In ancient times a relatively small number of men produced steel. They used small furnaces that delivered less than 25 pounds, an incredibly minute figure when one considers that there are numerous steelworks today, throughout the world, capable of production in excess of 1 million tons. In 1987, Nippon Steel in Japan produced 26 million tons of steel, 6% of the reported 430 million tons of steel produced.

   

     Steel, which is an alloy of iron and carbon in which the carbon content ranges up to 2 percent, is by far the most widely used material for building the world's infrastructure and industries. It is used to fabricate everything from bearings to automobiles. Additionally, the tools required to build and manufacture such articles are also made of steel. Indicative of the importance of this material, in 1989 total steel production was 43 times that of the next material, aluminum. The main reasons for the popularity of steel are the relatively low cost of making, forming, and processing it, the abundance of its two raw materials (iron ore and scrap), and its unparalleled range of mechanical properties.

 Iron Is the major component of steel, but in its pure state is not much harder than copper. It is also in a polycrystalline state, consisting of many crystals that join one another on their boundaries, in its solid state like all other metals. A crystal is a well-ordered arrangement of atoms that can best be pictured as spheres touching one another. They are ordered in planes, called lattices, which penetrate one another in specific ways. For iron, the lattice arrangement can best be visualized as a unit cube with eight iron atoms at its corners. Important for the uniqueness of steel is the allotropy of iron--that is, its existence in two crystalline forms. In the body-centered cubic (bcc) arrangement, there is an additional iron atom in the center of each cube. In the face-centered cubic (fcc) arrangement, there is one additional iron atom at the center of each of the six faces of the unit cube. It is significant that the sides of the face-centered cube, or the distances between neighboring lattices in the fcc arrangement, are about 25 percent larger than in the bcc arrangement.  The result is that there is more space in the fcc than in the bcc structure to keep foreign atoms in solid solution.

Iron has its bcc allotropy below 912º C (1,674º F) and from 1,394º C (2,541º F) up to its melting point of 1,538º C (2,800º F). Referred to as ferrite, iron in its bcc formation is also called alpha iron in the lower temperature range and delta iron in the higher temperature zone. Between 912º and 1,394º C iron is in its fcc order, which is called austenite or gamma iron. The allotropic behavior of iron is retained with few exceptions in steel, even when the alloy contains considerable amounts of other elements.

There is also the term beta iron, which refers not to mechanical properties but rather to the strong magnetic characteristics of iron. Below 770º C (1,420º F), iron is ferromagnetic; the temperature above which it loses this property is often called the Curie point.

 The principal method of strengthening iron and converting it into steel is by adding small amounts of carbon. In solid steel, carbon is generally found in two forms. Either it is in a solid solution as austenite and ferrite, or it is found as carbide. The carbide form can be iron carbide (Fe3C, known as cementite), or it can be carbide of an alloying element such as titanium.


     This diagram illustrates the effect of carbon on iron in the formation of steel.

 


The A-B-C line represents the liquidus points (i.e., the temperatures at which molten iron begins to solidify), and the H-J-E-C line represents the solidus points (at which solidification is completed). The A-B-C line indicates that solidification temperatures decrease as the carbon content of an iron melt is increased. (This explains why gray iron, which contains more than 2 percent carbon, is processed at much lower temperatures than steel.) Molten steel containing, for example, a carbon content of 0.77 percent (shown by the vertical dashed line in the figure) begins to solidify at about 1,475º C (2,660º F) and is completely solid at about 1,400º C (2,550º F). From this point down, the iron crystals are all in an austenitic--i.e. fcc--arrangement and contain all of the carbon in solid solution. Cooling further, a dramatic change takes place at about 727º C (1,341º F) when the austenite crystals transform into a fine lamellar structure consisting of alternating platelets of ferrite and iron carbide. This microstructure is called pearlite, and the change is called the eutectoidic transformation. Pearlite has a diamond pyramid hardness (DPH) of approximately 200 kilograms-force per square millimeter (285,000 pounds per square inch), compared to a DPH of 70 kilograms-force per square millimeter for pure iron. Cooling steel with a lower carbon content (e.g., 0.25 percent) results in a microstructure containing about 50 percent pearlite and 50 percent ferrite; this is softer than pearlite, with a DPH of about 130. Steel with more than 0.77 percent carbon--for instance, 1.05 percent--contains in its microstructure pearlite and cementite; it is harder than pearlite and may have a DPH of 250.

