the observation field contains five or more intermetallic compounds containing Cr, each having a diameter of 0.01 μm or more and less than 5.00 μm in equivalent circular diameter. (16) 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, the aluminum alloy contains 0.01 to 0.2% by mass of Ti, with the remainder being Al and unavoidable impurities.

[0014] (17) In an 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, the tube 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 internal 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 the composition of Al and unavoidable impurities.

[0015] (19) In the internal 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 further contains 0.1% or more of Cr, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.9. (20) In the pipe with an internal spiral groove according to (16) related to one embodiment of the present invention, in addition to the above composition, one or two of Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0% are contained by mass%. When Mn is contained and further Cr: 0.1% or more is contained, it is preferable that the addition amounts of Mn and Cr satisfy the relational expression of Mn ≦ -9×Cr + 3.2.

[0016] (21) In the pipe with an internal spiral groove according to (17) related to one embodiment of the present invention, in addition to the above composition, one or two of Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0% are contained by mass%. When Mn is contained and further Cr: 0.1% or more is contained, it is preferable that the addition amounts of Mn and Cr satisfy the relational expression of Mn ≦ -9×Cr + 2.9. (22) In the pipe with an internal spiral groove according to (18) related to one embodiment of the present invention, in addition to the above composition, one or two of Mn: 0.01 to 1.2% and Mg: 0.01 to 2.0% are contained by mass%. When Mn is contained and further Cr: 0.1% or more is contained, it is preferable that the addition amounts of Mn and Cr satisfy the relational expression of Mn ≦ -9×Cr + 3.2.

[0017] (23) In the pipe with an internal spiral groove according to (19) related to one embodiment of the present invention, in addition to the above composition, it is preferable to contain one or more of 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) In the pipe with an internal spiral groove according to (20) related to one embodiment of the present invention, in addition to the above composition, it is preferable to contain one or more of 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%.

[0018] (25) In the internally spiral grooved tube according to one embodiment of the present invention as described in (21), it is preferable 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) In the internally spiral grooved tube according to one embodiment of the present invention as described in (22), it is preferable 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%. [Effects of the Invention]

[0019] This 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, using minor corrosion of the material as the driving force to densely form a highly protective coating containing Cr. [Brief explanation of the drawing]

[0020] [Figure 1] A perspective view showing an example of an extruded tube with an internal straight groove according to the first embodiment of the present invention. [Figure 2] A longitudinal cross-sectional view of an extruded tube with a straight groove on its inner surface. [Figure 3] A perspective view showing an example of an internally spiral-grooved pipe according to the first embodiment of the present invention. [Figure 4] Longitudinal cross-sectional view of a pipe with internal spiral grooves. [Modes for carrying out the invention]

[0021] An example of an embodiment will be described in detail below with reference to the attached drawings. Note that, for convenience, the drawings used in the following description may show enlarged versions of key features to make them easier to understand. Figures 1 and 2 show an internally grooved extruded tube 14 used in the manufacture of the internally grooved spiral-grooved tube 1 shown in Figures 3 and 4. Multiple straight grooves 12 are formed on the inner surface of this internally 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.

[0022] The extruded tube 14 with an internal straight groove shown in Figures 1 and 2 consists of a tube body 14A with a circular cross-sectional contour. The outer diameter of the pipe body 14A (the diameter of the circle traced by the outer surface 14a of the pipe body 14A) is, for example, 3 mm or more and 15 mm or less. On the inner surface 14b of the pipe body 14A, as an example, multiple internal fins 13 are formed linearly along the length of the pipe body 14A at predetermined intervals in the inner circumferential direction of the pipe 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 pipe body 14A. By twisting and drawing an extruded tube 14 with an internal straight groove having a linear internal fin 13, an internal spiral grooved tube 1 with spiral fins 3 and spiral grooves 4, as shown in Figures 3 and 4, can be obtained.

