JPH0543974A - Aluminum alloy sheet excellent in baking hardenability of coating material and press formability and its production - Google Patents

Abstract

PURPOSE:To provide an aluminum alloy sheet excellent in press formability and excellent in hardenability by low temp. baking and suitable for an automobile body sheet and its manufacturing method. CONSTITUTION:This is an aluminum alloy sheet contg., by weight, 0.4 to 1.5% Mg, 0.24 to 1.6% Si, 0.12 to 1.5% Cu, <=1.0% Zn, 0.005 to 0.15% Ti and <=0.25% Fe, in which Si and Mg safisfy the relationship of Si >=0.6Mg (%) and the balance Al with inevitable impurities, in which the average aspect ratio H/L of the crystalline grains is regulated to >=0.7 and having >=20mum average grain size and is a manufacturing method for the above alloy sheet in which an aluminum ally ingot having the above compsn. is subjected to homogenizing treatment at 450 to 580 deg.C, is thereafter subjected to hot rolling and cold rolling into a desired sheet thickness, is heated to the range of 480 to 580 deg.C at >=3 deg.C/sec heating rate, is held for <=120sec and is cooled to 100 deg.C at >=2 deg.C/sec cooling 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 for press forming and a method for producing the same, and more particularly to an aluminum alloy sheet excellent in bake hardenability and press formability and suitable for automobile bodies and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a surface-treated cold-rolled steel sheet has been widely used as a sheet material for forming an automobile body sheet or the like. In order to meet the demand, aluminum alloy sheets have begun to be used for automobile body sheets and the like.

As an aluminum alloy for automobile body sheets, a non-heat treatment type Al-M represented by 5182 is used.
g-based alloy, heat treatment type Al-Cu system, Al-Mg-S
Divided into i system. As a non-heat-treatment type Al-Mg alloy, one that has been developed on the assumption that Cu or Zn is added in a trace amount and heat-treated (JP-A-57-12).
0648, JP-A-53-103914, etc.).

However, although they are slightly superior to the heat treatment type Al alloy in formability, they are inferior to the conventional surface-treated cold-rolled steel sheet, and further, the strength cannot be increased by the coating baking process. In addition, heat treatment type Al-Cu
System 2036, Al-Mg-Si system 6009, 601
0, 6011 is inferior in moldability, and further, in order to save energy against baking at 200 ° C in Europe and the United States, the temperature of 170 ° C or less, which is the mainstream in Japan, is maintained for 30 minutes at low temperature for a short time. There was also a problem that the strength did not increase in baking, but decreased in 2000 series. As described above, the conventional aluminum alloys are not sufficiently satisfied with the properties required for automobile body sheets, in particular, the formability and the bake hardenability.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has sufficient press formability for automobile bodies and the like, and has bake hardenability even at low temperature and short time baking. An object is to provide a good aluminum alloy plate and a method for manufacturing the same.

[0006]

Means and Actions for Solving the Problems As a result of various studies conducted by the inventors of the present application to achieve the above object,
By appropriately adjusting the chemical composition and optimizing the manufacturing conditions, the elongation after heat treatment is 30% or more, and 170
The yield strength after baking is 4 kgf / mm ² even before baking even at low temperature for a short time such as 20 minutes at ℃. The present invention has been completed by finding out that the composition is cured as described above. That is, the present invention has improved both the elongation at break as a material property and the yield strength after low temperature / short time baking in order to improve press formability and dent resistance after baking. Is.

In particular, regarding the chemical composition, from the viewpoint of increasing the strength after coating and baking, an appropriate amount of Cu is added to the Al-Mg-Si alloy to make the crystal grains equiaxed and coarse and to improve the yield strength before baking. In order to increase the yield strength increase due to baking, the recrystallization grain suppressing element is reduced.

That is, the aluminum alloy plate excellent in bake hardenability and press formability according to the present invention is
And 0.4 to 1.5% for Mg and 0.24 to 1.6 for Si.
%, Cu 0.12 to 1.5%, Zn 1.0% or less,
It contains Ti in the range of 0.005 to 0.15%, Fe in the range of 0.25% or less, and Si and Mg are Si ≧ 0.6Mg.
(%), The balance consists of Al and unavoidable impurities, and the average when the axial length of the crystal grain in the rolling direction is L and the axial length in the plate thickness direction perpendicular to L is H The aspect ratio H / L is 0.7 or more, and the average particle size is 20 μm or more. Further, with respect to this composition, Mn of 0.15% or less and C of 0.1% or less
r, 0.1% or less Zr, 0.1% or less V, and 0.
One or two or more of B of 0002 to 0.05% may be further contained.

