Claims:1. A method for producing a hot rolled steel, comprising steps of:
melting a steel having a composition comprising carbon (C) at a wt% of about 0.12 to 0.25, manganese (Mn) at a wt% of about 1.2 to 2.0, silicon (Si) at a wt% of about 0.5 to 1.2, chromium (Cr) at a wt% of about 0.8 to 1.3, aluminum (Al) at a wt% of about 0.09 to 0.3, molybdenum (Mo) at a wt% of about 0.1 to 0.3, niobium (Nb) at a wt% of about 0.015 to 0.04, titanium (Ti) at a wt% of about 0.02 to 0.06, sulphur (S) at a wt% of about 0.005 to 0.01, phosphorous (P) at a wt% of about 0.015 to 0.025 and nitrogen (N) at a wt% of about 0.004 to 0.01, to obtain molten steel;
casting the molten steel to obtain casted steel; and
hot rolling the casted steel to obtain the hot rolled steel;
wherein the obtained hot rolled steel has a tensile strength of about 1 GPa to 1.15 GPa and elongation of about 30% to 42%.

2. The method of claim 1, wherein the steel has a composition comprising C at a wt% of about 0.15 to 0.25, Mn at a wt% of about 1.4 to 1.8, Si at a wt% of about 0.5 to 1.2, Cr at a wt% of about 0.8 to 1.3, Al at a wt% of about 0.09 to 0.3, Mo at a wt% of about 0.1 to 0.3, Nb at a wt% of about 0.015 to 0.04, Ti at a wt% of about 0.02 to 0.06, S at a wt% of about 0.005 to 0.01, P at a wt% of about 0.015 to 0.025, and N at a wt% of about 0.004 to 0.01.

3. The method of any of the preceding claims, wherein prior to hot rolling, the casted steel is homogenized and subjected to roughening at a temperature range of about 1050 °C to 1150°C by applying deformation 60-95%, followed by reheating at a temperature range of about 1150 °C to 1300 °C at a heating rate of about 2 °C per minute to 20°C per minute for about 1.5 hours to 3 hours.

4. The method of any of the preceding claims, wherein a finish rolling temperature (FRT) in the austenite phase is applied during hot rolling.

5. The method of any of the preceding claims, wherein a FRT of about 850 °C to 1000 °C is applied during hot rolling.

6. The method of any of the preceding claims, wherein the hot rolled steel is cooled and subjected to coiling at a temperature range between martensitic start (Ms) temperature and bainitic start (Bs) temperature, preferably between 350 °C to 590 °C for about 0.1 hours to 2.5 hours, and followed by cooling the steel to ambient temperature.

7. The method of any of the preceding claims, wherein the hot rolled steel is an advanced high strength steel (AHSS), and possesses microstructure comprising bainite at a volume % of about 80 to 90 and a mixture of martensite and austenite at a volume % of about 10 to 20.

8. The method of any of the preceding claims, wherein the hot rolled steel is a steel sheet having a thickness of about 4 mm to 10 mm, and wherein the thickness of bainite phase in the steel ranges from about 350 to 500 nm.

9. The method of any of the preceding claims, wherein the hot rolled steel possesses yield to tensile ratio of about 0.6 to 0.7 and strain hardening exponent (n) in the range of about 0.15 to 0.21.

10. Hot rolled steel having a composition comprising carbon (C) at a wt% of about 0.12 to 0.25, manganese (Mn) at a wt% of about 1.2 to 2.0, silicon (Si) at a wt% of about 0.5 to 1.2, chromium (Cr) at a wt% of about 0.8 to 1.3, aluminum (Al) at a wt% of about 0.09 to 0.3, molybdenum (Mo) at a wt% of about 0.1 to 0.3, niobium (Nb) at a wt% of about 0.015 to 0.04, titanium (Ti) at a wt% of about 0.02 to 0.06, sulphur (S) at a wt% of about 0.005 to 0.01, phosphorous (P) at a wt% of about 0.015 to 0.025 and nitrogen (N) at a wt% of about 0.004 to 0.01, and possessing a tensile strength of about 1 GPa to 1.15 GPa and elongation of about 30% to 42%.

