Short answers, pulled from the story.
Steel is an alloy of iron and carbon, with a carbon content not far exceeding 2%. For plain carbon steel the carbon ranges between 0.02% and 2.14% by weight, and other elements such as chromium, manganese, nickel, and vanadium are added to produce different grades.
Steel keeps its carbon below about 2.1%, while plain iron-carbon alloys above 2.1% carbon are cast iron, which is not malleable even when hot. Wrought iron, now largely obsolete, holds less than 0.1% carbon but large amounts of slag, around 1 to 2%.
The earliest known steel appears in ironware excavated at Kaman-Kalehoyuk in Anatolia, nearly 4,000 years old and dating from 1800 BC. Wootz steel was later developed in Southern India and Sri Lanka in the 1st millennium BC.
Henry Bessemer introduced his process in 1855, beginning the modern era of mass-produced steel from pig iron. Later the Linz-Donawitz process of basic oxygen steelmaking, developed in 1952, made the same quantity of steel in one-twelfth the time of the open hearth process.
Stainless steel contains a minimum of 11% chromium, which forms a hard oxide on the metal surface that resists corrosion, and typical stainless steels contain 18% chromium. It is often combined with nickel, and corrosion-resistant steels are abbreviated CRES.
More than 1.6 billion tons of steel are produced annually, with China producing 54% of the world's steel in 2023. Steel is one of the most-recycled materials, with a global recycling rate over 60%, and steelmaking contributes about 8% of global emissions.
Low-background steel is steel manufactured before 1945, used for radiation-sensitive applications such as Geiger counters and radiation shielding. Steel made after World War II is contaminated with radionuclides because production uses air carrying radioactive dust from nuclear weapons testing.