WO2006005573A1

WO2006005573A1 - Process for producing aluminium alloy sheet material with improved bake-hardening response

Info

Publication number: WO2006005573A1
Application number: PCT/EP2005/007531
Authority: WO
Inventors: Peter De Smet; Jean Pierre Jules Baekelandt; Linzhong Zhuang; Marc-Jan De Haas
Original assignee: Corus Aluminium Nv
Priority date: 2004-07-09
Filing date: 2005-07-08
Publication date: 2006-01-19

Abstract

A process for producing aluminium alloy sheet material with improved bake-hardening response, comprising the steps of: (i). casting an aluminium alloy (ii). homogenising (iii). hot-rolling and/or cold-rolling (iv). solution heat treating followed by quenching characterised in that after the solution heat treating and quenching after a waiting time of at least 1 minute the alloy sheet material is subjected to a heat treatment involving heating the material to a spike temperature in the range of 100 to 250 °C at a first heating rate, holding the material at the spike temperature for a first period of time less than about 1 minute, and cooling the alloy from the spike temperature to a temperature of 85 °C or less at a first cooling rate.

Description

PROCESS FOR PRODUCING ALUMINIUM ALLOY SHEET MATERIAL WITH IMPROVED BAKE-HARDENING RESPONSE

This invention relates to a process for producing aluminium alloy sheet material with improved bake-hardening response.

Ideally, an aluminium alloy sheet should be soft and easily deformable prior to shaping it into a product, such as an automotive body part, and be harder and more difficult to deform after the product has been finished. Finishing in this respect may include painting and paint-baking. During paint-baking the metallurgical process of bake-hardening occurs. Bake-hardening requires creating a supersaturated solution of precipitating elements by bringing these elements into solid solution e.g. by a solution heat treatment wherein the material is heated to a temperature of about 560 ⁰C for a short period of time, typically about 10 to 30 seconds followed by rapid cooling or quenching to ambient temperatures. Upon holding the material, these precipitating elements will tend to cluster & precipitate. This may occur at ambient temperature in which case it is a so-called natural ageing process (and a so called T4 condition is reached), or it may be stimulated by heating the material (typically to temperatures in the range of 150-200⁰C), such as during paint-baking, in which case it is an artificial ageing process. Optionally, a pre-ageing treatment is applied between solution heat treatment & natural ageing to increase the paint bake response. In this case, a so called T4P temper is established.

An important property of a sheet material which is bake-hardenable is the so- called "paint bake response". This paint bake response represents the change in yield strength from the value after the solution heat treatment followed by an optional pre- ageing and a natural ageing process and the value after forming the product followed by a paint baking treatment. A high paint bake response means that the yield strength of the product increases considerably upon paint-baking thereby increasing its resistance to dents.

A disadvantage of the conventional solution heat treatment followed by quenching and an ageing process, natural or artificial, is that the mechanical properties after the ageing process are very difficult to control, resulting in an unstable microstructure.

Another disadvantage of the conventional solution heat treatment followed by quenching and an optional pre-ageirig and a natural ageing process is that the elongation in T4(P)-condition is insufficient for the material to undergo the deformation processes to form a product, such as an automotive body part. It is an object of the invention to provide a process for producing aluminium alloy sheet material with improved bake-hardening response wherein the mechanical properties after the ageing process are homogeneous and reproducible.

It is also an object of the invention to provide a process for producing aluminium alloy sheet material with improved bake-hardening response wherein the elongation in T4P-condition is increased.

According to the invention, one or more of these objectives can be achieved by a process for producing aluminium alloy sheet material with improved bake-hardening response, comprising the steps of: (i). casting an aluminium alloy

(ii). homogenising (iii). hot-rolling and/or cold-rolling (iv). solution heat treating followed by quenching characterised in that after the solution heat treating and quenching after a waiting time of at least 1 minute the alloy sheet material is subjected to a heat treatment involving heating the material to a spike temperature in the range of 100 to 250 ⁰C at a first heating rate, holding the material at the spike temperature for a first period of time less than about 1 minute, and cooling the alloy from the spike temperature to a temperature of 85 °C or less at a first cooling rate. This process, a single-peak or single-spike annealing process, involves a rapid heat treatment wherein the material is reheated at a well controlled first heating rate to a carefully selected spike temperature and holding the material at that spike temperature for a well-defined period of time, followed by cooling the material at a well controlled first cooling rate to a temperature of 85 ⁰C or less. By this carefully controlled process, the properties of the material obtained after the heat treatment are both homogeneous over the material and reproducible from batch to batch, coil to coil or product to product. It has proven to be beneficial to allow for a certain period of time to pass before starting the spike-annealing. It is believed that during this waiting time, which is defined as the time interval between quenching after solution heat treatment and the spike-anneal, a fine initial silicon-rich cluster distribution is formed, which may grow out further during a later spiking and/or paint-baking process. A relatively fine precipitate distribution after the spiking and/or paint-baking is then the result. When the spike-annealing is applied immediately after solution heat treating followed by quenching, the initial distribution is formed at the spike temperature and thus will be much coarser. This leads to a coarser precipitate distribution after paint-baking and thus to a lower paint-bake response. Consequently, the waiting time is needed to form the fine initial silicon-rich cluster distribution. A waiting time of at least 1 minute has proven to be required to achieve this.

