| 摘要 |
PROBLEM TO BE SOLVED: To provide a rolled steel bar having uniform cold workability in a cross section in an as-rolled state and capable of being manufactured without requiring any special equipment and also to provide its manufacturing method. SOLUTION: The rolled steel bar has a composition which consists of, by mass, 0.30 to 0.45% C, 0.10 to 0.40% Si, 0.50 to 0.90% Mn, 0.80 to 1.30% Cr, 0.10 to 0.30% Mo and the balance Fe with inevitable impurities and in which the value of Ceq represented by Ceq=[C]+[Si]/7+[Mn]/5+[Cr]/9+[Mo]/2 (where [X] is the content (mass%) of an element X) is made to 0.60 to 0.82. Moreover, this steel bar has a structure composed of ferrite and pearlite, and further, the difference between the average grain size of old austenite grains in a part at a depth, from the surface, of D/8 of an outer diameter D and that in a part at a depth of D/2 is made to 1.5 or less. COPYRIGHT: (C)2006,JPO&NCIPI
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