Note: Descriptions are shown in the official language in which they were submitted.
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Description
Title of Invention: STAINLESS STEEL PIPE WITH EXCELLENT
EROSION RESISTANCE AND MANUFACTURING METHOD
THEREOF
Technical Field
[1] The present invention is related to a stainless steel pipe with
excellent erosion
corrosion resistance and a manufacturing method thereof, and more
specifically, to a
stainless steel pipe with excellent erosion resistance, which improves erosion
re-
sistance in the inner area of the pipe while maintaining low temperature
toughness, and
a manufacturing method thereof.
[2]
Background Art
[31 Generally, stainless steels are classified according to chemical
components or metal
structures thereof. According to the metal structures, the stainless steels
are classified
into an austenitic type (300 series), a ferritic type (400 series), a
martensitic type and a
duplex type.
[4] Of these stainless steels, the ferritic type stainless steels are
broadly used as kitchen
utensils, electric equipments, materials for automobiles, internal and
external materials
for construction due to their excellent formability, corrosion resistance and
its
relatively low cost.
[51 However, since the ferritic type stainless steels have ferritic phase
structures at room
temperature, they have limits in using as a structual materials since they do
not show
neither a high strengthening phenomena caused by phase transformation, which
can be
observed at the austenitic type stainless steel, nor a strengthening phenomena
between
two phases, which can be observed at duplex stainless steels. Furthermore,
ferritic type
stainless steels also exhibit a phenomenon of showing brittleness at a low
temperature
(a ductile brittle transition temperature (DBTT)) and its effect becomes more
critical in
the cases of heavy gauges (referred to as a thick steel plate). On the other
hand, grain
growth upon welding operation might also affect the deterioration of ductility
and
strength of the material.
[6] Due to the characteristics, the ferritic type stainless steels have
been highly re-
strictively used as a structural material.
[71 However, the ferritic stainless steels have inherent cost benefit since
they contain
relatively low contents of expensive elements such as nickel, molybdenum and
the like
compared to other stainless grades.
[8] Consequently, if the ferritic type stainless steel can be used as a
structural material.
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users can widen the choice of material selection as well as saving the global
material
resources.
191 For the ferritic type stainless steels, in order to be used as a
structural material having
strength, weldability and DBTT features, the optimum alloy design as well as
manu-
facturing process control has to be controlled.
[10] In the case of the alloy design, only ferritic single phase is not
enough to get high
strength as well as problems of grain size coarsening upon welding process. Ac-
cordingly, to be used as a structural material, microstructural changes such
as mixed
phases has to be followed to overcome the handicap.
[11] Also, likewise carbon steels, the strength of ferritic type stainless
steel is proportionly
increased upon the increase of reduction ratio. The behavior is not much
exception
even though mixed phase concept is introduced in the ferritic type stainless
steels.
However, the change of heat treatment condition for mixed phase alloy might be
more
tricky than the single phase alloy that the quality features can be affected
much. Con-
sequently, to be a competitive structural material having both inherent
strength and
cost benefits, both alloy design and microstructural features has to be
optimized in this
ferritic type stainless steels.
[12] On the other hand, pipes for oil and gas treatment facilities face
lots of wear issues.
Particularly, when producing heavy oil such as oil sand, wear issue of
transport pipes
becomes critical due to diverse slurry movements. In other words, in the
process of
separating the bitumen from rock or sand ores, the movements of hot water
mixed
slurries normally accelerate the erosion corrosion damages of pipes that it
should be
minimized to save the maintenance cost. Accordingly, for these pipes, the
erosion
corrosion characteristics of pipe inner side becomes a hot issue.
[13] Therefore, generally, as the pipes for transferring the oil sand
slurry, steel pipes are
widely used. Particularly, the steel pipe itself has good abrasion resistance,
but a
technique for hardening the inner circumferential surface of the steel pipe by
heat
treatment has been suggested and used according to needs for higher abrasion
re-
sistance than that of the inner side of the steel pipe.
[14] As for an example, hardening technique of the inner surface of the
steel pipe by heat
treatment is disclosed in detail in "Method and apparatus for quenching inner
surface
of steel pipe (Japanese Patent Laid-Open Publication No. 2002-60834; Patent
Reference 1)." Patent Reference 1 discloses a technique for heating a steel
pipe by
induction heating from outside of the steel pipe till the entire thickness
reaches a target
temperature, and rapidly quenching the pipe inner surface by spraying waters,
which
hardens the inner surface of the pipe but not much notable hardening at the
outer
surface.
