METHOD OF CONTINUOUSLY HEAT-TREATING 7000-SERIES ALUMINIUM ALLOY SHEET MATERIAL

Abstract

A method for continuously solution heat-treating aluminium alloy sheet by continuously moving heat-treatable 7000-series aluminium alloy sheet through a continuous heat-treatment furnace arranged to heat the moving aluminium sheet to a set soaking temperature (T.sub.SET) in the temperature range of 370° C. to 560° C., the continuous heat-treatment furnace has an entry section and an exit section, the moving aluminium sheet moves substantially horizontally through the continuous heat-treatment furnace, and the moving aluminium sheet is rapidly cooled on leaving the exit section, and before or near the entry section of the continuous heat-treatment furnace the moving aluminium sheet is pre-heated to a temperature of 5° C. to 100° C. below the T.sub.SET using an average heat-up rate as function of the sheet thickness of at least Y=−31.Math.ln(X)+50, wherein Y is the heat-up rate in ° C./sec and X is the sheet thickness in mm.

Claims

1. A method for continuously annealing aluminium alloy sheet, comprising: continuously moving uncoiled heat-treatable 7000-series aluminium alloy sheet in the direction of its length through a continuous heat-treatment furnace arranged to heat the moving aluminium sheet to a set solution heat-treatment temperature (T.sub.SET) in the temperature range of 370° C. to 560° C., the continuous heat-treatment furnace has an entry section and an exit section, wherein the moving aluminium sheet moves substantially horizontally through the continuous heat-treatment furnace, wherein the continuous heat-treatment furnace is heated by convective heating, and wherein the moving aluminium sheet is rapidly cooled from T.sub.SET to below about 100° C. on leaving the exit section, and wherein before or near the entry section of the continuous heat-treatment furnace the moving aluminium sheet is pre-heated to a temperature of 5° C. to 100° C. below the T.sub.SET using an average heat-up rate as function of the sheet thickness of at least Y=−31.Math.ln(X)+50, wherein “Y” is the heat-up rate in ° C./sec and “X” is the sheet thickness in mm.

2. The method according to claim 1, wherein before or at the entry section of the continuous annealing furnace the moving aluminium sheet is pre-heated to a temperature of 5° C. to 100° C. below the T.sub.SET using an average heat-up rate as function of the sheet thickness of at least Y=−50.Math.ln(X)+80, wherein Y is the heat-up rate in ° C./sec and X is the sheet thickness in mm.

3. The method according to claim 1, wherein the pre-heating is performed inductively by means of induction heating.

4. The method according to claim 1, wherein the aluminium alloy sheet at final gauge has a thickness in the range of 0.3 to 4.5 mm.

5. The method according to claim 1, wherein the moving aluminium sheet moves substantially horizontally through the continuous heat-treatment furnace over a length of at least 20 meters.

6. The method according to claim 1, wherein the soaking time of the moving aluminium sheet at T.sub.SET is at least 1 seconds.

7. The method according to claim 1, wherein the moving aluminium sheet is pre-heated to a temperature of 5° C. to 75° C. below the T.sub.SET.

8. The method according to claim 1, wherein the aluminium sheet has been pre-treated by homogenisation, hot rolling, and optionally by cold rolling.

9. The method according to claim 1, wherein the 7000-series aluminium sheet has Zn in the range of 2.0% to 10.0%, and preferably in the range of 3.0% to 9.0%.

10. The method according to claim 1, wherein the 7000-series aluminium sheet has Mg in the range of 1.0% to 3.0%.

11. The method according to claim 1, wherein the 7000-series aluminium sheet has Cu is the range of <0.25%.

12. The method according to claim 1, wherein the 7000-series aluminium sheet has Cu in the range of 0.25% to 3.5%.

13. The method according to claim 9, wherein the 7000-series aluminium sheet further comprises: Fe<0.5%, Si<0.5%, 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, Mn at most 0.4, V at most 0.4, Ge at most 0.4, Ag at most 0.5, balance being aluminium and impurities.

