STEEL FOR HIGH STRENGTH SPRING, AND HIGH STRENGTH SPRING AND METHOD FOR MANUFACTURE THEREOF

摘要

A steel for a high strength spring which has a chemical composition, in mass %, that C: 0.36 to 0.48 %, Si: 1.80 to 2.80 %, Mn: 0.20 to 1.40 %, P: 0.015 % or less, S: 0.010 % or less, Cu: 0.10 to 0.50 %, Ni: 0.10 to 2.00 %, Cr: 0.05 to 1.20 %, s-Al: 0.005 to 0.040 %, N: 0.002 to 0.012 %, O: 0.002 % or less, and the balance: Fe and inevitable impurities, satisfies the following formulae (1), (2) and (3): 1.2 % = C(%) + Mn(%) + Cr(%) = 2.0 % (1) 1.4 % = Si(%)/3 + Cr(%)/2 + Mn(%) = 2.4 % (2) 0.4 % = Cu(%) + Ni(%) (3), and contains inclusions having a diameter of 10 µm or more in an amount of 10 pieces or less per 100 mm² of a visual field; and a method for manufacturing a high strength spring, characterized in that the above steel for a high strength spring having been refined to have a hardness of HRC52 or more is hot- or cold-formed into a shape of a spring and then the resultant spring is subjected to warm shotpeening, to manufacture a high strength spring exhibiting a maximum shearing stress of 1176 Ma or more. A high strength spring manufactured by the above method exhibits improved resistance to setting due to its hardness of HRC52 or more, and also good durability, and further can be manufactured practically.