Technical Field
[0001] The present disclosure relates to a steel sheet with
excellent surface quality, and a manufacturing method
5 therefor, and more particularly to a pickled steel sheet and
a cold-rolled steel sheet, with excellent surface quality,
the pickled steel sheet and the cold-rolled steel sheet
having a small deviation of a thickness of an internal oxide
layer and a decarburized layer in a length direction of the
10 steel sheet, and a manufacturing method therefor.
Background Art
[0002] In the case of carbon steel, the following Patent
Documents are known, such as suppressing formation of an
15 oxide or a decarburized layer, formed 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 the decarburized layer, formed on the surface layer,
are known.
20 [0003]
[0004] Patent Document 1 discloses a technique for applying
a decarburization inhibitor containing carbon to prevent
decarburization occurring during hot working of carbon steel,
and while this can prevent decarburization in a heating step,
25 it is not preferable to solve a problem of decarburization
3
having occurred during coiling after hot rolling.
[0005]
[0006] Patent Documents 2 and 3 disclose a technique for
improving pickling treatment capability by adding an
5 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
an internal oxide layer, and the like, in a length direction
of a coil.
10 [0007]
[0008] Patent Documents 4 and 5 disclose a technique for
removing scale using a heat treatment or induction heating
in a decarboxylation reducing atmosphere to effectively
remove scale generated on a surface of a steel material, but
15 it is also different from a technique for uniformly
controlling the internal oxide layer, and the like, in the
length direction of the coil, because there may be costs for
manufacturing and using an additional device.
[0009] [Prior art Document]
20 [0010] (Patent Document 1) Japanese Patent Publication No.
1993-123739
[0011] (Patent Document 2) Japanese Patent Publication No.
1998-072686
[0012] (Patent Document 3) Japanese Patent Publication No.
25 2004-331994
4
[0013] (Patent Document 4) Japanese Patent Publication No.
1995-070635
[0014] (Patent Document 5) Korean Patent Registration No.
10-1428311
5 [0015]
Summary of Invention
Technical Problem
[0016] An aspect of the present disclosure is to provide a
10 steel sheet with excellent surface quality and a
manufacturing method therefor.
[0017] The subject of the present invention is not limited
to the above. The subject of the present invention will be
understood from the overall content of the present
15 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
invention.
20 Solution to Problem
[0018] According to an aspect of the present disclosure,
[0019] a pickled steel sheet with excellent surface quality
is provided. The pickled steel sheet includes, by wt%, 0.05%
or more and less than 0.4% of carbon (C), 0.5% or less
25 (excluding 0%)of silicon (Si), 0.05% or less of phosphorus
5
(P), 0.03% or less of sulfur (S), 0.01% or less of boron (B),
0.1 to 2.5% of at least one of manganese (Mn) and chromium
(Cr), and a balance of iron (Fe) and inevitable impurities,
[0020] wherein an average thickness of an internal oxide
5 layer and/or a decarburized layer, formed in a surface layer
portion of the steel sheet, is 1 to 10 μm, and 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.
10 [0021]
[0022] According to another aspect of the present disclosure,
[0023] a cold-rolled steel sheet with excellent surface
quality is provided. The cold-rolled steel sheet includes,
by wt%: 0.05% or more and less than 0.4% of carbon (C), 0.5%
15 or less (excluding 0%) of silicon (Si), 0.05% or less of
phosphorus (P), 0.03% or less of sulfur (S), 0.01% or less
of boron (B), 0.1 to 2.5% of at least one of manganese (Mn)
and chromium (Cr), and a balance of iron (Fe) and inevitable
impurities,
20 [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 (%)] μm, and
[0025] a standard deviation of the thickness of the internal
25 oxide layer and/or the decarburized layer in a length
6
direction of the steel sheet is 2 μm or less.
