FIELD OF THE INVENTION

[001] The invention relates to a method for controlling continuous heat- treating and annealing of aluminum alloy sheet at final thickness continuously moving in a floating state substantially horizontally through a continuous convection floating furnace arranged to heat the moving aluminum sheet to a set peak metal tempera ture (TPMT). The controlling including controlling fan speeds and furnace air tem perature to accommodate variations in line speed of the aluminum alloy sheet con tinuously moving in the floating state horizontally through the continuous convec tion floating furnace.

BACKGROUND TO THE INVENTION

[002] As will be appreciated herein below, except as otherwise indicated, alumi num alloy designations and temper designations refer to the Aluminum Associa tion designations in Aluminum Standards and Data and the Teal Sheets Registra tion Record Series as published by the Aluminum Association in 2018 and fre quently updated, and well known to the persons skilled in the art.

[003] For any description of alloy compositions or preferred alloy compositions, all references to percentages are by weight percent unless otherwise indicated.

[004] In the production of motor vehicles in particular aluminum alloys the AA5000- and AA6000-series alloys like 5051 , 5182, 5454, 5754, 6009, 6016,

6022, and 61 1 1 , and various others, have been used to produce automotive struc tural parts and body-in-white (“BIW”) parts. [005] The industrial scale automotive sheet production of the heat-treatable AIMgSi-alloy series, also known as 6000-series aluminum alloys, typical examples include AA6005, AA6014, AA6016 and AA6022, comprises several discrete steps. A rolling slab or ingot is subjected to semi-continuous direct chill (DC)-casting or electromagnetic casting (EMC-casting), also continuous casting like belt or roll casting can be applied. The rolling slab or ingot may be preheated at about 500°C to 580°C for several hours for homogenization of the microstructure. Then the roll ing slab or ingot is hot rolled into hot rolled strip at a gauge of about 3 to 12 mm, the hot rolled strip is typically hot coiled and cooled down to ambient temperature. The hot rolled strip is cold rolled to final gauge in several passes, optionally an in termediate anneal is applied prior to the cold rolling or during the cold rolling pro cess, and at final gauge the strip is annealed to adjust the required material prop erties. The solution heat treating can be done either in a continuous heat treating furnace or in a batch type furnace.

[006] An economical attractive method of producing 6000-series aluminum sheet is by means of continuous solution heat treating at final gauge. At the end of a continuous solution heat treating furnace, the strip material is rapidly cooled or quenched to ambient temperature, for example by means of forced air cooling or spray cooling systems. By this solution heat treating the main alloying elements Mg and Si are dissolved, leading to a good formability, control of the yield strength and bake hardening behaviour, and brings the sheet material to a T4 temper.

[007] 7000-series aluminum alloys are heat treatable aluminum alloys contain ing zinc as the predominate alloying ingredient other than aluminum. For purposes of the present application, 7000-series aluminum alloys are aluminum alloys hav ing at least 2.0% Zn, and up to 10% Zn, with the zinc being the predominate alloy ing element other than aluminum.

[008] International patent application WO-2010/049445-A1 (Aleris) discloses a structural automotive component made from an aluminum alloy sheet product hav ing a gauge in a range of 0.5 to 4 mm, and having a composition consisting of, in wt.%: Zn 5.0-7.0%, Mg 1.5-2.3%, Cu max. 0.20%, Zr 0.05-0.25%, optionally Mn and/or Cr, Ti max. 0.15%, Fe max. 0.4%, Si max. 0.3%, and balance is made by impurities and aluminum. The sheet product has been solution heat treated (“SHT”) and cooled, artificially aged, after aging formed in a shaping operation to obtain a structural automotive component of predetermined shape, and subse quently assembled with one or more other metal parts to form an assembly form ing a motor vehicle component, and subjected a paint-bake cycle.

[009] Continuous annealing comprises continuously moving uncoiled non heat- treatable aluminum alloy sheet, for example AA5000 series aluminum sheets, in the direction of its length through a continuous annealing furnace and subse quently quenching the sheets after exiting the furnace.

[010] Continuous solution heat treating also comprises continuously moving un coiled heat-treatable aluminum alloy sheet, for example AA6000 sheet or AA7000 sheet, through a continuous heat treating furnace and subsequently quenching the sheets after exiting the furnace. After solution heat treatment, the alloy sheet can be hardened at room temperature (e.g., naturally aged) for a duration, hardened for a duration at a slightly elevated temperature (e.g., artificially aged or pre-aged), and/or otherwise further processed (e.g., cleaned, pretreated, coated, or otherwise processed).

[011] Published US patent application no. 20170253953 to Meyer et al discloses continuously heat treating by moving heat-treatable AA6000-series aluminum alloy sheet substantially horizontally through a convection floating furnace.

[012] Published US patent application no. 20170306466 to Meyer et al discloses continuously moving heat-treatable AA7000-series aluminum alloy sheet substan tially horizontally through a convection floating furnace.

[013] To provide longer production runs for continuous annealing or continuous solution heat treating of aluminum sheets the aluminum sheets are attached end to end in series and fed to the furnace. Typically, a first coil of the aluminum sheet is unrolled to form a sheet and fed to the furnace to be processed and a subse quent coil of the aluminum sheet is unrolled to form the next sheet to be pro cessed. Then the leading end of this second sheet is attached in series to the trail ing end of the previous sheet. Thus, a continuous aluminum sheet is fed to the fur- nace. The trailing edge of the sheet being processed is typically stopped to con nect this trailing edge to the leading edge of the new sheet to be processed to form a joint.

[014] When used for continuous annealing the continuous heat treatment lines of the invention anneal the aluminum sheet so it is important to control peak metal temperature.

[015] Solution heat treatment is similar to annealing, but it involves quenching, which is the rapid cooling of the alloy to preserve the distribution of the elements. When used for solution heat treating the continuous heat treatment lines heat the alloy to a temperature at which a particular constituent will enter into solid solution followed by cooling (quenching) at a rate fast enough to prevent the dissolved con stituent from precipitating. Maintaining the desired peak metal temperature during heating in both of these processes is important to obtain a product having the de sired properties. [016] Annealing and solution heat-treatment involve heating and cooling the sheet to specific temperatures and holding at those temperatures for specific dura tions of time. The temperature-time profile of sheet can greatly affect the resulting strength and ductility of the final sheet product. In a continuous annealing line as well as a continuous solution heat treating line deviation of nominal line speed leads to process conditions in the heat treatment furnace and quench that might lead to non-conforming product characteristics.

[017] Thus, temperature control of the set (target) peak metal temperature in a furnace for both annealing and solution heat treating is desired to be with a control accuracy of +/-3°C. or better. [018] For purposes of this specification, peak metal temperature (“PMT”) is the highest temperature that aluminum alloy achieves in the furnace of a continuous annealing line or a continuous solution heat treating line.

