DESCRIPTION
METHOD FOR MANUFACTURING HIGH-STRENGTH COLD-ROLLED/HOT-ROLLED
TRIP STEEL HAVING A TENSILE STRENGTH OF 590 MPA GRADE,
SUPERIOR WORKABILITY, AND LOW MECHANICAL-PROPERTY DEVIATION
Technical Field
The present invention relates to a method of
manufacturing high-strength cold-rolled TRIP 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 TRIP 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, and in order to ensure such
press formability, there is a need to manufacture
transformation enhanced steel products having superior
workability. Among types of steel having a transformed
structure, DP (Dual Phase) steel and TRIP (TRansfomation
Induced Plasticity) steel are well known to be high-strength
steel having superior workability. In particular, as is well
known in the art, TRIP steel is widely applied to parts having
a complicated shape which are required to have high
workability due to high elongation behavior caused by
transformation induced plasticity of residual austenite.
TRIP steel is residual austenite steel that has a three-
phase mixed structure comprising ferrite and bainite by
allowing austenite present at high temperature to remain at
room temperature. In this TRIP steel, appropriate heating and
cooling heat treatment with the addition of austenite
strengthening elements, such as C, Si, Mn, etc. enable 5 ~ 20%
of austenite to remain at room temperature. When such an
austenite phase, which is a metastable phase, undergoes
external deformation, it is transformed into martensite. When
transformation occurs in this way, a work-hardening exponent
is higher and necking resistance may increase, resulting in
superior workability, compared to typical steel.

In regard to methods of manufacturing such high-strength
cold-rolled TRIP steel, US Patent Nos. 5470529, 6319338,
6544354 and 6210496, and Korean Unexamined Patent Publication
No. 2003-0002581 are known, and methods of manufacturing high-
strength hot-rolled TRIP steel are disclosed in US Patent Nos.
5017248 and 5030298, and Japanese Patent Nos. 2015391,
2559272, 2820774 and 1871742. 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.
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. 02019314, US Patent
Application Nos. 2009-0214377 and 2009-0151821, and
PCT Publication No. WO00/055381, these inventions are mainly
directed to cooling techniques required to perform procedures
up to coiling after rolling, and do not propose methods of
manufacturing cold-rolled TRIP 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 TRIP steel
having a tensile strength of 590 MPa grade 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 TRIP steel by subjecting steel comprising, by
wt%, C: 0.05 ~ 0.20%, Si: 0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001
~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 2.0%, N: 0.001 ~ 0.02%,
Sb: 0.005 ~ 0.1%, a total of tramp elements (Cu+Cr+Ni+Sn+Pb):
0.18% or less, 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 slowly cooled to 620 - 690°C at a cooling rate of
1 ~ 20°C/s, immediately quenched at a cooling rate of 20 ~
100°C/s and then subjected to isothermal transformation heat
treatment at 310 ~ 420°C.
In addition, the present invention provides a method of
manufacturing a cold-rolled TRIP steel by subjecting steel
comprising, by wt%, C: 0.05 ~ 0.20%, Si: 0.8 ~ 2.0%, Mn: 1.2 ~
2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 2.0%, N:
0.001 ~ 0.02%, Sb: 0.005 ~ 0.1%, a total of tramp elements
(Cu+Cr+Ni+Sn+Pb): 0.18% or less, 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 - 225C - 65Mn + 15Si + 10P] ±
20°C, and the cooling heat treatment is performed in such a
manner that the strip after continuous annealing is slowly
cooled to 62-0 ~ 690°C at a cooling rate of 1 ~ 20°C/s,
immediately quenched at a cooling rate of 20 ~ 100°C/s and
then subjected to isothermal transformation heat treatment at
310 ~ 420°C.
In addition, the present invention provides a method of
manufacturing a cold-rolled TRIP steel by subjecting steel
comprising, by wt%, C: 0.05 ~ 0.20%, Si: 0.8 ~ 2.0%, Mn: 1.2 ~
2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 2.0%, N:
0.001 ~ 0.02%, Sb: 0.005 ~ 0.1%, a total of tramp elements
(Cu+Cr+Ni+Sn+Pb): 0.18% or less, 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 continuous
annealing is performed such that a continuous annealing
temperature falls in a range of a target temperature
calculated by a relation of [840 - 120C - 45Mn + 25Si + 34P -
45N -25Cu + 8Cr - 30Ni] ± 15°C, and the cooling heat treatment
is performed in such a manner that the strip after continuous

