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United States Patent Application 20170204503
Kind Code A1
ARUGA; Yasuhiro ;   et al. July 20, 2017

ALUMINUM ALLOY STRUCTURAL PART, METHOD FOR PRODUCING THE SAME, AND ALUMINUM ALLOY SHEET

Abstract

Disclosed are a structural part obtained from a 6000 series aluminum alloy sheet as a shaping raw material and having an improved crash performance; and a method for producing the sheet. For the sheet, a 6000 series aluminum alloy sheet is used which has a specified composition and is produced in the usual way. Even when this sheet is used, strain is given at a high level thereto by a cold work, thereby heightening the average dislocation density of a surface of the resultant structural part, which has been artificially aged. This density is measured by X-ray diffraction. Thus, the structural part is improved in strength and in crash performance, which is estimated in a VDA bending test, when the automobile collides.


Inventors: ARUGA; Yasuhiro; (Kobe-shi, JP) ; MATSUMOTO; Katsushi; (Kobe-shi, JP) ; SATO; Kazufumi; (Kobe-shi, JP)
Applicant:
Name City State Country Type

Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.)

Kobe-shi

JP
Assignee: Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.)
Kobe-shi
JP

Family ID: 1000002365093
Appl. No.: 15/380011
Filed: December 15, 2016


Current U.S. Class: 1/1
Current CPC Class: C22F 1/047 20130101; C22C 21/08 20130101; B21B 2003/001 20130101; C22F 1/043 20130101; B21B 3/00 20130101; C22C 21/02 20130101
International Class: C22F 1/047 20060101 C22F001/047; B21B 3/00 20060101 B21B003/00; C22F 1/043 20060101 C22F001/043; C22C 21/08 20060101 C22C021/08; C22C 21/02 20060101 C22C021/02

Foreign Application Data

DateCodeApplication Number
Jan 14, 2016JP2016-005510

Claims



1. An aluminum alloy structural part excellent in crash performance, comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, the percent symbols each representing % by mass, and Al and inevitable impurities as the balance of the part; and the structural part having an average dislocation density of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the density being measured by X-ray diffraction of the surface.

2. The aluminum alloy structural part excellent in crash performance according to claim 1, further comprising Cu: 0.05 to 1.0%, the percent symbol representing % by mass; and the amount of solute Cu in a solution separated from the structural part by a residue extracting method with hot phenol being from 0.05 to 1.0% by mass of the solution.

3. The aluminum alloy structural part excellent in crash performance according to claim 1, further comprising one or more of the following: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02 to 0.15%, the percent symbols each representing % by mass.

4. The aluminum alloy structural part excellent in crash performance according to claim 1, further comprising one or more of the following: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%, the percent symbols each representing % by mass.

5. A method for producing an aluminum alloy structural part excellent in crash performance, comprising: applying homogenization to an aluminum alloy ingot comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, the percent symbols each representing % by mass, and Al and inevitable impurities as the balance of the ingot, and subsequently rolling the ingot into a sheet; subjecting the sheet further to solutionizing and quenching treatments, and subsequently cold-working the treated sheet to be formed into a structural part while giving a strain of 5 to 20% to the sheet; thereby adjusting the artificially aged structural part to have a dislocation density of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the density being measured by X-ray diffraction of the surface.

6. The method for producing an aluminum alloy structural part excellent in crash performance according to claim 5, wherein the aluminum alloy structural part further comprises Cu: 0.05 to 1.0%, the percent symbol representing % by mass; and the amount of solute Cu in a solution separated from the structural part by a residue extracting method with hot phenol is from 0.05 to 1.0% of the solution.

7. The method for producing an aluminum alloy structural part excellent in crash performance according to claim 5, wherein the aluminum alloy structural part further comprises one or more of the following: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02 to 0.15%, the percent symbols each representing % by mass.

8. The method for producing an aluminum alloy structural part excellent in crash performance according to claim 5, wherein the aluminum alloy structural part further comprises one or more of the following: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%, the percent symbols each representing % by mass.

9. The method for producing an aluminum alloy structural part excellent in crash performance according to claim 5, wherein the strain is given to the sheet when the sheet is formed into the structural part.

10. An aluminum alloy sheet excellent in crash performance, for a structural part, comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, the percent symbols each representing % by mass, and Al and inevitable impurities as the balance of the sheet; and the following sheet having, as a microstructure, an average dislocation density of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the density being measured by X-ray diffraction of the surface: a surface of the sheet which is obtained, for simulating use of the structural part, by subjecting the sheet to solutionizing treatment of keeping the sheet at 550.degree. C. for 30 seconds, water quenching the sheet immediately down to room temperature at an average cooing rate of 30.degree. C./s, subjecting the sheet, immediately after the quenching, to a pre-aging treatment at 100.degree. C. for 5 hours, giving a strain of 10%, after the treatment, to the sheet through a tensile tester and further aging the sheet artificially at 210.degree. C. for 30 minutes.

11. The aluminum alloy sheet excellent in crash performance according to claim 10, further comprising Cu: 0.05 to 1.0%, the percent symbol representing % by mass; and the amount of solute Cu in a solution separated from the aluminum alloy sheet by a residue extracting method with hot phenol being from 0.05 to 1.0% by mass of the solution.

12. The aluminum alloy sheet excellent in crash performance according to claim 10, further comprising one or more of the following: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02 to 0.15%, the percent symbols each representing % by mass.

13. The aluminum alloy sheet excellent in crash performance according to claim 10, comprising one or more of the following: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%, the percent symbols each representing % by mass.
Description



BACKGROUND OF THE INVENTION

[0001] Field of the Invention

[0002] The present invention relates to a structural part which is obtained from a 6000 series aluminum alloy sheet (rolled sheet) as a shaping raw material and is excellent in crash performance (shock absorption performance); a method for producing the member; and an aluminum alloy sheet.

[0003] Description of Related Art

[0004] In recent years, social needs of making automotive bodies lighter have been further increasing in light of concern for the global environment and others. In order to respond to the needs, instead of steel material for steel sheets or others that have been hitherto used, aluminum alloy material has been used in, out of automotive body parts, panels (outer panels such as a hood, doors and a roof, and inner panels), reinforcing material such as a bumper reinforcement (bumper R/F) and door beams, and other parts.

[0005] In order to make automotive bodies lighter, it becomes necessary to enlarge the application of aluminum alloy material to, out of automotive members, also automotive structural parts contributing particularly to the weight saving thereof, such as side members or such members, frames, and pillars. However, it is necessary to give these automotive structural parts a new crash performance (crash resistance or crash characteristic), which results in a further weight saving of raw sheets for the structural parts, in a high shock absorption performance of the members when the automotive body collides, and in the protection of vehicle passengers.

[0006] About high strength reinforcing materials out of automotive structural parts as described above, it has been already popular to use, as raw material thereof, extruded shapes each produced by hot-extruding an JIS or AA7000 series aluminum alloy. In the meantime, about large-sized structural parts such as frames or pillars, it is preferred to use, as raw material thereof, rolled sheets each produced by an ordinary method, such as a method of homogenizing an ingot, and then hot-rolling the workpiece, or optionally cold-rolling the hot-rolled workpiece further. However, the 7000 series aluminum alloy is not easily produced into any rolled sheet because of high-level alloying thereof. Thus, the rolled sheets have not been very much put into practical use.

[0007] For this reason, as alloys for rolled sheets that are produced by an ordinary rolling method (in the usual way), attention has been paid to JIS or AA6000 series aluminum alloys, which are Al--Mg--Si aluminum alloys produced easily because these alloys are lower-level alloying metals than the 7000 series alloys.