[0026]
[0027] In addition, the pickled steel sheet and the coldrolled
steel sheet of the present disclosure may satisfy the
5 following Relational Expression 1,
[0028] [Relational Expression 1]
[0029] C(%) + Si(%)/6 + Mn(%)/20 + Cr(%)/20 + 2×P(%) + 4×S(%)
< 0.5.
[0030]
10 [0031] According to another aspect of the present disclosure,
[0032] a manufacturing method for a pickled steel sheet with
excellent surface quality is provided. The manufacturing
method for a pickled steel sheet includes operations of:
preparing a hot-rolled coil including, by wt%, 0.05% or more
15 and less than 0.4% 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), 0.01% or less of boron (B), 0.1 to
2.5% of at least one of manganese (Mn) and chromium (Cr),
and a balance of iron (Fe) and inevitable impurities; and
20 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,
[0033] wherein, when the hot-rolled coil is divided into a
first region, a second region, a third region, a fourth
25 region, and a fifth region, in a length direction, a pickling
7
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
5 the fifth region.
[0034]
[0035] According to an aspect of the present disclosure,
[0036] a manufacturing method for a cold-rolled steel sheet
with excellent surface quality is provided. The
10 manufacturing method for a cold-rolled steel sheet includes
operations of: preparing a hot-rolled coil including, by wt%,
0.05% or more and less than 0.4% 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), 0.01% or less of boron (B),
15 0.1 to 2.5% of at least one of manganese (Mn) and chromium
(Cr), and a balance of iron (Fe) and inevitable impurities;
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
20 cold rolling the hot-rolled steel sheet from which the
internal oxide layer and/or the decarburized layer have been
removed,
[0037] wherein, when the hot-rolled coil is divided into a
first region, a second region, a third region, a fourth
25 region, and a fifth region, a pickling tank passing speed of
8
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.
5 [0038]
Advantageous Effects of Invention
[0039] In the present disclosure having the configuration as
described above, it is possible to provide a carbon steel
sheet with excellent surface quality in which an internal
10 oxide layer, or the like, is uniformly formed in a length
direction of a steel sheet, and a manufacturing method
therefor. In particular, additional costs may not be incurred
through additional processes or equipment, but rather
productivity of pickling may be improved compared to the
15 existing methods, thereby reducing manufacturing costs.
[0040]
Best Mode for Invention
[0041] Hereinafter, the present disclosure will be described.
20 [0042] In general, as is well known, in a surface layer
portion of a hot-rolled coil manufactured through
conventional reheating, finishing rolling, cooling, and
coiling, there is an internal defect layer such as an
internal oxide layer and/or a decarburized layer. The
25 internal oxide layer may occur in a process in which
9
components such as chromium (Cr), manganese (Mn), silicon
(Si), zinc (Zn), magnesium (Mg), and aluminum (Al), which
have higher oxygen affinity, than iron (Fe) occurs oxidation
in a base material. The decarburized layer may occur in a
5 process of being discharged to an atmosphere in a form of a
gas after carbon in steel is combined with the atmosphere
and oxygen in scale, 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
10 wound with a hot-rolled coil (HC), a cooling time after
coiling, a width, a thickness, and a length of the hot-rolled
steel sheet, and the like, and may be within 50 μm.
[0043] Meanwhile, the internal defect layer also affects a
subsequent pickling process and a cold-rolling process,
15 thereby ultimately becoming a factor to deteriorate surface
properties of the finally manufactured steel sheet.
Accordingly, in the present disclosure, by providing optimum
pickling conditions, using a hot-rolled coil showing
thickness deviation such as the internal oxide layer, or the
20 like, a pickled steel sheet and a cold-rolled steel sheet
with excellent surface quality may be provided.
[0044]
[0045] Hereinafter, a pickled steel sheet and a cold-rolled
steel sheet of the present disclosure will be described.
