Field of Invention
The present invention relates to a high-strength hot-rolled steel sheet with a minimum
tensile strength of 540 MPa for automotive applications like wheel rims. 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. This necessitates the replacement of conventional low strength steels
with higher strength steels so that thinner sections can be used and reduction in the
weight of the auto components is possible without compromising on safety and
functional requirements. Among the various auto-components, substituting conventional
mild steel auto-wheels with high strength steels auto-wheels finds greatest possibility in
terms of weight reduction [T. Irie, K. Tsunoyama, M. Shinozaki and T. Kato: SAE Paper
No. 880695, 1988], and the corresponding energy savings is estimated to be 1.2-1.3
times the amount possible for non rotating parts [M. Mizui, T. Sekine, S. Soneda and T.
Herai: SAE Paper No. 850540, 1985].
The automotive wheel is composed of a disk and a rim. While the disc is press formed,
the rim is flared and then roll formed after flash butt welding. From the forming point of
view, the rim material needs to have good formability after welding. From the point of

view of application, the most important functional requirement is durability, which can
be increased by increasing the fatigue strength of the rim material. There is a trend
towards the use of hot-rolled sheets of a minimum tensile strength of 540 MPa for
wheel rim applications.
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.
European Patent EP1201780B1 discloses a method of producing a hot-rolled steel sheet
with a microstructure consisting of ferrite and martensite and a minimum tensile
strength of 540 MPa. However, this steel cannot be used for wheel rim applications, as
wheel rims are subjected to forming after welding and welding results in tempering of
martensite causing softening of the steel.
European Patent EP2243853A1 discloses a method of producing a high-strength hot-
rolled steel sheet of tensile strength varying in the range of 540 to 780 MPa. 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 high-strength hot-rolled
steel sheet having tensile strength of at least 540 MPa.
Another object of the present invention is to propose a high-strength hot-rolled steel
sheet having strength of at least 540 MPa which is adaptable to automotive industry in
particular for manufacturing of automotive wheel rims
Still another object of the present invention is to propose a high-strength hot-rolled
steel sheet having a minimum tensile strength of 540 MPa, which includes 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 540 MPa, which possesses a microstructure consisting of
ferrite and bainite, wherein the ferrite is precipitation strengthened and has a grain size
of 3 - 6 µm.
Another object of the present invention is to propose a manufacturing to produce a hot-
rolled steel sheet with a microstructure consisting of fine grained ferrite and bainite and
having a minimum tensile strength of 540 MPa.

