U.S. patent application number 11/791015 was filed with the patent office on 2008-09-04 for martensitic stainless steel.
This patent application is currently assigned to SUMITOMO METAL INDUSTRIES, LTD.. Invention is credited to Hideki Takabe.
Application Number | 20080213120 11/791015 |
Document ID | / |
Family ID | 36406975 |
Filed Date | 2008-09-04 |
United States Patent
Application |
20080213120 |
Kind Code |
A1 |
Takabe; Hideki |
September 4, 2008 |
Martensitic Stainless Steel
Abstract
Martensitic stainless steel according to the invention contains,
by mass, 0.001% to 0.01% C, at most 0.5% Si, 0.1% to 3.0% Mn, at
most 0.04% P, at most 0.01% S, 10% to 15% Cr, 4% to 8% Ni, 2.8% to
5.0% Mo, 0.001% to 0.10% Al, at most 0.07% N, 0% to 0.25% Ti, 0% to
0.25% V, 0% to 0.25% Nb, 0% to 0.25% Zr, 0% to 1.0% Cu, 0% to
0.005% Ca, 0% to 0.005% Mg, 0% to 0.005% La, and 0% to 0.005% Ce,
with the balance being Fe and impurities, and the steel satisfies
Expressions (1) and (2) and has a yield stress in the range from
758 MPa to 860 MPa. In this way, in the martensitic stainless steel
according to the invention, the tempering temperature range at
which a yield stress in the range from 758 MPa to 860 MPa can be
obtained is increased.
922.6-554.5C-50.9Mn+2944.8P+1.056Cr-81.1Ni+95.8Mo-125.1Ti-1584.9Al-376.1-
N.gtoreq.600 (1) 30C+0.5Mn+Ni+0.5Cu-1.5Si-Cr-Mo+7.9.gtoreq.0
(2)
Inventors: |
Takabe; Hideki; (Osaka-shi,
JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW, SUITE 250
WASHINGTON
DC
20005
US
|
Assignee: |
SUMITOMO METAL INDUSTRIES,
LTD.
Osaka-shi
JP
|
Family ID: |
36406975 |
Appl. No.: |
11/791015 |
Filed: |
October 26, 2005 |
PCT Filed: |
October 26, 2005 |
PCT NO: |
PCT/JP05/19685 |
371 Date: |
May 18, 2007 |
Current U.S.
Class: |
420/57 ; 420/40;
420/41; 420/61; 420/67 |
Current CPC
Class: |
C21D 2211/008 20130101;
C21D 1/25 20130101; C22C 38/04 20130101; C21D 6/004 20130101; C22C
38/44 20130101; C22C 38/02 20130101 |
Class at
Publication: |
420/57 ; 420/40;
420/41; 420/61; 420/67 |
International
Class: |
C22C 38/44 20060101
C22C038/44; C22C 38/40 20060101 C22C038/40 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 19, 2004 |
JP |
2004-335241 |
Claims
1. Martensitic stainless steel, comprising, by mass, 0.001% to
0.01% C, at most 0.5% Si, 0.1% to 3.0% Mn, at most 0.04% P, at most
0.01% S, 10% to 15% Cr, 4% to 8% Ni, 2.8% to 5.0% Mo, 0.001% to
0.10% Al, at most 0.07% N, 0% to 0.25% Ti, 0% to 0.25% V, 0% to
0.25% Nb, 0% to 0.25% Zr, 0% to 1.0% Cu, 0% to 0.005% Ca, 0% to
0.005% Mg, 0% to 0.005% La, and 0% to 0.005% Ce, the balance being
Fe and impurities, said steel satisfying Expressions (1) and (2)
and having a yield stress in the range from 758 MPa to 860 MPa.
922.6-554.5C-50.9Mn+2944.8P+1.056Cr-81.1Ni+95.8Mo-125.1Ti-1584.9Al-376.1N-
.gtoreq.600 (1) 30C+0.5Mn+Ni+0.5Cu-1.5Si-Cr-Mo+7.9.gtoreq.0 (2)
where the contents of the elements in percentage by mass are
substituted for the characters representing the elements.
2. The martensitic stainless steel according to claim 1, further
comprising at least one of 0.005% to 0.25% Ti, 0.005% to 0.25% V,
0.005% to 0.25% Nb, and 0.005% to 0.25% Zr.
3. The martensitic stainless steel according to claim 1, further
comprising 0.05% to 1.0% Cu.
4. The martensitic stainless steel according to claim 2, further
comprising 0.05% to 1.0% Cu.
5. The martensitic stainless steel according to claim 1, further
comprising at least one of 0.0002% to 0.005% Ca, 0.0002% to 0.005%
Mg, 0.0002% to 0.005% La, and 0.0002% to 0.005% Ce.