[019] Thus, the furnace line needs to continue to run sheet through the furnace during deviation of nominal line speed to maintain the sheet being processed at the desired peak metal temperature for the desired time. If the sheet being pro cessed merely stops in the furnace with no changes to heating by the furnace dur ing the attaching then the sheet would overheat. Thus, a first accumulator or looper upstream of the furnace provides a first buffer portion of the sheet to feed through the furnace to continue advancing a downstream portion of the aluminum sheet through the furnace while the attaching is being performed. However, this first buffer resource is finite.

[020] Also, after heat treating and quenching the sheet exiting from quenching is cut by shears (for example flying shears) to separate a product sheet from the re mainder of the sheet upstream of the cut. The portion of the sheet at the location of the cut is typically stopped during cutting. However, the furnace line needs to continue to run sheet through the furnace during this attaching to maintain the sheet being processed at the desired peak metal temperature. Thus, a second ac cumulator or looper downstream of the furnace provides sufficient space to accu mulate a second buffer portion of the sheet received from quenching to continue advancing an upstream portion of the sheet through the furnace while the cutting is being performed. However, this second buffer resource is finite.

[021] The buffers permit the sheet to continue moving through the furnace at its desired line speed. As a result, the target peak metal temperature for the target duration of the target peak metal temperature in the furnace can be maintained during the time permitted by the first and/or second buffer.

[022] However, if it is determined that the stoppage upstream or downstream of the furnace will occur for an extended time beyond that permitted for feeding from the first buffer or accumulating in the second buffer at the target line speed then the line speed must be reduced to extend the time for the first and/or second buffer to operate. Once the accumulator is empty (entrance) or full (exit) process section stoppage is unavoidable. Indeed, in addition to time expansion caused by issues at entrance and exit, there can be other reasons why a reduced line speed is desired to provide additional time or reduce scrap length, such as the following:

[023] Quality: e.g. dents: time to find and remove the source [024] Logistics: e.g. next coil not available or change of schedule

[025] Reducing line speed can harm the sheet product properties because the target PMT and time duration of the target PMT in the furnace are not maintained.

[026] Thus, if a portion of the line upstream or downstream of the furnace needs to be stopped for longer than can be compensated by the first and/or second buffer it would be beneficial to be able to control peak metal temperature during this time, while the line in the furnace slows down to maintain conforming con sistent product properties.

SUMMARY OF THE INVENTION

[027] It is an object of the invention to provide a method for continuously heat treating or annealing aluminum alloy sheet at final thickness to produce sheet that has consistent good mechanical properties.

[028] It is another object of the invention to control peak metal temperature (PMT) in a heat treating furnace, which is a continuous convection floating furnace, within +/- 5°C, preferably +/- 3°C, more preferably +/- 2°C of a target peak metal tempera ture by adjusting line speed, fan speeds, and furnace air temperature in at least one zone of the furnace when a portion of the sheet feeding the furnace or a portion of the sheet exiting the furnace is slowed down for an extended period of time.

[029] It is another object of the invention to control peak metal temperature (PMT) in a continuous furnace that can typically perform 1 ) annealing of non-heat treatable AA5000-series alloy; 2) solution heat treating of AA6000-series alloy; or 3) solution heat-treating aluminum AA7000-series alloy sheet.

[030] This and other objects and further advantages are met or exceeded by the present invention providing a method for continuously heating aluminum alloy sheet at final thickness in a continuous heat-treating furnace having an entry section and an exit section, wherein the heat treating furnace is a continuous convection floating furnace, comprising: continuously horizontally moving uncoiled aluminum alloy sheet in a floating state in a path along a direction of its length through a plurality of contiguous heat treatment zones of an elongated heat treatment chamber of the continuous heat- treating furnace arranged to heat the moving aluminum sheet to a set peak metal temperature (TPMT) in the temperature range of 350 to 590°C; wherein the contiguous heat treatment zones have independently controlla ble convection heaters along the path for heating the aluminum alloy sheet and in dependently controllable fans blowing above and below the aluminum alloy sheet along the path for guiding the aluminum alloy sheet along the path as the alumi num alloy sheet horizontally moves through the elongated heat treatment cham ber, wherein at least one said contiguous heat treatment zone is a peak metal temperature zone which has a target aluminum alloy sheet temperature which is the peak metal temperature of the aluminum alloy sheet in the elongated heat treatment chamber; taking measurements representative of heat transfer to the aluminum alloy sheet as the aluminum alloy sheet moves through the elongated heat treatment chamber, the measurements including speed of the aluminum alloy sheet through the elongated heat treatment chamber, speed of the fans blowing above and be low the aluminum alloy sheet, and furnace air temperature, and optionally surface temperature of the sheet, wherein said taking measurements comprises: continuously measuring the line speed of horizontal movement of the alumi num alloy sheet through the furnace and generating a speed signal proportional to the actual measured line speed vi e.ac _t of the aluminum alloy sheet through the fur nace, wherein the line speed of the aluminum alloy sheet through the furnace has at a line speed set point (vi e.set), continuously measuring the speeds of the furnace fans above and below the aluminum alloy sheet in said peak metal temperature zone, wherein the fan speeds of the fans above and below the aluminum alloy sheet have respective fan speed set points (Vf _an ,set), wherein the fan speed set point of the fans above the aluminum alloy sheet may be the same or different from the fan speed set point of the fans below the aluminum alloy sheet; continuously measuring the furnace air temperature in said peak metal tem perature zone, wherein the furnace air temperature in said peak metal temperature zone has a zone air temperature set point (T _ZO ne,set); and optionally continuously measuring the surface temperature of the sheet, wherein during normal operation the actual measured line speed, measured fan speeds in said peak metal temperature zone, and measured furnace air tem perature in said peak metal temperature zone simultaneously are respectively in the preset ranges for the line speed set point (vi e.set), the fan speed set points (Vfan.set), and the zone air temperature set point (T _ZO ne,set) in said peak metal tem perature zone; changing the fan speeds and the furnace air temperature in said peak metal temperature zone, in response to the actual measured line speed vime.act and the line speed set point vi _e .s _et according to equation (I) and equation (II):

wherein:

vii _ne .act is the actual measured line speed, for example in m/min,

viine.set is the line speed set point, for example in m/min,

V _fan.act is the actual fan speed in rpm in the peak metal temperature zone, x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75 to 0.95,

V _fan .s _et is the fan speed set point in rpm in the peak metal temperature zone, wherein the set point of each fan above the moving sheet may be the same or dif ferent from the set point of each fan below the moving sheet, Tzone.act is the actual zone air temperature in the peak metal temperature zone, Tzone.se _t is the zone air temperature set point in the peak metal temperature zone, y is between 0 and 0.2, preferably 0 and 0.1 .

[031] In the invention, the zone air temperature set point (T _ZO ne,set) for a respec tive zone lies within a respective preset temperature range. The line speed set point (viine.set) lies within a preset range for the speed of the aluminum alloy sheet. The respective fan speed set points (Vf _an ,set) lie within respective preset fan speed ranges. The zone air temperature set point (T _ZO ne,set) for a respective zone lies within a respective preset temperature range.

