DESCRIPTION
METHOD FOR MANUFACTURING HIGH-STRENGTH COLD-ROLLED/HOT-ROLLED
DP STEEL HAVING A TENSILE STRENGTH GRADE OF 590 MPA AND
EXCELLENT WORKABILITY, AS WELL AS LITTLE DEVIATION IN THE
MATERIAL PROPERTIES THEREOF
Technical Field
The present invention relates to a method of
manufacturing high-strength cold-rolled/hot-rolled DP steel
having a tensile strength of 590 MPa grade, superior
workability and low deviation in mechanical properties, and,
more particularly, to a method of manufacturing high-strength
cold-rolled/hot-rolled DP steel having superior elongation and
low deviation in mechanical properties using a thin-slab
casting technique.
Background Art
Recently, with the demand for improving fuel efficiency
of automobiles and reinforcing safety regulations of drivers
and passengers in automotive industries, thorough research
into low-weight and high-strength automobile bodies that can
provide enhanced impact resistance is ongoing.
Thus, in order to satisfy both low weight and high-
strength characteristics of automobile bodies, high-strength

steel sheets of 590 MPa grade or more are being intensively
developed and used. Also, because steel sheets for
automobiles are mainly processed via pressing, they should
have superior press formability. In order to ensure such
press formability, there is a need to manufacture products
having low yield strength, high ductility and uniform
mechanical properties.
Among types of steel having a transformed structure, a
high-strength product having low yield strength is typically
exemplified by DP (Dual-Phase) steel composed of two phases of
ferrite and martensite. DP steel has a composite
microstructure in which ferrite and martensite coexist, and
the yield ratio is decreased by operation potential adjacent
to grain boundaries of ferrite abutting on the martensite.
Hence, because elastic resilience upon processing is low and
thus shape fixability is high, and also because an elongation
is higher compared to deposition hardened steel sheets, DP
steel may be applied to high-strength parts requiring
workability to some degree.
Techniques for manufacturing high-strength cold-rolled DP
steel are disclosed in US Patent No. 4436561 and Japanese
Patent Nos. 1311609, 1922459, 2133123, 2940235 and 2658706,
and techniques for manufacturing high-strength hot-rolled DP
steel are disclosed in Japanese Patent Nos. 1170762 and
1202277, and US Patent Nos. 1397791, 4285741, and 4325751.

However, these prior inventions pertain to manufacturing
methods using a conventional mill process, undesirably and
unavoidably causing problems in that significant widthwise and
lengthwise deviations in mechanical properties occur in actual
production lines.
Also, in the case where DP steel is manufactured using a
conventional mill, because the finishing rolling rate in a
typical rolling process is as fast as 400 mpm or more, DP
steel should be coiled at a temperature equal to or lower than
an Ms temperature, making it difficult to stably ensure
desired mechanical properties.
Meanwhile, a mini-mill process, which manufactures steel
sheets by means of thin-slab casting corresponding to a novel
steelmaking process receiving much attention these days, is
spotlighted because a temperature difference is low in the
widthwise and lengthwise directions of a strip, making it
possible to manufacture steel having a transformed structure
with low deviation in mechanical properties. However, as
disclosed in European Patent No. 2020294, Japanese Unexamined
Patent Publication Nos. 2000-63955 and 2000-63956, and
PCT Publication No. WO00/055381, these inventions are mainly
directed to methods of manufacturing hot-rolled DP steel
including cooling techniques required to perform procedures up
to coiling after hot rolling, and do not propose methods of
manufacturing cold-rolled DP steel having higher mechanical

properties using a mini-mill process.
Disclosure
Technical Problem
Accordingly, the present invention has been made keeping
in mind the above problems occurring in the related art, and
an object of the present invention is to provide a method of
manufacturing high-strength cold —rolled/hot—rolled DP steel
having a tensile strength of 590 MPa grade, superior
workability and low deviation in mechanical properties, in
which a thin-slab casting technique may be adopted to ensure
superior workability and to remarkably lower the deviation in
mechanical properties in the widthwise and lengthwise
directions of a strip.
Technical Solution
In order to accomplish the above object, the present
invention provides the following manufacturing method.
The present invention provides a method of manufacturing
a cold-rolled DP steel by subjecting steel comprising, by wt%,
C: 0.05 ~ 0.11%, Si: 0.01 ~ 0.8%, Mn: 1.2 ~ 2.2%, P: 0.001 ~
0.1%, S:'0.001- ~ 0.02%, A1: 0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a
total of tramp elements (Cu+Ni+Sn+Pb): 0.18% or less, .one or
more selected from among B: 0.0002 ~ 0.005%, Cr: 0.01 ~ 2.0%,
Sb: 0.005 ~ 0.1%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, V: 0.001

~ 0.1% and Mo: 0.005 ~ 0.5%, and a balance being Fe and other
inevitable impurities to continuous casting to a thin slab
having a thickness of 30 ~ 150 mm, subjecting the thin slab to
roughing rolling, heating, finishing rolling and coiling, thus
producing a hot-rolled strip, and subjecting the hot-rolled
strip to pickling, cold rolling, continuous annealing and
cooling heat treatment, wherein the finishing rolling is
performed such that a difference in rolling rate in a single
strip is 15% or less, and the cooling heat treatment is
performed in such a manner that the strip after continuous
annealing is continuously cooled to 200 ~ 400°C at a cooling
rate of 10 ~ 150°C/s.
In addition, the present invention provides a method of
manufacturing a cold-rolled DP steel by subjecting steel
comprising, by wt%, C: 0.05 ~ 0.11%, Si: 0.01 ~ 0.8%, Mn: 1.2
~ 2.2%, P: 0.001 ~ 0.1%, S:0.001 ~ 0.02%, A1: 0.01 ~ 1.0%, N:
0.001 ~ 0.02%, a total of tramp elements (Cu+Ni+Sn+Pb): 0.18%
or less, one or more selected from among B: 0.0002 ~ 0.005%,
Cr: 0.01 ~ 2.0%, Sb: 0.005 ~ 0.1%, Ti: 0.001 ~ 0.1%, Nb: 0.001
~ 0.1%, V: 0.001 ~ 0.1% and Mo: 0.005 ~ 0.5%, and a balance
being Fe and other inevitable impurities to continuous casting
to a thin slab having a thickness of 30 ~ 150 mm, subjecting
the thin slab to roughing rolling, heating, finishing rolling
and coiling, thus - producing a hot-rolled strip, and subjecting
the hot-rolled strip to pickling, cold rolling, continuous

annealing and cooling heat treatment, wherein the finishing
rolling is performed such that a rolling temperature in a
final rolling stand falls in a range of a target temperature
calculated by a relation of [910 - 195C - 70Mn + 20Si + 30P -
25N - 15Cr - 40Mo] ± 20°C, and the cooling heat treatment is
performed in such a manner that the strip after continuous
annealing is continuously cooled to 200 ~ 400°C at a cooling
rate of 10 ~ 150°C/s.
In addition, the present invention provides a method of
manufacturing a cold-rolled DP steel by subjecting steel
comprising, by wt%, C: 0.05 ~ 0.11%, Si: 0.01 ~ 0.8%, Mn: 1.2
~ 2.2%, P: 0.001 ~ 0.1%, S:0.001 ~ 0.02%, A1: 0.01 ~ 1.0%, N:
0.001 ~ 0.02%, a total of tramp elements (Cu+Ni+Sn+Pb)': 0.18%
or less, one or more selected from among B: 0.0002 ~. 0.005%,
Cr: 0.01 ~ 2.0%, Sb: 0.005 ~ 0.1%, Ti: 0.001 ~ 0.1%, Nb: 0.001
~ 0.1%, V: 0.001 ~ 0.1% and Mo: 0.005 ~ 0.5%, and a balance
being Fe and other inevitable impurities to continuous casting
to a thin slab having a thickness of 30 -150 mm, subjecting
the thin slab to roughing rolling, heating, finishing rolling
and coiling, thus producing a hot-rolled strip, and subjecting
the hot-rolled strip to pickling, cold rolling, continuous
annealing and cooling heat treatment, wherein the finishing
rolling is performed such that a difference in rolling rate in
a single strip is 15% or less, the finishing rolling is

