U.S. patent application number 12/292922 was filed with the patent office on 2009-07-23 for cooling method of a steel pipe.
Invention is credited to Junji Nakata, Hajime Osako.
Application Number | 20090183805 12/292922 |
Document ID | / |
Family ID | 38778676 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090183805 |
Kind Code |
A1 |
Osako; Hajime ; et
al. |
July 23, 2009 |
Cooling method of a steel pipe
Abstract
A method of cooling a steel pipe which can effectively suppress
quenching-induced bending which occurs when quenching a thin-walled
steel pipe with a wall thickness/outer diameter ratio of at most
0.07 without decreasing the manufacturing efficiency of the steel
pipe comprises cooling the inner surface of the steel pipe by
spraying cooling water into the interior of a horizontally-disposed
steel pipe 2 while rotating the pipe in its circumferential
direction, and the outer surface is cooled by producing a downward
flow of cooling water streams 5a and 5b in a planar shape from
above onto the outer surface along the axial direction of the steel
pipe 2. Cooling of the inner surface is started at least 7 seconds
before cooling of the outer surface. Cooling of the outer surface
is carried out by producing downward flow in a planar shape of
cooling water 5a and 5b at two locations 4a and 4b at approximately
equal distances from the uppermost portion of the steel pipe 2, and
the flow rate of cooling water 5a which flows down at a location on
the upstream side in the rotational direction of the steel pipe 2
is made larger than the flow rate of cooling water 5b which flows
down at a location on the downstream side in the rotational
direction.
Inventors: |
Osako; Hajime;
(Wakayama-shi, JP) ; Nakata; Junji; (Wakayama-shi,
JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW, SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
38778676 |
Appl. No.: |
12/292922 |
Filed: |
December 1, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2007/061004 |
May 30, 2007 |
|
|
|
12292922 |
|
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Current U.S.
Class: |
148/590 |
Current CPC
Class: |
B21B 2045/0227 20130101;
C22C 38/46 20130101; C22C 38/42 20130101; C22C 38/44 20130101; C21D
9/08 20130101; C22C 38/02 20130101; C22C 38/58 20130101; C22C 38/04
20130101; C21D 1/18 20130101; C21D 1/667 20130101; C22C 38/24
20130101 |
Class at
Publication: |
148/590 |
International
Class: |
C21D 9/08 20060101
C21D009/08 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2006 |
JP |
2006-150248 |
Claims
1. A method of cooling a steel pipe in which the inner surface and
the outer surface of a horizontally-disposed steel pipe are cooled
while the steel pipe is rotated in its circumferential direction,
characterized in that cooling of the inner surface of the steel
pipe is carried out by spraying cooling water inside the steel
pipe, cooling of the outer surface of the steel pipe is carried out
by producing downwards flow of cooling water in a planar shape
along the axial direction onto the outer surface of the steel pipe
from above at two locations approximately equal distances from the
uppermost portion of the steel pipe on both sides of the uppermost
portion, wherein the flow rate of cooling water which flows
downwards at a location on the upstream side in the rotational
direction of the steel pipe is equal to or greater than the flow
rate of cooling water which flows downwards at a location on the
downstream side in the rotational direction, and cooling of the
inner surface of the steel pipe is started at least 7 seconds
before cooling of the outer surface of the steel pipe.
2. A method of cooling a steel pipe as set forth in claim 1 wherein
the ratio of the wall thickness to the outer diameter of the steel
pipe is at most 0.07.
Description
TECHNICAL FIELD
[0001] This invention relates to a method of cooling a steel pipe
capable of effectively suppressing bending of steel pipes which can
easily occur particularly when quenching thin-walled steel pipes,
thereby making it possible to manufacture steel pipes having
mechanical properties of increased uniformity.
BACKGROUND ART
[0002] Bending of steel pipes sometimes occurs at the time of
quenching. In the context of the present invention, "bending" of a
steel pipe means curvature in the axial direction of the steel
pipe. Below, bending which is observed at the time of quenching
will be referred to as "quenching-induced bending".