annealing is slowly cooled to 620 ~ 690°C at a cooling rate of
1 ~ 20°C/s, immediately quenched at a cooling rate of 20 ~
100°C/s and then subjected to isothermal transformation heat
treatment at 310 ~ 420°C.
In addition, the present invention provides a method of
manufacturing a cold-rolled TRIP steel by subjecting steel
comprising, by wt%, C: 0.05 ~ 0.20%, Si: 0.8 ~ 2.0%, Mn: 1.2 ~
2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 2.0%, N:
0.001 ~ 0.02%, Sb: 0.005 ~ 0.1%, a total of tramp elements
(Cu+Cr+Ni+Sn+Pb) : 0.18%' or less, 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 - 225C - 65Mn + 15Si + . 10P] ± 20°C, the
continuous annealing is performed such that a continuous
annealing temperature falls in a range of a target temperature
calculated by a relation of [840 - 120C - 45Mn + 25Si + 34P -

45N -25Cu + 8Cr - 30Ni] ± 15°C, and the cooling heat treatment
is performed in such a manner that the strip after continuous
annealing is slowly cooled to 620 ~ 690°C at a cooling rate of
1 ~ 20°C/s, immediately quenched at a cooling rate of 20 ~
100°C/s and then subjected to isothermal transformation heat
treatment at 310 ~ 420°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 920 ~ 1150°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 480 ~
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%.
The present invention provides a method of manufacturing
a hot-rolled TRIP steel by subjecting steel comprising, by
wt%, C: 0.06 ~ 0.20%, Si: 0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001
~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 2.0%, 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%,
and V: 0.001 ~ 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, reheating, finishing rolling 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 finishing rolling is cooled at a cooling rate of 25°C/s
or more on a run out table and then coiled at 350 ~ 470°C.
In addition, the present invention provides a method of
manufacturing a hot-rolled TRIP steel by subjecting steel
comprising, by wt%, C: 0.06 ~ 0.20%, Si: 0.8 ~ 2.0%, Mn: 1.2 ~
2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 2.0%, 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%, and V: 0.001 ~ 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, reheating, finishing rolling
and coiling, 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
- 225C - 80Mn + 15Si + 10P] ± 20°C, and the coiling is

performed in such a manner that the strip after finishing
rolling is cooled at a cooling rate of 25°C/s or more on a run
out table and then coiled at 350 ~ 470°C.
In addition, the present invention provides a method of
manufacturing a hot-rolled TRIP steel by subjecting steel
comprising, by wt%, C: 0.06 ~ 0.20%, Si: 0.8 ~ 2.0%, Mn: 1.2 ~
2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 2.0%, 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%, and V: 0.001 ~ 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, reheating, finishing rolling
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 rolling stand falls in a range of a
target temperature calculated by a relation of [910 - 225C -
80Mn + 15Si + 10P] + 20°C, and the coiling is performed in
such a manner that the strip after finishing rolling is cooled
at a cooling rate of 25°C/s or more on a run out table and
then coiled at 350 ~ 470°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 920 ~ 1150°C or its heat is maintained.
Advantageous Effects
In a method of manufacturing high-strength cold-
rolled/hot-rolled TRIP steel having a tensile strength of 590
MPa grade, superior workability, and low deviation in
mechanical properties according to the present invention, a
thin-slab casting technigue 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 TRIP 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 TRIP 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
TRIP 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, 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 TRIP steel according to the
present invention, manufactured via the mini-mill process and
the cold rolling process, comprises, by wt%, C: 0.05 ~ 0.20%,
Si: 0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~
0.02%, A1: 0.01 ~ 2.0%, N: 0.001 ~ 0.02%, Sb: 0.005 ~ 0.1%, a
total of tramp elements (Cu+Cr+Ni+Sn+Pb): 0.18% or less, and a
balance being Fe and other inevitable impurities. The
functions and the amounts of respective elements are described
below.
C determines the proportion of non-transformed austenite
in the annealing temperature range and also diffusively moves
to austenite during isothermal transformation heat treatment
so that austenite is stabilized, thus improving ductility. If
the amount of C is less than 0.05%, the proportion of residual
austenite may decrease, making it impossible to ensure
mechanical properties as desired in the present invention. In
contrast, if the amount thereof exceeds 0.20%, weldability may
decrease. Hence, the amount of C is preferably limited to
0.05 ~ 0.20%.
Si enhances the strength of a steel sheet due to solid
solution strengthening effects and suppresses the deposition
of cementite and thus causes C to be enriched in non-
transformed austenite, thereby stabilizing austenite. If the
amount of Si is less than 0.8%, it is difficult to ensure the

