Note: Descriptions are shown in the official language in which they were submitted.
CA 02566425 2006-11-10
Ultrahigh Strength UOE Steel Pipe and
a Process for its Manufacture
Technical Field
This invention relates to an ultrahigh strength UOE steel pipe having a
strength (TS) in the circumferential direction of the pipe of at least 750 MPa
and at
most 900 MPa, having a good balance of strength and toughness, and having
improved resistance to joint fracture and to a process for its manufacture.
Bacl~ground Art
In recent years, there has been a strong demand for a reduction in the cost of
to pipelines. For this purpose, as manufacturing techniques have progressed,
there has
been a marked tendency to increase the strength of steel pipes themselves used
to
lay pipelines. In the past, up to X80 grade of steel has been standardized by
the
American Petroleum Institute (API) and is actually being used in pipelines.
At present, standardization and practical utilization of even higher strength
t s X 100 grade (corresponding to a strength in the circumferential direction
of a pipe
of at least 750 MPa) are being actively investigated. When actually applying
such
an ultrahigh strength steel to a steel pipe for a pipeline, taking safety from
fracture
into consideration, a significantly higher level of toughness is demanded
compared
to the level which is realized with conventional steel. Accordingly, there is
a
2o demand for a steel pipe having both ultrahigh strength and ultrahigh
toughness and
a base metal steel which can be used to manufacture such a steel pipe.
JP H08-209290-A and JP H08-209291A disclose high strength steel pipes
having a high Mn + high Mo composition. The former discloses subjecting the
pipe to tempering treatment, and the latter discloses carrying out dual phase
rolling.
2s Similarly, JP H09-31536A discloses a high strength steel pipe having a Mn +
high Mo composition, but disclosed therein is an ultrahigh strength steel pipe
corresponding to X120 grade with a base metal strength of at least 950 MPa. JP
2000-199036A discloses an ultrahigh strength steel pipe with a steel pipe
strength
of at least 900 MPa. JP H08-199292A also discloses a high strength steel pipe
in
3o which the base metal structure has a martensite fraction of at least 90%,
and in the
examples, an ultrahigh strength steel having a base metal strength of at least
900
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2
MPa is used.
The steel pipe strength and the base metal steel strength are the same. The
I
steel pipe strength is a value measured in the circumferential direction of a
pipe,
i.e., the pipe circumferential strength.
Disclosure of the Invention
The above-described prior art documents are each aimed primarily at
increasing strength, and they do not sufficiently disclose the toughness of
the base
metal and the toughness of the heat affected zones (HAZ) of joints. Up to the
present time, a high strength steel which can adequately satisfy a balance
between
io strength and toughness and resistance to joint fracture which are
particularly
demanded in high strength steels of higher than X80 grade, has not existed. In
fact,
in the above-described patent documents, there is no mention of both joint
fracture
properties and toughness in the high strength region which is the area of
interest of
the present invention.
~s According to the present invention, in order to increase resistance to
joint
fracture in a UOE steel pipe, the carbon equivalent (Ceq) of steel is
increased to a
high range which has not been utilized in the past. As a result, HAZ softening
at
the time of welding, which is a phenomenon characteristic of UOE steel pipes
which are welded by submerged arc welding, can be markedly decreased.
2o On the other hand, taking into consideration the ability of on-site
circumferential welding which is performed at the time of laying of a pipeline
in the
field, there is a demand for a balanced composition design which can realize a
low
weld cracking parameter (Pcm).
As the strength of a steel increases, the level of toughness demanded of the
2s HAZ and the base metal increases. In this regard, it is essential to
decrease Ti and
N in order to increase HAZ toughness, and at the same time it is necessary to
decrease S in order to increase the toughness of the base metal.
When a UOE steel pipe having its strength controlled to at least 750 MPa
and at most 900 MPa (corresponding to X100 grade) by composition design taking
into consideration the above points was manufactured, it was found to have
extremely good resistance to joint fracture and good toughness. At the time of
manufacture, it was ascertained that if the temperature at the completion of
cooling
by water cooling after hot rolling was made 350° C or higher, an
extremely high
CA 02566425 2006-11-10
fracture toughness value of 150 J demanded of X100 grade could be satisfied.
