Non-tempered high tensile steel with excellent toughness of the base metal and weld heat-affected zone

摘要

PROBLEM TO BE SOLVED: To improve the toughness of the base material and weld heat-affected zone, by dispersing TiN into steel in a specified state and moreover dispersing oxide inclusions contg. specified amounts of Ti oxide, Ca oxide, REM oxide and Al2O3 therein.;SOLUTION: This steel has a compsn. in which the contents of C, Si, Mn, P and S are controlled to the optimum ones, moreover contg., by weight, 0.005 to 0.08% Ti, 0.0020 to 0.0100% N, 0.0010 to 0.0200% REM, 0.0010 to 0.0200% Ca and Al:Ti%/≤5, and, in which the value of Ceq(%) expressed by the formula is controlled to 0.36 to 0.45% as well. The content of each element in the formula is denoted by weight%. Then, into the steel, TiN of ≤0.05 μm size equivalent to a circle is dispersed by ≥1×103 pieces/mm, TiN of 0.03 to 0.20 μm size equivalent to a circle by 1×103 to 1×105 pieces/mm2, and moreover, oxide inclusions having a compsn. contg., by weight, 20 to 90% Ti oxide, Ca oxide and REM oxide by 5 to 50% in total and ≤70% Al2O3 are dispersed therein.;COPYRIGHT: (C)2001,JPO