above effects. In contrast, if the amount thereof exceeds
2.0%, coatability, corrosion resistance, and weldability may
decrease. Hence, the amount of Si is preferably limited to
0.8 ~ 2.0%.
Mn functions to form austenite, has solid solution
strengthening effects and is effective at decreasing the
diffusion rate of C, and also suppresses transformation in the
cooling process. If the amount of Mn is less than 1.2%, it is
difficult to ensure the residual austenite and to ensure
strength as desired in the present invention. In contrast, if
the amount thereof exceeds 2.2%, the diffusion of C may become
insufficient over isothermal retention time, and the stability
of austenite is rather deteriorated. Hence, the amount of Mn
is preferably limited to 1.2 ~ 2.2%.
P strengthens a steel sheet by virtue of solid solution
strengthening effects, and promotes the C enrichment in
austenite during isothermal transformation heat treatment when
added together with Si. If the amount of P is less than
0.001%, its effects cannot be ensured, and the manufacturing
cost may increase. In contrast, if the amount thereof exceeds
0.1%, spot weldability may deteriorate and brittleness may
increase. Hence, the amount of P is preferably limited to
0.001 ~ 0.1%.
S, which is an impurity element in steel, causes
segregation of the slab and 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%, problems such as segregation of the slab, etc.
may occur, 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 stabilization of
austenite 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 2.0%, the above effects are saturated,
and inclusions may increase along with the manufacturing cost.
Hence, the amount of acid-soluble A1 is preferably limited to
0.01 ~ 2.0%.
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 and the
manufacturing cost may be increased. Hence, the amount of N
is preferably limited to 0.001 ~ 0.02%.
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. 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 saturated,
and problems related to the manufacturing cost and the
deterioration of workability may occur. Hence, the amount of
Sb is preferably limited to 0.005 ~ 0.1%.
The tramp elements (Cu+Cr+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 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 TRIP
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 TRIP 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, 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, A1,
etc., which are important elements necessary for manufacturing
TRIP 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 920 ~ 1150°C or
its heat is maintained. If the surface temperature of the
roughing rolled strip is lower than 920°C, rolling deformation
resistance may remarkably increase. In contrast, if the
surface temperature thereof is higher than 1150°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 ~ 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 cold-rolled TRIP 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 rolling 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.
Furthermore, 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 - 225C - 65Mn + 15Si + 10P] ± 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 TRIP steel
having uniform mechanical properties as much as possible.
However, in the present invention, in the case where rolling
is performed such that the finishing rolling temperature of
the final stand is between Ar1 and Ar3 transformation
temperatures, that is, where rolling is performed in a two-

phase region in which austenite and ferrite coexist, an
elongation is improved at the same strength, and this was
confirmed by repeated tests.
In the case where a TRIP 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 the above temperature may vary
depending on the type of components, and the rolling
conditions in the range of a target temperature calculated by
the relation of [910 - 225C - 65Mn + 15Si + 10P] ± 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 austenite 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 480 ~
680°C. If the hot rolling coiling temperature is lower than
480°C, hot rolling strength is greatly increased, undesirably
causing cold rollability problems. In contrast, if the hot
rolling coiling temperature is higher than 680°C, hot-rolled
crayon coil may be created. Hence, such a temperature is
preferably limited to 480 ~ 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%. 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 is greatly increased, making it difficult to
perform rolling. Hence, the reduction ratio is preferably
limited to 40 ~ 75%.
Also, the continuous annealing is preferably performed
such' that the continuous annealing temperature falls in the
range of a target temperature- calculated by the relation of
[840 - 120C - 45Mn + 25Si + 34P - 45N -25Cu + 8Cr - 30Ni] ±
15°C. Upon manufacturing the TRIP steel, annealing is carried