11. The hot rolled steel of claim 10, wherein the steel is an advanced high strength steel (AHSS), and possesses microstructure comprising bainite at a volume % about 80 to 90 and a mixture of martensite and austenite at a volume % about 10 to 20.

12. The hot rolled steel of any of the preceding claims, wherein the steel is a steel sheet having a thickness of about 4 mm to 10 mm and possesses yield to tensile ratio of about 0.6 to 0.7 and strain hardening exponent (n) in the range of about 0.15 to 0.21.

13. An article comprising the hot rolled steel of claim 10.
, Description:TECHNICAL FIELD
The present disclosure is in the field of metallurgy. The disclosure provides an economical method to prepare steel having superior combination of ductility and strength. Said method and the corresponding steel product having excellent mechanical properties is highly suitable for industrial applications, for instance, in automotive and load bearing applications and applications where a combination of high elongation and strength is necessary.

BACKGROUND OF THE DISCLOSURE
Reduction in fuel consumption is one of major objective in automotive industry to help reduce emission. Another important objective is also to maintain high standard of safety. Both these objectives demand the use of stronger and tougher steel which has high strength high as well as very good elongation. Ultra high strength steel (UHSS) or Advanced high strength steel (AHSS) are known in the industry and there are various reports about AHSS or UHSS. However, the major issue with UHSS is poor forming capability and weak load bearing capability due to limited elongation. As strength and elongation are inversely proportional in metals and alloys, with the development of stronger or UHSS steel, the elongation also reduces or decreases significantly. As a result, the application of UHSS in various parts of motor vehicle gets significantly limited since forming becomes increasingly difficult.

Strong and tough steel also contributes to reduce air pollution. Light-weight vehicle design is essential now-a-days to address the problems of environmental pollution. Effective light-weight motor vehicles require utilization of advanced high strength steels (AHSS). However, because of its poor formability, the AHSS strip cannot be applied easily to a wide variety of motor vehicle components. Hence, the ductility and formability required for AHSS strip have become increasingly demanding.

Various attempts have been made to develop advanced high strength steel with very high elongation over the years. First such steel sheet with very high strength was reported by Bhadeshia, MSE-A, Volume 481 – 482, pp. 36 – 39, 2008; F. G. Caballero, H. K. D. H. Bhadeshia, K. J. A. Mawella, D. G. Jones and P. Brown, MST, Volume 18, pp. 279 – 284, 2002; & C. Garcia-Mateo, F. G. Caballero and H. K. D. Bhadeshia, ISIJ International, Volume 43, pp. 1238 – 1243, 2003. Although the steel developed by Bhadeshia et.al. has very high strength, however, the application scope in automotive and many other fields is very limited specially due to high alloy content, long production time (3-4 days) and limited elongation (<10%). The first two factors make the real production of steel highly difficult, whereas the last factor is not favorable in end applications. The higher carbon content (>0.7wt%) further makes the steel difficult for welding. Overall, the steel is expansive and has inadequate formability.

Further, various studies were made to develop steel with better properties including the combination of very high strength and elongation. However, such efforts still resulted in limitations including processing and economic difficulties along with non-achievement of desired quality of the developed steel.

Thus, there is a need to develop new processing route/parameters to fabricate advanced high strength steel having extraordinary strength and elongation, formability and weldability. Also, the steel must be processed through the existing facilities without needing significant investment. The present disclosure tries to address the aforesaid limitations and aims to achieve the present needs.

