STEEL MATERIAL SUPERIOR IN TOUGHNESS OF WELD HEAT-AFFECTED ZONE AND FATIGUE CHARACTERISTICS OF BASE METAL, AND METHOD FOR MANUFACTURING THE SAME

摘要

PPROBLEM TO BE SOLVED: To provide a steel material which shows superior HAZ toughness even when having been welded at such a high heat input as an amount of 50 kJ/mm or more, and besides, has improved fatigue characteristics of a base metal itself. PSOLUTION: The steel material has following characteristics of: (a) including an oxide containing Zr, a REM and Ca; (b) having the composition satisfying 5-50% ZrOSB2/SB, 10-50% oxide of a REM and 5.0-50% CaO, when the compositions of all oxides have been measured and converted into the composition of single oxide; (c) containing 120 pieces per 1 mmSP2/SPor more of oxides with circle equivalent diameters of 0.1-2.0 m, and 5 pieces per 1 mmSP2/SPor less of oxides with circle equivalent diameters of more than 5.0 m; (d) having a metal structure in which an average circle equivalent diameter D of crystal grains that are surrounded by a large-angle grain boundary having a crystal orientation difference of 15 degrees or more is 25 m or less, and a ratio R of random grain boundaries occupying in the large-angle grain boundaries is 70 area% or less, when observed with an EBSP method; and (e) having an average hardness of 170 Hv or more. PCOPYRIGHT: (C)2010,JPO&INPIT