TECHNICAL FIELD
[0001] The present invention relates to a steel material and a method for producing the
steel material, and more particularly relates to a steel material suitable for use
in a sour environment, and a method for producing the steel material.
BACKGROUND ART
[0002] Due to the deepening of oil wells and gas wells (hereunder, oil wells and gas wells
are collectively referred to as "oil wells"), there is a demand to enhance the strength
of oil-well steel pipes. Specifically, 80 ksi grade (yield strength is 80 to 95 ksi,
that is, 551 to 655 MPa) and 95 ksi grade (yield strength is 95 to 110 ksi, that is,
655 to 758 MPa) oil-well steel pipes are being widely utilized, and recently requests
are also starting to be made for 110 ksi grade (yield strength is 110 to 125 ksi,
that is, 758 to 862 MPa), 125 ksi grade (yield strength is 125 ksi to 140 ksi, that
is, 862 to 965 MPa) and 140 ksi grade (yield strength is 140 ksi to 155 ksi, that
is, 965 to 1069 MPa) oil-well steel pipes.
[0003] Most deep wells are in a sour environment containing corrosive hydrogen sulfide.
Oil-well steel pipes for use in such sour environments are required to have not only
high strength, but to also have sulfide stress cracking resistance (hereunder, referred
to as "SSC resistance").
[0004] Technology for enhancing the SSC resistance of steel materials as typified by oil-well
steel pipes is disclosed in Japanese Patent Application Publication No.
62-253720 (Patent Literature 1), Japanese Patent Application Publication No.
59-232220 (Patent Literature 2), Japanese Patent Application Publication No.
6-322478 (Patent Literature 3), Japanese Patent Application Publication No.
8-311551 (Patent Literature 4), Japanese Patent Application Publication No.
2000-256783 (Patent Literature 5), Japanese Patent Application Publication No.
2000-297344 (Patent Literature 6), Japanese Patent Application Publication No.
2005-350754 (Patent Literature 7), National Publication of International Patent Application No.
2012-519238 (Patent Literature 8) and Japanese Patent Application Publication No.
2012-26030 (Patent Literature 9).
[0005] Patent Literature 1 proposes a method for improving the SSC resistance of steel for
oil wells by reducing impurities such as Mn and P. Patent Literature 2 proposes a
method for improving the SSC resistance of steel by performing quenching twice to
refine the grains.
[0006] Patent Literature 3 proposes a method for improving the SSC resistance of a 125 ksi
grade steel material by refining the steel microstructure by a heat treatment using
induction heating. Patent Literature 4 proposes a method for improving the SSC resistance
of steel pipes of 110 to 140 ksi grade by enhancing the hardenability of the steel
by utilizing a direct quenching process and also increasing the tempering temperature.
[0007] Patent Literature 5 and Patent Literature 6 each propose a method for improving the
SSC resistance of a steel for low-alloy oil country tubular goods of 110 to 140 ksi
grade by controlling the shapes of carbides. Patent Literature 7 proposes a method
for improving the SSC resistance of steel material of 125 ksi (862 MPa) grade or higher
by controlling the dislocation density and the hydrogen diffusion coefficient to desired
values. Patent Literature 8 proposes a method for improving the SSC resistance of
steel of 125 ksi (862 MPa) grade by subjecting a low-alloy steel containing 0.3 to
0.5% of C to quenching multiple times. Patent Literature 9 proposes a method for controlling
the shapes or number of carbides by employing a tempering process composed of a two-stage
heat treatment. More specifically, in Patent Literature 9, a method is proposed that
enhances the SSC resistance of 125 ksi (862 MPa) grade steel by suppressing the number
density of large M
3C particles or M
2C particles.
CITATION LIST
PATENT LITERATURE
[0008]
Patent Literature 1: Japanese Patent Application Publication No. 62-253720
Patent Literature 2: Japanese Patent Application Publication No. 59-232220
Patent Literature 3: Japanese Patent Application Publication No. 6-322478
Patent Literature 4: Japanese Patent Application Publication No. 8-311551
Patent Literature 5: Japanese Patent Application Publication No. 2000-256783
Patent Literature 6: Japanese Patent Application Publication No. 2000-297344
Patent Literature 7: Japanese Patent Application Publication No. 2005-350754
Patent Literature 8: National Publication of International Patent Application No.
2012-519238
Patent Literature 9: Japanese Patent Application Publication No. 2012-26030
[0009] However, even if the techniques disclosed in the aforementioned Patent Literatures
1 to 9 are applied, in the case of steel material (for example, oil-well steel pipes)
having a yield strength of 140 ksi grade (965 to 1069 MPa), excellent SSC resistance
cannot be stably obtained in some cases.
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0010] An objective of the present disclosure is to provide a steel material that has a
yield strength within a range of 965 to 1069 MPa (140 to 155 ksi; 140 ksi grade) and
that also has excellent SSC resistance.
SOLUTION TO PROBLEM
[0011] A steel material according to the present disclosure contains a chemical composition
consisting of, in mass%, C: more than 0.50 to 0.80%, Si: 0.05 to 1.00%, Mn: 0.05 to
1.00%, P: 0.025% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%,
Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: 0.002 to 0.010%,
O: 0.0100% or less, V: 0 to 0.30%, Nb: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%,
Zr: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Ni: 0 to 0.50% and Cu: 0 to 0.50%,
with the balance being Fe and impurities. The steel material according to the present
disclosure also contains an amount of dissolved C within a range of 0.010 to 0.060
mass%. The steel material according to the present disclosure also has a yield strength
within a range of 965 to 1069 MPa, and a yield ratio of the steel material is 90%
or more.
[0012] A method for producing a steel material according to the present disclosure includes
a preparation process, a quenching process and a tempering process. In the preparation
process, an intermediate steel material containing the aforementioned chemical composition
is prepared. In the quenching process, after the preparation process, the intermediate
steel material that is at a temperature in a range of 800 to 1000°C is cooled at a
cooling rate of 50°C/min or more. In the tempering process, the intermediate steel
material after the quenching is held for 10 to 90 minutes at a temperature in a range
of 660°C to an A
c1 point, and thereafter is cooled from 600°C to 200°C at an average cooling rate of
5 to 300°C/sec.
ADVANTAGEOUS EFFECTS OF INVENTION
[0013] The steel material according to the present disclosure has a yield strength within
a range of 965 to 1069 MPa (140 ksi grade), and also has excellent SSC resistance.
BRIEF DESCRIPTION OF DRAWINGS
[0014]
[FIG. 1] FIG. 1 is a view illustrating the relation between the amount of dissolved
C and a fracture toughness value Kissc.
[FIG. 2A] FIG. 2A shows a side view and a cross-sectional view of a DCB test specimen
that is used in a DCB test in the examples.
[FIG. 2B] FIG. 2B is a perspective view of a wedge that is used in the DCB test in
the examples.
DESCRIPTION OF EMBODIMENTS
[0015] The present inventors conducted investigations and studies regarding a method for
obtaining both a yield strength in a range of 965 to 1069 MPa (140 ksi grade) and
SSC resistance in a steel material that it is assumed will be used in a sour environment,
and obtained the following findings.
[0016] If the dislocation density in the steel material is increased, the yield strength
of the steel material will increase. However, there is a possibility that dislocations
will occlude hydrogen. Therefore, if the dislocation density of the steel material
increases, there is a possibility that the amount of hydrogen that the steel material
occludes will also increase. If the hydrogen concentration in the steel material increases
as a result of increasing the dislocation density, even if high strength is obtained,
the SSC resistance of the steel material will decrease. Accordingly, at first glance
it seems that, in order to obtain both a high strength of 140 ksi grade (965 to 1069
MPa) and SSC resistance, utilizing the dislocation density to enhance the strength
is not preferable.
[0017] However, the present inventors discovered that by adjusting the amount of dissolved
C in a steel material, excellent SSC resistance can also be obtained while at the
same time raising the yield strength to 140 ksi grade (965 to 1069 MPa) by utilizing
the dislocation density. Although the reason is not certain, it is considered that
the reason may be as follows.
[0018] Dislocations include mobile dislocations and sessile dislocations, and it is considered
that dissolved C in a steel material immobilizes mobile dislocations to thereby form
sessile dislocations. When mobile dislocations are immobilized by dissolved C, the
disappearance of dislocations can be inhibited, and thus a decrease in the dislocation
density can be suppressed. In this case, the yield strength of the steel material
can be maintained.
[0019] In addition, it is considered that the sessile dislocations that are formed by dissolved
C reduce the amount of hydrogen that is occluded in the steel material more than mobile
dislocations. Therefore, it is considered that by increasing the density of sessile
dislocations that are formed by dissolved C, the amount of hydrogen that is occluded
in the steel material is reduced. As a result, the SSC resistance of the steel material
can be increased. It is considered that because of this mechanism, excellent SSC resistance
is obtained even when the steel material has high strength of 140 ksi grade.
[0020] As described above, the present inventors considered that by appropriately adjusting
the amount of dissolved C in a steel material, the SSC resistance of the steel material
can be increased while maintaining a yield strength of 140 ksi grade. Therefore, using
a steel material containing chemical composition consisting of, in mass%, C: more
than 0.50 to 0.80%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0100%
or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%,
B: 0.0001 to 0.0050%, N: 0.002 to 0.010%, O: 0.0100% or less, V: 0 to 0.30%, Nb: 0
to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 0.50%, W:
0 to 0.50%, Ni: 0 to 0.50% and Cu: 0 to 0.50%, with the balance being Fe and impurities,
the present inventors investigated the relation between the amount of dissolved C,
the yield strength, and a fracture toughness value Kissc that is an index of SSC resistance.
