DESCRIPTION
TITLE OF INVENTION
THICK-WALL OIL-WELL STEEL PIPE AND PRODUCTION METHOD
THEREOF
TECHNICAL FIELD
[0001]
The present invention relates to an oil-well steel pipe and a production
method thereof, and more particularly to a thick-wall oil-well steel pipe having a
wall thickness of 40 mm or more, and a production method thereof.
BACKGROUND ART
[0002]
As oil wells and gas wells (hereinafter, oil wells and gas wells are collectively
referred to as "oil wells") become deeper, higher strength is required for oil-well steel
pipes. Conventionally, oil-well steel pipes of 80 ksi grade (yield strength is 80 to
95 ksi, that is, 551 to 654 MPa), and of 95 ksi grade (yield strength is 95 to 110 ksi,
that is, 654 to 758 MPa) have been widely used. However, in recent years, oil-well
steel pipes of 110 ksi grade (yield strength is 110 to 125 ksi, that is, 758 to 862 MPa)
have been started to be used.
[0003]
Many of deep wells contain hydrogen sulfide which has corrosiveness. For
that reason, an oil-well steel pipe for use in deep wells is required to have not only
high strength but also sulfide stress cracking resistance (hereinafter referred to as
sse resistance).
[0004]
Conventionally, as a measure to improve the SSC resistance of an oil-well
steel pipe of95 to 110 ksi classes, there is known a method of cleaning steel or
refining steel structure. In the case ofthe steel proposed in Japanese Patent
Application Publication No. 62-253720 (Patent Literature 1), impurities such as Mn
and P are reduced to increase the level of cleanliness of steel, thereby improving the
SSe resistance of steel. The steel proposed in Japanese Patent Application
Publication No. 59-232220 (Patent Literature 2) is subjected to quenching twice to
refine crystal grains, thereby improving the sse resistance of steel.
[0005]
However, the SSe resistance of steel material significantly deteriorates as the
strength of steel material increases. Therefore, for practical oil-well steel pipes, a
stable production of an oil-well pipe of 120 ksi class (yield strength is 827 MPa or
more) having the sse resistance which can endure the standard condition (1 atm H2S
environment) of the constant load test ofNAeE TM0177 method A has not been
realized yet.
[0006]
Under the background described above, an attempt has been made to use
high-e low alloy steel having a e content of0.35% or more, which has not been put
into practical use, as an oil-well pipe to achieve high strength.
[0007]
The oil-well steel pipe disclosed in Japanese Patent Application Publication
No. 2006-265657 (Patent Literature 3) is produced by subjecting low alloy steel
containing e: 0.30 to 0.60%, er + Mo: 1.5 to 3.0% (Mo is 0.5% or more), and others
to tempering after oil-cooling quenching or austempering. This literature describes
that the above described production method allows to suppress quench cracking
which is likely to occur during quenching of high-e low alloy steel, thereby to obtain
an oil-well steel or oil-well steel pipe, which has excellent sse resistance.
[0008]
The oil-well steel disclosed in Japanese Patent No. 5333700 (Patent Literature
4) contains e: 0.56 to 1.00% and Mo: 0.40 to 1.00%, and exhibits not more than 0.50
deg of a half-peak width of(211) crystal plane obtained by X-ray diffractometry, and
yield strength of 862 MPa or more. This literature describes that SSe resistance is
improved by spheroidizing of grain boundary carbides, and the spheroidizing of
carbides during high temperature tempering is further facilitated by increasing thee
content. Patent Literature 4 also proposes a method of limiting a cooling rate
during quenching, or temporarily stopping cooling during quenching and performing
3
isothermal treatment to hold in a range of more than 100oe to 300°e, in order to
suppress quench cracking attributable to a high-e alloy.
[0009]
The steel for oil-well pipe disclosed in International Application Publication
No. W02013/191131 (Patent Literature 5) contains e: more than 0.35% to 1.00%,
Mo: more than 1.0% to 1 0%, and others in which the product of e content and Mo
content is 0.6 or more. Further in the above described steel for oil-well pipe, the
number of M2e carbide which has a circle equivalent diameter of 1 nm or more, and
has a hexagonal structure is 5 or more per 1 )..tm2, and the half-peak width of the
(211) crystal plane and the e concentration satisfy a specific relationship. In
addition, the above described steel for oil-well pipe has yield strength of 7 58 MPa or
more. In Patent Literature 5, a quenching method similar to that in Patent Literature
4 is adopted.
[001 0]
However, even with the techniques of Patent Literatures 3 to 5, it is difficult
to obtain excellent sse resistance and high strength in a thick-wall oil-well steel pipe,
more specifically in an oil-well steel pipe having a wall thickness of 40 mm or more.
In particular, in a thick-wall oil-well steel pipe, it is difficult to obtain high strength
and reduced variation in strength in the wall-thickness direction.
SUMMARY OF INVENTION
[00 11]
It is an object of the present invention to provide a thick-wall oil-well steel
pipe which has a wall thickness of 40 mm or more, and has excellent sse resistance
and high strength (827 MPa or more), in which variation in strength in the wallthickness
direction is small.
[0012]
A thick-wall oil-well steel pipe according to the present invention has a wall
thickness of 40 mm or more. The thick-wall oil-well steel pipe has a chemical
composition consisting of, in mass%, e: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.10
to 1.0%, P: 0.020% or less, S: 0.0020% or less, sol. AI: 0.005 to 0.1 0%, er: more
than 0.40 to 2.0%, Mo: more than 1.15 to 5.0%, eu: 0.50% or less, Ni: 0.50% or less,
4-
-I
N: 0.007% or less, 0: 0.005% or less, V: 0 to 0.25%, Nb: 0 to 0.1 0%, Ti: 0 to 0.05%,
Zr: 0 to 0.10%, W: 0 to 1.5%, B: 0 to 0.005%, Ca: 0 to 0.003%, Mg: 0 to 0.003%,
and rare earth metals: 0 to 0.003%, with the balance being Fe and impurities.
Further, a number of carbide which has a circle equivalent diameter of 1 00 nm or
more and contains 20 mass% or more of Mo is 2 or less per 100 J..!m 2. Furthermore,
the above described thick-wall oil-well steel pipe has yield strength of 827 MPa or
more, and the difference between a maximum value and a minimum value of the
yield strength in the wall-thickness direction is 45 MPa or less.
[00 13]
A method for producing a thick-wall oil-well steel pipe according to the
present invention includes the steps of: producing a steel pipe having the above
described chemical composition, subjecting the steel pipe to quenching once or
multiple times, wherein a quenching temperature in the quenching of at least once is
925 to 11 00°C, and subjecting the steel pipe to tempering after the quenching.
[0014]
A thick-wall oil-well steel pipe according to the present invention, which has
a wall thickness of 40 mm or more, has excellent SSC resistance and high strength
(827 MPa or more), as well as reduced variation in strength in the wall-thickness
direction.
BRIEF DESCRIPTION OF ORA WINGS
[0015]
[FIG. 1] FIG. 1 illustrates Rockwell hardness (HRC) in a wall-thickness direction of
a thick-wall oil-well steel pipe having a chemical composition shown in Table 1.
[FIG. 2] FIG. 2 illustrates a relationship between a tempering temperature for the
thick-wall oil-well steel pipe having the chemical composition shown in Table 1, and
yield strength in an outer surface portion, a wall-thickness central portion, and an
inner surface portion of the thick-wall oil-well steel pipe.
