JPH1112706A - Manufacture of aluminum alloy sheet excellent in baking hardenability and having controlled natural aging characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacture of an aluminum alloy sheet excellent in baking hardenability and having controlled natural aging characteristic, capable of inhibiting an increase in strength due to standing at ordinary temp. and attendant deterioration in formability while maintaining the improvement of strength due to baking finish. SOLUTION: An aluminum alloy stock sheet, composed essentially of Mg and Si, is heated at >=100 deg.C/min temp.-rise rate up to a temp. ranging from 480 to 580 deg.C and not higher than the burning temp. to undergo solution heat treatment and then subjected to hardening treatment at >=100 deg.C/min temp.-fall rate down to 50-90 deg.C to undergo solution hardening. Subsequently, the sheet is allowed to stand at 50 to 90 deg.C for 1 to 10 hr and further allowed to stand at room temp. The sheet is heated at >=100 deg.C/min temp.-rise rate, held in the temp. range of 200 to 250 deg.C for <=30 sec, and then subjected to hardening treatment at >=100 deg.C/min temp.-fall rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum alloy sheet suitable for a heat-treatable aluminum alloy sheet which is used for panels of automobiles, home electric appliances, machine parts, etc., and which increases the strength by utilizing the heating during baking. Regarding the production method, in particular, it is possible to obtain an aluminum alloy sheet material which has high baking hardenability and can suppress room temperature aging, in which high strength can be obtained by coating baking at low temperature and short time, and increase in strength due to room temperature aging can be suppressed. About the method.

[0002]

2. Description of the Related Art Conventionally, aluminum alloy plates have been used for panels and the like of automobiles, home electric appliances and mechanical parts for the purpose of weight reduction. When an aluminum alloy plate is used for these applications, first, forming processing such as pressing and bending is performed, and then, heat treatment (baking coating, baking) for imparting strength to the coating and the coating film is performed. Such an aluminum alloy plate is a material that has low strength during forming by pressing or the like, is easy to form, and after the forming process, the strength of the aluminum alloy plate itself is significantly improved by heat treatment of baking coating. Is considered ideal. Due to such demands, this type of aluminum alloy plate is mainly made of JIS which is an age hardening type alloy.
A 6000 series aluminum alloy plate is used, and various technical developments have been made.

However, with the energy saving and the diversification of resin materials, the conditions of baking coating tend to be shortened at a low temperature for a short time, and it is no longer possible to sufficiently improve the strength of the conventional aluminum alloy plate by baking coating. . Therefore, the present applicant has already proposed a method for producing an aluminum alloy plate which can obtain high strength even by baking at a low temperature for a short time (Japanese Patent Application Laid-Open Nos. 1-111851 and 62-8985).
No. 2). In these aluminum alloy sheets, a method of generating nuclei of precipitation in a matrix is adopted in order to increase the strength after baking at a low temperature.

However, when these aluminum alloy sheets are left at room temperature, the strength increases remarkably, and the formability deteriorates with the passage of time.

[0005] In recent years, for example, in the production of automobile panels and the like, extremely complicated forming processes such as press molding are often performed. At this time, if the moldability of the material to be used is reduced due to a change over time or the like, cracks or the like occur during press molding, which makes production difficult. Therefore, manufacturers of aluminum alloys respond to aging-type aluminum alloys, which are liable to change with time, by limiting the use period.

However, on the aluminum alloy user side of an automobile manufacturer or the like, the delivered aluminum alloy is rarely used within the service period, and is used after being left at room temperature for several months. Many. If the left aluminum alloy is press-molded under the conditions within the service period, cracks occur. For this reason, when press-forming an aluminum alloy that has been left as it is, the pressing conditions are set again before forming. When the conditions are changed in this way, cracking is suppressed,
Wrinkles and the like may occur, and productivity and production efficiency will be reduced.

Therefore, a method for producing an aluminum alloy material that suppresses age hardening at room temperature has been proposed (Japanese Patent Laid-Open Publication No. Hei 5 (1993) -520).
-70907). The technique described in this publication is to form a nucleus for precipitation at the time of coating baking and to decompose the GP zone by performing a two-stage heat treatment.

