Claims:WE CLAIM:
1. A high strength low alloy cold rolled steel strip comprising the following composition expressed in weight %:
Carbon (C): 0.3% - 0.35%,
Manganese (Mn): 1.2% - 1.4%,
Sulphur (S): = 0.02%,
Phosphorus (P): = 0.02%,
Nitrogen (N): = 0.01%,
Silicon (Si): 0.5% - 1.5%,
Aluminium (Al): 0.0% - 0.5%,
Chromium (Cr): = 0.2%,
Molybdenum (Mo): = 0.17%,
Copper (Cu): = 0.2%,
Nickel (Ni): = 0.5%, and the balance being Iron (Fe) and unavoidable impurities, wherein the steel strip comprises a structure including a bainitic-ferrite phase, martensite phase and a small fraction of retained austenite.

2. The high strength low alloy cold rolled steel strip as claimed in claim 1, wherein the high strength low alloy cold rolled steel strip exhibits tensile strength ranging from about 1200 MPa to 2200 MPa.

3. The high strength low alloy cold rolled steel strip as claimed in claim 1, wherein the high strength low alloy cold rolled steel strip exhibits a ductility ranging from 6 - 13%.

4. The high strength low alloy cold rolled steel strip as claimed in claim 3, wherein the high strength low alloy cold rolled steel strip exhibits a ductility of 12.5 %.

5. The high strength low alloy cold rolled steel strip as claimed in claim 1, wherein the microstructure of the steel strip is microstructure represented by, in area%, the bainitic ferrite ~ 80%, martensite ~15 % and retained austenite = 5%.
6. The high strength low alloy cold rolled steel strip as claimed in claim 1, wherein the high strength low alloy cold rolled steel strip exhibits yield stress varying in between 1100 - 2200 MPa.

7. A method (100) of producing a high strength low alloy cold rolled steel strip having a composition of Carbon (C) 0.3% - 0.35%, Manganese (Mn) 1.2% - 1.4%, Sulphur (S) = 0.02%, Phosphorus (P) = 0.02%, Nitrogen (N) = 0.01%, Silicon (Si) 0.5% - 1.5%, Aluminium (Al) 0.0% - 0.5%, Chromium (Cr) = 0.2%, Molybdenum (Mo) = 0.17%, Copper (Cu) = 0.2%, Nickel (Ni) = 0.5% and the balance being Iron (Fe) and unavoidable impurities, the method (100) comprising:
producing a molten steel having the said composition;
casting the molten steel in a casting apparatus;
heating the steel casting to a first predetermined temperature and soaking the steel casting at the first predetermined temperature for a first predetermined time;
deforming the steel casting in a first hot working process to obtain a steel slab;
cooling the steel slab obtained in the first hot working process to ambient temperature;
re-heating the steel slab to a second predetermined temperature and annealing the steel slab for a second predetermined time;
subjecting the steel slab to a second hot working process, at a third predetermined temperature to form a steel strip;
quenching the steel strip obtained during the second hot working process to a fourth predetermined temperature in a bath, and soaking in the steel strip in the bath at the fourth predetermined temperature for a third predetermined time;
cooling the steel strip to room temperature;
cold rolling the steel strip for further reduction in thickness; and
tempering the steel strip processed in the cold rolling process at a fifth predetermined temperature for a fourth predetermined time to obtain the steel strip comprising a structure including a bainitic-ferrite phase, martensite phase and small fraction of retained austenite phase.
8. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the high strength low alloy cold rolled steel strip exhibits tensile strength ranging from about 1200 MPa to 2200 MPa.

9. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the high strength low alloy cold rolled steel strip exhibits a ductility ranging from 6 – 13%.

10. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the microstructure of the steel strip is microstructure represented by, in area%, the bainitic-ferrite ~ 80%, martensite ~15 % and retained austenite = 5%.

11. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the high strength low alloy cold rolled steel strip exhibits yield stress varying in between 1100 - 2200 MPa.

12. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the first predetermined temperature is about 1250 °C and the first predetermined time is about three hours.

13. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the first hot working process is a hot forging process.

14. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the second predetermined temperature is about 1200 °C and the second predetermined time is about 45 minutes.

15. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the second hot working process is hot rolling process.

16. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 15, 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.

17. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 15, wherein temperature of the steel strip drops to the third predetermined temperature ranging from about 950 °C to 1000 °C, during the hot rolling process.

18. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the fourth predetermined temperature is about 390°C and the third predetermined time is about 12 hours.

19. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the quenching is an isothermal quenching process, and the bath is a salt bath.

20. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the cooling is air cooling.

21. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein during cold rolling process, shear stresses transform austenite in the steel sheet microstructure, into martensite.

22. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein thickness of the steel sheet after the second hot working process is about 5 mm, and thickness of the steel sheet after cold rolling process is about 1.5 mm.

23. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the fifth predetermined temperature ranges from about 200 °C to 500 °C and the fourth predetermined time is about 2 – 24 hours.

24. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 7, wherein the steel strip is grinded to remove scaling on a surface of the steel strip.

25. The method of producing a high strength low alloy cold rolled steel strip as claimed in claim 19, wherein the salt bath is an equivalent mixture of sodium nitrite (NaNO2) and sodium nitrate (NaNO3).
, Description:FIELD OF INVENTION
[0001] The present invention relates to a high strength low alloy cold rolled steel strip, sheet or blank, and more particularly to the high strength low alloy cold rolled steel strip, sheet or blank and method of manufacturing the high strength low alloy cold rolled steel strip, sheet or blank.

BACKGROUND
[0002] Metal such as steel is widely employed for various applications such as, automobile parts, construction materials, ship building, tools, machines, and numerous other applications, because of its high tensile strength and low cost. Steel is an alloy of iron (Fe), carbon (C), and other alloying elements such as Phosphorous (P), Sulphur (S), Manganese (Mn), Silicon (Si), Chromium (Cr), etc. Steel obtained from steel making process may not possess all the desired properties required for specific applications. Therefore, steel may be subjected to secondary processes such as heat treatment for controlling material properties to meet various needs in the intended applications.

[0003] Heat treatment may be carried out using techniques including but not limiting to annealing, normalizing, hot rolling, quenching, and the like. During heat treatment process, the material undergoes a sequence of heating and cooling operations, thus the microstructure of the steel may be modified during such operations. As a result of heat treatment, the steel undergoes phase transformation, influencing mechanical properties like strength, ductility, toughness, hardness, drawability and the like. The purpose of heat treatment is to increase service life of a product by improving its strength, hardness etc., or prepare the material for improved manufacturability.

[0004] Contemporarily, use of ultra-high strength steel sheet with tensile strength of 2 GPa has become increasingly popular in the manufacturing of electrical vehicles, to provide greater factor of safety. Conventional steels with tensile strength greater than 2GPa have been manufactured by hot forming process in mass scale production. However, steels formed by hot forming process pose problems such as low yield strength, generation of surface defects, requirement of suitable surface coating and other challenges such as surface defects, tool pollution which are undesirable. With advancement in technology, cold forming process may be adapted. However, it would be difficult to achieve 2GPa without incorporation of a good amount of martensite phase in the steel. Ductility of the initial steel has been provided by the austenite content in the bainitic microstructure. The phase hardness of bainitic-ferrite is higher than that of the common ferrite. During cold rolling, generation of dislocation in the bainitic-ferrite will further increase the strength of the steel. However, the maximum strength comes from the transformation of austenite in the bainitic microstructure during cold deformation. The austenite gradually transformed into the martensite depending on the percentage of cold-rolled reduction. In this regard, the presence of austenite with lower stacking fault energy will lead to the delayed transformation which in turn will allow extended number of rolling passes. 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
[0005] It is an object of the invention to solve the problems of the prior art and to provide a high strength low alloyed steel that can be cold rolled to a relatively thin gauge at wide dimensions, and made into HSLA strips, sheets and blanks, having tensile strength in the range of 1200 – 2200 MPa which also includes a modified chemical composition containing lower amount of chromium and moderate amount of manganese.

