EP3916118A1 - Method of curing a metal sheet or strip of 6xxx series aluminium alloy - Google Patents

Abstract

A method for hardening a sheet or strip made of an aluminum alloy of the 6xxx series and a sheet or strip produced thereby is described. In order to accelerate the curing process, a multi-stage heat treatment is proposed which includes a second holding at a second holding temperature (T2) in the range from 150 to 250 ° C during a second holding time (h2) and a subsequent, second accelerated cooling, wherein the second holding followed by a second accelerated cooling interrupts the first holding several times and this first holding thereby in holding sections, each at a first holding temperature (T1) in the range of 60 to 140 ° C and during a first holding period (h1a, h1b, h1c, h1d , h1e), which lasts longer than the second holding time (h2), divides.

Description

The invention relates to a method for hardening a sheet or strip made of an aluminum alloy of the 6xxx series and a sheet or strip produced therewith.

It is known from the prior art (cf. Ken Takata et al., "Improvement of Strength-Elongation Balance of Al-Mg-Si Sheet Alloy by Utilizing Mg-Si Cluster and Its Proposed Mechanism," Materials Transactions (2017 )) to improve the elongation at break and tensile strength values of Al-Mg-Si aluminum alloys of the 6xxx series by subjecting the aluminum alloy to a heat treatment that essentially forms Mg-Si clusters after a solution heat treatment and accelerated cooling (water quenching). This takes place, for example, at a holding temperature of 100 ° C. (degrees Celsius) during a holding time of 24 days, which represents a comparatively long process time, particularly with regard to industrial production.
The process time can be reduced with higher holding temperatures, for example at 180 ° C to 8 to 10 hours. The disadvantage is that essentially β ″ precipitates are formed at such higher holding temperatures, which means that the combination of elongation at break and tensile strength values that are known from heat treatments with reduced holding temperatures cannot be achieved.

The invention has therefore set itself the task of improving a method of the type described in such a way that the process time for curing with a heat treatment at comparatively low holding temperatures can be reduced with almost the same elongation at break and tensile strength values.

The invention solves the problem posed by the features of claim 1.

In that the heat treatment includes a second holding at a second holding temperature in the range from 150 to 250 ° C. during a second holding time as well includes subsequent, second accelerated cooling, it can initially be expected that - as is known with heat treatments at higher holding temperatures - on the one hand, process times are reduced, on the other hand, a poorer combination of elongation at break and tensile strength values is disadvantageously due to β "precipitations results.
The latter can, however, be avoided in a targeted manner by the provision for interrupting the first hold, since the second hold followed by a second accelerated cooling interrupts the first hold several times and this first hold thereby in holding sections, each at a first holding temperature in the range of 60 to 140 ° C and during a first hold period that lasts longer than the second hold time.
With the help of the interrupting second holding in conjunction with the second accelerated cooling, a comparatively high number of additional voids can be created and frightened in the 6xxx alloy, which supports the Mg-Si cluster formation during the subsequent first holding and thus the process time of the hardening under the first Hold significantly reduced.
According to the invention, therefore, with 6xxx aluminum alloys (i.e. AlMgSi alloys of the 6000 series) those elongation at break and tensile strength values can be achieved significantly faster which are known when hardening with the formation of essentially Mg-Si clusters with comparatively long process times.

The first holding temperature is preferably in the range from 80 to 120 ° C. in order to nevertheless be able to ensure a comparatively high Mg-Si cluster formation in the 6xxx aluminum alloy in the case of shorter first holding time segments. The above can be further improved by a first holding temperature in the range from 90 to 110 ° C.

The first holding time segments are less than or equal to 12 hours in order to use essentially all the trapped vacancies for the formation of Mg — Si clusters. The first holding time segments are preferably in the range from 2 to 8 hours in order to allow a sufficient number of trapped vacancies to form Use Mg-Si clusters. The above can be further improved by first holding periods in the range of 3 to 6 hours.

