FIELD OF INVENTION
[0001] The present invention relates to a high strength low alloy low cost hot rolled steel, and more particularly to the high strength low alloy low cost hot rolled steel having good ductility, and method of manufacturing the high strength low alloy low cost hot rolled steel strip, sheet or blank.

BACKGROUND
[0002] With ever increasing demand for sustainability, the steel industry has focused its research on development of steels having combination of properties. Therefore, addressing the present scenario has necessitated the development of a rolled steel, in hot rolled as well as cold rolled conditions, with high tensile strength coupled with excellent uniform elongation, total elongation, work hardening rate and toughness for automotive components such as lower suspensions, long members, cross members, and bumpers etc.
[0003] In this context, several works have been carried to meet the present and future requirement of steels for auto-sector and structural applications. For instance, to meet the combination of properties a new carbide steel or famously known as ‘nano-bainitic steel’ (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) has been developed successfully. Though the steel has been produced with highest strength ever achieved in any bulk material; production of the steel sheet takes about a week due to low temperature transformation mandatory to obtain the desired microstructure. This means it requires controlled low temperature cooling of the coil. Such a long duration of cooling time of the coil is not only not economically viable but also unsustainable for the commercial production. Another sustainability concern is related with the alloy composition where the amount of C in steel typically lies in the range of 0.8-1.0 wt. %. It is well known that high carbon decreases the weldability of the steel.
[0004] In another effort to meet the future demand of the automobile sector, US20140102600A1 attempted to obtain high strength and ductility combination. This work has successfully achieved minimum 1200 MPa tensile strength with 20% total elongation at laboratory and pilot scale. However, it has high C (>0.3 wt. %) and Silicon (>1.5 wt. %). High amount of C decreases the weldability and high Silicon causes surface scales due to formation of complex iron-silicon oxides during the process of hot rolled steel sheets. These problems are yet to be addressed.
[0005] To address the weldability issues in the above work, US10876184B2 attempted to limit the C to 0.3 wt.% and achieved above 1000 MPa with more than 15 % total elongation with high amount of uniform elongation. The properties are promising but the steel contains high amount of Cr and Mo which increases the cost of the steel. The decrease in weight reduction in using the steel may not compensate the high amount of alloying. Development of alloys with leaner composition is one of the aspects of ‘sustainable development’.
[0006] Work of the patent number KR20200075959A revealed development of High strength hot-rolled steel sheet having excellent workability. The steel contained microstructure with ferrite, bainite and retained austenite which provides a combination of ‘hole-expansion ratio’, elongation and strength. The strength was about 1200 MPa with minimum 18 % elongation besides hole expansion ratio of 29%. The properties are excellent, but the steel cannot be produced in the conventional hot strip mills due to presence of very high amount of Si ranging 2 to 3 wt.%. As mentioned above, Si in the steel causes complex scale problem and hence its usage should be limited to 0.5 to 1.0 wt.%.
[0007] In this regard, the recently developed bainitic grade steel of 1.5 GPa with ~ 30 % elongation can be produced in hot rolling mill setup that may not require controlled cooling of the coil (Debasis Poddar, Chiradeep Ghosh, Basudev Bhattacharya, and Vivek Kumar Singh. Materials Science and Engineering: A 762 (2019): 138079.). For this case, mill feasible cooling system (through ROT followed by coiling) will provide enough slow cooling (it may take ~32 hrs.) to serve the technical purpose to develop the required microstructure. This steel contains considerable amount of Si, Cr, Ni. Co like alloying elements and can be categorized as special steels for special applications considering its outstanding mechanical properties [Materials Science and Engineering: A 762 (2019): 138079]. For the general-purpose application, development of a low cost bainitic steel in the range of ~ 800 MPa with ~ 20 % elongation could be the high market demand. This steel will be commercially more sustainable if it can be produced in the existing rolling mill facility with normal cooling arrangements.
[0008] Thus, the prior art lacks the development of a steel with combination properties for sustainability which can deliver ~ 800 MPa UTS and at least 15% elongation without the addition of costly alloying addition like Cr, Ni and Mo.
[0009] In this context, new steel developed with leaner chemistry by excluding or reducing Ni, Cr, Mo, Co like expensive alloying elements to achieve the required properties by exploiting the existing hot rolling facilities in the integrated steel plants to get desired microstructure.
[0010] 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
[0011] It is an object of the invention to solve the problems of the prior art and to provide a high strength low alloyed low cost hot rolled steel that can be made into steel strips, sheets, and blanks, having tensile strength in the range of 700 – 1100 MPa which also includes a modified chemical composition excluding/reducing Ni, Cr and Mo like costly alloying elements.
