DESCRIPTION
METHOD OF MANUFACTURING HOT-ROLLED STEEL SHEET HAVING
EXCELLENT AGING RESISTANCE, AND HOT-ROLLED STEEL SHEET
MANUFACTURED THEREBY
Technical Field
The present invention relates to a method of
manufacturing a hot-rolled steel sheet having excellent aging
resistance, and a hot-rolled steel sheet manufactured by the
method. More particularly, the present invention relates to a
method of manufacturing a hot-rolled steel sheet having
excellent aging resistance and workability by optimizing the
ingredients of the steel sheet and the manufacturing process
thereof.
Background Art:
Steel sheets for household electric appliances and
automobiles are required to have excellent aging resistance
(fluting resistance), workability and coatability.
Fluting is the term used to describe a phenomenon whereby
the machined portion of a steel sheet is bent. When a fluting
phenomenon occurs, it is difficult to maintain the shape of
the molded portion of a steel sheet, so this fluting
phenomenon must be strictly restricted in a real process.
However, generally, since an aging phenomenon causing the
fluting phenomenon, the aging phenomenon being caused by

solid-solution elements, is difficult to adequately control,
carbonitride forming elements, such as titanium (Ti), niobium
(Nb) , etc., are added and precipitated in a steel-making
process to accomplish high cleanliness. The addition of such
carbonitride forming elements is helpful in the control of a
machining defect such as fluting or the like, but causes a"
productivity decline due to an increase in steel-making time
required to achieve high cleanliness and causes an increase in
manufacturing cost due to the addition of- expensive alloy
elements.
Further, it is known that it is difficult to control a
bending phenomenon due to fluting in medium or low carbon
steel. Therefore, in the case of household electric
appliances, automobiles and the like requiring shape
fixability and workability, it is required to establish
methods of controlling a bending phenomenon at the time of
processing a steel sheet.
Meanwhile, in order to improve the shape fixability of a
final product and to accomplish productivity improvement by
improving a manufacturing process, various processing
characteristics, such as extensibility, flanging property,
bendability, drawability and the like, as well as the
prevention of fluting caused by aging resistance are required.
Further, since these structures are exposed to an external
environment, a process of painting a steel sheet with an
organic material to improve weather resistance is performed,
and, in order to assure the above-mentioned characteristics,

it is required to develop a steal sheet for plating, which can
exhibit coatability, in terms of materials.
For example, Japanese Unexamined Patent Application
Publication No. 1989-282420 (invention title: method of
manufacturing hot-rolled steel sheet for working and method of
heat-treating hot-rolled steel sheet for working) discloses a
method of manufacturing a steel sheet suitable for high-
strength members for automobiles or industrial appliances,
wherein titanium (Ti), niobium (Nb) .and some rare-earth
elements are added to ultralow carbon steel base to
manufacture a hot-rolled steel sheet having excellent
workability and aging resistance. However, this method,, as
mentioned above, is helpful in the control of a machining
defect such as fluting or the like by the addition of
carbonitride forming elements, but is problematic in that it
causes a productivity decline due to an increase in steel-
making time for high cleanliness, and causes an increase in
manufacturing cost due to the addition of expensive alloy
elements.
Further, Korean Unexamined Patent Application Publication
No. 1996-23130 (invention title: method of manufacturing hot-
rolled steel sheet having excellent aging resistance)
discloses a method of improving the aging resistance of a hot-
rolled steel sheet by adding a very small amount of zirconium
(carbonitride forming element) to ultralow-carbon aluminum-
killed steel and then hot-rolling the ultralow-carbon
aluminum-killed steel in the transformation-temperature range

