Method of quenching alloy sheet to minimize distortion

Abstract

A method of producing a sheet article or other elongated product of solution heat treated aluminum alloy substantially free of permanent thermal distortion. The method involves subjecting an article made of a heat-treatable aluminum alloy to a solution heat treatment, cooling the article in a gas from the solutionizing temperature to an upper critical temperature below which precipitation of second phase particles of the alloy may occur, further cooling the article in a gas/liquid from the upper temperature to a lower critical temperature below which precipitation of the components may no longer occur, and optionally additionally cooling the article to a temperature below the lower critical temperature. The cooling of the article in the gas is carried out at a rate of cooling at which the yield strength of the article remains high enough to resist permanent deformation caused by thermal stress generated within the article by the cooling.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a Continuation-in-Part of our prior application Ser. No. 09/467,306, filed Dec. 17, 1999 (pending).

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The present invention relates to methods of quenching heat-treatable aluminum alloy sheet from solutionizing temperatures to fabricate alloy sheet in either the T4 or T4P tempers, which are the conditions in which such sheet articles are normally supplied to automobile manufactures. More particularly, the invention relates to methods of this kind intended to minimize distortions of the sheet articles caused by thermal stress generated during quenching.

[0004] II. Description of the Prior Art

[0005] Manufacturers of aluminum alloy sheet articles are faced with the challenge of providing thin gauge sheet materials that have both good formability in the supplied temper and high strength after a part has been formed and paint-cured. The automobile industry, in particular, demands such products for use as the raw material for producing body panels or structural members of reduced weight in the ongoing quest for improved vehicle economy and fuel efficiency.

[0006] It has become commonplace to make use of heat-treatable AA (Aluminum Association) 6000 series, or some AA2000 series, aluminum alloys for the production of such sheet articles. Heat-treatable alloys are generally those containing soluble alloying constituents in amounts that exceed their room temperature solubility limits. Such alloys may develop enhanced properties upon being subjected to working and/or heating, followed by a quenching step. These alloys usually contain hardening elements (e.g. magnesium, silicon and/or copper) to provide hardening during aging, and other elements, like Fe, Mn and possibly Cr, to control the formability and grain size. These alloys are generally direct chill (DC) or continuously cast and homogenized above the solvus temperature (which is defined as the temperature above which all the soluble particles, e.g. Mg.sub.2Si, Al.sub.wCu.sub.xMg.sub.ySi.sub.z (referred to as Q), and other particles depending on the alloy composition, become unstable and dissolve in the aluminum matrix), in order to dissolve the soluble particles present in the as-cast ingot or continuous strip, and to improve hot ductility for subsequent thermo-mechanical processing steps. The homogenized ingot is hot- and cold-rolled to the final gauge with or without an intermediate annealing step. The final gauge sheet material is then solutionized above the solvus temperature (usually in the range of 480 to 580.degree. C.) to dissolve the soluble particles that are formed during the hot- and cold-rolling, and then quenched to obtain the desired T4 or T4P temper.

[0007] The quenching process is one of the most critical steps in producing an acceptable sheet material in the supplied temper. The material is considered highly undesirable if it contains coarse grain boundary particles since they affect the mechanical properties of the sheet, such as its bendability, and to some extent the hardening response during the paint cure. Therefore, it is essential that the sheet material be rapidly quenched from the solutionizing temperature to avoid precipitation of the harmful secondary phase particles on the grain boundaries, and occasionally within the matrix, and to obtain the best combination of formability in the supplied temper and hardening response during the paint cure.

[0008] Unfortunately, rapid quenching from solutionizing temperature can result in permanent distortion of the sheet article, particularly for thin gauge products. When such distortion occurs, the sheet must be flattened mechanically more than usual by stretching and/or bending to remove the distortions. This decreases the formability of the material due to the cold working that is introduced during the stretching operation. It would therefore be advantageous to avoid such distortion during quenching procedures, and this goal has already been given consideration in the prior art. However, steps taken to avoid or minimize distortion can adversely affect the intended advantages of the quenching operation, or can introduce costly and inconvenient steps into the quenching process.

[0009] U.S. Pat. No. 4,784,921 which issued on Nov. 15, 1988 to M. E. Hyland, et al., discloses a method of producing aluminum sheet materials suitable for vehicle panels. The method involves a solution heat treatment followed by rapid quenching at a rate of at least 10.degree. F./sec (preferably at least 300.degree. F./sec) from the solutionizing temperature by means of liquid cooling. After the sheet has reached a temperature of 350.degree. F. or less, air cooling may be employed to ambient temperature.

[0010] Similarly, U.S. Pat. No. 5,061,327 which issued on Oct. 29, 1991 to D. K. Denzer, teaches of a method preferably involving a cold water quench directly from the solutionizing temperature. The preferred method involves sheet alloy quenching in water to approximately 93.degree. C. at a rate greater than or equal to 56.degree. C./sec, followed by air cooling to ambient temperature.

