Page 26 - MetalForming May 2011

Advanced
High Strength
Steels
Solve Growing
Demands for Formability
BY STUART KEELER AND PETER ULINTZ
Prior to 1960, requirements for higher-strength steels remains the same. However, stretchability and bendability
were met by higher-carbon or cold-worked steels.
These methods for increasing strength came at the expense of formability. Then came high-strength low-alloy (HSLA) steels, developed using thermal mechanical pro- cessing. Strength increased with minimum reduction in formability, due to steel modifications including smaller grain size, replacing an iron atom in the atomic structure with another element and adding other elements inside the atom- ic cell structure. Early HSLA steels boasted yield strengths of 35 to 50 ksi, and yield strengths of newer HSLA steels approach110 ksi.
(Note: For information on forming these steels, see the article titled, Forming Higher-Strength Steels, in the April 2009 issue of MetalForming).
Today, greater demands are being placed on higher- strength steels. These include:
1) Weight-reduction goals, met by reducing sheet thickness and then compensating by increasing yield strength. From a structural analysis, yield strength times sheet thickness
Stuart Keeler and Pete Ulintz author monthly columns in Met- alForming magazine and conduct seminars for the Precision Metalforming Association. Keeler is president of Keeler Tech- nologies LLC, Grosse Ile, MI; keeltech@comcast.net. Ulintz is advanced product engineering manager for Anchor Mfg. Group, Cleveland, OH; pete.ulintz@toolingbydesign.com.
decrease with reduced thickness and increased yield strength. 2) Part consolidation, often requiring the forming mode to change from deep draw to stretch. Consider an assembly of three separate and easily formed parts—after trimming, welding joins the three pieces. Now attempt to stamp the same final part from one blank in a three-cavity die. Without a binder between cavities to allow steel to flow and create adjacent walls, the forming mode becomes stretch. Stretch-
ability decreases as strength increases.
3) Designing high-strength steel parts with sharper fea-
tures and character lines that tend to severely localize defor- mation. This requires steels with increased stretchability.
AHSS, or “Designer Steels”
Responding to the demand for higher-strength steels with increased formability, steel researchers have taken a completely new approach to increase strength. Low-strength and HSLA steels have a single phase called ferrite—a crys- talline microstructure of pure iron with small amounts of car- bon. Advanced high strength steels (AHSS) boast microstruc- tures with one or more phases other than ferrite, such as martensite, bainite and retained austenite. The use of other phases allows steel mills to produce new steels with initial properties and response to deformation that more closely match the needs of the product and process designs.
Common AHSS grades fall into three formability types:
24 MetalForming/May 2011
www.metalformingmagazine.com