Description:
The following specification particularly describes the nature of the invention and the manner in which it is to be performed.

TECHNICAL FIELD

Present disclosure relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to an ultra-high strength steel sheet. Further embodiments of the disclosure disclose a method for manufacturing the ultra-high strength steel sheet which exhibits tensile strength greater than 2GPa.

BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron, carbon, and other elements such as Phosphorous (P), Sulphur (S), Nitrogen (N), Manganese (Mn), Silicon (Si), Chromium (Cr), etc. Because of its high tensile strength and low cost, steel is considered as a major component in wide variety of applications. Some of the applications of the steel includes construction, ship building tools, automobiles, machines, bridges and numerous other applications. The steel obtained from steel making process may not possess all the desired properties. Therefore, the steel may be subjected to secondary processes such as various heat treatment processes for controlling material properties to meet various needs in the intended applications.

Generally, heat treatment may be carried out using techniques including but not limiting to annealing, normalising, hot rolling, quenching, and the like. During heat treatment process, the material undergoes a sequence of heating and cooling operations, thus, the microstructures of the steel may be modified during such operation. As a result of heat treatment, the steel undergoes phase transformation, influencing mechanical properties like strength, ductility, toughness, hardness, drawability etc. 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.

With rising concerns over security, strengthening capability of troops is on demand. Providing safety gears for troops such as personal armour, armoured vehicles, and the like, are essential to ensure safety and improve combat capability of the troops. Strong, tough, and light-weight steel is one of the major requirements to resist any intrusions and ensure safety of the troops. Generally, material such as carbon fibre, glass fibre and the like have been used in armour applications. Though such materials are lighter, but they are expensive and possess inferior ballistic resistance, when compared to steel. Usually, ultra-high strength steel (UHSS) sheets are used to cater these requirements. Such steel includes high carbon %, to cater requirement of high strength and hardness. However, increase in carbon % decreases ductility and weldability of the steel, which is undesired. Additionally, developing such steels includes complex manufacturing cost, thus escalating costs of the steel.

One more patent literature known in the art has focussed on developing high strength steels. EP1052296B1 discloses a hot rolled armoured steel, which includes martensitic microstructure. The hot rolled armoured steel exhibits tensile strength of 1150 MPa, yield strength of 1100MPa and hardness of about 400 HB. Further, some patent literatures discloses a bainitic steel, exhibiting tensile strength of 2098 MPa, an armour-plate having steel whose hardness ranges from about 150 to 250 HB.

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

SUMMARY OF THE DISCLOSURE

One or more shortcomings of the prior art are overcome by method as disclosed 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, there is provided a method for manufacturing an ultra-high strength steel sheet. The method starts with melting the steel of composition [in weight percentage] comprising: carbon (C) about 0.31% to about 0.38%, manganese (Mn) at about 0.05% to about 0.08%, silicon (Si) at about 1.4% to about 1.6%, chromium (Cr) at about 1.4% to about 1.6%, aluminium (Al) at about 0.03% to about 0.05%, molybdenum (Mo) at about 0.38% to about 0.42%, vanadium (V) at about 0.28% to about 0.38%, nickel (Ni) at about 3.4% to about 3.6%, sulphur (S) at about 0.003% to about 0.006%, phosphorus (P) at about 0.003% to about 0.006%, nitrogen (N) up-to 0.005%, the balance being Iron (Fe) optionally along with incidental elements, in vacuum induction melting furnace and refining in vacuum arc re-melting. Further, the steel is casted in form of an ingot. Upon casting, the steel ingot is heated to a first predetermined temperature at a predetermined heating rate and soaking the steel ingot for a first predetermined time. The steel ingot is then deformed in a first working process at a second predetermined temperature, in an austenite region to impart deformation and reduce thickness. After first hot working process, the steel is subjected to a first cooling to an ambient temperature. The method further includes re-heating the steel above a third predetermined temperature and soaking the steel in the third predetermined temperature for a second predetermined time. Upon re-heating, the steel is subjected to a second hot working process at a fourth predetermined temperature, to impart deformation into austenite and, followed by a second cooling the steel sheet to room temperature. Further, the method includes heating the steel sheet to a fifth predetermined temperature and soaking the steel sheet at the fifth predetermined temperature for a third predetermined time. After, heating the steel sheet is again subjected to the second hot working process at the fourth predetermined temperature and followed by third cooling the steel sheet, to obtain the ultra-high strength steel sheet. The ultra-high strength steel sheet comprises primarily a martensitic microstructure.

