[Technical Field]
[0001] The present invention relates to a Cr containing steel pipe which is suitable as
a steel pipe for linepipe to be used for transporting crude oil or natural gas which
are produced at an oil well or a gas well, in particular, having excellent resistance
to intergranular stress corrosion cracking (or resistance to IGSCC) in a welded heat
affected zone.
[Background Art]
[0002] Nowadays, deep layer oil wells and gas wells to which consideration has not been
given due to their great depth or oil wells and gas wells whose development was abandoned
due to their intensely corrosive environment and so forth are being actively developed
from the viewpoint of skyrocketing crude oil prices and the depletion of oil resources
which is anticipated in the near future. Such oil wells and gas wells are generally
arranged deep in the ground and in an intensely corrosive environment, for example,
in a high temperature atmosphere containing carbon dioxide gas CO
2, chloride ions Cl
-, and so forth. In addition, oil wells and gas wells which are arranged in a severe
drilling environment, for example, at the bottom of the ocean are also being actively
developed. For pipelines transporting crude oil or natural gas produced at such oil
wells or gas wells, it is required to use a steel pipe having not only high strength
and high toughness but also excellent corrosion resistance, and further, excellent
weldability so as to decrease the laying cost of pipelines.
[0003] In order to meet this requirement, for example, Patent Literature 1 describes a martensitic
stainless steel pipe with excellent resistance to IGSCC in a welded heat affected
zone which is suitably used as a linepipe without performing a post weld heat treatment.
The martensitic stainless steel pipe described in Patent Literature 1 has a chemical
composition containing, by mass%, C: less than 0.0100%, N: less than 0.0100%, Cr:
10% to 14%, Ni: 3% to 8%, Si: 0.05% to 1.0%, Mn: 0.1% to 2.0%, P: 0.03% or less, S:
0.010% or less, and Al: 0.001% to 0.10%, and further containing at least one selected
from Cu: 4% or less, Co: 4% or less, Mo: 4% or less, and W: 4% or less, and at least
one selected from Ti: 0.15% or less, Nb: 0.10% or less, V: 0.10% or less, Zr: 0.10%
or less, Hf: 0.20% or less, and Ta: 0.20% or less, so that Csol (effective content
of dissolved carbon) is less than 0.0050%. According to Patent Literature 1, since
formation of Cr carbides at prior-austenite grain boundaries is prevented by controlling
the Csol, which is effective for forming Cr carbides, to be less than 0.0050%, formation
of Cr depleted zones which causes IGSCC in a welded heat affected zone is prevented
without performing a post weld heat treatment.
[0004] Patent Literature 3 describes a Cr containing steel pipe for linepipe having high
strength of X65 to X80 grade, and excellent in toughness, corrosion resistance, resistance
to sulfide stress cracking, and resistance to IGSCC in a welded heat affected zone.
The Cr containing steel pipe for linepipe described in Patent Literature 3 has a chemical
composition containing, by mass%, C: 0.001% to 0.015%, Si: 0.05% to 0.50%, Mn: 0.10%
to 2.0%, Al: 0.001% to 0.10%, Cr: 15.0% to 18.0%, Ni: 2.0% to 6.0%, Mo: 1.5% to 3.5%,
V: 0.001% to 0.20%, and N: 0.015% or less, under the condition that Cr+Mo+0.4W+0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N
is 11.5 to 13.3. With such a chemical composition, since a microstructure in a welded
heat affected zone, which is subjected to heating up to a temperature range for forming
ferrite single phase of 1300°C or higher and to cooling when welding is performed,
is formed such that 50% or more of prior-ferrite grain boundaries, in a ratio with
respect to the total length of the prior-ferrite grain boundaries, is occupied by
martensite phase and/or austenite phase and formation of Cr carbide depleted zones
is suppressed, a steel pipe having significantly increased resistance to IGSCC in
a welded heat affected zone can be obtained, which results in a significant decrease
in construction period of a welded steel pipe structure due to a post weld heat treatment
being unnecessary.
[0005] In addition, Patent Literature 2 describes a high-strength stainless steel pipe for
linepipe with excellent corrosion resistance. The high-strength stainless steel pipe
described in Patent Literature 2 has a chemical composition containing, by mass%,
C: 0.001% to 0.015%, N: 0.001% to 0.015%, Cr: 15% to 18%, Ni: 0.5% or more and less
than 5.5%, Mo: 0.5% to 3.5%, V: 0.02% to 0.2%, Si: 0.01% to 0.5%, Mn: 0.1% to 1.8%,
P: 0.03% or less, S: 0.005% or less, N: 0.001 to 0.015% and O: 0.006% or less, under
the condition that Cr+0.65Ni+0.6Mo+0.55Cu-20C≥18.5, Cr+Mo+0.3Si-43.5C-0.4Mn-Ni-0.3Cu-9N≥11.5,
and C+N≤0.025 are satisfied at the same time. According to Patent Literature 2, by
maintaining a ferrite-martensite dual phase microstructure containing an appropriate
amount of ferrite phase and by controlling Cr content to be in a rather high range
from 15% to 18%, a steel pipe having excellent hot workability, excellent low temperature
toughness, and sufficient strength for linepipe, and further having excellent corrosion
resistance in a corrosive high temperature environment of 200°C containing carbon
dioxide gas and chloride ions is obtained.
[Citation List]
[Patent Literature]
[Summary of Invention]
[Technical Problem]
[0007] However, in an intensely corrosive environment, even with the steel pipe described
in Patent Literature 1, there is a problem in that IGSCC occurring in a welded heat
affected zone cannot be completely suppressed, and IGSCC occurring in a welded heat
affected zone is at present prevented by performing a post weld heat treatment. The
steel pipe described in Patent Literature 1 was previously developed by the present
inventors, and the steel pipe according to Patent Literature 1 is a martensitic stainless
steel pipe whose microstructure does not include ferrite phase.
[0008] In the case of the steel pipe according to Patent Literature 2, no consideration
is given to IGSCC, and despite the fact that Cr content is increased, there is a problem
in that this steel pipe is rather poorer in terms of resistance to IGSCC than the
steel pipe described in Patent Literature 1 which has lower Cr content and that IGSCC
occurring in a welded heat affected zone cannot be completely suppressed.
[0009] In addition, in the case of the steel pipe according to Patent Literature 3, since
the amount of alloy added is comparatively large, there is a problem in that there
is an increase in material cost.
