Technical Field
[0001] The present disclosure relates to a high-carbon steel
sheet having good surface quality and a manufacturing method
5 therefor, and more particularly, to a high-carbon pickled
steel sheet and a high-carbon cold-rolled steel sheet, having
good surface quality, and a manufacturing method thereof.
Background Art
10 [0002] In the case of high carbon steel, the following Patent
Documents, such as suppressing formation of an oxide or a
decarburized layer on a surface layer in a manufacturing
step to improve surface quality, or using a heat treatment
or a special device to remove the generated oxide or
15 decarburized layer on the surface layer, are known.
[0003]
[0004] Patent Document 1 discloses a technique for applying
a decarburization inhibitor containing carbon to prevent
decarburization occurring during hot working of high-carbon
20 steel, and while this can prevent decarburization in a
heating step, it is not preferable to solve a problem of
decarburization having occurred during coiling after hot
rolling.
[0005]
25 [0006] Patent Documents 2 and 3 disclose a technique for
3
improving pickling treatment capability by adding an
additive containing sulfuric acid as a main component to
remove scale generated on a surface of a steel material, but
it is different from a technique for uniformly controlling
5 an internal oxide layer, and the like, in a length direction
of a coil.
[0007]
[0008] Patent Documents 4 and 5 disclose a technique for
removing scale using a heat treatment or induction heating
10 in a decarboxylation reducing atmosphere to effectively
remove scale generated on a surface of a steel material, but
a cost for manufacturing and using an additional device is
high, but it is different from a technique for uniformly
controlling an internal oxide layer, and the like, in a
15 length direction of a coil, because there may be costs for
manufacturing and using an additional device.
[0009] [Prior art Document]
[0010] (Patent Document 1) Japanese Patent Publication No.
1993-123739
20 [0011] (Patent Document 2) Japanese Patent Publication No.
1998-072686
[0012] (Patent Document 3) Japanese Patent Publication No.
2004-331994
[0013] (Patent Document 4) Japanese Patent Publication No.
25 1995-070635
4
[0014] (Patent Document 5) Korean Patent Registration No.
10-1428311
Summary of Invention
5 Technical Problem
[0015] An aspect of the present disclosure is to provide a
high-carbon steel sheet having good surface quality and a
manufacturing method therefor.
[0016] The subject of the present invention is not limited
10 to the above. The subject of the present invention will be
understood from the overall content of the present
specification, and those of ordinary skill in the art to
which the present invention pertains will have no difficulty
in understanding the additional subject of the present
15 invention.
Solution to Problem
[0017] According to an aspect of the present disclosure,
[0018] a high-carbon pickled steel sheet having good surface
20 quality is provided, the high-carbon pickled steel sheet
including, in weight%,0.4% or more and less than 1.2% of
carbon (C), 0.5% or less (excluding 0%) of silicon (Si),
0.05% or less of phosphorus (P), 0.03% or less of sulfur (S),
0.1 to 2.5% of at least one of manganese (Mn) and chromium
25 (Cr), and a balance of iron (Fe) and inevitable impurities,
5
[0019] wherein an average thickness of an internal oxide
layer and/or a decarburized layer formed in a surface layer
portion of the steel sheet is 1 to 10 μm, and
[0020] a standard deviation of the thickness of the internal
5 oxide layer and/or the decarburized layer in a length
direction of the steel sheet is 2 μm or less.
[0021]
[0022] According to another aspect of the present disclosure,
[0023] a high-carbon cold-rolled steel sheet having good
10 surface quality is provided, the high-carbon cold-rolled
steel sheet including, in weight%,0.4% or more and less than
1.2% of carbon (C), 0.05% or less of phosphorus (P), 0.03%
or less of sulfur (S), 0.1 to 2.5% of at least one of
manganese (Mn), silicon (Si), and chromium (Cr), and a
15 balance of iron (Fe) and inevitable impurities,
[0024] wherein an average thickness of an internal oxide
layer and/or a decarburized layer formed in a surface layer
portion of the steel sheet is 1×[1-cold reduction (%)]μm to
10×[1-cold reduction (%)], and
20 [0025] a standard deviation of the thickness of the internal
oxide layer and/or the decarburized layer in a length
direction of the steel sheet is 2 μm or less.
