Bake Hardenable Steels

Bake hardening is an advanced processing technique to produce low carbon steels, used for car bodies, with high strength. An optimized batch annealing treatment is necessary in order to have enough carbon in solution required for bake hardening. This makes the automotive bodies and panels strengthen after the paint baking treatment. This effect is due to pinning of dislocations by solute carbon atoms which refers to Cottrell’s atmosphere.

Using ultra-low-carbon vacuum-degassed steel, and precise alloying additions, partially stabilized steels can be produced that have a low amount of solute carbon available after precipitation reactions are completed on the galvanizing line.

Bake hardenable steel (BHS) takes advantage of the low solute carbon to produce controlled carbon strain aging to augment the yield strength of formed automotive panels, thus improving dent resistance or permitting some thickness reduction. The strain comes from press forming and the aging is accelerated by the paint baking treatment. BH steels contain enough supersaturated solute carbon that the aging reaction typically adds 4 to 8 ksi [27 to 55 MPa] to a stamped panel yield strength.

This approach to providing higher strength panels has the advantage of presenting formable low yield strength material to stamping operations so as to avoid panel shape problems due to elastic deflection associated with initial yield strengths exceeding 35 ksi [240 MPa]. BHS is the practical consequence of modern manufacturing technologies, which permit control of supersaturated solute carbon at a level which is just high enough to provide a useful amount of accelerated strain aging, without aging during transport/storage. The BHS process produces a coated product that will be free from stretcher strains for at least 2 to 3 months after its production, allowing stampers time to consume it before its mechanical properties begin to deteriorate due to aging.

Figure 1 illustrates the concept of bake hardening, with BH representing the flow stress increase on baking. This chart also represents the typical strain and baking conditions for the least formed areas of automotive panels.

Figure 1: Bake hardening phenomenon

Bake-Hardenable (BH) Steels

A bake-hardenable steel is any steel that exhibits a capacity for a significant increase in strength through the combination of work hardening during part formation and strain aging during a subsequent thermal cycle such as a paint-baking operation. These steels in USS Steel are made in the following grades:

• BH180
• BH210
• BH240

Any steel with adequate carbon and/or nitrogen in solution to cause strain-aging may be classified as bake-hardenable. In general, bake-hardenable steels are aluminum-killed steels with an adequate amount of aluminum to combine with the nitrogen as Aluminum Nitride (AlN).

A combination of relatively low yield strength prior to manufacturing and a high in-part strength after forming and paint baking makes bake-hardenable steels ideal for applications where dent and palm printing resistance is important. This material can be used in relatively deep draw or stretching operations. Due to the high in-part strength, bake-hardenable parts are also good candidates for downgaging, which is important for weight reduction efforts.

When using bake-hardenable steel, the amount of strain introduced during the forming process will largely dictate the final strength of the part. Since automotive parts, specifically exposed body panels, have a wide array of designs, there will be a corresponding disparity in the amount of strain introduced in these varying geometries. As a result, when using bake-hardenable steel, it is important to design an adequate amount of strain into a part in order to fully utilize this material’s dent resistant characteristics.

Tables 1 - 3 show the mechanical properties of the grades BH180, BH210 and BH240. The increase the values of yield strength during forming and baking of bake-hardenable steels is clearly evident.

Weldability – Low carbon level makes bake-hardenable steel a good welding candidate.

Fatigue Performance – If used properly, bake-hardenable steels have a high yield strength after forming and baking, which means it will have a good resistance to fatigue.

Denting – Bake-hardenable steels were designed for dent resistance.

Applications – Bake-hardenable materials provide customers with a material that is capable of reducing the amount of dents and dings found on today’s cars. These materials have the formability requirements needed to produce most exterior applications. These exterior parts benefit from the work and bake hardening kicks that are experienced during processing. These parts include doors, deck lids, quarter panels, fenders, hoods and roofs.