JPS6136064B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an aircraft stringer material with excellent corrosion resistance and a method for manufacturing the same. The stringers in the present invention refer to the longitudinal and circumferential reinforcing members 2 (stringers) and 3 (stringer frames) used inside the fuselage 1 of an aircraft, as shown in FIG. The shape is a-shaped, bZ as shown in Figure 2.
Typical examples include type and cJ type. AA7075 alloy is mainly used as a material for aircraft stringers, but since AA7075 alloy sometimes has problems with corrosion resistance, we have developed a 7075 alloy cladding with good corrosion resistance that uses AA7075 alloy as the core material and AA7072 alloy as the skin material. materials are also used. The typical conventional manufacturing method for this AA7075 alloy clad stringer is as follows.
AA7075 alloy is homogenized at about 460-480℃ for about 16-24 hours, and this AA7075 alloy is used as a core material.
AA7072 alloy is used as the skin material and the thickness is 4 to 8 at about 400℃.
After hot cladding rolling to a thickness of approximately 2 mm, intermediate annealing at approximately 410°C for approximately 1 hour, cooling in a furnace at a cooling rate of approximately 25°C per hour, and cold rolling into a plate approximately 2 to 4 mm thick. , the temperature was raised to about 420℃ over a heating time of 8 to 12 hours, and after being heated and softened at that temperature for about 2 hours, 1
The material is cooled to approximately 235°C at a cooling rate of approximately 25°C per hour, held at approximately 235°C for 6 hours, and then air cooled to form a stringer material, which is then subjected to stepped cold rolling processing (taper roll processing) with a processing rate of up to 90%. Alternatively, stringer materials were produced by solution treatment without step cold rolling. Among the above, the stepped cold rolling process is performed, for example, in a form as shown in FIG. It is processed into a form having a part C with a processing degree of , a part D with a high processing degree, etc. This is to reduce the overall weight of the aircraft by reducing the thickness of parts that do not require strength. The stringer material thus produced is formed into a hat shape, for example, as shown in FIG. 2(a), by section roll forming, and then subjected to T6 tempering to form a stringer. When producing stringers using the process described above, the following problems arise. In other words, the clad plate of AA7075 alloy O material produced as a stringer material by the conventional method has a coarse core material crystal grain size of about 150 to 250 μm,
When this material is subjected to solution treatment after cold working (taper rolling) with a low workability of about 10 to 30%, the crystal grains of the core become even coarser than the crystal grains of the material, resulting in a workability of about 20%. According to my experience, this part is the most coarsened. Of course, even when using such materials
In areas where solution treatment is applied after 50% or more cold working, fine crystal grains with a core grain size of approximately 50 μm are obtained, but within a single stringer, the degree of cold working varies from 0 to max. There are parts with various processing degrees up to 90%, so the total length of the stringer is approximately 10 m.
It is extremely difficult to reduce the crystal grain size of the core material to 100 μm or less throughout. Figure 4 shows the relationship between the working degree (upper row) and the core material grain size (lower row) when existing stringer materials are subjected to solution treatment after cold working at various working degrees.
Give an example. The crystal grains are fine in parts D, F, G etc. where the degree of work is high, but in areas A, where the degree of work is small.
In areas such as B, C, and E, the crystal grains are very large. In areas where the crystal grain size is 100 μm or more, such as A, B, C, and E, the mechanical properties, elongation, fracture toughness, etc. decrease, and roughness and cracks occur during section roll forming. Not only is the stringer extremely difficult to manufacture, but its functionality is also reduced. Furthermore, if the crystal grains are 100 μm or more, the surface roughness after chemical milling will be rough and the fatigue strength will be reduced. The present invention aims to solve the above-mentioned problems.
Even if solution treatment is performed after cold working with a low working degree of ~30%, the crystal grain size of the core material remains 100μ.
The object of the present invention is to provide a stringer material with excellent corrosion resistance and a method for producing such a material. That is, the first invention of the present invention provides Zn5.1 to 8.1
%, Mg1.8~3.4%, Cu1.2~2.6%, Ti0.20% or less, and further contains one or two of Cr0.18~0.35% or Zr0.05~0.25%, and the remaining Al and impurities. Zn0.8~1.3 with an alloy having the composition as a core material
