AMP_06_September_2021

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 1 2 5 a basic oxygen furnace; vacuum degas- sing or melting of steel scrap in an elec- tric arc furnace; continuous casting; hot milling; pickling; cold reduction; continuous annealing and quenching; and tension leveling for flatness. Cold- Stamp-Steel can be manufactured as either cold-rolled coils or sheets in un- coated, galvanized, and galvannealed product variants. The new steel possesses forma- bility that is suitable for cold stamp- ing. A comparison of ColdStamp-Steel with three commercial cold-rolled, high-strength steels for cold stamping— SSAB’s Docol 1200M-1700M marten- sitic grades [3] , ArcelorMittal’s MartIN- site grades [4] , and Kobo Steel’s Kobel- co grades [5] —shows it to be competitive with the lineup. MECHANICAL PROPERTIES Several compositions of Cold- Stamp-Steel have been melted, hot and cold rolled, heat treated, and tested. One composition in particular possess- es the most desirable mechanical prop- erties for EV component manufacturing. Figure 1 shows the engineering tensile stress diagrams of that composition af- ter two heat treatments: quenching and low tempering (Q+LT) at 450°F (232°C); and quenching and high tempering (Q+HT) at 1020°F (549 °C). The ASTM standard tensile specimenswith3.15-in. (~80-mm) gauge were cut from the un- coated cold-rolled sheets with 0.04-in. (~1.0-mm) thickness in the longitudinal direction, and the specimens were test- ed according to the ASTM standard at room temperature. The different tempering tem- peratures applied to ColdStamp-Steel produce high-strength mechanical properties suitable for safety and struc- tural components. This distinguishes it from commercial high-strength steels that possess only one strength and duc- tility level. COATINGS After quenching and low temper- ing, ColdStamp-Steel can be electro- galvanized without reducing strength and ductility. After quenching and high tempering, it can be hot-dip galvanized and galvannealed. A comparison of ColdStamp-Steel with commercial high-strength cold- rolled steels shows that after high tem- pering at 1000° to 1050°F (538° to 566 °C), only the proposed steel has a tensile strength of 180 ksi (1240 MPa) or higher, and elongation of 9-10%. This suggests that ColdStamp-Steel can be hot-dip galvanized or galvannealed without any reduction of its mechanical properties. Ambient corrosion resistance of the galvanized and galvannealed Cold- Stamp-Steel competes with the cor- rosion resistance of high-strength aluminum alloys, while high-tempera- ture corrosion and oxidation resistance is significantly higher for the new steel. WELDABILITY ColdStamp-Steel can be welded through conventional spot welding us- ing the adapted parameters. Given the increase in carbon concentration, it is necessary to increase the welding force and change welding cycles to achieve high-quality spot welding. The proposed composition of ColdStamp-Steel possesses the carbon equivalents CEVM = %C + (%Mn + %Si)/ 6 + (%Cr + %Mo+%W + %V+%Ti)/5 + (%Ni + %Cu)/15 of around 0.61, wherein the element concentrations are in wt%. By comparison, one widely used, com- mercial cold-rolled steel has the carbon equivalent of approximately 1.26. An initiative to improve ColdStamp-Steel by reducing the carbon equivalent be- low 0.60 without reducing its mechan- ical properties is now underway. COST COMPARISON The price quoted in June 2021 by a U.S. manufacturer of cold-rolled hot- dip galvanized ColdStamp-Steel sheets with 0.04-in. (~1.0-mm) thickness is $1980-$2080/ton, whereas the price of the 6082-T6 aluminum alloy sheet with 0.12-in. (~3.0-mm) thickness is $4100-$4200/ton. BATTERY ENCLOSURES A primary focus of EV developers is to increase battery capacity, but this in- creases theprobabilityof battery failure, including overheating and the possibil- ity of explosions. To eliminate potential harm to EV passengers, it is necessary to use material that is stronger at am- bient and elevated temperatures than aluminum alloys. Hot-dip galvanized and galvannealed ColdStamp-Steel is a suitable alternative for materials used in EV battery enclosures. With- out increasing the structure’s weight, ColdStamp-Steel offers a low-cost alter- Fig. 1 — Engineering tensile stress diagrams of a ColdStamp-Steel composition after two heat treatments: quenching and low tempering (Q+LT) at 450°F (232°C); and quenching and high tempering (Q+HT) at 1000°F (538°C).