Claims:1. A method for manufacturing high-strength ductile steel, the method comprising:
soaking, a steel comprising of a composition of:
Carbon (C) at about 0.4 wt% to about 0.42 wt%;
Manganese (Mn) at about 0.5 wt% to about 1wt %;
Sulphur (S) up-to 0.02 wt%;
Phosphorus (P) up-to 0.02 wt%;
Nitrogen (N) up-to 0.01 wt%;
Silicon (Si) at about 1.5 wt% to about 1.7 wt %;
Vanadium (V) at about 0.05 wt% to about 0.08 wt%;
Aluminum (Al) at about 0.1 wt% to about 0.3 wt %;
Chromium (Cr) at about 0.8 wt% to about 1 wt%;
Nickel (Ni) at about 2.8 wt% to about 3 wt %;
Cobalt (Co) at about 1.4 wt% to about 1.5 wt%;
Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%;
Copper (Cu) at about 0.2 wt% to about 0.3 wt%;
the balance being Iron (Fe) optionally along with incidental elements, at a first pre-determined temperature for a first pre-set period of time;
hot working, on the steel by a first hot working process, and cooling the steel;
re-heating, the steel to the first pre-determined temperature, and annealing the steel in the first predetermined temperature for a second pre-set period of time;
hot working, on the steel by a second hot working process;
isothermal quenching of the steel at a second predetermined temperature, and cooling the steel to a room temperature;
subjecting, the steel to a first tempering process at a third predetermined temperature for a third pre-set period of time and cooling the steel to a room temperature;
cold working, on the steel; and
subjecting, the steel to a second tempering process at fourth predetermined temperature for a fourth pre-set period of time to produce the high-strength ductile steel;

wherein, the high strength, ductile steel primarily comprises a martensitic microstructure.

2. The method as claimed in claim 1, wherein the high-strength, ductile steel exhibits ultimate tensile strength ranging from about of 2537 MPa to 2870 MPa.

3. The method as claimed in claim 1, wherein the high-strength ductile steel exhibits yield strength ranging from about of 2363 MPa to 2781 MPa.

4. The method as claimed in claim 1, wherein the high-strength ductile steel exhibits ductility ranging from about 5% to about 8%.

5. The method as claimed in claim 1, wherein the high-strength ductile steel comprises a martensitic microstructure and a retained austinite microstructure, and wherein during the cold working the steel experiences shear forces which augments a fraction of retained austinite microstructure to be transformed into martensitic microstructure.

6. The method as claimed in claim 1, wherein the re-heating, soaking, and first and second tempering processes are carried out in a furnace.

7. The method as claimed in claim 1, wherein the cooling is normal air cooling.

8. The method as claimed in claim 1, wherein the isothermal quenching is carried out in a hot water bath.

9. The method as claimed in claim 8, wherein the steel is maintained in the hot water bath for about 5 minutes to achieve equilibrium temperature.

10. The method as claimed in claim 1, wherein the first pre-determined temperature is about 1200 ºC.

11. The method as claimed in claim 1, wherein the second pre-determined temperature is about 100ºC.

12. The method as claimed in claim 1, wherein the third pre-determined temperature is about 160ºC.

13. The method as claimed in claim 1, wherein the fourth predetermined temperature ranges from about 200°C to 500°C.

14. The method as claimed in claim 1, wherein the first pre-set period of time is about 3 hours.

15. The method as claimed in claim 1, wherein the second pre-set period of time is about 45 minutes.

16. The method as claimed in claim 1, wherein the third pre-set period of time is about 24 hours.

17. The method as claimed in claim 1, wherein the fourth pre-set period of time ranges from about 2 hours to 5 hours.

18. The method as claimed in claim 1, wherein the first hot working process is a forging process.

19. The method as claimed in claim 1, wherein the second hot working process is a hot rolling process.

20. The method as claimed in claim 19, wherein the hot rolling process is carried out at least five to six times on the steel, and wherein, the temperature of the steel drops to a range of about 900ºC to about 950ºC during the hot rolling process.

21. The method as claimed in claim 1, wherein the cold working process is a cold rolling process.

22. The method as claimed in claim 1, wherein the method comprises, cooling the steel to a room temperature in air after the second tempering process.

23. The method as claimed in claim 1, wherein the second tempering process results in stress reliving in the steel, and reversal of minute amount austenite.

