METHOD FOR PRODUCING A SHEET METAL OR STRIP AND A SHEET METAL OR STRIP PRODUCED USING SAME

Abstract

A method for producing a sheet metal or strip and a sheet metal or strip produced using this method are described. For a comparatively high gloss level and a comparatively high durability, it is proposed for the sheet metal or strip made of an aluminum alloy of the EN AW-5xxx type to have a cold-rolled structure having a grain structure that is elongated in the rolling direction, with grains whose grain aspect ratio AIl=ll(0°)/ll(90°) is ≥10, more particularly ≥15, measured according to ASTM E112-13.

Description

TECHNICAL FIELD

The invention relates to a method for producing a sheet metal or strip and a sheet metal or strip produced using same.

PRIOR ART

It is known that in order to produce glossy parts from an anodized or electrolytically oxidized sheet metal or strip made of a 5xxx aluminum alloy, it is necessary to reduce optical scattering centers, for example foreign phases in the oxide layer, so that the directional light reflection responsible for the gloss level can take place unimpaired. This applies more particularly to those scattering centers whose size is comparable to or greater than the wavelength of visible light (400 to 750 nm).

For this reason, EP3011067B1 proposes for fine Mg₂Si precipitates that are produced during the manufacturing process to be reduced by means of an intermediate annealing with subsequent cooling during the cold rolling. This intermediate annealing takes place between the solvus temperature of the precipitates and the melting temperature of the aluminum alloy.

Disadvantageously, however, this method has no influence on scattering centers composed of Fe-containing precipitates, which reduce the gloss level of the sheet metal or strip due to their size and nonhomogeneous distribution in the structure. In addition, a sheet metal or strip that is heat-treated in this way is comparatively susceptible to corrosion, which has an adverse impact on the durability of the gloss level.

SUMMARY OF THE INVENTION

The object of the invention, therefore, is to modify the method of the type described at the beginning in a way that further improves both the gloss level and the durability thereof in a sheet metal or strip made of a 5xxx aluminum alloy.

By carrying out the intermediate annealing according to claim 1 (with a first holding temperature≥the solvus temperature of Mg-containing precipitates, more particularly of the Mg₂Si type, in the aluminum alloy, and with the first holding temperature<the melting temperature of the aluminum alloy) and with subsequent quenching of the intermediate annealed strip or sheet metal between the hot rolling and cold rolling, it is possible, by contrast with the prior art, to act on Mg-containing precipitates even before the cold rolling. Preferably, the quenching is carried out at a temperature below the precipitation temperature of Mg-containing precipitates. For example, the quenching is carried out at an average cooling rate of at least 10° C./s, for example at least 50° C./s.

If the subsequent cold rolling is carried out with a cold rolling degree of ≥70% from the hot strip thickness to the final thickness of the sheet metal or strip (which is also referred to as a metal strip or sheet metal strip), this leads to improved fragmentation of those precipitates that cannot be dissolved by the intermediate annealing, more particularly Fe-containing precipitates. Compared to the prior art, such precipitates are therefore more homogeneously present in the aluminum matrix and are less grouped so that it is also possible to significantly reduce their negative influence on the gloss level.

In addition, the grains of the aluminum matrix are considerably elongated, which leads to a higher corrosion resistance due to an extended path length for intercrystalline corrosion—this in turn can have a positive effect on the durability of the gloss level.

According to the invention, the intermediate annealing in combination with the comparatively high cold rolling degree results in an improved and more durable gloss level and an improved corrosion resistance of the sheet metal or strip.

The above can be further improved if the cold rolling is carried out with a cold rolling degree of ≥75% from the hot strip thickness to the final thickness.

According to the invention, this is provided in several roll passes and can optionally include a pickling step as an intermediate step between these roll passes.

In addition, the method according to the invention comprises a final annealing of the cold-rolled sheet metal or strip with a second holding temperature<the recrystallization temperature of the aluminum alloy. This means that the cold-rolled structure has undergone final annealing and is in a non-recrystallized state. The grain structure is elongated in the rolling direction. The sheet metal or strip that is produced in this way is in the H2X state, for example.

Preferably, the first holding temperature is in the range from ≥400° C. to ≤600° C. so that essentially all Mg-containing precipitates in the aluminum alloy are dissolved. This is more particularly achievable if the first holding temperature of the intermediate annealing is in the range from 450° C. to 560° C.