[0003] Quenching-induced bending is caused by factors such as
uneven cooling. In particular, when quenching a thin-walled steel
pipe in which the ratio (t/D) of the wall thickness (t) to the
outer diameter (D) has a low value such as at most 0.07, a large
amount of quenching-induced bending, which is considered a defect
in quality, can easily occur. There have been many proposals in the
past concerning cooling methods intended to suppress this
quenching-induced bending.
[0004] For example, JP H02-7372 B (1990) discloses a heat treatment
method which, during quenching of a metal pipe, suppresses
quenching-induced bending by performing slow cooling in the initial
stage of cooling the outer surface of the pipe so as to reduce the
temperature difference over the entire surface of the pipe followed
by usual rapid cooling.
[0005] In JP S61-4896 B (1986), a cooling method is disclosed in
which a pipe is cooled by spraying water into the interior of the
pipe from one end thereof while water sprayed from nozzles is
allowed to impinge on the outer surface of the pipe over
substantially the entire length thereof. In this method, toward the
end of the pipe which corresponds to the discharge end of water
sprayed into the pipe, the amount of water sprayed on the outer
surface of the pipe is increased, or the timing of the start of
outer surface cooling is made earlier, or the completion of outer
surface cooling is delayed, whereby the entire pipe is uniformly
cooled in a short period.
[0006] In the method disclosed in JP H02-7372 B, because slow
cooling is performed at the initial stage of cooling and only the
outer surface of a pipe is cooled, the cooling time is necessarily
elongated and the manufacturing efficiency of a pipe is
decreased.
[0007] In the method disclosed in JP S61-4896 B, it is necessary to
vary the amount of sprayed water for cooling the outer surface or
the timing of spraying (the timing of the start or completion of
spraying) in the axial direction of a pipe. As a result, the
structure and control of the apparatus become complicated. In
addition, although that document discloses that the entire pipe can
be uniformly cooled, there is no specific disclosure as to whether
quenching-induced bending can be suppressed. In that patent
document, the only specific example of an object which was cooled
is a steel pipe measuring 114.times.8.6.times.29,000 mm (see column
4, line 13), and the outer diameter/wall thickness ratio (t/D) of
this steel pipe is approximately 0.075 (=8.6/114). There is no
mention concerning thin-walled pipes having a t/D ratio of at most
0.07 which readily experience quenching-induced bending.
DISCLOSURE OF INVENTION
[0008] This invention provides a method of cooling a steel pipe
which can suppress quenching-induced bending during quenching of
thin-walled steel pipes having a t/D ratio of at most 0.07 and
which can solve the problems of the above-described prior art.
[0009] The present invention is a method of cooling a steel pipe in
which the inner surface and outer surface of a
horizontally-disposed steel pipe are cooled while rotating the
steel pipe in its circumferential direction, characterized in that
the ratio of the wall thickness to the outer diameter of the steel
pipe is preferably at most 0.07 and more preferably at most 0.06,
cooling of the inner surface of the steel pipe is carried out by
spraying cooling water inside the steel pipe and cooling of the
outer surface of the steel pipe is carried out by making cooling
water flow downwards in a planar shape in the axial direction onto
the outer surface of a steel pipe from above at two locations
approximately equally spaced from the uppermost portion of the
steel pipe on both sides thereof, the flow rate of cooling water
which flows downwards at a location on the upstream side in the
direction of rotation of the steel pipe is equal to or greater than
the flow rate of cooling water flowing downwards at a location on
the downstream side in the rotational direction, and cooling of the
inner surface of the steel pipe is commenced at least 7 seconds
prior to cooling of the outer surface of the steel pipe.
[0010] A method of cooling a steel pipe according to the present
invention can effectively suppress quenching-induced bending of
steel pipes without a decrease in the manufacturing efficiency of
steel pipes even when quenching thin-walled steel pipes for which
t/D is at most 0.07. In addition, the uniformity of cooling in both
the circumferential and axial directions of a steel pipe is
improved, leading to improvement in the uniformity of quenching and
accordingly uniformity of the mechanical properties of a steel
pipe. Thus, the steel pipe has improved toughness.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a vertical cross-sectional view schematically
showing the structure of a cooling apparatus for carrying out an
embodiment of a method of cooling a steel pipe according to the
present invention.
