WO2020157665A1

WO2020157665A1 - A high strength-high ductile steel and a method of manufacturing thereof

Info

Publication number: WO2020157665A1
Application number: PCT/IB2020/050670
Authority: WO
Inventors: Debasis PODDAR; Basudev Bhattacharya; Chiradeep Ghosh; Vivek Kumar Singh
Original assignee: Tata Steel Limited
Priority date: 2019-01-29
Filing date: 2020-01-29
Publication date: 2020-08-06

Abstract

The present disclosure discloses a method for producing high strength-high ductile steel. The process starts from soaking a steel of a defined composition, at a first pre-determined temperature for a first pre-set period of time. Then, hot working the steel ingot by a first hot working process, and cooling the steel. The method further comprises of 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. Then, hot working, on the steel by a second hot working process, followed by isothermal quenching of the steel at a second predetermined temperature, and cooling the steel to a room temperature.The method further includes tempering, the steel at a third predetermined temperature for a third pre-set period of time, to obtain high strength-high ductile steel. The high strength-high ductile steel primarily comprises a martensitic microstructure.

Description

TITLE:“A HIGH STRENGTH-HIGH DUCTILE STEEL AND A METHOD OF

MANUFACTURING THEREOF”

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-high ductile steel. Further embodiments of the disclosure disclose a method for manufacturing the a high strength-high ductile 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, application of ultra-high strength steel is limited to military and rail road application with its limited ductility. Researchers and technologists have been exploring the possibility of developing high ductile-high strength steel for the last several years. From the microstmctural point of view, bainitic steel consists of bainitic ferrite and austenite. On the other hands, TWIP steels (Twin Induced Plasticity) mainly consist of stable austenite which exhibits high strength and high ductility. However, the high manganese content in TWIP steel limits its mass industrial and commercial applications due to constraints associated with processing and higher production cost. It is well established that“twin” formation, in the stable austenite under the compressive and tensile loading process, is the fundamental aspect of achieving the high strength and high ductility in TWIP steel. In contrast, the austenite content in bainitic steel transforms into martensite under various compressive and tensile loading conditions. This transformation certainly is the key factor for achieving high strength at the cost of ductility. Usually, high strength bainitic steel is used for specific application like in annour vehicle, however, not being used for mass scale industrial manufacturing type 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 tiirough 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 strength- high ductile steel. The method comprising soaking, a steel 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 lwt %, 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 2 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.5 wt% to about 1 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 3 wt% to about 3.5 wt %, Cobalt (Co) at about 1.5 wt% to about 2 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. Then, hot working, on the steel by a first hot working process, and cooling the steel. Subsequently, the steel is subjected for re-heating to the first predetermined temperature, and the annealing the steel in the first predetemiined 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. Finally, tempering, the steel at a third predetermined temperature for a third pre-set period of time, to obtain high strength- high ductile steel. The high strength-high ductile steel primarily comprises a martensitic microstructure.

In an embodiment, the high strength-high ductile steel exhibits ultimate tensile strength ranging from about 1850 MPa to 1936 MPa, and the ductility ranging from about 17% to about 19%.

In an embodiment, the high-strength-high ductile steel comprises a martensitic microstructure and a retained austinite microstructure.

In an embodiment, the re-heating, soaking, and tempering are carried out in a furnace, and the cooling is normal air cooling. Further, the isothermal quenching is carried out in a hot water bath.

In an embodiment, 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 1250 °C, the second pre- detennined temperature is about 100°C, and the third pre-determined temperature ranging from about 100°C to about 160°C.

In an embodiment, volume fraction of the retained austinite in the microstructure of the high strength-high ductile steel increases gradually with increase in the third pre-determined temperature.

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, and the third pre-set period of time is about 24 hours.

In an embodiment, the first hot working process is a forging process, and the second hot working process is a rolling process. The 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 900°C to about 950°C during the rolling process. In an embodiment, the method comprises, cooling the steel to a room temperature in air after the tempering.

In an embodiment, the method comprises cleaning the steel by acid pickling at the temperature below 50°C, preferably at room temperature.

In an embodiment, the steel is produced by casting alloy in at least one of air-melting furnace, and vaccum furace.

In another non-limiting embodiment of the disclosure a high strength-high ductile steel is disclosed. The steel 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 lwt %, 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 2 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.5 wt% to about 1 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 3 wt% to about 3.5 wt %, Cobalt (Co) at about 1.5 wt% to about 2 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.

