FIELD OF THE INVENTION
The invention relates to manufacturing of hot-rolled high strength steel. Particularly the invention relates to manufacturing hot-rolled high strength steel by thin slab casting route (TSCR) and conventional hot strip mill route separately.
BACKGROUND OF THE INVENTION
Applications of Advanced High Strength Steels (AHSSs) gradually increasing in automotive industries due to its weight saving potential and improved crashworthiness. AHSS are generally produced by introducing other phases, such as martensite, bainite and retained austenite into the microstructure. Some of such important AHSS that have been developed are as follows:
Dual phase steel (DP Steel)
In DP steel hard martensite phase (>5% wt%) is uniformly distributed in the soft ferrite matrix. This can produce strength level up to 1200MPa. This steel is characterized by low yield ratio, high work hardening ratio and widely used for automobile components that demands high strength, good crashworthiness and good formability. The DP steel is mainly based on C-Mn composition. However, other alloying elements like Cr/Mo are added to enhance hardenability.
TRIP steel
TRIP steel microstructure consists of ferrite, bainite and some retained austenite (maximum 30%). This is also important member of AHSS family with strength level varies from 600-1200MPa. Due to presence of adequate amount of retained austenite TRIP steel exhibits high elongation and high work hardenability. Si and/Al plays key role in retaining the austenite at room temperature. However, Si is detrimental to surface quality.

Complex phase steel (CP)
Unlike TRIP steel complex phase steel doesn’t contain retained austenite. The microstructure of CP steel consists of ferrite, martensite, and bainite and further strengthened by precipitation hardening. This is compromise between DP and TRIP steel to produce AHSS (strength ranges from 800 to 1000MPa) but provide excellent properties to be used for automobile components like B pillar, anti-crash bar or bumper.
However, these steels possess poor stretch-flangeability, a property that is essential for certain automotive components, particularly the suspension, long members, wheel rim and disc etc. To meet this specific requirement the general belief is that a single phase ferrite microstructure is the optimum, and consequently, 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. One steel with Ti-Mo grades were developed and patented by Tata Steel (Patent No- IN241163). However, still there is a scope to find out an alternative composition which suited for both for Thin Slab Casting and Rolling (TSCR) and Hot Strip Mill (HSM) rolling condition.
OBJECTS OF THE INVENTION
In view of the foregoing limitations inherent in the prior-art, it is an object of the invention to propose a hot-rolled high strength steel for automotive.
Another objective of the invention is to propose a process for producing hot-rolled high strength steel having YS ≥ 700MPa, UTS ≥ 800MPa, %EI ≥ 17.
Still another objective of the invention is to propose a process for producing hot-rolled high strength steel having single phase ferrite with small amount of bainite/martensitic or both phases together microstructure.

Still another objective of the invention is to propose a process for producing hot-rolled high strength steel well suited for TSCR and hot strip mill.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a process for manufacturing a hot-rolled high strength steel comprising steps of casting a steel slab Carbon (C) < 0.10, Manganese (Mn): < 2.5, Sulphur (S) < 0.010, Phosphorus (P) < 0.025, Silicon (Si) < 0.60, Aluminum (Al) < 0.10, Nitrogen (N) < 0.009, Vanadium (V) < 0.3, Niobium (Nb): < 0.10, Molybdenum (Mo) < 0.30, rest: Iron (Fe) & incidental impurities (all in wt. percentage), heating the steel slab in a tunnel furnace at a temperature 1130 - 12000C, rolling the steel slab to thickness 20-30 mm from 210-220 mm in a roughing mill of the hot strip mill or rolling to 15-35mm from 60-70 mm in a first two finishing stand of TSCR mill, finish rolling the steel strip at finished rolling temperature (FRT) of <900 C, cooling the steel strip at run out table (ROT) at a rate >300C/sec; and coiling the steel strip at coiling temperature of 600-680 0C.
In another aspect, the invention provides a hot-rolled high strength steel comprising composition of carbon (C) < 0.10, manganese (Mn): < 2.5, sulphur (S) < 0.010, phosphorus (P) < 0.0250; silicon (Si) < 0.60, aluminum (Al) < 0.10, nitrogen (N) < 0.009, vanadium (V) < 0.3, niobium (Nb): < 0.10, molybdenum (Mo) < 0.30, rest: iron (Fe) & incidental impurities (all in wt. percentage); and ultimate tensile strength (UTS) > 800MPa.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIGURE 1 illustrates a flow diagram depicting various steps of a process for manufacturing hot-rolled high strength steel by conventional hot strip mill route and TSCR route in accordance with an embodiment of the invention.

