, 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 390 MPa and Lankford value (R-value) greater than 1.4.

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 strength, hardness etc., or prepare the material for improved manufacturability.

In the automotive industry, strict norms of legislation regarding fuel consumption and emission has forced the industry to develop lighter, more fuel-efficient vehicles. The two main objectives of the automobile industry are the reduction in vehicle weight and improvement in safety. In order to achieve this, automobile manufacturers prefer high strength materials coupled with better formability to make the parts such as side body in white, body inner panels, door inner panels and the like.

Conventionally, high strength cold rolled steel sheets having tensile strength less than 390 MPa and r values of approximately 1.2 have been progressively applied to side body panels or a door inner panels in the automobiles. However, such conventional steel sheets are difficult to be press-formed due to poor formability. When the body panels or a door inner panels are formed by employing conventional high strength cold rolled steel sheets, cracks formation tends to occur at the regions where the deep drawing is performed. The process of crack formation subsequently may lead to material failure during forming operations. Hence, the performances of conventional high strength steel may not sufficient to meet the contradicting requirement.

In some of the conventional arts, various steel compositions and heat treatment methods have been developed in order to obtain improved strength along with drawability and formability. However, Ultimate Tensile Strength (UTS) and r values obtained for such steels may be less than the desirable ranges to get optimal performance specially during forming processes.
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) up-to 0.005 wt.%, manganese (Mn) at about 0.5 wt.% to at about 1.2 wt.%, sulphur (S) up-to 0.015 wt.%, phosphorous (P) at about 0.05 wt.% to at about 0.09 wt.%, silicon (Si) up-to 0.5 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 50 ppm, titanium (Ti) at about 0.010 wt.% to at about 0.030 wt.%, niobium (Nb) at about 0.02 wt.% to at about 0.03 wt.%, chromium (Cr) at about 0.015 wt.% to at about 0.05 wt.%, boron (B) at about 5 ppm to about 15 ppm, and the balance being Iron (Fe) optionally along with incidental elements. The high-strength cold-rolled steel sheet exhibits Ultimate Tensile Strength (UTS) greater than 390 MPa and Lankford value (R-value) greater than 1.4.

In an embodiment, high-strength cold-rolled steel sheet comprises ferrite and pearlite microstructure.

In an embodiment, the high-strength cold-rolled steel sheet exhibits ductility above 30 %.

In an embodiment, the high-strength cold-rolled steel sheet exhibits yield strength from about 230 MPa to about 340 MPa.

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

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) up-to 0.005 wt.%, manganese (Mn) at about 0.5 wt.% to at about 1.2 wt.%, sulphur (S) up-to 0.015 wt.%, phosphorous (P) at about 0.05 wt.% to at about 0.09 wt.%, silicon (Si) up-to 0.5 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 50 ppm, titanium (Ti) at about 0.010 wt.% to at about 0.030 wt.%, niobium (Nb) at about 0.02 wt.% to at about 0.03 wt.%, chromium (Cr) at about 0.015 wt.% to at about 0.05 wt.%, boron (B) at about 5 ppm to about 15 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 steel sheet is coiled at a third predetermined temperature. The method may further involve cold rolling the steel sheet at room temperature, followed by soaking the cold worked 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 which exhibits Ultimate Tensile Strength (UTS) greater than 390 MPa and Lankford value (R-value) greater than 1.4.

In an embodiment, the casting is carried out in a continuous casting process.

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 higher than critical transformation temperature for austenite (Ar3). Further, the second predetermined temperature ranges from of about 900 ?C to about 940 ?C.

In an embodiment, the third predetermined temperature ranges 650 ?C to 720 ?C.

In an embodiment, the reduction in thickness of the steel sheet after cold rolling is above 40 %.

In an embodiment, soaking of cold-rolled steel sheet for a second predetermined time with a predetermined cooling rate is a continuous annealing process. The fourth predetermined temperature employed in continuous annealing process ranges from about 760 °C to about 800 °C. Further, the second predetermined time employed in continuous annealing process ranges from about 45 seconds to about 55 seconds.

In an embodiment, the continuous annealing 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 650 °C to about 680 °C and the first predetermined cooling rate ranges from about 5 °C/sec to about 13 °C/sec.

In an embodiment the continuous annealing 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 350 °C to about 400 °C and the second predetermined cooling rate is at least 30 °C/sec.

