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 2 GPa.

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. Steel obtained from steel making process may not possess all the desired properties. Therefore, 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 steel 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. 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.

In the recent past, use of ultra-high strength steel in automotive industry has increased over the conventional low strength steels, which have poor crash safety properties. Conventionally, steels are manufactured by hot forming process at higher temperatures . Further to improve hardenability, carbon, and boron content has been increased in the alloying mixture. Conventional Carbon-Manganese-Boron alloys such as 22MnB5, 27MnCrB5, and 37MnB5 steels are hot formed and die quenched to develop fully martensite structures which impart the strength. Formation of martensite is enabled by different alloying elements which improves the hardenability at the achievable cooling rate under dies after hot forming. However about 0.36 wt % of carbon is added to achieve a strength level of 2 GPa of hot formed steel. With such high carbon content in the steel, it will be difficult to fabricate the steel by welding process in modern light weight vehicles, which is undesired.

One of the patent literatures known in the art has focussed on developing high strength steels. CN106811689B discloses a method of forming steel, which annealing is a subsidiary process step and with carbon content of 1.2 to 1.7%. Increase in carbon % decreases ductility and weldability of the steel, which is undesired. Another patent literature CN106811681B discloses preparation of boron-free hot formed steel. Method includes annealing as one of the subsidiary steps, which escalates cost of manufacturing the steel.

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 arts 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 casting a steel of composition in weight percent including carbon (C) about 0.24% to about 0.32%, manganese (Mn) at about 1.5% to about 2%, silicon (Si) at about 0.05% to about 0.8%, chromium (Cr) at about 0.2% to about 0.9%, aluminium (Al) at about 0.02% to about 0.2%, molybdenum (Mo) at about 0.01% to about 0.6%, Niobium (Nb) at about 0.03% to about 0.08%, Titanium (Ti) at about 0.01% to about 0.05%, Boron (B) at about 0.0005% to about 0.005%, nickel (Ni) at about 0.1% to about 0.8%, sulphur (S) at about 0.01% to about 0.03%, phosphorus (P) at about 0.01% to about 0.03%, nitrogen (N) at about 0.0005% to about 0.02% and the balance being Iron (Fe) optionally along with incidental elements. The casted steel is then heated to a first predetermined temperature. Upon heating the steel, the steel may be subjected to a first hot working process for reducing thickness and to form steel sheet. Temperature of the steel sheet at end of the first hot working process reduces to a second predetermined temperature. Upon completion of the first working process, the steel sheet may be subjected to a first cooling for a first predetermined time. Further, the steel sheet may be subjected to a second cooling process to a third predetermined temperature. Upon cooling steel sheet, the steel sheet may be subjected to a cold working process to further reduce thickness of the steel sheet. The cold rolled steel sheet may be subjected to heating to a fourth predetermined temperature for a third predetermined time. After re-heating, the steel sheet is subjected to a hot forming process and followed by quenching the steel sheet, to obtain the ultra-high strength steel sheet. The ultra-high strength steel sheet exhibits a tensile strength greater than 2000MPa.

In an embodiment, the ultra-high strength steel sheet comprises primarily a martensitic microstructure and small amount of retained austenite.

In an embodiment, the ultra-high strength steel sheet exhibits yield strength greater than 1300 MPa, total elongation greater than 6.5% and hardness greater than 700HV.

In an embodiment, the first predetermined temperature ranges from about 1050 °C to 1250°C.

In an embodiment, the first hot working process is a hot rolling process. In the hot rolling process, reduction rate in thickness of the steel is high in initial six passes and reduction rate in thickness is less in subsequent passes.

In an embodiment, the first cooling process is water sprinkling process in a run out table and the first predetermined time ranges from about 2 seconds to 10 seconds.

In an embodiment, the second predetermined temperature is a finishing rolling temperature ranging from about 700 °C to 950 °C and the second predetermined time is about 24 hours

In an embodiment, the second cooling is a furnace cooling, and the third predetermined temperature is about 200 °C.