If the second holding temperature is in the range from 170 to 230 ° C, a temperature range can be specified in which, taking into account a comparatively short second holding time, with a reduced tendency to β "precipitations, a large number of voids can advantageously be generated Holding temperature in the range from 190 to 210 ° C can be further improved.

The second holding time of the second holding is preferably less than or equal to 15 minutes in order to create sufficient voids with a low tendency to precipitate in the 6xxx alloy.

$$\mathrm{h}\mathrm{2}\le \left(\mathrm{A}\mathrm{2}+\mathrm{B}\mathrm{2}\cdot \mathrm{KG}\right)\cdot \exp \left(-\frac{\mathrm{T}\mathrm{2}}{\mathrm{C}2}\right)+\left(\mathrm{D}\mathrm{2}+\mathrm{E}\mathrm{2}\cdot \mathrm{KG}\right)$$

mit folgenden Parametern

• A1= -26400, B1= 5000, C1= 22,00, D1= 1,25 und E1= 0,15
• A2= -125000, B2= 29700, C2= 20,75, D2= -0,55 und E2= 0,65

The process for hardening the 6xxx alloy can be further improved if the second holding time (h2) in seconds for a mean crystal grain size (KG), measured according to the ASTM E112 line-cutting method, in micrometers and at the second holding temperature (T2) in degrees Celsius, the following conditions Fulfills: $$\mathrm{H}\mathrm{2}\ge \left(\mathrm{A.}\mathrm{1}+\mathrm{B.}\mathrm{1}\cdot \mathrm{KG}\right)\cdot \exp \left(-\frac{\mathrm{T}\mathrm{2}}{\mathrm{C.}\mathrm{1}}\right)+\left(\mathrm{D.}\mathrm{1}+\mathrm{E.}\mathrm{1}\cdot \mathrm{KG}\right)$$

$$\mathrm{H}\mathrm{2}\le \left(\mathrm{A.}\mathrm{2}+\mathrm{B.}\mathrm{2}\cdot \mathrm{KG}\right)\cdot \exp \left(-\frac{\mathrm{T}\mathrm{2}}{\mathrm{C.}2}\right)+\left(\mathrm{D.}\mathrm{2}+\mathrm{E.}\mathrm{2}\cdot \mathrm{KG}\right)$$

with the following parameters

• A1 = -26400, B1 = 5000, C1 = 22.00, D1 = 1.25 and E1 = 0.15
• A2 = -125000, B2 = 29700, C2 = 20.75, D2 = -0.55 and E2 = 0.65

The first holding temperatures are preferably the same over the entire first holding. Preferably, several or all of the first holding time segments are the same. The repeated second holding at a second holding temperature is preferably the same during a second holding time. The first hold time segment of the first hold time segments preferably lasts longer than the subsequent first hold time segments.

In particular, the first and / or second accelerated cooling takes place at a cooling rate of at least 20 ° C / s, in particular at least 50 ° C / s, preferably at least 80 ° C / s, so that the voids formed during the second hold are reliable to be able to frighten you.

The heating from the first hold to the second hold preferably takes place at a heating rate of at least 10 ° C./s, in particular at least 50 ° C./s.

Preferably, the 6xxx aluminum alloy has from 0.2 to 1.5% by weight magnesium (Mg) and from 0.2 to 1.5% by weight silicon (Si) - which results in a large precipitation pressure when first held the formation of Mg-Si clusters and, on the second hold, can lead to an increased density of vacancies.
The 6xxx aluminum alloy can optionally have one or more elements: up to 1.1% by weight copper (Cu) and / or up to 0.7% by weight iron (Fe) and / or up to 1.0% by weight Manganese (Mn) and / or up to 0.35% by weight chromium (Cr) and / or up to 0.25% by weight zinc (Zn) and / or up to 0.15% by weight titanium (Ti) and / or up to 0.1% by weight of vanadium (V) and / or up to 0.2% by weight of zirconium (Zr) and / or up to 0.2% by weight of tin (Sn).
The remainder of the aluminum alloy has aluminum and impurities that are unavoidable due to the manufacturing process, each with a maximum of 0.05% by weight and a total of 0.15% by weight at most.
In addition, the aluminum alloy can contain from 0.2 to 1.2% by weight of magnesium (Mg) in order to further increase the formation of Mg — Si clusters.

