Page 31 - MetalForming July/August 2009

A
Grab and attempt to pull rug
Rug movement
B
Create and push hump
Rough Floor
Fig. 2—Schematic showing (A) almost impossible task to move a very large rug to the left by pulling, and (B) an easy move by creating and pushing a hump along the rug.
es can be used to strengthen steel, but the stretchability of the resultant product will be degraded compared to thermal- mechanical strengthening. In contrast, to create a lower strength steel, such as vacuum-degassed interstitial-free steel, the carbon content is greatly reduced, the steel is vacuum-degassed, and the effects of other elements present in the steel are minimized by combining them with titanium, columbium, and other additives.
All the different metal industries have conducted extensive research over many decades to understand how the chemistry and physics of the metals create different properties, experiment with appropriate processing, and to conduct consistency trials. The goal is develop products in a wide range of yield strengths to meet the ever-growing demands of the customer, who requires metal alloys with specific properties. MF
In the elastic portion of the defor- mation, the sheetmetal acted like a per- fect arrangement of unit cells through- out the entire sheet. Fortunately for the world, this perfect arrangement does not exist. The sheetmetal is full of dis- continuities or gaps at which a line of atoms is missing. Under sufficient exter- nal force, atoms on one side of the gap will jump from one side to the other, causing the line of missing atoms to move through the sheet. These moving gaps are called slip lines.
A rug analogy can illustrate how these slip lines create permanent defor- mation. Assume a huge rug in a room with a very rough floor needs to be moved one foot to the left (Fig. 2). One could pull and pull on the left edge (A) in an attempt to move the entire rug. Probably some of the threads of the rug will elongate elastically. Stop pulling on the rug and the threads will return to their original length (springback). The easy way to move the rug is to create a hump of rug one foot in width along the right edge of the rug (B). One can very easily push the hump (a slip line) across the room, thereby moving the rug one foot to the left.
Sheet steel coming out of the anneal- ing furnaces has nitrogen atoms “pin- ning” the slip lines. This requires an initial higher stress to initiate the plas- tic deformation observed at the upper yield stress in Fig. 1B. In our analogy, these nitrogen atoms are small nails tacking the rug to the floor. A higher force (upper yield strength) is required
to get the hump rolling. Once moving, the hump must continue to pull out additional nails as it moves across the room. This creates the almost constant load as the hump (now called a Lüder’s line) moves down the tensile sample. When the steel is temper passed after annealing, the effect of these nails is neutralized. In the old days of rimmed steel, the steel would age over time and the yield point elongation would return as the nitrogen atoms renailed the rug. The solution was to produce steel killed with aluminum. Now the nitride nails are tied to the aluminum and once pulled out are too big to be renailed. The aluminum-killed steels used today do not age.
How are different yield strengths created in steels? Common techniques include thermal-mechanical strength- ening. An atom of iron in the unit cell is replaced by an atom of another ele- ment that is either larger or smaller than the iron atom. This causes an expansion or contraction within the unit cell that makes it more difficult for slip lines to start moving. Atoms of other metals are placed as extra atoms inside the unit cells as further strength- eners.
Zones of unit cells with identical orientations are called grains. The grain boundaries are stronger than the grain interior. Therefore, smaller grains are stronger than larger grains. Cold work- ing the steel, significantly increasing the amount of carbon, quenching cer- tain microstructures, and other process-
www.metalformingmagazine.com
write no. 18 29