Hot off the press

Press hardening is helping OEMs to use innovative materials in the manufacture of lightweight vehicles

Ever stricter legislation in the EU, the US and other countries to cut greenhouse gas emissions has created a worldwide trend that increases the pressure on automotive manufacturers to reduce the weight of their vehicles. A need to balance this with the demands of crash safety, which tend to add weight, puts the industry in a tight spot.
 
Enter hot stamping, or press hardening. In the past decade, the hot stamping of automotive body parts has evolved from a niche technology into one that is now indispensable for weight reduction using high strength steel (HSS). Volkswagen (VW), for one, is claiming to have initiated this trend with the latest incarnation of its Golf model, which was made possible by a strict adherence to lightweight construction methods such as press hardening.
 
So how exactly does press hardening work? Jens Aspacher, sales manager at Schuler, a specialist in this technology area, offers an explanation: “Firstly, blanks or pre-formed parts are heated in a furnace to a temperature of 930°C. The heated blanks are then fed into the press as quickly as possible to prevent rapid air cooling; there is a time window of just ten to 12 seconds. The press remains closed for a few additional seconds in the forming process, during which time the parts are both cooled and hardened.”
   
Cycle times for press hardening range from eight to 30 seconds. However, in combination with a two-slide press and Schuler’s fast-feeder solutions, the company can offer enhanced productivity with its PCH (pressure-controlled hardening) concept, reducing the time to six seconds in some instances. Furthermore, even complicated part geometries and the more recent bonded sheet materials can be easily manufactured with this technology.

“A key factor in determining high quality and short cycle times is the fast heat transfer from hot part to cooling water,” stresses Aspacher. “The weakest link in this chain sets the speed.”

Dirk Haller, Schuler’s process development director, adds: “The determinants of the part’s cooling period include the transferred energy, which depends heavily on the thickness of the blank, as well as the heat transfer between blank and die.”

He continues: “Further factors are the die’s thermal conductivity and energy dissipation, namely the number of cooling channels, the temperature of the cooling medium and the flow in the cooling channels.” 

Haller’s work has so far led to the design, build and delivery of almost 50 dies for the press hardening process, for customers such as Fiat and VW, SSDT in China and Proton in Malaysia.

Making the right contacts
According to Schuler, the general rule with press hardening is: the higher the contact pressure, the faster the heat transfer and the shorter the cooling time – and the better the performance.

“This contact pressure is normally produced by spring assemblies, nitrogen gas springs or hydraulic cylinders in the die, or a draw cushion in the press,” states Haller. “By increasing the contact pressure using PCH cushions and dies, optimising the cooling channels and selecting the die steel according to the part, we have succeeded in further accelerating the heat transfer process and thus reduced cycle times.”
 
The parts are still about 200°C when they are finally removed from the press and ready for further processing. The scaling which forms on the surface can be shot blasted or else avoided completely by coating the blanks in advance.
 
Once they have cooled, the final parts offer a tensile strength of up to 1,500 MP, meaning that they have to be cut using a laser. However, due to their greater rigidity, less material is needed per part to ensure the same level of stability, thus reducing weight. Schuler also says that, in contrast with the cold forging of HSS, less press force is required: 400-1,200 tonnes, versus 2,500-3,000 tonnes. Furthermore, the spring-back effects which are notorious in the industry are also greatly reduced.

Press hardening on the up
All of these benefits have led to a dramatic increase in the number of press-hardened parts produced around the world, which now include bumper supports, roof frames, sills, B-columns and tunnels. From around 8m pieces in 1997, the total rose to 124m in 2010 and over the next two years through to the end of 2015, demand is expected to reach as many as 450m pieces.

This is partly due to the fact that component characteristics can now be determined more accurately. Examples include parts with patch blanks or the ‘tailored tempering’ method. Schuler has also collaborated recently on a joint project sponsored by the German Ministry for Science and Research, entitled ‘Flexible heat treatment for the tailored shaping of part characteristics and the increased energy efficiency of the hot forging process chain’.

The main focus was on parts such as B-pillars, which need to be as rigid as possible in the middle but able to be deformed at their ends in order to absorb energy in crash situations. This can be achieved using dies with varying temperature zones: heated, cooled or insulated. 
 
Schuler’s latest development in press hardening is a new cutting line that combines proven coil feeding technology with innovative laser technology, thus creating the first continual laser cutting process from coil. The company says that the ability to individually programme several laser cutting heads working in tandem ensures flexibility and offers new perspectives – with unprecedented throughput rates – for the tailored cutting of blanks.