[FIG. 3] FIG. 3 illustrates Jominy test results of a steel material having the chemical
composition shown in Table 1.
[FIG. 4] FIG. 4 is a transmission type electron microscope (TEM) image of a steel
material subjected to quenching at a quenching temperature of 850°C in FIG. 3.
5
[FIG. 5] FIG. 5 illustrates Jominy test results of a steel material having the chemical
composition shown in Table 2.
[FIG. 6] FIG. 6 illustrates Jominy test results when the number of quenching is
varied using the steel material having the chemical composition shown in Table 1.
DESCRIPTION OF EMBODIMENTS
[0016]
The present inventors have completed the present invention based on the
following findings.
[0017]
There is known a method of increasing Mn and Cr contents to ensure
hardenability. However, increasing the contents of those elements will result in
deterioration of SSC resistance. On the other hand, although C and Mo improve
hardenability as well as Mn and Cr do, they will not deteriorate SSC resistance.
Therefore, suppressing the Mn content to 1.0% or less and the Cr content to 2.0% or
less, and instead making the C content 0.40% or more and the Mo content more than
1.15% will make it possible to improve hardenability while maintaining sse
resistance. Higher hardenability will result in increase in the strength of steel.
[0018]
When the C content is 0.40% or more, carbides in steel are more likely to be
spheroidized. As a result of that, SSC resistance will be improved. Further, it is
possible to increase the strength of steel by precipitation strengthening of carbides.
[00 19]
In the case of an oil-well steel pipe having a normal thickness, adjusting the
chemical composition as describe above will make it possible to improve sse
resistance and hardenability at the same time. However, in an oil-well steel pipe
having a wall thickness of 40 mm or more, it is found that only adjusting the
chemical composition cannot ensure satisfactory hardenability.
[0020]
Under the circumstances, the present inventors have studied this problem.
As a result, the following findings have been obtained.
[0021]
In quenching, if quenching is performed with a carbide containing 20% or
more in mass% ofMo (hereinafter referred to as a Mo carbide) being undissolved,
hardenability will deteriorate. Specifically, when the Mo carbide is undissolved,
hardenability will not be improved since Mo and C are not sufficiently dissolved into
steel. Performing quenching in this state will only induce generation of bainite, and
martensite is not likely to be generated.
[0022]
Accordingly, a quenching temperature is set 925 to 11 00°C in the quenching
of at least once mnong quenching to be performed once or multiple times. In this
case, the Mo carbide will be dissolved sufficiently. As a result of that, hardenability
of steel is significantly improved, yield strength can be made 827 MPa or more, and
variation in yield strength (maximum value- minimum value) in the wall-thickness
direction can be suppressed to 45 MPa or less. Hereinafter, detailed description will
be made on this point.
[0023]
A seamless steel pipe having a wall thickness of 40 mm and having the
chemical composition shown in Table 1 was produced. The produced steel pipe
was heated at a quenching temperature of 900°C. Thereafter, quenching is
performed by applying mist cooling to the outer surface of the steel pipe.
[0024]
[Table 1]
Table 1
C I Si I Mn I p
~
Chemical composition (in mass%, and the balance being Fe and impurities
I s I Sol.Al I Cr I Mo I Cu I Ni I N I 0 I v
o.51 I o.26j 0.44 I o.oo6 I o.ooo6 I o.o31 I o.52 I 1.49 I o.o3 .I o.o2 I o.oo62 I o.ooo8 I o.o88
~
Nb I Ti I Ca
o.o32 I o.oo5 I o.ooo3
[0025]
Rockwell hardness (HRC) in the wall-thickness direction was measured in a
section normal to the axis direction ofthe steel pipe after quenching. Specifically,
Rockwell hardness (HRC) measurement test conforming to JIS 22245 (2011) was
performed in the above described section at 2 mm intervals from the inner surface
toward the outer surface.
[0026]
Measurement results are illustrated in FIG. 1. Referring to FIG. 1, a
reference line L 1 in FIG. I indicates HRCmin calculated from the following Formula
(1) specified by API Specification 5CT.
HRCmin =58 x C + 27 (1)
[0027]
Formula (1) means Rockwell hardness at a lower limit in which the amount of
martensite becomes 90% or more. In Formula (1), C means a C (carbon) content
(mass%) of steel. To ensure SSC resistance required as an oil-well pipe, hardness
after quenching is desirably not less than HRCmin specified by the above described
Formula (1).
[0028]
Referring to FIG. I, Rockwell hardness significantly decreased from the outer
surface toward the inner surface, and Rockwell hardness became less than HRCmin
of Formula (1) in a range from the wall thickness center to the inner surface.
[0029]
This steel pipe was subjected to tempering at various tempering temperatures.
Then, a round bar tensile test specimen having a diameter of 6 mm and a parallel
portion of 40 mm length was fabricated from each of a position of a 6 mm depth
from the outer surface (referred to as an outer surface first position), a wall-thickness
central position, and a position of a 6 mm depth from the inner surface (referred to as
an inner surface first position) of the steel pipe after tempering. Using the
fabricated tensile test specimens, tension test was performed at a normal temperature
(25°C) in the atmosphere to obtain yield strength (ksi).
[0030]
FIG. 2 is a diagram to illustrate the relationship between tempering
temperature (0 C) and yield strength YS. A triangle mark (6) in FIG. 2 indicates
yield strength YS (ksi) at the outer surface first position. A circle mark (0)
indicates yield strength YS (ksi) at the wall-thickness central position. A square
mark (D) indicates yield strength YS (ksi) at the inner surface first position.
[0031]
Referring to FIG. 2, the difference between the maximum value and the
minimum value of yield strength at the outer surface first position, the wall-thickness
central position, and the inner surface first position was large at any of tempering
temperatures. That is, hardness (strength) variation generated during quenching
was not resolved by tempering.
[0032]
Then, to investigate the effect of quenching temperature, Jominy test
conforming to JIS G0561 (2011) was performed using a steel material having the
chemical composition of Table I. FIG. 3 illustrates the Jominy test results.
[0033]
A rhombus (0) mark in FIG. 3 indicates a result at a quenching temperature
of950°C. A triangle (6) mark indicates a result at a quenching temperature of
920°C. A square (D) mark and a circle (0) mark indicate results at quenching
temperatures of900°C and 850°C, respectively. Referring to FIG. 3, the effect of a
quenching temperature on a quenching depth was significant in the case of steel
having a high C content and Mo content. Specifically, when a quenching
temperature was 950°C, Rockwell hardness was more than 60 HRC even at a
distance of30 mm from the water-cooling end, and thus excellent hardenability was
recognized compared with the case in which a quenching temperature was less than
925°C.
[0034]
Here, micro-structure observation ofthe steel material which had low
hardenability and was subjected quenching at a temperature of 850°C, was
performed. FIG. 4 illustrates a micro-structure photographic image (TEM image)
of the steel material subjected to quenching at 850°C. Referring to FIG. 4, there
were a large number of precipitates in the steel. As a result of performing Energy
(b
Dispersive X-ray Spectroscopy (EDX) on the precipitates, it was revealed that most
of the precipitates were undissolved Mo carbides (carbides containing 20 mass% of
Mo).
[0035]
In order to determine whether or not the same tendency was observed in a
high-C steel having a low Mo content, the following test was performed. A steel
material having the chemical composition shown in Table 2 was prepared. The Mo
content of this test specimen was 0.68% and lower than the Mo content in the
chemical composition ofTable 1.