Preferably the aluminum alloy is of the type AA6005, AA6016, AA6061, AA6063 or AA6082.

The sheet metal or strip preferably has a thickness of less than 5 mm, in particular 3 mm, in order to generate a sufficient number of voids and to prevent undesired excretions with the brief interruption of the first holding.

In addition, the invention has set itself the task of creating a sheet or strip subjected to hardening with comparatively high elongation at break and tensile strength values.

The invention solves the problem posed by the features of claim 13.

Because the sheet or strip has been subjected to the inventive method for curing, a sufficiently high 0.2% proof stress R _p0.2 > 200 MPa and a sufficiently high elongation at break A of> 20% can be achieved - this with essentially Mg-Si clusters in the aluminum matrix.

The aluminum alloy preferably has a cluster density of at least 2 × 10 ²⁴ clusters / m ³ with a Guinier radius> 1 nm (nanometer) and with a median Guinier radius of> 1.3 nm, measured with an atom probe tomography (LEAP) from Type LEAP 3000HR.

The width of the precipitation-free zones at the grain boundaries is preferably between 3 and 80 nm (nanometers) in order to limit a negative influence on the elongation values of the sheet or strip - all the more so if the width of the precipitation-free zones at the grain boundaries is between 5 and 50 nm.

The precipitates containing Mg and Si, in particular of the Mg ₂ Si type, preferably have an average size of 30 to 100 nm (nanometers) at the grain boundaries in order to be able to ensure sufficient strength at high elongation values - especially if the precipitates have an average size of 50 to 70 nm at the grain boundaries.

To demonstrate the effects achieved, rolled semi-finished products, namely sheets A, B and C designed as thin sheets, each with a sheet thickness of 1.7 mm (millimeters) and an AA6016 aluminum alloy were used Mg wt% Si wt% Cu weight% Fe wt% Mn wt% Zn wt% Ti wt% Cr wt% 0.65 1.16 0.17 0.17 0.10 0.04 0.02 0.04 and the remainder is aluminum as well as impurities that are unavoidable due to the manufacturing process, each with a maximum of 0.05% by weight and a total of 0.15% by weight.

These sheets A, B and C are subjected to different processes V1, V2, V3 for curing. These processes V1, V2, V3 all have other heat treatments that follow a solution heat treatment at 540 ° C. (degrees Celsius) for 2 minutes (minutes) and a subsequent, first accelerated cooling (namely water quenching).

Procedure V1 (state of the art):

This method is known from the prior art, in which a thin sheet A is subjected to a single-stage heat treatment after the solution heat treatment and the first accelerated cooling. This single-stage heat treatment consists of a first holding with a first holding temperature (T1) at 100 ° C. and a first holding time (h1) of 7 days.

In contrast to method V1, the heat treatments of methods V2, V3 according to the invention are multi-stage - as in FIG Fig. 1 can be seen.
This multistage is formed by interrupting a first hold four times by a second hold, including a second, accelerated cooling. This divides the first hold into holding sections.
These holding sections are different from one another on the one hand in the first holding time sections h1a or h1b, h1c, h1d, h1e, on the other hand they are the same in the first holding temperature T1 - although the latter does not necessarily have to be the case. Each holding section can maintain its individual holding temperature T1 during the individual first Hold time periods h1a, h1b, h1c, h1d and h1e, respectively. The first holding temperature T1 or first holding temperatures T1 of the first holding time segments h1a, h1b, h1c, h1d, h1e only have to meet the condition of 60 to 140.degree.

