U.S. patent application number 14/403731 was filed with the patent office on 2015-04-16 for high-strength seamless stainless steel tube for oil country tubular goods and method of manufacturing the same.
The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Yasuhide Ishiguro, Kazutoshi Ishikawa, Yukio Miyata, Tetsu Nakahashi.
Application Number | 20150101711 14/403731 |
Document ID | / |
Family ID | 49672883 |
Filed Date | 2015-04-16 |
United States Patent
Application |
20150101711 |
Kind Code |
A1 |
Miyata; Yukio ; et
al. |
April 16, 2015 |
HIGH-STRENGTH SEAMLESS STAINLESS STEEL TUBE FOR OIL COUNTRY TUBULAR
GOODS AND METHOD OF MANUFACTURING THE SAME
Abstract
A steel material has a chemical composition containing, by mass
%, C: 0.005% or more and 0.06% or less, Si: 0.05% or more and 0.5%
or less, Mn: 0.2% or more and 1.8% or less, Cr: 15.5% or more and
18.0% or less, Ni: 1.5% or more and 5.0% or less, V: 0.02% or more
and 0.2% or less, Al: 0.002% or more and 0.05% or less, N: 0.01% or
more and 0.15% or less, O: 0.006% or less, and further contains one
or more of Mo: 1.0% or more and 3.5% or less, W: 3.0% or less and
Cu: 3.5% or less, in which
Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C.gtoreq.19.5 and
Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn--Ni-0.3Cu-9N.gtoreq.11.5 are
satisfied, is made into a seamless steel tube by performing heating
and hot rolling.
Inventors: |
Miyata; Yukio; (Aichi,
JP) ; Ishiguro; Yasuhide; (Aichi, JP) ;
Ishikawa; Kazutoshi; (Aichi, JP) ; Nakahashi;
Tetsu; (Aichi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
49672883 |
Appl. No.: |
14/403731 |
Filed: |
May 30, 2013 |
PCT Filed: |
May 30, 2013 |
PCT NO: |
PCT/JP2013/003411 |
371 Date: |
November 25, 2014 |
Current U.S.
Class: |
148/506 ;
148/325 |
Current CPC
Class: |
C22C 38/44 20130101;
C22C 38/001 20130101; C21D 2211/008 20130101; C22C 38/002 20130101;
C21D 8/105 20130101; C21D 9/08 20130101; C22C 38/42 20130101; C21D
2211/005 20130101; C22C 38/04 20130101; C22C 38/02 20130101; C22C
38/50 20130101; C21D 8/10 20130101; C22C 38/48 20130101; C22C 38/06
20130101; C21D 9/085 20130101; C22C 38/46 20130101; C22C 38/54
20130101; C21D 2211/001 20130101 |
Class at
Publication: |
148/506 ;
148/325 |
International
Class: |
C22C 38/54 20060101
C22C038/54; C21D 9/08 20060101 C21D009/08; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/00 20060101
C22C038/00; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 8/10 20060101
C21D008/10; C22C 38/46 20060101 C22C038/46 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2012 |
JP |
2012-125126 |
Claims
1.-6. (canceled)
7. A method of manufacturing a high-strength seamless stainless
steel tube for oil country tubular goods having a wall thickness of
more than 25.4 mm, comprising: heating a steel material; hot
rolling including piercing rolling the steel material into a
seamless steel tube; and cooling the seamless steel tube to room
temperature at a cooling rate equal to or more than an air-cooling
rate, the steel material having a chemical composition containing,
by mass %, C: 0.005% or more and 0.06% or less, Si: 0.05% or more
and 0.5% or less, Mn: 0.2% or more and 1.8% or less, P: 0.03% or
less, S: 0.005% or less, Cr: 15.5% or more and 18.0% or less, Ni:
1.5% or more and 5.0% or less, V: 0.02% or more and 0.2% or less,
Al: 0.002% or more and 0.05% or less, N: 0.01% or more and 0.15% or
less, O: 0.006% or less, and further containing one, two or more
selected from among Mo: 1.0% or more and 3.5% or less, W: 3.0% or
less and Cu: 3.5% or less and the balance being Fe and inevitable
impurities, wherein expressions (1) and (2) are satisfied, the hot
rolling including piercing rolling is performed under conditions
such that the total rolling reduction in a temperature range of
1100.degree. C. to 900.degree. C. is 30% or more, and after the
rolled steel tube is cooled down to the room temperature,
quenching-tempering or tempering is performed:
Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C.gtoreq.19.5 (1),
Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn--Ni-0.3Cu-9N.gtoreq.11.5 (2), where
Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %) of chemical
elements respectively represented by corresponding atomic
symbols.
8. The method according to claim 7, wherein the chemical
composition further contains, by mass %, one or more selected from
among Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less and B:
0.01% or less.
9. The method according to claim 7, wherein the chemical
composition further contains, by mass %, Ca: 0.01% or less.
10. A high-strength seamless stainless steel tube for oil country
tubular goods having a wall thickness of more than 25.4 mm, the
steel tube having a chemical composition containing, by mass %, C:
0.005% or more and 0.06% or less, Si: 0.05% or more and 0.5% or
less, Mn: 0.2% or more and 1.8% or less, P: 0.03% or less, S:
0.005% or less, Cr: 15.5% or more and 18.0% or less, Ni: 1.5% or
more and 5.0% or less, V: 0.02% or more and 0.2% or less, Al:
0.002% or more and 0.05% or less, N: 0.01% or more and 0.15% or
less, O: 0.006% or less, and further containing one, two or more
selected from among Mo: 1.0% or more and 3.5% or less, W: 3.0% or
less and Cu: 3.5% or less and the balance being Fe and inevitable
impurities, in which expressions (1) and (2) are satisfied, having
a microstructure including a martensite phase as a main phase and a
second phase consisting of at volume ratio, 10% or more and 60% or
less of a ferrite phase and 0% or more and 10% or less of an
austenite phase, in which a GSI value, which is defined as a number
of ferrite-martensite grain boundaries per unit length of a line
segment drawn in a wall thickness direction, is 120 or more in a
central portion in the wall thickness direction, and having
excellent low-temperature toughness and excellent corrosion
resistance: Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C.gtoreq.19.5 (1),
Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn--Ni-0.3Cu-9N.gtoreq.11.5 (2), where
Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %) of chemical
elements respectively represented by corresponding atomic
symbols.
11. The high-strength seamless stainless steel tube according to
claim 10, wherein the chemical composition further contains, by
mass %, one or more selected from among Nb: 0.2% or less, Ti: 0.3%
or less, Zr: 0.2% or less and B: 0.01% or less.
12. The high-strength seamless stainless steel tube according to
claim 10, wherein the chemical composition further contains, by
mass %, Ca: 0.01% or less.
13. The method according to claim 8, wherein the chemical
composition further contains, by mass %, Ca: 0.01% or less.
