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.