Claims:WE CLAIM:
1. A method for producing bulk nanostructured steel, comprising steps of:
melting a steel having a composition comprising carbon at a wt% of about 0.001 to 0.2, manganese at a wt% of about 22 to 37, silicon at a wt% of about 1.5 to 5, aluminum at a wt% of about 1.2 to 5.2, sulphur at a wt% of about 0.002 to 0.012, phosphorous at a wt% of about 0.003 to 0.025 and nitrogen at a wt% of about 0.0035 to 0.009, to obtain molten steel;
casting the molten steel to obtain casted steel;
hot rolling the casted steel to obtain a hot rolled steel;
cold rolling the hot rolled steel to obtain a cold rolled steel; and
annealing the cold rolled steel and cooling to obtain the bulk nanostructured steel,
wherein the obtained bulk nanostructured steel is a completely recrystallized single phase austenitic steel.

2. The method of claim 1, wherein the steel has a composition comprising carbon at a wt% of about 0.004 to 0.01, manganese at a wt% of about 25 to 37, silicon at a wt% of about 2.3 to 3.6, aluminum at a wt% of about 2.4 to 3.4, sulphur at a wt% of about 0.005 to 0.01, phosphorous at a wt% of about 0.009 to 0.02 and nitrogen at a wt% of about 0.004 to 0.009.

3. The method of claim 1, wherein the steel has a composition comprising carbon at a wt% of about 0.001 to 0.01, manganese at a wt% of about 25 to 37, silicon at a wt% of about 1.5 to 5, aluminum at a wt% of about 1.2 to 5.2, sulphur at a wt% of about 0.008 to 0.01, phosphorous at a wt% of about 0.003 to 0.025 and nitrogen at a wt% of about 0.004 to 0.009.

4. The method of claim 1, wherein the casted steel is homogenized at a temperature range of about 1150 °C to 1250 °C for about 3 hours to 6 hours, prior to hot rolling.

5. The method of claim 1, wherein a reduction ratio of about 78% to 92.3% and a finishing temperature range of about 850 °C to 1000 °C is applied during hot rolling.

6. The method of claim 1, wherein the hot rolled steel is cooled before cold rolling to retain the austenite phase of steel.

7. The method of claim 1, wherein a reduction ratio of about 78% to 92.3% and equivalent strain of about 1.8 to 2.96 is applied during cold rolling.

8. The method of claim 1, wherein the cold rolling is performed at a temperature range of about 2°C to 50°C.

9. The method of claim 1, wherein the annealing is performed at a temperature range of about 500°C to 900°C for about 5 seconds to 3 hours.

10. The method of claim 1, wherein the annealing comprises holding the steel for about 0.005 ks to 10.8 ks.

11. The method of claim 1, wherein the steel post annealing is cooled at a cooling rate ranging from about 1 °C/minute to about 500 °C/second.

12. The method of claim 1, wherein the method produces bulk nanostructured steel by plain strain deformation of the steel, and wherein the reduction ratio in terms of equivalent strain is 1.8 to 2.96.

13. The method of claim 1, wherein the completely recrystallized bulk nanostructured single phase austenitic steel is strain-free, ultra-fine grained with equiaxed grain morphology and has an average grain size of less than 398 nm.

14. The method of claim 1, wherein the bulk nanostructured steel is a steel sheet having a thickness of about 0.8 to 2 mm.

15. A completely recrystallized bulk nanostructured single phase austenitic steel having a composition comprising carbon at a wt% of about 0.001 to 0.2, manganese at a wt% of about 22 to 37, silicon at a wt% of about 1.5 to 5, aluminum at a wt% of about 1.2 to 5.2, sulphur at a wt% of about 0.002 to 0.012, phosphorous at a wt% of about 0.003 to 0.025 and nitrogen at a wt% of about 0.0035 to 0.009.

16. The completely recrystallized bulk nanostructured steel of claim 15, wherein the steel is strain-free, ultra-fine grained with equiaxed grain morphology and has an average grain size of less than 398 nm.