out in the region in which austenite and ferrite coexist,
which is favorable in terms of ensuring desired mechanical
properties. The above relation is determined to obtain better
mechanical properties by empirically forming the coexisting
region varying depending on the main alloying elements such as
C, Mn, Si, etc., and the tramp elements, via repeated tests.
If the temperature is lower by 15°C than a value
calculated by the above relation, the proportion of austenite
is too low, or recrystallization may not occur. In contrast,
if the annealing heat treatment is performed at a temperature
higher by 15°C than the target temperature, the proportion of
austenite is too high, and the concentration of C in austenite
may be diluted, and thus the proportion of residual austenite
in the final structure may decrease, and the proportion of
martensite or bainite may increase. Furthermore, the mass
flow of the strip may become problematic due to the high
temperature. Hence, the continuous annealing temperature is
preferably limited to the above conditions.
Finally, the cooling heat treatment is preferably
performed in such a manner that the continuously annealed
strip is slowly cooled to 620 - 690°C at a cooling rate of 1 -
20°C/s, immediately quenched at a cooling rate of 20 ~ 100°C/s,
and then subjected to isothermal transformation heat treatment
at 310 ~ 420°C.
Specifically, upon slowly cooling the strip to 620 ~

690°C, if the cooling is performed at a temperature lower than
620°C, carbides may be deposited. In contrast, if the cooling
is performed at a temperature higher than 690°C, austenite is
not effectively stabilized. Also, in the cooling process, if
the cooling rate is less than l°C/s, productivity may decrease
remarkably. In contrast, if the cooling rate exceeds 20°C/s,
diffusion of C in austenite in the course of cooling becomes
insufficient.
Further, upon immediately quenching the strip at a
cooling rate of 20 ~ 100°C/s and subjecting it to isothermal
transformation heat treatment at 310 ~ 420°C, if the quenching
is carried out at a rate lower than 20°C/s, pearlite
transformation or bainite transformation in which cementite is
deposited may occur, making it difficult to obtain residual
austenite, undesirably lowering ductility. In contrast, if
the rate exceeds 100°C/s, bainite transformation in which
cementite is not deposited may be delayed upon the subsequent
isothermal transformation heat treatment, thus obtaining a
bulky coarse quenched structure, undesirably resulting in
lowered ductility.
Upon isothermal transformation heat treatment, if the
temperature is lower than 310°C, the C enrichment in austenite
becomes insufficient. In contrast, if the temperature is
higher than 420°C, cementite, etc., may be deposited,
undesirably resulting in lowered ductility. Hence, this

temperature 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 1
below, respective hot-rolled strips were manufactured under
operating conditions including the slab thickness, casting
rate, slab surface temperature, difference in rolling rate,
finishing rolling temperature and annealing temperature of
Table 2, and mechanical properties (tensile strength,
elongation and property deviation) and surface scale
generation thereof were measured. The results are shown in
Table 2 below.
In Table 1, Steel Nos. 1 -5 are hot-rolled strips
manufactured using a thin-slab casting technique (slab
thickness: 84 mm), and Steel Nos. 6 and 7 (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, 7 and 8 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 ~ 5 of Table 2, the
heating temperature of the strips after roughing rolling was
set to 1075°C, and in the conditions of Steel Nos. 6 and 7,
the reheating temperature was set to 1200°C, and the final
thickness of the hot-rolled strips was set to 3.2 mm.
The hot-rolled strips were pickled and then cold rolled
to a reduction ratio of 56.3%, thus producing cold-rolled
strips having a thickness of 1.4 mm, and each of the cold-
rolled strips was recrystallization annealed at the annealing
temperature of Table 2, slowly cooled to 650°C at a cooling
rate of 7°C/s, immediately cooled at a cooling rate of about
70°C/s, and subjected to isothermal transformation heat
treatment at 380 ~ 410°C.



