U.S. patent application number 15/013175 was filed with the patent office on 2016-08-04 for high strength aluminum alloy sheet.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). The applicant listed for this patent is Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Yasuhiro ARUGA, Katsushi Matsumoto, Hisao Shishido.
Application Number | 20160222491 15/013175 |
Document ID | / |
Family ID | 56552868 |
Filed Date | 2016-08-04 |
United States Patent
Application |
20160222491 |
Kind Code |
A1 |
ARUGA; Yasuhiro ; et
al. |
August 4, 2016 |
HIGH STRENGTH ALUMINUM ALLOY SHEET
Abstract
A high strength 6000-series aluminum alloy sheet improved with
good bendability is provided. Amounts of solute Mg and solute Si
are increased in a good balance, on the premise of not greatly
changing the composition and the manufacturing condition of the
aluminum alloy sheet, thereby promoting formation of Mg--Si
clusters in a good balance between the number of Mg and Si atoms to
increase the strength after BH without deteriorating the
bendability even after natural aging.
Inventors: |
ARUGA; Yasuhiro; (Kobe-shi,
JP) ; Shishido; Hisao; (Kobe-shi, JP) ;
Matsumoto; Katsushi; (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: |
56552868 |
Appl. No.: |
15/013175 |
Filed: |
February 2, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 21/02 20130101;
C22C 21/08 20130101 |
International
Class: |
C22C 21/08 20060101
C22C021/08; C22C 21/02 20060101 C22C021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 2, 2015 |
JP |
2015-018570 |
Claims
1. A high strength Al--Mg--Si alloy sheet comprising, based on mass
%, 0.6 to 2.0% of Mg, 0.6 to 2.0% of Si, 1.0% or less of Mn (not
including 0%), and 0.5% or less of Fe (not including 0%)
respectively, with the remainder consisting of Al and inevitable
impurities, wherein both of a Mg content and a Si content separated
in a solution by a residue extraction method with hot phenol are
0.6% or more as an amount of solute Mg and an amount of solute Si
of the sheet, the sum of the amount of solute Mg and the amount of
solute Si is 1.4% or more, and the ratio of the amount of solute Si
to the amount of solute Mg (solute Si/solute Mg) is 0.8 to 1.2.
2. The high strength aluminum alloy sheet according to claim 1,
wherein the aluminum alloy sheet further comprises one or more of,
1.0% or less (not including 0%) of Cu, 0.3% or less (not including
0%) of Cr, 0.2% or less (not including 0%) of Zr, 0.2% or less (not
including 0%) of V, 0.1% or less (not including 0%) of Ti, 0.5% or
less (not including 0%) of Zn, 0.2% or less (not including 0%) of
Ag, and 0.15% or less (not including 0%) of Sn.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an Al--Mg--Si alloy sheet.
The aluminum alloy sheet referred to in the present invention is a
rolled sheet such as a hot-rolled sheet or a cold-rolled sheet,
which is an aluminum alloy sheet subjected to tempering such as
solid solution treatment and quenching before being subjected to
bending fabrication and paint-bake treatment. Hereinafter, aluminum
may also be referred to as ALUMI or Al.
[0003] 2. Description of the Related Art
[0004] Recently, social need for weight reduction of vehicles such
as automobiles has increased more and more out of consideration for
global environment. To meet such social need, as a material of
automobiles, a lighter weight aluminum alloy material having
excellent formability and paint-bake hardenability (bake
hardenability, hereinafter also referred to as BH property) is used
increasingly in place of steel materials such as steel sheets.
[0005] Aluminum alloy sheets for large panel materials such as
outer panels and inner panels of automobiles include, for example,
Al--Mg--Si alloy sheets such as AA or JIS 6000-series (also simply
referred to hereinafter as 6000-series) are used. The 6000-series
aluminum alloy has a composition essentially containing Si and Mg,
ensures formability at a low proof stress during formation,
improves the proof stress (strength) by heating during artificial
aging (hardening) such as paint baking of the panel after
formation, and has excellent paint bake-hardenability capable of
ensuring necessary strength.
[0006] For further weight reduction of automobile bodies, extended
use of an aluminum alloy materials is desired for automobile
structural members such as skeleton materials, for example, frames
and pillars or reinforcing materials such as bumper reinforcements
and door beams for automobile members except for the panel
material.
[0007] However, further strengthening is required for the
automobile structural materials compared with the automobile
panels. Accordingly, for applying the 6000-series aluminum alloy
sheet used for the automobile panel materials to the skeleton
materials or the reinforcing materials needs further
strengthening.
[0008] However, it is not so easy to attain such high strengthening
without greatly changing the composition and the manufacturing
conditions of existent 6000-series aluminum alloy sheets and
without hindering the bendability, etc.
[0009] It has been variously proposed to control an amount of
solute Mg and an amount of solute Si in order to improve the
property such as a BH property of the 6000-series aluminum alloy
sheet as the panel materials.
[0010] For example, JP-A 2008-174797 intends to provide an
Al--Mg--Si alloy sheet of excellent room temperature stability
(suffering from less deterioration of the material by natural
aging) as the panel materials. For this purpose, the patent
literature proposes an Al--Mg--Si alloy sheet in which an amount of
solute Si is 0.55 to 0.80 mass % and an amount of solute Mg is 0.35
to 0.60 mass %, and the amount of solute Si/amount of solute Mg is
1.1 to 2. An example, having a high strength, of the aluminum alloy
sheet has a strength of about 210 MPa as 0.2% proof stress after
artificial aging of 170.degree. C..times.20 minutes after applying
2% strain to the sheet after natural aging for 15 days after the
manufacture of the sheet.