In an embodiment, the ultra-high strength steel sheet exhibits tensile strength greater than 2000 MPa, yield strength greater than 1300 MPa and hardness greater than 600 VHN.

In an embodiment, the ultra-high strength steel sheet comprises the martensitic microstructure greater than 95%, retained austenite lesser than 5%, and very small amount of bainite in scattered manner.

In an embodiment, the first predetermined temperature ranges from about 1100 °C to 1200°C, the heating rate is less than 2°C/s and the first predetermined time is 120 to 240 minutes.

In an embodiment, the first hot working process is hot forging, and the second predetermined temperature is a finishing forging temperature ranging from about 915°C to 975 °C.

In an embodiment, the third predetermined temperature is above 1100 °C, and steel is soaked in the third predetermined temperature for predetermined second time, depending on thickness of the steel sheet.

In an embodiment, the second hot working process is a hot rolling process, and the second hot working process is performed in two high plate mills and the rolling is performed by maintaining finish rolling temperature (FRT) in the austenite, below non-recrystallization temperature of the steel, which ranges from about 780 °C to 840 °C.

In an embodiment, the fifth predetermined temperature is about 1100 °C and the third predetermined time is about 10-30 minutes.

In an embodiment, the second hot working process is again performed in the austenite region with multiple passes, and with cumulative deformation of about 55% to 62%.
In an embodiment, the first cooling is a furnace cooling, the second cooling and the third cooling is an air cooling.
In an embodiment, thickness of the steel sheet after the second hot working process is 6mm to 8mm, and thickness of the steel sheet after the second hot working process, again is 2mm to 3mm.
In another non-limiting embodiment of the disclosure, an ultra-high strength steel sheet is disclosed. The steel sheet comprises composition in weight percentage (wt%) of: carbon (C) about 0.31% to about 0.38%, manganese (Mn) at about 0.05% to about 0.08%, silicon (Si) at about 1.4% to about 1.6%, chromium (Cr) at about 1.4% to about 1.6%, aluminium (Al) at about 0.03% to about 0.05%, molybdenum (Mo) at about 0.38% to about 0.42%, vanadium (V) at about 0.28% to about 0.38%, nickel (Ni) at about 3.4% to about 3.6%, sulphur (S) at about 0.003% to about 0.006%, phosphorus (P) at about 0.003% to about 0.006%, nitrogen (N) up-to 0.005%, the balance being Iron (Fe) optionally along with incidental elements. The ultra-high strength steel sheet comprises primarily a martensitic 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 together 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:

Figure. 1 is a flowchart illustrating a method for manufacturing an ultra-high strength steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 2 illustrates an optical microscopy image of the ultra-high strength steel sheet manufactured by the method of the present disclosure.

Figure. 3 illustrates a Scanning Electron Microscope [SEM] image showing microstructure of the ultra-high strength steel sheet manufactured by the method of the present disclosure.

Figure. 4 illustrates X-Ray Diffraction profile of the ultra-high strength steel sheet manufactured by the method of the present disclosure, 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 embodiment thereof has 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 forms 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 discloses an ultra-high strength steel sheet and a method for manufacturing or producing the ultra-high strength steel sheet. High strength and light weight are some of the important properties of the steel, for use in armor applications. As of now, high strength steel sheets with increased in carbon % content are used in armored applications. Though increased carbon % increases strength, but reduces ductility and weldability, which is undesired. Accordingly, the method for manufacturing ultra-high strength steel sheet, with tensile strength greater than 2000 MPa, yield strength greater than 1300 MPa and hardness greater than 600 HV, with less carbon content is described in the present disclosure. The ultra-high strength steel sheet may be widely employed in manufacturing armor gears, ballistic resistance armored vehicles and the like.

