Page 54 - MetalForming March 2016

The Science of Forming By Stuart Keeler
Questions Answered Regarding Advanced High-Strength Steels–Part 2
The use of dual-phase (DP) steels, the first successfully applied advanced high-strength steels (AHSS), triggered extensive research at steel companies around the world. The AHSS grades that followed are even more complex and powerful.
As explained in last month’s Science of Forming column, the key enabler for DP steels is the interaction between their ferrite phase and the islands of martensite (MS) that generate a high instantaneous n-value. Unfortunate- ly, after 6 to 8-percent deformation, the extra n-value interaction is exhaust- ed and the instantaneous n-value returns to that of a high-strength low- alloy (HSLA) steel.
Research metallurgists have found, however, that adding more islands of MS to the ferrite phase generates a continuous increase in instantaneous n-value. The process has proved most successful when creating the MS islands in a continuous sequence over the entire deformation range, from yielding to failure. That practice became the catalyst for new AHSS grades, called transformation-induced plasticity ( TRIP) steels. These steels contain islands of retained austenite (RA) inserted throughout their
Stuart Keeler (Keeler Technologies LLC) is known worldwide for his discovery of forming limit diagrams, development of circle-grid analysis and implementation of other press-shop analysis
tools. Keeler’s metalform- ing experience includes 24 years at National
Steel Corporation and 12 years at The Budd Com- pany Technical Center, enabling him to bring a very diverse background to this column and to the sem- inars he teaches for PMA.
Keeler Technologies LLC P.O. Box 283
Grosse Ile, MI 48138 keeltech@comcast.net
microstructure. Deforming the steel causes different islands of RA to convert to MS that in turn generates new levels of instantaneous n-values that ulti- mately promote greater stretchability.
• How good are the properties?
The process works so well that the instantaneous n-values for TRIP 350/600 steel (350-MPa YS, 600-MPa UTS) reach a plateau of 0.25. This TRIP steel has the same stretchability and forming-limit curve of a 200-MPa VD- IF mild steel at almost double the yield strength. In addition, TRIP steels have been called “designer steels” because, if a TRIP steel has a low-carbon retained austenite, the transforma- tion to a high n-value occurs during stamping. This allows for significant- ly increased part complexity. Howev- er, if a high-carbon RA is used, the transformation will activate only dur- ing a crash event to absorb the crash energy.
• How do TWIP (twinning-induced plasticity) steels differ from TRIP steels? The two steels, often confused, are completely different in microstructure. TWIP steels have high manganese con- tent that causes the steel to be austenitic at room temperature. When deformed, the grain microstructure splits in half. As these twin grains con- tinue to split into finer and finer grain sizes, the boundaries between them act to strengthen the steel. As a result, these steels exhibit very high strength along with very high instantaneous n- values (0.4), allowing for extremely high
stretchability.
TWIP steels remain in production
trials. Their cost is relatively high due to the added austenite. And, there are some potential welding problems with these grades. However, steel compa- nies continue to search for steels with a specific increase in properties above the current properties of DP and TRIP
steels, and TWIP steels often exceed those ranges.
• What other new steels are available?
The AHSS family contains a number of lesser-known steels.
• Ferritic-bainitic (FB) steels offer improved stretchability of sheared edges as measured by the hole-expan- sion test, higher n-values and increased total elongations.
• Complex-phase (CP) steels pro- vide high tensile strength generated by extreme grain refinement. They also provide high energy absorption and good capacity for hole expansion.
• Martensitic steels show the highest tensile strengths within the group of multiphase steels. Automakers form them into cross-members, side-intru- sion beams, bumper beams and vari- ous reinforcements. For more complex shapes, MS steel is heated, formed in a die and quickly quenched in the die. This avoids elastic springback problems.
To illustrate the growth of AHSS grades, in 2009 the AHSS Application Guidelines listed about 20 commercial AHSS grades. In 2014, the book (avail- able from www.worldautosteel.org) presents nearly 50 grades.
AHSS research at steel companies, automotive laboratories and universi- ties has been focused on creating and improving these steels for more than 50 yr. For the better part of the last decade, research has focused on:
• Increasing formability while main- taining a constant yield or tensile strength;
• Increasing strength while main- taining a constant formability; and
• Exchanging the hard islands of MS at the shear edge of DP steels with nano-size precipitates to protect the edge. The goal is to minimize the chance of edge tearing as DP steel undergoes stretching of the sheared edges. MF
52 MetalForming/March 2016
www.metalformingmagazine.com