Method V2 (according to the invention):

1. a. erstes Halten auf einer ersten Haltetemperatur T1 bei 100 °C während eines ersten Haltezeitabschnitts h1a von 3 Stunden;
2. b. Erwärmen unter Einbringung in ein Metallbad mit einer Aufheizrate von mehr als 100 °C/s (Grad Celsius pro Sekunde) auf eine zweite Haltetemperatur T2;
3. c. zweites Halten auf einer zweiten Haltetemperatur T2 bei 250 °C während einer zweiten Haltezeit h2 von 12 Sekunden, wobei damit die zweite Haltezeit h2 bei einer Korngröße KG von 50 µm die Bedingung 11.35 Sekunden ≤ h2 ≤ 39.91 Sekunden erfüllt wird;
4. d. zweites beschleunigtes Abkühlen unter Einbringung in ein Metallbad mit einer Abkühlrate von 80 °C/s auf eine Raumtemperatur (20 °C);
5. e. Erwärmen unter Einbringung in ein Ölbad mit einer Aufheizrate von 10 °C/s auf die erste Haltetemperatur T1;
6. f. erstes Halten auf einer ersten Haltetemperatur T1 bei 100 °C während eines ersten Haltezeitabschnitts h1b bzw. h1c bzw. h1d bzw. h1e von 1 Stunde;

wobei anschließend die Schritte b bis f dreimal wiederholt werden und nach diesen Wiederholungen anschließend das Feinblech B auf Raumtemperatur, gegebenenfalls beschleunigt, abgekühlt wird - wie dies in Fig. 1 zu erkennen ist.Thin sheet B was then subjected to the solution heat treatment and the first accelerated cooling to a multi-stage heat treatment in the following sequence:

1. a. first holding at a first holding temperature T1 at 100 ° C. for a first holding time segment h1a of 3 hours;
2. b. Heating with introduction into a metal bath at a heating rate of more than 100 ° C./s (degrees Celsius per second) to a second holding temperature T2;
3. c. second holding at a second holding temperature T2 at 250 ° C. during a second holding time h2 of 12 seconds, the second holding time h2 with a grain size KG of 50 μm thus fulfilling the condition 11.35 seconds h2 39.91 seconds;
4. d. second accelerated cooling with introduction into a metal bath at a cooling rate of 80 ° C./s to room temperature (20 ° C.);
5. e. Heating while placing in an oil bath at a heating rate of 10 ° C./s to the first holding temperature T1;
6. f. first holding at a first holding temperature T1 at 100 ° C. during a first holding time segment h1b or h1c or h1d or h1e of 1 hour;

steps b to f are then repeated three times and, after these repetitions, the thin sheet B is then cooled to room temperature, if necessary accelerated - as in FIG Fig. 1 can be seen.

In addition, in Fig. 1 it can be recognized that the first holding time segments h1a, h1b, h1c, h1d and h1e last significantly longer than the second holding time h2 (cf. hour in relation to seconds).

Method V3 (according to the invention):

Method V3 differs from method V2 only in point c, in that the second holding at a second holding temperature T2 at 205 ° C during a second holding time h2 of 45 seconds is carried out. This holding time also fulfills the condition 28.82 seconds ≤ h2 ≤ 101.60 seconds for a grain size KG of 50 µm.

The sheets A, B, C subjected to process V1, V2 or V3 for curing were examined by means of a tensile test with regard to their mechanical characteristic values 0.2% proof stress R _p0.2 , tensile strength R _m , uniform elongation A _g and elongation at break A. Table 1: Mechanical parameters including the heat treatment duration (WBD) sheet R _p0.2 [MPa] R _m [MPa] A _g [%] A [%] WBD A. 222 331 22.8 28.4 7 days B. 200 296 17.4 21.0 7 hours C. 234 329 20.2 28.5 7 hours

As can be seen in Table 1, the sheets B, C achieve almost the same mechanical parameters with significantly reduced curing process times - as is the case with sheet A and also in Fig. 2 is recognizable.