[0023] As an example of the twist drawing process, 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), a twist drawing process can be applied to the internal straight groove extruded tube 14 shown in Figures 1 and 2 to obtain the internal spiral groove pipe 1 shown in Figures 3 and 4. Therefore, the cross-sectional shapes of the spiral fins 3 and spiral grooves 4 formed in the internally spiral-grooved tube 1 are equivalent to the cross-sectional shapes of the internal fins 13 and straight grooves 12 formed in the internally straight-grooved extruded tube 14. The only difference is that the spiral fins 3 and spiral grooves 4 are formed spirally in the longitudinal direction of the tube, while the internal fins 13 and straight grooves 12 are formed linearly in the longitudinal direction of the tube.

[0024] The tip of the spiral fin 3 shown in 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-section 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 at the tip 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 a similar 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; various shapes such as rectangular or trapezoidal can be used.

[0025] The internally grooved extruded tube 14 is made of an aluminum alloy, which will be described later. For example, it is manufactured by placing a billet of the desired composition of aluminum alloy in the container of an extruder and extruding the billet through the die of the extruder. Therefore, the straight groove 12 and the internal fins 13 extend linearly in the longitudinal direction of the tube body 14A.

[0026] 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 that is larger than the tip of the spiral fin 3.

[0027] In this embodiment, the spiral fins 3 are intermittently extended in the inner circumferential direction of the tube body 1A, for example, in a number of 30 to 60. 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 tube body 1A (i.e., the thickness of the tube 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 tube 10 with an inner spiral groove. In this embodiment, by forming spiral fins 3 on the inner circumferential surface 1b, the heat exchange efficiency between the internal spiral grooved pipe 1 and the refrigerant liquid flowing inside it can be increased.

[0028] "Composition of aluminum alloys" The internally spiral-grooved tube 1 and the internally straight-grooved extruded tube 14 are, for example, made of an aluminum alloy containing, by mass%, Cr: 0.05~0.35%, Fe: 0.05~0.5%, Si: 0.05~0.8%, 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 the range including both the lower and upper limits unless otherwise noted. Therefore, when it is written as Cr:0.05~0.35%, it means that the Cr content is between 0.05% and 0.35%.

[0029] Cr: 0.05~0.35% In this embodiment, the aluminum alloy may contain 0.05% to 0.35% of Cr. The Cr is either 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, is 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.

[0030] 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 optimizing the manufacturing conditions, Cr can be doped into the Fe sites of these Fe-containing intermetallic compounds, allowing for a dense distribution of Cr-containing intermetallic compounds. If the Fe content is less than 0.05%, it affects the distribution of the intermetallic compounds described later and increases manufacturing costs, while if the content exceeds 0.5%, the extrusion processability of the raw tube decreases. Si: 0.05~0.8% In this embodiment, the aluminum alloy can contain 0.05% to 0.8% Si. When Si is included 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 improves the corrosion resistance of the aluminum alloy. If the Si content is less than 0.05%, a sufficient effect cannot be obtained, and if it exceeds 0.8%, the extrusion processability of the raw tube decreases.

[0031] "Relationship between Fe content and Si content" In the aluminum alloy of this embodiment, it is preferable that the relationship between the Fe content and Si content satisfies the expression 0.8Fe ≤ Si. By satisfying the aforementioned relationship, the formation of Al-Fe-Si intermetallic compounds, which have a smaller cathode reaction than Al-Fe intermetallic compounds, can be prioritized, thereby improving the corrosion resistance of the aluminum alloy. If the aforementioned relationship cannot be satisfied, for example, if the Fe content is high, the formation of Al-Fe intermetallic compounds will be prioritized, and the corrosion resistance will decrease. Regarding the Fe content and Si content, it is more preferable to have the relationship Fe ≤ Si.

[0032] 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: 0.01~0.2% In the aluminum alloy of this embodiment, 0.01 to 0.2% of Ti can be contained. By distributing layers with different Ti concentrations through the peritectic reaction during aluminum alloy casting and the extrusion process, corrosion progression in the wall thickness direction can be reduced. Regarding the Ti content, if it is less than 0.01%, a sufficient effect cannot be obtained, and if it exceeds 0.2%, the extrusion processability of the raw pipe 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 eluted from the matrix due to corrosion of the aluminum alloy of the aforementioned 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 Cu content is restricted to 0.05% or less.

