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The partial soft zone on the 2016 Civic’s rear frame rail varies from about 590-MPa advanced high-strength steel (pink) to 1500-MPa press-hardened steel (deep purple). The yellow depicts the transition zone between the two grades. (Photo by Ryan Gehm)

2016 Civic structure employs 'first' partial in-die soft zones

The 2016 Honda Civic employs selectively-tempered in-die “soft zone” technology in the B-pillars and rear frame rails, allowing zones within a single frame piece to have more ductile properties than the surrounding ultra-high-strength hot-stamped steel. The technology minimizes parts count and saves 15.6 lb (7 kg) compared to the previous-generation Civic.

“Our method of application for this [in the rear frame rail] is a world’s first,” according to Ryan Miller, Performance Development Leader for the 2016 Civic. “Full-band soft zones [such as in the B-pillar] have been done before on a few other cars, but what we’ve done that’s unique is we’ve created partial soft zones; this is much more difficult to do.”

Honda co-developed this specific application with supplier Gestamp over the past three years.

“These soft zones give us the ability to completely control the frame crush mode and prevent cracking of the frame at the same time. It’s one stamped piece of material and we control it by material property, not by thickness or any kind of tailor welding,” Miller explained.

The partial soft zones in the Civic’s rear frame rail varies from about 590-MPa advanced high-strength steel to 1500-MPa press-hardened steel.

“In a crash, we want this [part] to do an ‘accordion crush’—every bend absorbs energy. Without putting any geometric divots or anything like that in, it starts to crush here, then there, then there,” he said, pointing to the three varying soft zones. “So the whole thing ‘accordians’ up and prevents any cracking from happening in the high-strength steel. You can’t do that if you do full band, so that’s why we developed it this way.”

The process involves heating the sheet steel to 930°C (1706°F) before simultaneously stamping and cooling key areas of the part. A laser cutting trimming process then completes the component.

“During the stamping process, there’s cooling lines that are applied in just certain areas that cool the material at different rates and create a different amount of tempering. It actually creates a different material grade even though it’s still one part, one thickness, and one stamping,” he said.

The soft zones in the rear frame rail address two important issues: rigidity and weight.

“The traditional way of doing it is to use multiple parts. Body rigidity is based on section size or thickness of the material. If this was regular-grade material and we didn’t have these [soft zones], you’d have to put discontinuities in there—physical bend points to force it to bend where you want it to. Every time you do that you actually decrease the section in that area and then you lose rigidity,” Miller explained. “The other issue is that you have to absorb a given amount of energy, which means the material would have to be thicker, increasing weight. [This technology] solves those problems—it improves rigidity and reduces the weight.”

The next potential application for in-die soft zones is in the front frame rail, according to Miller. “It behaves very similarly to this [rear frame rail] except for the [crash] testing is a lot more extensive up there. Theoretically, that would be the next location.”

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