In an embodiment the waiting time is at least 2 minutes, and preferably at least 4 minutes. It was found that after a waiting time of at least 2 minutes after the solution heat treatment and quenching that the cluster distribution was already in an advanced phase, and that after a waiting time of at least 4 minutes after the solution heat treatment and quenching that the cluster distribution was fully developed. In an embodiment the waiting time is between 2 and 15 minutes, preferably between 2 and 12 minutes, more preferably between 2 and 9 minutes. In an embodiment the waiting time is between 4 and 15 minutes, preferably between 4 and 12 minutes, more preferably between 4 and 9 minutes.

It is believed that, when the time at room temperature after the solution heat treatment is too long (e.g. more than 15 minutes), the amount of available vacancies in the alloy decreases below a level that is needed for an acceptable degree of Si-rich cluster nucleation/growth during the spike-anneal. This results in a coarser distribution of the Si-rich clusters that are formed during the spike-anneal, resulting in a smaller paint-bake response.

The net-effect of the two counteracting processes described above reaches an optimum in paint-bake response when the spike is applied at a time in range 4-12 minutes after the solution heat treatment. This is presented schematically in table 1.

Very reproducible properties were obtained after a waiting time of between 4 and 9 minutes.

Table 1.

In an embodiment, before heating the material to the spike temperature, the alloy sheet material is subjected to heating the material to a peak temperature at a second heating rate, holding the material at the peak temperature for a second period of time, cooling the material at a second cooling rate to a cooling stop temperature, followed by heating the material to the spike temperature at the first heating rate.

This embodiment, a dual-peak or dual-spike annealing process, has proven to be beneficial for improving the elongation in T4P-condition. This is particularly beneficial for the production of automotive body sheet, where often a high formability is required. In general, the peak temperature is lower than the spike temperature. When the cluster distribution is mainly formed during the annealing at the peak temperature (i.e. the first spike of the dual-spike anneal), a more homogeneous distribution will be the result than when it is formed at the higher spike temperature of the single-step spike-anneal. The homogeneously distributed clusters formed at the peak temperature then grow out further during the annealing at the spike temperature second step (i.e. the second spike of the dual-spike anneal). This way, a dual-spike anneal yields a more homogeneous cluster distribution in T4P-condition than a single- spike anneal. By the influence of the cluster distribution the internal/local stress states of the alloy, more favourable uniform/total elongation and n-values are obtained by the dual-spike annealing process. It should be noted that it is of the utmost importance to carefully select and control the process parameters during the dual-spike annealing process in order to avoid inhomogeneous and/or irreproducible properties.

In an embodiment, the material is heated in said heat treatment to the spike temperature within the range of 100-220 ⁰C₁ preferably within the range of 110-170 ⁰C, more preferably within the range of 120-150⁰C. It was found that, particularly for those application where a low yield strength (Rp) is desirable, for instance for forming applications, that a spike temperature within these ranges is preferable. By carefully selecting the peak temperature from this range and careful control, the bake- hardening response has not only improved as compared to conventional artificial ageing processes, but also shows homogeneous and reproducible levels of bake- hardening response. When applied in a dual-spike annealing treatment, the elongation values show a small but significant improvement for this product application of body sheet.

In an embodiment, the material is heated in said heat treatment to the spike temperature within the range of 140-220 ⁰C. By carefully selecting and controlling the peak temperature from this range, the bake-hardening response has not only improved as compared to conventional artificial ageing processes, but also shows homogeneous and reproducible levels of bake-hardening response. When applied in a dual-spike annealing treatment, the elongation values show a small but significant improvement for this product application of body sheet. In an embodiment, the first heating rate is at least 5 °C/s, preferably at least 8

°C/s. By a careful selection and control of the heating rate the initial cluster distribution has been found to be fine and reproducible, thus yielding reproducible properties and bake-hardening response.

