ColdStamp-Steel

AMD Corp. is currently offering the production technologies and US patents of ColdStamp-Steel for sale or licensing.

Automotive 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. The proposed cold-roll high-strength steel for cold stamping ColdStamp-Steel, is ideal material for the production of the vehicle body-structure and safety components, including battery enclosures of electric vehicles (EV).

ColdStamp-Steel is a low-alloy composition with the total alloying elements (except carbon) < 3.0 wt. %. shows alloying elements of the steel.
Table 1

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 followed by 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.

Mechanical Properties
ColdStamp-Steel possesses formability suitable for cold stamping. A comparison of ColdStamp-Steel with the commercial cold-rolled, high-strength steels for cold stamping – SSAB’s Docol  martensitic grades, ArcelorMittal’s MartiNsite grades, and Kobo Steels’ Kobelco high strength grades – shows ColdStamp-Steel to be competitive.

Table 2 shows the ASTM standard tensile test results in the longitudinal/roll (L) and transverse (T) directions at room temperature (r.t.).
Table 2

The accompanying graph show the r.t. engineering tensile stress diagrams of ColdStamp-Steel after two heat treatments: quenching and low tempering ~450℉ (Q+LT) and quenching and high tempering ~1000℉ (Q+HT) . The ASTM standard tensile specimens with 2 in (50 mm) gauge were cut from the uncoated cold-rolled sheet of 0.04 in (1.0 mm) thickness in the L direction.

ColdStamp-Steel is coated by the commonly used processes, including galvanizing and aluminizing.  The steel is electro-galvanized without reducing its strength and ductility after quenching and low tempering (Q+LT) and hot-dip galvanized and galvannealed after quenching and high tempering (Q+HT).
Microstructure of ColdStamp-Steel consist of martensite, retained austenite, and small carbides (Q+LT) and ferrite and small carbides (Q+HT).

ColdStamp-Steel is 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. The steel possesses 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 a carbon equivalent of ~ 1.26.
An initiative to improve ColdStamp-Steel by reducing the carbon equivalent below 0.60 without the reduction of their mechanical properties is underway.

EV battery enclosures
Aluminum alloys have become the dominant material for battery enclosures used in EVs due to their low density and acceptable strength. Aluminum battery enclosures, or other platform parts, typically provide weight savings of ~40% compared to equivalent commercial steels. Traditionally, the best-suited aluminum alloys for battery enclosures are 6000-series and similar alloys.

Despite its lightweight and recyclability benefits, aluminum 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.

Increasing the battery capacity, a primary focus of EV developers increases the probability of battery failure, including overheating and the possibility of explosions. To eliminate the potential harm to EV passengers, it is necessary to utilize more robust material than aluminum alloys. Galvanized ColdStamp-Steel is an attractive material to be used in EV battery enclosures. Its sheets can substitute high-strength aluminum alloy sheets without increasing battery enclosure weight while improving their strength, safety, and lifetime.

Table 3 shows a comparison of the specific stiffness, specific yield strength, and specific ultimate tensile strength (ratios of stiffnesses and strengths to density) of the quenched and high-tempered and quenched and low-tempered ColdStamp-Steel and the heat-treated 6068-T6 aluminum alloy.
Table 3

ColdStamp-Steel can substitute high strength aluminum alloy without increasing the weight of the battery enclosures while strength and durability of the steel battery enclosure are significantly higher. The ambient corrosion resistance of the galvanized and aluminized ColdStamp-Steel competes with the corrosion resistance of high-strength aluminum alloys, while high-temperature corrosion and oxidation resistance are significantly higher.

The projected price of ColdStamp-Steel sheets is 45-55% less compared to the price of high-strength aluminum alloy sheets.

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AMD Corp. is currently offering the production technologies and US patents of ColdStamp-Steel for sale or licensing
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