WO2020234367A1

WO2020234367A1 - Method for hot rolling an aluminium alloy band

Info

Publication number: WO2020234367A1
Application number: PCT/EP2020/064099
Authority: WO
Inventors: Ali Asghar SARBAN
Original assignee: Aleris Aluminum Duffel Bvba
Priority date: 2019-05-21
Filing date: 2020-05-20
Publication date: 2020-11-26

Abstract

The present invention relates to a method for hot rolling an aluminium alloy band (2), wherein the hot rolling method comprises at least one rolling pass through a hot rolling mill, wherein an entry temperature of the aluminium alloy band (2) when entering said hot rolling mill and/or the hot rolling speed of said rolling pass is adjusted such that the Zener-Hollomon parameter (Z_def), or a function of the Zener-Hollomon parameter (Z_def), whereby Z_def is defined by formula (I) wherein έ is the mean strain rate, Q_defis the activation energy for deformation, R is the universal gas constant, and T_in is the entry temperature of the aluminium alloy band to the hot rolling mill, is kept at least approximately constant and/or within a pre-determined range.

Description

Method for hot rolling an aluminium alloy band

FIELD OF THE INVENTION

The present invention relates to a method for hot rolling an aluminium alloy band in a hot rolling mill, such as a reversing mill or preferably a tandem mill, a method for manufacturing an aluminium alloy band, and an aluminium alloy band manufactured according to this method.

BACKGROUND OF THE INVENTION

In the manufacturing process of aluminium alloy sheets or plates, hot rolling is an important processing step, in which an ingot is rolled into a band. The hot rolling practice has a strong effect on the mechanical properties as well as surface properties of the final product. For example, texture related properties of the sheet metal such as the r-value (Lankford value) are affected by the hot rolling process.

Hot rolling may be performed in a reversing mill with usually one rolling stand, where the aluminium alloy band is rolled back and forth several times through the rolling stand, until it has reached the desired thickness. According to another rolling practice, the cast ingot is first hot rolled to a transfer gauge in a (reversing) break down mill and is then transferred on a transfer table to a tandem hot rolling mill, which usually has several rolling stands. The tandem or finishing mill is passed only once. Because of relatively long length of the strip at transfer gauge, the tail end of the strip is left cooling in air for much longer time than the head end of the strip. Since the strip cools quickly at typical transfer gauges, the entry temperature to the finishing or tandem mill drops gradually over the length of the strip. In other words, while the head end enters the finishing mill, the tail end keeps cooling in the air. By the time the tail end enters the finishing mill, it is much cooler than the head end was when it entered the finishing mill. This temperature variation of up to typically 30-50°C along the length may lead to property variations along the length of the strip. Similar temperature variations between head end and tail end may also arise in a reversing mill, in particular during the last hot rolling passes, when the sheet thickness is reduced to an extent that significant cooling may take place in the time between the head end and the tail end entering the hot rolling mill.

Generally, poor hot rolling practice can lead to overall poor mechanical prop erties, especially low r-values (also called Lankford coefficient) in the entire rolled band, and also to variations of mechanical and surface properties, especially r-val- ues, roping and Wolkenbildung along the length of the finished band.

A standard industry practice in aluminium hot rolling is to try to keep the exit temperature of the finishing mill constant along the length. This implies that the speed of the rolling mill must increase at the tail end (which is cooler) in order to meet the requirement of constant exit temperature. This practice leads to consider able variation of properties along the length.

It is therefore an object of the invention to improve the mechanical and sur face properties of a hot-rolled band, especially r-values, in the entire rolled band. It is another object of the invention to reduce the variations of these properties along the length of the hot-rolled band.

SUMMARY OF THE INVENTION

These and other objects and further advantages are met or exceeded by the present invention providing methods according to claim 1 and 14 and a hot- rolled aluminium alloy band according to claim 13 as well as a finished band ac cording to claim 15. Advantageous embodiments are set out in the dependent claims.

DESCRIPTION OF THE INVENTION

As will be appreciated herein below, except as otherwise indicated, alumin ium alloy designations refer to the Aluminum Association designations in Alumi num Standards and Data and Registration Records as published by the Aluminum Association in 2018 and which are frequently updated, and are well known to the persons skilled in the art. For any description of alloy compositions or preferred al loy compositions, all references to percentages are by weight percent unless oth erwise indicated.

The present invention provides a method for hot rolling an aluminium alloy band, wherein the hot rolling method comprises at least one rolling pass through a hot rolling mill, wherein an entry temperature of the aluminium alloy band when en tering said hot rolling mill and/or the hot rolling speed of said rolling pass is ad justed such that the Zener-Flollomon parameter (Zdef), or a function of the Zener- Flollomon parameter (Zdef), or a parameter related to Zdef, is kept at least approxi mately constant and/or within a pre-determ ined range, whereby Z_de_f is defined by the following formula

(

1 ) wherein έ is the strain rate, Qdef is the activation energy for deformation, R is the universal gas constant, and Tin is the entry temperature of the aluminium alloy band to the hot rolling mill, generally given in °K. In the above equation,“exp(..)” stands for“e to the power

The Zener-Hollomon parameter (Zdef) can be interpreted as the amount of energy entered in the aluminium alloy material by hot rolling which in turn determines the recrystallization behaviour of the aluminium alloy material after deformation. Fur ther, this parameter also defines the deformation mechanism during a given hot deformation process. In the above formula when applied to hot rolling, έ is the strain rate which is directly proportional to the rolling speed and is generally given in s ¹, Qdef depends on the rolled aluminium alloy material and is generally given in J/mol and the universal gas constant R is 8.3144598 J/(mol °K). The temperature T,_n is the absolute temperature, generally given in °K.