SUMMARY OF THE DISCLOSURE
The present disclosure relates to a method for producing a hot rolled steel having excellent combination of strength and elongation.
In an embodiment, the method of preparing hot rolled steel comprises melting a steel composition to obtain molten steel, casting the molten steel to obtain casted steel, and hot rolling the casted steel to obtain the hot rolled steel.
In another embodiment, the steel composition employed for preparing the hot rolled steel comprises carbon (C) at a wt% of about 0.12 to 0.25, manganese (Mn) at a wt% of about 1.2 to 2.0, silicon (Si) at a wt% of about 0.5 to 1.2, chromium (Cr) at a wt% of about 0.8 to 1.3, aluminum (Al) at a wt% of about 0.09 to 0.3, molybdenum (Mo) at a wt% of about 0.1 to 0.3, niobium (Nb) at a wt% of about 0.015 to 0.04, titanium (Ti) at a wt% of about 0.02 to 0.06, sulphur (S) at a wt% of about 0.005 to 0.01, phosphorous (P) at a wt% of about 0.015 to 0.025 and nitrogen (N) at a wt% of about 0.004 to 0.01.
The present disclosure further provides a hot rolled steel product having a composition comprising carbon (C) at a wt% of about 0.12 to 0.25, manganese (Mn) at a wt% of about 1.2 to 2.0, silicon (Si) at a wt% of about 0.5 to 1.2, chromium (Cr) at a wt% of about 0.8 to 1.3, aluminum (Al) at a wt% of about 0.09 to 0.3, molybdenum (Mo) at a wt% of about 0.1 to 0.3, niobium (Nb) at a wt% of about 0.015 to 0.04, titanium (Ti) at a wt% of about 0.02 to 0.06, sulphur (S) at a wt% of about 0.005 to 0.01, phosphorous (P) at a wt% of about 0.015 to 0.025 and nitrogen (N) at a wt% of about 0.004 to 0.01, and corresponding article(s) thereof.

In an embodiment, the hot rolled steel is an advanced high strength steel (AHSS), and possesses microstructure comprising bainite at a volume % about 80 to 90 and a mixture of martensite and austenite at a volume % about 10 to 20.

In another embodiment, the hot rolled steel has excellent mechanical properties including a tensile strength of about 1 GPa to 1.15 GPa, elongation of about 30% to 42% along with strain hardening exponent (n) in the range of about 0.15 to 0.21.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 depicts Engineering Stress-Strain plot of the presently developed steel.

Figure 2 depicts Optical micrograph of the presently developed steel.

Figure 3 depicts Scanning Electron Microscope (SEM) microstructure of the presently developed steel.

Figure 4 depicts electron back scatter diffraction (EBSD) micrograph of the presently developed steel.

Figure 5 depicts X-ray diffraction (XRD) profile of the presently developed steel.

DETAILED DESCRIPTION OF THE DISCLOSURE
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. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The primary aim of the present disclosure is to develop steel possessing extraordinary strength and elongation properties. In particular, one of the objectives of the present disclosure is to fabricate advanced high strength steel (AHSS) having extraordinary strength and elongation, formability and weldability wherein said steel must be developed through existing hot rolling mill facilities without making significant/extra investment.

Another objective of the present disclosure is to develop a hot rolled advanced high strength bainitic steel product which eliminates the limitations of prior art as discussed above.

Yet another objective of the present disclosure is to develop hot rolled steel product having a thickness of about 4-10 mm with a tensile strength of at least 1 GPa and total elongation of at least 30%. It is another objective of the present disclosure to develop hot rolled steel product with yield to tensile ratio of about 0.6-0.7.

It is yet another objective of the present disclosure to develop a hot rolled steel product comprising microstructural constituents with bainite ranging from 80 to 90 volume % and the remaining volume comprising a mixture of retained austenite and martensite.

To meet the aforesaid objectives, the present invention provides a method of preparing advanced high strength hot rolled steel comprising melting a steel composition, casting the molten steel, and hot rolling the casted steel to obtain the advanced high strength hot rolled steel.

In an embodiment of the present disclosure, the hot rolled advanced high strength steel is developed from a liquid steel composition comprising the following primary components (wt%):
carbon (C): about 0.12 to 0.25, manganese (Mn): about 1.2 to 2.0, silicon (Si): about 0.5 to 1.2, chromium (Cr): about 0.8 to 1.3, aluminum (Al): about 0.09 to 0.3, molybdenum (Mo): about 0.1 to 0.3, niobium (Nb): about 0.015 to 0.04, titanium (Ti): about 0.02 to 0.06, sulphur (S): about 0.005 to 0.01, phosphorous (P): about 0.015 to 0.025 and nitrogen (N): about 0.004 to 0.01. Iron (Fe) of about 94.505 to 95.931 form rest of the steel composition.

