Note: Descriptions are shown in the official language in which they were submitted.
CA 03120271 2021-05-17
[DESCRIPTION]
[Invention Title]
HIGH-STRENGTH STEEL PLATE HAVING EXCELLENT LOW-TEMPERATURE
FRACTURE TOUGHNESS AND ELONGATION RATIO, AND MANUFACTURING
METHOD THEREFOR
[Technical Field]
[0001] The present disclosure relates to a high-strength steel
plate and a manufacturing method therefor, and more
particularly, to a high-strength steel plate for a pipeline
capable of being stably used even in a harsh environment by
having high strength characteristics through optimization of
a steel composition, a microstructure, and a manufacturing
process and having excellent low-temperature fracture
toughness and elongation ratio, and a manufacturing method
therefor.
[0002]
[Background Art]
[0003] Recently, as oilfield development has been carried out
in cold regions such as Siberia and Alaska where climatic
conditions are poor, projects to transport abundant gas
resources from oil-rich regions to consumption regions through
pipelines have been actively conducted. A steel used in such
a pipeline project should necessarily have durability against
deformation of the pipeline due to a cryogenic temperature and
Page 1
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
frost heave (a phenomenon of pushing up the surface of Earth
when the ground freezes at the change of seasons) as well as
a pressure of a transport gas, and is thus required to have high
strength characteristics and excellent drop weight tearing test
(DWTT) fracture toughness, and high elongation ratio
characteristics.
[0004]
[0005] A DWTT percent ductile fracture is a kind of index for
determining whether or not a steel for a pipeline has brittle
fracture arrestability for being safely used at a low
temperature. In general, the pipelines provided in the cold
regions are required to have a DWTT percent shear of 85% or more
at -20 C in a pipe state. In order to secure the DWTT percent
shear of 85% or more at -20 C in a pipe state, a DWTT percent
shear of a steel plate provided for manufacturing a pipe should
satisfy 85% or more at -30 C.
[0006]
[0007] In general, it has been known that DWTT property has
a deep association with an effective grain size of the steel
plate. The effective grain size is defined as a size of grains
having a high angle grain boundary, and as the effective grain
size is refined, crack arrestability increases. The reason
therefor that a propagation path of a crack changes at an
effective grain boundary when the crack is initiated and
propagated.
Page 2
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
[0008]
[0009] In order to refine the effective grain size, a method
of performing accelerated cooling immediately after rolling is
widely used. A mixed structure of acicular ferrite and bainite
may be implemented by accelerated cooling immediately after
rolling. However, a microstructure formed through usual
accelerated cooling has high hardness because carbon (C) is
supersaturated in grains, and accordingly, exhibits inferior
ductility such as a uniform elongation ratio less than 9% and
a total elongation ratio less than 20%. As a result,
formability at the time of forming a pipe is lowered, and local
stress concentration is easily generated at the time of applying
external deformation, and thus, stability of the pipe is
significantly reduced.
[0010]
[0011] Therefore, in the manufacture of a steel plate for a
pipeline, a manufacturing method for a steel plate for a
pipeline having excellent low-temperature fracture toughness
and having excellent ductility by having a uniform elongation
ratio of 9% or more and a total elongation ratio of 28% or more
by suppressing deterioration of an elongation ratio of the steel
plate manufactured by accelerated cooling has been demanded.
[0012]
[0013] In the related art, there were studies on a steel plate
having an excellent elongation ratio and low-temperature
Page 3
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
fracture toughness. In this regard, Patent Literature 1
proposes a method of manufacturing a steel containing a mixed
structure of 30 of 60% of equiaxed ferrite and 40 to 70% of
bainite in terms of area fraction as a microstructure by
non-recrystallized-region-rolling a steel containing nickel
(Ni), niobium (Nb), and molybdenum (Mo) at a rolling reduction
of 65% or more, primarily cooling the steel to a Bs temperature
at a cooling rate of 15 to 30 C/s, and secondarily cooling the
steel to a temperature range of 350 to 500 C at a cooling rate
of 30 to 60 C/s.
[0014]
[0015] However, in Patent Document 1 in which low-temperature
rolling is performed on a steel plate having a thickness of 20
mm or more, there is a technical difficulty in applying the
corresponding process condition to a steel plate having a
thickness less than 20 mm. The reason is that the steel plate
having the thickness less than 20 mm is subjected to
low-temperature rolling and then cooled rapidly, and it is thus
difficult to secure desired low-temperature fracture toughness,
strength, and elongation ratio in an entire length direction
of the steel plate, particularly, at a rear end portion of the
steel plate.
[0016]
[0017] (Related Art Document)
[0018] (Patent Document 1) Korean Patent Laid-Open Publication
Page 4
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
No. 10-2013-0073472 (published on 3 July, 2013)
[0019]
[Disclosure]
[Technical Problem]
[0020] An aspect of the present disclosure is to provide a
high-strength steel plate having excellent low-temperature
toughness, and a manufacturing method therefor.
[0021] An aspect of the present disclosure is not limited to
theabovementionedcontents. Those skilledinthe artwillhave
no difficulty in understanding an additional aspect of the
present disclosure from the general contents of the present
specification.
[0022]
[Technical Solution]
[0023] According to an aspect of the present disclosure, a
high-strength steel plate having excellent low-temperature
fracture toughness and elongation ratio contains: by wt%, 0.05
to 0.1% of carbon (C), 0.05 to 0.5% of silicon (Si), 1.4 to 2.0%
of manganese (Mn), 0.01 to 0 . 05% of aluminum (Al), 0 . 005 to 0.02%
of titanium (Ti), 0.002 to 0.01% of nitrogen (N), 0.04 to 0.07%
of niobium (Nb), 0.05 to 0.3% of chromium (Cr), 0.05 to 0.4%
of nickel (Ni), 0.02% or less of phosphorus (P), 0.005% or less
of sulfur (S), 0.0005 to 0.004% of calcium (Ca), remaining iron
(Fe), and inevitable impurities; and 20 to 60 area% of ferrite
and bainite as a microstructure, wherein a grain size of upper
Page 5
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
80% of high angle grain sizes based on 15 in a central portion
of the high-strength steel plate is 70 pm or less.
[0024] The high-strength steel plate may further contain 0.3
wt% or less of molybdenum (Mo).
[0025] A fraction of the bainite may be 35 to 75 area%.
[0026] The microstructure of the high-strength steel plate may
further contain 5 area% or less of martensite-austenite
constituent.
[0027] A yield strength of the high-strength steel plate may
be 485 MPa or more.
