Ferrite Evolution During Isothermal Process in a High Deformability Pipeline Steel

In order to ensure the safety of long-distance oil and natural gas transmission pipeline installed in seismic and/or permafrost region, high strength pipeline steel with excellent deformability has been developed. The ferrite and bainite dual phase pipeline steel is a very important kind of high deformability pipeline steel. Polygonal ferrite is a key microstructure in ferrite and bainite dual phase deformability pipeline steel. Ferrite evolution during isothermal process at 700 ℃ after 50% deformation at 800 ℃ was conducted by using a Gleeble-3800 thermal simulator, and microstructure was characterized by using an optical microscope, a scanning electron microscope and a transmission electron microscope. There are two types of ferrite, ferrite with high density dislocation and ferrite with a little dis location. There is about 7% （volume percent） deformation induced ferrite （DIF） for compression of 50% at 800 ℃ and strain rate of 1 s-1. During the isothermal process at 700 ℃, with the holding time increasing, ferrite volume percent, ferrite grain number and average ferrite grain size increase. As the holding time is prolonged, dislocation re-covery occurs in DIF. There are secondary phases in ferrite when the holding time is too long, and secondary phases and dislocation formation in dislocation pinning.

HREM Observation of Age-Precipitated Particles in Practical Cu-bearing Ultra-Low Carbon Steels

Morphology,distribution and crystal structures of age-precipitated particles in practical Cu-bearing steels with ultra-low carbon were studied by high resolution electron microscopy（HREM）.The age-precipitation behavior was explored when poly-microalloying elements Cu,Nb,Cr,Mo and Ni were used and ε-Cu particles coexisted with carbides（carbonitrides） in steels.The fine dispersed ε-Cu particles and carbides of microalloying element Nb,Cr,Mo being about 10 nm only in size were observed in peak aging specimens and over-aging specimens.The twin structures and stacking faults in the particles and the transient structures formed in the early stage of the particle precipitation were also observed.The crystal structure of three types of age-precipitation particles was firstly identified by lattice streaks in HREM images of the single particles.The experimental method identifying crystal structure of fine and dispersed particles using HREM images of single particle was proposed.