JP4222964B2 - Aluminum alloy plate for automobile outer plate, method for producing the same and automobile outer plate - Google Patents

Description

The present invention relates to an aluminum alloy plate for an automobile outer plate, a method for producing the same, and an automobile outer plate having excellent surface quality obtained by pressing the aluminum alloy plate.

  Conventionally, a 5000 series (Al-Mg series) alloy has been used as an aluminum alloy applied as an automobile outer plate. Although a 5000 series alloy is excellent in strength and ductility and exhibits good formability, When the content is increased, hot workability is deteriorated and an SS mark (stretcher / strain mark) is likely to be generated at the time of molding. The use of (Mg-Si) alloys is expanding.

  The 6000 series alloy is excellent in formability, increases in strength by performing paint baking, and exhibits excellent dent resistance, but is expensive compared to cold-rolled steel sheets that have been widely used as automobile outer plates, The range to be used is limited, and therefore, attempts have been made to reduce the manufacturing cost of 6000 series alloy plates.

  For example, in the production of a 6000 series alloy plate, the production of a plate material by a twin roll molten metal rolling method is proposed instead of the conventional IM method (DC casting-hot rolling method). It has been proposed to obtain excellent moldability and surface properties by setting the maximum length of continuous crystallized substances to 50 μm or less (see Patent Document 1).

In addition, a 6000 series alloy melt having a specific composition is continuously cast at a solidification rate of 150 ° C./second or more, and after a predetermined thickness is obtained by cold rolling, solution treatment is performed at a temperature of 520 to 560 ° C. A method for obtaining a plate material excellent in strength and formability by performing and rapidly cooling within 30 seconds has also been proposed (see Patent Document 2).
JP-A-10-130766 Japanese Patent Laid-Open No. 10-259464

  However, although the above proposed material is excellent in formability, when coating baking is performed after press working, ridging marks generated on the plate surface during press processing (streaks observed by unevenness generated on the surface). Pattern) may be observed visually, and there is a problem in quality as an automobile outer plate.

  In order to solve this problem, the inventors have conducted tests and examinations from various points of view regarding the mechanism of ridging mark generation in the molten rolled sheet and the suppression method thereof, and as a result, they exist inside the molten rolled sheet. It was found that the crystallized product was the cause of generation of ridging marks.

The present invention has been made as a result of further studies based on the above knowledge, and its purpose is to suppress ridging marks generated in press processing and to be applied as an automobile outer plate after press processing. An object of the present invention is to provide an aluminum alloy plate for an automobile outer plate excellent in surface quality, a manufacturing method thereof, and an automobile outer plate.

In order to achieve the above object, an aluminum alloy sheet for an automobile outer sheet according to claim 1 contains Si: 0.4 to 1.5%, Mg: 0.2 to 1.2%, and the balance Al and impurities. A hot-rolled sheet having a T4 tempered aluminum alloy in which Fe as an impurity is limited to 0.30% or less, and the maximum length of centerline segregation is 500 μm or less.

  An aluminum alloy plate for an automobile outer plate according to claim 2 is the aluminum alloy plate according to claim 1, wherein the aluminum alloy is further Mn: 0.3% or less (excluding 0%, the same shall apply hereinafter), Cr: 0.3% or less, It contains one or more of V: 0.2% or less and Zr: 0.15% or less.

  The aluminum alloy plate for an automobile outer plate according to claim 3 is characterized in that, in claim 1 or 2, the aluminum alloy further contains Zn: 0.5% or less.

An aluminum alloy plate for an automobile outer plate according to claim 4 is characterized in that in any one of claims 1 to 3, the aluminum alloy further contains Cu: 1.0% or less, and the automobile according to claim 5 The aluminum alloy plate for an outer plate according to any one of claims 1 to 4, wherein the aluminum alloy further contains at least one of Ti: 0.1% or less and B: 50 ppm or less. .

