Description: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 cold-rolled steel sheet. Further embodiments of the disclosure disclose a method for manufacturing the high-strength cold-rolled steel sheet with Ultimate Tensile Strength (UTS) greater than 980 MPa.

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 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 and microstructure 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, cold 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. One of the patent literatures discloses production of high strength steel involving soaking temperature of about 1220 °C to about 1250 °C for 120 minutes. This process may not be an energy efficient route for largescale manufacturing. Another patent involves an energy expensive process involving cooling rate exceeding 100 ° C/sec during final soaking to yield steel containing martensite rich (70-100 % of martensite) microstructure. Yet another patent discloses a high strength steel containing residual austenite (RA). Also, in some publications it is disclosed that steel composition with high levels of silicon. However, surface quality of these steels may be poor as silicon promotes formation of scale during hot rolling. Further, attempts have been made at developing steels with high strength by addition of high amount of titanium, chromium. However, this makes the steel expensive.

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 difficulties 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 improved 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 claimed 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 cold-rolled steel sheet. The steel sheet includes composition of carbon (C) of about 0.07 wt.% to about 0.09 wt.%, manganese (Mn) of about 1.8 % to about 1.9 %, sulphur (S) up-to 0.015 %, phosphorous (P) up-to 0.025 %, silicon (Si) of about 0.4 wt.% to about 0.5 wt.%, aluminium (Al) of about 0.02 wt.% to about 0.09 wt.%, nitrogen (N) up-to 50 ppm, titanium (Ti) of about 0.010 wt.% to about 0.03 wt.%, niobium (Nb) of about 0.010 wt.% to about 0.03 wt.%, molybdenum (Mo) of about 0.10 wt.% to about 0.30 wt.%, boron (B) of about 5 ppm to about 20 ppm, and the balance being Iron (Fe) optionally along with incidental elements. The high-strength cold-rolled steel sheet comprises martensite microstructure of about 35 % to about 45 % and balance being ferrite microstructure.

In an embodiment, the high-strength cold-rolled steel sheet exhibits ultimate tensile strength greater than 980 MPa. Further, the high-strength cold-rolled steel sheet exhibits ductility ranging from about 10 % to about 22 %, along with yield strength ranging from 580 MPa to about 750 MPa. The high-strength cold-rolled steel sheet exhibits a maximum yield ratio of 0.7.

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

In another non-limiting embodiment of the present disclosure, there is provided a method for manufacturing, high-strength cold-rolled steel sheet. The method includes steps of firstly casting a steel slab of a composition comprising: carbon (C) of about 0.07 wt.% to about 0.09 wt.%, manganese (Mn) of about 1.8 % to about 1.9 %, sulphur (S) up-to 0.015 %, phosphorous (P) up-to 0.025 %, silicon (Si) of about 0.4 wt.% to about 0.5 wt.%, aluminium (Al) of about 0.02 wt.% to about 0.09 wt.%, nitrogen (N) up-to 50 ppm, titanium (Ti) of about 0.010 wt.% to about 0.03 wt.%, niobium (Nb) of about 0.010 wt.% to about 0.03 wt.%, molybdenum (Mo) of about 0.10 wt.% to about 0.30 wt.%, boron (B) of about 5 ppm to about 20 ppm, and the balance being Iron (Fe) optionally along with incidental elements. Then, subjecting the steel slab to heating at 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 hot worked steel sheet is coiled at a third predetermined temperature. The method further involve cold rolling the steel sheet at room temperature, followed by soaking the cold-rolled steel sheet at a fourth predetermined temperature for a second predetermined time. Finally, the steel sheet is cooled to room temperature to obtain the high-strength cold-rolled steel sheet. The high-strength cold-rolled steel sheet comprises martensite microstructure of about 35 % to about 45 % and balance being ferrite microstructure.

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.

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

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 is 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 ranges 580 ?C to 660 ?C.

In an embodiment, the reduction in thickness of the steel sheet after cold rolling is above 35 %.

