US20150041027A1

US20150041027A1 - High Strength Aluminum Fin Stock for Heat Exchanger

Info

Publication number: US20150041027A1
Application number: US14/454,208
Authority: US
Inventors: Andrew D. Howells; Kevin Michael Gatenby; Hany Ahmed; Jyothi Kadali; Derek William Aluia; John Michael Baciak, III
Original assignee: Novelis Inc Canada; Denso Corp
Current assignee: Novelis Inc Canada; Denso Corp; Denso International America Inc
Priority date: 2013-08-08
Filing date: 2014-08-07
Publication date: 2015-02-12
Also published as: JP6673826B2; EP3030685B1; CN105593391A; EP3030685A1; BR112016002328A2; CA2919662C; MX2016001558A; KR20160042055A; CN110512124A; KR101988704B1; ES2779052T3; CA2919662A1; WO2015021244A1; JP2016531204A

Abstract

The present invention provides an aluminum alloy fin stock material with higher strength, and improved sag resistance for use in heat exchangers, such as automotive heat exchangers. The aluminum alloy fin stock material is produced from an aluminum alloy comprising about 0.8-1.4 wt % Si, 0.4-0.8 wt % Fe, 0.05-0.4 wt % Cu, 1.2-1.7 wt % Mn and 1.20-2.3 wt % Zn, with the remainder as Al. The aluminum alloy fin stock material is made by a process comprising direct chill casting the aluminum alloy into an ingot, preheating the ingot, hot rolling the preheated ingot, cold rolling the ingot and inter-annealing at a temperature of 275-400° C. After inter-annealing, the aluminum alloy fin stock material is a cold rolled in a final cold rolling step to achieve % cold work (% CW) of 20-35%.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. provisional patent application Ser. No. 61/863,572 filed Aug. 8, 2013, and U.S. provisional patent application Ser. No. 61/863,568 filed Aug. 8, 2013, which are both incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to the fields of material science, material chemistry, metallurgy, aluminum alloys, aluminum fabrication, and related fields. The present invention provides novel aluminum alloys for use in the production of heat exchanger fins, which are, in turn, employed in various heat exchanger devices, for example, motor vehicle radiators, condensers, evaporators and related devices.

BACKGROUND

There is a need for aluminum alloy fin stock material with high strength, for use in various heat exchanger applications, including radiators for automobiles. There is also a need to obtain aluminum alloy fin stock material with strong pre-braze mechanical properties, good behavior during brazing, i.e., enhanced brazed material sag resistance, and reduced fin erosion, as well as good strength and conductivity characteristics post-braze, for use in high performance heat exchanger applications.

SUMMARY

The present invention provides an aluminum alloy fin stock material for use in heat exchanger applications, such as automotive heat exchangers. This aluminum alloy fin stock alloy material was made by direct chill (DC) casting. The aluminum alloy fin stock material according to the embodiments of the present invention has one or more of the following properties: high strength, desirable post-braze mechanical properties, desirable sag resistance, desirable corrosion resistance and desirable conductivity. The aluminum alloy fin stock material according to some embodiments of the present invention displays larger grain dispersoids and improved strength before brazing. Some embodiments of the aluminum alloy fin stock material are produced in a desirable pre-braze temper, for example, H14.
The improved aluminum alloy fin stock material can be used in various applications, for example, heat exchangers. In one embodiment, the aluminum alloy fin stock material can be used in automotive heat exchangers, such as radiators, condensers and evaporators. In some embodiments, the aluminum alloy fin stock material is useful for high performance, light weight automotive heat exchangers. In some other embodiments, aluminum alloy fin stock material can be used for other brazed applications, including, but not limited to, HVAC applications. Other objects and advantages of the invention will be apparent from the following detailed description of the embodiments of the invention.

