JP6306123B2 - Aluminum alloy plate and method for producing aluminum alloy plate - Google Patents

Description

  The present invention relates to a 6000 series aluminum alloy plate produced by ordinary rolling and having excellent formability and bake coating curability.

  In recent years, due to consideration for the global environment, social demands for reducing the weight of automobile bodies are increasing. In order to meet such demands, an aluminum alloy material is applied to a large body panel (outer panel, inner panel) in an automobile body instead of a steel material such as a steel plate.

  Among the large body panels, panels such as hoods, fenders, doors, roofs, trunk lids, etc., such as outer panels (outer plates) and inner panels (inner plates), are thin and high-strength aluminum alloy plates. Al-Mg-Si-based AA to JIS6000-based (hereinafter also simply referred to as 6000-based) aluminum alloy plates are used.

  This 6000-based (Al-Mg-Si-based) aluminum alloy plate contains Si and Mg as essential components. In particular, the excess Si type 6000-based aluminum alloy has excellent age-hardening ability during artificial aging treatment. . For this reason, in press molding and bending processing, the moldability is ensured by reducing the yield strength, and even with relatively low-temperature artificial aging treatment such as paint baking treatment of the panel after molding, the yield strength is improved, and as a panel There is a bake hardenability (hereinafter also referred to as bake hard property = BH property, bake hardenability) that can secure the required strength of the resin.

  On the other hand, as is well known, an outer panel or the like 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, a large outer panel such as a hood or a door is formed into a molded product shape as an outer panel by press molding such as overhanging, and then the inner panel and Are joined to form a panel structure.

  The outer panel of the automobile or the like tends to be thinner for weight reduction, and is required to have high strength that is excellent in dent resistance after being thinned. Therefore, at the time of press forming, the aluminum alloy plate is made to have a lower yield strength, ensuring formability, and age hardening by heating at a relatively low temperature artificial aging treatment such as paint baking treatment of the panel after molding. There is a need for artificial age-hardening ability (baking paint curability) that can ensure the required strength even after being improved and thinned.

  Conventionally, various proposals have been made to control Mg-Si clusters for the bake hardenability of 6000 series aluminum alloy plates as materials for such automobile members. Recently, a technique has been proposed in which these Mg—Si-based clusters are measured after being measured at an endothermic peak or an exothermic peak of a differential scanning calorimetry curve (hereinafter also referred to as DSC) of a 6000-based aluminum alloy plate. (See Patent Documents 1 to 5).

  For example, in Patent Document 1, in the differential scanning calorimetry curve of a 6000 series aluminum alloy plate, the exothermic peak height W1 in the temperature range of 100 to 200 ° C. is 50 μW or more, and the exotherm in the temperature range of 200 to 300 ° C. The ratio between the peak height W2 and the exothermic peak height W1, W2 / W1, is specified to be 20 or less.

  In Patent Document 2, in the differential scanning calorimetry curve of a 6000 series aluminum alloy plate, the exothermic peak height in the temperature range of 230 to 270 ° C. is A and the exothermic peak in the temperature range of 280 to 320 ° C. in the differential scanning calorimetry curve. When the height is B and the exothermic peak height in the temperature range of 330 to 370 ° C. is C, the exothermic peak height B is 20 μW / mg or more, and the exothermic peak height with respect to the exothermic peak height B is The ratios of A and C are respectively defined as A / B of 0.45 or less and C / B of 0.6 or less.

  In patent document 3, although it is a 6000 series aluminum alloy plate, in the differential scanning calorimetry curve of the plate whose total amount of Mg and Si is 1.2 or less, there is one exothermic peak within a temperature range of 230 to 330 ° C. Or only two exothermic peaks having a temperature difference between the peaks of 50 ° C. or less, and the height of the one exothermic peak or the peak of the two exothermic peaks. It defines that the height of the exothermic peak with the larger height is in the range of 20 to 50 μW / mg.

In patent document 4, although it is a 6000 series aluminum alloy plate, in the differential scanning calorimetry curve of the plate which requires Sn addition, as an endothermic peak corresponding to dissolution of Mg-Si clusters, an endothermic peak in a temperature range of 150 to 230 ° C. The peak height of the exothermic peak in the temperature range of 240 to 255 ° C. is the exothermic peak corresponding to the formation of the Mg—Si cluster, while the peak height is 8 μW / mg or less (including 0 μW / mg). Is 20 μW / mg or more.

  In Patent Document 5, in a differential scanning calorimetry curve after tempering treatment including solution treatment and quenching treatment of an aluminum alloy material, in a temperature range of 150 to 250 ° C. corresponding to dissolution of Si / vacancy clusters (GPI). The negative endothermic peak height is set to 1000 μW or less, the positive exothermic peak height in the temperature range of 250 to 300 ° C. corresponding to the precipitation of Mg / Si clusters (GPII) is set to 2000 μW or less, and room temperature aging suppression and low temperature aging hardening ability are achieved. It is described that it is made excellent.

Japanese Patent No. 4117243 JP 2013-167004 A Japanese Patent Laying-Open No. 2015-196852 Japanese Patent Laying-Open No. 2015-196853 Japanese Patent No. 3819263

However, these conventional DSC endothermic peak and exothermic peak control techniques are intended to obtain high BH properties in response to the baking coating curing process for reducing the temperature and time of the automobile parts. The heating temperature is 175 ° C. at the highest, and 150 ° C. in the low example. In other words, the BH property in the baking finish hardening process at a high temperature such as 180 ° C. or higher is not intended.
For this reason, in such a baking coating curing process at a high temperature, an Al-Mg-Si aluminum alloy plate after long-term aging at room temperature is still improved in order to have both good formability and high BH properties. There was room for.
Moreover, after combining the good moldability and the high BH property in the baking coating curing process at the high temperature, it is still improved to obtain the high BH property in the baking coating curing process at the low temperature. There was room.
That is, there is still room for improvement in the technical problem that can be said to be mutually contradictory, and that has both elongation, high temperature BH property, and low temperature BH property.