of Ar3 to coarsen ferrite crystal grains. However, according
to this method, since a special element, such as zirconium, is
needed in order to increase an aging effect, workability for
steel making deteriorates, cost increases, and shape
fixability deteriorates because the strength of the
manufactured.steel is low.
Further, Korean Unexamined Patent Application Publication
No. 2001-60648 (invention title: method of manufacturing hot-
rolled steel sheet having excellent aging resistance and
extension uniformity) discloses a method of improving the
aging resistance and extension uniformity by reheating a steel
slab including carbon (C) : 0.02 ~ 0.05 wt%, manganese (Mn) :
0.10 ~ 0.30 wt%, boron (B): 10 ~ 30 ppm, phosphorus: 0.020 wt%
or less, sulfur (S) : 0.015 wt% or less, aluminum (Al) : 0.01 ~
0.04 wt%, nitrogen (N) : 40 ppm or less, and a balance of iron
(Fe) and inevitable impurities, hot-rolling the reheated steel
slab at a finish rolling temperature of Ar3 transformation
temperature or more to a thickness of 1.4 ~ 2.3 mm and then
rolling the hot-rolled steel slab at a temperature of 600 ~
. 700°C. However, this method is problematic in that it is
difficult to ensure high aging resistance because an aging
phenomenon caused by solid-solution elements such as carbon,
nitrogen and the like included in steel cannot be completely
prevented only by adding an extremely small amount of boron
and controlling rolling temperature.
Further, Japanese Unexamined Patent Application

Publication No. 2008-190008 (invention title: method of
manufacturing hot-rolled steel sheet having excellent aging
resistance) discloses a method of manufacturing hot-rolled
steel sheet having excellent aging resistance by hot-rolling a
slab including carbon (C): 0.04 ~ 0.25 wt%, silicon (Si): 0.01
~ 0.5 wt%, manganese (Mn) : 0.05 ~ 1.5 wt%, phosphorus: 0.09
wt% or less, sulfur (S) : 0.015 wt% or less, aluminum (Al) :
0.01 ~ 0.08 wt%, nitrogen (N) : 0.0005 ~ 0.015 wt%, and a
balance of iron (Fe) and inevitable impurities, cooling the
hot-rolled slab to 400°C or lower at an average cooling rate
of 60 °C/s or more, rolling the cooled slab and then skin-
pass-rolling the rolled slab at an elongation ratio of 0.1 ~
1.0% using a small diameter roll satisfying t/R£ 0.0055 (t is
thickness of sheet, R is diameter of roll) . However, this
method is problematic in that rolling temperature is lowered
to 400°C or less, so low-temperature precipitates are formed
and behave differently due to widthwise temperature non-
uniformity, causing a texture difference, with the result that
the shape of the hot-rolled steel sheet becomes poor, the slab
cannot be easily rolled, and post-process workability
deteriorates. Further, this method is problematic in that it
is difficult to use it to produce various sizes of materials
in a normal work line'because the diameter of a roll must be
controlled one by one in response to the thickness of a sheet

in order to induce a surface-operating electric potential.
Disclosure
Technical Problem
Accordingly, the present invention has been made to solve
the above-mentioned problems occurring in the prior art, and
an object of the present invention is to provide a method of
manufacturing a hot-rolled steel sheet, which can improve
workability and coatability as well as aging resistance by
adding cobalt (Co) and boron (B) to steel and optimizing a
manufacturing process and shot-blasting conditions, and a hot-
rolled steel sheet manufactured by this method.
Technical Solution
In order to accomplish the above object, an aspect of the
present invention provides a method of manufacturing a hot-
rolled steel sheet having excellent aging resistance,
including the step of: shot-blasting a surface of a steel
sheet manufactured by a steel including carbon (C) 0.01 ~ 0.12
wt%, manganese (Mn) 0.1 ~ 0.5 wt%, phosphorus 0.025 wt% or
less (excluding 0 wt%), sulfur (S) 0.02 wt% or less (excluding
0 wt%), aluminum (Al) 0.03 ~ 0.15 wt%, boron (B) 0.0005 ~
0.0020 wt%, cobalt (Co) 0.01 ~ 0.05 wt%, nitrogen (N) 0.002 ~.
0.008 wt%, and a balance of iron (Fe) and inevitable
impurities using shot balls having a size of 0.10 ~ 0.40 mm at
an injection speed of ,40 ~ 65 m/se.
In the method, the composition ratio of the steel may

