JPS6157385B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a superplastic high-strength aluminum alloy having fine grains of 25 μm or less. Rolled materials made from precipitation-hardening aluminum alloys through the usual manufacturing process of ingot formation → homogenization heat treatment → hot rolling → cold rolling → solution treatment have crystal grain sizes of 30 to 100 μm on the plate surface. If you want to obtain crystal grains of 25 μm or less, it is necessary to apply cold working higher than 90%. However, if the cold rolling process is applied to a degree higher than 90%, edge cracks may occur at the edge of the plate, or the plate may break at right angles to the rolling direction. Therefore, a method of obtaining fine crystal grains even with a low working degree has been proposed, as seen in, for example, Japanese Patent Application Laid-Open No. 132420/1983. However, this method requires overaging treatment. In view of the above-mentioned prior art, the present invention aims to obtain a superplastic high-strength aluminum alloy that does not require over-aging treatment and does not suffer from defects such as edge cracking and breakage even under high working conditions. The gist is Cr0.05~0.35% or Zr0.05~
A precipitation hardening aluminum alloy containing at least one of 0.25% and 0.2 to 0.001
It is characterized by being cooled at a cooling rate of ℃/sec, subjected to cold working of 60% or more, and then heated to a temperature of 400 to 530℃ at a heating rate of 1℃/sec or more to recrystallize to 25 μm or less. This is a method for producing superplastic high-strength aluminum alloys. First, the precipitation hardening aluminum alloy used in the present invention has Zn5.1~8.1%, Mg1.8~3.4%, and Cu1.2~2.6%.
%, Ti 0.2% or less, and as mentioned above, at least one of Cr 0.05 to 0.35 or Zr 0.05 to 0.25%. The reasons for limiting the composition of each component are as follows. Zn: If it is less than 5.1%, high strength cannot be obtained by tempering, and if it exceeds 8.1%, stress corrosion cracking is likely to occur. Mg: If it is less than 1.8%, high strength cannot be obtained by tempering, and if it exceeds 3.4%, rolling workability is poor.
Moreover, stress corrosion cracking becomes more likely to occur. Cu: If it is less than 1.2%, high strength cannot be obtained by tempering, and if it exceeds 2.6%, rolling workability is poor and toughness is reduced. Addition of Ti: 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%, large intermetallic compounds will crystallize. Addition of Cr: 0.05 to 0.35% has the effect of grain refinement and is effective in preventing stress corrosion cracking. Below 0.05%, these effects are absent, and 0.35%
%, it is not preferable because a huge intermetallic compound will crystallize. Zr: Addition of 0.05 to 0.25% has the effect of grain refinement and is effective in preventing stress corrosion cracking. If it is less than 0.05%, these effects will not be obtained, and if it exceeds 0.25%, a huge intermetallic compound will crystallize, which is not preferable. In the present invention, fine precipitated phases formed during hot rolling or cold rolling of an alloy having such a composition are heated to a solution treatment temperature to form a solid solution, and then treated at a rate of 0.2 to 0.001°C/sec. By cooling, supersaturated solute atoms precipitate during cooling, so age hardening at room temperature is small. For this reason, in the case of rolling, even when cold rolling exceeds 90%, there are few edge cracks or breaks, and rolling is possible. By recrystallizing the material that has been strongly processed in this way by raising the temperature at a heating rate of 1° C./sec or more, a material having crystal grains of 25 μm or less can be obtained. The reason for limiting the cooling conditions from the solution treatment temperature to room temperature in the present invention is that the cooling rate is 0.2%/se.
