Note: Descriptions are shown in the official language in which they were submitted.
CA 02403830 2002-08-27
DESCRIPTION
HIGH-WORKABILITY STEEL PIPE AND METHOD OF PRODUCING
SAME
Technical Field
The present invention relates to a steel pipe
having superior workability and a method of producing
the steel pipe.
Background Art
For the purpose of reducing the weight and cost,
the application of seam (electric resistance) welded
steel pipes to automobile parts has been considered.
Conventional seam welded stee.l pipes, however, have not
been sufficient in workability. Bending is employed to
manufacture, e.g., undercarriage or suspension parts of
automobiles. When the conventional seam welded steel
pipes are subjected to the bending, a problem has been
experienced in that a pipe wall is greatly thinned on
the outer side of a bent portion, and in the worst case
a pipe is ruptured. Everi in the case of not causing a
rupture, a large rate of thinning of the pipe wall
requires the use of a material having a greater
thickness to satisfy the design stress, and therefore a
1
CA 02403830 2002-08-27
sufficient reduction in weight: cannot be achieved.
As disclosed in Japanese Unexamined Patent
Application Publication No. 55-56624, for example, it
is known that improving an r-value (Lankford value) of
a pipe in the axial direction is effective to overcome
the problems described above. As a method for
increasing the r-value of a steel pipe, however, it is
only known to increase the r-value of strip steel as a
base material of a steel pipe as disclosed in, for
example, Japanese Unexamined Patent Application
Publication No. 6-41689. When producing seam welded
steel pipes, there has been a problem that the r-value
is reduced in a portion where melting or transformation
of a steel material has occurred during seam welding.
Another problem has arisen in that the seam welding
cannot be applied to steel plates not having a high r-
value, such as hot-rolled steel plates, high tensile
strength steel plates, and low, medium and high carbon
steel plates.
Accordingly, it is an object of the present
invention to provide a steel pipe being superior in
workability, particularly in bending workability, in
which an r-value of the pipe in the axial direction in
a portion where melting or transformation of a steel
material has occurred during seam welding is as high as
2
CA 02403830 2002-08-27
comparable to that in a portion where melting or
transformation of the steel ma.terial has not occurred,
and a method of producing the steel pipe.
Disclosure of Invention
With the view of overcoming the problems mentioned
above, the inventors have conducted studies based on a
consideration that working and heat treatment of seam
welded steel pipes are required to improve the r-value
in a welded portion near the seam. Then, the inventors
have studied a method of performing working and heat
treatment of a steel pipe evenly at any positions in
the circumferential direction, the steel pipe being
produced by seam-welding cold-rolled steel having a
high r-value. In the process of the studies, the
inventors have found that the r-value of the seam
welded steel pipe in the longitudinal direction (in the
axial direction of the pipe) is noticeably improved to
1.2 or above, in particular to 1.6 or above, at any
positions in the circumferential direction, including a
seamed portion, by a method of performing diameter-
reducing rolling on the seam welded steel pipe in a
temperature range of from 600 C to AcJ with a reduction
in diameter of not less than 30% (referred to as a
"method according to the present invention"
3
CA 02403830 2002-08-27
hereinafter).
As a result of applying the method according to
the present invention to seam welded steel pipes
produced using various kinds of steel plates as base-
material strip steel, the inventors have also found
that a high r-value can be obtained regardless of the
r-value of the original strip steel. Further, it has
been found that with the method according to the
present invention, the restriction of ingredients which
has hitherto been employed to obtain a high r-value in
steel sheets, i.e., a reduction of the C and N contents
and addition of stabilizing elements such as Ti and Nb,
are not required. As a result, seam welded steel pipes
having a high r-value can also be produced using, as
base-material strip steel, hot-rolled steel, high
tensile strength steel such as dual phase steel, and
low, medium and high carbon steel, which have a
difficulty in achieving a high r-value in the stage of
strip steel.
The views of the inventors regarding the reason
why a steel pipe having a high r-value can be obtained
from even a steel plate not having a high r-value are
as follows.
