DESCRIPTION
TITLE OF INVENTION: TITANIUM MATERIAL
5 TECHNICAL FIELD
C000ll
The present invention relates to a titanium material, more particularly to
a titanium material excellent in strength and workability.
10 BACKGROUND ART
[00021
Conventionally, plate-shaped and bar-shaped members formed from
materials such as titanium alloys and pure titanium have been widely used.
For example, a plate-shaped titanium material (hereinafter also referred
15 to as a "titanium plate") has been widely used for industrial products, wherein the
tita~kdrn plate i~ subjected t" varbus workings accompanied by piastic
deformation such as folding, bulging, and drawing to form various products.
The titanium plate which is subjected to such working is demanded to
have excellent workability.
20 [00031
Further, recently, the reduction in the thickness of a titanium plate has
been demanded in terms of reducing material cost, reducing the weight of a
product, and the like.
As a result, improvement in the strength of a titanium plate has
25 increasingly been demanded.
However, conventionally, the workability and the strength of a titanium
plate are in a trade-off relation, and it is difficult to simultaneously satisfy these
properties.
That is, conventional titanium plates have a problem that fabrication
becomes diilicult (poor in workability) with the increase in yield strength.
Coo041
5 With respect to the above subject, the following Patent Literature 1 shows
the results of the evaluation of workability of titanium thin plates having different
components and crystal grain sizes in a cupping test and describes that the finer
the crystal grain, the better the workability is (page 103, h m line 5).
Further, the following Patent Literature 1 discloses a method for
10 producing a pure titanium thin plate and describes the production of a pure
titanium thin plate having a reduced gloss surface, including performing final
annealing in the atmospheric air by the continuous annealing at (600 to 8 0 0 )x~ ~
(2 to 5) minutes, then performing pickling treatment, and adjusting the average
crystal grain size (hereinafter referred to as particle size) of the product to 3 to 60
15 pm.
:oooa:
Further, the following Patent Literature 2 discloses pure titanium for
building materials, a pure titanium plate, and a method for producing the same
and describes a titanium material for building materials which contains 900 ppm
20 or less of oxygen and 100 ppm or more and 600 ppm or less of Fe, wherein the
content of Ni and Cr is restrained.
Furthermore, Patent Literature 2 describes a titanium material for
building materials having an average crystal grain size of 70 pm or less which has
been subjected to pickling treatment with an aqueous nitric hydrofluoric acid
25 solution after cold rolling and annealing.
However, these Patent Literatures 1 and 2 show almost no data in which a
titanium material having a fine crystal grain size of 5 pm or less has been
evaluated, and Patent Literature 2 shows an Example in which the crystal grain
size is 3 pn but at the same time describes in the paragraph [0026] that "In actual
production, the lower limit will be about 5 p"w,hi ch is a negative description on
a crystal grain size of 5 pm or less.
5 This is probably because these literatures aim at obtaining an excellent
titanium material for building materials having a reduced gloss, and the
workability in bulging, deep drawing, and the like has not been sufficiently
investigated.
[OOO~]
10 Further, the following Patent Literature 4 discloses a titanium plate
excellent in workability, which has a low strength (yield strength) irrespective of
having excellent workability and cannot satisfy both workability and strength at
the same time.
15 CITATION LIST
PATENT LITEMTURE
[00071
Patent Literature 1: Japanese Patent Laid-Open No. 63-103056
Patent Literature 2: Japanese Patent Laid-Open No. 9-3573
20 Patent Literature 3: Japanese Patent Laid-Open No. 2006-316323
Patent Literature 4: Japanese Patent Laid-Open No. 63-60247
NON PATENT LITERATURE
[ooosl
Non Patent Literature 1: "Titanium", Vol. 57, No, 2 (issued by the Japan Titanium
'25 Society, in April, 2009)
SUMMARY OF INVENTION
TECHNIC& PROBLEM
[00091
An object of the present invention is to provide a titanium plate having
high strength and excellent workability.
