TECHNICAL FIELD
The present disclosure relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to a high strength hot rolled steel sheet. Further embodiments of the disclosure disclose a method for manufacturing a high strength hot rolled steel sheet with tensile strength of minimum 780 MPa.
BACKGROUND OF THE DISCLOSURE
Steel is an alloy of iron, carbon and other elements such as Phosphorous (P), Sulphur (S), Nitrogen (N), Manganese (Mn), Silicon (Si), Aluminium (Al) and any other incidental elements. Because of its high tensile strength and low cost, steel may be considered as a major component in wide variety of applications. Some applications of the steel may include buildings, ships, tools, automobiles, machines, bridges and numerous other applications. The steel obtained from steel making process may not possess all the desired properties for all application and therefore, the steel may be subjected to secondary processes such as heat treatment for controlling material properties to meet various needs in the intended applications.
Generally, heat treatment may be carried out using techniques including but not limiting to annealing, normalising, hot rolling, quenching, and the like. During heat treatment process, the material of steel undergoes a sequence of heating and cooling operations, thus, the microstructures of the steel may be modified during such operation. As a result of heat treatment, the steel may undergo phase transformation, influencing mechanical properties like strength, ductility, toughness, hardness, drawability etc. The purpose of heat treatment is to increase service life of a product by improving its strength, hardness etc., or prepare the material for improved manufacturability.
With rising concerns over global environmental problems and demand from automotive industry for higher collision safety of vehicles impose conflicting requirements on materials used for bodies of the vehicle. The vehicle bodies are required to be stronger yet lighter at the same time. Advancements in steel manufacturing may include commercialization of fuel cell vehicles and use of lighter materials like aluminium, composites etc. These materials meet the desired material properties, however associated problems such as formability, reliability and recyclability and higher cost of manufacturing make such materials commercially unattractive and hence, usage of such material may be limited to specific types of

components of the vehicles. Thus, it becomes inevitable to re-look at advanced high strength steels to meet the desired properties, as usage of steel mitigates some of above-mentioned issues with other materials.
There have been several developments in the field of advanced high strength steels, which poses good combination of tensile strength, elongation and stretch flangeability to address some of the afore-mentioned concerns. One such conventional process includes development of Titanium (Ti) and Molybdenum (Mo) based steel. The method to manufacture 780 MPa strength steel with fully ferrite microstructure by fixing carbon with Titanium (Ti) and Molybdenum (Mo) in the form of (Ti,Mo)C precipitates is disclosed. Though such steel is ascertained to have high strength and high ductility, requirement of significant quantity of Mo for manufacturing such steel adds on to production costs, indicating such steel to be expensive. Similarly, one of the patent literatures disclose DP780 steel and a method to create steel with minimum tensile strength of 780 MPa and having a ferrite-martensite microstructure. Due to significant difference in hardness between ferrite and martensite phases, hole expansion ratio of such dual phase steel is also low.
Also, another patent literature discloses 780 MPa strength steel with fully bainitic microstructure but it has high level of silicon. Surface quality of these steels may be poor as silicon promotes formation of scale during hot rolling.
Hence, there is a need for an economically attractive and technically viable way of developing high strength hot rolled steel with high ductility and good stretch flangeability grades without aforementioned limitations.
The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.
SUMMARY OF THE DISCLOSURE
One or more shortcomings of the prior art are overcome by method as disclosed and additional advantages are provided through the method as described in the present disclosure.
Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

In one non-limiting embodiment of the present disclosure, there is provided a high strength hot-rolled steel sheet. The steel sheet includes alloying composition comprising of carbon (C) at about 0.03 wt% to about 0.08 wt%, manganese (Mn) at about 1.3 wt% to about 1.6 wt%, silicon (Si) less than 0.15 wt%, Niobium (Nb) at about 0.01 wt% to 0.025 wt% titanium (Ti) at about 0.08 wt% to 0.14 wt%, vanadium 0.07 wt% to 0.09 wt.%, aluminium (Al) at about 0.2 wt% to 0.5 wt.%, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 wt%, nitrogen (N) up-to 0.007 wt%, the balance being Iron (Fe) optionally along with incidental elements. The high strength hot-rolled steel sheet comprises ferrite and bainite microstructure.
In an embodiment, the high strength hot-rolled steel sheet exhibits tensile strength of at least 780 MPa, ductility greater than 14% and a hole expansion ratio of greater than 40%.
In an embodiment, the area fraction of the ferrite-bainite microstructure includes ferrite content between 15% to 45% and balance being bainite with carbides or retained austenite-martensite.
In an embodiment, the ferrite phase in the ferrite-bainite microstructure of the high strength steel sheet includes Niobium, Titanium and Vanadium carbides precipitates.
In an embodiment, the high strength hot-rolled steel sheet, composition, consists of Manganese (Mn) preferably in the range of about 1.4 wt% to about 1.5 wt% and Titanium (Ti) preferably in the range of about 0.09 wt% to about 0.13 wt%.
In another non-limiting embodiment, a method for producing high strength hot rolled steel sheet is disclosed. The method includes steps of firstly casting a steel slab of composition comprising in weight percentage of: carbon (C) at about 0.03 wt% to about 0.08 wt%, manganese (Mn) at about 1.3 wt% to about 1.6 wt%, silicon (Si) less than 0.15 wt%, Niobium (Nb) at about 0.01 wt% to 0.025 wt% titanium (Ti) at about 0.08 wt% to 0.14 wt%, vanadium 0.07 wt% to 0.09 wt.%, aluminium (Al) at about 0.2 wt% to 0.5 wt.%, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 wt%, nitrogen (N) up-to 0.007 wt%, the balance being Iron (Fe) optionally along with incidental elements. The steel slab is subjected to heating at a first predetermined temperature for a first predetermined time, followed by subjecting the steel slab to a hot working to produce a steel sheet.