By performing the diameter-reducing rolling on a
seam welded steel pipe in a temperature range of from
4
CA 02403830 2002-08-27
600 C to Ac3 with a reduction in diameter of not less
than 30%, an ideal aggregation structure due to the
rolling, in which the <110> axis is parallel to the
longitudinal direction and the <111> to <110> axes are
parallel to the radial direction, is formed and then
further developed through restoration and
recrystallization. That aggregation structure provides
a high r-value. The aggregation structure due to the
rolling produces very great driving forces because
crystals are rotated by workirig strains. Unlike an
aggregation structure that is created through
recrystallization in the case of obtaining a high r-
value in steel sheets, the aggregation structure due to
the rolling is less affected by the second phase and
solid solution C. Consequently, even for the type of
strip steel which has a difficulty in obtaining a high
r-value in the stage of producing steel plates, a high
r-value can be obtained in the stage of producing steel
pipes.
Also, the reason why a high r-value is not
obtained by performing the diameter-reducing rolling at
low temperatures is that ideal crystal rotation is not
caused because of high work hardness, or that
restoration and recrystallization are not developed at
a sufficient level because of low temperatures.
5
CA 02403830 2002-08-27
Furthermore, the reason why a high r-value is not
obtained by a method of performing the diameter-
reducing rolling on a steel pipe at low temperatures
and then annealing the rolled steel pipe for
recrystallization is that the desired aggregation
structure is not developed through the cold rolling and
the recrystallization because of the effect of the
second phase and solid solution C.
In the field of producing steel sheets, there is
known a method of producizzg a steel sheet having a high
r-value by rolling steel into a sheet in the hot
ferrite range. This method of producing a steel sheet
having a high r-value is featured in that steel
containing C and N in reduced amounts and added with
stabilizing elements such as Ti and Nb is rolled at low
temperatures and then recrystallized. That sheet
rolling at low temperatures differs from the diameter-
reducing rolling at high temperatures intended by the
method according to the present invention. In fact, if
the known sheet rolling in the hot ferrite range is
carried out at 600 C or above, the r-value is not
improved, but rather noticeably lowered on the contrary.
This is because, in the sheet rolling in which draft is
applied in the thickness direction of a sheet, strain
occurs in a direction different from that in the
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CA 02403830 2002-08-27
diameter-reducing rolling of a steel pipe in which
draft is applied in the circumferential direction, and
hence the aggregation structure effective in increasing
the r-value is not developed.
As a result of further continuing the studies, the
inventors have found that, in the method according to
the present invention, the thickness deviation can be
noticeably reduced and the occurrence of wrinkles near
the seam can be suppressed by heating a seam welded
steel pipe to temperatures of not lower than Ac, before
the diameter-reducing rolling for austenitic
transformation of a part or the whole of a steel
structure, because the difference in mechanical
properties between the hardened structure of the seam
and the remaining portion is reduced. The present
invention has been accomplished based on the findings
set torth above. The features of the present invention
are as follows.
(1) A high-workability steel pipe wherein an r-value
in the longitudinal direction is not less than 1.2,
more preferably not less than 1.6, over an entire area
in the circumferential direction, including a seamed
portion.
(2) A method of producing a high-workability steel
pipe, the method comprising the step of performing
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CA 02403830 2008-02-28
diameter-reducing rolling on a steel pipe in a
temperature range of from 600 C to Ac3 with a reduction
in diameter of not less than 30%, the steel pipe being
produced by seam-welding strip steel.
(3) A method of producing a high-workability steel
pipe, wherein said method comprises the steps of heating
a steel pipe to temperatures of not lower than Acl, said
steel pipe being produced by seam-welding strip steel,
and then immediately or after cooling and reheating said
steel pipe, performing diameter- reducing rolling in a
temperature range of from 600 C to Ac3 with a reduction
in diameter of not less than 30% and after diameter-
reducing rolling of said steel pipe, heat treatment of
said rolled steel pipe by holding said pipe in a
temperature range of from 600 C to 900 C for a time of 1
second or longer is performed during cooling subsequent
to the diameter-reducing rolling or by reheating the
rolled steel pipe after said cooling, said steel pipe
being produced by seam-welding strip steel.
