WO1998035069A1

WO1998035069A1 - A process of reducing roping in automotive sheet products

Info

Publication number: WO1998035069A1
Application number: PCT/CA1998/000070
Authority: WO
Inventors: David James Lloyd; Gene Bruce Burger; Daniel Ronald Evans; Alok Kumar Gupta
Original assignee: Alcan International Limited
Priority date: 1997-02-05
Filing date: 1998-02-03
Publication date: 1998-08-13

Abstract

A process of producing an aluminum alloy sheet article having a reduced tendency to exhibit roping in products formed from those articles. The process involves the steps of direct chill casting a heat-treatable aluminum alloy, having a tendency to exhibit roping, to form a cast ingot, homogenizing the cast ingot, hot rolling the homogenized cast ingot, cold rolling the intermediate hot-rolled cast ingot to produce a sheet article of final gauge, and subjecting the sheet article of final gauge to a solutionizing step. Most importantly, the process includes a step, carried out during or after the cold rolling but before the solutionizing step, of subjecting the sheet article to a recovery anneal by heating the sheet article to a temperature in a range of 300 to 350 °C for 2 to 16 hours. The recovery anneal changes the crystallographic structure of the sheet article so that it has less tendency to exhibit roping when formed into product, e.g. automotive parts. The invention also relates to sheet articles produced by the process and formed products produced from the sheet articles.

Description

A Process of Reducing Roping in Automotive Sheet

Products

TECHNICAL FIELD

This invention relates to a process of reducing the tendency of aluminum alloy sheet articles to exhibit roping when formed into products, e.g. automotive parts. More particularly, the invention relates to the reduction of roping tendencies in aluminum-magnesium- silicon (i.e. 6000 series) alloy sheet products, and other heat-treatable aluminum alloys (e.g. the 2000 series alloys) typically used for fabricating automotive parts .

BACKGROUND ART

The use of aluminum alloys for automotive parts is increasing because of the desirable characteristics of such products, namely their light weight, corrosion resistance and high strength. Aluminum alloys of the 6000 series (alloys containing Al-Mg-Si) , and sometimes alloys of the 2000 series, are often chosen for automotive use, and the usual method of producing the sheet involves direct-chill (DC) casting, homogenising, hot rolling, cold rolling, solution heat treatment and possibly additional steps such as aging and levelling. This produces an aluminum sheet product in the so-called T4 temper.

Unfortunately, such sheet products often suffer from a tendency to exhibit a phenomenon known as roping, ridging or "paint brush" line formation (the term "roping" is used exclusively henceforth) when converted to formed products. Roping is a non-uniform pattern of surface relief caused by locally inhomogenous deformation which is heavily orientated in the rolling direction of the sheet article. The generation of a roped surface requires a component of stretch in the transverse direction, e.g. during subsequent transverse straining of the sheet products as they are being formed into automotive parts. Such roping is visible in, and detracts from, the final surface finish of the automotive product.

In the case of aluminum sheet products produced in the above manner, roping may be inhibited by modifying the production method so that complete recrystallisation occurs at an intermediate stage of processing. Several methods can potentially achieve this result, such as batch annealing for 1 hour at 400°C at an intermediate gauge. However, batch annealing tends to result in the formation of coarse Mg₂Si precipitates, which are difficult to redissolve during continuous heat treatment solutionizing (which involves heating characterized by a rapid thermal spike rather than sustained heating) .

The inhibition of roping is addressed in US Patent No. 5,480,498 issued on January 2, 1996 to Armand J. Beaudoin, et al . and assigned to Reynolds Metals Company. In this patent, cast aluminum alloy is homogenized and then hot rolled to form an intermediate product . Steps are taken to ensure that the exit temperature of the intermediate product resulting from the hot rolling process does not exceed 650°F (343 °C) . Either before cold rolling is commenced, or after cold rolling has commenced but has not yet been completed, the sheet product is subjected to an intermediate annealing step carried out at 600 to 1000°F (316 to 538°C) for periods of time up to 18 hours. This represents a full recrystallizing interanneal to change the crystallographic structure of the intermediate product . The intermediate annealing is then followed by cold rolling (or the remainder of the cold rolling steps) to final gauge, solution heat treatment, levelling and eventually forming into the desired automotive product.