  In view of such circumstances, the object of the present invention is to provide the conventional low-temperature baking coating curing, as well as good moldability and baking coating curing treatment at the high temperature, even after prolonged aging at room temperature. It is to provide a 6000 series aluminum alloy plate and a method for producing the same that can have high BH properties in processing.

  In order to achieve this object, the gist of the aluminum alloy plate excellent in formability and bake hardenability of the present invention is mass%, Mg: 0.3-1.5%, Si: 0.6-1 .5%, and the total of Mg content and Si content is more than 1.2%, and the balance is an aluminum alloy plate made of Al and unavoidable impurities. In the thermal analysis curve, an endothermic peak having a height A of 3 to 10 μW / mg exists in a temperature range of 150 to 230 ° C., and a height B of 20 to 20 in a temperature range of 230 ° C. or more and less than 330 ° C. An exothermic peak of 50 μW / mg is present, and the ratio B / A between the exothermic peak height B and the endothermic peak height A is more than 3.5 and less than 15.0.

  Moreover, in order to achieve the said objective, the summary of the manufacturing method of the aluminum alloy plate excellent in the moldability and baking coating curability of this invention is the mass%, Mg: 0.3-1.5%, Si: An aluminum alloy cold-rolled sheet containing 0.6 to 1.5% each, and the total of the Mg content and the Si content is more than 1.2%, the balance being Al and inevitable impurities, Within 1 hour after solution treatment and quenching treatment, pre-aging treatment is performed at a temperature range of 30 ° C. to 60 ° C. for 5 hours or more and 500 hours or less. In the differential scanning calorimetry curve, an endothermic peak having a height A of 3 to 10 μW / mg is present in a temperature range of 150 to 230 ° C., and a height B is in a temperature range of 230 ° C. or more and less than 330 ° C. An exothermic pin of 20-50 μW / mg And the ratio B / A between the exothermic peak height B and the endothermic peak height A is more than 3.5 and less than 15.0.

The present inventors have good moldability and high BH property even in the conventional low-temperature baking coating curing treatment as well as the high-temperature baking coating curing treatment, even after prolonged room temperature aging. We examined a cluster to combine the two.
As a result, in order to obtain high elongation even after prolonged aging at room temperature, it has been found that a certain number of clusters corresponding to the endothermic peak of the suggested heat curve in the temperature range of 150 to 230 ° C. is required. did.
Moreover, even if the baking temperature in the baking coating curing process varies greatly from high temperature to low temperature, in order to obtain a high BH amount at any baking temperature, the number of clusters corresponding to the endothermic peak of the suggested heat curve is reduced. It was also found that the number of clusters corresponding to exothermic peaks in the temperature range of 230 ° C. to 330 ° C. increases.
That is, by controlling precisely the endothermic peak of the suggested heat curve within the temperature range of 150 to 230 ° C. and the exothermic peak of the temperature range of 230 ° C. to 330 ° C., even after prolonged aging at room temperature As well as obtaining elongation, it has been found that even if the baking temperature varies greatly from high temperature to low temperature, high BH properties can be obtained at any baking temperature.
In addition, it was found that the differential thermal curve for obtaining high BH properties varies depending on the temperature especially in the baking coating curing conditions (artificial aging treatment conditions), and at a relatively low baking coating temperature of 175 ° C. or lower, 180 ° C. or higher. It was also found that it is necessary to control the differential heat curve more precisely than the relatively high baking coating temperature.

  Based on these findings, the present invention performs structure control to control the differential heat curve more precisely, and it can be said that the 6000 series aluminum alloy plates as materials for automobile members are mutually contradictory. Both have low-temperature BH properties.

It is explanatory drawing which shows the differential scanning calorimetry curve of this invention aluminum alloy plate.

  The aluminum alloy plate (molding material plate) referred to in the present invention is a rolled plate such as a hot rolled plate or a cold rolled plate, and is subjected to tempering (T4) such as solution treatment and quenching treatment. It is a raw material aluminum alloy plate before being formed into an automobile member to be used and before being subjected to artificial aging treatment (artificial age hardening treatment) such as paint baking hardening treatment. In the following description, aluminum is also referred to as aluminum or Al.

  Hereinafter, embodiments of the present invention will be specifically described for each requirement.

Aluminum alloy composition:
First, the chemical component composition of the aluminum alloy sheet of the present invention will be described below, including reasons for limiting each element. In addition,% display of content of each element means the mass% altogether.

The chemical component composition of the aluminum alloy plate of the present invention is determined so as to satisfy the required formability and bake hardenability from the composition of the 6000 series aluminum alloy as a material of an automobile member such as the automobile large body panel.
From this point of view, the chemical composition of the aluminum alloy sheet of the present invention is, by mass, Mg: 0.3 to 1.5%, Si: 0.6 to 1.5%, respectively, and the Mg content. And the Si content are over 1.2%, and the balance consists of Al and inevitable impurities.

  In addition to this composition, Cu: 0.02-0.8%, Fe: 0.05-0.5%, Mn: 0.05-0.3%, Zr: 0.04-0.1%, One or two of Cr: 0.04-0.3%, V: 0.02-0.1%, Ag: 0.01-0.1%, Zn: 0.01-0.3% The above may be included. In addition,% display of content of each element means the mass% altogether.

Si: 0.6 to 1.5%
Si, together with Mg, forms aging precipitates such as Mg-Si-based precipitates that contribute to strength improvement during solid solution strengthening and artificial aging treatment such as baking coating treatment, and exhibits age hardening ability and is necessary It is an essential element for obtaining strength (yield strength).
If the Si content is too small, the amount of solute Si before baking coating treatment (before artificial aging heat treatment) decreases, and the amount of Mg-Si-based precipitates is insufficient. Run short. On the other hand, when there is too much Si content, a coarse crystallized substance and a precipitate will be formed, ductility will fall, and it will become the cause of the crack in the case of raw-material plate rolling. Therefore, the Si content is in the range of 0.6 to 1.5%, preferably in the range of 0.7 to 1.5%.

