JP2002520486A - Heat treatment of aluminum alloy sheet products - Google Patents

Abstract

A process of heat treating a sheet article made of a 6000 series aluminum alloy to achieve good "paint-bake response" that is substantially unaffected by natural aging. The process comprises heating the alloy sheet article at a solutionizing temperature followed by cooling the alloy sheet article. The cooling involves the following steps: (1) cooling from said solutionizing temperature to a temperature in the range of 150 - 250 DEG C at a rate greater than or equal to about 4 DEG C per second; (2) further cooling the alloy to a temperature in the range of ambient to 100 DEG C at a rate of 20 to 30 DEG C (preferably about 25 DEG C) per minute; and (3) further cooling the alloy sheet article having a temperature of 55 DEG C or more after step (2) to ambient temperature at a rate of less than 2 DEG C per hour. Alloy sheet articles suitable for use in the fabrication of automobile skin part can be produced in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for producing a heat-treatable thin plate made of an aluminum alloy, and this alloy is particularly suitable as an assembly part such as a skin panel for an automobile. In particular, the present invention relates to a method for producing a sheet product that minimizes disadvantages associated with the sheet product due to natural aging.

(Prior Art) In the automotive industry, the use of aluminum alloy panels instead of steel panels is increasing in order to reduce the weight of vehicle bodies. Of course, a lightweight panel helps to reduce the weight of the vehicle body and lowers fuel efficiency. However, with the introduction of the aluminum alloy panel, predetermined requirements are imposed on the aluminum alloy itself. For aluminum alloy sheet products to be a useful automotive application, T4 delivered to the car manufacturer
It is necessary to have good moldability while maintaining the tempered condition. With good formability, the aluminum alloy can be bent at a desired angle and formed into a desired shape without forming cracks, tears or wrinkles. At the same time, after painting and baking (paint baking), the alloy panel must have sufficient strength to withstand an impact such as a collision.

In general, several aluminum alloys of the AA (Aluminum Association) 2000 series and 6000 series are considered for automotive panel applications. The AA 6000 series alloy contains magnesium and silicon and may or may not contain copper, but can be classified as an AA 2000 series alloy according to its copper content. These alloys are in the formable T4 tempered state and become more tough and T8 tempered after the painting / baking process. That is, the yield strength of the alloy is increased by the “paint baking action”. Among alloys of this type, particularly preferred alloys are:
AA 6111 alloy.

[0004] To facilitate understanding of the contents of the present invention, terms currently adopted for the temper of the alloy will be briefly described in order. A temper called T4 is well known (for example, Aluminum Standar published by The Aluminum Association)
ds and Data (1984) p. 11), solution treatment, quenching, 48
Related to alloys produced by natural aging for more than an hour. In this tempered state, the automotive sheet material is usually supplied to a parts manufacturer and used to form skin panels and the like. For example, T4 temper, which is a commercially available conventional product as an assembled part
For the AA 6111 sheet material, the cold-rolled material is solution-treated (solution heat treatment) by a continuous annealing furnace (annealing furnace) at 530 to 560 ° C, and the obtained alloy is rapidly cooled to a temperature of 35 to 45 ° C. It has been naturally aged for more than 2 days and has been subjected to normal final treatment. Alternatively, the material can be solution treated, coiled at 55-85 ° C, and the wound coil can be cooled to room temperature and subjected to conventional final processing. The material produced by this method exhibits the same performance in terms of formability and tensile strength as compared with the conventional T4 temper sheet material, and has a remarkable improvement in the baking action of the paint. The material produced by such a separate heat treatment step is called T4P tempered product in the country.

[0005] T8 temper relates to alloys obtained by solution heat treatment, cold working and then artificial aging. Artificial aging is a process in which an alloy is kept at a high temperature for a long time. T8X temper refers to a T4 temper material that has been 2% deformed by application of tension and then artificially aged at 170 ° C. for 20 minutes or 177 ° C. for 30 minutes (industrial forming process + process simulating paint baking).

[0006] An alloy that has reached peak strength only after solution heat treatment and artificial aging is called T6 temper.

