Claims:

1. An ultra-high strength high ductile hot-rolled steel sheet, comprising:
composition of:
carbon (C) at about 0.15 wt.% to at about 0.6 wt.%,
manganese (Mn) at about 3.5 wt.% to at about 7.5 wt.%,
sulphur (S) at about 0.001 wt.% to at about 0.003 wt.%,
phosphorous (P) at about 0.001 wt.% to at about 0.002 wt.%,
silicon (Si) at about 0.05 wt.% to at about 0.25 wt.%,
aluminium (Al) at about 0.5 wt.% to at about 1.5 wt.%,
nitrogen (N) at about 0.01 wt.% to at about 0.03 wt.%,
the balance being Iron (Fe) optionally along with incidental elements;

wherein, the ultra-high strength high ductile hot-rolled steel sheet comprises a dual phase lamellar ferrite-austenite microstructure.

2. The ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1, wherein dual phase lamellar ferrite-austenite microstructure comprises an alternate layers of austenite and ferrite.

3. The ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1, wherein the austenite microstructure is about 5 % to about 40 % and remaining being ferrite.

4. The ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1, wherein closed packed plane of the austenite lies parallel to the closed packed plane of the ferrite.

5. The ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1, wherein the microstructure of ultra-high strength high ductile hot-rolled steel sheet forms a twin in austenite when subjected to tensile straining.

6. The ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1, wherein the microstructure of ultra-high strength high ductile hot-rolled steel sheet undergoes a phase transformation from austenite to martensite when subjected to tensile straining.

7. The ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1, wherein the ultra-high strength high ductile hot-rolled steel sheet exhibits a yield strength of about 750 MPa to about 1100 MPa.

8. The ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1, wherein the ultra-high strength high ductile hot-rolled steel sheet exhibits Ultimate Tensile Strength (UTS) of about 950 MPa to about 1550 MPa.

9. The ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1, wherein the ultra-high strength high ductile hot-rolled steel sheet exhibits ductility (% elongation) of about 16 % to about 52 %.

10. A method for manufacturing an ultra-high strength high ductile hot-rolled steel sheet, the method comprising:

casting a steel slab of a composition, comprising:
carbon (C) at about 0.15 wt.% to at about 0.6 wt.%,
manganese (Mn) at about 3.5 wt.% to at about 7.5 wt.%,
sulphur (S) at about 0.001 wt.% to at about 0.003 wt.%,
phosphorous (P) at about 0.001 wt.% to at about 0.002 wt.%,
silicon (Si) at about 0.05 wt.% to at about 0.25 wt.%,
aluminium (Al) at about 0.5 wt.% to at about 1.5 wt.%,
nitrogen (N) at about 0.01 wt.% to at about 0.03 wt.%,
the balance being Iron (Fe) optionally along with incidental elements;

heating, the steel slab to a first predetermined temperature for a first predetermined time;
hot rolling, the steel slab at a second predetermined temperature to produce a steel sheet;
cooling, the steel sheet to a third predetermined temperature;
soaking, the hot rolled steel sheet at fourth predetermined temperature for a second predetermined time; and
cooling, the steel sheet with a predetermined cooling rate to form an ultra-high strength high ductile hot-rolled steel sheet;

wherein, the ultra-high strength high ductile hot-rolled steel sheet comprises a dual phase lamellar ferrite-austenite microstructure.

11. The method as claimed in claim 10, comprises melting of the alloys in an arc melting furnace before casting.

12. The method as claimed in claim 10, wherein the casting is performed in at least one of continuous caster and a thin slab caster.

13. The method as claimed in claim 10, wherein the first predetermined temperature ranges from about 800 ?C to about 1100 ?C, and the first predetermined time from about 2 hours to about 6 hours.

14. The method as claim in claim 10, wherein the hot rolling is performed by a multi-step rolling process in a finish rolling mill and the second predetermined temperature is above austenite finishing temperature (Af).

15. The method as claimed in claim 14, wherein the second predetermined temperature ranges from of about 500 ?C to about 1000 ?C, preferably about 625?C-675?C , more preferably at about 650 ?C.