It can prove advantageous for the above dissolution of Mg-containing precipitates if the intermediate annealing has a first holding time in the range from >2 s (seconds) to 6 min (minutes). This range of the first holding time can, among other things, prevent the formation of coarse grain due to excessive grain growth and thus can also prevent the occurrence of orange peel during the forming or bending of the sheet metal or strip.

The second holding temperature, i.e. the holding temperature of the final annealing, is, for example, in the range below the recrystallization temperature of the aluminum. The defect density or dislocation density in the aluminum matrix introduced by the cold rolling is reduced mainly by means of recovery processes while avoiding the formation of new grains. This makes it possible to adjust the required mechanical properties while retaining the elongated grain structure that is produced during the cold rolling and while increasing corrosion resistance. This is more particularly the case if the second holding temperature is in the range from 200 to 280° C., preferably in the range from 200° C. to 250° C.

The extent of the reduction in the defect density introduced by the cold rolling depends on the level of the thermal load introduced by the final annealing, for example due not only to the second holding temperature but also to the second holding time, i.e. the holding time of the final annealing. For example, the second holding time should be selected to be ≥6 h and/or ≤30 h in order to homogenize the mechanical characteristics over the entire length of the strip. Preferably, the second holding time is in the range from 8 to 24 h (hours).

If the final annealing of the sheet metal or strip is carried out under inert gas, for example, this can prevent a reaction of magnesium with oxygen from the environment and thus prevent a clouding of the surface. The gloss level of the sheet metal or strip can thus be further increased and/or made more reproducible.

Preferably, the hot rolling is carried out with a hot rolling degree of 95% or greater from the initial thickness to the hot strip thickness. This can subsequently provide a sufficiently high level of cold work hardening to ensure an elongated grain and to fragment Fe-containing phases. In this way, it is possible to further improve the gloss level as well as the corrosion resistance and thus the durability of the gloss level.

A sufficiently high hot strip thickness for the possibility of a comparatively high degree of forming during the cold rolling can be achieved, for example, if the sheet metal or strip is hot rolled to a hot strip thickness in the range from 5 to 12 mm, for example from 6 to 12 mm, or more particularly from 6 to 10 mm.

During the cold rolling, the sheet metal or strip is preferably rolled in rolling passes with a decreasing roll roughness of the rolls with each rolling pass. This allows, for example, the surface roughness of the strip or sheet metal to be further reduced, which according to the prior art, also has a positive effect on the gloss of the untreated aluminum surface.

Preferably, the sheet metal or strip is cold-rolled to a final thickness in the range from 0.5 to 1.5 mm (millimeters), more particularly to 0.8 to 1.3 mm. This thickness range is particularly suitable for applications in components in the decorative sector whose geometric design requires high degrees of forming and small bending radii.

The hot rolling can be further facilitated if, for example, the rolling ingot is kept at a heating temperature ≥400° C. for at least one hour before the hot rolling. More particularly, the heating temperature is in the range from 400° C. to 470° C. A heating temperature of 400° C. is a minimum temperature at which the hot rolling should take place since recrystallization processes become possible at this temperature and a reduction in ingot thickness can be facilitated. Since the heating temperature is ≤470° C., a coarsening of phases in the temperature range close to the melting temperature of the aluminum alloy is essentially prevented.

Another object of the invention is to further improve the gloss level and the durability thereof in a sheet metal or strip made of a 5xxx aluminum alloy of the type described at the beginning.

According to the invention, the sheet metal or strip is composed of an aluminum alloy of the EN AW-5xxx type with a cold-rolled structure that has a grain structure that is elongated in the rolling direction, with grains whose grain aspect ratio AI_l=l_l(0°)/l_l(90°)is ≥10, measured according to ASTM E112-13. Preferably, AI_l=l_l(0°)/l_l(90°) is ≥15.

Among other things, this significantly increases the path length for intercrystalline corrosion. It can therefore be assumed that the corrosion resistance and the durability of the gloss level are increased. This means that the sheet metal (often referred to as metal sheet) or strip (often referred to as metal strip) produced using the method according to the invention, with its cold-rolled and final annealed structure, can have an increased gloss level and increased durability.

In this connection, it can also be advantageous if l_l(90°) is <20 μm, more particularly <17 μm.

Preferably, the structure is not recrystallized in the H2X state and therefore has no recrystallized components. This is ensured, for example, by the final annealing of the cold-rolled sheet metal or strip with a second holding temperature<the recrystallization temperature of the aluminum alloy. This preserves the cold-rolled structure with the grain structure that is elongated in the rolling direction.

