[Technical Field of the Invention]
[0001] The present invention relates to a steel pipe and a steel plate.
[Related Art]
[0002] Recently, demands for petroleum, natural gas, and the like have increased, and the
diversification of energy supply sources has progressed. Therefore, in a harsh corrosive
environment at which development was abandoned in the past, for example, in a corrosive
environment including hydrogen sulfide, carbon dioxide gas, or chlorine ions, the
digging of crude oil or natural gas has been actively performed. Accordingly, improvement
of SSC resistance and HIC resistance is required for a steel pipe (steel pipe for
a line pipe) used for a pipeline that transports crude oil, natural gas, or the like.
[0003] In addition, high-strengthening is required for the steel pipe for a line pipe in
order to reduce the thickness for saving a material or to reduce the weight of a product.
However, when the addition amount of an alloying element is increased to improve strength
or the amount of heat input is increased for high-efficiency welding, the low-temperature
toughness of a heat-affected zone (HAZ) deteriorates.
[0004] For example, as disclosed in Patent Documents 1 and 2, a steel pipe having HIC resistance
has been manufactured using a technique such as an increase in the purity of steel,
a reduction in the amount of an inclusion, a control of the form of a sulfide inclusion
by addition of Ca, or suppression of center segregation by light rolling reduction
during casting or by accelerated cooling.
[0005] However, in the steel pipe disclosed in Patent Documents 1 and 2, SSC resistance
is not considered at all. Therefore, it is presumed that, in the steel pipe disclosed
in Patent Documents 1 and 2, HIC resistance is satisfactory but sulfide stress cracking
(SSC) resistance is not sufficient.
[Prior Art Document]
[Patent Document]
[0006]
[Patent Document 1] Japanese Examined Patent Application, Second Publication No. S63-001369
[Patent Document 2] Japanese Unexamined Patent Application, First Publication No.
S62-112722
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0007] An object of the present invention is to provide: a steel pipe that is suitable for
a line pipe and has a strength of API X52 to X70 grade and satisfactory SSC resistance
and HIC resistance; and a steel plate that is used as a base metal of the steel pipe.
[Means for Solving the Problem]
[0008] The present invention has been made based on the above-described object, and the
scope thereof is as follows.
- (1) According to one aspect of the present invention, there is provided a steel pipe
including:
a base metal that includes a cylindrical steel plate; and
a weld that is provided in a seam portion of the steel plate and extends in a longitudinal
direction of the steel plate,
in which the steel plate includes, as a chemical composition, by mass%,
C: 0.030% to 0.070%,
Si: 0.005% to 0.50%,
Mn: 1.05% to 1.65%,
Al: 0.010% to 0.070%,
Ti: 0.005% to 0.020%,
Nb: 0.005% to 0.045%,
Ca: 0.0010% to 0.0050%,
N: 0.0015% to 0.0070%,
Ni: 0% to 0.50%,
Mo: 0% to 0.50%,
Cr: 0% to 0.50%,
Cu: 0% to 0.50%,
V: 0% to 0.100%,
Mg: 0% to 0.0100%,
REM: 0% to 0.0100%,
P: limited to 0.015% or less,
S: limited to 0.0015% or less,
O: limited to 0.0040% or less, and
a remainder of Fe and impurities,
Ceq defined by the following Expression (i) in the chemical composition is 0.300 to
0.400,
a surface layer microstructure that is a microstructure in a range up to 1.0 mm from
a surface of the base metal in a depth direction includes a polygonal ferrite and
a granular bainite,
an area fraction of the polygonal ferrite in the surface layer microstructure is 0
to 70%,
a total area fraction of the polygonal ferrite and the granular bainite in the surface
layer microstructure is 50% or more,
a maximum hardness in the surface layer microstructure is 270 Hv or lower,
an area fraction of a polygonal ferrite in an internal microstructure that is a microstructure
in a range up to a thickness center from an area positioned at a distance of more
than 1.0 mm from the surface of the base metal in the depth direction is 40% or less,
a maximum hardness in the internal microstructure is 248 Hv or lower, and
an average hardness in the internal microstructure is 150 to 220 Hv,
where [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] represent the amounts of C, Mn, Ni,
Cu, Cr, Mo, and V by mass%.
- (2) In the steel pipe according to (1), the chemical composition may include, by mass%,
one or more selected from the group consisting of Ni: 0.05% to 0.50%, Mo: 0.05% to
0.50%, Cr: 0.05% to 0.50%, Cu: 0.05% to 0.50%, V: 0.010% to 0.100%, Mg: 0.0001% to
0.0100%, and REM: 0.0001% to 0.0100%.
- (3) In the steel pipe according to (1) or (2), a remainder of the surface layer microstructure
may consist of one or more selected from the group consisting of a bainite and a pseudo
pearlite, and a remainder of the internal microstructure may consist of one or more
selected from the group consisting of a granular bainite, a bainite, and a pseudo
pearlite.
- (4) According to another aspect of the present invention, there is provided a steel
plate that is used as the base metal of the steel pipe according to any one of (1)
to (3).
[Effects of the Invention]
[0009] According to the aspects of the present invention, it is possible to provide: a steel
plate that is suitable for a line pipe and has a strength of API X52 to X70 grade
and satisfactory SSC resistance and HIC resistance; and steel pipe that uses the steel
plate as a base metal and has satisfactory SSC resistance and HIC resistance. Specifically,
it is possible to provide a steel pipe having satisfactory sulfide stress cracking
resistance (SSC resistance) and hydrogen induced cracking resistance (HIC resistance)
and a steel plate that is used as a base metal of the steel pipe. The steel pipe having
satisfactory sour resistance (SSC resistance and HIC resistance) is suitable as a
line pipe that transports petroleum, natural gas, or the like.
[Brief Description of the Drawings]
[0010]
FIG. 1 is a schematic diagram showing a steel pipe according to an embodiment of the
present invention.
FIG. 2 is a diagram showing an example of cooling curves of a steel plate.
FIG. 3A is a diagram showing the results of measuring a hardness (load: 100 g) of
a surface layer microstructure in a range of 0.1 to 1.0 mm from the surface at a position
corresponding to 3 O'clock when a weld of the steel pipe is determined as a 0 O'clock
position.
FIG. 3B is a diagram showing the results of measuring a hardness (load: 100 g) of
the surface layer microstructure in a range of 0.1 to 1.0 mm from the surface at a
position corresponding to 6 O'clock when the weld of the steel pipe is determined
as the 0 O'clock position.
FIG. 3C is a diagram showing the results of measuring a hardness (load: 100 g) of
the surface layer microstructure in a range of 0.1 to 1.0 mm from the surface at a
position corresponding to 9 O'clock when the weld of the steel pipe is determined
as the 0 O'clock position.
FIG. 4 is a diagram showing an example of a SEM image of the surface layer microstructure.
FIG. 5 is a diagram showing an example of a SEM image of an internal microstructure.
[Embodiments of the Invention]
[0011] A steel pipe according to one embodiment of the present invention (hereinafter, also
referred to as the steel pipe according to the embodiment) includes:
a base metal that includes a cylindrical steel plate; and
a weld that is provided in a seam portion of the steel plate and extends in a longitudinal
direction of the steel plate,
in which the steel plate includes, as a chemical composition, by mass%,
C: 0.030% to 0.070%,
Si: 0.005% to 0.50%,
Mn: 1.05% to 1.65%,
Al: 0.010% to 0.070%,
Ti: 0.005% to 0.020%,
Nb: 0.005% to 0.045%,
Ca: 0.0010% to 0.0050%,
N: 0.0015% to 0.0070%,
P: limited to 0.015% or less,
S: limited to 0.0015% or less,
O: limited to 0.0040% or less,
the steel plate optionally further includes, by mass%, one or more selected from the
group consisting of Ni: 0.05% to 0.50%, Mo: 0.05% to 0.50%, Cr: 0.05% to 0.50%, Cu:
0.05% to 0.50%, V: 0.010% to 0.100%, Mg: 0.0001% to 0.0100%, and REM: 0.0001% to 0.0100%,
the steel plate includes a remainder of Fe and impurities,
Ceq represented by Ceq=[C]+[Mn]/6+([Ni]+[Cu])/15+([Cr]+[Mo]+[V])/5 is preferably 0.300
to 0.400,
a surface layer microstructure that is a microstructure in a range up to 1.0 mm from
a surface of the base metal in a depth direction includes polygonal ferrite and granular
bainite,
an area fraction of the polygonal ferrite in the surface layer microstructure is 0
to 70%,
a total area fraction of the polygonal ferrite and the granular bainite in the surface
layer microstructure is 50% or more,
the remainder may include bainite (including tempered bainite), pseudo pearlite, or
a mixture thereof,
a maximum hardness in the surface layer microstructure is 270 Hv or lower and preferably
250 Hv or lower,
an internal microstructure that is a microstructure in a range up to a thickness center
from an area positioned at a distance of more than 1.0 mm from the surface of the
base metal in the depth direction includes, by an area fraction, 40% or less of polygonal
ferrite,
the remainder may include granular bainite, bainite, pseudo pearlite, or a mixture
thereof,
a maximum hardness in the internal microstructure is 248 Hv or lower, and
an average hardness in the internal microstructure is 150 to 220 Hv.
[0012] In addition, a steel plate according to one embodiment of the present invention (hereinafter,
referred to as the steel plate according to the embodiment) is used as the base metal
of the steel pipe.
[0013] Hereinafter, the steel pipe according to the embodiment, the steel plate according
to the embodiment, and preferable methods of manufacturing the same will be described.
[0014] First, the base metal (that is, the steel plate according to the embodiment) of the
steel pipe according to the embodiment will be described.
(I) Chemical Composition
[0015] The reason for limiting the chemical composition of the base metal (the steel plate
according to the embodiment) of the steel pipe according to the embodiment will be
described. Hereinafter, "%" regarding the chemical composition represents "mass%".
C: 0.030% to 0.070%
[0016] C is an element that is required to improve the strength of the steel. When the C
content is less than 0.030%, the effect of improving the strength cannot be sufficiently
obtained. Therefore, the C content is 0.030% or more. The C content is preferably
0.040% or more.
[0017] On the other hand, when the C content is more than 0.070%, the strength of the steel
excessively increases, the hardness of the surface layer microstructure and the internal
microstructure, in particular, the center segregation portion is higher than 248 Hv,
and SSC resistance and HIC resistance deteriorate. Therefore, the C content is 0.070%
or less. From the viewpoint of suppressing deterioration in weldability, toughness,
and the like, the C content is preferably 0.050% or less.