25 [0046] First, in the pickled steel sheet and cold-rolled
10
steel sheet of the present disclosure, a steel sheet is used,
the steel sheet including, by weight %, 0.05% or more and
less than 0.4% of carbon (C), 0.5% or less (excluding 0%)of
silicon (Si), 0.05% or less of phosphorus (P), 0.03% or less
5 of sulfur (S), 0.01% or less of boron (B), 0.1 to 2.5% of
manganese (Mn) and/or chromium (Cr), and a balance of iron
(Fe) and inevitable impurities. Hereinafter, the steel
composition component of the present disclosure and the
reason for limiting its content will be described. Meanwhile,
10 "%" as used herein means "%" by weight, unless otherwise
specified.
[0047]
[0048] Carbon (C): 0.05% or more and less than 0.4%
[0049] Carbon (C) is an element that effectively contributes
15 to improving strength of steel, so that in the present
disclosure, a certain level or more of carbon (C) may be
included in order to secure strength of a high carbon steel
sheet. In addition, when a content of C is less than a
certain level, a desired strength, hardness, and durability
20 of a final part cannot be ensured and a function of the high
carbon steel sheet cannot be obtained, so in the present
disclosure, a lower limit of the content of carbon (C) may
be limited to 0.05 %. On the other hand, when carbon (C) is
excessively added, the strength is improved, but cracks occur
25 during a manufacturing process thereof or cracks also occur
11
on a surface thereof due to formation of excessive cementite,
which may cause a problem of deterioration of surface quality,
and deterioration of weldability. Therefore, in the present
disclosure, the content of carbon (C) may be limited to less
5 than 0.4%. Accordingly, the content of carbon (C) in the
present disclosure may be in a range of 0.05% or more and
less than 0.4%.
[0050]
[0051] Silicon (Si): 0.5% or less (excluding 0%)
10 [0052] Silicon (Si) is an element having a strong affinity
with oxygen, so when a large amount of Si is added, it is
not preferable because it may cause surface defects observed
with a naked eye by a surface scale such as a red scale.
Accordingly, in the present disclosure, an upper limit of a
15 content of silicon (Si) may be limited to 0.5%. However,
since silicon (Si) is an element not only acting as a
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).
20 [0053]
[0054] Phosphorus (P): 0.05% or less
[0055] Phosphorus (P) is a major element that segregates at
grain boundaries and causes a decrease in toughness of steel.
Therefore, it is preferable to control a content of
25 phosphorus (P) as low as possible. Therefore, it is
12
theoretically most advantageous to limit the content of
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
5 caused to control the content of phosphorus (P) to 0%.
Accordingly, in the present disclosure, in consideration of
this point, an upper limit of the content of phosphorus (P)
may be limited to 0.05%.
[0056]
10 [0057] Sulfur (S): 0.03% or less
[0058] Sulfur (S) is a major element forming Mns, increasing
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
15 advantageous to limit the content of sulfur (S) to 0%.
However, sulfur (S) is also 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 sulfur (S) to 0%. Accordingly, in the present disclosure,
20 an upper limit of the content of sulfur (S) may be limited
to 0.03% in consideration of this point.
[0059]
[0060] Boron (B): 0.01% or less
[0061] Boron (B) is an element that slows a transformation
25 rate, when being transformed from austenite to ferrite,
13
pearlite, bainite, or the like, and is an element that is
easy to control transformation through cooling. However,
when an excessive amount of boron (B) is added, B segregates
at grain boundaries and causes deterioration of strength,
5 ductility, toughness, and the like. Accordingly, a content
of boron (B) is preferably 0.01 % or less.
[0062]
[0063] At least one of manganese (Mn) and chromium (Cr): 0.1%
or more and less than 2.5%
10 [0064] Manganese (Mn) and chromium (Cr) are elements
contributing to forming hardenability of steel, so in the
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 expensive
15 elements, is not preferable from an economic point of view,
and when an excessive amount of manganese (Mn) and chromium
(Cr) is added, weldability may be deteriorated. 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%
20 or more and less than 2.5%.
