EP4674996A1

EP4674996A1 - Aluminum alloy sheet, method for producing same and heat exchanger

Info

Publication number: EP4674996A1
Application number: EP24784755.1A
Authority: EP
Inventors: Shinichi Nakamura; Yuki Totani; Takeyoshi Doko
Original assignee: UACJ Corp
Current assignee: UACJ Corp
Priority date: 2023-04-03
Filing date: 2024-03-22
Publication date: 2026-01-07
Also published as: JP2024147043A; WO2024209964A1; KR20250171268A; CN120958155A; MX2025011491A

Abstract

An aluminum alloy sheet having single-layer heat joining capability, the aluminum alloy sheet being formed of an aluminum alloy is provided. The aluminum alloy sheet comprises: Si of 2.00 to 3.00 mass %; Fe of 0.05 to 0.40 mass %; and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities. After a heating test in which a temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to a rolled surface and perpendicular to a rolling direction, a proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less.

Description

TECHNICAL FIELD

The present invention relates to an aluminum alloy sheet for single-layer heat joining and a heat exchanger manufactured by using the same.

BACKGROUND ART

Brazing is often used in methods for manufacturing products such as heat exchangers and heat sinks that are formed of aluminum material and have many metal joints. As an aluminum material for brazing, a brazing sheet on which a brazing filler metal has been clad to a core formed of aluminum material or a preplaced brazing filler metal has been used. However, the use of clad materials such as brazing sheets for joining a plurality of layers to each other after stacking them and additional joining materials such as preplaced brazing filler metals have contributed to the rising cost of heat exchangers and other equipment due to manufacturing costs and material costs thereof.
In view of this, an aluminum alloy material that can be heat joined in a single layer has been proposed in recent years (e.g., Patent Literature 1 and 2). The aluminum alloy material contains an Al-Si-based alloy, and the liquid phase generated inside the alloy material by heating is used for joining. With the aluminum alloy material, the liquid phase described above acts as a brazing filler metal, and thus it can be joined to other members without using a joining material such as a preplaced brazing filler metal although it has only a single layer. In the present invention, the capability of joining by heating even without a joining material in this manner is called "heat joining capability". The joining by such an aluminum alloy material having the heat joining capability in a single layer is called "heat joining", and the heating temperature at that time is called "heat joining temperature".
For the aluminum alloy material having the heat joining capability in a single layer, the material changes into a semi-molten state during the heat joining, and thus it is important to ensure deformation resistance at brazing temperature. As a method for improving deformation resistance in an aluminum alloy material, for example, Patent Literature 3 and 4 disclose an aluminum alloy material having excellent deformation resistance and the heat joining capability in a single layer, which is obtained by using a metal structure that allows grains to become coarse after heating for brazing and suppresses the formation of the liquid phase at grain boundaries.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: Japanese Patent 5436714
Patent Literature 2: WO 2022/176420
Patent Literature 3: Japanese Patent 5345264
Patent Literature 4: Japanese Patent 5732594

SUMMARY OF INVENTION

Technical Problem

Aluminum alloy materials that can be joined to other members by their own action (liquid phase bleeding from a base material) without using a joining material such as a brazing filler metal or a filler metal use less liquid phase for brazing joint than conventional brazing sheet fin materials clad with brazing filler metal.
Thus, the fillet area formed at the joint between a fin and a tube is smaller, which reduces the joining performance (brazeability) of the fin. If the fillet area is smaller, the corrosion wear time (corrosion protection life) when exposed to a corrosive environment is shortened, which causes the fin to peel off earlier, and the fin does not function adequately for sacrificial corrosion protection. Furthermore, there is a problem that the fin peeling causes the material to deform.
To increase the amount of the liquid phase, a method of increasing the Si content can be used, but a high Si content causes excessive bleeding from the base material during brazing heating, resulting in an extreme decrease in material strength, which does not provide sufficient strength and reduces corrosion resistance. A method of increasing the liquid phase ratio by increasing the Cu content can be also used, but there are problems that, for example, the corrosion resistance deteriorates significantly and the material strength of the material increases, resulting in reduced formability, and thus it is difficult to solve the problems by adjusting the composition.
Thus, the present invention has been made in view of the background described above, and it is an object thereof to provide an aluminum alloy sheet having excellent brazeability while ensuring material strength when manufacturing various types of aluminum alloy structural bodies. Solution to Problem
As a result of intensive studies, the inventors found that the distribution of grain boundaries and the average grain size of an aluminum alloy sheet after a predetermined heating test exert an influence on brazeability and material strength. The inventors then found an aluminum alloy material for an aluminum alloy sheet having joining ability and material strength and have completed the present invention, the aluminum alloy sheet comprising: Si of 2.00 to 3.00 mass %; Fe of 0.05 to 0.40 mass %, and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), wherein in a cross section perpendicular to a rolled surface and perpendicular to a rolling direction after a brazing heating test, the proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and the average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less.
That is, the present invention (1) provides an aluminum alloy sheet having single-layer heat joining capability, the aluminum alloy sheet being formed of an aluminum alloy comprising: Si of 2.00 to 3.00 mass %; Fe of 0.05 to 0.40 mass %; and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities, wherein
after a heating test in which a temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to a rolled surface and perpendicular to a rolling direction, a proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less.
The present invention (2) provides the aluminum alloy sheet according to (1), wherein in the cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, regions each having two or more grain boundaries in the sheet thickness direction are present at a proportion of 3% or more in the width direction.
The present invention (3) provides the aluminum alloy sheet according to (1) or (2), wherein in the cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, maximum width of regions having no grain boundaries in the sheet thickness direction is 2000 µm or less.
The present invention (4) provides the aluminum alloy sheet according to (1) or (2), wherein a sheet thickness is 0.08 mm or less.
The present invention (5) provides the aluminum alloy sheet according to (3), wherein a sheet thickness is 0.08 mm or less.
The present invention (6) provides a method for manufacturing an aluminum alloy sheet, the method comprising:

a casting step of casting, at a casting speed higher than 0.50 m/min and less than 0.70 m/min, a cast-rolled sheet of an aluminum alloy comprising Si of 2.00 to 3.00 mass %, Fe of 0.05 to 0.40 mass %, and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities; and
a cold-rolling step of cold rolling the cast-rolled sheet two or more times, wherein
annealing is performed one or more times between after the casting step and before a final cold rolling at the cold-rolling step, and
annealing conditions for all annealings are set such that an annealing temperature is 200 to 550°C and an annealing time is 1 to 10 hours.

The present invention (7) provides a heat exchanger comprising a tube made of an aluminum alloy through which a working fluid flows and a fin made of an aluminum alloy metallically joined to the tube, wherein

the tube is formed using a heat-exchanger tube material formed of an aluminum alloy,
the fin is formed using an aluminum alloy sheet formed of an aluminum alloy comprising Si of 2.00 to 3.00 mass %, Fe of 0.05 to 0.40 mass %, and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities, and having single-layer heat joining capability, and
in a cross section of the fin perpendicular to a rolled surface and perpendicular to a rolling direction, a proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less.

The present invention (8) provides the heat exchanger according to (7), wherein in the cross section of the fin perpendicular to the rolled surface and perpendicular to the rolling direction, regions each having two or more grain boundaries in the sheet thickness direction are present at a proportion of 3% or more in the width direction.
The present invention (9) provides the heat exchanger according to (7) or (8), wherein in the cross section of the fin perpendicular to the rolled surface and perpendicular to the rolling direction, maximum width of regions having no grain boundaries in the sheet thickness direction is 2000 µm or less.
The present invention (10) provides a heat exchanger comprising a tube made of an aluminum alloy through which a working fluid flows and a fin made of an aluminum alloy metallically joined to the tube, wherein

the heat exchanger is obtained by combining at least a heat-exchanger tube material formed of an aluminum alloy and a heat-exchanger fin material formed of an aluminum alloy, and then heating the resulting combined body to join the heat-exchanger tube material and the heat-exchanger fin material, and
the heat-exchanger fin material is a formed body of the aluminum alloy sheet according to any one of (1) to (5).

Advantageous Effect of Invention

According to the present invention, an aluminum alloy sheet having excellent brazeability while ensuring material strength when manufacturing various types of aluminum alloy structural bodies is provided.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a schematic phase diagram of an Al-Si alloy that is a typical binary-phase eutectic alloy.
Fig. 2 is an explanatory diagram illustrating a liquid phase formation mechanism in an aluminum alloy for forming an aluminum alloy sheet according to the present invention, in joining with the aluminum alloy sheet according to the present invention.
Fig. 3 is an explanatory diagram illustrating a liquid phase formation mechanism in an aluminum alloy for forming an aluminum alloy sheet according to the present invention, in joining with the aluminum alloy sheet according to the present invention.
Fig. 4 is a schematic phase diagram of an Al-Si alloy that is a typical binary-phase eutectic alloy.
Fig. 5 is a picture illustrating an example of measurement of grain boundaries in a cross section.
Fig. 6 is a picture illustrating an example of measurement of grain boundaries in a cross section.
Fig. 7 is a picture illustrating an example of measurement of the average grain size on a sheet surface.