[0006] Another objective of the present invention is to develop a method of manufacturing the high strength low alloyed steel strip, sheet or blank having the required elongation.

[0007] Another objective of present invention is to provide a new easier manufacturing method combining thermomechanical, cold rolling and heat treatment processes for the proposed chemical composition.

[0008] It is yet another objective of the present invention, to provide a high strength low alloy cold rolled steel strip, sheet or blank, having the following composition in weight%:Carbon (C): 0.3% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Sulphur (S): = 0.02%, Phosphorus (P): = 0.02%, Nitrogen (N): = 0.01%, Silicon (Si): 0.5% - 1.5%, Aluminium (Al): 0.0% - 0.5%, Chromium (Cr): = 0.2%, Molybdenum (Mo): = 0.17%, Copper (Cu): = 0.2%, Nickel (Ni): = 0.5%, and the balance being Iron (Fe) and unavoidable impurities.

SUMMARY OF INVENTION
[0009] This summary is provided to introduce concepts related to a high strength low alloyed steel and a method of manufacturing the high strength low alloyed 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.

[0010] In one aspect of the present invention, a high strength low alloy cold rolled steel strip is provided. The high strength low alloy cold rolled steel strip comprising the following composition expressed in weight %: Carbon (C): 0.3% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Sulphur (S): = 0.02%, Phosphorus (P): = 0.02%, Nitrogen (N): = 0.01%, Silicon (Si): 0.5% - 1.5%, Aluminium (Al): 0.0% - 0.5%, Chromium (Cr): = 0.2%, Molybdenum (Mo): = 0.17%, Copper (Cu): = 0.2%, Nickel (Ni): = 0.5%, and the balance being Iron (Fe) and unavoidable impurities. The steel strip comprises a structure including a bainitic-ferrite phase, martensite phase and a small fraction of retained austenite phase.

[0011] In an embodiment, the high strength low alloy cold rolled steel strip exhibits tensile strength ranging from about 1200 MPa to 2200 MPa.

[0012] In an embodiment, the high strength low alloy cold rolled steel strip exhibits a ductility ranging from 6 - 13%. In an embodiment, the high strength low alloy cold rolled steel strip exhibits a ductility of 12.5 %.

[0013] In an embodiment, the microstructure of the steel strip is microstructure represented by, in area%, the bainitic ferrite ~ 80%, martensite ~15 % and retained austenite = 5%.

[0014] In an embodiment, the high strength low alloy cold rolled steel strip exhibits yield stress varying in between 1100 - 2200 MPa.

[0015] In another aspect of the present invention, a method of producing a high strength low alloy cold rolled steel strip having a composition of Carbon (C) 0.3% - 0.35%, Manganese (Mn) 1.2% - 1.4%, Sulphur (S) = 0.02%, Phosphorus (P) = 0.02%, Nitrogen (N) = 0.01%, Silicon (Si) 0.5% - 1.5%, Aluminium (Al) 0.0% - 0.5%, Chromium (Cr) = 0.2%, Molybdenum (Mo) = 0.17%, Copper (Cu) = 0.2%, Nickel (Ni) = 0.5% and the balance being Iron (Fe) and unavoidable impurities is provided. The method comprises producing a molten steel having the said composition. The method also comprises casting the molten steel in a casting apparatus. The method further comprises heating the steel casting to a first predetermined temperature and soaking the steel casting at the first predetermined temperature for a first predetermined time. The method comprises deforming the steel casting in a first hot working process to obtain a steel slab. The method also comprises cooling the steel slab obtained in the first hot working process to ambient temperature. The method further comprises re-heating the steel slab to a second predetermined temperature and annealing the steel slab for a second predetermined time. The method comprises subjecting the steel slab to a second hot working process, at a third predetermined temperature to form a steel strip. The method also comprises quenching the steel strip obtained during the second hot working process to a fourth predetermined temperature in a bath and soaking in the steel strip in the bath at the fourth predetermined temperature for a third predetermined time. The method further comprises cooling the steel strip to room temperature. The method comprises grinding the steel strip to remove scaling on a surface of the steel strip. The method also comprises cold rolling the steel strip for further reduction in thickness. The method further comprises tempering the steel strip processed in the cold rolling process at a fifth predetermined temperature for a fourth predetermined time to obtain a steel strip comprising a structure including a bainitic-ferrite phase, martensite phase and small fraction of retained austenite phase.