[0007] While the conventional 6000 series alloy sheet shows a good paint baking action immediately after quenching, it has been observed that this baking action is slightly reduced by natural aging. Therefore, it would be advantageous if an alloy sheet could be manufactured that could prevent such a decrease in paint baking action and maintain the high yield strength of the T8X temper.

According to US Pat. No. 5,616,189 (issued April 1, 1997, same applicant as the present application),
A method of making an aluminum alloy sheet having 4 and an effective T8X temper is disclosed. According to this manufacturing method, heat treatment and controlled cooling are performed. However, the manufacturing method of this disclosure does not always provide good results, and therefore, it is necessary to develop another manufacturing method instead of this manufacturing method.

An object of the present invention is to provide a method for producing an alloy sheet made of a 6000 series aluminum alloy, which exhibits a desired paint baking action and a high yield strength of a T8X temper. It is to be.

Another object of the present invention is to provide a heat treatment method that avoids or reduces a decrease in the paint baking action of a 6000 series aluminum alloy.

One gist of the present invention is a method of heat-treating a thin plate made of a 6000 series aluminum alloy by heating the thin plate to a solution temperature and then cooling the thin plate. The step of cooling the thin plate article includes: (1) bringing the alloy from the solution temperature to a temperature of about 150 to 250 ° C. at about 4 ° C./sec or more (preferably 225 ° C./sec or more) (2) further cooling the alloy to a temperature range from ambient temperature (room temperature, eg, about 20 ° C.) to 100 ° C., at a rate of about 20 Cooling at a rate and then (3) further comprising cooling the alloy at a temperature of about 55 ° C. or higher cooled in the cooling step (2) to ambient temperature at a rate of less than about 2 ° C./hour. And providing a method characterized by:

With regard to the cooling step (3), the sheet product may be generally subjected to coil winding at the temperature indicated after the step (2), and then cooled at the rate indicated for the step (3). In the final step (3), artificial aging (pre-aging) of the alloy can be performed.

[0013] When solutionizing the alloy according to a continuous annealing line (heating section + quenching section, CAL), the cooling steps (1) and (2) are controlled and cooled in a furnace, whereby the required It is necessary to ensure a slower cooling rate. Process (3)
Can be carried out in a normal storage area in a manufacturing facility, provided that the ambient temperature can ensure the desired slow cooling rate.

[0014] The method of the present invention may be a part of a continuous production method of an alloy sheet product, and the continuous production method includes a casting method prior to the solution treatment and the multi-stage cooling treatment. Treatment, homogenization treatment, hot and cold rolling treatment are included.

The alloy used in the present invention may be any alloy as long as it is an AA 6000 series aluminum alloy. The most preferred alloy has the following composition. Cu: 0 to 1.0% by weight Mg: 0.4 to 1.1% by weight Si: 0.3 to 1.4% by weight Fe: 0.1 to 0.4% by weight Mn: 0 to 0.45% by weight Al: Remainder

If desired, suitable alloys can include Zr, Cr and / or Ti in small amounts not exceeding 0.15% by weight in total.

The most preferred alloy is AA 6111 alloy, which has the following composition (% by weight) (the balance being Al):

The method of the present invention is further improved by reducing the tendency of paint stoving action to decrease due to natural aging, compared to alloys of the same composition produced by conventional methods (solution treatment, quenching and natural aging). Sheet products can be produced that exhibit paint baking action (increased yield strength from T4 temper after painting and baking).

The mechanism of action of the required controlled cooling is not known exactly. However, although the present invention is not limited to the following theory, at present, step (1)
The controlled cooling of (2) and the formation of stable nuclei, which promote the cohesive fine particle deposition, which can be uniformly distributed in the alloy matrix during the artificial aging (pre-aging) step. ,Conceivable. In contrast, in the conventional method, nuclei formed during natural aging become unstable and dissolve during subsequent aging at high temperatures,
As a result, it is believed that the particle distribution in the matrix becomes coarse, whereby the strength increase of the T8X temper is reduced as compared to the strength increase expected from the material produced by the method of the present invention.

