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FIELP OF APPLICATION
The present invention relates to a process for producing ferritic bainitic dual
phase steel sheet for wheel rim / disc application. Generally a wheel is made up
of two parts, a rim which is like a doughnut or a ring and a disc that looks like a
dinner plate with different holes (holes for tube, vent holes and holes for nut
bolts) in it.
The disc is put inside the rim and is normally welded (sometimes fastened) to
the rim to make the complete wheel.
However, in case of larger wheels, there are other components like flange, lock
ring, driver, etc.
BACKGROUND OF THE INVENTION
In order to reduce fuel cost, there has been a continuous effort to reduce the
weight of the automobile by employing high strength steel material.
The thickness of the steel sheet for rim and disc application can be reduced by
using high strength material.
The conventional steel used at present has lower strength (450 MPa UTS max).

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A need therefore, exists for developing a high strength ferritic bainitic dual phase
(DP) steel sheet for use in rim and disc application in the automobile industry.
Due to higher strength of the steel the thickness of the steel sheet can be
reduced resulting in weight reduction of the wheel rim / disc.
SUMMARY OF THE INVENTION
The main object of the present invention therefore is to develop a high strength
hot rolled steel sheet having good combination of properties such as strength,
flash butt weldability, fatigue characteristics and formability for wheel rim and
disc application.
This object is achieved in the present invention by designing a new chemistry
and optimizing the hot rolling parameters for modifying the microstructure,
achieving grain refinement and precipitation strengthening.
The improved design of steel chemistry and processing parameters used in the
present invention for producing dual phase structure (ferritic bainitic) steel
sheets makes it of higher strength (ultimate tensile strength more than 590
MPa).
The designed steel chemistry for the present invention is shown in Table 1.

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Table 1
c Mn S P Si Al Nb Ca
(ppm) N2
(ppm)
0.05-
0.08 1.30-
1.50 0.005
(max) 0.03
(max) 0.35-
0.50 0.02-
0.05 0.035 -
0.045 25
(min) 70
(max)
The hot rolling parameters are designed in such a way that the microstructure of
steel produced consists of 80% ferrite and 20% bainite.
Thus the present invention provides a process for producing ferritic bainitic dual
phase steel sheet comprising the steps of processing steel through basic oxygen
method - ladle furnace - slab caster route with a suitably designed chemistry; hot
rolling the slab; and optimizing the hot rolling parameters to achieve a
microstructure of 80% ferritie and 20% bainite.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows the wheel assembly with wheel rim and wheel disc
Figure 2 shows the flow diagram for the process of the present invention
Figure 3 shows wheel rim making flow chart
Figure 4 shows wheel disc making flow chart
Figure 5 shows wheel assembly making flow chart
Figure 6 shows schematic view of a hot strip mill

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According to the present invention steel is processed through basic oxygen
process (BOF)- ladle furnace (LF)- slab caster route and then slabs are hot rolled
with designed hot rolling parameters in such a way that the microstructure of the
steel / preferably consists of 80% ferrite and 20% bainite, which is aimed in the
present invention. The material from the middle stage of rolling is collected and
tested on thermo-mechanical simulator for finalizing the final hot rolling and
cooling parameters.
The composition / steel chemistry used for the present invention has been
shown in Table 1.
The roles of various alloying elements used in the steel for the present invention
are given below:
Carbon content (0.05 - 0.08) in the steel has been reduced to improve the
weldability and toughness. A higher manganese content (1.30 - 1.50) is used
to achieve the strength level and to form bainitic structure. Silicon content (0.35
- 0.50) is useful for facilitating ferrite formation. It also enhances the strength
and ductility.
Niobium content (0.035 - 0.045) is indispensable to this invention. It increases
the strength by the mechanism of precipitation strengthening and grain
refinement. It resists the softening behaviour of heat affected zone (HAZ) due
to the precipitation of fine niobium carbide (NbC). It also accelerates the ferrite
formation.

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Aluminium (0.02 - 0.05) is used as deoxidizer and to fix nitrogen in solid solution
by formation of aluminium nitride (AIN). Low sulphur (0.005 max) to control the
elongated MnS, which has a harmful effect on the properties. Addition of
calcium (25 ppm min) is effective in spherodizing the elongated MnS particles.
To achieve the ferritic-bainitic structure, it is necessary to control the hot rolling
parameters such as finish rolling temperature (FRT), cooling strategy and coiling
temperature (CT). A lower FRT is used for accelerated ferrite formation.
Intermediate holding at around 770° C is necessary to achieve required amount
of ferrite. Faster cooling and low temperature coiling is essential to achieve the
ferritic-bainitic structure.
The advantages of steel sheet produced by the present invention over
conventional (ferritic pearlitic) steel are high strength, better weldability, better
formability, high fatigue strength and better stretch flangeability.
Its advantages over dual-phase ferritic martensitic steel are steel with better
heat affected zone (HAZ) characteristics due to ferritic bainitic structure than
ferritic martensitic structure, better fatigue properties and better stretch flanging
properties.
Dated this 23th day of August 2005.

This invention relates to a process for producing ferritic bainitic dual phase steel sheet for wheel nm/disc application, comprising the steps of: processing steel through basic oxygen method - ladle furnace - slab caster route with a suitably designed chemistry for high strength; hot rolling the slab; and optimizing the hot rolling parameters to achieve a microstructure of 80 % ferritic and 20 % bainitic steel sheet.