FIELD OF THE INVENTION
This invention relates to a process for manufacturing hot-rolled high strength steel. This invention particularly relates to a process for manufacturing hot-rolled high strength steel for automotive applications.
BACKGROUND OF THE INVENTION
Environment and passenger safety requirements for the automotive industry have led to a development of various Advanced High Strength Steels (AHSS). These steels provide increased strength with equivalent, or improved, ductility to the lower strength grades making it possible to reduce the weight of a component. AHSS are generally produced by combining various phases, such as martensite, bainite and retained austenite into the microstructure. However, introduction of second phases deteriorates the other forming properties like stretch-flageability. Therefore, in the present context efforts have been put to produce single phase ferrite matrix with nano precipitates dispersed in it. The same has been attempted earlier with addition of Molybdenum in the composition. However, as the required strength requirement is increased, the amount of molybdenum is very high which add to the cost of the material. Therefore, the need exists to provide steel with negligible molybdenum but which can retain a strength level of >950MPa with adequate ductility. In commercial scale 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 210-260 mm thickness is normally reheated at >1200C and subsequently reduced to 3-10 mm strips.
It is therefore extremely important to optimize the composition and processing conditions to obtain desired set of properties.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose a process for manufacturing hot-rolled high strength steel.
It is a further object of this invention to propose a process for manufacturing hot-rolled high strength steel without addition of more than 0.05 Mo.
Another object of this invention is to propose a process for manufacturing hot-rolled high strength steel having YS ≥ 800MPa, UTS ≥ 950MPa, %EI ≥ 15.

Yet another object of this invention is to propose a process for manufacturing hot-rolled high strength steel which is simple and cost- effective.
These and other objects and advantages of the invention will be apparent to a person skilled in the art on reading the ensuing description in conjunction with the accompanying drawings.
SUMMARY OF THE INVENTION
This invention relates to a process for manufacturing a hot-rolled high strength steel comprising the steps of casting a steel with certain composition in air induction furnace, followed by heating the steel in a furnace at a temperature >1200C, rolling the steel at finished rolling temperature (FRT) of 850-950 C, cooling the steel at run out table (ROT) at a rate of 30-100 C and coiling the steel at coiling temperature of 600-700 C.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 illustrates a flow diagram depicting various steps of the process for manufacturing hot-rolled high strength steel in laboratory according to an embodiment of the invention.
FIG. 2 illustrates Optical Microstructure of the steel sample.
FIG. 3 illustrates tensile stress strain curve of the steel sample.
DETAILED DESCRIPTION OF THE INVENTION
Thus according to this invention is provided a process for manufacturing a hot-rolled high
strength steel .
In accordance with this this invention the process for manufacturing a hot-rolled high
strength steel comprises the steps of casting a steel with a predetermined composition , in
an air induction furnace , followed by heating the steel in a furnace at a temperature
>1200C, rolling the steel at finished rolling temperature (FRT) of 850-950 C, cooling the steel
at run out table (ROT) at a rate of 30-100 C and coiling the steel at coiling temperature of
600-700 C.
In accordance with various embodiments of the invention is provided a process for
manufacturing a hot-rolled high strength steel, the process comprising steps of casting a
steel of composition Carbon (C): 0.08-0.15, Manganese (Mn): 1.00-2.50, Sulphur (S): 0.002-

0.010, Phosphorus (P): 0.005-0.1, Boron: 0 to 0.002, Silicon (Si): 0.05-0.60, Aluminum (Al): 0.02-0.10, Nitrogen (N): 0.004-0.007, Titanium (Ti): 0.06-0.12; Niobium (Nb): 0.005-0.100; Vanadium (V): 0.1-0.3; Molybdenum (Mo): <0.05, Iron (Fe) & unavoidable impurities: rest (all in wt. percentage), heating the steel in a furnace at a temperature ≥ 1200C, rolling the steel at finished rolling temperature (FRT) of 850-950 C, cooling the steel at run out table (ROT) at a rate of 30-100 C, coiling the steel at coiling temperature of 600-700 C. Shown in FIG. 1 is a flow diagram depicting various steps of a process for manufacturing a hot-rolled high strength steel (hereinafter “steel”) in accordance with an embodiment of the invention.
At step (104), the steel is cast with composition (all in wt. percentage) as provided in Table 1.

