Microstructure and Mechanical Properties of X70 Pipeline Steel with Excellent Deformability

The relationship between microstructure and properties after the critical zone accelerated cooling for X70 pipeline steel was investigated through thermal simulations,mechanical property tests and microscopic analysis methods. The results show that( B + F) dual-phases microstructure can be obtained by critical zone accelerated cooling. With the decrease of the starting cooling temperature,the content of bainite decreases and the content of ferrite increases,which leads to a decrease of the yield strength and a raise of plasticity. When the starting cooling temperature is 840 ℃,a lower yield ratio,a higher uniform elongation and a strain hardening index can be obtained,which accords with the technical requirements of high deformation pipeline steel. The bainite which has acquired fine laths,and the ferrite with a high dislocation density,can be obtained by critical zone accelerated cooling,which offers a higher strength-toughness and a good deformability of experimental steel.