[0008] Sheets of the 6000 aluminum alloys have been already used as large-sized automotive body panels (outer panels such as hoods, fenders, doors, a roof and a trunk lid; and inner panels). Thus, in order for the alloy sheets to have both of press formability and BH response (bake hardenability), which are required for these large-sized automotive body panels, or to be improved in both the properties, many suggestions have been made about metallurgically remedial measures about, e.g., component composition or microstructure.

[0009] For reinforcing materials as described above and others, 6000 series aluminum alloy extruded shapes have been hitherto suggested and put into practical use. However, for automotive structural parts, few examples of an aluminum alloy rolled sheet have been suggested.

[0010] Only Patent Literature 1 (JP 2001-294965 A) and other literatures suggest 6000 series aluminum alloy sheets in which controls are made about the size and the aspect ratio of crystal grains that are related to an aluminum alloy rolled sheet microstructure, whereby after the sheets are artificially aged, the sheets are improved to have a yield strength of 230 MPa or more and are heightened in crash performance.

[0011] In the meantime, as is well known, about means for controlling the composition or the microstructure of a 6000 series aluminum alloy raw sheet in order to improve the formability or strength properties of this sheet for a panel as described above, many suggestions have been hitherto made about controls of the grain diameter of crystal grains, controls of the texture, and controls of clusters of atoms in the sheet.

[0012] These microstructure-controlling means also include various means of controlling the amount of Mg, Si or Cu solid-solutionized in the alloy sheet, and of controlling the dislocation density thereof.

[0013] For example, Patent Literature 2 (JP 2008-174797 A) suggests that in order to gain, for a panel, a 6000 series aluminum alloy sheet which is excellent in stability at ordinary temperature and is not be easily lowered in BH response and other material-qualities by natural aging at room temperature, the solute Si amount and the solute Mg amount therein are set into the range of 0.55 to 0.80% by mass and that of 0.35 to 0.60% by mass, respectively, and the ratio of the solute Si amount to the solute Mg amount is set into the range of 1.1 to 2.

[0014] Patent Literature 3 (JP 2008-266684 A) also suggests, for a panel as described above, a 6000 series aluminum alloy sheet that is for being warm-formed and is excellent in BH response in which the amount of solute Cu that is measured by a residue extracting method is set into the range of 0.01 to 0.7% and further the average crystal grain diameter is set into the range of 10 to 50 .mu.m.

[0015] Furthermore, Non-Patent Literature 1 (Journal of Japan Institute of Metals and Materials, vol. 75, No. 5 (2011), pp. 283-290, "Experimental and Computationally Scientific Research on Competitive Precipitation Observed in Al--Mg--Si alloy Having High Dislocation Density and Ultrafine Grain Microstructure" Tetsuya Masuda, Shoichi Hirosawa, Zenji Hotta, and Kenji Matsuda) suggests that the following are forecasted in order to make a 6000 series aluminum alloy sheet higher in strength: microstructural parameters (dislocation density and crystal grain diameter) for combining dislocation strengthening or crystal grain refinement strengthening optimally with precipitation strengthening.

[0016] This literature states that: about samples each obtained by subjecting a 6000 series aluminum alloy sheet to cold rolling, or HPT working, which is a high-pressure torsion method, the dislocation densities thereof have been inspected. As a result, samples not subjected to the working have a dislocation density of about 10.sup.11 m.sup.-2, and the samples cold-rolled at a rolling ratio of 30% (corresponding strain: 0.36) have a dislocation density of about 10.sup.14 m.sup.-2. Measurements of the dislocation densities are made by an equal thickness interference method in which 5 visual fields in a 100000-magnification TEM photograph of each of the samples are used in an intersection analysis manner.

[0017] According to this Non-Patent Literature 1, an inspection has been made about conventional technique reports each stating that when the microstructure of a 6000 series aluminum alloy sheet is controlled for dislocation strengthening or crystal grain refinement strengthening, the artificial age-hardenability of the sheet is frequently restrained in a subsequent artificial aging of the sheet, so it is difficult to attain consistency between the two strengthening mechanisms.

[0018] Results of the test demonstrate that: as the artificial aging period elapses, the non-worked alloy sheets and the cold-rolled alloy sheets are increased in hardness; about the non-worked alloy sheets, the value obtained by subtracting the hardness thereof before the artificial aging treatment from the peak hardness thereof is 75 HV while about the cold-rolled alloy sheets, the value is 43 HV, which has become reversely smaller; thus, the cold rolling makes the artificial age-hardenability low. The literature states that as the aging period elapses, the HPT alloy sheets are monotonously decreased in hardness not to exhibit an aging hardening behavior.

[0019] Structural parts of automobiles and others in which the present invention is to be used are required to have properties peculiar to the use, for example, are required to be further heightened in strength, and to be newly caused to have a shock absorption performance, that is, crash resistance when the automotive body collides.

[0020] For example, according to a matter that collision safety standards of automobiles have been raised (or made severer) in recent years, in Europe and others, structural parts of an automobile, such as frames and pillars, have been required to satisfy crash performance (crash resistance or shock absorption performance) when the automobile collides, this property being evaluated in a "VDA 238.100 plate bending test for metallic materials (hereinafter referred to as a VDA bending test)", which is standardized by Verband der Automobilindustrie e.V. (VDA).

[0021] Against such severe safety standards, structural parts of any automobile that are obtained from 6000 series aluminum alloy sheets as shaping raw materials, these sheets being produced by an ordinary rolling method, are short in the following property when the automobile collides: crash performance that has been obtained by making the alloy sheets higher in strength. As means for causing such automotive structural parts, which are obtained from 6000 series aluminum alloy sheets as shaping raw materials, to satisfy the crash performance, an effective means has not yet been clear even when the existence of the above-mentioned non-patent literature is known. Thus, there remains a room for realizing such a means.

SUMMARY OF THE INVENTION

[0022] In light of such a situation, an object of the present invention is to provide a structural part which is obtained from a 6000 series aluminum alloy sheet as a shaping raw material and is improved in crash performance; a method for producing the member; and an aluminum alloy sheet.

[0023] For attaining this object, a subject matter of the aluminum alloy structural part of the present invention excellent in crash performance is a member, comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, the percent symbols each representing % by mass, and Al and inevitable impurities as the balance of the member; and this structural part having an average dislocation density of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the density being measured by X-ray diffraction of the surface.

[0024] For attaining this object, a subject matter of the method of the invention for producing an aluminum alloy structural part excellent in crash performance is a method including: applying homogenization to an aluminum alloy ingot comprising Mg: 0.30 to 1.5%.COPYRGT., and Si: 0.50 to 1.5%, the percent symbols each representing % by mass, and Al and inevitable impurities as the balance of the ingot, and subsequently rolling the ingot into a sheet; subjecting this sheet further to solutionizing and quenching treatments, and subsequently cold-working the treated sheet to be formed into a structural part while giving a strain of 5 to 20% to the sheet; thereby adjusting the artificially aged structural part to have a dislocation density of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the density being measured by X-ray diffraction of the surface.

[0025] For attaining this object, a subject matter of the aluminum alloy sheet of the invention excellent in crash performance is a sheet for a structural part comprising Mg: 0.30 to 1.5%, and Si: 0.50 to 1.5%, the percent symbols each representing % by mass, and Al and inevitable impurities as the balance of the sheet; and the following sheet having, as a microstructure, an average dislocation density of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the density being measured by X-ray diffraction of the surface: a surface of a sheet which is obtained, for simulating use of the structural part, by subjecting the sheet to solutionizing treatment of keeping the sheet at 550.degree. C. for 30 seconds, water quenching the sheet immediately down to room temperature at an average cooing rate of 30.degree. C./s, subjecting the sheet, immediately after the quenching, to a pre-aging treatment at 100.degree. C. for 5 hours, giving a strain of 10%, after the treatment, to the sheet through a tensile tester and further aging the sheet artificially at 210.degree. C. for 30 minutes.