[0028] A total elongation ratio of the high-strength steel
plate may be 28% or more, and a uniform elongation ratio of the
high-strength steel plate with respect to a rolling orthogonal
direction may be 9% or more.
[0029] A drop weight tearing test (DWTT) percent shear of the
high-strength steel plate at -30 C with respect to a rolling
orthogonal direction of the steel plate may be 85% or more.
[0030] A thickness of the high-strength steel plate may be less
than 20 mm.
[0031] According to another aspect of the present disclosure,
a manufacturing method for a high-strength steel plate having
excellent low-temperature fracture toughness and elongation
ratio includes: reheating a slab containing, by wt%, 0.05 to
0.1% of carbon (C), 0.05 to 0.5% of silicon (Si), 1.4 to 2.0%
of manganese (Mn), 0.01 to 0.05% of aluminum (Al), 0.005 to 0.02%
Page 6
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
of titanium (Ti), 0.002 to 0.01% of nitrogen (N), 0.04 to 0.07%
of niobium (Nb), 0.05 to 0.3% of chromium (Cr), 0.05 to 0.4%
of nickel (Ni), 0.02% or less of phosphorus (P), 0.005% or less
of sulfur (S), 0.0005 to 0.004% of calcium (Ca), remaining iron
(Fe) , and inevitable impurities; maintaining and extracting the
slab; recrystallized-region-rolling the maintained and
extracted slab in a temperature range of Tnr or higher;
non-recrystallized-region-rolling the
recrystallized-region-rolled material at a total reduction
ratio of 30% or more; and cooling the
non-recrystallized-region-rolled steel plate to a temperature
range of (Bs - 80 C) to Bs, wherein the
non-recrystallized-region-roll starts in a temperature range
of Tnr or lower and ends in a temperature range of (Ar3+ 100 C)
or higher.
[0032] The slab may further contain 0.3 wt% or less of
molybdenum (Mo).
[0033] A reheating temperature range of the slab may be 1140
to 1200 C.
[0034] A maintaining and extracting temperature range of the
slab may be 1140 to 1200 C.
[0035] The recrystallized-region-rolling may be performed in
an accumulation of passes, and an average reduction ratio of
each of the passes may be 10% or more.
[0036] The recrystallized-region-rolled material may be
Page 7
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
cooled to a temperature range of Tnr or lower by air cooling.
[0037] The non-recrystallized-region-rolled steel plate may
be cooled at a cooling rate of 10 to 50 C/s.
[0038] The cooling of the non-recrystallized-region-rolled
steel plate may start in the temperature range of (Ar3 + 30 C)
or higher.
[0039] A thickness of the high-strength steel plate may be less
than 20 mm.
[0040] The technical solution does not enumerate all of the
features of the present disclosure, and various features of the
present disclosure and advantages and effects according to the
various features will be understood in more detail with
reference to the following specific exemplary embodiments.
[0041]
[Advantageous Effects]
[0042] As set forth above, according to an exemplary embodiment
in the present disclosure, a steel plate particularly suitable
as a material for a pipeline by having high strength
characteristics and having excellent low-temperature fracture
toughness and elongation ratio, and a manufacturing method
therefor may be provided.
[0043]
[Description of Drawings]
[0044] FIG. 1 is a photograph of Specimen 2 of Inventive Example
observed with an optical microscope.
Page 8
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
[0045] FIG. 2 is graphs illustrating results obtained by
measuring high angle grain boundary grain sizes based on 15
of Specimen 2 using electron backscatter diffraction (EBSD).
[0046] FIG. 3 is a photograph of Specimen 18 of Inventive
Example observed with an optical microscope.
[0047] FIG. 4 is graphs illustrating results obtained by
measuring high angle grain boundary grain sizes based 15 of
Specimen 18 using an EBSD.
[0048]
[Best Mode for Invention]
[0049] The present disclosure relates to a high-strength steel
plate having excellent low-temperature fracture toughness and
elongation ratio, and a manufacturing method therefor, and
exemplary embodiments in the present disclosure will
hereinafter be described. Exemplary embodiments in the
present disclosure may be modified to have several forms, and
it is not to be interpreted that the scope of the present
disclosure is limited to exemplary embodiments described below.
The present exemplary embodiments are provided in order to
further describe the present disclosure in detail to those
skilled in the art to which the present disclosure pertains.
[0050]
[0051] Hereinafter, compositions of a steel according to the
present disclosure will be described in more detail.
Hereinafter, unless otherwise indicated, % indicating a content
Page 9
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
of each element is based on weight.
[0052]
[0053] A high-strength steel plate having excellent
low-temperature fracture toughness and elongation ratio
according to an exemplary embodiment in the present disclosure
contains: by wt%, 0.05 to 0.1% of carbon (C), 0.05 to 0.5% of
silicon (Si), 1.4 to 2.0% of manganese (Mn), 0.01 to 0.05% of
aluminum (Al), 0.005 to 0.02% of titanium (Ti), 0.002 to 0.01%
of nitrogen (N), 0.04 to 0.07% of niobium (Nb), 0.05 to 0.3%
of chromium (Cr), 0.05 to 0.4% of nickel (Ni), 0.02% or less
of phosphorus (P), 0.005% or less of sulfur (S), 0 . 0005 to 0 .004%
of calcium (Ca), remaining iron (Fe), and inevitable
impurities.
[0054]
[0055] In addition, the high-strength steel plate having
excellent low-temperature fracture toughness and elongation
ratio according to an exemplary embodiment in the present
disclosure may further contain, by wt%, 0.3% or less of
molybdenum (Mo).
[0056]
[0057] Carbon (C): 0.05 to 0.1%
[0058] Carbon (C) is an element that is the most effective in
improving strength of a steel. In addition, when an amount of
added carbon (C) is less than a predetermined level, expensive
alloying elements such as molybdenum (Mo) and nickel (Ni) need
Page 10
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
to be added in a large amount in order to secure the strength
of the steel, which is not preferable in terms of economy. In
the present disclosure, a lower limit of a content of carbon
(C) maybe limited to 0.05% in order to achieve such an effect.
However, when carbon (C) is added in an excessive amount, it
is not preferable in terms of weldability, formability,
toughness, and the like of the steel. Thus, in the present
disclosure, an upper limit of the content of carbon (C) may be
limited to 0.1%. Therefore, the content of carbon (C) of the
present disclosure maybe in the range of 0.05 to 0.1%, and may
be more preferably in the range of 0.05 to 0.095%.