[0016] In an embodiment, the high strength low alloy cold rolled steel strip exhibits tensile strength ranging from about 1200 MPa to 2200 MPa.

[0017] In an embodiment, the high strength low alloy cold rolled steel strip exhibits a ductility ranging from 6 – 13%.

[0018] In an embodiment, the microstructure of the steel strip is microstructure represented by, in area%, the bainitic-ferrite ~ 80%, martensite ~15 % and retained austenite = 5%.

[0019] In an embodiment, the high strength low alloy cold rolled steel strip exhibits yield stress varying in between 1100 - 2200 MPa.

[0020] In an embodiment, the first predetermined temperature is about 1250 °C and the first predetermined time is about three hours.

[0021] In an embodiment, the first hot working process is a hot forging process. In an embodiment, the second predetermined temperature is about 1200 °C and the second predetermined time is about 45 minutes.

[0022] In an embodiment, the second hot working process is hot rolling process. 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, temperature of the steel strip drops to the third predetermined temperature ranging from about 950 °C to 1000 °C, during the hot rolling process.

[0024] In an embodiment, the fourth predetermined temperature is about 390°C and the third predetermined time is about 12 hours.

[0025] In an embodiment, the quenching is an isothermal quenching process, and the bath is a salt bath. In an embodiment, the cooling is air cooling.

[0026] In an embodiment, during cold rolling process, shear stresses transform austenite in the steel sheet microstructure, into martensite.

[0027] In an embodiment, thickness of the steel sheet after the second hot working process is about 5 mm, and thickness of the steel sheet after cold rolling process is about 1.5 mm.

[0028] In an embodiment, the fifth predetermined temperature ranges from about 200 °C to 500 °C and the fourth predetermined time is about 2 – 24 hours.
[0029] In an embodiment, the salt bath is an equivalent mixture of sodium nitrite (NaNO2) and sodium nitrate (NaNO3).

[0030] Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Figure 1 illustrates a flowchart of a method of manufacturing a high strength low alloy cold rolled steel strip, according to an embodiment of the present invention;

[0032] Figures 2a and 2b illustrate a graphical representation of the method of manufacturing the high strength low alloyed steel strip, according to an embodiment of the present invention;

[0033] Figure 3 illustrates microstructure of hot rolled bainitic steel of composition 1;

[0034] Figure 4 illustrates microstructure of the high strength low alloyed cold rolled steel of composition 1, according to an embodiment of the present invention;

[0035] Figure 5 illustrates a graphical representation of results of X-ray Diffraction analysis carried out on a hot rolled bainitic steel sample of composition 1, according to an embodiment of the present invention;

[0036] Figure 6 illustrates a graphical representation of the results of X-ray Diffraction analysis carried out on the HSLA cold rolled steel sample tempered for a period of 24 hours at 500°C, according to an embodiment of the present invention;

[0037] Figure 7 illustrates a graphical representation of stress versus elongation, obtained during tensile test of the hot rolled bainitic steel sample of composition 1;

[0038] Figure 8 illustrates a graphical representation of stress versus elongation, obtained during tensile test of the HSLA cold rolled steel strip having composition 1, according to an embodiment of the present invention;
[0039] Figure 9 illustrates a graphical representation of stress versus elongation, obtained during tensile test of the hot rolled bainitic steel sample of composition 2;

[0040] Figure 10 illustrates microstructure of a hot rolled bainitic steel of composition 2;

[0041] Figure 11 illustrates a graphical representation of stress versus elongation, obtained during tensile test of the HSLA cold rolled steel strip having composition 2, according to an embodiment of the present invention;

[0042] Figure 12 illustrates a graphical representation of stress versus elongation, obtained during tensile test of the hot rolled bainitic steel sample of composition 3;

[0043] Figure 13 illustrates microstructure of a hot rolled bainitic steel of composition 3; and

[0044] Figure 14 illustrates a graphical representation of stress versus elongation, obtained during tensile test of the HSLA cold rolled steel strip having composition 3, according to an embodiment of the present invention.