5
SOLUTION TO PROBLEM
[oolol
Although the strength held strength) of a titanium material can be
increased by mainly adding oxygen (0)an d iron ( ~ e )b,u t when these are added,
10 ductility will be reduced to thereby reduce workability.
For example, since a titanium material specified in JIS class 1 has a low
content of oxygen and iron, a titanium plate using the material of JIS class 1
generally has a low strength held strength) but is excellent in ductility and
excellent in workability.
When a titanium material of JIS class 2 having a higher content of oxygen
and irnn t h a t~he tita::iurn rr.ateyi",! cf JIS class ? is used, the resiiltii~gt i t a i 6 ~
material will have a higher strength (yield strength) than the titanium material in
which the titanium material of JIS class 1 is used, whde it wdl tend to have a
reduced ductility to reduce workability.
20 Titanium materials of JIS class 3 and class 4 having a much higher
content of oxygen and iron have much higher strength (yield strength) but have a
much reduced ductility to greatly reduce workability.
That is, strength (yield strength) and workability have a certain
relationship (hereinafter, this relationship is also referred to as "strength (yield
25 strength)-workability" balance).
C00lll
Incidentally, plate materials and wire materials prepared by using
titanium materials are formed by subjecting the materials to working accompanied
by plastic deformation such as rolling and wire drawing.
These plate materials and wire materials subjected to working
accompanied by plastic deformation generally have the inner part in which a
5 worked structure is formed in the state as it is, and therefore, they are subjected to
a step called final annealing in order to recrystallize the structure before they are
supplied to the market.
For example, a titanium plate is subjected to working such as cold rolling
to adjust the thickness to a predetermined value and then subjected to batch
10 annealing, continuous annealing, or the like to recrystallize the worked structure
in the inner part to form equi-axed crystal grains (hereinafter, referred to as
"recrystallized grains").
These recrystallized grains greatly grow with the passage of annealing
time and the like, and in particular, in the period immediately after the initiation
15 of recrystallization where the particle size of the recrystallized grains is small, the
giowtti rate of the recrystaiiized grains wiiI be high and they wiii grow to a iarge
particle size exceeding 5 pm in a relatively short time.
When the recrystallized grains grow to such a size, a non-recrystallized
part (worked structure) will not remain, but only the equi-axed structure based on
20 the recrystallized grains will generally be formed in the inner part of the titanium
material.
~00121
As a result of extensive and intensive investigations to achieve the
above-described object, the present inventors have found that improvement in the
25 strenqth (yield strength) of a titanium material can be achieved by adjusting a
structure (refining of crystal grains by lcaving a non-recrystallized part) to which
attention has not been paid as a means for improving strength (yield strength).
Specifically, the present inventors have completed the present invention
by subjecting a commercially pure titanium plate which has been cold-rolled to a
predetermined thickness to final annealing in a vacuum using an electric furnace;
making various titanium plates having different structures on an experimental
5 basis by changing the temperature and time thereof: and evaluating the strength
(yield strength) and workability (ductility) thereof by a tensile test and an
Erichsen test.
[00131
As a result of the evaluation, it has been found that although strength
10 (yield strength) tends to increase and workability (Erichsen value) tends to be
reduced with the decrease in the size of crystal grains, the Erichsen value is not
significantly reduced provided that the average particle size of the recrystallized
grains is a predetermined size or less, and the "strength &eld
strength)-workability balance" can be improved compared with conventional
15 titanium materials.
:oo;4
Further, there has been a case where even if the average crystal grain size
of the recrystallized grains is a predetermined size or less, workability (Erichsen
value) is reduced, and therefore, the "strength (yield strength)-workability
20 balance" cannot be improved compared with conventional titanium materials.
As a result of the investigation of the microstructure of this titanium plate
in detail, many non-recrystallized parts have been observed in addition to the
grains recrystallized by final annealing.