Table 1
Elements Composition (by wt.) range Preferable Composition (by wt.) range Preferable Composition (by wt.)
Carbon (C): 0.080-0.15 0.080-0.120 0.045
Manganese (Mn) 1.00-2.50 1.5-2.0 1.5
Sulphur (S) 0.002-0.010 0.002-0.008 0.003
Phosphorus (P) 0.005-0.10 0.02-0.060 0.020
Silicon (Si) 0.05-0.60 0.2 to 0.6 0.400
Boron (B) 0-0.002 0-0.0015 0.001
Aluminium (Al) 0.02-0.10 0.04-0.08 0.040
Nitrogen (N) 0.004-0.007 0.004-0.005 0.004
Titanium (Ti) 0.06-0.12 0.06 to 0.10 0.090
Niobium (Nb) 0.005-0.100 0.02 to 0.060 0.020
Vanadium (V) 0.05-0.3 0.06-0.15 0.1
Molybdenum (Mo) 0 -0.05 <0.05 0.200
Iron (Fe) &
unavoidable
impurities Rest Rest Rest

The role of various alloying elements for the steel manufactured in accordance with present invention, is described below:
Carbon (C): Carbon is used for strengthening of steel mostly through formation of precipitates or facilitating favourable phase transformation. However, excessive carbon may lead to poor weldability and formation of undesired phases. Here adequate carbon is added to form sufficient precipitates to render required precipitate strengthening when used with other alloying elements such as Ti, V 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. 0.08-0.15 (wt. %). Manganese (Mn) : Manganese is a solid solution strengthener. It also increases the hardenability of the steel. High Mn also decreases the transformation temperature and thereby enhance formation of finer ferrite. Mn increases the carbon equivalent during welding. Therefore, a suitable amount of Mn , in 1.00-2.50 (wt. %), 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 in 0.05-0.60 (wt. %), to increase the strength of the steel through solid solution strengthening. However, higher amount of Si may lead to undesirable scale formation on the steel samples. Aluminium (Al): This is an Al killed steel and therefore small amount of Al (0.02-0.10 wt. %) is required.
Sulphur (S) and Phosphorous (P): S and P are detrimental alloying elements. S forms manganese sulphide (MnS), and is therefore kept as low as possible. A controlled amount of S (0.002-0.010 wt%) was added.
P is responsible for embrittlement when added in excess amount. However, controlled amount of addition of P may increase the solid solution strength significantly. Therefore, the controlled amount of P (0.005-0.10 wt%) was added. To tackle diffusion of phosphorous to grain boundaries, small amount of Boron is added. Diffusivity of Boron is faster than phosphorous and therefore, prevents phosphorous segregation at grain boundary.

Titanium (Ti), Vanadium (V) and Niobium (Nb): Ti, V and Nb are known for precipitation
strengthening. In accordance with an embodiment of the present invention, Ti, V along with
Nb has been added to increase the precipitation strengthening through formation of fine
precipitates. 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. However, keeping in view of cost, Mo has been eliminated
from the composition. However, small amount of Mo can be useful for facilitating formation
of fine precipitates. Therefore, small amount (<0.05 wt%) of Mo has been used in the
present invention.
Niobium (Nb): Addition of Nb is done to produce fine final ferrite grain size.
Nitrogen
The strength of the steel mainly depends 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 cooling/coiling of the steel strip. At step (108), the steel is heated in a muffle furnace at a temperature ≥1200C.
At step (112), the steel is deformed in Gleeble thermomechanical simulator to simulate the rolling mill conditions. Final pass of the deformation via rolling was given at a temperature which simulates finish rolling condition of hot rolling mill (FRT) of 850-950 C.
Once the desired properties are obtained in Gleeble thermomechanical simulator, similar conditionsaere replicated in a pilot scale rolling mill and the coiling temperature is simulated by holding in a furnace of predetermined temperature and time. The final thickness of the rolled material reached out to be 3-10 mm.
At step (116), the steel is cooled in the Gleeble simulator in such a way that the cooling rate simulates the run out table (ROT) at a rate of 30-100 C /sec.
At step (120), the steel is subjected to a predetermined formula to simulate the coiling conditions of 600-700 C.
Mechanical properties of the steel obtained by process (100) are given in Table 2.

Table 2
Sl. No. Property Value
1 YS, MPa >700 MPa
2 UTS, MPa >900 MPa
3 %El >15
4 Nature of tensile fracture Ductile
The results indicated in Table 2 shows that the steel with the optimum chemistry and manufactured by the process according to the present invention, 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 <15%.
The steel manufactured is also crack-less in nature.
However, the same chemistry can also be used for conventional hot strip rolling mill as well as Thin Slab and Casting Route.
Experimental Trial:
The above-mentioned steel making process can be validated by the following example. However, the following example should not be construed to limit the scope of the invention.
A new steel is cast with the composition as shown in Table 3.