% aluminum alloy as the skin material, the crystal grain size of the core material is 100μm or less, and the crystal grain size of the core material is 100μm or less, and even if solution treatment is performed after mild cold working, the crystal grains of the core material are This is an aircraft stringer material with excellent corrosion resistance and a diameter of 100μm or less. The reasons for limiting the core alloy components of the stringer material of the present invention are shown below. Zn...If it is less than 5.1%, the strength of the material after T6 treatment will be low, and if it exceeds 8.1%, there is a risk of decreased toughness and stress corrosion cracking. Mg...If it is less than 1.8%, the strength of the material after T6 treatment will be low, and if it exceeds 3.4%, the cold workability of the soft material will be poor and the toughness of the material after T6 treatment will be reduced. Cu...If it is less than 1.2%, the strength of the material after T6 treatment will be low, and if it exceeds 2.6%, the toughness of the material will be reduced. Addition of Ti at 0.20% or less is effective in refining the casting structure and preventing cracking of the ingot during casting, but if it exceeds 0.20%, huge intermetallic compounds will crystallize. Cr...If it is less than 0.18%, there is a risk of stress corrosion cracking, and if it exceeds 0.35%, a huge intermetallic compound will crystallize, which is not preferable. Zr……Addition of 0.05 to 0.25% is effective in preventing stress corrosion cracking and further refining crystal grains, but
If it is less than 0.25%, the effect will be small, and if it exceeds 0.25%, a huge intermetallic compound will crystallize, which is not preferable. Note that Fe, Si, and Mn as impurity elements need to be regulated as follows. Fe...Fe is effective in refining grains, but 0.50
%, the amount of insoluble compounds in the alloy increases and the toughness of the material decreases. Si...Si is effective in refining crystal grains, but 0.40
%, the amount of insoluble compounds in the alloy increases and the toughness of the material decreases. Mn...Mn is effective in preventing stress corrosion cracking, but if it exceeds 0.70%, hardenability and toughness decrease. As the alloy for the skin material, AA Alclad 7075 Sheet and Plate and
AA Alclad 7178 Sheet
and Plate) using AA7072 alloy. When manufacturing aircraft stringers using the material of the present invention, regardless of the degree of tapered roll processing, stringers with crystal grains in the core material after solution treatment over the entire length are 100 μm or less. If the crystal grain size after solution treatment is 100 μm or less, not only will no roughness or cracks occur during section roll forming, but
It becomes possible to obtain stringers with excellent mechanical properties, fracture toughness values, chemical milling properties, corrosion resistance, etc. A second invention of the present invention is a method for manufacturing an aircraft stringer material according to the first invention, which comprises homogenizing the core material and skin material of the aluminum alloy limited above, then hot clad rolling, and then cold rolling. Material rolled to a specified thickness by 320~
This method is characterized by softening by rapid heating to a temperature of 500° C. at an average heating rate of more than 11° C./min. The second invention homogenizes the aluminum alloy limited above, and this homogenization treatment involves sufficiently heating the core aluminum alloy ingot at 400 to 490°C for 2 to 48 hours. The elements are sufficiently dissolved in solid solution, and Cr and Zr are precipitated as fine intermetallic compounds. If homogenization treatment is insufficient due to low temperature or short time, hot workability of the aluminum alloy ingot will be poor, stress corrosion cracking resistance will decrease, and crystal grains will become coarse. . Furthermore, if the homogenization treatment temperature is higher than 490°C, eutectic melting will occur, which is not preferable. It is desirable that the alloy ingot of the skin material is also subjected to a homogenization treatment at a temperature of 400 to 560°C for 2 to 48 hours to sufficiently dissolve additional elements such as Zn. After the homogenization treatment, the skin material ingot is rolled to a predetermined thickness to form the skin material. Prior to hot rolling, the core material and skin material are degreased and washed, and the core material and skin material are joined by welding. Hot clad rolling is preferably started at a temperature of 350-470°C. If the temperature is less than 350°C, the deformation resistance is high, resulting in poor rolling workability, and if it exceeds 470°C, it becomes brittle and may cause processing cracks, which is undesirable. The cladding rate is preferably 0.05 to 10% on one side. When it is less than 0.05%, it is not only difficult to roll with skin, but also the skin material is easily damaged and there are problems in corrosion resistance. If the cladding ratio exceeds 10%, it is a problem because the strength of the laminated plate after heat treatment decreases. After hot rolling, softening is performed as necessary. For softening, it is necessary to maintain the temperature at 300 to 460°C and then cool it to about 260°C at a cooling rate of 30°C or less per hour. This softening step is particularly necessary when the subsequent cold rolling is to be performed at a high degree of working. The working degree in cold rolling is desirably 20% or more, and if the working degree is low, the crystal grain size of the core material of the stringer material becomes coarser to 100 μm or more. The cold-rolled material is softened by rapid heating to temperatures between 320 and 500°C with an average heating rate greater than 11°C/min. This process is particularly important for obtaining high quality stringer material. is important. Conventionally, the method of softening AA7075 alloy is 413~454
C., held at this temperature for 2 hours, cooled in air, reheated to 232.degree. C., held at this temperature for 6 hours, and then cooled to room temperature. This method is based on the U.S. Department of Defense's MIL-