17. The completely recrystallized bulk nanostructured steel of claim 15, wherein the steel has a composition comprising carbon at a wt% of about 0.004 to 0.01, manganese at a wt% of about 25 to 37, silicon at a wt% of about2.3 to 3.6, aluminum at a wt% of about 2.4 to 3.4, sulphur at a wt% of about 0.005 to 0.01, phosphorous at a wt% of about 0.009 to 0.02 and nitrogen at a wt% of about 0.004 to 0.009.

18. The completely recrystallized bulk nanostructured steel of claim 15, wherein the steel has a composition comprising carbon at a wt% of about 0.001 to 0.01, manganese at a wt% of about 25 to 37, silicon at a wt% of about 1.5 to 5, aluminum at a wt% of about 1.2 to 5.2, sulphur at a wt% of about 0.008 to 0.01, phosphorous at a wt% of about 0.003 to 0.025 and nitrogen at a wt% of about 0.004 to 0.009.

19. The completely recrystallized bulk nanostructured steel of claim 15, wherein the steel is a steel sheet having a thickness of about 0.8 to 2 mm and possess a hardness of about 300 HV to 330 HV.

20. An article comprising the completely recrystallized bulk nanostructured single phase austenitic steel of claim 15.

Dated this 03rd day of November, 2017

Durgesh Mukharya
IN/PA-1541
Of K&S Partners
Agent for the Applicant

To: The Controller of Patents,
The Patent Office,
at: Kolkata
, Description:TECHNICAL FIELD
The present disclosure is in the field of metallurgy, more particularly metal fabrication. The present invention provides a viable and simple method to fabricate bulk nanostructured steel. Said method of fabricating bulk nanostructured steel is highly suitable for industrial scale-up and the developed steel has excellent mechanical properties rendering it useful, especially in automotive and load bearing applications.

BACKGROUND OF THE DISCLOSURE
Strong and tough steel is one of the major contributors to control air pollution. Light-weight environmental friendly 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 and ultra-high strength steel (UHSS) sheets. Several methods have been developed to produce high strength steels through alloying, phase transformation, grain refinement. However, there are many limitations in such methods. For instance, due to poor formability, the UHSS sheet cannot be applied easily to a wide variety of motor vehicle components. Thus, the ductility and formability required for UHSS sheet becomes increasingly important and demanding.

In the automotive sector, reduction in fuel consumption thereby lowering emission and maintaining high standard of safety demands the use of stronger steel. Such needs could be fulfilled through various mechanisms of improving the strength of a steel such as grain refinement, precipitation strengthening, solid solution strengthening, dispersion strengthening. However, these mechanisms improve the strength of steel at the cost of ductility.

Further, conventional metals and alloys, especially carbon steels and austenitic steels was processed in the prior art by hot and cold rolling methods which generally produced steels with a grain size of nearly 10µm and above. But strength of said micrometre sized steel steels were not high since said property was compromised for achieving better ductility. Therefore, such processes are not able to provide necessary solution to meet the needs such as safety, light weight and environment-friendly. Therefore, fabrication of nanostructures was thought to provide the solution.

Accordingly, various complex/multi-step methods such as Accumulative Roll Bonding (ARB), Equi-channel angular extrusion (ECAE), high pressure torsion (HPT) have emerged to fabricate nanostructures in metals and alloys, preferably at laboratory scale. These methods have their own advantages and disadvantages. Though these methods are very much capable to produce nanostructures, these newer and complicated processing methods suffer from various drawbacks including application of severe strain and severe plastic deformation (SPD) during the process, failing to fabricate bulk nanostructures, difficulty in industrial scale-up and continuous production, and certainly cost intensive.

Hence, it will be of great importance and lucrative to the industry if simpler and cost-effective methods are developed for metal fabrication, especially steel, wherein the methods and fabricated products address the aforesaid concerns.