[Relation between amount of dissolved C and SSC resistance]
[0021] FIG. 1 is a view illustrating the relation between the amount of dissolved C and
a fracture toughness value Kissc for respective test numbers of the examples. FIG.
1 was obtained by the following method. FIG. 1 was created using the amount of dissolved
C (mass%) and the fracture toughness value Kissc (MPa √m) obtained with respect to
steel materials for which, among the steel materials of the examples that are described
later, conditions other than the amount of dissolved C satisfied the range of the
present embodiment.
[0022] The yield strength YS of each of the steel materials shown in FIG. 1 was within the
range of 965 to 1069 MPa (140 ksi grade). Adjustment of the yield strength YS was
performed by adjusting the tempering temperature. Further, with respect to the SSC
resistance, if the fracture toughness value Kissc that is an index of SSC resistance
was 30.0 MPa√m or more, it was determined that the SSC resistance was good.
[0023] Referring to FIG. 1, in a steel material in which the conditions of the aforementioned
chemical composition are satisfied, when the amount of dissolved C was 0.010 mass%
or more, the fracture toughness value Kissc became 30.0 MPa√m or more, indicating
excellent SSC resistance. On the other hand, in a steel material in which the conditions
of the aforementioned chemical composition are satisfied, when the amount of dissolved
C was more than 0.060 mass%, the fracture toughness value K
1SSC was less than 30.0 MPa√m. In other words, it was clarified that when the amount of
dissolved C is too high, conversely, the SSC resistance decreases.
[0024] The reason the SSC resistance decreases when the amount of dissolved C is too high
as described above has not been clarified. However, with respect to the range of the
chemical composition and yield strength YS of the present embodiment, excellent SSC
resistance can be obtained if the amount of dissolved C is made 0.060 mass% or less.
[0025] Therefore, by adjusting the chemical composition and tempering conditions to obtain
a yield strength YS within a range of 965 to 1069 MPa (140 ksi grade) and also making
the amount of dissolved C 0.010 to 0.060 mass%, the fracture toughness value K
1SSC becomes 30.0 MPa√m or more and excellent SSC resistance can be obtained.
[0026] Accordingly, in the present embodiment, the amount of dissolved C of the steel material
is set within the range of 0.010 to 0.060 mass%.
[0027] Note that, in order to appropriately control the amount of dissolved C and inhibit
the occurrence of mobile dislocations, the microstructure of the steel is made a microstructure
that is principally composed of tempered martensite and tempered bainite. The term
"principally composed of tempered martensite and tempered bainite" means that the
total volume ratio of tempered martensite and tempered bainite is 90% or more. When
the microstructure of the steel is principally composed of tempered martensite and
tempered bainite, in the steel material according to the present embodiment, the yield
strength YS is in a range of 965 to 1069 MPa (140 ksi grade), and a yield ratio YR
(ratio of the yield strength YS to the tensile strength TS; in other words, yield
ratio YR (%) = yield strength YS/tensile strength TS) is 90% or more.
[0028] A steel material according to the present embodiment that was completed based on
the above findings contains a chemical composition consisting of, in mass%, C: more
than 0.50 to 0.80%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0100%
or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%,
B: 0.0001 to 0.0050%, N: 0.002 to 0.010%, O: 0.0100% or less, V: 0 to 0.30%, Nb: 0
to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, Co: 0 to 0.50%, W:
0 to 0.50%, Ni: 0 to 0.50% and Cu: 0 to 0.50%, with the balance being Fe and impurities.
The steel material according to the present embodiment further contains an amount
of dissolved C within a range of 0.010 to 0.060 mass%. Further, in the steel material
according to the present embodiment, the yield strength is within a range of 965 to
1069 MPa, and the yield ratio is 90% or more.
[0029] In the present description, although not particularly limited, the steel material
is, for example, a steel pipe or a steel plate.
[0030] The aforementioned chemical composition may contain one or more types of element
selected from the group consisting of V: 0.01 to 0.30% and Nb: 0.002 to 0.100%.
[0031] The aforementioned chemical composition may contain one or more types of element
selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%
and Zr: 0.0001 to 0.0100%.
[0032] The aforementioned chemical composition may contain one or more types of element
selected from the group consisting of Co: 0.02 to 0.50% and W: 0.02 to 0.50%.
[0033] The aforementioned chemical composition may contain one or more types of element
selected from the group consisting of Ni: 0.02 to 0.50% and Cu: 0.01 to 0.50%.
[0034] The aforementioned steel material may be an oil-well steel pipe.
[0035] In the present description, the oil-well steel pipe may be a steel pipe that is used
for a line pipe or may be a steel pipe used for oil country tubular goods (OCTG).
The oil-well steel pipe may be a seamless steel pipe, or may be a welded steel pipe.
The oil country tubular goods are, for example, steel pipes that are used as casing
pipes or tubing pipes.
[0036] Preferably, an oil-well steel pipe according to the present embodiment is a seamless
steel pipe. If the oil-well steel pipe according to the present embodiment is a seamless
steel pipe, even if the wall thickness is 15 mm or more, the oil-well steel pipe will
have a yield strength within a range of 965 to 1069 MPa (140 ksi grade) and will also
have excellent SSC resistance.
[0037] The term "excellent SSC resistance" mentioned above means, specifically, that a value
of Kissc (MPa√m) is 30.0 MPa√m or more in a DCB test performed in accordance with
"Method D" described in NACE TM0177-2005 using an autoclave in which a solution obtained
by mixing a degassed 5% saline solution and 4g/L of sodium acetate and adjusting to
pH 3.5 using hydrochloric acid, and a gaseous mixture consisting of 10% H
2S gas and 90% CO
2 gas at a total pressure of 1 atm were sealed.
[0038] Further, the term "amount of dissolved C" mentioned above means the difference between
the amount of C (mass%) in carbides in the steel material and the C content of the
chemical composition of the steel material. The amount of C in carbides in the steel
material is determined by Formula (1) to Formula (5) using an Fe concentration <Fe>a,
a Cr concentration <Cr>a, an Mn concentration <Mn>a, an Mo concentration <Mo>a, a
V concentration <V>a and an Nb concentration <Nb>a in carbides (cementite and MC-type
carbides) obtained as residue when extraction residue analysis is performed on the
steel material, and an Fe concentration <Fe>b, a Cr concentration <Cr>b, an Mn concentration
<Mn>b and an Mo concentration <Mo>b in cementite obtained by performing point analysis
by EDS with respect to cementite identified by performing TEM observation of a replica
film obtained by an extraction replica method.
Note that, in the present description, the term "cementite" means carbides containing
an Fe content of 50 mass% or more.
[0039] A method for producing a steel material according to the present embodiment includes
a preparation process, a quenching process and a tempering process. In the preparation
process, an intermediate steel material containing the aforementioned chemical composition
is prepared. In the quenching process, after the preparation process, the intermediate
steel material that is at a temperature in a range of 800 to 1000°C is cooled at a
cooling rate of 50°C/min or more. In the tempering process, the intermediate steel
material after quenching is held at a temperature in a range of 660°C to the A
c1 point for 10 to 90 minutes, and thereafter the intermediate steel material is cooled
at an average cooling rate of 5 to 300°C/sec with respect to cooling from 600°C to
200°C.
[0040] In the present description, the term "intermediate steel material" refers to a hollow
shell in a case where the end product is a steel pipe, and refers to a plate-shaped
steel material in a case where the end product is a steel plate.
[0041] The preparation process of the aforementioned production method may include a starting
material preparation process of preparing a starting material containing the aforementioned
chemical composition, and a hot working process of subjecting the starting material
to hot working to produce an intermediate steel material.
[0042] Hereunder, the steel material according to the present embodiment is described in
detail. The symbol "%" in relation to an element means "mass percent" unless specifically
stated otherwise.
[Chemical Composition]
[0043] The chemical composition of the steel material according to the present embodiment
contains the following elements.
C: more than 0.50 to 0.80%
[0044] Carbon (C) enhances the hardenability and increases the strength of the steel material.
C also promotes spheroidization of carbides during tempering in the production process,
and increases the SSC resistance of the steel material. If the carbides are dispersed,
the strength of the steel material increases further. These effects will not be obtained
if the C content is too low. On the other hand, if the C content is too high, the
toughness of the steel material will decrease and quench cracking is liable to occur.
Therefore, the C content is within the range of more than 0.50 to 0.80%. A preferable
lower limit of the C content is 0.51%. A preferable upper limit of the C content is
0.70%, and more preferably is 0.62%.
Si: 0.05 to 1.00%
[0045] Silicon (Si) deoxidizes the steel. If the Si content is too low, this effect is not
obtained. On the other hand, if the Si content is too high, the SSC resistance of
the steel material decreases. Therefore, the Si content is within the range of 0.05
to 1.00%. A preferable lower limit of the Si content is 0.15%, and more preferably
is 0.20%. A preferable upper limit of the Si content is 0.85%, and more preferably
is 0.50%.
Mn: 0.05 to 1.00%
[0046] Manganese (Mn) deoxidizes the steel material. Mn also enhances the hardenability.
If the Mn content is too low, these effects are not obtained. On the other hand, if
the Mn content is too high, Mn segregates at grain boundaries together with impurities
such as P and S. In such a case, the SSC resistance of the steel material will decrease.