[0012] FIG. 2 shows graphs showing the results of numerical
calculation of the surface temperature, the yield strength YS, and
the axial stress .sigma.z of a steel pipe when the inner surface
and the outer surface of the steel pipe are cooled. FIG. 2(a) shows
the case in which cooling of the inner surface and the outer
surface of the steel pipe are started simultaneously (inner surface
advance time=0 seconds), and FIG. 2(b) shows the case in which only
cooling of the inner surface of the steel pipe is carried out
(inner surface advance seconds).
BEST MODE FOR CARRYING OUT THE INVENTION
[0013] An embodiment of a method of cooling a steel pipe according
to the present invention will be explained in detail while
referring when suitable to the accompanying drawings.
[0014] FIG. 1 is a vertical cross-sectional view schematically
showing the structure of a cooling apparatus for carrying out a
method of cooling a steel pipe according to this embodiment.
[0015] In FIG. 1, a cooling apparatus 1 includes a pair of rotating
rollers 3, 3 which support a horizontally-disposed steel pipe 2 and
rotate it in its circumferential direction. The cooling apparatus 1
additionally includes an inner surface cooling nozzle (not shown)
which is disposed near one end of the steel pipe 2 and which is
designed to spray cooling water into the interior of the steel pipe
2, and an outer surface cooling nozzle 7 which is installed above
the steel pipe 2. The inner surface cooling nozzle may be a
conventional spraying nozzle. The outer surface cooling nozzle 7
has slit-shaped discharge ports 6a and 6b for allowing streams of
cooling water 5a and 5b which have a planar shape in the pipe axial
direction to flow downwards from above at two locations 4a and 4b
which are approximately equally spaced from the uppermost (top)
portion of the outer peripheral surface of the steel pipe 2 on both
sides thereof (namely, at two locations which are approximately
symmetric with respect to the uppermost portion). The discharge
ports 6a and 6b preferably have a length extending over
substantially the entire length of the steel pipe 2. The cooling
water for cooling the outer surface preferably flows naturally
downwards in a laminar flow from the discharge ports 6a and 6b of
the nozzle 7, but it is also possible to apply pressure to the
cooling water.
[0016] A steel pipe 2 to which a cooling method according to this
embodiment can be advantageously applied is a thin-walled steel
pipe having a ratio t/D of the wall thickness t with respect to the
outer diameter D of at most 0.07 with which a significant amount of
quenching-induced bending which becomes a problem with respect to
quality can easily occur. This cooling method can be applied
particularly suitably to cooling of the inner and outer surfaces of
line pipe made from low carbon steel which is of low strength and
easily bends or line pipe of a grade not higher than API X60
(having a composition in mass percent of, for example, (a) C:
0.06%, Si: 0.26%, Mn: 1.24%, P: 0.013%, S: 0.001%, Cr: 0.16%, V:
0.06%, a remainder of Fe and impurities, with Ceq: 0.311%, or (b)
C: 0.06%, Si: 0.40%, Mn: 1.60%, P: 0.020%, S: 0.003%, Cu: 0.30%,
Ni: 0.50%, Cr: 0.28%, Mo: 0.23%, V: 0.08%, a remainder of Fe and
impurities, with Ceq: 0.498%). Even when this cooling method is
applied to a long steel pipe 2 with a length of at least 20 meters,
it can effectively suppress the occurrence of quenching-induced
bending.