[0026] In the present invention, a cold work, such as press forming to be applied to a structural part of an automobile or some other, is used to give (add) a large quantity of strain beforehand to a raw sheet (rolled sheet) subjected to a tempering treatment, such as solutionizing treatment, thereby making a surface of the formed structural part higher in dislocation density than in the prior art.

[0027] Such a structural part, the dislocation density of which has been made high, is artificially aged to cause the finally obtained structural part, which is to be used, to exhibit a high crash performance of showing a high 0.2% yield strength of 250 MPa or more and a bending angle of 90.degree. or more according to a VDA bending test.

[0028] Detailed mechanism of the exhibition (reasons therefor) have not yet been clear. However, it is presumed that: the structural part surface is made higher in dislocation density level than any raw sheet or structural part used for an ordinary panel, thereby increasing remarkably an effect of preventing dislocation-movement when the structural part is collapsed to be deformed, for example, when the automobile undergoes a collision accident; thus, the structural part is improved in balance between strength and crash performance.

[0029] This effect is increased also by an increase in the Cu amount solid-solutionized in the structural part, so that this member is increased in solute strengthening level to be heightened in strength and be further restrained from undergoing dislocation-localization when collapsed to be deformed, thereby being also improved in crash performance.

[0030] The present invention makes it possible to improve a 6000 series aluminum alloy raw rolled sheet, which has already been standardized as a structural part, in crash performance without changing the composition and the production process of the rolled sheet largely and without lowering the formability and other properties of the aluminum alloy raw rolled sheet.

[0031] Moreover, by giving (adding) a large quantity of strain beforehand to the aluminum alloy raw rolled sheet when the sheet is subjected to press forming or some other cold work to be made into a structural part, the structural part can be heightened in dislocation density in a production process of the structural part without increasing the number of steps for the cold work.

[0032] Thus, the present invention makes it possible to apply a raw rolled sheet to a structural part that is a security member important for automobiles even when this sheet is a 6000 series aluminum alloy raw rolled sheet used for an ordinary panel.

BRIEF DESCRIPTION OF THE DRAWING

[0033] FIG. 1 is a perspective view illustrating a manner of a VDA bending test for evaluating the shock absorption performance of a metallic test specimen.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0034] A structural part referred to in connection with the present invention denotes a structural part having a relatively large thickness of about 2 to 10 mm to function as a skeleton of, in particular, a transporting machine such as an automobile or a railroad vehicle, and is strictly distinguished from any large-sized body panel having a relatively small thickness less than 2 mm, such as an outer or inner panel.

[0035] The dislocation density of a surface of the structural part, which is specified in the present invention, is decreased by an artificial aging treatment of the structural part, such as paint-baking treatment thereof.

[0036] Accordingly, in order to ensure the crash performance of the structural part when the member is used, the dislocation density stipulated in the present invention is specified about the structural part after the member is artificially aged.

[0037] An aluminum alloy raw sheet referred to in connection with the present invention denotes an aluminum alloy raw sheet that is a hot-rolled sheet, a cold-rolled sheet or any other rolled sheet subjected to a tempering (T4) treatment, such as solutionizing treatment or quenching treatment, and that is a sheet which has not yet been formed into an automotive structural part to be used. In the following description, aluminum may be represented also as Al. Hereinafter, embodiments of the present invention will be specifically described in accordance with each requirement of the invention.

Aluminum Alloy Composition:

[0038] Initially, a description is made about not only the chemical component composition of an aluminum alloy sheet in the present invention, but also reasons why elements to be used therein and the respective contents by percentage of the elements are limited. The percent symbol(s) for showing the content of each of the elements (each) represent(s) % by mass.

[0039] The chemical component composition of the aluminum alloy sheet in the present invention functions as a presupposition for a purpose that a structural part obtained finally by aging a 6000 series aluminum alloy artificially, or a raw sheet to which strain or heat treatment is given or applied in such a manner that this raw sheet simulates the structural part described just above can satisfy a specified dislocation density and further gain a required strength and crash performance, and can preferably have, together therewith, formability into structural parts.

[0040] From these viewpoints, the chemical component composition of the aluminum alloy sheet in the present invention is rendered a composition including Mg: 0.30 to 1.5% and Si: 0.50 to 1.5%, and Al and inevitable impurities as the balance of the composition.

[0041] In order to improve the sheet in strength, this composition may further optionally include Cu in a proportion of 0.05 to 1.0%, or solute Cu in a solution separated from the alloy sheet by a residue extracting method with hot phenol in a proportion of 0.05 to 1.0% of the solution.

[0042] Moreover, in order to improve the strength, the composition may optionally include one or more of the following: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and Cr: 0.02 to 0.15%.

[0043] Furthermore, in order to improve the strength, each of the above-mentioned compositions may optionally include one or more of the following: Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%.

[0044] The percent symbol(s) for showing the content of each of the elements (each) represent(s) % by mass.

Si: 0.50 to 1.5%

[0045] Si is combined with Mg to produce a Mg--Si precipitate, which contributes to solute strengthening, and an improvement in the strength of the structural part when the aluminum alloy is subjected to an artificial aging treatment such as paint-bake treatment, thereby exhibiting aging hardenability. Thus, this element is an element essential for causing this structural part to gain a strength (yield strength) necessary for automobiles and others.

[0046] If the Si content by percentage is too small, the solute Si amount is decreased before the paint-bake treatment (before the artificial aging treatment) so that the amount of the produced Mg--Si precipitate is insufficient. Thus, the structural part is remarkably lowered in BH response to be short in strength or crash performance.

[0047] In the meantime, if the Si content by percentage is too large, coarse crystallized and precipitated products are produced so that the aluminum alloy is lowered in ductility to be cracked when rolled. Thus, the Si content is set into a range from 0.50 to 1.5%, preferably from 0.70 to 1.5%.

Mg: 0.30 to 1.5%

[0048] Mg is combined with Si to produce a Mg--Si precipitate, which contributes to solute strengthening, and an improvement in the strength of the structural part when the aluminum alloy is subjected to an artificial aging treatment such as paint-bake treatment, thereby exhibiting aging hardenability. Thus, this element is an element essential for causing this structural part to gain a yield strength necessary for automobiles and others.

[0049] If the Mg content by percentage is too small, the solute Mg amount is decreased before the artificial aging treatment so that the quantity of the produced Mg--Si precipitate is insufficient. Thus, the structural part is remarkably lowered in BH response to be short in strength or crash performance.

[0050] In the meantime, if the Mg content by percentage is too large, the aluminum alloy easily undergoes the formation of shear zones therein when cold-rolled, to be cracked in the rolling. Thus, the Mg content is set into a range from 0.3 to 1.5%, preferably from 0.7 to 1.5%.COPYRGT..

Cu: 0.05 to 1.0%

[0051] Cu makes the structural part high in strength by solute strengthening, and further improves the crash performance by a restraint of dislocation-localization when the member is collapsed to be deformed. If the Cu content by percentage is too small, this advantageous effect is small. If the content is too large, the advantageous effect is saturated, and the corrosion resistance and others are conversely deteriorated. Thus, Cu is optionally incorporated in a range from 0.05 to 1.0% into the aluminum alloy.

Solute Cu Amount: 0.05 to 1.0%

[0052] When Cu is incorporated to be caused to ensure (exhibit) the strength-heightening or the crash-performance-improving effect by the solute strengthening of Cu, the solute Cu amount in a solution separated from the structural part by a residue extracting method with hot phenol is set into a range from 0.05 to 1.0% of the solution. As the solute Cu amount is larger, the structural part is made better in work hardenability, smaller in yield ratio and larger in elongation to be improved in crash performance.

[0053] If the solute Cu amount is less than 0.05% regardless of the Cu content by percentage, the advantageous effect thereof is insufficient. The upper limit of the solute Cu amount is substantially equal to that of the added amount of Cu.

