【Invention Title】
HEAT TREATABLE STEEL, PRODUCT FORMED THEREOF HAVING
ULTRA HIGH STRENGTH AND EXCELLENT DURABILITY, AND METHOD
FOR MANUFACTURING SAME
【Technical Field】
The present disclosure relates to heat treatable
steel for automotive components or the like, and, more
particularly, to heat treatable steel, a product formed of
the heat treatable steel and having ultra high strength and
excellent durability, and a method for manufacturing the
product.
【Background Art】
Safety regulations for protecting vehicle passengers
and fuel efficiency regulations for protecting the
environment have recently been tightened, and thus there is
increasing interest in techniques for improving the
stiffness of automobiles and reducing the weight of
automobiles.
For example, components such as stabilizer bars or
tubular torsion beam axles of automotive chassis are
required to have both stiffness and durability because they
are used to support the weight of vehicles and are
constantly subjected to fatigue loads during driving.
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Moreover, the weight of vehicles has been gradually
increased because of the recent increasing use of comfort
components, and thus test conditions for guaranteeing
durability have been tightened. Accordingly, the
application of ultra high strength steels to heat treatable
steel components has been increase
ed for performance improvements and weight reduction.
The fatigue life of steel sheets for automotive
components is closely related with the yield strength and
elongation of the steel sheets, and the fatigue life of
heat treatable steel sheets is affected by surface
decarburization occurring during heat treatment processes
or surface scratches formed during steel pipe manufacturing
processes.
In particular, the influence of these factors
increases in proportion to the strength of steel, and thus
methods for manufacturing high strength automotive
components having a tensile strength grade of 1500 MPa or
greater, while solving problems arising during processes of
forming ultra high strength steels, have been proposed.
Examples of such methods include a hot press forming
method, in which high-temperature forming and die quenching
are performed simultaneously, and a post heat treatment
method in which cold forming, heating to an austenite
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region, and quenching by contact with a cooling medium
instead of contact with a die, are performed sequentially.
However, martensite obtained after quenching has low
toughness even though it has high strength. Thus, to
improve toughness, a method of performing a tempering
process after a quenching process has been commonly used.
The degree of strength obtainable by the hot press
forming method or the post heat treatment method is various,
and a method of manufacturing automotive components having
a tensile strength grade of 1500 MPa, using a heat treatedtype
steel pipe containing 22MnB5 or boron, was proposed in
the early 2000s.
Such automotive components are manufactured by producing an
electric resistance welding (ERW) steel pipe using a hotrolled
or cold-rolled coil, cutting the ERW steel pipe in
lengths, and heat treating the cut ERW steel pipe. That is,
such automotive components are manufactured by producing an
ERW steel pipe through a steel sheet slitting process,
performing a solution treatment on the ERW steel pipe by
heating the ERW steel pipe to an austenite region higher
than or equal to Ac3, and extracting the ERW steel pipe and
hot forming the ERW steel pipe using a press equipped with
a cooling device such that die quenching is performed
simultaneously with the hot forming. In some cases, after
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the hot forming, hot-formed products may be taken out from
a die and may then be quenched using a cooling medium.
In other methods, ultra high strength components
having a strength of 1500 MPa or greater and martensite or
a mixed phase of martensite and bainite as a final
microstructure may be manufactured by cold forming a steel
sheet in a shape similar to a component shape, performing a
solution treatment on the cold-formed steel sheet by
heating the cold-formed steel sheet to an austenite region
higher than or equal to Ac3, and extracting the heated
steel sheet and quenching the heated steel sheet using a
cooling medium, or such ultra high strength components may
be manufactured by hot forming a steel sheet in a final
product shape by using a die, and quenching the hot-formed
steel sheet by bringing the hot-formed steel sheet into
contact with a cooling medium.
In addition, a tempering process may be performed to
increase the durability life and toughness of the
components quenched, as described above.
In general, a tempering process is performed within a
temperature range of 500°C to 600°C and, as a result of the
tempering process, martensite transforms to ferrite, in
which cementite is precipitated. Thus, although tensile
strength decreases and a yield ratio increases to a range
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of 0.9 or greater, uniformity and total elongation are
improved as compared to a quenched state.
As the weight of automobiles increases, there is an
increasing need for higher-grade components made by heat
treated-type steel pipes.
In a strengthening method, the content of manganese
(Mn) and the content of chromium (Cr) in steel are fixed to
a range of 1.2% to 1.4% and to a range of 0.1% to 0.3%,
similar to the contents of Mn and Cr in heat treatable
steel of the related art containing boron (B), and the
content of carbon (C) in the steel is increased as a result
of considering post-heat treatment strength of the steel.
Based on the strengthening method, however, fatigue
cracking and sensitivity to crack propagation increase
because of an increase in strength, and thus the durability
of steel, that is, the fatigue life of steel, is not
increased in proportion to the increase in the strength of
the steel.
【Disclosure】
【Technical Problem】
An aspect of the present disclosure may provide heat
treatable steel for manufacturing a formed product having
ultra high strength and excellent durability.
An aspect of the present disclosure may also provide
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a formed product having ultra high strength and excellent
durability.
An aspect of the present disclosure may also provide
a method for manufacturing a formed product having ultra
high strength and excellent durability.
【Technical Solution】
According to an aspect of the present disclosure,
heat treatable steel may include, by wt%, carbon (C): 0.22%
to 0.42%, silicon (Si): 0.05% to 0.3%, manganese (Mn): 1.0%
to 1.5%, aluminum (Al): 0.01% to 0.1%, phosphorus (P):
0.01% or less (including 0%), sulfur (S): 0.005% or less,
molybdenum (Mo): 0.05% to 0.3%, titanium (Ti): 0.01% to
0.1%, chromium (Cr): 0.05% to 0.5%, boron (B): 0.0005% to
0.005%, nitrogen (N): 0.01% or less, and a balance of iron
(Fe) and inevitable impurities, wherein Mn and Si in the
heat treatable steel may satisfy Formula 1, below, and Mo/P
in the heat treatable steel may satisfy Formula 2, below:
[Formula 1]
Mn/Si ≥ 5
[Formula 2]
Mo/P ≥ 15
The heat treatable steel may further include at least
one or two selected from the group consisting of niobium
(Nb): 0.01% to 0.07%, copper (Cu): 0.05% to 1.0%, and
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nickel (Ni): 0.05% to 1.0%.
The heat treatable steel may have a microstructure
including ferrite and pearlite, or a microstructure
including ferrite, pearlite, and bainite.
The heat treatable steel may include one selected
from the group consisting of a hot-rolled steel sheet, a
pickled and oiled steel sheet, and a cold-rolled steel
sheet.
The heat treatable steel may include a steel pipe.
