JP4939088B2 - Manufacturing method of aluminum alloy sheet with excellent ridging mark property during forming - Google Patents

Description

  The present invention relates to a method for producing an aluminum alloy plate excellent in ridging mark properties during press forming on a panel (hereinafter, aluminum is also simply referred to as Al). The aluminum alloy plate referred to in the present invention refers to a plate that has been subjected to solution treatment and quenching after rolling.

  In recent years, with respect to global environmental problems caused by exhaust gas and the like, improvement in fuel efficiency has been pursued by reducing the weight of the body of a transport aircraft such as an automobile. For this reason, in particular, the application of lighter Al alloy materials such as rolled plate materials, extruded profiles, forged materials, and the like instead of steel materials that have been used in the past has been increasing for automobile bodies.

  Of these, panels such as outer panels (outer panels) and inner panels (inner panels) 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 increasing.

  Among these Al-Mg-Si Al alloy materials, panels such as outer panels (outer panels) and inner panels (inner panels) of panel structures such as automobile hoods, fenders, doors, roofs, trunk lids, etc. As the thin and high-strength Al alloy plate, it is preferable to use an excess Si type 6000 series Al alloy plate.

  This excess Si type 6000 series Al alloy is basically an Al—Mg—Si series aluminum alloy containing Si and Mg as essential components and having a Si / Mg ratio of 1 or more in mass ratio. And this excess Si type 6000 series Al alloy has excellent age-hardening ability, so at the time of press molding and bending processing, it secures formability by reducing the yield strength, and paint baking treatment of the panel after molding etc. It is age-hardened by heating at the time of relatively low-temperature artificial aging treatment to improve proof stress, and has age-hardening ability that can secure the required strength.

  Further, these excess Si type 6000 series Al alloy materials have 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 materials 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, as is well known, an outer panel of an automobile is manufactured by combining an aluminum alloy plate with a forming process such as an extension forming in a press forming or a bending forming. For example, an outer panel such as a hood or door is formed into a molded product shape as an outer panel by press molding such as overhanging, and then joined to the inner panel by hem (hemming) processing such as flat hem at the outer peripheral edge of the outer panel. Is performed to obtain a panel structure.

  At this time, there is a problem that a rough surface defect such as a ridging mark is likely to occur in a panel after press molding using an excess Si type 6000 series Al alloy plate as a raw material. The ridging mark is a phenomenon resulting from unevenness on the surface of the plate at the time of deformation such as press molding due to the texture arranged in the shape of stripes on the plate. For this reason, even if the crystal grain of the aluminum alloy plate is fine enough not to cause rough skin, it is troublesome to be caused by press forming.

  This ridging mark is particularly likely to occur when the press molding conditions become severe due to an increase in the size, complexity, or thickness of the panel structure. In addition, there is a problem that it becomes relatively inconspicuous immediately after press molding and becomes conspicuous after proceeding to the coating process as it is as a panel structure.

  When this ridging mark is generated, a panel structure for an outer plate (outer) or the like, which is required to have a particularly beautiful surface, has a problem in appearance and cannot be used.

In addition, the ingot is cooled after homogenization heat treatment at a temperature of 500 ° C. or higher, or cooled to room temperature and reheated, and hot rolling is started at a relatively low temperature of 350 to 450 ° C. It is also known to prevent ridging marks on the Al alloy plate (see, for example, Patent Documents 1 and 2).
Japanese Patent No. 2823797 (full text) JP-A-8-232052 (full text)

  However, even if cooling and initiation of hot rolling at a relatively low temperature of 350 to 450 ° C. are applied to the mass production hot rolling line after the homogenization heat treatment, depending on the hot rolling conditions, excess Si type 6000 Ridging marks on the aluminum alloy plate may not be prevented.

  That is, in a mass production hot rolling line, the aluminum alloy ingot is enlarged to a thickness of 400 mm or more. In a mass production hot rolling line, this large aluminum alloy ingot is composed of a reverse rough rolling mill and a tandem finishing rolling mill in which a plurality of rolling mills are arranged in series, Rolling consisting of a plurality of passes is performed.

  For this reason, in a mass-produced hot rolling line, the hot rolling conditions are different and complicated as compared to laboratory hot rolling conditions using a test rolling mill. For this reason, ridging marks may not be prevented only by cooling after the homogenization heat treatment or by lowering the hot rolling start temperature.

  On the other hand, in order to improve bending workability and press formability, in recent years, various proposals have been made to impart anisotropy to the texture of 6000 series Al alloy sheets. If the texture is made anisotropic, there is a problem that press formability is lowered.

  The present invention has been made paying attention to such circumstances, and its purpose is to prevent ridging marks during press forming even when a 6000 series Al alloy plate is produced by a mass-produced hot rolling line. A manufacturing method is to be provided.

In order to achieve this object, the gist of the manufacturing method of the aluminum alloy plate excellent in ridging mark property at the time of molding according to the present invention is mass%, Si: 0.4 to 1.3%, Mg: 0.2 to 1.2%, Mn: Homogeneous heat treatment of a large aluminum alloy ingot containing 0.01 to 0.65%, Cu: 0.001 to 1.0%, the balance consisting of Al and inevitable impurities and having a thickness of 400 mm or more at a temperature of 500 ° C or higher and lower than the melting point And then cooled to a temperature range of 350 to 450 ° C. at a cooling rate of 50 ° C./hr or more and 100 ° C./hr or less, and then composed of a reverse type rough rolling mill and a tandem type finishing rolling mill. Hot rolling is performed by setting the starting temperature in the rough rolling to a temperature range of 350 to 450 ° C. and the finishing temperature in the finish rolling to 350 ° C. or less by a hot rolling line on which rolling consisting of a plurality of passes is performed. After cold rolling, it is solution treatment and quenching treatment .