The method for producing an aluminum alloy sheet excellent in bake hardenability and press formability according to the present invention is 450 to 580 for an aluminum alloy ingot having the above composition.
After performing a single-stage or multi-stage homogenization treatment at a temperature in the range of ℃, the ingot is hot-rolled and cold-rolled to a desired plate thickness, and then a temperature in the range of 480 to 580 ℃. At a heating rate of 3 ° C / sec or more and at that temperature
It is characterized in that it is held for 120 seconds and then cooled to 100 ° C. at a cooling rate of 2 ° C./second or more. As a result, the average aspect ratio H / L is 0.7 or more and the average crystal grain size is 2
The aluminum alloy plate having crystal grains of 0 μm or more can be obtained.

In this case, between hot rolling and cold rolling,
Or between cold rolling and cold rolling, or both, 32
Intermediate annealing treatment at a temperature within the range of 0 to 580 ° C.
It is preferable to carry out once or twice or more. Hereinafter, the present invention will be described in detail. In addition, in the following description, "%" represents "% by weight". First, the reasons for limiting the component composition of the aluminum alloy according to the present invention will be described.

Mg: Mg is an essential basic component in the alloy according to the present invention, and forms a compound of Mg ₂ Si together with Si and contributes to the improvement of strength. However, if Mg is less than 0.4%, sufficient strength cannot be obtained, and conversely 1.5%.
If it exceeds, the elongation will decrease. Therefore, the content of Mg is specified in the range of 0.4 to 1.5%.

Si: Si, like Mg, is an essential basic component in the alloy according to the present invention.
_It forms a compound of ₂ Si and contributes to the improvement of strength. However, if Si exceeds 1.6%, coarse crystallized substances that adversely affect the formability tend to be generated, and if less than 0.24%, the strength is insufficient. Therefore, if the Si content is 0.
It is specified in the range of 24 to 1.6%. Also, Si ≧ 0.6
Mg (%) is effective for improving pressability. Therefore, Si is defined in a range that satisfies Si ≧ 0.6Mg (%).

Fe: Fe is usually contained in an aluminum alloy as an unavoidable impurity, and if the content exceeds 0.25%, coarse crystallized substances which adversely affect the formability due to coexistence with Al are formed. Easily, and Si
In combination with the above, the amount of Si useful as precipitation hardening is reduced. Therefore, the Fe content is specified to be 0.25% or less.

Cu: Cu is a component that improves strength and formability and contributes to curing by baking. However, if the content is less than 0.12%, the effect cannot be sufficiently obtained, and conversely, if it exceeds 1.5%, formability and corrosion resistance are deteriorated. Therefore, the Cu content is 0.12
Specify within the range of 1.5%.

Ti: Ti has the effect of improving the workability by refining the crystal grains of the ingot by adding a trace amount. However, if these are added excessively, a coarse crystallized product is generated and the moldability is deteriorated. Therefore, the Ti content is specified in the range of 0.005 to 0.15%.

Zn: Zn is an element contributing to the improvement of strength, but if it exceeds 1.0%, ductility and press formability are deteriorated. Therefore, the Zn content is specified to be 1.0% or less.

In the present invention, in addition to the above essential elements, if necessary, one or more of Mn, Cr, Zr, V and B may be added in an appropriate amount. Since these elements are recrystallization suppressing elements, they may be added in appropriate amounts for the purpose of suppressing abnormal grain growth. However, these alloy components have a negative effect on the equiaxing of the recrystallized grains and reduce the formability. Therefore, their contents must be specified within a range smaller than that of conventional aluminum alloys. Therefore, when these are added, the contents of Mn, Cr, Zr, V, and B are 0.15% or less, 0.10% or less, and 0.1% or less, respectively.
0% or less, 0.10% or less, 0.0002 to 0.05%
Prescribed in.

