Claims:We claim:

1. A high-strength steel sheet, comprising:
composition of:
carbon (C) in a range of about 0.09 wt.% to about 0.12 wt.%,
manganese (Mn) in a range of about 0.95 wt.% to about 1.15 wt.%,
silicon (Si) in a range of about 0.05 wt.% to about 0.15 wt.%,
phosphorous (P) up to 0.020 wt.%,
sulphur (S) up to 0.015 wt.%,
aluminium (Al) in a range of about 0.02 wt % to about 0.09 wt.%,
molybdenum (Mo) in a range of about 0.10 wt.% to about 0.25 wt.%,
nitrogen (N) up to 0.005 ppm, and
Iron (Fe) being remainder of the composition along with incidental elements; and
wherein, the high-strength steel sheet exhibits ductility greater than 30% and Ultimate Tensile Strength (UTS) greater than 490 MPa.
2. The high-strength steel sheet as claimed in claim 1, wherein the high-strength steel sheet comprises 10% -15% martensite microstructure and 85%-90 % ferrite microstructure.

3. The high-strength steel sheet as claimed in claim 1, wherein the high-strength steel sheet exhibits yield strength from about 290MPa to about 350MPa.

4. The high-strength steel sheet as claimed in claim 1, wherein the high-strength steel sheet exhibits a yield ratio of about 0.65 and less.

5. The high-strength steel sheet as claimed in claim 1, wherein the high-strength steel sheet exhibits a sum of work hardening index and bake hardening index more than 100MPa.

6. A method for manufacturing a high-strength steel sheet, the method comprising:
casting, a steel slab of a composition comprising:
carbon (C) in a range of about 0.09 wt.% to about 0.12 wt.%,
manganese (Mn) in a range of about 0.95 wt.% to about 1.15 wt.%,
silicon (Si) in a range of about 0.05 wt.% to about 0.15 wt.%,
phosphorous (P) up to 0.020 wt.%,
sulphur (S) up to 0.015 wt.%,
aluminium (Al) in a range of about 0.02 wt % to about 0.09 wt.%,
molybdenum (Mo) in a range of about 0.10 wt.% to about 0.25 wt.%,
nitrogen (N) up to 0.005 ppm,
Iron (Fe) being remainder of the composition along with incidental elements;
heating, the steel slab of the composition to a first predetermined temperature for a first predetermined interval of time;
hot working, the steel slab at a second predetermined temperature to produce steel sheet;
cooling, the hot worked steel sheet at a third predetermined temperature;
cold rolling, the steel sheet into a required thickness to form the cold-rolled steel sheet;
soaking, the cold- rolled steel sheet at a fourth predetermined temperature for a second predetermined interval of time; and
cooling, the steel sheet to room temperature to obtain a high-strength steel sheet,
wherein, the high-strength steel sheet exhibits ductility greater than 30% and Ultimate Tensile Strength (UTS) greater than 490 MPa.
7. The method as claimed in claim 6, wherein the high-strength steel sheet comprises 10% -15% martensite microstructure and 85%-90 % ferrite microstructure.

8. The method as claimed in claim 6, wherein the high-strength steel sheet exhibits yield strength from about 290MPa to about 350MPa.

9. The method as claimed in claim 6, wherein the high-strength steel sheet is suitable for a hot-dip galvanizing process.

10. The method as claimed in claim 9, wherein the hot-dip galvanizing process is suitable with zinc pot temperature raging from about 450 °C to about 470 °C.

11. The method as claimed in claim 9, wherein the hot-dip galvanizing process is suitable with galvannealing temperature ranging from about 490 °C to about 520 °C.

12. The method as claimed in claim 6, wherein the high-strength steel sheet exhibits a yield ratio to about 0.65 and less.

13. The method as claimed in claim 6, wherein the high-strength steel sheet exhibits a sum of work hardening index and bake hardening index from about100MPa and more.