If it is slower than c, coarse compounds (M phase [Mg
Nn ₂ ], etc.), and when the rate is faster than 0.2° C./sec, no M phase precipitation is observed, or even if it is observed, it is 1 μm or less. In this way, the amount of solute elements precipitated varies depending on the difference in cooling rate. If the cooling rate is fast, after quenching, the supersaturated solute atoms will
GP zones and precipitated phases are likely to occur. On the other hand, if the cooling rate is slow, supersaturated solute atoms precipitate within the grains or at the grain boundaries as M phase or other compounds during cooling. Furthermore, even after quenching, GP zones and precipitated phases are less likely to form. The ease of cold working differs depending on the difference in precipitation state as described above. Slow cooling at 0.2°C/sec or less produces coarse precipitates and reduces the deformation resistance of the matrix. Therefore, it is easy to deform. On the other hand, when cooling faster than 0.2℃/sec, the deformation resistance becomes large due to the GP zone and fine precipitates.
Easy to cause edge cracks and rolling cracks. Also, 0.001℃/
If it is slower than sec, the cooling rate is very slow and there is little economic benefit. Cold working facilitates recrystallization by imparting processing strain. If the degree of cold working is less than 66%, the grain size will be larger than 25 μm. In the case of this alloy, the size of the recrystallized grains becomes finer as the degree of cold working increases. This is because the higher the degree of cold working, the more regions undergo strong working, and at the same time the dislocation density also increases, so solute atoms tend to precipitate on more dislocations, hindering the movement of dislocations, and thus Crystal growth is also suppressed and recrystallized grains become smaller. After cold working, it is heated at 400-530℃ for recrystallization. It is difficult to recrystallize below 300℃;
When the temperature is lower than 400°C, solute atoms precipitated on dislocations tend to aggregate and form compounds. This is because the fixation of dislocations by solute atoms is reduced.
This is thought to be because dislocations become more mobile and recrystallized grains also become larger. At temperatures above 400°C, if the heating rate is fast, it is thought that recrystallization proceeds before the solute atoms aggregate. Of course, if the temperature exceeds the solution treatment temperature, the solute atoms become solid solution. Furthermore, if the temperature exceeds 530℃, the alloy will melt and recrystallization will not occur.
It is necessary to carry out at 400-530 °C. If the heating rate at that time is slower than 1℃/sec, the crystal grains will slowly pass through the grain coarsening region of 300 to 400℃ and become 25 μm or more, but if the heating rate is slower than 1℃/sec. The faster the speed, the finer the grains will be. Next, examples will be described. Example 1 [Cooling conditions from solution treatment temperature] Zn5.7%, Mg2.4%, Cu1.6%, Cr0.20%,
An aluminum alloy containing 0.05% Fe and 0.04% Si was formed into a 300 mm thick slab by continuous casting. After homogenization heat treatment at 470°C for 30 hours, the surface segregation layer was removed, and a 6 mm thick plate was formed by hot rolling at 400-450°C. After heating this to a solution treatment temperature of 482℃ and holding it for about 60 minutes,
The material was quenched to room temperature by changing the cooling rate. This heat-treated plate was subjected to 65% and 80% cold working, and then rapidly heated to a final temperature of 482°C. The heating rate is 50°C/sec. After holding for 10 minutes, water quenching was performed and the crystal grain size on the plate surface was examined. The results are shown in Table 1.