According to one aspect, the present invention is a UOE steel pipe having a
1 ~ ,
base metal chemical composition comprising, in mass percent, C: 0.03 - 0.08%,
Mn: 1.70 - 2.2%, S: at most 0.0020%, Ti: 0.005 - 0.025%, N: at most 0.0050%,
optionally at least one element selected from the following (i) through (iv),
and a
remainder of iron and unavoidable impurities, wherein the below-defined carbon
equivalent (Ceq) is at least 0.50%, the weld cracking parameter (Pcm) is at
most
0.24%, and the strength of the pipe in the circumferential direction is at
least 750
MPa and at most 900 MPa:
to Ceq = C + Mn/6 + (Cr + Mo + V)/S + (Cu + Ni)/15
Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + B
wherein Ceq = carbon equivalent, Pcm = weld cracking parameter, and the
symbol for each element in the above equations indicates the content of the
element
in mass percent,
is (i) one or two of Si: 0.05 - 0.50% and Al: at most 0.06%,
(ii) one or more of Cu: at most 1.0%, Ni: at most 2.0%, Cr: at most 1.0%,
Nb: at most 0.1%, and V: at most 0.1%,
(iii) Mo: at most 1.0%, and
(iv) Ca: at most 0.005%.
2o It is desired that a UOE steel pipe according to the present invention have
a
fracture toughness such that the Charily absorbed energy at -10° C is
at least 150 J
in both the base metal and heat affected zone (HAZ).
From another aspect, the present invention is a process for manufacturing a
UOE steel pipe having a carbon equivalent (Ceq) of at least 0.50% and a weld
2s cracking parameter (Pcm) of at most 0.24% as defined above and a strength
in the
circumferential direction of the pipe of at least 750 MPa and at most 900 MPa,
the
process comprising producing a steel plate by hot rolling of a steel having
the
above-described chemical composition followed by water cooling with a
temperature at the completion of water cooling of 350° C or higher,
applying U-
3o pressing and O-pressing to the resulting steel plate, and performing
welding and
pipe expanding to obtain a UOE steel pipe. Welding of the UOE steel pipe is
carried out by submerged (arc) welding according to a conventional manner.
According to the present invention, by manufacturing a steel pipe which is
controlled so as to have a high carbon equivalent (Ceq) and a strength of at
least
CA 02566425 2006-11-10
4
750 MPa and at most 900 MPa, HAZ softening of the welded joint which is
characteristic of UOE steel pipes which are welded by submerged arc welding is
a
diminished, and the resistance to joint fracture of the UOE steel pipe is
markedly
improved. At the same time, by decreasing the content of S, Ti, and N, the
s toughness of the base metal and HAZ can be maintained.
A UOE steel pipe according to the present invention can be manufactured
under the same conditions as a conventional UOE steel pipe of X80 grade or
below,
thereby making it possible to manufacture an ultrahigh strength UOE steel pipe
while maintaining productivity equivalent to that of a conventional UOE steel
pipe.
to Accordingly, the manufacturing costs of ultrahigh strength UOE steel pipes
can be
markedly decreased.
Brief Description of the Drawings
Figure 1 is a graph showing the relationship between the S content of steel
and the toughness of the base metal (the Charpy absorbed energy at -10°
C).
is Best Mode for Carrying Out the Invention
In order to apply an ultrahigh strength steel which is not prescribed by API
standards to an actual pipeline, it is necessary to provide a pipe having
properties
suited for the environment of use while taking into consideration ( 1 ) safety
from
fracture and (2) circumferential weldability.
2o Particularly in the case of a long distance pipeline for transporting
natural
gas or oil, occurrence of fracture of a pipe leads to a serious accident.
Modes of
fracture include brittle fracture and ductile fracture. In brittle fracture,
fracture
propagates at an ultrahigh speed of at least 500 m/sec, while in ductile
fracture, the
speed of propagation of fracture is lower and at most 300 m/sec. Accordingly,
2s when steel pipe is applied to an actual pipeline, it is essential that the
base metal
have a toughness such that it undergoes ductile fracture in the environment of
use.
Concerning the desired level of toughness, the HLP Committee (a Japanese
organization for fracture research) proposes that a higher fracture toughness
value
becomes necessary as the strength of a steel increases in order to restrain
the
3o propagation of fracture within a prescribed distance even when high speed
ductile
fracture occurs. The necessary fracture toughness value (the Charily absorbed
CA 02566425 2006-11-10
energy at -10° C) depends upon the strength grade of steel, the size of
a steel pipe,
the internal pressure, and other factors, but with X100 grade steel, it is not
40 to 50
J which is required of usual steel (API X70 grade and below) but becomes at
least
150 J. Accordingly, with X100 grade steel, in addition to high strength, a
high
fracture toughness value of this level is required.
