U.S. patent application number 12/241685 was filed with the patent office on 2009-02-05 for magnesium base alloy pipes and method of manufacturing the same.
This patent application is currently assigned to SUMITOMO (SEI) STEEL WIRE CORP.. Invention is credited to Nozomu Kawabe, Yukihiro OISHI, Hitoshi Takahashi, Katsumi Wakamatsu.
Application Number | 20090032151 12/241685 |
Document ID | / |
Family ID | 27792388 |
Filed Date | 2009-02-05 |
United States Patent
Application |
20090032151 |
Kind Code |
A1 |
OISHI; Yukihiro ; et
al. |
February 5, 2009 |
MAGNESIUM BASE ALLOY PIPES AND METHOD OF MANUFACTURING THE SAME
Abstract
A magnesium base alloy pipe having high strength and toughness
is provided along with a method of manufacturing such pipes. A
magnesium base alloy pipe, wherein the pipe is produced by drawing
a pipe blank of a magnesium base alloy comprising containing either
of the following ingredients (1) or (2): (1) about 0.1-12.0 mass %
of Al; or (2) about 1.0-10.0 mass % of Zn and about 0.1-2.0 mass %
of Zr. The novel alloy pipe is manufactured by a method comprising
steps of providing the above-described pipe blank, pointing the
pipe blank, and drawing the pointed pipe blank. The drawing step is
executed at a drawing temperature above approx. 50.degree. C.
Inventors: |
OISHI; Yukihiro; (Itami-shi,
JP) ; Kawabe; Nozomu; (Itami-shi, JP) ;
Takahashi; Hitoshi; (Itami-shi, JP) ; Wakamatsu;
Katsumi; (Itami-shi, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
SUMITOMO (SEI) STEEL WIRE
CORP.
Hyogo
JP
|
Family ID: |
27792388 |
Appl. No.: |
12/241685 |
Filed: |
September 30, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10506509 |
Sep 3, 2004 |
|
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|
PCT/JP03/02524 |
Mar 4, 2003 |
|
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12241685 |
|
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Current U.S.
Class: |
148/667 ;
148/420; 428/586; 72/286; 72/368 |
Current CPC
Class: |
Y10T 428/12292 20150115;
B21C 1/24 20130101; Y10T 428/12792 20150115; C22F 1/06 20130101;
B21C 9/00 20130101; B21C 1/003 20130101; Y10T 428/12764 20150115;
C22C 23/02 20130101; B21C 5/003 20130101; B21C 5/00 20130101; C22C
23/04 20130101 |
Class at
Publication: |
148/667 ;
148/420; 72/286; 72/368; 428/586 |
International
Class: |
C22F 1/06 20060101
C22F001/06; C22C 23/02 20060101 C22C023/02; B21C 29/04 20060101
B21C029/04; B21C 37/06 20060101 B21C037/06; B21K 21/02 20060101
B21K021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2002 |
JP |
2002-057861 |
Mar 4, 2002 |
JP |
2002-057870 |
Mar 7, 2002 |
JP |
2002-062367 |
Mar 7, 2002 |
JP |
2002-062432 |
Mar 25, 2002 |
JP |
2002-083131 |
Dec 2, 2002 |
JP |
2002-350061 |
Mar 3, 2003 |
JP |
2003-055502 |
Claims
1. A magnesium base alloy pipe characterized in that the pipe is
produced by drawing a pipe blank of a magnesium base alloy, the
pipe has a 0.75 or greater YP ratio, wherein the pipe contains
either of the following ingredients compositions (1) or (2): (1)
0.1-12.0 mass % of Al; or (2) 1.0-10.0 mass % of Zn and 0.1-2.0
mass % of Zr; and a tensile strength of the pipe is 250 MPa or
above.
2. The magnesium base alloy pipe according to claim 1 above,
wherein said pipe has a 3% or higher.
3. The magnesium base alloy pipe according to claim 2, wherein said
tensile strength is 350 MPa or above.
4. The magnesium base alloy pipe according to claim 2, wherein said
elongation is in the range of 15-20% and said tensile strength is
in the range of 250-350 MPa.
5. The magnesium base alloy pipe according to claim 2, wherein said
elongation is 5% or above and said tensile strength is 280 MPa or
above.
6. The magnesium base alloy pipe according to claim 5, wherein said
tensile strength is above 300 MPa.
7. The magnesium base alloy pipe according to claim 5, wherein said
elongation is 5% or above but below 12%.
8. The magnesium base alloy pipe according to claim 5, wherein said
elongation is 12% or above.
9. (canceled)
10. The magnesium base alloy pipe according to claim 1, wherein
said YP ratio is 0.75 or above but below 0.90.
11. The magnesium base alloy pipe according to claim 1, wherein
said YP ratio is 0.90 or above.
12. The magnesium base alloy pipe according to claim 1, wherein
said pipe has a 0.2% proof stress of 220 MPa or above.
13. The magnesium base alloy pipe according to claim 12, wherein
said 0.2% proof stress is 250 MPa or above.
14. The magnesium base alloy pipe according to claim 1, wherein
said alloy has a 10 .mu.m or smaller average grain size.
15. The magnesium base alloy pipe according to claim 1, wherein
said alloy has a duplex grain structure comprising fine grains and
coarse grains.
16. The magnesium base alloy pipe according to claim 15, wherein
said alloy has a duplex grain structure comprising grains having a
3 .mu.m or smaller average grain size and grains having a 15 .mu.m
or greater average grain size.
17. The magnesium base alloy pipe according to claim 16, wherein
said grains having a 3 .mu.m smaller average grain size have a 10%
or greater grains area share.
18. The magnesium base alloy pipe according to claim 1, wherein
said alloy has a mixed structure comprising twins and
recrystallized grains.
19. The magnesium base alloy pipe according to any one of the
preceding claims 1 through 8 and 10 through 18, wherein said pipe
has a surface roughness Rz defined by Rz.ltoreq.5 .mu.m on the
surface thereof.
20. The magnesium base alloy pipe according to any one of the
preceding claims 1 through 8 and 10 through 18, wherein said pipe
has a 80 MPa or smaller axial residual tensile stress in the
surface thereof.
21. The magnesium base alloy pipe according to any one of the
preceding claims 1 through 8 and 10 through 18, wherein said pipe
has a 0.02 mm or smaller differential outside diameter.
22. The magnesium base alloy pipe according to any one of the
preceding claims 1 through 8 and 10 through 18, wherein said pipe
has a noncircular cross-sectional shape.
23. The magnesium base alloy pipe according to any one of the
preceding claims 1 through 8 and 10 through 18, wherein said alloy
comprises 0.1-12.0 mass % of Al plus 0.1-2.0 mass % of Mn.
24. The magnesium base alloy pipe according to claim 23, wherein
said alloy comprises 0.1-12.0 mass % of Al plus at least one
ingredient to be selected from the group consisting of 0.1-5.0 mass
% of Zn and 0.1-5.0 mass % of Si.
25. The magnesium base alloy pipe according to any one of the
preceding claims 1 through 8 and 10 through 18, wherein said pipe
have a 0.5 mm or smaller wall thickness.
26. The magnesium base alloy pipe according to any one of the
preceding claims 1 through 8 and 10 through 18, wherein said pipe
comprise a butted pipe having longitudinally a uniform outside
diameter with its inside diameters at its opposite end portions
being smaller than that of its intermediate portion.
27. A method of manufacturing a magnesium base alloy pipe
comprising: a step of providing a pipe blank of any one of the
following magnesium base alloys (A) through (C): (A) a magnesium
base alloy containing 0.1-2.0 mass % of Al; (B) a magnesium base
alloy containing 0.1-12.0 mass % of Al plus at least one ingredient
to be selected from the group consisting of 0.1-2.0 mass % of Mn,
0.1-5.0 mass % of Zn and 0.1-5.0 mass % of Si; or (C) a magnesium
base alloy containing 1.0-10.0 mass % of Zn and 0.1-2.0 mass % of
Zr; a metal pointing step for pointing said pipe blank; and a
drawing step for drawing the resultant pointed pipe blank; wherein
said drawing step is executed at a drawing temperature of
50.degree. C. or above.
28. The method of manufacturing a magnesium base alloy pipe
according to claim 27, wherein heating to said drawing temperature
is accomplished by heating the pipe blank in an atmosphere furnace,
heating the same in a high-frequency heating furnace, or heating a
drawing die.
29. The method of manufacturing a magnesium base alloy pipe
according to claim 27, wherein said drawing temperature ranges from
100.degree. C. to 350.degree. C.
30. The method of manufacturing a magnesium base alloy pipe
according to claim 27, wherein area reduction ratio in one drawing
pass is 5% or above.
31. The method of manufacturing a magnesium base alloy pipe
according to claim 27, wherein said drawing step is accomplished in
a multistep process using plurality of dies.
32. The method of manufacturing a magnesium base alloy pipe
according to claim 27, wherein said drawing step is accomplished by
using at least a die, heating only an initial working portion of a
pointed pipe blank where it contacts said die, and drawing said
pointed pipe blank at the temperature of the thus heated initial
working portion or as it cools naturally therefrom.
33. The method of manufacturing a magnesium base alloy pipe
according to claim 32, wherein a heating temperature of said
initial working portion ranges of 150.degree. C. or more but below
400.degree. C.
34. A method of manufacturing a magnesium base alloy pipe
comprising: a step of providing a pipe blank of any one of the
following magnesium base alloys (A) through (C): (A) a magnesium
base alloy containing 0.1-12.0 mass % of Al; (B) a magnesium base
alloy containing 0.1-12.0 mass % of Al plus at least one ingredient
to be selected from the group consisting of 0.1-2.0 mass % of Mn,
0.1-5.0 mass % of Zn and 0.1-5.0 mass % of Si; or (C) a magnesium
base alloy containing 1.0-10.0 mass % of Zn and 0.1-2.0 mass % of
Zr; a metal pointing step for pointing said pipe blank; and a
drawing step for drawing the resultant pointed pipe blank; wherein
said pointing step is accomplished by heating at least a front
working end of the pipe blank entering a pointing machine.
35. The method of manufacturing a magnesium base alloy pipe
according to claim 34, wherein said front working end is heated at
its portion contacting said pointing machine.
36. The method of manufacturing a magnesium base alloy pipe
according to claim 34, wherein said pointing step is executed by
controlling at least the temperature of said front working end
entering said pointing machine to 50-450.degree. C.
37. The method of manufacturing a magnesium base alloy pipe
according to claim 34, wherein said pointing step is executed with
a heat insulating material inserted in the front end of the pipe
blank.
38. The method of manufacturing a magnesium base alloy pipe
according to claim 34, wherein said pointing step is executed on a
swaging machine by heating the front end of the pipe blank in a
heated liquid.
39. The method of manufacturing a magnesium base alloy pipe
according to claim 27, further comprising a lubrication step for
lubricating at least an initial working portion of the pipe blank
in advance of said drawing step.
40. The method of manufacturing a magnesium base alloy pipe
according to claim 39, wherein said lubrication step comprises
immersing the pipe blank in a preheated lubricant.
41. The method of manufacturing a magnesium base alloy pipe
according to claim 39, wherein said lubrication step forms a
lubricant coating on the pipe blank.
42. The method of manufacturing a magnesium base alloy pipe
according to claim 41, wherein said lubricant coating comprises a
fluorine-based resin.
43. The method of manufacturing a magnesium base alloy pipe
according to claim 42, wherein said fluorine-based resin comprises
a PTFE or PFA.
44. The method of manufacturing a magnesium base alloy pipe
according to claim 41, wherein said lubricant coating is formed by
dispersing a fluorine-based resin in water to prepare an aqueous
dispersion thereof, immersing the pipe blank in said aqueous
dispersion, and heating the pipe blank taken out of said aqueous
dispersion.
45. The method of manufacturing a magnesium base alloy pipe
according to claim 44, wherein the pipe blank taken out of said
aqueous dispersion at about 300-450.degree. C.
46. The method of manufacturing a magnesium base alloy pipe
according to claim 27, wherein said drawing step comprises mandrel
drawing using a mandrel passing through a die and a lubricant
coating is formed on said mandrel.
47. The method of manufacturing a magnesium base alloy pipe
according to claim 27, wherein said drawing step comprises: a first
plain drawing step in which one end of the pipe blank is passed
through a die inside and the pipe blank is drawn without squeezing
its wall between the inside of the die and a plug; a plug drawing
step for squeezing an intermediate portion of the pipe blank
between the inside of the die and the plug; and a second plain
drawing step in which the other end of the pipe blank is drawn
without squeezing its wall between the inside of the die and the
plug; to form a butted pipe having thick-walled opposite ends and a
thin-walled intermediate portion.
48. The method of manufacturing a magnesium base alloy pipe
according to claim 27, wherein said drawing step comprises mandrel
drawing using a mandrel having longitudinally varied outside
diameters to form a butted pipe.
49. The method of manufacturing a magnesium base alloy pipe
according to claim 48, wherein the pipe blank is drawn by grasping
its front working end extending out of a die exit.
50. The method of manufacturing a magnesium base alloy pipe
according to claim 48, wherein said drawing step is executed in
multiple passes by using dies having varied inside diameters.
51. The method of manufacturing a magnesium base alloy pipe
according to claim 27, further comprising a heat treatment step for
heating a drawn pipe at 150.degree. C. or higher temperatures.
52. The method of manufacturing a magnesium base alloy pipe
according to claim 51, wherein said heat treatment step is executed
at 300.degree. C. or lower temperatures.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to a magnesium base alloy
pipes or tubes and a method of manufacturing the same. More
specifically, the present invention relates to a magnesium base
alloy pipes or tubes having improved toughness or high strength and
to a method of manufacturing such pipes.
[0003] 2. Background Art
[0004] Magnesium base alloys, lighter than aluminum and superior to
steel and aluminum in specific strength and specific rigidity, are
extensively used for aircraft parts, automobile parts, etc. as well
as bodies of various electric appliances, or for many other
applications. Particularly, magnesium base alloys have been used
heretofore for press moldings or press-molded products, and as a
method for manufacturing sheet materials for press molding a
rolling process has been typically known (for example, see Patent
Document 1 and Patent Document 2).
[0005] Here, Patent Document 1 is Japanese Patent Provisional
Publication No. 2001-200349, and Patent Document 2 is Japanese
Patent Provisional Publication No. H06-293944.
[0006] For the magnesium base alloys having excellent properties as
described above, it is desired that they are used not only as sheet
materials but as pipes or tubular materials.
[0007] However, since magnesium and its alloys have a close-packed
hexagonal lattice structure, they lacks in ductility with their
marked inferiority in plastic workability. Therefore, it has been
very difficult so far to produce pipes of magnesium or its
alloys.
[0008] Although magnesium base alloy pipes may be hot-extruded into
pipes, the resultant pipes have been hardly used for structural
materials because of their very low strength. For example, such
hot-extruded pipes of magnesium alloy are not in the least superior
to pipes of aluminum alloy in strength.
[0009] Accordingly, it is a principal object of the present
invention to provide magnesium base alloy pipes or tubes having
high strength or improved toughness or tenacity and a method of
manufacturing such pipes.
[0010] Further, another object of the present invention is to
provide magnesium base alloy pipes having a high YP ratio (a ratio
of 0.2% proof stress vs. tensile strength, to be described herein
later) and method of manufacturing such pipes.
DISCLOSURE OF INVENTION
[0011] The inventors have studied from various aspects the drawing
technique of magnesium base alloys that had been deemed till then
difficult in the art to find out that drawn magnesium base alloy
pipes can be improved in their strength and ductility by using
specified processing conditions in drawing, and have accomplished
the present invention based on those findings.
