U.S. patent application number 14/764813 was filed with the patent office on 2015-12-24 for method and facility for manufacturing seamless steel pipe with excellent toughness.
The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Kenichiro Eguchi, Yasuhide Ishiguro, Takeshi Suzuki.
Application Number | 20150368734 14/764813 |
Document ID | / |
Family ID | 51261968 |
Filed Date | 2015-12-24 |
United States Patent
Application |
20150368734 |
Kind Code |
A1 |
Suzuki; Takeshi ; et
al. |
December 24, 2015 |
METHOD AND FACILITY FOR MANUFACTURING SEAMLESS STEEL PIPE WITH
EXCELLENT TOUGHNESS
Abstract
It is difficult in the related art to realize not only a
decrease in material variability due to non-uniform microstructure
distribution in the wall thickness direction of a pipe body but
also the maintenance of satisfactory productivity of the whole heat
treatment line at the same time. A method includes determining in
advance whether or not the pipe body is made of a steel grade
having an Ms point lower than 200.degree. C.; leaving the pipe body
of a steel grade having an Ms point lower than 200.degree. C.
additionally at room temperature (it is preferable to be
transported to a holding bed 6 and left) until the temperature
difference between the portion having the highest temperature and
the portion having the lowest temperature in a cross section in a
direction at a right angle to the pipe axis becomes less than
2.0.degree. C. after the quenching treatment has been performed,
and then performing the tempering treatment; and, on the other
hand, performing a tempering treatment on the pipe body of a steel
grade not having an Ms point lower than 200.degree. C. without
leaving the pipe body at room temperature after a quenching
treatment has been performed.
Inventors: |
Suzuki; Takeshi; (Handa,
JP) ; Eguchi; Kenichiro; (Handa, JP) ;
Ishiguro; Yasuhide; (Handa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Family ID: |
51261968 |
Appl. No.: |
14/764813 |
Filed: |
January 22, 2014 |
PCT Filed: |
January 22, 2014 |
PCT NO: |
PCT/JP2014/000297 |
371 Date: |
July 30, 2015 |
Current U.S.
Class: |
148/511 ;
266/99 |
Current CPC
Class: |
C22C 38/58 20130101;
C22C 38/001 20130101; C22C 38/00 20130101; C22C 38/002 20130101;
C22C 38/42 20130101; C22C 38/02 20130101; C22C 38/06 20130101; C22C
38/46 20130101; C21D 1/18 20130101; C21D 9/08 20130101; C21D 11/005
20130101; C22C 38/04 20130101; C22C 38/44 20130101 |
International
Class: |
C21D 9/08 20060101
C21D009/08; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/00 20060101 C22C038/00; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C21D 1/18 20060101 C21D001/18; C22C 38/42 20060101
C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2013 |
JP |
2013-016459 |
Claims
1. A method for manufacturing a seamless steel pipe including a
process of performing quenching and tempering on a pipe body which
is an intermediate product of a seamless steel pipe, the method
comprising determining in advance whether or not the pipe body is
made of a steel grade having an Ms point lower than 200.degree. C.;
additionally leaving the pipe body of a steel grade having an Ms
point lower than 200.degree. C. at room temperature until the
temperature difference between the portion having the highest
temperature and the portion having the lowest temperature in a
cross section in a direction at a right angle to the pipe axis
becomes less than 2.0.degree. C. after the quenching treatment has
been performed, and then performing the tempering treatment; and,
on the other hand, performing a tempering treatment on the pipe
body of a steel grade not having an Ms point lower than 200.degree.
C. without leaving the pipe body at room temperature after a
quenching treatment has been performed.
