Claims:

1. A high-strength bake-hardenable cold-rolled steel sheet, comprising:
composition of:
carbon (C) at about 0.0015 wt.% to at about 0.0035 wt.%,
manganese (Mn) up-to 0.80 wt.%
sulphur (S) up-to 0.015 wt.%,
phosphorous (P) up-to 0.06 wt.%,
silicon (Si) up-to 0.03 wt.%,
aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%,
nitrogen (N) up-to 0.004 ppm,
niobium (Nb) at about 0.005 wt.% to at about 0.025 wt.%,
molybdenum (Mo) at about 0.02 wt.% to at about 0.03 wt.%,
the balance being Iron (Fe) optionally along with incidental elements;

wherein, the high-strength bake-hardenable cold-rolled steel sheet exhibits Ultimate Tensile Strength (UTS) greater than 340 MPa and Bake-Hardening (BH) index greater than 30 MPa.

2. The high-strength bake-hardenable cold-rolled steel sheet as claimed in claim 1, wherein the high-strength bake-hardenable cold-rolled steel sheet comprises Ferrite and Pearlite microstructure.

3. The high-strength bake-hardenable cold-rolled steel sheet as claimed in claim 1, wherein the high-strength bake-hardenable cold-rolled steel sheet exhibits Lankford value (R-value) greater than 1.5.

4. The high-strength bake-hardenable cold-rolled steel sheet as claimed in claim 1, wherein the high-strength bake-hardenable cold-rolled steel sheet exhibits ductility (% elongation) of about 40%.

5. The high-strength bake-hardenable cold-rolled steel sheet as claimed in claim 1, wherein the high-strength bake-hardenable cold-rolled steel sheet exhibits good non-ageing behavior properties at room temperature.

6. The high high-strength bake-hardenable cold-rolled steel sheet as claimed in claim 5, wherein shelf life of high-strength bake-hardenable cold-rolled steel sheet with good non-ageing properties at room temperature is at least about 6 months.

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

8. A method for manufacturing a high-strength bake-hardenable cold-rolled steel sheet, the method comprising:
casting, a steel slab of a composition comprising:
carbon (C) at about 0.0015 wt.% to at about 0.0035 wt.%,
manganese (Mn) up-to 0.80 wt.%
sulphur (S) up-to 0.015 wt.%,
phosphorous (P) up-to 0.06 wt.%,
silicon (Si) up-to 0.03 wt.%,
aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%,
nitrogen (N) up-to 0.004 ppm,
niobium (Nb) at about 0.005 wt.% to at about 0.025 wt.%,
molybdenum (Mo) at about 0.02 wt.% to at about 0.03 wt.%,
the balance being Iron (Fe) optionally along with incidental elements;
heating, the steel slab to a first predetermined temperature for a first predetermined time;
hot working, the steel slab at a second predetermined temperature to produce a steel sheet;
cooling, the hot worked steel sheet to a third predetermined temperature;
cold rolling, the steel sheet at third predetermined temperature;
soaking, the cold-rolled steel sheet at fourth predetermined temperature for a second predetermined time with a predetermined cooling rate;
coiling, the steel sheet, to room temperature to obtain the high-strength bake-hardenable cold-rolled steel sheet;

wherein, the high-strength bake-hardenable cold-rolled steel sheet exhibits Ultimate Tensile Strength (UTS) greater than 340 MPa and Bake-Hardening (BH) index greater than 30 MPa.

9. The method as claimed in claim 8, wherein, the high-strength bake-hardenable cold-rolled steel sheet comprises Ferrite and pearlite microstructure.

10. The method as claimed in claim 8, wherein the high-strength bake-hardenable cold-rolled steel sheet exhibits Lankford value (R-value) greater than 1.5.

11. The method as claimed in claim 8, wherein the high-strength bake-hardenable cold-rolled steel sheet exhibits good non-ageing behavior properties at room temperature.

12. The method as claimed in claim 11, wherein the shelf life of high-strength bake-hardenable cold-rolled steel sheet with good non-ageing properties at room temperature is at least about 6 months.