[0010] An object of the present invention is, by solving the problems in conventional arts
described above, to provide a Cr containing steel pipe for linepipe having desired
high strength, and excellent in toughness, corrosion resistance, resistance to sulfide
stress cracking, and resistance to IGSCC in a welded heat affected zone. The steel
pipe which the present invention is intended for is a steel pipe of X65 to X80 grade
(steel pipe having a yield strength (YS) of 448 to 651 MPa). In addition, hereinafter,
"excellent toughness" refers to a case where absorbed energy E
-40 (J) at -40°C in a Charpy impact test is 50 J or more, and "excellent corrosion resistance"
refers to a case where a corrosion rate (mm/y) is 0.10 mm/y or less in a 200 g/liter
NaCl aqueous solution at a temperature of 150°C in which carbon dioxide gas of 3.0
MPa is dissolved in the saturated state. Hereinafter, "steel pipe" includes not only
seamless steel pipe and but also welded steel pipe.
[Solution to Problem]
[0011] The present inventors, in order to achieve the object described above, diligently
conducted investigations regarding various factors having influences on resistance
to IGSCC in a welded heat affected zone of a ferritic-martensitic stainless steel
pipe in a corrosive high temperature environment containing carbon dioxide gas and
chloride ions.
[0012] As a result, it was found that, in the case of such a ferritic-martensitic stainless
steel, since ferrite grains having a large grain diameter are formed in a heating
cycle when welding is performed, and since Cr carbides are precipitated at the grain
boundaries of the ferrite grains having a large grain diameter in a subsequent cooling
cycle, Cr depleted zones are formed at the grain boundaries, which causes IGSCC. Moreover,
the present inventors found that, in the case of this kind of steel, if almost all
the grain boundaries of the ferrite grains having a large grain diameter is occupied
by austenite phase at least by inducing ferrite (α) to austenite (γ) transformation
at the grain boundaries before Cr carbides are precipitated at the grain boundaries,
precipitation of Cr carbides at the grain boundaries is prevented, and thus formation
of Cr depleted zones is suppressed, which allows IGSCC to be prevented.
[0013] In addition, from the results of further investigations, it was found that, in order
to prevent IGSCC by inducing α to γ transformation at the grain boundaries before
Cr carbides being precipitated at the grain boundaries, it is necessary to adjust
a chemical composition so that P
1 defined by equation (1) below is 11.5 to 13.3 and that P
2 defined by equation (2) below is 0 or more:
[0014] From the investigations by the present inventors, it was newly found that, by controlling
a chemical composition so that P
1 is 13.3 or less and that P
2 is 0 or more, carbides (Cr carbides) are less likely to be precipitated at the grain
boundaries, and thus Cr depleted zones are also less likely to be formed, which allows
IGSCC to be prevented.
[0015] That is to say, in the case where P
1 is 13.3 or less, that is, in the case of a chemical composition in which the contents
of ferrite forming elements are small, single ferrite phase microstructure having
a large grain diameter is formed in a zone which is subjected to a high temperature
higher than 1200°C, which is nearly equal to the melting point, in a heating process
when a girth welding is performed, for example, when laying pipelines, and then, in
a cooling process, α to γ transformation is induced to form γ phase at the grain boundaries
or inside the grains. In such a case, since the solubility product of carbides is
larger in γ phase than in α phase, carbides (Cr carbides) are less likely to be precipitated
at the grain boundaries, which allows IGSCC to be prevented due to Cr depleted zones
being less likely to be formed. It is needless to say that most of or all of γ phase
transforms into martensite phase in the subsequent cooling process.
[0016] On the other hand, in the case where P
1 is more than 13.3, that is, in the case of a chemical composition in which the contents
of ferrite forming elements are large, since α to γ transformation is not induced
in the cooling process, Cr carbides are precipitated at the grain boundaries, which
tends to cause IGSCC due to formation of Cr depleted zones.
[0017] From the further investigations, it was found that, even in the case where the contents
of Cr and Ni are small, if it is possible to adjust a chemical composition so that
P
1 is 13.3 or less and that P
2 is 0 or more, it is possible to realize the microstructure change described above,
which allows IGSCC in a welded heat affected zone to be prevented.
[0018] The present invention has been completed on the basis of the findings described above
and further investigations. That is to say, the gist of the present invention is as
follows.
- (1) A Cr containing steel pipe for linepipe, characterized by having;
a chemical composition consisting of, by mass%, C: 0.001% to 0.015%, Si: 0.05% to
0.50%, Mn: 0.10% to 2.0%, P: 0.020% or less, S: 0.010% or less, Al: 0.001% to 0.10%,
Cr: 13% or more and less than 15%, Ni: 2.0% to 5.0%, Mo: 1.5% to 3.5%, V: 0.001% to
0.20%, N: 0.015% or less, and the balance being Fe and inevitable impurities, under
the condition that P1 defined by equation (1) below is 11.5 to 13.3 and that P2 defined by equation (2) below is 0 or more, and
a microstructure in a welded heat affected zone, which is subjected to heating up
to a temperature range for forming ferrite single phase of 1300°C or higher and to
cooling when welding is performed, is formed such that 50% or more of prior-ferrite
grain boundaries, in a ratio with respect to the total length of the prior-ferrite
grain boundaries, is occupied by martensite phase,
(where Cr, Mo, W, Si, C, Mn, Ni, Cu, N: the contents (mass%) of the chemical elements
represented respectively by the corresponding atomic symbols).
- (2) The Cr containing steel pipe for linepipe according to (1), characterized in that
the chemical composition further contains, by mass%, at least one selected from Cu:
0.01% to 3.5% and W: 0.01% to 3.5%.
- (3) The Cr containing steel pipe for linepipe according to (1) or (2), characterized
in that the chemical composition further contains, by mass%, at least one selected
from Ti: 0.01% to 0.20%, Nb: 0.01% to 0.20%, and Zr: 0.01% to 0.20%.
- (4) The Cr containing steel pipe for linepipe according to any one of (1) to (3),
characterized in that the chemical composition further contains, by mass%, at least
one selected from Ca: 0.0005% to 0.0100% and REM: 0.0005% to 0.0100%.
[Advantageous Effects of Invention]
[0019] According to the present invention, a Cr containing steel pipe for linepipe excellent
in resistance to IGSCC in a welded heat affected zone can be manufactured at low cost
without performing a post weld heat treatment, which results in a great industrial
effect. In addition, according to the present invention, since steel pipe structures
such as pipelines can be constructed without performing a post weld treatment, there
is also a merit of significantly decreasing construction cost due to, for example,
a decrease in construction period.
[Brief Description of Drawings]
[0020]
[Fig. 1] Fig. 1 is a schematic diagram illustrating a simulated welding thermal cycle
used in EXAMPLES.
[Fig. 2] Fig. 2 is a schematic diagram illustrating the test specimen bent in a U-shape
using a jig for U-bent test used in EXAMPLES.
[Description of Embodiments]
[0021] First, the reasons for limiting the chemical composition of the steel pipe according
to the present invention will be described. Hereinafter, mass% will be represented
simply by %, unless otherwise noted.
