Development of Nonleaded Free Machining Steel for Automotive Parts

We have developed a lead-free, free-machining 0.1 - 0.8% C steel for machine structural use, which demonstrates machinability almost equivalent to lead-containing free-machining steels. This free-machining steel is characterized by controlling the sulfide shape by Ca-Al deoxidation, and, unlike conventional Ca-deoxidized steels (1) and Al-deoxidized CaS free-machining steels (2), its machinability is improved with the aid of manganese-calcium sulphides which precipitate wrapping calcium-aluminate oxides. In order that these duplex inclusions generate, it is essential that the activity balance between [O] and [S] be controlled to 15 ≦ a_S/a_O ≦ 100. The free-machining steel which satisfies this activity balance has an outstanding machinability and fatigue strength, which are equivalent to lead containing steels. It has been under consideration to apply this novel free-machining steel to automotive parts such as crank shafts and connecting rods.