[0017] When cooling a steel pipe 2 with the cooling apparatus 1
according to this embodiment, first, the steel pipe 2 is rotated in
its circumferential direction by rotating the rotating rollers 3, 3
in the direction of the arrows. Then, cooling of the inner surface
of the steel pipe 2 is commenced by spraying cooling water from the
unillustrated inner surface cooling nozzle into the interior of the
steel pipe from one end thereof. The sprayed cooling water is
discharged from the other end of the steel pipe 2. Cooling of the
outer surface of the steel pipe 2 is then commenced by making
cooling water 5a and 5b from the discharge ports 6a and 6b of the
outer surface cooling nozzle 7 flow downwards towards the outer
peripheral surface of the steel pipe 2. The cooling water may if
necessary contain an additive such as a corrosion inhibitor as is
well known in the art.
[0018] The rotational speed of the steel pipe 2 is preferably at
least 30 rpm and at most 80 rpm. If the rotational speed of the
steel pipe 2 is less than 30 rpm, the condition of the steel pipe 2
after quenching can easily vary in the circumferential direction
thereof. On the other hand, if the rotational speed of the steel
pipe 2 exceeds 80 rpm, the necessary equipment becomes large in
size and complicated and equipment costs increase.
[0019] The rate at which cooling water is sprayed into the interior
of the steel pipe 2 from the inner surface cooling nozzle is
preferably at least 2,000 m.sup.3 per hour and at most 6,500
m.sup.3 per hour. If the flow rate of cooling water sprayed into
the steel pipe 2 is less than 2,000 m.sup.3 per hour, the cooling
ability is inadequate, whereas if it exceeds 6,500 m.sup.3 per
hour, the necessary equipment becomes large in size and complicated
and equipment costs increase.
[0020] In a cooling method according to this embodiment, cooling of
the inner surface of the steel pipe 2 begins at least 7 seconds
before cooling of the outer surface of the steel pipe 2 for the
following reasons.
[0021] FIG. 2 are graphs showing the results of numerical
calculation of the surface temperature, the yield strength YS, and
the axial stress oz of the steel pipe 2 when the inner surface and
the outer surface of the steel pipe 2 were cooled. FIG. 2(a) shows
the results when cooling of the inner surface and cooling of the
outer surface of the steel pipe 2 were commenced simultaneously
(advance time for the inner surface=0 seconds) and FIG. 2(b) shows
the results when only the inner surface of the steel pipe 2 was
cooled (advance time for the inner surface=.infin.seconds). The
results shown in the graphs of FIGS. 2(a) and 2(b) were observed
under conditions in which the outer diameter of the steel pipe 2
was 412.3 mm, the wall thickness was 8.30 mm, the length was 30 mm,
the material of the pipe was low carbon steel, the flow rate of
cooling water sprayed into the steel pipe 2 from the inner surface
cooling nozzle was 5,400 m.sup.3 per hour, the flow rate of cooling
water which flowed downwards onto the outer surface of the steel
pipe 2 from the outer surface cooling nozzle 7 was 2,700 m.sup.3
per hour, and the rotational speed of the steel pipe 2 was 65
rpm.
[0022] As shown in FIG. 2(a), if cooling of the inner surface and
cooling of the outer surface of a steel pipe P are commenced
simultaneously, the absolute value |.sigma.z| of the axial stress
produced by thermal expansion and contraction of the steel pipe 2
in the initial stage after the start of cooling, i.e., in the stage
in which the surface temperature of the steel pipe 2 is 550.degree.
C. or higher (the axial stress in the region indicated by symbol A
in the graph of FIG. 2(a)), or that of the axial stress produced
after the surface temperature of the steel pipe 2 decreases to
lower than 550.degree. C. (the axial stress in the region shown by
symbol B in the graph of FIG. 2(a)) and including the stress caused
by bainite transformation or martensite transformation or the like
is sometimes larger than the absolute value |YS| of the yield
stress.
[0023] In contrast, as shown in the graph of FIG. 2(b), when only
cooling of the inner surface of the steel pipe 2 is carried out,
the absolute value |.sigma.z| of the axial stress is always less
than the absolute value |YS| of the yield stress from the start to
the completion of cooling, namely, in the period until the surface
temperature of the steel pipe 2 decreases to room temperature.