Si: 0.005% to 0.50%
[0018] Si is an element that functions as a deoxidation material during steelmaking. In
addition, Si is an element that is unavoidably incorporated during steelmaking. When
the Si content is less than 0.005%, the effect cannot be sufficiently obtained. Therefore,
the Si content is 0.005% or more. From the viewpoint of sufficiently obtaining the
deoxidation effect, the Si content is 0.050% or more.
[0019] On the other hand, when the Si content is more than 0.50%, toughness of a heat-affected
zone (HAZ) deteriorates. Therefore, the Si content is 0.50% or less. The Si content
is preferably 0.35% or less.
Mn: 1.05% to 1.65%
[0020] Mn is an element contributing to the strength and toughness of the steel. When the
Mn content is less than 1.05%, the effect of improving the strength and toughness
cannot be sufficiently obtained. Therefore, the Mn content is 1.05% or more. The Mn
content is preferably 1.15% or more.
[0021] On the other hand, when the Mn content is more than 1.65%, a large amount of MnS
that deteriorates HIC resistance is formed, the hardness of the internal microstructure,
in particular, the center segregation portion is higher than 248 Hv, and HIC resistance
deteriorates. Therefore, the Mn content is set to be 1.65% or less. The Mn content
is preferably 1.50% or less.
Al: 0.010% to 0.070%
[0022] Al is an element that is added for deoxidation. When the Al content is less than
0.010%, the effect cannot be sufficiently obtained. Therefore, the Al content is 0.010%
or more. The Al content is 0.020% or more.
[0023] On the other hand, when the Al content is more than 0.070%, a cluster on which an
Al oxide accumulates is formed, and HIC resistance deteriorates. Therefore, the Al
content is 0.070% or less. The Al content is preferably 0.045% or less.
Ti: 0.005% to 0.020%
[0024] Ti is an element that is bonded to N and forms a nitride. This nitride contributes
refinement of crystal grains. When the Ti content is less than 0.005%, the effect
cannot be sufficiently obtained. Therefore, the Ti content is 0.005% or more. The
Ti content is preferably 0.008% or more.
[0025] On the other hand, when the Ti content is more than 0.020%, a coarse nitride is formed,
and HIC resistance deteriorates. Therefore, the Ti content is 0.020% or less. The
Ti content is 0.015% or less.
Nb: 0.005% to 0.045%
[0026] Nb is an element that refines crystal grains by widening non-recrystallization temperature
range, form a carbide or a nitride and contributes to improvement of the strength
of the steel. When the Nb content is less than 0.005%, the effect cannot be sufficiently
obtained. Therefore, the Nb content is 0.005% or more. The Nb content is preferably
0.010% or more.
[0027] On the other hand, when the Nb content is more than 0.045%, a coarse carbide or nitride
is formed, and HIC resistance deteriorates. Therefore, the Nb content is 0.045% or
less. The Nb content is 0.035% or less.
Ca: 0.0010% to 0.0050%
[0028] Ca is an element that is bonded to S to form CaS, suppresses formation of MnS stretched
in a rolling direction, and thus contributes improvement of HIC resistance. When the
Ca content is less than 0.0010%, the effect cannot be sufficiently obtained. Therefore,
the Ca content is 0.0010% or more. The Ca content is preferably 0.0020% or more.
[0029] On the other hand, when the Ca content is more than 0.0050%, a Ca oxide accumulates,
and HIC resistance deteriorates. Therefore, the Ca content is 0.0050% or less. The
Ca content is preferably 0.0040% or less.
N: 0.0015% to 0.0070%
[0030] N is an element that forms a nitride and contributes to suppressing the coarsening
of austenite grains during heating. When the N content is less than 0.0015%, the effect
cannot be sufficiently obtained. Therefore, the N content is 0.0015% or more. The
N content is preferably 0.0020% or more.
[0031] On the other hand, when the N content is more than 0.0070%, a coarse carbonitride
is formed, and HIC resistance deteriorates. Therefore, the N content is 0.0070% or
less. The N content is preferably 0.0050% or less.
[0032] In order to improve strength, toughness, and other characteristics, in addition to
the above-described elements, the chemical composition of the base metal (the steel
plate according to the embodiment) of the steel pipe according to the embodiment may
include one or more selected from the group consisting of Ni, Mo, Cr, Cu, V, Mg, and
REM instead of a part of Fe within a range where the characteristics of the steel
plate according to the embodiment do not deteriorate. These elements are optional
elements and are not necessarily included. That is, the lower limits of the amounts
of the elements are 0%.
Ni: 0% to 0.50%
[0033] Ni is an element that contributes to improvement of toughness and strength of the
steel and improvement of corrosion resistance. In order to obtain these effects, the
Ni content is preferably 0.05% or more. The Ni content is more preferably 0.10% or
more.
[0034] On the other hand, when the Ni content is more than 0.50%, strength excessively increases,
toughness deteriorates, and SSC resistance deteriorates due to grain boundary selective
corrosion of the surface. Therefore, even when Ni is included, the Ni content is preferably
0.50% or less. The Ni content is preferably 0.35% or less.
Mo: 0% to 0.50%
[0035] Mo is an element that contributes to improvement of the hardenability of the steel.
In order to obtain this effect, the Mo content is preferably 0.05% or more. The Mo
content is more preferably 0.10% or more. On the other hand, when the Mo content is
more than 0.50%, strength excessively increases, and toughness deteriorates. Therefore,
even when Mo is included, the Mo content is 0.50% or less. The Mo content is preferably
0.35% or less.
Cr: 0% to 0.50%
[0036] Cr is an element that contributes to improvement of the strength of the steel. In
order to obtain this effect, the Cr content is preferably 0.05% or more. The Cr content
is more preferably 0.10% or more. On the other hand, when the Cr content is more than
0.50%, strength excessively increases, and toughness deteriorates. Therefore, even
when Cr is included, the Cr content is 0.50% or less. The Cr content is preferably
0.35% or less.
Cu: 0% to 0.50%
[0037] Cu is an element that contributes to improvement of the strength of the steel and
improvement of corrosion resistance. In order to obtain these effects, the Cu content
is preferably 0.05% or more. The Cu content is more preferably 0.10% or more. On the
other hand, when the Cu content is more than 0.50%, strength excessively increases,
and toughness deteriorates. Therefore, even when Cu is included, the Cu content is
0.50% or less. The Cu content is preferably 0.35% or less.
V: 0% to 0.100%
[0038] V is an element that forms a carbide and/or a nitride and contributes improvement
of the strength of the steel. In order to obtain this effect, the V content is preferably
0.010% or more. The V content is more preferably 0.030% or more. On the other hand,
when the V content is more than 0.100%, toughness deteriorates. Therefore, even when
V is included, the V content is preferably 0.100% or less. The V content is preferably
0.080% or less.
Mg: 0% to 0.0100%
[0039] Mg is an element that forms a fine oxide to suppress the coarsening of crystal grains
and contributes improvement of the toughness of the steel. In order to obtain this
effect, the Mg content is preferably 0.0001% or more. The Mg content is more preferably
0.0010% or more.
[0040] On the other hand, when the Mg content is more than 0.0100%, an oxide aggregates
and is coarsened, and HIC resistance and toughness deteriorate. Therefore, even when
Mg is included, the Mg content is 0.0100% or less. The Mg content is preferably 0.0050%
or less.
REM: 0% to 0.0100%
[0041] REM is an element that contributes to improvement of SSC resistance, HIC resistance,
and toughness by controlling the form of a sulfide inclusion. In order to obtain these
effects, the REM content is preferably 0.0001 % or more. The REM content is more preferably
0.0010% or more.
[0042] On the other hand, when the REM content is more than 0.0100%, an oxide is formed,
the cleanliness of the steel deteriorates, and HIC resistance and toughness deteriorate.
Therefore, even when REM is included, the REM content is 0.0100% or less. The REM
content is preferably 0.0060% or less.
[0043] In the embodiment, REM refers to rare earth elements and is a collective term for
17 elements including Sc, Y, and lanthanoids. The REM content refers to the total
amount of the 17 elements.
[0044] As described above, the base metal (the steel plate according to the embodiment)
of the steel pipe according to the embodiment basically has the chemical composition
including the above-described essential elements and the remainder consisting of Fe
and impurities. However, the base metal may have a chemical composition including
the above-described essential elements, the above-described optional elements, and
the remainder consisting of Fe and impurities.
[0045] Here, the impurities refer to elements which are incorporated from raw materials
such as ore or scrap or incorporated in various environments of the production process
when the steel is industrially produced, and the impurities are allowed to be included
in the steel in a range where there are no adverse effects on the steel.
[0046] Among the impurities, it is preferable that P, S, O, Sb, Sn, Co, As, Pb, Bi, and
H are controlled to be in ranges described below.
P: 0.015% or less
[0047] P is an impurity element, and the less the P content, the better. When the P content
is more than 0.015%, HIC resistance significantly deteriorates. Therefore, the P content
is 0.015% or less. The P content is preferably 0.010% or less.
[0048] The less the P content, the better, and the lower limit thereof may be 0%. However,
when the P content is reduced to less than 0.003%, the manufacturing costs significantly
increase. Therefore, the lower limit of the P content in the actual steel plate is
substantially 0.003%.
S: 0.0015% or less
[0049] S is an element that forms MnS stretched in a rolling direction during hot rolling.
This stretched MnS deteriorates HIC resistance. When the S content is more than 0.0015%,
HIC resistance significantly deteriorates. Therefore, the S content is 0.0015% or
less. The S content is preferably 0.0010% or less.
[0050] The less the S content, the better, and the lower limit thereof may be 0%. However,
when the S content is reduced to less than 0.0001%, the manufacturing costs significantly
increase. Therefore, the upper limit of the S content in the actual steel plate is
substantially 0.0001%.
O: 0.0040% or less
[0051] O is an element that avoidably remains after deoxidation. The less the O content,
the better. When the O content is more than 0.0040%, a large amount of an oxide is
formed, and HIC resistance significantly deteriorates. Therefore, the O content is
0.0040% or less. The O content is preferably 0.0030% or less.
[0052] The less the O content, the better, and the lower limit thereof may be 0%. However,
when the O content is reduced to less than 0.0010%, the manufacturing costs significantly
increase. Therefore, the lower limit of the O content in the actual steel plate is
substantially 0.0010%.
[0053] In addition, in consideration of the influence on the steel plate characteristics
and the steel pipe characteristics, for example, the content each of Sb, Sn, Co, and
As is 0.10% or less, the amount of each of Pb and Bi is 0.005% or less, and the H
content is preferably 0.0005% or less.