[032] Generally, furnace air temperature at normal undisrupted operation is above peak metal temperature to provide the driving force for heat transfer to heat the moving sheet. Thus, in normal operation the sheet achieves a peak metal temperature initially in a designated zone and maintains temperature in a range from the peak metal temperature to the soaking temperature Tsoa _k which is the predetermined desired minimum temperature selected for annealing or solution heat treating, in subsequent downstream zones until it is cooled after exiting the furnace. By definition Tsoak is lower than peak metal temperature (TPMT).

[033] However, when line speed slows in the zone where peak metal tempera ture is initially achieved, and likewise the other zones of the furnace, the invention reduces the fan speed and reduces the furnace air temperature. Reducing fan speed while still keeping the moving aluminum sheet floating provide less hot air to the nozzles blowing hot air on the sheet to reduce convection heating of the ad vancing sheet. Reducing the furnace air temperature to a temperature closer to the peak metal temperature is achieved by admitting outside cooling air and / or reducing heating from the burner. This reducing of furnace air temperature re duces the temperature driving force to heat the advancing sheet. As a result, at the slower line speed the sheet will initially arrive at the desired peak metal tem perature in the same zone as it initially did during normal operation or in an earlier upstream zone. In either case, approximately the same peak metal temperature is achieved but it is held for a longer soak time. If it continues to be initially achieved in the same zone the soak time is longer because the sheet is moving slower. If it is achieved upstream of the zone in which it normally initially achieves peak metal temperature the soak time is longer not only because it is moving slower but also because the sheet is moving through more zones at peak metal temperature.

[034] The control of the peak metal temperature by fan speed and furnace air temperature compensation maintains temperature typically within +/- 5°C, prefera bly +/- 2°C of target peak metal temperature for all decelerating, constant, or ac celerating speed conditions.

[035] The invention may control the sheet temperature by fan speed and fur nace air temperature compensation, not only in one or more peak metal tempera ture zones, but also in additional zones of the furnace. Thus, taking measurements in the method of the invention may further comprise: continuously measuring the speeds of the furnace fans above and below the aluminum alloy sheet in one or more additional zones of said contiguous zones, said additional zones being in addition to said peak metal temperature zone, wherein for each additional zone the fan speeds of the fans above and be low the aluminum alloy sheet have respective fan speed set points (Vf _an, set), wherein the fan speed set point of each fan above the aluminum alloy sheet may be the same or different from the fan speed set point of each fan below the alumi num alloy sheet; continuously measuring the furnace air temperature in each additional zone, wherein the furnace air temperature in each additional zone has a respec tive zone air temperature set point (T _ZO ne,set); and optionally continuously measuring the surface temperature of the sheet, wherein during normal operation the actual measured line speed, measured fan speeds in each additional zone, and measured furnace air temperature in each additional zone simultaneously are respectively in the preset ranges for the line speed set point (vi e.set), the fan speed set points (Vf _an ,set), and the zone air tem perature set point (T _ZO ne,set) in the additional zone; respectively changing the fan speeds and the furnace air temperature in each additional zone, in response to the actual measured line speed vime.ac _t and the line speed set point vi e.se _t according to equation (I) and equation (II):

wherein:

viine.act is the actual measured line speed, for example in m/m in,

viine.set is the line speed set point, for example in m/min,

V _f an.act is the actual fan speed in rpm in the additional zone,

x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75 to 0.95,

V _f an.se _t is the fan speed set point in rpm in the additional zone, wherein the set point of the fans above the moving sheet may be the same or different from the set point of the fans below the moving sheet,

Tzone.act is the actual zone air temperature in the respective additional zone,

Tzone.set is the zone air temperature set point in the respective additional zone, y is between 0 and 0.2, preferably 0 and 0.1 .

[036] In the invention, the continuous measuring of the surface temperature of the sheet may be accomplished by an infrared temperature gun or other type of pyrometer, for instance, in the peak metal temperature zone and/or one or more additional zones. The invention may employ a feedback loop with sheet surface temperature measurement(s) instead of, or in addition to, using a model to deter mine the metal temperature, e.g., peak metal temperature, for example a com puter model based on monitoring and measuring fan speed and zone furnace air temperature to determine the heat transfer rate along with line speed measure ment to determine the time the metal is exposed to those conditions. BRIEF DESCRIPTION OF THE FIGURES

[037] FIG. 1 is a schematic representation of the method and the apparatus used.

[038] FIG. 2 shows a schematic drawing of the first looper accumulator. [039] FIG. 3 shows a schematic drawing of a zone of the continuous convection floating furnace.

[040] FIG. 4 schematically shows a portion of the upper nozzle header box to il lustrate nozzles that discharge into the space within the elongated heat treatment chamber of the furnace. [041] FIG. 5 schematically shows a portion of the lower nozzle header box to il lustrate nozzles that discharge into the space within the elongated heat treatment chamber of the furnace.

[042] FIG. 6 shows data from an example.

[043] DESCRIPTION OF THE INVENTION

[044] FIG. 1 provides a schematic representation of the method in accordance with the invention and the continuous heat-treatment furnace used. The continuous heat-treatment furnace (1 ) is arranged to transport and to heat-treat uncoiled alumi num sheet (2) moving in the direction of its length along direction of travel“T”. The aluminum sheet is uncoiled from coil (8). Typically, the aluminum alloy sheet (2) at final gauge has a thickness in the range of 0.3 mm to 4.5 mm, preferably of 0.7 mm to 4.5 mm. The sheet width is typically in the range of about 700 mm to 2700 mm.

[045] FIG. 1 shows the aluminum sheet (2) moving through a first looper accu mulator (12) upstream of the furnace (3). FIG. 1 also shows a joiner (16) upstream of looper 12 and a shearing station (18) downstream of second looper accumulator (14). The joiner (16) attaches a leading end of the roll (8) to the trailing end of the sheet (2). For example, joining may be by welding, e.g., by means of friction stir welding.

[046] Then the moving aluminum sheet (2) passes within the detection range of a line speed sensor (13) which detects the speed of the moving aluminum sheet (2) in its direction of travel“T”.

[047] Then the moving aluminum sheet (2) is gradually heated up from room temperature (RT) to the set peak metal temperature (TPMT) as it moves through the elongated heat treatment chamber (3) of the continuous heat-treatment furnace (1 ) having an entry portion (4) and an exit portion (5). The moving aluminum sheet (2) is heated in the chamber (3) of the furnace (1 ) to the set peak metal temperature and soaked for a number of seconds (tso _A x) in the chamber (3) of the furnace (1 ) at a temperature in the range from the set peak metal temperature to the soaking temperature Ts _oak which is the predetermined desired minimum temperature se lected for annealing or solution heat treating. By definition Ts _oak is lower than peak metal temperature (TPMT).