2. A facility for manufacturing a seamless steel pipe including a
facility where a pipe body which is an intermediate product of a
seamless steel pipe is subjected to quenching and tempering, the
facility comprising a means for determining in advance whether or
not the pipe body is made of a steel grade having an Ms point lower
than 200.degree. C. and a holding bed for leaving the pipe body
additionally at room temperature until the temperature difference
between the portion having the highest temperature and the portion
having the lowest temperature in a cross section in a direction at
a right angle to the pipe axis becomes less than 2.0.degree. C.
before performing the tempering treatment.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method and a facility for
manufacturing a seamless steel pipe excellent in toughness. These
are used in particular for manufacturing a product pipe with
excellent toughness by performing quenching and tempering, which is
thermal refining, on a pipe body, which is an intermediate product
(semimanufactured product) of a seamless steel pipe of a steel
grade such as stainless steel having a low Ms point (martensitic
transformation start temperature) and a low Mf point (martensitic
transformation finish temperature).
[0002] Here, "excellent in toughness" refers to, for example, the
quality of satisfying ISO standard 13680. That is to say, it means
that, when a Charpy impact test is performed on three transverse
test pieces (in C direction) which are taken from the central part
of the wall thickness of a product pipe at a test temperature of
-10.degree. C., the average absorbed energy (vE.sub.-10) of the
three test pieces is 40 J or more, and the number of test pieces
whose absorbed energy is less than 40 J is one or less, where the
absorbed energy thereof is 27 J or more (2/3 or more of the
required value of 40 J).
BACKGROUND ART
[0003] Examples of related art for manufacturing a seamless steel
pipe include the following techniques.
[0004] Patent Literature 1 discloses a technique for manufacturing
a product having high strength and high toughness by controlling a
heating temperature and a cooling rate when a quenching heat
treatment is performed in order to manufacture a 13Cr seamless
stainless steel pipe having a large wall thickness.
[0005] Patent Literature 2 discloses a facility for minimizing a
decrease in productive efficiency when a quenching treatment is
performed on a steel grade on which quenching treatment cannot be
performed at a high cooling rate. In the facility, however, the
heat treatment is performed on a first-in first-out basis as long
as no trouble occurs.
[0006] Patent Literature 3 discloses a method for manufacturing a
seamless steel pipe composed of a martensite-ferrite dual phase
steel.
[0007] Patent Literature 4 discloses a technique for decreasing a
variation in hardness in the longitudinal direction of a steel pipe
after quenching has been performed using a quenching method in
which a quenching liquid is made to flow in one direction through
the steel pipe, by controlling the flow rate of the liquid in
accordance with the measured values of temperatures of the liquid
which are determined on the entrance side and exit side of the
pipe.
CITATION LIST
Patent Literature
[0008] PTL 1: Japanese Unexamined Patent Application Publication
No. 2008-189945
[0009] PTL 2: Japanese Unexamined Patent Application Publication
No. 2009-242863
[0010] PTL 3: Japanese Unexamined Patent Application Publication
No. 2005-336595
[0011] PTL 4: Japanese Unexamined Patent Application Publication
No. 2001-032022
TECHNICAL FIELD
Technical Problem
[0012] A pipe body, which is an intermediate product of a seamless
steel pipe of a steel grade of, for example, martensitic stainless
steel, is subjected to a heat treatment for quenching and tempering
so as to be controlled to at the required level of strength and
toughness after having been made into a pipe having a predetermined
size by performing tube rolling using hot working. In an ordinary
heat treatment process, the pipe body is heated to a temperature
equal to or higher than the Ac.sub.1 point and equal to or lower
than the Ac.sub.3 point in a heating furnace and then rapidly
cooled to near room temperature by performing, for example, water
cooling first in a quenching treatment, and the rapidly cooled pipe
body is heated to a temperature equal to or lower than the Ac.sub.1
point in another heating furnace and then allowed to cool in a
subsequent tempering treatment (refer to, for example, Patent
Literature 1). Recently, facilities by which such heat treatments
are performed are continuously linable, and treatment conditions
such as heating temperatures and heating times are respectively set
in accordance with various kinds of products.