The problem with this process is that it requires very careful control of the temperature of the intermediate product during the hot rolling phase, and it tends to results in extensive coarsening of precipitate phases, which are difficult or impossible to fully dissolve in the short thermal cycle associated with continuous annealing. The lack of full solutionizing in this way decreases strength and makes it difficult to achieve a consistent paint bake response (an increase of hardness achieved by some alloys after forming, painting and baking typical of the process used to finish automotive panels) .

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an improved method of reducing or inhibiting roping tendencies in aluminum alloy sheet products . Another object of the invention is to provide an improved process of reducing or inhibiting roping tendencies in 6000 series or 2000 series aluminum alloy sheet products.

Another object of the invention is to provide a method of reducing or inhibiting roping tendencies in aluminum automotive sheet articles without disturbing conventional hot rolling and cold rolling treatment steps carried out during the conventional production of such sheet articles.

According to one aspect of the present invention there is provided a process of producing an aluminum alloy sheet article having a reduced tendency to exhibit roping in products formed from said sheet article, said process comprising the steps of: direct chill casting a heat-treatable aluminum alloy having a tendency to exhibit roping, to form a cast ingot; homogenizing said cast ingot to produce an homogenized cast ingot; hot rolling the homogenized cast ingot to produce an intermediate hot-rolled sheet article; cold rolling the intermediate hot-rolled sheet article to final gauge; and subjecting said sheet article of final gauge to a solutionizing step; wherein said process includes a step, carried out during or after said cold rolling and before said solutionizing step, of subjecting a sheet article to a recovery anneal comprising heating said sheet article to a temperature in a range of 300 to 350°C for 2 to 16 hours.

By the term "recovery anneal" we mean an annealing step that has the effect of recovering the deformation structure of the alloy to give a combination of texture components that minimize roping tendencies of the final sheet article. A recovery anneal differs from a "full anneal" in that the latter causes complete recrystallization to produce a stable grain structure, whereas the former does not . The recovery anneal may be carried out after the cold rolling has been completed, or if the cold rolling involves several rolling steps, the recovery anneal may be carried out after a first of the cold rolling steps but before the final cold rolling step.

The invention also relates to sheet articles of reduced roping tendency thus produced, and products formed from such sheet articles. Preferably, the alloy is a 6000 series aluminum alloy (containing aluminum, magnesium and silicon), e.g.

AA6111, AA2036, AA6010, AA6016 and AA6022, and is most preferably the AA6111 alloy. However, the process is also effective for heat-treatable 2000 series alloys (a heat-treatable alloy is one which exhibits increased strength due to precipitation when held at elevated temperature) .

The process of the invention has the advantage that the recovery anneal is carried out on sheet of the final gauge, or close to final gauge, so that the capacity of the rolling line does not have to be increased.

Moreover, the effective temperature and treatment time ranges are quite wide, so that variations likely to be encountered in commercial operations can be accommodated. The process of the invention is therefore effective and economical.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A, IB, 1C and ID are each photomicrographs showing the dislocation structures of aluminum sheet products following a recovery anneal carried out at 300°C for periods of time of 1 hour, 2 hours, 5 hours and 16 hours, respectively;

Figure 2 is a graph showing the volumes of various crystallographic components of aluminum sheet products (alloy AA6111) after various treatments, i.e. after conventional cold-rolling, in T4 temper and in T4 temper following a recovery anneal (Rec Ann) according to the present invention; and

Figure 3 is a photograph showing two aluminum alloy sheet products positioned side by side, one having been produced by a standard method (left hand product) and the other having been treated to a recovery anneal according to the present invention (right hand product) . Both sheets were strained at 10 to 15% in the transverse direction and then rubbed with a polishing stone to reveal high and low points in surface topography.

BEST MODES FOR CARRYING OUT THE INVENTION

As noted above, the present invention inhibits roping by employing a recovery anneal at final gauge, or close to final gauge, prior to the conventional solution heat treatment employed during the manufacture of sheet product suitable for automobile part manufacture. In this process, cold rolled sheet, produced by a standard hot and cold rolling route, is partially annealed (referred to as a recovery anneal) prior to the normal solution heat treatment (usually carried out on a continuous heat treatment line) .

