HEAT TREATMENT METHOD OF ALUMINUM ALLOY PANEL

Abstract

The present invention provides a heat treatment method of an aluminum alloy panel, which can prevent surface curvature of molded aluminum alloy panel. For this purpose, the present heat treatment methods comprise: cold rolling an aluminum alloy panel at a reduction ratio of 45 to 50% at a final pass in a cold rolling process; first heat-treating the cold-rolled aluminum alloy panel at 450 to 510° C. for 3 hours; rapidly cooling the heat-treated aluminum alloy panel at a rate 60° C./sec or higher after the first heat treatment; and second heat-treating the rapidly cooled aluminum alloy panel at 200 to 220° C.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2008-0070424 filed Jul. 21, 2008, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a heat treatment method of an aluminum alloy panel. More particularly, the present invention relates to a heat treatment method of an aluminum alloy panel, which can prevent the formation of surface curvature that can occur when the aluminum alloy panel is molded.

(b) Background Art

Aluminum alloys used for vehicle outer panels are 5xxx (aluminum-magnesium) and 6xxx (aluminum-magnesium-silicon) series alloys, and the panels are formed to have a thickness up to 2 mm to increase strength and hardness.

In terms of moldability, the aluminum-magnesium alloys are superior to the aluminum-magnesium-silicon alloys and are applied to inner and outer panels having a complicated shape.

However, the aluminum-magnesium alloy sheets have a problem in that they show stretcher-strain mark formed on the surfaces thereof due to the interaction between dislocations that cause plastic deformation and precipitates of Al—Si during stamping or other deformations, i.e. due to dynamic strain aging and inhomogeneous deformation around precipitates by the precipitation of the alloy elements added to obtain high elongation and high formability by straining during deformation, thus deteriorating the surface quality of the sheets.

As shown in FIG. 1, the surface waviness can be characterized by the occurrence of serrated flow on a tension curve from tensile test on panel materials.

A prior art method to solve the problem of surface waviness is to subject an outer panel having such surface waviness to a post-process of sanding the front surface of the outer panel. The method, however, reduces the productivity and increases the manufacturing cost.

In order to achieve high moldability, it is necessary to increase the added amount of Mg; however, higher Mg content increases surface waviness. Accordingly, in a prior art method, in the case of the inner panel having a complicated shape, an alloy containing 2.8% or more of Mg is used to ensure the moldability and, in the case of the outer panel, an alloy containing Mg of less than 2.8% is used to maintain the surface quality. However, the moldability of the outer panel is not enough to accommodate design needs. Accordingly, the outer panel is prepared by using an aluminum-magnesium alloy containing a high content of Mg coupled with a pose-process of sanding the front surface of the panel.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

In one aspect, the present invention provides a heat treatment method of an aluminum alloy panel, the method comprising: cold rolling an aluminum alloy panel at a reduction ratio of 45 to 50% at a final pass in a cold rolling process; first heat-treating the cold-rolled aluminum alloy panel at 450 to 510° C. for 3 hours; rapidly cooling the heat-treated aluminum alloy panel at a rate more than 60° C./sec after the first heat treatment; and second heat-treating the rapidly cooled aluminum alloy panel at 200 to 220° C.

In a preferred embodiment, the aluminum alloy panel is an AA5454 aluminum-magnesium alloy panel comprising 95.35-96.45 wt % of aluminum (Al), 3.0 to 3.8 wt % of magnesium (Mg), 0.2 to 0.5 wt % of manganese (Mn), 0.35 wt % of iron (Fe).

In another preferred embodiment, the aluminum alloy panel is cold-rolled at a reduction ratio of 45 to 50% such that shear stress is applied to the surface of the aluminum alloy panel and shear texture {001}<110> is developed.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

The above and other features of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a graph showing the formation of surface waviness, obtained as a result of a tensile test for an aluminum alloy panel prepared by a conventional heat treatment method;

FIG. 2 is a diagram showing textures shown as (111) pole figures after cold rolling of a panel during heat treatment of the present invention;

FIG. 3 is a graph showing the results of tensile deformation behavior test for an aluminum panel prepared by a heat treatment method of the present invention and an aluminum panel prepared by a conventional heat treatment method; and

FIG. 4 shows images that compare surface waviness of the aluminum panel prepared by the heat treatment method of the present invention and that of the aluminum panel prepared by the conventional heat treatment method.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

The present invention provides a heat treatment method in which a simple heat treatment process is applied to an aluminum-magnesium alloy panel in order to reduce surface waviness formed by dynamic strain aging, facilitate the molding process, and obtain high strength characteristics by work hardening after molding.

In other words, the present invention provides a heat treatment method, which can obtain high strength characteristics after heat treatment due to the formation of precipitates by magnesium (Mg) in an aluminum-magnesium alloy panel and reduce surface roughness during the molding of the panel by performing a heat treatment in a specific temperature range.