According to a sixth aspect of the present invention, there is provided a method for producing an aluminum alloy plate for an automobile outer plate, wherein the molten aluminum alloy according to any one of the first to fifth embodiments is introduced through a nozzle between a pair of rolls and solidified. In the molten metal rolling which is drawn while rolling with the pair of rolls, a line L connecting the final solidification position 4 of the molten metal and the center point of the pair of rolls during the molten metal rolling ₁ Distance D to ₁ Is the position P where the molten metal first contacts the roll ₁ , P ₂ Line L connecting ₂ And a line L connecting the center points of the pair of rolls ₁ Distance D to ₂ D ₁ ≧ 0.3 × D ₂ The molten metal sheet is rolled to satisfy the above condition, and the obtained molten rolled sheet is tempered by T4, and the maximum length of centerline segregation is 500 μm or less.

According to a seventh aspect of the present invention, an automobile outer plate is obtained by pressing the molten aluminum alloy plate according to any one of the first to fifth aspects.

  ADVANTAGE OF THE INVENTION According to this invention, generation | occurrence | production of the ridging mark in press work is suppressed and the aluminum alloy plate for motor vehicle outer plates and the motor vehicle outer plate which were excellent in the surface quality after press work are provided.

  The aluminum alloy plate of the present invention is a 6000 series aluminum alloy plate obtained by a molten metal rolling method (also referred to as a continuous casting rolling method) and subjected to T4 tempering (solution treatment, quenching, normal temperature aging). The significance and reasons for limitation are as follows.

  Si coexists with Mg to form a Mg-Si compound to improve the strength, and also functions to give high paint bake hardenability. The preferable content of Si is in the range of 0.4 to 1.5%. If it is less than 0.4%, it is difficult to obtain sufficient strength by heating during coating baking, and the moldability may be lowered. If it exceeds 5%, bending workability may be lowered, which may cause a practical problem. The more preferable content range of Si is 0.6 to 1.3%, and the most preferable content range is 0.8 to 1.2%.

  Mg functions to improve strength by coexisting with Si to form a Mg-Si compound. The preferable content of Mg is in the range of 0.2 to 1.2%. If the content is less than 0.2%, it is difficult to obtain sufficient strength by heating during coating baking. The yield strength after quenching is increased, and the formability and shape freezing properties during press working are reduced. The more preferable content range of Mg is 0.3 to 0.8%, and the most preferable content range is 0.4 to 0.7%.

  Fe is basically contained as an impurity, and forms an Al-Fe-Si-based or Al-Fe-based crystallized product at the time of casting, so that it is easy to form a continuous crystallized product inside the alloy plate. It is desirable to limit the Fe content to 0.30% or less. If the content exceeds 0.30%, a continuous crystallized product is easily formed inside the alloy plate, and the formability and bending workability are deteriorated, which may cause a practical problem.

  Mn, Cr, V, and Zr function so as to prevent rough skin at the time of molding by refining crystal grains. Preferable contents are Mn: 0.3% or less, Cr: 0.3% or less, V: 0.2% or less, Zr: 0.15% or less. As a result, a formable intermetallic compound is formed and the moldability is lowered. Further preferable content ranges are: Mn: 0.05 to 0.15%, Cr: 0.05 to 0.15%, V: 0.05 to 0.15%, Zr: 0.05 to 0.12% is there.

  Zn functions to improve surface treatability. The preferable content of Zn is in the range of 0.5% or less. If it exceeds 0.5%, the corrosion resistance after coating is reduced. A more preferable content range of Zn is 0.3% or less.

  Cu functions to improve formability. The preferable content of Cu is in the range of 1.0% or less, and if it exceeds 1.0%, the corrosion resistance after coating is reduced. A range of 0.3 to 0.8% is preferable from the viewpoint of moldability, and a range of 0.1% or less is preferable when corrosion resistance is important.

  Ti and B function to refine the cast structure and improve formability. Preferable contents are in the ranges of Ti: 0.1% or less and B: 50 ppm or less. When the content exceeds each upper limit, the formation of coarse intermetallic compounds increases and the formability tends to decrease. .