Safety from fracture can be evaluated by the location of fracture when a
force is applied in the circumferential direction of pipe. The location of
fracture
can be classified as being the base metal, the weld metal, or the weld heat
affected
zone (HAZ). When fracture occurs in the base metal, as stated above, if
sufficient
Io toughness is provided, ductile fracture occurs. When fracture occurs in the
weld
metal, ductile fracture occurs in some cases, but in the majority of cases,
brittle
fracture occurs. Accordingly, it is absolutely necessary to avoid fracture in
the weld
metal. In general, fracture in the weld metal is prevented by making the
strength of
the weld metal at least as high as that of the base metal (performing
overmatching).
1 s Fracture in the HAZ is a phenomenon which is observed particularly in high
strength steels with a strength of at least 700 MPa.
A steel according to the present invention is particularly effective at
preventing HAZ fracture. The following are conceivable as means of preventing
HAZ fracture:
20 ( 1 ) making the strength of the weld metal at least as high as that of the
base
metal (providing overmatching)
(2) limiting the weld heat input as low as possible in order to reduce the
area
of the HAZ,
(3) increasing the strength of the HAZ,
2s (4) controlling the shape of the weld, i.e., reducing stress concentrations
in
the toe portion of the weld.
In the present invention, Ceq is increased in order to increase the strength
of
the HAZ. The HAZ has a structure formed by melting due to the effect of heat
followed by resolidification or transformation. In order to increase the
strength of
3o the HAZ, it is effective to make the composition rich (increase both Ceq
and Pcm)
or to decrease the heat input. For this purpose, the heat input can be set to
the
lowest heat input which can provide the desired shape of the weld. However,
making the composition rich has the problem that it leads to a decrease in
circumferential weldability when joining steel pipes to each other in the
field.
CA 02566425 2006-11-10
In the present invention, a high strength is achieved by increasing Ceq so as
to suppress softening of the HAZ, while circumferential weldability is
maintained at
a good level by limiting Pcm up to a certain value.
In order to increased HAZ toughness, control of the content of N and Ti is
also important. It was found that by optimizing the balance of content of
these
elements, a deterioration in toughness accompanying an increase in strength
can be
prevented.
In the past, a TMCP (thermo-mechanical control process) was generally
applied to the manufacture of ultrahigh strength steel having a TS of 750 MPa
or
io higher in such a manner that the temperature at the completion of water
cooling
after hot rolling was at most 200 ° C (in many reports it is described
to be room
temperature). This cooling condition was employed in order to provide the
steel
with basic properties such as strength and toughness.
In the present invention, even though the steel has an ultrahigh strength of
at
is least 750 MPa, taking into consideration safety from fracture, it has a
chemical
composition for which Ceq >_ 0.50% and manufactured with the temperature at
the
completion of water cooling after hot rolling being 350° C or higher.
As a result,
fracture in the vicinity of a joint is prevented at the time of occurrence of
fracture,
and at the same time a high strength and high toughness can both be achieved.
2o By not employing an extremely low temperature for the temperature at the
completion of water cooling, the deformability of the base metal, i.e.,
uniform
elongation thereof can be greatly increased. Accordingly, a manufacturing
process
and a UOE steel pipe according to the present invention are extremely
effective
from the standpoint of safety from fracture.
2s Uniform elongation (degree of ultimate elongation) is the amount of plastic
deformation of a material occurring up to the maximum load in a tensile test.
Accordingly, the fact that a base metal has a large uniform elongation means
that if
the pressure abruptly increases during operation of a pipeline, the amount of
plastic
deformation up to the value of TS is large, and the safety from fracture is
high.
3o From this standpoint, it is desirable that the uniform elongation of the
base metal be
at least 5.0%.
Figure 1 is a graph showing the relationship between the S content and the
toughness (the Charpy absorbed energy at -10° C) of the base metal for
X100
grade steels. From Figure 1, it can be seen that the toughness of the base
metal is
CA 02566425 2006-11-10
markedly improved by reducing the S content. From this result, it can be found
that
it is effective to control the S content in an ultrahigh strength steel when a
high
fracture toughness value is desired.
