U.S. patent application number 11/387747 was filed with the patent office on 2006-10-19 for seamless steel tubes and pipes for use in oil well.
Invention is credited to Toshiharu Abe, Nobutoshi Murao, Keiichi Nakamura, Hajime Osako.
Application Number | 20060231168 11/387747 |
Document ID | / |
Family ID | 37014901 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060231168 |
Kind Code |
A1 |
Nakamura; Keiichi ; et
al. |
October 19, 2006 |
Seamless steel tubes and pipes for use in oil well
Abstract
Disclosed are seamless steel tubes for oil well use, comprising
C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo:
0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%,
wherein the concentration product by Al and N content, corrected by
Ti and V, is within the range of 0.00001 to 0.00050, and the
residuals are Fe and impurities including P: 0.025% or less, and S:
0.010% or less. Ti, V, Nb, or B is preferably contained to enhance
the quench hardenability as well as the resistance to sulfide
stress corrosion cracking, and further Ca, Mg and/or REM is
preferably contained to improve the shape of non-metallic
inclusions, enhancing the resistance to sulfide stress corrosion
cracking. Thus, said tubes by the invention can be produced by
efficient means realizing energy savings, and widely used as ones
having excellent stability in mechanical strength.
Inventors: |
Nakamura; Keiichi;
(Wakayama-shi, JP) ; Osako; Hajime; (Wakayama-shi,
JP) ; Murao; Nobutoshi; (Wakajamashi, JP) ;
Abe; Toshiharu; (Osaka-shi, JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW
SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
37014901 |
Appl. No.: |
11/387747 |
Filed: |
March 24, 2006 |
Current U.S.
Class: |
148/334 ;
420/110; 420/111 |
Current CPC
Class: |
C22C 38/22 20130101;
C22C 38/02 20130101; C22C 38/04 20130101 |
Class at
Publication: |
148/334 ;
420/110; 420/111 |
International
Class: |
C22C 38/22 20060101
C22C038/22 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2005 |
JP |
2005-087462 |
Claims
1. Seamless steel tubes for use in oil well, comprising, by mass %,
C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo:
0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%,
wherein each content of Al, N, Ti and V satisfies the relationship
expressed by the equation (i) below, and wherein the residuals are
Fe and impurities including P: not more than 0.025% and S: not more
than 0.010%,
0.00001.ltoreq.Al.times.{N-14.times.(Ti/144+V/153)}.ltoreq.0.00050
(i), where each symbol of elements in equation (i) denotes the
content (mass %).
2. Seamless steel tubes for use in oil well, comprising, by mass %,
C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo:
0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%,
further comprising one or two of Nb: 0.005-0.040% and B:
0.0003-0.005%, wherein each content of Al, N, Ti and V satisfies
the relationship expressed by the equation (i) below, and wherein
the residuals are Fe and impurities including P: not more than
0.025% and S: not more than 0.010%,
0.00001.ltoreq.Al.times.{N-14.times.(Ti/144+V/153)}.ltoreq.0.00050
(i), where each symbol of elements in equation (i) denotes the
content (mass %).
3. Seamless steel tubes for use in oil well, comprising, by mass %,
C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo:
0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%,
further comprising one or more of Ca: 0.0003-0.005%, Mg:
0.0003-0.005% and REM: 0.0003-0.005%, wherein each content of Al,
N, Ti and V satisfies the relationship expressed by the equation
(i) below, and wherein the residuals are Fe and impurities
including P: not more than 0.025% and S: not more than 0.010%,
0.00001.ltoreq.Al.times.{N-14.times.(Ti/144+V/153)}.ltoreq.0.00050
(i), where each symbol of elements in equation (@) denotes the
content (mass %).