The "strength (yield strength)-workability balance" has been investigated
25 based on the amount of the non-recrystallized part, and it has been found that
workability is extremely reduced if the area rate of the non-recrystallized part in
the cross-sectional area of the titanium plate exceeds 30%.
Note that, herein, the non-recrystallized part means a part in which a
worked structure subjected to plastic working remains.
Specifically, the present invention related to a titanium material for
5 achieving the above object is characterized in that the titanium material has an
iron content of 0.60% by mass or less and an oxygen content of 0.15% by mass or
less, with the balance being titanium and unavoidable impurities, the titanium
material having a worked structure formed by working accompanied by plastic
deformation and a recrystallized structure formed by annealing after the working,
10 wherein the titanium material is formed such that the average particle size of
crystal grains of the recrystallized structure is 1 pm or more and 5 pm or less, and
the area of a non-recrystallized part in the cross-sectional area of the titanium
material is more than 0% and 30% or less.
15 ADVANTAGEOUS EFFECT OF INVENTION
[on16!
The present invention can provide a titanium material having high
strength and excellent workability.
20 BRIEF DESCRIPTION OF DRAWINGS
[00171
FIG. 1 is a photomicrograph showing the microstructure of the titanium
plate of Example observed with a transmission electron microscope (a
non-recrystallized part is observed in a part between recrystallized grains).
FIG. 3 is a graph showing the relationship hetween the yield strength and
the Erichsen value.
DESCRIPTION OF EMBODIMENTS
[0018]
Hereinafter, a preferred embodiment of the titanium material according to
the present invention will be described taking a titanium plate as an example.
5 The titanium plate in the present embodiment is formed from a titanium
material having an iron (Fe) content of 0.60% by mass or less and an oxygen (0)
content of 0.15% by mass or less, with the balance being titanium (Ti) and
unavoidable impurities.
This titanium plate is formed by working accompanied by plastic
10 deformation followed by annealing and has, in the inner part thereof, a worked
structure accompanying the working and a recrystallized structure accompanying
the annealing, wherein the titanium plate is formed such that the average particle
size of crystal grains of the recrystallized structure is 1 pm or more and 5 pm or
less, and the area of a non-recrystallized part in the cross-sectional area of the
15 titanium plate is more than 0% and 30% or less.
iO0I.91
As described above, the iron (Fe) is contained at a percentage of 0.60% by
mass or less.
Note that the upper limit of Fe is 0.60% by mass because Fe is a
20 P-phase-stabilizing element in a titanium material, and if the content of Fe
exceeds 0.60% by mass, many P-phases may be produced in the structure
constituting the titanium plate in addition to the a-phase.
That is, since ductility is greatly reduced or corrosion resistance is
reduced depending on the size of the P-phase formed, it is important to keep the
25 content of Fe contained in the titanium material which forms the titanium plate of
the present cmbodirnent at 0.60% by mass or less in terms of forming a titaniuln
plate having high strength and excellent workability.
[00201
Note that although the lower limit of the Fe content is not necessarily
demanded in terms of forming a titanium plate having high strength and excellent
workability, an expensive and high purity titanium sponge must be used as a raw
5 material if a titanium plate having a Fe content of less than 0.01% by mass is
intended to be used, which may increase the material cost of the titanium plate.
Therefore, the content of Fe is preferably 0.01% by mass or more and
0.60% by mass or less in terms of the cost of the titanium plate and the like.
[00211
10 For example, in the Kroll process, a titanium material having an Fe
content of 0.60% by mass or more is generally formed only in a small region near
the vessel.
Therefore, most of the titanium sponge obtained by the Kroll process can
be used because the titanium plate in the present embodiment has a content of
15 iron as a component in the range of 0.01 to 0.60% by mass.
t " + 4 1 4 4 - - J - - - 1 - - - 1
bIICi b l b d ~~ L~Z~WVI ~ L ~L~~~~ C J G tIfLu ~~u ~ u ~ l l ~ t fLl l~l rI oIt : Y a l a to De
suitable as a consumption material in that almost no restriction is added to the
use part of the titanium sponge.
