JP2016084513A - Aluminum alloy having suppressed room temperature aging property and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress increase of yield strength by holding room temperature in an Al-Mg-Si alloy.SOLUTION: There is provided a manufacturing method of an alloy containing Mg:0.3 to 2.0% and Si:0.3 to 2.0%, further one of Cu:0.2 to 1.0% and Fe:0.05 to 0.3% and having yield stress value just after manufacturing of 150 MPa or less and a value of yield stress value when holding at 30°C for 20 days just after manufacturing minus yield stress value when holding for 10 days of 4 MPa or less and a manufacturing method by conducting a solution heat treatment at 500°C to the melting point with holding time of 1 minute to 1 hours, a heat treatment at 70 to 180°C with the holding time of 2 to 24 hours and then adding strain between plane strain and tensile strain of 0.2 to 2% by a tension leveler.SELECTED DRAWING: None

Description

The present invention relates to an Al-Mg-Si alloy that is applied to automobiles, and an aluminum alloy that suppresses an increase in strength due to aging from after production until press molding at the automobile production site, and a method for producing the same.
It is an Al—Mg—Si alloy in which the increase in strength (room temperature aging) after long-term room temperature retention is suppressed, and is particularly used for automotive materials.

  In order to reduce the weight of automobile bodies, aluminum alloys tend to be used for vehicle bodies. The alloys used are mainly Al—Mg—Si alloys. This alloy is expected to increase the yield stress after the paint baking process in the automobile production line. This is the main reason why Al-Mg-Si alloys are used for automotive materials.

  In this alloy, fine clusters (hereinafter referred to as clusters) in which additive elements Mg and Si are aggregated are formed by heat treatment at the time of manufacturing the alloy. A cluster is a form in which Mg atoms and Si atoms form a group without changing the crystal structure of aluminum, which is the parent phase. This cluster changes into a precipitate that does not significantly reduce the formability in press molding and contributes to an increase in yield stress in the paint baking process.

However, in an alloy in which clusters are formed, the yield stress increases due to the retention (room temperature aging) near the room temperature during manufacture and press forming. This is because the clusters grow at room temperature. Such an increase in yield stress before press forming is not desirable for press forming itself, and thus a restriction arises in that press forming must be performed during a predetermined period from manufacture.
Such limitation of the period from manufacture to press molding is not desirable from the viewpoint of the degree of production freedom in automobile manufacture. Furthermore, when exporting to a long distance such as overseas, it is necessary to suppress the increase in yield stress as much as possible while maintaining the room temperature for a long time. From this point of view, it is not desirable to increase the yield stress during holding at room temperature.

There have been reports of room temperature aging suppression, which means suppression of increase in yield stress during room temperature holding using various methods. In Patent Document 1 and Patent Document 2, adjustment of the additive element is performed, in Patent Document 3 and Patent Document 4 the preheat treatment temperature is lowered after the solution heat treatment, and in Patent Document 5 and Patent Document 6, the preheat treatment condition is adjusted. Patent Document 7 proposes the adjustment of the cluster number density to be formed, and Patent Document 8 and Patent Document 9 each propose a room temperature aging suppression method using texture formation.
On the other hand, Patent Document 10 proposes a technique for suppressing room temperature aging by introducing a strain of 0.5 to 5% to a material surface by a leveler after solution treatment and quenching treatment.

Japanese Patent Laid-Open No. 11-172390 JP 2008-174797 A JP 2000-273567 A JP 2003-321723 A JP-A-9-143644 JP 2013-167004 A JP 2009-242904 A JP 2004-10982 A JP 2003-321754 A JP 2003-247040 A

In an aluminum alloy containing Mg and Si, it is necessary to suppress the growth of clusters at the time of holding at room temperature as much as possible in order to suppress the increase in yield stress at the time of holding at room temperature. On the other hand, the cluster needs to have a function of changing to a precipitate by coating baking heat treatment and contributing to an increase in strength. That is, it is necessary to simultaneously maintain both the growth suppression of clusters and the function of changing to precipitates during paint baking heat treatment.
However, none of the effects of room temperature aging suppression by the prior art was satisfactory.