Relation 1 = [910 - 225C - 65Mn + 15Si + 10P]
Relation 2 = [840 - 120C - 45Mn + 25Si + 34P - 45N - 25Cu +
8Cr - 30Ni]
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. Also, TS×EI (tensile strength × 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 2, it is
possible to manufacture high-strength cold-rolled TRIP steel
having superior elongation and TS×EI with very low deviation
in mechanical properties according to the present invention.
Meanwhile, the composition of high-strength hot-rolled
TRIP steel according to the present invention comprises, by
wt%, C: 0.06 ~ 0.20%, Si: 0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001
~ 0.1%, S: 0.001 ~ 0.02%, A1: 0.01 ~ 2.0%, 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%,
and V: 0.001 ~ 0.1%, and a balance being Fe and other
inevitable impurities. The functions and the amounts of
respective elements are briefly described below.
C stabilizes austenite and thus increases the amount of
austenite which remains at room temperature, thereby enhancing
ductility. If the amount of C is less than 0.06%, the
proportion of austenite may decrease, making it impossible to
ensure mechanical properties as desired in the present
invention. In contrast, if the amount thereof exceeds 0.20%,
weldability may decrease. Hence, the amount of C is
preferably limited to 0.06 ~ 0.20%.
Si enhances the strength of a steel sheet due to solid

solution strengthening effects and suppresses the deposition
of cementite, thus facilitating C to be enriched in non-
transformed austenite, whereby austenite is stabilized. If
the amount of Si is less than 0.8%, it is difficult to ensure
the above effects. In contrast, if the amount thereof exceeds
2.0%, coatability, corrosion resistance and weldability may
decrease. Hence, the amount of Si is preferably limited to
0.8 ~ 2.0%.
Mn stabilizes austenite, has solid solution strengthening
effects, and suppresses transformation in the course of
cooling. If the amount of Mn is less than 1.2%, it is
difficult to ensure the amount of residual austenite and to
ensure the strength as desired in the present invention. In
contrast, if the amount thereof exceeds 2.2%, hot rollability
may become problematic. Hence, the amount of Mn is preferably
limited to 1.2 ~ 2.2%.
P strengthens a steel sheet by virtue of solid solution
strengthening, and facilitates C to be enriched in austenite
when added together with Si. If the amount of P is less than
0.001%, its effects cannot be ensured, and the manufacturing
cost may increase. In contrast, if the amount thereof exceeds
0.1%, spot weldability may deteriorate and brittleness may
increase. Hence, the amount of P is preferably limited to
0.001 ~ 0.1%.
S, which is an impurity element in steel, causes

segregation of the slab and 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%, problems such as segregation of the slab, etc.
may occur, and ductility and weldability of a steel sheet may
deteriorate. 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 stabilization of
austenite 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 2.0%, the above effects are saturated,
and inclusions may increase along with the manufacturing cost.
Hence, the amount of acid-soluble A1 is preferably limited to
0.01 ~ 2.0%.
N is effective at stabilizing austenite. If the amount
of N is less than 0.001%, it is difficult to expect 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 composition thus formed may be further added
with one or more selected from among Ti, Nb and V. Although
these elements have no decisive influence on ensuring basic
properties of high-strength hot-rolled TRIP 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, 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 thereof
is limited to 0.001 ~ 0.1%.
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 TRIP
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 invention is to
manufacture high-strength hot-rolled TRIP 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 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 take
place. 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, A1,
etc., which are important elements necessary for manufacturing
TRIP steel, becomes uniform, and also, a temperature gradient
in the widthwise and thickness directions of the strip may
decrease to thus effectively 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 920 ~ 1150°C and
its heat is maintained. If the surface temperature of the
roughing rolled strip is lower than 920°C, rolling deformation

resistance may remarkably increase. In contrast, if the
surface temperature thereof is higher than 1150°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 ~ 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 TRIP 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 rolling rate
upon finishing rolling. That is, 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 direction
or lengthwise direction of the strip may remarkably increase.
Furthermore, 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 - 225C - 80Mn + 15Si + 10P] ± 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 TRIP steel