Mn: 0.05 to 0.5%; Zr: 0.02 to 0.20%; and Cr: 0.02 to 0.15%

[0054] One or more of Mn, Zr and Cr may be optionally incorporated into the aluminum alloy, as the same advantageous-effect element(s) for making crystal grains in the ingot or raw sheet finer, to contribute to an improvement of the finally obtained structural part in strength.

[0055] These elements are present in a dispersed particle form to contribute to crystal grain refinement to produce also an advantageous effect of improving the raw sheet in formability. If the content by percentage of each of the elements is too small, the strength- or formability-improving effect based on the crystal grain refinement is insufficient. If the content is too large, coarse compound grains are produced to deteriorate the aluminum alloy in ductility.

[0056] Thus, when one or more of Mn, Zr and Cr are optionally incorporated, the incorporation is attained as follows: Mn: 0.05 to 0.5%, Zr: 0.02 to 0.20%, and/or Cr: 0.02 to 0.15%.

Ag: 0.01 to 0.2%, Sn: 0.001 to 0.1%, and Sc: 0.02 to 0.1%

[0057] One or more of Ag, Sn and Sc may be optionally incorporated into the aluminum alloy as the same advantageous-effect element(s) for improving the strength thereof.

[0058] Ag causes an aging precipitate contributing to an improvement of the structural part in strength to be densely and minutely precipitated by an artificial aging treatment after the raw sheet is formed into the structural part, thereby producing an advantageous effect of promoting the enhancement of the strength. Thus, as required, Ag is optionally incorporated. If the Ag content is less than 0.01%, the strength-improving effect is small. If the Ag content is too large, the aluminum alloy is conversely lowered in various properties such as rollability and weldability, and further the strength-improving advantageous effect is also saturated merely to increase costs. Thus, when Ag is optionally incorporated, the Ag content is set into a range from 0.01 to 0.2%.

[0059] Sn has an advantageous effect of restraining the production of clusters at room temperature to keep an excellent formability or workability of the raw sheet after a solutionizing/quenching treatments thereof, and further improving the strength when the sheet is subsequently subjected to an artificial aging treatment, such as paint-bake treatment. Sn is therefore an element essential for giving the structural part a yield strength and crash performance necessary for structural parts of automobiles. If the Sn content is less than 0.001%, the advantageous effects are small. If the content is more than 0.1%, the advantageous effects are saturated, and the aluminum alloy conversely undergoes hot brittleness to be remarkably deteriorated in hot workability (heat stretchability). Thus, when Sn is optionally incorporated, the Sn content is set into a range from 0.001 to 0.1%.

[0060] Sc makes crystal grains in the ingot and the finally obtained product fine to contribute to an improvement in the strength thereof. Moreover, Sc is dispersed in a dispersed particle form to contribute to crystal grain refinement to improve the raw sheet also in formability. If the Sc content by percentage is too small, these advantageous effects are short. If the content is too large, coarse compound grains are produced to deteriorate the aluminum alloy in ductility. Thus, when Sc is optionally incorporated, Sc is incorporated in a range from 0.02 to 0.1%.

Other Elements:

[0061] Elements other than the above-mentioned elements, for example, Ti, B, Fe, Zn and V are inevitable impurities. The aluminum alloy may contain each of these elements in a content range specified in the JIS Standard and others for 6000 series alloys.

Dislocation Density:

[0062] Under the presupposition of the above-mentioned alloy composition, about the microstructure of each surface (microstructure obtained by observing the surface) of the structural part subjected to an artificial aging treatment, or each surface of a raw sheet to which strain or heat treatment is given or applied in such a manner that the raw sheet simulates this structural part, the dislocation density measured by X-ray diffraction is set into a range from 3.0.times.10.sup.14 to 8.0.times.1024 m.sup.-2, preferably from 4.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2 on average.

[0063] About the surface of the artificially aged structural part, or the surface of the raw sheet to which the strain or heat treatment is given or applied in such a manner that the raw sheet simulates this structural part, the dislocation density is set into the specified range, whereby the structural part can have a 0.2% yield strength of 250 MPa or more, and further have such a crash performance that a bending angle of 90.degree. or more is obtained according to a VDA bending test thereof.

[0064] It can be presumed that: the dislocation density level of the structural part surface is made higher than that of any surface of a raw sheet for an ordinary panel, or a panel obtained from this raw sheet as raw material, whereby the structural part produces a remarkably increased effect of hindering dislocation-movement when collapsed to be deformed, for example, when the automobile meets with a collision accident; and thus, the member is improved in balance between strength and crash performance.

[0065] In connection with this point, if the dislocation density is less than 3.0.times.10.sup.14 m.sup.-2 to be too small, the structural part is equivalent to conventional members (such as panels) not to exhibit required strength and crash performance.

[0066] In the meantime, if the dislocation density exceeds 8.0.times.10.sup.14 m.sup.-2 on average to be too large, the aluminum alloy is lowered in elongation to be conversely lowered in crash performance.

[0067] In the present invention, when a raw sheet (rolled sheet) subjected to a tempering treatment, such as solutionizing treatment, is subjected to a cold work, such as press forming, so as to be made into a structural part, or before or after this press forming, the sheet is further cold-worked to add (give) strain beforehand to the structural part. In this way, the dislocation density of any surface of the formed structural part is heightened into the above-mentioned specified range.

[0068] For reference, when an aluminum alloy raw sheet is made into a panel by an ordinary press forming, strain given thereto is generally a small value less than 5%. Consequently, the dislocation density of any surface of a structural part obtained from the raw sheet cannot be adjusted into the range specified in the present invention after the member is artificially aged.

[0069] Considering conditions for an artificial aging treatment such as an ordinary paint-baking treatment at high temperature for a short period, in order to set the dislocation density of the structural part surface into the range specified in the present invention, it is necessary to set, into 5% or more, preferably 10% or more, the strain given to the structural part by press forming into this structural part, or the strain given thereto by a combination of the press forming with cold work performed before or after the press forming.

[0070] However, if the given strain is more than 20%, the dislocation density of the structural part surface may exceed 8.0.times.10.sup.14 m.sup.-2 on average to become too large under conditions for an artificial aging treatment of the structural part, such as an ordinary paint-baking treatment thereof at high temperature for a short period. Thus, the structural part may be lowered in elongation to be conversely lowered in crash performance.

[0071] Considering that strain is given to the raw sheet when the sheet is press-formed into a structural part, it is actually difficult to give strain or dislocation in a quantity more than described in the above.

[0072] Accordingly, in order to control the average dislocation density into the range specified in the present invention under conditions for the artificial aging treatment, such as the high-temperature and short-period ordinary paint-baking treatment, it is preferred to select an optimal strain to be given from a range from 5 to 20%, preferably from 10 to 20% while the artificial aging conditions are also considered.

[0073] For reference, the Non-Patent Literature 1 suggests dislocation (density) strengthening for making a 6000 series aluminum alloy sheet higher in strength. The 6000 series aluminum alloy sheet is artificially aged through cold rolling, or HPT working, which is a high-pressure torsion method.

[0074] However, as described in the conventional technique reports, the test results support only the fact that even when the 6000 series aluminum alloy sheet is dislocation-strengthened (increased in dislocation density), the sheet is restrained from having aging-hardenability while subsequently artificially aged. The literature never describes any relationship between this dislocation density and the crash performance of the structural part obtained by formation of the sheet and artificial aging treatment thereof.

[0075] The microstructure or mechanical properties of this (original) structural part subjected to the artificial aging treatment can be evaluated by examining microstructure or mechanical properties of the following even when a raw sheet therefor is actually subjected to formation into a structural part followed by an artificial aging treatment: the microstructure and mechanical properties of a product obtained, for simulating this original structural part, by subjecting a 6000 series aluminum alloy raw sheet subjected to tempering treatments, such as solutionizing and quenching treatment, to a cold work for giving strain thereto, such as press forming, followed by an artificial aging treatment.