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

15. The method as claimed in claim 14, comprises finishing rolling temperature during the hot rolling process at a temperature higher than a temperature of austenite (Ar3).

16. The method as claimed in claim 14 comprises coiling of the high-strength steel sheet at a coiling temperature ranging from about 580 °C to about 650°C.

17. The method as claimed in claim 6, wherein the first predetermined temperature ranges from about 1200 °C to 1250 °C, and the first predetermined time from about 150 to 250 minutes.

18. The method as claimed in claim 6, wherein the second predetermined temperature ranges from about 850°C to 940 °C.

19. The method as claimed in claim 6, wherein the third predetermined temperature ranges from about 580 °C to 650 °C.

20. The method as claimed in claim 6, wherein thickness of the steel sheet reduced during the cold rolling process is more than 40% of initial thickness.

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

22. The method as claimed in claim 21, wherein the fourth predetermined temperature employed in the continuous annealing process ranges from about 790 °C to about 820 °C.

23. The method as claimed in claim 21, wherein the second predetermined time employed in the continuous annealing process ranges from about 40 seconds to about 60 seconds.

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

25. The method as claimed in claim 24, wherein the fifth predetermined temperature ranges from about 640 °C to about 675 °C and the first predetermined cooling rate ranges from about 8 °C/sec to about 15 °C/sec.

26. The method as claimed in claim 21, wherein the continuous annealing process includes rapid cooling of the steel sheet to a sixth predetermined temperature at a second predetermined cooling rate.

27. The method as claimed in claim 26, 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.
28. The method as claimed in claim 21, wherein the continuous annealing process includes an over ageing step of the steel sheet at a seventh predetermined temperature for a third predetermined time.

29. The method as claimed in claim 28, wherein the seventh predetermined temperature ranges from about 250 °C to about 300 °C and the third predetermined time ranges from about 85 seconds to about 120 seconds.

30. The method as claimed in claim 28, wherein the continuous annealing process includes a cooling and coiling of the steel sheet to room temperature to form high strength steel sheet.

31. The method as claimed in claim 21 comprises a skin passing step after continuous annealing process, wherein the elongation of the steel sheet during skin passing is about 1.0 % to about 1.2 %.

32. An automotive body panel comprising a high-strength steel sheet as claimed in claim 1.
, Description:TECHNICAL FIELD
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 steel sheet. Further embodiments of the disclosure disclose a method for manufacturing the high-strength steel sheet having an Ultimate Tensile Strength (UTS) greater than 490 MPa and ductility greater than 30%, with 10% -15% martensite microstructure and 85%-90 % ferrite microstructure.

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. More particularly in the recent trend, steel and high strength steel has been excessively used in the automobile industry. However, 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, weight reduction of a vehicle body has been the recent trend. Conventionally, sheet metal thinning has been actively used for the weight reduction of the vehicle body to improve fuel efficiency. Since, strict norms of legislation regarding fuel consumption, emission and collision safety of automobiles have forced the automotive industry to develop lighter, more fuel-efficient vehicles, and it is not possible to respond simply by reducing the weight by reducing the thickness. To achieve this, automobile manufacturers prefer high strength steel metal sheets to make the structural components of the automobiles. Therefore, the demand for a high-strength steel metal sheets is ever increasing.

