FIELD OF INVENTION
[0001] The present invention relates to an advanced high strength hot rolled
steel, and more particularly to a method of manufacturing the advanced high strength hot rolled sheet or strip with excellent tensile ductility.
BACKGROUND
[0002] Reduction in fuel consumption and improved safety are the two major objectives to reduce emission and protect human life. Both these objectives demand use of stronger as well as tougher steel which has very high strength with exceptional elongation. Advanced high strength steel (AHSS) are known in the industry and there are various reports about AHSS. However, the major issue with existing AHSS is poor ductility as a result affect formability. As strength and elongation are inversely proportional in metals and alloys, with the development of stronger or AHSS steel, the elongation also reduces or decreases significantly. As a result, the application of AHSS in various parts of motor vehicle gets significantly limited since forming becomes increasingly difficult.
[0003] Strong and tough steel also contributes to reduce emission, therefore, reduce air pollution. Light-weight vehicle design is pre-requisite now-a-days to address the problems of environmental pollution. Effective light-weight motor vehicles require utilization of advanced high strength steels (AHSS). However, because of its poor formability, the AHSS strip cannot be applied easily to a wide variety of motor vehicle components. Hence, the ductility and formability required for AHSS strip have become increasingly demanding.
[0004] Various attempts have been made to develop advanced high strength steel
with very high elongation over the years. Some examples of these steels are given
in applications US2016333440A, US9115416B2, KR20120065464A,
WO2017131055A etc. Very high strength was reported also by Bhadeshia, MSE-A, Volume 481 – 482, pp. 36 – 39, 2008; F. G. Caballero, H. K. D. H. Bhadeshia, K. J. A. Mawella, D. G. Jones and P. Brown, MST, Volume 18, pp. 279 – 284, 2002; & C. Garcia-Mateo, F. G. Caballero and H. K. D. Bhadeshia, ISIJ International, Volume 43, pp. 1238 – 1243, 2003. Although the steel developed by Bhadeshia et.al. has very high strength, the application scope in automotive and

many other fields are very limited specially due to high alloy content, long production time (3-4 days) and limited elongation (<10%). The first two factors make the line production of steel highly difficult, whereas the last factor is not favorable in end applications. The higher carbon content (>0.7wt%) further makes the steel difficult for welding. Overall, the steel is expensive and has inadequate formability, which is undesirable.
[0005] The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the prior arts.
OBJECTIVE OF INVENTION
[0006] It is an object of the invention to solve the problems of the prior art and to develop steel with better properties including the combination of very high strength and elongation and to achieve the required properties by exploiting the existing hot rolling facilities in the integrated steel plants to get desired microstructure.
[0007] Another objective of the present invention is to develop a novel composite nano precipitate hardened steel having a microstructure of more than 95% ferrite, less than 5% pearlite and maximum 0.1 to 0.5% nano-sized micro alloyed complex precipitates and having minimum yield strength 700MPa along rolling direction for a specific automotive product application such as towing eye.
[0008] Another objective of present invention is to provide a new easier manufacturing method combining thermomechanical, hot rolling and heat treatment processes for the proposed chemical composition.
SUMMARY OF INVENTION
[0009] This summary is provided to introduce concepts related to an advanced high strength hot rolled steel, and a method of manufacturing the advanced high strength hot rolled steel sheet or strip. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0010] In one aspect of the present invention, a method (100) for manufacturing advanced high strength hot rolled steel sheet or strip having thickness in the range of 2.7 to 4.2mm is provided. The method (100) comprises casting molten steel having a composition expressed in weight %: C: 0.07-0.09, Mn: 1.7-1.9, S: 0.002 – 0.01, P: 0.02-0.04, Si: 0.05-0.11, Al: 0.02-0.04, Cr: 0.02-0.04, Mo: 0.03-0.08, Nb: 0.02-0.04, Ti: 0.07-0.09, V: 0.14-0.16, N: 0.004-0.01, and the balance being Iron (Fe) and unavoidable impurities to obtain a steel slab. The method also comprises homogenizing the steel slab to a temperature greater than 1200oC. The method further comprises roughing the steel slab in roughing mill with exit temperature in the range of 1050-1150oC. The method comprises hot rolling the roughed steel slab to produce a steel sheet such that finish rolling is done at a temperature (TFRT) varying in the range of 870oC to 950oC. The method also comprises cooling at a cooling rate in the range of 5oC/s - 75oC/s till a coiling temperature (TCT) is reached. TCT varies in the range 580 to 650oC. The method comprises coiling the steel sheet at the coiling temperature TCT and cooling till a room temperature is reached to obtain the advanced high strength hot rolled steel sheet.
[0011] In an embodiment, the advanced high strength hot rolled steel sheet comprises a microstructure of more than 95% ferrite, less than 5% pearlite and maximum 0.1 to 0.5% nano-sized micro alloyed complex precipitates. The nano-sized micro alloyed complex precipitates includes precipitates (at least one of carbide, nitride or carbonitride) of at least one of Ti, Nb, V and Mo.
[0012] In an embodiment, the ferrite is precipitation strengthened and has a grain size of 2.5 to 5 µm.
[0013] In an embodiment, the advanced high strength hot rolled steel exhibits an ultimate tensile strength greater than 750 MPa along the rolling direction.
[0014] In an embodiment, the advanced high strength hot rolled steel exhibits a yield strength greater than 700 MPa, minimum uniform elongation of 10%, minimum total elongation of 15%, and hardness 250-280 HV.
[0015] In an embodiment, the steel slab is homogenized at a temperature in the range of 1200-1275°C for a time duration in the range of 2.5-3.5 hours.

