JPH06136478A - Baking hardening type al alloy sheet excellent in formability and its production - Google Patents

Abstract

PURPOSE:To make excellent the formability of an Al alloy sheet even when the producing method for obtaining the increase of the strength of the stock corresponding to its thinning to accompany its lightening and for obtaining sufficient hardenability even under the baking conditions in baking/coating at a low temp. in a short time is applied thereto. CONSTITUTION:The baking hardening type Al-Mg-Cu-Mn alloy sheet having a compsn. contg., by weight, 0.3 to 1.0% Mg, 0.5 to 2.0% Si (contg. 0.4 to 1.5% MgSi and 0.4 to 1.2% residual Si), 0.3 to 2.0% Cu and 0.05 to 1.0% Mn, and the balance Al with impurities and excellent in formability is obtd. This Al alloy ingot is homogenized at the burning temp. or below, is hot-rolled, is thereafter annealed at 350 to 450 deg.C for 5 to 10hr and is then cold-rolled to regulate its sheet thickness into a desired one, and solution treatment is executed. After that, it is hardened at 50 to 120 deg.C at >=300 deg.C/min cooling rate and, as it is, holding is executed at 50 to 120 deg.C for 1 to 48hr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bake hardenable Al alloy sheet having excellent formability and a method for producing the same, and more specifically, Al used for panel materials for automobiles, home appliances, machine parts, etc. An alloy plate that has excellent formability during processing such as pressing and bending, and has improved strength during short-time heat treatment such as baking coating (baking) in these manufacturing processes.
The present invention relates to an alloy plate and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, the problems to be solved by the invention
In order to give strength to the coating film in the coating process after processing, the Al alloy plate, which is mainly used for automobiles, home appliances, mechanical parts, etc. for weight reduction, is subjected to forming processing such as pressing and bending. A heat treatment (baking coating: baking) is performed, and a method of improving the strength of an Al alloy plate by utilizing the heating temperature at that time is used.

It is ideal that such an Al alloy plate is a material that has a low strength during forming such as pressing, is easy to form, and whose strength is remarkably improved by heat treatment of baking after forming. Al-Mg-Si based aluminum alloys are mainly used. As such an Al alloy, the present applicant has previously proposed Japanese Patent Laid-Open No. 11-11851.

However, in the Al-Mg-Si system aluminum alloy and the manufacturing method thereof which have been conventionally used for this kind of application, T4 is emphasized due to the importance of formability or shape fixability.
The strength in the state is extremely low, and further, even if the strength is improved after bake-hardening, sufficient strength cannot be obtained, and there is a drawback that it is deformed only by applying a slight external force.

On the other hand, the weight of automobile parts tends to be further reduced due to the regulation of low fuel consumption of automobiles.
As a result, it is required to reduce the thickness of the Al alloy sheet. However, in the conventional Al alloy sheet and its manufacturing method, the material strength in the T4 state is low to improve the formability, or the material strength is reduced for thinning. There was a problem that the moldability was remarkably deteriorated when the value was increased.

Further, recent baking conditions at the time of baking coating have become lower due to the progress of coating materials and the like because of the increasing use of parts such as resin which cannot be treated at high temperature in order to save energy and improve productivity. ing. For example, the baking temperature of Al alloy used for automobile parts is about 20 in the past.
Although it was 0 degreeC, in recent years, it has come to be processed at the low temperature of 160-170 degreeC. Therefore, the conventional Al-Mg-
In the Si-based aluminum alloy plate and its manufacturing method,
Although not a conventional method, a manufacturing method has been proposed for improving the bake hardenability at a low temperature / short time treatment, but when this treatment method is performed, there is a problem that the formability of most Al alloys is significantly reduced. there were.

The present invention solves the above-mentioned problems of the prior art, increases the strength of the material corresponding to the thinning of the material with the weight reduction, and has sufficient bake hardening under the baking conditions of low temperature and short time bake coating. It is an object of the present invention to provide an Al alloy plate excellent in formability and a manufacturing method thereof even if a manufacturing method for obtaining the property is applied.