This is the method recommended in Sections 5, 2, 7, and 2 of the H6088E military standard, and all softening methods for aircraft 7075 alloy comply with the above method and are common knowledge to those skilled in the art. The above-mentioned softening step in the present invention breaks the common sense of those skilled in the art. If the heating temperature exceeds 500°C, the material may melt or abnormal crystal grain growth of the core material may occur, resulting in significantly coarse recrystallized grains, which is not preferable. If the heating temperature is lower than 320°C, the material will not completely soften and recrystallize, resulting in the problem of cracks occurring during stepped cold rolling (taper roll processing) when manufacturing stringers. After all, it is possible to produce a stringer material having fine core recrystallized grains of 100 μm or less only when it is heated and softened at a temperature of 320 to 500°C. Regarding the heating rate to the above temperature, the average is 11℃/
It is essential to perform rapid heating at a rate greater than 1 minute, and in this case, Mg in the core material during heating
- There is little precipitation of Nn-based compounds, and the dislocation structure introduced by cold rolling changes into a uniform fine cell structure by softening by rapid heating. When a material with such a structure is subjected to a weak taper roll process (10 to 30%) and then subjected to solution treatment, recrystallization proceeds with the uniform fine cell structure as the core, resulting in a uniform fine cell structure. Recrystallized grains of core material are obtained. If the heating rate is less than 11℃/min on average, Mg-
As the Zn-based compound precipitates non-uniformly, the dislocation structure either completely disappears or a coarse cell structure of non-uniform size remains. If such a material is subjected to solution treatment after weak tapering roll processing, the above-mentioned uniform and fine recrystallized grains cannot be obtained, and the crystal grains of the core material become extremely coarse. Regarding the cooling rate after softening due to rapid heating, if the cooling rate is less than 30℃ per hour, a complete O material can be obtained, so the cold workability of the material is good, and about 90% at a time. tapered roll rolling is possible. On the other hand, if the cooling rate after softening due to rapid heating is fast, quenching occurs and age hardening occurs, resulting in a material with higher strength than ordinary O material, so it cannot be used as a stringer material with a relatively low degree of processing. Although it is possible, there are problems in terms of processability when applied to stringers that require a high degree of processing. The third invention of the present invention is for the countermeasure,
In the second invention, when the cooling rate during softening is 30°C or more per hour, it is reheated to 200 to 500°C, and especially if the reheating temperature is less than 200 to 350°C, it is cooled in air or for 1 hour. This method is characterized by cooling at a rate of 30°C or less per hour, and when the reheating temperature is 350 to 500°C, cooling at a rate of 30°C or less per hour. In other words, the cooling rate after rapid heating is
When the temperature is 30°C or higher, it will harden and age harden, so it is necessary to reheat it to a predetermined temperature to make the O material. If the reheating temperature is lower than 200°C, it will not soften, and if it exceeds 500°C, the crystal grains may become coarse. In order to prevent reheating, the cooling rate after reheating should be air cooled if the reheating temperature is relatively low (less than 200 to 350℃), or the cooling rate should be 30℃ or less per hour. cooling at speed,
When the reheating temperature is relatively high, 350 to 500°C, cooling is performed at a cooling rate of 30°C or less per hour. If the reheating temperature is less than 200-350℃, the temperature is so low that baking will not occur even if air cooling is used. Further, when the reheating temperature is 350 to 500°C, it is not preferable that the cooling rate is higher than 30°C per hour because baking occurs. By doing so, even if the cooling rate after rapid softening is fast, a high degree of processing is possible. In addition, the reheating temperature affects the tensile strength of the obtained material and the crystal grain size of the core material of the material (hereinafter sometimes referred to as W material) that is solution-treated after processing the material into a stepped taper roll. It was found through experiments that An example of this relationship is shown in FIG. Figure 5 shows the tensile strength of O material obtained by reheating the material that has been softened by rapid heating at various temperatures, and the tensile strength of this O material.
The graph shows the relationship between the crystal grain size and reheating temperature of a W material that was water-quenched after 20% cold working and solution treatment at 470°C for 40 minutes. That is, a material that is air-cooled after being softened by rapid heating and left at room temperature has a high tensile strength because it is hardened, and when reheated, the tensile strength decreases as the reheating temperature increases. In addition, the crystal grain size of the core material of W material that has been subjected to 20% cold working after reheating varies depending on the reheating temperature.
℃, the crystal grain size is relatively small, about 25-35 μm, and when the reheating temperature is 350-440%, the crystal grain size is 35 μm.
~50μm, quite large, reheating temperature 440~500℃
The crystal grain size at this time becomes small again to 30 to 35 μm. Next, the present invention will be explained in more detail with reference to Examples.