4. Seamless steel tubes for use in oil well, comprising, by mass %,
C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%, Mo:
0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than 0.010%,
further comprising one or two of Nb: 0.005-0.040% and B:
0.0003-0.005%, and also one or more of Ca: 0.0003-0.005%, Mg:
0.0003-0.005% and REM: 0.0003-0.005%, wherein each content of Al,
N, Ti and V satisfies the relationship expressed by the equation
(i) below, and wherein the residuals are Fe and impurities
including P: not more than 0.025% and S: not more than 0.010%,
0.00001.ltoreq.Al.times.{N-14.times.(Ti/144+V/153)}.ltoreq.0.00050
(i), where each symbol of elements in equation (i) denotes the
content (mass %).
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to seamless steel tubes and
pipes (hereinafter, simply referred to as tube(s)) for use in oil
well, more particularly, to tubes having less variation of tensile
strength, i.e., excellent stability in mechanical strength.
[0003] 2. Description of the Related Art
[0004] Seamless steel tubes having higher reliability compared to
welded tubes, are frequently used in hostile oil well environment
and/or high temperature environment, so that the tubes having
higher mechanical strength in association with stable strength,
higher toughness and higher resistance to sour gas environment
degradation are required.
[0005] For instance, in Japanese Patent Application Publication No.
2001-73086, seamless steel tubes containing V, Nb, Ti, Cr and Mo
that are arranged to satisfy the relationship expressed by the
preset equation to thereby suppress the formation of
M.sub.23C.sub.6 type carbide, which are in general formed in
quenching and tempering treatment, are disclosed as having high
toughness as well as high corrosion resistance.
[0006] In order to meet these requirements, it is essential to
stabilize the mechanical strength, especially in steel tubes that
require the resistance to sour gas environment degradation, the
narrower scatter band of mechanical strength is mostly concerned.
Also, even ordinary oil well tubes are likely to be deformed
partially due to the outer pressure in oil well, which is
attributed to the variation of mechanical strength (scatter of
mechanical strength), so that the stability of mechanical strength
in steel tubes becomes an indispensable factor.
[0007] Conventionally, in order to stabilize the mechanical
strength, it is perceived that the uniform tempered martensite
microstructure is preferable for steel tubes, and, thus, C, Mn, Cr,
Mo and the like are required to be controlled within narrow
allowable range to precisely adapt the quench hardenability.
However, this much measure could not realize the stable mechanical
strength. What is more, an in-line heat treatment is taking place
of a conventional off-line heat treatment from the viewpoint of an
enhancement of productivity, the savings of energy in use and cost
reduction. In this in-line heat treatment, there is an issue that
the quench hardenability fluctuates according to the variation of
grain size of prior austenite, thus making it difficult to obtain
the stable mechanical strength.
[0008] As afore-mentioned, there is still left a problem to be
solved in order to produce seamless steel tubes for use in oil well
having stable mechanical strength, high toughness and resistance to
sour gas corrosion environment, wherein the variation of quench
hardenability attributable to the variation of prior austenite
grain size is suppressed.
SUMMARY OF THE INVENTION
[0009] The present invention is started in view of the above
circumstances, and the task thereof is to provide seamless steel
tubes for use in oil well having excellent stability in mechanical
strength, which can be produced by efficient means leading up to
realization of energy saving.
[0010] The present inventors, to achieve the above task, looked
into the problems thus far and made various investigations on the
production of seamless steel tubes having excellent stability in
mechanical strength to thereby obtain findings shown in (a)-(d) in
the followings, and, thus, the present invention is completed.
[0011] (a) In order to reduce the variation of mechanical strength
of steel due to the variation of quench hardenability of steel
tube, it is important to control the precipitation amount of AlN
within steel. In this regard, it is necessary to control the
concentration product (hereinafter, may be alternatively referred
to as "Al.times.{N-14.times.(Ti/144+V/153)}"), as an index of AlN
precipitation amount, to be expressed by Al content (mass %) and
the effective N content (mass %) corrected with Ti and V each as a
nitride former, within the range of 0.00001 to 0.00050.
