FERROUS METALS. Steel Structures Steel in Structures.

FERROUS METALSFERROUS METALS

Steel StructuresSteel Structures

Steel in StructuresSteel in Structures

Steel ProductionSteel Production

Ferrous metals are those metals Ferrous metals are those metals that contain Iron.that contain Iron.

The steel production process might be divided into three phases:– Reduction of iron to pig iron– Refining pig iron to steel– Forming the steel into products

IronIron

Iron is extracted from Iron is extracted from iron oresiron ores such as such as Hematite (FeHematite (Fe22OO33) and Magnetite (Fe) and Magnetite (Fe33OO44))

The iron ores contain 25% to 70% The iron ores contain 25% to 70% metallic iron. Sulfur, phosphorous, silica metallic iron. Sulfur, phosphorous, silica and clay are the principal impurities.and clay are the principal impurities.

Materials used to produce pig iron are coke, limestone and iron ore.

Iron ore, coke and limestone are heated Iron ore, coke and limestone are heated together at high temperatures in blast together at high temperatures in blast furnaces for the extraction process of furnaces for the extraction process of iron.iron.

Pig Iron (Raw Iron)Pig Iron (Raw Iron) Pure iron is a soft metal having a structure of

iron crystals. In metallurgy, pure iron is called “Ferrite”

CokeCoke → provides the heat & supplies carbon (C) to → provides the heat & supplies carbon (C) to extract ironextract ironC + OC + O22 → CO → CO22

COCO22 + C → 2CO + C → 2CO3CO + Fe3CO + Fe22OO33 → 2Fe + 3CO → 2Fe + 3CO22

Limestone is used to reduce the impurities. Ordinary pig iron as produced by blast furnaces Ordinary pig iron as produced by blast furnaces

contains iron, about 92 percent; carbon, 3 or 4 contains iron, about 92 percent; carbon, 3 or 4 percent; silicon, 0.5 to 3 percent; manganese, percent; silicon, 0.5 to 3 percent; manganese, 0.25 to 2.5 percent; phosphorus, 0.04 to 2 0.25 to 2.5 percent; phosphorus, 0.04 to 2 percent; and a trace of sulfurpercent; and a trace of sulfur..

CAST IRONCAST IRON

When pig iron is further processed by When pig iron is further processed by remelting to eliminate some of the carbon, remelting to eliminate some of the carbon, cast iron (having a carbon content of about cast iron (having a carbon content of about 1.5 to 4%) is produced. 1.5 to 4%) is produced.

The remelting process is usually performed in The remelting process is usually performed in a cupola (a smaller version of blast furnace).a cupola (a smaller version of blast furnace).

During the remelting operation in the cupola, During the remelting operation in the cupola, no particular chemical change in the iron is no particular chemical change in the iron is ecpected. Some of the impurities may be ecpected. Some of the impurities may be eliminated and a more uniform product is eliminated and a more uniform product is obtained.obtained.

CAST IRONCAST IRON

After the treatment in the cupola the After the treatment in the cupola the molten iron is cast into forms of molten iron is cast into forms of desired shape. Depending on the rate desired shape. Depending on the rate of cooling the final product is called as:of cooling the final product is called as:1. Gray Cast Iron1. Gray Cast Iron

2. White Cast Iron2. White Cast Iron

GRAY CAST IRONGRAY CAST IRON

When cast iron is allowed to cool slowly, most of When cast iron is allowed to cool slowly, most of the free carbon solidifies in large crystals known the free carbon solidifies in large crystals known as graphite. as graphite.

A small part of the carbon combines with iron to A small part of the carbon combines with iron to form cementite (Feform cementite (Fe33C).C).

– Iron and Carbon unit to form Iron Carbide (Fe3C) “Cementite” with the ratio of 1 Carbon : 14 Iron.

– Cementite: is very hard and brittle substance so the more cementite the iron contains the more it gets harder.

Dark spots in this microscopic view of steel are cementite

WHITE CAST IRONWHITE CAST IRON

When cast iron is not allowed to cool When cast iron is not allowed to cool slowly, the amount of cementite slowly, the amount of cementite increases and the amount of graphite increases and the amount of graphite decreases. decreases.

Most carbon is reacted with Fe to get Most carbon is reacted with Fe to get cementite (Fecementite (Fe33C). C).

Therefore, white cast iron is strong Therefore, white cast iron is strong and hard but brittle. and hard but brittle.

Moreover, since it is rapidly cooled it Moreover, since it is rapidly cooled it has high initial stresses.has high initial stresses.

