Advanced High-Strength Steels Application Guidelines Version 3

Version 3 of the Advanced High-Strength Steel (AHSS) Application Guidelines. AHSS Application Guidelines is WorldAutoSteel’s continuing effort to compile and disseminate global knowledge and experience in applying advanced high-strength steels to vehicle applications to meet weight reduction, safety, structural performance and manufacturing cost goals.

AHSS is relatively new to the materials world, but its application in automotive structures has increased since the steel industry’s UltraLight Steel Auto Body (ULSAB), UltraLight Steel Auto Closures (ULSAC) and ULSAB-AVC (Advanced Vehicle Concepts) programmes have successfully demonstrated weight savings and performance improvements at no cost penalty over similar applications using conventional steels.

One of most important additions to Version 3 of the Guidelines is the contribution to the breadth of forming knowledge (Section 2). All steel materials change with deformation, but AHSS steels exhibit unique elongation and work hardening qualities. Understanding what is taking place in the die as forming occurs has been a challenge for automakers. Stress-strain curves provide the information needed for this critical evaluation. Consequently, Version 3 includes a collection of typical stress-strain curves – both engineering and true – for different grades of high-strength low-alloy (HSLA), dual phase (DP), transformation-induced plasticity (TRIP), complex phase (CP), and martensitic (MS) steels. A typical stress-strain curve for Mild steel is included in each graph for reference purposes. This will permit users to compare potential forming parameters, press loads, press energy requirements, and other parameters when switching among different steel types and grades.

Said Dr. Stuart Keeler, technical editor of the AHSS Application Guidelines, “The real end-user of steel products is the forming die, and it’s in the forming die where all of the issues occur. To accurately simulate formation in these advanced steels, the die needs the property data. These stress-strain curves provide engineers with this valuable information.”

Also accomplished in Version 3 is a reorganization and supplementation of the springback information into three approaches: 1) Change the elastic stresses; 2) Reduce or minimize the elastic stresses; or 3) Lock in the elastic stresses. Die guidelines and other information are provided for each approach, which give engineers insight into design and manufacturing procedures for springback compensation.

“The wealth of information gathered in Version 3 of the AHSS Application Guidelines is an example of the global steel industry’s commitment to help our customers more quickly realise the extraordinary benefits of advanced steels ,” said Ed Opbroek, IISI Director, Automotive, and director of WorldAutoSteel. “Material experts around the world collaborated with Dr. Keeler to deliver this vital research that is central to effective design and manufacturing.”

Following is a brief summary of each section and related changes:

Section 1: General Descriptions of AHSS
Section 1 provides definitions for the Advanced High-Strength Steels and provides understandable metallurgical descriptions based on their unique microstructures. Included are explanations describing how these microstructures are created by the steel mills and how they generate the beneficial characteristics of these steels. New for Version 3 are definitions of newer AHSS, such as ferritic-bainitic (FB), twinning-induced plasticity (TWIP), hot-formed (HF) and post-forming heat-treatable (PFHT) steels.

Section 2: Forming
Two-thirds of the AHSS Application Guidelines is devoted to the many aspects of forming. Nine key topics are examined, ranging from mechanical properties and forming limits of the incoming steel to in-service requirements that must be met by the final stamping.

The improved capabilities the AHSS bring to the automotive industry do not bring new forming problems, but certainly they accentuate problems already existing with the application of any higher strength steel. These problems are being solved by increasing the database of knowledge in working with these steels, embodied in the AHSS Application Guidelines.

New emphasis is being placed on determining specific needs of the stamping, highlighting critical forming modes, and identifying essential mechanical properties. The interaction of all inputs to the forming system means the resulting higher loads and energy needs of AHSS also place new requirements on press capacity, tool construction and protection, lubricant capabilities, process design, and maintenance.
To this end, the AHSS Application Guidelines forming section addresses topics such as mechanical properties, forming limits, and forming modes before covering the more traditional areas of tooling, springback and press loads. Mentioned previously is the addition of the stress-strain curves. Other additions include more about the local elongation problems when sheared edges are subjected to tensile deformation, such as hole expansion, stretch flanging, and tight radius bends.

Section 3: Joining
Section 3 addresses the issue of joining Advanced High Strength Steels. A variety of joining techniques are discussed including different welding processes, brazing, adhesive bonding, mechanical joining and hybrid joining. Also addressed in this section are material issues for weld repair.

AHSS are satisfactorily joinable for automotive applications. Joining coated and uncoated AHSS is the same as conventional steels. AHSS differ from mild steels by chemical composition and microstructure, and higher strengths are achieved in AHSS by modifying the steel microstructure. The as-received microstructure will be changed while welding AHSS. The higher the heat input the greater the effect on the microstructure. At fast cooling rates there also is a tendency to form martensite and/or bainite microstructures in the weld metal and in the HAZ. These phase transformations depend on the chemical composition of the steel and joining process. Therefore, some precautions have to be taken during welding for a successful AHSS weld joint.

Production control is critical for joining of AHSS to the successful assembly. Manufacturers with highly developed joining control methodology will experience no major change in their operations. Others may require additional checks and maintenance. In certain instances modifications to equipment or processing methodologies may be required for successful joining of AHSS. These issues are addressed in the AHSS Application Guidelines, however, recognizing that more data are needed in some areas to be complete.

Section 4 and 5: Glossary and Reference
To assist in understanding the material contained in the Application Guidelines, a six-page glossary is included where many of the unfamiliar terms are defined using press shop language.

Likewise, emphasis in Section 5 is to list, where possible, the source documents of the figures, tables, and other graphics. This will allow the reader to go back to the original documents for more information about specific items covered in the AHSS Application Guidelines.

Interaction for Further Development
To further assist OEMs and their suppliers, WorldAutoSteel member companies continually seek to work with customers to apply Advanced High-Strength Steels to meet automotive application goals, whether that is in creating the right mix of AHSS properties for a specific application or helping with other design and manufacturing challenges. Interaction is encouraged as OEMs and suppliers work with this steel technology.

As well, WorldAutoSteel is committed to continuous improvement of the AHSS Application Guidelines to reflect the ever-increasing global body of AHSS vehicle applications knowledge. To this end, steel company feedback, as well as valuable feedback from OEMs and suppliers is actively sought, with the expectation to update and add to the current information over time. The next version of Advanced High-Strength Steel (AHSS) Application Guidelines is scheduled to be made publicly available in 2008.