In an embodiment, the continuous annealing includes an over ageing step of the steel sheet at the sixth predetermined temperature for a third predetermined time. Further, the third predetermined time is at least 90 seconds.

In an embodiment, the continuous annealing 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 wherein the elongation during skin passing is about 0.6 % to about 0.9 %.

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 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 cold-rolled steel sheet, according to an exemplary embodiment of the present disclosure.

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

Figure. 3is 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.

Figures. 6a and 6b illustrate the optical micrograph of the high-strength cold-rolled steel sheet of the present disclosure at a magnification of 100X and 400 Xrespectively according to an exemplary embodiment of the present disclosure.

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

DETAILED DESCRIPTION

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

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

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

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

Embodiments of the present disclosure discloses a high-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, steels with strength less than 390 MPa are produced by conventional heat treatments. However, these steels have poor formability and tend to produce cracks during forming process. Accordingly, the method of present disclosure discloses a production of high-strength cold-rolled steel sheet that exhibits Ultimate Tensile Strength (UTS) greater than 390 MPa and Lankford value (R-value) greater than 1.4. The cold-rolled steel sheet may be widely employed to make automotive components requiring high strength, high 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 in weight percentage of: carbon (C) up-to 0.005 wt.%, manganese (Mn) at about 0.5 wt.% to at about 1.2 wt.%, sulphur (S) up-to 0.015 wt.%, phosphorous (P) at about 0.05 wt.% to at about 0.09 wt.%, silicon (Si) up-to 0.5 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 50 ppm, titanium (Ti) at about 0.010 wt.% to at about 0.030 wt.%, niobium (Nb) at about 0.02 wt.% to at about 0.03 wt.%, chromium (Cr) at about 0.015 wt.% to at about 0.05 wt.%, boron (B) at about 5 ppm to about 15 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 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 900 °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 650 °C to 720 °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 760 °C to 800 °C for about 45 seconds to 55 seconds. The steel may be subsequently cooled slowly, to a fifth predetermined temperature of about 650 to 680 °C, at a first predetermined cooling rate of about 5 °C/s to about 13 °C/s. The steel may be then cooled to a sixth predetermined temperature of about 350 °C to 400 °C, at a second predetermined temperature of at least 35 °C/s. The steel may be further processed in the over ageing section at the sixth predetermined temperature of about 300 °C to 350 °C for a third predetermined time of about 90 seconds. The steel may be then cooled to a seventh predetermined temperature of about 150 °C at a third predetermined cooling rate of about 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 ferrite and pearlite 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 strength, R-value (formability) the steel may be improved. The steel produced by the method of the present disclosure, includes a ferrite-pearlite 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 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) up-to 0.005 wt.%, manganese (Mn) at about 0.5 wt.% to at about 1.2 wt.%, sulphur (S) up-to 0.015 wt.%, phosphorous (P) at about 0.05 wt.% to at about 0.09 wt.%, silicon (Si) up-to 0.5 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 50 ppm, titanium (Ti) at about 0.010 wt.% to at about 0.030 wt.%, niobium (Nb) at about 0.02 wt.% to at about 0.03 wt.%, chromium (Cr) at about 0.015 wt.% to at about 0.05 wt.%, boron (B) at about 5 ppm to about 15 ppm, and the balance being Iron (Fe) optionally along with incidental elements may 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 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 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 900 °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 650 °C to about 720 °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) to 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 40 %.

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 760 °C to 800 °C for about 45 seconds to about 55 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 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 5 °C/s to about 13 °C/s. The steel may be then fast cooled to a sixth predetermined temperature of about 350 °C to 400 °C at a second predetermined temperature of at least 35 °C/s [shown in block 107].

Now referring to block 108, after fast cooling, the steel sheet may be further processed in the over-aging section at a sixth predetermined temperature of about 300 °C to 350 °C for a third predetermined time of about 90 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 in order to improve the mechanical properties and surface texture and improve flatness. Finally, the steel sheet is cooled and coiled to 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 ensures that the microstructure consists of ferrite and pearlite microstructure.

Steel with ultra low carbon and nitrogen content is known as Interstitial Free steel (IF steel). In IF steel, there may be no interstitial solute atoms to strain the solid iron lattice, resulting in very soft steel. IF steels have interstitial free body centered cubic (bcc) ferrite matrix. IF steel has ultra-low carbon and nitrogen content may be achieved by removing carbon monoxide, hydrogen, nitrogen, and other gasses during steelmaking through a vacuum degassing process. The low carbon and nitrogen present in the steel may be ‘stabilized’ by small additions of Ti and Nb. Ti and Nb may be strong carbide/nitride formers, taking the remaining C and N out of solution in liquid iron, after which these two elements may be no longer available to reside in the interstices between solidified iron atoms. During continuous annealing IF steel has the ability, to form crystal orientations favorable to deep drawing. Hence, IF steels normally have high strength, high plastic strain ratio (R-value), high strain rate sensitivity and good formability.