Therefore, the Mn content is within a range of 0.05 to 1.00%. A preferable lower limit
of the Mn content is 0.25%, and more preferably is 0.30%. A preferable upper limit
of the Mn content is 0.90%, and more preferably is 0.80%.
P: 0.025% or less
[0047] Phosphorous (P) is an impurity. In other words, the P content is more than 0%. P
segregates at the grain boundaries and decreases the SSC resistance of the steel material.
Therefore, the P content is 0.025% or less. A preferable upper limit of the P content
is 0.020%, and more preferably is 0.015%. Preferably, the P content is as low as possible.
However, if the P content is excessively reduced, the production cost increases significantly.
Therefore, when taking industrial production into consideration, a preferable lower
limit of the P content is 0.0001%, more preferably is 0.0003%, and further preferably
is 0.001%.
S: 0.0100% or less
[0048] Sulfur (S) is an impurity. In other words, the S content is more than 0%. S segregates
at the grain boundaries and decreases the SSC resistance of the steel material. Therefore,
the S content is 0.0100% or less. A preferable upper limit of the S content is 0.0050%,
and more preferably is 0.0030%. Preferably, the S content is as low as possible. However,
if the S content is excessively reduced, the production cost increases significantly.
Therefore, when taking industrial production into consideration, a preferable lower
limit of the S content is 0.0001%, more preferably is 0.0002%, and further preferably
is 0.0003%.
Al: 0.005 to 0.100%
[0049] Aluminum (Al) deoxidizes the steel material. If the Al content is too low, this effect
is not obtained and the SSC resistance of the steel material decreases. On the other
hand, if the Al content is too high, coarse oxide-based inclusions are formed and
the SSC resistance of the steel material decreases. Therefore, the Al content is within
a range of 0.005 to 0.100%. A preferable lower limit of the Al content is 0.015%,
and more preferably is 0.020%. A preferable upper limit of the Al content is 0.080%,
and more preferably is 0.060%. In the present description, the "Al" content means
"acid-soluble Al", that is, the content of "sol. Al".
Cr: 0.20 to 1.50%
[0050] Chromium (Cr) enhances the hardenability of the steel material. Cr also increases
temper softening resistance of the steel material and enables high-temperature tempering.
As a result, the SSC resistance of the steel material increases. If the Cr content
is too low, aforementioned effects are not obtained. On the other hand, if the Cr
content is too high, the toughness and SSC resistance of the steel material decreases.
Therefore, the Cr content is within a range of 0.20 to 1.50%. A preferable lower limit
of the Cr content is 0.25%, and more preferably is 0.30%. A preferable upper limit
of the Cr content is 1.30%.
Mo: 0.25 to 1.50%
[0051] Molybdenum (Mo) enhances the hardenability of the steel material. Mo also forms fine
carbides and increases the temper softening resistance of the steel material. As a
result, Mo increases the SSC resistance of the steel material by high temperature
tempering. If the Mo content is too low, these effects are not obtained. On the other
hand, if the Mo content is too high, the aforementioned effects are saturated. Therefore,
the Mo content is within a range of 0.25 to 1.50%. A preferable lower limit of the
Mo content is 0.50%, and more preferably is 0.65%. A preferable upper limit of the
Mo content is 1.20%, and more preferably is 1.00%.
Ti: 0.002 to 0.050%
[0052] Titanium (Ti) forms nitrides, and refines crystal grains by the pinning effect. As
a result, the strength of the steel material increases. If the Ti content is too low,
this effect is not obtained. On the other hand, if the Ti content is too high, Ti
nitrides coarsen and the SSC resistance of the steel material decreases. Therefore,
the Ti content is within a range of 0.002 to 0.050%. A preferable lower limit of the
Ti content is 0.003%, and more preferably is 0.005%. A preferable upper limit of the
Ti content is 0.030%, and more preferably is 0.020%.
B: 0.0001 to 0.0050%
[0053] Boron (B) dissolves in the steel, enhances the hardenability of the steel material
and increases the steel material strength. This effect is not obtained if the B content
is too low. On the other hand, if the B content is too high, coarse nitrides form
and the SSC resistance of the steel material decreases. Therefore, the B content is
within a range of 0.0001 to 0.0050%. A preferable lower limit of the B content is
0.0003%, and more preferably is 0.0007%. A preferable upper limit of the B content
is 0.0035%, and more preferably is 0.0025%.
N: 0.002 to 0.010%
[0054] Nitrogen (N) is unavoidably contained. N combines with Ti to form fine nitrides and
thereby refines the grains. On the other hand, if the N content is too high, N will
form coarse nitrides and the SSC resistance of the steel material will decrease. Therefore,
the N content is within the range of 0.002 to 0.010%. A preferable upper limit of
the N content is 0.005%, and more preferably is 0.004%.
O: 0.0100% or less
[0055] Oxygen (O) is an impurity. In other words, the O content is more than 0%. O forms
coarse oxides and reduces the corrosion resistance of the steel material. Therefore,
the O content is 0.0100% or less. A preferable upper limit of the O content is 0.0030%,
and more preferably is 0.0020%. Preferably, the O content is as low as possible. However,
if the O content is excessively reduced, the production cost increases significantly.
Therefore, when taking industrial production into consideration, a preferable lower
limit of the O content is 0.0001%, more preferably is 0.0002%, and further preferably
is 0.0003%.
[0056] The balance of the chemical composition of the steel material according to the present
embodiment is Fe and impurities. Here, the term "impurities" refers to elements which,
during industrial production of the steel material, are mixed in from ore or scrap
that is used as a raw material of the steel material, or from the production environment
or the like, and which are allowed within a range that does not adversely affect the
steel material according to the present embodiment.
[Regarding optional elements]
[0057] The chemical composition of the steel material described above may further contain
one or more types of element selected from the group consisting of V and Nb in lieu
of a part of Fe. Each of these elements is an optional element, and increases the
SSC resistance of the steel material.
V: 0 to 0.30%
[0058] Vanadium (V) is an optional element, and need not be contained. In other words, the
V content may be 0%. If contained, V combines with C or N to form carbides, nitrides
or carbo-nitrides and the like (hereinafter, referred to as "carbo-nitrides and the
like"). These carbo-nitrides and the like refine the substructure of the steel material
by the pinning effect, and improve the SSC resistance of the steel. V also forms fine
carbides during tempering. The fine carbides increase the temper softening resistance
of the steel material, and increase the strength of the steel material. In addition,
because V also forms spherical MC-type carbides, V suppresses the formation of acicular
M2C-type carbides and thereby increases the SSC resistance of the steel material.
If even a small amount of V is contained, aforementioned effects are obtained to a
certain extent. However, if the V content is too high, the toughness of the steel
material decreases. Therefore, the V content is within the range of 0 to 0.30%. A
preferable lower limit of the V content is more than 0%, more preferably is 0.01%,
and further preferably is 0.02%. A preferable upper limit of the V content is 0.20%,
more preferably is 0.15%, and further preferably is 0.12%.
Nb: 0 to 0.100%
[0059] Niobium (Nb) is an optional element, and need not be contained. In other words, the
Nb content may be 0%. If contained, Nb forms carbo-nitrides and the like. These carbo-nitrides
and the like refine the substructure of the steel material by the pinning effect,
and increase the SSC resistance of the steel material. In addition, because Nb also
forms spherical MC-type carbides, Nb suppresses the formation of acicular M2C-type
carbides and thereby increases the SSC resistance of the steel material. If even a
small amount of Nb is contained, aforementioned effects are obtained to a certain
extent. However, if the Nb content is too high, carbo-nitrides and the like are excessively
formed and the SSC resistance of the steel material decreases. Therefore, the Nb content
is within the range of 0 to 0.100%. A preferable lower limit of the Nb content is
more than 0%, more preferably is 0.002%, further preferably is 0.003%, and further
preferably is 0.007%. A preferable upper limit of the Nb content is 0.025%, and more
preferably is 0.020%.
[0060] A total of the contents of the aforementioned V and Nb is preferably 0.30% or less,
and further preferably is 0.20% or less.
[0061] The chemical composition of the steel material described above may further contain
one or more types of element selected from the group consisting of Ca, Mg and Zr in
lieu of a part of Fe. Each of these elements is an optional element, and increases
the SSC resistance of the steel material.
Ca: 0 to 0.0100%
[0062] Calcium (Ca) is an optional element, and need not be contained. In other words, the
Ca content may be 0%. If contained, Ca refines sulfides in the steel material and
increases the SSC resistance of the steel material. If even a small amount of Ca is
contained, aforementioned effect is obtained to a certain extent. However, if the
Ca content is too high, oxides in the steel material coarsen and the SSC resistance
of the steel material decreases. Therefore, the Ca content is within the range of
0 to 0.0100%. A preferable lower limit of the Ca content is more than 0%, more preferably
is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0006%. A preferable
upper limit of the Ca content is 0.0025%, and more preferably is 0.0020%.
Mg: 0 to 0.0100%
[0063] Magnesium (Mg) is an optional element, and need not be contained. In other words,
the Mg content may be 0%. If contained, Mg renders S in the steel material harmless
by forming sulfides, and increases the SSC resistance of the steel material. If even
a small amount of Mg is contained, aforementioned effect is obtained to a certain
extent. However, if the Mg content is too high, oxides in the steel material coarsen
and decrease the SSC resistance of the steel material. Therefore, the Mg content is
within the range of 0 to 0.0100%. A preferable lower limit of the Mg content is more
than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably
is 0.0006%, and even further preferably is 0.0010%. A preferable upper limit of the
Mg content is 0.0025%, and more preferably is 0.0020%.