 Additional changes are made possible by heat-treating--for instance, by accelerating the rate of cooling through the austenite-to-ferrite transformation point, shown by the P-S-K line in the figure. (This transformation is also called the Ar1 transformation, r standing for refroidissement, or "cooling.") Increasing the cooling rate of pearlitic steel (0.77 percent carbon) to about 200º C per minute generates a DPH of about 300, and cooling at 400º C per minute raises the DPH to about 400. The reason for this increasing hardness is the formation of a finer pearlite and ferrite microstructure than can be obtained during slow cooling in ambient air. In principle, when steel cools quickly, there is less time for carbon atoms to move through the lattices and form larger carbides. Cooling even faster--for instance, by quenching the steel at about 1,000º C per minute--results in a complete depression of carbide formation and forces the undercooled ferrite to hold a large amount of carbon atoms in solution for which it actually has no room. This generates a new microstructure, martensite. The DPH of martensite is about 1,000, which makes it is the hardest and most brittle form of steel. Tempering martensitic steel--i.e., raising its temperature to a point such as 400º C and holding it for a time--decreases the hardness and brittleness and produces strong and tough steel. Quench-and-temper heat treatments are applied at many different cooling rates, holding times, and temperatures; they constitute a very important means of controlling steel's properties.

 A third way to change the properties of steel is by adding alloying elements other than carbon that produce characteristics not achievable in plain carbon steel. Each of the approximately 20 elements used for alloying steel has a distinct influence on microstructure and on the temperature, holding time, and cooling rates at which microstructures change. They alter the transformation points between ferrite and austenite, modify solution and diffusion rates, and compete with other elements in forming intermetallic compounds such as carbides and nitrides. There is a huge amount of empirical information on how alloying affects heat-treatment conditions, microstructures, and properties. In addition, there is a good theoretical understanding of principles, which, with the help of computers, enables engineers to predict the microstructures and properties of steel when alloying, hot-rolling, heat-treating, and cold-forming in any way.

     A good example of the effects of alloying is the making of high-strength steel that welds easily. This cannot be done by using only carbon to strengthen, because carbon creates brittle zones around the weld. However but it can be done by keeping carbon low and adding small amounts of other strengthening elements, such as nickel or manganese. In principle, the strengthening of metals is accomplished by increasing the resistance of lattice structures to the motion of dislocations. Dislocations are failures in the lattices of crystals that make it possible for metals to be formed. When elements such as nickel are kept in solid solution in ferrite, their atoms become embedded in the iron lattices and block the movements of dislocations. This phenomenon is called solution hardening. An even greater increase in strength is achieved by precipitation hardening. In this process certain elements (e.g., titanium, niobium, and vanadium) do not stay in solid solution in ferrite during the cooling of steel but instead form finely dispersed, extremely small carbide or nitride crystals, which also effectively restrict the flow of dislocations. In addition, most of these strong carbide or nitride formers generate a small grain size, because their precipitates have a nucleation effect and slow down crystal growth during recrystallization of the cooling metal. Producing a small grain size is another method of strengthening steel, since grain boundaries also restrain the flow of dislocations.

Alloying elements have a strong influence on heat-treating, because they tend to slow the diffusion of atoms through the iron lattices and thereby delay the allotropic transformations. This means, for example, that the extremely hard martensite, which is normally produced by fast quenching, can be produced at lower cooling rates. This results in less internal stress and, most important, a deeper hardened zone in the workpiece. Improved hardness is achieved by adding such elements as manganese, molybdenum, chromium, nickel, and boron. These alloying agents also permit tempering at higher temperatures, which generates better ductility at the same hardness and strength.