13. The method as claimed in claim 8, wherein the steel slab is hot charged into a furnace for heating.

14. The method as claimed in claim 8, wherein the casting is carried out in a continuous casting process.

15. The method as claimed in claim 14, wherein the continuous casting process is performed in at least one of continuous caster.

16. The method as claimed in claim 15, wherein the temperature of the steel slab at an exit of the thin slab caster is maintained above 1000 ?C.

17. The method as claimed in claim 8, 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.

18. The method as claimed in claim 8, wherein the hot working is a hot rolling process.

19. The method as claimed in claim 8, wherein the hot working is performed in a finish rolling mill, and the second predetermined temperature higher than critical transformation temperature for austenite (Ar3).

20. The method as claimed in claim 19, wherein the second predetermined temperature ranges from of about 900 ?C to about 940 ?C.

21. The method as claimed in claim 8, wherein the hot rolled steel sheet is cooled and optionally coiled at a temperature range of 650 ?C to 720 ?C.

22. The method as claimed in claim 8, wherein the third predetermined temperature is room temperature.

23. The method as claimed in claim 8, wherein the reduction in thickness of the steel sheet after cold rolling is about 70 % to 80 %.

24. The method as claimed in claim 8, wherein soaking of cold-rolled steel sheet for a second predetermined time with a predetermined cooling rate is a continuous annealing process.

25. The method as claimed in claim 24, wherein the fourth predetermined temperature employed in continuous annealing process ranges from about 750 °C to about 800 °C.

26. The method as claimed in claim 24, wherein the second predetermined time employed in continuous annealing process is ranging from about 50 seconds.

27. The method as claimed in claim 24, wherein the predetermined cooling rate employed in continuous annealing process is ranging from about 10 °C/second to about 35 °C/second.

28. Automotive body panel comprising a high-strength bake-hardenable cold-rolled steel sheet as claimed in claim 1.
, 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 bake-hardenable cold-rolled steel sheet. Further embodiments of the disclosure disclose a method for manufacturing the high-strength bake-hardenable cold-rolled steel sheet with Ultimate Tensile Strength (UTS) greater than 340 MPa and Bake-Hardening (BH) Index greater than 30 MPa with very good formability.

BACKGROUND OF THE DISCLOSURE

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

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

In the automotive industry, the threat to future 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 parts such as side body panels or a door inner panels prefer high strength materials coupled with better formability. The performances of conventional high strength steel may not sufficient to meet the contradicting requirement. It has been found out that bake-hardenable steel sheets are an excellent solution to the above problem. In case of outer body panels where good dent resistance may be required, high strength bake-hardenable steels have increasingly being used.

In case of bake-hardening steel, the parts of the vehicle body panel may pass through a paint baking cycle that is done after all the forming operations are completed. In this paint baking cycle, the painted vehicle body part may be treated in an oven to dry and allow the paint to bond to the steel substrate. Depending on the quality and the amount of the paint required, this process (painting and paint baking) may be repeated several times. The paint baking process may not only give the vehicle body an aesthetically pleasing look, but also positively influences on the strength of the material used for the structure. The increment of strength which happens during such paint baking process is known as Bake-Hardening (BH) effect. In case of bake-hardenable steel, there may be an increase in yield strength of the material during post paint baking operation. These bake-hardenable steels may offer good formability (low yield strength for good shape fixability) during press operations and also higher final yield strength during paint baking operations for good dent resistance. Such an increase in strength may provide the automotive manufacturers an option to improve the safety of the vehicle and to reduce the gauge thickness of the steel. The BH effect is important also because of the fact that it is easier and economical to get the steel with good formability (low yield strength, R-value greater than 1).

In of the conventional arts, various steel compositions and heat treatment methods have been developed in order to obtain desired strength and bake-hardenability. Ultimate Tensile Strength (UTS) and Bake-hardening (BH) Index values obtained for these steels may be less than the desirable ranges to get optimal performance. Reduced strength and poor bake-hardenability values limit the applications of these steels. More importantly, these bake-hardenable steels disclosed in conventional art undergoes ageing phenomena at room temperate. Ageing may drastically reduce the stability and shelf life of the steel at room temperature. Due to ageing phenomena ultimate tensile strength and bake hardening index may also decrease and make the steel less economical.