[0011] PCT publication WO 98/42885 of Oct. 1, 1998 filed in the name of Aluminum Company of America discloses a process for the manufacture of metal alloy sheet without distortion wherein a controllably variable liquid quenching means is used to control the time at which an alloy strip remains above a critical temperature (the so-called Leidenfrost temperature--the temperature above which a vapor layer is present between the sheet surface and the liquid cooling film). It is said that the liquid quench cools the metal through the critical temperature at a rate that can be controllably varied from each metal alloy composition and can increase or at least maintain the strength potential of the alloy without physical distortion. However, this reference links the cooling to variables that have nothing to do with the material being cooled. The publication states that the Leidenfrost temperature is functionally and operatively related to the specific spray orifice used for the cooling liquid, the flow rate, the physical and chemical properties of the liquid, and the pressure used to apply the liquid. It is therefore difficult to achieve the best results for any particular alloy without carrying out complicated adjustments. Also, while the material may have a gas layer separating the hot material from the cooling liquid, this gas layer is created by evaporation of the cooling liquid, which requires the extraction of heat of vaporization from the material, thus providing a significantly higher cooling rate than would be the case if a change of state from a liquid to a gas was not involved.

[0012] Consequently, the inventors of the present invention have found that methods involving liquid quenching from solution temperatures are difficult to control in a way that enables thermal distortion to be minimized while retaining good formability in the as-supplied tempers and high strength after the part has been formed and paint cured.

SUMMARY OF THE INVENTION

[0013] An object of the present invention is to provide a method of quenching a sheet article of solution heat treated aluminum alloy in a way that minimizes or avoids distortion of the article, while also avoiding undue precipitation of second phase particles from the metal.

[0014] Another object of the invention is to provide such a method that can be carried out in a practical and relatively inexpensive manner.

[0015] Another object of the invention is to provide a method of producing an article made of heat-treatable alloy in T4 or T4P temper by casting, homogenizing, rolling, and heat treating, while employing a solutionizing and quenching step that avoids undue distortion and particle precipitation.

[0016] The invention in one of its aspects provides a method of producing an article (preferably a sheet) of solution heat treated metal (preferably aluminum) alloy substantially free of permanent thermal distortion. The method comprises subjecting an article made of a heat-treatable aluminum alloy to a solution heat treatment at a solutionizing temperature to dissolve soluble particles present in the alloy, cooling the article in a gas from the solutionizing temperature to an upper critical temperature below which substantial precipitation of soluble second phase particles may occur, further cooling the article in a liquid or a liquid/gas mixture from the upper critical temperature to a lower critical temperature below which precipitation of the second phase particles may no longer occur, and optionally additionally cooling the article to a temperature below the lower critical temperature. The cooling of the article in the gas above the upper critical temperature is carried out at a rate of cooling at which the yield strength of the article remains high enough to resist permanent deformation caused by thermal stress generated within the article by the cooling effect, and the further cooling of the article between the upper and lower critical temperatures in the liquid takes place at a rate of cooling fast enough to avoid significant precipitation from the alloy.

[0017] The further cooling of the article in a liquid or a gas/liquid (i.e. below the upper critical temperature) is preferably carried out in at least two (and preferably three) stages at different cooling rates, with the cooling rates increasing from stage to stage as the cooling progresses. Most preferably, the further cooling of the article is such that, when cooled in a continuous process, it creates a parabolic or substantially parabolic temperature profile when the sheet article temperature is plotted against distance down the sheet. The further cooling of the sheet article is preferably effected by advancing the strip article past a plurality of spray nozzles for directing sprays of liquid or liquid/gas onto the strip article, the sprays causing the cooling in the at least two stages at the indicated different rates. The sheet article is preferably first advanced past at least one nozzle, which gives a relatively gentle cooling action, preferably using air or air/water mixtures, then past at least one nozzle directing a spray of liquid which gives a more aggressive cooling action, preferably also at a non-perpendicular angle to an adjacent surface of the sheet article, and then past at least one nozzle directing a spray of liquid giving the most aggressive cooling action and preferably also substantially perpendicular to the surface of the sheet article. Alternatively, or additionally, the flow rates of the spray nozzles may be adjusted so that the sheet article encounters sprays of different intensities or different fluid temperatures as the sheet article is advanced.

[0018] In another aspect, the invention provides a method of quenching a sheet article made of a heat-treatable aluminum alloy following solution heat treatment at a solutionizing temperature while avoiding substantial thermal distortion, in which variable cooling rates are employed at different stages of quenching. Specifically, the article is first cooled in a gas from the solutionizing temperature to an upper critical temperature, below which precipitation of second phase particles of the alloy may occur, at a rate of cooling at which the yield strength of the article remains high enough to resist permanent deformation caused by thermal stress generated within the article by the cooling, followed by further cooling from the upper critical temperature to a lower critical temperature below which precipitation of said second phase particles may no longer occur at a rate of cooling fast enough to avoid significant precipitation from the alloy.

[0019] According to yet another aspect of the invention, there is provided a process of producing a heat treatable aluminum alloy sheet article in T4 or T4P temper, which comprises direct chill or continuously casting an alloy to form a casting, homogenizing the casting above a solvus temperature of the alloy to dissolve soluble particles present in the casting, hot- and/or cold-rolling the casting to form a sheet article of final gauge, solutionizing the sheet article of final gauge at a solutionizing temperature above the solvus temperature to dissolve soluble particles formed during the hot- and/or cold-rolling, and quenching the sheet article of final gauge to form a sheet article in T4 or T4P temper, wherein the quenching is carried out by cooling the sheet article in a gas from the solutionizing temperature to an upper critical temperature below which precipitation of second phase particles of the alloy may occur, further cooling the article in a liquid or a gas/liquid mixture from the upper critical temperature to a lower critical temperature below which precipitation of the second phase particles may no longer occur, and optionally additionally cooling the article to a temperature below the lower critical temperature. Preferably, the cooling of the article above the upper critical temperature in the gas is carried out at a rate of cooling at which the yield strength of the article remains high enough to resist permanent deformation caused by thermal stress generated within the article by the cooling, and the further cooling of the article between the upper and lower critical temperatures in the liquid takes place at a rate of cooling fast enough to avoid significant precipitation from the alloy. In this process, the hot and/or cold rolling of the casting may be carried out with an intermediate annealing step.

