Lightweight Steel on a (Cold) Roll
A newly developed high-strength steel for cold stamping aims to beat aluminum for EV battery enclosures and other vehicle applications.
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. 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, bainite, ferrite, and retained austenite in different volume percent, depending on the steel composition and its 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.
ColdStamp-Steel 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 martensitic grades, ArcelorMittal’s MartiNsite grades, and Kobo Steels’ Kobelco grades – shows ColdStamp-Steel 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.).
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 the commonly used galvanizing and aluminizing processes that can be applied to SC1 and SC2 without reduction of its mechanical 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 to improve the SC1 and SC2 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- and 7000-series and similar alloys.
Despite its light weight 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. 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 aluminum alloys. Galvanized and aluminized ColdStamp-Steel, particularly SC2, is an attractive material to be used in EV battery enclosures. Steel sheets can be substituted for enclosures made from high-strength aluminum alloy sheets without increasing the structure’s weight, while improving safety, durability and longevity. Per-pound production cost of high-strength aluminum 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 aluminum 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.
ColdStamp-Steel can be coated in several ways. It can be galvanized by electroplate or hot-dip processes, which supply durability at long-term continuous exposures with the maximum temperature of up to 392°F (200°C). Continuous exposure to temperatures above this can cause the outer free zinc layer to peel from the underlying zinc-iron alloy layer. Galvanized ColdStamp-Steel (compositions SC1 and SC2) possesses the mechanical properties as shown in Table 3 with some corrections on the added galvanic layers; corrosion resistance of the galvanized ColdStamp-Steel competes with the corrosion resistance of high-strength aluminum alloys. Cost of production per pound of 6000-series and 7000-series aluminum alloy sheets is more than 100% higher than the cost per pound of the galvanized SC1 or SC2 sheets.
After quenching and high tempering at 1,000-1,050°F, SC1 and SC2 possess tensile strength of more than 175 ksi (1,200 MPa) and elongation of 9-10%; quenched and high-tempered SC1 and SC2 can be galvannealed without reduction of their mechanical properties. Cost of the galvannealed ColdStamp-Steel is slightly higher than the cost of the hot-dip galvanized alloy.
ColdStamp-Steel, coated by electroplating or hot-dip processes, offers definitive benefits for use in battery-enclosure applications. The aluminized, galvannealed and galvanized ColdStamp-Steel can prevent failure of battery enclosures at temperatures of up to 1,400°F (760°C) and withstand fire up to 2,200°F (1,205°C), allowing for more time for EV passengers to evacuate in the event of an emergency. Durability of battery enclosures made from the aluminum alloys cannot compete with the durability of enclosures of the same weight made from coated ColdStamp-Steel at ambient and elevated temperatures. Furthermore, per-pound production cost of 6000-series and 7000-series aluminum alloy sheet is currently more than 100% higher than the cost per pound of aluminized ColdStamp-Steel sheet.
Dr. Gregory Vartanov is chief engineer at Advanced Materials Development Corp.