[Table] Example 2 [Cold working degree] The alloy shown in Example 1 was hot rolled to a thickness of 8 mm, further cold rolled to a thickness of 4 mm, and then worked at 500°C.
After solution treatment for hours, it was cooled at a cooling rate of 0.2 and 0.005°C/sec. This sample was cold rolled to 1.8
The sheets were rolled to thicknesses of mm, 1.4 mm, 1 mm, 0.6 mm, and 0.4 mm. The rolling reduction ratios are 55, 65, 75, 85, and 90%, respectively. 482 at a heating rate of 50℃/sec.
The temperature was raised to ℃, held for 10 minutes, and then water quenched. Table 2 shows the grain size of the plate surface after water quenching.

[Table] Example 3 [Recrystallization temperature and heating rate] The alloy shown in Example 1 was hot rolled to a thickness of 4 mm, solution treated at 460°C for 1 hour, and then furnace aged at 0.005°C/sec. This sample was subjected to 8% cold working to yield 0.6
mm thickness. This board is 330, 380, 420, 450, 480
The temperature was raised at a heating rate of 50°C/sec to each temperature of 10°C. After being held at each temperature for 30 minutes, water quenching is performed to reach the final temperature.
Tempering was performed at 120°C for 24 hours. Table 3 shows the relationship between crystal grain size and tensile strength at this time. In addition, furnace-cooled materials are subjected to 65% and 85% cold working, and 1.4mm and 0.6mm thick plates are processed to 480%, 0.1, 0.5, 1, 10, 50, and 100%.
The temperature was raised at various heating rates of °C/sec, held for 30 minutes, and then water quenched. Table 4 shows the crystal grain size on the plate surface at this time.

【table】

[Table] Example 4 [Application to rods and pipes] Zn5.6%, Mg2.5%, Cu1.5%, Cr0.22%,
An aluminum alloy billet with a diameter of 0.8 mm containing 0.12% Fe and 0.06% Si is formed into ingots, subjected to homogenization heat treatment at 470℃ for 24 hours, the segregation layer removed, and then heated to 440℃.
Hot extruded. A round bar with a diameter of 80 mm was extruded, and part of the round bar was formed into a tube with an outer diameter of 80 mm, an inner diameter of 60 mm, and a wall thickness of 10 mm. Each piece was made to have a length of 200 mm, and solution treatment was performed at 480°C for 2 hours, followed by furnace cooling at a rate of 0.01°C/sec. Using a cold isostatic extruder, these samples were made into round bars with a diameter of 25 mm, an outer diameter of 52.5 mm, an inner diameter of 50 mm, and a wall thickness of 1.25 mm.
Extruded into mm tube. In both cases, the degree of cold working is approximately 90%.
It is. These are heated to a temperature of 480℃ for rods at 5℃/
sec, and in the case of tubes, the temperature was raised at a heating rate of 50°C/sec. After being held at 480°C for about 30 minutes, it was water quenched and the surface crystal grain size was measured. This result bar is 14μ
m, the particle size was 8 μm in the tube. Example 5 Alloys with varying addition amounts of Zn, Mg, Cu, Ti, Cr, and Zr were cast into a 30 x 175 x 175 mm mold and heated at 470℃ for 24 hours.
A time homogenization process was performed. Thereafter, the segregation layer was removed and cold rolled at 450°C to a thickness of 4 mm. This material was solution-treated at 480℃ for 30 minutes to produce a 0.01℃/sec
After quenching at a cooling rate of , it was cold worked to a thickness of 0.4 mm. These were heated to 482℃ at a heating rate of 100℃/sec, held for 10 minutes, and then water quenched, and the crystal grain size on the plate surface and the tensile strength after tempering at 120℃ for 24 hours were measured. . However, Mg3.4% or more, Cu2.6%
The crystal grains were not measured for the above materials because they had poor rolling workability. In order to compare the stress corrosion cracking resistance, a test piece with a width of 20 mm and a length of 100 mm was cut from the tempered material along the rolling direction, bent into a U-shape with an inner radius of 5 mm, and heated with a chromic acid mixture for 30 minutes. The samples were boiled for a minute and the occurrence of cracks was compared. In addition, regarding tensile strength, 54Kg/mm or more was considered to be a pass. The results are shown in Table 5.

【table】

Claims (1)

[Claims] 1 A precipitation hardening aluminum alloy containing at least one of 0.05 to 0.35% Cr or 0.05 to 0.25% Zr is hot-worked and cold-worked according to a conventional method, heated to a solution treatment temperature, and then Cooled at a cooling rate of 0.001°C/sec and subjected to over 60% cold working,
A method for producing a superplastic high-strength aluminum alloy, which comprises raising the temperature to a temperature of 400 to 530°C at a heating rate of 1°C/sec or more and recrystallizing it to a size of 25 μm or less.