JP6677584B2 - Method of manufacturing energy absorbing member - Google Patents

Description

  The present invention relates to a method of manufacturing an energy absorbing member that absorbs energy at the time of an automobile collision, and more particularly to a method of manufacturing an energy absorbing member made of an extruded 7000 (Al-Zn-Mg-Cu) aluminum alloy.

2. Description of the Related Art Conventionally, energy absorbing members such as bumper beams and door beams have been used in automobiles to improve collision safety. Since this energy absorbing member is required to have high energy absorbing properties, a 7000-series aluminum alloy extruded material of Zn-Mg-Cu-based material having excellent strength is used. However, in order to further increase the energy absorption, it is not sufficient to simply increase the strength of the energy absorbing member, and it is necessary to prevent the energy absorbing member from cracking at the time of collision.
In this regard, for example, Patent Documents 1 and 2 disclose that an energy-absorbing member made of an extruded 7000-series aluminum alloy is overaged to prevent the energy-absorbing member from cracking in a collision, It is described that the absorbency is enhanced.

JP 2001-140029 A JP 2006-233336 A

  If the 7000 series aluminum alloy extruded material is overaged, cracking of the energy absorbing member at the time of collision is prevented, that is, the pressure cracking resistance of the energy absorbing member is improved. However, on the other hand, the strength of the energy absorbing member is reduced. Therefore, there is a limit to the amount of energy absorption of the energy absorbing member that is increased by the overaging treatment, and it is difficult to further improve the energy absorbing property of the energy absorbing member made of the 7000 series aluminum alloy extruded material.

  The present invention further improves the pressure-resistant cracking resistance of the energy absorbing member without lowering the strength of the energy absorbing member made of the 7000 series aluminum alloy extruded material. The purpose of the present invention is to improve the energy absorptivity.

  The manufacturing method of the energy absorbing member according to the present invention is as follows: Zn: 5.5 to 7.0% by mass, Mg: 0.5 to 1.8% by mass, Cu: 0.1 to 0.5% by mass, Fe: 0.01 to 0.40% by mass, Si: 0.01 to 0.20% by mass, Ti: 0.005 to 0.2% by mass, and Zr: 0.01 to 0.25% by mass. Cr: 0.01 to 0.25% by mass, Mn: 0.01 to 0.25% by mass, at least one of which has a composition of Al and inevitable impurities, and is extruded and press-baked The extruded aluminum alloy material is heated at a heating rate of 10 ° C./sec or more within a range of 330 to 550 ° C., kept in the same range for more than 0 seconds and 5 minutes or less, and then from the same range. After performing a heat treatment for cooling at a cooling rate of 50 ° C./sec or more, an artificial aging treatment is performed.

In the present invention, press-quenched 7000 series aluminum alloy extruded material is heated at a heating rate of 10 ° C./sec or more within a range of 330 to 550 ° C., and is kept in the same range for more than 0 seconds and 5 minutes or less. Subsequently, after performing a heat treatment for cooling at a cooling rate of 50 ° C./sec or more from the same range, an aging treatment is performed. This aging processing includes overaging processing. Coarse MgZn ₂ precipitates are present at the crystal grain boundaries of the 7000 series aluminum alloy extruded material after press quenching, and the MgZn ₂ precipitates are resistant to pressure cracking of the energy absorbing member made of the 7000 series aluminum alloy extruded material. Lower. On the other hand, by performing the above-described heat treatment, the MgZn ₂ precipitate is dissolved again and disappears. Then, the MgZn ₂ precipitates even disappear, it does not affect the strength of the 7000 series aluminum alloy extruded product. As a result, according to the present invention, it is possible to further improve the pressure-resistant cracking resistance of the energy absorbing member without lowering the strength of the energy absorbing member, and thereby the energy absorbing member made of a 7000 series aluminum alloy extruded material Can improve the energy absorption.

FIG. 3A is a plan view of a test piece used in an example, and FIG. 3B is a side view illustrating a method of a U bending test. It is a figure showing other test methods for evaluating crush cracking nature.