In an embodiment, soaking of the cold-rolled steel sheet for a second predetermined time is a continuous annealing process. The fourth predetermined temperature employed in the continuous annealing process ranges from about 790 °C to about 820 °C. Further, the second predetermined time employed in the continuous annealing process is about 50 seconds.

In an embodiment, the continuous annealing process includes slow cooling of the steel sheet to a fifth predetermined temperature at a first predetermined cooling rate. Further, the fifth predetermined temperature ranges from about 640 °C to about 675 °C and the first predetermined cooling rate is at most 15 °C/sec.

In an embodiment, the continuous annealing process includes fast cooling of the steel sheet to a sixth predetermined temperature at a second predetermined cooling rate. Further, the sixth predetermined temperature ranges from about 250 °C to about 300 °C and the second predetermined cooling rate is at least 30 °C/sec.

In an embodiment, the continuous annealing process includes an over ageing step of the steel sheet at the sixth predetermined temperature for a third predetermined time. The third predetermined time is about 80 seconds, and the sixth predetermined temperature ranges from about 250 °C to about 300 °C.

In an embodiment, the continuous annealing process includes cooling of the steel sheet to a seventh predetermined temperature at a third predetermined cooling rate after the over ageing. Further, the seventh predetermined temperature is about 150 °C to and the third predetermined cooling rate is at most of 10 °C/sec.

In an embodiment, a skin passing step is being performed after continuous annealing process, wherein the elongation during skin passing is about 0.3 % to about 0.6 %.

In yet another non-limiting embodiment, automotive body panel comprising high-strength 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 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 cold-rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figure.2 illustrates a graphical representation of heat treatment steps followed during the continuous annealing process for producing high-strength cold-rolled steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 3 illustrates a pictorial representation of hot rolling process for producing steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 4 illustrates a pictorial representation of cold rolling process for producing steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 5 illustrates a pictorial representation of continuous annealing process for producing steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 6 illustrates a graphical representation of mechanical properties of high-strength cold-rolled steel sheet of the present disclosure.

Figures. 7a and 7b illustrate the optical and Scanning electron micrographs of the high-strength cold-rolled steel sheet of the present disclosure at a magnification of 1000X and 5000X respectively according to an exemplary embodiment of the present disclosure.

Figure. 8 illustrates Scanning Electron Microscopy (SEM) image of the high-strength cold-rolled steel sheet of the present disclosure 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 cold-rolled steel sheet and a method for manufacturing a high-strength 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 980 MPa are produced by the methods in which precipitation strengthening has been utilized as the key mechanism to obtain or improve the strength. However, it will be difficult to replicate similar level of precipitation strengthening in every grain of the microstructure due to the variation in processing conditions across width and length of the strip in actual plant scale production. Accordingly, the method of present disclosure, discloses a production of high-strength cold- rolled steel sheet, with ultimate tensile strength of minimum 980 MPa with improved ductility. The present disclosure is directed towards producing cold-rolled steel sheet with ultimate tensile strength greater than 980 MPa along with ductility ranging from about 10 % to about 22 % and yield strength ranging from 580 MPa to about 750 MPa. The cold rolled steel sheet may be widely employed to make automotive components requiring high strength with improved ductility, formability, and weldability.