The recovery anneal of the present invention does not have to be carried out on final gauge material to be effective, and instead may be carried out between two cold rolling steps. Such a treatment prevents roping provided the reduction in thickness is limited to about 30% or less (preferably about 20% or less) during the final cold rolling carried out between the recovery anneal and the continuous annealing and solution heat treatment. The final cold rolling to gauge after the recovery anneal produces a product which is stronger when it commences the continuous annealing and solution heat treatment, and so is less susceptible to dings and damage during handling. Moreover, a more desirable finer grained final product is produced than is the case when the recovery anneal is carried out on the final gauge material .

Experiments have shown that recovery annealing times of 2 to 16 hours, more preferably 2 to 5 hours (when the recovery anneal is carried out on final gauge material) , at 300°C and 350°C result in a significant reduction of roping, or no roping, in products formed from the resulting T4 sheet. In the case of alloy AA6111, for example, the most preferred conditions for the recovery anneal treatment are found to be approximately 4 hours at 300°C. An objective is always to minimize the treatment temperature and the time at the treatment temperature, while still achieving the desired recovery of the deformation structure and the elimination of roping. A typical example of a recovery anneal carried out on AA6111 alloy between cold rolling steps involves hot rolling a DC cast ingot to one tenth inch, cold rolling to an intermediate gauge, recovery annealing at the intermediate gauge for about 4 hours at about 300°C, subjecting the intermediate product to a 20% reduction cold roll, and then subjecting the alloy to a continuous annealing and solution heat treatment.

Other 6000 series alloys, and some 2000 series alloys, are very similar from the microstructural point of view, and thus react in the same way to the recovery anneal. However, slightly different temperatures and times may be required for different alloys. Suitable ranges of temperature and time can be found by simple trial and experimentation, while verifying a lack of roping in the final product .

The steps of direct chill casting, hot rolling, cold rolling and solutionizing also employed in the present invention may be conventional. For example, suitable steps are disclosed in US patent No. 5,616,189, issued on April 1, 1997 to Jin et al . and assigned to the same assignee as the present application. Suitable steps are also disclosed in US patent 5,480,498 mentioned above. The disclosure of both these patents are incorporated herein by reference.

As an example of the conventional steps, the alloy may be DC-cast using practices common in the industry. The resulting ingot is then homogenized at 540-580°C for a period of 4 to 16 hours, and then directly hot rolled to an intermediate gauge of 2.5 to 8 mm. This intermediate gauge sheet is then cold rolled to a final gauge of approximately 1 mm, and then solutionized on a continuous annealing line, achieving a maximum temperature of 540-570°C, followed by rapid cooling. When employing such methods, the recovery anneal is preferably carried out prior to the final cold rolling step.

In contrast, it is in some cases possible to produce sheet product of final gauge directly from hot mill exit gauge ingot in a single pass. In such cases, the recovery anneal would, of course, be carried out on the sheet product of final gauge before solutionizing. Since the phenomenon of roping is still not fully understood, it is not possible to give a reliable theoretical reason as to why the present invention results in the inhibition of roping. However, roping appears to be due to the formation of certain "hard" crystallographic texture components in the final sheet as well as their spatial distribution. The recovery annealing treatment of the present invention is believed to stabilize these components, so that the textural content and distribution after recrystallisation during solution treatment is modified, and the localization of plastic deformation, or roping, is inhibited.

Unlike the batch annealing procedure, which involves annealing at 400 °C to fully recrystallise the sheet product, the recovery anneal of the present invention involves a lower temperature heat treatment which is designed to recover the dislocation structure, and to generate texture components which will survive the subsequent solution heat treatment at the final gauge, and hence avoid the generation of spatial correlated "hard" texture components.