  In the present invention, the cooling after the homogenization heat treatment is preferably performed at a cooling rate of 50 ° C./hr to 100 ° C./hr to a temperature range of 350 to 450 ° C., and a temperature of 350 to 450 ° C. In the range, the hot rough rolling is started.

  In the present invention, the cooling after the homogenization heat treatment is preferably performed at a cooling rate of 50 ° C./hr or more and 100 ° C./hr or less to a temperature range of 350 ° C. or less, and then a temperature of 350 to 450 ° C. The hot rough rolling is started by reheating to the range.

  In the present invention, the ingot is cooled after homogenization heat treatment or cooled to room temperature and then reheated, and hot rolling is started at a relatively low temperature of 350 to 450 ° C., which is the same as in Patent Documents 1 and 2 above. It is.

  However, the relatively large ingot used in the mass-produced hot rolling line targeted by the present invention is an ingot having a thickness of 400 mm or more. The hot rolling line is also a hot rolling line that is composed of a reverse type rough rolling mill and a tandem type finishing rolling mill for each large ingot and performs rolling composed of a plurality of passes.

  In the present invention, even when producing a 6000 series Al alloy sheet by such a mass-produced hot rolling line, in order to prevent ridging marks during press forming, the ingot is cooled after homogenization heat treatment, As summarized, the cooling rate is specified to be 50 ° C / hr or more and 100 ° C / hr or less.

By setting this specific cooling rate range, the Mg ₂ Si compound in the ingot can be controlled to an appropriate size and distribution as a nucleation site for recrystallized grains during hot rolling even in a mass production hot rolling line. . As a result, the formation of coarse recrystallized grains (hot fibers) can be suppressed, the structure can be homogenized during recrystallization, and the ridging mark property during molding can be improved.

The specific cooling rate range after the homogenization heat treatment of the present invention does not eliminate the coarse Mg ₂ Si compound in the ingot, and does not thoroughly refine. In the present invention, an appropriate distribution (number) of the number of coarse Mg ₂ Si compounds having a diameter of 2 μm or more necessary as nucleation sites for recrystallized grains during hot rolling is ensured as described later. The specific cooling rate range after the homogenization heat treatment of the present invention is for this purpose.

  Further, in the present invention, the ingot after cooling in the specific cooling rate range after the homogenization heat treatment is started to be hot-rolled at a lower temperature as described above, and heated at a lower temperature below the recrystallization temperature. The hot rolling is finished. As described above, this suppresses the formation of coarse recrystallized grains, homogenizes the structure during recrystallization, and ensures that the ridging mark property during molding is improved.

  As a result, the coarse recrystallized grains can be suppressed even in the excess Si type 6000 series Al alloy plate, which is likely to produce the coarse recrystallized grains even at the low temperature of hot rolling at about 280 ° C. , Ridging marks can be suppressed.

  Incidentally, the cooling rate of course varies greatly depending on the size (mainly thickness) of the ingot. However, in the case of a relatively large ingot having a thickness of 400 mm or more, which is the subject of the present invention, when trying to obtain the specific cooling rate of 50 ° C./hr or more and 100 ° C./hr or less, soaking is performed. It is necessary to forcibly cool the ingot with a fan inside or outside the furnace.

  For example, in the case of the above-described relatively large ingot, the cooling rate is small in the soaking in the soaking furnace or outside the furnace. For this reason, a cooling rate above the lower limit of 50 ° C./hr cannot be obtained, and inevitably the cooling rate is lower than the lower limit of 50 ° C./hr. On the other hand, in forced air cooling or water cooling outside the soaking furnace, the cooling rate increases even in the case of the relatively large ingot. For this reason, the cooling rate does not reach the upper limit of 100 ° C./hr or lower, and may exceed the upper limit of 100 ° C./hr.

  Below, it demonstrates concretely about the embodiment of the manufacturing method of this invention Al alloy plate.

(Chemical composition)
First, the chemical component composition of the 6000 series Al alloy plate targeted by the present invention will be described. The 6000 series Al alloy plate targeted by the present invention is required to have various properties such as excellent formability, BH property, strength, weldability, and corrosion resistance as the above-mentioned automobile material.

  In order to satisfy such requirements, the composition of the Al alloy plate includes, by mass%, Si: 0.4 to 1.3%, Mg: 0.2 to 1.2%, Mn: 0.01 to 0.65%, Cu: 0.001 to 1.0%. The balance is made of Al and inevitable impurities.

  In addition, the 6000 series Al alloy plate targeted by the present invention is applied to an excess Si type 6000 series Al alloy plate in which the mass ratio Si / Mg between Si and Mg that is liable to generate ridging marks is 1 or more. preferable.

  For other elements, the content (allowable amount) of each impurity level in accordance with AA or JIS standards. Specifically, other alloy elements include 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: 0.2% or less. 1 type or 2 types or more may be included. In addition to or instead of these, one or two of Ag: 0.2% or less and Zn: 1.0% or less may be further included.