[0026]
[0027] According to another aspect of the present disclosure,
25 [0028] a manufacturing method for a high-carbon pickled
6
steel sheet having good surface quality is provided, the
manufacturing method, including operations of: preparing a
hot-rolled coil; and removing an internal oxide layer and/or
a decarburized layer in a surface layer portion by immersing
5 the hot-rolled coil in a pickling tank and passing the same
therethrough,
[0029] wherein, when the hot-rolled coil is divided into a
first region, a second region, a third region, a fourth
region, and a fifth region, in a length direction, a pickling
10 tank passing speed of a hot-rolled coil corresponding to the
second region, the third region, and the fourth region is
controlled to be slower than a pickling tank passing speed
of a hot-rolled coil corresponding to the first region and
the fifth region.
15 [0030]
[0031] According to another aspect of the present disclosure,
[0032] a manufacturing method for a high-carbon cold-rolled
steel sheet having good surface quality is provided, the
manufacturing method including operations of: preparing a
20 hot-rolled coil; removing an internal oxide layer and/or a
decarburized layer in a surface layer portion by immersing
the hot-rolled coil in a pickling tank and passing the same
therethrough; and cold rolling a hot-rolled steel sheet from
which the internal oxide layer and/or the decarburized layer
25 has been removed,
7
[0033] wherein, when the hot-rolled coil is divided into a
first region, a second region, a third region, a fourth
region, and a fifth region, in a length direction, a pickling
tank passing speed of a hot-rolled coil corresponding to the
5 second region, the third region, and the fourth region is
controlled to be slower than a pickling tank passing speed
of a hot-rolled coil corresponding to the first region and
the fifth region.
10 Advantageous Effects of Invention
[0034] In the present disclosure having the configuration as
described above, it is possible to provide a high-carbon
steel sheet having good surface quality in which internal
oxide layers are uniformly formed in a length direction of
15 the steel sheet, and a manufacturing method therefor. In
particular, the present disclosure does not incur additional
costs through additional processes, equipment, or the like,
but rather improves productivity of pickling compared to the
existing methods, thereby reducing manufacturing costs.
20
Best Mode for Invention
[0035] Hereinafter, the present disclosure will be described.
[0036] In general, as is well known, in a surface layer
portion of a hot-rolled coil manufactured through
25 conventional reheating, finishing rolling, cooling, and
8
coiling, there is an internal defect layer such as an
internal oxide layer and/or a decarburized layer. The
internal oxide layer may occur in a process in which
oxidation of components such as chromium (Cr), manganese
5 (Mn), silicon (Si), zinc (Zn), magnesium (Mg), and aluminum
(Al), which have higher oxygen affinity, than iron (Fe),
occurs in a base material. The decarburized layer may occur
in a process of being discharged to an atmosphere in a form
of a gas after carbon in steel is combined with oxygen in
10 scale along with the atmosphere, and a thickness of the
internal defect layer may vary depending on a composition of
a hot-rolled steel sheet, a temperature when a hot-rolled
steel sheet is coiled with a hot-rolled coil (HC), a cooling
time after coiling, a width, a thickness, a length, and the
15 like, of the hot-rolled steel sheet, and may be within 50
μm.
[0037] Meanwhile, the internal defect layer also affects a
subsequent pickling process and a cold-rolling process,
thereby ultimately becoming a factor to deteriorate surface
20 properties of the finally manufactured steel sheet. In
particular, in the case of high carbon steel containing 0.4%
C or more, a time required to complete microstructure
transformation due to cooling in a ROT after finishing
rolling becomes longer, and accordingly, a temperature of
25 the hot-rolled coil wound increases by transformation
9
heating, so that there may be a significant deviation in the
thickness of the internal defect layer such as the internal
oxide layer and/or the decarburized layer between front and
rear end portions and a middle end portion of the hot-rolled
5 coil. Accordingly, in the present disclosure, by providing
optimum pickling conditions, using a hot-rolled coil
exhibiting thickness deviations such as of the internal oxide
layer, or the like, a high-carbon pickled steel sheet and a
cold-rolled steel sheet having good surface quality may be
10 provided.