[0024] The reason for this is thought to be that compared to outer
surface cooling in which only the portion where the planar streams
of cooling water 5a and 5b flow down is cooled for an instant, in
the case of inner surface cooling, it is possible to substantially
uniformly cool the steel pipe 2 over its entire periphery, so
temperature unevenness of the steel pipe 2 does not readily
develop, and variation in the axial stress oz decreases.
[0025] In a cooling test performed on an actual steel pipe 2 under
the same conditions as were set for obtaining the results shown in
the graphs of FIGS. 2(a) and 2(b), a significant amount of
quenching-induced bending occurred when cooling of the inner
surface and cooling of the outer surface were simultaneously
carried out, whereas a significant amount of troublesome
quenching-induced bending did not occur when only inner surface
cooling was carried out.
[0026] Based on the above-described results from FIGS. 2(a) and
2(b) and from the cooling test, it is thought that
quenching-induced bending of a steel pipe 2 occurs when the
absolute value |.sigma.z| of the axial stress is greater than the
absolute value |YS| of the yield stress (i.w., |.sigma.z|>|YS|).
Accordingly, quenching-induced bending of a steel pipe 2 can be
suppressed by cooling a steel pipe 2 such that the relationship
|.sigma.z|<|YS| is always established. As shown in FIG. 2(b),
the relationship |.sigma.z|<|YS| is always established if only
inner surface cooling is carried out. However, with only inner
surface cooling, the cooling capacity of the steel pipe 2 per unit
time is inadequate and cooling takes a long time. As a result, the
manufacturing efficiency of a steel pipe 2 decreases, or the steel
pipe 2 cannot be sufficiently uniformly cooled due to the effect of
recuperation of heat from the steel pipe 2, whereby a steel pipe
having uniform mechanical properties cannot be obtained.
[0027] Therefore, according to an embodiment of the present
invention, in order to prevent a decrease in manufacturing
efficiency and guarantee uniform quenching, cooling is carried out
not only on the inner surface but also on the outer surface of a
steel pipe 2. In order to establish the relationship
|.sigma.z|<|YS| at least in the initial stage of cooling in
which the surface temperature of the steel pipe 2 is 550.degree. C.
or higher, it is effective to begin cooling of the inner surface of
the steel pipe 2 before cooling of the outer surface. Specifically,
by making the advance time at least 7 seconds, the relationship
|.sigma.z|<|YS| can be maintained throughout all the period of
cooling the steel pipe 2.
[0028] For the above-described reasons, in this embodiment, by
starting cooling of the inner surface of the steel pipe 2 at least
7 seconds in advance of cooling of the outer surface of the steel
pipe 2, i.e., by setting the timing of the start of spraying of
cooling water from the inner surface cooling nozzle to be at least
7 seconds before the timing of the start of allowing cooling water
5a and 5b to flow down from the discharge ports 6a and 6b of the
outer surface cooling nozzle, the relationship |.sigma.z|<|YS|
is maintained over substantially the entire cooling process. As a
result, quenching-induced bending of a steel pipe can be
effectively suppressed with certainty.
[0029] If the advance time of inner surface cooling exceeds 30
seconds, a long time is required for cooling of a steel pipe 2 and
operating efficiency decreases. Therefore, the advance time is
preferably at most 30 seconds.
[0030] In order to increase the cooling efficiency of the outer
surface of a steel pipe 2, it is conceivable to increase both the
flow rates of the cooling water 5a and 5b which flows down from the
discharge ports 6a and 6b, respectively. However, if the flow rates
of cooling water 5a and 5b are both too large, a water film which
accumulates on the outer surface of the steel pipe 2 between the
locations 4a and 4b where the cooling water 5a and 5b runs down
becomes thicker than necessary, and the rate of effective
utilization of cooling water (the proportion of cooling water which
contributes purely to cooling of the steel pipe 2) decreases, and
cooling water no longer smoothly flows in the rotational direction
of the steel pipe 2.
