| abstract |
Spring consisting of,% by mass, 0.5 to 0.7% of C, 1.0 to 2.0% of Si, 0.1 to 1.0% of Mn, 0.1 to 1.0% Cr, not more than 0.035% P and not more than 0.035% S and the rest of Fe and unavoidable impurities, where the spring has a structure that includes at least 65% bainite and 4 to 4 13% residual austenite by surface ratio in a cross section, where residual austenite contains carbon at an average concentration of 0.65 to 1.7%, where residual austenite has an average grain diameter equivalent to a maximum circle of 3 μm, the structure includes 5 to 30% martensite in surface relation in a cross section, the spring has a residual tension tension layer in a cross section from the surface to a depth of 0.35 mm at D / 4, where D (mm) is a diameter equivalent to a circle of the cross section, the residual compression stress layer has a maximum residual tension of pressure between 800 and 2,000 MPa, and the spring has a central part with a Vickers hardness of 550 to 650 HV in a cross section and has a layer of great hardness with a hardness greater than that of the central part at 50 to 500 HV from the surface to a depth of 0.05 to 0.3 mm. |