[0020] The invention also relates to a solution-heat-treated alloy article or sheet produced by the methods and processes described above.

[0021] The gas cooling step has the practical effect of lowering the quench-start temperature for the subsequent liquid cooling step that is carried out at a high rate of cooling and that consequently generates high thermal stresses. At the lower quench-start temperature, the alloy has an increased yield strength compared to that at the solutionizing temperature, and is therefore better able to resist thermal deformation.

[0022] The initial cooling of the article in the gas (referred to as slow cooling) may be accomplished by using heated air as the cooling medium in one or more successive zones of a heat treatment apparatus through which the article is passed immediately following the heating zone of a continuous heat treatment furnace. The heated air in the cooling zone is preferably directed onto one or both of the surfaces of the sheet article from suitable gas nozzles. The temperature within the cooling zones may be set at different (progressively lower) temperatures to successively increase the quenching rate in approximate proportion to the increase of yield strength of the metal that occurs with decreasing temperature. In this way, very low quenching stresses are initially introduced while the metal yield strength is low.

[0023] The quenching steps of the invention are acceptable metallurgically because no detrimental precipitation takes place above the upper critical temperature of the precipitation range. Therefore, the slow quenching at high temperature does not affect the eventual metallurgical structure of the alloy sheet product. Once the upper critical temperature has been reached, rapid quenching may be carried out to avoid precipitation without causing thermal distortion. The additional cooling to a temperature below the lower critical temperature (e.g. ambient or room temperature), if required at all, is then preferably carried out in a gas, such as air. However, liquid cooling may alternatively be employed for this step, or the metal may merely be allowed to stand and cool naturally.

[0024] The invention is particularly applicable to the treatment of AA6000 series aluminum alloys. In such cases, the upper critical temperature is about 450.degree. C. and the lower critical temperature is about 325.degree. C., and the step of cooling the sheet article with a gas from the solutionizing temperature to the upper critical temperature is preferably carried out at a rate of between 10.degree. C./sec and 200.degree. C./sec., and more preferably at approximately 20.degree. C./sec. The further cooling from the upper critical temperature to the lower critical temperature is preferably conducted at a rate of between 200.degree. C./sec and 2000.degree. C./sec., and preferably between 200.degree. C./sec and 450.degree. C./sec.

[0025] The present invention provides an efficient and productive method of minimizing thermal distortion during cooling of a sheet alloy from the solution heat treatment temperature. The method has the considerable economic advantage that it may be carried out in conventional quenching equipment (e.g. a continuous heat treatment line), without undue difficulty and normally without requiring expensive modifications.

[0026] The present invention is applicable to all heat treatable alloys that are subjected to solutionizing and quenching during processing, e.g. aluminum alloys of the AA6000 series, AA2000 series, AA7000 series and Al--Li type alloys. Although the critical temperature range (upper critical temperature and lower critical temperature) and the desired cooling rates may be different for different alloys, the appropriate ranges and cooling rates for each alloy may be determined by simple experimentation.

[0027] It should be realized that the term "sheet" as used herein refers to a generally flat (planar) article, often (but not necessarily) of indefinite length. The thickness of such articles is not particularly relevant, although automotive sheet articles (e.g. of AA6111) subjected to solutionizing and quenching normally have a thickness of 2.3 mm or less, typically 0.8 to 2.0 mm. However, the term "sheet" is intended to includes a product often referred to as "plate" (generally taken to mean a rolled product that is rectangular in cross-section having a thickness not less than 0.250 inch (6.35 mm) with sheared edges). While the present invention is primarily concerned with planar articles, such as metal sheet, the method may also be applied to shaped products, such as extrusions, in which distortion is a factor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a graph showing how yield strength (YS) of an alloy (in this case alloy AA6111) declines as the temperature of the alloy increases and, in particular, how the YS declines rapidly in the upper temperature range (e.g. above 450.degree. C.);

[0029] FIG. 2 is a graphical illustration of various quenching methods for AA6111 aluminum alloy according to the prior art and preferred embodiments of the present invention;

[0030] FIG. 3 is a schematic diagram showing a simplified cross-section of a part of a heat treatment apparatus suitable for carrying out one preferred form of the present invention;

[0031] FIGS. 4 and 5 are graphs representing typical temperature profiles of sheet articles passing through heating and cooling zones of the heat treatment apparatus. The cooling profile shown is in accordance with the present invention;

[0032] FIG. 6 is a diagram similar to that of FIG. 3, showing the part of a heat treatment apparatus used for liquid quenching according to a further preferred embodiment of the present invention; and

[0033] FIGS. 7 and 8 are graphs showing modeled temperature profile predictions for strip articles quenched according to preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] The present invention provides a method for minimizing or eliminating permanent distortion of alloy sheet or other heat-treatable materials caused by the thermal stresses associated with quenching practices following solution heat treatments, while retaining the desirable effects of such solutionizing to the greatest possible extent.