During soaking at the annealing temperature, the equilibrium solubility of carbon may be reached. The high cooling rate may prevent carbide precipitation and produce a supersaturated carbon concentration. This solute carbon may affect the mechanical properties such as formability. In continuous annealing, dissolution of precipitates may also be expected to occur at the high soaking temperature. IF steels may possess typically non aging properties. Because of their non ageing properties, IF steels may be the standard base for hot dipped galvanized products. During the zinc coating and galvannealing steps the strip may be reheated above the over ageing temperature. If normal low carbon steels are being used, carbon would re-dissolve, and may cause strain ageing. With IF steels, cooling and reheating may be irrelevant, since carbon and nitrogen may not be available to be re-dissolved and cause aging. In an embodiment of the present disclosure, the high strength cold-rolled steel sheet including ferrite and pearlite microstructure exhibits Ultimate Tensile Strength (UTS) greater than 390 MPa and Lankford value (R-value) greater than 1.4 using IF steel with ultra-low carbon content.

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 may help in strengthening phases, and may be often considered as a cheaper element to increase strength. When the content of C may be less than 0.005 wt.%, the ferrite grains may coarsen at annealing, so that surface defect such as “Orange peel” may tend to occur at press forming. If C content is more than 0.005 wt.%, it may affect the room temperature ageing property of the steel, depending on the amount of Ti or Nb available. Hence, the amount of carbon may be maintained in between to about 0.005 wt.%.

Manganese (Mn) may be the effective element for the solid solution hardening. In order to obtain the tensile strength of 390MPa or more, the content of Mn may be needed to set in between 0.5 wt.% to 1.2 wt.%. When the content of Mn may be more than 1.2 wt.%, the r value may be decreased.

Sulphur (S) exists as sulphides in steel. When the content of S is more than 0.015 wt.%, the ductility may be decreased, and hence the amount of S is set to 0.015 wt.% or preferably 0.01 wt.% or less.

Phosphorous (P) may be added to get desired tensile strength by solid solution strengthening. When the content of P is more than 0.09 wt.%, the r values may be decreased, and in addition, increased P may tend to decrease in the adhesion of the coating, and hence, the content of P is set from 0.05wt.% to 0.09 wt.%.

Silicon (Si) is an effective element for solid solution hardening. When the content of Si is more than 0.5 wt.%, the surface quality may be deteriorated due to scaling, and in addition, increased Si content may decrease the adhesion of the coating to the surface of steel, In order to obtain the tensile strength of 390 MPa or more, the content of Si is set up to 0.50 wt.%

Aluminium (Al) may be added in the range of 0.02 wt.% to 0.09 wt. %. Aluminium is used as a deoxidizer. Higher amount of Al causes casting issues.

Nitrogen (N) may be added up to 50 ppm. Higher nitrogen may fix up higher amount of titanium through formation of TiN and may be reduce the availability of Ti for TiC precipitates which otherwise may be led to strengthening process. Also, increase in nitrogen content may increase the size of the TiN leading to reduced ductility.

Titanium (Ti) and Niobium (Nb) may have the effect of improving the r value by the grain refinement of the hot rolled coils or by the decrease in the solute C and N with the formation of precipitates. In order to get higher r values, a stoichmetric quantity of Nb may be added. However, when the content of Nb may be more than 0.03 wt.%, cracking tends to occur in the slab surface at the casting stage due to the formation of NbN and Nb (C, N) precipitates at austenite grain boundaries. Hence, content of Nb is set to 0.02 wt.% to 0.03 wt.%, and Ti is set to 0.01 wt.% to 0.03wt.%.

Boron (B) may be added to prevent the secondary work embrittlement effect of P. Boron may be effective at an optimum range of 5 to 15 ppm, beyond which it may deteriorate drawability.

