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How Steel Is Made : Steel Products: Flat Rolled

Hot Rolled

A semifinished steel slab destined to be rolled into flat rolled products must first be slowly heated in a process called reheating (to about 2300 degrees Fahrenheit for carbon steel) so that the temperature throughout the slab is equalized and the slab is softened. If the slab is allowed to cool after being cast, the reheating process can take about four hours.

The heated slab is then rolled under tremendous pressure or hot rolled, which reduces the thickness of the slab from 4-12 inches to 0.10-2 inches and lengthens the slab from 30-40 feet up to one-half mile. The different types of mills or machines used to prepare slabs for hot rolling are referred to as scale breakers, descalers, roughers, or scarfers. Each set of rolls in the rolling mill are referred to as a stand.

After the hot rolling of the slab, the resulting “hot rolled” product can then be either coiled up or cut into single sheets or plates and allowed to cool.Flat rolled steel under 3/16 of an inch thick is considered sheet; strip is a sheet that is less than 2 feet wide. Flat rolled steel over 3/16 of an inch thick is considered plate.


Cold Rolled

Taking the hot rolled steel and turning it into cold rolled involves a few more steps. First, the steel is pickled to remove the black oxide scale. Once descaled,the steel enters a set or sets of rollers, which reduce the thickness of the steel further. The steel can either be passed through a series of rolling stands on a tandem mill or passed back and forth through a one- or two-stand reversing mill. A typical hot rolled thickness might be in the 0.08-0.13 inch range. Cold rolled ends up frequently in the 0.30-0.50 inch range. The cold rolled sheet can end up being four times as long as the original hot rolled substrate. After steel is rolled or worked in the cold stage, it becomes more brittle. To soften the steel and make it more malleable or ductile, the steel is frequently annealed.The annealing process slowly heats up the steel over many hours and then slowly cools it again. The physical properties of the steel are affected by the rate of heating, the degree of heating, and the rate of cooling. The annealing process, however, can again blacken the surface of the steel and may require another pickling step to clean the surface. A bright anneal, or hydrogen anneal process,  uses a controlled, oxygen-free atmosphere during annealing so that the discoloring oxide scale does not form on the surface of the steel. Because annealing and pickling are often done in sequence, a typical equipment arrangement is an an-neal and pickle, or A&P, line. In a continuous A&P line, each successive coil is welded to the tail of another and fed continuously into the annealing furnaces and pickling baths (making the sheets continuous reduces handling). After the steel is processed, the continuous sheet is cut back into coilable and shippable lengths. Other processes that may be performed on the steel include tempering,quenching, leveling, slitting, and coating. Tempering is a form of cold rolling that gives the steel a precise amount of hardness on the outer surface of the steel. After tempering, the steel is not annealed, so in effect the steelmaker is more precisely controlling the brittleness or hardness of the final product, particularly on its sur-face.In temper rolling, reduction is not the goal as it is in general cold rolling. Quenching, usually done to hot rolled steel, is the rapid cooling of steel, usually in a water bath, which imparts strength and hardness for specific applications. In a crude sense, quenching is a form of rapid cooling after an anneal or heatin step. Quenching steel in a bath of oil provides a different cooling rate and chemical environment, and thus, yields a steel with different properties. Leveling is just that and is performed on individual sections of steel to remove the tendency of the steel, particularly when heated, to “remember” the coiled shape it once was in. Slitting is cutting a sheet lengthwise to form narrow strips.


Coated Steel

Coatings can include zinc, aluminum, tin, terne, and/or paint. Zinc coatings, orgalvanizing, can be put on either by the hot dip process or electrolytically. Hot dip galvanizing involves dipping the steel into a molten zinc bath and wiping off the excess. The modern way to “wipe” the steel is with a steady blast of air or nitrogen to blow away any excess coating material. Either one or both sides of the steel can be coated in this way. To electrogalvanize (EG), an electrical charge is put on the steel, which attracts the zinc to bond to the surface. Electrogalvanizing is more expensive than hot dip galvanizing; as control over the coating layer has improved immensely with the hot dip process, electro galvanizing has lost market share to hot dip. The market for galvanized steel is divided into light and heavy categories. 

