[DESCRIPTION]
[Technical Field]
The present disclosure relates to a high carbon hot
rolled steel sheet having excellent material uniformity,
and more particularly, to a high carbon hot rolled steel
sheet having excellent material uniformity that may be used
in machine parts, tools, automobile parts, and the like,
and a method for manufacturing the same.
[Background Art]
High carbon hot rolled steel sheets using high carbon
steel have been used in various applications, e.g., machine
parts, tools, automobile parts, and the like. Such steel
sheets, suitable for the above-described applications, are
manufactured by forming hot rolled steel sheets having
corresponding target thicknesses, performing blanking,
bending and press-forming on the hot rolled steel sheets to
obtain desired shapes, and finally performing a heat
treatment process on the hot rolled steel sheets to impart
high hardness to the hot rolled steel sheets.
High carbon hot rolled steel sheets may require
excellent material uniformity because high material
deviations in the high carbon hot rolled steel sheets not
only worsen dimensional precision in a forming process and
cause defects during processing, but also lead to nonuniform
structure distribution even in a final heat
treatment process.
Although various inventions have been suggested to
improve the formability of high carbon hot rolled steel
sheets, most inventions have only focused on controlling
the sizes and distribution of carbides in microstructures
after a cold rolling process and an annealing process, no
invention regarding the formability and heat treatment
uniformity of hot rolled steel sheets has been proposed.
More specifically, patent document 1, related to the
formability of a high carbon annealed steel sheet obtained
after performing cold rolling and annealing discloses that
the formability of the steel sheet is improved if a carbide
distribution, in which an average carbide particle diameter
is 1 um or less and a fraction of carbides having a
particle diameter of 0.3 um or less is 20% or less, is
obtained by controlling annealing conditions. However,
there is no mention of the formability of a hot rolled
steel sheet. Moreover, carbides do not necessarily have to
be formed to have a particle diameter of 1 um or less after
annealing a hot rolled steel sheet having excellent
formability.
Further, even in patent document 2 in which a ferrite
particle diameter of 5 um or more and a carbide particle
diameter standard deviation of 0.5 or less are prescribed
by properly controlling annealing conditions, there is no
mention of hot rolled structure, and a hot rolled steel
sheet having excellent formability does not necessarily
have to maintain the same carbide distribution as in the
above-mentioned invention after being treated under
ordinary annealing conditions.
Patent document 3 discloses that fine blanking
workability increases when ferrite grain sizes satisfy a
range of 10 um to 20 um while maintaining fractions of
pearlite and cementite to levels of 10% or less. Although
the disclosed invention specifies the controlling of the
microstructure of an annealed steel sheet, the formability
of the disclosed invention is far from that of a hot rolled
structure. On the contrary, as a method of improving the
formability of a hot rolled structure, if the formation of
ferrite is suppressed and a uniform phase distribution is
obtained, material deviations may be minimized.
Patent document 4 suggests a hot rolled structureprescribing
method of obtaining a ferrite fraction of about
10% or less by adjusting a ferrite particle diameter to be
6 um or less after annealing and a carbide particle
diameter to be within the range of 0.1 um to 1.2 um after
annealing, and cooling a hot rolled steel sheet at a rate
of 120°C per second or higher. However, the disclosed
invention is for improving stretch-flangeability of an
annealed steel sheet, and a fast cooling rate of 120 °C/sec
is not always required to form a hot rolled steel sheet
having a ferrite fraction of about 10% or less.
Patent document 5 suggests a method of improving the
formability of an annealed steel sheet by adjusting
fractions of pro-eutectoid ferrite and pearlite to be 5% or
less respectively, forming a high carbon bainite structure
having a bainite fraction of 90% or more, and forming a
structure in which fine cementite is distributed after
annealing. However, the disclosed invention is only for
improving the formability of an annealed steel sheet by
finely adjusting an average carbide size to be 1 um or less
and a grain size to be 5 um or less, but is not related to
the formability of a hot rolled steel sheet.
(Patent document 1) Japanese Patent Application Laidopen
Publication No. 2005-344194
(Patent document 2) Japanese Patent Application Laidopen
Publication No. 2005-344196
(Patent document 3) Japanese Patent Application Laidopen
Publication No. 2001-140037
(Patent document 4) Japanese Patent Application Laidopen
Publication No. 2006-063394
(Patent document 5) Korean Patent Application Laidopen
Publication No. 2007-0068289
[Disclosure]
[Technical Problem]
In order to solve the above-described problems, an
aspect of the present disclosure may provide a high carbon
hot rolled steel sheet capable of securing excellent
material uniformity by controlling kinds and contents of
alloying elements and structures thereof, and a method for
manufacturing the high carbon hot rolled steel sheet.
