Steel material excellent in toughness and base metal fatigue characteristics of weld heat-affected zone and its manufacturing method

摘要

The invention provides a steel material, wherein (a) the steel material comprises oxides containing Zr, REM and Ca; (b) all the oxide compositions are measured and commuted as the single oxide, satisfying the following relationship: ZrO2: 5-50%, REM oxide: 10-50%, and CaO: 5.0-50%; (c) the number of the oxide with equivalent circle diameter of 0.1-2.0Mum in each 1mm2 is more than 120 and the number of the oxide with equivalent circle diameter exceeding 5.0Mum in each 1mm2 is more than 5; (d) when the metal structure of the steel material is observed using a back scattered electron diffractionmethod (EBSP method), the average equivalent circle diameter D of the granule encircled by a large corner grain boundary with crystal orientation difference of below 15 degree is below 25Mum and the rate R of the random grain boundary to the large corner grain boundary is below 70%; (e) the average hardness is above 170Hv. Even thought performing the master line energy welding with line energy ofabove 50kJ/mm, the HAZ toughness is also excellent and the self fatigue characteristic of the base material is improved.