Conventionally, high strength steel sheets having tensile strength less than 490 MPa have been progressively applied to structural components. However, such conventional steel sheets are difficult to be press-formed due to poor formability. When the automobiles components are formed by employing conventional high strength steel sheets, crack 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. The conventional steel sheets further require high weldability to weld parts of the automobiles. The poor weldability may lead to material break at the time of collision due to absorbing impact. Hence, the performances of conventional high strength steel may not be enough 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, formability and weldability. However, Ultimate Tensile Strength (UTS) 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 one non-limiting embodiment of the present disclosure, there is provided a high-strength steel sheet. The steel sheet includes a composition of carbon (C) in a range of about 0.09 wt.% to about 0.12 wt.%, manganese (Mn) in a range of about 0.95 wt.% to about 1.15 wt.%, silicon (Si) in a range of about 0.05 wt.% to about 0.15 wt.%, phosphorous (P) up to 0.020 wt.%, sulphur (S) up to 0.015 wt.%, aluminium (Al) in a range of about 0.02 wt % to about 0.09 wt.%, molybdenum (Mo) in a range of about 0.10 wt.% to about 0.25 wt.%, nitrogen (N) up to 0.005 ppm and Iron (Fe) being remainder of the composition along with incidental elements. The high-strength steel sheet exhibits Ultimate Tensile Strength (UTS) 490 MPa or more and ductility greater than 30 %.

In an embodiment, the high-strength steel sheet comprises 10% -15% martensite microstructure and 85%-90 % ferrite microstructure.

In an embodiment, the high-strength steel sheet exhibits yield strength from about 290MPa to about 350MPa.

In an embodiment, the high-strength steel sheet exhibits yield ratio of about 0.65 and less.

In an embodiment, the high-strength steel sheet exhibits a sum of work hardening index and bake hardening index more than100MPa.

In another non-limiting embodiment of the present disclosure, there is provided a method for manufacturing, high-strength steel sheet. The method includes steps of firstly casting a steel slab of a composition comprising: carbon (C) in a range of about 0.09 wt.% to about 0.12 wt.%, manganese (Mn) in a range of about 0.95 wt.% to about 1.15 wt.%, silicon (Si) in a range of about 0.05 wt.% to about 0.15 wt.%, phosphorous (P) up to 0.020 wt.%, sulphur (S) up to 0.015 wt.%, aluminium (Al) in a range of about 0.02 wt % to about 0.09 wt.%, molybdenum (Mo) in a range of about 0.10 wt.% to about 0.25 wt.%, nitrogen (N) up to 0.005 ppm and Iron (Fe) being remainder of the composition along with incidental elements. Subjecting the steel slab to heating at a first predetermined temperature for a first predetermined time. Performing hot working at a second predetermined temperature to produce a steel sheet, and then, performing cooling at a third predetermined temperature. The method further involves cold rolling the steel sheet into a required thickness to form the cold-rolled steel sheet, 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 steel sheet which exhibits ductility greater than 30% and Ultimate Tensile Strength (UTS) greater than 490 MPa, and that containing 10% -15% martensite microstructure and 85%-90 % ferrite microstructure.

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 high-strength steel sheet is suitable for a hot-dip galvanizing process.

In an embodiment, the high-strength steel sheet is suitable for the hot-dip galvanizing process is suitable with zinc pot temperature from about 450 °C to about 470 °C.

In an embodiment, the high-strength steel sheet is suitable for the hot-dip galvanizing process is suitable with galvannealing temperature from about 490 °C to about 520 °C.

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

In an embodiment, finishing rolling temperature during the hot rolling process at a temperature higher than a temperature of austenite (Ar3) i.e. around 850 °C to 940 °C where Ar3 is the critical transformation temperature for transformation of austenite to ferrite starts at equilibrium.

In an embodiment, coiling of the steel sheet at a temperature ranging from about 580 °C to about 650°C.

In an embodiment, the first predetermined temperature ranges from about 1200 °C to about 1250 °C, and the first predetermined time from about 150 to 250 minutes.

In an embodiment, the second predetermined temperature ranges from about 850 to 940 °C.

In an embodiment, the third predetermined temperature ranges from about 580 to 650 °C.

In an embodiment, the reduction in thickness of the steel sheet reduced after the cold rolling process is more than 40% of the initial thickness.

In an embodiment, soaking of the cold-rolled steel sheet for a second predetermined interval of time with a predetermined cooling rate 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 ranges from about 40 seconds to about 60 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 640 °C to about 675 °C and the first predetermined cooling rate ranges from about 8 °C/sec to about 15 °C/sec.