In an embodiment, the high strength-high ductile comprises martensitic microstructure and retained austinite microstructure. The microstructure of the high strength-high ductile steel forms a twin in stable austenite when subjected to tensile straining.

In yet another non-limiting embodiment, a method for manufacturing high strength-high ductile steel is disclosed. The method comprising: 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 lwt %, 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 2 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.5 wt% to about 1 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 3 wt% to about 3.5 wt %, Cobalt (Co) at about 1.5 wt% to about 2 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. Then, subjecting, the steel to a roughing process, and quenching the steel, and hot working on the steel by a second hot working process. The method further includes tempering, the steel at a third predetermined temperature for a third pre-set period of time, to obtain high strength-high ductile steel. The high strength-high ductile steel primarily comprises martensitic microstructure. Ill an embodiment, the high-strength-high ductile steel comprises martensitic microstructure and retained austinite microstructure.

In still another embodiment, an automotive light weight structural part comprising a high- strength-high ductile steel as described above is disclosed.

It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment of the disclosure.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which: Figure.1 is a flowchart illustrating a method for producing high strength-high ductile steel, according to an exemplary embodiment of the present disclosure.

Figure.2 is graphical flow diagram illustrating a method for producing high strength-high ductile steel, according to an exemplary embodiment of the present disclosure. Figure. 3 illustrates microstructure of the high strength-high ductile steel manufactured by the method of figure. 1, which is only quenched, according to an exemplary embodiment of the present disclosure. Figures. 4-7 illustrates microstructure of the high strength-high ductile steel manufactured by the method of figure. 1, which is quenched and tempered at different temperatures, in accordance with some embodiments of the present disclosure.

Figure. 8a illustrates a graphical representation of results of X-ray Diffraction analysis carried out on the high strength-high ductile steel sample hot rolled-quenched at 100°C, and tempered at 100°C for a period of 24 hours, according to an exemplary embodiment of the present disclosure.

Figures. 8b illustrates a graphical representation of results of X-ray Diffraction analysis carried out on the high strength-high ductile steel sample hot rolled-quenched at 100°C, and tempered at 160°C for a period of 24 hours, according to an exemplary embodiment of the present disclosure.

Figure. 9 is graphical representation of stress versus elongation, obtained during tensile test of the high-strength high ductile steel tempered for the time period of about 24 hours at various temperatures, 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 alterative 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 w'ord "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 high ductile steel and a method for manufacturing or producing a high strength and high ductile steel. Ductility is an important property for the mass industrial application of high strength material like steel. As of now, high strength and high ductile steels are produced as TWIP steel. However, high strength bainitic steel is used for specific application like in armor vehicle, however, not being used for mass scale industrial manufacturing type application because of its poor ductility. On the other hand high manganese content in TWIP steel limits its mass industrial and commercial applications due to constraints associated with processing and higher production cost. The present disclosure provides a method for manufacturing high strength- high ductile steel in which ferritic-bainite phase is replaced by fine martensitic strip which can incorporate higher strength value. It is expected that the fine martensite plates will contribute little positively to the ductility of the steel in comparison to the transformed martensite from blocky retained austenite. Additionally, the presence of mechanically stable thin-film retained austenite (RA) in-between the lath martensite plate and interior of the martensite strip play a major role to impart ductility to the martensite base current steel. Here, the austenite phase needs to be at lower SFE (stacking fault energy) range to promote twin in the retained austenite. This would be a more effective option to incorporate the additional strength without impairing ductility. Hence, a TWIP effect in the martensitic based steel might bring considerable changes in the conventional strength and ductility inverse relationship concept. Accordingly, by adopting the method of present disclosure, the steel with martensitic microstructure with retained austinite may be produced.

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

In the method of manufacturing high strength-high 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 is initially soaked at a first predetermined temperature for a first pre-set period of time in the furnace. In an embodiment, the first pre-determined temperature is about 1250°C. The hot steel 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 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 45 minutes. Further, the steel is subjected to a second hot working process like a rolling process. In an embodiment, the steel is rolled for at least five times between the one or more pair of rollers. The steel is then isothermally quenched to a second predetermined temperature of about 100°C, and cooled to a room temperature. Further, the steel is tempered in a third predetermined temperature for a third pre-set time of about 24 hours, and cooled to a room temperature. In an embodiment, the third predetermined temperature may be about 100°C to about 160°C. Hence, processing the steel by the method of present disclosure, results in microstructural changes to form a martensitic microstructure with retained austinite. Thus, imparts high ductility in the range of about 17% to 19%, and ultimate tensile strength of about 1850 MPa to 1936 MPa. Thus, the steel obtained by the method of the present disclosure, will have remarkably high plasticity and strength combination. Therefore, the 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 strength-high 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 2 are exemplary embodiments of the present disclosure illustrating a flowchart depicting and a process flow diagram of a method for manufacturing high strength-high ductile steel. 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 -high 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-high 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 furace, 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 Iwt %, 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 2 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.5 wt% to about 1 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%. Nickel (Ni) at about 3 wt% to about 3.5 wt %, Cobalt (Co) at about 1.5 wt% to about 2 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 °C to 1280 °C, preferably 1250 °C. 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 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 1220 °C to 1280 °C, preferably 1250 °C. 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.

