Microalloyed steels for high-strength forgings

In the past thirty-five years, two families of microalloyed (MA) steels have been developed for high strength barand forging applications. The first family was introduced in 1974 and represented the medium carbon steelsto which were added small amounts of niobium or vanadium. These early medium carbon contents steelsexhibited pearlite-ferrite microstructures and showed good strength and high-cycle fatigue resistance.About 15 years later, microalloyed multiphase steels were introduced, which had microstructures comprisedof mixtures of ferrite, bainite, martensite, and retained austenite, depending on the compositionand processing. These steels were capable of reaching very high strengths, with good fatigue resistanceand high fracture resistance. Prior to the early 1970s, high strength forgings could be obtained only by finalheat treatment, involving reheating, quenching and tempering (QT). It has been shown repeatedly that the aircooled forgings made from MA pearlite-ferrite steels can exhibit strengths and fatigue resistances similarto those of the more expensive heat treated forgings. This paper will follow the developmentof the microalloyed pearlite-ferrite steels over the past 35 years