6. The martensitic stainless steel according to claim 2, further
comprising at least one of 0.0002% to 0.005% Ca, 0.0002% to 0.005%
Mg, 0.0002% to 0.005% La, and 0.0002% to 0.005% Ce.
7. The martensitic stainless steel according to claim 3, further
comprising at least one of 0.0002% to 0.005% Ca, 0.0002% to 0.005%
Mg, 0.0002% to 0.005% La, and 0.0002% to 0.005% Ce.
8. The martensitic stainless steel according to claim 4, further
comprising at least one of 0.0002% to 0.005% Ca, 0.0002% to 0.005%
Mg, 0.0002% to 0.005% La, and 0.0002% to 0.005% Ce.
Description
TECHNICAL FIELD
[0001] The present invention relates to martensitic stainless
steel, and more specifically to martensitic stainless steel for use
in a corrosive environment including corrosive substances such as
hydrogen sulfide, carbon dioxide gas, and chloride ions.
BACKGROUND ART
[0002] As deeper oil wells and gas wells have come to be dug, there
has been a demand for stronger and tougher martensitic stainless
steel used as a steel material for oil well such as oil country
tubular goods. Martensitic stainless steel having a yield stress
(0.2% proof stress) in the range from 758 MPa to 860 MPa
(hereinafter referred to as "110 ksi grade") and martensitic
stainless steel having a strength equal to or higher than the 110
ksi grade have been developed.
[0003] Martensitic stainless steel for oil well must have high
corrosion resistance such as SCC (Stress Corrosion Cracking)
resistance and SSC (Sulfide Stress Cracking) resistance. This is
because oil wells and gas wells exist in corrosive environments
that include corrosive substances such as hydrogen sulfide, carbon
dioxide gas, and chloride ions. More specifically, martensitic
stainless steel for use in oil wells must have high strength, high
toughness, and high corrosion resistance.
[0004] Martensitic stainless steel having high strength and high
corrosion resistance is disclosed by JP 2003-3243 A. The disclosed
martensitic stainless steel contains at least 1.5% by mass of Mo
and allows higher SSC resistance than conventional martensitic
stainless steel to be obtained.
[0005] If the Mo content is high, the tempering temperature range
that allows the 110 ksi grade strength to be obtained (hereinafter
referred to as "tempering temperature range") is very small. FIG. 1
shows the relation between the yield stress of martensitic
stainless steel with a high Mo content (hereinafter referred to as
"high Mo martensitic stainless steel) and the tempering
temperature. The high Mo martensitic stainless steel in FIG. 1
contains, by mass, 0.016% C, 11.8% Cr, 7.2% Ni, and 2.9% Mo, with
the balance being Fe and impurities. With reference to FIG. 1, the
gradient of the tempering temperature curve C10 in the yield stress
range from 758 MPa to 860 MPa is large. Therefore, the tempering
temperature must be about in the range from 580.degree. C. to
600.degree. C. in order to obtain the 110 ksi grade strength for
the high Mo martensitic stainless steel. More specifically, the
tempering temperature range .DELTA.T that allows the 110 ksi grade
strength to be obtained is very small.
[0006] If the tempering temperature range .DELTA.T is small, the
productivity is reduced. In general, several hundred tons of such
high Mo martensitic stainless steel is successively produced. In
this case, the high Mo martensitic stainless steel is made by a
plurality of heats (molten steel produced by a single steel making
process) and the chemical compositions of the heats are not
completely the same and slightly vary among them. If the tempering
temperature range .DELTA.T is small, the tempering temperature must
be changed every time the chemical composition changes in order to
obtain the 110 ksi grade strength for the steel. In short, in order
to obtain the 110 ksi grade strength, the tempering temperature
must be changed for each of the heats. The necessity of changing
the tempering temperature setting in this manner lowers the
productivity.
[0007] Note that International Publication No. WO 2004/57050 is a
patent document related to the invention.
DISCLOSURE OF THE INVENTION
[0008] It is an object of the invention to provide martensitic
stainless steel having a large tempering temperature range that
allows a yield stress in the range from 758 MPa to 860 MPa to be
obtained.
[0009] The inventor conducted various experiments and examinations
and has obtained the following findings.
[0010] (A) If the martensitic stainless steel has such a chemical
composition that the transformation point A.sub.c1 of the steel is
high, the tempering temperature range that allows the yield stress
to be from 758 MPa to 860 MPa increases. If the transformation
point A.sub.c1 is lower, austenite forms during high temperature
tempering process, which reduces the strength.