According to another aspect of the present disclosure,
a formed product having ultra high strength and excellent
durability may include, by wt%, carbon (C): 0.22% to 0.42%,
silicon (Si): 0.05% to 0.3%, manganese (Mn): 1.0% to 1.5%,
aluminum (Al): 0.01% to 0.1%, phosphorus (P): 0.01% or less
(including 0%), sulfur (S): 0.005% or less, molybdenum
(Mo): 0.05% to 0.3%, titanium (Ti): 0.01% to 0.1%, chromium
(Cr): 0.05% to 0.5%, boron (B): 0.0005% to 0.005%, nitrogen
(N): 0.01% or less, and a balance of iron (Fe) and
inevitable impurities, wherein Mn and Si in the formed
product may satisfy Formula 1, below, Mo/P in the formed
product may satisfy Formula 2, below, and the formed
product may have a tempered martensite matrix,
[Formula 1]
Mn/Si ≥ 5
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[Formula 2]
Mo/P ≥ 15
According to another aspect of the present disclosure,
a method for manufacturing a formed product having ultra
high strength and excellent durability may include:
preparing the heat treatable steel; forming the heat
treatable steel to obtain a formed product; and tempering
the formed product.
The forming of the heat treatable steel may be
performed by heating the heat treatable steel and then hot
forming and cooling the heat treatable steel simultaneously,
using a cooling die.
The forming of the heat treatable steel may be
performed by heating the heat treatable steel, hot forming
the heat treatable steel, and cooling the heat treatable
steel, using a cooling medium.
The forming of the heat treatable steel may be
performed by cold forming the heat treatable steel, heating
the heat treatable steel to an austenite temperature range
and maintaining the heat treatable steel within the
austenite temperature range, and cooling the heat treatable
steel, using a cooling medium.
The above-described aspects of the present disclosure
do not include all aspects or features of the present
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disclosure. Other aspects or features, and effects of the
present disclosure, will be clearly understood from the
following descriptions of exemplary embodiments.
【Advantageous Effects】
The present disclosure provides heat treatable steel
for manufacturing a formed product having ultra high
strength and excellent durability, and a product formed of
the heat treatable steel and having ultra high strength and
excellent durability. Thus, the heat treatable steel or the
formed product may be used to manufacture heat treated-type
components of automotive chassis or frames to reduce the
weight of the components and improve the durability of the
components.
【Best Mode】
Embodiments of the present disclosure will now be
described in detail.
In general, the tensile strength above 1500 MPa may
be obtained by 22MnB5 steel. In order to get relatively
high tensile strength, it is necessary to increase the
carbon (C) content of steel. Boron-added heat treatable
steel, for example, such as 25MnB5 or 34MnB5, may be used.
Boron-added heat treatable steel may include silicon
(Si): 0.2% to 0.4%, manganese (Mn): 1.2% to 1.4%,
phosphorus (P): 0.01% to 0.02%, and sulfur (S): less than
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0.005%.
However, ultra high strength products formed of such
boron-added heat treatable steel are affected by
segregation of impurities such as P and S in proportion to
the strength thereof, and if the microstructure of the
ultra high strength products is not optimized after a
tempering process, the durability of the ultra high
strength products decreases.
Thus, the inventors have conducted research and
experiments so as to improve the durability of ultra high
strength products formed of boron-added heat treatable
steel and, based on the results of the research and
experiments, the inventors propose the present invention.
That is, according to the present disclosure, the
composition of steel and manufacturing conditions therefor
may be controlled to obtain a formed product having ultra
high strength and excellent durability. In particular, 1)
the content of phosphorus (P), deteriorating bendability or
fatigue characteristics while segregating along austenite
grain boundaries during a heat treatment process, is
adjusted to be as low as possible, and the ratio of
molybdenum (Mo)/phosphorus (P) is controlled, 2) the ratio
of manganese (Mn)/silicon (Si) is controlled to suppress
the formation of oxides in weld zones, and 3) tempering
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conditions are optimized to obtain excellent durability
characteristics.
Hereinafter, steel for forming will be described in
detail according to an aspect of the present disclosure.
According to an aspect of the present disclosure,
heat treatable steel having improved fatigue
characteristics includes, by wt%, carbon (C): 0.22% to
0.42%, silicon (Si): 0.05% to 0.3%, manganese (Mn): 1.0% to
1.5%, aluminum (Al): 0.01% to 0.1%, phosphorus (P): 0.01%
or less (including 0%), sulfur (S): 0.005% or less,
molybdenum (Mo): 0.05% to 0.3%, titanium (Ti): 0.01% to
0.1%, chromium (Cr): 0.05% to 0.5%, boron (B): 0.0005% to
0.005%, nitrogen (N): 0.01% or less, and the balance of
iron (Fe) and inevitable impurities, wherein Mn and Si in
the heat treatable steel satisfy Formula 1, below, and Mo/P
in the heat treatable steel satisfies Formula 2, below:
[Formula 1]
Mn/Si ≥ 5
[Formula 2]
Mo/P ≥ 15
First, reasons for limiting the chemical composition
of the heat treatable steel will be described according to
the present disclosure.
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Carbon (C): 0.22% to 0.42%
Carbon (C) is a key element for increasing the
hardenability of steel sheets used for forming and, after
steel sheets are die quenched or subjected to a quenching
treatment, the strength of the steel sheets is markedly
affected by the content of carbon (C). If the content of C
is less than 0.22%, it may be difficult to obtain a
strength of 1500 MPa or greater. If the content of C is
greater than 0.42%, strength may increase excessively, and
the possibility of stress concentration and cracking in
weld zones increases in a process of manufacturing steel
pipes for hot press forming. Therefore, the content of C
may preferably be limited to 0.42% or less.
To obtain intended tensile strength after quenching
and tempering, the content of C may be adjusted as follows:
0.23% to 0.27% for 1500 MPa grade, 0.33% to 0.37% for 1800
MPa grade, and 0.38% to 0.42% for 2000 MPa grade.
Silicon (Si): 0.05% to 0.3%
In addition to manganese (Mn), silicon (Si) is a key
element determining the quality of weld zones of steel
pipes for forming, rather than improving the hardenability
of steel sheets for forming. As the content of Si increases,
oxides may be more likely to remain in weld zones, and thus
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the process of flattening or expanding pipe may not be
satisfactory. Although a lower Si content is more
advantageous, the content of Si may be adjusted to be
greater than or equal to 0.05%, which is the minimum amount
of Si that may be contained as an impurity. However, if the
content of Si is greater than 0.3%, the quality of weld
zones may become unstable. Thus, preferably, the upper
limit of the content of Si may be set to be 0.3%, and more
preferably, the content of Si may be set to be within the
range of 0.10% to 0.25%.
Mn: 1.0% to 1.5%
Like carbon (C), manganese (Mn) improves the
hardenability of a steel sheet for forming and has the most
decisive effect, next to C, on the strength of the steel
sheet after the steel sheet is die quenched or subjected to
a quenching treatment. However, when a steel pipe for
forming is manufactured by an electric resistance welding
(ERW) method, the welding quality of the steel pipe is
dependent on the weight ratio of Si and Mn. If the content
of Mn is low, the fluidity of molten materials in weld
zones increases and thus oxides are easily removed, but
post-heat treatment strength reduces. Thus, the lower limit
of the content of Mn is set to be 1.0%. On the other hand,
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if the content of Mn is high, although strength increases,
the fluidity of molten materials in weld zones decreases,
and thus oxides are likely to remain in weld zones,
lowering post-heat treatment bendability. Thus, preferably,
the upper limit of the content of Mn may be set to be 1.5%,
and more preferably, the content of Mn may be set to be
within the range of 1.1% to 1.4%.