SUMMARY OF THE DISCLOSURE
The present disclosure relates to a method of producing/fabricating bulk nanostructured steel.
In an embodiment, the developed bulk nanostructured steel has a composition comprising carbon at a wt% of about 0.001 to 0.2, manganese at a wt% of about 22 to 37, silicon at a wt% of about 1.5 to 5, aluminum at a wt% of about 1.2 to 5.2, sulphur at a wt% of about 0.002 to 0.012, phosphorous at a wt% of about 0.003 to 0.025 and nitrogen at a wt% of about 0.0035 to 0.009. In another embodiment, the steel has a composition comprising carbon at a wt% of about 0.001 to 0.01, manganese at a wt% of about 25 to 37, silicon at a wt% of about 1.5 to 5, aluminum at a wt% of about 1.2 to 5.2, sulphur at a wt% of about 0.008 to 0.01, phosphorous at a wt% of about 0.003 to 0.025 and nitrogen at a wt% of about 0.004 to 0.009.
In another embodiment, the method of fabricating bulk nanostructured steel comprises melting a steel composition as described above, followed by casting, hot rolling, cold rolling and annealing to obtain the bulk nanostructured steel.
In yet another embodiment, the developed bulk nanostructured steel is a completely recrystallized, strain free, ultra-fine grained (UFG) single phase austenitic steel with equiaxed grain morphology and an average grain size of less than 398 nm. Said steel having large fraction of grains are within the nanostructure or submicron size regime, and therefore make the present steel as bulk nanostructured metals.
The present disclosure further provides bulk nanostructured steel having excellent mechanical properties and corresponding article(s) thereof.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 depicts various methods employed in the prior art to fabricate nanostructured materials - (a) Equi-channel angular extrusion (ECAE), (b) High pressure torsion (HPT), (c) Accumulative Roll-Bonding (ARB).
Figure 2 depicts schematic view of the present method to fabricate bulk nanostructured steel.
Figure 3 depicts microstructure of bulk nanostructured steel according to present method.
Figure 4 depicts electron micrograph of the rolled bulk nanostructured steel according to present method.
DETAILED DESCRIPTION OF THE DISCLOSURE
As used herein, the phrases ‘bulk nanostructured steel’ refers to the product of the present disclosure which comprises>95% grains within submicron range and comprises microstructure with a characteristic length scale in the order of a few nanometers.
As used herein, the phrase ‘completely recrystallized’ or ‘fully recrystallized’ refers to the steel product of the present disclosure containing defects-free or recrystallized grains which are relatively free of dislocations.
As used herein, the phrase ‘ultrafine grained (UFG)’ refers to polycrystals having very small grains with average grain sizes less than 398 nm.
As used herein, the phrase ‘equiaxed grain’ refers to equiaxed grains/crystals that have axes of approximately the same length. Further, equiaxed grains is an indication of recrystallization.
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 present disclosure relates to a method of fabricating bulk nanostructured steel and corresponding product thereof. The disclosure provides a simple and economical method of steel fabrication by employing conventional or existing mill/production setup and eliminating the complex processes involved in steel fabrication.

In particular, the present disclosure aims at preparing completely recrystallized ultra-fine grained nanostructured (UGF) bulk single phase austenitic steel. Said steel product is fabricated by method involving cold rolling and subsequent annealing steps. The present method applied to fabricate the completely recrystallized bulk nanostructured steel offers good mechanical properties [Table 2] and is also expected to provide high toughness desired ductility due to low stacking fault energy and significant twinning. Therefore, the present method applied to fabricate completely recrystallized bulk single phase nanostructured austenitic steel sheet can be used in areas/applications where very high toughness is essential, especially in automotive industry such as automotive bumper and other load bearing applications, cryogenic exposure, oil and gas, spring, and wires. The steel sheet fabricated by the present method is also thinner, and therefore, satisfies the criteria of light weight, fuel efficiency and environment friendliness which are usually essential in the automotive industry.

The method of preparing bulk nanostructured steel in the present disclosure comprises the steps of melting a steel (raw material/input) followed by casting the molten steel, hot rolling, cold rolling and subsequent annealing to obtain the steel product. The steel/raw material employed in the present method and the steel product i.e. completely recrystallized ultra-fine grained nanostructured (UGF) bulk single phase austenitic steel product obtained by the present method has a composition comprising primary components - carbon (C) at a wt% of about 0.001 to 0.01, manganese (Mn) at a wt% of about 25 to 37, silicon (Si) at a wt% of about 1.5 to 5, aluminum (Al) at a wt% of about 1.2 to 5.2, sulphur (S) at a wt% of about 0.008 to 0.015, phosphorous (P) at a wt% of about 0.003 to 0.025 and nitrogen (N) at a wt% of about 0.004 to 0.009. Said composition further comprises at least one or more additional elements selected from iron (Fe), nickel (Ni), chromium (Cr), copper (Cu), at various wt% to make up the final composition to 100 wt%. In an embodiment, the wt% of Fe is 60-65wt%, Ni is 0.004-0.01, Cr is 0.003-0.008 and Cu is 0.0035-0.0075.