In the method of manufacturing the ultra-high strength steel sheet, a first step may include melting and refining a steel of composition [in weight percentage], including carbon (C) about 0.31% to about 0.38%, manganese (Mn) at about 0.05% to about 0.08%, silicon (Si) at about 1.4% to about 1.6%, chromium (Cr) at about 1.4% to about 1.6%, aluminium (Al) at about 0.03% to about 0.05%, molybdenum (Mo) at about 0.38% to about 0.42%, vanadium (V) at about 0.28% to about 0.38%, nickel (Ni) at about 3.4% to about 3.6%, sulphur (S) at about 0.003% to about 0.006%, phosphorus (P) at about 0.003% to about 0.006%, nitrogen (N) up-to 0.005%, the balance being Iron (Fe) optionally along with incidental elements in a furnace. The melted steel is then casted in form an ingot. The casted steel ingot is then heated to a first predetermined temperature ranging from about 1100 °C to 1200°C at a predetermined heating rate of less than 2°C/s for a first predetermined time period of about 120 to 240 mins. Upon heating the steel ingot, the steel ingot may be deformed in a first working process in an austenite region to impart deformation and reduce thickness at a second predetermined temperature. In an embodiment, the first working process is hot forging and the second predetermined temperature is a finishing forging temperature ranging from about 915°C to 975 °C. Upon completion of the first working process, the steel may be subjected first cooling, which is a furnace cooling, to cool the steel to an ambient temperature. Upon cooling to the ambient temperature, the steel may be further re-heated to a third predetermined temperature which is above 1100 °C and soaked at the third predetermined temperature for few hours depending on thickness of steel. Further, the steel sheet is subjected to a second hot working process at a fourth predetermine temperature, which is below non-recrystallization temperature ranging from about 780 °C to 840 °C and followed by second cooling to room temperature.

Upon cooling steel sheet to the room temperature, the steel sheet is heated to a fifth predetermined temperature which is about 1100 °C and soaked at about 1100 °C for a third predetermined time, which is about 10 minutes to 30 minutes. Upon re-heating and soaking, the steel sheet is again subjected to the second hot working process at the fourth predetermine temperature, which is below non-recrystallization temperature, ranging from about 780 °C to 840 °C. Further, the steel sheet is subjected to a third cooling, which is air cooling to obtain the ultra-high strength steel sheet. The ultra-high strength steel sheet according to the present disclosure may have a microstructure constituted primarily of a martensitic microstructure which is greater than 95%, retained austenite lesser than 5% and very small amount of bainitic in scatter manner.

In an embodiment, the ultra-high strength steel sheet exhibits, tensile strength of more than 2000 MPa, yield strength of more than 1300 MPa and hardness of more than 600 HV. Further, the martensite structure includes high dislocation density and consists of very fine size and random orientation. This provides high strength and hardness to the ultra-high strength steel sheet. Therefore, the ultra-high strength steel sheet may be used in ballistic resistance applications such as but not limiting to armour gear, armoured vehicles, and the like.

Henceforth, the present disclosure is explained with the help of figures for a method of manufacturing ultra-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 depicting a method for manufacturing an ultra-high strength steel sheet. In the present disclosure, mechanical properties such as strength, tensile strength, yield strength and hardness of the final microstructure of the steel sheet may be improved. The steel ultra-high strength steel sheet produced by the method of the present disclosure, includes less % of carbon, and includes substantially a martensitic 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 matter described herein.

