Description:FIELD OF INVENTION
[0001] The present invention relates to a complex phase high strength hot rolled steel sheet or strip having thickness ranging from 2.0 mm to 6.0 mm, and more particularly to the method of manufacturing the complex phase high strength hot rolled steel sheet or strip adaptable for manufacturing automobile components that require moderate amount of forming.
BACKGROUND
[0002] In recent years, in view of global environment conservation, an improvement in fuel efficiency of automobiles has been strongly requested. Accordingly, by increasing the strength of materials used for forming automobile bodies, a decrease in thickness and a reduction in weight have been energetically carried out. As a material in response to the requirement as described above, advance high strength steel (AHSS) sheets, including, transformation hardening type DP steel (Dual Phase Steel), and TRIP steel using the TRIP (Transformation Induced Plasticity) phenomenon of retained austenite, have been developed.
[0003] Low carbon hot rolled steel strips are extensively used for the manufacturing of structural components of automobiles. Hot rolled low carbon steel strips with microstructures containing two or more phases, commonly known as advanced high strength steels, are widely used in making various structural components of vehicles. One very common example is dual phase (DP) steel, the microstructure of which comprises ferrite and martensite. While the presence of martensite accounts for the adequate strength, the ferrite, being the soft phase, ensures appreciable ductility. Some important features of dual phase steel grades are:
[0004] Strengthening through phase transformation
[0005] Appreciable strength without heavy alloying
[0006] Smooth stress-strain plot, without yield point elongation phenomenon, ensuring good formability.
[0007] However, problems related to the stamping, forming, and drawing of dual phase steel and TRIP (transformation induced plasticity) steel are well known, and significant hurdles exist for successful implementation using the existing manufacturing infrastructure.
[0008] 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
[0009] It is an object of the invention to solve the problems of the prior art and to provide a new chemical composition, for hot rolled complex phase (CP) grade steel, for obtaining a microstructure majorly comprising bainite, ferrite and martensite phases.
[0010] Another objective of the present invention is to develop a bendable steel grade with moderate amount of forming that can be made into steel strips, sheets, and blanks, having tensile strength above 980 MPa with tensile elongation value over 10%.
[0011] Another objective of the present invention is to develop the complex phase high strength hot rolled steel for manufacturing automobile components. The grade has low carbon-based steel composition, suitably alloyed with Mn and other microalloying elements.
[0012] 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
[0013] This summary is provided to introduce concepts related to a complex phase high strength hot rolled steel, and a method of manufacturing the complex phase high strength hot rolled 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.
[0014] In one aspect of the present invention, a method for manufacturing a complex phase high strength hot rolled steel sheet or strip having thickness in the range of 2 mm to 6 mm is provided. The method comprises casting molten steel having a composition expressed in weight %: Carbon (C): < 0.16%, Manganese (Mn): 1.2% - 2.8%, Chromium (Cr): 0.1-1.0%, Silicon (Si): 0.2%-0.8%, Sulphur (S): <0.006%, Phosphorus (P): < 0.02%, Niobium (Ni): 0.0 - 0.08%, Aluminium (Al): 0.02-0.06%, Titanium (Ti): <0.05%, Molybdenum (Mo): <0.5%, Nitrogen (N) < 80 ppm, and the remaining being substantially iron and incidental impurities, and unavoidable impurities to obtain a steel slab. The method also comprises reheating the steel slab to a temperature in the range of 1150 – 1280oC. The method further comprises roughing the steel slab in roughing mill with exit temperature in the range of 1010-1080oC. 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 820oC to 900oC, above Ar3 temperature of the steel. The method also comprises cooling the hot rolled steel to a first intermediate temperature in the range of 350-650oC at a first intermediate cooling rate in the range of 20oC/s - 100oC/s to obtain first intermediate hot rolled steel and coiling thereafter. The method further comprises cooling the hot rolled coil to room temperature to obtain complex phase high strength hot rolled steel sheet. The complex phase high strength hot rolled steel sheet comprises a microstructure of 5-20% ferrite, 5-20% martensite and about 60-90% bainite. The complex phase high strength hot rolled steel exhibits an ultimate tensile strength in the range of 980 - 1150 MPa.
