| 摘要 |
This invention provides an ultra-high strength low yield ratio line pipe steel which is excellent in HAZ toughness and field weldability and has a tensile strength of at least 950 MPa (exceeding x100 of the API standard). The steel is of a low carbon-high Mn-Ni-Mo-Nb-trace Ti type selectively containing B, Cu, Cr and V, whenever necessary, its micro-structure comprises a martensite/bainite and ferrite soft/hard two-phase mixed structure having a ferrite fraction of 20 to 90%, this ferrite contains 50 to 100% of worked ferrite, and the ferrite grain size is not greater than 5 .mu.m. The production of an ultra-high strength low yield ratio line pipe steel (exceeding x100) excellent in low temperature toughness and field weldability becomes possible. As a result, the safety of a pipeline can be remarkably improved, and execution efficiency and transportation efficiency of the pipeline can be drastically improved.
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