[oozzl
30 The oxygen (0) is contained in the titanium material in a content of 0.15%
by mass or less.
The 0 content of the titanium material forming the titanium plate of the
present embodiment is 0.15% by mass or less because if the 0 content exceeds
0.15% by mass, the strength of the titanium plate may be excessively improved to
15 prevent the workability thereof from being sufficiently imparted thereto even if
improvement in the "strength-workability balance" is intended to be achieved by
reducing the size of crystal grains, thus making it difficult to form a titanium plate
suitable for working such as bulging and deep drawing.
[00231
Note that although the lower limit of the 0 content is not particularly
provided, a titanium plate may have to be produced using an expensive and high
5 purity titanium sponge as a raw material if the 0 content of the titanium material
constituting the titanium plate is intended to be set to less than 0.015% by mass.
Therefore, the 0 content is preferably 0.015% by mass or more and 0.16%
by mass or less.
lo0241
Further, it is important that unavoidable impurities such as carbon (C),
nitrogen (N), and hydrogen (H) are each contained in a content corresponding to
JIS class 2 or less for the purpose of ensuring good workability in fabrication.
More specificdx it is important that the content of C, N, and H is each
less than 0.02% by mass.
15 Further, the content of C is preferably 0.01% by mass or less, the content
of N is preferabiy 0.01% by mass or less, and the content of H is preferably 0.01%
by mass or less.
Although a lower limit is not provided in the above content of C, N, and H
from the point of view of the workability of a titanium plate, the production cost of
20 the titanium plate may be significantly increased if the content is intended to be
extremely reduced.
From the point of view of preventing such cost increase, the C content is
preferably 0.0005% by mass or more, the content of N is preferably 0.0005% by
mass or more, and the content of H is preferably 0.0005% by mass or more.
25 i00251
'1s described above, the titanium plate of the present invention has a
worked structure and a recrystallized structure in the inner part thereof and is
formed such that the average particle size of crystal grains of the recrystallized
structure is 1 pm or more and 5 p or less, and the area of a non-recrystallized
part in the cross-sectional area of the titanium plate is more than 0% and 30% or
less.
5 [00261
The upper Limit of the average particle size of the recrystallized structure
is 5 pm because if the average crystal grain size of equi-axed a-grains produced by
recrystallization exceeds 5 pm, the effect of the refining of crystal grains will be
small, making it dif6cult to achieve excellent "strength-workability balance".
Further, the lower limit is 1 pm because if the titanium plate is subjected
to working (rolling, forging, or the like) in actual production (by an industrially
feasible method followed by annealing to obtain an average crystal grain size of
less than 1 p, the area rate of the non-recrystallized part (worked structure) to
be described below will increase, which extremely increases strength but greatly
15 reduces ductility, making it difficult to achieve excellent "strength-workability
l aiance'!.
lo0271
The non-recrystallized part is formed from a worked structure in which a
titanium plate is plastically deformed by working (cold rolling, forging, or the like)
30 to collapse crystal grains, and the strength of the titanium plate can be improved
by allowing the worked structure to remain in the titanium plate.
A titanium plate comprising a worked structure formed by cold rolling or
the like has high strength while its ductility is very small.
Therefore, the worked structure has conventionally been recrystallized by
25 annealing to form an equi-axed structure, and sufficient annealing time has been
provided to such an extent that the worked structure does not rernain in the
titanium plate.
On the other hand, with respect to the titanium plate in the present
embodiment, the worked structure is allowed to remain in the titanium plate by
employing annealing conditions to be described below, and, moreover, the particle
size of recrystallized grains is adjusted as described above.
