FIELD OF INVENTION
The present invention relates to a method of producing hot rolled high
strength steel (>600MPa) adaptable to wheel rim and disc application.
BACKGROUND OF THE INVENTION
In the present scenario, the increased fuel economy leading to
environmental protection have necessitated to develop hot-rolled thin steel
sheet with high tensile strength coupled with high elongation for automotive
underbody parts such as wheel rim and disc.
Such steel have been produced as advanced high strength steel for
automotive application with 800 MPa minimum tensile strength through
deformation of steel sheet of Ti and Mo added to low carbon steel
encouraging interphase precipitation to produce microstructure of fine
ferrite. (Tata Steel Reference application no. 958/KOL/2007 dated 3rd July,
2007)
Such automobile sheets/strips have been produced in the form of Dual
phase steel consisting of a ferrite microstructure and a second phase i.e;

pearlite, bainite, martensite or retained austenite as disclosed by JP-A-4-
329848 referred herein signifies "Unexamined Japanese patent
Publication"). However, due to existence of second hard phases in these
steels a good stretch flangeability can not be achieved.
Such steel sheet/strip have also been proposed with an acicular ferrite
microstructure and excellent stretch flangeability in which high tensile
strength is obtained by fine grain acicular ferrite structure along with
precipitates of TiC or NbC (Japanese Reference JP-A-7-11382). However,
this steel lacks in sufficient elongation due to existence of acicular ferrite
microstructure which contains high dislocation density.
Such steel sheet/strip can also be obtained by having a ferrite-bainite
structure (Japanese Reference, JP-A-6-172924). Although, this steel is
having higher stretch flangeability but do not show higher elongation due to
bainitic ferrite structure having high dislocation density.
Such steel sheet/strips can also be obtained with either solid solution
strengthening or precipitation strengthening by TiC in a matrix of polygonal
ferrite (Japanese Reference, JP-A-6-200351). However, high elongation and
excellent hole expansion ration can not be obtained due to coarseness of
this precipitates, which are not thermally stable at the conventional
processing temperatures.

The proposed invention has been developed to solve the difficulties of high
elongation and excellent stretch flangeability in hot-rolled high strength
steel sheet for fine grains (2-4 µm) automotive wheel rim and discs
application by developing a single phase fine grain ferritic steel strengthened
by precipitation hardening with tensile strength of 600 MPa minimum.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a method of
producing hot-rolled high strength steels which eliminates the
disadvantages of prior art.
Another object of the present invention is to propose a method of producing
hot-rolled high strength steels which generates a single phase
microstructure of fine ferrite (2-4 µm).
A further object of the present invention is to propose a method of
producing hot-rolled high strength steels which increases strength due to
addition of precipitation hardening elements like Ti and Mo.
A still further object of the present invention is to propose a method of
producing hot-rolled high strength steels which yields UTS > 600 MPa and
elongation >23%.

An yet further object of the present invention is to propose a method of
producing hot-rolled high strength steels which achieves an excellent hole
expansion percentage in hot rolled steel.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - shows a graph indicating the relationship between dilation and
transformation temperature
Fig. 2 - shows a graph indicating the CCT of steel under investigation
Fig. 3 - shows a microstructure of the steel as per invention that showing
single phase ferrite grains
Fig.4 - shows the increase in yield strength due to, solid solution, grain
refinement and precipitation strengthening
Fig.5 - shows the samples of the steel subjected to hole expansion test
Fig.6 - shows a fractography of fractured portion of sample showing entirely
simple mode
Fig.7 - shows a graph indicating the relationship between TS x El/t0-2 and
press formability
Fig.8 - shows S-N Curve in high cycle fatigue testing for the steel as per
invention

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The present invention relates to a process of preparing high strength steel
that comprising making of liquid steel in a basic oxygen furnace-transfer the
liquid steel to a ladle furnace wherein the heat was properly degassed to
restrict the nitrogen content in order to achieve maximum Ti, Mo carbide
precipitates.
The final composition in wt% of liquid steels are achieved before tapping as
follows:
C-0.04 to 0.08 Mn- 1.15 to 1.3 Si-0.1 to 0.2
S-0.008 max P - 0.025 max Al-0.01 to 0.07
N- 0.005 max Mo-0.08 to 0.12 Ti-0.04 to 0.05
The liquid steel melt having such compostion was continuously cast into
slab of 1100 mm width and 210 mm thickness with a casting speed of 1.0-
1.30 metre/minute. The casting speed was optimized to avoid any break out
during slab casting. The slabs were hot rolled into strip of 1100 mm width
and 3.2 mm thickness at FRT of 850-920°C and coiling temperature of 500-
630°C respectively. The design of the alloy and processing is done in such a
way that the transformation to ferrite is delayed and transferred to the end
of run out table. Accordingly, the rolling load experienced at first stand and

last stand of finishing mill was 14.0 MN maximum and 7.0 MN maximum
respectively. The rolling load in the mill for this steel was lowered as the
precipitations was avoided during roughing and finish rolling. The
mechanical properties of steel was shown in Table-1.
Table-1: Mechanical properties of steel