[15] The method disclosed in Patent Reference 1 is a technique applied to
carbon steel
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pipe. Here, the amount of water and quenching rate at the pipe inner surface
has to be
well controlled to get the uniform microstructures.
[16] On the other hand, since the heavy oil normally forms a toxic
condition due to the
containing of chloride ions or organic acids in the slurries, the material for
these pipe
also requires good corrosion resistance in this environment. It is one of the
critical
reason that economical stainless steel is needed in this industrial field.
Also, since the
environment is harsh in the sense of temperature and wear condition, both
toughness at
low temperature and good erosion resistance of the material is required.
[17] Accordingly, the present inventors have completed the present
invention based on the
fact that dual phase utility ferritic stainless steel pipe can improve erosion
corrosion re-
sistance by way of induction heating method.
[18] The description provided above as a related art of the present
invention is just for
helping to understand the background of the present invention and should not
be
construed as being included in the related art known by those skilled in the
art.
[19]
Disclosure of Invention
Technical Problem
[20] The present invention has been made in an effort to solve the above-
described
problems associated with the prior art. The present invention is objected to
provide a
stainless steel pipe with excellent erosion resistance, wherein erosion
resistance of the
inner area of the pipe is improved while maintaining corrosion resistance and
low tem-
perature toughness, and a manufacturing method thereof.
[21] Particularly, in order to improve the erosion resistance of the inner
area of the pipe,
the microstructure of pipe material has to be composed of dual phases combined
with
martensite and ferrite phases rather than ferritic single phase. The present
invention
provides a stainless steel pipe with excellent erosion resistance, wherein a
mixed phase
structure partially containing a martensite phase rather than a ferrite single
phase is
formed, and a manufacturing method thereof.
[22]
Solution to Problem
[23] In order to accomplish the above-described objects, the stainless
steel pipe with
excellent erosion resistance upon the embodiment of the present invention is
char-
acterized in that it is a steel pipe including Cr at 11.0 to 14.0 wt% and
having a ferrite
factor (FF) of 6 to 9 as expressed by the following formula 1, wherein, based
on a
thickness of the steel pipe, the inner area has a mixed phase structure of
marten site and
ferrite, the outer area has either ferrite phase structure or a mixed phase
structure of
ferrite and martensite, and a martensite fraction of the inner area is greater
than a
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martensite fraction of the outer area:
[24] [Formula 1]
[25] FF = [Cr + 6Si +8Ti + 4Mo + 2A1 +4Nb] - [2Mn + 4Ni + 40(C+N)1
[26] wherein Cr, Si, Ti. Mo, Al, Nb, Mn, Ni, C and N represent contents
(wt%) of re-
spective components.
[27] The stainless steel pipe is characterized in that it includes C at
0.01 to 0.05 wt%, N at
0.01 to 0.05 wt%, Ti at 0.25 wt% or less, and Si at 0.5 wt% or less.
[28] The stainless steel pipe is characterized in that it further includes
Nb at 0.25 wt% or
less, wherein the sum of the contents of Ti and Nb is 0.25 wt% or less.
[29] The stainless steel pipe is characterized in that it has a thickness
of 8 to 30 mm.
[30] On the other hand, a manufacturing method of the stainless steel pipe
with excellent
abrasion resistance according to one embodiment of the present invention is
char-
acterized in that it includes rapidly heating the inner area of the stainless
steel pipe,
which comprises Cr: 11.0 to 14.0 wt% and has a ferrite factor (FF) of 6 to 9
as
expressed by the following formula 1, to a temperature of the inner area
higher than a
Acl transformation temperature and heating the outer area of the stainless
steel pipe to
a temperature of the outer area lower than the Acl transformation temperature:
and
cooling the stainless steel pipe to form a mixed phase structure of martensite
and ferrite
in the inner area of the stainless steel pipe:
[31] [Formula 1]
[32] FF = [Cr + 6Si +8Ti + 4Mo + 2A1 +4Nb] - [2Mn + 4Ni + 40(C+N)]
[33] wherein, Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents
(wt%) of re-
spective components.