[0015] In an embodiment, the complex phase high strength hot rolled steel comprises the composition expressed in weight %: C – 0.12, Mn –1.9, Si –0.35, Al – 0.04, Nb – 0.045, Cr – 0.61, Ti – 0.013, Mo – 0.25 and the balance being Iron (Fe) and unavoidable impurities, wherein the carbon equivalent (Ceq) expressed by formula CE = (C) + (Mn+Si)/6 + (Cu+Ni)/15 + (Cr+Mo+V+Nb)/5 is 0.676, wherein each symbol in brackets represents the content (mass%) of the corresponding element.
[0016] In an embodiment, the obtained complex phase high strength hot rolled steel exhibits a yield strength (YS) of 915 MPa, ultimate tensile strength (UTS) of 1062 MPa, %Elongation – 11.2, strain hardening exponent (n) – 0.13, and yield ratio (YS/UTS) of 0.86.
[0017] In an embodiment, the complex phase high strength hot rolled steel comprises the composition expressed in weight %: C – 0.136, Mn –2.26, Si –0.31, Al – 0.05, Nb – 0.047, Cr – 0.56, Ti – 0.02, Mo – 0.28 and the balance being Iron (Fe) and unavoidable impurities, wherein the carbon equivalent (Ceq) expressed by formula CE = (C) + (Mn+Si)/6 + (Cu+Ni)/15 + (Cr+Mo+V+Nb)/5 is 0.741, wherein each symbol in brackets represents the content (mass%) of the corresponding element.
[0018] In an embodiment, the obtained complex phase high strength hot rolled steel exhibits a yield strength (YS) of 810 MPa, ultimate tensile strength (UTS) of 1061 MPa, %Elongation – 12.6, strain hardening exponent (n) – 0.13 and yield ratio (YS/UTS) of 0.76.
[0019] In another aspect of the present invention, a complex phase high strength hot rolled steel is provided. The complex phase high strength hot rolled steel comprises the following composition expressed in weight %: (C): < 0.16%, (Mn): 1.2% - 2.8%, (Cr): 0.1-1.0%, (Si): 0.2%-0.8%, (S): <0.006%, (P): < 0.02%, (Ni): 0.0 - 0.08%, (Al): 0.02-0.06%, (Ti): <0.05%, (Mo): <0.5%, (N) < 80 ppm, and the remaining being substantially iron and incidental impurities. The complex phase high strength hot rolled steel comprises a microstructure of 5-20% ferrite, 5-20% martensite, and 60-90% bainite.
[0020] In an embodiment, the complex phase high strength hot rolled steel exhibits ultimate tensile strength (UTS) in the range of 980-1150 MPa, yield strength (YS) > 750 MPa, and minimum %Elongation – 10.
[0021] In an embodiment, a component produced from the complex phase high strength hot rolled steel is used in automobile applications.
[0022] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Figure 1 illustrates a flowchart of a method of manufacturing a complex phase high strength hot rolled steel;
[0024] Figure 2a illustrates a SEM image of the complex phase high strength hot rolled steel having composition of Steel 1;
[0025] Figure 2b illustrates a high magnification SEM image of the complex phase high strength hot rolled steel having composition of Steel 1;
[0026] Figure 3a illustrates a SEM image of the complex phase high strength hot rolled steel having composition of Steel 2; and
[0027] Figure 3b illustrates a high magnification SEM image of the complex phase high strength hot rolled steel having composition of Steel 2.
[0028] 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
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] The present disclosure provides a complex phase high strength hot rolled steel that may be used to produce components for automobile applications. The complex phase high strength hot rolled steel comprises the following composition expressed in weight %: Carbon (C): < 0.16%, Manganese (Mn): 1.2% - 2.8%, Chromium (Cr): 0.1-1.0%, Silicon (Si): 0.2%-0.8%, Sulphur (S): <0.006%, Phosphorus (P): < 0.02%, Niobium (Ni): 0.0 - 0.08%, Aluminium (Al): 0.02-0.06%, Titanium (Ti): <0.05%, Molybdenum (Mo): <0.5%, Nitrogen (N) < 80 ppm, and the remaining being substantially iron and incidental impurities. The complex phase high strength hot rolled steel comprises a structure including a bainitic phase, ferrite phase, and martensite phase. In an embodiment, the complex phase high strength hot rolled steel comprises a microstructure of microstructure of 5-20% ferrite, 5-20% martensite, and 60-90% bainite. The complex phase high strength hot rolled steel exhibits an ultimate tensile strength in the range of 980-1150 MPa and a minimum %Elongation – 10.