   The testing of steel's properties often begins with checking hardness. This is measured by pressing a diamond pyramid or a hard steel ball into the steel at a specific load. The Vickers Diamond Pyramid Hardness tester, which measures the DPH mentioned above, uses an indenter with an included angle of 136º between opposite faces of a pyramid and usually a load of 10, 30, or 50 kilograms-force. The diagonal of the impression is measured optically, and the hardness expressed as the load in kilograms-force divided by the impressed area of the pyramid in square millimeters. Tensile and yield strength are determined by pulling a standardized machined sample in a special hydraulic press and recording the pulling force at increasing elongations until the sample breaks. The elongation at this point, and the way the fracture looks, are good indications of the steel's ductility. Measuring the pulling force at 0.20 percent elongation and dividing it by the test bar's cross section are a means of calculating the yield strength, a good indicator of cold formability. Impact toughness is determined by hitting a standardized, prismatic, notched sample with a test swing hammer and recording the work required to break it. This is performed at different temperatures, because brittleness increases as temperature falls.

     There are many other tests used in the industry to check a steel's mechanical properties, such as wear tests for rails, drawability tests for sheets, and bending tests for wire. Metallographic laboratories examine the microstructure of polished and etched steel samples, often on computerized and very powerful microscopes. Laboratories also check physical data such as thermal elongation and electromagnetic properties. Chemical composition is often checked by completely automated spectrometers. There are also several nondestructive tests as, for example, the ultrasonic test and magnaflux test used to check for internal and external flaws such as laminations or cracks.

 There are several thousand steel grades, either published, registered, or standardized worldwide, all of which have different chemical compositions, and special numbering systems have been developed in several countries to classify the huge number of alloys. In addition, all the different possible heat treatments, microstructures, cold-forming conditions, shapes, and surface finishes mean that there is an enormous number of options available to the steel user. Fortunately, steels can be classified reasonably well into a few major groups according to their chemical compositions, applications, shapes, and surface conditions.

     On the basis of chemical composition, steels can be grouped into three major classes: carbon steels, low-alloy steels, and high-alloy steels. All steels contain a small amount of incidental elements left over from steel making. These include manganese, silicon, or aluminum from the deoxidization process conducted in the ladle, as well as phosphorus and sulfur picked up from ore and fuel in the blast furnace. Copper and other metals, called residuals, are introduced by scrap used in the steelmaking furnace. Because all these elements together normally constitute less than 1 percent of the steel, they are not considered alloys.

Carbon steels are by far the most produced and used, accounting for about 90 percent of the world's steel production. They are usually grouped into high-carbon steels, with carbon above 0.5 percent; medium-carbon steels, with 0.2 to 0.49 percent carbon; low-carbon steels, with 0.05 to 0.19 percent carbon; extra-low-carbon steels, with 0.015 to 0.05 percent carbon; and ultra-low-carbon steels, with less than 0.015 percent carbon. Carbon steels are also defined as having less than 1.65- percent manganese, 0.6 percent silicon, and 0.6 percent copper, with the total of these other elements not exceeding 2 percent.

     Low-alloy steels have up to 8 percent alloying elements; any higher concentration is considered to constitute high-alloy steel. There are about 20 alloying elements besides carbon. These are manganese, silicon, aluminum, nickel, chromium, cobalt, molybdenum, vanadium, tungsten, titanium, niobium, zirconium, nitrogen, sulfur, copper, boron, lead, tellurium, and selenium. Several of these are often added simultaneously to achieve specific properties.

