PURPOSE: To produce a steel in which the toughness in the weld heat affected zone is more improved by specifying the compsn. of a steel and incorporating Ti oxides in which the grain size and the number of grains are specified therein.
CONSTITUTION: This steel has a compsn. contg., by weight, 0.03 to 0.18% C, ≤0.50% Si, 0.40 to 2.0% Mn, ≤0.035% P, 0.0010 to 0.0050% S, 0.004 to O.020% Al, 0.005 to 0.025% Ti, 0.0020 to 0.0060% N, and the balance Fe and contg. Ti oxides in which the grain size is regulated to ≤0.01μm and the number of grains is regulated to 2×107 to 5×109 pieces/mm3. In this steel, the sites in which TiN is precipitated is increased, and the number of TiN to be precipitated is increased. As a result, the refining of the T grains subjected to pinning by many TiN is made possible. Moreover, the oxides themselves pin the boundaries of γ to stop the movement of the boundaries. Furthermore, the steel may be incorporated with ≤1.0$ Cu, ≤3.8% Ni, ≤0.050% Nb, ≤0.1% V, each ≤0.6% Cr and Mo and 0.0002 to 0.0020% B as well.
ITO AKIRA
ISHIKAWA TADASHI