[0016] In an embodiment, the hot rolling is performed in the austenite phase using multi-pass deformation 10-40% each pass in the austenite.
[0017] In an embodiment, the addition of various microalloying elements such as Titanium, Vanadium and Niobium introduces different types of complex carbide precipitates which affects the strengthening depending upon their amount in the steel.
[0018] In an embodiment, the lower level of silicon in the steel improves the scale behavior, thereby improve the steel surface.
[0019] In another aspect of the present invention, an advanced high strength hot rolled steel is provided. The advanced high strength hot rolled steel comprises the following composition expressed in weight %: Carbon (C): 0.07% - 0.09%, Manganese (Mn): 1.7% - 1.9%, Chromium (Cr): 0.02-0.04%, Silicon (Si): 0.05%-0.11%, Niobium (Nb): 0.02%-0.04%, Aluminium (Al): 0.02-0.04%, Molybdenum (Mo): 0.03%-0.08%, Vanadium (V): 0.14-0.16%, Titanium (Ti): 0.07%- 0.09%, Sulphur (S): 0.002-0.01%, Phosphorus (P): 0.02-0.04%, Nitrogen (N): 0.004-0.01%, and the remaining being substantially iron and incidental impurities. The advanced high strength hot rolled steel comprises a microstructure of maximum 95 % or more ferrite, less than 5% pearlite and maximum 0.1 to 0.5% nano-sized micro alloyed complex precipitates.
[0020] In an embodiment, the advanced high strength hot rolled steel exhibits an ultimate tensile strength greater than 750 MPa along the rolling direction.
[0021] In an embodiment, the advanced high strength hot rolled steel exhibits a yield strength greater than 700 MPa, minimum uniform elongation of 10%, minimum total elongation of 15%, and hardness 250-280 HV.
[0022] In an embodiment, a component produced from the advanced high strength hot rolled steel is used in automobile applications.
[0023] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Figure 1 illustrates a flowchart of a method of manufacturing an advanced high strength hot rolled steel, according to an embodiment of the present invention;
[0025] Figure 2 illustrates a graphical representation of stress versus elongation, obtained during tensile test of the advanced high strength hot rolled steel having composition of sample “S1”, according to the embodiment of the present invention;
[0026] Figure 3 illustrates an optical micrograph of the advanced high strength hot rolled steel having composition of sample “S1”, according to the embodiment of the present invention;
[0027] Figure 4 illustrates a SEM image of the advanced high strength hot rolled steel having composition of sample “S1”, according to the embodiment of the present invention; and
[0028] Figure 5 illustrates an EBSD microstructure of the advanced high strength hot rolled steel having composition of sample “S1”, according to the embodiment of the present invention.
[0029] The drawings referred to in this description are not to be understood as being drawn to scale except if specifically noted, and such drawings are only exemplary in nature.
DETAILED DESCRIPTION
[0030] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0031] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and

embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0032] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0033] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0034] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0035] The present disclosure provides a method (100) of manufacturing advanced high strength hot rolled steel that may be used to produce components for automobile applications. The advanced high strength hot rolled steel comprises the following composition expressed in weight %: C: 0.07-0.09, Mn: 1.7-1.9, S: 0.002 – 0.01, P: 0.02-0.04, Si: 0.05-0.11, Al: 0.02-0.04, Cr: 0.02-0.04, Mo: 0.03-0.08, Nb: 0.02-0.04, Ti: 0.07-0.09, V: 0.14-0.16, N: 0.004-0.01, and the balance being Iron (Fe) and unavoidable impurities. The advanced high strength hot rolled steel comprises a structure including a ferrite phase, a pearlite phase, and nano-

micro alloyed complex precipitates. In an embodiment, the advanced high strength hot rolled steel sheet comprises a microstructure of more than 95% ferrite, less than 5% pearlite and maximum 0.1 to 0.5% nano-sized micro alloyed complex precipitates. The advanced high strength hot rolled steel exhibits an ultimate tensile strength above 750 MPa, minimum 700 MPa yield strength, hardness 250-280 HV and a minimum %Elongation – 15 along the rolling direction. The nano-sized micro alloyed complex precipitates includes precipitates (at least one of carbide, nitride or carbonitride) of at least one of Ti, Nb, V and Mo.
[0036] In one embodiment, the advanced high strength hot rolled steel comprises C - 0.08, Mn –1.88, Si – 0.104, V - 0.15, Mo – 0.047, Cr – 0.027, Al – 0.042, Ti – 0.084, Nb – 0.038, P – 0.014, S – 0.002, N – 0.006, and the balance being Iron (Fe) and unavoidable impurities. The advanced high strength hot rolled steel exhibits yield strength (YS) in the range of 777 MPa, ultimate tensile strength (UTS) in the range of 856 MPa, % uniform elongation – 11, and total elongation 19%.
[0037] In another embodiment, the advanced high strength hot rolled steel comprises C - 0.08, Mn –1.88, Si – 0.11, V - 0.15, Mo – 0.047, Cr – 0.028, Al – 0.045, Ti – 0.085, Nb – 0.040, P – 0.014, S – 0.002, N – 0.006, and the balance being Iron (Fe) and unavoidable impurities. The advanced high strength hot rolled steel exhibits yield strength (YS) in the range of 785 MPa, ultimate tensile strength (UTS) in the range of 848 MPa, % uniform elongation – 12, and total elongation 20%.
[0038] In yet another embodiment, the advanced high strength hot rolled steel comprises C - 0.078, Mn –1.89, Si – 0.108, V - 0.15, Mo – 0.047, Cr – 0.027, Al – 0.042, Ti – 0.086, Nb – 0.040, P – 0.014, S – 0.002, N – 0.006, and the balance being Iron (Fe) and unavoidable impurities. The advanced high strength hot rolled steel exhibits yield strength (YS) in the range of 736 MPa, ultimate tensile strength (UTS) in the range of 877 MPa, % uniform elongation – 11, and total elongation 19%.
[0039] In an embodiment, the advanced high strength hot rolled steel comprises C - 0.078, Mn –1.88, Si – 0.106, V - 0.16, Mo – 0.047, Cr – 0.027, Al – 0.042, Ti – 0.086, Nb – 0.040, P – 0.014, S – 0.002, N – 0.0055, and the balance being Iron

(Fe) and unavoidable impurities. The advanced high strength hot rolled steel exhibits yield strength (YS) in the range of 745 MPa, ultimate tensile strength (UTS) in the range of 862 MPa, % uniform elongation – 10, and total elongation 19%.
[0040] Referring to Figure 1, an exemplary thermo-mechanical method (100) of manufacturing the advanced high strength hot rolled steel strip, sheet or blank is illustrated. Each step shown in figure 1 represents one or more process, method or subroutine steps carried out in the method. Furthermore, the order of blocks is illustrative only and the blocks can change in accordance with the present disclosure. Additional blocks can be added, or fewer blocks can be utilized, without departing from this disclosure. The method (100) for manufacturing the advanced high strength hot rolled steel strip, sheet or blank begins at step (102). At step (102), molten steel having composition expressed in weight %: C: 0.07-0.09, Mn: 1.7-1.9, S: 0.002 – 0.01, P: 0.02-0.04, Si: 0.05-0.11, Al: 0.02-0.04, Cr: 0.02-0.04, Mo: 0.03-0.08, Nb: 0.02-0.04, Ti: 0.07-0.09, V: 0.14-0.16, N: 0.004-0.01, and the balance being Iron (Fe) and unavoidable impurities is cast in a casting apparatus to obtain a steel slab. In an embodiment, the molten steel is cast through continuous caster.
[0041] At step (104), the steel slab (cast ingots) is homogenized inside a furnace kept at a temperature of greater than 1200˚C. In one example, the furnace is a reheating furnace. In the preferred embodiment, the steel slab is reheated to a temperature in the range of 1200 - 1275oC for a time duration in the range of 2.5-3.5 hours.
[0042] At step (106), the homogenized steel slab obtained in the step (104) is rolled/deformed in the roughing mill with exit temperature in the range of 1050-1150oC. More particularly, the homogenized slabs are subjected to deformation in industrial roughening mill in the temperature range 1040-1150˚C to break the cast structure for further use in the hot rolling mill.
[0043] At step (108), the roughed steel slab obtained in step (106) is subjected to a second hot working process such as hot rolling process such that finish rolling is done at a temperature (TFRT) to obtain a hot rolled steel strip. TFRT varies in the