[0035] In one embodiment, the complex phase high strength hot rolled steel comprises the composition expressed in weight %: C – 0.12, Mn –1.9, Si –0.35, Al – 0.04, Nb – 0.045, Cr – 0.61, Ti – 0.013, Mo – 0.25 and the balance being Iron (Fe) and unavoidable impurities, wherein the carbon equivalent (Ceq) expressed by formula CE = (C) + (Mn+Si)/6 + (Cu+Ni)/15 + (Cr+Mo+V+Nb)/5 is 0.676. Each symbol in brackets represents the content (mass%) of the corresponding element. The complex phase high strength hot rolled steel exhibits a yield strength (YS) of 915 MPa, ultimate tensile strength (UTS) of 1062 MPa, %Elongation – 11.2, strain hardening exponent (n) – 0.13, and yield ratio (YS/UTS) of 0.86.
[0036] In another embodiment, the complex phase high strength hot rolled steel comprises the composition expressed in weight %: C – 0.136, Mn –2.26, Si –0.31, Al – 0.05, Nb – 0.047, Cr – 0.56, Ti – 0.02, Mo – 0.28 and the balance being Iron (Fe) and unavoidable impurities, wherein the carbon equivalent (Ceq) expressed by formula CE = (C) + (Mn+Si)/6 + (Cu+Ni)/15 + (Cr+Mo+V+Nb)/5 is 0.741. Each symbol in brackets represents the content (mass%) of the corresponding element. The complex phase high strength hot rolled steel exhibits a yield strength (YS) of 810 MPa, ultimate tensile strength (UTS) of 1061 MPa, %Elongation – 12.6, strain hardening exponent (n) – 0.13 and yield ratio (YS/UTS) of 0.76.
[0037] Referring to Figure 1, an exemplary thermo-mechanical method (100) of manufacturing the complex phase high strength hot rolled 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 complex phase high strength hot rolled strip, sheet or blank begins at step (102).
[0038] At step (102), molten steel having composition expressed in weight %: Carbon (C): < 0.16%, Manganese (Mn): 1.2% - 2.8%, Chromium (Cr): 0.1-1.0%, Silicon (Si): 0.2%-0.8%, Sulphur (S): <0.006%, Phosphorus (P): < 0.02%, Niobium (Ni): 0.0 - 0.08%, Aluminium (Al): 0.02-0.06%, Titanium (Ti): <0.05%, Molybdenum (Mo): <0.5%, Nitrogen (N) < 80 ppm, and the remaining being substantially iron and incidental impurities is cast in a casting apparatus to obtain a steel slab.
[0039] At step (104), the steel slab (cast ingots) is reheated inside a furnace kept at a temperature in the range of 1150 - 1280 oC. In the preferred embodiment, the steel slab is reheated to a temperature of 1200 oC.
[0040] At step (106), the heated steel slab obtained in the step (104) is rolled/deformed in the roughing mill with exit temperature in the range of 1010-1080oC.
[0041] At step (108), the roughed steel slab obtained in step (106) is subjected to a 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 820oC to 900oC, above Ar3 temperature of the steel. 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 passes to reduce the thickness of the steel to required size in the range of 2-6 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 passes.
[0042] At step (110), the hot rolled steel strip obtained in step (108) is cooled to a first intermediate temperature in the range of 350-650oC at a first intermediate cooling rate in the range of 20oC/s - 100oC/s to obtain first intermediate hot rolled steel sheet and coiling thereafter. In one example, the first intermediate hot rolled steel sheet is kept at the first intermediate temperature in the range of 350-650oC for a time duration in the range of 10 – 20 sec. More particularly, the first intermediate hot rolled steel sheet is kept at the first intermediate temperature in the range of 350-650oC for a time duration of 15 sec. In an example, the holding time may vary on the thickness of the steel sheet, length of the run-out table and the speed of the run-out table, without any limitations.