C: 0.001% to 0.015%
[0022] C is a chemical element which contributes to an increase in strength, and it is necessary
that the C content be 0.001% or more in the present invention.
[0023] On the other hand, in the case where the C content is more than 0.015%, there is
a deterioration in toughness in a welded heat affected zone. In particular, it is
difficult to prevent IGSCC in a welded heat affected zone. Therefore, the C content
is set to be 0.001% to 0.015%, preferably 0.002% to 0.010%.
Si: 0.05% to 0.50%
[0024] Si is a chemical element which functions as a deoxidizing agent and increases strength
due to solid solution hardening, and it is necessary that the Si content be 0.05%
or more in the present invention. However, in the case where the Si content is more
than 0.50%, there is a deterioration in the toughness not only in a base steel but
also in a welded heat affected zone. Therefore, the Si content is set to be 0.05%
to 0.50%, preferably 0.10% to 0.40%.
Mn: 0.10% to 2.0%
[0025] Mn contributes to an increase in strength due to solid solution hardening and is
an austenite forming element which increases the toughness in both a base steel and
a welded heat affected zone by suppressing formation of ferrite phase. Although it
is necessary that the Mn content be 0.10% or more in order to realize these effects,
the effects become saturated in the case where the Mn content is more than 2.0% and
the effect corresponding to the content cannot be expected. Therefore, the Mn content
is set to be 0.10% to 2.0%, preferably 0.20% to 1.5%.
P: 0.020% or less
[0026] Since P is a chemical element which deteriorates corrosion resistances such as CO
2 corrosion resistance and resistance to sulfide stress cracking, it is preferable
that the P content be as small as possible in the present invention, but there is
an increase in manufacturing cost in the case where the P content is excessively decreased.
As an industrially realizable range at comparatively low cost in which there is not
a deterioration in corrosion resistance, the P content is set to be 0.020% or less,
preferably 0.015% or less.
S: 0.010% or less
[0027] Since S is a chemical element which significantly deteriorates hot workability in
a pipe manufacturing process, it is preferable that the S content is as small as possible,
but, since it is possible to manufacture a pipe using an ordinary process in the case
where the S content is 0.010% or less, the S content is set to be 0.010% or less,
preferably 0.004% or less.
Al: 0.001% to 0.10%
[0028] Al is a chemical element which is strongly effective as a deoxidizing agent, and
it is necessary that the Al content be 0.001% or more in order to realize this effect,
but there is a negative effect on toughness in the case where the Al content is more
than 0.10%. Therefore, the Al content is set to be 0.10% or less, preferably 0.05%
or less.
Cr: 13% or more and less than 15%
[0029] Cr is a chemical element which increases corrosion resistance such as CO
2 corrosion resistance and resistance to sulfide stress cracking as a result of forming
a protective surface film. It is necessary in the present invention that the Cr content
be 13% or more in order to increase corrosion resistance in an intensely corrosive
environment. On the other hand, in the case where the Cr content is 15% or more excessively,
it is necessary that large amount of other alloy elements such as Ni be added in order
to control the value of P
1 to be within the specified range, and there is a significant increase in material
cost. Therefore, the Cr content is set to be 13% or more and less than 15%, preferably
more than 14% and less than 15%.
Ni: 2.0% to 5.0%
[0030] Ni is a chemical element which increases corrosion resistance such as CO
2 corrosion resistance and resistance to sulfide stress cracking as a result of strengthening
a protective surface film and contributes to an increase in strength. It is necessary
that the Ni content be 2.0% or more in order to realize these effects, but, in the
case where the Ni content is more than 5.0%, there is a tendency for hot workability
to deterioration and there is a significant increase in material cost. Therefore,
the Ni content is set to be 2.0% to 5.0%, preferably 2.5% to 5.0%.
Mo: 1.5% to 3.5%
[0031] Mo is a chemical element which is effective for increasing resistance to pitting
corrosion caused by Cl
- (chloride ions). It is necessary that the Mo content be 1.5% or more in order to
realize this effect. On the other hand, in the case where the Mo content is more than
3.5%, there is a deterioration in hot workability and there is a significant increase
in manufacturing cost. Therefore, the Mo content is set to be 1.5% to 3.5%, preferably
1.8% to 3.0%.
V: 0.001% to 0.20%
[0032] V is a chemical element which contributes to an increase in strength and is effective
for increasing resistance to stress corrosion cracking. These effects are markedly
realized in the case where the V content is 0.001% or more, but there is a deterioration
in toughness in the case where the V content is more than 0.20%. Therefore, the V
content is set to be 0.001% to 0.20%, preferably 0.010% to 0.10%.
N: 0.015% or less
[0033] Although N is effective for increasing pitting corrosion resistance, since N is a
chemical element which significantly deteriorates weldability, it is preferable that
the N content be as small as possible in the present invention. However, there is
an increase in manufacturing cost in the case where the N content is excessively decreased.
As an industrially realizable range at comparatively low cost in which there is not
a deterioration in weldability, the N content is set to be 0.015% or less.
[0034] Although the chemical composition described above is a basic chemical composition,
in addition to the basic chemical composition, at least one selected from Cu: 0.01%
to 3.5% and W: 0.01% to 3.5%; and/or at least one selected from Ti: 0.01% to 0.20%,
Nb: 0.01% to 0.20%, and Zr: 0.01% to 0.20%; and/or at least one selected from Ca:
0.0005% to 0.0100% and REM: 0.0005% to 0.0100% may be selectively added if necessary.
[0035] At least one selected from Cu: 0.01% to 3.5% and W: 0.01% to 3.5%
[0036] Since Cu and W are both chemical elements which increase CO
2 corrosion resistance, these chemical elements may be selectively added if necessary.
[0037] Cu is, moreover, also a chemical element which contributes to an increase in strength.
It is preferable that the Cu content be 0.01% or more in order to realize these effects,
but, since the effects become saturated in the case where the Cu content is more than
3.5%, effects corresponding to the content cannot be expected, which results in an
economic disadvantage. Therefore, in the case where Cu is added, it is preferable
that the Cu content be 0.01% to 3.5%, more preferably 0.30% to 2.0%.
[0038] W is, moreover, also a chemical element which increases resistance to stress corrosion
cracking, resistance to sulfide stress cracking, and pitting corrosion resistance.
It is preferable that the W content be 0.01% or more in order to realize these effects,
but, since the effects become saturated in the case where the W content is more than
3.5%, effects corresponding to the content cannot be expected, which results in an
economic disadvantage. Therefore, in the case where W is added, it is preferable that
the W content be 0.01% to 3.5%, more preferably 0.30% to 2.0%.
