Claims:WE CLAIM:

1. Medium carbon nano-structured bainitic steel with improved properties comprising C: 0.45 - 0.55 %, Mn: 1.40 – 1.50 %, Si: 1.70 – 1.80 %, Cr: 1.10 -1.12
%, Mo: 0.15 – 0.20 % , the balance being Fe and unavoidable impurities and having been bainitisedby austenitising at 8500 C for about 30 minutes, followed by isothermal annealing at about 285o C above Ms temperature for 7 to 10 days.

2. Bainitic steel as claimed in claim 1, wherein the improved properties in the steel include improved weldability, enforcement of transformation kinetics with Yield Strength of 1315 – 1471 MPa, Ultimate Tensile Strength: 1719 – 1919 MPa, % Elongation: 10 -14, Hardness (Hv30 ) : 520 -540 and Impact Toughness at – 40o C :
10 -12 J.

3. Bainitic steel as claimed in claims 1 and 2, wherein the steel has a percent composition of C: 0.49, Mn: 1.46, Si: 1.78, Cr: 1.12, Mo: 0.17, the balance being Fe and unavoidable impurities.

4. Bainitic steel as claimed in claim 3, wherein the steel has the Yield Strength:

1471 MPa, Ultimate Tensile Strength: 1919 MPa, % Elongation: 14 % and the Impact

Toughness at -40o C : 12 J.

5. Bainitic steelas claimed in claims 1 to 5, wherein the steel has nano-sized bainiticlath of ~90 -120 nm.

6. A process for producing medium carbon nano-structured bainitic steel with improved properties having composition C: 0.45 – 0.55 %, Mn: 1.40 – 1.50 %, Si:
1.70 – 1.80 %, Cr: 1.10 – 1.12 % and Mo: 0.15 – 0.20 %, the balance being Fe and unavoidable impurities comprising banitising the said steel by austenitising at 850o C for about 30 minutes, followed by isothermal annealing at about 285o C above Ms temperature for 7 to 10 days.

7. A process as claimed in claim 6, wherein the improved properties in the steel include improved weldability, enforcement of transformation kinetics with Yield

8

Strength: 1315 •1471 MPa, Ul1imate Tensie Strength: 1719 - 1919 MPa. %
Elongation: 10 - 14, Hardness (Hv,,): 520 -640 and Impact TOIJ!jlness at - 40" C
:10 -12 J.

8. A process as claimed in claims 6 and 7, Wherein the steel has a percent composition ofC: 0.49. Mn: 1.46, Si: 1.78. Cr: 1.12. Mo: 0.17 and the balance being Fe and unavoidable .mpunties.

9. A process as claimed in Claims 6 to 8. ¥.tlerein the steel has the nano- sited bailiticlath of 90 - 120 rm.

10. Rals manufactured from the nano-slrudured bain,tic steei as elained in Claims

1 to 5.

, Description:MEDIUM CARBON NANO–STRUCTURED BAINITIC STEEL WITH IMPROVED PROPERTIES AND A PROCESS FOR PRODUCING THE SAME

FIELD OF THE INVENTION:

The invention relates to bainitic steel. The invention particularly relates to medium carbon nano-structured bainitic steel with improved properties.
The invention includes a process for producing the same.

BACKGROUND OF THE INVENTION AND PRIOR ART:

Nano-bainitic carbide free microstructure is achieved in steels having carbon: ~1.0% and Si >1.5% through isothermal annealing by holding for a period ranging from days to weeks. The mechanical properties achieved in these steels are quite remarkable with tensile strength varying from 1.6-2.5 GPa, hardness in the range of 650–700 HV and fracture toughness: 80 – 130 MPa/m2, depending on the steel chemistry and isothermal transformation temperatures employed. Nano-bainitic steels with lath size
<100 nm are possible to be produced by maintaining low transformation temperature (~200oC), which is possible only by keeping C level to ~1 wt. %. However, enhancing carbon level beyond ~1% leads to weldability related problems. Therefore nano- bainitic steels with a much reduced carbon level needs to be developed.
Some prior-art patents on the subject have been reported which are mentioned herein by reference.

US 20110126946A1 – Bainite Steel and the methods of manufacture thereof. The said steel comprising between 90 % and 50 % bainite , the rest being austenite and has percent composition of C: 0.6 to 1.1, Si: 1.5 to 2.0, Mn: 0.5 to 1.8, Ni: up to
3, Cr: 1.0 to 1.5, Mo: 0.2 to 0.5, V: 0.1 to 0.2, balance being iron.