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 to 6 times. During rolling process of the steel ingot, the temperature of the steel ingot reduces below the first pre-detennined temperature i.e. the temperature reduces from about 900°C to about 950°C. After rolling process, the steel ingot is isothermally quenched [shown in block 107] to a second predetennined temperature. In an embodiment, the second pre-detennined temperature is about 100°C. In an embodiment, the isothermal quenching of the steel ingot may be carried out in hot water maintained at 100 °C and kept for 5 minutes to achieve equilibrium temperature. After quenching, the steel ingot is cooled to room temperature by air cooling as shown in step 108. Subsequently, the steel ingot is subjected for tempering at third predetermined temperature for a third predetermined time as shown in block 109. In an embodiment, the third predetennined temperature may range from about 100°C to about 160°C, 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 furace and cooled in air to room temperature. The steel ingot processed by the method of the present disclosure results in microstructural changes to form high strength-high ductile steel. In an embodiment, the steel comprises a substantially martensitic microstructure with retained austinite (RA).

The method optionally comprises cleaning the steel by acid pickling at the temperature below 50 °C, preferably at room temperature.

In an embodiment, the high strength-high ductile steel exhibits ultimate tensile strength ranging from about 1850 MPa tol936 MPa, and ductility ranging from about 17% to about 19%. Advantageously, it may be noted that the transfer from annealing furnace to rolling mill took about 7 seconds, the five reduction passes took about 30 seconds and transfer from rolling mill to water quench at 100° C took less than 2 seconds. This signifies the processing is much faster when compared to the conventional processes. 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 0.8 to 1 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 - 3.5 wt.% . 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 act 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 to 1.06 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%. Thus 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 tlie 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 tempering temperature in improvement of ductility and strength of the steel. The composition of the steel for which the tests are carried out is as shown in below table 1.

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 predetermined 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.

Figures. 3-7 show the microstructure of the quenched, and the quenched and tempered (at different temperature) sample. This consists of clear lath-martensite plates separated by thin retained austenite (RA) film with overall homogeneity in microstructure and granular the retained austenite (RA) in the interior of lath-m artensite .

Referring to Figure. 3 which shows microstructure of the steel which is quenched to 100 °C, and not tempered. This microstructure clearly shows the presence of lath-martensite with thin retained austenite (RA) and granular retained austenite (RA) in the interior of lath-martensite . On the other hand. Figure. 4 shows microstructure of the steel which is quenched to 100 °C and subjected to tempering for 24 hour at 100 °C. It is evident from Figure. 4 that, microstructure of lath-martensite with thin retained austenite (RA) and granular retained austenite (RA) is present in the interior of lath-martensite. Now referring to Figures. 5-7 which illustrates microstructure of the steel which is quenched to 100 °C and subjected to tempering for 24 hour at 120 °C, quenched to 100 °C and subjected to tempering for 24 hour at 140 °C, and quenched to 100 °C and subjected to tempering for 24 hour at 160 °C respectively. It is evident that all the figures clearly shows the microstructure of lath-martensite with thin retained austenite (RA) and retained austenite (RA) in the interior of lath-martensite. It is significant to note that retained austenite (RA) progressively increased with tempering temperature increased.

Hence, the final microstructure of the steel fonned by the method described above includes matristic microstructure and retained austinite (RA) microstructure. Thus, 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.

Referring to figures. 8a and 8b, which are exemplary embodiments of the present disclosure, illustrating graphical representation of the results of XRD analyses carried out on the steel sample i.e. hot rolled-quenched to 100 °C and tempered at 100 °C for 24 hours and tempered at 160 °C for 24 hours respectively. The parent material i.e. only quenched steel exhibits martensitic microstructure (see Figure.3) with low content of granular retained austenite (RA) in the interior of martensite plate. Thin film may be observed in between martensitic plate. The presence of retained austenite (RA) may be confirmed by the XRD results (Figures 8a and 8b). The results indicate that there is a gradual increase in the volume fraction of the retained austenite (RA) as the tempering temperature is changing from 100°C to 160°C temperature.