[0039] At least one selected from Ti: 0.01% to 0.20%, Nb: 0.01% to 0.20%, and Zr: 0.01%
to 0.20%
[0040] Since Ti, Nb, and Zr are all chemical elements which tend to form carbides more than
Cr, these chemical elements are effective for suppressing Cr carbides from being precipitated
at the grain boundaries in a cooling process. Therefore, at least one of these chemical
elements may be selectively added if necessary. It is preferable that the contents
of these chemical elements be respectively Ti: 0.01% or more, Nb: 0.01% or more, and
Zr: 0.01% or more in order to realize this effect, but there is a deterioration in
weldability and toughness in the case where the contents of these chemical elements
are respectively Ti: more than 0.20%, Nb: more than 0.20%, and Zr: more than 0.20%.
Therefore, in the case where these chemical elements are added, it is preferable that
the contents of these chemical elements be respectively Ti: 0.01% to 0.20%, Nb: 0.01%
to 0.20%, and Zr: 0.01% to 0.20%, more preferably Ti: 0.020% to 0.10%, Nb: 0.020%
to 0.10%, and Zr: 0.020% to 0.10%.
[0041] At least one selected from Ca: 0.0005% to 0.0100% and REM: 0.0005% to 0.0100%
[0042] Since Ca and REM are both chemical elements which increase hot workability and manufacturing
stability in a continuous casting process as a result of controlling the form of inclusions,
these chemical elements may be selectively added if necessary. It is preferable that
the contents of these chemical elements be respectively Ca: 0.0005% or more and REM:
0.0005% or more in order to realize these effects, but there is an increase in the
amount of inclusions in the case where the contents of these chemical elements are
respectively Ca: more than 0.0100% and REM: more than 0.0100%, which results in a
deterioration in the cleanliness of steel. Therefore, in the case where these chemical
elements are added, it is preferable that the contents of these chemical elements
be respectively Ca: 0.0005% to 0.0100% and REM: 0.0005% to 0.0100%, more preferably
Ca: 0.0010% to 0.0050% and REM: 0.0010% to 0.0050%.
[0043] In the present invention, within the range of the chemical composition described
above, the contents of chemical elements are controlled under the condition that P
1 defined by equation (1) below is 11.5 or more and 13.3 or less and that P
2 defined by equation (2) below is 0 or more :
(where Cr, Mo, W, Si, C, Mn, Ni, Cu, N: the contents (mass%) of the chemical elements
represented respectively by the corresponding atomic symbols),
(where Cr: Cr content (mass%)).
[0044] P
1 is an index for evaluating hot workability and resistance to IGSCC, and in the present
invention, the contents of chemical elements are controlled to be within the ranges
described above so that P
1 is 11.5 to 13.3. In the case where P
1 is less than 11.5, since hot workability is insufficient for the manufacture of seamless
steel pipes, it is difficult to manufacture seamless steel pipes. On the other hand,
in the case where P
1 is more than 13.3, there is a deterioration in resistance to IGSCC as described above.
Similarly, in the case where P
2 is less than 0, there is a deterioration in resistance to IGSCC. Therefore, the contents
of chemical elements are to be controlled to be within the ranges described above
and to satisfy the conditions that P
1: 11.5 to 13.3 and P
2: 0 or more.
[0045] The balance of the chemical composition other than the chemical elements described
above consists of Fe and inevitable impurities. Among inevitable impurities, O: 0.010%
or less is acceptable.
[0046] The steel pipe according to the present invention has the chemical composition described
above, and, moreover, has a microstructure including martensite phase as a base phase
and including, at volume percentage, 10% to 35% of ferrite phase and 30% or less of
austenite phase. Here, the martensite phase includes tempered martensite phase. It
is preferable that the volume percentage of martensite phase be 40% or more in order
to achieve desired strength. In addition, since ferrite phase is soft and effective
for increasing workability, it is preferable that the volume percentage of ferrite
phase be 10% or more in order to increase workability. On the other hand, in the case
where the volume percentage of ferrite phase is more than 35%, it is impossible to
achieve desired high strength (X65). In addition, austenite phase increases toughness.
It is preferable that the volume percentage of austenite phase be 15% or more in order
to achieve sufficient toughness. However, in the case where the volume percentage
of austenite phase is more than 30%, it is difficult to achieve sufficient strength.
[0047] Note that regarding austenite phase, there is a case where austenite phase does
not altogether undergo transformation to martensite phase when quenching is performed
and some portion of austenite phase is retained, and there is a case where some portions
of martensite phase and ferrite phase undergo reverse transformation when tempering
is performed and are stabilized so as to be retained as austenite phase even after
cooling.
[0048] In the case of the steel pipe according to the present invention having the chemical
composition described above and the microstructure described above, if a welded zone
is formed, a microstructure in a welded heat affected zone, which is subjected to
heating up to a temperature range for forming ferrite single phase of 1300°C or higher
and to cooling when welding is performed, is formed such that 50% or more of prior-ferrite
grain boundaries, in a ratio with respect to the total length of the prior-ferrite
grain boundaries, is occupied by martensite phase. As a result, since precipitation
of Cr carbides at grain boundaries of prior-ferrite grains having a large grain diameter
can be prevented, the occurrence of IGSCC can be suppressed, which results in an increase
in resistance to IGSCC in a welded heat affected zone.
[0049] Subsequently, a preferable method for manufacturing the steel pipe according to the
present invention will be described in the case of a seamless steel pipe as an example.
[0050] First, it is preferable that molten steel having the chemical composition described
above be smelted with an ordinary smelting method such as one using a converter, an
electric furnace, or a vacuum melting furnace and that the molten steel be made into
a steel material such as a billet with an ordinary method such as a continuous casting
method or a slabbing mill method for rolling an ingot. Subsequently, the steel material
is heated and hot rolled into a seamless steel pipe having a desired size using a
Mannesmann-plug mill method or a Mannesmann-mandrel mill method. It is preferable
that the seamless steel pipe after hot rolling be cooled down to a room temperature
by an accelerated cooling at a cooling rate equal to or larger than an air-cooling
rate, preferably at an average cooling rate of 0.5°C/s or more from 800°C to 500°C.
With this method, for the steel pipe having a chemical composition according to the
present invention, a microstructure having martensite phase as a base phase as described
above can be obtained. In the case where the cooling rate is less than 0.5°C/s, a
microstructure having martensite phase as a base phase as described above cannot be
obtained. Here, "a microstructure having martensite phase as a base phase" means a
microstructure in which martensite has the largest volume percentage or in which martensite
has almost the same volume percentage as another phase which has the largest volume
percentage.
[0051] Note that instead of performing accelerated cooling after hot rolling as described
above, reheating followed by quenching and tempering may be performed. Regarding quenching,
it is preferable that the seamless steel pipe be reheated up to a temperature of 800°C
or higher and held for 10 minutes or more and that the reheated pipe be cooled down
to a temperature of 100°C or lower at a cooling rate equal to or larger than an air-cooling
rate or at an average cooling rate of 0.5°C/s or more from 800°C to 500°C. In the
case where the reheating temperature is lower than 800°C, desired microstructure having
martensite phase as a base phase cannot be achieved.