Brief Description of the Drawings
Fig. 1 is a graph showing the relationship between
an r-value in the longitudinal direction of a steel pipe
having been subjected to diameter-reducing rolling
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CA 02403830 2002-08-27
and a reduction in diameter.
Fig. 2 is a graph showing the relationship between
an r-value in the longitudinal direction of a steel
pipe having been subjected to diameter-reducing rolling
and an outgoing-side temperature in the rolling process.
Fig. 3 is a graph showing the relationship between
a seam thickness deviation in a steel pipe having been
subjected to diameter-reducing rolling and a heating
temperature before the diameter-reducing rolling.
Best Mode for Carrying Out the Invention
In a high-workability steel pipe according to the
present invention, an r-value in the longitudinal
direction is not less than 1.2. The reason is that the
bending workability of the steel pipe is noticeably
improved when the r-value is not less than 1.2. More
preferably, the high-workability steel pipe has an r-
value of not less than 1.6 because the bending
workability is further improved when the r-value is not
less than 1.6.
The high-workability steel pipe according to the
present invention can be produced by performing
diameter-reducing rolling on a steel pipe in a
temperature range of from 600 C to Ac, with a reduction
in diameter of not less than 30%, the steel pipe being
9
CA 02403830 2002-08-27
produced by seam-welding strip steel and having a seam.
The r-value is affected by the reduction in diameterand
the temperature during the diameter-reducing rolling.
Fig. 1 is a graph showing the relationship between
the r-value in the longitudinal direction and the
reduction in diameterresulted at circumferential
positions 0 , 90 , 180 and 270 of each steel pipe
which was produced by performing the diameter-reducing
rolling on a seam welded steel pipe under a condition
of the outgoing-side temperature being set to 730 C
while changing the reduction in diameterthe seam welded
steel pipe being produced by an ordinary method from
strip steel having the same composition as steel A in
Table 1 given below. The seam position is assumed to
be at 0 (this is similarly applied to the following
description). From Fig. 1, it is understood that,
regardless of the circumferential positions, the r-
value of not less than 1.3 is obtained at the reduction
in diameterof not less than 30%, and the r-value of not
2G less than 1.6 is obtained at the reduction in
diameterof not less than 50%.
Fig. 2 is a graph showing the relationship between
the r-value in the longitudinal direction and the
outgoing-side temperature resulted at circumferential
positions 0 , 90 , 180 and 270 of each steel pipe
CA 02403830 2002-08-27
which was produced by performing the diameter-reducing
rolling on a seam welded steel pipe under a condition
of the reduction in diameter set to 30% while changing
the outgoing-side temperature, the seam welded steel
pipe being produced by an ordinary method from strip
steel having the same composition as steel A in Table 1
given below. From Fig. 2, it is understood that the r-
value of not less than 1.2 is obtained at the outgoing-
side temperature of not lower than 600 C.
Based on the experiment results mentioned above, a
lower limit of the temperature for the diameter-
reducing rolling was set to 600 C and a lower limit of
the reduction in diameterwas set to 30%. Also, an
upper limit of the temperature for the diameter-
reducing rolling was set to the same as an upper limit
of the temperature range in which the steel structure
contains ferrite, i.e., the temperature Ac,. The r-
value is not improved even by the diameter-reducing
rolling if it is performed on steel whose structure
contains no ferrite. The temperature Ac, depends on
the chemical composition of steel, and can be
determined based on experiments. A range of
temperature Ac3 is approximately not higher than 900 C.
In the present invention, so long as the steel
structure contains ferrite, the second phase (phase
11
CA 02403830 2002-08-27
other than ferrite) is not. limited to particular one.
For example, austeriite may be the second phase. More
preferably, the diameter-reducing rolling is performed
at temperatures where ferrite forms the main phase
(phase having a volume ratio of 50% or more).