In an embodiment, the steel of the present disclosure has a composition comprising ‘C’ at a wt% of about 0.004 to 0.01, ‘Mn’ at a wt% of about 25 to 37, ‘Si’ at a wt% of about 2.3 to 3.6, ‘Al’ at a wt% of about 2.4 to 3.4, ‘S’ at a wt% of about 0.005 to 0.01, ‘P’ at a wt% of about 0.009 to 0.02 and ‘N’ at a wt% of about 0.004 to 0.009.

In another embodiment, the steel of the present disclosure has a composition comprising ‘C’ at a wt% of about 0.004 to 0.015, ‘Mn’ at a wt% of about 24 to 29, ‘Si’ at a wt% of about 2.7 to 3.2, ‘Al’ at a wt% of about 2.4 to 3.15, ‘S’ at a wt% of about 0.002 to 0.006, ‘P’ at a wt% of about 0.003 to 0.01 and ‘N’ at a wt% of about 0.004 to 0.008.

In yet another embodiment, the steel of the present disclosure has a composition comprising ‘C’ at a wt% of about 0.003 to 0.008, ‘Mn’ at a wt% of about 24 to 33, ‘Si’ at a wt% of about 2.65 to 3.22, ‘Al’ at a wt% of about 2.60 to 3.15, ‘S’ at a wt% of about 0.002 to 0.0055, ‘P’ at a wt% of about 0.003 to 0.01 and ‘N’ at a wt% of about 0.0035 to 0.0075.

In a preferred embodiment, the wt% of C + Mn + S + P ranges from about 25.5 to 34.5, and the wt% of Al + Si + N ranges from about 3.2 to 8.5 in the steel composition.

In an embodiment, the significance of the primary components and their weight percentages in the steel composition of the present disclosure is as follows:

‘C’ with 0.001-0.2 wt% - this amount of carbon content helps in achieving microstructures, desired phase fractions, weldability and phase stability. The amount of carbon is preferably below 0.06 wt%.

‘Mn’ with 22-37 wt% - manganese helps to get austenite. The amount of manganese is preferably 25 wt%, or more preferably 27 wt% or more, and even more preferably 28 wt% or more. In another preferred embodiment, the manganese amount is more than 25 wt%.
‘Al’ with 1.2-5.2 wt.% - aluminum is a strong ferrite stabilizer. The Al content is preferably maintained above 1.5 w% and below 5 wt% to obtain suitable microstructure formation and phases. More preferably, the Al content is above 2 wt%.

‘Si’ with 1.5-5 wt% - silicon is also a ferrite stabiliser and is essential to control microstructure and phase formation. The Si content is preferably below 4.5 wt%. In another preferred embodiment, the Si amount is in the range of 1.8 to 4.5 wt%.

‘S’ with 0.002 to 0.012 wt% and ‘P’ with 0.003 to 0.025 wt% - sulphur and phosphorous content are kept within said ranges which help in minimizing the amount of inclusions which act as potential sites for premature failure during forming operations.

‘N’ with 0.0035 to 0.009 wt% - excess nitrogen may lead to hard inclusions such as TiN and AlN which deteriorate formability. Therefore, the nitrogen content is employed within the said range, preferably below 0.005 wt%.

The present method of fabricating/preparing bulk nanostructured steel by employing the composition as described above, comprises:
(a) melting a steel having said composition to obtain molten steel,
(b) casting the molten steel to obtain casted steel,
(c) hot rolling the casted steel to obtain a hot rolled steel,
(d) cold rolling the hot rolled steel to obtain a cold rolled steel, and
(e) annealing the cold rolled steel followed by cooling to obtain the bulk nanostructured steel product.