24. The method as claimed in claim 1, wherein the reheating is carried out in an inert atmosphere.

25. The method as claimed in claim 1, wherein the method comprises grinding and cleaning the steel before cold working.

26. The method as claimed in claim 1, wherein the steel is produced by casting alloy in an air-induction furnace.

27. A high strength ductile steel, comprising:
Carbon (C) at about 0.4 wt% to about 0.42 wt%;
Manganese (Mn) at about 0.5 wt% to about 1wt %;
Sulphur (S) up-to 0.02 wt%;
Phosphorus (P) up-to 0.02 wt%;
Nitrogen (N) up-to 0.01 wt%;
Silicon (Si) at about 1.5 wt% to about 1.7 wt %;
Vanadium (V) at about 0.05 wt% to about 0.08 wt%;
Aluminum (Al) at about 0.1 wt% to about 0.3 wt %;
Chromium (Cr) at about 0.8 wt% to about 1 wt%;
Nickel (Ni) at about 2.8 wt% to about 3 wt %;
Cobalt (Co) at about 1.4 wt% to about 1.5 wt%;
Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%;
Copper (Cu) at about 0.2 wt% to about 0.3 wt%;
the balance being Iron (Fe) optionally along with incidental elements;
wherein, the high strength-ductile steel primarily comprises a martensitic microstructure.

28. The high strength ductile steel as claimed in claim 27, wherein during the cold working the steel experiences shear forces which augments a fraction of retained austinite microstructure to be transformed into martensitic microstructure.

29. The high strength ductile steel as claimed in claim 27, wherein the high strength-ductile steel exhibits ultimate tensile strength ranging from about of 2537 MPa to 2870 MPa.

30. The high strength ductile steel as claimed in claim 27, wherein the high strength-ductile steel exhibits yield strength ranging from about of 2363 MPa to 2781 MPa.

31. The high-strength high-ductile steel as claimed in claim 27, wherein the high strength-ductile steel exhibits ductility ranging from about 5% to about 6%.

32. The high-strength high-ductile steel as claimed in claim 27, wherein the abrasive wear loss of the steel varies in the range of 0.124 – 0.144g as per ASTM G 65-04

33. A method for manufacturing high strength-ductile steel, the method comprising:
casting a steel of composition comprising:
Carbon (C) at about 0.4 wt% to about 0.42 wt%;
Manganese (Mn) at about 0.5 wt% to about 1wt %;
Sulphur (S) up-to 0.02 wt%;
Phosphorus (P) up-to 0.02 wt%;
Nitrogen (N) up-to 0.01 wt%;
Silicon (Si) at about 1.5 wt% to about 1.7 wt %;
Vanadium (V) at about 0.05 wt% to about 0.08 wt%;
Aluminum (Al) at about 0.1 wt% to about 0.3 wt %;
Chromium (Cr) at about 0.8 wt% to about 1 wt%;
Nickel (Ni) at about 2.8 wt% to about 3 wt %;
Cobalt (Co) at about 1.4 wt% to about 1.5 wt%;
Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%;
Copper (Cu) at about 0.2 wt% to about 0.3 wt%;
the balance being Iron (Fe) optionally along with incidental elements;
subjecting, the steel to a roughing process, and quenching the steel;
hot working on the steel by a second hot working process;
subjecting the steel to a first tempering at a third predetermined temperature for a third pre-set period of time;
cold working, on the steel; and
subjecting, the steel to a second tempering at fourth predetermined temperature for a fourth pre-set period of time to produce the high-strength ductile steel;

wherein, the high strength-ductile steel primarily comprises a martensitic microstructure.

34. The method as claimed in claim 33, wherein the high strength ductile steel exhibits ultimate tensile strength ranging from about of 2537 MPa to 2870 MPa.

35. The method as claimed in claim 33, wherein the high strength ductile steel exhibits yield strength ranging from about of 2363 MPa to 2781 MPa.

36. The method as claimed in claim 33, wherein the high strength ductile steel exhibits ductility ranging from about 5% to about 8%.

37. The method as claimed in claim 33, wherein the high-strength ductile steel comprises a martensitic microstructure and a retained austinite microstructure, and wherein during the cold working the steel experiences shear forces which augments a fraction of retained austinite microstructure to be transformed into martensitic microstructure.

38. A structural part of a machinery and automobile comprising a high-strength ductile steel as claimed in claim 27.
, 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 a high strength ductile steel. Further embodiments of the disclosure disclose a method for manufacturing the high strength- ductile steel which exhibits ~2.7 GPa of yield strength and ~2.9 GPa of ultimate tensile strength with reasonable ductility.

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 with greater factor of safety specially for automotive and defense applications is limited due to chances of catastrophic failures. This is due to the lack in achieving super high yield strength of the steel in combination with ductility.