Mg: 0.3 to 1.5%
Mg also forms aged precipitates such as Mg-Si-based precipitates that contribute to strength improvement during solid tempering and artificial aging heat treatment such as baking coating, together with Si, and exhibits age-hardening ability. It is an essential element for obtaining strength. If the Mg content is too small, the amount of solid solution Mg before baking coating treatment is reduced, and the amount of Mg-Si-based precipitates is insufficient, so that the BH property is remarkably lowered and the strength is insufficient. On the other hand, when there is too much Mg content, it will become easy to form a shear band at the time of cold rolling, and will cause the crack at the time of a raw material plate rolling. Therefore, the Mg content is in the range of 0.3 to 1.5%, preferably in the range of 0.4 to 0.8%.

  Moreover, in order to exhibit the outstanding artificial age hardening ability in the paint baking process after shaping | molding to a panel, the sum total of Mg content and Si content shall be more than 1.2%. When the total is 1.2% or less as in Patent Document 3, even if the production conditions of the board are within a preferable range described later, an endothermic peak or an exothermic peak defined in the present invention cannot be generated, Artificial age-hardening ability is insufficient, and the required strength cannot be obtained. However, the upper limit of the total of the Mg content and the Si content is determined by the limit at which the plate can be produced without causing hot rolling cracking, and is preferably 2.5%.

One or more of Cu, Fe, Mn, Zr, Cr, V, Ag, Zn These elements have the effect of increasing the strength of the plate in common. Of course, it is selectively contained as necessary, but there are of course a common part and a different part in the specific mechanism.

  Cu can improve strength by solid solution strengthening. If the Cu content is too small, the effect is small, and if it is too much, the effect is saturated, and on the contrary, the corrosion resistance is deteriorated.

  Fe forms a crystallized product, serves as a nucleus of recrystallized grains, plays a role of preventing coarsening of crystal grains and improving strength. If the content is too small, the effect is small. If the content is too large, a coarse compound is formed, which becomes a starting point of destruction, and the strength and formability are lowered.

  Mn, Zr, Cr, and V contribute to strength improvement by refining the crystal grains of the ingot and the final plate product. Further, these elements exist as dispersed particles, contribute to crystal grain refinement, and improve moldability. When each content is too small, the effect of improving strength and formability due to the refinement of crystal grains is insufficient. On the other hand, when there are too many of these elements, a coarse compound will be formed and ductility will deteriorate.

  Ag has the effect of precipitating finely aging precipitates that contribute to strength improvement by artificial aging heat treatment after molding to automobile members, thereby promoting high strength. If the content is too small, the effect of improving the strength is small, and if the content is too large, various properties such as rollability and weldability are reduced, and the effect of improving the strength is saturated and expensive.

  Zn is useful for improving artificial age-hardening ability (BH property), and has an effect of increasing the strength by promoting precipitation of a compound phase such as a GP zone in the crystal grains of the plate structure in the baking coating process. .

  Therefore, when Cu, Fe, Mn, Zr, Cr, V, Ag, and Zn are contained, as described above, Cu: 0.02 to 0.8%, Fe: 0.05 to 0.5 %, Mn: 0.05-0.3%, Zr: 0.04-0.1%, Cr: 0.04-0.3%, V: 0.02-0.1%, Ag: 0.0. In the range of 01 to 0.1% and Zn: 0.01 to 0.3%, one kind or two or more kinds are contained.

Other elements:
Other than these elements, other elements such as Ti and B are inevitable impurities. Ti, together with B, forms a coarse compound and degrades mechanical properties. However, since the inclusion of a small amount also has the effect of refining the crystal grains of the aluminum alloy ingot, each content in the range specified by the JIS standard is allowed as a 6000 series alloy. As an example of this allowable amount, Ti is 0.1% or less, preferably 0.05% or less. Further, B is set to 0.03% or less. Incidentally, in the present invention, Sn, which is essential in Patent Document 4, is not added. In the case where Sn is added, the Mg-Si cluster is reduced on the contrary when the production conditions of the preferred plate described later are used, the endothermic peak and the exothermic peak defined in the present invention cannot be generated, and the artificial age hardening ability is insufficient. As a result, the required strength may not be obtained.

(Material board structure)
Based on the above alloy composition, in the present invention, the structure of an aluminum alloy plate is used as an index indicating in advance the state of artificial aging precipitates in a member made of this plate, and DSC obtained by differential scanning calorimetry ( Differential scanning calorimetry curve: DSC profile).
In other words, the present invention is capable of baking at a relatively low temperature as well as good formability and high BH property in a relatively high temperature baking coating curing process, which is the main target, even after prolonged room temperature aging. In order to have high BH property also in the curing process, it is defined by DSC (differential scanning thermal analysis curve: DSC profile) obtained by differential scanning thermal analysis.

  Under relatively low-temperature baking coating conditions, if there is an endothermic peak in the temperature range of 150 to 230 ° C., it is difficult to obtain a high amount of BH, but the relatively high-temperature baking coating that is the main subject of the present invention. Under the conditions, even if the endothermic peak exists to some extent, a high BH amount can be obtained. This is because, under the conventional relatively low-temperature baking coating conditions, even if a cluster corresponding to the endothermic peak is dissolved during the baking coating process, it is difficult for a subsequent strengthening phase to be newly generated, so that a high BH property is obtained. It is assumed that it is not possible.

On the other hand, under relatively high-temperature baking coating conditions, the main subject of the present invention, the clusters corresponding to the endothermic peaks are dissolved in an extremely short time, and then a strengthening phase is easily generated. It is presumed that even if a peak is present in advance, a high BH property can be obtained. For this reason, the endothermic peak can be increased under the relatively high temperature baking finish curing conditions targeted by the present invention, and the work curability can be enhanced by the presence of clusters corresponding to the endothermic peak. Therefore, if there is also an exothermic peak in the temperature range of 230 ° C. or higher and lower than 330 ° C. that enhances BH properties, both high work curability (formability) and BH properties can be achieved.
This relatively high temperature baking coating curing treatment condition means, for example, that the baking coating curing treatment is performed under conditions of a heating temperature of 180 to 230 ° C. and a heating holding time of 10 to 30 minutes. The heating temperature of the baking coating curing process is distinguished from at most 175 ° C. in particular at the heating temperature.

