| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel plate which is superior in FB workability and further in formability after the FB working step, and to provide a manufacturing method. SOLUTION: A base steel material has a composition comprising, by mass%, 0.1-0.5% C, 0.5% or less Si, 0.7-2.0% Mn, 0.03% or less P, 0.02% or less S, and the balance Fe with unavoidable impurities; has a structure in which an average particle diameter of ferrites is 1 to 10μm, a spherodized rate of carbides is 80% or more, and an amount S<SB>gb</SB>of carbides in grain boundaries of ferrites defined by the expression of S<SB>gb</SB>(%)=äS<SB>on</SB>/(S<SB>on</SB>+S<SB>in</SB>)}×100 is more than 40%; and has such a hardness that the difference between mean hardness in the region between the surface and a 10% deep portion of plate thickness and mean hardness in a central part of the plate thickness is 5 to 20 points by Vickers hardness HV. The steel plate may also include 0.2 to 1.5% Mn and further one or two elements selected from Cr and Mo so that the total content of Mn, Cr, Mo and Al can be less than 5.0%. COPYRIGHT: (C)2008,JPO&INPIT
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