14. The method according to claim 1, wherein the 7000-series aluminium sheet has following solution heat treatment and cooling an equiaxed recrystallized microstructure.

15. The method according to claim 1, wherein before or at the entry section of the continuous annealing furnace the moving aluminium sheet is pre-heated to a temperature of 5° C. to 100° C. below the TSET using an average heat-up rate as function of the sheet thickness of at least Y=−62.Math.ln(X)+100, wherein Y is the heat-up rate in ° C./sec and X is the sheet thickness in mm.

16. The method according to claim 1, wherein the pre-heating is done inductively by means of a transverse flux induction heating device.

17. The method according to claim 1, wherein the moving aluminium sheet moves substantially horizontally through the continuous heat-treatment furnace over a length of at least 40 meters.

18. The method according to claim 1, wherein the soaking time of the moving aluminium sheet at TSET is at least 5 seconds.

19. The method according to claim 1, wherein the 7000-series aluminium sheet has Zn in the range of 3.0% to 9.0%.

20. The method according to claim 13, wherein the 7000-series aluminium sheet comprises: Fe in the range of <0.35%, Si in the range of <0.4%.

Description

DESCRIPTION OF THE DRAWINGS

[0067] The invention shall now be described with reference to the appended drawings, in which:

[0068] FIG. 1 is a schematic representation of the method and the apparatus used; and

[0069] FIG. 2A and FIG. 2B are a schematic representation of a temperature profile as function of the time of aluminium sheet travelling through a continuous heat-treatment furnace according to the state-of-the-art and according to the invention; and

[0070] FIG. 3 is a schematic representation of the required minimum heat-up rate as function of sheet thickness and with preferred embodiments.

[0071] 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 aluminium sheet (2) moving in the direction of its length. The aluminium sheet is being uncoiled from coil (8). It moves through the continuous heat-treatment furnace (3) having an entry portion (4) and an exit portion (5). On leaving the exit portion (5) the moving aluminium sheet is rapidly cooled in the cooling section (6) to below about 100° C., e.g. to about room temperature. An industrial continuous heat-treatment furnace represents a substantial capital investment; once commissioned and operational significant modifications such as making it longer in length are often not feasible due to lay-out constraints on the shop floor.

[0072] The moving or travelling aluminium sheet moves substantially horizontally through the continuous heat-treatment furnace over a length of at least about 20 meters, preferably over at least 55 meters. Hot-air nozzles (not shown) 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 sometimes also referred to 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 furnace 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 air is heated by burners that work preferably with combustion pre-heated air. Temperature control of the set soak temperature is with a control accuracy of +/−3° C. or better.

[0073] The moving sheet (2) enters the entry section (4) at high strip speed or line speed at ambient temperature and is gradually heated-up while travelling through the continuous heat-treatment furnace to a preset solution heat treatment temperature (e.g. about 510° C.) depending on the aluminium alloy. In a conventional continuous heat-treatment furnace the average heat-up rate of the aluminium sheet is typically in a range of about 10-15° C./sec for an about 1 mm sheet material. Depending on the strip speed the strip temperature may reach the actual preset 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, whereafter the moving sheet is leaving the heat-treatment furnace at the exit section (5) and is immediately quenched in the cooling section (6). This is also schematically shown in FIG. 2A where the moving aluminium sheet is gradually being heated up from room temperature (RT) to the solution heat treatment temperature (T.sub.SET) and is soaked for a number of seconds (t.sub.SOAK) at the set solution heat treatment temperature or set soak temperature. The soaking time (t.sub.SOAK) is defined at the time spent at the set solution heat treatment temperature or set soaking temperature (T.sub.SOAK)±5° C.

[0074] Depending on the aluminium alloy composition or sheet thickness a longer soaking time at the preset solution heat treatment temperature can be very desirable in order to achieve the desired balance of mechanical properties, but for many 7000-series alloys this can only be achieved at lower strip speed due to the defined dimensions of the heat treatment furnace, which makes it economically significantly less attractive where the strip speed or line speed has to be reduced from for example about 40 m/min to about 20 or 25 m/min.