SUMMARY OF THE INVENTION
[0011] In the existent control for the amount of solute Mg and the
amount of solute Si, for example, in JP-A No. 2008-174797, the
strength is insufficient for the application use of the skeleton
materials or reinforcing materials since this is originally
intended for use as the panel materials.
[0012] Further, while the skeleton materials or reinforcing
materials have no requirement for high press formability as that
for the panel material, when the material sheet is fabricated into
the skeleton material or the reinforcing material, since the
material sheet is mainly subjected to bending fabrication, a
bendability of such an extent as not causing cracking by V-bending
fabrication is required.
[0013] The present invention has been accomplished for solving such
a subject and intends to provide a high strength 6000-series
aluminum alloy sheet that can be manufactured without greatly
changing the composition and the manufacturing conditions of the
existent 6000-series aluminum alloy sheet as the skeleton material
or the reinforcing material, and can also be fabricated into
members.
[0014] According to an aspect of the present invention in order to
attain the object, there is provided a high strength Al--Mg--Si
alloy sheet comprising, based on mass %, 0.6 to 2.0% of Mg, 0.6 to
2.0% of Si, 1.0% or less of Mn (not including 0%), and 0.5% or less
of Fe (not including 0%) respectively with the remainder consisting
of Al and inevitable impurities, in which both of the Mg content
and the Si content in a solution separated by a residue extraction
method with hot phenol are 0.6% or more as an amount of solute Mg
and an amount of solute Si of the sheet and, the sum of the amount
of solute Mg and the amount of solute Si is 1.4% or more, and the
ratio of the amount of solute Si to the amount of solute Mg (solute
Si/solute Mg) is 0.8 to 1.2.
[0015] In the present invention, on the premise of not greatly
changing the existent aluminum alloy composition and manufacturing
condition, the relation between the amount of solute Mg and the
amount of solute Si and the strength of the 6000-series aluminum
alloy sheet has been reconsidered. As a result, it has been found
that formation of Mg--Si clusters in a good balance between the
number of Mg and Si atoms can be promoted by tempering the
manufactured sheet as will be described later. The Mg--Si clusters
in a good balance between Mg and Si tend to be transformed to
Mg--Si precipitates during paint-bake treatment to remarkably
contribute to the increase in the strength. As a result, it has
been found that the 0.2% proof stress after BH of 185.degree.
C..times.20 minutes can be increased to 260 MPa or more, preferably
280 MPa or more and, more preferably, 300 MPa or more also after
the natural aging without deteriorating the bendability.
[0016] Accordingly, the aluminum alloy of the present invention is
suitable, for example, as skeleton materials or reinforcing
materials requiring higher strength than the panel materials of
automobiles.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0017] Preferred embodiments of the present invention are to be
described specifically on every constitutions.
(Chemical Composition)
[0018] First, a chemical composition of an Al--Mg--Si based
(hereinafter also referred to as 6000-series) aluminum alloy sheet
of the present invention is to be described below. In the present
invention, skeleton materials or reinforcing materials except for
the panel materials are strengthened without deteriorating the
bendability and without greatly changing the existent composition
and the manufacturing conditions.
[0019] In order to satisfy such a subject in view of the
composition, the 6000-series aluminum alloy sheet has a composition
comprising, based on mass %, 0.6 to 2.0% of Mg, 0.6 to 2.0% of Si,
1.0 or less (not including 0%) of Mn and 0.5% or less (not
including 0%) of Fe respectively, with the remainder consisting of
Al and inevitable impurities. "%" expression for the content of
each of the elements means mass %.
[0020] The range of content and the meaning of each element or the
allowable amount thereof in the 6000-series aluminum alloy are to
be described.
Si: 0.6 to 2.0%
[0021] Si, together with Mg, is an essential element for obtaining
a necessary strength (proof stress) as outer panels of automobiles
by forming Mg--Si based precipitates that contribute to the
improvement of the strength upon solid solution strengthening and
artificial aging such as paint-bake treatment, thereby providing
aging hardenability. If the Si content is insufficient, since the
amount of solute Si is decreased and the amount of formed Mg--Si
based precipitates becomes insufficient before the paint-bake
treatment, the BH property is deteriorated remarkably. On the other
hand, if the Si content is excessive, coarse constituents and
precipitates are formed thereby causing remarkable cracks during
hot rolling Accordingly, the Si content is defined within a range
from 0.6 to 2.0%. A preferred lower limit of Si is 0.8% and a
preferred upper limit thereof is 1.5%.
Mg: 0.6 to 2.0%
[0022] Mg, together with Si, is an essential element for obtaining
a necessary proof stress as the panels, by forming Mg--Si based
precipitates that contribute to the improvement of the strength
upon solid solution strengthening and artificial aging such as
paint-bake treatment, thereby providing aging hardenability. If the
Mg content is insufficient, since the amount of solute Mg is
decreased and the amount of formed Mg--Si based precipitates
becomes insufficient before the paint-bake treatment, the BH
property is deteriorated remarkably. In contrast, if the Mg content
is excessive, coarse constituents and precipitates are formed
thereby causing remarkable cracks during the hot rolling
Accordingly, the Mg content is defined within a range from 0.6 to
2.0%. A preferred lower limit of Mg is 0.8% and a preferred upper
limit thereof is 1.5%.
