Study On Microstructure&Property Of Pipeline Steel With Excellent Deformability Based On The Method Of HOP

In this paper, by material micro structure analysis, mechanical properties test and thermal simulation technology, through the heat treatment process of Heating On-line partitioning (HOP) for the existing traditional pipeline steel to obtain the dual-phases micro structure of the bainite+M/A, which making the traditional pipeline steel has excellent deformability on the basis of maintaining the previous original strength.The relationship between technology micro structure and property of the pipeline steels with excellent deformability is established.Studies on heat treatment technology of the HOP under different stop-cooling temperatures show that the dual-phases micro structure of the bainite+M/A can be obtained under all kinds of stop-cooling temperatures. With increasing the stop-cooling temperature, the strength, hardness, impact toughness and yield/tensile ratio of test steel down, when the elongation at rupture and uniform elongation increase and the strain hardening exponent remains unchanged. When the stop-cooling temperature is350℃, the bainite lath in the dual-phases micro structure is fine, there is the M/A film between the bainite laths and the precipitates of dispersed alloy carbides in the bainite laths; therefore it has more powerful strength combination which meets the operational requirements of pipeline steels with excellent deformability.Studies on different partitioning temperatures show that, with the partitioning temperature of HOP rising, the yield strength, tensile strength and hardness of test steel decrease, and the uniform elongation and elongation at rupture increase. This is mainly because that the carbon supersaturation and lattice distortion in the bainitic matrix decreases with increasing partitioning temperature, when the dislocation density decreases, the bainite lath widens and the type and shape of the carbide precipitation in the bainite change,which cause the reduction in material strength and increasing in plasticity.Studies on mechanical properties of the three different composite micro structures of B+M/A, B+F and B+F+M/A show that, the three composite micro structures all have good strength, toughness and deformation capacity. Studies on structure-properties of pre-strain and strain aging of the the dual-phases micro structure of (B+M/A) show that:pre-strain makes the strength and yield/tensile ratio of test steel higher than that of the base metal and plasticity and toughness lower than the base metal; after strain, the dislocation density of test steel increases which prompting increased material strength and decreased ductility and toughness. The strain aging makes the strength and yield/tensile ratio higher than that of the base metal and plasticity and toughness lower than the base metal; with the strain aging temperature increases, the strength and hardness increase, the ductility and toughness decrease, the yield/tensile ratio increases and the strain hardening exponent decreases. Compared with ordinary X80steel, X80steel of (B+M/A) dual-phases has a lower tend of strain aging.Through regression analysis of the micro structure parameters and mechanical properties of the bainite+M/A dual-phases micro structure, the relationship between the microstructure parameters of the B+M/A and strength-ductility is established.