 The many applications of steel demonstrate best the great versatility of this material. Most often, carbon steels meet consumer’s needs. Good examples are sheets for deep-drawn automobile bodies and appliances made of low-carbon steels, medium-carbon structural steels and plates employed in all kinds of construction, high-carbon railroad rails, and wires at all carbon levels used for hundreds of items. The addition of costly alloys begins when combinations of properties are requested that exceed carbon steels ability.

     A group called the high-strength low-alloy (HSLA) steels meets the demand for high strength, good weldability, and higher resistance to atmospheric corrosion. These grades have low carbon levels (e.g., 0.05 percent) and contain small amounts of one or a combination of elements such as chromium, nickel, molybdenum, vanadium, titanium, and niobium. HSLA steels are used for oil or gas pipelines, ships, offshore structures, and storage tanks.

     This group, developed for good machinability and fabricated into bolts, screws, and nuts, contains up to 0.35 percent sulfur and 0.35 percent lead; also, it sometimes has small additions of tellurium or selenium. These elements form many inclusions, which are normally avoided but are desired in this application because they break the long, hazardous strings of metal that are usually formed during machining into small chips. This keeps tools and work-pieces clean, improves tool life, and permits machining at higher speeds.

     Another group is the wear-resistant steels, made into wear plates for rock-processing machinery, crushers, and power shovels. These are austenitic steels that contain about 1.2 percent carbon and 12 percent manganese. The latter element is a strong austenizer; that is, it keeps steel austenitic at room temperature. Manganese steels are often called Hadfield steels, after their inventor, Robert Hadfield.

     Wear resistance is brought about by the high work-hardening capabilities of these steels; this in turn is generated during the pounding (i.e., deforming) of the surface, when a large number of disturbances are created in the lattices of their crystals that effectively block the flow of dislocations. In other words, the more pounding the steel takes, the stronger it becomes. Such significant increases in strength by cold forming are also utilized in the production of high-strength, cold-drawn wire such as those used in pre-stressed concrete or automobile tires. A special case, piano wire drawn from 0.8-percent-carbon steel, can reach a tensile strength of 275 kilograms-force per square millimeter.

    In principle, steelmaking is a melting, purifying, and alloying process carried out at approximately 1,600º C (2,900º F) in molten conditions. Various chemical reactions are initiated, either in sequence or simultaneously, in order to arrive at specified chemical compositions and temperatures. Indeed, many of the reactions interfere with one another, requiring the use of process models to help in analyzing options, optimizing competing reactions, and designing efficient commercial practices.

     The major iron-bearing raw materials for steelmaking are blast-furnace iron, steel scrap, and direct-reduced iron (DRI). Liquid blast-furnace iron typically contains 3.8 to 4.5 percent carbon (C), 0.4 to 1.2 percent silicon (Si), 0.6 to 1.2 percent manganese (Mn), up to 0.2 percent phosphorus (P), and 0.04 percent sulfur (S). Its temperature is usually 1,400º to 1,500º C (2,550º to 2,700º F). The phosphorus content depends on the ore used, since phosphorus is not removed in the blast-furnace process, whereas sulfur is usually picked up during iron making from coke and other fuels. DRI is reduced from iron ore in the solid state by carbon monoxide (CO) and hydrogen (H2). It frequently contains about 3 percent-unreduced iron ore and 4 percent gangue, depending on the ore used. It is normally shipped in briquettes and charged into the steelmaking furnace like scrap. Steel scrap is metallic iron containing residuals, such as copper, tin, and chromium, which vary with its origin. Of the three major steelmaking processes--basic oxygen, open hearth, and electric arc--the first two, with few exceptions, use liquid blast-furnace iron and scrap as raw material and the latter uses a solid charge of scrap and DRI.