[0035] Rapid quenching of heat treatable alloys is required in order to maximize the formability in the as-supplied (to the user) temper while obtaining high strength, durability and good corrosion resistance in the final product to meet the strict requirements of the automotive industry and other users. However, the inventors of the present invention have realized that such rapid quenching is necessary only within the range of temperatures in which second phase precipitation may occur. At temperatures above this range, a slower rate of cooling may be adopted without significantly affecting the metallurgical structure of the metal, and a rate of cooling may be chosen at which the yield strength of the metal remains high enough to resist thermal stresses generated by the cooling procedure, thus minimizing or eliminating permanent deformation of the sheet product. Such deformation is, in fact, only likely at high temperatures where the yield strength of the alloy is extremely low compared to the yield strength at ambient temperature. For most heat-treatable aluminum alloys, once the temperature has been reduced to the precipitation range, high rates of cooling may be employed without causing deformation because the yield strength of the alloy at these temperatures has increased significantly compared to the yield strength at the solutionizing temperature.

[0036] Accordingly, the present invention divides the cooling operation from the solutionizing temperature into two or more distinct steps. The first step involves a slow rate of cooling (achieved by gas cooling in a heated gas) from the solutionizing temperature to the upper critical temperature below which precipitation may occur, and a second step involves rapid cooling (achieved by liquid quenching, or mixed liquid and gas quenching) through the precipitation range between the upper and lower critical temperatures. The nature of the further cooling (if any) carried out below the lower critical temperature of the precipitation range is less critical and may be carried out at any desired rate using either liquid cooling or gas cooling, although gas cooling employing air is preferred. At these lower temperatures the yield strength of the alloy is high enough to resist thermal deformation without allowing precipitation of detrimental particles on the grain boundaries.

[0037] The alloy thus obtained is generally aged, following quenching, to obtain the desired T4 or T4P tempers.

[0038] FIG. 1 is a graph showing how the yield strength (YS) of a typical AA6000 series alloy (specifically AA6111) falls as the temperature of the alloy is raised. The vertical axis (ordinate) shows the yield stress at a particular temperature expressed as a percentage of the room temperature yield stress (R.TempYS). It will be seen that the yield stress falls rapidly to zero as temperatures increase from about 450 to 600.degree. C. Solutionizing often takes place at temperatures up to about 600.degree. C., so the metal is very susceptible to deformation at these temperatures. On the other hand, at a temperature of about 450.degree. C., approximately 50% of the room temperature yield stress has been developed, so the material is quite deformation-resistant below this temperature. For AA6000 series alloys, this is generally the upper critical temperature of the precipitation range.

[0039] The present invention is described in particular as being a method of processing aluminum alloy sheets of the Aluminum Association AA6000 series aluminum alloys and some AA2000, AA7000 and Al--Li alloys. However, it is fully contemplated that the methods of the present invention provide thermal mechanical processing procedures which can be beneficial to the working of a variety of heat-treatable alloys, including other categories of aluminum alloy, provided the upper end of the temperature range at which significant precipitation may take place is sufficiently low that, for the alloy in question, rapid quenching may be initiated from that temperature without significant distortion of the alloy sheet product. That is to say, at the upper critical temperature of the alloy in question, the yield strength of the alloy should be high enough to resist thermal stresses caused by rapid quenching. At temperatures above the critical temperature, the cooling rate is slowed to allow the yield strength of the alloy to increase before significant thermal stresses are imposed.

[0040] FIG. 2 illustrates temperature-time correlation curves for AA6111 alloy sheet product. The figure includes regions A, B and C depicting temperature ranges of importance to the present invention. Region A is the temperature range between the solutionizing temperature (T.sub.sol) and the upper critical temperature (T.sub.upper) below which precipitation may commence. Region B represents the precipitation range (critical range) in which secondary phase precipitation may take place if the time and temperature conditions are appropriate. Region C is the range of temperatures below T.sub.lower, the temperature below which further secondary phase precipitation does not take place at any rate of cooling. It should be noted that these ranges in temperature are not absolute, and will vary for any given quenching method depending upon the physical properties of the subject alloy. The temperature ranges can, however, be determined experimentally for any given heat treatable alloy.

[0041] Three quenching procedures are illustrated in FIG. 2, namely Quench 1, Quench 2 and Quench 3. These procedures differ only prior to time "ts", i.e. the time from the termination of solutionizing at which the alloy temperature first reaches T.sub.upper. Following this time, the three procedures follow the same curve.

[0042] Quench 1 depicts a typical temperature gradient for quenching methods of the prior art, while Quench 2 and Quench 3 depict temperature gradients of the methods embodied by the present invention.

[0043] In accordance with common practices of the prior art, the quenching process is generally a single step process which provides rapid cooling directly from the heat solutionizing temperature (T.sub.sol) through the critical precipitation temperature range. As illustrated in FIG. 2, the temperature gradient of Quench 1 is generally uniform through regions A and B. In contrast, Quench 2 and Quench 3 provide at least one preliminary step of slow cooling through region A before initiating the rapid cooling step through region B. The initial cooling of the sheet alloy within region A of FIG. 1 is conducted by the application of heated air in a manner such that thermal stress in most or all of the sheet alloy does not exceed the yield strength of the sheet. This slow cooling using heated air allows the temperature of the sheet alloy to approach the critical T.sub.upper temperature at a rate sufficiently slow to avoid high thermal stresses while allowing the development of properties of strength and stability sufficient to withstand the high thermal stresses generated at the onset of rapid quenching.

