Claims:1. A method for manufacturing high ductile bainitic steel, the method comprising:
soaking, a steel comprising Carbon (C) at about 0.4 wt% to about 0.51 wt%; Manganese (Mn) at about 0.4 wt% to about 0.5 wt %; Silicon (Si) at about 1.5 wt% to about 2 wt %; Sulphur (S) at about 0.01 wt% to about 0.02 wt%; Phosphorus (P) at about 0.02 wt%; Nitrogen (N) at about 0.004 wt% to about 0.007 wt%; Chromium (Cr) at about 0.8 wt% to about 1 wt%; Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%; Cobalt (Co) at about 1.4 wt% to about 2 wt%; Vanadium (V) at about 0.05 wt% to about 0.08 wt%; Copper (Cu) at about 0.2 wt% to about 0.3 wt%; Nickel (Ni) at about 2.5 wt% to about 3 wt %; Aluminum (Al) at about 0.7 wt% to about 1.3 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, and isothermal quenching of the steel to a second predetermined temperature; and
soaking the steel in a salt bath at the second predetermined temperature for a third pre-set period of time, to obtain the high ductile bainitic steel,
wherein, the bainitic steel exhibits ductility ranging from about 30% to about 36%.
2. The method as claimed in claim 1, wherein the re-heating and soaking are carried out in a furnace.

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

4. The method as claimed in claim 1, wherein the isothermal quenching is carried out in a salt bath solution.

5. The method as claimed in claim 1, wherein the first pre-determined temperature ranges from about 1250 ºC to 1350ºC.

6. The method as claimed in claim 1, wherein the second pre-determined temperature ranges from about 300ºC to 400ºC.

7. The method as claimed in claim 1, wherein the first pre-set period of time is about 2.5 hours to 3.5 hours.

8. The method as claimed in claim 1, wherein the second pre-set period of time is about 0.5 hours to 1 hour.

9. The method as claimed in claim 1, wherein the third pre-set period of time is about 1 hour to 72 hours.

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

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

12. The method as claimed in claim 11, wherein the rolling process is carried out at least five times on the steel.

13. The method as claimed in claim 1, wherein the method comprises, cooling the steel to a room temperature in air after soaking for the third pre-set period of time.

14. The method as claimed in claim 1, wherein the method comprises cleaning the steel by cold acid pickling.

15. A bainitic steel comprising:
Carbon (C) at about 0.4 wt% to about 0.51 wt %;
Manganese (Mn) at about 0.4 wt% to about 0.5 wt %;
Silicon (Si) at about 1.5 wt% to about 2 wt %;
Sulphur (S) at about 0.01 wt% to about 0.02 wt%;
Phosphorus (P) at about 0.02 wt%;
Nitrogen (N) at about 0.004 wt% to about 0.007 wt%;
Chromium (Cr) at about 0.8 wt% to about 1 wt%;
Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%;
Cobalt (Co) at about 1.4 wt% to about 2 wt%;
Vanadium (V) at about 0.05 wt% to about 0.08 wt%;
Copper (Cu) at about 0.2 wt% to about 0.3 wt%;
Nickel (Ni) at about 2.5 wt% to about 3 wt %;
Aluminum (Al) at about 0.7 wt% to about 1.3 wt %, wherein the balance being Iron (Fe) optionally along with incidental elements.

16. The bainitic steel as claimed in claim 15, wherein the bainitic steel comprises primarily bainitic microstructure of at least 95%.

17. The bainitic steel as claimed in claim 16, wherein the microstructure of the bainitic steel forms a twin in stable austenite when subjected to tensile straining.

18. The bainitic steel as claimed in claim 15, wherein the bainitic steel exhibits an ultimate tensile strength ranging from about 1500 MPa to about 1525 MPa.

19. The bainitic steel as claimed in claim 15, wherein the bainitic steel exhibits ductility ranging from about 30% to 36%.