[Table] Example 1 Stringer material with crystal grain size of core material of 100 μm or less produced by the method of the present invention using alloys No. 1 and No. 4 shown in Table 1 as core materials and alloy No. 10 as skin material. Table 2 shows the performance comparison results of a 3 mm thick stringer material with a core grain size of 100 μm manufactured by the conventional method using No. 1 and No. 4 alloys as the core material and No. 10 alloy as the skin material. Shown below. The material of the present invention, which has fine crystal grains in the core material and also has fine crystal grains in the core material after cold working and quenching, does not suffer from rough skin and cracking during section roll forming, which is a problem with materials made using conventional methods. We were able to resolve the issues such as: The material of the present invention also has excellent fracture toughness and chemical milling properties, and because it is clad with AA7072 alloy, it has significantly better corrosion resistance than conventional materials.

[Table] Manufacturing method according to the present invention: Homogenization treatment of core material (460°C x 16hr) → Hot clad rolling (rolling from 300mm to 6mm at 400°C) → Cold rolling (6mm → 3mm) → Rapid heating (450°C) Heating rate in °C
Heating at 200℃/min and holding for 3 minutes) → Cooling (cooling at 5℃/min) → Reheating (300℃ x 1hr) → Cooling (20℃/min)
hr cooling rate to 200℃, then air cooling) Conventional manufacturing method: Core material homogenization treatment (460℃ x 16hr) → Hot clad rolling (rolled to 6 mm at 400℃) → Softening (420℃)
x 1hr heating → furnace cooling) → cold rolling (6 → 3mm) → softening (420℃ x 2hr heating) → cooling to 235℃ at 25℃/hr → holding at 235℃ for 6 hours → air cooling. Example 2 (Influence of temperature increase rate during rapid heating) After homogenizing alloy No. 1 shown in Table 1 at 470°C for 24 hours, hot clad rolling was started at 410°C using No. 10 alloy as a skin material. , 350mm was rolled into a 6mm thick plate. The finishing temperature of hot rolling was 340°C. Next, the 6 mm thick plate was cold rolled to a thickness of 3 mm, heated to 450°C at various heating rates shown in Table 3, held for 2 minutes, and then cooled at a cooling rate of 25°C per hour to form a 3 mm O material. And so. The grain size of the core material of the material (W material) obtained by cold working this plate at various working degrees, solution treatment using a salt bath for 40 minutes at 470°C, and water quenching and the temperature increase to 450°C. Table 3 shows the speed relationship. The cladding rate is 2.4% on one side and cladding on both sides.

