U.S. patent application number 16/476704 was filed with the patent office on 2019-12-12 for steel material and method for producing steel material.
The applicant listed for this patent is Nippon Steel Corporation. Invention is credited to Yuji Arai, Hiroki Kamitani, Atsushi Soma, Shinji Yoshida.
Application Number | 20190376167 16/476704 |
Document ID | / |
Family ID | 62978210 |
Filed Date | 2019-12-12 |
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United States Patent
Application |
20190376167 |
Kind Code |
A1 |
Yoshida; Shinji ; et
al. |
December 12, 2019 |
Steel Material and Method for Producing Steel Material
Abstract
The 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% and O: 0.0100% or less, with the balance being Fe and
impurities. The steel material contains an amount of dissolved C
within a range of 0.010 to 0.060 mass %. The steel material 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.
Inventors: |
Yoshida; Shinji;
(Chiyoda-ku, Tokyo, JP) ; Arai; Yuji; (Chiyoda-ku,
Tokyo, JP) ; Soma; Atsushi; (Chiyoda-ku, Tokyo,
JP) ; Kamitani; Hiroki; (Chiyoda-ku, Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
62978210 |
Appl. No.: |
16/476704 |
Filed: |
January 22, 2018 |
PCT Filed: |
January 22, 2018 |
PCT NO: |
PCT/JP2018/001750 |
371 Date: |
July 9, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/04 20130101;
C21D 9/085 20130101; C22C 38/42 20130101; C22C 38/06 20130101; C22C
38/001 20130101; C22C 38/26 20130101; C22C 38/44 20130101; C22C
38/46 20130101; C22C 38/50 20130101; C22C 38/52 20130101; C21D
6/008 20130101; C22C 38/54 20130101; C21D 6/005 20130101; C22C
38/48 20130101; C22C 38/28 20130101; C22C 38/22 20130101; C22C
38/02 20130101; C22C 38/32 20130101; C21D 8/105 20130101; C21D
6/004 20130101; C21D 8/005 20130101; C22C 38/24 20130101 |
International
Class: |
C22C 38/42 20060101
C22C038/42; C22C 38/02 20060101 C22C038/02; C22C 38/04 20060101
C22C038/04; C22C 38/06 20060101 C22C038/06; C22C 38/44 20060101
C22C038/44; C22C 38/46 20060101 C22C038/46; C22C 38/48 20060101
C22C038/48; C22C 38/50 20060101 C22C038/50; C22C 38/52 20060101
C22C038/52; C22C 38/54 20060101 C22C038/54; C21D 8/00 20060101
C21D008/00; C21D 8/10 20060101 C21D008/10; C21D 9/08 20060101
C21D009/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2017 |
JP |
2017-009977 |
Claims
1. A steel material comprising: 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, an amount
of dissolved C within a range of 0.010 to 0.060 mass %, a yield
strength within a range of 965 to 1069 MPa, and a yield ratio of
90% or more.
2. The steel material according to claim 1, wherein the chemical
composition contains 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%.
3. The steel material according to claim 1, wherein the chemical
composition contains 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%.
4-8. (canceled)
9. The steel material according to claim 2, wherein the chemical
composition contains 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%.
10. The steel material according to claim 1, wherein the chemical
composition contains 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%.
11. The steel material according to claim 2, wherein the chemical
composition contains 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%.
12. The steel material according to claim 3, wherein the chemical
composition contains 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%.
13. The steel material according to claim 9, wherein the chemical
composition contains 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%.
14. The steel material according to claim 1, wherein the chemical
composition contains 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%.
15. The steel material according to claim 2, wherein the chemical
composition contains 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%.
16. The steel material according to claim 3, wherein the chemical
composition contains 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%.
17. The steel material according to claim 9, wherein the chemical
composition contains 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%.
18. The steel material according to claim 10, wherein the chemical
composition contains 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%.
19. The steel material according to claim 11, wherein the chemical
composition contains 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%.
20. The steel material according to claim 12, wherein the chemical
composition contains 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%.
21. The steel material according to claim 13, wherein the chemical
composition contains 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%.
22. The steel material according to claim 1, wherein the steel
material is an oil-well steel pipe.