The head end of an aluminium alloy band (also referred to as hot band or strip) is the end that enters the hot rolling mill first, and the tail end is the end that enters the rolling mill last, in one particular rolling pass. In the present invention, the term hot rolling pass means that the whole band passes the hot rolling mill once. In a reversing mill having only one stand, the hot rolling pass is completed after the whole band has passed the stand once. In a multi-stand tandem mill hav ing e.g. two to three (or more) stands, the hot rolling pass is completed after the whole band has passed all stands. Generally, a hot rolling operation will comprise several hot rolling passes through a reversing mill, or alternatively one or several passes through a breakdown mill, followed by one pass through a multi-stand tan dem finishing mill.

The adjustment according to the invention may be done during one pass (e.g. by adjusting the rolling speed along the length of the band as a function of the entry temperature which varies along the length of the band). However, the ad justment may also be made generally once for each pass (without necessarily re quiring adjustments during the pass), e.g. by selecting a specific rolling speed de pending on the entry temperature measured only at the head end of the band.

The invention has realized that the Zener-Hollomon parameter Z_de_f during hot rolling is an important parameter determining the properties of the final product, in particular the r-value, also called Lankford coefficient. In particular, high values for Zdef - up to a certain limit - go together with high r-values, wherein the r-value or Lankford coefficient is a measure of the plastic anisotropy of a rolled band. There fore, it is used as an indicator of the formability, for example as measured by an Erichsen test, which is indicative of stretch forming capability of aluminium alloy rolled sheets. It has been found that the r-value is improved in the hot-rolled band, and is still improved after further processing steps such as cold rolling, solution heat treatment (SHT) and ageing, which of course may also influence the r-value. In this document, r-values have been measured in the final solution heat treated material, i.e. after the hot-rolled band has been cold rolled to final gauge, subjected to SHT and quenching, and ageing.

The r-value may be defined as follows: If x and y are the coordinate directions in the plane of rolling (x in rolling direction and y in 90° to rolling direction) and z is the thickness direction, then the r-value is given by

where ez is the plastic strain in x direction and e_z ^v is the plastic strain through- the-thickness in a tensile test performed in the transverse direction y. In the cur rent invention, r has been measured by tensile testing in the transverse direction at between 8% and 12% tensile plastic strain.

The invention further teaches that Z_de_f (or a parameter related thereto, which is e.g. a function of Zdef) - and therefore the r-value in the final product - may be influenced during a hot rolling operation by adjusting the entry temperature and/or hot rolling speed, preferably by adjusting one of these two parameters in relationship to the other. For example, if the entry temperature is measured at a position just before entry of the hot band into the rolling mill, the hot rolling speed may be ad justed accordingly in a way that Z_de_f remains (at least approximately) constant. This adjustment is exactly opposite to the known practice: In the conventional way of hot rolling of an aluminium alloy, i.e. aiming at a constant exit temperature along the length of the hot band, the Zener-Hollomon parameter is lower at the head end of the hot band as a result of higher entry temperature and lower speed and increases towards the tail end of the hot band, since the band cools down towards the tail and the rolling speed is increased in order to keep the exit temperature constant. The invention has recognized that this severe change of Z_de_f from the head end to the tail end of the aluminium alloy hot band results in a considerable variation of prop erties from the head end to the tail end, especially texture related properties like the r-values.

In contrast thereto, according to an embodiment of the invention, the rolling speed will be decreased when the band cools down, in order to keep the Zener- Hollomon parameter at least approximately constant and/or as high as it is needed to be. This may be performed during a single rolling pass, e.g. in a multi-stand tan dem mill, i.e. the rolling speed is adjusted so as to decrease towards the tail end, where the aluminium alloy band gets cooler. It is also possible to adjust the entry temperature so as to be able to work with a specific rolling speed. For example, if a high Z_de_f is desired, the entry temperature can be reduced by cooling while running at low rolling speed. In another example, in order to maintain a high rolling speed for high productivity, high entry temperature may be needed to obtain a comparably high Z_de_f. According to another embodiment, both entry temperature and hot rolling speed may be adjusted in order to aim at a relatively high and at least approximately constant Zdef.

The invention requires that rolling speed and/or entry temperature are ad justed in order to keep Z_de_f or a function of Z_de_f at least approximately constant and/or within a predetermined range during a hot rolling pass. Thus, Z_de_f or a function of Z_de_f is kept at least approximately constant, wherein approximately constant means within a variation in the range of ±20%, preferably within a variation in the range of ±10%, more preferred within a variation in the range of ±6%, most preferred within a variation in the range of ±3%. Zdef depends on the activation energy Qdef, which may vary between different alloys and the initial state of the material of the alumin ium alloy band. Therefore, the absolute value of Z_de_f may be different for different rolling operations. Therefore, embodiments of the invention require that it should be kept within a variation in the range of ±10%, of its initial value during the rolling pass. In further embodiments, a target value for Zd_ef or a function of Z_de_f is selected within a predetermined range, and rolling speed and/or entry temperature are adjusted in order to keep Z_de_f or a function of Z_de_f at least approximately constant at the target value.