【table】

[Table] As shown in Table 3, when the heating rate to 450°C is higher than an average of 11°C/min, the crystal grains of the core material of the material and the core material of the material subjected to solution treatment after cold working are The crystal grains are uniform fine grains of 100 μm or less,
When the average temperature is below 11°C/min, the crystal grains of the core material become significantly coarsened. Next, in order to examine the performance of typical 3mmO materials mentioned above in more detail, we performed 0 to 80% cold rolling, which corresponds to stringer processing, and rolled this at 470°C for 40 minutes using a salt bath. Table 4 shows the various performances of the W material, which was solution-treated and water-quenched, and the T6 material, which was aged at 120°C for 24 hours after quenching. Materials with a heating rate greater than 11° C./min on average have good performance as stringer materials. Furthermore, there is no problem with corrosion resistance as it is made of No. 10 AA7072 alloy.

【table】

[Table] Example 3 (Effect of heating temperature) A 3 mm cold-rolled clad plate was prepared in exactly the same manner as in Example 2 using Alloy No. 2 shown in Table 1 as the core material and No. 10 alloy as the skin material. Rolled to . This board is 320~
Heated to various temperatures between 520℃ at various heating rates,
After that, it was cooled at a cooling rate of 5°C per minute, and then
It was heated at 300°C for 1 hour and cooled at a cooling rate of 20°C per hour to form a 3 mmO material. In addition, the clud rate is
2.5%, and cladding was performed on only one side. After cold working the above 3mmO material, the crystal grain size of the core material of the W material was water-quenched after solution treatment at 470℃ for 40 minutes using a salt bath, and the heating temperature was increased with a heating time of 2 minutes. The relationship is shown in Table 5. Even if solution treatment is performed after mild working, a core material with fine grains can only be obtained when the cold-rolled plate is heated to 320~320
It is only O material that has been softened by rapid heating to a temperature of 500°C, and if the heating temperature is outside this range, a material with fine core grains cannot be obtained even if solution treatment is performed after weak working.

[Table] Three examples of 3 mm thick O material softened under the conditions shown in Table 5 were cold rolled to a maximum of 80% and heated to 470°C.
Tests were conducted on W material that was solution-treated for 40 minutes and then water-quenched, and T6 material that was aged at 122°C for 25 hours after quenching. The results are shown in Table 6, and each example had sufficient performance as a stringer material.

[Table] Example 4 (Effect of heating holding time) A 3 mm thick cold rolled plate was prepared in the same manner as in Example 2 using Alloy No. 3 shown in Table 1 as the core material and No. 10 alloy as the skin material. Clad rolled. This plate was heated to a temperature of 480 to 385°C at the heating rate shown in Table 7, held for each time, and then cooled at a cooling rate of 5°C per minute.
Then, after heating at 300°C for 1 hour, the material was cooled in air to obtain a 3 mmO material. The cladding rate was 2.5%, and cladding was performed on both sides. The relationship between the crystal grain size, heating temperature, and holding time of the W material core material, which was water-quenched after 20% cold rolling of this 3 mm O material and then solution treatment at 470℃ using a salt bath for 40 minutes. are shown in Table 7.

[Table] As can be seen from Table 7, it is clear that a material with fine crystal grains of the core material can be obtained over each holding time. The crystal grains of the core material of the W material, which was cold rolled from 0 to 90% of the above 3 mm material plate, solution treated and water quenched, are all less than 100 μm, and are 1.5 t (t = plate thickness).
Even when bent at 90 degrees with a bending radius of , no roughening or cracking occurred at all, making it suitable as a stringer material. Example 5 A 350 mm thick ingot of the No. 1 alloy shown in Table 1 was used as the core material, and No. 10 alloy was used as the skin material to be clad rolled under the production conditions shown in Table 8, and a final 2-5 mm thick ingot was rolled. It was made into a timber board. The cladding rate was 2.2% on one side, and double-sided cladding was performed. O manufactured under the manufacturing conditions of Nos. 1 to 17 in Table 8
After cold-rolling the material plate by 20%, it was solution-treated at 470°C for 40 minutes using a salt bath, and then water-quenched to 120%.
Table 9 shows the test results of various performances of T6 material aged at ℃ for 24 hours. As is clear from Table 9, the crystal grain size of the core material of the stringer material manufactured under the conditions of the present invention is 100.
The crystal grain size of the core material is 100 μm or less even for materials quenched after cold working, and does not become coarse, and both the W material and the T6 material exhibit good performance as stringer materials. Although Table 9 shows only the results when the degree of working is 20%, even when cold working is performed from 0 to 80%, the crystal grain size of the core material after solution treatment is all 100 μm or less, W material,
Both T6 materials had sufficient performance as stringer materials.