[0013] In the case of steel grades such as martensitic stainless
steel (refer to Patent Literature 1) and martensite-ferrite dual
phase steel (refer to Patent Literature 3), a desired area ratio of
a martensite phase is achieved by performing quenching and
tempering described above. Here, an Ms point and an Mf point widely
vary in accordance with steel chemical composition which
characterizes the steel grade, and there is even a steel grade
having an Ms point lower than 100.degree. C. and an Mf point lower
than room temperature. The temperature of the pipe body after
quenching has been performed is commonly confirmed by determining
the surface temperature thereof. In the case of the steel grade
having a low Ms point and a low Mf point as described above, the
influence of a difference in temperature between the surface and
the inside of the wall thickness of the pipe body (non-uniform
temperature distribution in the wall thickness direction) on a
martensitic transformation ratio is non-negligible. That is to say,
even if the surface temperature of the pipe body is almost equal to
room temperature after quenching has been performed, in the case
where a tempering treatment is started before the temperature
distribution in the wall thickness direction reaches a uniform and
steady state, an unintended microstructure distribution is formed,
which is one of the factors causing material variability (a
variation in mechanical properties, in particular, toughness) after
thermal refining has been performed.
[0014] On the other hand, in the case of the steel grade (referred
to as "specific steel grade" for convenience) for which it is
intended to achieve a desired area ratio of a martensite phase by
performing above described quenching and tempering, since,
martensite transformation per se occurs even when cooling is
performed at a low cooling rate such as that at which the pipe body
is allowed to cool after heating for quenching (heating in a
quenching treatment), there is a decrease in the material
variability described above by leaving the pipe body at room
temperature for a sufficient time after cooling has been performed,
to room temperature. However, in the case where the specific steel
grades and the other steel grades are subjected to a heat treatment
in the same heat treatment line on a first-in first-out basis
(refer to, for example, Patent Literature 2), there is a problem of
a decrease in the productive efficiency for the whole heat
treatment line due to an obstacle caused by the fact that it is
necessary to leave the specific steel grades at room temperature
for a duration of predetermined time or more.
[0015] In conclusion, a quenching method and a quenching facility
for decreasing a variation in hardness in the longitudinal
direction of a pipe body by performing flow control of a quenching
liquid are known (refer to, for example, Patent Literature 4),
however, there is a problem in that it is difficult to realize not
only a decrease in material variability due to non-uniform
microstructure distribution in the wall thickness direction of a
pipe body of specific steel grades but also the maintenance of a
satisfactory productive efficiency for the whole heat treatment
line at the same time in the case where the specific steel grades
and other steel grades are subjected to a heat treatment in the
same heat treatment line as described above.
Solution to Problem
[0016] The present inventors diligently conducted investigations in
order to solve the problems described above, and as a result, found
that there is a significant decrease in material variability
described above and that there is an improvement in the average
value of data (average value of vE.sub.-10) within the range of the
material variability described above, by discriminating pipe bodies
made of steel grades having an Ms point lower than 200.degree. C.
from pipe bodies made of the other steel grades, and in the case of
former, after water cooling has been performed for quenching, by
additionally leaving the pipe bodies at room temperature until the
temperature difference between the portion having the highest
temperature and the portion having the lowest temperature in a
cross section in a direction at a right angle to the pipe axis (in
the wall thickness direction) becomes less than 2.0.degree. C. In
the case of latter, it is appropriate that the pipe bodies be
subjected to ordinary quenching and tempering. The present
invention has been completed on the basis of the knowledge
described above, and the subject matter of the present invention is
as follows.
[0017] (1) A method for manufacturing a seamless steel pipe with
excellent toughness including a process of performing quenching and
tempering on a pipe body which is an intermediate product of a
seamless steel pipe, the method including determining in advance
whether or not the pipe body is made of a steel grade having an Ms
point lower than 200.degree. C.; additionally leaving the pipe body
of a steel grade having an Ms point lower than 200.degree. C. at
room temperature until the temperature difference between the
portion having the highest temperature and the portion having the
lowest temperature in a cross section in a direction at a right
angle to the pipe axis becomes less than 2.0.degree. C. after the
quenching treatment has been performed, and then performing the
tempering treatment; and, on the other hand, performing a tempering
treatment on the pipe body of a steel grade not having an Ms point
lower than 200.degree. C. without leaving the pipe body at room
temperature after a quenching treatment has been performed.