23. A method for producing a steel material, comprising: a
preparation process of preparing an intermediate steel material
containing a chemical composition according to claim 1, a quenching
process of, after the preparation process, cooling the intermediate
steel material that is at a temperature in a range of 800 to
1000.degree. C. at a cooling rate of 50.degree. C./min; and a
tempering process of holding the intermediate steel material after
quenching at a temperature in a range of 660.degree. C. to an
A.sub.c1 point for 10 to 90 minutes, and thereafter cooling the
intermediate steel material at an average cooling rate in a
temperature region from 600.degree. C. to 200.degree. C. of 5 to
300.degree. C./sec.
24. The method for producing a steel material according to claim
23, wherein the preparation process includes: a starting material
preparation process of preparing a starting material containing a
chemical composition according to claim 1, and a hot working
process of subjecting the starting material to hot working to
produce an intermediate steel material.
Description
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.sub.3C
particles or M.sub.2C particles.
CITATION LIST
Patent Literature
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
[0008] 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
[0009] 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
[0010] 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.
[0011] 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.degree. C. is cooled at a
cooling rate of 50.degree. 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.degree. C. to an A.sub.c1 point, and thereafter is cooled from
600.degree. C. to 200.degree. C. at an average cooling rate of 5 to
300.degree. C./sec.
Advantageous Effects of Invention
[0012] 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
[0013] FIG. 1 is a view illustrating the relation between the
amount of dissolved C and a fracture toughness value
K.sub.1SSC.
[0014] 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.
[0015] FIG. 2B is a perspective view of a wedge that is used in the
DCB test in the examples.
DESCRIPTION OF EMBODIMENTS
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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%, 0: 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 K.sub.1SSC
that is an index of SSC resistance.
[0022] [Relation Between Amount of Dissolved C and SSC
Resistance]
[0023] FIG. 1 is a view illustrating the relation between the
amount of dissolved C and a fracture toughness value K.sub.1SSC 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 K.sub.1SSC (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.
[0024] 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 K.sub.1SSC that is an index of SSC
resistance was 30.0 MPa m or more, it was determined that the SSC
resistance was good.
[0025] 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 K.sub.1SSC 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.sub.1SSC was less
than 30.0 MPaV/m. In other words, it was clarified that when the
amount of dissolved C is too high, conversely, the SSC resistance
decreases.
[0026] 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.
[0027] 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.sub.1SSC becomes 30.0 MPa m or more and excellent SSC resistance
can be obtained.
[0028] 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 %.
[0029] 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.
[0030] 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%, 0:
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.
[0031] In the present description, although not particularly
limited, the steel material is, for example, a steel pipe or a
steel plate.
[0032] 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%.
[0033] 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%.
[0034] 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%.
[0035] 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%.
[0036] The aforementioned steel material may be an oil-well steel
pipe.
[0037] 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.
[0038] 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.
[0039] The term "excellent SSC resistance" mentioned above means,
specifically, that a value of K.sub.1SSC (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 4 g/L
of sodium acetate and adjusting to pH 3.5 using hydrochloric acid,
and a gaseous mixture consisting of 10% H.sub.2S gas and 90%
CO.sub.2 gas at a total pressure of 1 atm were sealed.
[0040] 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.
<Mo>c=(<Fe>a+<Cr>a+<Mn>a)x<Mo>b/(<Fe>-
;b+<Cr>b+<Mn>b) (1)
<Mo>d=<Mo>a-<Mo>c (2)
<C>a=(<Fe>a/55.85+<Cr>a/52+<Mn>a/53.94+<Mo>-
;c/95.9)/3.times.12 (3)
<C>b=(<V>a/50.94+<Mo>d/95.9+<Nb>a/92.9).times.12
(4)
(amount of dissolved C)=<C>-(<C>a+<C>b) (5)
[0041] Note that, in the present description, the term "cementite"
means carbides containing an Fe content of 50 mass % or more.
[0042] 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.degree. C. is cooled at a
cooling rate of 50.degree. C./min or more. In the tempering
process, the intermediate steel material after quenching is held at
a temperature in a range of 660.degree. C. to the A.sub.c1 point
for 10 to 90 minutes, and thereafter the intermediate steel
material is cooled at an average cooling rate of 5 to 300.degree.
C./sec with respect to cooling from 600.degree. C. to 200.degree.