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, a method for manufacturing high strength-ductile steel is disclosed. The method comprising soaking, a steel comprising of a composition of: Carbon (C) at about 0.4 wt% to about 0.42 wt%, Manganese (Mn) at about 0.5 wt% to about 1wt %, Sulphur (S) up-to 0.02 wt%, Phosphorus (P) up-to 0.02 wt%, Nitrogen (N) up-to 0.01 wt%, Silicon (Si) at about 1.5 wt% to about 1.7 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.1 wt% to about 0.3 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 2.8 wt% to about 3 wt %, Cobalt (Co) at about 1.4 wt% to about 1.5 wt%, Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%, Copper (Cu) at about 0.2 wt% to about 0.3 wt%, the balance being Iron (Fe) optionally along with incidental elements, at a first pre-determined temperature for a first pre-set period of time. Further, hot working, on the steel by a first hot working process, and cooling the steel. Then, re-heating, the steel to the first pre-determined temperature, and annealing the steel in the first predetermined temperature for a second pre-set period of time. The method further includes hot working, on the steel by a second hot working process, and isothermal quenching of the steel at a second predetermined temperature and cooling the steel to a room temperature. Then subjecting, the steel to a first tempering process at a third predetermined temperature for a third pre-set period of time and cooling the steel to a room temperature and cold working, on the steel. The method further includes subjecting, the steel to a second tempering process at fourth predetermined temperature for a fourth pre-set period of time to produce the high-strength ductile steel, wherein, the high strength, ductile steel primarily comprises a martensitic microstructure.

In an embodiment, the high-strength, ductile steel exhibits ultimate tensile strength ranging from about of 2537 MPa to 2870 MPa. Further, the high-strength high-ductile steel exhibits high yield strength ranging from about of 2363 MPa to 2781 MPa. Also, the high-strength ductile steel exhibits ductility ranging from about 5% to about 6%.

In an embodiment, the high-strength ductile steel comprises a martensitic microstructure and a retained austinite microstructure, and wherein during the cold working the steel experiences shear forces which augments a fraction of retained austinite microstructure to be transformed into martensitic microstructure.

In an embodiment, the steps of re-heating, soaking, and first and second tempering processes are carried out in a furnace, and the cooling is normal air cooling.

In an embodiment, the isothermal quenching is carried out in a hot water bath, and the steel is maintained in the hot water bath for about 5 minutes to achieve equilibrium temperature.

In an embodiment, the first pre-determined temperature is about 1200 ºC, second pre-determined temperature is about 100ºC, the third pre-determined temperature is about 160ºC, and the fourth predetermined temperature ranges from about 200°C to 500°C.

In an embodiment, the first pre-set period of time is about 3 hours, the second pre-set period of time is about 45 minutes, the third pre-set period of time is about 24 hours, and the fourth pre-set period of time ranges from about 2 hours to 5 hours.

In an embodiment, the first hot working process is a forging process, and the second hot working process is a hot rolling process.

In an embodiment, the hot rolling process is carried out at least five to six times on the steel, and wherein, the temperature of the steel drops to a range of about 900ºC to about 950ºC during the hot rolling process.

In an embodiment, wherein the cold working process is a cold rolling process.

In an embodiment, the method comprises cooling the steel to a room temperature in air after the second tempering process.

In an embodiment, second tempering process results in stress reliving in the steel, carbon diffusion and reversal of minute amount austenite.

In an embodiment, the reheating is carried out in an inert atmosphere.

In an embodiment, the method comprises machining and cleaning the steel before cold working.

In an embodiment, the steel is produced by casting alloy in an air-induction furnace.

In another non-limiting embodiment of the disclosure, a high strength-ductile steel is disclosed. The steel comprising composition of Carbon (C) at about 0.4 wt% to about 0.42 wt%, Manganese (Mn) at about 0.5 wt% to about 1wt %, Sulphur (S) up-to 0.02 wt%, Phosphorus (P) up-to 0.02 wt%, Nitrogen (N) up-to 0.01 wt%, Silicon (Si) at about 1.5 wt% to about 1.7 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.1 wt% to about 0.3 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 2.8 wt% to about 3 wt %, Cobalt (Co) at about 1.4 wt% to about 1.5 wt%, Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%, Copper (Cu) at about 0.2 wt% to about 0.3 wt% the balance being Iron (Fe) optionally along with incidental elements. The high strength- ductile steel primarily comprises a martensitic microstructure.

In an embodiment, during the cold working the steel experiences shear forces which augments a fraction of retained austinite microstructure to be transformed into martensitic microstructure.