[0012] Another objective of the present invention is to develop a method of manufacturing the high strength low alloy low-cost hot rolled steel strip, sheet or blank having the required elongation.
[0013] 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.
[0014] It is yet another objective of the present invention, to provide a high strength low alloy low cost hot rolled steel strip, sheet or blank, having the following composition in weight%: Carbon (C): 0.25% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Silicon (Si): 0.3% - 0.7%, Aluminium (Al): 0.3% - 0.5%, Sulphur (S): 0.0% to 0.04%, Phosphorus (P): 0.0% to 0.04%, Nitrogen (N): 0.0% to 0.1%, Chromium (Cr): 0.0% - 0.25%, , Copper (Cu): 0.0% - 0.25%, Nickel (Ni): 0.0% - 0.7%, and the balance being Iron (Fe) and unavoidable impurities.
SUMMARY OF INVENTION
[0015] This summary is provided to introduce concepts related to a high strength low alloy low cost hot rolled steel and a method of manufacturing the high strength low alloy low cost hot rolled steel. 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.
[0016] In one aspect of the present invention, a high strength low alloy low cost hot rolled steel having good ductility is provided. The steel comprising the following composition expressed in weight %: Carbon (C): 0.25% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Silicon (Si): 0.3% - 0.7%, Aluminium (Al): 0.3% - 0.5%, Sulphur (S): 0.0% to 0.04%, Phosphorus (P): 0.0% to 0.04%, Nitrogen (N): 0.0% to 0.1%, Chromium (Cr): 0.0% - 0.25%, Copper (Cu): 0.0% - 0.25%, Nickel (Ni): 0.0% - 0.7%, and the balance being Iron (Fe) and unavoidable impurities. The high strength low alloy low cost hot rolled steel comprises a structure including a bainitic-ferrite phase, and martensite phase. The high strength low alloy low cost hot rolled steel exhibits ultimate tensile strength ranging from about 700 MPa to 1100 MPa, and elongation ranging from 18 - 25%.
[0017] In an embodiment, the microstructure of the steel is represented by the bainitic ferrite ~ 85%, and martensite ~ 10%, 4% max retained austenite and 1% max Fe3C.
[0018] In an embodiment, the high strength low alloy low cost hot rolled steel exhibits yield strength ranging from 500 MPa to 907 MPa.
[0019] In an embodiment, a method for manufacturing the high strength low alloy low cost hot rolled steel sheet or strip is provided. The method comprising casting steel having a composition expressed in weight %: Carbon (C): 0.25% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Silicon (Si): 0.3% - 0.7%, Aluminium (Al): 0.3% - 0.5%, Sulphur (S): 0.0% to 0.04%, Phosphorus (P): 0.0% to 0.04%, Nitrogen (N): 0.0% to 0.1%, Chromium (Cr): 0.0% - 0.25%, Copper (Cu): 0.0% - 0.25%, Nickel (Ni): 0.0% - 0.7%, and the balance being Iron (Fe) and unavoidable impurities. The method also comprises reheating the steel casting inside a furnace kept at a temperature about 1250 °C and soaking the steel casting at the 1250 °C temperature for a time duration of about three hours. The method further comprises deforming the steel casting in a hot forging process to obtain a steel slab. The method comprises cooling the steel slab obtained in the hot forging process to ambient temperature. The method also comprises reheating the steel slab to a temperature about 1200°C and annealing the steel slab for a time duration of about 45 minutes in argon gas atmosphere. The method further comprises subjecting the steel slab to a hot rolling process such that finish rolling is done at a temperature (TFRT) to obtain a steel strip. TFRT varies in the range 950oC to 1000oC. The method comprises quenching the steel strip obtained during the hot rolling process to a temperature about 390oC in a bath and soaking the steel strip in the bath at the 390oC temperature for a time duration of 17 hours. The method also comprises cooling the steel strip to room temperature to obtain the steel strip comprising a structure including a bainitic-ferrite phase, and martensite phase.
[0020] In an embodiment, the high strength low alloy low cost hot rolled steel strip exhibits ultimate tensile strength ranging from about 700 MPa to 1100 MPa, elongation ranging from 18 - 25% and yield strength ranging from 500 MPa to 907 MPa.
[0021] In an embodiment, the microstructure of the steel strip is microstructure represented by, the bainitic ferrite ~ 85%, and martensite ~ 10%, 4% max retained austenite and 1% max Fe3C.
[0022] 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.
