Claims:

1. A method for manufacturing a high strength steel sheet, the method comprising:
casting a steel slab of a composition comprising in weight percentage (wt.%) of:
carbon (C) at about 0.05% to about 0.1 %,
manganese (Mn) at about 1.2% to about 1.7 %,
silicon (Si) at about 0.05% to about 0.2 %,
chromium (Cr) at about 0.2% to 0.5%,
aluminium (Al) up-to 0.06 %,
sulphur (S) up-to 0.005 %,
phosphorous (P) up-to 0.025 %,
nitrogen (N) up-to 0.007 %,
balance being Iron (Fe) optionally along with incidental elements;
heating, the steel slab to a first predetermined temperature for a first predetermined time;
subjecting, the steel slab to a hot working to produce a steel sheet, wherein, the hot working includes:
deforming, the steel slab in a first hot working process, at a second predetermined temperature; and
deforming, the steel slab in a second hot working process, at a third predetermined temperature;
holding, the steel sheet for a second predetermined time after the hot working;
cooling, the steel sheet at a first predetermined cooling rate to a fourth predetermined temperature;
holding, the steel sheet for a third predetermined time at the fourth predetermined temperature;
cooling, the steel sheet at a second predetermined cooling rate to a fifth predetermined temperature; and
coiling, the steel sheet, at the fifth predetermined temperature to obtain the high strength steel sheet;
wherein, the high strength steel sheet comprises ferrite-bainite microstructure.

2. The method as claimed in claim 1, wherein the high strength steel sheet exhibits tensile strength greater than 590 MPa.
3. The method as claimed in claim 1, wherein the high strength steel sheet exhibits ductility ranging from about 18% to about 30%.

4. The method as claimed in claim 1, wherein the high strength steel sheet exhibits hole expansion ratio of greater than 100%.

5. The method as claimed in claim 1, wherein mass fraction of the ferrite-bainite microstructure is represented by ferrite content greater than 20% and less than 45 % and the balance being bainite with carbides.

6. The method as claimed in claim 5, wherein average size of carbides is less than 0.6 micron.

7. The method as claimed in claim 1, wherein the casting is carried out in a continuous casting process.

8. The method as claimed in claim 7, wherein the continuous casting process is performed in at least one of continuous caster and a thin slab caster.

9. The method as claimed in claim 8, wherein the temperature of the steel slab at exit of the thin slab caster is maintained above 1000 ?C.

10. The method as claimed in claim 1, wherein the first predetermined temperature is greater than 1150°C, preferably ranging from about 1200°C to 1220°C, and the first predetermined time ranging from about 30 minutes to about three hours.

11. The method as claimed in claim 1, wherein the hot working is a hot rolling process.

12. The method as claimed in claim 1, wherein the first hot working process is performed in a roughing mill, and the second predetermined temperature is greater than a recrystallization temperature of the steel slab.

13. The method as claimed in claim 1, wherein the second hot working process is performed in four or more than four stands of a finishing mill.

14. The method as claimed in claim 1, wherein the third predetermined temperature ranging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium.

15. The method as claimed in claim 1, wherein the second predetermined time ranges from 0.5 seconds to 3 seconds.

16. The method as claimed in claim 1, wherein the first predetermined cooling rate is greater than 60°C/second, and the cooling is performed with intensive or laminar water cooling on a run out table.

17. The method as claimed in claim 1, wherein the fourth predetermined temperature ranges from about 640°C to 700°C, preferably 660°C to 680°C.

18. The method as claimed in claim 1, wherein for the third predetermined period of time ranges from about 2 seconds to 10 seconds, preferably 3second to 6 seconds.

19. The method as claimed in claim 1, where second predetermined cooling rate is greater than 40?C/s, and the cooling is an intensive cooling-cum- laminar cooling carried out on the run-out-table..

20. The method as claimed in claim 1, wherein the fifth predetermined temperature is ranges from about 400°C to 480°C, preferably 430°C to 450°C.

21. The method as claimed in claim 1, further comprising performing a pickling and skin pass treatment on the steel sheet after the hot working.

22. The method as claimed in claim 21, wherein the pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.

23. A high strength steel sheet with a tensile strength greater than 590 MPa, comprising:
composition in weight percentage (wt.%) of:
carbon (C) at about 0.05% to about 0.1 %,
manganese (Mn) at about 1.2% to about 1.7 %,
silicon (Si) at about 0.05% to about 0.2 %,
chromium (Cr) at about 0.2% to 0.5%,
aluminium (Al) up-to 0.06 %,
sulphur (S) up-to 0.005 %,
phosphorous (P) up-to 0.025 %,
nitrogen (N) up-to 0.007 %,
balance being Iron (Fe) optionally along with incidental elements.