In particular, sheet C even has improved mechanical characteristics in comparison with sheet A at the 0.2% yield strength, which, as is known, can only be achieved with sheet A with an extremely long process time of 7 days.

• Blech B:
  • Breite der ausscheidungsfreien Zonen an den Korngrenzen 69 nm.
  • Mittlere Größe der Ausscheidungen vom Typ Mg₂Si an den Korngrenzen 70 nm.
• Blech C:
  • Breite der ausscheidungsfreien Zonen an den Korngrenzen 31 nm.
  • Mittlere Größe der Ausscheidungen vom Typ Mg₂Si an den Korngrenzen 56 nm.
    Clusterdichte von 2,55 x 10²⁴ Cluster/m³ mit einem Median-Guinier Radius von 1,7 nm.

A metallurgical examination of sheet B and C yielded the following results:

• Sheet B:
  • Width of the precipitation-free zones at the grain boundaries 69 nm.
  • Average size of the precipitates of the Mg ₂ Si type at the grain boundaries 70 nm.
• Sheet C:
  • Width of the precipitation-free zones at the grain boundaries 31 nm.
  • Average size of the precipitates of the Mg ₂ Si type at the grain boundaries 56 nm.
    Cluster density of 2.55 x 10 ²⁴ clusters / m ³ with a median Guinier radius of 1.7 nm.

Sheet C, compared to sheet B, has both a small mean size of the precipitates and a smaller width of the precipitation-free zones, which explains the increased strength values at higher elongation values according to Table 1.

According to the invention, a significantly faster method for hardening 6xxx aluminum alloys can therefore be created using methods V2 and V3, with which excellent elongation at break and tensile strength values can also be achieved due to the hardening of the aluminum alloy with the formation of essentially Mg-Si clusters.

In particular, if the second holding temperature (T2 at 205 ° C.) of method V3 is lower than that of method V2, a clear improvement in the direction of an optimum which lies in the range from 190 to 210 ° C. can be recognized.

In general, it is stated that "particularly" can be translated into English as "more particularly". A feature preceded by "particularly" is to be regarded as an optional feature that can be omitted and therefore does not represent a restriction, for example of the claims. The same applies to "preferably", translated into English as "preferably".

Claims (16)

Method for hardening a sheet or strip made of an aluminum alloy of the 6xxx series with the following steps: Solution annealing of sheet metal or strip, first accelerated cooling of the solution annealed sheet or strip and heat treatment of the quenched sheet or strip, which heat treatment comprises a first holding at a first holding temperature (T1) in the range from 60 to 140 ° C, characterized in that the heat treatment a second holding at a second holding temperature (T2) in the range from 150 to 250 ° C. during a second holding time (h2) and a subsequent, second accelerated cooling, wherein the second holding with subsequent second accelerated cooling interrupts the first holding several times and this first holding thereby in holding sections, each at a first holding temperature (T1) in the range of 60 to 140 ° C and during a first holding time segment (h1a, h1b, h1c, h1d, h1 e), which lasts longer than the second holding time (h2), divides. Method according to Claim 1, characterized in that the first holding temperature (T1) is in the range from 80 to 120 ° C, in particular from 90 to 110 ° C. Method according to Claim 1 or 2, characterized in that the first holding time segments (h1a, h1b, h1c, h1d, h1e) are less than or equal to 12 hours, in particular in the range from 2 to 8 hours, preferably in the range from 3 to 6 hours . Method according to Claim 1, 2 or 3, characterized in that the second holding temperature (T2) is in the range from 170 to 230 ° C, in particular in the range from 190 to 210 ° C. Method according to one of Claims 1 to 4, characterized in that the second holding time (h2) of the second holding is not equal to 15 minutes. Method according to one of Claims 1 to 5, characterized in that the second holding time (h2) in seconds at an average crystal grain size (KG) in micrometers and at the second holding temperature (T2) in degrees Celsius fulfills the following conditions: $$\mathrm{H}\mathrm{2}\ge \left(\mathrm{A.}\mathrm{1}+\mathrm{B.}\mathrm{1}\cdot \mathrm{KG}\right)\cdot \exp \left(-\frac{\mathrm{T}\mathrm{2}}{\mathrm{C.}\mathrm{1}}\right)+\left(\mathrm{D.}\mathrm{1}+\mathrm{E.}\mathrm{1}\cdot \mathrm{KG}\right)$$