[0052] Regarding tempering, it is preferable that the quenched pipe be heated up to a temperature
of 500°C or higher and 700°C or lower, preferably 500°C or higher and 680°C or lower,
and held for a specified time and that the heated pipe be air-cooled. With this method,
desired high strength, desired high toughness, and desired excellent corrosion resistance
are achieved at the same time.
[0053] Although a method for manufacturing a steel pipe has been described in the case of
a seamless steel pipe as an example, the present invention is not limited to this.
Using a steel material (steel plate) having the chemical composition described above,
an electric resistance welded steel pipe or a UOE steel pipe may be manufactured using
an ordinary process and used as a steel pipe for linepipe. In the case of an electric
resistance welded steel pipe or a UOE steel pipe, also it is preferable that the quenching
and tempering described above be performed to have the microstructure described above.
[0054] In addition, a welded structure (steel pipe structure) may be constructed by welding
the steel pipes according to the present invention described above. Note that welding
of the steel pipes according to the present invention includes welding of the steel
pipes according to the present invention with other kinds of steel pipes. Such a welded
structure which is constructed by welding the steel pipes according to the present
invention, have a welded zone in which a welded heat affected zone, which is subjected
to heating when welding is performed, preferably up to a temperature range for forming
ferrite single phase of 1300°C or higher and to cooling, has a microstructure in which
50% or more of prior-ferrite grain boundaries, in a ratio with respect to the total
length of the prior-ferrite grain boundaries, is occupied by martensite phase and/or
austenite phase. With this, since IGSCC is suppressed, there is an increase in resistance
to IGSCC in a welded heat affected zone without performing a post weld heat treatment.
[0055] Hereafter, the present invention will be further described on the basis of EXAMPLES.
[EXAMPLES]
[0056] After molten steels having a chemical composition given in Table 1-1 and Table 1-2
had been smelted using a vacuum melting furnace and degassed, the degassed molten
steels were cast into billets of 100 kgf, and then the billets were made into steel
pipe materials having specified sizes by hot forging. These steel pipe materials were
subjected to heating and pipe making in a hot working process using a model seamless
mill (small-scale experimental seamless mil) to obtain seamless steel pipes (having
an outer diameter of 72 mmφ and a thickness of 5.5 mm).
[0057] Regarding the obtained seamless steel pipe in the cooled state after pipe making,
it was investigated by performing a visual test whether or not there were cracks on
the inner and outer surfaces to evaluate hot workability. Here, a case where there
was a crack having a length of 5 mm or more in the longitudinal end of the pipe was
evaluated as "with crack: x" and other case was evaluated as "without crack: ○".
[0058] Subsequently, a test sample (steel pipe) was cut out of the obtained seamless steel
pipe, and the test sample (steel pipe) was subjected to quenching and tempering under
the conditions given in Table 2.
[0059] Test specimens were cut out of the test sample (steel pipe) which had been subjected
to quenching and tempering, and were subjected to microstructure observation, tensile
test, impact test, corrosion test, sulfide stress test and U-bent test. The microstructure
observation and the test methods will be described hereafter.
(1) Microstructure observation
[0060] A test specimen for microstructure observation was cut out of the obtained test sample
(steel pipe). After polishing and etching the test specimen, the microstructure was
identified by taking photographs using an optical microscope (at a magnification ratio
of 1000 times), and then the percentages of ferrite phase and martensite phase of
the base steel were determined using an image analyzer. Here, the amount of γ phase
was determined using an X-ray diffraction method.
(2) Tensile test
[0061] An arc-shaped test specimen for a tensile test specified in the API standards was
cut out of the obtained test sample (steel pipe) such that the tensile direction was
the direction of the pipe axis. After performing a tensile test using the test specimen,
tensile property (yield strength YS and tensile strength TS) was determined in order
to evaluate the strength of the base steel.
(3) Impact test
[0062] A V-notch test specimen having a thickness of 5.0 mm was cut out of the obtained
test sample (steel pipe) in accordance with JIS Z 2242. After performing a Charpy
impact test using the test specimen, absorbed energy
vE
-40 (J/cm
2) at -40°C was determined in order to evaluate the toughness of the base steel.
(4) Corrosion test
[0063] A test specimen for a corrosion test having a thickness of 3 mm, a width of 25 mm,
and a length of 50 mm was cut out of the obtained test sample (steel pipe) by performing
mechanical working. After performing a corrosion test using the test specimen, corrosion
resistance (CO
2 corrosion resistance and pitting corrosion resistance) was evaluated. In the corrosion
test, a 200 g/liter NaCl aqueous solution of a temperature of 150°C in which carbon
dioxide gas of 3.0 MPa was dissolved in the saturated state was contained in an autoclave,
and the test specimen was immersed in the aqueous solution for 30 days. After the
corrosion test, by determining the weight of the test specimen, and by calculating
a corrosion rate based on a change in weight (reduction in weight) before and after
the corrosion test, CO
2 corrosion resistance was evaluated. In addition, after the corrosion test, the test
specimen was observed using a loupe at a magnification ratio of 10 times in order
to investigate whether or not pitting corrosion occurred on the surface of the test
specimen. A case where pitting occurred was evaluated as x, and a case where pitting
did not occur was evaluated as ○.
(5) Sulfide stress cracking (SSC) test
[0064] A four-point bending test specimen having a thickness of 4 mm, a width of 15 mm,
and a length of 115 mm was cut out of the obtained test sample (steel pipe). After
performing a four-point bending test in accordance with EFC (European Federation of
Corrosion) No. 17 using the test specimen, resistance to sulfide stress cracking (resistance
to SSC) was evaluated by observing where or not cracks occurred. Here, a test solution
consisting of 50 g/liter NaCl + NaHCO
3 (pH: 4.5) was used, and a mixed gas consisting of 1 vol% H
2S + 99 vol% CO
2 was flowed during the test. Further, Applied stress was equal to the YS (yield strength)
of the base steel and the test period was 720 hours (hereinafter, abbreviated as h).
A case where cracks occurred was evaluated as x, and a case where cracks did not occur
was evaluated as ○.
(6) U-bent test
[0065] A test specimen having a thickness of 4 mm, a width of 15 mm, and a length of 115
mm was cut out of the obtained test sample (steel pipe), and the welding thermal cycle
illustrated in Fig. 1 was applied to the central portion of the test specimen. After
applying the welding thermal cycle, the test specimen was polished and etched for
microstructure observation in order to investigate whether or not there were transformation
phases (martensite phase and/or austenite phase) produced from prior-α grain boundaries.