[0012] Further, the inventors have found out that a drawing
process, when combined as required with a prescribed beat treatment
as a process subsequent to the drawing, is effective for producing
magnesium base alloy pipes that can compatibly satisfy both a high
YP ratio and a high ductility at high strength, and based on such
findings have accomplished the present invention.
(Magnesium Base Alloy Pipes)
[0013] Specifically, in one aspect, the present invention provides
a magnesium base alloy pipe, wherein the pipe is produced by
drawing a pipe blank of a magnesium base alloy containing either of
the following chemical ingredients (1) or (2):
[0014] (1) about 0.1-12.0 mass % of Al;
[0015] (2) about 1.0-10.0 mass % of Zn and about 0.1-2.0 mass % of
Zr.
[0016] For magnesium base alloy pipes of the present invention,
both casting magnesium base alloys and wrought magnesium base
alloys may be used alike. More specifically, magnesium base alloys
belonging to AZ, AS, AM, or ZK types in the ASTM Code may be used,
for example. Also, the magnesium base alloys may be subdivided in
terms of Al content into two groups, namely magnesium base alloys
containing 0.1 to less than 2.0 mass % of Al, and those containing
2.0 to 12.0 mass % of Al. Besides the above-mentioned chemical
ingredients, the magnesium base alloys, as practically used,
contain unavoidable impurities in addition to their main ingredient
Mg, as is known to those who skilled in the art. The unavoidable
impurities include Fe, Si, Cu, Ni, Ca, etc.
[0017] In the ASTM AZ type, alloys subject to AZ31, AZ61, AZ91, for
example, have an Al content in the range of 2.0-12.0% by mass. The
AZ31 represents, for example, magnesium base alloys containing
2.5-3.5 mass % of Al, 0.5-1.5 mass % of Zn, 0.15-0.5 mass % of Mn,
0.05 or less mass % of Cu, 0.1 or less mass % of Si, and 0.04 or
less mass % of Ca. The AZ61 represents, for example, magnesium base
alloys containing 5.5-7.2 mass % of Al, 0.4-1.5 mass % of Zn,
0.15-0.35 mass % of Mn, 0.05 or less mass % of Ni, and 0.1 or less
mass % of Si. The AZ91 represents, for example, magnesium base
alloys containing 8.1-9.7 mass % of Al, 0.35-1.0 mass % of Zn, 0.13
or more mass % of Mn, 0.1 or less mass % of Cu, 0.03 or less mass %
of Ni, and 0.5 or less mass % of Si. The magnesium base alloys of
the ASTM AZ type containing about 0.1 to less than-2.0 mass % of Al
are represented by AZ10 and AZ21, for example. The AZ10 represents,
for example, magnesium base alloys containing 1.0-1.5 mass % of Al,
0.2-0.6 mass % of Zn, 0.2 or more mass % of Mn, 0.1 or less mass %
of Cu, 0.1 or less mass % of Si, and 0.4 or less mass % of Ca. The
AZ21 represents, for example, magnesium base alloys containing
1.4-2.6 mass % of Al, 0.5-1.5 mass % of Zn, 0.15-0.35 mass % of Mn,
0.03 or less mass % of Ni, and 0.1 or less mass % of Si.
[0018] The magnesium base alloys of the ASTM AS type containing
2.0-12.0 mass % of Al are represented by AS41, for example. The
AS41 represents, for example, magnesium base alloys containing
3.7-4.8 mass % of Al, 0.1 or less mass % of Zn, 0.15 or less mass %
of Cu, 0.35-0.60 mass % of Mn, 0.001 or less mass % of Ni, and
0.6-1.4 mass % of Si. In the ASTM AS type, alloys subject to AS21,
for example, have an Al content in the range of 0.1 to less than
2.0% by mass. The AS21 represents, for example, magnesium base
alloys containing 1.4-2.6 mass % of Al, 0.1 or less mass % of Zn,
0.15 or less mass % of Cu, 0.35-0.60 mass % of Mn, 0.0401 or less
mass % of Ni, and 0.6-1.4 mass % of Si.
[0019] In the ASTM AM type, the AM60 represents, for example,
magnesium base alloys containing 5.5-6.5 mass % of Al, 0.22 or less
mass % of Zn, 0.35 or less mass % of Cu, 0.13 or more mass % of Mn,
0.03 or less mass % of Ni, and 0.5 or less mass % of Si. The AM100
represents, for example, magnesium base alloys containing 9.3-10.7
mass % of Al, 0.3 or less mass % of Zn, 0.1 or less than % of Cu,
0.1-0.35 mass % of Mn, 0.01 or less mass % of Ni, and 0.3 or less
mass % of Si.
[0020] In the ASTM ZK type, the ZK60 represents, for example,
magnesium base alloys containing about 4.8-6.2 mass % of Zn, and
0.45 or more mass % of Zr.
[0021] Although magnesium can hardly provide any sufficient
strength when used as a simple substance material, its alloy
containing either 0.1-2.0 mass % of Al or 1.0-10.0 mass % of Zn and
0.1-2.0 mass % of Zr, as described above, and processed through a
prescribed drawing can have desirable strength. It is preferred
that the magnesium base alloys containing about 0.1-12.0 mass % of
Al for the pipe according to the present invention contain about
0.1-2.0 mass % of Mn. Also, the magnesium base alloys containing
about 0.1-12.0 mass % of Al for the pipe according to the present
invention may preferably contain at least one of about 0.1-5.0 mass
% of Zn and 0.1-5.0 mass % of Si. In this regard, a more preferable
Zn content ranges from about 0.1 to 2.0 mass % and a more
preferable Si content ranges from 0.3 to 2.0 mass %. Drawing under
conditions described later a magnesium base alloy containing the
above-described elements as additives can yield a magnesium base
alloy pipe having not only mechanical strength but also toughness.
A more preferable Zr content is about 0.4-2.0% by mass.
[0022] Moreover, since the magnesium base alloy pipes of the
present invention combine high strength and outstanding toughness
such as a 3% or higher elongation (elongation after fracture) and
250 MPa or higher tensile strength, they exhibit higher specific
strength as compared with like materials in the prior art and thus
may be applicable to structural materials for lightweight-oriented
fields where such strength is particularly required. The magnesium
base alloy pipes according to the present invention, thus having
such high strength and toughness, can advantageously secure safety
when used as such structural materials.
[0023] According to the present invention, preferable minimum
tensile strength varies among 250, 280, 300, 320 and 350 MPa or
higher. Magnesium base alloy pipes having 350 MPa or higher tensile
strength with an 3% or larger elongation have larger specific
strength as compared with the conventional materials and may most
preferably used for structural materials in lightweight-oriented
applications where the strength matters particularly. Of course, it
goes without saying that magnesium base alloy pipes with tensile
strength 350 MPa or less may also be practically used in various
applications. A more preferable elongation is 8% or above and,
particularly preferably, 15% or above. Especially, magnesium base
alloy pipes having 250-350 MPa tensile strength with a 15-20%
elongation exhibit excellent toughness, and can be subjected to
bending at small radii and thus are expectedly applicable to
diverse structural materials. More specifically, if such magnesium
base alloy pipes have an outside diameter of D (mm), they can be
bent easily at a bending radius 3D or less. Also, according to the
present invention, the magnesium base alloy pipes may be subdivided
in terms of elongation into two groups, namely pipes having an
elongation of not less than 5% but less than 12% and pipes having a
12% or higher elongation. Typically, magnesium base alloys with a
20% or lower elongation may be practically used.
[0024] In another aspect, the present invention provides a
magnesium base alloy pipe having chemical compositions as described
herein above, wherein the pipe has a YP ratio not less than
0.75.
[0025] The YP ratio herein referred to is a ratio given by "0.2%
proof stress/tensile strength." For applying a magnesium base alloy
to a structural material, high strength is desired. In this
connection, since the working limit is determined not based on
tensile strength but on 0.2% proof stress, it is necessary to
increase not only the absolute tensile strength but also YP ratio
for providing high-strength magnesium base alloy. Magnesium base
alloy pipes obtained by the conventional hot extrusion technique
have a YP ratio in the range of 0.5 or more but less than 0.75,
which is not in the least high as compared with ordinary structural
materials, and thus it has been demanded to increase the YP ratio
of such pipes. Accordingly, the present invention provides a
magnesium base alloy pipe having a 0.75 or higher YP ratio that has
not been achieved in the prior art by using specified processing
conditions in drawing operation including drawing temperature,
working ratio or reduction ratio, heating rate to the drawing
temperature and drawing speed, and as required by applying a
prescribed heat treatment after drawing, as described herein
below.
[0026] For example, a magnesium base alloy pipe with a 0.90 or
higher YP ratio can be obtained by performing drawing at a 1 m/sec.
or higher drawing speed at a drawing temperature ranging from about
50.degree. C. to 300.degree. C. (more preferably from 100.degree.
C. to 200.degree. C., still more preferably from 100.degree. C. to
150.degree. C.) with a heating rate to drawing temperature of
1.degree. C./sec.-100.degree. C./sec. and a working ratio above 5%
per one drawing pass (more preferably above 10% or more,
particularly preferably above 20%. Further, a magnesium base alloy
pipe having a YP ratio in the range of 0.75 or more but less than
0.90 can be produced by providing a cooling step after the
aforesaid drawing step and then subjecting the pipe to a heat
treatment at a temperature ranging from 150.degree. C. (preferably
200.degree. C.) to 300.degree. C. over a retention time of 5 min or
longer. Although a higher YP ratio typically represents higher
strength, since such a high YP ratio naturally leads to a lower
workability when some postprocessing such as bending is involved,
magnesium base alloy pipes having a YP ratio in the range of 0.75
or more but less than 0.90 are favorably practical in view of
manufacturability or productivity. According to the present
invention, a more preferable YP ratio ranges from 0.80 or more but
less than 0.90.
[0027] In its third aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein the pipe has 0.2% proof stress not less than
220 MPa.
[0028] The working limit of a structural material is determined
depending on its 0.2% proof stress, as described above.
Accordingly, the present invention provides a magnesium base alloy
pipe having higher specific proof stress as compared with the
conventional materials, specifically 0.2% proof stress not less
than 220 MPa, by using specified processing conditions in drawing
operation including drawing temperature, working ratio or reduction
ratio, heating rate to the drawing temperature and drawing speed.
More preferably, the 0.2% proof stress is not less than 250
MPa.
[0029] In its fourth aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein its magnesium base alloy has an average grain
size of 10 .mu.m (micrometers) or below.
[0030] A magnesium base alloy pipe having both strength and
toughness in balance can be produced by making fine the average
grain size of its magnesium base alloy. The control of average
grain size is effected by adjusting processing conditions in
drawing operation such as working ratio, drawing temperature, or
heat treatment temperature after drawing. For reducing the average
grain size to 10 .mu.m or smaller, it is preferable to heat-treat
the pipe at 200.degree. C. or higher temperatures after
drawing.
[0031] Especially, by providing a fine crystal structure with an
average grain size of 5 .mu.m or below, a magnesium base alloy pipe
satisfying its strength and toughness requirements further in
balance. Such a fine crystal structure having an average grain size
of 5 .mu.m or below may be obtained by applying a heat treatment at
a temperature preferably in the range of 200.degree. C. to
250.degree. C. after drawing.
[0032] In its fifth aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein its magnesium base alloy has a mixed or duplex
grain structure comprising fine grains and coarse grains.
[0033] By providing the alloy with a duplex grain structure, a
magnesium base alloy pipe combining strength with toughness in
balance can be obtained. As an example, one typical duplex grain
structure in such a magnesium base alloy comprises fine grains with
an average grain size of 3 .mu.m or below and coarse grains having
an average grain size not smaller than 15 .mu.m. Especially, by
controlling to a 10% or grater level the ratio of the total area
shared by 3 .mu.m or smaller grains in any section of an alloy
sample (hereinafter shall be briefly referred to as "grains area
share"), a magnesium base alloy pipe having further improved
strength and toughness can be attained. Such a duplex grain
structure can be created by a combination of a drawing process with
a heat treatment subsequent thereto, as will be described herein
later. For this, the heat treatment is performed preferably at
150.degree. C. or above but below 200.degree. C.
[0034] In its sixth aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein its alloy has a mixed or hybrid structure
comprising twins and recrystallized grains.
[0035] By providing such a mixed structure, a magnesium base alloy
pipe can have improved strength and toughness in balance.
[0036] In its seventh aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein the surface roughness Rz of its alloy
satisfies a condition given by: Rz.ltoreq.5 .mu.m.
[0037] In its eighth aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein the pipe has in its surface an axial residual
tensile stress not greater than 80 MPa.
[0038] In its ninth aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein the pipe has a 0.02 mm or smaller differential
outside diameter. The "differential outside diameter" herein
referred to is the difference between the largest and the smallest
outside diameters in a cross section of the pipe.
[0039] For a magnesium base alloy pipe, a surface smoothness, an
axial residual tensile stress below a predetermined value or a
differential outside diameter below a predetermined value leads to
an excellent precision workability in that such factors are
effective for improving accuracy in bending and the like
processes.
[0040] The surface roughness of a pipe can be controlled mainly by
adjusting the working temperature in its drawing. In addition, the
surface roughness may be affected by the drawing speed, a lubricant
used for working, and so forth. The axial residual tensile stress
may be adjusted through the setting of drawing conditions
(temperature, working ratio), etc. The differential outside
diameter may be controlled through the drawing die configuration or
by controlling the drawing temperature, drawing direction, etc.
[0041] In its tenth aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein the pipe has a noncircular external
configuration in cross section.
[0042] Most typically, both the inner and outer walls of a pipe are
circular (concentric to each other) in cross section. However, the
magnesium base alloy pipe according to the present invention having
also improved toughness may be easily fabricated as various
odd-shaped pipes such as those having elliptical, rectangular,
polygonal, or other cross sections, not limited to circular pipes.
A process for forming a noncircular pipe may be readily
accommodated by modifying or changing the die configuration.
Further, there may be a case where it is desired to form such a
pipe for any structural material as having concavities and
convexities partially in its external surface with its cross
sections varying in shape locally along its longitudinal direction.
Such an odd-shaped pipe having cross sections varying in shape
locally along the longitudinal direction may be obtained by
subjecting a drawn pipe to form rolling. Even when embodied as
odd-shaped pipes, the magnesium base alloy pipe according to the
present invention can have similar desirable properties as
properties of those pipes having the same circular cross-section
through their entire lengths and may be applied also to structural
materials such as various kinds of frame materials including
materials for bicycles, motorcycles and so on.
[0043] In its eleventh aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein the pipe has a wall thickness of 0.5 mm or
below.
[0044] So far, the drawing technique has not been able to
substantially provide any magnesium base alloy pipes withstanding
their practical use, and even those magnesium base alloy pipes
produced by extrusion exceed 1.0 mm in wall thickness. According to
the present invention, a thin-walled magnesium base alloy pipe can
be obtained through drawing by using prescribed drawing conditions
to be described herein later. Especially, magnesium base alloy
pipes having a wall thickness of 0.7 mm or below, preferably 0.5 mm
or below, may be provided also.
[0045] Such thin-walled alloy pipes are obtained by drawing.
Heretofore, the production of magnesium base alloy pipes has been
limited to short-length products by extrusion or like processes at
the best due to difficulties involved in their working. The
elongation of the prior art magnesium base alloy pipes have varied
widely from 5 to 15% and their tensile strength were at the level
of about 240 MPa at the best. According to the present invention,
thin-walled alloy pipes having improved toughness and strength can
be produced by drawing.
[0046] In its twelfth aspect, the present invention provides a
magnesium base alloy pipe having the above-described chemical
composition, wherein the pipe comprises a butted pipe having
longitudinally a uniform outside diameter with its inside diameters
at its opposite end portions being smaller than that of its
intermediate portion.