Embodiments of the Invention According to one preferred embodiment of the present invention, the alloy is direct chill cast, skinned, homogenized at 480-580 ° C for less than 48 hours, and heat treated. Hot / cold rolling to form an intermediate-thick sheet product, cold-rolling the intermediate-thick sheet product to the final thickness, solution heat treatment at 480-580 ° C in a continuous heat treatment (CASH) furnace, and the present invention Quenched by the control method described above, coiled at a temperature of less than about 85 ° C, and then cooled to room temperature. Thereafter, it is generally subjected to various final operations such as leveling (smoothing process),
A smooth sheet is obtained and processed to obtain a part. The panel formed from the material of the present invention can achieve higher strength as compared with the conventional AA 6111 sheet material due to curing of the paint. The temper alloy obtained in this way is called T4CC in Japan.

The processing includes an intermediate annealing processing operation, and the processing operation includes:
It can be performed between hot rolling and a final cold rolling operation to produce high strength sheet products without roping [e.g., co-pending
PCT / CA98 / 00109, filed February 17, 1998, published August 27, 1998, WO 98/37251, the disclosure of which is incorporated herein by reference. ].

According to another preferred embodiment of the present invention, performed by a two-belt cast 11,
This is shown in the schematic diagram of FIG. A continuous metal strip 10 of a 6000 series alloy (preferably an AA 6111 alloy) is hot rolled in a rolling zone 12. In this hot rolling step, a small amount of precipitate is formed. The hot-rolled product is subjected to a coil winding process to form a coil 14. The hot rolled strip 10 is then unwound from the coil 14, cold rolled by a cold rolling mill 15 and coiled to form a coil 16. Next, the cold-rolled strip 10 is unwound from the coil 16 and subjected to a continuous solution heat treatment,
The mixture is rapidly cooled while being controlled by 17, and is again subjected to a solution treatment to precipitate component particles. Then, a coil is wound up to form a coil 18.

The solution heat treatment is a process for re-dissolving the precipitated alloy component in the alloy. In the solution heat treatment, the alloy sheet material is generally heated to a temperature of about 500 to 570 ° C. (preferably about 560 ° C.).
C.). An improved quenching or cooling treatment is then performed according to the method of the present invention. The coil take-up strip 18 is a T4 temper, which can be marketed to an automobile manufacturer or a component manufacturer, which deforms the strip to form a panel 20, which is then painted / baked. , Forming a painted panel 22 of T8X temper.

According to the present invention, a material having the characteristics of a known T4 temper product can be produced by controlling the cooling conditions immediately after the solution treatment as described above. In fact, when the controlled cooling process is combined with the heating coil winding process (about 55-80 ° C), the aging effect is significantly improved.

As described above, the controlled cooling process from the solution temperature is performed in two stages,
These steps are referred to as steps (1) and (2) or first and second steps.
During the first cooling step, the material is cooled to an intermediate temperature of about 150-250 ° C at a rate typically employed in industrial continuous heat treatment lines. In the next second cooling step, 85 ° C
Allow to cool (natural cooling) to a temperature of less than, if desired, coil winding, then cool the coil to room temperature.

It should be noted that the heat treatment method of the present invention can be used to solution treat the material, cool it to an intermediate temperature of about 150-250 ° C., and then cool it at a slower rate to form a stable nucleus. What
It can be easily implemented by using a long continuous annealing furnace.

EXAMPLES Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto. Example 1: Laboratory Experiments In the following laboratory experiments, a commercially available AA 6111 alloy sheet [composition (% by weight)
): Cu 0.74%, Fe 0.24%, Mg 0.79%, Mn 0.13%, Si 0.60%, Ti 0.06%, Cr
0.05% and the balance Al) were used.

The alloy was previously solution heat treated at 560 ° C. by a continuous annealing / solution heat treatment (CASH) line, quenched with cold water and stored at room temperature. Several samples prepared from this material were again solution treated by heating at 558 ° C. by means of a fluidized bed and then in two stages (steps (1) and (2) of the invention, namely the first and (The step simulating the second step).

