FIELD OF THE INVENTIQNl
This invention relates to method for preparing high strength steel with ultra fine grained microstructure.
BACKGROUND OF THE INVENTIQN:
There are various applications of steel such as line pipes, construction etc for which high yield strength are required even with a low or moderate elongation value. Among the various strengthening mechanism, the grain refinement is the only method to improve both strength and toughness simultaneously. The main benefit of this approach is to avoid costly alloying elements, avoid additional heat treatment and improve weldability owing to lower alloying content. Ferrite refinement achieved by the conventional controlled rolling method results in grain size more than 5 pm, whereas the efforts are on in many institutions and laboratories to produce ultra fine grain size below 3 urn using various techniques but yet to achieve in actual rolling condition. The objective of the present work was to study the formation of ultrafine grain microstructure by designing suitable thermo-mechanical parameters in a micro-alloyed steel in laboratory scale and apply the process to experimental rolling to achieve the same.

OBJECTS OF THE INVENTION:
An object of this invention is to propose a method for preparing high strength steel with ultra fine grained microstructure;
Another object of this invention is to study the formation of ultra fine grain microstructure by designing suitable thermo-mechanical parameters in a micro-alloyed steel;
Further object of this invention is to avoid costly alloying elements;
Still further object of this invention is to avoid additional heat treatment and improve weldability owing to lower alloying content.
SUMMA8Y OF THE INVENTION:
The present invention is provided with a method for preparing high strength steel with ultra fine grained microstructure comprising the steps of heating the steel material @ 5°-15°C/s to a temperature of 11400-11700C; soaking at 1140°-11700C for sufficient time (@ 1 inch thickness per hour); cooling the steel material @ 1°-5°C/s from 11400-1170°C to a temperature 780°-850°C; applying a compressive strain (≥ 1.3) at a slow strain rate (0.01-0.1/s); quenching the steel material from 780°-800°C in water at ambient temperature or air cooling.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1: Schematic diagram of the thermo-mechanical cycle applied.
Fig. 2: (a) Optical and (b) SEM microstructures of the steel after quenching in water without deformaiion.
Fig. 3: Optical microstructure after thermo-mechanical simulation deformed at a stain rate of O.l/s followed by quenching.
Fig. 4: Scanning electron microstructure after thermo-mechanical simulation deformed at the stain rate of O.01/s and 0.1/s followed by quenching.
Fig 5: Grain size distribution of the ferrite as estimated using SEM for steel 2 (a) 780°C-0.01/s.
Fig. 6: (a) Optical and (b) SEM microstructures of the rolled steel sheet (Rolling A).
Fig. 7: Optical microstructures of the rolled steel sheet (Conventional Rolling A).
BRIEF DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION~
The transformatinn temperatures of the steel were determined from these dilatation curves. Ac1 and AC3 temperatures were found to be 744°C and 877°C, whereas Ar3 and Ar1 temperatures at 2°C/s were found to be 748 and 653°C, respectively.

The method of thermo-mechanical treatment to achieve ultra-fine grained micro-structure is described as follows:
a) Heating the steel material @ 5°-15°C/s to a temperature of 1140°-1170°C.
b) Soaking at 11400-1170°C for sufficient time (@ 1 inch thickness per hour).
c) Cooling the steel material @ 1°-5°C/s from 11400-1170°C to a temperature 780°-850°C.
d) Applying a compressive strain (~ 1.3) at a slow strain rate (0.01-0.1/5).
e) Quenching the steel material from 780°-800°C in water at ambient temperature or air cooling.
The microstructure of the quenched sample without prior deformation, which reveals a fully coarse martensitic structure as shown in Fig. 2. High amount of deformation above AC3 temperature (782°-800°C) were applied during thermo-mechanical treatment, the temperature of deformation (Td) were decided as 780°C, 800°C and 850°C, which fall in the range of Ar3 and Ac3. The microstructures of the samples quenched after deformation reveals a very fine ferrite and martensite microstructure as shown in Fig. 3. Some optical microstructures for strain rates of 0.1/5, and 0.01/5, whereas the SEM microstructures. These reveal predominantly martensitic microstructure with a varying amount of fine ferrite phase as shown in Fig. 4. Formation of ultra fine grained ferrite during heavy deformation appears to be due to dynamic transformation. Heavy deformation at low temperature yields in high amount of dislocation density, deformation bands and defects leading to more nucleation site for transformation. Amount of ferrite varies with Td and strain rate. A maximum amount of ferrite was found to be around 50% for Td=780 C and 0.01/5 with grain size between 1-4 urn. A typical grain size distribution for Td=780 C for 0.01/s. It reveals an average ferritic grain

size of 2.7 Mm with a grain size distribuiion of ~30% with <2 μm, ~70% with <3 μm and ~95% with <4 μm. At higher strain rates or higher temperature of deformation, lower amount of ferrite was noted.
To explore the possibility of producing fine grained ferrite during rolling, the forged plate of this steel was hot rolled in experimental rolling mill followed by air cooling. The optical and SEM microstructures of the steel rolled (rolled in one pass with E=1.3 with a finish rolling just above to Ar3). It shows a mixed microstructure of very fine (more than 70% < 3 μm) with some coarse (4,5 μm) elongated pancake ferrite- pearlite microstructure as shown in Fig. 6. The average grain size of the ferrite was < 3 μm as against the coarser ferrite-pearlite microstructure with an average much higher grain size for convention rolling as shown in Fig. 7. The tenslle properties of the steel are compared in Table 2. The steel results in a high strength than that of conventional rolling. However, the elongation was found to be inferior than that of conventionally rolled materia..
There are various applications of steel such as line pipes, construction etc for which high yield strength are reqUired even with a low or moderate elongation value. Steel produced in this way can be used for this type of application due to very high yield strength alongwtth a good toughness values.
Table 1: Chemical analysis steel (25 kg induction furnace heat) taken under investigations.


Table 2: Mechanical properties of steel rolled in experimental rolling mill and then heat-treated inter-critical range.


WE CLAIM:
1. A method for preparing high strength steel with ultra fine grained
microstructure comprising the steps of:
- heating the steel material to a temperature range of 1140°-1170°C;
- soaking the heated steel for sufficient time;
- cooling the steel material to a temperature range of 780°-850°C;
- applying a compressive strain at a slow strain rate (0.01-0.1/s);
- quenching the steel material in water at ambient temperature or air cooling.

2. The method as claimed in claim 1 wherein the internal grain size produced ranging from 1-4 ~m.
3. The method as claimed in claim 1 wherein a grain size distribution of 30% with <2 μm, ~70% with <3 μm and 95% with <4 μm at Td 780°C for 0.01/s.
4. The method as claimed in claim 1 wherein the chemical composition of steel is as follows:
%C -1.06-0.08 %Mn -1.5-1.6 %Si - 0.2-0.4 %S - 0.01-0.015 %P - 0.01-0.015 %(Nb,Ti,V)- 0.04-0.12

5. The method as claimed in claim 1 wherein the steel material achieves a tensile strength of 640-661 Mpa.
6. The method as claimed in claim 1 wherein the steel material increases elasticity strength by 13-15%.
7. The method as claimed in claim 1, wherein the application of a mechanical strain at 780°-800°C converts the internal grain into a mixed microstructure of ultra fine ferrite and martensite after quenching or fine ferrite and pearlite after air cooling resulting a high strength property of steel material.
8. The method for preparing high strength steel with ultra fine grained microstructure as substantially described and illustrated with accompanying drawings.