DESCRIPTION

The present invention provides an aluminum alloy fin stock material. This aluminum alloy fin stock alloy material was made by direct chill (DC) casting. Some embodiments of the aluminum alloy fin stock material have one or more of improved strength, improved corrosion resistance or improved sag resistance. In some embodiments, the aluminum alloy fin stock material exhibits desirable pre-braze (H14) temper mechanical properties and desirable post-braze mechanical properties, sag resistance, corrosion resistance and conductivity. In some other embodiments, the aluminum alloy fin stock material displays larger grain size after brazing and improved strength pre-brazing. The aluminum alloy fin stock material can be used in various applications, for example, heat exchangers. In one example, the aluminum alloy fin stock material can be used in automotive heat exchangers, such as radiators, condensers and evaporators.
Compositions of an aluminum alloy fin stock material fall within the scope of the present invention. Some exemplary embodiments of the aluminum alloy fin stock material compositions are described below. All % values used below and throughout this document in reference to the amounts of constituents of the aluminum alloy fin stock material compositions are in weight % (wt %).
In one embodiment, the aluminum alloy fin stock material comprises about 0.8-1.4% Si, 0.4-0.8% Fe, 0.05-0.4% Cu, 1.2-1.7% Mn and 1.2-2.3% Zn, remainder aluminum.
In another embodiment, aluminum alloy fin stock material comprises about 0.9-1.3% Si, 0.45-0.75% Fe, 0.10-0.30% Cu, 1.3-1.7% Mn and 1.30-2.2% Zn, remainder aluminum.
In yet another embodiment, the aluminum alloy fin stock material comprises about 0.9-1.2% Si, 0.50-0.75% Fe, 0.15-0.30% Cu, 1.4-1.6% Mn and 1.4-2.1% Zn, remainder aluminum.
In one embodiment, the DC fin stock material comprises 0.9-1.1% Si, 0.10-0.25% Cu, 0.45-0.7% Fe, 1.4-1.6% Mn, and 1.4-1.7% Zn with the remainder Al.
In yet another embodiment, the aluminum alloy fin stock material comprises 0.90-1.0% Si, 0.15-0.25% Cu, 0.5-0.6% Fe, 1.5-1.6% Mn, and 1.5-1.6% Zn, remainder Al.
In yet another embodiment, the aluminum alloy fin stock material comprises 0.9-1% Si, 0.2% Cu, 0.5-0.6% Fe, 1.5-1.6% Mn, and 1.5-1.6% Zn, remainder Al.
In yet another embodiment, the aluminum alloy fin stock material comprises 0.9-0.95% Si, 0.2% Cu, 0.5-0.6% Fe, 1.5-1.6% Mn, and 1.5-1.6% Zn, remainder Al.
In another embodiment, the aluminum alloy fin stock material comprises 0.90-0.95% Si, 0.15-0.20% Cu, 0.55% Fe, 1.5% Mn, and 1.5% Zn, remainder Al.
In yet another embodiment, the aluminum alloy fin stock material comprises 0.95% Si, 0.15% Cu, 0.55% Fe, 1.5% Mn, and 1.5% Zn, remainder Al.
In yet another embodiment, the aluminum alloy fin stock material comprises 0.90-0.95% Si, 0.15-0.20% Cu, 0.5-0.6% Fe, 1.5% Mn and 1.5% Zn, remainder Al.
In yet another embodiment, the aluminum alloy fin stock material comprises 1.0-1.2% Si, 0.2-0.3% Cu, 0.5-0.6% Fe, 1.4-1.55% Mn, and 1.9-2.1% Zn, remainder Al.
In yet another embodiment, the aluminum alloy fin stock material comprises 0.95%±0.05 Si, 0.2%±0.05 Cu, 0.6%±0.1 Fe, 1.45%±0.05 Mn, and 1.55%±0.1 Zn, remainder Al.
In one more embodiment, the aluminum alloy fin stock material comprises 1.15%±0.05 Si, 0.25%±0.05 Cu, 0.6%±0.1 Fe, 1.5%±0.05 Mn, and 2.0%±0.1 Zn, remainder Al.
Optionally, Cr and/or Zr or other grain size controlling elements may be present in the aluminum alloy fin stock material compositions in an amount of up to 0.2% each, up to 0.15% each, up to 0.1%, each, up to 0.05% each, or up to 0.03% each. It is to be understood that the aluminum alloy fin stock material compositions described herein may contain other minor elements, sometimes referred to as unintentional elements, in an amount typically below 0.05%.
Some embodiments of the aluminum alloy fin stock materials of the present invention display a higher solidus temperature, referred to as onset of melting, leading to improved core shrinkage, a phenomenon in which brazed aluminum alloy units do not have the desired shape. While not wanting to be bound by the following statement, it is believed, based on differential scanning calorimetry (DSC) measurements and ThermoCalc® software (Stockholm, Sweden) simulations, that lowering the Si content and the Zn content and increasing the Mn content in aluminum alloy fin stock material compositions can lead to higher onset of melting temperature (solidus), which contributes to core shrinkage reduction. In one example, an aluminum alloy fin stock material composition according to the embodiments of the present invention displays a solidus temperature above 617° C. and a coarse post braze grain size of about 400 gm. In one more example, limiting the Si content of the alloy to 0.9-1% (preferably to 0.9-0.95%) and the Zn content to 1.5-1.6%, while maintaining the Mn content relatively high (for example, around 1.5%) raises the solidus temperature of the alloy, which, in turn, strengthens the material at the brazing temperature, so that it can resist sag or high temperature creep that can result in core shrinkage.