[0047] Since the magnesium base alloy pipe of the present invention
has high strength and toughness, it may be readily formed into a
butted pipe and is applicable to bicycle flames, etc. Generally,
the butted pipe has a uniform outside diameter longitudinally,
while its inside diameters being reduced at its opposite ends as
compared with its intermediate portion.
(Method of Manufacturing Magnesium Base Alloy Pipes)
[0048] In one aspect, the method of manufacturing the magnesium
base alloy pipes according to the present invention comprises:
[0049] a step of providing a pipe blank of any one of the following
magnesium base alloys (A) through (C): [0050] (A) a magnesium base
alloy containing about 0.1-12.0 mass % of Al; [0051] (B) a
magnesium base alloy containing about 0.1-12.0 mass % of Al plus at
least one ingredient to be selected from the group consisting of
about 0.1-2.0 mass % of Mn, 0.1-5.0 mass % of Zn and 0.1-5.0 mass %
of Si; or [0052] (C) a magnesium base alloy containing about
1.0-10.0 mass % of Zn and 0.1-2.0 mass % of Zr;
[0053] a metal pointing step for pointing said pipe blank; and
[0054] a drawing step for drawing the resultant pointed pipe blank,
wherein said drawing step is executed at a drawing temperature
50.degree. C. or above.
[0055] By executing the drawing the in such a temperature zone, the
magnesium base alloy pipe can be improved in at least one of
strength and toughness. Especially, a magnesium base alloy pipe
best-suited to structural materials requiring a light weight in
addition to strength can be obtained, such structural materials
including those pipes used for chairs, tables, pickels (ice axe),
or pipes for bicycle frames or like frames.
[0056] In another aspect, the method of manufacturing the magnesium
base alloy pipes according to the present invention comprises:
[0057] a step of providing a pipe blank of any one of the following
magnesium base alloys (A) through (C): [0058] (A) a magnesium base
alloy containing about 0.1-12.0 mass % of Al; [0059] (B) a
magnesium base alloy containing about 0.1-12.0 mass % of Al plus at
least one ingredient to be selected from the group consisting of
about 0.1-2.0 mass % of Mn, 0.1-5.0 mass % of Zn and 0.1-5.0 mass %
of Si; and [0060] (C) a magnesium base alloy containing about
1.0-10.0 mass % of Zn and 0.1-2.0 mass % of Zr;
[0061] a metal pointing step for pointing said pipe blank; and
[0062] a drawing step for drawing the resultant pointed pipe blank,
wherein said pointing step is executed by heating at least a front
working end of the pipe blank entering a pointing machine. It is
preferred that when the pipe blank is fed in the pointing machine
at least its front end portion is heated at 50-450.degree. C., more
preferably 100-250.degree. C.
[0063] The pointing step thus executed as involving heating is
effective for preventing the resultant pipe from undergoing
cracking.
[0064] According to the present invention, the magnesium base alloy
pipes are manufactured through the following process steps:
providing pipe blanks.fwdarw.(film
coating).fwdarw.pointing.fwdarw.drawing (heat
treatment).fwdarw.straightening. Among these steps, the film
coating and heat treatment are performed as required. Hereafter,
each of these process steps will be described in detail.
[0065] According to the present invention, pipes produced by
casting or extrusion may be used as the pipe blanks. Of course,
pipes drawn by the method of the present invention may be also used
as the pipe blanks for further processing.
[0066] It is preferred that the pipe blank is lubricated at least
its front end portion before drawing. The film coating, which is
one type of lubrication, is accomplished by coating the pipe blank
with a lubricant. For the lubricant coating, it is preferred to use
a material exhibiting in drawing an adequate thermal resistance at
the drawing temperature and having a small surface frictional
resistance as coating. As such materials, fluorine-based resins
such as a polytetrafluoroethylene (PTFE) and a
tetrafluoro-perfluoroalkyl vinyl ether resin (PFA) are preferred,
for example. More specifically, the film coating may be
accomplished, for example, by first dispersing a water-dispersible
PTFE or PFA in water to prepare its aqueous dispersion liquid, then
immersing a pipe blank in this dispersion liquid, and subsequently
heating the wet pipe blank at about 300-450.degree. C. to form PTFE
or PFA coating on the pipe surface. The lubricant coating formed by
this film coating remains in the drawing step to be described
herein later and acts to prevent seizing of the pipe blank. For the
pipe blanks subjected to the film coating, a separate immersion in
lubricant step as described herein later may or may not be applied
in combination therewith.
[0067] The pointing is done to reduce the diameter of an end of a
pipe blank so that the end of the pipe blank may be inserted into a
die hole in the succeeding drawing process. For this pointing, is
used a pointing machine such as a swaging machine. The pointing is
performed by heating at least the front working end of the pipe
blank at 50-450.degree. C. This temperature shall be referred to
herein after as an "inlet temperature." The "front working end"
herein referred to is a front end portion of the pipe blank where
it is reduced in diameter by a pointing machine. More preferably,
the inlet temperature ranges from 100 to 250.degree. C. The inlet
temperature is a temperature of the pipe blank at its front working
end just before it is fed in the pointing machine.
[0068] For this purpose, means for heating the pipe blank are not
particularly limited. For example, the pipe blank temperature at
its front end may be controlled by first heating it by means of a
suitable heater in advance and then appropriately varying the time
elapsing before it is fed in a swaging machine. However, it is
desired to minimize the temperature drop before the pipe blank is
fed in the pointing machine after its heating. Especially, it is
preferred to heat a part (usually die) of the pointing machine
contacting the pipe blank. It is also preferred that the pointing
is performed with a heat insulating material made of a magnesium
base alloy, or other alloy or a metal inserted in the front end of
the pipe blank. When the pipe blank is fed in the swaging machine,
the pipe blank begins to cool due to its contact with the die.
However, the existence of the heat insulating material acts to
deter the temperature drop at the end of the pipe blank during
pointing so as to permit the pointing step to be executed while
inhibiting cracking of the pipe. To cite a typical example of the
heat insulating materials, copper or like materials that are easier
to work than the magnesium base alloy may be employed.
[0069] Working ratio (outside diameter reduction ratio) in the
pointing step is preferably 30% or below. If the working ration
exceeds 30%, the pipe blank tends to undergo cracking in the course
of pointing. For inhibiting cracking more assuredly, the working
ratio should be preferably 15% or below or, more preferably, 10% or
below.
[0070] The thus pointed pipe blank is then subjected to the drawing
process. The drawing step is executed by passing the pipe blank
through a die or the like. For this, any time-proven technique for
pipe drawing involving copper alloys, aluminium alloys or other
alloys may be employed. Such techniques includes, for example, (1)
plain drawing in which a pipe blank is passed through a hole die
without providing any specific member inside the pipe blank, (2)
plug drawing using a plug provided inside the pipe blank, and (3)
mandrel drawing using a mandrel passing through a die. The plug
drawing includes fixed plug drawing in which a plug 2 having a
longer straight portion is fixed at the front end of a bearing rod
1 and the pipe blank 4 is subjected to drawing through a space
defined between the plug 2 and the die 3, as shown in FIG. 1 (A).
Besides, the plug drawing includes: floating plug drawing which
employs a plug 2 without using a bearing rod, as shown in FIG. 1
(B); and semi-floating plug drawing in which a plug 2 having a
shorter straight portion is fixed at the front end of the bearing
rod 1 and the drawing is performed through a space defined by the
plug 2 and the die 3, as shown in FIG. 1 (C). While, in the mandrel
drawing, a mandrel 5 passing through the die 3 is disposed through
the entire pipe blank length, and the drawing is accomplished
through a space defined by the die 3 and the mandrel 5, as shown in
FIG. 1 (D). In this connection, the drawing can be carried out more
smoothly if the mandrel is coated with a lubricant. Especially, the
mandrel drawing is suitable for producing an alloy pipe having a
wall thickness of 0.7 mm or below.
[0071] Among others, a combination of the plain drawing and the
plug drawing facilitates fabrication of butted pipes. That is, the
drawing process may be executed in the following manner, for
example. First, one end of a pipe blank is passed through a die and
the pipe blank is subjected to plain drawing without squeezing its
wall between the inner wall of the die and a plug. Then, the
intermediate portion of the pipe blank is subjected to plug drawing
so as to squeeze its pipe blank between the inner wall of the die
and the plug. Further, the other end of the pipe blank is subjected
also to plain drawing without squeezing its wall between the inner
wall of the die and the plug. This process can form a butted pipe
with thick-walled opposite end portions and a thin-walled
intermediate portion. Besides, in the mandrel drawing using a
mandrel passed through the die, a butted pipe may be formed by
using a mandrel having its diameter varied along its length. For
this, it may be appropriate that the pipe blank is drawn by
grasping the front working end of the pipe blank extending out of
the die exit. For drawing the pipe blank as it is grasped, a
drawbench or a like means may be used. Further, if this drawing
operation is repeated two or more times by using varied diameters
of die orifice, butted pipes may also be formed effectively. By
repeating the drawing more than once with varied diameters of die
orifice, a butted pipe having a great difference in thickness
between its thin-walled and thick-walled portions can be
manufactured.
[0072] Further, according to the present invention, the drawing is
executed at 50.degree. C. or higher temperatures. Drawing
temperatures above approx. 50.degree. C. allows easier working of
pipes. However, since the strength decreases as the drawing
temperature increases, the temperature is preferably below approx.
350.degree. C. The drawing temperature ranges preferably from
100.degree. C. to 300.degree. C., more preferably from 100.degree.
C. to 200.degree. C., or most preferably from 100.degree. C. to
150.degree. C.
[0073] The drawing temperature herein referred to represents a
temperature of the pipe blank before or after it is fed in the die
or temperature setting at a heating means. For example, the drawing
temperature may be a temperature of the pipe blank just before it
is fed in the die or a temperature of the pipe blank just after the
die exit (namely drawn pipe temperature), or a temperature set on a
heater when such a heater is provided at a place just before the
die. In any case, there is no substantial difference among them
practically. However, since the pipe blank temperature just after
the die exit tends to vary with such factors as working ratio,
working speed, die temperature, pipe configuration, and type of
drawing (mandrel drawing, plug drawing, etc.), it is easier to
specify the drawing temperature as a temperature of the pipe blank
just before the die inlet.
[0074] For heating to this drawing temperature, the pipe blank may
be heated only at its front end portion, or it may be heated
wholly. Whatever the case may be, the heating applied in the
above-described manner is effective for producing a magnesium base
alloy pipe excellent in both strength and toughness. Especially, it
is preferred to heat an initial working portion of the pipe blank
that first contacts at least the die. The initial working portion
differs from the aforementioned front working end for pointing.
That is to say, in the drawing operation, since the pipe blank
contacts a die (and a plug or a mandrel) first by the root portion
of its front working end and its effective drawing is started
thereat, the initial working portion means this starting point of
drawing, i.e. the root portion of the front working end. More
specifically, the initial working portion in the plain drawing is a
position on the pipe blank where it first contacts the die, while
for the plug drawing the initial working portion is a position on
the pipe blank where it first contacts the die and the plug, and
for the mandrel drawing the initial working portion is a point on
the pipe blank where it first contacts the die and the mandrel.
[0075] For heating a pipe blank, it is preferred that the pipe
blank is heated by immersing the same in a preheated lubricant, or
is heated in an atmosphere furnace or in a high-frequency heating
furnace. Alternatively, it may be heated by heating the drawing
die. Among others, the immersion of the pipe blank in preheated
lubricant is preferred in that heating is accomplished along with
lubrication. The temperature of the pipe blank at the die exit
(exit temperature) may be adjusted by varying the time (cooling
time) it stands to cool before it is fed in the drawing die after
heating. For lubricating the pipe blank, forced lubrication may be
employed in addition to film coating or immersion in lubricant
described above. For the forced lubrication, a pressurized
lubricant is forcedly supplied into a gap between the die and the
pipe blank during drawing. The lubricant may be of powder or
oil.
[0076] By drawing the pipe blank by applying thereto a combination
of its lubrication and heating as described above, it is possible
to inhibit seizing or fractures occurring in drawing. Especially,
it is preferred that the pipe blank is drawn under predetermined
heating conditions after the above-described pointing process.
[0077] For example, the pipe blank is drawn based on plug-drawing
using a die in combination with a plug, where only the initial
working portion of the pipe blank may be heated without otherwise
heating its remaining portion during the drawing, or the pipe blank
may be drawn as it cools down naturally from a temperature to which
it was heated beforehand. The heating temperature of the initial
working portion is preferably in the range of 150.degree. C. or
above but below 400.degree. C.
[0078] According to the present invention, the heating rate to the
aforementioned drawing temperature is preferably in the range of
1.degree. C./sec.-100.degree. C./sec. Meanwhile, the drawing speed
is preferably 1 m/min. or above.
[0079] The drawing may be executed in multiple passes and/or a
multistep process. By executing the drawing in such repeated
multiple passes, it is possible to obtain smaller-diameter
pipes.
[0080] Further, according to the present invention, it is preferred
that the reduction in cross-sectional area (hereinafter shall be
referred to as "area reduction ratio") in 1 drawing pass is 5% or
above. A low working ratio yields only a small increase in
strength, while drawing with an area reduction ratio of 5% or above
facilitates fabrication of magnesium base alloy pipes having
adequate strength and toughness. More preferably, the area
reduction ratio per drawing pass should be 10% or above or, most
preferably, 20% or above. However, since an excessively large
working ratio is impractical, the area reduction ratio per pass
ranges up to about 40%.
[0081] It is also preferred that the total area reduction ratio in
drawing is 15% or above. More preferably, the total area reduction
ratio is 25% or above. By drawing the pipe blank with the total
area reduction ratio not less than 15% as above, is allowed
production of magnesium base alloy pipes combining strength and
toughness in balance.
[0082] Preferably, the cooling rate of the work after drawing is
not less than 0.1.degree. C./sec. This is because a cooling rate
below this lower limit will act to accelerate grain growth. For
cooling, air cooling including air blast cooling, etc. may be used,
and the cooling rate may be controlled by adjusting the wind
velocity, air flow or the like.
[0083] By drawing a pipe blank having a chemical composition
according to the present invention by the above-described method, a
magnesium base alloy pipe having an elongation of 3% or above and
tensile strength of 350 MPa or above can be produced.
[0084] Further, by heating the drawn pipe at above approx.
150.degree. C. (preferably above approx. 200.degree. C.), the
relief of strain induced in drawing and the recrystallization in
the alloy are accelerated to permit an additional improvement in
toughness. This heat treatment is performed at temperatures
preferably 300.degree. C. or below. The retention time of this
heating temperature ranges preferably from about 5 to 60 minutes.
More preferably, the lower limit of the retention time is in the
range of about 5 to 15 minutes with its upper limit in the range of
about 20 to 30 minutes. By this heat treatment, a magnesium base
alloy pipe having approx. 15-20% tensile strength and approx.
250-350 MPa elongation can be produced. It is also to be noted here
that the pipe produced by the method of the present invention can
be used practically as intended without applying the heat treatment
at temperatures above approx. 150.degree. C. after drawing.
BRIEF DESCRIPTION OF DRAWINGS
[0085] FIG. 1 shows schematic diagrams (A) through (D) illustrating
typical methods of drawing pipes, respectively;
[0086] FIG. 2 shows a graph illustrating a relationship between the
outside diameter and the working ratio of pipes of an AZ31
alloy;
[0087] FIG. 3 is shows a similar graph illustrating a relationship
between the outside diameter and the working ratio of pipes of an
AZ61 alloy;
[0088] FIG. 4 shows a graph illustrating a relationship between the
working ratio and the drawing force;
[0089] FIG. 5 is a micrograph showing a structure of metal of the
alloy specimen No. 17-8 in the experimental example 2-3 according
to the present invention;
[0090] FIG. 6 shows schematic diagrams (A) and (B) illustrating
processes of manufacturing a butted tube, with (A) representing
plain drawing process and (B) a plug drawing process.
