TITLE:
A PROCESS FOR MANUFACTURING A HOT-ROLLED HIGH STRENGTH STEEL BY TSCR
ROUTE
REFERENCE TO RELATED APPLICATION:
The invention disclosed in the application is an improvement in or a modification of the
invention disclosed and claimed in the specification of the Patent No. 241163
(958/KOL/2007) and claims benefits of Section 54-56 of the Patents Act, 1970
FIELD OF THE INVENTION
The invention relates to manufacturing hot-rolled high strength steel. Particularly
the invention relates to manufacturing hot-rolled high strength steel by thin slab casting
route (TSCR).
BACKGROUND OF THE INVENTION
Applications of Advanced High Strength Steels (AHSSs) are gradually increasing
in automotive industries due to its weight saving potential and improved
crashworthiness. Higher strength in advanced high strength steels is obtained through
various mechanisms such as grain refinement, phase transformation and precipitation
strengthening. In recent times, nano precipitate strengthened steels are getting more
attention due to its superior strength–formability combination. This is due to single
phase ferrite microstructure with nano precipitates dispersed in the ferrite matrix.
High strength steel slabs are normally produced through basic oxygen furnace
followed by ladle furnace and continuous casting route. And finally rolled through
conventional hot strip mill route where a slab of 210 mm thickness is normally reheated
C and subsequently reduced to 3-10 mm strips.
Modern technology such as Thin Slab Casting and Rolling (TSCR) process is an
integrated route where a much thinner slab (70 mm) is reduced to 3-10 mm strip
through hot rolling. Since TSCR is an integrated process it saves energy.
Absence of conventional roughening mill, use of much thinner slab and lower
reheating temperature in tunnel furnace (- 1150 oC) requires special attention for steel
design and processing through TSCR.
OBJECTS OF THE INVENTION

In view of the foregoing limitations inherent in the prior-art, it is an object of the
invention to determine a process for manufacturing a hot-rolled high strength by TSCR
route.
Another objective of this invention is to propose a process for producing hot-
rolled high strength steel having YS ≥ 700MPa MP , UTS ≥ 8 MP , %EI ≥ 7.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a process for manufacturing a hot-rolled
high strength steel by TSCR method including steps of casting a steel of composition
Carbon with certain composition, then heating the steel in a tunnel furnace at a
temperature 1130-1200oC, rolling the steel at finished rolling temperature (FRT) of 850-
950oC, cooling the steel at run out table (ROT) at a rate of 30-100oC and coiling the steel
at coiling temperature of 600-700 oC.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 illustrates a flow diagram depicting various steps of a process for manufacturing
hot-rolled high strength steel by TSCR route in accordance with an embodiment of the
invention.
FIGS. 2a and 2b illustrate Optical and Scanning Electron Microstructure (SEM) of the
steel sample respectively.
FIG. 3 shows bend test of the steel sample illustrating no cracks observed.
FIG. 4 illustrates tensile stress strain curve for the steel sample.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a process for manufacturing a hot-
rolled high strength steel by TSCR method, the process comprising steps of casting a
steel of composition Carbon (C): 0.04-0.06, Manganese (Mn): 1.00-2.00, Sulphur (S):
0.002-0.010, Phosphorus (P): 0.005-0.050, Silicon (Si): 0.05-0.60, Aluminum (Al): 0.02-
0.10, Nitrogen (N): 0.004-0.007, Tin (Ti): 0.06-0.12; Niobium (Nb): 0.005-0.100;
Molybdenum (Mo): 0.15-0.25, Iron (Fe) & unavoidable impurities: rest (all in wt.

percentage), heating the steel in a tunnel furnace at a temperature 1130-1200 oC, rolling the steel at finished rolling temperature (FRT) of 850-950oC, cooling the steel at run out
table (ROT) at a rate of 30-100oC , coiling the steel at temperature of 600-700oC.
Shown in FIG. 1 is a flow diagram depicting various steps of a process (100) for
manufacturing a hot-rolled high strength steel (hereinafter "steel") by TSCR route in
accordance with an embodiment of the invention.
At step (104), the steel is casted with composition (all in wt. percentage) as
provided in Table 1.

The role of various alloying elements is described below for the steel present
embodiment:
Carbon (C): Carbon is used for strengthening of the material. Here adequate
carbon is added to form sufficient precipitates in order to render required precipitate
strengthening when used with other alloying elements such as Ti, Mo and Nb.

Higher amount of C increases the amount of second phases thereby reducing the
stretch-flangeability. Higher carbon is also harmful for welding, therefore keeping all
these in mind a suitable amount of carbon has been used for the present steel i.e. 0.04-
0.06 (wt. %).
Manganese (Mn): Manganese is a solid solution strengthener. It also increases
the hardenability of the steel. High Mn also increases the carbon equivalent during
welding. Therefore a suitable amount of Mn has been used so that along with solution
strengthening it helps in increasing the hardenability of the steel in such a manner that
all ferrite formation takes place during coiling only.
Silicon (Si): Si is a solid solution strengthener and ferrite stabilizer. It has been
added to increase the strength of the steel through solid solution strengthening.
Aluminium (Al): This is a Al killed steel and therefore small amount of Al is
required.
Sulphur (S) and Phosphorous (P): S and P are detrimental alloying elements. S
forms manganese sulphide (MnS) and P is responsible for embrittlement. Therefore, the
amount of S and P is kept as low as possible.
Titanium (Ti) and Niobium (Nb): Ti and Nb are known for precipitation
strengthening. In accordance with an embodiment of the present invention, Ti along
with Nb has been added to increase the precipitation strengthening through formation of
fine precipitates along with Mo. Nb is beneficial to increase the non-recrystallization
temperature (Tnr) and therefore reduce the final grain size of the steel.
Molybdenum (Mo): Mo is useful in present embodiment to increase the strength
of the steel through favouring formation of higher number of fine precipitates and also
reducing the coarsening of the precipitates formed.
Niobium (Nb): Addition of Nb is done in order to produce fine final ferrite grain
size.
The strength of the steel mainly relies on the sufficient alloying addition of
titanium, niobium, molybdenum, silicon and manganese. In addition, the carbon and
carbide forming micro alloys are added in such a way that single phase ferrite along with
fine nano precipitates form during coiling of the steel strip.
At step (108), the steel is heated in a tunnel furnace at a temperature 1130-