[0059]
[0060] Silicon (Si): 0.05 to 0.5%
[0061] Silicon (Si) is an element useful for deoxidation of
a molten steel, and is also an element that contributes to
improving strength of the steel by solid solution strengthening.
In the present disclosure, a lower limit of a content of silicon
(Si) maybe limited to 0.05% in order to achieve such an effect.
A more preferable lower limit of the content of silicon (Si)
may be 0.1%. However, since silicon (Si) is an element having
strong oxidation properties, it is preferable to limit an upper
limit of the content of silicon (Si) to a predetermined range.
That is, when silicon (Si) is added in an excessive amount, it
causes red scale formation at the time of hot rolling, which
is not preferable in terms of surface quality, and has an
Page 11
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
undesirable influence on toughness of a weld zone. Thus, in
the present disclosure, the upper limit of the content of
silicon (Si) may be limited to 0.5%. A more preferable upper
limit of the content of silicon (Si) may be 0.4%.
[0062]
[0063] Manganese (Mn): 1.4 to 2.0%
[0064] Manganese (Mn) is an element that is effective in solid
solution strengthening of the steel. In the present disclosure,
a lower limit of a content of manganese (Mn) may be limited to
1.4% in order to secure high strength properties of the steel.
However, when manganese (Mn) is added in an excessive amount,
a segregation portion may be formed over a wide range in a central
portion of a thickness at the time of casting a slab in a
steelmaking process, which is not preferable in terms of
weldability of a final product . Thus, in the present disclosure,
an upper limit of the content of manganese (Mn) may be limited
to 2.0%. A more preferable upper limit of the content of
manganese (Mn) may be 1.8%.
[0065]
[0066] Aluminum (Al): 0.01 to 0.05%
[0067] Aluminum (Al) is a representative element that is added
as a deoxidizing agent along with silicon (Si). In addition,
aluminum (Al) is an element that contributes to improving
strength of the steel by solid solution strengthening. In the
present disclosure, a lower limit of a content of aluminum (Al)
Page 12
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
may be limited to 0.01% in order to achieve such an effect. A
more preferable lower limit of the content of aluminum (Al) may
be 0.015%. However, when aluminum (Al) is excessively added,
it is not preferable in terms of impact toughness. Thus, in
the present disclosure, an upper limit of the content of
aluminum (Al) may be limited to 0.05%. A more preferable upper
limit of the content of aluminum (Al) may be 0.04%.
[0068]
[0069] Titanium (Ti) : 0.005 to 0.02%
[0070] Titanium (Ti) is an element that forms TIN precipitates
in a solidification process of the steel to suppress growth of
austenite grains in a slab heating and hot rolling process, and
thus refines a grain size of a final structure. In the present
disclosure, a lower limit of the content of titanium (Ti) may
be limited to 0.005% in order to achieve a toughness improvement
effect of the steel according to the refinement of the final
structure. A more preferable content of titanium (Ti) may be
0.008%. However, when titanium (Ti) is excessively added, TiN
is coarsely precipitated at the time of heating the slab, which
is not suitable for the refinement of the final structure. Thus,
in the present disclosure, an upper limit of the content of
titanium (Ti) may be limited to 0.02%. A more preferable upper
limit of the content of titanium (Ti) may be 0.018%.
[0071]
[0072] Nitrogen (N) : 0.002 to 0.01%
Page 13
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
[0073] Nitrogen (N) is solid-dissolved in the steel and then
precipitated to serve to increase strength of the steel, and
it is known that such a strength improvement effect is much
greater than that of carbon (C) . In addition, in the present
disclosure, TiN is formed through a reaction between titanium
(Ti) and nitrogen (N) and it is intended to suppress growth of
grains in a reheating process. Thus, a lower limit of a content
of nitrogen (N) may be limited to 0.002%. However, when
nitrogen (N) is excessively added, nitrogen (N) exists in a form
of solid solution nitrogen (N) rather than a form of TiN
precipitates, so that toughness of the steel may be
significantly reduced. Thus, in the present disclosure, an
upper limit of the content of nitrogen (N) may be limited to
0.01%. A preferable upper limit of the content of nitrogen (N)
may be 0.006 %, and a more preferable upper limit of the content
of nitrogen (N) may be 0.005%.
[0074]
[0075] Niobium (Nb) : 0.04 to 0.07%
[0076] Niobium (Nb) is an element that is very useful for
refining grains, and is an element that significantly
contributes to improving strength of the steel by promoting
formation of acicular ferrite or bainite, which is a
high-strength structure. In addition, since high-temperature
rolling is inevitable for a steel plate having a thickness less
than 20 mm, which is a target thickness in the present disclosure,
Page 14
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
niobium (Nb) that has the greatest effect on an increase in a
non-recrystallization temperature needs to be added in a
predetermined amount or more. Thus, in the present disclosure,
a lower limit of a content of niobium (Nb) may be limited to
0.04%. However, when niobium (Nb) is excessively added,
weldability of the steel may be deteriorated. Thus, in the
present disclosure, an upper limit of the content of niobium
(Nb) may be limited to 0.07%. A preferable upper limit of the
content of niobium (Nb) may be 0.06%.
[0077]
[0078] Chromium (Cr): 0.05 to 0.3%
[0079] Chromium (Cr) is an element that improves hardenability
and is an element that is effective in increasing strength of
the steel. In addition, chromium (Cr) is an element that
contributes to improving a uniform elongation ratio by
promoting formation of martensite-austenite constituent (MA)
at the time of accelerated cooling. In the present disclosure,
a lower limit of a content of chromium (Cr) may be limited to
0.05% in order to achieve such an effect. A more preferable
lower limit of the content of chromium (Cr) may be 0.08%.
However, when chromium (Cr) is excessively added, deterioration
of weldability of the steel maybe caused. Thus, in the present
disclosure, an upper limit of the content of chromium (Cr) may
be limited to 0.3%. A preferable upper limit of the content
of chromium (Cr) maybe 0.25%, and a more preferable upper limit
Page 15
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
of the content of chromium (Cr) may be 0.2%.
[0080]
[0081] Nickel (Ni): 0.05 to 0.4%
[0082] Nickel (Ni) is an element that effectively contributes
to improving toughness and strength of the steel. In the
present disclosure, a lower limit of a content of nickel (Ni)
may be limited to 0.05% in order to achieve such an effect.
However, nickel (Ni) is an expensive element, and excessive
addition of nickel (Ni) is not preferable in terms of economy
Thus, in the present disclosure, an upper limit of a content
of nickel (Ni) may be limited to 0.4%. A preferable upper limit
of the content of nickel (Ni) maybe 0.3%, and a more preferable
upper limit of the content of nickel (Ni) may be 0.25%.