[Technical Solution]
According to an aspect of the present disclosure, a
high carbon hot rolled steel sheet having excellent
material uniformity may include 0.2% by weight to 0.5% by
weight of carbon (C) , more than 0% by weight to 0.5% by
weight of silicon (Si), 0.2% by weight to 1.5% by weight of
manganese (Mn), more than 0% by weight to 1.0% by weight of
chromium (Cr), more than 0% by weight to 0.03% by weight of
phosphorous (P), more than 0% by weight to 0.015% by weight
of sulfur (S), more than 0% by weight to 0.05% by weight of
aluminum (Al), 0.0005% by weight to 0.005% by weight of
boron (B), 0.005% by weight to 0.05% by weight of titanium
(Ti), more than 0% by weight to 0.01% by weight of nitrogen
(N) , and the balance of iron (Fe) and unavoidable
impurities, wherein the high carbon hot rolled steel sheet
may include a pearlite phase having an area fraction of 95%
or more.
According to another aspect of the present disclosure,
a method for manufacturing a high carbon hot rolled steel
sheet having excellent material uniformity may include:
manufacturing a high carbon steel slab including 0.2% by
weight to 0.5% by weight of C, more than 0% by weight to
0.5% by weight of Si, 0.2% by weight to 1.5% by weight of
Mn, more than 0% by weight to 1.0% by weight of Cr, more
than 0% by weight to 0.03% by weight of P, more than 0% by
weight to 0.015% by weight of S, more than 0% by weight to
0.05% by weight of Al, 0.0005% by weight to 0.005% by
weight of B, 0.005% by weight to 0.05% by weight of Ti,
more than 0% by weight to 0.01% by weight of N, and the
balance of Fe and unavoidable impurities; reheating the
slab at a temperature of 1,100°C to 1,300°C; hot rolling
the reheated slab such that a finishing hot rolling
temperature is in a temperature range of 800°C to 1,000°C;
cooling the hot rolled steel sheet at a cooling rate CR1
satisfying the following formula 1 or 1' until a
temperature of the hot rolled steel sheet reaches 550°C
from the finishing hot rolling temperature; and coiling the
cooled steel sheet at a coiling temperature CT satisfying
the following formula 2,
[Formula 1]
Condi < CRl(°C/sec) < 100,
Condi = a larger value between 175 - 300*C(wt.%)
30xMn(wt.%) - 100xCr(wt.%) and 10
[Formula 1']
Condi < CRl(°C/sec) < Condi + 20,
Condi = a larger value between 175 - 300*C(wt.%)
30xMn(wt.%) - 100xCr(wt.%) and 10
[Formula 2]
Cond2 < CT(°C) < 650,
Cond2 = 640 - 237xC(wt.%) - 16.5xMn(wt.%)
8.5xCr(wt.%).
[Advantageous Effects]
According to embodiments of the present disclosure, a
high carbon hot rolled steel sheet having excellent
material uniformity and a method for manufacturing the same
are provided, wherein elements, microstructure, and process
conditions of the steel sheet are controlled to achieve
excellence in material uniformity among hot rolled
structures of the high carbon hot rolled steel sheet,
thereby guaranteeing excellent dimensional precision of
parts after formation, preventing defects during processing,
and guaranteeing uniform structure and hardness
distribution even after a final heat treatment process.
[Description of Drawings]
FIG. 1 is a graph illustrating transformation curves
of a hot rolled steel sheet with respect to a cooling rate.
[Best Mode]
The present inventors have conducted significant
research into devising a steel material having excellent
material uniformity that is a property required in a high
carbon hot rolled steel sheet. Using the results of the
research, the present inventors completed the present
disclosure after confirming that a steel material having
excellent material uniformity can be provided by precisely
controlling alloy element contents and process conditions,
particularly cooling conditions and coiling conditions as
functions of alloy elements, to obtain a pearlite structure
of 95% or more.
Hereinafter, a high carbon hot rolled steel sheet
having excellent material uniformity as an aspect of the
present disclosure will be described.