Zr: 0 to 0.0100%
[0064] Zirconium (Zr) is an optional element, and need not be contained. In other words,
the Zr content may be 0%. If contained, Zr refines sulfides in the steel material
and increases the SSC resistance of the steel material. If even a small amount of
Zr is contained, aforementioned effect is obtained to a certain extent. However, if
the Zr content is too high, oxides in the steel material coarsen. Therefore, the Zr
content is within the range of 0 to 0.0100%. A preferable lower limit of the Zr content
is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further
preferably is 0.0006%. A preferable upper limit of the Zr content is 0.0025%, and
more preferably is 0.0020%.
[0065] In a case where two or more types of element selected from the aforementioned group
containing Ca, Mg and Zr are contained in combination, the total of the contents of
these elements is preferably 0.0100% or less, and more preferably is 0.0050% or less.
[0066] The chemical composition of the steel material described above may further contain
one or more types of element selected from the group consisting of Co and W in lieu
of a part of Fe. Each of these elements is an optional element that forms a protective
corrosion coating in a hydrogen sulfide environment and suppresses hydrogen penetration.
By this means, each of these elements increases the SSC resistance of the steel material.
Co: 0 to 0.50%
[0067] Cobalt (Co) is an optional element, and need not be contained. In other words, the
Co content may be 0%. If contained, Co forms a protective corrosion coating in a hydrogen
sulfide environment and suppresses hydrogen penetration. By this means, Co increases
the SSC resistance of the steel material. If even a small amount of Co is contained,
aforementioned effect is obtained to a certain extent. However, if the Co content
is too high, the hardenability of the steel material will decrease, and the steel
material strength will decrease. Therefore, the Co content is within the range of
0 to 0.50%. A preferable lower limit of the Co content is more than 0%, more preferably
is 0.02%, and further preferably is 0.05%. A preferable upper limit of the Co content
is 0.45%, and more preferably is 0.40%.
W: 0 to 0.50%
[0068] Tungsten (W) is an optional element, and need not be contained. In other words, the
W content may be 0%. If contained, W forms a protective corrosion coating in a hydrogen
sulfide environment and suppresses hydrogen penetration. By this means, W increases
the SSC resistance of the steel material. If even a small amount of W is contained,
aforementioned effect is obtained to a certain extent. However, if the W content is
too high, coarse carbides form in the steel material and the SSC resistance of the
steel material decreases. Therefore, the W content is within the range of 0 to 0.50%.
A preferable lower limit of the W content is more than 0%, more preferably is 0.02%,
and further preferably is 0.05%. A preferable upper limit of the W content is 0.45%,
and more preferably is 0.40%.
[0069] The chemical composition of the steel material described above may further contain
one or more types of element selected from the group consisting of Ni and Cu in lieu
of a part of Fe. Each of these elements is an optional element, and increases the
hardenability of the steel.
Ni: 0 to 0.50%
[0070] Nickel (Ni) is an optional element, and need not be contained. In other words, the
Ni content may be 0%. If contained, Ni enhances the hardenability of the steel material
and increases the steel material strength. If even a small amount of Ni is contained,
aforementioned effect is obtained to a certain extent. However, if the Ni content
is too high, the Ni will promote local corrosion, and the SSC resistance of the steel
material will decrease. Therefore, the Ni content is within the range of 0 to 0.50%.
A preferable lower limit of the Ni content is more than 0%, more preferably is 0.02%,
and further preferably is 0.05%. A preferable upper limit of the Ni content is 0.35%,
and more preferably is 0.25%.
Cu: 0 to 0.50%
[0071] Copper (Cu) is an optional element, and need not be contained. In other words, the
Cu content may be 0%. If contained, Cu enhances the hardenability of the steel material
and increases the steel material strength. If even a small amount of Cu is contained,
aforementioned effect is obtained to a certain extent. However, if the Cu content
is too high, the hardenability of the steel material will be too high, and the SSC
resistance of the steel material will decrease. Therefore, the Cu content is within
the range of 0 to 0.50%. A preferable lower limit of the Cu content is more than 0%,
more preferably is 0.01%, further preferably is 0.02%, and further preferably is 0.05%.
A preferable upper limit of the Cu content is 0.35%, and more preferably is 0.25%.
[Amount of dissolved C]
[0072] The steel material according to the present embodiment contains an amount of dissolved
C which is within the range of 0.010 to 0.060 mass%. If the amount of dissolved C
is less than 0.010 mass%, the immobilization of dislocations in the steel material
will be insufficient and excellent SSC resistance of the steel material will not be
obtained. On the other hand, if the amount of dissolved C is more than 0.060 mass%,
conversely, the SSC resistance of the steel material will decrease. Therefore, the
amount of dissolved C is within the range of 0.010 to 0.060 mass%. A preferable lower
limit of the amount of dissolved C is 0.020 mass% and more preferably is 0.030 mass%
[0073] An amount of dissolved C within the aforementioned range is obtained by, for example,
controlling the holding time for tempering and controlling the cooling rate in the
tempering process. The reason is as described hereinafter.
[0074] The amount of dissolved C is highest immediately after quenching. Immediately after
quenching, C is dissolved except for a small amount thereof that precipitated as carbides
during quenching. In the tempering process thereafter, some of the C precipitates
as carbides as a result of being held for tempering. As a result, the amount of dissolved
C decreases toward the thermal equilibrium concentration with respect to the tempering
temperature. If the holding time for tempering is too short, this effect will not
be obtained and the amount of dissolved C will be too high. On the other hand, if
the holding time for tempering is too long, the amount of dissolved C will approach
the aforementioned thermal equilibrium concentration, and will hardly change. Therefore,
in the present embodiment the holding time for tempering is set within the range of
10 to 90 minutes.
[0075] If the cooling rate for cooling after tempering is slow, dissolved C will reprecipitate
while the temperature is decreasing. In the conventional methods for producing steel
material, because cooling after tempering has been performed by allowing the steel
material to cool, the cooling rate has been slow. Consequently, the amount of dissolved
C has been almost 0 mass%. Therefore, in the present embodiment, the cooling rate
after tempering is raised, and a dissolved C amount in the range of 0.010 to 0.060
mass% is obtained.
[0076] The cooling method is, for example, a method that performs forced cooling of the
steel material continuously from the tempering temperature to thereby continuously
decrease the surface temperature of the steel material. Examples of this kind of continuous
cooling treatment include a method that cools the steel material by immersion in a
water bath, and a method that cools the steel material in an accelerated manner by
shower water cooling, mist cooling or forced air cooling.
[0077] The cooling rate after tempering is measured at a region that is most slowly cooled
within a cross-section of the steel material that is tempered (for example, in the
case of forcedly cooling both surfaces, the cooling rate is measured at the center
portion of the steel material thickness). Specifically, in a case where the steel
material is a steel plate, the cooling rate after tempering can be measured by inserting
a sheath-type thermocouple into the center portion of the thickness of the steel plate
and measuring the temperature. In a case where the steel material is a steel pipe,
the cooling rate after tempering can be measured by inserting a sheath-type thermocouple
into the center portion of the wall thickness of the steel pipe and measuring the
temperature. Further, in a case of forcedly cooling only a surface on one side of
the steel material, the surface temperature on the non-forcedly cooled side of the
steel material can be measured by means of a non-contact type infrared thermometer.
[0078] The temperature region from 600°C to 200°C is a temperature region in which diffusion
of C is comparatively fast. Therefore, in the present embodiment, the average cooling
rate in the temperature region from 600°C to 200°C is made 5°C/sec or more.
[0079] On the other hand, if the cooling rate after tempering is too fast, very little
of the C that had dissolved after being held during tempering precipitates. As a result,
in some cases the amount of dissolved C is excessive. In this case, the SSC resistance
of the steel material decreases. Therefore, in the present embodiment, the cooling
rate after tempering is made 300°C/sec or less.
[0080] In this case, the amount of dissolved C can be made to fall within the range of 0.010
to 0.060 mass%. However, the amount of dissolved C in the steel material may be adjusted
to within a range of 0.010 to 0.060 mass% by another method.
[Method for calculating amount of dissolved C]
[0081] The term "amount of dissolved C" means the difference between the amount of C (mass%)
in carbides in the steel material and the C content of the chemical composition of
the steel material. The amount of C in carbides in the steel material is determined
by Formula (1) to Formula (5) using an Fe concentration <Fe>a, a Cr concentration
<Cr>a, an Mn concentration <Mn>a, an Mo concentration <Mo>a, a V concentration <V>a
and an Nb concentration <Nb>a in carbides (cementite and MC-type carbides) obtained
as residue when extraction residue analysis is performed on the steel material, and
an Fe concentration <Fe>b, a Cr concentration <Cr>b, an Mn concentration <Mn>b and
an Mo concentration <Mo>b in cementite obtained by performing point analysis by EDS
with respect to cementite identified by performing TEM observation of a replica film
obtained by an extraction replica method.
Note that, in the present description, the term "cementite" means carbides containing
an Fe content of 50 mass% or more. Hereunder, the method for calculating the amount
of dissolved C is described in detail.
