Page 44 - Metalforming Magazine April 2022

Cutting Edge
By Eren Billur, Ph.D.
To Heat or Not to Heat
For many years, metal forming professionals have discussed hot stamping vs. cold, and where Tier suppliers should invest. Hot stamping requires high capital expenditure, including typically a high-tonnage (800 to 1200 tons) hydraulic press with 3 by 2-m bolster, a roller-hearth furnace more than 30 m long or a multi-cham- ber furnace, and associated automation equipment. These lines are energy- intensive with installed power capaci- ties often exceeding 2 MW. Recently, even wider furnaces and higher-tonnage presses with larger bolster dimensions have become the norm, to accommo- date large laser-welded parts such as door rings. Typical cycle times can range from 8 to 20 sec.—considered inefficient by some. However, with two- or four-out systems, productivity can approach that of a cold-stamping line.
On the other side of the spectrum, cold forming typically occurs in a pro- gressive- or transfer-die press, servo or conventional mechanical drive. Pro- gressive-die presses have tonnage rat- ings commonly in the range of 630 to 1250 tons at relatively high stroke rates. Transfer presses, typically ranging from 800 to 2500 tons, operate at relatively lower stroke rates. Power requirements
Eren Billur is the founder of Billur Metal Form, a consulting, engineering and training company in Ankara, Turkey. He stud- ied at Baskent University and Virginia Common- wealth University, received a Ph.D. in Mechanical Engineering from The Ohio State Uni-
versity, and worked as a researcher at the Center for Precision Forming. His areas of expertise include material characterization, sheet metal forming processes and finite element simulations. He has authored/co-authored more than 20 scien- tific papers (including proceedings) and con- tributed to four books, including “Hot Stamping of Ultra High Strength Steels,” published in 2018. Eren Billur, Billur Metal Form, Founder eren@billur.com.tr
can vary between 75 kW (630 tons) to 350 kW (2500 tons). Recently, we’ve seen European companies installing transfer presses of 3000 tons or higher capacity. Steels for cold forming can range from the softest interstitial-free grades with ultimate tensile strength (UTS) as low as 250 MPa, to 3rd Gen advanced high-strength steels (AHSS) with 1500-MPa UTS.
In early 2000s, the only available steel for hot stamping was Type 22MnB5, commonly referred to as PHS 1500. Ini- tially, the grade was nominated with a minimum 1000-MPa yield strength ( YS) and a minimum 1500-MPa UTS, although many OEMs now classify this material with a minimum YS of 950 MPa and UTS of 1300 or 1350 MPa.
PHS Unrivaled, Until Cold- Formable Giga-Pascal Steels
When the 7th-generation Honda Civic rolled out in 2001, it was the first Honda to contain DP590 AHSS in its body-in-white. In 2005, the Honda Odyssey featured a rollformed marten- sitic rear bumper beam with a UTS of 1300 MPa. In 2007, several Honda and Acura models started using TRIP780 and DP980 steels. That same year Honda
began using hot-stamped steel. During the mid-2000s to early 2010s, many OEMs sought alternative cold-formable grades at relatively high strength to replace hot-stamped components.
Ten years ago, I counted approxi- mately 160 hot stamping lines around the world. Stamping-industry profes- sionals were discussing the more- formable AHSS grades on the horizon, promising tensile strength of 1.0 to 1.2 GPa. Around this time, Volvo presented a study in Germany showing that a DP980 steel could have a higher YS than PHS 1500 after cold forming and bake hardening, perhaps making hot stamping no longer feasible. However, during the last 10 years, the number of hot stamping lines worldwide actu- ally has more than quadrupled.
In 2017, I, along with a group of researchers, completed a study similar to Volvo’s with a 3rd Generation TBF ( TRIP-aided bainitic ferrite). Since the material had nearly twice the elonga- tion of DP980, we strained it to 10 per- cent and bake-hardened it. With 2 per- cent or more pre-strain (Fig. 2), the material could easily have a yield strength of 1000 MPa after paint baking, almost on par with PHS 1500.
1800 1600 1400 1200 1000
800 600 400 200
0
0 2 4 6 8 10 12 14 16 18 20 22 24
Engineering Strain (e) [%]
PHS 1500
MART 1300
DP 980
TRIP 780
DP
590
42 MetalForming/April 2022
www.metalformingmagazine.com
Fig. 1—Properties of advanced high-strength-steel (AHSS) grades commonly used dur- ing the mid-2000s. DP980 found use in basic shapes with limited draws, and martensitic grades were only rollformed.
Engineering Stress (ơe) [MPa]