[0036]
I I
[Table 2]
Table 2
~
Chemical composition (in mass%, and the balance being Fe and impurities)
C I Si I Mn I p I s J soLA! I Cr I Mo I Cu I Ni I N I 0 I v Nb I Ti I B I Ca
o.53 I 0.21 I 0.43 I o.oo1 I o.oo1 I o.o29 I o.52 I o.68 I - I o.o2 I o.oo38 I o.ooo9 I o.o88 o.o31 1 o.oo6 1 o.ooo1 I o.ooo2
{2_
[0037]
Jominy test conforming to JIS G0561 (20 11) was performed using the steel
material of Table 2. FIG. 5 illustrates the Jominy test results.
[0038]
A rhombus (0) mark in FIG. 5 indicates a result at a quenching temperature
of950°C. A triangle (.6) mark and a square (D) mark indicate results at quenching
temperatures of920°C and 900°C, respectively. Referring to FIG. 5, in the case of
a low Mo content, there was observed no effect of a quenching temperature on the
quenching depth. That is, it was found that the effect of the quenching temperature
on the quenching depth was a phenomenon peculiar to high-Mo, high-C low alloy
steel having a C content of 0.40% or more and a Mo content of more than 1.15%.
[0039]
Further, using the steel material of Table I, the effect of a quenching
temperature when quenching was performed multiple times was investigated.
[0040]
A black triangle (A) mark in FIG. 6 illustrates a Jominy test result when
quenching was performed two times, in which the quenching temperature was 950°C
and the soaking time was 30 minutes in the first quenching, and the quenching
temperature was 900°C and the soaking time was 30 minutes in the second
quenching. A white triangle (.6) mark in FIG. 6 illustrates a Jominy test result
when only the first quenching was performed in which the quenching temperature
was 950°C and the soaking time was 30 minutes. Referring to FIG. 6, it is seen that
when quenching is performed two times, hardenability will be improved ifthe
quenching temperature in the quenching of at least once is 925 oc or more.
[0041]
As described so far, if quenching is performed at a quenching temperature of
925°C or more (hereinafter, referred to as high temperature quenching) for high-Mo,
high-C low alloy steel, an undissolved Mo carbide will sufficiently dissolve, and
thereby hardenability will be significantly improved. As a result of that, it is
possible to obtain yield strength of 827 MPa or more and reduce the variation in
yield strength in the wall-thickness direction. Further, it is also possible to improve
SSC resistance since Cr content and Mn content can be suppressed.
13
[0042]
A thick-wall oil-well steel pipe according to the present embodiment, which
has been completed based on the above described findings, has a wall thickness of 40
mm or more. The thick-wall oil-well steel pipe has a chemical composition
consisting of, in mass%, C: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.0%, P:
0.020% or less, S: 0.0020% or less, sol. AI: 0.005 to 0.1 0%, Cr: more than 0.40 to
2.0%, Mo: more than 1.15 to 5.0%, Cu: 0.50% or less, Ni: 0.50% or less, N: 0.007%
or less, 0: 0.005% or less, V: 0 to 0.25%, Nb: 0 to 0.1 0%, Ti: 0 to 0.05%, Zr: 0 to
0.1 0%, W: 0 to 1.5%, B: 0 to 0.005%, Ca: 0 to 0.003%, Mg: 0 to 0.003%, and rare
earth metals: 0 to 0.003%, with the balance being Fe and impurities. Further, the
number of carbide which has a circle equivalent diameter of 100 nm or more and
contains 20 mass% or more of Mo is 2 or less per 100 ~-tm2 • Further, the above
described thick-wall oil-well steel pipe has yield strength of 827 MPa or more, in
which the difference between a maximum value and a minimum value of the yield
strength in the wall-thickness direction is 45 MPa or less.
[0043]
A method for producing a thick-wall oil-well steel pipe according to the
present embodiment includes the steps of: producing a steel pipe having the above
described chemical composition, subjecting the steel pipe to quenching once or
multiple times, wherein a quenching temperature in the quenching of at least once is
925 to II 00°C, and subjecting the steel pipe to tempering after the quenching.
[0044]
Hereinafter, the thick-wall oil-well steel pipe according to the present
embodiment and the production method thereofwill be described in detail.
Regarding chemical composition,"%" means "mass%."
[0045]
[Chemical composition]
The chemical composition of a low-alloy oil-well steel pipe according to the
present embodiment contains the following elements.
[0046]
C: 0.40 to 0.65%
The carbon (C) content of a low-alloy oil-well steel pipe according to the
present embodiment is higher than those of conventional low-alloy oil-well steel
pipes. e improves hardenability and increases strength of steel. A higher e
content further facilitates spheroidizing of carbides during tempering, thereby
improving sse resistance. Further, e combines with Mo or V to form carbides,
thereby improving temper softening resistance. Dispersion of carbides will result in
further increase in strength of steel. If the e content is too low, these effects cannot
be obtained. On the other hand, if the e content is too high, the toughness of steel
deteriorates so that quench cracking becomes more likely to occur. Therefore, the
e content is 0.40 to 0.65%. The lower limit ofthe e content is preferably 0.45%,
more preferably 0.48%, and further more preferably 0.51 %. The upper limit of e
content is preferably 0.60%, and more preferably 0.57%.
[0047]
Si: 0.05 to 0.50%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot
be obtained. On the other hand, if the Si content is too high, sse resistance will
deteriorate. Therefore, the Si content is 0.05 to 0.50%. The lower limit of the Si
content is preferably 0.1 0%, and more preferably 0.15%. The upper limit ofthe Si
content is preferably 0.40%, and more preferably 0.35%.
[0048]
Mn: 0.10 to 1.0%
Manganese (Mn) deoxidizes steel. Further, Mn improves hardenability of
steel. If the Mn content is too low, these effects cannot be obtained. On the other
hand, if the Mn content is too high, Mn, along with impurity elements such as
phosphorus (P) and sulfur (S), segregates at grain boundaries. In this case, the sse
resistance and toughness of steel will deteriorate. Therefore, the Mn content is 0.10
to 1.0%. The lower limit of the Mn content is preferably 0.20%, and more
preferably 0.30%. The upper limit of the Mn content is preferably 0.80%, and more
preferably 0.60%.
[0049]
P: 0.020% or less
Phosphorous (P) is an impurity. P segregates at grain boundaries, thereby
deteriorating the SSC resistance of steel. Therefore, the P content is 0.020% or less.
The P content is preferably 0.015% or less, and more preferably 0.012% or less.
The P content is preferably as low as possible.
[0050]
S: 0.0020% or less
Sulfur (S) is an impurity. S segregates at grain boundaries, thereby
deteriorating the SSC resistance of steel. Therefore, the S content is 0.0020% or
less. The S content is preferably 0.0015% or less, and more preferably 0.0010% or
less. The S content is preferably as low as possible.
[0051]
Sol. AI: 0.005 to 0.10%
Aluminum (AI) deoxidizes steel. If the AI content is too low, this effect
cannot be obtained and the SSC resistance of steel deteriorates. On the other hand,
if the AI content is too high, oxides are formed, thereby deteriorating the SSC
resistance of steel. Therefore, the AI content is 0.005 to 0.1 0%. The lower limit
ofthe AI content is preferably 0.010%, and more preferably 0.015%. The upper
limit of the AI content is preferably 0.08%, and more preferably 0.05%. The term
"AI" content as used herein means the content of "acid-soluble AI," that is "sol. AI."
