Microalloyed steels for high-strength forgings

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