Some embodiments of the present invention relate to aluminum alloy fin stock materials having a defined composition and obtained by processes that include defined process steps and conditions. A combination of defined composition and production process can lead to improved properties of the aluminum alloy fin stock materials. One example of such improved properties are improved pre-braze mechanical properties. Improved pre-braze mechanical properties (also referred to as properties “in pre-braze condition”) result in improved fin crush resistance during assembly, while maintaining suitable sag resistance and thermal conductivity after brazing (post-brazing).
The processes of producing aluminum alloy fin stock materials according to embodiments of the present invention involve the step of producing an ingot by a direct chill (DC) casting process, which is commonly used throughout the aluminum industry, whereby a large ingot ˜1.5 m×0.6 m×4 m is cast from a large holding furnace which supplies metal to a shallow mold or molds supplied with cooling water. The solidifying ingot is continuously cooled by the direct impingement of the cooling water and is withdrawn slowly from the base of the mold until the full ingot or ingots are completed. Once cooled from the casting process, the ingot rolling surfaces are machined to remove surface segregation and irregularities. The machined ingot is preheated for hot rolling. The preheating temperature and duration are controlled to low levels to preserve a large grain size and high strength after the finished fin stock is brazed. Several ingots (about 8 to 30) are charged to a furnace and preheated with gas or electricity to the rolling temperature. The period of maintaining a temperature achieved by pre-heating can also be referred to as “soak” or “soaking. In one embodiment, the minimum soak time at about 480° C. is about 2 hours (in other words, at least 2 hours). In another embodiment, the soak time is 4-16 hours at 480° C. Aluminum alloys are typically rolled in the range of about 450° C. to about 560° C. If the temperature is too cold, the roll loads are too high, and if the temperature is too hot, the metal may be too soft and break up in the mill.
The processes for making some embodiments of the aluminum alloy fin stock materials, may involve one or more cold rolling steps. Each of the cold rolling steps may, in turn, involve multiple cold rolling passes. A cold rolling step characterized by “% cold work” or % CW achieved. Generally, % CW can be defined as the degree of cold rolling applied to the aluminum alloy fin stock. As used in the present document, % CW is calculated as:
$% CW = \frac{initial gauge - final gague}{initial gauge} * 100 %$
Achieving a specified range or value of % CW may be desirable in order to attain the required strength range of the aluminum alloy fin stock material. Some embodiments of the of the aluminum alloy fin stock materials are produced by processes that involve a cold rolling step achieving 25-35% CW. In some examples, a cold rolling step achieving % CW of 25% or 29% may be employed. In some cases, increasing % CW, for example, to 35% leads to an increase in pre-braze tensile strength of the aluminum alloy fin stock material, which, in turn, beneficially reduces the fin crush during radiator assembly. In some other cases, increasing the % CW, however, may be undesirable, as it may lead to finer post braze grain size due to an increase in the driving force for recrystallization, resulting in reduced sag resistance.
The processes for making some embodiments of the aluminum alloy fin stock materials may also involve an inter-annealing step to attain desired properties of the aluminum alloy fin stock material according to the embodiments of the present invention. The term “inter-annealing” or “inter-anneal” (IA) refers to a heat treatment applied between cold rolling steps. IA temperature may affect the properties of the aluminum alloy fin stock materials according to the embodiments of the present invention. For example, an investigation of the IA temperature used in the processes for making certain embodiments of the aluminum alloy fin stock materials showed that reducing the IA temperature from 400° C. to 350° C. resulted in coarser post-braze grain size. In some embodiments of the aluminum alloy fin stock materials, a combination of % CW and IA temperature employed in the production process results in desirable properties. In one example, a combination of IA temperature of 350° C. and % CW of 35% led to beneficial combination of post-braze grain size and sag resistance the aluminum alloy fin stock material. In another example, a combination of IA temperature of 300° C. and % CW of 25% led to beneficial combination of post-braze grain size and sag resistance the aluminum alloy fin stock material. In another example, a combination of IA temperature and % CW during processing of the aluminum alloy fin stock material in H14 temper resulted in improved fin crush resistance. Accordingly, the processes of producing aluminum alloy fin stock materials employing specified IA temperature and % CW, which lead, in some examples, to higher pre-braze tensile strength and improved fin crush resistance during assembly, are included within the embodiments of the present invention.