[0083]
[0084] Phosphorus (P): 0.02% or less
[0085] Phosphorus (P) is a representative impurity element
that exists in the steel, and is mainly segregated in a central
portion of the steel plate to cause a decrease in toughness of
the steel, and it is thus preferable to manage phosphorus (P)
at a level as low as possible. However, in order to completely
remove phosphorus (P) in the steel, an excessive cost and time
are required in the steelmaking process, which is not preferable
in terms of economy. Thus, in the present disclosure, a content
of phosphorus (P) may be limited to 0.02% or less. A more
preferable content of phosphorus (P) may be 0.015% or less.
Page 16
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
[0086]
[0087] Sulfur (S): 0.005% or less
[0088] Sulfur (S) is also a representative impurity element
that exists in the steel, and is an element that combines with
manganese (Mn) or the like in the steel to form nonmetallic
inclusions such as MnS, and accordingly, significantly impairs
toughness and strength of the steel. Thus, it is preferable
to manage sulfur (S) at a level as low as possible. However,
in order to completely remove sulfur (S) in the steel, an
excessive cost and time are required in the steelmaking process,
which is not preferable in terms of economy. Thus, in the
present disclosure, a content of sulfur (S) may be limited to
0.005% or less. A more preferable content of sulfur (S) may
be 0.003% or less.
[0089]
[0090] Calcium (Ca): 0.0005 to 0.004%
[0091] Calcium (Ca) is an element that is effective in
suppressing crack formation around nonmetallic inclusions by
spheroidizing nonmetallic inclusions such as MnS. In the
present disclosure, a lower limit of a content of calcium (Ca)
may be limited to 0.0005% in order to achieve such an effect.
However, when calcium (Ca) is excessively added, a large amount
of CaO-based inclusions is formed to cause a decrease in impact
toughness. Thus, in the present disclosure, an upper limit of
a content of calcium (Ca) maybe limited to 0 . 004% . A preferable
Page 17
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
upper limit of the content of calcium (Ca) may be 0.002%.
[0092]
[0093] Molybdenum (Mo): 0.3% or less
[0094] Molybdenum (Mo) is an element that is effective in
securing both of high strength and high toughness by promoting
formation of bainite, which is a low-temperature transformation
structure. Therefore, in the present disclosure, molybdenum
(Mo) maybe selectively added in order to achieve such an effect.
However, molybdenum (Mo) is an expensive element and it is not
preferable in terms of economy to add molybdenum (Mo) in an
excessive amount. Thus, in the present disclosure, an upper
limit of a content of molybdenum (Mo) may be limited to 0.3%.
[0095]
[0096] In the present disclosure, in addition to the steel
compositions described above, the remainder may contain Fe and
inevitable impurities. The inevitable impurities may be
unintentionally mixed in a general steelmaking process and may
not be completely excluded, and those skilled in a general
steelmaking field may easily understand the meaning of the
inevitable impurities. In addition, the present disclosure
does not entirely exclude addition of a composition other than
the steel compositions described above.
[0097]
[0098] A microstructure according to the present disclosure
will hereinafter be described in more detail.
Page 18
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
[0099]
[00100] The steel plate according to an exemplary embodiment
in the present disclosure may contain ferrite and bainite as
a microstructure, and may further contain martensite-austenite
constituent. Fractions of the ferrite and the bainite may be
20 to 60 area% and 35 to 75 area%, respectively, and a fraction
of the martensite-austenite constituent may be 5 area% or less.
[00101]
[00102] The steel plate according to the present disclosure
contains ferrite having a fine high angle grain boundary in an
area of 20% or more, and may thus effectively secure
low-temperature drop weight tearing test (DWTT)
characteristics. In addition, the steel plate according to the
present disclosure contains the ferrite in an area of 60% or
less and contains the bainite in an area of 35% or more, and
may thus secure a yield strength of 485 MPa or more. However,
in the present disclosure, a fraction of the bainite may be
limited to 75 area% or less in order to prevent the high angle
grain boundary from becoming excessively coarse, and
accordingly, low-temperature DWTT characteristics may be
effectively secured. In addition, the martensite-austenite
constituent has an undesirable influence on the low-temperature
DWTT characteristics, and it is thus preferable to suppress a
fraction of the martensite-austenite constituent as much as
possible. Therefore, in the present disclosure, the fraction
Page 19
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
of the martensite-austenite constituent may be limited to 5
area% or less.
[00103]
[00104] In addition, the steel plate according to an
exemplary embodiment in the present disclosure may have a grain
size of 70 pm or less in upper 80% of high angle grain sizes
based on 15 in a central portion of the steel plate. That is,
in the present disclosure, effective grain sizes may be refined
by refining the high angle grain sizes, and accordingly,
low-temperature DWTT characteristics may be effectively
secured. Here, the central portion of the steel plate may be
interpreted as an area including a point of t/2, and may also
be interpreted as an area of a point of t/4 to 3*t/4 (t:
thickness (mm) of steel plate) .
[00105]
[00106] The steel plate according to an exemplary embodiment
in the present disclosure may have a thickness less than 20 mm,
and a more preferable thickness of the steel plate may be 16
mm or less. In addition, the steel plate according to an
exemplary embodiment in the present disclosure may have a yield
strength of 485 MPa or more, a total elongation ratio of 28%
or more, and a uniform elongation ratio of 9% or more with respect
to a rolling orthogonal direction, and may have a DWTT percent
ductile fracture of 35% or more at -30 C with respect to the
rolling orthogonal direction of the steel plate. Therefore,
Page 20
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
in the present disclosure, a steel plate particularly suitable
as a material for a pipeline by effectively securing strength,
low-temperature fracture toughness, and an elongation ratio in
spite of having the thickness less than 20 mm may be provided.
[00107]
[00108] A
manufacturing method according to the present
disclosure will hereinafter be described in more detail.
[00109]
[00110] The
high-strength steel plate having excellent
low-temperature fracture toughness and elongation ratio
according to an exemplary embodiment in the present disclosure
may be manufactured by reheating a slab containing, by wt%, 0.05
to 0.1% of carbon (C), 0.05 to 0.5% of silicon (Si), 1.4 to 2.0%
of manganese (Mn), 0.01 to 0.05% of aluminum (Al), 0 . 005 to 0.02%
of titanium (Ti), 0.002 to 0.01% of nitrogen (N), 0.04 to 0.07%
of niobium (Nb), 0.05 to 0.3% of chromium (Cr), 0.05 to 0.4%
of nickel (Ni), 0.02% or less of phosphorus (P), 0.005% or less
of sulfur (S), 0.0005 to 0.004% of calcium (Ca), remaining iron
(Fe), and inevitable impurities, maintaining and extracting the
slab, recrystallized-region-rolling the maintained and
extracted slab in a temperature range of Tnr or higher,
non-recrystallized-region-rolling the
recrystallized-region-rolled material at a total reduction
ratio of 30% or more, and cooling the
non-recrystallized-region-rolled steel plate to a temperature
Page 21
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
range of (Bs - 80 C) to Bs.
