Page 18 - MetalForming May 2013

How High is High for
Automotive Steels?
Are we seriously talking about forming automotive parts from steel products that are more than six times as strong as mild steel? And, if we now can formulate these extremely strong steels, what’s the limit?
BY RONALD P. KRUPITZER
It was quite surprising the other day ments required passenger cars to
when an engineer mentioned to
me that he was studying the use of an 1800-MPa tensile-strength steel for a part. Back in the 1970s when auto- motive steels were primarily mild steel with yield strengths of 170 MPa and tensile strengths of 275 MPa, that was all the strength required to make a car or light truck. The properties of great- est interest to the car makers were formability indices like r-value, which determine how deep a part can be drawn into a die; the steel’s related uni- form elongation, n-value, which deter- mines the amount of stretch a part can handle; delta r, which measures the unevenness of a draw, also known as the earing characteristics; and the forming-limit diagram (FLD).
Are we seriously talking about using steel products in automobiles that are more than six times as strong as mild steel? And, if we now can formulate steels six times stronger than mild steels, what’s the limit? For the stamper or the forming specialist, what new
Ronald Krupitzer is vice president, auto- motive market, for the Steel Market Devel- opment Institute: www.autosteel.org.
factors need to be looked at to handle this amazing shift in strength?
How Did We Get Here?
The quest for mass reduction in automobiles began in earnest in 1973, when carmakers reacted to the oil cri- sis and embargo as the United States experienced runaway gas pricing and shortages. Then, in 1975 Congress enacted the first Corporate Average Fuel Economy (CAFE) regulations. This first wave of fuel-economy require-
improve from about 18 miles/gal. to 27.5 miles/gal. during the period of 1978 to 1990. At the same time, dialog between car companies and steelmak- ers shifted to how the steel industry could contribute. This began a mis- sion that has never stopped.
What level of strength increase of the high-strength steels (HSS) was devel- oped during this first wave of mass- reduction efforts? During the 1970s and ’80s, the industry welcomed two types of HSS: dent-resistant steels, and high-strength low-alloy (HSLA) struc- tural grades, both of which now are classified as conventional HSS.
For dent-resistant steels, providing higher yield strength in the part and reducing the outer panel thickness became important objectives. Two types of dent-resistant steels were introduced—renitrogenized sheet steels that had accelerated strain-aging properties at room temperature, and bake-hardening steels (still used today) that also provided strain hardening, but with the majority of the hardening occurring during the baking of the stamped body panels occurring in car- company paint shops.
16 MetalForming/May 2013
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