satisfy 3.5 < [B(ppm) x Al(%) x Co(%) / N(%)] ^ 18.
Further, the steel having the composition ratio may be
finish-rolled at 860 ~ 950°C, the finish-rolled steel may be
cooled at a cooling rate of 30 ~ 100 °C/sec, and the cooled
finish-rolled steel may be rolled at 580 ~ 680°C.
Further, the surface roughness index ratio (Rmax/Ra) of
the shot-blasted steel may be 12 ~ 23.
Another aspect of the present invention provides a hot-
rolled steel sheet having excellent aging resistance,
manufactured by a steel that includes carbon (C) 0.01 ~ 0.12
wt%, manganese (Mn) 0.1 ~ 0.5 wt%, phosphorus 0.025 wt% or
less (excluding 0 wt%), sulfur (S) 0.02 wt% or less (excluding
0 wt%), aluminum (Al) 0.03 ~ 0.15 wt%, boron (B) 0.0005 ~
0.0020 wt%, cobalt (Co) 0.01 ~ 0.05 wt%, nitrogen (N) 0.002 ~
0.008 wt%, and a balance of iron (Fe) and inevitable
impurities, wherein the steel has a surface roughness index
ratio (Rmax/Ra) of 12 ~ 23 by shot-blasting.
In the hot-rolled steel sheet, the composition ratio of
the steel may satisfy 3.5 < [B(ppm) x Al(%) x Co(%) / N(%)] <
18.
Further, the transformed ferrite formed by shot-blasting
the steel may occupy 3 ~ 10% of the hot-rolled steel sheet in
a width direction of the hot-rolled steel sheet.
Advantageous Effects

As described above, according to the method of
manufacturing a hot-rolled steel sheet of the present
invention, workability and coatability, as well as aging
resistance, can be improved by optimizing the composition
ratio of a steel sheet and the manufacturing process thereof,
so high value-added steel sheets used in household electric
appliances, automobiles and the like can be manufactured.
Further, according to this method, the efficiency of an acid
washing work can be increased because of a high descaling
effect, environmental pollution can be prevented, and process
time can be shortened.
Description of Drawings
FIG. 1 is a photograph showing the texture of a hot-
rolled steel sheet according to the present invention.
FIG. 2 is a graph evaluating the characteristics of the
hot rolled steel sheet depending on the surface roughness
index ratio.
Best Mode
Hereinafter, preferred embodiments of the present
invention will be described in detail.
As described above, the hot-rolled steel sheet of the
present invention was manufactured by repeatedly conducting
research and experiments on high value-added steel sheets
satisfying aging resistance, workability and coatability to be
used for household electric appliances, automobiles and the

like. The composition ratio of ingredients constituting steel
may be controlled as follows.
Carbon (C) is an element added to improve the strength of
steel. As the amount of carbon increases, the tensile and
yield strength of steel increases. However, when carbon is
excessively added, the workability of a material deteriorates,
so the upper limit thereof is defined by 0.12%. Meanwhile,
when the amount of carbon is less than 0.01%, there is a
problem in that additional time is required to perform the
decarbonization during a steel making process, and the quality
of a material is rapidly changed, so it is difficult to secure
a material of consistent quality-. .Therefore, it is preferred
that the amount of carbon be 0.01 ~ 0.12%, and preferably 0.02
~ 0.08%.
Manganese (Mn) is an element widely used as a solid
solution enhancing element, and is an important element that
serves to increase the strength of steel and improve hot
workability, but it is also an element that deteriorates the
flexibility and workability of a material due to the formation
of MnS. When the amount of manganese is small, workability is
improved, but it is difficult to secure the strength of steel.
Therefore, in order to secure desired strength, manganese must
be added in an amount of 0.1% or more. In contrast, when
manganese is excessively added, economical efficiency becomes
low, and center segregation is caused. Therefore, the upper
limit thereof is defined by 0.5%.
Phosphorus (P) . is an element serving to improve the

