Working of steel

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The steel products we encounter everyday are polycrystalline materials consisting of many grains of steel. Iron atoms arrange themselves regularly in one crystal, and the direction of the arrangement of atoms differs among grains. The diameter of an iron atom is 0.25 nanometers, while that of a grain is usually 10 to 20m.
Iron atoms arrange themselves in one of two stable crystal structures called the body-centered cubic structure and the face-centered cubic structure. The body-centered cubic structure of iron, which is called ferrite, is stable at (i) a temperature of 1,665K (1,392) or above and (ii) at 1,184K (911) or below, the crystal forms being referred to as iron and a iron, respectively. The face-centered cubic structure, which is called austenite, is stable in a temperature range everywhere between the above-mentioned two temperature ranges, and the iron of this structure in this temperature range is called iron. The phenomenon by which a crystal structure changes to another due to a change in temperature is referred to as a phase transformation. The temperature at which this phenomenon occurs is called the transformation temperature. The transformation temperature depends upon both the nature and the amount of the alloying elements.
There are portions in actual grains where the regularity of the positions of the iron atoms is lost, these portions being called lattice defects. Particularly important lattice defects are (i) "vacancies", which are point-like defects in which an iron atom is missing at a lattice point, and (ii) "dislocations", which are linear defects. Vacancies play an important role in the diffusion of atoms, and plastic deformation occurs when dislocations move. Foreign atoms, with a size different from that of iron atoms, are present in a steel grain. These atoms exist in two different forms, i.e., as a "solid solution", in which they are present in the lattice structure of iron as shown in the figure, and as a "precipitate", in which they form another crystal structure and are present within the grain or at the grain boundaries. Solid solutions are divided into interstitial solid solutions and substitutional solid solutions. In the former type, carbon, nitrogen, and other atoms much smaller than iron atoms are located in the space between iron atoms. In the latter type, atoms larger in size (aluminum, titanium), atoms that have almost the same size (nickel, chromium), or atoms smaller in size (silicon, phosphorous) than iron atoms, take the place of some of the iron atoms.
A polycrystal is composed of many grains with different orientations. Although a polycrystal usually has no orientation as a whole, it can assume a texture that has many grains with specific orientations under some working and heat-treatment conditions. A grain boundary has excess energy; therefore, when it becomes possible for atoms to move, a change occurs in such a manner that the area of the grain boundary decreases; that is, grain growth occurs. The smaller the grain size, the higher the strength and toughness. In other words, the smaller, the grain size, the better. It is therefore necessary to reduce the size of grains. Grains can be newly generated by the two mechanisms of transformation and recrystallization. Transformation was discussed above. Recrystallization is the phenomenon in which, when a material is heated after being worked beyond its critical strain, the strain energy accumulated by working is released by diffusion which rearranges the position of the atoms, and new grains are formed. Thus, grain refinement is achieved by utilizing these mechanisms.
	
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When steel is heat treated, changes occur not only to the crystal structure and grain size but also in the state of the foreign atoms present in the steel. The minimum equilibrium concentration of a foreign atom at which the atom precipitates is defined as solubility limit. The foreign atoms form solid solution when the concentration of the atom is less than the solubility limit, and precipitate as a compound when the solubility limit is exceeded. Solubility limit is determined by the thermodynamic properties of entities which react to each other to form precipitates. When interaction between the entities is affirmative and Gibb's energy of the precipitate formation is negatively large, precipitates are formed even at low concentrations of the entities.
The figure shows an iron-carbon phase diagram, which is the most fundamental for steel, showing how the transformation temperature or solubility limit depends upon the carbon content. In the heat treatment of low-carbon steel, the line segment PQ, which represents the solubility limit of carbon in -ferrite, is important. The solubility limit represented by PQ increases as the temperature increases. Therefore, if, on heating, the solubility limit increases and, subsequently, exceeds the carbon concentration of the steel, all the carbides that have been precipitated will decompose and dissolve. Precipitation will occur again when the solubility limit decreases as the steel cools.