Ceq: 0.300 to 0.400
[0054] In the steel pipe according to the embodiment, in order to further improve strength,
SSC resistance, and HIC resistance, the chemical composition of the steel plate used
for the base metal of the steel pipe satisfies not only the amount of each of the
elements, but also Ceq (carbon equivalent) defined by the following Expression (1)
is 0.400 or less.
[0055] In the expression, [C], [Mn], [Ni], [Cu], [Cr], [Mo], and [V] represent the contents
(mass%) of C, Mn, Ni, Cu, Cr, Mo, and V.
[0056] When Ceq is higher than 0.400, hardenability excessively increases, the maximum hardness
of the surface layer microstructure of the base metal (steel plate) described below
is higher than 270 Hv, and thus SSC resistance deteriorates. In addition, the maximum
hardness of the internal microstructure is higher than 248 Hv, and HIC resistance
deteriorates. Therefore, Ceq is 0.400 or lower. Ceq is preferably 0.350 or lower.
In order to secure a predetermined strength, the lower limit of Ceq is 0.300 or higher.
(II) Microstructure
[0057] Next, the microstructure (the microstructure and the hardness thereof) of the base
metal of the steel pipe according to the embodiment will be described.
[0058] When controlled cooling is performed on the steel plate, the surface layer of the
steel plate is more rapidly cooled as compared to the inside of the steel plate. This
implies that a difference in mechanical properties is generated due to a difference
between the microstructure of the surface layer of the steel plate and the microstructure
of the inside of the steel plate. In particular, the hardness of the surface layer
of the steel plate is higher than that of the inside of the steel plate. The present
inventors found that, in the steel plate and the steel pipe having the above-described
microstructure, SSC resistance is poor in a range (surface layer) up to 1.0 mm from
the surface in the depth direction (through-thickness direction).
[0059] On the other hand, the present inventors found that, by using recuperating during
controlled cooling of the steel plate, the microstructure of the surface layer of
the steel plate and the microstructure of the inside of the steel plate can be controlled,
and thus an increase in the hardness of the surface layer of the steel plate can be
suppressed.
[0060] In the steel pipe according to the embodiment, in order to secure satisfactory SSC
resistance and HIC resistance, the microstructure of the steel plate of the base metal
is divided into (i) a microstructure (surface layer microstructure) in a range up
to 1.0 mm from the surface of the steel plate in the depth direction (through-thickness
direction) and (ii) a microstructure (internal microstructure) in a range up to a
thickness center from an area positioned at a distance of more than 1.0 mm from the
surface of the base metal in the depth direction. In each of the microstructures,
the kind, the fraction (area fraction), and the hardness of the microstructure are
defined.
[0061] In the steel pipe according to the embodiment, the range up to 1.0 mm from the surface
of the steel plate as the base metal in the depth direction will be referred to as
"surface layer" (hereinafter, also simply referred to as "the surface layer of the
steel plate). According to accelerated cooling, in particular, the hardness of the
range up to a depth of 1.0 mm from the surface becomes high. Therefore, the surface
layer microstructure is defined as the microstructure in a range up to 1.0 mm from
the steel plate surface in the depth direction.
[0062] The surface layer microstructure in a range up to a depth of 1.0 mm from the surface
of the steel plate as the base metal includes polygonal ferrite and granular bainite,
the area fraction of polygonal ferrite is 0% to 70%, the total area fraction of polygonal
ferrite and granular bainite is 50% or more, and the maximum hardness is 270 Hv or
lower.
[0063] When the area fraction of polygonal ferrite in the surface layer is more than 70%,
a high concentration of C accumulates on the remainder, a hardening region is formed,
and thus SSC resistance deteriorates. Therefore, the area fraction of polygonal ferrite
is 70% or less. The area fraction of polygonal ferrite is preferably 50% or less.
In addition, in order to secure SSC resistance, the total area fraction of polygonal
ferrite and granular bainite is 50% or more.
[0064] The remainder of the surface layer microstructure may include one or more selected
from the group consisting of bainite and pseudo pearlite. However, the remainder is
not necessarily included. That is, the total area fraction of polygonal ferrite and
granular bainite may be 100%.
[0065] When the maximum hardness of the surface layer microstructure is higher than 270
Hv, SSC resistance deteriorates. Therefore, the maximum hardness of the surface layer
microstructure is 270 Hv or lower. The maximum hardness of the surface layer microstructure
is preferably 250 Hv. From the viewpoint of SSC resistance, although the lower limit
of the maximum hardness of the surface layer microstructure is not necessarily determined,
the maximum hardness of the surface layer microstructure is substantially 160 Hv or
higher.
[0066] The area fraction of each of the microstructures can be obtained by observing the
microstructure with a scanning electron microscope (SEM), for example, at a magnification
of 1000-fold. The surface layer microstructure can be obtained by observing positions
of 0.1 mm, 0.2 mm, and 0.5 mm from the surface of the steel plate and obtaining the
average of the area fractions at the respective positions.
[0067] In the embodiment, polygonal ferrite is a microstructure that is observed as a massive
microstructure not including a coarse precipitate such as coarse cementite or MA in
grains.
[0068] Bainite is a microstructure in which a prior austenite grain boundary is clear, a
fine lath structure is developed in grains, and a fine carbide and an austenite-martensite
constituent mixture are scattered in and between laths. Here, bainite also includes
tempered bainite.
[0069] Granular bainite is a microstructure that is formed at an intermediate transformation
temperature between acicular ferrite and bainite, the acicular ferrite being a microstructure
in which a prior austenite grain boundary is not clear and acicular-shaped ferrite
(a carbide and an austenite-martensite constituent are not present) is formed in a
random crystal orientation in grains. In the granular bainite, a prior austenite grain
boundary partially appears, a coarse lath structure is present in grains, and a portion
where a fine carbide and an austenite-martensite constituent are scattered in and
between laths and a portion of acicular or amorphous ferrite where a prior austenite
grain boundary is not clear are mixed.
[0070] Pseudo pearlite is pearlite in which parallel row of cementite is arranged.
[0071] FIG. 4 shows an example of a microstructure (observed with a scanning electron microscope
at a magnification of 1000-fold) at a distance of 0.5 mm from the surface of the steel
plate. In FIG. 4, a portion, which is surrounded by a smooth curve and in which internal
portion is smooth, is polygonal ferrite, and a portion where white spots are present
in internal portion is granular bainite.
[0072] The maximum hardness of the surface layer microstructure is measured as follows.
[0073] First, 300 mm×300 mm steel plates are cut out by gas cutting from positions of 1/4,
1/2, and 3/4 of the width of the steel plate (positions of 3 o'clock, 6 o'clock, and
9 o'clock when the weld of the steel pipe is 0 o'clock) from a width-direction end
portion (corresponding to the seam portion in the case of the steel pipe) of the steel
plate in the width direction of the steel plate. Block test pieces having a length
of 20 mm and a width of 20 mm are collected by mechanical cutting from the centers
of the cut steel plates and are polished by mechanical polishing. Regarding each of
the block test pieces, the hardness is measured using a Vickers hardness meter (load:
100 g) at 100 points in total that are obtained by setting a point of 0.1 mm from
the surface as a starting point, setting 10 pints from the starting point in a through-thickness
direction at an interval of 0.1 mm, and setting 10 points at the same depth at an
interval of 1.0 mm in a width direction. Unless two or more measurement points having
a hardness of higher than 270 Hv among the test pieces continuously appear in the
through-thickness direction as a result of the above-described measurement in every
test piece, it is determined that the maximum hardness of the surface layer microstructure
is 270 Hv or lower.
[0074] When two or more measurement points having a hardness of higher than 270 Hv are continuously
present in the through-thickness direction, this hardness is not an abnormal value,
a microstructure having a high hardness is formed, SSC resistance deteriorates, which
is not allowable. Accordingly, in the embodiment, even when one measurement point
having a hardness of higher than 270 Hv is present, if two or more measurement points
do not continuously appear in the through-thickness direction, this point as an abnormal
point is not adopted, and the second highest value is obtained as the maximum hardness.
When two or more measurement points having a hardness of higher than 270 Hv are continuously
present in the through-thickness direction, the highest value is adopted as the maximum
hardness.
[0075] FIGS. 3A to 3C show the results of measuring the hardness of the surface layer microstructure
at three positions corresponding to 3 O'clock, 6 O'clock, and 9 O'clock when the weld
of the steel pipe is at a 0 O'clock position. Using a Vickers hardness meter, the
hardness of the surface layer microstructure is measured under a load of 100 g by
setting every 10 measurement points at the same depth at an interval of 0.1 mm in
a region from a depth of 0.1 mm to a depth of 1.0 mm from the surface layer. It can
be seen that, at all the points, the maximum hardness is 270 Hv or lower and SSC resistance
is excellent.
[0076] The microstructure (internal microstructure) in a range up to a thickness center
from an area positioned at a distance of more than 1.0 mm from the surface of the
steel plate as the base metal in the depth direction: the area fraction of polygonal
ferrite is 40% or less, the maximum hardness is 248 Hv or lower, and the average hardness
is 150 to 220 Hv.
[0077] When the area fraction of polygonal ferrite in the internal microstructure is more
than 40%, it is difficult to secure a required strength and HIC resistance. Therefore,
the area fraction of polygonal ferrite is 40% or less. The area fraction of polygonal
ferrite is preferably 30% or less and more preferably 25% or less.
[0078] The remainder of the internal microstructure consists of one or more selected from
the group consisting of granular bainite, bainite, and pseudo pearlite.
[0079] When the maximum hardness in the internal microstructure is higher than 248 Hv, HIC
resistance deteriorates. Therefore, the maximum hardness is 248 Hv or lower. In addition,
when the average hardness is lower than 150 Hv, required mechanical properties cannot
be secured. Therefore, the average hardness is 150 Hv or higher. the average hardness
is 160 Hv or higher. On the other hand, when the average hardness is higher than 220
Hv, HIC resistance and toughness deteriorate. Therefore, the average hardness is 220
Hv or lower. The average hardness is preferably 210 Hv or lower.
[0080] The microstructural fraction (area fraction) of the internal microstructure can be
obtained by observing a 1/4 thickness (t/4) position from the surface of the steel
plate with a scanning electron microscope (SEM), for example, at a magnification of
1000-fold. The reason why the observation position is the t/4 position is that the
microstructure of the t/4 position is a representative microstructure of the internal
microstructure.
