The weld heat affected zone having excellent toughness of steel

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Provided is a steel having excellent toughness in a heat affected zone (HAZ) even upon high heat input welding with a heat input of 60 kJ/mm or higher. (a) The steel includes an oxide containing Zr, at least one rare-earth element, and Ca. (b) Of all inclusions in the steel, inclusions with an equivalent circle diameter of 0.1 to 2 µm are present in a number density of 120 or greater per square millimeter of an observation field area, and oxides with an equivalent circle diameter greater than 3 µm are present in a number density of 5.0 or less per square millimeter of an observation field area. (c) The inclusions in the steel have chemical compositions meeting a condition specified by Formula (1) : Insol . Ti - 3.4 × Insol . N / Insol . Al = 1.0 to 8