Chromium (Cr) may increase solid solution strengthening effects while enhancing aging resistance at room temperature, ensuring the high strength of the steel sheet while reducing the in-plane anisotropy index of the steel sheet. A chromium content of 0.015 wt.% or more ensure the strength of the steel sheet, but a chromium content of more than 0.05 wt. % cause reduction in ductility and r values. Hence Cr value may be maintained about 0.015 wt.% to at about 0.05 wt.%.

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 compositions of the steel by using method of the present disclosure. The composition of the steel samples (A to E) 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 an embodiment of the present disclosure, tensile straining for all the steel samples were carried out in tensile tester machine. A tensile test may involve mounting the specimen in a machine, such that it is subjecting it to constant strain. For each set of compositions, obtained yield strength (YS), ultimate tensile strength (UTS), percentage elongation (%), R-values and yield ration values are also tabulated in table. 1. The Lankford value (R-value) is a measure of the plastic anisotropy of a rolled sheet metal. This scalar quantity may be used as an indicator of the formability of recrystallized low-carbon steel sheets.

In case of composition A to E, when the soaking temperature is about 800 °C to 840 °C during continues annealing process, the steel exhibits ultimate tensile strength less than 390 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 760 °C to 800 °C, a well balance between strength and percentage elongation (ductility) may be obtained by achieving a microstructure having ferrite and pearlite in the steel matrix. Further, higher the amount of Nb may lead to higher R-values. Hence, the cold-rolled steel sheets soaked 760 °C to 800 °C exhibit very good formability which may be result of high-strength and good ductility (percentage elongation).