In another embodiment, the composition for developing the steel product of the present disclosure comprises the following primary components (wt%):
C at a wt% of about 0.15 to 0.25, Mn at a wt% of about 1.4 to 1.8, Si at a wt% of about 0.5 to 1.2, Cr at a wt% of about 0.8 to 1.3, Al at a wt% of about 0.09 to 0.3, Mo at a wt% of about 0.1 to 0.3, Nb at a wt% of about 0.015 to 0.04, Ti at a wt% of about 0.02 to 0.06, S at a wt% of about 0.005 to 0.01, P at a wt% of about 0.015 to 0.025, and N at a wt% of about 0.004 to 0.01.

In yet another embodiment, the present method for producing hot rolled steel comprises steps of:
melting the steel having a composition comprising carbon (C) at a wt% of about 0.12 to 0.25, manganese (Mn) at a wt% of about 1.2 to 2.0, silicon (Si) at a wt% of about 0.5 to 1.2, chromium (Cr) at a wt% of about 0.8 to 1.3, aluminum (Al) at a wt% of about 0.09 to 0.3, molybdenum (Mo) at a wt% of about 0.1 to 0.3, niobium (Nb) at a wt% of about 0.015 to 0.04, titanium (Ti) at a wt% of about 0.02 to 0.06, sulphur (S) at a wt% of about 0.005 to 0.01, phosphorous (P) at a wt% of about 0.015 to 0.025 and nitrogen (N) at a wt% of about 0.004 to 0.01, to obtain molten steel,
casting the molten steel to obtain casted steel, and
hot rolling the casted steel to obtain the hot rolled steel.

In still another embodiment of the present method, prior to hot rolling, the casted steel is homogenized and subjected to roughening at a temperature range of about 1050 °C to 1150 °C by applying deformation 60-95%, and reheating at a temperature range of about 1150 °C to 1300 °C in a furnace at a heating rate of about 2 °C per minute to 20°C per minute for about 1.5 hours to 3 hours. The reheated steel taken out from the furnace is subjected to hot rolling deformation in the austenite phase with several passes.

In still another embodiment of the present method, a finish rolling temperature (FRT) in the austenite phase is applied during hot rolling step. In a preferred embodiment of the present method, a FRT of about 850 °C to 1000 °C is applied during hot rolling.

In still another embodiment of the present method, the hot rolled steel is cooled and subjected to coiling at a temperature range between martensitic start (Ms) temperature and bainitic start (Bs) temperature, preferably between 350 °C to 590 °C for about 0.1 hours to 2.5 hours. In an embodiment, coiling is followed by cooling the steel to ambient temperature. In an embodiment, cooling to ambient temperature is carried out by normal air cooling or vacuum or water.

In an exemplary embodiment of the disclosure, the hot rolled steel obtained by the present method has a tensile strength of about 1 GPa to 1.15 GPa and elongation of about 32% to 42%.

In another exemplary embodiment of the disclosure, the hot rolled steel obtained by the present method is an advanced high strength steel (AHSS), and possesses microstructure comprising bainite at a volume % of about 80 to 90 and a mixture of martensite and austenite at a volume % of about 10 to 20.

In yet another exemplary embodiment of the disclosure, the hot rolled steel obtained by the present method is a steel sheet having a thickness of about 4 mm to 10 mm, and wherein the thickness of bainite phase in the steel is below submicron level, and especially ranges from about 350 to 500 nm.

In an exemplary embodiment of the disclosure, the hot rolled steel obtained by the present method possesses yield to tensile ratio of about 0.6 to 0.7, and strain hardening exponent (n) in the range of about 0.15 to 0.21.

More particularly, the method according to the present disclosure for developing the steel product with the above specified composition consists of following steps: alloy melting or heat making, casting, roughening, reheating, hot rolling, cooling and coiling, followed by further cooling to ambient temperature. In an embodiment of the present disclosure, said processing steps of the method are detailed below:

The steel composition according to the present invention is melted in an induction melting furnace and subsequently cast in the form of thick bar or ingot. The ingot is homogenized at a temperature above 1200 °C and is held for sufficient time according to thickness, and roughening is subsequently applied to deform the austenite and break the cast structure and also to prepare the cast structure with suitable thickness as an input for hot rolling process. Prior to hot rolling, the steel is soaked/reheated at high temperature above 1130°C for several hours and subsequently hot rolled to a thickness of about 4-10 mm by maintaining a finish rolling temperature (FRT) in the austenite region. Subsequently, coiling is performed at a predetermined temperature in between martensite start (Ms) temperature and bainite start (Bs) temperature according to composition of the steel. The coiling simulation is performed either in salt bath or a similar kind of arrangement at a predetermined temperature which is above Ms or below Bs, and the hot rolled product is held for various time periods. The steel samples are then transferred to air and allowed to cool to ambient temperature.