In an embodiment, the continuous annealing process includes rapid 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 steel may be further processed in the over ageing section at the seventh predetermined temperature ranging from about 250 °C to about 300 °C for a third predetermined time ranges from about 85 seconds to about 120 seconds. The steel may be then cooled and coiled to room temperature to form high strength steel sheet.

In an embodiment, a skin passing step is being performed after continuous annealing wherein the elongation during skin passing is about 1.0 % to about 1.2 %.

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

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

Figure. 3 illustrates a graphical representation of a limit curve plotted based on experiments performed on the high strength steel sheet, according to an exemplary embodiment of the present disclosure.

Figures. 4a-4d illustrate graphical representation of weldability results and analysis performed on the high strength steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 5a illustrates a micrographic view of phosphate crystals, according to an exemplary embodiment of the present disclosure.

Figure. 5b illustrates a table of phosphate coating bath parameters with varying compositions, according to an exemplary embodiment of the present disclosure.

Figures. 6a and 6b illustrates a micrographic view of the high-strength steel sheet, according to an exemplary embodiment of the present disclosure.

Figure. 7 illustrates a bar graphical representation of yield ratio, BH index and work hardening plotted based on experiments performed on the high strength steel sheet, 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 steel sheet and a method for manufacturing a high-strength steel sheet. Conventional steel sheets used in the industry comprises strength which is less than 490 MPa and these high-strength steel sheets are produced by conventional heat treatment methods. However, these high- strength steels have poor formability and tend to produce cracks during forming process. Also, these high strength steels have poor weldability that may lead to material breakage at impact. Accordingly, the method of present disclosure discloses a production of high-strength steel sheet that exhibits Ultimate Tensile Strength (UTS) greater than 490 MPa and having 10% -15% martensite microstructure and 85%-90 % ferrite microstructure. Additionally, the high-strength steel sheet so manufactured may include characteristic of Strength, ductility, formability, weldability and phosphating. The high strength steel sheet may be widely employed to make automotive components requiring high strength, high ductility, formability and weldability. Additionally, the present disclosure discloses a production of high-strength steel sheet that exhibits high phosphating properties.

According to various embodiment of the disclosure, the method of manufacturing high-strength steel sheet, includes a first step of producing the steel slab of composition comprising in weight percentage of: carbon (C) in a range of about 0.09 wt.% to about 0.12 wt.%, manganese (Mn) in a range of about 0.95 wt.% to about 1.15 wt.%, silicon (Si) in a range of about 0.05 wt.% to about 0.15 wt.%, phosphorous (P) up to 0.020 wt.%, sulphur (S) up to 0.015 wt.%, aluminium (Al) in a range of about 0.02 wt % to about 0.09 wt.%, molybdenum (Mo) in a range of about 0.10 wt.% to about 0.25 wt.%, nitrogen (N) up to 0.005 ppm and Iron (Fe) being remainder of the composition along with incidental elements by any manufacturing process including but not limiting to casting. The steel slab is then hot charged and heated to a temperature of about 1200 °C to 1250 °C for about to about 150 to 250 minutes. 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 at a finish rolling temperature during the hot rolling process at a temperature higher than a temperature of austenite (herein referred to as Ar3) i.e. around 850 °C to 940 °C where Ar3 is the critical transformation temperature for 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 to 650 °C. Steel sheet may be further subjected to cold rolling process. The 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 ranges from about 790 °C to about 820 °C for about 40 seconds to about 60 seconds. In an embodiment, the steel may be subsequently slow cooled, to a fifth predetermined temperature ranging from about 640 °C to about 675 °C, at a first predetermined cooling rate which ranges from about 8 °C/sec to about 15 °C/sec. Further, the steel may then be subjected to fast cooling to a sixth predetermined temperature ranges from about 250 °C to about 300 °C, at a second predetermined temperature of at least 30°C/sec.