  Other alloy elements and gas components other than the above alloy elements are also 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, or the allowable amount will be described below.

Si: 0.4 to 1.3%.
Si, together with Mg, forms an aging precipitate that contributes to strength improvement during solid tempering and artificial aging treatment at low temperatures such as paint baking treatment, and exhibits age hardening ability, which is necessary as an outer panel for automobiles. For example, it is an essential element for obtaining the required strength (proof strength) of 170 MPa or more. Therefore, it is the most important element for combining various characteristics such as press formability and hemmability in the excess Si type 6000 series Al alloy plate of the present invention.

  In addition, in order to demonstrate the excellent low-temperature age-hardening ability of 170 MPa or more after the low-temperature paint baking treatment after molding to the panel (at the time of low-temperature aging treatment after applying 2% stretch at 170 ° C × 20 minutes) It is preferable to have an excess Si type 6000-based Al alloy composition in which Si / Mg is 1.0 or more by mass and Si is excessively contained with respect to Mg.

  When the Si content is less than 0.4%, the age-hardening ability and further various properties such as press formability and hemmability required for each application cannot be obtained. On the other hand, when Si exceeds 1.3%, hemmability and press formability are particularly hindered. Furthermore, weldability is significantly impaired. Therefore, Si is set in the range of 0.4 to 1.3%. In the outer panel, hem workability is particularly important. Therefore, in order to further improve flat hem workability as well as press formability, the Si content is preferably set to a lower range of 0.6 to 1.2%.

Mg: 0.2-1.2%.
Mg forms aging precipitates that contribute to strength improvement with Si during the above-mentioned artificial aging treatment such as solid solution strengthening and paint baking treatment, and exhibits age-hardening ability, and as a panel, for example, necessary proof stress of 170 MPa or more It is an essential element for obtaining.

  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 required proof stress of 170 MPa or more necessary for a panel cannot be obtained.

  On the other hand, if the Mg content exceeds 1.2%, the formability such as press formability and bending workability is significantly inhibited. Therefore, the Mg content is in the range of 0.2 to 1.2%, and the Si / Mg is such that the mass ratio is 1.0 or more. In order to further improve the flat heme workability, when the Si content is set to a lower range of 0.6 to 1.2%, in order to obtain an excess Si type 6000 series Al alloy composition correspondingly, Further, the Mg content is preferably in the lower range of 0.2 to 0.7%.

Cu: 0.001 to 1.0%
Cu has the effect of accelerating the formation of aging precipitates that contribute to the improvement of strength in the crystal grains of the Al alloy material structure under the conditions of artificial aging treatment at a relatively low temperature and short time of the present invention. Further, solid solution Cu has an effect of improving formability. This effect is not obtained when the Cu content is less than 0.001%. On the other hand, if it exceeds 1.0%, the stress corrosion cracking resistance, the thread rust resistance of the corrosion resistance after coating, and the weldability are significantly deteriorated. For this reason, it is 0.8% or less for structural material applications where corrosion resistance is important, and 0.1% or less for the appearance of yarn rust resistance in panel applications such as automotive exterior panels. It is preferable to do.

Mn: 0.01 to 0.65%
Mn produces dispersed particles (dispersed phase) during the homogenization heat treatment, and these dispersed particles have the effect of preventing grain boundary movement after recrystallization, so that fine crystal grains can be obtained. . As described above, the press formability and hem workability of the Al alloy plate of the present invention improve as the crystal grains of the Al alloy structure become finer. In this respect, when the Mn content is less than 0.01%, these effects are not obtained.

  On the other hand, when the Mn content is increased, coarse Al-Fe-Si- (Mn, Cr, Zr) -based intermetallic compounds and crystal precipitates are easily generated during melting and casting, and the mechanical properties of the Al alloy sheet Causes the properties to deteriorate. In particular, in flat hem processing where the processing conditions have become strict due to the complicated shape, thinning, or the presence of a gap between the inner panel edge and the inner edge of the outer panel edge, the Mn content is 0.15%. When it exceeds, heme workability will fall. For this reason, Mn is set in the range of 0.01 to 0.65%, and more preferably in the range of 0.01 to 0.15%, particularly in flat hem processing in which the processing conditions are severe.

(Production method)
Next, a method for producing the Al alloy plate of the present invention will be described below. In the present invention, the Al alloy ingot having the above-described component composition is subjected to homogenization heat treatment, hot rolling, and further cold rolling, followed by solution treatment and quenching treatment.

  However, as described above, the present invention is intended for mass-produced hot rolling lines using relatively large ingots. In the present invention, even when a 6000 series Al alloy plate is manufactured by such a mass-produced hot rolling line, ridging marks during press forming are prevented. Necessary or preferable conditions for this will be described below.

(Melting, casting)
First, in the melting and casting process, a normal melt casting method such as a continuous casting rolling method and a semi-continuous casting method (DC casting method) is appropriately selected for the molten Al alloy melt adjusted within the above-mentioned 6000 component standard range. And cast.

