Lean alloy design for high and highest strength steels

The production of thermomechanically rolled plates is closely related to the microalloying element niobium (Nb). This element ensures both a strength increase and also the much desired combination with high toughness. This paper presents a few examples that highlight the effects of grain refinement and precipitation hardening for line pipe grades in the range of X60 and X70 and shows that the onset of accelerated cooling reduces the effect of Nb on precipitation hardening. The application of a lower carbon, higher Nb-bearing steel influences grain refinement and toughness because rolling takes place at higher temperatures. The benefit of grain refinement for toughness is illustrated for several examples of steel grades in the range of X80 and X100. Furthermore, high strength structural steel grades with yield strengths up to 960MPa have been produced with different levels of Nb. The paper shows data from daily production as well as results from R&D work based on laboratory investigations. (AU)