In an embodiment, the steel may be further processed in the over ageing section at the seventh predetermined temperature ranging from about 250 °C to about 300 °C for a third predetermined time ranges from about 85 seconds to about 120 seconds. The steel may be then cooled and coiled to room temperature to form high strength steel sheet. The high strength steel sheet according to the present disclosure may have 10% -15% martensite microstructure and 85%-90 % ferrite microstructure.

Various embodiments of the high strength with manufacturing processes are explained referencing Figures 1 to 7.

Now referring Figures. 1 and 2 are exemplary embodiments of the present disclosure illustrating a flowchart of a method for producing high-strength steel sheet, and a graphical representation of heat treatment process. In the present disclosure, mechanical properties such as strength, formability, weldability of the steel may be improved. The steel produced by the method of the present disclosure, includes 10% -15% martensite microstructure and 85%-90 % ferrite microstructure. The method is now described with reference to the flowchart blocks. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method. Additionally, individual blocks may be deleted from the methods without departing from the scope of the subject. The method is particularly applicable to high-strength steel, and it may also be extended to other type of steels as well.

The method of manufacturing the high strength steel sheet according to the present disclosure consists of a casting step followed by a hot-rolling step. Other steps include, coiling, cold rolling, soaking, and a controlled cooling step using of the steel material. 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 an 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 a composition of: carbon (C) in a range of about 0.09 wt.% to about 0.12 wt.%, manganese (Mn) in a range of about 0.95 wt.% to about 1.15 wt.%, silicon (Si) in a range of about 0.05 wt.% to about 0.15 wt.%, phosphorous (P) up to 0.020 wt.%, sulphur (S) up to 0.015 wt.%, aluminium (Al) in a range of about 0.02 wt % to about 0.09 wt.%, molybdenum (Mo) in a range of about 0.10 wt.% to about 0.25 wt.%, nitrogen (N) up to 0.005 ppm and Iron (Fe) being remainder of the composition along with incidental elements may be casted in a continuous casting process.

The method further includes steps of hot charging 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, 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 from about 150 to 250 minutes.

The method further includes steps 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. During hot rolling the hot charged steel slab may be subjected to roughing milling process. The roughing mill 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 the milled steel sheet may be further subjected to finish rolling. In an embodiment, during finish rolling, the steel sheet surface may be subjected to further thickness reduction, along with surface finishing and dynamic recrystallization. The finish rolling temperature during hot rolling may vary in the range of 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 which ranges from about 580 to 650 °C. [shown in block 104].

Now referring to block 105, the method further includes the step of cold rolling the steel sheet. In an embodiment, the steel sheet subjected to cold rolling may be subjected to picking process for surface cleaning and modification. Later, the steel sheet may be subjected to cold rolling process in order to reduce the thickness and achieve a final thickness reduction without any external heat. During the cold rolling process, point defect density (vacancies, self-interstitials etc.) and dislocation density increases within the steel sheet. In contrast, 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 the cold rolling process, the steel sheet may be subjected to coiling process to form full hard cold rolled coil. In an embodiment, reduction in thickness of the steel sheet after the cold rolling process may be above 40 %.