(Homogenization heat treatment)
Next, the cast Al alloy ingot is subjected to homogenization heat treatment. As the temperature of the homogenization heat treatment itself, a homogenization temperature of 500 ° C. or higher and a temperature lower than the melting point are appropriately selected as usual. The purpose of this homogenization heat treatment is to homogenize the structure, that is, to eliminate segregation in the crystal grains in the ingot structure. When the heat treatment temperature is lower than 500 ° C., intragranular segregation of the ingot cannot be sufficiently eliminated, and this acts as a starting point of fracture, so that press formability and bendability deteriorate. Further, the heat treatment time is preferably 2 hours or more, although it depends on the thickness of the ingot. If it is lower than 2 hr, intragranular segregation of the ingot cannot be sufficiently eliminated, and this may act as a starting point of fracture.

  In the present invention, in order to prevent ridging marks during press forming even when producing a 6000 series Al alloy sheet by a mass-produced hot rolling line, a rough rolling start temperature of 350 to 450 ° C. is applied after homogenization heat treatment of the ingot. Cool to a range at a cooling rate of 50 ° C / hr or more and 100 ° C / hr or less.

By setting this specific cooling rate range, the Mg ₂ Si compound in the ingot can be controlled to an appropriate size and distribution as a nucleation site for recrystallized grains during hot rolling even in a mass production hot rolling line. . As a result, even with an excess Si type 6000 series Al alloy plate, the formation of coarse recrystallized grains (hot fibers) during hot rolling is suppressed, and the structure during recrystallization is homogenized and formed. The ridging mark property at the time can be improved.

As described above, the specific cooling rate range after the homogenization heat treatment according to the present invention does not eliminate the coarse Mg ₂ Si compound in the ingot, and does not thoroughly refine. This is to secure the appropriate number (number) of coarse Mg ₂ Si compounds with a diameter of 2 μm or more necessary as nucleation sites for recrystallized grains during hot rolling.

When the cooling rate is less than the lower limit of 50 ° C./hr, the Mg ₂ Si compound in the ingot is coarsened. Specifically, the Mg ₂ Si compound having a diameter of 2 μm or more (precipitation amount) has an average number density of 1000 / Too much beyond mm ² . For this reason, since it becomes difficult for a compound to fully dissolve at the time of solution treatment, it becomes impossible to obtain a required strength after artificial aging.

On the other hand, when the cooling rate exceeds the upper limit of 100 ° C./hr, the fine Mg ₂ Si compound (precipitation amount) having a diameter of 1 to 2 μm or less is excessively increased. Specifically, the fine Mg ₂ Si compound (precipitation amount) having a diameter of 1 to 2 μm is excessively increased with an average number density exceeding 8000 / mm ² . Then, the coarse Mg ₂ Si compound (precipitation amount) having a diameter of 2 μm or more becomes an average number density of less than 300 / mm ² and becomes too small. For this reason, the number of coarse Mg ₂ Si compounds having a diameter of 2 μm or more necessary as nucleation sites for recrystallized grains during hot rolling cannot be secured. As a result, the formation of coarse recrystallized grains (hot fibers) is suppressed, but on the contrary, the structure cannot be homogenized during recrystallization. As a result, the ridging mark property at the time of molding cannot be improved.

Therefore, in order to improve the ridging mark property, the Mg ₂ Si compound in the ingot used for hot rolling is a coarse Mg ₂ Si compound having a diameter of 2 μm or more and an average number density of 300 to 1000 / mm. ² is preferably present (precipitated). At the same time, the fine Mg ₂ Si compound (precipitation) having a diameter of 1 to 2 μm is preferably 8000 / mm ² or less in terms of average number density.

(conductivity)
In a plate structure in which the Mg ₂ Si compound is in such a range, the conductivity measured on the plate surface is 50 IACS% or less. Therefore, the conductivity measured on the surface of the plate is a standard indicating the precipitation state (structure) of the preferable Mg ₂ Si compound. The electrical conductivity (% IACS) of the plate surface is measured by polishing the plate surface with emery paper # (mesh) 1000 and then measuring the central portion of the plate with an eddy current conductivity measuring device. The number of measurements is arbitrary 10 places on the plate, and the conductivity is obtained as an average value of these.

(Average number density measurement of Mg ₂ Si compound)
Here, the diameter of the Mg ₂ Si compound refers to the maximum diameter among the diameters of the individual Mg ₂ Si compounds. The average number density of Mg ₂ Si compounds having a diameter of 2 μm or more and a diameter of 0.5 to 2 μm is measured by image analysis of SEM (scanning electron microscope) images. In order to make the measurement reproducible, the SEM measurement is carried out at a depth of 1/4 t from the surface of the aluminum alloy plate of thickness t, and image analysis is performed at an observation magnification of 1000 times and 20 arbitrary measurement points. Will be performed.

(Cooling means after homogenization heat treatment)
In the case of a relatively large ingot having a thickness of 400 mm or more, which is a subject of the present invention, the cooling rate of the ingot after the homogenization heat treatment is specified to be 50 ° C / hr or more and 100 ° C / hr or less. In order to achieve a cooling rate range, it is necessary to forcibly air-cool the ingot with a fan inside or outside the soaking furnace.

  The actual ingot (slab) size has a thickness of 400 to 600 mm, a width of 1000 to 2500 mm, and a length of 4 to 10 m, so that the ingot is uniformly cooled in the above specific cooling rate range. Depending on the size and arrangement of the lump, fans are arranged as appropriate, and forced air cooling is performed inside or outside the soaking furnace.

