Page 50 - MetalForming May 2016

The Science of Forming
By Stuart Keeler
How Heat Can Affect Steel Performance
Ferrite-Martensite DP
Grain boundary
Ferrite
Martensite
The microstructures of the different types of steel com- prise a variety of grain sizes, and grain size impacts an alloy’s mechanical properties. For example, the schematic in Fig. 1 portrays the grains found in a dual- phase (DP) advanced high-strength steel (AHSS). The grains making up the ferrite phase have two important, controlling features—the boundaries and cores of the grain. The grain boundaries are stronger than the cores, so to increase the strength of the steel, smaller grains are formed to reduce the size of the weaker cores. This increases the volume of the stronger grain boundaries. Grain size depends on the avail- able heat.
Steel types ranging from common mild steel to high- strength low-alloy (HSLA) grades are based on a ferrite microstructure. AHSS grades comprise one or more phases other than ferrite. DP steels (Fig. 1), for example, comprise a martensitic structure.
Excessive Annealing
Annealing of steels used to involve placing stacked coils onto a fire-resistant base. A metal tube with a closed upper end was placed over the stack to protect the steel from direct flames. Then, the annealing unit—another tube with attached burners—was placed over the original metal tube. With this process, the steel wraps in the middle of the coil under- went a different heating process than did the ends of the coil, the outer wraps and the inner wraps. This caused mechan- ical properties to vary along the coil and from edge to edge.
Steel companies now use continuous-annealing lines to achieve well-controlled and consistent grain size and other properties. The speed of the coil in the heat zone remains constant to achieve uniform properties. However, one day a real disaster occurred, when a steel supplier’s annealing rules were violated. As a result, when a metalforming com- pany attempted to stamp the material, very large tears formed. From the bottom edge was a wide tear in the sheet, which ran up through the center of the stamping. From the main tear were a number of side tears that turned into other
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 keeltech@comcast.net
Fig. 1—Schematic of a ferrite microstructure with a highlighted grain boundary. The 10-percent martensite grains exist in dual- phase (DP) steels. AHSS Application Guidelines, Ver. 5.0.
multiple tears. In between the long tears were groups of small tears. An examination of the surface showed rough pebbles over the flat steel. And, metallurgical examination of the stamping revealed that large grains had formed com- pletely out of control.
The cause of
the problem? The
steel assigned for the stamping had been heated in the anneal- ing furnace zone for more than an hour. When a continuous- annealing line stops for any length of time, the material with- in the annealing furnace must be cut out of the coil. That did not happen in this case.
Annealing Intermediate Stampings
Now consider a stamped part that must form in five dies. Attempts have been made to restore formability to a third die, allowing the stamped part to survive additional stretch in dies
No change
Smaller grains
Heat energy
Strain energy
Number of Grains
One grain
Fig. 2—High heat and strain energy are needed to recreate original smaller grain size in a partially formed stamping.
48 MetalForming/May 2016
www.metalformingmagazine.com
Grain Size