Bake hardening steels

The composition and processing of these steels are designed to promote a significant increase in yield strength during low-temperature heat treatment, particularly paint curing.

ArcelorMittal bake hardening steels can thus achieve higher strength in the finished part while retaining good forming performance. The gain in yield strength through the "bake hardening" (BH) effect is generally greater than 40 MPa. Thanks to this BH effect, ArcelorMittal steels offer two advantages compared to conventional drawing quality steels:

• Improved dent resistance in all finished parts in the case of low forming strains (hood, roof, doors and wings)
• Substantial weight reduction potential at equivalent dent resistance (the decrease in thickness is offset by increased yield strength resulting from the heat treatment process)

Bake hardening steels thus offer a suitable response to automotive bodywork requirements. By providing an excellent drawability-dent resistance combination, they enhance vehicle weight reduction and aesthetics.

Steels in the BH range are designed for visible (door, hood, tailgate, front wing, roof) and structural (underbody, reinforcement, cross member, lining) parts.

The table below summarizes the cross-references between ArcelorMittal product range, European standards and the VDA 239-100 standard.

The CR195BH grade refers to Japanese standards.

These cross-references are given for information only. ArcelorMittal BH grades generally offer guarantees of tighter mechanical properties (see table below).

A_80mm %: Percentage elongation after fracture using a specimen with gauge length L₀ = 80 mm (ISO 6892-1 type 2 (EN20x80))
A_50mm %: Percentage elongation after fracture using a specimen with gauge length L₀ = 50 mm (ISO 6892-1 type 1 (ASTM12.5x50) or type 3 (JIS25x50)
A%: Percentage elongation after fracture using a proportional specimen with L₀ = 5.65 (So)^1/2
Ag %: Percentage plastic extension at maximum force
BH₂: Increase in yield strength between a reference condition after a 2% plastic pre-strain and the condition obtained after heat treatment (170°C-20minutes)

"Bake hardening" is a controlled aging phenomenon related to the presence of carbon and/or nitrogen in solid solution in the steel. The BH₂ parameter is used to evaluate the resulting increase in dent resistance. It is given by: BH₂ = LYS - 2% PS, in which LYS is the lower yield stress measured after heat treatment and PS is the yield stress after initial 2% plastic pre-strain. BH₂ measurement is a reliable and reproducible way to quantify the metal's ability to harden during cataphoresis.

The diagram below illustrates the Bake hardening mechanism and shows the displacement of carbon atoms in solution during heat treatment-typically 20 minutes at 170°C to block the dislocations generated by forming. This ultimately increases the metal's yield strength.

The bake hardening family of steels exhibits good drawability in all strain modes; its drawability is essentially equivalent to that of IF (interstitial-free) steels of similar yield strength.

The figure below shows examples of forming limit curves for the bake hardening family of steels in a thickness of 1.0 mm.

ArcelorMittal has a database on the forming of BH grade steels. To integrate these steels at the design stage, a team of experts is available to perform specific studies based on modeling or shop-floor experience.

Due to their low alloy content, bake hardening steels can be readily welded by all conventional welding processes.