strength and corrosion resistance of steel. In order to
improve these characteristics, it is preferred that phosphorus
be added in a large amount. However, since phosphorus is an
element that causes center segregation during casting,
workability is deteriorated when it is added in a large
amount. Therefore, the amount of phosphorus is limited to
0.025% (excluding 0%), and preferably the amount thereof may
be 0.005 ~ 0.015%.
Sulfur (S) forms a nonmetallic intermediate serving as a
corrosion initiation point in combination with manganese (Mn),
and causes hot shortness, so, if possible, it is preferred
that the amount.thereof be reduced. Therefore, the amount of
sulfur is limited to 0.02% (excluding 0%), and preferably the
amount thereof may be 0.01% or less.
Aluminum (Al) is an element added to deoxidize molten
steel and to improve the aging characteristics of steel in
combination with solid solution elements in the molten steel.
Therefore, the lower limit thereof is defined by 0.03%. When
aluminum is excessively added, there is a problem in. that the
content of intermediates in steel increases, thus causing
surface defects and deteriorating workability. Therefore, the
upper limit thereof is defined by 0.15%, and preferably the
amount thereof may be 0.03 ~ 0.15%,
Boron (B) is an element improving the aging properties of
steel in combination with a solid-solution element in steel,
and is a curability-enhancing element increasing the strength
of a material when added in a small amount. Therefore, in

order to obtain desired material characteristics, it is
required that boron be added in an amount of 0.0005% or more.
However, when the amount of boron is more than 0.0020%, there
are problems in that deterioration of the steel quality and
the grain boundary cracking• of steel are caused and the
surface of a hot-rolled steel sheet becomes rough during
continuous casting. Therefore, it is preferred that the
amount thereof be 0.0005 ~ 0.0020%.
Cobalt (Co) is an element accelerating the formation of
precipitates in steel. In order to obtain such an effect,
cobalt must be added in an amount of 0.01% or more. However,
when cobalt is added in an amount of more than 0.05%, it
causes increase in manufacturing costs due to the addition of
an expensive alloy element in a large amount, rather than
contribution to acceleration of precipitation. Therefore, it
is preferred that the amount of cobalt be 0.01 ~ 0.05%.
Nitrogen (N) is an element useful for enhancing material
properties while existing in the state of solid solution, but
is a main element causing an aging phenomenon. Therefore, in
order to secure workability, it is required that the amount of
nitrogen be adjusted in a predetermined range, and that the
upper limit thereof be defined by 0.008%. Further, when the
amount thereof is less than 0.002%, sufficient strength cannot
be obtained, and the number of sites for forming precipitates
is reduced. Therefore, the lower limit thereof is defined by
0.002%, and ' the amount thereof is adjusted in the range of
0.002 ~ 0.008%.

In order to secure excellent workability by suitably
controlling the fraction of precipitates and solid solution,
it is preferred that the content ratio of alloy elements
satisfy 3.5 ≤ [B(ppm) x Al(%) x Co(%) / N(%)] ≤ 18. When
[B(ppm) x Al(%) x Co(%) / N(%)] is less than 3. 5, it is
difficult to secure high aging resistance and workability
because of expression of solid solution elements. In
contrast, when [B(ppm) x Al(%) x Co(%) / N(%)] is more than
18, workability can be secured, but partial surface defect and
productivity decline are caused by the addition of elements in
large amounts. For this reason, it is preferred that the
content ratio of alloy elements satisfy 3.5 ≤ [B(ppm) x Al(%)
x Co(%) / N(%)] ≤ 18.
Hereinafter, a method of manufacturing a hot-rolled steel
sheet using the steel including ingredients in the composition
ratio according to the present invention will be described in
detail.
First, a steel including carbon (C) 0.01 ~ 0.12 wt%,
manganese (Mn) 0.1 ~ 0.5 wt%, phosphorus 0.025 wt% or less
(excluding 0 wt%) , sulfur (S) 0.02 wt% or less (excluding 0
wt%), aluminum (Al) 0.03 ~ 0.15 wt%, boron (B) 0.0005 ~ 0.0020
wt%, cobalt (Co) 0.01 ~ 0.05 wt%, nitrogen (N) 0.002 ~ 0.00.8
wt%, a balance of iron (Fe) and inevitable impurities is hot-
rolled by a general hot-rolling process to obtain a hot-rolled
steel sheet, and then the surface of the hot-rolled -steel
sheet is shot-blasted using shot balls having a size of 0.10 ~
0.40 mm at an injection speed of 40 ~ 65 m/sec.

