Field of the Invention
The present invention relates to a high strength hot rolled steel sheet with a minimum
tensile strength of 590 MPa and good formability, for automotive structural applications.
The invention further relates to a method of manufacturing the hot rolled steel sheet.
Background of Invention
In the automotive industry there is an increasing demand for lighter vehicles to reduce
fuel consumption and carbon footprint. This necessitates the replacement of
conventional low strength steels with higher strength steels so that thinner gauges can
be used and reduction in the weight of the auto components is possible without
compromising on safety and functional requirements. There is a particular demand for
high strength steels for automotive structural parts like suspension and chassis
components and wheel rim applications. Typically the minimum strength requirement
for such applications is 590 MPa. These parts are quite complicated in shape and are
manufactured mainly by press forming. Hence, apart from the high strength
requirements, it is also essential that the steels used for manufacturing such parts have
good formability and especially stretch flangeability. However, it is a general
phenomena that increasing strength levels is accompanied with a concomitant decrease
in ductility (and hence formability), and moreover the ductility decreases almost linearly
with increasing strength.

Therefore, in order to replace the existing grades of steel used for automotive structural
applications, it is necessary to develop hot-rolled steel sheets which not only possess a
minimum tensile strength of 590 MPa but also have good formability, especially stretch
flangeability.
European Patent EP1398392A1 discloses a method of producing a hot rolled dual phase
(ferrite + martensite) steel of minimum tensile strength of 590 MPa for wheel disk
applications. Though the proposed steel is claimed to have excellent shape fixability and
fatigue durability after forming, considering that it has a ferrite + martensite
microstructure, its stretch flangeability is expected to be quite low.
European Patent EP2053139B1 discloses a method in which a hot rolled steel sheet is
subjected to heat treatment after forming so as to achieve a tensile strength varying in
the range of 440 to 640 MPa. However, the heat treatment after forming, which is an
essential part of the invention, is likely to add to the processing cost and hence is not
suitable for mass production.
European Patent EP2578714A1 discloses a method of producing hot-rolled steel sheets
with a minimum tensile strength of 590 MPa with excellent bake hardenability and
stretch-flangeability. According to the proposed method the steel must contain 1.7 to
2.5 wt% of Mn. When added in such large amounts, Mn tends to segregate in the
central portion in the thickness direction, which induces microstructural heterogeneity
that induces cracking during press forming and reduces the stretch-flangeability.

Moreover, additions of Cr/Mo/Ni/B for increasing hardenability and Nb/Ti/V for refining
austenite grain size and precipitation strengthening are incorporated, all of which leads
to an increase in cost of the steel.
European Patent EP2586886A1 discloses a method of producing a high-strength hot-
rolled steel sheet of a minimum tensile strength of 590 MPa and possessing excellent
stretch flangeability. The proposed steel relies on precipitation strengthening of ferrite
by Titanium. The dissolution temperature of Titanium is quite high and hence this
method can be applied only to conventional hot rolling facilities which are equipped
with a reheating furnace and not continuous strip production facilities.
In light of the said prior art, there is a need for an invention to overcome the limitations
of the prior art and disclose a process that does not need conventional hot rolling
facilities equipped with a reheating furnace.
Objects of the Invention
It is therefore an object of the present invention to propose a hot-rolled steel sheet
having a minimum tensile strength of 590 MPa which eliminates the disadvantages of
the prior art.
Another object of the present invention is to propose a hot-rolled steel sheet having a
minimum tensile strength of 590 MPa and a hole expansion ratio (HER) of ≥ 60%.

Another object of the present invention is to propose a hot-rolled steel sheet having a
minimum tensile strength of 590 MPa and good stretch flangeability, which is adaptable
to the automotive industry, particularly for structural parts like suspension and chassis
components and wheel rims.
A still another object of the present invention is to propose a hot-rolled steel sheet
having a minimum tensile strength of 590 MPa and good stretch flangeability, which
possesses a microstructure consisting of ferrite and bainite.
A further object of the present invention is to propose a hot-rolled steel sheet having a
minimum tensile strength of 590 MPa and good stretch flangeability, which possesses a
microstructure consisting of ferrite and bainite, wherein the ferrite is precipitation
strengthened and has a grain size of 3 to 6 μm.
Another object of the present invention is to propose a manufacturing process to
produce a hot-rolled steel sheet with a microstructure consisting of fine grained ferrite
and bainite which has good stretch flangeability and a minimum tensile strength of 590
MPa.
One more object of the present invention is to propose a manufacturing process in to
produce a hot rolled steel sheet with Ferrite and Bainite phases having tensile strength
of at least 590 MPa and HER >= 60 such that proposed process can be adopted in both
HSM (Hot Strip Mills) and CSP (Compact Strip Casting).