20. A method for manufacturing high ductile bainitic steel, the method comprises,
casting a steel comprising Carbon (C) at about 0.4 wt% to about 0.51 wt%; Manganese (Mn) at about 0.4 wt% to about 0.5 wt %; Silicon (Si) at about 1.5 wt% to about 2 wt %; Sulphur (S) at about 0.01 wt% to about 0.02 wt%; Phosphorus (P) at about 0.02 wt%; Nitrogen (N) at about 0.004 wt% to about 0.007 wt%; Chromium (Cr) at about 0.8 wt% to about 1 wt%; Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%; Cobalt (Co) at about 1.4 wt% to about 2 wt%; Vanadium (V) at about 0.05 wt% to about 0.08 wt%; Copper (Cu) at about 0.2 wt% to about 0.3 wt%; Nickel (Ni) at about 2.5 wt% to about 3 wt %; Aluminum (Al) at about 0.7 wt% to about 1.3 wt %; the balance being Iron (Fe) optionally along with incidental elements, by a continuous casting process;
subjecting, the cast steel to a roughing process, and quenching the cast steel in a second pre-set period of time; and
hot working on the cast steel by a second hot working process, to obtain the high ductile bainitic steel, wherein the bainitic steel exhibits ductility ranging from about 30% to 36%. , 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 ductile bainitic steel. Further embodiments of the disclosure disclose a method for manufacturing the high ductile bainitic steel.

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 undergo 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.

In the recent past, Twin Induced Plasticity (TWIP) steels are commonly used in variety of applications because of enhanced mechanical properties such as, but not limiting to high strength and ductility. The microstructure of the TWIP steel, mainly consist of stable austenite. The stable austenite under the action of compressive and tensile loads or tensile straining favours in twin formation, which results in exhibiting high ductility. Though TWIP steels possess high ductility, the presence of high manganese content of about 15 wt% -20 wt%, limits the TWIP steels from utilizing in mass industrial and commercial applications because of high processing and production cost. This limitation of the TWIP steel has shifted focus of the manufactures to use a bainitic steel.

Bainitic steel may be formed by heating the steel alloy above the austenization temperature and cooling at a rate more rapid than that required to form pearlite and less rapid than required to form martensite. Under compression and tensile loads or tensile straining, the austenite content in the microstructure of the bainitic steel transforms into martensite, resulting in imparting high strength, nevertheless fails to impart high ductility. High strength and poor ductile property of the bainitic steel limits the area of applicability of the bainitic steel. As an example, the use of bainitic steel is currently limited to military, rail road application and cannot be extended to mass scale industrial manufacturing and commercial application.

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

SUMMARY OF THE DISCLOSURE

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

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

In one non-limiting embodiment, there is provided a method for manufacturing high ductile bainitic steel. The process initially starts from shaping of steel ingot that includes a composition comprising Carbon (C) at about 0.4 wt% to about 0.51 wt%; Manganese (Mn) at about 0.4 wt% to about 0.5 wt %; Silicon (Si) at about 1.5 wt% to about 2 wt %; Sulphur (S) at about 0.01 wt% to about 0.02 wt%; Phosphorus (P) at about 0.02 wt%; Nitrogen (N) at about 0.004 wt% to about 0.007 wt%; Chromium (Cr) at about 0.8 wt% to about 1 wt%; Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%; Cobalt (Co) at about 1.4 wt% to about 2 wt%; Vanadium (V) at about 0.05 wt% to about 0.08 wt%; Copper (Cu) at about 0.2 wt% to about 0.3 wt%; Nickel (Ni) at about 2.5 wt% to about 3 wt %; Aluminum (Al) at about 0.7 wt% to about 1.3 wt %; the balance being Iron (Fe) optionally along with incidental elements. The steel ingot is initially soaked at a first pre-determined temperature for a first pre-set period of time, and subjected to hot working by a first hot working process. Then the steel ingot will be subjected for cooling. After the first hot working operation, the steel ingot will be re-heated to a first pre-determined temperature, and will be annealed for a second pre-set period of time. After annealing, the steel ingot will be subjected to a second hot working process, and then isothermal quenching and holding of the bainitic steel will be carried-out to second predetermined temperature. Lastly, the method involves soaking steel ingot in a slat bath at a second predetermined temperature for a third pre-set period of time, time, to obtain the high ductile bainitic steel. The bainitic steel processed by the method exhibits ductility ranging from about 30% to about 36%.

In an embodiment, the re-heating and soaking of the bainitic steel is carried out in a furnace.

In an embodiment, cooling the bainitic grade steel is normal air cooling and isothermal quenching is carried out in the salt bath solution to the second predetermined temperature.

In an embodiment, the first pre-determined temperature ranges from about 1250ºC to 1350ºC.