  In the case of the above-described relatively large ingot, the cooling rate becomes too low when the cooling is performed in the soaking furnace or outside the furnace without using a fan. Therefore, the cooling rate is inevitably lower than the lower limit of 50 ° C./hr. On the other hand, when the ingot is water-cooled with mist or spray, the cooling rate is too high even in the case of the relatively large ingot. For this reason, the cooling rate does not reach the upper limit of 100 ° C./hr or lower, and inevitably the cooling rate exceeds the upper limit of 100 ° C./hr even in summer. For this reason, there is a possibility that a new problem in which an abnormality occurs in the shape such as deformation or warping due to heat shrinkage of the ingot may occur.

(Mode of cooling after homogenization heat treatment)
Here, after the homogenization heat treatment, the mode when cooling to the temperature range of 350 to 450 ° C. where the hot rough rolling is started is the above cooling range of 50 ° C./hr or more and 100 ° C./hr or less. It is also possible to cool directly to this temperature and start hot rough rolling in this temperature range (hereinafter also referred to as two-stage soaking).

  In addition, it is cooled to a temperature range of 350 ° C. or less at a cooling rate of 50 ° C./hr or more and 100 ° C./hr or less, and then reheated to a temperature range of 350 to 450 ° C. where hot rough rolling is started. Then, hot rough rolling may be started in this temperature range (hereinafter also referred to as twice soaking).

(Hot rolling line)
In the present invention, a relatively large ingot cooled in the specific cooling rate range after the homogenization heat treatment is hot-rolled by a hot rolling line suitable for mass production of plates. This hot rolling line is usually composed of a reverse type rough rolling mill consisting of one unit and a tandem type finishing mill consisting of usually 3 to 5 units. In these rough rolling mill and finish rolling mill, rolling consisting of a plurality of passes is performed.

  In the present invention, hot rough rolling of the ingot after cooling in the specific cooling rate range after the homogenization heat treatment is started in this hot rolling line in a lower temperature range of 350 to 450 ° C. At the same time, the hot rolling in the finish rolling is finished at 350 ° C. or lower. As described above, this suppresses the formation of coarse recrystallized grains, homogenizes the structure during recrystallization, and ensures that the ridging mark property during molding is improved.

  As a result, the coarse recrystallized grains can be suppressed even in the excess Si type 6000 series Al alloy plate, which is likely to produce the coarse recrystallized grains even at the low temperature of hot rolling at about 280 ° C. , Ridging marks can be suppressed.

  When the hot rough rolling start temperature exceeds 450 ° C, recrystallization occurs and coarse recrystallized grains are formed during hot rolling, and the recrystallized grains in a specific orientation of the plate that cause ridging marks are streaked. It is often lined up in. Further, if the hot rough rolling start temperature is less than 350 ° C., the hot rolling itself becomes difficult.

  Further, the end temperature of the hot rolling is set to 350 ° C. or less, and it is processed into a plate shape such as a coil shape or a plate shape. When the hot rolling finish temperature exceeds 350 ° C., particularly coarse recrystallized grains are likely to be generated, and the recrystallized grains in a specific orientation of the plate that cause ridging marks are often arranged in a streak shape.

  As described above, in the present invention, the ingot after the homogenization heat treatment is cooled, and hot rolling is started at a lower temperature, and the hot rolling is finished at a lower temperature without generating coarse recrystallized grains. . For this reason, it is possible to suppress the formation of coarse recrystallized grains that cause ridging marks during hot rolling.

(Hot rolled sheet annealing)
The hot rolled sheet is annealed (roughened) before cold rolling as necessary. It may be omitted for the sake of manufacturing efficiency and reduction of manufacturing cost, and cold rolling may be performed without pre-annealing the hot-rolled sheet.

(Cold rolling)
After the roughening, cold rolling is subsequently performed to produce a cold-rolled sheet (including a coil) having a desired thickness.

(Solution and quenching)
Utilizing dispersed particles controlled to a size distribution and amount within the range of the present invention by soaking of the ingot, random orientation as a recrystallization nucleus for suppressing ridging marks in the final solution treatment and quenching treatment In order to obtain a recrystallization orientation having the above, it is preferable that the temperature raising rate of the final solution treatment is 100 ° C./min or more. In the temperature rising process of 100 ° C./min or more in the final solution treatment, the dispersed particles serve as nuclei for forming random recrystallized crystal orientations. The rate of temperature rise is more preferably 200 ° C./min or more, and more preferably 300 ° C./min or more.

  The solution treatment conditions are preferably at least 500 ° C. and a melting point in order to sufficiently precipitate aging precipitates that contribute to strength improvement by artificial aging treatment such as paint baking hardening after press molding of the plate. Perform in the following temperature range. More preferably, they are 510 degreeC or more and 570 degrees C or less, More preferably, they are 520 degreeC or more and 560 degrees C or less.

Next, in the quenching treatment from the solution treatment temperature, if the cooling rate is slow, Si, Mg ₂ Si, etc. are likely to precipitate on the grain boundary, which is likely to be the starting point of cracks during press forming and bending, and these formability Decreases. In order to ensure this cooling rate, the quenching treatment should be performed by selecting and using water cooling means and conditions such as air cooling such as fans, mist, spraying, immersion, etc., and quenching at a cooling rate of 300 ° C / min or more. preferable. More preferably, it is 600 ° C./min or more, more preferably 700 ° C./min or more, and still more preferably 800 ° C./min or more.

