FB steels sometimes are utilized to meet specific customer application requirements that require Stretch Flangeable (SF) or High Hole Expansion (HHE) capabilities for improved edge stretch capability.
FB steels have a microstructure of fine ferrite and bainite. Strengthening is obtained by both grain refinement and second phase hardening with bainite. Figure 2-14 shows the microstructure for FB 450/600. FB steels are available as hot-rolled products.
Figure 2-14: Photomicrograph of FB 450/600
The primary advantage of FB steels over HSLA and DP steels is the improved stretchability of sheared edges as measured by the hole expansion test (λ). Compared to HSLA steels with the same level of strength, FB steels also have a higher strain hardening exponent (n-value) and increased total elongation. Engineering and true stress-strain curves for FB steel grades are located in Figure 2-15 of the Advanced High-Strength Steel Application Guidelines.
Because of their good weldability, FB steels are considered for tailored blank applications. These steels also are characterized by both good crash performances and good fatigue properties.
Current production grades of FB steels and example automotive applications:
FB 330/450 Rim, brake pedal arm, seat cross member, suspension arm
FB 450/600 Lower control arm, rim, bumper beam, chassis parts, rear twist beam.