As evident from Figures 8a and 8b, volume fraction of retained austinite is about 5.5% and 8.1% respectively for tempered at 100 °C for 24 hours and tempered at 160 °C for 24 hours. Also, there are no such apparent microstructural changes that can be observed with the change in the tempering temperature. It is expected that carbon diffusion to austenite controls the ductility without lowering the considerable amount of strength of the material. It may be assumed that, the rule of phase mixture which may be applicable here, thus contribute to mechanical properties accordingly. The carbon diffusion from martensite may decrease the strength a little. This diffused carbon simultaneously harden the retained austenite (RA). Hence overall, there has not been any appreciable variation in the mechanical strength of the steel, nevertheless it affects the ductility as demonstrated in the current results (Table: 2) below. Carbon depleted martensite surely may contribute to the ductility a little and on the other hand carbon enriched austenite become more stable and thus may lower the SFE of austenite which is favourable for twin formation during straining. It plays a significant role in enhancing the ductility. To obtain the high strength, martensitic hard-phase may be considered as a key factor in the strengthening mechanism. Further, the extremely fine untransformed austenite is present in the matrix and thus stabilized mechanically so much so that during straining twin formation may be favoured. Additionally, the retained austinite (RA) will always be surrounded by hard martensitic phase which will not allow retained austinite (RA) to be transformed to martensite since it is limited by volume expansion and remain in highly stressed condition (as demonstrated by austenite peak-shift in the Figures. 8a and 8b). This may contribute to the mechanical stability of the retained austinite (RA).

Now, during straining the dislocation may get a chance to interact with twin contained austenite. As the amount of strain increases, there is a possibility that the dislocation will directly interact with the nano-twin boundaries. This may contribute more towards getting high ductility. This resembles to the TWIP effect. Therefore, from the experimental results it is evident that the low temperature tempering of the steel for the period of 24 hours may play crucial role in improving the ductility and strength of the steel.

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

Table: 2 wherein, the samples are:

a) M100T: Hot rolled-Quenched (100 °C water) and Tempered at 100 °C for 24 Hrs. b) M120T: Hot rolled-Quenched (100 °C water) and Tempered at 120 °C for 24 Hrs. c) M140T: Hot rolled-Quenched (100 °C water) and Tempered at 140 °C for 24 Hrs. d) M160T: Hot rolled-Quenched (100 °C water) and Tempered at 160 °C for 24 Hrs. Now referring to Figure. 9, which is an exemplary embodiment 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 of the steel tempered for 24 hours at various temperatures like 100 °C, 120 °C, 140 °C, and 160 °C. 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. 9, the steel which is tempered: at 100 °C possess around 1940 MPa UTS with around 7.8 % total elongation, at 120°C possess around 1936 MPa UTS with around 19 % total elongation, at 140°C possess around 1904 MPa UTS with around 18.5 % total elongation, and at 160 °C possess around 1853 MPa UTS with around 16.8 % total elongation. Hence overall, there has not been any appreciable variation in the mechanical strength of the steel, nevertheless it affects the ductility.

As explained in the aforementioned paragraphs, the method of the present disclosure is most easiest processing route compared to others available method, and the method achieves about 2 GPa strength and ductility raging between 17 to 19% in the 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-high 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 lwt %, 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 2 wt %, Vanadium (V) at about 0.05 wt% to about 0.08 wt%, Aluminum (Al) at about 0.5 wt% to about 1 wt %, Chromium (Cr) at about 0.8 wt% to about 1 wt%, Nickel (Ni) at about 3 wt% to about 3.5 wt %, Cobalt (Co) at about 1.5 wt% to about 2 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 defonnation, and then subsequently the steel slab may be subjected for quenching. The quenching may be done in a hot water for the second predetermined time. Then, the steel slab may be subjected for second hot working process such as rolling process. The method further includes tempering, the steel at a third predetermined temperature for a third pre-set period of time, to obtain high strength-high ductile steel. The high strength-high ductile steel primarily comprises martensitic microstructure.

In an embodiment of the present disclosure, the high strength-high 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-high ductile steel may be used in any other industrial structural applications.

