CH Steels for cold stamping

The family of CH steels are the new generation of the Complex Phase grade. These Advanced High Strength Steels provide an improved formability compared to conventional  complex phase steels thanks to their higher elongation. These steels allow the production of  structural elements by a cold forming method such as stamping. The elongation is significantly improved thanks to a small amount of residual austenite.

Thanks to their excellent mechanical properties, these grades are particularly suitable for automotive safety parts for strength in a crash. Typical parts are shown in the figure below:

The above table summarizes, purely for information, the European and VDA standards corresponding to the ArcelorMittal product range.

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)

Please contact us for products under development.

CH steels have good overall forming properties compared with their level of tensile strength.

The figure below shows an example of the forming limit curve for CR780Y980T-CH steel 1.5 mm thick, compared to a conventional lower Yield Stress DP steel with the same Tensile Strength. CH steel of the same strength has slightly better overall ease of forming despite its much higher elastic limit (+ 180MPa).

Forming limit curves for CR780Y980T-CH steel 1.5 mm thick in comparison with lower Yield Stress Dual Phase steel (ArcelorMittal model for Europe)

Forming limit curves for CR780Y980T-CH steel 1.5 mm thick in comparison with lower Yield Stress Dual Phase steel (Keeler model for North America)

The advantage of these steels is above all that they also have excellent local ease of forming/ductility. This high local ductility gives them good ease of forming in cut edges, as well as in ductility behaviour during a crash.

This local ductility was quantified using local measurement of thickness reduction on the section of the tensile test specimen after fracture. The graph below gives the comparison of this thickness reduction for different grades of the same strength (980 MPa).

The CH range of steels has good resistance spot weldability. The welding range, determined according to the ISO 18278-2 standard, is quite wide. The table below shows examples (indicative) of spot welding characteristics for DH steels matching joints, determined according to the ISO 18278-2 standards.