TECHNICAL FIELD

The present disclosure relates in general to a field of material science and metallurgy. Particularly, but not exclusively, the present disclosure relates to a high-strength high-ductile cold-rolled steel sheet. Further embodiments of the disclosure disclose a method for manufacturing the high-strength high-ductile cold-rolled steel sheet with tensile strength of minimum 780 MPa, with very good weldability and formability.

BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron, carbon and other elements such as Phosphorous (P), Sulphur (S), Nitrogen (N), Manganese (Mn), Silicon (Si), Chromium (Cr), etc. Because of its high tensile strength and low cost, steel may be considered as a major component in wide variety of applications. Some of the applications of the steel may include buildings, ships, tools, automobiles, machines, bridges and numerous other applications. The steel obtained from steel making process may not possess all the desired properties. Therefore, the steel may be subjected to secondary processes such as heat treatment for controlling material properties to meet various needs in the intended applications.

Generally, heat treatment may be carried out using techniques including but not limiting to annealing, normalising, hot rolling, quenching, and the like. During heat treatment process, the material undergoes a sequence of heating and cooling operations, thus, the microstructures of the steel may be modified during such operation. As a result of heat treatment, the steel undergoes phase transformation, influencing mechanical properties like strength, ductility, toughness, hardness, drawability etc. The purpose of heat treatment is to increase service life of a product by improving its mechanical properties or to prepare the material for improved manufacturability.

With rising concerns over global environmental problems and demand from automotive industry for higher collision safety of motor vehicles, impose conflicting requirements on materials used for the vehicle bodies. The vehicle bodies are required to be stronger yet lighter at the same time. In the recent past several technological advancements have been made, and such advancements may include commercialization of fuel cell vehicles and use of lighter materials like aluminium, composites etc. These materials meet the desired material properties but the associated problems like formability, reliability and recyclability and the higher cost make these materials commercially less attractive and hence, such materials are used only for specific types of components and vehicles. Thus, it becomes inevitable to re-look at advanced high strength steels to meet the desired properties, as usage of steel mitigates the above-mentioned issues with other materials.

There have been several developments in the field of advanced high strength steels, which poses good combination of tensile strength, elongation, formability and weldability to address some of the afore-mentioned concerns. One such process includes development dual phase (DP) steel. Some of the literatures known in the art teaches a method to manufacture 780 MPa strength DP steels with ferrite-bainite microstructure. Though these type of DP steels has high strength and high ductility, the stretch flangeability may be lower due to presence of ferrite-bainite two phase microstructure. One of the patent literatures disclose DP780 steel without the presence of molybdenum and niobium exhibiting 20-70% banite in the final morphology. Another patent discloses a high strength DP780 with 10-30% martensite in the microstructure exhibiting less than 20 per elongation. In yet another prior art, a method for manufacture steel with minimum tensile strength of 780 MPa and having a 20-80 % martensite remaining being ferrite in microstructure is disclosed. Due to significant difference in hardness between ferrite and martensite phases, the hole expansion ratio of dual phase steel may also be low. Further, attempts have been made at developing steels with high strength and high stretch flangeability by addition of high amount of Molybdenum. However, this makes the steel expensive.

One more patent literature known in the art has focussed on developing high strength steel with fully ferritic microstructure by fixing the carbon with high amount of titanium (Ti) to form TiC precipitates. Also, in some publications it is disclosed that chemistry with high levels of silicon. However, surface quality of these steels may be poor as silicon promotes formation of scale during hot rolling.

In most of the conventional process as described above, primarily, precipitation strengthening has been utilized as the key mechanism to obtain strength. However, it will be difficult to replicate similar level of precipitation strengthening in every grains of the microstructure due to the variation in processing conditions across width and length of strip in actual plant scale production. This creates inhomogeneity in deformation behaviour inside the material during hole expansion test and hence, poses difficulty in obtaining high formability in precipitation strengthened steels produced through large scale processing in hot strip mills.

Hence, there is a need for an economically attractive and technically viable way of developing high-strength cold-rolled steel with high ductility, formability and weldability without aforementioned limitations.

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

SUMMARY OF THE DISCLOSURE

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

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

In a non-limiting embodiment of the present disclosure, there is provided a high-strength high-ductile cold-rolled steel sheet. The steel sheet includes composition of carbon (C) at about 0.07 wt.% to at about 0.09 wt.%, manganese (Mn) at about 1.8 % to at about 1.9 %, sulphur (S) up-to 0.015 %, phosphorous (P) up-to 0.025 %, silicon (Si) at about 0.4 wt.% to at about 0.5 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 0.005 ppm, titanium (Ti) at about 0.015 wt.% to at about 0.030 wt.%, molybdenum (Mo) at about 0.15 wt.% to at about 0.30 wt.%, boron (B) 10 ppm to 20 ppm and the balance being Iron (Fe) optionally along with incidental elements. The high-strength high-ductile cold-rolled steel sheet comprises martensite microstructure of about 25 % to about 40 % and balance being ferrite microstructure.

In an embodiment, the high-strength high-ductile cold-rolled steel sheet exhibits ultimate tensile strength greater than 780 MPa, and ductility ranging from about 14 % to about 28 %.

