High-Strength Pipe Steel Structure Formation During Thermomechanical Treatment

The effect of low-alloy steel type X70-X80 composition and thermomechanical treatment (TMT) parameters on the main processes of austenite structure formation and final intergranular ferrite-bainite structure, and also sheet and coiled rolled product properties are studied. A favorable effect is demonstrated for an increase in Nb content in steel (up to 0.09–0.10%) in limiting grain growth and a shift in the temperature range for austenite partial recrystallization (T95–T5) into a higher temperature region. Rolling in the partial recrystallization range causes formation of a varied grain structure containing extended areas of lath bainitic ferrite, as a result of which there is a reduction in low-temperature toughness and cold resistance. The effect of adding Mo, Ni, Cr, and Cu on the nature of steel X70 phase transformations is provided. Different types of Nb(C, N) and (Nb, V)(C, N) particle precipitates within austenite and ferrite are shown. Results are summarized for previous studies.