Supported by stronger steels
It is fair to say that the success of press hardening is largely attributable to the extremely high strength of the steels involved, with the development efforts of steel-makers in the field of manganese-boron steels being particularly important.

For example, Sweden-based Uddeholm says that manufacturers using its purposely-formulated tool steels for hot stamping can avoid common problems and achieve longer, more predictable production runs and reduced cycle times. Until recently, cold forming was the dominant method for producing critical structural components in HSS, but it is being rapidly usurped by the game-changing hot stamping method, which places new demands on the steels used in the process.

Tool steel for hot stamping requires a particular set of properties in order to cope with the specific failure mechanisms involved in the process. Selecting steel with the right properties means that auto manufacturers have a clear opportunity to improve end product quality.

To function reliably at the temperatures involved, tool steels need to demonstrate excellent thermal conductivity and hot yield strength as well as high toughness and ductility. This is because the process brings an increased risk of premature tool failure, leading to unplanned production stops and delays. Put simply, a high-end tool steel minimises wear at elevated temperatures and provides improved dimensional tolerances and fewer scrapped parts due to tool scratches.

Uddeholm produces several steel grades suitable for tools used in hot stamping. For instance, QRO 90 Supreme is the company’s recommendation for optimising cycle times in production when the surface of the tool is subjected to excessive heat. This is thanks to its high thermal conductivity along with good strength and toughness, enabling efficient internal cooling. The dies used to form advanced HSS are heated and must be allowed time to cool between pressings. QRO 90 Supreme tool steel helps to increase productivity by reducing cycle times.
 
Another Uddenholm material for hot stamping is Unimax, which shows its true qualities when excessive wear is experienced in the die. At a recommended hardness of 56 HRc, Unimax resists abrasive wear, both hot and cold, and is said to increase the life of hot stamping dies significantly.
 
The company says that using its steels in combination with hot stamping processes can result in fewer structural parts per vehicle, and reduce individual component weight by 30-35%.
 
Helping Honda’s Acura MDX
One of the best illustrations of the use of hot-stamped material solutions involves the ArcelorMittal Usibor door ring, an innovative, life-saving solution which is used in the Honda Acura MDX, resulting in significant weight reduction and improved safety. “Honda came to us with an idea to improve their vehicles, particularly for the MDX,” says Todd Baker, president of ArcelorMittal Tailored Blanks (AMTB) Americas. “They wanted to achieve a single-piece door ring that would accomplish a number of their objectives.”
 
“The new MDX is lighter, faster, stronger and more fuel efficient,” adds Jim Keller, 2014 Acura MDX chief engineer. “The body structure is almost entirely new, but taking weight out of a car and making it stronger is not an easy thing. We wanted to design the car to distribute the loads around the passengers – through the floor, the roof and the door,” he says. “We had to rethink how we could distribute these even more concentrated loads in a way that keeps people safe, and one of the ways we did that was with the new hot-stamped door ring.” Keller claims that the use of HSS for the entire structure around the door is a unique application of hot stamping in the industry.
 

The hot-stamped door ring in the Honda Acura MDX offers up to five times greater strength than a cold-pressed component

The Honda Acura MDX was designed from day one with a new small offset barrier crash test as part of its design strategy. This test is where the car hits another vehicle or a pole with around 25% of the front of the vehicle. “Incidents of this type cause 25% of the fatalities and major injuries in frontal crashes,” says Baker. “Currently, only about 30% of vehicles are getting an acceptable or better rating in the small offset crash.”
 
“The Acura MDX got the highest ranking in this test using our door ring,” states Greg Ludkovsky, vice-president of ArcelorMittal Global R&D. “After this very severe frontal crash test, which forces the whole front end of the vehicle to collapse into an accordion, we were able to come to the vehicle, open the front door and get in and out of the driver’s compartment without any problems. Hot stamping the door ring allows us to achieve significantly higher strength levels than in cold press materials – typically up to four or five times more.”
 
The project is a great example of collaboration within ArcelorMittal. Once the Usibor steel is produced, it is shipped to ArcelorMittal’s blanker, a joint venture with Delaco in Dearborn, Michigan. The blank door rings are subsequently shipped to the company’s tailored blanks facility in Pioneer, Ohio, where laser ablation and laser welding occur. Parts are then transferred to Eagle Bend at Magna’s Cosma facility in Clinton, Tennessee, where it is hot stamped before finally being delivered to Honda’s plant in Alabama.
 
“This was and is the first hot-stamped, laser-welded, Usibor door ring in the industry,” concludes Baker. “We introduced in North America a brand new process called laser ablation just to be able to weld the Usibor material. I believe it creates an industry standard that other automakers are going to look at and use as the benchmark for the small offset crash test.”