In addition to the above-mentioned elements, inevitable impurities are contained as in the case of ordinary aluminum alloys, but the amount thereof is acceptable as long as the effects of the present invention are not impaired. For example, Be, Na, K, etc. do not cause a problem even if they are contained in an amount of about 0.001% or less. Next, the organization will be described.

The formability of an aluminum alloy largely depends on the crystal grain size and the crystal grain shape. Average particle size is 20μ
is less than m and the average aspect ratio of crystal grains is H / L
If (L: crystal grain axis length in the rolling direction, H: plate thickness direction axis length perpendicular to L) is less than 0.7, the formability is not sufficient, and the press pattern appears and the surface quality deteriorates. It is necessary that the average grain size is 20 μm or more and the average aspect ratio of the crystal grains is 0.7 or more. Next, the manufacturing conditions of the alloy of this invention will be described.

An aluminum alloy having the components and compositions defined in the above range is melted and cast by a conventional method, and the ingot is subjected to one-step or multi-step homogenization heat treatment at a temperature in the range of 450 to 580 ° C. .. By performing such homogenization treatment, diffusion solid solution of the eutectic compound crystallized during casting is promoted and local microsegregation is reduced. Further, by this treatment, it is possible to finely precipitate the compounds of Mn, Cr, Zr, and V that play an important role in suppressing the abnormal grain growth of the crystal grains of the final product and achieving uniformity. However, when the temperature of this treatment is lower than 450 ° C, the above-mentioned effects are insufficient, while when it exceeds 580 ° C, eutectic melting occurs. Therefore, the temperature of the homogenization treatment is 450 to 580 ° C.
And the range. Note that the holding time within this temperature range is 1
If the time is less than the time, the above effect cannot be sufficiently obtained, and if the heating for a long time exceeding 72 hours is saturated, the effect is saturated.
The holding time for this homogenization treatment is preferably 1 to 72 hours.

Then, the ingot subjected to such homogenization treatment is subjected to hot rolling and cold rolling in order to obtain a predetermined plate thickness according to a conventional method. Further, skin pass rolling of 5% or less may be carried out for strain correction or surface roughness adjustment.

After completion of rolling, for such rolled plate material,
Heating to a temperature in the range of 480 to 580 ° C at a heating rate of 3 ° C / sec or more, and immediately after reaching that temperature, or 12
After holding for a period of 0 seconds or less, heat treatment is performed under the condition that the material is rapidly cooled to 100 ° C. at a cooling rate of 2 ° C./second or more. By this treatment, the structure is homogenized, the average grain size of the crystal grains is adjusted to 20 μm or more, the average aspect ratio is adjusted to 0.7 or less, and further the processing strain is removed, and as a result, the press formability can be improved. Further, this heat treatment aims at solution treatment of an intermetallic compound such as Mg ₂ Si, which makes a large contribution to bake hardening, and achieves improvement of bake hardenability. In this case, if the heating temperature is lower than 480 ° C., the above effects cannot be sufficiently obtained. Further, if the heating temperature exceeds 580 ° C., the heating rate is too slow, or the holding time is too long, some of the crystal grains cause abnormal grain growth. Furthermore, the cooling rate up to 100 ° C is 2 ° C /
If it is less than 2 seconds, the above-mentioned compounds coarsely precipitate during cooling, which is not desirable in terms of press formability and bake hardenability. Therefore, the conditions are defined as described above.

In addition to the above steps, one or more intermediate annealings may be performed between the hot rolling and the cold rolling, or between the cold rolling and the cold rolling, or both of them. Is desirable. By performing this intermediate annealing, it is possible to prevent edge cracking during strong reduction in cold rolling, and the Mg compound that functions as a recrystallization nucleus precipitates to homogenize the structure, and as a result, formability is improved. Can be improved. However, if the temperature at this time is less than 320 ° C, the effect is not sufficient, and if it exceeds 580 ° C, eutectic melting occurs. Therefore, the intermediate annealing is performed in the range of 320 to 580 ° C. The intermediate annealing is not an essential process, and the intermediate annealing may be omitted from the viewpoint of process saving.

The aluminum alloy sheet thus obtained is excellent in bake hardenability and press formability, has an elongation at break of 30% or more, and is also excellent in hardenability by low temperature bake. Therefore, such an aluminum alloy plate is suitable for an automobile body sheet.

