Summary
Highlights
Structural steel is a key material in construction and automotive industries, its improvement driven by the need for lighter, more fuel-efficient vehicles. Physical metallurgy principles have been effectively used to enhance its strength, leading to new grades of steel since the 1980s.
Steels are categorized into killed, semi-killed, and rimming steels based on their deoxidation process. Rimming steel, with dissolved oxygen, exhibits a vigorous reaction during solidification, which can compensate for shrinkage but makes it brittle. Killed steels, fully deoxidized, prevent this reaction but result in a large shrinkage cavity. Semi-killed steels offer a balance between the two.
Increasing carbon content in steel boosts yield strength, ultimate tensile strength, and hardness, but reduces ductility. Steel specifications, like Indian Standard (IS), AISI, and British Standard (BS), use alphanumeric codes to denote composition and properties. These systems help identify the type and alloying elements present in steel.
While several strengthening mechanisms exist, such as solid solution strengthening, strain hardening, grain refinement, and precipitation hardening, structural steels often have limitations due to their low carbon and alloy content. Solid solution strengthening involves adding solute atoms to hinder dislocation movement, while strain hardening increases dislocation density. Grain refinement is crucial for improving strength without sacrificing toughness.
Most strengthening mechanisms that increase steel's strength tend to decrease its ductility and toughness, and raise its ductile-to-brittle transition temperature. However, grain refinement is unique as it improves yield strength while also lowering the ductile-to-brittle transition temperature, enhancing both strength and toughness.
Grain size in steel can be controlled through cold working followed by annealing, or through hot working processes. Cold working increases the surface area for nucleation, leading to finer grains after recrystallization. Hot working, where plastic deformation and recrystallization occur simultaneously, requires precise control of finishing temperatures to achieve a fine grain structure, thus improving both strength and toughness.