[0008]

However, even with the technique disclosed in Japanese Patent Application Laid-Open No. Hei 5-70907, the effect of suppressing aging is not sufficient, and the strength increases greatly when left at room temperature. For this reason, cracks and the like may occur during press molding.

The present invention has been made in view of such a problem, and it is possible to suppress an increase in strength due to standing at room temperature and a decrease in moldability due to standing at room temperature while maintaining the improvement in strength due to coating baking. An object of the present invention is to provide a method for producing an aluminum alloy sheet having excellent bake hardenability and suppressed aging at room temperature.

[0010]

SUMMARY OF THE INVENTION According to the present invention, there is provided a method for producing an aluminum alloy sheet having excellent bake hardenability and suppressed normal temperature aging, comprising the steps of: casting a molten aluminum alloy containing Mg and Si as main components; A step of obtaining an ingot, a step of homogenizing and heat-treating the ingot at a temperature equal to or lower than the burning temperature, and a step of hot-rolling the ingot after the homogenizing heat treatment, and further cold-rolling to a predetermined thickness and temper. And 10
Heating the solution at a temperature of 480 to 580 ° C. at a heating rate of 0 ° C./min or more to a temperature not higher than the burning temperature to perform a solution heat treatment; and quenching to 50 to 90 ° C. at a temperature decreasing rate of 100 ° C./min or more. Standing at 50 to 90 ° C. for 1 to 10 hours, aging at room temperature, 100 ° C.
Heating at a heating rate of at least 200 g / min.
Holding at a temperature range of 30 seconds or less,
A quenching process at a temperature lowering rate of at least one minute.

Thus, according to the present invention, an ingot of a JIS 6000-based aluminum alloy containing Mg and Si as main components is prepared, subjected to a homogeneous heating treatment at a burning temperature or lower, and subjected to hot rolling and cold rolling. After heating to a temperature of 480 to 580 ° C. at a heating rate of 100 ° C./min or more and below the burning temperature to perform solution heat treatment, quenching to 50 to 90 ° C. at a cooling rate of 100 ° C./min or more, 50 to 90
Aged at room temperature for 1 to 10 hours at 100 ° C
Heating at a heating rate of at least 200 g / min.
Temperature for 30 seconds or less, and then at 100 ° C. /
Quenching at a temperature lowering rate of more than one minute, thereby suppressing the aging at room temperature while maintaining the curability by baking at a low temperature and a short time, and maintaining the strength at room temperature for several months. An increase and a decrease in moldability can be prevented.

It is desirable to perform intermediate annealing before and / or during the cold rolling. By performing the intermediate annealing, the formability in cold rolling can be improved.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have conducted intensive studies in order to solve the above-mentioned problems. As a result of the conventional JIS 6000 series aluminum alloy, the cause of the deterioration of formability due to normal temperature aging is as follows. Because of the high vacancy concentration, the inventors conceived that an aggregate (cluster) of vacancies and solute atoms peculiar to the JIS 6000 alloy was precipitated at room temperature. Then, it has been found that by performing an appropriate heat treatment, clusters precipitated at room temperature can be eliminated, the vacancy concentration can be reduced, and normal-temperature aging can be suppressed.

Hereinafter, the heat treatment conditions of the present invention will be described. First, a JIS 6000 series aluminum alloy material is melted and cast to produce an aluminum alloy ingot. Next, a homogenizing heat treatment is performed at a temperature equal to or lower than the burning temperature, followed by hot rolling and, if necessary, intermediate annealing. Thereafter, cold rolling is performed to a predetermined thickness. Thereafter, a heat treatment, which is a feature of the present invention, is performed.

That is, first, a JIS 6000-based aluminum alloy material is heated at a cooling rate of 100 ° C./min or more for 50 to 6 minutes.
After solution quenching at 0 ° C., the solution is left at this temperature for 1 to 10 hours to generate nuclei for precipitation during baking coating. Next, clusters are deposited by leaving the mixture at room temperature. And maintained at a temperature rising rate of 100 ° C./min or more in a temperature range of 200 to 250 ° C. for 30 seconds or less,
By performing the quenching treatment at a temperature lowering rate of 100 ° C./min or more, the clusters precipitated by being left at room temperature are redissolved and the vacancy concentration is reduced. In this way, normal temperature aging is suppressed while maintaining bake hardenability.