In an embodiment, AA5454 aluminum-magnesium alloy may be used. The AA5454 aluminum-magnesium alloy is a commercially available alloy comprising 95.35-96.45 wt % of aluminum (Al), 3.0 to 3.8 wt % of magnesium (Mg), 0.2 to 0.5 wt % of manganese (Mn), 0.35 wt % of iron (Fe).

An AA5454 aluminum-magnesium alloy panel having the above-described composition may be manufactured by a known method. The method comprises: dissolving a raw material in an ingot state and forming an aluminum-magnesium alloy slab having a thickness of 150 mm by DC casting; performing hot rolling at 550° C. to be rolled into a thickness of 20 mm; cold rolling the hot-rolled coil at 420° C. to be rolled into a thickness of 1.0 mm; and performing aging treatment at 450-510° C. for 5 to 7 hours.

Here, the aging treatment contributes to increase the moldability of the panel and maximize the elongation; however, it is inevitable that the magnesium precipitates, which cause the formation of surface waviness, become coarse.

According to the present invention, a maximum shear stress is applied during the cold rolling in order to suppress the growth of magnesium precipitates, thereby preventing the magnesium precipitates from becoming coarse, and a multistage heat treatment is performed so that the precipitates are minutely distributed.

The cold rolling process is performed in five passes. The surface shear strain is applied at the final pass of the cold rolling process, what we called shear rolling, applying a rolling thickness strain corresponding to 45 to 50% with respect to the thickness of the previous pass (after fourth pass).

If the shear stress is applied in all passes, grain orientation is changed to {001}<110>, called shear texture, and thereby the anisotropy is increased. Accordingly, it is necessary that the shear stress should be applied in the final rolling pass.

In an embodiment, the AA5454 aluminum alloy panel is cold-rolled at a rolling reduction ratio of 45 to 50% at the final pass, the cold-rolled panel is subjected to a first heat treatment process which is performed at 450 to 510° C. for 3 hours, the panel then is rapidly cooled at a rate of 60° C./sec or higher, and the rapidly cooled panel is subjected to a second heat treatment process which is performed at 200 to 220° C.

That is, the AA5454 aluminum alloy panel is cold-rolled at a threshold temperature, where magnesium (Mg) precipitated at the grain boundary of the AA5454 aluminum alloy does not grow, any already-formed magnesium precipitates are dissolved by the first heat treatment process, and the magnesium precipitates are finely reprecipitated by the second heat treatment while being prevented from growing.

FIG. 2 is a diagram showing textures shown as (111) pole figures after cold rolling of the panel. It can be seen from FIG. 2 that while a panel prepared by a conventional heat treatment method (A) has ring-shaped texture on the surface and in the inside thereof, a panel prepared by the present heat treatment method (B) has a shear texture {001}<110> developed on the surface thereof, which delays the growth of magnesium precipitates during the heat treatment and a ring-shaped texture, a typical rolling texture, developed in the inside thereof.

FIG. 3 is a graph showing the results of tensile deformation behavior test for an aluminum panel prepared by a heat treatment method of the present invention and an aluminum panel prepared by a conventional heat treatment method. It can be seen from FIG. 3 that unlike the panels prepared by conventional methods (heat-treated at 450° C. or 510° C. for 5 hours), the panel prepared by the present heat to treatment method does not show strain aging, which may mean that the growth of magnesium precipitates is suppressed by the second heat treatment.

Here, the effect according to the heat treatment of the present invention will be described with respect to Test Examples below.

Test Examples

Strain aging was examined for the panels prepared by the heat treatment method according to the present invention and those prepared by conventional heat treatment methods. The formation of strain aging was checked on tensile curves and examined through surface roughness analysis. The test results are shown in the following Table 1.