[0033] Mn: 0.01~1.2% In this embodiment, the aluminum alloy can contain 0.01 to 1.2% 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.

[0034] 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 relationship Mn ≤ -9 × Cr + 2.9. In the aluminum alloy of this embodiment, if it contains Cr and Mn, and further contains Ti, it is preferable that the relationship between the Mn content and the Cr content satisfies the relationship Mn ≤ -9 × Cr + 3.2, when it contains 0.1% or more of Cr and 0.01% or more of Ti. When Mn is added to a material containing 0.1% or more Cr, if the relationship Mn ≤ -9 × Cr + 2.9 is not satisfied, a large amount of coarse intermetallic compounds may be formed, potentially reducing extrusion processability. When Cr and Ti are present as described above, and Mn is also added, 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.

[0035] Adding Mn to aluminum alloys improves material strength, but may reduce 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 it is preferable to maintain the relationship in the aforementioned relational expression when Ti is present at 0.01% or more.

[0036] Mg: 0.01~2.0% In the aluminum alloy of this embodiment, in addition to the aforementioned elements, Mg can be included in an amount of 0.01% to 2.0%. Mg is included in aluminum alloys to improve material strength by either dissolving in the matrix 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.

[0037] In addition, in the aluminum alloy of this embodiment, when Mn and Mg are included in the composition, one or more of the following Zr, V, Mo, Sr, Sc, Bi, Sn, and Zn may be included. The desirable content of each additive element is as follows. Zr: 0.3% or less Zr precipitates as intermetallic compounds such as Al-Zr, improving material strength. If the Zr content exceeds the upper limit of 0.3%, the extrudeability of the raw tube decreases. V: 0.3% or less V precipitates as intermetallic compounds such as Al-V, improving material strength. If the V content exceeds the upper limit of 0.3%, the extrudeability of the raw tube decreases. Mo: 0.3% or less Mo precipitates as intermetallic compounds such as Al-Mo, improving material strength. If the Mo content exceeds the upper limit of 0.3%, the extrudeability of the raw tube decreases.

[0038] Sr: 1.0% or less Sr precipitates as intermetallic compounds such as Al-Sr and Al-Si-Sr, improving material strength. If the Sr content exceeds the upper limit of 1.0%, the extrudeability of the raw tube decreases. Sc:0.3% or less Sc precipitates as intermetallic compounds such as Al-Sc, improving material strength. If the Sc content exceeds the upper limit of 1.0%, the extrudeability of the raw tube decreases. Bi: 0.5% or less Bi precipitates as intermetallic compounds such as Al-Mg-Bi, improving material strength. If the Bi content exceeds the upper limit of 0.5%, the extrudeability of the raw tube decreases. Sn: 0.5% or less Sn precipitates as intermetallic compounds such as Al-Sn and Al-Mg-Sn, improving material strength. If the Sn content exceeds the upper limit of 0.5%, the extrudeability of the raw tube decreases. Zn: 0.01% or more and 1.5% or less Zn, by dissolving in aluminum, lowers the potential of aluminum and destabilizes the surface oxide film, thereby increasing the number of corrosion initiation points and reducing the corrosion rate by creating a planar corrosion morphology. Below the lower limit of Zn content, the effect is insufficient, and above the upper limit, self-corrosion resistance deteriorates significantly.

[0039] • Inevitable impurities In addition, the aluminum alloy constituting the internally spiral-grooved tube 1 of this embodiment may contain other unavoidable impurities. It is desirable that the content of these impurities be 0.05% or less.

