High-strength steel for lightweight automotive design

High strength meets excellent formability

With its excellent formability combined with high strength, complex-phase steel HR660Y760T-CP is ideally suited for safety-critical structural and other high-strength vehicle components. A cutting clearance tailored to the material in the punching process is highly important to additionally increase the resistance of punched edges to edge cracking.

Developed for demanding forming processes 

The high-strength, hot-rolled multiphase steel is designed for complex applications where punched contours, extruded holes, and subsequent forming operations are standard. Not only do the properties of the material itself matter. The quality of the punched edge is equally critical, as damage introduced during punching has a significant impact on subsequent formability.

Hole expansion test as a measure of punched-edge formability

The hole expansion test according to ISO 16630 is used to evaluate the resistance of punched edges to cracking. This test indicates how well a material withstands local strains at the punched edge and how resistant it is to edge cracking. To determine hole expansion performance, a 10 mm diameter hole is punched into a test specimen which is then expanded using a conical punch tool until the first crack extends through the entire sheet thickness. The hole expansion ratio (HER) is determined from the difference between the expanded and original diameter, relative to the original diameter.

The optimal cutting clearance makes the difference
Tests by voestalpine have shown that both the material itself and the design of the punching process have a significant impact on subsequent forming performance. While the hole expansion test is typically performed using a cutting clearance of 12% of the sheet thickness, forming performance can be improved even further in practice by optimizing the cutting clearance. The following diagram illustrates the relationship between cutting clearance and hole expansion ratio for a series of test specimens cut from a 3 mm thick HR660Y760T-CP steel strip.

• Very small cutting clearances of around 5% promote the formation of a secondary shear zone. The high local deformations and local punching defects, which introduce notches, increase pre-existing material defects and worsen edge formability.
• Cutting clearances of around 25% or more result in burr formation at the punched edge. The burr also represents an area of reduced formability due to strain hardening, promoting premature crack initiation during forming. In addition, uneven burr formation can create a notch effect, further promoting crack initiation and reducing hole expansion performance.
• At the optimal mid-range cutting clearance of around 20%, both secondary shear zones and burr formation are avoided, minimizing damage to the punched edge. In the test series, hole expansion performance therefore increased significantly within this range, reaching approximately 80%. Even within the range of the optimal cutting clearance, however, attention must be paid to optimal formation of the shear-affected zone (SAZ).

In summary, the results show that HR660Y760T-CP can fully exploit its high edge formability within a suitably selected cutting clearance range of around 20%. For automotive manufacturers, this means improved process reliability in production, a lower risk of edge cracking, and reliable manufacturing of highly stressed components.