In an embodiment, the second pre-determined temperature is about 300ºC to 400ºC.

In an embodiment, the first pre-set period of time is about 2.5 hours to 3.5 hours.

In an embodiment, the second pre-set period of time is about 0.5 hours to 1 hour.

In an embodiment, the third pre-set period of time is about 1 hour to 72 hours.

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

In an embodiment, the second hot working process is a rolling process and the rolling operation is carried out at least 5 times on the bainitic steel.

In an embodiment, the method comprises of cooling the bainitic steel to a room temperature in air after soaking for the third pre-set period of time and cleaning the bainitic steel by cold acid pickling.
In another non-limiting embodiment, a bainitic steel with high ductility is disclosed. The bainitic steel comprises carbon (C) at about 0.4 wt% to about 0.51 wt %, Manganese (Mn) at about 0.4 wt% to about 0.5 wt %, Silicon (Si) at about 1.5 wt% to about 2 wt %, Sulphur (S) at about 0.01 wt% to about 0.02 wt%, Phosphorus (P) at about 0.02 wt%, Nitrogen (N) at about 0.004 wt% to about 0.007 wt%; Chromium (Cr) at about 0.8 wt% to about 1 wt%, Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%, Cobalt (Co) at about 1.4 wt% to about 2 wt%, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Copper (Cu) at about 0.2 wt% to about 0.3 wt%, Nickel (Ni) at about 2.5 wt% to about 3 wt %, Aluminum (Al) at about 0.7 wt% to about 1.3 wt % and the balance being Iron (Fe) optionally along with incidental elements.

In an embodiment, the bainitic steel comprises primarily bainitic microstructure of at least 95%.
In an embodiment, microstructure of the bainitic steel forms a twin in stable austenite when subjected to tensile straining.
In an embodiment, the bainitic steel exhibits an ultimate tensile strength ranging from about 1500 MPa to about 1525 MPa and ductility ranging from about 30% to about 36%.
In yet another non-limiting embodiment of the present disclosure, a method for manufacturing high ductile bainitic steel is disclosed. The method comprises producing a steel in air furnace that comprising Carbon (C) at about 0.4 wt% to about 0.51 wt%; Manganese (Mn) at about 0.4 wt% to about 0.5 wt %; Silicon (Si) at about 1.5 wt% to about 2 wt %; Sulphur (S) at about 0.01 wt% to about 0.02 wt%; Phosphorus (P) at about 0.02 wt%; Nitrogen (N) at about 0.004 wt% to about 0.007 wt%; Chromium (Cr) at about 0.8 wt% to about 1 wt%; Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%; Cobalt (Co) at about 1.4 wt% to about 2 wt%; Vanadium (V) at about 0.05 wt% to about 0.08 wt%; Copper (Cu) at about 0.2 wt% to about 0.3 wt%; Nickel (Ni) at about 2.5 wt% to about 3 wt %; Aluminum (Al) at about 0.7 wt% to about 1.3 wt %; the balance being Iron (Fe) optionally along with incidental elements, by a continuous casting process. The cast steel is subjected to a roughing process in the roughing mill and followed by quenching for a second pre-set period of time. Further, the method comprises hot working on the cast steel by a second hot working process, to obtain the high ductile bainitic steel, which exhibits ductility ranging from about 30% to 36%.

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 ductile bainitic steel, according to an exemplary embodiment of the present disclosure.

Figure. 2 illustrates microstructure of the bainitic steel manufactured by the method of figure. 1, according to an exemplary embodiment of the present disclosure.

Figure. 3 illustrates a fractography study of a fracture surface of the bainitic steel produced by the method of figure. 1, according to an exemplary embodiment of the present disclosure.

Figure. 4 illustrates a graphical representation of results of X-ray Diffraction analysis carried out on the bainitic steel sample soaked in the salt bath for a period of 24 hours, according to an exemplary embodiment of the present disclosure.

Figure. 5 illustrates a graphical representation of the results of X-ray Diffraction analysis carried out on the bainitic steel sample soaked in the salt bath for a period of 72 hours, according to an exemplary embodiment of the present disclosure.

Figure. 6 is graphical representation of stress versus elongation, obtained during tensile test of the bainitic steel soaked in the salt bath for a time period of about 24 hours, according to an exemplary embodiment of the present disclosure.