14. The high-strength seamless stainless steel tube according to
claim 11, wherein the chemical composition further contains, by
mass %, Ca: 0.01% or less.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a seamless steel tube for oil
country tubular goods, in particular, to a high-strength seamless
stainless steel tube with both excellent low-temperature toughness
and excellent corrosion resistance.
BACKGROUND
[0002] Nowadays, deep oil wells to which consideration has never
been given and sour gas fields whose development was abandoned due
to their intense corrosion environment and so forth are being
actively developed on a global scale from the viewpoint of a sharp
rise in the price of crude oil and the exhaustion of oil resources
which is anticipated in the near future. Such oil wells and gas
fields are generally found very deep in the ground and in an
intense corrosion environment in which the atmosphere has a high
temperature and contains and so forth. Therefore, steel tubes for
oil country tubular goods used to drill such oil wells and gas
fields have been required to have not only high strength but also
excellent corrosion resistance.
[0003] For oil wells and gas fields in an intense corrosion
environment containing CO.sub.2, Cl.sup.- and so forth, 13% Cr
martensitic stainless steel tubes have been used as steel tubes for
oil country tubular goods in the past. However, there has been a
problem in that ordinary 13% Cr martensitic stainless steel cannot
be used in an environment containing a large amount of Cl.sup.- and
having a high temperature of higher than 100.degree. C.
[0004] Therefore, in such a high-temperature corrosion environment,
duplex stainless tubes have been used. However, there is a problem
in that, since duplex stainless tubes contain a large amount of
alloying chemical elements and are poor in terms of hot
formability, duplex stainless tubes can be manufactured by only
using particular kinds of hot processing and are expensive.
[0005] To solve the problems described above, for example, Japanese
Unexamined Patent Application Publication No. 2005-336595 describes
a method of manufacturing a high-strength stainless steel tube for
oil country tubular goods with excellent corrosion resistance, the
method including making a steel tube material having a chemical
composition including, by mass %, C: 0.005% to 0.05%, Si: 0.05% to
0.5%, Mn: 0.2% to 1.8%, Cr: 15.5% to 18%, Ni: 1.5% to 5%, Mo: 1% to
3.5%, V: 0.02% to 0.2%, N: 0.01% to 0.15%, and O: 0.006% or less,
in which relational expressions (1) and (2) below are satisfied,
into a steel tube having a specified size by performing hot
processing for tube making, cooling the tube down to room
temperature at a cooling rate equal to or more than an air-cooling
rate after tube making has been performed and performing
quenching-tempering on the tube by reheating the tube up to a
temperature of 850.degree. C. or higher, subsequently cooling the
heated tube down to a temperature of 100.degree. C. or lower at a
cooling rate equal to or more than an air-cooling rate and then
heating the cooled tube up to a temperature of 700.degree. C. or
lower:
Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C.gtoreq.19.5 (1),
[0006] (where Cr, Ni, Mo, W, Cu and C: contents (mass %) of
chemical elements respectively represented by corresponding atomic
symbols)
Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn--Ni-0.3Cu-9N.gtoreq.11.5 (2),
[0007] (where Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %)
of chemical elements respectively represented by corresponding
atomic symbols). According to the technique described in Japanese
Unexamined Patent Application Publication No. 2005-336595, a
high-strength stainless steel tube for oil country tubular goods
having sufficient corrosion resistance effective even in an intense
corrosion environment having increased concentrations of CO.sub.2,
Cl.sup.- and so forth and an increased temperature of up to about
200.degree. C. in which 13% Cr martensitic stainless steel cannot
be used can be stably manufactured.
[0008] In addition, Japanese Patent No. 4577457 describes a method
of manufacturing a stainless steel tube, the method including
making a billet having a chemical composition containing, by mass
%, C: 0.001% to 0.05%, Si: 0.05% to 1%, Mn: 2% or less, Cr: 16% to
18%, Ni: 3.5% to 7%, Mo: more than 2% and 4% or less, Cu: 1.5% to
4%, rare-earth element: 0.001% to 0.3%, sol. Al: 0.001% to 0.1%,
Ca: 0.0001% to 0.3%, N: 0.05% or less and O: 0.05% or less, or
further containing one or more selected from the group consisting
of Ti: 0.5% or less, Zr: 0.5% or less, Hf: 0.5% or less and V: 0.5%
or less into a steel tube by performing hot processing and then
performing quenching-tempering on the steel tube. According to the
technique described in Japanese Patent No. 4577457, a stainless
steel tube for oil country tubular goods having not only sufficient
corrosion resistance effective even in an intense corrosion
environment having a high temperature of up to about 230.degree. C.
but also high strength can be manufactured.
[0009] Nowadays, since oil wells and gas fields found very deep in
the ground are being drilled more often than ever before, tubes for
oil country tubular goods having a thick wall are demanded to
prevent tubes for oil country tubular goods from being crushed due
to pressure from the geological stratum. In the technique described
in Japanese Patent No. 4577457, there is a problem in that, when a
tube has a wall thickness of more than 25.4 mm, toughness
deteriorates and thus the desired high toughness and high strength
cannot be achieved at the same time.
[0010] It could therefore be helpful to provide a high-strength
seamless stainless steel tube for oil country tubular goods having
a wall thickness of more than 25.4 mm, having not only a high
strength of a 110 ksi (758 MPa) grade yield stress or more but also
a high toughness of 40 J or more in terms of absorbed energy
vE.sub.-10 determined by performing a Charpy impact test at a test
temperature of -10.degree. C., and, further, having excellent
corrosion resistance and a method for manufacturing the steel tube.
"Excellent corrosion resistance" refers to when a tube has
excellent CO.sub.2 corrosion resistance effective even in a
corrosion environment having a high temperature of 230.degree. C.
or higher and containing CO.sub.2 and Cl.sup.-.
SUMMARY
[0011] We thus provide:
[0012] (1) A method of manufacturing a high-strength seamless
stainless steel tube for oil country tubular goods having a wall
thickness of more than 25.4 mm, the method including heating a
steel material; hot rolling including piercing rolling the steel
material into a seamless steel tube; and cooling the seamless steel
tube down to room temperature at a cooling rate equal to or more
than an air-cooling rate, the steel material having a chemical
composition containing, by mass %, C: 0.005% or more and 0.06% or
less, Si: 0.05% or more and 0.5% or less, Mn: 0.2% or more and 1.8%
or less, P: 0.03% or less, S: 0.005% or less, Cr: 15.5% or more and
18.0% or less, Ni: 1.5% or more and 5.0% or less, V: 0.02% or more
and 0.2% or less, Al: 0.002% or more and 0.05% or less, N: 0.01% or
more and 0.15% or less, O: 0.006% or less, and further containing
one, two or more selected from among Mo: 1.0% or more and 3.5% or
less, W: 3.0% or less and Cu: 3.5% or less and the balance being Fe
and inevitable impurities, in which relational expressions (1) and
(2) below are satisfied, the hot rolling including piercing rolling
is performed under conditions such that the total rolling reduction
in a temperature range of 1100.degree. C. to 900.degree. C. is 30%
or more, and after the rolled steel tube is cooled down to the room
temperature, quenching-tempering or tempering is performed:
Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C.gtoreq.19.5 (1),
[0013] (where Cr, Ni, Mo, W, Cu and C: contents (mass %) of
chemical elements respectively represented by corresponding atomic
symbols)
Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn--Ni-0.3Cu-9N.gtoreq.11.5 (2),
(where Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %) of
chemical elements respectively represented by corresponding atomic
symbols).