[00111]
[00112] Slab Reheating, Maintaining and Extracting
[00113] The slab according to the present disclosure has
the same alloy composition as the alloy composition of the steel
plate described above, and a description of the alloy
composition of the slab according to the present disclosure is
thus replaced by the description of the alloy composition of
the steel plate described above.
[00114]
[00115] Since the slab reheating is a process of heating
a steel in order to smoothly perform the subsequent rolling
processes and secure desired physical properties of the steel
plate, a heating process needs to be performed within an
appropriate temperature range according to a purpose. A lower
limit of a slab reheating temperature needs to be determined
in consideration of whether or not it is a temperature at which
precipitated elements may be sufficiently dissolved in the
steel. In particular, since the slab according to the present
disclosure essentially contains niobium (Nb) in order to secure
high strength properties, the lower limit of the slab reheating
temperature may be limited to 1140 C in consideration of a
resoluble temperature of niobium (Nb) . On the other hand, when
the slab reheating temperature is excessively high, the
austenite grains become excessively coarse, which may cause a
Page 22
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
problem that grains of a final steel plate are excessively
increased. Thus, in the present disclosure, an upper limit of
the slab reheating temperature may be limited to 1200 C.
[00116]
[00117] The reheated slab maybe subjected to a maintaining
and extracting process, if necessary, and a maintaining and
extracting temperature of the slab may be limited to a
temperature range of 1140 to 1200 C for reasons similar to those
of the slab reheating temperature.
[00118]
[00119] Recrystallized-Region-Rolling
[00120] The recrystallized-region-rolling may be performed
in a temperature range of Tnr or more . In the present disclosure,
Tnr refers to a lower limit of a temperature range at which
recrystallization of austenite occurs. That is, the
recrystallized-region-rolling may be performed in a
temperature range of an austenite recrystallized region. The
recrystallized-region-rolling may be performed in multiple
passes, and rolling may be performed at an average reduction
ratio of 10% or more per pass. The reason is that when the
average reduction ratio per pass is less than 10%, a grain size
of recrystallized austenite becomes coarse, which may cause a
decrease in toughness of the final steel plate.
[00121]
[00122] The recrystallized-region-rolled material may be
Page 23
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
cooled to a temperature range of Tnr or lower under a cooling
condition of air cooling. That is, the
recrystallized-region-rolled material is not immediately
subjected to non-recrystallized-region-rolling, and may wait
for a predetermined time to be cooled to a temperature range
of a non-recrystallized region by air cooling. The reason is
that when a rolling force is applied in the corresponding
section, partial recrystallization may occur, such that a
brittle fracture due to a coarse austenite grain size may occur.
[00123]
[00124] Non-recrystallized-Region-Rolling
[00125] The non-recrystallized-region-rolling is
performed on the recrystallized-region-rolled material. A
start temperature of the non-recrystallized-region-rolling
may be Tnr or lower, and an end temperature of the
non-recrystallized-region-rolling may be (Ar3+100 C). The
non-recrystallized-region-rolling is a process for elongating
austenite produced by recrystallized-region-rolling to be
elongate and forming a deformed structure in a grain to obtain
fine ferrite and bainite, and strength, an elongation ratio and
brittle fracture arrestability of the steel plate may be
effectively improved by the
non-recrystallized-region-rolling.
[00126]
[00127] The lower the end temperature of the
Page 24
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
non-recrystallized-zone-rolling, the higher the degree of
deformation of the austenite, which is effective in improving
low-temperature fracture toughness, but when the end
temperature of the non-recrystallized-zone-rolling is
excessively low, low-strength ferrite is produced, which is
disadvantageous in securing strength. Thus, in the present
disclosure, the end temperature of the
non-recrystallized-zone-rolling may be limited to (Ar3 + 5000)
or higher.
[00128]
[00129] In addition, a rolling reduction of the
non-recrystallized-region-rolling is an important factor in
securing low-temperature toughness of the steel. In the
present disclosure, the rolling reduction of the
non-recrystallized-region-rolling may be limited to 30% or more
in order to secure low-temperature DWTT percent ductile
fracture characteristics according to refinement of grain sizes
of a final steel. Since it is effective in improving
low-temperature toughness that the rolling reduction of the
non-recrystallized-region-rolling becomes larger, an upper
limit of the rolling reduction of the
non-recrystallized-region-rolling may not be limited.
However, when the rolling reduction of the
non-recrystallized-region-rolling exceeds a predetermined
level, an effect of the refinement of the grain size is saturated,
Page 25
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
but a rolling reduction of the recrystallized-region-rolling
is relatively decreased. Thus, in the present disclosure, the
rolling reduction in the non-recrystallized-region-rolling
may be limited to 90% or less.
[00130]
[00131] Cooling
[00132] The non-recrystallized-region-rolled steel plate
may be cooled from a cooling start temperature of (Ar3 + 30 C)
or higher to a cooling stop temperature of (Bs - 80 C) to Bs.
When the cooling start temperature is excessively low, a large
amount of ferrite having low strength is produced, and
accordingly, strength of the steel plate may be significantly
decreased. Thus, in the present disclosure, the cooling may
start in a temperature range of (Ar3 + 30 C) or higher.
[00133]
[00134] In addition, since the steel plate according to the
present disclosure has a final thickness less than 20 mm, it
is most preferable in terms of strength and an elongation ratio
to stop cooling in a temperature range of (Bs - 80 C) to Bs.
The reason is that when the cooling stop temperature is lower
than (Bs - 80 C), acicular ferrite and bainite having a high
angle grain boundary formed to be coarse and a low angle grain
boundary are formed in a large amount, such that an elongation
ratio may be decreased, and when the cooling stop temperature
exceeds Bs, an amount of bainite produced is small, such that
Page 26
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
strength of the steel plate may not be secured. The steel plate
may be quenched to the cooling stop temperature of (Bs - 80 C)
to Bs, then cooled to room temperature by air cooling or
radiation cooling.