In an embodiment, the high strength-ductile steel exhibits ultimate tensile strength ranging from about of 2537 MPa to 2870 MPa, yield strength ranging from about of 2363 MPa to 2781 MPa, and ductility ranging from about 5% to about 6%.

In an embodiment, the abrasive wear loss of the steel varies in the range of 0.124 – 0.144g as per ASTM G 65-04.

In yet another non-limiting embodiment, a method for manufacturing high strength-ductile steel is disclosed. The method includes casting a steel of composition comprising: Carbon (C) at about 0.4 wt% to about 0.42 wt%, Manganese (Mn) at about 0.5 wt% to about 1wt %, Sulphur (S) up-to 0.02 wt%, Phosphorus (P) up-to 0.02 wt%, Nitrogen (N) up-to 0.01 wt%, Silicon (Si) at about 1.5 wt% to about 1.7 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.1 wt% to about 0.3 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 2.8 wt% to about 3 wt %, Cobalt (Co) at about 1.4 wt% to about 1.5 wt%, Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%, Copper (Cu) at about 0.2 wt% to about 0.3 wt% the balance being Iron (Fe) optionally along with incidental elements. Then subjecting, the steel to a roughing process, and quenching the steel, and hot working on the steel by a second hot working process. Further, the method includes subjecting the steel to a first tempering at a third predetermined temperature for a third pre-set period of time, and cold working, on the steel. Then subjecting, the steel to a second tempering at fourth predetermined temperature for a fourth pre-set period of time to produce the high-strength ductile steel. The high strength ductile steel primarily comprises 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 producing high strength-ductile steel, according to an exemplary embodiment of the present disclosure.

Figure.2a is a graphical flow diagram illustrating hot working cycle of the method for producing high strength-ductile steel.

Figure. 2b is a graphical flow diagram of cold working cycle of the hot worked steel.

Figures. 3a and 3b illustrates a graphical representations of results of X-ray Diffraction analysis carried out on the high strength-ductile steel sample showing austenite peaks in Hot-rolled sample and non-presence of austenite peaks in the sample which is Cold-Rolled-Tempered after hot rolling, according to an exemplary embodiment of the present disclosure.

Figure. 4 illustrates microstructure of the steel, which is hot rolled, quenched and tempered, according to an exemplary embodiment of the present disclosure.

Figure. 5a is graphical representation of stress versus elongation, obtained during tensile test of the steel, which is just hot rolled, cold-rolled, and cold-rolled-tempered at 200 °C, according to an exemplary embodiment of the present disclosure.

Figure. 5b is graphical representation of stress versus elongation, obtained during tensile test of the high strength ductile steel cold-rolled-tempered at 300 °C and at 500 °C, 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 a high strength and ductile steel and a method for manufacturing or producing a high strength and ductile steel. The application of ultra-high strength steel with greater factor of safety specially for automotive and defense application is limited due to chances of catastrophic failure. This is due to the lack in achieving super high yield strength of the steel in combination with ductility. In this context, the present disclosure is expected to make an important contribution towards the futuristic and strategic light weight application of steel with greater factor of safety. A steel with superior factor of safety has been obtained by the sharp increase of yield strength with reasonable ductility that specifically required for the automotive and defense applications.
With advancements in technology and with efforts to develop high strength steel, patent application has been filed for ~2 GPa (~2000 MPa) hot-rolled grade steel which has shown elongation close to ~20% with reasonable yield strength [Indian Patent Application No.: 201931003490, Filing dt.: January 29, 2019]. From the microstructural point of view, this ~2 GPa steel consists of martensite (~93%) and carbon enriched stable austenite (~7%). It is expected that “twin” formation, in the stable austenite film under the compressive and tensile loading process, is the fundamental aspect of achieving high ductility in this steel.
Now, in the present disclosure, the yield strength of this ~2 GPa steel is further increased by generating excess dislocation and partial transformation of nanoscale retained austenite through cold rolling process. Therefore, the steel developed in the present disclosure has an exceedingly high strength in combination of reasonable ductility that will provide greater factor of safety. Therefore, the present disclosure opens a cost-effective option overall.