In the present invention, it is important to regulate the maximum length of continuous crystallized substances existing in the alloy plate to 500 μm or less. As a crystallized product, an Al—Fe—Si compound, an Al—Fe compound, and a Mg—Si compound are formed. As shown in FIG. 1, the production of the aluminum alloy plate by the molten metal rolling method is performed by introducing the molten aluminum alloy 1 through a nozzle 2 between a pair of rolls R ₁ and R ₂ , and rolling the solidified aluminum alloy plate 3 into a roll R _1. is carried out by drawing while rolling in R _2, since the final solidification position 4 of the molten metal 1 is positioned approximately at the center of the plate thickness, in the final solidification position 4 Fe, Si, component concentrations, such as Mg is the highest It is easy to form the continuous crystallized product in the vicinity of the center of the plate thickness. In FIG. 1, S indicates a solidification interface of the molten metal 1.

  Crystallized substances formed continuously in the vicinity of the center of the plate thickness are also called centerline segregation, but these crystallized substances are harder than matrix aluminum, so that they are crystallized when pressed after T4 refining. Is difficult to deform and only the surrounding aluminum is deformed, so that the shape of the crystallized material is raised on the surface of the plate, and it is observed as one kind of ridging mark (striated surface defect). In order to prevent generation of ridging marks, the maximum length of continuous crystallized substances formed in the vicinity of the center portion of the plate thickness is preferably 500 μm or less, and more preferably 300 μm or less.

In molten metal rolling, the optimum casting conditions differ depending on the equipment and alloy composition, and it is necessary to set the casting speed, molten metal temperature, etc. to optimum conditions. In order to make the thickness 500 μm or less, the distance D ₁ between the final solidification position 4 of the molten metal 1 during molten metal rolling and the line L ₁ connecting the center points O ₁ and O ₂ of the pair of rolls R ₁ and R ₂ is the molten metal. Is a distance D ₂ between the line L ₂ connecting the positions P ₁ and P ₂ where the roller first contacts the roll and the line L ₁ connecting the center points O ₁ and O ₂ of the pair of rolls R ₁ and R _2. Therefore, it is necessary to satisfy D ₁ ≧ 0.3 × D ₂ , and for that purpose, the casting speed, the distance D ₃ between the nozzle tip N and the rolls R ₁ and R _2, etc. are set in accordance with the molten metal rolling apparatus. Is set. In this case, the casting speed V (mm / s) and the distance D ₃ (mm) between the nozzle tip N and the rolls R ₁ and R ₂ must satisfy V ≦ 0.5D _3. The speed V (mm / s) and the sheet thickness t (mm) of the molten rolled material must satisfy V ≦ 10 t. By satisfying these conditions, casting that satisfies the above-mentioned D ₁ ≧ 0.3 × D ₂ becomes possible.

  The obtained molten rolled sheet is T4 tempered by a solution treatment and quenching process without performing cold rolling, or is T4 tempered by a solution treatment and quenching process after cold rolling. The When cold rolling is performed, intermediate annealing may be performed as necessary. The conditions for the solution treatment and quenching are not particularly defined, but the solution treatment is preferably performed at a temperature as high as possible, for example, in a temperature range of 500 to 580 ° C., and the holding time in the solution treatment is relatively high. A short time, for example 300 seconds or less, is sufficient. The quenching is preferably performed at a quenching rate of 5 ° C./second or more.

  After quenching, preliminary aging treatment may be performed for the purpose of imparting bake hardness. As processing conditions in this case, within 60 minutes after quenching, the temperature is preferably 40 to 120 ° C. and within 50 hours. In addition, after the preliminary aging treatment, the restoration treatment within 60 seconds can be performed at a temperature of 170 to 230 ° C. within 3 days, and this baking treatment can further improve the paint bake hardenability.

  By pressing the aluminum alloy plate for an automobile outer plate obtained as described above, an automobile outer plate having excellent surface quality and suppressed generation of ridging marks is manufactured, and the cost can be reduced.

  Examples of the present invention will be described below in comparison with comparative examples, and the effects will be demonstrated. In addition, these Examples show one embodiment of this invention, and this invention is not limited to these.

