Page 56 - MetalForming May 2012

The Science of Forming
By Stuart Keeler
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 reduc- tion 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.
From the 1960s through early 1990s, new information about these automotive steels and their capabilities devel- oped 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 com- prises 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
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementa- tion of other press-shop analysis tools. Keeler’s metal- forming experience includes 24 years at National Steel Corporation and 12 years at The Budd Company Technical Center, enabling him to bring a very diverse background to this column and to the seminars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
70% 60 50 40 30 20 10 0
Conventional Steels
Austenitic-Based Steels
Generation 2
54 MetalForming/May 2012
www.metalformingmagazine.com
IF
BH
HSLA
Generation 3
Current Area of Research
HS IF CMn
500
Fig. 1—Each ellipse approximates the range of properties provid- ed for different grades within a steel type. Overlap of ellipses shows possible options for specific applications. Courtesy of WorldAutoSteel.
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 sim- ilar format of DP 600T/350Y in SAE J2745—Surface Recom- mended 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) ref- erence steel.
Another feature of AHSS Application Guidelines is the banana curve that replaces the use of traditional boxes rep- resenting ranges of yield or tensile strengths for low-, medi- um-, 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 dif- ferent steel types with the same strength. The lower curve of ellipses shows the current production steels, known as Gen- eration 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 requir- ing less stretchability than Generation 2 steels, at less cost and with better joining capabilities.
0
200
Generation 1 MART 800 1100
Tensile Strength (MPa)
1400 1700
Mild
• Improved properties • Reduced cost
IS
TRIP DP,CP
• Improved weldability
AHSS Grades
Tooling Technology
Elongation