Paris - US President Donald Trump has raised fears of a trade war after he announced punitive tariffs on two the world's most strategic metals: steel and aluminium.

Following are facts about these metals and which countries are the key producers and purchasers.

- Steel -

-- The production of steel -- iron enriched with carbon -- virtually launched the industrial revolution at the end of the 19th century. Today there are at least 3,500 different types of steel, distinguished by the amount of carbon, and the addition of other chemical elements.

-- Among steel companies, Arcelor Mittal in 2016 was the world's leader producing 95 million tonnes. Five of the 10 largest companies were Chinese.

-- The main importer of steel is the United States, followed by Germany and China, according to United Nations data in 2016. The US mainly imports from Canada, Brazil and South Korea.

-- Steel is especially used in building construction and automobile manufacturing. It is also used in shipping, machinery and military equipment. Steel is found in tin cans and other objects in everyday life.

-- The cost of so-called hot rolled steel is around $700 per tonne in Europe, according to the latest estimates in the weekly Metal Bulletin of Unicredit bank.

-- The worldwide workforce in steel production is around five million people, according to the industry's main union Worldsteel.

- Aluminium -

-- Aluminium is made from the aluminia extracted from bauxite rock. Frenchman Paul Heroult and American Charles Martin Hall discovered separately in 1886 the process for producing aluminium on an industrial scale.

-- China produced nearly half of the world's 63 million tonnes of aluminium in 2017. The rest of Asia produced 3.9 milion tonnes and Western Europe 3.7 million tonnes, according to main producers' association World Aluminium.

Aluminium output broken down by continents, with changes since 1999

--  The world's largest aluminium companies are Russia's Rusal, US giant Alcoa and numerous Chinese firms.

-- Like steel, the United States if the number one importer of aluminium, followed by Germany and Japan, according to 2016 United Nations data. The US imports the metal from Canada as well as China and Russia.

-  Aluminium is a sought-after metal because it is light -- three times lighter than steel -- and it is resistant to corrosion. It is also a conductor of heat and electricity. It is mainly used in construction, manufacturing of cars and airplanes, in high tension lines, and also food packaging.

-- On the London Metal Exchange, a tonne of aluminium was worth nearly $2,078.50 on March 20.

By Yann Schreiber