range 870oC to 950oC. The hot rolling is performed in the austenite phase using multi-pass deformation 10-40% each pass in the austenite. The hot rolling process may be carried out by passing the steel through a pair of rolls and rolling may be carried out for at least five times to reduce the thickness of the steel to required size in the range of 2.7-4.2 mm. In an embodiment, the hot rolling process is performed by passing the steel through a pair of rolls and rolling is carried out for at least 5 times.
[0044] At step (110), the hot rolled steel strip obtained in step (108) is cooled to a cooling rate in the range of 5oC/s - 75oC/s till a coiling temperature (TCT) is reached, wherein TCT varies in the range 580 to 650oC.
[0045] At step (112), the steel sheet is coiled at the coiling temperature TCT and is cooled till a room temperature is reached to obtain the advanced high strength hot rolled steel sheet having thickness in the range of 2.7 mm to 4.2 mm. In an embodiment, the coiling was performed in controlled manned locations wise at predetermined temperature in the range as said to achieve precise coiling temperature. The steel after coiling in the down coiler was subjected to cooling in air to obtain the above said advanced high strength hot rolled steel with exceptional ductility.
[0046] The obtained advanced high strength hot rolled steel sheet comprises a
microstructure of more than 95% ferrite, less than 5% pearlite and maximum 0.1 to 0.5% nano-sized micro alloyed complex precipitates. The obtained advanced high strength hot rolled steel exhibits an ultimate tensile strength above 750 MPa, minimum 700 MPa yield strength, hardness 250-280 HV and a minimum %Elongation – 15 along the rolling direction. In an embodiment, the ferrite is precipitation strengthened and has a grain size of 2.5 to 5 µm. The nano-sized micro alloyed complex precipitates includes precipitates (at least one of carbide, nitride or carbonitride) of at least one of Ti, Nb, V and Mo.
[0047] The method (100) of the present disclosure includes melting, casting, heat treatment, thermomechanical hot-rolled routes, which are simple. Slab drop-out temperature, finish rolling temperature, and coiling temperature, all have significant effects on final mechanical properties.

[0048] Following portions of the present disclosure provides details about the proportion of each element in a composition of the advanced high strength hot rolled steel and their role in enhancing properties.
[0049] Carbon (C) may be used in the range of about 0.07 to 0.09 wt.%: Amount of Carbon is so selected such that it results in desired microstructure and strengthening. Good weldability is one of the required properties for the above steel, therefore, carbon level was preferred not to exceed 0.12, preferably 0.08.
[0050] Phosphorus (P) may be used below 0.025%: Phosphorus amount was be restricted to 0.025%, and preferably 0.020% or less, since high phosphorous was detrimental in steel.
[0051] Sulphur (S) may be used < 0.01 wt.% Similar to phosphorus, Sulphur was also detrimental to the steel. The Sulphur content was kept as low as possible, more preferably below 0.004 wt% to minimize the amount of inclusions which are potential sites for premature failure during forming operations.
[0052] Manganese (Mn): 1.7 to 1.9 wt.%: Manganese promotes solid solution strengthening and stabilizes austenite, thereby imparting strength in the steel. However, excessive amount of Mn is not recommended as it can deteriorate weldability of the steel, so the manganese content was restricted to below 2 wt.% or less more preferably less than 1.9 wt.%.
[0053] Silicon (Si): 0.05-0.11 wt.%: Silicon is also a ferrite stabilizer. Silicon strengthen ferrite and impart strength. However, excess amount of silicon addition in steel is detrimental due to varieties of scale formation during hot rolling and cooling. Such scale formation leads to surface deterioration and reduces coat-ability/ galvanizability. Hence, Si was restricted within the wt% range as mentioned herein and was more preferably kept below 0.11 wt%.
[0054] Molybdenum (Mo): 0.03 - 0.08 wt%: Molybdenum is added to steel to strengthen either through precipitation or hardenability. Mo by combining with other elements normally enhance strength. Further, since Mo is costly, its amount was restricted to lower level less than 0.1. taking cost advantage of Mo. However, depending on application and critically of application Mo content may be varied.