[0023] In an embodiment, the quenching is an isothermal quenching process, and the bath is a salt bath. In an embodiment, cooling is normal air cooling.
[0024] In an embodiment, thickness of the steel sheet after the second hot working process is about 5 mm.
[0025] In an embodiment, the steel strip is grinded to remove scaling on a surface of the steel strip.
[0026] In an embodiment, the salt bath is an equivalent mixture of sodium nitrite (NaNO2) and sodium nitrate (NaNO3). In an embodiment, the furnace is an induction air melting furnace.
[0027] In an embodiment, any retained austenite formed during quenching and soaking process is converted to martensite as the steel strip is cooled to room temperature to obtain the steel strip comprising a structure including a bainitic-ferrite phase, and martensite phase.
[0028] In an embodiment, the high strength low alloyed low cost hot rolled steel comprising the following composition expressed in weight %: Carbon (C): 0.27%, Manganese (Mn): 1.32%, Silicon (Si): 0.59%, Aluminium (Al): 0.38, Sulphur (S): 0.017%, Phosphorus (P): 0.028, Nitrogen (N): 0.07%, Chromium (Cr): 0.20%, Copper (Cu): 0.22%, Nickel (Ni): 0.52%, and the balance being Iron (Fe) and unavoidable impurities exhibits ultimate tensile strength ranging from about 1081 MPa, elongation ranging from 18% and yield strength ranging from 907 MPa.
[0029] In an embodiment, the high strength low alloyed low cost hot rolled steel comprising the following composition expressed in weight %: Carbon (C): 0.26%, Manganese (Mn): 1.33%, Silicon (Si): 0.65%, Aluminium (Al): 0.29, Sulphur (S): 0.037%, Phosphorus (P): 0.032, Nitrogen (N): 0.06%, Copper (Cu): 0.22%, Nickel (Ni): 0.54%, and the balance being Iron (Fe) and unavoidable impurities exhibits ultimate tensile strength ranging from about 785 MPa, elongation ranging from 19% and yield strength ranging from 578 MPa.
[0030] In an embodiment, the high strength low alloyed low cost hot rolled steel comprising the following composition expressed in weight %: Carbon (C): 0.27%, Manganese (Mn): 1.13%, Silicon (Si): 0.49%, Aluminium (Al): 0.31, Sulphur (S): 0.018%, Phosphorus (P): 0.030, Nitrogen (N): 0.065%, Copper (Cu): 0.20%, and the balance being Iron (Fe) and unavoidable impurities exhibits ultimate tensile strength ranging from about 733 MPa, elongation ranging from 23% and yield strength ranging from 525 MPa.
[0031] In an embodiment, the high strength low alloyed low cost hot rolled steel comprising the following composition expressed in weight %: Carbon (C): 0.28%, Manganese (Mn): 1.29%, Silicon (Si): 0.58%, Aluminium (Al): 0.33, Sulphur (S): 0.030%, Phosphorus (P): 0.029, Nitrogen (N): 0.069%, and the balance being Iron (Fe) and unavoidable impurities exhibits ultimate tensile strength ranging from about 828 MPa, elongation ranging from 18% and yield strength ranging from 616 MPa.
[0032] In an embodiment, a component produced from the high strength low alloy low cost hot rolled steel is provided. The component is used in structural as well as automotive applications.
[0033] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Figure 1 illustrates a flowchart of a method of manufacturing a high strength low alloy low cost hot rolled steel, according to an embodiment of the present invention;
[0035] Figure 2 illustrates a graphical representation of thermo-mechanical processing routes of the method of manufacturing the high strength low alloyed steel strip, according to an embodiment of the present invention;
[0036] Figure 3a illustrates a graphical representation of stress versus elongation, obtained during tensile test of the high strength low alloy low cost hot rolled steel having composition of sample 1(BCrNiCu);
[0037] Figure 3b illustrates a graphical representation of stress versus elongation, obtained during tensile test of the high strength low alloy low cost hot rolled steel having composition of sample 2 (BNiCu);
[0038] Figure 3c illustrates a graphical representation of stress versus elongation, obtained during tensile test of the high strength low alloy low cost hot rolled steel having composition of sample 3 (BCu);
[0039] Figure 3d illustrates a graphical representation of stress versus elongation, obtained during tensile test of the high strength low alloy low cost hot rolled steel having composition of sample 4 (B);
[0040] Figure 4a illustrates a graphical representation of results of X-ray Diffraction analysis carried out on the high strength low alloy low cost hot rolled steel having composition of sample 1(BCrNiCu);
[0041] Figure 4b illustrates a graphical representation of results of X-ray Diffraction analysis carried out on the high strength low alloy low cost hot rolled steel having composition of sample 2 (BNiCu);
[0042] Figure 4c illustrates a graphical representation of results of X-ray Diffraction analysis carried out on the high strength low alloy low cost hot rolled steel having composition of sample 3 (BCu);
[0043] Figure 4d illustrates a graphical representation of results of X-ray Diffraction analysis carried out on the high strength low alloy low cost hot rolled steel having composition of sample 4 (B);
[0044] Figure 5a illustrates microstructure of high strength low alloy low cost hot rolled steel having composition of sample 1(BCrNiCu);
[0045] Figure 5b illustrates microstructure of high strength low alloy low cost hot rolled steel having composition sample 2 (BNiCu);
[0046] Figure 5c illustrates microstructure of high strength low alloy low cost hot rolled steel having composition sample 3 (BCu);
[0047] Figure 5d illustrates microstructure of high strength low alloy low cost hot rolled steel having composition sample 4 (B); and
[0048] Figures 6a, 6b, and 6c illustrate TEM microstructure images of high strength low alloy low cost hot rolled steel having composition of sample 3 (BCu).
