A bit of both. We add small quantities of other metals (like manganese, chromium and aluminium) and these have an effect on the crystal structure of the steel. By playing around with different heat treatments such as heating up and cooling to different temperatures, with different heating and cooling rates (speed) and for different times, we can get different mixtures of the phases (parts of the structure with one type of crystal structure). These mixtures give us different mechanical properties like strength, ductility, fatigue resistance, crash resistance etc
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Flora commented on :
Thanks!