PURPOSE: To reduce abrasion even when high lead gasoline is used, by using a steel material such that it contains specified weight percentages of C and Cr, chrominum carbide is finely dispersed in its basic composition, and its hardness lies within a specified range.

CONSTITUTION: A side rail is constructed of a steel material such that it contains 0.16W1.40wt%, and Cr, 11W19wt%, chromium carbide, Hv 1300 or more, is finely dispersed in its basic composition, and its hardness lies within the range HRc 30W50. By this, it is possible to reduce abrasion since its hardness is high, HRc 30W50, due to the hard carbide of Hv 1300 or above, while in the case of chromium plating, abrasion is considerable due to the lapping action of lead particles since its hardness is insufficient, when high lead gasoline is used. Further, it is possible to adjust the hardness by using in combination surface treatment, such as soft nitriding.