Example 1
The aluminum alloy having the composition shown in Table 1 was melted, and the distance D ₁ between the final solidification position 4 and the line L ₁ connecting the center of the roll was determined by the twin roll type hot rolling mill shown in FIG. A line L connecting the roll centers so that D ₁ ≧ 0.3 × D ₂ with respect to a distance D ₂ between the line L ₂ connecting the contact portions of the roll and the line L ₁ connecting the roll centers. ₁ and the distance D ₃ at the tip N of the nozzle 2 is set to 60 mm, subjected to molten metal rolling casting speed as 1 m / min, to produce a molten metal rolled plate having a thickness of 2.0 mm.

  The obtained molten rolled sheet was subjected to a solution treatment for 5 seconds at 540 ° C., then quenched with tap water at room temperature to obtain a T4 tempered material, and the obtained T4 tempered material as a test material, By the method, the tensile properties after 7 days from quenching, the maximum length of continuous crystallized material inside the plate, and the presence or absence of ridging marks were evaluated. The results are shown in Table 2.

Evaluation of tensile properties: JIS No. 5 test pieces were molded, and tensile strength (σ _B ), proof stress (σ _0.2 ), and elongation (δ) were measured according to JIS Z2241.
Evaluation of the maximum length of the continuous crystallized material inside the plate: Cut the test material into a width of 15 mm and a length of 30 mm, fill the resin so that the rolled surface becomes the observation surface, and polish to # 1200 with emery paper Further, buffing was performed to expose the central portion of the plate thickness, etching was performed with Keller's solution, and the maximum length of a continuous crystallized product was measured with an optical microscope.
Evaluation of generation of ridging marks: The test material was cut to a width of 200 mm and a length of 30 mm, a tensile strain of 10% was introduced in the plate width direction, a 15 ± 5 μm coating film was applied with a spray paint, and ridging was visually observed. The presence or absence of the mark was observed.

As can be seen from Table 2, in all the test materials according to the present invention, the distance D ₁ between the final solidification position 4 and the line L ₁ connecting the center of the roll is the line L ₂ connecting the contact portion between the molten metal and the roll. And D ₁ ≧ 0.3 × D ₂ with respect to the distance D ₂ from the line L ₁ connecting the roll centers, and the maximum length of continuous crystallized material inside the plate is 9 to 12 μm. It was short and no ridging marks were observed.

Example 2
Using the three types of molten metal sheets produced in Example 1, 50% cold rolling was performed to obtain a sheet thickness of 1.0 mm, and a solution treatment was performed at 540 ° C. for 5 seconds, followed by room temperature tap water. And T4 tempered material. The same evaluation as Example 1 was performed by using the obtained T4 tempered material as a test material. The results are shown in Table 3.

As can be seen from Table 3, in all the test materials according to the present invention, the distance D ₁ between the final solidification position 4 and the line L ₁ connecting the center of the roll is the line L ₂ connecting the contact portion between the molten metal and the roll. And D ₁ ≧ 0.3 × D ₂ with respect to the distance D ₂ from the line L ₁ connecting the roll centers, and the maximum length of continuous crystallized material inside the plate is 16 to 25 μm. It was short and no ridging marks were observed.

Comparative Example 1
An aluminum alloy having the composition shown in Table 1 was melted, and, as in Example ₁ , the distance D ₁ between the final solidification position 4 and the line L ₁ connecting the center of the roll was determined by A line L connecting the centers of the rolls such that D ₁ <0.3 × D ₂ with respect to a distance D ₂ between a line L ₂ connecting the contact portions of the rolls and a line L ₁ connecting the roll centers. ₁ and the distance D ₃ at the tip N of the nozzle 2 is set to 30 mm, subjected to molten metal rolling casting speed as 2m / min, to produce a molten metal rolled plate having a thickness of 2.0 mm.

  About the obtained molten rolled sheet, after performing solution treatment for 5 seconds at 540 ° C., it was quenched with normal temperature tap water to obtain a T4 tempered material, and the obtained T4 tempered material was used as a test material. Evaluation similar to 1 was performed. The results are shown in Table 4.