[0076] About preferred treatment conditions for simulating this original structural part, in order to simulate a specific usage of the original structural part, a 6000 series raw aluminum alloy sheet is subjected to solutionizing treatment at a temperature selected from the range of a temperature of 550.degree. C. to the melt temperature of the sheet (both inclusive), this range being one out of preferred producing conditions that will be also described later, for about 0.1 sec to several tens of seconds in a continuous furnace, or for about several tens of minutes in a batch furnace; immediately, the sheet is rapidly cooled to room temperature at an average cooling rate of 20.degree. C./sec or more and, is subjected, immediately after the cooling, to a pre-aging treatment of keeping the sheet at 60 to 120.degree. C. for 2 to 10 hours; and then a strain of 10 to 20% is given to the resultant sheet through a tensile tester, and subsequently the sheet is further artificially aged at 210 to 270.degree. C. for 10 to 30 minutes. By examining the microstructure and mechanical properties of the product obtained in this way, the original structural part can be evaluated with a high correlation and a good reproducibility.

[0077] In order to make this reproducibility stricter in the aluminum alloy sheet of the present invention, specific treatment conditions for simulating the original structural part are rendered one-point conditions of subjecting an aluminum alloy sheet to a solutionizing treatment of keeping the sheet at 550.degree. C. in a batch furnace for 30 seconds; immediately water quenching the sheet to room temperature at an average cooling rate of 30.degree. C./sec; subjecting the sheet, immediately after the quenching, to a pre-aging treatment at 100.degree. C. for 5 hours; and subsequently giving the resultant sheet a strain of 10% through a tensile tester, and further aging this sheet artificially at 210.degree. C. for 30 minutes.

Method for Measuring Dislocation Density

[0078] As described in the Non-Patent Literature 1 and others, it is widely used to measure the dislocation density of, e.g., a metallic sheet through, e.g., a transmission electron microscope. In the present invention, the dislocation density is more easily measured with a better reproducibility by X-ray diffraction. Regions (cell walls and shear zones) where linear and streak dislocations, out of dislocations, gather densely are not easily discriminated through any transmission electron microscope, so that the regions cause measurement accidental errors when the dislocation density p is analyzed and gained. In contrast, X-ray diffraction produces an advantage that even such dislocations gathering in large numbers cause only small accidental errors since the dislocation densities p are calculated out from the respective half-value widths of diffraction peaks obtained from individual surfaces of the texture, as will be detailed later.

[0079] In the microstructure of a sheet into which dislocations are introduced by adding plastic deformations to the sheet by, e.g., cold rolling or a tensile test, lattice strains are generated to centralize the dislocations. Moreover, by the arrangement of the dislocations, low-angle grain boundaries, cell structures, and others are developed. When such dislocations, or domain structures following the dislocations are caught from the resultant X-ray diffraction pattern, in accordance with diffraction indexes thereof characteristic breadths and shapes make their appearance in the diffraction peaks of the pattern. When the shapes (line profile) of the diffraction peaks are analyzed (line profile analysis), the dislocation density can be gained.

[0080] Specifically, from a structural part artificially aged or a raw sheet simulating this structural part, a sample is collected in such a manner that surfaces of the member or sheet are to be observing surfaces. Microstructures of the surfaces of the sample are then subjected to X-ray diffraction. The half-value width of a diffraction peak of each of the (111), (200), (220), (311), (400), (331), (420) and (422) planes (orientation planes), which are main orientations of the texture of the surfaces of the structural part, are gained.

[0081] As the dislocation density p is higher, the half-value width of the diffraction peak of each of these planes is larger. The surfaces of the structural part as the sample, which are targets to be measured by X-ray diffraction, may be in the state of the surfaces of the sample not subjected to any further treatment, or may be washed without being etched.

[0082] Next, from the respective half-value widths of the diffraction peaks of these individual surfaces, the lattice strain (crystal strain) a is gained by the Williamson-Hall method. Furthermore, the dislocation density p of the sample can be calculated out in accordance with the following expression:

.rho.=16.1.epsilon..sup.2/b.sup.2

wherein .rho. represents the dislocation density; .epsilon., the lattice strain of the sample; and b, the magnitude of Burger's vector.

[0083] As the magnitude of the Burger's vector, 2.8635.times.10.sup.-10 m is used.

[0084] The Williamson-Hall method is a known line profile analyzing method used widely to gain the dislocation densities or crystal grain diameters of a metal sample from a relationship between respective half-value widths of plural diffraction peaks of the sample, and the diffraction angles thereof. Known is also a series of manners for gaining the dislocation densities by X-ray diffraction. The dislocation densities obtained by the manner series for gaining the dislocation densities by X-ray diffraction are generically named the "dislocation density measured by X-ray diffraction" in the present invention.

[0085] About the dislocation density of any structural part, 10 samples collected from arbitrarily-selected sites of the structural part are measured, and the resultant dislocation densities are averaged.

Producing Method:

[0086] The following describes a preferred method for producing the structural part of the present invention. Initially, a preferred method for producing a raw rolled sheet is described hereinafter in the order of steps thereof.

[0087] A 6000 series aluminum alloy sheet which is a raw material for the structural part is a hot-rolled sheet obtained by subjecting an ingot to homogenization followed by hot rolling, or a cold-rolled sheet obtained by subjecting the hot-rolled sheet to cold rolling, and is produced in a usual manner of subjecting the hot-rolled sheet or cold-rolled sheet further to a tempering treatment such as solutionizing treatment. Specifically, the raw material is an aluminum alloy hot-rolled sheet produced through ordinary individual producing steps composed of casting, homogenization and hot rolling, and having a sheet thickness of about 2 to 4 mm; or a cold-rolled sheet obtained by cold-rolling a hot-rolled sheet having a larger thickness and produced through the same steps into a thickness of about 2 to 4 mm.

[0088] The 6000 series aluminum alloy sheet in the present invention may be produced by an especial producing method or rolling method in which after continuous casting into a thin sheet in, e.g., a twin roll manner, the thin sheet is cold-rolled with an omission of any hot rolling, or is warm-rolled.

[0089] Accordingly, the producing method has an advantage that a raw sheet can be produced without making a large change of 6000 series aluminum alloy compositions standardized already for structural parts as described above, and without making a large change of a rolling step in the usual way.

Melting, and Casting:

[0090] A raw alloy is initially molten and cast. In the melting and casting steps, a molten aluminum alloy adjusted into the above-mentioned 6000 series component composition range, as the molten raw alloy, is cast by an appropriately selected ordinary melting and casting method such as a continuous casting method or a semi-continuous casting method (DC casting method).

Homogenization:

[0091] Next, the cast aluminum alloy ingot is subjected to homogenization in the usual way before subjected to hot rolling. A purpose of this homogenization is to homogenize the microstructure, that is, to remove segregation inside crystal grains in the ingot microstructure. Conditions for this homogenization are appropriately selected from the range of 500.degree. C. or higher and less than the melting point of the alloy, and the holding period range of 2 hours or longer.

Hot Rolling:

[0092] The sheet is then hot-rolled. Under a condition that the starting temperature of the hot rolling is higher than the solid phase line temperature of the alloy, burning is caused not to conduct the hot rolling itself easily. If the hot rolling starting temperature is lower than 350.degree. C., an excessively high load is generated in the hot rolling not to conduct the hot rolling itself easily. Thus, the hot rolling is performed at a hot rolling starting temperature selected from the range of 350.degree. C. to the solid phase line temperature to produce a hot-rolled sheet having a thickness of about 2 to 10 mm. This hot-rolled sheet is not necessarily annealed before cold-rolled; however, the sheet may be annealed.