Such a shot-blasting process is one of the most
characteristic technical constitutions for securing aging
resistance according to the present invention. In this shot-
blasting process, proper compression stress is applied to the
surface of a hot-rolled steel sheet to form transformed
ferrite grains having high movable electric potential density,
so an electric potential fixing phenomenon due to solid
solution elements causing an aging phenomenon is reduced,
thereby improving the aging resistance of the hot-rolled steel
sheet.
In order to accomplish such an effect, it is preferred
that the size of e,ach short ball used in ' shot-blasting be
adjusted in the range of 0.10 ~ 0.40 mm. The reasons for this
are that, when the size thereof is less than 0.10 mm, the
mechanical stripping effect of a surface layer of a hot-rolled
steel sheet is insufficient, and thus it is difficult to
obtain a proper residual stress effect, and that, when the
size thereof is more than 0.40 mm, the maximum roughness of a
surface layer of a hot-rolled steel sheet rapidly increases,
and thus the surface layer may be cracked during a
manufacturing process.
Meanwhile, it is preferred that the injection speed be
adjusted in the range of 40 ~ 65 m/sec. The reasons for this
are that, when the injection speed is less than 40 m/sec, the
impact pressure of short balls applied to the surface layer of
a hot-rolled steel sheet is low, and thus it is difficult to
secure desired aging resistance, and that, when the injection

speed is more than 65 m/sec, the depth of a surface-cured
layer of a hot-rolled steel sheet exceeds 10% of the thickness
of the hot rolled steel sheet, thus causing non-uniform
processing.
In order to secure the mechanical properties of the hot-
rolled steel sheet according to the present invention, after
the shot-blasting process, it is preferred that the surface
roughness index ratio (Rmax/Ra) of the hot-rolled steel sheet
be adjusted in the range of 12 ~ 23. Here, Rmax means a
height of the maximum point in a surface roughness curve of
the hot-rolled steel sheet, and Ra means an average surface
roughness in the center line of the surface roughness curve
thereof. When the surface roughness index ratio thereof is
less than 12, the ratio of convex portions to concave portions
is not appropriate, so the adsorptivity of organic matter to
the surface thereof becomes low, and the residual stress
distribution in the thickness direction of a material is
insufficient, and thus it is difficult to secure aging
resistance. In contrast, when the surface roughness index
ratio thereof is more' than 23, the hot-rolled steel sheet
reaches a saturation state in terms of adsorbing organic
matter, and simultaneously the surface thereof starts to
crack. Therefore, it is preferred that the surface roughness
index ratio thereof be adjusted in the range of 12 ~ 23.
FIG. 1 (a) shows normal ferrite grains- having low
electric potential density, and FIG. 1 (b) shows transformed
ferrite grains having a reticular structure- in which electric