C.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] [Chemical Composition]
[0047] The chemical composition of the steel material according to
the present embodiment contains the following elements.
[0048] C: More than 0.50 to 0.80%
[0049] 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%.
[0050] Si: 0.05 to 1.00%
[0051] 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%.
[0052] Mn: 0.05 to 1.00%
[0053] 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%.
[0054] P: 0.025% or Less
[0055] 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%.
[0056] S: 0.0100% or Less
[0057] 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%.
[0058] Al: 0.005 to 0.100%
[0059] 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".
[0060] Cr: 0.20 to 1.50%
[0061] 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%.
[0062] Mo: 0.25 to 1.50%
[0063] 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%.
[0064] Ti: 0.002 to 0.050%
[0065] 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%.
[0066] B: 0.0001 to 0.0050%
[0067] 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%.
[0068] N: 0.002 to 0.010%
[0069] 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%.
[0070] O: 0.0100% or Less
[0071] 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%.
[0072] 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.
[0073] [Regarding Optional Elements]
[0074] 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.
[0075] V: 0 to 0.30%
[0076] 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 M.sub.2C-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%.
[0077] Nb: 0 to 0.100%
[0078] 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 M.sub.2C-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%.
[0079] 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.
[0080] 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.
[0081] Ca: 0 to 0.0100%
[0082] 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%.
[0083] Mg: 0 to 0.0100%
[0084] 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%.
[0085] Zr: 0 to 0.0100%
[0086] 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%.
[0087] 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.
[0088] 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.
[0089] Co: 0 to 0.50%
[0090] 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%.
[0091] W: 0 to 0.50%
[0092] 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%.
[0093] 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.
[0094] Ni: 0 to 0.50%
[0095] 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%.
[0096] Cu: 0 to 0.50%
[0097] 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%.
[0098] [Amount of Dissolved C]
[0099] 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 %
[0100] 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.
[0101] 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.
[0102] 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.
[0103] 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.
[0104] 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.
[0105] The temperature region from 600.degree. C. to 200.degree. 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.degree. C. to 200.degree.
C. is made 5.degree. C./sec or more.
[0106] 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.degree. C./sec or less.
[0107] 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.
[0108] [Method for Calculating Amount of Dissolved C]
[0109] 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.
<Mo>c=(<Fe>a+<Cr>a+<Mn>a)x<Mo>b/(<Fe>-
;b+<Cr>b+<Mn>b) (1)
<Mo>d=<Mo>a-<Mo>c (2)
<C>a=(<Fe>a/55.85+<Cr>a/52+<Mn>a/53.94+<Mo>-
;c/95.9)/3.times.12 (3)
<C>b=(<V>a/50.94+<Mo>d/95.9+<Nb>a/92.9).times.12
(4)
(amount of dissolved C)=<C>-(<C>a+<C>b) (5)
[0110] 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.
[0111] [Determination of C Content of Steel Material]
[0112] 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.
[0113] [Calculation of C Amount that Precipitates as Carbides
(Precipitated C Amount)]
[0114] 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.
[0115] [Procedure 1. Determination of Residual Amounts of Fe, Cr,
Mn, Mo, V and Nb by Extraction Residue Analysis]
[0116] 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.sub.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 .mu.m 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-.mu.m 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.
[0117] [Procedure 2. Determination of Content of Fe, Cr, Mn and Mo
in Cementite by Extraction Replica Method and EDS]
[0118] 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.
[0119] [Procedure 3. Adjustment of Mo Amount]
[0120] 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.
[0121] Specifically, the amount of Mo precipitating as cementite
(<Mo>c) is calculated by Formula (1).
<Mo>c=(<Fe>a+<Cr>a+<Mn>a)x<Mo>b/(<Fe>-
;b+<Cr>b+<Mn>b) (1)
[0122] 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).
<Mo>d=<Mo>a-<Mo>c (2)
[0123] [Procedure 4. Calculation of Precipitated C Amount]
[0124] 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.sub.3C type structure (M include Fe, Cr, Mn and Mo).
<C>a=(<Fe>a/55.85+<Cr>a/52+<Mn>a/53.94+<Mo>-
;c/95.9)/3.times.12 (3)
<C>b=(<V>a/50.94+<Mo>d/95.9+<Nb>a/92.9).times.12
(4)
[0125] Thus, the precipitated C amount is
<C>a+<C>b.