[34] The stainless steel pipe is characterized in that it includes C at
0.01 to 0.05 wt%, N at
0.01 to 0.05 wt%, Ti at 0.25 wt% or less, Nb at 0.25 wt% or less, Si at 0.5
wt% or less,
Ni at 2.0 wt% or less, Mo at 2.0 wt% or less, Al at 0.1 wt% or less, Mn at 2.0
wt% or
less, the balance of Fe and inevitable impurities, and the sum of the contents
of Ti and
Nb is 0.25 wt% or less.
[35] The inner area of the stainless steel pipe is characterized in that it
is rapidly heated by
induction heating.
[36] The inner area of the stainless steel pipe is characterized in that it
is heated by
induction heating to a temperature of 100 to 300 C higher than the Acl
transformation
temperature.
[37] The temperature of the outer area of the stainless steel pipe heated
by induction
heating is characterized in that it maintained at a temperature lower than the
Acl trans-
formation temperature, and a difference in temperature between the inner area
and the
outer area of the stainless steel pipe has to be satisfied in a range of 100
to 300 C.
[38] The cooling of the stainless steel pipe is characterized in that it
includes the cooling
5
of the stainless steel pipe includes air-cooling or water spray cooling the
outer area of
the stainless steel pipe.
[39] On the other aspect, a manufacturing method of the stainless steel
pipe with
excellent erosion resistance upon the embodiment of the present invention is
characterized in that it includes entering an induction heating device into an
inner
hollow portion of the stainless steel pipe, which has an area in which a
ferrite phase
and an austenite phase coexist, at a temperature higher than a Ad l
transformation
temperature; rapidly heating the inner area of the stainless steel pipe based
on the
thickness thereof by induction heating by applying electricity to the
induction heating
device; cooling the inner area of the stainless steel pipe to form a mixed
phase
structure of martensite phase and ferrite phase in the inner area of the
stainless steel
pipe.
[40]The stainless steel pipe is characterized in that when the stainless steel
pipe is
heated by the induction heating, the stainless steel pipe has a temperature
gradient at
which the temperature gradually decreases from the inner area to the outer
area based
on the thickness thereof, and the temperature of the inner area of the
stainless steel
pipe reaches above the Ael transformation temperature and the temperature of
the
outer area of the stainless steel pipe does not reach the Ad l temperature.
[41] The inner area of the stainless steel pipe is characterized in that it
is heated by
induction heating to a temperature of 100 to 300 C higher than Ad l
transformation
temperature, and the outer area of the stainless steel pipe is maintained at a
temperature lower than the Ael transformation temperature.
[42] The cooling of the stainless steel pipe is characterized in that it
includes air-
cooling or water spray cooling the outer area of the stainless steel pipe.
[43] According to one aspect of the present invention, there is provided a
stainless steel
pipe with erosion resistance, said steel pipe comprising Cr at 11.0 to 14.0
wt% and
having a ferrite factor (FF) of 6 to 9 as, expressed by the following Formula
I,
wherein, based on a thickness of the steel pipe, the inner area has a mixed
phase
structure of martensite and ferrite, the outer area has either ferrite phase
structure or a
mixed phase structure of ferrite and martensite, and a martensite fraction of
the inner
area is greater than a martensite fraction of the outer area:
[Formula 1]
FF = [Cr + 6Si +8Ti + 4Mo + 2A1 +4Nb] - [2Mn + 4Ni + 40(C+N)]
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5a
wherein, Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents (wt%) of
respective components, and
wherein the stainless steel pipe comprises C at 0.01 to 0.05 wt%, N at 0.01 to
0.05
wt%, Ti at 0.25 wt% or less, Nb at 0.25 wt% or less, Si at 0.5 wt% or less, Ni
at 2.0
wt% or less, Mo at 2.0 wt% or less, Al at 0.1 wt% or less, Mn at 2.0 wt% or
less, the
balance of Fe and inevitable impurities, and the sum of the contents of Ti and
Nb is
0.25 wt% or less.