[0018] (2) A facility for manufacturing a seamless steel pipe with
excellent toughness including a facility where a pipe body which is
an intermediate product of a seamless steel pipe is subjected to
quenching and tempering, the facility including a means for
determining in advance whether or not the pipe body is made of a
steel grade having an Ms point lower than 200.degree. C. and a
holding bed for leaving only the pipe body of a steel grade having
an Ms point lower than 200.degree. C. among the pipe bodies which
have been subjected to the quenching treatment additionally at room
temperature until the temperature difference between the portion
having the highest temperature and the portion having the lowest
temperature in a cross section in a direction at a right angle to
the pipe axis becomes less than 2.0.degree. C. before performing
the tempering treatment.
Advantageous Effects of Invention
[0019] According to the present invention, since steel grades
having an. Ms point lower than 2.00.degree. C. are left
additionally at room temperature until the temperature distribution
in the wall thickness direction becomes sufficiently uniform after
a quenching treatment has been performed and before a tempering
treatment is performed so that a product pipe having a decreased
material variability and excellent toughness is obtained, and since
the other steel grades are normally subjected to a heat treatment
on a first-in first-out basis without being disturbed by the pipe
bodies left as described above, it is possible to manufacture
seamless steel pipes excellent in toughness while maintaining
satisfactory productivity of the whole heat treatment line.
BRIEF DESCRIPTION OF DRAWINGS
[0020] FIG. 1 is a schematic plan view illustrating an example of a
heat treatment line used for the present invention.
DESCRIPTION OF EMBODIMENTS
[0021] FIG. 1 is a schematic plan view illustrating an example of a
heat treatment line used for the present invention. Among pipe
bodies 1, which are intermediate products of seamless steel pipes,
a pipe body (also called pipe A for convenience), which has been
judged to be a pipe having an Ms point of 200.degree. C. or higher,
is heated to an appropriate temperature corresponding to its steel
grade using a heating furnace for quenching 2, and then cooled so
that the temperature of the outer surface of the pipe body becomes
almost equal to room temperature by immersing the pipe body in
cooling water in a quenching water tank 3. Subsequently, the pipe
body is transported through a cooling bed 4 to a heating furnace
for tempering 5 in which the pipe body is subjected to a tempering
treatment at an appropriate temperature corresponding to its steel
grade. Here, the Ms point is calculated using equation (1)
described later.
[0022] On the other hand, a pipe body (also called pipe B for
convenience), which has been judged to be a pipe having an Ms point
lower than 200.degree. C., is treated in the same pathway as a pipe
A is treated until the pipe body is transported to the cooling bed
4. However, only a pipe B is transported to a holding bed (also
called buffer line) 6, which is a different pathway from that to
which the pipe A is transported, and left at room temperature on
the buffer line 6 until the temperature difference (referred to as
.DELTA.T) between the portion having the highest temperature and
the portion having the lowest temperature in a cross section in a
direction at a right angle to the pipe axis becomes less than
2.0.degree. C. Subsequently, the pipe B is returned to the cooling
bed 4 and subjected to a tempering treatment in the same pathway as
the pipe A is treated.
[0023] Here, the cooling bed 4 and the holding bed 6 are different
facilities from each other in the example of the present invention.
However, in the case where there is sufficient room on the cooling
bed 4, some part of the cooling bed may be used as a holding
bed.
[0024] In the present invention, examples of the specific steel
grade described above (a specific steel grade for which it is
intended to achieve a desired area ratio of a martensite phase by
performing quenching and tempering) include, for example, a steel
grade having a chemical composition containing, by mass %, C:
0.005% to 0.05%, Si: 0.05% to 1.0%, Mn: 0.2% to 1.8%, P: 0.03% or
less, S: 0.005% or less, Cr: 11% to 20%, Ni: 1.5% to 10%, Mo: 1% to
5%, N: 0.15% or less, and the balance being Fe and inevitable
impurities. Here, the chemical composition may further contain, by
mass %, one, two, or more selected from among Al: 0.002% to 0.05%,
Cu: 3.5% or less, Nb: 0.5% or less, V: 0.5% or less, Ti: 0.3% or
less, Zr: 0.2% or less, W: 3% or less, B: 0.01% or less, Ca: 0.01%
or less, and REM: 0.01% or less instead of some portion of Fe.