In an embodiment, the steel (raw material) having the composition as described above is melted in a melting furnace to obtain molten steel. In another embodiment, the melting process includes melting the charge, refining the melt, adjusting the melt chemistry and tapping the molten steel into a vessel. In yet another embodiment, the furnace employed in the melting process includes known melting furnace, preferably selected from electric arc furnaces (EAF), induction furnaces, cupolas furnaces and combinations thereof.

In yet another embodiment, the casted steel is homogenized prior to hot rolling. In an embodiment, said homogenization is carried out at a temperature range of about 1100 °C to 1250 °C for about 2 hours to 6 hours.

In yet another embodiment, the casted and homogenized steel is hot rolled wherein said steel is fed into rolling mill and processed at a finishing temperature range of about 850 °C to 1000 °C and subsequently water quenched / air cooled to room temperature. In an embodiment, a reduction ratio of about 70% to 95% is applied during the hot rolling process. During hot rolling, the steel is processed applying several passes. The amount of reduction applied in every pass is in between 20-50% in terms of engineering strain. In another embodiment, the subsequent cooling helps in retaining the austenite phase of the steel.

In still another embodiment, the hot rolled stock is subjected to cold rolling wherein said hot rolled stock is cut into different shapes preferably, rectangular coupons, and fed into rolling mill and processed at a temperature range of about 2°C to 50°C and by applying various reduction ratios in terms of equivalent. In another embodiment, a reduction ratio of about 78% to 92.3% and equivalent strain of about 1.8 to 2.96 is applied during the cold rolling process.

In still another embodiment, the cold rolled stock is annealed at a temperature range of about 500°C to 900°C for about 5 seconds to 3 hours. In an embodiment, the annealing further comprises holding the cold rolled stock/steel for about 0.005 ks to 10.8 ks. In another embodiment, the annealing process comprises recovery stage resulting in softening of the steel through removal of primarily linear defects (dislocations), followed by recrystallization stage where new strain-free grains nucleate and grow, and finally grain growth leading to microstructure containing equi-axed grains.

In an embodiment, the steel post annealing is cooled at a cooling rate ranging from about 1 °C/minute to 500 °C/second. The microstructure formed during the annealing process is responsible for the production of nanostructure during this cooling stage.

In an exemplary embodiment, the present method of preparing bulk nanostructured steel comprises:
(a) melting a steel having the composition as described above in a melting furnace to obtain molten steel,
(b) casting the molten steel into bars to obtain casted steel followed by homogenization at a temperature range of about 1100 °C to 1250 °C for about 2 hours to 6 hours,
(c) hot rolling the casted and homogenized steel at a reduction ratio of about 70% to 95%. Hot rolling the steel was done applying several passes with reduction at each stage controlled between 20-50%. Finishing temperature range of about 850 °C to 1000 °C was applied to obtain the hot rolled steel, followed by cooling the hot rolled steel,
(d) cold rolling the hot rolled steel at a reduction ratio of about 78% to 92.3%, an equivalent strain of about 1.8 to 2.96 and at a temperature range of about 2°C to 50°C, to obtain a cold rolled steel, and
(e) annealing the cold rolled steel at a temperature range of about 500°C to 900°C for about 5 seconds to 3 hours, followed by cooling to obtain the bulk nanostructured steel.

In an embodiment, the present method employs the composition as described above such that the cold rolling step results in nanostructure formation post annealing. In another embodiment, the present method produces the bulk nanostructured steel by plain strain deformation, thus, avoiding severe strain/severe plastic deformation (SPD) as employed commonly in the prior art steel fabrication processes.

In another embodiment, the obtained bulk nanostructured steel by the present method is a completely recrystallized, strain-free, ultra-fine grained (UFG) equiaxed single phase austenitic steel.

In another embodiment, the obtained bulk nanostructured steel by the present method is in the form of a steel sheet having a thickness of about 0.8 to 2.0 mm.