Amount of Solute Si and Amount of Solute Mg
[0023] A typical concept in the prior art, for example, in JP-A
2008-174797 of controlling the amount of solute Mg and the amount
of solute Si of the 6000-series aluminum alloy sheet in the use of
panel materials for automobiles is that the amount of solute Mg and
the amount of solute Si are suppressed to the lowest limit
necessary for the BH property in order to suppress formation of
Mg--Si, Si--Si and Mg--Mg clusters which cause natural aging of the
sheet.
[0024] On the contrary, in the present invention, it has been found
that formation of Mg--Si clusters in a good balance between the
number of Mg and Si atoms can be promoted when the amount of solute
Mg and the amount of solute Si are increased in a good balance on
the premise of not greatly changing the existent compositions and
the manufacturing condition of the aluminum alloys.
[0025] That is, it has been found that when the amount of solute Mg
and the amount of solute Si are increased in a good balance, it is
possible to suppress the formation of Si enriched Mg--Si clusters
(aggregates of Mg atoms and Si atoms) that are formed during
tempering of the manufactured sheet (during pre-aging), and ensure
the amount of Mg--Si precipitates that are formed during the
paint-bake treatment, and contribute to the increase in the
strength.
[0026] The Si enriched Mg--Si clusters described above cause
natural aging, also deteriorate the bendability and are less
transformed to Mg--Si precipitates during the paint-bake treatment
and less contribute to the improvement of the BH property and the
increase in the strength.
[0027] On the contrary, the Mg--Si clusters in a good balance
between Mg and Si do not cause natural aging and do not deteriorate
the bendability, but tend to be transformed to the Mg--Si
precipitates during the paint-bake treatment, thereby remarkably
contributing to the increase in the strength.
[0028] Accordingly, when the Si enriched Mg--Si clusters described
above are suppressed and the Mg--Si clusters in a good balance
between Mg and Si are increased, the natural aging can be
suppressed and, in addition, the BH property can be improved to
remarkably increase the strength. That is, even after the natural
aging, 0.2% proof stress after BH of 185.degree. C..times.20
minutes can be increased to 260 MPa or more, preferably, 280 MPa or
more and, more preferably, 300 MPa or more without deteriorating
the bendability.
[0029] Therefore, in the present invention, the amount of solute Mg
and the amount of solute Si are increased and the Mg content and
the Si content separated in a solution by a residue extraction
method with hot phenol are increased as 0.6% or more. That is, both
of the amount of solute Mg and the amount of solute Si of the sheet
are increased as 0.6% or more thereby ensuring the amount of Mg--Si
precipitates that are formed during the paint-bake treatment and
contribute to the increase in the strength. In order to increase
the effect, both of the amount of solute Mg and the amount of
solute Si are defined preferably as 0.7% or more.
[0030] Naturally, the upper limits for the amount of solute Mg and
the amount of solute Si of the sheet are determined based on the Mg
content and the Si content of the sheet.
[0031] Concurrently, the sum of the amount of solute Mg and the
amount of solute Si (that is, the sum of the Mg content and the Si
content in the solution) is increased as 1.4% or more, preferably,
1.5% or more for ensuring the amount of the Mg--Si precipitates
that are formed during the paint-bake treatment and contribute to
the increase of the strength. Naturally, also the upper limit for
the sum of the amount of solute Mg and the amount of solute Si is
determined depending on the Mg content and the Si content of the
sheet.
[0032] Further, for keeping the balance between the amount of
solute Mg and the amount of solute Si, the ratio of the amount of
solute Si to the amount of solute Mg (solute Si/solute Mg), that
is, the ratio of the Si content to the Mg content in the solution)
is defined as 0.8 to 1.2.
[0033] The upper limit 1.2 for the ratio of the amount of solute Si
to the amount of solute Mg (solute Si/solute Mg) is defined for
suppressing the formation of the Si enriched Mg--Si clusters that
tend to be formed when the amount of solute Si is larger to the
amount of solute Mg during pre-aging and it is preferably 1.1 or
less.
[0034] In contrast, also in a case where the amount of solute Mg is
excessive to the amount of solute Si, formation of the Mg--Si
clusters in a good balance between the number of Mg and Si atoms is
suppressed. Accordingly, the lower limit of the ratio of the amount
of solute Si to the amount of solute Mg is defined as 0.8.
Mn: 1.0% or Less (Not Including 0%)
[0035] Mn improves the strength of the aluminum alloy by the effect
of solid solution strengthening and crystal grain refining.
However, if Mn is contained excessively by more than 1.0%, the
amount of Al-Mn-Fe based intermetallic compounds is increased
tending to form fracture origins, and the amount of the Mg--Si
based precipitates that contribute to the increase of the strength
is decreased. Accordingly, the Mn content is defined as 1.0% or
less (not including 0%).
Fe: 0.5% or Less (Not Including 0%)
[0036] Since Fe forms Al--Mn--Fe based intermetallic compounds in
the aluminum alloy, as the Fe content increases, the amount of the
compounds thereof is increased more tending to form fracture
origins and also decrease the amount of the Mg--Si based
precipitates that contribute to the increase of the strength. In
addition, since Fe is introduced in the aluminum alloys as bare
metal impurities and the content is increased as the amount of
aluminum alloy scraps increases as the melting material (ratio to
the aluminum ingot), the Fe content is preferably as less as
possible, and Fe is an element to be restricted. However, since
decrease of Fe below the detection limit needs a considerable cost,
it is necessary to more or less permit the presence of Fe.
Accordingly, the Fe content is defined as 0.5% or less (not
including 0%).