Once preheated, the ingot is hot rolled to form a coil which is then cold rolled. The cold rolling process takes place in several steps, and a step of inter-annealing is employed between cold-rolling steps to recrystallize the material prior to the final cold rolling step. IA temperature in the range of about 275-400° C., 300-400° C., 300-450° C., 340-460° C., or 325-375° C. may be employed. For example, IA temperature of about 300° C., 350° C. or 400° C. may be employed in the processes of producing aluminum alloy fin stock materials according to embodiments of the present invention. After inter-annealing, the aluminum alloy fin stock material is cold rolled in the final cold rolling step to obtain the desired final gauge or thickness. After the final cold rolling step, the aluminum alloy fin stock material can be slit into narrow strips suitable for the manufacture of radiators and other automotive heat exchangers. In some embodiments the processes of producing aluminum alloy fin stock materials according to embodiments of the present invention, % CW employed in the final cold rolling step is 20-35% or 25-35%, for example, about 25% or 29%.
Various combinations of production parameters may be beneficially employed in the processes for processes of producing aluminum alloy fin stock materials according to embodiments of the present invention. In one example, % CW in the range 25-35% is employed in the final rolling step, resulting in improved pre-braze yield strength and tensile strength of the aluminum alloy fin stock materials, which, in turn, leads to reduction in the fin crush occurrence during assembly. In another example, selecting IA temperature of about 350° C. results in larger post-braze grain size. In one more example, using % CW of about 29% during the final cold rolling step further increases post-braze grain size. In yet another example, inter-annealing at 350° C. for 4 hours is employed in combination with 29% CW in the final cold rolling step, which results in a material with desirable characteristics of good pre-braze strength and large post-braze grain size, high thermal conductivity and good sag behavior. In yet another example, inter-annealing at 400° C. for an average of about 3 hours is employed, followed by applying % cold work (CW) of about 29% to achieve final gauge. In yet another example, soaking at about 480° C. for an average of 4 hours is employed during the hot-rolling step, in combination with interannealing at about 300-400° C. and % CW in the final cold-rolling step of about 25-35% to final gauge. In yet another example, soaking at 480° C. for 4-16 hours in hot rolling step is employed in combination with interannealing at 350° C. and % CW of 29% in the final rolling step. In yet another example, soaking at 480° C. for 4-16 hours in hot rolling step is employed in combination with interannealing at 400° C. and % CW of 29% in the final rolling step. In one more example, soaking at 480° C. for an average of 4 hours in hot rolling step is employed in combination with interannealing at of 350° C. and % CW of 35% in the final rolling step. In one more example, inter-annealing at 325-375° C. and 20-35% CW, such as interannealing at 300° C. and CW 25% in the final cold rolling step is employed.
The aluminum alloy fin stock materials produced according to some embodiments of the present invention are produced as sheets varying in gauge (thickness) between 45 μm and 80 μm. The aluminum alloy fin stock material according to the embodiments of the present invention has one or more of the following properties: minimum ultimate tensile strength (UTS) of 130 MPa (in other words, 130 MPa or more, or at least 130 MPa) measured post-brazing (for example, 134 or 137 MPa); average conductivity value of about 43%, about 41.5%, about 42.7% or about 43.3% (International Annealed Copper Standard (IACS)); an open circuit potential corrosion value vs. Standard Calomel Electrode (SCE) of −680mV or less, −700 mV or less or −740 or less (for example, −710 mv, −720 mv, −724 mv, −725 mv, −743 mv, −740mV or −758 mV); a sag value between 7 mm, where the final gauge was 47.5 μm, and 5 mm, where the final gauge was 50 μm, with a cantilevered length of 35 mm. The above properties of aluminum alloy fin stock material sheets are measured after applying a faster braze cycle, whereby the material is heated to a temperature of 605° C. and cooled to room temperature in a period of about 20 minutes, to simulate the temperature time profile of a commercial brazing process. The aluminum alloy fin stock material according to the embodiments of the present invention can have UTS pre-brazing in the range of 180-220 MPa (for example, 185 or 190 MPa). The aluminum alloy fin stock material according to the embodiments of the present invention can also have grain size >200 μm for example, 200 or 400 μm
The following examples will serve to further illustrate the present invention without, at the same time, however, constituting any limitation thereof. On the contrary, it is to be clearly understood that resort may be had to various embodiments, modifications and equivalents thereof which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the invention.