[0044] The rate of cooling and the temperature of the heated air required to produce this rate of cooling can be found empirically. Alternatively, the rate of cooling may be determined by applying a mathematical model that uses temperature, pressure, speed, product gauge and the heat transfer coefficient to calculate the optimum temperature of the heated air based on the temperature of the metal following the solutionizing treatment. The temperature of the cooling gas may be determined using the same mathematical model and varies with the peak metal temperature targeted. A typical range of gas temperatures suitable for the AA6111 alloy would be from 300 to 350.degree. C.

[0045] The heated air may be applied in one or more successive zones of a continuous heat treatment apparatus to gradually cool the sheet alloy. Upon reaching T.sub.upper, rapid quenching using water or any other standard high heat capacity liquid (or mixture of gas and liquid), is initiated in further cooling zones to provide rapid cooling of the alloy sheet through a temperature range in which alloy precipitation would readily occur if the rate of cooling were sufficiently slow. At this stage of quenching, the yield strength of the sheet alloy has substantially increased from the yield strength at the solutionizing temperature. As such, the sheet alloy is capable of withstanding the stress of the quenching conducted through region B. In particular, the sheet alloy enters region B with sufficient strength to withstand the stresses from rapid cooling without suffering the consequence of permanent thermal distortion. The temperature of the cooling liquid used in this step may be much the same as the temperature of liquids used for conventional quenching steps. The temperature is generally between 18.degree. C. (ambient) and 30.degree. C. (warm).

[0046] Once cooled to a temperature in region C, which is below the range of precipitation (T.sub.lower), the sheet is strong and no harmful precipitation occurs. The details of any further cooling that may be initiated are therefore less critical to the present invention. The temperature of the cooling medium used for this step may be controlled by the application of heat or cooling using heat exchangers. Typically, the temperature varies from 20.degree. C. to about 100.degree. C.

[0047] The methods of the invention accordingly reduce distortion by exposing the sheet to a high quench rate only after the alloy is sufficiently cooled to withstand the associated stresses which accompany a rapid temperature drop. As a result, permanent distortion to the sheet alloy is minimized and often eliminated.

[0048] As noted, the first quenching step of the present invention occurs within region A of FIG. 2 where the sheet alloy is slowly cooled with air in one or more stages from the solutionizing temperature to a temperature just above the predetermined T.sub.upper temperature. The second quenching step occurs in region B and is indicative of the preferred rate of cooling through the critical temperature range. Table 1 further illustrates data on preferred and working temperatures associated with the present invention. It should be noted that the upper and lower range temperatures, as well as the working quenching rates, depend on the shape and position of the critical time curve (explained below), while preferred quenching rates are determined from the conditions which give the best combination of mechanical properties. Ideally, the closer the working temperatures come to the preferred temperature, the less thermal distortion expected in the sheet alloy product.

1TABLE 1 Working and Preferred Quenching Rates for Air and Water Cooling Phases Cooling Rate Range .degree. C./sec .sup.1st Step 2.sup.nd Step Cooling Range above Cooling to T.sub.upper .degree. C. below T.sub.upper .degree. C. ALLOY T.sub.upper .degree. C. Working Preferred Working Preferred AA6111 .gtoreq.450 10-200 .about.20 200-2000 200-450 (air cooling) AA6016 .gtoreq.450 10-200 .about.20 200-2000 340-575 (air cooling)

[0049] As illustrated in FIG. 2, the range of the critical time curve for AA6111 alloy is found to be within the range of 325.degree. C. to 450.degree. C. The region above the T.sub.upper temperature (region A) represents the temperature range wherein slow cooling is preferred. This slow cooling step allows the sheet alloy to become stronger and thus be able to withstand the thermal stress of the subsequent aggressive quenching step. The conventional process as depicted as Quench 1, illustrates the onset an occasion of rapid cooling from the solutionizing temperature. The sheet alloy resulting from this treatment will suffer the effects of permanent distortion as a result of temperature gradients, induced by the rapid quench, exceeding the yield strength of the sheet alloy at or near the initial quenching temperature. Resulting sheet alloy must then be flattened mechanically by stretching and/or bending to remove the distortion. Such labor-intensive cold working efforts add to the cost of processing and decrease the formability of the material.

[0050] As further illustrated in FIG. 2, it is not until the temperature drops below T.sub.upper at time ts that rapid quenching must be initiated. Beginning at time ts, the time required to quench below T.sub.lower directly determines the amount of precipitation which will occur. The figure shows a critical time curve S enclosing a shaded region E. At a temperature close to T.sub.upper, the driving force for precipitation of second phase particles is very small and nucleation and growth rates are slow thereby prolonging the start of the precipitation process. On the other hand, at temperatures around T.sub.lower the driving force is very large and diffusion rates are too slow thereby impeding the onset of the precipitation process. Thus, precipitation rapidly occurs only at the nose N of the critical time curve C where a large driving force and high diffusion rates are present. The critical time curve S, as illustrated in FIG. 2, represents the beginning of this precipitation of second phase particles as a function of time and temperature. Thus, if the quenching curve crosses the shaded region E, secondary phase precipitation will take place to an undesirable extent. The properties of such a critical time curve, including the number, shape and position of the curve is largely dependent on alloy composition and the processing history of the alloy.

[0051] Therefore, an objective of the thermal mechanical processing of a sheet alloy in region B is to avoid the nose N of the critical curve C via rapid quenching and thus to eliminate the possibility of adverse effects caused by second phase precipitation. The time before the nose of the curve is called the incubation period, which is less than one second for AA6111. The precipitation process does not occur provided the time at temperature during quenching or aging is less than the incubation period or lies to the left of the critical curve.