Other Elements
[0037] In addition, for increasing the strength of the aluminum
alloy sheet in the present invention, one or more of other
elements, for example, 1.0% or less (not including 0%) of Cu, 0.3%
or less (not including 0%) of Cr, 0.2% or less (not including 0%)
of Zr, 0.2% or less (not including 0%) of V, 0.1% or less (not
including 0%) of Ti, 0.5% or less (not including 0%) of Zn, 0.2% or
less (not including 0%) of Ag, and 0.15% or less (not including 0%)
of Sn may be contained.
[0038] Since the elements described above have an effect, in
common, of increasing the strength of the sheet, they can be
regarded as elements of providing an equivalent strengthening
effect and their specific mechanisms include, naturally, common
portions and different portions.
[0039] Cr, Zr, and V, like Mn, form dispersed particles (dispersion
phase) during a homogenizing heat treatment and such dispersed
particles have an effect of hindering the movement of grain
boundary after recrystallization and serving to refine crystal
grains. Further, Ti serves to form constituents as nuclei of
recrystallized grains, thereby suppressing crystal grains from
coarsening, and refining crystal grains. While Cu improves the
strength, since Cu deteriorates the bendability as the strength is
increased, the content is defined preferably as 0.7% or less. Zn
and Ag are useful for improving the artificial aging hardenability
(BH property) and have an effect of promoting precipitation of a
compound phase such as a GP zone into the crystal grains of the
sheet microstructure under the condition of relatively low
temperature and short time in the artificial aging. Sn has an
effect of suppressing the diffusion of Mg and Si at a room
temperature by capturing atom vacancy, suppressing the increase in
the strength at a room temperature (natural aging), and releasing
the vacancy captured during the artificial aging and promoting the
diffusion of Mg and Si, thereby increasing the BH property.
[0040] However, if the content of each of the elements is
excessive, manufacture of the sheet becomes difficult by formation
of coarse compounds, etc. and the strength, the bendability, and
the corrosion resistance are also deteriorated. Particularly, Cu
remarkably deteriorates the bendability if it is contained
excessively. Accordingly, when the elements are contained, their
contents are restricted to each of the upper limit values or less
as described above.
(Manufacturing Method)
[0041] Then, a method of manufacturing the aluminum alloy sheet of
the invention is to be described. The manufacturing process per se
of the aluminum alloy sheet of the invention is a customary or
known process, in which an aluminum alloy slab having the
6000-series composition is cast and then subjected to a
homogenizing heat treatment, hot-rolled and cold-rolled into a
sheet of a predetermined thickness and further subjected to
tempering such as solid solution treatment and quenching.
[0042] However, in the manufacturing steps, hot rolling conditions
and pre-aging conditions after the solid solution treatment and the
quenching are defined within preferred ranges as will be described
later in order to obtain the microstructure defined in the present
invention (amount of solute Mg and amount of solute Si).
(Melting and Casting Cooling Rate)
[0043] First, in the melting and casting process, a molten aluminum
alloy melted and adjusted within the range of the 6000-series
composition is cast by properly selecting usual melting and casting
process such as a continuous casting, a semi-continuous casting
method (DC casting), etc. For controlling the clusters within the
range defined in the present invention, an average cooling rate
during casting is preferably as high (fast) as possible from the
liquidus temperature to the solidus temperature, for example, at
30.degree. C./min or higher.
[0044] Without such temperature control (cooling rate) in a high
temperature region during casting, the cooling rate in the high
temperature region is inevitably lowered. If the average cooling
rate in the high temperature region is lowered, the amount of
constituents formed coarsely in the temperature range in the high
temperature region is increased, and the size and the amount of the
constituents vary greatly in the direction of the width and in the
direction of the thickness of the slab. As a result, there may be a
high possibility that clusters cannot be controlled as defined in
the range of the present invention.
(Homogenizing Heat Treatment)
[0045] Subsequently, the cast aluminum alloy slab is subjected to a
homogenizing heat treatment prior to hot rolling. The homogenizing
heat treatment (soaking) is important for sufficiently
solid-solutionizing Si and Mg in addition to homogenization of the
microstructure (eliminating segregation in the crystal grains in
the slab microstructure) as an ordinary purpose. So long as the
purpose is attained under the conditions, the conditions are not
particularly restricted and the treatment is usually applied for
once or in one step.
[0046] Si and Mg are solid-solutionized sufficiently by properly
selecting the homogenizing heat treatment temperature from a range
of 500.degree. C. or higher and 560.degree. C. or lower, and the
homogenizing (holding) time from a range of one hour or more. If
the homogenizing temperature is lower, amounts of solute Si and
solute Mg cannot be ensured and the microstructure defined by the
present invention (amount of solute Mg and amount of solute Si)
cannot be attained even by the pre-aging (reheating) after the
solid solution treatment and the quenching to be described later.
Further, since the segregation in the crystal grains cannot be
eliminated sufficiently and acts as fracture origins, the
bendability is deteriorated.
[0047] After the homogenizing heat treatment, hot rolling is
performed in which it is necessary not to lower the temperature of
the slab to 500.degree. C. or lower before the start of hot rough
rolling after the homogenizing heat treatment thereby ensuring the
amounts of solute Si and solute Mg. If the temperature of the slab
is lowered to 500.degree. C. or lower before the start of the hot
rough rolling, there may be a high possibility that Si and Mg are
precipitated failing to ensure the amounts of solute Si and solute
Mg for attaining the microstructure defined in the present
invention (amount of solute Mg and amount of solute Si).
