Description:
The following specification particularly describes the nature of the invention and the manner in which it is to be performed.

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 cold rolled steel sheet. Further embodiments of the disclosure disclose a method of manufacturing the high strength cold rolled steel sheet, which exhibits tensile strength greater than 1700 MPa.

BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron (Fe), carbon (C), and other alloying elements such as Phosphorous (P), Sulphur (S), Manganese (Mn), Silicon (Si), Chromium (Cr), Magnesium (Mg) 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, ship building, tools, automobiles, machines, bridges, and numerous other applications. Steel obtained from steel making process may not possess all the desired properties. Therefore, steel may be subjected to secondary processes such as 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 microstructure of the steel may be modified during such operations. As a result of heat treatment, the steel undergoes phase transformation, influencing mechanical properties like strength, ductility, toughness, hardness, drawability and the like. 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 recent past, use of ultra-high strength steel sheet with tensile strength of 2 GPa has been adapted in automotive applications to provide greater factor of safety. Conventionally, steels with tensile strength greater than 2GPa have been manufactured by hot forming process in mass scale production. However, steels formed by hot forming process pose problems such as low yield strength and other challenges such as surface defects, demanding for surface coating and the like, which is undesired. With advancement in technology, cold forming process may be adapted. However it would be difficult to achieve 2GPa without incorporation of a good amount of martensite phase in the steel. Ductility of the initial steel has been provided by the austenite content in the bainitic microstructure. The phase hardness of bainitic-ferrite is higher than that of the common ferrite. During cold rolling, generation of dislocation in the bainitic-ferrite will further increase the strength of the steel. However, the maximum strength comes from the transformation of austenite in the bainitic microstructure during cold deformation. This austenite has gradually transformed into the martensite depending on the percentage of cold-rolled reduction. In this regard, the presence of austenite with lower stacking fault energy will lead to the delayed transformation which in turn will allow extended number of rolling passes.

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 the 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 of the present disclosure, a high strength cold rolled steel sheet is disclosed. The high strength cold rolled steel sheet comprises composition in weight percentage (wt%) of carbon (C) about 0.3% to about 0.35%, manganese (Mn) at about 0.7% to about 1%, sulphur (S) up-to 0.02%, phosphorus (P) up-to 0.02%, nitrogen (N) up-to 0.001%, silicon (Si) at about 1.3% to about 1.5%, aluminium (Al) at about 0.1% to about 0.4%, chromium (Cr) at about 0.4% to about 0.5%, molybdenum (Mo) at about 0.1% to about 0.22%, copper (Cu) at about 0.l5% to about 0.2%, nickel (Ni) at about 2.3% to about 2.7%, cobalt (Co) at about 1% to about 1.5%, and the balance being Iron (Fe) optionally along with incidental elements. The high strength cold rolled steel sheet comprises primarily a bainitic microstructure.
In an embodiment, the high strength cold rolled steel sheet exhibits tensile strength ranging from about 1700 MPa to 2200 MPa, elongation ranging from about 6% to 10% and yield stress of about 2200 MPa.
In an embodiment, the microstructure of the steel sheet is microstructure represented by, in area%, the bainite of about 70% to 75%, martensite of about 20% and austenite less than 5%.
In another non-limiting embodiment, there is provided a method for manufacturing high strength cold rolled steel sheet. The method initially starts from casting steel comprising composition in weight percentage (wt%) of carbon (C) about 0.3% to about 0.35%, manganese (Mn) at about 0.7% to about 1%, sulphur (S) up-to 0.02%, phosphorus (P) up-to 0.02%, nitrogen (N) up-to 0.001%, silicon (Si) at about 1.3% to about 1.5%, aluminium (Al) at about 0.1% to about 0.4%, chromium (Cr) at about 0.4% to about 0.5%, molybdenum (Mo) at about 0.1% to about 0.22%, copper (Cu) at about 0.l5% to about 0.2%, nickel (Ni) at about 2.3% to about 2.7%, cobalt (Co) at about 1% to about 1.5%, and the balance being Iron (Fe) optionally along with incidental elements. The casted steel is then heated to a first predetermined temperature and is soaked at the first predetermined temperature for a first predetermined time. Upon heating the casted steel, the casted steel may be deformed in a first hot working process and followed by cooling the steel to an ambient temperature. The method further includes reheating the steel to a second predetermined temperature and annealing the steel for a second predetermined time. The reheated steel may be subjected to a second hot working process at the third predetermined temperature to form a steel sheet. Upon completion of the second hot working process, the steel sheet may be quenched to a fourth predetermined temperature in a bath and, followed by soaking the steel sheet in the bath at the fourth predetermined temperature for a third predetermined time. Furthermore, the method includes cooling the steel sheet to room temperature and subjecting the steel sheet to cold rolling process for further reduction in thickness of the steel sheet. The cold rolled steel sheet is then tempered at a fifth predetermined temperature for a fourth predetermined time, to produce high strength cold rolled steel sheet. The high strength cold rolled steel sheet primarily comprises a bainitic microstructure.
In an embodiment, the first predetermined temperature is about 1250 °C and the first predetermined time is about three hours.
In an embodiment, the first hot working process is a hot forging process.
In an embodiment, the second hot working process is a hot rolling process. The hot rolling process is performed by passing the steel through a pair of rolls and rolling is carried out for at least 5 times.
In an embodiment, temperature of the steel sheet drops to the third predetermined temperature ranging from about 950 °C to 1000 °C, during the hot rolling process.
In an embodiment, the fourth predetermined temperature is about 350 °C and the third predetermined time is about 24 hours.
In an embodiment, quenching is an isothermal quenching process, and the bath is a salt bath.
In an embodiment, cooling is air cooling.
In an embodiment, during cold rolling process, shear stresses transform austenite in the steel sheet microstructure, into martensite.
In an embodiment, thickness of the steel sheet after the second hot working process is about 5 mm, and thickness of the steel sheet after cold rolling process is about 1.5 mm.
In an embodiment, the method includes a step of grinding the steel sheet before cold rolling process.
In an embodiment, the fifth predetermined temperature ranges from about 200 °C to 500 °C and the fourth predetermined time is about 24 hours.
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 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 a high strength cold rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figures. 2a and 2b is a graphical representation of the method of Figure. 1, for producing the high strength cold rolled steel sheet.