potentials are accumulated by a shot-blasting process to have
high electric potential density. The transformed ferrite
structure formed by shot blasting may occupy 3 ~ 10% of the
hot-rolled steel sheet in the direction of a thickness
thereof. When the occupation ratio of the transformed ferrite
structure in the direction of the hot-rolled steel sheet is
less than 3%, the hot-rolled steel sheet does not exhibit
structural characteristics that are sufficient for fixing
solid solution elements, and thus does not secure targeted
aging resistance. Further, when the occupation ratio of the
transformed ferrite structure in the direction of the hot-
rolled steel, .sheet is more than 10%, the material of the hot-
rolled steel sheet is cured during working and post-treatment
processes. Therefore, it is preferred that the occupation
ratio of the transformed ferrite structure in the direction of
the hot-rolled steel sheet be adjusted in the range of 3 ~
10%.
As described above, when the surface of the steel sheet
manufactured by a general hot-rolling process is shot-blasted
under predetermined conditions, a hot-rolled, steel sheet
having excellent aging resistance and workability can be
manufactured. Moreover, when a hot-rolling process is
optimized, a hot-rolled steel sheet having more excellent
aging resistance, workability and coatability can be
manufactured. Hereinafter, an optimized hot-rolling process
will be described in detail.
First, the steel having the above composition ratio may

be finish-rolled at 860 ~ 950°C. When the finish-rolling
temperature is lower than 860°C, the steel is hot-rolled at a
low temperature region, so crystal grains are rapidly mixed,
thereby causing the deterioration of rollability and
workability. In contrast, when the finish-rolling temperature
is higher than 950°C, the steel is not uniformly hot-rolled
over the entire thickness of the steel, so crystal grains are
not sufficiently miniaturized, and thus crystal grains are
coarsened, thereby deteriorating the impact toughness of the
hot-rolled steel sheet. Therefore, it is preferred that the
finish-rolling temperature be adjusted in the range of 860 ~
950°C.
Subsequently, the finish-rolled steel may be cooled in a
run-out-table (ROT) at a cooling rate of 30 ~ 100 °C/sec.
When the cooling rate of the finish-rolled steel in the ROT is
less than 30 °C/sec, relatively coarse crystal grains are
formed by dynamic crystal grain growth, thus causing the
strength and workability of the hot-rolled steel sheet. In
contrast, when the cooling rate of the finish-rolled steel in
the ROT is more than 100 °C/sec, the finish-rolled steel is
non-uniformly cooled in the width direction thereof, thus
causing the texture of the hot-rolled steel sheet to be non-
uniform. Therefore, it is preferred that the cooling rate

thereof be adjusted in the range of 30 ~ 100 °C/sec.
Finally, the hot-rolled steel sheet obtained after the
finish-rolled steel is cooled in the ROT may be rolled at 580
~ 680°C. When the rolling temperature is lower than 580°C, the
temperature of the hot-rolled steel sheet in the width
direction thereof becomes non-uniform during cooling and
holding processes, so the behaviors of forming low-temperature
precipitates become varied, causing the texture of the hot-
rolled steel sheet to be non-uniform, thereby causing the
deterioration of workability. In contrast, when the rolling
temperature is higher than 680°C, a final product has a coarse
cementite structure, thus causing a problem of deteriorating
the workability and corrosion resistance of the hot-rolled
steel sheet. Therefore, it is preferred that the rolling
temperature be adjusted in the range of 580 ~ 680°C.
Mode for Invention
In order to evaluate the technical effects of the method
of manufacturing a hot-rolled steel sheet having excellent
aging resistance according to the present invention, the
following experiments were carried out.
First, several kinds of steel (three kinds of steel of
the present invention, three kinds of comparative steel)
having the composition .ratios given in Table 1 below were

provided, reheated to 1250°C for 2 hours in a heating furnace,
and then hot-rolled under hot-rolling conditions given in
Table 2 below. The physical properties and mechanical
properties of the several kinds of steel were respectively
measured, and the results thereof are given in Table 3 below.

- In the evaluation of coatability given in Table 3 above,
among the evaluation items of coating adhesion and surface
characteristics, when both coating adhesion and surface
characteristics are excellent, it is represented by "good",
when one of coating adhesion and surface characteristics is
excellent, it is represented by "normal", and when neither
coating adhesion nor surface characteristics are excellent, it
is represented by "poor".
In the evaluation of .workability given in Table 3 above,
the crack lengths of test pieces were measured, and then
classified into 5 steps as given in Table 4 below. Here, step

1 is represented by "good", steps 2 ~ 3 are represented by
"normal", and steps 4 ~ 5 are represented by "poor".