[0081] FIG. 5 shows an example of the microstructure of the t/4 position (observed with
a scanning electron microscope at a magnification of 1000-fold). In FIG. 5, a portion
which is surrounded by a smooth curve and in which internal portion is smooth is polygonal
ferrite. In addition, a portion where white spots or a white line appears is granular
bainite or pseudo pearlite, and a portion that is surrounded by a jagged white line
and where a thin pattern appears is bainite.
[0082] The maximum hardness and the average hardness in the internal microstructure can
be measured using the following method.
[0083] 300 mm×300 mm steel plates are cut out by gas cutting from positions of 1/4, 1/2,
and 3/4 (positions of 3 o'clock, 6 o'clock, and 9 o'clock when the weld of the steel
pipe is 0 o'clock) from a width-direction end portion (corresponding to the seam portion
in the case of the steel pipe) of the steel plate in the width direction of the steel
plate. Block test pieces having a length of 20 mm and a width of 20 mm are collected
by mechanical cutting from the centers of the cut steel plates and are polished by
mechanical polishing. Regarding each of the block test pieces, the hardness is measured
using a Vickers hardness meter (load: 1 kg) by setting a depth position of 1.2 mm
from the surface as a starting point, setting 10 points from the starting point in
a through-thickness direction at an interval of 0.2 mm, and setting 10 points at the
same depth at an interval of 1.0 mm in a width direction. Unless two or more measurement
points having a hardness of higher than 248 Hv continuously appear in the through-thickness
direction as a result of the above-described measurement, it is determined that the
maximum hardness of the surface layer microstructure is 248 Hv or lower.
[0084] In the base metal of the steel pipe according to the embodiment, a high hardness
value (abnormal value) may appear locally. However, even when this abnormal value
appears, HIC resistance can be secured. On the other hand, when two or more measurement
points having a hardness of higher than 248 Hv are continuously present in the through-thickness
direction, HIC resistance deteriorates, which is not allowable. Accordingly, in the
embodiment, even when one measurement point having a hardness of higher than 248 Hv
is present, unless two or more measurement points do not continuously appear in the
through-thickness direction, this point as an abnormal point is not adopted, and the
second highest value is obtained as the maximum hardness. On the other hand, when
two or more measurement points having a hardness of higher than 248 Hv are continuously
present in the through-thickness direction, the highest value is adopted as the maximum
hardness.
[0085] In addition, the average hardness is calculated by obtaining the average value of
the hardnesses at all the measurement points.
[0086] Next, the weld of the steel pipe according to the embodiment will be described.
[0087] The steel pipe according to the embodiment can be obtained by processing the steel
plate according to the embodiment into a pipe shape, making opposite end portions
(width-direction end portions of the steel pipe) of the cylindrical steel plate abut
against each other, and welding the end portions. Therefore, as shown in FIG. 1, the
steel pipe 1 according to the embodiment includes the weld 3 that is provided in the
seam portion of the steel plate 2 and extends in the longitudinal direction of the
steel plate. Typically, the weld 3 is continuously provided in a range from one end
portion of the steel plate 2 in the longitudinal direction to another end portion
thereof.
[0088] In general, during steel pipe welding, the weld is provided such that the thickness
is more than that of the base metal. In addition, the weld metal has a higher alloy
content than the base metal and also has high corrosion resistance. Therefore, the
weld does not substantially cause fracture to occur. Accordingly, the weld of the
steel pipe according to the embodiment is not particularly limited as long as it is
obtained by SAW welding or the like under typical conditions.
[0089] It is preferable that the steel pipe according to the embodiment has a strength that
satisfies X52 to X70 defined by API 5L in consideration of application to a line pipe.
[0090] Next, a preferable method of manufacturing the steel pipe according to the embodiment
will be described.
[0091] As long as the steel pipe according to the embodiment has the above-described configuration,
the effects thereof can be obtained irrespective of the manufacturing method thereof.
For example, a manufacturing method including the following processes is preferable
because the steel pipe according to the embodiment can be stably obtained.
[0092] That is, the steel plate according to the embodiment can be obtained using a manufacturing
method including:
- (i) a hot-rolling process of heating a slab having a predetermined chemical composition
at 1050°C to 1250°C, subjecting the slab to hot rolling and finishing hot rolling
at 830°C to 1000°C;
- (ii) an accelerated cooling process of performing accelerated cooling on the steel
plate after finishing hot rolling from a surface temperature range of 750°C to 950°C
to a surface temperature range of 400°C to 650°C at an average cooling rate of 15
to 100 °C/sec such that recuperating where an increase in temperature is 5°C to 65°C
is performed two or more times in the middle of the accelerated cooling.
In addition, the steel pipe according to the embodiment is obtained using a manufacturing
method including (i) and (ii) described above and further including:
- (iii) a forming process of forming the steel plate according to the embodiment obtained
as described above into a cylindrical shape; and
- (iv) a welding process of making opposite end portions of the cylindrical steel plate
abut against each other and welding the end portions.
[0093] Hereinafter, preferable conditions in each of the processes will be described.
<Hot-Rolling Process>
Slab Heating Temperature: 1050°C to 1250°C
[0094] A slab that is manufactured by casting molten steel having the same chemical composition
as that of the base metal of the steel pipe according to the embodiment is heated
to 1050°C to 1250°C and subjected to hot rolling. The casting of the molten steel
and the manufacturing of the slab before hot rolling may be performed using an ordinary
method.
[0095] When the slab heating temperature is lower than 1050°C, carbonitrides of non-solid-solubilized
Nb and Ti are formed, and HIC resistance deteriorates. Therefore, the slab heating
temperature is preferably 1050°C or higher. The slab heating temperature is more preferably
1100°C or higher. On the other hand, when the slab heating temperature is higher than
1250°C, the crystal grain size increases, and low-temperature toughness deteriorates.
In addition, the austenite grain size increases, and hardenability excessively increases.
As a result, a hard phase is formed in the surface layer microstructure and the internal
microstructure, and SSC resistance and HIC resistance deteriorate. Therefore, the
slab heating temperature is preferably 1250°C or lower. The slab heating temperature
is more preferably higher than 1200°C or lower.
[0096] During hot rolling, the slab heated to the above-described temperature is hot-rolled
at a typical rolling reduction ratio to obtain a steel plate. The plate thickness
may be set depending on the required thickness of a line pipe and thus is not particularly
limited.
Rolling Finishing Temperature: 830°C to 1000°C
[0097] In order to obtain the predetermined surface layer microstructure and the predetermined
internal microstructure by accelerated cooling after finish rolling, the rolling finishing
temperature (finishing temperature) is 830°C to 1000°C. When the rolling finishing
temperature is lower than 830°C, it is difficult to obtain the surface layer microstructure
and the internal microstructure. Therefore, the finish rolling temperature is preferably
830°C or higher. The finish rolling temperature is more preferably 850°C or higher.
[0098] On the other hand, when the rolling finishing temperature is higher than 1000°C,
crystal grains are coarsened, and low-temperature toughness deteriorates. Therefore,
the rolling finishing temperature is preferably 1000°C or lower. The rolling finishing
temperature is more preferably 900°C or lower.
<Accelerated Cooling Proess>
[0099]
Cooling Start Temperature Ts: 750°C to 950°C
Cooling Stop Temperature Tf: 400°C to 650°C
Average Cooling Rate Vc: 15 to 100 °C/sec
Number of Times of Recuperating: two or more
Increase in Temperature caused by Recuperating: 5°C to 65°C (excluding recuperating
after stopping final water cooling)
[0100] In the accelerated cooling process, accelerated cooling is performed on the steel
plate after finishing hot rolling from a surface temperature range of 750°C to 950°C
to a surface temperature range of 400°C to 650°C at an average cooling rate of 15
to 100 °C/sec such that two or more times of recuperating where an increase in temperature
from the start of cooling to the end of cooling is 5°C to 65°C is included.
[0101] The accelerated cooling including recuperating in the middle can be performed by
adjusting the amount of cooling water that is sprayed to the steel plate per cooling
zone in a cooling facility in which a plurality of divided cooling zones are arranged
in a longitudinal direction of the steel plate (conveyance direction).
[0102] FIG. 2 shows an example of cooling curves of the steel plate. Four cooling curves
include a cooling curve of the thickness middle portion (1/2 thickness portion), a
cooling curve of a 1/4 thickness position (t/4 portion) from the surface, a cooling
curve of a portion of a depth of 1.0 mm from the surface, and a cooling curve of the
steel plate surface. Accelerated cooling is performed on the entire steel plate from
the cooling start temperature (Ts) of 830°C to about 620°C for about 10 seconds such
that recuperating is performed three times in the middle.
[0103] During this cooling, the cooling start temperature Ts and the cooling stop temperature
Tf are points shown in the drawing, and the average cooling rate Vc can be obtained
by dividing a temperature change ΔT (cooling start temperature Ts-cooling stop temperature
Tf) by a cooling time Δt (the time for which water cooling is performed).
[0104] It can be seen from FIG. 2 that the temperature of the steel plate surface during
cooling is temporarily increased by recuperating due to sensible heat inside the steel
plate as a result of adjusting the amount of cooling water sprayed per cooling zone.
On the other hand, the cooling curve of the steel plate surface and the cooling curve
of the portion of a depth of 1.0 mm from the surface are affected by recuperating.
However, the cooling curve of the thickness middle portion (1/2 thickness portion)
and the cooling curve of the 1/4 thickness portion are not affected by recuperating,
and it can be seen that the inside of the steel plate is cooled at a substantially
constant cooling rate.
[0105] When the cooling start temperature Ts is lower than 750°C, in the surface layer microstructure,
coarse ferrite is formed after rolling, and a microstructure having a high hardness
such as martensite is formed as the remainder. As a result, SSC resistance deteriorates.
In addition, when the cooling start temperature Ts is lower than 750°C, the ferrite
fraction in the internal microstructure is excessively large, and the hardness of
a hard phase is also high. Therefore, the cooling start temperature Ts is preferably
750°C or higher. The cooling start temperature Ts is more preferably 780°C or higher.
[0106] On the other hand, in a case where the cooling start temperature Ts is higher than
950°C, even when recuperating is performed two or more times, the maximum hardness
of the surface layer microstructure is higher than 270 Hv, and SSC resistance deteriorates.
Therefore, the cooling start temperature Ts is preferably 950°C or lower. The cooling
start temperature Ts is more preferably 880°C or lower.
[0107] When the cooling stop temperature Tf is lower than 400°C, the average hardness of
the internal microstructure is higher than 220 Hv, and HIC resistance deteriorates.