[0065]
[0066] [Relational Expression 1]
[0067] It is preferable that the pickled steel sheet and the
cold-rolled steel sheet of the present disclosure contain C,
25 Si, Mn, Cr, P and S so as to satisfy the following Relational
14
Expression 1. A reason for defining the following Relational
Expression 1 in the present disclosure is that the elements
described above causes deterioration of weldability. If a
sum of the contents of the component elements defined by the
5 following Relational Expression 1 is 0.5 or more, there is
a problem in that weldability is deteriorated and cracks are
generated around a welded part.

WE CLAIMS:
1. A pickled steel sheet with excellent surface
quality, comprising, by wt%:
0.05% or more and less than 0.4% of carbon (C), 0.5%
5 or less (excluding 0%)of silicon (Si), 0.05% or less of
phosphorus (P), 0.03% or less of sulfur (S), 0.01% or less
of boron (B), 0.1 to 2.5% of at least one of manganese (Mn)
and chromium (Cr), and a balance of iron (Fe) and inevitable
impurities,
10 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
a standard deviation of the thickness of the internal
oxide layer and/or the decarburized layer in a length
15 direction of the steel sheet is 2 μm or less.
2. The pickled steel sheet with excellent surface
quality of claim 1, wherein the following Relational
Expression 1 is satisfied,
20 [Relational Expression 1]
C(%) + Si(%)/6 + Mn(%)/20 + Cr(%)/20 + 2×P(%) + 4×S(%)
< 0.5.
3. The high-carbon pickled steel sheet with
25 excellent surface quality of claim 1, wherein the standard
41
deviation of the thickness of the internal oxide layer and/or
the decarburized layer in the length direction of the steel
sheet is 1.6 μm or less.
5 4. A cold-rolled steel sheet with excellent surface
quality, comprising, by wt%:
0.05% or more and less than 0.4% 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), 0.01% or less
10 of boron (B), 0.1 to 2.5% of at least one of manganese (Mn)
and chromium (Cr), and a balance of iron (Fe) and inevitable
impurities,
wherein an average thickness of an internal oxide layer
and/or a decarburized layer, formed in a surface layer
15 portion of the steel sheet, is 1 × [1- cold reduction(%)] μm
to 10 × [1- cold reduction(%)] μm, and
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.
20
5. The cold-rolled steel sheet with excellent
surface quality of claim 4, wherein the average thickness of
the internal oxide layer and/or the decarburized layer formed
in the surface layer portion of the steel sheet is in a range
25 of 0.2 to 8 μm.
42
6. The cold-rolled steel sheet with excellent
surface quality of claim 4, wherein the following Relational
Expression 1 is satisfied,
5 [Relational Expression 1]
C(%) + Si(%)/6 + Mn(%)/20 + Cr(%)/20 + 2×P(%) + 4×S(%)
< 0.5.
7. The high-carbon cold-rolled steel sheet with
10 excellent surface quality of claim 4, wherein the standard
deviation of the thickness of the internal oxide layer and/or
the decarburized layer in the length direction of the steel
sheet is 1.6 μm or less.
15 8. In a manufacturing method for a pickled steel
sheet, comprising the steps of: preparing a hot-rolled coil
including, by wt%, 0.05% or more and less than 0.4% 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), 0.01%
20 or less of boron (B), 0.1 to 2.5% of at least one of manganese
(Mn) and chromium (Cr), and a balance of iron (Fe) and
inevitable impurities; and 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
25 the same therethrough, the manufacturing method of the
43
pickled steel sheet having excellent surface quality,
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
5 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.
10
9. The manufacturing method for a pickled steel sheet
with excellent surface quality of claim 8, wherein the hotrolled
coil satisfies the following Relational Expression 1,
[Relational Expression 1]
15 C(%) + Si(%)/6 + Mn(%)/20 + Cr(%)/20 + 2×P(%) + 4×S(%)
< 0.5.