US 8956470B2 - Bainite Steel and methods of manufacture thereof. Bainite steel containing between 90 % and 50 % bainite, the rest being austenite. The steel contains in wt. percent C: 0.6 to 1.1, Si: 1.5 to 2.0, Mn: 0.5 to 1.8, Ni: upto 3, Cr: 1.0 to 1.5, Mo: 0.2 to 0.5, V: 0.1 to 0.2, balance being iron.

US 20130167983A1- Super bainite steel and method for manufacturing it. The steel consisting of elements in wt. percent, C: 0.4 - 1.1, Mn: 0.4 - 2.1, Si: 0.15 – 1.2,
Al: 0.0 – 2.0, Cr: 0.0 -1.4, Ni: 0.0 - 2.5, Mo: 0.0 –0.6, V: 0.0 -0.3, Co: 0.0 – 3.0, P <

0.025, S: < 0.025, the balance being iron.

US 20140102600A1 – Bainitic Steel of High Strength and High Elongation and Method to manufacture said Bainitic Steel. The steel consists of the following elements in wt. %, C: 0.25 -0.55, Si: 0.5 – 1.8, Mn: 0.8 -3.8, Cr: 0.2 – 2.0, Ti: 0.0 –
0.1, Cu: 0.0 – 1.2, V: 0.0 -0.5, Nb: 0.0 -0.06, Al: 0.0 – 2.75, N: <0.004, P < 0.025, S: <0.025.

Though, the strength properties of nano -structured bainitic steels are excellent but the austempering time is often 10: s or 100: s of hours which makes their production more complicated. The transformation rate can be accelerated by enhancing the free energy change associated with austenite to ferrite transformation. However, this needs to be done in a cost- effective way (without addition of costly element like Co) and without affecting system hardenability. This necessitates further research for the development of low cost medium carbon bainitic steel.

OBJECTS OF THE INVENTION:

The object of the invention is to provide for a ultra-high strength medium carbon nano-structured bainitic steel with improved mechanical properties and enhanced transformation kinetics.

Another object is to provide for the process technology for producing the ultra-high strength medium carbon bainitic steel in a cost effective way.

Another object is to manufacture rails or like articles from the said steel.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:

The invention is described with reference to the accompanying drawings in which

Figure 1 illustrates schematic of typical heat treatment cycle followed during salt bath bainitising treatment.

Figure 2 illustrates the scanning electron micrograph (SEM) of plate sample of nano- structured bainitic structure observed in the developed steel.

Figure 3 illustrates the plate sample of transmission electronmicrograph (TEM) of nano-structured bainitic structure observed in the developed steel.

DETAILED DESCRIPTION OF THE INVENTION:

According to the invention there is provided a medium carbon content nano – structured bainitic steel with improved properties which comprises following percent composition of C: 0.45 – 0.55, Mn: 1.40 –1.50, Si: 1.70 – 1.80, Cr: 1.10 – 1.12 and Mo: 0.15 –0.20, the balance being Fe and unavoidable impurities and which has been bainitised by austenitising at about 850o C for about 30 minutes, followed by
isothermal annealing at about 285o C above Ms temperature for 7 to 10 days.

The said improved properties would include improved weldability as compared with the conventional bainitic steel, enforcement of transformation kinetics with Yield Strength: 1315 – 1471 Mpa, Ultimate Tensile Strength: 1719 -1919 MPa, % Elongation: 10 -14, Hardness (Hv30): 520 -540 and Impact Toughness at - 40oC: 10 -
12 J.

In a specific embodiment the said steel has a percent composition of C: 0.49, Mn:

1.46, Si: 1.78, Cr: 1.12 and Mo: 0.17, the balance being Fe and unavoidable impurities and the steel has the Yield Strength: 1919 Mpa, % Elongation: 14 % and the Impact Toughness at - 40o C : 12 J.

The invention includes a process for producing medium carbon nano -structured bainitised steel which comprises bainitising the steel with percent composition C:
0.45 –0.55, Mn: 1.40 -1.50, Si: 1.70 – 1.80, Cr: 1.10 -1.12, Mo: 0.15 – 0.20, the balance being Fe and unavoidable impurities and by austenitising at about 850o C

for about 30 minutes , followed by isothermal annealing at about 285oC above Ms temperature for 7 to 10 days.

The invention also includes rails manufactured from the said nano-structured bainitic steel.