[0091] FIG. 7 shows a longitudinal section of a butted tube.
BEST MODE FOR CARRYING OUT THE INVENTION
[0092] Hereinafter, the present invention will be described in
detail based on the preferred embodiments thereof such as those
included in the following experimental examples.
Experimental Example 1-1
[0093] An extruded pipe (outside diameter: 15.0 mm, wall thickness:
1.5 mm) of an alloy subject to the ASTM AZ31 (shall be referred to
as AZ31 alloy and like nomenclature shall apply hereinafter) and an
extruded pipe of an AZ61 alloy having the same configuration as
above were drawn, respectively, at varied temperatures to be
reduced to 12.0 mm in outside diameter, thus yielding various
specimens of drawn magnesium base alloy pipes. The AZ31 alloy of
the extruded pipe used was a magnesium base alloy containing 2.9
mass % of Al, 0.77 mass % of Zn and 0.40 mass % of Mn with the
balance composed of Mg as a base material and unavoidable
impurities, while the AZ61 alloy of the extruded pipe was a
magnesium base alloy containing 6.4 mass % of Al, 0.77 mass % of Zn
and 0.35 mass % of Mn with the balance likewise comprising Mg and
unavoidable impurities. The drawing was accomplished in two passes
of plain drawing, with the first pass reducing the outside diameter
to 13.5 mm and the second pass reducing the same to 12.0 mm. The
first and the second passes yielded area reduction ratios of 10.0%
and 12.3%, respectively, with the total area reduction ratio of
21.0%. After drawing, the pipes were air-cooled at a rate of
1-5.degree. C./sec. For heating the pipes in drawing, a heater was
provided before a die, and the working temperature (drawing
temperature) was observed in terms of the heating temperature set
on the heater. This applies also in experimental examples 1-2
through 1-10 to be described herein later. The heating rate to the
drawing temperature was in the range of 1-2.degree. C./sec., and
the drawing speed was approx. 10 m/min. The resultant drawn pipes
had the properties as shown in Table 1.
TABLE-US-00001 TABLE 1 Area reduction Elongation after 0.2% proof
Alloy Specimens Working temp. ratio Tensile strength fracture
stress type No. (.degree. C.) (%) (MPa) (%) (MPa) YP ratio AZ31 1-1
Not worked (extruded pipe) 245 9.0 169 0.69 1-2 20 21 Working
impossible 1-3 50 21 395 6.0 380 0.96 1-4 100 21 380 8.0 362 0.95
1-5 200 21 345 10.5 321 0.93 1-6 300 21 303 11.5 279 0.92 AZ61 1-7
Not worked (extruded pipe) 285 6.0 188 0.66 1-8 20 21 Working
impossible 1-9 50 21 462 6.0 432 0.94 1-10 100 21 451 8.0 422 0.94
1-11 200 21 439 8.5 408 0.93 1-12 300 21 412 10.5 382 0.93
[0094] As shown in Table 1, the extruded pipes (specimens No. 1-1
and 1-7 representing comparative examples) of AZ31 and AZ61 alloys,
respectively, have tensile strength of 290 MPa or below, 0.2% proof
stress of 190 MPa or below, a YP ratio of 0.70 or below, and an
elongation (elongation after fracture) of 6-9%. While, the drawn
pipe specimens No. 1-3 through 1-6 and the specimens 1-9 through
1-12 drawn at temperatures above 50.degree. C., embodying the
present invention, have tensile strength above 300 MPa, 0.2% proof
stress above 250 MPa, a YP ratio above 0.90 in addition to an
elongation duly above 5%. Thus, it will be clearly understood that
the specimens prepared according to the present invention are
improved in their strength without greatly reducing its toughness.
Among those last specimens, the No. 1-4 through 1-6 specimens and
No. 1-10 through 1-12 specimens which were drawn at temperatures
from 100.degree. C. to 300.degree. C. are improved particularly in
toughness with their further higher elongations above 8%.
Therefore, it will be understood that the drawing temperature is
preferably in the range of 100.degree. C. to 300.degree. C. in view
of elongation after fracture. When the drawing temperature exceeded
300.degree. C. the rate of increase in tensile strength became low,
while the specimens No. 1-2 and 1-8 subjected to drawing at a room
temperature, namely 20.degree. C., representing another comparative
examples, could not actually withstand working due to breakage.
Thus, it turns out that the balance of strength and toughness is
more improved by employing a drawing temperature in the range of
approx. 50.degree. C. to 300.degree. C. (preferably 100.degree. C.
to 300.degree. C.).
[0095] For the resultant specimens No. 1-3 through 1-6 and 1-9
through 1-12, it was possible to apply drawing even in 3 or more
passes. These specimens No. 1-3 through 1-6 and 1-9 through 1-12
had surface roughness Rz of 5 um or below and an axial residual
tensile stress of 80 MPa or below in their pipe surfaces as
determined by X-ray diffraction. Besides, these pipe specimens had
a differential outside diameter (difference between the largest and
the smallest outside diameters in a cross section of the pipe) of
0.02 mm or below.
Experimental Example 1-2
[0096] An extruded pipe (outside diameter: 15.0 mm, wall thickness:
1.5 mm) of an AZ31 alloy and an extruded pipe of an AZ61 alloy
having the same configuration as above were drawn, respectively,
with varied reduction ratios, consequently yielding various
specimens of drawn magnesium base alloy pipes having different
outside diameters. The AZ31 alloy of the extruded pipe used was a
magnesium base alloy containing 2.9 mass % of Al, 0.77 mass % of Zn
and 0.40 mass % of Mn with the balance composed of Mg as a base
material and unavoidable impurities, while the AZ61 alloy of the
extruded pipe was a magnesium base alloy containing 6.4 mass % of
Al, 0.77 mass % of Zn and 0.35 mass % of Mn with the balance
likewise comprising Mg and unavoidable impurities. The drawing was
accomplished in one pass of plain drawing by using varied area
reduction ratios of 5.5% (O.D. after a drawing: 14.20 mm), 10.0%
(O.D. after a drawing: 13.5 mm) and 21.0% (O.D. after a drawing:
12.0 mm), respectively. The drawing temperature was 150.degree. C.,
and the cooling rate after drawing was 1-5.degree. C./sec. The
heating rate to the drawing temperature was in the range of
1-2.degree. C./sec., and the drawing speed was 10 m/min. The
resultant drawn pipes had the properties as shown in Table 2.
TABLE-US-00002 TABLE 2 Area Working reduction Tensile Elongation
0.2% proof Alloy Specimen temp. ratio strength after fracture
stress type No. (.degree. C.) (%) (MPa) (%) (MPa) YP ratio AZ31 2-1
Not worked (extruded pipe) 245 9.0 169 0.69 2-2 150 5.5 302 10.5
275 0.91 2-3 150 10 325 9.5 302 0.93 2-4 150 21 362 8.0 342 0.94
AZ61 2-5 Not worked (extruded pipe) 285 6.0 188 0.66 2-6 150 5.5
362 10.5 327 0.90 2-7 150 10 408 9.5 382 0.94 2-8 150 21 445 8.0
425 0.96
[0097] As shown in Table 2, the extruded pipes (specimens No. 2-1
and 2-5 representing comparative examples) of AZ31 and AZ61 alloys,
respectively, have tensile strength of 290 MPa or below, 0.2% proof
stress of 190 MPa or below, a YP ratio of 0.70 or below, and an
elongation of 6-9%. While, the drawn pipe specimens No. 2-2 through
2-4 and the specimens 2-6 through 2-8 drawn with an area reduction
ratio above 5%, embodying the present invention, have tensile
strength above 300 MPa, 0.2% proof stress above 250 MPa, a YP ratio
above 0.90 in addition to a high elongation above 8%. Thus, it will
be clearly understood that by drawing with an area reduction ratio
above 5% the specimens prepared according to the present invention
are improved in their strength without greatly reducing its
toughness.
[0098] These specimens No. 2-2 through 2-4 and 2-6 through 2-8 had
surface roughness Rz of 5 .mu.m or below and an axial residual
tensile stress of 80 MPa or below in their pipe surfaces as
determined by X-ray diffraction. Besides, these pipe specimens had
a differential outside diameter of 0.2 mm or below.
Experimental Example 1-3
[0099] In this experimental example, to prepare drawn magnesium
base alloy pipes, three types of extruded pipes were drawn at
150.degree. C. to be reduced in outside diameter to 12.0 mm,
respectively, including an extruded pipe of a magnesium base alloy
(AZ10 alloy) containing 1.2 mass % of Al, 0.4 mass % of Zn and 0.3
mass % of Mn with the balance composed of Mg as a base material and
unavoidable impurities, an extruded pipe of an AS41 magnesium base
alloy containing 4.2 mass % of Al, 1.0 mass % of Si and 0.40 mass %
of Mn with the balance likewise comprising Mg and unavoidable
impurities and an extruded pipe of an AS21 magnesium base alloy
containing 1.9 mass % of Al, 1.0 mass % of Si and 0.45 mass % of Mn
with the balance likewise comprising Mg and unavoidable impurities.
Each extruded pipe subjected to drawing was 15.0 mm in outside
diameter and 1.5 mm in wall thickness. The drawing was performed in
the same manner and under the same conditions as in the
experimental example 1-1 above, except that the drawing temperature
was fixed at 150.degree. C. As comparative examples, specimens were
prepared by drawing the respective corresponding extruded pipes in
the same manner as above at 20.degree. C. The resultant drawn pipes
had the properties as shown in Table 3.
TABLE-US-00003 TABLE 3 Area Working reduction Tensile Elongation
0.2% proof Alloy Specimen temp. ratio strength after fracture
stress type No. (.degree. C.) (%) (MPa) (%) (MPa) YP ratio AZ10 3-1
Not worked (extruded pipe) 210 10 120 0.57 3-2 20 21 Working
impossible 3-3 150 21 325 9.0 304 0.94 AS41 3-4 Not worked
(extruded pipe) 251 9.0 148 0.59 3-5 20 21 Working impossible 3-6
150 21 371 9.0 345 0.93 AS21 3-7 Not worked (extruded pipe) 210
10.5 135 0.64 3-8 20 21 Working impossible 3-9 150 21 330 9.5 310
0.94
[0100] As shown in Table 3, the extruded pipes not subjected to
drawing (specimens No. 3-1, 3-4 and 3-7 representing comparative
examples) of any of AZ10, AS41 and AS21 alloys, respectively, have
tensile strength of 260 MPa or below, 0.2% proof stress of 150 MPa
or below, a YP ratio of 0.65 or below, and an elongation of
9-10.5%. While, the drawn pipe specimens No. 3-3, 3-6 and 3-9 drawn
with an area reduction ratio above 5%, embodying the present
invention, have tensile strength above 300 MPa, 0.2% proof stress
above 250 MPa, a YP ratio above 0.90 in addition to a high
elongation above 9.0%. Thus, it will be clearly understood that by
drawing with an area reduction ratio above 5% the specimens
prepared according to the present invention are improved in their
strength without greatly reducing its toughness. These specimens
No. 3-3, 3-6 and 3-9 had surface roughness Rz of 5 .mu.m or below
and an axial residual tensile stress of 80 MPa or below in their
pipe surfaces as determined by X-ray diffraction. Besides, these
pipe specimens had a differential outside diameter of 0.02 mm or
below.
Experimental Example 1-4
[0101] An extruded pipe (outside diameter: 15.0 mm, wall thickness:
1.5 mm) of an Az31 alloy and an extruded pipe of an AZ61 alloy
having the same configuration as above were drawn, respectively, to
be reduced to 12.0 mm in outside diameter and the resultant drawn
pipes were heat-treated at varied temperatures to obtain various
specimens of drawn magnesium base alloy pipes. The AZ31 alloy of
the extruded pipe used was a magnesium base alloy containing 2.9
mass % of Al, 0.77 mass % of Zn and 0.40 mass % of Mn with the
balance composed of Mg as a base material and unavoidable
impurities, while the AZ61 alloy of the extruded pipe was a
magnesium base alloy containing 6.4 mass % of Al, 0.77 mass % of Zn
and 0.35 mass % of Mn with the balance likewise comprising Mg and
unavoidable impurities. The drawing was accomplished at 150.degree.
C. in one pass of plain drawing. The area reduction ratio was
21.0%. For heating the pipes in drawing, a heater was provided
before a die, and the drawing temperature was observed in terms of
the heating temperature set on the heater. The heating rate to the
drawing temperature was in the range of 1-2.degree. C./sec., and
the drawing speed was 10 m/min. After drawing, the pipes were
air-cooled to a room temperature at a rate of about 1-5.degree.
C./sec. and thereafter heated again to be subjected to
heat-treatment at 100-300.degree. C. for 15 minutes.
[0102] The resultant pipe specimens were studied for their tensile
strength, 0.2% proof stress, elongation after fracture, YP ratio,
and the grain size. For determining the average grain size, a
texture in a cross section of the wall of each pipe specimen was
microscopically magnified, and sizes of multiple crystal grains
within the microscopic field were measured and averaged. The
resultant drawn pipes had the properties as shown in Tables 4 and
5.
TABLE-US-00004 TABLE 4 Heat treatment Tensile 0.2% proof Elongation
Average grain Alloy Specimen temp. strength stress after fracture
size type No. (.degree. C.) (MPa) (MPa) YP ratio (%) (.mu.m) AZ31
4-1 Without 362 342 0.94 7.5 17.5 4-2 100 360 335 0.93 7.0 17.2 4-3
150 335 298 0.89 12.5 Duplex grain 4-4 200 312 265 0.85 17.0 3.8
4-5 250 301 240 0.80 19.0 4.3 4-6 300 295 225 0.76 20.0 7.5 4-7
Extruded pipe 245 169 0.69 9.0 18.8
TABLE-US-00005 TABLE 5 Heat treatment Tensile 0.2% proof Elongation
Average grain Alloy Specimen temp. strength stress after fracture
size type No. (.degree. C.) (MPa) (MPa) YP ratio (%) (.mu.m) AZ61
5-1 Without 445 425 0.96 7.5 17.3 5-2 100 443 421 0.95 6.0 17.0 5-3
150 425 380 0.89 12.0 Duplex grain 5-4 200 375 325 0.87 18.0 3.9
5-5 250 359 292 0.80 19.0 4.6 5-6 300 338 261 0.77 18.0 7.8 5-7
Extruded pipe 285 188 0.66 6.0 20.3
[0103] As clearly understood from Tables 4 and 5, for both the AZ31
and AZ61 alloys, the specimens No. 4-3 through 4-6 and 5-3 through
5-6 subjected to heat treatment at temperatures above 150.degree.
C. after drawing, embodying the present invention, are shown to
have a substantially improved elongation after fracture and
strength, as compared with those extruded pipes which were
subjected to neither drawing nor heat treatment (specimens No. 4-7
and 5-7 representing comparative examples). Specifically, these
specimens No. 4-3 through 4-6 and 5-3 through 5-6 have tensile
strengths above 280 MPa, 0.2% proof stress above 220 MPa, a YP
ratio in the range of 0.75 to less than 0.90 and an elongation
above 12%, showing improvements in ductility and strength in
balance. Especially, it turns out that the specimens No. 4-4
through 4-6 and 5-4 through 5-6 for which the heat treatment was
carried out at temperatures above 200.degree. C. are further
improved in toughness with their elongations higher than 17%. Among
those, the specimens No. 4-4, 4-5 and 5-4, 5-5 involving a heat
treatment temperature in the range of 200.degree. C.-250.degree. C.
had tensile strength above 300 MPa, 0.2% proof stress above 240
MPa, a YP ratio in the range of 0.80 to less than 0.90 and an
elongation above 17%, showing improvements in strength and
ductility further in balance.
