5 new EV-optimized automotive steels: CR1900M, HR1500M, 1000CP, 800HER and 1000DH-GI
Here’s a quick update on the Docol AHSS/UHSS product pipeline: what we’re developing, what is now available for trials, and what new grades are ready for sales.
Many of these grades, at least partially, are in response to new demands from electric vehicles. EVs continue to increase in weight as OEMs add battery cells for longer driving range. That means more stress on the car structure, which will require greater strength – while trying to keep body-in-white weights the same. And while managing that heavier, battery-generated weight, the BIW will also need to increase stiffness by using new material in innovative and optimized designs.
SSAB recently announced that it is working on a Docol 1900 MPa martensitic automotive steel grade, according to José Puente Cabrero, Product Manager for Docol Cold Rolled AHSS Steels.
Initial production trials prove that SSAB’s current steelmaking lines can produce the 1900 megapascal martensitic steel. If development continues as planned, Docol 1900M will become the strongest AHSS steel for cold-forming body-in-white (BIW) components ever offered.
“The share of EVs is increasing rapidly and will continue to increase at a higher pace,” notes Puente Cabrero. “At the same time, the average weight of EVs has also increased. Reasons include increased driving range, as well as an increase in electronics for active safety, driving performance, and comfort.”
“The crash requirements have also become more stringent. SSAB sees a need for an automotive steel that can withstand higher forces – while keeping the vehicle weight at an optimum level.”
Promising BIW applications for 1900 martensitic include transversal and longitudinal beams, reinforcements, and EV battery pack protection.
“The space between battery cells and the BIW deformation zones is steadily decreasing as more cells are added to the battery pack,” says Puente Cabrero. “Designers of battery pack enclosures have to take ever higher forces into their modeling for impact protection — while keeping weight and cost in mind.”
“1900M UHSS could offer EV OEMs strong, yet cost-efficient, lightweight design options that aren’t currently available.”
Roll forming will likely be the common way to shape 1900 martensitic, although cold stamping might be possible if the part is not too complex and springback is accurately modeled and compensated for.
Should we expect automotive martensitic steels that are even stronger than 1900 MPa in the future?
“There is a trade-off between higher strengths, formability and ductility,” says Puente Cabrero. “How much a higher strength steel can be utilized in an automotive part is also dependent on the OEM’s design philosophy, their fabrication practices, and how early in the development phase SSAB, as material experts, gets involved.”
“Keeping in mind the exponential advances in digitalization – and the opportunities digitalization brings to the material design and production technologies – I would be surprised if 1900 MPa was the limit. Plus, it’s part of our Docol DNA: how high in steel strength we can go? As long as those higher, gigapascal strength steels continue to contribute to improved car performance, higher safety, and environmental sustainability, we will keep pushing the limits.”
Puente Cabrero concludes, “SSAB’s working name for this grade is ‘1900M’, but our development will determine how the final specification of the product we launch will look like.”
“Electrification of cars is definitely speeding up the development and adoption of new hot-rolled AHSS steels,” says Daniel Sund, Product Manager for Docol Hot Rolled AHSS steels.
Traditionally, increasing AHSS strength levels has led to some compromises in formability. “What our new, exclusive Docol HR800HER (hole expansion ratio) grades do is provide the ability for chassis designers to increase strength and reduce weight — but with less limitations in forming,” explains Sund.
The Docol HR800HER-75 and Docol HR800HER-100 provide typical hole expansion rates of 75% and 100% — and guaranteed 45% and 65% expansion rates — respectively. Both 800 MPa steels are now available for commercial sales and can be found grouped with Docol complex phase grades.
These 800 megapascal steels are for very advanced, complex chassis shapes — for example, designs with intricate stretched edges and extensive flanging,” note Sund. “I believe these grades will rapidly become standard for EV chassis designs, where OEMs and Tiers are looking for strong, lightweight material that can be formed into complex geometries for critical components.”