Figure. 3 illustrates microstructure of conventional hot rolled steel.

Figure. 4 illustrates microstructure of the high strength cold rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 5 illustrates a graphical representation of results of X-ray Diffraction analysis carried out on the high strength cold rolled steel sheet sample, which is tempered for a period of 24 hours at 200 °C, according to an exemplary embodiment of the present disclosure.

Figure. 6 illustrates a graphical representation of the results of X-ray Diffraction analysis carried out on the high strength cold rolled steel sheet sample tempered for a period of 24 hours at 500 °C, 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 high strength cold rolled steel sheet having composition 1, 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 high strength cold rolled steel sheet having composition 2, according to an exemplary embodiment of the present disclosure.

Figure. 9 is a graphical representation of stress versus elongation, obtained during tensile test of the high strength cold rolled steel sheet having composition 3, 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 alternatives falling within the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusions, such that a method that includes 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 cold rolled steel sheet and a method for manufacturing or producing the high strength cold rolled steel sheet. Conventionally, steels with tensile strength greater than 2GPa have been manufactured by hot forming process in mass scale production. However, steels formed by hot forming process pose problems such as lower yield strength and other challenges such as surface defects, demanding for surface coating and the like, which is undesired. Accordingly, the method for manufacturing the high strength cold rolled steel sheet, which exhibits a tensile strength ranging from about 1700 MPa to 2200 MPa is described in the present disclosure. The high strength cold rolled steel sheet may be widely employed in industrial applications such as but not limiting to automotive industry.