[0043] At step (112), cooling the hot rolled coil to room temperature to obtain complex phase high strength hot rolled steel strip or sheet having thickness in the range of 2 mm to 6 mm. In an embodiment, the cooling is air cooling. The obtained complex phase high strength hot rolled steel sheet comprises a microstructure of microstructure of 5-20% ferrite, 5-20% martensite, and 60-90% bainite. The obtained complex phase high strength hot rolled steel sheet or strip exhibits ultimate tensile strength ranging from 980 - 1150 MPa, yield strength (YS) > 750 MPa, and minimum %elongation of 10%.
[0044] In this method, a rapid cooling is applied after hot rolling to prevent austenite grain growth. There is only one step of cooling, after which the holding temperature in below the bainitic transformation temperature. This results in maximum transformation of austenite into bainite phase. Since bainitic transformation is a slow process, this cannot transform the entire austenite into bainite. After coiling, as the temperature gradually drops below martensitic transformation temperature, some portion of the remaining austenite transforms into martensite. In this way, finally, the complex microstructure is developed.
[0045] Austenite pancaking is achieved by substantial deformation of austenite below Tnr temperature. To ensure required amount of deformation under Tnr, it is important to raise the Tnr by suitable alloying additions. Nb is considered the most common and effective alloying element for this purpose. Addition of Mn facilitates stabilization of austenite, particularly in a situation when C content is maintained at around 0.1±0.02% to ensure a low carbon equivalent (CE) that can be accommodated for the purpose of industrial processing / application.
[0046] Addition of Cr slows down the transformation, which facilitates the formation of hard phases like martensite and bainite more conveniently under industrial conditions, which otherwise would have required extremely high cooling rates. A higher austenitization temperature is required to ensure complete dissolution of micro-alloying elements, mainly Nb.
[0047] The hot deformation schedule should ensure a substantial amount of deformation (about 50%) under Tnr. Immediately after finish rolling, there should be a rapid drop of temperature, to restrict grain growth. This is first stage of cooling, during which some amount of ferrite forms in the microstructure, and the temperature drops just below the bainitic transformation temperature. Holding at this temperature for certain time duration is required for the completion of bainitic transformation. The time length of this intermediate holding determines the bainite fraction in the microstructure. After the intermediate holding, another rapid cooling treatment (second stage cooling) transforms the remaining austenite into martensite.
[0048] 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, single step cooling rate after finish rolling, intermediate holding temperature and time, and coiling temperature, all have significant effects on final mechanical properties.
[0049] Following portions of the present disclosure provides details about the proportion of each element in a composition of the complex phase high strength hot rolled steel and their role in enhancing properties.
[0050] Carbon (C) may be used in the range of about 0.0 to 0.16 wt.%: Amount of Carbon is so selected such that it is just sufficient for generating bainite and martensite on cooling.
[0051] Sulphur (S) may be used in the range of about < 0.006 wt.%, & Phosphorus (P) may be used in the range of about < 0.02 wt.%: Amounts of Sulphur and phosphorus are kept as low as possible. S should preferably be less than 50 ppm, while P should be less than 0.02%.
[0052] Manganese (Mn): 1.2 to 2.8 wt.%: Manganese promotes solid solution strengthening and stabilizes austenite. However, excessive amount of Mn is not recommended as it can deteriorate weldability of the steel.
[0053] Chromium (Cr) may be used in the range of about 0.1 to 1.0 wt.%. Chromium (Cr) addition can substantially increase the strength and hardenability of the steel and facilitate the formation of bainite and martensite.
[0054] Niobium (Nb) may be used below 0.08 wt.%. Niobium (Nb) content is used for (a) maintaining strength at room temperature, (b) raising Tnr temperature, etc. The addition of Nb should be optimum, as excessive addition will increase the strength and cost of the material.
[0055] Molybdenum (Mo) may be used less than 0.5%. Addition of Mo in optimum quantity can improve the hardenability of steel.
[0056] Titanium (Ti) may be used less than 0.05%. Addition of Ti helps prevent transverse cracking during continuous casting.
[0057] The chemical composition of the proposed alloys, and their carbon equivalent (CE) values are shown in Table 1.
