Page 31 - MetalForming November 2010

Cold worked
Cut edge
Fig. 2—A crack usually occurs at the cut edge that has reduced stretchability due to work hardening and the notch effect of an intersecting inclusion.
Fig. 3—The MnS inclusion without ISC act as notches and crack starters. This danger is minimized by the rounded inclusions formed with ISC. (Y. Bilimoria & N. Lazaridis, Arcelormittal, 9-11-2000.)
effects of the cold working during cut- ting and the inclusions.
• Send a steel sample to a metallur- gical lab to assess the type, quantity and characteristics of all inclusions.
• Send a failed stamping to a metal- lurgical lab for fracture analysis.
• For a given die, collect formed stampings and flat blanks from coils that run well and from coils that break. This proves the die can make acceptable stampings. Comparisons of properties and microstructures between make and break coils can be made that will speed the failure investigation. This inves- tigative work is best accomplished in partnership with your steel supplier.
Remember that part design, tooling design, lubrication, press characteristics and other inputs to the forming system generate the amount of required defor- mation. Therefore, a coil of steel with a given array of MnS or other inclusions easily can make one stamping design but catastrophically explode with anoth- er design.
Likewise, system inputs can change daily within a given forming operation. This means a standard upper limit on MnS inclusions cannot be established that will cover all combinations of form- ing severity. In addition, the demand for higher strength steels is increasing each year. Utilization of these steels will require cleaner microstructures to generate the properties needed for even more complex and demanding stampings. MF
Without ISC
With ISC
edge has been highly degraded by the severe cold work created by the cut- ting, blanking, slitting, trimming or shearing operations. This greatly reduces the allowable edge stretch even for inclusion-free steel. When an inclu- sion intersects this cold worked edge (Fig. 2), a natural crack starter is pres- ent that reduces the allowable edge stretchability even more.
Early metallurgical procedures to minimize the adverse effects of MnS inclusions are called Inclusion Shape Control (ISC). Mischmetal (rare earth) or calcium is added to the molten steel to combine with the globs of MnS to change their final properties and shape of the inclusion segments from elon- gated to circular (Fig. 3).
The typical approach to MnS inclu- sions is attacking the source of the prob- lem. The amount of sulfur in the steel is dramatically reduced. Instead of sulfur values greater than 0.050 percent, low sulfur values now are around 0.012 per- cent and ultra-low values can be found at 0.004 percent or less. Most of the sulfur reduction is accomplished in the metallurgical control station located between the basic oxygen furnace and
the continuous caster. Some of the more notch-sensitive advanced high-strength steels (dual phase and Martensitic) often will have both very low sulfur and ISC.
In summary:
• Lower sulfur reduces the quantity of MnS inclusions.
• ISC changes the shape from elon- gated to circular reducing the propen- sity as a crack initiation point.
• For severe stretching or very high strength steels, reduced sulfur and ISC may be required.
How does a press shop identify MnS inclusions as the source of the problem? The options for collecting clues range from quick to sophisticated. Several are listed here:
• For deformation of a sheet, rotate the blank 90 deg.
• For a bend, turn the blank upside down to see if the inclusions are near only one face of the sheet.
• Look at the “Certs” to check the sul- fur level.
• Run longitudinal and transvers ten- sile samples to check for a major differ- ence between the two total elongations.
• Use a hole expansion test to docu- ment the individual and combined
This Science of Forming CD accumulates four years’ worth of Dr. Keeler’s published
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