$$\mathrm{H}\mathrm{2}\le \left(\mathrm{A.}2+\mathrm{B.}2\cdot \mathrm{KG}\right)\cdot \exp \left(-\frac{\mathrm{T}\mathrm{2}}{\mathrm{C.}2}\right)+\left(\mathrm{D.}2+\mathrm{E.}2\cdot \mathrm{KG}\right)$$

with the following parameters A1 = -26400, B1 = 5000, C1 = 22.00, D1 = 1.25 and E1 = 0.15 A2 = -125000, B2 = 29700, C2 = 20.75, D2 = -0.55 and E2 = 0.65 Method according to one of Claims 1 to 6, characterized in that the first holding temperatures (T1) are the same over the entire first holding and / or the several or all first holding time segments (h1a, h1b, h1c, h1d, h1e) are the same and / or the repeated second holding at a second holding temperature (T2) during a second holding time (h2) is the same. Method according to one of Claims 1 to 7, characterized in that the first and / or second accelerated cooling takes place at a cooling rate of at least 20 C / s, in particular at least 50 ° C / s, preferably at least 80 ° C / s . Method according to one of Claims 1 to 8, characterized in that the heating from the first hold to the second hold takes place at a heating rate of at least 10 ° C / s, in particular of at least 50 ° C / s. The method according to any one of claims 1 to 9, characterized in that the aluminum alloy
from 0.2 to 1.5% by weight, in particular from 0.2 to 1.2% by weight, magnesium (Mg), from 0.2 to 1.5% by weight silicon (Si),
optional to 1.1 Wt% Copper (Cu), up to 0.7 Wt% Iron (Fe), up to 1.0 Wt% Manganese (Mn), up to 0.35 Wt% Chromium (Cr), up to 0.25 Wt% Zinc (Zn), to 0.15 Wt% Titanium (Ti), up to 0.1 Wt% Vanadium (V), up to 0.2 Wt% Zircon (Zr), up to 0.2 Wt% Tin (Sn), and the remainder is aluminum as well as impurities that are unavoidable due to the manufacturing process. Process according to one of Claims 1 to 10, characterized in that the aluminum alloy is of the type AA6005, AA6016, AA6061, AA6063 or AA6082. Method according to one of Claims 1 to 11, characterized in that the sheet metal or strip has a thickness of less than 5 mm, in particular less than 3 mm. Sheet or strip made of an aluminum alloy of the 6xxx series hardened by a method according to one of claims 1 to 12, characterized in that the sheet or strip has a 0.2% yield strength (R _p0.2 )> 200 MPa and an elongation at break (A) of> 20%. Sheet metal or strip according to claim 13, characterized in that the aluminum alloy has a cluster density of at least 2 x 10 ²⁴ clusters / m ³ with a Guinier radius> 1 nm and with a median Guinier radius of> 1.3 nm. Sheet metal or strip according to Claim 13 or 14, characterized in that the width of the precipitation-free zones at the grain boundaries is between 3 and 80 nm, in particular between 5 and 50 nm. Sheet metal or strip according to claim 13, 14 or 15, characterized in that the Mg- and Si-containing precipitates, in particular of the Mg ₂ Si type, have an average size of 30 to 100 nm, in particular 50 to 70 nm, at the grain boundaries, exhibit.

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