Microstructure and strength in ultrastrong cold-drawn medium carbon steel

•A strongest (4.1 GPa) medium carbon ferrite-pearlite steel wire is produced.•Microstructural evolution and parameters with drawing strains are quantified.•Strengthening mechanisms of medium carbon ferrite-pearlite steel are identified.•Good agreement between estimated and measured flow stresses is found.•Rule of mixture works based on quantified microstructural parameters with strain.