JP5544591B2 - Copper alloy tube - Google Patents

Description

  The present invention relates to a Sn-P-based copper alloy tube (copper tube). In the following description, this “copper alloy tube” is also abbreviated as “copper tube”.

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

  In recent years, refrigerants used in heat exchangers have greatly changed to those having a small ozone depletion coefficient from the viewpoint of protecting the global environment. However, the operating pressure of the newly adopted refrigerant has increased to about 1.6 to 6 times that of the conventional refrigerant R22. On the other hand, in order to suppress the increase in the cost of copper pipes due to soaring copper bullion, there is an increasing demand for thinner copper pipes for reducing the amount of copper used.

  In contrast, the greater the tensile strength of the copper tube used, the thinner the wall thickness. However, in the conventional phosphorous-deoxidized copper pipe, since the tensile strength is small, in order to cope with the increase in the operating pressure, it is necessary to increase the thickness of the pipe on the contrary, such as 0.5 mm or less, The wall thickness cannot be reduced.

  In addition, the brazed portion of the copper tube that is brazed when the heat exchanger is assembled is inevitably heated to a temperature of 800 ° C. or higher for several seconds to several tens of seconds. For this reason, in the brazed portion and the vicinity thereof, there is a big problem that the crystal grains of the copper tube structure become coarser and the strength decreases due to softening as compared with other portions.

  Under these circumstances, it is necessary to increase the thickness of the conventional phosphorous deoxidized copper pipe, and it is not possible to meet the demand for thinning the copper pipe under the increased operating pressure. For this reason, instead of this phosphorous deoxidized copper, there is a strong demand for a copper tube that can meet these requirements.

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

  In addition, a method for improving the fracture pressure by controlling the texture such as the Goss orientation accumulation rate in the Sn-P-based copper pipe is also disclosed (see Patent Document 5). Furthermore, a copper tube that combines high fracture pressure and good bending workability by increasing the ratio of fracture strength and tensile strength (fracture strength / tensile strength) to that of phosphorous deoxidized copper has also been proposed. (See Patent Document 6).

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

  However, even Sn-P copper pipes that have high strength and respond to the demand for thinning are exposed to high temperatures of 800 ° C. or higher by brazing during assembly of the heat exchanger, and the crystal grains are coarse. The problem of softening and lowering the strength is also unavoidable like phosphorous deoxidized copper. Therefore, even with the Sn-P copper pipe, there is still room for improvement with respect to the problem of softening due to brazing.

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

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

  As described above, the Sn-P-based copper alloy pipe has been improved in properties and has been left with room for improvement in the problem of embrittlement cracking at a high temperature during the brazing process. That is, in the case of the brazing treatment of the Sn-P-based copper alloy tube, when the copper tube is exposed to a high temperature of 800 ° C. or higher and strain is applied to the copper tube, Brittle cracks may occur. Accordingly, the Sn-P-based copper alloy tube still has a technical problem that when it is thinned to about 1 mm or less, it does not cause embrittlement cracking during the brazing process.

  The present invention has been made in view of such a problem, and provides an Sn-P-based copper alloy tube excellent in high-temperature embrittlement resistance and free from the occurrence of embrittlement cracks during the brazing treatment. With the goal.

  For the above purpose, the most important gist of the copper alloy tube of the present invention is Sn: 0.3 to 2.0 mass%, P: 0.005 to 0.1 mass%, Mg: 0.005 to 0 0.5% ≦ 2.5 × “Mg mass%” + “Sn mass%” ≦ 2.0%, and the balance is composed of Cu and inevitable impurities. It consists of a copper alloy and has a structure in which the average crystal grain size in a cross section parallel to the axial direction of the tube is 20 μm or less.

  For the above purpose, the next important points of the copper alloy tube of the present invention are Sn: 0.4-2.0 mass%, P: 0.005-0.1 mass%, B: 0.001- It has a structure in which 0.06% by mass is contained and the balance is made of a copper alloy having a composition composed of Cu and inevitable impurities, and the average crystal grain size in a cross section parallel to the axial direction of the tube is 20 μm or less. is there.