Global texture analysis of batch annealed sheet shows that it produces an increase in the volume fraction of "cube" (001)<100> and "rotated cube" 001<310> texture components and essentially eliminates the "Goss" (011)<100>. Other recrystallisation studies indicated that the "cube" component may survive the solutionizing treatment, provided a sufficient volume fraction of cube can be generated, and the cold reduction minimized. These results lead the inventors to the concept of introducing a formal recovery treatment (recovery anneal) just prior to the final continuous annealing line solution treatment. Ideally, the recovery anneal should produce a structure in which the cube and rotated cube components are each present on a global scale in amounts of approximately 5% by volume or more and the Goss component is present similarly in amounts of less than approximately 3% by volume of the sheet article of final gauge following the solutionizing.

The concept underlying the present invention was tested by the inventors as explained in the Experimental Verification section below.

EXPERIMENTAL VERIFICATION

Three annealing temperatures were initially considered: 250°C, 300°C and 350°C for times up to 16 hours. Plant cold-rolled AA6111 sheet, designated coil 11035, was given a recovery anneal followed by a solution heat treatment at 560°C, then naturally aged for 7 days and pulled 15% in the transverse direction to assess roping. In addition, the T4 and T8X properties of the sheet given the recovery anneal were measured (T8X temper refers to a sheet product that has been deformed in tension by 2% followed by a 30 minute treatment at 177°C to represent the forming plus paint curing treatment typically experienced by automotive panels) . The results of the roping test referred to above are given in Table 1 below.

TABLE 1 Roping Tendency for Different Recovery Anneals

These results show that the use of a "recovery anneal" can eliminate, or greatly reduce, roping in AA6111 alloy, and that the associated increase in grain size does not appear to be extreme.

The mechanical properties after the different treatments are shown in Table 2 below.

TABLE 2 Mechanical Properties after Different Recovery Anneals

^XUTS stands for ultimate tensile strength. This is the maximum stress attained in a tensile sample during a uniaxial tensile test. At strains beyond this point, continued deformation is confined to a small region in the tensile sample, called the neck.

²Yield strength (YS) is the stress at which plastic flow initiates in a material. At this point, the entire volume of material is deforming plastically. It is usually defined by the 0.2% offset criterion, given by the stress at which the strain deviates from linear elastic stress-strain behaviour by 0.2%.

The mechanical properties, in the T4 and T8X tempers are very comparable for all the treatments.

A final experiment, carried out on 5 mm re-roll AA6111 alloy, coil 65779, was carried out to consider introducing the recovery anneal at intermediate gauge. The alloy was cold rolled to different reductions, given a recovery anneal of (a) 5 hours at 300°C and (b) 12 hours at 300°C, followed by cold rolling to 1 mm final gauge and solution treated. Table 3 shows the results for different reductions between the recovery anneal and the final solution treatment. The roping tendency and fine grain size were then measured.

TABLE 3

The influence of Cold Reduction after the Recovery Anneal on final Product Roping

The results in Table 3 show that a recovery anneal followed by a cold reduction of 30% or less can reduce the level of roping in the final T4 sheet.

As noted above, roping is thought to reflect the spatial distribution of "hard" and "soft" crystallographic texture components in the sheet. While at present there are no techniques available which can provide an exact description of the three-dimensional spatial distribution of texture components in a product, it is of interest to examine the microstructural changes occurring during the recovery anneal and the global texture distributions developed as a result of this processing route.

Figures 1A, IB, 1C and ID of the accompanying drawings show the dislocation structure after different recovery annealing times at 300°C.

The recovery anneal produces a rearrangement of the dislocation structure to a well recovered, fine scale sub-grain structure which is stabilized to some extent by the incoherent precipitates, the majority of which has been formed during the prior hot rolling. At the longest time of 16 hours at 300°C, the occasional sub- grain had coarsened significantly, indicating that recrystallisation to produce a stable grain structure is just beginning.