The method further includes, soaking of cold worked steel sheet at a fourth predetermined temperature for a second predetermined time. Prior to soaking, the cold worked steel sheet coil may be subjected to electrolytic cleaning process. Soaking of the steel sheet may be carried out in a continuous annealing process at a temperature ranges from about 790 °C to about 820 °C for about 40 seconds to about 60 seconds [shown in block 106]. The steel sheet may be further subjected to continues annealing process. Annealing is the process of relieving the internal stresses in the steel that may be built up during the cold rolling process. Steel sheet hardens after cold rolling due to the dislocation tangling generated by plastic deformation. Annealing is therefore carried out to soften the material. The annealing process comprises heating, holding of the material at an elevated temperature (soaking), and cooling of the material. Heating facilitates the movement of iron atoms, resulting in the disappearance of tangled dislocations and the formation and growth of new grains of various sizes, which depend on the heating and soaking conditions. 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 sheet may be subsequently cooled to a fifth predetermined temperature ranging from about 640 °C to about 675 °C at a first predetermined cooling rate ranging from about 8 °C/sec to about 15 °C/sec. The steel may be then fast/rapid cooled to a sixth predetermined temperature ranging from about 250 °C to about 300 °C at a second predetermined temperature of at least 330°C/sec [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 seventh predetermined temperature ranging from about 250 °C to about 300 °C for a third predetermined time ranging from about 85 seconds to about 120 seconds. The steel sheet may be then cooled to room temperature to form high strength steel sheet. After annealing process, the steel sheet may be subjected to skin passing process. In an embodiment, the skin passing process 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 steel sheet [as shown in block 109].

Referring to figure 2, illustrates a schematic diagram of the heat treatment employed in continuous annealing.

Dual-phase steels are characterized by the presence of uniformly distributed hard martensite phase in a soft ferrite matrix. Such a microstructure can be achieved using an appropriate annealing cycle. The steel is heated to an inter-critical temperature range, where a small fraction of the ferrite transforms to austenite. The steel is then cooled slowly by around 15 to 40 °C, to re-distribute the chemical elements present, particularly carbon. From this temperature, it is cooled rapidly, by around 300 to 400 °C, during which the austenite formed further transforms to the hard martensite phase. This finally results in the desired combination of ferrite and martensite phases in the steel.

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 depresses A3 thereby increasing the volume fraction of austenite that can be formed at a given top temperature. During the slow cooling on continuous annealing process, some of this austenite will re-transform to ferrite, ejecting carbon 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-hard and kinetics of both ferrite and pearlite formation are pushed to longer times.

In one embodiment of the present disclosure, the composition of carbon enriches the austenite which results in a stronger martensite forming with a lower Ms temperature, and as such a stronger overall dual phase product. If the Ms is suppressed below 470oC then martensite will form in the final cooling stage on a galvanising line, avoiding tempering during the overage section. Typical over aging temperatures at Continuous annealing process for dual phase steels are approximately 300° C, dropping to 250oC by the end of the overage, and some tempering of the martensitic phase is expected in these products. Carbon beyond 0.09% will hamper welding and forming characteristics of the material and is hence limited to 0.09 to 0.12% in the current invention.

In an embodiment, the composition of Manganese (Mn) contributes to the hardenability of austenite and retards the kinetics of bainite formation. In a further embodiment, the manganese (Mn) composition, reduces the expected volume fraction of bainite that would form during the long high temperature overage on some galvanising lines. In an embodiment, if the Mn content is more than 1.15%, it will affect the weldability of the steel as well as on strength and if Mn content is less than 0.95%, it will affect the properties, and hence the content of Mn is set to 0.95% or more and 1.15% or less.

In one embodiment of the present disclosure, the composition of silicon (Si) is a ferrite stabiliser and is insoluble in cementite. A small silicon addition will help promote ferrite formation during the slow cooling during continuous annealing process, this being more important if annealing top temperatures are such that a fully austenitic structure is formed. Silicon due to its insolubility in cementite, acts to suppress pearlite formation and the formation of bainitic carbides thereby reducing the critical cooling rate that is required to obtain martensite. Due to its insolubility in cementite, it also contributes to the resistance of martensite to tempering, and hence the content of Si is set to 0.05% or more and 0.150% or less. This level of Si also helps in improving weldability.

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

Further, in one embodiment of the present disclosure, the composition of sulphur (S) levels should be reduced as far as possible to limit the formation of MnS particles, which are sufficiently ductile to form long stringers elongated in the rolling direction and adversely affect transverse strength, ductility, edge flangeability and bendability, and hence the content of S is set to 0.015% or less.
In one embodiment of the present disclosure, the composition of 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% or more and 0.25% or less.