DESCRIPTION OF EMBODIMENTS

An aluminum alloy sheet according to the present invention is an aluminum alloy sheet having single-layer heat joining capability, the aluminum alloy sheet being formed of an aluminum alloy comprising: Si of 2.00 to 3.00 mass %; Fe of 0.05 to 0.40 mass %; and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities, wherein after a heating test in which a temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to a rolled surface and perpendicular to a rolling direction, a proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less.
The aluminum alloy sheet according to the present invention comprises Si, Fe, and Mn as essential elements. The aluminum alloy sheet according to the present invention is formed by the essential elements, optional additive elements added as necessary, and as the balance other than these, aluminum and inevitable impurities.
The aluminum alloy sheet according to the present invention is formed of an aluminum alloy comprising: Si of 2.00 to 3.00 mass %; Fe of 0.05 to 0.40 mass %; and Mn of 0.80 to 1.80 mass %, with the balance being Al and inevitable impurities. In other words, the aluminum alloy sheet according to the present invention is formed by the aluminum alloy.
The aluminum alloy for the aluminum alloy sheet according to the present invention may further comprise Cu of 0.20 mass % or less and Zn of 6.00 mass % or less as the optional additive elements. In other words, in the aluminum alloy for the aluminum alloy sheet according to the present invention, the content of Cu is 0.00 to 0.20 mass % and the content of Zn is 0.00 to 6.00 mass %.
The aluminum alloy for the aluminum alloy sheet according to the present invention may further comprise, as the optional additive elements, any one or two or more among Mg of 0.08 mass % or less, Ti of 3.00 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less. The aluminum alloy for the aluminum alloy sheet according to the present invention may also comprise In of 0.10 mass % or less, Sn of 0.10 mass % or less, and rare earth elements of 0.10 mass % or less as the optional additive elements.
Si is an element that forms an Al-Si-based liquid phase to contribute to joining. The Si content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 2.00 to 3.00 mass %, preferably 2.10 to 2.80 mass %, more preferably 2.20 to 2.60 mass %. When the Si content in the aluminum alloy is within this range, a sufficient amount of liquid phase can be formed, allowing satisfactory joining to be obtained because the amount of liquid phase bleeding is sufficient, and also the material strength does not decrease too much during heating, allowing the material to maintain its shape. Furthermore, when the Si content in the aluminum alloy is within the above-described range, the temperature difference between a solidus and a liquidus of the aluminum alloy is larger, resulting in a longer time for solidification to be completed during casting near the center of the sheet thickness. As a result, solute atoms are discharged from near the surface layer to the center, and second-phase particles are present densely due to the more concentrated solute atoms, whereby grain growth in the center of the sheet thickness is inhibited. Thus, during heat joining, the number of grains in the sheet thickness direction increases, and deformation due to grain boundary sliding is suppressed. The thicker a sheet and the higher a heating temperature, the larger is the amount of the bleeding liquid phase. Thus, the amount of the liquid phase required during the heat joining is adjusted in accordance with the structure or dimensions of fins of a heat exchanger to be manufactured, and the Si content in the aluminum alloy and the heat joining temperature are adjusted in accordance with the amount of the liquid phase required during the heat joining. If the Si content in the aluminum alloy is less than the above-described range, a sufficient amount of liquid phase cannot be formed, resulting in less liquid phase bleeding and incomplete joining. If the Si content exceeds the above-described range, Si particles in the aluminum alloy material increases and the amount of the formed liquid phase increases, and thus the material strength during the heating significantly decreases, which makes it very difficult to maintain the shape as a fin material.
Fe has an effect of increasing the strength by slightly dissolving into the matrix in a solid state, and also has an effect of preventing reduction of the strength at high temperature in particular by dispersing as crystallized substances or precipitates. The Fe content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.05 to 0.40 mass %, preferably 0.08 to 0.35 mass %. When the Fe content in the aluminum alloy is within this range, the strength is higher and reduction of the strength at high temperature can be prevented. If the Fe content in the aluminum alloy is less than this range, not only the above-described effect is small, but also costs increase due to the need to use an ingot with higher purity. If the Fe content in the aluminum alloy exceeds this range, coarse intermetallic compounds are formed during casting, causing manufactural problems, and corrosion resistance decreases when a joined body is exposed to a corrosive environment (especially a corrosive environment where liquid flows), and furthermore, recrystallized grains become finer by heating during joining, resulting in lower deformation resistance.
Mn dissolves into the aluminum matrix in a solid state during casting, and promotes the formation of Al-based intermetallic compounds having equivalent diameters of 0.01 to 0.50 µm in the subsequent machining process. The Mn content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.80 to 1.80 mass %, preferably 1.00 to 1.60 mass %. When the Mn content in the aluminum alloy is within this range, Al-based intermetallic compounds having equivalent diameters of 0.01 to 0.50 µm are present in a sufficient amount, a pinning effect of appropriate strength can be obtained, and only limited grains grow, resulting in coarse grains. Thus, grain boundary sliding is suppressed by the coarse grains, and deformation resistance increases. If the Mn content in the aluminum alloy is less than the above-described range, the above-described effect is not sufficiently obtained and the deformation resistance decreases. If the Mn content exceeds the above-described range, coarse intermetallic compounds are formed during casting, causing manufactural problems.
In addition to Si, Fe, and Mn, the aluminum alloy for the aluminum alloy sheet according to the present invention may further comprise any one or two or more among Cu, Zn, Mg, Ti, Zr, Cr, V, Be, Sr, Bi, Na, Ca, In, Sn, and rare earth elements as the optional additive elements if necessary.
Cu is an additive element that dissolves into the matrix in a solid state to increase the strength. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Cu, the Cu content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.20 mass % or less, preferably 0.01 to 0.18 mass %. When the Cu content in the aluminum alloy is within this range, the strength is higher. If the Cu content in the aluminum alloy exceeds this range, the corrosion resistance decreases.
Zn is an effective element for increasing corrosion resistance due to the sacrificial anti-corrosion action. Zn has the effect of setting the natural potential less-noble by almost uniformly dissolving into the matrix in a solid state. For example, when the aluminum alloy material according to the present invention is used for a fin material, by setting the potential thereof less-noble, the sacrificial anti-corrosion action for relatively inhibiting the corrosion of a tube joined to the fin can be exerted. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Zn, the Zn content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 6.00 mass % or less, preferably 0.05 to 6.00 mass %, and particularly preferably 0.10 to 5.00 mass %. When the Zn content in the aluminum alloy is within this range, the corrosion resistance is higher. If the Zn content in the aluminum alloy exceeds this range, the corrosion rate excessively increases, resulting in low self-corrosion resistance and low sacrificial anti-corrosion action.
After the heat joining, Mg forms Mg₂Si, which causes age hardening and increases the strength. Thus, Mg is an additive element that has a strength-increasing effect. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Mg, the Mg content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.08 mass % or less, preferably 0.005 to 0.07 mass %. When the Mg content in the aluminum alloy is within this range, the strength is higher. If the Mg content in the aluminum alloy exceeds this range, Mg reacts with flux to form a high-melting compound, which significantly reduces the joining performance. In the present invention, as for Mg or not only Mg but also other chemical compositions, a content that is a predetermined content or less includes a content of 0 mass %.
Cr increases the strength by solid-solution strengthening, and also precipitates Al-Cr-based intermetallic compounds, which act to coarsen grains after heating. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Cr, the Cr content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.30 mass % or less, preferably 0.05 to 0.30 mass %. When the Cr content in the aluminum alloy is within this range, the strength is higher. If the Cr content in the aluminum alloy exceeds this range, coarse intermetallic compounds are easily formed and plastic workability decreases.
Zr precipitates as an Al-Zr-based intermetallic compound and has the effect of increasing the strength after heat joining by dispersion strengthening. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Zr, the Zr content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.30 mass % or less, preferably 0.05 to 0.30 mass %. When the Zr content in the aluminum alloy is within this range, the strength is higher. If the Zr content in the aluminum alloy exceeds this range, coarse intermetallic compounds are easily formed and plastic workability decreases.
Ti and V have the effects of increasing the strength by dissolving into the matrix in a solid state, and also preventing the propagation of corrosion in the sheet thickness direction by being distributed in layers. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Ti, the Ti content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.30 mass % or less, preferably 0.05 to 0.30 mass %. When the aluminum alloy comprises V, the V content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.30 mass % or less, preferably 0.05 to 0.30 mass %. When the Ti content or the V content in the aluminum alloy is within this range, the strength is higher and the propagation of corrosion in the sheet thickness direction can be prevented. If the Ti content or the V content in the aluminum alloy exceeds this range, giant crystallized substances are formed, which degrade formability and corrosion resistance.
Be, Sr, Bi, Na, and Ca can increase the joining performance by finely dispersing Si particles and increasing the fluidity of the liquid phase, for example. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Be, the Be content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.10 mass % or less, preferably 0.0001 to 0.10 mass %. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Sr, the Sr content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.10 mass % or less, preferably 0.0001 to 0.10 mass %. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Bi, the Bi content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.30 mass % or less, preferably 0.0001 to 0.30 mass %. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Na, the Na content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.10 mass % or less, preferably 0.0001 to 0.10 mass %. When the aluminum alloy for the aluminum alloy sheet according to the present invention comprises Ca, the Ca content in the aluminum alloy for the aluminum alloy sheet according to the present invention is 0.05 mass % or less, preferably 0.0001 to 0.05 mass %. When the Be content, the Sr content, the Bi content, the Na content, or the Ca content in the aluminum alloy is within the above-described corresponding range, the joining performance is higher. If the Be content, the Sr content, the Bi content, the Na content, or the Ca content in the aluminum alloy exceeds the above-described range, adverse effects such as reduction of corrosion resistance may occur. When the aluminum alloy comprises one or two or more among Be, Sr, Bi, Na, and Ca, the respective additive elements all need to be within the above-described composition ranges. The aluminum alloy for the aluminum alloy sheet according to the present invention may also comprise In of 0.10 mass % or less, Sn of 0.10 mass % or less, and rare earth elements of 0.10 mass % or less.
