DESCRIPTION
METHOD FOR PRODUCING HIGH-STRENGTH STEEL MATERIAL
EXCELLENT IN SULFIDE STRESS CRACKING RESISTANCE
TECHNICAL FIELD
[0001]
The present invention relates to a method for producing a high-strength
steel material excellent in sulfide stress cracking resistance. More
particularly, the present invention relates to a method for producing a high strength
steel material excellent in sulfide stress cracking resistance, which
steel material is especially suitable for an oil-well steel pipe and the like such
as a casing and a tubing for oil well and gas well. Still more particularly, the
q
present invention relates to a low-cost method for producing a low-alloy highstrength
steel material which is excellent in strength and sulfide stress
cracking resistance, and by which the improvement in toughness due to the
refinement of prior-austenite grains can be expected.
BACKGROUND ART
[0002]
As oil wells and gas wells (hereinafter, as a general term of oil wells and
gas wells, referred simply to as "oil wells") become deeper, oil-well steel pipes
(hereinafter, referred to as "oil-well pipes") are required to have higher
strength.
[00031
To meet this requirement, conventionally, oil-well pipes of 80 ksi class,
that is, having a yield stress (hereinafter, abbreviated as "YS") of 551 to 655
MPa (80 to 95 ksi) or oil-well pipes of 95 ksi class, that is, having a YS of 655
to 758 MPa (95 to 110 ksi) have been used widely. Further, recently, oil-well
pipes of 110 ksi class, that is, having a YS of 758 to 862 MPa (110 to 125 ksi),
and further oil-well pipes of 125 ksi class, that is, having a YS of 862 to 965
MPa (125 to 140 ksi) have begun to be used.
[00041
Further, the oil and gas in most of the deep wells having been developed
recently contain corrosive hydrogen sulfide. In such an environment,
hydrogen embrittlement called sulfide stress cracking (hereinafter, referred
also to as "SSC") occurs, and resultantly the oil-well pipe is sometimes broken.
It is widely known that with the increase in strength of steel, the susceptibility
to SSC increases.
[00051
Therefore, in developing high-strength oil-well pipes, not only the
material design of high-strength steel is required to be made but also the steel
is required to have SSC resistance. Especially in developing high-strength
oil-well pipes, the prevention of SSC is the biggest problem. The sulfide
stress cracking is sometimes referred also to as sulfide stress corrosion
cracking ("SSCC").
[0006]
As the method for preventing SSC of low-alloy oil-well pipes, methods of
(1) high purification of steel, (2) mode control of carbides, and (3) refinement of
crystal grains have been known.
[OOO~]
Concerning the high purification of steel, for example, Patent Documents
1 and 2 propose methods for improving the SSC resistance by mean of
restriction of the sizes of nonmetallic inclusions to specific ones.
[00081
Concerning the mode control of carbides, for example, Patent Document
3 discloses a technique in which the ratio of MC-type carbides to total carbides
is 8 to 40 mass% in addition to the restriction of the total amount of carbides to
2 to 5 mass% to tremendously improve the SSC resistance.
[00091
Concerning the refinement of crystal grains, for example, Patent
Document 4 discloses a technique in which the crystal grains are made fine by
performing quenching treatment two times or more on a low-alloy steel to
improve the SSCC resistance. Patent Document 5 also discloses a technique
in which the crystal grains are made fine by the same treatment as that in
Patent Document 4 to improve the toughness.
Loo 101
Conventionally, in producing low-alloy steel materials in the field of
seamless steel pipes for oil well and the like pipes, to attain strength
properties and/or toughness, heat treatment of quenching and tempering has
often been performed after the finish of hot rolling such as hot pipe making.
As a method for heat treatment of quenching and tempering of the seamless
steel pipe for oil well, conventionally, a so-called "reheat quenching process"
has generally been performed, in which process, a steel pipe having been hot
rolled is reheated in an offline heat treatment furnace to a temperature not
lower than the Ac3 transformation point and is quenched, and further is
tempered at a temperature not higher than the Acl transformation point.
Loo 111
However, in recent years, from the viewpoints of process saving and
energy saving, there has also been performed a process in which a steel pipe
having been hot rolled is directly quenched from a temperature not lower than
the Arg transformation point and thereafter is tempered (a so-called "direct
quenching process") or further a process in which a steel pipe having been hot
rolled is sequentially soaked (hereinafter, especially referred also to as
"supplementarily heated") at a temperature not lower than the Ars
transformation point and thereafter is quenched from a temperature not lower
than the Ar3 transformation point and thereafter is tempered (a so-called
"inline heat treatment process" or "inline quenching process").
[00121
As disclosed in Patent Documents 4 and 5, it has been widely known
that a close relationship exists between the prior-austenite grains of low-alloy
steel and the SSC resistance and toughness, and the SSC resistance and
toughness are decreased remarkably by the coarsening of grains.
[OO 131
In the case where the "direct quenching process" is adopted for the
purpose of process saving and energy saving, the prior-austenite grains
coarsen, so that it sometimes becomes difficult to produce a seamless steel pipe
excellent in toughness and SSC resistance. The above-described "inline heat
treatment process" somewhat solves this'problem, but is not necessarily
comparable to the "reheat quenching process".
[OO1 41
The reason for this is thought to be that in the simple "direct quenching
process" and "inline heat treatment process", in the case where only tempering
is performed as the heat treatment of the postprocessing, there does not exist a
process of reverse transformation from ferrite of body-centered cubic structure
to austenite of face-centered cubic structure.
[00151
To solve the above-described problem of coarsening of crystal grains,
Patent Documents 6 and 7 propose methods in which a steel pipe having been
directly quenched and a steel pipe having been quenched by inline heat
treatment, respectively, are reheated and quenched &om a temperature not
lower than the A r 3 transformation point before the final tempering treatment.
100 161
In Patent Documents 4 and 5, tempering is performed at a temperature
not higher than the Acl transformation point in between the reheat quenching
treatments of plural times, and in Patent Documents 6 and 7, tempering is
performed at a temperature not higher than the Acl transformation point in
between the direct quenching treatment and quenching treatment performed
in inline heat treatment, respectively, and the reheat quenching treatment.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[00171
Patent Document 1: JP2001-172739A
Patent Document 2: JP2001-131698A
Patent Document 3: JP2000-178682A
Patent Document 4: JP59-232220A
Patent Document 5: JP60-009824A
Patent Document 6: JP6-220536A
Patent Document 7: W096136742
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[OO18 1
By the techniques for restricting the sizes of nonmetallic inclusions to
specific ones that are proposed in Patent Documents 1 and 2, an excellent SSC
resistance can be attained. However, since the steel must be purified, the
production cost sometimes increases.
