Description:TECHNICAL FIELD

Present disclosure relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to a method of producing high strength steel sheet. Further, embodiments of the disclosure /disclose the method for producing high strength steel sheet that exhibits an ultimate tensile strength greater than 1480 MPa with refined pearlitic microstructure.

BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron, carbon, and other alloying element. Because of its high tensile strength and low cost, steel may be considered as a most viable choice for major components manufacturing in a wide variety of applications. Some of the applications of the steel may include buildings, ships, tools, automobiles, machines, bridges, and numerous other applications.

Steel may be generally manufactured as steel slabs by processes such as casting including but not limiting to continuous casting, and then the steel is formed into various shapes depending on the application. One such common form of steel is a steel sheet which is obtained by converting the steel slab into steel sheet by series of metal forming processes to find its use in the sheet metal industry. For converting of the steel slab into steel sheet, processes such as hot rolling and cold rolling are carried out.

Conventionally, cold rolling may be an integral part of a steel plant. The primary objective of cold rolling is to impart intended properties in the sheets and may be done conducted after hot rolling. These stell sheets may be used in a variety of applications such as vehicle body manufacturing, home appliances etc. Further, these slabs are cold rolled at room temperature in the cold rolling process. Further, slabs are initially processed by a strip mill to obtain the sheet with a eutectoid and hypo/hyper-eutectoid steels containing pearlite. Pearlite is a two phased lamellar structure composed of alternative layers of ferrite and cementite. The pearlite microstructure in any strip often imparts hardness and strength to the strip. The sheets with the pearlite microstructure may be used for producing various products like cutting saws, automotive components, gardening tools, surgical blades, springs, measuring devices, wire rods, tire bead wires, deep drawn high strength wires, wires for suspension bridges, etc. High strength steel sheets are also used in jackets and body armour jackets due to the primary advantage of being an economical choice amongst various other alternate materials for safety.

More specifically, chest protection jackets and body armour jackets are made up of a variety of materials such as high strength steel plates or advanced ceramic based composite systems embedded in jackets. The chest protection jackets offer protection against threats such as thrusts, or stabs from sharp objects. The properties of the material used for manufacturing such protection jackets would vary depending on the application. Material for manufacturing the high strength steel used in protection jackets are selected based on various factors such as the required level of protection, cost of the material, comfort offered to the user wearing the jacket and the life of the material in service.

Conventionally, the high strength steel used in protection jackets are manufactured as hot rolled plates with a low to medium carbon containing martensitic or pearlitic microstructure to achieve the required hardness. Further, there exists a large variety of steel grades that may be used for manufacturing the high strength steel. The hot rolled plates manufactured from steel slab is cut from the steel coils and are provisioned in the protection jackets for imparting the threat resistance capability to the jackets. The high strength steel used in protection jackets generally includes a significant addition of costly alloying elements such as chromium, molybdenum, and nickel with the aim to impart high hardenability. Typical properties of these material include hardness of 360-400 BHN with yield strength of 1180 MPa and ultimate tensile strength of 1300 MPa and a total elongation of about 6-8%. The above-mentioned type of high strength steels with alloying elements form the most commonly used material for life protection jackets. However, the addition of the costly alloying elements increases the cost of the high strength steel used in protection jackets. Consequently, the overall cost of the protection jacket also increases drastically. Further, the weight of high strength steel due to alloying elements also increases drastically. The overall weight of the protection jacket also increases and renders discomfort to the user wearing the protection jacket.

Further, with advancement in material sciences, ceramic materials have gained increasing relevance and are often used as a solution to manufacture lightweight protection jackets. The modern ceramic based armour plates are generally manufactured and used as monolithic plates that impart impact resistance owing to the high hardness and compressive strength of the material. The most common examples of such advanced ceramics include alumina, silicon carbide or boron carbide and titanium diboride that vary to a great extent in the level of hardness. Further, Kevlar has also gained significance in protection jackets. Kevlar has become the material of choice in protection jackets since, Kevlar offers high degree of hardness and impact resistance. However, the ceramic material and the additional alloying elements used to manufacture the protection jacket are expensive and the high cost of the material particularly with respect to the steel plates often increases the overall cost of the protection jackets an unreasonable or unaffordable extent.

