Abstract: A hot work tool steel or hot work tool contains, in mass %, 0.25 to 0.45% of C, 0.1 to 0.4% of Si, 0.5 to 0.9% of Mn, 0.2 to 0.6% of Ni, 4.9 to 5.5% of Cr, 1.9 to 2.3% of Mo or 1/2W by itself or 1.9 to 2.3% of (Mo+1/2W) in combination, 0.6 to 0.9% of V, and a balance of Fe and impurities, and value A and value B calculated by the following Formula 1 and Formula 2 satisfy that value A is 6.00 or more and value B is 1.00 or less. Value A=?0.7(% Si)+1.5(% Mn)+1.3(% Ni)+0.9(% Cr)+0.6(%(Mo+1/2W))+0.3(% V)??Formula 1: Value B=1.9(% C)+0.043(% Si)+0.12(% Mn)+0.09(% Ni)+0.042(% Cr)+0.03(%(Mo+1/2W))?0.

Abstract: The hot work tool steel or hot work tool contains, in mass %, 0.25 to 0.45% of C, 0.1 to 0.4% of Si, 0.5 to 0.9% of Mn, 0 to 0.6% of Ni, 4.9 to 5.5% of Cr, 1.3 to 2.3% of Mo or 1/2W by itself or 1.3 to 2.3% of (Mo+1/2W) in combination, 0.6 to 0.9% of V, and a balance of Fe and impurities, and value A and value B calculated by the following Formula 1 and Formula 2 satisfy that value A is 6.00 or more and value B is 1.00 or less. Value A=?0.7(% Si)+1.5(% Mn)+1.3(% Ni)+0.9(% Cr)+0.6(%(Mo+1/2W))+0.3(% V) ??Formula 1 Value B=1.9(% C)+0.043(% Si)+0.12(% Mn)+0.09(% Ni)+0.042(% Cr)+0.03(%(Mo+1/2W))?0.

Abstract: A method for evaluating the corrosion-fatigue life of a steel material includes: bringing a corrosive medium into contact with a predetermined portion of the steel material; measuring strain on the steel material while applying cyclic loads to the steel material; measuring an N-th strain index from strain amplitude at the number of initial load applications and strain amplitude at the N-th load application (N is a natural number larger than the number of initial load applications); and estimating the corrosion-fatigue life of the steel material by comparing the N-th strain index with a previously certified life strain index.

Abstract: A method for evaluating the corrosion-fatigue life of a steel material includes: bringing a corrosive medium into contact with a predetermined portion of the steel material; measuring strain on the steel material while applying cyclic loads to the steel material; measuring an N-th strain index from strain amplitude at the number of initial load applications and strain amplitude at the N-th load application (N is a natural number larger than the number of initial load applications); and estimating the corrosion-fatigue life of the steel material by comparing the N-th strain index with a previously certified life strain index.

Abstract: Disclosed is a hot-working tool steel having improved toughness and high-temperature strength. Also disclosed is a method for producing the hot-working tool steel. The hot-working tool steel comprises the following components (by mass): C: 0.34-0.40%, Si: 0.3-0.5%, Mn: 0.45-0.75%, Ni: 0-0.5% (exclusive), Cr: 4.9-5.5%, (Mo+1/2W): 2.5-2.9% (provided that Mo and W are contained singly or in combination), and V: 0.5-0.7%, with the remainder being Fe and unavoidable impurities. Preferably, the cross-sectional structure of the hot-working tool steel upon quenching contains a granular structure and an acicular structure, wherein the granular structure (A %) accounts for 45 area % or less, the acicular structure (B %) accounts for 40 area % or less, and the remaining austenite (C %) accounts for 5 to 20 volume %.