| 摘要 |
PURPOSE:To obtain an unrefined steel bar having a ferrite-pearlite structure and showing high strength and toughness comparable to those of a conventional refined material as hot rolled by adding Mn, V, etc., to a medium carbon steel and regulating the carbon equiv. to a specified value. CONSTITUTION:A medium carbon steel ingot cong., by weight, 0.30-0.055% C, <0.30% Si, 0.60-2.00% Mn, 0.06-0.30% V, 0.020-0.060% sol. Al and 0.0020-0.060 N and having >=0.60% carbon equiv. represented by a formula Ceq=%C+Si/7+%Mn/5+%Mn/0+%V is heated to >=1,050%oC and hot rolled at 650-800 deg.C finishing temp. A high strength and high toughness unrefined steel bar having a ferrite-pearlite two-phase structure as hot rolled is obtd. The steel bar has ferrite grain size No. >=10, >=(0.5-0.63X%C) ferrite ratio related to the C content, >70kgf/mm<2> tensile strength and >=15kgfm/cm<2> Charpy impact value at ordinary temp.
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