having uniform mechanical properties as much as possible.
However, in the present invention, in the case where rolling
is performed such that the finishing rolling temperature of
the final stand is between Ar1 and Ar3 transformation
temperatures, that is, where rolling is performed in a two-
phase region in which austenite and ferrite coexist, an
elongation is improved at the same strength, and this was
confirmed by repeated tests.
In the case where a TRIP 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 type of components, and the rolling
conditions in the range of a target temperature calculated by
the relation of [910 - 225C - 80Mn + 15Si + 10P] ± 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
non-transformed austenite is regarded as being important. In
the case where finishing rolling is performed in a two-phase
region, distribution behavior of solute elements is improved,
and thus ferrite is purified while austenite is further
stabilized even in the presence of the same components.
Also, the coiling is preferably performed in such a
manner that the finishing rolled strip is cooled at a cooling
rate of 25°C/s or more on the run out table and then coiled
350 ~ 470°C. If the cooling rate on the run out table is less
than 25°C/s, the austenite stabilization elements such as C,
Mn, etc. may be deposited into pearlite from the austenite,
and thus enrichment effects may deteriorate. Hence, the
cooling rate is limited to 25°C/s or more.
If the hot rolling coiling temperature is lower than
350°C, martensite is formed and thus strength is greatly
increased, but an elongation may decrease. In contrast, if
the hot rolling coiling temperature is higher than 4 7 0°C,
cementite in steel is deposited, thus suppressing the
formation of residual austenite. Hence, the hot rolling
coiling temperature is preferably limited to 380 ~ 490°C.
The finishing rolling and the coiling are characteristic
technical constructions of the present invention, and two or
more among them are combined, thus manufacturing high-strength
hot-rolled TRIP steel having a tensile strength of 590 MPa

grade and low deviation in mechanical properties as desired in
the present invention.
To evaluate the technical effects of the present
invention, the following test was carried out.
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 2 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 and 8 (slab thickness: 230
mm) are hot-rolled strips manufactured under conventional mill
conditions.
In Table 4, the slab surface temperature indicates the
surface temperature measured immediately before roughing
rolling. The cumulative reduction ratio upon roughing rolling
is represented by a percentage of a value obtained by dividing
a difference between the slab thickness (84 mm) at the inlet
of the roughing mill and the slab thickness (mm) at the outlet
of the roughing mill by the slab thickness. 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 3 ± 20°C, and
thus Comparative Steels 5, 8 and 9 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 4, the
heating temperature of the strips after roughing rolling was
set to 1060°C, and in the conditions of Steel Nos. 7 and 8,
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
50°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, TS×EI (tensile strength × 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 TRIP steel
having superior 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
TRIP (TRansfomation Induced Plasticity) 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 ~ 0.20%, Si:
0.8 ~ 2.0%, Mn: 1.2 -2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%,
A1: 0.01 ~ 2.0%, N: 0.001 ~ 0.02%, Sb: 0.005 ~ 0.1%, a total
of tramp elements (Cu+Cr+Ni+Sn+Pb): 0.18% or less, 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 TRIP 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 slowly cooled to
620 ~ 690°C at a cooling rate of 1 ~ 20°C/s, immediately
quenched at a cooling rate of 20 ~ 100°C/s and then subjected
to isothermal transformation heat treatment at 310 ~ 420°C.

2. A method of manufacturing high-strength cold-rolled
TRIP 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 ~ 0.20%, Si:
0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%,
A1: 0.01 ~ 2.0%, N: 0.001 ~ 0.02%, Sb: 0.005 ~ 0.1%, a total
of tramp elements (Cu+Cr+Ni+Sn+Pb): 0.18% or less, 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 TRIP 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 -
225C - 65Mn + 15Si + 10P] ± 20°C, and
the cooling heat treatment is performed in such a manner
that the strip after continuous annealing is slowly cooled to
620 ~ 690°C at a cooling rate of 1 ~ 20°C/s, immediately
quenched at a cooling rate of 20 ~ 100°C/s and then subjected

to isothermal transformation heat treatment at 310 ~ 420°C.
3. A method of manufacturing high-strength cold-rolled
TRIP 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 ~ 0.20%, Si:
0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S:. 0.001 ~ 0.02%,
A1: 0.01 ~ 2.0%, N: 0.001 ~ 0.02%, Sb: 0.005 ~ 0.1%, a total
of tramp elements (Cu+Cr+Ni+Sn+Pb): 0.18% or less, 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 TRIP steel,
wherein the continuous annealing is performed such that a
continuous annealing temperature falls in a range of a target
temperature calculated by a relation of [840 - 120C - 45Mn +
25Si + 34P - 45N -25Cu + 8Cr - 30Ni] ± 15°C, and
the cooling heat treatment is performed in such a manner
that the strip after continuous annealing is slowly cooled to
620 ~ 690°C at a cooling rate of 1 - 20°C/s, immediately