In a preferred embodiment, the function of Z_de_f is the natural logarithm of Z_de_f, In(Zdef). In other embodiments, the function of Z_de_f may be a linear, polynomial and/or logarithmic function of Zdef. Preferably, the parameter related to Zdef is a function of the entry temperature, the rolling speed of the rolling pass and optionally the exit temperature. Z_de_f or a function thereof can be related to a certain property like r- value, and can be determined by simple experiments for a specific product on a specific rolling mill using a specific rolling reduction scheme, for example as ex plained below.

In such experiments, various entry rolling temperatures are combined with various rolling speeds. Exit temperature for every combination is registered. The rolling reduction is kept preferably constant in all experiments. Z_de_f or any parameter related to it or a function of Z_de_f may now be calculated for every experiment knowing the entry and exit temperatures and rolling speed. Once r-values on the final product are measured for every experiment, a relationship is sought between the r-values and the said parameter related to Zdef. The said parameter may be chosen in a way that the relationship is linear by preference. An example of such a relationship is given below in equations (4) and (5).

In detail, a model is developed that predicts the r-values of the final product based on the hot rolling conditions in the rolling mill while the rest of the process is fixed. The resulting model/relationship can be used in a method according to an embodiment of the invention to adjust rolling speed and/or entry temperature dur ing hot rolling:

The Zener-Hollomon parameter Zdef defines deformation mechanism and stored energy for recrystallization.

Where R is the universal gas constant: R=8.314 J/kg °K, and Qdef s activation en ergy for deformation, here assumed to be 279000 J/mol for an AA6016 series alu minium alloy.

A term relating to the rate of static recrystallization may be defined as

Where Qrex is the activation energy for recrystallization, here assumed to be 200000 J/mol for an AA6016 series aluminium alloy.

The function f combines Zdet and Zrex to correlate with final texture.

b= -0.45 (assumed to get best fit), and Tin and Texit are entry and exit tempera tures, respectively, in °K. r-values have been shown to correlate almost linearly with natural logarithm of f, thus

Once a target r-value is selected or predetermined (e.g. 0.60), the corresponding value of ln(f) can be determined experimentally. This value can now be taken as constant ln(fo) for the corresponding exit temperature T exit. Thus, a parameter B re lated to the Zener-Hollomon parameter (and a function of the Zener-Hollomon pa rameter) can be defined as

In this example, B is the natural logarithm of Zdef, i.e. B = In (Z_def). B can be cal culated using experimental data for a given r-value and a given alloy. For the 6xxx series aluminium alloy studied, it has been found that B - and thus the r-value - does not depend on the exit temperature T_exit. Now, a relationship between B and rolling speed V and entry temperature T,_n may be established.

According to equation (1 ), Z_de_f depends on the entry temperature T,_h, and the strain rate έ. If applied to a tandem finishing mill, the entry temperature T,_n typically varies between 300°C and 450°C for 6xxx-series alloys. If a reversing mill is used, the entry temperature is generally higher, typically between 400°C and 500°C. The strain rate έ depends on the partial reduction in the hot rolling stand and the rolling speed (V), wherein it is at least approximately proportional to the rolling speed V. The speed at which the sheet material leaves a hot rolling stand is the rolling speed V. In an industrial tandem finishing mill, exit speed in the final stand is typically be tween 70 and 250 m/min. Average plastic strain in one rolling stand can be calcu lated from the entry and exit thickness of that stand, according to the following equa tion

wherein ho and hi are the entry and exit thicknesses in a rolling stand, respec tively, and In is the natural logarithmic function. A typical final exit thickness from a tandem mill is between 3.0 to 15.0 mm. The strain rate is then a function of strain and speed of rolling, thus έ = Pe, V) (8)

An average strain rate during a rolling pass may be approximated by the following formula, wherein D is the diameter of the work roll.

^ _ 100 V In (h /h

3 3D (h^h /2

Where V is in m/min, D, h and /^are in mm.

Strain in a hot rolling stand is typically between 0.5 and 1 .5 depending on the entry and exit gauges. In the production of a specific product, the rolling reduc tion (or strain) in each hot rolling pass is generally determined once and kept con stant. Therefore, the invention teaches to adjust the rolling speed in order to adjust strain rate.

By inserting equation (9) and (6) into equation (5), rearranging and defining new constants, one obtains:

For the best fit with the experimental data, an offset value Vo has been introduced leading to:

Where

It should be noted here that choice of B determines the level of r-values obtained.

In preferred embodiments, B is selected within a predetermined range and is kept at least approximately constant during hot rolling. Accordingly, equation (1 1 ) is an example of a relationship between a parameter B, which is related to Zdef and is a function of Zdef, rolling speed V and entry temperature Tin_, which may be used in practicing the method according to the invention. A, C and Vo are constant for the product that was studied and its processing route. For the 6xxx series alloy tested, good results were obtained with the values Vo=60, A=5.9 and

C=279, 000/8, 314=33,557. However, for other compositions, and/or for processing on a different mill and/or using a different rolling reduction, other values may be used.

