Dual phase (DP) steels have been investigated using a new approach utilizing simple cold-rolling and subsequent intercritical annealing of a martensite-ferrite duplex starting structure. The ferrite grain size and volume fraction of martensite were varied by changing the rolling reduction and intercritical annealing time. Ultrafine grained DP (UFG-DP) steel with an average grain size of about 2 μm was achieved by short intercritical annealing of the 80% cold–rolled duplex microstructure. Tensile testing revealed superior mechanical properties (the ultimate tensile strength of 1100 MPa and elongation of 13%) for the new DP steel in comparison with the commercially used high strength DP980 steel. The variations in hardness, strength and elongation of the specimens with rolling reduction and intercritical holding time were correlated to microstructural characterizations. The inherent mechanism for strengthening of the DP steel was discussed and the contribution of each strengthening factor was quantitatively calculated. The results showed that the calculated yield strength (430.6 MPa) is very close to the measured value (422 MPa).
2015 by The Iron and Steel Institute of Japan