And determining the length of the prior-α grain boundaries occupied by the transformation
phases (martensite phase and/or austenite phase), a grain boundary occupancy ratio
was calculated with respect to the total length of the prior-α grain boundaries.
[0066] Moreover, a test specimen having a thickness of 2 mm, a width of 15 mm, and a length
of 75 mm was cut out of the central portion of the specimen which had been subjected
to the welding thermal cycle, and subjected to a U-bent test using the jig illustrated
in Fig. 2. In the U-bent test, as illustrated in Fig. 2, the test specimen was bent
in a U-shape having an internal radius of 8.0 mm and immersed in a corrosive solution.
Two kinds of corrosive solution below were used:
① 50 g/liter NaCl solution of temperature: 100°C, CO2 pressure: 0.1 MPa, pH: 2.0, and
② 200 g/liter NaCl solution of temperature: 150°C, CO2 pressure: 0.1 MPa, pH: 2.0.
Further, the test period was 168 h.
[0067] After the U-bent test, the cross section of the test specimen was observed using
an optical microscope at a magnification ratio of 100 times in order to investigate
whether or not cracks occurred. And resistance to IGSCC in a welded heat affected
zone (resistance to IGSCC in HAZ) was evaluated. A case where cracks occurred was
evaluated as x, and a case where cracks did not occur was evaluated as ○.
[0068] The obtained results are given in Table 3.
[0069] All of the examples of the present invention (pipe Nos. 1 through 19) were excellent
in terms of hot workability, and had high strength, that is, YS: 450 MPa or more,
high toughness, that is,
vE
-40: 50 J/cm
2 or more, high corrosion resistance, that is, corrosion rate: 0.10 mm/y or less, and
further, had no SSC and no IGSCC in HAZ which had been subjected to heating up to
a temperature of 1300°C or higher, which means that these pipes were excellent in
terms of resistance to IGSCC in HAZ.
[0070] In the case of the comparative examples (pipe Nos. 20 through 30), there was a deterioration
in hot workability, toughness, corrosion resistance, resistance to SSC, or resistance
to IGSCC in HAZ.
[0071] Specifically, in the case of pipe Nos. 20 through 23, since P2 was out of the range
according to the present invention, there was a deterioration in resistance to IGSCC
in HAZ.
[0072] In the case of pipe Nos. 24 and 25, since P1 was out of the range according to the
present invention, there was a deterioration in hot workability.
[0073] In the case of pipe No. 26, since the C content was more than the upper limit according
to the present invention, there was a deterioration in toughness.
[0074] In the case of pipe Nos. 28 through 30 which respectively corresponded to steel Nos.
F, K and M in Patent Literature 1, since the Cr content was less than the lower limit
according to the present invention, since the Ni content was more than the upper limit
according to the present invention, and since P1 was less than the lower limit according
to the present invention, the volume percentage of ferrite phase was 0% and there
was a deterioration in resistance to IGSCC in HAZ in the case of corrosive solution
② which was more intensely corrosive.
[0075] [Table 1-1]
Table 1-1
Steel No. |
Chemical Composition (mass%) |
P1 |
P2 |
Note |
C |
Si |
Mn |
P |
S |
Al |
Cr |
Ni |
Mo |
Cu |
W |
V |
Ti,Nb,Zr |
Ca,REM |
N |
A |
0.009 |
0.20 |
0.55 |
0.014 |
0.001 |
0.015 |
14.8 |
4.5 |
2.0 |
- |
- |
0.021 |
- |
- |
0.011 |
11.65 |
0.75 |
Example |
B |
0.006 |
0.15 |
0.45 |
0.012 |
0.001 |
0.021 |
14.5 |
4.2 |
1.8 |
- |
- |
0.021 |
- |
- |
0.009 |
11.62 |
0.63 |
Example |
C |
0.008 |
0.34 |
0.53 |
0.011 |
0.002 |
0.028 |
14.4 |
4.4 |
2.2 |
- |
- |
0.031 |
- |
- |
0.009 |
11.66 |
0.54 |
Example |
D |
0.010 |
0.20 |
0.68 |
0.013 |
0.002 |
0.017 |
14.2 |
4.6 |
3.2 |
1.04 |
- |
0.015 |
- |
- |
0.009 |
11.76 |
0.34 |
Example |
E |
0.010 |
0.21 |
1.21 |
0.009 |
0.002 |
0.030 |
14.0 |
4.4 |
2.8 |
- |
1.06 |
0.032 |
- |
- |
0.010 |
11.88 |
0.12 |
Example |
F |
0.010 |
0.19 |
1.30 |
0.014 |
0.002 |
0.018 |
13.5 |
3.2 |
2.1 |
1.21 |
1.53 |
0.028 |
- |
- |
0.011 |
11.65 |
0.10 |
Example |
G |
0.009 |
0.35 |
0.54 |
0.008 |
0.001 |
0.029 |
14.8 |
4.3 |
2.0 |
- |
- |
0.024 |
Ti/0.085 |
- |
0.012 |
11.89 |
0.51 |
Example |
H |
0.009 |
0.42 |
0.19 |
0.016 |
0.002 |
0.026 |
14.0 |
4.6 |
2.6 |
- |
- |
0.031 |
Nb/0.048 |
- |
0.008 |
11.59 |
0.41 |
Example |
I |
0.009 |
0.41 |
0.42 |
0.012 |
0.002 |
0.022 |
14.5 |
4.5 |
2.0 |
1.02 |
1.03 |
0.045 |
Zr/0.045 |
- |
0.011 |
11.57 |
0.68 |
Example |
J |
0.010 |
0.16 |
0.54 |
0.012 |
0.001 |
0.015 |
14.2 |
4.2 |
2.3 |
- |
- |
0.020 |
- |
Ca/0.0023 |
0.012 |
11.59 |
0.51 |
Example |
K |
0.009 |
0.20 |
0.49 |
0.011 |
0.001 |
0.027 |
13.5 |
3.1 |
1.9 |
- |
- |
0.016 |
- |
REM/0.0044 |
0.009 |
11.69 |
