A Briefing on Hot Stamping
One of the vehicles that globally anticipated in 2020 was the Volkswagen ID.3, the first vehicle in the marque’s drive toward high-volume electrification. As an electric vehicle, the ID.3 has to have reduced mass so as to facilitate range (less weight to move means less work for the motors which means less battery power is drawn), but it also has to be safe.
Producing the VW ID.3 at a factory in Zwickau, Germany. (Image: VW)
So to that end, the vehicle features A-pillars and side sills that are produced by a Czech supplier, Snop Automotive, on a hot stamping line that includes a 1,200-metric ton press, an oven that heats the blanks to 930°C, three-axis twin feeders for blank transport, a blank loader, and dies sourced from Schuler.
According to John Catterall, vice president, automotive program for AISI, hot stamping came into existence in order to address the need to be able to create complex shapes, like B-pillars, from high-strength materials, like boron steel.
This process, which is now become mainstream, actually goes back to the early 1980s, Catterall says, when there were side intrusion beams fitted into vehicle—but these were tubes, as more complex forming couldn’t then be accomplished. Roll forming could handle it.
But over time, that has changed.
So there are the sills and A-pillars of vehicles like the ID.3. B-pillars Complete body side rings. Need safety? This is the ticket. Or a ticket, as we’ll see.
One of the things that may not be widely understood about hot stamping is that the characteristics of the end part are not just a consequence of the heat. The heat provides formability of the otherwise difficult-to-form material. (Boron steel can be formed without being warmed.)
Schuler made its first hot stamping presses for automotive applications in 1993. Ford was its initial customer. The equipment was used to produce side impact beams and bumpers. Since then, Schuler has sold 100 hot stamping systems. And it isn’t the only company providing equipment for the process. Which means there is a supply base that has been built out for OEMs. (Image: Schuler)
In fact, when Catterall talks about the process he refers to it as a press-hardening process.
Here is the thing: The material is heated, then stamped. While it is in the die, it undergoes a material treatment: the austenite in the steel is transformed so a martensite microstructure is
The Ford Bronco will make use of a 3rd generation steel—that will be cold formed. (Image: Ford)
developed. The quenching of the steel is performed while it is still in the die.
And this is something that requires serious process control. Catteral points out that as soon as the hot blank goes into the die it begins to cool. So there are the considerations of making sure that the forming is done before it cools too much and that there is uniformity of the material in the die so that a homogeneous part produced. “It is all done in the die,” he says.
About the Equipment
Which leads to a question about the equipment.
Catterall says that assuming there is sufficient tonnage for the parts to be produced, a press that is used for non-press-hardened parts can be used for the operation.
As for dies, again, quite similar but—there needs to be cooling channels to carry away the heat. “You have to cool these dies because otherwise, the temperature won’t drop quickly enough to get the transformation into the martensite microstructure,” he says.
He says that due to the complexity of some dies it may be that drilling cooling channels won’t be sufficient, that additive manufacturing, which allows the creation of curved channels, may be necessary.
He says that surface treatment of a hot stamping die is different than for a conventional stamping die. But that because a softer material is being formed with hot stamping there isn’t as much friction on the surface of the die than would otherwise be the case.
Thermal expansion and contraction have an effect on the dies, which is something that Catterall says operators need to keep an eye on.
And Now a Change. . .
According to Catterall, the steel industry works to address the needs of OEMs.
And this rolls us back to the beginning, to when Schuler supplied Ford with the hot stamping equipment back in 1993, as the press manufacturer’s first customer for the system.
In October 2020 ArcelorMittal announced that is the sole source of 3rd generation advanced high-strength steel for the 2021 Ford Bronco. Which is putting Ford in a pioneering position, once again.
Why is this material, Fortiform 980, notable?
Because Ford was looking for a steel that would be both lightweight (the OEM was looking for a 10% weight-save) while providing the utmost in safety:
And because this Gen 3 material is cold stamped.
Does this mean the end of hot stamping?
Catterall doesn’t think so. But he suggests that now more OEMs are going to be running the numbers to determine the best business case for the type of material they select for specific applications.
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