According to various embodiment of the disclosure, the method of manufacturing high-strength cold-rolled steel sheet, includes first step of producing the steel slab of composition comprising: carbon (C) of about 0.07 wt.% to about 0.09 wt.%, manganese (Mn) of about 1.8 % to about 1.9 %, sulphur (S) up-to 0.015 %, phosphorous (P) up-to 0.025 %, silicon (Si) of about 0.4 wt.% to about 0.5 wt.%, aluminium (Al) of about 0.02 wt.% to about 0.09 wt.%, nitrogen (N) up-to 50 ppm, titanium (Ti) of about 0.010 wt.% to about 0.03 wt.%, niobium (Nb) of about 0.010 wt.% to about 0.03 wt.%, molybdenum (Mo) of about 0.10 wt.% to about 0.30 wt.%, boron (B) of about 5 ppm to about 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 hot charged to furnace for hearting to a temperature of about 1200 °C to 1250 °C for about 30 minutes to about three hours. 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 and coiled at a temperature which varies in the range of 580 °C to 660 °C. Steel sheet may be further subjected to cold working process including but not limited to cold rolling. Cold rolling may be carried out at the 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 fourth predetermined temperature of about 790 °C to 820 °C for about 50 seconds. The steel sheet may be subsequently cooled slowly, to a fifth predetermined temperature of about 640 to 675 °C, at a first predetermined cooling rate of about 15 °C/s. The steel sheet may be then cooled to a sixth predetermined temperature of about 250 °C to 300 °C, at a second predetermined temperature of at least 30 °C/s. The steel sheet may be further processed in the over ageing section at the sixth predetermined temperature of about 250 °C to 300 °C for a third predetermined time of about 80 seconds. The steel sheet may be then cooled to a seventh predetermined temperature of about 150 °C at a third predetermined cooling rate less 10 °C/s and finally cooled and coiled to room temperature to form high-strength cold-rolled steel sheet. The cold-rolled steel sheet according to the present disclosure may have 35 % to about 45 % martensite microstructure and balance being ferrite microstructure.

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 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 cold-rolled steel sheet, and a graphical representation of heat treatment followed during the continuous annealing process, respectively. In the present disclosure, mechanical properties such as ultimate tensile strength, yield strength 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 may 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 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, coiling, cold rolling, soaking, and a controlled cooling 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) of about 0.07 wt.% to about 0.09 wt.%, manganese (Mn) of about 1.8 % to about 1.9 %, sulphur (S) up-to 0.015 %, phosphorous (P) up-to 0.025 %, silicon (Si) of about 0.4 wt.% to about 0.5 wt.%, aluminium (Al) of about 0.02 wt.% to about 0.09 wt.%, nitrogen (N) up-to 50 ppm, titanium (Ti) of about 0.010 wt.% to about 0.03 wt.%, niobium (Nb) of about 0.010 wt.% to about 0.03 wt.%, molybdenum (Mo) of about 0.10 wt.% to about 0.30 wt.%, boron (B) of about 5 ppm to about 20 ppm, and the balance being Iron (Fe) optionally along with incidental elementsmay be casted in a continuous casting process.

The method then includes the step of hot charging of the steel slab 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 heating, 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 homogeneous composition by complete dissolution of any precipitates that may have formed in the preceding processing steps.

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 hot working process may be but not limited to hot rolling. Hot-rolling is a metal forming process in which metal stock/slab 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 prevent the metal from work hardening. In an embodiment, the hot charged steel slab may be hot rolled as shown in figure 3. During hot rolling hot charged steel slab may be subjected to roughing mill (201). The roughing mill (201) usually consists of one or two roughing stands in which the steel slab may be hot rolled back and forth few times repeatedly to reach the minimum thickness requirement. Roughing milled steel sheet may be further subjected to finish rolling (202). During finish rolling (202) sheet surface may be subjected to further thickness reduction, surface finishing and dynamic recrystallization. The finish rolling temperature during hot rolling may vary in the range of Ar3 i.e. around 850 °C to 940 °C (second predetermined temperature) 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 and coiled at a third predetermined temperature of about 580 °C to about 660 °C [shown in block 104].

Now referring to block 105, the method further includes the step of cold rolling the steel sheet at room temperature. 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 as shown in figure 4. During cold rolling steel sheet may be subjected to picking process (301) for surface cleaning and modification at around 90 °C. Later, steel sheet may be subjected to rolling process for final thickness reduction without any external heat. During cold rolling the point defect density (vacancies, self-interstitials etc.) and dislocation density increase within the steel sheet. This leads to increase in the internal energy (stored energy) of the steel sheet. The energy storage within the steel sheet during cold rolling process may be used as driving force for re-crystallization on subsequent annealing process. After cold rolling steel may be subjected to coiling process (302) to form full hard cold rolled coil. In an embodiment, reduction in thickness of the steel sheet after cold working may be above 35 %.