[0038]
[0039] Hereinafter, a pickled steel sheet and a cold-rolled
steel sheet of the present disclosure will be described.
[0040] First, the pickled steel sheet and the cold-rolled
15 steel sheet of the present disclosure are not limited to a
specific steel composition component, and carbon steel
having various composition components may be used.
Preferably, high carbon steel having 0.4% or more of C is
used.
20 [0041] More preferably, a steel sheet is used, the steel
sheet including, in weight %, 0.4% or more and less than 1.2%
of carbon (C), 0.5% or less (excluding 0%) of silicon (Si),
0.05% or less of phosphorus (P), 0.03% or less of sulfur (S),
0.1 to 2.5% of at least one of manganese (Mn) and chromium
25 (Cr), and a balance of iron (Fe) and inevitable impurities.
10
Hereinafter, the steel composition component of the present
disclosure and the reason for limiting a content thereof
will be described. Meanwhile, "%" as used herein means "%"
by weight, unless otherwise specified.
5 [0042]
[0043] Carbon (C): 0.4% or more and less than 1.2%
[0044] Carbon (C) is an element that effectively contributes
to improving strength of steel, so that, in the present
disclosure, a certain level or more of carbon (C) may be
10 included in order to secure strength of a high carbon steel
sheet. In addition, when a content of C is lower than a
certain level, desired strength, hardness, and durability of
a final part cannot be ensured and a function of the high
carbon steel sheet cannot be obtained, so in the present
15 disclosure, a lower limit of the content of carbon (C) may
be limited to 0.4 %. On the other hand, when carbon (C) is
excessively added, the strength is improved, but cracks may
occur during a manufacturing process thereof or cracks also
occur on a surface thereof due to formation of excessive
20 proeutectoid cementite, which may cause a problem of
deterioration of surface quality. Therefore, in the present
disclosure, the content of carbon (C) may be limited to less
than 1.2%. Accordingly, in the present disclosure, the
content of carbon (C) may be in a range of 0.4% or more and
25 less than 1.2%.
11
[0045]
[0046] Silicon (Si): 0.5% or less (excluding 0%)
[0047] Silicon (Si) is an element having a strong affinity
with oxygen, so when a large amount of Si is added, it is
5 not preferable because it may cause surface defects observed
with the naked eye such as surface scale, including red scale.
Accordingly, in the present disclosure, an upper limit of a
content of silicon (Si) may be limited to 0.5%. However,
since silicon (Si) is an element not only acting as a
10 deoxidizer but also contributing to improving strength of
steel, in the present disclosure, 0% may be excluded from a
lower limit of the content of silicon (Si).
[0048]
[0049] Phosphorus (P): 0.05% or less
15 [0050] Phosphorus (P) is a major element segregating at grain
boundaries and may cause a deterioration in toughness of
steel. Therefore, it is preferable to control a content of
phosphorus (P) as low as possible. Therefore, it is
theoretically most advantageous to limit the content of
20 phosphorus (P) to 0%. However, since phosphorus (P) is an
impurity that is unavoidably introduced into steel during a
steelmaking process, and an excessive process load may be
caused to control the content of phosphorus (P) to 0%.
Accordingly, in the present disclosure, in consideration of
25 this point, an upper limit of the content of phosphorus (P)
12
may be limited to 0.05%.
[0051]
[0052] Sulfur (S): 0.03% or less
[0053] Sulfur (S) is a major element forming Mns, increasing
5 an amount of precipitates, and embrittling steel. Therefore,
it is preferable to control a content of sulfur (S) as low
as possible. Therefore, it is theoretically most
advantageous to limit the content of sulfur (S) to 0%.
However, sulfur (S) is also an impurity that is unavoidably
10 introduced into steel during a steelmaking process, and an
excessive process load may be caused to control the content
of sulfur (S) to 0%. Accordingly, in the present disclosure,
in consideration of this point, an upper limit of the content
of sulfur (S) may be limited to 0.03%.
