Page 38 - MetalForming August 2013

The Science of Forming
By Stuart Keeler
The Call for Fresh Steel
Fifty years ago, “old steel” could have been blamed for a steel coil or blank failing to make an adequate stretch- formed stamped part. Back then, rimmed steel was common, processed by pouring molten steel from an open- hearth furnace into large, thick-walled iron molds (Fig. 1). Each mold produced an ingot that then was rolled into a slab and further processed to become coils. Steelmakers called this “rimmed steel” because its high quantity of internal oxygen and other gasses that caused an eruption of excess gas on solidification, forming a clean edge surface or “rim” on the ingot.
Unfortunately, most of the gasses remained trapped in the atomic structure of the steel. Rolling or temper-rolling the steel activated strain aging and, with time, the oxygen and nitrogen segregated to different atomic zones. This increased the steel’s strength and reduced its stretchability; the yield- point elongation returned. The coils should have been labeled, “Best Used Within 90 Days.”
To return some of the steel’s stretchability and eliminate yield-point elongation, the rimmed steel coils were again temper- or flex-rolled. This retriggered more strain aging and an increase in yield strength. As a result, metalformers used “old” rimmed steel for easier stampings and specified “fresh” steel for more difficult stampings.
Aluminum a Strain-Aging Cure
More recently, steel-mill metallurgists discovered that adding aluminum to the melt causes the gasses to be tied up as aluminum oxides and nitrides. This prevents the eruption of gasses when pouring the molten steel into the mold, and avoids the strain-aging problem. Since aluminum shot caus- es the steel to become dead in the mold, the steel is called alu- minum-killed—it essentially has an infinite shelf life without change of mechanical properties.
Today, most steel is made by the continuous-casting process. Here, molten steel fills an oscillating, water-chilled mold high on top of the casting machine. As the steel exits downward from the mold, a thin shell of solid steel forms and
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
Tooling Technology
Schematic shows molten steel being transferred to a mold for solidification into an ingot. Each ingot is hot- and/or cold-rolled to different dimensions to form a master coil. Cour- tesy of AISI- Sheet Metal Formability, Washington D.C., 1984.
enters a series of rollers. Water spray solidifies the continu- ous slab as it curves through a 90-deg. curve to exit the cast- ing machine on a horizontal runout table.
Obviously, the eruption of excess gasses of rimmed steel would form holes in the thin shell of molten steel and cause a disaster, as molten steel would pour all over the equipment and floor. Therefore, only aluminum-killed steel can be used in the continuous-casting process, eventually eliminating the production of rimmed steels.
Too often, accurate explanations of why old rimmed steel will not make difficult stampings are difficult to find in press shops. More likely some creative story has been generated and passed from one generation to another. Even today a given coil of steel can be called flawed because it aged too long in the back storeroom, generating a call for fresh steel. The demise of rimmed steel and the changeover to alu- minum-killed draw-quality (AK-DQ) steel is an untold story. Even worse, AK-DQ steels now are divided into drawing steel and forming steel—another level of confusion. For ref- erence, the steel world calls these mild steels.
Bake Hardenable, for Dent Resistance
As another example of only half of the information being explained, consider bake-hardenable (BH) steels. Intro- duced in the late 1980s, these steels offer maximum stretch- ability to form difficult parts, and then exhibit an increase in yield strength after forming. Automakers like these proper- ties for dent resistance. BH steels are made with extra carbon —a yield strengthener—in solution. After parts are formed, they receive a coating of paint and an approximate 20-min. paint bake. The heat from the baking furnace causes the
36 MetalForming/August 2013
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