[0040] "Number of Cr-containing intermetallic compounds" In the aluminum alloy applied to this embodiment, the ED-TD parallel surface is 10,000 μm 2 In the observation field, it is preferable to have 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. ED is the extrusion direction, TD is the transverse direction perpendicular to ED and the thickness direction (ND), and the ED-TD parallel surface is a surface parallel to ED and TD formed by grinding the surface of the extruded material. Whether or not an intermetallic compound contains Cr can be determined, for example, 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 but less than 5.00 μm). 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 can be determined by compositional analysis using EDS (energy-dispersive X-ray spectroscopy).

[0041] By densely distributing intermetallic compounds containing chromium (Cr), when the compounds decompose due to minor corrosion, Cr concentrates on the surface of the aluminum alloy, forming a corrosion-resistant oxide film containing Cr in the corroded area, thereby improving the corrosion resistance of the aluminum alloy. In other words, even if minor corrosion occurs, Cr seeps out from the intermetallic compounds, and a Cr-containing oxide film is formed to cover the corroded area, improving corrosion resistance. If the Cr content is less than 0.05%, it is not possible to form a coating with sufficient corrosion resistance. Regarding intermetallic compounds containing Cr, more preferably 10,000 μm 2 There are more than 10 in the observation field of view.

[0042] To distribute intermetallic compounds containing Cr in a predetermined number and size, it is possible to incorporate the appropriate amounts of Cr, Fe, and Si into the aluminum alloy, and then select an appropriate homogenization temperature and an appropriate extrusion temperature for the molten aluminum alloy ingot. For aluminum alloys with the desired number of intermetallic compounds, appropriate solution treatment, aging treatment, etc., can be applied as needed.

[0043] "Homogenization treatment temperature" It is preferable to subject aluminum alloy ingots to a homogenization treatment at a temperature between 400°C and 600°C. Homogenization can be performed by heat treatment at a predetermined temperature within the aforementioned range for a period of 1 to 12 hours. Heat treatment at the predetermined temperature causes the precipitation of Al-Fe intermetallic compounds and Al-Fe-Si intermetallic compounds, and further promotes the doping of Cr into 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 to 580°C is more desirable for homogenization.

[0044] "Extrusion temperature" It is preferable to use an aluminum alloy with the aforementioned composition and homogenized 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, friction between the aluminum alloy and the die generates processing heat, 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 or redissolution of the intermetallic compounds. For similar reasons, an extrusion temperature of less than 580°C is desirable. "Extrusion speed" The extrusion speed is preferably 0.5 m / s or higher. By setting the extrusion speed to a predetermined rate of 0.5 m / s or higher, processing stress is applied to the material, crushing coarse intermetallic compounds and allowing uniform intermetallic compounds to precipitate.

[0045] 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 spiral-grooved tube 1 is manufactured from the internally straight-grooved extruded tube 14 by twist drawing, then it is made of an aluminum alloy with the same composition as the internally straight-grooved extruded tube 14. If this internally grooved tube 1 is used as a heat transfer tube for a heat exchanger, good heat exchange characteristics can be obtained because it has spiral fins 3 and spiral grooves 4 on its inner surface. In other words, 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.

[0046] 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.

[0047] 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 tube 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 its inner surface, and then 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. [Examples]

[0048] Using aluminum alloys of one of the compositions 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 alloy used was subjected to a homogenization treatment for 8 hours at one of the temperatures shown in manufacturing methods A to F in Table 2 to adjust its microstructure. When manufacturing the extruded tubes with internal straight grooves, the extrusion was carried out using one of the extrusion speeds shown in manufacturing methods A to F in Table 2. Internally spiral-grooved tubes of samples No. 1 to No. 63 were manufactured using the material grade No. and manufacturing method shown in Tables 3 and 4, and their extrusion processability, number of intermetallic compounds, corrosion loss, and material strength were measured as follows.