  In the present invention, in order to further enhance the age-hardening property in the artificial age-hardening treatment such as the paint baking process of the molded panel, a pre-aging treatment is performed after the quenching treatment in order to promote the precipitation of the age-related precipitates that contribute to strength improvement. You may do it. This preliminary aging treatment is preferably held at a temperature range of 60 to 150 ° C., preferably 70 to 120 ° C. for a required time of 1 to 24 hours. As this preliminary aging treatment, after the cooling end temperature of the quenching treatment is increased to 60 to 150 ° C., it is immediately reheated or kept as it is. Alternatively, it is immediately reheated to 60-150 ° C. within 5 minutes after solution treatment and quenching to room temperature.

  Furthermore, in order to suppress aging at room temperature, heat treatment (artificial aging treatment) at a relatively low temperature may be performed after the preliminary aging treatment without time delay. When the time delay is present, room temperature aging (natural aging) occurs with time even after the preliminary aging treatment, and after the room temperature aging occurs, the effect of the heat treatment at the relatively low temperature is exhibited. It becomes difficult.

  Further, in the case of continuous solution quenching, the quenching process is completed within the temperature range of the preliminary aging, and the coil is wound around a coil at the same high temperature. In addition, you may reheat before winding up to a coil, and you may heat-retain after winding. Moreover, after the quenching process to room temperature, it may be reheated to the above temperature range and wound at a high temperature.

  In addition to this, it is of course possible to further increase the strength by performing aging treatment or stabilization treatment at a higher temperature according to the application or required characteristics.

(Average crystal grain size)
The Al alloy plate thus obtained is refined to an average crystal grain size of 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 average crystal grain size is larger than 50 μm, the press formability such as bending workability and overhang is remarkably deteriorated, and defects such as cracks and rough skin during forming tend to occur. Also, ridging marks are likely 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.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Next, examples of the present invention will be described. A 6000 series Al alloy sheet having the composition of A to I shown in Table 1 is subjected to homogenization heat treatment (abbreviated as soaking) and hot rolling (abbreviated as hot rolling) under the conditions shown in Table 2, and further cold rolled. Was manufactured by solution treatment and quenching. In addition, in the display of the content of each element in Table 1, “−” indicates that the content is below the detection limit.

  More specific production conditions for the test material Al alloy plate are as follows. An ingot of 500 mm thickness, 2000 mm width, and 7 m length having each composition shown in Table 1 was melted by a DC casting method and then subjected to homogenization heat treatment (first time) under the conditions shown in Table 2. After this homogenization heat treatment, two-stage soaking was performed to cool down to the hot rolling start temperature at the cooling rate shown in Table 2 (in Table 2, the second soaking was indicated as ``-'') . Alternatively, it was cooled twice to room temperature at the cooling rate shown in Table 2 and then reheated to the hot rolling start temperature (in Table 2, the reheating temperature and holding time for the second soaking were determined). ).

  After this soaking, hot rolling was performed to a thickness of 2.5 mmt at each start temperature and each end temperature of rough rolling shown in Table 2 and each end temperature of finish rolling. The hot-rolled sheet was subjected to direct cold rolling after omitting the roughening to obtain a cold-rolled sheet having a thickness of 1.2 mmt.

  The thickness of the final cold-rolled plate, which will be the test plate, is increased to 1.2 mm to reflect the increased thickness of the outer car panel for protecting pedestrians and occupants in recent collisions. It was. When the thickness of the test plate is increased in this way, it is advantageous for improving press formability, but disadvantageous for bending workability such as hem. Furthermore, the inner panel sandwiched between the hems is made thinner to suppress the increase in weight as the automobile outer panel becomes thicker, which is disadvantageous for 180 ° bending workability such as hems. It becomes.

  The cold-rolled sheet was heated in a continuous heat treatment facility at a rate of temperature increase of about 300 ° C./min in common with each example, and when the solution treatment temperature of 550 ° C. was reached (holding time 10 Immediately after quenching to room temperature, quenching was performed at a cooling rate of approximately 600 ° C./min. Immediately after this quenching, a preliminary aging treatment was carried out at a temperature of 100 ° C. for 2 hours.

(Test plate requirements)
A test plate (blank) is cut out from each final product plate after the tempering treatment, and each test plate structure has a diameter of 2 μm or more after aging at room temperature for 3 months (90 days). the average number density of mg ₂ Si compound particles were each measured by the measuring method described above and the average number density of mg ₂ Si compound particles size diameter 0.5 to 2 [mu] m.

  Similarly, the average conductivity (IACS%) and average crystal grain size (μm) as the structure of each test plate after room temperature aging for 3 months after the tempering treatment, and the characteristics of the test plate as ridging marks And 0.2% proof stress (MPa) in the direction of 45 ° with respect to the rolling direction were measured and evaluated. These results are shown in Table 3. The average conductivity (IACS%) and the average crystal grain size (μm) were each determined by the method described above.

(Riding mark)
The ridging mark property of the manufactured aluminum alloy sheet can be evaluated without actually applying the coating treatment after press forming. That is, it can be evaluated in a simulated manner by measuring the surface roughness Ra and the surface waviness Wa after 10% stretching by a tensile test in a direction perpendicular to the rolling direction of each test plate. Specifically, the surface roughness Ra and surface waviness Wa before stretching of each test plate are measured, and the ratio (ratio) of the surface roughness Ra after stretching to the surface roughness Ra before stretching, the stretch The magnification (ratio) of the subsequent surface waviness Wa to the surface waviness Wa before stretching is determined.

