HIGH STRENGTH STEEL PLATE EXCELLENT IN STRETCH-FLANGING PROPERTY AND DELAYED FRACTURE RESISTANCE

摘要

PROBLEM TO BE SOLVED: To provide a high tensile strength steel plate having sufficient required characteristics even in use requiring stretch-flanging properties and delayed fracture resistance like an automotive steel plate and free from surface defects caused by cluster-shaped inclusions. ;SOLUTION: This steel plate is the one having a compsn. contg., by weight, 0.05 to 0.20% C, ≤2.0% Si, 1.8 to 3.5% Mn, 0.015 to 0.2% Ti, ≤0.01% Al, ≤0.02% N and one or two kinds of Ca and REM by 0.0005 to 0.1% in total, in which the total content of S and one or two kinds of Ca and REM satisfies the relation of S-5×(32/40)Ca+(32/140)REM≤0.0014% as well, also, Mn and Ti satisfy the relation of Mn/Ti≤200, oxide inclusions of 1 to 50 μm maximum size contain Ti oxides and one or two kinds of CaO and REM oxides and having a composite structure in which the volume ratio of martensite and bainite is 30 to 100%.;COPYRIGHT: (C)2000,JPO