[0049] "Extrusion processability" The extrusion processability was evaluated by measuring the shape of the internal fins formed on the inner surface of extruded tubes with internal straight grooves obtained by extrusion processing. The method for measuring the shape of the internal fins involved cutting the extruded tube to obtain a cross-section, embedding it in resin, polishing and observing, and measuring the tip of the internal fin. When the aluminum alloy has poor extrudeability, insufficient aluminum flow into the fin section during the extrusion process of the raw tube can easily occur, resulting in a defect where the width of the internal fins becomes narrow. If the measured width of the internal fin tip is 70% or less of the design value, it is judged as an extrusion defect and is rated as C. If it is 90% or less, it is judged as somewhat good and is rated as B. If it exceeds 90%, it is judged as good and is rated as A. The results for each judgment are shown in Table 3.

[0050] "Distribution state 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 parallel surface between the ED-TD using a TEM (transmission electron microscope). 2Taking the (100 μm angle) as the observation field of view, the number of fine intermetallic compounds and coarse intermetallic compounds was counted. 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). 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.

[0051] "Corrosion weight loss" Specimens were cut out from the inner surface spiral groove pipes numbered 1 to 63 shown in Tables 3 and 4 with a length of 150 mm, both ends were masked, and they were subjected to a corrosion test with the inner surface protected. The corrosion test was carried out for 2000 h using the SWAAT (standard ASTM) test and evaluated by the weight change before and after the corrosion test. When the corrosion weight loss is less than 5 mg / cm 2 it was judged that the corrosion resistance was good and rated as A. When the corrosion weight loss is 5 mg / cm 2 or more and less than 10 mg / cm 2 it was judged that the corrosion resistance was slightly good and rated as B. When the corrosion weight loss is 10 mg / cm 2 or more, it was judged that the corrosion resistance was poor and rated as C. The respective judgment results were listed in Table 3.

[0052] "Material strength" Specimens were cut out from the inner surface spiral groove pipes numbered 1 to 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 and the cross-sectional area was calculated. When the strength was 92 MPa or more, it was judged to be good and rated as A. When the strength was 85 MPa or more and less than 92 MPa, it was judged to be slightly good and rated as B. When it was less than 85 MPa, it was judged to be poor and rated as C. The respective judgment results were listed in Table 3.

[0053]

Table 1

[0054] [Table 2]

[0055] [Table 3]

[0056] [Table 4]

[0057] As shown in Table 3, samples No. 1 to 20 are aluminum alloys containing Cr: 0.05 to 0.35%, Fe: 0.05 to 0.5%, and Si: 0.05 to 0.8% by mass, with the remainder being Al and unavoidable impurities. They are internally spiral-grooved tubes satisfying the relationship 0.8Fe ≤ Si between Fe and Si. These internally spiral-grooved tubes, corresponding to the examples, have a 10,000 μm parallel surface to the ED-TD. 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. Furthermore, samples No. 21 to 30 were obtained 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 of them exhibited properties equivalent to those of the aforementioned samples.

[0058] Samples No. 31 to 43 shown in Tables 3 and 4 are internally spiral-grooved tubes made using aluminum alloys of material types No. 31 to 43 shown in Table 1. Material No. 31 is an aluminum alloy with a Cr content of 0.4%, and No. 32 is an aluminum alloy with a Cr content of 0.03%. However, the extrudeability of aluminum alloy No. 31 was rated as C. The internally spiral-grooved tube using aluminum alloy No. 32 had a small number of intermetallic compound particles and also received a C rating for corrosion loss. Considering the Cr content of the aluminum alloy used in this example, a range of 0.05 to 0.35% is considered desirable.

[0059] 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.

[0060] 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%. However, 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 example to be in the range of 0.05 to 0.8%.

[0061] The aluminum alloy of material grade 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. The internal spiral grooved tube of sample No. 25, which uses aluminum alloy No. 37, shows significant corrosion weight loss. Therefore, it is considered necessary to satisfy the relationship 0.8Fe ≤ Si regarding the Fe and Si content.