[0052] Quench 2 of FIG. 2 employs a constant slow rate of cooling from the solutionizing temperature to T.sub.upper, the slope of the curve being gradual enough to avoid the generation of thermal stresses strong enough to exceed the yield strength of the alloy, and thus cause permanent deformation.

[0053] An alternative embodiment of the present invention is depicted in region A of FIG. 2 as Quench 3. In accordance with this embodiment a plurality of heated air phases are applied in the initial step of slow cooling to T.sub.upper to progressively increase the quenching rate in approximate proportion to the increase of yield strength with decreasing temperature. This can be accomplished by subjecting the alloy sheet to two or more heated air phases, operating at progressively lower air temperatures. In this manner, very low quenching stresses are initially introduced to the sheet alloy when the yield strength of the alloy is low.

[0054] FIG. 3 is a simplified cross-section of part of a heat treatment apparatus suitable for carrying out a preferred form of the present invention. The apparatus 10 is in the form of a tunnel 11 through which the strip article 12 is drawn on a continuous basis. The apparatus is divided internally into a number of zones. A first zone 11A forms a last step of the solutionizing procedure. High temperature gas is introduced into the zone from nozzles 14 located in the top and bottom of the tunnel 11. The strip article passing through this zone is heated to the solutionizing temperature.

[0055] A second zone 11B forms a region in which the temperature of the sheet article is cooled from the solutionizing temperature to T.sub.upper. Heated gas is injected into the zone through nozzles 15. As the strip article passes through this zone it may be cooled, for example, according to Quench 2 or Quench 3 of FIG. 2. Different temperature profiles in this zone can be obtained by varying the temperature of the gases passing through various nozzles in the zone.

[0056] The strip article then passes into a third zone 11C in which the article is quenched from T.sub.upper to T.sub.lower. In the first part 16 of this zone, so-called "Coanda" nozzles 18 are provided to deliver a cooling liquid. The nozzles are positioned at an angle to the surface of the strip article 12 in the direction of sheet travel and mixes the cooling liquid (water) with air to reduce the droplet size. The smaller droplet size and angled positioning of the nozzle serve to improve the cooling efficiency. In this part of the zone 11C, the temperature of the strip article is above the Leidenfrost temperature and there is a vapour barrier between the liquid coolant (water) and the sheet surface. This arrangement provides more efficient cooling than just spraying cooling liquid directly onto the strip article. Once the temperature of the strip article passes through the Leidenfrost point, the vapour barrier disappears and conventional spray nozzles may be employed. This further cooling to T.sub.lower is carried out in a second part 17 of the third zone 11C using conventional nozzles 20 oriented perpendicular to the surface of the strip article.

[0057] It should be noted here that, although the present invention makes use of liquid (or liquid/gas) cooling above the Leidenfrost temperature in the indicated embodiment, this is employed only below T.sub.upper and, in contrast, slow cooling by means of a gas is utilized above T.sub.upper. In this further respect, the present invention differs from the disclosure of WO 98/42885 (disclosed earlier) where there is no such slow gas cooling. WO 98/42885 commences with liquid cooling and follows with gas cooling, whereas the present invention does the opposite. Since the present invention starts the cooling with a gas, the heat transfer coefficient is inherently lower at this critical time than would be the case with liquid cooling as in WO 98/42885 (even taking into account the gas generation of the Leidenfrost effect because of the additional cooling effect due to a change of state of the cooling medium from liquid to gas).

[0058] In a fourth and final zone 11D, cooling below T.sub.lower takes place by means of a gas introduced through nozzles 21.

[0059] FIGS. 4 and 5 are examples of temperature profiles of metal sheet articles positioned in heat treatment lines operated according to the present invention. These are graphs showing the predicted temperature with elapsed time of a point on the advancing metal strip article in solutionizing and cooling apparatus of the type shown in FIG. 3. FIG. 4 shows the predicted profile for light gauge sheet (1.00 mm), and FIG. 5 shows the profile for heavy gauge sheet (2.00 mm). The Figures show the rates of change of temperature of the metal in the various heating and cooling zones of the heat treatment line. In both cases, it will be seen that, following the peak solutionizing temperature, there is a slow cooling period A in a gas (heated air) preceding a rapid liquid (water) quench B, and then a further cooling period C to low temperature (post-quench cooling).

[0060] In practice, sheet material subjected to the heat treatments of FIGS. 4 and 5 is found to be substantially free of thermal distortion while having good metallurgical properties suitable for use in the automotive industry.

[0061] According to a further preferred embodiment of the present invention, the cooling effected below the upper critical temperature is carried out in at least two (and more preferably three) stages. Rather than proceeding at an essentially constant rate throughout this cooling step as in the previous embodiments, the cooling commences in a first stage at a relatively slow rate and then proceeds after a short period of time in a subsequent stage at a much faster rate, or at an increasingly faster rate. A third even more aggressive cooling stage may be provided. When the strip is cooled in a continuous heat treatment furnace, the indicated different stages correspond to different zones or regions within the heat treatment furnace.

