Patent ID: 7220325

Claim:
High-strength steel suitable for making line pipe and pressure vessels, the steel characterized by: a) a steel composition comprising: at least about 0.01 and no more than about 0.1% wt. carbon; at least about 0.03 and no more than about 0.12% wt. niobium; at least about 0.008 and no more than about 0.03% wt. titanium; at least about 1.4 and no more than about 1.9% wt. manganese; at least about 0.1 and no more than about 0.5% wt. molybdenum; a maximum phosphorus content of about 0.02% wt.; a maximum sulfur content of about 0.015% wt.; a maximum nitrogen content of about 0.015% wt.; and the balance being iron (Fe) and incidental impurities; b) a two phase microstructure having an increased dislocation density comprising about 30% polygonal ferrite and about 70% acicular ferrite with an average grain size of no more than about 5 μm; and c) precipitates containing niobium with a precipitate particle size of no more than about 5 nm, and further characterized by a process for producing the steel, which comprises: d)heating the steel at a selected dissolving temperature to dissolve substantially all the niobium in the steel; e) deforming the steel a first time while the steel is in the austenite region; f) cooling the steel at an accelerated rate to a temperature of about 350° C. to about 450° C.; and g) deforming the steel a second time, at a temperature of about 350° C. to about 450° C., to introduce dislocations in the crystal structure of the steel so as to increase the kinetics of precipitation, to, in turn, maximize the precipitation of the niobium particles to a particle size of no more than about 5 nm.