Hereinafter, an energy absorbing member made of an extruded 7000-series aluminum alloy according to the present invention and a method for manufacturing the same will be specifically described.
(Composition of aluminum alloy)
First, the composition of the 7000 series aluminum alloy according to the present invention will be described. However, this composition itself is known as a 7000 series aluminum alloy.
Zn: 5.5 to 7.0 mass%
Mg: 0.5 to 1.8 mass%
Zn and Mg are elements that form MgZn ₂ which is an intermetallic compound to improve the strength of the 7000 series aluminum alloy. If the Zn content is less than 5.5% by mass or the Mg content is less than 0.5% by mass, the yield strength of 300 MPa or more required for an energy absorbing member cannot be obtained. On the other hand, if the Zn content exceeds 7.0% by mass or the Mg content exceeds 1.8% by mass, even if a predetermined heat treatment is performed after press quenching, the pressure cracking resistance cannot be improved, and the energy absorption Increase is not expected. From the viewpoint of improving the pressure crack resistance, the upper limits of the Zn content and the Mg content are preferably 6.5% by mass and 1.6% by mass, respectively.

Cu: 0.1 to 0.5% by mass
Cu is an element that improves the strength of the 7000 series aluminum alloy. If the Cu content is less than 0.1% by mass, there is no sufficient effect of improving the strength, while if it exceeds 0.5% by mass, the extrudability is lowered. The upper limit of the Cu content is preferably 0.4% by mass.
Ti: 0.005 to 0.2 mass%
Ti has an effect of refining crystal grains during casting of a 7000 series aluminum alloy to improve the formability (for example, bending workability) and pressure cracking resistance of an extruded material, and is added in an amount of 0.005% by mass or more. On the other hand, when the content exceeds 0.2% by mass, the effect is saturated, and a coarse intermetallic compound is crystallized, and the moldability is rather lowered.

Zr: 0.01 to 0.25% by mass
Cr: 0.01 to 0.25 mass%
Mn: 0.01 to 0.25 mass%
Mn, Cr and Zr have the effect of suppressing the recrystallization of the 7000 series aluminum alloy extruded material to make the crystal structure fine recrystallized or fibrous, and to improve the stress corrosion cracking resistance. . If the content of each of these elements is less than 0.01% by mass, the above effect is not sufficient. On the other hand, when the contents of Mn, Cr, and Zr each exceed 0.25% by mass, extrudability is reduced, quenching sensitivity is increased, and strength is reduced. In particular, when press quenching is performed by air cooling, the total content of two or more is preferably 0.4% by mass or less in order to prevent quenching sensitivity from increasing.

Inevitable impurities Fe and Si are main inevitable impurities of the 7000 series aluminum alloy, and are regulated to 0.40% by mass and 0.20% by mass, respectively, so as not to deteriorate various properties of the 7000 series aluminum alloy. On the other hand, reducing each of Fe and Si in the 7000 series aluminum alloy to less than 0.01% by mass imposes a heavy burden on costs. Therefore, the Fe content is 0.01 to 0.40% by mass, and the Si content is 0.01 to 0.20% by mass.
Inevitable impurities other than Fe and Si are set to 0.05% by mass or less in a simple substance, and 0.15% by mass or less in total.

(Method of manufacturing energy absorbing member)
First, a 7000 series aluminum alloy having the above composition is cast and homogenized, then hot-extruded, and press-quenched by water cooling or air cooling.
Press quenching may be either water cooling or air cooling. However, if press quenching is performed with water cooling, the aluminum alloy extruded material is distorted, it is difficult to ensure dimensional accuracy, and a straightening step is required. Therefore, it is preferable that the press hardening is performed by air cooling.

  Next, the aluminum alloy extruded material (hollow material) after the press quenching is cut into a predetermined length as necessary, and is heated at a heating rate of 10 ° C./sec or more within a range of 330 to 550 ° C. A heat treatment is performed in which the temperature is kept in the range for more than 0 second and not more than 5 minutes, and subsequently, the same range is cooled at a cooling rate of 50 ° C./second or more.

The reason why the heating rate is set to 10 ° C./sec or more in the heat treatment is to prevent coarse MgZn _{2 from} being precipitated in the temperature rising process. The reason why the holding temperature is set in the range of 330 to 550 ° C. is that MgZn ₂ does not re-dissolve at a temperature lower than 330 ° C., and local melting may occur at a temperature higher than 550 ° C. After the extruded material reaches this temperature range, even if the holding time within the temperature range is extremely short (time exceeding 0 seconds), the effect of re-solid solution can be obtained. Therefore, the extruded material may be cooled immediately after reaching the temperature range, or may be cooled after being kept in the same temperature range for a predetermined time. The upper limit of the holding time is not particularly limited, but is preferably 5 minutes or less from the viewpoint of production efficiency. The holding time is preferably within 20 seconds, more preferably within 10 seconds. As the heating means, for example, a high frequency induction heating device or a nitrite furnace can be used. The reason why the cooling rate is set to 50 ° C./sec or more in the heat treatment is to prevent MgZn ₂ from reprecipitating in the cooling process. Water cooling or air cooling can be used for cooling.

