Steelmakers are developing innovative new processes and coatings to meet changing demands
There is a significant need within the automotive industry to find ways of increasing safety in a crash situation, using lightweight designs,” explains Anke Meyer, marketing manager at SSAB. One of the solutions is electro-galvanised steels such as SSAB’s Docol 1400 MZE.
The development of AHSS has required the steel-making industry to adapt its processes at each step. “For instance, as these grades are more alloyed than conventional grades, the casting operations require specific precautions,” explains Jean-Luc Thirion, general manager at ArcelorMittal global R&D. “We have adapted our rolling facilities to be able to increase the dimensional capabilities of these products, which are lower gauge despite their higher strength. Reaching the very high mechanical properties necessitates specific adaptations of the annealing and cooling process. And finally, the coating processes have been adapted to provide a high level of corrosion protection in spite of the high alloying rate.”
A traditional practice of the automotive sector has been the use of tailored blanks. These flat steel sheets of different thicknesses, grades and coatings, welded together, enable carmakers to reduce vehicle weight and address specific design and safety issues by putting the right steel at the right place in the right quantity. The advantages of this technology have been increased by the recent development of laser-welded blanks for hot stamping grades.
Laser-welded blanks can now be made from an extensive range of steels, including AHSS grades with all types of coatings. The advantages of laser-welded blanks made of ordinary high-strength steels also apply to welded blanks made of very high-strength steels and of ArcelorMittal’s Usibor1500P for hot stamping.
“These steels are used to further reduce the weight and increase the strength of the welded blank,” Thirion says. “The advantages of high-strength steels are further enhanced when combined with milder steel grades in welded blanks to adjust local formability in deep-drawn parts. It has been demonstrated on many occasions that the Forming Limit Curve (FLC) of the weakest metal does not by itself accurately predict the appearance of necking phenomena close to butt welded joints, even though fracture occurs in the weakest metal. To overcome this, we have developed a dedicated digital analysis tool for these configurations to support accurate prediction of fracture risk when drawing from a laser welded blank.”
“It has already been demonstrated that solutions combining the use of very high-strength steels and welded blanks offer the advantages of both technologies,” Thirion continues. “Hot-stamped, laser-welded blank solutions have been developed for this purpose. These optimise thickness and material utilisation through the use of laserwelded blank technology, while maximising mechanical performance through the use of hot stamped Usibor 1500P.”
To cope with the specific properties of Usibor 1500P, in particular its aluminum coating, ArcelorMittal has developed a dedicated welding process that ensures optimum performance of the welded joint and functional performance of the final part. “It was essential, at the design stage, to be able to guarantee that the weld would under no circumstances constitute a weak point in the structure concerned,” Thirion says. “Given that guarantee, engineers designing body-in-white structures are able to consider these solutions using conventional methods, without having to introduce sophisticated weld fracture models when calculating crash performance.”
“Our forecasts anticipate a fast growth in the upcoming years, especially thanks to the combined use of laser-welded blanks with hot-stamping technologies,” Thirion adds. “ArcelorMittal Tailored Blanks has developed a new hotstamped door ring concept which combines the benefits of laser welding technology with the high performance of hot-stamped steel. The new door ring is stamped as one part instead of the usual four. Forming the part requires just one stamping tool and one stamping operation. No post-assembly tasks are required. By reducing the number of parts and operations required and the engaged (gross) weight, the overall cost of ownership for such a door ring is significantly lowered.
“By using Usibor 1500P and Ductibor 500P, the weight of the optimised door ring was reduced to just 12.7kg – a 19% saving compared to a baseline C-segment vehicle. Front, side and pole impact tests were performed on the door ring using Euro NCAP standards. In all cases the optimised door ring met the required standard and, in the case of the side, pole and roll-over tests, it outperformed the existing baseline solution.”
Press hardening technology offers new opportunities to produce lighter and stronger materials. This technology entails heating a sheet blank made of boron alloying steel being to 900°C, formed and then quickly cooled to room temperature. The process makes the material three times as hard as the original sheet, which enables production of parts that combine very high strength with low weight.
“Basically all automobile manufacturers use presshardened body parts today,” says Per Josefsson, vicepresident of global sales and service at AP&T. “In addition to safety-related parts such as bumpers and side impact beams, press hardening is used when manufacturing floor tunnels and B-pillars, as well as some chassis parts.”
A B-pillar produced using conventional methods has a blank thickness of 2-3mm and weighs 20kg; with press hardening, it is equally strong but half as thick and 50% lighter.
Several car manufacturers estimate that the proportion of press-hardened parts will increase to 35 to 40% of vehicle weight over the next few years, compared to the current 5-10%. According to Josefsson, this will result in a demand for more press-hardening production solutions as current capacity is far from sufficient for automotive needs.
Blanks made of boron alloying steel, with or without aluminium/silicon or zinc coatings, are used in press hardening, depending on the desired corrosion qualities of the final product. The basic principle is for the blank to be heated to approximately 930°C, then immediately pressed into the desired shape and quickly cooled at a speed of 80-100°C per second.
The steel’s structure changes during the process; first from ferrite/pearlight to austenite where the material is most formable, and then to the high-tensile martensite. The material’s strength more than triples during the process – from 400 to 1200-1400 megapascal – at the same time that elongation qualities are modified.
“The longer cycle times associated with the process constitute one disadvantage of press hardening compared to conventional cold pressing,” says Josefsson. “The automotive industry aims to use the technology to manufacture increasingly complex parts with quality-assured properties in a flexible process with short lead times and at a low cost. The technology is under constant development in order to satisfy future demands.”
Swedish firm AP&T has developed its Multi Cylinder System to enable a 66% increase in pressing speed. An operation that previously took three seconds to perform now takes one second. This means the sheet metal retains a sufficiently high temperature to enable deep-drawing in one single stroke, allowing more complex parts to be formed.
The automotive industry’s ambition to reduce vehicle weight by using a larger proportion of press-hardened parts means the technology will be used to manufacture increasingly complex parts in the future.
The metal parts surrounding the car’s door opening are one example. Generally consisting of four parts made of different materials, each individually pressed and then welded together into the body, it will soon be possible to join the parts before pressing, reducing finishing work and considerably speeding up the process.
Materials are also experiencing quick development; for example, a new type of zinc sheet is being tested that does not need to be pre-shaped like current alternatives.