[0011] (B) In addition to the increase in the transformation point
A.sub.c1, the C content is reduced. In this way, the tempering
temperature range that allows the yield stress to be from 758 MPa
to 860 MPa even more increases. This is because as the C content is
higher, the gradient of the tempering temperature curve in the
yield stress range of 758 MPa to 860 MPa increases.
[0012] (C) If the C content is lowered, .delta. ferrite is more
likely to be generated, which affects the strength and toughness of
the steel. The 110 ksi grade martensitic stainless steel for use in
environments where the atmospheric temperature is less than
0.degree. C. must have both high strength and high toughness. If
the steel has a composition that allows the structure of the steel
to become martensitic even though the transformation point A.sub.c1
is raised and the C content is lowered, .delta. ferrite can be
prevented from forming and the toughness can be prevented from
being lowered while the 110 ksi grade strength is maintained.
[0013] As a result of consideration based on these findings, it was
found that if the C content is 0.01% or less, and the following
Expressions (1) and (2) are satisfied, the tempering temperature
range that allows the yield stress to be from 758 MPa to 860 MPa
can be larger than conventional cases.
922.6-554.5C-50.9Mn+2944.8P+1.056Cr-81.1Ni+95.8Mo-125.1Ti-1584.9Al-376.1-
N.gtoreq.600 (1)
30C+0.5Mn+Ni+0.5Cu-1.5Si-Cr-Mo+7.9.gtoreq.0 (2)
where the contents of the elements in percentage by mass are
substituted for the characters representing the elements.
[0014] The left side of Expression (1) (hereinafter the left side
of Expression (1)=F1) is an expression used to estimate the
A.sub.c1 transformation point of the martensitic stainless steel
according to the invention. As described above, if the A.sub.c1
transformation point is high, retained austenite can be prevented
from being precipitated during tempering process, so that the yield
stress can be prevented from being abruptly lowered. Stated
differently, the gradient of the tempering temperature curve in the
yield stress range from 758 MPa to 860 MPa can be small.
[0015] Note that F1.gtoreq.600 because the tempering process is
carried out at 600.degree. C. or less. If the tempering temperature
is set to 600.degree. C. or more, microscope carbide or
intermetallic compounds in the steel become coarse, and this rather
reduces the strength and toughness. Since the tempering temperature
is 600.degree. C. or less, it is only necessary that F1 be
600.degree. C. or more. Expression (2) is an expression used to
make the steel after tempering martensitic. If the contents of C,
Mn, and Ni that are austenite forming elements and the contents of
Si, Cr, and Mo that are ferrite forming elements satisfy the
relation defined by Expression (2), the structure becomes
martensitic, and .delta. ferrite can be prevented from being
produced. Therefore, the strength can be prevented from being
lowered, and high toughness can be maintained.
[0016] Note that if the martensitic stainless steel does not
contain optional elements Ti and Cu, the "Ti" and "Cu" in
Expressions (1) and (2) are both "0."
[0017] If these expressions are satisfied, a tempering temperature
curve as curve C1 shown in FIG. 2 can be obtained, and the gradient
of the tempering temperature curve in the yield stress range from
758 MPa to 860 MPa can be smaller than that in the conventional
cases. Therefore, the tempering temperature range .DELTA.T1 that
allows the yield stress to be in the range from 758 MPa to 860 MPa
is larger than the tempering temperature range .DELTA.T2 of the
conventional tempering temperature curve C2. Therefore, the
decrease in the productivity because of temperature setting changes
during operation can be prevented.
[0018] The inventor completed the following invention based on the
above-described findings.
[0019] Martensitic stainless steel according to the invention
contains, by mass, 0.001% to 0.01% C, at most 0.5% Si, 0.1% to 3.0%
Mn, at most 0.04% P, at most 0.01% S, 10% to 15% Cr, 4% to 8% Ni,
2.8% to 5.0% Mo, 0.001% to 0.10% Al, at most 0.07% N, 0% to 0.25%
Ti, 0% to 0.25% V, 0% to 0.25% Nb, 0% to 0.25% Zr, 0% to 1.0% Cu,
0% to 0.005% Ca, 0% to 0.005% Mg, 0% to 0.005% La, and 0% to 0.005%
Ce, with the balance being Fe and impurities, the steel satisfies
Expressions (1) and (2) and has a yield stress in the range from
758 MPa to 860 MPa.
922.6-554.5C-50.9Mn+2944.8P+1.056Cr-81.1Ni+95.8Mo-125.1Ti-1584.9Al-376.1-
N.gtoreq.600 (1)
30C+0.5Mn+Ni+0.5Cu-1.5Si-Cr-Mo+7.9.gtoreq.0 (2)
where the contents of the elements in percentage by mass are
substituted for the characters representing the elements.
