JPH09263869A - Aluminum alloy sheet excellent in impact characteristic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an aluminum alloy sheet excellent in impact characteristic and its production, to enable the contribution, in terms of material, to the safety of automobile against collision. SOLUTION: In an aluminum alloy sheet having a composition consisting of, by weight, 0.4-2% Mg, 0.4-2% Si, and the balance Al with inevitable impurities, the average size, in the cross section in a rolling direction, of the crystallized substances crystallized in the alloy sheet is regulated so that it satisfies inequalities (Dl+Dc)/2<=5μm and Dl/Dc<=1.25, where Dl and Dc represent the length of the crystallized substances in a rolling direction and the length of the crystallized substances in a sheet-thickness direction, respectively. By this method, the aluminum alloy sheet excellent in impact characteristic can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum alloy plate having excellent impact characteristics suitable for use as a material for automobiles and a method for producing the same.

[0002]

BACKGROUND OF THE INVENTION Aluminum alloys are known to have inferior impact properties compared to iron (see, for example, Automotive Technology, 30
(No. 2) (1976), P. 117). That is,
It has been considered that even if the strain rate is higher than 100 / s, its strength hardly changes. By the way, recently, the application of aluminum alloy sheets as a material for automobiles has been expanding. The purpose of applying this aluminum alloy plate is to reduce the weight of the automobile body, but the environment surrounding the automobile becomes severe, and various measures have been taken to ensure the safety of passengers in the event of a collision. Therefore, in the case of an aluminum alloy plate that constitutes an automobile part, it is required to improve the collision characteristics from the viewpoint of collision safety. For extruded aluminum alloy profiles, a method of controlling the size of grain boundary precipitates to a predetermined size is disclosed as a method for improving impact absorption, as described in JP-A-7-54090. . This regulates the cooling rate after performing solution treatment and then extrusion molding, and particularly controls the size of grain boundary precipitates.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an aluminum alloy sheet having excellent impact characteristics and a method for producing the same, in order to allow the material to contribute to the collision safety of an automobile. To do.

[0004]

DISCLOSURE OF THE INVENTION As a result of studies on the influence of the morphology of crystallized substances in an aluminum alloy on the impact characteristics, the present inventors have found that (1) the morphology of the crystallized substances influences the impact properties. Given that the more isotropic the shape, the better the impact properties, and (2) In order to make the crystallized substance isotropic, to cool the aluminum alloy when solidifying it. It was found that increasing the speed is effective. The present invention is based on the above-mentioned findings, and in weight%, Mg: 0.4 to 2%, Si:
In an aluminum alloy plate containing 0.4 to 2%, the balance being Al and unavoidable impurities, the average size of the crystallized product crystallized in the alloy plate in the rolling direction cross section is expressed by formulas (1) and (2). An aluminum alloy plate with excellent impact characteristics, which satisfies the following requirements. (D1 + D _C ) / 2 ≦ 5 μm (1) D1 / D _C ≦ 1.25 (2) (where D1: crystallized product (Length in rolling direction, D _C : length in thickness direction of crystallized product)

In the above aluminum alloy plate, if necessary, one or more of Cu: 1% or less, Fe: 0.5% or less, or / and Ti: 0.2% or less, B: 0.01%.
Hereinafter, it is preferable to contain one or more of Be: 0.005% or less. Moreover, in the present invention, in the method for producing an aluminum alloy plate, the cooling rate during solidification is 5
It is a method for producing an aluminum alloy sheet having excellent impact characteristics, which comprises casting at a temperature of ℃ / s or more, reheating, performing predetermined hot rolling and cold rolling, and performing recrystallization annealing.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the component composition in the present invention will be described. Mg is an indispensable element that is added in order to form fine precipitates together with Si and obtain high paint bake hardenability. If its content is less than 0.4%, the strength is insufficient. Meanwhile, 2
Even if added in excess of%, the effect will be saturated. Therefore, Mg is set to 0.4 to 2%.

Si is an essential element for forming fine precipitates together with the above-mentioned Mg and obtaining high bake hardenability. If the added amount is less than 0.4%, the strength is insufficient, while if added more than 2%, a large amount of crystallized substances are formed, and even if the size is controlled, the improvement of impact properties is suppressed. In addition to the above, the bake hardening amount is saturated, so the upper limit is 2%. If necessary, the basic composition system of the steel according to the present invention in which the contents of Mg and Si are specified as described above is C
At least one of u and Fe is added.

Cu is an element added mainly for the purpose of increasing the strength by precipitation hardening and coating bake hardening. If the added amount is less than 0.1%, it has no effect, and if the added amount exceeds 1%, not only the corrosion resistance is extremely deteriorated, but also precipitation is promoted and the change with time during standing at room temperature is large. , Moldability deteriorates. Therefore, the addition amount of Cu is set to 0.1 to 1%. Fe is an unavoidable impurity element in an aluminum alloy, and forms a crystallized substance at the time of solidification to reduce the formability. Therefore, the content should be as low as possible, and therefore it should be 0.5% or less. On the other hand, if it is lower than 0.05%, the manufacturing cost is increased. Therefore, the lower limit is preferably 0.05%.