[00191
Also, by the technique for controlling the modes of carbides that is
proposed in Patent Document 3, a very excellent SSC resistance can be
attained. However, the contents of Cr and Mo are restricted to restrain the
formation of MssCs-type carbides. Therefore, the hardenability is restricted,
so that for a thick-wall material, there is a possibility of insufficient
hardenability.
[00201
A process comprising direct quenching process or inline heat treatment
process, and then reheating and quenching from a temperature not lower than
the AT3 transformation point before the final tempering makes the prior
austenite grains more refined, thereby improving the SSC resistance of the
steel, compared with the case where the final tempering is performed following
the direct quenching or the inline heat treatment, or the case where the steel
pipe is once air-cooled close to room temperature, and thereafter the steel pipe
is subjected to a reheat-and-quenching treatment and tempering treatment.
[00211
Even in the case where after being subjected to the direct quenching
treatment or the inline heat treatment, the steel pipe is reheated and
quenched from a temperature not lower than the A r 3 transformation point
before the final tempering treatment as described above, the refinement of
prior-austenite grains is still insufficient as compared with the case where the
reheat quenching treatment is performed two times as proposed in Patent
Documents 4 and 5.
[00221
Therefore, by the technique in which the steel pipe having been directly
quenched is reheated and quenched from a temperature not lower than the AT3
transformation point before the final tempering treatment, which technique is
disclosed in Patent Document 6, a sufficient SSC resistance cannot necessarily
be attained.
Lo0231
Similarly, even if the steel pipe having been quenched by inline heat
treatment is reheated and quenched from a temperature not lower than the
AT3 transformation point before the final tempering treatment as proposed in
Patent Document 7, a sufficient SSC resistance cannot sometimes be attained.
Lo0241
Therefore, when an attempt is made to realize the refinement of crystal
grains that is sufficient as a high-strength oil-well steel pipe, the reheat
quenching treatment performed two times or more as disclosed in Patent
Documents 4 and 5 is significant. However, the reheat quenching treatment
performed two times or more leads to the rise in production cost.
[00251
Patent Documents 4 and 7 propose techniques in which the crystal
grains are made ultrafine by increasing the temperature rising rate at the
time of reheat quenching. In the techniques, however, the equipment must be
modified on a large scale because the heating means comes to consist of
induction heating or the like.
[00261
The present invention was made in view of the above situation, and
accordingly an objective thereof is to provide a low-cost method for producing a
high-strength steel material excellent in SSC resistance. Particularly, the
objective of the present invention is to provide a method for producing a highstrength
steel material in which the refinement of prior-austenite grains is
realized by an economically efficient means, whereby the excellent SSC
resistance and the improvement in toughness can be expected. The term
"high strength" in the present invention means that the YS is 655 MPa (95 ksi)
or higher, preferably 758 MPa (110 ksi) or higher, and further preferably 862
MPa (125 ksi) or higher.
MEANS FOR SOLVING THE PROBLEMS
Loo271
As described above, after being subjected to the direct quenching
treatment or the quenching treatment of inline heat treatment, a steel is
further reheated to a temperature not lower than the Acs transformation point
and is quenched, whereby the prior-austenite pains can be made fine. In the
case where the steel having been quenched is further repeatedly quenched,
after the preceding quenching treatment, intermediate tempering is often
performed at a temperature not higher than the Acl transformation point.
This intermediate tempering treatment has an effect of preventing delayed
cracking such as so-called "season cracking" occurring in a quenched steel.
[00281
However, the intermediate tempering must be performed under proper
conditions. In the case where the temperature of intermediate tempering is
too low or the heating time is too short, a sufficient effect of restraining season
cracking cannot be achieved in some cases. Inversely, even if the temperature
is not higher than the Acl transformation point, in the case where the
temperature of intermediate tempering is too high or the heating time is too
long, the effect of making crystal grains fine is lost even if the reheat
quenching is performed after the intermediate tempering treatment, and
sometimes, the advantageous effect of improving the SSC resistance
disappears.
[00291
Accordingly, the present inventors carried out various studies on a lowcost
method for producing a high-strength steel material by which method the
steel material has a sufficient effect of restraining season cracking and
simultaneously has an excellent SSC resistance due to the realization of
refinement of prior-austenite grains.
[OO~O]
As the result, the present inventors obtained findings that if
intermediate tempering treatment, which has been supposed to have to be
performed at a temperature not higher than the Acl transformation point to
improve the properties of the quenched steel material, is performed at a
temperature in the two-phase region of ferrite and austenite exceeding the Acl
transformation point, the prior-austenite grains are made fine remarkably
when the next reheat quenching treatment is performed.
[003 11
Moreover, the present inventors obtained quite novel findings that if
heat treatment is performed at a temperature in the above-described twophase
region of ferrite and austenite, even for a steel that has not been
quenched, for example, a steel that has been cooled at a cooling rate of air
cooling or the like after being hot-worked into a desired shape, if the steel is
next heated to a temperature in a proper austenite zone and is quenched, the
prior-austenite grains are made fine remarkably.
Lo0321
The present invention was completed based on the above-described
findings, and involves the methods for producing a high-strength steel
material excellent in sulfide stress cracking resistance described below.
Hereinafter, in some cases, the methods are referred simply to as "the present
invention (1)" to "the present invention (7)". Also, in some cases, the present
inventions (1) to (7) are generally named "the present invention".
100331
(1) A method for producing a high-strength steel material excellent in
sulfide stress cracking resistance, wherein a steel that has a chemical
composition consisting of, by mass percent, C: 0.15 to 0.65%, Si: 0.05 to 0.5%,
Mn: 0.1 to 1.5%, Cr: 0.2 to 1.5%, Mo: 0.1 to 2.5%, Ti: 0.005 to 0.50%, Al: 0.001
to 0.50%, and the balance of Fe and impurities, wherein Ni, P, S, N and 0
among the impurities are Ni: 0.1% or less, P: 0.04% or less, S: 0.01% or less, N:
0.01% or less, and 0 : 0.01% or less, and that has been hot-worked into a
desired shape is sequentially subjected to the steps of the following 111 to [3]:
[I] A step of heating the steel to a temperature exceeding the Acl
transformation point and lower than the ACQ transformation point and cooling
the steel;
[2] A step of reheating the steel to a temperature not lower than the ACQ
transformation point and quenching the steel by rapid cooling; and
[31 A step of tempering the steel at a temperature not higher than the
Acl transformation point.
