The Effects of Microscopic Metallurgical Factors On Macroscopic Deformation Properties In Dual Phase Steels

Linepipes for high strain applications, such as in seismic or permafrost regions, are required to have sufficient deformability, in order to prevent buckling or weld fracture caused by compressive and/or tensile deformation induced by large ground movement. Extensive efforts have been made in developing linepipes, which have lower yield strength to tensile strength (Y/T) ratio, and higher strain hardening coefficient (nvalue) and uniform elongation. Dual phase (DP) microstructure control is one of the key technologies for obtaining these desired properties. These high strain linepipes, with a ferrite-bainite microstructure, are produced by applying an accelerated-cooling process from the α+γ region. It is well known that the volume fraction of the hard phase, the hardness difference between soft and hard phases (hardness ratio), and microscopic inhomogeneities, such as local strain distribution, affect macroscopic deformation properties in DP steels. However, the effects of microscopic metallurgical factors such as the second-phase (bainite or pearlite), dislocation structure and the micro hardness of each phase on macroscopic deformation properties (Y/T ratio, n-value, uniform elongation, etc.) in DP steels with polygonal ferrite are not as well understood. macroscopic deformation properties are discussed. In both of ferritebainite and ferrite-pearlite DP steels, as the volume fraction of secondphase increased, Y/T ratio decreased. Then, in the case of ferrite-bainite steels, Y/T ratio increased in the samples with more than 60% bainite. Ferrite-bainite steels showed lower Y/T ratios and higher n-values, compared with ferrite-pearlite steels. And they also showed stressstrain curve with round-house type without yield phenomenon. It is believed that these macroscopic deformation properties are related to the differences in movable dislocation densities in the ferrite, near the phase boundaries (ferrite/bainite or ferrite/pearlite). It is also believed that the amount of recovered dislocations, due to thermal annealing in ferrite-pearlite steels, affects the Y/R and n-value.