The gist of the present invention resides in that
a steel pipe is subjected to the diameter-reducing
rolling in a temperature range where the steel
structure has the ferrite phase. From the standpoint
of improving the r-value, there is no particular
restriction upon the history prior to the diameter-
reducing rolling. For example, the heating temperature
prior to the diameter-reducing rolling may be any of
the temperature at which the steel structure has the
single austenitic phase, the temperature at which the
steel structure has the two austenitic and ferrite
phases, and the temperature at which the steel
structure has the single ferrite phase. Further, prior
to the diameter-reducing rolling, the steel pipe may be
rolled at such temperatures as forming austenite as the
single phase or the main phase.
Fig. 3 is a graph showing the relationship between
a heating temperature and a thickness deviation
resulted for each steel pipe which was produced by
performing the diameter-reducing rolling on a seam
12
CA 02403830 2006-08-30
welded steel pipe under conditions of the reduction in
diameter set to 30% and the rolling temperature set to
700 C while changing the heating temperature, the seam
welded steel pipe being produced by an ordinary method
from strip steel having the same composition as steel D
in Table 1 given below. From Fig. 3, it is understood
that the heating prior to the diameter-reducing rolling
is preferably set to be not lower than the temperature
Acl from the standpoint of suppressing the thickness
deviation and wrinkles occurred near the seam. The
temperature Acl depends on the chemical composition of
the steel pipe, etc., and can be determined based on
experiments. A range of temperature Acl is approximately
not lower than 800 C. However, if the heating temperature
is too high, the crystal grain size would be excessively
increased, thus resulting in a problem of, for example,
increasing surface roughness during the working. For that
reason, the heating temperature is preferably set to be
not higher than 900 C.
There is no particular restriction upon the cooling
after the heating of the steel pipe. Subsequent to the
heating, the diameter-reducing rolling may be performed,
for example, after cooling the steel pipe down to
temperatures at which ferrite
13
CA 02403830 2002-08-27
forms the main phase, or by reheating the steel pipe
after cooling it down to the room temperature.
Further, preferably, after the diameter-reducing
rolling of the steel pipe, heat treatment of holding
the rolled steel pipe in a temperature range of from
600 C to 900 C for a time of 1 second or longer is
performed in the present invention.
In the present invention, since the diameter-
reducing rolling is performed at temperatures of not
lower than 600 C, the work hardness is low and a
sufficient level of workability is obtained with
additional treatment. Even so, by performing heat
treatment for holding the rolled steel pipe at a
certain temperature for a certain time in succession to
the diameter-reducing rolling, the elongation and the
r-value are further improved. This effect is developed
by holding the rolled steel pipe at temperatures of not
lower than 600 C for a time of 1 second or longer.
However, if the holding temperature exceeds 900 C, the
steel structure would be transformed into the single
austenitic phase and the r-value would be reduced
because of the randomized aggregation structure. For
that reason, the heat treatmerit is preferably performed
on conditions of the holding temperature in the range
of from 600 C to 900 C and the holding time of 1 second
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CA 02403830 2002-08-27
or longer. Additionally, the heat treatment may be
performed during cooling subsequent to the diameter-
reducing rolling or by reheating the rolled steel pipe
after the cooling_
(Example)
Seam welded steel pipes were produced by an
ordinary method from various }cinds of hot-rolled steel
plates having chemical compositions shown in Table 1,
and the diameter-reducing rol:ling was,performed on each
steel pipe under conditions shown in Table 2. Heating
of the steel pipe prior to the diameter-reducing
rolling was not held at all or held for a time of 1 to
600 seconds after reaching thE: temperature shown in
- Table 2. Tensile specimens of JIS No. 12-A were
sampled from circumferential positions 0 , 90 , 1$0 and
270 of each steel pipe obtained_ After bonding a
strain gauge with a gauge-length of 2 mm to each
specimen, a tensile test was carried out on the
specimen by applying a nominal strain of 6 to 7%. Then,
a ratio of a true strain F, in the longitudinal direction
to a true strain e~ in the width direction;was
measured. From a gradient p of that ratio, the r-value
was calculated based on the following formulae:
pE~
r-value = p/(-1 - p)
1.5
CA 02403830 2002-08-27
Further, a thickness deviation il was calculated by
measuring a pipe wall thickriess ts of a seamed portion
and an average pipe wall thickness tb of the remaining
portion. That is:
thickness deviation 11% = (ts - tb)/tb x 100%
Moreover, the presence or absence of wrinkles was
determined by observing an image of an area near the
seam in a cross-section perpendicular to the axis of
the steel pipe, the image being enlarged at a
magnification of 50 times.