Lo0341
(2) A method for producing a high-strength steel material excellent in
sulfide stress cracking resistance, wherein a steel that has a chemical
composition consisting of, by mass percent, C: 0.15 to 0.65%, Si: 0.05 to 0.5%,
Mn: 0.1 to 1.5%, Cr: 0.2 to 1.5%, Mo: 0.1 to 2.5%, Ti: 0.005 to 0.50%, Al: 0.001
to 0.50%, at least one selected from the elements shown in (a) and (b), and the
balance of Fe and impurities, wherein Ni, P, S, N and 0 among the impurities
are Ni: 0.1% or less, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, and 0 :
0.01% or less, and that has been hot-worked into a desired shape is
sequentially subjected to the steps of the following [I] to [3]:
[I] A step of heating the steel to a temperature exceeding the Acl
transformation point and lower than the Ac~tra nsformation point and cooling
the steel;
[21 A step of reheating the steel to a temperature not lower than the Acs
transformation point and quenching the steel by rapid cooling; and
[31 A step of tempering the steel at a temperature not higher than the
Acl transformation point.
(a) Nb: 0.4% or less, V: 0.5% or less, and B: 0.01% or less;
(b) Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.
[OO~S]
(3) The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to (1) or (2), wherein the steel
having the chemical composition according to (1) or (2) is hot-finished into a
sea~lesst eel pipe and is air cooled, and thereafter is sequentially subjected
to the steps of 111 to [3].
[00361
(4) The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to (1) or (2), wherein after the steel
having the chemical composition according to (1) or (2) has been hot-finished
into a seamless steel pipe, the steel is supplementarily heated at a
temperature not lower than the Ar3 transformation point and not higher than
1050°C in line, and after being quenched from a temperature not lower than
the AT3 transformation point, the steel is sequentially subjected to the steps of
ill to [31.
[00371
(5) The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to (1) or (2), wherein after the steel
having the chemical composition according to (1) or (2) has been hot-finished
into a seamless steel pipe, the steel is directly quenched from a temperature
not lower than the Ar3 transformation point, and thereafter is sequentially
subjected to the steps of [I] to [31.
[00381
(6) The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to (41, wherein the heating in step
[l] is performed by a heating apparatus connected to an apparatus for
quenching of inline heat treatment.
[00391
(7) The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to (51, wherein the heating in step
[l] is performed by a heating apparatus connected to a quenching apparatus
that performs direct quenching.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[00401
According to the present invention, since the refinement of prioraustenite
grains can be realized by an economically efficient means, a highstrength
steel material excellent in SSC resistance can be obtained at a low
cost. Also, by the present invention, a high-strength low-alloy steel seamless
oil-well pipe excellent in SSC resistance can be produced at a relatively low
cost. Further, according to the present invention, the improvement in
toughness due to the refinement of prior-austenite grains can be expected.
MODE FOR CARRYING OUT THE INVENTION
[00411
Hereunder, the requisites of the present invention are explained in
detail.
[00421
(A) Chemical composition
First, in item 0, explanation is given of the chemical composition of a
steel used in the production method of the present invention and the reasons
why the composition range is restricted. In the explanation below, symbol
"%" concerning the content of each element means "percent by mass".
loo431
C: 0.15 to 0.65%
C (Carbon) is an element necessary to enhance the hardenability and to
improve the strength. However, if the C content is less than 0.15%, the effect
of enhancing the hardenability is poor, and a sufficient strength cannot be
attained. On the other hand, if the C content exceeds 0.65%, the tendency for
a quenching crack to be generated at the quenching time is remarkable.
Therefore, the C content is 0.15 to 0.65%. The lower limit of the C content is
preferably 0.20%, further preferably 0.23%. Also, the upper limit of the C
content is preferably 0.45%, further preferably 0.30%.
loo441
Si: 0.05 to 0.5%
Si (Silicon) is necessary to deoxidize steel, and also has an action for
enhancing the temper softening resistance and for improving the SSC
resistance. For the purpose of deoxidation and improvement in SSC
resistance, 0.05% or more of Si must be contained. However, if Si is contained
excessively, steel is ernbrittled, and additionally the SSC resistance is rather
decreased. In particular, if the Si content exceeds 0.5%, the toughness and
SSC resistance are decreased significantly. Therefore, the Si content is 0.05
to 0.5%. The lower and upper limits of the Si content are preferably 0.15%
and 0.35%, respectively.
[00451
Mn: 0.1 to 1.5%
Mn (Manganese) is contained to deoxidize and desulfurize steel.
However, if the Mn content is less than 0.1%, the above-described effects are
poor. On the other hand, if the Mn content exceeds 1.5%, the toughness and
SSC resistance are decreased. Therefore, the Mn content is 0.1 to 1.5%. The
lower limit of the Mn content is preferably 0.15%, further preferably 0.20%.
Also, the upper limit of the Mn content is preferably 0.85%, further preferably
0.55%.
100461
Cr: 0.2 to 1.5%
Cr (Chromium) is an element for ensuring the hardenability and for
improving the strength and SSCresistance. However, if the Cr content is less
than 0.2%, sufficient effects cannot be achieved. On the other hand, if the Cr
content exceeds 1.5%, the SSC resistance is rather decreased, and further a
decrease in toughness is brought about. Therefore, the Cr content is 0.2 to
1.5%. The lower limit of the Cr content is preferably 0.35%, and more
preferably 0.45%. The upper limit of the Cr content is preferably 1.28%, and
more preferably 1.2%.
Lo0471
Mo: 0.1 to 2.5%
Mo (Molybdenum) enhances the hardenability and ensures the strength,
and also improves the temper softening resistance. Therefore, due to the
containing of Mo, tempering at high temperatures can be performed, and
resultantly, the shape of carbides turns spherical, and the SSC resistance is
improved. However, if the Mo content is less than 0.1%, these effects are poor.
On the other hand, if the Mo content exceeds 2.5%, despite the fact that the
raw material cost increases, the above-described effects somewhat saturates.
Therefore, the Mo content is 0.1 to 2.5%. The lower limit of the Mo content is
preferably 0.3%, further preferably 0.4%. Also, the upper limit of the Mo
content is preferably 1.5%, further preferably 1.0%.