[0104] Further, it is shown that as compared with the specimens No.
4-2 and 5-2 heat-treated at 100.degree. C. after drawing and with
the specimens No. 4-1 and 5-1 not subjected to heat treatment after
drawing, the specimens No. 4-3 through 4-6 and 5-3 through 5-6
which underwent heat treatment at temperatures above 150.degree. C.
after drawing are greatly improved in their elongation, although
their tensile strength, 0.2% proof stress and YP ratio is somewhat
reduced. On the other hand, since the rate of increase in tensile
strength decreases if the heat treatment temperature exceeds
300.degree. C., it is preferred that the heat treatment is
performed at a temperature of 300.degree. C. or below. Thus, it
turns out that by executing the heat treatment at a temperature in
the range of 150.degree. C. to 300.degree. C. (preferably
200.degree. C. to 300.degree. C.) after a drawing, magnesium base
alloy pipes having can have high strength as well as improved
toughness.
[0105] Regarding the average grain size of the specimens obtained
in the experiment, the extruded pipe specimens not subjected to
drawing (specimens No. 4-7 and 5-7) and the specimens heat-treated
at temperatures of 100.degree. C. or below (specimens No. 4-1, 4-2
and 5-1, 5-2) have a larger grain size above 15 .mu.m, as shown in
Tables 4 and 5. On the other hand, the specimens heat-treated at
temperatures above 200.degree. C. (specimens No. 4-4 through 4-6
and 5-4 through 5-6) have fine grains with an average grain size of
10 .mu.m or below. Among those, the specimens heat-treated at a
temperature of 200-250.degree. C. (specimens No. 4-4, 4-5 and 5-4,
5-5) have an average grain size of 5 .mu.m or below. Further, the
specimens heat-treated at 150.degree. C. (specimens No. 4-3 and
5-3) had a mixed texture comprising grains having a 3 .mu.m or
smaller average grain size and grains having a 15 .mu.m or larger
average grain size, in which the grains area share by 3 .mu.m or
smaller grains was above 10%. Thus, it turns out that an alloy
texture comprising fine grains or a mixed texture having fine
grains and coarse grains yields a magnesium base alloy pipe having
strength and toughness in balance as above.
[0106] For those specimens subjected to heat treatment (specimens
No. 4-3 through 4-6 and 5-3 through 5-6) at 150.degree.
C.-300.degree. C., it was possible to apply drawing in two or more
multiple passes. These specimens No. 4-3 through 4-6 and 5-3
through 5-6 had surface roughness Rz of 5 .mu.m or below. Further,
these specimens had an axial residual tensile stress of 80 MPa or
below in their pipe surfaces as determined by X-ray diffraction.
Besides, these pipe specimens had a differential outside diameter
(difference between the largest and the smallest outside diameters
in a cross section of the pipe) of 0.02 mm. or below.
Experimental Example 1-5
[0107] In this experimental example, three types of extruded pipes
were drawn at 150.degree. C. to be reduced in outside diameter to
12.0 mm, respectively, including an extruded pipe of a magnesium
base alloy (AZ10 alloy) containing 1.2 mass % of Al, 0.4 mass % of
Zn and 0.3 mass % of Mn with the balance composed of Mg as a base
material and unavoidable impurities, an extruded pipe of an AS41
magnesium base alloy containing 4.2 mass % of Al, 1.0 mass % of Si
and 0.40 mass % of Mn with the balance likewise comprising Mg and
unavoidable impurities and an extruded pipe of an AS21 magnesium
base alloy containing 1.9 mass % of Al, 1.0 mass % of Si and 0.45
mass % of Mn with the balance likewise comprising Mg and
unavoidable impurities. Then, the drawn pipes were subjected to
heat treatment at 200.degree. C. to prepare drawn magnesium base
alloy pipe specimens. Each extruded pipe subjected to drawing was
15.0 mm in outside diameter and 1.5 mm in wall thickness. The
drawing was performed in the same manner and under the same
conditions as in the experimental example 1-1 above, except that
the heat treatment after drawing was carried out at 200.degree. C.
As comparative examples, specimens were prepared in the same manner
by changing this heat treatment temperature to 100.degree. C.
Further, the resultant pipe specimens were studied for their grain
size in the same manner as in the experimental example 1-4 above.
The resultant drawn pipe specimens had tensile strength, 0.2% proof
stress, elongation after fracture, YP ratios and grain sizes as
shown in Table 6.
TABLE-US-00006 TABLE 6 Heat treatment Tensile 0.2% proof Elongation
Average grain Alloy Specimen temp. strength stress after fracture
size type No. (.degree. C.) (MPa) (MPa) YP ratio (%) (.mu.m) AZ10
6-1 Without 325 304 0.94 9.0 18.5 6-2 100 322 301 0.93 9.0 18.0 6-3
200 291 250 0.86 18.0 4.0 6-4 Extruded pipe 210 120 0.57 10.0 20.1
AS41 6-5 Without 371 345 0.93 9.0 19.3 6-6 100 368 340 0.92 9.0
19.2 6-7 200 325 276 0.85 18.5 3.8 6-8 Extruded pipe 251 148 0.59
9.0 21.2 AS21 6-9 Without 330 310 0.94 9.5 19.9 6-10 100 328 305
0.93 9.0 19.5 6-11 200 299 257 0.86 18.5 3.9 6-12 Extruded pipe 210
135 0.64 10.5 20.2
[0108] As clearly understood from Table 6, for any of AZ10, AS41
and AS21 alloys, the specimens No. 6-3, 6-7 and 6-11 subjected to
heat treatment at 200.degree. C. after drawing, embodying the
present invention, are shown to have a substantially improved
elongation after fracture and strength, as compared with those
extruded pipes which were subjected to neither drawing nor heat
treatment (specimens No. 6-4, 6-8 and 6-12 representing comparative
examples). Further, regarding the average grain size of the
specimens obtained in the experiment, the extruded pipe specimens
not subjected to drawing (specimens No. 6-4, 6-8 and 6-12), the
drawn specimens not subjected to heat treatment (specimens No. 6-1,
6-5 and 6-9) and the specimens heat-treated at 100.degree. C.
(specimens No. 6-2, 6-6 and 6-10) have a larger grain size above 15
.mu.m. On the other hand, the specimens heat-treated at
temperatures at 200.degree. C. (specimens No. 6-3, 6-7 and 6-11)
have fine grains with an average grain size of 5 .mu.m or below.
These specimens No. 6-3, 6-7 and 6-11 had surface roughness Rz of 5
.mu.m or below and an axial residual tensile stress of 80 MPa or
below in their pipe surfaces as determined by X-ray diffraction.
Besides, these pipe specimens had a differential outside diameter
of 0.02 mm or below.
Experimental Example 1-6
[0109] An extruded pipe (outside diameter: 15.0 mm, wall thickness:
1.5 mm) of an ZK40 alloy and an extruded pipe of an ZK60 alloy
having the same configuration as above were drawn, respectively, to
be reduced to 12.0 mm in outside diameter and the resultant drawn
pipes were heat-treated at varied temperatures to obtain various
specimens of drawn magnesium base alloy pipes. The ZK40 alloy of
the extruded pipe used was a magnesium base alloy containing 4.1
mass % of Zn and 0.5 mass % of Zr with the balance composed of Mg
as a base material and unavoidable impurities, while the ZK60 alloy
of the extruded pipe was a magnesium base alloy containing 5.5 mass
% of Zn and 0.5 mass % of Zr with the balance likewise comprising
Mg and unavoidable impurities. The drawing was accomplished at
150.degree. C. in one pass of plain drawing. The area reduction
ratio was 21.0%. For heating the pipes in drawing, a heater was
provided before a die, and the drawing temperature was observed in
terms of the heating temperature set on the heater. The heating
rate to the drawing temperature was in the range of 1-2.degree.
C./sec., and the drawing speed was 10 m/min. After drawing, the
pipes were air-cooled to a room temperature at a rate of about
1-5.degree. C./sec. and thereafter heated again to be subjected to
heat-treatment at 100-300.degree. C. for 15 minutes.
[0110] The resultant pipe specimens were studied for their tensile
strength, 0.2% proof stress, elongation after fracture, YP ratio,
and the grain size. For determining the average grain size, a
texture in a cross section of the wall of each pipe specimen was
microscopically magnified, and sizes of multiple crystal grains
within the microscopic field were measured and averaged. The
resultant drawn pipes had the properties as shown in Tables 7 and
8.
TABLE-US-00007 TABLE 7 Heat treatment Tensile 0.2% proof Elongation
Average grain Alloy Specimen temp. strength stress after fracture
size type No. (.degree. C.) (MPa) (MPa) YP ratio (%) (.mu.m) ZK40
7-1 Without 425 399 0.94 8.5 19.3 7-2 100 422 392 0.93 8.0 18.5 7-3
150 412 368 0.89 12.0 Duplex grain 7-4 200 352 301 0.86 18.0 3.6
7-5 250 341 276 0.81 19.0 4.4 7-6 300 332 260 0.78 21.0 7.8 7-7
Extruded pipe 275 201 0.73 8.0 19.8
TABLE-US-00008 TABLE 8 Heat treatment Tensile 0.2% proof Elongation
Average grain Alloy Specimen temp. strength stress after fracture
size type No. (.degree. C.) (MPa) (MPa) YP ratio (%) (.mu.m) ZK60
8-1 Without 458 431 0.94 9.5 18.8 8-2 100 452 422 0.93 9.0 18.9 8-3
150 428 381 0.89 12.5 Duplex grain 8-4 200 372 315 0.85 18.0 3.2
8-5 250 358 289 0.81 19.0 4.5 8-6 300 337 265 0.79 20.0 7.7 8-7
Extruded pipe 295 212 0.72 9.0 20.5
[0111] As clearly understood from Tables 7 and 8, for both the ZK40
and ZK60 alloys, the specimens No. 7-3 through 7-6 and 8-3 through
8-6 subjected to heat treatment at temperatures above 150.degree.
C. after drawing, embodying the present invention, are shown to
have a substantially improved elongation after fracture and
strength, as compared with those extruded pipes which were
subjected to neither drawing nor heat treatment (specimens No. 7-7
and 8-7 representing comparative examples). Specifically, these
specimens No. 7-3 through 7-6 and 8-3 through 8-6 have tensile
strengths above 300 MPa, 0.2% proof stress above 220 MPa, a YP
ratio in the range of 0.75 to less than 0.90 and an elongation
above 12%, showing improvements in ductility and strength in
balance. Especially, it turns out that the specimens No. 7-4
through 7-6 and 8-4 through 8-6 for which the heat treatment was
carried out at temperatures above 200.degree. C. are further
improved in toughness with their elongations higher than 18%. Among
those, the specimens No. 7-4, 7-5 and 8-4, 8-5 involving a heat
treatment temperature in the range of 200.degree. C.-250.degree. C.
had tensile strength above 340 MPa, 0.2% proof stress above 250
MPa, a YP ratio in the range of 0.80 to less than 0.90 and an
elongation above 18%, showing improvements in strength and
ductility further in balance.
[0112] Further, it is shown that as compared with the specimens No.
7-2 and 8-2 heat-treated at 100.degree. C. after drawing and with
the specimens No. 7-1 and 8-1 not subjected to heat treatment after
drawing, the specimens No. 7-3 through 7-6 and 8-3 through 8-6
which underwent heat treatment at temperatures above 150.degree. C.
after drawing are greatly improved in their elongation, although
their tensile strength, 0.2% proof stress and YP ratio are somewhat
reduced. On the other hand, since the rate of increase in tensile
strength decreases if the heat treatment temperature exceeds
300.degree. C., it is preferred that the heat treatment is
performed at a temperature of 300.degree. C. or below. Thus, it
turns out that by executing the heat treatment at a temperature in
the range of 150.degree. C. to 300.degree. C. (preferably
200.degree. C. to 300.degree. C.) after a drawing, magnesium base
alloy pipes can have high strength as well as improved
toughness.
[0113] Regarding the average grain size of the specimens obtained
in the experiment, the extruded pipe specimens not subjected to
drawing (specimens No. 7-7 and 8-7) and the specimens heat-treated
at temperatures of 100.degree. C. or below (specimens No. 7-1, 7-2
and 8-1, 8-2) have a larger grain size above 15 .mu.m, as shown in
Tables 7 and 8. On the other hand, the specimens heat-treated at
temperatures above 200.degree. C. (specimens No. 7-4 through 7-6
and 8-4 through 8-6) have fine grains with an average grain size of
10 .mu.m or below. Among those, the specimens heat-treated at a
temperature of 200-250.degree. C. (specimens No. 7-4, 7-5 and 8-4,
8-5) have an average grain size of 5 .mu.m or below. Further, the
specimens heat-treated at 150.degree. C. (specimens No. 7-3 and
8-3) had a mixed texture comprising grains having a 3 .mu.m or
smaller average grain size and grains having a 15 .mu.m or larger
average grain size, in which the grains area share by 3 .mu.m or
smaller grains was above 10%. Thus, it turns out that an alloy
texture comprising fine grains or a mixed texture having fine
grains and coarse grains yields a magnesium base alloy pipe having
strength and toughness in balance as above.
[0114] For those specimens subjected to heat treatment (specimens
No. 7-3 through 7-6 and 8-3 through 8-6) at 150.degree.
C.-300.degree. C., it was possible to apply drawing in two or more
multiple passes. These specimens No. 7-3 through 7-6 and 8-3
through 8-6 had surface roughness Rz of 5 .mu.m or below. Further,
these specimens had an axial residual tensile stress of 80 MPa or
below in their pipe surfaces as determined by X-ray diffraction.
Besides, these pipe specimens had a differential outside diameter
(difference between the largest and the smallest outside diameters
in a cross section of the pipe) of 0.02 mm or below.
Experimental Example 1-7
[0115] An extruded pipe (outside diameter: 15.0 mm, wall thickness:
1.5 mm) of an ZK 40 alloy and an extruded pipe of an ZK60 alloy
having the same configuration as above were drawn, respectively, at
varied temperatures to be reduced in outside diameter to 12.0 mm,
thus yielding various specimens of drawn magnesium base alloy
pipes. The ZK40 alloy of the extruded pipe used was a magnesium
base alloy containing 4.1 mass % of Zn and 0.5 mass % of Zr with
the balance composed of Mg as a base material and unavoidable
impurities, while the ZK60 alloy of the extruded pipe was a
magnesium base alloy containing 5.5 mass % of Zn and 0.5 mass % of
Zr with the balance likewise comprising Mg and unavoidable
impurities. The drawing was accomplished in two passes of plain
drawing, with the first pass reducing the outside diameter to 13.5
mm and the second pass reducing the same to 12.0 mm. The first and
the second passes yielded area reduction ratios of 10.0% and 12.3%,
respectively, with the total area reduction ratio of 21.0%. After
drawing, the pipes were air-cooled at a rate of 1-5.degree. C./sec.
For heating the pipes in drawing, a heater was provided before a
die, and the working temperature (drawing temperature) was observed
in terms of the heating temperature set on the heater. This applies
also in an experimental example 1-8 to be described herein later.
The heating rate to the drawing temperature was in the range of
1-2.degree. C./sec., and the drawing speed was 10 m/min. The
resultant drawn pipes had the properties as shown in Table 9.