Summary of the Invention
The invention discloses a high-strength hot-rolled steel sheet with a tensile strength of
at least 540 MPa and method of manufacturing the same. A steel slab of the
composition comprising, in weight per cent 0.05-0.07% of C, 1.0-1.5% of Mn, 0.1-0.3%
of Si, 0.03% or less (not including 0%) of Nb, maximum 0.005% of S, maximum
0.030% of P, maximum 0.005% 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 (TCT)-
TINT as per the current invention varies in the range Ae3 - 320 (°C) to Ae3 - 300 (°C)
and coiling temperature TCT 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 540 MPa according to
the present invention contains in weight percent 0.05-0.07% of C, 1.0-1.5% of Mn, 0.1-
0.3% of Si, 0.03% or less (not including 0%) of Nb, maximum 0.005% of S, maximum
0.030% of P, maximum 0.005% of N, the remaining being substantially iron and
incidental impurities. The hot-rolled steel sheet a minimum tensile strength of 540 MPa
according to the present invention has a microstructure comprising 70-90% of ferrite
and 10-30% 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 540 MPa includes the steps of
casting the slab in either a conventional or a thin slab caster 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.
The present invention produces a hot-rolled steel sheet with a minimum tensile strength
of 540 MPa, 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 which
demand a high strength, good formability and weldability, especially 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 540 MPa is achieved.
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%: Carbon is one of the most effective and economical strengthening
elements. Carbon combines with Nb 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%: 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%. At
high levels it enhances centerline segregation during continuous casting. The preferable
range for Mn is 1.0 - 1.3 wt. %.
Si: 0.1-0.5%: Silicon like Mn is a very efficient solid solution strengthening element.
However, additions of Si should be restricted to less than 0.5% or more specifically less
than 0.3% in order to prevent the formation of surface scales.
Nb: 0.03% 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.
P: 0.03% 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% maximum: The Sulphur content has to be limited otherwise it results in a
very high inclusion level that deteriorates formability.
N: 0.005% maximum: Reducing nitrogen levels 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. Increasing N content also raises the dissolution
temperature of Nb(CN) and hence reduces the effectiveness of Nb. Thus Nb levels
should be preferably below 0.005%.
Microstructure: In order to achieve the targeted strength properties, 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. 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(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.
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 greater than 1100 °C (preferably in the range 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 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 steel
sheet 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. 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 is
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. More specifically, this
temperature is given by Ae3 - 320 (°C) ≤ TINT ≤ Ae3 - 300 (°C). After this temperature
is attained, the steel sheet is subjected to natural air cooling which facilitates the
remaining austenite to transform to bainite.
Coiling: Coiling is carried out at a coiling temperature (TCT) given by 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
Two slabs as per the composition of present invention were continuously cast 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 a slower cooling rate and a
higher coiling temperature were used. The coiling temperatures for both steels as well
as their mechanical properties 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 the ROT
cooling parameters do not conform to the processing parameters of the invention.

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 540 MPa. Further,
the process of the current invention can be applied to hot rolling facilities which are not
equipped with a reheating furnace. The manufactured steel sheet comprises of 70-90%
ferrite and 10-30% bainite and can be used to manufacture automotive components
such as 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 Carbon, 1.0-1.5% of Manganese, 0.1-0.5% of Silicon, 0.03% or
less (not including 0%) of Niobium, maximum 0.005% of Sulphur,
maximum 0.030% of Phosphorous, maximum 0.005% of Nitrogen, 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 (Tcr ) is reached,
wherein TCT varies in the range 400°C to 500°C; and
and coiling the steel sheet at the coiling temperature Tcr.

2. The process as claimed in claim 1, wherein microstructure of the high-strength
hot-rolled steel sheet comprises of 70-90% ferrite and 10-30% bainite.
3. The process as claimed in claim 1 and claim 2, wherein the ferrite is precipitation
strengthened.
4. The process as claimed in claim 1 and claim 2, wherein the ferrite has a grain
size of 3 - 6µm.
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. The process as claimed in claim 1, wherein Manganese is preferably present in
the range of 1-1.3 wt.%.
7. The process as claimed in claim 1, wherein Silicon is preferably present in the
range of 0.1-0.3 wt.%.
8. A high-strength hot-rolled steel sheet with a tensile strength ≥ 540 MPa
comprising, on a weight per cent basis, 0.05-0.07% of Carbon, 1.0-1.5% of
Manganese, 0.1-0.5% of Silicon, 0.03% or less (not including 0%) of Niobium,
maximum 0.005% of Sulphur, maximum 0.030% of Phosphorous, maximum
0.005% of Nitrogen, the remaining being iron and incidental impurities; and a
microstructure comprising of 70-90% ferrite and 10-30% bainite.

9. The high-strength hot-rolled steel sheet as per the claim 8, wherein the ferrite is
precipitation strengthened.
10.The high-strength hot-rolled steel sheet as per the claim 8, wherein the ferrite
has a grain size of 3 - 6 µm.
11. The high-strength hot-rolled steel sheet as per the claim 8, wherein Manganese
is preferably present in the range of 1-1.3 wt. %.
12. The high-strength hot-rolled steel sheet as per the claim 8, wherein Silicon is
preferably present in the range of 0.1-0.3 wt. %.