In an embodiment, the high-strength high-ductile cold-rolled steel sheet suitable for a hot-dip galvanizing process.

In another non-limiting embodiment of the present disclosure, there is provided a method for manufacturing a high-strength high-ductile cold-rolled steel sheet. The method includes steps of firstly casting a steel slab of a composition comprising: carbon (C) at about 0.07 wt.% to at about 0.09 wt.%, manganese (Mn) at about 1.8 % to at about 1.9 %, sulphur (S) up-to 0.015 %, phosphorous (P) up-to 0.025 %, silicon (Si) at about 0.4 wt.% to at about 0.5 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 0.005 ppm, titanium (Ti) at about 0.015 wt.% to at about 0.030 wt.%, molybdenum (Mo) at about 0.15 wt.% to at about 0.30 wt.%, boron (B) 10 ppm to 20 ppm and the balance being Iron (Fe) optionally along with incidental elements. Then, subjecting the steel slab to heating in a first predetermined temperature for a first predetermined time and performing hot working at a second predetermined temperature to produce a steel sheet. Subsequently, the steel sheet is cooled to a third predetermined temperature. The method further involves cold working the steel sheet at third predetermined temperature, followed by soaking the cold worked steel sheet at fourth predetermined temperature for a second predetermined time with a predetermined cooling rate. Finally, the steel sheet is coiled at a fifth predetermined temperature to obtain the high-strength high-ductile cold-rolled steel sheet. The high-strength high-ductile cold-rolled steel sheet obtained by the method includes martensite microstructure of about 25 % to about 40 % and balance being ferrite microstructure.

In an embodiment, the steel slab is hot charged into a furnace for heating.

In an embodiment, the casting is carried out in a continuous casting process. The continuous casting process is performed in at least one of continuous caster and a thin slab caster. Further, the temperature of the steel slab at exit of the thin slab caster is maintained above 1000 °C.

In an embodiment, the first predetermined temperature ranges from about 1200 °C to about 1250 °C, and the first predetermined time from about 30 minutes to about three hours.

In an embodiment, the hot working is a hot rolling process.

In an embodiment, the hot working is performed in a finish rolling mill, and the second predetermined temperature higher than critical transformation temperature for austenite (Ar3). Further, the second predetermined temperature ranges from of about 850 °C to about 940 °C.

In an embodiment, the third predetermined temperature is room temperature.

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

In an embodiment, soaking of cold rolled steel sheet for a second predetermined time with a predetermined cooling rate is a continuous annealing process.

In an embodiment, the fourth predetermined temperature employed in continuous annealing process ranges from about 790 °C to about 820 °C.

In an embodiment, the second predetermined time employed in continuous annealing process is ranging from about 75 seconds.

In an embodiment, the predetermined cooling rate employed in continuous annealing process is ranging from about 30 °C/second to about 40 °C/second.

In an embodiment, the fifth predetermined temperature is about 600 °C to about 680 °C.

In an embodiment, the reduction in thickness of the steel sheet after cold- working is at least 40 %.

In an embodiment, the method includes performing a phosphating treatment on the steel sheet. The size of the phosphate crystals formed are about 2 µm to about 3 µm with P-ratio of the phosphate layer more than 90 %.

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

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

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

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

Figure.1 is a flowchart illustrating a method for producing high-strength high-ductile cold-rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figure.2 is a graphical representation of cooling profile followed during the continuous annealing process for producing high-strength high-ductile cold-rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 3 illustrates the optical micrograph of the high-strength high-ductile cold-rolled steel sheet at a magnification of 100X.

Figure. 4 illustrates the optical micrograph of the high-strength high-ductile cold-rolled steel sheet at a magnification of 400X.

Figure. 5 illustrates the Scanning Electron Microscopy (SEM) images (SEM) of the high-strength high-ductile cold-rolled steel sheet at a magnification of 2000X.

Figure. 6 illustrates graphical representation of formability test conducted on high-strength high-ductile cold-rolled steel sheet.

Figures. 7a, 7b,7c and 7d illustrates graphical representations of resistance spot weldability tests conducted on high-strength high-ductile cold-rolled steel sheet.

Figure. 8 illustrates Scanning Electron Microscopy (SEM) of the high-strength high-ductile cold-rolled steel sheet after phosphating process.

The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION

The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristic of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.

Embodiments of the present disclosure discloses a high-strength high-ductile cold-rolled steel sheet and a method for manufacturing a high-strength high-ductile cold-rolled steel sheet. Strength, ductility and formability are some of the important properties for the mass industrial application of high strength materials like steel. As of now, high strength steel sheets with tensile strength more than 780 MPa are produced by the methods in which precipitation strengthening has been utilized as the key mechanism to obtain strength. However, it will be difficult to replicate similar level of precipitation strengthening in every grains of the microstructure due to the variation in processing conditions across width and length of strip in actual plant scale production. Accordingly, the method of present disclosure, discloses a production of high-strength high-ductile cold- rolled steel sheet, with tensile strength of minimum 780 MPa with excellent formability and weldability. The present disclosure is directed towards producing a low carbon cold-rolled steel sheet with tensile strength greater than 780 MPa along with percentage elongation of about 14 % to about 28 %. The cold rolled steel sheet may be widely employed to make automotive components requiring high strength, high ductility, formability and weldability.