The existing methods for manufacturing high strength steels in protection jackets are often very expensive. Further, the pearlitic microstructure imparted to the high strength steels from the existing manufacturing methods is often unrefined. Consequently, the protection jackets may not absorb very large impacts. The existing protection jackets may be subjected to a very limited number of impacts after which they degrade drastically due to which the overall operational life of the jacket is also very limited. The above disclosed existing methods are also not economical for the mass production.

The present disclosure is directed to overcome one or more limitations stated above or any other limitation associated with the conventional arts.

SUMMARY OF THE DISCLOSURE

One or more shortcomings of the prior art are overcome by a method and a product as claimed and additional advantages are provided through the method as described in the present disclosure.

Additional features and advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

In one non limiting embodiment of the disclosure, a method for producing high strength steel sheet is disclosed. The method includes casting, a steel slab comprising a composition in weight% of carbon (C) of about 0.6 wt.% – 1.2 wt.%, manganese (Mn) of about 0.1 -1.5 wt.%, balance being Iron (Fe) optionally along with incidental elements. The steel slab is subjected to a hot working process form a steel sheet. The steel sheet is cooled to obtain a substantially pearlitic microstructure in the steel sheet. Further, the steel sheet is subjected to a cold rolling process by applying at least 50% of strain during cold rolling to obtain the high strength steel sheet where, cold rolling of the steel sheet refines the pearlitic microstructure of the steel sheet.

In an embodiment, the cold rolling refines interlamellar spacing of the pearlitic microstructure and resulting in directional alignment of the pearlitic microstructure in the steel sheet.

In an embodiment, the directional alignment of the pearlitic microstructure in the steel sheet along with the energy stored during the cold rolling increases the hardness and tensile strength of the steel sheet.

In an embodiment, the high strength steel sheet exhibits tensile strength greater than 1480 MPa with yield strength greater than 1350 MPa. The high strength steel also sheet exhibits total elongation greater than 4% and hardness greater than 400 Hv.

In an embodiment, the casting is a continuous casting process and hot working of the steel slab is a hot rolling process.

In an embodiment, the cooling of the steel sheet is subjected to a laminar cooling on a run out table with water at a predetermined cooling rate to generate the substantially pearlitic microstructure of the steel sheet.

In an embodiment, the predetermined cooling rate ranges from 1 °C/s – 15 °C/s and the steel slab is hot rolled at a temperature ranging from 1100 °C – 800 °C.

In one non limiting embodiment of the disclosure, a high strength cold rolled steel sheet, is disclosed. The steel sheet includes a composition in weight% of carbon (C) of about 0.6 wt.% – 1.2 wt.%, manganese (Mn) of about 0.1 -1.5 wt.%, balance being Iron (Fe) optionally along with incidental elements where, the high strength cold rolled steel sheet comprises a refined pearlitic microstructure.

It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined to form a further embodiment of the disclosure.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

Fig. 1 is a flowchart illustrating a method for producing high strength steel, according to an exemplary embodiment of the present disclosure.

Figure 2a shows micrographic view of a cold rolled steel subjected to a cold rolling process by applying 50% of strain during cold rolling.

Figure 2b shows micrographic view of a cold rolled steel subjected to the cold rolling process by applying 75% of strain during cold rolling.

Figure 3a shows a micrographic view from a scanning electron microscope of the steel sheet obtained by hot rolling.

Figure 3b shows a micrographic view from the scanning electron microscope of the cold rolled steel sheet subjected to cold rolling process by applying 50% of strain during cold rolling.