Figure 2 of the accompanying drawings shows the different global texture components developed in the test . The Figure presents volume fractions of texture components calculated from the global texture distributions evaluated for AA6111 sheet processed through two processing routes, and compares them with that of the cold-rolled sheet. The volume % calculations were carried out utilizing model texture distributions with a Gaussian spread (phi 0 = 11 deg) about the exact orientation of the texture component. The alloy in all cases was similar, i.e. at the same nominal composition within the AA6111 composition range. The texture of the cold-rolled sheet was obtained after cold rolling the sheet through a 63% reduction in thickness. Cold rolling was carried out directly after hot rolling. From this point in the process route, coil 11035 was directly passed through the solution heat treatment (SHT) line, and it is the texture in this T4 condition (i.e. following SHT and natural aging at room temperature) which was evaluated and presented in Figure 2. Coil 43522, following cold rolling, was subsequently given a recovery anneal ( "Rec Ann") treatment at 300°C for 4 hours. The coil was then passed through the solution heat treatment line. The texture of this coil in the T4 condition was evaluated and the results of the analysis given in Figure 2. The Figure shows that the recovery anneal generates a significant volume fraction of cube texture, which is retained after the solution treatment. As already noted, included in this Figure for comparison is the texture developed in material produced by conventional cold rolling, and material in conventional T4 temper.

The results show that a recovery anneal of about 5 hours at 300°C shortly prior to solution heat treatment on the continuous annealing line will reduce the tendency for roping.

Figure 3 compares the stoned surface of conventionally processed sheet with sheet subjected to a recovery anneal . The lack of roping in the latter product is apparent . The experiments also suggest that the process of the invention is quite robust in that the time window can be as large as 16 hours at 300°C, and for shorter times the temperature can be as high as 350°C. In addition, work done to date indicates that the process can tolerate up to a 30% cold reduction (more preferably a 25% cold reduction, and ideally about 20% cold reduction) between the recovery anneal and the solution treatment without introducing significant roping in the final T4 sheet. The tensile properties of the final T4 sheet are equivalent to those of standard AA6111-T4 sheet processed without any heat treatment at intermediate gauge .

The present invention is illustrated further by the following Examples, which are not intended to limit the scope of the present invention. EXAMPLE 1

A plant trial was carried out in the assignee's Kingston Works in Kingston, Ontario when coil 43522 was subjected to a recovery anneal of 4 hours at 300°C immediately prior to solution heat treating in a continuous heat treat line. The final T4 sheet was free of roping.

EXAMPLE 2

Cold rolled AA6111, coil 1085, was partially annealed at:

250°C - 2-16 hrs - roped

300°C - 2 hrs slight roping

300°C - 8 hrs no roping

300°C - 16 hrs no roping

350°C - 2 hrs slight roping

350°C - 8 hrs slight roping

350°C - 16 hrs slight roping

400°C - 2 hrs roped

prior to solutionizing at 560°C in a sand bed and then assessing the roping. The results are shown above.

Following this, a further series of experiments were carried out using recovery anneals of : 2, 8, and 16 hours at 300°C 2, 8, and 16 hours at 350 °C and the specimens examined for roping, as well as their tensile properties.

All of the recovery anneal treatments inhibited roping, with the recovery anneals at 300° C appearing to be most effective.

The subsequent tensile properties are shown in Table 4, and these are comparable to conventionally processed material from this coil.

TABLE 4

Note:

Material was treated as above, solution heat treated @ 560°C in sand bed for 2 minutes, FAQ (forced air quench) and naturally aged for 7 days before testing.

¹ and ² see definitions shown for Table 2.

³ Uniform Elongation is the strain in a tensile test at which the UTS is attained. Further strain beyond this point is contributed by only a small region in the test sample, called the neck region.

⁴ Total Elongation is the elongation achieved in a tensile sample up to the point of failure. This includes the contribution of both uniform elongation and the straining which occurs in the neck region of the sample.

⁵ n Value is the strain hardening exponent. This is obtained by fitting an exponential function to the experimental stress-strain curve, for stresses up to the UTS level.

⁶ The R value is the plastic strain ratio, given by the ratio of the width strain to thickness strain which develops during a tensile test on a sheet test sample. EXAMPLE 3

A full plant trial was carried out at Assignee's Kingston Works. Alloy AA6111 was subjected to standard processing to sheet having a final gauge of 1 mm. The sheet was subjected to a recovery anneal of 4 hours at 300°C, followed by conventional solution heat treatment on a continuous annealing and solution heat treatment (CASH) line to produce a T4P product. The product following the recovery anneal was designated T4PC. The sheet was then levelled and cut to length at Warren Works .