Next, each heat treatment condition will be described. First, a JIS 6000 series Al alloy is melted by an ordinary method and then cast to obtain an aluminum alloy ingot. Thereafter, the aluminum alloy ingot is subjected to a homogenizing heat treatment.
In the homogenization heat treatment, it is preferable to homogenize the aluminum alloy ingot under the following conditions.

Homogenization heat treatment temperature: 480 to 580 ° C. The homogenization heat treatment is a heat treatment performed to uniformly disperse the segregation of the added element and to control the size and volume content of the dispersed precipitate. If the temperature of the homogenization heat treatment is lower than 480 ° C., segregation of a coarse compound or coarse precipitation of an additional element may occur, so that moldability may be reduced. On the other hand, if the homogenization heat treatment temperature exceeds 580 ° C., burning may occur and cracks may occur during hot rolling. Therefore, the temperature in the homogenization heat treatment is preferably set to 480 to 580 ° C. In addition, depending on the composition of the aluminum alloy,
Burning may occur at 580 ° C. or lower, and the homogenizing heat treatment temperature is preferably set as high as possible according to the composition within a range in which burning does not occur. Further, the holding time is appropriately determined.

Next, a hot rolling process and a cold rolling process are performed. If an intermediate annealing process is performed between the hot rolling process and the cold rolling process, the formability can be further improved.

Intermediate annealing temperature: 300 to 450.degree. C. If the intermediate annealing temperature is lower than 300.degree. C., annealing is not sufficiently performed, and the effect of improving formability is small. On the other hand, if the intermediate annealing temperature exceeds 450 ° C., extra precipitates may be deposited, and it is difficult to obtain sufficient bake hardening properties.
Therefore, the temperature of the intermediate annealing treatment is preferably set to 300 to 450 ° C.

Then, after the aluminum alloy material is reduced to a predetermined thickness by cold rolling, a solution heat treatment is performed.

Next, the conditions of the solution heat treatment will be described.

Temperature rising rate: 100 ° C./min or more In the solution heat treatment, if the temperature rising rate is less than 100 ° C./min, the added elements do not form a solid solution, the strength is reduced, and the bake hardenability is not sufficient. Therefore, in the solution heat treatment, the rate of temperature rise is set to 100 ° C./min or more.

Solution heat treatment temperature: 480 to 580 ° C.
If the temperature of the solution heat treatment temperature below the burning temperature is less than 480 ° C., the intermetallic compound comprising the additional element does not form a solid solution and the intermetallic compound remains, so that sufficient strength or bake hardness is obtained. Absent.
On the other hand, if the solution temperature exceeds 580 ° C., burning occurs and plate cracks occur, and molding becomes difficult. Therefore, the temperature in the solution heat treatment is 480 to 580 ° C., which is lower than the burning temperature. Depending on the composition of the aluminum alloy, burning may occur at 580 ° C. or lower, and the solution heat treatment temperature is preferably as high as possible depending on the composition within a temperature range in which burning does not occur. Further, the holding time is appropriately determined.

Quenching / cooling rate: 100 ° C./min or more In quenching after solution heat treatment, the cooling rate is 100
If the temperature is lower than ℃ / minute, a solid solution element is precipitated, and sufficient strength cannot be obtained, and the moldability is lowered. Therefore, the cooling rate is set to 100 ° C./min or more.

Hardening temperature: 50 to 90 ° C. If the quenching temperature is less than 50 ° C., sufficient bake hardness cannot be obtained. On the other hand, when the quenching temperature exceeds 90 ° C., the moldability is reduced. Therefore, the quenching temperature is set to 50 to 90 ° C.

After quenching, it is left to improve bake hardenability and moldability.

Leaving temperature: 50 to 90 ° C. If the leaving temperature is lower than 50 ° C., sufficient bake hardness cannot be obtained. On the other hand, when the standing temperature exceeds 90 ° C., the moldability is reduced. Therefore, the leaving temperature is set to 50 to 90 ° C.