Preferably, the surface of the strip or sheet metal has a gloss level of ≥50 GU, measured according to ÖNORM EN ISO 7668 at an angle of 20° and transverse to the rolling direction. Preferably, this value is ≥55 GU. For example, a measuring device called the “micro-TRI-gloss S” made by the company BYK-Gardner GmbH can be used for this measurement.

Preferably, the sheet metal or strip has a surface layer that is polished or is polished and anodized, wherein the surface of the strip or sheet metal has a gloss level of ≥60 GU, measured according to ÖNORM EN ISO 7668 at an angle of 20° and transverse to the rolling direction.

Preferably, the aluminum alloy of the strip or sheet metal contains

• • from 0.50 to 1.1 wt % magnesium (Mg),
  • from 0.01 to 0.30 wt % silicon (Si),
  • from 0.01 to 0.7 wt % Iron (Fe)
  • and residual aluminum as well as inevitable production-related impurities, each totaling a maximum of 0.05 wt % and all together totaling at most 0.15 wt %.

Optionally, the aluminum alloy can also contain elements from the following group, either individually or in combination:

• • up to 0.20 wt % manganese (Mn)
  • up to 0.20 wt % copper (Cu)
  • up to 0.10 wt % chromium (Cr)
  • up to 0.25 wt % zinc (Zn)
  • up to 0.01 wt % titanium (Ti)
  • up to 0.03 wt % gallium (Ga)
  • up to 0.05 wt % vanadium (V).

DETAILED DESCRIPTION OF THE INVENTION

To demonstrate the effects achieved, several strips (also called metal strips or sheet metal strips) A, B, C, and D with a final thickness of 1.2 mm were produced. Strips A, B, C, and D are of the EN AW-5xxx type with the following chemical composition in wt %: Mg: 0.8%, Si: 0.03%, Fe: 0.03%, and residual aluminum as well as inevitable production-related impurities with a maximum of 0.05 wt % each and all together totaling at most 0.15 wt %.

Production sequence A: The strip A according to the invention was subjected to the following method steps in the indicated order:

• • a. Heating of the aluminum alloy rolling ingot to a temperature of 420° C. for 9 hours;
  • b. Hot rolling of the rolling ingot in several hot-rolling passes to a hot-rolled strip with a hot rolling degree of 98% and a hot strip thickness of 8 mm;
  • c. Intermediate annealing of the hot-rolled strip in a continuous strip furnace with a first holding temperature of 480° C. (≥the solvus temperature of Mg-containing precipitates) with a holding time of 3 min.
  • d. Quenching of the intermediate annealed strip with water to a temperature <100° C. at a cooling rate of 50° C./s.
  • e. Cold rolling of the quenched strip in several rolling passes with a pickling step as an intermediate step and with decreasing roll roughness and with a cold rolling degree of 85% to a final thickness of 1.2 mm.
  • f. Final annealing of the cold-rolled strip with a second holding temperature of 240° C. (i.e. <the recrystallization temperature of the aluminum alloy) in a coil furnace with a second holding time of 12 h.
  • g. Cooling of the final annealed strip to room temperature in still air.

Production sequence B: The strip B according to the invention was subjected to the following method steps in the indicated order:

• • a. Heating of the aluminum alloy rolling ingot to a temperature of 420° C. for 9 hours.
  • b. Hot rolling of the rolling ingot in several hot-rolling passes to a hot-rolled strip with a hot rolling degree of 98% and a hot strip thickness of 8 mm.
  • c. Intermediate annealing of the hot-rolled strip in a continuous strip furnace with a first holding temperature of 480° C. (≥the solvus temperature of Mg-containing precipitates) with a holding time of 3 min.
  • d. Quenching of the intermediate annealed strip with water to a temperature <100° C. at a cooling rate of 50° C./s.
  • e. Cold rolling of the quenched strip in several rolling passes with a pickling step as an intermediate step and with decreasing roll roughness, wherein a rougher roll is used in the last cold rolling pass by contrast with strip A according to the invention, and with a cold rolling degree of 85% to a final thickness of 1.2 mm.
  • f. Final annealing of the cold-rolled strip with a second holding temperature of 240° C. in a coil furnace with a second holding time of 12 hours.
  • g. Cooling of the final annealed strip to room temperature in still air.