Those results are listed in Table 3 along with the
tensile strength (TS) and the elongation (El).
The r-value is 1.2 or above at any positions in
the circumferential direction in Examples of the
present invention, whereas the r-value is below 1.2 in
Comparative Examples. Also, in the specimens heated to
temperatures of not lower thari Acx, the thickness
deviation is smaller and wrinkles are not caused.
Industrial Applicability
According to the present invention, a high-
workability steel pipe can be provided which has a high
r-vaiue over an entire area iri the circumferential
direction, including a seamed portion, and also has a
good shape. Limits in bending and expanding work of
16
CA 02403830 2002-08-27
the steel pipe are noticeably improved, whereby
omission of steps due to the integral forming and a
reduction in weight can be achieved. Further, seam
welded steel pipes having a high r-value can also be
produced using, as base materials, hot--rolled steel,
high tensile strength steel such as dual phase steel,
and low, medium and high carbon steel, which have a
difficulty in achieving a high r-value with a
conventional method of producing a steel pipe by simply
seam-welding a steel plate. As a result, the present
invention is able to remarkably enlarge the applicable
range of bending of steel pipes and hence greatly
contributes to development of the industry.
17
CA 02403830 2002-08-27
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18
CA 02403830 2002-08-27
Table 2
No Steel Heating Incoming- Outgoing- Total Effective Heat Remarks
Temperature side side Reduction Reduction Treatment
( C) Temperature Temperature in in
in Diameter- in Diameter- Diameter Diameter*
Reducing Reducing (k) (8)
Rolling ( C) Rollin ( C)
1 A 800 780 730 50 50 - Example
2 A 900 880 830 50 5 - Comparative
Example
3 A 630 610 560 50 10 - Comparative
Example
4 B 800 780 730 50 50 - Example
B 800 780 730 50 50 - Example
6 C 800 780 730 50 50 730 C x 5 Example
min.
7 D 900** 720 680 50 50 - Example
8 D 850 720 680 50 50 - Example
9 D 800 780 730 50 50 - Example
D 800 720 680 50 50 - Example
11 D 750 720 680 50 50 - Example
12 D 735 720 680 50 50 - Example
13 D 720 720 680 50 50 - Example
14 E 800 780 730 50 50 - Example
F 800 780 730 0 0 - Comparative
Example
1 16 F 800 780 730 15 15 - Comparative
Example
17 F 800 780 730 30 30 - Example
18 F 800 780 730 40 40 - Example
19 F 800 780 730 50 1 50 - Example
F 800 780 730 60 60 - Example
21 F 800 780 73070 70 - Example
22 F 900 890 850 30 2 - Comparative
Exa le
3 I F 850 840 780 30 30 - Example
24 F 750 730 680 30 30 - Example
F 700 680 600 30 30 - Example
26 F 630 610 560 50 10 - Comparative
Example
27 G 900 780 _730 50 50 - Example
28 G 850 780 730 50 50 - Example
29 G 800 780 730 30 30 - Example
30G 800 780 730 40 40 - Example
31 G 800 780 730 50 50 - Example
32 H 800 780 730 50 50 - Example