15 [0054]
[0055] At least one of manganese (Mn) and chromium (Cr): 0.1%
or more and less than 2.5%
[0056] Manganese (Mn) and chromium (Cr) are elements
contributing to forming hardenability of steel, so in the
20 present disclosure, manganese (Mn) and chromium (Cr) may be
included to achieve this effect. However, excessive addition
of manganese (Mn) and chromium (Cr), which are relatively
expensive elements, is not preferable from an economic point
of view, and if excessive amounts of manganese (Mn) and
25 chromium (Cr) are added, weldability may be deteriorated.
13
Therefore, in the present disclosure, a content of at least
one of manganese (Mn) and chromium (Cr) may be in a range of
0.1% or more and less than 2.5%.
[0057]
5 [0058] In the present disclosure, in addition to the steel
composition described above, a remainder may include Fe and
inevitable impurities. Inevitable impurities may be
inevitably added in a typical steel manufacturing process,
and it cannot be completely excluded, and those skilled in
10 the ordinary steel manufacturing field can easily understand
the meaning. In addition, in the present disclosure, the
addition of a composition, other than the steel composition
described above, should not be not entirely excluded.
[0059]
15 [0060] In the pickled steel sheet of the present disclosure,
an average thickness of an internal oxide layer and/or a
decarburized layer formed in a surface layer portion of a
steel sheet is required to be in a range of 1 to 10 μm. If
the thickness is less than 1 μm, the internal oxide layer
20 and/or the decarburized layer are removed in large amounts
or the internal oxide layer and/or the decarburized layer
are entirely removed so that an uncontrollable level thereof
is present. In this case, there is a problem in that pickling
productivity is deteriorated as well as consumption of the
25 steel sheet removed due to pickling increases. Meanwhile, if
14
the thickness thereof exceeds 10 μm, the internal oxide layer
and/or the decarburized layer remaining on the surface
thereof are left thick, so that there may be a problem of
deteriorating surface quality such as durability, or the
5 like.
[0061] Meanwhile, in the present disclosure, the thickness
of the internal oxide layer and/or the decarburized layer is
obtained by measuring a cross-section of the steel sheet
with an optical microscope or an scanning electron microscope
10 (SEM), and the average thickness is obtained by measuring at
least five locations in the length direction of the steel
sheet, to obtain an average value thereof. That is, in the
present disclosure, the thickness of the internal oxide layer
and/or the decarburized layer is obtained by measuring a
15 cross-section of the steel sheet with an optical microscope
or scanning electron microscope (SEM), and the decarburized
layer is divided into a base material layer and a
decarburized layer by measuring a cross-section corroded
using a corrosion solution such as nital, or the like, and
20 the internal oxide layer is divided into a base material
layer and an internal oxide layer by being directly observed
from the cross-section thereof without corrosion. In this
case, the average thickness of the internal oxide layer
and/or the decarburized layer is obtained by measuring at
25 least five locations in the length direction of the steel
15
sheet, to obtain an average value thereof. A measurement
position in the length direction of the steel sheet is
measured by taking one or more samples from each region,
when a coil is equally divided into 5 equal regions in the
5 length direction. In addition, the standard deviation is
obtained by calculating a standard deviation value for data
in at least five locations in the length direction of the
steel sheet measured thereabove.
[0062]
10 [0063] Meanwhile, in the cold-rolled steel sheet of the
present disclosure, the average thickness of the internal
oxide layer and/or the decarburized layer formed in the
surface layer portion of the steel sheet satisfies a range
of 1×[1-cold reduction(%)]μm to 10×[1- cold reduction (%)]μm.
15 That is, the thickness of the internal oxide layer and/or
the decarburized layer formed in the surface layer portion
of the steel sheet is also reduced according to a reduction
during cold rolling. Preferably, the average thickness of
the internal oxide layer and/or the decarburized layer formed
20 in the surface layer portion of the cold-rolled steel sheet
is managed in a range of 0.2 to 8 μm.