[0126] [Calculation of Amount of Dissolved C]
[0127] 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.
<C>c=<C>-(<C>a+<C>b) (5)
[0128] [Microstructure]
[0129] 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.
[0130] 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.
[0131] 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 m.sup.2 (magnification
of .times.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.
[0132] [Shape of Steel Material]
[0133] 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.
[0134] [Yield Strength YS and Yield Ratio YR of Steel Material]
[0135] 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.
[0136] [SSC Resistance of Steel Material]
[0137] 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 4 g/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.sub.2S gas and 90% CO.sub.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.degree. C. while agitating the liquid solution
and also continuously blowing in the aforementioned gaseous
mixture. The K.sub.1SSC (MPa m) value of the steel material
according to the present embodiment determined under the foregoing
conditions is 30.0 MPa m or more.
[0138] [Production Method]
[0139] 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.
[0140] [Preparation Process]
[0141] 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.
[0142] 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.
[0143] [Starting Material Preparation Process]
[0144] 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.
[0145] [Hot Working Process]
[0146] 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.degree. 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%.
[0147] 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.
[0148] 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.
[0149] 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.
[0150] 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.
[0151] [Quenching Process]
[0152] 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.sub.3 point. A preferable quenching temperature is 800 to
1000.degree. 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.
[0153] 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.
[0154] 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.degree. C. during quenching is defined as a cooling rate
during quenching CR.sub.800-500. The cooling rate during quenching
CR.sub.800-500 is 50.degree. C./min or higher. A preferable lower
limit of the cooling rate during quenching CR.sub.800-500 is
100.degree. C./min, and more preferably is 250.degree. C./min.
Although an upper limit of the cooling rate during quenching
CR.sub.800-500 is not particularly defined, for example, the upper
limit is 60000.degree. C./min.
[0155] 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.
[0156] [Tempering Process]
[0157] 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.
[0158] A preferable tempering temperature is in a range from
660.degree. C. to the A.sub.ct point. If the tempering temperature
is 660.degree. C. or more, carbides are sufficiently spheroidized
and the SSC resistance is further increased.
[0159] 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.
[0160] [Regarding Rapid Cooling after Tempering]
[0161] 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.
[0162] 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.degree.
C. after tempering is defined as a cooling rate after tempering
CR.sub.600-200. In the method for producing the steel material
according to the present embodiment, the cooling rate after
tempering CR.sub.600-200 is 5.degree. C./sec or higher. By this
means, the amount of dissolved C according to the present
embodiment is obtained. The temperature region from 600.degree. C.
to 200.degree. 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.
[0163] Therefore, the cooling rate after tempering CR.sub.600-200
is within the range of 5 to 300.degree. C./sec. A preferable lower
limit of the cooling rate after tempering CR.sub.600-200 is
10.degree. C./sec, and more preferably is 15.degree. C./sec. A
preferable upper limit of the cooling rate after tempering
CR.sub.600-200 is 100.degree. C./sec, and more preferably is
50.degree. C./sec.
[0164] A method for cooling so that the cooling rate after
tempering CR.sub.600-200 is within the range of 5 to 300.degree.
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.sub.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).
[0165] 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
[0166] Molten steels of a weight of 180 kg containing the chemical
compositions shown in Table 1 were produced.