Formula 1: Mn/Si ≥ 5.0
When a steel pipe for forming is manufactured by an
ERW method, the quality of the steel pipe is dependent on
the content ratio of Mn and Si. If the content of Si
increases and the content ratio of Mn/Si is less than 5,
there is a high possibility that oxides may not be removed
from weld zones but may remain in the weld zones, and in a
flattening test after a steel pipe manufacturing process,
the performance of a steel pipe may be low. Therefore, the
content ratio of Mn/Si may be set to be 5.0 or greater.
Aluminum (Al): 0.01% to 0.1%
Aluminum (Al) is an element functioning as a
deoxidizer.
If the content of Al is less than 0.01%, the
deoxidizing effect may be insufficient, and thus it may be
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preferable that the content of Al be 0.01% or greater.
However, if Al is added excessively, Al forms a precipitate
together with nitrogen (N) during a continuous casting
process, thereby resulting in surface defects and excessive
oxides remaining in weld zones when a steel pipe is
manufactured by the ERW method. Therefore, it may be
preferable that the content of Al be set to be 0.1% or less,
and, more preferably, to 0.02% to 0.06%.
Phosphorus (P): 0.01% or less (including 0%)
Phosphorus (P) is an inevitably added impurity and
has substantially no effect on strength after a forming
process. However, P deteriorates bendability or fatigue
characteristics because P precipitates along austenite
grain boundaries during heating in a solution treatment
before a forming process or during heating after a forming
process. Thus, according to the present disclosure, the
upper limit of the content of P may be set to be 0.01%, and
preferably the content of P may be set to be within the
range of 0.008% or less, and more preferably within the
range of 0.006% or less.
Sulfur (S): 0.005% or less
Sulfur (S) is an impurity contained in the steel. If
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S combines with Mn in the form of elongated sulfides,
cracks are easily formed along a metal flow inside a near
weld region surface during a steel pipe manufacturing
process, and S contained in a steel sheet deteriorates the
toughness of the steel sheet after a cooling or quenching
process. Thus, the content of S may preferably be set to be
0.005% or less. More preferably, the content of S may be
set to be 0.003% or less, and, even more preferably, to
0.002% or less.
Molybdenum (Mo): 0.05% to 0.3%
In addition to chromium (Cr), molybdenum (Mo)
improves the hardenability of a steel sheet and stabilizes
the strength of the steel sheet after quenching. In
addition, Mo is an effective element in widening an
austenite temperature range to include a lower temperature
and reducing segregation of P in steel during annealing in
a hot or cold rolling process and during heating in a
forming process.
If the content of Mo is less than 0.05%, the effect
of improving hardenability or widening an austenite
temperature range may not be obtained. Conversely, if the
content of Mo is greater than 0.3%, even though strength is
increased, it is not economical because the strength
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increasing effect is not high, compared to the amount of Mo
used. Thus, the upper limit of the content of Mo may
preferably be set to be 0.3%.
Mo/P ≥ 15.0
The ratio of Mo/P has an effect on segregation of P
along austenite grain boundaries when a steel pipe formed
of the heat treatable steel is subjected to heating during
a hot forming process or heating after a forming process.
Although it is important to reduce the content of P
as an impurity, the addition of Mo has an effect of
reducing segregation along grain boundaries.
To obtain this effect, the ratio of Mo/P may
preferably be set to be 15.0 or greater. Although a higher
ratio of Mo/P is more advantageous, the upper limit of the
ratio of Mo/P is determined by considering both the abovedescribed
effect and economic aspects.
Titanium (Ti): 0.01% to 0.1%
During heating in a forming process or heating after
a forming process, titanium (Ti) precipitates in the form
of TiN, TiC, or TiMoC and suppresses the growth of
austenite grains. In addition, if the precipitation of TiN
occurs sufficiently in steel, the effectiveness of boron
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(B) in improving the hardenability of austenite is
increased, and thus strength is stably improved after die
quenching or a quenching treatment.
If the content of Ti in the heat treatable steel is
less than 0.01%, the microstructure of the heat treatable
steel is not sufficiently refined, or the strength of the
heat treatable steel is not sufficiently improved.
Conversely, if the content of Ti is greater than 0.1%, the
effect of improvements in strength does not increase in
proportion to the content of Ti. Thus, preferably, the
upper limit of the content of Ti may be set to be 0.1%, and
more preferably, the content of Ti may be set to be within
the range of 0.02% to 0.06%.
Chromium (Cr): 0.05% to 0.5%
In addition to manganese (Mn) and carbon (C),
chromium (Cr) improves the hardenability of a steel sheet
for forming and increases the strength of the steel sheet
after die quenching or a quenching treatment.
In a process of adjusting martensite, Cr has an
effect on a critical cooling rate for easily obtaining
martensite. Furthermore, in a hot press forming process, Cr
lowers the A3 temperature.
Preferably, Cr may be added in an amount of 0.05% or
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greater to obtain these effects. However, if the content of
Cr is greater than 0.5%, hardenability required for a
formed product assembly process may be increased
excessively, and weldability may be decreased. Thus, the
content of Cr may preferably be set to be 0.5% or less, and,
more preferably, to 0.1% to 0.4%.
Boron (B): 0.0005% to 0.005%
Boron (B) is highly effective in improving the
hardenability of a steel sheet for forming. Even a very
small amount of B may markedly increase strength after die
quenching or a quenching treatment.
If the content of B is less than 0.0005%, these
effects may not be obtained, and thus it may be preferable
that the content of B be 0.0005% or greater.
However, if the content of B is greater than 0.005%,
the above-mentioned effects are saturated. Thus, the
content of B may preferably be set to be 0.005% or less and,
more preferably, to 0.001% to 0.004%.
Nitrogen (N): 0.01% or less
Nitrogen (N) is an inevitably added impurity
facilitating the precipitation of AlN during a continuous
casting process and causing cracks in corners of a
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continuously cast slab. However, it is known that N forms
precipitates such as TiN and functions as a source of
occlusion of diffusion hydrogen, and thus if the amount of
N precipitation is properly controlled, resistance to
hydrogen delayed fracture may be improved. Thus, preferably,
the upper limit of the content of N may be set to be 0.01%,
and more preferably, the content of N may be set to be
within the range of 0.07% or less.
At least one or two selected from the group
consisting of niobium (Nb): 0.01% to 0.07%, copper (Cu):
0.05% to 1.0%, and nickel (Ni): 0.05% to 1.0% may be added
to the heat treatable steel having the above-described
composition so as to improve the properties of the heat
treatable steel.
Niobium (Nb): 0.01% to 0.07%
Niobium (Nb) is an element effective in grain
refinement of steel.
Nb suppresses growth of austenite grains during
heating in a hot rolling process and increases a noncrystallization
temperature range in a hot rolling process,
thereby markedly contributing to the refinement of a final
microstructure.
In a later hot press forming process, such a refined
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microstructure has an effect of inducing grain refinement
and effectively dispersing impurities such as P.
If the content of Nb is less than 0.01%, these
effects may not be obtained, and thus it may be preferable
that the content of Nb be 0.01% or greater.
