Research on Heat Treatment Process of Low Mn High Nb Sour Resistance Pipeline Steel

The mechanical properties and microstructure of high Nb pipeline steel after tempering at 500 ~680 ℃ were studied. The results indicated that the mechanical properties achieved the optimal value after tempering at 600 ℃,compared with the steel at rolled status. The yield strength was increased by 105 ~130 MPa and tensile strength was enhanced about50~70 MPa. The toughness and elongation varied slightly. The microstructure type before and after tempering keep the same,which were mainly acicular ferrite;With tempering temperature increased, the ferrite lath gradually was merged,M/A islands and carbides were obviously decomposed, and polygonal ferrite was evolved. After 600 ℃ tempering,the pinning dislocation phenomenon was obvious,and the increase of fine precipitates are the main reason for strength increase and plasticity improvement.