Accordingly, the method of manufacturing high-strength high-ductile cold-rolled steel sheet, includes first step of producing the steel slab of composition comprising in weight percentage of: carbon (C) at about 0.07 wt.% to at about 0.09 wt.%, manganese (Mn) at about 1.8 % to at about 1.9 %, sulphur (S) up-to 0.015 %, phosphorous (P) up-to 0.025 %, silicon (Si) at about 0.4 wt.% to at about 0.5 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 0.005 ppm, titanium (Ti) at about 0.015 wt.% to at about 0.030 wt.%, molybdenum (Mo) at about 0.15 wt.% to at about 0.30 wt.%, boron (B) 10 ppm to 20 ppm and the balance being Iron (Fe) optionally along with incidental elements by any manufacturing process including but not limiting to casting. The steel slab is then reheated to a temperature of about 1200 °C to 1250 °C. The steel slab may be then subjected to hot working process including but not limited to hot-rolling process. The hot charged steel slab may be hot-rolled in finishing mill. The finish rolling temperature may vary in the range of Ar3 i.e. around 850 °C to 940 °C where Ar3 is the critical transformation temperature for austenite transformation of austenite to ferrite starts at equilibrium. After the hot rolling step, the steel sheet may be cooled to room temperature. Steel sheet may be further subjected to cold working process including but not limited to cold rolling. Cold rolling may be carried out at room temperature without the aid of any external energy. Cold-rolled steel sheet may be subjected to heat treatment process such as but not limited to continuous annealing process and soaked at about 790 °C to 820 °C for about 75 seconds. The predetermined cooling rate employed during continues annealing process may ranging from about 30 °C/second to about 40 °C/second. Then, the steel sheet may be coiled in the coiling temperature TCT which varies in the range 600 °C to 680 °C. The cold-rolled steel sheet according to the present disclosure may have a microstructure comprises martensite microstructure of about 25 % to about 40 % and balance being ferrite microstructure.

Strength of dual phase may be primarily obtained from strength of martensite structure.

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 cold-rolled steel sheet. However, such exemplary embodiments should not be construed as limitations of the present disclosure, since the method may be used on other types of steels where such need arises. A person skilled in the art may envisage various such embodiments without deviating from scope of the present disclosure.

Figures. 1 and 2 are exemplary embodiments of the present disclosure illustrating a flowchart of a method for producing high-strength high-ductile cold-rolled steel sheet, and a graphical representation of cooling profile followed during the continuous annealing process. In the present disclosure, mechanical properties such as strength, ductility, formability and weldability the final microstructure of the steel may be improved. The steel produced by the method of the present disclosure, includes a martensite-ferrite microstructure. The method is now described with reference to the flowchart blocks and is as below. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject. The method is particularly applicable to high-strength high ductile steel cold-rolled steel and it may also be extended to other type of steels as well.

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

At block 101, a steel of desired alloy composition is formed by any of the manufacturing process including but not limited to casting process. In embodiment, the steel is made in the form of slabs, and the alloy may be prepared in at least one of air-melting furnace, and vacuum furnace. The steel slab may have composition of: carbon (C) at about 0.07 wt.% to at about 0.09 wt.%, manganese (Mn) at about 1.8 % to at about 1.9 %, sulphur (S) up-to 0.015 %, phosphorous (P) up-to 0.025 %, silicon (Si) at about 0.4 wt.% to at about 0.5 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 0.005 ppm, titanium (Ti) at about 0.015 wt.% to at about 0.030 wt.%, molybdenum (Mo) at about 0.15 wt.% to at about 0.30 wt.%, boron (B) 10 ppm to 20 ppm and the balance being Iron (Fe) optionally along with incidental elements may be casted in at least one of continuous caster and a thin slab caster. When casted in a thin slab caster, the temperature of the cast slab may not be allowed to drop to a temperature below 1000 °C, as it might result in loss of ductility and cause formation of edge cracks. There may also be a possibility of the micro alloying elements to precipitate out and it may then become difficult to completely dissolve the precipitates in the subsequent reheating process rendering them ineffective for precipitation strengthening.

The method then includes the step of heating as shown in block 102. After casting the steel slab with the specified composition, the slabs may be heated in a furnace to a first predetermined temperature for a first predetermined time. In an embodiment, the steel slab may be hot charged into the furnace for hearing, and the first predetermined temperature may be greater than 1150 °C, preferably in the range of 1200 °C to 1250 °C, and the first predetermined time ranges from 30 minutes to about 3 hours. In an embodiment, the first predetermined temperature may be maintained at least above 1150 °C, to ensure complete dissolution of any precipitates that may have formed in the preceding processing steps. A first predetermined temperature greater than 1250 °C may also not desirable because it may lead to grain coarsening of austenite and/or excessive scale loss.