Figure 3c shows a micrographic view from the scanning electron microscope of the cold rolled steel sheet subjected to cold rolling process by applying 75% of strain during cold rolling.

Fig. 4a is a graphical representation showing tensile test results on a tensile stress v/s tensile strain curve of the of the cold rolled steel subjected to cold rolling process by applying 50% of strain during cold rolling, according to an exemplary embodiment of the present disclosure.

Fig. 4b is a graphical representation showing tensile test results on a tensile stress v/s tensile strain curve of the of the cold rolled steel subjected to cold rolling process by applying 75% of strain during cold rolling, according to an exemplary embodiment of the present disclosure.

The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION

The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristic of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular form disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.

Embodiments of the present disclosure disclose a high-strength cold rolled steel sheet and a method for manufacturing a high-strength steel sheet. Hardness, tensile strength, and yield strength of the steel are some of the important properties for high strength steels. For some of the applications including but not limiting to amor jackets, it is desirable to have the steel sheet with pearlitic microstructure. Pearlitic microstructure in the high strength steels produced by the existing manufacturing methods is often unrefined. Consequently, the high strength steels manufactured form conventional methods which are used in protection jackets may not absorb very large impacts. The existing protection jackets or body armour may be subjected to a very limited number of impacts after which they degrade drastically, and the overall operational life of the jacket is also very limited.

According to various embodiment of the disclosure, a method for producing high strength steel sheet. The method includes casting, a steel slab comprising a composition in weight% of carbon (C) of about 0.6 wt.% – 1.2 wt.%, manganese (Mn) of about 0.1 -1.5 wt.%, balance being Iron (Fe) optionally along with incidental elements. The steel slab is subjected to a hot working process form a steel sheet. The steel sheet is cooled to obtain a substantially pearlitic microstructure in the steel sheet. Further, the steel sheet is subjected to a cold rolling process by applying at least 50% of strain during cold rolling to obtain the high strength steel sheet where, cold rolling of the steel sheet refines the pearlitic microstructure of the steel sheet.

Henceforth, the present disclosure is explained with the help of figures of a method of manufacturing high-strength steel sheet. However, such exemplary embodiments should not be construed as limitations of the present disclosure since the method may be used on other types of steels where such need arises. A person skilled in the art may envisage various such embodiments without deviating from scope of the present disclosure.

Figure. 1 is an exemplary embodiment of the present disclosure illustrating a flowchart of a method for producing high strength steel. The steel produced by the method of the present disclosure, includes a refined pearlitic microstructure. The method is now described with reference to the flowchart blocks and is as below. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject. The method is particularly applicable to high strength cold rolled steel, and it may also be extended to other type of steels as well.

The method of manufacturing the high strength low alloy steel sheet according to the present disclosure consists of a casting step followed by hot rolling step, cooling and cold rolling. The various processing steps are described in their respective order below:

At block 101, a steel slab of desired alloy composition is formed by any of manufacturing process such as casting including but not limited to continuous casting process. In an embodiment, alloy may be prepared in at least one of air-melting furnace, and vacuum furnace. The steel slab may have composition of carbon (C) in a range of about 0.6 wt.% – 1.2 wt.% preferably 0.8 wt.%, manganese (Mn) in the range of about 0.1 -1.5 wt.% and iron (Fe) being remainder of the composition along with incidental elements may be casted in a continuous casting process.

The addition of each alloying element and the limitations imposed on each element are essential for achieving the required refined pearlitic microstructure and the same is described below in detail.

Carbon (C) acts as a strengthening element and carbon in the range of about 0.6 wt.% – 1.2 wt.% is generally used in medium and high carbon steels. Manganese (Mn) in the range of about 0.1 -1.5 wt.% not only imparts solid solution strengthening but it also lowers the austenite to ferrite transformation temperature thereby refining the grain size.