     The most important chemical reactions carried out on these materials (especially on blast-furnace iron) are the oxidation of carbon to carbon monoxide, silicon to silica, manganese to manganese oxide, and phosphorus to phosphate, as follows: 2C  + 02     = 2CO

            Si  + O2    = SiO2

            2Mn + O2    = 2MnO

            2P  + 2.5O2 = P2O5       

                         

Unfortunately, iron is also lost in this series of reactions, as it is oxidized to ferrous oxide:

            2Fe + O2 = 2FeO

The FeO, absorbed into the liquid slag, then acts as an oxidizer itself, as in the following reactions:

            C  + FeO  = CO + Fe;

         Or 2P + 5FeO = P2O5 + 5Fe

In the open-hearth furnace, oxidation also takes place when gases containing carbon dioxide (CO2) contact the melt and react as follows:  Fe + CO2 = FeO + CO;

              Or   C  + CO2 = 2CO      

     The products of the above reactions, the oxides silica, manganese oxide, phosphate, and ferrous oxide, together with burnt lime (calcium oxide; CaO) added as flux, form the slag. Burnt lime has by itself a high melting point of 2,570º C (4,660º F) and is therefore solid at steelmaking temperatures, but when it is mixed with the other oxides, they all melt together at lower temperatures and thus form the slag. A basic slag contains approximately 55 percent CaO, 15 percent SiO2, 5 percent MnO, 18 percent FeO, and other oxides plus sulfides and phosphates. The basicity of a slag is often simply expressed by the ratio of CaO to SiO2. In this case with CaO being the basic and SiO2 being the acidic component. Usually, a basicity above 3.5 provides good absorption and holding capacity for calcium phosphates and calcium sulfides.

    The majority of sulfur, present as ferrous sulfide (FeS), is removed from the melt not by oxidation but by the conversion of calcium oxide to calcium sulfide:

    FeS + CaO = CaS + FeO.

According to this equation, desulfurization is successful only when using a slag with plenty of calcium oxide--in other words, with a high basicity. A low iron oxide content is also essential, since oxygen and sulfur compete to combine with the calcium. For this reason, many steel plants desulfurize blast-furnace iron before it is refined into steel, since at that stage it contains practically no dissolved oxygen, owing to its high silicon and carbon content. Nevertheless, sulfur is often introduced by scrap and flux during steelmaking, so that, in order to meet low sulfur specifications (for example, less than 0.008 percent), it is necessary to desulfurize the steel as well.

     A very important chemical reaction during steelmaking is the oxidation of carbon. Its gaseous product, carbon monoxide, goes into the off-gas, but, before it does that, it generates the carbon monoxide boil, a phenomenon common to all steelmaking processes and very important for mixing. Mixing enhances chemical reactions, purges hydrogen and nitrogen, and improves heat transfer. Adjusting the carbon content is important, but it is often oxidized below specified levels, so that carbon powder must be injected to raise the carbon again.

Removing oxygen

As the carbon level is lowered in liquid steel, the level of dissolved oxygen theoretically increases according to the relationship %C %O = 0.0025. This means that, for instance, a steel with 0.1 percent carbon, at equilibrium, contains about 0.025 percent, or 250 parts per million, dissolved oxygen. The level of dissolved oxygen in liquid steel must be lowered because oxygen reacts with carbon during solidification and forms carbon monoxide and blowholes in the cast. This reaction can start earlier, too, resulting in a dangerous carbon monoxide boil in the ladle. In addition, a high oxygen level creates many oxide inclusions that are harmful for most steel products. Therefore, usually at the end of steelmaking, during the tapping stage, adding aluminum or silicon deoxidizes liquid steel. Both elements are strong oxide formers and react with dissolved oxygen to form alumina (Al2O3) or silica. These float to the surface of the steel, where they are absorbed by the slag. The upward movement of these inclusions is often slow because they are small (e.g., 0.05 millimeter), and combinations of various deoxidizers are sometimes used to form larger inclusions that float more readily. In addition, stirring the melt with argon or an electromagnetic field often serves to give them a lift.