[0093] The hot rolling of the ingot subjected to the homogenization is composed of a rough rolling step of the ingot (slab) and a finish rolling step. In these rough and finish rolling steps, a rolling machine of, e.g., a reverse type or a tandem type is appropriately used.

[0094] During the hot rolling from the start of the hot rough rolling to the end thereof, it is preferred to keep the solute amounts of Si and Mg surely without lowering the temperature to 450.degree. C. or lower. If the lowest temperature of the rough rolled sheet in the middle of the rolling path is lowered to 450.degree. C. or lower, for example, by making the rolling period long, one or more compound precipitate easily. Thus, even when strain is given thereto before the sheet is artificially aged, the dislocation density may not be sufficiently increased. Moreover, the possibility is great that the solute Cu amount is also lowered.

[0095] After the hot rough rolling, the sheet is subjected to hot finish rolling the end temperature of which is preferably set into the range of 300 to 360.degree. C. If the end temperature of the hot finish rolling is lower than 300.degree. C. to be too low, the rolling load becomes high to lower the producing performance of this method. In the meantime, in the case of heightening the end temperature of the hot finish rolling to make the alloy into a recrystallized phase without leaving a large quantity of the deformed microstructure, coarse transition-element-dispersed particles may probably precipitate if the end temperature is higher than 360.degree. C.

[0096] From the temperature of the material (sheet) just after the end of the hot finish rolling to a material temperature of 150.degree. C., the average cooling rate is controlled into at lowest 5.degree. C./hour or more by forcible cooling using, e.g., fans. If this average cooling rate is less than 5.degree. C./hour, the quantity of a precipitate produced during the cooling becomes large. Thus, even when strain is given to the sheet before the sheet is artificially aged, the dislocation density does not increase sufficiently. Moreover, the solute Cu amount is decreased in the resultant product sheet.

[0097] It is therefore preferred that the average cooling rate is larger just after the end of the hot finish rolling. The rate is set to at lowest 5.degree. C./hour or more, preferably to 8.degree. C./hour.

[0098] For reference, according to any ordinary hot finish rolling, after this rolling, the resultant sheet is wound into the form of a coil. Thus, when the coil diameter is an ordinary diameter, the average cooling rate according to natural cooling just after the end of the hot finish roll easily turns into less than 5.degree. C./hour as far as the rolled sheet is not forcibly cooled by, e.g., fans.

[0099] When the resultant hot-rolled sheet is further cold-rolled, the sheet does not need to be annealed before the cold rolling. However, the annealing may be performed.

Cold Rolling:

[0100] The sheet is then cold-rolled. In the cold rolling, the hot-rolled sheet is cold-rolled to produce a cold-rolled sheet (that may be in the form of a coil) having a desired final thickness. In order to make the crystal grain finer, the cold rolling ratio is desirably 30% or more. Moreover, to attain the same purpose that the above-mentioned annealing does, the hot-rolled sheet may be subjected to intermediate annealing in the middle of the cold rolling path.

Solutionizing and Quenching Treatments:

[0101] After the cold rolling, the rolled sheet is subjected to solutionizing treatment followed by quenching treatment down to room temperature. For the solutionizing and quenching treatments, an ordinary continuous heat treatment line may be used. In order for the treated sheet to gain a sufficient solute amount of each of Mg, Si, and other elements, it is preferred to perform the solutionizing treatment at the molten temperature of the sheet or lower, followed by the quenching down to room temperature at an average cooling rate of 20.degree. C./second or more. If the solutionizing treatment temperature is lower than 550.degree. C., compounds of Mg--Si and others that have been produced before this solutionizing treatment are insufficiently re-solid-solutionized so that the solute amounts of Mg and Si are lowered.

[0102] If the average cooling rate is less than 20.degree. C./second, the possibility becomes great that during the cooling, Mg--Si precipitates are produced to lower the solute amounts of Mg and Si so that sufficient solute amounts of Mg and Si cannot be ensured. In order to ensure the cooling rate in the quenching, a cooling means is selected from fans and other air-cooling means or manners, and mist, spraying, immersion and other water-cooling means or manners, as well as cooling conditions are selected.

Pre-Aging Treatment: Reheating Treatment:

[0103] After the solutionizing treatment followed by the quenching treatment, resulting in the cooling of the sheet to room temperature in this way, it is preferred to subject the cold-rolled sheet to pre-aging treatment (reheating treatment) within one hour of the cooling end. If the room temperature holding period from the end of the quenching down to room temperature to the pre-aging start (heating start) is too long, Mg--Si clusters rich in Si amount are unfavorably produced by natural aging at room temperature, so that an increase cannot easily be made in the amount of Mg--Si clusters good in balance between the Mg and Si proportions. It is therefore more preferred that this room temperature holding period is shorter. The solutionizing and quenching treatments, and the reheating treatment may be continuously conducted without having any time lag substantially therebetween. The lower limit of the period is not particularly specified.

[0104] About this pre-aging treatment, a holding period at 60 to 120.degree. C. is adjusted preferably into the range of 2 to 40 hours both inclusive. In this case, Mg--Si clusters good in balance between the Mg and Si proportions are produced.

[0105] If the pre-aging temperature is lower than 60.degree. C., or the holding period is shorter than 2 hours, the same results as in the case of not conducting this pre-aging are produced to restrain the production of Mg--Si clusters rich in Si amount. Thus, the Mg--Si clusters good in balance between the Mg and Si proportions are not easily increased in quantity, so that the alloy sheet is easily lowered in yield strength after paint-baked.

[0106] In the meantime, if the pre-aging temperature is higher than 120.degree. C., or the holding period is longer than 40 hours, the quantity of produced precipitation nuclei is too large so that the alloy sheet is too high in strength when subjected to bending work before the bake-painting. Thus, the sheet is easily deteriorated in bendability.

Production of Structural Part

Strain Addition:

[0107] The raw sheet subjected to these tempering treatments (T4) is formed into a product, such as a side member or such a member, a frame, a pillar or any other structural part, mainly by press forming.

[0108] At this time, the raw sheet is formed into a structural part while a strain of 5 to 20% is given thereto by cold work. In addition thereto, the resultant structural part is artificially aged, thereby making it possible to set, into the range of 3.0.times.10.sup.14 or 8.0.times.10.sup.14 m.sup.-2, the dislocation density of any surface of the structural part subjected to the artificial aging treatment. This density is measured by X-ray diffraction.

[0109] At this time, it is allowable before the artificial aging treatment to apply a cold work for giving the above-mentioned strain beforehand to the raw sheet when the raw sheet is press-formed into the structural part without conducting cold work in a separate step.

[0110] In accordance with the shape of the structural part, the strain may be added thereto by not only the press forming but also a method or means for the cold work, such as tension, cold rolling, a leveler or stretch. In this case, the total strain added by the press forming and the cold work is adjusted into the above-mentioned range.

[0111] The strain is made larger than that added at the time of press forming for producing, e.g., an automotive panel in the usual way, and then the strain is beforehand added (given) to the alloy sheet before the sheet is artificially aged. In order to adjust the average dislocation density into the above-mentioned range of 3.0.times.10.sup.14 to 8.0.times.10.sup.14 m.sup.-2, the strain is added thereto in a proportion of 5% or more, preferably of from 10 to 20% both inclusive.

[0112] As described above, if the strain is less than 5%, this strain, which depends on the artificial aging treatment conditions, is not largely different from that given when any conventional press forming or bending working is conducted. Thus, the average dislocation density cannot be adjusted to 3.0.times.10.sup.14 m.sup.-2 or more.

[0113] As the strain is larger, the average dislocation density can be made larger. However, if the strain is more than 20%, the average dislocation density exceeds 8.0.times.10.sup.14 m.sup.-2 so that the resultant structural part is remarkably lowered in elongation to be poor in crash performance.

