Nippon Steel Corporation

Isuzu Motors Limited

Kouji Oosato

Yutaka Neishi

Wataru Nagai

Yorimasa Tsubota

Hirohito Eto

Yoshitomi Yamada

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 mmor less, the number of coarse Mn sulfides having an equivalent circular diameter of 3.0 &#x3bc;m or more is within a range of 1.0 to 10.0 per mm, and the number of oxides is 15.0 per mmor less.0.42&#x2264;Mo+3V&#x2264;1.50&#x2003;&#x2003;(1)V/Mo&#x2265;0.50&#x2003;&#x2003;(2)

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Chitose, Japan

Kouji Oosato