[0014] (2) The method of manufacturing a high-strength seamless
stainless steel tube for oil country tubular goods according to
item (1), in which the chemical composition further contains, by
mass %, one or more selected from among Nb: 0.2% or less, Ti: 0.3%
or less, Zr: 0.2% or less and B: 0.01% or less.
[0015] (3) The method of manufacturing a high-strength seamless
stainless steel tube for oil country tubular goods according to
item (1) or (2), in which the chemical composition further
contains, by mass %, Ca: 0.01% or less.
[0016] (4) A high-strength seamless stainless steel tube for oil
country tubular goods having a wall thickness of more than 25.4 mm,
the steel tube having a chemical composition containing, by mass %,
C: 0.005% or more and 0.06% or less, Si: 0.05% or more and 0.5% or
less, Mn: 0.2% or more and 1.8% or less, P: 0.03% or less, S:
0.005% or less, Cr: 15.5% or more and 18.0% or less, Ni: 1.5% or
more and 5.0% or less, V: 0.02% or more and 0.2% or less, Al:
0.002% or more and 0.05% or less, N: 0.01% or more and 0.15% or
less, O: 0.006% or less, and further containing one, two or more
selected from among Mo: 1.0% or more and 3.5% or less, W: 3.0% or
less and Cu: 3.5% or less and the balance being Fe and inevitable
impurities, in which relational expressions (1) and (2) below are
satisfied, having a microstructure including a martensite phase as
a main phase and a second phase consisting of, at volume ratio, 10%
or more and 60% or less of a ferrite phase and 0% or more and 10%
or less of an austenite phase, in which a GSI value, which is
defined as the number of ferrite-martensite grain boundaries per
unit length of a line segment drawn in the wall thickness
direction, is 120 or more in the central portion in the wall
thickness direction, and having excellent low-temperature toughness
and excellent corrosion resistance:
Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C.gtoreq.19.5 (1),
[0017] (where Cr, Ni, Mo, W, Cu and C: contents (mass %) of
chemical elements respectively represented by corresponding atomic
symbols)
Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn--Ni-0.3Cu-9N.gtoreq.11.5 (2),
[0018] (where Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %)
of chemical elements respectively represented by corresponding
atomic symbols).
[0019] (5) The high-strength seamless stainless steel tube for oil
country tubular goods according to item (4), in which the chemical
composition further contains, by mass %, one or more selected from
among Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less and B:
0.01% or less.
[0020] (6) The high-strength seamless stainless steel tube for oil
country tubular goods according to item (4) or (5), in which the
chemical composition further contains, by mass %, Ca: 0.01% or
less.
[0021] A high-strength seamless stainless steel tube for oil
country tubular goods having a wall thickness of more than 25.4 mm,
having not only a high strength of a 110 ksi (758 MPa) grade yield
stress or more but also a high toughness of 40 J or more in terms
of absorbed energy vE.sub.-10 in a Charpy impact test and, further,
having excellent corrosion resistance can be manufactured easily
and at low cost, which results in a significant industrial
effect.
BRIEF DESCRIPTION OF THE DRAWING
[0022] FIG. 1 is a graph illustrating the relationship between
absorbed energy vE.sub.-10 in a Charpy impact test and a GSI
value.
DETAILED DESCRIPTION
[0023] We conducted investigations regarding various factors having
an influence on toughness, and, as a result, found that it is
necessary to form a microstructure having a decreased grain
diameter to enhance the toughness of a stainless steel tube having
a thick wall. In stainless steel having a chemical composition
containing 16% to 18% of Cr and about 2% to 6% of Ni to enhance
corrosion resistance, a ferrite phase crystallizes at a time of
solidification, and some of the ferrite phase transforms into an
austenite phase when the stainless steel is cooled down to room
temperature. However, since the ferrite phase is not completely
eliminated and some of the ferrite phase is retained, it is almost
impossible to decrease the grain diameter even by performing a heat
treatment afterward. Therefore, we thought of using a spacing GSI
(grain size index) value between various phases such as a ferrite
phase and an austenite phase (or a martensite phase) as an index
expressing the degree of a decrease in the grain diameter of a
microstructure and found that, in a stainless steel tube having a
chemical composition containing 16% to 18% of Cr and about 2% to 6%
of Ni, there is an enhancement in toughness by decreasing a GSI
value, that is, by decreasing the spacing between various
phases.
[0024] From the results of further investigations, we found that,
when hot processing including piercing rolling is performed, there
is a decrease in spacing GSI between various phases by performing
hot processing under conditions such that rolling reduction in a
specified temperature range is equal to or more than a certain
value, which results in a significant enhancement in toughness.
[0025] First, experimental results will be described.
[0026] Steel materials (billets) having a chemical composition
containing, by mass %, 0.026% C-0.20% Si-0.24% Mn-0.01% P-0.001%
S-16.7% Cr-4.11% Ni-0.027% V-2.13% Mo-1.06% W-0.51% Cu-0.02%
Al-0.051% N and the balance being Fe and inevitable impurities were
heated at various heating temperatures. Moreover, by performing hot
rolling using a piercer mill, an elongator mill, a plug mill and so
forth at various temperatures with various rolling reductions,
seamless steel tubes having an outer diameter of 297 mm.phi. and a
wall thickness of 26 to 34 mm were made and cooled down to room
temperature by performing air-cooling. Using a test piece for
microstructure observation which had been cut out of the obtained
steel tube, polished and etched with a vilella's reagent, a
microstructure was observed using an optical microscope (at a
magnification of 400 times). By performing image analysis on the
taken microstructure photograph, a GSI value was determined as an
index representing the degree of a decrease in the grain diameter
of a microstructure. The GSI value was determined by counting the
number of ferrite-martensite grain boundaries per unit length
(line/mm) in the wall thickness direction using the obtained
microstructure photograph. In addition, using a Charpy impact test
piece (having a thickness of 10 mm) cut out of the obtained steel
tube in the longitudinal direction of the steel tube, absorbed
energy vE.sub.-10 (J) at a test temperature of -10.degree. C. was
determined. The obtained results are illustrated in the form of the
relationship between vE.sub.-10 and a GSI value in FIG. 1.