16. The method as claimed in claim 10, wherein the third predetermined temperature is room temperature.

17. The method as claimed in claim 10, wherein soaking of hot rolled steel sheet at fourth predetermined temperature for a second predetermined time is a short time annealing process.

18. The method as claimed in claim 17, wherein the fourth predetermined temperature employed in the short time annealing process ranges from about 900 °C to about 1200 °C.

19. The method as claimed in claim 17, wherein the second predetermined time employed in the short time annealing process is ranging from about 5 seconds to about 3600 seconds.

20. The method as claimed in claim 10, wherein the cooling of the steel sheet is a quenching process.

21. The method as claimed in claim 20, wherein the predetermined cooling rate is about 5 °C /second to 100 °C/second.

22. The method as claimed in claim 20, wherein the quenching is carried out in at least one of air or water medium.

23. The method as claimed in claim 10, wherein the reduction in thickness of the steel sheet after hot working is about 70 % to 90 %.

24. The method as claimed in claim 10, wherein the dual phase lamellar ferrite-austenite microstructure comprises an alternate layer of austenite and ferrite.

25. The method as claimed in claim 24, wherein the austenite microstructure is about 5 % to about 40 % and remaining being ferrite.

26. The method as claimed in claim 10, wherein the closed packed plane of the austenite lies parallel to the closed packed plane of the ferrite.

27. The method as claimed in claim 10, wherein the microstructure of ultra-high strength high ductile hot-rolled steel sheet exhibits a phase transformation from austenite to martensite when subjected to tensile straining.

28. The method as claimed in claim 10, wherein the ultra-high strength high ductile hot-rolled steel sheet exhibits a yield strength of about 750 MPa to about 1100 MPa.

29. The method as claimed in claim 10, wherein the ultra-high strength high ductile hot-rolled steel sheet exhibits Ultimate Tensile Strength (UTS) of about 950 MPa to about 1550 MPa.

30. The method as claimed in claim 10, wherein the ultra-high strength high ductile hot-rolled steel sheet exhibits ductility of about 16 % to about 52 %.

31. An automotive structural part comprising an ultra-high strength high ductile hot-rolled steel sheet as claimed in claim 1.
, Description:TECHNICAL FIELD

The present disclosure in general relates to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to an ultra-high strength high ductile hot-rolled steel sheet. Further embodiments of the disclosure disclose a method for manufacturing the ultra-high strength high ductile hot-rolled steel sheet.

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 may be considered as a major component in wide variety of applications. Some of the applications of the steel may include buildings, ships, 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 heat treatment 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.

Conventionally, application of ultra-high strength steel is limited to military and railroad application with its limited ductility. Researchers and technologists have been exploring the possibility of developing ultra-high ductile-high strength steel for the last several years. Manganese steels may be one such important class of steels which are known for their excellent combination of ductility and strength. The steels containing low to high amount of Mn (3.5 wt.% to 7.5 wt. %), low to high amount of carbon (0.15 wt.% to 0.60 wt. %) and silicon (<0.5 wt.%) may be essential for desirable strength and elongation. However, the steel containing high amount of Mn i.e. high Mn steel may suffer from various drawbacks such as delayed cracking and melting of Mn. The high manganese content in steel further limits its mass industrial and commercial applications due to constraints associated with processing and higher production costs. Hence, it is very important to reduce Mn content for real application of this grade of steels. From the microstructural point of view, medium manganese steel consists of austenite and ferrite microstructure. It is well established that the austenite content in the medium manganese steel transforms into martensite under various compressive and tensile loading conditions. It is also known that twin and dislocation interaction may occur in the stable austenite under the compressive and tensile loading process. These are fundamental aspect of achieving the high strength and high ductility in Transformation induced plasticity (TRIP) or Twinning-Induced Plasticity (TWIP) steel. This transformation certainly is the key factor for achieving high strength at the cost of ductility.

Although the medium Mn steels have very high strength and moderate ductility, reduction of Mn in steel may reduce the fraction of austenite in two phase microstructure at room temperature leading to deterioration in mechanical properties. Therefore, additional deformation mechanism may be highly desirable with changing the chemistry to achieve sufficient ductility. In one of the conventional arts, various thermo-mechanical routes have been employed for the production of medium manganese steel. But stabilization of microstructure with low amount of manganese still remains as a challenging task due to ill effects such as manganese segregation at grain boundaries, inappropriate Mn partitioning between the phases, inhomogeneities etc. which affect the final microstructure and thereby the mechanical properties. In such cases, additional heat treatments may be needed to perform for redistributing manganese so as to tailor the amount, morphology and stability of the retained austenite. These additional processing steps make the whole process cumbersome and less economical during mass production.