0.06 |
Example |
L |
0.012 |
0.21 |
0.89 |
0.014 |
0.002 |
0.020 |
13.4 |
2.4 |
2.5 |
3.2 |
- |
0.045 |
- |
Ca/0.0046 |
0.012 |
11.62 |
0.08 |
Example |
M |
0.006 |
0.19 |
0.61 |
0.008 |
0.002 |
0.015 |
14.7 |
4.6 |
2.0 |
- |
1.15 |
0.023 |
Ti/0.098 |
Ca/0.0034 |
0.012 |
12.00 |
0.35 |
Example |
N |
0.010 |
0.21 |
0.53 |
0.013 |
0.001 |
0.020 |
14.8 |
3.4 |
2.1 |
- |
- |
0.041 |
- |
- |
0.011 |
12.82 |
-0.42 |
Comparative Example |
O |
0.009 |
0.32 |
0.40 |
0.011 |
0.001 |
0.030 |
14.0 |
3.1 |
2.0 |
- |
- |
0.020 |
- |
- |
0.010 |
12.35 |
-0.35 |
Comparative Example |
[0076] [Table 1-2]
Table 1-2
Steel No. |
Chemical Composition (mass%) |
P1 |
P2 |
Note |
C |
Si |
Mn |
P |
S |
Al |
Cr |
Ni |
Mo |
Cu |
W |
V |
Ti,Nb,Zr |
Ca,REM |
N |
P |
0.010 |
0.15 |
0.40 |
0.013 |
0.002 |
0.020 |
14.0 |
3.0 |
2.1 |
- |
0.65 |
0.026 |
Nb/0.035 |
- |
0.010 |
12.72 |
-0.72 |
Comparative Example |
Q |
0.012 |
0.15 |
1.23 |
0.017 |
0.002 |
0.034 |
13.6 |
2.6 |
2.1 |
- |
- |
0.027 |
- |
- |
0.012 |
12.02 |
-0.22 |
Comparative Example |
R |
0.010 |
0.25 |
1.27 |
0.013 |
0.002 |
0.031 |
14.2 |
4.2 |
2.0 |
- |
- |
0.022 |
- |
- |
0.012 |
11.02 |
1.08 |
Comparative Example |
S |
0.010 |
0.26 |
1.23 |
0.009 |
0.001 |
0.033 |
13.6 |
3.2 |
2.1 |
1.5 |
- |
0.019 |
- |
- |
0.012 |
11.09 |
0.71 |
Comparative Example |
T |
0.025 |
0.20 |
0.58 |
0.012 |
0.001 |
0.020 |
14.8 |
3.8 |
2.2 |
- |
- |
0.022 |
- |
- |
0.011 |
11.84 |
0.56 |
Comparative Example |
U |
0.009 |
0.20 |
0.45 |
0.012 |
0.001 |
0.022 |
14.8 |
3.7 |
1.2 |
- |
- |
0.021 |
- |
- |
0.011 |
11.69 |
0.71 |
Comparative Example |
V |
0.0068 |
0.24 |
0.61 |
0.017 |
0.002 |
0.018 |
12.6 |
6.1 |
2.3 |
- |
- |
0.051 |
Ti/0.072 |
Ca/0.0022 |
0.0078 |
8.26 |
3.04 |
Comparative Example (Steel F in Patent Literature 1) |
W |
0.0083 |
0.49 |
1.18 |
0.019 |
0.002 |
0.029 |
12.9 |
6.5 |
2.1 |
- |
- |
0.051 |
Ti/0.065, Nb/0.031, Zr/0.026 |
Ca/0.0010 |
0.0082 |
7.74 |
3.71 |
Comparative Example (Steel K in Patent Literature 1) |
X |
0.0085 |
0.13 |
0.46 |
0.015 |
0.001 |
0.031 |
12.5 |
5.6 |
2.6 |
- |
- |
0.064 |
Ti/0.059, Nb/0.021, Zr/0.026 |
Ca/0.0018 |
0.0062 |
8.93 |
2.32 |
Comparative Example (Steel M in Patent Literature 1) |
[0077] [Table 2]
Table 2
Pipe No. |
Steel No. |
Cooling after Hot Rolling |
Quenching |
Tempering |
Note |
Cooling Method |
Cooling Rate (°C/s) |
Quenching Temperature (°C) |
Holding Time (min) |
Cooling Rate (°C/s) |
Tempering Temperature (°C) |
1 |
A |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
600 |
Example |
2 |
A |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
3 |
A |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
640 |
Example |
4 |
B |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
600 |
Example |
5 |
B |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
6 |
B |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
640 |
Example |
7 |
C |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
600 |
Example |
8 |
C |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
9 |
C |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
640 |
Example |
10 |
D |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
11 |
E |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
12 |
F |
Air-cooling |
3.3 |
930 |
20 |
3.3 |
620 |
Example |
13 |
G |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
14 |
H |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
15 |
I |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
16 |
J |
Air-cooling |
3.3 |
900 |
20 |
50 |
620 |
Example |
17 |
K |
Air-cooling |
3.3 |
930 |
20 |
50 |
650 |
Example |
18 |
L |
Air-cooling |
3.3 |
930 |
20 |
3.3 |
650 |
Example |
19 |
M |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Example |
20 |
N |
Air-cooling |
3.3 |
930 |
20 |
3.3 |
650 |
Comparative Example |
21 |
O |
Air-cooling |
3.3 |
930 |
20 |
3.3 |
650 |
Comparative Example |
22 |
P |
Air-cooling |
3.3 |
930 |
20 |
3.3 |
650 |
Comparative Example |
23 |
Q |
Air-cooling |
3.3 |
930 |
20 |
3.3 |
650 |
Comparative Example |
24 |
R |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Comparative Example |
25 |
S |
Air-cooling |
3.3 |
930 |
20 |
3.3 |
620 |
Comparative Example |
26 |
T |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Comparative Example |
27 |
U |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
650 |
Comparative Example |
28 |
V |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Comparative Example (Steel F in Patent Literature 1) |
29 |
W |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Comparative Example (Steel K in Patent Literature 1) |
30 |
X |
Air-cooling |
3.3 |
900 |
20 |
3.3 |
620 |
Comparative Example (Steel M in Patent Literature 1) |
[0078] [Table 3-1]
Table 3-1
Pipe No. |
Steel No. |
P1 |
P2 |
Hot Workability |