[Determination of C content of steel material]
[0082] In a case where the steel material is a plate material, an analysis sample having
the shape of a machined chip is taken from a center portion of the thickness, and
in a case where the steel material is a pipe material, an analysis sample having the
shape of a machined chip is taken from a center portion of the wall thickness. The
C content (mass%) is analyzed by an oxygen-stream combustion-infrared absorption method.
The resulting value was taken to be the C content (<C>) of the steel material.
[Calculation of C amount that precipitates as carbides (precipitated C amount)]
[0083] The precipitated C amount is calculated by the following procedures 1 to 4. Specifically,
in procedure 1 an extraction residue analysis is performed. In procedure 2, an extraction
replica method using a transmission electron microscope (hereunder, referred to as
"TEM"), and an element concentration analysis (hereunder, referred to as "EDS analysis")
of elements in cementite is performed by energy dispersive X-ray spectrometry (hereunder,
referred to as "EDS"). In procedure 3, the Mo content is adjusted. In procedure 4,
the precipitated C amount is calculated.
[Procedure 1. Determination of residual amounts of Fe, Cr, Mn, Mo, V and Nb by extraction
residue analysis]
[0084] In procedure 1, carbides in the steel material are captured as residue, and the contents
of Fe, Cr, Mn, Mo, V and Nb in the residue are determined. Here, the term "carbides"
is a generic term for cementite (M
3C-type carbides) and MC-type carbides. The specific procedure is as follows. In a
case where the steel material is a plate material, a cylindrical test specimen having
a diameter of 6 mm and a length of 50 mm is extracted from a center portion of the
thickness. In a case where the steel material is a steel pipe, a cylindrical test
specimen having a diameter of 6 mm and a length of 50 mm is extracted from a center
portion of the wall thickness of the steel pipe in a manner so that the center of
the wall thickness becomes the center of the cross-section. The surface of the extracted
test specimen is polished to remove about 50 µm by preliminary electropolishing to
obtain a newly formed surface. The electropolished test specimen is subjected to electrolysis
in an electrolyte solution of 10% acetylacetone + 1% tetra-ammonium + methanol. The
electrolyte solution after electrolysis is passed through a 0.2-µm filter to capture
residue. The obtained residue is subjected to acid decomposition, and the concentrations
of Fe, Cr, Mn, Mo, V and Nb are determined in units of mass percent by ICP (inductively
coupled plasma) optical emission spectrometry. The concentrations are defined as <Fe>a,
<Cr>a, <Mn>a, <Mo>a, <V>a and <Nb>a, respectively.
[Procedure 2. Determination of content of Fe, Cr, Mn and Mo in cementite by extraction
replica method and EDS]
[0085] In procedure 2, the content of each of Fe, Cr, Mn and Mo in cementite is determined.
The specific procedure is as follows. A micro test specimen is cut out from a center
portion of the thickness in a case where the steel material is a plate material, and
is cut out from a center portion of the wall thickness in a case where the steel material
is a steel pipe, and the surface of the micro test specimen is finished by mirror
polishing. The test specimen is immersed for 10 minutes in a 3% nital etching reagent
to etch the surface. The surface thereof is covered with a carbon deposited film.
The test specimen whose surface is covered with the deposited film is immersed in
a 5% nital etching reagent, and held therein for 20 minutes to cause the deposited
film to peel off. The deposited film that peeled off is cleaned with ethanol, and
thereafter is scooped up with a sheet mesh and dried. The deposited film (replica
film) is observed using a TEM, and point analysis by EDS is performed with respect
to 20 particles of cementite. The concentration of each of Fe, Cr, Mn and Mo is determined
in units of mass percent when taking the total of the alloying elements excluding
carbon in the cementite as 100%. The concentrations are determined for 20 particles
of cementite, and the arithmetic average values for the respective elements are defined
as <Fe>b, <Cr>b, <Mn>b and <Mo>b.
[Procedure 3. Adjustment of Mo amount]
[0086] Next, the Mo concentration in the carbides is determined. In this case, Fe, Cr, Mn
and Mo concentrate in cementite. On the other hand, V, Nb and Mo concentrate in MC-type
carbides. In other words, Mo is caused to concentrate in both cementite and MC-type
carbides by tempering. Therefore, the Mo amount is calculated separately for cementite
and for MC-type carbides. Note that, in some cases a part of V also concentrates in
cementite. However, the amount of V that concentrates in cementite is negligibly small
in comparison to the amount of V that concentrates in MC-type carbides. Therefore,
when determining the amount of dissolved C, V is regarded as concentrating only in
MC-type carbides.
[0087] Specifically, the amount of Mo precipitating as cementite (<Mo>c) is calculated by
Formula (1).
[0088] On the other hand, the amount of Mo precipitating as MC-type carbides (<Mo>d) is
calculated in units of mass percent by Formula (2).
[Procedure 4. Calculation of precipitated C amount]
[0089] The precipitated C amount is calculated as the total of the C amount precipitating
as cementite (<C>a) and the C amount precipitating as MC-type carbides (<C>b). <C>a
and <C>b are calculated in units of mass percent by Formula (3) and Formula (4), respectively.
Note that, Formula (3) is a formula that is derived from the fact that the structure
of cementite is a M
3C type structure (M include Fe, Cr, Mn and Mo).
[0090] Thus, the precipitated C amount is <C>a+<C>b.
[Calculation of amount of dissolved C]
[0091] The amount of dissolved C (hereunder, also referred to as "<C>c") is calculated in
units of mass percent by Formula (5) as a difference between the C content (<C>) and
the precipitated C amount of the steel material.
[Micro structure]
[0092] The microstructure of the steel material according to the present embodiment is principally
composed of tempered martensite and tempered bainite. More specifically, the volume
ratio of tempered martensite and/or tempered bainite in the microstructure is 90%
or more. In other words, the total of the volume ratios of tempered martensite and
tempered bainite in the microstructure is 90% or more. The balance of the microstructure
is, for example, retained austenite or the like. If the microstructure of the steel
material containing the aforementioned chemical composition contains tempered martensite
and tempered bainite in an amount equivalent to a total volume ratio of 90% or more,
the yield strength will be within the range of 965 to 1069 MPa (140 ksi grade), and
the yield ratio will be 90% or more.
[0093] In the present embodiment, if the yield strength YS is within the range of 965 to
1069 MPa (140 ksi grade) and the YR is 90% or more, it is assumed that the total of
the volume ratios of tempered martensite and tempered bainite in the microstructure
is 90% or more. Preferably, the microstructure is composed of only tempered martensite
and/or tempered bainite.
[0094] Note that, the following method can be adopted in the case of determining the total
of the volume ratios of tempered martensite and tempered bainite by observation. In
a case where the steel material is a plate material, a small piece having an observation
surface with dimensions of 10 mm in the rolling direction and 10 mm in the plate width
direction is cut out from a center portion of the thickness. In a case where the steel
material is a steel pipe, a small piece having an observation surface with dimensions
of 10 mm in the pipe axis direction and 10 mm in the pipe circumferential direction
is cut out from a center portion of the wall thickness. After polishing the observation
surface to obtain a mirror surface, the small piece is immersed for about 10 seconds
in a nital etching reagent, to reveal the microstructure by etching. The etched observation
surface is observed by means of a secondary electron image obtained using a scanning
electron microscope (SEM), and observation is performed for 10 visual fields. The
area of each visual field is 400 µm
2 (magnification of ×5000). In each visual field, tempered martensite and tempered
bainite are identified based on the contrast. The total of the area fractions of tempered
martensite and tempered bainite that are identified is determined. In the present
embodiment, the arithmetic average value of the totals of the area fractions of tempered
martensite and tempered bainite determined in all visual fields is taken as the volume
ratio of tempered martensite and tempered bainite.
[Shape of steel material]
[0095] The shape of the steel material according to the present embodiment is not particularly
limited. The steel material is, for example, a steel pipe or a steel plate. In a case
where the steel material is an oil-well steel pipe, preferably the steel material
is a seamless steel pipe. In this case, a preferable wall thickness is 9 to 60 mm.
The steel material according to the present embodiment is, in particular, suitable
for use as a heavy-wall oil-well steel pipe. More specifically, even if the steel
material according to the present embodiment is an oil-well steel pipe having a thick
wall of 15 mm or more or, furthermore, 20 mm or more, the steel material exhibits
excellent strength and SSC resistance.
[Yield strength YS and yield ratio YR of steel material]
[0096] The yield strength YS of the steel material according to the present embodiment is
within a range of 965 to 1069 MPa (140 ksi grade), and the yield ratio YR of the steel
material is 90% or more. In the present description, the term "yield strength YS"
means the stress when elongation of 0.65% is obtained in a tensile test. In short,
the strength of the steel material according to the present embodiment is of 140 ksi
grade. Even though the steel material according to the present embodiment has such
high strength, the steel material also has excellent SSC resistance by satisfying
the conditions regarding the chemical composition, amount of dissolved C and microstructure,
which are described above.
[SSC resistance of steel material]
[0097] The SSC resistance of the steel material according to the present embodiment can
be evaluated by a DCB test performed in accordance with "Method D" described in NACE
TM0177-2005. The liquid solution used is obtained by mixing a degassed 5% saline solution
and 4g/L of sodium acetate and adjusting to pH 3.5 using hydrochloric acid. The gas
charged inside the autoclave is a gaseous mixture of 10% H
2S gas and 90% CO
2 gas at a total pressure of 1 atm. Thereafter, a DCB test specimen into which a wedge
was driven is enclosed inside the vessel, and is held for three weeks at 24°C while
agitating the liquid solution and also continuously blowing in the aforementioned
gaseous mixture. The Kissc (MPa√m) value of the steel material according to the present
embodiment determined under the foregoing conditions is 30.0 MPa√m or more.