The above-mentioned high heating rate and cooling rate may be performed in a temperature range of 200 to 330 ° C., and the heating rate and cooling rate in a temperature range of less than 200 ° C. or more than 330 ° C. And cooling may be allowed to cool. However, from the viewpoint of heating and cooling efficiency, within a temperature range of less than 200 ° C. or more than 330 ° C., both the heating rate and the cooling rate are preferably 10 ° C./sec or more. More preferably, it is performed at a speed.
The heat treatment is performed on the entire or a part of the extruded aluminum alloy.

After the heat treatment, the aluminum alloy extruded material is subjected to an artificial aging treatment. Thereby, the energy absorbing member according to the present invention can be manufactured.
The conditions of the artificial aging treatment are not particularly limited, and can be performed under general aging treatment conditions. Alternatively, aging treatment (overaging treatment) can be performed under conditions of higher temperature and longer time than general aging treatment. Specific aging treatment conditions are, for example, in the range of 85 to 95 ° C. × 2 to 4 hours + 130 to 140 ° C. × 5 to 10 hours, or 85 to 95 ° C. × 2 to 4 hours + 160 to 180 ° C. × 5 to 10 hours. What is necessary is just to select suitably. The effects of improving the pressure-resistant cracking resistance and increasing the amount of energy absorption by the heat treatment can be obtained regardless of whether the subsequent artificial aging treatment is a general aging treatment or an overaging treatment.

Although the manufacturing method described above does not include plastic working on the aluminum alloy extruded material, as one of the embodiments of the present invention, all or part of the aluminum alloy extruded material (one along the length direction) is used. (Part region) is further subjected to plastic working.
When performing plastic working on a part of the aluminum alloy extruded material, it is preferable that the plastic working be performed after press quenching and before the heat treatment.
When plastic working is performed after the heat treatment, in the subsequent aging treatment, aging of the part subjected to plastic working is accelerated (overaging) as compared with the part not subjected to plastic working, and uniformity of strength over the entire length of the member is maintained. And the performance of the energy absorbing member may be impaired. Further, if plastic working is performed after aging treatment, cracking may occur due to high strength, or stress corrosion cracking resistance may be reduced due to residual stress generated by plastic working. Therefore, when plastic working is performed on a part of the extruded aluminum alloy, it is desirable to perform the plastic working after press quenching and before the heat treatment.

In the energy absorbing member manufactured by the above method, the heat treatment causes the coarse MgZn ₂ precipitate present at the crystal grain boundaries of the aluminum alloy extruded material after press quenching to be dissolved again and disappear. . When the coarse MgZn ₂ precipitate disappears from the crystal grain boundary, the pressure-resistant cracking resistance of the energy absorbing member is improved. On the other hand, even when the coarse MgZn ₂ precipitate disappears from the crystal grain boundary, the strength of the energy absorbing member is reduced. Does not drop. As a result, the energy absorption amount of the energy absorbing member can be increased.

  No. described in Table 1 A 7000 series aluminum alloy billet having a composition of 1 to 12 was homogenized at 470 ° C. for 6 hours and extruded at an extrusion temperature (billet temperature) of 470 ° C. and an extrusion speed of 5 m / min. After the extrusion, press hardening was performed by air cooling with a fan (cooling rate: about 50 ° C./min). The cross-sectional shape of the extruded material is a rectangle with a contour of 60 mm x 120 mm, consisting of two flanges (long side of the contour) and three webs (short side and center of the contour). All are 2.0 mm. This example shows one embodiment of the present invention, and the present invention is not limited to this.

This extruded material was subjected to a heat treatment under the conditions shown in Table 2 (only No. 12 was not used). In this heat treatment, the heating rate shown in Table 2 is the heating rate from room temperature (25 ° C.) to the holding temperature, the holding time is the holding time at the holding temperature, and the cooling rate is the cooling rate from the holding temperature to 200 ° C. (After that, it is cooled to room temperature). Therefore, in Table 2, the holding temperatures of 330 to 550 ° C. For 1 to 8 and 11, the time kept in the temperature range of 330 to 550 ° C. is slightly longer than the holding time shown in Table 2.
Subsequently, aging treatment was performed under the conditions shown in Table 2. No. The aging processes Nos. 1, 4, 9, and 10 are general aging processes. The aging processes 5 to 8, 11, and 12 correspond to overaging processes.

