Tooling by Design



High Stress for Stampers and Die Shops

By: Peter Ulintz

Saturday, March 1, 2008
The application of advanced high-strength steel (AHSS) with ultimate tensile strengths of approximately 600 MPa now is well established within the automotive industry. Current trends point to growth in applications requiring even higher-strength grades, to 800 MPa.

Due to their higher strengths and greater springback after forming, producing high-strength-steel stampings and their tools requires special attention. Recent research has proven the effectiveness of several unique methods that address problems specific to AHSS, including the selection of optimum stamping processes, solving formability issues, controlling springback, selecting die steels for optimal tool life and establishing product-design strategies to assure manufacturability.

As the application of higher-strength steel continues to grow in popularity among automotive and consumer products, lower-tier suppliers will increasingly become more responsible for engineering tools and producing AHSS stampings. These companies, primarily contract stamping and tool and die shops, are well established and experienced with mild steel and high-strength low-alloy (HSLA) steels. However, most are not prepared for the challenges presented by the new advanced steels without an appropriate transfer of recently developed engineering practices and manufacturing know-how.

This is the premise of a two-hour seminar that I will present at Regional METALFORM in Birmingham, AL, in April. The presentation’s titled, Designing and Building Dies for High-Strength Steel Stampings.

According to data presented by Ducker Worldwide at the American Iron and Steel Institute’s Great Designs in Steel event in March 2007, the content of AHSS in North American light vehicles is projected to increase from the current level of 9.5 percent to nearly 35 percent over the next eight years (see pie charts). During this same period, bake-hardenable and medium high-strength steels are projected to increase in usage from current 6.6-percent levels to 23.5 percent, while mild-steel content diminishes to less than 30 percent. The use of more traditional HSLA grades also is expected to drop slightly.

Achieving this level of AHSS use presents many challenges for manufacturers and tool and die companies. The Auto/Steel Partnership (A/SP), a consortium involving the Detroit automakers and seven North American steel companies, has identified three major AHSS stamping and forming challenges:

Tool breakage resulting from the increased stress required to form parts from grades with double and triple the strength of conventional high-strength steels.

Tool wear such that hardened inserts and shear edges can wear out during a single production run as a result of the AHSS material hardness being nearly equal to the die hardness.

Part quality problems arising from the increased springback associated with the higher strengths and increased process sensitivity to material properties.

In addition, the Center for Automotive Research, a consulting partner with A/SP that maintains a working relationship with the tool and die industry, assembled tooling coalitions to tackle common tooling and metalforming problems associated with AHSS materials. The coalition of North American tooling shops identified additional challenges not initially raised by the automotive and steel companies in dealing with AHSS, including:

• The absence of any detailed metallurgical steel data early in die development to adequately support forming simulations and ensure continuity of material throughout die development and tryout.

• Numerous die recuts required due to extensive springback not predicted by forming simulation. Some tools had to be recut 10 times, driving up tool cost and reducing quality.

• Dimensional buy-off criteria being too difficult for AHSS. The unattainable goal of 100-percent print compliance and dimensional Cpk levels of 1.33 places nearly every tool program into crisis mode, resulting in late delivery.

High stress for stampers and die shops

The tooling coalition also has identified five general recommendations for improving tooling, stamping processes and product design for AHSS stamped parts:

Springback prediction accuracy and springback management of AHSS-grade materials still requires much improvement.

Product design features such as radii and part shape greatly affect springback, including twist and curl. Anticipating the impact of these product features during tooling design can help reduce springback problems.

Tool and die standards for HSLA and mild-steel stampings are not adequate for AHSS. Special tool standards, accounting for the higher forming stresses and wear characteristics of AHSS alloys, must be adopted.

Material availability is a significant problem affecting simulation accuracy, tool design and tool tryout, especially when it is specified as a last-minute change.

Tool buyoff standards are historically based on part-print compliance and Cpk results. These requirements usually are challenging enough for HSLA stampings and are proven to be much too difficult for AHSS stampings. A functional buyoff procedure, rather than the pursuit of print dimensions or unachievable Cpk, is needed, especially for tools capable of achieving high levels of repeatability but not part-print specification.

It also is important to understand that manufacturing AHSS stampings will affect the size, strength, power and overall configuration of almost every major piece of pressroom equipment. In many instances, coil handling, feeding, straightening and press equipment originally designed for mild-steel and HSLA applications will prove insufficient for AHSS. This fundamental lack of understanding already has led more than a few companies down a path to financial ruin.

Clearly, designing AHSS stampings, tools and processes with the same approach as designing for HSLA and mild steel, using the same GD&T methods, tooling standards and buyoff criteria, will be the source of stress for many companies dealing with AHSS for the first time. Prepare yourself and your company. Consider joining me at Regional METALFORM as we explore some key product and process requirements, recommended practices, and unique manufacturing methods that both stampers and die shops will need to consider, understand and employ when designing and building dies for high-strength-steel stampings. MF


Related Enterprise Zones: Tool & Die

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