it promotes precipitation of coarse carbide namely fe 23 cb 6 in the austenite the lower limit of b content is therefore defined as 5 mass ppm or 0 77 u00d7 n mass ppm u2212 17 4 mass ppm whichever is greater and the upper limit is defined as 50 mass ppm the contents of the impurities p and s are not particularly defined but from the viewpoint of achieving good ductility the content of each is preferably 0 02 mass or less similarly to in conventional extra fine steel wires although the steel wire rod used in the present invention has the aforesaid elements as its basic components one or more of the following optional additive elements can be positively included in addition for the purpose of improving strength toughness ductility and other mechanical properties cr 0 03 to 0 5 mass ni 0 5 mass or less co 0 5 mass or less v 0 03 to 0 5 mass cu 0 2 mass or less mo 0 2 mass or less w 0 2 mass or less and nb 0 1 mass or less where the content ranges of ni co cu mo w and nb do not include 0 mass explanation will now be made regarding these elements cr 0 03 to 0 5 mass as cr reduces lamellar spacing it.