A high carbon hot rolled steel sheet according to an
embodiment of the present disclosure may include 0.2% by
weight to 0.5% by weight of C, more than 0% by weight to
0.5% by weight of Si, 0.2% by weight to 1.5% by weight of
Mn, more than 0% by weight to 1.0% by weight of Cr, more
than 0% by weight to 0.03% by weight of P, more than 0% by
weight to 0.015% by weight of S, more than 0% by weight to
0.05% by weight of Al, 0.0005% by weight to 0.005% by
weight of B, 0.005% by weight to 0.05% by weight of Ti,
more than 0% by weight to 0.01% by weight of N, and the
balance of Fe and unavoidable impurities.
The high carbon hot rolled steel sheet may preferably
include 0.2% by weight to 0.4% by weight of C.
Further, the high carbon hot rolled steel sheet may
preferably include 0.4% by weight to 0.5% by weight of C.
Hereinafter, in the embodiment of the present
disclosure, reasons for specifying elements of the high
carbon hot rolled steel sheet as described above will be
described in detail. In the following description, the
contents of constitutional elements are given in percent by
weight (wt.%).
C: 0.2% by weight to 0.5% by weight
Carbon (C) is an element required for securing
hardenability during heat treatment and hardness after heat
treatment, and C is preferably contained in an amount of
0.2% by weight or more to secure hardenability during heat
treatment and hardness after heat treatment. However, if C
is contained in an amount of more than 0.5% by weight, it
may be difficult to obtain excellent material uniformity as
intended in the present disclosure because a very high hot
rolling hardness is maintained to result in an increase in
the absolute values of material deviations and
deterioration of formability.
If C is contained in an amount range of 0.2% by
weight to 0.4% by weight, since the steel sheet is soft
before a final heat treatment process, forming processes
such as pulling-out, forging, and drawing are easily
performed for manufacturing complicated machine parts.
Further, if C is contained in an amount range of 0.4%
by weight to 0.5% by weight, although processing is
relatively difficult in forming processes, abrasion
resistance and fatigue resistance of the high carbon hot
rolled steel sheet are excellent due to a high degree of
hardness of the steel sheet after final heat treatment, and
thus the steel sheet may be usefully used for manufacturing
groups of machine parts operating in high load conditions.
Si: more than 0% by weight to 0.5% by weight
Silicon (Si) is an element added along with Al for
the purpose of deoxidation. If Si is added, the adverse
effect of producing red scale may be suppressed, while
ferrite may be stabilized to result in increases of
material deviations. Therefore, the upper limit of the
content of C may preferably be set to 0.5% by weight.
Mn: 0.2% by weight to 1.5% by weight
Manganese (Mn) is an element contributing to
increasing hardenability and securing hardness after heat
treatment. If the content of Mn is very low to be within
the range of less than 0.2% by weight, the steel sheet may
become very vulnerable because a coarse FeS is formed. On
the other hand, if the content of Mn is greater than 1.5%
by weight, alloying costs may be increased, and residual
austenite may be formed.
Cr: more than 0% by weight to 1.0% by weight
Chromium (Cr) is an element contributing to
increasing hardenability and securing hardness after heat
treatment. Further, Cr contributes to improving
formability of the steel sheet by finely adjusting a
pearlite lamellar spacing. When Cr is contained in an
amount of more than 1.0% by weight, alloying costs are
increased, and phase transformation is excessively delayed
such that it may be difficult to obtain a sufficient phase
transformation when cooling the steel sheet in a run out
table (ROT). Therefore, the upper limit of the content of
Cr may preferably be set to be 0.1% by weight.
P: more than 0% by weight to 0.03% by weight
Phosphorous (P) is an impurity element in the steel
sheet. It may be preferable to set the upper limit of the
content of P to be 0.03% by weight. If P is contained in
an amount of more than 0.03% by weight, the weldability of
the steel sheet may be deteriorated, and the steel sheet
may become brittle.
S: more than 0% by weight to 0.015% by weight
Like phosphorous, sulfur (S) is an impurity element
worsening the ductility and weldability of the steel sheet.
Therefore, it may be preferable to set the upper limit of
content of S to be 0.015% by weight. If S is contained in
an amount of more than 0.015% by weight, the possibility of
lowering the ductility and weldability of the steel sheet
is increased.
Al: more than 0% by weight to 0.05% by weight
Aluminum (Al) is an element for deoxidation and
functions as a deoxidizer during a steelmaking process.