In another non-limiting embodiment of the disclosure, an ultra-high strength steel sheet is disclosed. The steel sheet comprises composition in weight percent including carbon (C) about 0.24% to about 0.32%, manganese (Mn) at about 1.5% to about 2%, silicon (Si) at about 0.05% to about 0.8%, chromium (Cr) at about 0.2% to about 0.9%, aluminium (Al) at about 0.02% to about 0.2%, molybdenum (Mo) at about 0.01% to about 0.6%, Niobium (Nb) at about 0.03% to about 0.08%, Titanium (Ti) at about 0.01% to about 0.05%, Boron (B) at about 0.0005% to about 0.005%, nickel (Ni) at about 0.1% to about 0.8%, sulphur (S) at about 0.01% to about 0.03%, phosphorus (P) at about 0.01% to about 0.03%, nitrogen (N) at about 0.0005% to about 0.02% and the balance being Iron (Fe) optionally along with incidental elements. The ultra-high strength steel sheet exhibits a tensile strength greater than 2000MPa.

In an embodiment, the carbon is preferably less than 0.3% and total alloying content is preferably less than 3.5%.

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 embodiments 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 is graphical representation of hardness values of the steel processed by conventional techniques and ultra-high strength steel manufactured by the method of the present disclosure.

Figures. 3a to 3d, and 3e to 3h illustrates microstructure of the steel processed by conventional techniques and manufactured by the method of the present disclosure, respectively.

Figure. 4 is a graphical representation of stress versus elongation, obtained during tensile test of the steel formed by conventional annealing process.

Figure. 5 is a graphical representation of stress versus elongation, obtained during tensile test of the steel formed by conventional annealing process.

Figure. 6 is a graphical representation of stress versus elongation, obtained during tensile test of the ultra-high strength steel which is manufactured by the method of the present disclosure, according to an exemplary embodiment of the present disclosure.

Figure. 7 is a graphical representation of stress versus elongation, obtained during tensile test of the ultra-high strength steel which is 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 inclusions, 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 automobile applications. As of now, high strength steel sheets with increased carbon % content are used in industrial 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 700 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 casting steel of composition [in weight percent] including carbon (C) about 0.24% to about 0.32%, manganese (Mn) at about 1.5% to about 2%, silicon (Si) at about 0.05% to about 0.8%, chromium (Cr) at about 0.2% to about 0.9%, aluminium (Al) at about 0.02% to about 0.2%, molybdenum (Mo) at about 0.01% to about 0.6%, Niobium (Nb) at about 0.03% to about 0.08%, Titanium (Ti) at about 0.01% to about 0.05%, Boron (B) at about 0.0005% to about 0.005%, nickel (Ni) at about 0.1% to about 0.8%, sulphur (S) at about 0.01% to about 0.03%, phosphorus (P) at about 0.01% to about 0.03%, nitrogen (N) at about 0.0005% to about 0.02% and the balance being Iron (Fe) optionally along with incidental elements. The casted steel is then heated to a first predetermined temperature ranging from about 1050 °C to 1250°C. Upon heating the steel, the steel may be subjected to a first hot working process to reduce thickness and to form a steel sheet. Temperature of the steel sheet at end of the first hot working process reduces to a second predetermined temperature. In an embodiment, the first hot working process may be a hot rolling process and the second predetermined temperature is a finish rolling temperature, which is about 700 °C to 950 °C. Upon completion of the first working process, the steel sheet may be subjected to a first cooling, which is water sprinkling process for a first predetermined time, which is about 2 to 10 seconds. Further, the steel sheet may be subjected to a second cooling process, which is about 24 hours, for reducing temperature to a third predetermined temperature, which is about 200 °C.

Upon cooling the steel sheet, the steel sheet may be subjected to a cold working process, which is a cold rolling process to further reduce thickness of the steel sheet. The cold rolled steel sheet may be subjected to heating for a third predetermined time to a fourth predetermined temperature, which is about 750 °C to 950 °C. Upon re-heating, the steel sheet may be subjected to a hot forming process, which is a hot stamping process and followed by quenching the steel sheet, 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 martensite and small amount of retained austenite.