【table】

【table】

【table】

[Table] Example 6 (Influence of alloy composition) A 400 mm thick ingot of alloy No. 3 to 7 shown in Table 1 was
After homogenizing at ℃ for 25 hours, it was used as a core material and No.10
Hot clad rolling was started at 400°C using the alloy as a skin material, and a plate with a thickness of 6 mm was rolled. The hot rolling finish temperature was 300°C. Next, the 6 mm thick plate was cold rolled to 3 mm thickness and heated to 460°C at an average heating rate of 300°C/min.
After heating at that temperature for 5 minutes, cooling at a cooling rate of 10°C per minute, and then cooling at 300°C for 1 minute.
After heating for a period of time, it was air cooled to obtain a 3 mmO material. For comparison, alloys No. 8 and No. 9 shown in Table 1 were used as the core material, and alloy No. 10 was used as the skin material, and a final 3 mm O plate material was prepared in the same manner. The crud rate is 2.6.
%, and cladding was performed on only one side. In order to examine the performance of these O material plates as stringer materials, each O material plate was cold rolled by 20% and then rolled at 470℃.
Tests were conducted on W material that was solution treated for 40 minutes and water quenched, and T6 material that was aged at 120°C for 24 hours after quenching. Its various performances are shown in Table 10. When alloys 3 to 7 are used as the core material, they have good performance as stringer materials, but when No. 8 alloy is used as the core material, the strength is low, and when No. 9 alloy is used as the core material, the stringer material has good performance. Its use as a stringer material is problematic because of the risk of stress corrosion cracking of the core material.

【table】 [Brief explanation of the drawing]

Figure 1 is a partial perspective view of the inside of the aircraft fuselage, Figures 2 a, b, and c are examples of cross-sectional shapes of stringers, and Figure 3
The figure is a perspective view showing the processing state of the stringer material.
FIG. 4 is an explanatory diagram showing an example of the relationship between the working degree and the grain size, and FIG. 5 is a graph showing the relationship between the tensile strength of the material, the grain size of the W material, and the reheating temperature. 1... Body, 2... Reinforcement material (stringer),
3...Reinforcement material (stringer frame).

Claims (1)

[Claims] 1 Zn5.1-8.1%, Mg1.8-3.4%, Cu1.2-2.6
%, Ti0.20% or less, further Cr0.18~0.35% or
One side or both sides with an alloy containing one or two types of Zr0.05~0.25% and the remainder consisting of Al and impurities as the core material and an Al alloy containing 0.8~1.3% Zn as the skin material. Clad material with a core grain size of 100μ
An aircraft stringer material with excellent corrosion resistance, in which the crystal grain size of the core material is 100 μm or less even after mild cold working and solution treatment. 2 Zn5.1-8.1%, Mg1.8-3.4%, Cu1.2-2.6
%, Ti0.20% or less, further Cr0.18~0.35% or
An alloy containing one or two types of Zr0.05~0.25% and the remaining Al and impurities is homogenized and used as a core material, and an Al alloy containing Zn0.8~1.3% is used as a skin material on one or both sides. Perform hot crud rolling of the crud,
The material is then cold-rolled to a predetermined thickness and then rapidly heated to a temperature of 320 to 500°C at an average temperature increase rate of greater than 11°C/min to soften it, and the crystal grain size of the core material is reduced to 100 μm. A method for producing an aircraft stringer material with excellent corrosion resistance, characterized in that the crystal grain size of the core material is 100 μm or less even after mild cold working and solution treatment. 3 Zn5.1~8.1%, Mg1.8~3.4%, Cu1.2~2.6
%, Ti0.20% or less, further Cr0.18~0.35% or
An alloy containing one or two types of Zr0.05~0.25% and the remaining Al and impurities is homogenized and used as a core material, and an Al alloy containing Zn0.8~1.3% is used as a skin material on one or both sides. The material is hot clad rolled and then cold rolled to a specified thickness.
It is softened by rapid heating to a temperature of 320 to 500°C at an average heating rate of more than 11°C/min, and when the cooling rate during this softening is 30°C or more per hour, it is heated to 200 to 500°C. Reheat and reheat temperature is 200 ~
If the temperature is less than 350°C, cool by air or at a rate of 30°C or less per hour, and if the reheating temperature is 350 to 500°C, cool at a rate of 30°C or less per hour to remove the crystals of the core material. A method for producing an aircraft stringer material with excellent corrosion resistance, characterized in that the grain size is 100 μm or less, and the crystal grain size of the core material is 100 μm or less even after mild cold working and then solution treatment.