The method further includes soaking of cold rolled steel sheet at a fourth predetermined temperature for a second predetermined time. Prior to soaking, full hard cold rolled coil may be subjected to electrolytic cleaning (401) process. Soaking may be carried out in a continuous annealing process at a temperature ranging from about 790 °C to 820 °C for about 50 seconds [shown in block 106]. Continues annealing process is as shown in figure 5. Annealing is the process of relieving the internal stresses in the steel sheet 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. This phenomenon makes 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.

After soaking, the steel may be subsequently cooled to a fifth predetermined temperature of about 640 to 675 °C at a first predetermined cooling rate of about 15 °C/s. The steel may be then fast cooled to a sixth predetermined temperature of about 250 °C to 300 °C at a second predetermined cooling rate of at least 30 °C/s [shown in block 107].

Now referring to block 108, after fast cooling, the steel sheet may be further processed in the overaging section at a sixth predetermined temperature of about 250 °C to 300 °C for a third predetermined time of about 80 seconds. The steel sheet may be then cooled to a seventh predetermined temperature of about 150 °C at a third predetermined cooling rate of about 10 °C/s. After annealing process steel sheet may be subjected to skin passing (402) process. Skin passing may be performed to improve the mechanical properties and surface texture and improve flatness. The elongation during skin passing is about 0.3 % to about 0.6 %. Finally, the steel sheet is cooled and coiled at room temperature to form the high-strength cold-rolled steel sheet [as shown in block 109].

A schematic diagram of the heat treatment employed in continuous annealing is shown in Figure. 2. This heat treatment process ensures that the microstructure consists of about 35 % to about 45 % martensite 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 35 % to 45 % 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. However, further increase in amount of martensite phase beyond 45 % may decrease the 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 ductility of the steel deteriorates. In an embodiment of the present disclosure, the cold-rolled steel sheet comprises martensite microstructure of about 35 % to about 45 % in ferrite microstructure exhibits the ultimate tensile strength above 980 MPa.

In an embodiment, the high-strength 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 Ms is 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 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 1.8 wt.% to about 1.9 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. 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 0.025 % 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 50 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 0.010 wt. % to 0.030 wt.%. Further, titanium forms carbides which when finely dispersed promotes strengthening.

Niobium (Nb) may promote grain refinement and improve the rate of transformation of austenite to martensite, both of which help in increasing the tensile properties. Hence, amount of Nb is maintained between 0.01 wt.% and 0.03 wt.%.

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 0.10 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 5 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 produced by using method of the present disclosure. The compositions of the steel samples (A to B) 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.

Sl No C (%) Mn (%) Si (%) P (%) S (%) Mo (%) Ti (%) Nb (%) Al (%) N (ppm) B (ppm) Soaking temperature (°C) Cooling rate (°C/s) % Cold reduction YS (MPa) UTS (MPa) El (%) Remarks
A 0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 678 1015 13.6 Example 1
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 682 1015 15.5
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 663 990 14.4
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 658 1013 15.1
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 687 1011 15.8
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 < 790 30 - 40 44 666 966 19.2 Comparative example
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 < 790 30 - 40 44 498 957 17.8
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 < 790 30 - 40 44 671 957 17.5
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 < 790 30 - 40 44 633 953 12.4
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 732 1065 12.2 Example 2
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 678 1027 14.3
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 690 1009 15.7
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 662 996 15.1
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 790 - 820 30 - 40 44 694 1044 14
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 < 790 30 - 40 44 595 921 14.6 Comparative example
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 < 790 30 - 40 44 579 907 15
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 < 790 30 - 40 44 648 971 15
0.08 1.92 0.42 0.013 0.002 0.24 0.022 0.028 0.058 33 18 < 790 30 - 40 44 629 968 15.1
B 0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 660 986 17.4 Example 3
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 663 998 14.8
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 689 1044 13.2
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 654 987 16.9
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 692 1003 14.4
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 < 790 30 - 40 36 587 929 16.7 Comparative example
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 < 790 30 - 40 36 659 955 14.6
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 < 790 30 - 40 36 615 930 16.5
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 < 790 30 - 40 36 593 915 18.5
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 706 1045 13.5 Example 4
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 663 989 16.2
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 683 1001 14.3
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 726 1063 11.7
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 30 - 40 44 670 1007 15.5
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 < 30 36 581 923 16.5 Comparative example
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 < 30 36 601 915 17.2
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 < 30 36 545 892 18.5
0.088 1.88 0.455 0.014 0.0015 0.178 0.026 0.018 0.069 55 15 790 - 820 < 30 36 553 892 19.1