In an embodiment, the ultra-high strength steel sheet exhibits tensile strength of more than 2000 MPa, yield strength of more than 1300 MPa, hardness of more than 700 HV and total elongation of greater than 6.5%. Therefore, the ultra-high strength steel sheet may be used in industrial applications such as but not limiting to automotive industry.

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 ultra-high strength steel sheet produced by the method of the present disclosure, includes less % of carbon and less alloying content 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 casted. In embodiment, the steel may have composition [in weight percent] including carbon (C) about 0.24% to about 0.32%, manganese (Mn) at about 1.5% to about 2%, silicon (Si) at about 0.05% to about 0.8%, chromium (Cr) at about 0.2% to about 0.9%, aluminium (Al) at about 0.02% to about 0.2%, molybdenum (Mo) at about 0.01% to about 0.6%, Niobium (Nb) at about 0.03% to about 0.08%, Titanium (Ti) at about 0.01% to about 0.05%, Boron (B) at about 0.0005% to about 0.005%, nickel (Ni) at about 0.1% to about 0.8%, sulphur (S) at about 0.01% to about 0.03%, phosphorus (P) at about 0.01% to about 0.03%, nitrogen (N) at about 0.0005% to about 0.02% and the balance being Iron (Fe) optionally along with incidental elements. In an embodiment, liquid steel with the above-mentioned composition and range of alloying elements may be formed in one of a blast furnace or an electric arc furnace. Liquid steel may be continuously casted into a slab. The liquid steel of the specified composition may be continuously casted either in a conventional continuous caster or a thin slab caster.
At block 102, the method may include heating the steel to a first predetermined temperature. As an example, steel may be heated in a reheating furnace. In an embodiment, the first predetermined temperature may range from around 1050 °C to 1250°C. Upon heating the steel, the steel may be subjected to a first hot working process to reduce thickness of the steel to form a steel sheet [as shown in block 103]. During the first hot working process the temperature of the steel may reduce and may reach to a second predetermined temperature, at end of the first hot working process. In an embodiment, the first hot working process is a hot rolling process. The hot rolling process may be performed in two plate mills to reduce thickness of the steel. Thickness of the steel may be reduced at higher rate for about 6 passes and thickness of the steel may be reduced at lower rate in subsequent passes. As an example, thickness of the steel may be reduced to around 6 mm to 2.5 mm. Hot rolling process 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.
Further, the method may include subjecting the steel to a first cooling process for a first predetermined time [as shown in block 104]. In an embodiment, the first cooling may be water sprinkling carried out on a runout table and the first predetermined time ranges from around about 2 seconds to 10 seconds, to simulate run out table condition after hot rolling process. Upon first cooling, the steel may be subjected to a second cooling process for reducing temperature to third predetermined temperature for a second predetermined time [as shown in block 104]. In an embodiment, the second cooling may be furnace cooling, the third predetermined temperature is about 200 °C and the second predetermined temperature is about 24 hours. This simulates the industrial run out table cooling process after hot rolling and slow coil cooling process.

At block 105, the method may include subjecting the steel sheet to a cold working process to further reduce thickness of the steel sheet. In an embodiment, the cold working process may be a cold rolling process and the thickness of steel sheet may be reduced to about 1 mm to 1.8mm. In the cold rolling process, the steel sheet may be introduced between rollers to induce shear stress, to carry out microstructural changes in the steel sheet such that the ferrite and pearlite microstructure may transform into the microstructure having an elongated ferrite pearlite grains. Upon subjecting the steel sheet to the cold working process, the steel sheet may be heated to a fourth predetermined temperature for a third predetermined time [as shown in block 106]. In an embodiment, the fourth predetermined temperature may range from about 750 °C to 950 °C and the third predetermined time may range from about 2 minutes to 6 minutes.

The method further includes, subjecting the steel sheet to a hot forming process and followed by quenching, to obtain the ultra-high strength steel sheet [as shown in block 107]. In an embodiment, the hot forming process may be a hot stamping process and quenching may be die quenching process. The hot forming process aids in transformation of austenite into martensitic microstructure in the steel and, thus improving mechanical properties of the steel.

