Perfecting the Mix
The molten steel filling the ladle is like a blank canvas; other elements, such as silicon, manganese, vanadium, and niobium, are added to create the specific chemistry desired for the final product. For structural beams, Nucor-Yamato makes a grade of steel known as A992, which, since it was first standardized in 1998, has been supplanting other grades (primarily one known as A36) as the standard for building frames because of its high yield strength and tensile strength, especially under seismic stress.
From the electric arc furnace, the ladle of molten steel is moved to the ladle metallurgy furnace for fine chemical tuning. The ladle has a porous plug at its base for pumping argon gas up through the liquid steel, causing it to bubble and stir, much as salad dressing is shaken to mix its ingredients.
When the batch is believed ready (after about 40 minutes), a sample about the size of a silver dollar is taken, cooled, and analyzed in an optical emission spectrometer. The spectrometer provides a kind of fingerprint showing the amounts of various elements. If they seem to fall in the correct range, the batch is ready to cast.
Jim Schoen, a plant metallurgist, has seen the ladle do its work countless times in more than 20 years of making steel. As he stands in the pulpit above the ladle and watches the steel agitate, he marvels at the consistency of the process, which runs 24 hours a day.
The steel mixtures “fall out of spec,” as he says, once maybe every two months.
Casting
When the ladle rolls away from the second furnace, a pair of giant hooks lift it—still full of liquid steel—40 feet high, beyond a layer of gray stairs and catwalks, to be poured into its molds for crude shaping. Steel is poured from the ladle into a tub that divides it into two streams (another caster in the plant has four streams), each of which flows down a long mold, forming as a strand, before being cut to length by automatic gas torches at the bottom. The steel is alive, red-hot, and now in the rough form of a beam, called a beam blank or bloom.
Members of the technical staff know the temperature of the steel at all times. Just after casting, the steel registers near 1,800 degrees Fahrenheit. Sprays of water hit the beam blanks to help cool them to a solid state within minutes.
Some of the cooled ones are stockpiled for finishing later. Some go back in for reheating and final milling. The beam blanks are sent into a 10-foot-deep gas oven and brought to about 2,100 degrees Fahrenheit. When hot again, they slide into the “breakdown mill,” where they are rolled violently back and forth like missiles within flatbed channels, and then through a series of fearsome machines that press them into the correct sizes.
Finishing Touches
The flanges of the nearly complete beams must be cooled to promote their ductility. A finishing mill evens them out, and a straightener prevents any bow, sweep, or camber along their 27-foot spans. With a terrific grind and a nebula of sparks, the beams are cut to lengths measuring anywhere from 30 feet to 80 feet. Finally, they are taken for shipping to steel fabricators, who ready them for construction.
The steel beam, mighty yet plastic, has been melted, muscled, and sculpted into a new life. “Remember,” Schoen points out, “this started out as a piece of scrap.”