  When this exothermic peak and endothermic peak are present together, the balance between the peak heights as well as the respective peak heights is important. For example, if the ratio of the exothermic peak / endothermic peak is too small, the presence of clusters corresponding to the endothermic peak is too large and the BH property is too low. On the other hand, if the exothermic peak / endothermic peak is too large, the presence of clusters corresponding to the endothermic peak is too small, and the work curability is poor.

Based on such knowledge, in the present invention, in order to achieve both high elongation and relatively high BH properties, the DSC before the artificial aging treatment of the aluminum alloy sheet is within a temperature range of 150 to 230 ° C. An endothermic peak having a height A of 3 to 10 μW / mg is present, an exothermic peak having a height B of 20 to 50 μW / mg is present in a temperature range of 230 ° C. or more and less than 330 ° C., and The ratio B / A between the exothermic peak height B and the endothermic peak height A is more than 3.5 and less than 15.0.
Moreover, in order to make it compatible with high elongation not only in the case of the comparatively high temperature but also in the comparatively low BH property, in the DSC before the artificial aging treatment of the aluminum alloy plate, a temperature range of 150 to 230 ° C. An endothermic peak with a height A of preferably 3 to 8 μW / mg is present in the inside, and an exotherm with a height B of preferably 20 to 40 μW / mg within a temperature range of 230 ° C. or higher and lower than 330 ° C. A peak is present, and the ratio B / A between the exothermic peak height B and the endothermic peak height A is more than 3.5 and less than 15.0. More preferably, the endothermic peak height A is 3 to 7 μW / mg, and the exothermic peak height B is 20 to 35 μW / mg.

  Regarding the endothermic peak, the fact that the endothermic peak is high on the minus side means that the clusters are dissolved during the differential thermal analysis, in other words, there are many clusters corresponding to the endothermic peaks. If the negative peak height A is as low as less than 3 μW / mg, the work curability is lowered and the moldability is lowered. On the other hand, if the negative peak height A is too high exceeding 10 μW / mg, the relatively high-temperature BH property decreases, and if it exceeds 7 μW / mg, the relatively low-temperature BH is decreased. The nature will decline.

  With respect to the exothermic peak, a high exothermic peak means that many clusters that are the core of the reinforcing phase or the reinforcing phase are generated during the differential thermal analysis, in other words, the strengthening phase or the strengthening phase. This means that there are few core clusters. If the positive peak height B exceeds 50 μW / mg, the reinforcing phase or the core of the reinforcing phase is too few, and the BH property of baking coating curing at a relatively high temperature becomes low, resulting in 40 μW. If it exceeds / mg and is too high, the BH property of baking coating curing at a relatively low temperature will be low. On the other hand, if the positive peak height B is too low, less than 20 μW / mg, the reinforcing phase or the core of the reinforcing phase is too many, and the elongation is lowered. Incidentally, these tendencies are based on the premise that the alloy composition of the plate satisfies the scope of the present invention.

  As described above, the structure defined by DSC at the stage of the material plate is very much affected by the generation behavior of the precipitated phase during the artificial aging treatment (at the time of BH) of the member such as the automobile panel manufactured from the material plate. There is a good correlation. As a result, it is possible to evaluate the formability and BH property of the material plate by controlling the DSC at the stage of the material plate without bothering manufacturing the member. In other words, the structure defined by the DSC at the stage of the material plate can be an index of formability and BH property in a member using the material plate as a molding material.

  For example, in Patent Document 1, there is no endothermic peak having a height of 3 to 10 μW / mg in the temperature range of 150 to 230 ° C., and conversely, an exothermic peak is present in the temperature range of 100 to 200 ° C. In the said patent document 2, there is no endothermic peak whose height is 3-10 microwatts / mg in the temperature range of 150-230 degreeC as the FIG. In Patent Document 3, the total amount of Mg and Si is 1.2% or less, the endothermic peak and the exothermic peak defined in the present invention cannot be generated, the artificial age hardening ability is insufficient, and the necessary strength is obtained. It becomes impossible. In Patent Document 4, as shown in FIG. 1, there is no endothermic peak having a height of 3 to 10 μW / mg within a temperature range of 150 to 230 ° C., and a temperature range of 260 ° C. or more and 330 ° C. or less. There is an exothermic peak whose height exceeds 20 μW / mg. In Patent Document 5, as shown in FIG. 1, there is an endothermic peak having a height of 3 to 10 μW / mg within a temperature range of 150 to 230 ° C., and an exothermic peak is present at a position of 260 ° C. The ratio of exothermic peak / endothermic peak is too small, less than 3.5, and the ratio of endothermic peak is too high.

  Therefore, the conventional DSC endothermic peak and exothermic peak control technique is different from the DSC endothermic peak and exothermic peak defined in the present invention. The high elongation (formability) and bake coating curability of the 6000 series aluminum alloy plate after long-term aging at room temperature cannot be achieved.

(Organization control method prescribed by DSC)
Control of the structure specified by the DSC exothermic peak is, as described later, the aluminum alloy cold-rolled sheet within 5 hours at a temperature range of 30 ° C. to 60 ° C. within 1 hour after solution treatment and quenching treatment. Preliminary aging treatment at a low temperature for a long time is performed.
In order to increase the BH property at a comparatively low temperature, as will be described later, within 1 hour after the solution treatment and quenching treatment, the temperature is maintained at a temperature range of 100 ° C. to 300 ° C. for 5 seconds or more and 300 seconds or less. After the preliminary aging treatment at a high temperature for a short time, the preliminary aging treatment is performed. For this reason, there is an advantage that it can be controlled without greatly changing the composition of the 6000 series aluminum alloy already standardized as the automobile member of the rolled plate and without greatly changing the rolling process by a conventional method.

(Production method)
The 6000 series aluminum alloy sheet of the present invention is a cold-rolled sheet obtained by subjecting an ingot to hot rolling after soaking and then cold rolling, and further subjected to tempering such as solution treatment. Manufactured. That is, an aluminum alloy hot rolled sheet having a thickness of about 2 to 10 mm is manufactured through normal manufacturing processes such as casting, homogenization heat treatment, and hot rolling. Subsequently, it is cold-rolled to obtain a cold-rolled sheet having a thickness of 3 mm or less.