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

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

SUMMARY OF THE DISCLOSURE

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

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

In a non-limiting embodiment of the present disclosure, there is provided a high-strength bake-hardenable cold-rolled steel sheet. The steel sheet includes composition of carbon (C) at about 0.0015 wt.% to at about 0.0035 wt.%, manganese (Mn) up-to 0.80 wt.%, sulphur (S) up-to 0.015 wt.%, phosphorous (P) up-to 0.06 wt.%, silicon (Si) up-to 0.03 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 0.004 ppm, niobium (Nb) at about 0.005 wt.% to at about 0.025 wt.%, molybdenum (Mo) at about 0.02 wt.% to at about 0.03 wt.%, the balance being Iron (Fe) optionally along with incidental elements. The high-strength bake-hardenable cold-rolled steel sheet exhibits Ultimate Tensile Strength (UTS) greater than 340 MPa and Bake-Hardening (BH) index greater than 30 MPa.

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

In an embodiment, the high-strength bake-hardenable cold-rolled steel sheet exhibits Lankford value (R-value) greater than 1.5.

In an embodiment, the high-strength bake-hardenable cold-rolled steel sheet exhibits ductility (% elongation) of about 40%.

In an embodiment, the high-strength bake-hardenable cold-rolled steel sheet exhibits exhibits good non-ageing behavior properties at room temperature for at least about 6 months.

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

In another non-limiting embodiment of the present disclosure, there is provided a method for manufacturing, high-strength bake-hardenable cold-rolled steel sheet. The method includes steps of firstly casting a steel slab of a composition comprising: carbon (C) at about 0.0015 wt.% to at about 0.0035 wt.%, manganese (Mn) up-to 0.80 wt.%, sulphur (S) up-to 0.015 wt.%, phosphorous (P) up-to 0.06 wt.%, silicon (Si) up-to 0.03 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 0.004 ppm, niobium (Nb) at about 0.005 wt.% to at about 0.025 wt.%, molybdenum (Mo) at about 0.02 wt.% to at about 0.03 wt.%, 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 cooled to a third predetermined temperature. The method may further involve cold-rolling the steel sheet to a desired thickness, followed by soaking the cold worked steel sheet at a fourth predetermined temperature for a second predetermined time. Finally, the steel sheet is coiled to room temperature to obtain the high-strength bake-hardenable cold-rolled steel sheet with Ultimate Tensile Strength (UTS) greater than 340 MPa and Bake-Hardening (BH) index greater than 30 MPa.

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. Further, 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 hot rolled steel sheet is cooled and optionally coiled at a temperature range of 650 ?C to 720 ?C.

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

In an embodiment, the reduction in thickness of the steel sheet after cold-rolling is about 70% to 80%.

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

In an embodiment, the fourth predetermined temperature employed in continuous annealing process ranges from about 750 °C to about 800 °C.

In an embodiment, the second predetermined time employed in continuous annealing process is ranging from about 50 seconds.

In an embodiment, the predetermined cooling rate employed in continuous annealing process is ranging from about 10 °C/second to about 35 °C/second.

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

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

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

Figure. 4 illustrates graphical representation of Yield Strength values (YS) high-strength bake-hardenable cold-rolled steel sheet during real-time test for ageing phenomena.

Figure. 5 illustrates graphical representation of Bake Hardening (BH) Index for the high-strength bake-hardenable cold-rolled steel sheet during real-time test for ageing phenomena.