Hence, there is a need for an economically attractive and technically viable way of developing ultra-high strength medium manganese steel with high ductility without aforementioned limitations.

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 and a product 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 a non-limiting embodiment of the present disclosure, there is provided an ultra-high strength high ductile hot-rolled steel sheet. The steel sheet includes composition of carbon (C) at about 0.15 wt.% to at about 0.6 wt.%, manganese (Mn) at about 3.5 wt.% to at about 7.5 wt.%, sulphur (S) at about 0.001 wt.% to at about 0.003 wt.%, phosphorous (P) at about 0.001 wt.% to at about 0.002 wt.%, silicon (Si) at about 0.05 wt.% to at about 0.25 wt.%, aluminium (Al) at about 0.5 wt.% to at about 1.5 wt.%, nitrogen (N) at about 0.01 wt.% to at about 0.03 wt.%, the balance being Iron (Fe) optionally along with incidental elements. The ultra-high strength high ductile hot-rolled steel sheet comprises a dual phase lamellar ferrite-austenite microstructure.

In an embodiment, the dual phase lamellar ferrite-austenite microstructure comprises an alternate layers of austenite and ferrite. Further, the austenite microstructure is about 5 % to about 40 % and remaining being ferrite with the closed packed plane of the austenite lying parallel to the closed packed plane of the ferrite.

In an embodiment, the microstructure of ultra-high strength high ductile hot-rolled steel sheet forms a twin in austenite when subjected to tensile straining. Further, the microstructure of ultra-high strength high ductile hot-rolled steel sheet undergoes a phase transformation from austenite to martensite when subjected to tensile straining.

In an embodiment, the ultra-high strength high ductile hot-rolled steel sheet exhibits a yield strength of about 750 MPa to about 1100 MPa.

In an embodiment, the ultra-high strength high ductile hot-rolled steel sheet exhibits Ultimate Tensile Strength (UTS) of about 950 MPa to about 1550 MPa.

In an embodiment, the ultra-high strength high ductile hot-rolled steel sheet exhibits ductility (% elongation) of about 16 % to about 52 %.

In another non-limiting embodiment of the present disclosure, there is provided a method for manufacturing an ultra-high strength high ductile hot-rolled steel sheet. The method includes steps of firstly casting a steel slab of a composition comprising carbon (C) at about 0.15 wt.% to at about 0.6 wt.%, manganese (Mn) at about 3.5 wt.% to at about 7.5 wt.%, sulphur (S) at about 0.001 wt.% to at about 0.003 wt.%, phosphorous (P) at about 0.001 wt.% to at about 0.002 wt.%, silicon (Si) at about 0.05 wt.% to at about 0.25 wt.%, aluminium (Al) at about 0.5 wt.% to at about 1.5 wt.%, nitrogen (N) at about 0.01 wt.% to at about 0.03 wt.%, the balance being Iron (Fe) optionally along with incidental elements. Then, subjecting the steel slab to heating in a first predetermined temperature for a first predetermined time and performing hot rolled at a second predetermined temperature to produce a steel sheet. Subsequently, the steel sheet is cooled to a third predetermined temperature. The method further involves soaking of the hot rolled steel sheet at fourth predetermined temperature for a second predetermined time. Finally, the steel sheet is cooled with a predetermined cooling rate to form an ultra-high strength high ductile hot-rolled steel sheet. The ultra-high strength high ductile hot-rolled steel sheet so obtained comprises a dual phase lamellar ferrite-austenite microstructure.

In an embodiment, melting of the alloys in an arc melting furnace before casting.

In an embodiment, the casting is carried out in a continuous casting process. The continuous casting process is performed in at least one of continuous caster and a thin slab caster.

In an embodiment, the first predetermined temperature ranges from about 800 ?C to about 1100 ?C, and the first predetermined time from about 2 hours to about 6 hours.

