Analysis on Microstructure and Properties of X70 Large Deformation Pipeline Steel after Delaying Accelerated Cooling

The change rule of microstructure and properties after delaying accelerated cooling(DAC) for X70 pipeline steel was investigated through thermal simulation, mechanical property test and microscopic analysis method. The results showed that bainite +ferrite(B+F) dual-phases microstructure can be obtained by DAC. With the increasing of starting cooling temperature, the content of bainite increases and content of ferrite decreases, which lead to increase of yield strength and reduce of plasticity. When starting cooling temperature is 530 ℃, lower yield ratio, higher uniform elongation and strain hardening index can be obtained, which accord with the technical requirements of high deformation pipeline steel.