In an embodiment, the hot working includes deforming, the steel slab in a first hot working process, at a second predetermined temperature, and subjecting the steel slab to deformation in a second hot working process, at a third predetermined temperature. The hot worked steel sheet is held for a second predetermined time, and then subjected to cooling at a first predetermined cooling rate to a fourth predetermined temperature. Further, the cooled steel sheet is held for a third predetermined time at the fourth predetermined temperature and cooled at a second predetermined cooling rate to a fifth predetermined temperature. The steel sheet is coiled at the fifth predetermined temperature to obtain the high strength steel sheet.
In an embodiment, the casting is carried out in a continuous casting process. The continuous casting process is performed in at least one of continuous caster and a slab caster.
In an embodiment, the temperature of the steel slab at exit of the slab caster is maintained above 1000 °C.
In an embodiment, the first predetermined temperature is greater than 1150°C, preferably ranging from about 1200°C to 1250°C, and the first predetermined time ranging from about 30 minutes to about three hours.
In an embodiment, the hot working is a hot rolling process. The first hot working process is performed in a roughing mill, and the second predetermined temperature is greater than a recrystallization temperature of the steel slab, ranges from about 1050°C to 1170°C and the second hot working process is performed in four or more than four strands of a finishing mill.
In an embodiment the third predetermined temperature ranging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium and wherein Ae3 is at least 850°C
In an embodiment, the second predetermined time ranges from 0.5 seconds to 3 seconds, and the first predetermined cooling rate is greater than 30°C/second, and the cooling is performed with intensive or laminar water cooling on a run out table.
In an embodiment, the fourth predetermined temperature ranges from about 650°C to 720°C, preferably 660°C to 690°C.

In an embodiment, the third predetermined period of time ranges from about 3 seconds to 12 seconds, preferably 4second to 7 seconds, and the second predetermined cooling rate is greater than 30°C/s, and the cooling is an intensive cooling-cum- laminar cooling carried out on the run-out-table.
In an embodiment, the fifth predetermined temperature is ranges from about 430°C to 530°C, preferably 480°C to 500°C.
In another non-limiting embodiment, a high strength steel sheet with a tensile strength greater than 780 MPa and hole expansion ratio of greater than 25%. The steel sheet comprising composition in weight percentage (wt.%) of carbon (C) at about 0.03 wt% to about 0.08 wt%, manganese (Mn) at about 1.3 wt% to about 1.6 wt%, silicon (Si) less than 0.15 wt%, Niobium (Nb) at about 0.01 wt% to 0.025 wt% titanium (Ti) at about 0.08 wt% to 0.14 wt%, vanadium 0.07 wt% to 0.09 wt.%, aluminium (Al) at about 0.2 wt% to 0.5 wt.%, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 wt%, nitrogen (N) up-to 0.007 wt%, the balance being Iron (Fe) optionally along with incidental elements.
In yet another non-limiting embodiment, a method for manufacturing a high strength hot rolled steel sheet is disclosed. The method includes steps of casting, a steel slab of a composition, comprising in weight percentage (wt.%) of: carbon (C) at about 0.03 wt% to about 0.08 wt%, manganese (Mn) at about 1.3 wt% to about 1.6 wt%, silicon (Si) less than 0.15 wt%, Niobium (Nb) at about 0.01 wt% to 0.025 wt% titanium (Ti) at about 0.08 wt% to 0.14 wt%, vanadium 0.07 wt% to 0.09 wt.%, aluminium (Al) at about 0.2 wt% to 0.5 wt.%, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 wt%, nitrogen (N) up-to 0.007 wt%, the balance being Iron (Fe) optionally along with incidental elements. Then, subjecting the steel slab to heating in a first predetermined temperature for a first predetermined time and then subjecting, the steel slab to a hot working to produce a steel sheet. The hot working includes deforming, the steel slab in a first hot working process, at a second predetermined temperature, and deforming, the steel slab in a second hot working process, at a third predetermined temperature. The hot worked steel sheet is held for a second predetermined time at a third predetermined temperature and then subjected to cooling at a second predetermined cooling rate to a fourth predetermined temperature. Further, the cooled steel sheet is coiled at the fourth predetermined temperature to obtain a high strength hot-rolled steel sheet.

