Automotive: Cutting the Strong Stuff
Manufacturing Engineering Magazine September 2019
By Frank Burke
If you’ve recently visited planet Earth after being away for several decades, you may be pleasantly surprised to learn how the steel industry, automakers and their suppliers have been working together collaboratively to develop steels that are stronger, lighter and able to meet stringent safety, emissions and fuel economy standards.
All of these factors add up to meeting the seemingly contradictory goals of reducing weight for fuel efficiency and environmental considerations and providing superior collision protection for auto occupants, and accomplishing these objectives while aiming at a moving regulatory target.
The future is now for Advanced High Strength Steels (AHSS), or at least will be now for several 2020 model year vehicles, a number of which will benefit from using the latest AHSS to meet collision impact safety targets, including Hyundai Motor’s 2020 Sonata, Volvo’s XC40 plug-in hybrid, and the Ram 1500 light duty truck. (Refer to “Steel Muscle in New Vehicles” at worldautosteel.org for a list of new vehicles using AHSS.) Sophisticated engineering and metallurgy have produced steels that have progressed from High Strength Steel (HSS) to AHSS for significant cost savings and improved performance. (See sidebar on page 88 for a discussion of AHSS diversity.)
Close Partnership Boosts AHSS
The benefits of a close partnership among three groups—automakers, steelmakers, and supply chain partners—can be seen in how these new steels affect processing at the OEMs and Tier One manufacturers. This includes machining, principally for the supply chain of tool and diemakers, cold formers and hot and cold stampers.
Forecasters are optimistic about the greater adoption and use of AHSS products. According to WorldAutoSteel.org, the “rapid increase in demand requires automakers to master the fundamentals in record time and quickly get up to speed on specific application knowledge.”
To aid the auto industry in meeting these challenges, the AHSS Guidelines Version 6.0 has been formulated to assist automotive designers, engineers and press shop personnel in applying this next generation of steels to vehicle manufacturing. The guidelines include a broad portfolio of materials, advanced fabrication technologies, optimized joining processes and a database that captures AHSS Metallurgy. The Guidelines Version 6.0 are downloadable at WorldAutoSteel.org (click on Projects/AHSS Guidelines).
New Grades, Improved Performance
Advances in new grades of steel and innovative manufacturing processes have multiplied steel grades from a half dozen different grades in the 1970s to today’s 200 or so steel grades that are up to ten times stronger, according to Jody Hall, vice president, automotive program (formerly SMDI), American Iron and Steel Institute, Southfield, Mich.
“These advanced steel grades have produced improvements in performance and lightweighting of vehicle body structures to meet new collision protection requirements, fuel economy targets and emission reduction without making cars heavier, and in fact making them lighter,” said Hall. “Our members’ focus is on flat products, including sheet metal for body structure and chassis using mostly stamping, hot stamping and roll forming.”
The industry’s challenge was to develop new Third Generation Advanced High-Strength Steels from 800 MPa to 1,400 MPa with better formability and the ability to stamp more complex geometries, according to Hall. The new stronger advanced steels were needed to meet requirements about how vehicles would resist collision impacts from front, rear, side and roof (rollover), she said.
“Automakers saw the requirements making cars and light duty trucks heavier because of the added steel needed to perform better in collisions, ultimately calling for the development of new stronger AHSS grades,” said Hall. “The challenge was to develop advanced steels with higher strength and better formability. The more complex geometries couldn’t be stamped with AHSS, but if you heat up boron steels used for hot stamping and put the sheet metal blank into the die when it’s hot and stamp it, the cooling water in the die extracts the heat. The result is the strongest phase of the metal, Martensite, forming the complex part geometry when it is hot and very pliable.”
She noted that The Auto/Steel Partnership, a joint collaborative effort among the automakers and steel industry, has been able to meet industrywide challenges like improving how dies stand up under the stresses of newer, stronger steels by developing better die materials and putting coatings on the dies to improve wear resistance and chipping resistance.
The Auto/Steel Partnership is a consortium of the automotive and steel industries to pursue research, validation and education that have helped automakers enhance vehicle safety and fuel economy and improve design and manufacturing. Auto/Steel Partnership initiatives include AHSS Repairability to develop automotive repair guidelines for AHSS grades having tensile strengths between 780 MPa and 1,800 MPa. Other AHSS initiatives include Stamping, Chassis Corrosion, Fracture Prediction, GMAW Welding and many others.
