JP2006291338A - Aluminum alloy sheet having excellent formability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aluminum alloy sheet having excellent formability based on evaluation of the hole expandability closely correlating with formability in actual press forming. <P>SOLUTION: The 6000-series aluminum alloy sheet having a 0.2% proof stress of ≥120 MPa is such that the hole expanding rate of the aluminum alloy sheet when the aluminum alloy sheet is subjected to punched hole expanding by changing a clearance/sheet thickness which is the ratio of the clearance of a die and a punch with respect to the sheet thickness, exhibits a maximal value of ≥45% within a range of 10 to 25% in the clearance ratio. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a 6000 series aluminum alloy plate excellent in formability (hereinafter, aluminum is also simply referred to as Al).

  Conventionally, Al alloy materials with excellent formability and bake hardenability have been used for transportation equipment such as automobiles, ships, aircraft or vehicles, machinery, electrical products, architecture, structures, optical equipment, and components and parts of equipment. Has been.

  In particular, in the field of the body of a transport device such as an automobile, in recent years, improvement in fuel efficiency has been pursued by reducing the weight in response to global environmental problems caused by exhaust gas and the like. For this reason, the application of lighter Al alloy materials such as rolled plates and extruded shapes instead of steel materials conventionally used for automobile bodies is increasing.

  Among these, panels such as outer panels (outer plates) and inner panels (inner plates) of panel structures such as automobile hoods, fenders, doors, roofs, and trunk lids are made of Al-Mg-Si AA to JIS. The use of 6000 series (hereinafter simply referred to as 6000 series) Al alloy sheets is being studied.

  The 6000 series Al alloy sheet basically contains Si and Mg as essential and has excellent age-hardening ability. BH properties (bake-hardness, artificial age-hardening ability) that can secure the required strength by aging hardening by heating at the time of relatively low-temperature artificial aging (curing) treatment such as paint baking treatment of the subsequent panel Paint bake hardenability).

  Further, the 6000 series Al alloy plate has a relatively small amount of alloy elements as compared with other 5000 series Al alloys having a large amount of alloy such as Mg. For this reason, when the scraps of these 6000 series Al alloy sheets are reused as an Al alloy melting material (melting raw material), the original 6000 series Al alloy ingot is easily obtained and the recyclability is also excellent.

  On the other hand, in the application field of the automotive panel structure, an Al alloy plate is formed into a panel by press forming such as overhanging, drawing or trimming. In recent years, with the adoption of Al alloy plates for automobile panels, the application to panels with more complex shapes and difficult to form has increased. For example, the press-molded panel shape tends to become complicated, such as the overhang height and the overhang area becoming large, and the shape has a curved portion that extends and undergoes flange deformation. For this reason, molding defects such as cracking during press molding and rough skin are more likely to occur.

  On the other hand, in order to improve various required characteristics such as formability of the 6000 series Al alloy plate, various measures have been conventionally taken by precisely controlling the components, the structure, or the manufacturing method.

  Here, a hole expansion test (stretch flangeability test, burring test) is known as a method for evaluating stretch flangeability among the formability of these Al alloy plates. In this method, after punching a hole in a test piece, burring (hole expansion) is performed on the punched hole by using a punch with the burr as the upper surface side of the die surface. Then, the hole expansion ratio λ (%) is obtained, and the higher the value of λ is, the higher the stretch flangeability is evaluated.

  Using this, the limit drawing ratio (pure deep drawing, ball head depth) of 1000 series Al alloy plates such as 1070, 1050 and 1100 for forming, 3000 series such as 3003 and 3004, and 5000 series such as 5052 and 5182 It is known that the hole expansion ratio λ is obtained together with the aperture and the Elksen value. According to this, the O material has a higher hole expansion ratio λ as well as the limit drawing ratio and the Elksen value than the H material hard material, and is excellent in formability (see Non-Patent Document 1).