[0020] If the optional elements Ti and Cu are not contained, the
"Ti" and "Cu" in Expressions (1) and (2) are both "0." The 0.2%
proof stress corresponds to the yield stress.
[0021] In the martensitic stainless steel according to the
invention, the gradient of the tempering temperature curve can be
reduced by setting the C content to 0.01% or less. In addition, the
A.sub.C1 transformation point can be higher than the conventional
examples if Expression (1) is satisfied. Therefore, the gradient of
the tempering temperature curve is reduced, and the tempering
temperature range that allows the yield stress to be in the range
from 758 MPa to 860 MPa is increased as compared to the
conventional examples.
[0022] The strength can be prevented from being less than 110 ksi
as Expression (2) is satisfied, and the high toughness can be
maintained. Since the Mo content is high, high corrosion resistance
is obtained.
[0023] The martensitic stainless steel according to the invention
preferably contains at least one of 0.005% to 0.25% Ti, 0.005% to
0.25% V, 0.005% to 0.25% Nb, and 0.005% to 0.25% Zr.
[0024] The martensitic stainless steel according to the invention
preferably contains 0.05% to 1.0% Cu.
[0025] The martensitic stainless steel according to the invention
preferably contains at least one of 0.0002% to 0.005% Ca, 0.0002%
to 0.005% Mg, 0.0002% to 0.005% La, and 0.0002% to 0.005% Ce.
[0026] In this way, the hot workability of the martensitic
stainless steel is improved. Note that these elements if contained
do not affect the effects of the invention described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows the relation between the yield stress of a
conventional high Mo martensitic stainless steel and the tempering
temperature; and
[0028] FIG. 2 shows the relation between the yield stresses of
sample materials 1 and 14 according to an embodiment of the
invention and the tempering temperature.
BEST MODE FOR CARRYING OUT THE INVENTION
[0029] Now, an embodiment of the invention will be described in
detail in conjunction with the accompanying drawings, in which the
same or corresponding portions are denoted by the same reference
characters, and their description will not be repeated.
[0030] 1. Chemical Composition
[0031] Martensitic stainless steel according to an embodiment of
the invention has the following composition. Hereinafter, "%"
related to each element refers to "% by mass."
[0032] C, 0.001% to 0.01%
[0033] If the C content is excessive, the gradient of the tempering
temperature curve becomes steep, and steel having a yield stress in
the range from 758 MPa to 860 MPa cannot stably be produced. The C
content should be limited to a small value. If the C content is
less than 0.001%, on the other hand, the manufacturing cost
increases. Therefore, the C content is in the range from 0.001% to
0.01%, preferably from 0.001% to 0.008%.
[0034] Si: 0.5% or less
[0035] Silicon is effectively applied as a deoxidizing agent. On
the other hand, Si hardens steel and therefore an excessive Si
content degrades the toughness and workability of the steel.
Silicon is a ferrite forming element and therefore prevents the
steel from becoming martensitic. Therefore, the Si content is not
more than 0.5%, preferably 0.3% or less.
[0036] Mn: 0.1% to 3.0%
[0037] Manganese contributes to improvement in the hot workability
of the steel. Furthermore, Mn is an austenite forming element and
contributes to formation of a martensitic structure. However, an
excessive Mn content degrades the toughness. Therefore, the Mn
content is in the range from 0.1% to 3.0%, preferably from 0.3% to
1.0%.
[0038] P: 0.04% or less
[0039] Phosphorus is an impurity and causes SSC to be generated,
and therefore the P content is limited as much as possible. The P
content is 0.04% or less.
[0040] S: 0.01% or less
[0041] Sulfur is an impurity and lowers the hot workability.
Therefore, the S content is limited as much as possible. The S
content is 0.01% or less.
[0042] Cr: 10% to 15%
[0043] Chromium contributes to improvement in corrosion resistance
in a wet carbon dioxide gas environment. On the other hand, Cr is a
ferrite forming element and an excessive Cr content makes it
difficult to form tempered martensite, which lowers the strength
and the toughness. Therefore, the Cr content is in the range from
10% to 15%, preferably from 11% to 14%.
[0044] Ni: 4% to 8%
[0045] Nickel is an austenite forming element and necessary for the
structure after tempering to become martensitic. If the Ni content
is too small, the structure after the tempering contains much
ferrite. On the other hand, an excessive Ni content causes the
structure after the tempering to have much austenite. Therefore,
the Ni content is in the range from 4% to 8%, preferably from 4% to
7%.
[0046] Mo: 2.8% to 5.0%
[0047] Molybdenum is a critical element that contributes to
improvement in SSC resistance and strength. In the martensitic
stainless steel according to the embodiment, the lower limit for
the Mo content to allow high SSC resistance to be obtained is 2.8%.