Lighter gauge galvanized is used in applications such as the underbody parts of automobiles, air ductwork, roofing and siding, garbage cans, metal building panels, and metal studs. Heavy gauge galvanized steel can be found in electrical boxes,casings for light fixtures, bumpers, grain bins, and highway guard rails. Once the steel is galvanized, it can also be annealed to increase its malleability.Such a galvannealed product is useful in increasingly demanding forming and drawing applications, such as complex-curve autobody panels, in which the steel must retain the integrity of its coating in the downstream fabricating steps.

Applying a layer of aluminum, or aluminizing, also offers an anticorrosive coating that is more effective than other coatings in certain atmospheres and conditions. Aluminizing can be accomplished in either a hot dipping or electrolytic process. Tin-coated steel, or tinplate, is used in tin cans because it is corrosion resistant (protection from oxidation and discoloration) and has less impact on the taste of foods than zinc. Tin coatings are usually applied electrolytically. Once on the steel, these coatings will take on a shine when passed through a heater, provid-ing a good adherence base for further lacquer and enamel coatings. Terne- coated sheet is produced by immersing the steel in a bath of molten terne metal, an alloy primarily of lead with some tin for its adherence properties. Terne,or “dull”-coated steel, gets its name because it is duller than tin-only steel. Although terne-coated steels only account for a small part of the overall steel mar-ket, they do offer performance characteristics that are useful in products such as fuel tanks and air cleaners. Steel can also be painted, perhaps after a zinc or tin coating. Painted galvanized sheets, for example, might be found in roofing, siding, gutters, interior cabinets, or appliances. Paint coating technology has evolved in quite sophisticated ways to coat more evenly and thoroughly as well as to enable greater bends in the painted steel without the paint cracking along the bend edge. More advanced steels are also being developed that are soft during the formation of a part but then harden as the steel is heated to dry a paint coating. Such bake-hardenable steels are increasingly in demand from customers in the automotive markets.



Plate is a flat rolled steel product that is more than 3/16 of an inch thick. Plate can be made on a flat or discrete plate mill (plate mill plate, or PMP), in which the steel is cut into salable lengths after a slab is rolled. Coiled plate, also called continuous mill plate or CMP, involves the coiling of longer lengths of plate up to one inch in thickness. Later, the coils can be cut into exact lengths for specific customer orders, reducing waste and increasing yields. Coiled plate can enable more efficient use of a rolling mill and further treatment steps on a more cost-effective, continuous basis. Plate is a hot rolled product. That is, the output of a plate mill is similar to the out-put of a hot rolled strip mill. A producer with cold rolling capacity will normally prefer to take slabs through a hot rolled sheet mill as well as a cold rolling mill to maximize value-added. The cold rolling more fully utilizes the steelmaker’s equipment and improves the product mix. When sheet markets are soft, however,it sometimes pays to move more slabs through the plate mill or to sell thicker hot rolled as coiled plate. In this way, plate is often considered dependent on strength in the sheet and strip markets even though it may have its own supply/demand fundamentals. Commodity steel plate is used in a variety of applications, such as the manufacture of welded girders, flooring, storage tanks, machinery parts, barges, and ships. More specialized steel plate, such as high-strength/low-alloy, heat treated,or alloy plate, can have superior strength and performance characteristics for particular applications, such as the manufacture of construction, mining and logging equipment, pressure vessels, oil and gas transmission lines, and the fabrication of bridges and buildings. If the plate has been quenched and tempered, the plate will be harder and stronger and can be used in places such as military armor or hard rock mining equipment.


Pipe and Tube

Large diameter pipe and tube products can be made from sheet or plate. Strips of steel can be bent into a tube and welded lengthwise (or twisted into a continuous spiral like a paper straw or paper-towel core and edge welded) to form welded pipe and welded tubing. Butt-welded pipe is found in standard plumbing, whereas electric-resistance welded (ERW) pipe, which is larger in diameter, is found in natural gas distribution lines.