In addition, as for defining the hole expandability itself of Al alloy plates, high hole expandability is defined for the 5000 series Al alloy plates containing Mg 2 to 8%, defining the area of the shear surface in the punched section of the hole. An Al alloy plate has been proposed (see Patent Document 1).
Aluminum Technical Handbook issued June 16, 1985: Light Metal Publishing Co., Ltd., page 786, Table 1.10 JP 2003-301232 A (claims, pages 1 to 4)

  However, there are few examples of evaluation of hole expansibility about the 6000 series Al alloy plate made into the object by this invention until now. This is because the application of 6000 series Al alloy sheets to the application field of the above-mentioned automobile panel structure is recent, the formability at the time of actual 6000 series Al alloy sheet press forming, and the 6000 series Al alloys This also depends on the fact that the relationship with the formability evaluation based on the hole expandability of the plate has not yet been established.

  On the other hand, due to its excellent age-hardening ability, the 6000 series Al alloy plate is prone to age-hardening at room temperature (room temperature) and is stable at room temperature after the production of the Al alloy plate until it is molded into the above-mentioned applications. There is a big problem of lacking. When such room temperature aging occurs, even after the manufacture, even if the 6000 series Al alloy sheet satisfies the formability, the formability decreases when it is formed for actual use after a certain period of time. In other words, it does not satisfy the demand.

  Therefore, if evaluation of press formability closely correlating with formability at the time of actual 6000 series Al alloy plate press forming can be made in advance, the above-described components, structures or production for improving formability of 6000 series Al alloy plate Method control is easier to do. Further, the press formability of a 6000 series Al alloy plate that has undergone room temperature aging can be evaluated, and it can be determined whether or not forming is possible, or whether or not a material formability improving process such as a recovery process is necessary.

  In addition, if a 6000 series Al alloy plate with excellent press formability can be produced in accordance with this press formability evaluation method, on the forming side, design changes such as the shape of the shape and efficiency of processes and time will be sacrificed. Thus, molding can be performed without taking measures such as relaxing the molding process conditions.

  The present invention has been made paying attention to such a situation, and the object thereof is an aluminum alloy having excellent formability based on evaluation of hole expansion property closely correlated with formability at the time of actual press forming. It is intended to provide a board.

  In order to achieve this object, the gist of the aluminum alloy plate excellent in formability of the present invention is a 6000 series aluminum alloy plate having a 0.2% proof stress of 120 MPa or more. The hole expansion rate of the aluminum alloy plate when the clearance / plate thickness, which is the clearance ratio with the punch, is changed, and the hole expansion rate of the aluminum alloy plate is 45% or more when the clearance ratio is in the range of 10-25%. It shows the maximum value.

  The inventors of the present invention have found that when a 6000 series Al alloy plate is punched and widened by changing the clearance, the 6000 series Al alloy exhibits a maximum value of a hole expansion ratio of 45% or more within the range of the above-mentioned constant clearance ratio. It was found that the plate exhibits excellent press formability.

  The reason for this is not clear, but the 6000 series Al alloy sheet showing a maximum value of 45% or more of the hole expansion rate within the above-mentioned range of the clearance ratio is excellent in stretch flangeability, and therefore has an actual press formability. It is considered excellent.

  As described above, the hole expansion ratio λ itself has been conventionally evaluated for stretch flangeability, and serves as a guideline for press formability together with the limit drawing ratio and the Elksen value. It is also known that a higher hole expansion ratio λ that correlates with a limit drawing ratio or an Elksen value tends to be excellent in press formability.

  However, for the 6000 series Al alloy plate, the size of the hole expansion ratio λ is not necessarily good enough to correspond to the actual press formability, and the hole expansion is related to the clearance between the die and punch in the hole expansion process. When looking at the value of the rate λ, it is also true that it corresponds to the actual press formability only for the first time.