[0008]

[Means for Solving the Problems] In order to achieve the above-mentioned object, the inventor of the present invention has conducted extensive studies to improve the method according to the above proposal. As a result, it was found that the strengthening mechanism of the conventional Al-Mg-Si based alloy is based on the following age hardening mechanism.

SS (solid solution) → GP zone (T4 state)
→ β'-Mg ₂ Si (after baking coating)

That is, the mechanism for increasing the strength of the material is due to this aging precipitate, and when the manufacturing method for increasing the bake hardenability in the low temperature and short time bake coating treatment is applied, the material is in the T4 state. Since the nucleus growth site or the GP zone is precipitated, the strength is increased, but the formability is significantly reduced. Therefore, if the amounts of Mg and Si are not proper, it is difficult to obtain good values for both the strength and the formability. Further, the addition of Cu to Al-Mg-Si alloys, known for 6009 and 6010, aging precipitates S'-CuMgAl ₂ is deposited, increasing the strength, further C
u has the effect of changing the texture of the Al-Mg-Si based alloy and making it an orientation with good formability. Moreover, as in the case of the 5000 series alloy, where Mg is fixed in the dislocation loop, Cu is also fixed in the dislocation loop and contributes to the improvement of formability.

The present invention has been completed on the basis of the investigation of the cause and the knowledge, and further detailed research on the contained components and manufacturing conditions.

That is, according to the present invention, Mg: 0.3 to 1.0.
%, Si: 0.5-2.0% (however, regarding the relationship between Mg and Si, the content of Mg ₂ Si is 0.4-1.5%, and the amount of residual Si is 0). .4 to 1.2%),
Further, it contains Cu: 0.3 to 2.0% and Mn: 0.05 to 1.0%, Ti: 0.1% or less, Cr: 0.0% if necessary.
4% or less, Fe: 0.5% or less, at least one of which is Al and impurities, and the balance is Al and Mg-Si-Cu-
The main point is Mn-based Al alloy plates.

Further, in the case of having higher formability, the structure of the plate material is controlled to have a crystal grain size of 30 μm or less, and the texture has a high peak in the (100) plane <001> direction. Is characterized by.

Further, the preferable manufacturing method is as follows. The Al alloy ingot having the above chemical composition is homogenized at a temperature not higher than the burning temperature, hot-rolled, and hot-rolled at 350 to 450 ° C. After annealing at a temperature of 5 to 10 hours and then cold rolling to a desired plate thickness,
After solution treatment, quenching is performed at a cooling rate of 300 ° C./min or more to a temperature of 50 to 120 ° C., and then 50 to
It is characterized in that it is maintained at a temperature of 120 ° C. for 1 to 48 hours.

The present invention will be described in more detail below.

[0016]

[Action]

First, the reasons for limiting the chemical components in the present invention will be explained.

Mg: Mg is the solid solution strengthening of itself and S
An element that imparts strength in cooperation with i. Aged precipitate β'-
Mg ₂ Si is deposited, and this amount depends on the amount of Mg added. Further, as described later, when combined with Cu, an aging precipitate S'-CuMgAl ₂ is precipitated and its precipitation hardening imparts strength. However, if it is less than 0.3%, sufficient strength (hereinafter, the strength means the material and the strength after baking at 170 ° C.) cannot be obtained. At the same time, the equilibrium phase Mg ₂ Si grows as a crystallized substance, and a decrease in elongation is observed, so that the formability is remarkably reduced. Therefore, the Mg content is set to the range of 0.3 to 1.0%.

Si: Si is an element which, together with Mg, imparts strength by precipitation hardening mainly by precipitation of the aging precipitate β'-Mg ₂ Si, and this amount depends on the amount of Si added. However, if it is less than 0.5%, sufficient strength cannot be obtained, and it is 2.0
If it exceeds 0.1%, the equilibrium phase Mg ₂ Si is crystallized and the elongation is greatly reduced, that is, the formability is deteriorated. Therefore, S
The i content is in the range of 0.5 to 2.0%.

