Decreasing the grain size increases the toughness of a steel, i.e. decreases the ductile-brittle transition temperature (DBTT), as can be seen in the diagram below for predominantly ferritic steels.

In fact decreasing the grain size is the only mechanism by which both the strength and the toughness of a steel is increased. Other mechanisms of increasing strength lead to a decrease in toughness.

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.

Click on the above factors to find out more about how they are used to control ferrite grain size.