Manufacturing Tooling for AHSS
The tensile properties of AHSS are important considerations when designing and manufacturing fabrication tooling, according to Steve Clark, director of engineering, TechSolve, Cincinnati, an organization focused on advancing manufacturers through technology.
“Blanking, punching and forming tooling and processes will need to accommodate the higher strength, work hardening, strain rate sensitivity and possible non-homogeneous behavior of these complex materials,” said Clark. “Since higher forming pressures and press energy are required, all elements of the forming equipment and tooling must accommodate the loads necessary to ensure success. That means higher strength and higher surface hardness tooling materials prepared to smoother finishes and often with additional coatings for lubricity,” he said.
Tooling trial and rework iterations can be reduced using FEA-type forming simulation with CAD solid models of the tooling design, according to Clark. “During the simulation process, it can be determined if the component will require additional ribs and/or bosses to help control springback,” he said. “The harder tooling materials and expensive coatings used to accommodate AHSS materials can become costly if tools are designed in a trial-and-error fashion without simulation. Ideally, the design of the component is performed in parallel with the design and manufacturing of the tooling to optimize manufacturability while ensuring functional needs are met.”
AHSS components should not be designed as if they were typical mild steel components, he warned. “AHSS materials require more careful design, planning and processing to obtain good fabrication results.”
Superior Flat Products Needed
Steel producers like ArcelorMittal work directly with automotive OEMs and their supply chain as well as through the Auto/Steel Partnership, the automotive and steel industries consortium mentioned earlier that researches solutions for vehicle safety, fuel economy, and improved design and manufacturing.
“My team’s focus is on flat products, coils for typical automotive applications like the most critical body safety cage structures,” said Bala Krishnan, director of automotive product applications for ArcelorMittal Global R&D. “All the components are manufactured by hot or cold stamping or roll forming—all manufacturing processes to form flat steel into high-performance shapes for best weight and safety balance.
“We work with equipment suppliers and have partnerships with hot stampers and cold stamping facilities as well as in-house OEM stamping and assembly plants,” Krishnan continued. “My team frequently visits these cold stamping facilities at the OEMs or the hot stamping facilities, mostly at the Tier Ones, to make sure that they are familiar with existing products, to help resolve issues, and to advise them about products in the pipeline. We want them prepared to understand how die wear can be an issue with the stronger steels and that they need to have the correct press capacity to accommodate higher loads, wear, springback, etc.”
Krishnan believes that it will take a transformation of the manufacturing infrastructure to handle the new, stronger AHSS products that are being developed. “Diemaking, welding (particularly innovative laser welding), and hot and cold stamping will all be affected,” he stated.
Diemaking will have to adjust to stronger materials and be able to resist wear and chipping. Also, welding will be more difficult with adjustments necessary for heat cycles, types of electrodes and dwell times, according to Krishnan. Roll formers will have to deal with higher strength steels reaching 2,000 MPa and make sure that their roll forming lines have the power to form these and that the pre-pierced hole locations, some of which could be mounting locations, are within specifications.
“We’ll inform them about the level of springback that they will have to deal with and how multiple rolls in their roll forming lines can benefit them,” he said.
ArcelorMittal will introduce new advanced cold stamping grades in 2021 that are typically used in the side structure, rocker reinforcement, pillars, and roof rails as well as bumpers and seat structures. “New hot stamping grades up to 2,000 MPa and a lower 1,000 MPa will be also be introduced; the 1,000 MPa grade would be even more formable after hot stamping and will be able to take the deformation after a crash like an accordion,” he said.
Krishnan added that typical applications for hot stamping grades would be similar to cold stamping grades for front and side structures. “Our hot stamping grades can be laser welded to one another. It’s a key aspect of the technology that ArcelorMittal is able to weld different strength levels, like 1,500 MPa to 500 MPa to 2,000 MPa, into one big piece that are then hot stamped. They have different strengths and are currently being used for door rings by Honda and Acura,” he said.
Springback is a challenge for cold stamping, but not a big issue for hot stamping, said Krishnan. Springback is directly related to the thickness of the product and yield strength of the products. “The whole point of weight reduction is to reduce the thickness of the component so springback is a key consideration for diemakers who must be aware of the parameters of the new grades we are developing, which we communicate to them,” he said.