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Reynolds 531 (pronounced 'five-three-one') is a brand name, registered to Reynolds Cycle Technology of Birmingham in the United Kingdom, for a manganese-molybdenum, medium carbon steel bicycle tubing.
Introduced in 1935 and for many years at the forefront of alloy steel tubing technology, 531 tubing has been superseded by more complex alloys and heat-treatment/cold work cycles as Reynolds continues to compete with other manufacturers of steel for the bicycle industry.
The approximate alloying composition of 531 tubing is 1.5% Mn, 0.25% Mo, 0.35% C, and is similar to the old British BS970 En 16/18 steel. Its mechanical properties and response to heat treatment are broadly similar to the AISI 4130 standard alloy steel, also used for bicycle frames, among other applications. This material was used to form the front subframes on the Jaguar E-Type of the 1960s.
Reynolds 531 is now only available to special order, but in the past was used by many bicycle manufacturers for their racing cycles.