Leaving time: 1 to 10 hours If the leaving time is less than 1 hour, sufficient bake hardness cannot be obtained. On the other hand, if the leaving time exceeds 10 hours,
Moldability decreases. Therefore, the leaving time is set to 1 to 10 hours.

Then, as an ordinary temperature aging treatment after the solution heat treatment and the quenching treatment, the aluminum alloy material is left at ordinary temperature for 24 hours or more to precipitate precipitates such as clusters. If the next step (heat treatment for re-solid solution) is performed without performing the normal-temperature aging treatment, the strength of the aluminum alloy material rapidly increases in the T4 state, and the formability and bendability are reduced. Connect. Therefore, normal temperature aging treatment is performed before heat treatment for re-solid solution.

Temperature rise rate during re-solid solution heat treatment: 100 ° C./min
After natural aging or, in order to melt the aging precipitation was subjected to heat treatment for redissolved. In this heat treatment, if the rate of temperature rise is less than 100 ° C./min, the precipitation is insufficiently melted, and a stable phase precipitate is generated depending on the conditions, and the bake hardenability at a low temperature becomes insufficient. I will. Therefore,
The rate of temperature rise during the re-solid solution heat treatment is 100 ° C./min or more.

Heat treatment temperature: 200 to 250 ° C. The heat treatment for re-solid solution is a heat treatment for melting the clusters (vacancies or solute atoms) precipitated by aging again in the aluminum alloy. If the heat treatment temperature is lower than 200 ° C., the clusters are not sufficiently melted and the strength increases due to normal temperature aging. On the other hand, if the heat treatment temperature exceeds 250 ° C., precipitates of a stable phase are generated, so that the bake hardenability at a low temperature becomes insufficient. Therefore, the heat treatment temperature for re-solid solution is set to 200 to 250 ° C.

Heat treatment holding time: 30 seconds or less If the heat treatment holding time exceeds 30 seconds, precipitates of stable phases are generated, so that the bake hardenability at low temperatures becomes insufficient. Therefore, the heat treatment holding time is set to 30 seconds or less. Note that the time for maintaining the heat treatment temperature may be 0 seconds, that is, it is not necessary to maintain the heat treatment temperature.

Temperature drop rate: In the quenching after heat treatment for re-solid solution of 100 ° C./min or more , if the cooling temperature is set to less than 100 ° C./min, the precipitation is insufficiently melted and the stable phase Precipitates are generated and bake hardening at low temperatures becomes insufficient. Therefore, the cooling rate is set to 100 ° C./min or more.

The aluminum alloy used in the present invention is not limited to the JIS 6000 type aluminum alloy, but may be any aluminum alloy containing Si and Mg as main components.

[0035]

EXAMPLES Examples of the present invention will be described below in comparison with comparative examples that fall outside the scope of the claims of the present invention.

First Embodiment First, Si: 1.0% by weight, Mg: 0.6% by weight, M
n: 0.06% by weight, Ti: 0.02% by weight, Fe:
A JIS 6000-based aluminum alloy material containing 0.15% by weight was melted and cast by an ordinary method to obtain an alloy ingot having a thickness of 50 mm. Next, the alloy ingot was heated at a temperature of 560 ° C. for 4 hours, subjected to a homogenizing heat treatment, and hot-rolled to obtain a hot-rolled material having a thickness of 5.0 mm. Subsequently, the hot-rolled material is allowed to stand at room temperature, then heat-treated at a heating rate of 200 ° C./min at a temperature of 500 ° C. for 5 seconds to perform an intermediate annealing treatment, and then cold-rolled at room temperature. To obtain a rolled material having a thickness of 1.0 mm. And
This rolled material was subjected to a solution heat treatment under the conditions shown in Table 1 below, and then quenched to 70 ° C. at a rate of 100 ° C./min.

[0037]

[Table 1]

Then, the temperature is maintained at 70 ° C. for 2 hours, and further left at normal temperature for 36 hours to perform aging treatment at normal temperature. Thereafter, as a heat treatment for re-solid solution, the temperature is raised to 250 ° C. at a rate of 200 ° C./min. And held for 5 seconds. And 200 ° C /
A quenching treatment was performed at a temperature lowering rate of 1 minute to obtain a T4 material. Then, this T4 material was left at room temperature for 7 days and 90 days to prepare an aluminum alloy material (Al alloy material), which was tested and evaluated under the following conditions.