(Melting, casting)
First, in the melting and casting process, an ordinary molten casting method such as a continuous casting method and a semi-continuous casting method (DC casting method) is appropriately selected for the molten aluminum alloy adjusted to be dissolved within the above-mentioned 6000 series component composition range. Cast. Here, in order to control the cluster within the specified range of the present invention, the average cooling rate at the time of casting is as large as possible (fast) from the liquidus temperature to the solidus temperature of 30 ° C./min. Is preferred. When such temperature (cooling rate) control in the high temperature region during casting is not performed, the cooling rate in this high temperature region is inevitably slow. As a result, the amount of crystallized material generated roughly in the temperature range in this high temperature region increases, and the size and amount of crystallized material vary in the plate width direction and thickness direction of the ingot. As a result, there is a high possibility that the DSC cannot be controlled within the scope of the present invention.

  Next, the cast aluminum alloy ingot is subjected to a homogenization heat treatment prior to hot rolling. This homogenization heat treatment (uniform heat treatment) is important for sufficiently dissolving Si and Mg in addition to the normal purpose of homogenizing the structure (eliminating segregation in crystal grains in the ingot structure). It is.

  The homogenization heat treatment temperature is 500 ° C. or more and 580 ° C. or less, and the homogenization (holding) time is appropriately selected from a range of 1 hour or more to sufficiently dissolve Si and Mg. If the homogenization temperature is low, the solid solution amount of Si and Mg cannot be secured, and the DSC exothermic peak can be defined by the pre-aging treatment (reheating treatment) after the solution treatment and quenching treatment described later. Disappear. Further, segregation in the crystal grains cannot be sufficiently eliminated, and this acts as a starting point of fracture, so that formability is lowered.

  After this homogenization heat treatment, hot rolling is performed, but until the start of the hot rough rolling after the homogenization heat treatment, the temperature of the ingot is not lowered to 450 ° C. or lower without lowering the temperature of the ingot. It is preferable to ensure the amount of solution. When the temperature of the ingot is lowered to 450 ° C. or less by the start of rough rolling, Si and Mg are likely to precipitate, and there is a high possibility that the solid solution amount of Si and Mg cannot be secured for the purpose of the DSC. Become.

(Hot rolling)
Hot rolling is composed of a rough rolling process of an ingot (slab) and a finish rolling process according to the thickness of the sheet to be rolled. In these rough rolling process and finish rolling process, a reverse or tandem rolling mill is appropriately used.

  During rolling from the start to the end of hot rough rolling, it is necessary to ensure the solid solution amount of Si and Mg without lowering the temperature to 400 ° C. or lower. During hot rough rolling, when the temperature of the rough rolled plate falls below 400 ° C., Si and Mg are precipitated, and it is not possible to secure a solid solution amount of Si or Mg for prescribing the DSC. Increases nature.

  After such hot rough rolling, hot finish rolling with an end temperature in the range of 250 to 360 ° C. is performed. If the soaking temperature or the finishing temperature of this finish rolling is too low, Mg and Si compounds are generated during soaking and hot rolling, and the balance of solid solution Mg / Si changes, It becomes difficult to make the DSC regulation.

(Hot rolled sheet annealing)
Annealing of the hot-rolled sheet before cold rolling is not necessary, but may be performed.

(Cold rolling)
In the cold rolling, the hot-rolled sheet is rolled to produce a cold-rolled sheet (including a coil) having a desired final thickness. However, in order to further refine the crystal grains, the cold rolling rate is desirably 60% or more, and intermediate annealing may be performed between cold rolling passes for the same purpose as the annealing.

(Solution and quenching)
After the cold rolling, solution treatment and subsequent quenching to room temperature are performed. In order to obtain a sufficient amount of solid solution of each element such as Mg and Si, it is desirable to heat to a solution treatment temperature not lower than 500 ° C. and not higher than a melting temperature.

Further, from the viewpoint of suppressing the formation of coarse grain boundary compounds that lower the moldability, the average cooling rate from the solution temperature to the quenching stop temperature at room temperature is desirably 20 ° C./s or more. If the average cooling rate of the quenching treatment to room temperature after the solution treatment is small, coarse Mg ₂ Si and simple substance Si are generated during cooling, and bending workability is deteriorated. Moreover, the amount of solid solution after solution forming falls, and BH property will fall. In order to ensure this cooling rate, the quenching treatment is performed by selecting water cooling means and conditions such as air cooling such as a fan, mist, spray, and immersion, respectively.

(Preliminary aging treatment: reheating treatment)
After such a solution treatment, it is preferable to quench the steel sheet and cool it to room temperature, and then subject the cold-rolled sheet to a pre-aging treatment (reheating treatment) within one hour. This preliminary aging treatment is held at a temperature range of 30 ° C. to 60 ° C. for 5 hours or more and 500 hours or less as a lower temperature and longer time than the conventional method in order to control the structure specified by the DSC peak. It is preferable to perform a low-temperature long-time preliminary aging treatment. As a result, Mg—Si clusters having a good balance between Mg and Si are formed, and the structure specified by the DSC peak is obtained. Therefore, the high aging and the BH property at a relatively high temperature can be combined by the low-temperature long-time preliminary aging treatment.

  If the room temperature holding time from the completion of the quenching treatment to room temperature until the start of the pre-aging treatment (heating start) is too long, too many clusters corresponding to endothermic peaks are generated due to room temperature aging, and the BH property tends to be low. . Accordingly, the shorter the room temperature holding time is better, the solution treatment and quenching treatment and the reheating treatment may be continued so that there is almost no time difference, and the lower limit time is not particularly set.

  Further, if the preliminary aging temperature is less than 30 ° C. or the holding time is less than 5 hours, as in the case where this preliminary aging treatment is not performed, too many clusters corresponding to endothermic peaks are generated, BH property tends to be low. On the other hand, if the pre-aging condition exceeds 60 ° C. or exceeds 500 hours, the amount of clusters that form the core of the strengthening phase and the strengthening phase corresponding to the exothermic peak is too much, and the press before baking coating. The strength at the time of molding becomes too high, and the moldability tends to deteriorate.