In the method of manufacturing the high strength cold rolled steel sheet, a first step may include casting the steel comprising composition in weight percentage (wt%) of carbon (C) about 0.3% to about 0.35%, manganese (Mn) at about 0.7% to about 1%, sulphur (S) up-to 0.02%, phosphorus (P) up-to 0.02%, nitrogen (N) up-to 0.001%, silicon (Si) at about 1.3% to about 1.5%, aluminium (Al) at about 0.1% to about 0.4%, chromium (Cr) at about 0.4% to about 0.5%, molybdenum (Mo) at about 0.1% to about 0.22%, copper (Cu) at about 0.l5% to about 0.2%, nickel (Ni) at about 2.3% to about 2.7%, cobalt (Co) at about 1% to about 1.5%, and the balance being Iron (Fe) optionally along with incidental elements. The casted steel is then heated to a first predetermined temperature and may be soaked at the first predetermined temperature for a first predetermined time. Upon heating the casted steel, the casted steel may be deformed in a first hot working process, and followed by cooling the steel to an ambient temperature. In an embodiment, the first hot working process may be a hot forging process, the first predetermined temperature is about 1250 °C and the first predetermined time is about three hours.
The method further includes reheating the steel to a second predetermined temperature and annealing the steel for a second predetermined time. In an embodiment, the second predetermined temperature is about 1200 °C and the second predetermined time is about 45 minutes. The reheated steel may be subjected to a second hot working process at the third predetermined temperature to form a steel sheet. In an embodiment, the second hot working process may be a hot rolling process and the third predetermined temperature is a temperature drop during the hot working process, which ranges from about 950 °C to 1000 °C. Upon completion of the second hot working process, the steel sheet may be quenched to a fourth predetermined temperature in a bath and, followed by soaking the steel sheet in the bath at the fourth predetermined temperature for a third predetermined time. In an embodiment, quenching is an isothermal quenching process, the bath is a salt bath, and the third predetermined time is about 24 hours. The method further includes cooling the steel sheet to room temperature and subjecting the steel sheet to cold rolling process for further reduction in thickness of the steel sheet. The cold rolled steel sheet is then tempered at a fifth predetermined temperature for a fourth predetermined time, to produce high strength cold rolled steel sheet. In an embodiment, the fifth predetermined temperature ranges from about 200 °C to 500 °C and the fourth predetermined time is about 24 hours.
The high strength cold rolled steel sheet processed by the method of the present disclosure includes bainitic microstructure of about 70% to 75%, austenite less than 5% and martensite of about 20%. Further, the high strength cold rolled steel sheet exhibits tensile strength ranging from about 1700MPa to 2200 MPa, elongation ranging from about 6% to 10% and yield strength of about 2200 MPa. Therefore, the high strength cold rolled steel sheet may be used in wide variety of industrial applications. As an example, the application may include but not limiting to an automotive industry.
Henceforth, method of manufacturing the high strength cold rolled steel sheet of the present disclosure is explained with the help of figures. 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 exemplary embodiment of the present disclosure which illustrates a flowchart depicting a method for manufacturing the high strength cold rolled steel sheet, and Figures. 2a and 2b are exemplary embodiments which illustrates schematic representation of the method of manufacturing the high strength cold rolled steel sheet. In the present disclosure, mechanical properties such as strength, tensile strength, yield strength and hardness of the final microstructure of the steel sheet may be improved. The high strength cold rolled steel sheet is produced by cold rolling process, aiding in reducing manufacturing complexity and reducing cost of production of the steel sheet. The method is now described with reference to the schematic representation and flowchart blocks. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject matter described herein.

At block 101, steel comprising a composition in weight percentage (wt%) of carbon (C) about 0.3% to about 0.35%, manganese (Mn) at about 0.7% to about 1%, sulphur (S) up-to 0.02%, phosphorus (P) up-to 0.02%, nitrogen (N) up-to 0.001%, silicon (Si) at about 1.3% to about 1.5%, aluminium (Al) at about 0.1% to about 0.4%, chromium (Cr) at about 0.4% to about 0.5%, molybdenum (Mo) at about 0.1% to about 0.22%, copper (Cu) at about 0.l5% to about 0.2%, nickel (Ni) at about 2.3% to about 2.7%, cobalt (Co) at about 1% to about 1.5%, and the balance being Iron (Fe) optionally along with incidental elements, may be casted. As an example, liquid steel with the above-mentioned composition and range of alloying elements may be continuously casted into a slab. The liquid steel of the specified composition is first continuously casted either in a conventional continuous caster or a thin slab caster.

At block 102 and as seen in Figure. 1, the method includes heating the casted steel to a first predetermined temperature and soaking the same at the first predetermined temperature for a first predetermined time. In an embodiment, the first pre-determined temperature is about 1250°C and the first predetermined time is about three hours. As an example, casted steel may be heated in a furnace.

Once, the steel is heated and soaked as per block 102, the steel may be deformed by subjecting the steel to a first working process and followed by cooling [as shown in block 103]. In an embodiment, the first working process may be a hot forging process. Forging is a mechanical process in which the structure may be deformed by applying localized compressive stresses. As an example, the localized compressive stresses may be induced using a motor driven 0.5 ton forging hammer. Blows are delivered by the hammer on to the steel ingot in order to induce localized compressive stresses, which may result in internal grain deformation, thus enhancing strength and stiffness of the steel. In an embodiment, cooling of the steel is normal air cooling.