Molybdenum is a ferrite forming element and excessive addition of
the element prevents the structure from becoming martensitic. The
upper limit for the Mo content is therefore 5.0%. The Mo content is
preferably in the range from 2.8% to 4.0%.
[0048] Al: 0.001% to 0.10%
[0049] Aluminum is effectively applicable as a deoxidizing agent.
On the other hand, an excessive Al content causes many inclusions
to be generated, and the corrosion resistance is lowered.
Therefore, the Al content is from 0.001% to 0.10%, preferably from
0.001% to 0.06%.
[0050] N: 0.07% or less
[0051] Nitrogen forms a nitride and lowers the corrosion
resistance. Therefore, the N content is 0.07% or less.
[0052] Note that the balance consists of Fe and impurities. The
impurities are mixed in the manufacturing process for various
reasons.
[0053] The martensitic stainless steel according to the embodiment
further contains at least one of Ti, V, Nb, and Zr if required.
[0054] Ti: 0% to 0.25%
[0055] V: 0% to 0.25%
[0056] Nb: 0% to 0.25%
[0057] Zr: 0% to 0.25%
[0058] Note that Ti, V, Nb, and Zr are optional elements. These
elements fix C and reduce variations in strength. On the other
hand, an excessive content of any of these elements prevents the
structure after tempering from becoming martensitic. Therefore, the
content of each of these elements is set to the range from 0% to
0.25%, preferably from 0.005% to 0.25%, more preferably from 0.005%
to 0.20%.
[0059] The martensitic stainless steel according to the embodiment
contains Cu if required.
[0060] Cu: 0% to 1.0%
[0061] Copper is an optional element and an austenite forming
element as with Ni suitable for making the structure after
tempering martensitic. On the other hand, an excessive Cu content
lowers the hot workability. Therefore, the Cu content is from 0% to
1.0%, preferably from 0.05% to 1.0%.
[0062] The martensitic stainless steel according to the embodiment
further contains at least one of Ca, Mg, La, and Ce if
required.
[0063] Ca: 0% to 0.005%
[0064] Mg: 0% to 0.005%
[0065] La: 0% to 0.005%
[0066] Ce: 0% to 0.005%
[0067] Note that Ca, Mg, La, and Ce are optional elements. These
optional elements contribute to improvement in hot workability. On
the other hand, excessive contents of these elements cause coarse
oxides to be generated, which lowers the corrosion resistance.
Therefore, the contents of these elements are all in the range from
0% to 0.005%, preferably from 0.0002% to 0.005%. Among these
elements, Ca and La are elements that particularly contribute to
improvement in hot workability.
[0068] 2. Manufacturing Method
[0069] Steel having the above-described chemical composition is
melted and refined by well-known refining process. Then, the molten
steel is formed into a continues casting material by a continues
casting method. The continuos casting material is for example a
slab, bloom, or billet. Alternatively, the molten steel may be made
into ingots by an ingot casting method.
[0070] The slab, bloom, or ingot is formed into billets by hot
working. At the time, the billets may be formed by hot rolling or
by hot forging.
[0071] The billets produced by the continues casting or hot working
are subjected to further hot working and formed into oil country
tubular goods. For example, Mannesmann process may be performed as
the hot working. Alternatively, Ugine-Sejournet hot extrusion
process may be employed as the hot working, while a forged pipe
making method such as Ehrhardt method may be employed. The oil
country tubular good after the hot working is subjected to
quenching process and tempering process. The quenching process is
carried out according to a well-known method. The quenching
temperature is for example from 900.degree. C. to 950.degree. C.,
while other temperature ranges may be employed.
[0072] In the tempering process, the lower limit for the tempering
temperature is preferably 500.degree. C. If the tempering
temperature is too high, retained austenite is precipitated, so
that the yield stress cannot be in the range from 758 MPa to 860
MPa. Therefore, the upper limit for the tempering temperature is
preferably 600.degree. C.
[0073] The martensitic stainless steel according to the embodiment
of the invention satisfies the following Expressions (1) and
(2):
922.6-554.5C-50.9Mn+2944.8P+1.056Cr-81.1Ni+95.8Mo-125.1Ti-1584.9Al-376.1-
N.gtoreq.600 (1)
30C+0.5Mn+Ni+0.5Cu-1.5Si-Cr-Mo+7.9.gtoreq.0 (2)
If Expression (1) is satisfied, the A.sub.c1 transformation point
is high, and therefore the gradient of the tempering curve in the
yield stress range from 758 MPa to 860 MPa can be reduced. If
Expression (2) is satisfied, the structure can become martensitic
in an accelerated manner. Therefore, if both Expressions (1) and
(2) are satisfied, the tempering temperature range that allows the
yield stress to be in the range from 758 MPa to 860 MPa can be
larger than those of the conventional examples. Therefore, a
decrease in productivity based on changes in the temperature
setting during operation can be reduced.