33 I 800 780 730 50 50 - Example
34 J 800 780 73050 50 - Example
351K 800 780 730 50 50 - Example
36 L 760 740 700 60 60 - Example
- _ ----'
* effective reduction in diameter: reduction in diameter in temperature range
of
600 C to Ao3
** rolling after cooling and reheating (for other types of steel, rolling
immediately
after heating)
19
CA 02403830 2002-08-27
Table 3
Seam Wrinklea
No 0 (Seam) 90 180 270 Thick 0 not Remarks
nees occurred
Devia
tion
TS/ E1* r- TS/ E1' r- TS/M El' r- TS/ E1* r- /4 x
MPa /i value MPa /t value Pa /t value MPa /! value occurred
1 -
1 300 55 2.0 303 54 2.0 307 54 2.1 301 55 2.1 0.3 O Example
2 300 45 0.8 309 45 0.9 307 45 0.8 308 45 0.8 0.3 0 Compara
tive
Example
3 450 35 1.0 450 35 1.1 459 36 1.0 451 34 1.1 10.D x Compara
tive
Example
4 350 50 2.0 356 51 2.0 356 50 2.0 350 51 2.0 0.5 O Example
350 50 2.4 358 51 2.4 351 49 2.5 356 49 2.4 0.5 O Example
6 620 25 1.8 624 24 1.8 625 25 1.8 629 25 1.9 0.3 O Example
7 640 27 1.7 646 27 1.7 641 27 1.7 647 26 1.7 0.5 0 Example
8 631 25 1.7 651 26 1.6 641 25 1.8 641 25 1.8 1.0 O Example
9 620 28 1.8 626 29 1.8 621 29 1.9 627 28 1.9 0.5 O Example
640 24 1.6 659 24 1.7 632 24 1.7 636 24 1.7 2.0 0 Example
11 644 22 1.6 650 22 1.7 635 22 1.7 632 22 1.8 3.0 O Example
12 653 20 1.6 657 21 1.6 640 21 1.8 623 21 1.8 8.0 x Example
13 644 19 1.7 650 19 1.7 637 19 1.9 614 19 1.8 15.0 x Example
14 650 25 1.8 652 25 1.9 651 25 1.8 651 26 1.9 0.5 O Example
500 25 0.7 508 26 0.8 503 24 0.8 501 25 0.8 0.3 O compara
tive
Exam le
16 590 28 1.0 593 28 1.1 599 29 1.1 595 28 1.0 0.3 O Co~pVea
Example
17 610 28 1.3 610 28 1.3 618 28 1.3 614 29 1.3 0.9 O Example
18 610 29 1.4 619 29 1.4 611 30 1.4 611 28 1.4 0.9 O Example
19 610 30 1.6 617 31 1.7 611 30 1.6 615 31 1.6 0.9 O Example
610 32 2.0 616 31 2.0 612 33 2.1 610 31 2.1 0.9 0 Example
21 610 35 2.5 615 35 2.6 613 35 2.6 618 36 2.6 0.8 O Example
22 590 28 0.8 593 27 0.8 599 28 0.8 593 28 0.9 0.2 0 Compara
tive
Example
23 610 29 1.4 612 30 1.4 614 30 1.5 616 29 1.5 0.2 O Example
24 610 28 1.3 613 29 1.3 615 28 1.4 612 28 1.4 0.0 O Example
650 27 1.2 651 26 1.2 650 27 1.2 658 26 1.2 3.0 x Example
26 630 22 0.9 680 21 1.0 687 22 1.0 685 23 0.9 15.0 x Compara
tive
Example
27 630 30 1.3 638 30 1.3 639 31 1.4 640 31 1.3 0.7 O Example
28 630 33 1.4 636 33 1.4 630 33 1.5 638 33 1.5 0.5 O Example
29 630 30 1.3 638 30 1.3 639 31 1.4 i 640 31 1.3 0.3 O Example
630 33 1.4 636 33 1.4 630 33 1.5 638 33 1.5 0.3 0 Example
31 630 35 1.8 637 34 1.9 635 35 1.8 633 34 1.9 0.4 O Example
32 600 30 1.8 606 30 1.8 609 30 1.9 600 30 1.8 0.5 0 Example
33 600 30 1.8 604 29 1.8 605 31 1.9 601 29 1.9 0.8 O Example
34 820 24 1.6 823 25 1.6 821 25 1.7 825 24 1.7 0.3 0 Example
820 22 1.6 821 22 1.6 823 1.7 830 22 1.7 0.8 0 Example
36 695 28 1.8 595 28 1.8 595 28 1,8 595 28 1.8 0.3 O Example
* yheet thickness - 1.6 mm