In an embodiment, the first period of time is 25 seconds or less, preferably 15 second or less. By limiting the annealing time at the peak temperature to a carefully controlled short time, the cluster distribution can be fine-tuned so as to result in good, homogeneous and reproducible properties.

In an embodiment the first cooling rate is less than 20 °C/s, preferably less than 15 °C/s, more preferably less than 10 °C/s. Again by carefully selecting and controlling the first cooling rate, the cluster distribution that was formed during the preceding annealing can be retained. It has proven to be advantageous to limit the first cooling rate to a maximum of 20 °C/s, although better control of the cooling rate and thus of the properties of the final product could be achieved by limiting the first cooling rate to a maximum of 15 °C/s or even rate to a maximum of 10°C/s. In an embodiment of the invention the alloy sheet material is cooled from the spike temperature in a two-stage cooling involving a first cooling step and a second cooling step, wherein the material is cooled in the first cooling step at a cooling rate of less than 3 °C/s to a change-over temperature and then cooled from the change-over temperature in the second cooling step at a cooling rate of less than 50 °C/s, wherein the time-average of the cooling rate of the first and second cooling step is less than 8 °C/s, preferably less than 4 °C/s. Preferably the cooling rate in the first cooling step is lower than the cooling rate in the second cooling step. When a slower cooling-rate is applied after the spike-anneal, it is believed that a more homogeneous cluster distribution can be expected than for a faster cooling-rate. This has effects on the internal/local stress states of the alloy and therefore on the elongation. By combining a slow initial cooling before the change-over temperature with a faster cooling thereafter the natural ageing at lower temperatures such as for example below 60⁰C is hampered or prevented. Since the time in the temperature range where natural ageing occurs during cooling is clearly shorter for the 2-stage cooling variant, its degree of enhanced natural ageing is smaller as well leading to better elongation values. It was found that the change-over temperature is preferably in the range of 40 to 130 ⁰C₁ more preferably in the range of 70 to 115 ⁰C. Preferably the cooling rate in the first cooling step is less than 1 °C/s and/or the cooling rate in the second cooling step is less than 30 °C/s. By careful selection and control of the process parameter an optimal combination of properties was achieved.

It should be noted that the last part of the cooling after annealing at the spike temperature could be cooling of the material on a coil. This type of cooling will not result in a constant cooling rate but the cooling rate will decrease exponentially with time and approach ambient temperature asymptotically. The cooling rate will be very low. More preferably; the second cooling step goes on till room temperature is reached and the material is reheated to a temperature in the range of 50-120⁰C before cooling of the material on a coil as a pre ageing step to further increase the paint bake response.

In a preferred embodiment of the invention the first peak temperature is in the range of 50 to 150 ⁰C, and/or the first and/or second heating rate is at least 5 °C/s, and/or the second period of time is less than about 5 seconds and/or the second cooling rate is less than 10 °C/s and/or the cooling-stop-temperature is in the range of 40 to 100 ⁰C. Careful selection and control of the process parameters of the temperature during the first spike of the dual-spike anneal proved to result in very reproducible and homogeneous properties. It also resulted in improved elongation values as compared to the single-spike anneal. Use of the stepped cooling after the second spike of the dual-spike anneal results in a further improvement.

In an embodiment the first peak temperature in the range of 70 to 120 ⁰C, preferably in the range of 70 to 100⁰C. This is an embodiment of the dual-spike annealing process and provides a further improvement in homogeneity and reproducibility of the properties of the sheet material.

In an embodiment of the invention said sheet material is an alloy of the AA6000- series. The process according to the invention proved to be particularly beneficial for this type of alloy. More preferably, the alloy is of the AA6016-type. The invention is also embodied in a product made from the sheet material produced according to the process of the invention. Its improved homogeneity and reproducibility in terms of the mechanical properties, improved paint-baking response and elongation values prior to paint-baking makes the material particularly suitable for products like automotive body sheet panel produced from an aluminium sheet material. In an embodiment of the invention the aluminium alloy sheet material produced according to the process of the invention has a total elongation of at least 26 % (A50), preferably at least 27%.

The invention will now be further explained by the following non-limitative examples with reference to the accompanying schematical drawings wherein: - Figure 1 shows a schematic drawing of a dual-spike and single-spike anneal;

Figure 2 shows a schematic drawing of a single-spike anneal in combination with different cooling procedures.

Figure 3 shows the influence of time of a single-spike 14O⁰C application on T4P yield strength and on 2% + 170°C/20min Paint Bake Response (PBR). Time is the waiting time or time interval between quenching after solution heat treatment and the spike-anneal.