The method further includes a step or a stage of hot working the steel slab by a hot working process [shown in block 103] immediately after heating. In an embodiment, the first hot working process may be but not limited to hot-rolling. Hot-rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform at high temperatures and hot-rolling is carried out above the recrystallization temperature of the steel. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. In an embodiment, the steel slab may be hot-rolled in finishing mill. The finish rolling temperature may vary in the range of Ar3 i.e. around 850 °C to 940 °C where Ar3 is the critical transformation temperature for austenite transformation of austenite to ferrite starts at equilibrium. After completion of hot rolling process, the hot-rolled steel sheet may be cooled to a third predetermined temperature which is generally room temperature. As shown in block 104, the cooling of the hot-rolled sheet to room temperature may be carried out by normal air cooling.

Now referring to block 105, the method further includes the step of cold working of sheet at room temperature. In an embodiment, the cold working may include but not limited to cold rolling process. Cold rolling is a metal forming process in which metal sheet is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform at low temperature and cold-rolling temperature will be well below the recrystallization temperature. In an embodiment, the cold rolling may be performed in room temperature without the aid of any external heat. During cold work the point defect density (vacancies, self-interstitials etc.) and dislocation density increase within the steel sheet. This leads to an increase in the internal energy (stored energy) of the steel sheet. The energy storage within the steel sheet during cold working process can be used as driving force for re-crystallization on subsequent annealing process. In an embodiment, reduction in thickness of the steel sheet after cold working may be at least 40 %.

The method further includes soaking of cold-rolled steel sheet at a fourth predetermined temperature for a second predetermined time using a predetermined cooling rate as show in block 106. Soaking may be carried out in a continuous annealing process at a temperature ranging from about 790 °C to 820 °C for about 75 seconds. The cooling rate employed in the continuous annealing process may be ranging from about 30 °C/ second to 40 °C/second. Continuous annealing is a more commonly used annealing process in modern steel heat treatment process. Annealing is the process of relieving the internal stresses in the steel that may be built up during the cold rolling process. Steel sheet hardens after cold rolling due to the dislocation tangling generated by plastic deformation. Annealing is therefore carried out to soften the material. The annealing process comprises heating, holding of the material at an elevated temperature (soaking), and cooling of the material. Heating facilitates the movement of iron atoms, resulting in the disappearance of tangled dislocations and the formation and growth of new grains of various sizes, which depend on the heating and soaking conditions. These phenomena make hardened steel crystals to recover and recrystallize into softened one. Furthermore, during annealing process precipitates decompose to solute atoms which subsequently dissolve into the steel matrix on heating and holding to get homogenous microstructure.

Now referring to block 107, the method further includes the step of coiling. Coiling may be carried out at a coiling temperature or a fifth predetermined temperature. In an embodiment, the fifth predetermined temperature ranges from about 600 °C to 680 °C. It is preferable to keep the fourth predetermined temperature at 600 °C to 680 °C in order to achieve optimized ductility and formability Coiling below 600 °C may be avoided as it limits to the formation of bainite microstructure in the steel, and coiling above 680 °C, may create second phase such as pearlite. throughout the microstructure.

A schematic diagram of the cooling profile employed in continuous annealing is shown in Figure. 2. This ensures that the microstructure consists of about 25 % to about 40 % and balance being ferrite microstructure.

Strength may be primary obtained from the strength of martensite structures. This dual-phase steels comprise microstructures of hard phase martensite about 25 % to 40 % in relatively soft ductile and fine-grained ferrite matrix. The increase in martensite percentage increases ultimate tensile strength of the Dual Phase (DP) steel due to increasing volume fraction of harder phase. Presence of uniformly distributed martensite may act as a hindrance for dislocations motions. During plastic deformation, dislocations may be pinned by hard martensite phase and crack propagation will be slowed down by the presence of soft ferrite phase and hence ultimate tensile strength and ductility of the steel may be improved. However, further increase in amount of martensite phase beyond 40 % may decrease the strength and ductility due to the increase in brittleness imparted by too much martensite phase. Presence of large percentage of martensite increases the brittleness and act as crack initiation-propagation points. Hence, the strength and ductility of the steel deteriorates. In an embodiment of the present disclosure, the cold-rolled steel sheet comprises martensite microstructure of about 25 % to about 40 % in ferrite microstructure exhibits the ultimate tensile strength above 780 MPa along with 14 % to 28 % percentage elongation.

In an embodiment, the high-strength high-ductile cold-rolled steel sheet is suitable for a hot-dip galvanizing process.

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

Carbon (C): Carbon is an inherent component in steel, carbon helps in strengthening phases, and is often considered as a cheaper element to increase strength. Strength will not be achieved with carbon less than 0.02 wt.%. Carbon depresses A3 (upper critical temperature) increasing the volume fraction of austenite. During the slow cooling in continuous annealing process, some of this austenite will re-transform to ferrite, ejecting carbon back into the parent austenite grains, as the solubility of carbon in ferrite is negligible. As the remaining austenite becomes increasingly carbon enriched, it becomes more hardenable and the kinetics of both ferrite and pearlite formation are pushed to longer times.