In the evaluation of aging resistance given in Table 3
above, the aging resistances of steel sheets are classified
into 5 steps depending on the degree of each steel sheet being
bent after processing. Here, step 1, in which each steel
sheet was comparatively slightly bent, is represented by
"good", steps 2 ~ 3, in which each steel sheet was partially
pleasant to the touch, are represented by "normal", and steps
4 ~ 5, in which each steel sheet was bent to such , as degree
that it can be observed by the naked eye, are represented by
"poor".
The test results given in Table 3 above are evaluated as
follows.
Examples 1 to 5 satisfy all the process conditions of
steel composition control, hot-rolling and shot-blasting. In
Examples 1 to 5, a yield point elongation phenomenon did not

occur, a surface roughness index ratio (Rmax/Ra) was adjusted
in the range of 12 ~ 23, and a bending phenomenon did not
occur during processing, thus securing excellent aging
resistance. Moreover, cracks did not occur even at the time
of bending processing, so high workability and coatability
were exhibited, thereby ensuring manufacture of an excellent
hot-rolled steel sheet and an excellent disk for plating.
Examples 6 to 8 satisfy the process conditions of steel
composition control (invention steel 1, invention steel 2) and
shot-blasting, but do not satisfy the process condition of
hot-rolling. More specifically, in Example 6, the finish-
rolling temperature was 750°C which is lower than the control
temperature range, in Example 7, the cooling rate of the
finish-rolled steel in ROT was 15 °C/s which is lower than the
control cooling rate range, and in Example 8, the rolling
temperature was 400°C which is lower than the control
temperature range. In Examples 6 to 8, a yield point
elongation phenomenon partially occurred, but excellent aging
resistance was exhibited. Further, workability was somewhat
deteriorated due to the non-uniformly mixed crystal grains,
the precipitation of solid solution elements and the
difference in material properties, but each of the hot-rolled
steel sheets satisfies high-quality conditions and has
excellent coatability. Therefore, these hot-rolled steel
sheets can be used as high value-added steel sheets for

household electric appliances, automobiles and the like.
Comparative Example 1 satisfies the process conditions of
steel composition control (invention steel 1) and hot-rolling,
but does not satisfy the process condition of shot-blasting.
More specifically, in the shot-blasting condition, shot balls
having a size of 0.91 mm, which is larger than the control
range, was used, and the shot-blasting speed was 90 m/sec,
which is faster than the control range. In this case, the
hot-rolled steel sheet of Comparative Example 1 exhibits good
aging resistance because a yield point elongation phenomenon
does not occur, but the workability and coatability thereof
are deteriorated due to the increase in surface roughness and
the increase in inner cured layer. Therefore, the hot-rolled
steel sheet of Comparative Example 1 cannot be suitably used
as a high-quality hot-rolled steel sheet.
Comparative Examples 2 to 7 satisfy the process
conditions of hot-rolling and shot-blasting, but do not
satisfy the process condition of steel composition control
(comparative steels 1 to 6) . In this case, most of the hot-
rolled steel sheets of Comparative Examples 2 to 7 have low
aging resistance because it is difficult to control a yield
point elongation phenomenon, and do not satisfy sufficient
workability and coatability because they are bent during
processing.
Moreover, in the hot-rolled steel sheet of Comparative
Example 2, in which the composition ration of elements [B(ppm)
■x Al(%) x Co(%) / N(%)] is 0.00 and titanium (Ti) is not