[Production method]
[0098] The method for producing a steel material according to the present embodiment includes
a preparation process, a quenching process and a tempering process. The preparation
process may include a starting material preparation process and a hot working process.
In the present embodiment, a method for producing an oil-well steel pipe will be described
as one example of a method for producing a steel material. The method for producing
an oil-well steel pipe includes a process of preparing a hollow shell (preparation
process), and a process of subjecting the hollow shell to quenching and tempering
to obtain an oil-well steel pipe (quenching process and tempering process). Each of
these processes is described in detail hereunder.
[Preparation process]
[0099] In the preparation process, an intermediate steel material containing the aforementioned
chemical composition is prepared. The method for producing the intermediate steel
material is not particularly limited as long as the intermediate steel material contains
the aforementioned chemical composition. As used here, the term "intermediate steel
material" refers to a plate-shaped steel material in a case where the end product
is a steel plate, and refers to a hollow shell in a case where the end product is
a steel pipe.
[0100] The preparation process may preferably include a process in which a starting material
is prepared (starting material preparation process), and a process in which the starting
material is subjected to hot working to produce an intermediate steel material (hot
working process). Hereunder, a case in which the preparation process includes the
starting material preparation process and the hot working process is described in
detail.
[Starting material preparation process]
[0101] In the starting material preparation process, a starting material is produced using
molten steel containing the aforementioned chemical composition. Specifically, a cast
piece (a slab, bloom or billet) is produced by a continuous casting process using
the molten steel. An ingot may also be produced by an ingot-making process using the
molten steel. As necessary, the slab, bloom or ingot may be subjected to blooming
to produce a billet. The starting material (a slab, bloom or billet) is produced by
the above described process.
[Hot working process]
[0102] In the hot working process, the starting material that was prepared is subjected
to hot working to produce an intermediate steel material. In a case where the steel
material is a steel pipe, the intermediate steel material corresponds to a hollow
shell. First, the billet is heated in a heating furnace. Although the heating temperature
is not particularly limited, for example, the heating temperature is within a range
of 1100 to 1300°C. The billet that is extracted from the heating furnace is subjected
to hot working to produce a hollow shell (seamless steel pipe). For example, the Mannesmann
process is performed as the hot working to produce the hollow shell. In this case,
a round billet is piercing-rolled using a piercing machine. When performing piercing-rolling,
although the piercing ratio is not particularly limited, the piercing ratio is, for
example, within a range of 1.0 to 4.0. The round billet that underwent piercing-rolling
is further hot-rolled to form a hollow shell using a mandrel mill, a reducer, a sizing
mill or the like. The cumulative reduction of area in the hot working process is,
for example, 20 to 70%.
[0103] A hollow shell may also be produced from the billet by another hot working method.
For example, in the case of a heavy-wall steel material of a short length such as
a coupling, a hollow shell may be produced by forging such as Ehrhardt process. A
hollow shell is produced by the above process. Although not particularly limited,
the wall thickness of the hollow shell is, for example, 9 to 60 mm.
[0104] The hollow shell produced by hot working may be air-cooled (as-rolled). The hollow
shell produced by hot working may be subjected to direct quenching after hot rolling
without being cooled to normal temperature, or may be subjected to quenching after
undergoing supplementary heating (reheating) after hot rolling. However, in the case
of performing direct quenching or quenching after supplementary heating, it is preferable
to stop the cooling midway through the quenching process and conduct slow cooling
for the purpose of suppressing quench cracking.
[0105] In a case where direct quenching is performed after hot rolling, or quenching is
performed after supplementary heating after hot rolling, for the purpose of eliminating
residual stress, a stress relief treatment (SR treatment) may be performed at a time
that is after quenching and before the heat treatment of the next process.
[0106] As described above, an intermediate steel material is prepared in the preparation
process. The intermediate steel material may be produced by the aforementioned preferable
process, or may be an intermediate steel material that was produced by a third party,
or an intermediate steel material that was produced in another factory other than
the factory in which a quenching process and a tempering process that are described
later are performed, or at a different works. The quenching process is described in
detail hereunder.
[Quenching process]
[0107] In the quenching process, the intermediate steel material (hollow shell) that was
prepared is subjected to quenching. In the present description, the term "quenching"
means rapidly cooling the intermediate steel material that is at a temperature not
less than the A
3 point. A preferable quenching temperature is 800 to 1000°C. In a case where direct
quenching is performed after hot working, the quenching temperature corresponds to
the surface temperature of the intermediate steel material that is measured by a thermometer
placed on the exit side of the apparatus that performs the final hot working. Further,
in a case where quenching is performed after supplementary heating is performed after
hot working, the quenching temperature corresponds to the temperature of the furnace
that performs the supplementary heating.
[0108] The quenching method, for example, continuously cools the hollow shell from the quenching
starting temperature, and continuously decreases the surface temperature of the hollow
shell. The method of performing the continuous cooling treatment is not particularly
limited. The method of performing the continuous cooling treatment is, for example,
a method that cools the hollow shell by immersing the hollow shell in a water bath,
or a method that cools the hollow shell in an accelerated manner by shower water cooling
or mist cooling.
[0109] If the cooling rate during quenching is too slow, the microstructure does not become
one that is principally composed of martensite and bainite, and the mechanical property
defined in the present embodiment is not obtained. Therefore, as described above,
in the method for producing the steel material according to the present embodiment,
the intermediate steel material is rapidly cooled during quenching. Specifically,
in the quenching process, the average cooling rate when the surface temperature of
the intermediate steel material (hollow shell) is within the range of 800 to 500°C
during quenching is defined as a cooling rate during quenching CR
800-500. The cooling rate during quenching CR
800-500 is 50°C/min or higher. A preferable lower limit of the cooling rate during quenching
CR
800-500 is 100°C/min, and more preferably is 250°C/min. Although an upper limit of the cooling
rate during quenching CR
800-500 is not particularly defined, for example, the upper limit is 60000°C/min.
[0110] Preferably, quenching is performed after performing heating of the hollow shell
in the austenite zone a plurality of times. In this case, the SSC resistance of the
steel material increases because austenite grains are refined prior to quenching.
Heating in the austenite zone may be repeated a plurality of times by performing quenching
a plurality of times, or heating in the austenite zone may be repeated a plurality
of times by performing normalizing and quenching. Hereunder, the tempering process
will be described in detail.
[Tempering process]
[0111] Tempering is performed after performing the aforementioned quenching. The tempering
temperature is appropriately adjusted in accordance with the chemical composition
of the steel material and the yield strength YS, which is to be obtained. In other
words, with respect to the hollow shell containing the chemical composition of the
present embodiment, the tempering temperature is adjusted so as to adjust the yield
strength YS of the steel material to within a range of 965 to 1069 MPa (140 ksi grade)
and to make the YR of the steel material 90% or more.
[0112] A preferable tempering temperature is in a range from 660°C to the A
c1 point. If the tempering temperature is 660°C or more, carbides are sufficiently spheroidized
and the SSC resistance is further increased.
[0113] If the holding time for tempering (tempering time) is too short, the amount of dissolved
C becomes excessive because precipitation of carbides does not proceed. Even if the
tempering time is overlong, there will be almost no change in the amount of dissolved
C. Therefore, in order to control the amount of dissolved C to be within an appropriate
range, the tempering time is set within a range of 10 to 90 minutes. A preferable
lower limit of the tempering time is 15 minutes. A preferable upper limit of the tempering
time is 70 minutes, and more preferably is 60 minutes. Note that, in a case where
the steel material is a steel pipe, in comparison to other shapes, temperature variations
with respect to the steel pipe are liable to occur during holding for tempering. Therefore,
in a case where the steel material is a steel pipe, the tempering time is preferably
set within a range of 15 to 90 minutes. A person skilled in the art will be sufficiently
capable of making the yield strength YS of the steel material containing the chemical
composition of the present embodiment fall within the range of 965 to less than 1069
MPa by appropriately adjusting the aforementioned holding time at the aforementioned
tempering temperature.
[Regarding rapid cooling after tempering]
[0114] Conventionally, cooling after tempering has not been controlled. However, if the
cooling rate of the steel material after tempering (that is, after being held for
the aforementioned holding time at the aforementioned tempering temperature) is slow,
almost all of the C that had dissolved will reprecipitate while the temperature is
decreasing. In other words, the amount of dissolved C will be approximately 0 mass%.
Therefore, in the present embodiment, the intermediate steel material (hollow shell)
after tempering is rapidly cooled.
[0115] Specifically, in the tempering process, the average cooling rate when the surface
temperature of the intermediate steel material (hollow shell) is within the range
of 600 to 200°C after tempering is defined as a cooling rate after tempering CR
600-200. In the method for producing the steel material according to the present embodiment,
the cooling rate after tempering CR
600-200 is 5°C/sec or higher. By this means, the amount of dissolved C according to the present
embodiment is obtained. The temperature region from 600°C to 200°C is a temperature
region in which diffusion of C is comparatively fast. On the other hand, if the cooling
rate after tempering is too fast, in some cases very little of the C that had dissolved
will precipitate, and the amount of dissolved C will be excessive. In such a case,
the SSC resistance of the steel material decreases. Furthermore, in such a case, the
low-temperature toughness of the steel material may decrease.