In another embodiment of the present disclosure, specimens for analysis of microstructure and mechanical properties are taken from the hot rolled sheet. Microstructural characterization is carried out using optical, scanning electron microscope (SEM) and electron back scatter diffraction (EBSD). X-Ray diffraction (XRD) is employed to confirm the microstructural constituents. Further, the mechanical properties are evaluated by Vickers hardness method and tensile tests are performed as per ASTM standard.

The present disclosure further provides a hot rolled steel product comprising the primary constituents carbon (C) at a wt% of about 0.12 to 0.25, manganese (Mn) at a wt% of about 1.2 to 2.0, silicon (Si) at a wt% of about 0.5 to 1.2, chromium (Cr) at a wt% of about 0.8 to 1.3, aluminum (Al) at a wt% of about 0.09 to 0.3, molybdenum (Mo) at a wt% of about 0.1 to 0.3, niobium (Nb) at a wt% of about 0.015 to 0.04, titanium (Ti) at a wt% of about 0.02 to 0.06, sulphur (S) at a wt% of about 0.005 to 0.01, phosphorous (P) at a wt% of about 0.015 to 0.025 and nitrogen (N) at a wt% of about 0.004 to 0.01. In an embodiment, Iron (Fe) of about 94.505 to 95.931 form rest of the hot rolled steel product.

In an embodiment of the present disclosure, importance of the primary components constituting the presently developed hot rolled steel product is described below:

C (0.12 to 0.25 wt %): Said wt% of carbon resulted in desired strengthening. Carbon also retained austenite stability, which plays important role affecting the tensile properties. Good weldability was also achieved by said carbon levels. Preferred carbon content was kept below 0.25% to achieve desired strength and elongation and also weldability. The more preferred carbon content was below 0.22%.

Mn (1.2 - 2.0) wt%: Said wt% of manganese helped in stabilizing the retained austenite which is beneficial with respect to tensile properties. However, the amount was preferably 1.2 or more, more preferably 1.5% or more. Further, manganese content was preferably kept less than 2.5% to avoid welding and casting cracks.

Al (0.09 - 0.3 wt %): Al is a stronger ferrite stabilizer. It does not allow the carbon to come out easily from the retained austenite, thereby, allowing more amount of retained austenite to be formed during bainite reaction. Further, Al addition was favorable over silicon addition from galvanizing point of view. However, the amount included was not excessive, since higher amounts could create problems during casting. In particular, excess Al might allow formation of hard oxides in the weld area, thereby, deteriorating weldability. Hence, to ensure beneficial effect of Al, the Al content in the presently developed steel was maintained up to 0.3 % or preferably above 0.09 wt%.

Si (0.5 - 1.2 wt %): Silicon is also a ferrite stabiliser. Silicon suppresses carbide precipitation during bainite transformation at constant temperature holding / coiling and alloy formation of greater amount of retained austenite in the microstructure. However, excess amount of silicon addition in steel is detrimental due to varieties of scale formation during hot rolling and cooling. Such scale formation leads to surface deterioration and reduces coatability/ gavanizibility. Hence, Si was restricted within the wt% range as mentioned herein and was more preferably kept below 0.7 wt%.

P (0.025 wt% maximum): Phosphorus amount was be restricted to 0.025%, and preferably 0.020% or less, since high phosphorous was detrimental in steel.

S (0.01 wt% maximum): Similar to phosphrous, sulphur was also detrimental to the steel. The sulphur content was kept as low as possible, more preferably below 0.01 wt% to minimize the amount of inclusions which are potential sites for premature failure during forming operations.

N (0.01 wt% maximum): Excess nitrogen in steel was also detrimental. Excess nitrogen may lead to hard inclusions such as TiN and AlN which deteriorate formability operations. Nitrogen content was restricted up to 0.01 wt%.