Accordingly, the method of manufacturing high strength ductile steel of the present disclosure, steel comprising desired composition may be, formed by any manufacturing process, including but not limiting to casting. Then the steel may be subjected for soaking at a first pre-determined temperature for a first pre-set period of time. In an embodiment, the first pre-determined temperature is about 1200 ºC and the first pre-set period of time is about 3 hours. Further, hot working is carried out on the steel by a first hot working process followed by cooling the steel. Then, the steel is subjected for re-heating to the first pre-determined temperature and annealing the steel in the first predetermined temperature for a second pre-set period of time. In an embodiment, second pre-set period of time is about 45 minutes. The method further includes hot working, on the steel by a second hot working process, and isothermal quenching of the steel at a second predetermined temperature and cooling the steel to a room temperature. The steel is then subjected to a first tempering process at a third predetermined temperature for a third pre-set period of time and cooling the steel to a room temperature and cold working, on the steel. In an embodiment, third pre-determined temperature is about 160ºC and third pre-set period of time is about 24 hours. The method further includes subjecting, the steel to a second tempering process at fourth predetermined temperature for a fourth pre-set period of time to produce the high-strength ductile steel. In an embodiment, the fourth predetermined temperature ranges from about 200°C to 500°C, and fourth pre-set period of time ranges from about 2 hours to 5 hours. The high strength, ductile steel obtained by the method primarily comprises a martensitic microstructure. Further, the high-strength, ductile steel exhibits ultimate tensile strength ranging from about of 2537 MPa to 2870 MPa. Further, the high-strength high-ductile steel exhibits yield strength ranging from about of 2363 MPa to 2781 MPa. Also, the high-strength high-ductile steel exhibits reasonable ductility ranging from about 5% to about 8%. Cold rolled sample exhibits 2000 MPa along with ~8% elongation after tempering at 500 °C.

Henceforth, the present disclosure is explained with the help of figures for a method of manufacturing high strength-ductile 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 can envisage various such embodiments without deviating from scope of the present disclosure.

Figures.1 and 2a and 2b are exemplary embodiments of the present disclosure illustrating a flowchart depicting a method for manufacturing high strength-ductile steel and a process flow diagram of a hot working and cold working process. In the present disclosure, mechanical properties such as strength and ductility of the final microstructure of the steel may be improved. The steel produced by the method of the present disclosure, includes a martensitic microstructure and a retained austinite 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. The method is particularly applicable to high-strength -ductile steel and it may also be extended to other type of steels as well.

At block 101, a steel of desired alloy composition is formed by any of the manufacturing process. The steel is further processed to form high strength-ductile steel. In embodiment, the steel is made in the form of ingots, and the alloy may be prepared in at least one of air-melting furnace, and vaccum furnace. The steel ingot may have composition of in weight percentage (wt%) of: Carbon (C) at about 0.4 wt% to about 0.42 wt%, Manganese (Mn) at about 0.5 wt% to about 1wt %, Sulphur (S) up-to 0.02 wt%, Phosphorus (P) up-to 0.02 wt%, Nitrogen (N) up-to 0.01 wt%, Silicon (Si) at about 1.5 wt% to about 1.7 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.1 wt% to about 0.3 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 2.8 wt% to about 3 wt %, Cobalt (Co) at about 1.4 wt% to about 1.5 wt%, Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%, Copper (Cu) at about 0.2 wt% to about 0.3 wt%, the balance being Iron (Fe) optionally along with incidental elements.

At block 102, the method comprises of soaking the steel in the form of ingots. The steel ingots may be subjected for soaking in the first predetermined temperature for a first pre-set period of time in an annealing furnace. In an embodiment, the first pre-determined temperature ranges from about 1220 ºC to 1280 ºC, preferably 1200 ºC. Further, in an embodiment, the first pre-set period of time is around 3 hours.

The method includes a step or a stage of hot working the steel ingot by a first hot working process [shown in block 103] immediately after soaking. In an embodiment, the first hot working process is a forging process. Forging is a mechanical process in which the components may be reshaped by applying localized compressive stresses. As an example, the localized compressive stresses may be induced using a 0.5-ton forging hammer. Blows are delivered by the hammer on to the steel ingot in order to induce localized compressive stresses, which may result in internal grain deformation, thus enhancing strength and stiffness of the structure. After carrying out the forging process, the steel ingot may be allowed to cool. In an embodiment of the present disclosure, the cooling of the steel ingot is carried out by normal air cooling.

At block 104, the method comprises re-heating the steel ingot to a first pre-determined temperature of around 1220 ºC to 1280 ºC, preferably 1250 ºC. At block 105, the method comprises of annealing the steel ingot for a second pre-set period of time. In an embodiment, the second pre-set period of time is about 45 minutes. Annealing is a heat treatment process, which involves heating the structure above the re-crystallization temperature and maintaining the temperature for a suitable time and cooling. During annealing, atoms migrate in the crystal lattice and the number of dislocations decreases, which result in improving mechanical properties of the structure. In an embodiment, the annealing is carried out in an argon gas atmosphere.