[0025] As described above, in the case of a steel grade whose Mf
point is lower than room temperature (which is a kind of specific
steel grade described above), a martensitic transformation ratio,
that is, the amount of residual austenite at each position in the
wall thickness direction of a pipe body is practically determined
by the temperature distribution in the wall thickness direction of
the pipe body (temperature distribution in a cross section in a
direction at a right angle to the pipe axis) when starting
tempering. Regarding such a temperature distribution, even in the
case where a temperature difference .DELTA.T between the portion
having the highest temperature and the portion having the lowest
temperature in the temperature distribution in the wall thickness
direction of the pipe body is less than 10.degree. C., a difference
(variation) in the amount of residual austenite at the positions in
the wall thickness direction of the pipe body is non-negligible.
Such a variation in the amount of residual austenite is one of the
factors which cause the material variability of a product.
[0026] In order to solve the problem of the variation, a pipe B is
left at room temperature until .DELTA.T becomes less than
2.0.degree. C. according to the present invention. With this
method, since there is a significant decrease in a variation in the
amount of residual austenite in the wall thickness direction of the
pipe body when starting tempering, there is a significant decrease
in material variability of the product after tempering has been
performed, and the average value of data (the average value of
vE.sub.-10) within the range of the material variability is
improved. In the case where tempering is started before .DELTA.T
becomes less than 2.0.degree. C., such effect cannot be realized.
Here, the reason why the discrimination criterion for a pipe B is
set to be that the pipe has an Ms point lower than 200.degree. C.
is because it was found that, from the results of the experiments
conducted by the present inventors, there is practically no problem
with considering that this criterion is almost equivalent to the
condition that the pipe has an Mf point lower than room
temperature.
[0027] In the present embodiment, the Ms point is calculated using
equation (1) below which has been derived using regression analysis
regarding the relationship between the contents [mass %] of the
constituents of the chemical composition of steel and the
experimental data of the Ms point which were determined using the
thermal expansion curves which were obtained in advance by
conducting continuous cooling transformation experiments using
thermal expansion test pieces having various chemical compositions
for the specified steel grade described above.
Ms[.degree.C.]=502-810[% C]-1230[% N]-13[% Mn]-30[% Ni]-12[%
Cr]-54[% C]-6[% Mo] (1),
[0028] where, under the assumption that M is an atomic symbol,
symbol [% M] represents the content of a constituent chemical
element represented by symbol M in equation (1), and where [% M] is
assigned a value of 0 in the case where symbol M represents a
chemical element which is not contained in the steel.
[0029] As a preferable embodiment, a waiting time (lead time) from
an end of quenching treatment (water cooling) to a start of
tempering treatment is set in accordance with the steel grade which
is going to be heat-treated. In order to set the lead time, it is
preferable that a Ms point be determined in advance using equation
(1) above, and a calculation device be prepared e by combining
measured data of an ambient temperature and surface temperature of
a pipe body, and heat-transfer calculation. In the case of a pipe
body (pipe B described above) of a steel grade having an Ms point
lower than 200.degree. C., if the lead time on the cooling bed 4 on
an ordinary first-in first-out basis is shorter than the time which
is required for temperature homogenization in order to decrease
.DELTA.T to less than 2.0.degree. C., the pipe body is transported
to the buffer line 6 temporarily, left there at room temperature
until .DELTA.T becomes less than 2.0.degree. C., and then subjected
anew to a tempering treatment.
EXAMPLES
[0030] Steel billets having the chemical compositions and the Ms
points, which were calculated using equation (1), given in Table 1
were formed into pipes by performing hot working, and thereafter
air-cooled to a temperature of 100.degree. C. to room temperature
to obtain 10 pipe bodies having an outer diameter of 195.0 mm and a
wall thickness of 27.0 mm which were used as starting materials of
seamless steel pipes.