Using the extruded material after the aging treatment, measurement of proof stress and evaluation of crush cracking were performed in the following manner. Table 2 shows the results. In addition, No. Since the extruded material of No. 9 caused local melting, measurement of proof stress and evaluation of crush cracking were not performed. In addition, No. Reference numeral 12 corresponds to the conventional material described in Patent Documents 1 and 2.
(Measurement of proof stress)
A JIS No. 13 B test piece was sampled in parallel with the extrusion direction from a web (short side of the contour) of the extruded material, and a tensile test was performed according to JIS Z2241 to measure the proof stress.

(Evaluation of crush cracking properties)
A test piece 1 having a length of 50 mm and a width of 30 mm is cut out from the web (short side of the contour) of the extruded material so that the direction perpendicular to the extrusion direction is the length direction of the test piece, and is used as a test material. The crush cracking property was evaluated by a test. The shape of the test piece 1 is shown in FIG. The U-bending test uses a U-bending jig and a 30-ton universal testing machine, and places the test piece 1 on the support 2 as shown in FIG. This is a test by the push-bending method of pushing down to the maximum. In this embodiment, the test is performed in order from the one having the largest R (radius) at the tip of the pressing jig 3, and the minimum bending R (critical bending) at which the crack does not occur outside the bending corner (tensile side portion) of the test piece 1. R) was determined. The U-bending test was performed three times for each bending R (n = 3). When a crack occurred in the test piece even in one of the three times, it was determined that a crack occurred in the test piece in the bending R. It is shown that the smaller the critical bending R is, the less the crack is generated when a force is applied to the material.

  In addition, as a test method for evaluation of the crush cracking property, the above-mentioned U bending test, a local plate bending test at the time of axial compression shown in FIG. 2A, and a cross section of the bending center shown in FIG. A local plate bending test accompanying the change is considered. The test shown in FIG. 2A is a test for measuring the length (height) of the extruded material 4 when a crack is generated by compressively deforming the extruded material 4 cut into a predetermined length in the axial direction. FIG. 2A shows the shape of the extruded material 4 before and after deformation. The test shown in FIG. 2B is a test in which a three-point bending test is performed on the extruded material 5 cut to a predetermined length, and the amount of pushing of the pushing jig 6 when a crack occurs is measured. FIG. 2B shows the shape of the extruded material 5 before and after deformation together with a cross-sectional view of the bending center. Since the same evaluation result can be obtained by any of the above three test methods, the U-bending test was employed in this example.

  As shown in Table 2, No. 1 was heat-treated under the conditions specified in the present invention. Nos. 1 to 8 were not subjected to the heat treatment specified in the present invention. As compared with No. 12 (conventional material), the critical bending R is as small as 2/3 or less, and the proof stress of 300 MPa or more required as an energy absorbing member is obtained. On the other hand, in No. 3 in which the holding temperature of the heat treatment was lower than the conditions specified in the present invention. No. 10 and No. 10 in which the cooling rate of the heat treatment was lower than the conditions specified in the present invention. No. 11 is a conventional material. Although the critical bending R is slightly smaller than that of No. 12, It is much larger than 1-8.

1 Test piece 2 Support 3 Pushing jig

Claims (3)

Zn: 5.5 to 7.0% by mass, Mg: 0.5 to 1.8% by mass, Cu: 0.1 to 0.5% by mass, Fe: 0.01 to 0.40% by mass, Si: 0.01 to 0.20% by mass, Ti: 0.005 to 0.2% by mass, Zr: 0.01 to 0.25% by mass, Cr: 0.01 to 0.25% by mass, Mn: 0.01 to 0.25 and containing one or more of mass%, having the balance consisting of Al and unavoidable impurities, to an aluminum alloy extruded product after subjected to press quenching extrusion, the After subjecting a part to plastic working, it is heated within a range of 330 to 550 ° C. at a heating rate of 10 ° C./sec or more, kept within the same range for more than 0 seconds and 5 minutes or less, and subsequently 50 An energy absorbing section characterized by performing an artificial aging treatment after performing a heat treatment for cooling at a cooling rate of at least ° C / sec. The method of manufacturing the material. The method according to claim 1 , wherein the temperature range is 400 to 550C. 3. The method according to claim 1, wherein the aging treatment is an overaging treatment.

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