In an embodiment, the casting is carried out in a continuous casting process. The continuous casting process is performed in at least one of continuous caster and a slab caster.
In an embodiment, the temperature of the steel slab at exit of the slab caster is maintained above 1000 °C.
In an embodiment, the first predetermined temperature is greater than 1150°C, preferably ranging from about 1200°C to 1250°C, and the first predetermined time ranging from about 30 minutes to about three hours.
In an embodiment, the hot working is a hot rolling process. The first hot working process is performed in a roughing mill, and the second predetermined temperature is greater than a recrystallization temperature of the steel slab, ranges from about 1050°C to 1170°C and the second hot working process is performed in four or more than four strands of a finishing mill.
In an embodiment, the third predetermined temperature ranging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium and wherein, Ae3 is at least 850°C
In an embodiment, the second predetermined time ranges from 0.5 seconds to 3 seconds, and the second predetermined cooling rate is greater than 30°C/s, and the cooling is an intensive cooling-cum- laminar cooling carried out on the run-out-table.
In an embodiment, the fourth predetermined temperature ranges from about 540°C to 620°C.
It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment of the disclosure.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:
Figure.1 is a flowchart illustrating a method for producing high strength hot rolled steel sheet, according to an exemplary embodiment of the present disclosure.
Figure.2 is graphical flow diagram of cooling profile followed during the method illustrated in Figure. 1.
Figures. 3a-3c illustrates microstructure of steel sheet manufactured by method of present disclosure in which (3a) is an Optical micrograph, (3b) is a SEM micrograph showing combination of equiaxed ferrite(polygonal), Bainitic ferrite, MA(martensite-austenite) phase of very small size, and (3c) EBSD image from which fraction of ferrite phase is calculated using grain orientation spread criteria (GOS<0.8) and area fraction of ferrite is in the range of 20 to 40%, according to an exemplary embodiments of the present disclosure.
Figures. 4a to 4c illustrates transmission electron microscope (TEM) micrographs of steel sheet manufactured by method of present disclosure in which (4a) micrograph shows clearly presence of bainite and polygonal ferrite (4b) micrograph shows presence of row of interphase precipitates in polygonal ferrite and (4c) micrograph shows size of interphase precipitates (~ 3nm)., in accordance with some embodiments of the present disclosure.
Figure. 5 illustrates S-N curve for the steel sheet- manufactured by method of present disclosure indicating endurance limit of 300 MPa, in accordance with some embodiments of the present disclosure.
Figure.6 is a flowchart illustrating a second method for producing high strength hot rolled steel sheet, according to another exemplary embodiment of the present disclosure.

Figure.7 is graphical flow diagram of cooling profile followed during the method for producing high strength hot rolled steel sheet of Figure. 6.
Figures. 8a-8c illustrates microstructure of steel sheet manufactured by method of Figure. 6 in which (8a) is an Optical micrograph, (8b) is a SEM micrograph showing primarily bainitic ferrite, MA (martensite-austenite) phase of very small size and (8c) EBSD image from which fraction of ferrite phase is calculated using grain orientation spread criteria (GOS<0.8) and area fraction of ferrite is in the range of less than 20%, according to an exemplary embodiment of the present disclosure.
Figures. 9a and 9b illustrates transmission electron microscope (TEM) micrographs of steel sheet manufactured by method of present disclosure in which (9a) micrograph shows clearly presence of bainitic ferrite, and (9b) micrograph shows presence of precipitates of size of interphase precipitates (~ 8-15 nm) in accordance with some embodiments of the present disclosure.
Figure. 10 illustrates S-N curve for the steel sheet- manufactured by method of present disclosure indicating endurance limit of 301 MPa, in accordance with some embodiments of the present disclosure.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristic of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to

be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.
In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.
While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.
Embodiments of the present disclosure discloses a high strength hot rolled steel sheet and a method for manufacturing or producing a high strength hot rolled steel sheet. Strength, ductility, hole expansion ratio, and stretch flangeability are some of the important properties for the mass industrial application of high strength material like steel. The method of present disclosure discloses a production of high-strength hot rolled steel sheet, with tensile strength of minimum 780 MPa with good ductility, hole expansion ratio, stretch flangeability and fatigue properties. The present disclosure is directed towards producing a low carbon hot rolled steel sheet with tensile strength greater than 780 MPa along with a hole expansion ratio more than 40%, and % elongation more than 14%. The hot rolled steel sheet may be widely employed to make automotive components requiring high strength, high ductility and high stretch flangeability, by optimizing composition of the steel sheet to ensure inexpensive manufacturing capabilities.

Henceforth, the present disclosure is explained with the help of figures for a method of manufacturing high strength-hot rolled steel sheet. However, such exemplary embodiments should not be construed as limitations of the present disclosure since the method may be used on other types of steels where such need arises. A person skilled in the art may envisage various such embodiments without deviating from scope of the present disclosure.
Figures.1 and 2 are exemplary embodiments of the present disclosure illustrating a flowchart depicting a method for manufacturing high strength-hot rolled steel sheet, and a graphical flow diagram of cooling profile, respectively. In the present disclosure, mechanical properties such as strength, ductility, hole expansion ratio (measure of stretch flangeability) and final microstructure of the steel may be improved. The steel produced by the method of the present disclosure, includes a microstructure which is primarily a mixture of polygonal ferrite and bainite or bainitic ferrite. A very small fraction of Martensite/Austenite phases (< 2%) also may be retained, however, these phases in such small fraction may not greatly affect properties of the steel sheet. The method is now described with reference to the flowchart blocks and is as below. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject matter described herein. The method is particularly applicable to high-strength hot rolled steel sheet and it may also be extended to other type of steels as well.
The method of manufacturing the high strength hot-rolled steel sheet according to the present disclosure consists of a casting step followed by a hot working and controlled cooling, where the steel sheet is then coiled to satisfy component composition described below. It should not be construed that the following steps to be limited, rather, coiling may be performed after hot working and prior cooling, based on variation in temperature and cooling rate, respectively. The various processing steps of Figures. 1 and 2 respective, are described below:
As shown in block 101, the method starts with the process of casting. In the method of present disclosure, the steel of the specified composition including in weight percentage of 0.03 - 0.08 % of carbon, 1.3– 1.6 % of manganese - preferably 1.4 -1.5 %, less than 0.15 % of silicon, 0.08-0.14 % of titanium- preferably 0.09– 0.13%, Niobium 0.01-0.025%, V 0.07 to 0.09%, 0.2 – 0.5 % of aluminum, maximum up to 0.005 % of sulphur, maximum up to 0.025 % of phosphorous and up to 0.007 % of nitrogen, the balance being iron and impurities by