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 and B, when the soaking temperature less than 790 °C during continuous s annealing process, the steel exhibits ultimate tensile strength less than 980 MPa; but has high percentage elongation may be due the presence of more amount of softer ferrite phase in the final microstructure. But when soaking during continuous annealing at around 790 °C to 820 °C, there is increase in the ultimate strength beyond 980Mpa and percentage of elongation (ductility) attains a balanced value. Improvement in strength beyond 980 MPa with balanced percentage elongation (ductility) has been obtained by achieving a microstructure having 35 % to 45 % martensite in ferrite matrix. Yield strength value of cold ranges from 580 MPa to about 750 MPa and the high-strength cold-rolled steel sheet exhibits a maximum yield ratio of 0.7. Hence, the cold-rolled steel sheets soaked 790 °C to 820 °C exhibit improved mechanical properties as predicted in figure 6.

Referring to figures 7a and 7b which illustrate the optical and scanning electron micrographs of high-strength cold-rolled steel sheet at a magnification of 1000X and 5000 X respectively. In the figure 7a, grey regions correspond to ferrite and black regions correspond to martensite phase, while in figure 7b grey regions correspond to martensite and black regions correspond to ferrite phase. Optical micrographs clearly depict 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 of the cold-rolled steel sheet.

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 % alkaline solution at about 38 °C to about 42 °C for a duration of around 120 seconds. Subsequent rinsing at room temperature is carried out for about 30 seconds. Surface activation may be carried out for 30 seconds at 30 °C using surface conditioner having a pH ranging from 8.5 to 11.5. 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 33 °C to about 37 °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. Figure 8 indicates the SEM surface morphology of high-strength cold-rolled steel sheet after phosphating 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 body panels such as side body panels or a door inner panel. The high strength 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
101-108 Flowchart blocks
101 Casting stage
102 Heating stage
103 Hot working stage
104 Coiling stage
105 Cold working stage
106 Soaking stage
107 Cooling stage
108 Overaging and final coiling stage
201 Roughing mill stage
202 Finish rolling stage
301 Picking process
302 Coiling stage after cold rolling
401 Electrolytic cleaning process
402 Skin passing process

Claims:1. A high-strength cold-rolled steel sheet, comprising:
composition of:
carbon (C) of about 0.07 wt.% to about 0.09 wt.%,
manganese (Mn) of about 1.8 % to about 1.9 %,
sulphur (S) up-to 0.015 %,
phosphorous (P) up-to 0.025 %,
silicon (Si) of about 0.4 wt.% to about 0.5 wt.%,
aluminium (Al) of about 0.02 wt.% to about 0.09 wt.%,
nitrogen (N) up-to 50 ppm,
titanium (Ti) of about 0.010 wt.% to about 0.03 wt.%,
niobium (Nb) of about 0.010 wt.% to about 0.03 wt.%
molybdenum (Mo) of about 0.10 wt.% to about 0.30 wt.%,
boron (B) of about 5 ppm to about 20 ppm, and
the balance being Iron (Fe) optionally along with incidental elements;
wherein, the high-strength cold-rolled steel sheet comprises martensite microstructure of about 35 % to about 45 % and balance being ferrite microstructure.

2. The high-strength cold-rolled steel sheet as claimed in claim 1, wherein the high-strength cold-rolled steel sheet exhibits ultimate tensile strength greater than 980 MPa.

3. The high-strength cold-rolled steel sheet as claimed in claim 1, wherein the high-strength cold-rolled steel sheet exhibits ductility ranging from about 10 % to about 22 %, along with yield strength ranging from 580 MPa to about 750 MPa.

4. The high-strength cold-rolled steel sheet as claimed in claim 3, wherein the high-strength cold-rolled steel sheet exhibits a maximum yield ratio of 0.7.