performed such that a rolling temperature in a final rolling
stand falls in a range of a target temperature calculated by a
relation of [910 - 195C - 70Mn + 20Si + 30P - 25N - 15Cr -
40Mo] ± 20°C, and the cooling heat treatment is performed in
such a manner that the strip after continuous annealing is
continuously cooled to 200 ~ 400°C at a cooling rate of 10 ~
150°C/s.
In the above methods, the continuous casting is
preferably performed at a casting rate of 4.5 mpm or more.
Also, the roughing rolling is preferably performed such that a
surface temperature of the thin slab at an inlet of a roughing
mill is 950 ~ 1100°C, and a cumulative reduction ratio upon
roughing rolling is 65 ~ 90%. Also, the heating is preferably
performed in such a manner that the strip after roughing
rolling is heated to 950 ~ 1100°C or its heat is maintained.
Also, the coiling is preferably performed in such a manner
that the strip after finishing rolling is coiled at 450 ~
680°C. Also, the cold rolling is preferably performed in such
a manner that the strip after pickling is rolled to a
reduction ratio of 40 ~ 75%. Also, the continuous annealing
is preferably performed in such a manner that the strip after
cold rolling is continuously annealed at 750 ~ 840-°C.
On the other hand, the present invention provides a
method of manufacturing a high-strength hot-rolled DP steel by

subjecting steel comprising, by wt%, C: 0.03 ~ 0.1%, Si: 0.01
~ 1.1%, Mn: 0.8 ~ 2.0%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1:
0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a total of tramp elements
(Cu+Ni+Sn+Pb): 0.18% or less, one or more selected from among
Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, B: 0.0002 ~ 0.005%, Cr:
0.01 ~ 2.0%, Mo: 0.005 ~ '0.5% and Sb: 0.005 ~ 0.1%, and a
balance being Fe and other inevitable impurities to continuous
casting to a thin slab having a thickness of 30 ~ 150 mm, and
subjecting the thin slab to roughing rolling, heating,
finishing rolling, cooling and coiling, wherein the finishing
rolling is performed such that a difference in rolling rate in
a single strip is 15% or less, and the coiling is performed in
such a manner that the strip after cooling is coiled in a
range of a target temperature calculated by a relation of [310
- 420C - 50Mn - 15Si - 12Cr - 7.5Mo] ± 30°C.
In addition, the present invention provides a method of
manufacturing a hot-rolled DP steel by subjecting steel
comprising, by wt%, C: 0.03 ~ 0.1%, Si: 0.01 ~ 1.1%, Mn: 0.8 ~
2.0%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 1.0%, N:
0.001 ~ 0.02%, a total of tramp elements (Cu+Ni+Sn+Pb) : 0.18%.
or less, one or more selected from among Ti: 0.001 ~ 0.1%, Nb:
0.001 ~ 0.1%, B: 0.0002 ~ 0.005%, Cr: 0.01 ~ 2.0%, Mo: 0.005 ~
0.5% and Sb: 0.005 ~ 0.1%, and a balance being Fe and other
inevitable impurities to continuous casting to a thin slab

having a thickness of 30 ~ 150 ram, and subjecting the thin
slab to roughing rolling, heating, finishing rolling, cooling
and coiling, wherein the finishing rolling is performed such
that a rolling temperature in a final stand is between Ar1 and
Ar3 transformation temperatures, and the coiling is performed
in such a manner that the strip after cooling is coiled in a
range of a target temperature calculated by a relation of [310
- 420C - 50Mn - 15Si - 12Cr - 7.5Mo] ± 30°C.
In addition, the present invention provides a method of
manufacturing a hot-rolled DP steel by - subjecting steel
comprising, by wt%, C: 0.03 ~ 0.1%, Si: 0.01 ~ 1.1%, Mn: 0.8 ~
2.0%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 1.0%, N:
0.001 ~ 0.02%, a total of tramp elements (Cu+Ni+Sn+Pb) : 0.18%
or less, one or more selected from among Ti: 0.001 ~ 0.1%, Nb:
0.001 ~ 0.1%, B: 0.0002 ~ 0.005%, Cr: 0.01 ~ 2.0%, Mo: 0.005 ~
0.5% and Sb: 0.005 ~ 0.1%, and a balance being Fe and other
inevitable impurities to continuous casting to a thin slab
having a thickness of 30 ~ 150 mm, and subjecting the thin
slab to roughing rolling, heating, finishing rolling, cooling
and coiling, wherein the cooling is performed in such a manner
that the strip after finishing rolling is cooled at a cooling
rate of 50°C/s or more on a run out table, and the coiling is
performed in such a manner that the strip after cooling is
coiled in a range of a target temperature calculated by a

relation of [310 - 420C - 50Mn - 15Si - 12Cr - 7.5Mo] ± 30°C.
In addition, the present invention provides a method of
manufacturing a hot-rolled DP steel by subjecting steel
comprising, by wt%, C: 0.03 ~ 0.1%, Si: 0.01 ~ 1.1%, Mn: 0.8 ~
2.0%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 1.0%, N:
0.001 ~ 0.02%, a total of tramp elements (Cu+Ni+Sn+Pb): 0.18%
or less, one or more selected from among Ti: 0.001 ~ 0.1%, Nb:
0.001 ~ 0.1%, B: 0.0002 ~ 0.005%, Cr: 0.01 ~ 2.0%, Mo: 0.005 ~
0.5% and Sb: 0.005 ~ 0.1%, and a balance being Fe and other
inevitable impurities to continuous casting to a thin slab
having a thickness of 30 ~ 150 mm, and subjecting the thin
slab to roughing rolling, heating, - finishing rolling, cooling
and coiling, wherein the finishing rolling is performed such
that a difference in rolling rate in a single strip is 15% or
less, the finishing rolling is performed such that a rolling
temperature in a final stand is between Ar1 and Ar3
transformation temperatures, the cooling is performed in such
a manner that the strip after finishing roughing is cooled at
a cooling rate of 50°C/s or more on a run out table, and the
coiling is performed in such a manner that the strip after
cooling is coiled in a range of a target temperature
calculated by a relation of [310 - 420C - 50Mn - 15Si - 12Cr -
7.5Mo] + 30°C.
In the above methods, the continuous casting is

preferably performed at a casting rate of 4.5 mpm or more.
Also, the roughing rolling is preferably performed such that a
surface temperature of the thin slab at an inlet of a roughing
mill is 950 ~ 1100°C, and a cumulative reduction ratio upon
roughing rolling is 65 ~ 90%. Also, the heating is preferably
performed in such a manner that the strip after roughing
rolling is heated to 1000 ~ 1150°C or its heat is maintained.
Advantageous Effects
In a method of manufacturing high-strength cold-
rolled/hot-rolled DP steel having a tensile strength of 590
MPa grade and low deviation in mechanical properties according
to the present invention, a thin-slab casting technique can be
employed to ensure superior workability and to remarkably
lower the deviation in mechanical properties in the widthwise
and lengthwise directions of a strip, thus manufacturing high-
strength cold-rolled/hot-rolled DP steel having high quality.
Also, the thin-slab casting technique can obviate a
reheating process in a conventional mill, thus saving energy
and improving productivity.
Also, the thin-slab casting technique enables use of
steel obtained by melting scrap such as scrap metal, etc., in
an electric furnace, thus increasing recyclability of
resources.