  Therefore, an object of the present invention is to delay the cluster growth in the aluminum alloy and lower the room temperature aging more than that of the conventional alloy, that is, to suppress the increase in the yield stress while maintaining the room temperature.

  As described above, the increase in yield stress due to room temperature aging is caused by cluster growth. Industrially, the increase in yield stress after 180 days is preferably 45 MPa or less. In order to produce such an aluminum alloy, it is first necessary to elucidate the increasing behavior of the yield stress when kept at room temperature.

As a result of investigating the increase behavior of yield stress at the time of holding the room temperature up to 180 days of various alloys, the present inventors have found that the increase rate of the yield stress changes after 10 days from the production. Based on this knowledge, the present inventors adjusted the yield stress after 180 days from production to 45 MPa or less by making the difference obtained by subtracting the yield stress value after 10 days from the yield stress value after 20 days from production 4 MPa or less. Succeeded in doing.
In addition, the yield stress value immediately after manufacture needs to be 110 MPa or less. When the yield stress immediately after production exceeds 110 MPa, the absolute value of the yield stress after 180 days increases to a level at which it is impossible to suppress the decrease in formability even if the increase in strength is suppressed.

  As a method of reducing the difference obtained by subtracting the yield stress value after 10 days from the yield stress value after 20 days from the production to 4 MPa or less, the present inventors have partially destroyed the clusters formed in the preliminary heat treatment stage, thereby increasing the room temperature. A method to suppress cluster growth under aging was devised.

If a part of the cluster is destroyed by strain due to external stress on the alloy, the broken cluster is first repaired before the growth of the cluster during the subsequent holding at room temperature. This repair takes time. For this reason, it was expected that the amount of increase in yield stress was suppressed as a result of the time required for the growth of the cluster to a predetermined size was longer in the partially destroyed cluster than in the non-destructed cluster.
In addition, even if a part of the cluster is destroyed, if the cluster is repaired at room temperature, it has the effect of expressing the specified yield stress at the time of paint baking as before the failure. It was thought that there would be no problem in securing.

  The present inventors devised the introduction of dislocations as a method for destroying this cluster. The cluster is cut by the introduced dislocation and partially destroyed. However, it is necessary to introduce dislocations almost uniformly into the material. For this purpose, it is necessary to impart a strain reaching the plastic region to the entire plate thickness.

The stress applied to impart strain may be either compressive stress or tensile stress. However, when compressive stress is applied to the plate material to which the present invention is applied, it is industrially necessary to deform the coil in the longitudinal direction or the plate width direction. Is difficult. In addition, rolling can be considered as an application of compressive stress in the thickness direction. However, in order to give sufficient strain to the center of the thickness, an appropriate rolling reduction is required, which lowers the ductility of the material.
For this reason, an extension strain was devised by applying a tensile stress in the coil longitudinal direction. Generally, even with a simple leveler or skin pass, elongation strain is imparted in the material. After holding for 180 days at 30 ° C., 0.2% tensile strain is applied and heat treatment is performed at 170 ° C. for 20 minutes in this order. Although the yield stress value is 170 MPa or more, when the tension in the coil longitudinal direction is not sufficient, the applied strain is limited to the surface layer, and only some of the clusters on the surface layer in the alloy plate are destroyed, and room temperature aging is achieved. The suppression effect is extremely low.

The present invention has been made on the basis of the above examination, and the gist thereof is as follows.
(1) In mass%, Mg: 0.3-2.0%, Si: 0.3-2.0% is contained, the yield stress value immediately after manufacture is 110 MPa or less, and 30 ° C. immediately after manufacture. The value obtained by subtracting the yield stress value when retained for 10 days from the yield stress value when retained at 20 days at 4 days is 4 MPa or less, and 0.2% tensile strain after being retained at 30 ° C. for 180 days immediately after production. An aluminum alloy that suppresses an increase in yield stress at room temperature retention, wherein the yield stress value after application and heat treatment at 170 ° C. for 20 minutes is 170 MPa or more.
Here, the yield stress value is a 0.2% proof stress value in a tensile test.
(2) By mass%, Cu: 0.2-1.0%, Fe: 0.05-0.3% is further contained, The room temperature holding | maintenance as described in (1) characterized by the above-mentioned An aluminum alloy that suppresses the increase in yield stress.
(3) The aluminum alloy processed into a plate shape is subjected to a solution heat treatment at 500 ° C. or higher and a melting point or lower at a holding time of 1 minute or more and 1 hour or less, and then 70 hours at a holding time of 2 hours or more and 24 hours or less. (1) or (2) above, wherein a strain between 0.3 to 2% plane strain to tensile strain is applied by a tension leveler after heat treatment at a temperature of from ℃ to 180 ℃. The manufacturing method of the aluminum alloy which suppressed the raise amount of the yield stress by holding at room temperature.
It is.

