The mad metallurgy of advanced high strength steel
The mad metallurgical voodoo of advanced high-strength steel. That’s right, brainiacs - strap on that propeller, and rack that slide rule: We’re goin’ in…
This line from the 2019 Hyundai Santa Fe press kit >> caught my eye:
“torsional stiffness is increased by 15.4 percent to 31.5ｘ104kgf-m2/radian” - 2019 Santa Fe press kit.
That's roughly the same as 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
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),
The body has 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…
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.
Check out the 2019 Hyundai Santa Fe buyer’s guide >>
True tech
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.
So here’s the thing - when people think about ‘steel’ they usually fail to acknowledge the astonishing variations in flavours available today.
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.
But it’s not very workable. If you bend it too much, it just breaks.
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.
The benefits of advanced high strength steel
Basically, increasing the use of advanced high-strength steels in cars allows manufacturers to achieve their strength targets without making those cars too heavy.
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.
But the car industry manufactures something like 65 million cars annually at present. So shifting to advanced high-strength steel could save as much as five or six million tonnes steel annually.
That’s hardly insignificant, in the context of the impost on the environment.
More on advanced high strength automotive steel >>
Download a detailed guide to advanced high strength steel >>