Malleable Cast Iron: is obtained by Malleable Cast Iron: is obtained by annealing the white cast iron.annealing the white cast iron.

Annealing is the process of heating and Annealing is the process of heating and cooling to induce softening, which will cooling to induce softening, which will eliminate the initial stresses. eliminate the initial stresses.

Moreover, this type of cast iron is also Moreover, this type of cast iron is also shaped by a hammer or by the pressure of shaped by a hammer or by the pressure of rollers.rollers.

It has some ductility.It has some ductility.

MALLEABLE CAST IRONMALLEABLE CAST IRON

CAST IRONCAST IRON

All cast irons, in general, are brittle All cast irons, in general, are brittle materials. materials.

They are easy to form shapes, by They are easy to form shapes, by casting into molds. casting into molds.

They are cheaper than forming steel They are cheaper than forming steel shapes.shapes.

STEELSTEEL

Steel is also produced from pig iron by Steel is also produced from pig iron by removing the impurities and by removing the impurities and by decreasing the carbon content.decreasing the carbon content.

Pig iron is again heated and the Pig iron is again heated and the excess carbon is removed as COexcess carbon is removed as CO22 gas gas and the oxides of other impurities and the oxides of other impurities form a slag on top of the molten steel.form a slag on top of the molten steel.

Composition of SteelComposition of Steel

The essential difference between cast iron and steel is in the amount of carbon contained in the constituency of the metal.

Steel is fundamentally an alloy of iron and carbon with carbon content less than 1.5% while cast iron is an alloy of iron and carbon with carbon content ranging between 1.5 to 4% .

They are first cast into simple shapes, They are first cast into simple shapes, “ingot”“ingot”

Later ingots are given a preliminary shaping by Later ingots are given a preliminary shaping by being rolled or forged into “billets”.being rolled or forged into “billets”.

Finally, desired shapes are obtained from billets Finally, desired shapes are obtained from billets by:by:• ForgingForging• RollingRolling• Extrusion Extrusion • DrawingDrawing

Shaping Structural SteelShaping Structural Steel

FORGING

Place a billet in a die and apply Place a billet in a die and apply pressure with a suitably pressure with a suitably shaped punch. shaped punch.

As the metal is forced into position, it is As the metal is forced into position, it is stressed above the proportional limit and stressed above the proportional limit and refinement in grain boundaries or goes into refinement in grain boundaries or goes into strain hardening.strain hardening.

Thick plates, sheetings and objects of Thick plates, sheetings and objects of irregular shape are produced by forging.irregular shape are produced by forging.

ROLLINGROLLING

Hot RollingHot Rolling: If the temperature of the : If the temperature of the metal is above its critical temperature metal is above its critical temperature which causes recrystallization of the which causes recrystallization of the crystallic structure. Properties will not crystallic structure. Properties will not be affected greatly.be affected greatly.

Cold RollingCold Rolling: If the temperature is : If the temperature is below the critical temperature the below the critical temperature the initial crystallic structure will be initial crystallic structure will be maintained but the properties will maintained but the properties will change. The strength is increased but change. The strength is increased but the ductility is decreased.the ductility is decreased.

EXTRUSIONEXTRUSION

Applying sufficient pressure to the Applying sufficient pressure to the material by forcing it through a die material by forcing it through a die which has the required constant cross-which has the required constant cross-section (I-beams, channel section)section (I-beams, channel section)

DRAWINGDRAWING

A rolled “rod” is pulled through a die having A rolled “rod” is pulled through a die having an opening smaller than the original cross-an opening smaller than the original cross-section of the rod. Wires and some rods are section of the rod. Wires and some rods are prepared by this method.prepared by this method.

FACTORS THAT AFFECT FACTORS THAT AFFECT PROPERTIES OF STEELSPROPERTIES OF STEELS

Carbon contentCarbon content Heat treatment and shaping methodHeat treatment and shaping method Presence of harmful elementsPresence of harmful elements Presence of alloying materials.Presence of alloying materials.

Carbon ContentCarbon Content For steel C For steel C → → ~~0.05-20.05-2%% usually usually <<1.51.5%% It affects both strength & ductility. As it It affects both strength & ductility. As it

increases, strength increases but ductility increases, strength increases but ductility decreases.decreases.

If C If C << 0.5% 0.5% →→ well defined yield point well defined yield point E is the same : 2.1x106 kgf/cmE is the same : 2.1x106 kgf/cm22

Low Carbon SteelsLow Carbon Steels: (C: (C<<0.20.2%%) : soft & very ductile ) : soft & very ductile “commercial steels” “commercial steels” →→ used for construction. used for construction.