any manufacturing process including but not limiting to casting. In an embodiment, the casting process is a continuous casting process. When the steel is cast in a slab caster such as a thin slab caster, where temperature of the steel slab may not be allowed to drop to a temperature below 1000 °C, as it might result in loss of ductility and cause formation of edge cracks.
The method then includes the step of heating as shown in block 102. After casting the steel slab with the specified composition, the slabs are heated to a first predetermined temperature for a first predetermined time. In an embodiment, the first predetermined temperature is greater than 1150 °C, preferably in the range of 1200 °C to 1250 °C, and the first predetermined time ranges from 30 minutes to 3 hours. In an embodiment, the first predetermined temperature is above 1150 °C, to ensure complete dissolution of precipitates that may have formed in the preceding processing steps. A first predetermined temperature greater than 1250 °C is also not desirable because it may lead to grain coarsening of austenite and/or excessive scale loss. In some embodiment, heating of the steel slab may be carried out in a furnace.
Once the steel slab is heated as per the block 102, it is subjected for hot working as shown in block 103 to form a steel sheet. In an embodiment, the hot working process is a hot rolling process. As shown in block 103, after casting and heating the steel slab with the specified composition, it is hot rolled. The hot rolling may constitute two steps of deformation via the first hot working process and the second hot working process. In an embodiment, the first hot working process is a deformation process of steel slab in a roughing mill at the second predetermined temperature.
In an embodiment, the second predetermined temperature is above recrystallisation temperature of the steel slab and ranges from about 1050°C to 1170°C. In the roughing stage, the cast structure may be broken down, and the new structures may be formed. Further, the second hot working process is carried out at a third predetermined temperature. In an embodiment, in the second hot working process is a further deformation process of the steel slab carried out in four or more strands of the finishing mill, and temperatures in all strands of the finishing mill are such that microstructure of the material consists of co-phase austenite. In an embodiment, the third predetermined temperature is in the range of Ae3 to Ae3 + 70 °C., wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium and Ae3 is at least 850°C.

After finish rolling, the rolled steel sheet may be held at the third predetermined temperature (T3) for a second predetermined time as shown in block 104. In an embodiment, the second predetermined time ranges from 0.5 seconds to 3 seconds. Then, the steel sheet is subjected to intensive cooling or laminar cooling (as shown in Figure. 2) at a predetermined cooling rate as shown in block 105. In an embodiment, the predetermined cooling rate is greater than 30 °C/s till a fourth predetermined temperature (T4) is reached. In an embodiment, the cooling is performed on a run-out-table. Cooling is stopped as the fourth temperature reaches 650 °C to 720°C, and the steel sheet is held at the fourth predetermined temperature for a third predetermined period of time as shown in block 106.
In an embodiment, the third predetermined period of time ranges from about 4 seconds to 12 seconds, preferably 5second to 7 seconds. As per block 107, the method further includes the second step of cooling (as shown in Figure. 2) . This cooling is done as fast as possible to a fifth predetermined temperature (T5) which is also called as coiling temperature in the range of 430 °C to 530 °C. Now referring to block 108, coiling may be carried out at a coiling temperature or a fifth predetermined temperature.
In an embodiment, the fifth predetermined temperature ranges from about 430°C to 530°C. It is preferable to keep the fifth predetermined temperature at 480°C to 500°C to achieve optimized ductility and stretch flangeability. Coiling below 430°C may be avoided to prevent the formation of harder phases such as martensite microstructure in the steel, and coiling above 530°C, may create more larger grain boundary carbides apart and may cause variation in properties along length of the coil. A schematic diagram of the cooling profile is shown in Figure. 2. This ensures that the microstructure consists of ferrite-bainite with interphase precipitates in ferrite phase only.
In an embodiment, the high strength hot rolled steel sheet exhibits tensile strength greater than 780 MPa along with a hole expansion ratio more than 40%, and % elongation more than 14%. In order to achieve the required mechanical properties with ferrite bainite microstructure without significant addition of silicon as proposed in the disclosure, it may be required to have precipitates in the ferrite formed. Carbide or Martensite/Austenite phase formation may be minimized as it is a very hard phase and often acts as a source for void formation during deformation and results in premature failure and very low stretch