[0045] 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
[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] Figures 1, 2a and 2b, illustrate an exemplary method (100) of manufacturing a high strength low alloyed steel strip, sheet or blank. The high strength low alloy cold rolled steel strip comprises the following composition expressed in weight %: Carbon (C): 0.3% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Sulphur (S): = 0.02%, Phosphorus (P): = 0.02%, Nitrogen (N): = 0.01%, Silicon (Si): 0.5% - 1.5%, Aluminium (Al): 0.0% - 0.5%, Chromium (Cr): = 0.2%, Molybdenum (Mo): = 0.17%, Copper (Cu): = 0.2%, Nickel (Ni): = 0.5%, and the balance being Iron (Fe) and unavoidable impurities. The steel strip comprises a structure including a bainitic-ferrite phase, martensite phase and a small fraction of retained austenite phase.
[0052] Referring to Figure 1, the method (100) of manufacturing a high strength low alloyed steel strip, sheet or blank of the desired composition is illustrated. At step (102), a molten steel having composition expressed in weight %: Carbon (C): 0.3% - 0.35%, Manganese (Mn): 1.2% - 1.4%, Sulphur (S): = 0.02%, Phosphorus (P): = 0.02%, Nitrogen (N): = 0.01%, Silicon (Si): 0.5% - 1.5%, Aluminium (Al): 0.0% - 0.5%, Chromium (Cr): = 0.2%, Molybdenum (Mo): = 0.17%, Copper (Cu): = 0.2%, Nickel (Ni): = 0.5%, and the balance being Iron (Fe) and unavoidable impurities is produced.

[0053] At step (104), the molten steel produced in the step (102) is casted in a casting apparatus (to obtain cast ingots). At step (106) the method (100) comprises heating the steel casting (cast ingots) to a first predetermined temperature and soaking the same at the first predetermined temperature for a first predetermined time. In an embodiment, the first pre-determined temperature is about 1250°C and the first predetermined time is about three hours. In one example, the casted steel may be heated in a furnace.

[0054] At step (108), the heated and soaked steel obtained in the step (106) 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 (110) the steel slab obtained in the step (108) is cooled to ambient temperature. In an embodiment, cooling of the steel is performed using normal air cooling.

[0055] At step (112), the method (100) comprises re-heating the steel slab to a second pre-determined temperature and annealing the steel for a second predetermined time. In an embodiment, the second pre-determined temperature may be around 1200 ºC and the second predetermined time may be about 45 minutes.

[0056] At step (114), the re-heated and annealed steel slab obtained in step (112) is subjected to a second hot working process at a third predetermined temperature to form a steel strip (also shown in Figure 2a). In an embodiment, the second hot working process may be hot rolling process. 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. In an embodiment, temperature of the steel strip drops to the third predetermined temperature ranging from about 950 °C to 1000 °C, during the hot rolling process.

[0057] At step (116), the hot rolled steel strip obtained in step (114) is quenched to a fourth predetermined temperature in a bath and followed by soaking the steel strip in the bath at the fourth predetermined temperature for a third predetermined time. 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). The quenched and soaked steel is cooled to room temperature. 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.

[0058] During quenching, the retained austenite becomes enriched in carbon and is stabilized mechanically such that, during straining new twin(s) formation may be favored, thus facilitating in improving ductility. In an embodiment, the fourth predetermined temperature is about 390 ° C and the third predetermined time is about 12 hours. In an embodiment, the steel strip is grinded after quenching process, to remove all scaling and to make both surfaces parallel to each other.