The aluminum alloy plate according to the present invention includes:
If necessary, one or more of Ti, B and Be are further added. To improve hot workability, T
The addition of i or B is effective. Ti and B refine the crystal grains in the ingot and improve the hot workability. However, crystallization of primary crystal Al ₃ Ti particles significantly deteriorates moldability, so that crystallization must be prevented.
Therefore, the upper limit of the Ti content is 0.2%. Also, when B is added excessively, a compound TiB ₂ with Ti is formed,
In order to deteriorate the workability, the upper limit is 0.01%.

Regarding Be, in particular, Mg is 1.5
When the content of alloying component exceeds%, it is added to prevent oxidation of the molten metal. Therefore, the effect is saturated by excessive addition, so 0.005% is made the upper limit. The aluminum alloy plate of the present invention is characterized by comprising the above-mentioned components, but other components may be contained in the components contained in a normal aluminum alloy plate. In particular, since Cr and Zn are elements that deteriorate the formability and corrosion resistance, it is better to add Cr as little as possible, and the total content is 0.1% or less.

Next, the form of the crystallized substance which is the most important in the present invention will be described. As a result of studying the morphology of crystallized substances in order to improve the impact properties of the aluminum alloy sheet, the present inventors found that when the size of the crystallized substance in the rolling direction cross section satisfies the following two equations in terms of average size, It has been found that the characteristics are dramatically improved. (D1 + D _C ) / 2 ≦ 5 μm D1 / D _C ≦ 1.25 (where D1: length of crystallized product in rolling direction, D _C : length of crystallized product in plate thickness direction)

FIG. 1 shows the experimental results obtained from the above findings. In the experiment, in a 6000 series alloy of Al-0.7% Si-0.7% Mg, the cooling rate at the time of solidification was changed to prepare a material in which the size and shape of the crystallized substance were changed, and the strain rate was the amount of increase in tensile strength in a tensile test of 10 ^-3 s ^-1 and 10s ^-1 was investigated. From the result of Figure 1, (D1 + D _C )
Under the conditions of 1/2 ≦ 5 μm and D1 / D _C ≦ 1.25, an increase in tensile strength due to a high strain rate was observed, and an effect of improving impact properties was obtained.

Next, a method for manufacturing the aluminum alloy sheet according to the present invention will be described. The alloy having the composition of the present invention is cast by a DC casting method or another casting method. Further, a thin slab continuous casting method or a twin roll casting method may be used. In the present invention, the cooling rate during solidification is the most important factor. 2 and 3 are results showing the relationship between the cooling rate during solidification and the morphology (isotropic) of the crystallized substance in the experiment of FIG. Here, the size of the crystallized substance is measured in an arbitrary L cross section (cross section parallel to the rolling direction), and the crystallized amount of 1000 crystallized substances in the rolling direction and the plate thickness direction are measured with an optical microscope structure of 500 times. It was calculated from the average size. It was found that the cooling rate at the time of solidification should be 5 ° C./s or more in order that the morphology of the crystallized substance should satisfy the above two expressions. Therefore,
The cooling rate during solidification is 5 ° C / s or more. The higher the cooling rate during solidification, the smaller the size of the crystallized substance and the more isotropic it is. However, when it exceeds 500 ° C / s,
A large amount of the solid solution element remains, which hardens and simultaneously reduces ductility. Therefore, preferably 5000 ℃
A cooling rate of / s or less is good.

Subsequent hot rolling may be a conventional method,
It is not specified in particular. That is, 450 to 58
After performing homogenization treatment within the range of 0 ° C., hot rolling is performed. At this time, when the thin slab continuous casting method or the twin roll casting method is used, hot rolling can be performed without cooling to room temperature. Next, after cold rolling, solution treatment recrystallization treatment is performed. The heat treatment conditions at that time are not particularly specified, and normal continuous annealing or box annealing may be performed. Further, subsequent temper rolling does not impair the effects of the present invention.