Equilibrium theories based on thermodynamics deal with the stable crystal structure and the state of foreign atoms. However, structures formed in practice by heat treatment are not determined solely by equilibrium theory. This is illustrated by the fact that the carbon in the steel is precipitated not as graphite (thermodynamically stable phase), but as the metastable cementite phase (Fe3C). For graphite to precipitate, it would be necessary to achieve complete diffusion of the carbon atoms to bring about the change predicted by equilibrium theory. When such diffusion is not achieved, for example in rapid cooling, the change is suspended while in progress. On the other hand, plastic deformation accelerates precipitation by increasing the precipitation sites available and by promoting diffusion. By making use of these phenomena, different structures can be obtained in steel of the same composition by control of the crystal structure and the size and distribution of the precipitated particles.
Fine precipitates induce strain in the surrounding crystal lattice of iron, and consequently provide great resistance to dislocation movements and increase the strength, even though they are present in only minute amounts. Hence, elements which cause dissolution and precipitation within the temperature range of heat treatment and hot working are suitable for the formation of fine precipitates. Typical elements like niobium and vanadium result in the formation of carbonitrides. When steels containing these elements are hot rolled, thermo-mechanical control processes are used practically to increase strength by precipitating fine particles and by refining the crystal grains, which is accomplished by controlling the conditions for rolling and cooling. Thus, the structure of such steels can be changed considerably by the heat treatment applied. This makes it possible to produce steel materials with diverse properties, and thus to select the properties suitable for specific applications.
	
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A material is worked by utilizing plastic deformation to give it a shape suitable for its application. In this process, a change occurs not only in the visible shape and size, but also at the atomic level in the interior of the material.
Plastic deformation of a metal occurs by slipping of atoms on specific planes of a crystal. This slipping of atoms does not occur at any one time over the entire crystal plane. In fact, it occurs by the movement of linear lattice defects called dislocations. The figure shows the atomic structure of an edge dislocation and the process by which plastic deformation occurs when the dislocation moves on the slip plane.
Materials in which dislocations can move easily are those which tend to be soft and subject to easy plastic deformation. On the other hand, hard and strong materials can be obtained if it is difficult for dislocations to move. Factors that make it difficultfor dislocations to move include foreign atoms in solid solutions and precipitated particles. Hardening by these factors is referred to as solid solution hardening and precipitation hardening, respectively. As plastic deformation proceeds, many dislocations accumulate in a crystal, which interact with each other and prevent movement of the dislocation. Therefore, a material becomes harder as plastic deformation continues. This is called work hardening. A work hardened material returns to its former soft state when the accumulated dislocations disappear. When the work-hardened material is heated in an annealing process, a large number of dislocations disappear through the diffusion of atoms. During hot rolling, the as-rolled material is soft because both work hardening and annealing occur simultaneously. However, during cold rolling, only a work hardening occurs and, therefore, cold-rolled material is hard and brittle.
Another change in the crystal as a result of plastic deformation is the rotation of a crystal, which occurs when plastic deformation is caused only on a specific plane and direction of slipping. The rotation of a crystal forms a texture in which the crystal grains are oriented in the direction of the mechanical working.
In cold rolling with a large amount of deformation, the crystal grains become elongated. If a material with elongated grains is heated after being subjected to plastic deformation above the critical value, new equiaxed grains with fewer dislocations nucleate and grow, and the material returns to the soft state that existed before deformation. This phenomenon, which is called recrystallization, is used for the refining and softening of crystal grains.
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Working methods include rolling, forging, extrusion, and drawing. The most basic of these is rolling. When a wide strip is rolled between two rolls, it is possible to neglect the deformation in the width direction and treat it merely as two-dimensional deformation in the thickness and length directions, except at the edges of the strip. Vertical stress P and horizontal stress Q are generated in the material between the rolls. P is a stress caused by the compressive load from the rolls, while Q is a stress generated when the deformation in the rolling direction is restrained by the portions of the strip before and after the strip in contact with the roll. Frictional force Pr is generated by the friction between the material surface and the roll surface. On the entry side, this frictional force acts in the direction of delivery, because the circumferential speed of the roll is higher than the material speed. On the delivery side, however, the frictional force acts in the direction of entry, because the material speed is higher. The point at which the two speeds become equal is called the neutral point. Taking a micro volume which has unit length in the width direction of the roll in the oblique-lined region of Fig.(a), if stress P and stress Q are assumed to be constant within the thickness, and the friction coefficient is assumed to be constant over the whole arc of contact, Eq. 1 can be derived by considering the force balance in the horizontal direction. Equation 2 is a yield criterion which shows that, in order for the material to develop plastic deformation, the shear stress generated by stresses P and Q must reach the shear yield stress of the material. P and Q can be calculated by solving Eqs. 1 and 2. The distribution of vertical stress P is shown in Fig.(b), where stress P has its peak at the neutral point. The rolling force per unit width is calculated by integrating stress P over the whole arc of contact. Furthermore, the rolling torque can be calculated by integrating the moment around the roll shaft caused by stress P.