In the present invention, the necessary least fracture toughness value is 150
J, so the S content is made at most 20 ppm. When a still higher fracture
toughness
value such as 200 J or greater is desired, the S content can be made 14 ppm or
less.
The present invention can provide a UOE steel pipe which can satisfy all of
prevention of HAZ fracture of a joint, a high uniform elongation of a base
metal,
and good circumferential weldability required at the time of laying of a
pipeline,
which could not be achieved by conventional manufacturing processes.
According to the present invention, with a UOE steel pipe manufactured by
TMCP with the temperature at the completion of water cooling being
350° C or
higher which is the same as for usual steel of API X80 grade or below, a
strength
corresponding to API X100 grade is satisfied by increasing the carbon
equivalent
is (Ceq) to 0.50% or greater, and circumferential weldability can be provided
by
limiting the weld cracking parameter (Pcm) to 0.24% or lower.
The chemical composition of the base metal in the present invention is as
follows.
C: 0.03 - 0.08%
2o C is an element which is effective at increasing strength of steel. In
order to
impart a strength of X100 grade to steel, its content is made at least 0.03%.
However, if the C content exceeds 0.08%, it leads to a marked decreases in
toughness so that it has an adverse effect on the mechanical properties of the
base
metal, and at the same time it promotes formation of surface defects on a
slab. A
2s preferred C content is 0.03 - 0.05%.
Mn: 1.70 - 2.2%
Mn is an element which is effective at increasing the strength and toughness
of steel, and its content is made at least 1.70% in order to impart sufficient
strength
and toughness. However, if the Mn content exceeds 2.2%, the toughness of a
weld
3o deteriorates. A preferred Mn content is 1.8 - 2.0%.
S: at most 0.0020%
S is one of the elements which it is necessary to limit their content in order
to
achieve the necessary toughness of a base metal. If the S content exceeds
0.0020%,
the fracture toughness value necessary for the base metal cannot be achieved.
As
CA 02566425 2006-11-10
g
previously explained with respect to Figure l, the S content may be further
limited
in accordance with the fracture toughness value required of the base metal,
such as
to at most 0.0014%.
Ti: 0.005 - 0.025%
Ti has an effect of suppressing grain growth in a HAZ by forming TiN and
thus increasing the toughness of the HAZ. For this purpose, it is necessary
for the
Ti content to be at least 0.005%. However, if the Ti content exceeds 0.025%,
the
amount of dissolved N increases, and HAZ toughness deteriorates. A preferred
Ti
content is 0.005 - 0.018%.
N: at most 0.0050%
N forms nitrides with V, Ti, and the like and thus has the effect of
increasing
high temperature strength of steel. However, if the content of N exceeds
0.0050%,
it forms carbonitrides with Nb, V, and Ti, thereby causing a decrease in the
toughness of the base metal and the HAZ. When a high level of HAZ toughness is
~s desired, N is preferably controlled at an extremely low value of at most
0.0035%.
In addition to the above-described basic components of composition, the
carbon equivalent (Ceq) and weld cracking parameter (Pcm) of the base metal
are
extremely important factors in order to achieve a high strength of at least X
100
grade and high toughness in the base metal and HAZ.
2o Ceq of the base metal: at least 0.50%
In order to ensure that a base metal strength of at least X100 grade is
achieved by TMCP in which the temperature at the completion of water cooling
is
set to 350° C or higher, the carbon equivalent (Ceq) of the base metal
is made at
least 0.50%. As long as a base metal strength of X100 grade or higher can be
2s achieved, there is no particular upper limit on the Ceq, but Ceq is
preferably at most
0.55%. Ceq is given by the following equation (the symbols for elements in the
equation indicate the content of those elements in mass percent):
Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15.
Pcm of the base metal: at most 0.24%
3o The steel composition is designed such that the weld cracking parameter
(Pcm) of the base metal is at most 0.24% in order to achieve high strength and
high
toughness even at the time of circumferential welding. There is no particular
lower
limit for Pcm, but normally it is at least 0.16%. Pcm is given by the
following
equation (the symbols for elements in the equation indicate the content of
those
CA 02566425 2006-11-10
elements in mass percent):
Pcm = C + Si/30 + Mn~20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + B.