TABLE-US-00009 TABLE 9 Area Working reduction Tensile Elongation
0.2% proof Alloy Specimen temp. ratio strength after fracture
stress type No. (.degree. C.) (%) (MPa) (%) (MPa) YP ratio ZK40 9-1
Not worked 275 8.0 201 0.73 (extruded pipe) 9-2 20 21 Working
impossible 9-3 50 21 448 6.0 419 0.94 9-4 100 21 432 9.0 405 0.94
9-5 200 21 421 10.0 389 0.92 9-6 300 21 395 11.5 362 0.92 ZK60 9-7
Not worked 295 9.0 212 0.72 (extruded pipe) 9-8 20 21 Working
impossible 9-9 50 21 477 6.0 446 0.94 9-10 100 21 464 9.0 435 0.94
9-11 200 21 452 10.0 419 0.93 9-12 300 21 426 10.5 392 0.92
[0116] As shown in Table 9, the extruded pipes (specimens No. 9-1
and 9-7 representing comparative examples) of ZK40 and ZK60 alloys,
respectively, have tensile strength below 300 MPa, 0.2% proof
stress below 220 MPa, a YP ratio below 0.75, and an elongation
(elongation after fracture) of 8-9%. While, the drawn pipe
specimens No. 9-3 through 9-6 and the specimens 9-9 through 9-12
drawn at temperatures above 50.degree. C., embodying the present
invention, have tensile strength above 300 MPa, 0.2% proof stress
above 250 MPa, a YP ratio above 0.90 in addition to an elongation
duly above 5%. Thus, it will be clearly understood that the
specimens prepared according to the present invention are improved
in their strength without greatly reducing its toughness. Among
those last specimens, the No. 9-4 through 9-6 specimens and No.
9-10 through 9-12 specimens which were drawn at temperatures from
100.degree. C. to 300.degree. C. are improved particularly in
toughness with their further higher elongations above 8%.
Therefore, it will be understood that the drawing temperature is
preferably in the range of 100.degree. C. to 300.degree. C. in view
of elongation after fracture. When the drawing temperature exceeded
300.degree. C. the rate of increase in tensile strength became low,
while the specimens No. 9-2 and 9-8 subjected to drawing at a room
temperature, namely 20.degree. C., representing another comparative
examples, could not actually withstand working due to breakage.
Thus, it turns out that the balance of strength and toughness is
more improved by employing a drawing temperature in the range of
50.degree. C. to 300.degree. C. (preferably 100.degree. C. to
300.degree. C.).
[0117] For the resultant specimens No. 9-3 through 9-6 and 9-9
through 9-12, it was possible to apply drawing even in 3 or more
passes. These specimens No. 9-3 through 9-6 and 9-9 through 9-12
had surface roughness Rz of 5 .mu.m or below and an axial residual
tensile stress of 80 MPa or below in their pipe surfaces as
determined by X-ray diffraction. Besides, these pipe specimens had
a differential outside diameter (difference between the largest and
the smallest outside diameters in a cross section of the pipe) of
0.02 mm or below.
Experimental Example 1-8
[0118] An extruded pipe (outside diameter: 15.0 mm, wall thickness:
1.5 mm) of an ZK40 alloy and an extruded pipe of an ZK60 alloy
having the same configuration as above were drawn, respectively,
with varied reduction ratios, thus yielding various specimens of
drawn magnesium base alloy pipes having different outside
diameters. The ZK40 alloy of the extruded pipe used was a magnesium
base alloy containing 4.1 mass % of Zn and 0.5 mass % of Zr with
the balance composed of Mg as a base material and unavoidable
impurities, while the ZK60 alloy of the extruded pipe was a
magnesium base alloy containing 5.5 mass % of Zn and 0.5 mass % of
Zr with the balance likewise comprising Mg and unavoidable
impurities. The drawing was accomplished in one pass of plain
drawing by using varied area reduction ratios of 5.5% (O.D. after a
drawing: 14.20 mm), 10.0% (O.D. after a drawing: 13.5 mm) and 21.0%
(O.D. after a drawing: 12.0 mm), respectively. The drawing
temperature was 150.degree. C., and the cooling rate after drawing
was 1-5.degree. C./sec. The heating rate to the drawing temperature
was in the range of 1-2.degree. C./sec., and the drawing speed was
10 m/min. The resultant drawn pipes had the properties as shown in
Table 10.
TABLE-US-00010 TABLE 10 Working Area Tensile Elongation 0.2% proof
Alloy Specimen temp. reduction ratio strength after fracture stress
type No. (.degree. C.) (%) (MPa) (%) (MPa) YP ratio ZK40 10-1 Not
worked (extruded pipe) 275 8.0 201 0.73 10-2 150 5.5 339 10.5 306
0.90 10-3 150 10 378 10.0 348 0.92 10-4 150 21 425 8.5 399 0.94
ZK60 10-5 Not worked (extruded pipe) 295 9.0 212 0.72 10-6 150 5.5
377 10.5 342 0.91 10-7 150 10 421 9.5 389 0.92 10-8 150 21 458 9.5
431 0.94
[0119] As shown in Table 10, the extruded pipes (specimens No. 10-1
and 10-5 representing comparative examples) of ZK40 and ZK60
alloys, respectively, have tensile strength below 300 MPa, 0.2%
proof stress below 220 MPa, a YP ratio below 0.75, and an
elongation of 8-9%. While, the drawn pipe specimens No. 10-2
through 10-4 and the specimens 10-6 through 10-8 drawn with an area
reduction ratio above 5%, embodying the present invention, have
tensile strength above 300 MPa, 0.2% proof stress above 250 MPa, a
YP ratio above 0.90 in addition to a high elongation above 8%.
Thus, it will be clearly understood that by drawing with an area
reduction ratio above 5% the specimens prepared according to the
present invention are improved in their strength without greatly
reducing its toughness.
[0120] These specimens No. 10-2 through 10-4 and 10-6 through 10-8
had surface roughness Rz of 5 .mu.m or below and an axial residual
tensile stress of 80 MPa or below in their pipe surfaces as
determined by X-ray diffraction. Besides, these pipe specimens had
a differential outside diameter of 0.02 mm or below.
Experimental Example 1-9
[0121] An extruded pipe (outside diameter: 15.0 mm, wall thickness:
1.5 mm) of a magnesium base alloy (AM60 alloy) containing 6.1 mass
% of Al and 0.44 mass % of Mn with the balance composed of Mg as a
base material and unavoidable impurities was drawn at 150.degree.
C. to be reduced in outside diameter to 12.0 mm to prepare drawn
magnesium base alloy pipe specimen. The drawing was performed in
the same manner and under the same conditions as in the
experimental example 1-1 above, except that the drawing temperature
was fixed at 150.degree. C. As comparative examples, a specimen was
prepared by drawing the extruded pipe in the same manner as above
at 20.degree. C. In this experiment, the resultant drawn pipes had
the properties as shown in Table 11.
TABLE-US-00011 TABLE 11 Working Area Tensile Elongation 0.2% proof
Alloy Specimen temp. reduction ratio strength after fracture stress
type No. (.degree. C.) (%) (MPa) (%) (MPa) YP ratio AM60 11-1 Not
worked (extruded pipe) 267 8.5 165 0.62 11-2 20 21 Working
impossible 11-3 150 21 375 8.0 348 0.93
[0122] As shown in Table 11, the extruded pipe not subjected to
drawing (specimen No. 11-1) had tensile strength of 267 MPa, 0.2%
proof stress of 165 MPa, YP ratio of 0.62 and elongation after
fracture of 8.5%. While, the drawn pipe specimen No. 11-3 drawn
with an area reduction ratio above 5%, embodying the present
invention, have tensile strength above 300 MPa, 0.2% proof stress
above 250 MPa, a YP ratio above 0.90 in addition to a high
elongation above 8%. Thus, it will be clearly understood that the
specimen prepared according to the present invention is improved in
its strength without greatly reducing its toughness. This specimen
had surface roughness Rz of 5 .mu.m or below and an axial residual
tensile stress of 80 MPa or below in their pipe surfaces as
determined by X-ray diffraction. Besides, these pipe specimens had
a differential outside diameter of 0.02 mm.
Experimental Example 1-10
[0123] An extruded pipe (outside diameter 15.0 mm, wall thickness
of 1.5 mm) of a magnesium base alloy (AM60) containing 6.1 mass %
of Al and 0.44 mass % of Mn with the balance composed of Mg as a
base material and unavoidable impurities was drawn at 150.degree.
C. to be reduced in outside diameter to 12.0 mm, and then the drawn
pipes were subjected to heat treatment at 200.degree. C. to prepare
a drawn magnesium base alloy pipe specimen. The pipe specimens were
prepared in the same manner and under the same conditions as in the
experimental example 1-1 above, except that the drawing temperature
was fixed at 150.degree. C. and a 200.degree. C. heat treatment was
applied after drawing. As comparative examples, were prepared in
the same manner as above a specimen which was heat-treated at
100.degree. C. after drawing and a specimen not subjected to heat
treatment. Further, the resultant pipe specimens were studied for
their average grain size in the same manner as in the experimental
example 1-4 above. These resultant pipe specimens had the
properties as shown in Table 12.
TABLE-US-00012 TABLE 12 Heat treatment Tensile 0.2% proof
Elongation Average Alloy Specimen temp. strength stress after
fracture grain size type No. (.degree. C.) (MPa) (MPa) YP ratio (%)
(.mu.m) AM60 12-1 Without 375 348 0.93 8.0 18.2 12-2 100 372 344
0.92 8.0 18.5 12-3 200 330 285 0.86 18.0 3.8 12-4 Extruded pipe 267
165 0.62 8.5 18.5
[0124] As shown in Table 12, the specimen No. 12-3 subjected to
heat treatment at temperatures at 200.degree. C. after drawing,
embodying the present invention, are shown to have a substantially
improved elongation after fracture and strength, as compared with
the extruded pipe which was subjected to neither drawing nor heat
treatment (specimen No. 12-4 a representing comparative example).
Further, regarding the average grain size of the specimens prepared
in the experiment, the extruded pipe specimen not subjected to
drawing (specimens No. 12-4), the drawn specimen not subjected to
heat treatment (specimen No. 12-1) and the specimen heat-treated at
100.degree. C. (specimens No. 12-2) have a larger grain size above
15 .mu.m. On the other hand, the specimen heat-treated at
temperatures at 200.degree. C. (specimen No. 12-3) has fine grains
with an average grain size below 5 .mu.m. This specimen No. 12-3
had surface roughness Rz below 5 .mu.m and an axial residual
tensile stress below 80 MPa in their pipe surfaces as determined by
X-ray diffraction. Besides, the No. 12-3 had a differential outside
diameter below 0.02 mm.
Experimental Example 2-1
[0125] Extruded pipe blanks (O.D.: 10-45 mm, wall thickness: 1.0-5
mm) of an Az31 alloy and an AZ61 alloy were pointed, respectively,
with different working ratios at varied temperatures. The AZ31
alloy of the extruded pipes used was a magnesium base alloy
containing 2.9 mass % of Al, 0.77 mass % of Zn and 0.40 mass % of
Mn with the balance composed of Mg as a base material and
unavoidable impurities, while the AZ61 alloy of the extruded pipes
was a magnesium base alloy containing 6.4 mass % of Al, 0.77 mass %
of Zn and 0.35 mass % of Mn with the balance likewise comprising Mg
and unavoidable impurities.
[0126] For the pointing operation, a front end of a pipe blank was
heated at 350.degree. C. and the time elapsing before the front end
was fed in a die of a swaging machine thereafter (cooling time) was
varied to control the temperature of the pipe blank at its front
working end just before it was fed in a swaging machine (inlet
temperature). The inlet temperature was determined by calculation
from the heating temperature (350.degree. C.) and the cooling time.
For heating some pipe blanks, the die of the swaging machine was
also heated. The die was heated at 150.degree. C. Further, some
pipe blanks were heated with a cylindrical copper block (heat
insulating material) inserted in their front ends. For each pipe
blank, the inlet temperature, use of die heating, use of a heat
insulating material, and the workability at each working ratio are
shown in Tables 13 and 14. The working ratio is given by {(pipe
O.D. before working-pipe O.D. after working)/pipe O.D. before
working}.times.100, and the workability is marked with
".smallcircle." for specimens worked without cracking, while it is
marked with "x" for specimens that underwent cracking. The pointed
specimens exhibited a certain relationship between the outside
diameters of pipes before working and their working ratios, as
shown on the graphs of FIGS. 2 and 3. FIG. 2 shows the test results
for AZ31 specimens, and FIG. 3 for AZ61 specimens.
TABLE-US-00013 TABLE 13 Inlet Heat Workability vs. Specimen Alloy
temp. Die insulating working ratio No. type (.degree. C.) heating
material 3% 5% 10% Notes 13-1 AZ31 20 No No X X X 13-2 AZ31 50 No
No .largecircle. X X 13-3 AZ31 100 No No .largecircle.
.largecircle. .largecircle. 13-4 AZ31 450 No No .largecircle.
.largecircle. .largecircle. 13-5 AZ31 480 No No .largecircle.
.largecircle. .largecircle. *1 13-6 AZ31 20 Yes No .largecircle. X
X 13-7 AZ31 50 Yes No .largecircle. .largecircle. X 13-8 AZ31 100
Yes No .largecircle. .largecircle. .largecircle. 13-9 AZ31 450 Yes
No .largecircle. .largecircle. .largecircle. 13-10 AZ31 480 Yes No
.largecircle. .largecircle. .largecircle. *1 13-11 AZ31 20 No Yes X
X X 13-12 AZ31 50 No Yes .largecircle. .largecircle. X 13-13 AZ31
100 No Yes .largecircle. .largecircle. .largecircle. 13-14 AZ31 450
No Yes .largecircle. .largecircle. .largecircle. 13-15 AZ31 480 No
Yes .largecircle. .largecircle. .largecircle. *1 *1: Unusable due
to severe surface oxidation
TABLE-US-00014 TABLE 14 Inlet Heat Workability vs. Specimen Alloy
temp. Die insulating working ratio No. type (.degree. C.) heating
material 2% 3% 5% Notes 14-1 AZ61 20 No No X X X 14-2 AZ61 50 No No
.largecircle. X X 14-3 AZ61 100 No No .largecircle. .largecircle.
.largecircle. 14-4 AZ61 450 No No .largecircle. .largecircle.
.largecircle. 14-5 AZ61 480 No No .largecircle. .largecircle.
.largecircle. *1 14-6 AZ61 20 Yes No .largecircle. X X 14-7 AZ61 50
Yes No .largecircle. .largecircle. X 14-8 AZ61 100 Yes No
.largecircle. .largecircle. .largecircle. 14-9 AZ61 450 Yes No
.largecircle. .largecircle. .largecircle. 14-10 AZ61 480 Yes No
.largecircle. .largecircle. .largecircle. *1 14-11 AZ61 20 No Yes X
X X 14-12 AZ61 50 No Yes .largecircle. .largecircle. X 14-13 AZ61
100 No Yes .largecircle. .largecircle. .largecircle. 14-14 AZ61 450
No Yes .largecircle. .largecircle. .largecircle. 14-15 AZ61 480 No
Yes .largecircle. .largecircle. .largecircle. *1 *1: Unusable due
to severe surface oxidation
[0127] From Tables 13, 14 and graphs in FIGS. 2, 3, it turns out
that the pipe blank specimens having 50.degree. C. inlet
temperature at their front end can be pointed without undergoing
cracking if their working ratios are in the rage of about 2-3%. The
specimens having a 50.degree. C. inlet temperature can be pointed
with a higher working ratio, if either the die heating or the heat
insulating material is combined therewith. Further, those specimens
involving an inlet temperature of 100-450.degree. C. allow pointing
with a high working ratio above 5%. Although the specimens
involving an inlet temperature above 480.degree. C. could be
pointed, it was determined that such specimens could not bear
commercial applications due to their remarkable surface oxidation.
