FORMING METHOD FOR ALUMINUM ALLOY SHEET

Abstract

 In a first step S101, a plate material made of a 7000-series aluminum alloy having undergone T6 treatment is heated to a first temperature that enables hot press forming. In a second step S102, a molded body is formed by performing hot press forming for the plate material heated to the first temperature. In a third step S103, the molded body is heated at a second temperature for 20 to 30 min to increase an electrical conductivity and a hardness. The second temperature can be a temperature within a range of 170°C to 200°C.

Description

Technical Field

[0001] The present invention relates to a method of forming an aluminum alloy plate.

Background Art

[0002] Currently, to improve the fuel consumption of an automobile, weight reduction is important. Weights have been reduced mainly by thinning a plate material using an ultra-high tensile strength steel plate. However, the weight exponentially affects rigidity. For this reason, there is a limit to making a plate thin because it is difficult to ensure component rigidity. On the other hand, an aluminum alloy has not only a high strength but also a low specific gravity. Hence, it is possible to achieve weight reduction while ensuring a plate thickness. In particular, Al-Zn-Mg-based alloys or Al-Zn-Mg-Cu-based alloys (7000-series aluminum alloys) are effective because of their high strengths.

Related Art Literature

Patent Literature

[0003] Patent Literature 1: Japanese Patent Laid-Open No. 2010-159489

Disclosure of Invention

Problem to be Solved by the Invention

[0004] However, concerning the high-strength aluminum alloys such as 7000-series aluminum alloys, the ductility of an age-hardened plate material at room temperature is as low as about 10%, and cold press forming is difficult. Hence, in the conventional technique, the plate material undergoes a solution heat treatment and is then press-formed in an annealed state (see patent literature 1). In this technique, however, for example, in a production site of plate materials of high-strength aluminum alloys such as 7000-series aluminum alloys for automobiles, there have been confirmed problems that the strength of a formed plate material cannot be set to a desired state, and stress corrosion cracking (SCC) readily occurs (the SCC resistance is low).

[0005] The present invention has been made to solve the above-described problems, and has as its object to further improve the strength and SCC resistance of a press-formed aluminum alloy.

Means of Solution to the Problem

[0006] According to the present invention, there is provided a method of forming an aluminum alloy plate, the method comprising a first step of heating a plate material made of a 7000-series aluminum alloy having undergone T6 treatment to a first temperature that enables hot press forming, a second step of performing hot press forming for the plate material heated to the first temperature to form a molded body, and a third step of heating the molded body at a second temperature for 20 to 30 min to increase an electrical conductivity and a hardness.

[0007] In a configuration example of the method of forming an aluminum alloy plate, the first temperature is a temperature within a range of 250°C to 270°C.

[0008] In a configuration example of the method of forming an aluminum alloy plate, in the first step, the plate material is heated to the first temperature for 150 sec at maximum.

[0009] In a configuration example of the method of forming an aluminum alloy plate, the second temperature is a temperature within a range of 170°C to 200°C.

[0010] In a configuration example of the method of forming an aluminum alloy plate, the plate material is made of an A7075 aluminum alloy.

Effect of the Invention

[0011] As described above, according to the present invention, the molded body after hot press is heated at the second temperature that is a temperature within the range of, for example, 170°C to 200°C for 20 or 30 min to increase the electrical conductivity and the hardness. It is therefore possible to further improve the strength and the SCC resistance of the press-formed plate material of an aluminum alloy.

Brief Description of Drawings

[0012] 

Fig. 1 is a flowchart for explaining a method of forming an aluminum alloy plate according to the embodiment of the present invention;

Fig. 2 is a timing chart showing the change of the temperature of a plate material or a molded body;

Fig. 3A is a graph showing the result of an experiment that simulates the first step, the second step, and the third step of the method of forming an aluminum alloy plate according to the embodiment of the present invention while setting the first temperature to 250°C;

Fig. 3B is a graph showing the result of an experiment that simulates the first step, the second step, and the third step of the method of forming an aluminum alloy plate according to the embodiment of the present invention while setting the first temperature to 255°C;

Fig. 3C is a graph showing the result of an experiment that simulates the first step, the second step, and the third step of the method of forming an aluminum alloy plate according to the embodiment of the present invention while setting the first temperature to 260°C;

Fig. 3D is a graph showing the result of an experiment that simulates the first step, the second step, and the third step of the method of forming an aluminum alloy plate according to the embodiment of the present invention while setting the first temperature to 265°C;

Fig. 4A is a graph showing the differences of a Vickers hardness and an electrical conductivity after the third step in an experiment that simulates the first step, the second step, and the third step of the method of forming an aluminum alloy plate according to the embodiment of the present invention while setting the conditions of the third step to 170°C and 20 min; and

Fig. 4B is a graph showing the differences of the Vickers hardness and the electrical conductivity after the third step in an experiment that simulates the first step, the second step, and the third step of the method of forming an aluminum alloy plate according to the embodiment of the present invention while setting the conditions of the third step to 200°C and 30 min. Best Mode for Carrying Out the Invention

[0013] A method of forming an aluminum alloy plate according to the embodiment of the present invention will now be described with reference to Figs. 1 and 2. Fig. 2 shows the change of the temperature (the temperature of a plate material or a molded body) in each step to be described below.