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 bake-hardenable cold-rolled steel sheet and a method for manufacturing a high-strength bake-hardenable 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, bake-hardenable steels with strength less than 340 MPa are produced by conventional heat treatments. However, these steels have reduced shelf life due to ageing phenomena and the mechanical properties decay along with time. Accordingly, the method of present disclosure, discloses a production of high-strength bake-hardenable cold-rolled steel sheet, exhibits Ultimate Tensile Strength (UTS) greater than 340 MPa and Bake-Hardening (BH) index greater than 30 MPa. 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 bake-hardenable cold-rolled steel sheet, includes first step of producing the steel slab of composition comprising in weight percentage of: carbon (C) at about 0.0015 wt.% to at about 0.0035 wt.%, manganese (Mn) up-to 0.80 wt.%, sulphur (S) up-to 0.015 wt.%, phosphorous (P) up-to 0.06 wt.%, silicon (Si) up-to 0.03 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 0.004 ppm, niobium (Nb) at about 0.005 wt.% to at about 0.025 wt.%, molybdenum (Mo) at about 0.02 wt.% to at about 0.03 wt.%, the balance being Iron (Fe) optionally along with incidental elements by any manufacturing process including but not limiting to casting. The steel slab is then reheated to a temperature of about 1200 °C to 1250 °C. The steel slab may be then subjected to hot working process including but not limited to hot-rolling process. The hot charged steel slab may be hot-rolled in finishing mill. The finish rolling temperature may vary in the range of Ar3 i.e. around 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 optionally 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 750 °C to 800 °C for about 50 seconds. The steel may be subsequently cooled slowly, to a fifth predetermined temperature of about 640 to 675 °C, at a first predetermined cooling rate of less than 15 °C/s. The steel may be then cooled to a sixth predetermined temperature of about 300 °C to 350 °C, at a second predetermined temperature of more than 35 °C/s. The steel may be further processed in the over ageing section at a seventh predetermined temperature of about 300 °C to 350 °C for a third predetermined time of about 80 seconds. The steel may be then cooled to an eighth predetermined temperature of about 150 °C at a third predetermined cooling rate of about 10 °C/s and coiled to room temperature. The cold-rolled steel sheet according to the present disclosure may have a microstructure comprises ferrite and pearlite microstructure.

Strength of the steel may be primarily obtained from bake-hardening process.

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 bake-hardenable 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 bake-hardenable cold-rolled steel sheet, and a graphical representation of heat treatment followed during the continuous annealing process. In the present disclosure, mechanical properties such as strength, hardenability index and 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 bake-hardenable cold-rolled steel, and it may also be extended to other type of steels as well.

The method of manufacturing the high strength bake-harbenable 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) at about 0.0015 wt.% to at about 0.0035 wt.%, manganese (Mn) up-to 0.80 wt.%, sulphur (S) up-to 0.015 wt.%, phosphorous (P) up-to 0.06 wt.%, silicon (Si) up-to 0.03 wt.%, aluminium (Al) at about 0.02 wt.% to at about 0.09 wt.%, nitrogen (N) up-to 0.004 ppm, niobium (Nb) at about 0.005 wt.% to at about 0.025 wt.%, molybdenum (Mo) at about 0.02 wt.% to at about 0.03 wt.%, 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 re-heating as shown in block 102. After casting the steel slab with the specified composition, the slabs may be heated in a furnace to a first predetermined temperature for a first predetermined time. In an embodiment, the steel slab may be hot charged into the furnace for 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 first hot working process may be but not limited to hot rolling. Hot-rolling is a metal forming process in which metal stock is passed through one or more pairs of rolls to reduce the thickness and to make the thickness uniform at high temperatures and hot-rolling is carried out above the recrystallization temperature of the steel. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. In an embodiment, the steel slab may be hot rolled in finishing mill. The finish rolling temperature may vary in the range of Ar3 i.e. around 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 optionally coiled at a 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 sheet at third predetermined temperature i.e. 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. During cold working 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 working process can be used as driving force for re-crystallization on subsequent annealing process. In an embodiment, reduction in thickness of the steel sheet after cold working may be at least 40 %.

The method further includes soaking of cold-rolled steel sheet at a fourth predetermined temperature for a second predetermined time. Soaking may be carried out in a continuous annealing process at a temperature ranging from about 750 °C to 800 °C for about 50 seconds [shown in block 106]. 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 less than 15 °C/s. The steel may be then cooled to a sixth predetermined temperature of about 300 °C to 350 °C at a second predetermined temperature of more than 35 °C/s [shown in block 107].

Now referring to block 108, the steel may be further processed in the overaging section at a seventh predetermined temperature of about 300 °C to 350 °C for a third predetermined time of about 80 seconds. The steel may be then cooled to an eighth predetermined temperature of about 150 °C at a third predetermined cooling rate of about 10 °C/s and finally coiled to room temperature to form the high-strength bake-hardenable cold-rolled steel sheet.
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.