From the Al alloy material subjected to the above-described steps, the JIS
No. 5 test piece was cut out, and a tensile test was performed on each test piece using an autograph (manufactured by Shimadzu). Regarding the evaluation of the strength, Al which had been subjected to room temperature aging treatment for 7 days after the production described above was used.
About the alloy material (7-day elapsed material) and the Al alloy material (90-day elapsed material) subjected to the normal temperature aging treatment for 90 days,
0.2% proof stress (hereinafter, simply referred to as yield strength) were measured, the values of these Strength (sigma _0.2) ((sigma _0.2 90 days elapsed material)
The change amount Δσ _0.2 of the proof stress value calculated by − (σ _{0.2 of the} material after 7 days) was evaluated. Note that Δσ _0.2 is 15 N / mm ²
The following cases were considered good. In addition, 7 days old material and 9
170 days while applying 2% tension to the 0-day-old material
After aging treatment at a temperature of 20 ° C. for 20 minutes, a tensile test was performed, and the values of these proof stresses were evaluated as bake hardness (hereinafter, this proof stress is referred to as BH proof stress). The case where the BH proof stress value was 190 N / mm ² or more was regarded as good.

The results of these tests are shown in Table 2 below.
The proof stress value, the amount of change in proof stress value, and the BH proof stress value of the material aged 90 days and the material aged 90 days are shown. In addition, when the change amount of the proof stress value and the evaluation of the BH proof stress value are good, it is set to “O”,
When at least one of the variation in the proof stress value and the BH proof stress value was defective, the overall evaluation was “×”.

[0041]

[Table 2]

As shown in Table 2 above, in Examples 1 to 3, since the solution heat treatment was performed under appropriate conditions, the variation in proof stress and the BH proof stress were good.

On the other hand, in Comparative Example 10, since the heating rate was less than the lower limit of the range of the present invention, the BH proof stress was low,
Sufficient bake hard was not obtained.

In Comparative Example 11, since the solution heat treatment temperature was lower than the lower limit of the present invention, the BH proof stress was low and sufficient bake hard was not obtained.

In Comparative Example 12, since the solution heat treatment temperature exceeded the upper limit of the present invention, cracking occurred due to burning.

Second Embodiment An Al alloy material having the same composition as that of the first embodiment was subjected to a cold rolling process in the same manner as in the first embodiment to a thickness of 1.0 mm.
Rolled material was obtained. Next, the rolled material was heated to a temperature of 530 ° C. at a rate of 200 ° C./min, held for 30 seconds to perform solution heat treatment, and then cooled to a temperature shown in Table 3 at a rate of 100 ° C./min. Hardened. Then, the quenching temperature was maintained for the time shown in Table 3 below.

[0047]

[Table 3]

Then, after standing at normal temperature for 36 hours, the temperature was raised to 250 ° C. at a rate of 200 ° C./min as a heat treatment for re-solid solution and held for 5 seconds. And 200 ° C /
A quenching treatment was performed at a temperature lowering rate of 1 minute to obtain a T4 material. Then, the T4 material was left at room temperature for 7 days and 90 days to produce an Al alloy material, which was tested and evaluated under the conditions shown in the first example.

Table 4 below shows the results of these tests.
The proof stress value, the amount of change in proof stress value, and the BH proof stress value of the material aged 90 days and the material aged 90 days are shown. In addition, when the change amount of the proof stress value and the evaluation of the BH proof stress value are good, it is set to “O”,
When at least one of the variation in the proof stress value and the BH proof stress value was defective, the overall evaluation was “×”.

[0050]

[Table 4]

As shown in Table 4 above, in Examples 4 to 6, the amount of change in the proof stress and the BH proof stress were good because they were held under appropriate conditions.

On the other hand, in Comparative Example 13, the holding time (quenching temperature) was less than the lower limit of the range of the present invention.
The H proof stress was low, and sufficient bake hard was not obtained.

In Comparative Example 14, since the holding temperature exceeded the upper limit of the range of the present invention, the amount of change in the proof stress was large, and the moldability was reduced.

