A Disruptive Approach to Strip Steel

Sometimes the time is right for a change. And right now might be one of those times when it comes to producing sheet steel.

Leonardo da Vinci isn’t the only inventor whose imagination was well ahead of what the technology of his day made possible (e.g., think only of the helicopter and the years between da Vinci’s drawing and Sikorsky making a real product of it). Sir Henry Bessemer, of Bessemer Furnace fame, is another. Back in 1856, he patented a steel-making process for casting strip steel. Unlike conventional steel-making methods (that he helped develop) that employ a multitude of steps, this one was much simpler and provided a more near-net approach to achieving strip. There was a slight problem, however. Which is that there weren’t the tools and methods available in Bessemer’s day to make this undertaking practical, to say nothing of commercial. In fact, Bessemer wasn’t alone in his quest for thin-roll casting capability. It was a process chased by metallurgists and steel makers throughout the better part of the 20th century.

It sounds like something that might have been in The X Files or a William Gibson novel:

1993...Port Kembla, Australia. . .Broken Hill Proprietary of Australia and Ishikawajima-Harima Heavy Industries (IHI) of Japan collaborate on...Project M.

Project M was a full-size plant dedicated to determining whether something could be reliably done that had only been shown to be possible on a pilot basis: Casting commercial quality low-carbon steel through the twin-roll strip casting process. It was something that was more than 100 years in the making. In this case, BHP and IHI had been working together since 1988. They’d proven that they could cast steel as their work proceeded between 1990 and 1992. They’d done stainless. They’d done low-carbon. But Project M had another requirement. It was to prove the technical viability of the process for carbon steels on a large scale.

But even Project M, though successful, could go only so far. A fundamental challenge was to determine whether what became known as the “Castrip” technology (a clever neologism if there ever was one) could be performed on a commercial basis as in: Make money while making steel.

Project M came to a close in 1999. BHP and IHI, which had invested hundreds of millions in the process, went looking for some assistance. Which they found in Nucor Corp. (Charlotte, NC), the steel company known for its innovative approach to, well, everything. In March 2000, Castrip LLC (BHP, 47.5%; Nucor, 47.5%; IHI, 5%) was established.

Project M gave way to Project C. This is beyond an experiment. This is the real deal. It is a full-blown Nucor facility in Crawfordsville, Indiana, that is dedicated to directly casting carbon sheet steel. And they are making product in Crawfordsville with the twin-roll technique.

(Castrip owns the technology. Nucor has the exclusive license to the process in the U.S.)

“The biggest excitement is around the fact that we’re able to get rid of the need for the huge capital-intensive slab caster and hot strip mill,” says Peter Campbell, director of Technology and Marketing for Castrip LLC. And, he adds, the energy requirement for the Castrip process is significantly less, which is not a minor issue in terms of steel-making technology.

At the risk of grossly simplifying the Castrip process, it goes like this:

  • Liquid steel (there is an electric arc furnace on the Crawfordsville site) is provided to a ladle metallurgy furnace. The ladle metallurgy furnace is used to adjust both temperature and chemistry
  • Steel travels from the ladle, through a tundish, into a transition piece that leads to a nozzle
  • The nozzle then delivers the liquid steel between two 500-mm diameter opposed rolls. The casting speed is typically on the order of 80 m/min
  • The solidified strip—generally 1,345-mm wide (though it can go up to 2,000 mm)—is fed between two pinch rolls
  • From the pinch rolls, the strip goes through a hot rolling stand that reduces the strip thickness by as much as 40%. The thickness that results is generally on the order of 0.7 to 2.0 mm
  • A cooling table. A shear. Coilers. Voilá!

It should be noted that while there are some proprietary pieces of equipment involved, there is a not-insignificant portion of standard equipment. And it is important to note that whereas the Castrip process and its associated equipment requires a length of approximately 60 m, the length for thin slab casting is on the order of from 300 to 400 m, and for conventional slab casting, it is 500 to 800 m.

Think about this: As the strip thickness decreases during the Castrip process, the throughput increases. That’s the opposite of conventional strip processing. That means that the Castrip advantage really comes into its own when you’re talking about thinner material. Peter Campbell points out that the cost of liquid steel is the same no matter what process you’re talking about. But the casting and rolling costs for Castrip are significantly less as you 
go thinner with the material. (It must be admitted that right now, they’re talking about non-exposed automotive applications for the strip that’s being produced. Campbell says that the material can be used to replace conventional cold- and light gage hot-rolled materials. And refinements are on the way, so body panel material is certainly not out of the question.)

Think about this, too: A Castrip mill can be operated (profitably) at about 500,000 tons per year. Even a mini-mill (for which Nucor, of course, is famous) needs to operate at three times that level.

And this leads to a huge possibility. Remember the original Ford Rouge Complex that integrated steel making with the rest of the elements of vehicle manufacture? Clearly, the necessary economies of scale for steel making don’t lend themselves to setting up a steel mill next to an assembly plant. At least not the economies of scale for conventional steel making.

“Major users of flat-rolled steel could have their own steel-making and sheet-making facilities,” Campbell speculates. Although he’s talking about the construction market, he does admit that it is conceivable that one day this could be the case for the auto industry, as well. (Campbell notes that he comes from BHP Steel, which has a concentration on the construction market, so they’re examining the opportunities there first.)

Which brings us back to Bessemer and everyone else who has tried to cast strip. What’s the issue? While there are several aspects, Campbell notes that one of the issues is related to understanding solidification at the micron scale. Realize that while the previous explanation of the process is elementary, getting the liquid/transitional/solid steel to flow isn’t a simple task. So on the one hand, there needed to be precise process parameters developed. That, in turn, necessitated sensors and high-speed computers that can provide the required process adjustments for the operation—things that certainly didn’t exist in the 19th century.

Campbell acknowledges that Castrip is a “disruptive technology.” And like other disruptive technologies, it will take time to have an effect.

But consider this: Nucor built its first steel mill in 1968. Today it is the largest steel producer in the United States.

Going from liquid steel to strip in a single setup: Castrip. (Image: Castrip Corp.)