[0025]

Embodiments of the present invention will be described below. (Example 1)

Alloys having the components and compositions shown in Tables 1 and 2 were melted and continuously cast, the obtained ingots were chamfered, and then homogenized at 520 ° C. for 8 hours, then, The slab is heated to 460 ° C and hot rolled to a plate thickness of 4 mm,
After cooling to room temperature, cold rolling was performed at a rolling ratio of 75% to obtain a plate material having a thickness of 1 mm. The finish temperature of hot rolling is 2
It was 80 ° C. This 1mm thick plate material can be used up to 550 ℃ 1
After heating at a rate of 0 ° C / sec and holding for 60 seconds, forced air cooling was performed up to 100 ° C at a cooling rate of 20 ° C / sec.

The plate material produced in this manner is used at room temperature for 30 minutes.
After leaving for a day, it is cut into a predetermined shape and subjected to a tensile test (JIS5
No., tensile direction: rolling direction) and conical cup test (J
ISZ2249: test tool type 17) was carried out, and the average crystal grain size and the average aspect ratio H / L of the crystal grains were measured. The conical cup test was performed as a press-molding simulation, and the composite moldability of overhanging and deep drawing was evaluated by CC.
It was evaluated by V (mm) (the smaller the CCV, the better the moldability). Further, the crystal grain shape was determined by a cutting method using a Ga treatment to reveal a microstructure and 50 samples. Furthermore, in order to simulate coating baking after press molding, heat treatment (corresponding to baking) was performed at 170 ° C. for 30 minutes, and then a tensile test was performed again under the same conditions as the above-mentioned test after heat treatment.

The results of these tests are shown in Tables 3 and 4. The column of "bake hardening" shows a value obtained by subtracting the yield strength after the final heat treatment from the yield strength after the baking simulation. The surface properties after the conical cup test are also shown.

Alloy Nos. 1 to 16 in Table 1 are working examples within the composition range of the present invention, and Alloy Nos. 17 to 33 in Table 2 are examples.
Is a comparative example outside the range. Alloy number 3
Reference numerals 1 to 33 are alloys conventionally used for body sheets and correspond to 2036, 5182 and 6010, respectively.

As is clear from Table 3, the alloy Nos. 1 to 16 of the examples have an elongation of 30% or more and an average particle size of 20.
It was confirmed that the moldability was not less than μm, the average aspect ratio was not less than 0.7, the CCV was also good, and excellent moldability was obtained. Also, the bake hardening yield strength is 4kgf / mm ² It was confirmed that it has a high value as above and has excellent moldability and low temperature bake hardenability.

On the other hand, alloy numbers 17 to 33 of the comparative examples shown in Table 2 have an average particle size of 20 μm or more and an average aspect ratio of 0.7 or more, as is clear from Table 4. Also, both or one of the moldability and the bake hardenability was inferior to the examples. For example, alloy Nos. 17, 18, and 20 having a low content of Mg, Si, or Cu, which are components contributing to bake hardening, have a low bake hardenability of 2 kgf.
/ Mm ² It was about. On the contrary, Alloy Nos. 19 and 21 containing a large amount of Si and Cu had low formability. In addition, Mn,
Alloy Nos. 23, 24, 25, 26, 27, 2 in which the amounts of Cr, Zr, V, Ti-B and Fe are higher than the range of the present invention.
Nos. 8 and 29 have low elongation, and the average crystal grain size is 20 μm.
In the following, since the average aspect ratio of the crystal grains was 0.7 or less, the moldability was poor, and the flow marks were generated, and the surface properties were also poor.

FIG. 1 shows the relationship between the CCV characteristics and the average aspect ratio of crystal grains. From this figure, it is seen that the average aspect ratio is 0.7 or less, which is outside the range of the present invention (alloy No. 29, It is apparent that the CCV characteristics of 30) are poor and the moldability is poor.

Alloy No. 22 had an amount of Ti-B smaller than the range of the present invention, but the elongation value was insufficient. Further, it was confirmed that the conventional alloy numbers 31 to 33 are inferior in both the bake hardenability and the formability.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4] (Example 2)

Next, among the alloys shown in Table 1, ingots having the composition of alloy No. 2 were used to produce alloy sheet materials under the production conditions shown in Table 5. The conditions of Example 1 were adopted for the treatments not particularly described in Table 5 (rolling conditions, etc.). In Table 3, symbols A to E are examples within the range of the manufacturing method according to the present invention, and symbols F to M are comparative examples outside the range. The same evaluation test as in Example 1 was performed on the plate material manufactured in this manner. The results are also shown in Table 5.
Also described in. As is clear from Table 5, it was confirmed that the comparative examples not satisfying the conditions of the present invention had insufficient elongation and moldability, or bake hardenability.