24. The high strength steel sheet as claimed in claim 23, wherein the high strength steel sheet comprises ferrite-bainite microstructure, mass fraction of the ferrite-bainite microstructure is represented by ferrite content greater than 20% and less than 45 % and the balance being bainite with carbides.

25. The high strength steel sheet as claimed in claim 24, wherein the average size of carbides is less than 0.6 micron.

26. The high strength steel sheet as claimed in claim 23, wherein the high strength steel sheet exhibits ductility ranging from about 18% to about 30%.

27. The high strength steel sheet as claimed in claim 23, wherein the high strength steel sheet exhibits hole expansion ratio of greater than 100%.

28. The high-strength hot-rolled steel sheet as claimed in claim 23, wherein the manganese (Mn) is preferably in the range of about 1.2% to about 1.6%.

29. The high-strength hot-rolled steel sheet as claimed in claim 23, wherein the chromium (Cr) is preferably in the range of about 0.3% to about 0.4%.

30. A method for manufacturing a high strength hot rolled steel sheet, the method comprising:
heating, a steel slab of a composition comprising in weight percentage of:
composition in weight percentage (wt.%) of:
carbon (C) at about 0.05% to about 0.1 %,
manganese (Mn) at about 1.2% to about 1.7 %,
silicon (Si) at about 0.05% to about 0.2 %,
chromium (Cr) at about 0.2% to 0.5%,
aluminium (Al) up-to 0.06 %,
sulphur (S) up-to 0.005 %,
phosphorous (P) up-to 0.025 %,
nitrogen (N) up-to 0.007 %,
balance being Iron (Fe) optionally along with incidental elements, to a temperature greater than 1150°C for a time ranging from about 30 minutes to about three hours;
subjecting the steel slab to the hot rolling to produce a steel sheet, wherein, the hot rolling includes:
deforming, the steel slab in a roughing mill, at a temperature above the recrystallisation temperature of the steel slab; and
deforming, the steel slab in four or more than four stands of a finishing mill, at a temperature raging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium;
holding, the steel sheet for 0.5 seconds to 3 seconds after hot rolling, to allow recrystallization of austenite;
cooling, the steel sheet at a cooling rate greater than 60°C/second to an intermediate temperature, to prevent formation of larger size polygonal ferrite;
holding, the steel sheet at the intermediate temperature for a period of 2 seconds to 10 seconds;
cooling, the steel sheet at a cooling rate is greater than 40?C/s to a coiling temperature; and
coiling, the steel sheet, at the coiling temperature ranging from about 400°C to 480°C to obtain a high-strength hot-rolled steel sheet;
wherein, the high-strength hot-rolled steel sheet comprises ferrite bainite microstructure.

31. The method as claimed in claim 30, further comprising: performing a pickling and skin pass treatment on the steel sheet after the hot rolling.

32. The method as claimed in claim 31, wherein pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.

33. The method as claimed in claim 30, wherein the intermediate temperature ranges from about 640°C to 700°C, preferably 660°C to 680°C

34. Automotive chassis part, and suspension parts comprising a high-strength steel sheet as claimed in claim 23.
, 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 hot rolled steel sheet. Further embodiments of the disclosure disclose a method for manufacturing a high strength hot rolled steel sheet with tensile strength of minimum 590 MPa, excellent stretch flangeability with > 100%-hole expansion ratio, and high ductility.

BACKGROUND OF THE DISCLOSURE

Steel is an alloy of iron, carbon, and other alloying elements. 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. 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.

With rising concerns over global environmental problems and demand from automotive industry for higher collision safety of motor vehicles, impose conflicting requirements on materials used for the vehicle bodies. The vehicle bodies are required to be stronger yet lighter at the same time. However, with increase in strength, manufacturability becomes more challenging as ductility of the material comes down. In addition, higher strength materials in general, also shows poorer stretch flangeability. To ensure that high strength steels are shaped to components such as suspension parts, hinges etc, the material should possess adequate ductility and stretch flangeability. Automotive manufacturers look for steels with isotropic properties though rolled steels easily develops anisotropy in properties because of crystallographic texture.