In an embodiment, the hot rolling is performed by a multi-step rolling process in a finish rolling mill and the second predetermined temperature is above austenite finishing temperature (Af). Further, second predetermined temperature ranges from of about 500 ?C to about 1000 ?C, preferably about 625?C-675?C, more preferably at about 650 ?C.

In an embodiment, the third predetermined temperature is room temperature.
In an embodiment, soaking of hot rolled steel sheet at fourth predetermined temperature for a second predetermined time is a short time annealing process.

In an embodiment, the fourth predetermined temperature employed in the short time annealing process ranges from about 900 °C to about 1200 °C.

In an embodiment, the second predetermined time employed in the short time annealing process is ranging from about 5 seconds to about 3600 seconds.

In an embodiment, the cooling of the steel sheet is a quenching process and the quenching is carried out in at least one of air or water medium.

In an embodiment, the predetermined cooling rate of about 5 °C/second to 100°C/second

In an embodiment, the reduction in thickness of the steel sheet after hot working is about 70 % to 90 %.

In yet another non-limiting embodiment, automotive part comprising automotive structural part comprising an ultra-high strength high ductile hot-rolled steel sheet as per the above composition is disclosed.

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 producing ultra-high strength high ductile hot-rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figure.2 is a graphical representation of thermo-mechanical processing profile for producing the ultra-high strength high ductile hot-rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figures. 3a, 3b and 3c illustrates the Scanning Electron Microscopy (SEM) images of the ultra-high strength high ductile hot-rolled steel sheet with varying compositions at a magnification of 20,000X, according to an exemplary embodiment of the present disclosure.

Figure. 4 illustrates a graphical representation of results of X-ray Diffraction analysis performed on the ultra-high strength high ductile hot-rolled steel sheet prior to tensile straining, according to an exemplary embodiment of the present disclosure.
Figure. 5 illustrates a graphical representation of results of X-ray Diffraction analysis performed on the ultra-high strength high ductile hot-rolled steel sheet after tensile straining, 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 high ductile hot-rolled steel sheet and a method for manufacturing the ultra-high strength high ductile hot-rolled steel sheet. Strength and ductility are the important properties for the mass industrial application of high strength materials like steel. As of now, high strength and high ductile steels are produced as Transformation induced plasticity (TRIP) steel or Twinning-Induced Plasticity (TWIP) steel with high Mn content. However, high manganese content in TWIP/TRIP steel limits its mass industrial and commercial applications due to constraints associated with processing and higher production cost. Accordingly, the present disclosure provides a method for manufacturing the ultra-high strength high-ductile hot-rolled steel sheet with medium manganese content through a thermo-mechanical processing route. The fine-grained medium manganese hot-rolled steel sheet obtained through thermo-mechanical processing route exhibits Ultimate Tensile Strength (UTS) of about 950 MPa to about 1550 MPa along with ductility (percentage elongation) of about 16 % to about 52 %. The hot rolled steel sheet may be widely employed to make automotive structural components requiring high strength, high ductility, and easy formability.

In majority of the industrial and commercial applications, steels with twin formation in the microstructure are preferred due to their enhanced mechanical properties. The mechanical properties include but are not limited to strength, ductility, torsion, hardness and toughness. The present disclosure forms a twin in stable austenite when subjected to tensile straining. This improves strength and ductility of the steel. Further, phase transformation of austenite to martensite adds up to improvement in strength and ductility.