However, if the content of Nb is greater than 0.07%,
the sensitivity of a slab to cracks may increase in a
continuous casting process, and the anisotropy of a hotrolled
or cold-rolled steel sheet may increase. Thus, the
content of Nb may preferably be set to be 0.07% or less and,
more preferably, to 0.02% to 0.05%.
Copper (Cu): 0.05% to 1.0%
Copper (Cu) is an element improving the corrosion
resistance of steel. In addition, when a tempering process
is performed to improve toughness after a forming process,
supersaturated copper (Cu) leads to the precipitation of ε-
carbide and thus age-hardening.
If the content of Cu is less than 0.05%, these
effects may not be obtained, and thus the lower limit of
the content of Cu may preferably be set to be 0.05%.
However, if the content of Cu is excessive, surface
defects are caused during steel sheet manufacturing
processes, and it is uneconomical because corrosion
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resistance does not increase as much as the amount of Cu.
Thus, preferably, the upper limit of the content of Cu may
be set to be 1.0%, and more preferably, the content of Cu
may be set to be within the range of 0.2% to 0.8%.
Nickel (Ni): 0.05% to 1.0%
Nickel (Ni) is effective in improving the strength
and toughness of a steel sheet for forming and the
hardenability of the steel sheet, as well. In addition, Ni
is effective in decreasing susceptibility to hot shortening
caused when only copper (Cu) is added.
In addition, Ni widens an austenite temperature range
to include a lower temperature and may thus effectively
broaden a process window during annealing in a hot rolling
process and a cold rolling process and during heating in a
forming process.
If the content of Ni is less than 0.05%, these
effects may not be obtained. Conversely, if the content of
Ni is greater than 1.0%, although hardenability improves or
strength increases, it is uneconomical because the effect
of improving hardenability may not be proportional to the
amount of Ni required. Thus, preferably, the upper limit of
the content of Ni may be set to be 1.0%, and more
preferably the content of Ni may be set to be within the
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range of 0.1% to 0.5%.
When the heat treatable steel is a raw material, that
is, when the heat treatable steel is not heat treated, the
heat treatable steel may have a microstructure including
ferrite and pearlite or a microstructure including ferrite,
pearlite, and bainite.
The heat treatable steel may be one selected from the
group consisting of a hot-rolled steel sheet, a pickled and
oiled steel sheet, and a cold-rolled steel sheet.
Alternatively, the heat treatable steel may be a
steel pipe.
Hereinafter, a method for manufacturing a formed
product using the heat treatable steel having improved
fatigue characteristics will be described.
According to another aspect of the present disclosure,
the method for manufacturing a formed product includes a
process of preparing the heat treatable steel; a process of
forming the heat treatable steel to obtain a formed
product; and a process of tempering the formed product.
The heat treatable steel may be one selected from the
group consisting of a hot-rolled steel sheet, a pickled and
oiled steel sheet, and a cold-rolled steel sheet.
The process of forming the heat treatable steel to
obtain a formed product may be performed as follows.
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1) The process of forming the heat treatable steel to
obtain a formed product may be performed by heating the
heat treatable steel and then simultaneously hot forming
and cooling the heat treatable steel using a cooling die.
For example, the hot forming may be hot press
forming.
2) Alternatively, the process of forming the heat
treatable steel to obtain a formed product may be performed
by heating the heat treatable steel, hot forming the heat
treatable steel, and cooling the hot formed, heat treatable
steel using a cooling medium.
For example, the hot forming may be hot press forming.
For example, the cooling using a cooling medium may
be water cooling or oil cooling.
After heating the heat treatable steel to an
austenite temperature range and extracting and hot forming
the heat treatable steel, the heat treatable steel may be
water cooled or oil cooled. Here, if the heat treatable
steel is cooled in the hot forming process, the heat
treatable steel may be reheated and then water cooled or
oil cooled.
3) Alternatively, the process of forming the heat
treatable steel to obtain a formed product may be performed
by cold forming the heat treatable steel, heating the heat
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treatable steel to an austenite temperature range and
maintaining the heat treatable steel within the austenite
temperature range, and cooling the heat treatable steel,
using a cooling medium.
For example, the cold forming may be cold press
forming.
For example, the cooling using a cooling medium may
be water cooling or oil cooling.
The formed product obtained by cold forming the heat
treatable steel may be heated to an austenite temperature
range and maintained within the austenite temperature range,
and then the formed product may be extracted and water
cooled or oil cooled.
In the method of simultaneously performing hot
forming and cooling using a die, and the method of
performing hot forming and then cooling using a cooling
medium the heat treatable steel may be heated to a
temperature range of 850°C to 950°C and maintained within
the temperature range for 100 seconds to 1,000 seconds, for
example.
In the method of simultaneously performing hot
forming and cooling, the heat treatable steel heated and
maintained as described above may be extracted, hot formed
using a prepared die, and cooled directly in the die to
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200°C or less, at a cooling rate ranging from a critical
cooling rate of martensite to 300°C/s, for example.
In the method of performing hot forming and then
cooling using a cooling medium, the heat treatable steel
heated and maintained as described above may be extracted,
hot formed, and water or oil cooled to 200°C or lower, at a
cooling rate ranging from a critical cooling rate of
martensite to 300°C/s, for example.
In the method of performing cold forming and then a
heat treatment, the formed product may be heated to a
temperature of 850°C to 950°C in a high frequency induction
heating furnace or in a batch heating furnace and may be
maintained at the temperature for 100 seconds to 1,000
seconds, for example. Then, the formed products may be
cooled using a proper cooling medium to 200°C or less at a
cooling ratio ranging from a critical cooling rate of
martensite to 300°C/s.
If the heating temperature is less than 850°C,
ferrite transformation may proceed from the surface of the
heat treatable steel because of a temperature decrease
while the heat treatable steel is being extracted from a
heating furnace and hot formed, and thus martensite may not
be sufficiently formed across the thickness of the heat
treatable steel, making it difficult to obtain an intended
Page 28
degree of strength.
Conversely, if the heating temperature is greater
than 950°C, austenite grains may coarsen, manufacturing
costs may increase because of heating costs, and durability
may deteriorate after a final heat treatment because of
accelerated surface decarbonization.
Therefore, it may be preferable that the heating
temperature of the heat treatable steel be within the range
of 850°C to 950°C.
The cooling rate after the hot forming may be set to
obtain a final microstructure having a martensite matrix.
To this end, the cooling rate may be set to be higher than
a critical cooling rate of martensite. That is, the lower
limit of the cooling rate may be set to be the critical
cooling rate of martensite.
However, if the cooling rate is excessively high, the
effect of strengthening is saturated, and additional
cooling equipment may be required. Thus, the upper limit of
the cooling rate may preferably be set to be 300°C/s.
If the cooling temperature is greater than 200°C,
martensite transformation may not completely occur, and
thus an intended martensite structure may not be obtained.
As a result, it may be difficult to obtain an intended
degree of strength.
Page 29
Next in this process, the formed product manufactured
as described above is tempered.
The formed product having a martensite matrix is
tempered to impart toughness to the formed product and to
determine the durability of the formed product according to
tempering conditions.
A key factor of tempering conditions is a tempering
temperature.
