Alekk: Motorcycle helmets are made from different materials but steel is not one of them.
The conventional motorcycle helmet has two principal protective components: a thin, hard, outer shell typically made from polycarbonate plastic, fiberglass, or Kevlar and a soft, thick, inner liner usually made of expanded polystyrene or polypropylene "EPS" foam.
The idea behind the helmet is to absorb energy of fall, that means some crumple like energy absorbing bumper on cars nowadays. Fancy plastics, fiberglass and foam fillings that are dent resistant to a point, them crack while absorbing enery in cracking are better protection for falling off riders. There are a few 'Vanity' helmets with a metal outer shell and some riders will use the old army surplus helmets with their inner web shock absorber netting, a few police depts many years back issued metal riot helmet to their trike riders and 2 wheeler traffic officers- but some at the time had their motor officers wearing soft uniform caps like foot patrol officer – same uniform as patrol cars. Protection from bullets some, low speed drops also- but higher speed crash meant energy directed into skull or broken necks. Current safety laws mean non metal motorcycle helmets are approved and therefore legal in states requiring 'approved' by safety foundation or DOT for rider. Locally the helmets are optional for licensed riders- permit students are require to have helmet but older full license it is optional and the metal Viking Helmet with metal or cow horns, army surplus and old Wehrmacht helmets sometimes seen- Russian, French, Prussian, and the WW1 British Tommies as well. Better protection than bare head or leather WW1 flying caps also legal. your head, your state laws, your choice
No, the job of a helmet is to slow the speed of your head and consequently your brain at impact. The impact with a steel helmet would stop your head too quickly and the brain smashes into the inside of the skull – killing you or worse turning you into a non-edible vegetable.
In a sword fight with another motorcyclist but anymore swords are not that popular so your best protection would be full-body armor and a bullet-proof helmet.
The squishy stuff inside your skull offers the best protection.
Never seen one.