There have been several developments in the field of advanced high strength steels, which possess good combination of tensile strength, elongation and stretch flangeability to address some of the afore-mentioned concerns. One such conventional process includes development of Nb-V based SPFH590 steel [IN201400279]. The method to manufacture 590 MPa strength steel with ferrite-bainite microstructure is disclosed. Though the steel has high strength and high ductility, the stretch flangeability may be lower due to directionality in properties. Because of addition of microalloying elements, stronger crystallographic texture gets developed and because of which properties become significantly different in different direction of measurement. Less is the directionality in properties, better is the stretch flangeability. With a ferrite-bainite two phase microstructure in this development, hardness difference exists between both phases. This also plays some role in reducing the hole expansion ratio. In addition, the chemistry includes > 0.3 wt.% silicon which is detrimental to surface quality due to scale issues.

Similarly, one of the patent literatures [US16070605] disclose DP590 steel and a method to create steel with minimum tensile strength of 590 MPa and having a ferrite martensite microstructure. Due to significant difference in hardness between ferrite and martensite phases, the hole expansion ratio of dual phase steel is also low. Further, attempts have been made at developing steels with high strength and high stretch flangeability by addition of elements like Mo [EP1577412A1]. However, this makes the steel expensive. One more patent literature known in the art has focussed on developing fully ferritic microstructure by fixing the carbon with Titanium (Ti) (TiC precipitates) [US 2013/0087252 A1]. This is again a micro-alloyed chemistry with sufficient level of Ti. This steel is expected to have directionality in properties, which have not been disclosed in this patent. In patent publication [WO 2009028515], significant Si has been added to the chemistry, which may result in poor surface finish due to the sticky silicon scales. Also, in open literature, it is disclosed that chemistry with high levels of silicon for steels with such level of strength. However, surface quality of these steels will again be poor as silicon promotes formation of scale during hot rolling.

Hence, there is a need for an economically attractive and technically viable way of developing high strength steel sheet with high ductility, better surface finish, less anisotropy in strength and very high stretch flangeability without 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 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 one non-limiting embodiment, there is provided a method for producing high strength hot rolled steel sheet. The process starts from casting a steel slab of a composition comprising in weight percentage (wt.%) of carbon (C) at about 0.05% to about 0.1 %, manganese (Mn) at about 1.2% to about 1.7 %, silicon (Si) at about 0.05% to about 0.2 %, chromium (Cr) at about 0.2% to 0.5%, aluminium (Al) up-to 0.06 %, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 %, nitrogen (N) up-to 0.007 %, balance being Iron (Fe) optionally along with incidental elements. Heating, the steel slab to a first predetermined temperature for a first predetermined time, and subjecting, the steel slab to a hot working to produce a steel sheet. The hot working includes deforming, the steel slab in a first hot working process, at a second predetermined temperature, and deforming, the steel slab in a second hot working process, at a third predetermined temperature. After hot rolling, the steel sheet is held for a second predetermined time, and then subject for cooling at a first predetermined cooling rate to a fourth predetermined temperature. Once the steel sheet is cooled, it is again held for a third predetermined time at the fourth predetermined temperature and cooled at a second predetermined cooling rate to a fifth predetermined temperature. Then the steel sheet is coiled at the fifth predetermined temperature to obtain the high strength steel sheet, such that it comprises ferrite-bainite microstructure.

In an embodiment, the high strength steel sheet exhibits tensile strength greater than 590 MPa, ductility ranging from about 18% to about 30%, and hole expansion ratio of greater than 100%.
In an embodiment, mass fraction of the ferrite-bainite microstructure is represented by ferrite content greater than 20% and less than 45% and the balance being bainite with carbides, and average size of carbides is less than 0.6 micron.

In an embodiment, the casting is carried out in a continuous casting process. The continuous casting process is performed in at least one of continuous caster and a thin slab caster.

In an embodiment, the temperature of the steel slab at exit of the thin slab caster is maintained above 1000 ?C.

In an embodiment, the first predetermined temperature is greater than 1150°C, preferably ranging from about 1200°C to 1220°C, and the first predetermined time ranging from about 30 minutes to about three hours.

In an embodiment, the hot working is a hot rolling process. The first hot working process is performed in a roughing mill, and the second predetermined temperature is greater than a recrystallization temperature of the steel slab and the second hot working process is performed in four or more than four stands of a finishing mill. Further, the third predetermined temperature ranging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium.

In an embodiment, the second predetermined time ranges from 0.5 seconds to 3 seconds, and the first predetermined cooling rate is greater than 60°C/second, and the cooling is performed with intensive or laminar water cooling on a run out table. Further, the fourth predetermined temperature ranges from about 640°C to 700°C, preferably 660°C to 680°C.

In an embodiment, the third predetermined period of time ranges from about 2 seconds to 10 seconds, preferably 3second to 6 seconds, and the second predetermined cooling rate is greater than 40?C/s, and the cooling is an intensive cooling-cum- laminar cooling carried out on the run-out-table. Further, the fifth predetermined temperature is ranges from about 400°C to 480°C, preferably 430°C to 450°C.