5 [00281
It is important in terms of obtaining excellent "strength-workability
balance" that the non-recrystallized part (worked structure) is provided so that the
ratio of the area thereof to the cross sectional area of the titanium plate is 30% or
less.
10 If the area rate of the non-recrystallized part is higher than 30%, the
strength of the titanium plate will be higher, but the ductility will be reduced,
making it difficult to allow the titanium plate to exhibit excellent workability.
As a result, it may be impossible to obtain excellent "strength-workability
balance."
The area rate of the non-recrystallized part is preferably 10% or less in
terms of Ecre reliably i ~ p a r t i gth e excellent "~treii@h--w~kilbilbitdya nce" to
the titanium plate.
iqote that although the lower limit is not particularly limited, the particle
size of recrystallized grains will rapidly increase if the non-recrystallized part is
20 lost (the area rate is 0%).
Therefore, the area rate of the non-recrystallized part is preferably 0.1%
or more in that the particle size of recrystallized grains can be more reliably
adjusted within the range as described above.
[00291
'The method for adjusting the particle size of the recrystallized grains and
forming the non-recrystallized part as described above includes a method in which
the titanium plate is adjusted to a desired thickness in a common rolling process
and the like and then subjected to final annealing in a predetermined condition.
roo301
The annealing technique which can be employed in the ha1 annealing
can be roughly classified into a continuous type and a batch type.
5 Among these, a continuous type final annealing is a method of annealing
by spreading a cold-rolled coil and passing a titanium plate at a constant speed
through an annealing furnace, and the method can control the holding time of
heating temperature by the plate-passing speed.
In the final annealing of conventional titanium platee, in the case of the
10 continuous type, the heating temperature is 700 to 800°C, and the heating time is
from several tens of seconds to about 2 minutes.
On the other hand, a batch type final annealing is a method of heating the
coil of a titanium plate in an annealing furnace in the state of a coil as it is,
wherein the titanium plate is slowly heated, in order to reduce the difference in
15 the application of heat between the outer part and the inner part of the coil. and
its cooiing rate is aiso very slow.
In the final annealing of conventional titanium plates, in the case of the
batch type, the heating temperature is 550 to 650°C, and the heating time is from
about 3 hours to 30 hours.
20 [00311
On the other hand, the final annealing performed when producing the
titanium plate of the present embodiment is preferably performed, for example, in
a continuous system, under heating conditions at a temperature of 580°C or more
and less than 600°C for 1 minute or more and 10 minutes and less, or under
25 heating conditions at 3 temperature of 600°C or more and 650°C or less for 10
seconds or more and 2 minutes or less.
The time period of 10 seconds or more is selected as a preferred heating
condition because if the time for holding the temperature is shorter than 10
seconds, a proper range of operation conditions such as plate-passing speed and
heating temperature to perform predetermined annealing to a titanium plate will
be very narrow, which requires highly accurate control of an apparatus or its
5 operation.
On the other hand, a condition of 10 minutes or less is preferred as the
heating time because if the holding time exceeds 10 minutes, the plate-passing
speed must be reduced, thus reducing the productivity.
to0321
10 Further, a temperature of 580°C or more is selected as a preferred
condition of heating temperature because if the heating temperature is lower than
580°C, it will be difficult to cause a predetermined recrystallization in a titanium
plate in a holding time of 10 minutes or less, and the area rate of the
non-recrystallized part will exceed 30% in many cases.
15 Furthermore, the heating temperature of 650°C or less ia selected because
if the teirrperature is higher than 6GOgC, ihe recrystallization of a titanium piate
may have been completed even in a heating time of 10 seconds, and recrystallized
grains may grow to an average particle size of 5 pm or more.
loo331
20 Further, the final annealing performed when producing the titanium plate
of the present embodiment is preferably performed under heating conditions at a
temperature of 420°C or more and less than 650°C for 3 hours or more and 50
hours or less, when it is a batch type.