Production sequence C: The strip C not according to the invention was produced with the following method steps:

• • a. Heating of the aluminum alloy rolling ingot to a heating temperature of 420° C. for 9 hours.
  • b. Hot rolling of the rolling ingot in several hot-rolling passes to a hot-rolled strip with a hot rolling degree of 98% and a hot strip thickness of 9 mm.
  • c. Cold rolling of the quenched strip in several rolling passes with a pickling step as an intermediate step, with decreasing roll roughness and with a cold rolling degree of 87% to a final thickness of 1.2 mm
  • d. Final annealing of the cold-rolled strip with a second holding temperature of 245° C. in a coil furnace with a second holding time of 12 hours.
  • e. Cooling of the final annealed strip to room temperature in still air.

Production sequence D: The strip D not according to the invention was produced with the following method steps:

• • a. Heating of the aluminum alloy rolling ingot to a temperature of 420° C. for 9 hours.
  • b. Hot rolling of the rolling ingot in several hot-rolling passes to a hot-rolled strip with a hot rolling degree of 98% and a hot strip thickness of 9 mm.
  • c. Cold rolling of the quenched strip in several cold rolling passes with a pickling step as an intermediate step and with a cold rolling degree of 82% to a material thickness of 1.6 mm.
  • d. Intermediate annealing of the cold-rolled strip in a continuous strip furnace with a first holding temperature of 450° C. with a holding time of 2 min and immediate quenching to a temperature <100° C. at a cooling rate of 50° C./s.
  • e. Cold rolling of the quenched strip in several cold rolling passes with a cold rolling degree of 25% to a material thickness of 1.2 mm.
  • f. Final annealing of the cold-rolled strip in a coil furnace with a second holding temperature of 240° C. and a holding time of 12 hours.
  • g. Cooling of the final annealed strip to room temperature in still air.

To compare the gloss levels achieved according to the different production variants A, B, C and D, samples were treated in an acidic brightening bath (consisting of distilled water, sulfuric acid, and phosphoric acid). After subsequent application of an artificial anodizing layer, the gloss levels were compared to one another.

The determined grain elongation and the corresponding gloss levels (measured according to ÖNORM EN ISO 7668 at an angle of 20° and transverse to the rolling direction) of strips A and B, which were treated according to the invention, and strips C and D, which were not treated according to the invention, are listed in Table 1 below.

TABLE 1
Characteristic values of strips A to D
		Grain		Gloss level GU
	Grain	elongation	Gloss level GU	surface
	thickness	ratio	surface untreated	treated
Strip	l l(90°)[μm]	Al	(raw surface)	(polished + anodized)
A	15	20	62	67
B	16	16	57	64
C	32	2	53	62
D	48	2	44	57

As Table 1 illustrates, the strips A and B, which undergo intermediate annealing in the continuous strip furnace following the hot rolling process according to the invention, have a gloss level that is up to 10 GU higher than strips C and D. This is the case for both the untreated strip and the strip that is surface-treated, i.e. (chemically or electrolytically) polished and then anodized.

The positive effect of the intermediate annealing of the hot-rolled strip or sheet metal is illustrated by comparing production variants A and B with C.

In the case of production sequences A and B, the implementation of the intermediate annealing prior to the cold rolling according to the invention can be used to influence the soluble Mg-containing phases. By means of the Mg-containing phases being brought into solution during the intermediate annealing of the hot strip and the associated rapid cooling of the material, the number of scattering centers is minimized in a lasting way. This leads to an increase in the gloss level compared to production variant C, which was carried out without intermediate annealing.

In addition, the influence of a high cold rolling degree of ≥70%, 85% in the exemplary embodiments, is illustrated by directly comparing the samples A, B, and C with sample D. The production sequences according to A, B, and C result in highly elongated grain. This can be seen in Table 1.

The production sequences A, B, and C were cold-rolled with decreasing roll roughness in order to thus reduce the surface roughness. A comparison of the production sequences A and B shows higher gloss levels for sheet metal A than is the case for sheet metal B, for which a rougher roll was used. Sheet metal B according to the invention is nevertheless significantly higher in gloss level than sheet metals C and D that are not according to the invention, sheet metal C of which, for example, was also cold-rolled with a decreasing roll roughness.

In this case, the grain thickness l_(90°) (according to ASTM E112-13) for strips A and B in the H2X state is significantly lower in the structure than that of strips D and C in the H2X state. This difference is also reflected in the grain expansion ratio AI_l=l_l(0°)/l_l(90°) (measured according to ASTM E112-13), which, at over 15, differs significantly from the grain expansion ratio AI_l of strips C and D.

Applying such a cold rolling degree of at least 70% influences not only the corrosion resistance but also the predominantly Fe-containing phases that are present in the material after the intermediate annealing. Fragmentation of these phases reduces the number of scattering centers in the size range comparable to the wavelength of visible light. According to the invention, this results in an increase in the gloss level, as can be seen in strips A and B in comparison with strips C and D, which are not according to the invention, according to Table 1, and has already been explained above.