10. The manufacturing method for a pickled steel
sheet with excellent surface quality of claim 8, wherein the
20 hot-rolled coil is prepared from a process comprising the
steps of:
reheating a steel slab in a temperature range of 1050
to 1350 °C and then rough rolling, and then finishing hot
rolling the rough-rolled steel slab in a temperature range
25 of 800 to 950 °C;
44
cooling the finishing hot-rolled hot-rolled steel
sheet to a temperature range of 500 to 750 °C at a cooling
rate of 10 to 1000 °C/s, and then coiling the hot-rolled
steel sheet; and
5 air cooling the coiled hot-rolled steel sheet.
11. The manufacturing method for a pickled steel
sheet with excellent surface quality of claim 8, wherein a
pickling tank passing speed of the hot-rolled coil
10 corresponding to the third region is controlled to be slower
than a pickling tank passing speed of the hot-rolled coil
corresponding to the second region and the fourth region.
12. The manufacturing method for a pickled steel
15 sheet with excellent surface quality of claim 8, wherein the
pickling tank passing speed of 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
20 the hot-rolled coil in the third region]×1/2 to 5×[pickling
tank passing speed of the hot-rolled coil in the third
region]×2, and an average pickling tank passing speed thereof
in the second region and the fourth region is controlled to
be 5×[pickling tank passing speed of the hot-rolled coil in
25 the third region/2]×1/2 to 5×[pickling tank passing speed of
45
the hot-rolled coil in the third region/2]×2.
13. The manufacturing method for a pickled steel
sheet with excellent surface quality of claim 8, wherein,
5 when the hot-rolled coil is divided into n regions in a
length direction, a picking tank passing speed of a hotrolled
coil corresponding to a (n/2)th region, the region in
which the thickness of the internal oxide layer and/or the
decarburized layer is the thickest, is 5 mpm to 50 mpm, in
10 the case of t≤(n/2), a picking tank passing speed of the
hot-rolled coil corresponding to each region is controlled
by the following Relational Expression 2, and in the case of
t＞(n/2), a picking tank passing speed of the hot-rolled coil
corresponding to each region is controlled by the following
15 Relational Expression 3,
[Relational Expression 2]
Pickling tank passing speed of the hot-rolled coil
corresponding to a tth region = n×[pickling tank passing speed
of the hot-rolled coil corresponding to the (n/2)th
20 region/t]×1/2 to n×[pickling tank passing speed of the hotrolled
coil corresponding to the (n/2)th region/t]×2
[Relational Expression 3]
Pickling tank passing speed of the hot-rolled coil
corresponding to the tth region = n×[pickling tank passing
25 speed of the hot-rolled coil corresponding to the (n/2)th
46
region/(n-t+1)]×1/2 to n×[pickling tank passing speed of the
hot-rolled coil corresponding to the (n/2)th region/(n-t+1)]
×2,
where, in Relational Expressions 2 to 3, n is a natural
5 number, and the tth refers to an order sequentially assigned
to correspond to each region divided in a length direction
of the hot-rolled coil.
14. The manufacturing method for a pickled steel
10 sheet with excellent surface quality of claim 8, wherein a
concentration of hydrochloric acid of a pickling solution in
the pickling tank is 5 to 25%.
15. The manufacturing method for a pickled steel
15 sheet with excellent surface quality of claim 8, wherein a
temperature of the pickling solution in the pickling tank is
in a range of 70°C to 90°C.
16. The manufacturing method for a pickled steel
20 sheet with excellent surface quality of claim 8, wherein,
after the pickling, 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 a standard
deviation of the thickness of the internal oxide layer and/or
25 the decarburized layer in a length direction of the pickled
47
steel sheet is 2 μm or less.