[0059] At step (118), the steel strip is subjected to cold rolling process, for further reduction in thickness of the steel sheet (shown in Figure 2b). As an example, the thickness of the steel sheet may be reduced to about 1.5 mm. In an embodiment, during cold rolling the shear stress developed may transform the austenite into martensite, which contributes to increase tensile strength and elongation. The obtained ductility after cold rolling is due to presence of remaining untransformed and highly mechanically stable retained austenite. This small stable austenite may contain twin.

[0060] At step (120), the cold worked steel strip obtained in step (118) is tempered at a fifth predetermined temperature for a fourth predetermined time to obtain the steel strip comprising a structure including a bainitic-ferrite phase, martensite phase and small fraction of retained austenite phase. Tempering is a mechanical process, which involves heating the steel to a high temperature, which is below the melting point and then cooling the steel in air. The effect of tempering is quite significant in increasing the strength without impairing elongation. The cold-rolled steel contains huge numbers of dislocation and dislocation substructure. Tempering the steel sheet may contribute to improve elongation due to evolution of austenite.

[0061] In an embodiment, the high strength low alloy cold rolled steel strip has microstructure represented by, in area%, the bainitic-ferrite ~ 80%, martensite ~15 % and retained austenite = 5%. In an embodiment, the high strength low alloy cold rolled steel strip exhibits tensile strength ranging from about 1200MPa to 2200 MPa, elongation ranging from about 6% ~ 13%, more preferer ably 12.5% and yield stress varying in between 1100 - 2200 MPa. In an embodiment, the fifth predetermined temperature ranges from around 200 °C to 500 °C and the fourth predetermined time varies in the range of 2-24 hours. Steel processed by the method of the present disclosure results in microstructural changes to form the high strength cold rolled low alloy steel strip.

[0062] Figures 3 and 5 illustrates microstructure and X-ray Diffraction analysis of a hot rolled bainitic steel. The microstructure of the hot rolled bainitic steel consists of bainitic-ferrite and austenite. The mechanical properties are very remarkable; about 1150 MPa UTS, accompanied with maximum 20 % of elongation. Here, the key factors to achieve high strength are solid solution strengthening, bainitic-ferrite, and some retained austenite (RA) -transformed martensite formed during air cooling to room temperature from holding temperature. After 12 hours holding at 390 °C, the retained austenite becomes enriched in carbon and stabilized mechanically so much so that during straining new twin(s) formation may be favored. This will perhaps contribute more towards getting high ductility. It is expected that during tensile straining the thin mechanically stable austenite plates do not transform into martensite, rather there are greater chances of formation of twin in stable austenite. However, the XRD results of hot-rolled (HR) in Figure 5 doesn’t demonstrate any austenite presence in the HR sample. It is possible that the carbon enriched austenite in the bainitic ferrite have transformed into martensite while cooling from the holding temperature as soon as it reached to the martensite start temperature. However, the tensile curve clearly illustrates a considerable ductility which indicates that a low fraction of highly stable austenite is certainly present in the microstructure. The volume fraction of that stable austenite is so low that the XRD was not able to interpret the same. This highly potential HR steel can be used as a starting parent material for the production of ultra-high strength cold-rolled steel strip as a low strength HR steel is always economical to cold-rolled at lower capacity CR mill.

[0063] Figures 4 and 6 illustrate microstructure and X-ray Diffraction analysis of the high strength low alloy cold rolled steel manufactured by the method of the present disclosure. The Cold Rolling of HR steel demonstrates 1200 MPa – 2200 MPa UTS and 6 - 13% of elongation. During cold rolling, shear stresses transform austenite of the hot-rolled bainitic microstructure into martensite. This contributes to observed increase in the UTS up to 2200 MPa in combination of the presence dislocation forest through cold rolling. The reduction of retained austenite is clearly visible in X-ray Diffraction analysis of the HSLA cold rolled steel (as seen in Figure 6). The obtained ductility after cold rolling is due to presence of remaining untransformed and highly mechanically stable retained austenite, which may contain twin(s).