[0074] High toughness necessary for steel for use in an oil well
can be obtained by satisfying Expression (2).
[0075] Note that if the martensitic stainless steel does not
contain Ti and Cu as optional elements, "Ti" and "Cu" in
Expressions (1) and (2) are "zero."
[0076] In the above-described example, the martensitic stainless
steel is made into a steel pipe, but the steel may be formed into a
steel plate.
EXAMPLE 1
[0077] Sample materials having chemical compositions given in Table
1 were produced and each examined for the tempering temperature
range that allows the yield stress to be in the range from 758 MPa
to 860 MPa. The sample materials were also examined for toughness
and corrosion resistance.
TABLE-US-00001 TABLE 1 Chemical Compositions (unit: % by mass, the
balance is Fe and impurities) No. C Si Mn P S Cu Cr Ni Mo sol. AL N
Inventive 1 0.006 0.14 0.46 0.012 0.0008 0.02 11.79 6.79 2.91 0.020
0.0053 Steel 2 0.006 0.14 0.47 0.012 0.0008 0.02 11.75 6.78 2.90
0.027 0.0030 3 0.007 0.17 0.40 0.014 0.0011 0.02 11.88 6.90 2.95
0.029 0.0075 4 0.006 0.15 0.47 0.012 0.0008 0.26 11.79 6.72 2.91
0.025 0.0038 5 0.006 0.20 0.46 0.011 0.0010 0.03 11.88 7.01 3.12
0.035 0.0053 6 0.007 0.16 0.47 0.012 0.0009 0.93 11.78 6.37 2.90
0.025 0.0037 7 0.008 0.14 0.30 0.017 0.0010 0 11.00 7.90 4.80 0.035
0.0060 8 0.004 0.15 0.45 0.011 0.0010 0 11.90 7.98 3.80 0.025
0.0099 9 0.006 0.16 0.46 0.012 0.0008 0.03 11.90 6.79 2.91 0.020
0.0501 10 0.006 0.14 0.46 0.012 0.0010 0.02 11.70 6.56 2.91 0.020
0.0450 11 0.007 0.14 0.35 0.012 0.0008 0 11.91 6.81 2.94 0.020
0.0065 Comparative 12 0.008 0.14 0.46 0.013 0.0010 0.02 11.84 7.21
2.91 0.025 0.0061 Steel 13 0.007 0.20 0.45 0.010 0.0010 0.02 11.96
6.93 2.89 0.028 0.0051 14 0.016 0.17 0.71 0.015 0.0010 0.03 11.80
7.20 2.92 0.030 0.0055 15 0.013 0.22 0.44 0.011 0.0010 0.02 12.55
6.12 3.08 0.026 0.0050 16 0.013 0.15 0.71 0.012 0.0009 0.26 11.83
6.54 2.92 0.022 0.0053 Chemical Compositions (unit: % by mass, the
balance is Fe and impurities) No. Nb V Ti Zr Ca Mg La Ce F1 F2
Inventive 1 0.002 0.11 0.075 0 0 0 0 0 628.7 0.20 Steel 2 0.002
0.06 0.081 0 0 0 0 0 617.0 0.25 3 0.002 0.05 0 0 0.0007 0 0 0 626.4
0.14 4 0.002 0.06 0.079 0 0 0 0 0 626.0 0.24 5 0 0.06 0.093 0 0 0 0
0 602.1 0.03 6 0.002 0.06 0.079 0.0003 0 0 0 0 652.9 0.26 7 0 0.04
0.100 0 0 0.0006 0 0 713.5 0.18 8 0 0.05 0.088 0 0 0 0 0 604.9 0.30
9 0.002 0.11 0.075 0 0 0 0.0009 0 611.9 0.06 10 0 0.04 0.082 0 0 0
0 0.0008 631.4 0.06 11 0 0 0 0 0 0 0 0 644.0 0.04 Comparative 12
0.002 0.06 0.084 0 0.0006 0 0 0 587.2 0.63 Steel 13 0 0.06 0.089 0
0 0 0 0 595.3 0.12 14 0.004 0.06 0.001 0 0 0 0 0 580.3 0.98 15 0
0.06 0.088 0 0 0 0 0 683.3 -1.32 16 0.002 0.06 0.079 0 0 0 0 0
629.7 0.34 F1 = 922.6 - 554.5C - 50.9Mn + 2944.8P + 1.056Cr -
81.1Ni + 95.8Mo - 125.1Ti - 1584.9Al - 376.1N F2 = 30C + 0.5Mn + Ni
+ 0.5Cu - 1.5Si - Cr - Mo + 7.9
[0078] Steel having the chemical compositions given in Table 1 were
melted. As shown in Table 1, the chemical compositions of the
sample materials 1 to 11 were within the range of the chemical
compositions according to the invention.
