Effect of Precipitation Characteristics on Mechanical Properties and Stretch Flangeability of Nano‑Dispersion Strengthened High Strength Ferritic Steel

Hot-rolled high strength steel with ferrite matrix and nano-size precipitation is one of the novel advanced ferrous grades at present. This grade of steel, namely NPS800 (Nano-precipitation strengthened 800 grade) fnds application in automobile industry for the manufacture of long member parts of large-capacity vehicles. Apart from the minimum strength level of 800 MPa and ductility over 16%, the steel requires adequate stretch fangeability to prevent a catastrophic failure in service. While the grade achieved the desired tensile strength and ductility as per the stipulated specifcation, there were incidences of erratic stretch fangeability. Detailed microstructural characterization has been carried out to explore the reasons for variations
in stretch fangeability. The study reveals that there is a direct correlation between nature of precipitate distribution and size with stretch fangeability. Pre-dominant interphase-controlled precipitation below~12 nm average size is desirable to obtain desired stretch fangeability. To achieve the typical characteristic of precipitation, Ti/Mo atomic ratio plays an important role.