Nb (0.015 - 0.04 wt%): Niobium was added to increase the strength of the steel by various mechanisms such as grain refinement and precipitation. Nb addition was also useful for having larger amount of retained austenite in the microstructure. Nb was added very carefully and was optimized suitably to reduce costs (since Nb is costly).

Mo (0.1 - 0.3 wt%): Molybdenum was added to enhance the hardenability in steel, thereby, favoring easy formation of bainite. Due to excess hardenability, softer ferrite and relatively harder pearlite phase formation could be suppressed during bainitic reaction. Further, since Mo is costly, its amount was restricted below 0.3 wt% to make the developed steel economical and still taking processing advantage of Mo during hot rolling.

Cr (0.8 - 1.3 wt%): Function of Chromium is very much similar to Mo, wherein it avoids formation of polygonal ferrite and pearlite. Cr addition was more economical in advanced high strength steel development. However, care should be taken since Cr could be harmful if excessive amount is added form various kind of carbides.

Ti (0.02 - 0.1 wt%): Ti is beneficial to restrict austenite grain growth. In addition, Ti also forms very fine carbonitride in the presence of Nb, vanadium (V) and increases strength. However, excess amount of Ti could be harmful as Ti has the tendency to form hard TiN inclusions. Accordingly, the amount of Ti was restricted below 0.1 wt%.

The amount of carbon and manganese in the steel composition is restricted according to the amounts described above for better weldability. Silicon is also restricted below certain levels as described above to improve the product characteristics. Further, the hot rolling, cooling and coiling parameters are employed in a way to ensure that the developed steel could be produced under conventional mill operating conditions in the same run out table to obtain steel strip in the thickness range of 4-10 mm and having high strength & elongation. The high strength and elongation is achieved through formation of optimum combination of phase mixture such as austenite, bainite and martensite in the microstructure.

In an embodiment, the hot rolled steel product of the present disclosure possesses extraordinary tensile strength along with elongation properties. In an exemplary embodiment, the hot rolled steel has a tensile strength of about 1 GPa to 1.15 GPa and elongation of about 30% to 42%.

In another exemplary embodiment, the hot rolled steel of the present disclosure is an advanced high strength steel (AHSS) and possesses microstructure comprising bainite at a volume % of about 80 to 90 and a mixture of martensite and austenite at a volume % of about 10 to 20 at ambient temperature. The bainite (80-90%) present in the microstructure is essentially very fine with high dislocation density, and leads to higher strength and good ductility combination. Further, the mixture of retained austenite and martensite constituents (10-20%) of the microstructure is also essential in the developed steel, wherein said mixture also improves the tensile properties including ductility and strength.

The present disclosure also relates to articles manufactured from the hot rolled steel strip/sheets described herein. The hot rolled steel of the present disclosure and corresponding article(s) manufactured using said steel find applications where a combination of high tensile strength and elongation is important, particularly in automotive structural and load bearing applications. Exemplary embodiments of articles manufactured using said steel include but are not limited to automotive components such as lower suspension, long and cross member bumpers, tipper body, A and B Pillar, pressure vessel, lifting and excavation, Oil and Gas boiler. The hot rolled steel of the present disclosure are also useful in applications related to lifting and excavation, defense equipments and in areas of high strength steel based applications.

The present disclosure is thus successful in developing advanced high strength hot rolled steel product possessing an extraordinary combination of elongation and strength. The developed steel has advantages including but not limiting to the following:
- the process employed for developing the steel is simple.
- since the developed steel has excellent combination of tensile strength and ductility, it is attractive for automotive structural applications and in several other areas where good combination of tensile strength and elongation is required.
- since the steel is microstructured, it is useful in applications where toughness and load bearing capability is required.
- the alloying composition employed for developing the steel favors processing of the steel in existing conventional hot strip mills, without requiring additional/significant investment.
- the alloying composition employed for developing the steel also ensures easy process control and better product surface quality after hot rolling.
- the alloying composition employed for developing the steel also minimizes surface related issues such as coating etc.
- the alloying composition employed for developing the steel is also beneficial for good and easy weldability and for better castability.