[0027] FIG. 1 indicates that it is necessary to decrease the grain
diameter of a microstructure to GSI: 120 or more to achieve
toughness of vE.sub.-10: 40 J or more. From the results of other
experiments, we confirmed that a decrease in the grain diameter of
a microstructure to GSI: 120 or more can be achieved by performing
hot rolling under conditions such that the total rolling reduction
in a temperature of 1100.degree. C. to 900.degree. C. is 30% or
more. In hot rolling including piercing rolling where a slab is
heated at an ordinary heating temperature (1100.degree. C. to
1250.degree. C.), a temperature of 1100.degree. C. to 900.degree.
C. corresponds to rolling using an elongator mill and a plug mill
or an mandrel mill. That is to say, we found that, to enhance the
low-temperature toughness of a seamless steel tube, that is, to
decrease the grain diameter of a microstructure, it is necessary
that rolling using an elongator mill, a plug mill and so forth be
performed under conditions such that the temperature is low and the
rolling reduction is high, that is, the total rolling reduction is
30% or more.
[0028] The method of manufacturing a high-strength seamless
stainless steel tube for oil country tubular goods will be
described. A seamless steel tube is manufactured by heating a steel
material and by performing hot rolling including piercing
rolling.
[0029] The reasons for limitations on a chemical composition of a
steel material will be described hereafter. Hereinafter, mass %
used when describing a chemical composition is simply represented
by %, unless otherwise noted.
[0030] The steel material has a chemical composition containing C:
0.005% or more and 0.06% or less, Si: 0.05% or more and 0.5% or
less, Mn: 0.2% or more and 1.8% or less, P: 0.03% or less, S:
0.005% or less, Cr: 15.5% or more and 18.0% or less, Ni: 1.5% or
more and 5.0% or less, V: 0.02% or more and 0.2% or less, Al:
0.002% or more and 0.05% or less, N: 0.01% or more and 0.15% or
less, O: 0.006% or less, and further containing one, two or more
selected from among Mo: 1.0% or more and 3.5% or less, W: 3.0% or
less and Cu: 3.5% or less and the balance being Fe and inevitable
impurities, in which (1) and (2) are satisfied:
Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C.gtoreq.19.5 (1),
[0031] (where Cr, Ni, Mo, W, Cu and C: contents (mass %) of
chemical elements respectively represented by corresponding atomic
symbols)
Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn--Ni-0.3Cu-9N.gtoreq.11.5 (2),
[0032] (where Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %)
of chemical elements respectively represented by corresponding
atomic symbols).
C: 0.005% or More and 0.06% or Less
[0033] C is a chemical element related to an increase in the
strength of martensitic stainless steel. It is necessary that the C
content be 0.005% or more in the present invention. On the other
hand, when the C content is more than 0.06%, there is a significant
deteriorate in corrosion resistance. Therefore, the C content is
limited to 0.005% or more and 0.06% or less, preferably 0.01% or
more and 0.04% or less.
Si: 0.05% or More and 0.5% or Less
[0034] Si is a chemical element which functions as a deoxidation
agent, and Si is added in the amount of 0.05% or more. However,
when the Si content is more than 0.5%, there is a deteriorate in
CO.sub.2 corrosion resistance and there is a deteriorate in hot
formability. Therefore, the Si content is limited to 0.05% or more
and 0.5% or less, preferably 0.1% or more and 0.4% or less.
Mn: 0.2% or More and 1.8% or Less
[0035] Mn is a chemical element which increases strength, and Mn is
added in the amount of 0.2% or more to achieve the desired high
strength. On the other hand, when the Mn content is more than 1.8%,
there is a negative influence on toughness. Therefore, the Mn
content is limited to 0.2% or more and 1.8% or less, preferably
0.2% or more and 0.8% or less.
P: 0.03% or Less
[0036] Since P is a chemical element which deteriorates corrosion
resistance, it is preferable that the P content be as small as
possible. However, since the P content is controlled at
comparatively low cost without deteriorating corrosion resistance
when the P content is 0.03% or less, it is acceptable that the P
content is about 0.03% or less. Therefore, the P content is limited
to 0.03% or less. Since there is an increase in manufacturing cost
when the P content is excessively small, it is preferable that the
P content be 0.005% or more.
S: 0.005% or Less
[0037] Since S is a chemical element which significantly
deteriorates hot formability, it is preferable that the S content
be as small as possible. However, it is acceptable that the S
content is 0.005% or less, because it is possible to manufacture a
pipe using normal processes when the S content is 0.005% or less.
Therefore, the S content is limited to 0.005% or less. Since there
is an increase in manufacturing cost when the S content is
excessively small, it is preferable that the S content be 0.0005%
or more.
Cr: 15.5% or More and 18.0% or Less
[0038] Cr is a chemical element which enhances corrosion resistance
as a result of forming a protective film and, in particular
contributes to an enhancement in CO.sub.2 corrosion resistance. It
is necessary that the Cr content be 15.5% or more to enhance
corrosion resistance at a high temperature. On the other hand, when
the Cr content is more than 18%, there is a deterioration in hot
formability and there is a decrease in strength. Therefore, the Cr
content is limited to 15.5% or more and 18.0% or less, preferably
16.0% or more and 17.5% or less, more preferably 16.5% or more and
17.0% or less.
Ni: 1.5% or More and 5.0% or Less
[0039] Ni is a chemical element effective in increasing corrosion
resistance by strengthening a protective film and which increases
the strength of steel as a result of forming a solid solution.
These effects become noticeable when the Ni content is 1.5% or
more. On the other hand, when the Ni content is more than 5.0%,
since there is a decrease in the stability of a martensite phase,
there is a decrease in strength. Therefore, the Ni content is
limited to 1.5% or more and 5.0% or less, preferably 3.0% or more
and 4.5% or less.
V: 0.02% or More and 0.2% or Less
[0040] V contributes to an increase in strength through solid
solution strengthening and is effective in increasing resistance to
stress corrosion cracking. It is necessary that the V content be
0.02% or more to realize these effects. On the other hand, when the
V content is more than 0.2%, there is a deterioration in toughness.
Therefore, the V content is limited to 0.02% or more and 0.2% or
less, preferably 0.03% or more and 0.08% or less.
Al: 0.002% or More and 0.05% or Less
[0041] Al is a chemical element which functions as a deoxidation
agent, and it is necessary that the Al content be 0.002% or more to
realize this effect. On the other hand, when the Al content is more
than 0.05%, since there is an increase in the amount of alumina
containing inclusions, there is a deterioration in ductility and
toughness. Therefore, the Al content is limited to 0.002% or more
and 0.05% or less, preferably 0.01% or more and 0.04% or less.
N: 0.01% or More and 0.15% or Less
[0042] N is a chemical element which markedly enhances pitting
corrosion resistance, and it is necessary that the N content be
0.01% or more. On the other hand, when the N content is more than
0.15%, various nitrides are formed and there is a deterioration in
toughness. Therefore, the N content is limited to 0.01% or more and
0.15% or less, preferably 0.02% or more and 0.08% or less.