The rolling force is the most basic value used in the determination of the deformation induced by a rolling mill and the resulting strip thickness on the delivery side. It must be evaluated as accurately as possible. When dealing theoretically with improvements in the thickness accuracy and profile of a rolled strip, it is necessary to reflect, in the rolling force, both the distribution of stresses in the thickness direction and the deformation of strip in the width direction. A finite element method which permits three-dimensional analysis can be used for this purpose.
Heat is generated by the deformation of the material and the friction between the material and the rolls, consequently, the temperature of the rolls and of the material rises, and roll wear also occurs. This results in the occasional sticking of the rolls to the material. Water and/or rolling oil are supplied to the contact area between the rolls and the material as a means of lubrication to reduce the friction, and hence the rolling force and rolling torque, thereby minimizing these problems.
Taking the above factors into account, several methods of determining rolling force and its widthwise distribution have been developed. The optimum choice will depend upon the local conditions under consideration.
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All the parts that compose a rolling mill are subjected to elastic deformation by the rolling force. The amount of deformation of the rolls by the rolling force is the largest component of the vertical deformation of the whole rolling mill, accounting for 60-70% of the total amount of the deformation. The amount of deformation of the housing and screw-down device each account for 10-20%.
As shown in the figure, rolls in a 4-high rolling mill are subjected to four kinds of deformation: (i) deflection of the back-up rolls, (ii) deflection of the work rolls, (iii) flattening of the work rolls caused by contact with the back-up rolls and material, and (iv) flattening of the back-up rolls caused by contact with the work rolls. The amounts of these four types of deformation have been analyzed theoretically.
The ratio of the rolling force to the amount of vertical deformation of the whole rolling mill, including the deformation of the roll, screw-down device, and housing, is called the mill modulus. The mill modulus is 500-1,000 ton/mm for plate rolling mills and 400-600 ton/mm for cold rolling mills. The larger the diameter of the back-up rolls, the higher the mill modulus. A rolling force of the order of 1,000 tons is generated during rolling, so that mill deformation of more than 1mm occurs. Unless this deformation is taken into account, thickness accuracy cannot be ensured. Furthermore, because the mill modulus has a finite value, there exists a minimum thickness below which the rolling mill cannot reach.
The flattening deformation of the work rolls during rolling requires corrections to the calculations of the rolling force derived from deformation theory. The deflection of the work rolls results in a widthwise distribution of strip thickness in the form of a convex crown, in which the thickness is greater at the center of the width and smaller at the edges. This widthwise difference in thickness is called the strip crown. In addition, a steep decrease in thickness occurs at both edges of the strip due to the combined effects of plastic deformation in the width direction, roll flattening, and roll abrasion. This phenomenon is called edge drop. Reducing the strip crown and edge drop is the greatest challenge for materializing accurate profile in strip rolling.
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Strip thickness h2 on the delivery side of the rolls is equal to the work-roll gap under load if the elastic deformation of the material is excluded. As given by Eq. 1, h2 is the sum of S0 and S, S0 being the work roll gap without load and S the amount of deformation of the rolling mill under load. The S is given by rolling force P divided by mill modulus M, so the delivery side strip thickness is dependent upon the rolling force. The S, which is of the order of millimeters, cannot be neglected when calculating the delivery side strip thickness. Equation 1 is shown by curve (a) in the figure, which iscalled the elastic deformation curve of the mill.