[0062] This further embodiment is based on the realization that, in the case of a sheet article cooled uniformly across the width of the sheet article (the preferred arrangement), thermal stresses in the sheet article are proportional to the second derivative of temperature at points along the long axis of the sheet article. The second derivative of temperature is the sheet article temperature differentiated twice with respect to the distance along the sheet. It is the gradient of the temperature gradient along the sheet article length (mathematically expressed as .delta..sup.2T/.delta.x.sup.2 where T is temperature and x is distance along the strip). The temperature profile which minimizes the second derivative of temperature is a parabolic profile with zero initial gradient. The embodiments described above which employ pre-cooling with air exploit this by giving an approximation to a parabola and also cooling gently when the metal is hot. The same can be done over a much shorter length during the water (or water and air) part of the quench below the critical temperature, so that the entire cooling process has a temperature profile (longitudinally of the strip article) that is, or closely approximates, a parabola.

[0063] This can be achieved by cooling the strip article by passing it through heat treatment apparatus of the type shown in part in FIG. 6. This figure shows an apparatus corresponding generally to the right hand side of the apparatus of FIG. 3 (the section labeled "Quench 1 to T lower"), but with modifications as described. The apparatus of FIG. 6 is the same as the earlier apparatus in that it has a tunnel 11' through which a strip article 12' is advanced on a continuous basis in the direction of arrow A over supporting rollers 30 and 32. In the illustrated part of the apparatus, the strip article is quenched from T.sub.upper to T.sub.lower, first by the action of "Coanda" nozzles 18', which cool by the combined action of water and air, and then by water spray nozzles. The first pair of spray nozzles 20' (which may be located about 800 mm, for example, from the Coanda nozzles 18') are orientated at an angle (e.g. in the range of 25 to 75.degree.) to the strip article 12' in the direction of advancement of the strip article, as shown in the drawing, whereas the succeeding pairs of nozzles 20" (which are preferably spaced about 300 mm from each other) are orientated perpendicular to the surface of the strip article. The quench is commenced by the Coanda nozzles 18' (causing a first stage of cooling in a first zone of this region of the apparatus). The Coanda nozzles also act as air knives to prevent water from running back towards the earlier zone (not shown). The angled water spray nozzles 20' bring about a second stage of the quench (in a second zone of the apparatus) which causes cooling at a faster rate than that of the Coanda nozzles 18', but at a slower rate than the succeeding nozzles 20" by use of a lower spray intensity. This can be referred to as "gentle water cooling" followed by "aggressive water cooling" of the perpendicular nozzles 20". Thus, the overall cooling operation following the solution heat treatment may be referred to as "air cooling to the upper critical temperature, then water and air cooling, gentle water cooling and aggressive water cooling." During the air or air/water cooling part of the cooling procedure below the upper critical temperature, the cooling heat transfer coefficient optimally, for 1 mm thick sheet, falls within the range of 150-300 W/m.sup.2K. During the subsequent stages of the cooling, the rate of cooling optimally falls within the range of 300 to 2000 W/m.sup.2K. The method still achieves high overall quench rates between about 470.degree. C. and 250.degree. C. and thus satisfies the metallurgical requirements (no substantial precipitation), while minimizing thermal distortion.

[0064] To achieve a heat transfer coefficient of 150 W/m.sup.2K or more in the first cooling stage, it is usually necessary to use both top and bottom air Coanda nozzles 18', as shown. As a guide to the volume flows required to achieve both the coefficients and avoid excessive air temperature rise, the total volume (top and bottom) would preferably be about 3.0 m.sup.3/sec per meter width. The air velocity would need to be about 40 meters/second which implies a nozzle width of about 35 mm with 1.5 m.sup.3/sec per meter of width of air flow on the top and bottom.

[0065] It is also important to size the air extraction system to maintain a lower air static pressure in the quench chamber than in the up and down-stream oven section, in order to contain the water spray effectively.

[0066] If necessary, the perpendicular nozzles 20" may be of adjustable spray intensity so that the cooling rate may be caused to increase further as the strip article passes through the zones of the quenching region.

[0067] FIG. 7 shows modeled results for the further preferred embodiment which achieves the same average quench rate down to 250.degree. C. (140.degree. C. per second) as the apparatus of FIG. 3, but in which the second derivative of temperature with respect to sheet article length at the start of the quench is reduced by a factor of three. There are higher values of the second derivative further down the strip article, but in these locations the temperature of the strip article is lower and the yield strength is higher.

[0068] FIG. 8 shows modeled results for spray intensities set to give an average quench rate of 200.degree. C. per second down to 250.degree. C. and, again, to restrict the second derivative of temperature when the strip is hot. This shows the importance of being able to grade the spray intensities along the length of the strip article.

[0069] It will be seen that the temperature profiles of FIGS. 7 and 8 more accurately represent a part of a parabola, thus minimizing distortion effects during the quenching operation.

[0070] In all cases, of course, the temperature curve or profile achieved in the quenching zone should avoid the precipitation zone (shaded region of FIG. 2) to avoid the formation of precipitates. However, as will be seen from FIG. 2, for most alloys there is room at the top of the region B that allows the quenching rate to be a little slower at higher temperatures.

[0071] In accordance with the method of the present invention, detrimental alloy precipitation is avoided prior to rapid quenching. Accordingly, the present invention provides a sheet alloy of superior quality having preferred characteristics of workability, strength and durability. In addition, the present invention provides a method whereby thermal distortion is minimized and often eliminated. Therefore, the need for addition working after the performance of step-wise cooling of the present invention is obviated. Thus, there is provided a method to minimize distortion with improved overall efficiency. Accordingly, the sheet alloy produced by this method may have better strength and formability than sheet alloys of the prior art.