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

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

casting a steel slab of a composition comprising:
carbon (C) of about 0.07 wt.% to about 0.09 wt.%,
manganese (Mn) of about 1.8 % to about 1.9 %,
sulphur (S) up-to 0.015 %,
phosphorous (P) up-to 0.025 %,
silicon (Si) of about 0.4 wt.% to about 0.5 wt.%,
aluminium (Al) of about 0.02 wt.% to about 0.09 wt.%,
nitrogen (N) up-to 50 ppm,
titanium (Ti) of about 0.010 wt.% to about 0.03 wt.%,
niobium (Nb) of about 0.010 wt.% to about 0.03 wt.%
molybdenum (Mo) of about 0.10 wt.% to about 0.30 wt.%,
boron (B) of about 5 ppm to about 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;
coiling, the hot worked steel sheet at a third predetermined temperature;
cold rolling, the steel sheet at room temperature;
soaking, the cold-rolled steel sheet at fourth predetermined temperature for a second predetermined time; and
cooling, the steel sheet to room temperature to obtain a high-strength cold-rolled steel sheet;
wherein, the high-strength cold-rolled steel sheet comprises martensite microstructure of about 35 % to about 45 % and balance being ferrite microstructure.

7. The method as claimed in claim 6, wherein the high-strength cold-rolled steel sheet exhibits ultimate tensile strength greater than 980 MPa.

8. The method as claimed in claim 6, wherein the high-strength cold-rolled steel sheet exhibits ductility ranging from about 10 % to about 22 %, along with yield strength ranging from 580 MPa to about 750 MPa.

9. The method as claimed in claim 6, wherein the casting is carried out in a continuous casting process.

10. The method as claimed in claim 6, wherein the continuous casting process is performed in at least one of continuous caster.

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

12. The method as claimed in claim 6, 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 6, wherein the hot working is a hot rolling process.

14. The method as claimed in claim 6, wherein the hot working is performed in a finish rolling mill, and the second predetermined temperature is 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 6, wherein the third predetermined temperature ranges 580 ?C to 660 ?C.

17. The method as claimed in claim 6, wherein the reduction in thickness of the steel sheet after cold rolling is above 35 %.

18. The method as claimed in claim 6, wherein the soaking of the cold-rolled steel sheet for a second predetermined time is a continuous annealing process.

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

20. The method as claimed in claim 18, wherein the second predetermined time employed in the continuous annealing process is about 50 seconds.

21. The method as claimed in claim 18, wherein the continuous annealing process includes slow cooling of the steel sheet to a fifth predetermined temperature at a first predetermined cooling rate.

22. The method as claimed in claim 21, wherein the fifth predetermined temperature ranges from about 640 °C to about 675 °C and the first predetermined cooling rate is at most 15 °C/sec.

23. The method as claimed in claim 18, wherein the continuous annealing process includes fast cooling of the steel sheet to a sixth predetermined temperature at a second predetermined cooling rate.

24. The method as claimed in claim 23, wherein the sixth predetermined temperature ranges from about 250 °C to about 300 °C and the second predetermined cooling rate is at least 30 °C/sec.

25. The method as claimed in claim 18, wherein the continuous annealing process includes an over ageing step of the steel sheet at the sixth predetermined temperature for a third predetermined time.

26. The method as claimed in claim 25, wherein the third predetermined time is about 80 seconds, and the sixth predetermined temperature ranges from about 250 °C to about 300 °C.
27. The method as claimed in claim 18, wherein the continuous annealing process includes cooling of the steel sheet to a seventh predetermined temperature at a third predetermined cooling rate after the over ageing.

28. The method as claimed in claim 27, wherein the seventh predetermined temperature is about 150 °C to and the third predetermined cooling rate is at most of 10 °C/sec.

29. The method as claimed in claim 18 comprises a skin passing step after continuous annealing process, wherein the elongation during skin passing is about 0.3 % to about 0.6 %.

30. Automotive body panel comprising a high-strength cold-rolled steel sheet as claimed in claim 1.