3. Digital data With regard to traction characteristics, the following categories will be considered: ferritic or ferrite-perlite grades (predominantly ferrite) ; perlitic or perlite ferrite grades (predominantly perlite) ; interstitial-free steels ; double-phase steels (ferrite-martensite) ; ferrite-bainite shades. 3.1 Ferritic steels Ferritic and ferrito-perlitic steels account for a very large proportion of general-purpose steels. The well-accepted physical law for these steels is the Hall-Petch law ( and ) :Re=σ0+kd−1/2