In an embodiment, the method comprises performing a pickling and skin pass treatment on the steel sheet after the hot working. The pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.

In another non-limiting embodiment, a high strength steel sheet with a tensile strength greater than 590 MPa. The steel sheet comprising composition in weight percentage (wt.%) of carbon (C) at about 0.05% to about 0.1 %, manganese (Mn) at about 1.2% to about 1.7 %, silicon (Si) at about 0.05% to about 0.2 %, chromium (Cr) at about 0.2% to 0.5%, aluminium (Al) up-to 0.06 %, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 %, nitrogen (N) up-to 0.007 %, and the balance being Iron (Fe) optionally along with incidental elements.

In an embodiment, the high strength steel sheet exhibits tensile strength greater than 590 MPa, ductility ranging from about 18% to about 30%, and hole expansion ratio of greater than 100%.

In an embodiment, mass fraction of the ferrite-bainite microstructure is represented by ferrite content greater than 20% and less than 45 % and the balance being bainite with carbides, and average size of carbides is less than 0.6 micron.

In yet another non-limiting embodiment, a method for manufacturing a high strength hot rolled steel sheet is disclosed. The method includes heating, a steel slab of a composition comprising in weight percentage of composition in weight percentage (wt.%) of: carbon (C) at about 0.05% to about 0.1 %, manganese (Mn) at about 1.2% to about 1.7 %, silicon (Si) at about 0.05% to about 0.2 %, chromium (Cr) at about 0.2% to 0.5%, aluminium (Al) up-to 0.06 %, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 %, nitrogen (N) up-to 0.007 %, and balance being Iron (Fe) optionally along with incidental elements, to a temperature greater than 1150°C for a time ranging from about 30 minutes to about three hours. Then, subjecting the steel slab to the hot rolling to produce a steel sheet. The hot rolling includes deforming, the steel slab in a roughing mill, at a temperature above the recrystallisation temperature of the steel slab; and deforming, the steel slab in four or more than four stands of a finishing mill, at a temperature raging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium. After hot rolling, the method includes step of holding, the steel sheet for 0.5 seconds to 3 seconds after hot rolling, to allow recrystallization of austenite. Then, cooling, the steel sheet at a cooling rate greater than 60°C/second to an intermediate temperature, to prevent formation of larger size polygonal ferrite. Further, the method includes holding, the steel sheet at the intermediate temperature for a period of 2 seconds to 10 seconds and cooling, the steel sheet at a cooling rate is greater than 40?C/s to a coiling temperature. After cooling, the steel sheet is coiled at the coiling temperature ranging from about 400°C to 480°C to obtain a high-strength hot-rolled steel sheet. The high-strength hot-rolled steel sheet comprises ferrite bainite microstructure.

In an embodiment, the method comprising performing a pickling and skin pass treatment on the steel sheet after the hot working. The pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.

In still another embodiment, automotive chassis part, and suspension parts comprising a high-strength hot-rolled steel sheet as described above 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 is graphical flow diagram of cooling profile followed during the method for producing high strength steel sheet, according to an exemplary embodiment of the present disclosure.

Figures. 3a and 3b illustrates SEM microstructure of steel sheet-sample-1 manufactured by method of present disclosure.

Figures. 4a and 4b illustrates gleeble simulated microstructure of steel sheet-sample-1 manufactured by method of present disclosure, corresponding to two different coiling temperatures.

Figure. 5 illustrates microstructure of cross-section a steel sheet manufactured by method of present disclosure, after subjecting to hole expansion ratio test.

Figure 6 illustrates macro image of cross section of steel sheet manufactured by method of present disclosure, showing no center line segregation.

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 or producing a high strength steel sheet. Strength, ductility and stretch flangeability are some of the important properties for the mass industrial application of high strength materials like steel. As of now, high strength steel sheets with tensile strength more than 590MPa are produced by the methods in which precipitation strengthening has been utilized as the key mechanism to obtain strength. However, it will be difficult to replicate similar level of precipitation strengthening in every grains of the microstructure due to the variation in processing conditions across width and length of sheet in actual plant scale production. Such a level of strength is alternatively obtained by having ferrite bainite microstructure where microalloying elements such as Nb, Ti, V are added. Such addition develops inhomogeneity or anisotropy in properties as a result of which lower stretch flangeability is obtained. Accordingly, the method of present disclosure, discloses a production of high strength steel sheet, with tensile strength of minimum 590 MPa with excellent stretch flangeability. The present disclosure is directed towards producing a low carbon hot rolled steel sheet with tensile strength greater than 590 MPa along with a hole expansion ratio more than 100%, and % elongation more than 18% to about 30%. The hot rolled steel sheet may be widely employed to make automotive components requiring high strength, adequate ductility, and very high stretch flangeability.