The inventors have observed variations in elongation
with respect to the tempering temperature and found that
elongation increases in proportion to the tempering
temperature up to a certain point, and then elongation
decreases, even though the tempering temperature increases.
The inventors found that if tempering is performed at
a temperature (Ttempering) at which elongation has a peak,
the durability life of the formed product increases
markedly, and found that the Ttempering has a relationship
with the content of C, as expressed by Formula 3, below:
[Formula 3]
Ttempering (°C) = 111*[C]-0.633
According to the present disclosure, the formed
product manufactured as described above is tempered by
maintaining the formed product at a tempering temperature
satisfying the following Formula 4 for 15 minutes to 60
Page 30
minutes.
[Formula 4]
Tempering temperature (°C) = Ttempering (°C) ± 30
[where Ttempering (°C) = 111*[C]-0.633]
As described above, the formed product is tempered to
improve the toughness and durability of the formed product.
After the tempering, the formed product may have a
tempered martensite single phase microstructure or a
microstructure including tempered martensite in an amount
of 90% or more and at least one or two from the group
consisting of ferrite, bainite, and retained austenite as a
remainder.
The formed product manufactured as described above
may have a tensile strength of 1500 MPa or greater.
For example, the formed product may have a tensile
strength of 1600 MPa or greater.
The formed product may have a yield ratio of 0.7 to
0.9.
In general, a martensite matrix obtained through a
quenching process has a high degree of tensile strength but
a low degree of elongation, and a yield ratio of 0.7 or
less. If tempering is performed under conventional
tempering conditions, that is, at a temperature of 500°C to
600°C, yield strength and tensile strength decrease
Page 31
markedly, elongation is increased, and a yield ratio of 0.9
or higher is obtained.
Thus, the inventors have evaluated tensile strength
characteristics and low-frequency fatigue characteristics
while varying the temperature of a tempering process
performed after a quenching process and have found an
interesting phenomenon.
That is, as the temperature of a tempering process
increases, yield strength increases and peaks at a
temperature of 200°C to 300°C. Then, with a further
increase of the tempering temperature, yield strength
decreases linearly and constantly, and with the increase of
the tempering temperature, tensile strength decreases
constantly. Elongation, particularly uniform elongation,
decreases markedly when the tempering temperature is 250°C
or greater, and then increases when the tempering
temperature is 400°C or greater.
In terms of microstructure, C dissolved in martensite
by a quenching process undergoes a change of state when a
tempering process is performed. If the temperature of the
tempering process is low, ε-carbide exists. However, if the
temperature of the tempering process is high, ε-carbide
converts to cementite, and this precipitation of cementite
explains why yield strength and tensile strength decrease.
Page 32
A low-frequency fatigue test (Δε/2=±0.5%) was
performed while controlling stain, with respect to a
tempering temperature, so as to evaluate fatigue life.
According to the test, fatigue life increased and peaked in
a tempering temperature range of 200°C to 250°C, and when
the tempering temperature was higher than this range,
fatigue life decreased. In other words, it can be found
that low-frequency fatigue life increases markedly if yield
strength is increased and a yield ratio of 0.7 to 0.9 is
obtained without a decrease in elongation, particularly
uniform elongation, as a result of a tempering process
performed after a quenching process.
The formed product has a long fatigue life.
The formed product has a low-frequency fatigue life
preferably within the range of 5,000 cycles or more (where
the number of cycles refers to a cycle number at which
fracture occurs under a strain application condition of
Δε/2=±0.5%).
Hereinafter, an example method for manufacturing heat
treatable steel as a starting material for forming a formed
product will be described according to the present
disclosure.
The heat treatable steel may be at least one selected
from the group consisting of a hot-rolled steel sheet, a
Page 33
pickled and oiled steel sheet, and a cold-rolled steel
sheet, and example methods for manufacturing such steel
sheets will now be described according to the present
disclosure.
A hot-rolled steel sheet may be manufactured through
the following processes:
heating a steel slab having the same composition as
the composition of the heat treatable steel of the present
disclosure to a temperature range of 1150°C to 1300°C;
manufacturing a steel sheet by rough rolling and hot
rolling the heated steel slab; and
coiling the steel sheet at a temperature of 500°C to
700°C.
Since the steel slab is heated to a temperature range
of 1150°C to 1300°C, the microstructure of the steel slab
may become homogenized, and even though some of the
carbonitride precipitates, such as Nb and Ti precipitates,
are dissolved, growth of grains of the steel slab may be
suppressed, thereby preventing the excessive growth of
grains.
The hot rolling may include finish hot rolling at a
temperature of Ar3 or greater.
If the temperature of finish hot rolling is lower
than Ar3, some austenite may be transformed into ferrite,
Page 34
to result in a dual phase region (in which ferrite and
austenite exist together), and hot rolling may be performed
in this state. In this case, resistance to deformation is
not uniform, and thus the mass flow of the steel slab may
be negatively affected. In addition, if stress concentrates
on ferrite, slab fracture may occur.
Conversely, if the temperature of finish hot rolling
is excessively high, surface defects such as sand-like
scale may be formed. Thus, the temperature of hot finish
rolling may preferably be set to be 950°C or less.
In addition, when the steel sheet is cooled and
coiled using a run-out table after the hot rolling, the
coiling temperature may be adjusted so as to reduce widthwise
material property variations of the steel sheet and
prevent the formation of a low-temperature phase such as
martensite, which may have a negative influence on the mass
flow of the steel sheet in a subsequent cold rolling
process.
If the coiling temperature is lower than 500°C, a
low-temperature microstructure such as martensite may be
formed, and thus the strength of the steel sheet may be
increased excessively. Particularly if the steel sheet is
over-cooled in a width direction of a coil, material
properties of the steel sheet may be varied in the width
Page 35
direction, and the mass flow of the steel sheet may be
negatively affected in a subsequent cold rolling process,
thereby making it difficult to control the thickness of the
steel sheet.
Conversely, if the coiling temperature is greater
than 700°C, internal oxidation may occur in the surface of
the steel sheet, and thus cracks that are formed as
internal oxides are removed in a pickling process may
develop as notches. As a result, it may be difficult to
flatten or expand a final product such as a steel pipe.
Thus, the upper limit of the coiling temperature may
preferably be limited to 700°C.
The steel sheet formed by hot rolling may be cold
rolled to form a cold-rolled steel sheet. In this case, the
cold rolling is not limited to particular conditions or
methods, and the reduction ratio of the cold rolling may be
within the range of 40% to 70%.
According to an example method of forming a coldrolled
steel sheet, the hot-rolled steel sheet manufactured
by the above-described method of the present disclosure is
pickled to remove surface oxides and is cold rolled to form
a cold-rolled steel sheet, and the cold-rolled steel sheet
(fully hardened material) is continuously annealed.
The temperature of the annealing may range from 750°C
Page 36
to 850°C.
If the annealing temperature is lower than 750°C,
recrystallization may occur insufficiently, and if the
annealing temperature is higher than 850°C, grain
coarsening may occur and costs for annealing may increase.
After the annealing, overaging may be performed
within the temperature range of 400°C to 600°C to obtain a
ferrite matrix in which pearlite or bainite is partially
included.