  According to the present invention, the rate of increase in yield stress due to holding at room temperature from production to press molding is reduced, and long distances are maintained while maintaining the current properties including strength after long-term alloy management and paint baking heat treatment. Alloy transportation (export) to is possible.

The aluminum alloy of the present invention that suppresses the cluster growth at room temperature aging by partially destroying the cluster formed in the preliminary heat treatment stage and the manufacturing method thereof in detail about individual requirements and preferable requirements constituting the present invention. explain.
First, the components of the aluminum alloy will be described. In addition,% of content of each component means the mass%.

(component)
Mg is a main element constituting the cluster, and its addition amount depends on the Si addition amount, but at least 0.3% or more is necessary for cluster formation. However, addition exceeding 2.0% forms a precipitate of Mg alone. This precipitate becomes a starting point of fracture and reduces moldability. Therefore, the upper limit was set to 2.0%.

Si is also a main element constituting the cluster, and its addition amount is determined for the same reason as Mg atoms. 0.3% or more is necessary for forming a cluster, and addition exceeding 2.0% causes Si precipitates to be the starting point of fracture to be formed as in the case of Mg, thereby reducing press formability.
Note that Mg / Si is desirably 1 or less. When the Si addition amount is larger than the Mg addition amount, the distribution density of the formed clusters increases, and both molding and strength are expected to be improved.

  The above are the basic components of the aluminum alloy of the present invention, and the balance is Al and inevitable impurities. Furthermore, it can contain the following components as needed.

Cu is an element effective for formability in a solid solution state. For that purpose, addition of 0.2% or more is necessary. However, a large amount of addition exceeding 1.0% forms a Cu-based precipitate. Therefore, the strength increases but the moldability decreases. Therefore, the addition amount is set to 0.2% to 1.0%.
When Fe exceeds 0.3%, which is an element mixed at the time of casting, Fe—Al-based precipitates are formed, and formability, particularly ductility, is significantly reduced. However, addition of 0.05% or more is effective for strengthening crystal grains. Therefore, the addition amount is set to 0.05 to 0.3%.

  For the purpose of increasing the strength, one or more of Mn, Cr and Ti may be added. However, a large amount of addition increases the yield stress and decreases the ductility before the formability is lowered. Under the constraint that the yield stress value immediately after production is 110 MPa or less, the upper limits of the addition amount of Mn, Cr, and Ti are 2.0%, 1.0%, and 0.2%, respectively.

Next, a manufacturing method will be described. The reason for limiting the change in yield stress over time will be explained in relation to the manufacturing method.
(Production method)
First, in the melting and casting process, a melt casting method such as a continuous casting rolling method or a semi-continuous casting method (DC casting method) is selectively performed on the molten alloy.

In the next homogenization heat treatment, we aim to homogenize the material according to a standard method. The main purpose of the homogenizing heat treatment is to eliminate segregation of additive elements. For this purpose, heat treatment at a temperature of 560 ° C. or higher and a melting point or lower is required.
Although the heat treatment time depends on the amount of added elements, it is sufficient if it is 20 minutes or longer and 8 hours or shorter within the above temperature range. If it is shorter than 20 minutes, it is difficult to sufficiently eliminate segregation. On the other hand, if it is 8 hours or more, the production cost increases.
Further, after being held within the above temperature range for the above heating time, it is necessary to cool at a cooling rate of 20 ° C./sec or more. When the cooling rate is less than 20 ° C./sec, the amount of solid solution Mg and the amount of solid solution Si are reduced by forming precipitates, and the strength and ductility of the product are lowered. Means for increasing the cooling rate include forced air cooling and water cooling, but the means is not particularly limited.