For example, when the intermediate annealing temperature or the heat treatment temperature is out of the range of the present invention as in Comparative Examples F, G, and L, the formability and bake hardenability are poor, and the solution hardening is performed as in Comparative Example K. It was confirmed that the bake hardenability was inferior when the cooling rate under the conditions was small. Further, Comparative Example M has a low heat holding temperature for solution hardening, an average particle size of less than 20 μm and an average aspect ratio of less than 0.7, and has low elongation, resulting in poor formability and rough surface texture. It was confirmed that it was not good, and that sufficient bake hardening could not be obtained. Comparative Example I, which has a high heating and holding temperature for solution hardening, undergoes eutectic melting and the like, and its strength decreases. Further, Comparative Example J, which has a long heating and holding time for solution hardening, causes abnormal grain growth. , Flow mark appeared, and it was confirmed that the moldability was inferior. It was confirmed that in the case of Comparative Example H in which the heating rate of solution hardening was small, the elongation tended to be low.

[0040]

[Table 5]

[0041]

EFFECTS OF THE INVENTION According to the present invention, the elongation, press formability, and bake hardenability during low temperature and short time coating baking are superior to those of conventional aluminum alloy sheets. There is provided an aluminum alloy sheet suitable for an automobile body sheet or the like, which requires dent resistance, and a method for producing the same.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an average aspect ratio of crystal grains and CCV characteristics.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Naotake Yoshihara Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Shinji Mitao 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd.

Claims (5)

[Claims] 1. Mg in an amount of 0.4 to 1.5% by weight and S
i is 0.24 to 1.6%, Cu is 0.12 to 1.5%,
Zn is 1.0% or less, Ti is 0.005 to 0.15%,
Fe in the range of 0.25% or less, and Si and M
g satisfies the relationship of Si ≧ 0.6Mg (%), and the balance is A
When L is the axial length of the crystal grain in the rolling direction and H is the axial length in the plate thickness direction perpendicular to L, the average aspect ratio H / L is 0.7 or more. And an average grain size of 20 μm or more, which is excellent in bake hardenability and press formability. 2. A Mn content of 0.15% or less by weight% and an O.V.
1% or less of Cr, 0.1% or less of Zr, 0.1% or less of V, and 0.0002 to 0.05% of B are further included in one or more kinds. Claim 1
An aluminum alloy sheet excellent in bake hardenability and press formability as described in 1. 3. Mg in an amount of 0.4 to 1.5% by weight and S
i is 0.24 to 1.6%, Cu is 0.12 to 1.5%,
Zn is 1.0% or less, Ti is 0.005 to 0.15%,
Fe in the range of 0.25% or less, and Si and M
g satisfies the relationship of Si ≧ 0.6Mg (%), and the balance is A
1 and multi-stage homogenization treatment at a temperature in the range of 450 to 580 ° C. for an aluminum alloy ingot composed of 1 and inevitable impurities, and then hot rolling and cold rolling this ingot. To the desired thickness, then 480-
It is heated to a temperature in the range of 580 ° C. at a heating rate of 3 ° C./sec or more and held at that temperature for 0 to 120 seconds, then 10
A method for producing an aluminum alloy sheet excellent in bake hardenability and press formability, which comprises cooling to 0 ° C. at a cooling rate of 2 ° C./second or more. 4. The aluminum alloy ingot has a weight% of
, 0.15% or less of Mn, 0.1% or less of Cr, 0.
1% or less Zr, 0.1% or less V, and 0.0002
The method for producing an aluminum alloy sheet having excellent bake hardenability and press formability according to claim 3, further comprising one or more of B of 0.05 to 0.05%. 5. 320 to 580 between hot rolling and cold rolling, or between cold rolling and cold rolling, or both.
The method for producing an aluminum alloy sheet excellent in bake hardenability and press formability according to claim 3 or 4, wherein the intermediate annealing treatment is performed once or twice or more at a temperature within a range of ° C.