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 D) for which the tests are carried out is as shown in below table 1. The compositions of table 1 were continuously cast in the 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. Further, for each set of compositions, obtained yield strength (YS), ultimate tensile strength (UTS), percentage elongation (%), and yield ratio values are also tabulated in table. 1. This scalar quantity may be used as an indicator of the formability of recrystallized low-carbon steel sheets.

In case of composition A to D, when the soaking temperature is about 790 °C to about 820 °C during continuous annealing process, the steel exhibits ultimate tensile strength greater than 490 MPa and high percentage elongation, due the presence of 10% -15% martensite microstructure and 85%-90 % ferrite microstructure, thereby exhibiting very good formability. However, when soaking the steel sheet during continuous annealing at around less than 790 °C, it leads to insufficient martensite formation, leading to a lower ultimate tensile strength. Similarly, a soaking temperature more than 820 °C is expected to give very high properties due to a large fraction of martensite, which is also not desirable.

Sl No %C %Mn %Si %P %S %Al N, ppm %Mo YS (Mpa) UTS (Mpa) EL Strain(%) Yield Ratio
(YS/UTS) Soaking Temperature Cooling rate Remarks
A 0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 324 505 30.6 0.64 790 - 820 > 30 Example 1
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 307 496 33.1 0.62 790 - 820 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 329 506 30.9 0.65 790 - 820 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 330 504 30.1 0.65 790 - 820 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 329 503 33.4 0.65 790 - 820 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 328 504 32.3 0.65 790 - 820 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 336 515 33.0 0.65 790 - 820 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 334 518 31.4 0.64 790 - 820 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 330 505 31.0 0.65 790 - 820 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 315 479 33.3 0.66 790 - 820 < 30 Comp. Example
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 332 455 35.8 0.73 790 - 820 < 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 322 468 32.9 0.69 < 790 > 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 332 485 32.8 0.68 < 790 < 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 311 489 33.3 0.64 < 790 < 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 309 484 33.2 0.64 < 790 < 30
0.099 1.051 0.078 0.021 0.005 0.058 30 0.11 298 488 33.5 0.61 < 790 < 30
B 0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 321 502 31.9 0.64 790 - 820 > 30 Example 2
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 317 493 33 0.64 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 310 493 33.6 0.63 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 325 501 30.5 0.65 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 322 492 32.8 0.65 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 313 493 33.8 0.63 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 326 498 34.5 0.65 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 323 501 32.1 0.64 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 329 512 31.2 0.64 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 317 500 30.9 0.63 790 - 820 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 297 486 33.5 0.61 790 - 820 < 30 Comp. Example
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 303 484 33.8 0.63 790 - 820 < 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 299 486 33.8 0.62 < 790 < 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 304 488 34.2 0.62 < 790 < 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 301 487 34.6 0.62 < 790 < 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 304 486 34.2 0.63 < 790 > 30
0.11 1.007 0.085 0.019 0.006 0.041 23 0.18 305 488 34.6 0.63 < 790 > 30
C 0.090 1.04 0.09 0.017 0.006 0.054 32 0.25 297 504 36.1 0.59 790 - 820 > 30 Example 3
0.090 1.04 0.09 0.017 0.006 0.054 32 0.25 295 517 35.9 0.57 790 - 820 > 30
0.090 1.04 0.09 0.017 0.006 0.054 32 0.25 296 513 37.4 0.58 790 - 820 > 30
0.090 1.04 0.09 0.017 0.006 0.054 32 0.25 292 510 38.6 0.57 790 - 820 > 30
0.090 1.04 0.09 0.017 0.006 0.054 32 0.25 352 524 33.6 0.67 < 790 > 30 Comp. Example
0.090 1.04 0.09 0.017 0.006 0.054 32 0.25 354 518 35.8 0.68 < 790 > 30
0.090 1.04 0.09 0.017 0.006 0.054 32 0.25 297 456 36.0 0.65 790 - 820 < 30
0.090 1.04 0.09 0.017 0.006 0.054 32 0.25 290 448 37.0 0.65 790 - 820 < 30
D 0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 339 519 31.6 0.65 790 - 820 > 30 Example 4
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 320 505 32.8 0.63 790 - 820 > 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 327 510 32.5 0.64 790 - 820 > 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 325 508 30.3 0.64 790 - 820 > 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 333 515 33.1 0.65 790 - 820 > 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 311 494 32 0.63 790 - 820 > 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 303 491 34.7 0.62 790 - 820 > 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 352 524 32.4 0.67 < 790 > 30 Comp. Example
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 354 518 32.9 0.68 < 790 > 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 370 530 29.8 0.70 < 790 > 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 336 492 32.8 0.68 790 - 820 < 30
0.110 1.06 0.11 0.020 0.004 0.048 35 0.21 347 491 31.2 0.71 790 - 820 < 30