However, the strength and formability depend on the added amounts of Mg and Si, and it is the Mg ₂ Si produced by these that contributes to the increase in strength and strength in baking coating. However, if the Mg ₂ Si content is less than 0.4%, the strength is very low and the bake hardenability is almost zero. On the other hand, if it exceeds 1.5%, the elongation is lowered and the formability is remarkably lowered. Therefore, the amounts of Mg and Si are mixed so as to be in the range of 0.4 to 1.5% as Mg ₂ Si.

Further, if Si is added excessively, Si that has not been produced as Mg ₂ Si and has the effect of sticking to the dislocation loops and increasing the formability, and this residual Si forms a solid solution in the T4 state. If so, the strength increases due to solid solution hardening.
However, if the residual Si content is less than 0.4%, sufficient strength cannot be obtained, and if it exceeds 1.2%, the strength increases and the formability deteriorates. Therefore, the amount of residual Si is set in the range of 0.5 to 1.2%. The Mg and Si amounts are properly mixed in consideration of the Mg ₂ Si amount and the residual Si amount.

Cu: Cu is an element which imparts strength by the aging precipitate S'-CuMgAl ₂ . In the present invention, Mg is Si
Not only does it bond with Cu to bring about a complex precipitation hardening action, which improves strength and bake hardenability by low temperature baking. However, if it is less than 0.3%, sufficient strength cannot be obtained at low temperature baking, and if it exceeds 2.0%, S'-based nucleus growth sites and GP zones are formed, so that it is mixed with those of β'-based. , The strength rapidly increases, and the elongation and the formability decrease.
Therefore, the Cu content is set in the range of 0.3 to 2.0%, and more preferably in the range of 0.6 to 1.0%. Furthermore, Cu has the effect of sticking to dislocation loops and increasing formability, and when Cu is added, the texture of the Al-Mg-Si alloy changes to the cubic position ((100) plane, <001> direction). And improve moldability.

Mn: Mn is similar to Cu, and MnAl ₆ is deposited as a second-layer precipitate, which can be sufficiently solution-treated to form a solid solution to increase the strength and to re-create the alloy structure. It has the effect of suppressing the crystals and refining the crystal grains. Therefore, it is an element imparted to the improvement of molding. But 0.0
If it is less than 5%, the effect of refining crystal grains does not appear, and further, the precipitation of the second layer precipitate MnAl ₆ is not remarkable, so that the improvement of the molding workability is not recognized. Further, if the content is more than 1.0%, coarse crystallized substances are formed, and the formability is lowered. Therefore, the content of Mn is set to the range of 0.05 to 1.0%. By adding Mn in this range, even if the solution treatment is sufficiently performed and the strength of the material is increased, the crystal grain becomes 30 μm or less, and therefore the moldability is not deteriorated.

In the present invention, Al-Mg-Si-Cu is used.
If the above-mentioned Mg, Si, Cu and Mn are essential components, the effect of the -Mn-based aluminum alloy is sufficiently obtained,
Other elements may be contained as necessary as long as the effects of the present invention are not impaired. For example, at least one of Ti, Cr and Fe can be contained.

Ti: Ti is an element that makes the crystal grains of the ingot finer and improves the formability, but if it is contained in excess of 0.1%, coarse crystallized substances are formed and the formability is lowered. Let Therefore, the content of Ti is set to 0.1% or less.

Cr: Cr is an element which has the effect of improving the strength, but when the content increases, coarse crystallized substances are formed and the formability is lowered. Therefore, the Cr content is 0.4% or less.

Fe: Fe has a small effect of improving the strength, but when the content thereof is large, the formation of crystallized substances is remarkable and the formability is deteriorated. Therefore, the Fe content is set to 0.5% or less.

Next, preferable manufacturing conditions for the alloy sheet of the present invention will be described.