[0062] Aluminum alloy No. 38 is a sample containing more Ti than the aforementioned range, aluminum alloy No. 39 is a sample containing more Cu than the aforementioned range, and aluminum alloy No. 40 is a sample containing more Mn than the aforementioned range. The internally spiral-grooved tube of sample No. 38, which used aluminum alloy No. 38, had problems with extrusion processability. The internally spiral-grooved tube of sample No. 39, which used aluminum alloy No. 39, suffered from significant corrosion loss. The internally spiral-grooved tube of sample No. 40, which used aluminum alloy No. 40, also exhibited problems with extrusion processability. Regarding the Ti content, the internally spiral-grooved tubes of samples 8-10, 14, and 20, which use aluminum alloys of grades 8-10, 14, and 20, contain 0.01-0.2% Ti and have not shown any problems. Regarding the Mn content, the internally spiral-grooved tubes of grades 11-14, which use grades 11-14 containing 0.01-1.2% Mn, have not shown any problems. Therefore, when the aluminum alloy used in this embodiment contains Ti, it is preferable that it contains 0.01 to 0.2%, when it contains Cu, it is preferable that it contains 0.05% or less, and when it contains Mn, it is preferable that it contains 0.01 to 1.2%.

[0063] 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 alloy samples No. 41 and 42 experienced problems during the extrusion process.

[0064] Aluminum alloy No. 43 is a sample containing a high amount of magnesium. Aluminum alloy sample No. 43 exhibited problems with extrusion processability. The internally spiral-grooved tubes for samples No. 44-48, 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 were adopted, and the desired amounts of Cr, Fe, and Si were used as in material grades No. 1, 8, 11, and 14, the number of intermetallic compound particles in the distribution decreased to 1 to 4, resulting in increased corrosion loss. [Explanation of Symbols]

[0065] 1...Tube with internal spiral groove, 3...Spiral fin, 4...Spiral groove, 12...Straight groove, 13...Internal fins, 14...Extruded tube with straight grooves on the inside.

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 satisfies 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 equivalent to 0.01 μm or more and less than 5.00 μm in the observation field of view. 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 with an inner 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 an aluminum alloy having a composition of Al and unavoidable impurities.

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, it contains one or two of the following in mass%: Mn: 0.01-1.2% and Mg: 0.01-2.0%. An extruded tube with an inner surface straight groove according to claim 1 or 2, characterized in that it contains Mn and, further, contains 0.1% or more of Cr, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.

9.

7. In addition to the above composition, the extruded tube with an inner surface straight groove according to claim 3, characterized in 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 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.

8. In addition to the above composition, it contains one or two of the following in mass%: Mn: 0.01-1.2% and Mg: 0.01-2.0%. The extruded tube with an inner surface straight groove according to claim 4, characterized in that it contains Mn and, if it also 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, it contains one or two of the following in mass%: Mn: 0.01-1.2% and Mg: 0.01-2.0%. The extruded tube with an inner surface straight groove according to claim 5, characterized in that it contains Mn, and if it also 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, by mass%, Cr: 0.05-0.35%, 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 equivalent to 0.01 μm or more and less than 5.00 μm in the observation field of view.

16. The tube with an internal spiral groove according to claim 14 or 15, characterized in that, in addition to the above composition, it contains 0.01 to 0.2% Ti by mass, 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 claim 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, it contains one or two of the following in mass%: Mn: 0.01-1.2% and Mg: 0.01-2.0%. The internal spiral grooved tube according to claim 14 or 15, characterized in that it contains Mn and further contains 0.1% or more of Cr, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.

9.

20. The internal spiral grooved tube according to claim 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, it contains one or two of the following in mass%: Mn: 0.01-1.2% and Mg: 0.01-2.0%. The internal spiral grooved tube according to claim 17, characterized in that it contains Mn and, further, contains 0.1% or more of Cr, so that the amount of Mn and Cr added satisfies the relationship Mn ≤ -9 × Cr + 2.

9.

22. In addition to the above composition, it contains one or two of the following in mass%: Mn: 0.01-1.2% and Mg: 0.01-2.0%. The internal spiral grooved tube according to claim 18, characterized in 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 + 3.

2.

23. The internal spiral grooved tube according to claim 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 claim 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 claim 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 claim 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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