Figure 4 shows a comparison of elongation (uniform & total) in T4P-state between a single-step spike-anneal and a 2 step spike-anneal.

Figure 5 shows a comparison of elongation in T4P-state for different cooling- procedures from the spike temperature to ambient temperature (RT).

Figure 6 shows the influence of spike temperature on T4P yield strength and on

2% + 170°C/20min paint bake response after a waiting time between solution heat treatment and the spike-anneal of 6 minutes.

In Figure 1 , a dual-spike anneal is compared to a single-spike anneal. It should be noted that the values of the process parameters are given as an example of an embodiment of the invention, and are by no means meant to be limitative.

In Figure 2, examples of three different cooling-rate variants of a single-spike anneal are given. The spike was applied 6 minutes after solution heat treatment. In the first variant, the alloy is cooled at 25°C/s from the spike temperature to room temperature, in the second variant the alloy is cooled at 0.3°C/s from the spike temperature to room temperature, in the third variant the alloy is cooled in two stages (first 0.3°C/s to 100⁰C and subsequently 25°C/s to room temperature).

Experiments were performed on industrial alloys of the AA6016 type comprising (all weight percentages) Si 1.0-1.5; Fe max 0.50; Cu max 0.20; Mn max 0.20; Mg 0.25-0.6; Cr max 0.10; Zn max 0.20; Ti max 0.15, balance aluminium and inevitable impurities. A typical alloy of this type has the following composition : 1.02 % Si, 0.23 % Fe, 0.17 % Cu, 0.07 % Mn, 0.43 % Mg, 0.03 % Cr, balance aluminium and inevitable impurities.

The alloy was cast, preheated, hot-rolled, cold-rolled, inter-annealed and further cold-rolled to a gauge of 1 mm. On a continuous annealing line, the cold-rolled material is given a solution heat treatment at 56O⁰C followed by rapidly cooling to room temperature.

The effects of incorporation of a spike-anneal solution heat treatment and cooling to ambient temperature will be described.

The single-spike anneal consists of the following steps: heating at 10°C/s to a temperature of 120, 130 or 14O⁰C soaking for a short time (in this work 5s) at said temperature of 120, 130 or 140⁰C - cooling to room temperature (cooling-rates of 0.3°C/s, 25°C/s and a combination of 0.3°C/s to 100⁰C and 25°C/s to room temperature are tested). The dual-spike anneal consists of the following steps: heating at 10°C/s to 9O⁰C, - immediate cooling at 1°C/s to 7O⁰C heating at 10°C/s to 14O⁰C keeping for 5s at 14O⁰C finally cooling at 25°C/s to room temperature.

In Figure 3, the yield-strength in T4P-condition and the paint-bake response (i.e. the increase in yield-strength due to application of 2%+170°C/20min on the alloy in

T4P-state) are presented for the single-spike anneal at 14O⁰C applied at different times after solution heat treatment. The highest paint-bake responses are achieved for the variants that are spike-annealed at least 1 minute after solution heat treatment.

When the single-spike anneal is applied directly after solution heat treatment the paint- bake response is lower. The same holds for spike-annealing after 12 minutes and 21 minutes. The effect of the spike is also shown by comparing the results with the 'No spike'-experiment. For the same alloy but with a copper content of less than 0.03% similar results were obtained. A significant increase in paint-bake response is obtained after a waiting time of at least 1 minutes, and a further increase is obtained at longer waiting times. From our experiments, it thus has become clear that in order to obtain a the best paint-bake response, a substantial amount of natural age hardening is needed before application of the spike anneal. The spike anneal is to be applied after a waiting time of at least 1 minute, preferably of at least 2 minutes, or even at a waiting time of more than 4 minutes after solution heat treatment (preferably in the range 4-12 minutes and more preferably in the range 4-9 minutes). In Figure 4, the uniform and total elongation in T4P-condition for a dual-spike anneal is compared to that of a single-spike anneal. It can be seen that the elongation values of the dual-spike annealed variant are about 1 % higher than that of the single- spike annealed variant. In addition, the n value of the dual-spike annealed variant is 0.01 higher (i.e. 0.30 versus 0.29 for the single-spike annealed variant). Both types of spike-anneal are applied 6 minutes after solution heat treatment.