[0064]
[0065] In addition, in the pickled steel sheet and coldrolled steel sheet of the present disclosure, the standard
25 deviation of the thickness of the internal oxide layer and/or
16
the decarburized layer in the length direction of the steel
sheet satisfies 2 μm or less. If the standard deviation of
the thickness thereof exceeds 2 μm, a deviation in surface
quality occurs for each location, and a deviation in an
5 amount removed through pickling occurs, so that there may be
a problem in that an amount of consumption of the steel sheet
removed through pickling is increased or is not sufficiently
removed, resulting in lowering surface quality. More
preferably, the standard deviation of the thickness thereof
10 is limited to 1.6 μm or less.
[0066]
[0067] Next, a manufacturing method for a pickled steel sheet
and a cold-rolled steel sheet having good surface quality
according to the present disclosure will be described.
15 [0068] First, in the present disclosure, a hot-rolled coil
is prepared.
[0069] First, as described above, the present disclosure is
not limited to the steel composition component of the hotrolled coil. Preferably, it is high-carbon steel having 0.4%
20 or more of C, and more preferably, a steel sheet including,
in wt%, 0.4% or more and less than 1.2% of carbon (C), 0.5%
or less (excluding 0%) of silicon (Si), 0.05% or less of
phosphorus (P), 0.03% or less of sulfur (S), 0.1 to 2.5% of
at least one of manganese (Mn) and chromium (Cr), and a
25 balance of iron (Fe) and inevitable impurities, is used.
17
[0070] In addition, the present disclosure is not limited to
a specific manufacturing process for manufacturing the hotrolled coil, and a general manufacturing process may be used.
Specifically, the general manufacturing process of the hot5 rolled coil includes operations of: reheating a steel slab
provided with the above-described steel composition;
providing a hot-rolled steel sheet by hot rolling the
reheated slab; cooling the hot-rolled hot-rolled steel sheet;
coiling the cooled hot-rolled steel sheet; and cooling the
10 coiled coil.
[0071] As an example, a hot-rolled coil may be manufactured
using the following manufacturing processes.
[0072] Reheating and hot rolling a slab
[0073] A slab manufactured by the conventional slab
15 manufacturing process may be reheated in a certain
temperature range. For a sufficient homogenization treatment,
a lower limit of a reheating temperature may be limited to
1050°C, and an upper limit of the reheating temperature may
be limited to 1350°C in consideration of economic feasibility
20 and surface quality.
[0074] Then, the reheated slab may be rough-rolled by a
conventional method, and the rough-rolled steel slab may be
hot rolled to a thickness of 1.5 mm to 10 mm by finishing
hot-rolling. In the present disclosure, hot rolling may be
25 performed under conventional conditions, but a finishing
18
rolling temperature for controlling a rolling load and
reducing a surface scale may be in a range of 800 to 950°C.
[0075]
[0076] Cooling and coiling
5 [0077] Control cooling may be performed on a hot-rolled steel
sheet immediately after hot rolling.
[0078] In the present disclosure, since surface quality of
the hot-rolled steel sheet is strictly controlled, it is
preferable that cooling in the present disclosure is started
10 within 5 seconds. When a time from hot rolling to a start of
cooling exceeds 5 seconds, an internal oxide layer and/or a
decarburized layer, not intended by the present disclosure,
may be formed in a surface layer portion of the steel sheet,
by air cooling in an atmosphere. A more preferable time from
15 hot rolling to the start of cooling may be within 3 seconds.
[0079] In addition, the hot-rolled steel sheet immediately
after hot rolling may be cooled to a coiling temperature of
500°C or more and 750°C or less at a cooling rate of 10 to
1000°C/s. When the cooling rate is less than 10°C/s, an
20 internal oxide layer and/or a decarburized layer may be
formed in a surface layer portion of the steel sheet during
cooling, so there may be a problem in that surface quality
desired by the present disclosure cannot be secured. Although,
in the present disclosure, an upper limit of the cooling
25 rate is not specifically limited to secure the desired
19
surface quality, the upper limit of the cooling rate may be
limited to 1000°C/s in consideration of facility limitations
and economic feasibility. In addition, when the coiling
temperature is less than 500°C, a low-temperature
5 transformation structure such as bainite or martensite may
be formed to cause cracks in the steel sheet. When the
coiling temperature exceeds 750°C, an excessively large
amount of the internal oxide layer and/or the decarburized
layer may be formed in the surface layer portion of the steel
10 sheet, so that there may be a problem in that the surface
quality desired by the present disclosure cannot be secured.