Figure. 7 is a graphical representation of stress versus elongation, obtained during tensile test of the bainitic steel soaked in the salt bath for a time period of about 72 hours, according to an exemplary embodiment of the present disclosure.

Figure. 8 is a graphical representation of stress versus elongation, obtained during tensile test of the bainitic steel soaked in the salt bath for a period of one hour, 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 ductile bainitic steel and a method for manufacturing or producing high ductile bainitic steel. Ductility is an important property for the mass industrial application of high strength material like the bainitic steel. As of now, the bainitic steels with ultimate tensile strength range between 1500 MPa and 2000 MPa have been developed with ductility ranging from 9 to 20%. Poor ductility properties, limits the application of the bainitic steel. The present disclosure provides a method for manufacturing high ductile bainitic steel, by inhibiting phase transformation of austenite into martensite in the bainitic steel during tensile straining. Accordingly, by adopting the method of present disclosure, the fine austenite in the bainitic steel becomes enriched carbon and mechanically stabilized, to favour twin formation in stable austenite in the microstructure of the bainitic steel, during tensile straining. 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 inhibits formation of martensite during phase transition of the austenite in the bainitic steel during tensile straining and induce twin formation in the bainitic steels during tensile straining. This improves ductility of the bainitic steel.

In the method of manufacturing high ductile bainitic steel of the present disclosure, steel ingot comprising desired composition may be, formed by any manufacturing process, including but not limiting to continuous casting steel slab process. Then the steel ingot is initially soaked at a first pre-determined temperature for a first pre-set period of time in the furnace. In an embodiment, the first pre-determined temperature is about 1300ºC. The hot steel ingot is then subjected to a first hot working process and followed by normal air cooling. In an embodiment, the first hot working process is a forging process. The cooled steel ingot is reheated to the first pre-determined temperature and annealed for a second pre-set period of time. In an embodiment, the time period for which the steel ingot is annealed is about 0.5 hours. Further, the steel ingot is subjected to a second hot working process like a rolling process. In an embodiment, the steel ingot is rolled for at least five times between the one or more pair of rollers. The steel ingot is then isothermally quenched to a second predetermined temperature of about 350ºC. Further, the steel ingot is soaked in a salt bath at the second pre-determined temperature for a third pre-set period of time ranging from about 1 hour to 72 hours. Hence, processing the steel ingot by the method of present disclosure, results in microstructural changes to form a bainitic steel with bainitic microstructure of at least 95%. The bainitic steel formed by the process of the present disclosure, favours in twin formation in stable austenite under tensile straining. The twin formation in austenite imparts high ductility in the bainitic steel which may be in the range of about 30% to 36%. Thus, the bainitic steel obtained by the method of the present disclosure, will have remarkably high plasticity. Therefore, the bainitic steel may be used in wide variety of industrial applications where structural components require some amount of formability. 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 high ductile bainitic 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.

Figure.1 is an exemplary embodiment of the present disclosure which illustrates a flowchart depicting a method for manufacturing high ductile bainitic steel. In the present disclosure, mechanical properties such as ductility of the final microstructure of bainitic steel may be improved by inhibiting the transformation of stable austenite to martensite during tensile straining. The bainitic steel produced by the method of the present disclosure, favours in twin formation in stable austenite during tensile straining, and thereby improves ductility. 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 bainitic steel and it can also be extended to other type of steels as well.

At block 101, a steel of desired composition is formed by any of the manufacturing process. The cast steel ingot is further processed to form high ductile bainitic steel. In embodiment, the steel ingot may have composition of carbon (C) at about 0.4 wt% to about 0.51 wt%, Manganese (Mn) at about 0.4 wt% to about 0.5 wt %, Silicon (Si) at about 1.5 wt% to about 2 wt%, Sulphur (S) at about 0.01 wt% to about 0.02 wt%, Phosphorus (P) at about 0.02 wt%, Nitrogen (N) at about 0.004 wt% to about 0.007 wt%, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%, Cobalt (Co) at about 1.4 wt% to about 2 wt%, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Copper (Cu) at about 0.2 wt% to about 0.3 wt%, Nickel (Ni) at about 2.5 wt% to about 3 wt%, Aluminum (Al) at about 0.7 wt% to about 1.3 wt% and the balance being Iron (Fe) optionally along with incidental elements.

