Stretch Flangeability of Low Carbon Micro-alloyed Ferrite-Pearlite and Ferrite-Bainite Steel

The application of low carbon micro-alloyed steel sheets in chassis and frame parts of automobiles demands high formability during hot or cold forming operations to produce various intricate shapes. In view of the forming applications, stretch flangeability is considered as one of the most important critical parameters for these steel grades. The stretch-flangeability of micro-alloyed steels, with three different types of microstructure consisting of mainly single-phase ferrite, ferrite-pearlite and ferrite-bainite micro-constituents, is evaluated in this investigation based on hole expansion ratio (HER). The desired microstructures of the low carbon steels micro-alloyed with Nb, Nb-V and Nb-V-Ti steels were obtained at three different coiling temperatures by systematically varying the plant operating process parameters. While Micro-alloying elements largely affect the mechanical strength and ductility of the steel, its direct impact on HER value and fracture behavior are not correlated. The correlation of microstructure with tensile strength and ductility have been attempted for the studied low carbon micro-alloyed steels and described in this paper. It is observed that single-phase steel consisting of soft ferritic matrix as well as steel with 5 to 15 pct pearlite uniformly distributed in ferrite matrix has better stretch flangeability and strength to hole expansion ratio correlation in comparison to ferrite-bainite steel.