[0012] (b) The reason why the concentration product,
Al.times.{N-14.times.(Ti/144+V/153)}, is controlled within the
range indicated in the above (a) is shown as below. Namely, when
the above concentration product factor exceeds 0.00050 and gets
higher, the AlN precipitation amount increases to suppress the
coarsening of grain size to thereby reduce the quench hardenability
of steel tube. On the other hand, when the above concentration
product is less than 0.00001, the AlN precipitation amount
decreases to coarsen the grain size to thereby enhance the quench
hardenability of steel tube. That such a possible fluctuation of
quench hardenability of steel tube to be created as above shall be
suppressed to stabilize the mechanical strength shall necessitate
to control the above concentration product within the range
indicated in the above (a).
[0013] (c) In order to secure the mechanical strength of steel tube
upon enhancing the quench hardenability and to prevent the
toughness from decreasing, it is effective to control each content
of C, Si, Mn and Cr, while it is effective to control Mo content in
order to enhance the quench hardenability and the resistance to
sulfide stress corrosion cracking.
[0014] (d) When Ti is contained, N is immobilized as nitride to
thereby leave B in the dissolved state, so that the quench
hardenability can be enhanced. Also, when V is contained, fine
carbide precipitates during tempering treatment to thereby enhance
the mechanical strength. Nb contributes to enhance the resistance
to sulfide stress corrosion cracking by forming carbonitride, and
so does B by enhancing the quench hardenability which leading up to
increasing martensite. Further, by containing one or more of Ca, Mg
and REM, the shape of non-metallic inclusions can be improved to
enhance the resistance to sulfide stress corrosion cracking.
[0015] The present invention is completed based on the above
findings and the gist thereof pertains to seamless steel tubes for
use in oil well shown in the following (1)-(4).
[0016] (1) Seamless steel tubes for use in oil well, comprising, by
mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%,
Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than
0.010%, wherein each content of Al, N, Ti and V satisfies the
relationship expressed by the equation (i) below, and wherein the
residuals are Fe and impurities including P: not more than 0.025%
and S: not more than 0.010%,
0.00001.ltoreq.Al.times.{N-14.times.(Ti/144+V/153)}.ltoreq.0.00050
(i),
[0017] where each symbol of elements in equation (i) denotes the
content (mass %).
[0018] (2) Seamless steel tubes for use in oil well, comprising, by
mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%,
Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than
0.010%, further comprising one or two of Nb: 0.005-0.040% and B:
0.0003-0.005%, wherein each content of Al, N, Ti and V satisfies
the relationship expressed by the equation (i) above, and wherein
the residuals are Fe and impurities including P: not more than
0.025% and S: not more than 0.010%.
[0019] (3) Seamless steel tubes for use in oil well, comprising, by
mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%,
Mo: 0.05-2.0%, Th: 0-0.05%, V: 0-0.1%, and Al: not less than
0.010%, further comprising one or more of Ca: 0.0003-0.005%, Mg:
0.0003-0.005% and REM: 0.0003-0.005%, wherein each content of Al,
N, Ti and V satisfies the relationship expressed by the above
equation (i), and wherein the residuals are Fe and impurities
including P: not more than 0.025% and S: not more than 0.010%.
[0020] (4) Seamless steel tubes for use in oil well, comprising, by
mass %, C: 0.14-0.35%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Cr: 0.05-1.5%,
Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%, and Al: not less than
0.010%, further comprising one or two of Nb: 0.005-0.040% and B:
0.0003-0.005%, and also one or more of Ca: 0.0003-0.005%, Mg:
0.0003-0.005% and REM: 0.0003-0.005%, wherein each content of Al,
N, Ti and V satisfies the relationship expressed by the above
equation (i), and wherein the residuals are Fe and impurities
including P: not more than 0.025% and S: not more than 0.010%.
[0021] In the present invention, "excellent stability in mechanical
strength" is meant to that there is less variation of strength like
tensile strength of steel, which will be recited at the later
stage.