In the heating test in which the temperature of the aluminum alloy sheet according to the present invention is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is the heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to a rolled surface and perpendicular to the rolling direction after the heating test, the proportion of regions each having one or more grain boundaries in the sheet thickness direction is 25% or more and less than 90%, preferably 30 to 85%, more preferably 35 to 80% in the width direction, and the average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less, preferably 200 to 900 µm, more preferably 300 to 850 µm. In other words, the aluminum alloy sheet according to the present invention has such a metal structure that, by heating in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is heating in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably raised at 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, the proportion of regions each having one or more grain boundaries in the sheet thickness direction is 25% or more and less than 90%, preferably 30 to 85%, more preferably 35 to 80% in the width direction, and the average grain size on the sheet surface in a direction perpendicular to the rolling direction is 950 µm or less, preferably 200 to 900 µm, more preferably 300 to 850 µm. The inventors have found that the aluminum alloy sheet having such a metal structure that, by heating in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is heating in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably raised at 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction, the grain boundaries in the sheet thickness direction fall within the above-described ranges and the average grain size on the sheet surface in a direction perpendicular to the rolling direction falls within the above-described ranges can satisfy both material strength and brazeability in heat joining in a single layer.
In the heating test in which the temperature of the aluminum alloy sheet according to the present invention is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is the heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, the proportion of regions each having one or more grain boundaries in the sheet thickness direction is 25% or more and less than 90%, preferably 30 to 85%, more preferably 35 to 80% in the width direction. The aluminum alloy sheet according to the present invention has many channels, or grain boundaries, for the liquid phase bleeding from the base material to flow. The fillet area can be increased in the aluminum alloy sheet according to the present invention because of these many channels for the liquid phase. Thus, in such an aluminum alloy sheet that, in the heating test in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is the heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, regions each having one or more grain boundaries in the sheet thickness direction fall within the above-described range, many channels for the liquid phase to flow are present and the fillet area can be increased. Herein, the number of grain boundaries in the sheet thickness direction in a sheet cross section is determined after heating because it is very difficult to observe the number of grain boundaries in the sheet thickness direction in a sheet cross section during the heating.
In the heating test in which the temperature of the aluminum alloy sheet according to the present invention is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is the heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, the average grain size on the sheet surface in a direction perpendicular to the rolling direction after the heating test is 950 µm or less, preferably 900 µm or less, more preferably 850 µm or less. A finer grain size increases the number of channels for the liquid phase to flow, thereby being able to increase the fillet area. Thus, in such an aluminum alloy sheet that, in the heating test in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is the heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, the average grain size on the sheet surface in a direction perpendicular to the rolling direction after the heating test falls within the above-described range, the fillet area can be increased. Herein, the average grain size is determined after heating because it is very difficult to observe the average grain during the heating.
In the heating test in which the temperature of the aluminum alloy sheet according to the present invention is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is the heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, the average grain size on the sheet surface in a direction perpendicular to the rolling direction after the heating test is preferably 200 µm or more, more preferably 300 µm or more. If the grain size is too small, grain boundary sliding and a significant reduction in material strength will occur. Thus, in such an aluminum alloy sheet that, in the heating test in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is the heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, the average grain size on the sheet surface in a direction perpendicular to the rolling direction after the heating test falls within the above-described range, grain boundary sliding and a significant reduction in material strength are less likely to occur.
In the aluminum alloy sheet according to the present invention, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, regions each having two or more grain boundaries in the sheet thickness direction are present at a proportion of 3% or more in the width direction, preferably 5% or more. When the regions each having two or more grain boundaries are present in the above-described range, more channels for liquid phase bleeding from the base material to flow are present, resulting in excellent brazeability.
In a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test of the aluminum alloy sheet according to the present invention, the maximum width of regions having no grain boundaries in the sheet thickness direction is 2000 µm or less, preferably 1800 µm or less. When the regions having no grain boundaries fall within the above-described range, more channels for liquid phase bleeding from the base material to flow are present, resulting in excellent brazeability.
The heating test in the present invention in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes is the heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes. Under these heating conditions, the heating test is performed on a test sample of the aluminum alloy sheet according to the present invention in an atmosphere of inert gas first. Subsequently, in the test sample after the heating test, the number of grain boundaries in the sheet thickness direction in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction is measured, and the average grain size on the sheet surface in a direction perpendicular to the rolling direction is measured. The temperature rise conditions for the heating test are conditions in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then raised up to 600°C, which are conditions in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, and raised from 580°C to the holding temperature in 8±3 minutes.
In the present invention, the number of grain boundaries in the sheet thickness direction in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction is measured as follows. Specifically, the sample after brazing heating is cut out perpendicularly to the rolled surface and perpendicularly to the rolling direction, is embedded in a resin, and is mirror polished. The polished embedding resin is immersed in a SWAAT test solution for 12 hours. The resin surface is then lightly polished to remove contaminants. This process is intended to corrode grain boundary portions and make it easier to identify grain boundaries. Subsequently, the structure in the cross section is photographed at 200-fold magnification using a microscope for metallographic observation. In the picture of the photographed cross section, boxes are drawn around regions each having one or more grain boundaries, the length of each portion enclosed by the corresponding box in the width direction is determined, all the lengths of the enclosed portions are summed, and the sum is divided by the total length of the photographed cross section in the width direction to calculate the proportion (percentage) of the regions each having one or more grain boundaries. In the picture of the photographed cross section, boxes are drawn around regions each having two or more grain boundaries, the length of each portion enclosed by the corresponding box in the width direction is determined, all the lengths of the enclosed portions are summed, and the sum is divided by the total length of the photographed cross section in the width direction to calculate the proportion (percentage) of the regions each having two or more grain boundaries. Among regions having no grain boundaries, that is, portions that are not enclosed as regions each having one or more grain boundaries, the length, in the width direction, of a portion the length of which in the width direction is the largest is measured to determine the maximum width of the regions having no grain boundaries in the sheet thickness direction. Herein, the number of grain boundaries in the sheet thickness direction refers to the number of grain boundary lines that intersect a line drawn in a sheet thickness direction (in a direction perpendicular to the rolled surface) on the picture of the photographed cross section.
Fig. 5 illustrates a measurement example of grain boundaries on a cross section of the aluminum alloy sheet according to the present invention. In Fig. 5, Fig. 5A is a photomicrograph of the cross section before image processing, and Fig. 5B is a picture obtained by image processing the photomicrograph in Fig. 5A, in which grain boundaries are indicated as black lines. In Fig. 5B, grain boundaries are indicated as black lines, and regions each having one or more grain boundaries in the sheet thickness direction are enclosed by white boxes. In the structural example in Fig. 5B, three portions of regions each having one or more grain boundaries in the sheet thickness direction are present. The lengths of the respective enclosed portions in the width direction are measured as regions each having one or more grain boundaries in the sheet thickness direction. Subsequently, the lengths, in the width direction, of the enclosed portions of the regions each having one or more grain boundaries in the sheet thickness direction, which are present in the entire area photographed are summed up, and the proportion thereof is calculated by the following formula: The proportion of regions each having one or more grain boundaries in the sheet thickness direction (%) = ((Total length, in the width direction, of the enclosed portions of the regions each having one or more grain boundaries in the sheet thickness direction, which are present in the entire area photographed) / (Total length of the area photographed)) × 100.
In Fig. 5A, the portions that are not enclosed are regions each having zero or more grain boundaries in the sheet thickness direction. The length, in the width direction, of a region the length of which in the width direction is the largest (the length indicated by the white double-pointed arrow in Fig. 5A) out of the regions not enclosed that are present in the entire area photographed is defined as the maximum width of regions having no grain boundaries in the sheet thickness direction.
Fig. 6 illustrates a measurement example of grain boundaries on a cross section of an aluminum alloy sheet according to the present invention. In Fig. 6, Fig. 6A is a photomicrograph of the cross section before image processing, and Fig. 6B is a picture obtained by image processing the photomicrograph in Fig. 6A, in which grain boundaries are indicated as black lines. In Fig. 6B, regions each having two or more grain boundaries in the sheet thickness direction are enclosed by white boxes, and in the structural example in Fig. 6B, two portions of regions each having two or more grain boundaries in the sheet thickness direction are present. The lengths of the respective enclosed portions in the width direction are measured as regions each having two or more grain boundaries in the sheet thickness direction. Subsequently, the lengths, in the width direction, of the enclosed portions of the regions each having two or more grain boundaries in the sheet thickness direction, which are present in the entire area photographed are summed up, and the proportion thereof is calculated by the following formula: The proportion of regions each having two or more grain boundaries in the sheet thickness direction (%) = ((Total length, in the width direction, of the enclosed portions of the regions each having two or more grain boundaries in the sheet thickness direction, which are present in the entire area photographed) / (Total length of the area photographed)) × 100.
In the present invention, the average grain size on the sheet surface in a direction perpendicular to the rolling direction is measured as follows. Specifically, a sample after brazing heating is cut in a size of 20 mm × 30 mm and polished to be faced from the sheet surface, whereby the center of the sheet thickness is exposed. Subsequently, mirror polishing and Parker etching are performed, and polarization observation is performed under a metallurgical microscope. Pictures of six views are taken at 20-fold magnification, and grain structures in the six views are observed at 20-fold magnification in a connected manner (observed such that they are parallel to the rolling direction). 10 lines of 6 mm are drawn at a pitch of 1 mm perpendicularly to the rolling direction, the number of grains on the 10 lines is counted, the total number of grains is measured, and the average grain size is calculated by the following formula: $Average grain size (μm) = (6000 \times 10) / Total number of grains on the 10 lines .$