[0049] 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
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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.
[0055] The present disclosure provides a high strength low alloy low cost hot rolled steel having good ductility comprising the following composition expressed in weight %: Carbon (C): 0.25% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Silicon (Si): 0.3% - 0.7%, Aluminium (Al): 0.3% - 0.5%, Sulphur (S): 0.0% to 0.04%, Phosphorus (P): 0.0% to 0.04%, Nitrogen (N): 0.0% to 0.1%, Chromium (Cr): 0.0% - 0.25%, Copper (Cu): 0.0% - 0.25%, Nickel (Ni): 0.0% - 0.7%, and the balance being Iron (Fe) and unavoidable impurities. The high strength low alloy low cost hot rolled steel comprises a structure including a bainitic-ferrite phase, and martensite phase. In an embodiment, the high strength low alloy low cost hot rolled steel comprises a microstructure represented by, the bainitic ferrite ~ 85%, and martensite ~ 10%, 4% max retained austenite and 1% max Fe3C.
[0056] The high strength low alloy low cost hot rolled steel exhibits ultimate tensile strength ranging from about 700 MPa to 1100 MPa, and elongation ranging from 18 - 25%. The high strength low alloy low cost hot rolled steel exhibits yield strength ranging from 500 MPa to 907 MPa. In an embodiment, the high strength low alloy low cost hot rolled steel exhibits ultimate tensile strength ranging from about 700 MPa to 800 MPa that demonstrates a reasonable ductility in combination with high strength.
[0057] Referring to Figures 1 and 2, an exemplary thermo-mechanical method (100) of manufacturing the high strength low alloy low cost 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 high strength low alloy low cost hot rolled steel strip, sheet or blank begins at step (102). At step (102), molten steel having composition expressed in weight %: Carbon (C): 0.25% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Silicon (Si): 0.3% - 0.7%, Aluminium (Al): 0.3% - 0.5%, Sulphur (S): 0.0% to 0.04%, Phosphorus (P): 0.0% to 0.04%, Nitrogen (N): 0.0% to 0.1%, Chromium (Cr): 0.0% - 0.25%, Copper (Cu): 0.0% - 0.25%, Nickel (Ni): 0.0% - 0.7%, and the balance being Iron (Fe) and unavoidable impurities is cast in a casting apparatus (to obtain cast ingots).
[0058] At step (104), the steel casting (cast ingots) is reheated inside a furnace kept at a temperature of about 1250°C and thereafter the heated steel casting is soaked at the temperature 1250°C for a time duration of about three hours. In another example, the steel cast may be reheated to a temperature above 1100°C and may be soak at temperature more than 1100°C for a time duration of more than 2 hours, without limiting the scope of the invention. In one example, the furnace is an induction air melting furnace.
[0059] The steel slab is soaked at temperature about 1250°C. Steel is held at this temperature for sufficient time for the formation of homogenous structure and composition throughout its mass. The soaking time depends on the thickness of the work piece and the steel composition. Higher temperatures and longer soaking times are required for larger cross sections.
[0060] At step (106), the heated and soaked steel obtained in the step (104) is deformed by subjecting the steel to a first working process to obtain a steel slab. In an embodiment, the first working process is a hot forging process. Forging process is a mechanical process in which the structure may be deformed by applying localized compressive stresses. In an example, the localized compressive stresses may be induced using a motor driven 0.5 ton forging hammer. Blows are delivered by the hammer on to the steel in order to induce localized compressive stresses, which may result in internal grain deformation, thus enhancing strength and stiffness of the steel. At step (108), the steel slab obtained in the step (106) is cooled to ambient temperature. In an embodiment, cooling of the steel is performed using normal air cooling.