(Hot Rolling)
[0048] Hot rolling includes a rough rolling step of a slab and a
finish rolling step depending on the thickness of the sheet to be
rolled. In the rough rolling step or finish rolling step, reversed
type, tandem type rolling mill, etc are used properly.
[0049] During rolling from the start to the end of hot rough
rolling, it is necessary not to lower the temperature to
450.degree. C. or lower, thereby ensuring the amounts of solute Si
and solute Mg. If the lowest temperature of the roughly rolled
sheet between passes is lowered to 450.degree. C. or lower due to
increase of the rolling time, etc., Mg--Si based compounds tend to
be precipitated to decrease the amount of solute Mg and the amount
of solute Si. Accordingly, there may be a high possibility that the
amounts of solute Si and solute Mg in order to attain the
microstructure defined in the present invention (amount of solute
Mg and amount of solute Si) cannot be ensured.
[0050] After the hot rough rolling described above, a hot finish
rolling with an end temperature in a range of 300 to 360.degree. C.
is performed. If the end temperature of the hot finish rolling is
excessively low, e.g., lower than 300.degree. C., the rolling load
is increased to lower the productivity. In contrast, when the end
temperature of the hot finish rolling is made higher for obtaining
a recrystallized microstructure without leaving much fabrication
microstructure, if the temperature exceeds 360.degree. C., Mg--Si
based compounds tend to be precipitated to decrease the amount of
solute Mg and the amount of solute Si. Accordingly, there may be a
high possibility that the amounts of solute Si and solute Mg cannot
be ensured for attaining the microstructure defined in the present
invention (amount of solute Mg and amount of Si).
(Annealing of Hot Rolled Sheet)
[0051] Annealing before cold rolling of the hot rolled sheet (rough
annealing) may be applied although this is not always
necessary.
(Cold Rolling)
[0052] In the cold rolling, the hot rolled sheet is rolled to
manufacture a cold rolled sheet (also including coil) of a desired
sheet thickness. For refining the crystal grains further, the cold
rolling compression reduction is preferably 30% or more, and
intermediate annealing may be performed between cold rolling passes
with the same purpose as that for rough annealing.
(Solid Solution Treatment and Quenching)
[0053] After the cold rolling, solid solution treatment and
successive quenching to a room temperature are performed. For the
solid solution treatment and the quenching, a usual continuous heat
treatment line may be used. However, for obtaining a sufficient
amount in solid solution for each of the elements such as Mg and
Si, it is preferred to hold the rolled sheet at a solid solution
treatment temperature, that is, from 550.degree. C. or higher to a
melting temperature or lower for 10 seconds or more and then cool
the sheet at an average cooling rate of 20.degree. C./sec or more
from the holding temperature to 100.degree. C. If the temperature
is lower than 550.degree. C. or the holding time is shorter than 10
seconds, re-solid solution of compounds, for example, Al-Mn-Fe
based or Mg--Si based that are formed before the solid solution
treatment is insufficient and the amount of solute Mg and the
amount of solute Si are decreased. Accordingly, there is a high
possibility that the amounts of solute Si or solute Mg for
attaining the microstructure defined in the present invention
(amount of solute Mg and amount of solute Si) cannot be
ensured.
[0054] If the average cooling rate is less than 20.degree. C./sec,
precipitates mainly based on Mg--Si are formed during cooling to
decrease the amount of solute Mg and the amount of solute Si and
there is also a high possibility that an amount of solute Si and an
amount of solute Mg cannot be ensured. In order to ensure the
cooling rate, air cooling means such as a blower, water cooling
means such as mist, spray or clipping, as well as conditions
therefor are selected and used respectively for the quenching.
(Pre-Aging: Reheating)
[0055] After cooling to a room temperature by quenching after the
solid solution treatment, the cold rolled sheet is subjected to
pre-aging (reheating) within one hour. If the room temperature
holding time after the end of the quenching to the room temperature
till the start of the pre-aging (start of heating) is excessively
long, the Si enriched Mg--Si clusters are formed by the natural
aging in which Mg--Si clusters in a good balance between Mg and Si
can be less increased. Accordingly, it is preferred that the room
temperature holding time is preferably as short as possible, and
the solid solution treatment and the quenching may be in continuous
with scarce time interval. The lower limit for the time is not
particularly defined.
[0056] In the pre-aging, the holding time of the sheet at 60 to
120.degree. C. is defined as 10 hours or more and 40 hours or less.
Thus, the Mg--Si clusters in a good balance between Mg and Si
defined in the present invention are formed.
[0057] If the pre-aging temperature is lower than 60.degree. C., or
the holding time is less than 10 hours, the result of the treatment
is identical with that in a case without the pre-aging, in which
the Mg--Si clusters that suppress the Si enriched Mg--Si clusters
and in a good balance between Mg and Si are less increased, thereby
tending to lower the proof stress after the paint-bake
treatment.
[0058] In contrast, if the temperature is higher than 120.degree.
C. or the holding time is longer than 40 hours in the pre-aging
conditions, the precipitated nuclei are formed excessively in which
the strength during bending fabrication before the paint-bake
treatment is excessively high tending to deteriorate the
bendability.
[0059] The present invention is to be described more specifically
with reference to examples but it will be apparent that the present
invention no way undergoes restriction by the following examples
and can be practiced with appropriate modification in a range
adaptable to the gists of the invention as described before and to
be described later, any of which is included within the technical
scope of the present invention.