Summary of the Invention:
The invention discloses a high-strength hot-rolled steel sheet with a tensile strength of
at least 590 MPa and method of manufacturing the same. A steel slab of the
composition comprising, in weight percent 0.05-0.07% of C, 1.0-1.5% of Mn, 0.3-0.5%
of Si, 0.03% or less (but not including 0%) of Nb, 0.06% or less (but not including 0%)
of V, maximum 0.005% of S, maximum 0.030% of P, 0.007 - 0.012% of N, the
remaining being substantially iron and incidental impurities,
is reheated to a temperature greater than 1100°C. The steel slab is then hot-rolled such
that finish rolling is done at a temperature (TFRT). The finish rolling temperature as per
the current invention varies in the range Ae3 - 50 (°C) to Ae3 + 50 (°C), where Ae3 is
the temperature at which the transformation of austenite to ferrite starts at equilibrium.
After the hot rolling step, the steel slab is cooled at a cooling rate of 50 - 70°C/s till an
intermediate temperature, TINT is reached, followed by performing natural cooling and
then coiling the steel sheet at coiling temperature (Tcr). TINT as per the current
invention varies in the range Ae3 - 320 (°C) to Ae3 - 300 (°C) and coiling temperature
Tcr varies in the range 400°C to 500°C.
Brief description of drawings
Fig. 1: Schematic diagram of cooling profile as per the current invention
Fig. 2: Microstructures of (a) Steel 1 (Inventive Example) and (b) Steel 2 (Comparative
Example).

Detailed description of the invention
The hot rolled steel sheet having a minimum tensile strength of 590 MPa and good
stretch flangeability, according to the present invention contains in weight percent 0.05-
0.07% of C, 1.0-1.5% of Mn, 0.3-0.5% of Si, 0.03% or less (not including 0%) of Nb,
0.06% or less (not including 0%) of V, maximum 0.005% of S, maximum 0.030% of P,
0.007 - 0.012% of N, the remaining being substantially iron and incidental impurities.
The hot rolled steel sheet having a minimum tensile strength of 590 MPa and good
stretch flangeability, according to the present invention has a microstructure comprising
70 to 90% of ferrite and 10 to 30% of bainite. The hot rolled steel sheet according to
the present invention has a microstructure comprising 70 to 90% of ferrite and 10 to
30% of bainite, wherein the ferrite is precipitation strengthened and has a grain size of
3-6 μm.
The method of manufacturing the hot rolled steel sheet with a ferrite + bainite
microstructure with a minimum tensile strength of 590 MPa and good stretch
flangeability includes the steps of casting the slab in either a conventional or a thin slab
caster and then and then reheating the cast slab to a temperature greater than 1100°C
(preferably in the temperature range of 1100°C-1200 °C), hot rolling the slab such that
finish rolling is done at a temperature (TFRT) such that Ae3 - 50 (°C) ≤ TFRT ≤ Ae3 + 50
(°C), where Ae3 is the temperature at which the transformation of austenite to ferrite
starts at equilibrium, and then cooling at a cooling rate of 50 - 70°C/s till an

intermediate temperature, TINT, given by Ae3 - 320 (°C) ≤ TINT ≤ Ae3 - 300 (°C) is
reached, followed by performing natural cooling till the coiling temperature (TCT) given
by 400°C < TCT < 500°C is reached, and then coiling the steel sheet at TCT-
According to the present invention, it is possible to produce a hot rolled steel sheet with
a minimum tensile strength of 590 MPa which also has good stretch flangeability,
consisting of a ferrite + bainite microstructure. Such a steel sheet is adaptable in a wide
spectrum of industrial fields including the automobile industry, the electric industry and
the machinery industry. It is particularly suited to manufacture automotive parts and
components and other industrial parts and components of complex shapes which
demand high strength, good formability and weldability, especially structural parts like
suspension and chassis components and wheel rims.
The present invention relates to a hot rolled steel sheet which has a specific alloying
composition and is manufactured with a precise control of the rolling and cooling
parameters in order to produce the target microstructure, such that a minimum tensile
strength of 590 MPa as well as good stretch flangeability is achieved.
The basic components constituting the hot rolled steel sheet produced according to the
present invention are described below.
Alloying additions: The addition of each alloying element and the limitations imposed
on each element are essential for achieving the target microstructure and properties.
C: 0.05-0.07 weight%: Carbon is one of the most effective and economical
strengthening elements. Carbon combines with Nb and V to form carbides or