Sl no %C
(= 0.0050) %Mn
(0.50 – 1.2) %P
(0.05 – 0.09) %S
(= 0.015) %Si
(= 0.50) %Al
(0.02 – 0.09) N
(= 50ppm) %Cr
(0.015 – 0.05) %Ti
(0.010-0.030) %Nb
(0.02-0.03) Bppm
( 5 - 15) YS UTS %El R-value Yield Ratio Soaking Temperature Remarks
A 1 0.0017 0.70 0.05 0.01 0.003 0.041 18 0.016 0.024 0.023 0.0009 247 403 34.0 1.757 0.61 760 -800 Example 1
2 0.0017 0.70 0.05 0.01 0.003 0.041 18 0.016 0.024 0.023 0.0009 256 410 38.8 1.785 0.62 760 -800
3 0.0017 0.70 0.05 0.01 0.003 0.041 18 0.016 0.024 0.023 0.0009 251 414 36.1 1.483 0.61 760 -800
4 0.0017 0.70 0.06 0.01 0.004 0.041 18 0.016 0.024 0.023 0.0009 269 429 35.2 1.605 0.63 760 -800
5 0.0017 0.70 0.06 P.01 0.004 0.041 18 0.016 0.024 0.023 0.0009 263 417 38.4 1.618 0.63 760 -800
6 0.0017 0.70 0.05 0.01 0.003 0.041 18 0.016 0.024 0.023 0.0009 238 386 39.0 1.665 0.62 800 - 840 Comp. Example
7 0.0017 0.70 0.05 0.01 0.003 0.041 18 0.016 0.024 0.023 0.0009 249 388 39.7 1.667 0.64 800 - 840
8 0.0017 0.70 0.05 0.01 0.003 0.041 18 0.016 0.024 0.023 0.0009 246 385 39.8 1.672 0.64 800 - 840
9 0.0017 0.70 0.05 0.01 0.003 0.041 18 0.016 0.024 0.023 0.0009 244 387 39.1 1.796 0.63 800 - 840
10 0.0017 0.70 0.06 0.01 0.004 0.041 18 0.016 0.024 0.023 0.0009 240 388 37.0 1.691 0.62 800 - 840
B 11 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 269 434 36.0 1.480 0.62 760 -800 Example 2
12 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 291 434 35.0 1.490 0.67 760 -800
17 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 268 430 36.0 1.460 0.62 760 -800
18 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 294 432 36.0 1.500 0.68 760 -800
19 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 291 425 37.0 1.480 0.68 760 -800
20 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 269 436 35.3 1.458 0.62 760 -800
21 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 291 438 38.2 1.596 0.66 760 -800
22 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 279 424 38.0 1.513 0.66 760 -800
23 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 282 431 35.6 1.573 0.65 760 -800
24 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 300 432 36.8 1.629 0.69 760 -800
25 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 296 423 36.8 1.472 0.70 760 -800
26 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 292 437 34.1 1.507 0.67 760 -800
27 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 305 447 36.0 1.716 0.68 760 -800
28 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 292 424 37.0 1.508 0.69 760 -800
13 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 283 378 37.0 1.458 0.75 800 - 840 Comp. Example
14 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 249 365 36.0 1.629 0.68 800 - 840
15 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 282 388 37.0 1.513 0.73 800 - 840
16 0.0035 0.97 0.06 0.012 0.225 0.033 33 0.035 0.03 0.022 0.0015 270 369 37.0 1.472 0.73 800 - 840
C 29 0.0033 0.95 0.086 0.008 0.341 0.021 25 0.021 0.022 0.026 0.0014 313 454 33.1 1.550 0.69 760 -800 Example 3
30 0.0033 0.95 0.086 0.008 0.341 0.021 25 0.021 0.022 0.026 0.0014 322 483 32.1 1.410 0.67 760 -800
31 0.0033 0.95 0.086 0.008 0.341 0.021 25 0.021 0.022 0.026 0.0014 329 488 33.4 1.450 0.67 760 -800
32 0.0033 0.95 0.086 0.008 0.341 0.021 25 0.021 0.022 0.026 0.0014 325 485 31.8 1.550 0.67 760 -800
33 0.0033 0.95 0.086 0.008 0.341 0.021 25 0.021 0.022 0.026 0.0014 329 490 32.9 1.460 0.67 760 -800
D 46 0.0028 0.58 0.052 0.009 0.005 0.034 21 0.016 0.022 0.022 0.0007 260 397 39.6 1.842 0.65 760 - 800 Example 4
47 0.0028 0.58 0.052 0.009 0.005 0.034 21 0.016 0.022 0.022 0.0007 238 395 39.4 1.799 0.60 760 - 800
41 0.003 0.56 0.065 0.009 0.005 0.038 18 0.017 0.019 0.022 0.0007 244 399 40.4 1.352 0.61 720 - 760 Comp. Example
42 0.003 0.56 0.065 0.009 0.005 0.038 18 0.017 0.019 0.022 0.0007 251 401 39.5 1.294 0.63 720 - 760
43 0.003 0.56 0.065 0.009 0.005 0.038 18 0.017 0.019 0.022 0.0007 245 405 39.7 1.337 0.60 720 - 760
44 0.003 0.56 0.065 0.009 0.005 0.038 18 0.017 0.019 0.022 0.0007 239 409 38.5 1.323 0.58 720 - 760
45 0.003 0.56 0.065 0.009 0.005 0.038 18 0.017 0.019 0.022 0.0007 246 398 39.8 1.390 0.62 720 - 760
E 48 0.0022 0.74 0.05 0.01 0.005 0.055 18 0.02 0.027 0.025 0.0009 241 394 38.0 1.763 0.61 760 - 800 Example 5
49 0.0022 0.65 0.05 0.008 0.006 0.04 20 0.02 0.025 0.022 0.0008 237 393 39.9 1.611 0.60 760 - 800
50 0.0022 0.65 0.05 0.008 0.006 0.04 20 0.02 0.025 0.022 0.0008 239 392 40.2 1.769 0.61 760 - 800
51 0.0024 0.65 0.047 0.004 0.005 0.047 29 0.016 0.023 0.025 0.0005 244 390 39.6 1.738 0.63 760 - 800
52 0.0021 0.75 0.05 0.005 0.006 0.057 20 0.017 0.025 0.029 0.0005 268 421 36.9 1.570 0.64 760 - 800
53 0.0021 0.75 0.05 0.005 0.006 0.057 20 0.017 0.025 0.029 0.0005 267 422 37.9 1.700 0.63 760 - 800

Table-1

Referring to figures 6a and 6b which illustrate the optical micrograph of high-strength cold-rolled steel sheet at a magnification of 100X and 400x respectively. In the figures, black regions correspond to pearlite to and grey regions corresponds to ferrite. It may be also noted that, steel obtained via continuous annealing has fine grained ferrite uniformly distributed throughout the microstructure. The grain size appears to determine the maximum strength obtained. The smaller the grain size becomes, the higher the strength and larger the R-values. Hence, steel shows improved strength and R-values with finer grain microstructure.

