clase de matalurgia

7/30/2019 Clase de Matalurgia

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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 thedirection 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 20 m.

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 changein 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 "solidsolution", 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 isexceeded. 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 thesteel, all the carbides that have been precipitated will decompose and dissolve.


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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), butas 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 thestrength, 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 crystalplane. 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 difficult for

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


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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 ofcrystal 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, thisfrictional 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. Thedistribution 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 permitsthree-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.


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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 istaken 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. Thisphenomenon 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, h 2 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 is

called 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


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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 itis 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 resultof accurate prediction of the rolling force from rolling theory, the improved accuracy of


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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.

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 edgesof 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


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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 ofspecial 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 obliquely to 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 therolls.

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 constantbetween 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. Thesephenomena 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 stripscontinuously through a heating furnace, and then dividing and recoiling the strips. The


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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 dipgalvanizing 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 tothe 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 and galvannealed 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 thepretreatment 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, platingcells of the various types shown in the figure have been developed and put into practical


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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 methodof 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

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 andnames of the parts of an H-beam.


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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 verticalrolls 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 techniquescapable 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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clase de matalurgia

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