The necessity of containing Al in an amount of more than
0.05% by weight is low, and nozzles may be clogged during a
continuous casting process if Al is contained in an
excessive amount. Therefore, it may be preferable to set
the upper limit of the content of Al to be 0.05% by weight.
B: 0.0005% by weight to 0.005% by weight
Boron (B) is an element greatly contributing to
securing hardenability of the steel sheet and thus may be
added in an amount of 0.0005% by weight or more to obtain a
hardenability-reinforcing effect. However, if B is added
in an excessive amount, boron carbide may be formed on
grain boundaries to form nucleus forming sites and rather
worsen hardenability. Therefore, it may be preferable to
set the upper limit of the content of B to be 0.005% by
weight.
Ti: 0.005% by weight to 0.05% by weight
Since titanium (Ti) forms TiN by reacting with
nitrogen (N) , titanium (Ti) is added as an element for
suppressing the formation of BN, so-called boron protection
If the content of Ti is less than 0.005% by weight,
nitrogen contained in the steel sheet may not be
effectively fixated. On the other hand, if the content of
Ti is excessive, the steel sheet may become vulnerable due
to the formation of coarse TiN. Therefore, the content of
Ti may be adjusted to be within a range in which nitrogen
contained in the steel sheet is sufficiently fixed.
Therefore, it may be preferable to set the upper limit of
Ti to be 0.05% by weight.
N: more than 0% by weight to 0.01% by weight
Nitrogen (N) is an element that contributes to the
hardness of a steel material, but N is an element that is
difficult to be controlled. If N is contained in an amount
of more than 0.01% by weight, brittleness may be greatly
increased, and B contributing to hardenability may be
consumed in the form of BN by surplus N remaining after the
formation of TiN. Therefore, it may be preferable to set
the upper limit of N to be 0.01% by weight.
The high carbon hot rolled steel sheet of the
embodiment of the present disclosure includes Fe and
unavoidable impurities in addition to the above-described
constituent elements.
It is required to additionally limit the type and
shape of the internal structure of the steel sheet having
the above-described components so that the steel sheet may
become a high carbon hot rolled steel sheet having
excellent material uniformity.
Namely, according to an embodiment of the present
disclosure, it may be preferable that the microstructure of
the high carbon hot rolled steel sheet may have pearlite in
an area fraction of 95% or more.
If the fraction of pearlite phase is less than 95%,
i.e., if a pro-eutectoid ferrite phase, a bainite phase or
a martensite phase is formed to a fraction of 5% or more,
the material deviation of the steel sheet may be increase,
and thus it may be difficult to impart material uniformity
to the steel sheet.
Further, it may be preferable that the area fraction
of pearlite phase be 75% or more before coiling. The
pearlite phase imparts material uniformity to the hot
rolled steel sheet. If the area fraction of pearlite is
75% or more before coiling, pearlite colonies surrounded by
tilt grain boundaries having a misorientation angle of 15°
or more may be formed to an average size of 15 um or less,
and thus a fine and uniform structure may be obtained.
Accordingly, the fine and uniform structure enables the hot
rolled steel sheet to have a more uniform material
deviation.
If the pearlite phase formed before coiling has an
insufficient fraction of less than 75%, a large amount of
latent heat of transformation is accumulated in a coil
after coiling such that partial spheroidizing of a pearlite
structure proceeds to cause a high hardness deviation and
coarsen a lamella structure due to heat of transformation.
Therefore, a low hardness structure is partially formed.
Further, a ferrite phase or a bainite phase may be formed
during transformation.
As described above, according to the present
disclosure, most pearlite transformation occurs in a
relatively low temperature range before coiling such that a
small average interlamellar spacing of 0.1 um or less may
be obtained in the final microstructure of the steel sheet,
and thus the material uniformity of the steel sheet may
further be improved.
In order to manufacture a high carbon hot rolled
steel sheet satisfying the purpose of the embodiment of the
present disclosure as described above, an example devised
by the present inventors will be described hereinafter in
detail. However, the embodiments of the present disclosure
are not limited to the example.
A method for manufacturing a high carbon hot rolled
steel sheet according to an embodiment of the present
disclosure may generally include heating a steel slab
satisfying the above-described element system and
microstructure, rolling the heated slab, performing
finishing rolling on the rolled slab in a temperature range
of 800°C to 1,000°C, and cooling and coiling the finish
rolled steel sheet.
