Are there lightweight metals usable as bearings instead as steel.?

Bearings can be, and are, made of of lightweight metals.  Even plastic and glass.  Depends on the application.

No, ball/roller bearings are very hard steel.

Bronze bushings are also used. They are softer than the steel shaft. They must be lubed sufficiently.

Cobalt bearings. Beware that you don't use Cobalt 60, highly radioactive. Uranium 238 is the shutters.

Journal bearings typically have an alloy called babbit or something similar that is soft and corrosion resistant. 

steel makes a poor bearing material, you want something softer than steel. Then the bearing can be replaced as it wears, instead of the shaft.

I'm excluding rolling bearings, as I assume that is not what this is about.

Plain bearing, consisting of a shaft rotating in a hole. There are several specific styles: bushing, journal bearing, sleeve bearing, rifle bearing, composite bearing;

Aluminum and titanium.  Google "lightweight bearings" to find ads.