Automotive Engineering - June 2021 - 22

Lightweight steel on a
(COLD)
ROLL
Table 1.
properties. Comparisons of SC1 and SC2 with the commercial
high strength cold-rolled steels show that after
high tempering at 1,000-1,050°F, only SC1 and SC2 possess
the tensile strength of more than 175 ksi/1200 MPa
and elongation of 9-10%. Quenched and high-tempered
SC1 and SC2 can be galvannealed without reduction of
their mechanical properties.
SC1 and SC2 can be welded by the conventional
spot welding with the adapted parameters. Given the
increase in carbon concentration, it is necessary to
increase the welding force and adapt welding cycles
to achieve high quality spot welding. SC1 and SC2 possess
the carbon equivalents CEVM=%C + (%Mn +
%Si)/6 + (%Cr + %Mo+%W + %V+%Ti)/5 + (%Ni +
%Cu)/15 of ~ 0.975 and ~0.61. Cold-rolled Docol 1700M
steel, by comparison, has the carbon equivalent of
~1.26. An initiative is underway to improve the SC1 and
SC2 by reducing the carbon equivalent below 0.60
without the reduction of their mechanical properties.
EV battery enclosures
Aluminum (AL) alloys have become the dominant material
for battery enclosures used in EVs due to their low
density and acceptable strength. AL battery enclosures,
or other platform parts, typically provide weight savings
of ~40% compared to equivalent commercial steels.
Traditionally, the best-suited AL alloys for battery enclosures
are 6000- and 7000-series and similar alloys.
Despite its light weight and recyclability benefits,
AL alloys have a crucial disadvantage if the heat generated
by the battery cells raises the temperature of
battery enclosures above 600°F (315°C). At more than
300-sec exposure at 600°F or higher, the yield
strength drops by more than 70%, especially for parts
that are in direct contact with the battery cells.
Furthermore, in critical situations of fire at about
2,200°F (1,205°C), the battery enclosures fail within ~5
sec, creating a paramount safety concern for EV occupants.
Regarding thermoplastics and composite materials,
their use in battery enclosures is challenged by
cost and by temperatures much less than 600°F.
Increasing the battery capacity, a primary focus of EV
developers, increases the probability of battery failure,
including overheating and possibility of explosions. To
eliminate the potential harm to EV passengers, it is necessary
to utilize more robust material than AL alloys.
Galvanized and aluminized ColdStamp-Steel, particularly
SC2, is an attractive material to be used in EV battery
22 June 2021
Table 3, heat-treat specifications of the SC1 and SC2 materials.
Table 2, wherein E, YS, UTS, and El are a modulus of elasticity, a yield strength
(0.2% offset), an ultimate tensile strength, and a total elongation.
Table 4, comparison of the r.t. mechanical properties of the SC2 sheets and the
7075 AL alloy sheets hardened to the highest strength.
enclosures. Steel sheet can be substituted for enclosures made from
high-strength AL alloy sheet without increasing the structure's weight,
while improving safety, durability and longevity. Per-pound production
cost of high-strength AL alloy sheets is more than 100% higher than the
cost per pound of galvanized and aluminized ColdStamp-Steel sheets.
A comparison of the specific stiffness, specific yield strength and
specific ultimate tensile strength (ratios of stiffnesses and strengths
to density) of SC2 and 7075-T6 (Table 4) shows that the steel can be
substituted for any high-strength AL alloy without increasing the
weight of the battery enclosures, as the thickness of SC2 sheet is 2.8
times less than the thickness of 7075-T6 sheet.
AUTOMOTIVE ENGINEERING
ALL TABLES: AMD CORP.

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