Table-1

Referring to figure 3 which illustrates the experiments have been performed on a 0.8 mm thick steel sheet.

Referring now to Figures 4a to 4d which illustrate graphical representation of weldability that may be performed on the high strength steel sheet. The following graphical representations shows experimental analysis on the high strength steel sheet with respect to weld current, TS (tensile strength), CTS (cross tensile strength) and nugget diameter. In an example, the steel sheet thickness may be around 0.83 mm. Further, a constant pressure of around 2.5 kN may be applied on the steel sheet wherein the tip diameter of the weld tip is at around 6mm. During the welding process, an initially, a pressure is applied for about 24 weld time cycles. At step b, the weld current (I) is gradually increased for about 1.2 weld time cycles, and then again maintained at a constant current (I) in step c, for about 14 weld time cycles, after which the weld current (I) is reduced to zero. Finally, at step d, the weld is allowed to cool for about 5 weld time cycles. From the graphs and experiments, it is evident that the high-strength steel exhibits improved weldability.

Referring now to figures 5a illustrates a micrographic view of phosphate crystals. As shown in figure, the phosphate crystals observed on sample substrate is less than 5 µm, which is considered as good grade of phosphating. This gives the surface a uniform coverage of the phosphate crystals with a globular shape. In one embodiment, the average size of the crystals of phosphate coating may be 2-3 micron, with a P-ratio of the phosphate layer more than 90 %. Figure 5b illustrates phosphate coating bath parameters with varying compositions and the corresponding temperature respectively.

Referring to figures 6a and 6b which illustrates optical micrographic views of the high-strength steel sheet. In the figure 6a, black regions correspond to martensite and grey regions correspond to ferrite, and in figure 6b, grey regions correspond to martensite and black regions correspond to ferrite, wherein the high-strength steel sheet containing 10% -15% martensite microstructure and 85%-90 % ferrite microstructure. 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. Hence, steel shows improved strength with finer grain microstructure.

Referring to figures 7, the following bar graphical representations shows experimental analysis on the high strength steel sheet with respect to yield ratio, BH index and work hardening. In an example, the yield ratio of the steel sheet may be less than 0.65. Further, the work hardening and the BH index of the steel sheet may be 61 and 55 respectively, and the yield ratio of the steel sheet after component forming and paint baking may be less than 0.80.

In an embodiment of the present disclosure, the high strength steel sheet of the present disclosure may be used any application including but not limiting to automotive body components such as structural components for automotive components/parts. The high strength steel sheet may be used in any other industrial structural applications based on requirement.

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
101-109 Flowchart blocks
101 Casting stage
102 Heating stage
103 Hot working stage
104 Cooling stage at third predetermined temperature
105 Cold rolling stage
106 Soaking stage
107 Cooling stage at fifth predetermined temperature
108 Overaging and coiling stage
109 Cooling and final coiling stage at room temperature