Hereinafter, detailed conditions for the respective
processes will be described.
Reheating: 1,100°C to 1,300°C
Since the heating of the slab is a heating process
for smoothly performing a succeeding rolling process and
sufficiently obtaining target physical properties of a
steel sheet, the heating process is carried out within a
proper temperature range to obtain target physical
properties.
When reheating the slab, there is a problem that a
hot rolling load is rapidly increased if the heating
temperature is less than 1,100°C. On the other hand, if
the heating temperature is higher than 1,300°C, an
increased amount of scale may be on the surface of the slab
to increase the amount of material loss and heating costs.
Rolling conditions
When the reheated slab is hot-rolled to form a steel
sheet, the temperature of finish hot rolling is set to be
within the range of 800°C to 1,000°C.
During the hot rolling, a rolling load may be greatly
increased if the finish hot rolling temperature is lower
than 800°C. On the other hand, if the finish hot rolling
temperature is higher than 1,000°C, the structure of the
steel sheet may be coarsened and rendered brittle, and a
thick layer of scale may be formed on the steel sheet to
worsen the surface quality of the steel sheet.
Cooling conditions
When cooling the hot rolled steel sheet, the hot
rolled steel sheet is cooled in a water-cooling ROT until
the temperature of the steel sheet reaches 550°C from the
finish hot rolling temperature.
At this time, the steel sheet is cooled at a cooling
rate CRl lower than 100 °C/sec but equal to or higher than
Condi as represented by Formula 1 below. If the cooling
rate CRl is lower than the Condi calculated by Formula 1
below, a ferrite phase is formed during cooling, resulting
in a hardness difference of 30 Hv or greater. On the other
hand, if the cooling rate CRl exceeds 100 °C/sec, the shape
of the steel sheet deteriorates markedly.
In the embodiment of the present disclosure, Boron
(B) is added, and the contents of C, Mn and Cr are
controlled. Therefore, a target degree of material
uniformity may be obtained even at a usual cooling rate.
[Formula 1]
Condi < CR1 (°C/sec) < 100,
Condi = a larger value between 175 - 300*C (wt.%)
30xMn (wt.%) - lOOxCr (wt.%) and 10
Further, the cooling rate CR1 may be adjusted to be
within a range of not less than Condi to not more than
Condl+20 °C/sec as represented by Formula 1' below. If the
cooling rate CR1 is controlled as represented by Formula 1',
the formation of a ferrite phase is prevented, and along
with this the temperature of the steel sheet is not far
deviated from a nose temperature of phase transformation to
facilitate pearlite transformation in the subsequent
process.
[Formula 1']
Condi < CR1 (°C/sec) < Condi + 20,
Condi = a larger value between 175 - 300*C (wt.%)
30xMn (wt.%) - lOOxCr (wt.%) and 10
Coiling conditions
After the steel sheet passes through the watercooling
ROT, the steel sheet is coiled into a roll. At
this time, the temperature of the steel sheet is adjusted
to a coiling temperature CT satisfying Formula 2 by means
of recuperative heat or additional cooling.
If the coiling temperature exceeds 650°C, a ferrite
phase may be formed in a retention stage after the coiling
process although manufacturing conditions such as the
above-described cooling conditions are satisfied. On the
other hand, if the coiling temperature is less than Cond2
calculated by Formula 2, a bainite phase may be formed to
increase the hardness difference of the steel sheet
[Formula (2)]
Cond2 < CT (°C) < 650,
Cond2 = 640 - 237xC(wt.%) - 16.5xMn(wt.%) - 8.5xCr
(wt.%)
When manufacturing a high carbon hot rolled steel
sheet, constituent elements are controlled, and at the same
time, the rate of cooling and the temperature of coiling
are controlled as shown in FIG. 1. Then, a pearlite phase
may be formed to an area fraction of 75% or more prior to a
coiling process. If a pearlite phase is formed to an area
fraction of 75% or more before a coiling process, the area
fraction of the pearlite phase in the steel sheet may
become 95% or more after the coiling process.
Further, manufacturing conditions such as constituent
elements and cooling rates are controlled so as to form
pearlite colonies having an average size of 15 um or less
and adjust an average interlamellar spacing to be 0.1 um or
less, thereby reducing a hardness difference between
microstructures of the hot rolled steel sheet to 30 HV or
less and imparting excellent material uniformity to the hot
rolled steel sheet. At this time, the hardness difference
is defined as a difference between a 95% hardness level and
a 5% hardness level when a maximum hardness value and a
minimum hardness value measured in the hot rolled steel
sheet are set as 100% and 0% respectively.