In the method of manufacturing high strength steel sheet, includes first step of producing the steel slab of composition including in weight percentage [wt.%] of 0.05 - 0.1 % of carbon, 1.2 – 1.7 % of manganese - preferably 1.3 -1.5 %, 0.05- 0.2 % of silicon, 0.2-0.50 % of Chromium- preferably 0.3 – 0.4%, 0 – 0.06 % of aluminum, maximum up to 0.005 % of sulphur, maximum up to 0.025 % of phosphorous and up to 0.007 % of nitrogen, the balance being iron and impurities by any manufacturing process including but not limiting to casting. The steel slab is then reheated to a temperature greater than 1150 °C. The steel slab is then subjected to deformation through first and second hot working process. In an embodiment, the first and second hot working process is hot rolling in roughing mill and finishing mill, and the finish rolling temperature may vary in the range of Ae3 to Ae3 + 70 °C, where Ae3 is the temperature at which the transformation of austenite to ferrite starts at equilibrium. After the hot rolling step, a small-time delay of 0.5s to 3s may be given to allow recrystallization of austenite before water cooling is initiated. This is important as it decreases the texture development in room temperature phases and hence, planar anisotropy in properties is minimized. The steel sheet may be cooled in an intensive cooling zone of run out table at a cooling rate of greater than 60 °C/s and then cooling is stopped at a temperature of 640 ?C to 700 ?C, preferably 660 ?C to 680 ?C for a period of 2 to 10 s, preferably 3 to 6 s. After this brief period, cooling is again started till coiling temperature is reached. Coiling temperature TCT varies in the range of 400 ?C to 480 ?C preferably in the range of 430°C to 450°C. The hot rolled steel sheet according to the present disclosure may have a microstructure comprising of 20 to 45 % ferrite with remaining phase as bainite. Size of carbide is controlled to less than 0.6 micron. Such fine carbides resist void formation better. This is critical to achieve very high stretch flangeability.

In an embodiment, high strength hot rolled steel sheet exhibits, tensile strength of more than 590MPa along with excellent stretch flangeability, hole expansion ration more than 100%, and ductility more than 18% to suit automotive applications and its manufacturing process.

In an embodiment, the ferrite bainite microstructure with extremely fine carbides with weaker texture in the steel sheet enable in obtaining a high stretch flangeability.

In an embodiment, the alloying composition of the hot rolled steel sheet includes very low amount of Si, this avoids the possibility of detrimental scales post hot rolling. Further, alloying composition of the hot rolled steel sheet includes Cr, which is less expensive element than Nb, V, Mo. Addition of Cr is primarily for achieving stronger bainite and minimizes microstructural banding.

Henceforth, the present disclosure is explained with the help of figures for a method of manufacturing high strength 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 depicting a method for manufacturing high strength-hot rolled steel sheet, and a graphical flow diagram of cooling profile, respectively. In the present disclosure, mechanical properties such as strength, ductility, and hole expansion ratio of the final microstructure of the steel may be improved. The steel produced by the method of the present disclosure, includes a ferrite-bainite 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 matter described herein. The method is particularly applicable to high strength steel sheet and it may also be extended to other type of steels as well.
The method of manufacturing the high strength hot-rolled steel sheet according to the present disclosure consists of a casting step followed by a hot working in first and second hot working process, holding, two stage cooling step and a coiling step using a steel material which satisfies the component composition described below. The various processing steps are described in their respective order below:

As shown in block 101, the method starts with the process of casting. In the method of present disclosure, the steel of the specified composition including in weight percentage of 0.05 - 0.1 % of carbon, 1.2 – 1.7 % of manganese - preferably 1.3 -1.5 %, 0.05- 0.2 % of silicon, 0.2-0.5 % of Chromium- preferably 0.3 – 0.4%, 0 – 0.06 % of aluminum, maximum up to 0.005 % of sulphur, maximum up to 0.025 % of phosphorous and up to 0.007 % of nitrogen, the balance being iron and other incidental elements like impurities, is first continuously cast either in a conventional continuous caster or a thin slab caster. When cast in a thin slab caster, the temperature of the cast slab may not be allowed to drop to a temperature below 1000 °C, as it might result in loss of ductility and cause formation of edge cracks.