O: 0.006% or Less
[0043] O is present in the form of an oxide in steel and has a
negative effect on ductility, toughness and so forth. Therefore, it
is preferable that the O content be as small as possible. In
particular, when the O content is more than 0.006%, there is a
significant deterioration in hot formability, toughness and
corrosion resistance. Therefore, the O content is limited to 0.006%
or less.
One, Two or More Selected from Mo: 1.0% or More and 3.5% or Less,
W: 3.0% or Less and Cu: 3.5% or Less
[0044] Since Mo, W and Cu are all chemical elements which enhance
corrosion resistance, one, two or more selected from among these
chemical elements are added.
[0045] Mo is a chemical element which contributes to an enhance in
corrosion resistance by increasing resistance to pitting corrosion
caused by and it is necessary that the Mo content be 1.0% or more.
On the other hand, when the Mo content is more than 3.5%, there is
a deterioration in strength and toughness and an increase in
material cost. Therefore, when Mo is added, the Mo content is
limited to 1.0% or more and 3.5% or less, preferably 1.5% or more
and 3.0% or less.
[0046] W is a chemical element which contributes to an enhance in
corrosion resistance like Mo, and it is preferable that the W
content be 0.5% or more. However, when the W content is more than
3.0%, there is a deterioration in toughness and there is an
increase in material cost. Therefore, when W is added, the W
content is limited to 3.0% or less, preferably 0.5% or more and
2.5% or less.
[0047] Since Cu is effective in suppressing penetration of hydrogen
into steel by strengthening a protective film, Cu contributes to an
enhancement in corrosion resistance. It is preferable that the Cu
content be 0.5% or more to realize these effects. However, when the
Cu content is more than 3.5%, there is a deterioration in hot
formability. Therefore, when Cu is added, the Cu content is limited
to 3.5% or less, preferably 0.5% or more and 2.5% or less.
[0048] The contents of the constituent chemical elements described
above are controlled to be within the ranges described above, in
which expressions (1) and (2) are satisfied:
Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C.gtoreq.19.5 (1),
[0049] (where Cr, Ni, Mo, W, Cu and C: contents (mass %) of
chemical elements respectively represented by corresponding atomic
symbols)
Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn--Ni-0.3Cu-9N.gtoreq.11.5 (2),
[0050] (where Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %)
of chemical elements respectively represented by corresponding
atomic symbols). Note that, when the values of the left-hand sides
of expressions (1) and (2) are calculated, a symbol is assigned a
value of 0 when the corresponding chemical element is not
contained.
[0051] By controlling the contents of Cr, Ni, Mo, W, Cu and C so
that expression (1) is satisfied, there is a significant enhance in
corrosion resistance (CO.sub.2 corrosion resistance) at a high
temperature (up to 230.degree. C.) in a corrosion environment
containing CO.sub.2 and Cl.sup.-. It is preferable that the value
of the left-hand side of expression (1) be 20.0 or more from the
viewpoint of high-temperature corrosion resistance.
[0052] By controlling the contents of Cr, Mo, W, Si, C, Mn, Ni, Cu
and N so that expression (2) is satisfied, there is an enhancement
in hot workability, and hot workability necessary to manufacture a
martensitic stainless steel tube can be achieved. It is preferable
that the value of the left-hand side of expression (2) be 12.5 or
more.
[0053] The chemical composition described above is a base chemical
composition and, in addition to the base chemical composition, one
or more selected from among Nb: 0.2% or less, Ti: 0.3% or less, Zr:
0.2% or less and B: 0.01% or less and/or Ca: 0.01% or less may be
added.
One or More Selected from Among Nb: 0.2% or Less, Ti: 0.3% or Less,
Zr: 0.2% or Less and B: 0.01% or Less
[0054] Since Nb, Ti, Zr and B are all chemical elements which
increase the strength of steel and enhance resistance to stress
corrosion cracking, one or more selected from among these chemical
elements may be added as needed. It is preferable that the contents
of these chemical elements be respectively Nb: 0.02% or more, Ti:
0.04% or more, Zr: 0.02% or more and B: 0.001% or more to realize
these effects. On the other hand, when the contents of these
chemical elements are respectively Nb: more than 0.2%, Ti: more
than 0.3%, Zr: more than 0.2% and B: more than 0.01%, there is a
deterioration in toughness. Therefore, the contents of these
chemical elements are respectively limited to Nb: 0.2% or less, Ti:
0.3% or less, Zr: 0.2% or less and B: 0.01% or less.
Ca: 0.01% or Less
[0055] Since Ca is a chemical element which contributes to a
morphology control function of sulfides as a result of
spheroidizing sulfide containing inclusions, Ca may be added as
needed. By spheroidizing sulfide containing inclusions, there is a
decrease in the lattice distortion in a matrix in the vicinity of
the inclusions to obtain an effect of decreasing the hydrogen
trapping capability of the inclusions. It is preferable that the Ca
content be 0.0005% or more to realize this effect. On the other
hand, when the Ca content is more than 0.01%, there is an increase
in the amount of oxide containing inclusions, which deteriorates
corrosion resistance. Therefore, when Ca is added, it is preferable
that the Ca content be 0.01% or less.
[0056] The remainder of the chemical composition other than the
constituent chemical elements described above consists of Fe and
inevitable impurities. As an inevitable impurity, O: 0.010% or less
is acceptable.
[0057] There is no particular limitation on what method is used to
manufacture a steel tube material. However, it is preferable that
molten steel having a specified chemical composition be smelted
using a common refining method such as one using a steel converter
and that the smelted steel be made into a cast material such as a
billet using a common casting method such as a continuous casting
method. Note that, other than a continuous casting method, a cast
material such as a billet may be manufactured using an ingot
casting-blooming method.
[0058] A seamless steel tube is manufactured by heating a steel
material having the chemical composition described above, by
performing ordinary hot rolling including piercing rolling using a
Mannesmann-plug mill method or a Mannesmann-mandrel mill method,
and further performing cooling down to room temperature at a
cooling rate equal to or more than an air-cooling rate. Herein, the
wall thickness of the seamless steel tube is set to be more than
25.4 mm. The size of a steel material which is a starting material
is controlled to be within an appropriate range to achieve a
seamless steel tube having such a wall thickness. Heating
temperature of a steel material: 1100.degree. C. or higher and
1300.degree. C. or lower
[0059] When the heating temperature of a steel material is lower
than 1100.degree. C., there is an enhancement in deformation
resistance due to the heating temperature being excessively low and
it is difficult to perform hot rolling due to a load on rolling
mills being excessively large. On the other hand, when the heating
temperature is higher than 1300.degree. C., there is a
deterioration in toughness due to an increase in crystal grain
diameter and there is a decrease in yield due to an increase in the
amount of scale loss. Therefore, it is preferable that the heating
temperature of a steel material be 1100.degree. C. or higher and
1300.degree. C. or lower, more preferably 1200.degree. C. or higher
and 1280.degree. C. or lower.