[0022] Besides, mass % may be designated by simply %,
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a diagram showing the effect of Al and the
effective N on the variation of yield strength of steel tubes that
were subjected to quenching and tempering treatment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] The present invention pertains to seamless steel tubes for
use in oil well, comprising C: 0.14-0.35%, Si: 0.05-1.0%, Mn:
0.05-2.0%, Cr: 0.05-1.5%, Mo: 0.05-2.0%, Ti: 0-0.05%, V: 0-0.1%,
and Al: not less than 0.010%, wherein each content of Al, N, Ti and
V satisfies the relationship expressed by foregoing equation (i),
and wherein the residuals are Fe and impurities including P: not
more than 0.025% and S: not more than 0.010%. Herein, the reason
for such limitation as above in the invention and a preferable
range is recited further.
[0025] C:
[0026] Carbon (C) is contained for the purpose of ensuring the
mechanical strength of seamless steel tubes for use in oil well.
When its content is less than 0.14%, the quench hardenabiliy
becomes deficient. Thus, the temperature of tempering treatment can
not be raised, so that it becomes difficult to ensure the required
property of steel. Meanwhile, when its content exceeds 0.35%,
quench cracking likely occurs and the toughness is also
deteriorated. By reason of the above, the proper range of C content
is set to 0.14-0.35%. Incidentally, the preferable range of C
content is 0.16-0.28%, and more preferably 0.20-0.28%.
[0027] Si:
[0028] Silicon (Si) has not only a function of a deoxidizer but
also a function of enhancing the quench hardenability of steel to
heighten the mechanical strength.
[0029] In order to utilize the above function, it is necessary for
Si of not less than 0.05% to be contained. However, when its
content exceeds 1.0% and gets higher, the resistance to sulfide
stress corrosion cracking deteriorates. Thus, the proper range of
Si content is set to 0.05-1.0%. Further, the preferable range of Si
content is 0.1-0.5%.
[0030] Mn:
[0031] Manganese (Mn) has not only a function of a deoxidizer but
also a function of enhancing the quench hardenability of steel to
heighten the mechanical strength. In order to utilize the above
function, it is necessary for Mn of not less than 0.05% to be
contained. However, when the content exceeds 2.0% and gets higher,
the compositional segregation aggravates to thereby decrease the
toughness. Thus, the proper range of Mn content is set to
0.05-2.0%.
[0032] P:
[0033] Phosphor (P) is an impurity in steel and tends to segregate
along the grain boundaries to thereby reduce the toughness.
Accordingly, the proper range of P content is set to not more than
0.025%. And a preferable P content is not more than 0.020%.
S:
[0034] Sulfur (S) is also an impurity in steel, and tends to unite
with Mn or Ca to form non-metallic inclusions. When S content
exceeds 0.010%, the deterioration of the toughness as well as the
resistance to sulfide stress corrosion cracking in steel worsens,
which is attributable to non-metallic inclusions. Thus, the proper
range of S content is set to not more than 0.010%. And a preferable
S content is not more than 0.005%.
[0035] Cr:
[0036] Chromium (Cr) is an effective element in enhancing the
quench hardenability of steel, and needs to be contained by not
less than 0.05% in order to achieve the above effect. However, when
its content exceeds 1.5% and gets higher, the toughness as well as
the resistance to sulfide stress corrosion cracking in steel
decreases. Thus, the proper range of Cr content is set to
0.05-1.5%. And a preferable Cr content is 0.2-1.2%.
[0037] Mo:
[0038] Molybdenum (Mo) is an effective element not only in
enhancing the quench hardenability to thereby ensure the high
mechanical strength but also in increasing the resistance to
sulfide stress corrosion cracking. In order to achieve the above
effect, Mo should be contained by not less than 0.05%. However,
when its content exceeds 2.0% and gets higher, the toughness as
well as the resistance to sulfide stress corrosion cracking in
steel decreases. By reason of the above, the proper range of Mo
content is set to 0.05-2.0%. And a preferable Mo content is
0.1-0.8%.