Referring to block 106 in combination with Figure. 2a, the method comprises of hot working the steel ingot by a second hot working process. In an embodiment, the second hot working process may be a hot rolling process. Rolling is a mechanical process, which involves passing the metal stock through one or more pairs of rolls to refine the grain size in the structure. As an example, the steel ingot may be passed through the one or more pair of rolls for at least 5 to 6 times [shown on Figure. 2a]. During rolling process of the steel ingot, the temperature of the steel ingot reduces below the first pre-determined temperature i.e. the temperature reduces from about 900ºC to about 950ºC. After rolling process, the steel ingot is isothermally quenched [shown in block 107] at a second pre-determined temperature. In an embodiment, the second pre-determined temperature is about 100ºC. In an embodiment, the isothermal quenching of the steel ingot may be carried out in hot water bath maintained at 100 °C and kept for 5 minutes to achieve equilibrium temperature. After quenching, the steel sample is cooled to room temperature by air cooling as shown in step [108] [shown in Figure.2a].

Subsequently, the steel ingot is subjected for first tempering process at third predetermined temperature for a third predetermined time as shown in block 109. In an embodiment, the third predetermined temperature may be about 160ºC, and the third predetermined time may be about 24 hours. In an embodiment, tempering may be carried out in a muffle furnace. After completion of 24 hours of low temperature tempering, the steel ingot may be taken out of the muffle furnace and cooled in air to room temperature.

Referring to block 110 in combination with Figure. 2b, the method comprises of cold working the hot rolled steel sheet by a cold working process. In an embodiment, the cold working process may be a cold rolling process. As an example, the steel sheet may be passed through the one or more pair of rolls in room temperature, and during cold rolling process the thickness of the steel sheet may be reduced up-to 70%. After the cold rolling process, steel sheet may be subjected for a second tempering process at fourth predetermined temperature for a fourth predetermined time as shown in block 111. In an embodiment, the fourth predetermined temperature may be in the range of 200°C to 500°C, and the fourth pre-set period of time may range from about 2 hours to 5 hours.

The steel sheet processed by the method of the present disclosure results in microstructural changes to form high strength-ductile steel. In an embodiment, the steel sheet comprises a substantially martensitic microstructure with retained austinite (RA).

In an embodiment, the method optionally comprises grinding process to make the two sides parallel in the thickness direction after tempering of the hot rolled steel sheet and before cold rolling.
In an embodiment, the high-strength, high-ductile steel exhibits ultimate tensile strength ranging from about of 2537 MPa to 2870 MPa. Further, the high-strength high-ductile steel exhibits yield strength ranging from about of 2363 MPa to 2781 MPa. Also, the high-strength high-ductile steel exhibits ductility ranging from about 5% to about 8%.
In an embodiment, during cold rolling, the hot-rolled steel sheet has experiences sever shear force and that augments the fraction of existing RA content to be transformed into martensite. This transformation plays a role to sharply rise the yield strength up to 2300 MPa (~2.3 GPa), and the ultimate tensile strength to 2500 MPa (~2.5 GPa). Further, the steel demonstrates 5 - 6% total elongation i.e. ductility which is an indication of the presence of remaining stable RA after cold rolling.
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 used in the range between 0.4 to 0.42 wt%. Carbon is an austenite stabilizer and controls the martensite formation. Excessive carbon may promote carbide precipitates in the interface of martensite and austenite and may vary the precipitation formation as the cooling rate varies, this may affect the constant strength over a wide range of cooling rate.

Silicon (Si) may be used minimum in the range of 1.5 to 2 wt%. Silicon suppress the carbide formation in the steel when it is added to about 1.5 wt%. It leads to carbide free matrix which eventually improve the ductility and impact toughness of the steel.

Manganese (Mn) may be used in the range of 0.5 to 1.0 wt%. 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. However, the hardenability may decrease as a result of reducing (Mn). The (Mn) content limit may be considered low or high as per extent of hardenability required with carbide free matrix.