Fig. 7 illustrates a measurement example of the average grain size on a sheet surface of the aluminum alloy sheet according to the present invention. As illustrated in Fig. 7, 10 lines of 6 mm are drawn at a pitch of 1 mm perpendicularly to the rolling direction. The number of grains on each of the 10 lines is then counted. The total number of grains on each of the 10 lines is then measured, and the average grain size is calculated by the following formula: $Average grain size (μm) = (6000 \times 10) / Total number of grains on the 10 lines .$ In Fig. 7, the grains on each line are marked with black circles near their centers. The number on the upper side of each line in Fig. 7 is a line number, and the number on the lower side of each line indicates the number of grains on each line.
The thickness of the aluminum alloy sheet according to the present invention is preferably 0.08 mm or less. The preferable sheet thickness for a fin material of a heat exchanger is 0.08 mm or less. The aluminum alloy sheet according to the present invention has excellent deformation resistance even when the sheet thickness is as small as 0.08 mm or less.
The aluminum alloy sheet according to the present invention is an aluminum alloy sheet having the single-layer heat joining capability at a temperature at which the liquid phase ratio is 5.0% or more and 35.0% or less (aluminum alloy sheet having a single-layer heat joining capability). In other words, the aluminum alloy sheet according to the present invention is a single-layer brazing sheet.
The following describes the aluminum alloy sheet having the single-layer heat joining capability (hereinafter, also referred to as "single-layer brazing sheet") at a temperature at which the liquid phase ratio is 5.0% or more and 35.0% or less.
The single-layer brazing sheet needs to be joined at a temperature at which the ratio of the mass of the liquid phase formed in the aluminum alloy material to the total mass of the aluminum alloy material (hereinafter, referred to as "liquid phase ratio") is 5% or more and 35% or less. If the liquid phase ratio exceeds 35%, the amount of the liquid phase formed is so large that the aluminum alloy material cannot maintain its shape, resulting in large deformation. If the liquid phase ratio is less than 5%, the joining becomes difficult. The liquid phase ratio is preferably 5 to 30%, and the liquid phase ratio is more preferably 10 to 20%.
The mechanism of formation of the liquid phase will be described. Fig. 1 schematically illustrates a phase diagram of an Al-Si alloy that is a typical binary-phase eutectic alloy. When an aluminum alloy material having a Si concentration of c1 is heated, a liquid phase starts to be formed at a temperature T1 near but above a eutectic temperature (solidus temperature) Te. At the eutectic temperature Te or less, crystalline precipitates are distributed in a matrix partitioned by grain boundaries, as illustrated in Fig. 2(a). When the liquid phase starts to be formed in this state, the grain boundaries in which the crystalline precipitates are distributed in larger amount due to segregation are melted to form liquid phases, as illustrated in Fig. 2(b). Subsequently, as illustrated in Fig. 2(c), the surroundings of crystalline precipitate particles and intermetallic compounds of Si, which is a main additive element composition dispersed in the matrix of the aluminum alloy, are melted in spherical shapes to form liquid phases. Furthermore, as illustrated in Fig. 2(d), these spherical liquid phases formed in the matrix dissolves again into the matrix in a solid state due to interface energy with the lapse of time or a rise of temperature, and move to the grain boundaries or surfaces by diffusion in the solid phase. Subsequently, when the temperature rises to T2 as illustrated in Fig. 1, the amount of liquid phases increases in the phase diagram. As illustrated in Fig. 1, if the Si concentration of the aluminum alloy material is c2, which is less than the maximum solid-solubility limit concentration, a liquid phase starts to be formed near but above a solidus temperature Ts2. However, unlike the case of c1, in the structure immediately before melting, crystalline precipitates do not always exist in the matrix as illustrated in Fig. 3(a). In this case, the grain boundaries are first melted to form liquid phases as illustrated in Fig. 3(b), and then a liquid phase starts to be formed in a location where the concentration of solute elements is locally high in the matrix as illustrated in Fig. 3(c). As illustrated in Fig. 3(d), these spherical liquid phases formed in the matrix dissolve again into the matrix in a solid state due to interface energy with the lapse of time or a rise of temperature, and move to the grain boundaries or surfaces by diffusion in the solid phase, in the same manner as in the case of c1. When the temperature rises to T3, the amount of the liquid phases increases more than that illustrated in the phase diagram. Thus, the joining in the present invention utilizes the liquid phase formed by local melting inside the single-layer brazing sheet (the fin material for a heat exchanger according to the present invention), which can achieve both the joining and shape keeping.
A behavior of the metal structure from the formation of a liquid phase to the joining will be described. A single-layer brazing sheet that forms a liquid phase and an aluminum alloy opposing material to be joined to this sheet are combined, and these are heated at a temperature in which the liquid phase ratio is 5.0% or more and 35.0% or less. When a joint is observed with a microscope, a very small amount of liquid phase, which is formed on a surface of the single-layer brazing sheet during joining as described above, fills a clearance between the sheet and the aluminum alloy opposing material with an oxide coating that has been broken by the action of flux, for example. Subsequently, the liquid phase near the joining interface between both alloy materials moves into the aluminum alloy opposing material, and grains of the solid phase α-phase in the single-layer brazing sheet being in contact with the joining interface accordingly grow toward inside of the aluminum alloy opposing material. Meanwhile, grains in the aluminum alloy opposing material also grow toward the single-layer brazing sheet. A structure in which the structure of the single-layer brazing sheet is embedded in the aluminum alloy opposing material near the joining interface is formed for the joining. Thus, a metal structure other than that of the single-layer brazing sheet and the aluminum alloy opposing material is not formed at the joining interface.
In contrast, when a brazing sheet on which a brazing filler metal has been clad is used and joined to an aluminum alloy opposing material by brazing heating, a fillet is formed in a joint and a eutectic structure appears. Thus, a joining structure different from that of the case in which the single-layer brazing sheet is used and joined to an aluminum alloy opposing material by brazing heating is formed. In other words, when a brazing sheet on which a brazing filler metal has been clad is used and joined to an aluminum alloy opposing material by brazing heating, the liquid-phase brazing filler metal fills the joint to form a fillet. Thus, a eutectic structure different from the surroundings is formed in the joint. Also in the welding method, the joined portion melts locally, resulting in a metal structure that is different from that of other areas.
Thus, when the single-layer brazing sheet is used to be heat joined to an aluminum alloy opposing material, the joining structure is different from that of the case using a brazing sheet on which a brazing filler metal has been clad and the case of welding, in that the metal structure of the joined portion consists of both materials to be joined, or includes a material into which both materials to be joined are integrated.
Because of such a joining behavior, when a single-layer brazing sheet is used to be heat joined to an aluminum alloy opposing material, almost no change in shape occurs near the joined area after the joining process. In other words, shape changes after joining, such as beads in welding and fillets in brazing, hardly occur when the single-layer brazing sheet is used to be heat joined to an aluminum alloy opposing material. Nevertheless, joining can be achieved by metal joining as well as welding and brazing. For example, when a drawn cup-type stacked type heat exchanger is assembled using a brazing sheet clad with a brazing filler metal (the brazing clad ratio is 5% on each side), the height of the stacked type heat exchanger decreases by 5 to 10% after brazing heating because the molten brazing filler metal gathers at the joined portion. Thus, this decrease needs to be taken into account in product design. In contrast, when a single-layer brazing sheet is used to be heat joined to an aluminum alloy opposing material, dimensional changes after the joining are very small, which enables high-precision product design.
In the present invention, it is very difficult to measure the actual liquid phase ratio during heating of the single-layer brazing sheet. Thus, the liquid phase ratio specified in the present invention is determined by equilibrium calculation. Specifically, it is calculated based on the chemical composition and the maximum reached temperature during heating by using thermodynamic equilibrium calculation software such as Thermo-Calc (registered trademark) made by Thermo-Calc Software AB.
The relation between the liquid phase ratio and the temperature will be described with reference to the phase diagram illustrated in Fig. 4. Fig. 4 is a modified diagram of Fig. 1. In Fig. 4, a line extending parallel to the horizontal axis through the temperature Te (hereinafter, referred to as "solidus 1") and a line extending toward the upper left from the left end of solidus 1 to 660°C on the vertical axis while delineating a boundary with the α-phase (hereinafter, referred to as "solidus 2") both represent solidi. A line extending toward the lower right from 660°C on the vertical axis and being in contact with the solidus 1 (hereinafter, referred to as "liquidus 1") and a line extending toward the upper right from this contact position while delineating a boundary with (Si + liquid phase) both represent liquidi.
It is assumed here that P0 is a point at a temperature T2, a line parallel to the horizontal axis of the diagram is drawn through P0, P1 is an intersection with the liquidus 1, and P2 is an intersection with the solidus 2. An Al-Si alloy having a Si concentration of C1 is in a state in which the liquid phase and the solid phase coexist at the temperature T2, and the Si concentration in the liquid phase is a concentration C_P1 at the point P1, and the Si concentration in the solid phase is a concentration C_P2 at the point P2. The ratio of the mass of the liquid phase to the total mass at the temperature T2, that is, the liquid phase ratio is a ratio of the length of a line segment P0 to P2 to the length of a line segment P1 to P2.
As described above, based on the phase diagrams of a binary-phase alloy as illustrated in Fig. 1 and Fig. 4, the liquid phase ratio is obtained by drawing from the chemical composition and temperature. Similarly, even in a ternary or more multicomponent system, the liquid phase ratio can be obtained for the ternary or more multicomponent system by drawing from the chemical composition and temperature based on a phase diagram. Although it is difficult to represent a phase diagram of a ternary or more multicomponent system as a simple X-Y plane diagram as illustrated in Fig. 4, the liquid phase ratio can be obtained by computer calculation using Thermo-Calc's thermodynamic equilibrium calculation software.
The aluminum alloy sheet according to the present invention has a such metal structure that, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after a heating test in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, the proportion of regions each having one or more grain boundaries in the sheet thickness direction is 25% or more and less than 90% in the width direction, and the average grain size on the sheet surface in a direction perpendicular to the rolling direction is 950 µm or less. Thus, many channels, or grain boundaries, for the liquid phase bleeding from the base material to flow are present during brazing heating, which can increase the fillet area by heat joining. Consequently, the aluminum alloy sheet according to the present invention has excellent brazeability even with a low Si content, and thus can satisfy both brazeability and material strength.
The aluminum alloy sheet according to the present invention is suitably used as a material for manufacturing a heat exchanger. In other words, the aluminum alloy sheet according to the present invention is suitable as an aluminum alloy sheet for a heat exchanger.
The aluminum alloy sheet according to the present invention may be manufactured by any manufacturing method. For example, the aluminum alloy sheet according to the present invention is preferably manufactured by a method for manufacturing an aluminum alloy sheet according to the present invention described below.
The method for manufacturing an aluminum alloy sheet according to the present invention is a method for manufacturing an aluminum alloy sheet, the method comprising:

The method for manufacturing an aluminum alloy sheet according to the present invention comprises at least the casting step, the cold-rolling step, and the annealing.
The casting step is a step of casting the cast-rolled sheet of the aluminum alloy having a predetermined chemical composition by the continuous casting rolling. In a continuous casting method, the cooling rate during solidification is fast, and thus coarse crystals are less likely to be formed and the formation of Si-based intermetallic compounds having equivalent diameters of 5.0 to 10 µm is suppressed. As a result, the number of recrystallized nuclei can be reduced, and thus only certain grains grow and coarse grains can be obtained. Furthermore, in the continuous casting method, difference in cooling rate in the width direction is smaller and increase in concentration due to discharge of solute atoms easily becomes more uniform in the width direction than in a DC (Direct Chill) casting method of water-cooling an ingot having a large thickness, and thus the quality of the aluminum alloy material is stable. The continuous casting method is not limited to a particular one insofar as the method can continuously cast a plate ingot like twin-roll continuous casting rolling or twin-belt continuous casting. The twin-roll continuous casting rolling is a method of supplying molten aluminum to between a pair of water-cooled rolls from a molten metal nozzle made of a refractory, thereby continuously casing and rolling a thin plate, and the Hunter process, the 3C process, and the like are known as examples thereof. The twin-belt continuous casing method is a continuous casing method of feeding a molten metal to between water-cooled rotating belts that are vertically opposed, solidifying the molten metal into a slab under cooling from belt surfaces, continuously pulling out the slab from a side of the belts opposite to the molten-metal feeding side, and winding the slab into a coiled form. In the twin-roll continuous casting rolling, the cooling rate during the casting is as high as several times to several hundred times that in a semi-continuous casting method. For example, the cooling rate in the semi-continuous casting method is 0.5 to 20°C/s, while the cooling rate in the twin-roll continuous casting rolling is 100 to 1000°C/s. Thus, the twin-roll continuous casting rolling is characterized in that dispersed particles formed during the casting are more finely distributed with higher density than in the semi-continuous casting method. This suppresses the formation of coarse crystallites, resulting in coarser grains during heating for joining. This high cooling rate also allows the amount of additive elements dissolving in a solid state to be increased. Thus, fine precipitates are formed by subsequent heat treatment, which can contribute to grain coarsening during the heating for joining.
At the casting step, the cooling rate for casting by the twin-roll continuous casting rolling is preferably 100 to 1000°C/s. If the cooling rate is less than 100°C/s, it is difficult to obtain a desired metal structure, and if it exceeds 1000°C/s, stable manufacture is difficult. The speed of the rolled sheet for casting by the twin-roll continuous casting rolling is preferably 0.3 to 3 m/min. The casting speed exerts an influence on the cooling rate. If the casting speed is less than 0.3 m/min, the compound becomes coarse because the cooling rate is not sufficient as described above. If it exceeds 3 m/min, the aluminum material does not solidify sufficiently between the rolls during casting, and a normal plate ingot cannot be obtained. The temperature of the molten metal for the casting by the twin-roll continuous casting rolling method is preferably 650 to 800°C, more preferably 680 to 750°C. The temperature of the molten metal is a temperature of a head box disposed immediately upstream of the molten-metal feed nozzle. If the temperature of the molten metal is less than the above-described range, dispersed particles of coarse intermetallic compounds are formed in the molten-metal feed nozzle, and these particles get mixed into the ingot, thereby causing sheet breakage during cold rolling. If the temperature of the molten metal exceeds the above-described range, the aluminum material does not solidify sufficiently between the rolls during the casting, and a normal plate ingot cannot be obtained.
The thickness of the plate ingot to be cast by the twin-roll continuous casting rolling is preferably 2 to 10 mm, particularly preferably 4 to 8 mm. In this thickness range, the solidification rate in the center of the plate thickness is also fast, and a uniform structure can be easily obtained. If the thickness is less than this range, the amount of aluminum passing through a casting machine per unit time is small, which makes it difficult to stably feed the molten metal in the plate width direction. If the thickness exceeds this range, winding by the rolls is difficult.
The casting speed is higher than 0.50 m/min and less than 0.70 m/min, preferably 0.53 to 0.69 m/min, more preferably 0.58 to 0.68 m/min. When the casting speed is within this range, it is easy to obtain such an aluminum alloy sheet that "in a heating test in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is a heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably raised at 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, the proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and the average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less".
At the casting step, the cast-rolled sheet formed of an aluminum alloy comprising: Si of 2.00 to 3.00 mass %, preferably 2.10 to 2.80 mass %, more preferably 2.20 to 2.60 mass %; Fe of 0.05 to 0.40 mass %, preferably 0.08 to 0.35 mass %; and Mn of 0.80 to 1.80 mass %, preferably 1.00 to 1.60 mass %, with the balance being Al and inevitable impurities is cast. The cast-rolled sheet obtained by performing the casting step may further comprise, if necessary, as optional additive elements, any one or two or more among: Cu of 0.20 mass % or less, preferably 0.01 to 0.18 mass %; Zn of 6.00 mass % or less, preferably 0.05 to 6.00 mass %, particularly preferably 0.10 to 5.00 mass %; Mg of 0.08 mass % or less, preferably 0.005 to 0.07 mass %; Ti of 0.30 mass % or less, preferably 0.05 to 0.30 mass %; Zr of 0.30 mass % or less, preferably 0.05 to 0.30 mass %; Cr of 0.30 mass % or less, preferably 0.05 to 0.30 mass %; V of 0.30 mass % or less, preferably 0.05 to 0.30 mass %; Be of 0.10 mass % or less, preferably 0.0001 to 0.10 mass %; Sr of 0.10 mass % or less, preferably 0.0001 to 0.10 mass %; Bi of 0.30 mass % or less, preferably 0.0001 to 0.30 mass %; Na of 0.10 mass % or less, preferably 0.0001 to 0.10 mass %; and Ca of 0.05 mass % or less, preferably 0.0001 to 0.05 mass %. The cast-rolled sheet obtained by performing the casting step may comprise In of 0.10 mass % or less, Sn of 0.10 mass % or less, and rare earth elements of 0.10 mass % or less of as optional additive elements, if necessary. At the casting step, the molten metal of the aluminum alloy having the above-described chemical composition is prepared, and the continuous casting rolling is performed using the molten metal, whereby the chemical composition of the cast-rolled sheet can be set to the above-described chemical composition.
The cold-rolling step is a step of cold rolling the cast the cast-rolled sheet obtained by performing the casting step. At the cold-rolling step, the cold rolling is performed two or more times. In other words, at the cold-rolling step, a cold-rolling pass is performed two or more times. The number of cold rollings at the cold-rolling step is selected as appropriate. At the cold-rolling step, the aluminum alloy sheet is cold rolled until the thickness of a final sheet is obtained. In other words, the thickness of the aluminum alloy sheet after the final cold rolling at the cold-rolling step is the thickness of the final sheet.
In the method for manufacturing an aluminum alloy sheet according to the present invention, annealing is performed one or more times between after the casting step and before the final cold rolling at the cold-rolling step. In the method for manufacturing an aluminum alloy sheet according to the present invention, the timing for performing the annealing includes timings: (1) after performing the casting step and before performing the cold-rolling step; and (2) between a cold rolling and a cold rolling when cold rolling is performed two or more times at the cold-rolling step. In either or both of (1) and (2), one or more times, preferably one to three times, more preferably one to two times, annealing is performed. When cold rolling is performed three or more times at the cold-rolling step, there are two or more intervals between cold rollings, and in such cases, annealing may be performed two or more times at the cold-rolling step. The annealing is performed to soften the aluminum alloy sheet to facilitate obtaining the desired strength in the final cold rolling. This annealing can optimally adjust the size and density of intermetallic compounds in the aluminum alloy sheet and the amount of additive elements dissolving therein in a solid state. In the method for manufacturing an aluminum alloy sheet according to the present invention, no annealing is performed after the last cold rolling at the cold-rolling step has been performed.
As the annealing conditions in the annealing, the annealing temperature is 200 to 550°C, preferably 250 to 450°C, and the annealing time is 1 to 10 hours. In other words, the annealing involves heating at an annealing temperature of 200 to 550°C, preferably 250 to 450°C, for an annealing time of 1 to 10 hours. If the annealing temperature is less than this range, the aluminum alloy sheet is not sufficiently softened, resulting in high tensile strength before heat joining. High tensile strength before heat joining results in poor formability, which deteriorates core dimensions, and consequently durability decreases. If the annealing temperature exceeds the above-described range, annealing is performed at an excessive temperature above the softening temperature of the aluminum alloy sheet, which is economically disadvantageous.
In the method for manufacturing an aluminum alloy sheet according to the present invention, the total reduction of cold rolling to be performed after the last annealing has been performed is preferably 20 to 50%, and particularly preferably 25 to 40%. When the total reduction of the cold rolling to be performed after the last annealing has been performed is within this range, it is easy to obtain the aluminum alloy sheet having "such a metal structure that, in a heating test in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is a heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably raised at 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, the average grain size in a plane parallel to a rolled surface after the heating test is 370 µm or more, preferably 370 to 1500 µm, particularly preferably 400 to 1500 µm, and the average number of grains in the sheet thickness direction after the heating test is 1.5 pieces or more, preferably 1.5 to 10.0 pieces". In the present invention, when annealing is performed only once, the last annealing refers to this one annealing, and when annealing is performed two or more times, it refers to the annealing that is performed at the very end among these two or more annealings. The total reduction A (%) of the cold rolling to be performed after the last annealing has been performed is a value calculated by the following formula. $A (%) = ((B - C) / B) \times 100$