Elements Steel 1 Steel 2
C % 0.12 0.136
Mn % 1.9 2.26
Si % 0.35 0.31
Al % 0.04 0.05
Nb % 0.045 0.047
Ti % 0.013 0.02
Cr % 0.61 0.56
Mo % 0.25 0.28
CE* 0.676 0.741

Table -1: Chemical composition, and carbon equivalent (CE) of Steel 1 & Steel 2
[0058] The carbon equivalent (Ceq) is expressed by formula CE = (C) + (Mn+Si)/6 + (Cu+Ni)/15 + (Cr+Mo+V+Nb)/5, wherein each symbol in brackets represents the content (mass%) of the corresponding element.
[0059] To investigate the properties of the steel, experiments were carried out for specific composition which is reported in Table 1. Tensile specimens were prepared according to ASTM E8 specification, applicable for specimens with thickness 12.5 mm or less, and gauge length 50 mm. SEM samples were prepared by following standard methods.
[0060] Different samples of the complex phase high strength hot rolled steel were prepared by utilizing the different compositions as mentioned in table 1. Other parameters of were kept constant (reheating – 1200 deg C, finish rolling – 870 deg C, cooling rate ~ 60 deg C / s, holding at 580 deg C, and coiling temperature – 580 deg C).
[0061] Standard metallography techniques were followed to prepare the samples for different mechanical properties measurements. The measured properties for the steel samples are depicted in Table 2.
YS (MPa) UTS (MPa) %El N YR (YS/UTS)
Steel 1 915 1062 11.2 0.11 0.86
Steel 2 810 1061 12.6 0.13 0.76

Table 2: Variation in mechanical properties with varying composition.
[0062] Figures 2a, 2b, 3a & 3b illustrate SEM images of the obtained complex phase high strength hot rolled steels having compositions (Steel 1 and Steel 2) reported in Table 1 and by varying the holding times at the first intermediate temperature followed during the method (100). From the figures it can be observed that the developed steels exhibit microstructure having bainite, ferrite, and martensite.
[0063] Figures 2a and 3a present the SEM micrographs of the Steels 1 and 2, while Figures 2b and 3b present high magnification pictures of Steels 1 and 2, clearly revealing the bainite phase, with small fractions of martensite and ferrite. The microstructures also reveal that Steel 2 has a lower volume of bainite but slightly higher volume of martensite, which accounts for the lower YS/UTS ratio and higher n-value.
[0064] The present invention provides the complex phase high strength hot rolled steel and the method (100) of manufacturing the complex phase high strength hot rolled steel having higher strength with bare minimum addition of alloying elements. The complex phase high strength hot rolled steel microstructure majorly comprises bainite, ferrite and martensite phases having tensile strength above 980 MPa with tensile elongation value over 10%. The complex phase 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.
[0065] The complex phase high strength hot rolled steel having high strength and superior tensile ductility at room temperature, is 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 new steel developed with leaner chemistry to achieve the required properties by exploiting the existing hot rolling facilities in the integrated steel plants to get desired microstructure.
[0066] The method (100) may employ additional processes such as pickling to remove scaling and to make both surfaces parallel to each other, without limiting the scope of the invention.
[0067] It should be understood that the experiments are carried out for particular compositions of the complex phase 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 complex phase high strength hot rolled steel strip, as well.
[0068] 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.
[0069] 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.
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 a complex phase high strength hot rolled steel sheet or strip having thickness in the range of 2 mm to 6 mm, the method (100) comprising:
casting molten steel having a composition expressed in weight %: Carbon (C): < 0.16%, Manganese (Mn): 1.2% - 2.8%, Chromium (Cr): 0.1-1.0%, Silicon (Si): 0.2%-0.8%, Sulphur (S): <0.006%, Phosphorus (P): < 0.02%, Niobium (Ni): 0.0 - 0.08%, Aluminium (Al): 0.02-0.06%, Titanium (Ti): <0.05%, Molybdenum (Mo): <0.5%, Nitrogen (N) < 80 ppm, and the remaining being substantially iron and incidental impurities, and unavoidable impurities to obtain a steel slab;
reheating the steel slab to a temperature in the range of 1150 – 1280oC;
roughing the steel slab in roughing mill with exit temperature in the range of 1010-1080oC;
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 820oC to 900oC, above Ar3 temperature of the steel;
cooling the hot rolled steel to a first intermediate temperature in the range of 350-650oC at a first intermediate cooling rate in the range of 20oC/s - 100oC/s to obtain first intermediate hot rolled steel sheet and coiling thereafter; and
cooling the hot rolled coil to room temperature to obtain complex phase high strength hot rolled steel sheet, wherein the complex phase high strength hot rolled steel sheet comprises a microstructure of 5-20% ferrite, 5-20% martensite and about 60-90% bainite, wherein the complex phase high strength hot rolled steel exhibits an ultimate tensile strength in the range of 980 - 1150 MPa.