Rolling force P, which can also be determined by deformation theory, is expressed in Eq. 2 as a function of material factors and rolling conditions. Mean deformation resistance km is a function of the rolling reduction, rolling speed, rolling temperature, and material chemistry. In terms of geometrical relationships, the contact arc length L is related to both the roll radius and rolling reduction, as shown in Eq. 3. Equations 2 and 3 indicate that the rolling force increases as the mean deformation resistance of the material, entry side strip thickness, and amount of rolling reduction increase. This relationship is represented by curve (b) in the figure. This curve is called the plastic deformation curve of the material in rolling. The delivery side strip thickness is determined by solving Eqs. 1, 2, and 3, and corresponds to the point of intersection of the two curves in the figure.
During rolling, the rolling force and the delivery side strip thickness change if some variation occurs in the roll gap, the mean deformation resistance caused by a variation in speed and temperature, or the entry side strip thickness. In other words, a change in the delivery side strip thickness can be instantaneously detected by monitoring the rolling force. When the rolling force changes, the delivery strip thickness can always be kept constant by adjusting So in Eq. 1 by the amount required to compensate for the rolling force difference. This is the principle of automatic gauge control (AGC).
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The figure shows the manufacturing process for hot rolled and cold rolled coils of strips. A slab about 250mm in thickness is heated in the continuous reheating furnace. After the scale breaker has removed the scale from the surface of the slab, the slab is then hot rolled by a hot strip mill which contains both roughing mills and finishing mills. The roughing mills, which are 2-high or 4-high mills of 2 to 6 stands, carry out either reversing or one-direction rolling. The finishing mills, which are 4-high or 6-high tandem mills of 5 to 7 stands, carry out continuous rolling to the final strip thickness. The thickness of strips rolled on the hot strip mill ranges from 0.8 to 25.4mm, the maximum strip width is 1.3 to 2.2m, and the rolling speed of the final stand is about 1.3 km/min. After hot rolling, hot rolled strips are cooled and coiled. For products other than as-hot rolled strips, scale is removed from the surface of the hot rolled strip in a continuous pickling line, and the hot rolled strip is then cold rolled by a tandem cold rolling mill or a reversing mill. The tandem cold rolling mills, which are 4-high or 6-high mills of 4 to 6 stands, roll strips to a minimum thickness of 0.1mm at a rolling speed of 2.5 km/min. Coils in the as-cold rolled condition become work hardened, so it is necessary to anneal the strip to the required hardness. There are two kinds of annealing: continuous annealing, in which coils are uncoiled and passed continuously through the annealing furnace, and batch annealing, in which coils are stacked and annealed in bell-type furnace. Continuous annealing is now the mainstream practice, since its productivity is much higher, and the heating and cooling rates are much faster and more controllable, and rapid cooling is possible. As the cooling rate is slow in batch annealing, a larger amount of solute carbon in a material precipitates in larger sizes and coils become softer than in continuous annealing. Since yield-point elongation occurs in annealed materials, it is necessary to apply skin pass rolling, which is called temper rolling, to prevent this problem with annealed materials.
The manufacturing process for strips achieves the target thickness. At the same time, the properties suited to the application are obtained by controlling the grain size, precipitates, and texture through the hot rolling, cold rolling, and heating and cooling processes.
Plates are usually produced by a hot reverse rolling mill, comprising a single stand roughing mill and a single stand finishing mill. Although these rolling mills are basically the same as those used for producing strips, they differ in the following points: (i) they are wider and more powerful; (ii) forward and reverse rotation of the rolls is possible; and (iii) a mechanism for rotating the slab 90 is provided before and after rolling so that products of larger width than the slab width can be produced.
	
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The most important considerations for rolling plate and strip are size control to obtain the target dimensions, and profile control to obtain flatness. Of the dimensions of flat rolled products, the target width and length can be obtained by shearing and cutting the surplus portions after rolling. However, the rolling operation itself is the sole and final means of ensuring the target thickness and profile.
With a unit coil weight of 40 tons, a hot rolled coil 3.2mm in thickness and 1m in width has a total length of 1,600m; a cold rolled coil 0.8mm in thickness and 1m in width has a total length of 6,400m. The current tandem hot strip mills or tandem cold rolling mills can roll coils of these sizes in about 1 to 3 minutes.
According to the present standard, the thickness variations in the longitudinal and width directions of the coil are within plus or minus tens of micrometer for hot rolled strip and several micrometers for cold rolled strip, as shown in the figure.