[0055] Titanium (Ti): 0.07 - 0.09 wt%: Ti is beneficial to restrict austenite grain growth. In addition, Ti also forms very fine carbonitride in the presence of Nb, vanadium (V) and increases strength. However, Ti could be harmful as Ti has the tendency to form hard TiN inclusions. Accordingly, the amount of Ti was restricted below 0.1wt% or less or minimum level.
[0056] Nitrogen (N): 0.01 wt% maximum: Excess nitrogen in steel was also detrimental. Excess nitrogen may lead to hard inclusions such as TiN and AlN which deteriorate formability operations. Nitrogen content was restricted up to 0.01 wt%.
[0057] Vanadium (V): 0.14 - 0.17 wt%: Vanadium is added to steel to strengthen through thermo-mechanical processing and precipitation. Vanadium combining with other elements normally enhance strength. Further, since V is costly, its amount was restricted preferably below 0.16 wt% to make the developed steel economical and still taking processing advantage of V during hot rolling.
[0058] Aluminium (Al): 0.01 - 0.05 wt%: Al is added to steel to deoxidize the steel and also helps to restrict austenite grain growth.
[0059] Niobium (Nb) may be used below 0.06 wt.%: Niobium helps in austenite strengthening through alternation of austenite recrystallization temperature. Niobium was added to increase the strength of the steel by various mechanisms such as grain refinement and precipitation. Nb was added very carefully and was optimized suitably to reduce costs (since Nb is costly).
[0060] The amount of carbon and manganese in the steel composition is restricted according to the amounts described above for better weldability and formability. Silicon is also restricted below certain levels as described above to improve the product characteristics. In an embodiment, the lower level of silicon in the steel improves the scale behavior, thereby improve the steel surface. Titanium, Vanadium and Niobium introduces different types of complex carbide precipitates which affects the strengthening depending upon their amount in the steel.

[0061] Further, the hot rolling, cooling and coiling parameters are employed in a way to ensure that the developed steel could be produced under conventional mill operating conditions in the industrial run out table to obtain steel strip in the thickness range of 2.7-4.2 mm and having very high strength & exceptional elongation.
[0062] The chemical composition of the proposed alloys are shown in Table 1.

Sample C Mn Si V Mo Cr Al Ti Nb P S N
S1 0.08 1.88 0.10 4 0.15 0.047 0.027 0.042 0.084 0.038 0.014 0.002 0.006
S2 0.08 1.88 0.11 0.15 0.047 0.028 0.045 0.085 0.040 0.014 0.002 0.006
S3 0.07 8 1.89 0.10 8 0.15 0.047 0.027 0.045 0.086 0.040 0.014 0.002 0.006
S4 0.07 8 1.88 0.10 6 0.16 0.047 0.027 0.045 0.086 0.040 0.014 0.002 0.0055
Table -1: Chemical composition of Steel 1, 2, 3 & 4
[0063] To investigate the properties of the steels, experiments were carried out for specific compositions which are reported in Table 1 of advanced high strength hot rolled steel obtained by using the method (100) of the present disclosure. Tensile specimens were prepared along the rolling direction of the rolled sheet. Tensile tests were performed as per ASTM standard with gauge length 50mm. Microstructural characterization was carried out using optical, scanning electron microscope and electron back scatter diffraction and also by transmission electron microscope (TEM). Mechanical properties were evaluated by tensile testing, Vickers hardness. SEM and TEM samples were prepared by following standard methods. EBSD was employed to confirm the phases.
[0064] Different samples of the advanced high strength hot rolled steel were prepared by utilizing the compositions as mentioned in table 1 and using the method (100). Standard metallography techniques were followed to prepare the samples for different mechanical properties measurements. The measured properties for the steel samples having compositions as mentioned in table 1 are depicted in Table 2. The measured properties for the steel samples having compositions as mentioned in table 1 with varying thickness are depicted in Table 3.