At block 101, a steel of desired composition may be melted and refined in vacuum induction melting furnace and refining in vacuum arc re-melting, respectively. In embodiment, the steel may have composition [in weight percent] including carbon (C) about 0.31% to about 0.38%, manganese (Mn) at about 0.05% to about 0.08%, silicon (Si) at about 1.4% to about 1.6%, chromium (Cr) at about 1.4% to about 1.6%, aluminum (Al) at about 0.03% to about 0.05%, molybdenum (Mo) at about 0.38% to about 0.42%, vanadium (V) at about 0.28% to about 0.38%, nickel (Ni) at about 3.4% to about 3.6%, sulphur (S) at about 0.003% to about 0.006%, phosphorus (P) at about 0.003% to about 0.006%, nitrogen (N) up-to 0.005%, the balance being Iron (Fe) optionally along with incidental elements.

At block 102, the method may include casting the steel in form of an ingot. Liquid steel with the above-mentioned composition and range of alloying elements may be 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. Further, the method includes a step or stage of heating the steel ingot to a first predetermined temperature at a predetermined heating rate for a first predetermined time to achieve homogenization [as shown in block 103]. In an embodiment, the first predetermined temperature ranges from about 1100°C to 1200°C, and the heating rate is less than 2°C/s. Further, in an embodiment, the first pre-determined time is about 120-240 mins.
Once the steel ingot is heated and soaked as per block 103, the steel ingot may be subjected to a first hot working process at a second predetermined temperature, as shown in block 104. In an embodiment, the hot working process is a hot forging process, which is done in an austenite region to impart deformation and reduce thickness of the steel ingot. Further, in an embodiment, the second predetermined temperature is a finishing forging temperature ranging from about 915°C - 975 °C. As shown in block 104, after forging the steel ingot, the steel may be subjected to a first cooling to an ambient temperature. In an embodiment, first cooling may be a furnace cooling, which aids in minimizing temperature gradient and thermal stresses within the steel.
After cooling the steel to ambient temperature as per block 104, the steel may be heated above a third predetermined temperature, to austenite region and may be soaked at the third predetermined temperature for a second predetermined time [as shown in block 105]. In an embodiment, the third predetermined temperature is above 1100 °C. As an example, the third predetermined temperature may be in the range of about 1100 °C to 1200 °C. Further, in an embodiment, the second predetermined time is about 60 minutes to 180 minutes, which depends on thickness of the steel after forging process.

Now referring to block 106, the method further includes the step of subjecting the steel to a second hot working process at a fourth predetermined temperature, to impart deformation into austenite and, followed by second cooling the steel to room temperature. In an embodiment, the second hot working process may be a hot rolling process. The hot rolling process may be performed in two plate mills for several number of passes to impart deformation into austenite and to reduce thickness of the steel to around 6mm to 8mm. 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. In an embodiment, the fourth predetermined temperature is a finish rolling temperature (FRT) in the austenite. The finish rolling temperature (FRT) is below non-recrystallization temperature ranging from about 780°C to 840 °C. In an embodiment, the second cooling may be air cooling.

Further, the method includes heating the steel sheet processed in the second hot working process to a fifth determined temperature and soaking the steel at the fifth pre-determined temperature for a third predetermined time [as shown in block 107]. In an embodiment, the fifth predetermined temperature may be around 1100 °C and the third pre-determined time may range from about 10 minutes to 30 minutes. Upon heating and the soaking the steel sheet as per block 107, the steel sheet may be again subjected to the second hot working process [i.e., the hot rolling process], at the fourth predetermined temperature for further reducing the thickness of the steel sheet and followed by a third cooling [as shown in block 108]. In an embodiment, the hot rolling process may be again performed in the austenite region with multiple passes, and with cumulative deformation of about 55% to 62% and, thickness of the steel sheet reduced to about 2mm to 3mm. In an embodiment, the third cooling may be air cooling.

In an embodiment, the steel processed by the method of the present disclosure, results in microstructural changes to form the ultra-high strength steel sheet. The ultra-high strength steel sheet includes martensitic microstructure greater than 95%, retained austenite lesser than 5%, and very small amount of bainite in scattered manner.