Description of Drawing
FIG. 1 is a schematic view illustrating a mini-mill
process according to the present invention.

10: continuous caster 20: roughing mill
30: heater 40: coil box
50: finishing mill 60: run out table
70: coiler
Mode for Invention
Hereinafter, a detailed description will be given of the
present invention.
As mentioned above, the present invention pertains to a
method of manufacturing high-strength cold-rolled DP steel via
a mini-mill process using a thin-slab casting technique, and
the mini-mill process according to the present invention is
briefly described with reference to FIG. 1. The hot-rolled
strip produced by the mini-mill process undergoes a known cold
rolling process (pickling, cold rolling, continuous annealing,
cooling heat treatment) to thus result in final cold-rolled DP
steel, and a description of the cold rolling process is
omitted.
Specifically, a thin slab (a) having a thickness of 30 ~
150 mm is manufactured using a continuous caster 10. This
slab is thinner and thus refers to a thin slab, compared to

slabs having a thickness of 200 mm or more produced using a
continuous caster of a conventional mill. As a conventional
slab having a thickness of 200 mm or more is completely cooled
in an open-air yard, etc., it' has to be sufficiently reheated
so as to have a surface temperature of 1100°C or more in a
reheating furnace before performing the hot rolling. However,
because the thin slab is transferred directly to a roughing
mill 20 without passing through the reheating furnace, heat of
the casting process may be utilized, thus saving energy and
greatly improving productivity.
The thin slab is rolled into a hot-rolled strip having a
predetermined thickness or less using the roughing mill 20.
The temperature of the strip lowered in this procedure is
supplemented using a heater 30, after which the heated hot-
rolled strip (b) is rolled to a desired final thickness using
a finishing mill 50, cooled via ROT (Run Out Table) 60
(hereinafter referred to as a "run out table") , and then
finally coiled at a predetermined temperature using a coiler
70, thereby manufacturing a hot-rolled steel sheet having
desired mechanical properties.
As such, in order to compensate for a difference between
the casting rate and the rolling rate, a coil box 40 is
disposed before the finishing mill 50, so that the hot-rolled
strip (b) passed through the inductive heater 30 is primarily
coiled. As a high-speed casting technique at 6 mpm or more

has recently been actualized, an endless hot rolling process
which does not use the coil box 40 is now being developed.
The high-strength cold-rolled DP steel according to the
present invention, manufactured via the mini-mill process and
the cold rolling process, comprises, by wt%, C: 0.05 ~ 0.11%,
Si: 0.01 ~ 0.8%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S:0.001 ~
0.02%, A1: 0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a total of tramp
elements (Cu+Ni+Sn+Pb): 0.18% or less, one or more selected
from among B: 0.0002 - 0.005%, Cr: 0.01 ~ 2.0%, Sb: 0.005 ~
0.1%, Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, V: 0.001 ~ 0.1% and
Mo: 0.005 ~ 0.5%, and a balance being Fe and other inevitable
impurities. The functions and the amounts of respective
elements are described below.
C increases the strength of a steel sheet, and is very
important to ensure a composite structure of ferrite and
martensite. If the amount of C is less than 0.05%, strength
as desired in the present invention cannot be ensured. In
contrast, if the amount thereof exceeds 0.11%, toughness and
weldability may decrease, and also, cracking on a strand upon
continuous casting may occur more frequently. Hence, the
amount of C is preferably limited to 0.05 ~ 0.11%.
Si is useful in ensuring strength while ductility of a
steel sheet is not decreased. Furthermore, the formation of
ferrite is promoted, and C enrichment in non-transformed
austenite is facilitated, thus easily accelerating the

formation of martensite. If the amount of Si is less than
0.01%, it is difficult to ensure the above, effects. In
contrast, if the amount thereof exceeds 0.8%, surface
properties and weldability may decrease. Hence, the amount of
Si is preferably limited to 0.01 ~ 1.0%.
Mn exhibits very large solid solution strengthening
effects and promotes the formation of a composite structure of
ferrite and martensite. If the amount of Mn is less than
1.2%, it is difficult to ensure strength as desired in the
present invention. In contrast, if the amount thereof exceeds
2.2%, a thin-slab cast strip is difficult to form and
segregation problems may occur. Hence, the amount of Mn is
preferably limited to 1.2 ~ 2.2%.
P is effective at strengthening a steel sheet. If the
amount of P is less than 0.001%, its effect cannot be ensured,
and the manufacturing cost may increase. In contrast, if the
amount thereof exceeds 0.1%, press formability may
deteriorate. Hence, the amount of P is preferably limited to
0.001 ~ 0.1%.
S, which is an impurity element in steel, decreases
ductility and weldability of a steel sheet, as well as surface
defects of a slab. It is difficult to control the amount of S
to be less than 0.001%. If the amount thereof exceeds 0.02%,
slab defects may be caused, and ductility and weldability of a
steel sheet may decrease. Hence, the amount of S is

preferably limited to 0.001 ~ 0.02%.
Acid-soluble A1 is bonded with 0 in steel to thereby
generate deoxidation and is effective at improving
hardenability of martensite via the distribution of C in
ferrite to austenite, like Si. If the amount of acid-soluble
A1 is less than 0.01%, the above effects cannot be ensured.
In contrast, if the amount thereof exceeds 1.0%, the above
effects are saturated, and the manufacturing cost may
increase. Hence, the amount of acid-soluble A1 is preferably
limited to 0.01 ~ 1.0%.
N is effective at stabilizing austenite. If the amount
of N is less than 0.001%, it is difficult to ensure the above
effect. In contrast, if the amount thereof exceeds 0.02%, the
above effect is saturated, and edge cracking of a thin-slab
cast strand may occur. Hence, the amount of N is preferably
limited to 0.001 ~ 0.02%.
The tramp elements (Cu+Ni+Sn+Pb) are impurity elements
resulting from a scrap used as a feed in a steel making
process. If the total amount thereof exceeds 0.18%, surface
cracking of the thin-slab cast strand may be caused. Hence,
the total amount of these elements is preferably limited to
0.18% or less.
The steel having such a composition may be further added
with one or more selected from among B, Cr, Sb, Ti, Nb, V and
Mo. Although these elements have no decisive influence on

ensuring basic properties of high-strength cold-rolled DP
steel as desired in the present invention, one or more among
them are preferably added to precisely control tensile
strength, yield strength, and surface quality of products.
B delays transformation from austenite to pearlite in the
course of cooling during annealing. If the amount of B is
less than 0.0002%, the above effect cannot be expected. In
contrast, if the amount thereof exceeds 0.005%, hardenability
is greatly increased, and thus an elongation may remarkably -
decrease. Hence, the amount of B is preferably limited to
0.0002 ~ 0.005%.
Cr is added to improve hardenability of steel and to
ensure high strength. If the amount of Cr is less than 0.01%,
it is difficult to ensure the above effects. In contrast, if
the amount thereof exceeds 2.0%, the above effects are
saturated, and ductility may decrease. Hence, the amount of
Cr is preferably limited to 0.01 ~ 2.0%.
Sb suppresses the surface enrichment of an oxide and thus
decreases surface defects, and is very effective at
suppressing the formation of a coarse surface enriched product
due to an increased temperature and changes in a hot rolling
process. If the amount of Sb is less than 0.005%, it is
difficult to ensure the above effects. In contrast, if the
amount thereof exceeds 0.1%, the above effects are not
significantly increased, and problems related to the

