Characteristics of Ferritic-Bainitic (FB) Steel

Applications where High Hole Expansion (HHE) or Stretch Flangeable (SF) capabilities are required for enhanced edge stretch capabilities involve the use of FB steels. The microstructure of the FB steels consists of fine ferrite and bainite. Grain refinement followed by second phase hardening with bainite provides strengthening to the steel. The microstructure for FB 450/600 is depicted in Figure 1. Hot-rolled FB steel products are also available.

Photomicrograph of FB 450/600

Figure 1. Photomicrograph of FB 450/600

Key Characteristics of FB Steels

The hole expansion test (λ) reveals that FB steels have improved stretchability of sheared edges when compared to DP and HSLA steels. Another key advantage of FB steels over HSLA steels is the increased total elongation and higher strain hardening exponent (n-value) at the same level of strength. FB steel is used in customized tailored blank applications, thanks to its good weldability. Good fatigue properties and crash performances are other key characteristics of the FB steels. Figure 2 shows the tensile strength-elongation graph of the FB steels.

The tensile strength-elongation graph of FB steel

Figure 2. The tensile strength-elongation graph of FB steel

Current production grades of FB steels and the corresponding automotive applications are given in the following table:

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.

Download the Advanced High-Strength Steels Applications Guidelines free here, to learn more about the metallurgy, forming and joining of these new steels.

This information has been sourced, reviewed and adapted from materials provided by WorldAutoSteel (World Auto Steel).

For more information on this source, please visit WorldAutoSteel (World Auto Steel).

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