100481
Ti: 0.005 to 0.50%
Ti (~itanium) has an action for improving the . hardenability by
immobilizing N, which is an impurity in steel, and by causing B to exist in a
dissolved state in steel at the time of quenching. Also, Ti has an effect of
preventing the coarsening of crystal grains and the abnormal grain growth at
the time of reheat quenching by precipitating as fine carbo-nitrides in the
process of temperature rise for reheat quenching. However, if the Ti content
isless than 0.005%, these effects are low. On the other hand, if the Ti content
exceeds 0.50%, a decrease in toughness is brought about. Therefore, the Ti
content is 0.005 to 0.50%. The lower limit of the Ti content is preferably
0.010%, further preferably 0.012%. Also, the upper limit of the Ti content is
preferably 0.10%, further preferably 0.030%.
[0049]
Al: 0.001 to 0.50%
Al (Aluminum) is an element effective in deoxidizing steel. However, if
the Al content is less than 0.001%, a desired effect cannot be achieved, and if
the A1 content exceeds 0.50%, the amount of inclusions increases and the
toughness decreases, and also the SSC resistance is decreased by the
coarsening of inclusions. Therefore, the A1 content is 0.001 to 0.50%. The
lower and upper limits of the Al content are preferably 0.005% and 0.05%,
respectively. The above-described A1 content means the amount of sol.Al
(acid-soluble Al).
[00501
A chemical composition of the steel used in the production method of the
present invention (specifically, the chemical composition of the steel according
to the present invention (1)) consists of the above-described elements and the
balance of Fe and impurities, wherein Ni, P, S, N and 0 among the impurities
are Ni: 0.1% or less, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, and 0:
0.01% or less.
[00511
The "impurities" described herein mean elements that mixedly enter on
account of various factors in the production process including raw materials
such as ore or scrap when a steel is produced on an industrial scale, and are
allowed to be contained within the range such that the elements do not exert
an adverse influence on the present invention.
loo521
Hereunder, explanation is given of Ni, P, S, N and 0 (oxygen) in the
impurities.
Lo0531
Ni: 0.1% or less
Ni (Nickel) decreases the SSC resistance. In particular, if the Ni
content exceeds 0.1%, the decrease in SSC resistance is remarkable.
Therefore, the content of Ni in the impurities is 0.1% or less. The Ni content
is preferably 0.05% or less, and more preferably 0.03% or less.
Lo0541
P: 0.04% or less
P (Phosphorus) segregates at the grain boundary, and decreases the
toughness and SSC resistance. In particular, if the P content exceeds 0.04%,
the decrease in toughness and SSC resistance is remarkable. Therefore, the
content of P in the impurities is 0.04% or less. The upper limit of the content
of P in the impurities is preferably 0.025%, further preferably 0.015%.
[00551
S: 0.01% or less
S (Sulfur) produces coarse inclusions, and decreases the toughness and
SSC resistance. In particular, if the S content exceeds 0.01%, the decrease in
toughness and SSC resistance is remarkable. Therefore, the content of S in
the impurities is 0.01% or less. The upper limit of the content of S in the
impurities is preferably 0.005%, further preferably 0.002%.
[00561
N: 0.01% or less
N (Nitrogen) combines with B, and prevents the advantageous effect of
improving the hardenability of B. Also, if N is contained excessively, N
produces coarse inclusions together with Al, Ti, Nb, etc., and has a tendency to
decrease the toughness and SSC resistance. In particular, if the N content
exceeds 0.01%, the decrease in toughness and SSC resistance is remarkable.
Therefore, the content of N in the impurities is 0.01% or less. The upper limit
of the content of N in the impurities is preferably 0.005%.
Loo571
0: 0.01% or less
0 (Oxygen) produces inclusions together with Al, Si, etc. By the
coarsening of inclusions, the toughness and SSC resistance are decreased. In
particular, if the 0 content exceeds 0.01%, the decrease in toughness and SSC
resistance is remarkable. Therefore, the content of 0 in the impurities is
0.01% or less. The upper limit of the content of 0 in the impurities is
preferably 0.005%.
[00581
Another chemical composition of the steel used in the production method
of the present invention (specifically, the chemical composition of the steel
according to the present invention (2)) further comprises at least one element
of Nb, V, B, Ca, Mg and REM (rare earth metal).
[00591
The "REM" described herein is a general term of a total of 17 elements of
Sc, Y and lanthanoids, and the content of REM means the total content of one
or more element(s) of REM.
[OO~O]
Hereunder, explanation is given of the operational advantages of Nb, V,
B, Ca, Mg and REM and the reasons why the composition range is restricted.
[OO~II
(a) Nb: 0.4% or less, V: 0.5% or less, and B: 0.01% or less
All of Nb, V and B have an action for improving the SSC resistance.
Therefore, in the case where it is desired to attain a higher SSC resistance,
these elements may be_ contained. Hereunder, Nb, V and B are explained.
lo0621
Nb: 0.4% or less
Nb (Niobium) is an element that precipitates as fine carbo-nitrides, and
has an effect of making the prior-austenite grains fine and thereby improving
the SSC resistance. Therefore, Nb may be contained as necessary. However,
if the Nb content exceeds 0.4%, the toughness deteriorates. Therefore, the
content of Nb, if contained, is 0.4% or less. The content of Nb, if contained, is
preferably 0.1% or less.
[00631
On the other hand, in order to stably achieve the above-described effect
of Nb, the content of Nb, if contained, is preferably 0.005% or more, and
further preferably 0.01% or more.
(00641
V: 0.5% or less
V (vanadium) precipitates as carbides (VC) when tempering is
performed, and enhances the temper softening resistance, so that V enables
tempering to be performed at high temperatures. As the result, V has an
effect of improving the SSC resistance. Also, V has an effect of restraining
the production of needle-form Mo2C, which becomes the starting point of
occurrence of SSC when the Mo content is high. Further, by containing V in
complex with Nb, a greater SSC resistance can be attained. Therefore, V may
be contained as necessary. However, if the V content exceeds 0.5%, the
toughness decreases. Therefore, the content of V, if contained, is 0.5% or less.
The content of V, if contained, is preferably 0.2% or less.
[0065]
On the other hand, in order to stably achieve the above-described effect
of V, the content of V, if contained, is preferably 0.02% or more. In particular,
in the case where the steel contairis. 0.68% or more of Mo, to restrain the
production of needle-form Mo2C, the above-described amount of V is preferably
contained complexly.