In an embodiment, the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

EXAMPLES
Example 1: Preparation of steel using various alloy compositions
The steel compositions described in table 1 were melted in an induction melting furnace and was subsequently cast in the form of 65-80 mm thick bar or ingot. The ingot was homogenized at temperature above 1200 °C and was held for 90-360 minutes according to thickness. Roughening was subsequently applied to deform the austenite and break the cast structure and also to prepare input having suitable thickness for the hot rolling step. Prior to hot rolling, the steel was soaked/reheated at high temperature 1130-1200 °C in a rehearing furnace for several hours (about 1.5 – 4 hours) and was subsequently cooled using cooling rate of about 5-20 °C/s. This was followed by hot rolling wherein rolling deformation was applied above Tnr of the steel, which is in the range 1000-1050 °C and subsequently deformation was applied below the Tnr of the steel with at a temperature range of 850-925 °C with thickness of about 4-9 mm. The steel was rolled using multiple passes applying deformation each pass 15-50%. The finish rolling temperature (FRT) was kept in the austenite region. Subsequently, the steel was cooled from FRT to coiling temperature using cooling rate of about 5 to 75 °C/s using air or water. The coiling was performed at predetermined temperature in the range of 400-500 °C in between martensite start (Ms) and bainite start (Bs) temperature according to compositions of the steel. The coiling simulation was performed using either salt bath or a similar equivalent set-up at predetermined temperature (400-500 °C) above Ms or below Bs and was held for various time periods (about 15 seconds to 300 minutes). The steel samples were then transferred to air and were allowed to cool to ambient temperature in presence of water or air to obtain hot rolled steel products.

Example 2: Analysis of structural and mechanical properties
Specimens for microstructure and mechanical properties were taken from the hot rolled sheets prepared in Example 1. Microstructural characterization was carried out using optical, scanning electron microscope and electron back scatter diffraction. Mechanical properties were evaluated by Vickers hardness method and tensile tests were performed as per ASTM standard. X-Ray diffraction was employed to confirm the microstructural constituents.

Tensile testing was carried out to assess the performance of the new developed steel. For example, the engineering stress-strain plot of one of the presently prepared steels is depicted in Figure 1, which shows that the steel has superior combination of tensile behavior. The tensile strength and elongation obtained with the present hot rolled steel products is in the range 1015-1150 MPa (1 GPa - 1.15 GPa) and elongation 32-41% [table 1], respectively. The strain hardening exponent (n) value is in the range of 0.15 to 0.21. The optical micrograph as shown in Figure 2 confirms that the developed steel has microstructural constituents bainite, martensite and retained austenite. The scanning electron microscopy examination in detail further confirmed the presence of bainite along with martensite and retained austenite (Figure 3). The SEM examination also ensured that the structure is very fine. EBSD examination further ensured the presence of above said phases in the microstructure (Figure 4). The observation in the EBSD was further corroborated by XRD examination which confirmed the major presence of bainite phase. The fineness of bainite determined the strength and toughness of the steel. The thickness of bainite structure was found below submicron level. X-ray diffraction carried out on the developed steel showed the presence diffraction peaks from body centre cubic (BCC) indicated by abcc and face centre cubic (FCC) austenite indicated by ?fcc peak in the plot shown in Figure 5. The intensity of the bcc phase peak is several times higher than the intensity of the fcc peak clearly confirming that the amount of bcc bainite phase is the major phase in the developed steel. This analysis confirms that the presently developed steel major comprises bainite structure and some amount of martensite along with little amounts of retained austenite. EBSD also confirms the presence of small amounts of retained austenite.

Table 1: Steel compositions and their properties
Compositions C Mn Si Cr Mo Al Ti Nb P S N Tensile Strength
(MPa) TEL (%) n
1 0.25 1.8 0.8 1.10 0.15 0.25 0.05 0.04 0.021 0.01 0.009 1090 32 0.18
2 0.20 1.6 0.6 1.0 0.11 0.2 0.03 0.02 0.02 0.01 0.01 1020 40 0.18
3 0.15 1.4 1.2 0.91 0.2 0.1 0.04 0.03 0.019 0.01 0.01 1005 41 0.15

The above results confirm the extraordinary properties of the presently developed hot rolled steel products including tensile strength between 1015-1150 MPa (1 GPa - 1.15 GPa), total elongation (TEL) between 32-41% and strain hardening exponent (n) in the range of 0.15 to 0.21.