At step (112), the steel is s s , s s
s T 8 -9
The thickness of the strip being rolled out from the stands is 3-10mm.
At step (116), the steel is cooled at r OT -
/sec.
At step (120), s s -7
Mechanical properties of the steel obtained by process (100) are given in Table
2.

The results indicated in Table 2 shows that the steel with the optimum
chemistry and process imparts excellent combination of strength and ductility.
The steel also possesses single phase ferritic microstructure, with small amount
of bainite/martensitic as second phase, wherein the second phase is <10%.
The steel manufactured is also crack-less in nature.
However, the same chemistry can also be used for conventional hot strip rolling
mill.

Experimental Trial:
The above mentioned steel making process can be validated by the following
example. The following example should not be construed to limit the scope of invention.
A new steel is casted with composition shown in Table 3.

The steel is heated in a tunnel furnace at a temperature 1150±20. The steel is
next s T 88 T s s
OT T s s oled. The steel is finally
Various samples were taken for steel measuring the properties. The properties of
various samples are shown in Table 4.


The tensile properties were measured using test specimens with 50 mm gauge
length (prepared according to ASTM E8 specification), fitted with an extensometer. All
tests were performed at room temperature.
The results indicate that the new steel with the optimum chemistry recorded
excellent combination of strength and ductility. The material also possesses
predominantly single phase ferritic microstructure, with small amount of
bainite/martensitic as second phases (Figure 2).
Bend test of samples with different thicknesses were performed as per BIS 1599
standard. 1T, 2T and closed bend tests were done and no cracks were found along the
bend axis of the sample (shown in Figure 3).
The stress-strain curve of the sample is shown in Figure 4.

Advantages:
The high tensile strength of the steel helps to reduce the gauge automotive
component or increase load carrying performance of the auto component. High tensile
strength of the material also allows usage of thinner gauge material and allow reducing
the weight of the car body. The high strength steel produced can be used for
automotive application such as long member of truck, chassis, base of tipper body truck
etc.

WE CLAIM
1. A process for manufacturing a hot-rolled high strength steel by TSCR method,
the process comprising steps of:
casting a steel of composition Carbon (C): 0.04-0.06, Manganese (Mn): 1.00-
2.00, Sulphur (S): 0.002-0.010, Phosphorus (P): 0.005-0.050, Silicon (Si): 0.05-
0.60, Aluminum (Al): 0.02-0.10, Nitrogen (N): 0.004-0.007, Tin (Ti): 0.06-0.12;
Niobium (Nb): 0.005-0.100; Molybdenum (Mo): 0.15-0.25, Iron (Fe) &
unavoidable impurities: rest (all in wt. percentage);
heating the steel in a tunnel furnace at a temperature 1130-1200oC;
rolling the steel at finished rolling temperature (FRT) of 850-950 oC;
cooling the steel at run out table (ROT) at a rate of 30-100oC and
coiling the steel at coiling temperature of 600-700 oC.
2. The process as claimed in claim 1, wherein the composition of the hot-rolled high
strength steel is Carbon (C): 0.04-0.06, Manganese (Mn): 1.3-1.7, Sulphur (S):
0.002-0.008, Phosphorus (P): 0.005-0.05, Silicon (Si): 0.3-0.6, Aluminum (Al):
0.04-0.08, Nitrogen (N): 0.004-0.005, Tin (Ti): 0.08-0.10; Niobium (Nb): 0.015-
0.030; Molybdenum (Mo) 0.186-0.220; Iron (Fe) & unavoidable impurities: Rest
(all in wt. percentage).
3. The process as claimed in claim 1, wherein the composition of the hot-rolled high
strength steel is Carbon (C): 0.0450, Manganese (Mn): 1.50, Sulphur (S): 0.003,
Phosphorus (P): 0.020, Silicon (Si): 0.400, Aluminum (Al): 0.040, Nitrogen (N):
0.004, Tin (Ti): 0.090; Niobium (Nb): 0.020; Molybdenum (Mo): 0.200, Iron (Fe)
& unavoidable impurities: rest (all in wt. percentage).
4. The process as claimed in claim 1, wherein the steel has yield strength (YS) >
700MPa.
5. The process as claimed in claim 1, wherein the steel has ultimate tensile strength
(UTS) > 800MPa.

6. The process as claimed in claim 1, wherein the steel has % elongation (%EL) >
17.
7. The process as claimed in claim 1, wherein the steel has single phase ferritic
microstructure with bainite/martensitic as second phase, wherein the
bainite/martensitic < 10%.