Strength may be primary obtained through bake-hardening process. Bake hardening (BH) is a phenomenon where the strain aging is exploited in a positive way to improve the strength of formed automotive components. Bake hardening of steel may be controlled by static strain aging mechanism which may involve three stages namely, i) Snoek rearrangement ii) Cottrell atmospheres formation and iii) Precipitation of Coherent Carbides.

The first stage of strain aging may be attributed to Snoek rearrangement of interstitials in the stress field of dislocation. Initially, there may be a random distribution of interstitials in the microstructure. During pre-straining, dislocations may attempt to minimize the strain energy in the region of the dislocation by moving from random to minimum energy site positions such as grain boundaries. Only a minute amount of interstitials may take part in the ‘Snoek rearrangement’ process and the time for required ‘Snoek rearrangement’ may be less than the time required for a normal interstitial jump. The second and slower stage, which may be formation of ‘Cottrell atmospheres’ by long-range diffusion of interstitial solutes from outside the strained region. It may be a form of long-range diffusion of interstitial solute atoms to dislocation cores, which immobilize the dislocation motion by anchoring them. In this mechanism, solute nitrogen and carbon may diffuse and interact with the strain fields of mobile dislocations to form ‘Cottrell atmosphere’. These atmospheres may constitute regions in which the elastic strain field of the dislocation may be partially relaxed, and hence its energy may be reduced, so that the solutes may effectively lock/anchor the dislocations movement. The driving force for pinning/anchoring may be reduction in lattice energy. Both impurity atoms and dislocations induce lattice strains in the iron matrix and these strains can be relaxed if the interstitial atoms diffuse to the vicinity of dislocations. Therefore, the consequent decrease in strain energy may increase the stress required for subsequent dislocation movement and ultimately increase strength of the steel. The last stage of the bake hardening process may be ‘precipitation of carbides’. Carbide particles may be nucleated by segregation of solute atoms to the core regions of dislocation, which causes an increase in yield strength and ultimate tensile strength. With continued solute segregation to dislocation cores, the increased local solute concentration may lead to the formation of clusters and then precipitates which can eventually saturate the dislocation sites. Hence, finally, the strength of steel may increase after bake-hardening process. In an embodiment of the present disclosure, the high strength bake-harbenable cold-rolled steel sheet including ferrite and pearlite microstructure exhibits Ultimate Tensile Strength (UTS) greater than 340 MPa and Bake-Hardening (BH) index greater than 30 MPa.

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 0.0005%, the ferrite grains may coarsen at annealing, so that surface defect such as “Orange peel” tends to occur at press forming. Additionally, if the C content is less than 0.0015%, it may affect the bake-hardening index adversely. If C content is more than 0.0035%, 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 is set in between to 0.0015 wt. % to 0.0035 wt.%

Manganese (Mn) may be the effective element for the solid solution hardening. In order to obtain the tensile strength of 340MPa or more, the content of Mn may be needed to set to 0.8 wt.% less. When the content of Mn may be more than 0.8 wt.%, the r value is decreased.

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

Phosphorous (P) may be added to get desired tensile strength by solid solution strengthening. When the content of P is more than 0.06 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 to 0.06 wt.% or less.

Silicon (Si) is an effective element for solid solution hardening. When the content of Si is more than 0.05 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, and hence the content of Si is set to 0.03 wt.% or less.

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 0.004 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.

Niobium (Nb) has the role in improving the R-value by grain refinement for steel sheets and in decreasing the solute C and N with the formation of precipitates. In order to get a combination of high average r values and good bake-hardening index, a calculated stoichiometric amount of Nb may be added so that a very little level of C remains free in solute form. When the content of Nb is more than this calculated amount, it may affect the bake-hardening property of the steel. Hence, the Nb content is set to 0.005 wt.% to 0.025 wt.%.

Molybdenum (Mo) has potential to improve the shelf life at room temperature, without significantly affecting the yield strength of the steel. This may be achieved by forming a weak short-range interaction between Mo and C, which in turn may reduce the mobility of C at room temperature. Hence, Mo amount is set to 0.02 wt.% to 0.03 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 specific composition of the steel by using method of the present disclosure. The composition of the steel samples (A to G) 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.