The first cooling condition is a condition for obtaining various intermediate cooling temperatures (ICT), and the first cooling condition was measured by performing several calibration experiments. A 1.0 mm thick tensile test sample embedded with a thermocouple was heated to 558 ° C. in a sand fluidized bed, held for 30 seconds, and cooled to room temperature (RT) by forced air. FIG. 2 shows the heating / cooling characteristics of the sample. By repeating such an experiment several times, it was found that the obtained heating / cooling curve was very reproducible. Using the cooling curves of FIG. 2, the time required to reach various intermediate cooling temperatures was measured. The second cooling condition was simulated by cooling from an intermediate cooling temperature to room temperature or pre-aging temperature with air.

To examine the effect of the first cooling conditions on the tensile strength properties, several samples were cooled to various intermediate temperatures (about 100-250 ° C.) and then allowed to cool to room temperature (second cooling). The sample was solution treated at 560 ° C., cooled with forced air for a predetermined period to obtain a predetermined intermediate temperature, and further air-cooled to room temperature. The sample took up to 8 minutes to cool to room temperature.

An additional heat treatment was performed to measure the combined effect of the controlled cooling process and the heated cooling coil winding process. Solution treated sample, intermediate cooling temperature about 150 ~ 250 ℃
And then air cooled. The samples were then heat treated at 60, 80 or 100 ° C. for 5 hours and subjected to a treatment simulating pre-aging.

After one week, two tensile tests were performed on T4 temper and T8X temper (2% tensile deformation +
177 ° C. for 30 minutes). Without natural aging, the stability of the tensile strength properties was examined and similar results were obtained. The material properties of the present invention were then compared with those of materials produced by conventional methods.

Experimental Results Effect of First Cooling Treatment on Intermediate Cooling Temperature Table 1 below summarizes the average tensile strength properties of AA 6111 with various tempers. In Table 1, UTS is the ultimate tensile strength, YS is the yield strength, and% El.
In terms of elongation, ksi is klb / in and MPa is megapascal.

[Table 1]: Tensile strength properties of AA 6111 alloy (decomposition heat treatment (560 ° C × 1 minute, sand fluidized bed) + forced air cooling to intermediate temperature + air cooling + pre-aging) * Tested immediately after heat treatment

FIG. 3 shows the change in yield strength as a function of the intermediate cooling temperature. In FIG.
Curve (a) is the yield strength value after cooling, curve (b) is the yield strength value after one week at room temperature, curve (c) is the T8X temper yield strength value after one week at room temperature, Curve (d) is
It is the T8X temper yield strength value after cooling.

The following technical matters have been found from FIG. (I) The yield strength of the alloy immediately after the heat treatment is about 82.7 to 89.6 MPa (about 12 to 13 ksi). These values are approximately equal to the yield strength of the quenched conventional material. (Ii) The yield strength of T4 temper does not change depending on the change of the intermediate cooling temperature (
Curve (a) in FIG. 3, see). (Iii) The yield strength (curve (d) in FIG. 3) of the T8X temper immediately after cooling shows significantly different aging behavior. Materials cooled by conventional methods have an intermediate cooling temperature of less than
It shows a yield strength of MPa (about 40 ksi). Above this intermediate cooling temperature, the yield strength increases, approaching 293.5 MPa (43.5 ksi) at 250 ° C. (Iv) After one week of natural aging, the yield strength of the material (curve (b) in Figure 3) is approximately 132.4 MPa
(About 19.2 ksi). As before, the yield strength of the T4 temper does not depend on the intermediate cooling temperature. (V) As expected, natural aging reduced the yield strength of the T8X temper. The decrease in yield strength is related to the intermediate cooling temperature. For example, the material exhibits a yield strength of 206.8 MPa (30 ksi) when cooled by the first cooling to a temperature below 150 ° C. Higher yield strengths can be achieved by increasing the intermediate cooling temperature to a maximum of 250 ° C. At this temperature, the material exhibits a yield strength of 256.5 MPa (37.2 ksi),
This yield strength is approximately equal to about 25% higher yield strength than conventionally produced materials.