[0052]
Cr: more than 0.40 to 2.0%
Chromium (Cr) improves hardenability of steel and increases its strength. If
the Cr content is too low, the aforementioned effect cannot be obtained. On the
other hand, if the Cr content is too high, the toughness and SSC resistance of steel
will deteriorate. Therefore, the Cr content is more than 0.40 to 2.0%. The lower
limit of the Cr content is preferably 0.48%, more preferably 0.50%, and further more
preferably 0.51 %. The upper limit of the Cr content is preferably 1.25%, and more
preferably 1.15%.
[0053]
Mo: more than 1.15 to 5.0%
Molybdenum (Mo) significantly improves hardenability when the quenching
temperature is 925°C or more. Further, Mo produces fine carbides, thereby
-;/-
improving temper softening resistance of steel. As a result, Mo contributes to the
improvement of SSC resistance through high temperature tempering. If the Mo
content is too low, this effect cannot be obtained. On the other hand, if the Mo
content is too high, the aforementioned effect will be saturated. Therefore, the Mo
content is more than 1.15 to 5.0%. The lower limit ofthe Mo content is preferably
1.20%, and more preferably 1.25%. The upper limit of the Mo content is preferably
4.2%, and more preferably 3.5%.
[0054]
Cu: 0.50% or less
Copper (Cu) is an impurity. Cu deteriorates SSC resistance. Therefore, the
Cu content is 0.50% or less. The Cu content is preferably 0. I 0% or less, and more
preferably 0.02% or less.
[0055]
Ni: 0.50% or less
Nickel (Ni) is an impurity. Ni deteriorates SSC resistance. Therefore, the
Ni content is 0.50% or less. The Ni content is preferably 0.10% or less, and more
preferably 0.02% or less.
[0056]
N: 0.007% or less
Nitrogen (N) is an impurity. N forms nitrides, thereby destabilizing the SSC
resistance of steel. Therefore, theN content is 0.007% or less. TheN content is
preferably 0.005% or less. TheN content is preferably as low as possible.
[0057]
0: 0.005% or less
Oxygen (0) is an impurity. 0 produces coarse oxides, thereby deteriorating
the sse resistance of steel. Therefore, the 0 content is 0.005% or less. The 0
content is preferably 0.002% or less. The 0 content is preferably as low as possible.
[0058]
The balance ofthe chemical composition ofthe thick-wall oil-well steel pipe
ofthe present embodiment consists of Fe and impurities. Impurities as used herein
refer to elements which are mixed in from ores and scraps which are used as the raw
material of steel, or from environments of the production process, etc.
[0059]
The chemical composition of the thick-wall oil-well steel pipe of the present
embodiment may further contain one or more kinds selected from the group
consisting ofV, Nb, Ti, Zr, and Win place of a part of Fe.
[0060]
V: 0 to 0.25%
Vanadium (V) is an optional element, and may not be contained. If
contained, V forms carbides, thereby improving the temper softening resistance of
steel. As a result, V contributes to the improvement of SSe resistance through high
temperature tempering. However, if the V content is too high, the toughness of
steel deteriorates. Therefore, the V content is 0 to 0.25%. The lower limit of the
V content is preferably 0.07%. The upper limit of the V content is preferably
0.20%, and more preferably 0. I 5%.
[006 I]
Nb: 0 to 0. I 0%
Niobium (Nb) is an optional element, and may not be contained. If
contained, Nb combines with e and/or N to form carbides, nitrides, or carbonitrides.
These precipitates (carbides, nitrides, and carbonitrides) refine the sub-structure of
steel through a pinning effect, thereby improving the sse resistance of steel.
However, if the Nb content is too high, nitrides are excessively produced, thereby
destabilizing the SSe resistance of steel. Therefore, the Nb content is 0 to 0.1 0%.
The lower limit of the Nb content is preferably 0.01 %, and more preferably 0.013%.
The upper limit of the Nb content is preferably 0.07%, and more preferably 0.04%.
[0062]
Ti: 0 to 0.05%
Titanium (Ti) is an optional element, and may not be contained. If contained,
Ti forms nitrides, and refines crystal grains through a pinning effect. However, if
the Ti content is too high, Ti nitrides become coarser, thereby deteriorating the SSe
resistance of steel. Therefore, the Ti content is 0 to 0.05%. The lower limit of the
Ti content is preferably 0.005%, and more preferably 0.008%. The upper limit of
the Ti content is preferably 0.02%, and more preferably 0.015%.
[0063]
Zr: 0 to 0.10%
Zirconium (Zr) is an optional element, and may not be contained. As in the
case of Ti, Zr forms nitrides, and refines crystal grains through a pinning effect.
However, if the Zr content is too high, Zr nitrides become coarser, thereby
deteriorating the sse resistance of steel. Therefore, the Zr content is 0 to 0.1 0%.
The lower limit of the Zr content is preferably 0.005%, and more preferably 0.008%.
The upper limit of the Zr content is preferably 0.02%, and more preferably 0.015%.
[0064]
W: 0 to 1.5%
Tungsten (W) is an optional element, and may not be contained. If
contained, W forms carbides, thereby improving the temper softening resistance of
steel. As a result, W contributes to the improvement of sse resistance through
high temperature tempering. Further, as in the case of Mo, W improves
hardenability of steel, and particularly, significantly improves hardenability when the
quenching temperature is 925oe or more. Thus, W supplements the effect of Mo.
However, if theW content is too high, its effect will be saturated. Further, W is
expensive. Therefore, theW content is 0 to 1.5%. The lower limit of theW
content is preferably 0.05%, and more preferably 0.1 %. The upper limit of theW
content is preferably 1.3%, and more preferably 1.0%.
[0065]
The thick-wall oil-well steel pipe according to the present embodiment may
further contain B in place of a part ofF e.
[0066]
B: 0 to 0.005%
Boron (B) is an optional element, and may not be contained. If contained, B
improves hardenability. This effect appears even if a small amount of B which is
not immobilized by N exists in steel. However, if the B content is too high, M23
(eB)6 is formed at grain boundaries, thereby deteriorating the sse resistance of steel.
Therefore, the B content is 0 to 0.005%. The lower limit of the B content is
preferably 0.0005%. The upper limit ofthe B content is preferably 0.003%, and
more preferably 0.002%.
[0067]
The chemical composition of the thick-wall oil-well steel pipe according to
the present embodiment may further contain one or more kinds selected from the
group consisting of Ca, Mg, and rare earth metal (REM) in place of a part of Fe.
Any of these elements improves the shape of sulfide, thereby improving the SSC
resistance of steel.
Ca: 0 to 0.003%
Mg: 0 to 0.003%
Rare Earth Metal (REM): 0 to 0.003%
Calcium (Ca), Magnesium (Mg), and Rare Earth Metal (REM) are all optional
elements, and may not be contained. If contained, these elements combine with S
in steel to form sulfides. As a result of this, the shapes of sulfides are improved,
thus improving the sse resistance of steel.
[0068]
Further, REM combines with P in steel, and suppresses the segregation of P at
grain boundaries. As a result, deterioration of the SSC resistance of steel
attributable to the segregation ofP will be suppressed.