~00661
B: 0.01% or less
B (Boron) is an element having effects of increasing the hardenability
and improving the SSC resistance. Therefore, B may be contained as
necessary. However, if the B content exceeds 0.01%, the SSC resistance
rather decreases, and further the toughness also decreases. Therefore, the
content of B, if contained, is 0.01% or less. The content of B, if contained, is
preferably 0.005% or less, and further preferably 0.0025% or less.
Lo0671
On the other hand, in order to stably achieve the above-described effects
of B, the content of B, if contained, is preferably 0.0001% or more, and further
preferably 0.0005% or more.
However, the above-described effects of B appear in the case where B is
caused to exist in a dissolved state in steel. Therefore, in the case where B is
contained, the chemical composition is preferably regulated so that, for
example, Ti of an amount such as to be capable of immobilizing N having a
high affinity with B as nitrides is contained.
[00691
(b) Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less
All of Ca, Mg and REM react with S existing as an impurity in steel to
form sulfides, and has an action for improving the shapes of inclusions and
thereby increasing the SSC resistance. Therefore, these elements may be
contained as necessary. However, if either element is contained exceeding
0.005%, the SSC resistance rather decreases, also a decrease in toughness is
br-o-u gh.t about, and fkrther defects are liable to occur often on the surface of
steel. Therefore, the content of any of Ca, Mg .and REM, if contained, is
0.005% or less. The content of any of these elements, if contained, is
preferably 0.003% or less.
[00701
On the other hand, in order to stably achieve the above-described effect
of Ca, Mg and REM, the content of any of these elements, if contained, is
preferably 0.001% or more.
[00711
As already described, the "REM" is a general term of a total of 17
elements of Sc, Y and lanthanoids, and the content of REM means the total
content of one or more element(s) of REM.
Lo0721
The REM is generally contained in a form of misch metal. Therefore,
REM may be added, for example, in a form of misch metal, and may be
contained so that the amount of REM is in the above-described range.
[00731
Only one element of any of Ca, Mg and REM can be contained, or two or
more elements can be contained complexly. The total content of these
elements is preferably 0.006% or less, and further preferably 0.004% or less.
[00741
(B) Production method
Next, in item (B), detailed explanation is given of the method for
producing a high-strength steel material excellent in sulfide stress cracking
resistance of the present invention.
[00751
In the method for producing a high-strength steel material excellent in
sulfide stress cracking resistance in accordance with the present invention, the
steel that has the chemical composition described in item 0 and that has
been hot-worked into a desired shape is subjected to the following steps
sequentially:
[I] A step of heating the steel to a temperature exceeding the Acl
transformation point and lower than the Ac3 transformation point and cooling
the steel;
[21 A step of reheating the steel to a temperature not lower than the ACB
transformation point and quenching the steel by rapid cooling; and
[31 A step of tempering the steel at a temperature not higher than the
Acl transformation point.
[00761
By perforining the steps of items [I] to [3] sequentially, the refinement of
prior-austenite grains can be realized, the high-strength steel material
excellent in SSC resistance can be obtained at a low cost, and further the
improvement in toughness due to the refinement of prior-austenite grains can
be expected.
[00771
If the steel has the chemical composition described in item 00 and has
been hot-worked into a desired shape, the production history before the
performance of step [I] is not subject to any specific restriction. For example,
if the steel is produced by the ordinary process in which an ingot or a cast piece
is formed after melting, and the steel is hot-worked into a desired shape by
any method such as hot-rolling or hot-forging, after the hot working for
forming a desired shape, the steel may be cooled at a low cooling rate as in air
cooling, or may be cooled at a high cooling rate as in water cooling.
[00781
The reason for this is as described below. Even if any treatment is
performed after the hot working for forming a desired shape, by sequentially
performing the steps [l] to [31 thereafter, a micro-structure consisting mainly
of fine tempered martensite is formed after the tempering treatment at a
temperature not higher than the Acl transformation point in step [3] has been
finished.
[00791
The heating in step [I] must be performed at a temperature exceeding
the Acl transformation point and lower than the Ac3 transformation point. In
the case where the heating temperature deviates from the above-described
temperature range, even if reheat quenching is performed in the next step [z],
sufficient refinement of prior-austenite grains cannot be realized in some cases.
[00801
The step [I] need not necessarily be restricted specifically except that
the heating is performed at a temperature exceeding the Acl transformation
point and lower than the ACQtr ansformation point, that is, at a temperature in
the two-phase region of ferrite and austenite.
[0081]
Even if the heating treatment is performed under the condition that the
value of PL expressed by
PL = (T + 273) x (20 + loglot)
in which T is heating temperature (OC) and t is heating time (h), exceeds
23,500, the refinement of austenite grains quenched in the next step 121 tends
to saturate, and the cost merely increases. Therefore, the heating treatment
is preferably performed under the condition that the value of PL is 23,500 or
smaller. Concerning the heating time, depending on the furnace type used for
heating, at least 10s is desirable. Also, the cooling after the heating
treatment is preferably air cooling.
100821 - - -
After step [I], the steel is subjected to a step of being reheated to a
temperature not lower than the ACQtr ansformation point in step 121, that is, to
a temperature in the austenite temperature range and being quenched by
rapid cooling, whereby the refinement of austenite grains is achieved.
100831
If the reheating temperature in step 121 exceeds (Ac3 transformation
point + 10o°C), the prior-austenite grains are sometimes coarsened.
Therefore, the reheating temperature in step [2] is preferably (Ac3
transformation point + 100°C) or lower.
100841
The quenching method need not necessarily be subject to any specific
restriction. A water quenching method is used generally, however, as long as
martensitic transformation occurs in the quenching treatment, the steel may
be rapidly cooled by an appropriate method such as a mist quenching method.
[0085]
After step 121, the steel is subjected to a step of being tempered at a
temperature not higher than the Acl transformation point in step [3], that is,
at a temperature in the temperature range in which reverse transformation
into austenite does not occwr, whereby the high-strength steel material
excellent in sulfide stress cracking resistance can be obtained. The lower
limit of the tempering temperature may be determined appropriately by the
chemical composition of steel and the strength required for the steel material.
For example, the tempering may be performed at a higher temperature to
decrease the strength, and on the other hand, at a lower temperature to
increase the strength. As the cooling method after tempering, air cooling is
desirable.
[00861
Hereunder, the method for producing a steel material in accordance with
the present invention is explained in more detail by taking the case where a
seamless steel pipe is manufactured as an example.