Steel YS (MPa) TS (MPa) UEL (%) TEL (%)
S1 777 856 11 19
S2 785 848 12 20
S3 736 877 11 19
S4 745 862 10 19
Table 2: Variation in mechanical properties with varying composition.
Steel variant YS (MPa) TS (MPa) UEL (%) TEL (%)
3mm 750-775 800-850 10-12 17-20
4mm 750-780 820-850 11-12 18-20
Table 3: Variation in mechanical properties with varying compositions of table 1 and varying thickness of final product.
[0065] Figure 2 illustrates the engineering tensile properties of the obtained advanced high strength hot rolled steel having composition of sample “S1” reported in Table 1 using the method (100). The mechanical properties are very remarkable considering the leaner chemistry of the current steels; superior yield strength (minimum 700MPa) along rolling direction with tensile strength more than 750MPa and elongation above 15%. The uniform elongation was above 10%.
[0066] Figure 3 illustrates an optical micrograph of the advanced high strength hot rolled steel of the steel having composition of sample “S1”. From the figures it can be observed that the developed steels exhibits mainly BCC structure consisting of mainly ferritic microstructure. The above observation is also corroborated through scanning electron micrograph (Figure 4). The SEM microstructure shows mainly ferritic structures with finer grains. EBSD micrograph presented in figure 5 shows presence of different orientation of the ferrite grains. Some of the advanced analysis carried out for precipitation studied and confirmed the steel has very fine nanoprecipitates of dispersed in the matrix.
[0067] The above results confirm the extraordinary properties along the rolling direction of the presently developed hot rolled steel products with yield strength minimum 700MPa, tensile strength min 750MPa, total elongation (TEL) above 15% required for specific automotive component such as towing eye.

[0068] The present invention provides advanced high strength hot rolled steel and the method (100) of manufacturing the advanced high strength hot rolled steel having higher strength with bare minimum addition of alloying elements. The advanced high strength hot rolled steels makes an important contribution towards the cost effective, futuristic, and strategic light weight application of steel with greater factor of safety. A superior factor of safety may be obtained by achieving the sharp increase of yield strength with reasonable ductility, specifically required for the automotive and structural applications. The thermomechanical / hot-rolling process is quite simple and does not require huge energy consumption. Therefore, the method (100) of the present disclosure aids in reducing energy consumption and thus a cost-effective steel manufacturing process. Further, the method (100) provides a nano precipitate hardened hot rolled advanced high strength ferritic steel developed with leaner chemistry having yield strength, tensile strength, and total elongation in the range of 700-780MPa, 750-850MPa and 15-20%, respectively along the rolling direction of the steel. The high strength and elongation is achieved through formation primarily single-phase microstructure with the help of judicious combination of alloying to make economical.
[0069] The method (100) may employ additional processes such as grinding to remove scaling and to make both surfaces parallel to each other, without limiting the scope of the invention.
[0070] It should be understood that the experiments are carried out for particular compositions of the advanced high strength hot rolled steel reported in Table 1 and the results brought out are reported in Table 2. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the advanced high strength hot rolled steel strip, as well.
[0071] Furthermore, the terminology used herein is for describing embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without

departing from the scope of the present disclosure as encompassed by the following claims.
[0072] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[0073] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions, or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