quenched at a cooling rate of 20 ~ 100°C/s and then subjected
to isothermal transformation heat treatment at 310 ~ 420°C.
4. A method of manufacturing" high-strength cold-rolled
TRIP 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 ~ 0.20%, Si:
0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%,
A1: 0.01 ~ 2.0%, N: 0.001 ~ 0.02%, Sb: 0.005 ~ 0.1%, a total
of tramp elements (Cu+Cr+Ni+Sn+Pb): 0.18% or less, 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 TRIP 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 - 225C -
65Mn + 15Si + 10P] ± 20°C,

the continuous annealing is performed such that a
continuous annealing temperature falls in a range of a target
temperature calculated by a relation of [840 - 120C - 45Mn +
25Si + 34P - 45N -25Cu + 8Cr - 30Ni] ± 15°C, and
the cooling heat treatment is performed in such a manner
that the strip after continuous annealing is slowly cooled to
620 ~ 690°C at a cooling rate of 1 ~ 20°C/s, immediately
quenched at a cooling rate of 20 ~ 100°C/s and then subjected
to isothermal transformation heat treatment at .310 ~ 420°C.
5. The method of any one of claims 1 to 4, wherein the
continuous casting is performed at a casting rate of 4.5 mpm
or more.
6. The method of any one of claims 1 to 4, 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%.
7. The method of any one of claims 1 to 4, wherein the
heating is performed in such a manner that the strip after
roughing rolling is heated to 920 ~ 1150°C or its heat is
maintained.

8. The method of any one of claims 1 to 4, wherein the
coiling is performed in such a manner that the strip after
finishing rolling is coiled at 480- ~ 680°C.
9. The method of any one of claims 1 to 4, wherein the
cold rolling is performed in such a manner that the strip
after pickling is rolled to a reduction ratio of 40 ~ 75%.
10. A method of manufacturing high-strength hot-rolled
TRIP steel having a tensile strength of 590 MPa grade-and low
deviation in mechanical properties, comprising:
subjecting steel comprising, by wt%, C: 0.06 ~ 0.20%, Si:
0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%,
A1: 0.01 ~ 2.0%, 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%, and V: 0.001 ~ 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, reheating,
finishing rolling and coiling, thus manufacturing a hot-rolled
TRIP 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 finishing rolling is cooled at a cooling rate of 25°C/s
or more on a run out table and then coiled at 350 ~ 470°C.
11. A method of manufacturing high-strength hot-rolled
TRIP steel having a tensile strength of 590 MPa grade and low
deviation in mechanical properties, comprising:
subjecting steel comprising, by wt%, C: 0.06 ~ 0.20%, Si:
0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001 - 0.1%, S: 0.001 ~ 0.02%,
A1: .0.01 ~ 2.0%, 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%, and V: 0.001 ~ 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, reheating,
finishing rolling and coiling, thus manufacturing a hot-rolled
TRIP 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 -
225C - 80Mn + 15Si + 10P] ± 20°C, and
the coiling is performed in such a manner that the strip
after finishing rolling is cooled at a cooling rate of 25°C/s
or more on a run out table and then coiled at 350 ~ 470°C.

12. A method of manufacturing high-strength hot-rolled
TRIP steel having a tensile strength of 590 MPa grade and low
deviation in mechanical properties, comprising:
subjecting steel comprising, by wt%, C: 0.06 ~ 0.20%, Si:
0.8 ~ 2.0%, Mn: 1.2 ~ 2.2%, P: 0.001 ~ 0.1%, S: 0.001 ~ 0.02%,
A1: 0.01 ~ 2.0%, 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%, and V: 0.001 ~ 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, reheating,
finishing rolling and coiling, thus manufacturing a hot-rolled
TRIP 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 - 225C -
80Mn + 15Si + 10P] ± 20°C, and
the coiling is performed in such a manner that the strip
after finishing rolling is cooled at a cooling rate of 25°C/s
or more on a run out table and then coiled at 350 ~ 470°C.

13. The method of any one of claims 10 to 12, wherein the
continuous casting is performed at a casting rate of 4.5 mpm
or more.
14. The method of any one of claims 10 to 12, 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%.
15. The method of any one of claims 10 to 12, wherein the
reheating is performed in such a manner that the strip after
roughing rolling is heated to 920 ~ 1150°C of its heat is maintained.