[44] According to another aspect of the present invention, there is
provided a
manufacturing method of a stainless steel pipe with erosion resistance,
comprising:
rapidly heating the inner area of the stainless steel pipe, which comprises
Cr: 11.0
to 14.0 wt% and has a ferrite factor (FF) of 6 to 9 as expressed by the
following
Formula 1, to a temperature of the inner area higher than a Ac 1
transformation
temperature and heating the outer area of the stainless steel pipe to a
temperature of
the outer area lower than the Ad l transformation temperature; and cooling the
stainless
steel pipe to form a mixed phase structure of martensite and ferrite in the
inner area of
the stainless steel pipe:
[Formula 1]
FF = [Cr + 6Si +8Ti + 4Mo + 2A1 +4Nb] - [2Mn + 4Ni + 40(C+N)]
wherein, Cr, Si, Ti, Mo, Al, Nb, Mn, Ni, C and N represent contents (wt%) of
respective components, and
wherein the stainless steel pipe comprises C at 0.01 to 0.05 wt%, N at 0.01 to
0.05
Ti at 0.25 wt% or less, Nb at 0.25 wt% or less, Si at 0.5 wt% or less, Ni at
2.0
wt% or less, Mo at 2.0 wt% or less, Al at 0.1 wt% or less, Mn at 2.0 wt% or
less, the
balance of Fe and inevitable impurities, and the sum of the contents of Ti and
Nb is
0.25 wt% or less.
Advantageous Effects of Invention
[45] According to the embodiments of the present invention, a low chromium-
containing ferritic type stainless steel pipe with excellent corrosion
resistance and low-
temperature toughness can be used to ensure erosion resistance required upon
transport of slurries.
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5b
Brief Description of Drawings
[46] FIG. 1 is a phase diagram of a low-carbon ferritic type stainless
steel as a material
of the present invention according to the Cr content;
[47] FIG. 2 is a graph showing changes in temperature of the materials
according to
heat treatment temperature and heat treatment time;
[48] FIG. 3 is an image of a microstructure of the low carbon ferritic type
stainless steel
as the material of the present invention before heat treatment;
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[49] FIG. 4 is an image of a microstructure of the low carbon ferritic type
stainless steel as
the material of the present invention after heat treatment at 720 C;
[50] FIG. 5 is an image of a microstructure of the low carbon ferritic type
stainless steel as
the material of the present invention after heat treatment at 780 C;
[51] FIG. 6 is an image of a microstructure of the low carbon ferritic type
stainless steel as
the material of the present invention after heat treatment at 800 C;
[52] FIG. 7 is an image of a microstructure of the low carbon ferritic type
stainless steel as
the material of the present invention after heat treatment at 870 C;
[53] FIG. 8 is a graph showing the relationship of the erosion resistance
depending upon
the change of materials hardness; and
[54] FIG. 9 is a graph showing the relationship of low-temperature impact
toughness
depending upon changes in heat treatment temperatures.
[55]
Mode for the Invention
[56] Hereinafter, the embodiments of the present invention will be
described in detail with
reference to the accompanying drawings.
[57] FIG. 1 is a phase diagram of a low-carbon ferritic type stainless
steel as a material of
the present invention according to the Cr content, FIG. 2 is a graph showing
changes in
temperature of the materials according to heat treatment temperature and its
time. FIG.
3 is an image of a microstructure of the low carbon ferritic type stainless
steel as the
material of the present invention before heat treatment, FIG. 4 is an image of
a mi-
crostructure of the low carbon ferritic type stainless steel as the material
of the present
invention after heat treatment at 720 C, FIG. 5 is an image of a
microstructure of the
low carbon ferritic type stainless steel as the material of the present
invention after heat
treatment at 780 , FIG. 6 is an image of a microstructure of the low carbon
ferritic type
stainless steel as the material of the present invention after heat treatment
at 800 C,
FIG. 7 is an image of a microstructure of the low carbon ferritic type
stainless steel as
the material of the present invention after heat treatment at 870 C, FIG. 8 is
a graph
showing the relationship of the erosion resistance according to changes in
hardness of
the materials, and FIG. 9 is a graph showing the relationship of low-
temperature
impact toughness depending upon a change in temperature.
[58] In order to improve the erosion resistance of an inner part (inner
area based on
thickness) of a steel pipe manufactured with a ferritic type stainless steel,
which has a
dual phase microstructure and excellent corrosion resistance and low-
temperature
toughness characteristics, in the present invention, the inner area of the
stainless steel
pipe is induction heated to form enough volume of martensite phase at the
inner area.
[59] The material of the present invention is a low Cr ferritic type
stainless steel
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containing Cr at 11 to 14 wt%, and a steel pipe type steel, which has a dual
phase
structure (ferrite + tempered martensite) at room temperature to ensure
strength and
ductility. Generally, this type of steel is called a "utility ferritic
stainless steel
(hereinafter, referred to as a 'UF stainless steer)" is used.