Carbon enrichment in the austenite also results in a stronger martensite formation with a lower Ms (martensite start temperature) and leading to a stronger overall dual phase steel. If the Ms may be suppressed below 470 °C then martensite will form in the final cooling stage on a galvanising line, avoiding tempering process during the overage section. Typical over aging temperatures at continuous annealing process for dual phase steels may be approximately 300 °C, drops to 250 °C by the end of the overage, and some tempering of the martensitic phase is expected in these products. Steel with carbon beyond 0.09 % may require hamper welding and is hence limited to 0.07 to 0.09 % in the current invention.

Manganese (Mn) may be added in the range of about 0.18 wt.% to about 0.19 wt.%. Manganese contribute to the hardenability of austenite, and retard the kinetics of bainite formation, reducing the expected volume fraction of bainite that would form during the long high temperature overage on some galvanising lines. If Mn content is more than 1.9 wt.%, it will affect the weldability of the steel as well as on strength and if Mn content is less than 1.8 wt.%, it will affect the microstructure properties.

Sulphur (S) may be set to 0.015% or less. Sulphur levels may be reduced as far as possible to limit the formation of MnS particles, which are sufficiently brittle to form long stringers elongated in the rolling direction and adversely affect transverse strength, ductility, edge flangeability and bendability.

Phosphorus (P) may be added up to 0.025 wt.% maximum. Phosphorus content may be restricted to 0.025 wt.% maximum as higher phosphorus levels may lead to reduction in toughness and weldability due to segregation of phosphorus into columnar grain boundaries. Sulphur content has to be also limited otherwise it results in a very high inclusion level that deteriorates formability.

Present at residual levels. In combination with carbon and manganese, phosphorus is detrimental to weldability, and particularly cross tension strength and the ability to achieve ‘plug’ failures, as phosphorus will segregate to the columnar grain boundaries in the weldment weakening them. For this reason, phosphorus levels may be minimised, and hence the content of P is set to 0.020 % or less.

Silicon (Si) is a ferrite stabiliser and is insoluble in cementite. A small silicon addition may help to promote ferrite formation during the slow cool on continuous annealing process, this being more important if annealing top temperatures are such that a fully austenitic structure is formed. Due to its insolubility in cementite, silicon may act to suppress pearlite formation and the formation of bainitic carbides reducing the critical cooling rate that is required to obtain martensite. Due to its insolubility in cementite, it may also contribute to the resistance of martensite to undergo tempering, and hence the content of Si may be set to 0.4 wt.% or more and 0.5 wt.% or less. This level of Si in steel may also help in improving weldability.

Aluminium (Al) may be added in the range of 0.02 wt.% to 0.09 wt. %. Aluminium is used as a deoxidizer. It limits growth of austenite grains. Higher amount of Al causes casting issues and hence, should be restricted.

Nitrogen (N) may be added up to 0.005 ppm. Higher nitrogen may fix up higher amount of titanium through formation of TiN, which may not be effective strengthener. It reduces availability of Ti and hence reduces the effectiveness of Ti which is utilized for strengthening purposes through transformation strengthening and TiC precipitates. Also, increase in nitrogen content increases size of the TiN. Larger sized TiN will reduce both ductility and hole expansion ratio.

Titanium (Ti) may form nitrides with nitrogen content and may improve formability and hence the content of Ti is set to 0.015 wt. % to 0.030 wt.%. Further, titanium forms carbides which when finely dispersed promotes strengthening.

Molybdenum (Mo) is an alloying element, which improves hardenability of austenite by suppressing the pearlitic reaction, lowering the bainite transformation temperature, and hence the content of Mo is set to 0.15 wt.% to 0.30 wt.%.

Boron (B) is added to improve hardenability of austenite reducing sensitivity of final microstructure and tensile strength to cooling rate or to the temperature of annealing in the intercritical region, and hence the content of B may be set to 10 ppm to 20 ppm.

Example:

Further embodiments of the present disclosure will now be described with examples of particular composition of the steel. Experiments have been carried out for various set of specific composition of the steel by using method of the present disclosure. The compositions of the steel samples (A to H) for which the tests are carried out is as shown in below table 1. The compositions of table 1 were continuously cast in a slab caster and the slabs were hot-rolled followed by final cold rolling and continuous annealing. In order to, optimize the annealing process, varying continuous annealing temperature parameters were used as tabulated in in below table.1.