[0014] First, in a first step S101, a plate material made of a 7000-series aluminum alloy having undergone T6 treatment is heated to a first temperature that enables hot press forming. The first temperature can be a temperature within the range of 250°C to 270°C. Also, in this step, the plate material can be heated to the first temperature for 150 sec at maximum.

[0015] In this step, for example, the heating rate is preferably set to 10°C/sec or more. For example, when rapid heating is performed up to 250°C at 10°C/sec as heat treatment conditions in the first step, the deformation resistance lowers (300 MPa) and the formability improves (a breaking elongation is 170), and additionally, the hardness at room temperature after forming is 153 HV, which can satisfy a target value (> 140) .

[0016] On the other hand, if slow heating is performed up to 250°C at 1°C/sec as heat treatment conditions in the first step, lowering of the deformation resistance (208 MPa) and improvement of the formability (a breaking elongation is 17%) can be achieved, but the hardness at room temperature after forming is 107 HV, which cannot satisfy the target value (> 140). Note that the target value of the hardness will be described later.

[0017] Also, the heat treatment is preferably executed by well-known contact heating from the viewpoint of the heating rate. This heating enables press forming to be described later. Note that the plate material is made of an Al-Zn-Mg-based alloy or Al-Zn-Mg-Cu-based alloy such as A7075 (JIS).

[0018] Next, in a second step S102, the plate material heated to the first temperature is formed by hot press forming, thereby forming a molded body. In the hot press forming, the plate material heated to the forming enable temperature is press-formed using a die, and simultaneously rapidly cooled by the die. By this rapid cooling, the temperature of the plate material lowers from the first temperature to, for example, 20°C to 25°C. For example, when a die including a cooling mechanism by water cooling or the like is used, the above-described hot press forming can be executed. Alternatively, the above-described hot press forming can be executed by a direct water cooling method in which a press target is directly cooled by discharging cooling water from the inner surface of a die to the press target and making the cooling water flow between the die surface and the press target.

[0019] Here, to prevent the temperature of the plate material from lowering, the plate material is preferably put in a heat insulating box or a heating box and conveyed in a warmed or heated state from a heating furnace where the heat treatment is executed to a press machine that executes hot press forming.

[0020] Next, in a third step S103, the molded body is heated at a second temperature for 20 to 30 min, thereby increasing the electrical conductivity and the hardness (strength). The second temperature can be a temperature within the range of 170°C to 200°C.

[0021] According to the method of forming an aluminum alloy plate of the above-described embodiment, it is possible to further improve the strength and the strength of a press-formed plate material (molded body) of an aluminum alloy. In addition, since the plate material made of a 7000-series aluminum alloy having undergone T6 treatment is used, a parts manufacturer can easily handle it. According to the embodiment, since the plate material is heated to the first temperature that enables hot press forming, press forming of a part having a hat-shaped section, like the body frame of an automobile, can be performed. Since the part can be held at 20°C for one day after the press forming, the conveyance time from the parts manufacturer to a car manufacturer can be ensured.

[0022] The heat treatment (re-aging treatment; third step) at a temperature of 170°C or 200°C for 20 or 30 min exhibits the same heat history as coating-baking treatment conditions generally employed in car manufacturers in Japan, Europe, and the United States of America. For this reason, the heat treatment using a coating drying oven after the conveyed plate material (part) is assembled to the vehicle body of an automobile can be executed as the third step. Note that the heat treatment (third step) need not use a coating drying oven. It is also possible to attach the part to a vehicle body using a thermosetting adhesive and simultaneously set the adhesive.

[0023] The results of experiments simulating the first step, the second step, and the third step will be described next. As an experiment, first, a test piece (20 mm × 20 mm, thickness: 2 mm) made of an A7075 plate material having undergone T6 treatment was prepared.