-4 condition of 3 hours or more is preferred as the heating time because if
25 the heating time is shorter than 3 hours, the temperature in the inner part of a
coil may not reach a predetermined temperature depending on the size of the coil.
On the other hand, a condition of 50 hours or less is preferred as the
heating time because if the heating time exceeds 60 hours, the time required for
annealing will be excessively long, thus reducing the productivity of the titanium
plate.
Lo0341
Further, a heating temperature of 420°C or more is preferred because if
the heating temperature is lower than 420°CI it will be diEcult to cause e
predetermined recrystallization in a titanium plate in a holding time of 50 hours
or less, and the area rate of the non-recrystallized part will exceed 30% in many
cases.
Or it is because several annealing furnaces (heating equipment) must be
possessed in order to ensure a predetermined production volume, which increases
the cost of equipment and requires a large space for installing the annealing
furnaces.
Note that, in a batch type, since the titanium plate is heated in the state
of a coil, the temperature increasing rate is different between the outer part and
the inner part of the coil, and the time until the temperature reaches a target
temperature is also different.
Depending on the size of the coil, the heating temperature, and the
heating capacity of the annealing furnace, the time until the temperature reaches
20 a target temperature generally differs by tens of minutes to several hours.
Therefore, it is important to heat the coil to a temperature range where
the size of recrystallized grains does not greatly differ even if the heating time
differs to some extent, that is, important to a temperature range where the growth
rate of recrystallized grains is slow.
[00351
Further, ths heating temperature is preferably less lhan 550°C because
since the growth rate of recrystallized clystal grains is high at a temperature of
560°C or more, when the heating time is ehortened in accordance with the outer
part of the coil, a target temperature may not be reached in the inner part of the
coil, leading to a state where a non-recrystallized part which is not recrystallized
may be in an amount exceeding 30%; conversely, when the heating time is
5 lengthened in accordance with the inner part of the coil, the recrystallized grains
may excessively grow in the outer part of the coilt leading to an average cqatal
grain size of 5 pm or more.
100361
Note that the final annealing of either a continuous type or a batch type ie
10 desirably performed in a vacuum or in an inert gas atmosphere.
A titanium plate having excellent "strength-workability balance" can be
obtained by adjusting the average particle size of recrystallization and the residual
percentage of the non-recrystallized part (worked structure) with the annealing
conditions as described above.
16 100371
Note that aithough not described in detail here, a known matter in a
conventional titanium plate and titanium plate production method can also be
employed in the present invention in the range which does not significantly impair
the effect of the present invention.
20 Further, although a titanium plate is mentioned as an example of a
titanium material in the present embodiment, a titanium material of various
forms such as a wire material, a bar material, and a tubing material is the same
as the titanium plate in that excellent "strength-workability balance" is exhibited,
and these titanium materials also fall within the scope intended by the present
2 5 invention.
EXAMPLES
[oosal
Next, the present invention will be described in more detail with reference
to Examples, but the present invention is not limited to these.
[00391
5
(Sample Nos. 1 to 45)
(Preparation of Test Pieces)
An ingot (140 mm in diameter) was prepared by small-sized vacuum arc
melting, and the ingot was heated to 1050°C and then forged to prepare a slab
10 having a thickness of 50 mm.
The slab was hot-rolled at 850°C to a thickness of 5 mm and then
annealed at 750°C, and the scale on the surface of the annealed slab was removed
by shot peening and pickling to prepare a plate material.
The plate material was further cold-rolled to prepare a plate-shaped
15 sample (titanium plate) having a thickness of 0.5 mm.
The titanimi phte having a thickiiess of 3.5 mm was siitrjecteb b find
annealing at a temperature of 400 to 800°C for 48 hours or less in an argon gas
atmosphere to prepare a test piece in which crystal grains have been adjusted.
[OO~O]
20 (Measurement of Components)
The amounts of iron and oxygen contained in the titanium plate were
measured using the plate material after hot-rolling from which the surface scale
was cut.