[0031] Five pipe bodies (P1 through P5) which were selected as the
examples of the present invention by conducting a random sampling
from among the pipe bodies prepared as described above were
subjected to a heat treatment (quenching and tempering) as
described hereafter. The heat treatment line illustrated in FIG. 1
was used. In a quenching treatment, the pipe bodies were heated to
a temperature of 950.degree. C., and then water-cooled. After water
cooling and recuperation had been performed, the surface
temperature (measured value) of the pipe bodies was 30.degree. C.
to 36.degree. C. The pipe bodies were left at room temperature (in
atmospheric air) for 8 hours or more, then charged into a heating
furnace for tempering when .DELTA.T (calculated value) became
1.2.degree. C. to 1.8.degree. C. , and subjected to a tempering
treatment at a temperature of 600.degree. C.
[0032] The other five pipe bodies (P6 through P10), which were used
as comparative examples, were subjected to a quenching treatment
under the same conditions for the examples of the present
invention, then charged into a heating furnace for tempering on an
ordinary first-in first-out basis without performing time
management for decreasing .DELTA.T to less than 2.0.degree. C. and
subjected to a tempering treatment at a temperature of 600.degree.
C. In this case, .DELTA.T (calculated value) was 6.0.degree. C.
when the pipe bodies were charged into the heating furnace for
tempering.
[0033] Using three V-notched test pieces (S1, S2, and S3) which
were taken from each of the tempered pipe bodies in accordance with
JIS Z 2202 (sampling position was the central part of the wall
thickness of the pipe body, the length of the test piece was 10 mm,
the longitudinal direction of the test piece was the
circumferential direction of the pipe body (C direction), and the
depth direction of the V notch was the longitudinal direction of
the pipe body (L direction)), a Charpy impact test was conducted in
accordance with JIS Z 2242 and vE.sub.-10 was obtained.
[0034] The obtained results are given in Table 2. As Table 2
indicates, in the case of the examples of the present invention,
the average value of vE.sub.-10 values (the number of the samples
was 15) was 87.7 J, where there was no test piece having a
vE.sub.-10 value of less than 40 J. In addition, the variation in
the vE.sub.-10 value was very small as indicated by a standard
deviation of 3.8 J. On the other hand, in the case of the
comparative examples, the average value of the vE.sub.-10 values
(the number of the samples was 15) was 81.7 J. However, there were
two test pieces having a vE.sub.-10 value of less than 40 J. In
addition, in the case of the comparative examples, there was a
decrease in the average value and there was an increase in
variation as indicated by a standard deviation of 17.9 J. By
checking the results for each pipe body, there are pipe bodies
having a vE.sub.-10 value equivalent to that of the present
invention among the comparative examples. On the other hand, there
are pipe bodies having a significantly decreased vE.sub.-10 value,
which results in a decrease in the average value and an increase in
variation.
[0035] As described above, according to the present invention,
mechanical properties with increased stability can be obtained.
TABLE-US-00001 TABLE 1 Chemical Composition (mass %) Ms Point C Si
Mn P S Cr Ni Mo V N O Cu Al (.degree. C.) 0.027 0.29 0.37 0.017
0.0009 16.7 3.8 2.4 0.047 0.051 0.0027 0.94 0.0015 33
TABLE-US-00002 TABLE 2 vE.sub.-10 (J) Pipe Test Test Test Average
Standard Body Piece Piece Piece Value Deviation Code S1 S2 S3 Ave.
.sigma. Example P1 91.2 89.3 82.4 87.7 3.8 P2 84.9 93.1 87.2 P3
85.4 80.1 89.7 P4 85.6 91.5 92.4 P5 85.3 86.7 90.4 Comparative P6
92.5 89.4 72.4 81.7 17.9 Example P7 91.2 89.6 92.3 P8 39.8 90.2
39.3 P9 91.5 89.7 90.5 P10 82.3 90.1 84.6
REFERENCE SIGNS LIST
[0036] 1 pipe body
[0037] 2 heating furnace for quenching
[0038] 3 quenching water tank
[0039] 4 cooling bed
[0040] 5 heating furnace for tempering
[0041] 6 holding bed (buffer line)
* * * * *