[048] The moving or travelling aluminum sheet moves substantially horizontally in a floating state through the elongated heat treatment chamber (3) over a length of typically at least about 20 meters, preferably over at least 55 meters.

[049] On leaving the exit portion (5) the moving aluminum sheet (2) is rapidly cooled in the cooling section (6) to below about 150°C, e.g. to about room temper ature.

[050] Then the aluminum sheet (2) passes through a second looper accumulator (14) downstream of the furnace (3) and then proceeds to a shearing station (18). The shearing station (18) cuts the heat treated aluminum sheet 2 into product sheets 20. For example, flying shears may cut the heat treated aluminum sheet 2 into product sheets 20.

[051] FIG. 2 shows a schematic drawing of details of the first looper accumulator (12). The first looper accumulator (12) has a series of rollers defining a path that can be expanded or contracted to accommodate a stoppage of the trailing end of the sheet (2) while the joiner attaches a leading end of the roll (8) to the trailing end of the sheet (2). The second looper accumulator (14) would have the same or similar structure as the first looper accumulator (12) to accommodate the sheet (2) while a portion of sheet (2) downstream of the second looper accumulator (14) is stopped or slowed at the shearing station (18) while the shearing station (18) cuts the heat treated aluminum sheet (2) into the product sheets (20) and being re coiled into individual coils.

[052] The continuous heat-treatment furnace (1 ) is a continuous convection floating furnace arranged to heat the moving aluminum sheet to a set peak metal temperature (TPMT). The furnace (1 ) has a series of contiguous zones (10) in its chamber (3) arranged to heat the moving sheet (2) such that during normal opera tion at least one zone (10) heats the moving sheet (2) to the set peak metal tem perature (TPMT).

[053] FIG. 3 shows a schematic drawing of details of a zone (10) of the continu ous convection floating furnace (1 ). Each zone (10) has at least one fan (30) above the aluminum alloy sheet (2) and at least one fan (32) below the aluminum alloy sheet (2). The fans (30), (32) blow recirculated hot furnace air into respective upper and lower nozzle header boxes (34), (36) which include and feed a respec tive plurality of nozzles which blow the recirculated hot furnace air onto the sheet (2). The upper nozzle header box (34) blows the recirculated furnace air down wardly onto the sheet (2) to heat and stabilize the moving aluminum alloy sheet 2. The lower nozzle header box (36) blows the recirculated furnace air upwardly onto the sheet 2 to heat, float and stabilize the moving aluminum alloy sheet (2) as it travels in direction of travel”T”.

[054] Each zone (10) typically has at least one convection heater, for example burner (40), above the sheet (2) and at least one burner (42) below the sheet (2). Typically, the burners (40), (42) are fed by combustible gas, typically natural gas, lines (44), (46). Each zone (10) also has at least one fresh air feed duct (50) above the sheet (2) and/or below the sheet (2) fed by fresh air intake conduit 51. [055] FIG. 3 illustrates gas firing burners with multiple air circulation fans. These burners are convective heaters. Preferably, gas firing burners with multiple air cir culation fans perform the convective heating. However, various other convective heating means can be applied, e.g. resistance heating, in the continuous heat treat ment furnace.

[056] The moving aluminum sheet moves substantially horizontally through the elongated heat treatment chamber (3) of the continuous furnace over a length of at least about 20 meters, preferably at least 40 meters, and more preferably of at least about 55 meters. A practical maximum length is about 125 meters, but the invention is not limited to this maximum length.

[057] FIG. 4 schematically shows a portion of the upper nozzle header box (34) to illustrate nozzles (35) which discharge into the space within the elongated heat treatment chamber (3) of the furnace.

[058] FIG. 5 schematically shows a portion of the lower nozzle header box (36) to illustrate nozzles (37) which discharge into the space within the elongated heat treatment chamber (3).

[059] The hot-recirculating furnace air nozzles throughout the furnace length heat the strip and keep it afloat on an air cushion. Thus, the strip is travelling in a floating state. Such a furnace is also known as convection floating furnace. The elimination of mechanical contact at elevated temperature in the heat-treatment furnace translates into a fault-free strip surface. The continuous heat-treatment fur nace can be modular in design; as such the furnace comprises several heating zones that use turbines (not shown) to generate an air channel consisting of top and bottom airflows. The burners that heat the air preferably work with combustion pre-heated air.

[060] The moving sheet (2) enters the entry section (4) at Vi e.se _t at ambient temperature and is gradually heated-up while travelling through the continuous heat-treatment furnace to a pre-set heat treatment temperature in the temperature range of 350°C to 590°C, preferably 450°C to 590°C. In the continuous heat-treat ment furnace the average heat-up rate of the aluminum sheet is typically in a range of about 10-15°C/sec for an about 1 mm thick sheet material. Depending on the strip speed the strip temperature may reach the actual pre-set solution heat treatment temperature only far into the second-half of the furnace length or even near the end of the continuous heat-treatment furnace and it is actually soaked at the solution heat treatment temperature for a very short period of time, e.g. a few seconds. Thereafter the moving sheet leaves the heat-treatment furnace at the exit section (5) and is immediately quenched in the cooling section (6).

[061] The soaking temperature Ts _oak is the predetermined desired minimum temperature selected for annealing or solution heat treating. By definition Ts _oak is lower than peak metal temperature (TPMT). The soaking time (tsoAx) is the time the sheet is held at or above Tsoak. The soaking time (tsoAx) of the moving aluminum sheet is at least one second, typically at least 5 seconds, more typically 5 to 30 seconds, for example 10 seconds.

[062] Generally, aluminum sheet speed (also known as line speed) through the furnace is at least 3 meters/minute. Typical aluminum sheet speed is about 20 to about 140 m/min.

[063] Optionally, the quenched and moving aluminum sheet is stretched up to about 0.7%, typically in a range of about 0.1 % to 0.5%, by means of tension level ling. Preferably, the stretched and moving aluminum sheet is subsequently cleaned and provided with a coating, e.g., a passivation coating, or otherwise processed.

[064] Optionally, a pre-bake heat treatment heat-treats the stretched aluminum sheet having a passivation coating. The pre-bake treatment increases in particular the paint-bake response of the AA6000-series aluminum sheet material.

[065] Furnace Control

[066] In accordance with the invention this balance of properties and process economy has been improved by implementing a control mechanism and apparatus to maintain peak metal temperature in the furnace in the event of a reduction in line speed.

[067] To cope with different line speeds, the invention controls heat treatment furnace based on the actual line speed.

[068] During continuous heat treating in the convection floating furnace to mini mize the effect of line speed variations on peak metal temperature, the invention controls the speed of the top and bottom circulation fans and the zone temperature based upon line speed. The top and bottom circulation fans in each zone circulate the air (controlling heat transfer) and floats the sheet through the furnace.