  Here, in order to further increase the BH property at a relatively low temperature, first, within 1 hour after the solution treatment and quenching treatment, first, the temperature is maintained at a temperature range of 100 ° C. to 300 ° C. for 5 seconds or more and 300 seconds or less. It is preferable to perform the preliminary aging treatment for a long time at a low temperature and immediately after the preliminary aging treatment for a short time at a high temperature. Thereby, control of DSC by the low-temperature long-time preliminary aging treatment can be ensured, and the endothermic peak height A in the temperature range of 150 to 230 ° C. in the DSC is more preferably in the range of 3 to 8 μW / mg. It can be controlled within a preferable range of 3 to 7 μW / mg. Similarly, the height B of the exothermic peak in the temperature range of 230 ° C. or higher and lower than 330 ° C. in DSC can be controlled to be a preferable range of 20 to 40 μW / mg, and a more preferable range of 20 to 35 μW / mg. If this high temperature and short time pre-aging treatment is not performed or if the above conditions are not met, the structure specified by the DSC peak may not be obtained or the structure may be relatively low. There is a possibility that the BH property of the resin becomes low.

  As described above, the aluminum alloy plate of the present invention manufactured with the structure specified by the DSC peak is press-molded as a material on a large body panel or the like of an automobile and then baked after being painted. The coating is hardened (artificial aging treatment) to increase the strength. As described above, this baking coating curing treatment is preferably performed at a high temperature in order to achieve the effects of the present invention, and examples include conditions of a heating temperature of 180 to 230 ° C. and a heating and holding time of 10 to 30 minutes. If the heating temperature is too low, for example, if the heating temperature is too low, the structure indicated by the differential heat curve needs to be more precisely controlled as described above.

  Next, examples of the present invention will be described. 6000 series aluminum alloy plates having different structures defined by the DSC of the present invention were produced by making different compositions and production conditions. And after Assembling the board, after holding at room temperature for 100 days, As proof strength (proof strength before baking coating hardening treatment), AB proof strength (proof strength after baking coating hardening treatment), elongation at break, BH property (paint bake hardening property), respectively Measurement and evaluation. These results are shown in Tables 1 and 2.

  Specifically, the 6000 series aluminum alloy plate having the composition shown in Table 1 was subjected to various pre-aging treatment conditions after solution treatment and quenching treatment, as shown in Table 2. Here, in the display of the content of each element in Table 1, the display in which the numerical value of each element is blank indicates that the content is below the detection limit.

(Production conditions for aluminum alloy sheets)
The specific production conditions of the aluminum alloy plate were the same (same) as follows in each example except for the preliminary aging treatment conditions. Aluminum alloy ingots having respective compositions shown in Table 1 were commonly melted by DC casting. At this time, in common with each example, the average cooling rate during casting was set to 50 ° C./min from the liquidus temperature to the solidus temperature. Subsequently, if necessary, the ingot after chamfering is subjected to soaking treatment at 550 ° C. for 10 hours, and then hot rough rolling is started at that temperature, and then the end temperature is between 250 to 360 ° C. A hot finish rolling was performed to obtain a hot rolled sheet. This hot-rolled sheet was cold-rolled at a processing rate of 67% to obtain a cold-rolled sheet having a thickness of 1.0 mm.

  Furthermore, each cold-rolled sheet was subjected to a solution treatment for 1 minute at 550 ° C. using a glass stone furnace, and then cooled to room temperature by water cooling. Within 1 hour after this cooling, high temperature short time pre-aging using an oil bath and low temperature long time pre-aging using an atmospheric furnace are performed at the temperature (° C) and holding time (hr) shown in Table 2. After the preliminary aging treatment, air cooling was performed.

  After these tempering treatments, from each final product plate after standing at room temperature for 100 days, a test plate (blank) 300 mm × 300 mm was cut out from the longitudinal end portion and the width center portion of the product, and the DSC of each test plate was cut. And properties were measured and evaluated. These results are shown in Table 2.

(DSC measurement)
The DSC of the structure at the three locations in the central part of the thickness of the test plate was measured, and the average value of these three locations was the endothermic peak temperature (° C) in the DSC (differential scanning calorimetry curve) of this plate. And height (μW / mg), exothermic peak temperature (° C.) and height (μW / mg) were measured. In Table 2 showing the results, for the sake of convenience, the endothermic peak in the temperature range of 150 to 230 ° C. is simply referred to as “endothermic peak”, and the exothermic peak in the temperature range of 230 ° C. or higher and lower than 330 ° C. is referred to as “exothermic peak”. "

  In differential thermal analysis at each measurement location of these test plates, test equipment: TG / DTA6300 manufactured by Seiko Instruments Inc., standard material: aluminum, sample container: aluminum, temperature rising condition: 10 ° C./min, atmosphere: argon (50 ml / min) and sample weight: 39.0 to 41.0 mg, respectively. The obtained differential thermal analysis profile (μW) was divided by the sample weight (μW / mg), and then the differential heat In the section of 0 to 100 ° C. in the analysis profile, the region where the profile of differential thermal analysis becomes horizontal was defined as a reference level of 0, and the exothermic peak height from this reference level was measured.

Paint bake hardenability As mechanical properties of the test plate, 0.2% yield strength (As yield strength) and elongation at break (%) were determined by a tensile test. In addition, each of these test plates is commonly used, and after 2% stretching simulating press molding on the automobile member, the coating baking and curing treatment at high temperature is 185 ° C. × 20 minutes, and the coating is performed at low temperature. The 0.2% yield strength (AB yield) of the test plate after each artificial age hardening treatment (after BH) of 170 ° C. × 20 minutes as the bake hardening treatment was determined by a tensile test. And the BH property of each test plate was evaluated from the difference (increased yield strength) between these 0.2% proof stresses. The 0.2% proof stress after BH is 190 MPa or more in high temperature paint baking and curing (185 ° C. × 20 minutes), and at least 160 MPa or higher in low temperature paint baking and hardening (170 ° C. × 20 minutes), preferably The pass was evaluated as 180 MPa or more, and the elongation at break was 25% or more as an evaluation of press formability. Note that the elongation at break, which is the evaluation of press formability, is only 1% difference between 24% and 25%. For example, corners and character lines with sharpened or complicated outer panel shapes of automobiles are distorted. It has a great influence on whether or not it can be molded with a beautiful and sharp curved surface structure without wrinkles.