Therefore, the cooling stop temperature Tf is preferably 400°C or higher. The cooling
stop temperature Tf is more preferably 480°C or higher. On the other hand, when the
cooling stop temperature Tf is higher than 650°C, the average hardness of the internal
microstructure is lower than 150 Hv, and there may be a case where the predetermined
strength cannot be satisfied. In addition, a microstructure having a high hardness
is locally formed, and SSC resistance and HIC resistance may deteriorate. Therefore,
the cooling stop temperature Tf is preferably 650°C or lower. The cooling stop temperature
Tf is more preferably 580°C or lower.
[0108] When the average cooling rate Vc is slower than 15 °C/sec, polygonal ferrite having
an area fraction of more than 70% is formed in the surface layer microstructure. In
addition, in the internal microstructure polygonal ferrite having an area fraction
of more than 40% is formed. In this case, the strength as a line pipe cannot be secured.
Therefore, the average cooling rate Vc is preferably 15 °C/sec or faster. The average
cooling rate Vc is more preferably 25 °C/sec or faster.
[0109] On the other hand, when the average cooling rate Vc is faster than 100 °C/sec, martensite
transformation occurs, the hardness of the surface layer microstructure is higher
than 270 Hv, and SSC resistance deteriorates. In addition, the maximum hardness of
the internal microstructure is higher than 248 Hv, and HIC resistance deteriorates.
Therefore, the average cooling rate Vc is preferably 100 °C/sec or slower. The average
cooling rate Vc is more preferably 80 °C/sec or slower.
[0110] When the number of times of recuperating where the recuperated temperature during
accelerated cooling is in a predetermined range is one or less, the hardness of the
surface layer microstructure is higher than 270 Hv, and SSC resistance deteriorates.
Therefore, the number of times of recuperating is two or more.
[0111] FIG. 2 shows the cooling curve when the number of times of recuperating is three.
However, the number of times of recuperating may be appropriately determined between
the cooling start temperature and the cooling stop temperature depending on the kind
of steel or the plate threading speed.
[0112] In the steel plate according to the embodiment, cooling is performed in a film boiling
state in order to form a predetermined microstructure. In order to perform cooling
in the film boiling state, recuperating is not completed during water cooling, and
an increase in the temperature caused by recuperating is 65°C or lower. When the temperature
increase caused by recuperating is higher than 65°C, coarse ferrite is formed, and
a predetermined microstructure cannot be obtained. On the other hand, when the temperature
increase caused by recuperating is lower than 5°C, the effect of recuperating cannot
be obtained. Therefore, the width of the temperature increase caused by recuperating
is preferably 5°C to 65°C. The width of the temperature increase caused by recuperating
is preferably 10°C to 65°C. However, regarding the final recuperating after stopping
water cooling, the width of the temperature increase caused by recuperating is not
necessarily 5°C to 65°C.
[0113] When the temperature of the steel plate is increased during cooling by induction
heating or the like instead of recuperating, the temperature of the inside of the
steel plate also increases. Therefore, even when heating is performed by induction
heating or the like instead of recuperating, a predetermined microstructure cannot
be obtained.
[0114] When recuperating where a temperature increase is 5°C to 65°C is performed two or
more times, it is preferable that the first recuperating is performed such that the
steel plate surface temperature after recuperating is 500°C or higher. Even when the
steel plate surface temperature after the first recuperating is lower than 500°C,
the surface layer microstructure having satisfactory SSC resistance and the internal
microstructure having satisfactory HIC resistance can be secured. However, in order
to stably secure the surface layer microstructure having satisfactory SSC resistance
and the internal microstructure having satisfactory HIC resistance, it is preferable
that the first recuperating is performed such that the steel plate surface temperature
after recuperating is 500°C or higher.
[0115] After a short time from the stop of water cooling, a temperature difference between
the surface temperature and the center temperature is eliminated. For example, in
FIG. 2, at about 620°C, there is no temperature difference between the surface layer
of the steel plate (surface temperature) and the inside of the steel plate (center
temperature), and the steel plate temperature is stable. Subsequently, it is preferable
to perform cooling up to 300°C or lower at an average cooling rate of 0.5 °C/sec to
5.0 °C/sec. As long as the average cooling rate is 0.5 °C/sec to 5.0 °C/sec, air cooling
may be performed. When the average cooling rate is slower than 0.5 °C/sec, the predetermined
strength cannot be obtained. On the other hand, when the average cooling rate is faster
than 5.0 °C/sec, the toughness of the center portion deteriorates.
<Forming Process>
[0116] The formation of the steel plate according to the embodiment into the steel pipe
is not limited to a specific forming method. Warm working can also be used, but cold
working is preferable from the viewpoint of dimensional accuracy.
<Welding Process>
[0117] Next, opposite end portions of the steel plate formed into a cylindrical shape are
made to abut against each other, and the end portions are welded. Welding is not limited
to a specific welding method, but submerged arc welding (SAW) is preferable. Welding
conditions may be well-known conditions depending on the plate thickness and the like.
[0118] In the method of manufacturing the steel pipe according to the embodiment, a heat
treatment (seam heat treatment) may be performed such that a microstructure (ferrite
and pearlite having an area fraction of more than 10%) that deteriorates the toughness
of the weld is not formed. The heat treatment temperature may be a typical temperature
range and is preferably in a range of 300°C to the Ac 1 point.
[0119] A heat treatment is not performed on the base metal of the steel pipe according to
the embodiment. Therefore, the microstructure of the base metal is the same as the
microstructure of the steel plate according to the embodiment. The base metal of the
steel pipe according to the embodiment has the same microstructure as that of the
steel plate according to the embodiment, and thus mechanical properties for use in
a line pipe and satisfactory local weldability. In addition, since the weldability
of the steel plate according to the embodiment is satisfactory, the weld of the steel
pipe according to the embodiment has satisfactory mechanical properties. Accordingly,
the steel pipe according to the embodiment is suitable as a steel pipe for a line
pipe.
[Examples]
[0120] Next, examples of the present invention will be described. However, the conditions
of the examples are merely exemplary examples to confirm the operability and the effects
of the present invention, and the present invention is not limited to these condition
examples. The present invention can adopt various conditions within a range not departing
from the scope of the present invention as long as the object of the present invention
can be achieved under the conditions.
(Example 1)
[0121] A slab having a chemical composition and Ceq shown in Table 1 was hot-rolled and
cooled under conditions shown in Table 2. As a result, a steel plate was manufactured.
In Table 2, the number of times of recuperating is the number of times of recuperating
where a temperature increase was 5°C or higher. In addition, the maximum width of
recuperating temperature is the width of a temperature increase during recuperating
where the width of the temperature increase was the maximum.
[Table 1]
Kind of Steel |
Chemical Composition (Remainder: Fe and Im purities) |
Ceq |
C |
Si |
Mn |
P |
S |
Al |
Ti |
Nb |
Ca |
N |
O |
Ni |
Mo |
Cr |
Cu |
V |
Mg |
REM |
1 |
0.046 |
0.22 |
1.39 |
0.006 |
0.0002 |
0.019 |
0.012 |
0.018 |
0.0016 |
0.0021 |
0.0014 |