High gauge accuracy can now be maintained over the whole length of a coil as a result of accurate prediction of the rolling force from rolling theory, the improved accuracy of the elastic deformation curve of the mill based on deformation analysis, and the practical use of high-level computer control techniques with high-sensitivity sensors and quick-response actuators. Progress in theory, operational techniques, and equipment as an integrated system is expected to lead to further improvements in accuracy.
	
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Flatness failures include center buckles and edge waves. With the former, the center of the width is excessively elongated, resulting in waviness, while with the latter, the edges of the strip are excessively elongated,resulting again in waviness. Flatness failures and strip crowns are caused by widthwise differences in the thickness reduction rate.
General methods for reducing strip crown include the 4-high rolling mill, in which the work rolls are supported by back-up rolls; the use of a roll crown, in which the work roll is given a convex shape; and the use of the roll bender to deflect the roll in the direction opposite that of the predicted strip crown. All these methods have the following aims: Before entry of the strip into the rolls, the work roll gap is arranged to have a concave shape so that the center of the strip width is thinner than edges, and after entry of the strip, the surfaces of the top and bottom work rolls become parallel as a consequence of rolling. These methods are in practical use, and strip crown has progressively improved. New methods have also been developed for further improvements. These include the roll-shifting mill, in which the rolls are shifted in the widthwise direction, and the roll-cross mill, in which the roll axes are crossed.
There are two types of roll-shifting mill. One shifts the work rolls, and the other shifts the intermediate rolls. The work-roll shifting type aims to make the strip thickness uniform and improve the flatness over a wide range of strip widths by shifting rolls of special shapes, as shown in the figure. The intermediate-roll shifting type aims at greater efficiency in achieving the same objectives by shifting the intermediate rolls so that their barrel ends approach the edges of the material being rolled. In some cases, a special shape is also given to the intermediate rolls.
In the roll-cross mill, the top and bottom roll axes are positioned obliquelyto each other to adjust the roll gap. A large effect can be obtained with a crossing angle as small as 1.5. The three types of roll-cross mill are shown in the figure. The pair-roll-cross mill is commonly used in hot rolling mills for plate and strip.
	
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The figure shows an example of the sensors installed in a standard tandem cold rolling mill and the control functions of these sensors.
A rolling mill for flat products must produce products free from camber and bends while obtaining the desired thickness and width. Thickness control is carried out by repeating a process that involves (i) measuring the strip thickness with a sensor, (ii) calculating the difference between the measured and target thicknesses, (iii) converting the difference into the desired roll gap to compensate for this difference by a static control model, and (iv) adjusting the screw-down device to this roll gap. In practice, rolling is started by setting the rolling conditions given by the model so that the target values are met. During rolling, additional control is conducted to correct by the sensors and control units the deviation of measured values from the target value. This additional control is called dynamic control. Modern rolling mills are equipped with numerous sensors and control units, in addition to the basic hardware used to support and drive the rolls.
Strip thickness is adjusted by controlling the amount of rolling reduction of the work rolls, using thickness gauges as sensors installed before and after each roll stand. Control of the profile by decreasing the strip crown and edge drop is achieved by profile control units installed in the rolling mill, such as those associated with bending, shifting, or crossing the intermediate rolls and work rolls on the basis of the thickness distribution and the output of the profile detectors.
In tandem mills, rolling is usually conducted with a tensile force exerted on the material. It is therefore necessary to maintain the volumetric flow rate of the material constant between the stands as well as control the tensile force. Failure to maintain the constancy will result in the strip breaking or looping between the rolling stands. For this purpose, the rotational speed of the rolls is controlled based on the results of strip speed measurement. In recent hot rolling practice, it has become common to carry out controlled rolling, for achieving required microstructure by controlling the temperature of the material being rolled for controlled cooling in accordance with measured temperatures.
	
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Steel material hardens after cold rolling due to the dislocation tangling generated by plastic deformation. Annealing is therefore carried out to soften the material. The annealing process comprises heating, holding of the material at an elevated temperature (soaking), and cooling of the material. Heating facilitates the movement of iron atoms, resulting in the disappearance of tangled dislocations and the formation and growth of new grains of various sizes, which depend on the heating and soaking conditions. These phenomena make hardened steel crystals recover and recrystallize to be softened.
