Page 52 - MetalForming Magazine February 2023 - Metal Forming for the Automotive Industry

plexity, as achieving the necessary microstructure in some grades requires quench, hold and reheat capabilities prior to a second quench.
The Horizon
Global production of 3rd Gen steels began during the past few years and continues to welcome new grades as process optimization continues. One new family under development—medi- um-manganese (MedMn) steels—likely is the next to become commercially available. Unlike the higher-manganese TWIP steels, these new grades likely will contain 3- to 10-percent Mn. One study reveals a combination of 1400- MPa tensile strength and 18-percent total elongation.
One grade within the MedMn fam- ily—deformed and partitioned (D&P) steels—has a yield strength greater than 2000 MPa and a uniform elongation of 16 percent. These alloys require anneal- ing before cold rolling to increase dis-
location density, followed by a tem- pering process to attain the targeted microstructural balance.
Parting Thoughts
Global standards for 3rd Gen steels do not yet exist, with requirements contained within individual OEM stan- dards. While approved suppliers will meet the minimum and maximum property requirements for each grade, each mill takes a different approach to meet these ranges, as the achieved properties are based on the specific equipment and capabilities at each mill. Rephrased: Approved suppliers will meet the necessary property win- dows but will take up a different por- tion of the allowed range, compared with other suppliers. Unlike mild steel and conventional high-strength steels, AHSS grades are engineered products and no longer are commodities where users can swap between suppliers with little due diligence.
In addition, we tend to describe these steels based only on their strength and elongation in a tensile test and refer to this ductility as a meas- ure of “global formability” (see the accompanying figure). This represen- tation fails to account for characteris- tics grouped under local formability, specifically enhanced bendability and enhanced cut-edge expandability. For example, it’s hard to see the benefit of CP steels solely from looking at the global formability diagram. However, these grades have a microstructural balance that gives them excellent stretch flangability (as measured in a hole-expansion test).
Similarly, other grades offer excep- tional bendability without cracking, making them good candidates for crush members in crash-energy-management applications. After compiling more data on bendability and hole expansion of different grades, readers can look for a local formability diagram. MF
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