flangeability. Low carbon content is hence maintained in the desired range to obtain very low amount of carbide.
In an embodiment, figure 5 indicates the plot between stress amplitude and number of cycles to failure and shows that the fatigue strength of the high strength hot rolled steel sheet is 300 MPa and corresponding to the endurance limit at 300 MPa, the steel endures 20 lakhs cycles without failure when tested at fatigue ratio, R=-1.
Strength may be primarily obtained from both strengthened ferrite precipitate and strength of bainitic structures. It is intended to have interphase precipitates in polygonal ferrite in the microstructure as large hardness difference between polygonal ferrite and bainitic ferrite may be detrimental for stretch flangeability. Hence, less strength difference may be desired so that the stretch flangeability is improved. Inhomogeneity in microstructure may cause inhomogeneous strain partitioning and may affect local ductility. Amount of ferrite phase is kept greater than 15% to retain enough ductility in the steel, where such composition of ferrite may require high holding time at the third predetermined temperature and such a holding time may decrease rolling speed and hence may need slowing down of the mill.
Hence, a microstructure consisting of 15 to 45% of precipitate strengthened polygonal ferrite and remaining bainite (and less 2% Martensite/Austenite phase) has been developed to achieve strength more than 780 MPa along with a HER more than 40% and fatigue strength (Endurance limit) of 300 MPa at fatigue ratio, R=-1.
In accordance with another embodiment, the method of manufacturing the high strength hot-rolled steel sheet may also include a casting step followed by a hot rolling step, a controlled cooling step and a coiling step using a steel material which satisfies the component composition described below. The various processing steps are described in their respective order below:
As shown in block 201 of figure 6, the method starts with the process of casting. In the method of present disclosure, the steel of the specified composition including in weight percentage of 0.03 - 0.08 % of carbon, 1.3– 1.6 % of manganese - preferably 1.4 -1.5 %, 0.15 % of silicon, 0.08-0.14 % of titanium- preferably 0.09– 0.13%, Niobium 0.01-0.025%, V 0.07 to 0.09%, 0.2 – 0.5 % of aluminum, maximum up to 0.005 % of sulphur, maximum up to 0.025 % of phosphorous and up to 0.007 % of nitrogen, the balance being iron and

impurities by any manufacturing process including but not limiting to casting. In an embodiment, casting is performed in a continuous caster or a thin slab caster. When the steel is cast in a slab caster such as a thin slab caster where temperature of the steel slab may not be allowed to drop to a temperature below 1000 °C, as it might result in loss of ductility and cause formation of edge cracks.
The method then includes the step of heating as shown in block 202. After casting the steel slab with the specified composition, the slabs are heated to a first predetermined temperature for a first predetermined time. In an embodiment, the first predetermined temperature is greater than 1150 °C, preferably in the range of 1200 °C to 1250 °C, and the first predetermined time ranges from 30 minutes to 3 hours. In an embodiment, the first predetermined temperature should be above 1150 °C, to ensure complete dissolution of precipitates that may have formed in the preceding processing steps. A first predetermined temperature greater than 1250 °C is also not desirable because it may lead to grain coarsening of austenite and/or excessive scale loss. In some embodiment, heating of the steel slab may be carried out in a furnace.
Once the steel slab is heated as per the block 202, it may be subjected for hot working as shown in block 203 to form a steel sheet. In an embodiment, the hot working process is a hot rolling process. As shown in block 203, after casting and heating the steel slab with the specified composition, it is hot rolled. The hot rolling may constitute two steps of deformation via the first hot working process and the second hot working process. In an embodiment, the first hot working process is a deformation process of steel slab in a roughing mill at the second predetermined temperature. In an embodiment, the second predetermined temperature is above recrystallisation temperature of the steel slab and ranges from about 1050°C to 1170°C. In the roughing stage, the cast structure may be broken down, and the new structures may be formed. Further, the second hot working process is carried out at a third predetermined temperature.
In an embodiment, in the second hot working process, is a further deformation process of the steel slab carried out in four or more strands of the finishing mill, and temperatures in all strands of the finishing mill are such that microstructure of the material consists of co-phase austenite. In an embodiment, the third predetermined temperature is in the range of Ae3 to Ae3 + 70 °C., wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium and Ae3 is at least 850°C.

After finish rolling, the rolled steel sheet may be held at the third predetermined temperature (T3) for a second predetermined time as shown in block 204. In an embodiment, the second predetermined time ranges from 0.5 seconds to 3 seconds. Then, the steel sheet is subjected to intensive cooling or laminar cooling (as shown in Figure. 7) at a second predetermined cooling rate as shown in block 205.
In an embodiment, the second predetermined cooling rate is greater than 30 °C/s till a fourth predetermined temperature (T4) is reached. In an embodiment, the cooling is performed on a run-out-table. Cooling is stopped as the fourth predetermined temperature reaches 540 °C to 620 °C which is also called as coiling temperature. Now referring to block 206, coiling may be carried out at a coiling temperature or a fourth predetermined temperature. A schematic diagram of the cooling profile is shown in Figure 7 and shows that the cooling of the steel sheet takes place from T3 to T4 with a cooling rate greater than 30 °C/s. This ensures that the microstructure consists of ferrite-bainite with precipitates in ferrite phase only.
In an embodiment, figure 10 shows the plot between stress amplitude and number of cycles to failure and indicates that the fatigue strength (endurance limit) of the high strength hot-rolled steel sheet is 301 MPa and corresponding to the endurance limit at 301 MPa, the steel endures 20 lakhs cycles without failure when tested at fatigue ratio, R=-1.
The following portions of the present disclosure provides details about the proportion of each alloying element in a composition of the steel and their role in enhancing properties.
Carbon (C) may be added in the range of about 0.03 wt% to about 0.08 wt%. Carbon is an inherent component in steel, carbon helps in strengthening phases, and is often considered as a cheaper element to increase strength. Strength will not be achieved with carbon less than 0.03 wt%. But carbon content in steel beyond 0.08 wt% may have detrimental effects like it will lead to formation of more amount of undesirable second phases such as carbides, martensite/austenite islands and thereby, ductility and hole expansion ratio may get deteriorated.
Manganese (Mn) may be added in the range of about 1.3 wt% to about 1.6 wt%. Manganese not only imparts solid solution strengthening to ferrite but also lowers the austenite to ferrite transformation temperature. However, Mn level cannot be increased beyond 1.6 wt% as at such high levels Mn may enhance centerline segregation during continuous casting and hence