EXAMPLE 1

An aluminum alloy fin stock material was made by a process that involved DC casting, preheating the ingot to 480° C. for about 8 hours, followed by hot rolling to about 2.5 mm, cold rolling, and inter-annealing at 350° C. for about 2 hours prior to final cold rolling step. The composition range of the aluminum alloy fin stock material was within the following specification: 1.1±0.1% Si, 0.6±0.1% Fe, 0.2±0.05% Cu, 1.4±0.1% Mn and 1.50±0.1% Zn, with the remainder Al. The aluminum alloy fin stock material produced varied in gauge between 49 and 83 μm. The aluminum alloy fin stock material had a minimum ultimate tensile strength of ˜130 MPa. The aluminum alloy fin stock material had an average conductivity after brazing of ˜43 IACS and an open circuit potential corrosion value vs. SCE of −741 mV. These values were measured after applying a simulated brazing cycle, whereby the sample was heated to a temperature of 605° C. and cooled to room temperature in a period of about 20 minutes to simulate the temperature time profile of a commercial brazing process.

EXAMPLE 2

Two samples of aluminum alloy fin stock material were made by a process that involved DC casting, followed by hot rolling with pre-heating at 480° C. for 4-16 hours, cold rolling, and inter-annealing at 350° C. for the first sample and at 400° C. for the second sample, prior to final cold rolling to 29% % CW . The composition of the first sample was: 0.95% Si, 0.6% Fe, 0.2% Cu, 1.45% Mn and 1.55% Zn, with the remainder Al. The composition of the second sample was: 1.15% Si, 0.6% Fe, 0.25% Cu, 1.5% Mn and 2% Zn, with the remainder Al. The aluminum alloy fin stock material had a post-braze ultimate tensile strength of ˜134 MPa for the first sample and ˜137 MPa for the second sample. The aluminum alloy fin stock material had an average conductivity after brazing of ˜42.7 IACS for the first sample and ˜43.3 IACS for the second sample. The aluminum alloy fin stock material had an open circuit potential corrosion value vs. SCE of −710 mV for the first sample and −743 mV for the second sample. The aluminum alloy fin stock material had a grain size of 400 μm for the first sample and 200 μm for the second sample. The aluminum alloy fin stock material exhibited pre-braze UTS of 185 MPa for the first sample and 190 MPa for the second sample. The comparison between the two samples revealed that both samples produced attractive mechanical properties, but the open circuit potential corrosion value of the first sample was lower, indicating that increase in Zn content may be desirable. The second sample had advantageously lower open circuit potential corrosion value.
All patents, patent applications, publications, and abstracts cited above are incorporated herein by reference in their entirety. Various embodiments of the invention have been described in fulfillment of the various objectives of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those of skill in the art without departing from the spirit and scope of the invention as defined in the following claims.

Claims

1. An aluminum alloy comprising about 0.8-1.4 wt % Si, 0.4-0.8 wt % Fe, 0.05-0.4 wt % Cu, 1.2-1.7 wt % Mn and 1.20-2.3 wt % Zn, with the remainder as Al.

2. The aluminum alloy of claim 1, comprising about 0.9-1.3 wt % Si, 0.45-0.75 wt % Fe, 0.10-0.3 wt % Cu, 1.3-1.7 wt % Mn and 1.30-2.2 wt % Zn, with the remainder as Al.

