| 摘要 |
PURPOSE: A manufacturing method of a steel for spring is provided to meet confirm Rockwell Hardness of less than 25 HRC, which is hardness standard required in machining operation. CONSTITUTION: The manufacturing method of a steel for spring includes the steps of (i) heat treating a steel wire rod comprising C 0.4-0.7wt.%, Si 2.0-3.5wt.%, Mn 0.3-0.8wt.%, Cr 0.3-0.8wt.%, Nb or V 0.01-0.3wt.%, Ni 0.1-5.0wt.%, O 0.0015wt.% or less, N 0.005-0.03wt.%, P 0.01wt.% or less, S 0.01wt.% or less, a balance of Fe and other inevitable impurities at a temperature range of 780 to 820°C for 1-5 hours; and (ii) cooling the steel wire rod at a cooling rate of 5°C/sec or less. The steel for spring is characterized in that a portion of spheroidized structure is 30 to 60% in the steel texture and the nodule size of ferrite structure is less than 15μm.
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