[0061] At step (110), the method (100) comprises re-heating the steel slab to a temperature greater than 1100ºC and annealing the steel for a time duration greater than 30 minutes in an inert gas atmosphere. In an embodiment, the temperature may be around 1200 ºC and the time may be about 45 minutes in argon gas atmosphere.
[0062] At step (112), the re-heated and annealed steel slab obtained in step (110) 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 steel strip. TFRT varies in the range 950oC to 1000oC (also shown in Figure 2). 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 about 5 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.
[0063] At step (114), the hot rolled steel strip obtained in step (112) is quenched to a predetermined temperature of about 390°C in a bath, followed by soaking the steel strip in the bath at the 390°C temperature for a time duration of 17 hours. In an embodiment, the quenching is an isothermal quenching process, and the bath is a salt bath. In an embodiment, the salt bath is an equivalent mixture of sodium nitrite (NaNO2) and sodium nitrate (NaNO3).
[0064] At step (116), the quenched and soaked steel is cooled to room temperature to obtain the high strength low alloy low-cost hot rolled steel strip. The high strength low alloy low-cost hot rolled steel obtained has the bainitic-ferrite as the predominant phase, and martensite as secondary phase. Some amount of unavoidable 4% max retained austenite and 1% max Fe3C is also present in the steel. In an embodiment, the steel strip may be grinded after quenching process, to remove all scaling and to make both surfaces parallel to each other.
[0065] The obtained high strength low alloy low cost hot rolled steel strip exhibits ultimate tensile strength ranging from about 700 MPa to 1100 MPa, elongation ranging from 18 - 25% and yield strength ranging from 500 MPa to 907 MPa.
[0066] It may be noted that the transfer from annealing furnace to rolling mill took about 3 seconds, the five reduction passes took about 24 seconds and transfer from rolling mill to salt bath took less than 3 seconds.
[0067] In an embodiment, the method (100) of the present disclosure includes melting, casting, heat treatment, thermomechanical hot-rolled routes, which are simple. For the current work, impressive result was achieved by employing induction air melting furnace. Hence, this a cost-effective invention in respect to the most of similar inventions. No special casting routes were used. Further, in conventional techniques, cast ingots were homogenized for 2 days at 1200 °C to 1300 °C. Whereas, in the method of the present disclosure, the steel is homogenized for 3 hours, thus conserving energy. The thermomechanical / hot-rolling process is quite simple and can be straight way start from homogenization temperature and is an industry feasible production process. It includes rough passing (950 – 1000 °C) in the hot-rolling mill before quenching to 390 °C. Quenching at 390 °C is a low temperature, that does not require huge energy consumption and this temperature can be readily maintained by deliberate use of waste heat. Therefore, the method (100) of the present disclosure aids in reducing energy consumption and thus a cost-effective high strength steel manufacturing process.
[0068] In an embodiment, the method (100) employs normal air cooling and eliminates use of vacuum melting furnace, unlike conventional methods. This aids in reducing cost of the steel manufacturing process.
[0069] Following portions of the present disclosure provides details about the proportion of each element in a composition of the high strength low alloy low cost hot rolled steel sheet and their role in enhancing properties.
[0070] Carbon (C) may be used in the range of 0.25 to 0.35 wt.%. Carbon is an austenite stabilizer and provides a single phase of bainite in between Bs and Bf temperature. Excessive carbon can promote carbide precipitates in an inner portion of the bainite texture and can vary the precipitation formation as the cooling rate varies, which may affect the constant strength over a wide range of cooling rate. Carbon (C) below the above range, it may decrease the solute solution strengthening of bainitic-ferrite.
[0071] Silicon (Si) may be used in the range of about 0.3 to 0.7 wt.%. Silicon (Si) suppresses the formation of carbide that need minimum 1.5% Si, which leads to carbide free bainitic matrix to improve ductility and impact toughness.
[0072] Manganese (Mn) may be used in the range of about 1.2 to 1.4 wt.%. Lower content of manganese (Mn) may retain the toughness and lower the possibility of carbide formation aiming to produce carbide free bainitic matrix to improve the ductility. However, the hardenability may decrease as a result of reducing manganese. Manganese content limit may be considered low or high as per extent of hardenability required with carbide free bainitic matrix.