	TABLE 1
	Reduction	First		Second		Surface	Cooling rate
	ratio (%) in	annealing	First	annealing	Second	waviness	(° C./sec)
	final cold	temperature	annealing	temperature	annealing	after tensile	after first
	rolling	(° C.)	time (hr)	(° C.)	time (hr)	test	annealing
Example 1	45	450	3	200	2	Not occurred	60
Example 2	45	480	3	200	2	Not occurred	60
Example 3	45	510	3	200	2	Not occurred	60
Example 4	45	450	3	200	3	Not occurred	60
Example 5	45	480	3	200	3	Not occurred	60
Example 6	45	510	3	200	3	Not occurred	60
Example 7	45	450	3	220	2	Not occurred	60
Example 8	45	480	3	220	2	Not occurred	60
Example 9	45	510	3	220	2	Not occurred	60
Example 10	45	450	3	220	3	Not occurred	60
Example 11	45	480	3	220	3	Not occurred	60
Example 12	45	510	3	220	3	Not occurred	60
Example 13	50	450	3	200	2	Not occurred	60
Example 14	50	480	3	200	2	Not occurred	60
Example 15	50	510	3	200	2	Not occurred	60
Example 16	50	450	3	200	3	Not occurred	60
Example 17	50	480	3	200	3	Not occurred	60
Example 18	50	510	3	200	3	Not occurred	60
Example 19	50	450	3	220	2	Not occurred	60
Example 20	50	480	3	220	2	Not occurred	60
Example 21	50	510	3	220	2	Not occurred	60
Example 22	50	450	3	220	3	Not occurred	60
Example 23	50	480	3	220	3	Not occurred	60
Example 24	50	510	3	220	3	Not occurred	60
Comparative	30	450	5	—	—	Occurred
Example 1
Comparative	30	480	5	—	—	Occurred
Example 2
Comparative	30	510	5	—	—	Occurred
Example 3
Comparative	30	450	7	—	—	Occurred
Example 4
Comparative	30	480	7	—	—	Occurred
Example 5
Comparative	30	510	7	—	—	Occurred
Example 6
Comparative	30	450	3	200	2	Occurred	60
Example 7
Comparative	30	510	3	200	2	Occurred	60
Example 8
Comparative	30	450	3	200	3	Occurred	60
Example 9
Comparative	30	510	3	200	3	Occurred	60
Example 10
Comparative	30	450	3	220	2	Occurred	60
Example 11
Comparative	30	510	3	220	2	Occurred	60
Example 12
Comparative	30	450	3	220	3	Occurred	60
Example 13
Comparative	30	510	3	220	3	Occurred	60
Example 14
Comparative	45	430	3	200	2	Occurred	60
Example 15
Comparative	45	430	3	200	3	Occurred	60
Example 16
Comparative	45	530	3	200	2	Occurred	60
Example 17
Comparative	45	530	3	200	3	Occurred	60
Example 18
Comparative	50	430	3	220	2	Occurred	60
Example 19
Comparative	50	430	3	220	3	Occurred	60
Example 20
Comparative	50	530	3	220	2	Occurred	60
Example 21
Comparative	50	530	3	220	3	Occurred	60
Example 22
Comparative	55	450	3	200	2	Surface	60
Example 23						defect
Comparative	55	450	3	200	3	Surface	60
Example 24						defect
Comparative	55	510	3	200	2	Surface	60
Example 25						defect
Comparative	55	510	3	200	3	Surface	60
Example 26						defect
Comparative	55	450	3	220	2	Surface	60
Example 27						defect
Comparative	55	450	3	220	3	Surface	60
Example 28						defect
Comparative	55	510	3	220	2	Surface	60
Example 29						defect
Comparative	55	510	3	220	3	Surface	60
Example 30						defect
Comparative	45	450	3	200	2	Occurred	50
Example 31
Comparative	45	450	3	200	3	Occurred	50
Example 32
Comparative	45	510	3	200	2	Occurred	50
Example 33
Comparative	45	510	3	200	3	Occurred	50
Example 34
Comparative	50	450	3	220	2	Occurred	50
Example 35
Comparative	50	450	3	220	3	Occurred	50
Example 36
Comparative	50	510	3	220	2	Occurred	50
Example 37
Comparative	50	510	3	220	3	Occurred	50
Example 38

It can be seen from Table 1 that no surface waviness occurred in Examples 1 to 24 in accordance with the present invention (B) whereas surface defects or surface waviness occurred in Comparative Examples 1 to 38 (A).

As described above, according to the present methods, magnesium precipitates can be prevented from becoming coarse, surface waviness formed by dynamic strain aging during molding of the panel can thereby be reduced, which makes it possible to facilitate the molding process and obtain high strength characteristics by work hardening after molding.

The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

1. A heat treatment method of an aluminum alloy panel, the method comprising:

cold rolling an aluminum alloy panel at a reduction ratio of 45 to 50% at a final pass in a cold rolling process;

first heat-treating the cold-rolled aluminum alloy panel at 450 to 510° C. for 3 hours;

rapidly cooling the heat-treated aluminum alloy panel at a rate of 60° C./sec or higher after the first heat treatment; and

second heat-treating the rapidly cooled aluminum alloy panel at 200 to 220° C.

2. The method of claim 1, wherein the aluminum alloy panel is an AA5454 aluminum-magnesium alloy panel comprising 95.35-96.45 wt % aluminum (Al), 3.0 to 3.8 wt % of magnesium (Mg), 0.2 to 0.5 wt % of manganese (Mn), and 0.35 wt % of iron (Fe).

3. The method of claim 1, wherein the aluminum alloy panel is cold-rolled at a reduction ratio of 45 to 50% such that shear stress is applied to the surface of the aluminum alloy panel and shear texture {001}110> is developed.