EXAMPLE
[0060] 6000-series aluminum alloy sheets of different
microstructures defined by the amount of solute Mg and the amount
of solute Si in the present invention were separately fabricated by
changing the composition and manufacturing condition, and As proof
stress, BH property (paint bake-hardenability), and bendability
were measured and evaluated respectively after holding at a room
temperature for 100 days after manufacture of the sheets. The
results are shown in Tables 1 and 2.
[0061] Specifically, 6000-series aluminum alloy sheets having
compositions as shown in Table 1 were manufactured separately while
variously changing the manufacturing conditions, for example, the
soaking temperature, the lowest temperature of the roughly rolled
sheet between passes of hot rough rolling (described as the lowest
temperature in Table 2), the end temperature of the hot finish
rolling, the holding temperature and the holding time of solid
solution treatment, the average cooling rate, the temperature and
the holding time in the pre-aging. In the expression for the
content of each of the elements in Table 1, columns for respective
elements which are left blank with no numerical values indicate
that the contents are below the detection limit
[0062] Specific manufacturing conditions of the aluminum alloy
sheets are as described below. Aluminum alloy slabs of respective
compositions shown in Table 1 were melted in common by a DC casting
method. In this case, an average cooling rate during casting from a
liquidus temperature to a solidus temperature was defined as
50.degree. C./min in common with each of examples. Successively,
after subjecting the slabs to soaking for 6 hours in common with
each of examples under the temperature conditions shown in Table 2,
hot rough rolling was started at that temperature. Table 2 also
shows the lowest (pass) temperature in the hot rough rolling.
[0063] Then, in common with each of the examples, the sheets were
hot rolled in the succeeding finish rolling at an end temperature
shown in Table 2 to a thickness of 4.0 mm to form hot rolled
sheets. After subjecting the aluminum alloy sheets after hot
rolling to rough annealing of 500.degree. C..times.1 minute in
common with each of the examples, the sheets were subjected to cold
rolling at a compression reduction of 50% with no intermediate
annealing in the course of cold rolling pass to obtain cold rolled
sheets of 2.0 mm thickness.
[0064] Each of the cold rolled sheets was continuously subjected to
tempering (T4) while being recoiled and coiled in a continuous heat
treatment facility in common with each of the examples.
Specifically, solid solution treatment was performed at an average
heating rate of 10.degree. C./sec up to 500.degree. C. and holding
the sheet for 20 seconds after reaching the aimed temperature of
540.degree. C., and then the sheet was cooled to a room temperature
by water cooling at an average cooling rate of 100.degree. C./sec.
Just after the cooling, pre-aging was performed at temperature
(.degree. C.) and for holding time (hr) shown in Table 2. After the
pre-aging, gradual cooling (spontaneous cooling) was performed.
[0065] Sheet specimens (blanks) were cut out from respective final
sheet products which were left for 100 days at a room temperature
after the tempering and each of the sheet specimens was measured
and evaluated for the microstructure defined by the amount of
solute Mg and the amount of solute Si and the properties. The
result is shown in Table 2.
(Measurement of the Amount of Solute Mg and the Amount of Solute
Si)
[0066] The amount of solute Mg and the amount of solute Si in each
of the specimens were measured by dissolving sheet specimens as the
object of measurement according to a residue extraction method with
hot phenol, separating under filtration the solid solution by using
a filter of 0.1 .mu.m mesh, and measuring the contents of Mg and Si
separated in the solution as the amount of solute Mg and the amount
of solute Si respectively.
[0067] The residue extraction method with hot phenol was performed
specifically as described below. First, after heating phenol
contained in a separable flask, each of specimen sheets as the
object of measurement was transferred into the separable flask and
thermally decomposed. Then, after adding benzyl alcohol, the
contents were filtered and separated under suction, and the total
content of the separated Mg and Si in the solution was
quantitatively analyzed. For the quantitative analysis, atomic
absorption spectrometry (AAS), inductively coupled plasma atomic
spectrometry (ICP-OES), or the like was used optionally. For the
filtration under suction, a membrane filter having 0.1 .mu.m mesh
size (size of particle to be captured) and 47 mm.phi. was used as
described above.
[0068] Measurement and calculation were performed for the three
specimens each sampled from the sheet specimen at three points in
total, i.e., at one point in a central portion in the direction of
the thickness and at two points on both ends in the direction of
the width of the sheet specimen, and the amounts of solute Mg and
solute Si (mass %) for respective specimens were averaged. Then,
the sum of the amount of solute Si and the amount of solute Mg, and
the ratio of the amount of solute Si to the amount of solute Mg
(amount of solute Si/amount of solute Mg) were calculated based on
the amount of solute Si and the amount of solute Mg.
(Paint Bake-Harden Ability)
[0069] As the mechanical properties of the sheet specimen, 0.2%
proof stress (proof stress after BH) of the sheet specimen after 2%
stretch that simulated the bending fabrication after artificial
aging of 185.degree. C..times.20 minutes (after BH) was determined
by a tensile test in common with each of the sheet specimens. The
BH property of the respective sheet specimens was evaluated based
on the difference between 0.2% proof stress to each other
(increment of the proof stress).
[0070] In the tensile test, No. 5 test specimen of JIS Z 2201 (25
mm.times.50 mmGL.times.sheet thickness) was sampled from each of
the sheet specimens and subjected to the tensile test at a room
temperature. The tensile direction of the test specimen was in a
direction orthogonal to the rolling direction. The tensile speed
was set to 5 mm/min up to the 0.2% proof stress and to 20 mm/min
after applying the proof stress. The number of times N of the
mechanical property measurement was 5 and each of result was
calculated as an average value. For the test specimen for measuring
the proof stress after the BH. The BH treatment was performed after
applying 2% preliminary strain that simulated the press forming of
the sheet by a tensile tester.