  The present inventors have reexamined the mechanism of the occurrence of embrittlement cracks during the brazing treatment of Sn—P based copper alloy tubes. As a result, as a result of investigating the mechanical properties at high temperature, the elongation of the Sn-P-based copper alloy tube decreases to a level less than half of the elongation at room temperature at a high temperature of 800 ° C. or higher, which is a brazing equivalent temperature. I found out. Moreover, when the fracture | rupture part which the embrittlement crack produced was observed, it discovered that it was a grain boundary embrittlement crack.

  For this reason, it is assumed that the cause of embrittlement cracking during brazing of the Sn-P-based copper alloy tube is that the grain boundaries become brittle due to preferential oxidation of the grain boundaries at high temperatures. Is done. In particular, compared to conventional thick copper pipes, thinned copper pipes have a large thickness of brittle oxide layers (grain layers with grain boundaries oxidized) with respect to the thickness, which increases the effect. For this reason, when a strain is applied to the copper tube, it tends to cause embrittlement cracking.

  On the other hand, the present inventors have found that the addition of a trace amount of Mg or B improves the elongation at high temperatures of the Sn—P based copper alloy tube. That is, Mg and B are elements that have a stronger tendency to oxidize than Sn and P. When these elements are added, oxidation of these elements occurs preferentially, and oxidation on the grain boundary side is caused. It is thought to be greatly suppressed.

  Therefore, according to this invention, even if it is a thin Sn-P type copper alloy pipe | tube, the thickness of the brittle oxide layer with respect to thickness is suppressed without becoming large at the time of brazing. For this reason, even when a strain is applied to a copper tube thinned to 1.0 mm or less, an embrittlement crack at a high temperature can be suppressed, and an Sn-P-based copper alloy tube excellent in high temperature embrittlement resistance is provided. be able to.

  Hereinafter, the embodiments of the present invention will be specifically described in order for each requirement.

Copper alloy composition:
The copper alloy composition of the copper tube in the present invention is excellent in various properties such as softening resistance, fracture strength and bending workability required for the copper alloy tube, As important compositions, Sn: 0.3-2.0% by mass, P: 0.005-0.1% by mass, Mg: 0.005-0.5% by mass, and 0.5% ≦ 2.5 × “Mg mass%” + “Sn mass%” ≦ 2.0% is satisfied, and the balance is Cu and inevitable impurities.

  Next, although it is inferior to Mg in terms of ease of manufacture and cost, it is the next most important composition in order to have excellent properties such as softening resistance, fracture strength and bending workability required for copper alloy pipes. As follows: Sn: 0.4-2.0% by mass, P: 0.005-0.1% by mass, B: 0.001-0.06% by mass, and the remainder from Cu and inevitable impurities The composition is as follows.

  Here, the difference in the composition of the copper alloy between the present invention and the prior art will be described. Patent Document 1 does not contain Mg or B, and Patent Documents 2 to 8 do not contain B. In Patent Documents 2 to 4, Mg is treated as the same impurity as Fe, Ni, Co, Mn, Mg, Cr, Ti, Zr, Ag, and the like, and the total amount of these elements is 0.03% by mass or less. It is regulated. On the other hand, in Patent Documents 5 to 7, Mg, like Fe, Ni, Mn, Mg, Cr, Ti, Zr, and Ag, has the strength, softening resistance, pressure breakdown strength, and heat resistance of the copper alloy. It is treated as an element that improves and refines crystal grains to improve bending workability. However, the Mg content is less than 0.07% by mass in total of one or more elements selected from the group consisting of Fe, Ni, Mn, Mg, Cr, Ti, and Ag. And the content of Mg in the Example Table 1 of these patent documents is about 0.02% at the maximum in common. Moreover, in Example Table 1 containing 0.02% by mass of Mg, the Sn content is 0.4% by mass in common, and 2.5 × “Mg mass%” + “Sn mass%” 0.45, which deviates slightly from the relationship of 0.5% ≦ 2.5 × “Mg mass%” + “Sn mass%” ≦ 2.0%, which is essential in the present invention (see Table 1 described later). Corresponds to Comparative Example 3).

  Next, the reason for adding (containing) each element and the reason for limiting the composition will be described.

Sn: 0.3-2.0 mass%
Sn (tin, tin) has the effect of improving the tensile strength of the copper tube by solid solution strengthening and reducing the coarsening of crystal grains without reducing the elongation at room temperature. For this reason, when it contains in the copper alloy of the heat exchanger tube which uses a various refrigerant | coolant, it becomes possible to make the wall thickness of a pipe | tube thin compared with a phosphorus deoxidation copper pipe.