2. The method (100) for manufacturing complex phase high strength hot rolled steel sheet or strip having thickness in the range of 2 mm to 6 mm as claimed in the claim 1, wherein the complex phase high strength hot rolled steel comprises the composition expressed in weight %: C – 0.12, Mn –1.9, Si –0.35, Al – 0.04, Nb – 0.045, Cr – 0.61, Ti – 0.013, Mo – 0.25 and the balance being Iron (Fe) and unavoidable impurities, wherein the carbon equivalent (Ceq) expressed by formula CE = (C) + (Mn+Si)/6 + (Cu+Ni)/15 + (Cr+Mo+V+Nb)/5 is 0.676, wherein each symbol in brackets represents the content (mass%) of the corresponding element.
3. The method (100) for manufacturing complex phase high strength hot rolled steel sheet or strip having thickness in the range of 2 mm to 6 mm as claimed in the claim 2, wherein the complex phase high strength hot rolled steel exhibits a yield strength (YS) of 915 MPa, ultimate tensile strength (UTS) of 1062 MPa, %Elongation – 11.2, strain hardening exponent (n) – 0.13, and yield ratio (YS/UTS) of 0.86.
4. The method (100) for manufacturing complex phase high strength hot rolled steel sheet or strip having thickness in the range of 2 mm to 6 mm as claimed in the claim 1, wherein the complex phase high strength hot rolled steel comprises the composition expressed in weight %: C – 0.136, Mn –2.26, Si –0.31, Al – 0.05, Nb – 0.047, Cr – 0.56, Ti – 0.02, Mo – 0.28 and the balance being Iron (Fe) and unavoidable impurities, wherein the carbon equivalent (Ceq) expressed by formula CE = (C) + (Mn+Si)/6 + (Cu+Ni)/15 + (Cr+Mo+V+Nb)/5 is 0.741, wherein each symbol in brackets represents the content (mass%) of the corresponding element.
5. The method (100) for manufacturing complex phase high strength hot rolled steel sheet or strip having thickness in the range of 2 mm to 6 mm as claimed in the claim 4, wherein the obtained complex phase high strength hot rolled steel exhibits a yield strength (YS) of 810 MPa, ultimate tensile strength (UTS) of 1061 MPa, %Elongation – 12.6, strain hardening exponent (n) – 0.13 and yield ratio (YS/UTS) of 0.76.
6. A complex phase high strength hot rolled steel comprising the following composition expressed in weight %:
Carbon (C): < 0.16%,
Manganese (Mn): 1.2% - 2.8%,
Chromium (Cr): 0.1-1.0%,
Silicon (Si): 0.2%-0.8%,
Sulphur (S): <0.006%,
Phosphorus (P): < 0.02%,
Niobium (Ni): 0.0 - 0.08%,
Aluminium (Al): 0.02-0.06%,
Titanium (Ti): <0.05%,
Molybdenum (Mo): <0.5%,
Nitrogen (N) < 80 ppm, and the remaining being substantially iron and incidental impurities, wherein the complex phase high strength hot rolled steel comprises a microstructure of 5-20% ferrite, 5-20% martensite, and 60-90% bainite.
7. The complex phase high strength hot rolled steel as claimed in the claim 1, wherein the complex phase high strength hot rolled steel exhibits ultimate tensile strength (UTS) in the range of 980-1150 MPa, yield strength (YS) > 750 MPa, and minimum %Elongation – 10.
8. A component produced from the complex phase high strength hot rolled steel as claimed in the claims 1 to 7, wherein the component is used in automobile applications.