For other alloys, also a parameter B related to Z_de_f may be determined which corresponds to a predetermined target r-value, and formula (1 1 ) above may be used for adjusting V and Tin. However, the invention is not limited to this formula, which is based on various assumptions and experimental data, and the invention can also be executed by using a different relationship between Z_de_f or a function thereof V and Tin_.

Preferably, the at least one rolling pass in which the entry temperature and/or the hot rolling speed is adjusted according to the invention, is the last pass or among the last passes of the hot rolling operation. Thus, if the hot rolling mill is a reversing mill, the inventive method may be applied to the last or at least one of the last 1 to 4 rolling passes, or to each of the 1 to 4 last hot rolling passes. In the case of a tandem finishing mill, the Zener-Hollomon parameter will preferably be that of the last stand, or alternatively an average over all stands of the tandem mill. For practi cal reasons, in a tandem mill, the entry temperature may be measured at the entry to the first stand, whereas the strain rate is determined by the rolling speed of the last rolling stand. Thus, using these parameters, effectively an average Z_de_f is calcu lated for the entire tandem mill. However, preferably the relationship between entry temperature, hot rolling speed and the resulting r-value will be determined experi mentally for each product, so as to yield best results.

The method of the present invention is preferably applied to AA6XXX-series alloys, for example AA6016. In addition, the method may be applied to AA2xxx- series, AA3xxx-series or AA5xxx-series alloys. The method may be used to pro duce aluminium alloy sheet for use in the automotive industry, for example for use as automobile body sheet or other automobile structural parts. The aluminium al loy sheet produced using the method of the invention may be formed into automo bile body panels, for example by deep drawing.

According to an embodiment of the invention, in the hot rolling method of the present invention, the energy entered into the material is maintained approxi mately constant during hot rolling by keeping the Zener-Hollomon parameter (Zdef) or a function thereof (or a parameter related thereto) at least approximately con stant along the length of the hot rolled product. The term "approximately constant" - because of the exponential relationship between entry temperature and Z_de_f - may yet allow for variations of Z_de_f while entry temperature may experience small variations. For example,“approximately constant” in the context of this invention means within a variation in the range of ±20%, preferably within a variation in the range of ±10%, more preferred within a variation in the range of ±6%, most pre ferred within a variation in the range of ±3%. In an embodiment of the invention, a function of Zdef such as f(Zdef) = In (Zdef) is kept approximately constant during said rolling pass, namely within a variation in the range of ±20%, preferably within a variation in the range of ±10%, more preferred within a variation in the range of ±6%, most preferred within a variation in the range of ±3%. In an embodiment, this means that Z_de_f or the function of Z_de_f is kept within this range, starting from the value it has at the beginning of the rolling pass. Keeping Z_de_f or a function of Z_de_f within this range may be achieved by adjusting the rolling speed along the length of the aluminium alloy band, depending on the entry temperature variation along the length of the band. The rolling speed may for example be adjusted continually or stepwise. In the case of a tandem mill, preferably Tj_n will be the entry tempera ture before entering the first stand. Preferably the entry temperature is measured continuously during the hot rolling operation, preferably at a position just before the band enters the hot rolling stand, e.g. using pyrometers. For example, when the entry temperature changes, the rolling speed is adjusted to the new tempera ture, i.e. the rolling speed will be high if the temperature of the sheet is high (e.g. at the head end of the hot band) and the rolling speed is reduced if the tempera ture of the sheet is reduced (e.g. at the tail end of the hot band). This method is quite contrary to the method of constant exit temperature, which is the conventional industrial way of working. In the proposed method, the speed of the moving aluminium alloy band through the tandem/finishing mill would have to be reduced as the hot band gets colder towards the tail end.

It has been demonstrated that this aspect of the invention leads to signifi cantly lower variations of mechanical and surface properties, especially r-values and Wolkenbildung, along the length of the rolled band, both in the hot band after hot rolling, and in the final product, i.e. after further processing steps such as cold rolling SHT and quenching, and ageing.

In an embodiment, if the entry temperature of the aluminium alloy band in a rolling stand varies along the length of the band, the hot rolling speed in that rolling stand is adjusted such that the hot rolling speed is higher at a position along the length of the band where the entry temperature is higher, and is lower at a position where the entry temperature of the aluminium or aluminium alloy band is lower. Thereby, the Zener-Hollomon parameter may be kept approximately constant along the length of the band, leading to lower variations of mechanical and surface properties.

Preferably, the entry temperature of the aluminium alloy band is measured at an entry position of the hot rolling mill during the rolling pass, for example by a pyrometer. In useful embodiments, the entry temperature is measured continually along the length of the band during the at least one pass.

According to an embodiment of the invention, for a given alloy and for a given tandem entry temperature (Tin), the tandem rolling speed should be kept above a minimum value to obtain a predetermined target level of r-values, wherein preferably the target r-value is measured in the final product, e.g. in the band after cold rolling to final gauge, SHT followed by quenching and optionally ageing. The minimum speed may be calculated from the following relationship, which is ob tained by solving equation (5) for έ, and replacing f by fo for the minimum desired r- value and using equation (6) in it.