Claims

What we claim is:

1. A method of producing an article of solution heat treated aluminum alloy substantially free of permanent thermal distortion, comprising: subjecting an article made of a heat-treatable aluminum alloy to a solution heat treatment at a solutionizing temperature to dissolve soluble precipitates present in said alloy, cooling said article in a gas from said solutionizing temperature to an upper critical temperature below which precipitation of second phase particles of said alloy may occur, further cooling said article in a liquid or gas/liquid mixture from said upper critical temperature to a lower critical temperature below which precipitation of said second phase particles may no longer occur, and optionally additionally cooling said article to a temperature below said lower critical temperature, wherein said cooling of said article above the upper critical temperature in said gas is carried out at a rate of cooling at which the yield strength of the article remains high enough to resist permanent deformation caused by thermal stress generated within the article by said cooling, and said further cooling of said article between the upper and lower critical temperatures in said liquid takes place at a rate of cooling fast enough to avoid significant precipitation from said alloy.

2. The method of claim 1, wherein said cooling of said article below said lower critical temperature is carried out in a gas.

3. The method of claim 1, wherein said cooling of said article below said lower critical temperature is carried out at a rate slower than said further cooling in said liquid or gas/liquid mixture.

4. The method of claim 1, wherein the cooling of the article in the gas from said solutionizing temperature to the upper critical temperature is carried out in a single cooling zone.

5. The method of claim 1, wherein the cooling of the article in the gas from said solutionizing temperature to the upper critical temperature is carried out in a plurality of cooling zones.

6. The method of claim 5, wherein the temperature of the gas in each successive cooling zone is progressively lower.

7. The method of claim 6, wherein the lower temperatures are chosen to progressively increase the quenching rate in each zone in proportion to an increase of yield strength of the alloy in each zone.

8. The method of claim 1, wherein said alloy of said sheet article is an AA6000 series aluminum alloy.

9. The method of to claim 8, wherein said upper critical temperature range is about 450.degree. C. and said lower critical temperature is about 325.degree. C.

10. The method of claim 8, wherein said step of cooling said sheet article with a gas from said solutionizing temperature to said upper critical temperature is carried out at a rate of between 10.degree. C./sec and 200.degree. C./sec.

11. The method of claim 8, wherein said step of cooling said sheet article with a gas from said solutionizing temperature to said upper critical temperature is conducted at a rate of approximately 20.degree. C./s.

12. The method of claim 8, wherein said step of further cooling from the upper critical temperature to the lower critical temperature is conducted at a rate of between 200.degree. C./sec and 2000.degree. C./sec.

13. The method of claim 8, wherein said step of further cooling said sheet article with a liquid or a gas/liquid mixture from the upper critical temperature to the lower critical temperature is carried out at a rate of between 200.degree. C./sec and 450.degree. C./sec.

14. The method of claim 1, wherein the article is in the form of a sheet.

15. The method of claim 1, wherein said further cooling of said article in a liquid or gas/liquid is carried out in at least two stages at different cooling rates, said cooling rates increasing from stage to stage as said cooling progresses.

16. The method of claim 15, wherein said further cooling of said article exhibits a parabolic or substantially parabolic temperature profile when sheet article temperature is plotted against distance down the article.

17. The method of claim 15, wherein said further cooling of said article is effected by advancing said article past a plurality of spray nozzles for directing sprays of liquid or liquid/gas onto said article, said sprays causing said cooling in said at least two stages at said different rates.

18. The method of claim 17, wherein said article is first advanced past at least one Coanda nozzle, then past at least one nozzle directing a spray of liquid at a non-perpendicular angle to an adjacent surface of said article, and then past at least one nozzle directing a spray of liquid perpendicular to said surface of said article.

19. The method of claim 17, which includes adjusting flow rates of said spray nozzles so that said article encounters sprays of different intensities as said article is advanced.

20. A method of quenching an article made of aluminum or an aluminum alloy following solution heat treatment at a solutionizing temperature while avoiding substantial thermal distortion, wherein variable cooling rates are employed at different stages of quenching, characterized in that the article is first cooled in a gas from the solutionizing temperature to an upper critical temperature, below which precipitation of second phase particles of the alloy may occur, at a rate of cooling at which the yield strength of the article remains high enough to resist permanent deformation caused by thermal stress generated within the article by the cooling, followed by further cooling from the upper critical temperature to a lower critical temperature below which precipitation of said second phase particles may no longer occur at a rate of cooling fast enough to avoid significant precipitation from the alloy.

21. A method according to claim 20, characterized in that the article is in the form of a sheet.

22. A process of producing a heat treatable aluminum alloy sheet article in T4 or T4P temper, which comprises direct chill or continuously casting an alloy to form a casting, homogenizing the casting above a solvus temperature of the alloy to dissolve soluble particles present in the casting, hot- and/or cold-rolling the casting to form a sheet article of final gauge, solutionizing the sheet article of final gauge at a solutionizing temperature above the solvus temperature to dissolve soluble particles formed during the hot- and/or cold-rolling, and quenching the sheet article of final gauge to form a sheet article in T4 or T4P temper, wherein the quenching is carried out by cooling the sheet article in a gas from the solutionizing temperature to an upper critical temperature below which precipitation of second phase particles of the alloy may occur, further cooling the article in a liquid or a gas/liquid mixture from the upper critical temperature to a lower critical temperature below which precipitation of the second phase particles may no longer occur, and optionally additionally cooling the article to a temperature below the lower critical temperature.

23. The process of claim 22, wherein the hot and/or cold rolling of the casting is carried out with an intermediate annealing step.