[0064] The effect of tempering is quite significant in increasing the strength without impairing elongation. The cold-rolled steel contains huge numbers of dislocations and dislocation substructures. During tempering the carbon partitions into the dislocation thus to the substructure. At 200°C, short range diffusion of carbon occurs and reaches nearby dislocation. This will lock the dislocation movement thus increases the yield-strength of the observed HSLA steel in Figure 2b (compare to Figure 2a). However, no such reduction of ductility demonstrates due to the absence of austenite-based transformation product i.e. martensite. While increasing the tempering temperature, the little evolution of austenite in the bainitic-martensite microstructure is expected and thus the strength and yield stress have decreased over the increase of tempering temperature (400 and 500 °C). However, the steel demonstrated a high stability in the mechanical properties even on the 24 hours of long tempering duration. The steel has demonstrated a range of strength from 1200 to 2200 MPa with 6 – 13 % elongation (result of all three compositions which will be explained below), respectively. Therefore, thin gauge HSLA steel strip, sheet, or blank has a great potential for automobile application and a great futuristic possibility to replace the hot-forming process that will provide a cost-effective option.

[0065] In an embodiment, the method of the present disclosure includes melting, casting, heat treatment, thermomechanical and cold-rolled routes, which are simple. 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. Additionally, the thermomechanical process (i.e., the hot rolling process) is simple and can be directly started from homogenization temperature. It includes rough passing in the 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 of the present disclosure aids in reducing energy consumption and thus a cost-effective high strength steel manufacturing process.

[0066] In an embodiment, the method employs normal air cooling and eliminates use of vacuum melting furnace, unlike conventional methods. This aids in reducing cost of the steel manufacturing process.

[0067] Following portions of the present disclosure, provides details about the proportion of each element in a composition of the high strength cold rolled steel sheet and their role in enhancing properties.

[0068] Carbon (C) may be used in the range of 0.3 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

[0069] Silicon (Si) may be used in the range of about 0.5 to 1.5 wt.%. Silicon (Si) suppresses the formation of carbide, which leads to carbide free bainitic matrix to improve ductility and impact toughness.

[0070] 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.

[0071] Chromium (Cr) may be used in the range of about 0.4 to 0.5 wt.%. Chromium (Cr) of this proportion can substantially increase the strength and hardenability of the steel. It can vary beyond above range for customized strength and hardenability requirement

[0072] Nickel (Ni) may be used below 0.5 wt.%. Nickle (Ni) content limited to maximum to increase the residual austenite carbon without sacrificing the hardenability. It increases the strength and toughness.

[0073] Molybdenum (Mo) may be used below 0.2 wt.%. Molybdenum (Mo) addition of small quantity reduces the impurity embrittlement and to increases hardenability. Excess addition may reduce the carbon content in austenite. It increases the room temperature strength in steel.

[0074] Aluminium (Al) may be used in the range of about 0.01 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.

[0075] Copper (Cu) may be used below 0.2 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.

Examples
[0076] Further embodiments of the present disclosure will be now described with an example of particular compositions of the HSLA (cold rolled) steel, which are illustrated in Tables 1, 2 and 3. Experiments have been carried out for specific compositions of the HSLA cold rolled steel formed by using the method of the present disclosure. Results have been compared on various fronts to show the contribution of tempering temperature in improving mechanical properties of the HSLA steel.

Composition 1: Quenching at 390°C for 12 hrs.

C Mn S P Si Al Cr Mo Ni Cu N
0.29 1.35 0.011 0.017 0.55 0.53 0.19 0.17 0.49 0.20 70 ppm

Table: 1

Composition 2: Quenching at 390°C for 12 hrs.