  However, if the Sn content of the copper alloy tube exceeds 2.0% by mass, solidification segregation in the ingot becomes severe and segregation may not be completely eliminated by normal hot extrusion and / or processing heat treatment. The metal structure, mechanical properties, bending workability, structure after brazing, and mechanical properties of the alloy pipe become non-uniform. Further, in order to perform extrusion molding at the same extrusion pressure as that of a copper alloy having a Sn content of 2.0% by mass or less, the extrusion temperature needs to be raised, thereby increasing the surface of the extruded material. Oxidation increases, resulting in decreased productivity and increased surface defects in copper alloy tubes. For this reason, Sn content shall be 2.0 mass% or less, Preferably it is 1.20 mass% or less.

  On the other hand, if the Sn content is too small, the tensile strength of the copper tube cannot be improved by solid solution strengthening, and the above-described sufficient strength and fine crystal grain size cannot be obtained. For this reason, when it contains B mentioned later, the minimum of content of Sn shall be 0.4 mass% or more. On the other hand, Mg, which will be described later, has a high solid-solution strengthening ability and a high effect of improving the tensile strength, similarly to Sn. For this reason, when it contains Mg, since content of Sn can be decreased, the minimum of content of Sn shall be 0.3 mass% or more. Therefore, when Mg is contained, the Sn content range is preferably in the range of 0.3 to 2.0 mass%, and when B is contained, it is preferably in the range of 0.4 to 2.0 mass%. .

P: 0.005 to 0.1% by mass
P (phosphorus), like Sn, has the effect of improving the tensile strength of the copper alloy tube and suppressing the coarsening of crystal grains. If the P content of the copper tube exceeds 0.1% by mass, cracking is likely to occur during hot extrusion, the stress corrosion cracking susceptibility increases, and the thermal conductivity decreases greatly. On the other hand, if the P content is less than 0.005% by mass, the amount of oxygen is increased due to insufficient deoxidation, Sn oxide is generated, the soundness of the ingot is lowered, and bending workability as a copper pipe is reduced. descend. Therefore, the range of P content shall be 0.005-0.1 mass%.

  In the present invention, in addition to these, any one of Mg: 0.005 to 0.5 mass% and B: 0.001 to 0.06 mass% is further contained. However, in the present invention, Mg and B are not contained simultaneously. That is, Mg and B are not contained at the same time when the amount is not less than the respective lower limit amounts. However, when containing either Mg or B (more than the respective lower limit amounts), the content of the other element is allowed to be less than the lower limit amount or at the impurity level, and the content of the other element is 0 mass. % May be sufficient. This is because both Mg and B have the effect of solid solution strengthening, like Sn and P, and the simultaneous positive addition and inclusion offset each other's inclusion effect from the limit of solid solution. is there.

Mg: 0.005 to 0.5 mass%
Mg (magnesium) is an element having a stronger oxidation tendency than Sn and P, and has an effect of suppressing preferential grain boundary oxidation at a high temperature during the brazing treatment. As a result, even when the Sn-P-based copper alloy tube is thinned, the thickness of the brittle oxide layer with respect to the wall thickness is not increased during the brazing process, and the copper tube is strained. However, even if embrittlement cracking can be suppressed and the thickness is reduced to 1.0 mm or less, a Sn-P-based copper alloy tube excellent in high temperature embrittlement resistance can be obtained.

  When the Mg content is less than 0.005 mass%, the effect of suppressing the grain boundary oxidation is small, the elongation at high temperature is small, and the elongation in a high temperature tensile test at 800 ° C. of the copper alloy tube is less than 25%. Become. On the other hand, when the Mg content exceeds 0.5% by mass, a large amount of coarse Mg phosphide is generated, and the elongation at room temperature is reduced. Therefore, the Mg content is in the range of 0.005 to 0.5% by mass, preferably more than 0.02% by mass, less than 0.1% by mass, more preferably more than 0.03% by mass, and 0.1%. The range is not more than mass%.