Artificial Aging Treatment:

[0114] The artificial aging treatment of the strain-given structural part or a sheet to which a strain of 5 to 20% is given to simulate this structural part may be conducted by paint-bake treatment or an ordinary artificial aging treatment (T6 or T7).

[0115] Conditions for the heating temperature and the holding period are freely decided in accordance with, e.g., a desired strength of the structural part or the natural-aging-advancing degree thereof at room temperature. When the artificial aging treatment is, for example, a one-stage treatment, the artificial aging treatment is conducted preferably at a heating temperature of 200 to 270.degree. C. for a holding period of 5 to 30 minutes.

[0116] If the heating temperature is too low or the holding period is too short, the structural part may undergo insufficient aging hardening not to have a strength or crash performance that is a target of the present invention. Also if the heating temperature is too high or the holding period is too long, the structural part may undergo over-aging not to have a strength or crash performance that is a target of the present invention.

EXAMPLES

[0117] In each of invention examples and comparative examples, as shown in Table 2 described below, one out of variously changed strains was added to a cold-rolled sheet of a tempering-applied 6000 series aluminum alloy having one out of component compositions shown in Table 1 to simulate the structural part concerned. The resultant was artificially aged, and then measurements and evaluations were made about the microstructure of any surface of this artificially-aged test material (the average dislocation density thereof and the solute amount of Cu therein), and the strength and the crash performance evaluated in a VDA bending test. The results are shown in Table 2. In the indication of the content by percentage of each element in Table 1, the symbol "-" shown in a numerical value cell about the element denotes that the content by percentage is not more than the limit of detection.

[0118] The above-mentioned cold-rolled sheet as a raw sheet was specifically produced as follows:

[0119] An aluminum alloy ingot having the composition shown in Table 1 was molten and cast into an ingot by a DC casting method. Subsequently, the ingot was subjected to homogenization at a temperature-raising rate of 150.degree. C./hour and at a homogenization temperature of 550.degree. C. for a holding period of 3 hours.

[0120] Thereafter, hot rough rolling of the workpiece was started at 500 to 520.degree. C., and the lowest temperature of the hot rough rolling was set to one out of variously changed temperatures shown in Table 2. Furthermore, the workpiece was subjected to hot finish rolling the end temperature of which was set into the range of 300 to 350.degree. C. to produce a hot-rolled sheet of 4.0 mm thickness.

[0121] At this time, the average cooling rate (.degree. C./hour) from the material (sheet) temperature just after the end of the hot finish rolling to a material temperature of 150.degree. C. was set to one out of variously changed values as shown in Table 2.

[0122] This hot-rolled sheet was cold-rolled at a rolling ratio of 50% without being subjected to heat treatment after the hot rolling and intermediate annealing in the middle of the cold rolling path. Thus, the cold-rolled sheet was obtained, which had a thickness of 2.0 mm.

[0123] Furthermore, the respective cold-rolled sheets in the examples were subjected to a tempering treatment (T4) under conditions common to the examples in heat treatment facilities. Specifically, the sheets were each subjected to solutionizing treatment at 550.degree. C. for a holding period of 30 seconds. At this time, the average heating rate up to the solutionizing treatment temperature was set to 50.degree. C./second. After the solutionizing treatment, at an average cooling rate of 30.degree. C./second, the workpiece was subjected to water quenching down to room temperature. Just after the quenching, the workpiece was subjected to pre-aging treatment at 100.degree. C. for a holding period of 5 hours. After the pre-aging treatment, the workpiece was gradually (naturally) cooled to yield a T4 material.

[0124] In each of the examples, from the T4 material, a #5 tensile test specimen (25 mm.times.25 mm GL.times.sheet thickness) according to a JIS Z 2201 was collected. In order to simulate the addition of strain to the T4 material when the material is formed into the structural part concerned, one out of variously changed pre strains was added to the #5 test specimen in a tensile test that will be detailed later. The strain-added #5 test specimen was artificially aged under conditions shown in Table 2 to make a tensile test. Thereafter, from this test specimen, sheet-form test specimens each having a required size were cut out, and then evaluated about the solute amount of Cu therein, and the dislocation density and the shock absorption performance thereof as follows:

Measurement of Solute Amount of Cu:

[0125] In a measurement of the solute amount of Cu, one of the above-mentioned sheet-form test specimens, which was a target to be measured, was dissolved by a residue extracting method with hot phenol. The resultant solid and solution were filtrated and separated from each other through a filter having a mesh (particle catching diameter) of 0.1 .mu.m. The Cu content by percentage in the separated solution was measured as the solute Cu amount.

[0126] This residue extracting method with hot phenol was specifically performed as follows: Initially, phenol was put into a decomposing flask and then heated. The sheet-form test specimen to be measured was transferred into the decomposing flask to be heated and decomposed. Next, benzyl alcohol was added thereto, and then the content was filtrated under reduced pressure to be separated into a solid and a solution. The Cu content by percentage in the separated solution was quantitatively determined.

[0127] This quantitative determination appropriately made use of, e.g., atomic absorption spectrophotometry (AAS), or inductively coupled plasma emission spectrometry (ICP-OES). As described above, for the filtration under reduced pressure, a membrane filter was used which had a mesh of 0.1 .mu.m and a diameter of 47 mm.

[0128] The measurement and calculation were made about three samples collected from arbitrarily-selected three sites of the sheet-form test specimen. The respective solute amounts (% by mass) of Cu in these samples were averaged and the resultant value was defined as the solute Cu amount.

Measurement of Dislocation Density:

[0129] A surface of one of the sheet-form test specimens was caused to simulate a surface of the structural part concerned, and the dislocation density (.times.10.sup.14 m.sup.-2) of the sheet-form test specimen surface was measured by X-ray diffraction under the above-mentioned conditions. The measurement was made about arbitrarily-selected 5 sites of the sheet-form test specimen. The respective dislocation densities of these sites were averaged and the resultant value was defined as the average dislocation density (.times.10.sup.14 m.sup.-2).

Tensile Test:

[0130] The artificially-aged #5 tensile test specimen was used to make a tensile test at room temperature. At this time, the tensile direction of the test specimen was made parallel to the rolling direction. The test was made at room temperature, 20.degree. C., on the basis of JIS Z 2241 (1980) at a supporting point distance of 50 mm and a constant tensile speed of 5 mm/minute until the test specimen was broken. If the test specimen had a 0.2% yield strength of 250 MPa or more, the specimen was judged to be acceptable as an artificially-aged structural part.

Shock Absorption Performance:

[0131] A bending test for evaluating shock absorption performance was made in accordance with the following VDA bending test: "VDA 238-100 plate bending test for metallic materials", which is standardized by Verband der Automobilindustrie e.V. (VDA). This test method is illustrated in FIG. 1 as a perspective view.

[0132] As represented by dot lines in FIG. 1, initially, one of the sheet-form test specimens is put onto two rolls arranged in parallel to each other and having a roll gap therebetween, so as to make right and left parts of the specimen equal in length to each other and be horizontally stretched.

[0133] Specifically, the sheet-form test specimen is put onto the two rolls, so as to make right and left parts of the specimen equal in length to each other and be horizontally stretched in such a manner that the rolling direction of the specimen is made perpendicular to the extending direction of a plate-form pushing/bending member arranged to stand upward and vertically, and that the center of the specimen is positioned at the center of the narrow roll gap.

[0134] The pushing/bending member is pushed from the above onto the center of the sheet-form test specimen to apply a load thereto. In this way, this sheet-form test specimen is pushed (thrusted) toward the narrow roll gap to be bent. Thus, the bent and deformed center of the sheet-form test specimen is pushed into the narrow roll gap.

