| 摘要 |
PROBLEM TO BE SOLVED: To provide a low alloy steel sheet excellent in bore expandability, workability for fine blanking and heat treatment property and used for various machine parts. SOLUTION: This low alloy steel sheet has a composition by weight of 0.15-0.50% C, <=0.30% Si, 0.3-1.0% Mn, <=0.03% P, <=0.01% S, 0.01-0.15% Ti, 0.0005-0.0050% B, <=0.01% N, 0.02-0.10% Al, and 0-0.5% Cr and in which P content and B content satisfy the relation of [%p]<=6×[%B]+0.005. The average grain size of carbides which are dispersedly precipitated in the steel is regulated to 0.4-1.0μm and also their spheroidizing ratio is regulated preferably to >=90%. Further, the surface roughness Ra of the steel sheet is <=5μm.
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