  Subsequent hot rolling also follows the regular method. First, it is necessary to set a starting temperature, and the temperature should be 450 ° C. or higher. At temperatures below 450 ° C., the frequency of recrystallization during hot rolling decreases sharply, which increases the possibility of non-recrystallization in the final product. Preferably, if the starting temperature is 560 ° C. or higher, the precipitate remaining in the homogenization heat treatment can be dissolved to increase the solid solution Mg amount and the solid solution Si amount. The final thickness is not particularly limited and is preferably 5 mm or less from the viewpoint of the ease of the subsequent cold rolling process.

  In order to obtain reliable recrystallization, the hot-rolled sheet may be annealed before cold rolling. In that case, a condition of 20 minutes or more at a temperature of 400 ° C. or more is sufficient, but long-term annealing exceeding 8 hours is a disadvantage of increasing the production cost. In consideration of the entire manufacturing cost, this hot-rolled sheet annealing may be omitted.

Subsequent cold rolling may be performed by a standard method to a desired plate thickness.
Similar to hot-rolled sheet annealing, in order to obtain reliable recrystallization, one or more heat treatments (intermediate annealing) may be performed during the cold rolling. This heat treatment is also carried out according to a conventional method. The temperature at this time may be 400 ° C. or higher as in the case of hot rolling annealing before cold rolling, and the holding time may be 20 minutes or longer and 8 hours or shorter without increasing the manufacturing cost.
Follow the regular method for cold rolling. When performing the total cold rolling reduction, particularly intermediate annealing, it is preferable that the cold rolling rate from the intermediate annealing to the final sheet thickness is large. By setting the total cold rolling rate to 75% or more, recrystallized grains at the time of final annealing are refined. When performing the intermediate annealing, it is desirable that the cold rolling rate from the intermediate annealing to the final sheet thickness is 75% or more.

After the cold rolling, solution heat treatment is performed.
The solution heat treatment is performed in order to bring the added Mg and Si into a solid solution state in which sufficient solid solution Mg amount and solid solution Si amount exist together with recrystallization of the rolling structure, and for that purpose, at least 500 ° C. It is necessary to heat more. If a solid solution state with a sufficient amount of solid solution Mg and solid solution Si cannot be obtained, a predetermined baking yield stress value cannot be obtained. The upper limit temperature is not higher than the melting point, but is preferably not higher than 580 ° C. in order to suppress deflection of the plate during cooling after solution treatment.

The holding time is required to be 1 minute or longer in the heating temperature range in order to completely dissolve both elements added. The upper limit is 1 hour. A time longer than this is not preferable because it increases the production cost and decreases the industrial value.
The cooling rate after the solution heat treatment is desirably high in order to avoid the formation of precipitates other than clusters before the subsequent pre-heat treatment and the formation of clusters formed at a low temperature, which is undesirable for the present application as described later. On the other hand, deflection that lowers the plate shape accuracy is likely to occur due to rapid cooling, resulting in a decrease in yield. Therefore, it is preferable to set the upper limit of the cooling rate to 50 ° C./sec, at which deflection is unlikely to occur. The lower limit is 1 ° C./sec. If it is less than that, an undesirable precipitate is formed in order to obtain strength during cooling.

  After the solution heat treatment, a preliminary heat treatment for forming the obtained solid solution Mg and solid solution Si in clusters is performed, but the preliminary heat treatment in the cluster formation temperature range needs to be started immediately after the solution heat treatment. In the case of keeping at room temperature after solution heat treatment, preliminary heat treatment is performed that lasts within 6 hours. If the room temperature holding time exceeds 6 hours, the solid solution Mg and solid solution Si secured by solution heat treatment are consumed to form another cluster (low temperature cluster) that does not contribute to securing the yield stress value during coating baking. Therefore, it becomes difficult to ensure the yield stress value at the time of desired coating baking.

  In the preliminary heat treatment performed after the solution heat treatment, high-temperature clusters are formed by solid solution Mg and solid solution Si. For that purpose, it is necessary to hold | maintain at 70 to 180 degreeC. Below 70 ° C., low temperature clusters are formed, which are another type of cluster that does not contribute to the yield stress value due to desired paint baking. On the other hand, when it exceeds 180 degreeC, a precipitate will be formed. The holding time in this temperature range is determined by the diffusion of solid solution atoms. Diffusion depends on the holding temperature. Accordingly, in the high temperature holding, a predetermined high temperature cluster is formed with a holding time shorter than that in the low temperature holding. In the case of holding at 180 ° C., if the holding is performed for at least 2 hours, a high temperature cluster is surely formed. On the other hand, 6 hours are required for holding at 70 ° C.