From Figure 5, it can be seen that a spike-anneal with a slow cooling-rate (0.3°C/s) to room temperature already leads to a 0.7 % better elongation in T4P- condition compared to that of the spike-anneal with a relatively fast (25°C/s) cooling- rate. When the alloy is cooled in two stages to room temperature, the total elongation is even 1.2 % better than that of the variant that is cooled at 25°C/s over the whole range to room temperature.

From Figure 6, it can be seen that the T4P yield strength after a single-spike anneal consisting of heating at 10°C/s to a spike temperature of 120, 130 or 140⁰C, soaking for a short time (in this work 5s) at the spike temperature and cooling to room temperature at 25°C/s decreases with decreasing spike temperature. It appears that a maximum paint-bake response is obtained at a spike temperature of 13O⁰C.

From the abovementioned examples, it can be seen that the aluminium sheet material produced according to the process as disclosed in this application has a total elongation of at least 26%, or even 27% when measured over a gauge length of 50 mm (A50) according to EN10002.

It is of course to be understood that the present invention is not limited to the described embodiments and examples described above, but encompasses any and all embodiments within the scope of the description and the following claims.

Claims

1. A process for producing aluminium alloy sheet material with improved bake- hardening response, comprising the steps of: (i). casting an aluminium alloy

(ii). homogenising (iii). hot-rolling and/or cold-rolling (iv). solution heat treating followed by quenching characterised in that after the solution heat treating and quenching after a waiting time of at least 1 minute the alloy sheet material is subjected to a heat treatment involving heating the material to a spike temperature in the range of

100 to 250 ⁰C at a first heating rate, holding the material at the spike temperature for a first period of time less than about 1 minute, and cooling the alloy from the spike temperature to a temperature of 85 ⁰C or less at a first cooling rate.

2. A process according to claim 1 wherein, before heating the material to the spike temperature, the alloy sheet material is subjected to heating the material to a peak temperature at a second heating rate, holding the material at the peak temperature for a second period of time, cooling the material at a second cooling rate to a cooling stop temperature, followed by heating the material to the spike temperature at the first heating rate.

3. A process according to claim 1 or 2, wherein the material is heated in said heat treatment to the spike temperature within the range of 100 - 220 ⁰C, preferably within the range of 100 - 170 ⁰C, more preferably within the range of 110-160 ⁰C, even more preferably within the range of 120-150⁰C .

4. A process according to any one of claims 1 to 3, wherein the waiting time is at least 2 minutes, preferably at least 4 minutes.

5. A process according to any one of claims 1 to 4, wherein the waiting time is at most 15 minutes, preferably at most 12 minutes, more preferably at most 9 minutes .

6. A process according to any one of the claims 1 to 5 wherein the first heating rate is at least 5 °C/s, preferably at least 8 °C/s.

7. A process according to any one of the claims 1 to 6 wherein the first period of time is 25 seconds or less, preferably 15 seconds or less.

8. A process according to any one of claims 1 to 7 wherein the first cooling rate is less than 20 °C/s, preferably less than 15 °C/s, more preferably less than 10 °C/s.

9. A process according to any one of claims 1 to 8 wherein the alloy sheet material is cooled from the spike temperature in a two-stage cooling involving a first cooling step and a second cooling step, wherein the material is cooled in the first cooling step at a cooling rate of less than 3 °C/s to a change-over temperature and then cooled from the change-over temperature in the second cooling step at a cooling rate of less than 50 °C/s, wherein the time-average of the cooling rate of the first and second cooling step is less than 8 °C/s, preferably less than 4 °C/s.

10. A process according to claim 9 wherein the change-over temperature is in the range of 40 to 130 ⁰C.

11. A process according to claim 9 or 10 wherein the cooling rate in the first cooling step is less than 1 °C/s and/or the cooling rate in the second cooling step is less than 30 °C/s.

12. A process according to any one of claims 2 to 11 wherein the first peak temperature is in the range of 50 to 150 ⁰C, and/or the first and/or second heating rate is at least 5 °C/s, and/or the second period of time is less than about 5 seconds and/or the second cooling rate is less than 10 °C/s and/or the cooling-stop-temperature is in the range of 40 to 100 ⁰C.

13. A process according to any one of the claims 2 to 12 wherein the alloy sheet material is subjected to heating the material to the first peak temperature in the range of 70 to 120 ⁰C, preferably 70 to 100⁰C.

14. A process according to any one of claims 1 to 13 characterized in that said sheet material is an alloy of the AA6000-series

15. Aluminium alloy sheet material produced according to any of the claims 1 to 14 having a total elongation of at least 26 % (A50), preferably at least 27%.

16. Automotive body sheet panel produced from an aluminium sheet material produced according to any of the claims 1 to 14.