Liquid steel with the above-mentioned composition and range of alloying elements is continuously casted into a slab. The liquid steel of the specified composition is first continuously casted either in a conventional continuous caster or a thin slab caster. In an embodiment, when casted in a thin slab caster, the temperature of the cast slab may be restricted below 950 °C. Further, transverse cracks will develop in the slab if casted at low temperature below 950 °C. Consequently, the temperature of the cast slab may be restricted below 950 °C.

The method then includes the step of hot working the steel slab by a hot working process [shown in block 102] after casting the steel slab. In an embodiment, the hot working process may be but not limited to hot rolling. Hot rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform at high temperatures and hot rolling is carried out above the recrystallization temperature of the steel. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. In an embodiment, the hot charged steel slab may be hot rolled using hot mill strip. During hot rolling, hot charged steel slab may be subjected to roughing mill. The roughing mill usually consists of one or two roughing stands in which the steel slab may be hot rolled back and forth few times repeatedly to reach the thickness requirement. Roughing milled steel sheet may be further subjected to finish rolling. During finish rolling, the sheet surface may be subjected to further thickness reduction, surface finishing and dynamic recrystallization. The slab may be hot rolled into a steel sheet at a temperature ranging from 800 °C –1100 °C. The hot rolling may include the roughing step above the recrystallization temperature and a finishing step below the recrystallization temperature, when rolling is done in a conventional hot strip mill.

The next step 103 involves cooling the hot rolled sheet to obtain a pearlitic microstructure in the hot rolled sheet. The hot rolled steel sheet may be subjected to laminar cooling on a run out table with water at a cooling rate ranges from 1 °C/s – 15 °C/s. The run-out table usually includes a plurality of rollers for traversing the hot roller strip. The run-out table may be configured with a plurality of nozzles at a significant height from the rollers. The nozzles may generate or spray a thin curtain of water or any other coolant on the hot rolled strip traversing on the rollers. The coolant flow from the nozzles may be adjusted to control the cooling rate of the hot rolled steel sheet. The hot rolled steel sheet may be cooled by the run-out table and the cooling rate may range from 1 °C/s – 15 °C/s. The cooling rate is determined based on multiple experiments such that the above-mentioned range of cooling rate assists in developing substantial or fully pearlitic microstructure in the steel sheet. The above-mentioned range of cooling rate ensures the formation of the pearlite structure. The steel sheet majorly comprises of the pearlite microstructure.

Referring to block 104, the method further includes the step of cold rolling the steel sheet at a predetermined cold rolling strain. The cold rolling process is similar to the above-mentioned hot rolling process however, the steel sheet is rolled at a substantially lesser temperature or at the room temperature in cold rolling process. Cold rolling involves forcing the steel sheet through two rolls that have a gap in between each other. When the steel sheet is cold rolled, it is plastically deformed as it is forced between the two rolls. The metal is compressed by the rolls and the plastic deformation occurs in the direction of the rolling. Cold working plastically deforms the steel sheet at a temperature below its recrystallization temperature. The steel sheet may be subjected to the cold rolling process by applying at least 50% of strain during cold rolling to obtain the high strength steel sheet. Multiple experiments are conducted, and it is determined that subjecting the steel sheet to the cold rolling process by applying strain greater than or equal to 50% of strain leads to the refinement of the pearlite microstructure in the steel sheet. The steel sheet may be cold rolled in multiple small passes and a lubricant may be used during cold rolling for minimizing heat generated due to friction. The cold rolling of the steel sheet improves the pearlitic microstructure of the steel sheet. The cold rolling refines the pearlitic microstructure, and a fine scale of the microstructure is created. Consequently, the hardness, tensile strength, yield strength are drastically improves and the pearlite microstructure is refined. Further, steel sheets that were subjected to 50% and 75% of strain during cold rolling and the mechanical properties of the cold rolled sheets we determined by standard tests. The tests conducted on the cold rolled sheets and the corresponding results are indicates by means of an example below.