manufacturing cost and the deterioration of workability may be
caused. Hence, the amount of Sb is preferably limited to
0.005-0.1%.
Ti, Nb and V are effective at increasing yield strength
of a steel sheet and achieving a fine particle size. If the
amount of these elements is less than 0.001%, it is difficult
to ensure the above effects. In contrast, if the amount
thereof exceeds 0.1%, the manufacturing cost may increase and
the excessive deposits may be formed, undesirably
deteriorating ductility of ferrite. Hence, the amount of Ti,
Nb and V is limited to 0.001 ~ 0.1%.
Mo delays transformation from austenite to pearlite and
is added to achieve ferrite fineness and high strength. If
the amount of Mo is less than 0.005%, the above effects cannot
be obtained. In contrast, if the amount thereof exceeds 0.5%,
the above effects are saturated and ductility may decrease.
Hence, the amount of Mo is preferably limited to 0.005 ~ 0.5%
The present invention includes Fe and other inevitable
impurities as the balance, in addition to the - above
components.
A method of manufacturing high-strength cold-rolled DP
steel according to the present invention using molten steel
comprising the above components is described in detail below.
As mentioned above with reference to FIG. 1, the present
invention includes a mini-mill hot rolling process comprising

continuous casting, roughing rolling, heating, finishing
rolling, cooling and coiling, and a cold rolling process
comprising pickling, cold rolling, continuous annealing, and
cooling heat treatment, and the characteristic technical
construction of the invention is to manufacture high-strength
cold-rolled DP steel having low deviation in mechanical
properties by newly controlling the operating conditions of
respective steps.
Specifically, the continuous casting is preferably
performed at a casting rate of 4.5 mpm or more. Typically,
steel having a tensile strength of 590 MPa grade or more
contains larger amounts of elements such as C, Mn, Si, etc.,
added to ensure strength, compared to soft products, and thus
segregation of a strand is more likely to occur at a lower
casting rate. When segregation occurs in this way, it is
difficult to ensure strength and a widthwise or lengthwise
deviation in mechanical properties may occur. Hence, the
casting rate is set to 4.5 mpm or more.
The roughing rolling is performed by subjecting the
continuously cast thin slab to roughing rolling using a
roughing mill equipped with 2-4 stands. As such, this
process is preferably conducted such that the surface
temperature of the thin slab at the inlet of the roughing mill
is 950 ~ 1100°C and the cumulative reduction ratio upon
roughing rolling is 65 ~ 90%.-

If the surface temperature of the thin slab at the inlet
of the roughing mill is lower than 950°C, a roughing rolling
load may greatly increase, and also edge cracking may occur.
In contrast, if the surface temperature thereof is higher than
1100°C, so called San-Su type scale may be generated. Hence,
such a surface temperature is limited to 950 ~ 1100°C.
Furthermore, the cumulative reduction ratio upon roughing
rolling is regarded as being important in order to obtain a
desired product having uniform mechanical properties in the
present invention. As the reduction ratio upon roughing
rolling increases, microscopic distribution of Mn, Si, Al,
etc., which are important elements necessary for manufacturing
DP steel, becomes uniform, and also, a temperature gradient in
the widthwise and thickness directions of the strip may
decrease to thus attain uniform mechanical properties.
However, if the cumulative reduction ratio is less than 65%,
the above effects are not sufficiently exhibited. In
contrast, if the cumulative reduction ratio exceeds 90%,
rolling deformation resistance considerably increases, thus
raising the manufacturing cost. Hence, the rolling is
preferably carried out such that the cumulative reduction
ratio is 65 ~ 90%.
The heating is preferably performed in such a manner that
the roughing rolled strip is heated again to 950 ~ 1100°C or
its heat is maintained. If the surface temperature of the

roughing rolled strip is lower than 950°C, rolling deformation
resistance may remarkably increase. In contrast, if the
surface temperature thereof is higher than 1100°C, high energy
cost is required to increase the temperature, and surface
scale defects may occur more frequently. Hence, the heating
temperature is preferably limited to 920 ~ 1100°C.
The finishing rolling is preferably conducted such that a
difference in the rolling rate in a single strip is 15% or
less. Because the high-strength cold-rolled DP steel of a 590
MPa grade according to the present invention uses the
transformed structure as the strengthening means, ' the
mechanical properties may vary depending on the deformation
rate upon finishing rolling. If the difference in rolling
rate in the finishing mill having stands exceeds 15%, it is
difficult to obtain a uniform cooling rate on the subsequent
run out table and a desired coiling temperature, and thus
deviation in mechanical properties in the widthwise- or
lengthwise direction of the strip may remarkably increase.
Also, in the finishing rolling process, the rolling
temperature of the final rolling stand is preferably set to
fall in the range of a target temperature calculated by the
relation of [910 - 195C - 70Mn + 20Si + 30P - 25N - 15Cr -
40Mo] ± 20°C. In a conventional hot rolling process,
finishing rolling is typically completed at a temperature

equal to or higher than an Ar3 transformation temperature to
manufacture DP steel having uniform mechanical properties as
much as possible.
However, in the present invention, in the case where
rolling is performed in a two-phase region in which austenite
and ferrite coexist such that the finishing rolling
temperature of the final stand is between Ar1 and Ar3
transformation temperatures, an elongation is improved at the
same strength, and this was confirmed by repeated tests.
Also, in the case where a DP steel sheet is manufactured using
a thin-slab casting technique, the finishing rolling
temperature is preferably set to be between Ar1 and Ar3
transformation temperatures by virtue of having the advantage
that the temperature control of the strip is easier, compared
to a conventional hot rolling process. In the present
invention, it is noted that above temperature may vary
depending on the amounts of elements, and the rolling
conditions in the range of a target temperature calculated by
the relation of [910 - 195C - 70Mn + 20Si + 30P - 25N - 15Cr -
40Mo] ± 20°C facilitate rolling in a two-phase region, and
this was confirmed by repeated tests.
The results confirmed by the repeated tests are construed
through the following theoretical description. For example,
in the case of steel having a transformed structure, to

improve both strength and ductility, how austenite
stabilization elements, such as C, Mn, etc., are enriched in
the non-transformed austenite is regarded as being important.
In the case where finishing rolling is performed in the two-
phase region, distribution behavior of solute elements is
improved, and thus ferrite is purified while martensite is
further stabilized even in the presence of the same
components, and these effects continue even after cold rolling
and annealing,.
Also, the coiling is preferably performed in such a
manner that the finishing rolled strip is coiled at 450 ~
680°C. If the hot rolling coiling temperature is lower than
450°C, hot rolling strength is greatly increased, undesirably
causing cold rollability problems. In contrast, if this
coiling temperature is higher than 680°C, hot-rolled crayon
coil may be created. Hence, such a temperature is preferably
limited to 450 ~ 680°C.
The cold rolling is preferably conducted in such a manner
that the pickled strip is rolled to a reduction ratio of 40 ~
75%. The cold rolling is preferably performed in such a
manner that the pickled strip is rolled to a reduction ratio
of 40 ~ 75%'. If the reduction ratio is less than 40%,
recrystallization may not occur upon annealing. In contrast,
if the reduction ratio exceeds 75%, rolling deformation
resistance greatly increases, making it difficult to perform

