Dilatation curve of ferritic-bainitic dual-phase (FBDP) steel alloy

The proposed Ferrite- Bainite Dual Phase (FBDP) steel is suitable for automotive industries. The steel satisfies high specific strength (strength/weight ratio), and high strength-ductility which are positively reflected on both fuel consumption and crashing resistance. FBDP steel is supposed to satisfy not only the successful properties of FMDP steel but also fulfills the flanging formability and excellent toughness especially on the heat affected zone (HAZ). Both properties became essential for processing hinged and reinforcement parts in the automotive. The microstructural constituents of FMDP are the main reasons for such property deficiencies. The present article is dealing with the optimum parameters controlling processing of the FBDP steel and characterization of the proposed steel.
Three steel alloys were prepared by melting and casting as Y-blocks (35x200x300 mm). The alloys contain 0.1% C. Silicon varies between 0.26 to 0.3%, while chromium varies between 0.7 to 0.9%. Specimens for hot forging were prepared to simulate the roughening deformation step. Forging was carried out at the temperature range 1200-800 °C. Round bars with 12 mm diameter and 10 mm thickness flat strips were obtained by forging.
The microstructure changes and allotropic transformations during thermo-mechanical simulation are investigated. A series of heating – cooling cycles to detect the critical and allotropic transformation temperature by dilatation were carried out on the thermo-mechanical simulator (Gleeble 3500). On the other hand, five – consecutive hits were used during the physical simulation of hot rolling process. Two hits were representing the roughening stage followed by three ones representing finish rolling. Holding at 500°C for 5, 7, 10, 12 and 15 min after last hit has been applied and then followed by air cooling.
Dilation curves appear that Ac1= 766 °C, while Ac3 was detected as 883 °C. Baintic allotropic transformation temperatures were clearly noticed as 618 °C for Bs and 542 °C for Bf. The recrystallization temperature was also detected as 1035 °C. Holding for 5-7 min. at 500 °C was concluded as the optimum for creation a bainite volume fraction. Rough hot deformation at higher temperature above the recrystallization temperature is essential, where no strain hardening and possibility for achieving high strains without excessive loads. Finishing deformation at temperature lower than Tr would create fine bainitic structure.

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