PORTAGE | Metal that just received rust-proofing piles up in the cavernous warehouse of U.S. Steel's plant in Portage, ready to be trucked off to factories throughout the Midwest.

A few workers wrap plastic around massive steel coils that can weigh more than a M4 Sherman Tank. The men in hard hats look like distant boats in a silver and grey matte sea of banded coil.

The rolling line whirs so loudly that shouts sound like whispers from a few feet away. A thin ribbon of steel sheet runs continuously through the rollers. The stretched-out metal speeds by at several hundred feet a minute – so fast it looks motionless from some angles.

U.S. Steel's Midwest Plant lately has been cranking out steel to keep pace with the best auto sales in more than half a decade.

The former National Steel finishing facility, which the Pittsburgh-based steelmaker bought out of bankruptcy a decade ago, processes metal for cars, cans, refrigerators, highway signs, utility boxes, doors and roof shingles. The plant makes the steel -- forged 10 miles away at Gary Works -- more valuable with treatments, such as zinc coating to keep it from rusting for years.

Though the steel industry as a whole has been struggling since the economic downturn, the plant on Lake Michigan's southern shore finishes steel for automakers who have been posting their strongest sales since 2007. The newly emboldened automakers are planning to ratchet up production to the highest level in a fourth quarter since 1999, said Tim Roper, owner of Smith Motors Auto Group.

U.S. Steel's Portage facility fills orders from most major domestic and foreign automakers, said Robert Minto, a quality manager for sheet at the Midwest Plant. The plant's 72-inch, hot-dipped galvanized steel line runs 24-7, barring an occasional brief shutdown for maintenance.

Nationally, shipments of hot-dipped galvanized steel products rose 6 percent in August over July, according to the American Iron and Steel Institute.

"In terms of global manufacturing, the U.S. and other global economies have been doing better," said Anthony Bridge, vice president of engineering and research and development. "They've been shaking off some of the lethargy of the second quarter. Overall, the economic performance has been choppy and sub-par, which has induced considerable uncertainty in investing and hiring. But we're moving toward more consistent growth in the coming quarter."

Domestic steel production has bounced back after a precipitous drop in 2009 but still not risen back to its pre-recession level, Bridge said.

Cars and light trucks, however, have been a bright spot for the steel industry. Historically, the automotive sector accounts for about 15 percent of the demand for steel, so it is a crucial market, Bridge said.

On a recent Monday, the Midwest Plant had to fulfill several orders for a Japanese automaker, which has posted record U.S. sales this year. Carmakers put the metal they processed in Portage into exposed body panels and doors.

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At the plant, workers drove sledgehammers into welded seams to test for splitting. They wrapped weatherproofing plastic around huge coils of steel that – if unfurled – could stretch for miles.

Much of the production is fully automated.

An unending ribbon of steel sheet wound through massive machinery towering a few stories tall. The sheet can be stretched out to a fraction of an inch, to a point it looks more like aluminum foil than the sturdy stuff that goes into trucks, fridges and bridges.

The metal wound its way through several stations – cleaning chemicals that rid the surface of oils, automated scrubbers that scraped off dirt and water that rinsed it off.

After getting dried off and baking in a furnace, the steel was immersed in a bath of molten zinc and raised vertically though the cooling tower that juts out of the boxy factory like an exclamation point.

"It's the tallest building at the plant," Minto said. "It has to rise up that high, or it won't dry."

The production line makes both hot-dipped galvanized sheet and galvannealed steel. A big difference between those steel types is that galvanized metal gets painted before it's formed and galvannealed steel can be painted later by the customer, such as at an auto factory, Minto said.

Both types of steel are intended for products that are built to last, such as buses, trucks and vending machines. Grain bins, for instance, need the anti-corrosive coating to keep their metallic sheen after being exposed to the elements year after year.

Finished coils range in size from 6 tons to 35 tons, depending on customers' needs.

U.S. Steel alters the thickness of the sheet and other dimensions, based on what orders it has to fill that day.

Lately, more of the steel has been going into cars.

"We serve all the big automakers," Minto said. "It was small, but it's been growing."