The effect of grain size on yield strength, s_y, is given by the Hall Petch equation for structural steels:

where s_o is the lattice resistance, i.e. friction stress which opposes dislocation motion
k is a constant, sometimes called the dislocation locking term
d is the ferrite grain size

What grain size is required to give a yield strength of 225 MPa in a ferritic steel?

To control the ferrite grain size two different approaches can be used (separately or in combination)

• Controlled rolling schedules, for example thermo-mechanical controlled rolling
• Controlled alloying element additions