[0015]

【Example】

Example 1 A 6000 series aluminum alloy containing Mg: 1.31% and Si: 1.30% was melted and solidified at various cooling rates as shown in Table 1 to obtain a cast material. After that, hot rolling and cold rolling are performed according to the usual method, and the plate thickness is 1 mm.
Cold rolled sheet. This cold rolled sheet is continuously annealed at 520 ° C x 1
After the solution treatment for 0 seconds, it was cooled to room temperature. Regarding the material evaluation, first, the tensile test was performed using JIS Z 220
Processed into No. 5 test piece according to No. 1, JIS Z 224
It carried out according to the test method of 1. Further, in order to evaluate the impact properties, a tensile test was conducted with a tensile speed of 100 / s as a strain rate. The bake hardening amount (BH) is 2
%, A heat treatment was carried out at 170 ° C. for 20 minutes, and the yield point was evaluated again by the tensile test. As shown in Table 1, No. 1 of the present invention example. In Nos. 3, 4, and 5, the strength increased in the tensile test at a strain rate of 100 / s due to the finely uniformed (isotropic) morphology of the crystallized substance. Further, the local elongation (1-El) is also improved. On the other hand, in Comparative Example No. In Nos. 1 and 2, the cooling rate at the time of solidification was out of the specified range of the present invention, and the size of the crystallized substance was too large, so that the strength did not increase in the tensile test at the strain rate of 100 / s.

[0016]

[Table 1]

Example 2 Various aluminum alloys as shown in Table 2 were melted and solidified at a cooling rate within the range of the present invention to produce cast pieces. Then, hot rolling and cold rolling were performed by a usual method to obtain a 1 mm cold rolled sheet. In addition, what was cast by the thin slab continuous casting method was hot-rolled directly and then cold-rolled. Further, in the twin roll method, it was cast to 3 mm and cold-rolled without hot-rolling. The cold-rolled sheet thus obtained was subjected to recrystallization treatment by continuous annealing or box annealing, and then the material quality was evaluated in the same manner as in Example 1. Furthermore, an evaluation test of corrosion resistance was also conducted. The evaluation is
Each of the zinc phosphate-treated alloys was subjected to commercial electrodeposition coating, intermediate coating and top coating, and the coated surface was cut with a cutter in the L direction and the C direction, and the coating was performed according to ASTM D2083 method. When the swollen width was 2 mm or less, it was evaluated as ◯, and when it was more than 2 mm, it was evaluated as x.

[0018]

[Table 2]

The results are shown in Table 3. Examples of the present invention (No. 6 to
In 12), an increase in strength was observed in a region where the strain rate was high at 100 / s, and the impact characteristics were excellent. On the other hand, in the case of No. 13 is 6000
The high bake hardenability required for alloys cannot be obtained. Si
Which deviates from the scope of the present invention. 14 is a coarse S
At the same time as causing the deterioration of ductility due to the precipitation of the i phase, the strength does not increase at the strain rate of 100 / s, and the impact characteristics are poor. If Cu is added beyond the scope of the invention, N
o. No. 15 has poor corrosion resistance. Furthermore, the Fe. In No. 16, the amount of crystallized substances formed is large, the ductility is reduced, the strength is not high at a strain rate of 100 / s, and the impact properties are poor. In addition, No. In 17,
Since Cu deviates from the range of the present invention to a low level, the bake hardenability, which is a characteristic of the 6000 series alloy, required for automobile plate materials cannot be obtained.

[0020]

[Table 3]

[0021]

EFFECT OF THE INVENTION The aluminum alloy sheet of the present invention isotropically adjusts the morphology of crystallized substances, so that it has excellent impact characteristics as compared with conventional aluminum alloys and is suitable for automobile bodies. It becomes possible to provide.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the morphology of crystallized substances and the amount of increase in tensile strength due to changes in strain rate,

FIG. 2 is a diagram showing a relationship between a cooling rate during solidification and (D1 + D _C ) / 2,

FIG. 3 is a diagram showing a relationship between a cooling rate during solidification and D1 / D _C.

Claims (4)

[Claims] 1. By weight%, Mg: 0.4-2%, Si:
In an aluminum alloy plate containing 0.4 to 2%, the balance being Al and unavoidable impurities, the average size of the crystallized product crystallized in the alloy plate in the rolling direction cross section is expressed by formulas (1) and (2). An aluminum alloy plate with excellent impact characteristics, which satisfies the following requirements. (D1 + D _C ) / 2 ≦ 5 μm (1) D1 / D _C ≦ 1.25 (2) (where D1: crystallized product (Length in rolling direction, D _C : length in thickness direction of crystallized product) 2. By weight%, Mg: 0.4-2%, Si:
2. The aluminum alloy according to claim 1, wherein the aluminum alloy contains 0.4 to 2%, and the balance contains Al and unavoidable impurities and one or more of Cu: 1% or less and Fe: 0.5% or less. Board. 3. By weight%, Mg: 0.4-2%, Si:
0.4 to 2%, the balance to Al and unavoidable impurities, Ti: 0.2% or less, B: 0.01% or less,
Be: 0.005% or less is included, and 1 or more types are included, The aluminum alloy plate of any one of Claim 1 or 2 characterized by the above-mentioned. 4. The method for producing an aluminum alloy plate according to claim 1, 2 or 3, wherein the cooling rate during solidification is 5 ° C.
A method for producing an aluminum alloy sheet having excellent impact characteristics, which comprises casting at a rate of not less than 1 s / s, reheating, performing predetermined hot rolling and cold rolling, and performing recrystallization annealing.