[0039] Al:
[0040] Aluminum (Al) has a function of a deoxidizer and is an
effective element in enhancing the toughness as well as the
workability of steel. And when Al content is less than 0.010%, the
dissolved state C increases, resulting in the marked increase of
mechanical strength. Thus, the proper range of Al content is set to
not less than 0.010%. And a preferable upper limit of Al content is
0.080%.
[0041] Reason for limitation of each of Al, N, Ti and V content by
the relationship expressed by the equation (i):
[0042] The reason why the concentration product,
Al.times.{N-14.times.(Ti/144+V/153)}, to be given by Al content
(mass %) and the effective N content, which is afore-mentioned, is
controlled to fall within the range expressed by the equation (i)
below is described in the followings.
0.00001.ltoreq.Al.times.{N-14.times.(Ti/144+V/153)}.ltoreq.0.00050
(i)
[0043] When the above concentration product,
Al.times.{N-14.times.(Ti/144+V/153)}, which is an index to indicate
the extent how much AlN precipitation is present in steel, exceeds
0.00050 and gets higher, the amount of AlN precipitation in steel
increases. Consequently, the coarsening of grain size is suppressed
and the quench hardenability decreases. On the other hand, the
concentration product is less than 0.00001, the amount of AlN
precipitation in steel decreases and grain size is coarsened,
resulting in the increase of quench hardenability. By reason of the
above, in order to suppress the fluctuation of quench hardenability
in association with the decrease or excessive increase of quench
hardenability to thereby stabilize the mechanical strength, a
proper range of the concentration product is set to 0.00001-0.00050
as expressed by the above equation (i). And a preferable range of
the concentration product is 0.00003-0.0003.
[0044] FIG. 1 is the diagram showing the effect of Al and the
effective N on the variation of yield strength of steel tubes that
were subjected to quenching and tempering treatment.
[0045] In this diagram, the valuation whether the variation of
yield strength of steel tubes is large or small is made in
accordance with the equation (ii) as described hereinbelow. Namely,
the symbol .smallcircle. in the diagram designates that the
variation of mechanical strength is small, while the symbol
.circle-solid. designates that the variation of mechanical strength
is large.
[0046] As seen from said diagram, the proper range of the
concentration product to be given by Al content (mass %) and the
effective N content is the region where the above equation (@) is
satisfied, and also Al content is not less than 0.010%.
[0047] In the followings, a first and a second group of chemical
elements and compositions which are to be contained, when
necessary, are set forth.
[0048] A first group consists of Ti, V, Nb and B, wherein one or
more of these elements is contained, when necessary.
[0049] Ti:
[0050] Titanium (Ti) is an element having the function that it
immobilizes N in steel as nitride to thereby leave B in steel as
the dissolved state during quenching treatment, thus contributing
to enhance the quench hardenability. This element may not be
contained, but the above function can be utilized by containing
it.
[0051] However, when Ti content exceeds 0.05% and gets higher, it
remains in steel as coarse nitride to thereby reduce the resistance
to sulfide stress corrosion cracking.
[0052] By reason of the above, the range of the content of Ti when
it is contained is set to 0-0.05%. And a preferable range of the
content is 0.005-0.025%.
[0053] B:
[0054] Boron (B) is an element having the function that it
contributes to enhance the quench hardenability to thereby increase
the amount of martensite, thus enhancing the resistance to sulfide
stress corrosion cracking. Even if its content should be the level
of an impurity, its function can be exerted. It does not matter
whether it may be contained or not, but it is preferable that its
content is not less than 0.0003% in order to exhibit the more
distinct effect. When B content exceeds 0.005% and gets higher,
however, the steel toughness decreases. Thus, a content range when
B is contained is set to 0.0003-0.005%. And a preferable content
range is 0.0003-0.003%.
[0055] V:
[0056] Vanadium (V) is an element having the function that it
precipitates as carbide during tempering treatment to thereby
heighten the mechanical strength of steel. This element may not be
contained, but the above function can be utilized by containing it.