[069] The zone temperatures are the furnace air temperatures of the zones of the convection floating furnace. The convection floating furnace typically has more than one zone, for example 4 to 28 zones. The zones are typically heated by burning natural gas or other combustible gas.

[070] The fan speed and temperature have direct relation to the heat transfer coefficient (HTC) and line speed has a direct relation to the energy input to the strip over the furnace length. Thus, a fan speed and zone temperature control based on line speed is employed to have a peak metal temperature (PMT) (and soak time) controlled system. Time in the furnace will typically change when line speed changes, so maintaining the peak metal temperature and minimizing soak time changes is beneficial.

[071] The inventive method for continuously heat-treating aluminum alloy sheet at final thickness in a continuous heat treating furnace having an entry section and an exit section, wherein the heat treating furnace is a continuous convection float ing furnace, comprising continuously horizontally moving uncoiled aluminum alloy sheet in a floating state in a path along a direction of its length through a plurality of contiguous heat treatment zones of an elongated heat treatment chamber of the continuous heat- treating furnace arranged to heat the moving aluminum sheet to a set peak metal temperature (TPMT) in the temperature range of 350°C to 590°C; wherein the contiguous heat treatment zones have independently controlla ble convection heaters along the path for heating the aluminum alloy sheet and in dependently controllable fans blowing above and below the aluminum alloy sheet along the path for guiding the aluminum alloy sheet along the path as the alumi num alloy sheet horizontally moves through the elongated heat treatment cham ber, wherein at least one said contiguous heat treatment zone is a peak metal temperature zone which has a target aluminum alloy sheet temperature which is the peak metal temperature of the aluminum alloy sheet in the elongated heat treatment chamber; taking measurements representative of heat transfer to the aluminum alloy sheet as the aluminum alloy sheet moves through the elongated heat treatment chamber, the measurements including speed of the aluminum alloy sheet through the elongated heat treatment chamber, speed of the fans blowing above and be low the aluminum alloy sheet, and furnace air temperature, and optionally surface temperature of the sheet, wherein said taking measurements comprises: continuously measuring the line speed of horizontal movement of the alumi num alloy sheet through the furnace and generating a speed signal proportional to the actual measured line speed vi e.ac _t of the aluminum alloy sheet through the fur nace, wherein the line speed of the aluminum alloy sheet through the furnace has at a line speed set point (vi e.set), continuously measuring the speeds of the furnace fans above and below the aluminum alloy sheet in said peak metal temperature zone, wherein the fan speeds of the fans above and below the aluminum alloy sheet have respective fan speed set points (Vf _an, set), wherein the fan speed set point of the fans above the aluminum alloy sheet may be the same or different from the fan speed set point of the fans below the aluminum alloy sheet; continuously measuring the furnace air temperature in said peak metal tem perature zone, wherein the furnace air temperature in said peak metal temperature zone has a zone air temperature set point (T _ZO ne,set); and optionally continuously measuring the surface temperature of the sheet, wherein during normal operation the actual measured line speed, measured fan speeds in said peak metal temperature zone, and measured furnace air tem perature in said peak metal temperature zone simultaneously are respectively in the preset ranges for the line speed set point (vi e.set), the fan speed set points (vfan.set), and the zone air temperature set point (T _ZO ne,set) in said peak metal tem perature zone; changing the fan speeds and the furnace air temperature in said peak metal temperature zone, in response to the actual measured line speed vi _me.act and the line speed set point vi _e .s _et according to equation (I) and equation (II):

wherein:

vii _ne .act is the actual measured line speed, for example in m/m in,

viine.set is the line speed set point, for example in m/min,

V _fan.act is the actual fan speed in rpm in the peak metal temperature zone, x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75 to 0.95,

V _fan .s _et is the fan speed set point in rpm in the peak metal temperature zone, wherein the set point of each fan above the moving sheet may be the same or dif ferent from the set point of each fan below the moving sheet,

T _zone .act is the actual zone air temperature in the peak metal temperature zone, Tzone.set is the zone air temperature set point in the peak metal temperature zone, y is between 0 and 0.2, preferably 0 and 0.1. [072] Preferably the measurements are taken for the first zone to achieve peak metal temperature and all the contiguous heat treatment zones in the furnace downstream of this peak metal temperature zone, more preferably all the contigu ous heat treatment zones in the furnace, and the fan speeds and furnace air tem peratures adjusted for the first zone to achieve peak metal temperature and all contiguous heat treatment zones in the furnace downstream of this peak metal temperature zone, more preferably all the contiguous heat treatment zones in the furnace.

[073] Thus, the invention may control the sheet temperature by fan speed and furnace air temperature compensation, not only in one or more peak metal temper ature zones, but also in additional zones of the furnace. Thus, taking measure ments in the method of the invention may further comprise: continuously measuring the speeds of the furnace fans above and below the aluminum alloy sheet in one or more additional zones of said contiguous zones, said additional zones being in addition to said peak metal temperature zone, wherein for each additional zone the fan speeds of the fans above and be low the aluminum alloy sheet have respective fan speed set points (Vf _an, set), wherein the fan speed set point of each fan above the aluminum alloy sheet may be the same or different from the fan speed set point of each fan below the alumi num alloy sheet; continuously measuring the furnace air temperature in each additional zone, wherein the furnace air temperature in each additional zone has a respec tive zone air temperature set point (T _ZO ne,set); and optionally continuously measuring the surface temperature of the sheet, wherein during normal operation the actual measured line speed, measured fan speeds in each additional zone, and measured furnace air temperature in each additional zone simultaneously are respectively in the preset ranges for the line speed set point (vi e.set), the fan speed set points (Vf _an ,set), and the zone air tem perature set point (T _ZO ne,set) in the additional zone; respectively changing the fan speeds and the furnace air temperature in each additional zone, in response to the actual measured line speed vime.ac _t and the line speed set point vi e.se _t according to equation (I) and equation (II):

wherein:

viine.act is the actual measured line speed, for example in m/min,

viine.set is the line speed set point, for example in m/min,

V _f an.act is the actual fan speed in rpm in the additional zone,

x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75 to 0.95,

V _f an.se _t is the fan speed set point in rpm in the additional zone, wherein the set point of the fans above the moving sheet may be the same or different from the set point of the fans below the moving sheet,

Tzone.act is the actual zone air temperature in the respective additional zone,

Tzone.set is the zone air temperature set point in the respective additional zone, y is between 0 and 0.2, preferably 0 and 0.1 .