  The reason for this is not clear, but when punching holes are expanded, the stretch flangeability of the 6000 series Al alloy plate is greatly affected by the clearance. Further, the 6000 series Al alloy plate whose stretch flangeability is lowered due to aging at room temperature or the like and the actual press formability is lowered is more affected by this clearance. For this reason, it is presumed that the value of the hole expansion rate λ decreases as a whole, and it becomes difficult to produce a peak value showing a maximum value of 45% or more.

  According to the present invention, evaluation of press formability closely correlated with formability at the time of actual 6000 series Al alloy sheet press molding can be performed in advance, and the above-described components for improving formability of 6000 series Al alloy sheet And the control of the organization or manufacturing method becomes easier.

  Further, the press formability of a 6000 series Al alloy plate that has undergone room temperature aging can be evaluated, and it can be determined whether or not forming is possible, or whether or not a material formability improving process such as a recovery process is necessary.

  Furthermore, according to the present invention, since a 6000 series Al alloy plate excellent in press formability can be formed, on the side to be molded, molding processing is sacrificed at the expense of design changes such as molding shape and efficiency of process and time. Molding can be performed without taking measures such as relaxing the conditions.

  Hereinafter, embodiments of the 6000 series Al alloy plate of the present invention will be described in detail.

(Hole expansion ratio)
The hole expansion ratio λ (%) itself referred to in the present invention is obtained by the same method as the conventional hole expansion test. That is, after punching a hole of a predetermined diameter in a test piece of a 6000 series Al alloy plate, burring (hole expansion) is performed on the punched hole using a punch with the burr as the upper surface side of the die surface. Then, the punch is stopped when the edge of the punched hole is broken (cracked), and the hole expansion ratio λ () is calculated from the hole inner diameter (ds) at the time of breaking and the initial hole diameter (d0) before burring by the following formula. %).
λ = [(ds−d0) / d0] × 100 (%)

  However, according to the present invention, the hole expansion ratio λ (%) is not obtained under a constant (one condition) clearance condition as in the prior art. In the present invention, the ratio of the clearance between the die and the punch with respect to the thickness of the 6000 series Al alloy plate = clearance / thickness of the aluminum alloy plate when the punched hole is expanded by varying the plate thickness λ (%) Are obtained sequentially.

  Based on the relationship between the hole expansion ratio λ (%) and the clearance ratio, a 6000 series Al alloy plate showing a maximum value with the clearance ratio in the range of 10 to 25% and a hole expansion ratio λ of 45% or more is obtained. The 6000 series Al alloy plate within the scope of the present invention with good press formability is evaluated.

  Moreover, even if the hole expansion ratio λ shows a maximum value when the clearance ratio is in the range of 10 to 25%, if it is less than 45%, the press formability is lowered. Further, even if the maximum value of the hole expansion ratio λ is 45% or more, if the clearance ratio showing the maximum value is out of the range of 10 to 25%, the press formability is also lowered.

  FIG. 1 shows the relationship between the clearance ratio (horizontal axis) and the hole expansion rate (vertical axis) of the Al alloy sheet. In FIG. 1, Invention Example 3 indicated by black rhombus plots and Invention Example 4 indicated by black square plots are examples of inventions of 6000 series Al alloy sheets having a 0.2% proof stress of 120 MPa or more in Example Table 2 described later, respectively. It is. As can be seen from FIG. 1, Invention Example 3 shows a maximum value with a hole expansion ratio λ of about 50% at a clearance ratio of about 19%, which is within the range of 10 to 25% of the present invention. On the other hand, Invention Example 4 shows a maximum value of about 46% near the lower limit with a clearance ratio of about 12.5% near the lower limit and a hole expansion ratio λ of about 46%. Therefore, as in the examples described later, Invention Example 3 is superior to Invention Example 4 in press formability.

(Alloy plate proof stress)
The 0.2% proof stress of the 6000 series Al alloy sheet of the present invention is at least 120 MPa. If the 0.2% yield strength is less than 120 MPa, the necessary strength as an automobile panel will be insufficient even if the 0.2% yield strength is improved by artificially age-hardening later for the aforementioned automotive panel applications and the like. On the other hand, press formability also decreases.