In this case, the cold-rolled steel sheet may have a
strength of 800 MPa or less, similar to the hot-rolled
steel sheet.
Furthermore, in the present disclosure, a steel pipe
being used as a starting material for manufacturing a
formed product may be manufactured by any method without
limitations.
The steel pipe may be manufactured using the abovedescribed
steel sheet of the present disclosure by an ERW
method. In this case, ERW conditions are not limited.
A drawing process may be performed to reduce the
diameter of the steel pipe or to ensure the straightness of
the steel pipe. Before the drawing process, it may be
necessary to pretreat the steel pipe by heating the steel
pipe to a temperature range of 500°C to Ac1 and cooling the
Page 37
steel pipe in air, so as to reduce the hardness of weld
zones formed after ERW, and form a microstructure suitable
for drawing. If the drawing ratio, that is, the difference
between the initial outer diameter and the final outer
diameter expressed in a percentage, is greater than 40%,
drawing defects may be formed because of excessive
deformation. Thus, it may be preferable that the drawing
ratio be set to be within the range of 10% to 35%.
【Mode for Invention】
Hereinafter, the present disclosure will be described
more specifically according to examples.
However, the following examples should be considered
in a descriptive sense only and not for purposes of
limitation. The scope of the present invention is defined
by the appended claims, and modifications and variations
may be reasonably made therefrom.
(Example 1)
Steel slabs having compositions shown in Table 1,
below, were hot rolled to obtain hot-rolled steel sheets,
and the hot-rolled steel sheets were pickled and oiled.
The hot rolling was performed on the steel slabs to
obtain hot-rolled steel sheets having a thickness of 4.5 mm
by heating the steel slabs within the temperature range of
1200°C ±30°C for 180 minutes to homogenize the steel slabs,
Page 38
performing rough rolling and finish rolling on the steel
slabs to obtain hot-rolled steel sheets, and coiling the
hot-rolled steel sheets at temperatures shown in Table 2,
below.
Steel pipes having an outer diameter of 28 mm were
produced using the picked hot-rolled steel sheets by an
electric resistance welding (ERW) method.
The quality of weld zones of the steel pipes was
evaluated by a flattening test in which the weld lines of
the steel pipes were aligned in a 3 o'clock direction, and
cracking in the weld zones of the steel pipes was checked
after compressing the steel pipes. Results of the
flattening test are shown in Table 2, below. In Table 2,
"O" denotes no cracking, and "X" denotes cracking in
welding zones.
New specimens (steel sheets) were prepared under
conditions allowing the steel sheets to pass the flattening
test. Then, JIS 5 tensile test specimens (parallel portion
width 25 mm, gauge length 25 mm), and low-frequency fatigue
test specimens (parallel portion width 12.5 mm, gauge
length 25 mm) were taken from the new specimens in a
direction parallel to the rolling direction of the new
specimens.
The specimens were maintained at 900°C for 7 minutes
Page 39
and quenched in a water bath while maintaining the
temperature of the water bath at 20°C.
The quenched specimens were heat treated within a
temperature range of 200°C to 330°C for one hour, according
to C contents thereof, as shown in Table 2, below, and then
tensile characteristics and fatigue characteristics of the
specimens were evaluated. Fatigue life was evaluated by
applying a stain of Δε/2 = ±0.5% in a triangular wave form
at a deformation frequency of 0.2 Hz.
In addition, Table 2, below, shows tensile
characteristics of the hot-rolled steel sheets.
In Table 2, YS, TS, and El refer to yield strength,
tensile strength, and elongation, respectively, and fatigue
life refers to the number of cycles at which fracture
occurred under a strain application condition of Δε/2=±0.5%.
[Table 1]
No Pro
duc
ts
Chemical composition (wt%) Mn/Si Mo/P Steels
C Si Mn P S s-Al Ti Cr B* Mo **AE N*
1 *PO 0.34 0.2
0
1.2
9
0.013 0.0025 0.025 0.03 0.15 0.1
5
- 42 6.5 11.5 ***CS
2 PO 0.35 0.1
5
1.3 0.007
1
0.0027 0.029 0.029 0.16 20 0.1
4
- 45 8.7 19.7 ****IS
3 PO 0.35 0.1
5
1.3 0.007
0
0.0027 0.031 0.025 0.17 19 0.1
5
Nb:
0.05
42 8.7 21.4 IS
4 PO 0.26 0.2
5
1.1 0.005
8
0.0012 0.03 0.033 0.4 22 0.1 - 41 4.4 17.2 CS
5 PO 0.25 0.1
5
1.2
5
0.005
8
0.0012 0.03 0.033 0.4 22 0.1 - 50 8.3 17.2 IS
6 PO 0.35 0.2
0
1.4 0.007
1
0.0025 0.025 0.023 0.17 19 0.1
5
Cu:
0.2
38 7.0 21.1 IS
7 PO 0.35 0.2
1
1.3 0.006
6
0.0021 0.023 0.03 0.18 18 0.1
9
Cu:
0.5
55 6.2 28.8 IS
Page 40
Ni:
0.3
8 PO 0.20 0.1
1
1.3 0.008 0.0015 0.031 0.029 0.4 26 0.2
1
- 57 11.8 26.3 IS
9 PO 0.35 0.2
5
1.2 0.013 0.0011 0.029 0.032 0.38 25 0.2 - 60 4.8 15.4 CS
10 PO 0.4 0.1
6
1.3 0.007
8
0.0009 0.027 0.029 0.15 17 0.1
8
- 38 8.1 23.1 IS
11 PO 0.35 0.3
0
1.2 0.015 0.0011 0.029 0.032 0.38 25 0.1 - 40 4.0 6.7 CS
12 PO 0.35 0.4
0
1 0.008
2
0.0023 0.025 0.023 0.17 24 0.2
5
- 45 2.5 30.5 CS
*PO: pickled and oiled steel sheet, **AE: Additional
Elements, ***CS: Comparative Steel, ****IS: Inventive Steel
(In Table 1 above, the contents of B and N are in ppm)
[Table 2]
No Pro
duc
ts
Tensile
characteristics of
starting materials
**
FT
Tensile characteristics
after tempering
Fatigue
Life
(cycles
)
Steels
Coiling
(°C)
YS
(Mpa)
TS
(Mpa)
El
(%
)
Tempering
(°C)
YS
(Mpa)
TS
(Mpa)
El
(%)
Yield
Ratio
(YR)
1 *PO 650 442 640 23 O 220 1450 1807 9.9 0.802 5540 ***CS
2 PO 650 428 620 22 O 220 1460 1800 10.1 0.811 6445 ****IS
3 CR 600 477 658 20 O 220 1490 1820 11.0 0.819 6910 IS
4 PO 650 400 567 26 X - 1310 1640 12 0.799 - CS
5 PO 680 410 570 27 O 250 1270 1605 11.6 0.791 6320 IS
6 PO 650 454 655 23 O 220 1445 1840 9.5 0.785 6700 IS
7 PO 650 448 637 24 O 220 1455 1820 9.9 0.799 6819 IS
8 PO 650 387 520 28 O 330 1050 1430 13 0.734 6510 CS
9 PO 650 431 620 22 X 220 1450 1803 10 0.804 - CS
10 PO 650 472 688 20 O 200 1654 2070 8.8 0.799 6990 IS
11 PO 650 442 620 22 X 220 1438 1817 10.5 0.791 5020 CS
12 PO 650 415 614 24 X 220 1430 1801 10.7 0.794 - CS
*PO: pickled and oiled steel sheet, **FT: Flattening Test,
***CS: Comparative Steel, ****IS: Inventive Steel
As shown in Tables 1 and 2, above, tensile strength
was measured after tempering was performed in a range of
1430 MPa to 2070 MPa, depending mainly on the content of C.
Specimen 8, having a low C content, has a low posttempering
tensile strength, at the level of 1430 MPa, and
Page 41
Specimen 10, having a C content of 0.4%, has a high posttempering
tensile strength, at the level of 2070 MPa.
