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FIELD OF THE INVENTION
The present invention relates to producing an ultra high strength steel with
minimal ductility from ferrite-bainite-dual phase steel and the steel sheet / strip
produced thereof.
BACKGROUND OF THE INVENTION
In automobile and traction application problems are associated to generate high
horse power motors for higher weight of auto-traction body, tractions parts and
various locking and coupling parts of strips and sheets.
Attempts have been therefore made in automobile industry to develop auto body
and its associated parts with reduced weight with higher strength maintenance
for optimal and smooth running of auto and traction equipment.

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In general such steels as referred in the prior state of art have been developed
with strength ductility relationship having inverse relationship in steel gamete.
The present invention has proposed to develop advance high strength steel
(AHSS) from Carbon-Manganese-Silicon based steel to result very high strength
steel with nominal ductility in a new approach.
DESCRIPTION OF THE INVENTION
One of the object of the invention is to prepare a steel composition in weight %
C - 0.1-0.3, Mn - 1.1 - 1.7, Si - 0.2 - 0.6, S - < 0.015, P - < 0.02, Ti - < 0.01,
V - 0.1 - 0.3, Cr - 0.05 - 0.1, in an air induction furnace.
Another object of the invention is to develop a dual phase ferrite-bainite steel
with homogenized microstructure by forging a steel bloom of C-Mn-Si based steel
at 900 - 1100° C with different amount of deformation and temperature within
the said range at each stroke and hot rolling the forged bloom just above Ar3
(860° C - 920° C) temperature followed by normalizing to obtain a hot rolled
steel sheet / strip.

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A yet another object of the invention is cold deforming the ferrite-bainite-steel
slab to different percentages of 20 to 85 and finally recrystallised annealing the
resulted strip or sheet at temperature ranges from 200-700° C for 20 - 60
minutes.
The hot rolled structure of ferrite bainite has YS - 400 MPa, UTS - 550 MPa and
elongation - 20 %.
The said hot rolled ferrite - bainite structure is further transformed with further
reduction of ductility on cold working the slab at different percentages of 20 to
85 % and on final recrystallisation annealing at 200° C to 700° C to result
advance high strength steel with YS-750 MPa, UTS - 850 MPa and elongation -
15 %.

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The microstructures of the hot rolled ferrite-bainite steel slab and final cold
worked recrystallised annealed slab of transformed ferrite-bainite structure at
different magnifications of 50 to 500 are represented in Figures 1 and 2. The
initial microstructure of hot rolled slab constituents of ~ 75 vol % of bainite and
~ 25 vol % of ferrite. It results in YS: ~ 400 MPa, UTS: ~ 550 MPa and
Elongation: ~ 20 %. The final microstructure also constitutes the same amount
of bainite and ferrite. The cold rolling and subsequent annealing results in YS: ~
750 MPa, UTS: ~ 850 MPa and Elongation: ~ 15 %.
More oriented grains in the final structure and more fine grained structures
obtained after giving cold deformation and recrystallisation annealing of the hot
rolled slab of ferrite-bainite structure.
The invention is illustrated with an example in Table-1 below showing high
strength-steel obtained of 750 MPa with 15 % elongation on 20 to 80 % cold
working and final recrystallisation annealing at 200° C to 700°
C of the steel slabs. The steel samples of tensile strength resulted above UTS
850 MPa with less than 15 % elongation of the slab samples break under the
conditions used in automobile or traction parts and therefore discarded.

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% Cold Rolledreduction Annealingtemperature, ° C Strength, MPa % Elongation
200 870 9
300 800 11
20 400 790 13
500 750 15
600 680 16
700 550 19
30 200 850 9
300 845 10
400 840 12
500 790 14
600 700 18
700 600 20
50 200 880 7
300 870 12
400 850 15
500 820 16
600 750 19
700 700 20
200 1100 4
300 1085 7
80 400 1000 9
500 990 9
600 800 14
700 600 20

-7-
Though the invention has been narrated with a specific embodiment, it should
not be read and construed in a restrictive manner as various modifications,
alterations and adaptations are possible within the scope and ambit of the
invention as defined in the encompassed appended claims.

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WE CLAIM
1. A method of producing an ultra high strength steel with minimal ductility
from ferrite-bainite dual phase steel comprising the steps of:
producing C-Mn-Si based steel having composition in weight %
C - 0.1-0.3, Mn - 1.1 - 1.7, Si - 0.2 - 0.6, S - < 0.015, P - <
.02, 71 - < 0.01, V - 0.1 - 0.3, Cr - 0.05 - 0.1, in an air
induction furnace, forging a bloom of said steel at 900-1100° C
with different amount of deformation at each stroke to achieve
a homogenized structure and conveying the same to hot rolling
mill, and hot rolling the bloom at just above Ar3 (860o C - 920°
C) temperature and normalising to obtain slab of ferrite-bainite
steel slab,
cold rolling the said hot rolled slab to 20-85 % deformation and
annealing the cold reduced steel sheet / strip at 200-700° C for
20 - 60 minutes to result ultra high strength steel with minimal
ductility.

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2. A method of producing an ultra high strength steel as claimed in claim 1,
wherein on hot rolling at final recrystallisation temperature just above A3,
results ferrite-bainite microstructure steel having YS-400 MPa, UTS - 550
MPa and elongation - 20 %.
3. A method as claimed in claim 1, wherein the high strength end product
steel sheet / strip has YS - 750 MPa, UTS - 850 MPa and elongation - 15
%.
4. A steel sheet / strip of composition in weight % C - 0.1-0.3, Mn - 1.1 -
1.7, Si - 0.2 - 0.6, S - < 0.015, P - < 0.02, Ti - < 0.01, V - 0.1 - 0.3, Cr -
0.05 - 0.1, with ultra high strength and minimal ductility produced
according to the method of claim 1.

This invention relates to a method of producing an ultra high strength steel with minimal ductility from ferrite-bainite dual phase steel comprising the steps of: producing C-Mn-Si based steel having composition in weight % of C - 0.1-0.3, Mn - 1.1 - 1.7, Si - 0.2 - 0.6, S - ≤ 0.015, P - ≤ 0.02, Ti - ≤ 0.01, V - 0.1 - 0.3, Cr - 0.05 - 0.1, 1 in an air induction furnace, forging a bloom of said steel at 900-1100°C with different amount of deformation at each stroke to achieve a homogenized structure and conveying the same to hot rolling mill, and hot rolling the bloom at just above Ar3 temperature and normalising to obtain slab of ferrite-bainite steel slab, cold rolling the said hot rolled slab to 20-85 % deformation and annealing the cold reduced steel sheet / strip at 200-700°C for 20 - 60 minutes to result ultra high strength steel with minimal ductility.