[60] As shown in FIG. 1, the UF stainless steel has an area in which a
ferrite phase and an
austenite phase coexist at a temperature of 800 C or higher. The austenite
phase can
increase the hardness of the material by transforming into the martensite
phase if
cooled to the room temperature except a very slow cooling condition.
[61] In order to confirm this fact, FIG. 2 represents real heat treatment
conditions at
various temperatures, and FIGS. 3 to 7 are images showing microstructures
before and
after heat treatment at 720 C, 780 C, 800 C and 870 C, respectively. As shown
in FIG.
6, it could be confirmed that the martensite phase appeared at a temperature
of 800 C
or more, and thereby the hardness increased. On the other hand, as shown in
FIG. 8, it
could be confirmed that the erosion resistance of the UF stainless steel is
increased in
proportion to the material hardness.
[62] Further, FIG. 9 represents the low-temperature impact toughness
according to a
change in temperature. In FIG. 9, it could be confirmed that impact toughness
was
rapidly decreased when the martensite phase was produced at a high
temperature,
higher than the Acl transformation temperature. This deterioration of the
impact
toughness is a general phenomenon as long as the martensite phase is produced
all
through the thickness of the steel pipe material. However, if the
microstructure is
formed differently depending on the thickness, the situation may be changed.
This is
related with the fact that generally overall ductility of the plate tend to be
more in-
fluenced by the ductile phase part rather than hard phase part, maintaining
similar
ductiltiy of the ductile phase only feature.
[631 The UF steel is a steel characterized in that the ferrite phase and
the austenite phase
coexist at a temperature of 800 C or higher in terms of stability, and the
ferritic type
stainless steel pipe according to one embodiment of the present invention
includes Cr
at 11.0 to 14.0 wt%, C at 0.0110 0.05 wt%, N at 0.01 to 0.05 wt%, Ti at 0.25
wt% or
less (exclusive of 0 wt%), Nb at 0.25 wt% or less (exclusive of 0 wt%), Si at
0.5 wt%
or less (exclusive of 0 wt%), Ni at 2.0 wt% or less (exclusive of 0 wt%), Mo
at 2.0
wt% or less (exclusive of 0 wt%), Al at 0.1 wt% or less (exclusive of 0 wt%),
Mn at
2.0 wt% or less (exclusive of 0 wt%), the balance of Fe and inevitable
impurities, and
the sum of the contents of Ti and Nb is in a range of 0.25 wt% or less.
[64] The reasons of restricting the compositional range of each element as
described
above will be described in detail.
[65] It is preferred that the Cr content is in a range of 11.0 to 14.0 wt%.
When the Cr
content is less than 11 wt%, it may be difficult to form an autogenous
protecting film
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due to degraded corrosion resistance and oxidation resistance. and when it
excesses 14
wt%, the ductility may be lowered and the production cost may increase.
[66] It is preferred that the C content is in a range of 0.01 to 0.05 wt%.
When the C
content is less than 0.01 wt%, the hardness and the strength of the material
may be
lowered, and when it excesses 0.05 wt%, the probabilities of deterioration of
the low-
temperature toughness and the occurrence of intergranular corrosion may
increase.
[67] It is preferred that the N content is in a range of 0.01 to 0.05 wt%.
When the N
content is less than 0.01 wt%, the hardness and the strength of the material
may be
lowered, and when it excesses 0.05 wt%, the deterioration of the low-
temperature
toughness may be intensified.
[68] It is preferred that the Ti content is in a range of 0.25 wt% or less
(exclusive of 0
wt%). When the Ti content excesses 0.25 wt%, surface defects may be caused and
the
probability of lowering the low-temperature toughness may be increased due to
formation of TiN precipitates in the material.
[69] It is preferred that the Nb content is in a range of 0.25 wt% or less
(exclusive of 0
wt%). When the Nb content excesses 0.25 wt%, the material cost may be
increased,
and the probabilities of lowering the low-temperature toughness may be
increased due
to Nb(C, N) precipitates.
[70] It is preferred that the Si content is in a range of 0.5 wt% or less
(exclusive of 0
wt%). When the Si content excesses 0.5 wt%, the ferrite factor (FF) may
increase so as
to soften the material.
[71] It is preferred that the Ni content is in a range of 2.0 wt% or less
(exclusive of 0
wt%). When the Ni excesses 2.0 wt%, the production cost of the material may
increase
and the probability of the low-temperature toughness feature may be affected
much by
reducing the ferrite factor (FF), which will be described later.
