Study On Microstructural Evolution And Mechanical Properties Of X100 High Grade Pipeline Steel

Adopting X100 pipeline steel which has high strength and toughness can effectively increase operation pressure, decrease diameter and wall thickness. It can guarantee transport safety and meanwhile reduce the transport costs effectively. So it is significantly to develop and research on X100 high grade pipeline steel.Under this background, the paper systematically research the TMCP, micro structures and properties of X100 pipeline steel by means of thermal simulation technique, hot rolling experiment, mechanical properties test and microscopic analysis method, to provide practice production with experimental data and new concepts of processing control. The mainly results involved as follows:(1) Grain growth behavior of austenite at different heating temperature and holding time was investigated by thermal simulation test. As the result shows, with the increase of reheating temperature and holding time, the grain size of origin austenite is increasing and is more uniform. Vast particles of secondary phase are in solid solution when the heating temperature rises to 1200℃. Comprehensively considering of the grain size, degree of uniformity and solid solution of secondary phase, the best reheating temperature and holding time to get the fine origin austenite microstructure is 1200℃ and 3min respectively under experimental condition.(2) Thermal simulation test was employed to determine the phase transformation behavior under continuously cooling condition. The static and dynamic CCT curves were drawn based on thermal expansion method and metallographic analysis. As the result shows, with the increase of cooling rate, the microstructures, Quasi-polygonal Ferrite(QF), Acicular Ferrite(AF), Granular Bainite(GB), Lath Bainite(LB), Martensite(M), successively present in matrix. Deformation increases the start temperature of phase transformation, expanses the temperature range of phase transformation and promotes the phase transformation of AF and GB which cause the C curve to move left.(3) Through the thermal simulate experiment and metallographic analysis, the effect of deforming temperature, amount of deformation, cooling rate and finish cooling temperature on the microstructure of test steel were investigated. As the result shows, Deformation temperature and the amount of deformation have much less impact on the final microstructure. With deformation temperature decrease, or the amount of deformation increase, or cooling rate increase, or finish cooling temperature decrease, M/A island dispersed and the size decrease.(4) Adopting two regions rolling process combined with accelerated cooling technique, hot rolling process trial test of X100 pipeline steel was carried out in laboratory. The X100 pipeline steel plate which satisfy the properties requirement can be obtained by controlling of the finish rolling temperature in the range of 810～820℃, cooling rate in range of 25～35℃/s, finish cooling temperature in range of 520～570℃. And the complex microstructure comprise of AF, GB and LB.