|
0.11 |
0.19 |
|
|
|
|
0.338 |
2 |
0.044 |
0.24 |
1.38 |
0.009 |
0.0003 |
0.040 |
0.011 |
0.018 |
0.0022 |
0.0032 |
0.0021 |
0.16 |
|
0.23 |
0.21 |
0.040 |
|
|
0.353 |
3 |
0.042 |
0.21 |
1.37 |
0.005 |
0.0002 |
0,032 |
0.012 |
0.021 |
0.0018 |
0.0028 |
0.0017 |
|
|
0.17 |
|
|
|
|
0.304 |
4 |
0.055 |
0.31 |
1.62 |
0.008 |
0.0002 |
0.026 |
0.011 |
0.044 |
0.0019 |
0.0022 |
0.0011 |
|
|
|
|
|
|
|
0.325 |
5 |
0.067 |
0.11 |
1.40 |
0.009 |
0.0006 |
0.019 |
0.009 |
0.032 |
0.0039 |
0.0029 |
0.0032 |
|
|
|
|
|
|
|
0.300 |
6 |
0.031 |
0.22 |
1.64 |
0.005 |
0.0002 |
0.013 |
0.011 |
0.007 |
0.0028 |
0.0021 |
0.0013 |
|
|
|
|
|
|
|
0.304 |
7 |
0.042 |
0.32 |
1.45 |
0.007 |
0.0003 |
0.021 |
0.019 |
0.009 |
0.0022 |
0.0029 |
0.0017 |
|
0.15 |
0.20 |
|
|
|
|
0.354 |
8 |
0.051 |
0.22 |
1.47 |
0.011 |
0.0005 |
0.027 |
0.011 |
0.026 |
0.0024 |
0.0043 |
0.0019 |
0.20 |
|
|
|
0.012 |
|
|
0.312 |
9 |
0.039 |
0.34 |
1.43 |
0.009 |
0.0004 |
0.032 |
0.013 |
0.028 |
0.0049 |
0.0031 |
0.0039 |
0.20 |
0.10 |
0.27 |
|
0.040 |
|
|
0.373 |
10 |
0.033 |
0.01 |
1.38 |
0.012 |
0.0009 |
0.019 |
0.017 |
0.021 |
0.0031 |
0.0016 |
0.0021 |
|
0.10 |
|
|
0.098 |
|
0.0032 |
0.303 |
11 |
0.027 |
0.19 |
1.43 |
0.009 |
0.0012 |
0.028 |
0.012 |
0.029 |
0.0033 |
0.0032 |
0.0019 |
0.15 |
|
|
0.15 |
|
0.0025 |
|
0.285 |
12 |
0.075 |
0.21 |
1.64 |
0.017 |
0.0008 |
0.021 |
0.022 |
0.018 |
0.0029 |
0.0033 |
0.0018 |
0.20 |
0.25 |
|
|
|
|
|
0.412 |
13 |
0.041 |
0.004 |
1.43 |
0.009 |
0.0008 |
0.021 |
0.011 |
0.021 |
0.0032 |
0.0031 |
0.0043 |
0.20 |
|
0.13 |
|
|
|
|
0.319 |
14 |
0.048 |
0.29 |
1.03 |
0.008 |
0.0005 |
0.029 |
0.011 |
0.029 |
0.0033 |
0.0032 |
0.0018 |
0.20 |
|
0.20 |
|
|
|
|
0.273 |
15 |
0.051 |
0.29 |
1.68 |
0.008 |
0.0004 |
0.019 |
0.003 |
0.018 |
0.0026 |
0.0032 |
0.0018 |
|
|
|
|
|
|
|
0.331 |
16 |
0.048 |
0.29 |
1.44 |
0.012 |
0.0017 |
0.031 |
0.009 |
0.044 |
0.0007 |
0.0038 |
0.0017 |
0.20 |
|
|
0.20 |
|
|
|
0.315 |
17 |
0.041 |
0.31 |
1.45 |
0.003 |
0.0005 |
0.008 |
0.014 |
0.042 |
0.0057 |
0.0029 |
0.0023 |
0.20 |
|
|
0.20 |
|
|
|
0.309 |
18 |
0.039 |
0.11 |
1.48 |
0.004 |
0.0005 |
0.043 |
0.013 |
0.044 |
0.0031 |
0.0029 |
0.0019 |
|
0.10 |
|
|
|
|
|
0.306 |
19 |
0.044 |
0.29 |
1.39 |
0.007 |
0.0006 |
0.033 |
0.012 |
0.003 |
0.0036 |
0.0031 |
0.0029 |
|
0.10 |
|
|
0.120 |
|
|
0.320 |
20 |
0.058 |
0.31 |
1.11 |
0.008 |
0.0007 |
0.029 |
0.008 |
0.021 |
0.0031 |
0.0033 |
0.0021 |
0.35 |
0.15 |
0.48 |
0.35 |
|
|
|
0.416 |
21 |
0.051 |
0.29 |
1.21 |
0.008 |
0.0004 |
0.027 |
0.013 |
0.031 |
0.0029 |
0.0075 |
0.0021 |
|
0.54 |
0.30 |
|
|
0.0120 |
|
0.421 |
22 |
0.044 |
0.11 |
1.41 |
0.008 |
0.0002 |
0.026 |
0.011 |
0.039 |
0.0024 |
0.0042 |
0.0023 |
0.60 |
0.25 |
|
0.60 |
|
|
0.0120 |
0.409 |
23 |
0.072 |
0.21 |
1.38 |
0.008 |
0.0004 |
0.040 |
0.011 |
0.042 |
0.0022 |
0.0032 |
0.0019 |
|
0.10 |
0.20 |
|
|
|
|
0.362 |
24 |
0.048 |
0.22 |
1.37 |
0.017 |
0.0005 |
0.032 |
0.012 |
0.038 |
0.0023 |
0.0021 |
0.0018 |
|
0.21 |
0.21 |
|
|
|
|
0.360 |
25 |
0.042 |
0.21 |
1.42 |
0.008 |
0.0002 |
0.032 |
0.022 |
0.032 |
0.0021 |
0.0031 |
0.0018 |
|
|
0.20 |
|
|
|
|
0.319 |
26 |
0.038 |
0.20 |
1.68 |
0.007 |
0.0002 |
0.022 |
0.009 |
0.021 |
0.0025 |
0.0033 |
0.0019 |
|
|
0.22 |
|
|
|
|
0.362 |
27 |
0.032 |
0.23 |
1.43 |
0.007 |
0 .0017 |
0.022 |
0.010 |
0.015 |
0.0014 |
0.0042 |
0.0021 |
|
|
0.19 |
|
|
|
|
0.308 |
28 |
0.038 |
0.24 |
1.29 |
0.008 |
0.0008 |
0.019 |
0.011 |
0.015 |
0.0053 |
0.0032 |
0.0018 |
0.30 |
|
0.20 |
0.30 |
|
|
|
0.333 |
29 |
0.042 |
0.21 |
1.29 |
0.005 |
0.0005 |
0.038 |
0.012 |
0.022 |
0.0008 |
0.0034 |
0.0019 |
|
0.20 |
0.17 |
|
|
|
|
0.331 |
30 |
0.048 |
0.19 |
1.39 |
0.006 |
0.0005 |
0.009 |
0.014 |
0.028 |
0.0016 |
0.0038 |
0.0015 |
|
|
0.23 |
|
|
|
|
0.326 |
31 |
0.046 |
0.13 |
1.44 |
0.004 |
0.0004 |
0.023 |
0.017 |
0.003 |
0.0018 |
0.0022 |
0.0015 |
|
|
0.24 |
|
|
|
|
0.334 |
32 |
0.045 |
0.23 |
1.42 |
0.008 |
0.0004 |
0.021 |
0.011 |
0.048 |
0.0021 |
0.0021 |
0.0016 |
|
0.10 |
0.31 |
|
|
|
|
0.364 |
33 |
0.052 |
0.24 |
1.33 |
0.007 |
0.0002 |
0.026 |
0.008 |
0.036 |
0.0028 |
0.0072 |
0.0025 |
|
0.10 |
0.21 |
|
|
|
|
0.336 |
34 |
0.039 |
0.22 |
1.35 |
0.007 |
0.0004 |
0.030 |
0.007 |
0.032 |
0.0021 |
0.0034 |
0.0029 |
0.60 |
0.10 |
|
|
|
|
|
0.324 |
[Table 2]
Manufacturing No. |
Kind of Steel |
Product Thickness |
Heating Temperature |
Rolling Finishing Temperature |
Cooling Start Temperature |
Average Cooling Rate |
Cooling Stop Temperature |
Number of Times of Recuperating |
Maximum Width of Recuperated Temperature |
mm |
°C |
°C |
°C |
°C/sec |
°C |
Times |
°C |
S1 |
1 |
8 |
1250 |
990 |
948 |
45 |
550 |
3 |
35 |
S2 |
2 |
8 |
1250 |
950 |
940 |
99 |
455 |
2 |
65 |
S3 |
3 |
20 |
1180 |
880 |
800 |
45 |
530 |
3 |
35 |
S4 |
4 |
11 |
1200 |
944 |
812 |
21 |
602 |
4 |
35 |
S5 |
5 |
42 |
1140 |
830 |
785 |
70 |
520 |
4 |
34 |
S6 |
6 |
13 |
1060 |
880 |
810 |
25 |
570 |
4 |
28 |
S7 |
7 |
20 |
1150 |
859 |
790 |
45 |
495 |
4 |
33 |
S8 |
8 |
13 |
1200 |
835 |
780 |
60 |
539 |
2 |
45 |
S9 |
9 |
32 |
1150 |
830 |
760 |
88 |
451 |
3 |
64 |
S10 |
10 |
10 |
1200 |
930 |
813 |
65 |
470 |
3 |
48 |
S11 |
1 |
20 |
1180 |
840 |
800 |
35 |
475 |
3 |
42 |
S12 |
1 |
20 |
1180 |
830 |
775 |
42 |
495 |
4 |
26 |
S13 |
1 |
10 |
1180 |
880 |
825 |
38 |
465 |
2 |
15 |
S14 |
11 |
10 |
1200 |
935 |
800 |
26 |
580 |
4 |
33 |
S15 |
12 |
10 |
1200 |
935 |
800 |
26 |
580 |
4 |
33 |
S16 |
13 |
10 |
1200 |
935 |
800 |
26 |
580 |
4 |
33 |
S17 |
14 |
10 |
1200 |
935 |
800 |
26 |
580 |
4 |
33 |
S18 |
15 |
10 |
1200 |
935 |
800 |
26 |
580 |
4 |
33 |
S19 |
16 |
10 |
1200 |
935 |
800 |
26 |
580 |
4 |
33 |
S20 |
17 |
10 |
1200 |
935 |
800 |
26 |
580 |
4 |
33 |
S21 |
18 |
13 |
1200 |
935 |
824 |
18 |
605 |
4 |
39 |
S22 |
19 |
13 |
1200 |
935 |
824 |
18 |
605 |
4 |
39 |
S23 |
20 |
13 |
1200 |
935 |
824 |
18 |
605 |
4 |
39 |
S24 |
21 |
13 |
1200 |
935 |
824 |
18 |
605 |
4 |
39 |
S25 |
22 |
13 |
1200 |
935 |
824 |
18 |
605 |
4 |
39 |
S26 |
23 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S27 |
24 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S28 |
25 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S29 |
26 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S30 |
27 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S31 |
28 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S32 |
29 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S33 |
30 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S34 |
31 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S35 |
32 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S36 |
33 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S37 |
34 |
20 |
1160 |
855 |
805 |
32 |
475 |
3 |
27 |
S38 |
1 |
11 |
1000 |
930 |
790 |
35 |
600 |
4 |
35 |
S39 |
1 |
11 |
1280 |
930 |
790 |
35 |
600 |
4 |
35 |
S40 |
1 |
11 |
1250 |
1030 |
790 |
120 |
600 |
4 |
35 |
S41 |
1 |
11 |
1200 |
800 |
730 |
35 |
600 |
4 |
35 |
S42 |
1 |
11 |
1200 |
930 |
790 |
35 |
670 |
4 |
35 |
S43 |
1 |
20 |
1200 |
930 |
790 |
85 |
370 |
1 |
35 |
S44 |
1 |
32 |
1200 |
930 |
790 |
90 |
400 |
0 |
- |
S45 |
1 |
40 |
1200 |
930 |
790 |
140 |
300 |
1 |
30 |
S46 |
1 |
20 |
1180 |
855 |
805 |
135 |
425 |
2 |
27 |
S47 |
1 |
20 |
1180 |
825 |
765 |
40 |
435 |
3 |
32 |
S48 |
1 |
20 |
1180 |
860 |
745 |
37 |
485 |
3 |
28 |
S49 |
1 |
20 |
1180 |
985 |
960 |
44 |
465 |
3 |
38 |
S50 |
1 |
20 |
1180 |
865 |
800 |
38 |
375 |
3 |
32 |
S51 |
1 |
20 |
1180 |
840 |
800 |
35 |
475 |
1 |
42 |
S52 |
1 |
20 |
1180 |
930 |
813 |
65 |
470 |
2 |
75 |
[0122] A test piece was collected from the manufactured steel plate, the surface layer microstructure
(positions of 0.1 mm, 0.2 mm, and 0.5 mm) and the internal microstructure (t/4 position)
were observed with a SEM at a magnification of 1000-fold and the fractions (area fractions)
of polygonal ferrite, granular bainite, and the remainder were calculated. The remainder
of the surface layer microstructure consisted of one or more selected from the group
consisting of bainite and pseudo pearlite, and the remainder of the internal microstructure
consisted of one or more selected from the group consisting of granular bainite, bainite,
and pseudo pearlite.
[0123] In addition, a JIS No.5 tensile test piece was prepared, and a tensile test according
to JIS Z 2241 was performed to measure a yield strength and a tensile strength.