The iron content was measured according to JIS H1614, and the oxygen
r,l5 content was measured according to JIS H1620.
[004 11
(Measurement of Tensile strength)
Further, the tensile strength of the test piece (titanium plate) in which the
crystal grain size has been adjusted as described above was measured according to
JIS Z 2241.
LO0421
5 (Evaluation of Workability)
Furthermore, the workability of the test piece (titanium plate) in which
the crystal grain size has been adjusted as described above was evaluated.
The evaluation was performed by the measurement of the Erichsen value
using graphite grease as a lubricant according to JIS 22247.
10 100431
(Investigation of Structure)
The microstructure of the titanium plate was observed to obtain
structural photographs of crystal grains (recrystallized a-grains) and a
non-recrystallized part (worked structure).
15 Note that an optical microscope or a transmission electron microscope was
used for the observation.
,4n example of the structural photograph observed with a transmission
electron microscope is shown in FIG. 1 (microstructure of sample No. 28).
In this structural photograph, recrystallized a-grains and a
20 non-recrystallized part are observed.
(In the photograph shown in FIG. 1, a place as indicated by "A" is the
non-recrystallized part.)
This photograph was determined for the area other than the
non-recrystallized part using image analysis software to determine the average
? 5 :mea of recrystallized a-prains. and the diameter of a circle having the same area
3s the average area was determined by calculation to define the average particle
size of recrystallized grains.
Further, the area rate of the non-recrystallized part was determined from
the area of the non-recrystallized part.
The results of the above are shown in Table 1.
lo0441
5 [Tablell
1- - I1 0.160 0.065 7- -J O 10 min 1) :I46 83.2
/ !2 l!.209 0.104 (20 LO min rl 0 411 7.6
1 43 0.030 0.022 150 1 hr 1.8 -43 263 3.6
44 0.066 0.977 500 1 hr 2.3 :I5 238 10.4
45 0.042 0.024 -400 24 hr 1.5 45 414 6.9
The above Sample Nos. 1 to 30 each have an average particle size of
recrystallized grains of 5 pm or less, and in each of these samples, a
non-recrystallized part is observed at an area rate of less than 30% in the cross
section of the titanium plate; and Sample Nos. 31 to 42 are in the state where the
5 non-recrystallized part does not remain, like conventional titanium plates.
Further, Sample Nos. 43 to 45 have been obtained by adjusting annealing
conditions so that the non-recrystallized part is intentionally allowed to remain,
wherein the non-recrystallized part has been allowed to remain in the state where
the area rate exceeds 30%.
10 The above Sample Nos. 1 to 30 and Nos. 31 to 42 have been obtained by
adjusting the size of crystal grains (circle-equivalent average grain size of the
wphase) and the amount of the non-recrystallized part with the difference
between annealing conditions, irrespective of using titanium materials in which
oxygen content and iron content are almost the same.
15 As shown in Table 1, the average particle size can be suppressed small
and high yield strength is exhbiteci by containing the non-recrystallized part.
In the above evaluation, workability (Erichsen value) generally tends to
decrease as the yield strength increases, but when the samples having comparable
workability (Erichsen value) are compared with each other, it is found that the
20 yield strength of these samples is increased and these samples have high strength
by allowing the non-recrystallized part to be present (for example, refer to the
comparison of Sample No. 1 with No. 31, No. 9 with No. 34, and No. 15 with No.
39).
That is, it is found that when crystal grains have a size of 5 pm or less and
2 5 .I non-recrystallized part is present in an amount of 3014 or less, the "yield
strength-workability balance" is good.
On the other hand, when the area of the non-recrystallized part is more
than 30% after the final annealing, workability (Erichsen value) is greatly reduced,
as shown in Sample Nos. 43 to 45.
These results have also shown that the present invention can provide a
titanium plate having high strength and excellent workability.
5 [00461