Example:

Further embodiments of the present disclosure will now be described with examples of composition of the steel. Experiments have been carried out on the steel by using method of the present disclosure.

The steel sheets may initially be subjected to 50% and 75% of strain during cold rolling. Further, small samples may be cut from the cold rolled steel sheets and may be prepared further into metallographic specimen by mounting, grinding followed by coarse and fine polishing using standard techniques. The final polished specimens may be etched with standard 2 vol% nital solution. An optical microscope as well as a Field Emission Gun Scanning Electron Microscope (FEGSEM) may be used to study the microstructure of the high strength steel sheet sample.

Further, mechanical properties of the material were determined by standard hardness and tensile tests. Hardness of the samples were measured in Vickers scale and tensile tests of the cold rolled samples were carried out at room temperature using ASTM E8 standard specimen with a slow cross head speed of 2 mm/min.

Figure 2a shows micrographic view of a cold rolled steel subjected to a cold rolling process by applying 50% of strain during cold rolling and Figure 2b shows micrographic view of the sample of cold rolled steel sheet subjected to the cold rolling process by applying 75% of strain. Further, though the pearlite is not clearly resolved in the optical micrograph of Figure 2a, it is apparent that the microstructure in Figure 2a exhibits the signature of the cold rolling deformation. The directional orientation of the pearlite microstructure is more apparent for steel sheet cold rolled by applying 75% of strain than the steel sheet that is cold rolled by applying 50% strain. Cold rolling by applying 75% of strain results in larger degree of the deformation causing a more favourable condition for the alignment of the pearlite colonies along the rolling direction. Thus, it is evident that, cold rolling the steel sheet at larger rolling strains ensures better alignment and refinement of the pearlitic microstructure along the rolling direction.

Figure 3a shows a micrographic view from a scanning electron microscope of the steel sheet obtained by hot rolling. It is evident from the Figure 3a that the pearlitic microstructure is randomly oriented when the steel sheet is only subjected to hot rolling. Further, hot rolling results in formation of random pearlitic colonies all over the steel sheet and the pearlitic microstructure in the steel sheet is completely unrefined in nature.

Figure 3b shows a micrographic view from the scanning electron microscope of the cold rolled steel sheet subjected to cold rolling process by applying 50% of strain during cold rolling and Figure 3c shows a micrographic view from the scanning electron microscope of the cold rolled steel sheet subjected to cold rolling process by applying 75% of strain during cold rolling. It is evident from the Figures 3b and 3c that the cold rolled samples show a specific alignment of the pearlitic microstructure when compared to the microstructure from Figure 3a. Apart from the directional alignment of the pearlite in the cold rolled samples, the refinement in the scale of the microstructure is also improved as seen along the region “A” of Figure 3b. Whereas hot rolling only results in the formation of the random colonies of pearlite. Further, the interlamellar spacing of the pearlitic microstructure is highly refined in sample subjected to cold rolling at 75% of strain when compared with the interlamellar spacing of the pearlite microstructure of the sample subjected to cold rolling by applying 50% of strain. This fine scale of the pearlite microstructure along with the defects generated due to the cold deformation increases the hardness and tensile strength of the material. Therefore, the cold rolled high strength steel sheets improve the impact resistance capability required for protection jackets.

Further, the hardness values of the samples subjected to hot rolling, cold rolling at 50% of strain and cold rolling at 75% strain are recorded in Vickers scale using a nominal load of 1 kg. The variation in hardness between these samples is presented in the below Table 1.

Sample #1 #2 #3 #4 #5 Average
Hot rolled 306 304 309 315 301 310
Cold rolled with 50% strain 362 370 368 371 362 371
Cold rolled with 75% strain 423 412 424 417 414 417

Table 1: Vickers Hardness of the hot rolled steel sheet, cold rolled steel sheet at 50% of strain and cold rolled steel sheet at 75% strain.