[0124] In addition, the hardness was measured using a Vickers hardness meter. The hardness
of the surface layer microstructure was measured under a load of 100 g by setting
every 10 points at the same depth at an interval of 0.1 mm in a region from a depth
of 0.1 mm to a depth of 1.0 mm from the surface layer. On the other hand, the hardness
of the internal microstructure was measured under a load of 1 kg by setting every
10 points at the same depth at an interval of 0.2 mm in a region from a depth of 1.2
mm from the surface layer to the thickness center. Based on the results, the maximum
hardness of the surface layer microstructure was obtained, and the maximum hardness
and the average hardness of the internal microstructure were obtained.
[0125] Further, a test piece was collected from the manufactured steel plate, and the following
test was performed to evaluate HIC resistance and SSC resistance.
Evaluation of HIC Resistance
[0126] A test according to TM0284 of NACE (National Association of Corrosion and Engineer)
was performed to observe whether or not hydrogen induced cracking (HIC) occurred.
When the HIC area fraction was 5% or less, HIC resistance was evaluated to be satisfactory
(OK). When the HIC area fraction was more than 5%, HIC resistance was evaluated to
be poor (NG).
[0127] The NACE test is a test in which hydrogen sulfide gas is saturated in a solution
including 5% NaCl solution+0.5 acetic acid and having a pH of 2.7 and the steel plate
is dipped in the solution for 96 hours to observe whether or not cracking occur.
Evaluation of SSC Resistance
[0128] A full thickness test piece having a width of 15 mm and a length of 115 mm was collected
from the steel plate in a width direction, and SSC resistance was evaluated in a 4
point bending test according to TM0284m ASTM (American Society for Testing and Materials)
G39 of NACE.
[0129] In the 4 point bending test, the test piece to which a stress corresponding to 90%
of 0.2% proof stress derived from the tensile test was applied was dipped for 720
hours in an aqueous solution including 5% sodium chloride +0.5 acetic acid at normal
temperature (24°C) and having a pH of 2.7 in which hydrogen sulfide gas of 1 atm was
saturated, and the test piece surface was observed at a magnification of 10-fold to
determine whether or not SSC occurred.
[0130] A test piece where SSC did not occur was evaluated as "Pass (OK)", and a test piece
where SSC occurred was evaluated as "Fail (NG)". The results are shown in Table 3.
[Table 3]
Manufacturing No. |
Kind of Steel |
Surface laver Microstructure |
Internal Microstructure |
Properties |
Area Fraction of Polygonal Ferrite (%) |
Total Area Fraction of Polygonal Ferrite and Granular Bainite (%) |
Maximum Hardness (Load: 100 g) (Hv) |
Area Fraction of Polygonal Ferrite (%) |
Maximum Hardness (Load: 1 kg) (Hv) |
Average Hardness (Load: 1 kg) (Hv) |
Yield Strength (MPa) |
Tensile Strength (MPa) |
SSC Test |
HIC Test |
S1 |
1 |
15 |
97 |
248 |
5 |
225 |
195 |
476 |
595 |
OK |
OK |
S2 |
2 |
55 |
94 |
222 |
12 |
212 |
193 |
473 |
588 |
OK |
OK |
S3 |
3 |
45 |
94 |
190 |
35 |
165 |
154 |
393 |
482 |
OK |
OK |
S4 |
4 |
31 |
97 |
225 |
15 |
212 |
193 |
473 |
588 |
OK |
OK |
S5 |
5 |
12 |
95 |
232 |
13 |
205 |
193 |
520 |
583 |
OK |
OK |
S6 |
6 |
65 |
88 |
205 |
28 |
199 |
178 |
444 |
543 |
OK |
OK |
S7 |
7 |
13 |
91 |
218 |
6 |
186 |
178 |
452 |
535 |
OK |
OK |
S8 |
8 |
35 |
91 |
222 |
29 |
188 |
178 |
430 |
544 |
OK |
OK |
S9 |
9 |
55 |
87 |
268 |
22 |
196 |
184 |
472 |
565 |
OK |
OK |
S10 |
10 |
20 |
95 |
232 |
20 |
208 |
189 |
460 |
575 |
OK |
OK |
S11 |
1 |
55 |
92 |
232 |
5 |
218 |
202 |
533 |
627 |
OK |
OK |
S12 |
1 |
65 |
93 |
235 |
28 |
205 |
182 |
461 |
565 |
OK |
OK |
S13 |
1 |
0 |
95 |
248 |
0 |
227 |
205 |
547 |
634 |
OK |
OK |
S14 |
11 |
68 |
89 |
266 |
62 |
288 |
168 |
407 |
512 |
OK |
NG |
S15 |
12 |
0 |
43 |
278 |
0 |
277 |
223 |
578 |
680 |
NG |
NG |
S16 |
13 |
35 |
99 |
242 |
12 |
240 |
193 |
478 |
590 |
OK |
NG |
S17 |
14 |
82 |
95 |
288 |
75 |
293 |
159 |
390 |
480 |
NG |
NG |
S18 |
15 |
43 |
94 |
232 |
10 |
278 |
196 |
489 |
599 |
OK |
NG |
S19 |
16 |
35 |
94 |
243 |
15 |
236 |
190 |
466 |
580 |
OK |
NG |
S20 |
17 |
45 |
85 |
240 |
21 |
232 |
190 |
470 |
578 |
OK |
NG |
S21 |
18 |
25 |
94 |
249 |
14 |
229 |
198 |
485 |
605 |
OK |
NG |
S22 |
19 |
38 |
85 |
222 |
25 |
225 |
193 |
470 |
588 |
OK |
NG |
S23 |
20 |
0 |
44 |
272 |
0 |
286 |
240 |
599 |
729 |
NG |
NG |
S24 |
21 |
0 |
47 |
288 |
0 |
301 |
235 |
605 |
713 |
NG |
NG |
S25 |
22 |
0 |
41 |
292 |
0 |
312 |
236 |
570 |
720 |
NG |
NG |
S26 |
23 |
12 |
49 |
278 |
13 |
265 |
212 |
512 |
645 |
NG |
NG |
S27 |
24 |
43 |
85 |
242 |
22 |
243 |
183 |
465 |
576 |
OK |
NG |
S28 |
25 |
50 |
87 |
236 |
19 |
229 |
178 |
439 |
558 |
OK |
NG |
S29 |
26 |
21 |
85 |
244 |
9 |
258 |
192 |
478 |
598 |
OK |
NG |
S30 |
27 |
52 |
88 |
233 |
15 |
212 |
182 |
448 |
553 |
OK |
NG |
S31 |
28 |
49 |
85 |
238 |
12 |
228 |
179 |
445 |
558 |
OK |
NG |
S32 |
29 |
43 |
99 |
233 |
13 |
222 |
180 |
449 |
558 |
OK |
NG |
S33 |
30 |
42 |
85 |
234 |
9 |
214 |
182 |
448 |
559 |
OK |
NG |
S34 |
31 |
68 |
85 |
243 |
33 |
258 |
158 |
448 |
498 |
OK |
NG |
S35 |
32 |
48 |
85 |
238 |
12 |
222 |
182 |
444 |
561 |
OK |
NG |
S36 |
33 |
44 |
89 |
248 |
18 |
228 |
186 |
448 |
563 |
OK |
NG |
S37 |
34 |
45 |
85 |
253 |
14 |
225 |
184 |
443 |
561 |
NG |
OK |
S38 |
1 |
65 |
93 |
248 |
55 |
277 |
174 |
420 |
530 |
OK |
NG |
S39 |
1 |
0 |
34 |
333 |
0 |
298 |
233 |
565 |
710 |
NG |
NG |
S40 |
1 |
0 |
24 |
323 |
0 |
289 |
239 |
589 |
729 |
NG |
NG |
S41 |
1 |
75 |
95 |
308 |
55 |
253 |
190 |
519 |
578 |
NG |
NG |
S42 |
1 |
78 |
95 |
322 |
60 |
323 |
172 |
435 |
525 |
NG |
NG |
S43 |
1 |
10 |
35 |
288 |
19 |
278 |
190 |
471 |
578 |
NG |
NG |
S44 |
1 |
5 |
91 |
299 |
17 |
196 |
172 |
428 |
535 |
NG |
OK |
S45 |
1 |
10 |
45 |
305 |
20 |
255 |
191 |
466 |
582 |
NG |
NG |
S46 |
1 |
8 |
16 |
312 |
4 |
290 |
220 |
583 |
665 |
NG |
NG |
S47 |
1 |
73 |
85 |
277 |
18 |
232 |
188 |
461 |
563 |
NG |
OK |
S48 |
1 |
78 |
99 |
292 |
35 |
261 |
180 |
418 |
542 |
NG |
NG |
S49 |
1 |
44 |
85 |
273 |
18 |
242 |
203 |
540 |
633 |
NG |
OK |
S50 |
1 |
54 |
48 |
282 |
12 |
258 |
208 |
544 |
634 |
NG |
NG |
S51 |
1 |
38 |
85 |
272 |
5 |
214 |
201 |
533 |
622 |
NG |
OK |
S52 |
1 |
72 |
91 |
282 |
16 |
207 |
199 |
503 |
612 |
NG |
OK |
(Example 2)
[0131] The steel plate shown in Table 3 was formed into a pipe shape by C-press, U-press,
and O-press, end surfaces were temporarily welded, main welding was performed from
internal and external surfaces, and the steel pipe was expanded. As a result, a steel
pipe for a line pipe was obtained. As the main welding, submerged arc welding was
adopted. Manufacturing No. of the steel plate relates to Manufacturing No. of the
steel pipe. For example, the steel pipe of Manufacturing No. T1 was manufactured using
the steel plate of Manufacturing No. S1. the steel pipe of Manufacturing No. T2 was
manufactured using the steel plate of Manufacturing No. S2.
[0132] A test piece was collected from the manufactured steel plate, the surface layer microstructure
(positions of 0.1 mm, 0.2 mm, and 0.5 mm) and the internal microstructure (t/4 position)
were observed with a scanning electron microscope at a magnification of 1000-fold
to calculate the fractions (area fractions) of polygonal ferrite, granular bainite,
and the remainder.
[0133] In addition, a JIS No.5 tensile test piece was prepared, and a tensile test according
to JIS Z 2241 was performed to measure a yield strength and a tensile strength.
[0134] In addition, the hardness was measured using a Vickers hardness meter. The hardness
of the surface layer microstructure was measured under a load of 100 g by setting
every 10 points at the same depth at an interval of 0.1 mm in a region from a depth
of 0.1 mm to a depth of 1.0 mm from the surface layer. On the other hand, the hardness
of the internal microstructure was measured under a load of 1 kg by setting every
10 points at the same depth at an interval of 0.2 mm in a region from a depth of 1.2
mm from the surface layer to the thickness center.