[0087]
In the case where the high-strength steel material excellent in sulfide
stress cracking resistance is a seamless steel pipe, a billet having the chemical
composition described in item 0 is prepared.
[0088]
The billet may be bloomed from a steel block such as a bloom or a slab,
or may be cast by round CC. Needless to say, the billet may also be formed
from an ingot.
[0089l
From the billet, a pipe is hot-rolled. In particular, first, the billet is
heated to a temperature in the temperature range in which piercing can be
performed, and is subjected to hot piercing process. The billet heating
temperature before piercing is usually in the range of 1100 to 1300°C.
Eoo9ol
The means for hot piercing is not necessarily restricted. For example, a
hollow shell can be obtained by the Mannesmann piercing process or the like.
[00911
The obtained hollow shell is subjected to elongation working and finish
working.
Coo921
The elongation working is a step for manufacturing a seamless steel pipe
having a desired shape and size by elongating the hollow shell having been
pierced by a piercing machine and regulating the size. This step can be
performed by using, for example, a mandrel mill or a plug mill. Also, the
finish working can be performed by using a sizer or the like.
[00931
The working ratio of elongation working and finish working is not
necessarily restricted. The finishing temperature in the finish working is
preferably 1100°C or lower. However, if the finishing temperature exceeds
1050°C, a tendency for coarsening of crystal grains is sometimes developed.
Therefore, the finishing temperature in the finish working is further
preferably 1050°C or lower. At a temperature not higher than 900°C,
working is difficult to do on account of the increase in deformation resistance,
so that the pipe-making is preferably performed at a temperature exceeding
900°C.
[00941
As shown in the present invention (31, the seamless steel pipe having
been subjected to hot finish working may be air-cooled as it is. The "air
cooling" described herein includes so-called "natural cooling" or "being allowed
to cool".
[00951
Additionally, as shown in the present invention (4), the seamless steel
pipe having been subjected to hot finish working may be supplementarily
heated at a temperature not lower than the Ar3 transformation point and not
higher than 1050°C in line, and quenched from a temperature not lower than
the Ar3 transformation point, that is, at a temperature in the austenite
temperature range. In this case, since two quenching treatment including the
reheat quenching treatment is performed in the subsequent step [2], the
refinement of crystal grains can be realized.
[00961
If the seamless steel pipe is supplementarily heated at a temperature
exceeding 1050°C, the coarsening of austenite grains becomes remarkable, and
even if reheat quenching treatment is performed in the subsequent step 121,
the refinement of prior-austenite grains becomes difficult to do in some cases.
The upper limit of the supplemental heating temperature is preferably 1000°C.
As the method for quenching from a temperature not lower than the Ar3
transformation point, a general water quenching method is economical,
however, any quenching method in which martensitic transformation occurs
can be used, and, for example, a mist quenching method may be used.
[00971
Moreover, as shown in the present invention (5), the seamless steel pipe
having been subjected to hot finish working may be directly quenched from a
temperature not lower than the Ar3 transformation point, that is, from a
temperature in the austenite temperature range. In this case, since two
quenching treatment including the reheat quenching treatment is performed
in the subsequent step [2], the refinement of crystal grains can be realized.
As the method for quenching from a temperature not lower than the Ar3
transformation point, a general water quenching method is economical,
however, any quenching method in which martensitic transformation occurs
can be used, and, for example, a mist quenching method may be used.
[0098]
In the above-described methods, the seamless steel pipe having finished
being hot-worked and subsequently cooled is subjected to "the step of heating
the steel to a temperature exceeding the Acl transformation point and lower
than the ACQtr ansformation point and cooling the steel" in step 111, which is a
characteristic step of the present invention.
[00991
In the explanation below, the heating performed before step [2], that is,
the heating in step 111 is sometimes referred to as "intermediate heat
treatment".
[01001
The intermediate heat treatment is preferably performed by a heating
apparatus connected to an apparatus for quenching of inline heat treatment
when the seamless steel pipe having been subjected to hot finish working is
supplementarily heated at a temperature not lower than the Ar3
transformation point and not higher than 1050°C in line, quenched from a
temperature not lower than the A r 3 transformation point, and subsequently
subjected to the intermediate heat treatment, as shown in the present
invention (6). Besides, the intermediate heat treatment is preferably
performed by a heating apparatus connected to a quenching apparatus that
performs direct quenching when the seamless steel pipe having been subjected
to hot finish working is directly quenched from a temperature not lower than
the Ar3 transformation point, and subsequently subjected to the intermediate
heat treatment, as shown in the present invention (7). By using the heating
apparatuses, a sufficient effect of restraining season cracking is achieved.
[01011
As already described, the heating conditions in step 111 need not
necessarily be restricted specifically except that the heating is performed at a
temperature exceeding the Acl transformation point and lower than the A c ~
transformation point, that is, at a temperature in the two-phase region of
ferrite and austenite.
[01021
The seamless steel pipe having been subjected to step [I] is reheated and
quenched in step [2], and further is tempered in step [3].
[01031
By the above-described methods, there can be obtained a high-strength
seamless steel pipe which is excellent in SSC resistance, and by which the
improvement in toughness can also be expected.
[01041
Hereunder, the present invention is explained more specifically by
reference to examples. The present invention is not limited to the examples.
E,XAMPLES
[01051
(Example 1)
The components of each of steels A to L having the chemical
compositions given in Table 1 were regulated in a converter, and each of the
steels A to L was subjected to continuous casting, whereby a billet having a
diameter of 310 mm was prepared. Table 1 additionally gives the Acl
transformation point and Ac3 transformation point that were calculated by
using the Andrews formulas [I] and [2] (K. W. Andrews: JISI, 203 (19651, pp.
721 - 727) described below. For each steel, Cu, W and As were not detected in
a concentration of such a degree as to exert an influence on the calculated
value.
[O 1061
Acl point (OC) = 723 + 29.1 x Si - 10.7 x Mn - 16.9 x Ni + 16.9 x Cr + 6.38
x W+ 290xAs ... 111
ACQ point ('C) = 910 - 203 x C0.6+ 44.7 x Si - 15.2 x Ni + 31.5 x Mo + 104
xV+13.1xW-(30xMn+11xCr+20xCu-700xP-400xAl-120xAs-
400 x Ti) ... [21
where, each of C, Si, Mn, Cu, Ni, Cr, Mo, V, Ti, Al, W, As and P in the formulas
means the content by mass percent of that element.