A: Total reduction (%) of cold rolling to be performed after the last annealing has been performed
B: Thickness of the rolled sheet immediately after the last annealing has been performed
C: Thickness of the rolled sheet after the last cold rolling has been performed

When only one cold rolling is performed after the last annealing has been performed, the thickness of the rolled sheet before this cold rolling is B, and the thickness of the rolled sheet after the cold rolling is C. When a plurality of cold rollings are performed after the last annealing has been performed, the thickness of the rolled sheet before the first cold rolling among the cold rollings after the last annealing has been performed is B, and the thickness of the rolled sheet after the last cold rolling is C.
The temper of the aluminum alloy sheet obtained by performing the method for manufacturing an aluminum alloy sheet according to the present invention may be O material or may be H material. When the aluminum alloy sheet is to be H1n material or H2n material, the final cold-rolling ratio is 50% or less, preferably 5 to 50%. If the final cold-rolling ratio exceeds 50%, many recrystallized nuclei are formed during heating, resulting in finer grain size after heating for joining. If the final cold-rolling ratio is less than 5%, manufacture may be substantially difficult.
The aluminum alloy sheet obtained by performing the method for manufacturing an aluminum alloy sheet according to the present invention has the single-layer heat joining capability at a temperature at which the liquid phase ratio is 5.0% or more and 35.0% or less.
In the method for manufacturing an aluminum alloy sheet according to the present invention, a cast-rolled sheet is cast at the casting step by continuous casting and rolling, preferably by the twin-roll continuous casting and rolling, annealing is performed one or more times between after the casting step and obtaining of the final sheet, and the annealing conditions in all annealings are set such that the annealing temperature is 200 to 550°C, preferably 250 to 450°C, and the annealing time is 1 to 10 hours, and preferably the total reduction in the cold rolling after the last annealing is 20 to 50%, preferably 25 to 40%. This enables manufacture of such an aluminum alloy sheet that "in a heating test in which the temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, which is a heating test in which the temperature is preferably raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less, preferably raised at 45°C/min or less, raised from 400°C to 580°C in 8±3 minutes, raised from 580°C to the holding temperature in 8±3 minutes, and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, the proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and the average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less".
In the case of heat joining using the aluminum alloy sheet according to the present invention, the aluminum alloy sheet according to the present invention is formed into a predetermined shape, further combined with an opposing material to be joined, and then heated at the heat joining temperature to perform heat joining. In the heat joining using the aluminum alloy sheet according to the present invention, the appropriate heat joining temperature is within a temperature range in which the liquid phase ratio is 5 to 35%, and a time for which the liquid phase ratio is held at 5% or more is preferably 30 to 3600 seconds. The liquid phase ratio is preferably 5% or more because a small amount of liquid phase may make joining difficult. If the liquid phase ratio exceeds 35%, the amount of liquid phase formed is too large and the aluminum alloy material is greatly deformed during heat joining and cannot retain its shape. If the time for which the liquid phase ratio is 5% or more is less than 30 seconds, the joint may not be sufficiently filled with liquid phase, and if the time exceeds 3600 seconds, the aluminum material may be more deformed. To achieve these conditions, the heating temperature is set to 580°C to 640°C and the holding time at the heating temperature only needs to be set to be 0 to about 10 minutes during heat joining. Here, 0 minute means that cooling is started as soon as the temperature of the material reaches a predetermined joining temperature. As the heating conditions for heat joining, conditions that have been adjusted to an appropriate range to achieve a sound joining state without deformation may be used. A heating atmosphere during heat joining is preferably, for example, a non-oxidizing atmosphere in which air is replaced with nitrogen, argon, or the like. When a joined body is obtained in heat joining, even better joining performance can be obtained by using non-corrosive flux. Furthermore, in the heat joining, the joining can be performed by heating in a vacuum or under reduced pressure.
A heat exchanger according to the present invention is a heat exchanger comprising a tube made of aluminum alloy through which a working fluid flows and a fin made of aluminum alloy metallically joined to the tube, wherein

the heat exchanger is obtained by combining at least a heat-exchanger tube material formed of an aluminum alloy and a heat-exchanger fin material formed of an aluminum alloy, and then heating the resulting combined body to join the heat-exchanger tube material and the heat-exchanger fin material, and
the heat-exchanger fin material is a formed body of the aluminum alloy sheet according to the present invention.