[0073] Chromium (Cr) may be used in the range of about 0.0 to 0.25 wt.%. Chromium (Cr) addition can substantially increase the strength and hardenability of the steel. It can vary beyond above range for customized strength requirement.
[0074] Nickel (Ni) may be used below 0.7 wt.%. Nickle (Ni) content limited to maximum to increase the residual austenite carbon without sacrificing the hardenability. It increases the strength and toughness.
[0075] Aluminium (Al) may be used in the range of about 0.3 to 0.5 wt.%. Aluminium (Al) of this proportion improves strength and ductility. It can also be added more or less as a solid solution strengthener.
[0076] Copper (Cu) may be used below 0.25 wt.%. Copper (Cu) of this proportion increase the solid solution strengthening and aiming to boost up the toughness. This can be added more to increase the strength and toughness.
[0077] In order to determine the chemical composition of the steel with which it is possible to obtain the desired results, some trials were made with samples S1, S2, S3 and S4 having compositions reported in Table 1, in % by weight:
Sample Type C Mn Si Al Cr Ni Cu
S
P N
S1 BCrNiCu
0.27 1.32 0.59 0.38 0.20 0.52 0.22 0.017
0.028
0.07
S2 BNiCu
0.26 1.33 0.65 0.29 - 0.54 0.22 0.037
0.032
0.06
S3 BCu 0.27 1.13 0.49 0.31 - - 0.20
0.018
0.030
0.065
S4 B 0.28 1.29 0.58 0.33 - - -
0.030 0.029 0.069
Table: 1
Sample ID Key alloying elements
BCrNiCu Cr, Ni, Cu added Bainite-martensite
BNiCu Ni, Cu added Bainite-martensite
BCu Cu added Bainite-martensite
B Bainite-martensite without adding Cr, Ni, Cu
Table: 2
[0078] The chemical compositions were chosen in order to obtain a structure having bainitic-ferrite phase and martensite phase with minimum amount of retained austenite, and to obtain the high strength low alloy low cost hot rolled steel having good ductility.
[0079] The steels having the above chemical compositions were produced using the method (100). The obtained results: yield strength YS, ultimate tensile strength UTS, and total elongation are reported in table 3:
Sample Type YS (MPa) UTS (MPa) Elongation %
S1 BCrNiCu 907 1081 18
S2 BNiCu 578 785 19
S3 BCu 525 733 23
S4 B 616 828 18

Table: 3
[0080] To investigate the properties of the steels, experiments were carried out for specific compositions which are reported in Table 1 of the high strength low alloy low cost hot rolled steel formed by using the method (100) of the present disclosure. The standard tensile samples (sub-size of gauge length 25 mm following E8/E8M-09, ASTM) were prepared keeping the tensile axis parallel to rolling direction. XRD samples were prepared by following standard methods. SEM and TEM samples were prepared by following standard methods.
[0081] Figures 3a to 3d illustrate the tensile results of the high strength low alloy low cost hot rolled steel having compositions reported in Table 1. Figures 4a to 4d illustrate X-ray Diffraction analysis of high strength low alloy low cost hot rolled steel having compositions reported in Table 1. From the figures it can be observed that the developed steels exhibit bainitic microstructure having bainitic-ferrite and martensite (instead of retained austenite). The mechanical properties are very remarkable considering the leaner chemistry of the current steels; ranging from 700 – 1100 MPa UTS, accompanied with maximum 18 – 23 % of elongation. However, the XRD results of steels (illustrated in figure 4a to 4d) demonstrate retained austenite in the range of 0 – 3% (vol.) max. For the current work, overall chemistry of the steels is very slender. Thus, the Ms (martensite starts temperature) temperature of Retained Austenite (RA) lies in between holding (390 °C) to room temperature. It is expected that majority of the retained austenite in the bainite microstructure have transformed into martensite while cooling from the holding temperature (at 390 °C) as soon as it reached to the Ms temperature and final microstructure became martensite containing bainite. The major strength of the steels is coming from the phase mixture of bainitic-ferrite and martensite.