CLAIMS
We Claim:
1. A method (100) for manufacturing advanced high strength hot rolled steel
sheet or strip having thickness in the range of 2.7 to 4.2mm, the method
(100) comprising:
casting molten steel having a composition expressed in weight %: C: 0.07-0.09, Mn: 1.7-1.9, S: 0.002 – 0.01, P: 0.02-0.04, Si: 0.05-0.11, Al: 0.02-0.04, Cr: 0.02-0.04, Mo: 0.03-0.08, Nb: 0.02-0.04, Ti: 0.07-0.09, V: 0.14-0.16, N: 0.004-0.01, and the balance being Iron (Fe) and unavoidable impurities to obtain a steel slab;
homogenizing the steel slab to a temperature greater than 1200oC;
roughing the steel slab in roughing mill with exit temperature in the range of 1050-1150oC;
hot rolling the roughed steel slab to produce a steel sheet such that finish rolling is done at a temperature (TFRT), wherein TFRT varies in the range of 870oC to 950oC;
cooling at a cooling rate in the range of 5oC/s - 75oC/s till a coiling temperature (TCT) is reached, wherein TCT varies in the range 580 to 650oC;
coiling the steel sheet at the coiling temperature TCT and cooling till a room temperature is reached to obtain the advanced high strength hot rolled steel sheet.
2. The method (100) for manufacturing advanced high strength hot rolled steel sheet or strip as claimed in the claim 1, wherein the advanced high strength hot rolled steel sheet comprises a microstructure of more than 95% ferrite, less than 5% pearlite and maximum 0.1 to 0.5% nano-sized micro alloyed complex precipitates.
3. The method (100) for manufacturing advanced high strength hot rolled steel sheet or strip as claimed in the claim 1, wherein the ferrite is precipitation strengthened and has a grain size of 2.5 to 5 µm.
4. The method (100) for manufacturing advanced high strength hot rolled steel sheet or strip as claimed in the claim 1, wherein the advanced high strength hot rolled steel exhibits an ultimate tensile strength greater than 750 MPa

along the rolling direction, a yield strength greater than 700 MPa, minimum uniform elongation of 10%, minimum total elongation of 15%, and hardness 250-280 HV.
5. The method (100) for manufacturing advanced high strength hot rolled steel sheet or strip as claimed in the claim 2, wherein the nano-sized micro alloyed complex precipitates includes at least one of carbide, nitride or carbonitride precipitates of at least one of Ti, Nb, V and Mo.
6. The method (100) for manufacturing advanced high strength hot rolled steel sheet or strip as claimed in the claim 1, wherein the steel slab is homogenized at a temperature in the range of 1200-1275°C for a time duration in the range of 2.5-3.5 hours.
7. The method (100) for manufacturing advanced high strength hot rolled steel sheet or strip as claimed in the claim 1, wherein the hot rolling is performed in the austenite phase using multi-pass deformation 10-40% each pass in the austenite.
8. The method (100) for manufacturing advanced high strength hot rolled steel sheet or strip as claimed in the claims 1 and 2, wherein the addition of various microalloying elements such as Titanium, Vanadium and Niobium introduces different types of complex carbide precipitates which affects the strengthening depending upon their amount in the steel.
9. The method (100) for manufacturing advanced high strength hot rolled steel sheet or strip as claimed in the claim 1, wherein the lower level of silicon in the steel improves the scale behavior, thereby improve the steel surface.
10. An advanced high strength hot rolled steel comprising the following composition expressed in weight %:
Carbon (C): 0.07% - 0.09%, Manganese (Mn): 1.7% - 1.9%, Chromium (Cr): 0.02-0.04%, Silicon (Si): 0.05%-0.11%, Niobium (Nb): 0.02%-0.04%, Aluminium (Al): 0.02-0.04%,

Molybdenum (Mo): 0.03%-0.08%, Vanadium (V): 0.14-0.16%, Titanium (Ti): 0.07%- 0.09%, Sulphur (S): 0.002-0.01%, Phosphorus (P): 0.02-0.04%, Nitrogen (N): 0.004-0.01%, and the remaining being substantially iron and incidental impurities, wherein the advanced high strength hot rolled steel comprises a microstructure of maximum 95 % or more ferrite, less than 5% pearlite and maximum 0.1 to 0.5% nano-sized micro alloyed complex precipitates.
11. The advanced high strength hot rolled steel as claimed in the claim 10, wherein the advanced high strength hot rolled steel exhibits an ultimate tensile strength greater than 750 MPa along the rolling direction.
12. The advanced high strength hot rolled steel as claimed in the claim 10, wherein the advanced high strength hot rolled steel exhibits a yield strength greater than 700 MPa, minimum uniform elongation of 10%, minimum total elongation of 15%, and hardness 250-280 HV.
13. The advanced high strength hot rolled steel as claimed in the claim 10, wherein the nano-sized micro alloyed complex precipitates includes at least one of carbide, nitride or carbonitride precipitates of at least one of Ti, Nb, V and Mo.
14. A component produced from the advanced high strength hot rolled steel as claimed in the claims 1 to 13, wherein the component is used in automobile applications.