.,
In a UOE steel pipe according to the present invention, there are no
particular restrictions on the Ceq and Pcm of the weld metal.
In this specification, when Ceq and Pcm appear by themselves, they refer to
the Ceq and Pcm of the base metal including the HAZ, i.e., that of the entire
steel
pipe except for the weld metal.
The strength in the circumferential direction of a UOE steel pipe according
to the present invention is at least 750 MPa and at most 900 MPa. This
strength
level of a steel pipe is defined to indicate that it is the level of X 100
grade. In the
present invention, by controlling the chemical composition of steel as
described
above, an ultrahigh strength UOE steel pipe of X 100 grade strength can be
manufactured by the same process as for a conventional low strength UOE steel
pipe in which the temperature at the completion of water cooling after hot
rolling is
is 350° C or higher, and the pipe can be provided with the fracture
toughness value
required in the base metal and HAZ.
The base metal of a UOE steel pipe according to the present invention may
further contain one or more optional elements selected from the group listed
below
as (i) - (iv).
(i) Si: 0.05 - 0.50%, Al: at most 0.060%
Si and Al both have a deoxidizing effect, and preferably at least one of them
is included.
Si is effective not only as a deoxidizing agent but also at increasing the
strength of steel. If the Si content is less than 0.05%, deoxidization is
inadequate.
2s If the Si content exceeds 0.5%, a large amount of martensite-austenite
constituent is
formed in the HAZ, thereby causing the toughness of the HAZ to deteriorate
extremely and thus leading to a decrease in the mechanical properties of a
steel
pipe. The Si content can be selected within the range of 0.05 - 0.50% taking
into
consideration a balance with the plate thickness of the steel plate.
3o Like Si, A1 functions as a deoxidizing agent. Its effects can be adequately
attained when its content is at most 0.06%. Addition in excess of this amount
adversely affects circumferential weldability in the field and is also not
desirable
from the standpoint of economy.
(ii) Cu: at most 1.0%, Ni: at most 2.0%, Cr: at most 1.0%, Nb: at most 0.1 %,
CA 02566425 2006-11-10
to
V: at most 0.1%
These elements serve to improve hardenability of steel when added in a
small amount and thus have an effect of improving various properties.
Cu can increase strength without significantly impairing toughness as a
result of a change in microstructure due to solid solution strengthening and
the
effect of increasing hardenability. If Cu exceeds 1.0%, the Cu checking
phenomenon which is harmful in that it causes the formation of slab surface
defects
may occur. In order to prevent such defects, it becomes necessary for the slab
to be
heated at a low temperature, thereby imposing limitations on the range in
which
1 o manufacture can be performed.
In the same manner as Cu, Ni also can increase strength without significantly
impairing toughness by a microstructural change due to solid solution
strengthening
and the effect of increasing hardenability. At the same time, it serves to
suppress a
deterioration in the toughness of the base metal and HAZ after hot bending.
is However, addition of more than 2.0% of Ni increases costs so is not
practical, and it
also adversely affects ability of on-site circumferential welding.
Like Cu and Ni, Cr also can increase strength without significantly
deteriorating toughness by a microstructural change due to solid solution
hardening
and the effect of increasing hardenability. However, if Cr exceeds 1.0%, the
2o toughness of the HAZ decreases.
Nb and V have a great effect on increasing strength by precipitation
strengthening and the effect of increasing hardenability, or on improving
toughness
by crystal grain refinement. However, if either is added in excess of 0.1 %,
it causes
a decrease in the toughness of HAZ.
2s When at least one of these elements is added, a more preferred content is
Cu:
at most 0.50%, Ni: at most 0.80%, Cr: at most 0.40%, Nb: at most 0.06%, and V:
at
most 0.06%.
(iii) Mo: at most 1.0%
Mo is effective at increasing the strength of the base metal and of welds. If
3o too much Mo is added, it causes a deterioration in ability of on-site
circumferential
welding and the toughness of the HAZ. Therefore, its upper limit is made 1.0%.
When Mo is added, a more preferred content is at most 0.50%.
(iv) Ca: at most 0.005%
Ca has an effect on shape control and specifically spheroidizing of inclusions
CA 02566425 2006-11-10
11
in steel, thereby preventing hydrogen-induced cracking or lamellar tears.
However,
these effects saturate at a Ca Content of 0.005%.