TABLE-US-00001 TABLE 1 Test Chemical Composition (Unit is mass %;
balance is Fe and impurities) Number C Si Mn P S Al Cr Mo Ti B N 1
0.51 0.31 0.42 0.008 0.0007 0.036 0.49 0.71 0.015 0.0013 0.005 2
0.52 0.29 0.46 0.009 0.0005 0.055 0.48 0.70 0.015 0.0011 0.003 3
0.53 0.31 0.46 0.009 0.0008 0.036 1.05 0.70 0.014 0.0013 0.002 4
0.51 0.30 0.43 0.008 0.0007 0.043 1.04 0.69 0.016 0.0014 0.005 5
0.53 0.31 0.44 0.009 0.0008 0.049 0.50 0.70 0.014 0.0014 0.003 6
0.55 0.29 0.45 0.011 0.0008 0.041 0.48 1.25 0.013 0.0015 0.002 7
0.58 0.23 0.41 0.012 0.0006 0.045 1.01 0.68 0.013 0.0014 0.005 8
0.60 0.27 0.43 0.010 0.0008 0.039 0.99 0.70 0.014 0.0012 0.004 9
0.62 0.26 0.42 0.010 0.0011 0.041 0.53 0.72 0.007 0.0012 0.003 10
0.61 0.26 0.44 0.011 0.0008 0.035 0.53 0.68 0.007 0.0013 0.004 11
0.62 0.26 0.44 0.009 0.0008 0.049 0.65 0.70 0.006 0.0012 0.003 12
0.67 0.26 0.44 0.010 0.0008 0.045 1.35 0.50 0.007 0.0013 0.003 13
0.68 0.24 0.41 0.011 0.0006 0.038 0.49 0.70 0.011 0.0013 0.004 14
0.51 0.26 0.43 0.012 0.0007 0.046 0.49 0.71 0.006 0.0005 0.003 15
0.51 0.29 0.40 0.010 0.0005 0.047 1.10 0.71 0.015 0.0023 0.004 16
0.52 0.29 0.43 0.011 0.0010 0.050 0.65 0.70 0.018 0.0021 0.005 17
0.51 0.31 0.47 0.012 0.0006 0.049 1.00 0.69 0.019 0.0014 0.004 18
0.55 0.29 0.40 0.009 0.0008 0.036 0.12 0.68 0.015 0.0014 0.004 19
0.56 0.28 0.44 0.009 0.0008 0.046 1.30 0.19 0.007 0.0015 0.003 20
0.54 0.28 1.20 0.010 0.0006 0.043 0.52 0.57 0.006 0.0012 0.005 21
0.58 0.28 0.44 0.011 0.0007 0.033 0.51 0.88 0.027 0.0015 0.015 22
0.62 1.50 0.44 0.010 0.0010 0.045 0.52 0.98 0.027 0.0015 0.003 23
0.52 0.27 0.38 0.009 0.0008 0.041 0.50 0.66 0.008 0.0013 0.004 24
0.60 0.22 0.45 0.011 0.0008 0.031 0.51 0.71 0.012 0.0012 0.003 25
0.55 0.30 0.42 0.009 0.0010 0.049 0.67 0.71 0.011 0.0015 0.004 Test
Chemical Composition (Unit is mass %; balance is Fe and impurities)
Number O V Nb Ca Mg Zr Co W Ni Cu 1 0.0006 0.10 0.010 -- -- -- --
-- -- -- 2 0.0008 -- -- -- -- -- -- -- -- -- 3 0.0007 0.09 -- -- --
-- -- -- -- -- 4 0.0011 -- 0.011 -- -- -- -- -- -- -- 5 0.0013 --
-- 0.0019 -- -- -- -- -- -- 6 0.0015 -- -- -- 0.0015 -- -- -- -- --
7 0.0007 -- -- -- -- 0.0018 -- -- -- -- 8 0.0006 -- -- -- -- --
0.32 -- -- -- 9 0.0010 -- -- -- -- -- -- 0.31 -- -- 10 0.0009 -- --
-- -- -- -- -- 0.06 -- 11 0.0008 -- -- -- -- -- -- -- -- 0.24 12
0.0016 -- -- -- -- -- -- -- -- -- 13 0.0015 -- -- -- -- -- -- -- --
-- 14 0.0012 -- -- -- -- -- -- -- -- -- 15 0.0011 -- -- -- -- -- --
-- -- -- 16 0.0018 -- -- -- -- -- -- -- -- -- 17 0.0014 -- -- -- --
-- -- -- -- -- 18 0.0014 -- -- -- -- -- -- -- -- -- 19 0.0011 0.10
-- -- -- -- -- -- -- -- 20 0.0014 -- -- -- -- -- -- -- -- -- 21
0.0015 -- -- -- -- -- -- -- -- -- 22 0.0017 -- -- -- -- -- -- -- --
-- 23 0.0014 -- -- -- -- -- -- -- -- -- 24 0.0016 0.10 -- -- -- --
-- -- -- -- 25 0.0009 -- -- -- -- -- -- -- -- --
[0167] Ingots were produced using the aforementioned molten steels.