The martensite microstructure includes high dislocation density, and very fine size and random orientation. This ensures exceptional strength and hardness of the steel. Further, retained austenite helps to enhance the ductility.
In an embodiment, the ultra-high strength steel sheet exhibits, tensile strength of more than 2000 MPa i.e., in the range of about 2000 MPa to 2100 MPa, yield strength of more than 1300 MPa i.e., in the range of about 1300 MPa to about 1370 MPa and hardness of more than 600 HV i.e., in the range of about 600 HV to 660 HV.

Figure. 2 is an exemplary embodiment of the disclosure, which illustrates an optical micrograph image of the ultra-high strength steel sheet, which is formed by using method of the present disclosure. The micrograph confirms that, the developed by the ultra-high strength steel sheet has mainly martensite microstructure as indicated, with negligible amount of retained austenite and/or bainite. The micrograph further confirms flavour of structural pancaking of austenite and presence of fine martensitic plates across the microstructure. Referring now to Figure. 3, which illustrates Scanning Electron Microscope [SEM] microstructure. The SEM microstructure confirms that, the ultra-high strength steel sheet mainly includes fine martensite microstructure. Further, referring to Figure. 4, which illustrates an X-Ray Diffraction profile of the ultra-high strength steel sheet. The diffraction profile shows very strong BCC and weak FCC peaks indicating presence of mainly martensite and with very small amount austenite. Additionally, Electron backscatter diffraction [EBSD] examination also confirms presence of mainly martensite structure and negligible amount of austenite.

Hence, the final microstructure of the ultra-high strength steel sheet formed by the method described above, exhibits high combination of yield and tensile strength with very high hardness. This makes the steel ultra-strong and light weight. These properties make the ultra-high strength steel sheet suitable for, but not limiting to personal armor applications which light weight, strong and hard steel.
The ultra-high strength steel sheet is a hot rolled air cooled product, with no additional heat-treatment or post processing necessary. Thus, makes the method of present disclosure efficient and production efficient.
The ultra-high strength steel relatively has lower carbon, hence possessing better weldability
The following portions of the present disclosure provides details about the proportion of each alloying element in a composition of the steel and their role in enhancing properties.

Carbon (C) may be added in the range of about 0.31 wt% - 0.38 wt%. Carbon is an essential element to achieve desired strengthening. Carbon level must also be controlled to ensure good weldability. Preferable carbon content may be below 0.40% to achieve desired strength and weldability. Therefore, carbon content below 0.4% is preferable.

Vanadium (V) may be added about 0.35wt% maximum. Vanadium may be added to increase strength of the steel by various mechanisms such as grain refinement, precipitation, and the like. Vanadium may be added carefully and optimized to take advantage of economic advantage as Vanadium is costly. Therefore, Vanadium level should be below 0.35% or more preferably, below 0.33%.

Silicon (Si) may be added in the range of about 1.4 wt% to about 1.6 wt%. Silicon is a ferrite stabiliser. Silicon may suppress carbide precipitation during bainite or martensite transformation after hot rolling during constant temperature holding or coiling. Excess amount of silicon addition in steel may be detrimental due to varieties of scale formation during hot rolling and cooling. Further, excess silicon may not be desirable from scale point of view. Hence, silicon may be restricted within certain range as mentioned above and preferably may be below 1.6wt%.

Phosphorus (P) may be added at about 0.01wt% maximum. Phosphorus is considered detrimental in steel. Therefore, content of phosphorus may be restricted to a maximum of 0.09% and preferably 0.008% or less.

Sulphur may be added about 0.008wt% maximum. Similar to phosphorus, sulphur is also considered detrimental in steel. Therefore, content to be kept as low as possible, preferably below 0.007 wt%. More preferably sulphur content may be below 0.006 wt%, to ensure better toughness.