[00135]
[00136] In addition, the cooling of the present disclosure
maybe performed at a cooling rate of 10 to 100 C/s. The reason
is that when the cooling rate is less than 10 C/s, a fraction
of equiaxed ferrite is significantly increased, such that high
strength characteristics of the steel plate may not be
effectively secured. In terms of a facility condition and
economy, an upper limit of the cooling rate may be limited to
100 C/s, and a more preferable upper limit of the cooling rate
may be 50 C/s.
[00137]
[00138] The steel plate manufactured by the manufacturing
method described above may contain ferrite and bainite as a
microstructure, and may further contain martensite-austenite
constituent. Fractions of the ferrite and the bainite may be
20 to 60 area% and 35 to 75 area%, respectively, and a fraction
of the martensite-austenite constituent may be 5 area% or less.
In addition, the steel plate manufactured by the manufacturing
method described above may have a grain size of 70 pm or less
in upper 80% of high angle grain sizes based on 15 in a central
portion of the steel plate.
Page 27
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
[00139]
[00140] Therefore, the steel plate manufactured by the
manufacturing method described above may have a thickness less
than 20 mm, and may have a yield strength of 485 MPa or more,
a total elongation ratio of 28% or more, and a uniform elongation
ratio of 9% or more with respect to a rolling orthogonal
direction, and may have a DWTT percent ductile fracture of 85%
or more at -30 C with respect to the rolling orthogonal
direction of the steel plate. Therefore, according to the
manufacturing method according to an exemplary embodiment in
the present disclosure, a steel plate particularly suitable as
a material for a pipeline by effectively securing strength,
low-temperature fracture toughness, and an elongation ratio in
spite of having the thickness less than 20 mm may be provided.
[00141]
[Mode for Invention]
[00142] Hereinafter, the present disclosure will be
described in more detail through Inventive Example. However,
it is to be noted that Inventive Example to be described later
is for illustrating and embodying the present disclosure and
is not intended to limit the scope of the present disclosure.
[00143]
[00144] (Inventive Example)
[00145] Slabs having alloy compositions of Table 1 and
having a thickness of 250 mm were manufactured, and steel plate
Page 28
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
specimens having thicknesses of 11 mm, 11.5 mm, and 22 mm,
respectively, were manufactured by applying process conditions
of Table 3. In this case, the slabs were manufactured by
applying process conditions used for manufacturing a general
slab, and recrystallized-region-rolling was performed by
applying a condition of an average rolling reduction per pass
of 10% or more in a temperature range of Tnr or higher for all
specimens. In addition, air cooling to a non-recrystallized
region temperature range after the
recrystallized-region-rolling was applied to all specimens.
An Tnr temperature, an Ar3 temperature, and a Bs temperature
were calculated on the basis of each alloy composition in Table
1 and shown in Table 2, and Equations used for calculating the
Tnr temperature, the Ar3 temperature, and the Bs temperature
of Table 2 were separately described below Table 2.
[00146]
[00147] [Table 1]
Ste Alloy Composition (wt)
Div
el c Si Mn P S Ni Cr Mo Nb Al Ca Ti
N isi
Typ
on
A 0.070 0.27 1.57 0.01 0.0020 0.10 0.10 - 0.049 0.023 0.0010 0.011
0.0032 Inv
B 0.074 0.25 1.66 0.013 0.0020 0.10 0.10 - 0.055 0.034 0.0011 0.014
0.0044 ent
C 0.050 0.25 1.58 0.012 0.0019 0.10 0.11 0.08 0.048 0.023 0.0010 0.013 0.0043
ive
D 0.060 0.24 1.55 0.011 0.0019 0.10 0.10 0.08 0.041 0.026 0.0013 0.014 0.0041
Ste
Page 29
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
E 0.050 0.25 1.52 0.012 0.0018 0.10 0.10 0.08 0.041 0.024 0.0010 0.014 0.0044
el
F 0.09 0.34 1.45 0.010 0.0011 0.17 0.1 - 0.045
0.027 0.0010 0.016 0.0041
G 0.050 0.26 1.57 0.010 0.0020 0.10 0.11 0.08 0.028 0.028 0.0010 0.012 0.0046
Corn
-
H 0.040 0.26 1.24 0.008 0.0010 0.14 0.2 0.07 0.04 0.030 0.0007 0.012
0.0046 Par
I 0.060 0.25 2.10 0.0075 0.0015 0.3 0.15 0.10 0.050 0.030 0.0016 0.015 0.0050
ati
ye
Ste
el
[00148]
[00149] [Table 2]
Steel Tnr ( C) Ar3 ( C) Ar3 + 30 ( C)
Ar3 + 100 ( C) Bs ( C) Bs - 80 ( C)
Type
A 1014 767 797 867 659 579
B 1060 760 790 860 650 570
,
C 1009 771 801 871 656 576
D 985 771 801 871 657 577
8 976 776 806 876 662 582
F 985 767 797 867 662 582
G 911 772 802 872 657 577
_ _
H 963 796 826 896 683 603
I 1028 718 748 818 595 515
[00150]
[00151] Equation 1: Tnr ( C) = 887 + 464*[C] + 6445*[Nb]
- 644* [Nb] (1/2) + 732* [V] - 230* [V] (1/2) + 890* [Ti] + 363* [Al] -
357* [Si]
Page 30
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
[00152] Equation 2: Ar3 (C) = 910 - 273*[C] - 74*[Mn] -
57*[Ni] - 16* [Cr] - 9* [Mo] - 5 [Cu]
[00153] Equation 3 : Bs (r) = 830 - 270* [C] - 90* [Mn] - 37* [Ni]
- 70* [Cr] - 83* [Mo]
[00154] (In Equations 1 to 3, [C], [Si], [Mn], [Al], [Ti] ,
[Nb] , [V] , [Cr] , [Mo], and [Cu] refer to wt% of respective alloy
compositions, and when a corresponding alloy composition is not
contained, calculation is performed by replacing a value of the
corresponding alloy composition with 0.)