(Bendability)
[0071] Bendability was measured for each of sheet specimens. In the
test, a test specimen of 30 mm width.times.35 mm length was
prepared while taking a major axis in the rolling direction, and
90.degree. V-bending was performed at a bending radius of 2.0 mm
while applying a load of 2000 kgf according to JIS Z 2248.
[0072] The surface state of the V-bent portion, for example,
generation of roughening, fine cracks, and large cracks was
observed visually and evaluated visually according to the following
criteria, and those of 6 or more scores were evaluated as
satisfactory (in Table 2, only the acceptance (.largecircle.,
.times.) is described). [0073] 9: no cracks, no roughening, [0074]
8: no cracks, slight roughening, [0075] 7: no cracks, roughening,
[0076] 6: slight fine cracks, [0077] 5: fine cracks, [0078] 4: fine
cracks over the entire surface, [0079] 3: large cracks, [0080] 2:
large cracks, immediately to fail, [0081] 1: failed.
[0082] As shown in Tables 1 and 2 respectively, Inventive Examples
1 to 8 are manufactured within the range of the chemical
composition and within the range of preferred conditions of the
present invention. Accordingly, in each of the inventive examples,
as shown in Table 2, both of the amount of solute Mg and the amount
of solute Si in the solution separated by the hot phenol residue
extraction method are 0.6% or more, the sum of the amount of solute
Mg and the amount of solute Si is 1.4% or more, and the ratio of
the amount of solute Si to the amount of solute Mg (solute
Si/solute Mg) is 0.8 to 1.2.
[0083] As a result, each of the inventive examples 1 to 8 has high
0.2% proof stress after BH and high strength and is excellent in
the bendability even after the natural aging as shown in Table
2.
[0084] On the contrary, Comparative Examples 9 to 14 in Table 2 use
an alloy example 1 identical with that of the inventive example in
Table 1. However, in each of the comparative examples,
manufacturing conditions such as the soaking temperature, the
lowest temperature in the hot rough rolling, the end temperature of
the hot finish rolling, the holding temperature and the holding
time in the solid solution treatment, the average cooling rate, and
the temperature and the holding time in the pre-aging are out of
the preferred conditions. As a result, the amount of solute Mg and
the amount of solute Si are out of the range defined in the present
invention and, the BH property and the bendability after the
natural aging are deteriorated when compared with the Inventive
Example 1 using the identical alloy composition.
[0085] Among them, in Comparative Example 9, the lowest temperature
in the hot rough rolling and the end temperature of the hot finish
rolling are excessively low. Accordingly, the sum of the amount of
solute Mg and the amount of solute Si is insufficient being out of
the lower limit on one hand, and the ratio of the amount of solute
Si and the amount of solute Mg (solute Si/solute Mg) is excessive
being out of the upper limit on the other hand, so that the BH
property was low and 0.2% proof stress after BH is
insufficient.
[0086] In Comparative Example 10, the lowest temperature in the hot
rough rolling is excessively low. Accordingly, the amount of solute
Mg and the sum of the amount of solute Mg and the amount of solute
Si is insufficient being out of the lower limit on one hand, and
the ratio (solute Si/solute Mg) is excessive being out of the upper
limit on the other hand, so that the BH property is low and 0.2%
proof stress after BH is insufficient.
[0087] In Comparative Example 11, the holding temperature in the
solid solution treatment is excessively low. Accordingly, the
amount of solute Mg, the amount of solute Si, and the sum of the
amount of solute Mg and the amount of solute Si are insufficient
being out of the lower limit, so that the BH property is low and
the 0.2% proof stress after BH is insufficient.
[0088] In Comparative Example 12, the holding time in the solid
solution treatment is excessively short. Accordingly, the amount of
solute Mg, the amount of solute Si, and the sum of the amount of
solute Mg and the amount of solute Si is insufficient being out of
the lower limit, so that the BH property is low and the 0.2% proof
stress after BH is insufficient.
[0089] In Comparative Example 13, the average cooling rate after
the solid solution treatment is insufficient. Accordingly, the
amount of solute Mg and the sum of the amount of solute Mg and the
amount of solute Si are insufficient being out of the lower limit
on one hand and the ratio (solute Si/solute Mg) is excessive being
out of the upper limit on the other hand, so that the BH property
is insufficient and the 0.2% proof stress after BH is
insufficient.
[0090] In Comparative Example 14, the soaking temperature and the
lowest temperature in the hot rough rolling are excessively low.
Accordingly, the amount of solute Mg and the sum of the amount of
solute Mg and the amount of solute Si are insufficient on one hand
and ratio (solute Si/solute Mg) is excessive being out of the upper
limit on the other hand, so that the BH property is low and the
0.2% proof stress after BH is insufficient. The bendability is also
poor.
[0091] While Comparative Examples 15 to 20 in Table 2 are
manufactured in a range of preferred conditions but use alloys Nos.
10 to 15 in Table 1 and the contents for Mg, Si, Mn, and Fe as
essential elements are out of the range defined in the invention
respectively. Accordingly, in such comparative examples, the amount
of solute Mg and the amount of solute Si, or relation thereof are
out of the ranges defined in the present invention as shown in
Table 2, and the strength after BH is deteriorated when compared
with that of the inventive examples.