Chromium (Cr) may be used in the range 0.97 to 0.98 wt%. This addition may substantially increase the strength and hardenability of the steel. It may vary beyond above range for customized strength and hardenability requirement.
Nickel (Ni) may be used in the range 3. This increases the strength and toughness.
Molybdenum (Mo) may be used in the range of 0.24 to 0.27 wt%. The addition of small quantity reduces the impurity embrittlement and to increases hardenability. Excess addition may reduce the carbon content in austenite. It increases the room temperature strength in steel.
Vanadium (V) may be added in range 0.06 to 0.07 wt%. This addition may reduce the stacking faults in the austenite. It also acts as solid solution strengthener.
Cobalt (Co) may be used in the range in between to 1.4 to 1.6 wt%. The addition of cobalt may effectively decrease the stacking fault energy of austenite.
Aluminum (Al) may be added in between 1.4 to 1.6 wt%. This addition improves strength and ductility. It can also be added more or less as a solid solution strengthener.
Copper (Cu) may be added in the range of 0.18 to 0.20wt%. This addition may increase the solid solution strengthening and aiming to boost up the toughness. This can be added more to increase the strength and toughness.

Example:

Further embodiments of the present disclosure will be now 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. Results have been compared on various fronts to show the contribution of cold working and tempering in improvement of strength of the steel. The composition of the steel for which the tests are carried out is as shown in below table 1.

C Mn S P Si Al Cr Co Mo Ni V Cu N
0.41 0.7 0.012 0.017 1.6 0.61 1.01 1.5 0.29 2.9 0.08 0.21 73 ppm

Table - 1

In an embodiment of the present disclosure, various experiments were carried out on the steel sample for composition as mentioned in Table - 1 for different tempering temperatures during formation of the steel. For conducting the experiment, the steel specimens of pre-determined dimensions may be prepared by the method of the present disclosure. In an embodiment, XRD samples ae prepared by following standard methods, and the standard tensile samples (sub-size of gauge length 25 mm following ASTM E8/E8M-09 standard) and wear test samples (65mm X 20mm X 2.5mm) are prepared keeping the tensile axis parallel to rolling direction. After the experiments, test results have been compared. In subsequent paragraphs of the disclosure, the method of carrying out the experiment and test results in conjunction with the figures is disclosed.
In an exemplary embodiment, dry sand rubber wheel (DSRW) abrasion experiments is carried out as per ASTM G 65-04 using dry abrasion test rig AR-50 machine. The test involves abrading a standard test specimen with a grit of controlled size and composition. The abrasive is introduced between the test specimen and a rotating wheel. This test specimen is pressed against the rotating wheel at a specified force (130 N) by means of a lever arm while a controlled flow of grit abrades the test surface, and the test is conducted using quartz grain sand (AFS 50/70) as abrasive. The sand flow rate is maintained between 320-350 g/min. The rpm of the wheel and the total number of revolutions were fixed at 200. The abrasion-tested samples are cleaned with acetone in an ultrasonic cleaning bath before and after the tests, and weight loss is measured. In an embodiment, the measured abrasive wear loss (i.e. weight loss) values for the investigated steel varies in the range of 0.124 – 0.144g as per ASTM G 65-04.
Referring to figures. 3a and 3b, which are exemplary embodiments of the present disclosure, illustrating graphical representation of the results of XRD analysis carried out on the steel sample which is just hot rolled and the sample which is subjected for cold rolling after hot rolling and tempering at 300 °C. The steel sample which only hot rolled demonstrates 6 – 8% of retained austenite (RA) volume fraction. Where after cold rolling the volume fraction of retained austenite decreased below the detectable limit of 5% and did not display any RA peaks. It indicates that considerable amount of RA has been transformed to martensite during cold rolling.
As evident from the graphs of Figure. 4, steel which is only quenched and tempered steel exhibits martensitic microstructure with low volume fraction of retained austenite (RA). The presence of RA has been confirmed by the XRD results (Figure 3a). The results indicate that there is a considerable decrease in the volume fraction of the RA after cold rolling. During cold rolling, the hot-rolled steel sheet experience severe shear force and that augmented the fraction of existing RA content to be transformed into martensite. This transformation results in sharply rising the yield strength up to 2300 MPa (~2.3 GPa), and the UTS reached to 2500 MPa (~2.5 GPa). It is also evident that, after achieving exceedingly high strength still it demonstrates 5 - 6% total elongation which is an indication of the presence of remaining stable RA after cold rolling (Figure. 3b).
It is significant to note that the strength of the cold-rolled steel has further been increased in the range of 2700 to 2900 MPa (2.7 to 2.9 GPa) with a yield strength 2600 to 2700 MPa (2.6 to 2.7 GPa) due to tempering. This is because of temperature induced martensite transformation of a minute amount of RA in the cold-rolled steel. In an embodiment, due to tempering the additional advantages may be stress reliving and reversal of minute amount austenite.

Referring to Table 2 below, mechanical properties of hot-rolled and tempered steel samples manufactured using the method of the present disclosure is depicted.