Samples % C % Mn % Si % P % S % Mo % Ti % Al N ppm B ppm Soaking temperature
(°C) YS, (Mpa) UTS, (Mpa) %El (50 GL)
A 0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 < 790 438 731 24
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 < 790 451 736 28.2
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 < 790 435 710 24.4
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 < 790 480 761 23.4
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 790 - 820 515 823 20.2
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 790 - 820 500 796 21.6
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 790 - 820 528 798 19.9
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 790 - 820 507 819 21.8
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 790 - 820 517 818 20.4
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 790 - 820 529 823 19.4
0.081 1.81 0.4 0.021 0.003 0.180 0.02 0.035 43 14 790 - 820 496 794 22
B 0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 < 790 460 755 22.7
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 < 790 428 707 25.9
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 < 790 505 779 20.5
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 < 790 436 720 28.7
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 < 790 464 750 23.8
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 790 - 820 508 808 21.8
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 790 - 820 514 799 21.4
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 790 - 820 511 811 21.6
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 790 - 820 502 833 21.2
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 790 - 820 537 809 19.7
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 790 - 820 515 835 19.9
0.082 1.82 0.4 0.022 0.003 0.183 0.022 0.035 45 18 790 - 820 531 832 19.7
C 0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 790 - 820 604 862 18.1
0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 790 - 820 565 867 16.4
0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 790 - 820 560 842 17.1
0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 790 - 820 533 844 14.1
0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 790 - 820 516 841 18.6
0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 < 790 441 725 23.1
0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 < 790 461 759 23.2
0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 < 790 450 734 25.1
0.088 1.89 0.45 0.022 0.0045 0.188 0.020 0.031 42 14 < 790 440 718 24.2
D 0.083 1.84 0.4 0.022 0.003 0.188 0.02 0.041 43 19 790 - 820 518 800 21
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 790 - 820 500 816 20
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 790 - 820 512 814 20.9
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 790 - 820 500 826 20.4
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 790 - 820 515 824 21.3
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 790 - 820 506 825 19.6
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 < 790 445 733 25.3
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 < 790 439 720 24.2
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 < 790 457 751 23.4
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 < 790 431 704 26.7
0.083 1.84 0.4 0.022 0.003 0.185 0.02 0.041 43 19 < 790 502 773 21.2
E 0.071 1.81 0.4 0.021 0.003 0.282 0.02 0.035 43 14 790 - 820 474 781 21
F 0.074 1.91 0.5 0.022 0.003 0.240 0.021 0.055 45 18 790 - 820 493 788 21.7
G 0.080 1.84 0.49 0.029 0.003 0.160 0.02 0.033 44 14 790 - 820 536 808 20.4
H 0.084 1.81 0.4 0.022 0.003 0.188 0.021 0.035 42 18 790 - 820 503 810 20.1
0.084 1.81 0.4 0.022 0.003 0.188 0.021 0.035 42 18 790 - 820 500 805 21.4
0.084 1.81 0.4 0.022 0.003 0.188 0.021 0.035 42 18 790 - 820 496 825 23.7
0.084 1.81 0.4 0.022 0.003 0.188 0.021 0.035 42 18 790 - 820 508 813 22.2
0.084 1.81 0.4 0.022 0.003 0.188 0.021 0.035 42 18 790 - 820 511 791 20.6
0.084 1.81 0.4 0.022 0.003 0.188 0.021 0.035 42 18 790 - 820 500 804 20.1
0.084 1.81 0.4 0.022 0.003 0.188 0.021 0.035 42 18 790 - 820 504 811 21.4
0.084 1.81 0.4 0.022 0.003 0.188 0.021 0.035 42 18 790 - 820 522 824 20.3
Table. 1

For each set of compositions, obtained yield strength (YS), ultimate tensile strength (UTS), percentage elongation (%) values are also tabulated in table. 1.In case of composition A to H, when the soaking temperature less than 790 °C during continues annealing process, the steel exhibits ultimate tensile strength less than 780 MPa but has large percentage elongation may be due the presence of more soft ferrite phase in the final microstructure. But when soaking during continuous annealing at around 790 °C to 820 °C, a well balance between strength and percentage elongation (ductility) has been obtained by achieving a microstructure having 20 % to 40 % martensite in ferrite matrix. Hence, the cold-rolled steel sheets soaked 790 °C to 820 °C exhibit very good formability which is a result of high-strength and good ductility (percentage elongation).

Referring to figure 3 and figure 4 which illustrate the optical micrograph of high-strength high-ductile cold-rolled steel sheet at a magnification of 100X and 400 X respectively. In the figures, grey regions correspond to ferrite and black regions correspond to martensite phase.

Reference now made to figure 5 which illustrates Scanning Electron Microscopy (SEM) image of high-strength high-ductile cold-rolled steel sheet at a magnification of 2000X. A focused electron beam of 20kv is made to fall on the surface of the metallographically polished steel sheet sample. The secondary electrons (SE2) which are emitted from very close to the specimen surface are collected using a secondary electron detector equipment attached with SEM to obtain the information for surface morphology. SEM images clearly depicts the uniform distribution and dispersion of martensite and ferrite phase with in cold-rolled steel sheet provide a composite effect leading to synergistic increase the strength and ductility of the cold-rolled steel sheet.

Referring to figure 6 which is an exemplary embodiment of the present disclosure, illustrating graphical representation of formability test conducted on high-strength high-ductile cold-rolled steel sheet of 1.44 mm thickness. Formability is the ability of a given steel sheet to undergo stretching deformation without being damaged. The extent to which the steel sheet can be stretched before failure occurs is known as the forming limit. From the forming limit graph, it is clear that major stain varies from about 0.2 % to about 0.45 % and minor stain varies from about to -0.2 % to about -0.5 %. These values clearly indicate that high-strength high-ductile cold-rolled steel sheet has good formability.

Thickness
(mm) Pressure
(kN) Tip diameter
(mm) a
Hold
(cycle) b
Upslope
(cycle) First step d
Cool
(cycle) First step F
Hold
(cycle)
c
Time
(cycle) I
Current
(kA) e
Time
(cycle) II
Current
(kA)
1.4 4.4 7.0 18 1.2 10 8 2 10 4 10
Table. 2

Referring to table 2 which is an exemplary embodiment of the present disclosure, tabulating the process parameters employed in resistance spot weldability test for high-strength high-ductile cold-rolled steel sheet. Figure 7a indicates the welding cycle employed in resistance spot weldability test.