The method of manufacturing an ultra-high strength high ductile hot-rolled steel, includes first step of producing the steel slab of composition comprising in weight percentage of: carbon (C) at about 0.15 wt.% to at about 0.6 wt.%, manganese (Mn) at about 3.5 wt.% to at about 7.5 wt.%, sulphur (S) at about 0.001 wt.% to at about 0.003 wt.%, phosphorous (P) at about 0.001 wt.% to at about 0.002 wt.%, silicon (Si) at about 0.05 wt.% to at about 0.25 wt.%, aluminium (Al) at about 0.5 wt.% to at about 1.5 wt.%, nitrogen (N) at about 0.01 wt.% to at about 0.03 wt.%, the balance being Iron (Fe) optionally along with incidental elements by any manufacturing process including but not limiting to casting. The steel slab is then reheated to a temperature of about 800 °C to 1100 °C for about 2 hours to 6 hours. The steel slab may be then subjected to hot working process including but not limited to hot-rolling process. The hot charged steel slab may be hot-rolled in finishing mill. The finish rolling temperature may be above austenite finishing temperature (Af) i.e. around 500 °C to 1000 °C, more preferably at around 650 °C. After the hot rolling step, the steel sheet may be cooled to room temperature. Hot-rolled steel sheet may be subjected to heat treatment process such as but not limited to short time annealing process and soaked at about 900 °C to 1200 °C for about 5 seconds to 3600 seconds. Then, the steel sheet may be cooled at the predetermined cooling rate of about 5 °C/second to 100 °C/second to obtain ultra-high strength high ductile hot-rolled steel. The hot-rolled steel sheet according to the present disclosure may have a microstructure comprises austenite about 5% to about 40 % and remaining being ferrite microstructure.

As an example, the application may include 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 high ductile hot-rolled steel. 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.

Figures. 1 and 2 are exemplary embodiments of the present disclosure illustrating a flowchart of a method for producing ultra-high strength high ductile hot-rolled steel, and a graphical representation of thermo-mechanical processing route for producing ultra-high strength high ductile hot-rolled steel sheet. In the present disclosure, mechanical properties such as strength, ductility of the final microstructure of the steel may be improved. The steel produced by the method of the present disclosure, includes austenite-ferrite 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 ultra-high strength high ductile hot-rolled steel and it may also be extended to other type of steels as well.

The method of manufacturing the ultra-high strength high ductile hot-rolled steel according to the present disclosure consists of a casting step followed by a hot rolling step, a controlled cooling step using a steel material which satisfies the component composition described below. The various processing steps are described in their respective order below:

At block 101, a steel of desired alloy composition is formed by any of the manufacturing process including but not limited to casting process. In embodiment, the steel is made in the form of slabs, and the alloy may be prepared in a high temperate furnace but not limited to air-melting furnace. The steel slab may have composition of: carbon (C) at about 0.15 wt.% to at about 0.6 wt.%, manganese (Mn) at about 3.5 wt.% to at about 7.5 wt.%, sulphur (S) at about 0.001 wt.% to at about 0.003 wt.%, phosphorous (P) at about 0.001 wt.% to at about 0.002 wt.%, silicon (Si) at about 0.05 wt.% to at about 0.25 wt.%, aluminium (Al) at about 0.5 wt.% to at about 1.5 wt.%, nitrogen (N) at about 0.01 wt.% to at about 0.03 wt.%, the balance being Iron (Fe) optionally along with incidental elements may be casted in at least one of continuous caster and a thin slab caster. When casted in a thin slab caster, the temperature of the cast slab may not be allowed to drop to a temperature below 1000 °C, as it might result in loss of ductility and cause formation of edge cracks.

The method then includes the step of heating as shown in block 102. After casting the steel slab with the specified composition, the slabs may be reheated in a furnace to a first predetermined temperature for a first predetermined time. In an embodiment, the steel slab may be hot charged into the furnace for heating, and the first predetermined temperature may be preferably in the range of 800 °C to 1100 °C to ensure complete dissolution of all elements and precipitates that have formed in the preceding processing steps, and the first predetermined time ranges from 2 hours to about 6 hours. A first predetermined temperature greater than 1100 °C may also not desirable because it may lead to grain coarsening of austenite and/or excessive scale loss.

The method further includes a step or a stage of hot working the steel slab by a hot rolling process [shown in block 103] immediately after heating. 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 steel slab may be hot-rolled in finishing mill. The finish rolling temperature may vary above austenite finishing temperature (Af) i.e. around 500 °C to 1000 °C, preferably about 625?C-675?C, more preferably at about 650 °C. During hot rolling, the hot slab may be subjected but not limited to 15 to about 20 number of passes in the finishing mill. After completion of hot rolling process, the hot-rolled steel sheet may be cooled to a third predetermined temperature which is generally room temperature. As shown in block 104, the cooling of the hot-rolled sheet to room temperature may be carried out by normal air cooling. In an embodiment, reduction in thickness of the steel sheet after hot working may be about 70 % to 90 %.