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 primarily martensitic microstructure and small amount of retained austenite.

In an embodiment, the ultra-high strength steel sheet exhibits, tensile strength of more than 2000 MPa (preferably in the range of 2050 MPa to 2500 MPa), yield strength of more than 1300 MPa, hardness of more than 700 HV and total elongation of more than 7.5%.

In an embodiment, the ultra-high strength steel sheet includes less carbon and alloying contents. This improves weldability of the steel, thus making suitable for industrial application such as but not limiting to automotive industry.

In the ultra-high strength steel sheet, martensitic transformation can occur at very low cooling rates, therefore, simple dies could be used for hot forming. Some retained austenite may be present after hot forming, which will improve the elongation and energy absorption during crash.

In an embodiment, the process of customizing to the low temperature heating (<800o C) before hot forming, aids in saving cost of the manufacturing process.

In an embodiment, the method of the present disclosure eliminates the need of coating and annealing, unlike conventional process.

In an embodiment, the method of the present disclosure adapts lesser heating temperature which results in less scale formation, leading to easy cleaning of the final steel.

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 0.24% to about 0.32%. Carbon is a main strengthening element in the steel. Carbon in this range increases hardenability and hardness of martensite phase.

Manganese (Mn) may be added in the range of about 1.5% to about 2%. Manganese may improve hardenability and reduce austenite transformation temperature. This helps in lowering hot forming temperature. At low hot forming temperature, the grain growth after transformation will be low. With lower grain size, the tensile strength and toughness of steel is increased.

Silicon (Si) may be added in the range of about 0.05% to about 0.8%. Silicon helps in reducing bonding during hot rolling.

Chromium (Cr) may be added in the range of about 0.2% to about 0.9%. Chromium helps in increasing hardenability and helps in avoiding unwanted phases during martensite transformation.

Aluminium (Al) may be added in the range of about 0.02% to about 0.2%. Aluminium reduces oxygen and nitrogen from steel melt.

Molybdenum (Mo) may be added in the range of about 0.01% to about 0.6%. Molybdenum may increase hardenability and aids in forming precipitate to increase strength.

Niobium (Nb) may be added in the range of about 0.03% to about 0.08%. Niobium’s carbide precipitation help in controlling the grain size during hot rolling.

Titanium (Ti) may be added in the range of about 0.01% to about 0.05%. Titanium form precipitates to control grain size, reduce solute nitrogen level in steel.

Boron (B) may be added about 0.0005%. Boron helps in increasing hardenability.

Nickel (Ni) may be added about 0.0005%. Nickel helps in improving toughness of the steel.

Sulphur (S) may be added in the range of about 0.01% to about 0.03%. Sulphur increases brittleness of the steel and low level of sulphur is desired.

Nitrogen (N) may be added in the range of about 0.0005% to about 0.02%. Nitrogen increases brittleness of the steel and low level of sulphur is desired.

Phosphorous (P) may be added in the range of about 0.01% to about 0.03%. Phosphorous increases brittleness of the steel and low level of sulphur is desired.

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.

Sample No. C Mn S P Si Al Cu Cr Ni Mo V Nb Ti N B
1 0.27 1.61 0.015 0.019 0.074 0.04 0.009 0.346 0.029 0.332 0.003 0.03 0.003 58 0.0007
2 0.25 1.89 0.009 0.018 0.278 0.06 0.01 0.418 0.03 0.329 0.011 0.02 0.003 77 0.0012
3 0.28 2.05 0.007 0.019 0.452 0.148 0.0121 0.4 0.057 0.412 0.008 0.057 0.0163 87 0.0060
4 0.29 1.84 0.006 0.013 0.393 0.1 0.008 0.418 0.03 0.036 0.002 0.02 0.004 280 0.0029

Table 1

Different compositions of steel (1 to 4) were processed by the method of the present disclosure to obtain ultra-high strength steel. The processed steel may be subjected to hardness testing at each processing step and the hardness of the steel at each processing step is depicted in Figure. 2.
From Figure. 2 it is evident that, the hardness of the steel subjected to conventional process such as annealing, and quenching is substantially less than the hardness of the steel processed by the method of the present disclosure. Further, from Figure. 2 it may be seen that the steel processed by the method of the present disclosure exhibits more hardness, which is about 730 HV.