[0024] Next, the test piece was heated to 250°C to 265°C for 60 to 150 sec (first step) and cooled to 20°C by water cooling (second step). The heat treatment was executed by immersing the test piece in an oil bath or salt bath at 250°C to 265°C. The temperature of the heat treatment corresponds to the first temperature. Also, the above-described water cooling processing was regarded as rapid cooling using a die in the hot press of the second step.

[0025] Next, after the test piece was held at 20°C for one day (natural aging), heat treatment was executed at a temperature of 170°C or 200°C for a treatment time of 20 min or 30 min (re-aging treatment). This process corresponds to the above-described third step, and exhibits the same heat history as coating-baking treatment conditions generally employed in car manufacturers in Japan, Europe, and the United States of America. The temperature of the treatment corresponds to the second temperature.

[0026] At the stage of each heat treatment, a Vickers hardness test and electrical conductivity measurement were executed. Figs. 3A, 3B, 3C, and 3D show the results of the test and the measurement. Fig. 3A shows a result in a case where the heat treatment condition in the first step was set to 250°C, Fig. 3B shows a result in a case where the heat treatment condition in the first step was set to 265°C, Fig. 3C shows a result in a case where the heat treatment condition in the first step was set to 260°C, and Fig. 3D shows a result in a case where the heat treatment condition in the first step was set to 255°C. In these treatments, it is considered that the test piece reaches the temperature (first temperature) of each heat treatment condition. Also, a solid line indicates a case where the second temperature is 170°C, and a broken line indicates a case where the second temperature is 200°C. In an ellipse of a dotted line indicating the third step S103, a plot point at the final end indicates a treatment time of 30 min, and a plot point before that indicates a treatment time of 20 min.

[0027] In each graph, at the stage of the second step S102, the hardness decreases from the initial state (T6 treatment is performed). Then, the hardness increases depending on the treatment condition of the third step S103. At all temperatures (250°C, 255°C, 260°C, and 265°C) in the first step S101, the hardness after the second step S102 greatly decreases, and the electrical conductivity greatly increases as compared to the values (195 HV and 32 IACS%) of A7075 that has undergone T6 treatment. Also, the higher the temperature (≈ first temperature) in the first step S101 is, and the longer the time of the first step S101 is, the larger the change amounts are.

[0028] On the other hand, as for the hardness and the electrical conductivity after the third step S103, under all conditions when the second temperature is 170°C, both values are larger than those after the second step S102. When the second temperature is 200°C, under many conditions in the first step S101, the electrical conductivity increases, but the hardness is unchanged or decreases. Some test pieces have the same hardness and electrical conductivity as, for example, a T76 material (167 HV and 38 IACS%) or a T73 material (153 HV and 40 IACS%) obtained by improving the SCC resistance of the A7075 material having undergone T6 treatment in the process of the third step S103. Note that as product characteristics, the hardness is preferably 140 HV or more, and the electrical conductivity is preferably 38 IACS% or more. The hardness is more preferably 153 HV or more. According to these results, it is considered that, by the present invention, a molded body can be obtained using a plate material made of a 7000-series aluminum alloy having not only a high strength but also an excellent SCC resistance.

[0029] Next, Figs. 4A and 4B show the differences, depending on conditions, of the Vickers hardness and the electrical conductivity after the third step in the above-described experiments. In Fig. 4A, the conditions of the third step are 170°C and 20 min. In Fig. 4B, the conditions of the third step are 200°C and 30 min. A number in each plot point indicates the treatment time of the first step. Also, each temperature shown in Figs. 4A and 4B indicates the result of measuring the temperature of the test piece in the first step using a thermocouple (corresponding to the first temperature). It can be found from these results that for each treatment time considering a temporal tolerance in an actual work, there are conditions that can further improve the strength (hardness) and the SCC resistance and can be provided for actual use.

[0030] Next, Table 1 shows the measurement results of the Vickers hardness and the electrical conductivity in a case where the first step was executed under various conditions for a test piece (20 mm × 20 mm, thickness: 2 mm) made of an A7075 plate material having undergone T6 treatment, the test piece was held at 20°C for one day, and after that, the third step was performed under the conditions of 170°C and 20 min. Also, Table 2 shows the measurement results of the Vickers hardness and the electrical conductivity in a case where the first step was executed under various conditions for a test piece (20 mm × 20 mm, thickness: 2 mm) made of an A7075 plate material having undergone T6 treatment, the test piece was held at 20°C for one day, and after that, the third step was performed under the conditions of 200°C and 30 min.

[0031] Note that the heat treatment was executed by immersing the test piece in an oil bath. As treatment corresponding to rapid cooling using a die in the hot press of the second step, water cooling was performed to cool the test piece to 20°C. "Temperature" in each table indicates a value (corresponding to the first temperature) obtained by measuring the temperature of the test piece using a thermocouple. It can be found from these results as well that for each treatment time considering a temporal tolerance in an actual work, there are conditions that can further improve the strength (hardness) and the SCC resistance and can be provided for actual use.

