Microstructure Control&Property Optimization Of Pipeline Steel With Excellent Deformability Based On The Different Accelerated Cooling Method

In this paper, the high strength-toughness and excellent deformability of a X80pipeline steel could be acquired by the heat treatment methods of the mild accelerated cooling, critical region accelerated cooling and delayed accelerated cooling. The relationship of techno logy-micro structure-properties of pipeline steel with excellent deformability was established by material microscopic analysis method, mechanical properties test and thermal simulation techniques.The research indicated that the control of accelerated cooling process might improve the microstructure and strength-toughness of X80pipeline steel, and its excellent deformation capacity can be also gained. Using the mild accelerated cooling method of oil cooling medium, the critical region accelerated cooling method of840℃and the delayed accelerated cooling method of530℃respectively,(B+F) dual-phases microstructure could be obtained.(B+F) Microstructure of X80pipeline steel based on the different accelerated cooling method was mainly made up of fine bainite and a certain amount of fine ferrite with high density of dislocations. The (B+F) dual-phases microstructure has high strength-toughness, lower yield/tensile ratio, bigger uniform elongation and higher strain hardening exponent which could meet the requirements of the pipeline steel with excellent deformability.The observation of fracture process of (B+F) pipeline steel showed that there were usually three ways of crack nucleation:inclusion (or second phase particle) nucleation, phase interface nucleation and nucleation in the matrix of ferrite or bainite. Cracks propagate mainly in ferrite. When cracks pass through ferrite, the path of cracks is rectilinear. When cracks pass through bainite, the path of cracks is zigzag. Based on the experimental observation and theoretical analysis, the deformation and fracture models of (B+F) pipeline steel were established.The regression analysis of microstructure and properties of (B+F) pipeline steel with excellent deformability indicated that the strength raised and plasticity decreased with the decreasing of the average free path of the bainite and the increasing of interfacial length of bainite in unit area, or with the increasing of volume fraction of bainite and the decreasing of the average grain diameter of bainite.