The increase in yield stress at room temperature retention after the preheat treatment is due to the growth of high temperature clusters formed by this preheat treatment. Therefore, if the growth rate of this high-temperature cluster is reduced, the rate of increase in yield stress during holding at room temperature is reduced.
As described above, when the growth rate of this high-temperature cluster is reduced and the yield stress value after 10 days is subtracted from the yield stress value after 10 days from the production at room temperature, the yield after 180 days is reduced. The stress value is 45 MPa or less.

For this purpose, it is effective to impart strain after the preliminary heat treatment. The strain of the present invention means a plastic strain because it is obtained from the shape difference before and after the load is applied to generate the strain.
The amount of strain to be applied is 0.3% or more. On the other hand, an increase in the amount of strain increases the strength, further reduces the ductility, and is undesirable for press formability. Therefore, it is necessary to set an upper limit of the strain amount. The amount of strain that does not impair the press formability is 2%. Therefore, the strain amount is 0.3% or more and 2% or less.

The mechanism for producing such an effect is considered as follows.
The strain introduced into the aluminum alloy material moves as dislocations in the material, deforms the aluminum alloy phase that is a parent phase, and breaks in a form that cuts high-temperature clusters present in the alloy phase. In the vicinity of the cut high-temperature cluster, due to the cutting and subsequent further dislocation action, Mg atoms and a part of the Si atoms constituting the high-temperature cluster become solute Mg and solute Si. The solid solution Mg atom and the solid solution Si atom are unevenly distributed around the cut high temperature cluster, and are different from the uniform state like the solid solution atom after the solution heat treatment. That is, in the subsequent room temperature holding, instead of newly forming a low temperature cluster, the movement is performed so as to restore the shape of the original high temperature cluster. Since this repair takes time, the growth of the cut and destroyed high-temperature cluster to a predetermined size is slower than the case where it is not destroyed, the rate of increase in yield stress is reduced, and the low-temperature cluster is not formed. It does not hinder the curing ability during baking.

  The stress that introduces dislocations that break high temperature clusters may be either tensile or compression, but in order to obtain this effect sufficiently, it is preferable to introduce dislocations uniformly in the material. In order to achieve this industrially, as described above, it is advantageous to apply a tensile stress (tension) in the longitudinal direction of the coil. In general, even with a simple leveler or skin pass, elongation strain is imparted in the material. However, if the tension is not enough, the strain imparted is limited to the surface layer, and only some of the clusters on the surface layer in the alloy plate. It is not destroyed and the effect of suppressing aging at room temperature is extremely low. Considering the effect of the invention and the basic formability change due to strain, the tension leveler is suitable.

  The amount of strain in the present invention is defined by the change in the material length before and after loading the deformation load. In general, changes in length in a plurality of directions are observed, but the maximum one of them may be defined. In the case of a coil shape, it can be evaluated by a change in length in the longitudinal direction.

  The feasibility and effects of the present invention as described above will be further described using examples.

Table 1 shows the aluminum alloys used for the tests and their components. These alloys were subjected to DC casting, uniform heat treatment at 580 ° C. for 24 hours, hot rolling up to a plate thickness of 4 mm starting from 500 ° C., and cold rolling to a plate thickness of 1 mm. A sample of was prepared. Here, a JIS No. 5 tensile test piece was prepared from the sample and subjected to solution heat treatment using an atmospheric furnace and preliminary heat treatment using an oil bath. Table 2 shows the process conditions after the solution heat treatment. In addition, the holding time of the solution heat treatment was 1 hour, and after the completion, air cooling and water cooling were sequentially performed and cooled to room temperature. After preliminary heat treatment, tensile strain was applied to the test piece with a tension leveler.
Thereafter, a tensile test at room temperature was performed. The room temperature was maintained by holding in a drying oven set at 30 ° C. Five types of conditions were set as combinations of solution heat treatment and pre-heat treatment conditions.