At block 104, the method includes of re-heating the steel to a second pre-determined temperature and annealing the steel for a second predetermined time. In an embodiment, the second pre-determined temperature may be around 1200 ºC and the second predetermined time may be about 45 minutes. Further, the re-heated and annealed steel may be subjected to a second hot working process at a third predetermined temperature to form a steel sheet [as shown in block 105 and Figure. 2a]. In an embodiment, the second hot working process may be hot rolling process. The hot rolling process may be carried out by passing the steel through a pair of rolls and rolling may be carried out for at least five times to reduce the thickness of the steel to about 5 mm.

After hot rolling the steel as per block 105, the steel sheet may be quenched to a fourth predetermined temperature in a bath and followed by soaking the steel sheet in the bath at the fourth predetermined temperature for a third predetermined time and cooling the steel to room temperature [as shown in block 106]. During quenching, the retained austenite becomes enriched in carbon and stabilize mechanically such that, during straining new twin(s) formation may be favored, thus facilitating in improving ductility. In an embodiment, the bath is a salt bath, the fourth predetermined temperature is about 350 ° C and the third predetermined time is about 24 hours.

In an embodiment, the steel sheet may be grinded after quenching process, to remove all scaling and to make both surfaces parallel to each other.

Now referring to block 107 and Figure. 2b, the steel sheet is subjected to cold rolling process, for further reduction in thickness of the steel sheet. As an example, the thickness of the steel sheet may be reduced to about 1.5 mm. In an embodiment, during cold rolling the shear stress developed may transform the austenite into martensite, which contributes to increase tensile strength greater than 2000 MPa and elongation of about 5%. The obtained ductility after cold rolling is due to presence of remaining untransformed and highly mechanically stable retained austenite. This small stable austenite may contain twin.

After subjecting the steel to cold working process (as per block 107), the steel sheet may be tempered at a fifth predetermined temperature for a fourth predetermined time [as shown in block 108 and best seen in Figure. 2b]. Tempering is a mechanical process, which involves heating the steel to a high temperature, which is below the melting point and then cooling the steel in air. Tempering the steel sheet may contribute to improve elongation due to evolution of austenite. In an embodiment, the fifth predetermined temperature ranges from around 200 °C to 500 °C and the fourth predetermined time is about 24 hours. Steel processed by the method of the present disclosure results in microstructural changes to form the high strength cold rolled steel sheet. In an embodiment, the high strength cold rolled steel comprises bainitic microstructure in a range of about 70% to 75%, martensite in the range of about 20% and austenite lesser than 5%.

In an embodiment, the high strength cold rolled steel sheet exhibits tensile strength ranging from about 1700MPa to 2200 MPa, elongation ranging from about 6% to 10% and yield strength of about 2200 MPa.

Figure. 3 illustrates microstructure of a hot rolled bainitic steel, formed by conventional methods. The microstructure of the hot rolled bainitic steel consists of bainitic-ferrite and austenite. The hot rolled steel exhibits tensile strength of about 1300 MPa and elongation of about 28%. It is expected that during tensile straining of the hot rolled bainitic steel, the thin mechanically stable austenite plates may not transform into martensite, rather there are greater chances of formation of twin in stable austenite. Referring to Figures. 4 to 6, which illustrates microstructure and X-ray Diffraction analysis of the high strength cold rolled steel manufactured by the method of the present disclosure. During cold rolling, shear stresses transform austenite of the hot-rolled bainitic microstructure into martensite. This contributed to observed increased in the UTS to greater than 2000 MPa. The reduction of retained austenite is clearly visible in X-ray Diffraction analysis of the cold rolled steel [seen in Figures. 5 and 6]. The obtained ductility after cold rolling is due to presence of remaining untransformed and highly mechanically stable retained austenite, which may contain twin.