C Mn S P Si Al Cr Mo Ni Cu N
0.29 1.24 0.011 0.017 0.92 0.02 0.19 0.16 0.50 0.20 71 ppm

Table: 2

Composition 3: Quenching at 390°C for 12 hrs.
C Mn S P Si Al Cr Mo Ni Cu N
0.29 1.22 0.011 0.017 1.48 0.01 0.47 0.17 0.49 0.2 71 ppm

Table: 3

[0077] In an embodiment of the present disclosure, various experiments were carried out on the HSLA steel strip sample for different compositions as mentioned in Table-1, Table-2, and Table-3, which may be subjected to quenching and tempering at different temperatures, during formation of the HSLA steel strip. For conducting the experiment, HSLA steel sheet samples were prepared for conducting microstructural investigation and conducting tensile test. As an example, tensile testing may be performed using Instron machine as per ASTM standard and XRD, SEM tests were conducted to investigate microstructure of the high strength cold rolled steel sheets

[0078] Accordingly, Table 4 illustrates mechanical properties of steel having composition 1 and subjected to quenching at 390 °C and tempered at different temperatures.
Sample Detail Sample
ID 0.2% Yield Stress (MPa) UTS (MPa) % Elongation
Hot-Rolled Bainite LB 855 1141 ~ 20%
Cold-Rolled Bainite LBC 1487 1675 ~ 6%
Cold-Rolled-200 Tempered LBC200 1720 1753 ~ 5%
Cold-Rolled-400 Tempered LBC400 1387 1350 ~ 6%
Cold-Rolled-500 Tempered LBC500 1249 1259 ~13%

Table: 4

[0079] Accordingly, Table 5 illustrates mechanical properties of steel having composition 2 and subjected to quenching at 390 °C and tempered at different temperatures.
Sample Detail Sample
ID 0.2% Yield
Stress (MPa) UTS (MPa) % Elongation
Hot-Rolled Bainite LPB 810 1047 ~ 19 %
Cold-Rolled Bainite LPBC 1189 1394 ~ 5 %
Cold-Rolled-200 Tempered LPBC200 1508 1542 ~ 4 %
Cold-Rolled-400 Tempered LPBC400 1290 1339 ~ 5 %
Cold-Rolled-500 Tempered LPBC500 1156 1197 ~ 7 %
Table: 5

[0080] Accordingly, Table 6 illustrates mechanical properties of steel having composition 3 and subjected to quenching at 390 °C and tempered at different temperatures.
Sample Detail Sample
ID 0.2 % Yield
Stress (MPa) UTS (MPa) % Elongation
Hot-Rolled Bainite LQB 882 1174 ~ 27 %
Cold-Rolled Bainite LQBC 1417 1938 ~ 6 %
Cold-Rolled-200 Tempered LQBC200 2030 2102 ~ 5 %
Cold-Rolled-400 Tempered LQBC400 1531 1614 ~ 7 %
Cold-Rolled-500 Tempered LQBC500 1364 1450 ~ 10 %
Table: 6
[0081] As explained in the result tables 4, 5, 6 and as seen in Figures 8 to 14, the HSLA steel strip exhibits better mechanical properties when tempered at 200 °C for a time period of about 24 hours.

[0082] It is evident that, the effect of tempering is quite significant in increasing the elongation. This is due to the little evolution of austenite in the bainitic-martensite microstructure. Though, the strength and yield stress have decreased over the increase of tempering temperature. The steel demonstrates a high stability in the mechanical properties even on the 24 hours of long tempering duration. The steel exhibits strength from 1200 to 2200 MPa with 6% to 13 % elongation, respectively. Therefore, thin gauge HSLA steel strip, sheet or blank will have a great potential for automobile applications and a great futuristic possibility to replace the hot-forming process that will provide a cost-effective option.

[0083] The present invention provides HSLA cold rolled steel and a method of manufacturing HSLA cold rolled steel. The HSLA cold 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 defense applications. Therefore, the present invention opens the avenue for a cost effective and feasible option for the processing of a cold rolled ultra-high strength steel aiming to develop a cold-stamping grade steel.

[0084] It should be understood that the experiments are carried out for particular compositions of the HSLA cold rolled steel strip and the results brought out in the previous paragraphs are for the composition shown in Tables 1, 2 and 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 HSLA cold rolled steel strip, as well.
[0085] In the above description, the terms high strength low alloyed steel, or HSLA or high strength low alloy cold rolled steel have been interchangeably used throughout the description.

[0086] 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.

[0087] 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.

[0088] 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.