The above Al-Mg-Si-Cu-Mn type aluminum alloy is melted, cast, homogenized, and hot-rolled by a conventional method. After hot-rolling, an annealing treatment is performed to control the structure. In the present invention, by performing this annealing, the precipitates in the alloy are finely and uniformly dispersed, and by suppressing recrystallization, the grain structure after cold rolling → solution treatment is made fine. This is an effective treatment for obtaining an increase in moldability due to the refinement of crystal grains. However, if the temperature is less than 350 ° C., there is no effect of refining the crystal grains and the formability is not improved. On the other hand, if the temperature exceeds 450 ° C., the solution heat treatment temperature is approached, solid solution strengthening occurs, and the strength increases, which causes ear cracks and the like in the subsequent cold rolling. Therefore,
The annealing temperature after hot rolling is in the range of 350 to 450 ° C. Further, the holding time at this annealing temperature is appropriately 5 to 10 hours.

After that, cold rolling is performed to a predetermined plate thickness, and then solution treatment is performed. The conditions of the step of cold rolling → solution treatment are not particularly limited.

Conventionally, after the solution treatment, quenching was performed by water cooling or air cooling to room temperature, but in the present invention, a new quenching-holding process is adopted. The cooling rate at the time of quenching and the holding time at the quenching temperature are controlled.

The quenching temperature (that is, the quenching end temperature) is 5
If it is less than 0 ° C., it has almost no bake hardenability by baking at a very low temperature of about 150 ° C., and it disappears with the passage of time at room temperature. On the other hand, when the quenching temperature exceeds 120 ° C., the strength at T4 is too high due to the precipitation of Mg ₂ Si, the moldability is deteriorated and the bake hardenability is not observed.
Therefore, the quenching temperature range is 50 to 120 ° C.

When the cooling rate at the quenching temperature (50 to 120 ° C.) is less than 300 ° C./min, the strength after quenching is low and the bake hardenability at low temperature (150 ° C.) is low. Will not be recognized. Therefore, this cooling rate is 300
℃ / min or more.

As for the holding temperature for quenching, in the case of low temperature quenching of 70 ° C., improvement in bake hardenability at the target low temperature is not recognized by holding for a short time, and when holding at 120 ° C. for a long time. Mg ₂ Si precipitates, the strength at T4 increases too much, and bake hardenability is not observed. Further, when the quenching temperature is lower than 50 ° C., the low temperature bake hardenability disappears when it is held for a long time exceeding 48 hours, and even if the temperature is kept higher than 120 ° C. for a short time of less than 0.5 hours, Mg _No bake hardenability is observed because 2Si has already precipitated. Therefore, the quenching conditions are a temperature of 50 to 120 ° C. and a holding time of 0.5 to 48 hours.

Further, the structure of the Al alloy sheet material having high formability will be described.

The crystal grain size is an important factor that influences moldability, SS mark, surface roughness, bending workability and the like.
Formability, especially bulging property, is improved when the crystal grain size is small, and skin roughness and bending workability are also better as the crystal grain size is smaller. Regarding the SS mark, deterioration due to fine crystal grains, which is seen in Al-Mg alloys and the like, is hardly recognized in the Al-Mg-Si alloys of the present invention, and the crystal grain size is reduced. However, the SS mark does not occur. Therefore, the crystal grain size is 30 μm or less.

The texture shows the preferential planes and directions of the alloy, and is one of the factors that govern the formability of this texture. For metals with a face-centered cubic structure such as Al, if the slip surface (111) has high strength in its structure, formability is the best, but for rolled sheet material,
Normally, strength is not obtained in the (111) plane, and the structure is cubic, that is, has a peak in the <001> direction of the (100) plane. However, in the Al-Mg-Si system alloy (10
There may be a peak in the <001> direction of the (0) plane, and the formability at this time is low. Therefore, the Al-Mg-S of the present invention
The i-Cu-Mn-based alloy is characterized by having a structure having a peak in the (001) plane <001> direction in this alloy system.

Next, examples of the present invention will be described.

[0039]

Example 1 A 50 mm thick ingot obtained by melting and casting an aluminum alloy having the chemical composition shown in Table 1 by a conventional method was subjected to a homogenizing treatment at 510 ° C. for 4 hours and then at a temperature of 480 ° C. or lower. Hot rolling was performed up to a plate thickness of 5 mm. After leaving the hot-rolled material to room temperature, the temperature rise rate is 40 ℃ / hr and 450 ℃
Annealing was performed under the condition of × 5 hr, and then cold rolling was performed at room temperature to a plate thickness of 1 mm, which was then subjected to an experiment.