In an embodiment, the method of the present disclosure includes melting, casting, heat treatment, thermomechanical and cold-rolled routes, which are simple. Further, in conventional techniques, cast ingots were homogenized for 2 days at 1200 °C to 1300 °C. Whereas, in the method of the present disclosure, the steel is homogenized for 3 hours, thus conserving energy. Additionally, the thermomechanical process (i.e., the hot rolling process) is simple and can be directly started from homogenization temperature. It includes rough passing in the rolling mill before quenching to 350 °C. Quenching at 350 °C is a low temperature, that does not require huge energy consumption and this temperature can be readily maintained by deliberate use of waste heat. Therefore, the method of the present disclosure aids in reducing energy consumption and thus a cost-effective high strength steel manufacturing process.

In an embodiment, the method employs normal air cooling and eliminates use of vacuum melting furnace, unlike conventional methods. This aids in reducing cost of the steel manufacturing process.

Following portions of the present disclosure, provides details about the proportion of each element in a composition of the high strength cold rolled steel sheet and their role in enhancing properties.

Carbon (C) may be used in the range of 0.3 to 0.35 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. Carbon (C) below the above range, it may decrease the solute solution strengthening of bainitic-ferrite.
Silicon (Si) may be used in the range of about 1.3 to 1.5 wt%. Silicon (Si) suppresses the formation of carbide, which leads to carbide free bainitic matrix to improve ductility and impact toughness.
Manganese (Mn) may be used in the range of about 0.7 to 1 wt%. Lower content of manganese (Mn) may retain the toughness and lower the possibility of carbide formation aiming to produce carbide free bainitic matrix to improve the ductility. However, the hardenability may decrease as a result of reducing manganese. Manganese content limit may be considered low or high as per extent of hardenability required with carbide free bainitic matrix.
Chromium (Cr) may be used in the range of about 0.4 to 0.5 wt%. Chromium (Cr) of this proportion can substantially increase the strength and hardenability of the steel. It can vary beyond above range for customized strength and hardenability requirement.
Nickel (Ni) may be used below 2.3 to 2.7 wt%. Nickle (Ni) content limited to maximum to increase the residual austenite carbon without sacrificing the hardenability. It increases the strength and toughness.
Molybdenum (Mo) may be used in the range of about 0.1 to 0.22 wt%. Molybdenum (Mo) 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.
Cobalt (Co) may be used in the range of about 1 to 1.5 wt%. Cobalt (Co) can accelerate the rate of reaction by increasing free energy difference between the ferrite and austenite phases. The tentative range is 1.4 to 1.6 but can vary on the basis of mechanical properties requirement.
Aluminium (Al) may be used in the range of about 0.1 to 0.4 wt%. Aluminium (Al) of this proportion improves strength and ductility. It can also be added more or less as a solid solution strengthener.
Copper (Cu) may be used in the range of about 0.15 to 0.2 wt%. Copper (Cu) of this proportion increase the solid solution strengthening and aiming to boost up the toughness. This can be added more to increase the strength and toughness.
Example:

Further embodiments of the present disclosure will be now described with an example of particular compositions of the high strength cold rolled steel, which are illustrated in Tables 1, 2 and 3. Experiments have been carried out for specific compositions of the high strength cold rolled steel formed by using the method of the present disclosure. Results have been compared on various fronts to show the contribution of tempering temperature in improving mechanical properties of the high strength cold rolled steel.
Composition 1: Quenching at 350 °C for 24 hrs.
C Mn S P Si Al Cr Co Mo Ni Cu N
0.33 1 0.011 0.017 1.6 0.3 0.46 1.41 0.23 2.45 0.24 71 ppm

Table: 1

Composition 2: Quenching at 395 °C for 24 hrs.
C Mn S P Si Al Cr Co Mo Ni Cu N
0.12 0.56 0.011 0.017 1.61 0.85 0.9 1.6 0.22 1.54 0.22 71 ppm

Table: 2

Composition 3: Quenching at 410 °C for 24 hrs.
C Mn S P Si Al Cr Co Mo Ni Cu N
0.37 1 0.011 0.017 1.55 0.13 0.47 0.51 0.21 1.02 0.22 71 ppm

Table: 3

In an embodiment of the present disclosure, various experiments were carried out on the high strength cold rolled steel sheet sample for different compositions as mentioned in Table-1, Table-2, and Table-3, which may be subjected to quenching and tempering at temperatures, during formation of the high strength cold rolled steel sheet. For conducting the experiment, the high strength cold rolled steel sheet samples were prepared for conducting microstructural investigation and conducting tensile test. As an example, tensile testing may be performed using Instron machine as per ASTM standard and XRD, SEM tests were conducted to investigate microstructure of the high strength cold rolled steel sheets.