The steel (having composition 3 ad 4) processed by conventional technique (i.e., cold rolled and annealing), and the steel (having compositions 3 and 4), processed by the method of the present disclosure, may be subjected to testing to determine mechanical properties of the ultra-high strength steel sheet. As an example, tensile testing may be performed as per ASTM standards with E8 sub-size sample having gauge length of 25 mm. Tensile test may be performed in a rigid servo hydraulic 100 T capacity universal tensile testing machine. Results from the tensile test have been illustrated in Figures. 4 to 7.

Figures 4 and 5, illustrates tensile test results of steel subjected to conventional process (i.e., cold rolling and annealing) and having compositions 3 and 4, respectively. As seen in Figure. 4, yield strength obtained is about 624 MPa, Ultimate Tensile strength is about 739 MPa and total elongation is about 17 %. Further, as seen in Figure. 5, the yield strength obtained was 411 MPa, Ultimate Tensile strength obtained 543 MPa and total elongation is 21%. The yield point elongation in these steels indicate that, all carbon in the steel has not been precipitated after annealing.

Figures. 6 and 7, illustrates tensile test results of steel processed by the method of the present disclosure and includes compositions 3 and 4, respectively. As seen in Figure. 6, yield strength obtained is about 1434 MPa, ultimate tensile strength obtained is about 2012 MPa and total elongation is about 6 %. Further as seen in Figure. 7, the yield strength obtained is about 1453 MPa, ultimate tensile strength obtained is about 2008 MPa and total elongation is about 7.3 %. The hot forming process step in the method of the present disclosure aids in producing martensitic microstructure in the steel.

It should be understood that the experiments are carried out for particular compositions of the steel and the results brought out in Figures. 6 and 7 are for the compositions 3 and 4, as 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
101-107 Flowchart blocks
101 Casting stage
102 Heating stage
103 First hot work process
104 First cooling and second cooling process
105 Cold working process
106 Reheating process
107 Hot forming and quenching