In an exemplary embodiment, the method adapted to fabricate bulk nanostructured steel is presented schematically in Figure 2. The figure shows the steps comprising melting of the steel and casting into bar. The cast bar was homogenized for several hours. The casted bar is subjected to hot rolling and cooling. The hot rolled steel was subjected to cold rolling at ambient temperature with equivalent strain reduction ratio at least 2.5. The schematic figure further indicates that the cold rolled steel sheet was allowed to go through annealing treatment near the recrystallization temperature followed by cooling to fabricate strain free bulk nanostructured steel sheet with thickness in the range of about 0.8 mm to 2 mm.

The present invention further relates to a steel product including an advanced high strength steel sheet (AHSS), preferably Twinning-Induced Plasticity (TWIP) steel, and articles thereof. In particular, the disclosure provides completely recrystallized bulk nanostructured single phase austenitic steel sheet having a composition comprising primary components - carbon at a wt% of about 0.001 to 0.2, manganese at a wt% of about 22 to 37, silicon at a wt% of about 1.5 to 5, aluminum at a wt% of about 1.2 to 5.2, sulphur at a wt% of about 0.002 to 0.012, phosphorous at a wt% of about 0.003 to 0.025 and nitrogen at a wt% of about 0.0035 to 0.009. In an embodiment, the steel sheet has a composition comprising carbon at a wt% of about 0.001 to 0.01, manganese at a wt% of about 25 to 37, silicon at a wt% of about 1.5 to 5, aluminum at a wt% of about 1.2 to 5.2, sulphur at a wt% of about 0.008 to 0.01, phosphorous at a wt% of about 0.003 to 0.025 and nitrogen at a wt% of about 0.004 to 0.009. In another embodiment, the steel sheet has a composition comprising carbon at a wt% of about 0.004 to 0.01, manganese at a wt% of about 25 to 37, silicon at a wt% of about2.3 to 3.6, aluminum at a wt% of about 2.4 to 3.4, sulphur at a wt% of about 0.005 to 0.01, phosphorous at a wt% of about 0.009 to 0.02 and nitrogen at a wt% of about 0.004 to 0.009.

In an embodiment, the completely recrystallized bulk nanostructured steel product of the present disclosure is strain-free, ultra-fine grained (UFG) / nanostructured with equiaxed grain morphology. In another embodiment, the steel has an average grain size of less than 398 nm.

In another embodiment, completely recrystallized bulk nanostructured steel sheet of the present disclosure has a thickness of about 0.8 to 2 mm.

The completely recrystallized bulk nanostructured steel of the present disclosure and the corresponding article(s) exhibit excellent mechanical properties including high strength, hardness and expected to offer good toughness. Achievement of said properties may be attributed to the steel composition and the method employed by the present disclosure. In an embodiment, the steel product possesses a hardness of about 300 HV to 330 HV.

The completely recrystallized bulk nanostructured steel of the present disclosure and corresponding article(s) manufactured using said steel find applications where high steel strength is important, particularly in automotive and load bearing applications. Exemplary embodiments of articles manufactured using said steel include but are not limited to automotive bumpers, cryogenic applications, oil and gas, spring, wires and numerous other steel based applications.

The present disclosure is thus successful in fabricating completely recrystallized bulk nanostructured steel by the method described herein which involves melting, casting, hot rolling, cold rolling and subsequent annealing processes. The achievement of the described structural and mechanical properties of the steel product by the present method is also be attributed to the steel composition described herein. The presently developed bulk nanostructured steel has smallest average grain size in the completely recrystallized condition, finest recrystallized grain size with equiaxed grain morphology and finest equiaxed recrystallized grain size with single phase.

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: Fabrication of bulk nanostructured steel
The method of the present disclosure was employed to fabricate bulk nanostructured strain free steel with following compositions of Table 1:

Table 1: Steel compositions employed to fabricate bulk nanostructured steel
Composition C Mn Si Al P S N
A 0.004 32 2.5 3 0.021 0.012 0.009
B 0.01 27 3.1 2.5 0.02 0.008 0.012
C 0.005 31 3.05 3.1 0.019 0.009 0.01
D 0.005 31 4.25 2.0 0.018 0.009 0.005
E 0.2 22 4.0 1.5 0.017 0.01 0.006