[0060] The steel material which has been heated up to the heating
temperature descried above is subjected to hot rolling including
piercing rolling. Regarding hot rolling, any of an ordinary
Mannesmann-plug mill method, in which the steel material is
subjected to processing using a piercer mill for performing
piercing rolling, a subsequent elongator mill, a plug mill and a
realer mill or, further, a sizing mill in this order, and an
ordinary Mannesmann-mandrel mill method, in which the steel
material is subjected to processing using a piercer mill to perform
piercing rolling, a subsequent mandrel mill and reducer mill in
this order, may be used.
[0061] The hot rolling including piercing rolling described above
is performed under conditions such that the total rolling reduction
in a temperature range of 1100.degree. C. to 900.degree. C. is 30%
or more. By controlling rolling reduction in this temperature range
to be within an appropriate range, the spacing between
ferrite-austenite (martensite) grain boundaries can be controlled
to be small and a decrease in grain diameter can be achieved, which
results in an enhancement in toughness. Even when rolling reduction
is controlled in a temperature range out of 1100.degree. C. to
900.degree. C., if rolling reduction in the temperature range of
1100.degree. C. to 900.degree. C. is out of the appropriate range
described above, a decrease in grain diameter cannot be achieved.
When the total rolling reduction in this temperature range is less
than 30%, it is difficult to achieve a decrease in grain diameter,
that is, it is difficult to control the number GSI of
ferrite-austenite (martensite) grain boundaries per unit length in
the wall thickness direction to be 120 or more. Therefore, the
rolling reduction in the temperature range of 1100.degree. C. to
900.degree. C. is 30% or more. With this method, since it is
possible to control the spacing between ferrite-austenite
(martensite) grain boundaries to be equal to or less than the
specified value, a decrease in grain diameter can be realized even
in a steel tube having a thick wall, which results in an enhance in
toughness. Note that there is no particular limitation on the upper
limit of rolling reduction in this temperature range.
[0062] In addition, there is no particular limitation on what
rolling conditions are used out of the temperature range of
1100.degree. C. to 900.degree. C. as long as a seamless steel tube
having a specified size and shape can be manufactured.
[0063] The seamless steel tube which has been manufactured by
performing hot rolling for tube making as described above is
subsequently subjected to cooling down to room temperature at a
cooling rate equal to or more than an air-cooling rate. In a steel
tube having the range of chemical composition, a microstructure
including a martensite phase as a main phase can be achieved by
performing cooling at a cooling rate equal to or more than an
air-cooling rate.
[0064] After tube making has been performed, the cooled steel tube
is subsequently subjected to a heat treatment including
quenching-tempering.
[0065] In quenching, the steel tube is heated up to a heating
temperature for quenching at 850.degree. C. or higher and
1000.degree. C. or lower, and then cooled with water. When the
heating temperature for quenching is lower than 850.degree. C.,
transformation into a martensite does not sufficiently progress,
and the desired high strength cannot be achieved. Further, there is
concern that intermetallic compounds may be formed and toughness
and corrosion resistance may deteriorate. On the other hand, when
the heating temperature for quenching is higher than 1000.degree.
C., the fraction of a martensite formed becomes excessively high,
and strength becomes excessively high. Therefore, it is preferable
that the heating temperature for quenching be 850.degree. C. or
higher and 1000.degree. C. or lower. There is no particular
limitation on a holding time when heating is performed for
quenching. However, it is preferable that the holding time be 10 to
30 minutes from the viewpoint of productivity. Further, it is more
preferable that the heating temperature for quenching be
920.degree. C. or higher and 980.degree. C. or lower.
[0066] After quenching has been performed, tempering is further
performed. In tempering, the steel tube is heated up to a tempering
temperature of 400.degree. C. or higher and 700.degree. C. or
lower, and then cooled at a cooling rate equal to or more than an
air-cooling rate. When the tempering temperature is lower than
400.degree. C., a sufficient tempering effect cannot be realized.
On the other hand, when the tempering temperature is higher than
700.degree. C., there is a tendency for intermetallic compounds to
precipitate, which may deteriorate toughness and corrosion
resistance. Therefore, it is preferable that the tempering
temperature be 400.degree. C. or higher and 700.degree. C. or
lower. Note that there is no particular limitation on a holding
time when heating for tempering is performed. However, it is
preferable that the holding time be 20 to 60 minutes from the
viewpoint of productivity. Further, it is more preferable that the
tempering temperature be 550.degree. C. or higher and 650.degree.
C. or lower.
[0067] Further, only tempering described above may be performed
without performing quenching on the steel tube which has been
subjected to tube making.
[0068] The seamless steel tube manufactured using the manufacturing
method described above has a chemical composition described above
and a microstructure including a martensite phase as a main phase
and a second phase consisting of, at volume ratio, 10% or more and
60% or less of a ferrite phase and 0% or more and 10% or less of an
austenite phase. Also, the steel tube is a thick high-strength
seamless stainless steel tube for oil country tubular goods having
a wall thickness of more than 25.4 mm and having a microstructure
in which a GSI value, which is defined as the number of
ferrite-martensite grain boundaries per unit length of a line
segment drawn in the wall thickness direction, is 120 or more in
the central portion in the wall thickness direction.
[0069] A microstructure includes a martensite phase as a main phase
and a second phase consisting of, at volume ratio, 10% or more and
60% or less of a ferrite phase and 0% or more and 10% or less of an
austenite phase in order to achieve the desired high strength.
[0070] When the volume ratio of a ferrite phase is less than 10%,
there is a deterioration in hot formability. On the other hand,
when the volume ratio of a ferrite phase is more than 60%, there is
a deterioration in strength and toughness. In addition, although
the second phase may include 10% or less of an austenite phase
other than a ferrite phase, it is preferable that the volume ratio
of an austenite phase be as small as possible, including 0%, to
achieve sufficient strength. When the volume ratio of an austenite
phase is more than 10%, it is difficult to achieve the desired high
strength.
[0071] The steel tube has a microstructure including a martensite
and a ferrite phase and, further, a retained austenite phase as
described above, in which a GSI value, which is defined as the
number of ferrite-martensite grain boundaries per unit length of a
line segment drawn in the wall thickness direction, is 120 or more
in the central portion in the wall thickness direction. When the
GSI value is less than 120, since it is difficult to achieve a
decrease in the grain diameter of a microstructure, it is difficult
to stably achieve the desired toughness.
[0072] Note that a GSI value (line/mm) is a value which can be
determined by counting the number (line/mm) of ferrite-martensite
grain boundaries in the wall thickness direction using a
microstructure photograph taken through the observation of a
sample, which has been etched with a vilella's reagent, using an
optical microscope (magnification of 100 to 1000 times).