Equivalents:

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both temis. 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

Claims

We claim:

1. A method for manufacturing high strength-high ductile steel, the method comprising:

soaking, a steel 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 lwt %;

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 2 wt %;

Vanadium (V) at about 0.05 wt% to about 0.08 wt%;

Aluminum (Al) at about 0.5 wt% to about 1 wt %;

Chromium (Cr) at about 0.8 wt% to about 1 wt%;

Nickel (Ni) at about 3 wt% to about 3.5 wt %;

Cobalt (Co) at about 1.5 wt% to about 2 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; and

tempering, the steel at a third predetennined temperature for a third pre-set period of time, to obtain high strength-high ductile steel; wherein, the high strength-high ductile steel primarily comprises a martensitic microstructure.

2. The method as claimed in claim 1, wherein the high strength-high ductile steel exhibits ultimate tensile strength ranging from about 1850 MPa to 1936 MPa.

3. The method as claimed in claim 1, wherein the high strength-high ductile steel exhibits ductility ranging from about 17% to about 19%.

4. The method as claimed in claim 1, wherein the high-strength-high ductile steel comprises a martensitic microstructure and a retained austinite microstructure.

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

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

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

8. The method as claimed in claim 7, wherein the steel is maintained in the hot water bath for about 5 minutes to achieve equilibrium temperature.

9. The method as claimed in claim 1, wherein the first pre-determined temperature is about 1250 °C.

10. The method as claimed in claim 1, wherein the second pre-determined temperature is about 100°C.

11. The method as claimed in claim 1 , wherein the third pre-determined temperature ranging from about 100°C to about 160°C.

12. The method as claimed in claim 11, wherein a volume fraction of the retained austinite in the microstructure of the high strength-high ductile steel increases gradually with increase in the third pre-determined temperature.

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

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

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

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

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

18. The method as claimed in claim 17, wherein the 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 900°C to about 950°C during the rolling process.

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

20. the method as claimed in claim 1, wherein the method comprises cleaning the steel by acid pickling at the temperature below 50 °C, preferably at room temperature.

21. The method as claimed in claim 1, wherein the steel is produced by casting alloy in at least one of air-melting furnace, and vaccum furace.

22. A high strength-high ductile steel, comprising:

Carbon (C) at about 0.4 wt% to about 0.42 wt%;

Manganese (Mn) at about 0.5 wt% to about lwt %;

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 2 wt %;

Vanadium (V) at about 0.05 wt% to about 0.08 wt%;

Aluminum (Al) at about 0.5 wt% to about 1 wt %;

Chromium (Cr) at about 0.8 wt% to about 1 wt%;

Nickel (Ni) at about 3 wt% to about 3.5 wt %;

Cobalt (Co) at about 1.5 wt% to about 2 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.

23. The high strength-high ductile steel as claimed in claim 22, wherein the high-strength-high ductile steel comprises martensitic microstructure and retained austinite microstructure.

24. The high strength-high ductile steel as claimed in claim 22, wherein the microstructure of the high strength-high ductile steel fomis a twin in stable austenite when subjected to tensile straining.

25. The high strength-high ductile steel as claimed in claim 22, wherein the high strength-high ductile steel exhibits ultimate tensile strength ranging from about 1850 MPato1936MPa.

26. The high strength-high ductile steel as claimed in claim 22, wherein the high strength-high ductile steel exhibits ductility ranging from about 17% to about 19%.

27. A method for manufacturing high strength-high 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 Iwt %;

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 2 wt %;

Vanadium (V) at about 0.05 wt% to about 0.08 wt%;

Aluminum (Al) at about 0.5 wt% to about 1 wt %;

Chromium (Cr) at about 0.8 wt% to about 1 wt%;

Nickel (Ni) at about 3 wt% to about 3.5 wt %;

Cobalt (Co) at about 1.5 wt% to about 2 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;

subjecting, the steel to a roughing process, and quenching the steel:

hot working on the steel by a second hot working process;

tempering, the steel at a third predetermined temperature for a third pre-set period of time, to obtain high strength-high ductile steel;

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

28. The method as claimed in claim 27, wherein the high strength-high ductile steel exhibits ultimate tensile strength ranging from about 1850 MPa to 1936MPa.

29. The method as claimed in claim 27, wherein the high strength-high ductile steel exhibits ductility ranging from about 17% to about 19%.

30. The method as claimed in claim 27, wherein the high-strength-high ductile steel comprises martensitic microstructure and retained austinite microstructure.

31. An automotive structural part comprising a high-strength-high ductile steel as claimed in claim 22.