3. The aluminum alloy of claim 1, comprising about 0.9-1.2 wt % Si, 0.5-0.75 wt % Fe, 0.15-0.3 wt % Cu, 1.4-1.6 wt % Mn and 1.4-2.1 wt % Zn, with the remainder as Al.

4. The aluminum alloy of claim 1, comprising about 0.9-1.1 wt % Si, 0.5-0.6 wt % Fe, 0.15-0.25 wt % Cu, 1.5-1.6 wt % Mn and 1.5-1.6 wt % Zn, with the remainder as Al.

5. The aluminum alloy of claim 1, comprising about 0.90-1.0 wt % Si, 0.55 wt % Fe, 0.15-0.20 wt % Cu, 1.5 wt % Mn and 1.5 wt % Zn, and with the remainder as Al.

6. The aluminum alloy of claim 5, comprising about 0.95 wt % Si and 0.15 wt % Cu.

7. The aluminum alloy of claim 1, comprising about 1.0-1.2 wt % Si, 0.5-0.6 wt % Fe, 0.2-0.3 wt % Cu, 1.4-1.55 wt % Mn and 1.9-2.1 wt % Zn, with the remainder as Al.

8. The aluminum alloy of claim 1, comprising about 0.95% Si, 0.2% Cu, 0.6% Fe, 1.45% Mn, and 1.55% Zn, remainder Al.

9. The aluminum alloy of claim 1, comprising about 1.15 wt % Si, 0.25 wt % Cu, 0.6 wt % Fe, 1.5 wt % Mn, and 2.0 wt % Zn, remainder Al.

10. The aluminum alloy of claim 1, further comprising up to 0.2% each of one or both of Cr or Zr.

11. An aluminum alloy fin stock material produced from the aluminum alloy of claim 1 by a process, comprising:

direct chill casting the aluminum alloy into an ingot;

preheating the ingot to 450 to 500° C. for 2 to 16 hours;

hot rolling the preheated ingot;

cold rolling the ingot;

inter-annealing at a temperature of 275-400° C.;

after inter-annealing, performing a final cold rolling step to achieve % cold work (% CW) of 20 to 35%.

12. The aluminum alloy fin stock material of claim 11, wherein the ingot is preheated at 480° C. for 2-16 hours.

13. The aluminum alloy fin stock material of claim 11, wherein the ingot is preheated for 2-12 hours.

14. The aluminum alloy fin stock material of claim 11, wherein the interannealing temperature is 325 to 375° C.

15. The aluminum alloy fin stock material of claim 11, wherein the interannealing temperature is 300, 350 or 400° C.

16. The aluminum alloy fin stock material of claim 11, wherein the interannealing temperature is 300° C. and % CW is about 25%.

17. The aluminum alloy fin stock material of claim 11, wherein the interannealing temperature is 350° C. or 400° C. and % CW is about 29%.

18. The aluminum alloy fin stock material of claim 11, wherein the gauge achieved in the final cold rolling step is between 45 μm and 80 μm.

19. The aluminum alloy fin stock material of claim 18, having a minimum ultimate tensile strength of about 130 MPa, measured post-brazing.

20. The aluminum alloy fin stock material of claim 18, having an ultimate tensile strength of about 137 MPa, measured post-brazing.

21. The aluminum alloy fin stock material of claim 18, having a corrosion potential of −680 mV or lower vs. standard calomel electrode (SCE).

22. The aluminum alloy fin stock material of claim 18, having an electrical conductivity of about 41.5% or 43.3% International Annealed Copper Standard (IACS).

23. A heat exchanger comprising the aluminum alloy fin stock material of claim 11.

24. The heat exchanger of claim 23, wherein the heat exchanger is an automotive heat exchanger.

25. The heat exchanger of claim 23, wherein the heat exchanger is a radiator, a condenser or an evaporator

26. A process for making an aluminum alloy fin stock material, comprising direct chill casting the aluminum alloy of claim 1 into an ingot;

preheating the ingot to 450-500 ° C. for 2 to 16 hours;

hot rolling the preheated ingot;

cold rolling the ingot;

inter-annealing at a temperature of 275 to 400° C.; and,

after inter-annealing, performing a final cold rolling step to achieve % cold work (% CW) of 20-35%.