[0116] Therefore, the cooling rate after tempering CR
600-200 is within the range of 5 to 300°C/sec. A preferable lower limit of the cooling rate
after tempering CR
600-200 is 10°C/sec, and more preferably is 15°C/sec. A preferable upper limit of the cooling
rate after tempering CR
600-200 is 100°C/sec, and more preferably is 50°C/sec.
[0117] A method for cooling so that the cooling rate after tempering CR
600-200 is within the range of 5 to 300°C/sec is not particularly limited, and a well-known
method can be used. The cooling method, for example, is a method that performs forced
cooling of a hollow shell continuously from the tempering temperature to thereby continuously
decrease the surface temperature of the hollow shell. Examples of this kind of continuous
cooling treatment include a method that cools the hollow shell by immersion in a water
bath, and a method that cools the hollow shell in an accelerated manner by shower
water cooling, mist cooling or forced air cooling. Note that, the cooling rate after
tempering CR
600-200 is measured at a region that is most slowly cooled within a cross-section of the
intermediate steel material that is tempered (for example, in the case of forcedly
cooling both surfaces, the cooling rate is measured at the center portion of the thickness
of the intermediate steel material).
[0118] A method for producing a steel pipe has been described as one example of the aforementioned
production method. However, the steel material according to the present embodiment
may be a steel plate or another shape. An example of a method for producing a steel
plate or a steel material of another shape also includes, for example, a preparation
process, a quenching process and a tempering process, similarly to the production
method described above. However, the aforementioned production method is one example,
and the steel material according to the present embodiment may be produced by another
production method.
EXAMPLES
[0119] Molten steels of a weight of 180 kg containing the chemical compositions shown in
Table 1 were produced.
[0120] Ingots were produced using the aforementioned molten steels. The ingots were hot
rolled to produce steel plates having a thickness of 20 mm.
[0121] After hot rolling, the steel plate of each test number was allowed to cool to bring
the steel plate temperature to normal temperature (25°C).
[0122] After being allowed to cool, the steel plates of each test number were reheated to
bring the steel plate temperature to the quenching temperature (920°C, which is in
the austenite single-phase zone), and were held for 20 minutes. After being held,
the steel plates were immersed in a water bath and quenched. At this time, the cooling
rate during quenching (CR
800-500) was 400°C/min. With respect to Test Number 23, after holding at the quenching temperature,
the steel plate was cooled by immersion in an oil bath. At this time, the average
cooling rate from 800°C to 500°C was 40°C/min.
[0123] After quenching, the steel plates of each test number were subjected to tempering.
In the tempering, the tempering temperature was adjusted so that the steel plates
became 140 ksi grade as specified in the API standards (yield strength of 965 to 1069
MPa). After performing a heat treatment at the respective tempering temperatures,
the steel plates were cooled. For the cooling, controlled cooling by mist water cooling
from both sides of the steel plate was performed. Note that, a type K thermocouple
of a sheath type was inserted into a center portion of the thickness of the steel
plate in advance, and the temperature was measured with respect to tempering and the
cooling thereafter. The tempering temperature (°C) and tempering time (min) and the
cooling rate (CR
600-200) (°C/sec) after tempering are shown in Table 2. Note that, the A
c1 point of the steel material in each of Test Number 1 to Test Number 25 was 750°C,
and the tempering temperature was set so as to be lower than the A
c1 point.
[Table 2]
[0124]
TABLE 2
Test Number |
Tempering Temperature (°C) |
Tempering Time (min) |
Cooling Rate After Tempering (°C/sec) |
YS (MPa) |
TS (MPa) |
YR (%) |
Dissolved C Amount (mass%) |
K1SSC (MPa√m) |
1 |
2 |
3 |
Average Value |
1 |
700 |
15 |
25 |
1025 |
1087 |
94 |
0.043 |
31.4 |
32.0 |
33.9 |
32.4 |
2 |
690 |
15 |
10 |
973 |
1050 |
93 |
0.041 |
35.1 |
32.3 |
32.9 |
33.4 |
3 |
690 |
30 |
10 |
1059 |
1148 |
92 |
0.041 |
30.6 |
31.5 |
31.4 |
31.2 |
4 |
690 |
40 |
5 |
1065 |
1153 |
92 |
0.025 |
30.9 |
31.6 |
31.1 |
31.2 |
5 |
680 |
40 |
10 |
983 |
1059 |
93 |
0.041 |
33.3 |
32.0 |
31.2 |
32.2 |
6 |
680 |
35 |
10 |
988 |
1068 |
93 |
0.046 |
33.2 |
32.6 |
31.7 |
32.5 |
7 |
680 |
35 |
35 |
993 |
1073 |
93 |
0.044 |
32.6 |
34.0 |
35.2 |
33.9 |
8 |
680 |
35 |
35 |
991 |
1071 |
93 |
0.044 |
33.9 |
32.7 |
33.2 |
33.3 |
9 |
680 |
60 |
35 |
980 |
1056 |
93 |
0.048 |
32.9 |
33.6 |
33.8 |
33.4 |
10 |
680 |
30 |
15 |
984 |
1060 |
93 |
0.050 |
34.8 |
36.3 |
36.2 |
35.8 |
11 |
680 |
30 |
10 |
989 |
1062 |
93 |
0.037 |
31.6 |
33.6 |
33.2 |
32.8 |
12 |
680 |
30 |
10 |
986 |
1060 |
93 |
0.045 |
34.4 |
34.2 |
35.9 |
34.8 |
13 |
680 |
60 |
25 |
974 |
1048 |
93 |
0.047 |
35.9 |
34.7 |
34.8 |
35.1 |
14 |
670 |
30 |
2 |
987 |
1065 |
93 |
0.005 |
24.7 |
26.3 |
26.7 |
25.9 |
15 |
670 |
40 |
2 |
980 |
1053 |
93 |
0.003 |
26.2 |
21.5 |
22.7 |
23.5 |
16 |
690 |
5 |
10 |
990 |
1084 |
91 |
0.083 |
22.7 |
23.7 |
25.1 |
23.8 |
17 |
670 |
10 |
600 |
992 |
1078 |
92 |
0.074 |
23.5 |
24.5 |
24.8 |
24.3 |
18 |
660 |
20 |
15 |
973 |
1042 |
93 |
0.048 |
25.8 |
20.6 |
23.0 |
23.1 |
19 |
690 |
20 |
25 |
1056 |
1137 |
93 |
0.044 |
21.0 |
21.4 |
23.7 |
22.0 |
20 |
690 |
30 |
5 |
976 |
1045 |
93 |
0.033 |
20.5 |
19.5 |
21.4 |
20.5 |
21 |
690 |
30 |
25 |
973 |
1044 |
93 |
0.052 |
25.1 |
25.8 |
27.2 |
26.0 |
22 |
690 |
30 |
15 |
974 |
1047 |
93 |
0.046 |
22.4 |
24.2 |
20.8 |
22.5 |
23 |
670 |
10 |
15 |
966 |
1094 |
88 |
0.043 |
24.3 |
21.2 |
21.0 |
22.2 |
24 |
690 |
30 |
2 |
1041 |
1124 |
93 |
0.003 |
24.1 |
25.1 |
23.4 |
24.2 |
25 |
600 |
30 |
10 |
1345 |
1465 |
92 |
0.046 |
10.1 |
5.3 |
4.2 |
6.5 |
[Evaluation results]
[YS and TS tests]
[0125] A tensile test was performed in accordance with ASTM E8. Round bar tensile test specimens
having a diameter of 6.35 mm and a parallel portion length of 35 mm were prepared
from the center parts of the thickness of the steel plates of each test number after
the quenching and tempering described above. The axial direction of each of the tensile
test specimens was parallel to the rolling direction of the steel plate. A tensile
test was performed in the atmosphere at normal temperature (25°C) using each round
bar test specimen, and the yield strength YS (MPa) and tensile strength TS (MPa) were
obtained. Note that, in the present examples, the stress at the time of 0.65% elongation
obtained in the tensile test defined as the YS for each test number. Further, the
largest stress during uniform elongation was taken as the TS. A ratio between the
YS and the TS was adopted as the yield ratio YR (%).
[Microstructure determination test]
[0126] With respect to the microstructures of the steel plates of each test number, apart
from Test Number 23 and 25, because the YS was in a range of 965 to 1069 MPa (140
ksi grade) and the YR was 90% or more, it was determined that the total of the volume
ratios of tempered martensite and tempered bainite was 90% or more. In the case of
Test Number 23, it is considered that ferrite formed.
[Amount of dissolved C measurement test]
[0127] With respect to the steel plates of each test number, the amount of dissolved C (mass%)
was measured and calculated by the measurement method described above. Note that,
the TEM used was JEM-2010 manufactured by JEOL Ltd., the acceleration voltage was
set to 200 kV, and for the EDS point analysis the irradiation current was 2.56 nA,
and measurement was performed for 60 seconds at each point. The observation regions
for the TEM observation were 8 µm × 8 µm, and observation was performed with respect
to an arbitrary 10 visual fields. The residual amounts of each element and the concentrations
of each element in cementite that were used to calculate the amount of dissolved C
were as listed in Table 3.
