Free cutting
steel consists of two types, one for machine
structural use and the other based on low
carbon steel for hard cutting, such as SAE12L14.
Conventionally, Pb has been added to free
cutting steels to improve their machinability,
but Pb in steel is thought to have a harmful
effect on the human body. Therefore, as alternatives
to Pb addition in free cutting steels, JFE
Steel and NKK BARS & SHAPES studied graphite
precipitation in the steel and control of
the sulfide morphology by the chemical composition
and developed new Pb-free free cutting steels
based on these respective concepts. In the
graphite-type free cutting steel for machine
structural use, JFE Steel realized machinability
equal to that of the Pb-added steel by controlling
the state of existence of C in the steel,
and achieved an excellent balance of cold
workability and high fatigue strength after
quenching and tempering. As a substitute for
the low carbon Pb-added free cutting steel
(SAE12L14), NKK BARS & SHAPES developed
a steel with improved machinability by crystallizing
large sulfide inclusions, which is realized
by increasing S addition while also adding
Cr on the basis of the estimation from the
calculated phase diagrams. |