  Mg, like Sn, has a high solid solution strengthening ability, and has the effect of improving the tensile strength of the copper alloy tube and suppressing the coarsening of crystal grains. For this reason, when Mg is contained, the relationship of 0.5% ≦ 2.5 × “Mg mass%” + “Sn mass%” ≦ 2.0% is satisfied in relation to the Sn content. . When 2.5 × “Mg mass%” + “Sn mass%” is smaller than 0.5%, the solid solution strengthening amount of Mg or Sn is too small, and sufficient strength cannot be obtained. Moreover, the elongation in the 800 degreeC high temperature tensile test of a copper alloy pipe | tube becomes low. On the other hand, when 2.5 × “Mg mass%” + “Sn mass%” is greater than 2.0%, the amount of Mg / Sn solid solution strengthening is too large, so that the strength becomes too high and bending is performed instead. The nature will decline.

B: 0.001 to 0.06% by mass
B (boron, boron) is an element having a stronger oxidation tendency than Sn and P like Mg, and has the effect of suppressing preferential grain boundary oxidation at a high temperature during the brazing treatment, similarly to Mg. As a result, even when the Sn-P-based copper alloy tube is thinned, the thickness of the brittle oxide layer with respect to the wall thickness is not increased during the brazing process, and the copper tube is strained. However, even if embrittlement cracking can be suppressed and the thickness is reduced to 1.0 mm or less, a Sn-P-based copper alloy tube excellent in high temperature embrittlement resistance can be obtained.

  When the B content is less than 0.001% by mass, the effect of suppressing grain boundary oxidation is small, the elongation at the high temperature is small, and the elongation in a high-temperature tensile test at 800 ° C. of the copper alloy tube is less than 25%. Become. On the other hand, if the B content exceeds 0.06% by mass, solidification segregation in the ingot becomes violent, and segregation may not be completely eliminated by normal hot extrusion and / or processing heat treatment. For this reason, there arises a problem that the metal structure, mechanical properties, bending workability, structure after brazing, and mechanical properties of the copper alloy tube are not uniform depending on the part.

  B, like Sn, Mg and P, has the effect of improving the tensile strength of the copper alloy tube and suppressing the coarsening of the crystal grains. In order to exert this effect, when B is contained, control is performed so as to satisfy 0.5% ≦ “B mass%” + “Sn mass%” ≦ 2.0% in relation to the Sn content. It is preferable. When “B mass%” + “Sn mass%” is smaller than 0.5%, the amount of solid solution strengthening of B or Sn is too small, and sufficient strength cannot be obtained. On the other hand, when “B mass%” + “Sn mass%” is larger than 2.0%, the amount of solid solution strengthening of B and Sn is too large, so that the strength becomes too high, and the bending workability decreases. End up.

Other elements:
Other elements are impurities, and the content is preferably as small as possible in order to improve the breaking strength of the thin Sn-P copper pipe. However, in order to reduce these impurities, there is a trade-off between the cost of using a raw metal melting raw material and the refining cost in melting and casting, and the practical allowable amount (upper limit amount) is shown below. Zn, Fe, Ni, Mn, Cr, Ti, Zr, Ag, etc. are allowed to contain up to 1.0% by mass in total. S is allowed to contain up to 0.005% by mass. As, Bi, Sb, Pb, Se, Te and the like are allowed to contain up to 0.0015% by mass in total. O is allowed to be contained up to 0.005% by mass. H is allowed to contain up to 0.0002% by mass.

Copper tube grain structure:
It is known that the smaller the crystal grain size in the copper tube structure, the better the fracture strength and the bending workability balance. Also in the present invention, this mechanism is utilized to refine the average crystal grain size of the copper alloy tube. That is, in the measurement result by the SEM-EBSP method described later, the average crystal grain size in the cross section parallel to the axial direction of the Sn-P-based copper alloy tube is refined to 20 μm or less, and the balance between fracture strength and bending workability is achieved. Improve.