The method further includes soaking of hot-rolled steel sheet at a fourth predetermined temperature for a second predetermined time as show in block 105. Soaking may be carried out in a short time annealing process at a temperature ranging from about 900 °C to 1100 °C for about 5 seconds to about 3600 seconds. Short time annealing is used annealing process in modern steel heat treatment process. Annealing is the process of relieving the internal stresses in the steel that may be built up during the hot rolling process. Steel sheet hardens after hot rolling due to the dislocation tangling generated by plastic deformation. Annealing is therefore carried out to soften the material. The annealing process comprises heating, holding of the material at an elevated temperature (soaking). Heating facilitates the movement of atoms, resulting in the disappearance of tangled dislocations and the formation and growth of new grains of various sizes, which depend on the heating and soaking conditions. These phenomena make hardened steel crystals to recover and recrystallize into softened one. Furthermore, during annealing process precipitates decompose to solute atoms which subsequently dissolve and redistribute into the steel microstructure on heating and holding to get homogenous microstructure.

Now referring to block 106, the method further includes final the step of cooling. Cooling is performed through a quenching process. Quenching is a rapid way of bringing steel back to room temperature after heat treatment to prevent the cooling process which dramatically changing the steel’s microstructure. Cooling during quenching may be carried out at a predetermined rate of 5 °C/second to 100 °C/second. Quenching may be performed including but not limited to air or water medium. Quenching may bed performed to retain austenite and ferrite microstructure at room temperature by freezing the diffusion and rearrangement of carbon and iron atoms. Further, quenching performed from a temperature of about 900 °C to 1100 °C at a rate of rate of 5 °C/second to 100 °C/second may bypass and avoid the formation of stable pearlite and bainite microstructure.

A graphical representation of thermo-mechanical processing profile for producing ultra-high strength high ductile hot-rolled steel sheet is shown in Figure. 2. This ensures that the dual phase microstructure consists austenite of about 5 % to about 40 % and remaining being ferrite. In dual phase microstructure austenite and ferrite phase may be arranged as lamellae with alternative layers of austenite and ferrite phase arranged together. The two phases may follow an orientation relationship such that the closed packed plane of austenite phase remains parallel to the closed packed plane of ferrite phase.

Austenite is the normal phase of steel at high temperatures, but not at room temperature. Because retained austenite exists outside of its normal temperature range, it is metastable. This clearly implies that when given the opportunity, it may change or transform from austenite into martensite. Hence, in order to obtain the ultra-high strength, transformation of austenite to martensitic hard-phase during tensile straining may be considered as a key factor in the strengthening mechanism. This effect is known as Transformation Induced Plasticity (TRIP). Further, during straining dislocation may get a chance to interact with twin contained retained austenite. As the amount of strain increases, there is a possibility that the dislocation will directly interact with the nano-twin boundaries. This may contribute more towards getting high ductility. This resembles to the Twinning-Induced Plasticity (TWIP) effect. Hence, in an embodiment of the present disclosure, microstructure consists austenite microstructure of about 5 % to about 40 % and remaining being ferrite microstructure leading to ultra-high-strength of about 950 MPa to about 1550 MPa and high ductility (% elongation) of about 16 % to 52 % due to TRIP/TWIP effect.

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): Carbon is an inherent component in steel, carbon helps in strengthening phases, and is often considered as a cheaper element to increase strength. Strength will not be achieved with carbon less than 0.02 wt.%. Carbon is an austenite stabilizer and carbon enrichment in the austenite also results in a stronger martensite formation. Excessive carbon may promote carbide precipitates in the interfaces which may affect the mechanical properties as wells as creates problems during welding. Hence, carbon content may be limited to 0.15 wt.% to about 0.6 wt.%.

Manganese (Mn) may be used in the range of 3.5 wt.% to 7.5 wt.%. Manganese is an austenite stabilizer. Hence, manganese not only imparts solid solution strengthening to the ferrite, but it also lowers the austenite to ferrite transformation temperature. Lower content of manganese may retain the toughness and lower the possibility of carbide formation aiming to produce carbide free matrix to improve the ductility.

