Abstract:

In the investigation, the microstructural observation and quasistatic tension experiments of X80 pipeline steel were conducted so as to reveal the effects of the cooling rate on its quenched dual-phase microstructure formation and strain-hardening behavior. The results show that (1) a dual-phase microstructure including ferrite and bainite is obtained after X80 pipeline steel is quenched; (2) the ferrite volume fraction and the uniform elongation increase with the decrease of the cooling rate while the strength decreases; (3) when the ferrite volume fraction is not less than 5.27%, the strain-hardening exponent has a positive correlation with the uniform elongation but a negative correlation with the yield ratio; (4) when the ferrite volume fraction reaches up to 22.8%, the work-hardening rate decreases with a fast stage and then a slow stage, and the latter stagethe later stage delays the necking point; and (5) with the decrease of the cooling rate, the lath-like bainite in the quenched microstructure turns to be granular and MA phase forms, simultaneously, the strain-hardening performance of the quenched microstructure is guaranteed and the uniform plastic deformation ability is improved by means of the conversion of the dislocation strengthening to the second phase strengthening.

Key words: X80 pipeline steel, quenching, ferrite, bainite, dual-phase microstructure, strain-hardening characteristic, uniform plastic deformation