[72] It is preferred that the Mo content is in a range of 0.3 wt% or less
(exclusive of 0
wt%). When the Mo content excesses 0.3 wt%, the production cost of the
material may
be increased and the low-temperature toughness may be degraded due to
formation of
Mo precipitates.
[73] It is preferred that the Al content is in a range of 0.1 wt% or less
(exclusive of 0
wt%). When the Al content excesses 0.1 wt%, the probability of nozzle clogging
may
be increased during continuous casting process and also surface defects may be
issued
due to AIN precipitates.
[74] It is preferred that the Mn content is in a range of 2.0 wt% or less
(exclusive of 0
wt%). When the Mn content excesses 2.0 wt%, the quality feature of welding
area may
be degraded due to increased formation of MnS precipitates.
[75] In the ferritic type stainless steel of the present invention, the Ti
and the Nb, which
have higher affinity to C and N than Cr. were added as a dual stabilization
purpose, in
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order to reduce the sensitivity of intergranular corrosion caused by
precipitation of
chrome carbides at the grain boundary upon welding. The Ti can be used alone
to
obtain the effect of improving the intergranular corrosion, but when adding Ti
only to a
large quantity, large inclusions such as TiN, TiC and the like may be easily
formed,
thereby causing the surface defects due to the presence of Ti stripes upon
rolling. Ac-
cordingly, it is preferred to add both Ti and the Nb as for a dual
stabilization. Here, the
sum of the Ti and Nb contents is recommended not to be higher than 0.25 wt%.
Sum-
merizing it, the reason of limiting Ti and Nb contents upto 0.25 wt% or less
is that the
content is enough to prevent the intergranular corrosion of the material
considering the
carbon contents, and too much Ti and Nb addition may cause the occurrence of
surface
defects and degeneration of the low-temperature toughness.
[76] On the other hand, it is preferred to control the compositional range
of the stainless
steel pipe according to the present invention, having the ferrite factor (FF)
of 6 to 9, as
expressed by the following Formula 1 .
[77] [Formula 1]
[78] FF = [Cr + 6Si +8Ti + 4Mo + 2A1 +4Nb] - [2Mn + 4Ni + 40(C+N)1
[79] wherein Cr, Si, Ti. Mo, Al, Nb, Mn, Ni, C and N represent contents
(wt%) of re-
spective composition.
[80] The term ferrite factor (FF) refers to a ratio of ferrite stabilizing
elements and
austenite stabilizing elements of a material, the ferrite stabilizing elements
such as Cr,
Si and the like and the austenite stabilizing elements such as Mn, Ni and the
like. If the
contents of the austenite stabilizing elements are increased, the ferrite
factor (FF) value
become smaller and the martensite content is increased, which increases the
material
hardness under the control of the heat treatment conditions.
[81] Accordingly, it is preferred to control the ferrite factor (FF) to 6
to 9, and it is more
preferred to satisfy the range of 8 to 9. The reason is that when the ferrite
factor (FF) is
less than 6, the low-temperature toughness may not be maintained to the
desired level
because the amount of the martensite increases after the heat treatment, and
when the
ferrite factor (FF) is over 9, the erosion resistance may not be maintained to
the desired
level because the amount of the martensite decreases even after the heat
treatment.
[82] As seen from the above-described ferrite factor (FF)-related formula,
the UF stainless
steel is characterized that it can control the microstructure at high
temperature range of
heat treatment reflecting a proper combination of a ferrite stabilizing
element group
such as Cr and an austenite stabilizing element group such as Ni. Namely, when
the
material is heated to the higher temperature, the austenite phase starts to be
stabilized
at a certain temperature, and this temperature is called as a Acl
transformation tem-
perature. When cooled in the air after being kept at a temperature higher than
this tem-
perature for a certain period, the produced austenite phase is characterized
in that it is
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transformed to a hard martensite phase. As such, the formation range of
austenite
phase is called Gamma loop in phase diagram, and if the material temperature
reaches
in this range and cooled either air or water atmosphere, hardness of the
material is
increased due to the formation of martensite phase.
[83] However, there is a tendency that as the hardness of the material
increases, the
toughness of the material may be deteriorated in inverse proportion to an
increase in
hardness of the material. Accordingly, for the steel material used for a steel
pipe for
transporting oil sand slurry, the erosion resistance is important, but the
hardness may
not be blindly increased because the low-temperature toughness should be
considered.