On the other hand, when its content exceeds 0.3% and gets higher,
the steel toughness decreases. From this reason, a content range
when V is contained is set to 0-0.3%.
[0057] Nb:
[0058] Niobium (Nb) is an element having the function that it forms
carbonitride at an elevated temperature range to thereby prevent
the grain size from coarsening, thus enhancing the steel toughness
as well as the resistance to sulfide stress corrosion cracking.
This element may not be contained, but the above function can be
utilized by containing it by not less than 0.005%. On the other
hand, when its content exceeds 0.040% and gets higher, carbonitride
gets excessively coarsened to thereby deteriorate the resistance to
sulfide stress corrosion cracking. Thus, a content range when Nb is
contained is set to 0.005-0.040%. And a preferable content range is
0.010-0.030%.
[0059] A second group consists of Ca, Mg and REM.
[0060] It does not matter whether these elements may be contained
or not, but when contained, either of these has the function that
it reacts with S in steel to form sulfide to thereby improve the
shape of the non-metallic inclusion, thus enhancing the resistance
to sulfide stress corrosion cracking. When the above function needs
to can be utilized, one or more elements being selected out of Ca,
Mg and REM (Rare Earth Metal element such as Ce, La and Y) is
contained by an amount of not less than 0.0005% in each. On the
other hand, when either content exceeds 0.005% and gets higher,
non-metallic inclusions in steel increase to thereby deteriorate
the resistance to sulfide stress corrosion cracking. By reason of
the above, the content range when either element is contained is
set to 0.0005-0.005%.
EXAMPLES
[0061] In order to confirm the effect of seamless steel tubes for
use in oil well according to the present invention, following tests
were carried out and the results were evaluated.
[0062] The billet of 225 mm in diameter was made for each of 22
grades of test steel whose chemical compositions are shown in
Tables 1 and 2. Then, the billet was heated at 1250.degree. C., and
subjected to Mannesmann Mandrel Tube Making Process to obtain a
seamless steel tube of 244.5 mm in outside diameter with 13.8 mm in
thickness. Thereafter, the seamless steel tube was subjected to
quenching and tempering treatment and the tensile test specimen was
sampled therefrom. TABLE-US-00001 TABLE 1 Inventive or Chemical
Compositions (mass %, Residuals of Fe and Impurities) Test No.
Steel No. Comparative C Si Mn P S Cr Mo Al N 1 1 Inventive Steel
0.24 0.25 0.92 0.012 0.002 0.51 0.48 0.010 0.0050 2 2 Inventive
Steel 0.27 0.09 0.81 0.023 0.006 0.60 1.93 0.010 0.0100 3 3
Inventive Steel 0.24 0.35 1.21 0.018 0.009 0.75 0.52 0.010 0.0500 4
4 Inventive Steel 0.15 0.75 1.30 0.018 0.006 1.23 0.31 0.030 0.0020
5 5 Inventive Steel 0.25 0.44 0.63 0.018 0.004 0.06 0.52 0.030
0.0050 6 6 Inventive Steel 0.21 0.35 1.12 0.009 0.006 0.60 0.56
0.030 0.0160 7 7 Inventive Steel 0.24 0.35 0.89 0.018 0.006 0.98