[0069]
However, if the contents of these elements are too high, not only are these
effects saturated, but also inclusions increase. Therefore, the Ca content is 0 to
0.003%, the Mg content is 0 to 0.003%, and REM is 0 to 0.003%. The lower limit
of theCa content is preferably 0.0005%. The lower limit of the Mg content is
preferably 0.0005%. The lower limit of the REM content is preferably 0.0005%.
[0070]
The term REM as used herein is a general term including 15 elements of
lanthanoide series, and Sc andY. The expression, REM is contained, means that
one or more kinds of these elements are contained. The REM content means a total
content of these elements.
[0071]
[Coarse carbides in steel and yield strength]
In the steel of a thick-wall oil-well steel pipe according to the present
embodiment, the number of carbide which has a circle equivalent diameter of 100
nm or more and contains 20 mass% or more of Mo is 2 or less per I 00 ~-tm2 •
Hereinafter, a carbide having a circle equivalent diameter of 100 nm or more is
referred to as a "coarse carbide." A carbide containing 20 mass% or more ofMo is
referred to as a "Mo carbide." Here, the content ofMo in a carbide refers to a Mo
content with the total amount of metal elements being 1 00 mass%. The total
amount of metal elements excludes carbon (C) and nitrogen (N). A Mo carbide
having a circle equivalent diameter of 100 nm or more is referred to as a "coarse Mo
carbide." The circle equivalent diameter means a diameter of the circle which is
obtained by converting the area of the above described carbide into a circle having
the same area.
[0072]
As described above, in a thick-wall oil-well steel pipe of the present
embodiment, as a result of performing "high temperature quenching" in which the
quenching temperature is 925°C or more, the number of undissolved coarse Mo
carbide is decreased and more Mo and C dissolve into steel. As a result of that, Mo
and C improve hardenability, and thus high strength can be obtained. Further, by
increasing the dissolved amount ofMo and C, the variation in strength in the wallthickness
direction is reduced. If the number N of coarse Mo carbide is 2 or less
per 100 )..tm2
, the yield strength will become 827 MPa or more, and the difference
between a maximum value and a minimum value of yield strength in the wallthickness
direction (hereinafter, referred to as yield strength difference ~ YS) will
become 45 MPa or less in a thick-wall oil-well steel pipe having a wall thickness of
40 mm or more.
[0073]
The number of coarse Mo carbide is measured by the following method. A
sample for microstructure observation is sampled from any position in a wallthickness
central portion. A replica film is sampled for the sample. The sampling
of the replica film can be performed at the following conditions. First, an
observation face of the sample is subjected to mirror polishing. Next, the polished
observation face is eroded by soaking in a 3% Nita! for 10 seconds at normal
temperature. After that, carbon shadowing is performed to form replica film on the
observation face. The sample of which the replica film is formed on the surface is
soaked in a 5% Nita! for 10 seconds at normal temperature to separate the replica
film from the sample by eroding an interface between the replica film and the sample.
After being washed in ethanol solution, the replica film is skimmed from the ethanol
solution with sheet mesh. The replica film is dried and observed. Using a
transmission type electron microscope (TEM) of a magnification of 10000,
photographic images of 10 visual fields are produced. The area of each visual field
is made 10 f..Lm x 10 f..Lm = 100 f..Lm2
•
[0074]
In each visual field, a Mo carbide among carbides is determined.
Specifically, Energy Dispersive X-ray Spectroscopy (EDX) is performed for the
carbides in each visual field. From this result, the content of each metal element
(including Mo) in carbides is measured. Among the carbides, one containing 20
mass% or more of Mo, with the total amount of metal elements being 1 00% is
regarded as a Mo carbide. The total amount of metal elements excludes C and N.
[0075]
A circle equivalent diameter of each determined Mo carbide is measured. A
general-purpose image processing application (lmageJ 1.47v) is used for the
measurement. A Mo carbide whose measured circle equivalent diameter is 1 00 nm
or more is determined as a coarse Mo carbide.
[0076]
The number of coarse Mo carbide in each visual field is counted. An
average number of coarse Mo carbide in 10 visual fields is defined as a coarse Mocarbide
number N (per 100 f..Lm 2
).
[0077]
Note that yield strength and yield strength difference ~ YS are measured by
the following method. A round bar tensile test specimen having a diameter of 6
mm and a parallel portion of 40 mm length is fabricated in a position of a 6 mm
depth from the outer surface (an outer surface first position), a wall-thickness central
position, and a position of a 6 mm depth from the inner surface (an inner surface first
position) of a section normal to the axial direction of the oil-well steel pipe. The
longitudinal direction ofthe specimen is parallel with the axial direction of the steel
pipe. With use of the specimen, tension test is performed at a normal temperature
(25°C) in the atmospheric pressure to obtain yield strength YS at each position. In
a thick-wall oil-well steel pipe of the present embodiment, the yield strength YS is
827 MPa or more at any position, as described above. Further, the difference
between the maximum value and the minimum value of yield strength YS at the
above described three positions is defined as yield strength difference L1 YS (MPa).
In a thick-wall oil-well steel pipe according to the present embodiment, the yield
strength difference L1 YS is 45 MPa or less, as described above.
[0078]
Note that the upper limit of the yield strength is not particularly limited.
However, in the case of the above described chemical composition, the upper limit of
the yield strength is preferably 930 MPa.
[0079]
[Production method]
An example of production method of the above described thick-wall oil-well
steel pipe will be described. In this example, description will be made on a
production method of a seamless steel pipe. The production method of a seamless
steel pipe includes a pipe-making step, a quenching step, and a tempering step.
[0080]
[Pipe-making step]
Steel having the above described chemical composition is melted and refined
in a well-known method. Next, molten steel is formed into a continuously cast
material by a continuous casting process. Examples of the continuously cast
material include a slab, a bloom, and a billet. Alternatively, molten steel may be
formed into an ingot by an ingot-making process.
[0081]
A slab, a bloom, or an ingot is subjected to hot working to form a round billet.
A round billet may be formed by hot rolling or hot forging.
[0082]
The billet is subjected to hot working to produce a hollow shell. First, the
billet is heated in a heating furnace. The billet withdrawn from the heating furnace
is subjected to hot working to produce a hollow shell (seamless steel pipe). For
example, a Mannesmann process is performed as the hot working to produce a
hollow shell. In this case, a round billet is piercing-rolled by a piercing machine.
The piercing-rolled round billet is further hot rolled by a mandrel mill, a reducer, and
a sizing mill, etc. to form a hollow shell. The hollow shell may be produced from a
billet by another hot working method. For example, in the case of a short thick-wall
oil-well steel pipe such as a coupling, the hollow shell may be produced by forging.
[0083]
By the above described steps, a steel pipe having a wall thickness of 40 mm
or more is produced. Although the upper limit of the wall thickness is not
particularly limited, it is preferably 65 mm or less in the viewpoint of the control of a
cooling rate in the quenching step described later. The outer diameter of the steel
pipe is not particularly limited. The outer diameter of the steel pipe is, for example,
250 to 500 mm.
[0084]
The steel pipe produced by hot working may be air cooled (as-rolled). The
steel pipe produced by hot working.may also be subjected to direct quenching after
hot pipe-making without being cooled to a normal temperature, or may be subjected
to quenching after supplementary heating (reheating) is performed after hot pipemaking.
However, when performing direct quenching or quenching after
supplementary heating (so-called in-line quenching), it is preferable that cooling be
stopped in the midway of quenching, or slow cooling be performed for the purpose
of suppressing quench cracking.