At block 102, the method comprises of soaking the bainitic grade steel alloy in the form of ingots. The steel ingots may be heated to the first pre-determined temperature in the air furnace, and will be subjected for soaking in the first predetermined temperature for a first pre-set period of time. In an embodiment, the first pre-determined temperature ranges from about 1250 ºC to 1350 ºC. As an example, the first pre-determined temperature is around 1300ºC. 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]. 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 motor driven 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 is 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 of re-heating the steel ingot to a first pre-determined temperature of around 1250ºC to 1350ºC. As an example, the first pre-determined temperature is of around 1300ºC. In an embodiment, re-heating the steel ingot is carried out in a furnace. 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 0. 5 hours to 1 hour. As an example, the second pre-set period of time is around 0.5 hours. Annealing is a mechanical 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.

At block 106, 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 rolling process. The 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 times. 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 1300ºC to about 1150ºC. After rolling process, the steel ingot is isothermally quenched [shown in block 107] to a second pre-determined temperature. In an embodiment, the second pre-determined temperature is about 350ºC. In an embodiment, the isothermal quenching of the steel ingot is carried out in a salt bath. After quenching, the steel ingot may be soaked at 350ºC in the salt bath solution [shown in block 108] for a third pre-set period of time and cooled to room temperature. In an embodiment, the third pre-set period of time ranges from about 1 hour to 72 hours. Steel ingot processed by the method of the present disclosure results in microstructural changes to form high ductile bainitic steel. In an embodiment, the bainitic steel comprises of about 95% bainitic microstructure.

Figure. 2 is an exemplary embodiment of the disclosure which illustrates microstructure of the bainitic steel, which is formed by using method of the present disclosure. The microstructure of the bainitic steel is a carbide free matrix and shows the presence of clear bainitic shaves with overall homogeneity and unstable austenite. Now referring to figure. 3, which is an exemplary embodiment of the present disclosure, illustrate a fractography study of a fracture surface of the bainitic steel obtained during tensile testing of the bainitic steel. The fractography study of the fracture surface shows the presence of fine dimple features that corroborates the achieved high ductility in the current bainitic steel. The untransformed fine stable austenite becomes enriched carbon and mechanically stabilized and favours in formation of new twins during tensile straining. The fractography study of the fracture surface confirms that, during straining condition, the dislocation in the matrix directly interact with the nano-twin boundaries in the stable austenite and thus contributes in enhancing ductility.

Hence, the final microstructure of the bainitic steel formed by the method described above, exhibits improved mechanical properties such as but not limited to strength, hardness, toughness, drawability, ductility and torsion. With these improved characteristics, the final product could then be used in a wide range of industrial and commercial applications with optimum processing cost.

The high ductile bainitic steel formed by the method of the present disclosure, exhibit following properties: final microstructure comprises ultra-thin stabilized austenite in between bainitic ferrite, Ultimate tensile strength of about 1500 MPa to 1525 MPa and ductility of around 30% to 36%.

The following portions of the present disclosure, provides details about the proportion of each element in a composition of the bainitic steel and their role in enhancing properties.

Carbon (C) may be used in the range of 0.45 to 0.5 wt%. Carbon is an austenite stabilizer and provides a single phase of bainite in between Bs and Bf temperature. Excessive carbon can promote carbide precipitates in an inner portion of the bainite texture and can vary the precipitation formation as the cooling rate varies, which may affect the constant strength over a wide range of cooling rate. Below the above range, it may decrease the solute solution strengthening of bainitic-ferrite.
Silicon (Si) may be used in the range of about 1.5 to 2 wt%. Silicon (Si) of this proportion suppress the formation of carbide, which leads to carbide free bainitic matrix to improve the ductility and impact toughness.
Manganese (Mn) may be used in the range of about 0.5 to 0.6 wt%. Manganese of this proportion assists in retaining the toughness and diminishes the possibility of carbide formation in the bainitic matrix to improve the ductility. However, manganese content below the range may decrease hardenability.
Chromium (Cr) may be used in the range of about 0.97 to 0.98 wt%. Chromium of this proportion substantially increase the strength and hardenability of the bainitic steel. Further, the range can be varied beyond the above range depending upon customized strength and hardenability requirement.
Nickel (Ni) may be used below 3 wt%. Nickel of this proportion increases the residual austenite carbon, thus increasing the strength and toughness.
Molybdenum (Mo) may be used in the range of about 0.24 to 0.27 wt%. Molybdenum of this proportion reduces the impurity embrittlement and increases hardenability. Excess addition may reduce the carbon content in austenite and also increases the room temperature strength in steel.
Vanadium (V) may be used in the range of about 0.06 to 0.07 wt%. Vanadium of this proportion reduces the stacking faults in the austenite and imparts solid solution strengthening.
Cobalt (Co) may be used in the range of about 1.4 to 1.6 wt%. Cobalt of this proportion accelerate the rate of reaction by increasing the free energy difference between the ferrite and austenite phases.
Aluminium (Al) may be used in the range of about 1 to 1.06 wt%. Aluminium of this proportion improves strength and ductility. It also acts as a solid solution strengthener.
Copper (Cu) may be used in the range of about 0.18 to 0.20 wt%. Copper of this proportion increase the solid solution strengthening and boost up the toughness.
Example:

Further embodiments of the present disclosure will be now described with an example of a particular composition of the bainitic steel. Experiments have been carried out for a specific composition of the bainitic steel formed by using method of the present disclosure. Results have been compared on various fronts to show the contribution of soaking time period in improvement of ductility of the bainitic steel. The composition of the bainitic 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 0.53 0.012 0.017 2.05 1.05 1.01 1.46 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 bainitic steel sample for composition as mentioned in Table - 1 for different soaking time periods during formation of bainitic steel. For conducting the experiment, the bainitic steel specimens of pre-determined dimensions may be prepared by the method of the present disclosure. 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.

Referring to figure. 4 and 5, which are exemplary embodiment of the present disclosure, illustrates graphical representation of the results of XRD analyses carried out on the bainitic steel sample i.e. the processed sample soaked in the salt bath for a period of 24 hours and for a period of 72 hours respectively. As seen from the graph illustrated in figure. 4, there is no perceptible volume fraction of austenite in the bainitic matrix. Further, the volume fraction of austenite is too low to generate enough intensity to form a visible XRD peak. The microstructure of the bainitic steel soaked for 24 hours contains extremely fine untransformed austenite in the matrix. As seen from the graph illustrated in figure. 5, there is some volume fraction of austenite, which generates enough intensity to form a visible XRD peak. The generation of intensity curve indicates formation of detectable amount of austenite over 72 hours soaking period. This might be the indication of little detectable amount of austenite grows over the three days holding period.

The mechanical properties are very impressive in case of soaking for 24 hours. As an example, for a soaking time period of about 24 hours, the bainitic steel exhibits an ultimate tensile strength of 1500 MPa, accompanied with maximum 36 % of elongation. In an embodiment, the microstructural changes occurring over the soaking time controls the ductility without lowering the strength of the materials. As described earlier, the matrix of the bainitic steel possess extremely fine untransformed austenite. After soaking for third pre-set period of time i.e. 24 hours at a second predetermined temperature i.e. 350 °C, this very fine austenite becomes enriched carbon and stabilized mechanically so much so that during straining new twins formation favored. Nevertheless, the volume fraction of the newly generated nano-twins may be extremely low thus does not contribute to the strengthening mechanism to a greater extent, rather this austenite contained nano-twins acts harder precipitate. During straining the dislocation may get a chance to interact with twin contained austenite. With further increase in strain value, there may be a possibility that the dislocation will directly interact with nano-twin boundaries. The fracture microstructure of a 24 hours soaking bainitic sample, demonstrates uniform distribution of fine-dimples that corroborates the achieved high ductility of the current steel. Martensitic feature was not observed on the rolling surface at near fracture zone. It is expected that during tensile straining the ultra-thin mechanically stable austenite plates did not transform to martensite, rather forms a twin in stable austenite. This result resembles to the high strength and high ductile TWIP steel properties, despite much leaner chemistry of the present material compared to that of TWIP steel and therefore the bainitic steel has a great potential for commercial manufacturing and providing a cost-effective option. Therefore, from the experimental results it is evident that the soaking of the bainitic steel in the salt bath for the period of 24 hours may play crucial role in improving the ductility of the bainitic steel.