Wherein T,_n is given in °K and Vmin is given in m/min. In an embodiment using a tandem mill for a AA6xxx series alloy, Vo=60, C=33,557 and A=5.9, but other val ues may be used for other alloys. Bmin determines the level of desired Zener-Hollo- mon parameter and therefore also the r-value. Thus, the above equation (12) is an example of a relationship between rolling speed, entry temperature and a function of the Zener-Hollomon parameter, here termed Bmin. In useful embodiments of the invention for an AA6xxx, in particular an AA6016 alloy, Bmin is at least 45, prefera bly 47 and more preferably 48.

However, the rolling speed should also be kept below a certain maximum in order to keep Z_de_f within the pre-determ ined range. The upper limit may be defined as follows:

wherein in the embodiment for a tandem mill for an AA6016 series alloy, Vo=60, C=33,557 and A=5.9, wherein B_max is 55, preferably 53 and more preferably 52. In other words, by using the above values for Vo, A and C for a 6xxx series alloy, V is calculated from equation (1 1 ) at the given temperature T,_n and at a desired (and thus pre-determ ined) value for B, which is in the range between 45 and 55, prefer ably between 47 and 53, more preferred between 48 and 52, most preferred be tween 48 and 50, depending on the target level of r-value. In an embodiment, the rolling speed is regulated to be higher than Vmin and up to 20%, preferably up to 10%, more preferred up to 3% above Vmin calculated according to equation (12). In an embodiment, Bmin is pre-determ ined and may be in a range between 45 and 53, more preferably between 47 and 52, and most preferred between 48 and 50.

For the alloy studied here (as described in the examples), Bmin or B=48 (preferably 49) is required to reach a desired target r-value of 0.6. Increasing B will raise the r-value, but only up to a certain limit. By continuous measurement of en try temperature along the length of the hot band and calculating the instantaneous speed from the above equation and applying it to the mill, a constant and high r- value may be produced along the length of the band. According to another embodiment of the invention, an entry temperature of the aluminium alloy band and/or the hot rolling speed is adjusted such that the Ze- ner-Hollomon parameter and/or a function thereof is kept above a certain value, in particular above a pre-determ ined minimum value, wherein this value may be ex perimentally determined depending on the requirements on the properties, in par ticular on the target r-value. The invention has recognized that high r-values are associated with higher values of Zener-Hollomon parameter, up to a certain limit. Thus, to increase r-values up to a certain maximum value, higher strain rates (roll ing speed) and lower entry temperatures (Tin) are preferred. For example, upon an increase in entry temperature to the rolling mill the rolling speed will be increased in order to keep the r-values high, and preferably at least approximately constant. This may be along the length of a single hot band, or may be in-between several hot bands. If higher r-values are desired, for a given entry temperature, the hot rolling speed may be increased. At a constant hot rolling speed, a decrease in en try temperature will make the r-values to increase up to a certain maximum value. For example, in equation (12), the entry temperature and/or the hot rolling speed may be adjusted such that B_min is in a range between 45 and 53, more preferably between 47 and 52, and most preferred between 48 and 50.

According to a further embodiment, the entry temperature T,_n of the alumin ium alloy band when entering said hot rolling mill is measured, and the hot rolling speed (V) of said rolling pass is adjusted such that the Zener-Flollomon parameter (Zdef), or a function of the Zener-Flollomon parameter, is kept within a pre-deter- mined range (and preferably at least approximately constant). This may be per formed by determining a relationship such as equation (11 ) between Zdef, T,_n and V, experimentally for a specific product and hot rolling reduction scheme, and ad justing V depending on the entry temperature of a particular band. In one embodi ment, the entry temperature at the head end of the band is measured and used to determine one (constant or approximately constant and minimum) rolling speed for this pass. In other embodiments, the entry temperature is continually measured during the pass and the rolling speed continually adjusted accordingly. According to another embodiment, the entry temperature of the aluminium alloy band before at least one rolling pass is adjusted in relation to a rolling speed of said rolling pass. Thus, the relationship between Ti_n and V may also be used to calculate the desired T,_n in relationship to the rolling speed. This may be done by cooling the aluminium alloy band before the said rolling pass or by pre-heating the hot rolling stock to a desired temperature. In some embodiments, the hot rolling speed of a rolling stand may be calculated using the entry temperature actually measured only once at the head end of said stand. The hot rolling speed is then kept constant along the length. In some embodiments, the hot rolling speed is kept constant as long as the continually measured entry temperature stays within a pre determined range, in which Z_def is within an acceptable, pre-determ ined range.

According to a further preferred embodiment, the entry temperature, T,_h, at the head end of the aluminium alloy band before the at least one rolling pass is rel atively low, in particular <420°C, preferably < 400°C. By limiting the entry tempera ture to the tandem or finishing hot mill to maximum 420°C or preferably to 400°C, temperature variations along the length of the hot band are reduced significantly, making the speed variations along the length unnecessary (in order to maintain a constant Zener-Hollomon parameter). This leads to the concept of constant-speed methodology for operation of the hot tandem mill or finishing mill. Limiting the max imum entry temperature, in addition to allowing to use a constant speed, has the advantage that exit temperature also stays constant.