[0082] Figures 5a to 5d illustrate the SEM microstructure of the high strength low alloy low cost hot rolled steels having compositions reported in Table 1. The SEM microstructures illustrate that steels exhibit very fine-thin martensite distributed homogeneously in the bainite matrix and subdivide the grains i.e., refinement of the individual grains. It is expected that the obtained ductility is due to the refinements of microstructure and thin martensite contributes to the strengthening of the steel. No such noticeable presence of blocky martensite observed, and finer martensite may not have great role in impairing the ductility of the steels, rather it may contribute to the strengthen the steels. Figures 6a to 6c illustrate TEM microstructure images of the obtained high strength low alloy low cost hot rolled steel having composition expressed in weight %: Carbon (C): 0.27%, Manganese (Mn): 1.13%, Silicon (Si): 0.49%, Aluminium (Al): 0.31, Sulphur (S): 0.018%, Phosphorus (P): 0.030, Nitrogen (N): 0.065%, Copper (Cu): 0.20%, and the balance being Iron (Fe) and unavoidable impurities. The microstructure comprises bainitic-ferrite, RA transformed martensite and corresponding diffraction pattern. It can be observed that the steel having the composition of sample 3 (BCu) exhibits ultimate tensile strength ranging from about 733 MPa, elongation ranging from 23% and yield strength ranging from 525 MPa.
[0083] The present invention provides high strength low alloy low cost hot rolled steel and the method (100) of manufacturing the high strength low alloy low cost hot rolled steel. The high strength low alloy low cost 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. Further, the steel provides higher vehicle occupant safety and higher fuel and energy savings.
[0084] It should be understood that the experiments are carried out for particular compositions of the high strength low alloy low cost hot rolled steel strip reported in Table 1 and the results brought out are reported in Table 3. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the high strength low alloy low cost rolled steel strip, as well.
[0085] 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.
[0086] 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.
[0087] 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:1. A high strength low alloy low cost hot rolled steel having good ductility comprising the following composition expressed in weight %:
Carbon (C): 0.25% - 0.35%,
Manganese (Mn): 1.2% - 1.4%,
Silicon (Si): 0.3% - 0.7%,
Aluminium (Al): 0.3% - 0.5%,
Sulphur (S): 0.0% to 0.04%,
Phosphorus (P): 0.0% to 0.04%,
Nitrogen (N): 0.0% to 0.1%,
Chromium (Cr): 0.0% - 0.25%,
Copper (Cu): 0.0% - 0.25%,
Nickel (Ni): 0.0% - 0.7%, and the balance being Iron (Fe) and unavoidable impurities, wherein the high strength low alloy low cost hot rolled steel comprises a structure including a bainitic-ferrite phase, and martensite phase, wherein the high strength low alloy low cost hot rolled steel exhibits ultimate tensile strength ranging from about 700 MPa to 1100 MPa, and elongation ranging from 18 - 25%.
2. The high strength low alloy low cost hot rolled steel as claimed in claim 1, wherein the microstructure of the steel is represented by the bainitic-ferrite ~ 85%, and martensite ~ 10%, 4% max retained austenite and 1% max Fe3C.
3. The high strength low alloy low cost hot rolled steel as claimed in claim 1, wherein the high strength low alloy low cost hot rolled steel exhibits yield strength ranging from 500 MPa to 907 MPa.
4. A method (100) for manufacturing the high strength low alloy low cost hot rolled steel sheet or strip as claimed in the claim 1, the method (100) comprising:
casting steel having a composition expressed in weight %: Carbon (C): 0.25% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Silicon (Si): 0.3% - 0.7%, Aluminium (Al): 0.3% - 0.5%, Sulphur (S): 0.0% to 0.04%, Phosphorus (P): 0.0% to 0.04%, Nitrogen (N): 0.0% to 0.1%, Chromium (Cr): 0.0% - 0.25%, Copper (Cu): 0.0% - 0.25%, Nickel (Ni): 0.0% - 0.7%, and the balance being Iron (Fe) and unavoidable impurities;
reheating the steel casting inside a furnace kept at a temperature about 1250 °C and soaking the steel casting at the temperature 1250 °C for a time duration of about three hours;
deforming the heated and casted steel in a hot forging process to obtain a steel slab;
cooling the steel slab obtained in the hot forging process to ambient temperature;
reheating the steel slab to a temperature about 1200°C and annealing the steel slab for a time duration of about 45 minutes in argon gas atmosphere;
subjecting the steel slab to a hot rolling process such that finish rolling is done at a temperature (TFRT) to obtain a steel strip, wherein TFRT varies in the range 950oC to 1000oC;
quenching the steel strip obtained during the hot rolling process to a temperature about 390oC in a bath, and soaking the steel strip in the bath at the 390oC temperature for a time duration of 17 hours;
cooling the steel strip to room temperature to obtain the steel strip comprising a structure including a bainitic-ferrite phase, and martensite phase.
5. The method (100) of producing the high strength low alloy low cost hot rolled steel sheet or strip as claimed in claim 4, wherein the high strength low alloy low-cost steel strip exhibits ultimate tensile strength ranging from about 700 MPa to 1100 MPa, elongation ranging from 18 - 25% and yield strength ranging from 500 MPa to 907 MPa.
