Patent ID: 7429301

Claim:
An ultra clean spring steel provided in the form of a wire, wherein: the wire comprises oxide inclusions with a sulfur concentration no more than 10 mass %; no less than 70% of the inclusions, in terms of numbers, that exist in an outer layer outside one quarter of a diameter of the wire and have a width no smaller than 3 μm, satisfy formula (1) CaO+Al 2 O 3 +SiO 2 +MnO+MgO>80mass % (1) and satisfy two or three of formulae (A) to (C) (A) SiO 2 :40-70 mass %, Al 2 O 3 : 0-20 mass %, CaO: 20-60 mass %; (B) SiO 2 : 30-65 mass %, Al 2 O 3 : 25-50 mass %, CaO: 10-30 mass %; (C) SiO 2 : 10-30 mass %, Al 2 O 3 : 25-50 mass %, CaO: 30-55 mass %; the width of an inclusion is a diameter of the inclusion measured in a direction perpendicular to a long axis of the inclusion, when the inclusion is observed in a cross section including a longitudinal axis of the wire; and concentrations in formulae (A) to (C) are normalized so that a total concentration of the three components SiO 2 , Al 2 O 3 , and CaO amounts to 100 mass %.