[074] Taking the measurements and adjusting fan speeds and furnace air tem perature in all the contiguous heat treatment zones involves a method for continu ously heat treating aluminum alloy sheet at final thickness in a continuous heat treating furnace having an entry section and an exit section, wherein the heat treating furnace is a continuous convection floating furnace, comprising: continuously horizontally moving uncoiled aluminum alloy sheet continu ously moving in a floating state in a path along a direction of its length through a plurality of contiguous heat treatment zones of an elongated heat treatment cham ber of the continuous heat-treating furnace arranged to heat the moving aluminum sheet to a set peak metal temperature (TPMT) in the temperature range of 350°C to 590°C; wherein the contiguous heat treatment zones have independently controlla ble convection heaters along the path for heating the aluminum alloy sheet and in dependently controllable fans blowing above and below the aluminum alloy sheet along the path for guiding the aluminum alloy sheet along the path as the alumi num alloy sheet horizontally moves through the elongated heat treatment cham ber, wherein at least one said contiguous heat treatment zone is a peak metal temperature zone which has a target aluminum alloy sheet temperature which is the peak metal temperature of the aluminum alloy sheet in the elongated heat treatment chamber; taking measurements representative of heat transfer to the aluminum alloy sheet as the aluminum alloy sheet moves through the elongated heat treatment chamber, the measurements including speed of the aluminum alloy sheet through the elongated heat treatment chamber, speed of the fans blowing above and be low the aluminum alloy sheet in each heat treatment zone, and furnace air temper ature in each heat treatment zone, and optionally surface temperature of the sheet in each heat treatment zone, wherein said taking measurements comprises: continuously measuring the line speed of horizontal movement of the alumi num alloy sheet through the furnace and generating a speed signal proportional to the measured line speed vime.act of the aluminum alloy sheet through the furnace, wherein the line speed of the aluminum alloy sheet through the furnace has at a line speed set point (vi e.set), continuously measuring the speeds of the furnace fans above and below the aluminum alloy sheet in each heat treatment zone, wherein the fan speeds of the fans above and below the aluminum alloy sheet have respective fan speed set points (Vfan.set) which lie within respective preset fan speed ranges, wherein the fan speed set point of the fans above the aluminum alloy sheet may be the same or different from the fan speed set point of the fans below the aluminum alloy sheet; continuously measuring the furnace air temperature in each heat treatment zone, such that the furnace air temperature in each heat treatment zone is at a re spective zone air temperature set point (T _ZO ne,set); changing the fan speeds and the furnace air temperature in each heat treat ment zone, in response to the actual line speed vime.act and the line speed set point viine.set according to equation (I) and equation (II) (per zone):

wherein: viine.act is the actual line speed, for example in m/min,

viine.set is the line speed set point, for example in m/min,

V _f an.ac _t is the actual fan speed in rpm in the respective heat treatment zone, x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75 to 0.95,

V _f an.se _t is the fan speed set point in rpm, wherein the set point of the fans above the moving sheet may be the same or different from the set point of the fans below the moving sheet in each respective heat treatment zone,

Tzone.ac _t is the actual zone air temperature in the respective heat treatment zone Tzone.set is the zone air temperature set point in the respective heat treatment zone y is between y is between 0 and 0.2, preferably 0 and 0.1 .

[075] In the invention, the zone air temperature set point (T _ZO ne,set) for a respec tive zone lies within a respective preset temperature range. The line speed set point (viine.set) lies within a preset range for the speed of the aluminum alloy sheet. The respective fan speed set points (Vf _an ,set) lie within respective preset fan speed ranges. The zone air temperature set point (T _ZO ne,set) for a respective zone lies within a respective preset temperature range.

[076] The values for“x” and“y” can be empirically determined by a user by run ning computer simulation furnace models that calculate the effect of varying the line speed on peak metal temperature. Then to validate these findings actual fur nace trials can be conducted varying the line speed and sampling the products from these trials. Changes in product properties indicate the changes in PMT re sulting from these line speed changes.

[077] Parameters x, y for an actual furnace are measured empirically by meas uring actual aluminum sheet temperatures with sensors over multiple furnace test runs at varying conditions of fan speed and zone temperature and time. To esti mate temperature during normal operation the invention monitors and measures fan speed and zone furnace air temperature to determine the heat transfer rate. In addition, the invention monitors and measures line speed to determine the time the metal sheet is exposed to those conditions. Then an online computer model based upon these measurements uses this information in heat transfer equations to calculate a peak metal temperature. Heat transfer equations for convective heat transfer are known to those skilled in the art. See for example, Engineering Heat Transfer, M.M. Rathore, Jones & Bartlett Learning, 2d Ed. (2011 ); or Heat Transfer, J.P. Holman, McGraw-Hill, 10 ^th ed. (2010).

[078] Typically, during furnace operation automated and continuous readings are taken of fan rpm and zone furnace air temperature. This coupled with line speed are used in a mathematic model to calculate peak metal temperature (PMT). For example, the model may be based on monitoring and measuring fan speed (rpm) and zone furnace air temperature to determine the heat transfer rate along with line speed measurement to determine the time the metal is exposed to those conditions

[079] However, in the invention, the continuous measuring of the surface tem perature of the sheet may also be accomplished by an infrared temperature gun or other type of pyrometer, for instance, in the peak metal temperature zone and/or one or more additional zones. The invention may employ a feedback loop with sheet surface temperature measurement(s) instead of, or in addition to, using a model to determine the metal temperature, e.g., peak metal temperature.

[080] This control of fan speed and furnace air temperature according to equa tions (I) and (II) should be activated once the actual line speed differs from the line speed set point vi e.se _t by more than a settable tolerance, e.g., 0 to 10% deviation from line speed set point, typically 1 % to 5% deviation from line speed set point.

[081] Fan speed parameter x is user selectable per furnace zone (in machine parameters) based on empirical data as explained above. However, in general x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75 to 0.95, and may be the same or different for each zone.

[082] There are fans above and below the sheet. The fans above may be at a different rpm than the fans below. However, the invention typically controls/adjusts the actual fan speed Vf _an ,act rpm of both sets of fans by the same ratio relative to their respective fan speed set point Vf _an ,set.

[083] Reducing the speed of the fans above and below the sheet to cope with line speed reductions is limited because safe floating of the sheet must be guaran teed all time. Thus, the formula above is limited to values for safely floating the sheet. In particular, the reduction of fan speed is limited by: Minimum fan speeds to maintain flotation of the sheet above furnace components to avoid damage, such as scratching, to the sheet, Maximum soaking time; and Minimum heating rates. This is generally no more than a 35% reduction in rpm, preferably no more than a 30 % reduction in rpm.

[084] Furnace air temperature parameter y is user selectable per furnace zone (in machine parameters) based on empirical data as explained above. However, in general y is between 0 and 0.2, preferably 0 and 0.1 , more preferably between 0.02 and 0.1 , and may be the same or different for each zone.