[Table 1]

200°C, 30 min/T6
Temperature-time	Hardness	Electrical conductivity
250°C, 120 s	154.1	39.2
250°C, 150 s	149.9	39.3
255°C, 90 s	148.6	39.1
255°C, 120 s	146.1	39.5
255°C, 150 s	142.4	39.7
260°C, 60 s	155.6	38.9
260°C, 90 s	152	39.4
260°C, 120 s	148.8	39.5
260°C, 150 s	140.2	39.8
265°C, 60 s	149.9	39
265°C, 90 s	144.2	39.8
270°C, 60 s	144.6	39.8

[Table 2]

200°C, 30 min/T6
Temperature-time	Hardness	Electrical conductivity
250°C, 120 s	154.1	39.2
250°C, 150 s	149.9	39.3
255°C, 90 s	148.6	39.1
255°C, 120 s	146.1	39.5
255°C, 150 s	142.4	39.7
260°C, 60 s	155.6	38.9
260°C, 90 s	152	39.4
260°C, 120 s	148.8	39.5
260°C, 150 s	140.2	39.8
265°C, 60 s	149.9	39
265°C, 90 s	144.2	39.8
270°C, 60 s	144.6	39.8

[0032] In the above description, the Vickers hardness test is used to roughly estimate the strength, and electrical conductivity measurement by an eddy current method is used to roughly estimate the SCC resistance. However, it is obvious that these can roughly be estimated using a similar strength/hardness test method or electrical conductivity/electric resistivity measurement method. Note that if the first temperature is 270°C or more, the hardness undesirably becomes too low. If the first temperature is 250°C or less, it is not easy to obtain a target electrical conductivity within a practical treatment time considering mass production.

[0033] As described above, according to the present invention, the molded body after hot press is heated at the second temperature that is a temperature within the range of, for example, 170°C to 200°C for 20 or 30 min to increase the electrical conductivity and the hardness. It is therefore possible to further improve the strength and the SCC resistance of the press-formed plate material of an aluminum alloy.

[0034] Some or all of the above-described embodiments can also be described as in the following supplementary notes but are not limited to the followings.

[Supplementary Note 1]

[0035] There is provided a method of forming an aluminum alloy plate, the method comprising:

a first step of heating a plate material made of a 7000-series aluminum alloy having undergone T6 treatment to a first temperature that enables hot press forming;

a second step of performing hot press forming for the plate material heated to the first temperature to form a molded body; and

a third step of heating the molded body at a second temperature for 20 to 30 min to increase an electrical conductivity and a hardness.

[Supplementary Note 2]

[0036] In the method of forming an aluminum alloy plate according to Supplementary Note 1,
the first temperature is a temperature within a range of 250°C to 270°C.

[Supplementary Note 3]

[0037] In the method of forming an aluminum alloy plate according to Supplementary Note 1 or 2,
in the first step, the plate material is heated to the first temperature for 150 sec at maximum.

[Supplementary Note 4]

[0038] In the method of forming an aluminum alloy plate according to any one of Supplementary Notes 1 to 3,
the second temperature is a temperature within a range of 170°C to 200°C.

[Supplementary Note 5]

[0039] In the method of forming an aluminum alloy plate according to any one of Supplementary Notes 1 to 4,
the plate material is made of an A7075 aluminum alloy.

[0040] Note that the present invention is not limited to the above-described embodiments, and various modifications and combinations can be implemented by those who have ordinary knowledge in the field without departing the technical scope of the present invention.

Claims

1. A method of forming an aluminum alloy plate, the method comprising:

a first step of heating a plate material made of a 7000-series aluminum alloy having undergone T6 treatment to a first temperature that enables hot press forming;

a second step of performing hot press forming for the plate material heated to the first temperature to form a molded body; and

a third step of heating the molded body at a second temperature for 20 to 30 min to increase an electrical conductivity and a hardness.

2. The method of forming an aluminum alloy plate according to claim 1, wherein the first temperature is a temperature within a range of 250°C to 270°C.

3. The method of forming an aluminum alloy plate according to claim 1, wherein in the first step, the plate material is heated to the first temperature for 150 sec at maximum.

4. The method of forming an aluminum alloy plate according to claim 1, wherein the second temperature is a temperature within a range of 170°C to 200°C.

5. The method of forming an aluminum alloy plate according to any one of claims 1 to 4, wherein the plate material is made of an A7075 aluminum alloy.

Drawing