Field of Invention
The present invention relates to a high strength thick (4.0 mm ≤ thickness ≤ 7.0mm)
hot rolled steel sheet with a minimum tensile strength of 600 MPa and good formability,
for automotive wheel disk manufacturing. 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, the weight reduction
achievable by substituting conventional mild steel with high strength steels is found to
be the largest for auto-wheels [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. Apart from the high
strength requirements, it is also essential that the steels used for manufacturing such
parts have good formability. However, it is a general phenomenon 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. There is a trend towards the use of hot rolled sheets of a minimum tensile
strength of 600 MPa for wheel rim applications.
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 600 MPa but also have good formability and fatigue
strength. Dual phase steel with 80-85% ferrite and 10-15% martensite is the suitable
material for the above mentioned purpose.
Japanese Patent WO2005005670-A1 discloses a method of producing a hot rolled dual
phase steel (ferrite/bainite + martensite) to be produced through hot strip mill route,
having minimum tensile strength range of 490-500 MPa for automotive application in
the thickness range of 2.5mm. Though the proposed steel is claimed to have excellent
shape fixability, no spring back phenomenon and no wall camber, the farigue
performance is expected to be quite low considering the low strength of the steel. Also
the proposed steel has low thickness.
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. The steel is subjected to baking after press forming. According to the
proposed method the steel must contain 0.5 to 2.0 wt% Si and 1.0 to 3.0% Mn. The
addition of this large amount of alloying elements is costly and the alloying elements
segregate at the center of the hot rolled strip.

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 pr°Cessing 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 not only induces cracking during press
forming but also leads to inconsistency in achieving the desired 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.

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 600 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 600 MPa which also has good formability especially stretch
flangeability or more specifically a hole expansion ratio or HER of ≥ 80% according to
the specification laid down by ISO 16630:2009(en).
Another object of the present invention is to propose a hot-rolled steel sheet having a
minimum tensile strength of 600 MPa and good stretch flangeability, which is adaptable
to the automotive industry, particularly for automotive wheel disk manufacturing.
A still another object of the present invention is to propose a hot-rolled steel sheet
having a minimum tensile strength of 600 MPa and good stretch flangeability, which
possesses a microstructure consisting of ferrite and martensite.
A further object of the present invention is to propose a hot-rolled steel sheet having a
minimum tensile strength of 600 MPa and good stretch flangeability, which possesses a
microstructure consisting of ferrite and martensite, wherein the ferrite has a grain size
of 3 - 6 µm.
Detailed description of the invention
The hot rolled steel sheet having a minimum tensile strength of 600 MPa and good
stretch flangeability, according to the present invention contains in weight percent

0.045-0.065% of C, 1.3-1.4% of Mn, 0.3-0.4% of Si, 0.013-0.018% of Nb, 0.5-0.6% of
Cr, maximum 0.005% of S, maximum 0.030% of P, 0.007 - 0.01% of N, maximum
0.05% of Al, maximum 0.01% of Cu, the remaining being substantially iron and
incidental impurities.
The hot rolled steel sheet having a minimum tensile strength of 600 MPa and good
stretch flangeability, according to the present invention has a microstructure comprising
80-84% of ferrite, 12-16% of martensite and less than 4% bainite.
The hot rolled steel sheet having a minimum tensile strength of 600 MPa and good
stretch flangeability, according to the present invention has a microstructure comprising
80-84% of ferrite, 12-16% of martensite and less than 4% bainite, wherein the ferrite
is strengthened by solid solution strengthening due to addition of Si, Mn and has a
grain size of 3 - 6nm.
The method of manufacturing the hot rolled steel sheet with a ferrite + martensite
microstructure with a minimum tensile strength of 600 MPa and good stretch
flangeability includes the steps of casting the slab in 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 810°C≤ TFRT ≤ 835°C, and
then cooling at a cooling rate of 20 - 50°C/s till an intermediate temperature, TINT,
given by 670°C ≤ TINT ≤ 700°C is reached and holding there for 5-6 seconds, followed
by cooling at a cooling rate of 35 - 50°C/s till the coiling temperature given by TCT <
250°C is reached, and then coiling the strip at TCT.

According to the present invention, it is possible to produce a hot rolled steel sheet with
a minimum tensile strength of 600 MPa which also has good stretch flangeability,
consisting of a ferrite + martensite microstructure. Such a steel sheet is adaptable in a
wide spectrum of industrial fields including the automobile 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.
Brief description of drawings
Fig. 1: Schematic diagram of cooling profile
Fig. 2: Microstructures of (a) Steel 1 (Inventive Example) and (b) Steel 2 (Comparative
Example).
Brief description of tables
Table 1: Mechanical properties
Best mode for carrying out the invention
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 600 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.
1. 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: 0.045-0.065%: 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 and grain refinement. This requires a minimum of 0.045%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 0.065%.
Mn: 1.30-1.4%: 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 1.4% as at
such high levels it enhances centerline segregation during continuous casting.
Si: 0.3-0.4%: Silicon like Mn 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 0.4% in order to prevent the formation of surface scales.
Cr: 0.5-0.6%: Chromium is a very effective alloying element to improve the
hardenability of the steel. A minimum of 0.5% of Cr is required to impart the
hardenability such that austenite to martensite transformation can take place during
cooling at RoT of TSCR mill. However, the Cr level cannot be increased to beyond 0.6%
as it adds to difficulty in the weldability of steel.
Nb: 0.013-0.018%: 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 and refines the ferrite grain
size. 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
0.018%. This limit has been specifically set keeping in mind the low equalization
temperatures possible in CSP pr°Cesses.