This cold rolled material was heated to a solution treatment temperature of 530 ° C. and held for 20 seconds, and then the quenching conditions shown in Table 2
That is, the average cooling rate at the time of cooling to a temperature (quenching temperature) in the range of 530 ° C. to room temperature is changed in the range of 50 to 800 ° C./min, and then the quenching temperature is kept at 0.5 to 6
After holding in the range of 0 hours, it was cooled to room temperature.

The obtained material was examined for mechanical properties after being left to stand at room temperature for 5 days after quenching and at 170 ° C.
The mechanical properties (bake hardenability) when baked for 20 minutes were examined. The results are shown in Table 3.

As is apparent from Table 3, the examples of the present invention were obtained by carrying out the quenching method under the conditions (Table 2) within the scope of the present invention.
It can be seen that the baking treatment at 70 ° C. is very excellent in bake hardenability. Furthermore, it can be seen that the inventive examples have excellent formability even when treated under the quenching conditions shown in Table 2. On the other hand, with chemical components outside the scope of the present invention, bake hardenability is not observed at all even when the quenching conditions within the scope of the present invention are adopted. From the above, the alloy of the present invention, the quenching conditions within the scope of the present invention
By applying (the method of the present invention), the bake hardenability at a low temperature (170 ° C.) is large.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

Example 2 Al alloy No. 3 shown in Table 1 of Example 1
(Chemical composition within the range of the present invention) and Al alloy No. 11 (Chemical composition outside the range of the present invention) were melted and cast by a conventional method, and the obtained ingot was heated at a heating rate of 40 ° C./hr for 510 minutes. After carrying out a homogenizing heat treatment of holding at a temperature of 4 ° C. for 4 hours, hot rolling was performed to obtain a plate having a thickness of 5 mm. The obtained hot-rolled material was annealed as shown in Table 4 and then cold-rolled to a thickness of 1.0 mm.
It was a plate.

Then, the obtained plate was heated at a heating rate of 400 ° C. /
Hold at 530 ℃ for 20 seconds at 800 ℃ / min
Quenching at a temperature of 50 ℃ at a cooling rate of
The temperature was maintained for 24 hours and used for the experiment. The strength, formability, and crystal grains of the heat-treated material were measured. As for the crystal grains, a plate having a thickness of 1.0 mm that has been subjected to the above heat treatment is mirror-polished with emery paper (No. 320 to 1200) and buff (alumina particle size 50 μm), and then electrolytically corroded by hydrofluoric acid, and then, The tissue was observed with an optical microscope, and the size was measured by the section method.

Characteristic and crystal grain size of the obtained material and baking
The bake hardenability after (170 ° C x 20 minutes) is shown in Table 5, and the alloy No. 3 (Table 1) was annealed (i) (Table 4) and the alloy No. 11 was used. Crystal structure photographs of the material annealed in (iv) are shown in FIGS. 1 and 2, respectively.

As is clear from Table 5 and FIGS. 1 and 2,
It can be seen that the alloy of the present invention has a grain size of 20 μm or less when annealed to the hot-rolled material under the conditions within the scope of the present invention and cold-rolled, which is finer than that of the comparative alloy and the comparative annealing method. Further, the alloy of the present invention has excellent formability and is effective in bake hardenability.

[0050]

[Table 4]

[0051]

[Table 5]

[0052]

Example 3 Al alloy No. 3 shown in Table 1 of Example 1
(Chemical composition within the range of the present invention) and Al alloy No. 11 (Chemical composition outside the range of the present invention) were melted and cast by a conventional method, and the obtained ingot was heated at a heating rate of 40 ° C./hr for 510 minutes. After carrying out a homogenizing heat treatment of holding at a temperature of 4 ° C. for 4 hours, hot rolling was performed to obtain a plate having a thickness of 5 mm. After leaving the obtained hot-rolled material up to room temperature, the temperature rising rate is 40 ° C / hr and 450 ° C x 5 hr.
Annealing treatment was performed under the conditions described above, and then cold rolling was performed at room temperature to obtain a plate thickness of 1 mm, which was then subjected to an experiment.