It is evident from Table 1 that the cold rolling deformation leads to a significant rise in the hardness value compared with the hot rolled steel sheets. The hot rolled samples recorded lower hardness values as compared with other standard steel sheet samples that were subjected to cold rolling at 50% of strain and 75% of strain. The 50% cold rolled sample showed almost a similar scale of pearlite spacing as that of the hot rolled material but the gain in hardness is significant over the hot rolled sample. The significant difference in hardness between the hot rolled and cold rolled samples is due to the residual strain energy that is stored in the material due to the cold deformation. The stored residual strain energy further increases in the cold rolled sample with 75% strain. Consequently, the sample subjected to cold rolling at 75% strain exhibits greater hardness. It is therefore evident that cold rolling at larger strains contributes to an even larger rise in the hardness value. The residual strain, the fine scale of the pearlite, also contributed to the rise of bulk hardness. Further, the average hardness of the hot rolled sample is around 310 Hv. Whereas the hardness of the cold rolled samples subjected to 50% of strain and 75% of strain is 314 Hv and 417 Hv respectively. It is therefore evident that the cold rolled samples of the steel sheet exhibit better hardness than the samples from the hot rolled sheets. It is also evident form the experimental results that, greater cold rolling strain also imparts greater hardness to the steel sheet.

Further, tensile tests were carried out on the samples from the cold rolled steel sheet at 50% of strain and samples from the cold rolled steel sheet at 75% strain. Fig. 4a is a graphical representation showing tensile test results on a tensile stress v/s tensile strain curve of the of the cold rolled steel subjected to cold rolling process by applying 50% of strain. Fig. 4b is a graphical representation showing tensile test results on a tensile stress v/s tensile strain curve of the of the cold rolled steel subjected to cold rolling process by applying 75% of strain. The below table 2 indicates the result of the tensile test on the samples from the cold rolled sheets subjected to 50% strain and 75% strain.

Sample Yield strength, MPa Ultimate tensile strength, MPa % elongation
Cold rolled with 50% strain 1208 1354 6.4
Cold rolled with 75% strain 1299 1491 4.4

Table 2: Yield strength, tensile strength, and elongation values of the cold rolled samples

With reference to the above table 2, the cold rolled with 50% strain sample recorded typical yield strength of 1208 MPa and ultimate tensile strength of 1354 MPa. Further, the 75% cold rolled material found to possess yield strength close to 1300 MPa and ultimate tensile strength of 1500 MPa. With further reference to Figure 4a, the sample that was subjected to cold rolling at 50% strain, undergoes failure after an elongation of around 6.4% whereas, the sample that was subjected to cold rolling at 50% strain, undergoes failure after an elongation of around 4.4%. The above results from the tensile tests again illustrate that the mechanical properties of the cold rolled steel sheet at the strain of 75% is superior to the mechanical properties of the cold rolled steel sheet at the strain of 50%. It is therefore evident that higher cold rolling strain imparts greater hardness, yield strength and ultimate tensile strength to the steel sheet.

In an embodiment, the method disclosed in the present disclosure including the additional step of cold rolling the steel sheets imparts refined pearlite microstructure to the steel sheet. In an embodiment, the method disclosed in the present disclosure including the additional step of cold rolling the steel sheets imparts superior tensile strength greater than 1480 MPa, yield strength greater than 1350 MPa and exhibits hardness greater than 400 Hv. In an embodiment, the high strength steel of the present disclosure is manufactured with near eutectoid chemical composition i.e., carbon in the range of 0.6 – 1.2 wt% and Mn in the range of 0.1 – 1.5 wt% without any other alloying elements. In an embodiment, no additional alloying elements are used in the method of the present disclosure for manufacturing the high strength steel sheet. The high strength steel sheet of the present disclosure is manufactured by relying only on carbon and manganese as the element for strength instead of other commonly used costly additions such as chromium, molybdenum, nickel etc. Thus, a cost-effective method of manufacturing high strength steel sheets is enabled by the method of the present disclosure. In an embodiment, the high bulk hardness, and the tensile strength of the cold rolled steel sheet with the near eutectoid chemical composition is achieved due to the refinement in the interlamellar spacing of the pearlitic microstructure as well as the stored energy of cold rolling deformation. In an embodiment, mechanical properties such as hardness, toughness, tensile strength, and yield strength are improved by manufacturing high strength steel in the above-mentioned method.