6. The method (100) of producing the high strength low alloy low cost hot rolled steel sheet or strip as claimed in claim 4, wherein the microstructure of the steel strip is microstructure represented by, the bainitic ferrite ~ 85%, and martensite ~ 10%, 4% max retained austenite and max 1% Fe3C.
7. The method (100) of producing the high strength low alloy low cost hot rolled steel sheet or strip as claimed in claim 4, wherein 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.
8. The method (100) of producing the high strength low alloy low cost hot rolled steel sheet or strip as claimed in claim 4, wherein the quenching is an isothermal quenching process, and the bath is a salt bath.
9. The method (100) of producing the high strength low alloy low cost hot rolled steel sheet or strip as claimed in claim 4, wherein cooling is normal air cooling.
10. The method (100) of producing the high strength low alloy low cost hot rolled steel sheet or strip as claimed in claim 4, wherein thickness of the steel sheet after the second hot working process is about 5 mm.
11. The method (100) of producing the high strength low alloy low cost hot rolled steel sheet or strip as claimed in claim 4, wherein the steel strip is grinded to remove scaling on a surface of the steel strip.
12. The method (100) of producing the high strength low alloy low cost hot rolled steel as claimed in claim 4, wherein the salt bath is an equivalent mixture of sodium nitrite (NaNO2) and sodium nitrate (NaNO3).
13. The method (100) of producing the high strength low alloy low cost hot rolled steel as claimed in claim 4, wherein the furnace is an induction air melting furnace.
14. The method (100) of producing the high strength low alloy low cost hot rolled steel as claimed in claim 4, wherein any retained austenite formed during quenching and soaking process is converted to martensite as the steel strip is cooled to room temperature to obtain the steel strip comprising a structure including a bainitic-ferrite phase, and martensite phase.
15. The high strength low alloyed low cost hot rolled steel as claimed in the claims 1 and 4, wherein the steel comprising the following composition expressed in weight %: Carbon (C): 0.27%, Manganese (Mn): 1.32%, Silicon (Si): 0.59%, Aluminium (Al): 0.38, Sulphur (S): 0.017%, Phosphorus (P): 0.028, Nitrogen (N): 0.07%, Chromium (Cr): 0.20%, Copper (Cu): 0.22%, Nickel (Ni): 0.52%, and the balance being Iron (Fe) and unavoidable impurities exhibits ultimate tensile strength ranging from about 1081 MPa, elongation ranging from 18% and yield strength ranging from 907 MPa.
16. The high strength low alloyed low cost hot rolled steel as claimed in the claims 1 and 4, wherein the steel comprising the following composition expressed in weight %: Carbon (C): 0.26%, Manganese (Mn): 1.33%, Silicon (Si): 0.65%, Aluminium (Al): 0.29, Sulphur (S): 0.037%, Phosphorus (P): 0.032, Nitrogen (N): 0.06%, Copper (Cu): 0.22%, Nickel (Ni): 0.54%, and the balance being Iron (Fe) and unavoidable impurities exhibits ultimate tensile strength ranging from about 785 MPa, elongation ranging from 19% and yield strength ranging from 578 MPa.
17. The high strength low alloyed low cost hot rolled steel as claimed in the claims 1 and 4, wherein the steel comprising the following composition expressed in weight %: Carbon (C): 0.27%, Manganese (Mn): 1.13%, Silicon (Si): 0.49%, Aluminium (Al): 0.31, Sulphur (S): 0.018%, Phosphorus (P): 0.030, Nitrogen (N): 0.065%, Copper (Cu): 0.20%, and the balance being Iron (Fe) and unavoidable impurities exhibits ultimate tensile strength ranging from about 733 MPa, elongation ranging from 23% and yield strength ranging from 525 MPa.
18. The high strength low alloyed low cost hot rolled steel as claimed in the claims 1 and 4, wherein the steel comprising the following composition expressed in weight %: Carbon (C): 0.28%, Manganese (Mn): 1.29%, Silicon (Si): 0.58%, Aluminium (Al): 0.33, Sulphur (S): 0.030%, Phosphorus (P): 0.029, Nitrogen (N): 0.069%, and the balance being Iron (Fe) and unavoidable impurities exhibits ultimate tensile strength ranging from about 828 MPa, elongation ranging from 18% and yield strength ranging from 616 MPa.
19. A component produced from the high strength low alloyed low cost hot rolled steel as claimed in the claims 1 to 18, wherein the component is used in structural as well as automotive applications.