Nippon Steel Corporation

Isuzu Motors Limited

Isuzu Motors Limited and Nippon Steel Corporation

Hirohito Eto

Yutaka Neishi

Hiromasa Takada

Yorimasa Tsubota

Wataru Nagai

Osamu Ohyama

Kouji Oosato

Tetsuroh Hashiguchi

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)

Steel piston

A steel for machine structural use, essentially having the following chemical composition:

Steel for machine structural use and machine parts made from such steel

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Fujisawa, Japan

Hirohito Eto