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

Once the steel slab is heated as per the block 102, it may be subjected for hot working as shown in block 103 to form a steel sheet. In an embodiment, the hot working process is a hot rolling process. As shown in block 103, after casting and heating the steel slab with the specified composition, it is hot rolled. The hot rolling may constitute two steps of deformation via the first hot working process and the second hot working process. In an embodiment, the first hot working process is a deformation process of steel slab in a roughing mill at the second predetermined temperature. In an embodiment, the second predetermined temperature is above recrystallisation temperature of the steel slab. In the roughing stage, the cast structure may be broken down, and the new structures may be formed. Further, the second hot working process is carried out at a third predetermined temperature. In an embodiment, in the second hot working process, is a further deformation process of the steel slab carried out in four or more strands of the finishing mill, and temperatures in all strands of the finishing mill are such that microstructure of the material consists of single-phase austenite. In an embodiment, the third predetermined temperature is in the range of Ae3 to Ae3 + 70 °C., wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium.
After finish rolling, the rolled steel sheet may be held at the third predetermined temperature (T3) for a second predetermined time. In an embodiment, the second predetermined time ranges from 0.5 seconds to 3 seconds. Then, the steel sheet is subjected to intensive cooling or laminar cooling (as shown in Figure. 2) at a predetermined cooling rate as shown in block 104. In an embodiment, the predetermined cooling rate is greater than 60 °C/s till a fourth predetermined temperature (T4) is reached. The cooling is performed on a run-out-table. In an embodiment, the predetermined cooling rate may be higher than specified to prevent formation of larger size polygonal ferrite before the temperature reaches less than 700 ?C. Cooling is stopped as the fourth temperature reaches 640 ?C to 700 ?C, and the steel sheet is held at the fourth predetermined temperature for a third predetermined period of time. In an embodiment, the third predetermined period of time ranges from about 2 seconds to 10 seconds, preferably 3 second to 6 seconds. The method further includes the second step of cooling (as shown in Figure. 2) . This cooling is done as fast as possible to a fifth predetermined temperature (T5) which is also called as coiling temperature in the range of 400 ?C to 480 ?C. coiling. Now referring to block 105, coiling may be carried out at a coiling temperature or a fifth predetermined temperature. In an embodiment, the fifth predetermined temperature ranges from about 400°C to 480°C. It is preferable to keep the fifth predetermined temperature at 430°C to 450°C to achieve optimized ductility and stretch flangeability. Coiling below 400°C may be avoided to prevent the formation of harder phases such as martensite microstructure in the steel, and coiling above 480°C, may create more and larger grain boundary carbides. A schematic diagram of the cooling profile is shown in Figure. 2. This ensures that the microstructure consists of ferrite-bainite only.

The method optionally comprises cleaning the steel sheet by acid pickling and skin pass treatment. In an embodiment, the steel sheet may be uncoiled and pickled in a pickling line and then skin passed in a skin pass mill and then coiled. The pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.
In an embodiment, the high strength steel sheet exhibits tensile strength greater than 590 MPa along with a hole expansion ratio more than 100%, and % elongation more than 18%. In order to achieve the required mechanical properties as proposed in the disclosure, it may be required to obtain a very homogenous microstructure. In the present disclosure, the microstructure consists of ferrite-bainite structures with very fine size carbides. Size of carbides matter as larger elongated grain boundary carbides become preferential site for void formation and later, they become initiation site for crack formation. Smaller carbides offer more resistance to crack initiation. And hence increases stretch flangeability.

Strength is primarily obtained from the phases. Appropriate ferrite fraction of 20 to 45% is maintained to achieve the strength and ductility. Microalloying is avoided to minimize development of crystallographic texture which is primarily responsible for directionality in properties. Hence, a homogenous property in all directions, desired for achieving high hole expansion ratio [HER] are achieved. Hence, a weaker texture is preferable to achieve HER more than 100%. Such a texture is obtained due to (i) chemistry without microalloying elements (ii) delay in start of cooling i.e., holding the steel sheet before initiation of cooling.

The following portion 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) may be added in the range of about 0.05 wt.% to about 0.1 wt.%. Carbon is an inherent component in steel, carbon helps in strengthening phases, and is often considered as a cheaper element to increase strength. Strength will not be achieved with carbon less than 0.05 wt.%. But carbon content in steel beyond 0.1 wt.% may have detrimental effects, like it will lead to formation of more amount of undesirable second phases such as carbides, martensite/austenite islands and thereby, ductility and hole expansion ratio may get deteriorated.