may create inhomogeneity in microstructure which may cause detrimental effect on stretch flangeability as well as result in increase in cost implications.
Silicon is a very cheap solid solution strengthening element and it has more solid solution strengthening potential than like manganese. However, Si content beyond 0.15 wt% may promote formation of scales during high temperature soaking and which may be undesirable.
Phosphorus content may be restricted to 0.025 wt% maximum as higher phosphorus levels may lead to reduction in toughness and weldability due to segregation of phosphorus into grain boundaries.
Sulphur content may be limited otherwise it results in a very high inclusion level that deteriorates formability.
Nitrogen (N) may be kept 0.007wt% maximum. Higher nitrogen may fix up higher amount of titanium through formation of TiN, which may not be effective strengthener. It reduces availability of Ti and hence reduces the effectiveness of Ti which is utilized for strengthening purposes through transformation strengthening and TiC precipitates. Also, increase in nitrogen content increases size of the TiN. Larger sized TiN will reduce both ductility and hole expansion ratio.
Aluminium (Al) may be added in the range of 0.2-0.5 wt %. Aluminium is used as a deoxidizer and killing of steel. It limits growth of austenite grains. However, main purpose of adding Al is to ensure formation of some ferrite in order to achieve desired ductility. Additional benefit of Al is to enhance the transformation kinetics of bainite so that less amount of M/A (martensite-austenite) phase form.
Titanium (Ti) may be added in the range of about 0.08 wt% to about 0.14 wt%. Titanium improves strength by limiting austenite grain size. More importantly, titanium forms carbides which when finely dispersed promotes strengthening.
Niobium (Nb) may be added in the range of about 0.01 wt% to about 0.03 wt%. Nb improves strength by grain refinement. More importantly, Nb forms carbides which also promotes strengthening.

Vanadium (V) may be added in the range of about 0.07 wt% to about 0.09 wt%. V improves strength through formation of fine carbides which also promotes strengthening.
Chromium may be added in the range of 0.3 to 0.5 wt%. It enhances the hardenability and enhances strength of bainite.
Example:
Further embodiments of the present disclosure will be now described with an example of a particular composition of the steel. Experiments have been carried out for a specific composition of the steel formed by using method of the present disclosure. The composition of the steel for which the tests are carried out is as shown in below table 1.

Chemical composition (wt. %)
C Mn Si Al Ti Nb V N S P
A 0.052 1.52 0.069 0.335 0.13 0.024 0.085 0.0063 0.001 0.015
Table – 1
The compositions of table-1 were continuously cast in a slab caster and the slabs were hot-rolled in a hot rolling mill. However, varying processing parameters were used in the mill and cooling as shown in Table 2 for two step cooling process and Table 4 for single step cooling process- which shows hot rolling process parameters used for hot rolling.

Steel Thickness, mm FRT (°C) IMT (°C) Holding time (s) CT (°C)
Steel 1 4.7 900 680 7 500
Steel 2 4.7 860 680 7 500
Steel 3 4.7 800 650 7 500
Table-2
Referring to the above table of two step cooling process, abbreviation for FRT refers to “Finish rolling temp” which corresponds to the second predetermined temperature, abbreviation for IMT refers to intermediate temperature which corresponds to fourth predetermined temperature and abbreviation for CT refers to coiling temperature which corresponds to fifth predetermined temperature.
As evident from Table-2 above, for steel sheet samples 1-2, the processing was done in accordance with the present invention, whereas for steel sheet sample 3, FRT temperature of lower than 850 °C was used. Effectiveness of Ti came down and hence, it missed the target strength (as shown in Table 3). Also, because of lower FRT, hole expansion ratio is relatively lower. Lower FRT creates more anisotropy or directionality in properties and hence, it resulted in correspondingly lower hole expansion ratio.
The microstructures of steel sheet sample 1 is shown in Figures 3a-3c, 4a-4c. Steel show Ferrite-bainite microstructure with very small fraction of Martensite/Austenite phases. As the carbon content in their chemistry is very low, the start temperature of bainitic transformation is high (~ 623 °C). TEM micrographs confirm presence of bainite and polygonal ferrite. Bainite is identified as the region with comparatively higher dislocation density and appear comparatively darker than ferrite region which appear bright in the micrographs. Presence of interphase precipitates are confirmed from the presence of rows of precipitates in the ferrite phase, as best seen in figures 3a and 3b. Size of the precipitates are also low (~3nm). Thus, the ferrite owing to the presence of interphase precipitates are stronger as a result of which

the hardness difference between ferrite and bainite phase is lower. Using an image analyser and/or processer, the ferrite fraction is calculated from the EBSD micrographs shown in Figure 3c. Grain orientation spread criteria is used for analysis and processing of the image in Figure 3c. Grains having less than 0.8 degree of GOS is considered as polygonal ferrite grains. Thus, the fraction of ferrite grains have accordingly been calculated and the value is 30.1%.
Thus, it is clear from the mechanical properties and the microstructures achieved, that the target properties cannot be achieved when the processing is not done as per the present disclosure.