The alloys comprising compositions A to E containing primary components as shown in Table 1 were melted in melting furnace and subsequently cast in the form of bar. The bar was homogenized at 1100-1250°C from 2-6 hours and hot rolled with several passes with reduction ratio between 70-95% and subsequently cooled to room temperature. Rectangular coupons were cut and subjected to cold rolling by applying various reduction ratio between 75-92.3% in terms of strain. The cold rolled sheets were subjected to annealing around recrystallization temperature of the steel in the range 500-900°C and held for various time periods between 5-3 hours followed by cooling at various rates between 1°C/min to 500°C/s to introduce strain free nanocrystals in the steel sheet. The steel was cooled using various cooling rates to obtain the bulk completely recrystallized strain free nanostructured steel sheets possessing structural features including smallest average grain size of less than 398 nm, finest recrystallized grain size with equiaxed grain morphology and finest equiaxed recrystallized grain size with single phase. The present method adapted to fabricate the bulk nanostructured steel is presented schematically in Figure 2.

EXAMPLE 2: Analysis of structural and mechanical properties
Specimens for analyzing microstructure and mechanical properties were prepared from different stages of the fabrication method described in Example 1. Microstructural characterization was carried out using optical microscope, scanning electron microscope (SEM), electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). Microstructural quantification was also performed to quantify the grain size and their distribution. Mechanical properties and structural analysis was done by Vickers hardness method and X-Ray diffraction (XRD).

Various prior art methods used to refine grain size or fabricate nanostructures are shown in Figure 1. All these methods fabricate nanostructures by applying severe plastic deformation (SPD). These methods have several limitations including difficulty in scale-up. However, fabrication of bulk completely recrystallized nanostructures is achieved by the present method which is simple, easy to scale-up and uses conventional melting, hot rolling and cold rolling method and subsequent annealing with relatively lesser deformation particularly in the cold rolling stage to achieve strain free grains after recrystallization annealing . The present method is unique and simple also from the point of view that bulk nanostructured steel sheet is fabricated with a thickness of about 0.8 mm to 2 mm by applying relatively less equivalent strain prior to annealing and a large steel sheet can be fabricated through an existing industrial set up without requiring additional infrastructure or burden or very large investment.

Figure 3 shows the microstructure of the as-deformed sheet which confirmed the presence of nanolamellar boundaries which are responsible for the production of nanostructures in the post annealing condition. Figure 4 show the microstructure of the bulk completely recrystallized nanostructured steel fabricated using the present method. This figure confirms that the bulk completely recrystallized steel sheet fabricated by the present method is filled up with single phase austenitic nano/ultrafine grains. The average grain size determined was well below submicron size or in the nanostructure regime, more particularly less than 398 nm. The fully recrystallized microstructure also contains many twins. The alloy design/composition was prepared in a manner to keep the stacking fault energy certain regime and retaining single phase austenite, which has face centered cubic (FCC) crystal structure. The stacking fault energy helps to control the dislocations in the alloy, therefore, facilitate to achieve the right kind of microstructure during rolling [Fig. 3]. The microstructure developed during rolling with the designed alloy, plays significant role in developing the nanocystalline/ultra-fine grained structure with equiaxed morphology during recrystallization [Fig. 4]. The dislocation structure and defects act as driving forces to form nanometer-sized grains. Recrystallization and grain growth during annealing are employed to develop nanostructures of diversified grain sizes.

Since mechanical properties of steel is very important from application point of view, mechanical characterization of the steel was carried out and the hardness in the as deformed condition was found in the range 450 to 500HV which was very encouraging. The hardness properties evaluated for the completely recrystallized nanostructured steel developed using the method of Example 1 was found be about 300 to 330HV (Table 2).

Table 2: Mechanical property of fabricated bulk nanostructured steel
Composition Hardness (HV)
A 300
B 310
C 305

The steel thus produced according to the present method is bulk, completely recrystallized, nanostructured, strain free, and possess smallest average grain size, finest recrystallized grain size with equiaxed grain morphology and finest equiaxed recrystallized grain size with single phase. The developed steel further has excellent mechanical properties as described above. The present method and the steel products therefore can be useful in automotive and load bearing applications and several other areas such as cryogenic, oil and gas, spring, wires where a good combination property of strength and light weight is needed. The present method can also be directly translated to large industrial scale economically and without much complication.