[085] However, furnace air temperature should be reduced no more than 10 °C, preferably no more than 5 °C, such that the inventive method can cool and heat up the furnace air in the same time period as the duration of the speed change. [086] Reducing the zone temperature to cope with line speed reductions is lim ited to a maximum deviation below recipe temperature set point. Every zone (10) typically has at least one burner (two shown in FIG. 3) and at least one fresh air cooling intake (two shown in FIG. 3). Thus, to raise temperature the feed of com bustible gas to the burner (40), (42) can be increased and / or the influx of air from the fresh air cooling intake (51 ) to the fresh air feed duct (50) can be adjusted. For example, to raise the zone temperature the feed of combustible gas to the burner (40), (42) can be increased and / or the influx of air from the fresh air cooling in take can be decreased. Also for example, to lower the zone temperature the feed of combustible gas to the burner (40), (42) can be decreased and / or the influx of air from the fresh air cooling intake can be increased.

[087] The control of the peak metal temperature by means of fan speed and tem perature compensation maintains temperature typically within +/- 5°C, preferably +/- 3°C, more preferably +/- 2°C of target peak metal temperature for all decelerating, constant, or accelerating speed conditions.

[088] Control scheme details

[089] FIG. 3 also schematically shows control scheme details of the zone (10) of the continuous convection floating furnace (1 ) that may be employed to implement this inventive method. Each zone (10) also has at least one furnace air tempera ture sensor (52) in communication with a temperature controller (54). 12. The fur nace air temperature sensors in the invention may be, for example, thermocou ples.

[090] The temperature controller (54) controlling the combustion gas feed valves (56) and the fresh air intake valves (58). The line speed sensor (13) is also in com munication with the temperature controller (54) and fan speed controllers (60),

(62). Thus, when the line speed sensor (13) senses a change in the line speed of the sheet (2) it sends signals to the fan speed controller (60) and to the tempera ture controller (54) to control the respective speed of the fans (30), (32) and the furnace air temperature according to according to the above listed equation (I) and equation (II).

[091] When line speed sensor (13) senses a change in the line speed of the sheet (2) it sends at least one signal to the fan speed controller (60) to adjust the speeds of the respective fans (30), (32) according to equation (I). Also, when the line speed sensor (13) senses a change in the line speed of the sheet (2) it sends signals to the furnace air temperature controller (54) to control furnace air temper ature according to equation (II). The furnace air temperature is controlled by con trolling the air intake valves (58) which admit cooling air and the combustion gas valves 56 which control feed of combustion gas to the burners (40), (42).

[092] Additional Operations

[093] If desired additional operations may be performed based on measure ments and control of line speed, furnace air temperature, and fan speeds.

[094] The method may further include taking measurements representative of heat transfer to the sheet as the sheet advances through the elongated heat treat ment chamber of the furnace which comprises measuring line speed of the sheet through the elongated heat treatment chamber, measuring the fan speeds of fans above and below the moving sheet in each contiguous heat treatment zone and measuring the furnace air temperature in each contiguous heat treatment zone; comparing the measured fan speeds with a preset fan speed range in the form of specified values; comparing the measured furnace air temperature with a preset air tempera ture range in the form of specified values; providing a plurality of different independently-controllable burners at differ ent positions along the pathway, providing a plurality of different independently- controllable fans above and below the moving aluminum sheet at different posi tions along the pathway; generating a real-time sheet temperature estimate of a single point wherein the sheet temperature estimate is established as a function of the sheet line speed in the elongated heat treatment chamber, the furnace air temperature in each con tiguous heat treatment zone, and the fan speeds in each contiguous heat treat ment zone (typically the invention validates with the properties of the metal in times of line speed flux); adjusting the sheet temperature estimate in response to a change in at least one of the sheet line speed, the furnace air temperature in at least one said zone, and the fan speeds in at least one said contiguous heat treatment zone.

[095] The sheet temperature estimates are sheet surface temperature esti mates.

[096] If desired the method may further include taking measurements repre sentative of heat transfer to the aluminum alloy sheet as the aluminum alloy sheet moves through the elongated heat treatment chamber of the furnace, which com prises from time to time taking measurements including speed of the aluminum al loy sheet through the elongated heat treatment chamber, speed of the fans above and below, and air temperature in each of the contiguous heat treatment zones, generating a real-time sheet temperature estimate along the path wherein the sheet temperature estimate is established as a function of speed of the alumi num alloy sheet through the elongated heat treatment chamber, speed of the fans above and below, and air temperature in each contiguous heat treatment zone; wherein said generating the real-time sheet temperature estimate along the path comprises estimating the aluminum alloy sheet temperature in the peak metal temperature zone at the line speed set point vime.set, and the fan speed set point

Vfan.Set, adjusting the sheet temperature estimate in response to a change in at least one of the sheet speed, speed of the fans above and below, and air tempera ture in each contiguous heat treatment zone; comparing the sheet temperature estimate to a desired temperature distri bution to determine any differences between the sheet temperature estimate and the desired temperature distribution, wherein the desired temperature distribution includes a plurality of temperature setpoints for the sheet along the pathway, each setpoint representing a target value; for each one of the burners and fans, regulating operation as a function of the differences with a closed-loop, feedback controller; estimating, in real-time, a sheet temperature profile at a point along the pathway for the sheet based on the sheet temperature estimate and the length of the sheet, and visually displaying a real-time representation of the sheet temperature estimate and soak time, and, if desired, the sheet thickness profile and the length of the sheet.

[097] Solution Heat Treating Process and Annealing Processes

[098] A conventional process for producing aluminum alloy products in rolled form with solution heat treating includes the processing steps wherein an alumi num alloy body is cast, after which it is homogenized and then hot rolled to an in termediate gauge. Next, the aluminum alloy body may be cold rolled. Next it is so lution heat treated and quenched, for example by means of water such as water quenching or water spray quenching. "Solution heat treating and quenching" and the like, generally referred to herein as "solutionizing", means heating an alumi num alloy body to a suitable temperature, generally above the solvus temperature, holding at that temperature long enough to allow soluble elements to enter into solid solution, and cooling rapidly enough to hold the elements in solid solution.

[099] The following are typical solution heat treating and annealing processes which may be performed in a furnace being controlled according to the method of the invention. [0100] To solution heat treat according to the method of the present invention the process includes continuously moving uncoiled heat treatable aluminum alloy sheet in the direction of its length horizontally through the continuous heat treat ment furnace arranged to heat the moving aluminum sheet to heat the product at a heating rate in the range of 2 to 200 °C./sec to a solution heat treating temperature of typically 350°C to 590°C, preferably 370°C to 590°C, more preferably 460°C to 580°C, or more preferably 500°C to 590°C, furthermore preferably 480°C to 580°C and soak the sheet at this temperature for a soaking time (tsoAK). Then the sheet exiting the furnace is quenched typically by quench water at a cooling rate in the range of 10 to 500 °C./sec to below a temperature of 150°C. Soaking temperature Ts _oak is the predetermined desired minimum temperature selected for the solution heat treating. By definition Tsoak is lower than peak metal temperature (TPMT). The soaking time (tsoAx) is the time the sheet is held at or above Tsoak. The soaking time (tsoAx) of the moving aluminum sheet is at least one second, for example at least 1 to at most 100 sec, typically at least 5 seconds, more typically 5 to 30 sec onds, e.g., 10 seconds.

