What is "mushet steel"

Mushet steel, also known as Robert Mushet's Special Steel (RMS), self-hardening steel and air-hardening steel, is considered the first tool steel and air-hardening steel. It was invented in 1868 by Robert Forester Mushet. Prior to Mushet steel, steel had to be quenched to harden it. It later led to the discovery of high speed steel.