| 摘要 |
PROBLEM TO BE SOLVED: To obtain excellent strength, toughness, and cold workability by specifying a composition and forming a graphite-containing fine mixed structure after rolling and annealing. SOLUTION: This steel material has a composition consisting of, by weight ratio, 0.20-0.80% C, <=0.15% Si, 0.60% Mn, <=0.03% P, <=0.2% Ni, <=0.2% Cr, <=0.3% Mo, <=0.1% Al, <=0.01% B, <=0.02% N, <=0.1 Nb, <=0.5% V, and the balance Fe with inevitable impurities and also has a structure which consists of ferrite of JIS crystalline grain size No.5 to 10, graphite of <=20μm maximum diameter, and cementite and in which graphite is distributed by >=100pieces/mm<2> . At the time of producing this steel material, the steel is heated to 900-1200 deg.C, rolled at >=20% draft at 700-1100 deg.C finishing temp., cold-drawn at 10-30% reduction of area, and then annealed in a temp. region between 650 deg.C and the Ac3 point for >=1hr, by which a fine mixed structure can be obtained.
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