The heat-exchanger tube material formed of an aluminum alloy for the heat exchanger according to the present invention is not limited to a particular one insofar as it is an aluminum alloy material, which is commonly used as a heat-exchanger tube material made of aluminum alloy, formed into a tubular shape.
The chemical composition of the aluminum alloy that forms the heat-exchanger tube material is not limited to a particular one, but examples of an aluminum alloy that forms a typical heat-exchanger tube material include 1000-series and 3000-series aluminum. Specifically, the examples include pure aluminum and an aluminum alloy comprising, with respect to the pure aluminum, one or two or more among: Si of 0.60 mass % or less; Fe of 0.70 mass % or less; Cu of 0.70 mass % or less; and Mn of 2.00 mass % or less, with the balance being Al and inevitable impurities.
The heat-exchanger fin material formed of an aluminum alloy for the heat exchanger according to the present invention is a formed body of the aluminum alloy sheet according to the present invention. The aluminum alloy sheet to be used for the heat-exchanger fin material of the heat exchanger according to the present invention is the same as the aluminum alloy sheet according to the present invention described above.
The heat exchanger according to the present invention is obtained by combining at least the heat-exchanger tube material formed of an aluminum alloy and the heat-exchanger fin material formed of an aluminum alloy, in addition to them, further combining necessary components such as a header, a tank, and a piping material, and heat joining their combined body.
The heating temperature during heat joining of the combined body is appropriately selected depending on the Si content. In addition to Si, Zn and Cu also exert an influence on the solidus temperature. Thus, when the aluminum alloy sheet according to the present invention comprises Zn and/or Cu in addition to Si, the heating temperature during heat joining of the combined body is appropriately selected depending on the contents of Si and Zn and/or Cu. The heating temperature for heat joining of the combined body is set within a temperature range in which the liquid phase ratio of the aluminum alloy sheet according to the present invention is 5 to 35%, and the time for which the liquid phase ratio is held at 5% or more is preferably 30 to 3600 seconds. The temperature rising rate during heat joining of the combined body is not uniquely specified and is appropriately selected in accordance with furnace structure and product design, but is generally 20 to 300°C/min.
In other words, the heat exchanger according to the present invention is a heat exchanger comprising a tube made of aluminum alloy through which a working fluid flows and a fin made of aluminum alloy metallically joined to the tube, wherein

the tube is formed using a heat-exchanger tube material formed of an aluminum alloy,
the fin is formed using an aluminum alloy sheet formed of an aluminum alloy comprising Si of 2.00 to 3.00 mass %; Fe of 0.05 to 0.40 mass %; and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities, and having single-layer heat joining capability, and
in a cross section of the fin perpendicular to a rolled surface and perpendicular to a rolling direction, a proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less.

The heat exchanger according to the present invention is obtained by using the aluminum alloy sheet having the single-layer heat joining capability as the fin material, and using the heat-exchanger tube material formed of an aluminum alloy as the opposing material, and heat joining them.
The fin for the heat exchanger according to the present invention is formed using the aluminum alloy sheet having the single-layer heat joining capability. For example, the fin for the heat exchanger according to the present invention is formed using the above-described aluminum alloy sheet according to the present invention.
The aluminum alloy that forms the fin for the heat exchanger according to the present invention is an aluminum alloy comprising: Si of 2.00 to 3.00 mass %, preferably 2.10 to 2.80 mass %, more preferably 2.20 to 2.60 mass %; Fe of 0.05 to 0.40 mass %, preferably 0.08 to 0.35 mass %; and Mn of 0.80 to 1.80 mass %, preferably 1.00 to 1.60 mass %, with the balance being Al and inevitable impurities. The aluminum alloy that the fin for the heat exchanger according to the present invention further comprise, if necessary, as optional additive elements, any one or two or more among: Cu of 0.20 mass % or less, preferably 0.01 to 0.18 mass %; Zn of 6.00 mass % or less, preferably 0.05 to 6.00 mass %, particularly preferably 0.10 to 5.00 mass %; Mg of 0.08 mass % or less, preferably 0.005 to 0.07 mass %; Ti of 0.30 mass % or less, preferably 0.05 to 0.30 mass %; Zr of 0.30 mass % or less, preferably 0.05 to 0.30 mass %; Cr of 0.30 mass % or less, preferably 0.05 to 0.30 mass %; V of 0.30 mass % or less, preferably 0.05 to 0.30 mass %; Be of 0.10 mass % or less, preferably 0.0001 to 0.10 mass %; Sr of 0.10 mass % or less, preferably 0.0001 to 0.10 mass %; Bi of 0.30 mass % or less, preferably 0.0001 to 0.30 mass %; Na of 0.10 mass % or less, preferably 0.0001 to 0.10 mass %; and Ca of 0.05 mass % or less, preferably 0.0001 to 0.05 mass %. The aluminum alloy of the aluminum alloy sheet according to the present invention may comprise In of 0.10 mass % or less, preferably 0.0001 to 0.10 mass %, Sn of 0.10 mass % or less, preferably 0.0001 to 0.10 mass %, Be of 0.10 mass %, preferably 0.0001 to 0.10 mass %, and rare earth elements of 0.10 mass % or less, preferably 0.0001 to 0.10 mass% of as optional additive elements.
In such a fin for the heat exchanger according to the present invention that, in a cross section thereof perpendicular to the rolled surface and perpendicular to the rolling direction, the proportion of regions each having one or more grain boundaries in the sheet thickness direction is 25% or more and less than 90% in the width direction and the average grain size on the sheet surface in a direction perpendicular to the rolling direction is 950 µm or less, many channels, or grain boundaries, for the liquid phase bleeding from the base material to flow are present during brazing heating, thereby increasing the fillet area by heat joining. Consequently, the fin for the heat exchanger according to the present invention has excellent brazeability even with a low Si content, and thus the heat exchanger according to the present invention is a heat exchanger that satisfies both brazeability and material strength.
In the fin for the heat exchanger according to the present invention, in the cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, regions each having two or more grain boundaries in the sheet thickness direction are present at a proportion of 3% or more, preferably 5% or more in the width direction. When the regions each having two or more grain boundaries are present in the above-described range, more channels for liquid phase bleeding from the base material to flow are present, resulting in excellent brazeability.
In the fin for the heat exchanger according to the present invention, in the cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, the maximum width of regions having no grain boundaries in the sheet thickness direction is 2000 µm or less, preferably 1800 µm or less. When the regions having no grain boundaries fall within the above-described range, more channels for liquid phase bleeding from the base material to flow are present, resulting in excellent brazeability.
The aluminum alloy that forms the tube for the heat exchanger according to the present invention is not limited to a particular one insofar as it is an aluminum alloy that is commonly used as a tube for a heat exchanger made of aluminum alloy.
The chemical composition of the aluminum alloy that forms the tube is not limited to a particular one, but examples of an aluminum alloy that forms a typical heat-exchanger tube material include 1000-series and 3000-series aluminum. Specifically, the examples include pure aluminum and an aluminum alloy comprising, with respect to the pure aluminum, one or two or more among: Si of 0.60 mass % or less, Fe of 0.70 mass % or less, Cu of 0.70 mass % or less, Mn of 2.00 mass % or less, with the balance being Al and inevitable impurities.
Examples will be given below to specifically describe the present invention, but the present invention is not limited to the Examples given below.