Sulphur (S) may be set to about 0.001 wt.% to about 0.003 wt.%. Sulphur levels may be reduced as far as possible to limit the formation of MnS particles, which are sufficiently brittle to form long stringers elongated in the rolling direction and adversely affect transverse strength, ductility, edge flangeability and bendability.

Phosphorus (P) may be added about 0.001 wt.% to about 0.002 wt. % maximum. Phosphorus content may be restricted to lower extent as higher phosphorus levels may lead to reduction in mechanical properties due to segregation of phosphorus into grain boundaries.

Silicon (Si) may be used minimum in the range of 0.05 wt.% to 0.25 wt.%. Silicon suppress the carbide formation in the steel and leads to carbide free matrix which eventually improve the ductility and impact toughness of the steel. Further, silicon is a very cheap solid solution strengthening element and it has more solid solution strengthening potential than like manganese. However, Si content beyond 0.25 wt.% promotes formation of scales during high temp soaking and which is often undesirable by the end customers.

Aluminium (Al) may be added in the range of 0.5 wt.% to 1.5 wt. %. Aluminium is used as a deoxidizer. It limits growth of austenite grains and leads to fine-grained austenite microstructure. Higher amount of Al causes casting issues and hence, should be restricted.

Nitrogen (N) may be added in the arrange of 0.01 wt.% to about 0.03 wt.%. Nitrogen is known to be a strong austenite stabilizer.

Example:

Steel Chemical composition (wt.%)
C Mn Al Si P S N Fe
1 0.2 6 1.0 0.2 0.002 0.003 0.006 92.389
2 0.3 6 1.0 0.2 0.002 0.003 0.006 92.289
3 0.5 6 1.0 0.2 0.002 0.003 0.006 92.089

Table-1

Further embodiments of the present disclosure will now be described with examples of particular composition of the steel. Experiments have been carried out for various set of specific composition of the steel by using method of the present disclosure. The compositions of the steel samples (steel-1 to steel-3) for which the tests are carried out is as shown in above table 1. The compositions of table 1 were continuously cast in a slab caster and the slabs were subjected to thermo-mechanical processing involving hot-rolling, a short time annealing and cooling to room temperature to get a dual phase microstructure of consisting austenite of about 5 % to about 40 % and balance being ferrite.

Referring to figures 3a, 3b and 3c which are exemplary embodiments of the present disclosure, illustrating the Scanning Electron Microscopy (SEM) images of the ultra-high strength high ductile hot-rolled steel sheet with varying composition at a magnification of 20,000X. During SEM analysis, high energy beam (25 kV) of electrons may be made to irradiate through a very thin steel sheet sample. The electron beams may obtain due to the interactions between the electrons and the atoms present in the steel sample are collected to provide the details about the microstructure with high magnification and resolution. Figure 3a depicts the microstructure of steel-1 reveling austenite and ferrite phase arranged in lamellar manner with large amount of ferrite phase (white region) and less amount of austenite phase (grey region). Figure 3b shows the microstructure of steel-2 with increased amount of austenite phase. Further, both austenite and ferrite form fine grained microstructure with well distributed austenite and ferrite phase which in turn may enhance the tensile strength and ductility. Figure 3c illustrates the aggregate microstructure of austenite and ferrite with alternative layer of austenite and ferrite phase are arranged together. Further, TEM examination confirmed the presence of twins in the steel. These twins may lead to enhanced mechanical properties due to TWIP on tensile straining. This dual phase TWIP steel not only present excellent strength, but also have outstanding ductility due to twinning.

Referring to figures 4, which is an exemplary embodiment of the present disclosure, illustrating a graphical representation of the results of XRD analysis carried out on the steel sample prior to tensile straining. This test method may be performed by directing an x-ray beam at a sample and measuring the diffracted x-ray beam intensity as a function of the diffracted angle. Planes oriented at different angle may diffract the x-rays at that particular angle. Position of the planes and their intensity respectively may reveal the information about the phase and amount of phase present in the microstructure. In XRD profile, set of planes (111) and (200) corresponding to austenite phase. Steel-2 has a very prominent, high intense (111) and (200) peaks indicating high amount of austenite in the microstructure. Steel-3 exhibits (111) and (200) peaks with decreased intensity indicating reduced amount of austenite compare to steel-3. Further, steel-1 shows very low intensity for (111) and (200) peaks indicating very low fraction of austenite in the microstructure.