[0073] Our steel tubes and methods will be further described on the
basis of EXAMPLES hereafter.
EXAMPLES
[0074] Molten steels having the chemical compositions given in
Table 1 were smelted using a steel converter, and then cast into
billets (steel materials having an outer diameter of 260 mm) using
a continuous casting method. The obtained steel materials were
heated at the temperatures given in Table 2, and then made into
seamless steel tubes (having an outer diameter of 168.3 to 297
mm.phi. and a wall thickness of 26 to 34 mm) by performing hot
rolling using an ordinary Mannesmann-plug mill method in which the
steel material is subjected to hot processing using a piercing
mill, an elongator mill, a plug mill and realer mill or, further, a
sizing mill in this order under conditions such that the rolling
reduction in a temperature range of 1100.degree. C. to 900.degree.
C. satisfied the conditions given in Table 2. Further, after hot
rolling had been performed, cooling was performed under the
conditions given in Table 2. The obtained seamless steel tubes were
subjected to quenching-tempering under the conditions given in
Table 2.
[0075] Using test pieces cut out of the obtained steel tubes, a
microstructure was observed, and tensile properties, toughness and
corrosion resistance were investigated. Investigation methods will
be described hereafter.
(1) Microstructure Observation
[0076] Using a test piece for microstructure observation cut out of
the central portion in the wall thickness direction of the obtained
steel tube, a microstructure in a cross section in the wall
thickness direction, which had been polished and etched with a
vilella's reagent, was observed using an optical microscope (at a
magnification of 100 to 1000 times). Using the taken photograph,
the kinds of microstructures were identified, and the fraction
(volume ratio) of a ferrite phase was calculated by performing
image analysis.
[0077] That of an austenite phase (.gamma.) was determined using an
X-ray diffraction method. The integrated intensities of diffracted
X-ray for the (220) plane of a .gamma. phase and the (211) plane of
a ferrite phase (.alpha.) were determined, and conversion was
performed using the following equation:
.gamma.(volume
ratio)=100/(1+(I.alpha.R.gamma./I.gamma.R.alpha.)),
[0078] where I.alpha.: integrated intensity of a .alpha. phase
[0079] I.gamma.: integrated intensity of a .gamma. phase [0080]
R.alpha.: theoretically calculated value of .alpha. on the basis of
crystallography [0081] R.gamma.: theoretically calculated value of
.gamma. on the basis of crystallography. Here, the phase fraction
of a martensite phase was derived as the remainder other than these
phases.
[0082] In addition, the test piece for microstructure observation
was etched with a vilella's reagent and observed using an optical
microscope (at a magnification of 400 times). Using the taken
photograph, the number (line/mm) of ferrite-martensite grain
boundaries was counted in the wall thickness direction in order to
calculate a GSI value.
(2) Tensile Properties
[0083] A strip specimen specified by API standard (having a gage
length of 50.8 mm) was cut out of the central portion in the wall
thickness direction of the obtained steel tube in accordance with
API standard so that the tensile direction is the direction of the
tube axis. By performing a tensile test based on API standard,
tensile properties (yield strength YS, tensile strength TS and
elongation El) were determined.
(3) Toughness
[0084] Using a V-notch test piece (having a thickness of 10 mm)
which was cut out of the central portion in the wall thickness
direction of the obtained steel tube in accordance with ISO
standard so that the longitudinal direction of the test piece was
the circumferential direction of the tube, a Charpy impact test was
performed under a condition of a test temperature of -10.degree. C.
in order to determine absorbed energy vE.sub.-10 (J). The number of
the test pieces was 3 for each steel tube, the average value of the
three was used as the value for the steel tube.
(4) Corrosion Resistance
[0085] A test specimen for a corrosion test (having a thickness of
3 mm, a width of 25 mm and a length of 50 mm) was cut out of the
central portion in the wall thickness direction of the obtained
steel tube and used for a corrosion test.
[0086] In the corrosion test, the test specimen was immersed in a
20% NaCl aqueous solution (having a temperature of 230.degree. C.
with carbon dioxide gas of 3.0 MPa being dissolved in the saturated
state) which was contained in an autoclave, for 14 days. After the
corrosion test had been performed, by determining the weight of the
test specimen, a corrosion rate was calculated from a decrease in
weight. In addition, after the corrosion test had been performed,
the test specimen was observed using a loupe at a magnification
ratio of 50 times to observe whether or not pitting corrosion
occurred. Pitting corrosion of a diameter of 0.2 mm or more was
observed and evaluated.
[0087] The obtained results are given in Table 3.
TABLE-US-00001 TABLE 1 Chemical Composition (mass %) Steel No. C Si
Mn P S Cr Ni V Al N O A 0.026 0.2 0.24 0.01 0.001 16.7 4.11 0.027
0.02 0.051 0.0025 B 0.019 0.18 0.49 0.01 0.001 17 3.88 0.045 0.02
0.049 0.0033 C 0.034 0.26 0.77 0.01 0.001 17.2 4.31 0.036 0.02
0.057 0.0029 D 0.023 0.33 0.66 0.01 0.001 16.1 3.59 0.054 0.02
0.047 0.0041 E 0.018 0.23 0.31 0.02 0.001 17.5 4 0.046 0.01 0.05
0.0019 F 0.012 0.27 0.45 0.02 0.001 16.7 2.6 0.46 0.01 0.056 0.0028
G 0.035 0.28 0.39 0.02 0.001 16.1 4.6 0.043 0.02 0.042 0.0024
Chemical Composition (mass %) Left-Hand Left-Hand Side Value
Satisfaction Side Value Satisfaction of of of of Relational
Relational Relational Relational Steel Expression Expression
Expression Expression No. Mo, W, Cu Nb, Ti, Zr, B Ca (1) *) (1) (2)
**) (2) Note A Mo: 2.13, Nb: 0.043 -- 20.73 Yes 13.47 Yes Example
W: 1.05, Cu: 0.51 B Mo: 2.59 -- -- 20.7 Yes 14.3 Yes Example C Mo:
2.34, Nb: 0.035, -- 21.33 Yes 12.84 Yes Example Cu: 0.56 Ti: 0.075,
Zr: 0.0087, B: 0.0013 D Mo: 2.01, Ti: 0.076 -- 20.65 Yes 12.95 Yes
Example W: 1.21, Cu: 2.02 E Mo: 2.40, Nb: 0.044 -- 21.59 Yes 14.39
Yes Example Cu: 0.75 F Mo: 1.90 -- -- 19.29 No 14.83 Yes
Comparative Ti: 0.025 Example G Mo: 1.90, -- 19.87 Yes 11.24 No
Comparative Cu: 0.62 Example *) Cr + 0.65 Ni + 0.60 Mo + 0.30 W +
0.55 Cu - 20 C .gtoreq. 19.5 . . . (1) **) Cr + Mo + 0.50 W + 0.30
Si - 43.5 C - 0.4 Mu - Ni - 0.3 Cu - 9 N .gtoreq. 11.5 . . . (2)
Underlined value is out of the range according to the present
invention.