[01071
[Table 11

[O~OB]
The billet was heated to 1250°C, and thereafter was hot-worked and
finished into a seamless steel pipe having a desired shape. In particular, the
billet having been heated to 1250°C was first pierced by using a Mannesmann
piercing mill to obtain a hollow shell. Then, the hollow shell was subjected to
elongation working by using a mandrel mill and finish working by using a
stretch reducing mill, and was finished into a seamless steel pipe having an
outside diameter of 244.48 mm, a wall thickness of 13.84 mm, and a length of
12 m. The finishing temperature in the diameter-reducing working using the
stretch reducing mill was about 950°C in all cases.
[01091
The seamless steel pipe having been finished so as to have the abovedescribed
dimensions was cooled under the conditions given in Table 2.
[01101
The "ILQ" in Table 2 indicates that the finished seamless steel pipe was
supplementarily heated under the conditions of 950°C x 10 min in line, and
was quenched by water cooling. The "DQ" indicates that the finished
seamless steel pipe was water-cooled from a temperature not lower than 900°C,
which is a temperature not lower than the A r 3 transformation point, without
being supplementarily heated, and was directly quenched. The "AR"
indicates that the finished seamless steel pipe was air-cooled to room
temperature.
[01111
The seamless steel pipe thus obtained was cut in pieces, and was
subjected to intermediate heat treatment experimentally under the conditions
given in Table 2. The cooling after the intermediate heat treatment was air
cooling. The symbol "-" in the intermediate heat treatment column of Table 2
indicates that the intermediate heat treatment was not performed.
From the steel pipe having been air-cooled after intermediate heat
treatment, a test specimen for measuring hardness was cut out, and the
Rockwell C hardness (hereinafter, abbreviated as "HRC") was measured. The
measurement of HRC was made from the viewpoint of evaluation of season
cracking resistance. If the HRC is 41 or less, especially 40 or less, it can be
judged that the occurrence of season cracking can be suppressed. For the
seamless steel pipe of "AR", that is, the steel pipe that was air-cooled to room
temperature after being finished, season cracking will not occur because the
steel pipe was not quenched. Therefore, for the steel pipe subjected to
intermediate heat treatment as well, the measurement of HRC was omitted.
[01131
Next, the steel pipe having been air-cooled after the intermediate heat
treatment was subjected to reheat quenching experimentally in step [21, in
which the steel pipe was heated at 920°C for 20 minutes and was quenched.
Concerning the reheat quenching, for the steel pipes using steels A to F and L,
the steel pipe was quenched by being dipped in tank or was rapidly cooled by
using jet water, and for the steel pipes using steels G to K, the steel pipe was
cooled by mist water spraying.
101 141
After the reheat quenching, the prior-austenite grain size number was
examined. That is, a test specimen was cut out of the reheat-quenched steel
pipe so that the cross section thereof perpendicular to the length direction of
pipe (pipe-making direction) is a.surface to be examined, and was embedded in
a resin. Thereby, the prior-austenite grain boundary was revealed by the
Bechet-Beaujard method, in which the test specimen was corroded by picric
acid saturated aqueous solution, and the prior-austenite grain size number
was examined in conformity to ASTM E112-10.
[01151
Table 2 additionally gives the HRC in the case where the steel pipe was
air-cooled after the intermediate heat treatment and the measurement result
of prior-austenite grain size number after reheat quenching. In Table 2, for
ease of description, the above-described HRC was described as "HRC after
intermediate heat treatment".
[0116]
[Table 21
[where T is heating temperature (oC) and t is heating time (h)]
".'I in the intermediate heat treatment column indicates that the intermediate heat treatment was not performed.
"--" in column of HRC after intermediate heat treatment indicates that HRC measurement was not performed.
* indicates that conditions do not satisfy those defined by the present invention.
Table 2 clearly demonstrates that regardless of the cooling conditions of
seamless steel pipe, in the test numbers of example embodiments of the
present invention in which the steel pipe was cooled after being heated at a
temperature exceeding the Acl transformation point and lower than the Ac3
transformation point as defined in the present invention, that is, at a
temperature in the two-phase region of ferrite and austenite, the prioraustenite
grain size number after reheat quenching was 9.5 in test number 47
even in the case of the coarsest grains, and in most cases, was 10 or more,
indicating fine grains.
[01181
While the prior-austenite grain size numbers of test numbers 9, 34, and
40 to 47 of example embodiments of the present invention were 9.5 to 11.2, the
prior-austenite grain size numbers of test numbers 6 and 12 of comparative
examples were 8.4 and 8.3, respectively. It is apparent that even in the case
where the seamless steel pipe is air-cooled and is not quenched after finish
working, if the steel pipe is manufactured by the method in accordance with
the present invention, an excellent refinement effect can be achieved.
[o 1191
Moreover, in example embodiments of the present invention, the HRC in
the case where the steel pipe was air-cooled after intermediate heat treatment
was 30.3 or less, so that season cracking will not occur.
10 1201
In contrast, in test numbers of comparative examples in which the steel
pipe was cooled after being heated at a temperature not higher than the Acl
transformation point deviating from the condition defined in the present
invention, the prior-austenite grain size numbers after reheat quenching were
at most 9.1 (test number ll), and the grains were coarse as compared with
example embodiments of the present invention.
[01211
As described above, it is apparent that by subjecting the steel, which has
the chemical composition defined in the present invention and has been hotworked
into a desired shape, to the steps [I] and [2] defined in the present
invention sequentially, that is, by cooling the steel having been heated at a
temperature exceeding the Acl transformation point and lower than the Ac3
transformation point and then by reheating the steel to a temperature not
lower than the Ac3 transformation point and quenching it by rapid cooling, the
prior-austenite grains can be made fine. By the refinement of prior-austenite
grains, the improvement in SSC resistance and toughness can be expected.
Lo1221
(Example 2)
To confirm the improvement in SSC resistance due to the refinement of
prior-austenite grains, which improvement was achieved by the method of the
present invention, some of the steel pipes subjected to the reheat quenching
described above (example 1) were subjected to tempering in step [3]. The
tempering was performed by heating the steel pipe at a temperature of 650 to
710°C for 30 to 60 minutes so that the YS is set to about 655 to 862 MPa (95 to
125 ksi), and the cooling after the tempering was air cooling.
101231
Table 3 gives the specific tempering conditions together with the cooling
conditions after the finish working of seamless steel pipe and the prioraustenite
grain size number after reheat quenching. The test numbers in
Table 3 correspond to the test numbers in Table 2 described above (example 1).