Tensile tests were performed on specimens after room temperature retention for 10 days, 20 days, and 180 days. They were marked as 10 days, 20 days, and 180 days, respectively. On the other hand, the test piece that was not kept at room temperature was immediately subjected to a tensile test and marked as after 0 date.
Table 3 shows the amount of tensile strain applied after the preliminary heat treatment in each alloy and the yield stress value (0.2% yield strength value) obtained after an aging time of 30 ° C. In addition, the yield stress value (hereinafter referred to as BHYS) after sequentially applying 0.2% tensile strain and heat treatment held at 170 ° C. for 20 minutes after 180 days is also shown.

  Necessary room temperature aging characteristics are based on the yield stress value when held at room temperature (30 ° C.) for 10 days, and the increase in yield stress value when held for 20 days is 4 MPa or less, and also holds for 180 days. The amount of increase in yield stress is 45 MPa or less, and the yield stress value at 180 days at room temperature is 150 MPa or less. Furthermore, BHYS needs to be 170 MPa or more.

  In the case of an alloy within the scope of the claims specified in the present invention, an increase amount for 20 days is 4 MPa or less and an increase amount for 180 days is 45 MPa or less at a tensile strain of 0.3% to 2%, and BHYS is 170 MPa. That's it. However, even in the case of alloys within the scope of claims, when the strain is less than 0.3%, the increase in 180 days exceeds 45 MPa, and the increase in strength due to long-term room temperature retention becomes extremely large, which adversely affects the formability. However, BHYS is 170 MPa or more, and the coating bake strength is maintained. In the case of 3.0%, the increase in strength for 180 days is 45 MPa or less, but the yield strength on day 0 is 110 MPa or more, and the absolute value of strength increases. In addition, since the amount of strain is large, the cluster is largely destroyed and BHYS is less than 170 MPa.

  Furthermore, if the heat treatment conditions are out of the claims, the predetermined characteristics cannot be obtained. If the solution heat treatment temperature is low (P2), the amount of solid solution is small, and the amount of increase in strength due to holding at room temperature is small, but the strength is increased by the presence of precipitates before solution heat treatment. Moreover, all the strains introduced by cold rolling remain without recovering, which also contributes to an increase in strength. Furthermore, since the amount of solid solution is small, BHYS is less than 170 MPa. When the pre-heat treatment temperature is high (P5) and the pre-heat treatment temperature within the claimed range but when the holding time is long (P3), the amount of strengthening becomes large, so the initial yield stress becomes large. On the other hand, when the preliminary heat treatment temperature is low (P4), the initial yield stress is small, but since the amount of solid solution is large, the increase in strength at room temperature is large. However, BHYS is small because clusters that do not contribute to BHYS are formed. On the other hand, if the preliminary heat treatment temperature is high, clusters are not formed, the initial yield stress is large, and as a result, BHYS is also large.

Claims (3)

  It contains Mg: 0.3-2.0% and Si: 0.3-2.0% in mass%, the yield stress value immediately after production is 110 MPa or less, and 20% at 30 ° C. immediately after production. The value obtained by subtracting the yield stress value when retained for 10 days from the yield stress value when retained for 10 days is 4 MPa or less, and 0.2% tensile strain was imparted after maintaining for 180 days at 30 ° C. immediately after production. An aluminum alloy that suppresses an increase in yield stress at room temperature, characterized in that a yield stress value after applying heat treatment at 20 ° C. for 20 minutes in order is 170 MPa or more.   It further contains any one of Cu: 0.2 to 1.0% and Fe: 0.05 to 0.3% by mass%. Aluminum alloy with reduced rise.   The aluminum alloy processed into a plate shape is subjected to a solution heat treatment at 500 ° C. or higher and a melting point or lower at a holding time of 1 minute or longer and 1 hour or shorter, and subsequently at 70 ° C. or higher and 180 hours at a holding time of 2 hours or longer and 24 hours or shorter. The yield at room temperature retention according to claim 1 or 2, wherein a strain between 0.2 to 2% plane strain to tensile strain is applied by a tension leveler after heat treatment at ℃ or lower. A method for producing an aluminum alloy that suppresses an increase in stress.