To provide a material for spring having high strength, such as ≥1,900 MPa tensile strength and high ductility, and a manufacturing method thereof.
The material for spring is composed of an iron-based alloy containing prescibed components, and has the area-ratio of the internal structure in any cross section, whose tempered martensite is 30-80%, lower bainite is 5-70% and retained austenite is 8-15%, and the average carbon concentration in the retained austenite is 1.0-2.0 wt.%. The manufacturing method has the following steps in order in which an austenizing step with the temperature of >Ac3 point to ≤(Ac3 point+250°C), a quenching step of cooling at ≥20°C/sec cooling rate and holding at (Ms point-200°C) to ≤Ms point during 10-60 sec, an isothermal transformation step of heating at ≥10°C/sec rate and holding at >Ms point to ≤(Ms point+70°C) during 90 to 3,600sec, and a cooling step of cooling down to the room-temperature.
ONO YOSHIKI
KUROKAWA SHINPEI
SHIBAIRI KOSUKE
