Heat treatment modifies flat steel's microstructure, altering strength, hardness, and toughness. Annealing (slow cooling from 800-900°C) softens steel by forming coarse ferrite-pearlite grains, increasing ductility (elongation up to 30%) but reducing strength. Normalizing (air cooling after heating) refines grains, balancing strength (tensile 450-550 MPa) and toughness. Quenching and tempering (heating to 850°C, water-quenching, then tempering at 200-600°C) produces martensite, boosting hardness (up to 50 HRC) and tensile strength (1000+ MPa) while reducing brittleness. These processes are critical for tailoring flat steel to applications: annealed for forming, normalized for structural use, and quenched/tempered for wear-resistant parts like machinery guides.