According to a further embodiment, the rolling speed is calculated only once based on the T,_n at the head end and a pre-determ ined minimum Z_def value and kept constant further on along the length. It has been found that the rolling speed V may be kept constant or at least constant within normal operating variations, i.e. without deliberately changing V during the rolling pass, when the entry tempera ture of the said pass is low, in particular <420°C, preferably < 400°C. Constant speed of rolling is calculated only once based on the entry temperature of the said rolling pass, which may be tandem or finish rolling and the desired level of Zener- Hollomon parameter. Although this will lead to some variation in Z_def preferably the variation will be within the definition of“at least at least approximately constant”, i.e. within ±20%, preferably within ±10%. The advantage of this embodiment is that operation of the mill is simplified, in addition to maintaining constant properties along the length.

The invention is also directed to a method for manufacturing an aluminium al loy band, comprising the following steps:

(a) casting an aluminium alloy ingot to provide a rolling feedstock;

(b) homogenizing and/or preheating the rolling feedstock;

(c) hot rolling the rolling feedstock into an aluminium alloy band according to the hot-rolling method described herein, preferably to a thickness in a range of 3.0 to 15.0 mm;

(d) optionally coiling the aluminium alloy band;

(e) optionally cold rolling the aluminium alloy band to a final thickness in a range of 0.5 to 5.0 mm;

(f) optionally solution heat treating and quenching the cold-rolled aluminium al loy band,

(g) optionally ageing the quenched aluminium alloy band to obtain a finished band.

The finished band may be used as aluminium alloy sheet product. The age ing may be natural ageing or artificial ageing. The cast aluminium alloy is prefera bly of the 6XXX-series, and more preferred an alloy used for automotive applica tions, such as automobile panels.

The aluminium alloy as described herein can be provided as an ingot or slab for fabrication into a rolling feedstock using semi-continuous casting tech niques regular in the art for cast products, e.g. DC-casting, EMC-casting, EMS- casting, and preferably having an ingot thickness in a range of about 300 mm or more, e.g. 400 mm, 500 mm or 600 mm. The rolling feedstock may be about 1000 mm or more in width and by about 3.5 meters or more in length. In another em bodiment thinner gauge slabs resulting from continuous casting, e.g. belt casters or roll casters, also may be used, in particular when producing thinner gauge end products. After casting the rolling feedstock, the thick as-cast ingot is commonly scalped to remove segregation zones near the cast surface of the cast ingot. Next, the rolling feedstock is homogenized and/or preheated. It is known in the art that the purpose of a homogenisation heat treatment has at least the follow ing objectives: (i) to dissolve as much as possible coarse soluble phases formed during solidification, and (ii) to reduce concentration gradients to facilitate the dis solution step. A preheat treatment achieves also some of these objectives.

Following the homogenization and/or preheat practice the rolling feedstock is hot rolled as described herein. The final thickness of the hot rolled band is pref erably between 3.0 and 15.0 mm.

Furthermore, the aluminium alloy band manufactured by the hot rolling method according to the invention can be, if desired, cold rolled, inter-annealed or pre-stretched to improve flatness, solution heat treated (SHT), cooled, preferably by means of quenching, stretched or cold rolled, and aged after solution heat treat ment, to obtain a finished band. The thickness of the finished band is preferably between 0.5 and 5.0 mm.

The invention is also directed to a hot rolled band produced by the hot roll ing method described herein, and to a finished band, i.e. a band produced from the hot-rolled band by the further processing steps recited herein, in particular cold rolling, optionally SHT followed by quenching, and optionally ageing.

In a preferred embodiment of the present invention, the r-values for the fin ished band manufactured according to the invention and made of an AA6xxx- series aluminium alloy are in the range of 0.55-0.9, preferably at least 0.6 when measured by a tensile test according to ISO 10113 in transverse direction. These r-values are measured in the middle of the width, as there is variation of the r- value across the width of an aluminium alloy band. However, such r-values are preferably achieved at all positions along the length of the band.

BRIEF DESCRIPTION OF THE FIGURES

The invention shall now be described by means of embodiments with refer ence to the attached drawings in which:

Fig. 1 shows a graph of r-values along the length of a hot rolled aluminium alloy band of the AA6xxx series depending on the distance from the head end and the tail end, wherein the band was obtained with conventional tandem hot rolling practice (constant exit temperature), and afterwards subjected to cold-rolling, SHT followed by quenching and ageing.

Fig. 2 shows a graph of the relationship of the r-value with an inverse func tion Of Zdef.

Fig. 3 shows an example of a relationship between entry temperature and rolling speed required to obtain a certain minimum r-value.

Fig. 4 shows an example of rolling speed adjusted to Tin during one rolling pass at a constant Zdef.

Fig. 5 shows a schematic representation of a tandem rolling mill.

DESCRIPTION OF EMBODIMENTS

Fig. 1 (prior art) is a plot of r-values measured at various distances between 20m and 160m from the tail end (circles) and from the head end (crosses) of a fin ished aluminium alloy band having a length of more than 2000 m. The r-values measured very close to the ends are thus shown on the left, whereas the r-values measured closer to the middle of the band are depicted on the right. The graph demonstrates that the r-value is low at the head end of the band as a result of low speed and high entry temperature and is very high at the tail end of the band as a result of low temperature and high speed. This severe change of stored energy from head to tail of a hot band results in variation of properties from head to tail in the finished band, especially texture related properties like r-values.