Elements 1st Composition (by wt.) 2nd Composition (by wt.)
Carbon (C): 0.08 0.083
Manganese (Mn) 2.0 1.8
Sulphur (S) 0.004 0.005
Phosphorus (P) 0.050 0.015

Silicon (Si) 0.500 0.46
Aluminium (Al) 0.040 0.05
Nitrogen (N) 0.005 0.0042
Titanium (Ti) 0.100 0.091
Niobium (Nb) 0.050 0.047
Vanadium (V) 0.1 0.093
Molybdenum (Mo) 0.05 0.004
Boron (B) 0.001 -
Iron (Fe) & unavoidable impurities Rest Rest
The steel is cast in an air induction 25kgs furnace, and heated in a furnace at a temperature 1230±20. The steel is next rolled at finished rolling temperature (FRT) of 880±10 C followed by cooling at a rate of 50±10 C. Then the steel is air cooled. The steel is finally coiled at coiling temperature of 650±20 C.
Various samples were taken for steel measuring the properties. The properties of various samples are shown in Table 4.

Thickness, (in mm) YS, MPa UTS, MPa %El Nature of tensile fracture
5 (1st composition) 794 958 19 Ductile
5 (1st composition) 779 962 20 Ductile
7 (1st composition) 754 954 18 Ductile
7 (1st composition) 742 959 19 Ductile
5 (1st composition) 806 910 17 Ductile
5 (1st composition) 799 916 18 Ductile
5 (1st composition) 827 940 20 Ductile

7 (2nd composition) 815 933 18 Ductile
7 (2nd composition) 778 922 18 Ductile
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).
The stress-strain curve of the sample is shown in Figure 3.
The high strength steel manufactured by the process according to the present invention shows several advantageous properties. 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, structural parts, front/back/side underrun protection device etc replacing 800MPa strength or lower strength material.

We claim:
1. A process for manufacturing a hot-rolled high strength steel, the process comprising:
casting a steel of composition Carbon (C): 0.08-0.15, Manganese (Mn): 1.00-2.50, Sulphur
(S): 0.002-0.010, Phosphorus (P): 0.00 to 0.060, Silicon (Si): 0.05-0.60, Aluminum (Al):
0.02-0.10, Nitrogen (N): 0.004-0.007, Titanium (Ti): 0.06-0.12; Niobium (Nb): 0.005-0.100;
Vanadium (V): 0.1-0.3; Molybdenum (Mo): 0-0.05, Boron (B): 0-20 ppm; Iron (Fe) &
unavoidable impurities: rest (all in wt. percentage);
heating the steel in a furnace at a temperature 1200-1300 C; rolling the steel at finished rolling temperature (FRT) of 850-950 C; cooling the steel at run out table (ROT) at a rate of 30-100 C/sec; and coiling the steel at coiling temperature of 600-700 C.
2. The process as claimed in claim 1, wherein the composition of the hot-rolled high strength steel is Carbon (C): 0.0450, Manganese (Mn): 2.0, Sulphur (S): 0.003, Phosphorus (P): 0.050, Silicon (Si): 0.400, Aluminum (Al): 0.040, Nitrogen (N): 0.004, Tin (Ti): 0.10; Niobium (Nb): 0.050; Molybdenum (Mo): 0.050, Boron (B): 0.001; Iron (Fe) & unavoidable impurities: rest (all in wt. percentage).
3. The hot-rolled high strength steel manufactured by the process as claimed in claim 4, wherein the steel has yield strength (YS) > 700MPa.
4. The hot-rolled high strength steel as claimed in claim 3, wherein the steel has ultimate tensile strength (UTS) > 900MPa.
5. The hot-rolled high strength steel as claimed in claim 3, wherein the steel has % elongation (%EL) > 15.
6. The hot-rolled high strength steel as claimed in claim 3, wherein the steel has single phase ferritic microstructure with bainite/martensitic as second phase, wherein the bainite/martensitic < 15%.

7. A hot-rolled high strength steel comprising:
a composition of Carbon (C): 0.08-0.15, Manganese (Mn): 1.00-2.50, Sulphur (S): 0.002-0.010, Phosphorus (P): 0.00 to 0.060, Silicon (Si): 0.05-0.60, Aluminum (Al): 0.02-0.10, Nitrogen (N): 0.004-0.007, Titanium (Ti): 0.06-0.12; Niobium (Nb): 0.005-0.100; Vanadium (V): 0.1-0.3; Molybdenum (Mo): 0-0.05, Boron (B): 0-20 ppm; Iron (Fe) & unavoidable impurities: rest (all in wt. percentage), and having a yield strength (YS) > 700MPa, an ultimate tensile strength (UTS) > 900MPa; a % elongation (%EL) > 15, and having single phase ferritic microstructure with bainite/martensitic as second phase, the bainite/martensitic < 15%.