  In the tensile test, No. 13A test piece (20 mm × 80 mmGL × plate thickness) of JISZ2201 was sampled from each of the test plates, and a tensile test was performed at room temperature. The tensile direction of the test piece at this time was the 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 N number for the measurement of mechanical properties was 5, and each was calculated as an average value. The test piece for measuring the yield strength after BH was subjected to the BH treatment after giving a pre-strain of 2% to the test piece by the tensile tester.

As shown in Tables 1 and 2, Invention Examples 1 to 8 are produced within the component composition range of the present invention and in a preferable condition range, and are subjected to a pre-aging treatment at a low temperature and a long time in a preferable range. Therefore, as shown in Table 2, these invention examples are as defined by DSC in the present invention. Even after long-term room temperature aging, as shown in Table 2, the moldability and BH properties are excellent. ing.
Specifically, it has a high elongation at break of at least 26%, a BH property of at least 192 MPa or higher (185 ° C. × 20 minutes), and a BH property of at least 162 MPa or more (170 ° C. × 20 minutes). have.
Furthermore, in the comparison between the inventive examples in Table 2, after the preliminary aging treatment at a high temperature and a short time was performed, the inventive example 2 which was immediately subjected to the low temperature and the long time preliminary aging treatment was subjected to the preliminary aging treatment at a high temperature and a short time. The BH property at a relatively low temperature is higher than that of Invention Example 1 in which only the preliminary aging treatment at a low temperature and a long time was performed without applying. Similarly, the invention examples 6, 7, and 8 that were subjected to the pre-aging treatment for a short time at a high temperature and immediately subjected to the pre-aging treatment for a long time at a low temperature were not subjected to the pre-aging treatment for a short time at a high temperature. Compared to Inventive Examples 3, 4 and 5 where only the preliminary aging treatment was performed, the BH property at a relatively low temperature is high on average although it depends on the difference in the alloy composition.
This is because the invention examples 2, 6, 7, and 8 further added a pre-aging treatment at a high temperature and a short time, and thus the endothermic peak height A in a temperature range of 150 to 230 ° C. in DSC was changed to a preferable range (3 ~ 8 μW / mg) or more preferable range (3 to 7 μW / mg), also the exothermic peak height B in the temperature range of 230 ° C. or higher and lower than 330 ° C. in DSC is preferably in the preferable range (20 to 40 μW / mg). In addition, it is because it can be controlled more precisely within a more preferable range (20 to 35 μW / mg).

  On the other hand, Comparative Examples 1 to 6 in Table 2 use the same alloy example 1 as the invention example. However, in each of these comparative examples, as shown in Table 2, the production conditions such as the temperature of the preliminary aging treatment and the holding time are out of the preferable conditions. As a result, DSC deviates from the range defined in the present invention, and either BH property or formability after long-term aging at room temperature is inferior to that of Invention Example 1 having the same alloy composition, and can be combined. Not. Specifically, even when the elongation at break is 26% or more, the BH property at a high temperature (185 ° C. × 20 minutes) is less than 190 MPa, or the BH property at a high temperature (185 ° C. × 20 minutes) is 190 MPa or more. However, the elongation at break is less than 25%, which does not satisfy the above acceptance criteria.

Of these, Comparative Example 1 is not subjected to preliminary aging treatment. For this reason, although an endothermic peak exists in the temperature range of 150-230 degreeC, the height A is too high exceeding 10 microwatt / mg, and exothermic peak height B in the temperature range of 230 degreeC or more and less than 330 degreeC Is too high, exceeding 50 μW / mg.
In Comparative Example 2, the time for the preliminary aging treatment on the low temperature side is too short. For this reason, although an endothermic peak exists in the temperature range of 150-230 degreeC, the height A is too high exceeding 10 microwatt / mg, and exothermic peak height B in the temperature range of 230 degreeC or more and less than 330 degreeC Is too high, exceeding 50 μW / mg.
In Comparative Example 3, the temperature of the preliminary aging treatment on the low temperature side is too high. Therefore, although an endothermic peak having a height A of 3 to 10 μW / mg exists in a temperature range of 150 to 230 ° C., an exothermic peak height B in a temperature range of 230 ° C. or more and less than 330 ° C. is 20 μW / mg. It is too low with less than mg.
In Comparative Example 4, the time for the preliminary aging treatment on the low temperature side is too long. For this reason, although an endothermic peak exists in the temperature range of 150-230 degreeC, the height A is too low as less than 3 microwatt / mg, and the exothermic peak height B in the temperature range of 230 degreeC or more and less than 330 degreeC is also It is too low, less than 20 μW / mg. In Comparative Example 5, the time for the pre-aging treatment on the high temperature side is too long. For this reason, although an endothermic peak exists in the temperature range of 150-230 degreeC, the height A is too low as less than 3 microwatt / mg, and the exothermic peak height B in the temperature range of 230 degreeC or more and less than 330 degreeC is also It is too low, less than 20 μW / mg.
In Comparative Example 6, the temperature of the pre-aging treatment on the low temperature side is too high. For this reason, although there is an endothermic peak in the temperature range of 150 to 230 ° C., the height A is too low as less than 3 μW / mg, and the exothermic peak height B in the temperature range of 230 ° C. or higher and lower than 330 ° C. The ratio B / A exceeds 15.0 and is too large.