     Deoxidization is also important before alloying steel with easy oxidizable metals such as chromium, titanium, and vanadium, in order to minimize losses and improve process control. Metals that do not oxidize readily, such as nickel, cobalt, molybdenum, and copper, can be added in the furnace to take advantage of high heating rates. In fact, alloying always has thermal effects on steelmaking--for example, the use of energy to heat and melt the alloying agents, or the heat of reaction or solution when they combine with other elements. Fortunately, there exists a large amount of empirical data, obtained from thousands of thermodynamic experiments, that, when supported by theoretical principles, allows steelmakers to predict such temperature changes.

Most alloys are added in the form of ferroalloys, which are iron-based alloys that are cheaper to produce than the pure metals. Many different grades are available. For example, ferrosilicon is supplied with levels of 50, 75, and 90 percent silicon and with varying levels of carbon and other additions.

     Also important for steelmaking is the absorption and removal of the two gases hydrogen and nitrogen. Hydrogen can enter liquid steel from moist air, damp refractories, and wet flux and alloy additions. It causes brittleness of solidified steel--especially in large pieces, such as heavy forging, that do not permit the gas to diffuse to the surface. Hydrogen can also form blowholes in castings. Nitrogen does not move into and out of liquid steel as easily as hydrogen, but it is well absorbed by liquid steel in the high-temperature zones of an electric arc or oxygen jet, where nitrogen molecules (N2) are broken up into atoms (N). Like hydrogen, nitrogen substantially decreases the ductility of steel.

     Basic steelmaking takes place in containers lined with basic refractories. These may be bricks or ram material made of highly stable oxides, such as magnesite, alumina, or the double oxides chrome-magnesite and dolomite. It is desirable that the refractories not participate in the steelmaking reactions, but unfortunately they do erode and corrode. Refractory bricks are produced in all shapes and grades by a highly specialized industry.

     Testing and sampling are an important part of liquid steelmaking. They are carried out by mechanized and often automated facilities, which immerse lances that are equipped with sensors for rapid computation of temperature and dissolved carbon, oxygen, and hydrogen. Test lances also take samples for analysis in laboratories. All results are usually fed automatically into a process-control computer.

    In principle, heat-treating already takes place when steel is hot-rolled at a particular temperature and cooled afterward at a certain rate, but there are also many heat-treating process facilities specifically designed to produce particular microstructures and properties. The simplest heat-treating process is normalizing. This consists of holding steel for a short time at a temperature 20º to 40º C above and then cooling it afterward in still air. Holding the steel in the gamma zone transforms the as-rolled or as-cast microstructure into austenite, which dissolves carbides. Then, during cooling, a very uniform grain is formed, consisting of both pearlite and ferrite or pearlite and cementite, depending on carbon content.

In all heat-treatment operations, the temperatures, holding times, and heating and cooling rates are varied according to the chemical composition, size, and shape of the steel. In general, alloy steels, which have a lower heat conductivity than carbon steels, are heated more slowly to avoid internal stresses.

 

 

 

    

 

 


  

 


     To make steel ductile for subsequent forming operations, an annealing treatment is applied. In annealing, the steel is usually held for several hours at several degrees below Ar1 and then slowly cooled. This precipitates and coagulates the carbides and results in large ferrite crystals. Cold-formed steel is usually annealed and recrystallized in this manner, holding it for several hours at about 680º C (1,260º F).

Annealing is performed in an inert or reducing atmosphere to prevent any oxidation of the steel surface. In batch annealing of cold-rolled strip, for example, several coils are set on a base and on top of one another. Then they are covered with a shell made of heat-resistant steel, which is sealed on the bottom and holds the inert gas during annealing. A crane then lowers a gas-fired bell furnace over this cover for heating. The total processing time, including its cooling, may be 50 to 120 hours, depending on furnace load and steel grade.