Resistance spot welding (RSW) is a process in which metal surfaces are joined in one or more spots by resistance to the flow of electric current through work pieces that are held together under force by electrodes. The weld may be made by a combination of heat, pressure, and time. The process is used for joining sheet materials and uses shaped copper alloy electrodes to apply pressure and convey the electrical current through the work piece. Heat may be developed mainly at the interface between two sheets, eventually causing the material being welded to melt, forming a molten pool, the weld nugget. The molten pool may be contained by the pressure applied by the electrode tip and the surrounding solid metal. Resistance spot welding test is commonly used in the automotive industry, because it has the advantage which is high speed, high-production assembly lines and suitability for automation. The welded joints are exposed to the variables of load and pressure, these conditions made the welded joint to fracture.

Figure 7b depicts the plot of effect of current on welding nugget diameter. The welding current to be the most significant parameter controlling the weld tensile strength as well as the nugget diameter. From the graph, it is clear that high-strength high-ductile cold-rolled steel sheet withstands a sufficiently wide weldable current range.

In order to study the effects of the nugget diameter on the tensile shear strength (TS) and cross tensile strength (CTS) of the resistance spot welded joints on the steel sheets metal, and to estimate the amount of load could be apply to each different nugget, the plots of nugget dia. versus TS and nugget dia. versus CTS are plotted respectively. Figure 7c and 7d represents the plots of nugget dia. versus TS and nugget dia. versus CTS are plotted respectively. Table 3 shows all the values of the plots of nugget dia. versus TS and nugget dia. versus CTS respectively. Fracture may be caused due to interface failure (IF) or Button pullout (BP). From the values tabulated below in table 3, it is clear that high-strength high-ductile cold-rolled steel sheet has exhibits a good joint strength over a wide range of load and hence has a good weldability properties.

Current (kA) Nugget Dia (mm) Fracture Mode TS (kN) Fracture Mode CTS (kN) Fracture Mode
5.0 0 IF 7.771 IF 4.564 IF
5.3 0 IF 8.764 IF 7.123 IF
5.6 1.705 IF 11.164 IF 8.205 IF
5.9 2.0825 IF 11.618 IF 7.826 BP
6.2 3.7225 IF 13.718 IF 7.488 BP
6.5 3.22 IF 15.758 IF 8.246 BP
6.8 4.645 BP with partial IF 17.314 IF 9.137 BP
7.1 5.3025 BP with partial IF 18.079 IF 10.112 BP
7.4 5.515 BP with partial IF 19.242 BP 10.085 BP
7.7 6.77 BP 19.192 BP 10.892 BP
8.0 7.0725 BP 19.709 BP 7.8325 BP
8.3 7.0475 BP 19.673 BP 11.305 BP
8.6 7.1475 BP 16.892 IF 8.922 BP
8.9 7.545 BP 20.805 BP 9.887 BP
9.2 5.865 BP with expulsion 20.524 BP with expulsion 9.144 BP with expulsion
9.5 6.215 BP with expulsion 21.305 BP with expulsion 11.836 BP with expulsion
9.8 5.9925 BP with expulsion 18.244 BP 9.181 BP with expulsion
Table. 3

In an embodiment, the method optionally comprises phosphating the steel sheet. Phosphate coatings are used on steel parts for corrosion resistance and lubricity. During phosphating process, the alkaline cleaning may be carried out in 2.5 % degreasing solution (NaOH/KOH based solution) at about 55 °C to about 60 °C for a duration of around 180 seconds. Subsequent rinsing is carried out for about 30-60 seconds. Surface Activation may be carried out for 60 seconds using 0.1 % Titania solution. The chemicals activate the metal to obtain fine crystalline structure during phosphate coating which will increase corrosion resistance and adhesion properties. Phosphate coating may be carried out using phosphoric acid at around 50 °C for about 120 seconds followed by DM water rinsing for about 30 seconds at room temperature. Phosphate crystals observed on sample substrate are less than 5 µm, which is considered as good phosphating. The average size of the crystals of phosphate coating are about 2 t o3 µm with a P-ratio of the phosphate layer more than 90 %. The surface has uniform coverage of the phosphate crystals and has globular shape. Figure 8 indicates the surface morphology of phosphate coated steel sheet.

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

Equivalents:

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

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

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Referral Numerals

Referral Numerals Description
101-107 Flowchart blocks
101 Casting stage
102 Heating stage
103 Hot working stage
104 Cooling stage
105 Cold working stage
106 Soaking and cooling
107 Coiling

Claims:

1.A high-strength high-ductile cold-rolled steel sheet, comprising:
composition of:
carbon (C) at about 0.07 wt.% to at about 0.09 wt.%,
manganese (Mn) at about 1.8 % to at about 1.9 %,
sulphur (S) up-to 0.015 %,
phosphorous (P) up-to 0.025 %,
silicon (Si) at about 0.4 wt.% to at about 0.5 wt.%,
aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%,
nitrogen (N) up-to 0.005 ppm,
titanium (Ti) at about 0.015 wt.% to at about 0.030 wt.%,
molybdenum (Mo) at about 0.15 wt.% to at about 0.30 wt.%,
boron (B) 10 ppm to 20 ppm, and
the balance being Iron (Fe) optionally along with incidental elements;
wherein, the high-strength high-ductile cold-rolled steel sheet comprises martensite microstructure of about 25 % to about 40 % and balance being ferrite microstructure.