A UOE steel pipe according to the present invention can be manufactured by
subjecting a steel slab which is adjusted to have the above-described chemical
s composition to hot rolling, and after the completion of finish rolling,
water cooling
is performed thereon such that the temperature at the completion of water
cooling is
350° C or higher. The resulting hot rolled steel plate is formed into a
tubular shape
by usual U-pressing and O-pressing, and then the abutting edges are bonded by
welding on the inner and outer surfaces. This welding is carried out by
submerged
arc welding. After the welded pipe is formed, it is subjected to pipe
expanding so
as to increase the roundness. Pipe expanding can be carried out by mechanical
pipe
expanding or hydraulic pipe expanding.
There are no particular restrictions on the steps of manufacture of a UOE
steel pipe in a manufacturing process for a UOE steel pipe according to the
present
1 s invention except for the water cooling conditions after hot rolling.
Manufacture
may be carried out in the same manner as for the manufacture of a conventional
UOE steel pipe of X80 grade or below. Nevertheless, a UOE steel pipe having an
ultrahigh strength of X100 grade (a strength in the pipe circumferential
direction of
at least 750 MPa and at most 900 MPa) and at the same time having improved
2o resistance to fracture can be manufactured.
The following example is intended to illustrate the present invention more
specifically, but it is merely for illustration purpose and does not restrict
the
invention in any way.
Example
2s Hot rolled steel plates for use as a base metal was prepared from steel
slabs
having the chemical compositions shown in Table 1 by heating and retaining
them
at a temperature of 1100 - 1200° C, then subjecting them to hot rolling
with a finish
rolling temperature irl the range of 700 - 850° C so as to give a plate
thickness of
20 mm. The hot-rolled plates were water cooled with the temperatures at the
3o completion of water cooling shown in Table 1 and then air cooled to room
temperature. The base metal steel plates were formed into a tubular shape by U-
pressing and then O-pressing in cold conditions. Then, the abutting edges of
the
shapes were welded by usual submerged arc welding, and the resulting pipes
were
CA 02566425 2006-11-10
12
subjected to mechanical pipe expanding. In this manner, UOE steel pipes having
an outer diameter of 910 mm,(36 inches), a wall thickness of 20 mm, and a
length
of 1200 mm were manufactured.
Table 1 also shows the strength and toughness of the base metal, the tensile
properties of the joint, and results of a circumferential welding test
performed on
the resulting UOE steel pipes. The base metal strength and the position at
which
joint tensile fracture occurs are particularly important parameters for
ascertaining
the effects of the present invention.
The toughness and strength of a base metal were evaluated by taking an
to impact test piece (JIS No. 4) and a tensile test piece (an ASTM rod-shaped
test
piece with a diameter of 6.35 mm) from the circumferential direction of each
UOE
steel pipe so as not to include the weld or the HAZ and determining the
Charily
absorbed energy at -10° C (indicated as VE-10° C), the tensile
strength (TS), and
the uniform elongation (degree of ultimate elongation).
is A tensile test of the joint was carried out by taking a tensile test piece
in the
circumferential direction such that the weld of each UOE steel pipe was in the
center of the test piece, and performing a tensile test on the test piece
having the
reinforcement of weld as it was to determine the tensile strength and
ascertain the
location of fracture. An impact test piece (JIS No. 4) was taken from the HAZ
20 (weld heat affected zone) of each UOE steel pipe and used to determine the
Charily
absorbed energy at -10° C (VE-10° C). Weldability was evaluated
by actually
performing circumferential welding of the UOE steel pipes and determining
whether cracking occurred at -10° C in y slit cracking test. Cases in
which
cracking was observed are indicated by an X and cases in which it were not
2s observed are indicated by an O.
Image
CA 02566425 2006-11-10
14
In Nos. 1 and 20 - 24 which are working examples of the present invention,
the strength and toughness of,th~ base metal satisfy the prescribed
conditions, and
at the same time, due to optimization of the chemical composition, the
resistance to
joint fracture was excellent as evidenced by the fact that fracture at the
base metal
could be achieved in the joint tensile test. In addition, circumferential
weldability
was also excellent.
In contrast, in the comparative examples, appropriate level of strength or
toughness or other properties could not be achieved. In particular, in Nos.
10, 12,
and 16 - 18, there was an extreme decrease in the toughness of the HAZ.