[0101] When solution heat treating continuously moving uncoiled heat treatable AIMgSi aluminum alloy sheet (also known as AA6000-series alloy sheet) at final gauge the sheet is heated to and held (soaked) at solution heat treating tempera ture in a range of 500°C to 590°C, preferably 520°C to 580°C, to dissolve in partic ular Mg and Si. The furnace being controlled according to the method of the inven tion. Then the sheet exiting the furnace is quenched typically by quench water at a cooling rate in the range of 10 to 500 °C./sec to below a temperature of 150°C. Typical AA6000 alloys treatable according to the invention include 6005, 6009,

6010, 6111 , 6014, 6016, 6022, 6061 , 6181 , 6082, 6182, and various others. Solu tion heat treating of AA6000 series alloy involves recovery in the upstream zone or zones in which the metal softens by rearranging the cold worked structure, recrys tallization of the metal in the middle zone or zones, and grain growth in the metal in the downstream zone or zones of the furnace where the metal achieves and soaks at soaking temperature during which the small recrystallized grains will grow to the desired size. [0102] When solution heat treating continuously moving uncoiled heat treatable aluminum alloy sheet of AA7000-series alloy at final gauge the sheet is heated to and held at solution heat treating temperature commonly in a range of about 430°C to 560°C, preferably 450°C to 560°C. The solid solution formed at high tem perature may be retained in a supersaturated state by cooling with sufficient rapid ity to restrict the precipitation of the solute atoms as coarse, incoherent particles, typically by quench water at a cooling rate in the range of 10 to 500 °C./sec to be low a temperature of 150°C.

[0103] If desired, the AA7000-series aluminum alloy has a Cu-content of less than 0.25% and is one of the following AA7000-series aluminum alloys, as defined by the Aluminum Association: 7003, 7004, 7204, 7005, 7108, 7108A, 7015, 7017, 7018, 7019, 7019A, 7020, 7021 , 7024, 7025, 7028, 7030, 7031 , 7033, 7035, 7035A, 7039, 7046, and 7046A. For AA7000-series alloys have no purposive addi tion of Cu, the solution heat treatment temperature should be at least 370°C. A preferred minimum temperature is 400°C, more preferably 430°C, furthermore preferably 450°C, and most preferably 470°C. The solution heat-treatment temper ature should not exceed 560°C. A preferred maximum temperature is 545°C, and preferably not more than 530°C.

[0104] If desired, the AA7000-series aluminum alloy has a Cu-content of 0.25% or more and is one of the following AA7000-series aluminum alloys, as defined by the Aluminum Association: 7009, 7010, 7012, 7014, 7016, 7116, 7022, 7122,

7023, 7026, 7029, 7129, 7229, 7032, 7033, 7034, 7036, 7136, 7037, 7040, 7140, 7041 , 7049, 7049A, 7149, 7249, 7349, 7449, 7050, 7050A, 7150, 7250, 7055, 7155, 7255, 7056, 7060, 7064, 7065, 7068, 7168, 7075, 7175, 7475, 7076, 7178, 7278, 7278A, 7081 , 7181 , 7085, 7185, 7090, 7093, 7095 and 7099. For AA7000- series alloys have a purposive addition of Cu, the solution heat treatment tempera ture should be at least 400°C. A preferred minimum temperature is 450°C, further more preferably 460°C, and most preferably 470°C. The solution heat-treatment temperature should not exceed 560°C. A preferred maximum temperature is 530°C, and preferably not more than 520°C. [0105] The AA7000-series aluminum sheet may have Zn in the range of 2.0% to 10.0%, and preferably in the range of 3.0% to 9.0%. The AA7000-series alumi num sheet may have Mg in the range of 1.0% to 3.0%. The AA7000-series alumi num sheet may have Cu is the range of <0.25%, preferably Cu in the range of 0.25% to 3.5%.

[0106] The AA7000-series aluminum sheet may further comprise

Fe < 0.5%, preferably < 0.35%,

Si < 0.5%, preferably < 0.4%, and

one or more elements selected from the group consisting of:

Zr at most 0.5,

Ti at most 0.3,

Cr at most 0.4,

Sc at most 0.5,

Hf at most 0.3,

Mnat most 0.4,

V at most 0.4,

Ge at most 0.4,

Ag at most 0.5,

balance being aluminum and impurities.

[0107] Following solution heat treatment and cooling the AA7000-series alumi num sheet may have a recrystallized microstructure.

[0108] After solutionizing, the AA7000-series aluminum alloy body may be option ally stretched a small amount (e.g., about 1 -5%) for flatness, thermally treated (e.g. by natural ageing or artificial ageing) and optionally subjected to final treat ment practices (e.g. a forming operation, paint-bake cycle in case of an automotive application).

[0109] Also, the method and apparatus of the invention can be applied to a broad range of heat-treatable aluminum alloys to be annealed or solution heat treated. For example, depending on the actual alloy composition, the invention may be employed with lower solution heat treatment temperatures, e.g., in the range of 460°C to 480°C. [0110] The heat treatment process includes continuously moving uncoiled heat treatable aluminum alloy sheet in the direction of its length horizontally through a continuous annealing furnace arranged to heat the moving aluminum sheet to heat the product to a temperature in the range of 370°C to 560°C, more typically 480°C to 560°C, for a duration of 1 to at most 100 sec. The moving aluminum sheet is rapidly cooled on leaving the furnace. The cooling is typically by quench water at a cooling rate in the range of 10 to 500 °C./sec to below a temperature below 150°C.

[0111] For annealing AA5000 series aluminum alloy, holding the product at suita ble temperature in the typical multi zone continuous heat-treating furnace has the following three stages:

1 ) Recovery - in the upstream zone or zones in which the metal softens by rear ranging the cold worked structure at a temperature in the range of 350°C to 450°C,

2) Recrystallization - in the middle zone or zones at an annealing temperature in the range of 350°C to 500°C, preferably 350°C to 450 °C wherein the metal is fully soft by removing dislocations and starts forming small grains, and

3) Grain growth - in the downstream zone or zones of the furnace that achieves peak metal temperature (PMT) and soaks the metal at temperature in the range of 450°C to 590°C, preferably 460°C to 550 °C wherein the small recrystallized grains will grow to the desired size. Then the sheet exits the furnace.

[0112] Typical AA5000 alloys treatable according to the present invention include 5030, 5051 , 5182, 5454, 5754, and various others.

[0113] The invention will now be illustrated with reference to a non-limiting exam ple according to the invention. [0114] EXAMPLE !

[0115] FIG. 6 shows a chart illustrating the control of peak metal temperature (PMT) and line speed according to the present invention. This shows as aluminum alloy sheet material went through a speed reduction from 50 to 35 m/m in PMT (°C) in a furnace the peak metal temperature varied an average +/- 1 ° C. The fan speed and air temperature were adjusted according to the invention.