[0079] The left sides of Expressions (1) and (2) are represented as
F1 and F2, respectively, and F1 and F2 were obtained for each of
the sample materials. For the sample materials without Ti, "0" is
entered in the box for "Ti" in F1 and for those without Cu, "0" is
entered in the box for "Cu" in F2.
[0080] As for all the sample materials 1 to 11, F1 and F2 were both
within the range according to the invention. More specifically, F1
was 600 or more and F2 was zero or more.
[0081] As for the sample materials 12 and 13, while the chemical
compositions were within the range according to the invention, F1
was less than 600. As for the sample materials 14 to 16, the C
content exceeded the upper limit according to the invention.
Furthermore, as for the sample material 14, F1 was less than 600,
and as for the sample material 15, F2 was less than zero.
[0082] The molten steel for the sample materials 1 to 16 were cast
into continuous casting materials. The produced continuous casting
materials were subjected to hot forging and hot rolling and made
into a plurality of steel plates each having a thickness of 15 mm,
a width of 120 mm, and a length of 1000 mm. The steel plates after
the hot forging and hot rolling were cooled by air to room
temperatures. Using the obtained steel plates, the following tests
were conducted.
[0083] 1. Tempering Temperature Range
[0084] To start with, the obtained plurality of steel plates were
quenched. At the time, the quenching temperature was 910.degree. C.
Then, the quenched steel plates were subjected to tempering. At the
time, the tempering temperature was varied within the temperature
range from 450.degree. C. to 650.degree. C. The steel plates after
the tempering at various temperatures were subjected to tensile
tests. More specifically, a round-bar test piece having a diameter
of 6.35 mm and a length of 25.4 mm for the parallel part was
produced from each of the steel plates. Using the produced
round-bar test pieces, tensile tests were conducted at room
temperatures based on JIS Z2241 and the yield stresses were
obtained. After the tensile tests, the tempering temperature range
.DELTA.T in which the yield stress was in the range from 758 MPa to
860 MPa was obtained for each of the sample materials. Note that
the 0.2% proof stress was set as the yield stress.
[0085] The tempering temperature ranges of the sample materials
that allow the yield stress to be in the range from 758 MPa to 860
MPa are given in Table 2.
TABLE-US-00002 TABLE 2 No. .DELTA.T(.degree. C.) Inventive 1 110
Steel 2 80 3 100 4 110 5 45 6 80 7 50 8 55 9 40 10 110 11 100
Comparative 12 10 Steel 13 10 14 20 15 25 16 20
[0086] In Table 2, .DELTA.T represents the difference between the
maximum temperature and the minimum temperature among the tempering
temperatures at which the yield stresses of the sample materials
are from 758 MPa to 860 MPa. The unit is ".degree. C."
[0087] As shown in Table 2, .DELTA.T was 40.degree. C. or more for
each of the sample materials 1 to 11. Meanwhile, .DELTA.T was less
than 40.degree. C. for the sample materials 12 and 13 because F1
was less than 600 for them. The sample material 14 had a high C
content, F1 was less than 600, and therefore .DELTA.T was less than
40.degree. C. The sample materials 15 and 16 each had a high C
content, and therefore .DELTA.T was less than 40.degree. C.
[0088] FIG. 2 shows the relation between the tempering temperature
and the yield stress in the sample materials 1 and 14. As shown in
FIG. 2, the gradient of the tempering temperature curve C1 of the
sample material 1 whose F1 was 600 or more was small in the yield
stress range of 758 MPa to 860 MPa and the tempering temperature
range .DELTA.T1 was 110.degree. C. Meanwhile, for the sample
material 14 whose F1 was less than 600, the gradient of the
tempering temperature curve C2 was large in the yield stress range
from 758 MPa to 860 MPa, and the tempering temperature range
.DELTA.T2 was as small as 20.degree. C.
[0089] 2. Toughness
[0090] The toughness values of the sample materials are given in
Table 3.