  Incidentally, when the thickness of the copper tube is relatively thick, the average crystal grain size has little influence on the fracture strength and the bending workability balance. However, when the thickness of the heat transfer tube is reduced to 1.0 mm or less due to demands for weight reduction and thinning, the influence of the crystal grain size on the fracture strength and bending workability balance is significant. growing. When the average crystal grain size is too large exceeding the upper limit, “strain concentration” when cracks are generated by the tensile force applied to the heat transfer tubes cannot be avoided, and cracks tend to occur in the heat transfer tubes. For this reason, the reliability when the copper tube for heat exchangers using the above-described alternative refrigerant having a high operating pressure is lowered. Moreover, when the crystal grain size becomes coarse, there is a problem that cracks are likely to occur in the bent portion when the copper tube is bent. In addition, from the viewpoint of the effect of crystal grain refinement, the average crystal grain size is preferably as small as possible. However, depending on manufacturing conditions, etc., refinement to about 2 to 3 μm is also conceivable.

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

Grain control method:
In order to control the crystal grain of the copper pipe structure as defined in the present invention, in the middle of the drawing process of the extruded element pipe, it is not usually performed, and intermediate annealing is performed, and recrystallization is performed once by this intermediate annealing. The final annealing may be performed after reducing the particle size. Generally, the crystal grain size after hot extrusion is as large as several tens to 100 μm. For this reason, there are few crystal grain boundaries used as the nucleation site at the time of final annealing, and the crystal grain size becomes large. On the other hand, as in the present invention, by performing intermediate annealing in the middle of drawing and recrystallizing, the grain boundaries that become nucleation sites during final annealing increase, and as a result, the crystal grains after final annealing The diameter can be reduced. According to the present invention, even if the thickness is reduced to 1.0 mm or less as described above, there is an advantage that it is easy to manufacture an Sn-P-based copper alloy tube with improved high temperature embrittlement resistance.

Method for measuring average grain size:
The average crystal grain size is determined using a crystal orientation analysis method in which a field emission scanning electron microscope (FESEM) is equipped with an EBSP (Electron Back Scattering (Scattered) Pattern) system. And measure each. Since the material of the copper tube in the longitudinal direction of the copper tube is uniform, these measurement positions may be measured at any part. However, it is preferable to avoid only both ends in the longitudinal direction of the copper pipe to be manufactured.

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

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

  In the present invention, basically, those whose orientations deviate within ± 15 ° from these crystal planes belong to the same crystal plane (orientation factor), and the orientation difference between adjacent crystal grains is 5 °. The above crystal grain boundary is defined as a crystal grain boundary. In addition, in the present invention, an electron beam is irradiated at a pitch of 1.0 μm with respect to a measurement area, tube axis direction 1000 μm × tube circumferential direction 800 μm, and each of these wide measurement ranges (regions) is measured. Then, when the number of crystal grains measured by the crystal orientation analysis method is n and each measured crystal grain size is x, the average crystal grain size is calculated as (Σx) / n.

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

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

  Thereafter, the billet is perforated by piercing and hot extruded at 750 to 950 ° C. At this time, it is necessary to eliminate Sn segregation in the Sn—P based copper pipe and to refine the structure of the product pipe. For this purpose, the reduction rate of the cross-section of the Sn-P-based copper tube by hot extrusion ([perforated billet donut area-cross-sectional area of the base tube after hot extrusion] / [perforated billet donut shape] Area] × 100%) is 88% or more, desirably 93% or more.

  Furthermore, the cooling rate until the surface temperature reaches 300 ° C. is 10 ° C./second or more, preferably 15 ° C./second or more, more preferably 20 ° C./second or more, by a method such as water cooling. It is preferable to cool so that.

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

  During or after the drawing process, intermediate annealing is performed at a temperature range of 400 ° C. to 700 ° C. for 2 minutes to 1 hour. When the intermediate annealing temperature is lower than 400 ° C., recrystallization does not occur in the intermediate annealing step, and the final annealing step is carried over with the crystal grain size being large. For this reason, like the conventional Sn-P-based copper alloy tube, the average crystal grain size tends to be 20 μm or more, and the fracture pressure is lowered.

  On the other hand, if the intermediate annealing temperature is higher than 700 ° C., the crystal grain size after the intermediate annealing becomes large, and it is carried over to the final annealing process while the crystal grain size remains large. For this reason, like the conventional Sn-P system copper alloy pipe, the crystal grain produced | generated at the last annealing process becomes comparatively large easily, and a fracture pressure becomes low.