The graph of Fig.2 shows how r-values are correlated with an inverse function of Zdef. The inverse function / can be defined in such a way as to have a linear relationship with the r-values. According to an example of the invention, it may be defined as follows:

Wherein a and b may be determined experimentally for a specific alloy product. Clearly, f {Zdef) decreases as Zdef increases. Any other type of function relating Zdef to the r-values can be used to obtain the same objective. The application of the graph of Fig. 2 is as follows. In order to obtain r-val- ues above a certain minimum value, the inverse function /(Zdef) has to stay below a certain maximum value or f{Z_der) £ C (15) or

Zdef ³ C (16)

Accordingly, a desired maximum value for the function / can be found by defining a desired minimum r-value. Constant C for the product of concern can be determined experimentally. When C is known, the desired relationship between speed of rolling ( έ ) and entry temperature to the mill (T_in may be found from equa tion (1 ) and the above calculations and rearrangements.

An example of such relationship is equation (11 ) and is shown in Fig. 3. The non-linear relationship between Ti_n and rolling speed V is plotted for Z_de = C. Any possible speed-temperature combination below the curve of Fig. 3 guarantees r- values above the target value from Fig. 2.

From Figs. 2 and 3 it can be observed that

1. An increase in entry temperature to the hot rolling mill requires an increase of speed in order to keep the r-values constant. This may be along the length of hot band, or in between different hot bands.

2. At a constant hot rolling mill entry temperature, an increase of speed will make the r-values to increase (to stay below the curve of Fig.3).

3. At a constant hot rolling speed, a decrease of entry temperature will make the r-values to increase (to stay below the curve of Fig.3).

The relationship of equations (16) or (11 ),(12), (13) can be implemented in a hot rolling mill, especially a hot finishing mill where entry temperature to the hot rolling mill is feed forwarded to calculate the speed of the hot rolling mill. The choice of constant C determines the overall level of r-values and exit temperature. A tandem finishing mill which may be used in the invention is shown in Fig.

5. In this case, the tandem mill 1 has three rolling stands, S1 , S2, S3. Where the hot band 2 enters the first rolling stand S1 , the temperature Tin is measured by an entry pyrometer 4. Temperature acquisition in this example is performed by a pair of ratio pyrometers, one pyro 4 on the entry side of the mill, which may be used for feedforward control of the rolling speed, and an exit pyro 6 between the last stand S3 and the coiler 16. Each pyro 4, 6 has an associated dual thermocouple assem bly 5, 7, respectively, used for calibration. The band 2 may be cooled in between the stands by action of the coolant spray system 10. Further, an air blow-off sys tem 12 and an air-coolant blow-off system 14 are present, which when in operation keep the band dry in the inter-stand area and thus minimize strip cooling. After coiling, the coil 18 is removed from the coiler 16.

EXAMPLES

Example 1

Aluminium alloy rolling stock of composition 1 .23 wt% Si, 0.40 wt% Mg, 0.23 wt% Fe, 0.06 wt% Cu, 0.13 wt% Mn, 0.018 wt% Cr, 0.019 wt% Ti, remainder Al and unavoidable impurities, was produced by DC-casting followed by homogenization. The hot rolling stock was first rolled to 28 mm in a breakdown mill and then trans ferred to a three-stand tandem mill, where the hot band was reduced from 28 mm to 7.5 mm, resulting in a total reduction of 73% in the tandem mill. The entry tem perature of the hot band to the tandem mill was measured along the length for each band, wherein the temperature was measured at the entry of the first rolling stand, and the rolling speed after the last rolling stand was kept constant for each band according to the below table 1 . After hot rolling, the band was coiled, cooled down, cold rolled to an inter-annealing gauge of 4.5mm, inter-annealed, cold rolled to a final gauge of 1.0 mm and finally solution heat treated. SHT was done in a continu ous furnace. All process steps after the hot rolling, and in particular the cold rolling practice, were the same for each band, only the hot rolling speed and entry temper ature were varied for each band, in order to study the effect of different hot rolling practices. From the finished bands, samples were collected from head and tail, and from the exact position of the said samples in the coil, entry tandem temperatures were traced back to the band at tandem gauge (7.5 mm). Finally, the r-values were measured by a tensile test according to ISO 10113 in transverse direction between 8% and 12% plastic strain.

Table 1 shows some numerical examples for C (i.e. Zdef) and the correspond- ing values for hot rolling speed and hot mill entry temperature and the resulting r- values in the final product.

Table 1

As can be seen, the higher the C, the higher is the r-value. It is also seen that, the higher the entry temperature, the higher should the speed be to guarantee a minimum_r-value.

