PURPOSE: To prevent the strain ageing of N and C and to obtain cold forging steel excellent in strain ageing resistance by applying the prescribed heat treat ment to a steel in which respective contents of C,Si, Mn, P, S, N, SolAl, B, Ti, and Fe are specified.

CONSTITUTION: A steel having a composition consisting of, by weight, ≤0.45% C, ≤0.35% Si, 0.2-2% Mn, ≤0.04% P, ≤0.025% S, ≤100ppm N, 0.03-0.09% SolAl, either of ≥0.001% B and ≥0.02% Ti or a combination of ≤0.0005% B and 0.01% Ti, and the balance Fe is refined. After the above steel is hot- rolled, overageing treatment is carried out at 200-500°C in the course of cooling or after cooling, by which age hardening ratio is regulated to ≤5% and strain ageing resistance is improved. The above age hardening ratio can be represented by an equation [where T.S' means tensile strength (kgf/mm2) of age-treated material after wiredrawing and T.S means tensile strength (kgf/mm2) in as wiredrawn state].