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 panels. 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
202 Finish rolling
301 Pickling 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) up-to 0.005 wt.%,
manganese (Mn) at about 0.5 wt.% to at about 1.2 wt.%,
sulphur (S) up-to 0.015 wt.%,
phosphorous (P) at about 0.05 wt.% to at about 0.09 wt.%,
silicon (Si) up-to 0.5 wt.%,
aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%,
nitrogen (N) up-to 50 ppm,
titanium (Ti) at about 0.010 wt.% to at about 0.030 wt.%,
niobium (Nb) at about 0.02 wt.% to at about 0.03 wt.%,
chromium (Cr) at about 0.015 wt.% to at about 0.05 wt.%,
boron (B) at about 5 ppm to about 15 ppm, and
the balance being Iron (Fe) optionally along with incidental elements;

wherein, the high-strength cold-rolled steel sheet exhibits Ultimate Tensile Strength (UTS) greater than 390 MPa and Lankford value (R-value) greater than 1.4.

2. The high-strength cold-rolled steel sheet as claimed in claim 1, wherein the high-strength cold-rolled steel sheet comprises ferrite and pearlite microstructure.

3. The high-strength cold-rolled steel sheet as claimed in claim 1, wherein the high-strength cold-rolled steel sheet exhibits ductility above 30 %.

4. The high-strength cold-rolled steel sheet as claimed in claim 1, wherein the high-strength cold-rolled steel sheet exhibits yield strength from about 230 MPa to about 340 MPa.

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

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) up-to 0.005 wt.%
manganese (Mn) at about 0.5 wt.% to at about 1.2 wt.%,
sulphur (S) up-to 0.015 wt.%,
phosphorous (P) at about 0.05 wt.% to at about 0.09 wt.%
silicon (Si) up-to 0.5 wt.%,
aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%,
nitrogen (N) up-to 50 ppm,
titanium (Ti) at about 0.010 wt.% to at about 0.030 wt.%
niobium (Nb) at about 0.02 wt.% to at about 0.03 wt.%,
chromium (Cr) at about 0.015 wt.% to at about 0.05 wt.%,
boron (B) at about 5 ppm to about 15 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 exhibits Ultimate Tensile Strength (UTS) greater than 390 MPa and Lankford value (R-value) greater than 1.4.

7. The method as claimed in claim 6, wherein, the high-strength cold-rolled steel sheet comprises ferrite and pearlite microstructure.

8. The method as claimed in claim 6, wherein the high-strength cold-rolled steel sheet exhibits ductility above 30 %.

9. The method as claimed in claim 6, wherein the high-strength cold-rolled steel sheet exhibits yield strength from about 230 MPa to about 340 MPa.

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

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

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

13. 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.

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

15. The method as claimed in claim 6, wherein the hot working is performed in a finish rolling mill, and the second predetermined temperature higher than critical transformation temperature for austenite (Ar3).

16. The method as claimed in claim 15, wherein the second predetermined temperature ranges from of about 900 ?C to about 940 ?C.

17. The method as claimed in claim 6, wherein the third predetermined temperature ranges 650 ?C to 720 ?C.

18. The method as claimed in claim 6, wherein the reduction in thickness of the steel sheet after cold rolling is above 40 %.

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

20. The method as claimed in claim 19, wherein the fourth predetermined temperature employed in the continuous annealing process ranges from about 760 °C to about 800 °C.

21. The method as claimed in claim 19, wherein the second predetermined time employed in the continuous annealing process ranges from about 45 seconds to about 55 seconds.

22. The method as claimed in claim 19, wherein the continuous annealing includes slow cooling of the steel sheet to a fifth predetermined temperature at a first predetermined cooling rate.

23. The method as claimed in claim 22, wherein the fifth predetermined temperature ranges from about 650 °C to about 680 °C and the first predetermined cooling rate ranges from about 5 °C/sec to about 13 °C/sec.

24. The method as claimed in claim 19, wherein the continuous annealing includes fast cooling of the steel sheet to a sixth predetermined temperature at a second predetermined cooling rate.

25. The method as claimed in claim 24, wherein the sixth predetermined temperature ranges from about 350 °C to about 400 °C and the second predetermined cooling rate is at least 30°C/sec.

26. The method as claimed in claim 19, wherein the continuous annealing includes an over ageing step of the steel sheet at the sixth predetermined temperature for a third predetermined time.

27. The method as claimed in claim 26, wherein the third predetermined time is at least 90 seconds.

28. The method as claimed in claim 19, wherein the continuous annealing includes cooling of the steel sheet to a seventh predetermined temperature at a third predetermined cooling rate after the over ageing.

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

30. The method as claimed in claim 19 comprises a skin passing step after continuous annealing process, wherein the elongation during skin passing is about 0.6 % to about 0.9 %.

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