Abstract: An upper member of a steel piston has a chemical composition which consists of, in mass %, C: 0.15 to 0.30%, Si: 0.02 to 1.00%, Mn: 0.20 to 0.80%, P: 0.020% or less, S: 0.028% or less, Cr: 0.80 to 1.50%, Mo: 0.08 to 0.40%, V: 0.10 to 0.40%, Al: 0.005 to 0.060%, N: 0.0150% or less, O: 0.0030% or less, and the balance: Fe and impurities, and satisfies Formula (1) and Formula (2), in which, at a cross section parallel to the axial direction of the upper member, the number of Mn sulfides is 100.0 per mm2 or less, the number of coarse Mn sulfides having an equivalent circular diameter of 3.0 ?m or more is within a range of 1.0 to 10.0 per mm2, and the number of oxides is 15.0 per mm2 or less. 0.42?Mo+3V?1.50??(1) V/Mo?0.

Abstract: An upper member of a steel piston has a chemical composition which consists of, in mass %, C: 0.15 to 0.30%, Si: 0.02 to 1.00%, Mn: 0.20 to 0.80%, P: 0.020% or less, S: 0.028% or less, Cr: 0.80 to 1.50%, Mo: 0.08 to 0.40%, V: 0.10 to 0.40%, Al: 0.005 to 0.060%, N: 0.0150% or less, O: 0.0030% or less, and the balance: Fe and impurities, and satisfies Formula (1) and Formula (2), in which, at a cross section parallel to the axial direction of the upper member, the number of Mn sulfides is 100.0 per mm2 or less, the number of coarse Mn sulfides having an equivalent circular diameter of 3.0 ?m or more is within a range of 1.0 to 10.0 per mm2, and the number of oxides is 15.0 per mm2 or less. 0.42?Mo+3V?1.50??(1) V/Mo?0.

Abstract: A steel for machine structural use, essentially having the following chemical composition:C: 0.45-0.60 wt %,Si: 0.50-2.00 wt %,Mn: 0.10-0.30 (0.30 not inclusive) wt %,P: 0.01-0.10 wt %,S: 0.01-0.20 wt %,V: 0.08-0.15 wt %, andN: 0.0020-0.0050 (0.0050 not inclusive) wt %. The remainder is Fe and impurities inevitably included. The inner structure of the steel is a ferrite-pearlite structure.