The hot rolled steel sheet manufactured by the method
of the embodiment of the present disclosure may be used
without performing additional processes thereon, or may be
used after performing processes such as an annealing
process thereon.
Hereinafter, the embodiments of the present
disclosure will be described in more detail through
examples. However, the embodiments of the present
disclosure are not limited thereto.
[Mode for Invention]
(Examples)
After steels having alloy compositions as represented
by Table 1 below were vacuum melted into 30 Kg ingots, a
sizing rolling process was performed on the vacuum melted
ingots to manufacture slabs having a thickness of 30 mm.
After the slabs were reheated at 1,200°C for one hour, a
hot rolling process was carried out on the reheated slabs,
wherein a finish hot rolling process was conducted on the
reheated slabs at 900°C to manufacture hot rolled steel
sheets having a final thickness of 3 mm.
After the finish hot rolling process, the steel
sheets were cooled to 550°C at cooling rates CRl in a
water-cooling ROT. The cooled steel sheets were charged
into a furnace that had already been heated to a target
coiling temperature, and retained in the furnace for one
hour. Then, after furnace cooling, an experimental hotrolling
coiling process was performed on the steel sheets.
At that time, cooling rates CRl and coiling temperatures CT
shown in Table 2 below were used for the steel sheets.
Further, microstructures of final hot rolled steel
sheets obtained by completing the coiling process were
analyzed, and Vickers hardness values of the final hot
rolled steel sheets were measured as shown in Table 2 below
At that time, the hardness values were measured in Vickers
hardness using a 500 g weight, and a hardness difference
was defined as a difference between a 95% hardness level
and a 5% hardness level when the maximum hardness value and
the minimum hardness value among hardness values measured
by repeating the measurement 30 or more times were set as
100% and 0% respectively.
(In Table 2, the remainders except for pearlite fractions
are consisted of pro-eutectoid ferrite)
As results of measurement, in the case of Comparative
Examples C and L using Comparative Steels C and L of Table
1 in which contents of boron (B) do not satisfy ranges
provided by the embodiments of the present disclosure,
although manufacturing conditions such as cooling
conditions and coiling conditions satisfy the embodiments
of the present disclosure, pearlite fractions were 83% and
87% respectively, i.e., the pearlite fractions do not
satisfy ranges suggested by the embodiments of the present
disclosure, and hardness deviations of 30 Hv or more were
also measured.
Further, in the case of Comparative Example I of
Table 2 in which coiling temperature conditions do not
satisfy the embodiments of the present disclosure, it can
be seen that, as ferrite phase are formed at high coiling
temperatures, pearlite fractions are 95% or less, and
hardness deviations are 79 Hv, i.e., material uniformity of
the steel sheets are inferior.
On the other hand, particularly in the case of
Inventive Example F among Inventive Examples satisfying
both composition ranges and manufacturing conditions
provided by the embodiments of the present disclosure, a
pearlite fraction was 99%, and a hardness deviation of 16
Hv was also measured.
Further, as results of measuring interlamellar
spacings of Inventive Examples, the measured interlamellar
spacings were all 0.1 um or less. Therefore, it was
confirmed that very fine structures were formed.
It can be seen through the above-described results
that a high strength hot rolled steel sheet having
excellent material uniformity may be obtained when both
composition ranges and manufacturing conditions provided by
the embodiments of the present disclosure are satisfied.
We Claim:
[Claim 1]
A high carbon hot rolled steel sheet having excellent
material uniformity comprising 0.2% by weight to 0.5% by
weight of carbon (C) , more than 0% by weight to 0.5% by
weight of silicon (Si), 0.2% by weight to 1.5% by weight of
manganese (Mn), more than 0% by weight to 1.0% by weight of
chromium (Cr), more than 0% by weight to 0.03% by weight of
phosphorous (P), more than 0% by weight to 0.015% by weight
of sulfur (S), more than 0% by weight to 0.05% by weight of
aluminum (Al), 0.0005% by weight to 0.005% by weight of
boron (B), 0.005% by weight to 0.05% by weight of titanium
(Ti), more than 0% by weight to 0.01% by weight of nitrogen
(N) , and the balance of iron (Fe) and unavoidable
impurities,
wherein the high carbon hot rolled steel sheet
comprises a pearlite phase having an area fraction of 95%
or more.
