Page 19 - MetalForming May 2014

ment to produce and form and fabri- cate the steels. In 2008, WorldAutoSteel launched its Future Steel Vehicle (FSV ) program, mating advanced designs to electric and fuel-cell powertrains to achieve an ultra-low carbon footprint. To balance crash and environmental performance, steelmakers accelerated the development of new AHSS grades, further stretching the envelope for strength and ductility. Through engi- neering optimization, FSV revealed that weight reductions of as much as 39 percent were attainable, allowing AHSS products to compete with other low-density lightweighting materials.
In 2002, WorldAutoSteel (the auto- motive arm of worldsteel) formed a working group of major steel compa- nies around the world to create the AHSS Application Guidelines. This doc- ument explains how and why AHSS steels differ from traditional higher- strength steels in terms of their ability to be formed, fabricated and welded. And as of May 14, 2014, WorldAutoSteel will offer an update of its AHSS Appli- cation Guidelines, creating Version 5.0 to capture additional knowledge and lessons learned from the FSV program and global steel suppliers. While pre- vious versions of the guidelines covered metallurgy, forming and joining, Ver- sion 5.0 also reflects new content high- lighting the broader materials portfolio, advanced fabrication technologies and optimized joining processes.
Note: Download the guidelines free of charge at www.worldautosteel.org, beginning May 14.
Version 5.0 Loaded with Forming and Fabricating Advice
Version 5.0 of the AHSS Application Guidelines describes technological advances achieved during the past 5 years; identifies new solutions to fab- rication and welding problems (includ- ing joining AHSS and other high- strength steels; and provides basic property information for tier press shops.
The number of commercially avail- able AHSS grades has increased from the 28 listed in the 2009 Version 4.1 of
Fig. 1—The relationship between percent total elongation and tensile strength.
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the guidelines to 51 grades specified in Version 5.0 (Table 1). This allows design- ers to more closely match steels to the requirements of the stampings. Referred to as “designer steels,” their properties and performance can be tuned for spe- cific applications. This requires the user to understand the differences among the steel types and their prop- erties. Greater cooperation and exchange of information between steel suppliers and users becomes critical.
Fig. 1 helps illustrate the expansion in AHSS material properties, and the potential for additional developments. The lower arc of the graph compares first-generation AHSS to conventional steels. The vertical axis represents per- cent total elongation, a measure of formability. The TWIP ellipse represents the second-generation AHSS grades, while the third-generation AHSS ellipse introduces the scope of research under- way to develop steels more formable than first-generation AHSS grades but less costly and more weldable than generation-two grades. The boundaries of each ellipse indicate the range of properties for that type of AHSS.
Welding Advice
from Around the World
Approximately 100 pages—a small book—of new information on welding AHSS reside within the new AHSS Appli- cation Guidelines. Including newly developed hybrid-welding processes, manufacturers now have at their dis- posal more than 100 welding processes from which to choose, each with its unique advantages and disadvantages
based on the application. Arc-welding processes, for example, offer equipment portability and low cost, but operate at relatively slow speeds and generate a considerable amount of heat. Mean- while, high-energy-density processes such as laser welding produce low heat input and fast welding speeds, but the equipment is expensive and joint fitup must be ideal. And, solid-state welding processes avoid many of the weld dis- continuities produced by fusion welding, but they often are restricted to limited joint designs. Lastly, while resistance- welding processes typically run very fast and require no additional filler materials, they often are limited to thin sheet applications or very high-pro- duction jobs.
Note: Version 5.0 continues to use the WorldAutoSteel steel designation of Type Min YS/Min TS in MPa to avoid misunderstandings. An example: DP 350/600, noting the steel’s minimum yield (350) and tensile (600) strengths in MPa (megapascal); one MPa equals 6900 psi. Note the very high tensile strengths plotted in Fig. 1—the 2000- MPa steels now are identified as 2-GPa (gigapascal) steels.
AHSS Application Guidelines, Version 5—in Summary
Opening Section: An introduction defines AHSS and their role in helping the automotive companies satisfy light- weighting, safety, efficiency, emissions, manufacturability, durability and cost requirements. Also covered, due to growing environmental concerrns: life- cycle assessment for CO2 emissions