rolling. Hence, the reduction ratio is preferably limited to
40 ~ 75%.
Also, the continuous annealing is preferably performed in
such a manner that the cold-rolled strip ' is continuously
annealed at 750 ~ 840°C. If the annealing temperature is
lower than 750°C, recrystallization may not occur. In
contrast, if the annealing temperature is higher than 840°C,
it is difficult to obtain a two-phase structure of ferrite and
martensite as the main phase in the present invention, and the
mass flow of the strip may become problematic. Hence, the
annealing temperature is preferably limited to 750 ~ 840°C.
Also, the cooling heat treatment is preferably performed
in such a manner that the continuous annealed strip is
continuously cooled to 200 ~ 400°C at a cooling rate of 10 ~
150°C/s.
If the cooling rate is less than 10°C/s, pearlite may be
formed during the cooling, making it difficult to obtain a DP
structure. In contrast, if the cooling rate exceeds 150°C/s,
ductility may decrease, and the shape of the sheet may become
poor. Hence, the cooling rate is preferably limited to 10 ~
150°C/s. Furthermore, if the cooling termination temperature
is lower than 200°C, it is difficult to control the shape of
the sheet as in the case where the cooling rate is too fast.
In contrast, if this temperature is higher than 400°C, it is
difficult to obtain a DP structure. Hence, the cooling

termination temperature is preferably limited to 200 ~ 400°C.
To evaluate the technical effects of the present
invention, the following test was conducted.
Using steel having the composition shown in Table 1
below, respective cold-rolled strips were manufactured under
operating conditions including the slab thickness, casting
rate, slab surface temperature, difference in rolling rate,
coiling temperature, annealing temperature, cooling rate,
etc., of Table 2, and mechanical properties (tensile strength,
elongation and property deviation) and surface shape of the
strips were measured. The results are shown in Table 2 below.
In Table 1, the total amount of tramp elements
(Cu+Ni+Sn+Pb) was controlled to 0.18% or less in all of Steel
Nos. Steel Nos. 1-6 are hot-rolled strips manufactured
using a thin-slab casting technique (slab thickness: 84 mm),
and Steel Nos. 7-9 (slab thickness: 230 mm) are hot-rolled
strips manufactured under conventional mill conditions.
In Table 2, the slab surface temperature indicates the
surface temperature measured immediately before roughing
rolling. The difference in rolling rate is represented by a
percentage of a value obtained by dividing a difference
between the maximum mass-flow rate and the minimum mass-flow
rate in a single strip upon final finishing rolling by the
average mass-flow rate, wherein a low difference in rolling
rate means that a change in the rolling rate is small. The

finishing rolling temperature shows whether rolling was
performed in the range of the target temperature calculated by
Relation 1 ± 20°C, and thus Comparative Steels 4, 11, 12 and
13 were obtained by performing rolling at the temperature
corresponding to a single phase region directly above the
Ar3 transformation temperature.
In the conditions of Steel Nos. 1 ~ 6 of Table 2, the
heating temperature of the strips after roughing rolling was
set to 1075°C, and in the conditions' of Steel Nos. 7-9, the
heating temperature was set to 1200°C, and the thickness of
the hot-rolled strips was set to 3.0 mm.
The hot-rolled strips were pickled and then cold rolled
to a reduction ratio of 60%, thus producing cold-rolled strips
having a thickness of 1.2 mm, and each of the cold-rolled
strips was cooled to 270°C using the annealing temperature and
the cooling rate of Table 2, thus manufacturing test samples.



In Table 2, tensile strength and elongation are values
measured at a position of w/4 of a JIS No. 5 test sample in a
direction perpendicular to the rolling direction. The
elongation is represented by a percentage of tensile strain
applied until a tensile sample breaks, and the property
deviation indicates a value obtained by subtracting the
minimum value from the maximum value among the property values
measured in the lengthwise and widthwise directions of the
coil.
As is apparent from the results of Table 2, it is
possible to manufacture high-strength cold-rolled DP steel
having superior workability (elongation) and very low

deviation in mechanical properties according to the present
invention.
Meanwhile, the composition of high-strength hot-rolled DP
steel according to the present invention manufactured using
the above mini-mill process, comprises, by wt%, C: 0.03 ~
0.1%, Si: 0.01 ~ 1.1%, Mn: 0.8 ~ 2.0%, P: 0.001 ~ 0.1%, S:
0.001 ~ 0.02%, A1: 0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a total of
tramp elements (Cu+Ni+Sn+Pb): 0.18% or less, one or more
selected from among Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, B:
0.0002 ~ 0.005%, Cr: 0.01 ~ 2.0%, Mo: 0.005 ~ 0.5% and Sb:
0.005 ~ 0.1%, and a balance being Fe and other inevitable
impurities. The functions and the amounts of respective
elements are briefly described below.
C increases the strength of a steel sheet, and is very
important to ensure a composite structure of ferrite and
martensite. If the amount of C is less than 0.03%, strength
as desired in the present invention cannot be ensured. In
contrast, if the amount thereof exceeds 0.1%, toughness and
weldability may decrease, and also, surface defects on a cast
strand upon thin-slab casting may occur more frequently.
Hence, the amount of C is preferably limited to 0.03 ~ 0.1%.
Si is useful in ensuring strength while ductility of a
steel sheet is not decreased. Furthermore, the- formation of
ferrite is promoted, and C enrichment in non-transformed
austenite is facilitated, thus easily accelerating the

formation of martensite. If the amount of Si is less than
0.01%, it is difficult to ensure the above effects. In
contrast, if the amount thereof exceeds 1.1%, surface
properties and weldability may decrease. Hence, the amount of
Si is preferably limited to 0.01 ~ 1.1%.
Mn exhibits very large solid solution strengthening
effects and promotes the formation of a composite structure of
ferrite and martensite. If the amount of Mn is less than
0.8%, it is difficult to ensure strength as desired in the
present invention. In contrast, if the amount thereof exceeds
2.0%, weldability and hot rollability may become problematic.
Hence, the amount of Mn is preferably limited to 0.8 ~ 2.0%.
P is effective at strengthening a steel sheet. If the
amount of P is less than 0.001%, its effect cannot be ensured,
and the manufacturing cost may increase. In contrast, if P is
excessively added, press formability may deteriorate. Hence,
the amount of P is preferably limited to 0.001 ~ 0.1%.
S, which is an impurity element in steel, deteriorates
ductility and weldability of a steel sheet. It is difficult
to control the amount of S to be less than 0.001%. If the
amount thereof exceeds 0.02%, ductility and weldability of a
steel sheet may decrease, and edge cracking of the strand may
occur. Hence, the amount of S is preferably limited to 0.001
- 0.02%.
Acid-soluble A1 is bonded with 0 in steel to thereby

generate deoxidation and is effective at improving
hardenability of martensite via the distribution of C in
ferrite to austenite, like Si. If the amount of this element
is less than 0.01%, the above effects cannot be ensured. In
contrast, if the amount thereof exceeds 1.0%, the above
effects are saturated, and only the manufacturing cost may
increase. Hence, the amount of acid-soluble A1 is preferably
limited to 0.01 ~ 1.0%.
N is effective at stabilizing austenite. If the amount
of N is less than 0.001%, it is difficult to ensure the above
effect. In contrast, if the amount thereof exceeds 0.02%, the
above effect is saturated, and weldability may decrease and
the manufacturing cost may increase. Hence, the amount of N
is preferably limited to 0.001 ~ 0.02%.
The tramp elements (Cu+Ni+Sn+Pb) are impurity elements
resulting from a scrap used as a feed in a steel making
process. If the total amount thereof exceeds 0.18%, surface
cracking of the thin-slab cast strand may be caused. Hence,
the total amount of these elements is preferably limited to
0.18% or less.
The steel having such a composition may be further added
with one or more selected from among Ti, Nb, B, Cr, Mo and Sb.
Although these elements have no decisive influence on ensuring
basic properties of high-strength hot-rolled DP steel desired
in the present invention, one or more among them are