[0031] A considerable portion of the cooling water 5a which flows
down at position 4a on the upstream side in the rotational
direction of the steel pipe 2, i.e., a considerable portion of the
cooling water which runs down from discharge port 6a flows in the
rotational direction on the outer surface of the steel pipe 2 as it
rotates. In contrast, a certain amount of the cooling water 5b
which flows down at position 4b on the downstream side in the
direction of rotation, i.e., of the cooling water which runs down
from discharge port 6b flows backwards against the direction of
rotation of the steel pipe 2, but almost all of it flows to the
downward side and then drops immediately after it flows down.
Namely, the contribution to cooling of the outer surface of the
steel pipe 2 is greater for cooling water 5a than for cooling water
5b.
[0032] Therefore, in this embodiment, the flow rate of cooling
water which flows down at location 4a on the upstream side in the
rotational direction of the steel pipe 2 is made equal to or larger
than the flow rate of cooling water 5b which flows down at location
4b on the downstream side in the rotational direction of the steel
pipe 2. The flow rates of cooling water 5a and 5b can be set by
adjusting the width of the slits of the discharge ports 6a and 6b,
respectively.
[0033] As a result, the amount of cooling water which flows in the
rotational direction along the outer surface of the steel pipe 2
can be increased as needed, and the water film which accumulates
between positions 4a and 4b on the outer surface of the steel pipe
where cooling water streams 5a and 5b, respectively, flow down can
be made a suitable thickness, thereby making it possible to further
increase the cooling efficiency of the outer surface of the steel
pipe 2.
[0034] The ratio of the flow rate of cooling water 5b which flows
down at location 4b on the downstream side in the rotational
direction of the steel pipe 2 with respect to the flow rate of
cooling water 5a which flows down at location 4a on the upstream
side in the rotational direction of the steel pipe 2 is preferably
in the range of 1-0.6 and more preferably in the range of 1-0.8. By
making this ratio somewhat smaller than 1, the amount of bending
can be decreased compared to when the ratio is 1 (namely, when the
flow rates of cooling water streams 5a and 5b are the same).
However, if this ratio is too small, the amounts of cooling water
on both sides of the outer peripheral surface of the steel pipe
become significantly unequal and the amount of bending ends up
increasing.
[0035] The angle .theta. between positions 4a and 4b where the two
streams of cooling water 5a and 5b impact the outer peripheral
surface of the steel pipe 2 as measured from the center of the
steel pipe 2 is preferably at least 12.degree. and at most
95.degree.. If this angle .theta. is less than 12.degree., the
region formed by the water film on the surface of the steel pipe 2
(the region between positions 4a and 4b) becomes extremely narrow.
If this angle exceeds 95.degree., except for the case in which the
outer diameter of the steel pipe 2 is extremely large, it is
difficult to supply a sufficient amount of water for cooling
between positions 4a and 4b of cooling water 5a and 5b on the outer
surface of the steel pipe, and cooling sometimes becomes
insufficient particularly at the uppermost portion of the steel
pipe 2.
[0036] Particularly when the angle .theta. is large, a third
discharge port for cooling water which flows downwards in a planar
shape (not shown) may be installed preferably in a position
immediately above the uppermost portion of the steel pipe 2. The
flow rate of cooling water which flows down from this third
discharge port is preferably smaller than the flow rates of cooling
water from the discharge ports 6a and 6b on both sides.
[0037] Although the cooling apparatus becomes complicated, it is
possible to have two rows of third streams of cooling water in a
planar shape. For example, it is possible to install two pairs of
two rows of discharge ports (namely, an inner pair and an outer
pair) for cooling water which flows down on the outer peripheral
surface of the steel pipe at roughly equal distances from the
uppermost portion on both sides of the uppermost portion of the
steel pipe. In this case, for the discharge ports of each pair, the
flow rate of cooling water which flows down in a position on the
upstream side in the rotational direction of the steel pipe 2 is
preferably set to be equal to or greater than the flow rate of
cooling water which flows down at a position on the downstream side
in the rotational direction of the steel pipe 2.