(Chemical composition)
Next, a preferable chemical component composition of the 6000 series Al alloy plate targeted by the present invention will be described. As the above-mentioned automobile panel material, in addition to excellent formability, various properties such as BH property, strength, weldability, and corrosion resistance are required. In order to satisfy such a requirement, the composition of the Al alloy plate includes, by mass%, Mg: 0.2 to 2.0%, Si: 0.3 to 2.0%, Mn: 0.01 to 0.65%, Cu: 0.001 to 1.0%. In addition, a 6000 series Al alloy sheet having a Si / Mg ratio of 1 or more in terms of mass ratio and the balance Al and inevitable impurities is preferable.

  For other elements, the content (allowable amount) of each impurity level is in line with AA or JIS standards. Specifically, other alloy elements are Fe: 1.0% or less, Mn: 1.0% or less, Cr: 0.3% or less, Zr: 0.3% or less, V: 0.3% or less, Ti:% or less, Ag: 0.2 % Or less, Zn: 1.0% or less.

  Other alloy elements and gas components other than the above alloy elements are impurities. However, from the viewpoint of recycling, not only high-purity Al ingots but also 6000 series alloys and other Al alloy scrap materials, low-purity Al ingots, etc. are used as melting raw materials as melting materials. In the case of melting, these other alloy elements are necessarily included. Therefore, in the present invention, these impurity elements are allowed to be contained within a range that does not hinder the intended effect of the present invention.

  The preferable content range and significance of each element in the 6000 series Al alloy composition will be described below.

Si: 0.3-2.0%
Si forms a compound phase (β ") together with Mg during solid-solution strengthening and artificial aging treatment at relatively low temperatures, such as paint baking after molding, and exhibits age-hardening ability. Therefore, it is an indispensable element for obtaining the necessary strength as an element, and is therefore the most important element for combining the necessary properties as a panel, such as press formability. Furthermore, it cannot combine various properties required for each application, such as press formability, etc. On the other hand, if Si exceeds 2.0%, press formability and bending workability are significantly hindered. In addition, it significantly impairs weldability, so Si should be in the range of 0.3-2.0%.

Mg: 0.2-2.0%
Mg is an indispensable element for forming the compound phase together with Si during the artificial aging treatment such as solid solution strengthening and paint baking treatment, to exhibit age hardening ability and to obtain the required strength as the panel . If the Mg content is less than 0.2%, the absolute amount is insufficient, so that the compound phase cannot be formed during the artificial aging treatment, and the age hardening ability cannot be exhibited. For this reason, the said required intensity | strength required as a board cannot be obtained. On the other hand, if the Mg content exceeds 2.0%, the formability such as press formability and bending workability is significantly inhibited. Therefore, the Mg content is in the range of 0.2 to 2.0%.

Cu: 0.001 to 1.0%
Cu is useful for improving the age hardening rate in 6000 series Al alloys. That is, it has the effect of promoting the precipitation of a compound phase such as a GP zone into the crystal grains of the Al alloy material structure under the conditions of artificial aging (hardening) treatment such as a paint baking process. Moreover, Cu dissolved in the artificial aging treatment state also has an effect of improving formability. If the Cu content is less than 0.001%, these effects are insufficient. However, if the Cu content exceeds 1.0% and is too large, there is a high possibility that a coarse compound is formed and the moldability is deteriorated. In addition, there is a high possibility that the corrosion resistance such as yarn rust resistance required for an automobile outer panel will deteriorate. Therefore, the upper limit of the Cu content is preferably 1.0% or less.

Mn: 0.01 to 0.65%
Mn is useful for refining crystal grains and can improve moldability. Mn generates dispersed particles (dispersed phase) during the homogenization heat treatment, and these dispersed particles have an effect of hindering grain boundary movement after recrystallization. However, if each content is too large, a coarse compound is formed, which becomes a starting point of destruction, and the moldability deteriorates instead. Therefore, it is preferable to contain Mn in the range of 0.01 to 0.65%.