[0135] When the load F from the above through the pushing/bending member turns maximum in this case, the angle of the outside of the bent center of the sheet-form test specimen is measured as the bending angle (.degree.) of the specimen. In accordance with the value of the bending angle, the shock absorbance performance is evaluated. As this bending angle is larger, the sheet-form test specimen is higher in shock absorbance performance (crash performance) to continue to have the bending deformation without being collapsed in the middle.

[0136] Test conditions of this VDA bending test are shown hereinafter, using symbols described in FIG. 1. The sheet-form test specimen is made into a square shape having a width "b" of 60 mm and a length "1" of 60 mm. The diameter D of each of the two rolls is set to 30 mm; and the roll gap L, to 4 mm, which is two times the sheet thickness of the sheet-form test specimen. The symbol S represents the pushed-depth of the center of the sheet-form test specimen into the roll gap when the load F becomes maximum.

[0137] As illustrated in FIG. 1, about the plate-form pushing/bending member, a side of the member at the low-end-side thereof, which is to be pushed onto the center of the sheet-form test specimen, is made into a tapered form such that the tip (lower end) thereof has a radius of 0.2 mm.

[0138] In each of the examples, the VDA bending test was made about three of the sheet-form test specimens (made three times). The average thereof was used as the bending angle (.degree.) of the example. The results are shown in Table 2.

[0139] As is evident from Table 2, the above-specified preferred strain or artificial aging treatment is applied to each of Invention Examples 1 to 13, which make use of respective aluminum alloys represented by alloy numbers 1 to 10 (within the composition range in the present invention) in Table 1.

[0140] Thus, these examples are in an artificially aged sheet state which simulates the structural part concerned, and satisfy the average dislocation density specified in the invention.

[0141] As a result, also about the crash performance evaluated in the VDA bending test, the bending angle is 90.degree. or more to satisfy an excellent property required for the structural part. Moreover, the 0.2% yield strength thereof is also a high strength of 250 MPa or more to satisfy a property required for the structural part.

[0142] In contrast, about each of the comparative examples, the alloy composition thereof is out of the range in the present invention, or the strain requirement thereof is out of the preferred range although the alloy composition is within the range in the invention.

[0143] Thus, each of the comparative examples does not satisfy the average dislocation density specified in the invention.

[0144] As a result, about the comparative example, the 0.2% yield strength or the crash performance evaluated in the VDA bending test is poorer than that about the invention examples not to satisfy a property required for the structural part.

[0145] About Comparative Examples 14 to 20, the alloy composition is within the range in the present invention as shown about alloy number 2 or 5 in Table 1, but producing conditions for their raw sheet are out of the preferred producing-conditions, or the preferred pre-strain is never or insufficiently added to the structural part that has not yet been artificially aged. Thus, on the whole, the average dislocation density range thereof is downwards out of the range specified in the present invention.

[0146] Comparative Example 14 does not satisfy the average dislocation density specified in the present invention since the lowest temperature in the hot rough rolling is too low in the production of the raw sheet.

[0147] About Comparative Example 15, the average dislocation density range thereof is downwards out of the range specified in the present invention because the average cooling rate is too small from the material (sheet) temperature just after the hot finish rolling to a material temperature of 150.degree. C. in the production of the raw sheet.

[0148] About Comparative Examples 16 and 17, the pre-strain is never or insufficiently added thereto.

[0149] About Comparative Example 18, the added pre-strain is too large.

[0150] About Comparative Examples 19 and 20, the artificial aging treatment temperature is too high or the holding period is too long relatively to the added pre-strain.

[0151] About Comparative Examples 21 and 22, their raw sheet is produced under the above-mentioned preferred conditions, and the strain is added thereto under the preferred conditions. However, their alloy composition is downwards out of the range about the Mg or Si content by percentage in the present invention, as shown about the alloy numbers 11 and 12 in Table 1.

[0152] Thus, their average dislocation density range is downwards out of the range specified in the invention, so that the BH response is remarkably lowered and the strength and the crash performance are too low.

[0153] The above-mentioned results support respective critical significances of the requirements of the present invention for causing the aluminum alloy structural part of the present invention to have both of a crash performance estimated in the VDA bending test, and a high strength.

TABLE-US-00001 TABLE 1 6000 Series aluminum alloy chemical-component-composition (% by mass) (balance: Al) Alloy No. Mg Si Fe Cu Me Zr Cr Ag Sn Sc Ti 1 0.58 0.95 0.17 -- -- -- -- -- -- -- -- 2 0.45 1.1 0.15 -- -- -- -- -- -- -- 0.02 3 0.35 1.2 0.08 -- -- -- -- -- -- -- 0.02 4 0.95 0.54 -- -- 0.35 -- -- -- -- -- 0.02 5 0.43 1.1 -- 0.08 0.12 -- -- -- -- -- -- 6 0.60 1.0 -- 0.72 -- -- -- -- -- -- 0.02 7 1.0 1.4 0.20 -- 0.08 -- 0.12 -- -- -- -- 8 0.70 0.95 0.12 0.18 -- 0.15 0.05 -- -- -- 0.02 9 1.4 1.1 0.14 -- -- -- -- 0.15 -- 0.07 0.02 10 0.45 0.66 0.15 0.45 0.07 -- 0.05 -- 0.05 -- 0.02 11 0.22 0.88 0.14 -- 0.08 0.05 -- -- -- -- 0.02 12 0.75 0.43 -- 0.12 0.08 -- -- -- -- -- 0.02

TABLE-US-00002 TABLE 2 Producing method Microstructure and properties after pre-stain Hot finish rolling addition and artificial aging treatment (T6) Hot rough Average cooling rate Artificial aging Microstructure Properties rolling (.degree. C./hr.) treatment Average 0.2% VDA Lowest from material Pre- End-point Holding dislocation Solute Cu Yield bending No. in temperature temperature just after strain temperature period density quality strength angle Classification No. Table 1 (.degree. C.) rolling-end to 150.degree. C. % (.degree. C.) (min) (.times.10.sup.14 m.sup.-2) (% by mass) (MPa) (.degree.) Invention 1 1 480 10 10 210 30 4.5 -- 253 98 Examples 2 2 470 7 5 250 10 3.4 -- 255 107 3 2 470 8 10 230 20 4.6 -- 258 101 4 2 470 7 15 230 15 5.6 -- 265 94 5 2 460 10 20 200 10 7.3 -- 276 90 6 3 460 7 10 230 20 4.1 -- 257 103 7 4 470 6 20 270 5 5.2 -- 254 95 8 5 460 10 10 230 20 5.2 0.07 265 96 9 6 460 15 10 230 20 6.7 0.63 280 97 10 7 450 7 15 230 20 7.6 -- 277 90 11 8 460 10 10 230 20 6.0 0.16 272 98 12 9 450 7 5 210 20 6.2 -- 270 92 13 10 470 15 10 250 20 3.1 0.42 254 110 Comparative 14 2 430 7 5 250 10 2.7 -- 244 88 Examples 15 5 460 4 10 250 20 2.9 0.03 248 85 16 2 470 7 -- 230 20 1.5 -- 228 98 17 2 470 7 2 230 20 2.4 -- 239 93 18 2 470 7 25 230 20 8.5 -- 290 75 19 5 460 10 10 280 15 2.6 0.03 243 89 20 5 460 10 10 230 40 2.8 0.04 247 88 21 11 480 10 10 210 30 2.7 -- 242 88 22 12 480 10 10 210 30 2.8 0.11 245 86

INDUSTRIAL APPLICABILITY

[0154] As described above, the present invention can provide a structural part obtained from a 6000 series aluminum alloy sheet as a shaping raw material, and having an improved crash performance; and a method for producing the sheet. The present invention is therefore suitable for a structural part contributing to weight saving for, e.g., an automobile, a bicycle or a railroad vehicle.

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