[84] Therefore, in this embodiment, an induction heating method was applied
in order to
improve the erosion resistance by increasing the hardness of the inner area
through
which the slurry is transported, based on the thickness of the steel pipe, and
to secure
the low-temperature toughness by controlling the outer area relatively softly.
Here, it is
also because the UF stainless steel is a steel whose metallurgical
characteristic is very
suitable for the application of the induction heating method as mentioned
above.
[85] In other words, in order to improve the erosion resistance of the
inner area based on
the thickness of the stainless steel pipe having enough ductility through the
previous
primary heat treatment at a temperature lower than the Acl transformation tem-
perature, an additional heat treatment is needed on the inner area based on
the
thickness of the stainless steel pipe. This heat treatment can be embodied by
various
methods, but here the solution was proposed by the induction heating method.
In
further detail, induction heating is directly applied to the inner area, whose
hardness is
needed to increase, at a temperature higher than the Acl transformation
temperature,
but in order to maintain the ductility of the material as it is, when
induction heating the
inner area, the stainless steel pipe has to have a temperature gradient at
which the tem-
perature gradually decreases from the inner area to the outer area based on
the
thickness thereof, and the temperature of the outer area based on the
thickness of the
stainless steel pipe is kept at a temperature lower than the Acl
transformation tem-
perature.
[86] Preferably, the temperature of the inner area of the stainless steel
pipe heated by the
induction heating is rapidly heated to a temperature of 100 to 300 C higher
than the
transformation point Ac 1, and then cooled in the air or water spray
condition. The tem-
perature of the outer area of the stainless steel pipe is kept at a
temperature lower than
the Acl transformation temperature having a difference in temperature of 100
to 300 C
or more between the outer area and the inner area of the stainless steel pipe.
[87] However, in the case of the thin material, the difference in
temperature between the
inner and outer faces may not be easily realized by the heat treatment method,
but in
the case of thick plate having a thickness of 8 mm or higher, it is more
easily realized
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by the heat transfer gradient in the thickness. For this reason, in this
embodiment, it is
preferred to limit the thickness of the stainless steel pipe to a range of 8
mm or more.
However, in order to have the preferable temperature gradient between the
inner area
(a part whose temperature reaches the Acl transformation temperature) and the
outer
area (a part whose temperature does not reach the Acl transformation
temperature) of
the stainless steel pipe, it is preferred that the minimum thickness of the
stainless steel
pipe is 8 mm or more and the maximum thickness thereof is 30 mm or less.
[88] In this embodiment, in order to induction heating the inner area of
the stainless steel
pipe, an induction heating device is inserted into an internal hollow portion
of the
stainless steel pipe, and electricity is applied to the induction heating
device followed
by rapidly heating the inner area of the stainless steel.
[89] To control the temperature between inner and outer area of the pipe,
with a time gap
of 0-10 second, air or water spray cooling might be followed along either pipe
inside
or pipe outside just after the narrow band area of the indution heating. In
this case, a
mixed phase of martensite and ferrite is formed in the inner area of the
stainless steel
pipe, while a ferrite single phase structure or a mixed phase structure of the
ferrite and
the martensite is formed in the outer area. Following this step, to be sure,
the
martensite fraction of the inner area will be higher than the martensite
fraction of the
outer area.
[90] When the stainless steel pipe is being heated by the induction
heating, the stainless
steel pipe has a temperature gradient at which the temperature gradually
decreases
from the inner area to the outer area based on the thickness thereof, and the
tem-
perature of the inner area of the stainless steel pipe reaches above the Acl
trans-
formation temperature but the outer area of the stainless steel pipe should
not much
higher(less than 100 C) than the Acl transformation temperature.
[91] In the case of the UF stainless steel, when the temperature is raised
to a temperature
higher than the Acl transformation temperature as described above, the
austenite phase
is observed in the steel structure, and the amount of formation is correlated
with the
heat treatment temperature and holding time. Namely, when the holding time is
as
short as several seconds, the phase may be slightly transformed at a
temperature higher
than the Acl transformation temperature, and only if when the temperature and
the
holding time satisfy at least certain optimum conditions, sufficient
martensite phase
formation becomes satisfied upon cooling. When the formation of the martensite
phase
is activated as described above, the hardness of the material should be
increased,
thereby enhancing the erosion resistance of the material.