carbonitrides which bring about precipitation strengthening. This requires a minimum of
0.05%C in the steel. However, in order to avoid the preitectic reaction during casting
(especially for continuous strip production or CSP facilities) and considering weldability
issues, the carbon content has to be restricted to less than 0.07%.
Mn: 1.0-1.5 weight%: Manganese not only imparts solid solution strengthening to the
ferrite but it also lowers the austenite to ferrite transformation temperature thereby
refining the ferrite grain size. However, the Mn level cannot be increased to beyond
1.5% as at such high levels it enhances centerline segregation during continuous
casting and also causes microstructural heterogeneities.
Si: 0.3-0.5 weight%: Silicon is a very efficient solid solution strengthening element. A
minimum of 0.3% Si is required for this purpose. However, additions of Si should be
restricted to less than 0.5% in order to prevent the formation of surface scales.
1Mb: 0.03 weight% maximum: Niobium is the most potent microalloying element for
grain refinement even when it is added in very small amounts. When in solid solution it
lowers the austenite to ferrite transformation temperature which not only refines the
ferrite grain size but also promotes the formation of lower transformation products like
bainite. However, to ensure the effectiveness of Nb, it should not be allowed to
precipitate before the transformation temperature is reached. To ensure that the entire
Nb content remains in solution before rolling commences, the maximum Nb content is
restricted to 0.03%. This limit has been specifically set keeping in mind the low
equalization temperatures possible in CSP processes.

V: 0.06 weight% maximum: Microalloying by Vanadium leads to precipitation
strengthening as well as grain refinement. The solubility of Vanadium in austenite is
more than that of other microalloying elements and so it is more likely to remain in
solution prior to transformation. During phase transformation, vanadium precipitates as
carbides and/or nitrides, depending on the relative carbon and nitrogen contents, at
grain boundaries and advancing phase boundaries, resulting in precipitation
strengthening as well as grain refinement. In order to achieve the desired
strengthening, it is required that up to 0.06% V is added. Addition of V in amounts
greater than 0.06 wt% would lead to the increase in cost.
P: 0.03 weight% maximum: Phosphorus content should be restricted to 0.03%
maximum as higher phosphorus levels can lead to reduction in toughness and
weldability due to segregation of P into grain boundaries.
S: 0.005 weight% maximum: The Sulphur content has to be limited otherwise it results
in a very high inclusion level that deteriorates formability.
N: 0.007 - 0.01 weight%: In order to achieve adequate precipitation strengthening by
Nb and V, it is essential a minimum of 0.007% N is present in the steel and more
preferably a minimum of 0.008% N is maintained. On the contrary, too high a N
content raises the dissolution temperature of Nb(CN) and hence reduces the
effectiveness of Nb. Reducing nitrogen levels also positively affects ageing stability and
toughness in the heat-affected zone of the weld seam, as well as resistance to inter-

crystalline stress-corrosion cracking. Thus N levels should be preferably kept below
0.01% or more specifically below 0.009%.
Microstructure: In order to achieve the target strength, various possible
strengthening mechanisms, have to be effectively utilized. As outlined above, the
strengthening contributions from solid solution elements and microalloying elements are
restricted. Also, the extent of possible grain refinement, by controlled rolling and
cooling is limited to 3-6 μm. In view of the above, the only way by which the target
strength can be achieved is by tailoring the microstructure and hence a microstructure
consisting of precipitation strengthened fine grained ferrite as matrix and bainite as the
second phase, was targeted in the present invention. In order to achieve the desired
stretch flangeability, the microstructure should be uniform or in other words the
strength difference between the matrix and the second phase should be low. Hence, a
fine and precipitation strengthened ferrite matrix with second phase bainite is a
preferred combination. Furthermore, martensite, degenerate pearlite/ pearlite and grain
boundary cementite should be avoided in order to achieve good stretch flangeability.
The contribution of each of the microstructural components is described below:
Ferrite: The hot rolled steel sheet according to the present invention has 70-90 %
ferrite. The ferrite is strengthened by solid solution strengthening contributions from Mn
and Si. Using suitable processing conditions, the grain size is restricted to 3-6 μm. This
grain refinement of ferrite leads to strengthening of the ferrite, the amount of which is