As shown in Table 4, none of the test material 7-9, the distance D ₁ of the a line L ₁ connecting the center of the final solidification position 4 and roll, the line L ₂ connecting the contact portion of the molten metal and the roll Since the distance D ₂ with the line L ₁ connecting the roll centers does not satisfy D ₁ ≧ 0.3 × D ₂ , the maximum length of continuous crystallized material inside the plate exceeds 500 μm. A ridging mark was generated.

Comparative Example 2
Using the three types of molten metal sheets produced in Comparative Example 1, 50% cold rolling was performed to obtain a sheet thickness of 1.0 mm, and a solution treatment was performed at 540 ° C. for 5 seconds, followed by room temperature tap water. And T4 tempered material. The same evaluation as Example 1 was performed by using the obtained T4 tempered material as a test material. The results are shown in Table 5.

As shown in Table 5, both test materials 10 to 12, the distance D ₁ of the a line L ₁ connecting the center of the final solidification position 4 and roll, the line L ₂ connecting the contact portion of the molten metal and the roll Since the distance D ₂ with the line L ₁ connecting the roll centers does not satisfy D ₁ ≧ 0.3 × D ₂ , the maximum length of continuous crystallized material inside the plate exceeds 500 μm. A ridging mark was generated.

It is a figure which shows the outline of the principal part of a molten metal rolling apparatus.

Explanation of symbols

1 Aluminum alloy molten metal 2 Nozzle 3 Molten metal rolling plate 4 Final solidification position (solidification tip)
R ₁ molten metal roll R ₂ molten metal roll O ₁ center of R ₁ O ₂ center of R ₂ S solidification interface P ₁ contact portion between molten metal and P ₂ contact portion between molten metal and roll N nozzle tip L ₁ O ₁ , O ₂ connected line L ₂ P ₁ , P ₂ connected line L ₃ Nozzle tip connected line D ₁ Solidification tip and L ₁ distance D ₂ L ₁ and L ₂ distance D ₃ L ₁ and L Distance of ₃

Claims (7)

Si: 0.4-1.5% (mass%, the same shall apply hereinafter), Mg: 0.2-1.2%, consisting of the balance Al and impurities, Fe as impurities being 0.30% or less An aluminum alloy sheet for an automobile outer sheet, which is a restricted aluminum alloy T4 tempered molten rolled sheet, wherein a maximum length of centerline segregation is 500 μm or less. Of the aluminum alloy, Mn: 0.3% or less (excluding 0%, the same shall apply hereinafter), Cr: 0.3% or less, V: 0.2% or less, Zr: 0.15% or less The aluminum alloy plate for an automobile outer plate according to claim 1, comprising at least one kind. The aluminum alloy plate for an automobile outer plate according to claim 1 or 2, wherein the aluminum alloy further contains Zn: 0.5% or less. The said aluminum alloy contains Cu: 1.0% or less further, The aluminum alloy plate for motor vehicle outer plates in any one of Claims 1-3 characterized by the above-mentioned. 5. The aluminum alloy plate for an automobile outer plate according to claim 1, wherein the aluminum alloy further contains at least one of Ti: 0.1% or less and B: 50 ppm or less. . In the molten metal rolling in which the molten aluminum alloy according to any one of claims 1 to 5 is introduced from a nozzle between a pair of rolls and the solidified aluminum alloy sheet is drawn while being rolled by the pair of rolls, Line L connecting the final solidification position 4 of the molten metal and the center point of the pair of rolls ₁ Distance D to ₁ Is the position P where the molten metal first contacts the roll ₁ , P ₂ Line L connecting ₂ And a line L connecting the center points of the pair of rolls ₁ Distance D to ₂ D ₁ ≧ 0.3 × D ₂ A method for producing an aluminum alloy sheet for an automobile outer sheet, wherein the molten sheet rolled to satisfy the above condition, T4 tempered and the maximum length of centerline segregation is 500 μm or less. An automobile outer plate, which is obtained by pressing the molten aluminum alloy sheet according to any one of claims 1 to 5.

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