Steel Thickness, mm YS (MPa) TS (MPa) Elongation (%) HER (%)
Steel 1 4.7 761 813 18 55
Steel 2 4.7 745 813 20 41
Steel 3 4.7 715 771 19 34
Table-3
Referring to the above table of two step cooling process, abbreviation for YS refers to yield strength and abbreviation for TS refers to tensile strength and abbreviation for HER refers to hole expansion ratio.

Steel Thickness, mm FRT (℃) CT (℃)
Steel 4 4.7 890 570
Steel 5 4 900 500
Table-4
Referring to the above table of single step cooling process, abbreviation for FRT refers to finish rolling temperature which corresponds to second predetermined temperature and

abbreviation for CT refers to coiling temperature which corresponds to fourth predetermined temperature.
The compositions of table-1 were continuously cast in a slab caster and the slabs were hot-rolled in a hot rolling mill. However, varying processing parameters were used in the mill and cooling as shown in Table 4 which shows hot rolling process parameters used for hot rolling.
As evident from the table 5, with relatively higher CT, steel sheet samples achieved better strength properties but the steel sheet sample 4 shows lower hole expansion ratio.

Steel Thickness, mm YS (MPa) TS (MPa) Elongation (%) HER (%)
Steel 4 4.7 762 843 20 27
Steel 5 4 767 806 17 51
Table 5
Referring to the above table of single step cooling process, abbreviation for YS refers to yield strength and abbreviation for TS refers to tensile strength and abbreviation for HER refers to hole expansion ratio.
The microstructures of the steel sheet sample 4 is shown in Figures 8a-8c, 9a-9b. Steel show Ferrite-bainite microstructure with very small fraction of Martensite/Austenite phases. As the carbon content in their chemistry is very low, the start temperature of bainitic transformation is high (~ 623 °C). TEM micrographs confirm presence of bainite and polygonal ferrite. Bainite is identified as the region with comparatively higher dislocation density and look comparatively darker than ferrite region which appear bright in the micrographs. Presence of precipitates are confirmed as best seen in Figures 9a and 9b. Size of the precipitates are also low (~8-15nm). Using an image analyzer and/or processor, the ferrite fraction is calculated from the EBSD micrographs shown in Figure 8c. Grain orientation spread criteria is used for analysis and processing of the image in Figure 8c. Grains having less than 0.8 degree of GOS is considered as polygonal ferrite grains. Thus, the fraction of ferrite grains have accordingly been calculated and the value is 15%.

It should be understood that the experiments are carried out for a particular composition of the steel and the results brought out in the previous paragraphs are for the composition shown in Table – 1. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the steel as well.
In an embodiment of the present disclosure, the high strength-high hot rolled steel sheet of the present disclosure may be used any application including but not limiting to automotive applications to manufacture structural components like wheel discs, chassis, pillars, outer and inner panels, suspension parts and the like. The high strength-hot rolled steel sheet may be used in any other industrial structural application.
Equivalents:
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.
It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that

such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."
While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Referral Numerals:

Referral Numerals Description
101-108 Flowchart blocks
101 Forming stage
102 Heating stage
103 Hot working stage
104 Holding stage
105 Cooling stage
106 Holding stage
107 Cooling stage
108 Coiling stage
201-206 Flowchart blocks
201 Forming stage
202 Heating stage
203 Hot working stage
204 Holding stage
205 Cooling stage
206 Coiling stage

We claim:
1. A high-strength hot-rolled steel sheet, comprising:
composition in weight percentage (wt%) of:
Carbon (C) at about 0.03 wt% to about 0.08 wt%,
Manganese (Mn) at about 1.3 wt% to about 1.6 wt%,
Silicon (Si) less than 0.15 wt%,
Niobium (Nb) at about 0.01wt% to 0. 025 wt%,
Titanium (Ti) at about 0.08wt% to 0.14 wt%,
Vanadium (V) at about 0.07 to 0.09 wt%
Aluminium (Al) at about 0.2 wt% to 0.5 wt%,
Sulphur (S) up-to 0.005 wt%,
Phosphorous (P) up-to 0.025 wt%,
Nitrogen (N) up-to 0.007 wt%, the balance being Iron (Fe) along with incidental elements, wherein, the high-strength hot-rolled steel sheet comprises of ferrite and bainite microstructure.
2. The high strength hot-rolled steel sheet as claimed in claim 1 wherein, the high strength hot-rolled steel sheet exhibits tensile strength of at least 780 MPa.
3. The high strength steel sheet as claimed in claim 1, wherein the area fraction of the ferrite-bainite microstructure includes ferrite content between 15% to 45% and balance being bainite with carbides or retained austenite-martensite.
4. The high strength steel sheet as claimed in claim 3, wherein ferrite phase includes Niobium, Titanium and Vanadium carbides precipitates.
5. The high strength steel sheet as claimed in claim 1, wherein the high strength steel sheet exhibits ductility greater than 14%.
6. The high strength steel sheet as claimed in claim 1, wherein the high strength steel sheet exhibits a hole expansion ratio of greater than 40%.
7. The high-strength hot-rolled steel sheet as claimed in claim 1, wherein the Manganese (Mn) is preferably in the range of about 1.4wt% to about 1.5wt%.