Example 2

In another example, rolling stock was produced as in example 1. The princi ple of constant Z_def was used to calculate the desired rolling speed from equation (12) for an experimentally processed coil at constant speed using the instantane ous entry temperature Ti_n. In this example, Z_def was kept constant at 1.3955E+21 (which corresponds to B=48,68). The resulting rolling speed V_min and the instanta- neous entry temperature T,_n are plotted in Fig. 4. Fig. 4 depicts the measured entry temperature along the length of a band of length 270m after hot rolling, and the corresponding hot rolling speed, calculated for a Z_def of 1.3955 10²¹ by using equa tion (12). The head end is shown at 0m on the left of the graph. As seen, gradually decreasing T,_n leads to gradually decreasing V_min that guarantees a constant r- value (here 0.60) along the length. By contrast, the conventional way of steering (constant exit temperature) would here lead to a gradually increasing speed (espe cially during the last 50 meters) and therefore higher r-values at the end than the beginning of the band. The invention is not limited to the embodiments described above, and which may be varied widely within the scope of the invention as defined by the appended claims.

Claims

1 . Method for hot rolling an aluminium alloy band (2), wherein the hot rolling method comprises at least one rolling pass through a hot rolling mill (1 ), wherein an entry temperature of the aluminium alloy band (2) when entering said hot rolling mill and/or the hot rolling speed of said rolling pass is adjusted such that the Zener-Hollomon parameter (Zdef), or a function of the Zener-Hollomon parameter (Zdef), is kept at least approximately con stant, wherein approximately constant means within a variation in the range of ±20%, preferably within a variation in the range of ±10%, more preferred within a variation in the range of ±6%,

whereby Z_de_f is defined by the following formula

wherein έ is the strain rate, Q_de_f is the activation energy for defor mation, R is the universal gas constant, and Tin is the entry temperature of the aluminium alloy sheet to the hot rolling mill.

2. Method according to any one of the preceding claims, wherein the rolling speed during said at least one hot rolling pass is adjusted depending on the entry temperature.

3. Method according to any one of the preceding claims, wherein the rolling speed during said at least one hot rolling pass is adjusted along the length of the aluminium alloy band according to the measured entry temperature so that the Zener-Hollomon parameter (Zdef) is kept at least approximately constant along the length of the aluminium alloy band, wherein approxi mately constant means within a variation in the range of ±20%, preferably within a variation in the range of ±10%, more preferred within a variation in the range of ±6%.

4. Method according to any one of the preceding claims, wherein an entry temperature of the aluminium alloy band (2) when entering said hot rolling mill varies along the length of the aluminium alloy band,

wherein the hot rolling speed during said at least one hot rolling pass is adjusted such that the hot rolling speed is higher at a position along the length of the band where the entry temperature is higher, and is lower at a position where the entry temperature of the aluminium alloy band (2) is lower.

5. Method according to any one of the preceding claims, wherein the entry temperature of the aluminium alloy band (2) is measured at an entry posi tion (4) of the hot rolling mill (1 ) during the rolling pass.

6. Method according to any one of the preceding claims, wherein an entry temperature of the aluminium alloy band (2) when entering said hot rolling mill and/or the hot rolling speed of said rolling pass is adjusted such that the Zener-Hollomon parameter (Zdef), or a function of the Zener-Hollomon pa rameter, is kept above a pre-determ ined minimum value.

7. Method according to any one of the preceding claims, wherein the entry temperature of the aluminium alloy band (2) when entering said hot rolling mill is measured, and the hot rolling speed of said rolling pass is adjusted such that the Zener-Hollomon parameter (Zdef), or a function of the Zener- Hollomon parameter, is kept at least approximately constant.

8. Method according to any one of the preceding claims, wherein the entry temperature of the aluminium alloy band (2) before the at least one rolling pass is adjusted in relation to a rolling speed of said rolling pass.

9. Method according to any one of the preceding claims, wherein the entry temperature, Ti_n, of the aluminium alloy band (2) before the at least one roll ing pass is < 420°C, preferably < 400°C.

10. Method according to any one of the preceding claims 5 to 9, wherein the rolling speed is calculated only once based on the T,_n at the head end and a pre-determ ined minimum value of Z_def or a function of Z_def and kept constant further on along the length.

11. Method according to any one of the preceding claims, wherein the alumin ium alloy is an AA6xxx-series alloy.

12. Method according to any one of the preceding claims, wherein for a given entry temperature (Tin), the rolling speed V in m/min is adjusted to be above a minimum value Vmin, wherein Vmin is determined as follows:

Wherein Ti_n is given in °K, and for an alloy of the AA6016 series, preferably Vo=60, A=5.9, C=33,557, wherein B_min is pre-determ ined and preferably in the range between 45 and 53, more preferably between 47 and 52, and most preferred between 48 and 50.

13. Hot rolled aluminium alloy band produced according to the method of any one of claims 1 to 12.

14. Method for manufacturing an aluminium alloy band (2), comprising the fol lowing steps:

(a) casting an aluminium alloy ingot to provide a rolling feedstock;

(b) homogenizing and/or preheating the rolling feedstock;

(c) hot rolling the rolling feedstock into an aluminium alloy band (2) ac cording to any one of the preceding claims 1 to 12, preferably to a thickness in a range of 3.0 to 15.0 mm; (d) optionally coiling the aluminium alloy band;

(f) optionally solution heat treating and quenching the cold-rolled alu minium alloy band,

(g) optionally ageing the quenched aluminium alloy band into a finished band.

15. Aluminium alloy band produced according to the method of claim 14,

wherein the finished band preferably has an r-value of >0.55, more pre ferred >0.6 along the length of the finished band.