[0085]
When direct quenching is performed after hot pipe-making, or quenching is
performed after performing supplementary heating after hot pipe-making, it is
preferable that stress removing annealing (SR treatment) be performed after
quenching and before heat treatment in the next step for the purpose of removing of
residual stress. Hereinafter, quenching step will be described in detail.
[0086]
[Quenching step]
The hollow shell after hot working is subjected to quenching. Quenching
may be performed multiple times. However, high temperature quenching
(quenching at a quenching temperature of925 to 11 00°C) shown next is performed
at least once.
[0087]
In the high temperature quenching, soaking is performed with the quenching
temperature being 925 to 11 00°C. If the quenching temperature is less than 925°e,
an undissolved Mo carbide wiii not dissolve sufficiently. As a result, the number N
of coarse Mo carbide becomes more than 2 per 100 j..tm2
• In such a case, the yield
strength of a thick-wail oil-weil steel pipe may become less than 827 MPa, and the
yield strength difference f1 YS in the wail-thickness direction may exceed 45 MPa.
On the other hand, when the quenching temperature exceeds 11 oooe, the sse
resistance deteriorates since y grains become significantly coarse. If the quenching
temperature in the high temperature quenching is 925 to 11 oooe, a Mo carbide
dissolves sufficiently, and the number N of coarse Mo carbide will become 2 or less
per 100 j..tm2
• As a result, hardenability is significantly improved. As a result, the
yield strength of a thick-wall oil-well steel pipe after tempering wiii become 827
MPa or more, and the yield strength difference f1 YS in the wail-thickness direction
wiii become 45 MPa or less. The lower limit of the quenching temperature in the
high temperature quenching is preferably 930°e, more preferably 940°e, and further
preferably 950°e. The upper limit of the quenching temperature is preferably
1050°e.
[0088]
The soaking time in the high temperature quenching is preferably 15 minutes
or more. If the soaking time is 15 minutes or more, a Mo carbide becomes more
likely to dissolve. The lower limit of the soaking time is preferably 20 minutes.
The upper limit of the soaking time is preferably 90 minutes. Even when the
heating temperature is I000°e or more, if the soaking time is 90 minutes or less,
coarsening ofy grains is suppressed and sse resistance is further improved.
However, even if the soaking time exceeds 90 minutes, a certain level of sse
resistance can be obtained.
[0089]
When quenching is performed multiple times, the first quenching is preferably
a high temperature quenching. In this case, a Mo carbide dissolves sufficiently by
the first high temperature quenching. As a result, even if the quenching temperature
in quenching of the foilowing stage is a low temperature less than 925°e, high
hardenability can be obtained. As a result, it is possible to further increase the yield
strength.
[0090]
Further, in the cooling in the final quenching when performing quenching
once or multiple times, it is preferable that the cooling rate be 0.5 to 5°C/sec in a
temperature range of 500 to 1 00°C at a position where the cooling rate becomes
minimum (hereinafter, referred to as a slowest cooling point) among positions in the
wall-thickness direction. When the above described cooling rate is less than
0.5°C/sec, the proportion of martensite is likely to become deficient. On the other
hand, when the above described cooling rate is more than 5°C/sec, quench cracking
may occur. When the above described cooling rate is 0.5 to 5°C/sec, the proportion
of martensite in steel sufficiently increases, resulting in increase in the yield strength.
The cooling means is not particularly limited. For example, mist water cooling may
be performed for the outer surface or both the outer and inner surfaces of the steel
pipe, or the cooling may be performed by using a medium, which has lower heat
transferring capability than that of water, such as oil or polymer.
[0091]
Preferably, forced cooling at the above described cooling rate is started before
the temperature at the slowest cooling position of the steel material becomes 600°C
or Jess. In this case, the yield strength is more likely to be increased.
[0092]
[Hardness (HRC) after quenching and before tempering]
When the above described thick-wall oil-well steel pipe is a coupling, as
specified by API Specification 5CT, the Rockwell hardness (HRC) ofthe steel pipe
after quenching and before tempering (that is, as quenched material) is preferably not
less than HRCmin specified by Formula (1) in the whole area ofthe steel pipe.
HRCmin =58 x C + 27 (1)
where "C" in Formula (1) is substituted by a C content (mass%).
[0093]
If the cooling rate in a range of 500 to 1 00°C at the above described slowest
cooling position is less than 0.5°C/sec, Rockwell hardness (HRC) will become Jess
than HRCmin of Formula (1). Ifthe cooling rate is 0.5 to 5°C/sec, Rockwell
-/-
hardness (HRC) will become not less than HRCmin specified by Formula (1). The
lower limit of the above described cooling rate is preferably 1.2°C/sec. The upper
limit of the above described cooling rate is preferably 4.0°C/sec.
[0094]
As described above, quenching may be performed two or more times. In this
case, quenching of at least once may be high temperature quenching. When
quenching is performed multiple times, as described above, it is preferable to
perform SR treatment after quenching and before performing quenching in the next
stage for the purpose of removing residual stress generated by quenching.
[0095]
When the SR treatment is performed, the treatment temperature is 600°C or
less. It is possible to prevent occurrence of delayed cracking after quenching by the
SR treatment. If the treatment temperature exceeds 600°C, prior-austenite grains
after final quenching may become coarse.
[0096]
[Tempering step]
Tempering is performed after the above described quenching is performed.
The tempering temperature is 650°C to Ac1 point. If the tempering temperature is
less than 650°C, spheroidizing of carbides will become insufficient, and sse
resistance will deteriorate. The lower limit of the tempering temperature is
preferably 660°C. The upper limit of the tempering temperature is preferably
700°C. The soaking time of the tempering temperature is preferably 15 to 120
minutes.
Examples
[0097]
Molten steel weighing 180 kg and having the chemical compositions shown in
Table 3 was produced.
[0098]
[Table 3]
Table 3
Mark c
A 0.51
B 0.50
c 0.51
D 0.51
E 0.52
F 0.61
G 0.49
H 0.52
I 0.55
J 0.53
K 0.56
Si Mn
0.24 0.44
0.24 0.44
0.24 0.31
0.24 0.31
0.24 0.29
0.19 0.44
0.20 0.45
0.31 0.62
0.22 0.28
0.19 0.42
0.33 0.35
~
Chemical composition (in mass%, and the balance being Fe and impurities)
p s sol-Al Cr Mo Cu Ni N 0 v
0.009 0.0009 0.031 0.51 1.20 0.02 0.02 0.0046 0.0013 0.10
0.008 0.0008 0.031 1.02 1.50 0.02 0.02 0.0045 0.0014 0.10
0.010 0.0011 0.031 0.51 2.02 - - 0.0047 0.0008 -
0.011 0.0010 0.030 0.52 2.01 - - 0.0051 0.0009 0.10
0.012 0.0009 0.032 1.01 1.49 - - 0.0048 0.0009 0.10
0.010 0.0007 0.033 1.02 1.20 - - 0.0039 0.0010 0.10
0.008 0.0010 0.021 0.65 3.50 - - 0.0025 0.0007 0.06
0.007 0.0007 0.034 0.63 1.76 0.01 0.02 0.0033 0.0012 -
0.009 0.0011 0.043 0.61 1.55 0.01 0.02 0.0029 0.0007 -
0.010 0.0012 0.038 0.64 1.25 0.01 0.02 0.0030 0.0011 -
0.007 0.0013 0.040 0.55 1.59 0.02 0.01 0.0035 0.0009 -
Q_~
Others
Nb Ti Ca -
- 0.005 0.0002 -
- 0.008 0.0003 -
0.030 0.006 0.0010 -
0.030 0.006 0.0014 -
0.030 0.006 0.0005 -
0.013 0.009 0.0003 -
0.027 0.005 0.0004 -
- - - -
- - - B 0.0015
- - - w 0.5
- - - Zr 0.0021
[0099]
Molten steel of each mark was used to produce an ingot. The ingot was hot
rolled to produce a steel plate supposing the use for a thick-wall oil-well steel pipe.