Steel Products: Long Shapes

Feeding billets and blooms through rollers that pinch and push the steel into various cross-sectional shapes and sizes is how long products or bars are made. The subset of long products that are rails or structural I-beams (called “H”-beams in Europe), for example, are pinched in the middle while being constrained on the top and bottom, causing the “I”-shaped cross section to form as the square steel billet is forced through a set of rollers. Other finished shapes might be rounds, bars, angles, piling, channel, “Z”-angles, and hex shapes. Rebar, merchant bar (MBQ), special bar quality bar (SBQ), and structurals are the four main categories of long products. Rebar is a round bar with hash-mark indentations along the side. Special bar quality bars are the bar subset with the most consistent metallurgical qualities. Small diameter rounds, called rods, are frequently an SBQ product and are often used for making drawn wire. Other product shapes not categorized elsewhere are referred to as merchant bars,MBQs, or merchants. Moving production among any of these products is possible,but the economics of clean steel production, roll changes, inventory management,and equipment pacing has frequently led companies to focus on one grade or product set. Any of the long products can further be heat treated, pickled,straightened, cut to length, or coated, depending on the eventual application.



All rebar in this country is made via the minimill process from scrap. The gross spread above scrap costs is the principal driver of profitability absent meaningful operating cost improvements. Few new rebar mills have come on-line in recent years, although there has been some capacity creep as existing mills have improved efficiencies. Other mills have moved to upgrade their products by converting from rebar to higher value-added bar and rod capability. Some mills can switch from one product to another depending on the season and on the order book.

For these reasons, strength or weakness in one subsegment of the bar markets can indirectly spill into the other markets over time. Every $1 million of highway spending equates to 45-50 tons of rebar consumption. In the past, as the government has proposed massive infrastructure plans, there have been thematic rallies in the rebar-oriented equities. Unfortunately, however, the infrastructure theme has generally disappointed because of a lack of funding for the plans. On the positive side, with state governments now funding projects, demand from the infrastructure-oriented rebar market can remain strong for a number of years.


Merchant Bar Products (MBQ)

Merchant products include long bars with round, square, flat, angled, and channeled cross sections. Merchant products are generally sold to fabricators, service centers, and manufacturers who cut, bend, shape, weld, and fabricate the shapes into other products. Joists, primarily for construction, represent 27% of the market and are the largest end use for merchant shapes. Other construction applications account for an additional 13% of the merchant market. Mine bolts represent approximately 10% of MBQ use, with the remaining 50% including a wide variety of construction equipment, industrial equipment, material handling, and transportation uses. Merchant products usually require more specialized handling and processing than rebar, such as straightening, and they usually carry $15-25 per ton higher operating margins than rebar. Merchant products sell for more than rebar because the greater number of product shapes forces more batch production runs, higher inventories, and more mill equipment adjustments (roll changes) in addition to the more specialized finishing.


Special Quality Bar (SBQ)

At the higher end of the quality spectrum in both metallurgical consistency and dimensional accuracy are the special quality bar and rod (SBQ) products. In order to achieve metallurgical quality, the steel has to be particularly clean. Steel cleanliness can be achieved through the integrated route (some integrated steel makers participate in the SBQ market) or through an additional refining step and cleaner iron inputs into an EAF minimill. Internal metallurgical quality and consistency can be achieved by a meaningful reduction during the rolling process.

Thus, finished bar size and required starting billet size are related; smaller bars from the same initial billet will have greater internal quality. These internal qualities are important when it comes to bending and machining the steel, and they ultimately contribute to product strength. Some typical end-use products for SBQ include motor shafts, engine bolts, screws, rivets, wrenches, bolts, springs, cable wire, chains, tire beads, and welding wire. Most producers see ties to the automotive industry as the key drivers of the SBQ grades, with oil and gas, agricultural equipment, and capital goods serving as other important markets.


Pipe and Tube

In the flat rolled product section we discussed the making of pipe and tube via the lengthwise welding of sheet, strip, and plate. Pipe and tube can also be made without welds.  So-called seamless pipe and tube is made as a rotating heated bloom or billet is pierced by a long-armed, pointed piece of steel called a mandrel.As the mandrel penetrates the length of the steel, a tube is formed. That pipe can then be worked with rollers into a longer pipe with a smaller diameter.

Seamless tubing is often used by process industries and in boiler tubing. Special grades of steel and larger diameters of pipe and tube go into oil country tubular goods (OCTG), an essential product for downhole oil and gas drilling activity.

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