0.52 0.050 0.0012 8 8 Inventive Steel 0.22 0.93 1.95 0.014 0.006
0.60 0.05 0.050 0.0050 9 9 Inventive Steel 0.24 0.36 1.30 0.018
0.006 1.10 0.58 0.050 0.0100 10 10 Inventive Steel 0.31 0.35 0.12
0.018 0.006 0.60 0.72 0.070 0.0010 11 11 Inventive Steel 0.23 0.55
0.71 0.024 0.006 1.47 0.52 0.070 0.0020 12 12 Inventive Steel 0.24
0.31 1.30 0.018 0.006 0.60 0.52 0.070 0.0070 13 13 Comparative
Steel 0.19 0.35 1.30 0.018 0.006 0.60 0.52 0.009* 0.0060 14 14
Comparative Steel 0.22 0.35 1.30 0.023 0.006 0.60 0.74 0.009*
0.0110 15 15 Comparative Steel 0.27 0.35 1.30 0.018 0.004 0.53 0.52
0.009* 0.0560 16 16 Comparative Steel 0.21 0.35 1.30 0.018 0.006
0.60 0.59 0.009* 0.0670 17 17 Comparative Steel 0.24 0.35 1.30
0.016 0.006 0.60 0.97 0.010 0.0600 18 18 Comparative Steel 0.30
0.35 1.30 0.019 0.006 0.60 0.75 0.030 0.0200 19 19 Comparative
Steel 0.26 0.35 1.30 0.018 0.003 1.23 0.23 0.050 0.0120 20 20
Comparative Steel 0.24 0.35 1.30 0.017 0.006 0.89 0.52 0.070 0.0090
21 21 Comparative Steel 0.28 0.35 1.30 0.018 0.005 0.75 0.89 0.038
0.0043 22 22 Comparative Steel 0.24 0.35 1.30 0.018 0.006 0.60 0.52
0.026 0.0046 Note: The symbol * denotes the deviation from the
specified range by the present invention.
[0063] TABLE-US-00002 TABLE 2 Inventive or Chemical Compositions
(mass %, Residuals of Fe and Impurities) Value of Variation Test
No. Steel No. Comparative Ti V Nb B Ca Mg REM Equation (i) of YS 1
1 Inventive Steel -- -- -- -- -- -- -- 0.000050 .smallcircle. 2 2
Inventive Steel -- -- -- -- -- -- -- 0.000100 .smallcircle. 3 3
Inventive Steel -- -- -- -- -- -- -- 0.000500 .smallcircle. 4 4
Inventive Steel -- -- -- -- -- -- -- 0.000060 .smallcircle. 5 5
Inventive Steel -- -- -- -- -- -- -- 0.000150 .smallcircle. 6 6
Inventive Steel 0.002 -- -- 0.0030 -- 0.0030 -- 0.000474
.smallcircle. 7 7 Inventive Steel -- 0.01 0.010 -- -- -- --
0.000014 .smallcircle. 8 8 Inventive Steel -- 0.05 -- -- 0.0003 --
-- 0.000021 .smallcircle. 9 9 Inventive Steel -- -- -- -- 0.0031 --
0.001 0.000500 .smallcircle. 10 10 Inventive Steel 0.008 -- 0.036
0.0003 -- 0.0003 -- 0.000016 .smallcircle. 11 11 Inventive Steel --
-- -- -- -- -- -- 0.000140 .smallcircle. 12 12 Inventive Steel --
-- -- -- -- -- -- 0.000490 .smallcircle. 13 13 Comparative Steel --
-- -- -- -- -- -- 0.000054 x 14 14 Comparative Steel -- -- -- -- --
-- -- 0.000099 x 15 15 Comparative Steel 0.015 -- -- -- -- -- 0.004
0.000491 x 16 16 Comparative Steel -- -- 0.012 -- -- -- --
0.000603* x 17 17 Comparative Steel -- 0.12 -- 0.0008 0.0024 -- --
0.000510* x 18 18 Comparative Steel -- -- -- -- -- 0.0006 --
0.000600* x 19 19 Comparative Steel -- -- -- 0.0020 -- -- 0.004
0.000600* x 20 20 Comparative Steel -- -- -- -- -- -- -- 0.000630*
x 21 21 Comparative Steel 0.010 0.04 -- -- -- -- -- -0.000013* x 22
22 Comparative Steel 0.020 0.10 -- -- -- -- -- -0.000169* x Note:
The symbol * denotes the deviation from the specified range by the
present invention. The definition of symbols .smallcircle. and x is
described in the text of this specification.