The plate thickness (corresponding to wall thickness) ofthe steel plate of each Test
number was as shown in Table 4.
[01 00]
[Table 4]
Table 4
Test
number
Mark
I A
2 A
3 B
4 B
5 c
6 c
7 D
8 D
9 E
10 E
Plate
thickness
40mm
53 mm
40mm
53 mm
40mm
53 mm
40mm
53 mm
40mm
53 mm
Heat treatment
950°C 30 minutes Mist Q
(Cooling rate 3°C/s)
950°C 30 minutes Mist Q
+580°C I 0 minutes SR
+900°C 30 minutes Mist Q
I (Cooling rate 2°C/s)
950°C 30 minutes Mist Q
(Cooling rate 3°C/s)
950°C 30 minutes Mist Q
+580°C I 0 minutes SR
+900°C 30 minutes Mist Q
(Cooling rate 2°C/s)
950°C 30 minutes Mist Q
+600°C 15 minutes SR
+900°C 30 minutes Mist Q
I (Cooling rate 3°C/s)
970°C 30 minutes Mist Q
+600°C 15 minutes SR
+900°C 30 minutes Mist Q
I (Cooling rate 2°C/s)
980°C 30 minutes Mist Q
+600°C 15 minutes SR
+900°C 30 minutes Mist Q
I (Cooling rate 2°C/s)
I 000°C 30 minutes Mist Q
+600°C 15 minutes SR
+900°C 30 minutes Mist Q
I (Cooling rate 1.5°C/s)
950°C 30 minutes Mist Q
+600°C 15 minutes SR
+900°C 30 minutes Mist Q
ICCooling rate 2°C/s)
950°C 30 minutes Mist Q
+600°C 15 minutes SR
+900°C 30 minutes Mist Q
As-quenched hardness (HRC)
Outer Wall- Inner
surface thickness surface
HRCmin
second central second
position position position
57.8 58.6 58.3 56.6
57 57.5 56.9 56.6
56.9 57 56.6 56.0
57.4 58.9 58.1 56.0
57.3 58 57 56.6
58 59.8 57.3 56.6
59.1 59.2 57.5 56.6
58.1 57.2 57.2 56.6
59.5 60 58 57.2
59.8 60.4 58.3 57.2
(Cooling rate 3°C/s)
950°C 30 minutes Mist Q
11 F 40mm
+600°C 15 minutes SR
62.7 63.2 63.3 62.4
+900°C 30 minutes Mist Q
!(Cooling rate 1.5°C/s)
950°C 30 minutes Mist Q
12 F 53 mm
+600°C 15 minutes SR
62.7 62.8 62.6 62.4
+900°C 30 minutes Mist Q
!(Cooling rate 1.5°C/s)
13 G 40mm
I 050°C 30 minutes Mist Q
60.1 59.6 60 55.4
ICCooling rate 2°C/s)
I 050°C 30 minutes Mist Q
14 G 53 mm
+550°C 15 minutes SR
58.5 57.9 57.5 55.4
+960°C 30 minutes Mist Q
I (Cooling rate 2°C/s)
15 c 40mm
900°C 30 minutes Mist Q
60.5 51.5 52 56.6
!(Cooling rate 3°C/s)
900°C 30 minutes Mist Q
16 c 53 mm
+550°C 15 minutes SR
58.7 50.3 51.3 56.6
+900°C 30 minutes Mist Q
(Cooling rate 3°C/s)
17 H 40mm
950°C 30 minutes Mist Q
59.1 58.5 58.3 57.2
I (Cooling rate.3°C/s)
18 I 45mm
950°C 30 minutes Mist Q
62.0 61.5 61.0 58.9
!(Cooling rate 2.5°C/s)
19 J 45 mm
950°C 30 minutes Mist Q
59.1 58.5 58.3 57.7
(Cooling rate 2.5°C/s)
20 K 53 mm
950°C 30 minutes Mist Q
61.5 61.0 61.0 59.5
lrcooling rate 2°C/s)
[0101]
Heat treatment (quenching and SR treatment) was performed at heat treatment
conditions shown in Table 4 for steel plates of each Test number after hot rolling.
Referring to Table 4, it is indicated that in Test No. 1, quenching by mist cooling
(mist Q) was performed once, the quenching temperature was 950°C, the soaking
time was 30 minutes, and the cooling rate of the steel plate in a temperature range of
500 to 100°C was 3°C/sec (denoted as "Cooling rate 3°C/sec" in Table 4).
[01 02]
It is indicated that in Test No. 2, quenching by mist cooling was performed in
the quenching of the first time, in which the quenching temperature was 950°C, and
the soaking time was 30 minutes. It is indicated that, thereafter, SR treatment
(denoted by "SR" in Table 4) was performed, in which the heat treatment
temperature was 580°C and the soaking time was 10 minutes. It means that,
thereafter, quenching by mist cooling of the second time was performed, in which the
quenching temperature was 900°C, the soaking time was 30 minutes, and the cooling
rate was 2°C/sec. Note that in the quenching by mist cooling, mist water was
sprayed onto only one ofthe surfaces (2 surfaces) of the steel plate. Then, the
surface onto which mist water had been sprayed was supposed to be the outer surface
of the steel pipe, and the surface on the other side was supposed to be the inner
surface ofthe steel pipe.
[0103]
The cooling rates shown in Table 4 are each an average cooling rate in a range
of 500 to 100°C at the slowest cooling position of the steel plate of each Test number.
[0 104]
After the above described heat treatment was performed, tempering was
performed. In tempering of each Test number, the tempering temperature was 680
to 720°C, and the soaking time was·l 0 to 120 minutes.
[0 1 05]
[Rockwell hardiness measurement test after quenching and before tempering]
Rockwell hardness was measured as shown below for the steel plate (as
quenched material) of each Test number after the above described heat treatment
(after the final quenching). Rockwell hardness (HRC) test conforming to JIS Z2245
(20 11) was performed in a position of a 1.0 mm depth from the outer surface (the
surface onto which mist water had been sprayed) (hereinafter referred to as an "outer
surface second position"), a plate thickness central position corresponding to the
wall-thickness center (wall-thickness central position), and a position of a 1.0 mm
depth from the inner surface (the surface opposite to the surface onto which mist
water had been sprayed) (hereinafter referred to as an "inner surface second
position") ofthe steel plate. Specifically, Rockwell hardness (HRC) of arbitrary
three locations was determined at each of the outer surface second position, the wallthickness
central position, and the inner surface second position, and an average
thereof was defined as Rockwall hardness (HRC) of each position (the outer surface
second position, the wall-thickness central position, and the inner surface second
position).
[01 06]
31
-Y-
[Measurement test of coarse Mo-carbide number N]
The coarse Mo-carbide number N (per 1 00 ~m2 ) was determined by the above
described method for the steel plate of each Test number after tempering.