preferably added to precisely control tensile strength, yield
strength, and surface quality of products.
Ti and Nb are effective at increasing strength of a steel
sheet and achieving a fine particle size. If the amount of
these elements is less than 0.001%, it is difficult to ensure
the above effects. In contrast, if the amount thereof exceeds
0.1%, the excessive deposits may be formed, undesirably
deteriorating ductility of ferrite. Hence, the amount of Ti
and Nb is limited to 0.001 ~ 0.1%.
B delays transformation from austenite to pearlite in the
course of cooling during annealing. If the amount of B is
less than 0.0002%, the above effect cannot be obtained. In
contrast, if the amount thereof exceeds 0.01%, hardenability
may greatly increase, undesirably deteriorating workability.
Hence, the amount of B is preferably limited to 0.0002 ~
0.01%.
Cr is added to improve hardenability of steel and to
ensure high strength. If the amount of Cr is less than 0.01%,
it is difficult to ensure the above effects. In contrast, if
the amount thereof exceeds 2.0%, the above effects are
saturated, and ductility may decrease. Hence, the amount of
Cr is preferably limited to 0.01 ~ 2.0%.
Mo delays transformation from austenite to pearlite and
is added to achieve ferrite fineness and high strength. If
the amount of Mo is less than 0.001%, the above effects cannot

be obtained. In contrast, if the amount thereof exceeds 1.0%,
the above effects are saturated and ductility may decrease.
Hence, the amount of Mo is limited to 0.001 ~ 1.0%
Sb suppresses the formation of hot-rolled scale. If the
amount of Sb is less than 0.005%, it is difficult to ensure
the above effect. In contrast, if the amount thereof exceeds
1.0%, the above effect is not further increased despite the
excessive addition thereof, and the manufacturing cost and
workability problems may be caused. Hence, the amount of Sb
is preferably limited to 0.005 -1.0%.
The present invention includes Fe and other inevitable
impurities as the balance, in addition to the above
components.
A method of manufacturing high-strength hot-rolled DP
steel according to the present invention using molten steel
comprising the above components is described in detail below.
As mentioned above with reference to FIG. 1, the mini-
mill process includes continuous casting, roughing rolling,
heating, finishing rolling, cooling and coiling, and the
characteristic technical construction of the present invention
is to manufacture high-strength hot-rolled DP steel having low
deviation in mechanical properties by newly controlling the
operating conditions of respective steps.
Specifically, the continuous casting is preferably
conducted at a casting rate of 4.5 mpm or more. Typically

steel having a tensile strength of 590 MPa grade or more
contains larger amounts of elements such as C, Mn, Si, etc.,
added to ensure strength, compared to soft products, and thus
segregation of a strand is more likely to occur at a lower
casting rate. When segregation occurs in this way, it is
difficult to ensure strength and the widthwise or lengthwise
deviation in mechanical properties may occur. Hence, the
casting rate is set to 4.5 mpm or more.
The roughing rolling is performed by subjecting the
continuously cast thin slab to roughing rolling using a
roughing mill equipped with 2-4 stands. As such, this
process is preferably conducted such that the surface
temperature of the thin slab at the inlet of the roughing mill
is 950 ~ 1100°C and the cumulative reduction ratio upon
roughing rolling is 65 ~ 90%.
If the surface temperature of the thin slab at the inlet
of the roughing mill is lower than 950°C, a roughing rolling
load may greatly increase, and also edge cracking may occur.
In contrast, if the surface temperature thereof is higher than
1100°C, San-Su type scale may be generated. Hence, such a
surface temperature is limited to 950 - 1100°C.
Furthermore, the cumulative reduction ratio upon roughing
rolling is regarded as being important in order to obtain a
desired product having uniform mechanical properties in the
present invention. As the reduction ratio upon roughing

rolling increases, microscopic distribution of Mn, Si, Al,
etc., which are important elements necessary for manufacturing
DP steel, becomes uniform, and also, a temperature gradient in
the widthwise and thickness directions of the strip may
decrease to thus attain uniform mechanical properties.
However, if the cumulative reduction ratio is less than 65%,
the above effects are not sufficiently exhibited. In
contrast, if the cumulative reduction ratio exceeds 90%,
rolling deformation resistance considerably increases, thus
raising the manufacturing cost. Hence, the rolling process is
preferably carried out such that the cumulative reduction
ratio is 65 ~ 90%.
The heating is preferably performed in such a manner that
the roughing rolled strip is heated again to 950 ~ 1150°C or
its heat is maintained. If the surface temperature of the
roughing rolled strip is lower than 950°C, rolling deformation
resistance may remarkably increase. In contrast, if the
surface temperature thereof is higher than 1100°C, high energy
cost is required to increase the temperature, and surface
scale defects may occur more frequently. Hence, the heating
temperature is preferably limited to 950 ~ 1150°C.
The finishing rolling is preferably conducted such that a
difference in the rolling rate in a single strip is 15% or
less. Because the high-strength hot-rolled DP steel of a 590
MPa grade according . to the present invention uses the

transformed structure as the strengthening means, the
mechanical properties may vary depending on the deformation
rate upon finishing rolling. If the difference in rolling
rate in the finishing mill having stands exceeds 15%, it is
difficult to obtain a uniform cooling rate on the subsequent
run out table and a desired coiling temperature, and thus the
deviation in mechanical properties in the widthwise direction
or lengthwise direction of the strip may remarkably increase.
Also, in the finishing rolling process, the rolling
temperature of the final stand is preferably set to be between
Ar1 and Ar3 transformation temperatures. In a conventional hot
rolling process, finishing rolling is typically completed at a
temperature equal to or higher than an Ar3 transformation
temperature to manufacture DP steel having uniform mechanical
properties as much as possible. However, in the present
invention, in the case where rolling is performed in a two-
phase region in which austenite and ferrite coexist such that
the finishing rolling temperature of the final stand is
between Ar1 and Ar3 transformation temperatures, an elongation
is improved at the same strength, and this was confirmed by
repeated tests. In the case where a hot-rolled steel sheet is
manufactured using a thin-slab casting technique, the
finishing rolling temperature is set to be between Ar1 and Ar3
transformation temperatures by virtue of having the advantage
that the temperature control of the strip is easier, compared

to a conventional hot rolling process.
The results confirmed by the repeated tests are construed
through the following theoretical description. For example,
in the case of steel having a transformed structure, to
improve both strength and ductility, how austenite
stabilization elements, such as C, Mn, etc., are enriched in
non-transformed austenite is regarded as being important. In
the case where finishing rolling is performed in the two-phase
region, distribution behavior of solute elements is improved,
and thus ferrite is purified while martensite is further
stabilized even in the presence of the same components.
Also, the cooling is performed in such a manner that the
finishing rolled strip is cooled at a cooling rate of 50°C/s
or more on the run out table, and the coiling is performed in
such a manner that the cooled strip is coiled in the range of
a target temperature calculated by the relation of [310 - 420C
- 50Mn - 15Si - 12Cr - 7.5Mo] ± 30°C.
If the cooling rate is less than 50°C/sec, ferrite
transformation is promoted, and cementite is formed, making it
difficult to obtain desired mechanical properties.
The above relation is empirically determined to ensure
desired strength and workability depending on the coiling
temperature and the amounts of the alloying elements, whereby
it is easy to ensure good mechanical properties upon coiling