Sample Code Yield Strength UTS Ductility
M160T-CR 2363 MPa 2537 MPa 5 %
2348 MPa 2548 MPa 5 %
M160T-CR-200HT 2781 MPa 2870 MPa 6 %
2608 MPa 2749 MPa 6 %
M160T-CR-300HT 2669 MPa 2751 MPa 5 %
2558 MPa 2632 MPa 5 %
M160T-CR-500HT 1909 MPa 2029 MPa 8 %
Table: 2
wherein, the samples are:
a) M160T-CR: Cold rolled steel sheet.
b) M160T-CR-200HT: Cold rolled steel sheet tempered at 200 °C.
c) M160T-CR-300HT: Cold rolled steel sheet tempered at 300 °C.
d) M160T-CR-500HT: Cold rolled steel sheet tempered at 500 °C.

Now referring to Figures. 5a and 5b, which are exemplary embodiments of the present disclosure illustrating a graph with stress versus elongation plot obtained during tensile test of the steel samples as per table-2 above. The tensile test may be carried using standard tensile test samples. As an example, the test sample may be with a gauge length of 25mm and E8/E8M-09 configuration, in accordance to ASTM standards. As evident from the graph illustrated in Figure. 5a, the steel which is cold rolled possess around 2537-2548 MPa of UTS, 2363-2348MPa of yield strength with around 5 % total elongation. Whereas, the steel sheet which is cold rolled and tempered at 200 °C possess around 2749-2870 MPa of UTS, 2608-2781 MPa of yield strength with around 6 % total elongation. Now referring to, Figure. 5b, the steel sheet which is cold rolled and tempered at 300 °C possess around 2632 -2751 MPa of UTS, 2558-2669 MPa of yield strength with around 5 % total elongation, and the steel sheet which is cold rolled and tempered at 500 °C possess around 2029 MPa of UTS, 1909 MPa of yield strength with around 8 % total elongation. From, the table-2 and Figures. 5a and 5b, it is significant to note that the strength of the cold-rolled steel has further been increased in the range of 2700 to 2900 MPa (2.7 to 2.9 GPa) with the yield strength 2600 to 2700 MPa (2.6 to 2.7 GPa) due to tempering. This might be because of temperature induced martensite transformation of a minute amount of RA in the cold-rolled steel.

It should be understood that the experiments are carried out for a particular composition of the steel and the results brought out in the previous paragraphs are for the composition shown in Table – 1. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the steel as well.
In another embodiment, the present disclosure discloses a method for producing a high strength-ductile steel is disclosed. The method may be useful from industrial point. The method according to this embodiment includes steps of casting a steel of composition comprising: of a composition in weight percentage (wt%) of: Carbon (C) at about 0.4 wt% to about 0.42 wt%, Manganese (Mn) at about 0.5 wt% to about 1wt %, Sulphur (S) up-to 0.02 wt%, Phosphorus (P) up-to 0.02 wt%, Nitrogen (N) up-to 0.01 wt%, Silicon (Si) at about 1.5 wt% to about 1.7 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.1 wt% to about 0.3 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 2.8 wt% to about 3 wt %, Cobalt (Co) at about 1.4 wt% to about 1.5 wt%, Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%, Copper (Cu) at about 0.2 wt% to about 0.3 wt%, the balance being Iron (Fe) optionally along with incidental elements by a continuous casting process. This forms a continuously casted steel slab, which may be subjected to roughing mill for a roughing operation. During this stage, the steel slab may undergo deformation, and then subsequently the steel slab may be subjected for quenching. The quenching may be isothermal quenching in a hot water for the second predetermined time. Then, the steel slab may be subjected for second hot working process such as hot rolling process. The method further includes first tempering process of the steel at a third predetermined temperature for a third pre-set period of time and cold working on the steel. Then subjecting, the steel to a second tempering at fourth predetermined temperature for a fourth pre-set period of time to produce the high-strength ductile steel. The high strength-ductile steel primarily comprises a martensitic microstructure.

In an embodiment of the present disclosure, the high strength-ductile steel 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, and the like. The high strength- ductile steel may be used in any other industrial structural applications including defence sector. It also has great futuristic probability of replacing the hot stamping grade steel and an in making Lifting and Excavation machineries, and body structures for future electric vehicles.

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
101-110 Flowchart blocks
101 Forming stage
102 Soaking stage
103 First hot working and cooling stage
104 Re-heating stage
105 Annealing stage
106 Second hot working stage
107 Isothermal quenching stage
108 Cooling stage
109 First tempering stage
110 Cold working
111 Second tempering