Specimens 4, 9, 11, and 12, having a high Si content
and a Mn/Si ratio of 5 or less, had cracks in the steel
pipe flattening test. However, the other specimens, having
a satisfactory Mn/Si ratio even though having a high C
content, did not have cracks in weld zones.
As described above, if tempering is performed after
quenching, a tensile strength of 1500 MPa or greater is
obtained. However, Specimen 8 has a tensile strength of
1500 MPa or less because of a high C content. As shown in
Tables 1 and 2, low-frequency fatigue lives measured after
tempering were different according to Mo/P ratios. That is,
Specimens 1 and 11, having a low Mo/P ratio, had a fatigue
life of less than 5500 cycles, for example. However,
specimens having a Mo/P ratio of 15 or greater had a
fatigue life of 6,000 cycles or greater.
(Example 2)
Steel slabs, having compositions shown in Table 3,
below, were hot rolled to obtain hot-rolled steel sheets,
and the hot-rolled steel sheets were pickled and oiled.
The hot rolling was performed on the steel slabs to
obtain hot-rolled steel sheets having a thickness of 3.0 mm
Page 42
by heating the steel slabs within the temperature range of
1200°C ±20°C for 180 minutes to homogenize the steel slabs,
performing rough rolling and finish rolling on the steel
slabs to obtain hot-rolled steel sheets, and coiling the
hot-rolled steel sheets at temperatures shown in Table 4,
below.
In Table 3, below, Ttempering (°C) refers to a
temperature calculated by Formula 3, below.
[Formula 3]
Ttempering (°C) = 111*[C]-0.633
The pickled and oiled hot-rolled steel sheets were
quenched and tempered.
The hot-rolled steel sheets were heated at 930°C for
6 minutes and then quenched in a water bath, while
maintaining the temperature of the water bath at 20°C.
The tempering was performed at a temperature of 200°C
to 500°C for 30 minutes to 60 minutes, and then tensile
characteristics and fatigue life characteristics were
evaluated. Results of the evaluation are shown in Table 4,
below. Here, the tensile characteristics and fatigue life
characteristics were evaluated in the same manner as in
Example 1.
In addition, Table 4, below, shows tensile
characteristics of the hot-rolled steel sheets.
Page 43
In Table 4, YS, TS, and El refer to yield strength,
tensile strength, and elongation, respectively, and fatigue
life refers to the number of cycles at which facture
occurred under a strain application condition of Δε/2=±0.5%.
[Table 3]
No Pro
duc
ts
Chemical Composition (wt%) Mn/Si Mo/P Ttempering
C Si Mn P S s-Al Ti Cr B* Mo (°C)
N*
2 *PO 0.3
5
0.15 1.3 0.0071 0.002
7
0.029 0.029 0.16 20 0.14
45 8.7 19.7 215.7
5 PO 0.2
5
0.15 1.2
5
0.0058 0.001
2
0.03 0.033 0.4 22 0.1
50 8.3 17.2 266.9
10 PO 0.4 0.16 1.3 0.0078 0.000
9
0.027 0.029 0.15 17 0.18
38 8.1 23.1 198.3
*PO: pickled and oiled steel sheet
(In Table 3 above, the contents of B and N are in ppm)
[Table 4]
No Pro
duc
ts
Tensile
characteristics of
starting materials
Tensile characteristics
after tempering
Yield
ratio
Lowfrequency
fatigue
life
(cycles)
Notes
Coiling
(°C)
YS
(Mpa)
TS
(Mpa)
El
(%
)
Tempering
(°C)
YS
(Mpa)
TS
(Mpa)
El
(%)
(YR)
2-0 *PO 650 428 620 22 Quenching 1186 1951 6.6 0.608 4560 -
2-1 PO 650 428 620 22 220 1460 1800 10.1 0.811 6445 **IR
2-2 PO 650 428 620 22 240 1428 1643 8.0 0.869 5690 IR
2-3 PO 650 428 620 22 330 1370 1500 9.0 0.913 3300 -
2-4 PO 650 428 620 22 500 1034 1100 13.0 0.94 3580 -
5-0 PO 680 410 570 27 Quenching 1018 1670 6.9 0.610 4250 -
5-1 PO 680 410 570 27 250 1270 1605 11.6 0.791 6320 IR
5-2 PO 680 410 570 27 330 1190 1310 9.7 0.908 4310 -
10-0 PO 650 472 688 20 Quenching 1302 2160 5.9 0.603 4900 -
10-1 PO 650 472 688 20 200 1650 2070 8.8 0.797 6990 IR
10-2 PO 650 472 688 20 330 1600 1700 7.5 0.941 4705 -
*PO: pickled and oiled steel sheet, **IR: Inventive Range
In Table 4, above, No. 2-0, 5-0, and 10-0 refer to
specimens that were heated at 930°C for 6 minutes and
quenched in a water bath having a temperature of 20°C but
Page 44
were not tempered. As shown in Table 4, Specimens 2-0, 5-0,
and 10-0 have a yield ratio close to 0.6 and a relatively
low fatigue life, compared to the case in which tempering
was performed at 200°C, 220°C, 240°C, and 250°C.
In addition, as shown in Tables 3 and 4, when a heat
treatment was performed in a tempering temperature range
satisfying Formula 4, below, high yield strength was
obtained, and a long fatigue life was obtained in the case
of the yield ratio being within the range of 0.7 to 0.9.
[Formula 4]
Tempering temperature (°C) = Ttempering (°C) ± 30°C
[where Ttempering (°C) = 111*[C]-0.633]
When tempering was performed under conditions not
satisfying Formula 4, fatigue lives were 5,000 cycles or
less. In particular, Specimens 2-3 and 2-4 had a fatigue
life of 5,000 cycles or less, despite having high
elongation.

【WE CLAIM:】
【Claim 1】
Heat treatable steel comprising, by wt%, carbon (C):
0.22% to 0.42%, silicon (Si): 0.05% to 0.3%, manganese
(Mn): 1.0% to 1.5%, aluminum (Al): 0.01% to 0.1%,
phosphorus (P): 0.01% or less (including 0%), sulfur (S):
0.005% or less, molybdenum (Mo): 0.05% to 0.3%, titanium
(Ti): 0.01% to 0.1%, chromium (Cr): 0.05% to 0.5%, boron
(B): 0.0005% to 0.005%, nitrogen (N): 0.01% or less, and a
balance of iron (Fe) and inevitable impurities, wherein Mn
and Si in the heat treatable steel satisfy Formula 1, below,
and Mo/P in the heat treatable steel satisfies Formula 2,
below:
[Formula 1]
Mn/Si ≥ 5
[Formula 2]
Mo/P ≥ 15
【Claim 2】
The heat treatable steel of claim 1, wherein the heat
treatable steel further comprises at least one or two
selected from the group consisting of niobium (Nb): 0.01%
to 0.07%, copper (Cu): 0.05% to 1.0%, and nickel (Ni):
0.05% to 1.0%.