[01 07]
[Yield strength (YS) and tensile strength (TS) test]
A round bar tensile test specimen having a diameter of 6 mm and a parallel
portion of 40 mm length was fabricated in a position of a 6.0 mm depth from the
outer surface (the surface onto which mist water had been sprayed) (an outer surface
first position), a wall-thickness central position, and a position of a 6.0 mm depth
from the inner surface (the surface opposite to the surface onto which mist water had
been sprayed) (an inner surface first position) of the steel plate of each Test number
after tempering. The axial direction of the tensile test specimen was parallel with
the rolling direction of the steel plate.
[0 1 08]
Using each round bar test specimen, tension test was performed at a normal
temperature (25°C) in the atmosphere to obtain yield strength YS (MPa) and tensile
strength (TS) at each position. Further, yield strength difference~ YS (MPa), which
is the difference between a maximum value and a minimum value of yield strength
YS (MPa) at each position, was determined.
[01 09]
[SSC resistance test]
A round bar tensile test specimen having a diameter of 6.3 mm and a parallel
portion of25.4 mm length was fabricated from the outer surface first position, the
wall-thickness central position, and the inner surface first position of the steel plate
of each Test number after tempering.
[0 11 0]
Using each test specimen, a constant-load type SSC resistance test
conforming to A method ofNACE-TMO 177 (2005 version) was performed.
Specifically, the test specimen was immersed into NACE-A bath of24°C (partial
pressure of H2S was 1 bar), and the immersed test specimen was subjected to a load
corresponding to 90% of the yield strength obtained by the above described yield
strength test. After elapse of 720 hours, whether or not cracking had occurred in the
test specimen was observed. When no cracking was observed, it was determined
that sse resistance was excellent ("NF" in Table 5), and when cracking was
observed, it was determined that SSe resistance was poor ("F" in Table 5).
[0 111]
[Test results]
Table 5 shows test results.
[0 112]
[Table 5]
Table 5
Test
number
I
2
3
4
5
6
7
8
9
10
II
I2
13
I4
15
16
17
18
19
20
Mark
A
A
B
B
c
c
D
D
E
E
F
F
G
G
c
c
H
I
J
K
Coarse Mo-
Wall carbide thickness numberN (per IOO
jlffi2)
40mm 1.3
53 mm 0.0
40mm 1.6
53 mm 1.3
40mm 1.0
53mm 1.0
40mm 0.0
53 mm 0.0
40mm 1.2
53 mm 1.8
40mm 1.0
53 mm 1.2
40mm 1.0
53mm 1.5
40mm 4.5
53mm 4.0
40mm 1.8
45mm 1.9
45mm 0.5
53 mm 1.5
~
YS
(MPa)
Outer Wall- Inner
surface first thickness surface first ~YS
position pcoesnittrioaln position
890 885 880 10
875 878 870 8
922 920 920 2
893 888 885 8
894 884 869 25
913 910 874 39
9I3 879 875 38
890 887 873 I7
968 965 965 3
898 873 879 25
885 900 873 27
9IO 899 878 32
906 907 905 2
9I2 9I2 911 I
894 854 826 68
89I 838 803 88
850 843 830 20
875 863 850 25
9Il 900 890 2I
888 862 854 34
:34-
TS sse resistance (MPa)
Outer Wall- Inner Outer Wallsurface
first thickness surface first surface first thickness Inner surface
position central position position central first position
position position
977 975 970 NF NF NF
959 962 955 NF NF NF
986 982 985 NF NF NF
965, 958 955 NF NF NF
954 942 937 NF NF NF
970 967 946 NF NF NF
980 950 933 NF NF NF
947 944 943 NF NF NF
1023 IOI6 I020 NF NF NF
947 946 950 NF NF NF
975 976 952 NF NF NF
96I 954 950 NF NF NF
964 975 964 NF NF NF
973 971 973 NF NF NF
954 959 958 NF F F
977 963 924 NF F F
923 9I7 912 NF NF NF
940 948 943 NF NF NF
969 967 970 NF NF NF
975 947 938 NF NF NF
-Y
[0113]
"!\ YS" in Table 5 shows yield strength difference of each Test number.
Referring to Table 5, in Test numbers I to 14 and Test numbers 17 to 20, the
chemical composition was appropriate, and also production conditions (quenching
conditions) were appropriate. As a result, the coarse Mo-carbide number N for Test
numbers 1 to 14 and Test numbers 17 to 20 was 2 or less per 100 f.!m 2
• As a result,
the yield strength was 827 MPa or more at any positions, and the yield strength
difference !\ YS was 45 MPa or less. Further, in the SSe resistance test, no cracking
was observed at any positions (outer face first position, wail-thickness central
position, and inner surface first position), exhibiting exceilent sse resistance. Note
that Rockweil hardness before tempering (HRe, see Table 4) for Test numbers I to
14 and Test numbers 17 to 20 was ail more than HRemin calculated from the above
described Formula (I).
[0 114]
On the other hand, the chemical compositions of Test numbers 15 and 16
were both appropriate. However, the quenching temperatures in the quenching
were both less than 925°e. As a result, the coarse Mo-carbide number N was 2 or
more per 100 f.!m2 for both Test numbers 15 and 16. As a result, the yield strength
at the inner surface first position was less than 827 MPa. Further, the yield strength
difference !\ YS exceeded 45 MPa. Furthermore, SSe was confirmed at the wailthickness
central position and the inner surface first position.
[0 115]
Embodiments ofthe present invention have been described. However, the
above described embodiments are merely examples to practice the present invention.
Therefore, the present invention will not be limited to the above described
embodiments and can be practiced by appropriately modifYing the above described
embodiments within the range not departing from the spirit of the present invention.

We claim:
1. A thick-wall oil-well steel pipe characterized by having a wall thickness of 40
mm or more, and having a chemical composition consisting of, in mass%,
C: 0.40 to 0.65%,
Si: 0.05 to 0.50%,
Mn: 0.10 to 1.0%,
P: 0.020% or less,
S: 0.0020% or less,
sol. AI: 0.005 to 0.10%,
Cr: more than 0.40 to 2.0%,
Mo: more than 1.15 to 5.0%,
Cu: 0.50% or less,
Ni: 0.50% or less,
N: 0.007% or less,
0: 0.005% or less,
V: 0 to 0.25%,
Nb: 0 to 0.10%,
Ti: 0 to 0.05%,
Zr: 0 to 0.10%,
W: 0 to 1.5%,
B: 0 to 0.005%,
Ca: 0 to 0.003%,
Mg: 0 to 0.003%, and
rare earth metal: 0 to 0.003%, with the balance being Fe and impurities,
wherein
the number of carbide which has a circle equivalent diameter of 1 00 nm or
more and contains 20 mass% or more ofMo is 2 or less per 100 J-Lm2, and wherein
the thick-wall oil-well steel pipe has yield strength of 827 MPa or more, and a
difference between a maximum value and a minimum value of the yield strength in a
wall-thickness direction is 45 MPa or less.
3G
2. A method for producing a thick-wall oil-well steel pipe, characterized by
comprising the steps of:
producing a steel pipe having the chemical composition according to claim 1,
subjecting the steel pipe to quenching once or multiple times, wherein a
quenching temperature in the quenching of at least once is 925 to 11 00°C, and
subjecting the steel pipe to tempering after the quenching.