(Average crystal grain size)
In addition to these regulations, it is preferable to further refine the average crystal grain size of the Al alloy plate to 50 μm or less. By making the crystal grain size fine or small within this range, bending workability and press formability can be ensured or improved. When the crystal grain size exceeds 50 μm and becomes coarse, bending workability and press formability are remarkably deteriorated, and defects such as cracks and rough skin during forming tend to occur.

  The crystal grain size referred to here is the maximum diameter of crystal grains in the longitudinal (L) direction of the plate. The crystal grain size is measured by a line intercept method in the L direction by observing the surface of the Al alloy plate that has been mechanically polished by 0.05 to 0.1 mm and then electrolytically etched using an optical microscope. 1 The measurement line length is 0.95mm, and the total measurement line length is 0.95 x 15mm by observing a total of 5 fields with 3 lines per field.

(Production method)
The 6000 series Al alloy plate can be manufactured by a conventional method. In the melting and casting process of Al alloy, select a normal melting and casting method such as continuous casting and rolling, semi-continuous casting (DC casting), etc. And cast. Next, the Al alloy ingot is subjected to homogenization heat treatment by a conventional method, and then hot rolled to form a hot rolled sheet such as a coil or plate, or further subjected to intermediate annealing as necessary. Cold-rolled and processed into a cold rolled sheet such as a coil or plate.

  The processed Al alloy sheets such as hot rolled sheets and cold rolled sheets are tempered by solution treatment and quenching as tempering treatments to obtain product plates. Of course, it is possible to add a tempering treatment such as an artificial age hardening treatment or a stabilization treatment according to the use and required characteristics to obtain a product plate.

  Next, examples of the present invention will be described. 6000 series Al alloy sheets of A and B compositions in Table 1, each of which was aged at room temperature for 2 days to 4 months after production, and were made into dies and punches with respect to the plate thickness. Clearance ratio = Clearance / Board widening (testing) with various changes in plate thickness. Then, the maximum value of the hole expansion rate λ (%) and the ratio of the clearance between the die and the punch with respect to the plate thickness when the hole expansion rate λ (%) reaches the maximum value were obtained. The hole expansion ratio λ (%) was calculated according to the above-described conditions. The hole expansion processing conditions are: punch: Φ50, shoulder R4.5, die: Φ60, shoulder R12.5, wrinkle holding force: 10tonf, lubricant: R303P (punch side oil), processing speed: 10-15mm / Min.

  The tensile test characteristics of these Al alloy sheets were also investigated. The tensile test characteristics were measured by setting the proof stress parallel to the rolling direction of the Al alloy sheet (in the L direction) as 0.2% proof stress (MPa). The tensile test was performed according to JIS Z 2201 and the shape of the test piece was a JIS No. 5 test piece. The crosshead speed was 5 mm / min, and the test piece was run at a constant speed until the test piece broke. These results are shown in Table 2. The average crystal grain size of the Al alloy plate was 50 μm or less in both the inventive example and the comparative example.

  The manufacturing conditions for the Al alloy sheets of these test materials are as follows. A 400 mm thick ingot of each composition A and B shown in Table 1 was melted by a DC casting method, subjected to homogenization heat treatment at 550 ° C., and hot-rolled to a thickness of 5 mm at an end temperature of 300 ° C. This hot-rolled sheet was subjected to intermediate annealing at 400 ° C. for 4 hours in a batch-type heat treatment facility, and then cold-rolled at a cold rolling rate of 80% to obtain a cold-rolled sheet having a thickness of 1.0 mm. The cold-rolled sheet is then subjected to solution treatment at 450 to 550 ° C. in a continuous heat treatment facility and then quenched to room temperature, and within 30 minutes after this quenching, a pre-aging treatment of 70 ° C. × 1 hour is performed. It was.