     In a different system, the cold-rolled strip is pulled through an 80-metre-high furnace with the strip moving up and down between many top and bottom rolls. Gas-fired radiation tubes usually heat these continuous-annealing furnaces in order to separate combustion gases from the inert atmosphere surrounding the strip. In this dynamic annealing process, the strip is heated to higher temperatures (for example, 780º C, or 1,440º F), held for only a few seconds, and immediately cooled by fast-circulating inert gas. The entry and exit sections of continuous-annealing lines are built, as on other strip-processing lines, to allow an uninterrupted and constant travel of the strip through the process section--in this case, the heating and cooling zones. The entry group has two uncoiling reels, a cross-shear, welding equipment for joining two strips, and a strip accumulator. The latter is often a looping tower, which supplies the process section above with strip at constant speed while welding is done at the entry section. The exit group works in a similar fashion, with a looping tower and two reels; it also cuts samples and substandard portions out of the strip.

Continuous-annealing lines are often 200 meters long, and the strip between uncoiler and recoiler is more than one kilometer in length. Strip annealed this way is not as soft as batch-annealed steel--a disadvantage compensated for by using ultra-low carbon steels--but it does have operating advantages in that annealing of one coil may take only one hour and the mechanical and surface properties of the strip are very uniform.

    The most common heat treatment for plates, tubular products, and rails is the quench-and-temper process. Large plates are heated in roller-type or walking-beam furnaces, quenched in special chambers, and then tempered in a separate low-temperature furnace. Uniform heating and quenching is crucial; otherwise, residual stresses will distort and warp the plate. Tubes made for very demanding services, such as oil drilling, are usually heat-treated in walking-beam furnaces and special quench-and-temper systems.

The heads of rails are sometimes heat-treated in-line by induction heating coils, air quenching, and tempering by a controlled use of the heat retained in the rail after quenching. Heavy-walled structural shapes are sometimes water-quenched directly after the last pass at the rolling mill and also tempered by the heat retained in the steel. In-line heat-treating results in cost savings because it eliminates extra heat-treating processes and facilities.

The quenching media and the type of agitation during quenching are carefully selected to obtain specified physical properties with minimum internal stresses and distortions. Oil is the mildest medium, and salt brine has the strongest quenching effect; water is between the two. In special cases, steel is cooled and held for some time in a molten salt bath, which is kept at a temperature either just above or just below the temperature where martensite begins to form. These two heat treatments are called martempering and austempering, and both result in even less distortion of the metal.

     The surface treatment of steel also begins during hot-rolling, because reheating conditions, in-line scale removal, rolling temperature, and cooling rate all determine the type and thickness of scale formed on the product, and this affects atmospheric corrosion, paintability, and subsequent scale-removal operations. Sometimes the final pass in hot rolling generates specific surface patterns--for example, the protrusions on reinforcing bars or floor plates. In cold rolling a specific surface roughness is rolled into the strip at the temper mill to improve the deep-drawing operation and to assure a good surface finish on the final product--for instance, on the roof of an automobile.

     Before cold forming, hot-rolled steel is always descaled, most commonly in an operation known as pickling. Scale consists of thin layers of iron oxide crystals, of which the chemical compositions, structures, and densities vary according to the temperature, oxidizing conditions, and steel properties that are present during their formation. Acids can dissolve these crystals; normally, hot hydrochloric or sulfuric acid is used, but for some alloy steels a different acid, such as nitric acid, is needed. In addition, inhibitors are added to the acid to protect the steel from being dissolved as well.

     The pickling of hot-rolled strip is carried out in continuous pickle lines, which are sometimes 300 meters long. The strip is pulled through three to five consecutive pickling tanks, each one 25 to 30 meters long, at a constant speed of about 300 meters per minute. Like other continuous strip-processing lines, pickle lines also have an entry and exit group to establish constant pickling conditions. After the last acid tank, there are sections that rinse, neutralize, dry, inspect, and oil the strip.

Long products, such as bars and wire rods, are normally pickled in batch operations by placing them on racks and immersing them in long, acid containing vats. Sometimes shot blasting is used instead of pickling; this removes scale from heavy hot-rolled products by directing high-velocity abrasives onto the surface of the steel.