  After this intermediate annealing, drawing is further performed to manufacture a smooth tube. The cross-sectional area reduction ratio after this intermediate annealing is set to 35% or more and 80% or less. If the area reduction is lower than 35%, the amount of accumulated strain is too small to increase the driving force necessary for generating recrystallized nuclei. For this reason, in the subsequent final annealing, equiaxed crystal grains free from distortion due to the formation of recrystallized nuclei are hardly generated, tend to be elongated grains, and softening resistance is inferior. On the other hand, if the area reduction rate exceeds 80% and is too high, the outer diameter of the final copper tube becomes too small, and the thickness of the copper tube becomes too thin, so that it cannot withstand the internal pressure of the refrigerant.

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

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

  In this final annealing, it is desirable that the drawing element tube has an actual temperature of 350 to 700 ° C., and the drawing element tube is preferably annealed at that temperature to be heated for about 1 to 120 minutes. When the body temperature of the drawn element tube is lower than 350 ° C., a complete recrystallized structure is not formed, and a fibrous processed structure remains, which makes it difficult for a customer to perform bending. Moreover, when the temperature exceeds 700 ° C., the crystal grains become coarse, and the bending workability of the tube is lowered. Therefore, it is desirable to anneal the drawing tube at a solid temperature of 350 to 700 ° C.

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

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

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

  The average crystal grain size, tensile strength, fracture strength, bending workability, and high temperature embrittlement characteristics in the cross section parallel to the axial direction of these copper alloy tubes were measured and evaluated. These results are also shown in Table 1. Specific manufacturing methods, measurements, and evaluation methods for these Sn-P-based copper tubes (smooth tubes) are as follows.

(Smooth tube manufacturing conditions)
Dissolution:
(A) Using electrolytic copper as a raw material, Sn—P-based copper pipe is to add a predetermined Sn to the molten metal, and optionally add additional additive elements, and then add a Cu—P master alloy. Thus, a molten metal having a predetermined composition was produced. The component compositions of these melted copper alloys were the component compositions of the copper pipe shown in Table 1. In Table 1, the contents of Zn, Fe, Ni, Mn, Cr, Ti, Zr, and Ag in the copper tube are 0% by mass and S is contained in common in both the inventive examples and the comparative examples. The amount is less than 0.005% by mass, the total content (total amount) of As, Bi, Sb, Pb, Se, Te is 0% by mass, the O content is less than 0.003% by mass, and the H content is 0. It was less than 0.0001% by mass.

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

Hot extrusion:
(C) This billet is heated to 650 ° C. with a billet heater, then heated to 950 ° C. with a heating furnace (induction heater), and after reaching 950 ° C., 2 minutes have passed and removed from the heating furnace, with a hot extruder After the piercing process with a diameter of 80 mm at the center of the billet, immediately (without delay), an extruded element tube having an outer diameter of 96 mm and a wall thickness of 9.5 mm was produced with the same hot extruder (cross-sectional reduction rate: 96. 6%). The extruded tube after hot extrusion was water-cooled to an average cooling rate of 40 ° C./second up to 300 ° C.

Rolling, drawing (first half):
(D) The extruded element tube is rolled to produce a rolled element tube having an outer diameter of 35 mm and a wall thickness of 2.3 mm, and the rolling element tube has a cross-sectional reduction rate of 35% or less in one drawing process. Then, drawing and drawing were performed to obtain an outer diameter of 22 mm, a wall thickness of 1.2 mm to an outer diameter of 10 mm, and a wall thickness of 0.91 mm. The area reduction rate until the subsequent intermediate annealing is 80 to 97%.

Intermediate annealing:
(E) Thereafter, as intermediate annealing, heating was performed at a temperature shown in Table 1 at a temperature rising rate in the range of 300 to 400 ° C. at 150 ° C./min or more in a heating furnace (induction heater). The sample was held in common for 30 minutes, passed through a cooling zone and gradually cooled to room temperature, and used as a test material.

Drawing (second half):
(F) After this intermediate annealing, the drawing drawing process was subsequently performed to obtain an outer diameter of 9.52 mm and a wall thickness of 0.80 mm, and copper pipes with various changes in the cross-sectional area reduction ratio in the latter drawing process were created. . Table 1 shows the cross-sectional area reduction ratio (%).

Final annealing:
(G) As the final annealing, the drawing tube was annealed in a reducing gas atmosphere by the roller hearth furnace at 500 ° C. for 60 minutes (the average heating rate was 12 ° C./minute in common), and then water-cooled Thus, a test material was obtained.