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: 0.007 - 0.01%: In order to achieve adequate grain refinement by Nb, 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 0.001% or more specifically below 0.009%.
2. 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 grain refinement 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 fine grained ferrite as matrix and martensite 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. In order to achieve required CFT test
performance ferrite strengthening by addition of Si and Mn is done. Hence, a fine
grained and solid solution strengthened ferrite matrix with second phase martensite is a

preferred combination. Furthermore, 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 80-84 %
ferrite. The ferrite is strengthened by solid solution strengthening contributions from Mn
and Si. Using suitable pr°Cessing 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.
Martensite: The hot rolled sheet according to the present invention has 12-16%
martensite. The martensite is hard phase and adds to the overall strength of the dual
phase steel. A minimum of 12% martensite is required to achieve 600 MPa UTS.
Bainite: The amount of bainite in the microstructure is less than 4%. It is essentially
low carbon bainite. Presence of more than 4% bainite is not desirable in the steel.
3. Production pr°Cess: 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 pr°Cessing steps are
described in their respective order below:
Casting: In the present invention, the steel of the specified composition is first
continuously cast in 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 °Ccurs. It then becomes
difficult to completely dissolve the precipitates in the subsequent reheating pr°Cess
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 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 pr°Cessing steps. A reheating
temperature greater than 1200°C is also not desirable because it leads 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. For producing this steel in CSP facility, 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 810°C≤ TFRT ≤ 835°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 20 - 50°C/s till a desired
intermediate temperature is reached. The cooling rate should be higher than 20°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 um can be achieved. The cooling rate should not be more than 50°C/s

because then the desired amount of ferrite will not form. This fast cooling is continued
up to an intermediate temperature, TINT. More specifically, this temperature is given by
670°C ≤ TINT ≤ 700°C. After this temperature is attained, the strip is subjected to
natural air cooling which facilitates the austenite to transform to ferrite. The holding
time at the intermediate temperature (TINT) has to be precisely controlled for 5-6
seconds. Less than 5 sec of holding time, causes less austenite to ferrite transformation
and subsequently formation of a higher amount of martensite and/or bainite at the
subsequent cooling stages. Whereas more than 6 seconds of holding at TINT causes
high austenite to ferrite transformation and subsequently results in less amount of
second phase martensite. After holding at the TINT, the steel is rapidly cooled at a
cooling rate of 35 - 50°C/s till the coiling temperature given by TCT < 250°C is reached,
and then coiling the strip at TCT. At this stage a cooling rate of 35 - 50°C/s is required
to transform the remaining austenite to transform to martensite. Less than 35°C/s leads
to a undesired microstructure of higher amount of bainite and less of martensite.
Coiling: Coiling is carried out at a temperature TCT < 250°C. Coiling above 250°C is
avoided to prevent the formation of bainite. 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) as well as another of a slightly deviant chemistry (Steel 2) was
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
though the cooling rate was maintained, the intermediate temperature was lower
(625°C) than that specified in the invention. The holding time for steel 2 is 10 seconds
which is higher than the steel 1. In case of the steel 1 holding time at the TINT is

accordance with the present 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 the chemistry and ROT cooling parameters do not
conform to the requirements of the invention.

A full component wheel disk was fabricated to experimentally measure the cornered
fatigue test (CFT) value of the present invention according to the specification laid
down by IS:9436-1980. The values are reported in the following table.


One of the objectives of the present invention is to reduce the thickness of the AHSS to
be used for manufacturing automotive wheel disk. Use of thinner section wheel in
automotive shall reduce the weight of the vehicle and shall increase the fuel efficiency.

We Claim:
1. A high strength hot rolled steel, the high strength steel comprising in weight
percent: 0.045-0.065% of C; 1.3-1.4% of Mn; 0.3-0.4% of Si; 0.013-0.018% of
Nb; 0.5-0.6% of Cr; maximum 0.005% of S; maximum 0.030% of P; 0.007 -
0.01% of N; maximum 0.05% of Al; maximum 0.01% of Cu; and the remaining
being substantially iron and incidental impurities, wherein the high-strength hot-
rolled steel sheet has a tensile strength of at least 600 MPa.
2. A pr°Cess of manufacturing a high strength hot-rolled steel sheet as claimed in
claim 1, the pr°Cess comprising the steps:
Continuous casting; (slab thickness: 60-70mm)
Reheating the cast slab to a temperature greater than 1100°C;
Hot rolling the slab, wherein finish rolling is done at a temperature, TFRT, such
that 810°C≤ TFRT ≤ 835°C;
Cooling the steel strip at a cooling rate of 20 - 50°C/s till an intermediate
temperature, TINT, given by 670°C ≤ TINT ≤ 700°C is reached;
Holding for 5-6 seconds and cooling the strip at a cooling rate of 35 - 50°C/s till
the coiling temperature, TCT < 250°C is reached; and
Coiling the strip at TCT.
3. The high strength hot rolled steel as claimed in claim 1, wherein the
microstructure of the produced steel sheet comprises 80-84% ferrite, 12-16%
martensite and less than 4% bainite.
4. The high strength hot rolled steel as claimed in claim 3, wherein the ferrite is
solid solution strengthened and has a grain size of 3 - 6 µm.

5. The high strength hot rolled steel as claimed in claim 1, wherein high strength
hot rolled steel has stretch flangeability with HER% (in drilled hole condition)
greater than 80%.
6. A wheel disc produced from the high strength steel claimed in claim 1 to claim 5.
7. The wheel disc as claimed in claim 6, wherein the wheel achieves cornered
fatigue test (CFT) value of at least 2.67 lacs.