Forming high-strength steel: what changes from mild steel

Why parts move to stronger steel

Stronger steel lets a body part carry the same load with a thinner sheet, which saves weight and helps crash performance. That is why 590, 780, 980 MPa and above are now common in structural parts. The cost is paid in the press shop: the same shapes are harder to form.

1. More springback

The stored elastic strain rises with strength, so the part leaves the die with more shape change. The fix is to predict it and compensate the die. See What is springback and how is it reduced?

2. Cracks at the edge

Cut and punched edges carry shear damage, and the edge is often where a flange or hole is stretched. High-strength steel with fine tensile elongation can still crack there, because edge ductility is a different property, measured with a hole expansion test. A standard forming limit diagram does not see it. See Stretch flange cracking prediction.

3. Cracks that appear later

At very high strength, steel becomes sensitive to hydrogen. A part with residual stress can crack hours or days after forming, in a moist environment, with nothing visible when it left the press. See Delayed fracture prediction.

4. Cracks the FLD calls safe, and higher press load

Bent, plane-strain areas can crack in narrow bands where a two-dimensional FLD shows no problem, because contact pressure with the tool changes the limit. See 3D-FLD. Higher strength also raises the forming force and tool wear, so the press and die have to be checked for the load as well as the part for the shape.

Checking it before tryout

Each of these problems can be checked in the forming simulation, but only if the material is described well. A reliable result needs measured material data and a model that suits the steel. The JESOLVA-Forming option for JSTAMP covers the three crack types above, and a good material model underlies the springback and strain results.