Besides, it was shown that according to the method of the present
invention a magnesium base alloy pipe having a wall thickness of
0.5 mm may be produced.
Experimental Example 2-2
[0128] In this experiment, pipe blanks were prepared by providing a
film coating on extruded pipes of alloys having the same chemical
compositions as those used in the experimental example 2-1 above.
The film coating was accomplished by first dispersing a PTFE resin
in water to prepare its aqueous dispersion liquid, then immersing a
pipe blank in this dispersion liquid, and subsequently heating the
wet pipe blank at 400.degree. C. to form PTFE coating on the pipe
surface. Thereafter, the thus coated pipe blank was pointed in the
same manner as in the specimen No. 13-3 in the experimental example
2-1 and then subjected to drawing.
[0129] Using a drawbench, the drawing was carried out by plug
drawing in one pass. The pipe blanks were subjected to drawing
along with their heating by any appropriate means including
immersion in a preheated lubricant, an atmosphere furnace, a
high-frequency heating furnace or drawing die heating. The exit
temperature of the pipe blank was adjusted by varying the time
elapsing before it was fed in the drawing die after it was taken
out from a lubricant bath, an atmosphere furnace or a
high-frequency furnace. The exit temperature is a temperature of
the drawn pipe just behind the exit of the drawing die. The heating
rate to exit temperature was 1-2.degree. C./sec. After drawing, the
pipes were air-cooled at a rate of 1-5.degree. C./sec. The drawing
speed was 10 m/min.
[0130] The exit temperature, heating method, lubrication method,
and the workability at each working ratio for the AZ31 and AZ61
specimens are shown in Tables 15 and Table 16, respectively. The
working ratio is given by {(pipe cross-sectional area before
working-pipe cross-sectional area after working)/pipe
cross-sectional area before working}.times.100. The workability is
marked with ".smallcircle." for specimens worked without cracking,
while it is marked with "x" for specimens that underwent cracking
and with "seized" for specimens undergoing seizing. In the
"lubrication method" column of Tables 15 and 16, "oil" indicates
application of a lubricating oil to a pipe blank, with "film
coating+oil" indicating application of a lubricating oil to a PTFE
resin-coated pipe blank, "film coating" indicating that a PTFE
resin-coated pipe blank is drawn without using a lubricating oil,
and "forced lubrication" indicating that drawing is done by
forcedly supplying a lubricating oil into a gap between a die and
the pipe blank.
[0131] Further, for the drawing process, a relationship between the
working ratio and the drawing force was studied. The drawing force
was measured by means of a load cell provided on the exit side of
the drawing die. The observed relationship between the working
ratio and the drawing force is shown on the graph of FIG. 4. In the
graph of FIG. 4, the white circle, triangle and diamond represent
data for AZ31 specimens, with (PTFE) (black circle) representing
data for film-coated AZ61 specimens immersed in a lubricant, and
AZ61 (typical) (black triangle) representing data for AZ61
specimens that were merely immersed in lubricant without film
coating, while "x" mark indicating calculated data.
TABLE-US-00015 TABLE 15 Exit Workability vs. Specimen Alloy temp.
working ratio No. type (.degree. C.) Heating method Lubrication
method 5% 10% 20% 5-1 AZ31 20 Immersed in lubricant Oil
.largecircle. X X 15-2 AZ31 50 '' '' .largecircle. .largecircle. X
15-3 AZ31 100 '' '' .largecircle. .largecircle. .largecircle. 15-4
AZ31 200 '' '' .largecircle. .largecircle. .largecircle. 15-5 AZ31
250 '' '' .largecircle. .largecircle. X 15-6 AZ31 20 Immersed in
lubricant Film coating + oil .largecircle. X X 15-7 AZ31 50 '' ''
.largecircle. .largecircle. X 15-8 AZ31 100 '' '' .largecircle.
.largecircle. .largecircle. 15-9 AZ31 200 '' '' .largecircle.
.largecircle. .largecircle. 15-10 AZ31 250 '' '' .largecircle.
.largecircle. X 15-11 AZ31 200 Atmosphere furnace Forced
lubrication .largecircle. .largecircle. .largecircle. 15-12 AZ31
200 '' Film coating + oil .largecircle. .largecircle. .largecircle.
15-13 AZ31 300 '' Film coating .largecircle. .largecircle. X 15-14
AZ31 200 High-frequency furnace Forced lubrication .largecircle.
.largecircle. .largecircle. 15-15 AZ31 200 '' Film coating + oil
.largecircle. .largecircle. .largecircle. 15-16 AZ31 300 '' Film
coating .largecircle. .largecircle. X 15-17 AZ31 100 Die heating
Forced lubrication .largecircle. .largecircle. .largecircle. 15-18
AZ31 100 '' Film coating + oil .largecircle. .largecircle.
.largecircle. 15-19 AZ31 300 '' Film coating .largecircle.
.largecircle. X
TABLE-US-00016 TABLE 16 Exit Workability vs. Specimen temp. working
ratio No. Alloy type (.degree. C.) Heating method Lubrication
method 5% 10% 20% 16-1 AZ61 20 Immersed in lubricant Oil
.largecircle. X X 16-2 AZ61 50 '' '' .largecircle. Seized X 16-3
AZ61 100 '' '' .largecircle. '' Seized 16-4 AZ61 200 '' ''
.largecircle. '' '' 16-5 AZ61 250 '' '' .largecircle. '' '' 16-6
AZ61 20 Immersed in lubricant Film coating + oil .largecircle. X X
16-7 AZ61 50 '' '' .largecircle. .largecircle. X 16-8 AZ61 100 ''
'' .largecircle. .largecircle. .largecircle. 16-9 AZ61 200 '' ''
.largecircle. .largecircle. .largecircle. 16-10 AZ61 250 '' ''
.largecircle. .largecircle. X 16-11 AZ61 200 Atmosphere furnace
Forced lubrication .largecircle. Seized Seized 16-12 AZ61 200 ''
Film coating + oil .largecircle. .largecircle. .largecircle. 16-13
AZ61 300 '' Film coating .largecircle. .largecircle. X 16-14 AZ61
200 High-frequency furnace Forced lubrication .largecircle. Seized
Seized 16-15 AZ61 200 '' Film coating + oil .largecircle.
.largecircle. .largecircle. 16-16 AZ61 300 '' Film coating
.largecircle. .largecircle. X 16-17 AZ61 100 Die heating Forced
lubrication .largecircle. Seized Seized 16-18 AZ61 100 '' Film
coating + oil .largecircle. .largecircle. .largecircle. 16-19 AZ61
300 '' Film coating .largecircle. .largecircle. X
[0132] From Tables 15, 16 and the graph of FIG. 4, it turns out
that desirable results were obtained at exit temperatures ranging
from 50 to 300.degree. C. Especially, it is understood that the
specimens combining the film coating and the lubrication by
lubricating oil can be subjected to drawing with a higher working
ratio.
Experimental Example 2-3
[0133] In this experimental example, some extruded pipes used in
the experimental example 2-2 above were subjected to drawing in
multiple passes so as to effect a varied total working ratio, and
some of the drawn pipes were heat-treated after drawing. The
"heating method" for drawing was the immersion in a preheated
lubricant, and the "lubrication method" is accomplished using a
lubricating oil. For drawing, the specimens with a 15% total
working ratio were drawn in one pass, the specimens with a 30%
total working ratio were drawn in two passes, while those with a
45% total working ratio were worked in three passes. For each pass,
the pipe blank was immersed in the lubricating oil to be heated to
an exit temperature. The total working ratio is given by {(pipe
cross-sectional area before working-pipe cross-sectional area after
the last working)/pipe cross-sectional area before
working}.times.100. The heat treatment after drawing was applied at
250.degree. C. for 30 minutes. The elongation and the tensile
strength were measured on all the resultant drawn pipe specimens.
The exit temperature, total working ratio, use of heat treatment
after drawing, elongation and tensile strength are shown in Table
17 for the respective specimens.
TABLE-US-00017 TABLE 17 Total Heat Exit working treatment
Elongation Tensile Specimen Alloy temp. ratio after after fracture
strength No. type (.degree. C.) (%) drawing (%) (MPa) 17-1 AZ31 200
15 No 3 280 17-2 AZ31 200 30 No 4 320 17-3 AZ31 200 45 No 3 370
17-4 AZ31 200 45 Yes 20 280 17-5 AZ61 200 15 No 3 300 17-6 AZ61 200
30 No 2 340 17-7 AZ61 200 45 No 4 380 17-8 AZ61 200 45 Yes 15
330
[0134] As is clear from Table 17, it turns out that the specimens
subjected to heat treatment after drawing have a high
elongation.
[0135] Further, the metal texture of the specimen No. 17-8 was
observed through an optical microscope. Its micrograph is shown in
FIG. 5. The observed metal texture exhibited a very unique
structure in which twins and recrystallized grains were contained
mixedly.
Experimental Example 2-4
[0136] In this experimental example, bending was applied using the
same specimen No. 15-4 as fabricated in the experimental example
2-2. By a rotating bending method, bending was imparted to the
drawn pipe specimen of 21.5 mm in outside diameter D and 1 mm in
wall thickness at room temperatures to cause its bend of 2.8 D in
radius of curvature. As a result, it was shown that the magnesium
base alloy pipe prepared according to the present invention can be
bent well in such a small bending radius.
Experimental Example 2-5
[0137] An extruded pipe of an Az31 alloy was fabricated into a
butted pipe in the following way. First, the starting extruded pipe
of 28 mm in O.D. and 2.5 mm in wall thickness was drawn by plug
drawing into a pipe of 24 mm in O.D. and 2.2 mm in wall thickness.
Then, the drawn pipe was heat-treated at 250.degree. C. for 30
minutes after drawing. For this drawing, pointing was done under
the same conditions as for the specimen No. 13-3 in the
experimental example 2-1 above, and the pointed pipe was drawn
under the same conditions as for the specimen No. 15-4 in the
experimental example 2-2 above. This applies likewise in the plain
drawing and the plug drawing to be described below.
[0138] Using the resultant drawn pipe, a butted pipe was fabricated
by combining the plain drawing and the plug drawing, as illustrated
in FIG. 6. First, one end of a drawn pipe 4 was passed through a
die 3, and then the drawn pipe 4 was subjected to plain drawing
without squeezing its wall between the plug 2 and the inside of the
die 3 (FIG. 6A). Thereafter, the middle portion of the drawn pipe 4
was subjected to plug drawing, in which the drawn pipe 4 had its
wall squeezed between the inside of the die 3 and the plug 2
reaching the inside of the die 3 (FIG. 6B). Then, the plug 2 was
retracted, and the other end of the drawn pipe 4 was subjected also
to plain drawing without squeezing the wall of the drawn pipe 4
between the plug 2 and the inside of the die 3 (FIG. 6A). By this
process, a butted pipe 10 having thick-walled opposite end portions
and a thin-walled intermediate portion could be formed, as shown in
FIG. 7. The resultant butted pipe 10 was 23 mm in O.D., 2.3 mm in
wall thickness at its opposite ends and 2.0 mm in wall thickness at
its middle portion.
Experimental Example 3-1
[0139] Extruded pipe blanks (O.D.: 10-45 mm, wall thickness: 1.0-5
mm) of an ZK60 alloy was pointed with different working ratios at
varied temperatures in the same manner as in the experimental
example 2-1. The ZK60 alloy of the extruded pipe was a magnesium
base alloy containing 5.9 mass % of Zn and 0.70 mass % of Zr with
the balance comprising Mg and unavoidable impurities.
[0140] For the pointing operation, a front end of a pipe blank was
heated at 350.degree. C. and the time elapsing before the front end
was fed in a die of a swaging machine thereafter (cooling time) was
varied to control the temperature of the pipe blank at its front
working end just before it was fed in a swaging machine (inlet
temperature). The inlet temperature was determined by calculation
from the heating temperature (350.degree. C.) and the cooling time.
For heating some pipe blanks, the die of the swaging machine was
also heated. The die was heated at 150.degree. C. Further, some
pipe blanks were heated with a cylindrical copper block (heat
insulating material) inserted in their front ends. For each pipe
blank, the inlet temperature, use of die heating, use of a heat
insulating material, and the workability at each working ratio are
shown in Table 18. The working ratio is given by {(pipe O.D. before
working-pipe O.D. after working)/pipe O.D. before
working}.times.100, and the workability is marked with
".smallcircle." for specimens worked without cracking, while it is
marked with "x" for specimens that underwent cracking.
TABLE-US-00018 TABLE 18 Inlet Heat Workability vs. Specimen Alloy
temp. Die insulating working ratio No. type (.degree. C.) heating
material 3% 5% 10% Notes 18-1 ZK60 20 No No X X X 18-2 ZK60 50 No
No .largecircle. X X 18-3 ZK60 100 No No .largecircle.
.largecircle. .largecircle. 18-4 ZK60 450 No No .largecircle.
.largecircle. .largecircle. 18-5 ZK60 480 No No .largecircle.
.largecircle. .largecircle. 1 18-6 ZK60 20 Yes No .largecircle. X X
18-7 ZK60 50 Yes No .largecircle. .largecircle. X 18-8 ZK60 100 Yes
No .largecircle. .largecircle. .largecircle. 18-9 ZK60 450 Yes No
.largecircle. .largecircle. .largecircle. 18-10 ZK60 480 Yes No
.largecircle. .largecircle. .largecircle. 1 18-11 ZK60 20 No Yes X
X X 18-12 ZK60 50 No Yes .largecircle. .largecircle. X 18-13 ZK60
100 No Yes .largecircle. .largecircle. .largecircle. 18-14 ZK60 450
No Yes .largecircle. .largecircle. .largecircle. 18-15 ZK60 480 No
Yes .largecircle. .largecircle. .largecircle. 1 *1: Unusable due to
severe surface oxidation
[0141] From Table 18, it turns out that the pipe blank specimens
having 50.degree. C. inlet temperature at their front ends can be
pointed without undergoing cracking if their working ratios are in
the rage of about 2-3%. The specimens having a 50.degree. C. inlet
temperature can be pointed with a higher working ratio, if either
the die heating or the heat insulating material is combined
therewith. Further, those specimens involving an inlet temperature
of 100-450.degree. C. allow pointing with a high working ratio
above 5%. Although the specimens involving an inlet temperature
above 480.degree. C. could be pointed, it was determined that such
specimens could not bear commercial applications due to their
remarkable surface oxidation. Besides, it was shown that according
to the method of the present invention a magnesium base alloy pipe
having a wall thickness of 0.5 mm may be produced.
Experimental Example 3-2
[0142] In this experiment, pipe blanks were prepared by providing a
film coating on extruded pipes of alloys having the same chemical
compositions as those used in the experimental example 3-1 above.
The film coating was accomplished by first dispersing a PTFE resin
in water to prepare its aqueous dispersion liquid, then immersing a
pipe blank in this dispersion liquid, and subsequently taking out
and heating the wet pipe blank at 400.degree. C. to form PTFE
coating on the pipe surface. Thereafter, the thus coated pipe blank
was pointed in the same manner as in the specimen No. 18-3 in the
experimental example 3-1 and then subjected to drawing.
[0143] Using a drawbench, the drawing was carried out by plug
drawing in one pass. The pipe blanks were subjected to drawing
along with their heating by any appropriate means including
immersion in a preheated lubricant, an atmosphere furnace, a
high-frequency heating furnace or drawing die heating. The exit
temperature of the pipe blank was adjusted by varying the time
elapsing before it was fed in the drawing die after it was taken
out from a lubricant bath, an atmosphere furnace or a
high-frequency furnace. The exit temperature is a temperature of
the drawn pipe just behind the exit of the drawing die. The heating
rate to exit temperature was 1-2.degree. C./sec. After drawing, the
pipes were air-cooled at a rate of 1-5.degree. C./sec. The drawing
speed was 10 m/min.