Page 46
【Claim 3】
The heat treatable steel of claim 1, wherein the heat
treatable steel has a microstructure comprising ferrite and
pearlite, or a microstructure comprising ferrite, pearlite,
and bainite.
【Claim 4】
The heat treatable steel of claim 1, wherein the heat
treatable steel comprises one selected from the group
consisting of a hot-rolled steel sheet, a pickled and oiled
steel sheet, and a cold-rolled steel sheet.
【Claim 5】
The heat treatable steel of claim 1, wherein the heat
treatable steel comprises a steel pipe.
【Claim 6】
A method for manufacturing a formed product having
ultra high strength and excellent durability, the method
comprising:
preparing heat treatable steel, the heat treatable
steel comprising, by wt%, carbon (C): 0.22% to 0.42%,
silicon (Si): 0.05% to 0.3%, manganese (Mn): 1.0% to 1.5%,
aluminum (Al): 0.01% to 0.1%, phosphorus (P): 0.01% or less
Page 47
(including 0%), sulfur (S): 0.005% or less, molybdenum
(Mo): 0.05% to 0.3%, titanium (Ti): 0.01% to 0.1%, chromium
(Cr): 0.05% to 0.5%, boron (B): 0.0005% to 0.005%, nitrogen
(N): 0.01% or less, and a balance of iron (Fe) and
inevitable impurities, wherein Mn and Si in the heat
treatable steel satisfy Formula 1, below, and Mo/P in the
heat treatable steel satisfies Formula 2, below,
[Formula 1]
Mn/Si ≥ 5
[Formula 2]
Mo/P ≥ 15;
forming the heat treatable steel to obtain a formed
product; and
tempering the formed product.
【Claim 7】
The method of claim 6, wherein the heat treatable
steel further comprises at least one or two selected from
the group consisting of niobium (Nb): 0.01% to 0.07%,
copper (Cu): 0.05% to 1.0%, and nickel (Ni): 0.05% to 1.0%.
【Claim 8】
The method of claim 6, wherein the heat treatable
steel comprises one selected from the group consisting of a
Page 48
hot-rolled steel sheet, a pickled and oiled steel sheet,
and a cold-rolled steel sheet.
【Claim 9】
The method of claim 6, wherein the heat treatable
steel comprises a steel pipe.
【Claim 10】
The method of claim 6, wherein the forming of the
heat treatable steel is performed by heating the heat
treatable steel and then hot forming and cooling the heat
treatable steel simultaneously, using a cooling die.
【Claim 11】
The method of claim 10, wherein, in the heating of
the heat treatable steel before the hot forming of the heat
treatable steel, the heat treatable steel is heated to a
temperature of 850°C to 950°C and maintained at the
temperature for 100 seconds to 1,000 seconds, and in the
cooling of the heat treatable steel after the hot forming
of the heat treatable steel, the heat treatable steel is
cooled to a temperature of 200°C or less at a cooling rate
ranging from a critical cooling rate of martensite to
300°C/s.
Page 49
【Claim 12】
The method of claim 6, wherein the forming of the
heat treatable steel is performed by heating the heat
treatable steel, hot forming the heat treatable steel, and
cooling the heat treatable steel using a cooling medium.
【Claim 13】
The method of claim 12, wherein, in the heating of
the heat treatable steel before the hot forming of the heat
treatable steel, the heat treatable steel is heated to a
temperature of 850°C to 950°C and maintained at the
temperature for 100 seconds to 1,000 seconds, and in the
cooling of the heat treatable steel after the hot forming
of the heat treatable steel, the heat treatable steel is
cooled to a temperature of 200°C or less at a cooling rate
ranging from a critical cooling rate of martensite to
300°C/s.
【Claim 14】
The method of claim 6, wherein the forming of the
heat treatable steel is performed by cold forming the heat
treatable steel, heating the heat treatable steel to an
austenite temperature range and maintaining the heat
treatable steel within the austenite temperature range, and
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cooling the heat treatable steel using a cooling medium.
【Claim 15】
The method of claim 14, wherein the heating,
maintaining, and cooling of the heat treatable steel are
performed by heating the heat treatable steel to a
temperature of 850°C to 950°C, maintaining the heat
treatable steel at the temperature for 100 seconds to 1,000
seconds, and cooling the heat treatable steel to a
temperature of 200°C or less, at a cooling rate ranging
from a critical cooling rate of martensite to 300°C/s.
【Claim 16】
The method of any one of claims 6 to 12, wherein the
tempering of the formed product is performed by maintaining
the formed product at a tempering temperature satisfying
Formula 4, below, for 15 minutes to 60 minutes:
[Formula 4]
Tempering temperature (°C) = Ttempering (°C) ± 30°C
[where Ttempering (°C) = 111*[C]-0.633]
【Claim 17】
A formed product having ultra high strength and
excellent durability, the formed product comprising, by wt%,
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carbon (C): 0.22% to 0.42%, silicon (Si): 0.05% to 0.3%,
manganese (Mn): 1.0% to 1.5%, aluminum (Al): 0.01% to 0.1%,
phosphorus (P): 0.01% or less (including 0%), sulfur (S):
0.005% or less, molybdenum (Mo): 0.05% to 0.3%, titanium
(Ti): 0.01% to 0.1%, chromium (Cr): 0.05% to 0.5%, boron
(B): 0.0005% to 0.005%, nitrogen (N): 0.01% or less, and a
balance of iron (Fe) and inevitable impurities, wherein Mn
and Si in the formed product satisfy Formula 1, below, Mo/P
in the formed product satisfies Formula 2, below, and the
formed product has a tempered martensite single phase
microstructure or a microstructure comprising tempered
martensite in an amount of 90% or greater and at least one
from a group consisting of ferrite, bainite, and retained
austenite as a remainder,
[Formula 1]
Mn/Si ≥ 5
[Formula 2]
Mo/P ≥ 15
【Claim 18】
The formed product of claim 17, wherein the formed
product further comprises at least one or two selected from
the group consisting of niobium (Nb): 0.01% to 0.07%,
copper (Cu): 0.05% to 1.0%, and nickel (Ni): 0.05% to 1.0%.
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【Claim 19】
The formed product of claim 17, wherein the formed
product has a low-frequency fatigue life, within a range of
5,000 cycles or greater, where the number of cycles refers
to a cycle number at which fracture occurs under a ±0.5%
strain application condition.
【Claim 20】
The formed product of claim 17, wherein the formed
product has a tensile strength of 1,500 MPa or greater.
【Claim 21】
The formed product of claim 17, wherein the formed
product has a yield ratio of 0.7 to 0.9.