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Thermomechanical processing of a Nb–Ti–V pipeline steel

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In this research, thermomechanical processing of a high-strength low-alloy steel was simulated through hot torsion testing. Average schedule and continuous cooling torsion after the actual schedule were used to determine the critical temperatures of the tested steel. Also, a simplified actual schedule with 79 % reduction in the austenite pancaking region was applied to investigate the effects of the temperature range of the finish rolling stage and the cooling rate on the microstructure and mechanical properties of the produced steels. Two different rolling conditions (with the temperature ranges of finish rolling of 950 to 840 °C and 910 to 800 °C) with two different cooling rates of 7.5 and 15 °C/s were examined. In this way, four different steels were produced and characterized by metallography. It was observed that a decrease in the temperature range of finish rolling and an increase in the cooling rate lead to an increase in the volume fraction of acicular ferrite and a decrease in the grain size. Repeated thermomechanical processing schedules followed by room temperature torsion testing were conducted to evaluate the strength of the steels produced. The results demonstrated that a decrease of 40 °C in the temperature range of finish rolling leads to about a 77-MPa increase in yield strength. Also, it was observed that by increasing the cooling rate from 7.5 to 15 °C/s, the yield strength increases by about 45 MPa. Finally, the relationship between yield strength and grain size of the produced steels was determined according to the Hall–Petch relation.

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Correspondence to M. Rakhshkhorshid.

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Rakhshkhorshid, M., Monajati Zadeh, H. & Hashemi, S.H. Thermomechanical processing of a Nb–Ti–V pipeline steel. Int J Adv Manuf Technol 79, 1623–1631 (2015). https://doi.org/10.1007/s00170-015-6952-2

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  • Microalloyed steel
  • Controlled rolling
  • Accelerated cooling
  • Austenite transformation
  • Hot torsion testing