(H) Measure copper tube properties such as average crystal grain size, tensile strength, fracture strength, bending workability, and high temperature embrittlement resistance of these manufactured copper tubes (outer diameter 9.52 mm, wall thickness 0.80 mm) did.

(Grain structure)
The average crystal grain size in the cross-sectional structure parallel to the axial direction of the central part of the manufactured copper tube was measured by the crystal orientation analysis method in which an EBSP system was mounted on an SEM. The measurement range was a rectangular region of 0.80 mm in the copper tube thickness direction × 1.5 mm in the axial direction of the tube.

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

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

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

(Softening resistance)
The produced copper alloy pipe is subjected to a tensile test at a high temperature of 800 ° C., and the elongation at break (%) is measured. The copper alloy pipe thinned to 1 mm or less does not cause embrittlement cracking during the brazing treatment. The scale is excellent in high temperature embrittlement resistance. Specifically, using a test piece in which both ends of the manufactured copper alloy tube (length: 260 mmL) are punched at a length of 80 mmL, using an AG-G type autograph tensile tester, the heating rate is 50 ° C. The temperature was raised at / min and held for 15 minutes after reaching 800 ° C. Thereafter, the elongation at break (%) of this high-temperature test piece was measured under the conditions of a test speed of 10.0 mm / min and GL = 50 mm. Three test pieces under the same conditions were tested, and the average value thereof was adopted.

(Invention example)
As shown in Table 1, since Invention Examples 1 to 12 have a copper alloy composition within the scope of the present invention and the drawing (intermediate annealing) conditions are appropriate, the cross section of this copper alloy tube structure is parallel to the copper alloy tube axis direction. The average crystal grain size in is 20 μm or less. For this reason, bending workability is good in spite of high tensile strength and breaking pressure. Moreover, the elongation in a high temperature tensile test at 800 ° C. of the copper alloy tube, which is equivalent to brazing heating, is 25% or more, and the high temperature embrittlement resistance is excellent. This is presumed that the effect of suppressing grain boundary oxidation by the copper alloy composition of the present invention is large.

(Comparative example)
In Comparative Examples 1 to 12, as shown in Table 1, the composition of the copper tube is out of the scope of the present invention.

  Comparative Example 1 does not contain Mg or B. For this reason, although the drawing (intermediate annealing) conditions are in an appropriate range and the average crystal grain size is 20 μm or less, the elongation in a high-temperature tensile test at 800 ° C. of the copper alloy tube is as low as about 21%, High temperature embrittlement resistance is poor. This is presumed that the copper alloy composition of Comparative Example 1 has no effect of suppressing grain boundary oxidation.

  The comparative example 2 has too little Sn content. For this reason, although the drawing (intermediate annealing) conditions are in an appropriate range and the average crystal grain size is 20 μm or less, the strength is low and the breaking pressure is too low.

  In Comparative Example 3, although each content of Mg and Sn is within each specified range, 2.5 × “Mg mass%” + “Sn mass%” does not satisfy 0.5% or more. For this reason, the tensile strength and breaking pressure required for a copper alloy pipe are too low. Moreover, the intermediate annealing in the middle of drawing is not performed. For this reason, the average crystal grain size becomes too large, and the elongation in a high temperature tensile test at 800 ° C. of the copper alloy tube is as low as about 24%.

  Comparative Example 4 also has the same composition as Comparative Example 3, and 2.5 × “Mg mass%” + “Sn mass%” does not satisfy 0.5% or more, but unlike Comparative Example 3, it is an intermediate during drawing. Annealed. For this reason, the average crystal grain size in a cross section parallel to the copper alloy tube axis direction of the copper alloy tube structure is 20 μm or less, and the bending workability is also good. Moreover, the elongation in the 800 degreeC high temperature tensile test of the copper alloy pipe | tube equivalent to the heating of brazing is 25% or more, and the high temperature embrittlement resistance is excellent. However, the tensile strength and breaking pressure required for copper alloy tubes are too low.

  In Comparative Example 5, the Mg content is too high, and 2.5 × “Mg mass%” + “Sn mass%” does not satisfy 2.0% or less. For this reason, intensity | strength becomes too high and bending workability will deteriorate. In addition, since the strength is too high, the drawing (intermediate annealing) conditions are in an appropriate range, and the elongation of the copper alloy tube in the high-temperature tensile test at 800 ° C. is 18% even though the average crystal grain size is 20 μm or less. About low.