[Examples]

(Example 1, Comparative Examples 1 and 2)

Aluminum alloys having chemical compositions given in Table 1 were used to cast cast-rolled sheets by twin-roll continuous casting and rolling. In the chemical compositions in Table 1, "-" represents that the content is at a detection limit or less, and "balance" includes inevitable impurities. The molten metal temperature during casting by the twin-roll continuous casting and rolling was 600 to 800°C, and the thickness of the cast-rolled sheets was 6.0 mm.
Subsequently, the resulting plate-like cast-rolled sheets were annealed at 420°C for 2 hours, and then cold rolled to the thicknesses given in Table 2 (after the first cold rolling). Subsequently, annealing was performed at 370°C for 2 hours, and then the resulting sheets were cold rolled to a thickness of 0.070 mm, whereby sample materials (final sheets) were obtained.
For each of the above sample materials, in a cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, the proportions of regions each having two or more grain boundaries in the sheet thickness direction and regions each having one or more and the maximum width of regions having none were measured, and the average grain size in a direction perpendicular to the rolling direction when viewed from the sheet surface, the fillet area, and the tensile strength after brazing heating were measured. The results are given in Table 2.

The sample materials were heated in an atmosphere of inert gas such that the temperature thereof was raised from 300°C to 400°C at a temperature rising rate of 41°C/min, from 400°C to 580°C in 7.2 minutes, from 580°C to 600°C in 7.4 minutes, and to a holding temperature of 600±3°C, and was then held at 600±3°C for 4.7 minutes. Subsequently, they were cooled to room temperature, whereby test materials after the heating test were obtained.

After brazing heating, each sample was cut out perpendicularly to the rolled surface and perpendicularly to the rolling direction, was embedded in a resin, and was mirror polished.
The polished embedding resin was then immersed in a SWAAT test solution for 12 hours. The resin surface was then lightly mirror polished to remove contaminants.
Subsequently, the structure of the cross section was photographed at 200-fold magnification using a microscope for metallographic observation. At this time, the observation length was set at about 25 mm in total length.
In the picture of the photographed cross section, the grain boundaries were colored in red to make them more visible.
Boxes are drawn around regions each having one or more grain boundaries, the length of each portion enclosed by the corresponding box in the width direction was determined, all were summed, and the sum was divided by the total length of the observed area in the width direction to calculate the proportion of the regions each having one or more grain boundaries. Boxes are also drawn around regions each having two or more grain boundaries, the length of each portion enclosed by the corresponding box in the width direction was determined, all were summed, and the sum was divided by the total length of the observed area in the width direction to calculate the proportion of the regions each having two or more grain boundaries.
For regions having no grain boundaries, among portions that were not enclosed as the regions each having one or more, the length, in the width direction, of a portion the length of which in the width direction was the largest was measured to determine the maximum width of regions having no grain boundaries in the sheet thickness direction.

Each sample after brazing heating was cut in a size of 20 mm × 30 mm and polished to be faced from the sheet surface, whereby the center of the sheet thickness was exposed.
Subsequently, mirror polishing and Parker etching were performed, and polarization observation was performed under a metallurgical microscope. Pictures of six views were taken at 20-fold magnification, and grain structures in the six views were observed at 20-fold magnification in a connected manner (observed such that they were parallel to the rolling direction).

10 lines of 6 mm were drawn at a pitch of 1 mm perpendicularly to the rolling direction, the number of grains on each of the ten lines was counted, the total number of grains was determined, and the average grain size was calculated by the following formula:

Average grain size (μm) = (6000 \times 10) / Total number of grains on the ten lines .

[Table 1]

No.	Chemical composition (mass %)
No.	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Al
1	2.50	0.20	0.02	1.20	0.01	0.00	1.50	0.021	Balance
2	2.50	0.20	-	-	-	-	1.40	-	Balance

[Table 2]

	Comparative Example 1	Example 1	Comparative Example 2
Alloy	1	1	2
Casting method	CC	CC	CC
Casting speed (m/min)	0.50	0.63	0.70
Cast-rolled sheet thickness (mm)	6.0	6.0	6.0
First annealing conditions
Temperature (°C)	420	420	420
Time (hours)	2	2	2
First cold rolling	0.093	0.093	0.093
Sheet thickness after rolling (mm)	0.093	0.093	0.093
Second annealing conditions
Temperature (°C)	370	370	370
Time (hours)	2	2	2
Second cold rolling	0.070	0.070	0.070
Sheet thickness after rolling (mm)	0.070	0.070	0.070

[Table 3]

	Comparative Example 1	Example 1	Comparative Example 2
Proportion of regions each having two or more grain boundaries (%)	0	7	43
Proportion of regions each having one or more grain boundaries (%)	21	39	100
Maximum width of regions having no grain boundaries (µm)	2619	1436	0
Average grain size (µm)	953	807	763
Fillet area (µm²)	6731	8568	6638
Tensile strength after brazing heating (MPa)	151	158	-

Casting method CC: Twin-roll continuous casting and rolling
Average grain size: Average grain size on the sheet surface in a direction perpendicular to the rolling direction after the heating test

Claims

An aluminum alloy sheet having single-layer heat joining capability, the aluminum alloy sheet being formed of an aluminum alloy comprising: Si of 2.00 to 3.00 mass %; Fe of 0.05 to 0.40 mass %; and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities, wherein
after a heating test in which a temperature is raised from 300°C to 400°C at an average temperature rising rate of 60°C/min or less and then held at 600±3°C for 5±3 minutes, in a cross section perpendicular to a rolled surface and perpendicular to a rolling direction, a proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less.
The aluminum alloy sheet according to claim 1, wherein in the cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, regions each having two or more grain boundaries in the sheet thickness direction are present at a proportion of 3% or more in the width direction.
The aluminum alloy sheet according to claim 1 or 2, wherein in the cross section perpendicular to the rolled surface and perpendicular to the rolling direction after the heating test, maximum width of regions having no grain boundaries in the sheet thickness direction is 2000 µm or less.
The aluminum alloy sheet according to claim 1 or 2, wherein a sheet thickness is 0.08 mm or less.
The aluminum alloy sheet according to claim 3, wherein a sheet thickness is 0.08 mm or less.
A method for manufacturing an aluminum alloy sheet, the method comprising:
a casting step of casting, at a casting speed higher than 0.50 m/min and less than 0.70 m/min, a cast-rolled sheet of an aluminum alloy comprising Si of 2.00 to 3.00 mass %, Fe of 0.05 to 0.40 mass %, and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities; and

a cold-rolling step of cold rolling the cast-rolled sheet two or more times, wherein

annealing is performed one or more times between after the casting step and before a final cold rolling at the cold-rolling step, and

annealing conditions for all annealings are set such that an annealing temperature is 200 to 550°C and an annealing time is 1 to 10 hours.
A heat exchanger comprising a tube made of an aluminum alloy through which a working fluid flows and a fin made of an aluminum alloy metallically joined to the tube, wherein
the tube is formed using a heat-exchanger tube material formed of an aluminum alloy,

the fin is formed using an aluminum alloy sheet formed of an aluminum alloy comprising Si of 2.00 to 3.00 mass %, Fe of 0.05 to 0.40 mass %, and Mn of 0.80 to 1.80 mass %, with a Cu content of 0.20 mass % or less (including 0.00 mass %) and a Zn content of 6.00 mass % or less (including 0.00 mass %), and optionally comprising any one or two or more among Mg of 0.08 mass % or less, Ti of 0.30 mass % or less, Zr of 0.30 mass % or less, Cr of 0.30 mass % or less, V of 0.30 mass % or less, Be of 0.10 mass % or less, Sr of 0.10 mass % or less, Bi of 0.30 mass % or less, Na of 0.10 mass % or less, and Ca of 0.05 mass % or less, with the balance being Al and inevitable impurities, and having single-layer heat joining capability, and

in a cross section of the fin perpendicular to a rolled surface and perpendicular to a rolling direction, a proportion of regions each having one or more grain boundaries in a sheet thickness direction is 25% or more and less than 90% in a width direction, and average grain size on a sheet surface in a direction perpendicular to the rolling direction is 950 µm or less.
The heat exchanger according to claim 7, wherein in the cross section of the fin perpendicular to the rolled surface and perpendicular to the rolling direction, regions each having two or more grain boundaries in the sheet thickness direction are present at a proportion of 3% or more in the width direction.
The heat exchanger according to claim 7 or 8, wherein in the cross section of the fin perpendicular to the rolled surface and perpendicular to the rolling direction, maximum width of regions having no grain boundaries in the sheet thickness direction is 2000 µm or less.
A heat exchanger comprising a tube made of an aluminum alloy through which a working fluid flows and a fin made of an aluminum alloy metallically joined to the tube, wherein
the heat exchanger is obtained by combining at least a heat-exchanger tube material formed of an aluminum alloy and a heat-exchanger fin material formed of an aluminum alloy, and then heating the resulting combined body to join the heat-exchanger tube material and the heat-exchanger fin material, and

the heat-exchanger fin material is a formed body of the aluminum alloy sheet according to any one of claims 1 to 5.