Part Design Freedom
Optimized part geometry is to key automotive lightweighting
Over the past 25 years, steels have gotten consistently stronger resulting in many materials > 1000 MPa, but at a fairly consistent tradeoff in formability. The challenge with trading off formability for strength in a material is that it limits the part designs that can be affordably constructed and part design is one of the most influential factors in lightweighting.
Part design is important because it affects stiffness, which is necessary to provide the smooth, confident, and quiet ride consumers expect. Stiffness is a function of both the geometry of the part and the modulus of the material. Engineers optimize geometry by designing parts with the most complexity putting the material exactly where needed to maximize stiffness—while using the minimum amount of raw material. The higher the formability or ductility of a material, the more optimized the part geometry and the thinner and lighter the sheet required to form the part.
Once stiffness performance is satisfied, the part must next be able to carry the ultimate loads being placed on it without permanent deformation or failure. Stiffness helps a part resist bending under load, but at some point, the stresses that build up could reach the strength limit of the material. Auto engineers would avoid this in three ways. First, they could simply use thicker material which better “shares” the stress load– however this would increase mass which is counter to the goal of lightweighting. The traditional preference is the second choice which is to pick a higher strength material. However, selecting higher strength material would limit part geometry due to reduced formability, making it difficult to form the original design with the correct stiffness level.
The third choice for solving a load carrying shortfall is to add even more optimized geometry to the part. When you optimize geometry the stresses in the part while under load are distributed as smoothly and evenly as possible. By doing this, peak load or “hot spots” are minimized, so that engineers can use the thinnest material. These “optimized” geometries can be achieved with highly formable, low strength steel, but typically this will revert back to square one; a material that cannot survive the ultimate load.
This is why a material that offers only high formability or high strength won’t solve today’s lightweighting challenges. The ideal scenario would be a material with high strength, but that also features enough formability to create parts with finely detailed geometry.
This is the challenge that NanoSteel sheet steels are designed to resolve – creating efficiencies by offering among the highest strength available with the formability of a mild steel. The following video further explains the advanced formability of NanoSteel AHSS.
VIDEO: Increasing Formability with NanoSteel 1:25