  Although Comparative Examples 7 and 8 in Table 2 are manufactured in a preferable range including the pre-aging conditions, Alloy Nos. 7 and 8 in Table 1 are used, respectively, and the alloy compositions are out of the scope of the present invention. ing. For this reason, as shown in Table 2, these comparative examples, as a result, DSC and the like are out of the range defined in the present invention. Either is inferior and cannot be combined. Specifically, even when the elongation at break is 25% or more, the BH property at a high temperature (185 ° C. × 20 minutes) is about 138 to 146 MPa, and the BH property at a low temperature (170 ° C. × 20 minutes) is 133--3. Only about 139 MPa.

The comparative example 7 is the alloy 7 of Table 1, and there is too little Mg and the quantity of the total content of Mg and Si is also too small. For this reason, although an endothermic peak exists in the temperature range of 150-230 degreeC, the height A is too low as less than 3 microwatt / mg, and the exothermic peak height B in the temperature range of 230 degreeC or more and less than 330 degreeC is also It is too low, less than 20 μW / mg.
The comparative example 8 is the alloy 8 of Table 1, Si is too little, and the total content of Mg and Si is too little. For this reason, although an endothermic peak exists in the temperature range of 150-230 degreeC, the height A is too low as less than 3 microwatt / mg, and the exothermic peak height B in the temperature range of 230 degreeC or more and less than 330 degreeC is also It is too low, less than 20 μW / mg.

  A DSC selected from these inventive examples and comparative examples is shown in FIG. In FIG. 1, the unit of the vertical axis indicated as “HeatFlow” is μW / m, the thick solid line indicates Invention Example 1 in Table 2, the thick dotted line (dashed line) indicates Invention Example 2, and the thin dotted line indicates Comparative Example 3. As shown in FIG. 1, in these inventive examples, an endothermic peak having a height A of 3 to 10 μW / mg exists in a temperature range of 150 to 230 ° C., and an exothermic peak having a height B of 20 to 50 μW / mg. Is present in a temperature range of 230 ° C. or more and less than 260 ° C., and in the temperature range of 260 ° C. or more and less than 330 ° C., no exothermic peak having a height of 20 μW / mg or more is present.

  From the results of the above examples, the composition and DSC specified in the present invention are required to combine good moldability and high BH properties even after baking at room temperature for a long time or at a high temperature by baking. The critical significance of each of these conditions is supported.

According to the present invention, it is possible to provide a 6000 series aluminum alloy plate that has both good formability and high BH properties even after baking at room temperature for a long time or at a high temperature by baking. That is, even after long-time aging at room temperature, it is possible to have both good moldability and high BH properties in the conventional low-temperature baking coating curing treatment as well as the conventional high-temperature baking coating curing treatment. As a result, the application of a 6000 series aluminum alloy plate can be expanded as an automobile member including a panel material.

Claims (7)

  In mass%, Mg: 0.3-1.5% and Si: 0.6-1.5%, respectively, and the total of the Mg content and Si content is over 1.2% The balance is an aluminum alloy plate made of Al and inevitable impurities, and in the differential scanning calorimetry curve of this plate, an endothermic peak having a height A of 3 to 10 μW / mg within a temperature range of 150 to 230 ° C. And an exothermic peak having a height B of 20 to 50 μW / mg exists within a temperature range of 230 ° C. or higher and lower than 330 ° C., and the ratio between the exothermic peak height B and the endothermic peak height A B / A is more than 3.5 and less than 15.0, an aluminum alloy plate excellent in formability and bake hardenability.   The aluminum alloy plate having excellent formability and bake coating curability according to claim 1, wherein the endothermic peak height A is 3 to 8 µW / mg, and the exothermic peak height B is 20 to 40 µW / mg. .   Further, the aluminum alloy plate is, in mass%, Cu: 0.02-0.8%, Fe: 0.05-0.5%, Mn: 0.05-0.3%, Zr: 0.04. -0.1%, Cr: 0.04-0.3%, V: 0.02-0.1%, Ag: 0.01-0.1%, Zn: 0.01-0.3% The aluminum alloy plate excellent in formability and baking coating curability according to claim 1 or 2, containing one or more of them. In mass%, Mg: 0.3-1.5% and Si: 0.6-1.5%, respectively, and the total of the Mg content and Si content is over 1.2% , The remainder of the aluminum alloy ingot consisting of Al and inevitable impurities is hot-rolled at a start temperature of over 400 ° C., the hot-rolled sheet after hot rolling is cold-rolled, and the aluminum alloy cold- rolled after cold rolling By subjecting the plate to a low-temperature long-time preliminary aging treatment in which the plate is kept within a temperature range of 30 ° C. to 60 ° C. for 5 hours or more and 500 hours or less within 1 hour after solution treatment and quenching treatment, In the differential scanning calorimetry curve before processing, an endothermic peak having a height A of 3 to 10 μW / mg is present in a temperature range of 150 to 230 ° C., and is within a temperature range of 230 ° C. or more and less than 330 ° C. Exotherm whose height B is 20-50 μW / mg Formability and bake coating curability characterized by having a peak and having a ratio B / A between the exothermic peak height B and the endothermic peak height A of more than 3.5 and less than 15.0 A method for producing an excellent aluminum alloy sheet.   Further, the aluminum alloy plate is, in mass%, Cu: 0.02-0.8%, Fe: 0.05-0.5%, Mn: 0.05-0.3%, Zr: 0.04. -0.1%, Cr: 0.04-0.3%, V: 0.02-0.1%, Ag: 0.01-0.1%, Zn: 0.01-0.3% The manufacturing method of the aluminum alloy plate excellent in the moldability and baking coating curability of Claim 4 containing 1 type or 2 types or more of them.   Within 1 hour after the solution treatment and quenching treatment, after performing a high-temperature short-time pre-aging treatment for holding at a temperature range of 100 ° C. to 300 ° C. for 5 seconds or more and 300 seconds or less, the low-temperature long-time pre-aging treatment is performed. The method for producing an aluminum alloy plate excellent in formability and bake hardenability according to claim 4 or 5, wherein the processing is performed. The aluminum alloy plate is coated after being formed, and is baked and hardened under conditions of a heating temperature of 180 to 230 ° C and a heating and holding time of 10 to 30 minutes. A method for producing an aluminum alloy sheet excellent in formability and bake coating curability.

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