[0064] Herein, quenching was performed in such a way that, after
holding 5 min at 950.degree. C. for uniform heat distribution,
water quenching was applied, and tempering was performed by holding
the steel tube thus quenched at 650.degree. C. for 30 min. The
above heat treatment condition is one example, and the quenching
and tempering treatment to be applied for seamless steel tubes
according to the present invention is not limited to the above.
[0065] And, with regard to tensile test, the tensile test specimen
with circular arc strip shape cross-section at its parallel
portion, being specified in 5CT in API Standard, is prepared from
the longitudinal direction of the sampled tube coupon for tensile
test purpose, and then, the tensile test was conducted to measure
yield strength YS (MPa).
[0066] Further, the variation of yield strength YS based on the
measured results as above was assessed to evaluate the stability in
mechanical strength, which was shown in Table 2 also.
[0067] In addition, in Table 2, the variation of yield strength YS
was checked by following method to evaluate by two-step technique.
Namely, 10 tensile tests (N frequency=10) as above were conducted
for each test steel designated by Steel No., and the test result
was checked with the below equation (ii). When the equation was
satisfied, it was judged that the variation of yield strength was
small and satisfactory (.smallcircle.), while, when the equation
was not satisfied, it was judged that the variation of YS was large
and defective (x). Each YS.ltoreq.Average YS-3.times.Standard
Deviation of Each YS (ii)
[0068] Herein, "Average YS" is defined to designate the average YS
in tensile tests across all test steels (22 grades) to be used for
testing, "Each YS" is defined to designate the individual average
YS in tensile tests for the test steel (one grade) of interest, and
"Standard Deviation of Each YS" is defined to designate the
standard deviation for the test steel of interest (one grade).
[0069] Test Nos. 1-12 are Inventive Examples which employed Steel
Nos. 1-12 according the present invention, while Test Nos. 13-22
are Comparative Examples which employed Steel Nos. 13-22 that are
comparative steels.
[0070] In Test Nos. 13-15 that employed Steel Nos. 13-15 of less Al
content, and in Test No. 16 that employed Steel No. 16 that had
less Al content and high value of the concentration product factor
or Al.times.{N-14.times.(Ti/144+V/153)}, each variation of yield
strength YS was large, and the stability in mechanical strength was
poor and unsatisfactory. Also, in Test Nos. 17-22 that employed
Steel Nos. 17-22 where Al content was within the specified range by
the present invention, but the concentration product or
Al.times.{N-14.times.(Ti/144+V/153)} was high, the variation of
yield strength YS in either test was large, and the stability in
mechanical strength was poor and unsatisfactory.
[0071] By contrast, in Test Nos. 1-12 that employed Steel Nos. 1-12
that conform to all conditions specified by the present invention,
the variation of yield strength YS was small, and the stability in
mechanical strength was excellent. Especially, in Test Nos. 6, 7, 8
and 10 that employed Steel Nos. 6, 7, 8 and 10 that contained Ti,
V, Nb or B, much higher mechanical strength as well as the
resistance to sulfide stress corrosion cracking were obtained, due
to enhancement of quench hardenability, while in Test Nos. 6, 8, 9
and 10 that employed steels with one or more of Ca, Mg and REM,
much higher resistance to sulfide stress corrosion cracking were
obtained.
INDUSRIAL APPLICABILITY
[0072] The present invention provides seamless steel tubes for oil
well use having excellent stability in mechanical strength, which
can be produced by efficient means leading up to realization of
energy saving. In particular, even when a process utilizing in-line
heat treatment is applied, the fluctuation of quench hardenability
due to the variation of prior austenite grain size is suppressed to
thereby enable the stable and high mechanical strength to be
obtained. Therefore, seamless steel tubes for oil well use
according to the present invention is highly appreciated as ones
having the stability in mechanical strength which can be produced
under the circumstances where energy savings along with
productivity is mostly concerned, and can be widely used from the
viewpoint of both streamlining of production process and the
expansion of application.
* * * * *