Furthermore, precipitates decompose to solute atoms which subsequently dissolve into the steel matrix on heating and holding, then reprecipitate in various sizes and distributions, depending on the rate of cooling. These changes in the size and distribution of the grains and precipitates also affect the hardness of the material.
The annealing of cold rolled coils has conventionally been conducted by grouping and annealing the coils in batches stacked in a bell-type furnace. This process is called batch annealing. However, continuous annealing is now more commonly used. This type of annealing involves uncoiling, and welding strips together, passing the welded strips continuously through a heating furnace, and then dividing and recoiling the strips. The figure shows a continuous annealing line, which is composed of the entry-side equipment, furnace section, and delivery-side equipment.
The main entry-side equipment comprises payoff reels, a welder, an electrolytic cleaning tank, and an entry looper.
The furnace section comprises a heating zone, soaking zone, and cooling zone. The cooling zone is divided into three sub-zones so that complex cooling patterns such as cooling-heating-holding-cooling can be performed. The delivery equipment comprises a delivery looper, shears, and coilers, and may be linked to a temper rolling mill and plating equipment as part of a larger continuous line.
The heating cycle applied to strips by continuous annealing differs from product to product, but the three patterns shown in the figure are typical. For cold-rolled strips for general use, it is normal practice to adopt a heating pattern in which the strip is heated to 973K (700) for about 1 minute, rapidly cooled, held at about 673K (400) for 1 to 3 minutes to precipitate the solute carbon, and then cooled to room temperature.
Although the total equipment length is 150 to 300m, the total length of the strip in the line is as much as 2,000m. The travel speed of the strip is 200 to 700 m/min. However, a recently developed line for can material passes strip 0.15mm in thickness at a maximum speed of 1,000 m/min. To operate such lines, speed control, tension control, and tracking control of the strip are necessary, in addition to a high level of automatic temperature and atmosphere control.
	
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Flat products which must provide corrosion resistance are coated after annealing. Typical hot-dip coated products include galvanized strips for automobiles, building materials and home electrical appliances, and tin- and chrome-plated strips for food and beverage cans and other containers. For reasons of efficiency, coating of continuous strip is more common than coating of cut sheets.
Coating processes are broadly divided into hot dipping and electroplating. The hot dip process is more suitable for heavy coating weights, and electroplating for lighter coatings. Electroplating is often used to apply a thin coat of expensive tin, and hot dip is used for heavy coatings of inexpensive zinc. The figure shows an example of a hot dip galvanizing line.
After passing through the pretreatment tanks for degreasing, pickling, and cleansing, the strip passes through the annealing furnace and a pot containing molten zinc. The annealing furnace is used to apply the heat cycle needed to obtain the required mechanical properties and activate the surface with a reducing gas, which makes it easy to coat zinc on the strip surface.
The coating weight is controlled by a purge gas jet blown on both surfaces of the strip from a nozzle above the pot, to remove excessive molten zinc.
The cross section of a galvanized strip is composed of the steel substrate, iron-zinc alloy layers, and a zinc layer. Because the paint adhesion and weldability of the surface of this zinc layer are not necessarily good, galvannealing has been developed to improve these properties. In the basic process for galvannealed strip, the zinc-coated strip emerges from the pot and is heated in a galvannealing furnace, forming an iron-zinc alloy layer by the interdiffusion of iron and zinc coating layer, so that the surface of the zinc layer also contains some amount of iron. The galvannealing line is usually equipped with a skinpass mill, a tension leveler, and chemical treatment equipment for chromating, following the galvannealing furnace.
For automotive steel strips, a thin iron plating is sometimes applied electrolytically to the iron-containing zinc layer to improve the sliding property between the die and material during press forming and adhesion of paints in electrostatic coating. In this case, electrolytic plating equipment is installed in the line.
Typical products from a hot dipping line are galvanized sheets and zinc-aluminum plated sheets for building materials, galvanized sheets andgalvannealed sheets for automobiles. Special products are aluminum coated sheets for car mufflers and lead-tin alloy-coated sheets (terne plates) for fuel tanks.