  In Comparative Example 6, although each content of Mg and Sn is within each specified range, 2.5 × “Mg mass%” + “Sn mass%” does not satisfy 2.0% or less. For this reason, intensity | strength becomes too high and bending workability will deteriorate. Further, since the strength is too high, the drawing (intermediate annealing) conditions are in an appropriate range, and the elongation of the copper alloy tube in the high-temperature tensile test at 800 ° C. is 17% despite the average grain size being 20 μm or less. About low.

  As shown in Table 1, in Comparative Examples 7 to 10, the composition of the copper tube is within the range of the present invention, but the drawing (intermediate annealing) conditions are outside the appropriate range.

  In Comparative Example 7, intermediate annealing in the middle of drawing is not performed, and in Comparative Example 8, the intermediate annealing temperature in the middle of drawing is too low. For this reason, in Comparative Examples 7 and 8, the average crystal grain size is too large, the elongation of the copper alloy tube in the high temperature tensile test at 800 ° C. is as low as about 20% and 19%, respectively, and the high temperature embrittlement resistance is inferior. .

  In Comparative Example 9, the intermediate annealing temperature during drawing is too high. For this reason, the average crystal grain size becomes too large, the elongation of the copper alloy tube in the high temperature tensile test at 800 ° C. is as low as 19% and 20%, respectively, and the high temperature embrittlement resistance is inferior.

  In Comparative Example 10, the area reduction rate in the drawing after the intermediate annealing is too small. For this reason, the amount of accumulated strain is too small to increase the driving force necessary for the formation of recrystallized nuclei, so that the average crystal grain size becomes too large in the subsequent final annealing. Therefore, the elongation of the copper alloy tube in the high temperature tensile test at 800 ° C. is as low as about 17%, and the high temperature embrittlement resistance is inferior.

  In Comparative Examples 11 to 13, as shown in Table 1, the drawing (intermediate annealing) conditions are in an appropriate range, but the composition of the copper tube is outside the scope of the present invention.

  Since the comparative example 11 had too high Sn content, it was not able to extrude and a copper pipe could not be manufactured.

  In Comparative Example 12, since the P content was too high, cracks occurred after extrusion, and a copper tube could not be produced.

  In Comparative Example 13, since the B content was too high, cracks occurred after extrusion, and a copper tube could not be produced.

  From the above results, even when the thickness is reduced to 1.0 mm or less, the present invention for obtaining a Sn-P-based copper alloy tube having excellent high-temperature embrittlement resistance and excellent fracture strength and bending workability is provided. The definition of the component composition, the crystal grain structure, and the significance of preferable production conditions for obtaining such a structure are supported. Further, the effect of suppressing grain boundary oxidation by the copper alloy composition of the present invention due to excellent high temperature embrittlement resistance is also supported.

  As described above, according to the present invention, even if the thickness is reduced to 1.0 mm or less, the Sn-P-based copper alloy tube having excellent high temperature embrittlement resistance and excellent fracture strength and bending workability is obtained. Can be provided. As a result, it can be suitably applied to a heat exchanger tube for a heat exchanger that is used after being thinned to a high operating pressure by a new alternative refrigerant.

Claims (3)

  Sn: 0.3-2.0% by mass, P: 0.005-0.1% by mass, Mg: 0.005-0.5% by mass, and 0.5% ≦ 2.5 × “Mg mass%” + “Sn mass%” ≦ 2.0%, the balance is made of a copper alloy having a composition consisting of Cu and inevitable impurities, and the average crystal grain size in the cross section parallel to the axial direction of the tube is A copper alloy tube having a structure of 20 μm or less.   Sn: 0.4 to 2.0% by mass, P: 0.005 to 0.1% by mass, B: 0.001 to 0.06% by mass, with the balance being Cu and inevitable impurities A copper alloy tube having a structure in which an average crystal grain size in a cross section parallel to the axial direction of the tube is 20 μm or less.   The copper alloy pipe according to claim 1 or 2, wherein an elongation of the copper alloy pipe in a high-temperature tensile test at 800 ° C is 25% or more.