Description:PLEASE SEE THE ATTACHMENTS. , Claims:We claim:
1.
A high stretch-flangeability hot rolled steel sheet, comprising:
alloying composition of:
carbon (C) more than of 0.03 wt.%, but less than 0.08 wt.%,
manganese (Mn) more than of 0.4 wt.%, but less than 1.1 wt.%,
silicon (Si) less than 0 to 0.2 wt.%,
sulphur (S) less than 0 to 0.005 wt.%,
phosphorus (P) less than 0 to 0.02 wt.%,
aluminum (Al) more than 0.02 wt.%, but less than 0.07 wt.%,
nitrogen (N) less than or equal to120ppm,
niobium (Nb) more than 0.03 wt.%, but less than 0.06 wt.%, and
iron (Fe) being remainder of the composition along with incidental elements; and
wherein, the high stretch-flangeability hot rolled steel sheet exhibits ultimate tensile strength 540-600 MPa and stretch-flangeability with more than or equal to 100% Hole Expansion Ratio (HER).
2.
The high stretch-flangeability hot rolled steel sheet as claimed in claim 1, wherein microstructure of the high stretch-flangeability hot rolled steel sheet comprises at least 50% Bainite phase and, less than 5% of a secondary phase in the microstructure, and remaining being ferrite phase along with traces of Nb (C, N) precipitates.
3.
The high stretch-flangeability hot rolled steel sheet as claimed in claim 2, wherein average grain size of combination of the ferrite phase and the bainite phase microstructure of the high stretch-flangeability hot rolled steel sheet ranges from 5-8 microns in size.
4.
The high stretch-flangeability hot rolled steel sheet as claimed in claim 2, wherein the ferrite phase of the microstructure includes mixed type structure being at least one of polygonal ferrite and Baintic ferrite.
5.
The high stretch-flangeability hot rolled steel sheet as claimed in claim 1, wherein the high stretch-flangeability hot rolled steel sheet exhibits total elongation 28.0 26 %-31 % and tensile Strength (TS) 540-600 MPa.
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6.
The high stretch-flangeability hot rolled steel sheet as claimed in claim 1, wherein the high stretch-flangeability hot rolled steel sheet comprises gamma fibre oriented in {554} <225>, {332} <113>, {111} <121>, (112) [5-92] orientations.
7.
A method for manufacturing a high stretch-flangeability hot rolled steel sheet, the method comprising:
casting, a steel slab comprising an alloying composition of:
carbon (C) more than of 0.03 wt.%, but less than 0.08 wt.%,
manganese (Mn) more than of 0.4 wt.%, but less than 1.1 wt.%,
silicon (Si) less than 0.2 wt.%,
sulphur (S) less than 0.005 wt.%,
phosphorus (P) less than 0.02 wt.%,
aluminum (Al) more than of 0.02 wt.%, but less than 0.07 wt.%,
nitrogen (N) less than or equal to120ppm,
niobium (Nb) more than of 0.03 wt.%, but less than 0.06 wt.%,
iron (Fe) being remainder of the composition 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 rolling to a second predetermined temperature to produce a steel sheet;
rapid cooling, the hot rolled steel sheet to a third predetermined temperature at a predetermined cooling rate; and
coiling, the steel sheet at a third predetermined temperature, to obtain a high stretch-flangeability hot rolled steel sheet,
wherein, the high stretch-flangeability hot rolled steel sheet exhibits ultimate tensile strength 540-600 MPa and stretch-flangeability with more than 100% of Hole Expansion Ratio (HER).
8.
The method as claimed in claim 7, wherein microstructure of the high stretch-flangeability hot rolled steel sheet comprises at least 50% Bainite phase, less than 5% of any other phases in the micro-structure of a secondary phase and remaining ferrite phase along with traces of Nb (C, N) precipitates.
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9. The method as claimed in claim 7, wherein average grain size of combination of the
ferrite phase and the bainite phase microstructure of the high stretch-flangeability hot
rolled steel sheet ranges from 5-8 microns in size.
10. The method as claimed in claim 7, wherein the high stretch-flangeability hot rolled steel
sheet exhibits total elongation 26% - 31% and tensile Strength (TS) 540-600 MPa.
11. The method as claimed in claim 7, wherein the casting is a continuous casting process.
12. The method as claimed in claim 7, wherein the steel slab is hot charged into a furnace
for heating.
13. The method as claimed in claim 7, wherein the first predetermined temperature ranges
from 1100 °C to 1200 °C, and the first predetermined time is in a range of 15 minutes
-30 minutes.
14. The method as claimed in claim 7, wherein the second predetermined temperature of
the steel slab for the hot rolling process is higher than critical transformation
temperature for austenite (Ar3).
15. The method as claimed in claim 14, wherein the second predetermined temperature
ranges from 850℃ to 900℃.
16. The method as claimed in claim 7, wherein thickness of the steel sheet reduced after
the hot rolling process is 90% - 96% of initial thickness.
17. The method as claimed in claim 7, wherein the third predetermined temperature range
is less than 470-400 ℃ and the predetermined cooling rate ranges from 20-`40℃/s.
18. Automotive body panel comprising a high stretch-flangeability hot rolled steel sheet as
claimed in claim 1.
Dated this on 29th day of February 2024