FIGURE 2 shows a microstructure of the steel after hot rolling during experimental trial in accordance with an embodiment of the invention.
FIGURE 3a and 3b shows a bend test of samples during experimental trial in accordance with an embodiment of the invention.
FIGURE 4 shows a tensile stress strain curve of sample during experimental trial in accordance with an embodiment of the invention.
FIGURE 5 shows high cycle fatigue test of sample during experimental trial in accordance with an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a process for manufacturing a process for manufacturing a hot-rolled high strength steel, the process comprising steps of: casting a steel slab of following composition Carbon (C) < 0.10, preferably 0.04 to 0.05, Manganese (Mn): < 2.5, preferably 1.4 to 1.6, Sulphur (S) < 0.010 preferably 0.003 to 0.08, Phosphorus (P) < 0.025 preferably 0.01 to 0.02; Silicon (Si) < 0.60, preferably 0.300 to 0.500, Aluminum (Al) < 0.10 preferably 0.04 to 0.06, Nitrogen (N) < 0.009, preferably 0.003 to 0.007, Vanadium (V) < 0.3, preferably 0.18 to 0.25, Niobium (Nb): < 0.10, preferably 0.015 to 0.025, Molybdenum (Mo) < 0.30, preferably 0.18 to 0.25, rest: Iron (Fe) & incidental impurities (all in wt. percentage); heating the steel slab in a tunnel furnace at a temperature 1130 - 12000C; rolling the steel slab to thickness 20-30 mm from 210-220 mm in a roughing mill of the hot strip mill or rolling to 15-35mm from 60-70mm in a first two finishing stand of TSCR mill, finish rolling the steel strip at finished rolling temperature (FRT) of <900 0C; cooling the steel strip at run out table (ROT) at a rate >300C/sec; and coiling the steel strip at coiling temperature of 600-680 0C.

Another embodiment the invention provide a hot-rolled high strength steel comprising: composition of carbon (C) < 0.10, manganese (Mn): < 2.5, sulphur (S) < 0.010, phosphorus (P) < 0.0250; silicon (Si) < 0.60, aluminum (Al) < 0.10, nitrogen (N) < 0.009, vanadium (V) < 0.3, niobium (Nb): < 0.10, molybdenum (Mo) < 0.30, rest: iron (Fe) & incidental impurities (all in wt. percentage) and ultimate tensile strength (UTS) > 800MPa.
Shown in FIGURE 1 is a flow diagram depicting series of steps for a process (100) for manufacturing a hot-rolled high strength steel (hereinafter “steel”). The process (100) can be performed by thin slab casting route (TSCR) and conventional hot strip mill separately At step (104), the steel slab 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 V, Mo and Nb. Higher amount of C will increase the amount of second phases and 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.
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 an Al killed steel and therefore the Al addition is not
intentional.
Sulphur (S) and Phosphorous (P): S and P are detrimental alloying elements.
S forms MnS and P responsible for embrittlement. Therefore, the amount of
S and P kept as low as possible.
Vanadium (V) and Niobium (Nb): Vanadium and Niobium are known for precipitation strengthening. V along with Nb has been added to increase the precipitation strengthening through formation of fine precipitates along with Mo. Vanadium precipitates are soluble at lower temperature as compared to Ti/Nb, therefore the formation of vanadium precipitates in ferrite is also high if processing is planned accordingly. 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 here to increase the strength of the steel through favoring formation of higher number of fine precipitates and also reducing the coarsening of the precipitates formed during coiling of steel coil. Mo also increases the hardenability of the steel and thereby reduces the chances of ferrite formation at higher temperature.
The strength of the steel mainly relies on the sufficient alloying addition of vanadium, niobium, molybdenum, silicon, carbon and manganese.
The carbide forming micro alloys such as niobium, molybdenum and vanadium are added in such a way that single phase ferrite along with fine nano precipitates and clusters form during cooling/coiling of the steel strip
At step (108), the steel slab is heated in a tunnel furnace at temperature 1130-1200C.
At step (112), the steel slab is rolled for thickness reduction.
In the hot strip mill a roughing stand is used to reduce the thickness of the steel slab. The steel slab of thickness 210-220 mm is fed, which is subsequently rolled to 20-30 mm in size.
Alternatively, route for the step (112) through TSCR can also be taken for the process (100). In the thin slab caster route (TSCR) mill which generally comprises of a 6 stands and no roughing mill. The first two finishing stands are used as roughing stands and the steel slab of thickness 60-70mm is reduced to normally 15-35 mm in the first two stands in the first two stands.
At step (116), the steel slab is rolled at finished rolling temperature (FRT) below the non-recrystallization temperature (Tnr). The Tnr in the present steel is ~930oC. Therefore, the FRT is selected < 900 C in order to have greater amount of accumulated strains to produce final ferrite grain size.
The thickness of the strip being rolled out from the stands is 1.5-12mm.

At step (120), the steel strip is cooled at run out table (ROT) at a rate >300 C /sec.
At step (124), the steel strip is coiled at coiling temperature of 600-680 0C.
Mechanical properties of the steel obtained by the 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 or both phases together as second phase, wherein the second phase is <10%.
The steel manufactured is also crack less in nature.
Experimental Trial:
The properties obtained in the steel from above mentioned steel making process (100) can be validated by the following example. The following experiments should not be construed to limit the scope of invention. The experiments have been performed via TSCR route.
A steel slab is casted with composition shown in Table 3.