[0075] In accordance with the invention this balance of properties and process economy has been improved by implementing a pre-heating device immediately before the entry section (4) or at the entry section (4) of the heat-treatment furnace. The pre-heat device (7) is arranged to enable a very fast heat-up rate defined by the equation of the heat-up rate as function of the sheet thickness of at least Y=−31.Math.ln(X)+50, wherein Y is the cooling rate in ° C./sec and X is the sheet thickness in mm, and with preferred higher heat-up rates, and which can be achieved for example by means of a transverse flux induction heating device, for example as disclosed in U.S. Pat. No. 5,739,506 (Ajax Magnethermic). It is desirable that the pre-heating of the aluminium sheet in the pre-heating device (7) observes a safety margin to avoid an overshoot in the temperature of the moving strip and thereby adversely affecting relevant engineering properties due to local melting of microstructural components in the aluminium alloy. Preferably the pre-heat is to a temperature of about 5° C. to 100° C., more preferably of about 5° C. to 75° C., below the preset solution heat treatment temperature at which the heat treatment of the aluminium alloy sheet material should be carried out. Thus for example preheating of the moving aluminium sheet to about 480° C. where the preset solution heat treatment temperature is 510° C. Further heating-up of the moving sheet occurs in the continuous heat-treatment furnace by convective heating. This is also schematically shown in FIG. 2B where the moving aluminium sheet is rapidly pre-heated from room temperature (RT) to the pre-heat temperature (T.sub.PRE) and then further heated to the set solution heat treatment temperature (T.sub.SET). The heat-up rate from RT to T.sub.PRE will in practice not be exactly linear and for that reason the average heat-up rate is used being the temperature difference between T.sub.PRE minus RT divided by the time required to reach T.sub.PRE; thus for example for 1 mm sheet material when from a room temperature of 25° C. a T.sub.PRE of about 480° C. is reached in about 5 sec, the average heat-up rate is about 91° C./sec. This allows, compared to the situation where there is no rapid pre-heating applied, a significantly longer soaking time at the set solution heat treatment temperature while maintaining about the same strip speed. Alternatively, it allows for a significantly increased line speed while having an about the same soaking time (t.sub.SOAK) compared to the state-of-the art situation. Thus for a given continuous annealing furnace, depending on the specific 7000-series alloy there is now significantly more flexibility in optimising soaking time in combination with the line speed in order to arrive at an improved balance of process economy and sheet properties.

[0076] By the method according to the invention and the use of the corresponding apparatus also thicker gauge sheet material can be processed at relative high strip speeds. Where for example a 1 mm sheet material can be processed with line speeds of up to about 70 m/min, a 2 mm sheet material of the same alloy can be processed only with a line speed of up to about 35 m/min due to the significantly longer heat-up time when heated in a convection furnace. With the method and apparatus according to the invention wherein the sheet material is preheated very rapidly to about 480° C. and the solution heat treatment temperature is about 510° C. the 2 mm sheet material can now be continuously heat treated at significantly higher line speeds in the range about 55 to 65 m/min while having an about similar t.sub.SOAK as the 1 mm sheet material.

[0077] FIG. 3 is a schematic representation of the required minimum average heat-up rate as function of the sheet thickness (line 1) and with preferred embodiments (lines 2-4) for the method according to this invention and also for the apparatus and kit-of-parts. The relationship is shown for sheet gauges in the preferred gauge range of 0.3 to 4.5 mm. For lines 1 to 4 the following natural logarithmic equations apply:

Y=−31.Math.ln(X)+50  Line 1:

Y=−50.Math.ln(X)+80  Line 2:

Y=−62.Math.ln(X)+100  Line 3:

Y=−93.Math.ln(X)+150  Line 4:

and wherein “Y” represents the average heat-up rate in ° C./sec and “X” represents the sheet thickness in mm.