Manganese (Mn) may be added in the range of about 1.2 wt.% to about 1.7 wt.%. Manganese not only imparts solid solution strengthening to the ferrite, but it also lowers the austenite to ferrite transformation temperature thereby reduces ferrite grain size. However, the Mn level cannot be increased to beyond 1.7 wt.% as at such high levels, there is more chances of occurrence of centreline segregation (CLS) during continuous casting. CLS will create inhomogeneity in microstructure which will have detrimental effect on stretch flangeability.

Silicon (Si) is a very cheap solid solution strengthening element and it has more solid solution strengthening potential than manganese. However, Si content beyond 0.2 wt.% promotes formation of scales during high temp soaking and which is often undesirable by the end customers.

Phosphorus (P) content should be restricted to 0.025 wt.% maximum as higher phosphorus levels may lead to reduction in toughness and weldability due to segregation of phosphorus into grain boundaries.

Sulphur (S) content must be limited otherwise it results in a very high inclusion level that deteriorates formability. Nitrogen (N) may be kept 0.007wt% maximum.

Aluminium (Al) may be added in the range of 0.02-0.06 wt. %. Aluminium is used as a deoxidizer and killing of steel. It limits growth of austenite grains.

Chromium (Cr) may be added in the range of 0.2 to 0.5 wt.%. Cr enhances hardenability and enhances strength of bainite. However, Cr also helps in dispersing microstructural banding.

In the steel composition, no microalloying elements such as Nb, Ti, V are added. As they help in development of crystallographic texture, which in turn influences the directionality in properties. They thus adversely affect stretch flangeability.

Examples:

Further embodiments of the present disclosure will be now described with an example of a particular composition of the steel. Experiments have been carried out for a specific composition of the steel formed by using method of the present disclosure. The composition of the steel for which the tests are carried out is as shown in below table 1.

Steel Chemical composition (wt. %)
C Mn Si Al Cr N S P
A 0.07 1.42 0.085 0.04 0.4. 45 0.0015 0.018

Table – 1

The compositions of table-1 were continuously cast in a slab caster and the slabs were hot-rolled in a hot rolling mill. Processing parameters are used in the mill and cooling as shown in Table 2- which shows hot rolling process parameters used for hot rolling
Steel Composition Thickness, mm Holding time for start of cooling, ts
FRT (°C)

IMT (°C)

Holding time (s)
CT (°C)
Steel 1 A 2.9 2 870 650 4 425

Table-2
As evident from Table-2 above, for steel sheet sample 1, the processing was done in accordance with the present disclosure.
The microstructures of steel sheet sample 1 is shown in Figures 3a-3b. It shows ferrite-bainite microstructure. Figure 3a shows ferrite-bainite ferrite grain microstructure. Figure 3b shows the magnified view of bainite having features of extremely fine carbide. Average size of carbide is less than 0.6 micro, according to an exemplary embodiment of the present disclosure. If the coiling is done at higher temperature, the carbides become larger in size as shown in figure 4b. In addition, they become located at the grain boundaries. Such carbides generate cracks at early stage of deformation and hence, the stretch flangeability comes down. Carbides of lower dimensions are not that harmful. They resist greater amount of deformation before formation of cracks or voids at its interface with matrix phase. The sample is cooled from an austenitizing temperature of 1230 ?C to temperatures where they are held for 1 hr before cooling the samples to room temperature. Figure 4a corresponds to a lower coiling temperature of 425 ?C, where very fine carbides of less than 0.4-micron size can be observed. Figure 4b corresponds to relatively higher coiling temperature of 480 ?C, where very coarser grain boundary carbides can be observed.
In addition, crystallographic texture has a major role in developing directionality in properties. Normally, strength in the transverse direction is the largest and it becomes minimum at an intermediate angle between the rolling and transverse direction. Such directionality in properties also reduces hole expansion ratio. Though rolling at below recrystallization temperature for austenite is advantageous in terms of refinement of ferrite grains, it unfortunately creates stronger crystallographic texture in room temperature microstructure, which in turn increases directionality in properties. Microalloying element such as Nb increases the no recrystallization temperature or in other words, it suppresses austenite recrystallization. In the present disclosure, such element has been avoided to minimize crystallographic texture. Texture index which is expressed as TI=?¦f_g^2 dg is a measure of amount of texture in the hot rolled steel sheet of this material. TI values calculated from bulk texture measurement (X-ray diffraction-based measurement) data on strip surface and centre plane of the strip are 1.5 and 1.88, respectively. Such low texture index suggests development of weak crystallographic texture, which is beneficial from the point of view of improving directionality properties and HER. Less directionality in properties have been observed in the steel as shown in table 3. Weak crystallographic texture in this material is a result of both chemistry and the processing. Being a chemistry without having microalloying elements, austenite recrystallization in both stages of hot working is very fast. In addition, a time gap of 0.5 to 3s also results in formation of recrystallized austenite. Such austenite results in ferrite grains or bainite grains without any basis for specific orientations. Thus, it helps in developing weaker texture. This helps in less directionality in properties, and it in turn helps in achieving lower HER.
Thus, even though, two phase ferrite-bainite microstructure earlier considered not good for HER due to hardness difference between the phases, higher HER can still be obtained through the method of the present disclosure. Small carbide size combined with weak texture index primarily results in significantly high hole expansion ratio. In addition, no significant banding is observed in the cross section of strip. Cr addition facilitates dispersion of banded structure. Thus, avoiding microstructural banding also helped in achieving improved hole expansion ratio.
Now referring to Tables 3, mechanical properties of hot rolled and skin passed sheet has been illustrated, respectively.
Steel
Direction to RD Thickness, mm YS
(MPa) TS
(MPa) Elongation
(%) HER
(%)
Steel 1 0 2.9 552 623 21 159
Steel 1 45 2.9 543 629 20
Steel 1 90 2.9 554 636 18