TABLE-US-00003 TABLE 3 yield stress absorbed energy No. F2 (MPa) at
-40.degree. C. (J) Inventive 1 0.20 804.0 185 Steel 2 0.25 782.0
182 3 0.14 812.0 176 4 0.24 805.0 185 5 0.03 813.0 188 6 0.26 838.0
187 7 0.18 854.0 192 8 0.30 808.0 190 9 0.06 841.0 188 10 0.06
807.0 190 11 0.04 773.0 194 Comparative 12 0.63 848.0 160 Steel 13
0.12 815.0 165 14 0.98 851.0 167 15 -1.32 844.0 81 16 0.34 841.0
171
[0091] The toughness tests were conducted as follows. The obtained
steel plates were quenched at 910.degree. C. and tempered so that
the yield stresses become values given in Table 3. From each of the
tempered steel plates, a V-notch test piece as wide as 10 mm
according to JISZ2202 was produced.
[0092] The produced V-notch test pieces were subjected to Charpy
impact tests according to JISZ2242 at -40.degree. C. and examined
for absorbed energy.
[0093] The unit of the absorbed energy in Table 3 is J. Since the
F2 values of the sample materials 1 to 11 are all at least zero,
the values of the absorbed energy exceeded 100 J, in other words,
high toughness resulted. Meanwhile, the F2 value of the sample
material 15 was less than zero, and therefore the absorbed energy
was low.
[0094] 3. Corrosion Resistance
[0095] Corrosion resistance in a wet carbon dioxide gas environment
was evaluated by conducting carbon dioxide gas corrosion tests. A
test piece having a width of 20 mm, a thickness of 3 mm, and a
length of 50 mm was cut from each of steel plates quenched and
tempered in the same conditions as the above toughness evaluation.
The surface of each test piece was polished with No. 600 emery
paper, then degreased, and dried.
[0096] The produced test pieces were immersed for 720 hours in a
25% NaCl aqueous solution in which CO.sub.2 gas at 9.73 atm and
H.sub.2S at 0.014 atm were saturated. Note that the aqueous
solution was kept at 165.degree. C. during the tests.
[0097] After the tests, the test pieces were each examined for
quantity loss caused by corrosion. More specifically, the corrosion
loss was obtained as a value produced by subtracting the weight of
a test piece after the test from the weight of the test piece
before the test. The presence/absence of local corrosion at the
surfaces of the test pieces was visually inspected. It was
determined that the piece had high corrosion resistance in a wet
carbon dioxide gas environment if the corrosion loss was less than
7.7 g and there was no local corrosion found.
[0098] The following SSC tests were conducted to examine SSC
resistance in a wet hydrogen sulfide environment. Tensile test
pieces each having a diameter of 6.3 mm and a length of 25.4 mm for
the parallel part were cut from steel plates quenched and tempered
in the same conditions as the above toughness evaluation. The
produced tensile test pieces were subjected to proof ring tests
based on NACE TM0177-96 Method A. At the time, the test pieces were
immersed for 720 hours in a 20% NaCl aqueous solution in which
H.sub.2S(CO.sub.2 bal.) at 0.03 atm was saturated. The pH of the
NaCl aqueous solution was 4.5 and the temperature of the aqueous
solution was kept at 25.degree. C. After the tests, the
presence/absence of crackings was visually inspected.
[0099] The result of the corrosion resistance tests is given in
Table 4.
TABLE-US-00004 TABLE 4 carbon dioxide gas corrosion SSC No. tests
tests 1 .largecircle. .largecircle. 2 .largecircle. .largecircle. 3
.largecircle. .largecircle. 4 .largecircle. .largecircle. 5
.largecircle. .largecircle. 6 .largecircle. .largecircle. 7
.largecircle. .largecircle. 8 .largecircle. .largecircle. 9
.largecircle. .largecircle. 10 .largecircle. .largecircle. 11
.largecircle. .largecircle.
[0100] In Table 4, ".largecircle." in the carbon dioxide gas
corrosion tests indicates that the corrosion loss was less than 7.7
g and there was no local corrosion. In the SSC corrosion tests,
".largecircle." indicates that there was no cracking generated. The
sample materials 1 to 11 each had high corrosion resistance.
[0101] Although the embodiment of the present invention has been
described and illustrated in detail, it is clearly understood that
the same is by way of illustration and example only and is not to
be taken by way of limitation. The invention may be embodied in
various modified forms as required without departing from the
spirit and scope of the invention.
INDUSTRIAL APPLICABILITY
[0102] The martensitic stainless steel according to the invention
is applicable as a steel material for use in a corrosive
environment including corrosive substances such as hydrogen
sulfide, carbon dioxide gas, and chloride ions. The steel is
particularly applicable to steel materials for production
facilities, geothermal power generation facilities, and carbon
dioxide gas removing facilities, and steel pipes used as oil
country tubular goods in a wet hydrogen sulfide environment and a
wet carbon dioxide gas environment such as oil wells or gas
wells.
* * * * *