[Claim 2]
The high carbon hot rolled steel sheet having
excellent material uniformity of claim 1, wherein the
pearlite phase has a colony size of 15 um or less and an
average interlamellar spacing of 0.1 um or less.
[Claim 3]
The high carbon hot rolled steel sheet having
excellent material uniformity of claim 1, wherein the hot
rolled steel sheet has a hardness difference of 30 HV or
less between a 95% hardness level and a 5% hardness level
when a maximum hardness value and a minimum hardness value
of the hot rolled steel sheet are set as 100% and 0%
respectively.
Claim 4
The high carbon hot rolled steel sheet having
excellent material uniformity of claim 1, wherein 75% or
more of the pearlite phase is formed prior to a coiling
process.
Claim 5
The high carbon hot rolled steel sheet having
excellent material uniformity of claim 1, comprising 0.2%
by weight to 0.4% by weight of C.
Claim 6
The high carbon hot rolled steel sheet having
excellent material uniformity of claim 1, comprising 0.4%
by weight to 0.5% by weight of C.
Claim 7
A method for manufacturing a high carbon hot rolled
steel sheet having excellent material uniformity
comprising:
29
manufacturing a high carbon steel slab comprising
0.2% by weight to 0.5% by weight of C, more than 0% by
weight to 0.5% by weight of Si, 0.2% by weight to 1.5% by
weight of Mn, more than 0% by weight to 1.0% by weight of
Cr, more than 0% by weight to 0.03% by weight of P, more
than 0% by weight to 0.015% by weight of S, more than 0% by
weight to 0.05% by weight of Al, 0.0005% by weight to
0.005% by weight of B, 0.005% by weight to 0.05% by weight
of Ti, more than 0% by weight to 0.01% by weight of N, and
the balance of Fe and unavoidable impurities;
reheating the slab at a temperature of 1,100°C to
1, 3 0 0 ° C ;
hot rolling the reheated slab such that a finishing
hot rolling temperature is in a temperature range of 800°C
to 1,000°C;
cooling the hot rolled steel sheet at a cooling rate
CR1 satisfying the following formula 1 until a temperature
of the hot rolled steel sheet reaches 550°C from the
finishing hot rolling temperature,
[Formula 1]
Condi < CRl(°C/sec) < 100,
Condi = a larger value between 175 - 300*C(wt.%)
30xMn(wt.%) - 100xCr(wt.%) and 10; and
coiling the cooled steel sheet at a coiling
temperature CT satisfying the following formula 2,
[Formula 2]
Cond2 < CT (°C) < 650,
Cond2 = 640 - 237xC (wt.%) - 16.5xMn (wt.%)
8.5xCr(wt.%).
[Claim 8]
A method for manufacturing a high carbon hot rolled
steel sheet having excellent material uniformity
comprising:
manufacturing a high carbon steel slab comprising
0.2% by weight to 0.5% by weight of C, more than 0% by
weight to 0.5% by weight of Si, 0.2% by weight to 1.5% by
weight of Mn, more than 0% by weight to 1.0% by weight of
Cr, more than 0% by weight to 0.03% by weight of P, more
than 0% by weight to 0.015% by weight of S, more than 0% by
weight to 0.05% by weight of Al, 0.0005% by weight to
0.005% by weight of B, 0.005% by weight to 0.05% by weight
of Ti, more than 0% by weight to 0.01% by weight of N, and
the balance of Fe and unavoidable impurities;
reheating the slab at a temperature of 1,100°C to
1, 3 0 0 ° C ;
hot rolling the reheated slab such that a finishing
hot rolling temperature is in a temperature range of 800°C
to 1,000°C;
cooling the hot rolled steel sheet at a cooling rate
CRl satisfying the following formula 1' until a temperature
of the hot rolled steel sheet reaches 550°C from the
finishing hot rolling temperature,
[Formula 1']
Condi < CRl (°C/sec) < Condl+20,
Condi = a larger value between 175-300xC (wt.%)
30xMn (wt.%) - lOOxCr (wt.%) and 10; and
coiling the cooled steel sheet at a coiling
temperature CT satisfying the following formula 2:
[Formula 2]
Cond2 < CT (°C) < 650,
Cond2 = 640 - 237xC (wt.%) - 16.5xMn (wt.%)
8.5xCr(wt.%).