Page 15 - MetalForming Magazine February 2023 - Metal Forming for the Automotive Industry

Cutting Edge
By Eren Billur, Ph.D.
Cold Forming of 1.5-GPa Steel: Is it Feasible?
Recent high-strength cold stamping: (a) TBF1180 in 2013 Infiniti Q50; (b) TM1320 in 2019 Mazda3; (c) TM1470 in 2021 Nissan Note; (d) TM1470 in 2022 Mazda CX60.
abcd
Hot Stamping of PHS 1500
Cold Stamping
of 3rd Gen-1500 MPa
Cost of Coil
Globally available, relatively easy to make steel
Blanking
Blanking is done in soft condition
Higher press tonnage required. Edge cracks may be observed
Heating
maintenance cost
No need for heating
Forming
(typically) 100-1200T press force, negligible springback
>2000T press force, hard to control springback
Productivity
10-25 seconds cycle time, with a 4-out die, 2.4 to 24 parts per minute
10-25 SPM press speeds may be possible
Trimming
space, logistics of parts
Trimming requirements should be similar
For many years, one of the strongest automotive steels was 22MnB5, a man- ganese-boron grade developed specifically for hot stamping applications. The steel also is known as PHS 1500 (press-hard- ened with 1500-MPa tensile strength), and in Europe a new nomenclature has been agreed upon by automakers to refer to the steel as CR1500T-MB—cold- rolled, 1500-MPa tensile strength, manganese-boron alloyed.
Comparison of the advantages and disadvantages of cold vs. hot stamping of 1500-MPa tensile- strength steel.
As discussed in a previous Cutting Edge column (To Heat or
Not to Heat, April 2022), automotive OEMs and Tier suppliers long have compared the benefits and the costs of the hot stamping and cold stamping processes—and some still do so. Until a decade ago, cold forming only applied
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
to steels with a maximum tensile strength of 980 MPa. Higher-strength materials only could be stamped into simple shapes such as door beams, or rollformed to constant cross sections— roof crossmembers or rocker reinforce- ments, for example.
Advancements in steelmaking have led to the development of the so-called 3rd Gen advanced high-strength steels (AHSS):
• TBF—TRIP-aided bainitic ferrite
• DH/DP-HD—Dual-phase high- ductility
• Q&P—Quenched and partitioned • TM—Tempered martensitic. These grades made cold forming of
higher-strength steels possible. In 2013, stampers began to cold-stamp A-pillars from TBF1180 steel, a record surpassed in 2019 with 1320-MPa TM steel. (Note: You may have read this as 1310 MPa in several online sources, although this grade has a tensile strength of 135
kgf/mm2, which should be rounded to 1320 MPa; at least one steelmaker calls this grade 1320.)
In December 2020, the first use of cold-stamped 1470-MPa TM steel was seen in a seat crossmember, and by mid-2022 A-pillar reinforcements were being stamped from 1470-MPa TM steel.
Advantages of Cold Stamping
Cold stamping gains favor for its (highly believed) lower costs. Most Tier suppliers and OEMs already have invested in cold stamping lines—trans- fer or tandem-transfer. Piercing and trimming traditionally occurs within the press, reducing the need for large capital investments and floorspace requirements for the laser trimming lines that typically accompany hot stamping lines.
In recent applications with higher- strength steels, higher-tonnage presses get the call. For example, one supplier
12 MetalForming/February 2023
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