decided by the Hall-Petch relationship. Also it is precipitation strengthened by the
formation of fine Nb,V(CN) precipitates.
Bainite: The amount of bainite in the microstructure is 10-30%. It is essentially low
carbon bainite. The strengthening from bainite comes from its fine structure and higher
dislocation density.
1. Production process: The method of manufacturing the hot rolled steel sheet
according to the present invention consists of a casting step followed by a hot rolling
step, a controlled cooling step and a coiling step using a steel material which satisfies
the component composition described above. The various processing steps are
described in their respective order below:
Casting: In the present invention, the steel of the specified composition is first
continuously cast either in a conventional continuous caster or a thin slab caster. When
cast in a thin slab caster, the temperature of the cast slab is not allowed to drop to a
temperature below 1000°C. This is because if the thin slab temperature falls below
1000°C, Nb precipitation occurs. It then becomes difficult to completely dissolve the
precipitates in the subsequent reheating process rendering them ineffective for
precipitation strengthening.
Reheating: After casting the slab with the specified composition, the slabs are reheated
to a temperature of 1100 to 1200°C for a duration of 20 minutes to 2 hours. The
reheating temperature should be above 1100°C, to ensure complete dissolution of any
precipitates of Nb and V that may have formed in the preceding processing steps. A

reheating temperature greater than 1200°C is also not desirable because it may lead to
grain coarsening of austenite and/or excessive scale loss.
Hot Rolling: After casting and reheating the steel slab with the specified composition, it
is hot rolled. The hot rolling should constitute of a roughing step above the
recrystallization temperature and a finishing step below the recrystallization
temperature, when rolling is done in a conventional hot strip mill. In case a CSP is used
for producing this steel, where there is no separate roughing mill, the deformation
schedule should be designed in such a manner that the cast structure is destroyed in
the initial stands and finishing is done below the recrystallization temperature. More
specifically the finish rolling in either set up should be done at a temperature, TFRT given
by Ae3 - 50 (°C) ≤ TFRT ≤ Ae3 + 50 (°C).
Laminar cooling on the Run-Out-Table (ROT): After finish rolling, the rolled strip is
subjected to laminar cooling on the ROT at a cooling rate of 50 - 70°C/s till a desired
intermediate temperature is reached. The cooling rate should be higher than 50°C/s to
prevent formation of pearlite. Any pearlite or degenerate pearlite if formed leads to
deterioration of both, tensile strength as well as stretch flangeability. High cooling rate
also results in lowering the ferrite start temperature which leads to refinement of the
ferrite grain size. It also prevents the growth of the ferrite. In this way the desired grain
size of 3-6 μm can be achieved. The cooling rate should not be more than 70°C/s
because then the desired amount of ferrite will not form. This fast cooling is continued
up to an intermediate temperature, TINT below the bainite start temperature. This is

again to ensure that no pearlite or degenerate pearlite is formed. More specifically, this
temperature is given by Ae3 - 320 (°C) ≤ TINT ≤ Ae3 - 300 (°C). After this temperature
is attained, the strip is subjected to natural air cooling which facilitates the remaining
austenite to transform to bainite.
Coiling: Coiling is carried out at a temperature 400°C < TCT < 500°C. Coiling below
400°C is avoided to prevent the formation of martensite. A schematic diagram of the
cooling profile is shown in Fig. 1.
EXAMPLES For the purpose of example only, a slab of the composition according to the
present invention (Steel 1) and (Steel 2) were continuously casted in a CSP mill. Both
slabs were hot rolled according to the present invention. However, the ROT cooling for
both samples were different. For Steel 1, the ROT cooling was done in accordance with
the present invention, whereas for Steel 2 though the cooling rate was maintained, the
intermediate temperature was higher than that specified in the invention. The
mechanical properties of both steels are listed in Table 1. The microstructures of the
two steels are shown in Fig. 2. It is clear from the mechanical properties and the
microstructures achieved, that the target properties cannot be achieved when ROT
cooling parameters do not conform to the requirements of the invention.
Table 1: Coiling temperatures and mechanical properties


The invention as per the current invention provides a method of manufacturing a high
strength hot-rolled steel sheet with a minimum strength of at least 590 MPa. The
manufactured steel sheet comprises of 70-90% ferrite and 10-30% bainite and can be
used to manufacture automotive components such as suspension and chassis
components and wheel rims.