In an embodiment, the high strength steel produced by the above-mentioned method may be used in protection jackets, armor jackets, other lifesaving jackets etc. The high strength steel produced by the above-mentioned method may been an ideal and cost-effective substitute to the expensive alloying elements used to impart strength to the steels used in the protection jackets.

Equivalents:

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Referral Numerals

Referral Numerals Description
101 Casting stage
102 Hot rolling stage
103 Cooling stage
104 Cold rolling stage

Claims:1. A method of producing high strength steel sheet, the method comprising:
casting, a steel slab comprising a composition in weight% of:
carbon (C) of about 0.6 wt.% – 1.2 wt.%,
manganese (Mn) of about 0.1 -1.5 wt.%,
balance being Iron (Fe) optionally along with incidental elements;
subjecting, the steel slab to a hot working process form a steel sheet;
cooling, the steel sheet, such that the steel sheet includes substantially a pearlitic microstructure;
subjecting, the steel sheet to a cold rolling process by applying at least 50% of strain during cold rolling to obtain the high strength steel sheet,
wherein, cold rolling of the steel sheet refines the pearlitic microstructure of the steel sheet.

2. The method as claimed in claim 1, wherein the cold rolling refines interlamellar spacing of the pearlitic microstructure resulting in directional alignment of the pearlitic microstructure in the steel sheet.

3. The method as claim in claim 2, wherein directional alignment of the pearlitic microstructure in the steel sheet along with the energy stored during the cold rolling increases the hardness and tensile strength of the steel sheet.

4. The method as claimed in claim 1, wherein the high strength steel sheet exhibits tensile strength greater than 1480 MPa.

5. The method as claimed in claim 1, wherein the high strength steel sheet exhibits yield strength greater than 1350 MPa.

6. The method as claimed in claim 1, wherein the high strength steel sheet exhibits total elongation greater than 4%.

7. The method as claimed in claim 1, wherein the high strength steel sheet exhibits hardness greater than 400 Hv.

8. The method as claimed in claim 1, wherein the casting is a continuous casting process.

9. The method as claimed in claim 1, wherein hot working of the steel slab is a hot rolling process.

10. The method as claimed in claim 1, wherein the cooling of the steel sheet is a laminar cooling on a run out table with water, at a predetermined cooling rate to generate the substantially pearlitic microstructure in the steel sheet.

11. The method as claimed in claim 1, wherein the predetermined cooling rate ranges from 1 °C/s – 15 °C/s.

12. The method as claimed in claim 1, wherein the steel slab is hot rolled at a temperature ranging from 1100 °C – 800 °C.

13. A high strength cold rolled steel sheet, comprising:
a composition in weight% of:
carbon (C) of about 0.6 wt.% – 1.2 wt.%,
manganese (Mn) of about 0.1 -1.5 wt.%,
balance being Iron (Fe) optionally along with incidental elements;
wherein, high strength cold rolled steel sheet comprises a refined pearlitic microstructure.

14. The steel sheet as claimed in claim 13, wherein the high strength steel sheet exhibits tensile strength greater than 1480 MPa.

15. The steel sheet as claimed in claim 13, wherein the high strength steel sheet exhibits yield strength greater than 1350 MPa.

16. The steel sheet as claimed in claim 13, wherein the high strength steel sheet exhibits total elongation greater than 4%.

17. The steel sheet as claimed in claim 13, wherein the high strength steel sheet exhibits hardness greater than 400 Hv.

18. An automotive component made of high strength cold rolled steel sheet as claimed in claim 13.