Medium Carbon SteelsMedium Carbon Steels: (C: (C<<0.50.5%%) : used in ) : used in machine parts machine parts && reinforcing bars reinforcing bars

High Carbon SteelsHigh Carbon Steels: (C: (C>>0.50.5%%) : used in ) : used in production of tools such as drills, saw blades. production of tools such as drills, saw blades. They are very hard.They are very hard.

Carbon ContentCarbon Content

Cold Drawn & Cold Rolled Cold Drawn & Cold Rolled → No → No microstructural changes. Changes are microstructural changes. Changes are plastic deformations in the form of plastic deformations in the form of elongation of grains.elongation of grains.

Hot Rolled → Microstructural changesHot Rolled → Microstructural changes Drawn & Annealed → Initial stresses are Drawn & Annealed → Initial stresses are

releaved.releaved.

Heat Treatment and Heat Treatment and Shaping MethodsShaping Methods

Heat Treatment and Heat Treatment and Shaping MethodsShaping Methods

Harmful ElementsHarmful Elements

SulfurSulfur: makes steel brittle at high : makes steel brittle at high temperature (limited to temperature (limited to → → <<0.050.05%%). ). May become important in hot-rolling.May become important in hot-rolling.

PhosphorousPhosphorous: makes steel brittle at low : makes steel brittle at low temperature (limited to temperature (limited to → → <<0.050.05%%). ). May become important in cold-rolling.May become important in cold-rolling.

Oxygen, Hydrogen & NitrogenOxygen, Hydrogen & Nitrogen

Alloying ElementsAlloying Elements

Alloying elements are added to steel:Alloying elements are added to steel:– Higher strength with ductilityHigher strength with ductility– Higher resistance to corrosionHigher resistance to corrosion– Higher resistance to heatHigher resistance to heat

Chromium & Nickel are the most Chromium & Nickel are the most important alloying elements. important alloying elements. “Stainless Steel” has “Stainless Steel” has ~~2020%% Chromium Chromium & 8% Nickel. & 8% Nickel.

GENERAL USES OF STEEL GENERAL USES OF STEEL FOR CONSTRUCTIONFOR CONSTRUCTION

Rolled Steel SectionsRolled Steel Sections: (I-beams, Wide : (I-beams, Wide Flange I-beams, Channel sections...) Flange I-beams, Channel sections...) are used for beams, columns and in are used for beams, columns and in trusses.trusses.

Round Steel BarsRound Steel Bars: are used : are used extensively in R/C structures as extensively in R/C structures as reinforcement and ties.reinforcement and ties.

STEEL BARS FOR STEEL BARS FOR CONCRETE REINFORCEMENTCONCRETE REINFORCEMENT

Concrete Concrete → for compression→ for compression Steel Bar → for tension therefore used in Steel Bar → for tension therefore used in

tension zones.tension zones.→ → Plain Bars: smooth surfacesPlain Bars: smooth surfaces→ → Deformed Bars: to increase the Deformed Bars: to increase the

bond characteristics they have some bond characteristics they have some deformation on the surfacesdeformation on the surfaces

→ → Wire Mesh: Welded at joints & used Wire Mesh: Welded at joints & used in slabs.in slabs.

They are produced usually by a hot rolling They are produced usually by a hot rolling operation, although some steel bars are operation, although some steel bars are cold rolled. Small diameter bars are cold rolled. Small diameter bars are usually cold drawn.usually cold drawn.

A cold rolled or cold drawn steel has A cold rolled or cold drawn steel has higher strengths but less ductility higher strengths but less ductility (Because of strain hardening!). Therefore, (Because of strain hardening!). Therefore, not desired.not desired.

Nominal diameter of plain bars Nominal diameter of plain bars → can be → can be measured.measured.

Nominal diameter of deformed bars → Nominal diameter of deformed bars →

D = 12.74

W

L

For reinforcing bars For reinforcing bars → yield strength, → yield strength, ultimate strength & ductility are important.ultimate strength & ductility are important.

TS 708 → C should be TS 708 → C should be <<0.25% →plain0.25% →plain<<0.4% → def.0.4% → def.

In TS 500 & TS 708 → S220a, S420a, In TS 500 & TS 708 → S220a, S420a, S420b, S500aS420b, S500a

S:steel a: hot rolled b: cold worked S:steel a: hot rolled b: cold worked 220: minimum yield point (MPa)220: minimum yield point (MPa)