The steel slab is heated in a tunnel furnace at a temperature 1150±20oC.
The final rolling is mentioned in the process step (112). Since there is no roughing stands for TSCR mill, the first two stands acts as roughing stands and therefore large deformation given in order to reduce the slab from thickness of 60-70mm to 15-35mm. The deformation gets reduced in the subsequent stands and sixth stand gives the lowest reduction. This is because as the temperature gets reduced in the second stand onwards the rolling load increases and higher deformation become challenging.
The steel slab is next rolled at finished rolling temperature (FRT) of 880±10 0C. The steel slab is cooled at run out table (ROT) at a rate of 50±10 0C/sec. The steel is finally coiled at coiling temperature of 650±20 0C.
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 (<10%) of bainite + martensitic as second phases (FIG. 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 FIGS. 3a and 3b).
The stress-strain curve of the sample is shown in FIG. 4.

The high cycle fatigue tests were performed as per ASTM standards under stress control mode. All the tests were performed till the fracture of the sample. The S-N curve generated through this method is shown in FIG. 5. It is evident from the S-N curve that the endurance limit of the HS 800 is much higher as compared to other commercially available grades for similar application.
Advantages:
The newly developed hot-rolled high strength steel used can be used for manufacturing of long members of truck, chassis, base of tipper body etc.
The newly developed hot-rolled high strength steel has YS ≥ 700MPa, UTS ≥ 800MPa, %EI ≥ 17.
High tensile strength of the material would allow usage of thinner gauge material and allow reducing the weight of the car body
The newly developed hot-rolled high strength steel has single phase ferrite
with small amount of bainite/martensitic or both microstructures. The ferrite
matrix is strengthened by fine interphase precipitates and nano clusters.

WE CLAIM:
1. A process for manufacturing a hot-rolled high strength steel, the
process comprising steps of:
casting a steel slab of following composition
Carbon (C) < 0.10, preferably 0.04 to 0.05, Manganese (Mn): < 2.5, preferably 1.4 to 1.6, Sulphur (S) < 0.010 preferably 0.003 to 0.08, Phosphorus (P) < 0.025 preferably 0.01 to 0.02; Silicon (Si) < 0.60, preferably 0.300 to 0.500, Aluminum (Al) < 0.10 preferably 0.04 to 0.06, Nitrogen (N) < 0.009, preferably 0.003 to 0.007, Vanadium (V) < 0.3, preferably 0.18 to 0.25, Niobium (Nb): < 0.10, preferably 0.015 to 0.025, Molybdenum (Mo) < 0.30, preferably 0.18 to 0.25, rest: Iron (Fe) & incidental impurities (all in wt. percentage); heating the steel slab in a tunnel furnace at a temperature 1130 -1200C;
rolling the steel slab to thickness 20-30 mm from 210-220 mm in a roughing mill of the hot strip mill or rolling to 15-35mm from 60¬70mm in a first two finishing stand of TSCR mill;
finish rolling the steel strip at finished rolling temperature (FRT) of <900 C;
cooling the steel strip at run out table (ROT) at a rate >300C/sec; and coiling the steel strip at coiling temperature of 600-6800 C.
2. The process as claimed in claim 1, wherein the composition of the hot-
rolled high strength steel is Carbon (C): 0.040, Manganese (Mn): 1.5,
Sulphur (S): 0.003, Phosphorus (P): 0.015, Silicon (Si): 0.45,
Aluminum (Al): 0.04, Nitrogen (N): 0.004, Vanadium (V): 0.2; Niobium
(Nb): 0.02; Molybdenum (Mo): 0.20; Iron (Fe) & unavoidable
impurities: Rest (all in wt. percentage).

3. The process as claimed in claim 1, wherein thickness of a strip after the steel slab being finish rolled out is 1.5-12mm.
4. A hot-rolled high strength steel having yield strength (YS) > 700MPa manufactured by the process as claimed in anyone of the claims 1-3.
5. A hot-rolled high strength steel having ultimate tensile strength (UTS) > 800MPa manufactured by the process as claimed in anyone of the claims 1-3.
6. A hot-rolled high strength steel having % elongation (%EL) > 17 manufactured by the process as claimed in anyone of the claims 1-3.
7. A hot-rolled high strength steel having ferritic microstructure with bainite/martensitic or both as second phase, wherein the second phase < 10% manufactured by the process as claimed in anyone of the claims 1-3.
8. A hot-rolled high strength steel comprising:
composition of carbon (C) < 0.10, manganese (Mn): < 2.5, sulphur (S) < 0.010, phosphorus (P) < 0.0250; silicon (Si) < 0.60, aluminum (Al) < 0.10, nitrogen (N) < 0.009, vanadium (V) < 0.3, niobium (Nb): < 0.10, molybdenum (Mo) < 0.30, rest: iron (Fe) & incidental impurities (all in wt. percentage); and ultimate tensile strength (UTS) > 800MPa.

9. The hot-rolled high strength steel as claimed in claim 8, wherein yield strength (YS) > 700MPa.
10. The hot-rolled high strength steel as claimed in claim 8, wherein % elongation (%EL) > 17.
11. The hot-rolled high strength steel as claimed in claim 8, wherein the hot-rolled high strength steel comprises ferritic microstructure with bainite / martensitic or both as second phase, wherein the second phase < 10%.