Abstract
Advance high strength steel with minimum UTS of 780 MPa is industrially developed utilizing continuous annealing line (CAL) and Gleeble thermo mechanical simulation. An outline of superior elongation, improved strain hardenability, enhanced strength of developed Fe – C – Mn – Si TRIP assisted steel is described. Correlation amid Simulated result and industrially annealed steel are stabilized for microstructure and mechanical property. Annealing condition is optimized for best combination of strength and formability accordingly. CCT diagram for the selected composition from JMatPro is utilized to optimize rapid cooling rate and over aging section temperature. Final microstructure of developed steel comprises tempered martensite, granular bainite with retained austenite distributed in polygonal ferrite matrix. An evaluation of developed TRIP steel is carried out with the help of microstructure and XRD analysis. It was concluded that strain hardening coefficient of new steel is comparable to that of drawing grades attributable to about 13 % retained austenite in microstructure.
Highlights
TRIP assisted steel are in extensive demand for automotive application owing to their excellent strength ductility combination
The strength deformation combination is attributed to TRIP phenomenon where strain-induced phase transformation of retained austenite (RA) renders excellent strain hardenability (n-value) [3]
The improved fatigue life is attributed to relaxation of high stressed field during fatigue by transformation of retained austenite to Martensite [7 – 10]
Summary
Pearlitic microstructure at HR stage is due to high coiling temperature (>550 °C) during hot rolling which is above the bainite start temperature as given in CCT diagram of sleeted steel composition
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