8. The high-strength hot-rolled steel sheet as claimed in claim 1, Titanium (Ti) is preferably in the range of 0.09 wt% to 0.13 wt%.
9. A method for manufacturing a high-strength hot-rolled steel sheet, the method comprising:
casting a steel slab of a composition comprising in weight percentage (wt%) of:
Carbon (C) at about 0.03 wt% to about 0.08 wt%, Manganese (Mn) at about 1.3 wt% to about 1.6 wt%, Silicon (Si) less than 0.15 wt%,
Niobium (Nb) at about 0.01 wt% to 0.025 wt%, Titanium (Ti) at about 0.08 wt% to 0.14 wt%, Vanadium (V) at about 0.07 wt% to 0.09 wt.% Aluminium (Al) at about 0.2 wt% to 0.5 wt%, Sulphur (S) up-to 0.005 wt%, Phosphorous (P) up-to 0.025 wt%, Nitrogen (N) up-to 0.007 wt%,
the balance being Iron (Fe) along with incidental elements; heating, the steel slab to a first predetermined temperature for a first predetermined time;
subjecting, the steel slab to a hot working to form a steel sheet, wherein, the hot working includes:
deforming, the steel slab in a first hot working process, at a second predetermined temperature; and
deforming, the steel slab in a second hot working process, at a third predetermined temperature, to form the steel sheet;
holding, the steel sheet for a second predetermined time after the hot working; cooling, the steel sheet at a first predetermined cooling rate to a fourth predetermined temperature;
holding, the steel sheet for a third predetermined time at the fourth predetermined temperature;
cooling, the steel sheet at a second predetermined cooling rate to a fifth predetermined temperature; and

coiling, the steel sheet at the fifth predetermined temperature to obtain the high strength steel sheet,
wherein, the high strength hot-rolled steel sheet comprises ferrite and bainite microstructure.
10. A method for manufacturing a high strength hot rolled steel sheet, the method comprising:
casting, a steel slab of a composition comprising in weight percentage (wt%) of:
Carbon (C) at about 0.03 wt% to about 0.08 wt%, Manganese (Mn) at about 1.3 wt% to about 1.6 wt%, Silicon (Si) less than 0.15 wt%,
Niobium (Nb) at about 0.01 wt% to about 0.025 wt% Titanium (Ti) at about 0.08 wt% to about 0.14 wt% Vanadium (V) at about 0.07 wt% to about 0.09 wt% Aluminium (Al) at about 0.2 wt% to 0.5 wt%, Sulphur (S) up-to 0.005 wt%, Phosphorous (P) up-to 0.025 wt%, Nitrogen (N) up-to 0.007 wt%,
balance being Iron (Fe) along with incidental elements, heating the steel slab to a first predetermined temperature for a first predetermined time;
subjecting the steel slab to the hot rolling to produce a steel sheet, wherein, the hot working includes:
deforming, the steel slab in a first hot working process, at a second predetermined temperature; and,
deforming, the steel slab in a second hot working process, at a third predetermined temperature, to form a steel sheet
holding, the steel sheet at a third predetermined temperature for a second predetermined time after hot rolling, to allow recrystallization of austenite;
cooling, the steel sheet at a second predetermined cooling rate to a fourth predetermined temperature; and

coiling, the steel sheet, at the fourth predetermined temperature to obtain a high-strength hot-rolled steel sheet;
wherein, the high-strength hot-rolled steel sheet comprises ferrite bainite microstructure.
11. The method as claimed in claims 9 and 10, wherein area fraction of the ferrite-bainite microstructure is represented by ferrite microstructure content being greater than 15% and less than 45% and, balance being bainite microstructure with carbides or retained martensite-austenite.
12. The method as claimed in claims 9 and 10, wherein casting is a continuous casting process.
13. The method as claimed in claims 12, wherein the temperature of the steel slab at an end of the continuous casting process is above 1000 °C.
14. The method as claimed in claims 9 and 10, wherein the first predetermined temperature is greater than 1150°C. , preferably ranging from about 1200 °C. to 1250 °C., and the first predetermined time ranging from about 30 minutes to about three hours.
15. The method as claimed in claims 9 and 10, wherein the hot working is a hot rolling process.
16. The method as claimed in claims 9 and 10, wherein the first hot working process is performed in a roughing mill, and the second predetermined temperature ranges from about 1050 °C. to 1170 °C..
17. The method as claimed in claims 9 and 10, wherein the second hot working process is performed in a finishing mill including at least four finishing mill stands.
18. The method as claimed in claims 9 and 10, wherein the third predetermined temperature ranges from Ae3 to Ae3+ 70°C, wherein Ae3 is temperature at which transformation of austenite to ferrite starts at equilibrium and wherein Ae3 is at least 850°C..

19. The method as claimed in claims 9 and 10, wherein the second predetermined time ranges from 0.5 seconds to 3 seconds.
20. The method as claimed in claim 9, wherein the first predetermined cooling rate is greater than 30°C/second.
21. The method as claimed in claims 9 and 10, wherein the second predetermined cooling rate is greater than 30 °C/second.
22. The method as claimed in claims 9 and 10, wherein cooling at the fourth predetermined temperature and the fifth predetermined temperature is performed by at least one of intensive water-cooling process, and laminar water-cooling process on a run out table.
23. The method as claimed in claims 9, wherein the fourth predetermined temperature ranges from about 650°C to 720°C.
24. The method as claimed in claims 9, wherein the third predetermined time ranges from about 3 seconds to 12 seconds.
25. The method as claimed in claims 9, wherein the fifth predetermined temperature ranges from about 430°C to 530°C.
26. The method as claimed in claim 10, wherein the fourth predetermined temperature ranges from 540 °C to 620 °C.
27. An automotive component manufactured from a high-strength steel sheet as claimed in claim 1.