TABLE-US-00002 TABLE 2 Hot Rolling Seamless Rolling Steel Tube
Reduction Size Heat Treatment from Hot Cooling (Outer Quenching
Steel Heating 1100.degree. C. Rolling after diameter Heating Tube
Steel Temperature to 900.degree. C. Method Tube mm.phi. .times.
Mall Temperature No. No. (.degree. C.) (%) *) Making Thickness mm)
(.degree. C.) 1 A 1250 21 a Water Cooling 297.phi. .times. 34 960 2
A 1250 26 a Water Cooling 297.phi. .times. 34 960 3 A 1250 34 a
Water Cooling 297.phi. .times. 34 960 4 A 1250 40 a Water Cooling
297.phi. .times. 34 960 5 A 1250 38 a Water Cooling 297.phi.
.times. 26 960 6 A 1250 45 a Water Cooling 297.phi. .times. 26 960
7 B 1250 23 a Water Cooling 297.phi. .times. 32 1000 8 B 1250 29 a
Water Cooling 297.phi. .times. 32 1000 9 B 1250 33 a Water Cooling
297.phi. .times. 32 1000 10 B 1250 37 a Water Cooling 297.phi.
.times. 32 1000 11 C 1260 35 a Water Cooling 297.phi. .times. 32
980 12 C 1260 40 b Water Cooling 168.3.phi. .times. 26 980 13 D
1240 33 a Water Cooling 297.phi. .times. 32 980 14 E 1240 36 a
Water Cooling 297.phi. .times. 32 980 15 E 1240 32 b Water Cooling
168.3.phi. .times. 26 980 16 E 1240 28 a Water Cooling 297.phi.
.times. 32 980 17 E 1240 33 a Water Cooling 297.phi. .times. 32 980
18 F 1240 35 a Water Cooling 297.phi. .times. 32 980 19 G 1260 34 a
Water Cooling 297.phi. .times. 26 980 Heat Treatment Quenching
Cooling Tempering Steel Holding Stop Heating Holding Tube Steel
Time Temperature Temperature Time No. No. (min) Cooling (.degree.
C.) (.degree. C.) (min) Note 1 A 30 Water Cooling 25 620 60
Comparative Example 2 A 30 Water Cooling 25 620 60 Comparative
Example 3 A 30 Water Cooling 25 620 60 Example 4 A 30 Water Cooling
25 620 60 Example 5 A 30 Water Cooling 25 620 60 Example 6 A 30
Water Cooling 25 620 60 Example 7 B 30 Water Cooling 25 630 60
Comparative Example 8 B 30 Water Cooling 25 630 60 Comparative
Example 9 B 30 Water Cooling 25 630 60 Example 10 B 30 Water
Cooling 25 630 60 Example 11 C 30 Water Cooling 25 600 60 Example
12 C 30 Water Cooling 25 600 60 Example 13 D 30 Water Cooling 25
600 60 Example 14 E 30 Water Cooling 25 590 60 Example 15 E 30
Water Cooling 25 600 60 Example 16 E 30 Water Cooling 25 610 60
Example 17 E 30 Water Cooling 25 610 60 Example 18 F 30 Water
Cooling 25 600 60 Comparative Example 19 G 30 Water Cooling 25 600
60 Comparative Example *) a: Mannesmann-Plug mill method, b:
Mannesmann-mandrel mill method
TABLE-US-00003 TABLE 3 Corrosion Resistance Steel Microstructure
Tensile Property Toughness Corrosion Existence Tube Steel M F
.gamma. GSI YS TS El vE.sub.-10 Rate of Pitting No. No. Kind* (vol
%) (vol %) (vol %) (line/mm) (MPa) (MPa) (%) (J) (mm/year)
Corrosion Note 1 A M + F + .gamma. 58 35 7 88 812 987 25.4 25 0.105
No Comparative Example 2 A M + F + .gamma. 58 34 8 113 833 974 23
38 0.098 No Comparative Example 3 A M + F + .gamma. 59 33 8 131 786
913 24.1 62 0.102 No Example 4 A M + F + .gamma. 55 37 8 145 823
952 22.8 105 0.978 No Example 5 A M + F + .gamma. 57 38 5 182 811
965 23.4 145 0.965 No Example 6 A M + F + .gamma. 59 33 8 194 798
987 24.4 152 0.104 No Example 7 B M + F + .gamma. 63 29 8 79 799
944 24.1 23 0.108 No Comparative Example 8 B M + F + .gamma. 55 39
6 105 783 916 23.6 32 0.093 No Comparative Example 9 B M + F +
.gamma. 57 41 2 132 800 933 21.5 82 0.078 No Example 10 B M + F +
.gamma. 60 32 8 140 812 945 25.4 125 0.069 No Example 11 C M + F +
.gamma. 65 28 7 135 868 1025 22.9 45 0.099 No Example 12 C M + F +
.gamma. 62 31 7 142 876 1033 22 79 0.087 No Example 13 D M + F +
.gamma. 59 36 5 136 796 987 23.2 84 0.082 No Example 14 E M + F +
.gamma. 59 33 8 140 823 976 24.4 105 0.093 No Example 15 E M + F +
.gamma. 61 33 6 133 856 1001 23 55 0.103 No Example 16 E M + F +
.gamma. 59 33 8 137 889 1052 23.5 69 0.11 No Example 17 E M + F +
.gamma. 61 31 8 141 901 1085 23.2 89 0.09 No Example 18 F M + F +
.gamma. 47 49 4 132 812 974 24.9 29 0.101 No Comparative Example 19
G M + F + .gamma. 62 31 7 135 796 966 23.9 65 0.179 Yes Comparative
Example *) M; martensite, F: ferrite, .gamma.: austenite Underlined
value is out of the range according to the present invention.
[0088] All of Examples had a high strength of 758 MPa (110 ksi) or
more and a high toughness of vE.sub.-10 (J): 40 J or more despite
having a large wall thickness. In addition, even in the intense
corrosion environment having a high temperature and containing
CO.sub.2 and Cl.sup.-, a decrease in weight due to corrosion was
0.127 mm/year or less and pitting corrosion did not occur, which
means these steel tubes were excellent in terms of corrosion
resistance.
[0089] On the other hand, in the Comparative Examples, corresponded
to one or more of when the desired high strength was not achieved,
when a GSI value was less than 120 and vE.sub.-10 (J) was less than
40 J, which means high toughness was not stably achieved, and when
a decrease in weight due to corrosion was more than 0.127 mm/year,
which means there was a deteriorate in corrosion resistance.
* * * * *