under the above conditions. More specifically, if the
temperature is lower by 30°C than the value calculated by the
above relation, the proportion of martensite may increase,
thus decreasing an elongation, making it difficult to ensure
desired strength. In contrast, if the temperature is higher
by 30°C than the value calculated by the above relation, the
proportion of ferrite or cementite may increase, undesirably
deteriorating the strength. Hence, the coiling temperature in
the present invention is preferably limited to the above
conditions.
To evaluate the technical effects of the present
invention, the following test was conducted.
Using steel having the composition shown in Table 3
below, respective hot-rolled strips were manufactured under
operating conditions including the slab thickness, casting
rate, difference in rolling rate, etc., of Table 4, and
mechanical properties (tensile strength, elongation and
property deviation) and surface scale generation thereof were
measured. The results are shown in Table 4 below.
In Table 3, the total amount of tramp elements
(Cu+Ni+Sn+Pb) was controlled to 0.18% or less in all of Steel
Nos. Also, Steel Nos. 1-6 are hot-rolled strips
manufactured using a thin-slab casting technique (slab
thickness: 84 mm), and steel Nos. 7 to 9 (slab thickness: 230
mm) are hot-rolled strips manufactured under conventional mill

conditions.
In Table 4, the difference in rolling rate is represented
by a percentage of a value obtained by dividing a difference
between the maximum mass-flow rate and the minimum mass-flow
rate in a single strip upon final finishing rolling by the
average mass-flow rate, wherein a low difference in rolling
rate means that a change in the rolling rate is small. In the
case of the finishing rolling temperature, Comparative Steels
3 and 4 manufactured using a mini-mill process and Comparative
Steels 6, 7 and 8 obtained using a conventional mill process
correspond to the cases where finishing rolling was performed
at the temperature equal to or higher than the
Ar3 transformation temperature.
The coiling temperature shows whether rolling was
performed in the range of a target temperature calculated by
Relation 3 ± 30°C, and Comparative Steel 5 was obtained by
performing coiling at a temperature by at least 30°C higher
than the target temperature.
In the conditions of Steel Nos. 1 ~ 6 of Table 4, the
surface temperature of the slabs upon roughing rolling was set
to 1080°C, the cumulative reduction ration upon roughing
rolling was 78%, and the heating temperature of the strips
after roughing rolling was set to 1080°C, and in the
conditions of Steel Nos. 7 to 9, the reheating temperature was

set to 1200°C. The cooling rate on the run out table in all
of Steel Nos. was set to about 70°C/s, and the final thickness
of the hot-rolled strips was set to 3.0 mm.

In Table 4, tensile strength and elongation are values
measured at a position of w/4 of a JIS No. 5 test sample in a
direction perpendicular to the rolling direction. The
elongation is represented by a percentage of tensile strain

applied until a tensile sample breaks, and the property
deviation indicates a value obtained by subtracting the
minimum value from the maximum value among the property values
measured in the lengthwise and- widthwise directions of the
coil. Also, TSXEI (tensile x elongation) is a parameter which
shows the superiority of the elongation property of high-
strength steel in which the elongation decreases as the
strength increases, wherein high TS×EI means that both the
tensile strength and the elongation are high.
As is apparent from the results of Table 4, it is
possible to manufacture high-strength hot-rolled DP steel
having superior workability (elongation and TS×EI) with very
low deviation in mechanical properties according to the
present invention.

CLAIMS
1. A method of manufacturing high-strength cold-rolled DP
(Dual-Phase) steel having a tensile strength of 590 MPa grade,
superior workability, and low deviation in mechanical
properties, comprising:
subjecting steel comprising, by wt%, C: 0.05 ~ 0.11%, Si:
0.01 ~ 0.8%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S:0.001 ~ 0.02%,
A1: 0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a total of tramp elements
(Cu+Ni+Sn+Pb) : 0.18% or less, one or more selected from among
B: 0.0002 ~ 0.005%, Cr: 0.01 ~ 2.0%, Sb: 0.005 ~ 0.1%, Ti:
0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, V: 0.001 ~ 0.1% and Mo: 0.005
~ 0.5%, and a balance being Fe and other inevitable impurities
to continuous casting to a thin slab having a thickness of 30
150 mm, subjecting the thin slab to roughing rolling,
heating, finishing rolling and coiling, thus producing a hot-
rolled strip, and subjecting the hot-rolled strip to pickling,
cold rolling, continuous annealing and cooling heat treatment,
thus manufacturing a cold-rolled DP steel,
wherein the finishing rolling is performed such that a
difference in rolling rate in a single strip is 15% or less,
and
the cooling heat treatment is performed in such a manner
that the strip after continuous annealing is continuously
cooled to 200 ~ 400°C at a cooling rate of 10 ~ 150°C/s.

2. A method of manufacturing high-strength cold-rolled DP
steel having a tensile strength of 590 MPa grade, superior
workability, and low deviation in mechanical properties,
comprising:
subjecting steel comprising, by wt%, C: 0.05 ~ 0.11%, Si:
0.01 - 0.8%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S:0.001 ~ 0.02%,
A1: 0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a total of tramp elements
(Cu+Ni+Sn+Pb) : 0.18% or less, one or more selected from among
B: 0.0002 ~ 0.005%, Cr: 0.01 - 2.0%, Sb: 0.005 ~ 0.1%, Ti:
0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, V: 0.001 ~ 0.1% and Mo: 0.0.05
~ 0.5%, and a balance being Fe and other inevitable impurities
to continuous casting to a thin slab having a thickness of 30
150 mm, subjecting the thin slab to roughing rolling,
heating, finishing rolling and coiling, thus producing a hot-
rolled strip, and subjecting the hot-rolled strip to pickling,
cold rolling, continuous annealing and cooling heat treatment,
thus manufacturing a cold-rolled DP steel,
wherein the finishing rolling is performed such that a
rolling temperature in a final rolling stand falls in a range
of a target temperature calculated by a relation of [910 -
195C - 70Mn + 20Si + 30P - 25N - 15Cr - 40Mo] ± 20°C, and
the cooling heat treatment is performed in such a manner
that the strip after continuous annealing is continuously

cooled to 200 ~ 400°C at a cooling rate of 10 ~ 150°C/s.
3. A method of manufacturing high-strength cold-rolled DP
steel having a tensile strength of 590 MPa grade, superior
workability, and low deviation in mechanical properties,
comprising:
subjecting steel comprising, by wt%, C: 0.05 ~ 0.11%, Si:
0.01 ~ 0.8%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S:0.001 ~ 0.02%,
A1: 0.01 ~ 1.0%, N: 0.001 - 0.02%, a total of tramp elements
(Cu+Ni+Sn+Pb): 0.18% or less, one or more selected from among
B: 0.0002 ~ 0.005%, Cr: 0.01 ~ 2.0%, Sb: 0.005 ~ 0.1%, Ti:
0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, V: 0.001 ~ 0.1% and Mo: 0.005
~ 0.5%, and a balance being Fe and other inevitable impurities
to continuous casting to a thin slab having a thickness of 30
150 mm, subjecting the thin slab to roughing rolling,
heating, finishing rolling and coiling, thus producing a hot-
rolled strip, and subjecting the hot-rolled strip to pickling,
cold rolling, continuous annealing and cooling heat treatment,
thus manufacturing a cold-rolled DP steel,
wherein the finishing rolling is performed such that a
difference in rolling rate in a single strip is 15% or less,
the finishing rolling is performed such that a rolling
temperature in a final rolling stand falls in a range of a
target temperature calculated' by a relation of [910 - 195C -

70Mn + 20Si + 30P - 25N - 15Cr - 40Mo] ± 20°C, and
the cooling heat treatment is performed in such a manner
that the strip after continuous annealing is continuously
cooled to 200 ~ 400°C at a cooling rate of 10 ~ 150°C/s.
4. The method of any one of claims 1 to 3, wherein the
continuous casting is performed at a casting rate of 4.5 mpm
or more.
5. The method of any one of claims 1 to 3, wherein the
roughing rolling is performed such that a surface temperature
of the thin slab at an inlet of a roughing mill is 950 ~
1100°C, and a cumulative reduction ratio upon roughing rolling
is 65 ~ 90%.
6. The method of any one of claims 1 to 3, wherein the
heating is performed in such a manner that the strip after
roughing rolling is heated to 950 ~ 1100°C or its heat is
maintained.
7. The method of any one of claims 1 to 3, wherein the
coiling is performed in such a manner that the strip after
finishing rolling is coiled at 450 ~ 680°C.

