07 March 2024

Overcoming the formability challenges of dual phase steels

Steel sheets for forming

Any automotive engineer knows that specifying the correct material is vitally important to the production, performance, durability and cost-effectiveness of a vehicle or component.

When considering vehicle chassis, structures and body panels, dual phase steels, like Tata Steel’s DP800, offer several significant benefits. As we have covered before, DP800 can enhance productivity and press shop efficiency owing to its consistent and stable mechanical properties. It also exhibits higher tensile strength than conventional (HSLA) steels, providing further opportunities for lightweighting and improving efficiency of the end product.

However, for all their benefits, dual phase steels are often seen as more challenging to form compared to high strength low alloy (HSLA) products. 

The truth is that dual phase steels simply behave differently on press, requiring an alternative tooling and process set up to HSLAs. When the stamping process is approached with this in mind, dual phase steels have been proven to be a dependable and valuable part of a vehicle’s material mix.

Here, we look at the tooling considerations that need to be made when stamping DP800 and other dual phase steels.

The importance of the first pressing

The World Auto Steel AHSS application guidelines state that Dual Phase (DP) steels have a microstructure consisting of a ferritic matrix with martensitic islands as a hard second phase. The soft ferrite phase is generally continuous, giving these steels excellent ductility and forming capability. While this is key to their make up as high-strength, lightweight steels, it also requires a different approach to processing than a typical HSLA.

One of the main benefits of a dual phase steel is its capacity for work hardening. When these steels deform, strain is concentrated in the lower-strength ferrite phase surrounding the islands of martensite, creating the unique high initial work-hardening rate (n-value) exhibited by these steels This means that after undergoing initial pressing or forming, the steel hardens and makes further forming difficult. Therefore, it is important to get as close to the final shape of the component as possible in the first stage of pressing. 

During the forming process, this hardening can lead to high levels of springback after the load is removed. This can be addressed by over crowning to allow for relaxation of the material, or designing wall bead or other geometry into the component, which will also reduce side wall curl without compromising formability.  An alternative method is to impart high strain into the component at the end of the forming stage to reduce the compressive levels and reduce the springback that way.

Simplified design

While dual phase steels are well suited for use where high strength is required in automotive body panels and chassis components, simplicity is key when deciding how these shapes are formed. 

For example, height changes across the component should be kept to a minimum, with any required changes being as gentle as possible. This again reduces localised deformation  and reduces high-work-hardening , meaning that excessive height changes can lead to localised strain concentrations and higher failure rates and more springback. 

For a similar reason, excessive steps in the flange of the component should also be avoided, as this can cause high levels of stress. Instead, the step should be pressed into the more formable material being joined to it to create a flat mating for welding. 

Generally, as the strength of any steel increases formability will decrease. This means that less complicated geometries will be easier and more reliable to form, which again reduce failure rates.  It also provides capacity left in the material to absorb crash loads and springback compensation measures. 

Punch clearance

Punch clearance refers to the horizontal distance between the leading edge of the punch, and the rearmost edge of the bottom die.

Depending on the design of the stamping process, it is common for HSLA steel tooling to operate at a relatively small punch clearance.

In the move from high strength steels to dual phase steels, it is not uncommon for production engineers to attempt to retain a similar punch clearance, which can lead to edge splitting and higher reject rates. 

In a recent collaboration with Tata Steel, Gestamp sought to understand the issue more fully, when a pair of press parts made from DP800 saw an average split rate of 22% rising as high as 35%. The tool had been moved to Gestamp from another supplier, who had similar problems. Having tried dual phased steels from four separate suppliers, Gestamp turned to Tata Steel’s Customer Technical Services (CTS) team for help. 

WorldAutoSteel guidelines recommend a 14% punch clearance for dual phase steels, while Tata Steel has found that a 12% clearance is preferable for its own DP800. The team found that the punch clearance was 9% on the areas affected by splitting on Gestamp’s part. 

As a result, trials were run increasing the punch clearance to 10%, 12% and 14% on 500 pairs of parts. These trials improved reject rates from an average of 22% to 0% on the left-hand part and 8% on the right-hand part and led to a recommendation to change the punch clearance on the part from 9% to 12%.