Sl No C (%) Mn (%) S (%) P (%) Si (%) Al (%) N (ppm) Mo (%) Nb (%) Soaking temperature (°C) Cooling rate
(° C/s) YS (Mpa) UTS (Mpa) EL (%) n value r-bar BH Index (MPa) Remarks
A 0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 750 - 800 > 35 233 360 40.40 0.227 1.66 32 Example 1
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 750 - 800 > 35 233 360 40.40 0.227 1.66 32
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 750 - 800 > 35 244 366 41.90 0.234 1.52 55
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 750 - 800 > 35 229 362 40.60 0.238 1.74 35
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 750 - 800 > 35 231 363 40.70 0.239 1.75 33
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 750 - 800 > 35 232 366 40.50 0.233 1.72 37
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 < 750 > 35 255 366 40.30 0.213 1.39 28 Comp. Example
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 < 750 > 35 240 360 41.00 0.232 1.33 25
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 < 750 < 35 244 362 42.30 0.234 1.29 26
0.0018 0.493 0.007 0.048 0.008 0.05 14 0.028 0.010 < 750 < 35 251 363 40.40 0.232 1.35 25
B 0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 750 - 800 > 35 220 355 41.60 0.251 1.72 35 Example 2
0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 750 - 800 > 35 220 355 41.60 0.251 1.72 35
0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 750 - 800 > 35 230 359 40.80 0.228 1.77 33
0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 750 - 800 > 35 234 364 40.10 0.234 1.65 41
0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 750 - 800 > 35 216 353 41.00 0.234 1.78 41
0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 < 750 > 35 252 366 41.90 0.253 1.29 22 Comp. Example
0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 < 750 > 35 239 357 40.50 0.219 1.34 22
0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 < 750 < 35 236 354 42.00 0.220 1.36 24
0.0019 0.424 0.008 0.047 0.005 0.05 20 0.029 0.010 < 750 < 35 252 366 41.70 0.218 1.32 24
C 0.0020 0.460 0.008 0.041 0.003 0.04 20 0.029 0.009 750 - 800 > 35 219 349 41.70 0.234 1.67 33 Example 3
0.0020 0.460 0.008 0.041 0.003 0.04 20 0.029 0.009 750 - 800 > 35 217 349 42.20 0.239 1.77 33
D 0.0021 0.420 0.006 0.045 0.003 0.04 21 0.001 0.009 750 - 800 > 35 224 348 41.90 0.229 1.69 39 Example 4
0.0021 0.420 0.006 0.045 0.003 0.04 21 0.001 0.009 750 - 800 > 35 234 357 40.10 0.231 1.56 41
0.0021 0.420 0.006 0.045 0.003 0.04 21 0.001 0.009 750 - 800 > 35 214 339 40.40 0.242 1.53 60
0.0021 0.420 0.006 0.045 0.003 0.04 21 0.001 0.009 750 - 800 > 35 250 362 38.60 0.241 1.57 46
E 0.0023 0.426 0.008 0.047 0.003 0.04 20 0.030 0.007 750 - 800 > 35 218 354 41.20 0.239 1.60 38 Example 5
0.0023 0.426 0.008 0.047 0.003 0.04 20 0.030 0.007 750 - 800 > 35 217 348 41.50 0.232 1.75 36
0.0023 0.426 0.008 0.047 0.003 0.04 20 0.030 0.007 750 - 800 > 35 223 352 41.60 0.240 1.68 45
0.0023 0.426 0.008 0.047 0.003 0.04 20 0.030 0.007 750 - 800 > 35 225 354 41.70 0.238 1.65 34
F 0.0025 0.399 0.007 0.028 0.003 0.04 35 0.026 0.008 750 - 800 > 35 232 346 42.20 0.235 1.53 43 Example 6
0.0025 0.399 0.007 0.028 0.003 0.04 35 0.026 0.008 750 - 800 > 35 232 347 41.70 0.234 1.53 49
0.0025 0.399 0.007 0.028 0.003 0.04 35 0.026 0.008 > 800 > 35 225 320 43.30 0.269 1.53 48 Comp. Example
0.0025 0.399 0.007 0.028 0.003 0.04 35 0.026 0.008 > 800 > 35 197 316 42.70 0.242 1.63 42
0.0025 0.399 0.007 0.028 0.003 0.04 35 0.026 0.008 > 800 > 35 197 317 42.90 0.251 1.62 45
0.0025 0.399 0.007 0.028 0.003 0.04 35 0.026 0.008 > 800 > 35 199 320 41.60 0.241 1.59 38
0.0025 0.399 0.007 0.028 0.003 0.04 35 0.026 0.008 < 750 > 35 234 349 42.70 0.244 1.32 59
G 0.0030 0.440 0.007 0.043 0.006 0.05 15 0.010 0.009 750 - 800 > 35 223 348 42.60 0.233 1.63 42 Example 7
0.0030 0.440 0.007 0.043 0.006 0.05 15 0.010 0.009 750 - 800 < 35 207 347 41.80 0.229 1.37 55 Comp. Example
0.0030 0.440 0.007 0.043 0.006 0.05 15 0.010 0.009 < 750 > 35 204 341 41.40 0.223 1.32 52
0.0030 0.440 0.007 0.043 0.006 0.05 15 0.010 0.009 < 750 < 35 205 340 41.50 0.230 1.39 43
0.0030 0.440 0.007 0.043 0.006 0.05 15 0.010 0.009 < 750 < 35 212 350 41.80 0.235 1.35 47