Relationship between Intermediate Cooling Temperature and Pre-Aging Conditions The yield strength of a material can be significantly increased by performing a pre-aging step after controlled cooling (FIG. 3). In general, the lower the intermediate cooling temperature, the lower the strength of both tempers. Increasing the pre-aging temperature increases the strength slightly, and for a material cooled to 250 ° C, the absolute value of its T8X yield strength is
Increase by 20.7 (Table 1). As can be seen from these data, higher pre-aging temperatures are preferred for achieving high strength, especially with respect to the intercooling temperature of 250 ° C. It should be noted, however, that selecting a higher pre-aging temperature range of about 60-100 ° C. has little effect on the T8X temper properties (see curve (d) of FIG. 3 and Table 1). .

The consolidated material is cooled to about 150-250 ° C. by a first cooling, and then
It has been found that with very slow cooling to a temperature below 0 ° C., a significant degree of paint baking action can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing steps performed according to a preferred embodiment of the method of the present invention.

FIG. 2 is a graph showing heating and cooling curves for the sample described in Example 1.

FIG. 3 is a graph showing the relationship between yield strength and intermediate cooling temperature for the sample described in Example 1.

[Explanation of symbols]

 10: Metal strip 11: Double belt casting 12: Rolling area 14: Coil 15: Cold rolling mill 16: Coil 17: Area 18: Coil 20: Panel 22: Panel

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22F 1/00 630 C22F 1/00 630A 691 691B 692 692A 692B (72) Inventor Pierre Henri Marois Canada, K7M4J5, Ontario, Kingston, Crescent Drive 38B

Claims (9)

[Claims] 1. A method for heat-treating a thin plate made of a 6000 series aluminum alloy by heating the thin plate at a solution heat temperature and then cooling the thin plate, wherein the step of cooling the thin plate includes: Cooling the alloy from the solution temperature to a temperature of 150-250 ° C. at a rate of about 4 ° C./sec or more; (2) further cooling the cooled alloy to a temperature range of ambient temperature to 100 ° C. Cooling at a rate of about 20-30 ° C / min, then (3) further cooling the alloy at 55 ° C or higher in the cooling step (2) to ambient temperature at a rate of less than 2 ° C / hour Cooling in the method. 2. The method according to claim 1, wherein the cooling rate in the cooling step (1) is 225 ° C./sec or more. 3. The method according to claim 1, wherein the cooling step (2) is performed at a temperature of about 25 ° C. 4. The method according to claim 1, wherein the sheet material is coiled at a temperature indicated before the cooling step (3) and after the cooling step (2). 5. The method according to claim 1, wherein the alloy treated by the method has the following composition: Cu: 0 to 1.0% by weight Mg: 0.4 to 1.1% by weight Si: 0.3 to 1.4% by weight Fe : 0.1 to 0.4 wt% Mn: 0 to 0.4 wt% Zr and / or Cr and / or Ti: 0 to 0.15 wt% Al: balance. 6. The method according to claim 1, wherein the alloy treated by the method has the following composition: Cu: 0.5 to 0.9% by weight Fe: 0.1 to 0.40% by weight Mg: 0.5 to 1.0% by weight Mn : 0 to 0.45% by weight Si: 0.6 to 1.1% by weight Ti: 0.10% by weight Cr: 0.10% by weight 7. The method according to claim 1, wherein the alloy treated in the method is an AA 6111 alloy. 8. The thin plate product to be subjected to the solution treatment is obtained by casting an ingot, subjecting the cast ingot to skin peeling, and making the skin peeled ingot uniform in a temperature range of 480 to 580 ° C. to be uniform. 2. The method of claim 1, prepared by hot / cold rolling the ingot to form an intermediate thickness sheet product, and cold rolling the intermediate thickness sheet product to a final thickness. 9. The thin sheet having an intermediate thickness is subjected to an intermediate annealing treatment.
The described method.