[0080]
[0081] Cooling the coiled coil
[0082] The coiled coil is air cooled. In this case, in a
15 high-carbon hot-rolled steel sheet, an oxide and/or a
decarburized layer may be additionally formed directly below
a surface thereof as well as a scale layer formed on a
surface layer. The oxide and/or the decarburized layer formed
directly below the surface layer are formed to have different
20 depths in front and rear end portions and in a central
portion in a length direction of the hot-rolled steel sheet.
This is because temperatures in the front and rear end
portions and in the central portion may be different, when
the hot-rolled coil is cooled in a coiled state. The oxide
25 and decarburized layers, directly below the surface in the
20
front and rear end portions and in the central portion may
have a depth of 0 to 5 μm and 3 to 20 μm, respectively.
[0083] In the hot-rolled steel sheet prepared by the above
manufacturing method, the internal oxide layer and/or the
5 decarburized layer formed in the surface layer portion may
be formed to have an average thickness of 2 to 20 μm.
[0084]
[0085] In the present disclosure, the internal oxide layer
and/or the decarburized layer of the surface layer are
10 removed by immersing the hot-rolled coil in a pickling
solution of the pickling tank and passing the same
therethrough.
[0086] In this case, in the present disclosure, when the
hot-rolled coil is divided into a first region, a second
15 region, a third region, a fourth region, and a fifth region,
in a length direction, a pickling tank passing speed of a
hot-rolled coil corresponding to the second region, the third
region, and the fourth region is controlled to be slower
than a pickling tank passing speed of a hot-rolled coil
20 corresponding to the first region and the fifth region. In
addition, it is preferable to control the pickling tank
passing speed of the hot-rolled coil corresponding to the
third region to be slower than the pickling tank passing
speed of the hot-rolled coil corresponding to the second and
25 fourth regions. Thereby, it is possible to obtain a pickled
21
steel sheet having a reduced thickness deviation in the
length direction through pickling treatment despite the
thickness deviation by length of the internal oxide layer
and/or the decarburized layer formed on the hot-rolled coil.
5 In the present disclosure, the thickness of the internal
oxide layer and/or the decarburized layer in the third region
is the thickest, and the division may be equal division.
[0087]
[0088] More preferably, the pickling tank passing speed of
10 the hot-rolled coil in the third region is 5 mpm to 50 mpm,
an average pickling tank passing speed thereof in the first
region and the fifth region is controlled to be 5×[pickling
tank passing speed of the hot-rolled coil in the third
region]×1/2 to 5×[pickling tank passing speed of the hot15 rolled coil in the third region]×2, and a pickling tank
passing speed of the hot-rolled coil in the second region
and the fourth region is controlled to be 5×[pickling tank
passing speed of the hot-rolled coil in the third region
/2]×1/2 to 5×[pickling tank passing speed of the hot-rolled
20 coil in the third region /2]×2.
[0089] The pickling tank passing speed of the hot-rolled
coil in the third region needs to be maintained at 50 mpm or
less in order to effectively remove the oxide and the
decarburized layer directly below the surface. Meanwhile, if
25 the passing speed thereof is too low, an amount of steel
22
sheet removed through pickling increases due to overpickling, and a pickling rate is slow and productivity is
deteriorated, so that it is preferable that the speed is
controlled to be 5 mpm or more.
5 [0090] The pickling tank passing speed of the hot-rolled
coil in the first region and fifth region may be controlled
to be faster than that in the third region, and the speed
thereof should be controlled to be 5 ×[pickling tank passing
speed of the hot-rolled coil in the third region]×1/2 to
10 5×[pickling tank passing speed of the hot-rolled coil in the
third region]×2, based on the pickling tank passing speed of
the hot-rolled coil in the third region. It is preferable to
be controlled to a range in which the oxide and the
decarburized layer directly below the surface is effectively
15 removed and productivity is not reduced.