included as a carbonitride forming element, there is a problem
in that the shape fixability of the hot-rolled steel sheet is
deteriorated because it is difficult to secure the workability
thereof due to the occurrence of an aging phenomenon and the
'strength thereof is low. In contrast, in the hot-rolled steel
sheet of Comparative Example 3, in which the composition
ration of elements [B(ppm) x Al(%) x Co(%) / N(%)] is 0.00 and
titanium (Ti) is included as a carbonitride forming element,
the hot-rolled steel sheet can exhibit desired strength, but
cannot exhibit targeted aging resistance and coatability even
when the hot-rolling and shot-blasting conditions according to
the present invention are applied.
Finally, FIG. 2 is a graph measuring the aging resistance
(represented by fluting index) and workability (represented by
sensitivity to crack occurrence) of the hot rolled steel sheet
manufactured using invention steel 1 depending on the surface
roughness index ratio. In order to satisfy targeted aging
resistance and workability, it is preferred that the fluting
index be adjusted to 2 or less, and the sensitivity to crack
occurrence be adjusted to 0.5 or less. When the surface
roughness index ratio of the present invention is adjusted in
the range of 12 ~ 23, the fluting index and the sensitivity to
crack occurrence are improved. In contrast, when the surface
roughness index ratio is lower than the control range, there
is a problem in that the hot-rolled steel sheet is easily bent
when the fluting index is 2 or more, .and when the surface
roughness index ratio is higher than the control range, there

is a problem in that aging resistance is saturated, but cracks
occur at the time of processing.

CLAIMS
1. A method of manufacturing a hot-rolled steel sheet
having excellent aging resistance, comprising the step of:
shot-blasting a surface of a steel sheet manufactured by a
steel including carbon (C) 0.01 ~ 0.12 wt%, manganese (Mn) 0.1
~ 0.5 wt%, phosphorus 0.025 wt% or less (excluding 0 wt%),
sulfur (S) 0.02 wt% or less (excluding 0 wt%) , aluminum (Al)
0.03 ~ 0.15 wt%, boron (B) 0.0005 ~ 0.0020 wt%, cobalt (Co)
0.01 ~ 0.05 wt%, nitrogen (N) 0.002 ~ 0.008 wt%, and a balance
of iron (Fe) and inevitable impurities, using shot balls
having a size of 0.10 ~ 0.40 mm at an injection speed of 40 ~
65 m/se.
2. The method of claim 1, wherein a composition ratio of
the steel satisfies 3.5 ≤ [B(ppm) x Al(%) x Co(%) / N(%)] ≤
18.
3. The method of claim 1 or 2, wherein the steel having
the composition ratio is finish-rolled at 860 ~ 950°C.
4. The method of claim 3, wherein the finish-rolled steel
is cooled at a cooling rate of 30 ~ 100 °C/sec.
5. The method of claim 4, wherein the cooled finish-

rolled steel is rolled at 580 ~ 680°C.
6. The method of claim 1, wherein a surface roughness
index ratio (Rmax/Ra) of the shot-blasted steel is 12 ~ 23.
7. A hot-rolled steel sheet having excellent aging
resistance, manufactured by a steel that includes carbon (C)
0.01 ~ 0.12 wt%, manganese (Mn) 0.1 ~ 0.5 wt%, phosphorus
0.025 wt% or less (excluding 0 wt%) , sulfur (S) 0.02 wt% or
less (excluding 0 wt%) , aluminum (Al) 0.03 ~ 0.15 wt%, boron
(B) 0.0005 ~ 0.0020 wt%, cobalt (Co) 0.01 ~ 0.05 wt%, nitrogen
(N) 0.002 ~ 0.008 wt%, and a balance of iron (Fe) and
inevitable impurities, wherein the steel has a surface
roughness index ratio (Rmax/Ra) of 12 ~ 23 by shot-blasting.
8. The hot-rolled steel sheet of claim 7, wherein a
composition ratio of the steel satisfies 3.5 ≤ [B (ppm) x Al(%)
x Co(%) / N(%)] ≤ 18.
9. The hot-rolled steel sheet of claim 7, wherein a
transformed ferrite formed by shot-blasting the steel occupies
3'~ 10% of the hot-rolled steel, sheet in a width direction of
the hot-rolled steel sheet.