[0092] Among them, in Comparative Examples 16 and 18, coarse
constituents and precipitates are formed, remarkable sheet cracks
are formed during the hot rolling, and the sheet per se could not
be manufactured, so that the microstructure and the property could
not be evaluated.
[0093] In Comparative Example 15, alloy 10 in Table 1 is used in
which Mg is insufficient.
[0094] In Comparative Example 16, alloy 11 in Table 1 is used in
which Mg is excessive.
[0095] In Comparative Example 17, alloy 12 in Table 1 is used in
which Si is insufficient.
[0096] In Comparative Example 18, alloy 13 in Table 1 is used in
which Si is excessive.
[0097] In Comparative Example 19, alloy 14 in Table 1 is used in
which Fe is excessive.
[0098] In Comparative Example 20, alloy 15 in Table 1 is used in
which Mn is excessive.
[0099] Accordingly, in view of the result of the examples described
above, it is supported that all the composition and the
microstructure defined in the present invention should be satisfied
in order to increase the strength without deteriorating the
bendability also after natural aging.
TABLE-US-00001 TABLE 1 Alloy Chemical composition of Al--Mg--Si
alloy sheet (mass %, remainder Al) No. Mg Si Mn Fe Cu Cr Zr V Ti Zn
Ag Sn 1 0.85 1.03 0.08 0.14 2 0.86 1.02 0.45 0.16 0.10 3 0.67 0.80
0.20 0.09 0.05 0.07 4 0.80 0.66 0.07 0.21 0.70 5 1.45 1.22 0.34
0.17 0.15 0.10 6 1.09 1.50 0.08 0.43 0.15 0.35 7 0.85 1.25 0.90
0.19 0.15 0.02 9 0.84 1.03 0.10 0.13 0.25 0.03 0.02 10 0.52 0.89
0.08 0.15 11 2.25 1.22 0.20 0.25 12 1.20 0.54 0.10 0.14 13 0.70
2.25 0.06 0.23 0.10 14 0.85 1.00 0.15 0.72 15 0.85 1.03 1.15
0.15
TABLE-US-00002 TABLE 2 Aluminum alloy sheet after holding
Manufacturing conditions of aluminum alloy sheet at room
temperature for 100 days Hot Hot Property rough finish Solid
solution treatment 0.2% Alloy Soaking rolling rolling Average proof
Bend- No. Soaking Lowest End Holding Hold- cooling Pre-aging Solid
solution amount stress ability in temper- temper- temper- temper-
ing rate Temper- (mass %) after 90.degree. Table ature ature ature
ature time (.degree. C./ ature Time Mg + Si/ BH V- Section No. 1
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) (sec) sec)
.degree. C. hr Mg Si Si Mg MPa bending Inventive 1 1 540 470 330
570 15 25 100 20 0.83 0.94 1.77 1.13 285 .smallcircle. Example
Comparative 9 1 530 410 280 570 15 25 100 20 0.61 0.75 1.36 1.23
252 .smallcircle. Example Comparative 10 1 540 420 300 570 15 25
100 20 0.59 0.73 1.32 1.24 244 .smallcircle. Example Comparative 11
1 540 490 330 530 15 25 100 20 0.49 0.52 1.01 1.06 220
.smallcircle. Example Comparative 12 1 540 490 330 570 3 25 100 20
0.50 0.54 1.04 1.08 223 .smallcircle. Example Comparative 13 1 540
490 330 570 15 10 100 20 0.54 0.68 1.22 1.26 239 .smallcircle.
Example Comparative 14 1 480 430 300 570 15 25 100 20 0.53 0.66
1.19 1.25 234 x Example Inventive 2 2 540 470 330 560 20 25 100 20
0.84 0.84 1.68 1.00 304 .smallcircle. Example Inventive 3 3 550 470
330 570 15 25 120 10 0.65 0.76 1.41 1.17 263 .smallcircle. Example
Inventive 4 4 540 460 320 560 20 25 60 40 0.77 0.64 1.41 0.83 275
.smallcircle. Example Inventive 5 5 520 450 310 550 15 20 80 30
1.10 0.98 2.08 0.89 298 .smallcircle. Example Inventive 6 6 540 460
320 560 20 25 100 20 1.02 1.15 2.17 1.13 312 .smallcircle. Example
Inventive 7 7 540 470 330 560 15 20 70 30 0.79 0.88 1.57 1.11 308
.smallcircle. Example Inventive 8 9 540 480 350 570 10 25 100 20
0.78 0.90 1.68 1.15 305 .smallcircle. Example Comparative 15 10 550
470 330 570 15 25 100 20 0.48 0.84 1.32 1.75 237 .smallcircle.
Example Comparative 16 11 510 Crack in hot oiling -- Example
Comparative 17 12 530 470 330 570 15 25 100 20 1.08 0.48 1.56 0.44
209 .smallcircle. Example Comparative 18 13 520 Crack in hot oiling
-- Example Comparative 19 14 540 470 330 560 15 25 100 20 0.84 0.75
1.59 0.89 253 x Example Comparative 20 15 520 460 320 560 15 20 100
20 0.75 0.57 1.32 0.76 246 x Example
[0100] According to the present invention, a 6000-series aluminum
alloy sheet increased in the strength without deteriorating
bendability can be provided. As a result, application use of the
6000-series aluminum alloy sheet can be extended as automobile
structural materials except for panel materials, for example,
skeleton materials such as frames and pillars or reinforcing
materials such as bumper reinforcements and door beams.
* * * * *