Automotive Engineering - June 2021 - 20

LIGHTWEIGHTING | COLD-STAMP STEEL FEATURE
Lightweight steel on a
(COLD)
ROLL
Lightweight steels will compete
with aluminum extrusions that
have been an early choice for EV
battery enclosures, such as seen
on the 2020 Audi e-tron.
A newly developed high-strength steel for cold stamping aims to beat aluminum
for EV battery enclosures and other vehicle applications.
by Dr. Gregory Vartanov
A
utomotive OEMs are faced with the difficult task of significantly
improving fuel economy and safety, while maintaining a competitive
position in the market - as well as investing in the electrified
future. This, among other benefits, can be accomplished
by utilizing higher-strength steel in the production process. A newly developed
and patented cold-roll high-strength steel for cold stamping, known
as ColdStamp-Steel, is ideal for producing vehicle body-structure and
safety components, including battery enclosures of electric vehicles (EV).
The microstructure of ColdStamp-Steel consists of one or more of
martensite, ferrite, and retained austenite in different percentages,
depending on the steel composition and heat treatment. Also, one or
more of carbides, nitrides and carbonitrides are part of the microstructure.
The resulting material offers high strength at moderate
ductility and ColdStamp-Steel features a higher strength-to-weight
ratio than commercial cold-rolled steels. Applications include vehicle
bumper reinforcement beams, pillars, door impact beams, rocker
panel inners and reinforcements, side-sill reinforcements, roof frame,
beltline reinforcements and clips.
ColdStamp-Steel is offered in Grades 1-3, which are low-alloy compositions
with the total alloying elements (except carbon) < 3.0 wt. %. Table 1
shows that Grade 1 has the lowest and Grade 3 the highest carbon concentrations.
The manufacturing process principally consists of the following
steps: melting of molten pig iron in a basic oxygen furnace, followed
by vacuum degassing or melting of steel scrap in an electric arc furnace;
continuous casting; hot milling; pickling; cold reduction; continuous anneal
and quenching and tension leveling for flatness. ColdStamp-Steel
can be manufactured as cold-rolled coils or cold-rolled sheets.
The product possesses formability that is suitable for cold stamping.
A comparison of ColdStamp-Steel with three commercial cold-rolled,
high-strength steels for cold stamping - SSAB's Docol 900M-1700M
20 June 2021
martensitic grades, ArcelorMittal's MartiNsite grades,
and Kobo Steels' Kobelco grades - shows ColdStampSteel
to be competitive. Table 2 shows the mechanical
properties of Grade 1-3 of the ASTM standard tensile
test in the longitudinal/roll (L) and transverse (T) directions
at room temperature (r.t.).
Heat treatments
Several compositions of ColdStamp-Steel have been
melted, hot- and cold-rolled, heat-treated, and tested.
Two compositions of Grade 3 have the most desirable
properties ( " SC1 and SC2 compositions " ). Table 3
shows that different heat treatments supply a wide
range of mechanical properties to SC1 and SC2. These
are applicable for different vehicle structural and safety
components, as well as electric vehicle (EV) battery-enclosure
structures.
The accompanying graphs show the r.t. engineering
tensile stress diagrams of SC1 and SC2 after heat treatments.
The ASTM standard tensile specimens with 2 in
(50 mm) gauge were cut from the uncoated cold
rolled SC1 of 0.06 in/1.50 mm thickness in the L direction.
Those with 3.15-in (80 mm) gauge were cut from
the uncoated cold-rolled SC2 of 0.04 in (1.0 mm)
thickness in the L direction.
SC1 and SC2 can be coated by the commonly used
processes, including galvanizing and aluminizing. Table 3
shows the heat treatments followed by those processes
that can be applied without reduction of mechanical
AUTOMOTIVE ENGINEERING
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