“But Docol HR800HER grades are also ‘problem-solvers’ for existing part designs suffering from cracked edges during production,” reports Sund. “75% and 100% typical hole expansion rates clearly demonstrate their extremely high edge ductility, providing a superior margin of safety from micro-cracking.”
Another Docol exclusive: hot-rolled 1500 MPa martensitic grades up to 4.0mm thick are now available for trial testing.
“Again, heavier vehicles – think EVs – are going to need both stronger and thicker structural AHSS,” says Sund. “Imagine EV battery pack protection for heavier passenger cars, light commercial vehicles, and trucks. New door sill designs aiming for 5-star ratings using the updated, more rigorous side impact tests. Reinforcements for truck frames. And bumpers and cross members for all types of vehicles. ”
“SSAB’s cold-rolled Docol 1500M has been available for several years now,” explains Sund. “We started with the Docol CR1500M and, for the new hot-rolled version, we extended the dimension range so we could match the mechanical properties of our CR in a HR steel. We even guarantee the same mechanical properties for both cold-rolled and hot-rolled Docol 1500M.”
Trial testing from OEMs and Tier 1s will be critical in the finalization of Docol HR1500M. “We want to be sure the OEMs are getting the material they need for advanced forming of four millimeter-thick hot-rolled 1500 martensitic. Real-world forming verification is, of course, what matters in the end.”
“HR1000CP is the highest strength, hot-rolled complex phase steel available on the market today,” notes Sund. Docol HR1000CP was developed with invaluable input from Toyota and Gestamp, who were looking to reduce the weight of the lower control arm in the 2020 Yaris model.
“Toyota developed their own 1000CP specifications, which included very rigorous fatigue standards that, in turn, were tested and verified by Gestamp,” reports Sund. There is currently a draft VDA standard for 1000CP – plus SSAB’s own standard, as well as SSAB’s ability to meet specific OEM standards, as in the case of Toyota.
“Jointly, we went through several different concepts for the Docol 1000CP steel, with the outcome – the currently available grade – being the best in terms of fatigue performance and formability.”
Sund continues, “If Docol HR1000 complex phase meets stringent fatigue requirements for control arms, then, in my opinion, it’s a good candidate for many other high-end chassis components that need to be strong, enable both lightweighting and durability, and are cost-efficient.”
Docol HR1000CP is currently commercially available in 2.5mm to 3.8mm thicknesses, with future targeted thicknesses to range from 2.0mm to 4.5mm.
“Automotive OEMs increasingly want the option to use hot-dip galvanized 1000 MPa AHSS steels for body-in-white parts,” states Matti Säily, Product Manager for Docol Metal Coated Automotive Steels.
But at the same time, they need high formability for more complex shapes. “Docol CR1000DH’s formability makes it a straightforward upgrade from CR800DP,” says Säily. “CR1000DH enables significant weight savings and designer flexibility – because it’s readily stamped into complex, deep-drawn components.”
“And – when using state-of-the-art component forming techniques – Docol CR1000DH can even replace CR600DP (dual phase).”
Docol dual phase steel with high formability accommodates optimized BIW designs as they become more space efficient while also reducing the number of parts. Its 1000 MPa tensile strength can be used to reduce weight, increase load bearing, or improve impact resistance — or a combination of all those goals.
Customer trials for Docol CR1000DH-GI have just opened, with both 590 MPa and 700 MPa yield strength versions available. First commercial deliveries of Docol CR1000DH-GI are planned for September 2022. The preliminary data sheet for Docol CR1000DH-GI is now available.
“For the Docol CR590Y980T-DH, in addition to our hot-dip galvanized version, we have started to develop a galvannealed grade in response to market interest,” reports Säily. “And we’re also working on a new, hot-dip galvanized 1200 MPa dual phase high formability grade as well.”
Säily is convinced that better steel – higher strength with formability – enables better BIW designs. “This is about design freedom: we’re removing many of the designer’s forming concerns, opening up options for complex parts, while simultaneously allowing for strengthening and lightweighting of the body-in-white.”