The final sheet was roping-free as evaluated by a 15% stretch and stoning. It was also free of roping when tested in plane strain and in balanced biaxial dome tests .

The mechanical properties of the sheet product are shown in Table 5 below.

TABLE 5 Mechanical Properties of AA6111-T4PC, Ex-Warren (coil

43522)

¹ The Erichsen test is a ball punch deformation test used to evaluate the ductility of metallic sheet materials . It involves near-biaxial stretching of a constrained test specimen, utilizing a ball indentor diameter and die geometry as given in ASTM specification E643-84 (Reapproved 1995) . Erichson cup heights are given in millimeters.

² The balanced biaxial dome test is used to evaluate the ductility of metallic sheet materials under conditions of biaxial stress (in the plane of the sheet) in which the stresses in the two principal stretching directions are equal . The testing procedure involves a large punch with a spherical contact geometry. Dome heights are given in millimeters, as the displacement which the punch moves through from contact with the sheet until initial fracture occurs. This displacement is dependent upon the dimensions of the punch and sample fixture.

These results demonstrate that the T4PC treatment produces sheet with comparable mechanical properties, but without roping exhibited by conventional AA6111-T4P.

Claims

CLAIMS :

1. A process of producing an aluminum alloy sheet article having a reduced tendency to exhibit roping in products formed from said sheet article, said process comprising the steps of: direct chill casting a heat- treatable aluminum alloy having a tendency to exhibit roping, to form a cast ingot; homogenizing said cast ingot to produce an homogenized cast ingot; hot rolling the homogenized cast ingot to produce an intermediate hot-rolled sheet article; cold rolling the intermediate hot-rolled sheet article to final gauge; and subjecting said sheet article of final gauge to a solutionizing step; characterized in that said process includes a step, carried out during or after said cold rolling and before said solutionizing step, of subjecting a sheet article to a recovery anneal comprising heating said sheet article to a temperature in a range of 300 to 350┬░C for 2 to 16 hours.

2. A process according to claim 1, characterized in that said recovery anneal is carried out after said cold rolling.

3. A process according to claim 1, characterized in that said cold rolling involves a plurality of cold rolling steps progressively reducing sheet thickness to final gauge, and wherein said recovery anneal is carried out after a first of said cold rolling steps but before a final cold rolling step.

4. A process according to claim 3, characterized in that said sheet thickness is reduced by 30% or less to final gauge after said recovery anneal.

5. A process according to claim 3, characterized in that said sheet thickness is reduced by 20% or less to final gauge after said recovery anneal.

6. A process according to claim 1, characterized in that said aluminum alloy is an alloy of the 6000 series.

7. A process according to claim 1, characterized in that said alloy is selected from the group consisting of AA6111, AA2036, AA6010, AA6016 and AA6022.

8. A process according to claim 1, characterized in that said alloy is alloy AA6111.

9. A process according to claim 6, characterized in that said recovery anneal is carried out by heating said article at said temperature for 2 to 5 hours.

10. A process according to claim 8, characterized in that said recovery anneal is carried out by heating said article at about 300┬░C for about 4 hours.

11. A process according to claim 1, characterized in that said recovery anneal results in increased amounts of cube and rotated cube components, and reduces Goss component, in said alloy after solutionizing as compared with sheet article produced by said process but omitting said recovery anneal.

12. A process according to claim 11, characterized in that said cube and rotated cube components are each present in amounts of 5% by volume or more and said Goss component is present in amounts of less than 3% by volume of said sheet article of final gauge following said solutionizing.

13. A process according to claim 1, characterized by hot rolling a direct chill cast ingot of AA6111 alloy to a thickness of about one tenth inch, cold rolling to an intermediate gauge article, recovery annealing at said intermediate gauge article for about 4 hours at about 300┬░C, subjecting the intermediate gauge article to a 20% reduction cold roll, and then subjecting the alloy to a continuous annealing and solution heat treatment.

14. An aluminum alloy sheet article having reduced tendency to exhibit roping, produced by a process according to any one of claims 1 to 13.

15. A shaped product exhibiting no visible surface roping, produced by forming a sheet article produced by a process according to any one of claims 1 to 13.