Advanced High Strength Steels in Modern Cars


This episode: The mad metallurgical voodoo of advanced high-strength steel. That’s right, brainiacs - strap on that propeller cap, and rack that slide rule: We’re goin’ in...


Full review of the 2019 Hyundai Santa Fe: Click here >>

If you’ve ever wanted to review cars for a living, I should tell you it’s not all 911s and hot tubs full of cheerleaders. One of the least sexy aspects of reviewing new cars is reading the press kit. Kind of obligatory, but a miracle cure for insomnia, right there.

So here I was, a few days previously, pleasantly succumbing to narcolepsy on page four of the new Hyundai Santa Fe press kit - only 18 pages and half a bottle of Vodka to go. Yessss!

I’d already learned that 57 per cent of the new body shell was made of advanced high-strength steel - and they have to hot stamp 15 of the major parts now, which is also kind of a big deal, structurally. (That was on page three.)

So I flick to page four, my eyelids are making like MH 370 over the Indian Ocean, incipient la petite mort, and I see this:

“torsional stiffness is increased by 15.4 percent to 31.5x104kgf-m2/radian”

Suddenly I’m wide awake, shouting ‘holy nude Twister, Batman!’ 

Because that’s interesting.

That body shell is stiffer than Bill Cosby riding Aspiring Starlet in the Benzodiazepine Cup. (The odds-on favourite.)

Putting a number on it - quantifying this torsional rigidity, which press kits seldom do - opens the window on the mad voodoo of structural design in cars, and why this advanced high-strength steel business really matters.

Despite the fact that it’s not the slightest bit sexy.

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How stiff is that?

That's 54,000 Newton-metres per degree of deflection. 54,000 Newton-metres is 120 times the peak torque of the Santa Fe’s diesel engine. Per degree of body twist. 

(Of course, the real trick engines pull off is not the torque - it’s the ability to generate that torque while rotating at 50 revolutions per second. So there’s that.)

Even so, the rigidity of the body is especially impressive when you consider:

  1. The shape of the body is not designed primarily to resist deflection (it’s designed to be a car, to carry seven people, and not look like a mad structural science experiment)
  2. The body hast these big holes all over it - we call them doors and windows - kinda necessary, but not all that helpful when you’re engineering-in torsional rigidity. The tailgate in particular, very unhelpful in torsion, and…
  3. The body’s just not that heavy. I’m ballparking it here, but it’s probably only about 500 kilos, as a bare shell. And cost is also a factor - you have to be able to build it, and not out of unobtainium.

When people think of tech - they think CarPlay and Android Auto. Wireless charging. Head-up display. I’d humbly submit that the mad voodoo of advanced steel is proper engineering porn, not one of these mildly titillating tawdry tech sideshows.

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Steel ain't steel

So here’s the thing - when people think about ‘steel’ they usually fail to acknowledge the astonishing variations in flavours available today.

Download the bible on AHSS >>

Mild steel has a tensile strength of about 250 megapascals - and before you curse and invoke the name of some fake deity, that just means you really won’t be hanging more than about 25 kilos off it (in tension) for every square millimetre of cross-sectional area. Bad idea.

High-speed steel (the stuff drills and cutting tools generally are made out of) has a tensile strength about 2500-3000 megapascals - that’s 10-15 times stronger, ballpark. It’s a lot harder, tougher, etc. So it’s just right for cutting the plasticene of steel.

But it’s not very workable. If you bend it too much, it just breaks.

Advanced high strength steel

Advanced high-strength steel is in the middle - up to about 1500 megapascals, but it will deform without breaking. It’ll endure up to 25 per cent elongation for manufacturing, unlike high-speed steel, which endures, basically, bugger-all plastic deformation.

So advanced high strength steel in the body of a car is up to six times stronger than mild steel, but about 35 per cent less workable - which is a worthwhile tradeoff. A lot more strength for a little bit less workability.

If you want to work it more than that, jam it into some complex shape, you’ll have to heat it up before you stamp it, which obviously costs extra. And they’ve had to do that a bit with the new Santa Fe.

Basically, increasing the use of advanced high-strength steels in cars allows manufacturers to achieve their strength targets without making those cars too heavy.

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The benefits

For you, the car owner, this means better ride and safer handling, because the body is no longer acting like a retarded and at times vaguely malicious secondary spring.

If you’re in a crash it means you’re sitting in a more survivable cage in the passenger compartment, while the ends of the car can be engineered to crumple and absorb the crash energy before it gets to you. That’s good.

It means the car weighs less, too. A huge advantage. That’s fuel economy, right there, plus better handling, more acceleration and better braking. Less wear and tear on things like tyres and brakes.

And finally there’s the environment. They might save about 100 kilos, potentially, in a design like Santa Fe, over building it in mild steel. That’s roughly the same as one big, fat passenger.

You might think that’s not such a big saving in isolation… I mean, cars don’t feel much different with 100 kilos of passenger aboard, and the fuel consumption isn’t noticeably different.

But the car industry manufactures something like 65 million cars annually at present.

Shifting to advanced high-strength steel could save as much as five or six million tonnes steel annually. More on mass reduction at WorldAutoSteel >>

That’s hardly insignificant, in the context of the impost on the environment.

More on AHSS at AZO Materials >>

Versus composites & aluminium

Yeah, composites would be lighter. But they’re a bastard to mass produce. They’re expensive. They don’t last as long as steel. When they fail, it’s catastrophic. And, their recyclability is shit. So there’s that.

Aluminium can slash weight, and it’s reasonably strong and recyclable - but it requires a huge energy input to manufacture, so the carbon footprint of aluminium is about 10 per cent higher than plastics and six times higher than steel.

More on steel V aluminium at Automotive News >>

Oddly enough, if you ever read a press kit from the four-ringed monkey spankers or the three pointed swastika about the wonders of their all-aluminium wanker express, they always fail to mention the environmental cost. Go figure.

Greenhouse implications

In a conventional internal combustion car, manufacturing accounts for about 20 per cent of lifetime emissions. But as the industry transitions increasingly towards so-called zero-emission vehicles, manufacturing rises to something like 80 per cent of lifetime emissions (depending on where you get the energy for propulsion).

So, although it might be nice to think you might be driving a composite or aluminium ‘clean’ EV in the future (and I’m looking at you, Tesla cult members) there’s a solid ecological case for advanced steels that extends well over the foreseeable event horizon.

Especially when you consider how easy it is to recycle steel.

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When a car hits a massive, rigid obstacle - as in this crash test - it crushes significantly, but then it bounces off. Every time. I want to talk to you about that bounce.

Why the bounce?

The structure crumples to protect the occupants. That absorbs kinetic energy, right? Momentarily, the car comes to a stop. Even if it rotates, there’s a point where the forward component of its momentum is zero. Job done.

Then it bounces backwards, off the block. And the block has nothing to do with that bounce. Nothing. What magic force pushes the car back?

The answer is locked somewhere in the mad metallurgical voodoo of metal. It’s something to do with the structure of the car. See if you can crack the kooky code on this and let me know why the car bounces back, in the comments feed.

The correct answer is here >>


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