[00155]
[00156] [Table 3]
Spe Steel Thickness Slab Start Cumulative End
Cooli Cooli Cooling Remar
I
cim Type (mm) of Heating Temperature Rolling
Temperature ng ng Rate k
en Steel Tempera (CC) of Reduction (%) (CC)
of Start Stop (CC/s)
Plate ture Non-recryst of Non-recryst Tempe Tempe
I
( C) allized-Reg
Non-recrysta allized-Reg ratur ratur
Ion-Rolling llized-Regio ion-Rolling e e
n-Rolling ( C) ( C)
1 A 11.5 1150 1000 42.5 915 810 590 36
Inven
2 A 11.5 1170 1000 42.5 915 820 640 36
tive
3 A 11.5 1160 1000 50 875 800 580 43
Examp
,
4 B 11.5 1160 1035 56 915 805 615 32
le
5 B 11.5 1165 1035 56 915 820 610 35
, 6 B 11.5 1150 1025 56 875 800 625 35
7 C 11 1160 990 39 925 830 580 30
Page 31
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
8 C 11 1150 995 39 920 825 635 30
9 D 11 1160 985 31 920 830 620 30
E 11 1150 975 31 920 825 590 30
. . .
11 E 11 1160 975 31 920 830 620 30
12 F 11 1150 981 39 875 805 620 35
. .
.
13 C 11 1160 1000 50 875 795 460 42 Compa
. .
14 D 11 1150 975 31 920 825 550 30 rativ
. _ . . 15 D 11 1150 970 31 920 825 510
30 e
. .
16 D 11 1160 983 50 863 771 685 30 Examp
17 E 11 1150 975 31 920 820 530 30 le
18 G 11 1170 910 12 910 830 470 30
19 G 11 1150 910 12 910 830 580 30
H 11 1159 954 0 954 819 491 47
21 I 11 1160 1020 45 920 830 570 30
. . .
22 A 22 1150 945 98 835 775 610 23
23 D 22 1144 935 78 827 767 617 21
[00157]
[00158] For each specimen of Table 3, a microstructure, a
yield strength and a tensile strength, an elongation ratio, and
a DWTT percent shear at -30 C were measured and shown in Table
5 4. The microstructure of each specimen was evaluated using an
optical microscope structure photograph and an electron
backscatter diffraction (EBSD) grain size distribution chart.
The yield strength, the tensile strength, and the elongation
ratio were evaluated by performing a room temperature tensile
Page 32
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
test on each specimen. A yield strength and a tensile strength
shown in Table 4 refer to measured values with respect to a
rolling orthogonal direction, respectively. In addition,
tensile properties and a percent ductile fracture were
evaluated by performing a DWTT test at -30 C on each specimen.
[00159]
[00160] [Table 4]
Spe Ste Ferrite Bainite M/A Grain
Yield Tensile Yield Elong Unifor DWTT Rem
I
cim el (area%) (area%) (area%) Size
Strengt Strengt Ratio ation a (%) at ark
en Typ (pm) of h (MPa) h (MI's) (%) Ratio Elonga -30 C
e Upper (%) tion
80% of Ratio
High (%)
Angle
Grains
_
1 A 25 71.0 4.0 52.0 524 667 79 33 9.0
100 Inv
2 A 42 54.5 3.5 40.5 508 635 80 36 10.5 100 ent
. _
3 A 28 67.5 4.5 46.5 541 671 81 35 9.6
100 ive
4 B 32 63.6 4.4 62.1 525 671 78 33 9.0 100 Exa
. ,
5 B 43 52.8 4.2 57.9 529 667 79 34 9.5 100 mPl
6 B 52 43.5 4.5 56.6 528 653 81 36 9
e.9 99
_ .
7 C 23 72.8 4.2 67.1 487 643 76 33 9.2
98
8 C 30 65.7 4.3 48.5 489 620 79 34 9.7
99
. .
9 D 23 72.7 4.3 60.1 505 638 79 33 9.1
100
Page 33
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
E 24 71.5 4.5 48.5 485 614 79 31
9.7 100
11 E 28 67.7 4.3 42.0 495 613 81 36
10.9 100
12 F 31 64.8 4.2 45.1 536 625
86 36 10.5 95
13 C 17 78.3 4.7 86.1 620 730 85 28
7.7 100 Corn
14 D 15 80.5 4.5 89.5 501 651 77 28
7.6 100 par
D 18 77.7 4.3 75.2 500 628 80 32 8.3
100 ati
16 D 75 22.9 2.1 90.5 455 570 80 42
13.0 92 Ye
17 E 22 74.0 4.0 76.5 492 632 78 28
8.2 100 Exa
_
18 G 24 72 4.0 93 524 656 80 28
6.3 75 mpl
e
19 G 26 69.8 4.2 82.5 502 620 81 32
7.9 80
. _
H 28 67.9 4.1 78.0 528 606 87 30
8.0 50
21 I 12 81.8 6.2 89.1 624 796
78 28 6.8 91
22 A 65 31.2 3.8 58.6 475 568 .. 84 47
13.0 100
23 D 66 30.5 3.5 65.1 470 565 83 48
13.5 100
[00161]
[00162] It may
be confirmed that Specimens 1 to 12 satisfying
the alloy composition and process conditions of the present
disclosure contain 20 to 60 area% of ferrite and 35 to 75 area%
5 of bainite as a microstructure, contain 5 area% or less of island
martensite, and have a grain size of 70 pm or less in upper 80%
of high angle grain sizes based on 15 in a central portion of
a steel plate, a yield strength of 4851'4Pa or more, a total
elongation ratio of 28% or more, a uniform elongation ratio of
10 9% or more with respect to a rolling orthogonal direction, and
a DWTT percent shear of 85% or more at -30 C with respect to
Page 34
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
the rolling orthogonal direction, and thus, has physical
properties particularly suitable as a material for a pipeline
provided in a cryogenic environment.
[00163]
[00164] Specimens 13 to 15 and 17 are specimens in a case
where the alloy composition of the present disclosure is
satisfied, but cooling is performed in a temperature range lower
than the cooling start temperature or the cooling end
temperature of the present disclosure. It may be confirmed that
in a case of Specimens 13 to 15 and 17, ferrite less than 20
area% and bainite more than 75 area% were formed, a grain size
of upper 80% of high angle grain sizes based on 15 in a central
portion of a steel plate exceeded 70 pm, and a uniform elongation
ratio was thus less than 9%.
[00165]
[00166] Specimen 16 is a specimen in a case where the alloy
composition of the present disclosure is satisfied, but
non-recrystallized-region-rolling was performed in a
temperature range lower than the end temperature of the
non-recrystallized-region-rolling of the present disclosure,
and cooling started in a temperature range lower than the
cooling start temperature of the present disclosure, such that
cooling ended in a temperature range higher than the cooling
stop temperature of the present disclosure. It may be confirmed
that in a case of Specimen 16, ferrite more than 60 area% was
Page 35
Date Recue/Date Received 2021-05-17
CA 03120271 2021-05-17
formed, such that a yield strength was less than 485 MPa.