Phase Percentage of Base steel (%) *) |
Tensile Property |
Toughness |
CO2 Corrosion Resistance |
Resistance to SSC |
Resistance to IGSCC in HAZ |
Note |
M |
F |
γ |
YS (MPa) |
TS (MPa) |
vE-40 (J/cm2) |
Corrosion Rate (mm/y) |
with or without Pitting |
with or without Crack |
Grain Boundary Occupancy Ratio**) |
with or without Crack ① |
with or without Crack ② |
1 |
A |
11.65 |
0.75 |
○ |
64 |
18 |
18 |
631 |
761 |
123 |
0.05 |
○ |
○ |
100 |
○ |
○ |
Example |
2 |
A |
11.65 |
0.75 |
○ |
61 |
17 |
22 |
573 |
738 |
140 |
0.06 |
○ |
○ |
100 |
○ |
○ |
Example |
3 |
A |
11.65 |
0.75 |
○ |
56 |
18 |
26 |
504 |
700 |
211 |
0.07 |
○ |
○ |
92 |
○ |
○ |
Example |
4 |
B |
11.91 |
0.44 |
○ |
71 |
12 |
17 |
604 |
765 |
168 |
0.05 |
○ |
○ |
97 |
○ |
○ |
Example |
5 |
B |
11.91 |
0.44 |
○ |
66 |
12 |
22 |
564 |
735 |
172 |
0.06 |
○ |
○ |
85 |
○ |
○ |
Example |
6 |
B |
11.91 |
0.44 |
○ |
62 |
13 |
25 |
514 |
703 |
225 |
0.08 |
○ |
○ |
100 |
○ |
○ |
Example |
7 |
C |
11.66 |
0.54 |
○ |
67 |
18 |
15 |
593 |
740 |
154 |
0.05 |
○ |
○ |
100 |
○ |
○ |
Example |
8 |
C |
11.66 |
0.54 |
○ |
62 |
17 |
21 |
553 |
729 |
165 |
0.07 |
○ |
○ |
95 |
○ |
○ |
Example |
9 |
C |
11.66 |
0.54 |
○ |
59 |
17 |
24 |
517 |
718 |
183 |
0.08 |
○ |
○ |
100 |
○ |
○ |
Example |
10 |
D |
11.76 |
0.34 |
○ |
63 |
22 |
15 |
596 |
785 |
231 |
0.03 |
○ |
○ |
100 |
○ |
○ |
Example |
11 |
E |
11.88 |
0.12 |
○ |
64 |
20 |
16 |
611 |
839 |
172 |
0.05 |
○ |
○ |
92 |
○ |
○ |
Example |
12 |
F |
11.65 |
0.10 |
○ |
60 |
28 |
12 |
600 |
773 |
99 |
0.07 |
○ |
○ |
100 |
○ |
○ |
Example |
13 |
G |
11.89 |
0.51 |
○ |
65 |
21 |
14 |
617 |
823 |
127 |
0.06 |
○ |
○ |
100 |
○ |
○ |
Example |
14 |
H |
11.59 |
0.41 |
○ |
53 |
25 |
22 |
589 |
773 |
160 |
0.06 |
○ |
○ |
100 |
○ |
○ |
Example |
15 |
I |
11.57 |
0.68 |
○ |
67 |
17 |
16 |
573 |
769 |
154 |
0.05 |
○ |
○ |
98 |
○ |
○ |
Example |
16 |
J |
11.59 |
0.51 |
○ |
56 |
26 |
18 |
637 |
857 |
137 |
0.05 |
○ |
○ |
99 |
○ |
○ |
Example |
17 |
K |
11.69 |
0.06 |
○ |
70 |
16 |
14 |
599 |
807 |
118 |
0.09 |
○ |
○ |
96 |
○ |
○ |
Example |
18 |
L |
11.62 |
0.08 |
○ |
64 |
26 |
10 |
594 |
764 |
74 |
0.09 |
○ |
○ |
100 |
○ |
○ |
Example |
*) F: ferrite, M: martensite, γ: austenite
**) the percentage of prior-ferrite grain boundaries occupied by martensite phase
or austenite phase with respect to the total length of the prior-ferrite grain boundaries
Resistance to IGSCC ① NaCl: 50 g/liter, 100°C, CO2 pressure: 0.1 MPa, pH: 2.0
Resistance to IGSCC ② NaCl: 200 g/liter, 100°C, CO2 pressure: 0.1 MPa, pH: 2.0 |
[0079] [Table 3-2]
Table 3-2
Pipe No. |
Steel No. |
P1 |
P2 |
Hot Workability |
Phase Percentage of Base steel (%) *) |
Tensile Property |
Toughness |
Resistance to CO2 Corrosion |
Resistance to SSC |
Resistance to IGSCC in HAZ |
Note |
M |
F |
γ |
YS (MPa) |
TS (MPa) |
vE-40 (J/cm2) |
Corrosion Rate (mm/y) |
with or without Pitting |
with or without Crack |
Grain Boundary Occupancy Ratio **) |
with or without Crack ① |
with or without Crack ② |
19 |
M |
12.00 |
0.35 |
○ |
55 |
27 |
18 |
606 |
789 |
205 |
0.06 |
○ |
○ |
99 |
○ |
○ |
Example |
20 |
N |
12.82 |
-0.42 |
○ |
65 |
20 |
15 |
561 |
739 |
131 |
0.07 |
○ |
○ |
59 |
× |
× |
Comparative Example |
21 |
O |
12.35 |
-0.35 |
○ |
65 |
20 |
15 |
594 |
787 |
118 |
0.07 |
○ |
○ |
69 |
× |
× |
Comparative Example |
22 |
P |
12.72 |
-0.72 |
○ |
63 |
25 |
12 |
583 |
801 |
111 |
0.07 |
○ |
○ |
71 |
× |
× |
Comparative Example |
23 |
Q |
12.02 |
-0.22 |
○ |
77 |
16 |
7 |
590 |
800 |
75 |
0.09 |
○ |
○ |
84 |
× |
× |
Comparative Example |
24 |
R |
11.02 |
1.08 |
× |
56 |
26 |
18 |
592 |
799 |
119 |
0.08 |
○ |
○ |
100 |
○ |
○ |
Comparative Example |
25 |
S |
11.09 |
0.71 |
× |
61 |
27 |
12 |
616 |
795 |
100 |
0.09 |
○ |
○ |
100 |
○ |
○ |
Comparative Example |
26 |
T |
11.84 |
0.56 |
○ |
55 |
31 |
14 |
598 |
790 |
32 |
0.06 |
○ |
○ |
87 |
○ |
○ |
Comparative Example |
27 |
U |
11.69 |
0.71 |
○ |
56 |
27 |
17 |
580 |
774 |
117 |
0.07 |
× |
× |
97 |
○ |
○ |
Comparative Example |
28 |
V |
8.26 |
3.04 |
○ |
83 |
0 |
17 |
608 |
770 |
204 |
0.048 |
○ |
○ |
- |
○ |
× |
Comparative Example (Steel F in Patent Literature 1) |
29 |
W |
7.74 |
3.71 |
○ |
86 |
0 |
14 |
619 |
814 |
219 |
0.060 |
○ |
○ |
- |
○ |
× |
Comparative Example (Steel K in Patent Literature 1) |
30 |
X |
8.93 |
2.32 |
○ |
88 |
0 |
12 |
639 |
864 |
250 |
0.092 |
○ |
○ |
- |
○ |
× |
Comparative Example (Steel M in Patent Literature 1) |
*) F: ferrite, M: martensite, y: austenite
**) the percentage of prior-ferrite grain boundaries occupied by martensite phase
or austenite phase with respect to the total length of the prior-ferrite grain boundaries
Resistance to IGSCC ① NaCl: 50 g/liter, 100°C, CO2 pressure: 0.1 MPa, pH: 2.0
Resistance to IGSCC ② NaCl: 200 g/liter, 100°C, CO2 pressure: 0.1 MPa, pH: 2.0 |