  Both of these magnifications (ratio) are 5 times or less, and even after stretching simulating press molding, a so-called increase in surface roughness between surface roughness Ra and surface waviness Wa is small. It can be evaluated that the ridging mark property is excellent.

  In the tensile test for imparting 10% stretch, a No. 5 test piece (25 mm × 50 mmGL × plate thickness) of JISZ2201 was collected from the Al alloy plate obtained as described above, and subjected to room temperature tension. At this time, the specimens were sampled in a direction perpendicular to the rolling direction and a direction perpendicular to the rolling direction. The tensile speed was 5 mm / min up to 0.2% proof stress and 20 mm / min after proof stress.

  The surface roughness Ra (arithmetic mean roughness) and surface waviness Wa (arithmetic mean waviness) of the plate are based on the definition and measurement method defined in JIS B0601, respectively. It was determined by measuring with a meter.

(As proof stress)
A JISZ2201 No. 5 test piece (25 mm × 50 mmGL × plate thickness) perpendicular to the rolling direction was taken from the Al alloy plate immediately after the tempering treatment, and a room temperature tensile test was performed. The room temperature tensile test was performed at room temperature of 20 ° C. based on JISZ2241 (1980) (metal material tensile test method). The crosshead speed was 5 mm / min, and the test was performed at a constant speed until the test piece broke. By this method, 0.2% proof stress was evaluated, and AS proof strength was set (N number = average value of 5).

(Yield strength after BH)
In order to investigate the artificial aging treatment ability (BH property), the above JIS No. 5 test piece was preliminarily given a strain of 2% and simulated at 170 ° C. , 20 minutes of artificial age hardening treatment was performed, and the proof stress (MPa) of each test plate after treatment was measured as the post-BH proof stress (MPa) under the tensile test conditions described above. Are shown in Table 3.

(Formability)
As the formability of the test plate, the crack limit height (LDH ₀ ) and the limit drawing ratio (LDR) for evaluating the stretch formability, and the bendability in the direction parallel to the rolling direction were tested. These results are also shown in Table 3.

For the crack limit height (LDH ₀ ) test, the test plate was cut into a test piece with a length of 180 mm and a width of 110 mm, using a spherical overhang punch with a diameter of 101.6 mm, and using R-303P as a lubricant, Stretching was performed at a pressure of 200 kN and a punching speed of 4 mm / S, and the height (mm) at which the test piece cracked was determined. Each sample was tested 3 times and the average value was adopted. The larger the crack limit height, the better the stretch formability. For example, in order to satisfy the stretch formability required for a molded panel for automobiles, it may be 27.0 mm or more.

  The limit drawing ratio (LDR) is obtained by punching test pieces of various diameters from the test plate, punching: 50mmφ-shoulder R8mm, die: 53mmφ-shoulder R8mm, and using the lubricant R-303P. A deep drawing test was performed under the conditions of a pressing pressure of 300 to 600 kgf and a test speed of 20 mm / min. And the shaping | molding limit blank diameter which cannot be deep-drawn was determined, and the limit drawing ratio was computed by the following formula | equation. Limit drawing ratio = forming limit blank diameter / punch diameter. The larger the limit drawing ratio, the better the deep drawability. For example, in order to satisfy the deep drawability required for a molded panel for automobiles, it may be 1.8 or more.

For the evaluation of bendability, a bend test specimen having a length of 150 mm and a width of 30 mm was taken from a test plate, and bendability assuming flat hemming was evaluated. That is, a 15% strain was preliminarily applied to the test piece, and then contact bending at an angle of 180 ° (inner bending radius R = about 0.25 mm) was performed. The evaluation of bendability was evaluated in five stages based on the following criteria by visually confirming the degree of cracking at the bent edge of the test piece after bending.
0: No rough skin or fine cracks.
1: Rough skin has occurred.
2: Although there is rough skin, there are no cracks including minute ones.
3: Small cracks occur.
4: Large cracks occur.
5: Multiple or many large cracks occurred.
Of the above ranks, 0 to 2 stages are acceptable and 3 to 5 are unacceptable. In order to reflect the severe hemming conditions of the inner panel sandwiched between the hem portions described above, the inner panel was not sandwiched into the hem portions.

As shown in Tables 1 and 2, Invention Examples 1 to 12 are subjected to homogenization heat treatment and hot rolling within the composition range of the present invention and within the conditions of the present invention. For this reason, as shown in Table 3, the Mg ₂ Si compound particles have a preferred average density and average conductivity (IACS%) or average crystal grain size (μm).

  As a result, even in the example of a 6000 series Al alloy plate with an excess Si type composition that has been aged at room temperature for 3 months (90 days) after the tempering treatment and the formability has deteriorated, excellent BH properties, press formability, Excellent ridging mark property while maintaining bending workability.

  In comparison with the same Invention Examples 1 to 6, Invention Example 2 has a relatively high cooling rate after soaking (close to the upper limit). In Invention Example 3, the cooling rate after soaking is relatively small (close to the lower limit). In Invention Example 4, the reheating temperature after soaking is relatively low (close to the lower limit). In Invention Example 6, the reheating temperature after soaking is relatively high (close to the upper limit). Therefore, Invention Examples 2 and 3 are in comparison with Invention Example 1 where the cooling rate after soaking is optimal, and Invention Examples 4 and 6 are in comparison with Invention Example 5 where the reheating temperature after soaking is optimal. , But relatively low characteristics.