Specially designed steel having chemical composition in the range of C: 0.45-0.55%, Mn: 1.40-1.50%, Si: 1.70-1.80%, Cr: 1.10-1.12%, Mo: 0.15-0.20%, the balance being Fe and unavoidable impurities and was melted and hot rolled to 8 & 12 mm thick plates using the experimental rolling mill. Samples of size 10 mm dia. x 85 mm length were prepared from 12 mm thick plates for dilatometry test in Thermo-mechanical simulator (Gleeble-3500 C) to determine critical transformation temperatures (Table I).Salt bath bainitising treatment of plates was done by austenistising followed by isothermal annealing above Ms temperature for 7 to 10 days (schematic of typical heat treatment cycle followed is shown in Figure 1).

Subsequent to isothermal annealing treatment, samples were air-cooled till room temperature. Heat treatment parameters followed for salt bath bainitising treatment are shown in Table II. Properties of the plates (Table III) were found to vary in the range of YS: 1315-1471 MPa, UTS: 1719-1919 MPa, % El: 10-14%, Impact toughness at -40o C : 10-12 J, Hardness (Hv30) : 520-540.Scanning Electron Micrograph (SEM) of plate sample is shown in Figure 2.

It can be observed that there is an alternate layer of bainitic ferrite and austenite phase, inside a austenite grain. Since it is difficult to distinguish interspersed bainitic ferrite and austenite phase in SEM micrograph because of refinement of the structure, TEM analysis (Figure 3) of the plate samples was also done to measure the bainitic ferrite lath size. Bainiticlath size was found to vary in the range of 90 –
120 nm. The finest lath size of ~90 nm was observed for plate, which was bainitised at 280o C for 10 days, while coarse bainitic lath of size 120 nm was observed for the plate, which was subjected to bainitising treatment at 285o C for 7 days. Best combination of properties was achieved for steel plate, which was austenitised at
850o C for 30 min. followed by salt bath bainitising treatment at 285o C for 10 days

resulting in formation of bainitic lath size of ~90 nm leading to YS : 1471 MPa, UTS:

1919 MPa, % El: 14%, Impact toughness at -40o C: 12 J, Hardness (HV30) : 540.

Table I: Transformation temperatures determined through dilatometry

Ac1 temp., o C Ac3 temp., o C Ms temp.. o C
773 834 270

Table II: Heat treatment parameters for Salt bath bainitising treatment

Austenitisation

Temp., o C Austenitisatio

n Time, min. Bainitising

Temp., o C Bainitising

Time., days
850 30 285 7-10 days

Table III: Properties achieved in plates after heat treatment

Property Salt bath bainitising at 285o C for
7 days 8 days 9 days 10 days
YS, MPa 1315 1326 1346 1471
UTS, MPa 1719 1737 1800 1919
% El (G.L=25mm) 10 12 14 14
Impact toughnes at -40o C 10 10 12 12
Hardness, HV30 520 528 532 540

HIGHLIGHTS OF THE INVENTION:

? Through proper design of steel chemistry and salt bath bainitising parameters, nano-structured bainitic structure interspersed with austenite phase

could be engineered in a medium carbon steel, which resulted in ultra high strength properties.

? Design of suitable steel chemistry (without addition of costly elements like Co) and proper austenitisation temperature and time resulted in enhancing the transformation kinetics (7-10 days) of nano-bainitic structure vis-a–vis conventional high carbon nanobainitic steels (10-15 days).

? Innovative alloy and process design to achieve comparable mechanical properties in a medium carbon (~0.50%) nano-structured bainitic steel vis-à-vis that achieved in conventional nano-baintic steel having carbon content of ~1.0%

Production of the above steel can be made in a steel plant having facilities like plate rolling mill and heat treatment facilities comprising austenitisation furnace, quenching facilities and salt bath furnace.

ACHIEVEMENTS OFTHE INVENTION

I) Establishment of the process technology for the production of ultra-high strength nano-structured bainite in a medium carbon steel in a cost-effective way which has applications in manufacturing rails and other articles.

II)Innovative alloy and heat treatment cycle design to achieve formation of bainitic lath size of ~90 nm leading to YS: 1471 MPa, UTS: 1919 MPa, % El: 14%, Impact toughness at -40o C : 12 J, Hardness (HV30) : 540in steel with percent composition
C: 0.49, Mn: 1.46, Si: -1.78, Cr: -1.12 and Mo: -0.17.

III) Enhancement of transformation kinetics through judicious selection of alloying elements and refinement of austenite grain structure.

IV) Improved weldability of the developed steel due to lower carbon content of the steel vis-à-vis conventional nano-baintic steel.