PURPOSE: To obtain a steel having excellent cold workability as hot-rolled by subjecting a steel in which C, Si, Mn and Al are specified to finish rolling and the subsequent gradual cooling under prescribed conditions.
CONSTITUTION: Steel constituted of, by weight, 0.1 to 0.4% C, ≤0.3% Si, 0.2 to 1.7% Mn and 0.01 to 0.1% Al, and the balance Fe is melted. This steel is subjected to finish rolling at Tc+30°C to Tc-30°C (where Tc(°C)=865-155×C %+-11.5×Si%-33.8×Mn%-8×Cr%+2.8×Mo%-21.8×Ni%+13.7×V%) and at 79 to 95% reduction of area. Next, it is gradually cooled from the start of ferritic transformation to the completion of pearlitic transformation at ≤15°C/min cooling rate. Then, the average grain size of ferritic grains and pearlitic grains over the wire size (mm) ×0.1 to 8 (mm) wire sizeX0.3 from the surface layer is regulated to ≤10μm.
TASHIRO HITOSHI
SATO HIROSHI
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