Accordingly, table 4 illustrates mechanical properties of steel having composition 1 and subjected to quenching at 350 °C and tempered at different temperatures.

Sample Detail 0.2 % Yield
Stress (MPa) UTS (MPa) % Elongation
Hot-Rolled Bainite 870 1318 28%
Cold-Rolled Bainite 1765 2066 5.37
Cold-Rolled-200 Tempered 2145 2177 6.7
Cold-Rolled-300 Tempered 2067 2133 6.19
Cold-Rolled-400 Tempered 1789 1883 8.7
Cold-Rolled-500 Tempered 1610 1700 10.26

Table: 4

Table 5 illustrates mechanical properties of steel having composition 2 and subjected to quenching at 395 °C and tempered at different temperatures.

Sample Detail 0.2 % Yield
Stress (MPa) UTS (MPa) % Elongation
Cold-Rolled Bainite 1144 1416 6
Cold-Rolled-200 Tempered 1635 1682 5.4
Cold-Rolled-300 Tempered 1604 1646 5
Cold-Rolled-400 Tempered 1489 1534 5.4
Cold-Rolled-500 Tempered 1326 1412 7

Table: 5
Table 6 illustrates mechanical properties of steel having composition 3 and subjected to quenching at 410 °C and tempered at different temperatures.

Sample Detail 0.2% Yield
Stress (MPa) UTS (MPa) % Elongation
Cold-Rolled Bainite 1417 1876 6.7
Cold-Rolled-200 Tempered 2074 2155 6
Cold-Rolled-300 Tempered 1966 2036 6.5
Cold-Rolled-400 Tempered 1644 1728 7.6
Cold-Rolled-500 Tempered 1468 1511 9.9

Table: 6

As explained in the aforementioned result tables and as seen in Figures. 7 to 9, the high strength cold rolled steel sheet exhibits better mechanical properties when tempered at 200 °C for a time period of about 24 hours.

It is evident that, the effect of tempering is quite significant in increasing the elongation. This is due to the little evolution of austenite in the bainitic-martensite microstructure. Though, the strength and yield stress have decreased over the increased of tempering temperature. However, the steel demonstrates a high stability in the mechanical properties even on the 24 hours of long tempering duration. The steel also exhibits strength from 1.7 to 2 GPa with 6% to 10 % elongation, respectively. Therefore, thin gauge sheet steel will have a great potential for automobile applications and a great futuristic possibility to replace the hot-forming process that will provide a cost-effective option.

It should be understood that the experiments are carried out for particular compositions of the high strength cold rolled steel sheet and the results brought out in the previous paragraphs are for the composition shown in Tables 1, 2 and 3. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the high strength cold rolled steel sheet, as well.

In an embodiment, the method employs normal air cooling and eliminates use of vacuum melting furnace, unlike conventional methods. This aids in reducing cost of the steel manufacturing process.
In an embodiment of the present disclosure, the high strength cold rolled steel sheet 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 Casting stage
102 Heating stage
103 Deforming stage
104 Re-heating and annealing stage
105 Second hot working stage
106 Quenching stage
107 Cold rolling stage
108 Tempering stage

Claims:We Claim:

1. A high strength cold rolled steel sheet, comprising:
composition in weight percentage of:
carbon (C) at about 0.3% to about 0.35%,
manganese (Mn) at about 0.7% to about 1%,
sulphur (S) up-to 0.02%,
phosphorus (P) up-to 0.02%,
nitrogen (N) up-to 0.01%,
silicon (Si) at about 1.3% to about 1.5%,
aluminium (Al) at about 0.1% to about 0.4%,
chromium (Cr) at about 0.4% to about 0.5%,
molybdenum (Mo) at about 0.1% to about 0.22%,
Copper (Cu) at about 0.l5% to about 0.2%,
nickel (Ni) at about 2.3% to about 2.7%,
cobalt (Co) at about 1% to about 1.5%, and
the balance being Iron (Fe) optionally along with incidental elements;
wherein, the steel sheet comprises primarily a bainitic microstructure.
2. The high strength cold rolled steel sheet as claimed in claim 1, wherein the microstructure of the steel sheet is microstructure represented by, in area%, the bainite of about 70% to 75%, martensite of about 20% and austenite less than 5%.
3. The high strength cold rolled steel sheet as claimed in claim 1, wherein the high strength cold rolled steel sheet exhibits tensile strength ranging from about 1700MPa to 2200 MPa.
4. The high strength cold rolled steel sheet as claimed in claim 1, wherein the high strength cold rolled steel sheet exhibits elongation ranging from about 6% to 10%.
5. The high strength cold rolled steel sheet as claimed in claim 1, wherein the high strength cold rolled steel sheet exhibits yield stress of about 2200 MPa.
6. A method for manufacturing a high strength cold rolled steel sheet, the method comprising:
casting a steel of a composition comprising in weight percentage (wt%) of:
carbon (C) about 0.3% to about 0.35%,
manganese (Mn) at about 0.7% to about 1%,
sulphur (S) up-to 0.02%,
phosphorus (P) up-to 0.02%,
nitrogen (N) up-to 0.001%,
silicon (Si) at about 1.3% to about 1.5%,
aluminium (Al) at about 0.1% to about 0.4%,
chromium (Cr) at about 0.4% to about 0.5%,
molybdenum (Mo) at about 0.1% to about 0.22%,
Copper (Cu) at about 0.l5% to about 0.2%,
nickel (Ni) at about 2.3% to about 2.7%,
cobalt (Co) at about 1% to about 1.5%, and
the balance being Iron (Fe) optionally along with incidental elements;
heating, the steel to a first predetermined temperature and soaking at the first predetermined temperature for a first predetermined time;
deforming, the steel in a first hot working process;
cooling, the steel processed in the first hot working process to ambient temperature;
re-heating, the steel to a second predetermined temperature and annealing the steel for a second predetermined time;
subjecting the steel to a second hot working process, at a third predetermined temperature to from a steel sheet;
quenching, the steel sheet processed by the second hot working process to a fourth predetermined temperature in a bath, and soaking in the steel sheet in the bath at the fourth predetermined temperature for a third predetermined time;
cooling, the steel sheet to room temperature;
subjecting, the steel sheet to cold rolling process for further reduction in thickness; and
tempering, the steel sheet processed in the cold rolling process at a fifth predetermined temperature for a fourth predetermined time;
wherein, the high strength cold rolled steel sheet comprises primarily a bainitic microstructure.
7. The method as claimed in claim 6, wherein the high strength cold rolled steel sheet exhibits tensile strength ranging from about 1700MPa to 2200 MPa.
8. The method as claimed in claim 6, wherein the high strength cold rolled steel sheet exhibits elongation in ranging from about 6% to 10%.
9. The method as claimed in claim 6, wherein the high strength cold rolled steel sheet exhibits yield stress of about 2200 MPa.
10. The method as claimed in claim 6, wherein the microstructure of the steel sheet is microstructure represented by, in area%, the bainite of about 70% to 75%, martensite of about 20% and austenite less than 5%.
11. The method as claimed in claim 6, wherein the first predetermined temperature is about 1250 °C and the first predetermined time is about three hours.
12. The method as claimed in claim 6, wherein the first hot working process is a hot forging process.
13. The method as claimed in claim 6, wherein the second predetermined temperature is about 1200 °C and the second predetermined time is about 45 minutes.
14. The method as claimed in claim 6, wherein the second hot working process is hot rolling process.
15. The method as claimed in claim 14, wherein the hot rolling process is performed by passing the steel through a pair of rolls and rolling is carried out for at least 5 times.
16. The method as claimed in claim 14, wherein temperature of the steel sheet drops to the third predetermined temperature ranging from about 950 °C to 1000 °C, during the hot rolling process.
17. The method as claimed in claim 6, wherein the fourth predetermined temperature is about 350 °C and the third predetermined time is about 24 hours.
18. The method as claimed in claim 6, wherein the quenching is an isothermal quenching process, and the bath is a salt bath.
19. The method as claimed in claim 6, wherein the cooling is air cooling.
20. The method as claimed in claim 6, wherein during cold rolling process, shear stresses transform austenite in the steel sheet microstructure, into martensite.
21. The method as claimed in claim 6, wherein thickness of the steel sheet after the second hot working process is about 5 mm, and thickness of the steel sheet after cold rolling process is about 1.5 mm.
22. The method as claimed in claim 6, comprises grinding the steel sheet before cold rolling process.
23. The method as claimed in claim 6, wherein the fifth predetermined temperature ranges from about 200 °C to 500 °C and the fourth predetermined time is about 24 hours.