Nitrogen (N) may be added about 0.005wt% maximum. Higher nitrogen content in steels may also be detrimental. Excess nitrogen content may lead to hard inclusions such as TiN and AlN which may deteriorate formability. Therefore, content of nitrogen may be restricted to a maximum of 0.005wt%.
Molybdenum (Mo) may be added about 0.45wt% maximum. Molybdenum may be added to enhance the hardenability in steel and, thereby favours easy formation of martensite or bainite. Due to excess hardenability softer ferrite and relatively harder pearlite phase formation could be suppressed. Further, since Molybdenum is costly, its amount is preferable below 0.45 wt%, to make the steel economical and take processing advantage during hot rolling.

Chromium (Cr) may be added about 1.6 wt% maximum. Like Molybdenum, Chromium may avoid formation of polygonal ferrite and pearlite. Chromium addition may be more economical in advanced ultra-high strength steel. However, Chromium may be harmful if added excessive amount, as Chromium may form various kind of carbides.
Example:

Embodiments of the present disclosure will now be described with an example of a particular composition of the steel. Experiments have been carried out for a specific composition of the steel formed by using method of the present disclosure. The composition of the steel for which the tests are carried out is as shown in below table 1.

Chemical composition (wt. %)
Elements C Si Cr Mo V Ni P S N
A 0.35 1.5 1.5 0.4 0.08 3.5 0.006 0.005 0.005
B 0.34 1.6 1.45 0.35 0.32 3.45 0.005 0.005 0.005
C 0.36 1.45 1.6 0.41 0.29 3.60 0.005 0.006 0.005

Table – 1

Different compositions of steel A, B and C were processed by the method of the present disclosure to obtain ultra-high strength steel. The processed steel may be subjected to testing to determine mechanical properties of the ultra-high strength steel sheet. As an example, tensile testing may be performed using Instron machine as per ASTM standard and hardness test may be performed by Vickers hardness. Accordingly, table 2 illustrates mechanical properties of steel having compositions A, B and C.

Composition Yield Strength
(MPa) Tensile strength
(MPa) Total Elongation
(%)
A 1300 2070 5
B 1363 2071 4.5
C 1350 2060 4.5

Table-2

It should be understood that the experiments are carried out for particular compositions of the steel and the results brought out in Table 2 are for the compositions shown in Table – 1. However, the said compositions should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the steel as well.

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
101-108 Flowchart blocks
101 Melting and refining stage
102 Casting stage
103 Heating stage
104 First hot working and first cooling
105 Heating stage
106 Second hot working stage and second cooling
107 Heating stage
108 Second hot working stage, again and third cooling

Claims:We Claim:

1. A method for manufacturing an ultra-high strength steel sheet, the method comprising:
melting steel of a composition comprising in weight percentage (wt%) of:
carbon (C) about 0.31% to about 0.38%,
manganese (Mn) at about 0.05% to about 0.08%,
silicon (Si) at about 1.4% to about 1.6%,
chromium (Cr) at about 1.4% to about 1.6%,
aluminium (Al) at about 0.03% to about 0.05%,
molybdenum (Mo) at about 0.38% to about 0.42%,
vanadium (V) at about 0.28% to about 0.38%,
nickel (Ni) at about 3.4% to about 3.6%,
sulphur (S) at about 0.003% to about 0.006%,
phosphorus (P) at about 0.003% to about 0.006%,
nitrogen (N) up-to 0.005%,
the balance being Iron (Fe) optionally along with incidental elements, in vacuum induction melting furnace and refining in vacuum arc re-melting;
casting, a steel ingot;
heating, the steel ingot to a first predetermined temperature at predetermined heating rate and soaking for a first predetermined time;
deforming, the steel ingot in a first hot working process in an austenite region to impart deformation and reduce thickness at a second predetermined temperature;
first cooling, the steel processed in the first hot working process to an ambient temperature;
re-heating, the steel above a third predetermined temperature and soaking the steel in the third predetermined temperature for a second predetermined time;
subjecting, the steel to a second hot working process, at a fourth predetermined temperature, to impart deformation into austenite, and second cooling the steel to room temperature;
heating, the steel sheet processed in the second hot working process to a fifth predetermined temperature, and soaking at the fifth pre-determined temperature for a third predetermined time;
subjecting, the steel sheet to the second hot working process again, at the fourth predetermined temperature; and
third cooling the steel sheet to room temperature, to obtain the ultra-high strength steel sheet;
wherein, the ultra-high strength steel sheet comprises primarily a martensitic microstructure.
2. The method as claimed in claim 1, wherein the ultra-high strength steel sheet exhibits a tensile strength greater than 2000 MPa.
3. The method as claimed in claim 1, wherein the ultra-high strength steel sheet exhibits a yield strength greater than 1300MPa.
4. The method as claimed in claim 1, wherein the ultra-high strength steel sheet exhibits a hardness greater than 600 HV.
5. The method as claimed in claim 1, wherein the ultra-high strength steel sheet comprises the martensitic microstructure greater than 95%, retained austenite lesser than 5%, and very small amount of bainite in scattered manner.
6. The method as claimed in claim 1, wherein the first predetermined temperature ranges from about 1100 °C -1200°C, the heating rate is less than 2°C/s and the first predetermined time is about 120 to 240 minutes.
7. The method as claimed in claim 1, wherein the first hot working process is hot forging, and the second predetermined temperature is a finishing forging temperature ranging from about 915°C - 975 °C.
8. The method as claimed in claim 1, wherein the third predetermined temperature is above 1100 °C.
9. The method as claimed in claim 1, wherein the steel is soaked in the third predetermined temperature for second predetermined time depending on thickness of the steel sheet.
10. The method as claimed in claim 1, wherein the second hot working process is a hot rolling process.
11. The method as claimed in claim 1, wherein the second hot working process is performed in two high plate mills and the hot rolling is performed by maintaining finish rolling temperature (FRT) in the austenite region, below non-recrystallization temperature of the steel.
12. The method as claimed in claim 10, wherein the non-recrystallization temperature of the steel ranges from about 780 °C to 840 °C.
13. The method as claimed in claim 1, wherein the fifth predetermined temperature is about 1100 °C and the third predetermined time is about 10-30 minutes.
14. The method as claimed in claim 1, wherein the second hot working process is again performed in the austenite region with multiple passes with cumulative deformation 55-62%.
15. The method as claimed in claim 1, wherein the first cooling is a furnace cooling process.
16. The method as claimed in claim 1, wherein the second cooling and third cooling is an air cooling process.
17. The method as claimed in claim 1, wherein thickness of the steel sheet after the second hot working process is 6mm to 8mm, and thickness of the steel sheet after the second hot working process, again is 2mm to 3mm.
18. An ultra-high strength steel sheet, comprising:
composition in weight percentage of:
carbon (C) about 0.31% to about .38%,
manganese (Mn) at about 0.05% to about 0.08%,
silicon (Si) at about 1.4% to about 1.6%,
chromium (Cr) at about 1.4% to about 1.6%,
aluminium (Al) at about 0.03% to about 0.05%,
molybdenum (Mo) at about 0.38% to about 0.42%,
vanadium (V) at about 0.28% to about 0.38%,
nickel (Ni) at about 3.4% to about 3.6%,
sulphur (S) at about 0.003% to about 0.006%,
phosphorus (P) at about 0.003% to about 0.006%,
nitrogen (N) up-to 0.005%, and
the balance being Iron (Fe) optionally along with incidental elements;
wherein, the ultra-high strength steel sheet comprises primarily a martensitic microstructure.
19. The ultra-high strength steel sheet as claimed in claim 18, comprises martensitic microstructure greater than 95%, retained austenite lesser than 5%, and very small amount of bainite in scattered manner.
20. The ultra-high strength steel sheet as claimed in claim 18, exhibits tensile strength greater than 2000 MPa.
21. The ultra-high strength steel sheet as claimed in claim 18, exhibits yield greater than 1300 MPa.
22. The ultra-high strength steel sheet as claimed in claim 18, exhibits hardness greater than 600 HV.
23. A body armor and light weight combat vehicle part comprising an ultra-high strength steel sheet as claimed in claim 18.

Dated this 01st February 2021