[0144] The exit temperature, heating method, lubrication method,
and the workability at each working ratio for the ZK60 specimens
are shown in Table 19. The working ratio is given by {(pipe
cross-sectional area before working-pipe cross-sectional area after
working)/pipe cross-sectional area before working}.times.100. The
workability is marked with ".smallcircle." for specimens worked
without cracking, while it is marked with "x" for specimens that
underwent cracking and with "seized" for specimens undergoing
seizing. In the "lubrication method" column of Table 19, "oil"
indicates application of a lubricating oil to a pipe blank, with
"film coating+oil" indicating application of a lubricating oil to a
PTFE resin-coated pipe blank, "film coating" indicating that a PTFE
resin-coated pipe blank is drawn without using a lubricating oil,
and "forced lubrication" indicating that drawing is done by
forcedly supplying a lubricating oil into a gap between a die and
the pipe blank.
TABLE-US-00019 TABLE 19 Workability vs. Specimen Exit temp. working
ratio No. Alloy type (.degree. C.) Heating method Lubrication
method 5% 10% 20% 19-1 ZK60 20 Immersed in lubricant Oil
.largecircle. X X 19-2 ZK60 50 '' '' .largecircle. .largecircle. X
19-3 ZK60 100 '' '' .largecircle. .largecircle. .largecircle. 19-4
ZK60 200 '' '' .largecircle. .largecircle. .largecircle. 19-5 ZK60
250 '' '' .largecircle. .largecircle. X 19-6 ZK60 20 Immersed in
lubricant Film coating + oil .largecircle. X X 19-7 ZK60 50 '' ''
.largecircle. .largecircle. X 19-8 ZK60 100 '' '' .largecircle.
.largecircle. .largecircle. 19-9 ZK60 200 '' '' .largecircle.
.largecircle. .largecircle. 19-10 ZK60 250 '' '' .largecircle.
.largecircle. X 19-11 ZK60 200 Atmosphere furnace Forced
lubrication .largecircle. .largecircle. .largecircle. 19-12 ZK60
200 '' Film coating + oil .largecircle. .largecircle. .largecircle.
19-13 ZK60 300 '' Film coating .largecircle. .largecircle. X 19-14
ZK60 200 High-frequency furnace Forced lubrication .largecircle.
.largecircle. .largecircle. 19-15 ZK60 200 '' Film coating + oil
.largecircle. .largecircle. .largecircle. 19-16 ZK60 300 '' Film
coating .largecircle. .largecircle. X 19-17 ZK60 100 Die heating
Forced lubrication .largecircle. .largecircle. .largecircle. 19-18
ZK60 100 '' Film coating + oil .largecircle. .largecircle.
.largecircle. 19-19 ZK60 300 '' Film coating .largecircle.
.largecircle. X
[0145] From Table 19, it turns out that desirable results were
obtained at exit temperatures ranging from 50 to 300.degree. C.
Especially, it is understood that the specimens combining the film
coating and the lubrication by lubricating oil can be subjected to
drawing with a higher working ratio.
Experimental Example 3-3
[0146] In this experimental example, some extruded pipes used in
the experimental example 3-2 above were subjected to drawing in
multiple passes so as to effect a varied total working ratio, and
some of the drawn pipes were heat-treated after drawing. The
"heating method" for drawing was the immersion in a preheated
lubricant, and the "lubrication method" is accomplished using a
lubricating oil. For drawing, the specimens with a 15% total
working ratio were drawn in one pass, the specimens with a 30%
total working ratio were drawn in two passes, while those with a
45% total working ratio were worked in three passes. For each pass,
the pipe blank was immersed in the lubricating oil to be heated to
an exit temperature. The working total ratio is given by {(pipe
cross-sectional area before working-pipe cross-sectional area after
the last working)/pipe cross-sectional area before
working}.times.100. The heat treatment after drawing was applied at
250.degree. C. for 30 minutes. The elongation and the tensile
strength were measured on all the resultant drawn pipe specimens.
The exit temperature, total working ratio, use of heat treatment
after drawing, elongation and tensile strength are shown in Table
20 for the respective specimens.
TABLE-US-00020 TABLE 20 Total Heat Elongation working treatment
after Tensile Specimen Alloy Exit temp. ratio after fracture
strength No. type (.degree. C.) (%) drawing (%) (MPa) 20-1 ZK60 200
15 No 4 321 20-2 ZK60 200 30 No 4 338 20-3 ZK60 200 45 No 3 372
20-4 ZK60 200 45 Yes 18 301
[0147] As is clear from Table 20, it turns out that the specimens
subjected to heat treatment after drawing have a high
elongation.
Experimental Example 3-4
[0148] In this experimental example, bending was applied using the
same specimen No. 19-4 as fabricated in the experimental example
3-2. By a rotating bending method, bending was imparted to the
drawn pipe specimen of 21.5 mm in outside diameter D and 1 mm in
wall thickness at room temperatures to cause its bend of 2.8 D in
radius of curvature. As a result, it was shown that the magnesium
base alloy pipe prepared according to the present invention can be
bent well in such a small bending radius.
Experimental Example 3-5
[0149] An extruded pipe of a ZK61 alloy was fabricated into a
butted pipe in the following way. First, the starting extruded pipe
of 28 mm in O.D. and 2.5 mm in wall thickness was drawn by plug
drawing into a pipe of 24 mm in O.D. and 2.2 mm in wall thickness.
Then, the drawn pipe was heat-treated at 250.degree. C. for 30
minutes after drawing. For this drawing, pointing was done under
the same conditions as for the specimen No. 18-3 in the
experimental example 3-1 above, and drawing was under the same
conditions as for the specimen No. 19-4 in the experimental example
3-2 above. This applies likewise in the plain drawing and the plug
drawing to be described below.
[0150] Using the resultant drawn pipe, a butted pipe was fabricated
by combining the plain drawing and the plug drawing, as illustrated
in FIG. 6. First, one end of a drawn pipe 4 was passed through a
die 3, and then the drawn pipe 4 was subjected to plain drawing
without squeezing its wall between the plug 2 and the inside of the
die 3 (FIG. 6A). Thereafter, the middle portion of the drawn pipe 4
was subjected to plug drawing, in which the drawn pipe 4 had its
wall squeezed between the inside of the die 3 and the plug 2
reaching the inside of the die 3 (FIG. 6B). Then, the plug 2 was
retracted, and the other end of the drawn pipe 4 was subjected also
to plain drawing without squeezing the wall of the drawn pipe 4
between the plug 2 and the inside of the die 3 (FIG. 6A). By this
process, a butted pipe 10 having thick-walled opposite end portions
and a thin-walled intermediate portion could be formed, as shown in
FIG. 7. The resultant butted pipe 10 was 23 mm in O.D., 2.3 mm in
wall thickness at its opposite ends and 2.0 mm in wall thickness at
its middle portion.
Experimental Example 4-1
[0151] Extruded pipes (O.D.: 26.0 mm, wall thickness: 1.5 mm,
length: 2000 mm) of AM60, AZ31, AZ61 and ZK60 alloys were set as
starting materials for the experiment. After being pointed for
drawing, each pointed pipe was heat-treated at 350.degree. C. for 1
hour to remove its work hardening or strain hardening due to
pointing and then the pipe was subjected to drawing under the
following conditions.
[0152] The drawing was performed in plug drawing method, and a high
frequency heating unit provided just before the die was set up so
that the temperature of the pipe just before it is inserted in the
die was 150.degree. C. The die had a 24.5 mm bore and the plug had
a 21.7 mm O.D. For all the pipes, the area reduction ratio applied
was 15.0%. In this experiment, the working turned out to be
successful regardless of alloy types. It was shown that
high-frequency heating is a very effective heating method.
Experimental Example 4-2
[0153] Extruded pipes (O.D.: 26.0 mm, wall thickness: 1.5 mm,
length: 2000 mm) of AM60, AZ31, AZ61 and ZK60 alloys were set as
starting materials for the experiment. To point the pipe for
drawing, its front end was heated by immersing it in a lubricant at
200.degree. C. and then introduced into a swaging machine for
pointing. This heating was effective for pointing the pipe without
causing cracks or like defects. Sufficient heating was attained by
2 minutes, the immersion in lubricant turned out to be effective as
a heating means. Besides, it was shown that according to the method
of the present invention a magnesium base alloy pipe having a wall
thickness of 0.5 mm may be produced.
Experimental Example 4-3
[0154] Twenty (20) extruded pipes (O.D.: 26.0 mm, wall thickness:
1.5 mm, length: 2000 mm) of an AZ61 alloy were set as starting
materials for the experiment. After pointing the pipes for drawing,
10 extruded pipes were subjected to film coating at their initial
working portions for drawing. The film coating was accomplished by
first dispersing a PTFE resin in water to prepare its aqueous
dispersion liquid, then immersing the pipes in this dispersion
liquid only at their initial working portions and their
peripheries, and subsequently heating the wet portions at
400.degree. C. for about 5 minutes.
[0155] Then, the thus processed 10 pipes and remaining 10
unprocessed pipes were subjected to drawing. The drawing was
performed in plug drawing method, and the pipes were heated by
immersing them in a lubricant heated at 180.degree. C. After taking
out from the lubricant bath, the pipes were drawn on a drawing
bench before they are naturally cooled. The temperature of the
pipes just before entering the die was about 150.degree. C. The die
had a 24.5 mm bore and the plug had a 21.7 mm O.D. The area
reduction ratio applied was 15.0%.
[0156] Seizing was observed in 6 out of 10 pipe specimens not
subjected to film coating, while seizing was not observed at all in
any of the coated pipe specimens. Thus, it turns out that even if
limited to initial working portions and their peripheries the film
coating is substantially effective for prevention of seizing.
Experimental Example 4-4
[0157] Twenty (20) extruded pipes (O.D.: 26.0 mm, wall thickness:
1.5 mm, length: 2000 mm) of an AZ61 alloy were set as starting
materials for the experiment. After pointing, the extruded pipes
were drawn once into pipes of 24.5 mm in. O.D. and 1.5 mm in wall
thickness and then heat-treated at 350.degree. C. for 1 hour.
[0158] The resultant primary-drawn pipes were pointed again for
drawing and then further subjected to drawing in the following
manner. The drawing was performed in plug drawing method. Among the
total 20 pipe specimens, 10 specimens had their front end portions
(initial working portions where the pipe walls first contact the
die and the plug before the drawing starts) heated in an atmosphere
heating furnace heated at 350.degree. C. and were drawn on a
drawing bench before they are naturally cooled to room
temperatures. The temperature of the pipes just before entering the
die was about 200.degree. C. The remaining 10 pipe specimens were
subjected to drawing without heating. That is, those remaining
specimens were drawn without having their front end portions
heated. The die had a 23.1 mm bore and the plug had a 20.4 mm O.D.
The area reduction ratio applied was 14.9%.
[0159] Seizing was observed in 9 pipe specimens out of 10 specimens
which were not heated at their front end portions, while no seizing
was observed in those specimens heated at their front end portions.
Thus, it turns out that even if limited to front end portions the
heating is substantially effective for prevention of seizing.
[0160] Further, in a similar experiment where the heating
temperature at the front end portion of the pipe was varied, no
desirable effectiveness was observed at temperatures below
150.degree. C., while the working was feasible but oxidation
occurred at temperatures above 400.degree. C.
Experimental Example 4-5
[0161] Extruded pipes (O.D.: 34.0 mm, wall thickness: 3.0 mm,
length: 2000 mm) of an AZ61 alloy were set as a starting material
for the experiment. After being pointed for drawing, each pointed
pipe was heat-treated at 350.degree. C. for 1 hour to remove its
work hardening or strain hardening due to pointing and then the
pipe was subjected to drawing under the following conditions. Ten
such extruded pipes were drawn in plug drawing method, using a die
having a 31 mm bore and a plug of O.D.25 mm. The area reduction
ratio was 9.7%. By immersing in a lubricant heated at 180.degree.
C., the pipes were heated before working so that their working
temperature becomes 140.degree. C. The working temperature here is
the temperature of the pipes just before entering the die.
[0162] The resultant drawn pipes were heat-treated at 350.degree.
C. for 1 hour. The heat-treated drawn pipes were formed into butted
pipes by using a mandrel under the following conditions. The
thick-walled portions (O.D.: 30 mm) at the opposite ends of the
pipes were worked using a mandrel of O.D. 24.2 mm, while their
thin-walled portions were worked by using a mandrel locally having
a larger outside diameter. For the mandrel drawing, the following
conditions were selectively applied: (1) working temperature at
room temperatures, and pipes coated with a fluororesin; (2) working
temperature at room temperatures, and the mandrel coated with a
fluororesin; (3) working temperature at room temperatures, without
film coating; (4) working temperature at 140.degree. C., pipes
coated with a fluororesin; (5) working temperature at 140.degree.
C., the mandrel coated with a fluororesin; and (6) working
temperature at 140.degree. C., without film coating. For the
fluororesin coating, a water-dispersible PFA was used. The
experimental results are shown in Table 21.
TABLE-US-00021 TABLE 21 Working ratio at Worked at room
temperatures Worked at 140.degree. C. Die Thin-walled thin-walled
Fluororesin Fluororesin Fluororesin Fluororesin I.D. portion I.D.
portion coated coated coated coated Pass (mm) (mm) (%) pipe mandrel
Uncoated pipe mandrel Uncoated 1 29.0 23.2 9.9 .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. 2 29.0 23.5 14.1 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. 3 29.0 23.8
18.3 .largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. 4 29.0 24.0 21.1 .largecircle.
.largecircle. X .largecircle. .largecircle. .largecircle. 5 29.0
24.5 28.3 X X X .largecircle. .largecircle. .largecircle.
[0163] As is clear from Table 21, a magnesium base alloy pipe may
be worked into a butted pipe by using a mandrel, and such a butted
pipe can be given a larger difference in its wall thickness by
coating the pipe or mandrel with a fluororesin. Further, it is
possible to fabricate a butted pipe with a further larger
difference in its wall thickness by increasing the working
temperature.
[0164] A working temperature below 100.degree. C. did not yield any
appreciable effectiveness, while the specimens underwent breakage
when the working temperature applied was above 350.degree. C. This
is due to a reduced material strength.
[0165] Further, mandrel drawing was executed by using a mandrel of
O.D. 22.0 mm for thick-walled portions of the pipe and a mandrel of
O.D. 24.5 mm for the thin-walled portion. The drawing was done at
room temperatures by coating the mandrels with a fluororesin.
Further, for the drawing, 3 dies of I.D. 29.6 mm, 28.7 mm and 28.0
mm were used in the cited sequence in 3 passes, respectively, while
applying annealing at about 350.degree. C. after each drawing pass.
Consequently, a butted pipe could be fabricated as having a large
difference in wall thickness, with a 3.0 mm thickness at its
thick-walled portions and a 1.75 mm thickness at its thin-walled
portion.
INDUSTRIAL APPLICABILITY
[0166] As fully described hereinbefore, the magnesium base alloy
pipe manufactured by the method according to the present invention
can combine high strength and toughness in balance by using
specified pointing conditions and/or drawing conditions.
Especially, this pipe is improved in properties including high
tensile strength, high YP ratio or high 0.2% proof stress in
addition to a high elongation after fracture representing its
toughness. Accordingly, the magnesium base alloy pipe of the
present invention are used effectively for applications requiring a
light weight in addition to strength, including pipes for chairs,
tables, wheelchairs, stretchers, pickels, or pipes for automobile
frames or like frames.
* * * * *