[Table 3]
[0128]
TABLE 3
Test Number |
Residual Amount (mass%) |
Concentration In Cementite (mass%) |
Dissolved C Amount (mass%) |
Fe |
Mn |
Cr |
Mo |
V |
Nb |
Fe |
Mn |
Cr |
Mo |
1 |
5.6 |
0.25 |
0.27 |
0.23 |
0.088 |
0.010 |
88.8 |
2.7 |
3.9 |
4.6 |
0.043 |
2 |
5.9 |
0.25 |
0.28 |
0.36 |
- |
- |
83.7 |
3.8 |
7.5 |
5.0 |
0.041 |
3 |
5.6 |
0.22 |
0.52 |
0.33 |
0.075 |
- |
77.6 |
2.8 |
14.6 |
5.0 |
0.041 |
4 |
5.7 |
0.21 |
0.47 |
0.40 |
- |
0.011 |
78.0 |
2.9 |
14.4 |
4.7 |
0.025 |
5 |
6.1 |
0.24 |
0.23 |
0.36 |
- |
- |
86.5 |
2.9 |
5.6 |
5.0 |
0.041 |
6 |
6.0 |
0.29 |
0.24 |
0.68 |
- |
- |
84.8 |
3.0 |
3.7 |
8.5 |
0.046 |
7 |
6.5 |
0.23 |
0.44 |
0.44 |
- |
- |
81.2 |
2.5 |
10.7 |
5.6 |
0.044 |
8 |
6.7 |
0.24 |
0.48 |
0.43 |
- |
- |
77.8 |
2.6 |
14.6 |
5.0 |
0.044 |
9 |
7.1 |
0.32 |
0.25 |
0.46 |
- |
- |
83.6 |
2.8 |
8.1 |
5.5 |
0.048 |
10 |
6.9 |
0.27 |
0.33 |
0.44 |
- |
- |
84.2 |
2.9 |
7.7 |
5.2 |
0.050 |
11 |
7.2 |
0.29 |
0.32 |
0.47 |
- |
- |
80.8 |
2.9 |
10.8 |
5.5 |
0.037 |
12 |
7.4 |
0.32 |
0.67 |
0.42 |
- |
- |
72.1 |
2.5 |
20.9 |
4.5 |
0.045 |
13 |
7.8 |
0.35 |
0.36 |
0.46 |
- |
- |
85.6 |
2.6 |
6.8 |
5.0 |
0.047 |
14 |
6.3 |
0.25 |
0.29 |
0.29 |
- |
- |
85.9 |
2.9 |
7.2 |
4.0 |
0.005 |
15 |
6.0 |
0.26 |
0.55 |
0.35 |
- |
- |
78.0 |
2.2 |
15.0 |
4.8 |
0.003 |
16 |
5.4 |
0.20 |
0.33 |
0.24 |
- |
- |
88.7 |
3.0 |
4.5 |
3.8 |
0.083 |
17 |
5.2 |
0.17 |
0.43 |
0.40 |
- |
- |
80.4 |
3.3 |
10.0 |
6.3 |
0.074 |
18 |
6.4 |
0.25 |
0.08 |
0.39 |
- |
- |
90.5 |
2.5 |
2.0 |
5.0 |
0.048 |
19 |
6.0 |
0.23 |
0.68 |
0.11 |
0.058 |
- |
78.0 |
3.2 |
17.0 |
1.8 |
0.044 |
20 |
6.1 |
0.49 |
0.15 |
0.51 |
- |
- |
81.3 |
5.0 |
6.8 |
6.9 |
0.033 |
21 |
6.5 |
0.26 |
0.30 |
0.46 |
- |
- |
84.0 |
3.0 |
7.0 |
6.0 |
0.052 |
22 |
7.2 |
0.26 |
0.28 |
0.38 |
- |
- |
86.0 |
1.5 |
8.0 |
4.5 |
0.046 |
23 |
5.9 |
0.24 |
0.26 |
0.38 |
- |
- |
84.5 |
2.9 |
7.1 |
5.5 |
0.043 |
24 |
7.2 |
0.26 |
0.28 |
0.41 |
0.091 |
- |
86.2 |
3.2 |
5.8 |
4.8 |
0.003 |
25 |
6.1 |
0.28 |
0.31 |
0.42 |
- |
- |
85.8 |
2.8 |
6.3 |
5.1 |
0.046 |
[DCB test]
[0129] With respect to the steel plates of each test number, a DCB test was conducted in
accordance with "Method D" of NACE TM0177-2005, and the SSC resistance was evaluated.
Specifically, three of the DCB test specimen illustrated in FIG. 2A were taken from
a center portion of the thickness of each steel plate. The DCB test specimens were
taken in a manner such that the longitudinal direction of each DCB test specimen was
parallel with the rolling direction. A wedge illustrated in FIG. 2B was further prepared
from each steel plate. A thickness t of the wedge was 3.10 mm.
[0130] The wedge was driven in between the arms of the DCB test specimen. Thereafter, the
DCB test specimen into which the wedge was driven was enclosed in a vessel. A liquid
solution obtained by mixing a degassed 5% saline solution and 4g/L of sodium acetate
and adjusting to pH 3.5 with hydrochloric acid was poured into the vessel so that
a gas portion remained in the vessel. Thereafter, a gaseous mixture consisting of
10% H
2S gas and 90% CO
2 gas was charged at a total pressure of 1 atm inside the autoclave to stir the liquid
phase, and the gaseous mixture was saturated in the liquid solution.
[0131] After sealing the vessel that had undergone the above described process, the vessel
was held for three weeks at 24°C while stirring the liquid solution and also continuously
blowing in the aforementioned gaseous mixture. Thereafter, the DCB test specimens
were taken out from inside the vessel.
[0132] A pin was inserted into a hole formed in the tip of the arms of each DCB test specimen
that was taken out and a notch portion was opened with a tensile testing machine,
and a wedge releasing stress P was measured. In addition, the notch in the DCB test
specimen was released in liquid nitrogen, and a crack propagation length "a" with
respect to crack propagation that occurred during immersion was measured. The crack
propagation length "a" was measured visually using vernier calipers. A fracture toughness
value Kissc (MPa√m) was determined using Formula (6) based on the obtained wedge releasing
stress P and the crack propagation length "a". For each steel, the fracture toughness
value Kissc (MPa√m) of the three DCB test specimens was determined. For each steel,
the arithmetic average of the three fracture toughness values was defined as the fracture
toughness value K
1SSC (MPa√m) of the relevant steel.
In Formula (6), h represents the height (mm) of each arm of the DCB test specimen,
B represents the thickness (mm) of the DCB test specimen, and Bn represents the web
thickness (mm) of the DCB test specimen. These are defined in "Method D" of NACE TM0177-96.
[0133] For each test number, the obtained fracture toughness values K
1SSC are shown in Table 2. If the fracture toughness value K
1SSC that was defined as described above was 30.0 MPa√m or more, it was determined that
the SSC resistance was good. Note that, the clearance between the arms when the wedge
is driven in prior to immersion in the test bath influences the K
1SSC value. Accordingly, actual measurement of the clearance between the arms was performed
in advance using a micrometer, and it was also confirmed that the clearance was within
the range in the API standards.
[Test results]
[0134] The test results are shown in Table 2.
[0135] Referring to Table 1 and Table 2, the chemical compositions of the steel plates of
Test Numbers 1 to 13 were appropriate, the YS was in the range of 965 to 1069 MPa
(140 ksi grade), and the YR was 90% or more. In addition, the amount of dissolved
C was in the range of 0.010 to 0.060 mass%. As a result, the K
1SSC value was 30.0 MPa√m or more and excellent SSC resistance was exhibited.
[0136] On the other hand, for the steel plates of Test Numbers 14 and 15, the cooling rate
after tempering was too slow. Consequently, the amount of dissolved C was less than
0.010 mass%. As a result, the fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0137] For the steel plate of Test Number 16, the tempering time was too short. Consequently,
the amount of dissolved C was more than 0.060 mass%. As a result, the fracture toughness
value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0138] For the steel plate of Test Number 17, the cooling rate after tempering was too
fast. Consequently, the amount of dissolved C was more than 0.060 mass%. As a result,
the fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0139] In the steel plate of Test Number 18, the Cr content was too low. As a result, the
fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0140] For the steel plate of Test Number 19, the Mo content was too low. As a result, the
fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0141] In the steel plate of Test Number 20, the Mn content was too high. As a result, the
fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0142] For the steel plate of Test Number 21, the N content was too high. As a result, the
fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0143] For the steel plate of Test Number 22, the Si content was too high. As a result,
the fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0144] For the steel plate of Test Number 23, the YR was less than 90%. As a result, the
fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited. It is considered
that the reason was that ferrite mixed into the microstructure because the cooling
rate during quenching was slow.
[0145] For the steel plate of Test Number 24, the cooling rate after tempering was too slow.
Consequently, the amount of dissolved C was less than 0.010 mass%. As a result, the
fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0146] For the steel plate of Test Number 25, the tempering temperature was too low. Consequently,
the YS was more than 1069 MPa. As a result, the fracture toughness value K
1SSC was less than 30.0 MPa√m and excellent SSC resistance was not exhibited.
[0147] An embodiment of the present invention has been described above. However, the embodiment
described above is merely an example for implementing the present invention. Accordingly,
the present invention is not limited to the above embodiment, and the above embodiment
can be appropriately modified and performed within a range that does not deviate from
the gist of the present invention.
INDUSTRIAL APPLICABILITY
[0148] The steel material according to the present invention is widely applicable to steel
materials to be utilized in a sour environment, and preferably can be utilized as
a steel material for oil wells that is utilized in an oil well environment, and further
preferably can be utilized as oil-well steel pipes, such as casing, tubing and line
pipes.