2. The high-strength high-ductile cold-rolled steel sheet as claimed in claim 1, wherein the high-strength high ductile cold-rolled steel sheet exhibits ultimate tensile strength greater than 780 MPa.

3. The high-strength high-ductile cold-rolled steel sheet as claimed in claim 1, wherein the high-strength high-ductile cold-rolled steel sheet exhibits ductility ranging from about 14 % to about 28 %.

4. The high-strength high-ductile cold-rolled steel sheet as claimed in 1, wherein the high-strength high-ductile cold-rolled steel sheet is suitable for a hot-dip galvanizing process.

5. A method for manufacturing a high-strength high-ductile cold-rolled steel sheet, the method comprising:

casting a steel slab of a composition comprising:
carbon (C) at about 0.07 wt.% to at about 0.09 wt.%,
manganese (Mn) at about 1.8 % to at about 1.9 %,
sulphur (S) up-to 0.015 %,
phosphorous (P) up-to 0.025 %,
silicon (Si) at about 0.4 wt.% to at about 0.5 wt.%,
aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%,
nitrogen (N) up-to 0.005 ppm,
titanium (Ti) at about 0.015 wt.% to at about 0.030 wt.%,
molybdenum (Mo) at about 0.15 wt.% to at about 0.30 wt.%,
boron (B) 10 ppm to 20 ppm, and
the balance being Iron (Fe) optionally along with incidental elements;

heating, the steel slab to a first predetermined temperature for a first predetermined time;
hot working, the steel slab at a second predetermined temperature to produce a steel sheet;
cooling, the hot worked steel sheet to a third predetermined temperature;
cold working, the steel sheet at third predetermined temperature;
soaking, the cold worked steel sheet at fourth predetermined temperature for a second predetermined time with a predetermined cooling rate;
coiling, the steel sheet, at a fifth predetermined temperature to obtain the high-strength high-ductile cold-rolled steel sheet;

wherein the high-strength high-ductile cold-rolled steel sheet comprises martensite microstructure of about 25 % to about 40 % and balance being ferrite microstructure.

6. The method as claimed in claim 5, wherein the steel slab is hot charged into a furnace for heating.

7. The method as claimed in claim 5, wherein the high-strength high ductile cold-rolled steel sheet exhibits ultimate tensile strength greater than 780 MPa.

8. The method as claimed in claim 5, wherein the high-strength high-ductile cold-rolled steel sheet exhibits ductility ranging from about 14 % to about 28 %.

9. The method as claimed in claim 5, wherein the casting is carried out in a continuous casting process.

10. The method as claimed in claim 9, wherein the continuous casting process is performed in at least one of continuous caster and a thin slab caster.

11. The method as claimed in claim 10, wherein the temperature of the steel slab at an exit of the thin slab caster is maintained above 1000 ?C.

12. The method as claimed in claim 5, wherein the first predetermined temperature ranges from about 1200 ?C to about 1250 ?C, and the first predetermined time from about 30 minutes to about three hours.

13. The method as claimed in claim 5, wherein the hot working is a hot rolling process.

14. The method as claimed in claim 5, wherein the hot working is performed in a finish rolling mill, and the second predetermined temperature higher than critical transformation temperature for austenite (Ar3).

15. The method as claimed in claim 14, wherein the second predetermined temperature ranges from of about 850 ?C to about 940 ?C.

16. The method as claimed in claim 5, wherein the third predetermined temperature is room temperature.

17. The method as claimed in claim 5, wherein the cold working is a cold rolling process.

18. The method as claimed in claim 5, wherein soaking of cold rolled steel sheet for a second predetermined time with a predetermined cooling rate is a continuous annealing process.

19. The method as claimed in claim 18, wherein the fourth predetermined temperature employed in continuous annealing process ranges from about 790 °C to about 820 °C.

20. The method as claimed in claim18, wherein the second predetermined time employed in continuous annealing process is ranging from about 75 seconds.

21. The method as claimed in claim 18, wherein the predetermined cooling rate employed in continuous annealing process is ranging from about 30 °C/second to about 40 °C/second.

22. The method as claimed in claim 5, wherein the fifth predetermined temperature is about 600 °C to about 680 °C.

23. The method as claimed in claim 5, wherein the reduction in thickness of the steel sheet after cold- working is at least 40 %.

24. The method as claimed in any of the preceding claims, further comprising: performing a phosphating treatment on the steel sheet.

25. The method of phosphating as claimed in claim 24, wherein the size of the phosphate crystals formed are about 2 µm to about 3 µm with P-ratio of the phosphate layer more than 90 %.

26. Automotive part comprising a high-strength high-ductile cold-rolled steel sheet as claimed in claim 1.