Page 32 - MetalForming February 2011

The Science of Forming
By Stuart Keeler
Why Sheetmetal Grain Size is Important
Cut a small coupon of sheetmetal. Grind and then fine polish any of the resulting surfaces—exposed top or bottom, edge parallel to rolling direction, or edge perpendicular to the rolling direction. Etch the surface of inter- est and examine under a microscope to study the microstructure or metallurgical components of the material, and perhaps discover some pre- cipitates or inclusions.
Fig.1—Surface grain boundaries can remain high while the inner core of the grain depletes during severe stretching. This visual defect is called orange peel.
Sheet surface
Grain
Grain boundary
The most common observed components are
grains with a mostly equiaxial geometry for sheetmetal as- received from the producing mill. The size of the grains plays an important role in the characteristics of the materi- al, ranging from increasing yield strength to causing visual surface defects.
To illustrate the fundamentals of grains and grain size, let’s consider low-carbon steel as an example. Steel is composed of atomic structures called unit cells, constructed from iron atoms. The iron cell structure is alloyed with carbon to make steel. As molten steel begins to solidify, groups of iron cells with identical cubic orientations coalesce into grains or crystals. To minimize surface area, the grains grow as spheres.
Grain size depends on composition, temperature history and other processing requirements. If a large number of grains begin to grow from the melt at about the same time, the solidified metal will have a smaller grain size. If only a few grains begin to grow, they will have more space in the melt to reach a much larger grain size. Unlike the interior of the grains, the boundaries of adjacent grains are disorganized intersections and are stronger than the interior of the grains. One of the first techniques to increase the strength of high- strength low-alloy (HSLA) steels is to generate a finer grain size with more of the stronger boundaries.
Grain size is reported as an ASTM grain-size number, n (ASTM E112 Standard Test Methods for Determination of Average Grain Size). This number then is used to calculate the
Stuart Keeler (Keeler Technologies LLC) is best known worldwide for his discovery of forming limit diagrams, development of circle grid analysis and implementa- tion of other press shop analysis tools. Stuart’s sheet- 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 the many seminars he teaches for PMA.
Keeler Technologies LLC
P.O. Box 283 | Grosse Ile, MI 48138 Fax: 734/671-2271 keeltech@comcast.net
number of grains per square inch at 100x magnification: Number of grains = 2(n-1)
It’s evident why HSLA steels often have grain-size numbers
ranging from 10 to 12 as one process used to increase their strength. What about low grain-size numbers? Traditional low-strength steels such as aluminum-killed draw-quality (AKDQ) steels or the newer designations of drawing steels (DS) and forming steels (FS) have grain-size numbers around 6or7.
Grain-size numbers of 5 and lower can create a visual sur- face problem called orange peel. Remember that the grain boundaries are stronger than the grain interior. When the steel is stretched to large strain levels, the grain boundaries resist deformation and allow the core of the grain to deform (Fig. 1). This obviously is not acceptable for a Class A surface, so some companies will specify a grain-size number of 6 or finer on their purchase orders.
When Accidents Happen
Continuous annealing lines (replacing older box-anneal furnaces) will sometimes experience a stoppage, for any number of reasons. The portion of the coil stopped in the heating section of the furnace continues to heat, causing the existing grain size to grow to a 00 or larger. Steel-mill proce- dures call for this affected area of the steel to be cut out and scrapped. But what if the overheated section of the coil remains in the coil and is sent to a press shop to make large automotive floor pans? Any stretched features in the panel
Grain Size Number
No. of grains per sq. in.
1
1 Very large grain
3
4
6
32
10
512
12
2048
14
8192 Very small grains
30 MetalForming/February 2011
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