On the other hand, Comparative Examples 13 to 18 use the same alloy example A as Invention Examples 1 to 6. However, the manufacturing conditions of each comparative example are out of the scope of the invention.
In Comparative Example 13, the cooling rate after soaking is too small outside the range of the invention.
In Comparative Example 14, the cooling rate after soaking is too large outside the range of the invention.
In Comparative Example 15, the cooling rate after the soaking is appropriate, but the hot rolling start temperature is too high outside the range of the invention.
In Comparative Example 16, the cooling rate after soaking is appropriate, but the hot rolling start temperature is too low outside the range of the invention.
In Comparative Example 17, although the cooling rate after soaking is appropriate, the reheating temperature is too high outside the range of the invention, and the hot rolling start temperature is too high.
In Comparative Example 18, although the cooling rate after soaking is appropriate, the reheating temperature is too low outside the range of the invention, and the hot rolling start temperature is too low.

Therefore, as shown in Table 3, it is difficult to perform hot rolling in Comparative Examples 16 and 18, and Comparative Examples 13, 14, 15, and 17 are the average number density and average conductivity (IACS%) of Mg ₂ Si compound particles. ) Or the average crystal grain size (μm) is out of the preferred range. As a result, in Comparative Examples 13 to 18, the examples of 6000 series Al alloy plates with an excess Si type composition that was aged at room temperature for 3 months after the tempering treatment and the formability was lowered, BH property, press formability In addition to any of the bending workability, the ridging mark property is remarkably inferior as compared with the inventive examples.

  Comparative Example 19 using an alloy H with an Si content of less than 0.4% and deviating from the component composition has BH properties and press formability in spite of performing homogenization heat treatment and hot rolling within the range of the present invention. The ridging mark property is remarkably inferior to that of the inventive examples.

  Comparative Example 20, which uses Alloy I with an Mg content of less than 0.2% and a component composition deviating, has BH properties and press formability in spite of performing homogenization heat treatment and hot rolling within the range of the present invention. The ridging mark property is remarkably inferior to that of the inventive examples.

  Therefore, the results of the above examples support the critical significance or effect of each requirement of the present invention.

  ADVANTAGE OF THE INVENTION According to this invention, even when manufacturing a 6000 series Al alloy plate with a mass-production hot rolling line, the manufacturing method which prevented the ridging mark at the time of press molding can be provided. In addition, it can cope with the bending process of a thickened automobile outer panel for protecting pedestrians and passengers at the time of collision in recent years. As a result, it is possible to expand the application of 6000 series Al alloy plates for transportation equipment such as automobiles, ships or vehicles, household electrical appliances, buildings, structural members and parts, and especially for transportation equipment such as automobiles. .

Claims (7)

In mass%, Si: 0.4-1.3%, Mg: 0.2-1.2%, Mn: 0.01-0.65%, Cu: 0.001-1.0%, the balance is made of Al and inevitable impurities, and has a thickness of 400 mm or more A large aluminum alloy ingot is subjected to homogenization heat treatment at a temperature of 500 ° C. or higher and lower than the melting point, and then cooled to a temperature range of 350 to 450 ° C. at a cooling rate of 50 ° C./hr or more and 100 ° C./hr or less. With a hot rolling line composed of a reverse type rough rolling mill and a tandem type finishing rolling mill, each of which is subjected to rolling consisting of a plurality of passes, the starting temperature in the rough rolling is set to a temperature range of 350 to 450 ° C., Production of an aluminum alloy sheet excellent in ridging mark properties during forming, characterized by performing hot rolling with an end temperature in the finish rolling of 350 ° C. or lower, further cold rolling, and then performing solution treatment and quenching treatment Method.   The molding according to claim 1, wherein after the homogenization heat treatment, the hot rough rolling is started by cooling to a temperature range of 350 to 450 ° C at a cooling rate of 50 ° C / hr or more and 100 ° C / hr or less. The manufacturing method of the aluminum alloy plate excellent in the ridging mark property of. After the homogenization heat treatment, it is cooled at a cooling rate of 50 ° C./hr or more and 100 ° C./hr or less to a temperature range of 350 ° C. or less, and then reheated to a temperature range of 350 to 450 ° C. It starts rolling method for manufacturing an aluminum alloy plate having excellent ridging mark resistance during molding of claim 1.   4. The cooling rate of 50 ° C./hr or more and 100 ° C./hr or less after the homogenization heat treatment is obtained by forced air cooling with an ingot fan inside or outside the soaking furnace. The manufacturing method of the aluminum alloy plate excellent in the ridging mark property at the time of shaping | molding as described in 1 ..   5. The method for producing an aluminum alloy plate having excellent ridging mark property at the time of molding according to claim 1, wherein the mass ratio Si / Mg between Si and Mg is 1 or more.   The method for producing an aluminum alloy plate excellent in ridging mark property at the time of molding according to any one of claims 1 to 5, wherein the Cu content is regulated to 0.1% or less.   The method for producing an aluminum alloy plate excellent in ridging mark property at the time of molding according to any one of claims 1 to 6, wherein the aluminum alloy plate is for an automobile outer plate.