The Science of Forming
Automotive Steels--Online Technical Resources
Starting in the 1960s, high-strength low-alloy (HSLA) steels were the hot topic for making vehicles stronger and safer. Over the next two decades the maximum strength of HSLA steels increased, while lower sulfur content reduced the number and size of inclusions that limited stretching and bending. Still, some labeled these steels a mundane commodity.
During the 1990s, the need for automotive weight reduction ignited research centers throughout the world to create completely different processes to produce stronger steels with added formability. Parallel research developed computerized die tryout (virtual press shops) to allow design of parts and tooling before hard dies were cut. Validation of the virtual results came when the physical parts performed identically to the virtual parts.
Fig. 1—Each ellipse approximates the range of properties provided for different grades within a steel type. Overlap of ellipses shows possible options for specific applications. Courtesy of WorldAutoSteel.
From the 1960s through early 1990s, new information about these automotive steels and their capabilities developed slowly. Knowledge was transferred through seminars, textbooks and magazines. Access usually was limited and often expensive. Today, a vast amount of information about new automotive steels is available at no cost via Internet downloads, webcasts and magazine subscriptions—most of them free. Here we describe some of the better sources of information.
The most comprehensive, free document is AHSS (Advanced High Strength Steel) Application Guidelines, available at www.worldautosteel.org. The document comprises six sections:
1) General description
2) Forming, including discussion of properties, springback and hydroforming
3) Joining—including resistance welding, laser welding and adhesive joining
4) Glossary—definitions from WorldAutoSteel and the Auto/Steel Partnership
5) References—sources of figures, graphs and technical papers
6) Appendix—details of case studies conducted by the Auto/Steel Partnership
Information in AHSS Application Guidelines compares these new steels to HSLA and conventional low-strength steels. For example, the section on springback applies to all steels. Some higher-strength steels are designated by yield strength or tensile strength, depending on global location and strength values. For example, the document uses the format HSLA 350/450 that designates type of steel, yield strength (MPa)/tensile strength (MPa). The SAE has proposed a similar format of DP 600T/350Y in SAE J2745—Surface Recommended Practice for Advanced High Strength Steels. AHSS Application Guidelines also includes typical engineering and true stress-strain curves for the different types and grades of higher-strength steels compared to mild (AKDQ) reference steel.
Fig. 2—Segments of the Fig. 1 curve can be used to describe hot forming and other steel processes. Courtesy of WorldAutoSteel.
Another feature of AHSS Application Guidelines is the banana curve that replaces the use of traditional boxes representing ranges of yield or tensile strengths for low-, medium-, high-, and ultra-high-strength steels. The banana curve (Fig. 1) shows a continuum of properties as steel increases from lowest to highest strength levels. The ellipses represent the range of grades available within each type of steel. Where the ellipses overlap, engineers often have a choice among different steel types with the same strength. The lower curve of ellipses shows the current production steels, known as Generation 1. The curve in the upper right hand corner represents Generation 2 austenitic steels (3XX stainless and TWIP steels). Between the two lie the Generation 3 steels under development. These steels will be designed for parts requiring less stretchability than Generation 2 steels, at less cost and with better joining capabilities.
The banana curves also provide a better understanding of processing changes to achieve forming and end-product benefits (Fig. 2). The gray ellipses define the AHSS. The numbered ellipses define the hot forming of boron-based steels. This processing provides exceptional stretchability to generate complex shapes with high final tensile strengths. The typical as-received tensile strength (1) is 400 MPa (58 Ksi). The blank is heated to 850 C (2) and formed with a tensile strength of 100 MPa (15 ksi) and a total elongation of 50 to 60 percent. The formed part is quenched in the die (3) to form martensite with an approximate 1550-MPa (225 ksi) tensile strength.
Stainless steel has been called the “formable high-strength steel.” This reference applies almost exclusively to the 3xx-series of stainless steels. Properties and other stainless-steel information can be found at www.ssina.com. One important document, under the category of information handbooks, is Design Guidelines for the Selection and Use of Stainless Steel.
An excellent website for information on automotive steels is that of the Auto/Steel Partnership—www.a-sp.org. This organization brings together the Detroit Big 3 automakers and six major North American steel companies to conduct automotive research and provide useful technical information for the automotive industry and its suppliers. The AHSS case studies published in the WorldAutoSteel AHSS Application Guidelines were conducted by the Auto/Steel Partnership, and are downloadable from the website. In all, the website contains more than 100 useful documents, including the Automotive Steel Design Manual and Material Uniformity.
Lastly, seminars continue to provide timely updates on the capabilities of new steels, and seminar papers often can be accessed online free of charge. The seminar most focused on automotive steels is Great Designs in Steel, held each year in mid-May in Detroit by the American Iron and Steel Institute’s Steel Market Development Institute. All of the papers presented during the 2003-2011 seminars are available at www.autosteel.org. MFOn July 18-19 in Grand Rapids, MI, Stuart Keeler will present a double seminar covering Sheetmetal 101—Understanding the Metal, and Forming Sheetmetal—Understanding the Properties. Learn more at www.pma.org.
Related Enterprise Zones: Materials/Coatings
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