This cold rolled material was heated to a solution treatment temperature of 530 ° C. and held for 20 seconds, and then the obtained plate was held at a temperature of 530 ° C. for 20 seconds at a heating rate of 400 ° C./min.
Quench at a temperature of 50 ° C at a cooling rate of 800 ° C / min,
The sample was kept at the temperature of 50 ° C. for 24 hours and used for the experiment. The texture of the obtained material was observed by the diffractometer method. The results are shown in FIGS. 3 and 4.
The alloys containing Cu and Mn have a high distribution in the (100) plane <001> orientation, and the orientations are completely different from those of these additive-free alloys. Looking at the formability of the material of the present invention having this texture and the comparative material having a heteromorphic texture, it can be seen that the alloy of the present invention has good formability.

[0054]

As described in detail above, according to the present invention,
Al-Mg-Si-Cu- Mn alloy plate Mg, and adjusting the content of Si, the optimal Mg ₂ Si content and the residual Si content, further strength and an effect of improving the formability Cu, addition of Mn By doing so, it is possible to obtain an Al alloy sheet material having excellent formability and bake hardenability at low temperature. Furthermore, the crystal grain is set to 20 μm or less, and the texture is (100) plane, <0.
By providing the azimuth in the 01> direction, the moldability can be further improved. Further, an annealing treatment is performed after hot rolling, and by controlling this annealing treatment, the crystal grain is set to 20 μm or less, so that further excellent formability can be obtained, and further, the quenching treatment after the solution treatment is performed. By controlling, it is possible to obtain an Al alloy plate material having excellent bake hardenability at low temperature. Therefore, it becomes possible to reduce the thickness of the Al alloy plate, and since the formability is good,
It contributes to the weight reduction of home electric appliances and mechanical parts, and the frequency of use can be increased industrially, and the effect is extremely high.

[Brief description of drawings]

FIG. 1 is a photograph showing a metallographic structure of an alloy of the present invention in Example 2.

2 is a photograph showing a metallographic structure of a comparative alloy in Example 2. FIG.

FIG. 3 is a diagram showing the texture and formability of the alloy of the present invention in Example 3.

FIG. 4 is a diagram showing a texture and formability of a comparative alloy in Example 3.

Claims (4)

[Claims] 1. In weight% (hereinafter the same), Mg: 0.3-
1.0%, Si: 0.5-2.0% (however, regarding the relationship between Mg and Si, the content of Mg ₂ Si is 0.4-1.5% and the remaining Si The amount is 0.4 to 1.2%), and further Cu: 0.3 to 2.0% and Mn: 0.05 to 1.
Bake hardening type Al-Mg-Si excellent in moldability characterized by containing 0% and the balance Al and impurities.
-Cu-Mn Al alloy plate. 2. Ti: 0.1% or less, Cr: 0.4% or less,
Fe: The Al alloy plate according to claim 1, containing at least one of Fe: 0.5% or less. 3. The sheet material according to claim 1, wherein the texture of the plate material is controlled to be 30 μm or less in terms of crystal grain size, and the texture has a high peak in the (100) plane <001> direction. The described Al alloy plate. 4. The Al alloy ingot having the chemical composition according to claim 1 or 2 is homogenized at a temperature not higher than the burning temperature, then hot-rolled, and after hot-rolling, 350 to
Annealing treatment is performed at a temperature of 450 ° C. for 5 to 10 hours, then cold rolling is performed to obtain a desired plate thickness, solution treatment is performed, and then a cooling rate of 300 to 50 ° C./min.
Bake hardening type Al-Mg-Si-Cu-Mn excellent in molding processability, characterized by quenching to a temperature of 20 ° C and holding at a temperature of 50 to 120 ° C for 1 to 48 hours.
For manufacturing a series Al alloy plate.