Steel Yield strength
MPa Ultimate tensile strength
MPa Ductility
(%) elongation
1 900 955 50
2 1100 1550 52
3 750 1040 16
Table-2

In an embodiment of the present disclosure, tensile straining for all the steel samples were carried out in tensile tester machine. A tensile test may involve mounting the specimen in a machine, such as it is subjecting it to constant strain. A sample having dimensions of 6 mm gauge length, 2 mm width and 0.8 mm thickness was prepared from each set of composition (Steel 1 to Steel 3). A constant strain of rate of 0.001/second is applied at room temperature. When a solid material is subjected to small straining, the bonds between the atoms are stretched and material undergoes plastic deformation revealing information regarding yield strength, ultimate tensile strength and ductility. For each set of compositions (Steel 1 to Steel 3), obtained yield strength (YS), ultimate tensile strength (UTS), ductility (% elongation) values are also tabulated in table 2.

In an embodiment of the present disclosure, when austenite containing dual phase steel is subjected to tensile straining, retained austenite phase may transform into martensite phase. This martensitic transformation is a non-diffusional mechanism that may be completely independent of time and dependent only on strain-induced at ambient temperatures. The martensite embryos may form at new nucleation sites generated by the deformation. The occurrence of this phase transformation may be closely linked to the plastic deformation (the martensite transformation kinetics as a function of the plastic strain) takes place in the austenite phase, which in turn may be connected to the stacking fault energy (SFE) of the steel.

In another embodiment, the present disclosure steel-2 exhibits superior yield strength, ultra-high tensile strength and high ductility due to high fraction of fine grained austenite (as evidenced in figure 4) which may transform into large fraction of martensite due to TRIP mechanism on tensile straining and hence mechanical properties may be enhanced (as evidenced in table 2). Steel-1 may exhibit decreased mechanical properties due to reduce of amount austenite and there by reduced fraction of martensite during TRIP on tensile straining. Furthermore, steel-3 has low mechanical properties due to low amount of austinite and hence low fraction of martensite after tensile straining leading. Further, as the amount of strain increases during tensile straining, large amount of dislocations may be generated and they may interact with twins present in the austenite phase. This TWIP effect further contributes towards getting high ductility. Therefore, the combined TRIP and TWIP effects may lead to the formation of the ultra-high tensile strength of about 950 MPa to 1550 MPa and ductility of about 16 % to about 52 %.

Referring to figures 5, which are exemplary embodiments of the present disclosure, illustrating graphical representation of the results of XRD analysis carried out on the steel sample after tensile straining. The fraction of martensite formed during the application of tensile straining may also depends on the response of the material during metal forming process such as but not limited to hot rolling process. High amount of hot rolling may induce excessive work hardening in austenite and thereby leading to higher amount of transformation to yield martensite phase i.e. higher the hot rolling, higher may be the amount of martensite formed and superior may be the mechanical properties. As an evidence, steel without hot rolling (starting of hot rolling) exhibits some fraction of austenite even after tensile straining. Presence of (111) and (200) peaks in figure 5 may confirm the presence of austenite which is not being transformed into martensite. In the steel which is subjected to 20 % hot rolling, peak at (111) and (200) corresponding to austenite phase decreased to large extent after tensile straining indicating the increased transformation of austenite to martensite Furthermore, in steel which is subjected to 40 % rolling, peak at (111) and (200) are completely disappeared after tensile straining indicating complete transformation of austenite into martensite imparting ultra-high tensile strength and ductility to the hot rolled steel sheet.

In an embodiment of the present disclosure, the ultra-high strength high-ductile hot rolled steel sheet of the present disclosure may be used any application including but not limiting to automotive applications to manufacture structural components like chassis, pillars, outer and inner panels, suspension parts and the like. The ultra-high strength high-ductile hot rolled steel sheet may be used in any other industrial structural applications.

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
101-106 Flowchart blocks
101 Casting stage
102 Heating stage
103 Hot working stage
104 Cooling stage
105 Soaking and cooling
106 Cooling state