We Claim:
1. A process of manufacturing a high-strength hot-rolled steel sheet, the process
comprising:
continuous casting a steel slab of composition comprising (wt. %) 0.05-
0.07% of C, 1.0-1.5% of Mn, 0.3-0.5% of Si, 0.03% or less of Nb, 0.06%
or less of V, maximum 0.005% of S, maximum 0.030% of P, 0.007 -
0.01% of N, the remaining being iron and incidental impurities;
reheating the steel slab to a temperature greater than 1100°C;
hot rolling the steel slab to produce a steel sheet such that finish rolling is
done at a temperature (TFRT), wherein TFRT varies in the range Ae3 - 50
(°C) to Ae3 + 50 (°C), where Ae3 is the temperature at which the
transformation of austenite to ferrite starts at equilibrium;
cooling the steel sheet at a cooling rate of 50 - 70°C/s till an intermediate
temperature (TINT) is reached, wherein TINT varies in the range Ae3 - 320
(°C) to Ae3 - 300 (°C);
performing cooling in air till a coiling temperature (TCT ) is reached,
wherein TCT varies in the range 400°C to 500°C; and
and coiling the steel sheet at the coiling temperature TCT.
2. The process as claimed in claim 1, wherein the microstructure of the steel sheet
comprises of: 70-90% ferrite and 10-30% bainite.

3. The process as claimed in claim 2, wherein the ferrite is precipitation
strengthened and has a grain size of 3 - 6 μm.
4. The process as claimed in claims 1, wherein the steel sheet has a tensile
strength > 590 MPa and good stretch flangeability with hole expansion ratio
(HER) greater than 60%.
5. The process as claimed in claim 1 , wherein the steel slab is reheated to a
temperature in the range of 1100°C-1200 °C.
6. A high-strength hot-rolled steel sheet with a tensile strength ≥ 590 MPa
comprising, on a weight per cent basis, 0.05-0.07% of C, 1.0-1.5% of Mn, 0.3-
0.5% of Si, 0.03% of Nb, 0.06% or less of V, maximum 0.005% of S, maximum
0.030% of P, 0.007 - 0.01% of N, the remaining being substantially iron and
incidental impurities, wherein the high-strength hot-rolled steel sheet comprises
a microstructure of 70-90% ferrite and 10-30% bainite .
7. The high-strength hot-rolled steel sheet as per the claim 6, wherein the ferrite is
precipitation strengthened.
8. The high-strength hot-rolled steel sheet as per the claim 6, wherein the ferrite
has a grain size of 3 - 6 μm.

9. The high-strength hot-rolled steel sheet as per the claim 6, wherein the steel
sheet has a good stretch flangeability with hole expansion ratio (HER) greater
than 60%.

ABSTRACT

The present invention relates to a high-strength hot-rolled steel sheet with a minimum
tensile strength of 590 MPa and method of manufacturing the same. The high-strength
hot-rolled steel sheet of the current invention finds use in manufacturing automotive
components such as suspension and chassis components and wheel rims. The method
of manufacturing the hot-rolled steel sheet includes the steps of casting a steel slab in
either a conventional or a thin slab caster and then reheating the cast slab to a
temperature greater than 1100°C, hot rolling the slab such that finish rolling is done at
a temperature (TFRT), such that TFRT varies in the range Ae3 - 50 (°C) to Ae3 + 50 (°C),
where Ae3 is the temperature at which the transformation of austenite to ferrite starts
at equilibrium, and then cooling at a cooling rate of 50 - 70°C/s till an intermediate
temperature (TINT = Ae3 - 320 (°C) to Ae3 - 300 (°C)) is reached, followed by
performing natural cooling till the coiling temperature (Tcr) is reached, and then coiling
the steel sheet at TCT.