Table-3
As evident from Figure. 5 and the Table-3 above, the high strength steel sheet of the present disclosure exhibits excellent hole expansion ratio [HER] of 159%. Figure. 5 shows specimen after testing, and location of image is close to the expanded hole. It shows that finer carbides are not prone to early void formation, and white arrow marks show regions where voids are clearly visible. Further, Figure 6 illustrates macro image of cross section of steel sheet showing no center line segregation. Centre Line Segregation will create inhomogeneity in microstructure which will have detrimental effect on stretch flangeability, and hence the same is avoided in the present disclosure by precise control of Mn content, and the process steps.

It should be understood that the experiments are carried out for a particular composition of the steel and the results brought out in the previous paragraphs are for the composition shown in Table – 1. However, this composition should not be construed as a limitation to the present disclosure as it could be extended to other compositions of the steel as well.

In another embodiment, the present disclosure discloses, a method for manufacturing high strength hot rolled steel sheet is disclosed. The method comprising: heating a steel slab of composition comprising in weight percentage of: carbon (C) at about 0.05% to about 0.1 %, manganese (Mn) at about 1.2% to about 1.7 %, silicon (Si) at about 0.05% to about 0.2 %, Chromium (Cr) at about 0.2% to 0.50 %, aluminium (Al) up-to 0.06 %, sulphur (S) up-to 0.005 %, phosphorous (P) up-to 0.025 %, nitrogen (N) up-to 0.007 %, the balance being Iron (Fe) optionally along with incidental elements, to a temperature greater than 11500C for a time ranging from about 30 minutes to about three hours. Then, hot rolling, the steel slab to produce a steel sheet. The hot rolling includes deforming the steel slab in a roughing mill, at a temperature above the recrystallisation temperature of the steel slab; and deforming the steel slab in one or more stages of a finishing mill, at a temperature raging from Ae3 to Ae3+ 70°C, wherein Ae3 is the temperature at which transformation of austenite to ferrite starts at equilibrium. After finish rolling, the rolled steel sheet is held for a time-period of 0.5 seconds to 3 seconds, so that cooling is delayed. Then the rolled steel sheet may be subjected to intensive cooling or laminar cooling at a cooling rate at greater than 60 °C/s till a desired coiling temperature is reached. The cooling is performed on a run-out-table. In an embodiment, the predetermined cooling rate may be higher than specified to prevent formation of larger size polygonal ferrite before the temperature reaches less than 700 ?C. Cooling is stopped as the fourth temperature reaches 640 ?C to 700 ?C for a period of 2 to 10s. The method further includes the second step of cooling. This cooling is done as fast as possible to a coiling temperature in the range of 400 ?C to 480 ?C. Coiling, the steel sheet, at the coiling temperature ranging from about 400°C to 480 °C to obtain a high-strength hot-rolled steel sheet. The high-strength hot-rolled steel sheet comprises Ferrite-bainite microstructure.

In an embodiment, the method further includes performing a pickling and skin pass treatment on the steel sheet after hot rolling. The pickling is performed in a pickling line to remove oxides and the skin pass is performed by a compressive deformation of about 0.4 to about 0.6% reduction.
In an embodiment of the present disclosure, the high strength-high steel sheet of the present disclosure may be used any application including but not limiting to automotive applications to manufacture structural components like chassis, suspension parts and the like. The high strength-hot 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 Forming stage
102 Heating stage
103 Hot working stage
104 Holding stage
105 Cooling stage
106 Holding stage
107 Cooling stage
108 Coiling stage