17. In a manufacturing method for a cold-rolled steel
sheet, comprising the steps of: preparing a hot-rolled coil
5 including, by wt%, 0.05% or more and less than 0.4% 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), 0.01%
or less of boron (B), 0.1 to 2.5% of at least one of manganese
(Mn) and chromium (Cr), and a balance of iron (Fe) and
10 inevitable impurities; 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 the hot-rolled steel
sheet from which the internal oxide layer and/or the
15 decarburized layer have been removed, the manufacturing
method for the cold-rolled steel sheet having excellent
surface quality,
wherein, when the hot-rolled coil is divided into a
first region, a second region, a third region, a fourth
20 region, and a fifth region, 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 corresponding to the first region and the fifth region.
25
48
18. The manufacturing method for a cold-rolled steel
sheet with excellent surface quality of claim 17, wherein
the hot-rolled coil satisfies the following Relational
Expression 1,
5 [Relational Expression 1]
C(%) + Si(%)/6 + Mn(%)/20 + Cr(%)/20 + 2×P(%) + 4×S(%)
< 0.5.
19. The manufacturing method for a cold-rolled steel
10 sheet of claim 17, wherein the hot-rolled coil is prepared
by a process comprising the steps of:
reheating a steel slab in a temperature range of 1050
to 1350°C and then rough rolling, and then finishing hot
rolling the rough-rolled steel slab in a temperature range
15 of 800 to 950°C;
cooling the finishing hot-rolled hot-rolled steel
sheet to a temperature range of 500 to 750°C at a cooling
rate of 10 to 1000 °C / s, and then coiling the hot-rolled
steel sheet; and
20 air cooling the coiled hot-rolled coil.
20. The manufacturing method for a cold-rolled steel
sheet with excellent surface quality of claim 17, wherein a
pickling tank passing speed of the hot-rolled coil
25 corresponding to the third region is controlled to be slower
49
than a pickling tank passing speed of the hot-rolled coil
corresponding to the second region and the fourth region.
21. The manufacturing method for a cold-rolled steel
5 sheet with excellent surface quality of claim 17, wherein
the pickling tank passing speed of the hot-rolled coil in
the third region is 5 mpm to 50 mpm, an average pickling
tank passing speed in the first region and the fifth region
is controlled to be 5×[pickling tank passing speed of the
10 hot-rolled coil in the third region]×1/2 to 5×[pickling tank
passing speed of the hot-rolled coil in the third region]×2,
and the 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
15 the third region /2]×1/2 to 5×[pickling tank passing speed
of the hot-rolled coil in the third region /2]×2.
22. The manufacturing method for a cold-rolled steel
sheet with excellent surface quality of claim 17, wherein,
20 when the hot-rolled coil is divided into n regions in a
length direction, a picking tank passing speed of a hotrolled
coil corresponding to a (n/2)th region, the region in
which the thickness of the internal oxide layer and/or the
decarburized layer is the thickest, is 5 mpm to 50 mpm, in
25 the case of t≤(n/2), a picking tank passing speed of a hot50
rolled coil corresponding to each region, is controlled by
the following Relational Expression 2, and in the case of
t＞(n/2), a picking tank passing speed of the hot-rolled coil
corresponding to each region, is controlled by the following
5 Relational Expression 3,
[Relational Expression 2]
Pickling tank passing speed of the hot-rolled coil
corresponding to a tth region = n×[pickling tank passing speed
of the hot-rolled coil corresponding to the (n/2)th
10 region/t]×1/2 to n×[pickling tank passing speed of the hotrolled
coil corresponding to the (n/2)th region/t]×2
[Relational Expression 3]
Pickling tank passing speed of the hot-rolled coil
corresponding to the tth region = n×[pickling tank passing
15 speed of the hot-rolled coil corresponding to the (n/2)th
region/(n-t+1)]×1/2 to n×[pickling tank passing speed of the
hot-rolled coil corresponding to the (n/2)th region/(nt+
1)]×2,
where, in Relational Expressions 2 to 3, n is a natural
20 number, and the tth refers to an order sequentially assigned
to correspond to each region divided in a length direction
of the hot-rolled coil.
23. The manufacturing method for a cold-rolled steel
25 sheet with excellent surface quality of claim 17, wherein,