8. The method of any one of claims 1 to 3, wherein the
cold rolling is performed in such a manner that the strip
after pickling is rolled to a reduction ratio of 40 ~ 75%.
9. The method of any one of claims 1 to 3, wherein the
continuous annealing is performed in such a manner that the
strip after cold rolling is continuously annealed at 750 ~
840°C.
10. A method of manufacturing high-strength hot-rolled DP
steel having a tensile strength of 590 MPa grade, superior
workability, and low deviation in mechanical properties,
comprising:
subjecting steel comprising, by wt%, C: 0.03 ~ 0.1%, Si:
0.01 ~ 1.1%, Mn: 0.8 ~ 2.0%, P: 0.001 ~ 0.1%, S: 0.001 ~
0.02%, A1: 0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a total of tramp
elements (Cu+Ni+Sn+Pb): 0.18% or less, one or more selected
from among Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, B: 0.0002 ~
0.005%, Cr: 0.01 ~ 2.0%, Mo: 0.005 ~ 0.5% and Sb: 0.005 ~
0.1%, and a balance being Fe and other inevitable impurities
to continuous casting to a thin slab having a thickness of 30
~ 150 mm, and subjecting the thin slab to roughing rolling,
heating, finishing rolling, cooling and coiling, thus
manufacturing a hot-rolled DP steel,
wherein the finishing rolling is performed such that a

difference in rolling rate in a single strip is 15% or less,
and
the coiling is performed in such a manner that the strip
after cooling is coiled in a range of a target temperature
calculated by a relation of [310 - 420C - 50Mn - 15Si - 12Cr -
7.5Mo] ± 30°C.
11. A method of manufacturing high-strength hot-rolled DP
steel having a tensile strength of 590 MPa grade, superior
workability, and low deviation in mechanical properties,
comprising:
subjecting steel comprising, by wt%, C: 0.03 ~ 0.1%, Si:
0.01 ~ 1.1%, Mn: 0.8 ~ 2.0%, P: 0.001 .- 0.1%, S: 0.001 ~
0.02%, A1: 0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a total of tramp
elements (Cu+Ni+Sn+Pb): 0.18% or less, one or more selected
from among Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, B: 0.0002 ~
0.005%, Cr: 0.01 ~ 2.0%, Mo: 0.005 ~ 0.5% and Sb: 0.005 ~
0.1%, and a balance being Fe and other inevitable impurities
to continuous casting to a thin slab having a thickness of 30
~ 150 mm, and subjecting the thin slab to roughing rolling,
heating, finishing rolling, cooling and coiling, thus
manufacturing a hot-rolled DP steel,
wherein the finishing rolling is performed such that a
rolling temperature in a final stand is between Ar1 and Ar3

transformation temperatures, and
the coiling is performed in such a manner that the strip
after cooling is coiled in a range of a target temperature
calculated by a relation of [310 - 420C - 50Mn - 15Si - 12Cr -
7.5Mo] ± 30°C.
12. A method of manufacturing high-strength hot-rolled DP
steel having a tensile strength of 590 MPa grade, superior
workability, and low deviation in mechanical properties,
comprising:
subjecting steel comprising, by wt%, C: 0.03 ~ 0.1%, Si:
0.01 ~ 1.1%, Mn: 0.8 ~ 2.0%, P: 0.001 ~ 0.1%, S: 0.001 ~
0.02%, A1: 0.01 ~ 1.0%, N: 0.001 ~ 0.02%, a total of tramp
elements (Cu+Ni+Sn+Pb): 0.18% or less, one or more selected
from among Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, B: 0.0002 ~
0.005%, Cr: 0.01 ~ 2.0%, Mo: 0.005 ~ 0.5% and Sb: 0.005 ~
0.1%, and a balance being Fe and other inevitable impurities
to continuous casting to a thin slab having a thickness of 30
~ 150 mm, and subjecting the thin slab to roughing rolling,
heating, finishing rolling, cooling and coiling, thus
manufacturing a hot-rolled DP steel,
wherein the cooling is performed in such a manner that
the strip after finishing rolling is cooled at a cooling rate
of 50°C/s or more on a run out table, and

the coiling is performed in such a manner that the strip
after cooling is coiled in a range of a target temperature
calculated by a relation of [310 - 420C - 50Mn - 15Si - 12Cr -
7.5Mo] ± 30°C.
13. A method of manufacturing high-strength hot-rolled DP
steel having a tensile strength of 590 MPa grade, superior
workability, and low deviation in mechanical properties,
comprising:
subjecting steel comprising, by wt%, C: 0.03 ~ 0.1%, Si:
0.01 ~ 1.1%, Mn: 0.8 ~ 2.0%, P: 0.001 ~ 0.1%, S: 0.001 ~
0.02%, A1: 0.01 - 1.0%, N: 0.001 ~ 0.02%, a total of tramp
elements (Cu+Ni+Sn+Pb): 0.18% or less, one or more selected
from among Ti: 0.001 ~ 0.1%, Nb: 0.001 ~ 0.1%, B: 0.0002 ~
0.005%, Cr: 0.01 ~ 2.0%, Mo: 0.005 ~ 0.5% and Sb: 0.005 ~
0.1%, and a balance being Fe and other inevitable impurities
to continuous casting to a thin slab having a thickness of 30
~ 150 mm, and subjecting the thin slab to roughing rolling,
heating, finishing rolling, cooling and coiling, thus
manufacturing a hot-rolled DP steel,
wherein the finishing rolling is performed such that a
difference in rolling rate in a single strip is 15% or less,
the finishing rolling is performed such that a rolling
temperature in a final stand is between Ar1 and Ar3

transformation temperatures,
the cooling is performed in such a manner that the strip
after finishing roughing is cooled at a cooling rate of 50°C/s
or more on a run out table, and
the coiling is performed in such a manner that the strip
after cooling is coiled in a range of a target temperature
calculated by a relation of. [310 - 420C - 50Mn - 15Si - 12Cr -
7.5Mo] ± 30°C.
14. The method of any one of claims 10 to 13, wherein the
continuous casting is performed at a casting rate of 4.5 mpm
or more.
15. The method of any one of claims 10 to 13, wherein the
roughing rolling is performed such that a surface temperature
of the thin slab at an inlet of a roughing mill is 950 ~
1100°C, and a cumulative reduction ratio upon roughing rolling
is 65 ~ 90%.
16. The method of any one of claims 10 to 13, wherein the
heating is performed in such a manner that the strip after
roughing rolling is heated to 1000 ~ 1150°C or its heat is
maintained.