[0038] In this manner, in this embodiment, the amount of
quenching-induced bending which develops when quenching a
thin-walled steel pipe P for which the ratio t/D is at most 0.07
can be made such that the maximum overall bending in a lot of pipes
is effectively suppressed without a decrease in the manufacturing
efficiency of steel pipes. As a result, the quenched steel pipes
have improved toughness. In contrast to the method disclosed in JP
S61-4896 B, cooling of the outer surface can be carried out under
the same conditions over the entire length of the steel pipe
without varying the starting time and the ending time in the axial
direction of the steel pipe, so complexity of the structure of
equipment and of control can be avoided. However, the timing of
start of cooling of the outer surface is delayed relative to
cooling of the inner surface over the entire length of the steel
pipe.
EXAMPLES
[0039] Using the cooling apparatus 1 shown in FIG. 1, cooling was
carried out on API X60 grade steel pipes 2 (in mass %, C: 0.06%,
Si: 0.26%, Mn: 1.24%, P: 0.013%, S: 0.001%, Cr: 0.16%, V: 0.06%, a
remainder of Fe and impurities, and Ceq: 0.311%) having the outer
diameter D, wall thickness t, ratio t/D, and length shown in Table
1 while rotating it at a rotational speed of 60 rpm with the inner
surface flow rate (the flow rate of cooling water for cooling the
inner surface), the total flow rate on the outer surface (the total
flow rate of cooling water for cooling the outer surface), the
inner surface advance time (the time interval from the start of
inner surface cooling to the start of outer surface cooling), the
separation between the streams of outer surface cooling water (the
distance in the circumferential direction between 4a and 4b in FIG.
1), and the angle .theta. having the values shown in Table 1. The
heating temperature of the steel pipe 2 prior to the start of
cooling was 920.degree. C. The discharge ports 6a and 6b for
cooling the outer surface extended over the entire length of the
steel pipe. Cooling was carried out until the inner and outer
surfaces of the steel pipe reached room temperature.
[0040] For comparison, cooling of the steel pipe 2 was carried out
using one stream of cooling water which flowed downwards in a
planar shape on the outer surface of the steel pipe 2. In this
case, the discharge port for cooling water was disposed immediately
above the uppermost portion of the steel pipe 2.
[0041] The amount of quenching-induced bending which was produced
in the steel pipe 2 after the completion of cooling (in unit of
mm/10 m; determined by measuring the amount of bending (mm) with a
thread stretched over the overall length of a pipe for the pipe
having the largest amount of bending in a lot of pipes undergoing
the same heat treatment, and converting this value into the amount
of bending per 10 meters) and the maximum fracture appearance
transition temperature vTs (the maximum value measured at 4
locations in the circumferential direction of the steel pipe) in a
Charpy impact test were determined.
[0042] Bending amounts of at most 10 mm are indicated by DOUBLE
CIRCLE (.circleincircle.), bending amounts of greater than 10 mm
and at most 20 mm are indicated by CIRCLE (.smallcircle.), bending
amounts of greater than 20 mm and at most 30 mm are indicated by
TRIANGLE (.DELTA.), and bending amounts exceeding 30 mm are
indicated by X. For the maximum fracture appearance transition
temperature vTs in a Charpy impact test, a value of -40.degree. C.
or below is indicated by CIRCLE, a value of greater than
-40.degree. C. and at most 0.degree. C. is indicated by TRIANGLE,
and a value exceeding 0.degree. C. is indicated by X. The overall
evaluation was whichever of the above two evaluations was the
worst, with the highest evaluation being CIRCLE. The results are
shown in Table 1.
TABLE-US-00001 TABLE 1 Outer surface Outer Inner cooling water
Outer Wall Inner surface surface in planar shape diam- thick- Rota-
surface overall cooling Number of eter ness tional flow flow
advance Amount of Charpy streams Angle Overall D t Length speed
rate rate time bending max. vTs (flow rate Spacing .theta. evalu-
Run [mm] [mm] t/D [m] [rpm] [t/hr] [t/hr] [sec] [mm/10 m] (.degree.