  At this time, as shown in Table 2, samples that were not subjected to the preliminary aging treatment, or those that were subjected to the preliminary aging treatment but had different room temperature aging periods after the plate production were prepared as test materials. Then, a molding test was performed with a relatively large blank by simulating that these Al alloy plates were press-molded as an automobile panel.

  More specifically, an overhang forming test was performed to evaluate moldability. These results are also shown in Table 2. The conditions of the stretch forming test were that a plurality of 1000 mm square test plates (blanks) were cut out from the test plate after aging at room temperature and stretched using a die with a bead by a mechanical press. The shape of the molded product was a hat type with a square tube-like overhang part with a side of 500 mm at the center and a height of 50 mm, and flat flanges around the four sides of the overhang part. .

  In the molding test, the wrinkle holding force was 490 kN, the lubricating oil was general rust preventive oil, and the molding speed was 20 mm / min. The evaluation was performed separately for the corner crack and the flange (end) crack, and the corner crack of the hat-shaped molded product did not occur three times. Things were evaluated as x.

  In addition, the maximum height (mm) of the generated `` wrinkle '' was measured regardless of cracking during this overhang forming, and the average of the wrinkle height during the three moldings was 1.0 mm or less. Was evaluated as ◯, when exceeding 1.0 mm and not more than 2.0 mm, Δ and when exceeding 2.0 mm as ×.

  As shown in Table 2, Invention Examples 2 to 4, in which 0.2% proof stress is 120 MPa or more, and the hole expansion rate of the Al alloy plate exhibits a maximum value of 45% or more in the range of the clearance ratio of 10 to 25%, Invention Examples 7 to 8 are excellent in press formability simulating actual press forming.

  In contrast, Comparative Examples 1 and 6 in which the 0.2% proof stress is less than 120 MPa even though the hole expansion rate of the Al alloy plate shows a maximum value of 45% or more in the clearance ratio range of 10 to 25%, The press formability is inferior.

  Further, even when the 0.2% proof stress is 120 MPa or more, a comparative example in which the hole expansion rate of the Al alloy sheet does not show a maximum value of 45% or more within the range of the clearance ratio of 10 to 25% due to room temperature age hardening or the like. 5 and 10 are inferior in press formability simulating actual press forming.

  Further, as shown in FIG. 1, the invention example 4 in which the hole expansion ratio λ shows a local maximum value near the lower limit with the clearance ratio near the lower limit is changed to other invention examples 2, 3, 7, 8, 9 In comparison, press formability is inferior.

  Therefore, from these results, the critical significance of the proof stress and the hole expansion rate defined in the present invention can be understood.

  ADVANTAGE OF THE INVENTION According to this invention, the aluminum alloy plate excellent in the moldability based on evaluation of the hole expansibility closely correlated with the moldability at the time of actual press molding can be provided. As a result, the application of the Al alloy material can be expanded to use for press-molded members.

It is explanatory drawing which shows the relationship between the clearance ratio at the time of the hole expansion process of an Al alloy plate, and a hole expansion rate.

Claims (2)

  A 6000 series aluminum alloy sheet with a 0.2% proof stress of 120 MPa or more. When this aluminum alloy sheet is punched and widened by changing the clearance / sheet thickness, which is the ratio of the clearance between the die and punch relative to the sheet thickness. An aluminum alloy plate having excellent formability, wherein the hole expansion rate of the aluminum alloy plate exhibits a maximum value of 45% or more when the clearance ratio is in the range of 10 to 25%. The 6000 series aluminum alloy plate contains, by mass, Mg: 0.2-2.0%, Si: 0.3-2.0%, Mn: 0.01-0.65%, Cu: 0.001-1.0%, and Si / Mg is a mass ratio. The aluminum alloy plate having excellent formability according to claim 1, wherein the aluminum alloy plate is composed of the balance Al and inevitable impurities.

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