Study On Microstructure And Properties Of （B+M/A） Pipeline Steel With Excellent Deformability

High deformability pipeline steel is a preferred material of steel pipe for transporting oil and gas in geological disaster area, such as earthquake, landslide, permafrost regions and so on which is a significant development within oil and gas pipelines in recent years. X80 pipeline steel containing bainite and M/A constituent can be obtained by a novel HOP(heating online partitioning) technology. Compared with traditional X80 pipeline steel, the(B+M/A)X80 pipeline steel has good combination of high strength-toughness and the favorable ductility with a lower yield/tensile ratio and higher uniform elongation, which meets the technical requirements of high deformability linepipe and is suitable for the environmentally severe regions such as permafrost and seismic regions. The relationship between technological parameters microstructurcs and mechanical properties of(B+M/A)X80 pipeline steel with excellent deformability is established. The full scale test of <I> 1219mmx22.0mm(B+M/A)X80 submerged arc welded pipe was applied, which provide theoretical basis for engineering application of the(B+M/A)X80 pipeline steel with large deformability.Research results indicate that the multiphase structure of bainite and M/A constituent can be obtained for X80 pipeline steel through different ACC stop temperature of HOP. With the ACC stop temperature increasing, the strength decreases and the elongation raises of the trail steels, which arc ascribable to the coarsening of bainite laths, the lower bainite content and dislocation density in bainite and higher the residual austenite content. After normalization at higher ACC stop temperature, martensite is forming, moreover, both carbides precipitation and retained austenite decomposition arc the key microstructure factors that result in the strength raises and the elongation falls of the steel. This(B+M/A) dual-phase structure at difierent ACC stop temperature makes the lower yield ratio, higher uniform elongation and strain hardening index for the experimental steel, which meets the technical requirements for large deformation.As the partitioning temperature increasing, the strength of the experimental steel decreases and the plasticity increases. When the partitioning temperature is high, the slight increase of strength and the decrease of plasticity. Good ductility, toughness and excellent deformability can be obtained at different partitioning temperatures.With the partitioning temperature increasing, the wider bainite lath, the lower dislocation density, the more content of retained austenite and carbides coarsening can be obtained, leading to the decrease of strength and the increase of ductility. When the partitioning temperature is high, the decrease of the volume content and stability of retained austenite is the key factor influencing the increase of strength and the decrease of plasticity.Based on the theory of the HOP, a new type of coiling continuous partitioning process was developed recently. The results show that with the coiling temperature increasing, the strength of the experimental steel decreases and the ductility increases because of the decrease of bainite content and dislocation density, and the increase of retained austenite content. When the coiling temperature is high, the precipitation of carbides and the decomposition of retained austenite consist the key factor influencing the increase of strength and the decrease of plasticity. This(B+M/A) dual-phase structure at different coiling temperature makes the lower yield ratio, higher uniform elongation and strain hardening index for the experimental steel, which meets the technical requirements of high deformability linepipe.The regression equations between structural parameters and mechanical properties of(B+M/A)X80 pipeline steel were established by Excel based on experimental results. Proven for these regression equations coincide with objective laws. They leads to the decrees of strength and the increases of plasticity by increasing the distribution of M/A(decreasing λ) and the length of grain boundary(increasing k) within certain range. The influence of the distribution of M/A on the mechanical properties of the steel with excellent deformability is more obvious and it is an important factor to improve the properties of the steel. The smaller diameter of M/A(globular or punctuate) endow the experiment steel with excellent deformation mechanical properties.The full scale test of Ф1219mm×22.0mm(B+M/A)X80 submerged arc welded pipe was applied. The(B+M/A)X80 welded pipe has good combination of high strength-toughness and deformability as a result of the bainitic, M/A component and the dispersed precipitation of carbides, which meets the technical requirements of large deformability linepipe.