Grain Refinement Behavior of Pipeline Steel in Welding Heat Affected Zone

Self refining grain behavior of X80 pipeline steel in the heat affected zone was studied systematically by physical simulation method. The sizes and distribution of the inclusions which promote intra-granular ferrite nucleation were measured using the automatic image analyzer, and the figures of the intra-granular ferrite which has self refining grain behavior were observed using transmission electron microscopes with energy dispersive X-ray spectroscopy, the fracture toughness values of the coarse grain zone in the HAZ were also measured. The results show that the welding thermal cycle has nearly no effect on the sizes and distribution of the inclusions in the HAZ. The reasons of self-refining grain in the HAZ are that the small non-metallic inclusions such as MnO、 TiO、MnS、CuS and （Mn，Cu）S in X80 pipeline steel ，which can induce the nucleation of intra-granular ferrite in the welding process, and sympathetic nucleation of intra-granular ferrite makes the coarse grains refined in the HAZ. First intra-granular ferrite has large boundary energy and high density dislocation, which induce the sympathetic nucleation of intra-granular ferrite.

Effect of Vanadium on Development of Acicular Ferrite Microstructure in Low Carbon Steel

The influence of vanadium on the development of an acicular ferrite microstructure has been investigated in a low carbon steel. Optical and electron microscopy were carried out to identify the precipitates, inclusions and constituents of the acicular ferrite microstructures. By the addition of vanadium, the main microstructural constituent was changed from a side plate ferrite to an acicular ferrite. VCN precipitates, which were known to favor the nucleation of acicular ferrite, were formed on the (Mn,Si) oxide and MnS particles. The presence of vanadium in alloys suppressed the formation of a side plate ferrite and reduced the transformation of ferrite during an isothermal transformation. Nucleation of intragranular ferrite and a subsequent sympathetic nucleation of ferrite within austenite grains were favored in the vanadium containing steel and an acicular ferrite microstructure was developed.