It should be noted in general that the German expression “insbesondere” can be translated as “more particularly” in English. A feature that is preceded by “more particularly” is to be considered an optional feature, which can be omitted and does not thereby constitute a limitation, for example, of the claims. The same is true for the German expression “vorzugsweise”, which is translated as “preferably” in English.

Claims

1. A method for producing a sheet metal or strip, wherein the method comprises the following steps in the indicated order: Hot rolling a rolling ingot made of an aluminum alloy of EN AW-5xxx type and with an initial thickness to a hot-rolled strip or sheet metal with a hot strip thickness,Intermediate annealing of the hot-rolled strip or sheet metal with a first holding temperature, which is ≥a solvus temperature of Mg-containing precipitates of the aluminum alloy and is<a melting temperature of the aluminum alloy, and subsequent quenching of the intermediate annealed strip or sheet metal,Cold rolling the quenched strip or sheet metal to a final thickness in a plurality of rolling passes, optionally with a pickling step as an intermediate step between the plurality of rolling passes, wherein the cold rolling is carried out with a cold rolling degree of ≥70% of a hot strip final thickness to the final thickness, andFinal annealing of the cold-rolled sheet metal or strip with a second holding temperature<a recrystallization temperature of the aluminum alloy.

2. The method according to claim 1, wherein the first holding temperature is in a range from ≥400° C. to ≤600° C.

3. The method according to claim 1, wherein the intermediate annealing has a first holding time in a range from >2 seconds(s) to 6 minutes (min).

4. The method according to claim 1, wherein the second holding temperature is in a range from 200° C. to 280° C.

5. The method according to claim 1, wherein the final annealing has a second holding time of ≥6 hours (h) and/or ≤30 h.

6. The method according to claim 1, wherein the final annealing of the sheet metal or strip is carried out under inert gas.

7. The method according to claim 1, wherein the hot rolling is carried out with a hot rolling degree of ≥95% or greater from the initial thickness to the hot strip thickness.

8. The method according to claim 1, wherein the sheet metal or strip is hot-rolled to the hot strip thickness in a range from 5 to 12 mm.

9. The method according to claim 1, wherein in the cold rolling, the sheet metal or strip is rolled in rolling passes with decreasing roll roughness of the rolls in each rolling pass.

10. The method according to claim 1, wherein the sheet metal or strip is cold-rolled to the final thickness in a range from 0.5 to 1.5 mm.

11. The method according to claim 1, wherein before the hot rolling, the rolling ingot is kept at a heating temperature ≥400° C. for at least one hour.

12. A sheet metal or strip made of an aluminum alloy of EN AW-5xxx type with a cold-rolled structure that has a grain structure that is elongated in a rolling direction, with grains whose grain aspect ratio AIl=ll(0°)/ll(90°) is ≥10, measured according to ASTM E112-13.

13. The sheet metal or strip according to claim 12, wherein ll(90°) is <20 μm.

14. The sheet metal or strip according to claim 12, wherein the structure is not recrystallized in an H2X state.

15. The sheet metal or strip according to claim 12, wherein a surface of the strip or sheet metal has a gloss level of ≥50 GU, measured according to ÖNORM EN ISO 7668 at an angle of 20° and transverse to the rolling direction.

16. The sheet metal or strip according to claim 12, wherein the sheet metal or strip has a surface layer that is polished or is polished and anodized, and wherein a surface of the strip or sheet metal has a gloss level of ≥60 GU, measured according to ÖNORM EN ISO 7668 at an angle of 20° and transverse to the rolling direction.

17. The sheet metal or strip according to claim 12, wherein the aluminum alloy contains from 0.50 to 1.1 wt % magnesium (Mg),from 0.01 to 0.30 wt % silicon (Si),from 0.01 to 0.7 wt % iron (Fe),

18. The method according to claim 1, wherein the first holding temperature is greater than or equal to a solvus temperature of Mg2Si type precipitates.

19. The method according to claim 1, wherein the cold rolling is carried out with a cold rolling degree of ≥75% of the hot strip final thickness to the final thickness.

20. The sheet metal or strip according to claim 12, wherein the sheet metal or strip is made of an aluminum alloy of the EN AW-5xxx type with a cold-rolled structure that has a grain structure that is elongated in the rolling direction, with grains whose grain aspect ratio AIl=ll(0°)/ll(90°) is ≥15 measured according to ASTM E112-13.