	
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	When electric current is properly supplied to a cathodic steel strip immersed in an electrolytic solution containing metallic ions, the metallic ions lose their electrical charges by combining with electrons and precipitate on the cathodic surface as metallic atoms.
A continuous electroplating line is composed of pretreatment equipment, plating equipment, and post-treatment equipment. The functions and construction of the pretreatment and post-treatment equipment are almost identical to those used in a hot-dip line.
To ensure uniform, efficient plating, it is important to supply the plating solution to the whole strip surface at high speed and with uniformity. As plating proceeds, metallic ions are lost from the plating solution. Quick resupply of these lost metallic ions is essential for uniform, high efficiency plating. In order to obtain a uniform plating thickness, it is necessary to ensure that the composition of the plating solution supplied to the whole strip surface should be uniform. It is also necessary with electroplating equipment to minimize the electrode distance between the strip and the anode in order to reduce electric resistance and hence power consumption. For this purpose, plating cells of the various types shown in the figure have been developed and put into practical use. Multiple cells are typically arranged in series to obtain high productivity.
Examples of electroplating are zinc, tin and chromium plating. Zinc plating is divided into pure zinc plating and alloy zinc plating such as zinc-iron and zinc-nickel, which are applied to produce electrogalvanized sheets for automobiles, home electrical appliances, and building materials. Tin plate is mainly used in food and beverage cans. The method of joining the can body has changed from soldering to cementing and welding. To reduce the consumption of expensive tin, the production of tin-free steel has increased. Tin-free steel is a surface-treated steel strip with a coating comprising an under layer of metallic chromium and an upper layer of chromium hydrate.
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H-beams have a large geometrical moment of inertia per unit weight and resist bending and twisting, and are therefore used as columns, beams, and bridge girders in architectural and civil construction. 
Products such as H-beams, whose cross-sectional shape is not rectangular, can also be produced by rolling. The figure shows the rolling equipment, forming process and names of the parts of an H-beam. 
Here, caliber rolling is conducted in the roughing stage. The materials are rolled by caliber rolls in order to obtain the same cross-sectional shape as that of the rolls. After producing a near H shape by caliber rolling, the product is finished by a universal mill and an edging mill. An H-shaped cross-section is formed when the material passes through four rolls, making the universal mill, which is equipped with a pair of vertical rolls and a pair of horizontal rolls, suitable for rolling H-beams. The edging mill is equipped with caliber rolls as shown in the figure, and has the function of adjusting the flange widths of products. 
In the universal mill, variations of flange- and web- thickness can be made easily by adjusting the roll gap. However, when products with different web heights and flange widths are to be rolled, it is necessary to employ exclusive-use rolls for these sizes, necessitating roll changes. In particular, since the web heights are determined by sum of the width of the horizontal rolls and flange thickness, it has to date been necessary to have the same number of horizontal roll sizes as product web heights. Development to overcome this problem has resulted in recent rolling mills and rolling techniques capable of adjusting the web heights by one roll with changeable width without changing rolls. 
By combining caliber rolling with universal rolling, it is also possible to roll steel products of non-H shape, such as sheet piles, channels, angles, and rails. 
	
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Steel tubes and pipe can be broadly categorized according to manufacturing method as seamless tubes and pipe made by hot rolling or hot extrusion, and welded pipe and butt-welded pipe made by bending and welding sheets or plates.
When seamless pipe is made by rolling, the rolling method involves piercing the material while it is being rolled, and is suitable for mass production. The figure shows the manufacturing process used in the Mannesmann plug mill, which is a typical rolling process. The Mannesmann-type piercer reduces the material by rolls that are inclined obliquely to each other. When the round billet is rotated while being compressed in the diametric direction, the central part of the billet becomes loose, which makes it easy to pierce a hole through the center. This is called the Mannesmann effect. The pierced portion is expanded by the elongator, and the wall thickness is then thinned and elongated by the plug mill. The internal and external surfaces are smoothed by the reeler, and the final dimensional adjustments are made by the sizer.
The hot-extrusion method involves working in the compressive-stress field. Therefore, it is characteristic of this method that high-alloy steel pipe of low deformability can be produced, as well as heavy-wall and large-diameter pipes.