C.) ratio) [mm] [.degree.] ation No. 323.9 12.7 0.039 25 60 5400
2800 0 50(X) -30.degree. C.(.DELTA.) 1 0 0 X 1 10 25(.DELTA.)
-30.degree. C.(.DELTA.) 1 0 0 .DELTA. 2 20 15(.largecircle.)
-30.degree. C.(.DELTA.) 1 0 0 .DELTA. 3 0 47(X) -50.degree.
C.(.largecircle.) 2(1:1) 100 18 X 4 10 20(.largecircle.)
-50.degree. C.(.largecircle.) 2(1:1) 100 18 .largecircle. 5 20
10(.circleincircle.) -50.degree. C.(.largecircle.) 2(1:1) 100 18
.largecircle. 6 20 6(.circleincircle.) -50.degree.
C.(.largecircle.) 2(5:4) 100 18 .largecircle. 7 406.4 12.7 0.031 25
60 6500 2800 20 10(.circleincircle.) -30.degree. C.(.DELTA.) 1 0 0
.DELTA. 8 5 65(X) -30.degree. C.(.DELTA.) 2(1:1) 100 14 X 9 7 20(X)
-50.degree. C.(.largecircle.) 2(1:1) 100 14 .largecircle. 10 20
9(.circleincircle.) -50.degree. C.(.largecircle.) 2(1:1) 100 14
.largecircle. 11 219.1 12.7 0.058 25 60 4500 2800 12
9(.circleincircle.) -30.degree. C.(.DELTA.) 1 0 0 .DELTA. 12 4
17(.largecircle.) -30.degree. C.(.DELTA.) 2(1:1) 100 26 .DELTA. 13
7 8(.circleincircle.) -50.degree. C.(.largecircle.) 2(1:1) 100 26
.largecircle. 14 12 4(.circleincircle.) -50.degree.
C.(.largecircle.) 2(1:1) 100 26 .largecircle. 15 Note: Flow rate
ratio = flow rate on upstream side: flow rate on downstream
side
[0043] Runs Nos. 5-7, 10, 11, 14, and 15 in Table 1 are examples of
carrying out cooling by the method according to the present
invention (namely, there were two streams of outer surface cooling
water, and inner surface cooling was carried out at least 7 seconds
in advance). For each example, the amount of bending was CIRCLE or
DOUBLE CIRCLE, and even with a thin-walled steel pipe having a t/D
ratio of at most 0.07 (i.e., 0.031 to 0.058), quenching-induced
bending could be effectively suppressed without decreasing the
manufacturing efficiency of a steel pipe. In addition, the Charpy
maximum fracture appearance transition temperature (maximum vTs)
was -40.degree. C. or below, so the toughness was good.
[0044] Runs Nos. 6 and 7 had the same cooling conditions as each
other except that the distribution of the flow rate of the two
streams of outer surface cooling water was different. Whereas the
amount of quenching-induced bending was 10 mm for Run No. 6 in
which the flow rates of the two streams of outer surface cooling
water were the same, for Run No. 7 in which the flow rate for the
stream on the upstream side in the rotational direction of the
steel pipe was made larger than the flow rate for the stream on the
downstream side, the amount of quenching-induced bending was
further decreased to 6 mm.
[0045] In contrast, in Run No. 1 in which there was one stream of
outer surface cooling water and inner surface cooling and outer
surface cooling were started simultaneously, the amount of
quenching-induced bending was too large, and toughness was poor
with a maximum vTs of -30.degree. C. In Runs Nos. 2, 3, 8, and 12
in which inner surface cooling began earlier than outer surface
cooling but there was one stream of outer surface cooling water,
toughness was poor with a maximum vTs of -30.degree. C. In Run No.
4 in which there were two streams of outer surface cooling water
but inner surface cooling and outer surface cooling were started
simultaneously, the amount of quenching-induced bending was too
large. In Runs Nos. 9 and 13 in which there were two streams of
outer surface cooling but the advance time of inner surface cooling
was shorter than 7 seconds, the amount of quenching-induced bending
was relatively large and the toughness was poor with a maximum vTs
of -30.degree. C.
* * * * *