[92] Of course, as described in the suggested embodiments, the method is
not limited to
induction-heating the inner area by inserting the induction heating device
into the
internal hollow portion of the stainless steel pipe, but various method, which
can
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partially heat only the inner area of the stainless steel pipe, can be
embodied with
changes and modifications
[93] On the other hand, the emplementation activity of UF stainless steel
induction
heating can be embodied as the following. The Acl transformation temperature
of the
UF stainless steel at quasi-static condition having the above-described
components(FF:
6-9) is around 800 C. Considering that the induction heating method is more
dynamic
condition, the temperature control of induction heating method has to be
differently
applied. Namely, Table 1 shows the hardness variation after sufficient soaking
time but
it is difficult to obtain the same microstructure after dynamic induction
heating
method.
[94] Table 1
[Table 1]
Soaking temp. Y.S(Mpa) T.S(Mpa) El(%) Hardnes s(HRB)
25 C(As-rolled) 814.4 926.4 17.3 99.7
720 C 591.4 662.2 28.0 91.2
740 C 414.5 538.6 33.9 79.6
760 C 323.9 479.1 39.3 79.0
780 C 350.2 503.3 37.6 81.9
800 C 404.1 541.6 34.6 86.9
830 C 467.2 599.1 30.4 94.4
850 C 508.5 635.4 27.8 96.6
870 C 548.2 671.2 25.4 98.5
[95] Thus, considering that the induction heating method is a rapid
heating, the optimum
temperature ranges to get enough phase transformation are to be conducted in
the
range of 100 to 300 C higher than the Acl transformation temperature, and then
the
cooling is conducted either in the air or water spray condition. Consequently,
the
difference in temperature between the inner area, to which the induction
heating is
applied, and the outer area of the stainless steel which is in an opposite
direction of the
inner area based on the thickness, should be 100 to 300 C or higher, as
described
above.
[96]
[97] Hereinafter, the effects of maintaining the low-temperature toughness
while
improving the hardness of inner pipe will be proved using the UF stainless
steel by
way of induction heating method as suggested above.
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[98] (Execution Example)
[99] The current investigation was conducted to see the changes of hardness
and me-
chanical characteristics depending on heat treatment temperatures with a UF
steel plate
including Cr at 11.0 to 14 wt%(gauge is 12mm).
[100] The Acl transformation temperature of the corresponding material
having a ferrite
factor of 6 to 9 was around 800 C. In this Execution Example, in order to
increase the
hardness of the inner part of the stainless steel, the induction heating was
conducted on
the inner area of the stainless steel pipe, heating the inner part to a
temperature higher
than the Ac1 transformation temperature. Then, the outer area of the stainless
steel
pipe was kept at a temperature lower than the Acl transformation temperature.
In this
case, the outer area of the stainless steel pipe showed an initial soft
microstructure,
thereby maintaining the similar low-temperature toughness compared to the
material
tested before the induction heating.
[101] The induction heating used for the inner area of the stainless steel
pipe was a rapid
heating method, and the heated area had such a characteristic that it was
relatively
rapidly cooled after the rapid heating. Therefore, compared to the quasi-
static heating
condition, the effective temperature for phase transformation in this
condition has to be
quite higher than the Acl transformation temperature. Accordingly, the actual
induction heating condition should be at least 100 C higher than the Acl trans-
formation temperature.
[102] Then, the formation of martensite phase in the inner area of the
stainless steel pipe
was activated, and an example of hardness variation is shown in the following
Table 2.
[103] Table 2
[Table 2]
Induction heating temp. Hardness(HRB)
800 C 190
850 C 200
900 C 220
950 C 250
1000 C 270
1050 C >270
[104] Compared with the hardness of Table 1, which was obtained by normal
heat
treatment in a furnace, it could be confirmed that there was a difference in
temperature
of approximately 100 to 300 C.
[105] The following Table 3 shows low-temperature toughness data of the
materials used
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in induction heating treatment. Here, it could be confirmed that the
difference in low-
temperature toughness values was not distinct compared to the materials used
in
normal soaking heat treatment condition at 750 C as shown in FIG. 9.
[106] Table 3
[Table 3]
Usage temp. Toughness(CVN, J)
20 C > 60
0 C >40
-20 C > 30
-40 C > 20
[107]
[108] The invention has been described in detail with reference to
preferred embodiments
thereof. However, it will be appreciated by those skilled in the art that
changes or mod-
ifications may be made in these embodiments without departing from the
principles
and spirit of the invention, the scope of which is defined in the appended
claims and
their equivalents.