[00167]
[00168] It may be confirmed that Specimens 18 to 21, which
are specimens that do not satisfy the alloy composition and the
process condition of the present disclosure, do not secure a
microstructure and physical properties desired by the present
disclosure.
[00169]
[00170] It may be confirmed that Specimens 22 and 23 satisfy
the alloy composition of the present disclosure, but have a
thickness of a steel plate exceeding 20 mm, such that ferrite
is excessively formed.
[00171]
[00172] FIG. 1 is a photograph of Specimen 2 observed with
an optical microscope, while FIG. 2 is graphs illustrating
results obtained by measuring high angle grain boundary grain
sizes based on 15 of Specimen 2 using an EBSD. As illustrated
in the graphs of FIG. 2, it may be confirmed that an average
grain size of high angle grain boundaries of Specimen 2 is 22.3
pm and a grain size of upper 80% of the high angle grain
boundaries is 40.5 pm.
[00173]
[00174] FIG. 3 is a photograph of Specimen 18 observed with
an optical microscope, and FIG. 4 is graphs illustrating results
obtained by measuring high angle grain boundary grain sizes
Page 36
Date Recue/Date Received 2021-05-17
based on 15 of Specimen 18 using an EBSD. As illustrated in
the graphs of FIG. 4, it may be confirmed that an average grain
size of high angle grain boundaries of Specimen 18 is 38 pm and
a grain size of upper 80% of the high angle grain boundaries
is 93 pm.
[00175]
[00176] Therefore, according to an exemplary embodiment in
the present disclosure, the steel plate particularly suitable
as the material for a pipeline by having the yield strength of
485MPa or more, the total elongation ratio of 28% or more, the
uniform elongation ratio of 9% or more with respect to the
rolling orthogonal direction, and the DWTT percent ductile
fracture of 85% or more at -30 C with respect to the rolling
orthogonal direction of the steel plate in spite of having a
thickness less than 20 mm, and the manufacturing method therefor
may be provided.
[00177]
[00178] While the present disclosure has been described in
detail through exemplary embodiment, other types of exemplary
embodiments are also possible. Therefore, the technical
spirit and scope of the claims set forth below are not limited
to exemplary embodiments.
****
In some aspects, embodiments of the present invention as
described herein include the following items:
Page 37
Date Regue/Date Received 2022-07-15
Item 1. A steel plate having low-temperature fracture
toughness and elongation ratio, comprising:
by wt%, 0.05 to 0.1% of carbon (C), 0.05 to 0.5% of silicon
(Si) , 1.4 to 2.0% of manganese (Mn) , 0.01 to 0.05% of aluminum
(Al) , 0.005 to 0.02% of titanium (Ti) , 0.002 to 0.01% of nitrogen
(N) , 0.04 to 0.07% of niobium (Nb) , 0.05 to 0.3% of chromium
(Cr) , 0.05 to 0.4% of nickel (Ni) , 0.02% or less of phosphorus
(P), 0.005% or less of sulfur (S), 0.0005 to 0.004% of calcium
(Ca) , remaining iron (Fe) , and inevitable impurities; and
20 to 43 area% of ferrite, 35 to 75 area% of bainite and
5 area% or less of martensite-austenite constituent as a
microstructure,
wherein a grain size of upper 80% of high angle grain sizes
based on 15 in a central portion of the steel plate is 70 pm
or less.
Item 2. The steel plate of item 1, further comprising 0.3
wt% or less of molybdenum (Mo) .
Item 3. The steel plate of item 1, wherein a yield strength
of the steel plate is 485 MPa or more.
Item 4. The steel plate of item 1, wherein a total
elongation ratio of the steel plate is 28% or more, and
a uniform elongation ratio of the steel plate with respect
to a rolling orthogonal direction is 9% or more.
Item 5. The steel plate of item 1, wherein a drop weight
Page 38
Date Regue/Date Received 2022-07-15
tearing test (DWTT) percent ductile shear of the steel plate
at -30 C with respect to a rolling orthogonal direction of the
steel plate is 85% or more.
Item 6. The steel plate of item 1, wherein a thickness
of the steel plate is less than 20 mm.
Item 7. A manufacturing method for a steel plate having
low-temperature fracture toughness and elongation ratio,
comprising:
reheating a slab comprising, by wt%, 0.05 to 0.1% of carbon
(C), 0.05 to 0.5% of silicon (Si), 1.4 to 2.0% of manganese (Mn),
0.01 to 0.05% of aluminum (Al) , 0.005 to 0.02% of titanium (Ti) ,
0.002 to 0.01% of nitrogen (N) , 0.04 to 0.07% of niobium (Nb) ,
0.05 to 0.3% of chromium (Cr), 0.05 to 0.4% of nickel (Ni), 0.02%
or less of phosphorus (P) , 0.005% or less of sulfur (S) , 0.0005
to 0.004% of calcium (Ca) , remaining iron (Fe) , and inevitable
impurities, at a temperature range of 1140 to 1200 C;
maintaining and extracting the slab at a temperature range
of 1140 to 1200 C to obtain a maintained and extracted slab;
recrystallized-region-rolling the maintained and
extracted slab in a temperature range of Tnr or higher to obtain
a recrystallized-region-rolled material;
non-recrystallized-region-rolling the recrystallized-
region-rolled material at a total reduction ratio of 30% or more
to obtain a non-recrystallized-region-rolled steel plate; and
cooling the non-recrystallized-region-rolled steel
Page 39
Date Regue/Date Received 2022-07-15
plate to a temperature range of (Bs - 80 C) to Bs at a cooling
rate of 10 to 50 C/s,
wherein the non-recrystallized-region-roll starts in a
temperature range of Tnr or lower and ends in a temperature range
of (Ar3 + 100 C) or higher,
wherein the recrystallized-region-rolling is performed
in an accumulation of passes, and an average reduction ratio
of each of the passes is 10% or more,
wherein the cooling of the non-recrystallized-
region-rolled steel plate starts in the temperature range of
(Ar3 + 30 C) or higher, and
wherein the steel plate comprises 20 to 43 area% of ferrite,
35 to 75 area% of bainite and 5 area% or less of
martensite-austenite constituent as a microstructure.
Item 8. The manufacturing method of item 7, wherein the
slab further comprises, 0.3 wt% or less of molybdenum (Mo).
Item 9. The manufacturing method of item 7, wherein the
recrystallized-region-rolled material is cooled to a
temperature range of Tnr or lower by air cooling.
Item 10. The manufacturing method of item 7, wherein a
thickness of the steel plate is less than 20 mm.
Page 40
Date Regue/Date Received 2022-07-15