Now referring to Figure. 6, 7 and 8, which are exemplary embodiments of the present disclosure illustrating a graph with stress versus elongation plot obtained during tensile test of the bainitic steel i.e. the processed sample soaked in the salt bath for a time period of about 24 hours,72 hours and 1 hour respectively. The tensile test may be carried using standard tensile test samples of the bainitic steel soaked in the slat bath for 24 hours, 72 hours and 1 hour. 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. 6, the bainitic steel sample soaked for 24 hours exhibits much linear (elastic) properties up to an yield stress of about 1300 MPa. Further, with increase in load, the sample fractures at an ultimate tensile stress value of around 1500 MPa with an elongation of around 36%. Now, as evident from the graph illustrated in figure. 7, the bainitic steel sample soaked for 72 hours exhibits much linear (elastic) properties up to an yield stress of about 920 MPa. Further, with increase in load, the sample fractures at an ultimate tensile stress value of around 1500 MPa with an elongation of around 19.5%. Further, as evident from the graph illustrated in figure. 8, the bainitic sample soaked for 1 hour exhibits much linear (elastic) properties up to an yield stress of about 950 MPa. Further, with increase in load, the sample fractures at an ultimate tensile stress value of around 1520 MPa with an elongation of around 11%.

Test results for different trials of the tensile test carried out on the bainitic steel samples soaked for 24 hours, 72 hours and 1 hour are shown in the below tables 2, 3 and 4:

Bainitic sample soaked for 24 hours:

Test No. Yield Stress (MPa) Ultimate Tensile Strength (MPa) % Elongation
Test 1 1131 1510 36 %
Test 2 1120 1513 31 %

Table-2

Bainitic steel sample soaked for 72 hours:

Test No. Yield Stress (MPa) Ultimate Tensile Strength (MPa) % Elongation
Test 3 930 1509 19.5 %
Test 4 925 1536 16 %

Table-3

Bainitic steel sample soaked for 1 hour:

Test No. Yield Stress (MPa) Ultimate Tensile Strength (MPa) % Elongation
Test 5 949 1499 11 %
Test 6 955 1520 12 %

Table-4
As explained in the aforementioned paragraphs, microstructure of the bainitic steel exhibits better mechanical properties when soaked for a time period of about 24 hours. The mechanical property considered was % elongation i.e. ductility, which has drastically increased to a range of about 30% - 36%.

It should be understood that the experiments are carried out for a particular composition of the bainitic 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 bainitic steel as well.

In an embodiment of the disclosure, a method of the present disclosure aids in producing high ductile bainitic steel. The method is intended to make a carbide free bainitic matrix and with a very low volume of enriched stable (stable) austenite in the microstructure of the bainitic steel. The stable (stable) austenite during tensile straining favours twin formation, which results in exhibiting high ductile property.

In another embodiment, the present disclosure discloses a method for producing a high ductile steel is disclosed. The method may be useful from industrial point. The method according to this embodiment, the steel comprising Carbon (C) at about 0.4 wt% to about 0.51 wt%; Manganese (Mn) at about 0.4 wt% to about 0.5 wt %; Silicon (Si) at about 1.5 wt% to about 2 wt %; Sulphur (S) at about 0.01 wt% to about 0.02 wt%; Phosphorus (P) at about 0.02 wt%; Nitrogen (N) at about 0.004 wt% to about 0.007 wt%; Chromium (Cr) at about 0.8 wt% to about 1 wt%; Molybdenum (Mo) at about 0.2 wt% to about 0.3 wt%; Cobalt (Co) at about 1.4 wt% to about 2 wt%; Vanadium (V) at about 0.05 wt% to about 0.08 wt%; Copper (Cu) at about 0.2 wt% to about 0.3 wt%; Nickel (Ni) at about 2.5 wt% to about 3 wt %; Aluminum (Al) at about 0.7 wt% to about 1.3 wt %; the balance being Iron (Fe) optionally along with incidental elements, may be produced 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 done in a salt bath for the second predetermined time. Then, the steel slab may be subjected for second hot working process such as rolling process, to obtain high ductile bainitic steel. The steel processed by this method may have bainitic microstructure of at least 95%, and may exhibit ductility of 30% to 36%.

In an embodiment of the present disclosure, the high ductile bainitic steel of the present disclosure may be used any application including but not limiting to automotive 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
101-108 Flowchart blocks
101 Forming stage
102 Soaking in furnace 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 Soaking in salt bath stage