In Comparative Example 15, since the holding time was shorter than the lower limit of the present invention, the BH proof stress was low, and sufficient bake hardness was not obtained.

In Comparative Example 16, since the holding time exceeded the upper limit of the range of the present invention, the amount of change in the proof stress value was large, and the moldability was reduced.

Third Embodiment An Al alloy material containing the same chemical components as in the first embodiment was subjected to the cold rolling process in the same manner as in the first embodiment, to a thickness of 1.
A rolled material of 0 mm was obtained. Next, this rolled material is heated at 200 ° C /
The temperature was raised to 530 ° C. at a rate of 30 minutes, held for 30 seconds to perform solution heat treatment, and then quenched to 70 ° C. at a rate of 100 ° C./min. Then, after being kept at 70 ° C. for 2 hours, it was left at room temperature for 36 hours. Next, it was heated at a heating rate of 200 ° C./min, and heat-treated for solid solution again under the conditions shown in Table 5 below.

[0057]

[Table 5]

Then, quenching was performed at a temperature reduction rate of 200 ° C./min to obtain a T4 material. For 7 days and 90 days,
An Al alloy material left at room temperature was prepared and tested and evaluated under the conditions shown in the first example.

Table 6 below shows the results of these tests.
The proof stress value, the amount of change in proof stress value, and the BH proof stress value of the material aged 90 days and the material aged 90 days are shown. In addition, when the change amount of the proof stress value and the evaluation of the BH proof stress value are good, it is set to “O”,
When at least one of the variation in the proof stress value and the BH proof stress value was defective, the overall evaluation was “×”.

[0060]

[Table 6]

As shown in Table 6 above, in Examples 7 to 9, the heat treatment for re-solid solution was performed under appropriate conditions, so that the variation in proof stress and the BH proof stress were good. Met.

On the other hand, in Comparative Example 17, since the heat treatment temperature was lower than the lower limit of the range of the present invention, the amount of change in the proof stress value was large, and the moldability was reduced.

In Comparative Example 18, since the heat treatment temperature exceeded the upper limit of the range of the present invention, the BH proof stress was low, and sufficient bake hard was not obtained.

In Comparative Example 19, since the heat treatment time exceeded the upper limit of the range of the present invention, the BH proof stress was low and sufficient bake hard was not obtained.

[0065]

As described above, according to the present invention,
By performing the predetermined heat treatment, it is possible to prevent an increase in strength due to normal temperature aging while maintaining excellent bake hardness at a low temperature for a short time, and to prevent a decrease in formability. Therefore, after manufacturing the Al alloy plate, it is not necessary to change the processing conditions even after being left at room temperature for a long period of time, so that the material can be stocked, the manufacturing process is simplified, and the yield is improved. Cost is reduced. Furthermore, since this Al alloy plate has excellent low-temperature and short-time bake hardening properties, when this alloy plate is used for automobiles, home electric appliances, machine parts, and the like,
Home appliances and mechanical parts can be reduced in weight.
Then, application to general-purpose industrial products is possible.

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Claims (2)

[Claims] 1. A step of casting a molten aluminum alloy containing Mg and Si as main components to obtain an aluminum alloy ingot, a step of performing a homogenizing heat treatment at a temperature equal to or lower than a burning temperature, and a step of performing a homogenizing heat treatment. The ingot after hot rolling,
Further cold rolling to a predetermined thickness and temper;
A step of heating at 480 to 580 ° C. at a heating rate of at least 00 ° C./min to a temperature below the burning temperature to perform a solution heat treatment; and a step of quenching to 50 to 90 ° C. at a rate of temperature drop of at least 100 ° C./min. Standing at 50 to 90 ° C. for 1 to 10 hours, aging at room temperature, 100 ° C.
Heating at a heating rate of at least 200 g / min.
Holding at a temperature range of 30 seconds or less,
And a step of quenching at a temperature lowering rate of not less than one minute. The method for producing an aluminum alloy sheet having excellent bake hardenability and suppressed normal temperature aging. 2. The method according to claim 1, wherein intermediate annealing is performed before and / or during the cold rolling. .