Also, a to d affixed to test numbers 7 and 8 are marks meaning that the
tempering conditions were changed.
From each of the tempered steel pipes, a test specimen for measuring
hardness was cut out to measure the HRC.
[01251
Also, from the steel pipe, a round-bar tensile test specimen specified in
NACE TM0177 Method A, which test specimen has a parallel part having an
outside diameter of 6.35 mm and a length of 25.4 mm, was cut out so that the
longitudinal direction thereof is the length direction of steel pipe (pipe-making
direction), and the tensile properties at room temperature were examined.
Based on the result of this examination, the constant load test specified in
NACE TM0177 Method A was conducted to examine the SSC resistance.
[01261
As the test solution for the SSC resistance examination, an aqueous
solution of 0.5% acetic acid + 5% sodium chloride was used. While hydrogen
sulfide gas of 0.1 MPa was fed into this solution, a stress of 90% of the actually
measured YS (hereinafter, referred to as a "90%AYS") or a stress of 85% of the
nominal lower-limit YS (hereinafter, referred to as a "85%SMYSWw) as imposed,
whereby the constant load test was conducted.
lo1271
Specifically, in test numbers 1 to 5, 14, 21, 23, 26, 38, 42, and 44 to 47
given in Table 3, the constant load test was conducted by imposing the
9O%AYS. Also, in test numbers 7a to 12 and 33 to 35, the constant load test
was conducted by imposing 645 MPa as the 85%SMYS considering the
strength level as 110 ksi class in which the YS is 758 to 862 MPa (110 to 125
ksi) from the examination result of tensile properties. In each of test numbers,
the SSC resistance was evaluated by the shortest rupture time by making the
number of tests 2 or 3. When rupture did not occur at the test of 720 hours,
the constant load test was discontinued at that time.
[O 1281
Table 3 additionally gives the examination results of HRC, tensile
properties, and SSC resistance. The shortest rupture time ">720" in the SSC
resistance column of Table 3 indicates that all of the test specimens were not
ruptured at the test of 720 hours. In the above-described case, in Table 3, "0"
mark was described in the judgment column considering the SSC resistance as
being excellent. On the other hand, in the case where the rupture time is not
longer than 720 hours, "xu mark was described in the judgment column
considering the SSC resistance as being poor.
[01291
[Table 31

of steps 111 and [2] defined in the present invention, to tempering treatment in
, .
step 131, an excellent SSC resistance can be attained.
INDUSTRIAL APPLICABILITY
[o 13 11
According to the present invention, since the refinement of prioraustenite
grains can be realized by an economically efficient means, a highstrength
steel material excellent in SSC resistance can be obtained at a low
cost. Also, by the present inventior,, a high-strength low-alloy steel seamless
oil-well pipe excellent in SSC resistance can be produced at a relatively low
cost. Further, ac. co- rding to the present invention, the improvement in
toughness due to the refinement of prior-austenite grains can be expected.

We claim:
1. A method for producing a high-strength steel material excellent in
sulfide stress cracking resistance, wherein a steel that has a chemical
composition consisting of, by mass percent, C: 0.15 to 0.65%, Si: 0.05 to 0.5%,
Mn: 0.1 to 1.5%, Cr: 0.2 to 1.5%, Mo: 0.1 to 2.5%, Ti: 0.005 to 0.50%, Al: 0.001
to 0.50%, and the balance of Fe and impurities, wherein Ni, P, S, N and 0
among the impurities are Ni: 0.1% or less, P: 0.04% or less, S: 0.01% or less, N:
0.01% or less, and 0: 0.01% or less, and that has been hot-worked into a
desired shape is sequentially subjected to the steps of the following [I] to [31:
[I] A step of heating the steel to a temperature exceeding the Acl
transformation point and lower than the Ac3 transformation point and cooling
the steel;
[21 A step of reheating the steel to a temperature not lower than the Ac3
transformation point and quenching the steel by rapid cooling; and
[31 A step of tempering the steel at a temperature not higher than the
Acl transformation point.
2. A method for producing a high-strength steel material excellent in
sulfide stress cracking resistance, wherein a steel that has a chemical
composition consisting of, by mass percent, C: 0.15 to 0.65%, Si: 0.05 to 0.5%,
Mn: 0.1 to 1.5%, Cr: 0.2 to 1.5%, Mo: 0.1 to 2.5%, Ti: 0.005 to 0.50%, Al: 0.001
to 0.50%, at least one selected from the elements shown in (a) and (b), and the
balance of Fe and impurities, wherein Ni, P, S, N and 0 among the impurities
are Ni: 0.1% or less, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, and 0:
0.01% or less, and that has been hot-worked into a desired shape is
sequentially subjected to the steps of the following [I] to [3]:
111 A step of heating the steel to a temperature exceeding the Acl
transformation point and lower than the Ac3 transformation point and cooling
the steel;
[2] A step of reheating the steel to a temperature not lower than the A c ~
transformation point and quenching the steel by rapid cooling; and
[3] A step of tempering the steel at a temperature not higher than the
Acl transformation point;
(a) Nb: 0.4% or less, V: 0.5% or less, and B: 0.01% or less;
(b) Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.
3. The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to claim 1 or 2, wherein the steel
having the chemical composition according to claim 1 or 2 is hot-finished into a
seamless steel pipe and is air cooled, and thereafter is sequentially subjected
to the steps of [I] to [31.
4. The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to claim 1 or 2, wherein after the
steel having the chemical composition according to claim 1 or 2 has been hotfinished
into a seamless steel pipe, the steel is supplementarily heated at a
temperature not lower than the AT3 transformation point and not higher than
1050°C in line, and after being quenched from a temperature not lower than
the A r 3 transformation point, the steel is sequentially subjected to the steps of
[ll to C31.
5. The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to claim 1 or 2, wherein after the
steel having the chemical composition according to claim 1 or 2 has been hotfinished
into a seamless steel pipe, the steel is directly quenched from a
temperature not lower than the Ars transformation point, and thereafter is
sequentially subjected to the steps of (11 to- (31.
6. The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to claim 4, wherein the heating in
step [I] is performed by a heating apparatus connected to an apparatus for
quenching of inline heat treatment.
7. The method for producing a high-strength steel material excellent in
sulfide stress cracking resistance according to claim 5, wherein the heating in
step (11 is performed by a heating apparatus connected to a quenching
apparatus that performs direct quenching.