Claims:We Claim:
1. A method for manufacturing an ultra-high strength steel sheet, the method comprising:
casting a steel of a composition comprising in weight percentage (wt%) of:
carbon (C) about 0.24% to about 0.32%,
manganese (Mn) at about 1.5% to about 2%,
silicon (Si) at about 0.05% to about 0.8%,
chromium (Cr) at about 0.2% to about 0.9%,
aluminium (Al) at about 0.02% to about 0.2%,
molybdenum (Mo) at about 0.01% to about 0.6%,
Niobium (Nb) at about 0.03% to about 0.08%,
Titanium (Ti) at about 0.01% to about 0.05%,
Boron (B) at about 0.0005% to about 0.005%,
nickel (Ni) at about 0.1% to about 0.8%,
sulphur (S) at about 0.01% to about 0.03%,
phosphorus (P) at about 0.01% to about 0.03%,
nitrogen (N) at about 0.0005% to about 0.02%,
the balance being Iron (Fe) optionally along with incidental elements;
heating, the steel to a first predetermined temperature;
subjecting, the steel to a first hot working process to reduce thickness to form the steel sheet, such that the temperature of the steel sheet at the end of the first hot working process is at a second predetermined temperature;
subjecting, the steel sheet to a first cooling process for a first predetermined time and then to a second cooling process for a second predetermined time to reduce temperature to a third predetermined temperature;
subjecting, the steel sheet to a cold working process to further reduce thickness of the steel sheet;
heating, the steel sheet processed in the cold working process to a fourth predetermined temperature for a third predetermined time;
subjecting, the steel sheet to a hot forming process; and
quenching the steel sheet, to obtain the ultra-high strength steel sheet;
wherein, the ultra-high strength steel sheet exhibits a tensile strength greater than 2000MPa.
2. The method as claimed in claim 1, wherein the ultra-high strength steel sheet comprises primarily a martensitic microstructure and small amount of retained austenite.
3. The method as claimed in claim 1, wherein the ultra-high strength steel sheet exhibits a yield strength greater than 1300 MPa.
4. The method as claimed in claim 1, wherein the ultra-high strength steel sheet exhibits total elongation of greater than 6.5%.
5. The method as claimed in claim 1, wherein the ultra-high strength steel sheet exhibits hardness greater than 700 HV.
6. The method as claimed in claim 1, wherein the first predetermined temperature ranges from about 1050 °C to 1250°C.
7. The method as claimed in claim 1, wherein the first hot working process is a hot rolling process.
8. The method as claimed in claim 7, wherein in the hot rolling process, reduction rate in thickness of the steel is high in initial six passes and reduction rate in thickness is less in subsequent passes.
9. The method as claimed in claim 1, wherein the second predetermined temperature is a finishing rolling temperature ranging from about 700 °C to 950 °C.
10. The method as claimed in claim 1, wherein the first cooling process is water sprinkling process carried out on a run out table and the first predetermined time ranges from about 2 seconds to 10 seconds.
11. The method as claimed in claim 1, wherein the second cooling is a furnace cooling, and the third predetermined temperature is about 200 °C.
12. The method as claimed in claim 1, wherein the second predetermined time is about 24 hours.
13. The method as claimed in claim 1, wherein the cold working process is a cold rolling process.
14. The method as claimed in claim 1, wherein thickness of the steel sheet after the first hot working process is 2.5mm to 6mm, and thickness of the steel sheet after the cold working process is 1mm to 1.8mm.
15. The method as claimed in claim 1, wherein the fourth predetermined temperature ranges from about 750 °C to 950 °C and the third predetermined time ranges from about 2 minutes to 6 minutes.
16. The method as claimed in claim 1, wherein the hot forming process is a hot stamping process, and the quenching is a die quenching process.
17. An ultra-high strength steel sheet, comprising:
composition in weight percentage of:
carbon (C) about 0.24% to about 0.32%,
manganese (Mn) at about 1.5% to about 2%,
silicon (Si) at about 0.05% to about 0.8%,
chromium (Cr) at about 0.2% to about 0.9%,
aluminium (Al) at about 0.02% to about 0.2%,
molybdenum (Mo) at about 0.01% to about 0.6%,
Niobium (Nb) at about 0.03% to about 0.08%,
Titanium (Ti) at about 0.01% to about 0.05%,
Boron (B) at about 0.0005% to about 0.005%,
nickel (Ni) at about 0.1% to about 0.8%,
sulphur (S) at about 0.01% to about 0.03%,
phosphorus (P) at about 0.01% to about 0.03%,
nitrogen (N) at about 0.0005% to about 0.02%,
the balance being Iron (Fe) optionally along with incidental elements;
wherein, the ultra-high strength steel sheet exhibits a tensile strength greater than 2000MPa.
18. The ultra-high strength steel sheet as claimed in claim 17, comprises majorly martensite microstructure and small amount of retained austenite.
19. The ultra-high strength steel sheet as claimed in claim 17, wherein the ultra-high strength steel sheet exhibits a yield strength greater than 1300 MPa.
20. The ultra-high strength steel sheet as claimed in claim 17, wherein the ultra-high strength steel sheet exhibits total elongation of greater than 6.5%.
21. The ultra-high strength steel sheet as claimed in claim 17, wherein the ultra-high strength steel sheet exhibits hardness greater than 700 HV.
22. The ultra-high strength steel sheet as claimed in claim 17, wherein the carbon is preferably less than 0.3%.
23. The ultra-high strength steel sheet as claimed in claim 17, wherein total alloying content is preferably less than 3.5%.
24. An automotive component made of ultra-high strength steel sheet as claimed in claim 17.