Table-1
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 as 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 Bake Hardening Index values are also tabulated in table. 1. In order to measure the bake-hardening effect during the experiments, the samples may be subjected to a tensile strain of 2 % at room temperature, unloaded, aged at the required temperature (usually 170 °C to 200 °C) for the necessary time (usually 20 minutes) and then strained again in tension until failure. The difference between the lower YS after strain aging and the flow stress at 2 % pre-strain is the Bake hardening Index. The steel exhibits a unique property of an increase of yield strength when subjected to the paint-baking process. 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.

Referring to figure 3 which illustrate the optical micrograph of high-strength bake-hardenable cold-rolled steel sheet at a magnification of 100X. 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 bake hardenability attainable. The smaller the grain size becomes, the higher the bake hardenability. Hence, steel shows improved Bake Hardening (BH) index greater than 30 MPa.

Referring to figure 4 and 5 which are exemplary embodiment of the present disclosure, illustrating graphical representation of Yield Strength values (YS) and Bake Hardening (BH) Index for high-strength bake-hardenable cold-rolled steel sheet during real-time test for ageing phenomena. During real-time test for ageing phenomena, the high strength bake-hardenable cold-rolled steel sheet samples are subjected to stress under a constant stain for prolonged time at room temperature. Mechanical properties are measured during a time interval of one month. Obtained yield strength (YS), ultimate tensile strength (UTS), percentage elongation (%), R-values and Bake Hardening Index values for the steel sample is tabulated in table 2 given below.

Parameter Immediately after production After 1 month After 2 months After 3 months After 4 months After 5 months After 6 months After 7 months
YS (MPa) 212 213 210 214 218 212 228 220
BH index (MPa) 39 31 36 35 36 41 39 38
UTS (MPa) 345 345 341 341 346 341 355 340
% El. 44.9 42.7 41.8 41.6 39.9 41.5 42.3 41.4
n - 0.209 0.203 0.202 0.201 0.205 0.234 0.202

Table-2

Mechanical properties values tableted in table 2 clearly indicate that exhibits good non-ageing behavior properties at room temperature. Cold-rolled steel sheet samples exhibit stable mechanical properties for at least about 6 months. The slower diffusion rate of carbon at room temperature allows the steel to remain at storage temperatures for up to six months without aging. Furthermore, presence of Mo element in the composition enhance the shelf-life for the steel samples.

In an embodiment of the present disclosure, the high strength bake-hardenable cold-rolled steel sheet of the present disclosure may be used any application including but not limiting to automotive body panels. The high strength bake-hardenable 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 Cooling stage
105 Cold working stage
106 Soaking
107 Cooling
108 Overaging and final coiling