Production of high tensile Nb-V microalloyed, 20-40mm thick pipe steel plates through controlled rolling

Abstract

The authors have examined a Nb- V microalloyed steel annealed at 1120°C and controlled rolled to 20-40 mm thick, the finish rolling temperatures T_F ranging between 805° and 680°C. Mechanical properties were studied as well as microstructure. The steel plates showed high strength combined with good low-temperature toughness. The 40 mm plates met the requirements of grade X60, American Petroleum Institute Standard 5LX. The transverse impact toughness in the Charpy V-notch test was >50J at -60° C and the 50% fracture appearance transition temperature was -75°C. A distinct change in microstructure and mechanical properties occurred when the finish rolling temperature was reduced from about 800° to 680° C. Depending upon the temperature for the final passes, the following terms were adopted in the discussion of the results:

(i) normal controlled rolling (T_F > 800° C)

(ii) slight thermomechanical rolling (T_F =800°-750°C)

(iii) heavy thermomechanical rolling (T_F < 750°C).

The third type of rolling produced the best strength-toughness combination.