The ingots were hot rolled to produce steel plates having a
thickness of 20 mm.
[0168] After hot rolling, the steel plate of each test number was
allowed to cool to bring the steel plate temperature to normal
temperature (25.degree. C.).
[0169] 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.degree. 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.sub.800-500) was
400.degree. 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.degree. C. to 500.degree. C. was 40.degree. C./min.
[0170] 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 (.degree. C.) and tempering time (min) and the cooling
rate (CR.sub.600-200) (.degree. C./sec) after tempering are shown
in Table 2. Note that, the A.sub.c1 point of the steel material in
each of Test Number 1 to Test Number 25 was 750.degree. C., and the
tempering temperature was set so as to be lower than the A.sub.c1
point.
TABLE-US-00002 TABLE 2 Cooling Tempering Tempering Rate After
Dissolved C K.sub.ISSC (MPa m) Test Temperature Time Tempering YS
TS YR Amount Average Number (.degree. C.) (min) (.degree. C./sec)
(MPa) (MPa) (%) (mass %) 1 2 3 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
[0171] [Evaluation Results]
[0172] [YS and TS Tests]
[0173] 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.degree. 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
(%).
[0174] [Microstructure Determination Test]
[0175] 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.
[0176] [Amount of Dissolved C Measurement Test]
[0177] 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 .mu.m.times.8 .mu.m, 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-US-00003 TABLE 3 Residual Amount Concentration In Dissolved C
Test (mass %) Cementite (mass %) Amount Number Fe Mn Cr Mo V Nb Fe
Mn Cr Mo (mass %) 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
[0178] [DCB Test]
[0179] 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.
[0180] 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 4 g/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.sub.2S gas and 90% CO.sub.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.
[0181] After sealing the vessel that had undergone the above
described process, the vessel was held for three weeks at
24.degree. 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.
[0182] 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 K.sub.1SSC (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 K.sub.1SSC
(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.sub.1SSC (MPa
m) of the relevant steel.
K 1 SSC = Pa ( 2 3 + 2.38 h / a ) ( B / Bn ) 1 / 3 Bh 3 / 2 ( 6 )
##EQU00001##
[0183] 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.
[0184] For each test number, the obtained fracture toughness values
K.sub.1SSC are shown in Table 2. If the fracture toughness value
K.sub.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.sub.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.
[0185] [Test Results]
[0186] The test results are shown in Table 2.
[0187] 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.sub.1SSC value
was 30.0 MPa m or more and excellent SSC resistance was
exhibited.
[0188] 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.sub.1SSC was less than
30.0 MPa m and excellent SSC resistance was not exhibited.
[0189] 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.sub.1SSC was less than 30.0 MPa m and excellent SSC resistance
was not exhibited.
[0190] 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.sub.1SSC was less than 30.0 MPa m and excellent SSC
resistance was not exhibited.
[0191] In the steel plate of Test Number 18, the Cr content was too
low. As a result, the fracture toughness value K.sub.1SSC was less
than 30.0 MPa m and excellent SSC resistance was not exhibited.
[0192] For the steel plate of Test Number 19, the Mo content was
too low. As a result, the fracture toughness value K.sub.1SSC was
less than 30.0 MPa m and excellent SSC resistance was not
exhibited.
[0193] In the steel plate of Test Number 20, the Mn content was too
high. As a result, the fracture toughness value K.sub.1SSC was less
than 30.0 MPa m and excellent SSC resistance was not exhibited.
[0194] For the steel plate of Test Number 21, the N content was too
high. As a result, the fracture toughness value K.sub.1SSC was less
than 30.0 MPa m and excellent SSC resistance was not exhibited.
[0195] For the steel plate of Test Number 22, the Si content was
too high. As a result, the fracture toughness value K.sub.1SSC was
less than 30.0 MPa m and excellent SSC resistance was not
exhibited.
[0196] For the steel plate of Test Number 23, the YR was less than
90%. As a result, the fracture toughness value K.sub.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.
[0197] 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.sub.1SSC was less than 30.0 MPa m and excellent SSC
resistance was not exhibited.
[0198] 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.sub.1SSC was less
than 30.0 MPa m and excellent SSC resistance was not exhibited.
[0199] 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
[0200] 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.
* * * * *