The Science of Forming
Why Sheetmetal Grain Size is Important
The most common observed components are grains with a
|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.|
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).
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 6 or 7.
Grain-size numbers of 5 and lower can create a visual surface 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 procedures 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 will be
|Fig. 2—Energy of cold working plus heat energy from an annealing furnace can cause a wide range of recrystallized grain sizes in a part. The areas with very large grains exhibit poor stretchability.|
As the steel is formed and cold worked, the equiaxed grains elongate in the direction of the deformation, become thinner and are cold worked. Some companies have tried to anneal the partially formed part back to its as-received grain structure in hopes of restoring the original stretchability for further forming. This annealing can backfire. When steel mills anneal their cold-rolled steel, the entire coil has been uniformly cold worked by about 70 percent, and then given a thorough anneal. When a metal stamper forms a part, some areas receive very little cold work while other areas receive heavy cold work.
To anneal or recrystallize the microstructure (Fig. 2) requires sufficient energy, provided by a combination of the cold work already in the part and the heat from the annealing furnace. For much of the stamped part, insufficient energy is available to recrystallize the grain size—nothing happens in those areas. However, at some part locations, enough energy is available to crystallize one grain, which will grow by absorbing its neighbor grains. Other areas of the part may have sufficient cold work to trigger different amounts of properly sized grains. MF
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