| 摘要 |
PROBLEM TO BE SOLVED: To obtain a bar wire rod excellent in punching workability by hot- rolling and cooling a cast or steel slab specifying component composition, a graphitization index and the degree of central segregation of carbon under specified conditions. SOLUTION: A cast or steel slab which has a composition consisting of by weight, >1.00-1.50% C, 1.00-2.80% Si, 0.01-2.00% Mn, <=0.050% P, <=0.10% S, <=0.0050% O and the balance Fe with inevitable impurities and in which a graphitization index CE determined by the equation is >=1.30 and also the degree of central segregation of carbon, [C]/[C]0 (where [C] is carbon content in an object position and [C]0 is carbon content at ingot steel analysis), is 1.01-2.00, is prepared. The slab is then heated to 850 to 1,150 deg.C, hot rolled and cooled down to 800 to 600 deg.C in >=5 min. By this method, graphite of >=1.0μm average grain size is precipitated by >=100 pieces/mm2, the metallic structure is composed of >=20% ferrite and the balance cementite, further, the hardness in the cross-sectional central part becomes not higher than that in the cross-sectional intermediate part and the hardness in the cross-sectional intermediate part becomes <=HY300.
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