Seamless pipe has outstanding homogeneity in the circumferential direction and is thus highly resistant to internal pressure and torsion. Taking advantage of this feature, seamless pipe is widely used for drilling and pumping petroleum and natural gas.
Welded pipe is divided into electric-resistance welded (denoted ERW hereinafter) pipe, spiral pipe, and UO pipe according to the forming and welding method. ERW pipe and butt-welded pipe are produced by continuously forming a hot-rolled coil into a tubular shape by forming mills. ERW pipe is produced by cold forming, and the seam is welded by electric-resistance welding. This type of steel pipe is used in large quantities as line pipe for transporting petroleum and gas. Butt-welded pipe is produced by hot forming after the whole material has been heated, and seams are then butt welded. This type of pipe is hot-dip galvanized and used for carrying water and gas.
The outer diameter of ERW pipe and butt-welded pipe is determined by the width of the material coil. However, as shown in the figure, spiral pipe is made by forming the coil into a spiral shape, which makes it possible to obtain a large outer diameter regardless of the width of the material. UO pipe is usually large in diameter and produced one piece at a time by forming plates. The plate is first pressed into a U shape by the U-press, and then into an O shape by the O-press. 
Because relatively thick material is used for making spiral and UO pipes, submerged arc welding is used for joining. The principal application of spiral pipe is pipe piles. UO pipe, as mentioned above, is mainly used as line pipe for transporting petroleum and natural gas in large quantity over long distances.
	
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	In the conventional process for hot strip production, slabs about 250mm in thickness are reheated in a furnace, and hot rolled one after another by a roughing mill to 30 to 60mm in thickness, and then rolled by a finishing mill to target thicknesses with a range of 0.8- 25.4mm.
In this process, the head and tail ends of each strip are finish-rolled and both ends run out to the coiler without tension between the coiler and finishing mill stands, and hence tend to pass unstably on the roller table, for instance, jumping and/or weaving. This tendency is larger with thinner thicknesses, and is one reason that hot strip with a thickness of less than 0.8mm cannot be produced by the conventional process.The two ends of the strip are different from the middle in size, profile, and properties because of the instability in the finish rolling. The ends are generally cut off to produce a uniform strip, but this results in a yield loss equal to the amount of the discarded ends. Furthermore, the interruption of the rolling operation caused by the rolling interval between slabs and slow starting and finishing speeds of finish rolling to avoid unstable rolling reduce productivity.
Yield and productivity can be improved by continuous rolling of several slabs, which is termed "endless rolling". Endless rolling was realized for the first time in the world in the new Hot Strip Mill at Kawasaki Steel Corporation's Chiba works in 1996. The layout of the plant is shown in the figure. The main equipment of the plant comprises three reheating furnaces, a sizing press, which reduces the slab width, three roughing mills, a coil box, a joining device (sheet bar welder), a 7-stand finishing mill, a strip shear, and two down coilers.
To conduct endless rolling, it is necessary to continuously supply sheet bars to the finishing mill by joining the tail end of the preceding sheet bar to the head end of the succeeding sheet bar at the joining device. Joining must be finished before the tail end of the preceding sheet bar is fed into the first stand of the finishing mill. The permissible duration for joining is therefore shorter than the finish rolling duration of one sheet bar, and usually is about 20 seconds. Furthermore, the head end of the succeeding sheet bar must arrive at the joining device at the proper timing for joining. A coil box with three positions of retained sheet bar coils is provided to perform the following functions: Sheet bar rolled by the roughing mill is coiled at the leftmost coiling position, held for adjustment of the joining timing at the waiting position in the middle, and then is uncoiled at the rightmost uncoiling position.
After finish rolling followed by cooling, the strip is divided by the high speed strip shear installed before the down coilers. In dividing and coiling strips, it is important to instantaneously stop the tail end of the preceding strip and to guide the head end of the succeeding strip running at a high speed to the next coiler.
In endless rolling, joined sheet bars can be finish-rolled stably at a constant speed over their whole length without interruption, improving the uniformity of strip qualities such as dimension, profile, and microstructure through the whole length. Yield is also improved because crops are not cut. Yield and productivity are estimated to increase by 0.5-1.0% and by 20%, respectively. Furthermore, with this new process, hot strip with a thickness of less than 0.8mm can be produced.
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