They are steels with a minimum sulfur content of 0.1%. They are carbon steels that are alloyed with sulfur or with sulfur and lead. They have a high content of manganese to form manganese sulphides (SMn).
Both SMn and lead get the lubrication of the cutting tool and the breaking of the chip that comes off the steel in the machining process, so that it allows high speeds of the tool in the machining by chip removal.
Their most common applications are for screws and bolts