Tensile testing was carried out in longitudinal as well as transverse
direction on 15 number of samples for each as per ASTM E8. The tensile
strength more than 600 MPa with a total elongation 23% minimum was
easily achieved.
It is possible to measure precipitation strengthening from knowledge of
grain size and composition, assuming that the strengthening due to
dislocation is minimal. As shown in Fig.3 the microstructure is single phase
ferrite having more than 90% grain in the size range of 3.4 to 3.6 um while
remaining grains are still finer. Since the amount of fine precipitates and
grain size of ferrite exert the great influence on the strength, the amount of
precipitation strengthening was estimated by subtracting the strengthening
due to solid solution hardening and grain size strengthening from the
measured yield strength. The yield strength of steel can be expressed by the
following equation:


Assuming solid solution strengthening for Mn 32.34 MPa%, for Si 83.16
MPa%, for P 680 MPa% and for MA (microalloying) 11 MPa%, the σSSs value
is 85.57 MPa. The strength of pure iron σo is around 52.39 MPa. The value
of σgs as per Hall-Petch equation of 18.1 MPa/mnr1/2, is 290 MPa. The
estimated precipitation strength of the steel under investigation, based on
longitudinal yield strength is approximately 145 MPa. This is much higher
than that of conventional hot-rolled steel sheet reported in past. The
different component of strengthening is shown in a Figure 4.
Stretch-flange-formability was evaluated by the hole expanding test. The
100 mm square sheet with a drilled hole of 10 mm in diameter machined by
Lathe at the center of the steel sheet. The hole was expanded by a conical
punch having 50 mm dia until a fracture occurred at the hole edge. The hole
expansion ratio was calculated using the following formula:
% hole expansion = [(Final dia - initial dia)] X 100
The results of hole expanding test of the steel sheet under investigation is
shown in Table 2 and the hole expanded samples are shown in Figure 5.
The fractography of specimen cut from the crack portion area of hole

expanded sample was studied under Scanning Electron microscope revealed
the ductile mode of fracture as shown in Figure 6.
Table-2: Hole expansion test results of steel under investigation

In general, a high strength hot-rolled steel having tensile strength 600 MPa
or more and possessing good hole expanding ratio is suitable for wheel rim
and discs during press forming without any crack provided it satisfies the
equation of (TS x E1) /t0.2> 12000 where TS is tensile strength MPa, E1 is total
elongation % and t is sheet material thickness, and is effective as an index
representing the difficulty in developing press cracking. The E1 of steel sheet
with different thickness (t) can be converted by using Oliver's equation, and
is proportional to t0.2. As shown in Figure 7, if (TS x El)/t0.2 exceeds 12200,
cracking does not occur at all.
High fatigue strength is also required for hot-rolled steel sheets for wheel
rim and disc. Figure 8 shows the stress-life curve on a log-log plot for steel
under investigation. At long lives most steels have only a small component

of cyclic strain which is plastic and the S-N approach is valid. In general the
fatigue limit increases as the strength of the base material increases.
However, when the strength level exceeds 600 MPa, conventional hot-rolled
sheets show a smaller increase in the fatigue limit (endurance limit/tensile
strength) relative to the increase in the strength of base material. In
contrast, high fatigue strength corresponding to the strength of base
material can be obtained with the steel under investigation. The endurance
limit of this grade is 310 MPa at 5 X 106 cycles. The fatigue strength ratio of
the steel is 0.51 which is much higher than any available grade in this
range of strength.

WE CLAIM
1. A method of producing high strength hot-rolled steel sheets for
automotive wheel rim and disc application comprising the steps of:
- making a liquid steel in a basic oxygen furnace (BOF) having
composition in wt%
C-0.04 to 0.08 Mn- 1.15 to 1.3 Si-0.1 to 0.2
S-0.008 max P - 0.025 max Al-0.01 to 0.07
N- 0.005 max Mo- 0.08 to 0.12 Ti- 0.04 to 0.05
- refining and degassing of nitrogen of the said steel in a ladle furnace;
- continuous casting the liquid steel into slab casting;
- hot rolling the slab casting into strip of 1100 mm width and 3.2 mm
thickness at FRT 850-920°C; and
- coiling the strips at coiling temperature 500-630°C
characterised in that the said steel achieves high strength due to
presence of precipitation hardening elements (Ti and Mo) that develops a
single phase microstructure of fine ferrite (2-4 um) after deformation.
2. The method of producing the high strength hot rolled steel as claimed
in claim 1, wherein the steel possess 615 to 630 MPa UTS and
elongation 23 to 30%.

3. The method of producing the high strength hot rolled steel as claimed
in claim 1, wherein the steel possess 175 to 178% hole expansion
after deformation.
4. The method of producing the high strength hot rolled steel as claimed
in claim 1, wherein the steel possess a fatigue strength ratio of 0.50 -
0.60.

A method of producing high strength hot-rolled steel sheets for automotive wheel rim and disc application comprising the steps of making a liquid steel in a basic oxygen furnace (BOF) having composition in wt% C- 0.04 to 0.08,
Mn- 1.15 to 1.3, Si- 0.1 to 0.2, S- 0.008 max, P - 0.025 max, Al- 0.01 to 0.07, N- 0.005 max, Mo- 0.08 to 0.12 and Ti- 0.04 to 0.05; refining and degassing of nitrogen of the said steel is carried in a ladle furnace; continuous casting the liquid steel into slab casting; hot rolling the slab casting into strip of 1100 mm width and 3.2 mm thickness at FRT 850-920°C and coiling the strips at coiling temperature 500-630°C to obtain high tensile (600 MPa minimum) with excellent elongation, hole expansion ratio and high endurance fatigue strength.