[0135] Further, a test piece was collected from the manufactured steel plate, and the following
test was performed to evaluate HIC resistance and SSC resistance.
Evaluation of HIC Resistance
[0136] A test according to TM0284 of NACE (National Association of Corrosion and Engineer)
was performed to observe whether or not hydrogen induced cracking (HIC) occurred.
When the HIC area fraction was 5% or less, HIC resistance was evaluated to be satisfactory
(OK). When the HIC area fraction was more than 5%, HIC resistance was evaluated to
be poor (NG).
[0137] The NACE test is a test in which hydrogen sulfide gas is saturated in a solution
including 5% NaCl solution+0.5 acetic acid and having a pH of 2.7 and the steel plate
is dipped in the solution for 96 hours to observe whether or not cracking occur.
Evaluation of SSC Resistance
[0138] A full thickness test piece having a width of 15 mm and a length of 115 mm was collected
from the steel plate in a width direction (direction perpendicular to a rolling direction),
and SSC resistance was evaluated in a 4 point bending test according to TM0284mASTM
(American Society for Testing and Materials) G39 of NACE.
[0139] In the 4 point bending test, the test piece to which a stress corresponding to 90%
of 0.2% proof stress derived from the tensile test was applied was dipped for 720
hours in an aqueous solution including 5% sodium chloride +0.5 acetic acid at normal
temperature (24°C) and having a pH of 2.7 in which hydrogen sulfide gas of 1 atm was
saturated, and the test piece surface was observed at a magnification of 10-fold to
determine whether or not SSC occurred. A test piece where SSC did not occur was evaluated
as "Pass (OK)", and a test piece where SSC occurred was evaluated as "Fail (NG)".
The results are shown in Table 4.
[Table 4]
Manufacturing No. |
Kind of Steel |
Outer Diameter (mm) |
Inner Diameter (mm) |
Surface layer Microstructure |
Internal Microstructure |
Properties |
Note |
Area Fraction of Polygonal Ferrite (%) |
Total Area Fraction of Polygonal Ferrite and Granular Bainite (%) |
Maximum Hardness (Load: 100 g) (Hv) |
Area Fraction of Polygonal Ferrite (%) |
Maximum Hardness (Load: 1 kg) (Hv) |
Average Hardness (Load: 1 kg) (Hv) |
Yield Strength (MPa) |
Tensile Strength (MPa) |
SSC Test |
HIC Test |
T1 |
1 |
457.2 |
8 |
15 |
97 |
249 |
5 |
226 |
202 |
561 |
623 |
OK |
OK |
Example |
T2 |
2 |
457.2 |
8 |
55 |
94 |
226 |
12 |
212 |
196 |
554 |
616 |
OK |
OK |
T3 |
3 |
609.6 |
20 |
45 |
94 |
192 |
35 |
165 |
159 |
467 |
519 |
OK |
OK |
T4 |
4 |
609.6 |
11 |
31 |
97 |
227 |
15 |
213 |
196 |
555 |
616 |
OK |
OK |
T5 |
5 |
609.6 |
42 |
12 |
95 |
234 |
13 |
207 |
206 |
632 |
670 |
OK |
OK |
T6 |
6 |
508.0 |
13 |
65 |
88 |
206 |
28 |
201 |
183 |
520 |
577 |
OK |
OK |
T7 |
7 |
609.6 |
20 |
13 |
91 |
219 |
6 |
186 |
184 |
523 |
577 |
OK |
OK |
T8 |
8 |
609.6 |
13 |
35 |
91 |
223 |
29 |
188 |
182 |
516 |
574 |
OK |
OK |
T9 |
9 |
609.6 |
32 |
55 |
87 |
268 |
22 |
198 |
194 |
588 |
631 |
OK |
OK |
T10 |
10 |
609.6 |
10 |
20 |
95 |
233 |
20 |
209 |
192 |
541 |
601 |
OK |
OK |
T11 |
1 |
812.8 |
20 |
55 |
92 |
235 |
5 |
221 |
207 |
599 |
665 |
OK |
OK |
T12 |
1 |
7620 |
20 |
65 |
93 |
237 |
28 |
207 |
187 |
541 |
601 |
OK |
OK |
T13 |
1 |
457.2 |
10 |
0 |
95 |
248 |
0 |
227 |
209 |
602 |
669 |
OK |
OK |
T14 |
11 |
457.2 |
10 |
68 |
89 |
268 |
62 |
288 |
172 |
486 |
541 |
OK |
NG |
Comparative Example |
T15 |
12 |
457.2 |
10 |
0 |
43 |
280 |
0 |
279 |
228 |
646 |
718 |
NG |
NG |
T16 |
13 |
457.2 |
10 |
35 |
99 |
244 |
12 |
241 |
197 |
561 |
623 |
OK |
NG |
T17 |
14 |
457.2 |
10 |
82 |
95 |
292 |
75 |
296 |
162 |
456 |
507 |
NG |
NG |
T18 |
15 |
457.2 |
10 |
43 |
94 |
233 |
10 |
279 |
200 |
569 |
632 |
OK |
NG |
T19 |
16 |
457.2 |
10 |
35 |
94 |
244 |
15 |
236 |
194 |
551 |
612 |
OK |
NG |
T20 |
17 |
457.2 |
10 |
45 |
85 |
242 |
21 |
232 |
194 |
549 |
610 |
OK |
NG |
T21 |
18 |
508.0 |
13 |
25 |
94 |
250 |
14 |
231 |
203 |
579 |
643 |
OK |
NG |
T22 |
19 |
508.0 |
13 |
38 |
85 |
223 |
25 |
225 |
198 |
563 |
625 |
OK |
NG |
T23 |
20 |
508.0 |
13 |
0 |
44 |
276 |
0 |
286 |
246 |
698 |
775 |
NG |
NG |
T24 |
21 |
508.0 |
13 |
0 |
47 |
291 |
0 |
303 |
241 |
682 |
758 |
NG |
NG |
T25 |
22 |
508.0 |
13 |
0 |
41 |
299 |
0 |
312 |
242 |
689 |
765 |
NG |
NG |
T26 |
23 |
609.6 |
20 |
12 |
49 |
283 |
13 |
268 |
219 |
626 |
695 |
NG |
NG |
T27 |
24 |
609.6 |
20 |
43 |
85 |
243 |
22 |
243 |
189 |
559 |
621 |
OK |
NG |
T28 |
25 |
609.6 |
20 |
50 |
87 |
239 |
19 |
231 |
184 |
541 |
601 |
OK |
NG |
T29 |
26 |
609.6 |
20 |
21 |
85 |
244 |
9 |
258 |
198 |
580 |
644 |
OK |
NG |
T30 |
27 |
609.6 |
20 |
52 |
88 |
234 |
15 |
221 |
188 |
536 |
596 |
OK |
NG |
T31 |
28 |
609.6 |
20 |
49 |
85 |
241 |
12 |
228 |
185 |
541 |
601 |
OK |
NG |
T32 |
29 |
609.6 |
20 |
43 |
99 |
235 |
13 |
224 |
186 |
541 |
601 |
OK |
NG |
T33 |
30 |
609.6 |
20 |
42 |
85 |
234 |
9 |
218 |
188 |
542 |
602 |
OK |
NG |
T34 |
31 |
609.6 |
20 |
68 |
85 |
248 |
33 |
260 |
163 |
483 |
537 |
OK |
NG |
T35 |
32 |
609.6 |
20 |
48 |
85 |
239 |
12 |
224 |
188 |
544 |
605 |
OK |
NG |
T36 |
33 |
609.6 |
20 |
44 |
89 |
248 |
18 |
231 |
192 |
546 |
607 |
OK |
NG |
T37 |
34 |
609.6 |
20 |
45 |
85 |
255 |
14 |
225 |
190 |
544 |
605 |
NG |
OK' |
|
T38 |
1 |
609.6 |
11 |
65 |
93 |
251 |
55 |
277 |
177 |
500 |
555 |
OK |
NG |
T39 |
1 |
609.6 |
11 |
0 |
34 |
334 |
0 |
301 |
237 |
670 |
744 |
NG |
NG |
T40 |
1 |
609.6 |
11 |
0 |
24 |
323 |
0 |
289 |
243 |
688 |
764 |
NG |
NG |
T41 |
1 |
609.6 |
11 |
75 |
95 |
308 |
55 |
255 |
193 |
545 |
606 |
NG |
NG |
T42 |
1 |
609.6 |
11 |
78 |
95 |
323 |
60 |
323 |
175 |
495 |
550 |
NG |
NG |
T43 |
1 |
609.6 |
20 |
10 |
35 |
291 |
19 |
280 |
196 |
561 |
623 |
NG |
NG |
T44 |
1 |
812.8 |
32 |
5 |
91 |
301 |
17 |
199 |
179 |
525 |
584 |
NG |
OK |
T45 |
1 |
812.8 |
40 |
10 |
45 |
309 |
20 |
260 |
200 |
582 |
646 |
NG |
NG |
T46 |
1 |
609.6 |
20 |
8 |
16 |
316 |
4 |
295 |
227 |
645 |
717 |
NG |
NG |
T47 |
1 |
609.6 |
20 |
73 |
85 |
279 |
18 |
234 |
194 |
546 |
607 |
NG |
OK |
T48 |
1 |
609.6 |
20 |
78 |
99 |
297 |
35 |
261 |
186 |
526 |
584 |
NG |
NG |
T49 |
1 |
609.6 |
20 |
44 |
85 |
278 |
18 |
242 |
210 |
614 |
682 |
NG |
OK |
T50 |
1 |
609.6 |
20 |
54 |
48 |
285 |
12 |
261 |
215 |
615 |
683 |
NG |
NG |
T51 |
1 |
609.6 |
20 |
38 |
85 |
278 |
5 |
214 |
208 |
603 |
670 |
NG |
OK |
T52 |
1 |
609.6 |
20 |
72 |
91 |
286 |
16 |
209 |
206 |
594 |
660 |
NG |
OK |
[Industrial Applicability]
[0140] According to the present invention, it is possible to provide: a steel pipe for a
line pipe that is suitable for a line pipe and has a strength of API X52 to X70 grade
and satisfactory SSC resistance and HIC resistance; and a steel plate having satisfactory
SSC resistance and HIC resistance that is used as a base metal of the steel pipe.
Accordingly, the present invention is highly applicable to the steel plate manufacturing
industry and the energy industry.
[Brief Description of the Reference Symbols]
[0141]
1: STEEL PIPE
2: STEEL PLATE (BASE METAL)
3: WELD